Influence of Vibrational Excitation of Triplet Molecules of Anthraquinone on the Photochemical Processes in Gas Mixtures with Amines

By the quenching of the delayed fluorescence (DF) of anthraquinone vapors by aliphatic amines (diethylamine, dibutylamine, cyclohexylamine) and pyridine the photoinduced processes proceeding with the participation of vibrationally excited triplet molecules of anthraquinone have been investigated. The DF quenchingrate constants K _q varying from 1·10⁶ sec^–1·torr^–1 in mixtures with diethylamine to 7·10³ sec^–1·torr^–1 in mixtures with pyridine have been estimated. A correlation between the values of K _q and the ionization potentials of foreign gases confirming the important role of interactions with charge transfer in the quenching of triplet molecules in the gas phase has been established. The influence of other relaxation processes on the DF quenching is considered. It is shown that the intermolecular vibrational relaxation in the T ₁ triplet state leading to the establishment of relaxation equilibrium at a vibrational temperature T _vib considerably increasing the medium temperature is the fastest process among the biomolecular processes (rate constants K _col ^V > 10⁶ sec^–1·torr^–1 > K _q). The values of T _vib and the vibrational energies E _vib of the triplet molecules after the energy exchange in the collisional complex have been estimated. It has been concluded that the photochemical reaction yield is determined by the intermolecular processes proceeding in the T ₁ state at a vibrational equilibrium characterized by high values of T _vib. The influence of E _vib of triplet molecules on the DF quenching rates at a photoinduced electron transfer is considered.     

Determination of the Constants of Binding of Acridine Dyes with Micelles of Sodium Dodecyl Sulfate Using the Quenching of Delayed Fluorescence by Heavy Atoms

The constants of binding dye molecules with the micelles of sodium dodecyl sulfate are determined using quenching of delayed fluorescence of acridine dyes by sodium iodide in aqueous–micellar solutions. Kinetic equations have been composed that describe the processes of deactivation of the excited states of dyes. By solving these equations at the concentration of the quencher sodium iodide corresponding to the minimum lifetime of triplet states and at the concentration of micelles corresponding to the least value of the delayed fluorescence quenching rate constants, we obtained the constants of binding dyes with micelles equal to 1.3·10⁷, 2.9·10⁷, and 3.1·10⁷ M^–1 for trypaflavine, acridine orange, and acridine yellow, respectively. We calculated the rate constants of quenching of the triplet states of the molecules of dyes by iodide ions (I ^–) that decreased in transition from trypaflavine to acridine orange and acridine yellow.     

Role of charge-transfer complexes in oxygen quenching of excited states of anthracene derivatives in the gas phase

Oxygen quenching of excited triplet and singlet states of gas-phase anthracene and its derivatives that have similar energies of the lower triplet levels but widely different oxidation potentials (0.44 < E_ox < 1.89 V) was studied. Quenching rate constants for singlet (k_S^O2) and triplet (k_T^O2) states in addition to the fraction of oxygen-quenched singlet and triplet states q_{S 1}(T₁^O2 were determined from the decay rates, fluorescence intensities, and delayed fluorescence as functions of oxygen pressure. It was found that k_S^O2 values vary from 2·10⁴ (9,10-dicyanoanthracene) to 1.2·10⁷ sec⁻¹·torr⁻¹ (anthracene, 9-methylanthracene, 2-aminoanthracene) and k_S^O2 values from 5·10² to 1·10⁵ sec⁻¹·torr⁻¹. The k_S^O2 values for anthracene, 9-methylanthracene, and 2-aminoanthracene, which have fast rates of interconversion from S₁ to T₁, are close to the rate constants for gas-kinetic collisions and are independent of the oxidation potentials (E_ox). The quenching rate constants k_S^O2 for the other anthracene derivatives and k_T^O2 for all studied compounds decrease with increasing free energy of electron transfer ΔG_ET, which indicates the important role of charge-transfer interactions in the oxygen quenching of singlet S_1- and triplet T₁ states. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 1, pp. 36–42, January–February, 2008.     

