Structure of the photocolored 2-(2',4'-dinitrobenzyl)pyridine crystal: two-photon induced solid-state proton transfer with minor structural perturbation

Unusual motion of the n-methoxypropyl moiety observed in the photochromic crystals of an organorhodium dithionite complex with n-methoxypropyltetramethylcyclopentadienyl ligands

The crystalline-state photochromism of a new organorhodium dinuclear complex having n-methoxypropyltetramethylcyclopentadienyl (η⁵-C₅Me₄n-C₃H₆OCH₃) and photoresponsive dithionite (μ-O₂SSO₂) ligands was investigated directly by performing single-crystal X-ray diffraction experiments; a reversible conformational change of the n-methoxypropyl moiety was intriguingly observed during the course of a thermal back-reaction.

Reversible Multicolor Photochromism of Dihydroazulene Crystals

The photochemical conversion of 1,8a-dihydroazulene-1,1-dicarbonitrile (DHA) to vinylheptafulvene (VHF) is a positive T-type photoswitch that is well understood in solution, but has not been explored in the solid state. Upon excitation with UV light, DHA is converted into VHF in the solid state, with a distinct color change from yellow to deep-red, and retention of crystallinity. The structure of the ring-opened product was assigned to syn-VHF using variable-temperature infrared spectroscopy, and determined by X-ray photodiffraction in a crystal enriched with the product by two-photon excitation. A radical pathway becomes an observable photoreaction channel at low temperatures, and includes a strongly colored, short-lived diradical intermediate.

Perspective: On the relevance of slower-than-femtosecond time scales in chemical structural-dynamics studies

A number of examples illustrate structural-dynamics studies of picosecond and slower photo-induced processes. They include molecular rearrangements and excitations. The information that can be obtained from such studies is discussed. The results are complementary to the information obtained from femtosecond studies. The point is made that all pertinent time scales should be covered to obtain comprehensive insight in dynamic processes of chemical and biological importance.

Experimental and theoretical study of enol–keto prototropic tautomerism and photophysics of azomethine–BODIPY dyads

Enol ↔ keto prototropic tautomerism can be exploited to modulate the photophysics of BODIPY chromophores based on proton-coupled photoinduced electron transfer processes.

Chemical x-ray photodiffraction: principles, examples, and perspectives

X-ray photodiffraction (in the chemical literature also referred to as photocrystallography), which is based on the combination of X-ray diffraction methods with samples excited by UV or visible light to solve fundamental photochemical or photophysical issues, has developed in the last couple of decades into a very promising technique for direct observation of photoinduced chemical species in the solid state. The capability of providing direct information on very small perturbations in atomic positions and thus on the minute changes in molecular geometry during (or as a consequence of) photoexcitation appears to be the most important asset of this emerging analytical technique. When combined with other physicochemical methods, X-ray photodiffraction can be a powerful tool for analysis of steady-state photoinduced structures as well as slow or very fast time-dependent phenomena. Despite being a very useful approach, however, due to a number of practical requirements that it places with regard to the system to be studied, at the present stage of developments the technique is not widely and indiscriminately applicable to any photoinduced process. In some particular chemical systems the inherent pitfalls could be practically overcome by practical or theoretical means. In this short chapter, the basic principles of X-ray photodiffraction are briefly summarized, and the prospects of its application to "physical" and "chemical" problems is illustrated with selected examples from recent literature. Some possible future developments and alternative approaches with this and related methods are also presented.

2-(2,4-Dinitrobenzyl)pyridinium 2-hydroxy-3,5-dinitrobenzoate

In the structure of the title salt, C₁₂H₁₀N₃O₄⁺·C₇H₃N₂O₇⁻, the cations and the anions are linked by a single N⁺—H⋯O_carbox­yl hydrogen bond, the discrete cation–anion unit having no inter­molecular associations other than weak cation–anion aromatic ring π–π inter­actions [ring centroid separation = 3.7320 (14) Å] and a number of weak inter-unit aromatic C—H⋯O contacts. An intramolecular C—H⋯O hydrox­yl–carboxyl hydrogen bond occurs in the anion.

