Solid-state photochemistry

This themed issue of CrystEngComm on Solid-State Photochemistry will focus on photochemical reactions, as well as on the photophysical structure and properties of solid-state materials, ranging from the crystalline state to thin-film nano-aggregates.

In order to enclose this large field, this themed issue has been arranged into the areas of solid-state photochromic materials, photomechanical materials, photo-induced reactions of organic or organometallic molecules, and optical and optoelectronic materials and processes.

Photochromic materials

Photochromic materials have become key elements in sensors, memory devices, and bioimaging applications. The optical properties of these materials are governed by the changing nature of the molecular structure upon photochemical stimulation. The structural origins of this solid-state photochemistry range from crystal polymorphism, to phase changes or perturbations in intermolecular interactions, as several articles in this themed issue highlight.

Mitoraj (Jagiellonian University) and Garcia (Université catholique de Louvain) present a complementary experimental and theoretical study on a crystalline anil dye, whose optical properties are characterised by polymorphism. Their examination of two polymorphs of N,N-bis(3-methoxysalicylidene)-1,5-diiminonaphthalene shows that both polymorphs are thermochromic, while only one of them is susceptible to photochromism upon irradiation with UV light (DOI: 10.1039/c6ce00266h).

Garcia et al. (Université catholique de Louvain) also explored the prospect of N-salicylidene-4-amino-1,2,4-triazoles as molecular switches. Their study includes the determination of crystal structures, with associated Hirshfeld surface analyses, and the evaluation of optical properties for a series of such compounds. Thermochromism is discovered in one of the subject materials (DOI: 10.1039/c6ce00749j).

Ito et al. (Hokkaido University) report an unprecedented example of a mechanically-induced solid-state phase change in a gold(I) isocyanide complex, from a blue- to yellow-emitting powder. This photochromic effect appears to originate from changes in Au⋯Au intermolecular interactions caused by the mechanical stimulation (DOI: 10.1039/c5ce02565f).

The solid-state luminescence of organic molecules is one of the most important aspects of photochromism, both from a fundamentally scientific and from a technological perspective. This critical importance is attributed to the fact that luminescence relates to the chemical structure of the constituent molecules and to their orientation with respect to each other. The molecular packing in the crystalline state may lead to excimers with unusual forms of emission.

Matsuda et al. (Kyoto University) contribute to the discussion in this area by presenting a study of the fluorescence behaviour of 2,6,10-trisubstituted 4,8,12-triazatriangulene cations (TATA⁺) in solution and in the solid state. The strong fluorescence quenching that was observed in the solid state upon direct functionalisation of the TATA core was ascribed to an increased level of intermolecular interactions (DOI: 10.1039/c6ce00703a).

Morimoto and Irie (Rikkyo University) report on reversible photo-switching of the fluorescence of diarylethene single crystals that contain benzo[b]thiophene S,S-dioxide groups upon alternate irradiation with UV and visible light. Moreover, the switching can occur in selective regions of the crystal, which shows potential for these photo-switchable fluorescent molecular crystals in optical memory and display device applications (DOI: 10.1039/c6ce00725b).

The group of Uchida (Ryukoku University) also studied a family of diarylethene derivatives, placing their focus on photo-induced topographical changes to the microcrystalline surfaces of four diarylethene derivatives. Exposure of the diarylethene derivatives to UV irradiation resulted in the formation of sample surfaces bearing microcrystals with cubic, needle-like, or plate-like habits. Coupling epitaxial crystal growth methods with such high levels of control over the surface morphology could ultimately benefit the fabrication of photonic crystals (DOI: 10.1039/c6ce00718j).

The examples that we have just seen on materials that exhibit fluorescence-based photo-switching are potentially useful for optoelectronic applications requiring a fast photo-response mechanism. Materials that display slow, or even metastable, light-induced switching via a non-fluorescent photo-response mechanism are of interest in other areas of optoelectronics. Photo-response mechanisms on this timescale are typically found in light-induced solid-state linkage isomerisation reactions in metal–organic coordination complexes, as exemplified by the following two studies. In addition, a photochromic change, which occurs over a period of seconds, is demonstrated for a novel metal–organic coordination polymer.

Schaniel et al. (Université de Lorraine) examined the photo-induced linkage isomerism of the NO ligands in the [Ru(NO)₂(PCy₃)₂Cl][BF₄] complex, using photocrystallography and IR-spectroscopy (DOI: 10.1039/c6ce00735j).

