-
[show abstract]
[hide abstract]
ABSTRACT: In this study, we report on the effects of solvent viscosity and polarity on the photochromic salicylaldehyde azine (SAA) molecule by examining the steady-state and UV-visible absorption results in the time scale from nanoseconds to hours, in solution and in a polymer film. For the neutral structure, the viscosity strongly affects the lifetime of the photochromic (trans-keto) tautomer by suppressing the second order quenching process, and thus increasing the photochrome lifetimes in highly viscous solvents to 500 μs in polar triacetine, and to 65 μs in non-polar squalane. Trapping SAA in a non-polar polymer film (polyethylene) results in further elongation of the photochromic lifetime (700 μs) by one order of magnitude (with respect to that in squalane), due to the retardation of the intramolecular back-isomerization. Another species, living significantly longer and absorbing more in the UV comparing to the photochrome, was identified as the syn-enol tautomer. The lifetime of this tautomer, created in a competitive mechanism to the photochrome creation, is much longer in non-polar solvents (hundreds of minutes) than in polar ones (tens of minutes), opposite to the trend observed for the photochrome. For the SAA anion, the transient living on the ns-μs time scale can be exclusively assigned to the triplet state, which is not observed for the neutral form at room temperature.
Photochemical and Photobiological Sciences 06/2012; 11(8):1389-400. · 2.58 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In this work, we report on photophysical studies of the anticancer drug topotecan (TPT) in aqueous solutions at different pHs. We used steady-state (UV-visible absorption and emission) and time-resolved picosecond (ps) emission spectroscopies to investigate the role of the H-bonding interactions as well as the proton concentration (pH = 0.48-7.40) on the behavior of topotecan (TPT) in its ground- (S(0)) and electronically first (S(1)) excited-states. At physiological conditions (pH = 7.40), the drug exists at S(0) in equilibrium between the enol (E), cation (C), and zwitterion (Z) forms. The photoformation of Z* (τ(Z) = 5.83 ns) occurs from directly excited (λ(exc) = 371 nm) E as the two-step reaction: E*→C*→Z*. In this process, a very fast (less than 10 ps) protonation of E* leads to C*, which subsequently undergoes fast (580 ps) deprotonation to give Z* as the final photoproduct. At higher proton concentrations (pH = 0.48-1.31), a ground-state equilibrium exists between three different cationic species (C1, C2, and C3). The proton motion from the acidic solution to the C forms of TPT to give the reactions C1*→C2*→C3* is governed by the proton diffusion. In these conditions, both dynamic and static quenching occurs. The rate constant values k*(DPT1) and k*(DPT2) for the direct protonation of C1* and C2*, respectively, depend on the pH of the surrounding. The number of protons implicated in the reaction changes from ∼2 (pH = 0.48-0.78) to ∼1 (pH = 0.78-1.31), thus indicating the existence of two different reactions and proton-transfer dynamics. These results evidence the conformational, structural, and dynamical changes of aqueous solutions of TPT with the pH of the environment. They should help to understand the molecular structure/activity of TPT at cellular level.
