Exploring the boundaries of a light-driven molecular motor design: new sterically overcrowded alkenes with preferred direction of rotation

Insight in the steric and electronic parameters governing isomerization processes in artificial molecular motors is essential in order to design more advanced motor systems. A subtle balance of steric parameters and the combination of helical and central chirality are key features of light-driven unidirectional rotary molecular motors constructed so far. In an approach to decrease the steric hindrance around the central olefinic bond (rotary axis) and thereby lowering the energy barrier for helix inversion resulting in an increased rotation rate, the boundaries of our molecular motor design are explored. In a new design of a light-driven molecular motor based on a sterically overcrowded alkene the methyl substituent adjacent to the stereogenic center, which is responsible for the control of the direction of rotation, is shifted one position away from the fjord region of the molecule compared to the second-generation motor systems. In contrast to previously developed light-driven molecular motors, there is a preference for the methyl substituent to adopt a pseudo-equatorial orientation. Nevertheless, this new type of motor is capable of functioning as a rotary molecular motor, albeit not with full unidirectionality. Under the combined influence of light and heat, there is a preferred clockwise rotation of one half of the molecule. Surprisingly, the effect of shifting the methyl substituent on the energy barrier for helix inversion is small and even a slight increase in the barrier is observed.     

Fine tuning of the rotary motion by structural modification in light-driven unidirectional molecular motors

The introduction of bulky substituents at the stereogenic center of light-driven second-generation molecular motors results in an acceleration of the speed of rotation. This is due to a more strained structure with elongated C=C bonds and a higher energy level of the ground state relative to the transition state for the rate-limiting thermal isomerization step. Understanding the profound influence that variation of the substituent at the stereogenic center holds over the rotational speed of the light-driven molecular motor has enabled the development of the fastest molecular motor reported thus far.     

Effect of charge-transfer enhancement on the efficiency and rotary mechanism of an oxindole-based molecular motor

Harvesting energy and converting it into mechanical motion forms the basis for both natural and artificial molecular motors. Overcrowded alkene-based light-driven rotary motors are powered through sequential photochemical and thermal steps. The thermal helix inversion steps are well characterised and can be manipulated through adjustment of the chemical structure, however, the insights into the photochemical isomerisation steps still remain elusive. Here we report a novel oxindole-based molecular motor featuring pronounced electronic push–pull character and a four-fold increase of the photoisomerization quantum yield in comparison to previous motors of its class. A multidisciplinary approach including synthesis, steady-state and transient absorption spectroscopies, and electronic structure modelling was implemented to elucidate the excited state dynamics and rotary mechanism. We conclude that the charge-transfer character of the excited state diminishes the degree of pyramidalisation at the alkene bond during isomerisation, such that the rotational properties of this oxindole-based motor stand in between the precessional motion of fluorene-based molecular motors and the axial motion of biomimetic photoswitches.

A novel oxindole-based light-driven molecular motor with pronounced push–pull character was investigated. The rotary mechanism stands in between the precessional motion of fluorene-based motors and the axial motion of biomimetic photoswitches.     

Next-generation quantum theory of atoms in molecules for the S1/S0 conical intersections in dynamics trajectories of a light-driven rotary molecular motor

Next-generation quantum theory of atoms in molecules was applied to analyze, along an entire bond path, intramolecular interactions known to influence the photoisomerization dynamics of a light-driven rotary molecular motor. The 3D bond-path framework set B_0,1 constructed from the least and most preferred directions of electronic motion, provided new insights into the bonding leading to different S₁ state lifetimes including the first quantification of covalent character of a closed-shell intramolecular bond path. We undertook the first use of the stress tensor trajectory T_σ(s) analysis on selected nonadiabatic molecular dynamics trajectories with the electron densities obtained using the ensemble density functional theory method. The stress tensor T_σ(s) analysis was found to be well suited to follow the dynamics trajectories that included the S₀ and S₁ electronic states through the conical intersection and also provided to a new measure to assess the degree of purity of the axial bond rotation for the design of rotary molecular motors.     

Control of rotor function in light-driven molecular motors

A study is presented on the control of rotary motion of an appending rotor unit in a light-driven molecular motor. Two new light driven molecular motors were synthesized that contain aryl groups connected to the stereogenic centers. The aryl groups behave as bidirectional free rotors in three of the four isomers of the 360° rotation cycle, but rotation of the rotors is hindered in the fourth isomer. Kinetic studies of both motor and rotor functions of the two new compounds are given, using (1)H NMR, 2D-EXSY NMR, and UV-vis spectroscopy. In addition, we present the development of a new method for introducing a range of aryl substituents at the α-carbon of precursors for molecular motors. The present study shows how the molecular system can be photochemically switched between a state of free rotor rotation and a state of hindered rotation and reveals the dynamics of coupled rotary systems.     

