The processes of radiationless conversion in aromatic and heteroaromatic molecules are investigated theoretically. The values of constant rates of internal conversion are calculated. The theoretical estimation of constant rates of S-T conversion aromatic hydrocarbon molecules and in their carbonyl-, thiocarbonyl-, and nitroderivatives and in azaheterocyclic molecules are given. The S-T conversion probability between the states of different orbital nature (nr* and r r * ) is equal to ca. 101o-lO1l sec-' that is two to four orders higher than the conversion probability between the states of the same orbital nature. It is shown that the process of T-S conversion may be described in the second and in the highest orders of perturbation theory. The luminescent characteristics of molecules are connected with the relative position of electron states of different orbital nature and multiplicity and conversion characteristics.One of the principal problems in molecular spectroscopy is to establish and explain the relation between the luminescent properties of organic molecules and their structure. Apart from theoretical interest, the problem is very important practically owing to the extensive use of luminescence in photochemistry, photobiology, radiation chemistry, and the chemistry of organic luminophores and scintillators. The problem is rendered even more urgent by the wide utilization of organic molecules in laser engineering.The radical progress of recent years in understanding the luminescent properties of molecules as related to their structure has been stimulated by the advent of electronic molecular structure theories and new knowledge on electronic transitions and states that have been used to interpret the experimental results. The following three points were critical to this progress: (i) determination of possible positions of electronic states of different orbital nature and multiplicity; (ii) determination of the relation between electronic state positions and system parameters; (iii) establishment of the effect of orbital nature on radiation and radiationless transitions rate constants.The theoretical aspects of the results will be discussed here with special emphasis on radiationless conversion processes. The results of experimental investigation of molecular luminescence spectra as related to their structure are systematically reported in Ref. 1 and specifically in Refs. 2 and 3.It will be assumed that a photochemically stable luminophore molecule is in a solvent at a sufficiently low temperature such that thermal energy is less than molecular oscillations. This means that due to the fast (about 13-" sec) vibrational relaxation processes, the starting excited electronic state level in conversion
A detailed account of quantum chemical procedures for estimating the rate constants of the photophysical processes at work in polyatomic organic molecules is given. The results obtained from combined experimental and theoretical research into the spectral-luminescent properties of acridine, 9-aminoacridine, 2,7-dimethyl-9-diphenylaminoacridine, and of their protonated forms are reported. The electronic absorption and fluorescence spectra of acridine have been investigated at room temperature in ethanol solution of varying pH and in other solvents of different chemical nature and polarity. The energy of excited states, the rate constants of the deactivation of the excited states, and the dipole moments obtained by quantum chemical methods for the examined compounds are presented. The findings of the investigations are discussed.
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