Pathways of spontaneous and photoinduced hydrogen tunnelling in organic crystals

Optical excitation at liquid helium temperature of pentacene molecules embedded in benzoic acid crystals gives rise to defects in the environment of pentacene. By deuteration of the acid proton of the matrix and/or selective deuteration of the pentacene molecules, it has been proven that these defects are due to the displacement of acid protons of the matrix and are produced by reversible proton transfer between the benzoic acid host and the pentacene guest. The defect sites are unstable and decay with lifetimes ranging from subseconds to hours. Different defect sites also interconvert spontaneously in the dark or after absorption of a second photon.     

Delocalized electronic excitations of pentacene dimers in a p-terphenyl host: picosecond photon echo experiments

Photon echo experiments are performed on delocalized electronic excitations of optical dimer states. The delocalized dimer states are found to have very long coherence times, close to the coherent limit The dimer dephasing is qualitatively similar to dephasing of pentacene monomers.     

Optical dephasing of molecular dimers in mixed crystals: Picosecond photon echo experiments

Picosecond photon echo experiments are used to examine optical dephasing of substitutional dimers and monomers of tetracene and pentacene in p-terphenyl host crystals. A comparison of experiments on tetracene and pentacene dimers permits the mechanism responsible for temperature-dependent optical dephasing to be determined. It is shown that excitation of librations rather than scattering between delocalized dimer states is the principal mechanism.     

A resonance raman scattering study of vibrational dephasing in the mixed crystal of pentacene in naphthalene

Resonance Raman scattering is used to investigate vibrational dephasing in the mixed crystal of pentacene in naphthalene. It is shown that, as for the pure electronic transition, uncorrelated resonant phonon scattering processes in the ground and vibrationally excited state induce vibrational dephasing in this system.     

Ultrafast optical dephasing in a low-temperature organic glass

The optical dephasing of pentacene in an o-terphenyl glass at 1.5 K is examined with picosecond photon echo and fluorescence line narrowing experiments. The homogeneous T₂ is found to be 1–3 ps, in contrast to all previous measurements on glass—chromophore systems in which the homogeneous T₂are two to three orders of magnitude longer.     

Temperature-dependent dephasing of delocalized dimer states of pentacene in p-terphenyl: picosecond photon echo experiments

Optical dephasing of pentacene dimers in p-terphenyl is studied. Dimer statesR₃ and R₄ exhibit an exponential temperature activation of T′₂ which is inconsistent with a mechanism involving scattering between delocalized dimer states. Other dephasing mechanisms are discussed. An upper limit is placed on the relaxation rate from the upper dimer state, R₁, to R₃     

Picosecond photon echoes detected by optical mixing

Picosecond photon echoes are shown to be easily detected by optical mixing. The synchronized picosecond excitation and probe pulses are generated by amplifying pulses from two dye lasers, synchronously pumped by a mode-locked argonion laser. The technique is used to study optical dephasing in the organic mixed crystal of pentacene in p-terphenyl.     

Thermally induced excited-state coherent raman spectra of solids

A difference frequency resonance has been observed for the 747 cm^?1 vibration in the first excited singlet state of pentacene in benzoic acid. The resonance is absent at low temperature (4.5 K) and its appearance is exponentially activated with an activation energy of 13.8 cm^?1. These observations are compared to theoretical expectations.     

Measurements of molecular dephasing and radiationless decay by laser-acoustic diffraction spectroscopy

In this paper we present a method for the measurements of molecular dephasing and radiationless decay in gases and solids. The technique utilizes a single-mode of a tunable dye laser and an extracavity acousto-optic element. The latter diffracts the laser beam into or out of the sampe for finite times when a train of radio frequency pulses is fed into the transducer. Using this laser-acoustic diffraction spectroscopy (LADS) we report some new results on iodine gas, and pentacene in a p-terphenyl host at 1.8 K. Combining these new results of LADS on pentacene and our earlier findings using the frequency switching method we present an analysis for the effect of laser bandwidth on the decay of the prepared excited states.     

Photochemical synthesis of pentacene and its derivatives

A novel alpha-diketone precursor of pentacene, 6,13-dihydro-6,13-ethanopentacene-15,16-dione, was prepared and converted successfully to pentacene in 74 % yield by photolysis of the precursor in toluene: Irradiation of the diketone solution in toluene with light of 460 nm under an Ar atmosphere caused the solution to change from yellow to fluorescent orange-pink within a few minutes, after which, purple precipitates appeared. After 35 min, the solution changed to colorless and the purple precipitates were filtered to give pentacene in 74 % yield. By contrast, in the presence of oxygen, the color of the solution changed from yellow to pale yellow, and only 6,13-endoperoxide of pentacene was quantitatively obtained. The rate of the reaction upon photolysis was measured by observing the decay of n-pi* absorption of the precursor at 460 nm, and was found to be similar in both the presence and absence of oxygen. Therefore, the photoreaction of the alpha-diketone precursor seemed to occur via the singlet excited state. Because the T-T absorption of pentacene was observed upon photolysis of the precursor in the nanosecond transient absorption measurement under an Ar atmosphere, the excited triplet state of the pentacene generated singlet oxygen by sensitization, and it reacted with the ground-state pentacene to give the 6,13-endoperoxide. The alpha-diketone deposited on glass was also converted successfully to pentacene film by photoirradiation. In addition, diketone precursors of a mixture of 2,8- and 2,9-dibromopentacene and 2,6-trianthrylene were also prepared and their photoconversion was performed.     

