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I have a compound which I recently studied its photophysical properties. I have observed that when I use a non-polar medium, like DCM, the quantum yield is higher than when I use a polar medium like DMF. Moreover, the fluorescence lifetime is also nearly twice in non-polar medium than in the polar medium. What could be the prospective cause for this observation? Any possible suggestions?
- Bidyut
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Dear Bidyut,
Beyond rechecking your fluorescence yield measurements, you should ask yourself about the solvatochromism of the molecule. By changing the solvent, the nature of the excited state may change. That can be seen in the absorption and emission spectra and that will change the fluorescence rate constant kf.
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I'm a PhD student in polymer physics particularly on photophysics of conjugated polymers. While reviewing literature I come across an article by Shu Hu and coworkers on 'Effect of Thermal Annealing on Conformation of MEH-PPV Chains in Polymer Matrix: Coexistence of H- and J-Aggregates' in which they assign the emission peak (I00) of H-type aggregates at lower energy than J-type aggregate when they coexist, which, I think, contradicts Kasha theory.
Some body help me explain this.
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Thanks a lot!
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I have synthesized a compound which is pure as well as no other possible diastereomers are present in it, as evident from ¹H NMR. On inspection of its photophysical properties an anomaly was noted as described below:
- when the compound was excited in the region (~400 - 450 nm) of its highest absorbance a low intensity emission peak was observed at about 500-550 nm with indication of excimer like entity formation (600 - 650 nm) on varying the concentrations.
- On the other hand , excitation of the molecule at a peak of lower absorbance, near to the λmax around 350 nm a very high intensity and sharp single emission peak was observed around 380 nm with no excimer like entity at higher concentrations.
What could be the plausible reasons behind these observations?
I would be grateful for valued suggestions, if any.
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If your fluorescence spectrometer allows, I would suggest doing a full EEM (excitation emission matrix) measurement. Or at least measure excitation spectra for your different emission bands. If excitation spectra are the same for two bands, there must be an excited state process leading to those two emissions. If they are differing, there must be two ground state compounds present with different absorbance spectra. A highly fluorescent impurity might get unnoticed from measuring absorbance spectrum or NMR, but still exist in small amounts.
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I know that transition metal complexes have attractive photophysical properties. But, Why pepole mostly working with Ruthenium. Secondly, How ligand affect the specificity to organelles.
Moreover, how we can say that this kind of ligand with metal specific to Mitochondria or lysosomes etc.
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Dear Abdul,
this is a very interesting technical question. We did only very little work with ruthenium complexes in the past. However, I think that the main advantages of ruthium complexes are that (a) ruthenium has different stable oxidation states, (b) most ruthenium complexes are kinetically stable (i.e. they don't easily decompose in solution), and (c) through the use of specially designed ligands they can be made water-soluble so that they can be utilized under physiological conditions. Various ruthenium complexes are now well established in the diagnosis and treatment of cancer. For a good overview about this topic please have a look at the following very interesting review article:
Applications of Ruthenium Complex in Tumor Diagnosis and Therapy
Fortunately this review has been posted by the authors as public full text on RG. Thus you can freely download it as pdf file.
Good luck with your research work!
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Hello RGers,
I am trying to model the reaction of triplet oxygen with organics. The reaction starts as a triplet overall, but I believe it would end as two doublets at infinite separation (HOO and an organic radical), which might further react to give a singlet. Clearly the spin is not being conserved, so I think I'm missing some important point. I have modelled this using HF, B3LYP, and MP2 to start.
My experience is predominantly in closed -shell inorganic species. Are there some key references that I should consult? My hunch is that there is some "photophysical" process like intersystem crossing that I will need to model.
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Hi Cory
No specific answer from me; instead, I would recommend searching the GAMESS (US) group, https://groups.google.com/g/gamess which has proved an excellent resource for me in the past.
All the best
Darren.
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Hi, can anyone discuss how to do multivariate curve resolution for the transient absorption spectra of bulk heterojunction organic solar cells? Thank you in advance.
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I have not done it myself, but see for example:
The time evolution can be tracked using the kinetic constraints:
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I am working on making nanocomposites from silicon nanocrystals and thiolate. I get a massive change in the photophysical properties of some composites. However, other composites did not show me any change in photophysical properties. I hope anyone can go more in-depth and explain how the interaction between radical thiolate and the surface of silicon nanocrystals works. Let us focus on the chemistry point of view.
I appreciate your time in answering my question.
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would you please tell us what is the optophysiacl property which you assess after building the composites?
As Yuri hinted, the change of complex building is possibility of passivating the dangling bonds on the silicon surface.
It is known in order to improve the optoelectronic properties of the silicon grains , one passivates their active surface states by for example hydrogenation of oxidation. The mechanism is to saturate the dangling bonds.
Best wishes
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I have seen a number of publications state that organic radicals often exhibit quenched photo-luminescence, yet offer no explanation as to why. Can someone offer a physical explanation as to why this would be the case? This question is particularly puzzling to me as you can clearly see polaronic or excitonic absorption bands in an absorption spectrum corresponding to the excitation of the radical anion. Why then does it not luminesce when excited at these wavelengths?
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All recombination mechanisms can be defined as radiative or non-radiative.
Photoluminescence measurements reveal radiative recombination processes. However, if they're intentionally or unintentionally added impurities or intrinsic defects they can form deep localized energy levels in the bandgap (gap of forbidden energies between conduction and valance bands or LUMO and HOMO). These deep traps act as efficient recombination centers according to Shockley-Read-Hall (SRH) statistics. As opposite to exciton or band-to-band bimolecular recombination, trap assisted recombination is non-radiative (usually becomes radiative only at very low temperatures in the form of additional PL peak at lower photon energy than bandgap). Since one additional non-radiative recombination path is added the photoluminescence signal is quenched because the net radiative recombination rate is reduced.
This paper is a nice example.
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Hello everyone,
i need to know, why there is an red shift in linear absorbance spectra as it measured from different solvents (Methanol,acetone and DMF). is is due to polarity of solvents? or any other kind of reasons?
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Hey Vinay,
yes, polarity is the main reason effecting the general shape and band position in absorbance. If you observe significant bathochromic shifts with increasing solvent polarity you might have a charge transfer absorbance. You will always observe it in cases were the excited state shows a higher degree of charge delocalization than the ground state.
To be sure about the charge transfer character, it is advisable to check for emission spectroscopy as well - there a similar bathochromic shift should be observed (in general with decreasing emission intensity for higher polarity).
Also, if soluble, I would recommend to measure again in low polarity solvents - like toluene or slightly higher in polarity anisole or chloro-benzene.
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Dear Colleagues,
I am relatively a newcomer to the amazing fields of photophysics and photochemistry.
From the available scientific literature, we may read that induced chlorophyll a fluorescence is mainly emitted by chlorophyll a molecules, located in Photosystem II (PSII), upon illumination onset. It has been reported about 300-500 chlorophyll a molecules in a single Photosystem II.
PSI fluorescence is constant and much lower than fluorescence from PSII. Its contribution to emitted plant fluorescence is considered negligible.
