Chiral Hydrogen Bond Environment Providing Unidirectional Rotation in Photoactive Molecular Motors

One-way rotation of a molecule-rotor driven by a shot noise

The shot noise of a tunneling current passing through a molecule-motor can sustain a one-way rotation when populating the molecular excited states by tunneling inelastic excitations. We demonstrate that a ratchet-like ground state rotation potential energy curve is not necessary for the rotation to occur. A relative shift in energy difference between the maxima of this ground state and the minima of the excited states is the necessary condition to get to a unidirectional rotation. The rotor speed of rotation and its rotation direction are both controlled by this shift, indicating the necessity of a careful design of both the ground and excited states of the next generation of molecule-motors to be able to generate a motive power at the nanoscale.

Learning from photobiology how to design molecular devices using a computer

Learning how to model photo-responsive proteins may open the way to the design of lightpowered biomimetic molecular devices.

Theoretical study of topographical features around the conical intersections of 9-(2-cyclopenten-1-ylidene)-9H-fluorene

Ab initio molecular orbital calculations have been performed to examine the topographical features around the conical intersections (CIXs) for the photoisomerization of 9-(2-cyclopenten-1-ylidene)-9H-fluorene (CPF). The present study is motivated by the computational findings of the topographical features in the CIX region of a fluorene-based light-driven molecular motor with smooth rotation (denoted by M5-PCPF). CPF is a parent molecule of M5-PCPF but exhibits neither helicity nor unidirectionality. A stable geometry in S1 (S1-geometry) is located in the region where the ethylenic rotary axis is perpendicularly twisted, but the fluorene part wags little against the rotary axis, as is the case of M5-PCPF. However, the topographical features around the CIXs of CPF are different from those of M5-PCPF. The wagging motion of the fluorene stator to the negative direction from S1-geometry leads straight to a CIX which implements forward and backward rotations. On the other hand, only the wagging motion to the positive direction does not lead to a CIX, and additional geometrical deformations are needed. Depending on the directions of additional geometrical deformations, two CIXs, which play the roles of respective exit channels for forward and backward rotations, are located in the positive wagging region. The difference in the topographical features in the CIX region between CPF and M5-PCPF is ascribed to the effect of the pentamethylene chain. By virtue of much less computational labor of CPF as well as the electronic structures being similar to those of M5-PCPF, the intrinsic reaction coordinate was followed from the ethylenic ππ* state at the stable geometry in S0 into S1-geometry. Thereby, it was confirmed that a spectroscopically dark state due to the π(fluorene)π*(ethylene) excitation contributes less to the photochemical process of the ethylenic bond torsion, as is the case of M5-PCPF.