Brominated short-chain basket-handle porphyrins and their copper(II) derivatives: spectral and electrochemical studies on the effect of β-substitution versus distortion

Abstract

Pyrrole-brominated short-chain basket-handle porphyrins and their copper(II) derivatives have been synthesized and characterized. These porphyrins are significantly distorted and the β-substitution by bromines further enhances the degree of distortion. Optical absorption and electrochemical sudies reveal considerable shifts in the energies of absorption maxima and electrode potentials. An analysis of these shifts indicates significant changes in the energies of the highest-occupied (HOMO)[a_1u(π) and a_2u(π)] and lowest-unoccupied molecular orbitals (LUMOs)[e_g(π^∗)] of the porphyrin ring upon β-substitution and distortion. Specifically: (a) both distortion and β-substitution decreases the energy gap between the HOMO and LUMO due to different stabilization/destabilization mechanisms and (b) the magnitude of separation between a_1u(π) and a_2u(π) is large for β-substitution while it is small for distortion relative to the corresponding planar unsubstituted derivative. The absorption and redox potential shifts show a non-linear behaviour with the number of bromine substituents and this is ascribed to the combined effect of antagonistic inductive interactions and steric hindrance. ESR spectral studies indicate weakening of the CuN σ bond without much change in the electronic structure of Cu²⁺.