Position- and region-isomerized derivatives of a V-shaped fluorophore: the unique solution-state dual emission and the unusual force-induced solid-state turn-on emission

A V-shaped parent fluorophore as well as its position- and region-isomerized derivatives with a phenyl introduced on either the electron-donating or the electron-withdrawing moiety in the ortho-, meta- or para-linkage is prepared. Compared with the parent, these derivatives exhibit unique solvent-dependent dual emission in solutions presumably due to the considerably enhanced rotation energy barrier triggered by the insertion of the phenyl, which results in competitive relaxation between the LE and the TICT states. The intrinsically differentiated electron effect induced by the linkage position, which is strengthened by the twisted conformations, should be responsible for the faint or fully quenched fluorescence of the ortho- and meta-isomers in both solution and solid states. The rare turn-on solid-state emission of such isomers under force stimuli is caused by the enhanced π-conjugation effect on excited molecules, which are released from broken lattices to possess more planarized geometries. Moreover, the red shifts of emission wavelengths before and after force application are remarkably reduced from the parent to the derivatives. X-ray crystallography helps us gain a deep insight on the reasons for such a reduction.