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

Area of Science:

• Photochemistry
• Computational Chemistry
• Molecular Dynamics

Background:

• Paracyclophanes possess rigid, 3D structures with stacked benzene rings.
• Through-space π-π interactions significantly impact their excited-state properties.

Purpose of the Study:

• Investigate the ultrafast excited-state dynamics of [3.3]paracyclophane ([3.3]PCP).
• Elucidate the interplay between electronic and nuclear motions post-photoexcitation.
• Provide a benchmark for trajectory-based dynamics methods.

Main Methods:

• Time-dependent density functional theory (TD-DFT).
• Nonadiabatic molecular dynamics via surface hopping simulations.
• 2 ps simulation timescale.

Main Results:

• [3.3]PCP rapidly relaxes from S3 to S1, undergoing kinetic trapping.
• Electronic structure evolves from excitonic to a mixed excitonic/charge-transfer (CT) state (CT ≈ 0.5).
• Structural contraction and inter-ring breathing motion govern dynamics.

Conclusions:

• [3.3]PCP demonstrates rapid relaxation and long-lived excited-state trapping.
• The study provides a mechanistic picture of excimer formation.
• [3.3]PCP serves as a stringent test for dynamics simulation capabilities.