The fluorescence and electronic structure of phenyl-substituted tetraazachlorin molecules

Abstract

The effect of the addition of phenyl groups to pyrrole rings of tetraazachlorins, a new class of tetrapyrroles, on the photophysical properties and electronic structure of the molecules has been investigated by a complex of experimental and theoretical methods. Characteristics of fluorescence at 293 and 77 K have been determined for phenyl-substituted tetraazachlorins. The objects of this study include unsubstituted tetraazaporphine. The introduction of phenyl groups affords a marked increase in the fluorescence quantum yield. For tetraazaporphine and phenyl-substituted tetraazachlorins, fluorescence buildup occurs as the temperature is decreased from 293 to 77 K, but to a lesser extent than for tetraazachlorins having no phenyl groups, which were earlier studied by the authors. The fluorescence buildup mechanism is discussed. The singlet oxygen generation quantum yield has been determined for the tetrapyrroles examined. This characteristic increases upon tetrapyrrole is phenylation. The electronic structure and absorption spectra of unsubstituted porphine and chlorin, tetraazaporphine, tetraazachlorin, octaphenyltetraazaporphine, and tetramethylhexaphenyltetraazachlorin have been calculated by the INDO/Sm method (original modification of the INDO/S method) with molecular geometry optimization using DFT. The results of the quantum-chemical calculation of the absorption spectra are in good agreement with experimental data for transitions to the lowest excited electronic states Qx(S1) and Qy(S2).