Do twisting and pyramidalization contribute to the reaction coordinate of charge-transfer formation in DMABN and derivatives?

The dual fluorescence spectra of various model compounds of DMABN varying in acceptor strength are reported. Dual fluorescence occurs irrespective of whether the more allowed 1La-type state is situated above or below the 1Lb-type state. Chemical bridging of the dimethylamino group makes the charge-transfer (CT) fluorescence band disappear in both cases. Pyramidalization effects are investigated using quantum-chemical modelling. The ionization potential of the dimethyl-amino donor group strongly increases with pyramidalization although the observed CT fluorescence is little affected. It is concluded that both twisting and `anti-pyramidalizationʹ (rehybridization to sp2) are important to explain the emission properties of the CT state.