Understanding the dynamics behind the photoisomerization of light-driven molecular rotary motors and switches
 project financed by the EU

Nature has ingeniously designed molecular motors, which play a central role in biology as the means of movement in living organisms. Chemistry is at an early stage of building artificial molecular motors at the bench. Currently, there is a high interest among the synthetic chemistry community in developing molecular motors and applying them in specific molecular devices ( functionalised surfaces, nano-cars, etc.). Rational synthetic design of molecular motors, however, implies a thorough understanding of their functionality. The thermochemical aspects of their operation are well understood, whereas their photo-dynamics still remains obscure and less amenable to chemical modification. It is my primary objective to investigate the photochemistry of molecular light driven devices and by this to fill the gap in our understanding of their mode of function. The outcome of this project will have far-reaching implications for achieving a rational synthetic design of future molecular photo driven systems and will help to improve the principles used for their design. This is a key to their application in future molecular devices in nanotechnology and synthetic biology.

Working cycle of synthetic molecular motors

The mode of action of these motors is based on the periodic repetition of photo-isomerization and thermal relaxation steps, which lead to a uni-directional rotation of one part of the molecule (rotor) with respect to another (stator). Diagram to the right shows the sequence of steps that the motor molecule undergoes to achieve a full rotation cycle: The photoisomerization about the central double bond brings conformer with the P-helicity to the M-form, which then undergoes a thermal helix inversion (M to P); after that the photoisomerization and helix inversion steps are repeated to complete the loop. Photoisomerization step is the most important, as it leads to a large scale rotational motion (half-loop). This step occurs on a time-scale of ca. 1.5 picosecond (ps), as suggested by both theory and experiment. It takes a few microseconds to complete the thermal helix inversion step.

The currently available synthetic motors display fairly low quantum efficiency (quantum yield of isomerization), ca. 14%. Roughly speaking only one out of seven light quanta results in rotation of the motor. Photoisomerization of the motor occurs via special features on the ground and excited electronic state potential energy surfaces known as conical intersections. The motor molecule in the scheme above reaches conical intersection at ca. 90° of twist about the central double bond and an additional geometric distortion known as pyramidalization. As a consequence of pyramidalization, the motor molecule performs not a pure axial rotation about the central double bond, but a sort of precessional motion as illustrated in the cartoons below.

precession

axial rotation

Furthermore, the need for pyramidalization leads to low accessibility of conical intersections and is the cause of the low quantum efficiency of the existing molecular motors. It is therefore necessary to modify the motor molecule in such a way that it could reach conical intersection without pyramidalization. In this way not only an axial rotation will be achieved by the motor, but its quantum efficiency will be increased as a consequence of improved accessibility of conical intersections.

What has been done so far
1. A new computational method for investigation of conical intersections in large molecules has been developed. For more information, see publications 103  and 102.

2. The effect of strongly electron withdrawing substituents and heteroatoms on the geometry at which S₀/S₁ conical intersection is reached has been modelled computationally and it was established that introducing substituents such as the cationic nitrogen removes the necessity to pyramidalize. Hence, a pure axial rotation can be achieved in molecular motors shown in figures below.

3. A novel class of molecular motors based on N-alkylated indanylidene pyrrolinium (NAIP) design has been proposed. These motors are capable of performing a continuous axial rotation (at least in theory) and operating at low temperatures. An example of a NAIP motor and its rotation cycle are shown below.

NAIP motor: the cartoon presents results of theoretical calculations of all intermediate structures along the rotation pathway of the motor molecule.

4. There is more to come...


  For more information, see publications:  118,  111,  109,  103,  102,  99,  85,  79.



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