The Journal of Physical Chemistry B 06/2012; · 3.70 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report on the role of H-bonding interactions on the UV-visible absorption and emission (steady-state and time-resolved) spectroscopy of topotecan (TPT) in solution. In aprotic solvents, a very fast (less than 10 ps) excited-state intramolecular proton-transfer reaction occurs in the absorbing enol (E) form to give a zwitterion (Z) form, emitting with a large Stokes shift. In protic solvents like methanol, the time constant of Z* formation is longer (32 ps) due to the participation of solvent molecules in the proton-transfer reaction. In aqueous solution at near-neutral pH (6.24), a ground-state equilibrium is established between E, cation (C), and Z forms. Direct excitation of E leads to Z* through two channels: a very fast one (less than 10 ps) involving an intramolecular proton-tranfer and a slower one (680 ps) with the C* intermediate formation and reaction. A fast (42 ps) deprotonation of E* to give the excited anion (A*) also competes with the photoformation of Z* at the S(1) state. At pH =12.15, the A structures are the principal emitting species (τ(A) ~ 0.41 ns), showing the largest Stokes shift. In aqueous solutions, we cannot exclude the existence of an equilibrium between the lactone and carboxylate forms of TPT, whose spectroscopic (absorption and emission spectra) and dynamical behaviors should not be very different. Time-resolved emission anisotropy measurements in solvents of different viscosities suggest that the rotational relaxation time (φ) of TPT is mainly governed by the viscosity of the medium, increasing from 104 ps (in tetrahydrofuran, THF) to 156 ps (in water) and 338 ps (in dimethyl sulfoxide, DMSO). These results give spectroscopic and dynamical information on the structures, stability, and dynamics (picosecond to nanosecond time scale) of TPT in solution. They provide insights on the role of the intermolecular H-bonding surrounding medium on the ground- and excited-state structure and reaction of TPT. The finding should contribute to a better understanding of the relationship between the structures of the drug and its surroundings.
The Journal of Physical Chemistry B 06/2012; 116(25):7522-30. · 3.70 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In this feature article, we discuss recent advances in studying ultrafast dynamic and structural aspects of host-guest interactions. Steady-state and time-resolved techniques exploring events from the femto- to nanosecond regime were used to examine the ultrafast photodynamics and subsequent events in selected nanostructures of the formed complexes. These consist of aromatic systems, biologically relevant molecules, and drugs trapped within cyclodextrins (CD) and human serum albumin (HSA) protein pockets. We examine the effects exerted by these chemical and biological cavitands on internal twisting motions, proton transfer and charge transfer, and cis-trans isomerization reactions that may occur in the confined molecular systems. In addition, the influence of a restricting environment on the interaction of guest molecules with biological water is considered. The dynamic details of the complexes (diffusion, early interactions, formation, stability, internal guest diffusion, and conformational changes) and the excited-state relaxation pathways, rate constants of the involved processes, and changes in the electronic distribution within encapsulated guests gave clues to elucidate their photobehavior and are relevant to the photostability and delivery of drugs when using nanocarriers.
Langmuir 03/2012; 28(17):6746-59. · 4.19 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The interactions of 5,10,15,20-tetrakis(4-sulfonatophenyl)-porphyrin (TSPP) with a quaternary ammonium modified β-cyclodextrin (QA-β-CD) and human serum albumin (HSA) protein in aqueous solutions at pH 7 were studied using steady-state, stopped-flow, and femtosecond to millisecond spectroscopy. TSPP forms 1:1 and 1:2 complexes with QA-β-CD (K(1) = 1.9 × 10(5) M(-1) and K(2) = 7 × 10(3) M(-1)) at 293 K, whereas with the HSA protein only 1:1 complex (K(1) = 1.7 × 10(6) M(-1)) has been found. The chemical and biological nanocavities have notable effects on the fluorescence lifetimes of the Q(x) state (from 9.3 to 11.1 ns in QA-β-CD and 11.6 ns in HSA). Furthermore, the rotational times (400 ps for the free TSPP, 1.6 and 19 ns for QA-β-CD and HSA protein complexes, respectively) clearly indicate the robustness of the formed entities. The confined environment does not affect much the fs dynamics (0.1-0.2 ps) of the encapsulated molecule. However, it clearly affect the ps one (1-2 ps (H(2)O) and 5-10 ps (QA-β-CD and HSA)). The effect of O(2) on the relaxation of the triplet state of the free and encapsulated TSPP is also studied and the obtained results are discussed in light of the shielding effect provided by the chemical and biological cavities. The observed difference, longer triplet lifetime upon encapsulation, might be relevant to the efficiency of this porphyrin in photodynamic therapy. The presteady-state kinetics of the TSPP:HSA has been studied by the stopped-flow spectrometer, and a two-step model was proposed for the complexation processes. The results show the importance of the initial association step for the overall ligand recognition process. This first step occurs with rate constant of ~4 × 10(5) M(-1) s(-1), which is about 5 orders of magnitude larger than the rate constant of the consecutive relaxation processes. We believe that our observations of molecular interaction between TSPP, QA-β-CD, and HSA protein from femtosecond to second at both ground and electronically first excited state give detailed information to improve our understanding of this kind of system and thus for a better design of drug delivery nanocarriers.