Some authors speak about P680 (a pigment named P680, located in Photosystem II), the reaction center RC or the special pair or the special chlorophyll dimers pigments PD1 or PD2, as the only source of fluorescence. I have a bit of confusion because it is not clear what chemical species is emitting the fluorescence that we can sense with our portable fluorometers.
1) If the Special Pair or RC is closed (it has been chemically switched to its reduced state), during the time that the Special pair is in that state, is the full bunch of chlorophyll a molecules in PSII going to dissipate their excitonic energy as fluorescence?
2) Why fluorescence emitted from PSI is not variable but constant? Does it has this fact something to do with the ratio [Chl a] to [Chl b] ???
Thank you so much in advance for your precious and kind help!
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Howdy Stancho,
First Chlrorphyll fluorescence is not a photophysical or photochemical phenomenon, but a photobiological one.
Chlorophyll fluorescence is light re-emitted by chlorophyll molecules (yes
chlorophyll!) during return from excited to non-excited states. It is used as an indicator of photosynthetic energy conversion in higher plants, algae and bacteria. Excited chlorophyll dissipates the absorbed light energy by driving photosynthesis (photochemical energy conversion), as heat in non-photochemical quenching or by emission as fluorescence radiation. As these processes are complementary processes, the analysis of chlorophyll fluorescence is an important tool in plant research with a wide spectra of applications.
The Kautsky effect
Upon illumination of a dark-adapted leaf, there is a rapid rise in fluorescence from Photosystem II (PSII), followed by a slow decline. First observed by Kautsky et al., 1960, this is called the Kautsky Effect. This variable rise in chlorophyll fluorescence rise is due to photosystem II. Fluorescence from photosystem I is not variable, but constant.
The increase in fluorescence is due to PSII reaction centers being in a "closed" or chemically reduced state. Reaction centers are "closed" when unable to accept further electrons. This occurs when electron acceptors downstream of PSII have not yet passed their electrons to a subsequent electron carrier, so are unable to accept another electron. Closed reaction centres reduce the overall photochemical efficiency, and so increases the level of fluorescence. Transferring a leaf from dark into light increases the proportion of closed PSII reaction centres, so fluorescence levels increase for 1–2 seconds. Subsequently, fluorescence decreases over a few minutes. This is due to; 1. more "photochemical quenching" in which electrons are transported away from PSII due to enzymes involved in carbon fixation; and 2. more "non-photochemical quenching" in which more energy is converted to heat.
PSII yield as a measure of photosynthesis
Chlorophyll fluorescence appears to be a measure of photosynthesis, but this is an over-simplification. Fluorescence can measure the efficiency of PSII photochemistry, which can be used to estimate the rate of linear electron transport by multiplying with light intensity. However, researchers generally mean carbon fixation when they refer to photosynthesis. Electron transport and CO2 fixation correlate well, but may not correlate in the field due to processes such as photorespiration, nitrogen metabolism and the Mehler reaction.
Relating electron transport to carbon fixation
A powerful research technique is to simultaneously measure chlorophyll fluorescence and gas exchange to obtain a full picture of the response of plants to their environment. One technique is to simultaneously measure CO2 fixation and PSII photochemistry at different light intensities, in non-photorespiratory conditions. A plot of CO2 fixation and PSII photochemistry indicates the electron requirement per molecule CO2 fixed. From this estimation, the extent of photorespiration may be estimated. This has been used to explore the significance of photorespiration as a photoprotective mechanism during drought.
Fluorescence analysis can also be applied to understanding the effects of low and high temperatures.
  • Sobrado (2008) investigated gas exchange and chlorophyll a fluorescence responses to high intensity light, of pioneer species and forest species. Midday leaf gas exchange was measured using a photosynthesis system, which measured net photosynthetic rate, gs, and intercellular CO2 concentration. His results show that despite pioneer species and forest species occupying different habitats, both showed similar vulnerability to midday photoinhibition in sun-exposed leaves.
Measuring stress and stress tolerance
Chlorophyll fluorescence can measure most types of plant stress. Chlorophyll fluorescence can be used as a proxy of plant stress because environmental stresses, e.g. extremes of temperature, light and water availability, can reduce the ability of a plant to metabolise normally. This can mean an imbalance between the absorption of light energy by chlorophyll and the use of energy in photosynthesis.
  • Favaretto et al. (2010) investigated adaptation to a strong light environment in pioneer and late successional species, grown under 100% and 10% light. Numerous parameters, including chlorophyll a fluorescence, were measured. Overall, their results show that pioneer species perform better under high-sun light than late- successional species, suggesting that pioneer plants have more potential tolerance to photo-oxidative damage.
  • Neocleous and Vasilakakis (2009) investigated the response of raspberry to Boron and salt stress. Leaf chlorophyll fluorescence was not significantly affected by NaCl concentration when Boron concentration was low. When Boron was increased, leaf chlorophyll fluorescence was reduced under saline conditions. They be concluded that the combined effect of Boron and NaCl on raspberries induces a toxic effect in photochemical parameters.
  • Lu and Zhang (1999) studied heat stress in wheat plants and found that temperature stability in Photosystem II of water-stressed leaves correlates positively and well, to the resistance in metabolism during photosynthesis.
Nitrogen Balance Index
A portable multiparametric fluorometer using the ratio between chlorophyll and flavonols can be applied to detect nitrogen deficiency in plants Because of the link between chlorophyll content and nitrogen content in leaves, chlorophyll fluorometers can be used to detect nitrogen deficiency in plants, by several methods.
Based on several years of research and experimentation, polyphenols can be assigned as indicators of the nitrogen status of a plant. For instance, when a plant is under optimal conditions, it favours its primary metabolism and synthesises the proteins (nitrogen molecules) containing chlorophyll, and few flavonols (carbon-based secondary compounds). On the other hand, in case of lack of nitrogen, we will observe an increased production of flavonols by the plant.
The NBI (Nitrogen Balance Index), allows the assessment of nitrogen conditions of a plant by calculating the ratio between Chlorophyll and Flavonols (related to Nitrogen/Carbon allocation) .
Hence Stancho,
I hope this short summary elucidates soma aspects of the photobiology of plant chlorophyll fluorescence. A lot more information is available. For example models to simulate plant fluorescence. An interesting one can be found at:
Success with your studies,
Frank
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Many photosensitizers have a singlet oxygen quantum yield (ΦΔ) equal 0.99 (or 1). Does it mean that these photosensitizers are exclusively acting via type II photochemical reactions?
(ΦΔ=1 → exculsive singlet oxygen producer ???)
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No.The example of m-THPC (Temoporphin,active ingredient of Foscan) shows experimentally that even if ISC~1 the competitive reactions are taking place in bulk media and both pathways exist and coexist.The equilibrium of O radical and O2 Superoxide when established is controlled by thermodynamics at formation and thenafter by distribution to products.Actually ur statement IN QUESTION is erroneous since for this to happen ISC Quantu,m yield should equal singlet oxygen yield.This is an ideal figure in practice!Oxygen Quantum yield doesn't measure ISC yield,it is allways a fraction of:if it`s over 70%-80% than Pathway II predominates Pathway I AND VICE-VERSA.You posed the question like an axiome in math."accordingly":" many photosensitizers have...",these are either erroneous results=artifacts...while in practice no photosensitizer has or can have quantum yield of 1 in singlet oxygen quantum yield !
