A combination of cryogenic UV-vis and CD spectroscopy and transient absorption spectroscopy at ambient temperature is used to study a new class of unidirectional rotary molecular motors. Stabilization of unstable intermediates is achieved below 95 K in propane solution for the structure with the fastest rotation rate, and below this temperature measurements on the rate limiting step in the rotation cycle can be performed to obtain activation parameters. The results are compared to measurements at ambient temperature using transient absorption spectroscopy, which show that behavior of these motors is similar over the full temperature range investigated, thereby allowing a maximum rotation rate of 3 MHz at room temperature under suitable irradiation conditions.

One of the key challenges in taking light‐driven unidirectional rotary motors from discovery to application is to increase the rate of rotation. Herein, we review our ongoing efforts to address this issue by meticulous improvement to the molecular design. To accelerate the rotary cycle, we have focused primarily on the rate‐limiting thermal isomerization step. This has been a fascinating and formidable objective, given that the first system we reported had a half‐life of over one week at room temperature! Our research has ultimately led to the construction of a unidirectional rotary molecular motor with a cycle 108 times faster than the original; that is, it can in principle function at 44 rotations per second.

Molecular rotary motors, though common in nature, were first synthesized rather recently. One of the most promising categories of light‐driven rotary molecular motors which allow for optical control is based on helical overcrowded alkenes. In this category of motors, the rotation of the motor's rotor involves four discrete steps: fast light‐induced cis‐trans isomerisation is followed by a thermally activated step, the same steps are repeated once more to complete the full 360° cycle. Though the rotation rate of the motors is mainly limited by the duration of thermally activated steps, many important factors, such as efficiency and directionality are determined by the light‐induced isomerisation. For the development of ultrafast molecular motors it is crucial to understand the mechanism and dynamics of the optically induced isomerisation steps. In this paper we will review recent time‐resolved optical pump‐probe experiments on chiral molecular motors and discuss the results in terms of potential energy landscapes for the ground and excited states of the motor molecules. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultrafast Light-Driven Nanomotors Based on an Acridane Stator Kulago, Artem A.; Mes, Emile M.; Klok, Martin; Meetsma, Auke; Brouwer, A.M.; Feringa, B.L.

The introduction of dibenzocyclohepten-5-ylidene as part of a unidirectional light-driven molecular motor allows a more complete picture of the pathway of thermal helix inversion to be developed. The most stable conformation is similar to that found in related motors in that it has, overall, an anti-folded structure with the substituent at the stereogenic centre adopting an axial orientation. Photochemical cis/trans isomerisation at -40 degrees C results in the formation of an isomer in a syn-folded conformation with the methyl group in an axial orientation. This contrasts with previous studies on related molecular rotary motors. The conformation of the higher energy intermediate typically observed for this class of compound is the anti-folded conformation, in which the methyl group is in an equatorial orientation. This conformation is available through an energetically uphill upper half ring inversion of the observed photochemical product. However, this pathway competes with a second process that leads to the more stable anti-folded conformation in which the methyl group is oriented axially. It has been shown that the conformations and pathways available for second-generation molecular motors can be described by using similar overall geometries. Differences in the metastable high-energy species are attributable to the relative energy and position on the reaction coordinate of the transition states. Kinetic studies on these new molecular motors thus provide important insights into the conformational dynamics of the rotation cycle.

The combination of a photochemical and a thermal equilibrium in overcrowded alkenes, which is the basis for unidirectional rotation of light-driven molecular rotary motors, is analysed in relation to the actual average rotation rates of such structures. Experimental parameters such as temperature, concentration and irradiation intensity could be related directly to the effective rates of rotation that are achieved in solution by means of photochemical and thermal reaction rate theory. It is found that molecular properties, including absorption characteristics and photochemical quantum yields, are of less importance to the overall rate of rotation than the experimental parameters. This analysis holds considerable implications in the design of experimental conditions for functional molecular systems that will rely on high rates of rotation, and shows that average rotation rates comparable to ATPase or flagella motors are within reach assuming common experimental parameters.

Light driven molecular motors based on sterically overcrowded alkenes achieve repetitive unidirectional rotation through a sequential series of photochemical and thermal steps. The influence of highly viscous environments on the functioning of unidirectional light driven molecular motors is established in the present report using three distinct media. Liquefied propane, a reference medium due to its low viscosity even at 85 K, and two binary solvent systems that undergo a well-defined glass-transition are used to evaluate the influence of glass-like viscosities on both photochemical equilibria and on thermal helix inversion in the molecular motors. It is found that the greater molecular volume of the stable conformations relative to the unstable conformations is responsible for a shift of the photochemical equilibrium from 20% to 100% in favour of the unstable form when irradiated in high viscosity media. These results demonstrate the critical role excluded solvent volume can play in immobilized photo responsive molecular systems. The volume expansion associated with the thermal reversal to the most stable isomer can be used to determine the maximum possible power output of the rotary cycle.

The cover picture of this issue of Phys. Status Solidi C has been taken from the article [1]. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)