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Abstract The optimization of the quantum efficiency of single-molecule light-driven rotary motors typically relies on chemical modifications. While, in isolated conditions, computational methods have been frequently used to design more efficient motors, the role played by the solvent environment has not been satisfactorily investigated. In this study, we used multiscale nonadiabatic molecular dynamics simulations of the working cycle of a 2-stroke photon-only molecular rotary motor. The results, which display dynamics consistent with the available transient spectroscopy measurements, predict a considerable decrease in the isomerisation quantum efficiency in methanol solution with respect to the gas phase. The origin of such a decrease is traced back to the ability of the motor to establish hydrogen bonds with solvent molecules. The analysis suggests that a modified motor with a reduced ability to form hydrogen bonds will display increased quantum efficiency, therefore extending the set of engineering rules available for designing light-driven rotary motors. 

Abstract RNA‐based tools for biological and pharmacological research are raising an increasing interest. Among these, RNA aptamers whose biological activity can be controlled via illumination with specific wavelengths represent an important target. Here, we report on a proof‐of‐principle study supporting the viability of a systematic use of Morita‐Baylis‐Hillman adducts (MBHAs) for the synthesis of light‐responsive RNA building blocks. Accordingly, a specific acetylated MBHA derivative was employed in the functionalization of the four natural RNA bases as well as two unnatural bases (5‐aminomethyl uracil and 5‐methylaminomethyl uracil). The results reveal a highly selective functionalization for both unnatural bases. The conjugation products were then investigated spectroscopically, photochemically and computationally. It is shown that when a single light‐responsive unit is present (i. e. when using 5‐methylaminomethyl uracil), the generated unnatural uracil behaves like a cinnamic‐framework‐based photochemical switch that isomerizes upon illumination through a biomimetic light‐induced intramolecular charge transfer mechanism driving a barrierless and, therefore, ultrafast reaction path. 

UV-Vis transient absorption (TA) spectroscopy is used to carry out a systematic investigation of the ultrafast CC double photoisomerization dynamics and quantum yield of each isomer of a set of six chromophores based on the same retinal-inspired, indanylidene pyrrolinium (IP) molecular framework. All compounds undergo a sub-picosecond photoisomerization, and can be categorized within two photoisomerization scenarios. Scenario I corresponds to compounds which display the signatures of a vibrationally coherent reactive motion through the conical intersection, with different degrees of vibrational coherence. Qualitatively distinct TA signatures are observed for other compounds which are therefore proposed to obey scenario II, referring to an intermediate regime between scenario I and a thermally-equilibrated, fully stochastic photoreaction. Remarkably, the photoisomerization scenario is observed to correlate with the computed distortion from planarity of the ground state equilibrium geometry, reflecting the torsional strain that would be released after photoexcitation. The most planar compounds – i.e. those having a CC double bond pre-twist of less than 10° – obey scenario II, while compounds obeying scenario I have larger pre-twists. The most pre-twisted compounds (>15°) show pronounced oscillatory signatures of a reaction-induced, low-frequency vibrational wavepacket observed in the S 0 photoproduct and assigned to the torsion mode of the reaction coordinate, thus mimicking the vibrationally coherent photoisomerization dynamics of the rhodopsin protein. Importantly, the systematic comparison of all photoisomerization quantum yields does however not reveal any correlation with observables such as excited state life time, vibrational coherence, absorption wavelengths or degree of pre-twisting.