Structural and solvent control of nonadiabatic photochemical bond formation: photocyclization of o-terphenyl in solution

J Phys Chem A. 2014 Jun 5;118(22):3913-25. doi: 10.1021/jp501988g. Epub 2014 May 22.

Abstract

Elucidating the molecular dynamics that underlie photoinduced electrocyclization is a critical step toward controlling nonadiabatic photochemistry that enables bond formation. Here we present a comprehensive examination of the photochemical dynamics of o-terphenyl (OTP) in solution. Ultrafast transient absorption measurements demonstrate that OTP cyclizes upon 266 nm photoexcitation to form 4a,4b-dihydrotriphenylene (DHT) on a solvent-dependent time scale of 1.5-4 ps, considerably slower than the nonadiabatic cyclization of related diarylethenes. Correlations in these time scales versus bulk solvent properties reveal that mechanical rather than electrostatic solvent-solute interactions impact the excited-state relaxation rate, impeding nuclear dynamics leading toward the conical intersection for cyclization. In contrast, solvent-dependent mechanical interactions are observed to facilitate vibrational relaxation of DHT on time scales of 10-25 ps. DHT decays via thermally activated ring-opening with a lifetime of ∼46 ns in tetrahydrofuran, 12 orders of magnitude faster than dihydrophenanthrenes. We conclude that the differences in excited-state dynamics of OTP and diarylethenes and the relative stability of their cyclized products are determined by the relative strain induced by twisting the central carbon-carbon bond that bridges the terminal phenyl rings in each to enable bond formation. We relate these structure-dynamics relationships to the feasibility of photoinduced cyclodehydrogenation of o-arenes and design considerations for molecular photoswitches.