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Time-resolved thermodynamic parameters in donor-acceptor super-molecules and in photo-switches, analysed by laser-induced opto-acoustic spectroscopy


Photoexcitation of biomimetic molecules, e.g. undergoing an electron transfer (ET) reaction or an isomerization, leads to optically well characterized intermediates. However, the phototransformation thermodynamic parameters, e.g. the enthalpy and entropy changes are not readily obtained. Laser-induced optoacoustic spectroscopy (LIOAS) is a suitable technique to study time-resolved thermodynamic profiles of photoreactions and has been successfully applied to molecular systems and biological photoreceptors. We plan to apply LIOAS to (1) carotene-porphyrin-acceptor 'supermolecules' (important in artificial photosynthesis), which on excitation produce relatively long-lived (hundreds of nanoseconds) charge separated states with large dipole moment changes and ( 2) closed-ring photoswitches which on excitation lead to long-lived (up to milliseconds) open states with smaller dipole moment changes. The measurements will be carried out with the molecules dissolved in a polar solvent incorporated into detergent micelles acting as nano-reactors, in turn suspended in aqueous medium. Varying the temperature permits separating the contributions from the heat and the structural volume change to the signal. The rates of the ET reactions are determined by the reorganization energy (a vibrational plus a solvent term). LIOAS measures the structural volume changes upon excitation, as a consequence of changes in bond lengths and angles plus interactions with the medium. The possible relationship between these volume changes and the Marcus reorganization energy will be analysed. The study of the photoswitches will allow the comparison between systems undergoing large and smaller dipole moment changes. These studies will also permit to solve methodological problems, e.g. the determination of the reaction quantum yields in the micelles by optical methods and the calibration of the size of the molecules that can be incorporated in the micelles without perturbing their physical properties.

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