Photophysics - Science topic

There is a whole chapter on effects of solvents on florescence emission spectra in Joseph Lakowicz's textbook "Principles of Fluorescence Spectroscopy" (Chapter 7 in the 1st edition). He divides solvent effects into general and specific types. The following is an excerpt from p. 189.

"By general solvent effects we mean those which result from the refractive index (n) and dielectric constant (epsilon)....Specific solvent effects refer to specific chemical interactions between the fluorophore and the solvent molecule, such as hydrogen bonding and complexation."

The rest of the chapter goes into details.

Thank you Prof. Pansu . I shall have a look.

Another potential scenario is that the DCM and Acetonitrile do not have the capability to create a hydrogen bond, while DMSO can form a hydrogen bond depending on the structure of your solute. If there is a complex formed in the ground state, it is likely that you will observe multiple lifetimes.

TTA emission will have a quadratic dependence on incident light intensity, where other (1 photon) types of emission will have a linear dependence.

It sounds like excimer (excited dimer) formation, meaning that the compound is self-associating. The classic example of this is pyrene.

Edward Sanders thank you.

Dear Bidyut,

It is generally accepted that the fluorescence rate kf of the Charge Transfer state is lower than that of the Locally Excited state since the transition moment <So|µ|S1> is less good for <So|µ|LE> compared to <So|µ|CT>. But often the non-radiative deactivation rate knr of CT states is higher than that of LE state. Then everything is possible for the lifetime T=1/(kf+knr)

Mohamed Khedawy

Thanks a lot!

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.

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:

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!

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.

I have not done it myself, but see for example:

The time evolution can be tracked using the kinetic constraints:

Dear Mahmud Sefannaser

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

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.

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.

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.

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.

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.

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.

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

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 !

more precisely:

" The net change regarding momentum and energy exchange with EM field

is identical to SPONTANEOUS emission, isn't it?"

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.

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.

@ Kumaran

Yes Sir.. We have tried in different solvent systems... But we didn't observe the reduction peak..

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.

Start with first year UV-spectroscopy. Do you have a tutor who knows UV-spectroscopy?

In terms of fluorescence spin-orbit coupling decreases the fluorescence quantum yield and singlet excited state lifetime. This is due to efficient k_ISC which competes with radiative transition. In the same time large k_ISC 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

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).

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.

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.

Dear Sir,

Your sentence  shows hat you are not familiar with internet searching.

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

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.

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...

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).

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:

I hope that helps.

The size obtained from TEM of Carbon Dots includes the length of surface functionalization.  XRD - not  includes.

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.

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. 

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

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).

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? 

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.

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.

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.

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!

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

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.

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.

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

Yu can try and see the results. I think the best way to do experiment.

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

Thanks! It seems that it is really tricky to do electrochemistry on this compound. That's probably why there is no data to find.

if it is a dynamic quenching process the lifetime will change as a function of the quencher concentration.  look up Stern-Volmer plots 

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.

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. 

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-(tau_DA / tau_D)

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

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.

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.  

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.

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.

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.

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

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.

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).

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.

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

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.

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.

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.

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:

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.

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.