Prolonging the Emissive Lifetimes of Copper(I) Complexes with 3 MLCT and 3 (π–π*) State Equilibria – A Fluorene Moiety as an “Energy Reservoir”

Two Cu I complexes, [Cu(imPhen)(POP)]PF 6 ( 1 , imPhen = 1 H ‐imidazo[4,5‐ f ][1,10]phenanthroline, POP = bis[2‐diphenylphosphino]phenyl ether) and [Cu(Flu‐imPhen)(POP)]PF 6 { 2 , Flu‐imPhen = 2‐(9 H ‐fluoren‐2‐yl)‐1 H ‐imidazo[4,5‐ f ][1,10]phenanthroline} were synthesized and characterized. a weak metal‐to‐ligand charge‐transfer (MLCT) absorption band at λ = 401 nm with a relatively low molar extinction coefficient ( ε = 3170 m –1  cm –1 ) was observed for 1 . In contrast, 2 displayed a higher MLCT absorption band at λ = 405 nm ( ε = 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 ( λ em = 564 nm, ϕ = 11.2 % for 1 and λ em = 568 nm, ϕ = 9.0 % for 2 ); however, the excited‐state lifetime of 2 ( τ = 51.2 µs) was almost four times longer than that of 1 ( τ = 13.8 µs). The prolonged luminescence lifetime of 2 was rationalized by the reversible energy transfer between the 3 MLCT state of the Cu atom and the 3 π→π* 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 3 MLCT state of a Cu I complex.