QM/MM Studies on Photoisomerization Dynamics of Azobenzene Chromophore Tethered to a DNA Duplex: Local Unpaired Nucleobase Plays a Crucial Role

The photoresponsive azobenzene‐tethered DNAs have received growing experimental attention because of their potential applications in biotechnology and nanotechnology; however, little is known about the initial photoisomerization of azobenzene in these systems. Herein we have employed quantum mechanics/molecular mechanics (QM/MM) methods to explore the photoisomerization dynamics of an azobenzene‐tethered DNA duplex. We find that in the S₁ state the trans–cis photoisomerization path is much steeper in DNA than in vacuo, which makes the photoisomerization much faster in the DNA environment. This acceleration is primarily caused by complex steric interactions between azobenzene and the nearby unpaired thymine nucleobase, which also change the photoisomerization mechanism of azobenzene in the DNA duplex.     

Are azobenzenophanes rotation-restricted?

We simulated the photoisomerization dynamics of an azobenzenophane with a semiclassical surface hopping approach and a semiempirical reparametrized quantum mechanics/molecular mechanics Hamiltonian. Only one of the two azobenzene chromophores in the molecule is taken into account quantum mechanically: the other one is treated by molecular mechanics. Both n-->pi* and pi-->pi* excitations are considered. Our results show that the photoisomerization reaction mainly involves the rotation around the N=N double bond. The excited state relaxation features are in qualitative agreement with experimental time-resolved fluorescence results.     

A pentiptycene-derived molecular brake: photochemical E→Z and electrochemical Z→E switching of an enone module

The synthesis and brakelike performance of a new molecular system (1) consisting of a pentiptycene rotor and a 2-methyleneindanone brake are reported. The rotation kinetics of the rotor was probed by both variable-temperature (1)H and (13)C NMR spectroscopy and DFT calculations, and the switching between the brake-on and brake-off states was conducted by a combination of photochemical and electrochemical isomerization. Because of the greater steric hindrance between the rotor and the brake units in the Z form ((Z)-1) than in the E form ((E)-1), rotation of the rotor is slowed down 500-fold at room temperature (298 K) on going from (E)-1 to (Z)-1, corresponding to the brake-off and brake-on states, respectively. The (E)-1→(Z)-1 photoisomerization in acetonitrile is efficient and reaches an (E)-1/(Z)-1 ratio of 11:89 in the photostationary state upon excitation at 290 nm, attributable to a much larger isomerization quantum efficiency for (E)-1 versus (Z)-1. An efficient (Z)-1→(E)-1 isomerization (96%) was also achieved by electrochemical treatment through the radical anionic intermediates. Consequently, the reversibility of the E-Z switching of 1 is as high as 85%. The repeated E-Z switching of 1 with alternating photochemical and electrochemical treatments is also demonstrated.     

Molecular Shape Analysis of a Maillard Reaction Intermediate

Abstract

The Maillard (browning) reaction involving the polycondensation of sugars and amino acids is believed to be an important abiotic pathway for humic substance formation in nature. However, a major drawback is that the Maillard reaction is extremely slow at temperatures encountered under normal environmental conditions. In order to elucidate some details of this process molecular shape analysis was applied to investigate the initial reaction between D-glucose and glycine to form the Amadori compound fructosylglycine which is an intermediate product in the Maillard reaction. The structure of the Amadori compound was optimized at a quantum mechanical level and its ground state electron energy calculated. Molecular Iso-Density Contours (MIDCO's), electron density contour surfaces of constant electron density, were constructed for D-glucose, glycine and fructosylglycine in order to study the steric conditions for the reaction. The calculations indicate that the Amadori compound and water on one hand and the separate entities D-glucose and glycine on the other hand are very similiar to each other in terms of their ground state energy. This agrees with the experimental observation that the reaction between D-glucose and glycine to form the Amadori compound is slow.     

On the effect of donor and acceptor substituents on the behaviour of light-driven rotary molecular motors

Light-driven rotary molecular motors based on overcrowded alkenes can be substituted with electron-donating and electron-withdrawing substituents (R = OMe, Cl and CN) in direct conjugation with the central double bond (the axis of rotation) without having a significant influence on the rate-limiting, thermal isomerisation step of their rotary cycle. This indicates that in this system, it is predominantly steric factors that determine the barrier to the thermal helix inversion. In contrast, the quantum yield and photoequilibria in the photochemical step were found to be quite sensitive to the combination of substituent and solvent employed.     