2,2′-[(1E,1′E)-2,2′-(2,5-Dibutoxy-1,4-phenylene)bis(ethene-2,1-diyl)]dipyridine

The centrosymmetric title mol­ecule, C₂₈H₃₂N₂O₂, has a central benzene ring subsituted in the 1- and 4-positions by (ethene-2,1-di­yl)pyridine groups, and in the 2- and 5-positions by but­oxy groups. The whole mol­ecule is X-shaped and relatively flat, the dihedral angle between the pyridine and the central benzene ring being 11.29 (10)°. In the crystal, neighboring mol­ecules are linked by weak C—H⋯N inter­actions, forming a two-dimensional undulating network.

Bis[2-(2,4-dinitro-benz-yl)pyridinium] biphenyl-4,4'-disulfonate trihydrate

In the structure of the title salt, 2C₁₂H₁₀N₃O₄⁺·C₁₂H₈O₆S₂²⁻·3H₂O, determined at 173 K, the biphenyl-4,4′-disulfonate dianions lie across crystallographic inversion centres with the sulfonate groups inter­acting head-to-head through centrosymmetric cyclic bis­(water)-bridged hydrogen-bonding associations [graph set R₄⁴(11)], forming chains. The 2-(2,4-dinitro­benz­yl)pyridinium cations are linked to these chains through pyridinium–water N—H⋯O hydrogen bonds and a two-dimensional network is formed through water bridges between sulfonate and 2-nitro O atoms, while the structure also has weak cation–anion π–π aromatic ring inter­actions [minimum ring centroid separation = 3.8441 (13) Å].

4-(4-Nitro-benz-yl)pyridinium 5-nitro-salicylate

In the title salt, C(12)H(11)N(2)O(2) (+)·C(7)H(4)NO(5) (-), the cations and anions inter-act through asymmetric cyclic pyridinium-carboxyl-ate N-H⋯O,O' hydrogen-bonding associations [graph set R(1) (2)(4)], giving discrete heterodimers having weak cation-anion π-π aromatic ring inter-actions [minimum ring centroid separation = 3.7116 (9) Å].

Kinetics of the single-crystal to single-crystal two-photon photodimerization of alpha-trans-cinnamic acid to alpha-truxillic acid

The quadratic dependence of the rate of photodimerization of alpha-trans-cinnamic acid induced by 532 nm pulsed laser light confirms a two-photon mechanism in the single crystals. Single crystals in well-defined orientations and circularly polarized light were used in the experiments. The reaction rate deviates from first-order kinetics, but fits the JMAK expression with a coefficient indicating a mechanism intermediate between a random distribution of product molecules in the crystal and the existence of growing nuclei. The reaction is accompanied by a large change in the monoclinic beta-angle of the unit cell, corresponding to increased pi-overlap between adjacent molecules and therefore to an increased reaction rate. The improved penetration of the light in the crystal and more homogeneous product formation are significant advantages in studies of mechanisms of chemical reactions in single crystals.

Direct Observation of Aminyl Radical Intermediate during Single-Crystal to Single-Crystal Photoinduced Orton Rearrangement

A photoinduced analogue of the thermal Orton rearrangement reaction by which an N-chlorine atom from a side amino group is transferred to a phenyl ring was studied in the solid state. Contrary to the mixture of products obtained in solution, in the N-chloro-N-acetylaminobenzene crystals the photoreaction proceeds with complete preservation of crystallinity, affording selectively and quantitatively the para isomer of chloroacetanilide. Study of the reaction mechanism by in situ steady-state photodiffraction, a combination of photoexcitation by UV light and single-crystal X-ray diffraction analysis, provided evidence for creation of N-acetyl-N-phenylaminyl (AcPhN*) radical as a metastable reaction intermediate. The structure of the aminyl radical produced in 9.2% yield from the major disordered component in the statically 85.6:14.4 disordered crystal was directly observed for the first time. The unprecedented stability of the radical is prescribed to the solid-state cage effect, the reactive center of the radical species being locked away from the reactive target molecules. The creation of the radical and its head-to-tail chain reaction within the undulated hydrogen-bonded ribbons involving the acetyl carbonyl group are employed to explain the high selectivity of the photoinduced single-crystal to single-crystal Orton rearrangement. On the basis of the change of the crystal structure and the physicochemical data, a three-center five-atom mechanism involving homolytic cleavage of the N-Cl bond followed by hydrogen abstraction by the carbonyl group is suggested for the solid-state photoinduced Orton rearrangement.

