Photochromic Product Research Articles

Photochromic Porous and Nonporous Viologen-Based Metal–Organic Frameworks for Visually Detecting Oxygen

The detection of the oxygen molecule is of particular interest due to various applications in chemical, biological, and environmental areas, and the typical materials of detecting O2 such as Clark-type electodes and optical oxygen sensors suffer from disadvantages such as insensitivity or usage of expensive instrument. In this paper, we present a nonporous two-dimensional viologen-based metal–organic frameworks consisting of pseudo-left- and right-handed helical chains [Cd(CPBPY)(o-BDC)(H2O)]·H2O (CPBPY = N-(3-carboxyphenyl)-4,4′-bipyridinium, o-BDC = o-benzenedicarboxylate) (1) that exhibits a slow photochromism under a vacuum, inert atmosphere, and even exposure to oxygen, and a three-dimensional porous 6-connected pcu topological metal–organic framework [Cd3(CPBPY)2(BDC)3]·DMF·H2O (BDC = 1,4-benzenedicarboxylate) (2), which displays rapid photochromism only under a vacuum or inert atmosphere. The photochromic product 2′ can quickly and conveniently detect O2 by naked eye recognition of color change. Th...

Investigation of ultrafast photoinduced processes for salicylidene aniline in solution and gas phase: toward a general photo-dynamical scheme

Photodynamics of 2-hydroxybenzylideneaniline (photochromic salicylidene aniline SAOH) and N-(2-methoxybenzylidene)aniline (SAOMe) are studied by steady state and transient optical spectroscopy in solution and gas phase at different excitation wavelengths (266, 355 and 390 nm). Two competitive processes are observed from the enol* excited state: on one hand a rotation to get a twisted-enol, and on the other hand an excited state intramolecular proton transfer (ESIPT) followed by a cis-trans isomerisation to get the trans-keto photochromic product. For the first time both processes are characterized at an ultrashort time scale for salicylidene aniline. Resolution of the spectrokinetic data is achieved by multivariate curve resolution and attribution of the intermediate species recovered is performed in comparison with the results obtained for SAOMe, which can only undergo enol rotational isomerisation. It shows that ESIPT and rotation to the twisted-enol for SAOH occur within 100 fs, as predicted by recent quantum dynamical simulations, with an efficiency ratio dependent on the excitation wavelength. Therefore a general photoinduced mechanism for salicylidene aniline is drawn.

Electronic Tera-Order Stabilization of Photoinduced Metastable Species: Structure of the Photochromic Product of Spiropyran Determined with in situ Single Crystal X-ray Photodiffraction

The extraordinary stability of the photoinduced red form of a cationic spiropyran ( k approximately 10 (-6) s (-1) in water and approximately 10 (-6) to less than 10 (-8) s (-1) in the solid state) was employed to obtain in situ X-ray diffraction evidence of its molecular structure. By UV excitation under selected experimental conditions, on average, approximately one third of the cations in a single crystal of spiropyran iodide salt was converted and retained as the red form during the experiment. According to the structure of the mixed crystal, the ring opening, which is due to increased distance between the spiro oxygen and carbon atoms, is associated with slight molecular flattening caused by concurrent out-of-plane shift (11.2(5) degrees ) of the pyranopyridinium half and in-plane shift (4.8(7) degrees ) of the indoline half. The overall geometry change of the cation fits the steric requirements imposed by the ion packing in the crystal and can be viewed as molecular flattening caused by breaking of the spiroconjugation. The structure of the cation confirms that (at least in the case of cationic spiropyrans) the product is confined in the crystal mainly as a zwitterionic resonance structure in cis configuration similar to the (early) transition state. Although the positive charge of the closed form facilitates the ring-opening reaction by moving the reactant closer to the transition state, neither the weakening of the spiropyran C-O bond nor the space provided by the iodide alone can account for the stability of the product. Instead, the density functional theory calculations indicate that the stabilization of the red form of the cationic relative to the neutral spiropyran is thermodynamically controlled, probably through compensation of the charge within the zwitterion by the methylpyridinium group.

