Designing light-driven rotary molecular motors

The ability to induce and amplify motion at the molecular scale has seen tremendous progress ranging from simple molecular rotors to responsive materials. In the two decades since the discovery of light-driven rotary molecular motors, the development of these molecules has been extensive; moving from the realm of molecular chemistry to integration into dynamic molecular systems. They have been identified as actuators holding great potential to precisely control the dynamics of nanoscale devices, but integrating molecular motors effectively into evermore complex artificial molecular machinery is not trivial. Maximising efficiency without compromising function requires conscious and judicious selection of the structures used. In this perspective, we focus on the key aspects of motor design and discuss how to manipulate these properties without impeding motor integrity. Herein, we describe these principles in the context of molecular rotary motors featuring a central double bond axle and emphasise the strengths and weaknesses of each design, providing a comprehensive evaluation of all artificial light-driven rotary motor scaffolds currently present in the literature. Based on this discussion, we will explore the trajectory of research into the field of molecular motors in the coming years, including challenges to be addressed, potential applications, and future prospects.</p

Designing light-driven rotary molecular motors

The ability to induce and amplify motion at the molecular scale has seen tremendous progress ranging from simple molecular rotors to responsive materials. In the two decades since the discovery of light-driven rotary molecular motors, the development of these molecules has been extensive; moving from the realm of molecular chemistry to integration into dynamic molecular systems. They have been identified as actuators holding great potential to precisely control the dynamics of nanoscale devices, but integrating molecular motors effectively into evermore complex artificial molecular machinery is not trivial. Maximising efficiency without compromising function requires conscious and judicious selection of the structures used. In this perspective, we focus on the key aspects of motor design and discuss how to manipulate these properties without impeding motor integrity. Herein, we describe these principles in the context of molecular rotary motors featuring a central double bond axle and emphasise the strengths and weaknesses of each design, providing a comprehensive evaluation of all artificial light-driven rotary motor scaffolds currently present in the literature. Based on this discussion, we will explore the trajectory of research into the field of molecular motors in the coming years, including challenges to be addressed, potential applications, and future prospects

Formylation boosts the performance of light-driven overcrowded alkene-derived rotary molecular motors

Artificial molecular motors and machines constitute a critical element in the transition from individual molecular motion to the creation of collective dynamic molecular systems and responsive materials. The design of artificial light-driven molecular motors operating with high efficiency and selectivity constitutes an ongoing fundamental challenge. Here we present a highly versatile synthetic approach based on Rieche formylation that boosts the quantum yield of the forward photoisomerization reaction while reaching near-perfect selectivity in the steps involved in the unidirectional rotary cycle and drastically reducing competing photoreactions. This motor is readily accessible in its enantiopure form and operates with nearly quantitative photoconversions. It can easily be functionalized further and outperforms its direct predecessor as a reconfigurable chiral dopant in cholesteric liquid crystal materials.</p

Impact of solvation on the photoisomerisation dynamics of a photon-only rotary molecular motor

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.Increasing the rotational efficiency of single-molecule light-driven rotary motors often relies on chemical modifications aimed at eliminating the factors that hinder rotation. Using multiscale nonadiabatic simulations, the authors investigate the transient conformations assumed by the motor molecule during its operation in a solvent and examine possibilities for enhancing the motor's efficiency by blocking certain solvent-solute interactions that restrain successful completion of the rotational movement

Photochemical and Electrochemical Switching of Overcrowded Alkenes

This thesis contains the research into the synthesis and functioning of molecular electromotors with quaternary centres; the novel synthesis and electrochmistry of bisthioxanthylidene electrochemical switches. Further research delves into new molecular motors with helical chirality and computational studies of isoindigo

