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.     

Temperature Dependence of the Efficiency of Collisional Transfer of Vibrational Energy in Mixtures of Vibrationally Excited Triplet Molecules with Foreign Gases

By the pressure dependences of the decay rates of delayed fluorescence activated by vibrational excitation of triplet molecules of benzophenone and anthraquinone, the efficiencies of collisional transfer of vibrational energy (V–V-transfer) in the vibrational quasi-continuum of the triplet state have been estimated. It is shown that the efficiencies of the process in mixtures with foreign gases increase with increasing dipole moment and polarizability of colliding molecules. In the temperature range 433–513 K, we obtained an inverse temperature dependence of the V–V-transfer efficiency, which is satisfactorily described by empirical relations taking into account long-range attractive forces. The results obtained point to the determining role of long-range attractive forces in quasi-resonance V–V-transfer of vibrational energy by molecules excited in vibrational quasi-continuum.     

Photophysical Processes in the Gas Phase at High Levels of Vibrational Excitation of Triplet Molecules of Benzophenone and Fluorenone

By the delayed fluorescence activated by direct multiphoton excitation of triplet molecules by CO₂–laser radiation we have studied the prevailing deactivation pathways of triplet molecules with a high store of vibrational energy E _vib. The dependences of the kinetic characteristics of delayed fluorescence on the presence of vapors and foreign gases have been used to estimate the rates and efficiencies of intermolecular vibrational relaxation in the vibrational quasi–continuum of the triplet state T ₁. By the changes in the intensities and decay rates over a wide range of vibrational energies we have established the E _vib dependences of reversible intercombination conversion between the states T ₁ and S ₁ and interconversion from T ₁ to the ground electronic state S ₀ for both the case of isolated excited molecules and at a steady vibrational temperature. It is shown that at high vibrational temperatures the radiationless transition from the T ₁ state to S ₀ has an activation character and is accomplished through the energy barrier. The conditions for going to an exponential dependence have been determined. It has been found that the obtained dependences are in good agreement with the known experimental results. The influence of molecular and environmental characteristics on the decay rate of triplet molecules is compared.     

Effect of Supercollisions on Energy Transfer in Mixtures of Bath Gases and Laser Excited Complex Molecules

Intensities and decay rates of delayed luminescence (DL) initiated by a pulse of N₂ laser were employed to probe collisional relaxation of complex molecules (benzophenone, acetophenone) diluted with bath gases Ar, Kr, Xe, C₂H₄, SF₆, C₅H₁₂. It was shown that vibrational relaxation can be interpreted in terms of two consecutive processes: vibration-vibration (V-V) and vibration-translation (V-T). The results clearly demonstrated that fast component of DL can be used to study V-V energy transfer. It was found that at relatively small internal energy the collisional efficiences of V-V process had the values typical for molecular processes in which supercollisions contribute. The average energies transferred per collision, (ΔE), well correlated with predictions of the simple ergodic collision theory of intermolecular energy transfer.     

Collisional Energy Transfer of Highly Vibrationally Excited Polyatomic Molecules. The Effect of Excited Molecule Complexity

Collisional relaxation was probed by CO₂ laser activated delayed fluorescence. The experimental information was adopted to determine the average energies transferred per collision (ΔE) from highly vibrationally excited polyatomic molecules to parent collider. The values of (ΔE) decreased with increasing the number of atoms in the excited molecules in line: biacetyl, acetophenone, benzophenone, antraquinone. The dependences of (ΔE) on the number of factors such as: 1) the average vibrational energy residing in the vibrational modes of excited molecules; 2) the potential of intermolecular interaction; 3) the reduced mass, and others were analyzed in details. The general interplay was noticed between (ΔE) and the molecular parameters which determined the increasing interaction strength and the decreasing energy transfer efficiency due to the adiabatic constraints on the energy transfer.     

Determination of the Probability of Intermolecular Energy Transfer of the Triplet State of a Number of Tetrapyrrole Molecules in Liquid Solutions

The quenching of the triplet state of a number of Pd porphyrins and chlorophyll a by other complexes of porphyrins with Pd and Fe in liquid solutions was investigated with the use of pulsed photoexcitation. It is shown that the experimental data are consistent with the results of a kinetic analysis of the mechanism of quenching of the triplet state through the formation of excited complexes of interacting molecules. On the basis of analytical and experimental data, the probability values of intermolecular tripletstate energy transfer have been determined: K _ET = 2·10⁴3·10⁵ sec^–1. A conclusion about the character of the excited complexes has been drawn; in particular, the distance between the molecules in the complexes has been estimated to be 5 .     

Direct Measurement of Collisional Energy Transfer Between Highly Vibrationally Excited Triplet Fluorenone and Bath Gases

Intensities and decay rates of CO₂-laser induced delayed fluorescence are used to probe collisional relaxation of vibrationally excited fluorenone diluted with bath gases: He, N₂, Kr. The average energies 〈ΔE〉 transferred per collision are found to vary with vibrational energy, the energy dependences of the collisional efficiency eventually level off.     