Probing interfacial characteristics of rubrene/pentacene and pentacene/rubrene bilayers with soft X-ray spectroscopy

The electronic structure of rubrene/pentacene and pentacene/rubrene bilayers has been investigated using soft X-ray absorption spectroscopy, resonant X-ray emission spectroscopy, and density-functional theory calculations. X-ray absorption and emission measurements reveal that it has been possible to alter the lowest unoccupied and the highest occupied molecular orbital states of rubrene in rubrene/pentacene bilayer. In the reverse case, one gets p* molecular orbital states originating from the pentacene layer. Resonant X-ray emission spectra suggest a reduction in the hole-transition probabilities for the pentacene/rubrene bilayer in comparison to reference pentacene layer. For the rubrenepentacene structure, the hole-transition probability shows an increase in comparison to the rubrene reference. We also determined the energy level alignment of the pentacene-rubrene interface by using X-ray and ultraviolet photoelectron spectroscopy. From these comparisons, it is found that the electronic structure of the pentacene-rubrene interface has a strong dependence on interface characteristics which depends on the order of the layers used.     

离散小波变换-支持向量回归方法同时测定苯酚、苯胺和苯甲酸

在波长范围200～400nm测定苯酚、苯胺和苯甲酸混合液的吸收光谱,用离散小波变换（DWT）对光谱数据进行处理,再用支持向量回归SVR方法进行建模,建立了离散小波变换一支持向量回归方法（DWT—SVR）。方法用于模拟样品和污染水样中苯酚、苯胺和苯甲酸的同时测定,结果满意。     

Orientation of pentacene molecules on SiO2: from a monolayer to the bulk

Near edge x-ray absorption fine structure (NEXAFS) spectroscopy is used to study the orientation of pentacene molecules within thin films on SiO2 for thicknesses ranging from monolayers to the bulk (150 nm). The spectra exhibit a strong polarization dependence of the pi* orbitals for all films, which indicates that the pentacene molecules are highly oriented. At all film thicknesses the orientation varies with the rate at which pentacene molecules are deposited, with faster rates favoring a thin film phase with different tilt angles and slower rates leading to a more bulklike orientation. Our NEXAFS results extend previous structural observations to the monolayer regime and to lower deposition rates. The NEXAFS results match crystallographic data if a finite distribution of the molecular orientations is included. Damage to the molecules by hot electrons from soft x-ray irradiation eliminates the splitting between nonequivalent pi* orbitals, indicating a breakup of the pentacene molecule.     

Electronic states of a single layer of pentacene: standing-up and flat-lying configurations

The electronic properties of a single layer (SL) of pentacene molecules are investigated by high-resolution UV photoemission and near-edge X-ray absorption spectroscopy in different configurations of the SL, either standing up on an aromatic self-assembled monolayer or planar on a bare Cu(001) substrate. The weakly interacting pentacene molecules in the standing-up SL present a semiconducting character, and the empty states distribution reflects that of gas-phase pentacene, while the planar pentacene-Cu system shows a metallic interface with redistribution of the empty molecular states. The highest-occupied molecular orbital lineshape in the weakly interacting SL shows a double structure, attributed to two nonequivalent molecules in the ordered configuration.     

Synthesis and spectral studies of copper(II) complexes with amide group ligands

Complexes of copper(II) with 2-(acetylamino)benzoic acid, 2-(benzoylamino) benzoic acid, 2-(aminocarbonyl)benzoic acid, 2-[(phenylamino)carbonyl]benzoic acid, 2-[(1-naphthalenylamino)carbonyl]benzoic acid, 2-[(2-aminophenylamino)carbonyl]benzoic acid, 2-aminobenzanilide, 2(aminobenzoyl)benzoic acid, maleanilic acid and malea-1-naphthalanilic acid have been prepared and characterized by chemical analyses, molar conductivity, magnetic susceptibility measurements, thermal data, IR, electronic and ESR spectra. The visible and ESR spectral studies of these complexes (except those of maleanilic acid and malea-1-naphthalanilic acid) indicate that they are monomeric having either square planar or distorted octahedral geometry around Cu(II). The Cu(II) complexes of maleanilic acid and malea-1-naphthalanilic acid have been tentatively assigned dimeric structures. From the ESR spectra of Cu(II) complexes various parameters have been calculated.     

