Photophysical Characterization of the Naturally Occurring Dioxobacteriochlorin Tolyporphin A and Synthetic Oxobacteriochlorin Analogues.

Abstract

Tolyporphins are tetrapyrrole macrocycles produced by a cyanobacterium-containing culture known as HT-58-2. Tolyporphins A-J are free base dioxobacteriochlorins, whereas tolyporphin K is an oxochlorin. Here, the photophysical characterization is reported of tolyporphin A and two synthetic analogues, an oxobacteriochlorin and a dioxobacteriochlorin. The characterization (in toluene, diethyl ether, ethyl acetate, dichloromethane, 1-pentanol, 2-butanone, ethanol, methanol, N,N-dimethylformamide and dimethylsulfoxide) includes static absorption and fluorescence spectra, fluorescence quantum yields and time-resolved data. The data afford the lifetime of the lowest singlet excited state and the yields of the nonradiative decay pathways (intersystem crossing and internal conversion). The three macrocycles exhibit only modest variation in spectroscopic and excited-state photophysical parameters across the solvents. The long-wavelength (Qy ) absorption band of tolyporphin A appears at ~680nm and is remarkably narrow (full-width-at-half-maximum ~7nm). The position of the long-wavelength (Qy ) absorption band of tolyporphin A (~680nm) more closely resembles that of chlorophyll a (662nm) than bacteriochlorophyll a (772nm). The absorption spectra of tolyporphins B-I, K (which were available in minute quantities) are also reported in methanol; the spectra of B-I closely resemble that of tolyporphin A. Taken together, tolyporphin A generally exhibits spectral and photophysical features resembling those of chlorophyll a.