Mechanofluorochromism of Triphenylamine-BODIPY: Effect of twisted intramolecular charge transfer and restriction in rotation on fluorescence

Abstract

The majority of the studies in the field of mechanofluorochromism (MFC), based on donor (D)-acceptor (A) complexes, have reported that MFC was strongly associated with the intramolecular charge transfer (ICT) character and aggregation-induced emission (AIE). However, the mechanism of MFC has been inconclusively demonstrated. In this study, a triphenylamine (TPA) introduced BODIPY complex was designed and synthesized to demonstrate the mechanism of MFC based on the D-A complex. We identified the effect of an external stimuli on the molecular geometry by employing solid state NMR, UV–vis spectroscopy, and density functional theory (DFT) calculations; this was because geometry, depending on the stimuli, has been established as a key for determining emission properties. The relationship between the identified geometry and the emission properties was experimentally investigated with regard to the ICT and restriction in rotation (RIR) characteristics. Under external stress, fluorescence quenching was suppressed by RIR, and the molecular planarization-induced weak ICT character hypsochromically shifted the emission spectrum. However, as the external stimuli were not applied, fluorescence quenching occurred due to the freely rotating molecules; further, the emission spectrum was bathochromically shifted due to the strong ICT character that was induced by the high torsional angle between D and A.