D-A type tetraphenylethylene functionalized dicyanoethylene derivatives exhibiting aggregation-induced emission and halogen effect on enhanced mechanofluorochromic behaviors

Two novel tetraphenylethylene functionalized dicyanoethylene derivatives, named TPE-DCN-F and TPE-DCN-Br, are designed and successfully prepared to investigate the influence of different halogen atoms on their mechanofluorochromic (MFC) properties. Optical spectra and theoretical calculations indicated that the two were typical donor-acceptor molecules. As a result, both fluorophores showed unique intramolecular charge transfer (ICT) features. Besides, the presence of tetraphenylethylene unit and dicyanoethylene segment endowed them with significant aggregation-induced emission (AIE) properties. The combination of ICT and AIE behaviors not only enabled TPE-DCN-F and TPE-DCN-Br to show intense solid-state fluorescence emission, but also provided them with reversible and high-contrast mechanofluorochromic behavior. The emission wavelengths of TPE-DCN-F and TPE-DCN-Br red-shifted from 535 nm and 530 nm to 571 nm and 580 nm, respectively, with wavelength variations of 36 nm and 50 nm. The high contrast MFC performance induced by bromine-substituted derivative should be due to more effective ICT process and AIE performance. The current work suggested that the introduction of bromide atom to the dicyanoethylene derivative could be an effective strategy for constructing high-contrast MFC materials.