Langmuir 03/2012; 28(9):4363-72. · 4.19 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Time-resolved emission and absorption spectroscopy are used to study the photoinduced dynamics of forward and back electron transfer processes taking place between a recently synthesized series of donor-(π-spacer)-acceptor organic dyes and semiconductor films. Results are obtained for vertically oriented titania nanotube arrays (inner diameters 36 nm and 70 nm), standard titania nanoparticles (25 nm diameter) and, as a reference, alumina nanoparticle (13 nm diameter) films. The studied dyes contain a triphenylamine group as an electron donor, cyanoacrylic acid part as an electron acceptor, and differ by the substituents in a spacer group that causes a shift of its absorption spectra. Despite a red-shift of the dye absorption band resulting in an improved response to the solar spectrum, smaller electron injection rates and smaller extinction coefficients result in reduced dye sensitized solar cell (DSSC) conversion efficiencies. For the most efficient dye, TPC1, electron injection from the hot locally excited state to titania on a time scale of about 100 fs is suggested, while from the relaxed charge transfer state it proceeds in a non-exponential way with time constants from 1 ps to 50 ps. Our results imply that the latter process involves the trap states below the conduction band edge (or the sub-bandgap tail of the acceptor states), localized close to the dye radical cation, and is accompanied by fast electron recombination to the parent dye's ground state. This process should limit the efficiency of DSSCs made using these types of organic dyes. The residual, slower recombination can be described by a stretched exponential decay with a characteristic time of 0.5 μs and a dispersion parameter of 0.33. Both the electron injection and back electron transfer dynamics are similar in titania nanoparticles and nanotubes. Variations between the two film types are only found in the time resolved emission transients, which are explained in terms of the difference in local electric fields affecting the position of the emission bands.
Physical Chemistry Chemical Physics 02/2012; 14(8):2816-31. · 3.57 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The photophysics of N,N'-bis(salicylidene)-p-phenylenediamine (BSP) is analyzed both theoretically and experimentally. The alternative intramolecular proton-transfer reactions lead to three different tautomers. We performed DFT and TDDFT calculations to analyze the topography of the reactions connecting the three tautomers. Deactivation paths through a Conical Intersection (CI) region are also analyzed to explain the low fluorescence quantum yield of the phototautomers. The complex molecular structure of BSP provides a large number of deactivation paths, almost all of them energetically available following the initial photoexcitation. Femtosecond (fs) time-resolved emission studies in solution and flash photolysis experiments (nano to millisecond regime) were performed to get detailed information on the time domain of the full photocycle. The picture that emerges by combining theoretical and experimental results shows a very fast (less than 100 fs) photoinduced single proton transfer process leading to a phototautomer where a single proton has moved. This species may deactivate through a low-energy CI leading in about 20 ps to a rotameric form in the ground state that has a lifetime of several tens of microseconds in solution. This process competes with another deactivation path taking place prior to the proton-transfer reaction which involves a low-energy CI leading to a rotamer of the enol structure. In the flash photolysis studies, the rotamer of the enol structure was directly identified by the positive transient absorption band in the 250-260 nm and its lifetime in n-hexane (10 ms) is almost 3 orders of magnitude longer than the lifetime of the photochrome (around 40 μs). Our findings do not exclude a double proton transfer reaction in the excited enol form to give a tautomer in less than 100 fs during the first (impulsive) phase of the reaction which reverts back to the photoproducts of the simple proton transfer in 1-3 ps.
