Electronic structure and photophysics of the aza-analogues of hydroporphyrins

Abstract

Abstract The spectra of fluorescence and fluorescence excitation of tetraazachlorin and tetraazabacteriochlorin substituted at the reduced pyrrole rings have been investigated at 290 and 77 K; the fluorescence quantum yield and life time have been measured at 290 K; the fluorescence polarization spectra have been obtained at 77 K. Quantum-chemical calculations have been performed for a series of porphin derivatives: porphin–chlorin–bacteriochlorin–tetraazaporphin (porphyrazine)–tetraazachlorin– tetraazabacteriochlorin by the CNDO/S method with the use of molecular geometry acquired by the AM1 and PM3 optimization. The visible and UV absorption spectra and the fluorescence polarization spectra are adequately described by the CNDO/S calculation results, as well as the spectral changes in the series porphyrazine–tetraazachlorin–tetraazabacteriochlorin and from bacteriochlorin to tetraazabacteriochlorin. Strong quenching of fluorescence on the pyrrole rings reduction in the porphyrazine macrocycle has been found. It is shown that this quenching cannot be wholly explained by the increase in the rate constant of the S 1 ⇝ S 0 internal conversion which may be due to the lowering of the S1 level. Supposedly, the increase in the rate constant of the S 1 ⇝ T 1 intersystem crossing is due to participation of intermediate triplet levels in this process.