Probing the Biexponential Dynamics of Ring-Opening in 7-Dehydrocholesterol

Abstract

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.

Ultrafast Pump–Repump–Probe Photochemical Hole Burning as a Probe of Excited-State Reaction Pathway Branching

“…35−37 Hence, the dynamics observed should be associated with a relatively structurally uniform ensemble of reactants. Therefore, do the two time scales observed reflect the initial relaxation of the excited reactant, S 1 * → S 1 , followed by electrocyclization, spectral dynamics associated with an intrinsically biexponential/ nonexponential process, 6,7 or rather branching between distinct relaxation pathways, including a competing funnel to the ground state or the formation of other photoproducts (e.g., Figure 1a)? Motivated in part by recent successful applications of threepulse "pump−repump−probe" (PRP) and "pump−dump− probe" spectroscopies to interrogate and manipulate photoinduced dynamics in light-activated materials, 10,38−41 biomolecules, 6,42−44 and charge-transfer events, 45−47 we have developed PRP transient photochemical hole burning, generalized schematically in Figure 1c, to unravel photochemical pathway branching for systems like OTP.…”

“…10,41 Similarly, Smith et al observed no repumpinduced changes in the ring-opening efficiency of the 7dehydrocholesterol. 6 In contrast, the 340 nm repump is resonant with both species, but the PRP signal is dominated by the repump's action on the photoproduct, as the efficiency of depleting the DHT population with the 340 nm repump pulse maximizes by time scales on which the photochemical formation of DHT has concluded (based on comparison with TAS data). The slight signal decay on the 10s of ps time scale reflects that vibrational cooling impacts (decreases) the efficiency for photochemical depletion of the photoproduct and is consistent with observations from Figures 2b and 3b.…”

The role of tachysterol in vitamin D photosynthesis – a non-adiabatic molecular dynamics study

“…Pre itself plays the central role, as it reacts to four distinct main Vita photo isomers (DPI) Pro, Lumi, and Tachy and toxisterols (Toxi). While the provitamin D ringopening and its analogous reaction in cyclohexadiene has been studied extensively, both theoretically [17][18][19][20] and experimentally, 18,[21][22][23][24][25][26] little research has been done to assess the influence of tachysterol on photochemical vitamin D production. In particular, its excited state dynamics has neither been investigated by experiments nor by simulations.…”

Multiphoton Control of 6π Photocyclization via State-Dependent Reactant–Product Correlations

“…Photoisomerizations underlie a variety of biological phenomena and applications, including the retinal photoresponse that drives vision, − the photochemistry of provitamin D, , and bilirubin detoxification through blue-light therapy. − Photoisomerization also underlies the responses of molecular photoswitches that can be used to modulate material properties, , as exemplified by molecular photochromism and memory, , light-activated motors and mechanical responses, ,− and rapidly photoswitchable molecular conductivity and electronics. , Furthermore, photoisomerizations (specifically, photocyclizations) have significant value for preparatory carbon–carbon bond formation − and are particularly useful where harsh reagents may damage reactive functional groups. − In all of these cases, isomerization occurs in one of the many steps through which photoexcited molecules deactivate energetically. The nature of isomerization products and the efficiency with which they are formed is governed by the shapes and interactions between potential-energy surfaces of two or more molecular electronic states. − There has been great interest in using multiphoton inputs in order to open new photoreaction channels along these surfaces or to optimize reaction efficiencies for generating a desired photoisomerization product. − Herein, we illustrate how multiphotonic input, selectively chosen, opens a new photoreaction channel for a simple photocyclizing reactant by taking advantage of state-dependent correlations between high-lying excited electronic states.…”

“…P hotoisomerizations underlie a variety of biological phenomena and applications, including the retinal photoresponse that drives vision, 1−4 the photochemistry of provitamin D, 5,6 and bilirubin detoxification through bluelight therapy. 7−9 Photoisomerization also underlies the responses of molecular photoswitches that can be used to modulate material properties, 10,11 as exemplified by molecular photochromism and memory, 12,13 light-activated motors and mechanical responses, 11,14−16 and rapidly photoswitchable molecular conductivity and electronics.…”