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The recoil force of radiation is known for spontaneous emission (for the radiation of an accelerating charge or dipole), when the photon field is empty. Is there any difference when stimulated emission is considered? Would it be enough to add an external force to the original radiative reaction-force without changing the original form of the radiative reaction?
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more precisely:
" The net change regarding momentum and energy exchange with EM field
is identical to SPONTANEOUS emission, isn't it?"
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What will be the sign of zeta potential for amine functionalized nanoparticles? The amines are neutral (not protonated). Due to the electron rich character of amine functional group shall I expect net negative charge?
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Zeta potential for amine functionalized nanoparticles can be negative. This depends on the effect of other side group positioned with amines. As zeta potential also depicts adsorption mechanism of ligands around nanoparticles.
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Did anyone observe the suppression of low fluorescence signals from one organelle due to high signals from other organelles?
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The word you are looking for is "dynamic range". The dynamic range in your images seems to be too small to see the weak signal (peroxisome) without saturating your strong signal (plastid). If I understood you correctly.
There are several ways you can try to improve on this. First, what bit-depth are you using, usually you can set the readout from the PMT to 8 or 12 bit, 16 bit on some systems. While this not increase the dynamic range as such, it subdivides it into more steps (256 for 8 bit, 4096 steps for 12 bit). Higher bit will make the files larger and might affect acquisition speed, but might give you more details on your weak structures.
Another option is HDRi (high dynamic range imaging), which you might know from your phone. In principle this can also be applied to microscopy images. A series of exposures is taken with different settings and then combined into a single image by software. Some microscope software has this function integrated but it can be done externally (e.g. using imageJ). However, be aware that this is a non-linear operation and the resulting image will not be quantitative anymore.
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We are working on the photophysical and electrochemical properties of some acridinedione derivatives. The dyes are acts as very good electron acceptor in its excited state. So, we are interested to determine the reduction potential of the dyes. When performing cyclic voltammetry, I was unable to observe the reduction peak but the reduction potential of the same dye was reported using pulse radiolysis technique. 
Can there be a system which undergo reduction but the corresponding process is not observable in cyclic voltammetry? What may be the possible reasons for this phenomena? 
Please anyone explain.
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@ Kumaran
Yes Sir.. We have tried in different solvent systems... But we didn't observe the reduction peak..
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Hi all, 
Photophysical studies of a molecule which is under development was done by using uv-vis spectoscopy. By inctreasing concentration gradually, the intensity of one of the signal was observed. This falls under the category of "hyperchromic shift". In literature, bathochromic (for J-aggregation)  and hypsochromic (for H-aggregation) are well known. But hyperchromic shifts are not well documented. 
Can Hyperchromic shift  cause these type of agglomerations also?
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At higher concentration due to intermolecular interaction (agglomeration) the absorption coefficient of the molecule/ aggregate might change. At what concentration you are working?  At higher concentration Lambert-Beer's plot will show non-linearity. For a equilibrium between two species (monomer and aggregation) you should also see an isobestic point. A temperature dependent study will shift the equilibrium and an isobestic point should be observed (assuming temperature itself doesn't change the absorption coefficient). You also need to independently confirm that aggregates are formed at that concentration. A DOSY NMR will help here. I would also like to see the spectra.
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Why we get no absorbance at higher energies compared to band gap energy of Rhodamine 6G?
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Start with first year UV-spectroscopy. Do you have a tutor who knows UV-spectroscopy?
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Hello!
I want to get some information about spin-forbidden phosphorescence emission. 
Some articles say that strong spin-orbit coupling would mix triplet and singlet energy and increase the ISC (intersystem crossing). What is the result of mixing triplet and singlet energy? How this mix promote the triplet excited state deactivating through phosphorescence? 
Thank you in advance!
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In terms of fluorescence spin-orbit coupling decreases the fluorescence quantum yield and singlet excited state lifetime. This is due to efficient kISC which competes with radiative transition. In the same time large kISC increases the concentration of the triplet state (triplet yield) but not necessarily the phosphorescence quantum yield. In fact the phosphorescence lifetime will decrease. This is due to the fact that efficient spin orbit coupling relaxed the forbidden nature of the phosphorescence transition (triplet to singlet).
There should be no doubt that spin-orbit coupling promotes ISC.
Reference:
K. K. Rohatgi-Mukherjee: Fundamentals of Photochemistry. Chapter 5
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I used Rhodamine B to find the quantum yield of pyrene, but I got it wrong. I was reading some literature and it says that reference dye should resemble your dye ( expected QY and absorption band).
 And I am also trying to find the quantum yield of benzopyrene.
Thanks.
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Additionally to the comments added above, I could also point you out to the PhotoChemCAD website, where you'll find the absorption and emission spectra of many compounds (http://omlc.org/spectra/PhotochemCAD/index.html). The papers from Würth et al. (http://pubs.acs.org/doi/abs/10.1021/ac2000303 and http://www.nature.com/nprot/journal/v8/n8/abs/nprot.2013.087.html) describe the method and discuss standards covering the whole spectral region. I also agree with Lerner and will go on with Quinine Sulfate. Make sure your water is free from chloride, as Cl- is a good quencher of QS, and chek the concentration of sulfuric acid you'll use (the standard values are given in the IUPAC report quoted in the answer by Demas).
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Hello everyone,
i cant find a reliable source explaining the consequences of the sulfonate modifications in alexa fluor dyes. why does the modification lead to an increase in brightness?
Thank you for your answer.
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I don't know why, or whether, the sulfate group increases brightness. Sulfate groups are certainly not required for fluorescent dyes to have high quantum yield. One thing that the sulfate group is good for is increasing water solubility of the dyes. This is a very valuable feature, because many fluorophores are quite hydrophobic and cause insolubility of the proteins they are attached to.
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Hi everybody I have the following questions. It would be very nice if some of you could help understanding them.
1. What is the significance of chi-square in life time measurement?. Why should the value be lower than 1.2 if it has to be acceptable data? or can it be higher also depending upon the system under consideration, say for example aggregated species.
2. Why do the organic aggregates (say organic compound dissolved in THF and added to water to get the dispersion) give multiple decay values (for example 2 or 3) in the lifetime measurements?
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To elaborate on Peter's answer on aggregates, when you have aggregates there are several effects that can be present. 1.) Some of you molecules are likely not aggregated due to aggregation equilibrium so you should see a lifetime consistent with individual molecules (if the aggregate particles don't change the mechanism/rate of quenching), additionally you may see two separate aggregate-based effects: Particles in aggregates may have longer lifetimes due to the fact that non-radiative decay rates may decrease, additionally if the aggregates can quench one another due to proximity you may see a short lifetime due to a new path (kq) competing. Finally something to note is that the aggregates may have slightly different environments on the molecular level due to how they are packed into these aggregates, this should mean that the lifetime of the aggregates is not a single value but a distribution of lifetime values. That being said, aggregates are still an incredibly way on predicting solid-state characteristics but they can be quite complicated.
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I am not able to locate any reference where aminocoumarins have been studied at low temperatures in non-polar solvents?
