Proton-induced Conversion from Non-Aufbau to Aufbau Electronic Structure of an Organic Radical with Turn-on Fluorescence

We report a donor-acceptor(D-A) type non-luminescent neutral radical, tris-2,4,6-trichlorophenylmethyl-N, N-dimethyl-9H-carbazol-3-amine(TTM-Cz-DMA). The results of cyclic voltammetry and quantum chemistry calculation confirm TTM-Cz-DMA has the non-Aufbau electronic structure, which means the singly occupied molecular orbital(SOMO) lies below the highest doubly occupied molecular orbital(HOMO). The non-Aufbau electronic structure changes to the Aufbau electronic structure after protonation and exhibits proton-responsive turn-on fluorescence, which is totally reversible by deprotonation. The dihedral angle between donor and acceptor moieties of TTM-Cz-DMA in excited state reduces from 88° to 62° after protonation, causing the turn-on fluorescence. Our results offer a viewing angle to understand the luminescence of radicals and provide a possible application of proton detection.