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Photomechanical crystals composed of three-dimensionally ordered and densely packed photochromes hold promise for high-performance photochemical actuators. However, bulk crystals with high structural ordering are severely limited in their flexibility, resulting in poor processibility and a tendency to fragment upon light exposure, while previous nano- or microcrystalline composites have lacked global alignment. Here we demonstrate a photon-fuelled macroscopic actuator consisting of diarylethene microcrystals in a polyethylene terephthalate host matrix. These microcrystals survive large deformations and show a high degree of three-dimensional ordering dictated by the anisotropic polyethylene terephthalate, which critically also has a similar stiffness. Overall, these ordered and compliant composites exhibit rapid response times, sustain a performance of over at least hundreds of cycles and generate work densities exceeding those of single crystals. Our composites represent the state-of-the-art for photochemical actuators and enable properties unattainable by single crystals, such as controllable, reversible and abrupt jumping (photosalient behaviour).Photomechanical crystals are promising materials for converting photon energy into macroscopic work via reversible structural changes when exposed to light. Here the authors demonstrate highly ordered and compliant microcrystalline composites with a photomechanical performance exceeding that of single crystals.

This article provides an overview of the basic aspects of the structure and dynamics of excitons in molecular crystals that give rise to their unique spectroscopic behavior. The two different types of optically accessible excitons, charge-transfer and Frenkel, are described and their different properties discussed. Particular attention is paid to the spin properties of Frenkel excitons (i.e., singlet and triplet) and also to their coupling to intramolecular vibrations. Experimental challenges in the study of molecular crystal optical properties are also reviewed, including their high optical density, complex refractive index behavior, and issues with sample crystallinity and chemical purity. Once created, excitons in molecular crystals can exhibit interesting dynamical behavior, including diffusion over large length scales and ionization into electron–hole pairs. Exciton–exciton interactions are also important, ranging from fusion or annihilation (two excitons combine into one exciton) to fission (one exciton splits into two excitons). The long-range diffusion and exciton fission effects have particular relevance for the design of organic photovoltaic materials.

The fission of singlet excitons into triplet pairs in organic materials holds great technological promise, but the rational application of this phenomenon is hampered by a lack of understanding of its complex photophysics. Here, we use the controlled introduction of vacancies by means of spacer molecules in tetracene and pentacene thin films as a tuning parameter complementing experimental observables to identify the operating principles of different singlet fission pathways. Time-resolved spectroscopic measurements in combination with microscopic modelling enables us to demonstrate distinct scenarios, resulting from different singlet-to-triplet pair energy alignments. For pentacene, where fission is exothermic, coherent mixing between the photoexcited singlet and triplet-pair states is promoted by vibronic resonances, which drives the fission process with little sensitivity to the vacancy concentration. Such vibronic resonances do not occur for endothermic materials such as tetracene, for which we find fission to be fully incoherent; a process that is shown to slow down with increasing vacancy concentration. A complete understanding of singlet fission (SF) in molecular materials will enable the design of optimised optoelectronic devices. Here, the authors use vacancy control in acene-based blends to link coherent and incoherent SF pathways to energetics.

Photomechanical materials exhibit mechanical motion in response to light as an external stimulus. They have attracted much attention because they can convert light energy directly to mechanical energy, and their motions can be controlled without any physical contact. This review paper introduces the photomechanical motions of photoresponsive molecular crystals, especially bending and twisting behaviors, from the viewpoint of symmetry breaking. The bending (right–left symmetry breaking) and twisting (chiral symmetry breaking) of photomechanical crystals are based on both intrinsic and extrinsic factors like molecular orientation in the crystal and illumination conditions. The ability to design and control this symmetry breaking will be vital for generating new science and new technological applications for organic crystalline materials.

