H.P. Trommsdorff

The vibrational spectrum of crystalline benzoic acid: Inelastic neutron scattering and density functional theory calculations

Vibrational spectra of several isotopomers of benzoic acid (BA) crystals have been recorded by inelastic neutron scattering and are compared with spectra calculated for different potential energy surfaces (PES). These PES were obtained within the harmonic approximation from quantum chemical density functional theory (DFT) calculations made for the monomer, the isolated dimer, and the crystal using different codes and different levels of basis functions. Without refinement of the force constants, agreement between calculated and observed spectra is already sufficient for an unambiguous assignment of all vibrational modes. The best agreement was obtained with periodic DFT calculations. The most prominent discrepancy between calculated and observed frequencies was found for the out-of-plane O–H bending modes. For these modes (as well as for the in-plane bending and the O–H stretching modes) the anharmonicity of the potential was calculated, and the anharmonic correction was shown to account for about one-thi...

Poly(di‐n‐hexyl‐silane) in solid solutions: Experimental and theoretical studies of electronic excitations of a disordered linear chain

The results of a study of the electronic structure and energy transfer dynamics of poly(di‐n‐hexyl‐silane), representative of a large class of conjugated polymers, in glassy solution at low temperature are reported. Optical excitation at ca. 3500 A yields a Frenkel exciton with possibly some small admixture of charge transfer states. The exciton is only weakly coupled to nuclear motions and small polaron effects are absent except for a slight expansion in the direction perpendicular to the chain axis on excitation. The excitation is localized by a continuous disorder distributed along the chain and apparently not by a lumped disorder in which segments of essentially perfectly ordered regions are separated by defects. The results are consistent with an exciton bandwidth of 4.4 eV. Energy transport between localized states can be understood in terms of a simple kinetic model which permits simulation of results from fluorescence and hole‐burning experiments. A novel mechanism of hole burning occurring in one...

A high resolution, inelastic neutron scattering investigation of tunnelling methyl groups in aspirin

Abstract The tunnel frequency of protonated methyl groups in aspirin has been measured, by inelastic neutron scattering, at 2 K, to be 1.22 μeV. This result and the temperature dependence up to 42 K are in poor agreement with NMR measurements of deuterated methyl groups which conclude that the rotational potential is purely three-fold symmetric. The discrepancy is attributed to a six-fold contribution in the rotational potential for which justification is provided by a calculation of the rotational potential based on the room temperature crystal structure.

Excitation dynamics in polysilanes

Abstract The spectral properties of poly-(di-n-hexyl)-silane, of different molecular weights, dissolved in low temperature glasses are presented and discussed. Laser photolysis produces both narrow holes at the exciting frequency as well as overall changes of the spectrum. The non-local spectral changes reflect the competing processes governing the dynamics of the extended excitations in the linear polymer chain. Numerical modelling of the spectra and the holeburning qualitatively reproduces the observed changes and gives insight in the polymer chain structure and properties.

Tunneling dynamics of delocalized protons in hydrogen bonds at low temperatures

Abstract High-resolution optical holeburning is used to characterize the tunneling potential of delocalized protons in benzoic acid dimers coupled to a dye molecule. The effect of pressure on the system is measured and the relaxation rates between the tunneling levels are determined from the homogeneous linewidths.

Singlet—triplet level crossings in matrix-isolated pentacene

Abstract Level crossings between the first excited singlet and a higher triplet level of pentacene in a single crystal matrix of benzoic acid are observed by monitoring the fluorescence intensity as a function of magnetic field. The resonances are quantitatively analyzed: the spin—orbit coupling matrix elements between the singlet and the triplet sublevels are of the order of 0.1 cm−1 and the decay time of the triplet level exceeds 1.7 ps.

Spectral hole burning: Spontaneous and photoinduced tunneling reactions in low temperature solids

Spectral hole burning is frequently observed in molecular solids when a solute dye molecule in a solvent matrix at very low temperature is irradiated with light having a narrow fiequency bandwidth. This phenomenon relies on photo induced changes of the absorption spectrum of the solute. All photochemical reactions lead in principle to such changes, but the extreme sensitivity of the method makes it particularly suitable for the characterization of reactions that occur at cryogenic temperatures with very small yield and that would be difEcult to observe by conventional methods. Here we will discuss in more detail the contribution of proton tunneling reactions to spectral hole burning and in particular new results obtained in pentacene doped benzoic acid crystals, which is an excellent model system,

Photoinduced proton disorder in pentacene-doped benzoic acid crystals. A model system for hole burning in glasses

Abstract Electronic excitation of dye molecules dissolved in benzoic acid host crystals leads to the appearance of a great variety of metastable defect structures in which the acid protons of the host are displaced from their stable position. Detailed investigations using selectively deuterated pentacene molecules as guests show that reversible hydrogen transfer reactions between the host and the guest are responsible for the appearance of these defects. The ensemble of pentacene molecules in defect environments can be regarded as a disordered solid. This ensemble exhibits many of the characteristics which are commonly observed for dye molecules dissolved in glasses, such as photoinduced and spontaneous site relaxation processes (hole burning, spectral diffusion, etc.). This ensemble, therefore, represents in many regards a model of these more complex amorphous systems.

Deuteration effect on photoinduced hydrogen motions in low temperature mixed molecular crystals

Abstract Irradiation at liquid helium temperatures of benzoic acid crystals doped with pentacene leads to a reversible bleaching of the pentacene absorption and the appearance of defect sites related to a rearrangement of the acid protons of the host. Upon deuteration of these acid protons a partial H/D exchange in pentacene during the crystal growth is observed. Different partially deuterated pentacene molecules give rise to distinct 0-0 bands. Selective and non-selective laser irradiations show that the yield of the reaction leading to the appearance of the defect sites varies strongly as a function of this partial deuteration of the guest pentacene molecule.

Excited states of polysilanes: High-resolution spectroscopy and molecular modeling

High-resolution spectroscopy at low temperatures (1.5–77 K) is used to study the electronic properties of the low-temperature phase of poly-di-n-hexyl-silane in solution. The absorption spectra exhibit an unexpected sensitivity to sample preparation. Experiments probing the energy transfer dynamics are reported and a model for the electronic excitation and the disorder reigning in the polymer is presented.