Spectral-Luminescent Properties and Electronic Structure of the Chlorin p 6 Derivatives with an Extended Conjugated Bond System

On the basis of spectral-luminescent investigations it is shown that the introduction of an additional hexatomic imidic ring into the chlorin p ₆ derivatives substantially shifts the absorption bands to the longwave spectral region but causes no decrease in the fluorescence quantum yield, i.e., it is not accompanied by an increase in the nonradiative deactivation rate of the excited singlet state S ₁. It follows from quantum-chemical calculations and also from the experimental data that the presence of this cycle in free bases results in the extension of the conjugated bond system and in the reduction of the singlet–triplet interval E(S ₁–T ₁) as compared to chlorin p ₆. In contrast to the majority of earlier investigated porphyrins and chlorins, the introduction of the central ion Zn into the chlorin p ₆ cycloimide derivative causes an increase in the quantum yield of fluorescence, lowering of the energy of the excited triplet state T ₁, and a noticeable (by 600 cm^–1) increase in the E(S ₁–T ₁) interval in comparison with the corresponding free bases. The data obtained are associated with the characteristic features of the extension of conjugated bond systems for the free bases of cycloimidoderivatives of chlorin p ₆ (22-membered cyclic polyene) and of their Zn complexes (20-membered cyclic polyene).