Twisted Donor-Acceptor Cruciform Luminophores Possessing Substituent-Dependent Properties of Aggregation-Induced Emission and Mechanofluorochromism

Three donor-acceptor (D-A)-containing cruciform luminophores, named DHCS-BC, DMCS-BC, and DTCS-BC, with twisted molecular conformation have been synthesized. The D-A-type cross-conjugated compounds showed unique intramolecular charge transfer (ICT) properties. Interestingly, they showed substituent-dependent aggregation induced emission (AIE) and mechanofluorochromic (MFC) behavior. The three cruciforms possessed evident ME nature with the AIE factors of 9, 19, and >492 and high solid-state fluorescence quantum yields of 0.332, 0.425, and 0.492, respectively. Moreover, their AIE activities increased in the order DHCS-BC < DMCS-BC < DTCS-BC, which was consistent with the torsional degree of molecular spatial conformations induced by the steric hindrance between carbazole and substituted cyanostyryl and the ICT degrees of the three compounds. The solid emission studies illustrated that their fluorescence color could change reversibly between bright blue-green (485 nm), bright blue-green (479 nm), bright yellow (526 nm) and light yellow-green (524 nm), yellow-green (534 nm), orange-red (606 nm) through grinding and dichloromethane (DCM) fuming treatment, giving the large spectral red-shifts of 39, 55, and 80 nm, respectively. The MFC activities of the three compounds are increased with the sequence of DHCS-BC < DMCS-BC < DTCS-BC, which was the result of the planarization of the molecular conformation and subsequent planar intramolecular charge transfer (PICT) under the external force stimuli.