Photoisomerization of disperse red 1 studied with transient absorption spectroscopy and quantum chemical calculations

The photoisomerization of the push-pull substituted azo dye Disperse Red 1 is studied using femtosecond time-resolved absorption spectroscopy and other spectroscopic and computational techniques. In comparison with azobenzene, the pipi* state is more stabilized by the effects of push-pull substitution than the npi* state, but the latter is probably still the lowest in energy. This conclusion is based on the kinetics, anisotropy of the excited state absorption spectrum, the spectra of the ground states, and quantum chemical calculations. The S(1)(npi*) state is formed from the initially excited pipi* state in <0.2 ps, and decays to the ground state with time constants of 0.9 ps in toluene, 0.5 ps in acetonitrile, and 1.4 ps in ethylene glycol. Thermal isomerization transforms the Z isomer produced to the more stable E isomer with time constants of 29 s (toluene), 28 ms (acetonitrile), and 2.7 ms (ethylene glycol). The pathway of photoisomerization is likely to be rotation about the N=N bond. Quantum chemical calculations indicate that along the inversion pathway ground and excited state energy surfaces remain well separated, whereas rotation leads to a region where conical intersections can occur. For the ground-state Z to E isomerization, conclusive evidence is lacking, but inversion is more probably the favored pathway in the push-pull substituted systems than in the parent azobenzene.     

MO Study of the photochemical behavior of the imine bond

The photoisomerization of imine compounds is studied in terms of an ab initio MO CI calculation. The potential curves of the syn-anti isomerization via the rotation and the inversion are examined for benzaldimine. It is suggested that the photoisomerization is initiated through the rotation around the C? N bond in both singlet and triplet states. The ease of the photoisomerization is found to be determined by both the conformation of phenyl ring in the ground state and the energy difference of vertically excited states between two isomers.     

Computational Insight to Improve the Thermal Isomerisation Performance of Overcrowded Alkene‐Based Molecular Motors through Structural Redesign

Synthetic overcrowded alkene‐based molecular motors achieve 360° unidirectional rotary motion of one motor half (rotator) relative to the other (stator) through sequential photochemical and thermal isomerisation steps. In order to facilitate and expand the use of these motors for various applications, it is important to investigate ways to increase the rates and efficiencies of the reactions governing the rotary motion. Here, we use computational methods to explore whether the thermal isomerisation performance of some of the fastest available motors of this type can be further improved by reducing the sizes of the motor halves. Presenting three new redesigned motors that combine an indanylidene rotator with a cyclohexadiene, pyran or thiopyran stator, we first use multiconfigurational quantum chemical methods to verify that the photoisomerisations of these motors sustain unidirectional rotary motion. Then, by performing density functional calculations, we identify both stepwise and concerted mechanisms for the thermal isomerisations of the motors and show that the rate‐determining free‐energy barriers of these processes are up to 25 kJ mol^?1 smaller than those of the original motors. Furthermore, the thermal isomerisations of the redesigned motors proceed in fewer steps. Altogether, the results suggest that the redesigned motors are useful templates for improving the thermal isomerisation performance of existing overcrowded alkene‐based motors.     

Mechanistic Origin of the Vibrational Coherence Accompanying the Photoreaction of Biomimetic Molecular Switches

The coherent photoisomerization of a chromophore in condensed phase is a rare process in which light energy is funneled into specific molecular vibrations during electronic relaxation from the excited to the ground state. In this work, we employed ultrafast spectroscopy and computational methods to investigate the molecular origin of the coherent motion accompanying the photoisomerization of indanylidene–pyrroline (IP) molecular switches. UV/Vis femtosecond transient absorption gave evidence for an excited‐ and ground‐state vibrational wave packet, which appears as a general feature of the IP compounds investigated. In close resemblance to the coherent photoisomerization of rhodopsin, the sudden onset of a far‐red‐detuned and rapidly blue‐shifting photoproduct signature indicated that the population arriving on the electronic ground state after nonadiabatic decay through the conical intersection (CI) is still very focused in the form of a vibrational wave packet. Semiclassical trajectories were employed to investigate the reaction mechanism. Their analysis showed that coupled double‐bond twisting and ring inversions, already populated during the excited‐state reactive motion, induced periodic changes in π‐conjugation that modulate the ground‐state absorption after the non‐adiabatic decay. This prediction further supports that the observed ground‐state oscillation results from the reactive motion, which is in line with a biomimetic, coherent photoisomerization scenario. The IP compounds thus appear as a model system to investigate the mechanism of mode‐selective photomechanical energy transduction. The presented mechanism opens new perspectives for energy transduction at the molecular level, with applications to the design of efficient molecular devices.     

