Absorption and fluorescent properties of pyrylium compounds: I. The natureof electronic transitions and structural rearrangement in the excited state

The localization of molecular orbitals in 2,4,6-substituted derivatives of pyrylium is studied. The conformation of three asymmetrical molecules with oxyethyl substituents in positions 2 and 4 and different substituents in po sition 6 of the pyrylium ring is calculated by the AM1 method. The localiza tion of the four upper occupied and two lower unoccupied MOs is determined, the fragment localization numbers are found, and the energies of five opti cal transitions, localization numbers, and the numbers of charge transfer b etween fragments are calculated. The conformation analysis of molecules in the S-0 and S-1 states is performed. Solid and liquid pyrylium solutions of different viscosity and polarity are experimentally investigated. The abso rption spectra are recorded and absorption cross sections are measured, as well as fluorescence spectra and fluorescence anisotropy spectra. The follo wing conclusions are made. In nonplanar molecules of pyrylium salts, four a bsorption transitions are localized at different parts of the molecule cont aining the pyrylium ring and one of the substituents. Upon excitation of mo lecules with complex substituents in position 6, the molecular fragment in position 2 turns around. This results in a flattening of the molecular frag ment containing the pyrylium ring and substituents 2 and 6 on which the flu orescence transition is localized. The rearrangement involves the low-ampli tude motion; it occurs almost without a loss of the excitation energy and o nly slightly affects the localization of molecular orbitals. As a result, t wo excited conformers are formed that possess close absorption and fluoresc ence properties. The radiative transitions in these conformers completely d etermine fluorescence of liquid solutions of any viscosity, including glyce rol solutions. Strong solvatochromism is related to the nonplanar structure of stable pyrylium molecules, whereas the weak solvatochromism of liquid s olutions is caused by localization of radiative transitions on a planar fra gment of unstable fluorescing conformers. (C) 2000 MAIK "Nauka/Interperiodi ca".