Fumaronitrile-Based Fluorogen: Red to Near-Infrared Fluorescence, Aggregation-Induced Emission, Solvatochromism, and Twisted Intramolecular Charge Transfer

By integrating N,N-dimethyl (donor, D) and fumaronitrile (acceptor, A) groups with the biphenyl fluorogen, which has an aggregation-induced emission (AIE) property, we have obtained the target molecule 2-(4-bromophenyl)-3-(4′-(dimethylamino)-biphenyl-4-yl) fumaronitrile (BDABFN). BDABFN is a red-to-near-infrared-emitting fluorogen with emission peaks at 653 and 710 nm for its amorphous and crystal solids, respectively. BDABFN shows an evident aggregation-induced emission property, and the fluorescence quantum efficiency of its solid is 26.5%. Cystallographic data indicate that there is no π–π stacking, and neither J- nor H-aggregates are observed between the adjacent molecules. The existence of multiple C–H···π bonds between the adjacent molecules restricts the intramolecular rotation of the D and A moieties and enables the fluorogen to emit intensely in the solid states. Meanwhile, because of strong intramolecular D–A interaction, BDABFN exhibits pronounced solvatochromism, and the fluorescence peak red-shifts from 552 nm in hexane (nonpolar and hydrophobic solvent) to 750 nm in tetrahydrofuran (polar and hydrophilic solvent. BDABFN also displays a typical twisted intramolecular charge transfer property in polar solvents because of the interaction between the N,N-dimethyl and fumaronitrile moieties.