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Dear Sir,
Your sentence  shows hat you are not familiar with internet searching.
I am not able to locate any reference where aminocoumarins have been studied at low temperatures in non-polar solvents?
Please consult Google Scholar and not Google and give your question in the mask .  . On other part, your posted question is too vague (quite bad formulated) . Why??
- What a kind of aminocoumarins? Define  the other substituents!. Do you mean free amino group like Coumarin 151 or monoalkylamino  like Coumarin 500 or dialkylamino like Coumarin 30?
If you want to study the photophysics of aminocoumarin like a scientist and not like a student, please define  low temperature (e.g. liquid nitrogen)  and define non polar solvents . Please note that you must have a solution that freezes transparent like a glass.
The concentration of your amino coumarin must be low to avoid crystallization when the solution is going in the frozen state.
Please order the Kodak optical products booklet (It is free of charge) for a lot of infos.
JRG
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Hi all. I'm doing some TIRF analysis of lipid membranes and layers, with and without dyes. I've long been pondering about some "strange" phenomena I've encountered. The internet isn't much help. It seems TIRF in practice is a closely guarded secret...
When I excite my sample (could be anything, even something that shouldn't fluoresce at all!) at either 488 nm or 640 nm, I get two very different results. At 488 I get a hazy outline of the sample morphology. However, if I excite with 640 I get a myriad of vibrating, glowing spots, as if I'm inducing fluorescence in a small % of the molecules (although there aren't any obviously fluorescent compounds involved). The density of the spots also roughly correlates with the surface morphology. Why does this happen?
And secondly, why are small impurities on the surface so extremely bright? Does it have something to do with the way it interacts with the light? 
And finally, how come I can excite the dye Atto655 perfectly well with low intensity at 488 nm, although spectra suggest that it's impossible? Is there some configuration with the microscope I'm overlooking (aside from using the wrong laser entirely. But I'm pretty sure I'm not. It looks pretty blue to my eyes)?
All solvents and samples are of highest purity grade, and equipment strictly cleaned and used for specific purposes only to avoid any type of contamination.
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1) Along with absorption and fluorescence, there is also another very important optical effect - scattering. This is what you see with fluorescence microscope in (presumably) absence of any fluorophors. You observe it at 488 as a haze image and at 640 as a bright image because you seem to be using the dichroic red filter (makes sense for your dye) - the scattered light at 640nm passes very well through it, but not the light scattered at 488 nm.
2) Scattering on inhomogeneities is very strong, that's why the intensity of scattered light closely follows the surface morphology (roughness). Scattering is quite a complex phenomena, please, check the literature.
3) One possible reason is the the absorption of dye at 488 nm - it might be very low compared to the main absorption bands, but it is not zero. Note that bands may shift depending on the local environment of dye molecules. Another reason could be the fluorescence of the organic matter - most biological matter will fluorescence green when illuminated by blue light. This fluorescence can be reabsorbed by dye and cause its excitation.
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How to calculate the concentration of OH radicals generated ?
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I am far from being an expert on neither -OH nor radicals, but is seems to me that since it is a radical, it is highly unstable and therefore any attempt to measure the concentration would yield a nonsense number. So I guess an (color changing?) indicator that would be highly preferred by the radicals generated in your system? It's just a thought though...
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I have got the enhancement of UV/VIS value (RTPL emission at 380 nm/550 nm) (from 2.2 to 9.5) for sol-gel derived doped zinc oxide thin films. What might be the possible explanations?
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Following articles may be helpful:
1. Enhancement of band-edge photoluminescence of bulk ZnO single crystals coated with alkali halide (http://journals.aps.org/prb/abstract/10.1103/PhysRevB.68.045421).
2. Effect of Addition of KI on the Hydrothermal Growth of ZnO Nanostructures Towards Hybrid Optoelectronic Device Applications (http://www.ingentaconnect.com/content/asp/jnn/2016/00000016/00000004/art00025).
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Hi, Folks!
I got a tough question concerning the pile-up effect of TCSPC technique.
As a book says, pile-up effects result from the fact that a TCSPC device can detect only one photon per signal period. If the detection rate is so high that the detection of a second photon within the recorded time interval becomes likely the signal waveform is distorted.
There is a strange words for me that the second photon could be probably detected within a pulse period if the detection rate is high enough. I wonder how it could happen if we set an upper threshold for TAC that can only detect one photon.
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Yin,
You have to decrease the number of excited molecules per excitation pulse so that at each TAC window you will get a single emitted photon at most. You can do it by decreasing the number of molecules that are excited per unit time.This depends on absorption coefficient at the excitation laser wavelength, the excitation laser intensity/power, the fluorophore quantum yield. Now, you do not want to alter the emission properties in order not to bias the fluorescence decay, so you can change the rest:
  1. Decrease the concentration of the fluorophore under measurement
  2. Decrease the excitation intensity
  3. Maybe add OD on the emission path
I hope that helps.
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The size obtained from TEM of Carbon Dots are the size of carbon core alone or it includes the length of surface functionalization too?
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The size obtained from TEM of Carbon Dots includes the length of surface functionalization.  XRD - not  includes.
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Looking back on about 30y of femtosecond optics and spectroscopy, I would like to start a discussion on which achievements active researchers today consider as, e.g. most valuable, as a break-through, most visionary etc. To be specific, I'm not looking for top-ranked papers but a more general idea/concept/invention/technique/method that has in your opinion revolutionalized the field, has paved the way for succeeding studies and/or has been picked up by other branches / has inspired researchers in other fields such as biology, chemistry or even industry.
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I'd like to add three of my favorites:
1) ultrafast spectroscopy and coherent control of single molecules. This goes hand in hand with the already mentioned pulse-shaping techniques, ultrafast nanoscopy and in-depth characterization of ultrafast pulses. I'm still amazed by the fact that it is possible to detect and manipulate the optical response of single particles and to compare the result to theoretical predictions. 
2) 4D ultrafast electron microscopy. It extends the concept of ultrashort pulse generation to electrons and opens up completely new insights into structure and dynamics of matter under various physical and chemical conditions, for instance phase transitions, particles in optical near fields or spatially-corellated systems.
3) although already fairly old and not very surprising, starting out from the photon-echo experiments, I still consider the development of multi-dimensional optical spectroscopy as ground-breaking. Clear visualization of chemical bond transformation, energy transfer between molecules and relation to chemical structure is in my opinion a very important development that still holds great potential for instance in medical research and energy related science.
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Measured excited state life time for terephthalic acid (2.4 ns) and 2-hdyroxyl terephthalic acid (7.6ns) by TCSPC method in acetonitrile which was obtained after monoexponential fitting of the decay curve. I was wondering how hydroxy group can stabilize the excited state?
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Peter thanks for your feedback and explanations. I think you raised a very relevant point about about the role of inter-system crossing (ISC) and its influence on lifetime. I am pretty sure based on singlet oxygen generation measurements, ISC decay channel is very efficient for terephthalic acid compared to hydroxy-terephthalic acid. 
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Is it possible to quantify and to follow the transformations of the iron-oxalate complex under UV irradiation? Photo-Fenton-like system
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Thank you
We quantified the iron by this methode but now we need to identify and to quantify the various species produced during the photo-transformation of this complex
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On what basis these are different? Do same applied to GSOP ( ground state oxidation potential) and HOMO level of a molecule? 
