Absorption and fluorescent properties of pyrylium compounds: I. The nature of 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 position 6 of the pyrylium ring is calculated by the AM1 method. The localization of the four upper occupied and two lower unoccupied MOs is determined, the fragment localization numbers are found, and the energies of five optical transitions, localization numbers, and the numbers of charge transfer between 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 absorption spectra are recorded and absorption cross sections are measured, as well as fluorescence spectra and fluorescence anisotropy spectra. The following conclusions are made. In nonplanar molecules of pyrylium salts, four absorption transitions are localized at different parts of the molecule containing the pyrylium ring and one of the substituents. Upon excitation of molecules with complex substituents in position 6, the molecular fragment in position 2 turns around. This results in a flattening of the molecular fragment containing the pyrylium ring and substituents 2 and 6 on which the fluorescence transition is localized. The rearrangement involves the lowamplitude motion; it occurs almost without a loss of the excitation energy and only slightly affects the localization of molecular orbitals. As a result, two excited conformers are formed that possess close absorption and fluorescence properties. The radiative transitions in these conformers completely determine fluorescence of liquid solutions of any viscosity, including glycerol solutions. Strong solvatochromism is related to the nonplanar structure of stable pyrylium molecules, whereas the weak solvatochromism of liquid solutions is caused by localization of radiative transitions on a planar fragment of unstable fluorescing conformers.