Physical Chemistry Chemical Physics 09/2011; 13(33):14960-72. · 3.57 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In this study, we report on the steady-state UV–visible absorption, reflectance, and emission spectra and femtosecond dynamics of 7-hydroxyquinoline (7HQ) interacting with purely siliceous (R-MCM41) and Al-doped (Al-MCM41) mesoporous silicate material in dichloromethane solution, as well as its photoconversion upon 320-nm irradiation. The results show that changes in the electrostatic field inside the nanochannels induced by Al ions significantly affect the ground- and excited-state photobehavior of the 7HQ structures H-bonded to the framework. For the Al-MCM41 sample, the obtained decay times (0.47–0.60 and 5.8–6.6 ps and a rise time of 3.3 ps) in the ultrafast dynamics are assigned to the formation and relaxation processes of anionic (A) and zwitterionic (Z) structures of 7HQ. These times are longer than those observed for the regular MCM-41 sample (0.26–0.32 and 3.5 ps and a rise time of 0.75 ps). The difference is explained in terms of different nonspecific (electrostatic) interactions of 7HQ with the nanohosts. In contrast, we found that the influence of the channel diameter (in the 25–40-Å range) is weak. Additionally, the 320-nm irradiation study showed that the main conversion route leads excited E to form A, which is stabilized in the ground state, and lowers the transformation into the Z and neutral keto (K) populations. For 7HQ/R-MCM41, the rate constants of the E → A → (Z, K) photoconversions were found to be 8.8 (±0.23) × 10–3 and 3.2 (±0.18) × 10–3 min–1, respectively. The K form is not produced when unstructured silica particles were used, indicating the effect of nanoconfinement on its formation and stability. We observed the same trend in the photoconversion of E to A and Z (K) in a solid-state sample, suggesting that the solvent does not play a key role in their formation. Our results demonstrate how the confinement and interactions of 7HQ with the MCM-41 nanomaterial affect the steady-state and time-resolved emission spectroscopy of a dye, as well as the equilibrium, stability, and photoconversion of its tautomeric and ionic structures. Such a rich photobehavior might be used in designing H-bonded guest:MCM-41 photoactive materials, such as light-emitting diodes, memories, and gates to cite a few possible applications in nanophotonics.
07/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: We report on femto- to nanosecond studies of the excited state intermolecular proton transfer (ESPT) reaction of trisodium 8-hydroxypyrene-1,3,6-trisulfonate (pyranine, HPTS) with the human serum albumin (HSA) protein. The formed robust 1:1 complexes (K(eq) = (2.6 ± 0.1) × 10(6) M(-1)) show both photoacid (∼430 nm) and conjugated photobase (∼500 nm) emissions of the caged HPTS in its protonated structure. The proton-transfer reactions in these complexes proceed in a large time window, spanning from 150 fs to ∼1.2 ns. The ultrafast component reflects a direct H-bond breaking and making in the robust complexes, involving the carboxylate groups of the amino acids, while the slowest one is arising from the slow dynamics of the so-called biological water. Additional time constants of the caged photoacid to give the conjugated photobase are observed, assigned to the ESPT reaction within "loose" complexes (3 to tens of picoseconds), and 130 ps and 1.2 ns due to the slow dynamics of the water molecules around the protein residues and involved in the proton transfer. The fs-ns anisotropy measurements confirm the robustness of the HPTS:HSA complexes. Our results indicate that, even though robust 1:1 complexes between HPTS and the HSA are formed, the system is heterogeneous, due to different possible interactions of the dye with the inside/outside parts of the protein. Furthermore, we find lower values of the initial anisotropy (r(0)) in the protein (0.33) and in γ-CD (0.28) in comparison with buffered aqueous solution (0.385). We propose that caging HPTS by the HSA protein and by the cyclodextrin affects the electronic redistribution in a different degree of mixing between the (1)L(a) and (1)L(b) states in the formed deprotonated form, for which the interactions of the sulfonate groups with the surroundings should play a key role.
