Effects of alkyl chain length on aggregation-induced emission, self-assembly and mechanofluorochromism of tetraphenylethene modified multifunctional beta-diketonate boron complexes

Two tetraphenylethene functionalized beta-diketonate boron complexes C2TPE-CAR and C16TPE-CAR that were bridged by carbazole bearing alkyl chains of different lengths, have been designed and successfully synthesized. The two donor-acceptor luminogens showed typical twisted intramolecular charge-transfer (TICT) emission. In particular, they exhibited alkyl-dependent aggregation-induced emission (AIE), self-assembly and mechano-fluorochromic (MFC) properties. Results showed that C16TPE-CAR with a long hexadecyl chain formed gels in certain solvents and also exhibited gelation-induced emission enhancement. H-aggregates were formed driven by the synergistic effect of pi-stacking interaction and van der Waals force in the gel phase. However, C2TPE-CAR, which possessed the short ethyl group, exhibited no self-assembly behavior. Simple mechanical force could change the emission color of C2TPE-CAR and C16TPE-CAR powders from initial bright yellow (571 and 558 nm) to final red (617 and 610 nm), thus high contrast mechanofluorochromism with large spectral shifts of up to 46 and 52 nm, respectively, was observed. The powder XRD studies showed that the MFC behavior might be switched reversibly under the stimulus of external force because of crystalline-amorphous phase transformation.