“Watching” the Dark State in Ultrafast Nonadiabatic Photoisomerization Process of a Light-Driven Molecular Rotary Motor

Abstract

Photoisomerization dynamics of a light-driven molecular rotary motor, 9-(2-methyl-2,3-dihydro-1H-cyclopenta[a]naphthalen-1-ylidene)-9H-fluorene, is investigated with trajectory surface-hopping dynamics at the semiempirical OM2/MRCI level. The rapid population decay of the S excited state for the M isomer is observed, with two different decay time scales (500 fs and 1.0 ps). By weighting the contributions of fast and slow decay trajectories, the averaged lifetime of the S excited state is about 710 fs. The calculated quantum yield of the M-to-P photoisomerization of this molecular rotary motor is about 59.9%. After the S → S excitation, the dynamical process of electronic decay is followed by twisting about the central C═C double bond and the motion of pyramidalization at the carbon atom of the stator-axle linkage. Although two S/S minimum-energy conical intersections are obtained at the OM2/MRCI level, only one conical intersection is found to be responsible for the nonadiabatic dynamics. The existence of "dark state" in the molecular rotary motor is confirmed through the simulated time-resolved fluorescence emission spectrum. Both quenching and red shift of fluorescence emission spectrum observed by Conyard et al. [ Conyard, J.; Addison, K.; Heisler, I. A.; Cnossen, A.; Browne, W. R.; Feringa, B. L.; Meech, S. R. Nat. Chem. 2012 , 4 , 547 - 551 ; Conyard, J.; Conssen, A.; Browne, W. R.; Feringa, B. L.; Meech, S. R. J. Am. Chem. Soc. 2014 , 136 , 9692 - 9700 ] are well understood. We find that this "dark state" in the molecular rotary motor is not a new electronic state, but the "dark region" with low oscillator strength on the initial S state.

Mapping the Excited‐State Potential Energy Surface of a Photomolecular Motor

“…However, the calculations are not entirely self‐consistent. Two suggest that dynamics involve a crossing of two excited states, while two others find that the dynamics occur on a single PES ,. Furthermore, two have investigated the entire 1 a / 1 b PES in both ground and excited states, which has not yet been characterized experimentally.…”

“…Pang et al. found that the S 1 surface had a single minimum, independent of the initial geometry . If that were the case then the observed difference in lifetime for that common state must reflect dynamical effects, that is, the pathway from the different initial geometries, rather than the final state reached.…”

“…These studies of 1 a excited‐state dynamics stimulated high quality quantum chemical calculations . Three recent examples point to a reaction coordinate that involves both pyramidalization at the fluorenyl–axle carbon (C9) and torsional motion about the axle; consistent with the observed solvent friction dependence ,,. These calculations reveal the fundamentally multidimensional nature of the reaction coordinate (in contrast to the 1D Figure ).…”

“…calculated excited‐state decay dynamics following both 1 a and 1 b excitation, reporting an overall slower decay for 1 b , while Pang et al. made a detailed simulation of excited‐state dynamics following 1 b excitation, but did not track the pathway for 1 a excitation ,…”

Modeling and analyzing a photo-driven molecular motor system: Ratchet dynamics and non-linear optical spectra

“…Theoretically, many studies have been carried out with the aim of understanding the details of the potential energy surface (PES) structures and non-adiabatic coupling (NAC) profiles to determine the rotational degrees of freedom and to simulate fast photoisomerization processes in molecular motors systems [12][13][14][15][16][17][18][19]. In contrast, the thermalization process, * ikeda.tatsushi.37u@kyoto-u.jp † A.G.Dijkstra@leeds.ac.uk ‡ tanimura.yoshitaka.5w@kyoto-u.jp which is known as a rate-limiting step of molecular motors [6][7][8], has not been well explored.…”