A. Corval

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.

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.

Proton Motions and Holeburning Mechanisms in Molecular Solids

Abstract Holeburning mechanisms based on proton motions in organic solids are discussed. Studies in model crystalline systems are used to identify and to characterize in detail processes involving methyl group spin conversion, tautomerization, and reversible hydrogen transfer reactions. All these mechanisms should also be operative in glasses.

Hydrogen bond dynamics in benzoic acid crystals

Abstract We report quasi-elastic neutron scattering (QENS) and optical spectroscopic measurements of the proton motion in the hydrogen bond of benzoic acid crystals, a system in which proton transfer occurs in a near-symmetric double-well potential and both coherent and incoherent proton tunneling have been measured. Inter-well relaxation is characterized by a correlation time that determines the width of the QENS line. Our new measurements cover the transition from the classical to the quantum regime. The optical spectroscopic data, using new dopant molecules, extend measurements of the proton dynamics to very low temperatures, where coherent tunneling is observed. Theoretical methods, based on a perturbative instanton approach, have been developed to describe tunneling in multidimensional potential energy surfaces and are used to model our observations.

Long-lived photoinduced proton transfer processes

Irradiation of the phenanthroline–dinitrobenzyl compound 3 generates, via a photoinduced proton transfer process, a long-lived tautomer having a lifetime about 5 × 103 times longer than that of the parent compound 1a.

Relaxation and photochemistry in pentacene doped benzoic acid crystals

Abstract Recent results on the mixed crystal system of pentacene dissolved in benzoic acid provide compelling support for the mechanism that leads, subsequent to light absorption, to the creation of metastable defects in these crystals.

PATHWAYS OF SPONTANEOUS AND PHOTOINDUCED HYDROGEN TUNNELLING IN ORGANIC CRYSTALS

Abstract 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.