Proton tunnelling reactions in pentacene doped benzoic acid crystals Original Research Article

Optical excitation of pentacene, doped into a benzoic acid crystal matrix, induces a reversible proton transfer reaction between the host and the guest. This reaction occurs at low temperatures and leads to the creation of defects corresponding to the displacement, from its regular position, of an acid proton of the host matrix. The formation and evolution of these defects is monitored via the electronic S0 → S1 transition of the pentacene guest. Here we report and discuss a variety of measurements made with the aim of obtaining information about the first step of the reaction. It is shown that the rate for the first step of this reaction is reduced by a factor of about 104 upon deuteration of the host matrix, demonstrating that this step occurs by tunnelling. Other unsuccessful experiments (optical, ESR, magnetic field effects) made to identify the first intermediate of the reaction as well as molecular orbital calculations of potential intermediates are also reported briefly. It is shown that the formation of the pentacene cation, protonated in the centre position, is consistent with all observations. This species is proposed as the most likely first intermediate of the reaction.