Mechanical Efficiency of Photochromic Nanomotors, From First Principles

Photochromic molecular motors hold promise for a multitude of potential applications in fields ranging from medicine to communications and structural repair. Yet, it is still a challenge to predict their mechanical efficiency. Here, azobenzene is explored as a representative light-driven nanomotor and estimate its quantum yield of photoisomerization and maximum mechanical efficiency. This is based on first-principles mapping of the 3D potential energy surfaces for the ground and excited states of the trans and cis configurations and identifying the minimum energy pathway for isomerization. A work cycle is devised and identifies force constant as the parameter that resembles temperature in the Carnot heat engine, but with very different efficiencies. The results show that the optomechanical efficiency of azobenzene at constant load is about 5% albeit under ideal conditions. To test the hypothesis, the study also explores the optomechanical efficiency of stilbene and 2-butene and shows that their efficiency does not exceed 5%.