Solid-State Photodimerization Reaction with Photosalient Effect and Photophysical and Electrochemical Properties of N‑Methylated 1‑Naphthylvinyl-4-Quinoline

Crystal packing-dependent photochemical reactions of several functional olefins have been explored, but not for 1-naphthylvinyl-4-quinoline (NVQ) and analogous N-methylated 1-naphthylvinyl-4-quinoline cation (MNVQ). Herein, crystal structure elucidation and photophysical and electrochemical properties for both NVQ and MNVQ, and photodimerization reaction with a photosalient effect for MNVQ have been investigated for the first time. The neutral compound NVQ stacks parallel in its crystal; however, having a distance of 4.43 Å between the CC bonds, it remained photoinert. Upon methylation at the quinoline-N, the resulting MNVQ cations stack parallel in a head-to-tail fashion with a distance of 3.6 Å between the CC bonds, and consequently it underwent [2 + 2] photocycloaddition reaction quantitatively to MQNCB dication, when irradiated under visible light (∼410 nm). Monitoring closely the single crystals of MNVQ under blue light showed that they burst upon exposure for 1 min, signifying a photosalient behavior. Measurements of photophysical properties showed that MNVQ exhibited a bathochromic shift in both absorption (by 4772 cm^–1) and emission (by 6198 cm^–1) compared to the neutral NVQ. Upon photodimerization, the MQNCB dication exhibited a hypsochromic and hypochromic shift both in absorption and emission compared to the MNVQ cation, a signature of cessation of conjugation. Cyclic voltammetry (CV) measurements showed that all three compounds are redox active and exhibit a reversible reduction event in the range of −0.6 to −0.9 V (vs Ag/AgCl). Experimental results have been substantiated by theoretical calculations.