Highly efficient luminescent E- and Z-isomers with stable configurations under photoirradiation induced by their charge transfer excited states†
Highly efficient luminescent E-/Z-isomers based on double bonds with stable E/Z configurations in the excited state under photoirradiated conditions are of great significance to organic photochemistry and photoelectronics. Herein we report the highly efficient luminescent E-/Z-isomers of a donor–acceptor triphenylamine–cyanostilbene molecule, TPNCF, which were synthesized simultaneously via a Knoevenagel reaction, with their E/Z configurations confirmed by the obtained single crystal structure. The NMR spectra showed that both E-TPNCF and Z-TPNCF maintained their stable E/Z configurations in DMSO-d6 solvent and films under photoirradiation conditions. The in situ UV spectra in different polar solvents showed that polarity played a positive role in stabilizing the E/Z isomers in their configurations. PL solvatochromism experiment and theoretical calculation results indicated that the S1 state has charge-transfer (CT) character in both E-TPNCF and Z-TPNCF. As the solvent polarity increased, the energy of the S1 state gradually decreased, and the E/Z isomerization reaction at the higher energy S2 state was effectively suppressed by a rapid internal conversion process from the S2 to the S1 state. This was supposed to be the main reason for the observed stable E-/Z-isomers under photoirradiated conditions. Photophysical investigations demonstrated that both E-TPNCF and Z-TPNCF exhibited an obvious aggregation-induced emission (AIE) with high fluorescence quantum efficiencies of 70% and 50% in films respectively. Further research revealed that the AIE phenomenon in the TPNCF molecules was mainly dominated by their CT excited state characteristic but not the E/Z isomerization or the restricted intramolecular rotation effect. Our work will contribute considerably to the basic photochemistry studies on E/Z isomerization reactions and the material design of stable E/Z isomers for organic photo-electronic applications.