Proton transfer along molecular wires

A major challenge to generate green energy from sunlight would be to split water which is an ubiquitous (on the Earth) molecule to produce H₂. However VUV light (6.66 eV) is needed to dissociate the H-OH covalent bond. In this work we have shown that it is possible to dissociate water via photo-sensitisation, with UV light, of a simple organic catalyst. Recently, ab-initio calculations predicted that pyridine (Py) can act as a photo-catalyst to split water by absorption of a UV photon. To test this prediction, we study the Py-H₂O complex in isolated cold molecular cluster. The setup is composed of a pulsed supersonic expansion combined with a time of flight mass spectrometer. We performed two different types of experiments. In one, we characterized the electronic spectroscopy of the pyridinyl (PyH^.) radical in the gas phase, produced by a pulsed high voltage discharge placed after the supersonic expansion nozzle. In the second one, we evidenced the reaction through UV excitation of Py-(H₂O)_n clusters monitoring the PyH^. reaction product. For this, we performed a two lasers (pump-probe) experiment, where the pump laser excites Py and the PyH^. is probed by ionization. We have evidenced that the reaction leads to PyH^. as photo-product of the reaction. We have also evidenced that the photo-product does not come from the 1-1 complex, but larger clusters are needed to trap the excess of energy of the photo-product released during the radicals production, in a competition with the stabilization of PyH^. the recombination to Py-H₂O. We have also tested the system trapped in cryogenic matrices in order to check the environment effect. These experiments showed that, whatever the matrix, UV irradiation leads the 'exotic' isomer of the Py, called Dewar Py. Even when we cannot dismiss the formation of PyH^., the fact we are not able to record time-resolved (ps) spectra avoid any observation of this latter species. The cage effect certainly favor fragments recombination, and then isomerization of parent Py toward Dewar form, or recovering of the initial state. We have demonstrated that the UV excitation of Py-(H₂O)n clusters leads to the formation of PyH^., thus we have evidenced the water splitting reaction. The last step of the process, that is the regeneration of the catalyst by absorption of a second UV photon, has been studied and it seems that only hot PyH^. molecules are able to follow this channel. At the present, we are studying bigger aromatic systems which are expected to lead to the photochemical indirect dissociation of water using less energy. (author)