Photodriven molecular motors are able to convert light energy into directional motion and hold great promise as miniaturized powering units for future nanomachines. In the current state of the art, considerable efforts have still to be made to increase the efficiency of energy transduction and devise systems that allow operation in ambient and non-damaging conditions with high rates of directional motions. The need for ultraviolet light to induce the motion of virtually all available light-driven motors especially hampers the broad applicability of these systems. We describe here a hemithioindigo-based molecular motor, which is powered exclusively by nondestructive visible light (up to 500 nm) and rotates completely directionally with kHz frequency at 20 °C. This is the fastest directional motion of a synthetic system driven by visible light to date permitting materials and biocompatible irradiation conditions to establish similarly high speeds as natural molecular motors.
Hemithioindigo-based molecular motors are powered by nondamaging visible light and provide very fast directional rotations at ambient conditions. Their ground state energy profile has been probed in detail, but the crucial excited state processes are completely unknown so far. In addition, very fast processes in the ground state are also still elusive to date and thus knowledge of the whole operational mechanism remains to a large extent in the dark. In this work we elucidate the complete lightdriven rotation mechanism by a combination of multiscale broadband transient absorption measurements covering a time scale from fs to ms in conjunction with a high level theoretical description of the excited state. In addition to a full description of the excited state dynamics in the various time regimes, we also provide the first experimental evidence for the elusive fourth intermediate ground state of the original HTI motor. The fate of this intermediate also is followed directly proving complete unidirectionality for both 180°rotation steps. At the same time, we uncover the hitherto unknown involvement of an unproductive triplet state pathway, which slightly diminishes the quantum yield of the E to Z photoisomerization. A rate model analysis shows that increasing the speed of motor rotation is most effectively done by increasing the photoisomerization quantum yields instead of barrier reduction for the thermal ratcheting steps. Our findings are of crucial importance for improved future designs of any light-driven molecular motor in general to yield better efficiencies and applicability.
Photoisomerization reactions are quintessential processes driving molecular machines and motors, govern smart materials, catalytic processes, and photopharmacology, and lie at the heart of vision, phototaxis, or vitamin production. Despite this plethora of applications fundamental photoisomerization mechanisms are not well understood at present. The famous hula-twist motion—a coupled single and double-bond rotation—was proposed to explain proficient photoswitching in restricted environments but fast thermal follow-up reactions hamper identification of primary photo products. Herein we describe an asymmetric chromophore possessing four geometrically distinct diastereomeric states that do not interconvert thermally and can be crystallized separately. Employing this molecular setup direct and unequivocal evidence for the hula-twist photoreaction and for photoinduced single-bond rotation is obtained. The influences of the surrounding medium and temperature are quantified and used to favor unusual photoreactions. Based on our findings molecular engineers will be able to implement photo control of complex molecular motions more consciously.
Baumeister FAM, Egger J, Schildhauer MT, Stengel‐Rutkowski S. Ambras syndrome: delineation of a unique hypertrichosis universalis congenita and association with a balanced pericentric inversion (8) (p11.2; q22). Clin Genet 1993: 44: 121–128. © Munksgaard, 1993 Congenital hypertrichosis universalis is a rare autosomal dominant disease. We report the further development of a Greek girl, now aged 3 years, the first case associated with a balanced structural chromosomal aberration. She was described as a neonate by Sigalas et al. (1990). Her persistent generalized hypertrichosis is most excessive on the face, ears and shoulders. Her fine silky hair is of the vellus, not the lanugo type. The syndrome features are characterized, referring to nine further published case reports. It is distinguished from other types of congenital hypertrichoses, which have been described in the literature under different synonyms. To avoid confusion in the terminology, we propose to name this type of hypertrichosis Ambras syndrome in reference to the first documented family with congenital hypertrichosis universalis in the 16th century.
Efficiency and performance of light triggered molecular motors are crucial features that need to be mechanistically understood to improve the performance and enable conscious property tailoringf or specific applications. In this work, three differenth emithioindigo-based molecular motors are investigated and all four steps in their complete unidirectionalr otation are unraveled fully quantitatively. Transient absorption spectroscopy across twelveo rders of magnitude in time is used to probe the fs nuclear motions up to the ms thermal kinetics, covering the timeframe of the whole motor rotation. The newly knownf ull mechanisms allow simulation of the motor systems to scrutinize their performance at realistic illumination conditions. This highlights the importance of photoisomerization quantum yields for the rotationspeed. The substitution pattern in close proximity to the rotation axle influences the excited and ground state properties. Reduction of electron donation and concomitant increaseo fs teric hindrance leads to faster photoisomerization reactions with quasi-ballistic behavior,b ut also to as light decrease in the quantume fficiency. The expected decelerating effects of increased stericsa re primarily manifested in the ground state. Ap romising approach for nextgeneration hemithioindigom otors is to elevate electron donation at the rotor fragment followed by an increaseo f steric hindrance.
The maximum speed of light‐driven molecular motors is an important key‐property governing not only their overall performances but also many advanced functions. Currently, special emphasis lies on increasing the rate of unidirectional rotations to surpass natural systems and harness the full potential of artificial motors. Herein, we report a new molecular setup for a prospective light‐powered three‐step motor based on the hemithioindigo chromophore. Comprehensive quantum chemical treatment predicts a very low energy barrier for the only thermal ratcheting step in the unidirectional 360° rotation. Thus an ultrafast motion in the THz range could be possible with this motor at high light intensities and consequently a precise control of rotation speeds solely by light intensity variations could potentially be achieved. Experimental analyses using X‐ray crystallography and solution spectroscopy deliver first insights into the working mechanism and show that visible‐light photoswitching is feasible in both stable switching states. Additionally, significant alterations of the ground‐state energies can be induced by pH changes without hampering photoswitching capabilities.
The photophysical and photochemical properties of sulfoxide and sulfone derivatives of hemithioindigo photoswitches are scrutinized and compared to the unoxidized parent chromophores. Oxidation results in significantly blue‐shifted absorptions and mostly reduction of photochromism while thermal stabilities of individual isomers remain largely unaltered. Effective photoswitching takes place at shorter wavelengths compared to parent hemithioindigos and high isomeric yields can be obtained reversibly in the respective photostationary states. Reversible solid‐state photoswitching is observed for a twisted sulfone derivative accompanied by visible color changes. These results establish oxidized hemithioindigo photoswitches as promising and versatile tools for robust light‐control of molecular behavior for a wide range of applications.
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