Effect of N-quaternization on crystal and electronic structures of E-pyridylvinyl benzothiazoles – Photophysical and electrochemical properties, and cytotoxicity studies

Abstract

The syntheses and structural characterization by single-crystal X-ray diffraction and various spectroscopic methods of benzothiazole-derived pyridyl-based fluorophores, namely, E-3-pyridylvinyl benzothiazole (3-PVBT), E-3-N-methylpyridinium vinyl benzothiazole (3-MPVBT), E-4-pyridylvinyl benzothiazole (4-PVBT), and E-4-N-methylpyridinium vinyl benzothiazole (4-MPVBT) have been presented to derive a comprehensive structure–property correlation. Notably, 3-MPVBT with Z’ = 2 belongs to a rare class of crystals. Upon N-quaternization, the cationic compounds exhibited red-shifted absorption and emission. Significantly larger Stokes shifts of 8700 and 8000 cm⁻¹, respectively, for 3-MPVBT and 4-MPVBT were observed due to intramolecular charge transfer (ICT), compared to Stokes shifts of 6100 and 7700 cm⁻¹ for their neutral analogues. The experimental results have been rationalized by DFT and TD-DFT calculations. Enhanced electrochemical reversibility was also observed in both the N-methylated compounds. Furthermore, the cytotoxicity of two selected fluorophores, 3-MPVBT and 4-MPVBT, was evaluated against breast cancer cell lines. Remarkably, N-quaternization is associated with multiple advantages, including red-shifted absorption and emission, larger Stokes shifts, stabilized HOMOs and LUMOs, diminished band gap, solubility in physiological pH, and advantageous redox characteristics.