Prolonging The Emissive Lifetimes Of Copper(I) Complexes With (Mlct)-M-3 And (3)(-*) State Equilibria - A Fluorene Moiety As An "Energy Reservoir"

Two Cu-I complexes, [Cu(imPhen)(POP)]PF6 (1, imPhen = 1H-imidazo[4,5-f][1,10]phenanthroline, POP = bis[2-diphenylphosphino]phenyl ether) and [Cu(Flu-imPhen)(POP)]PF6 {2, Flu-imPhen = 2-(9H-fluoren-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline} were synthesized and characterized. a weak metal-to-ligand charge-transfer (MLCT) absorption band at = 401 nm with a relatively low molar extinction coefficient (epsilon = 3170 m(-1)cm(-1)) was observed for 1. In contrast, 2 displayed a higher MLCT absorption band at = 405 nm (epsilon = 5400 m(-1)cm(-1)) owing to the incorporation of a fluorene group to imPhen ring. Complexes 1 and 2 exhibited similar emission wavelengths and quantum yields ((em) = 564 nm, phi = 11.2% for 1 and (em) = 568 nm, phi = 9.0% for 2); however, the excited-state lifetime of 2 ( = 51.2 mu s) was almost four times longer than that of 1 ( = 13.8 mu s). The prolonged luminescence lifetime of 2 was rationalized by the reversible energy transfer between the (MLCT)-M-3 state of the Cu atom and the (3)* state of fluorene moiety. To the best of our knowledge, this is the first use of a fluorene group as a triplet energy reservoir to extend the excited-state lifetime of an emissive (MLCT)-M-3 state of a Cu-I complex.