Direct observation of excited-state dynamics by hot UV absorption spectroscopy after IR multiphoton excitation

Fluoroethylcycloheptatriene has been irradiated by pulses from a TEA CO₂ laser. During and after the pulses, the hot UV absorption of the excited molecules was monitored. At very low gas pressures, time-resolved observation of the rate of unimolecular isomerization of the excited molecules was possible. By adding collision partners, stepwise collisional deactivation of excited molecules was also observed. By analysis of the transient spectra, the intra- and inter-molecular dynamics of the excited molecules was found to be quantitatively consistent with data from single-photon excitation experiments. The dependence of the observed dynamics on the laser fluence is demonstrated.     

Picosecond infrared probing of the vibrational spectra of transients formed upon UV excitation of stacked G-tetrad structures

The photophysical properties of stacked G-tetrads in diverse systems, including concentrated solutions of 5'-guanosine monophosphate (5'-GMP), polyguanylic acid (poly(G)) and the G-rich oligodeoxynucleotide sequence characteristic of human telomeric DNA, are probed by ps-TRIR and compared to those of the monomeric 5'-GMP.     

Ultrafast IR spectroscopy of polymeric cytosine nucleic acids reveal the long-lived species is due to a localised state

The decay pathways of UV-excited cytosine polymers are investigated using picosecond time-resolved infrared spectroscopy. Similar yields of a non-emissive (1)nπ* state are found in the single-stranded dC(30) polymer as in the dCMP monomer, but with a longer lifetime in the polymer (80 ps vs. 39 ps). A longer lifetime is also found in the d(CpC) dinucleotide. No evidence of excimer states is observed, suggesting that localised (1)nπ* excited states are the most significant intermediates present on the picosecond timescale.     

Kinetic study of light-induced polymerization by real-time UV and IR spectroscopy

Real time ultraviolet (RTUV) spectroscopy was used to study the photolysis kinetics of a radical-type morpholino initiator, during the polymerization of a multiacrylate monomer exposed to UV radiation in bulk, in solution, in a polyurethane-acrylate resin, and in a poly(methyl methacrylate) matrix. The photolysis rate constant k was determined from the exponential loss profile recorded; it was found to vary between 0.1 and 3s^?1, depending on the light intensity and on the monomer concentration. The quenching of the photoinitiator excited states by the acrylate monomer was shown to be an important deactivation pathway which substantially reduces the rate of initiation. The observed influence of the film thickness and photoinitiator concentration on the k value were accounted for by the internal filter effect. Conversion versus time curves were recorded by real time infrared (RTIR) spectroscopy for the various systems examined, thus allowing a direct comparison of both the actual polymerization rate and the residual unsaturation content of the cured polymer. Various factors were shown to be responsible for the early stop of the polymerization, such as depletion of the photoinitiator, O₂ inhibition, or vitrification of the polymer. The photoinitiated cationic ring-opening polymerization of a cycloaliphatic diepoxy monomer was also studied in real time by RTUV and RTIR spectroscopy. Despite a very fast photolysis of the triarylsulphonium initiator, the polymerization of the epoxy monomer developed less rapidly than for the acrylic monomer, with shorter kinetic chain lengths. A linear relationship was found to exist between the decay rate constant and the light intensity, for both the radical and the cationic photoinitiators, as expected for a direct photolysis process.     

UV and IR derivative spectroscopy of novolac layers

Thin solid layers of halophenol and cresol novolacs were investigated by UV derivative spectroscopy. Structural differences can be identified. Derivative spectroscopy was useful for the analysis of the OH stretching region of cresol novolac/PMMA blends. Three sub-bands were discussed and a high amount of intramolecular H-bonding for p-cresol novolac was deduced.     