On Electron Spin Polarization Created in the Excited Triplet State of Accessory Chlorophyll via Photoinduced Charge-Recombination of the Photosystem II Reaction Center

We present a theoretical approach to investigate the electron spin polarization (ESP) of the excited triplet state that has been detected using the time-resolved electron paramagnetic resonance (TREPR) method in the photosystem II center of the plants. We show, using the stochastic Liouville equation, that the ESP pattern created in the accessory chlorophyll (Chl_accD1) which reside near the P_D1 chlorophyll of the active branch is explained by one-step, concerted double electron transfer model, initiating from the singlet–triplet conversion of the light-induced charge-separated state composed of P_D1 radical cation and pheophytin radical anion. We also considered the sequential ESP transfer model via the triplet charge-recombination (CR) and the triplet–triplet energy transfer processes. It has been clearly shown that the ESP created in the ³Chl_accD1* is dependent on the rate constant (k _TT) of the triplet–triplet energy transfer from the intermediate triplet state created by the CR. Also we show that the relative orientation of the principal axes of the spin dipolar interaction in the intermediate triplet state (³P_D1*, as an example) may play a role in the ESP pattern, when the k _TT is smaller than the angular frequency of the Zeeman energy. We have theoretically shown that the TREPR measurement of the ESP is very powerful to investigate the primary chemical process and to characterize the intermediate as a signature of the stepwise ESP transfer.     

Reductive Fluorescence Quenching of the Photoexcited Free Base <Emphasis Type="Italic">meso</Emphasis>-Tetrakis (Pentafluorophenyl) Porphyrin by Amines

Steady state and time resolved fluorescence quenching behaviors of meso-Tetrakis (pentafluorophenyl) porphyrin (H₂F₂₀TPP) in presence of different aliphatic and aromatic amines have been executed in homogeneous dichloromethane (DCM) solution. At room temperature in DCM, free base (H₂F₂₀TPP) shows fluorescence with two distinct peaks at 640 and 711 nm and natural lifetime τ _f = 9.8 ns which are very similar to that of meso-tetraphenyl porphyrin (TPP). Unlike TPP, addition of both aliphatic and aromatic amines to a solution containing H₂F₂₀TPP results in an efficient decrease in fluorescence intensity without altering the shape and peak position of fluorescence emission. Upon addition of amines there was no change in optical absorption spectra of H₂F₂₀TPP. The fluorescence quenching rate constants ranged from 1 × 10⁹ to 4 × 10⁹ s⁻¹, which are one order below to the diffusion control limit, and temperature dependent quenching rate constants yield the activation energies which are found to be order of 0.1 eV. Femto second transient absorption studies reveal the existence of amine cation radical and porphyrin anion radicals with very short decay time (15 ps). The fluorescence quenching reaction follows Stern–Volmer kinetics. Steady state and time-resolved data are interpreted within general kinetic scheme of Marcus semi-classical model which attributes bimolecular electron transfer process between amines and the lowest excited singlet state of H₂F₂₀TPP. Calculated internal reorganization energies are found to be in between 0.04 and 0.22 ev. Variation of electron transfer rate as function of free energy change (∆G⁰) points the ET reactions in the present systems are in Marcus normal region. This is the first example of reductive fluorescence quenching of free base neutral porphyrins in homogeneous organic solvent ever known.     

Quenching of ππ* triplet states of vaporous polycyclic aromatic compounds by oxygen

The oxygen quenching rate constants k _T ^O2 of the triplet state T ₁ of vapors of polycyclic aromatic hydrocarbons (PAHs) with strongly different oxidation potentials 0.44 eV < E _OX < 1.61 eV and energies of the triplet levels 14800 cm^?1 < E _T < 24500 cm^?1 (anthracene, 2-aminoanthracene, 9-nitroanthracene, chrysene, phenanthrene, fluoranthene, and carbazole) are estimated from the measured dependences of the decay rates and intensities of delayed fluorescence on the oxygen pressure P _O2. It is found that the rate constants k _T ^O2 vary from 4 × 10³ (9-nitroanthracene) to 4 × 10⁵ s^?1 Torr^?1 (2-aminoanthracene) and increase with decreasing oxidation potentials E _OX of PAHs. The rate constants k _T ^O2 for vapors and solutions are compared. The dependences of k _T ^O2 on the free energy of two intermolecular processes, namely, triplet energy transfer to oxygen and electron transfer, are analyzed. It is shown that the rate constants k _T ^O2 increase with decreasing electron transfer free energy, which proves that, along with energy transfer, charge-transfer interactions contribute to the quenching of the triplet states of PAH vapors.     

