Photoexcited Molecules Research Articles

Photopolymerization of N,N‐dimethylaminoethylmethacrylate studied by photocalorimetry

AbstractPhotopolymerization of dimethylaminoethylmethacrylate (DMAEMA) is studied in bulk and in solutions in the presence of different photoinitiators using differential photocalorimetry (DPC). The rate of DMAEMA photopolymerization is slow compared to that of alkylmethacrylates. Bimodal DPC curves of DMAEMA photopolymerization in bulk are obtained. The type I photoinitiators (IRGACURE® 651 and IRGACURE® 1700), which produce free radicals by homolytic fragmentation of photoexcited molecules, are more effective in promoting photopolymerization of DMAEMA. The type II photoinitiators (benzophenone and IRGACURE® 500), which initiate DMAEMA photopolymerization through an H‐abstraction mechanism involving an amino group from the monomer (polymer), are less efficient and favor the formation of partly crosslinked products. The effects of the photoinitiator concentration, temperature, and solvent on the kinetic data are evaluated and discussed. A scheme of photopolymerization of DMAEMA, including the formation of intermediate DMAEMA based macromonomers, is proposed. The main point of the suggested scheme is a high chain transfer to the DMAEMA monomer. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 579–588, 2002

Excited state molecular structure determination in disordered media using laser pump/X-ray probe time-domain X-ray absorption spectroscopy.

Advances in X-ray technologies provide opportunities for solving structures of photoexcited state molecules with short lifetimes. Using X-ray pulses from a modern synchrotron source, the structure of a metal-to-ligand-charge-transfer (MLCT) excited state of Cu1(dmp)2+ (dmp = 2,9-dimethyl-1,10-phenanthroline) was investigated by laser pump/X-ray probe X-ray absorption fine structure (LPXP-XAFS) in fluid solution at room temperature on a nanosecond time scale. The experimental requirements for such pump probe XAFS are described in terms of technical challenges: (1) conversion of optimal excited state population, (2) synchronization of the pump laser pulse and probe X-ray pulse, and (3) timing of the detection. Using a laser pump pulse for the photoexcitation, a photoluminescent MLCT excited state of Cu1(dmp)2(BArF), (dmp = 2,9-dimethyl-1,10-phenanthroline, BArF = tetrakis(3,5-bis(trifluoromethylphenyl)borate) with a lifetime of 98 +/- 5 ns was created. Probing the structure of this state at its optimal concentration using an X-ray pulse cluster with a total duration of 14.2 ns revealed that (1) a CuII center was generated via a whole charge transfer; (2) the copper in the MLCT state bound an additional ligand to form a penta-coordinate complex with a likely trigonal bipyramidal geometry; and (3) the average Cu-N bond length increases in the MLCT excited state by 0.07 A. In contrast to previously reported literature, the photoluminescence of this pentacoordinate MLCT state was not quenched upon ligation with the fifth ligand. On the basis of experimental results, we propose that the absorptive and emissive states have distinct geometries. The results represent X-ray characterization of a molecular excited state in fluid solution on a nanosecond time scale.

Solvent reorganization controls the rate of proton transfer from neat alcohol solvents to singlet diphenylcarbene.

