Microcrystals of a diarylethene {1,2-bis[5'-methyl-2'-(2"-pyridyl)thiazolyl]perfluorocyclo-pentene} undergo jumps upon photoirradiation. These photochromic crystals present molecular structural changes upon irradiation with ultraviolet light because of reversible photocyclization reactions. When the energy absorbed by crystals reaches about 10 microJ, the uniaxial stress induced in the crystal lattice relaxes through directional jumps. If one prevents crystals from jumping, then parallel, equidistant cracks appear on crystal surfaces. These photomechanical effects could result from a Grinfeld surface instability.

Photomechanical effects are observed in single crystals of a thiazole-based diarylethene. Crystalline state photochromism has been characterized by absorption microspectroscopy. Clear single crystals turn colored under ultra-violet irradiation. When the energy absorbed by crystals reaches about ten microjoules, they jump. When we prevent crystals from jumping, parallel, equidistant cracks appear due to elastic energy dissipation. These phenomena are associated with a phototransformation ratio of only a few percent.

Spectroscopic and kinetic properties of a new photochromic medium, consisting of nanocrystals of spyropyran molecules (1,3-dihydro-1,3,3,5',6',pentamethyl-spiro[2H-indole-2,2'-[2H]pyrano [3,2-b]pyridinium] iodide) embedded in an organo-silicate sol-gel film, are presented and compared to microcrystals obtained by slow evaporation of a solvent. High photoconversion efficiencies for both kinds of crystals have been observed. In microcrystals, the photomerocyanine form absorbs at 570 nm with a fading rate of 5 h, in nanocrystals the photomerocyanine form absorbs at 535 nm with a fading rate of 41 h. Therefore, the crystalline structure of nanocrystals is different from the microcrystal one.

C24H19NO4S2, triclinic, P1̅ (no. 2), a = 10.319(4) Å, b = 10.275(7) Å, c = 11.31(1) Å, α = 103.01(2)°, β = 96.40(2)°, γ = 64.37(3)°, V = 1053.7 Å3, Z = 2, Rgt(F) = 0.055, wRref(F) = 0.056, T = 296 K.

The conformational changes of the photochromic 3-furylfulgide molecule have been studied in relation to its crystal structure. Intense two-photon irradiation of an initial pale yellow single crystal led to a red material that is no longer crystalline but still photochromic. Conversely, starting from a colored single crystal grown from an irradiated solution, X-ray diffraction gave the first determination of the molecular conformation and crystal structure of the colored form and showed that after visible irradiation the bleached material also lost its crystalline structure. The photoinduced molecular displacements responsible for this behavior are pointed out.

The photochromic behavior of 2-(2',4'-dinitrobenzyl)pyridine (alpha-DNBP) has been followed in poly(methyl methacrylate) (PMMA) films and benzene solutions to clarify the behavior of a precursor state, previously identified in studies on crystalline alpha-DNBP at low temperatures. In PMMA films, photolysis at temperatures less than or equal to 50 K led to the concurrent formation of a NH tautomer and a colorless intermediate, which was stable for several hours. On irradiation at low temperatures and warming the sample, the colorless intermediate was seen to react to produce the NH tautomer in a higher yield than that found in the direct photolysis. Further information on this intermediate has come from flash photolysis studies in benzene solution, in which a new transient absorption has been observed at 335 nm and assigned to this species. This decays within a few microseconds at room temperature to form an OH tautomer, which then interconverts to the NH tautomer. The precursor state is not quenched by oxygen or naphthalene. From consideration of the kinetic and spectral data, it is suggested that this new species corresponds to a nonrelaxed tautomeric form of the OH state of alpha-DNBP.

The quantum yields of the photoinduced ring-closure and ring-opening reactions of a series of 1,2-dithienylperfluorocyclopentene derivatives have been determined in view of the development of materials for photonic applications based on these molecules. The reaction quantum yields as a function of the S0–S1 excitation energy exhibit clear threshold behavior and are otherwise little sensitive to the chemical nature of the substituent. A ground state energy difference between the two isomers of >1eV is deduced from this data. Additional information on the reaction surface is obtained from the temperature dependence of the quantum yields of the ring-opening reaction.

Summary form only. We have prepared a new type of nanocomposite hybrid organo-mineral material that is composed of organic nanocrystals embedded in sol-gel glasses. Organic molecules display a large variety of optical properties (e.g. nonlinearities, fluorescence, photorefractive, photochromism, or photomagnetism). Our technique makes it possible to include these molecules in a solid inorganic media which affords easy elaboration, cutting and polishing, mechanical stability and low cost. We have demonstrated the interest of these new materials for optical power limiting devices.

Photochromism in crystalline 2-(2′,4′-dinitrobenzyl)pyridine is associated with a photoinduced proton transfer reaction leading to a metastable colored tautomer of several hours lifetime at room temperature. Transient absorption experiments performed between 10 and 300 K show two competitive proton transfer routes: a fast direct and temperature-independent one, assigned to an excited state process, and a much slower one, thermally activated, involving a multistep proton transfer in the ground state via a precursor state, assigned to an additional tautomer. An energy level diagram, including energy barriers, is afforded by a temperature-dependent study of the photochromic reaction.

Imidodiphosphinate ligands form a hydrophobic shell around terbium and europium ions leading to long-lived, highly luminescent complexes.

We report quasi-elastic neutron scattering (QENS) and optical spectroscopic measurements of the proton motion in the hydrogen bond of benzoic acid crystals, a system in which proton transfer occurs in a near-symmetric double-well potential and both coherent and incoherent proton tunneling have been measured. Inter-well relaxation is characterized by a correlation time that determines the width of the QENS line. Our new measurements cover the transition from the classical to the quantum regime. The optical spectroscopic data, using new dopant molecules, extend measurements of the proton dynamics to very low temperatures, where coherent tunneling is observed. Theoretical methods, based on a perturbative instanton approach, have been developed to describe tunneling in multidimensional potential energy surfaces and are used to model our observations.