Optical and time-resolved EPR studies of the excited states of azastilbenes and their protonated cations

The effects of protonation on the excited states oftrans-3-styrylpyridine (StP) andtrans-4,4′-dipyridylethylene (DPE) have been studied through measurements of the time-resolved electron paramagnetic resonance (EPR), ultraviolet absorption, and fluorescence spectra in methanol-water mixtures at 77 K. The assignment of the transient EPR signals was carried out with the aid of the stretched poly(vinyl alcohol) films method. From the analysis of these spectra it is concluded that the single protonation appears to have little effect on the zero-field splitting parameters and the anisotropy in the sublevel populating rates of the lowest excited triplet (T₁) states of StP and DPE. However, the decay rate constants of the fluorescent states decrease and fluorescence quantum yields increase on single protonation. These experimental results suggest that the single protonation causes a decrease in the intersystem crossing (ISC) rates for the three T₁ sublevels. These results are explained in terms of the vibronic mixing between the¹nπ* and¹ππ* states in the lowest excited singlet state. The assignment of StP to the specified conformer was carried out through the analysis of the anisotropic ISC processes.     

Combined Quenching Mechanism of Anthracene Fluorescence by Cetylpyridinium Chloride in Sodium Dodecyl Sulfate Micelles

The Stern-Volmer quenching constant (K_SV) for quenching of anthracene fluorescence in sodium dodecyl sulfate (SDS) micelles by pyridinium chloride has been reported previously to be 520 M^?1 based on steady state fluorescence measurements. However, such measurements cannot distinguish static versus dynamic contributions to the overall quenching. In the work reported here, the quenching dynamics of anthracene in SDS micelles by cetylpyridinium chloride (CPC), an analogue of pyridinium chloride, were investigated using both steady state and time resolved fluorescence quenching. Concurrent measurement of the decrease in fluorescence intensity and lifetime of anthracene provide a quantitative evaluation of collision induced (i.e. dynamic) versus complex formation (i.e. static) quenching of the anthracene fluorophore. The results reveal that a combined quenching mechanism is operative with approximately equal constants of 249?±?6 M^?1 and 225?±?12 M^?1 for dynamic and static quenching, respectively.     

Photophysics of the triple state of metal complexes of 9-anthracenecarboxylic acid

The triplet-triplet (T-T) absorption spectra and the T-T absorption decay kinetics are measured for solutions of 9-anthracenecarboxylic acid (ACA) and its complexes with metal ions (Cd³⁺ and Ln³⁺=Y³⁺, La³⁺, Ce³⁺, Eu³⁺, Gd³⁺, and Tb³⁺) in dimethylsulfoxide (DMSO) by the methods of flashlamp and laser pulse photolysis. The rate constants k _T of intracomplex quenching of the triplet state are measured for ACA complexes with ions Gd³⁺, Ce³⁺, Tb³⁺, and Eu³⁺. Larger values of k _T in complexes of ACA with paramagnetic ions Ce³⁺, Tb³⁺, and Eu³⁺, which have low-lying energy levels, compared to the values of k _T for complexes with other ligands (pyrene-3-sulfonate, pyrene-1,3,6,8-tetrasulfonate, and benzo[ghi]perylene-1,2-dicarboxylate) were explained by the lower energy of the triplet state of ACA (14400 cm^?1). For a complex with a paramagnetic ion Gd³⁺, which has no low-lying energy levels, the value of k _T is close to that measured by us earlier for the inner-sphere complex of pyrene-1,3,6,8-tetrasulfonate with the same ion. These results confirm our earlier assumption about the inner-sphere complexing of ACA with Ln³⁺ ions in DMSO.     

