SF6-sensitized dissociation of UF6 with a pulsed CO2 laser

The dissociation of UF₆ sensitized by SF₆ excited with a pulsed CO₂ laser in the presence of H₂ and CO as scavengers has been investigated. In the SF₆-UF₆-H₂ system the dissociation yields have been determined as a function of the laser frequency, the fluence, and H₂ partial pressure. A maximum dissociation yield has been found at a laser frequency of 935 cm^?1. No obvious dissociation of UF₆ was observed in the UF₆-SF₆ system without F-atom scavengers.     

Triplet-sensitized photobehaviour of the three stereoisomers of 1,4-distyrylbenzene and some aza-analogues

The triplet spectral properties of the three stereoisomers (EE, ZE and ZZ) of 1,4-distyrylbenzene and some symmetric EE-aza-analogues, bearing a nitrogen heteroatom in the side (2′-pyridyl, 4′-pyridyl or 2′-quinolyl) or in the central (2,5-pyridine) arene rings, have been investigated by laser flash photolysis in different solvents and compared with those of the parent hydrocarbon. The quantum yields of the triplet-sensitized photoisomerization have also been measured. Adiabatic formation of the excited ³EE^* isomer by irradiation of the other geometrical isomers has been detected by transient spectroscopy. The effect of the substrate concentration on the sensitized photoisomerization quantum yield revealed the occurrence of a quantum chain process, as known for similar arylolefins.     

Laser photolysis study of trans→cis photoisomerization of trans-1-phenyl-2-(2-naphthyl) ethylene

The direct trans→cis photoisomerization of trans-1-phenyl-2-(2-naphthyl) ethylene (trans-PNE) in liquid solution at room temperature was studied by the nanosecond laser photolysis technique. The time-resolved S_n←S₁ and T_n←T₁ absorption spectra were observed with trans-PNE at 300 K and 77 K. The lifetime of the triplet state of trans-PNE was found to be much shorter in liquid solution at room temperature than in rigid solution at 77 K. This fact and the effect of a triplet quencher shows that the photoisomerization of trans-PNE occurs mainly via the triplet state.     

Two-photon absorption of photochromic diarylethene and its application to rewritable holographic recording

A new photochromic diarylethene (1a) has been prepared. Both its photochromic behavior and nonlinear optical properties are investigated. 1a shows excellent ring-opening (λ_max = 386 nm) and ring-closing (λ_max = 652 nm) photoisomerization with UV-Vis light irradiation. With 800 nm femtosecond pulsed laser irradiation, 1a shows two-photon-induced photoisomerization, and a two-photon absorption cross-section (σ = 423×10⁵⁰ cm⁴·s per photon) is obtained by using two-photon induced fluorescence method. The applications of two-photon absorption of 1a to holographic recording has been also investigated. A two-photon induced micro-pattern is recorded on the diarylethene 1a-PMMA film with an femtosecond laser of 800 nm, 100 fs, 1 kHz, 50 mW.     

Phototransformations in nonhomogeneous media: cis-trans-isomerization of olefins in inclusion compounds

In order to elucidate photophysical and steric factors influencing extent and reaction pathway of a reaction process involving inclusion compounds the photoisomerization of cis- or trans-stilbene and cis- or trans-methylcinnamate has been studied in cavities of deoxycholic acid or Ni(II)-(4-picoline)₄ -(NCS)₂. The observed isomerization yields have been correlated with the excitation energy transfer processes which occur between host and guest molecules as well as with the symmetry properties of the various species involved during the isomerization process.     

Photochemische Umwandlungen. XV [1] Photochemische Isomerisierung des exo-Tricyclo[4.2.1.02′5]nona-3,7-dien-Systems. Vorläufige Mitteilung

The acetone sensitized isomerization of two exo-tricyclo[4.2.1.0^2,5]nonadiene derivatives, of the corresponding tricyclo[4.3.0.0^2,5]nonadienes, and the photoisomerization of two bicyclo[4.3.0]nonatrienes by direct excitation are described.     

Isotopic selectivity in the sensitized dissociation of SF5Cl and CF3I by multiphoton excitation of SF6

The dissociation of SF₅Cl and CF₃I sensitized by multiphoton excitation of SF₆ by a pulsed CO₂ laser has been studied versus pressure, laser fluence, inert gas and optical frequency. Isotopic effects have been observed between ³²SF₅Cl/³³SF₅Cl and between ¹³CF₃I/¹²CF₃I and selectivity factors as high as α^?1_{$\text{33}\text{32}$} = 1.57 or α_{$\text{13}\text{12}$} = 1.23 obtained.     

IR-laser-induced sensitized organic reactions: An example of rate control by vibrational energy transfer

The CO₂-laser-induced decomposition of cyclohexene has been studied in the presence of SiF₄, SF₆ and C₆F₆ as sensitizers. The pressure range investigated is between 10 and 30 Torr at laser fluences between 0.20 and 1.20 J cm⁻². At the same time measurements of the IR multiphoton absorption of the three sensitizers were also performed.The results have been analyzed within the framework of a kinetic model based on the assumption that the rate is controlled by vibrational energy transfer from the multiphoton-excited sensitizer molecules to the reactant. Evidence is presented to show that these sensitized reactions proceed under non-equilibrium conditions.     

