Structural and Solvent Control of Photochemical Bond Formation: Nonadiabatic Photocyclization of ortho-Terphenyls

Light-driven changes in molecular structure, including photoisomerization and electrocyclic bond formation, are involved in many chemical reactions prevalent in nature. Understanding the underlying nonadiabatic dynamics that allow such light-induced routes to proceed rapidly and efficiently is a critical first step toward harnessing their power for chemical applications. Of particular interest is how the structure of the reactant and the nature of its interactions with the local chemical environment impact the progress and outcome of photochemical dynamics. In this work, time-resolved ultrafast broadband transient absorption spectroscopy has been used to study ultrafast dynamics associated with photoinduced bond formation via 6π photocyclization in detail, with a particular focus on how reactant structure impacts photochemical dynamics of ortho-terphenyl (OTP) and a number of its structural analogues. Calculations of molecular geometries and excitation energies are used to assist with interpretation of experimental data. We find that bond formation is strongly influenced by the flexibility of the central ring and by intramolecular steric interactions that hinder structural evolution towards relevant conical intersections, allowing us to deduce critical nuclear coordinates in excited-state relaxation. Studies with structural analogues decorated with various substituents demonstrate how structure controls photocyclization dynamics and yields. Photocyclization dynamics were also found to have a strong dependence on mechanical solvent-solute interactions (viscosity and density), and very little to do with electrostatic affects (dipole moment and dielectric constant); this provide a means for exploring the nature of solvent friction on chemical dynamics in solution