Effect of porphyrin ring distortion on the singlet-excited state properties of alkyl-bridged, basket-handle porphyrins

Abstract

The effect of porphyrin ring distortion on the electrochemical and emission characteristics of a series of alkyl-bridged, basket-handle porphyrins and their Zn2+ derivatives are reported. The degree of distortion can be controlled by varying the chain length of the bridging group. The effects of distortion are apparent in quenching, broadening and red shifts of the emission bands relative to the corresponding planar derivative, and the maximum effects are observed for the most distorted isomer. The energy of the emission maxima of both Q(0,0) and B(0,0) transitions show a linear correlation with the chain length of the bridging group. Redox potential data suggest easier oxidations and harder reductions both in the ground and first singlet-excited states relative to planar H2TPP. In addition to Q band emission, the Zn2+ derivatives show emission from the upper excited state (S2) and the decreased quantum yield (φS2f) is accounted for in terms of a decreased energy gap between the S1 and S2 excited states in the distorted Zn2+ porphyrins.