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yes, of course they are different, because the energy level of single-occupied excited state (S1) is different from the energy of virtual unoccupied state (LUMO), (on excitation all energy levels are change).
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UV-Stabilizers are very useful additive for engineering thermoplastics. Since 1164, goes through well studies ESIPT (excited state intramolecular proton transfer) where as 3638 do not have adjacent -OH group to go through this mechanism. Please see attached structure and uv absorption profile. 3638 shows fluorescence where as 1164 is very very little. Any suggestion on understanding the working mechanism of 3638 will be highly appreciated. 
Thanks,
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Hi Hammad. As far as I understand, UV-stabilizers usually involve fast deactivation back to the ground state. If 3638 shows fluorescence, it doesn't seem to fit to the catergory? 
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When  a phosphorescence emission occurs, Singlet spin state is converted to triplet spin state and when it comes to ground state after phosphorescence emission of energy  it again in singlet state-----What is the driving force for such spin inversion  ?
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The reason is the same as the fluorescence, but the mechanism is slightly different due to the spin forbidden process.  The ground and the electronically excited states are stationary states, the electron density of a molecule is time-independent in these states (because they are the solution of the time independent Schrödinger equation). They should not emit(lost) light(energy), but they do. The guilty is the electromagnetic field that surrounds the molecules. The spontaneous emission (fluorescence and phosphorescence) arises from the interaction of molecules with the vacuum fluctuation of the quantized electromagnetic field. Triplet and singlet states are only assumptions. The real atomic/molecular states are spin-mixed states (real state = c1*triplet + c2*singlet). The degree of mixing  (spin-orbit coupling) is more important in the case of heavy atoms, there are no pure triplet states. The transition between singlet and triplet states are feasible due to the spin-orbit coupling. So..there are two driving forces:
1. coupling of radiation field  to the matter (interaction of vacuum fluctuation of the electromagnetic field with molecular states)
2. Spin-orbit coupling dissolves the forbidden nature of singlet <--> triplet transitions, because it is mixing the different spin sates. The real excited "triplet" state is not a pure triplet state, it has some singlet character --> transition to singlet groundstate is no more forbidden.
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I made some perovskite solar cells with ZnO as ETL, P3HT as HTL and gold as the back contact. Then I wanted to store them for longer period outside the glove box, So, I encapsulated them with UV curable optical cement and a glass slide and used black UV light for the curing. But when I stored the cells out side the glove box for 24 hours, their efficiency decreased significantly. If anyone will please advise me what to do here. 
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Dear Ghada, Hope you are well!
There are some thoughts concerning this issue. 
You can investigate yourself the effect of the uv curable resin on the solar cell be comparing the iv characteristics before and after the encapsulation to get out the effect of the encapsulation on the dark current, the short circuit current and the open circuit voltage and the fill factor. Would you please display your your iv curves before and after the encapsulation.
- Is the exposure to the environment has changed the active material properties. The resin may not be sealed against the environment and consequently chemical structure changing from the environment may penetrate the resin and react with the active material.   
- You can perform the encapsulation by an another method. You can use sealing ring and stick the back glass to this sealing ring without any coating on the active solar cell materials.  The rein will be used only to the sealing ring to stick it to the glass plates.
Moisture, humidity,and  UV cause rapid degradation to the perovskite solar cells  Therefore you have to avoid using UV , and moisture. The resin itself is not sealed against water vapor. To have to avoid curing by UV. Please see the link: http://surfacemeasurementsystems.com/applications/effects-humidity-moisture-degradation-high-efficiency-perovskite-solar-cells/
Bet wishes.
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Most of the literature search for me result into hits which are either done in 1970's or late 80's. I am interested mainly in knowing about transient absorption and photophysical nature singlet, triplet, luminescence etc. Sharing any useful literature will also be highly appreciated.
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Hello,
You may look this two works in which transient absorption of terephthalic acid is compared with some Metalorganic Frameworks (MOF-5 and MIL-88B(Fe)) obtained by a synthesis with TA and a metal precursor (Zn and Fe, respectively).
Álvaro et al: 
Laurier et al: 
I hope this would be an useful start point if you still need this information.
Best regards.
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I measured the Nano-second transient absorption spectroscopy and singlet oxygen emission for a terephthalate based system (attached), in ACN it shows weak fluorescence emission intensity, strong triplet-triplet transient absorption (lifetime in micro second) and strong singlet oxygen emission as well. Same systems in toluene, shows strong fluorescence emission but no triplet-triplet absorption and singlet oxygen emission, i was wondering what might be possibly leading to such a different behavior in these two solvents?
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I agree with the statements above.
However I may see another potential problem.   Based on the structure of the compound that you provided I am assuming that you are pumping (exciting) your sample in the UV region of the electromagnetic spectrum.
Based on that I am wondering, is is possible that when you use toluene that you are exciting your solvent and that you are seeing a signal from you solvent as well as the sample.  Maybe one signal blocks the other.  (also a stimulated emission can sometimes be seen for toluene by transient absorption). 
Acetonitrile absorbs much farther in the UV thank toluene does so it may bot cause re same problem. 
You could always tru a solvent like Dichloromethane (DCM) which is far less polar than ACN but still absorbs far in the UV.  
Good luck!
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Suppose I reverse bias my solar cell diode and measure external quantum efficiency as a function of applied reverse bias. Then the EQE is increasing with the applied reverse bias. After certain reverse bias the EQE is not changed very significantly? Can I get any idea regarding interface traps from the reverse bias dependent EQE for my device?
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Dear Sujit,
Would you please display an I-V curve to observe the effect with you and see how the reverse electric field increase the reverse current till saturation.
To explain the possibility of occurrence of this phenomenon, it is so that the active materiel in the organic solar cell has relatively dielectric constant, so the absorbed photons produce excitons, with relatively strong attraction bonding of its electron and hole.In order to be separated they must be dissociated. This is achieved by the electron and the hole transport layers at the sides of the active layer. If the excitons can not reach the transport layer at their lifetime they will recombine and gets lost for conduction. It results that the collection efficiency will be relatively small.
Reverse biasing the cell will be an electric field in the active layer which will assist the separation of the excitons to the Transport layers. As the reverse voltage increases the the excitons will be separated with the electric fild. Ultimately when the electric field is high enough it can separate all the excitins rendering the highest collection efficiency and the reverse current saturates to the the photocurrent.
This IS a qualitative explanation of the effect.
wish you success
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Sometimes I get a strong PL peak for my Methyl ammonium lead iodide perovskite samples around 770nm and sometimes I don't get anything at all. All my samples are prepared using the same precursors and prepared with the same method and looked exactly the same. Also, they all were excited at 532 nm. 
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Dear Ghada,
welcome again. The photo luminescence of a semiconductor material depends on the excess holes and electrons generated by the incident primary photons. This excess say dn = dp depends on the intensity of the incident  photo flux and the lifetime time of the generated electron hole pairs. On the other side the generated secondary photons depends on the presence of an appreciable radiative recombination rate. If the nonradiative recombination rate is dominating the recombination process, you will not observe the PL. 
