Ultrafast dynamics and excited-state trapping in [3.3]paracyclophane.

Paracyclophanes are rigid, three-dimensional frameworks in which two benzene rings are held in a parallel, stacked arrangement by short aliphatic linkers. Their derivatives display pronounced through-space π-π interactions that strongly influence their excited-state behavior. Here, we investigate the ultrafast excited-state dynamics of [3.3]paracyclophane ([3.3]PCP) using time-dependent density functional theory combined with nonadiabatic molecular dynamics via surface hopping. The 2 ps simulations provide a detailed picture of how electronic and nuclear motions evolve in concert after photoexcitation. Following excitation to the weak S3 band shoulder, [3.3]PCP rapidly relaxes into S1, where it becomes kinetically trapped. Analysis of the fragment-based transition density matrix reveals a concurrent transformation of the electronic structure-from an excitonic-resonance state with minor charge-transfer (CT) character (≈0.2) to a mixed excitonic/charge-resonance regime with CT ≈ 0.5. This evolution is accompanied by a structural contraction of the π-stack, as the two benzene rings approach each other, activating an inter-ring breathing motion that governs the subsequent dynamics. Because [3.3]PCP combines rapid nonadiabatic relaxation with long-lived excited-state trapping, it serves as a particularly demanding benchmark for trajectory-based dynamics methods, offering both a mechanistic picture of excimer formation and a stringent test of their current capabilities.