Spacer group-controlled luminescence and response of C3-symmetric triphenylamine derivatives towards force stimuli

Abstract

Two C₃-symmetric triphenylamine derivatives with three terminal cyano units as electron acceptors were prepared to investigate the effect of the spacer group on their photophysical properties and responses towards force. Their electronic transitions were carefully studied by electrochemistry, solvent-dependent spectroscopy and quantum chemical calculations. The results suggested that introducing a double bond between the donor and acceptor results in the longer absorption and emission wavelengths of TPAVCN owing to elevated HOMO and lowered LUMO energy levels and induces a larger excited state dipole moment because of the extended conjugated length. In polar solvents, both TPACN and TPAVCN possessed a longer emission wavelength. Theoretical calculations suggested that bathochromic shifts in emission bands could be ascribed to the large polar excited states owing to the light excitation-induced intramolecular charge transfer. Moreover, TPAVCN had a larger charge transfer length and average degree of the spatial extension of hole and electron distribution because of its longer molecular length. In crystals, TPAVCN had a longer emission wavelength relative to that of TPACN. Moreover, both compounds could reversibly change their fluorescence under force and solvent annealing stimuli, and their mechanochromic properties were regulated by spacer groups. TPACN changed its fluorescence from blue to cyan with a spectral shift of 12 nm after grinding, but a large spectral shift of 30 nm, and an obvious fluorescent color change from green to yellow were observed while grinding pristine TPAVCN solids.