It is a normal practice that the recombination centers for nonradiative recombination may change from sample to sample. The Bulk properties may be reproducable from sample to sample but the surface and interface properties are hard to reproduce because they are sensitive to the environmental conditions. The field effect transistors are delayed about three decades till the the interface between silicon an silicon oxide is fully controlled. It is the success story of the MOS transistors and the era of the integrated circuits.
So, it is the interface and the surface of the semiconductor that may change in an unpredictable manner till one control them by proper techniques such as passivation.  
My question is did you observve homogeneous PL in the specimens with such emissions?
I would like to add that the absence of the PL does not mean that the material will not work as a solar cell. Only it may have a lower conversion efficiency as the nonradiative lifetime is smaller.
Wish you success.
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e.g. one attached Fig. 1 (broad) vs. Fig. 2 (slightly sharp) does it tell anything about nature of triplet state? it was measured by using Laser Flash Photolysis nanosecond transient absorption (LP 920, Edinburgh instruments)
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Two experts on the field gave answers to this question: this website is awesome!!! (greetings, Joe).
Just wanted to add that the difference between broad vs. sharp absorption signals (plotted in energy/wavenumbers scale), coupled with signal intensity, is usually evidence of how well vibronic transitions happen between states (vibronic transitions are those happening simultaneously between electronic and vibrational levels). This is the heart of the Franck-Condon principle, which states "...electronic transitions occur most favorably when the nuclear structure of the initial and final states are most similar." (N. Turro). The broader the signal, the more access to many vibrational levels on the upper electronic state at the expense of signal intensity. The sharper the signal, the more "selective" the transition is and is usually accompanied by large intensity.
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I am stuying the photophysics of a system based on CdSe QD, and a cyanine dye , cardiogreen, CG. 
Till now we have some preliminary results , let me resume :
When we have the pure dye in a meOH solution, and we use an excitation source at 530nm, we dont see any emission from the dye .
as soon as we start adding QD to the same solution, we start to see an accumulation of both signals, one of the QD and the other of CG 
Then , we probe also, the quenching of the PL by adding CG, and we have a big decrease on the steady state emission signal. With 1 equivalent CG:QD we have 40 % of quqenching of PL intensity , at  5 eq , the quenching is close to 95%. 
At the same time that we perfomr these experiments we measure the TCSPC histograms. We start with the pure QD and the PL lifetime is around 20ns. As soon as we start to ad CG, the lifetime is decreased, but the relative change is not as big as the Intensity decrease. I mean 1 eq is just 8 % of change ion the lifetime, and when we reach the 5 eq , we have at most 40 %
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We propose that we could have a competitive mechanism  which happends before the emissive state is formed, which is responsable for decrease the total amount of emmissive state formed, this would lead to a decrease in the Intensity of the PL . 
We have propose the idea of haviong an energy tranfer or an electron transfer process from one of the higher excitonic states of the quantum dots. 
However how can we probe either one or the other?
I am now performming experiments on tour femtosecond fluorescence up conversion technique , an dmy idea is to measure the Internal Conversion time that Glaasbek (JPC  C 2006. 10.1021/jp055795g) has measured before
AND SEE WHAT HAPPENDS TO THE RISE TIME OF THE EMMISSIVE STATE WHEN WE ADD CG. 
Even we see afaster rise time due to the presence of CG, still we can not decide if we have energy transfer or electron transfer. 
So I ask again tricks and tips to discriminate between these two phenomena?
Thanks for you contributions 
Pedro Navarro PhD
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Dear Pedro,
To the best of my knowledge both process, energy and electron transfer, occur in most cases of interaction between QD and organic dyes. You should consider electron transfer process by Dexter's Theory ("A Theory of Sensitized Luminescence in Solids" http://dx.doi.org/10.1063/1.1699044).
One static PL experiment to determine if energy transfer by Förster's resonance (FRET) occurs, consists in high concentration of your organic dye (100:1, for example), exciting the QD in a wavelength that the organic dye have no absorption, and look at rise in the PL signal of the organic dye. If the PL spectrum of the organic dye rises up systematically, it is a strong evidence that FRET occurs.
Please, I invite you to look inside our paper "Cooperative effects in CdSe/ZnS-PEGOH quantum dot luminescence quenching by a water soluble porphyrin". We suggested more than one mechanism of energy transfer.
Best,
Gustavo
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The Commercially available Ti-Nanoxide-HT from solaronix brand containing an organic binder. If we use it for quenching studies, will the binder influence our result? What I am thinking is that we are coating the Ti-Nanoxide-HT (containing the binder) on FTO or on glass plate either by using doctor blade or spin coating method followed by annealing. After sensitizing it we are carrying out thin film photophysical studies. For investigation of solution state photophysical properties between the sensitizer and the nanoxide it is logic to use the same material i.e. with binder. It should be the actual condition. Am I right? Please give me your suggestions.
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Yu can try and see the results. I think the best way to do experiment.
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How far the quantum yield of quinine sulphate (0.54) will be linear. What is the Quantum yield at 300-320 nm excitations?
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In case you are using the relative methode in dilute solutions from Parker & Rees (C. A. Parker, W. T. Rees. Analyst 85, 587 (1960) ), the following holds true:
 With respect to optical density, it is constant up to fairly high values, but only if corrected for inner filter effects. With respect to wavelength, it is also constant over the complete range (think about Kasha rule...), but only if you correct the fluorescence spectra for excitation intensity.
Please us perchloric acid als solvent, as irregularities have been observed with quinine sulphate dissolved in sulfuric acid.
In general, the 1971 review on fluorescence quantum yield determination from Crosby and Demas is still worth reading:
Of course there are also newer reviews on this topic, please have a look for articles from Knut Rurack and Ute Resch-Genger
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Does anyone know what the redox energies are for the pacific blue dye?
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Thanks! It seems that it is really tricky to do electrochemistry on this compound. That's probably why there is no data to find.
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Decay time of a molecule having charge transfer character shows decrease in life time with increasing amount of water in THF/Water system. Quantum yield also decrease along with red shift. But the molecule shows longer decay time with increasing solvent polarity (Tol.<DCM<DMSO) along with red shift and decrease in quantum yield.
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if it is a dynamic quenching process the lifetime will change as a function of the quencher concentration.  look up Stern-Volmer plots 
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I have a FRET sample that shows expected fluorescence when the sample is stored in 2mL eppendorf tubes (Fisherbrand 05-408-138). However, the fluorescence intensity drops drastically and immediately when the samples are transferred to 0.5mL tubes (Fisherbrand 02-681-311). Both were sterilized using the same autoclave procedure. According to the company website, the 2mL tubes are DNase and RNase free but the 0.5mL ones are not. I was under the impression that autoclaving took care of contaminations.
Does anyone know what's going on? I have a ton of 0.5mL tubes so would like to avoid buying a different kind if I can.
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If the problem is surface adsorption onto the tubes, I doubt that rigorous pipetting will be beneficial. There are some approaches you could consider to reduce surface adsorption: use a different type of container, use a different solvent, coat the surface of the container with an agent to block adsorption (silicon, PEG), include some non-ionic detergent in the sample, maximize the volume-to-surface area ratio.
