Pressure-induced remarkable luminescence switch of a dimer form of donor-acceptor-donor triphenylamine (TPA) derivative

Two newly synthesized molecules with a twisted donor-acceptor-donor (D-A-D) structure were achieved, noted as TPA-Ph-CN and TPA-Py-CN, and their solvatochromic effects and mechanochromic properties upon grinding and/or under hydrostatic pressure were investigated deeply. The comparison of their solvatochromic behaviors demonstrates that TPA-Py-CN shows a stronger intramolecular charge transfer (ICT) effect than TPA-Ph-CN due to the substitution of electron-deficient acceptor from Ph-CN to Py-CN. Photophysical characterizations together with the analysis of crystal structures indicate that TPA-Py-CN exhibits a distinct dimer structure with C-HMIDLINE HORIZONTAL ELLIPSIS pi interactions, but TPA-Ph-CN does not. As a result, TPA-Py-CN shows green emission with three bands and the peak at 538 nm is assigned to excimer, while TPA-Ph-CN presents blue emission with only one band. Upon grinding, TPA-Py-CN shows more sensitive responsiveness to pressure than TPA-Ph-CN and exhibits a size-dependent emission property. TPA-Ph-CN only displays continuous red-shifted and broadened emission under hydrostatic pressure. In contrast, high pressure experiments and theoretical calculations of TPA-Py-CN demonstrate that the excimer fluorescence wavelength is pressure-independent. This study is helpful for better understanding intrinsic essences of the molecular structure and the properties of mechano-responsive organic molecules.