Femtosecond transient absorption spectroscopy was used to study singlet diphenylcarbene generated by photodissociation of diphenyldiazomethane with a UV pulse at 266 nm. Absorption by singlet diphenylcarbene was detected and characterized for the first time. Similar band shapes were observed in acetonitrile and in cyclohexane with lambda(max) approximately 370 nm. The singlet absorption decays by intersystem crossing to triplet diphenylcarbene at rates that agree with previous measurements. The singlet absorption band is completely formed 1 ps after the pump pulse. It is preceded by a strong and broad absorption band, which is tentatively assigned to excited-state absorption by a singlet diazo excited state. In neat alcohol solvents the growth and decay of the diphenylmethyl cation was observed. This species is formed by proton transfer from an alcohol molecule to singlet diphenylcarbene. Since a shell of solvent molecules surrounds each nascent carbene, the intrinsic rate of protonation in the absence of diffusion could be measured. In methanol, proton transfer occurs with a time constant of 9.0 ps, making this the fastest known intermolecular proton-transfer reaction to carbon. In O-deuterated methanol proton transfer occurs in 15.0 ps. Slower rates were observed in the longer alcohols. The protonation times correlate reasonably well with solvation times in these alcohols, suggesting that solvent fluctuations are the rate-limiting step. In all alcohols studied, the carbocations decay on a somewhat slower time scale to yield diphenylalkyl ethers. In methanol and ethanol the rate of decay is determined by reaction with neutral solvent nucleophiles. There is evidence in 2-propanol that geminate reaction within the initial ion pair is faster than reaction with solvent. No isotope effect was observed for the reaction of the diphenylmethyl carbocation in methanol. Using comparative actinometry the quantum yield of protonation was measured. In methanol, the quantum yield of carbocations reaches a maximum value of 0.18 approximately 18 ps after the pump pulse. According to our analysis, 30% of the photoexcited diazo precursor molecules are eventually protonated. Somewhat lower protonation efficiencies are observed in the other alcohols. Because the primary quantum yield for formation of singlet diphenylcarbene is still unknown, the importance of reaction channels that might exist in addition to protonation cannot be determined at present. Singlet carbenes are powerful, photogenerated bases that open new possibilities for fundamental studies of proton transfer in solution.

Experimental evidence for sub-3-fs charge transfer from an aromatic adsorbate to a semiconductor

The ultrafast timescale of electron transfer processes is crucial to their role in many biological systems and technological devices. In dye-sensitized solar cells, the electron transfer from photo-excited dye molecules to nanostructured semiconductor substrates needs to be sufficiently fast to compete effectively against loss processes and thus achieve high solar energy conversion efficiencies. Time-resolved laser techniques indicate an upper limit of 20 to 100 femtoseconds for the time needed to inject an electron from a dye into a semiconductor, which corresponds to the timescale on which competing processes such as charge redistribution and intramolecular thermalization of excited states occur. Here we use resonant photoemission spectroscopy, which has previously been used to monitor electron transfer in simple systems with an order-of-magnitude improvement in time resolution, to show that electron transfer from an aromatic adsorbate to a TiO(2) semiconductor surface can occur in less than 3 fs. These results directly confirm that electronic coupling of the aromatic molecule to its substrate is sufficiently strong to suppress competing processes.

Photooxidation of n-heptanal in air: Norrish type I and II processes and quantum yield total pressure dependency

Dilute mixtures of n-heptanal in synthetic air (up to 100 ppm) were photolyzed with fluorescent UV lamps (275–380 nm) at 298 K. The main photooxidation products, identified and quantitatively analyzed by FTIR spectroscopy, were pent-1-ene, CO, vinyl alcohol and ethanal. The photolysis rates and the absolute quantum yields Φ were found to be slightly dependent on the total pressure. At 100 Torr, Φ100 = 0.36 ± 0.03, whereas at 700 Torr the total quantum yield was Φ700 = 0.31 ± 0.01. The results may be explained by the collisional deactivation of photoexcited molecules. Two decomposition channels were identified: the radical channel C6H13CHO → C6H13 + HCO, and the molecular channel C6H13CHO → C5H10 + CH2CHOH, having the absolute quantum yields of 0.031 and 0.118 at 700 Torr. The product CH2CHOH tautomerizes to ethanal.

Pressure dependence of the photooxidation of selected carbonyl compounds in air: n-butanal and n-pentanal

