Cooperative Substituent Effects on the Excited States of Copper Phenanthrolines.

Abstract

Copper phenanthrolines are attractive as potential photosensitizers because of the ready availability of the metal, but efficient nonradiative decay including a solvent-induced quenching phenomenon ordinarily limits their utility. However, the present studies show that the addition of methyl substituents in the 3,8-positions of 1,10-phenanthroline can enhance the protective effect that bulky groups in the 2,9-positions have on the reactive charge-transfer excited state of a bis-ligand copper(I) derivative. Thus, the photoexcited Cu(dbtmp)(2)(+) complex has a lifetime of 920 ns in dichloromethane, whereas the parent complex without the methyl substituents has a lifetime of only 150 ns under the same conditions (dbtmp = 2,9-di-n-butyl-3,4,7,8-tetramethyl-1,10-phenanthroline). In dichloromethane, the complex with the 2,9-diphenyl-3,4,7,8-tetramethyl-1,10-phenanthroline ligand also exhibits a long lifetime (480 ns). Even more importantly, the latter combination of substituents appears to eliminate the problem of solvent-induced exciplex quenching.