Mechanochromism and crystallization-induced emission enhancement of carbazole derivatives with different terminal groups

Two D-A carbazole derivatives (TBCA and NCA) were synthesized to investigate the influence of terminal group on the photophysical and mechanofluorochromic (MFC) properties in solution and crystal state. Results suggest that TBCA with t-butylphenyl as terminal group has a slightly smaller optical bandgap than that of NCA with naphthyl as terminal group because NCA adopts a more distorted conformation. Both compounds exhibited crystallization-induced emission enhancement (CIEE). TBCA crystals are rod-like and emitted strong blue fluorescence and the lamellar NCA crystals have a red-shifted emission in comparison with that of TBCA. The single-crystal structure analysis indicates that the two molecules adopt a more distorted conformation in their crystal. CN⋯H–CC hydrogen bonds help TBCA molecules to form a 1D chain stacking, but no strong π-stacking between TBCA molecules exists. Obvious π-π interaction is present between the carbazole units in NCA crystal, which results in a longer emission wavelength that of TBCA. Furthermore, two compounds exhibited reversible MFC behaviors. Force stimuli promoted a larger bathochromic shift for TBCA than that of NCA because a more pronounced planarization for TBCA existed after force damaged ordered stacking in crystal.