Sekine and co-workers (Tokyo Institute of Technology) report the presence of a twofold photo-isomerisation in cobaloxime complexes with salicylidene-3-aminopyridine and 2-cyanoethyl groups, and show that this can be used to control the photochromic behaviour of these compounds (DOI: 10.1039/c6ce01005a).

The group of Fu (South China University of Technology, Fuzhou University) report the results of a study concerning a host–guest coordination polymer, based on a phenyl-substituted 4,4′-bipyridyl derivative that is embedded in a layered zinc–carboxylate framework. The densely-packed stacking structure observed was associated with a photochromic change (DOI: 10.1039/c6ce00727a).

Photomechanical materials

Photomechanical crystals convert light into mechanical work, and this process is as interesting as it is challenging. Photomechanical materials usually contain ordered photo-responsive molecules, and mechanical motion is induced by molecular strain that arises from a photo-reaction. Large photomechanical displacements are especially attractive for applications in light-controlled actuators and devices. A strong advantage of such photomechanical crystals is that they can be affected by an external stimulus without making any contact with the crystal; this paves the way for optically-based wireless applications.

The photomechanical motion of crystalline 1,2-bis(2-methyl-5-(4-(p-toluyloxymethyl)phenyl)-3-thienyl)perfluorocyclopentene is reported by the Kobatake group (Osaka City University). Most remarkably, these crystals exhibit a stepwise bending behaviour upon the application of UV light, as well as delayed bending after exposure to light irradiation (DOI: 10.1039/c6ce00607h).

Norikane et al. (National Institute of Advanced Industrial Science and Technology – AIST) show how irradiation with UV light can induce crystals of 4-methoxyazobenzene into executing a swimming motion on the surface of water, and how the direction of the movement can be controlled by the orientation of the light source (DOI: 10.1039/c6ce00738d).

Bardeen and Mueller (UC Riverside) discuss the photochemically induced crystal-to-crystal reaction of 9-tert-butyl anthracene ester. The photo-dimerisation of this ester can cause nanorod expansions by up to 15% as a result of the formation of a metastable crystalline intermediate (solid-state reacted dimer). The authors reveal the structure of the solid-state reacted dimer, which was assigned on the basis of a combination of powder X-ray diffraction analysis, solid-state nuclear magnetic resonance, and computational modelling (DOI: 10.1039/c6ce00742b).

Sidelnikov and colleagues (Institute of Solid State Chemistry and Mechanochemistry SB RAS) examine the effects of anisotropic structural strain arising from thermal expansion as a result of photo-isomerisation in crystalline [Co(NH₃)₅NO₂]Cl(NO₃). The authors show that this anisotropic structural strain is the result of temperature variations during the photo-isomerisation, and that its nature differs from the effect of strain that is generated by elastic external loading (DOI: 10.1039/c6ce00840b).

Koshima et al. (Waseda University) report that the exposure of a narrow plate-like microcrystal of an anthracene- and indanone-substituted stilbene derivative to UV light results in a reversible gradual bending of the crystal away from the light source. An ¹H NMR analysis revealed that the predominant contribution to the bending arises from the intermolecular [4 + 4] photo-dimerisation of the two anthracene planes (DOI: 10.1039/c6ce00848h).

Friščić and Barrett (McGill University) review the photo-induced motion of azo dyes in organised media (DOI: 10.1039/c6ce01128d). Their Highlight article is concerned with photomechanical azobenzene crystals, and focuses on new types of photomechanical behaviour, crystal engineering routes to a variety of crystalline photomechanical materials, in situ and real-time monitoring (based on X-ray diffraction) of structural changes arising from photomechanical effects, and new supramolecular interactions that permit the generation of photomechanical azobenzene crystals with controlled molecular stacking and crystal morphology.

Photo-induced reactions of organic or organometallic molecules

There is a historical precedent for solid-state photo-reactions. However, reports of them have become much more abundant over recent years, owing to various materials characterisation developments that are enabling the quantification of their molecular structures. Photo-dimerisation is a case in point, and the two articles mentioned below illustrate how the crystal structures of photo-reaction products can be realised by single-crystal and powder diffraction methods, with supporting spectroscopy measurements. These photo-reactions are either solution-assisted or occur while the sample maintains its solid-state form; the reactions may be partial or proceed to completion, and may be reversible or irreversible. The underlying mechanisms behind the photo-reactions are also evidenced, presenting examples of cooperative effects and order–disorder transitions.