Photochromism in p-methylbenzoylthioacetone and related β-thioxoketones

Irradiation of p-methylbenzoylthioacetone with UV or visible light brings about spectral changes characteristic for the photochromic behavior of β-thioxoketones. The nature of the initial species and of the photochromic product can be assigned based on spectral studies of the electronic and IR spectra in rare gas matrices combined with quantum chemical calculations. Additional arguments for the vibrational assignments are provided by using polarized light to induce the phototransformation, and by subsequent measurements of linear dichroism on partially aligned samples. Comparison with the results previously obtained for three related molecules: thioacetylacetone, p-methyl(thiobenzoyl)acetone, and monothiodibenzoylmethane reveals a common pattern of the photochromic reaction. In all four molecules, the initial species corresponds to an intramolecularly hydrogen-bonded enolic molecular structure, and the main photochromic product to an “open”, nonchelated enethiolic counterpart. On the basis of a computational TD-DFT study of ground and excited electronic states, possible mechanisms of the photochromic transformation are discussed.

Nitration of Photochromic Spirophenanthroxazine.

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Photochromism and polarization spectroscopy of p-methyl(thiobenzoyl)acetone

Photochromism of p-methyl(thiobenzoyl)acetone (1) has been studied in argon and xenon cryogenic matrices. Application of linearly polarized light to induce the phototransformation resulted in partial alignment of both the initial structure and the photochromic product. Different orientations were achieved by using irradiation wavelengths corresponding to differently polarized electronic transitions. This was followed by measurements of linear dichroism (LD) in the IR region. The analysis of the IR spectra, combined with the results of DFT B3LYP/cc-pVDZ calculations enabled determining the structures of the most stable ground state species and of the photoproduct. Similarly to the recently reported cases of thioacetylacetone and monothiodibenzoylmethane, the initial structure of 1 corresponds to an intramolecularly hydrogen-bonded enol form, and the photochromic species to an “open”, nonchelated enethiolic form. Vibrational assignments have been made for both species, greatly helped by the analysis of the LD spectra. It is concluded that DFT calculations for 1 quite reliably predict not only vibrational frequencies and intensities, but also transition moment directions, in both IR and electronic spectra.

Nitration of photochromic spirophenanthroxazine

Nitration of photochromic spirophenanthrooxazine with three different reagents (cupric nitrate, NaNO2 in acetic acid, and HNO3-H2SO4) was studied. The major reaction products were specified and characterized by 1H and 13C NMR and mass spectra. The mechanisms of their formation were proposed. Only the nitration with a mixture of nitric and sulfuric acids yielded a photochromic product containing the nitro group in the phenanthrene fragment.

Dynamics of photochromism in salicylideneaniline: A femtosecond spectroscopic study

Excited state intramolecular proton transfer (ESIPT) of salicylideneaniline enol and subsequent isomerization of the proton transferred keto structure to the final long-lived photochromic product has been studied in different solvents using femtosecond transient absorption and picosecond fluorescence spectroscopy. Three different types of transient absorption have been identified, viz. (a) from initially excited enol form, (b) proton transferred keto form and (c) corresponding to the final photochromic product. The ESIPT rate was determined from the rise time of the characteristic transient absorption originated from the proton transferred keto form and was found to be in the order of (200–300 fs)−1 in most of the solvents. Photochromic product formation is completed within a few tens of picoseconds. The time dependent spectral change observed at the early time delay after excitation was considered to be due to cooling of the vibrationally “hot” proton transferred keto form towards the formation of the final photochromic product. The cooling process leads to the formation of some “metastable” state within a few hundred of femtoseconds in the ground state potential energy surface of the final trans product.

Femtosecond spectroscopic study on photochromic salicylideneaniline

Photochromic reactions of salicylideneaniline (SA) have been studied in solution using femtosecond transient absorption spectroscopy and picosecond fluorescence measurements. In the excited state, SA undergoes intramolecular enol–keto tautomerism and subsequent isomerization to give a colored photoproduct. Both the proton transfer and photochromic product formation were found to occur within few hundreds of femtosecond. The mechanism of these two ultrafast processes and their solvent dependence are discussed.