FLUORESCENCE PROPERTIES OF DISTORTED SHORT-CHAIN BASKET HANDLE PORPHYRINS

The singlet excited state properties of a series of basket handle porp hyrins and their protonated and Zn2+ derivatives, determined by fluore scence spectroscopy, are described. The introduction of a short chain across the periphery of the porphyrin ring induces distortion in the p orphyrin plane. The decreased quantum yields and the ease of oxidation relative to the corresponding planar porphyrin have been attributed t o the induced distortion. On protonation, the emission bands experienc e a blue shift due to a lack of the expected resonance interaction. Th is is ascribed to the restricted rotation of the porphyrin-phenyl bond imposed by the ortho substituent. The Zn2+ porphyrins show emission f rom the S1 and S2 excited states with quenching of fluorescence. The d ecreased energy gap between the S1 and S2 excited states in the distor ted Zn2+ porphyrins promotes the S2 --> S1 internal conversion, thereb y decreasing the S2 --> S0 fluorescence yield.