Excimer-to-excimer adiabatic ring opening of dipyrenylcyclobutane via predissociation

Pyrene, a well-known excimer-forming arene, was used as one of the aryl substituents in a diarylethylene photochrome, pyrenylvinylquinoline (PVQ). Using this photochrome, a novel bichromophoric dyad DPVQ was designed, synthesized, and studied. As a result of [2 + 2] photocycloaddition, the dyad produced a cyclobutane CB with two vicinal pyrenyl substituents. Luminescent properties of the compounds were studied by steady-state and time-resolved methods. Both dyad DPVQ and cyclobutane CB displayed excimer emission, red-shifted relative to the emission of the corresponding monomers. Thus, for the first time, excimers were experimentally observed both in a bichromophoric dyad with two diarylethylene photochromes and in the corresponding cyclobutane. Time-resolved emission spectroscopy (TRES) detected that the disappearance of the cyclobutane pyrene excimer occurs within 1 ns and is accompanied by the simultaneous formation of a dyad excimer, indicating an adiabatic cyclobutane ring-opening reaction. It is suggested that the reaction proceeds via a predissociation mechanism with the intermediate formation of a tetramethylene biradical. Quantum-chemical density functional theory (DFT) calculations at the M06-2×/6-31G* level predicted the existence of the folded conformers with π-stacking interaction of PVQ photochromes in the dyad and the pyrenyl substituents in the cyclobutane. These pre-organized ground-state dimers can serve as precursors for excimer formation in the excited state.