title: Understanding the dynamics behind the photoisomerization of a light-driven fluorene molecular rotary motor.
abstract: Light-driven molecular rotary motors derived from chiral overcrowded alkenes represent a broad class of compounds for which photochemical rearrangements lead to large scale motion of one part of the molecule with respect to another. It is this motion/change in molecular shape that is employed in many of their applications. A key group in this class are the molecular rotary motors that undergo unidirectional light-driven rotation about a double bond through a series of photochemical and thermal steps. In the present contribution we report a combined quantum chemical and molecular dynamics study of the mechanism of the rotational cycle of the fluorene-based molecular rotary motor 9-(2,4,7-trimethyl-2,3-dihydro-1H-inden-1-ylidene)-9H-fluorene (1). The potential energy surfaces of the ground and excited singlet states of 1 were calculated, and it was found that conical intersections play a central role in the mechanism of photo conversion between the stable conformer of 1 and its metastable conformer. Molecular dynamics simulations indicate that the average lifetime of the fluorene motor in the excited state is 1.40 +/- 0.10 ps when starting from the stable conformer, which increases to 1.77 +/- 0.13 ps for the reverse photoisomerization. These simulations indicate that the quantum yield of photoisomerization of the stable conformer is 0.92, whereas it is only 0.40 for the reverse photoisomerization. For the first time, a theoretical understanding of the experimentally observed photostationary state of 1 is reported that provides a detailed picture of the photoisomerization dynamics in overcrowded alkene-based molecular motor 1. The analysis of the electronic structure of the fluorene molecular motor holds considerable implications for the design of molecular motors. Importantly, the role of pyramidalization and conical intersections offer new insight into the factors that dominate the photostationary state achieved in these systems.
authors.affiliation: Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, Netherlands.
authors.forename: Andranik
authors.initials: A
authors.lastname: Kazaryan
authors.forename: Jos C M
authors.initials: JC
authors.lastname: Kistemaker
authors.forename: Lars V
authors.initials: LV
authors.lastname: Schäfer
authors.forename: Wesley R
authors.initials: WR
authors.lastname: Browne
authors.forename: Ben L
authors.initials: BL
authors.lastname: Feringa
authors.forename: Michael
authors.initials: M
authors.lastname: Filatov
mesh_terms: D005449:Fluorenes / Q000737:chemistry*; D007536:Isomerism; D008027:Light; D008968:Molecular Conformation; D056004:Molecular Dynamics Simulation; D055668:Photochemical Processes; D011789:Quantum Theory; D012399:Rotation
chemical_list: D005449:Fluorenes; C041509:fluorene