Efficient fluorescence of alkali metal carbazolides

Abstract

The sterically demanding 1,8-bis(3,5-ditertbutylphenyl)-3,6-di-tert-butylcarbazole (dtbpCbz)-H is deprotonated with various metal bases to obtain a complete series of thermally robust alkali metal compounds of the type [(dtbpCbz)M] (M = Li, Na, K, Rb, Cs). These carbazolides can either be solely coordinated by carbazole ligands (1a-5a) or bear additional toluene ligands (1b-5b). In the cases of [(dtbpCbz)Li] (1a), [(dtbpCbz)K] (3a) and [(dtbpCbz)Rb] (4a), monomeric species were identified by single crystal XRD. Additionally, a carbazolide anion was incorporated into [NBu4][dtbpCbz] (6). All compounds were strongly luminescent, so PL and PLE measurements were conducted, revealing emissions in the range of 460-580 nm. The excited states possess a lifetime of up to 21 ns, so fluorescence phenomena were observed. Quantum yields at ambient temperature were measured up to 29% in the solid state and up to 100% in solution. In the series of 1a-4a, the emission maxima shift to higher energy as the alkaline earth metal cation becomes heavier, and the lifetimes of the excited state decrease. The Cs compounds show low quantum yields and lifetimes of the excited state.