Time-resolved transient absorption spectroscopy was used to investigate the primary geminate recombination and cage escape of alkyl radicals in solution over a temperature range from 0 to 80 degrees C. Radical pairs were produced by photoexcitation of methyl, ethyl, propyl, hexylnitrile, and adenosylcobalamin in water, ethylene glycol, mixtures of water and ethylene glycol, and sucrose solutions. In contrast to previous studies of cage escape and geminate recombination, these experiments demonstrate that cage escape for these radical pairs occurs on time scales ranging from a hundred picoseconds to over a nanosecond as a function of solvent fluidity and radical size. Ultrafast cage escape (<100 ps) is only observed for the methyl radical where the radical pair is produced through excitation to a directly dissociative electronic state. The data are interpreted using a unimolecular model with competition between geminate recombination and cage escape. The escape rate constant, k(e), is not a simple function of the solvent fluidity (T/eta) but depends on the nature of the solvent as well. The slope of k(e) as a function of T/eta for the adenosyl radical in water is in near quantitative agreement with the slope calculated using a hydrodynamic model and the Stokes-Einstein equation for the diffusion coefficients. The solvent dependence is reproduced when diffusion constants are calculated taking into account the relative volume and mass of both solvent and solute using the expression proposed by Akgerman (Akgerman, A.; Gainer, J. L. Ind. Eng. Chem. Fundam. 1972, 11, 373-379). Rate constants for cage escape of the other radicals investigated are consistently smaller than the calculated values suggesting a systematic correction for radical size or coupled radical pair motion.

Ultrafast broadband UV-visible transient absorption spectroscopy is used to characterize the photochemistry of α-terpinene, a 1,4-disubstituted-1,3-cyclohexadiene natural product. These results are compared with experiments probing the analogous ring-opening reaction of 7-dehydrocholesterol (DHC, provitamin D3) and the subsequent relaxation of previtamin D3. The major experimental results are as follows: (1) Like DHC, but unlike 1,3-cyclohexadiene, α-terpinene exhibits a broad excited state absorption (ESA) spectrum in the visible. The lifetime of the excited state is ca. 0.16 ps in 1-butanol and 0.12 ps in hexane. (2) The state responsible for the ESA is the initially excited state. Fluorescence from this state has a quantum yield of ~2 × 10(-5). The fluorescence quantum yield is an order of magnitude smaller, and the excited state lifetime is an order of magnitude shorter than that observed for DHC. (3) The initial gZg-triene photoproduct absorbs to the red, and the relaxed tZg-triene product absorbs to the blue of α-terpinene. The gZg→tZg reaction of the vibrationally hot photoproduct requires ca. 6.5 ps with no significant dependence on solvent polarity or viscosity. Thermalization occurs on a time scale of 2-4 ps depending on solvent, but shows no particular trends within the solvent series. (4) The conformational relaxation of previtamin D3 occurs on a similar time scale of ca. 5-8 ps with a modest dependence on the solvent viscosity.

Our prior discovery of a novel biexponential photochemical ring-opening in 7-dehydrocholesterol (DHC) to previtamin D3 [ Tang J. Chem. Phys. 2011 , 134 , 104503 ] is further explored with ultrafast transient absorption spectroscopy, and the results are compared with recently reported high-level theoretical calculations. Three types of experiments are reported. First, variation of the excitation wavelength from 297 to 266 nm leaves the excited state dynamics unaffected. The biexponential decay of the excited state absorption is independent of excitation wavelength with time constants of 0.57 ± 0.06 and 1.88 ± 0.09 ps, in excellent agreement with the results reported earlier (0.56 ± 0.06 and 1.81 ± 0.15 ps) following excitation at 266 nm. Second, variation of the chirp of the excitation pulse influences the relative amplitude of the fast and slow decay components but has no influence on the photoproduct yield. Third, a 545 nm pulse delayed by 0.64 ps with respect to the initial 266 nm pulse was used to perturb the "slow" population and probe the influence of additional electronic or vibrational energy on the reaction process. The results show ultrafast internal conversion Sn → S1 on a ca. 150 fs time scale but no subsequent effect on the reaction dynamics. The experiments reported here are consistent with the recent state averaged complete active space self-consistent field ab initio multiple spawning (SA-CASSCF-AIMS) calculations of Snyder et al. [ J. Phys. Chem. Lett. 2016 , 7 , 2444 ] that assign the biexponential decay to nonequilibrium dynamics related to the opening and closing motion of the cyclohexadiene ring moiety on the excited state surface. These new experiments support the model prediction that the biexponential dynamics does not involve multiple minima and demonstrate the direction for new experimental designs to manipulate the product yields and pathways.

The excited and ground state dynamics of a series of 1,3-cyclohexadiene derivatives and their hexatriene photoproducts are studied using ultrafast broadband UV-visible transient absorption spectroscopy. The substitution pattern around the cyclohexadiene backbone alters the excited state potential energy surface in the Franck-Condon region as evidenced by changes in the excited state absorption and fluorescence properties of the systems. Unsubstituted 1,3-cyclohexadiene and alpha-phellandrene exhibit no excited state absorption while a strong excited state absorption in the visible spectral region is observed for both alpha-terpinene and Provitamin D3. Steric factors introduced by the ring substitutions determine the dominant rotational isomer at equilibrium for the hexatriene photoproducts. Coupling to the solvent during the conformational relaxation from the initial helical all-cis hexatriene is unique to each photoproduct, but the relaxation process occurs on a 6-10 ps timescale regardless of the size or substitution pattern on the triene.