Position-isomerization-induced switch effect of mechanofluorochromism and mechanoluminescence on carbazolylbenzo [d] imidazoles

Two positional isomers BIM-C-CZ and BIM-N-CZ with N-ethylcarbazole and 4-cyanophenyl alternatively introduced at N-1 and C-2 site on benzo [d]imidazole are found to be MFC-/ML-inert and MFC-/ML-active respectively. The considerable reduction of ICT effect on BIM-C-CZ molecules with planarized conformation under force stimuli may well be responsible for the MFC inactivity of this isomer. Although the crystal of BIM-N-CZ-8, a homologue to BIM-N-CZ in which ethyl is replaced by n-octyl, holds a centrosymmetric space group, it still exhibits good ML behaviour similar to that of the non-centrosymmetric BIM-N-CZ crystal. The theoretical calculations of couples with strong intermolecular interactions reveal that large net dipole moments occur on couples in BIM-N-CZ and BIM-N-CZ-8 crystals rather than those in BIM-C-CZ crystal, which presumably accounts for the ML inertia of the latter. Thanks to the interchange of group positions, BIM-N-CZ and BIM-N-CZ-8 hold much larger dipole moments than BIM-C-CZ, which make great contribution to their excellent ML performance. Moreover, it is verified by our work that consideration of packing mode instead of individual molecules in a crystal should be a proper way to investigate the solid-state emission property of a fluorophore.