The Zhang group (Chinese Academy of Sciences) have quantitatively analysed the solid-state [2 + 2] photo-cycloaddition reaction of a photoactive olefin-containing pyridinium-based Cd(II) complex using ¹H NMR, powder X-ray diffraction, IR, and Raman spectroscopy; the isomerisation of its cyclobutane product upon recrystallisation is also described (DOI: 10.1039/c6ce00724d). This demonstrates that such a reaction may lead to novel and more efficient synthetic pathways that yield new cyclobutane derivatives which are otherwise difficult to access.

Fernandes and Levendis (University of the Witwatersrand) show that the photo-dimerisation of the α′-polymorph of o-ethoxy-trans-cinnamic acid proceeds in two stages at 343 K and yields α-truxillic acid (DOI: 10.1039/c6ce00809g). Single-crystal and powder X-ray diffraction are used to quantify these solid-state transitions.

It is also becoming possible to study ultrafast photo-reactions in the solid state using the latest advances in femtosecond pump-probe technology. Two articles in this themed issue demonstrate how this technology has been applied to capture ephemeral photophysical and photochemical responses.

Collet (Université Rennes) and co-workers employ femtosecond laser-pulse excitation to probe the ultra-fast photophysical response in single crystals of the spin-crossover complex [Fe(phen)₂(NCS)₂]. The authors reveal a cooperative transformation, which is induced by lattice expansion in the crystal via elastic coupling. This process is found to transform up to three molecules per photon (DOI: 10.1039/c6ce00659k).

Miller et al. (Max Planck Institute for the Structure and Dynamics of Matter; University of Hamburg; University of Toronto) describe a method based on transient absorption spectroscopy to probe the ultrafast dynamics of a ring-opening reaction in crystalline spironaphthooxazine. Such ultrafast studies of solid-state chemical reactions are usually hampered by product accumulation. However, the authors present an approach that circumvents this obstacle by using spectrally selected, post-excitation, ultrashort laser pulses that minimise the accretion of photoproducts (DOI: 10.1039/c6ce01049k).

Optical and optoelectronic materials and processes

Optical industries have long sought new organic-based materials to replace their traditional inorganic counterparts. Aside from growing environmental regulations that favour organic-based materials, such compounds possess key material attributes that are superior to those of inorganic materials. For example, organic materials can exhibit a much faster non-linear optical (NLO) response than inorganic media. This is because the NLO phenomenon is based upon electronic charge transfer in organic chromophores, while NLO processes in inorganic compounds are controlled by the much slower process of ionic displacement. Moreover, organic materials possess much greater diversity and versatility in molecular design, given their myriad synthetic possibilities. This has led the way for establishing new methods and protocols to systematically design and predict materials with tailor-made optical properties. Such product design at the molecular scale is featured in three articles.

Jazbinsek (Zurich University of Applied Sciences) and Kwon (Ajou University) apply a new crystal engineering approach to organic electro-optical materials. The approach is demonstrated for two model compounds: an ionic quinolinium chromophore and a non-ionic polyene. The authors show how to generate thickness-controlled large-area organic electro-optic crystals of both materials, by confining their growth projection with a bounded environment. THz spectroscopy is used to compare the extent to which the resulting crystals, relative to those grown by conventional crystallisation methods, have optimal characteristics for THz photonic applications (DOI: 10.1039/c6ce00958a).

Jazbinsek (Zurich University of Applied Sciences) and Kwon (Ajou University) also review the latest developments in organic NLO materials that are designed with a core of cognate acentric molecular fragments. The discussion highlights that use of these core fragments in the materials design process leads to NLO molecules which align in a common fashion and exhibit similar types of intermolecular interactions. Given that NLO properties are governed by molecular alignment and supramolecular interactions in organic materials, and that these cores are based on parent NLO chromophores that exhibit high second-order NLO effects and desirable THz properties, it follows that the materials discovery workflow which uses these core acentric structures is successful. The underpinning goal behind these material developments is to find a way to supersede the one compound that has dominated the organic NLO market for several decades: a stilbazolium-based ionic salt, known as DAST, 4-(4-(N,N-dimethylamino)styryl)-1-methylpyridinium 4-methylbenzenesulfonate (DOI: 10.1039/c6ce00707d).

Macchi (University of Bern) and co-workers propose a new computational method to predict dielectric constants and optical indicatrices for metal–organic networks. Atomic polarizability coefficients are determined for key structural components of these networks, known as secondary-building units, from which dielectric constants and optical indicatrices are derived using the assumption that the oriented gas model holds. The performance of this new method is tested against that of traditional first-principles calculations which employ periodic boundary conditions. In relative terms, this new method provides speedy calculations for materials discovery efforts that target metal–organic coordination complexes with useful optical properties. To this end, the authors consider optical transparency as an important property attribute, and conclude that zinc-based metal–organic coordination polymers hold good prospects. In broader terms, the authors forecast that its simplicity and computational speed will allow this method to become a useful screening tool for the discovery of new solid-state optical media (DOI: 10.1039/c6ce00918b).

