Cooperative substituent effects on the excited states of copper phenanthrolines

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-3,3,7,8-tetramethyl- 1, 10-phenanthroline). In dichloromethane, the complex with the 2,9-diphenyl-3,4,7,8-tetramethyl- I,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.