Effect of electron-withdrawing moieties on mechanochromism of phenothiazine derivatives
Two phenothiazine derivatives with two (CPBM) or four (CPDBM) electron-withdrawing moieties were synthesized, and their responsive properties to mechanical force were investigated and compared. CPDBM had absorption and emission spectra with longer wavelengths than CPBM in hexane. Quantum chemical calculation and electrochemical measurement were conducted. Longer CPDBM wavelength could be ascribed to the decrease in LUMO energy level owing to the existence of additional electron-withdrawing groups. In the crystal state, CPBM aggregated into dimer with a short distance and exhibited reversible mechanofluorochromism (MFC). Fluorescence color possessed reversible conversion between orange and red under alternative force and fuming stimuli. Long and thin dark red crystals of CPDBM with weak red fluorescence were obtained, and the fluorescence did not change after grinding. Thermal annealing of the dark red crystal produced red solid with orange fluorescence and strong red emission after grinding. Fuming and thermal annealing led to orange fluorescence, indicating MFC behavior.