Control of Photoconversion Yield in Unidirectional Photomolecular Motors by Push-Pull Substituents

Unidirectional molecular rotary motor with remotely switchable rotation direction

Light-driven rotary motors allow direct transformation of light energy into rotary motion at the nanoscale, giving rise to countless emerging applications in molecular engineering. The key feature enabling the unidirectional rotation and controlling its direction is the motor chirality, a factor hard to modify postsynthetically. Here, we propose a motor architecture, E-motor, whose operation direction can be switched remotely with an electric field pulse, without the need for chemical intervention. Our study relies on quantum chemical calculations and nonadiabatic molecular dynamics simulations performed for a specifically tailored system, PFCN, designed to provide illustration for the proposed motor type. We show that the PFCN chirality depends on the orientation of a covalently bound polar switching unit, which can be controlled with the electric field. At the same time, the proposed system manifests all characteristic photophysical properties of a molecular motor, and its set chirality is preserved during operation in the absence of the field.

Tuning the Photoisomerization Mechanism of Oxindole Switches with Electron-Donating Substituents

This study investigates the photoreaction mechanism of a hydroxy-substituted oxindole photoswitch using femtosecond transient absorption, fluorescence up-conversion, and computational chemistry. Deprotonation of the hydroxyl group enhances the push-pull character in the molecule, allowing tuning of the photoisomerization mechanism from a precessional to an axial motion. The neutral form of the switch exhibits longer excited-state lifetimes, while the anionic form decays rapidly within 200 fs. Computational models show that deprotonation increases the charge transfer and accessibility to conical intersections. This work highlights how varying the electron-donating strength of a substituent in a push-pull photoswitch tunes the photoreaction mechanism in designing photoswitches.

Quantum-Classical Simulations Reveal the Photoisomerization Mechanism of a Prototypical First-Generation Molecular Motor

Light-driven molecular rotary motors convert the energy of absorbed light into unidirectional rotational motion and are key components in the design of molecular machines. The archetypal class of light-driven rotary motors is chiral overcrowded alkenes, where the rotational movement is achieved through consecutive cis-trans photoisomerization reactions and thermal helix inversion steps. While the thermal steps have been rather well understood by now, our understanding of the photoisomerization reactions of overcrowded alkene-based motors still misses key points that would explain the striking differences in operation efficiency of the known systems. Here, we employ quantum-chemical calculations and nonadiabatic molecular dynamics simulations to investigate the excited-state decay and photoisomerization mechanism in a prototypical alkene-based first-generation rotary motor. We show that the initially excited bright state undergoes an ultrafast relaxation to multiple excited-state minima separated by low energy barriers and reveal a slow picosecond-timescale decay to the ground state, which only occurs from a largely twisted dark excited-state minimum, far from any conical-intersection point. Additionally, we attribute the origin of the high yields of forward photoisomerization in our investigated motor to the favorable topography of the ground-state potential energy surface, which is controlled by the conformation of the central cyclopentene rings.

Non-adiabatic Couplings in Surface Hopping with Tight Binding Density Functional Theory: The Case of Molecular Motors

Nonadiabatic molecular dynamics (NAMD) has become an essential computational technique for studying the photophysical relaxation of molecular systems after light absorption. These phenomena require approximations that go beyond the Born-Oppenheimer approximation, and the accuracy of the results heavily depends on the electronic structure theory employed. Sophisticated electronic methods, however, make these techniques computationally expensive, even for medium size systems. Consequently, simulations are often performed on simplified models to interpret the experimental results. In this context, a variety of techniques have been developed to perform NAMD using approximate methods, particularly density functional tight binding (DFTB). Despite the use of these techniques on large systems, where ab initio methods are computationally prohibitive, a comprehensive validation has been lacking. In this work, we present a new implementation of trajectory surface hopping combined with DFTB, utilizing nonadiabatic coupling vectors. We selected the methaniminium cation and furan systems for validation, providing an exhaustive comparison with the higher-level electronic structure methods. As a case study, we simulated a system from the class of molecular motors, which has been extensively studied experimentally but remains challenging to simulate with ab initio methods due to its inherent complexity. Our approach effectively captures the key photophysical mechanism of dihedral rotation after the absorption of light. Additionally, we successfully reproduced the transition from the bright to dark states observed in the time-dependent fluorescence experiments, providing valuable insights into this critical part of the photophysical behavior in molecular motors.

Kinetic Basis for the Design of Azobenzene-Based Photoswitchable A2a Adenosine Receptor Ligands

Photoisomerization of ligands is a key process in the field of photopharmacology. Thus, the kinetics and efficiency of this initial photoreaction are of great importance but can be influenced by the molecular environment of the binding pocket and the resulting confinement of the reaction pathway. In this study, we investigated the photoisomerization of an azobenzene derivative of the anti-Parkinson's drug istradefylline. To identify the impact of the binding pocket, the ligand was examined in solution and bound to its target protein, the A2a adenosine receptor (A2aR), belonging to the family of G protein-coupled receptors (GPCRs). Although the overall efficiency of isomerization is reduced when the ligand is bound, the initial photoreaction experiences little influence from the binding pocket. However, protein-coupled motion promotes a longer-lived excited-state population and thus leads to a reduction in efficiency. The results provide the kinetic basis for a photoswitchable GPCR ligand and demonstrate the influence of the binding pocket on fundamental photochemistry.