Z/E PHOTOISOMERIZATION OF UROCANIC ACID*

The E ? Z photoisomerization of the title compound (UA) (a naturally occurring sunscreen) has been studied in aqueous solution. At a UA concentration of 6mM and using 313nm excitation, φ_E→z= 0.52, φ_Z→E= 0.47 and the photostationary state is 34% E. Under these conditions, loss of UA is minimal. Low energy triplet quenchers fail to impede the isomerization, but the reaction can be induced by several triplet sensitizers. The E_T for UA is estimated to be approximately 55 kcal/mol.     

Molecular Beam Studies on the Laser-Enhanced Surface Reaction of GaAs(100) with Chlorine

A cw supersonic Cl₂ molecular beam coupled with an angle-resolved time-of-flight (TOF) technique has been used to investigate the laser-enhanced surface reaction of GaAs(100) with chlorine. The mass and velocity distributions of the major reaction products under 1064 nm laser irradiation have been measured as a function of laser fluence, detection angle, surface temperature and normal component of the translational energy of the incident chlorine molecules. It has been found that increasing both laser fluence and the translational energy of incident chlorine molecules markedly enhance mis surface reaction. The measured flux angular distributions of major reaction products can be fit satisfactorily with a bi-cosine function. Measurements of the mass and angular distributions of reaction products by a modulated molecular beam mass spectrometry show that the surface temperature effect is obvious for the Cl₂/GaAs(100) “dark” thermal reaction. A direct activated dissociative chemisorption is proposed for the mechanism of Cl₂ chemisorption on the GaAs(100) surface.     

Direct laser-chemical sensitized synthesis of formaldehyde from methane

An original method is proposed for the direct oxidation of methane to formaldehyde at room temperature and atmospheric pressure by atmospheric oxygen, under the action of a continuous tunable CO₂ laser. The synthesis is accompalished by a sensitized laser-chemical reaction due to intermodal vibrational activation of methane by V-V exchange with the sensitizer.Institute of General and Inorganic Chemistry, Ukrainian Academy of Sciences, Kiev. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 6, pp. 730–733, November–December, 1991. Original submitted March 27, 1991.     

Laser induced dielectric breakdown studies of the reaction UF6 + H2

Laser induced dielectric breakdown (LIDB) has been documented in UF₆ at pressures ranging from 8–100 torr. A high power, line selectable TEA CO₂ laser has been used as the source to induce the dielectric breakdown (DB). Reactions of the fragmented UF₆ with H₂ have been studied at various pressure ratios. In all cases a rapid and large pressure drop and heavy deposits of suspended particulates were observed and attributed to the LIDB driven reaction 2UF₆ + H₂ → 2UF₆ + H₂ → 2UF₅ + 2HF.     

Photodissociation of CF3NO: observation of NO (υ=1)

That excitation of CF₃NO with wavelengths between 580 and 660 nm yields CF₃ + NO has been shown by two direct techniques. In the first, the CF₃ and NO radicals have been scavenged by their reaction with Cl₂ to yield CF₃Cl and NOCl. as detected by both infrared and mass spectrometry. In the second technique, NO (υ=1) vibrational fluorescence has been observed following tunable dye laser excitation of CF₃NO. The rate of vibrational relaxation of NO (υ=1) in collisions with CF₃NO has been found to be (2.19 ± 0.19) × 10³ s^?1 torr ^?1.     

Dynamics of trans-cis photoisomerization of hetarylazo dyes

Dynamics of trans-cis photoisomerization of novel hetarylazo dyes containing hydrogenated quinoline and triazole or tetrazole moieties has been studied by femtosecond laser photolysis with spectrophotometric detection. For all the dyes under study, the absorbance dynamics after photoexcitation in the long-wavelength absorption band (λ_pump = 550 nm) is described by three fast processes with characteristic times of 0.07–0.27, 0.4–1.0, and 3–7 ps. The effect of the solvent and the azo dye structure on the dynamics of transient species has been investigated.     

Substituent and structural effects on the photochemistry of bicyclic βγ-unsaturated ketones

The direct and sensitized irradiations of the βγ-unsaturated ketones1(a-c) and2 have been investigated.1a,1c and2, on direct irradiation give rise to ODPM rearrangement in addition to the expected [1,3]-acyl shift. These represent the first examples of ODPM rearrangements arising from some state other than T₁(π-π^*). [1,3]-Acyl shifts observed in the sensitized reactions can involve direct absorption of light and singlet energy transfer from acetone in addition to population of the T₂(n-π^*) state.     

