Structural and Solvent Control of Nonadiabatic Photochemical Bond Formation: Photocyclization of o-Terphenyl in Solution

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• 作者：Molly S. Molloy ; Joshua A. Snyder ; Arthur E. Bragg
• 刊名：Journal of Physical Chemistry A
• 出版年：2014
• 出版时间：June 5, 2014
• 年：2014
• 卷：118
• 期：22
• 页码：3913-3925
• 全文大小：481K
• 年卷期：v.118,no.22(June 5, 2014)
• ISSN：1520-5215

文摘

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鈥? 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鈥搒olute 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鈥?5 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鈥揷arbon bond that bridges the terminal phenyl rings in each to enable bond formation. We relate these structure鈥揹ynamics relationships to the feasibility of photoinduced cyclodehydrogenation of o-arenes and design considerations for molecular photoswitches.