I agree with Dennis Klier's suggestion to check for loss of material from the solution. Absorbance is the simplest way to do it. Other ways depend on what the substance is, but might include mass spectrometry, HPLC, and drying/weighing.
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In the photophysical discussion of the UV-vis spectra of  reported phosphorescent iridium complexs for OLEDs, the absorption bands are commonly divided into to "two major absorption bands". Absorption bands with high absorbance  are usually ascribed to the absorption of ligand-centered transition, such as pi-pi* transition. And absorption bands with lower absorbance are usually correlated to the spin-forbidden metal-to-ligand charge transer transition. This kind of correlation is overly simplified. I want to know how  can I correlate the absorption bands more detail?  Also, how can I distinguish the pi-pi* transiton with charge transfer transition of the ligands if they are both possible to happen?
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Metal to ligand charge transfer transitions (MLCT) are not only weaker in absorption but  very much lower in energy also as compared to pi-pi* transitions of ligand. So they are easily identified as lowest energy transitions in the complex. These transitions will be absent in free ligand absorption spectra. 
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How to measure the efficiency of Dexter Energy Transfer? Is there a way by fluorescence decays? 
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Forster resonance energy transfer (FRET) is a non-radiative (radiationless) energy transfer. 
Dexter energy transfer involves transfer of excitation from donor to acceptor due to hopping of electrons. For this to happen the wavefunctions of both donor and acceptor must overlap.
As a consequence FRET occurs over large distance as compared to Dexter energy transfer.
Typical distance range for FRET is 10-100 Angstroms and for Dexter energy transfer it is <10 Angstroms.
It is not important to distinguish Dexter energy transfer from FRET. It does not matter by what mechanism energy transfer occurs. It is just another processes by which excitation energy is lost from the donor.
Dexter energy transfer efficiency can be measured in the same manner as one measures FRET efficiency. 
It can be measured from the ratio of fluorescence lifetime of donor in the presence of acceptor to the fluorescence lifetime of donor in the absence of acceptor using the formula E = 1-(tauDA / tauD)
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While in photophysical (absorption and emission) studies I found that the rhodamine derivatives towards metal ions chemosensing was more favorable in acetonitrile solution and not in other solvent such as DMF, DMSO etc. what's the reason behind this?
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In order to give a good answer, please give some infos about your rhodamine derivatives (rhodamine derivatives in your question is too vague). Have you a free COOH or more or esterified  like COOR...? What a kind of substituents at  NR2  (NH2, CH3,C2H5...a.s.o).
What a kind of metal ions . Acetonitrile is a polar aprotic solvent and  it is used as solvent and as  easily displaceable ligand  . You cannot compare it with DMF or DMSO.
Waiting for your answer .
JRG
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How can I determine fluorescent quantum yield using UV-vis spectroscopy of perylene derivatives? Which parameters do we need?
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One important thing is not explained in the above description, namely what is emission intensity. And by this term a number of photons is assumed. Thus, one needs to calculate integrals under the emission spectra of the reference and studied molecules in proper coordinates.
In the article this is not an issue, since the spectrum does not change with quenching in the particular case, and the integral is directly proportional e.g. to maximum intensity.
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The sample has strong luminescence around 630nm in solution while no stimulated emissions were detected in the femtosecond transient absorption spectra during the whole measurement. Is there any other possible origins besides overlap between ESAs and stimulated emissions? 
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If the timescale of your experiments is too short for internal conversion to occur, shouldn't your estimulated emission be of the same wavelength as your excitation (pump) light? In that case, you'll never be able to distinguish between the pump light and the estimulated emission light. Internal conversion takes at least a few dozens of femtoseconds to occur, maybe even picoseconds, depending on the chromophore and its medium.  
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I observed continuous redshift of the PL emission spectra while increasing the excitation wavelength of semiconductor polydisperse QD. I would like to understand the underlying physics responsible for this redshift? How does increase in excitation causes redshift in NP?
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Polydisperse QD have a distribution in band gaps. If you excite with higher wavelengths (lower energies), you become less and less able to excite the small QDs with high band gap. As a result, you lose the emission at short wavelengths and the overall emission redshifts.
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Is it similar to fluorescence anisotropy?
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If you are studying excitonic materials, then a major anisotropy arises through exciton migration. The polarised excitation beam selectively excites those molecules that are aligned with the polarisation, as the excitons migrate they then loose this polarisation yielding a time dependent anisotropy. If the excited state migration is slow compared to the excited state lifetime then the effect is small. Depending on the orientation of the polarisation of the pump and probe beams you can observe anisotropy decay or grow in.
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Also, please provide some books for the study of lanthanide spectroscopy.
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Mr. Devakumar,
Despite that the below paper is on actinide, and has mainly experimental character in the theoretical part you may found briefly on the quantum chemical approaches and corresponding softwares enabling accurate prediction of the optical properties of f-elements. They were used for the prediction of the absorption and fluorescence spectra as well as the vibrational ones in our study. For more details you could pay attention on the citated references on the theoretical background:
B.Ivanova, M. Spiteller, Uranyl–water-containing complexes: solid-state UV-MALDI mass spectrometric and IR spectroscopic approach for selective quantitation, Environmental Science and Pollution Research, 2014, 21, 1548-1563.
Unfortunately on lanthanides we have also useful results exactly on the topic of your question, but they are still under reviewering in the Journals and, therefore, I cannot provide you details. But it have lot abailable in the literature theoretical quantum chemical studies on physical optical properties of lanthanides. Therefore you could perform alone literature -esearch on this topic.
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This is quite a specific question, I know. I'm currently determining the absorption coefficients of a range of fluorescent materials and am a bit stuck with this one because I can't find its molecular weight anywhere. From literature I know Lumogen F Red 305 has Mw = 963.956 and Lumogen F Orange 240 has Mw = 710.873. It is necessary to know these so that the molar concentration can be determined which is essential in applying the Beer-Lambert law to determine the extinction coefficient. I've emailed BASF but the lumbering giant hasn't registered my existence.
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Hi Adam, did you get your answer?? I have the mass absorptivities (L g-1 cm-1) for quite a few of the Lumogen F dyes, and from these you can get to an absorption coefficient with a known plate thickness and optical density, .... however I don't have this for Pink 285:-( Regards, Bryce
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Is this something that can be replaced manually or do I just have to accept that this is the window provided for the PMT I'm using (UV-transmitting glass on an R446)?
I could employ more filters to block out as much light as possible but I was just wondering if this approach is possible.
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Derek, to remove stray and scattered light you don't need a filter or alter the window of the PMT.
1) collect the light with a lens or mirror with a long focal length
2) then focus the light through a fairly long tube (you can use a cardboard tube from a paper roll or something similar)
3) place your PMT on the other end of the tube
4) IF you need to reject light from your source (a laser I am guessing), place a long pass filter (with a cut-off wavelength longer than your source) at the entrance of the tube or in front of the PMT.
This method will remove tremendously the stray/scattered light from hitting your PMT.
Please see Figure 1 in the attached paper. We did not draw the tube in front of the PMT on the Figure but there is one in our set up which is about 40 cm long. We spray painted it black to adsorbed as much as possible any stray light.