The Journal of Physical Chemistry B 06/2011; 115(23):7637-47. · 3.70 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Steady-state and time-resolved picosecond emission studies were carried out to study the role of the proton concentration in the acid-base properties of the anticancer drug camptothecin (CPT) in its ground and electronically first excited states. The results show that, under acidic conditions, the excited-state proton-transfer (ESPT) reaction is irreversible, in contrast to previous literature data. We found that the prototropic species are equilibrated at the excited state (pK(a)* = 1.85) only in a restricted range of pH (1.5 < pH < 3), whereas only one species, either the neutral form (τ(N) = 3.76 ns) or the protonated form (τ(C) = 2.83 ns), can be detected at pH > 3 and pH < 1.5, respectively. The proton motion from the acidic solution to the neutral form in the pH 1-2 domain is diffusion-controlled. Within the range of pH 1-2, the reaction rate constant for the formation (k(d)) of the encounter complex between the proton and the neutral form ranges from 1.17 × 10(10) to 7.33 × 10(10) M(-1) s(-1), respectively. Under more acidic conditions (pH 0.9-0.95), the protonation of CPT does not depend on the diffusive step, because of the large amount of protons. The direct proton-transfer rate constant (k(DPT)*) increases with the proton concentration (time constants change from 24 ps to ∼1 ns at pH 0.9 and 2, respectively). The number of protons involved in the proton transfer changes from approximately one, for the diffusive regime, to approximately four, for the static regime. We found good agreement between the Birks model for equilibrated flourophores and the Debye-Smoluchowski equation (DSE) to accurately explain the ESPT reaction of CPT with acidic water in the reversible range. The proton motion at pH 2 (equilibrium range) exhibits diffusion-controlled behavior and can be explained using the Smoluchowski model. Our results show that the interaction of CPT with acidic water depends on the concentration of the acid, which changes the nature of both the structure and dynamics.
The Journal of Physical Chemistry A 05/2011; 115(20):5094-104. · 2.95 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report on femtosecond-nanosecond (fs-ns) studies of the triphenylamine organic dye (TPC1) interacting with titania nanoparticles of different sizes, nanotubes and nanorods. We used time-resolved emission and absorption spectroscopy to measure the photoinduced dynamics of forward and back electron transfer processes taking place in TPC1-titania complexes in acetonitrile (ACN) and dichloromethane (DCM) solutions. We observed that the electron injection from the dye to titania occurs in a multi-exponential way with the main contribution of 100 fs from the hot excited charge-transfer state of anchored TPC1. This process competes with the relaxation of the excited state, mainly governed by solvation, that takes place with average time constants of 400 fs in ACN and 1.3 ps in DCM solutions. A minor contribution to the electron injection process takes place with longer time constants of about 1-10 ps from the relaxed excited state of TPC1. The latter times and their contribution do not depend on the size of the nanoparticles, but are substantially smaller in the case of nanotubes (1-3 ps), probably due to the caging effect. The contribution is also smaller in DCM than in ACN. The efficient back recombination takes place also in a multi-exponential way with times of 1 ps, 15 ps and 1 ns, and only 20-30% of the initial injected electrons in the conduction band are left within the first 1 ns after excitation. The faster recombination rates are suggested due to those originating from the free electrons in the conduction band of titania or the electrons in the shallow trap states, while the slower recombination is due to the electrons in the deep trap states. The results reported here should be relevant to a better understanding of the photobehaviour of an organic dye with promising potential for use in solar cells. They should also help to determine the important factors that limit the efficiency of solar cells based on the triphenylamine-based dyes for solar energy conversion.