Photochromism of Mixed Crystals Containing Bisthienyl-, Bisthiazolyl-, and Bisoxazolylethene Derivatives

Single crystals composed of two or three different kinds of diarylethenes, having similar geometrical structures but different colors in the closed-ring isomers, 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene (1a), 1,2-bis(5-methyl-2-phenyl- 4-thiazolyl)perfluorocyclopentene (2a), and 1,2-bis(5-methyl-2-phenyl-4-oxazolyl)perfluorocyclopentene (3a) have been prepared in an attempt to form single crystals which exhibit different colors, depending on illumination wavelengths. When 1a and 2a are mixed in ethanol and the content of 2a in the feed is higher than that of 1a, only a needle-shaped crystal is obtained. The crystal shape is similar to that of 2a. On the other hand, high content of 1a in the feed leads to formation of a block-shaped crystal, of which the shape is similar to that of 1a. At a feed ratio of 1:1 for 1a and 2a, two types of crystals such as needle- and block-shapes are obtained from the same batch. Mixed crystals composed of three kinds of derivatives 1a/2a/3a have also been prepared. A crystal composed of the three derivatives in the molar ratio of 32 (1a):53 (2a):15 (3a) was obtained. X-ray analysis proved that 1a and 3a are packed in the crystal lattice of 2a. The mixed crystal 1a/2a/3a changed color from colorless to yellow, red, and blue upon irradiation with light of appropriate wavelengths.

N-salicylideneanilines: tautomers for formation of hydrogen-bonded capsules, clefts, and chains

[structure: see text] The synthesis, characterization, and solid-state structures of new salicylaldimines are reported. Bis(N-salicylideneaniline)s (BSANs) and tris(N-salicylideneaniline)s (TSANs) are sterically encumbered compounds featuring a central six-membered ring in the keto-enamine tautomer. When extended with additional functional groups, these molecules may form hydrogen-bonded capsules, clefts, and extended structures. A TSAN with N-(t)()BOC-o-phenylenediamine groups has been structurally investigated. The complementary hydrogen-bonding motif in this molecule leads it to form dimers in solution and in the solid state. A BSAN with N-(t)()BOC-o-phenylenediamine substituents forms a hydrogen-bonded cleft in solution but forms an extended hydrogen-bonded ladder assembly of cofacial dimers in the solid state. When N-(t)()BOC-1,8-naphthalenediamine was utilized to extend the cleft, an unusual perimidine structure was obtained with the central core in the enol tautomer. In addition, ab initio calculations have been used to support the assignment of the keto-enamine or enol-imine tautomers of the BSANs and TSANs and to predict tautomerization in related BSANs and TSANs.

Intramolecular Nitro-Assisted Proton Transfer in Photoirradiated 2-(2‘,4‘-Dinitrobenzyl)pyridine: Polarized Optical Spectroscopic Study and Electronic Structure Calculations

The nitro-assisted proton transfer (NAPT), responsible for the photoactivity of ortho-nitrobenzylpyridines and a model for the nitro-based caged compounds, is studied along with the parent compound 2-(2',4'-dinitrobenzyl)pyridine (DNBP) with polarized optical spectroscopy and theoretical calculations. The transition dipole moments of a DNBP single-crystal identified oriented molecules of the long-lived enamine tautomer (NH), rather than of the aci-nitro tautomer (OH), as carriers of the photoinduced blue coloration. It is clarified that the blue second singlet transition owes to intramolecular charge transfer from the allyl-pyridinium part to the dinitrophenyl fragment of NH. The theoretical modeling of the ground-state potential energy surface showed that while NH and OH can interconvert by means of direct proton transfer, such a process between the initial form CH and either OH and NH would require significant rotation of the aromatic rings. In the ground state, OH is less stable but the kinetically preferred product over NH. Once created, regardless of whether via ground-state or excited-state routes, the aci-nitro group of OH undergoes energetically inexpensive rotation to deliver the proton to the nitrogen acceptor. The "softening" of the energy surface around OH due to its structural flexibility, that is, mediation of the proton transfer by the nitro group, is crucial to overcome the high barrier for a direct proton jump from CH to NH, even in cases of unfavorable donor-acceptor geometry. The very small structural change experienced by the surrounding of a molecule undergoing NAPT is promising for the design of photoactive systems which retain their crystallinity during a prolonged operation.

Highly Stable Keto-Enamine Salicylideneanilines

[reaction: see text] Highly stable NH salicylideneanilines have been prepared by reaction of 1,3,5-triformylphloroglucinol with aniline derivatives. The NH form was confirmed by X-ray crystallographic data, as well as by NMR studies. A convenient one-step synthesis of triformylphloroglucinol is also reported.