Photophysical studies of xanthene dye in alkanols and in presence of inorganic ions

Eosin Y belongs to a xanthene group. It is an anionic fluorescent dye. The absorbance and fluorescence of Eosin Y have been investigated in a series of alkanols (methanol to propanol). When the solvents are added to the aqueous solution of Eosin Y (EY) the absorbance and fluorescence intensity are enhanced. The alkanols are found to affect the absorption and fluorescence spectra of the dye. On the basis of solvent adsorption model the binding constants of the dye with alkanols have been estimated. The interaction of solvent molecule with dye in aqueous solution is specific in nature. The fluorescence quenching of Eosin Y by the inorganic ions [Fe(CN)₆]⁻³, [Fe(CN)₆]⁻⁴ and Cl⁻ was also observed. The ions influenced the quenching process to different extents. The rate constants of quenching were calculated using the Stern-Volmer equation. The equilibrium constant of dye in presence of inorganic ions are determined by Scott equation.     

A study on the fluorescence quenching of 9-Aminoacridine by certain antioxidants

The fluorescence quenching of 9-Aminoacridine by certain estrogens and flavonoids in water was studied using absorption, steady state and time-resolved measurements. The bimolecular quenching rate constants for the chosen estrogens and flavonoids were found to be in the range of 3.2-9.2×10¹¹ and 0.36-14.46×10¹¹ M⁻¹s⁻¹, respectively. From lifetime measurement we observed that the quenching was mainly due to static mechanism through ground state complex formation. The binding constant (K) and the number of binding sites (n) were calculated based on the fluorescence quenching data. The free energy change (ΔG_et) for electron transfer process was calculated by Rehm-Weller equation.     

Energy transfer from the singlet levels of diketones and dyes to lanthanide ions in nanoparticles consisting of their diketonate complexes

The energy transfer from the S ₁ levels of p-phenylbenzoyltrifluoroacetone (PhBTA) and dyes to different Ln³⁺ ions is studied in nanoparticles (NPs) composed of complexes of this diketone with Ln³⁺ and 1,10-phenanthroline (phen) and doped with dye molecules. The quenching rate constants in the NPs consisting from complexes of Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Ho³⁺, Er³⁺, and Tm³⁺ are determined from the data on the quenching of sensitized (cofluorescence) and ordinary fluorescence of coumarin 30 (C30) and rhodamine 6G (R6G). The quenching rate constants vary from ≤5 × 10¹¹ to 10¹³ s^?1 for the fluorescence quenching of PhBTA by different Ln³⁺ ions, while the quenching of dye fluorescence occurs at rates of the order of 10⁹ s^?1. In the case of complexes with the Pr³⁺ ions, the fluorescence quenching of PhBTA in NPs composed of its complexes is accompanied by sensitized luminescence of Pr³⁺. The quenching observed is due to a nonradiative energy transfer from the S ₁ states of ligands and dyes to these ions. It is shown that in NPs composed of complexes with Eu³⁺, Yb³⁺, and Sm³⁺ the cofluorescence of C30 is quenched via the electron-transfer mechanism. The study of quenching of cofluorescence and fluorescence of dyes in NPs composed of mixed complexes of La³⁺ and Nd³⁺ (Ho³⁺) shows that the observed quenching of fluorescence and cofluorescence is governed mainly by the quenching of the S ₁ state of dyes when the Nd³⁺ (Ho³⁺) content does not exceed 5–10% and by the quenching of the S ₁ state of a ligand when the Nd³⁺ (Ho³⁺) content exceeds 50%. It is assumed that the high rate constant of energy transfer from the S ₁ level of ligands to ions Pr³⁺, Nd³⁺, Ho³⁺, Er³⁺, and Tm³⁺ in NPs composed of beta-diketonate complexes is caused by exchange interactions.     