Relaxation Processes at High Levels of Vibrational Excitation of Polyatomic Molecules in the Ground Electronic State

By the spectral and kinetic characteristics of the luminescence of vapors of polyatomic molecules (anthracene, anthraquinone, fluorenone) initiated by selective IR multiphoton excitation (IR MPE) of molecules in the ground electronic state S ₀ the relaxation processes proceeding under vibrational excitation of molecules to energies exceeding the energies of the lower excited electronic states have been investigated. The changes in the spectral and kinetic characteristics with increasing CO₂ laser energy density and vapor P _v and foreign gas pressure P _FG are analyzed. They are similar to the characteristics obtained for normal fluorescence of these molecules with changing vibrational energy E _vib content. On the basis of experimental data and model calculations it has been concluded that at the laser radiation densities used in the case of IR MPE the molecules reach energies considerably exceeding the energies of the electronic levels. It is shown that a nonadiabatic connection between the electronic states leads to the population of mixed electronic states isoenergetic to the vibrational levels of the ground electronic state and to emission of delayed luminescence spectrally identical to the normal luminescence of these molecules. It has been found that when high vibrational levels are populated, new relaxation channels, such as reverse electron relaxation, emission from high vibrational levels of the ground electronic state, and multiquantum vibrational energy transfer at collisions leading to a rapid establishment of vibrational equilibrium become important.     

有机发光材料中的三重激发态

有机发光材料有望广泛应用于新一代柔性光电子器件。由于自旋多重性,有机分子发光材料 中单重激发态和三重激发态转换较慢,限制有机发光器件特别是电注入荧光器件的效率。我们介绍 下近年来通过分子设计操控激发不同时间尺度三重态的动力学来突破这一限制的策略,通过控制激 发单重态和激发三线态之间的电子耦合,利用热激子系间窜越、反向系间窜越、激发三重态稳定化 等过程能够有效提高有机发光材料的发光效率。在此基础上实现的热活化延迟荧光、有机长余辉发 光等在有机发光二极管、传感器、生物成像等领域有重要潜在应用价值。     

Role of Mode-mode Coupling in Short-time Excited State Decay

Master equation of a relevant electronic and vibrational system is derived for a special diabatic basis corresponding to vertical processes. It is shown that bath modes contribute dynamically to the inter-state coupling only at short times. For long times the bath-induced inter-state coupling is static and increases with the contribution of bath modes to the Stokes shift and to the Herzberg-Teller correction of the excited state. Simultaneously, the time evolution of excited state population is studied numerically for the system consisting of two electronic levels interacting with two vibrational modes, coupled to a heat bath. A mutual coupling of the vibrational modes in the excited state is taken into account (Duschinsky effect). Excited state population relaxes faster if interacting vibrational mode dissipates its energy via vibrational mode of a smaller eigenfrequency. Fast component of excited state depopulation cannot be achieved via coherent mode-mode coupling, if the second mode is not directly coupled to the electronic inter-state transition.     

Nature of the Electron Excited States and Energy Transfer in Bichromophore Coumarin Molecules

Results of experimental and theoretical research for three bichromophore molecules, trans-stilbene-CH₂-coumarin 120 (I), 4-methylumbelliferone-CH₂-UC 17, and 4-(3-fluoro)-methylumbelliferone-CH₂-UC 17 (II, III), are presented. Schemes of photophysical processes in the bichromophore molecules based on quantum chemical calculations by the INDO method and theory of radiationless transitions in polyatomic organic molecules are suggested. After optical excitation to the strong donor absorption band, the fast internal conversion processes develop there. As a result, the molecule is found in the S ₁ ^* -state localized on the acceptor moiety. It is shown that a mechanism of intramolecular transfer energy in bichromophores different from that proposed by Förster may be realized. Excitation energy, initially located on D, will be transferred from the donor moiety to the acceptor chromophore in convenience of the internal conversion process. The intramolecular electronic energy transfer from energy donor to energy acceptor may be interpreted as the internal conversion process. The rate constants of internal conversion are calculated.     

Vibrational Structure of the Absorption Spectra and a Model of the HNO and DNO Molecules in the Excited State

The vibrational structure of the absorption spectra of the first n–^*–electron transitions of the HNO and DNO molecules is calculated in the Franck–Condon approximation. A structural model of the molecules in the excited electronic state is constructed on the basis of correlations and with the aid of a method of hybrid atomic orbitals. Evaluation of the influence of deuterium substitution on the intensities of the vibrational components upon electronic excitation is made. A comparison of the experimental and theoretical absorption spectra calculated for different models of the molecules is carried out.     

