Steric interactions influence on electron transfer efficiency in meso-nitrophenylporphyrins and their chemical dimers

Abstract

Steric interactions upon mono- and di-meso-phenyl substitution in octaethylporphyrins (OEP) and their chemical dimers with the phenyl spacer manifests itself in the dramatical T₁-state lifetimes shortening at 293 K (from ms down to μs in degassed toluene solutions) without any influence on spectral-kinetic parameters of S₀- and S₁-states. This effect is explained by non-planar dynamic conformations in excited T₁-states caused by the phenyl ring torsional librations around a single C-C bond. For meso-ortho-nitrophenyl substituted OEPs the S₁-state quenching is caused by the direct “through space” electron transfer to low-lying CT state while for the corresponding Pd-complexes the direct electron transfer takes place from the locally excited T₁-state. For the last two cases steric interactions provide the optimal geometry with high electronic coupling between porphyrin macrocycle and nitro-group.