9-Methylanthracene (9MA) undergoes a concerted [4 + 4] photodimerization in its crystal form that can be harnessed in order to generate photomechanical motions such as bending, twisting, and expansion. As described in this paper, 9MA nanowires were grown in anodic aluminum oxide (AAO) templates with the goal of using the crystal expansion to generate a net increase in the height of the composite disk. The growth conditions were optimized in order to raise the filling amount from 28% to 77% of the available volume in the porous AAO. A new experimental method for detecting motion, based on the analysis of data from a dynamically misaligned Michelson interferometer, was developed. Template bending was observed, showing that the photodimerization of the confined nanowires generated mechanical work, but no conclusive evidence for surface disruption or vertical translation was observed. Optical measurements, as well as atomic force and scanning electron microscopy, showed that incomplete filling, crystal orientation, and debris from template polishing likely prevented the observation of vertical actuation in these nanocrystal composites. This work highlights some of the practical challenges that are involved in creating photomechanical actuators using the organic–inorganic composite approach, with the two most significant being (1) the uniform filling of the porous template with the organic active material and (2) the removal of excess organic material from the template’s surface.

Nanosecond laser impulsive heating is used to generate a liquid→gas phase transition that performs mechanical work. A simple laser flyer apparatus is designed where a metal plate is placed on top of a variable volume of liquid. When a high-energy nanosecond laser pulse impacts the bottom of the plate, explosive vaporization of the liquid propels it upward. The initial velocity of the plate, and thus its kinetic energy, is measured as a function of several parameters, including liquid volume, laser wavelength, pulse energy, and chemical composition of the liquid. Light-to-kinetic energy conversion efficiencies that approach 2% are measured. Although surface discoloration of the metal is observed, there was no measurable decrease in the initial velocity even after 200 cycles. For protic liquids like water and ethylene glycol, the initial velocity is independent of the chemical composition of the liquid, possibly due to the contribution of exothermic oxidative reactions at high temperatures. For alkane liquids, the initial velocity decreases with increasing molecular weight, consistent with a simple model that relates it to physical properties like the heat of vaporization. The results suggest that nanosecond pulsed laser heating for liquid→gas actuation may be a relatively efficient way to transform photons into mechanical energy.

Semiconducting quantum dots have been used to harvest triplet excitons produced through singlet fission in organic semiconductors. These hybrid organic-inorganic materials may boost the efficiency of solar cells.

The production of hydrogen from water with semiconductor photocatalysts can be promoted by adding small amounts of metals to their surfaces. The resulting enhancement in photocatalytic activity is commonly attributed to a fast transfer of the excited electrons generated by photon absorption from the semiconductor to the metal, a step that prevents deexcitation back to the ground electronic state. Here we provide experimental evidence that suggests an alternative pathway that does not involve electron transfer to the metal but requires it to act as a catalyst for the recombination of the hydrogen atoms made via the reduction of protons on the surface of the semiconductor instead.

Through diversity of composition, sequence, and interfacial structure, hybrid materials greatly expand the palette of materials available to access novel functionality. The NSF Division of Materials Research recently supported a workshop (October 17–18, 2019) aiming to (1) identify fundamental questions and potential solutions common to multiple disciplines within the hybrid materials community; (2) initiate interfield collaborations between hybrid materials researchers; and (3) raise awareness in the wider community about experimental toolsets, simulation capabilities, and shared facilities that can accelerate this research. This article reports on the outcomes of the workshop as a basis for cross-community discussion. The interdisciplinary challenges and opportunities are presented, and followed with a discussion of current areas of progress in subdisciplines including hybrid synthesis, functional surfaces, and functional interfaces.

Recent advances in the use of nonlinear optical microscopy (NLOM) in skin microscopy are presented. Nonresonant spectroscopies including second harmonic generation, coherent anti‐Stokes Raman and two‐photon absorption are described and applications to problems in skin biology are detailed. These nonlinear techniques have several advantages over traditional microscopy methods that rely on one‐photon excitation: intrinsic 3D imaging with <1 μm spatial resolution, decreased photodamage to tissue samples and penetration depths up to 1000 μm with the use of near‐infrared lasers. Thanks to these advantages, nonlinear optical spectroscopy has become a powerful tool to study the physical and biochemical properties of the skin. Structural information can be obtained using the response of endogenous chemical species in the skin, such as collagen or lipids, indicating that optical biopsy may replace current invasive, time‐consuming traditional histology methods. Insertion of specific probe molecules into the skin provides the opportunity to monitor specific biochemical processes such as skin transport, molecular penetration, barrier homeostasis and ultraviolet radiation‐induced reactive oxygen species generation. While the field is quite new, it seems likely that the use of NLOM to probe structure and biochemistry of live skin samples will only continue to grow.