Two-proton exchange along the two hydrogen bonds mediates the tautomerization in benzoic acid (BA) dimers. Optical spectroscopy and quasi-elastic neutron scattering (QENS) have been employed to characterize the proton dynamics in doped and pure BA crystals. The proton motion in BA is governed by a multidimensional potential energy surface (PES), and recent theoretical methods, based on a perturbative instanton approach, to describe tunneling in such PES are presented. This PES is also modulated by the interaction with the solid state environment as manifest by the energy difference between the otherwise equivalent tautomers. The value of this energy difference in pure crystals is an important parameter in the data analysis of NMR and QENS. Both methods give mutually consistent values that differ significantly from earlier determinations via infrared and C-13 NMR as well as a recent evaluation using neutron diffraction data. The energy difference between tautomers is altered for dimers in the vicinity of impurity molecules. This is the basis for the optical spectroscopic methods, which enable a direct and accurate determination of the level structure and tautomerization dynamics of these coupled dimers in the limit of very low temperatures, where coherent tunneling is also observed in some cases. Measurements with new impurity molecules make it possible to monitor simultaneously at least 10 different tautomer configurations and prove that the influence of the probe molecules on the proton dynamics is small. The transition to thermally activated barrier crossing at higher temperatures is accessed via the width of the QENS line that is determined by the inverse of the proton correlation time. The quantitative data analysis of the scattered intensity as a function of temperature and scattering angle yields the energy difference, A, between the two wells (A/k(B)=90+/-20 K), the length (0.686 Angstrom, and direction of the proton jump vector. These measurements complement NMR investigations presented in the preceding paper.

Photo-induced proton transfer in crystalline 2-(2′,4′-dinitrobenzyl)pyridine is characterized by transient absorption between 10 and 300 K. Temperature independent and thermally activated processes are identified. The long-lived colored tautomer is shown to be formed by two competing routes, one on short time scales in the excited state and the other in the ground state via intermediate tautomers.

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Photoinduced proton transfer from 2-(2',4'-dinitrobenzyl)pyridine (alpha-DNBP) to [2.1,1] cryptand (C211) is demonstrated and characterized by transient absorption spectroscopy. Proton transfer is shown to occur from two tautomers reached after optical excitation of alpha-DNBP. Protonation of C211 is expected to form a proton cryptate in which the proton is included in the intramolecular cavity and stabilized, so that the back-transfer reaction is considerably slowed. As a consequence, the alpha-DNBP anion, formed by deprotonation, survives on time scales that are at least 10(3) times longer than those observed in the presence of another proton acceptor of similar basicity, triethylamine, which was studied for comparison and which does not equally stabilize the proton. As deprotonation by C211 intercepts different tautomers of alpha-DNBP, information about the reaction pathways of photocoloration in this compound is gained.

The photoinduced proton transfer reaction taking place in 2-(2',4'-dinitrobenzyl)pyridine (alpha-DNBP) and in some of its derivatives is characterized by LR, visible, and NMR spectroscopy. The enamine ''NH'' structure of the blue phototautomer is confirmed by the analysis of the IR spectra of alpha-DNBP and its deuterated analogue. 2D NOESY H-1 NMR data indicate that this tautomer is predominantly in the cis configuration. In the 2-(2',4'-dinitrobenzyl)phenanthroline derivative, the stabilization of the phototautomer is sufficient to make it thermally accessible. A quantitative analysis of the resulting thermochromism indicates that in toluene solutions the ground state energy of the ''NH'' form is lowered to 2.9 kcal mol(-1) above the thermodynamically stable ''CH'' tautomer compared to more than 8 kcal mol(-1) in the parent alpha-DNBP compound.

Low-temperature optical spectroscopy of guest-host molecular crystal systems provides a very sensitive tool to study proton tunneling processes. Examples include the light-induced creation and evolution of “proton defects”, translational tunneling along hydrogen bonds as well as rotational tunneling of methyl groups. Absorption spectra at 2 K of γ-picoline single crystals evolve over time scales of days due to the slow spin conversion of the methyl groups which leads to an ordering of the crystal.

Optical excitation of pentacene, doped into a benzoic acid crystal matrix, induces a reversible proton transfer reaction between the host and the guest. This reaction occurs at low temperatures and leads to the creation of defects corresponding to the displacement, from its regular position, of an acid proton of the host matrix. The formation and evolution of these defects is monitored via the electronic S0 → S1 transition of the pentacene guest. Here we report and discuss a variety of measurements made with the aim of obtaining information about the first step of the reaction. It is shown that the rate for the first step of this reaction is reduced by a factor of about 104 upon deuteration of the host matrix, demonstrating that this step occurs by tunnelling. Other unsuccessful experiments (optical, ESR, magnetic field effects) made to identify the first intermediate of the reaction as well as molecular orbital calculations of potential intermediates are also reported briefly. It is shown that the formation of the pentacene cation, protonated in the centre position, is consistent with all observations. This species is proposed as the most likely first intermediate of the reaction.

Despite structural similarities, different photoreactivities are displayed by the two crystalline phases of 6-(2,4-dinitrobenzyl)-2,2′-bipyridine. Whereas one phase (the arrangement of the molecules in the unit cell of this phase is shown on the right) undergoes a photoinduced proton transfer process, the other is photoinert. Deuteration causes a marked increase in the lifetime of the colored phototautomer.