Unravelling the details of vitamin D photosynthesis by non-adiabatic molecular dynamics simulations

Abstract

We investigate the photodynamics of vitamin D derivatives by a fully analytical implementation of the linear response time-dependent density functional theory surface hopping method (LR-TDDFT-SH). Our study elucidates the dynamics of the processes involved in vitamin D formation at the molecular level and with femtosecond resolution. We explain the major experimental findings and provide new insights that cannot directly be obtained from experiments: firstly, we investigate the dynamics of the photoinduced ring-opening of provitamin D (Pro) and cyclohexadiene (CHD) and the subsequent rotational isomerization. In agreement with recent experiments and CC2 calculations, only the bright S(1) state is involved in the ring-opening reaction. Our calculations confirm the experimentally reported 5 : 1 ratio between the excited state lifetimes of Pro and CHD. The longer lifetimes of Pro are attributed to steric constraints of the steroid skeleton and to temperature effects, both emerging directly from our simulations. For CHD and Pro, we present an explanation of the biexponential decay recently reported by Sension and coworkers [Tang et al., J. Phys. Chem., 2011, 134, 104503]: our calculations suggest that the fast and slow components arise from a reactive and an unreactive reaction pathway, respectively. Secondly, we assess the wavelength dependent photochemistry of previtamin D (Pre). Using replica exchange molecular dynamics we sample the Pre conformers present at thermal equilibrium. Based on this ensemble we explain the conformation dependent absorption and the essential features of Pre photochemistry. Consistent with the experiments, we find ring-closure to occur mostly after excitation of the cZc conformers and at lower energies, whereas Z/E isomerization of the central double bond preferably occurs after excitation at higher energies. For the isomerization we provide the first theoretical evidence of the proposed hula-twist mechanism. Our results show that LR-TDDFT-SH is a highly valuable tool for studying the photochemistry of moderately large systems, even though challenges remain in the vicinity of conical intersections.

Modelling materials for solar fuel synthesis by artificial photosynthesis; predicting the optical, electronic and redox properties of photocatalysts

“…presence of defects), finally, can be approximated when using TD-DFT, and when neglecting radiationless de-excitation via internal conversion, from the oscillator strength of the corresponding excitation via Einstein's equation for spontaneous emission. Calculation of the dark contribution to the exciton lifetime and of (exciton) dynamics in general requires non-adiabatic dynamics calculations, which explicitly take into account the coupling between the different energy surfaces [128][129][130][131][132][133][134][135][136]. Such calculations, however, are for the moment rather far from routine.…”

The exact forces on classical nuclei in non-adiabatic charge transfer

“…Moreover, the fundamental constructs of the BO approach, the BOPESs, form the launching ground for methods that describe processes beyond the adiabatic regime where the BO approximation itself fails. Prominent examples of such electronic non-adiabatic processes appear throughout physics, chemistry and biology, for example, vision, [2][3][4] photosynthesis, 5,6 photo-voltaics, [7][8][9] and proton-transfer/hydrogen storage. [10][11][12][13] The standard approaches to describe non-adiabatic molecular processes are in terms of coupled BOPESs and transitions between the corresponding adiabatic electronic states induced by the nuclear motion.…”

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

“…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). [65][66] Upon photoexcitation in S 1 , the molecule can either visit S 2 or reach the ground state (S 0 ) via CIs.…”