The dark current is the current out of the PMT when there is no light. Cutting out any light such as you suggest will not reduce your dark current.
If you are struggling with low signal to noise ratio, use a chopper and detect the signal with a lock-in amplifier set up at the frequency of the chopper. This will select out the noise and will allow you to see very small signals from your samples. This method is common for infra red photo luminescence. Not so much for visible light as one normally can got good signal from source/detector combination in this wavelength range.
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A blue shift of the emission peak of my compound upon the addition of quencher was observed. I guess there is an electron transfer between it and quencher. But I don't know how to explain this blue shift. Anybody could give me some suggestions?
P.S.: There is no change in UV absorbance curve of the compound upon addition of quencher.
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The fluorescence spectrum may depend on the presence of quencher for a variety of reasons but in general this requires the presence of more than one emitting species. Even in the case of a single fluorophore in the presence of the excited-state solvent relaxation one may observe the blue shift of the fluorescence band upon addition of a dynamic quencher which shortens the excited-state lifetime and therefore gives more weight to the emission from only partially relaxed excited-states (which are higher in energy than totally solvent-relaxed states).
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I want a method other than time correlated spectrofluorometer to establish ESIPT.
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Compare the fluorescence and absorption spectra. The fluorescence band of compounds with ESIPT shifts very strong to long wavelength absorption band. The difference between the band positions can be more that 6000 cm-1. The comparison of the bands for the compound in the solvents without OH group.
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I have labeled a protein with Cy3B and immobilized it on a surface. I chose the dye over Cy3 because of its higher brigthness. However, I get fast photobleaching and cannot detect as many photons from my molecule as the people in the literature get with a regular Cy3 dye. Photoprotection and laser power are already optimized.
So, my question: is there an inherent difference in photostability (meaning the number of photons you can get out of a single dye molecule before it bleaches) between the two dyes?
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Dear Franziska,
there is a comparison of photostability between Cy3 and Cy3B in Link et al., European Journal of Medicinal Chemistry 45 (2010) 5561-5566, citing also product literature from GE Life Sciences / formerly Amersham (Fluorescence Screening Reagents Guide). Both papers state that Cy3B is less photostable than Cy3.
Best regards,
Frank
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I have not found any article reproducing the results reported by:
B. Oliveira dos Santos, C. E. Fellows, J. B. de Oliveira e Souza, and C. A. Massone
“A 3% Efficiency Nitrogen Laser”
Applied Physics B (Photophysics and Laser Chemistry) 41 (1986), pp. 241-244
If anybody knows about this please, let me know!
In our laboratory we built a laser according to these specifications but we never reached the claimed 3% efficiency. We obtained only a 0.11% efficiency (Revista Mexicana de Fisica, 37(3), (1991) pp 391-395)
Latter on we used that laser to build a MOPA system :
Thanks for any information!
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I in detail investigated efficiency of the nitrogen laser in article: V.V. Apollonov, V.A. Yamshchikov " Efficiency of an electic-discharge N2 laser" Quantum Electronics 27(6), p/469-442 (1997). In it it was shown that almost difficult to obtain efficiency more than 0,1%. Efficiency of 3% is an error of measurements! Don't spend your force to prove the return.
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The Lippert-Mataga equation (see attached image) describes how the stokes shift changes as a function of solvent properties (epsilon and n) and the fluorescent molecule properties (dipole moments and Onsager radius, a). I can't see how this equation deals with the blueshifts that can occur in solvation phenomenon in which energy is essentially given to the singlet state, before fluorescence occurs, by an interaction with the host medium. Can someone explain this to me?
(In the equation the LHS is Stoke's shift in wavenumbers, on the RHS epsilon is relative permittivity, n is refractive index, mu is dipole moment with subscript E = excited state and G = ground state, a is the Onsager radius and C is a constant equal to the unperturbed Stoke's shift in vacuum).
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The lippert-mataga equation is typically used to determine the difference in the dipole moments, it does not really deal with the specific solvent effects that can influence the relative energy levels (and hence the emission wavelength). The blue/red shifts observed can be the result of conformational changes that give rise to new bands (TICT states).
take a look at this review: Z. Grabowski, K. Rotkiewicz, W. Rettig, Chem. Rev. 2003, 103, 3899–4031.
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Absorption spectra of complexes could be classified either ligand-center pi-pi (LC), metal-to-ligand charge transfer (MLCT), or ligand-to-metal charge transfer (LMCT). In most cases, LC band is easily to assigned, but the assignment of MLCT and LMCT is sometimes confusing. TDDFT method might be a good approach, but is there other methods to give a better assignment?
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Thanks for your kindly answer. Charge transfer band would be characterized by varying solvent polarity, but is it possible to distinguish whether the CT band is MLCT or LMCT? For TDDFT calculation, I found wB97XD might be a good long-range correlated functional.
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I just got a weird, broadened emission band for my poly-ala-helix (21 residues incl. some arginines for solubility) which has two labels (Xanthone and Naphtaline) that I'm using for triplet-triplet-energy transfer (TTET, similar to FRET, but different). The labels are attached on the 10th and 12th positions, which means they are facing different sides of the helix and can not attach to each other in the folded state. At 7M urea the unfolded state is stronger populated and the interaction is more likely and will also stabilize the unfolded state. This is why I expected dimer formation at 7M urea.
Well, this is the spectrum that I got and - according to literature - this seems to be an excited dimer. What should the spectrum look like for a regular dimer? How can I if show it's one or the other? Is this spectrum already proof enough?
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You should measure excitation spectra of both emissions (best would be a full excitation emission spectrum, EEM). The spectral shape of the excitation spectrum resembles the absorbance spectrum, because the emission intensity is proportional to the light intensity absorbed by the ground state.
If the excitation spectra of both emissions are the same and identical to the absorbance spectrum, it is a clear indication of an excited-state process (Excimer formation in your case). If the excitation spectrum of the long-wavelength emission is different, the emitting entity already exists in ground state.
The excitation spectrum of the short-wavelength emission should resemble the absorbance spectrum in any case...
Be aware, there are more processes that can lead to dual emission behaviour (ESIPT, TICT, phosphorescence, impurities)
And, xanthone is already special by itself, you might have to do some literature search, there is much of it out there... Just as a starting point:
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Is there any explanation if I observe only (or mainly) the decay in fluorescence as opposed to absorbance?
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Gomez almost correct but the the molecules derived after photobleaching may have significantly different peak absorption wavelengths, therefore you need to identify the peak absorptions of both photobleached product and as well as your original fluorophore. Initially before photoirradiation your fluorophone may alone yield single peak as photobleaching progressing you MAY find 2 or more peaks with progressively changing absorption intensities.
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Can anybody suggest molecules (dyes) with interesting solvatochromic properties in ground state?
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Mainly organic aromatic molecules having donor moiety like dialkylamino group in one part and the acceptor moiety at the other extreme will show very intersting solvatochromic propety in both ground state as well as in excited state (If intamolecular hydrogen bond or proton tranfer possiblity will be there then also it will be quite good). However I dont know why u r interested for ground state, generally these types of fluoroscents probes shows more interesting behavior in excited state rather than in groud state.