Strategies for Red-Light Photoswitching

Vuorovaikutteiset, muotoutuvat ja jopa älykkäät molekyylirakenteet ovat avain uuden sukupolven lääkeaineisiin ja toiminnallisiin materiaaleihin. Valokytkimet eli yhdisteet, jotka isomeroituvat reversiibelisti valon vaikutuksesta johtaen makroskooppisten ominaisuuksien muutoksiin, ovat erottamaton osa tätä tulevaisuutta. Mahdolliset sovelluskohteet ulottuvat lääketieteestä elektroniikkaan ja robotiikkaan. Valitettavasti useimmat valokytkinrakenteet, esimerkiksi laajalti käytetyt atsobentseenit, absorboivat ultraviolettivaloa, joka on vahingollista monille materiaaleille ja erityisesti eläville soluille. Jotta valokytkinten koko potentiaali voidaan hyödyntää, tarvitaan harmittomalla näkyvällä valolla toimivia yhdisteitä. Puna- tai infrapunavalo olisi ihanteellinen ärsyke biologian alalla käytettäville kytkimille. Sama pätee myös molekyylimoottoreihin eli yhdisteisiin, jotka pyörivät valon vaikutuksesta yksisuuntaisesti. Lisäksi sekä kytkinten että moottorien tulisi isomerisoitua valon vaikutuksesta tehokkaasti ja nopeasti, termisten isomerisaatioreaktioiden tulisi olla sovelluskohteesta riippuen hitaita tai nopeita ja yhdisteiden tulisi toimia hyvin erilaisissa ympäristöissä. Näiden ominaisuuksien hallitsemiseksi on tärkeää ymmärtää niiden taustalla olevat mekanismit. Tässä väitöskirjassa tutkimme kolmea keinoa toteuttaa valokytkentä punaisella valolla: (i) atsobentseenien absorptiospektrin siirtäminen rakennetta muokkaamalla, (ii) uusien, valmiiksi punaista valoa absorboivien rakenteiden hyödyntäminen ja (iii) epäsuora valokytkentä punavalolla aktivoitavia katalyyttejä hyödyntäen. Tarkastelemme strategioita teoreettiselta kannalta ja osoitamme, että niistä jokainen mahdollistaa valokytkennän punaista valoa käyttäen. Kullakin strategialla on etunsa ja haasteensa tehokkaan, nopean ja kestävän valokytkennän toteuttamiseksi. Tästä johtuen yksi ihanteellinen valokytkinmalli ei voi saavuttaa kaikkia eri sovelluksille asetettuja tavoitteita, vaan tulevaisuuden haaste on löytää kuhunkin käyttöön paras ratkaisu. Samoja periaatteita voidaan soveltaa myös molekyylimoottoreihin, jolloin molekulaarisen tason yksisuuntainen kiertoliike voidaan saada aikaan näkyvällä valolla. Lisäksi punaisella valolla toimivien valokytkinten rakenteita hyödyntämällä moottorien rotaatiota saadaan tehostettua.Responsive, adaptive and even intelligent molecular systems have been identified as the key to next-generation pharmaceuticals and functional materials. Photoswitches, compounds that isomerise reversibly between two distinct ground-state species upon excitation with light and consequently give rise to a macroscopic effect, are an integral part of this future. Their potential application areas range from photopharmacology to optoelectronics and soft robotics. However, most conventional photoswitch structures such as azobenzenes absorb ultraviolet light, high-energy photons that are detrimental to many artificial materials and especially to living systems. To harness their full potential, photoswitches should function efficiently with visible light that is benign to the environment. Red or near-infrared light would be the ideal stimulus for switches utilised in biological context, as these wavelengths are least absorbed by living tissue. The same applies to light-driven molecular motors, compounds that exhibit unidirectional rotation upon photoexcitation. In addition to absorption in the red part of the visible spectrum, both switches and motors should exhibit efficient and fast photoisomerisation, favourable thermal isomerisation kinetics and tolerance towards different environments in order to be useful in real-life applications. In this light, it is crucial to understand the underlying fundamental mechanisms that govern these attributes. In this thesis, we explore three different approaches to realise photoswitching with red light: (i) synthetic modifications of azobenzenes, (ii) utilisation of new photoswitch cores that inherently absorb low-energy photons, and (iii) indirect isomerisation with red-light photocatalysts. We study each strategy from a theoretical viewpoint and demonstrate that they all provide means to induce isomerisation with red light, each with unique advantages and challenges in terms of promoting efficient, fast and robust switching. As a result, a single optimal photoswitch system cannot be designed; instead, the challenge lies in identifying the best design for each application. The same principles can also be applied to molecular motors, giving rise to visible-light-powered unidirectional rotary motion on a molecular level. We show that drawing inspiration from red-light-absorbing photoswitches has repercussions not only on the visible-light absorption but also on enhanced rotation dynamics

Photo- and Redox-Driven Artificial Molecular Motors

Directed motion at the nanoscale is a central attribute of life, and chemically driven motor proteins are nature's choice to accomplish it. Motivated and inspired by such bionanodevices, in the past few decades chemists have developed artificial prototypes of molecular motors, namely, multicomponent synthetic species that exhibit directionally controlled, stimuli-induced movements of their parts. In this context, photonic and redox stimuli represent highly appealing modes of activation, particularly from a technological viewpoint. Here we describe the evolution of the field of photo- and redox-driven artificial molecular motors, and we provide a comprehensive review of the work published in the past 5 years. After an analysis of the general principles that govern controlled and directed movement at the molecular scale, we describe the fundamental photochemical and redox processes that can enable its realization. The main classes of light- and redox-driven molecular motors are illustrated, with a particular focus on recent designs, and a thorough description of the functions performed by these kinds of devices according to literature reports is presented. Limitations, challenges, and future perspectives of the field are critically discussed