Moiety effect on the luminescent property of star-shaped triphenylamine (TPA) derivatives as mechanochromic materials

Abstract

Two TPA-based star-shaped mechanochromic materials, namely ester-TPA and COOH-TPA, were designed and synthesized using TPA as the core and benzoic acid and methyl benzoate as the arms, and then importantly the moiety effect on the luminescent property of the materials was investigated. They displayed similar solvatochromic effects in different solvents for the D–π–A motif with charge transfer from TPA to the benzoic acid or methyl benzoate moiety, which was confirmed by the consistent UV-vis absorption and large differences displayed when dropped into different solvents as well as by orbital distribution analysis using density functional theory. However, they showed different mechanochromism performances, which were investigated by analysing the fluorescent spectrum, fluorescent quantum yield and fluorescence lifetime. The COOH-based emitter showed a better performance with a relatively obvious red-shift for CIE (0.17, 0.35) to (0.26, 0.49) together with transformation from crystalline to amorphous states, indicating the molecular packing pattern had been destroyed, which could be speculated from single-crystal structure analysis and powder X-ray diffraction analysis. In addition, COOH-TPA could also respond to alkalis, leading to a blue-shift and band broadening of the spectra, which might be ascribed to a reduced charge transfer from TPA to benzoic acid owing to deprotonation.