Unusual pathways of triplet state dynamic relaxation in meso-aryl-substituted porphyrins and their chemical dimers at 295 K

It was found that mono- and di-meso-phenyl substitution in octaethylporphyrins (OEP)and their chemical dimers with the phenyl ring as a spacer manifests itself in the dramatical shortening of T-1 state lifetimes at 295 K (from similar to 1.5 ms down to 2-5 mu s in degassed toluene solutions). On the other hand, this substitution does not influence spectral-kinetic parameters of S-0 and S-1 states. The enhancement of the T-1 state non-radiative deactivation is explained by torsional librations of the phenyl ring around a single C-C bond in sterically encumbered OEP molecules leading to nonplanar dynamic distorted conformations in the excited T-1 states. For these compounds with electron-accepting NO2-groups in the meso-phenyl ring the strong non-radiative deactivation of S-1 and T-1 states (by similar to 2-3 orders of magnitude) is observed upon the displacement of NO2-group from para- to ortho-position of the phenyl ring. The S-1 state quenching is caused by the direct intramolecular electron transfer to low-lying CT state of the radical ion pair (the normal region, non-adiabatic case presumably, V = 130-190 cm(-1) in dimethylformamide). The additional deactivation of the T-1 state is connected with thermally activated transitions to upper-lying CT states as well as the strengthening of intersystem crossing probabilities.