Ultrafast transient mid IR to visible spectroscopy of fully reduced flavins

The light sensing apparatus of many organisms includes a flavoprotein. In any spectroscopic analysis of the photocycle of flavoproteins a detailed knowledge of the spectroscopy and excited state dynamics of potential intermediates is required. Here we correlate transient vibrational and electronic spectra of the two fully reduced forms of flavin adenine dinucleotide (FAD): FADH(-) and FADH(2). Ground and excited state frequencies of the characteristic carbonyl modes are observed and assigned with the aid of DFT calculations. Excited state decay and ground state recovery dynamics of the two states are reported. Excited state decay occurs on the picosecond timescale, in agreement with the low fluorescence yield, and is markedly non single exponential in FADH(-). Further, an unusual 'inverse' isotope effect is observed in the decay time of FADH(-), suggesting the involvement in the radiationless relaxation coordinate of an NH or hydrogen bond mode that strengthens in the excited electronic state. Ground state recovery also occurs on the picosecond time scale, consistent with radiationless decay by internal conversion, but is slower than the excited state decay.     

IR multiple-photon excitation distribution probed by IR fluorescence

Vibrational fluorescence induced by IR multiple-photon excitation of hexafluorocyclobutene was measured as a function of laser frequencies. The width and shift of the inhomogeneous broadened fluorescence bands represent the excitation energy. The contrasting shapes of the excitation and previously measured chemical yield spectra are explained by the Poisson distribution of the molecules in the quasicontinuum.     

Determination of the tautomeric structures of uracil thiosubstituted derivatives by IR and RRS spectroscopy

The force fields, vibration frequencies, and relative intensities of the RRS spectra of uracil thiosubstituted derivatives are calculated and analyzed. It is shown that 2S-, 4S-and 2S,4S-thiouracils as well as 2-mercapto-4-thio-and 2,4-dimercapto-tautumers of dithiouracil occur in a neutral water solution.     

Chemistry of naphthazarin derivatives. 7. Determination of structures of substituted 2,6(7)-dihydroxynaphthazarins by UV and IR spectroscopy

A set of substituted 2,6- and 2,7-dihydroxynaphthazarins were synthesized. The difference in the UV spectra of alkaline alcoholic solutions of 2,6- and 2,7-dihydroxynaphthazarins allows the reliable differentiation of the structures of these compounds. The IR spectra of 2,6- and 2,7-dihydroxynaphthazarins have characteristic nonoverlapping intervals of stretching vibration frequencies of the -hydroxy groups. Based on these spectral regularities, the data on cuculoquinone, which has been isolated previously from lichen Cetraria cucullata and has been identified as 7,7"-bis(3-ethyl-2,5,6,8-tetrahydroxynaphthalene-1,4-dione), were revised and the structure of 6,6"-bis(3-ethyl-2,5,7,8-tetrahydroxynaphthalene-1,4-dione) was assigned to this compound.     

A study of pyridine adsorption on supported vanadium-aluminium catalysts by IR and UV spectroscopy

The acidic properties of supported vanadium-aluminum catalysts have been studied by the IR and UV spectroscopy of adsorbed pyridine. Introduction of vanadium ions does not result in the appearance of acidity of the Brönsted type in the temperature range under study (25–500°C). The adsorption of pyridine on samples containing large amounts of V₂O₅ reversibly increases the transmission in the IR region, which is due to the decrease of current carrier concentration. On pure Al₂O₃, pyridine is oxidized beginning with 400°C; V₂O₅ decreases the temperature of the beginning of oxidation to 200–300°C.

---

- . (25°–500°C). , V₂O₅, -, . Al₂O₃ 400°C , V₂O₅ 200–300°C.

     

Ultrafast formation of transients in spiropyran photochromism

Transient absorption spectra in the photocolouration and photobleaching of the indoline 6-nitrosubstituted spiropyran have been investigated with picosecond spectroscopy. The cis—cisoid intermediate stereoisomer at 440 nm and some of the final coloured forms are produced in less than 10 ps in the photocolouration.     

Langmuir-Blodgett films formed by continuously varying surface pressure. Characterization by IR spectroscopy and epifluorescence microscopy

