Photobleaching of 5,10,15,20-tetrakis (m-hydroxyphenyl) porphyrin (m-THPP)and the corresponding chlorin (m-THPC) and bacteriochlorin (m-THPBC). A comparative study

The photobleaching of compounds of the 5,10,15,20-tetrakis(m-hydroxyphenyl) porphyrin series at different reduction levels (m-THPP 1, m-THPC 2, m-THPBC 3) has been studied in methanol and in methanol-water (3:2, v/v) using an argon laser (514 nm) by observing the diminution of absorbance of band I (f or 2 and 3) and of band IV (for 1) with time. Under the conditions studied here, true photobleaching only occurs for m-THPC (2) and m-THPBC (3), with photomodification being the major process for m-THPP (1). The rates for the photobleaching of 2 and 3 are presented in different solvents. The photobl eaching rate of the bacteriochlorin 3 is found to be 90 times higher than t hat of the chlorin 2 in methanol-water (3:2, v/v). Singlet oxygen appears t o be the reactive species responsible for the photobleaching of 2 and 3 and the photomodification of 1. (C)1999 Elsevier Science S.A. All rights reser ved.