Influence of donor and acceptor groups on the S-T energy gap for thermally activated delayed fluorescence emitters

Molecules composed of different donors and acceptors are theoretically designed as potential thermally activated delayed fluorescence emitters, and their singlet–triplet (S-T) energy gap is studied using the optimal Hartree–Fork method. It is found that the S-T energy gap is in reverse proportional to the electron-donating ability. Stronger electron-donating ability of donors will induce smaller highest occupied molecular orbital–lowest unoccupied molecular orbital overlap and also a smaller S-T energy gap. Based on our calculation results, three molecules are proposed to have great potential to be used as thermally activated delayed fluorescence emitters in organic light-emitting diodes.     

Vibrational energy redistribution and vibrational dynamics of methanol mixed with Rhodamine 101 dye

Vibrational energy transfer that occurs after photoexcitation can be tracked in the first several femtoseconds by ultrafast time- and frequency-resolved CARS (coherent anti-Stokes Raman scattering) spectroscopy. Vibrational energy transfer from high-frequency modes to lower ones through chemical bond of pure methanol and methanol/Rhodamine 101 (Rh101) solution is detected. Through comparison and analysis of the experimental results, it is found that surrounding molecules have a significant influence on vibrational energy transfer and vibrational couplings among relevant modes.     

Intermolecular energy transfer in mixed laser dyes: photophysical properties of triplet states

Triplet-singlet energy transfer in laser dyes have been studied in EPA at 77K using N₂ laser as an excitation source. Phosphorescence of the donor (D) and the delayed fluorescence of the acceptor (A) and their lifetimes have been measured for coumarin 102 (D)-rhodamine B(A) and 9(10H)-acridone (D)-rhodamine 6G(A) dye systems as a function of acceptor concentration. These data yield energy transfer rate constants of ∼10³ dm³ mol⁻¹ s⁻¹ for the donor acceptor combinations, consistent with the Forster mechanism. The phosphorescence quantum efficiency and other spectral parameters are also reported.     

Nature of delayed luminescence of N-methylindole and indole in the gas phase

We have studied the spectral characteristics and the kinetics of delayed (long-lived) luminescence of N-methylindole and indole in the gas phase. For N-methylindole, we observed delayed annihilation fluorescence, spectrally matching fast fluorescence. There is no delayed annihilation fluorescence for indole, but we observed a delayed luminescence band with wavelength at the maximum 535 nm, which we interpreted as luminescence of free radicals formed as a result of dissociation of the N-H bond. We hypothesize that the excited states of the free radicals arise as a result of nonradiative energy transfer from indole in the triplet state to indole free radicals in the doublet state. The lifetimes of the triplet states of N-methylindole and indole in the gas phase at T = 373 K, obtained from analysis of the delayed luminescence kinetics, are 2.5 msec and 1.0 msec. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 3, pp. 341–345, May–June, 2007.     

Homo- and Heteroannihilation of Triplet-Excited Bengali Rose and Anthracene Molecules on Silica in a Wide Temperature Range

The processes of triplet-triplet annihilation of the triplet-energy donor of Bengali rose dye and anthracene acceptor adsorbed on the surface of wide-pore silica have been investigated in the temperature range 150–290 K. The rate constants of homo- and heteroannihilation of the molecules of luminophors have been determined in a wide temperature range. It has been established that the processes of energy transfer in the initial (after photoexcitation) periods of phosphorescence decay are described by the Inokuti–Hirayama equations modified in the present work for a two-dimensional problem, whereas in the mean-time and long-time periods the kinetics of phosphorescence decay becomes similar to the fractal one.     

Probability estimate for intersystem crossings in naphthalene and acenaphthene in the presence of benzophenone

To a first approximation in perturbation theory, we have obtained expressions for the rate constants for intersystem crossings in acceptor molecules. We show that exchange interactions between the components of the donor-acceptor pair can change the probability of intersystem crossings for both radiative T → S₀ and nonradiative S₁ → T transitions. The theoretical conclusions are supported by the results of experimental studies. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 4, pp. 550–553, July–August, 2006.     

Long-lived luminescence of complex molecules

Results of investigations of all types of long-lived luminescence of organic molecules in the gas phase and condensed media are presented. Methods for identification and separation of contributions of phosphorescence and thermally activated fluorescence in long-lived luminescence of organic-molecule vapors are proposed. Energy transfer and migration processes in the case of the inductive-resonant mechanism of intermolecular interaction leading to the appearance of sensitized phosphorescence and annihilation-induced delayed fluorescence are considered. Experimental results on energy migration obtained for solid solutions of organic compounds are analyzed within the framework of the concepts of percolation theory with account for the microscopic inhomogeneity of the systems under investigation and the fractal properties of the clusters of activator molecules. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, F. Skorina Ave., 70, Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 5, pp. 662–674, September–October, 1998.