Fluorescence properties of distorted short-chain basket handle porphyrins

Abstract

The singlet excited state properties of a series of basket handle porphyrins and their protonated and Zn²⁺ derivatives, determined by fluorescence spectroscopy, are described. The introduction of a short chain across the periphery of the porphyrin ring induces distortion in the porphyrin plane. The decreased quantum yields and the ease of oxidation relative to the corresponding planar porphyrin have been attributed to the induced distortion. On protonation, the emission bands experience a blue shift due to a lack of the expected resonance interaction. This is ascribed to the restricted rotation of the porphyrin-phenyl bond imposed by the ortho substituent. The Zn²⁺ porphyrins show emission from the S₁ and S₂ excited states with quenching of fluorescence. The decreased energy gap between the S₁ and S₂ excited states in the distorted Zn²⁺ porphyrins promotes the S₂→S₁ internal conversion, thereby decreasing the S₂→ S₀ fluorescence yield.