TRANSFERT PHOTOINDUIT DE PROTONS DANS DES CRISTAUX MIXTES PENTACENE/ACIDE BENZOIEQUE CARACTERISE PAR SPECTROSCOPIE OPTIQUE Ⅱ. MISE EN EVIDENCE ET CARACTERISATION DE LA REACTION PHOTOINDUITE DE TRANSFERT DE PROTONS. EFFETS DE LA DEUTERATION

摘要

Optical excitation at low temperature of pentacene guest molecules in benzoic acid crystals leads to the appearance of new absorption lines corresponding to defect sites (photosites). These defects, which disappear spontaneously in the dark, are due to a displacement from their equilibrium position of the acid protons of the host. We show that the deuteration of the acid host protons lowers the yield of forming photosites by a factor of ca. 10~4 and reduces the rate of decay by factors of 10~3 to 10~5, indicating that these processes are dominated by tunneling. The reaction mechanism of creating defects has been elucidated by measurement on partially deuterated pentacene molecules: the reaction takes place out of the first excited singlet state and the reactivity varies strongly as a function of the number and the position of H/D replacements in pentacene. The reaction mechanism proposed is the transfer of a proton from the matrix to the dye forming a very unstable proto-nated complex of pentacene, which relaxes to ground state pentacene and proton in a defect position of the matrix. This crystalline system may serve as a model for the study of photophysical holeburning and spectral diffusion processes in amophous solids.