Transient absorption spectra of tetraazaporphine and its reduced derivatives upon picosecond excitation

Transient absorption spectra of tetraazaporphine and substituted derivatives of tetraazachlorin and tetraazabacteriochlorin were obtained upon picosecond excitation with various probe pulse delay times in order to resolve the open question about the reasons for fluorescence quenching in tetraazaporphine hydrogenated derivatives. The quantum yield of triplet state T ₁ formation has been estimated. It has been shown that in all investigated cases radiationless de-excitation of the fluorescent level S ₁ occurs by two channels: S ₁ ⤳ T ₁ and S ₁ ⤳ S ₀, the latter being predominant. As the S ₁ level becomes lower, the transition rate for this channel increases. For tetraazaporphine and its derivatives, a quasi-photochemical mechanism is proposed that accounts for the anomalous efficiency of the S ₁ ⤳ S ₀ channel in the dissipation of the electronic excitation energy.