Dilute mixtures of n-butanal and n-pentanal in synthetic air (up to 100ppm) were photolyzed with fluorescent UV lamps (275–380nm) in air at 298K. The main photooxidation products, identified and quantitatively analyzed by FT-IR spectroscopy, were ethene (n-butanal) or propene (n-pentanal), CO, vinylalcohol and ethanal. The photolysis rates and the absolute quantum yields Φ of both compounds were found to be slightly dependent on the total pressure. At 100Torr, in the photolysis of n-butanal and n-pentanal Φ100=0.48±0.02 and 0.40±0.04, whereas at 700Torr, the total quantum yields were Φ700=0.38±0.02 and 0.32±0.01, respectively. These results could be explained by the collisional deactivation of photoexcited molecules. Two decomposition channels were identified in both cases: the radical channel RCHO→R+HCO, and the molecular channel CxHyCHO→Cx−1Hy−3+CH2CHOH, having the relative yields of 68±3 and 32±3% (n-butanal), and 20±4 and 80±4% (n-pentanal) at 700Torr. The product, CH2CHOH, tautomerizes to ethanal. UV absorption cross-sections were measured prior to photolysis. For the comparison, the products were also analyzed after photolysis of both compounds in teflon bags using direct sunlight.

Cold rubidium molecule formation through photoassociation: A spectroscopic study of the 0g- long-range state of 87Rb 2

We report the detailed analysis of translationally cold rubidium molecule formation through photoassociation. Cold molecules are formed after spontaneous decay of photoexcited molecules from a laser cooled atomic sample, and are detected by selective mass spectroscopy after two-photon ionization into Rb 2 + ions. A spectroscopic study of the 0 g - (5 S + 5 P 3/2 ) pure long-range state of 87Rb2 is performed by detecting the ion yield as a function of the photoassociation laser frequency; the spectral data are theoretically analyzed within the semiclassical RKR approach. Molecular ionization is resonantly enhanced through either the 2 3 Π g or the 2 3 Σ + g intermediate molecular states. Some vibrational levels of the latter electronic state are observed and assigned here for the first time. Finally, cold molecules formation rates are calculated and compared to the experimentally measured ones, and the vibrational distribution of the formed molecules in the a 3 Σ + u ground triplet state is discussed.

Supra Optical Nonlinearities (SON) of Methyl Red- and Azobenzene Liquid Crystal - Doped Nematic Liquid Crystals

In nematic liquid crystal films doped with methyl-red dye or azobenzene liquid crystal, we have observed extremely efficient optically induced director axis reorientation effects, and refractive index change coefficients >> 1 cm2/W. The basic mechanisms involved are determined to be optically induced space charge fields and flow reorientation, order parameter modification and molecular torques by the photo-excited dopant molecules. These supra-optical nonlinearities (SONs) enable nonlinear image conversion, optical limiting and holographic grating formation with μW power lasers.

Photooxidation of n-Hexanal in Air

Dilute mixtures of hexanal in synthetic air (up to 100 ppm) were photolyzed with fluorescent UV lamps (275-380 nm) in air at 298 K. The main photooxidation products, identified and quantitatively analyzed by FTIR spectroscopy, were butene, CO, vinylalcohol and ethanal. The photolysis rates and the absolute quantum yield Φ were found to be slightly dependent on the total pressure. At 100 Torr, Φ100 = 0.43 ± 0.02, whereas at 700 Torr the total quantum yield was Φ700 = 0.38 ± 0.02. These results may be explained by the collisional deactivations of photoexcited molecules. Two decomposition channels were identified: the radical channel C5H11CHO → C5H11 + HCO, and the molecular channel C5H11CHO → C4H8 + CH2=CHOH, having the relative yields of 27 and 73 % at 700 Torr. The product CH2=CHOH tautomerizes to ethanal.

Optical transitions in para-phenylenes under hydrostatic pressure

Photo excited para-phenylene molecules produce strong emission from singlet excitons, bleaching of singlet exciton absorption, and induced absorption from triplet excitons and polarons. Using hydrostatic pressure, which decreases the intermolecular separation and changes the molecular geometry, we probe these states to unravel the effects of changing inter-ring and interchain separations.

Sequence specificity of alkali-labile DNA damage photosensitized by suprofen.

