Alkali Blues: Blue-Emissive Alkali Metal Pyrrolates

Abstract

2-Iminopyrroles [H tBu L, 4-tert-butyl phenyl(pyrrol-2-ylmethylene)amine] are non-fluorescent π systems. However, they display blue fluorescence after deprotonation with alkali metal bases in the solid state and in solution at room temperature. In the solid state, the alkali metal 2-imino pyrrolates, M( tBu L), aggregate to dimers, [M( tBu L)(NCR)] 2 (M=Li, R=CH 3 , CH(CH 3 )CNH 2 ), or polymers, [M( tBu L)] n (M=Na, K). In solution (solv=CH 3 CN, DMSO, THF, and toluene), solvated, uncharged monomeric species M( tBu L)(solv) m with N,N′-chelated alkali metal ions are present. Due to the electron-rich pyrrolate and the electron-poor arylimino moiety, the M( tBu L) chromophore possesses a low-energy intraligand charge-transfer (ILCT) excited state. The chelated alkali cations rigidify the chromophore, restricting intramolecular motions (RIM) by the chelation-enhanced fluorescence (CHEF) effect in solution and, consequently, switch-on a blue fluorescence emission.