In search of the dark state of 5-methyl-2-hydroxypyrimidine using a numerical DFT/MRCI gradient

In a recent publication, Lobsiger et al. [Phys. Chem. Chem. Phys. 12, 5032 (2010)] presented infrared and electronic absorption spectra of supersonic jet-cooled 5-methyl-2-hydroxypyrimidine (5M2HP), the enol form of deoxythymine. In addition, they reported on the fast nonradiative decay of the S₁ population to a dark state. In the present paper, we have investigated the mechanism and rate constants of this nonradiative decay by means of quantum chemical multi-configuration methods. To this end, minima of the lowest excited singlet and triplet states as well as the minimum-energy crossing point of singlet and triplet potential energy hypersurfaces (PEHs) have been determined employing a numerical DFT/MRCI gradient where DFT/MRCI stands for a combination of density functional theory (DFT) and a semi-empirical multi-reference configuration interaction (MRCI) approach. Rate constants have been calculated in the Condon approximation using a time-dependent approach based on harmonic oscillator functions and electronic spin–orbit coupling matrix elements evaluated at the DFT/MRCI level. It is shown that the first excited triplet state possesses ³(n?→?π*) character in the gas phase. Fast intersystem crossing is mediated by the low-lying ³(π?→?π*) state whose PEH crosses both, the S₁ ¹(n?→?π*) and T₁ ³(n?→?π*) PEHs.     

Photoinduced intramolecular electron-transfer reactions in carbazole-fullerene and phenothiazine-fullerene linked compounds in benzene and benzonitrile as studied by fluorescence, transient absorption, time-resolved EPR, and magnetic field effects

Photoinduced intramolecular electron-transfer reactions in carbazole (Cz)-fullerene (C₆₀) (Cz(8)C₆₀) and phenothiazine (Ph)-C₆₀ (Ph(n)C₆₀ (n=8, 10, 12)) linked compounds have been investigated in benzene and benzonitrile by fluorescence, transient absorption, and time-resolved electron paramagnetic resonance measurements, and by magnetic field effects on the decay rate constants of the photogenerated biradicals. In benzonitrile, photoinduced intramolecular electron transfer from Cz to the singlet excited state of C₆₀ (¹C₆₀ ^*) occurred in Cz(8)C₆₀, but not to the triplet excited state (³C₆₀ ^*), while the intramolecular electron-transfer to both¹C₆₀ ^* and³C₆₀ ^* occurred in Ph(n)C₆₀ (n=8, 10, 12). In benzene, on the other hand, no electron transfer to both¹C₆₀ ^* and³C₆₀ ^* took place in all linked compounds. These results were interpreted in terms of the different Gibbs free energy changes in the two solvents.     

CIDEP Created by the Quenching of Photo-Excited Tryptophan at Protein Surface: A Challenge to CIDEP Probing of Protein Structural Changes

Quenching of the triplet excited state of molecular tryptophan by nitroxide radical in 1,4-dioxane and water solutions was investigated by means of time-resolved electron paramagnetic resonance (EPR) and Fourier-transform (FT)-EPR. The chemically induced dynamic electron polarization (CIDEP) signals with net emissive phase were recorded at these quenching events and were analyzed through radical-triplet pair mechanism. The CIDEP time profiles were well reproduced by Bloch and kinetic equations, assuming radical-triplet pair mechanism with the appropriate quenching rate constants. From a comparison of the simulation and the experiment, CIDEP enhancement factor in 1,4-dioxane was determined to be −30 × P _eq, where P _eq is the spin polarization of nitroxide at thermal equilibrium. Net emissive CIDEP was also observed by FT-EPR measurements on the nitroxide quenching of the triplet excited state of tryptophan residue in α-lactalbumin. Magnitude of CIDEP created in α-lactalbumin/nitroxide system depends on the pH condition of α-lactalbumin solution, which is related to protein folding dynamics. We argue the CIDEP mechanism at the α-lactalbumin surface and propose a possibility of a novel CIDEP method to probe a protein surface and structural changes.     