On irradiation at UVB wavelengths, in aerated neutral aqueous solution, the anti-inflammatory drug suprofen (SP) photosensitizes the production of alkali-labile cleavage sites in DNA much more efficiently than direct strand breaks. It is active at submillimolar concentrations despite having no significant binding affinity for DNA. Gel sequencing studies utilizing 32P-end-labeled oligonucleotides have revealed that piperidine-sensitive lesions are formed predominantly at the positions of guanine (G) bases, with the extent of modification being UV dose- and SP concentration-dependent. Quite distinct patterns of G-specific damage are observed in single-stranded and duplex DNA molecules. The uniform attack at all G residues in single-stranded DNA, which is enhanced in D2O, is compatible with a Type-II mechanism. SP is a known generator of singlet oxygen whose participation in the reaction is supported by the effects of quenchers and scavengers. In duplex DNA, piperidine-induced cleavage occurs with high selectivity at the 5'-G of GG and (less prominently) GA doublets. This behavior is characteristic of a Type-I process involving electron transfer from DNA to photoexcited SP molecules. The ability of SP to sensitize the formation of Type-I and Type-II photo-oxidation products from 2'-deoxyguanosine attests to the feasibility of competing mechanisms in DNA.

Bis-N-nitroso-caged nitric oxides: photochemistry and biological performance test by rat aorta vasorelaxation

Three new caged nitric oxides (NOs)—BNN3, BNN5Na, and BNN5M—were tested for biological use. BNNs have a strong ultraviolet (UV) absorption band (λmax: 300 nm, ε: 13.5mM−1cm−1) extended to 420nm and produce NO upon irradiation with 300–360nm light in quantum yields about 2. A photoexcited BNN molecule yields two NOs with time constants of less than 10ns for phase 1 and less than 20μs for phase 2 at 37°C, suggesting usefulness of BNNs for measuring in vivo and in vitro fast NO reactions. Upon irradiating with UV light, caged nitric oxides-loaded rat aortic strips maintained in a state of active tonic contraction effectively relaxed (<3μM BNN5M loading solution concentration). BNN3 is incorporated in the lipid membrane. BNN5Na, insoluble in organic solvents but water soluble, localizes in the water phase. BNN5M, is muscle-cell-permeable and hydrolysed to BNN5Na to remain in cytosol. BNNs were thermally stable and demonstrated no observable toxicity.

Dye sensitization of synthetic p-type diamond electrode

A synthetic semiconductive p-type diamond electrode was photosensitized by the Ru(bpy)32+ molecule in an electrolyte solution. The photoexcited molecule injects a hole into the valence band. However, under a certain potential, charge recombination occurred through interband states; the hole on the top of the valence band recombined with the photoreduced Ru(bpy)31+. The mechanism of this charge recombination was studied by measuring the transient photopotential and the intrinsic sub-band gap photoresponse of the electrode.

Anisotropy of quasiatomic Auger electrons in fast dissociation of photoexcited molecules

The angular distribution of photoproduced quasiatomic Auger electrons following fast dissociation of core-excited diatomic molecules is studied theoretically. Within the two-step approach the anisotropy of the Auger electrons is determined by the alignment of the atomic fragment, expressed in terms of photodissociation amplitudes. Numerical calculations are carried out for hydrogen halide molecules, and large alignment transfer from the transient molecule to the fragment core-excited atom is predicted. The results are compared with recent experimental data for HCl.

Fluorescence quenching induced by injected carriers in organic thin films

The effect of injected charges on fluorescence of organic thin films consisting of the donor molecule tetra(N,N-diphenyl-4-aminophenyl)ethylene (TTPAE) was investigated. Fluorescence was efficiently quenched by charge injection. By comparison of fluorescence measurements and current measurements, it was found that hole injection had essential role in fluorescence quenching and electron injection did not. Coulomb interaction between photoexcited molecules and injected carriers was found to cause fluorescence quenching.

Optical Nonlinearity by Photoinduced Variation of Intermolecular Forces in Liquids and Liquid Crystals

Recent experimental and theoretical results on the optical nonlinearity associated with molecular reorientation in dye-doped liquids and liquid crystals are reviewed. The variation of intermolecular forces experienced by a photo-excited dye molecule and the resulting change of molecular-statistical parameters such as rotational friction and orientational energy appear to be at the core of this effect. Photoinduced intramolecular charge transfer could be the cause of such variation. The angular momentum conservation is also discussed.