Effect of load-resisting force on photoisomerization mechanism of a single second generation light-driven molecular rotary motor

A pivotal aspect of molecular motors is their capability to generate load capacity from a single entity. However, few studies have directly characterized the load-resisting force of a single light-driven molecular motor. This research provides a simulation analysis of the load-resisting force for a highly efficient, second-generation molecular motor developed by Feringa et al. We investigate the M-to-P photoinduced nonadiabatic molecular dynamics of 9-(2,3-dihydro-2-methyl-1H-benz[e]inden-1-ylidene)-9H-fluorene utilizing Tully's surface hopping method at the semi-empirical OM2/MRCI level under varying load-resisting forces. The findings indicate that the quantum yield remains relatively stable under forces up to 0.003 a.u., with the photoisomerization mechanism functioning typically. Beyond this threshold, the quantum yield declines, and an alternative photoisomerization mechanism emerges, characterized by an inversion of the central double bond's twisting direction. The photoisomerization process stalls when the force attains a critical value of 0.012 a.u. Moreover, the average lifetime of the excited state oscillates around that of the unperturbed system. The quantum yield and mean lifetime of the S1 excited state in the absence of external force are recorded at 0.54 and 877.9 fs, respectively. In addition, we analyze a time-dependent fluorescence radiation spectrum, confirming the presence of a dark state and significant vibrations, as previously observed experimentally by Conyard et al.

Substituent effects on first generation photochemical molecular motors probed by femtosecond stimulated Raman

Unidirectional photochemical molecular motors can act as a power source for molecular machines. The motors operate by successive excited state isomerization and ground state helix inversion reactions, attaining unidirectionality from an interplay of steric strain and stereochemistry. Optimizing the yield of the excited state isomerization reaction is an important goal that requires detailed knowledge of excited state dynamics. Here, we investigate the effect of electron withdrawing and donating substituents on excited state structure and ultrafast dynamics in a series of newly synthesized first generation photochemical molecular motors. All substituents red-shift the absorption spectra, while some modify the Stokes shift and render the fluorescence quantum yield solvent polarity dependent. Raman spectra and density functional theory calculations reveal that the stretching mode of the C=C "axle" in the electronic ground state shows a small red-shift when conjugated with electron withdrawing substituents. Ultrafast fluorescence measurements reveal substituent and solvent polarity effects, with the excited state decay being accelerated by both polar solvent environment and electron withdrawing substituents. Excited state structural dynamics are investigated by fluorescence coherence spectroscopy and femtosecond stimulated Raman spectroscopy. The time resolved Raman measurements are shown to provide structural data specifically on the Franck-Condon excited state. The C=C localized modes have a different substituent dependence compared to the ground state, with the unsubstituted motor having the most red-shifted mode. Such measurements provide valuable new insights into pathways to optimize photochemical molecular motor performance, especially if they can be coupled with high-quality quantum molecular dynamics calculations.

Solar Azo-Switches for Effective E→Z Photoisomerization by Sunlight

Natural photoactive systems have evolved to harness broad-spectrum light from solar radiation for critical functions such as light perception and photosynthetic energy conversion. Molecular photoswitches, which undergo structural changes upon light absorption, are artificial photoactive tools widely used for developing photoresponsive systems and converting light energy. However, photoswitches generally need to be activated by light of specific narrow wavelength ranges for effective photoconversion, which limits their ability to directly work under sunlight and to efficiently harvest solar energy. Here, focusing on azo-switches-the most extensively studied photoswitches, we demonstrate effective solar E→Z photoisomerization with photoconversions exceeding 80 % under unfiltered sunlight. These sunlight-driven azo-switches are developed by rendering the absorption of E isomers overwhelmingly stronger than that of Z isomers across a broad ultraviolet to visible spectrum. This unusual type of spectral profile is realized by a simple yet highly adjustable molecular design strategy, enabling the fine-tuning of spectral window that extends light absorption beyond 600 nm. Notably, back-photoconversion can be achieved without impairing the forward solar isomerization, resulting in unique light-reversible solar switches. Such exceptional solar chemistry of photoswitches provides unprecedented opportunities for developing sustainable light-driven systems and efficient solar energy technologies.

Excited State Dynamics in Unidirectional Photochemical Molecular Motors

Unidirectional photochemically driven molecular motors (PMMs) convert the energy of absorbed light into continuous rotational motion. As such they are key components in the design of molecular machines. The prototypical and most widely employed class of PMMs is the overcrowded alkenes, where rotational motion is driven by successive photoisomerization and thermal helix inversion steps. The efficiency of such PMMs depends upon the speed of rotation, determined by the rate of ground state thermal helix inversion, and the quantum yield of photoisomerization, which is dependent on the excited state energy landscape. The former has been optimized by synthetic modification across three generations of overcrowded alkene PMMs. These improvements have often been at the expense of photoisomerization yield, where there remains room for improvement. In this perspective we review the application of ultrafast spectroscopy to characterize the excited state dynamics in PMMs. These measurements lead to a general mechanism for all generations of PMMs, involving subpicosecond decay of a Franck-Condon excited state to populate a dark excited state which decays within picoseconds via conical intersections with the electronic ground state. The model is discussed in the context of excited state dynamics calculations. Studies of PMM photochemical dynamics as a function of solvent suggest exploitation of intramolecular charge transfer and solvent polarity as a route to controlling photoisomerization yield. A test of these ideas for a first generation motor reveals a high degree of solvent control over isomerization yield. These results suggest a pathway to fine control over the performance of future PMMs.