9-Anthracenecarboxylic acid, a molecule that undergoes a reversible [4 + 4] photodimerization, is prepared in the form of oriented crystalline microribbons. When exposed to spatially uniform light irradiation, these photoreactive ribbons rapidly twist. After the light is turned off, they relax back to their original shape over the course of minutes. This photoinduced motion can be repeated for multiple cycles. The final twist period and cross-sectional dimensions of individual microribbons are measured using a combination of atomic force and optical microscopies. Analysis of this data suggests that the reversible twisting involves the generation of interfacial strain within the ribbons between unreacted monomer and photoreacted dimer regions, with an interaction energy on the order of 3.4 kJ/mol. The demonstration of reversible twisting without the need for specialized irradiation conditions represents a new type of photoinduced motion in molecular crystals and may provide new modes of operation for photomechanical actuators.

Nanorods composed of 9-tert-butylanthroate (9-TBAE) are synthesized using an Al2O3 template and solvent annealing. The rods consist of micron-scale crystalline domains, and UV light induces a [4 + 4] photodimerization that results in a uniform 15% expansion along the rod axis. This is in contrast to random 9-TBAE crystals, which disintegrate under the same conditions. Transmission electron microscopy, atomic force microscopy, and comparison of the X-ray crystal structures of the monomer and photodimer all provide evidence for a mechanism based on a crystal-to-crystal photoreaction leading to an increase in molecular volume. It is likely that the high surface-to-volume ratio in the nanorods provides a strain relief pathway that is absent in larger crystals. Preliminary attempts to reverse the reaction using shorter wavelength light to photodissociate the dimers were only partly successful. These results suggest that crystalline organic nanostructures may provide an efficient way to transform photochemical energy into mechanical motion on the nanometer scale.

Organic molecules can transform photons into Angstrom-scale motions by undergoing photochemical reactions. Ordered media, for example, liquid crystals or molecular crystals, can align these molecular-scale motions to produce motion on much larger (micron to millimeter) length scales. In this Review, we describe the basic principles that underlie organic photomechanical materials, starting with a brief survey of molecular photochromic systems that have been used as elements of photomechanical materials. We then describe various options for incorporating these active elements into a solid-state material, including dispersal in a polymer matrix, covalent attachment to a polymer chain, or self-assembly into molecular crystals. Particular emphasis is placed on ordered media, such as liquid-crystal elastomers and molecular crystals, that have been shown to produce motion on large (micron to millimeter) length scales. We also discuss other mechanisms for generating photomechanical motion that do not involve photochemical reactions, such as photothermal expansion and photoinduced charge transfer. Finally, we identify areas for future research, ranging from the study of basic phenomena in solid-state photochemistry, to molecular and host matrix design, and the optimization of photoexcitation conditions. The ultimate realization of photon-fueled micromachines will likely involve advances spanning the disciplines of chemistry, physics and engineering.

Mechanical devices that function on sub-micrometer length scales are expected to find applications in fields ranging from medicine to manufacturing. Biology provides many examples of nanoscale machines, for example, the adenosine triphosphate (ATP)-fueled motion of myosin along the actin filament. [1] In such systems, the motion is fueled by random encounters with high-energy chemical species in the surrounding medium and occurs stochastically, because there is no way to externally control diffusive encounters at the molecular level. An alternative is to use external energy sources (e.g., photons) to activate motion at the nanoscale. Photochemically powered mechanical motion is attractive because it does not require the presence of a second chemical species and external control of the motion can be achieved by manipulating the illumination conditions. These advantages have propelled research into photoactivated mechanical motion on the molecular level, and there have been many examples of photoactivated molecular switching, translation, and geometrical transformations. [2][3][4] In general, these schemes rely on intramolecular events-bond formation, cyclization, cis-trans isomerization-to drive a structural change on the order of 1 Å or less. This molecular-level change can be amplified by coupling multiple molecules together using a liquid-crystal polymeric host. Recent progress in the development of photodeformable polymer fibers and strips has led to micrometer-scale materials that exhibit light-induced shape memory effects, [5,6] reversible bending under anisotropic illumination conditions, [7][8][9][10][11][12][13] and photocontrol of the crosslink density. [14] Recently, we demonstrated the use of an intermolecular photochemical reaction to achieve large physical displacements in a different type of structure: organic molecular crystal nanorods.[15] By taking advantage of the well-known anthracene [4+4] photocycloaddition reaction in 9-tertbutyl-anthracene ester (9-TBAE) molecular crystal nanorods, a large (15 %) change in rod length could be generated. In most cases, the photochemical changes drive a reconstructive crystal-to-crystal phase transition that leads to crystal fragmentation and disintegration. [16][17][18] Apparently, the high surface-to-volume ratio of the nanorods provides sufficient strain relief to avoid cracking and fragmentation during this transition. Although the expansion of the 9-TBAE rods was ca. 100 times larger than that observed in other photoisomerizable molecular crystals, [19] it was largely irreversible. If a reversible system could be found, significant amounts of useful work could be generated by repeatedly inducing such mechanical motion. In the following, we demonstrate that molecular crystal nanorods composed of a related molecule, 9-anthracene carboxylic acid (9-AC), can undergo reversible photoinduced cycling between well-defined shapes after spatially localized excitation. The kinetics of recovery, its dependence on illumination conditions, and the long-term stability ...