Monolayer films of phospholipids at the air-water interface have been transferred to solid substrates under conditions of continuously varying surface pressure, an approach termed COVASP. The molecular and supramolecular properties of the film constituents have been characterized with two complementary techniques. IR spectroscopy was used to monitor chain conformation as a function of transfer surface pressure. Results were compared to those from Langmuir films determined directly at the A/W interface by IR reflection-absorption spectroscopy (IRRAS). The methylene stretching frequencies for both proteated and acyl chain perdeuterated 1,2-dipalmitoylphosphatidylcholine (DPPC and DPPC-d62) in the transferred molecules indicate that the phospholipids retain at least, in part, their surface pressure-dependent chain-conformational order characteristics. The line widths of these modes are somewhat reduced, suggestive of slower rates of reorientational motion in the Langmuir-Blodgett (LB) films. Epifluorescence microscopy reveals a progressive condensation gradient, including nucleation and growth of probe-excluding condensed domains along the transfer line. DPPC condensation, observed along a single LB film, was qualitatively comparable to compression-driven condensation as observed in situ or in conventional LB films transferred at constant pressures. However, condensation along the compression isotherm in COVASP-LB films was reduced by 15-20% as compared to films equilibrated at different constant pressures, probably the result of kinetic differences in equilibration processes. As a preliminary demonstration of the utility of this new approach, the monolayer --> multilayer transition known to occur (Eur. Biophys. J. 2005, 34, 243) in a four-component model for pulmonary surfactant has been examined. IR parameters from both the lipid and the protein constituents of the film all indicate that the transition persists during the transfer process. This new approach for the study of transferred films will permit the efficient characterization of lipid-protein interactions and structural transitions occurring in pulmonary surfactant films subjected to dynamic compression.     

Probing the glycosidic linkage: UV and IR ion-dip spectroscopy of a lactoside

The beta(1-->4) glycosidic linkage found in lactose is a prevalent structural motif in many carbohydrates and glycoconjugates. Using UV and IR ion-dip spectroscopies to probe benzyl lactoside isolated in the gas phase, we find that the disaccharide unit adopts only a single, rigid structure. Its fully resolved infrared ion-dip spectrum is in excellent agreement with that of the global minimum structure computed ab initio. This has glycosidic torsion angles of phi(H) (H1-C1-O-C4') approximately 180 degrees and psi(H) (C1-O-C4'-H4') approximately 0 degrees which correspond to a rotation of approximately 150 degrees about the glycosidic bond compared to the accepted solution-phase conformation. We discuss the biological implications of this discovery and the generality of the strategies employed in making it.     

Ion selectivity of crown ethers investigated by UV and IR spectroscopy in a cold ion trap

Electronic and vibrational spectra of benzo-15-crown-5 (B15C5) and benzo-18-crown-6 (B18C6) complexes with alkali metal ions, M(+)?B15C5 and M(+)?B18C6 (M = Li, Na, K, Rb, and Cs), are measured using UV photodissociation (UVPD) and IR-UV double resonance spectroscopy in a cold, 22-pole ion trap. We determine the structure of conformers with the aid of density functional theory calculations. In the Na(+)?B15C5 and K(+)?B18C6 complexes, the crown ethers open the most and hold the metal ions at the center of the ether ring, demonstrating an optimum matching in size between the cavity of the crown ethers and the metal ions. For smaller ions, the crown ethers deform the ether ring to decrease the distance and increase the interaction between the metal ions and oxygen atoms; the metal ions are completely surrounded by the ether ring. In the case of larger ions, the metal ions are too large to enter the crown cavity and are positioned on it, leaving one of its sides open for further solvation. Thermochemistry data calculated on the basis of the stable conformers of the complexes suggest that the ion selectivity of crown ethers is controlled primarily by the enthalpy change for the complex formation in solution, which depends strongly on the complex structure.     

UV spectroscopy of monosubstituted derivatives of 1,2-dihydro-C60-fullerenes

UV spectroscopy is used to determine the molar absorption coefficients of C₆₀ fullerene and monosubstituted 1,2-dihydro-C₆₀-fullerenes in different solvents. It is found that the extinction coefficient of C₆₀ at 330 nm (the main absorption band most frequently used for qualitative and quantitative determination of the C₆₀ content) is independent of the nature of the solvent and is ～54400 M^?1·cm^?1. The molar absorption coefficients of a series of monosubstituted 1,2-dihydro-C₆₀-fullerenes are practically independent of the chemical structure and the length of the substituent and are 35700 M^?1·cm^?1 (λ ～ 328 nm) and 115250 M^?1·cm^?1 (λ ～ 257 nm). It is shown that the substitution in fullerene proceeds via the double 6,6 bond, as evidenced by the absorption band at 424 nm in the spectra of these compounds, which is characteristic of monosubstituted methanofullerenes.