Salts of trans-3-Styryl Pyridine: The Effect of N-Quaternization on Solid-State Photoreactivity, and Photochemical Crystal Melting.

Investigating solid-state photoreactivity, driven by crystal packing, has been a major enduring research theme in Crystal Engineering. Trans-3-styryl pyridine (3-StPy), an unsymmetric olefin, is photo-stable. However, when converted to a series of salts, they exhibited solid-state photoreactivity under UV irradiation. Crystal structures of 3-StPy, its protonated salts, namely, (3-StPyH)(HSO) ⋅ HO (1), (3-StPyH)(CFCO) (2), and (3-StPyH)(p-Tol-SO) (3), and methylated salt (3-StPyMe)I (4) were determined by X-ray diffraction. 3-StPy molecules were found not to align in the parallel arrangement required to undergo solid-state [2+2] photocycloaddition reaction; however, upon protonation and methylation, the unsymmetric 3-StPyH and [3-StPyMe] cations aligned in a head-to-tail parallel arrangement, predominantly governed by the cation⋅⋅⋅π interactions. Various structural features, e. g., the patterns of hydrogen bonding, etc. have been addressed, and established by Hirshfeld surface analysis. The salt with p-tolyl sulfonate anion (3) with Z'>1 represents a rare crystal class. It was also noted that the crystals of 1 and 2 melted into a liquid state upon photodimerization reaction. UV-vis absorption and fluorescence properties have been explored. The electronic structures of 3-StPy, 3-StPyH, [3-StPyMe], and the dimer [3,3'-MPyPhCB] cations have been elucidated by DFT calculations, and the effect of N-quaternization on crystal structures and photophysical properties has been discussed.