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

Two CuI complexes, [Cu(imPhen)(POP)]PF6 ( 1, imPhen = 1 H-imidazo[4,5- f][1,10]phenanthroline, POP = bis[2-diphenylphosphino]phenyl ether) and [Cu(Flu-imPhen)(POP)]PF6 { 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 ( λem = 564 nm, ϕ = 11.2 % for 1 and λem = 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 3MLCT state of the Cu atom and the 3π→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 3MLCT state of a CuI complex. [ABSTRACT FROM AUTHOR]