Oriented growth of crystalline pentacene films with preferred a-b in-plane alignment on a rubbed crystalline polymethylene surface

We have achieved a growth of highly oriented crystalline pentacene thin films, with preferred a-b in-plane orientation with respect to the rubbing direction of a rubbed polymethylene surface. The polymethylene thin film, generated on a gold surface by gold-catalyzed decomposition of diazomethane, was annealed and gently rubbed in a fixed direction by a flannelette cloth to serve as an alignment layer during the deposition of pentacene molecules. Various surface analysis techniques, including reflection absorption IR spectroscopy (RAIRS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, grazing incidence X-ray diffraction (GIXD), and atomic force microscopy were used to elucidate the structural details of the polymethylene and the pentacene thin films deposited on it. Two crystalline morphologies of pentacene thin film were observed: the minor one of rod-like molecular crystals having their long axes of the crystals perpendicular to the rubbing direction, and the dominant one of platelet-like and layered crystals having the molecular axes stand near vertical to the surface. Moreover, GIXD revealed that the rubbing on polymethylene indeed induced a preferential azimuthal alignment of pentacene crystallites. The deposition of pentacene at 25 degrees C led to a twin growth of crystallites with the [110] direction predominately aligned perpendicular to the rubbing direction. In contrast, the pentacene deposition at 50 degrees C produced twinned crystallites of lower twin angle and the [120] direction aligned parallel to the rubbing direction.     

Synthesis,characterization and liquid chromatographic behaviours of a new chemically bonded liquid crystal

A new bonded liquid crystal stationary phase (2OC12) for high-performance liquid chromatography was studied. It resulted from coupling of LiChrospher Si 100 NH2 and a mesogenic carboxylic acid, 4-(4-(4-(3,4-didoceyloxystyrenyl)phenyl-diazenyl)phenyloxy-methylene) benzoic acid (ILC). ILC was characterized with proton NMR and differential scanning calorimetry, while 2OC12 was characterized by solid state 13C NMR and elemental analysis. 2OC12 surface area was determined by the BET method. The chromatographic behaviour of 2OC12 was investigated under both normal- and reversed-phase conditions. The plots of ln k against 1/T showed transition temperatures at 325 and 337 K. Polyaromatic hydrocarbons (PAHs) were separated using hexane, isooctane or hexane-chloroform. Above the transition temperatures, the bonded material exhibited a liquid crystal-like behaviour: (i) the plate number N was always highest possible, and (ii) the more retained the solute the more elongated it was (anthracene is eluted after phenanthrene, chrysene before tetracene, pentacene after dibenzo-a,h-anthracene). Using acetonitrile/water (60/40), reversed-phase data of aromatic hydrocarbons are similar (highest values of N, better resolution below than during the transitions).     

A study into the effect of subtle structural details and disorder on the terahertz spectrum of crystalline benzoic acid

The phonon modes of crystalline benzoic acid have been investigated using terahertz time-domain spectroscopy, rigid molecule atom-atom model potential and plane-wave density functional theory lattice dynamics calculations. The simulation results show good agreement with the measured terahertz spectra and an assignment of the terahertz absorption features of benzoic acid is made with the help of both computational methods. Focussing on the strongest interactions in the crystal, we describe each vibration in terms of distortions of the benzoic acid hydrogen bonded dimers that are present in the crystal structure. The terahertz spectrum is also shown to be highly sensitive to the location of the carboxylic acid hydrogen atoms in the cyclic hydrogen-bonded dimers and we have systematically explored the influence of the observed disorder in the hydrogen atom positions on the lattice dynamics.     

苯甲酸铽络合物发光性能的研究

本文应用紫外、红外吸收光谱、稳态和时间分辨的荧光光谱研究了苯甲酸及它的(邻、间、对)羟基衍生物与铽离子的络合物的发光性能和能量传递关系;探讨了羧酸的结构、络合物的形成,以及溶剂、pH值对发光强度和能量传递的影响。     

Singlet and triplet transitions in UV absorption spectra of pentacene

The UV absorption spectrum of pentacene in hexane solution and the diffuse UV reflectance spectrum for its solid sample have been obtained. A spectral band due to structural features resulting from intermolecular interactions in the solid state was detected for the solid sample at the energy of 1.51 eV. The types of detected electronically excited states for pentacene have been determined, and dominating electronic configurations have been interpreted using TD B3LYP/6-31G quantum-chemical calculations. The types of pentacene occupied and unoccupied molecular orbitals involved in electronic transitions have been determined from published photoelectron spectroscopy data for pentacene with the preliminary assignment of the relevant photoionization bands using the B3LYP/6-31G calculation method.