Aryl Imidazoisoindoles for Intrinsic 10π Pericyclic Photochromism with Nonlinear Acid Responsiveness

Abstract

Photochemical and thermal peri-cyclization and cycloreversion reactions of organic polyenes are typically governed by orbital symmetry considerations. Here, we report two aryl-imidazoisoindole derivatives featuring benzene and thiophene rings that undergo reversible photochromic reactions via 10π cyclization and cycloreversion. The synthetic study was fully characterized with NMR, Mass, and X-ray analyses, which are further supported by quantum chemical calculations based on the density functional theory, DFT. Both molecules exhibit a reversible color change under UV and visible light irradiation. Photochemical quantum yields are presented, showing suppression with solvent polarity. The colored ring-closed form displayed a spontaneous fading reaction, whose thermodynamic parameters are well characterized. The thiophene-based substance exhibited a lifetime of 67 hr, which was markedly reduced upon protonation of the imidazole ring. Notably, small amounts of acid markedly enhance the spontaneous fading reaction through a catalytic mechanism. DFT calculations clearly supported the ground-state structures and the major contribution of the neutral mesomeric form in both substances. The reaction pathway and transition state for the thermal reaction are also characterized, which rationally support the major contribution of reversible 10π-photocyclization/cycloreversion reactions.