Putting Photomechanical Switches to Work: An Ab Initio Multiple Spawning Study of Donor-Acceptor Stenhouse Adducts

Photomechanical switches are light sensitive molecules capable of transducing the energy of a photon into mechanical work via photodynamics. In this Letter, we present the first atomistic investigation of the photodynamics of a novel class of photochromes called donor-acceptor Stenhouse adducts (DASA) using state-of-the-art ab initio multiple spawning interfaced with state-averaged complete active-space self-consistent field theory. Understanding the Z/E photoisomerization mechanism in DASAs at the molecular level is crucial in designing new derivatives with improved photoswitching capabilities. Our dynamics simulations show that the actinic step consists of competing nonradiative relaxation pathways that collectively contribute to DASAs' low (21% in toluene) photoisomerization quantum yield. Furthermore, we highlight the important role the intramolecular hydrogen bond plays in the selectivity of photoisomerization in DASAs, identifying it as a possible structural element to tune DASA properties. Our fully ab initio simulations reveal the key degrees of freedom involved in the actinic step, paving the way for the rational design of new generations of DASAs with improved quantum yield and efficiency.