Porphyrins: XIII: Fluorescence spectra and quantum yields

Abstract

Fluorescence spectra, quantum yields, natural radiative lifetimes, and absorption oscillator strengths are reported for a number of porphyrins in benzene solution. Emission yields for free base, Mg-, and Zn-phthalocyanine in 1-chloronaphthalene solution are also reported. The phthalocyanines are considerably more fluorescent than the porphyrins, and this is attributed mainly to their shorter radiative lifetimes. The heavy atom effect on fluorescence is strong with yields in the order H 2 ∼ Mg > Zn ⪢ Cd. The effect of peripheral substituents is small, both on absorption and fluorescence. Four free base porphyrins studied all showed yields of about 10%, natural radiative lifetimes about 120 nsec, and oscillator strengths about 0.03. Closed-shell metal derivatives have natural radiative lifetimes roughly half those for the free bases, independent of the atomic number of the metal. A simple theoretical model of borrowing from the B band gives a good explanation for the difference between metal and free-base radiative lifetimes.