Ultrafast study of interfacial electron transfer between 9-anthracene-carboxylate and TiO2 semiconductor particles

The excited state dynamics of 9-anthracene-carboxylic acid adsorbed onto the surface of TiO2 semiconductor particles were examined with ca. 250 fs time resolution. A combination of transient absorption and time-resolved anisotropy measurements show that approximately 76% of the photo-excited dye molecules transfer an electron to the TiO2 particles. The time scale for the forward electron transfer reaction was determined to be ⩽1 ps. The 9-anthracene-carboxylate radical cations produced by this reaction undergo back electron transfer on a 54 ps time scale. A more accurate estimate of the forward electron transfer reaction time is not possible, due to the contribution to the transient absorption signal from adsorbed dye molecules that do not transfer electrons to TiO2. These nonreactive species are deactivated by either nonradiative decay or fluorescence emission. The fluorescence spectrum from the dye molecules bound to the TiO2 particles is very different to that of the free dye in solution. The free dye has a broad red-shifted spectrum, whereas, the adsorbed molecules have a structured spectrum that displays a small Stokes shift. The red shift in the free dye fluorescence spectrum is due to stabilization of the excited electronic state through torsional motion of the carboxylate group. This motion cannot occur when the dye is bound to the particle surface. Thus, the excited molecules emit from a nuclear configuration that is similar to the ground-state geometry, producing a structured fluorescence spectrum. The dual behavior of the adsorbed dye molecules (electron transfer versus nonradiative decay/fluorescence) is attributed to the existence of two different sites for adsorption at the surface of the TiO2 particles: electron transfer can occur from one site but not the other.

A kinetic model for photochemical vapor deposition from germane and silane

A kinetic model based on the collision theory of chemical reactions is proposed for laser-induced photochemical vapor deposition from germane and silane. Photochemical deposition is composed of two processes: extrinsic photochemical deposition from photo-excited molecules and intrinsic thermal deposition from normal reactant molecules. The growth rates of both photochemical deposition and thermal deposition are derived by means of statistical physics. Photochemical deposition dominates at low temperatures, and thermal deposition becomes prominent as the temperature rises. The transition temperature from photochemical deposition to thermal deposition is obtained as a function of fraction of photo-excited molecules and activation energies of photochemical deposition and thermal deposition. Finally, the model is extended to lamp-induced photochemical vapor deposition from silane.

Vibronic effects as a clue to the solution of the selectivity problem of semiconductor gas sensors

The equilibrium electrophysical parameters (electroconductivity work function) of semiconductor gas sensor surfaces are insufficient for the characterization of adsorption selectivity. The energy spectrum of surface electron states corresponding to the gas and vapour adsorption is quasicontinuous and does not reflect the individual properties of detecting molecules properly. Some types of vibronic (electron–vibrational) interactions arising in adsorption charging of the sensor surface are proposed to produce selectivity. Some vibronic effects are examined in the following situations: (1) At the adsorption of excited gas molecules (O v=1 2, Zn/ZnO); (2) at the vibrational excitation of detecting molecules in deactivation events of previously adsorbed photoexcited dye molecules (H 2O, D 2O rhodamine/Ge); and (3) at metal or semiconductor clusters deposited on sensor surface during the adsorption of detecting molecules (CO.Rh/SnO 2 Pd/SnO 2).

Effect of an external electric field on photoinduced interlayer electron transfer in Langmuir–Blodgett films as revealed by the effect of the field on fluorescence: a D–A system of thiacyanine and viologen

Electron donor and acceptor layers separated by a spacer layer were prepared with thiacyanine and viologen using the Langmuir–Blodgett technique, and the effect of an external electric field on the fluorescence of thiacyanine was examined. Interlayer electron transfer from the photoexcited molecules of thiacyanine to viologen was affected by the external electric field. The magnitude and direction of the field-induced change in the electron transfer rate were evaluated on the basis of measurements of the field-induced change in the fluorescence intensity combined with fluorescence decay measurements. © 1997 Elsevier Science S.A.