Spectral and kinetic parameters of transient species in the photolysis of naphthylmethylideneiminospironaphthopyran by excitation at different wavelengths: Nano- and femtosecond laser photolysis

Intramolecular processes occurring in a photobifunctional compound (PBC) comprising the spironaphthopyran and hydroxyazomethine moieties have been studied in methanol solutions by femtosecond laser photolysis using light with wavelengths of 340 and 490 nm. At the excitation wavelength of 490 nm, the cis-trans photoisomerization in the azomethine moiety occurs in the S₁ state. In the case of PBC photolysis with 340-nm light, the opening of the spiro bond of the spiropyran moiety (formation of the X form) also takes place during relaxation of the S_n state to the S₁ state followed by isomerization to the merocyanine form. The spectral and kinetic characteristics of different electronically excited have been were determined. The data have been compared with those of nanosecond laser photolysis.     

Sized dependence and microstructural defects on highly photocatalytic activity based on multisized CdTe quantum dots sensitized TiO2

A series of sensitized TiO₂ NPs with enhanced photocatalytic activity were prepared by different sized CdTe QDs with bifunctional ligand interlinking. The microstructures and morphologies of four different sizes of CdTe QDs and CdTe QDs sensitized TiO₂ NPs were characterized systematically by fluorescence spectroscopy, UV-Vis diffuse reflectance spectroscopy, FT-IR, XRD, XPS, FE-SEM, HRTEM, BET, and positron annihilation lifetime spectroscopy (PALS). The photocatalytic performance of CdTe QDs sensitized TiO₂ NPs was investigated by degrading MO and MB in aqueous solution under visible light irradiation. The process of degradation complied with the first-order reaction kinetics. Four different sized CdTe QDs sensitized TiO₂ exhibited higher photocatalytic activities than TiO₂. Particularly, yellow emitting CdTe QDs (3.28 nm) sensitized TiO₂ (CT_1.5) showed the best photocatalytic performance, which may be attributed to the optimal NP size, and has a moderate light absorption and electron injection. The effect of microstructural defects on CdTe QDs sensitized TiO₂ has been confirmed effectively by positron lifetime measurements.     

Spectral and photochemical properties of covalently linked dyads based on 2-styrylquinoline and 6-hydroxy-2-naphthoic acid

Photophysical and photochemical properties of bichromophoric covalently linked SnN dyads (n = 3, 5, 9), in which the 2-styrylquinoline (SQ) and 6-hydroxy-2-naphthoic acid (Np) moieties are linked by the dioxypolymethylene bridge–O–(CH₂)_n–O–have been studied. The properties of the dyads have been compared with those of the model compounds 2-(4-methoxystyryl)quinoline and methyl 6-hydroxy-2-naphthoate. Inductive-resonance (Förster) energy transfer (FRET) from the Np to the SQ unit with an efficiency up to 99.6% is observed in the S₁ state of the dyads. The Np unit in the neutral form does not affect the photoisomerization of the SQ unit regardless of the form of the latter, neutral or protonated (cationic). The Np moiety in the anionic (deprotonated) form hinders the photoisomerization of the SQ moiety, presumably, as a result to a combined action of several factors.     

Diarylethene Self‐Assembled Monolayers: Cocrystallization and Mixing‐Induced Cooperativity Highlighted by Scanning Tunneling Microscopy at the Liquid/Solid Interface

Stimulus control over 2D multicomponent molecular ordering on surfaces is a key technique for realizing advanced materials with stimuli‐responsive surface properties. The formation of 2D molecular ordering along with photoisomerization was monitored by scanning tunneling microscopy at the octanoic acid/highly oriented pyrolytic graphite interface for a synthesized amide‐containing diarylethene, which underwent photoisomerization between the open‐ and closed‐ring isomers and also a side‐reaction to give the annulated isomer. The nucleation (K_n) and elongation (K_e) equilibrium constants were determined by analysis of the concentration dependence of the surface coverage by using a cooperative model at the liquid/solid interface. It was found that the annulated isomer has a very large equilibrium constant, which explains the predominantly observed ordering of the annulated isomer. It was also found that the presence of the closed‐ring isomer induces cooperativity into the formation of molecular ordering composed of the open‐ring isomer. A quantitative analysis of the formation of ordering by using the cooperative model has provided a new view of the formation of 2D multicomponent molecular ordering.     

The CW-CO2 laser induced reaction of 1,1-difluoroethane with chlorine

The CW–CO₂ laser induced reaction of CF₂HCH₃ (1) with Cl₂ yields vinylidene fluoride (2) as the main product; other products are CH₃CF₂Cl (3), CF₂ClCH₂Cl (4), CF₂ = CHCl (5), CF₂ = CCl₂ (6) and CF₂ Cl₂ (7). The yield dependence of 2 on the CF₂HCH₃/Cl₂ ratio, the laser irradiation time, the laser power and the pressure of the gaseous reactants have been investigated. Furthermore, the TEA–CO₂ laser induced reaction of 1 with Cl₂. the CW–CO₂ laser induced reaction of 2 and 2 with Cl₂ have also been studied in order to gain more mechanistic insights for this complicated reaction system. Apparently, CF₂ClCH₃ but not CF₂HCH₂Cl. is the main precursor to 2. Interestingly, it has been found that the relatively strong double bond of CF₂ = CH₂ can be broken by laser irradiation. The possibility of applying this laser methodology to the production of vinylidene fluoride has been discussed.