Ultrafast electrocyclic ring opening of 7-dehydrocholesterol in solution: The influence of solvent on excited state dynamics

Abstract

Broadband UV-visible femtosecond transient absorption spectroscopy and steady-state integrated fluorescence were used to study the excited state dynamics of 7-dehydrocholesterol (provitamin D(3), DHC) in solution following excitation at 266 nm. The major results from these experiments are: (1) The excited state absorption spectrum is broad and structureless spanning the visible from 400 to 800 nm. (2) The state responsible for the excited state absorption is the initially excited state. Fluorescence from this state has a quantum yield of ∼2.5 × 10(-4) in room temperature solution. (3) The decay of the excited state absorption is biexponential, with a fast component of ∼0.4-0.65 ps and a slow component 1.0-1.8 ps depending on the solvent. The spectral profiles of the two components are similar, with the fast component redshifted with respect to the slow component. The relative amplitudes of the fast and slow components are influenced by the solvent. These data are discussed in the context of sequential and parallel models for the excited state internal conversion from the optically excited 1(1)B state. Although both models are possible, the more likely explanation is fast bifurcation between two excited state geometries leading to parallel decay channels. The relative yield of each conformation is dependent on details of the potential energy surface. Models for the temperature dependence of the excited state decay yield an intrinsic activation barrier of ∼2 kJ/mol for internal conversion and ring opening. This model for the excited state behavior of DHC suggests new experiments to further understand the photochemistry and perhaps control the excited state pathways with optical pulse shaping.

GPU-Accelerated State-Averaged Complete Active Space Self-Consistent Field Interfaced with Ab Initio Multiple Spawning Unravels the Photodynamics of Provitamin D3

“…Aside from its obvious biological importance, Pro is a model system for understanding how to optically control polyene chromophores, which has potential applications in nanoscience. 52 This reaction has been studied both experimentally [53][54][55][56][57][58][59][60][61][62] and theoretically. [63][64] Critical points on Pro potential energy surfaces are represented in Figure 1 (SA-3-CAS(6,4)/6-31G, see SI for details and validation of the electronic structure employed).…”

“…67 More recently, Tang et al employed ultrafast UV-vis transient absorption spectroscopy to demonstrate that the photoconversion of Pro to Pre exhibits biexponential decay, with time constants ranging from 0.39-0.65ps and 1.06-1.81ps depending on the solvent. 59 Tapavicza et al comprehensively studied the photochemistry of Pro and Pre using TSH coupled to LR-TDDFT. 64 At variance with experimental results, the computed reaction rates are too fast and the population decay is not biexponential.…”

Uncovering molecular relaxation processes with nonlinear spectroscopies in the deep UV

“…Interestingly, nonexponential depopulation of state 1B was suggested more than 20 years ago based on the analysis of the absorption line shapes and resonance Raman intensities in CHD [129]. Sension and co-workers have also raised the possibility that non-exponential relaxation dynamics are at work in provitamin D [134]. Pronounced non-exponential internal conversion dynamics is apparently a general property of this class of molecules.…”

“…The framework for thinking about these two molecules is owed to earlier work in which femtosecond transient absorption spectroscopies were employed [67,[121][122][123][124][125][126][127][128][129][130][131]. In particular, Sension and co-workers have conducted rigorous studies on CHD and a number of its derivatives in solution (e.g., alphaphellandrene, a-TP) [126,128,134,135]. This group has also shown that the basic physics translates to larger systems with cyclic diene moieties such as provitamin D [135][136][137].…”

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.…”