Molecular crystal nanowires composed of an anthracene‐9‐(1,3‐butadiene) derivative exhibit a rapid transition from straight to highly coiled structures when exposed to a pulse of visible light. The curling does not depend on the direction of light illumination and occurs for nanowires composed of either the E or Z isomer. The shape change is driven by an E⇄Z photoisomerization reaction that generates a mixture of isomers within a single nanowire.

A new method is presented for analyzing the effects of self-absorption on photoluminescence integrating sphere quantum yield measurements. Both the observed quantum yield and luminescence spectrum are used to determine the self-absorption probability, taking into account both the initial emission and subsequent absorption and reemission processes. The analysis is experimentally validated using the model system of the laser dye perylene red dispersed in a polymer film. This approach represents an improvement over previous methods that tend to overestimate the true quantum yield, especially in cases with high sample absorbance or quantum yield values.

The spectroscopy of solid anthracene is examined both experimentally and theoretically. To avoid experimental complications such as self-absorption and polariton effects, ultrathin polycrystalline films deposited on transparent substrates are studied. To separate the contributions from different emitting species, the emission is resolved in both time and wavelength. The spectroscopic data are interpreted in terms of a three-state kinetic model, where two excited states, a high energy state 1 and a low energy state 2, both contribute to the luminescence and are kinetically coupled. Using this model, we analyze the spectral lineshape, relative quantum yield, and relaxation rates as a function of temperature. For state 1, we find that the ratio of the 0-0 vibronic peak to the 0-1 peak is enhanced by roughly a factor of 3.5 at low temperature, while the quantum yield and decay rates also increase by a similar factor. These observations are explained using a theoretical model previously developed for herringbone polyacene crystals. The early-time emission lineshape is consistent with that expected for a linear aggregate corresponding to an edge-dislocation defect. The results of experiment and theory are quantitatively compared at different temperatures in order to estimate that the singlet exciton in our polycrystalline films is delocalized over about ten molecules. Within these domains, the exciton's coherence length steadily increases as the temperature drops, until it reaches the limits of the domain, whereupon it saturates and remains constant as the temperature is lowered further. While the theoretical modeling correctly reproduces the temperature dependence of the fluorescence spectral lineshape, the decay of the singlet exciton appears to be determined by a trapping process that becomes more rapid as the temperature is lowered. This more rapid decay is consistent with accelerated trapping due to increased delocalization of the exciton at lower temperatures. These observations suggest that exciton coherence can play an important role in both radiative and nonradiative decay channels in these materials. Our results show that the spectroscopy of polyacene solids can be analyzed in a self-consistent fashion to obtain information about electronic delocalization and domain sizes.

Molecular crystals composed of 9-anthracene carboxylic acid (9AC) can undergo reversible light-induced mechanical motions driven by a [4 + 4] photodimerization reaction. This paper explores the structure, photophysics, and photomechanical response of a family of anthracene carboxylic acid derivatives, with the goal of finding materials that have comparable or improved photomechanical properties. We find that methyl or phenyl substitution at the 10-position leads to a complete loss of photoreactivity due to changes in crystal packing. A series of halogen (F, Cl, Br) 10-substituted 9AC molecules all showed a similar stacked packing motif, but only the fluoro-substituted molecule was photoreactive in the solid. Its photomechanical response was similar to that of 9AC but with a much longer recovery time. Extending the carboxylic acid by adding a vinylene group at the 9-position resulted in crystals that showed good photoreactivity and a lack of fracture but no reversibility. Attempts to self-consistently rationalize observed trends in terms of excited state lifetimes or steric effects were only partly successful. Balancing factors such as electronic relaxation, steric interactions, and crystal packing present a challenge for engineering photoactive solid-state materials based on molecular crystals.