Physical Chemistry Chemical Physics 03/2011; 13(9):4032-44. · 3.57 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Single molecule studies of the free DY-630-MI and interacting with MCM-41 and (Al)MCM-41, show the conformational diversity of the molecule. The free dye is characterized by a single broad (fwhm = 0.7 ns) lifetime distribution histogram centered on 1.47 ns, which is also reflected in the broadness of the polarization value distribution histogram, covering almost the full range of values from -1 to 1. The fluorescence intensity traces of the free DY-630-MI show strong blinking behavior and weak photostability. Upon interaction with the mesoporous silica nanomaterials, MCM-41 and (Al)MCM-41, the dye molecule becomes more stable, with less blinking present in the fluorescence traces. The lifetime distribution histogram in the case of DY-630-MI/MCM-41 complexes is fitted by 3 Gaussians, indicating 3 distinct interaction sites. The Gaussian with the largest amplitude is centered on 2.19 ns, consistent with the confinement effect of MCM-41 and in agreement with the ensemble average studies. The polarization value distribution histogram becomes narrower in comparison with the free molecule and is more biased towards the positive limit. Replacing few Si(4+) ions with Al(3+) ones in the regular MCM-41 changes the local electrostatic field within the nanotube. This atomic substitution in the nanohosts results in a more selective orientation of the dye molecules, giving two populations with time constants 1.56 and 2.10 ns.
Physical Chemistry Chemical Physics 02/2011; 13(5):1819-26. · 3.57 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report on photophysical studies of lumichrome (Lc) in water at different pHs, and interacting with the human serum albumin (HSA) protein and β-cyclodextrin (β-CD) in neutral aqueous solutions. We used steady-state and picosecond time-resolved emission spectroscopy to investigate the structural changes of Lc at the ground and excited states, as well as the rotational dynamics of the complexes with HSA and β-CD. In neutral water, the predominant neutral alloxazine-type structure of Lc coexists with a small population of the anionic form. In the presence of HSA, we observed an increase in the absorption band intensity at 450 nm. This increase is due to a preferential complexation (1:1 stoichiometry, K=8600 M(-1)) of the Lc anion structures within the protein. This change is not observed when β-CD is added, in which the Lc neutral form is exclusively complexed, giving a 1:1 stoichiometry. The fluorescence lifetimes of Lc in neutral water solutions are 4.2 and 2.3 ns, assigned to anionic and neutral alloxazinic forms, respectively. Using β-CD, the lifetime of the 1:1 complexes is 0.74 ns, while in the case of HSA complexes we observed two lifetimes (0.83 and 0.14 ns), which we explained in terms of different interactions of the anions with the protein. The rotational relaxation time of free Lc in neutral water is 75 ps. For Lc:β-CD complexes this time is 0.44 ns, in full agreement with the expected value from the hydrodynamic theory. For HSA solutions, we obtained a distribution of values between ∼1 and 4.5 ns, suggesting a site heterogeneity of complexation and a different strength of binding for the involved Lc anionic forms. Our results give information about the different photorelaxation behavior of Lc within chemical and biological cavities, and might help in a better design of nanosystems for drug carriers and delivery.
The Journal of Physical Chemistry B 02/2011; 115(10):2424-35. · 3.70 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The relaxation dynamics of 5,10,15,20-tetrakis(4-hydroxyphenyl)-porphyrin (p-THPP) in tetrahydrofuran (THF) and encapsulated within the human serum albumin (HSA) protein in water solution was investigated. The protein environment affects the B→Q(y) and Q(x)→Q(y) transition dynamics (from 80 and 140-200 fs in THF to 50 and 100 fs in HSA, respectively) as well as the lifetime of the relaxed Q(x) state (9.1 vs 9.9 ns). The most prominent differences are observed in the relaxation dynamics in the hot Q(x) state in HSA, which includes the energy transfer to the protein in ∼1 ps and much slower solvent-assisted thermal equilibration component of about 20-30 ps.
