Effect of porphyrin ring distortion on the spectral and electrochemical properties of short-chain basket-handle porphyrins

Abstract

The effect of porphyrin ring distortion on the spectral and electrochemical properties of short-chain basket-handle porphyrins has been investigated. The covalent attachment of opposite phenyl rings of meso-5,10,15,20-tetraphenylporphyrin (H₂tpp) by short bridging groups of varying nature and length induces distortion in the porphyrin ring. Optical absorption studies indicate large bathochromic shifts (502-1139 cm¹) for both Q and Soret bands and the magnitude of these shifts are linearly correlated with the degree of distortion. The potential values for the porphyrin ring oxidation and reduction were shifted to less-positive (180260 mV) and to more-negative (280390 mV) values respectively relative to planar H₂tpp. The magnitude of the shifts in the optical absorption bands and redox potentials are independent of the nature of the bridging chain. The origin of these shifts are traced to structural effects caused due to the introduction of a short bridging group rather than to the electronic effect of the bridging group. Introduction of Cu²⁺ into the porphyrin core does not affect the degree of distortion and ESR studies indicate no significant change in the electronic structure of the Cu²⁺ ion relative to [Cu(tpp)].