Coherent Intramolecular Excimer Formation in Solid [2,2]-Paracyclophane: Time-Resolved Springing of a Molecular “Trap”

Impulsive UV excitation unleashes violent wave packet motions in [2,2]-paracyclophane (2PC) due to relief of ground state inter-ring repulsion. Contrary to theoretical predictions for the benzene excimer, transient absorption spectroscopy in oriented single crystals and in liquid THF demonstrates that excited 2PC absorbs visible light polarized along all three molecular axes. Absorption polarized perpendicular to the aromatic rings is deeply modulated in time by the ∼0.5 Å of light induced inter-ring breathing providing changes in potential energy gaps and dipole strengths during coherent intramolecular excimer formation. Overlapping transitions to several higher electronic states and strong non-Condon effects contribute to the modulating spectra. These insights prove current understanding of the simplest archetypal organic excimer is fundamentally flawed and provide the basis for testing improved electronic structure models. It is hard to overestimate the significance of achieving such improved understanding given the impact of excimers and exciplexes on all aspects of organic molecular photophysics.