Aggregation-enhanced emissive mechanofluorochromic carbazole-halogen positional isomers: tunable fluorescence via conformational polymorphism and crystallization-induced fluorescence switching

Carbazole-based donor-acceptor derivatives of halogen-substituted positional isomers were synthesised and the effect of the halogen and position on the solid-state fluorescence and mechanofluorochromism was explored. All compounds showed good solid-state fluorescence (phi(f) = 15.67 to 31.32%). Interestingly, the crystallization of the positional isomers produced concomitant blue and green fluorescent polymorphs and tunable solid-state fluorescence. Solid-state structural analysis revealed a subtle conformational change in the acceptor unit, which led to polymorphism and tunable fluorescence. Halogen substitution induced a higher conformational twist and a large fluorescence shift between the polymorphs. The supramolecular interactions (H-bonding, C-HMIDLINE HORIZONTAL ELLIPSIS pi and pi MIDLINE HORIZONTAL ELLIPSIS pi) observed in the crystal lattice rigidified the fluorophores and enhanced the solid-state fluorescence. Computational studies were performed to gain insights into the electronic energy level of the fluorophore and fluorescence modulation. All fluorophores showed mechanofluorochromism (MFC), reversible fluorescence switching, upon crushing and heating. The crushed solids also showed self-reversible fluorescence switching with a longer time. Interestingly, OFF-ON reversible fluorescence switching was also demonstrated by melting and warming/scratching. The powder X-ray diffraction studies supported self-reversible and crystallization-induced fluorescence switching.