H-Bond assisted mechanoluminescence of borylated aryl amines: Tunable emission and polymorphism

Design, synthesis and structural characterization of borylated aryl amines, Mes₂BAr {Ar = C₆(CH₃)₄NR₂ (1, 4); C₆H₄NR₂ (2, 5); C₆H₃(NR₂)₂ (3, 6); R = H or CH₃}, were reported. The intriguing optical signatures undoubtedly revealed their donor-acceptor characteristics and a bright tunable solid state emission is readily realized. The solid state luminescence characteristics of 1 and 3 were sensitive to mechanical stress with distinguishable emission color changes. Multiple strong intermolecular hydrogen bonds (N-H⋯N and N-H⋯π) accompanied by subtle conformational changes play a significant role in the piezochromic response. PXRD and FT-IR spectroscopic studies and insensitivity of substituted derivatives to mechanical stress support the above inference. Interestingly, compound 3 crystallized in two different polymorphic forms 3BP and 3GP, which showed distinct luminescence, i.e., green and blue color under UV light. Such changes are due to their distinct hydrogen-bond network assembly in the solid state. Quantum mechanical calculations were performed in order to corroborate the optical properties.