Photoisomerization dynamics of a rhodopsin-based molecule (potential molecular switch) with high quantum yields*

It is worthwhile to explore the detailed reaction dynamics of various candidates for molecular switches, in order to understand, e.g., the differences in quantum yields and switching times. Here we report density-functional-based simulations for the rhodopsin-based molecule 4-[4-Methylbenzylidene]-5-p-tolyl-3,4-dihydro-2H-pyrrole (MDP), synthesized by Sampedro et al. We find that the photoisomerization quantum yields are remarkably high: 82\% for \textit{cis}-to-\textit{trans}, and 68\% for \textit{trans}-to-\textit{cis}. The lifetimes of the S$_{1}$ excited state in \textit{cis}-MDP in our calculations are in the range of 900-1800 fs, with a mean value of 1270 fs, while the range of times required for full \textit{cis}-to-\textit{trans} isomerization are 1100-2000 fs, with a mean value of 1530 fs. In \textit{trans}-MDP, the calculated S$_{1}$ excited state lifetimes are 860-2140 fs, with a mean value of 1330 fs, and with the full \textit{trans}-to-\textit{cis} isomerization completed about 200 fs later. In both cases, the dominant reaction mechanism is rotation around the central C$=$C bond (connected to the pyrroline ring), and de-excitation occurs at an avoided crossing between the ground state and the lowest singlet state, near the midpoint of the rotational pathway.