The Journal of Physical Chemistry B 12/2010; 114(49):16567-73. · 3.70 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Exploring the relationship between the structure and dynamics of a molecular system is fundamental to a better understanding of its function. Here, we report on studies of femtosecond dynamics of the most stable molecular structures of a cardiovascular drug, levosimendan (LSM), in water at three different pHs, in chemical (β-cyclodextrin, β-CD) and biological (human serum albumin protein, HSA) nanocavities, and in two organic solvents with different viscosities. In the used organic solvents, the structural dynamics, ranging from 50 fs to 3 ps, depends on the viscosity of the solvent, reflecting the involvement of a twisting motion in the excited molecule. In water solutions at pH 3 and 5, the excited neutral form is decaying in a time of ∼0.4 ps, undergoing an ultrafast (<50 fs) intramolecular charge transfer (ICT) to generate charge transfer species decaying in ∼1 ps. In neutral (pH 7) and alkaline water (pH 12), the LSM is present in its anion structure at the ground state. In these media, the experiments reveal, in addition to the ultrafast decay of the anionic structure (1.3 ps), the formation of an ICT state having (n, π*) character, produced in ∼0.3 ps and decaying in ∼0.5 ps. Encapsulation by β-CD and HSA protein leads to a 1:1 stoichiometry complex, which shows longer decaying times (4 and 7 ps, respectively) of the caged anionic forms due to the nanoconfinement. Our results show a structural diversity of the LSM dynamics, reflecting its intimate interaction with its surrounding. We believe that the reported findings and the related discussion and conclusions bring new knowledge for a better understanding of the molecular activity of this drug, taking into account its rich structural dynamics. Furthermore, the results might be relevant for a better drug design and nanodelivery involving CDs and proteins.
The Journal of Physical Chemistry B 11/2010; 114(45):14787-95. · 3.70 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report on studies of the recently synthesized compound (TPC1) with a promising potential use in dye-sensitized solar cells. We used steady-state as well as femtosecond (fs) to nanosecond (ns) time-resolved emission techniques to understand its behaviour under different conditions of solvation and light excitation. In polar solvents the equilibrium between TPC1 normal and anion structures was found to depend on solvent H-bond acceptor ability and concentration of the dye. We observed a correlation between the contribution of the normal form in the total absorption spectrum and solar energy conversion efficiency of the photovoltaic devices prepared in different baths, which are high in dichloromethane and low in tetrahydrofurane. Both forms exhibit a large charge transfer character in the excited state manifested by a large Stokes shift between absorption and emission maxima (up to 9000 cm(-1) in acetonitrile). The lifetime of the relaxed state of the normal structure varies significantly with the solvent polarity (from 80 ps in acetonitrile to 1.8 ns in n-hexane), and it is considerably shorter than that of the anion one (1.2-2.6 ns). The ultrafast relaxation processes are dominated by the solvation dynamics which is the fastest in acetonitrile (below 1 ps) and the slowest in ethanol (about 25 ps, the amplitude-averaged time). The results reported here should be relevant to a better understanding of the photobehaviour of metal-free dyes for solar cells and help in the design of new and more efficient dyes for conversion of light to electricity.
Physical Chemistry Chemical Physics 07/2010; 12(28):8098-107. · 3.57 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report on studies of salicylaldehyde azine (SAA) dissolved in dichloromethane solution and within the cages of the faujasite zeolite (NaX) using steady-state and femtosecond to nanosecond time-resolved spectroscopy. In solution, an excited-state intramolecular proton-transfer reaction takes place in less than 80 fs, leading to a keto-type tautomer. In contrast within NaX zeolite, a zwitterionic (Z) form is present both at S0 and S1 states, and a large hypsochromic shift of the stationary emission spectrum is observed. The increase in fluorescence lifetime upon encapsulation (from 54 ps to 0.2−2.8 ns) is mainly due to hindrance in twisting motion of the confined Z structure imposed by the nanocage. A significant dependence of the lifetimes on the guest concentration inside the zeolite indicates an interaction between neighboring guest molecules leading to a quenching of the fluorescence. The analysis of emission decays using stretched-exponential model suggests that the excited-state interactions between neighboring dyes play a key role in the deactivation of the trapped Z fluorophores. For the ultrafast relaxation dynamics of the SAA/NaX composite, intramolecular vibrational-energy redistribution and vibrational cooling process occur in longer times (up to 360 fs and 5 ps, respectively). Additionally, the presence of nonfluorescent twisted (n,π*) state is suggested to form in 6−10 ps. We believe that our results are important for a better understanding of the photocycle of azine-based photochromic material when interacting with nanomaterials.