Ultrafast Excited State Dynamics in a First Generation Photomolecular Motor

Efficient photomolecular motors will be critical elements in the design and development of molecular machines. Optimisation of the quantum yield for photoisomerisation requires a detailed understanding of molecular dynamics in the excited electronic state. Here we probe the primary photophysical processes in the archetypal first generation photomolecular motor, with sub-50 fs time resolved fluorescence spectroscopy. A bimodal relaxation is observed with a 100 fs relaxation of the Franck-Condon state to populate a red-shifted state with a reduced transition moment, which then undergoes multi-exponential decay on a picosecond timescale. Oscillations due to the excitation of vibrational coherences in the S₁ state are seen to survive the ultrafast structural relaxation. The picosecond relaxation reveals a strong solvent friction effect which is thus ascribed to torsion about the C−C axle. This behaviour is contrasted with second generation photomolecular motors; the principal differences are explained by the existence of a barrier on the excited state surface in the case of the first-generation motors which is absent in the second generation. These results will help to provide a basis for designing more efficient molecular motors in the future.     

Exploring the Mechanism of a Chiral N-Alkyl Imine-Based Light-Driven Molecular Rotary Motor at MS-CASPT2//CASSCF and MS-CASPT2//(TD) DFT Levels

The working mechanism including the photoisomerization and thermal isomerization steps of a chiral N-alkyl imine-based motor synthesized by Lehn et al. are revealed by MS-CASPT2//CASSCF and MS-CASPT2//(TD-)DFT methods. For the photoisomerization process of the imine-based motor, it involves both the bright (π,π*) state and the dark (n,π*) state. In addition, the MECI has similar geometry and energy to the minimum of the S₁ state, which shows that the process is barrierless and keeps the unidirectionality of rotation well; the result confirms the imine-based motor is a good candidate for a light-driven molecular rotary motor. For the thermal isomerization process of the imine-based motor, there are two even isomerization paths: one with the mechanism of the in-plane N inversion, the energy barriers of which are 29.6 kcal mol⁻¹ at MS3-CASPT2//CAM-B3LYP level and 29.2 kcal mol⁻¹ at MS3-CASPT2//CASSCF level; the other with the mechanism of ring inversion of the cycloheptatriene moiety, with energy barriers of 28.1 kcal mol⁻¹ at MS3-CASPT2//CAM-B3LYP level and 18.1 kcal mol⁻¹ at MS3-CASPT2//CASSCF level. According to the structural feature of the stator moiety, the imine molecule can be used as a two-step or a four-step light-driven rotary motor.     

Direct Observation of Hemithioindigo‐Motor Unidirectionality

Hemithioindigo molecular motors undergo very fast unidirectional rotation upon irradiation with visible light, which has prevented a complete analysis of their working mechanism. In this work, we have considerably slowed down their motion by using a new synthesis for sterically hindered motor derivatives. This method allowed the first observation of all four intermediate states populated during rotation. The exact order in which each isomeric state is formed under irradiation conditions was elucidated using low temperature ¹H NMR spectroscopy in conjunction with other analytical methods. At the same time, complete unidirectionality could also be directly shown. Access to slowly rotating hemithioindigo motors opens up a plethora of new applications for visible‐light‐induced unidirectional motions, especially in areas such as catalysis, smart materials, and supramolecular chemistry.     

Mapping the Excited‐State Potential Energy Surface of a Photomolecular Motor

A detailed understanding of the operation and efficiency of unidirectional photomolecular rotary motors is essential for their effective exploitation in molecular nanomachines. Unidirectional motion relies on light‐driven conversion from a stable ( 1 a ) to a metastable ( 1 b ) conformation, which then relaxes through a thermally driven helix inversion in the ground state. The excited‐state surface has thus far only been experimentally characterised for 1 a . Here we probe the metastable, 1 b , excited state, utilising ultrafast transient absorption and femtosecond stimulated Raman spectroscopy. These reveal that the “dark” excited‐state intermediate between 1 a and 1 b has a different lifetime and structure depending on the initial ground‐state conformation excited. This suggests that the reaction coordinate connecting 1 a to 1 b differs to that for the reverse photochemical process. The result is contrasted with earlier calculations.