Effects of electron donor on luminescence and mechanochromism of D-π-A benzothiazole derivatives

Abstract

Two benzothiazole derivatives with triphenylamine (CBVD) and phenothiazine (CBVP) moieties as electron donor groups were prepared, and their solid luminescence and mechanofluorochromism (MFC) were investigated and compared. Solvent-dependent fluorescence spectra and quantum chemical calculation indicate that the luminescence of both compounds originates from the intramolecular charge transfer transition. Although the two compounds have similar absorption spectra in solution, CBVP possesses red-shifted fluorescence as compared with CBVD. Similar to that in solution, CBVP in crystal state has longer emission wavelength than CBVD in crystal state. Crystal structures indicate that their molecular stacking models in crystal phases are different. J-aggregate and H-aggregate coexist in CBVD crystal, whereas only H-aggregate was observed in CBVP crystal. Moreover, CBVP exhibits reversible luminescence color change from yellow to orange-red with a spectral shift of 36 nm under mechanical force stimuli, whereas CBVD has a spectral shift of only 20 nm, indicating poor color contrast. UV–Vis absorption spectra show that the mechanical force induces the disappearance of H-aggregate in CBVD crystal and leads to stacking model transition from H-aggregation to J-aggregation in the CBVP system.