Features of the Quenching of the Triplet States of Water-Soluble Porphyrins by Molecular Oxygen

Using the methods of time-resolved absorption spectroscopy, we have investigated the features of quenching, by molecular oxygen, of the excited triplet states of water-soluble 5,10,15,20-tetrakis-(4-N-methylpyridyl)-porphyrin (H₂TMPyP) and 5,10,15,20-tetrakis-(4-sulfonatophenyl)-porphyrin (H₂TSPP) in water–ethanol solutions. It has been revealed that for both compounds the rate constant of quenching of the triplet states increases with increasing viscosity of the medium. Quenching of the excited triplet states of the dissociated (in water) and undissociated (in ethanol) forms of water-soluble porphyrins occurs with a different efficiency, and the rate constant of quenching the triplet states by molecular oxygen k _T thereby is higher for the dissociated form. It has been shown by means of mathematical modeling that the experimental results obtained can be described in terms of the change in the rate constants of intracomplex transitions in the porphyrin–oxygen collisional complex at varied solution viscosity and their difference for the dissociated and undissociated forms of water-soluble porphyrin.     

Photoinduced interaction between MPA capped CdTe QDs and certain anthraquinone dyes

Photoinduced interaction of mercapto propionic acid (MPA) capped CdTe quantum dots (QDs) with certain anthraquinone dyes namely alizarin, alizarin red S, acid blue 129 and uniblue has been studied by steady state and time resolved fluorescence measurements. Addition of anthraquinone dyes to CdTe QDs results in the reduction of electron hole recombination has been observed (i.e., fluorescence quenching). The Stern-Volmer constant (K_SV), quenching rate constant (k_q) and association constants (K) were obtained from fluorescence quenching data. The interaction of anthraquinone dyes with QDs occurs through static quenching was confirmed by unaltered fluorescence lifetime. The occurrence of electron transfer quenching mechanism has been proved by the negative free energy change (ΔG_et) obtained as per the Rehm-Weller equation.     

Oxygen fluorescence quenching studies with single tryptophan-containing proteins

The work of Lakowicz and Weber [Biochemistry 12, 4161 (1973)] demonstrated that molecular oxygen is a powerful quencher of tryptophan fluorescence in proteins. Here we report studies of the oxygen quenching of several proteins that have a single, internal tryptophan residue. Among these are apoazurin (Pseudomonas aeruginosa), asparaginase (Escherichia coli), ribonuclease T₁ (Aspergillus oryzae), and cod parvalbumin. Both fluorescence intensity and phase lifetime quenching data are reported. By comparison of these data we find that there is a significant degree of apparent static quenching in these proteins. The dynamic quenching rate constants,k _q, that we find are low compared to those for tryptophan residues in other proteins. For example, for apoazurin we find an apparentk _q of 0.59×10⁹ M ^–1 s^–1 at 25°C. This value is the lowest that has been reported for the oxygen quenching of tryptophan fluorescence.     

Photoluminescence Electron-Transfer Quenching of Rhenium(I) Complexes with Organic Sulfides

Electron-transfer(ET) from organic sulfides to excited state rhenium(I)-based heteroleptic tricarbonyl complexes [Re(bpy)(CO)₃(py)]⁺ (I) and [Re(bpy)(CO)₃(ind))]⁺ (II) in acetonitrile solution is facile and luminescence quenching constants, k_q, are in the range 10⁵–10⁸ M⁻¹s⁻¹. The detection of the sulfide radical cation in this system using time-resolved absorption spectroscopy is a direct evidence for the ET nature of the reaction. The k_q values for the quenching of Re(I)-complexes with organic sulfides are analyzed with a scheme involving rate controlling electron transfer process. The measured rate constants for the electron transfer (ET) reaction are close to the values calculated from Marcus theory.     

Kinetic studies of Kr2 Cl in electron-beam excited gas mixtures

Gas mixtures containing atomic krypton and carbon-tetrachloride were excited using intense electron beam pulses. The fluorescence produced by Kr₂Cl¹ was studied using time-resolved spectroscopic techniques. The lifetime (470 ns) and the quenching rate constants of Kr₂Cl¹ were deduced from the measurements.