Photochemistry also lies at the heart of the photovoltaics industry, given the central role of optical absorption and emission processes in the function of solar-cell devices, including recent conjectures about single fission optical phenomena. Quantum-chemical calculations that are able to relate the associated excitonic processes to the material structures that control these devices are desperately needed. In this regard, two articles, summarised below, present computational approaches that offer important insights into light-harvesting materials that are used in organic photovoltaic (OPV) technology. On the one hand, these materials take the form of nano-aggregates within a thin-film medium. On the other hand, the single-crystal state is studied as a model system to understand the possible options by which molecules may pack together in a material to yield the most desirable attributes for photovoltaic performance.

Wang (Zhejiang University) and Beljonne (University of Mons) present a theoretical study on the light-harvesting material poly(3-hexylthiophene), known as P3HT. This is a conjugated polymer that acts as an archetypal donor constituent of an OPV device. The authors show how to model the conformational variation, positional disorder, and excitonic properties exhibited by nano-aggregates of this conjugated polymer via the concerted use of molecular dynamics simulations and quantum-chemical calculations, and adoption of Frenkel–Holstein theory. The results show that these nano-aggregates are ordered on the local scale but reside within larger material domains that feature disordered polymer chains. Furthermore, the model reveals how these ordered and disordered regions of P3HT partition the nature of the optical absorption and emission properties in this polymer (DOI: 10.1039/c6ce00645k).

Marom et al. (Carnegie Mellon University) discuss the effect of crystal packing on the excitonic properties of three known polymorphs of rubrene. This is a material that exhibits singlet fission in its single-crystal form, making it very attractive to the OPV community, owing to the potential of singlet fission in generating significant improvements in OPV device efficiency. The electronic structures and optical properties of each polymorph are determined using dispersion-inclusive density functional theory and many-body perturbation theory. The calculations predict that the monoclinic polymorph of rubrene stands to exhibit the highest singlet fission efficiency, to the extent that its efficiency may even challenge that of pentacene, the archetypal material for singlet-fission (DOI: 10.1039/c6ce00873a).

In summary, the collection of papers in this themed issue of CrystEngComm showcases the state-of-the-art in solid-state photochemistry, ranging from fundamental studies that show how crystals can bend upon the application of light, to helping solve the latest applied challenges which currently face the photovoltaics industry. The intertwining of experimental and computational research articles is deliberate since it demonstrates a growing trend in understanding material properties from both perspectives. This is particularly true in the field of optics, in which articles presented herein employ the latest computational tools to tackle problems involving excitonic properties that manifest in solid-state forms (e.g. nano-aggregates, buried interfaces) which cannot yet be characterised experimentally. Indeed, computational tools are becoming sufficiently powerful that they can now be employed in the prediction of material properties, as a paper herein illustrates for functionalising metal–organic networks. More generally, materials discovery for solid-state photochemistry is represented strongly in this themed issue, and features in various solid-state forms: from single crystals, to thin films to powders. The revelations in light-driven physical manipulations of single crystals demonstrated by a number of articles are particularly exciting, since this challenges the traditional concept of a single crystal being a rigid state of solid-state matter. Such articles fascinate the reader by showing that single crystals can not only change colour, but they can also change shape. While single-crystal photochromism has been widely known for some time, developments in materials characterisation techniques have now reached a point that the detailed understanding of compounds exhibiting photochromic phenomena is possible, thereby leading it to the forefront of research. A whole new area of solid-state photo-reaction chemistry is unfolding in tandem. Similarly, solid-state photo-reactions are not a new concept per se, but this themed issue exemplifies how a new generation of materials characterisation techniques is being employed to understand and thus promote them. Such techniques range from traditional diffraction and spectroscopy methods, which have been adapted to enable in situ experiments on light-activated compounds, to the latest advances in ultrafast femtosecond pump-probe spectroscopy, which has been used in two articles to capture ephemeral light-induced states of matter.

In short, this themed issue reveals that solid-state photochemistry has surpassed the bygone age when photochemistry was carried out primarily in the solution state; it also highlights an emerging area of single-crystal technologies based on optical actuators or THz modulators, and brand new areas of chemistry, some of which can only be accessed by photochemically-stimulated states of matter.

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