04/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: 2-[5′-N-(3-triethoxysilyl)propylurea-2′-hydroxyphenyl]-benzothiazole (HBTNH2) was covalently bonded to the inner surface of amorphous silica nanoparticles and unmodified and Al-doped mesoporous structured silicate, MCM-41 and (Al)MCM-41, respectively. The photodynamics of these materials was investigated using steady-state and femtosecond fluorescence up-conversion techniques. The guest molecule shows emission dependence on the excitation wavelength. Covalently bonding HBTNH2 to silica nanoparticles affects both the spectral and the dynamical behavior of the dye. The dual-band fluorescence and ultrafast dynamics indicate the possibilities for an excited-state intermolecular proton-transfer reaction between the dye and the silica framework. The observed behavior is assigned to contribution from excited populations of enol, keto, and anion forms. The excited-state dynamics and the emission spectra of the HBTNH2−MCM-41 material clearly reflect the confinement effect imposed by the MCM-41 nanochannels. Furthermore, we find that the photophysical behavior of HBTNH2 is strongly influenced by the Al content in the MCM-41 framework. We also show that, in the mesoporous silica materials with a low Al/Si ratio, the electronically excited-state intramolecular proton-transfer process is prevented and the resulting signals are due to the phenolate-type anion formed in the ground state. Our results demonstrate how the nature of the confinement affects both the steady-state and the time-resolved emission properties of a dye, which will shape the nanophotonics of this kind of nanomaterial.
03/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: We report on steady-state and time (ns to fs regime) resolved studies of H-bonding interactions and proton-transfer reaction dynamics of silica-based mesoporous material MCM-41 with an H-bond donor and acceptor guest aromatic molecule (7-hydroxyquinoline, 7HQ). We observed the ground state reaction which leads to the formation of intermediates and products of the confined molecular probe. We compare this behavior with the observed one for the dye adsorbed on the surface of silica particles, lacking the nanotubes. The result clearly shows that the formation of keto (or zwitterionic) tautomers at the ground state is enhanced by the confinement provided by the channels of MCM-41. Introduction of hydrophobic groups (by silylation of the OH groups in regular MCM-41 host) changes the ground state tautomeric equilibria and the emission behavior. A new lifetime (3.19 ns, suggested being due to a more stabilized anion of the guest) was observed in addition to the ones due to confined bound enol (0.26 ns), anion (1.5 ns), and zwitterionic (5.5 ns) structures. Both steady-state and ps-data show the importance of solvation of 7HQ structures inside MCM-41, when compared with the solid-state result. We investigated the intermolecular proton-transfer reaction dynamics in the confined structures using femtosecond-resolved emission spectroscopy, and we got the reaction times needed to produce the anion intermediates (0.3 ps) and zwitterion products (3 ps) upon electronic excitation of bound enol forms of the guest, in addition to the cooling times of the final zwitterionic form. We believe that our results might be useful for designing new nanophotonics sensors based on mesoporous materials, and open the window for further studies to better understand the chemical reactivity of silica-based nanohosts, at a short time scale.
03/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: The normal and anion structures of salicylaldehyde azine (SAA) in solvents of different viscosities and polarities have been studied by means of femto- to nanosecond time-resolved emission techniques. In the normal form, an excited-state intramolecular proton-transfer (ESIPT) reaction takes place with a time constant shorter than 80 fs to produce an excited keto-type tautomer in which intramolecular-vibrational energy redistribution and vibrational cooling occur in 100 fs to 2 ps. The viscosity-dependent emission decay in the red part of the spectrum with 5-11 ps reflects a twisting motion leading to rotamers of these keto-type structures, most probably of (n,pi*) nature. For the anion type, the viscosity dependent rise-times (3 to 400 ps) at the red part of the emission, and the wavelength-dependent fluorescence lifetimes (20 to 1100 ps) indicate a stepwise formation of different conformers of the anions. The results reported here should be relevant to a better understanding of the photobehaviour of photochromic compounds and charged chromophores in biological systems.
Physical Chemistry Chemical Physics 03/2010; 12(9):2107-15. · 3.57 Impact Factor
