Synthesis and photophysical properties of phenyl-sulfanylated chlororophyll derivatives

Chlorophyll derivatives that possessed a phenylsulfanyl group at the C31- or C32-position were synthesized and their optical properties were investigated. Methyl 31-phenylsulfanyl-mesopyropheophorbide-a was prepared by substitutions of the corresponding C31-hydroxy-chlorin, methyl bacteriopheophorbide-d, with thiophenol in the presence of zinc iodide or of the corresponding C31-bromo-chlorin with thiophenol. The regioisomeric C32-phenylsulfanyl-chlorin was obtained by addition of thiophenol to the C3-vinyl group of methyl pyropheophorbide-a in the presence of AIBN. Both the synthetic compounds gave similar electronic absorption and emission spectra in chloroform, but fluorescence quantum yield of the C31-sulfanyl-chlorin (0.18) was ca. 30% smaller than those of the C32-sulfanyl-chlorin (0.25) and the C3-ethyl-chlorin (0.24). These observations were consistent with their fluorescence lifetime data. It is suggested that the heavy atom effect of a sulfur atom at the C31-position can tune photophysical properties of the chlorophyll derivatives.