Ground and excited state electron transfer dynamics
Brennan, Jennifer L. (2002) Ground and excited state electron transfer dynamics. PhD thesis, Dublin City University.
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The focus of this work is the investigation of the factors controlling electron transfer in molecular electronic systems, in particular those affecting electron transfer to and from electronically excited states. To achieve this, a number of mono- and trimetallic osmium and ruthenium complexes were synthesised and characterised. Monolayers of an osmium polypyridyl complex bound to a platinum microelectrode via a ¿rara-l,2-bis-(4-pyridyl)ethylene bridge were formed to probe ground state electron transfer dynamics. This is compared to the rate of photoinduced oxidative electron transfer quenching which occurs in a trimetallic osmium complex where the metal centres are linked by the same bridging ligand. The rate constant for this quenching is 1.3 xlO8 s '1, compared to 2 x 106 s '1 for the ground state process with the same driving force. These investigations show that the strength of coupling across the bpe ligand is higher when it links two metal centres as opposed to when it bridges a metal centre and an electrode. Extensive experiments were carried out to quantify the effect of laser pulses on an unmodified electrode surface. Laser activation improves the heterogeneous kinetics of a solution phase redox probe by removing polishing debris and other adsorbed impurities. Laser-induced current transients observed following a single laser pulse are due to a rapid (jas) restructuring of the double-layer followed by a slow (ms) thermal decay within the metal electrode. A mathematical model has yielded values of the thermal diffusion coefficient as a function of applied potential. To investigate excited state heterogeneous electron transfer, monolayers of a ruthenium polypyridyl complex containing the bridging ligand, 2,2':4,4":4',4"- Quarterpyridyl are used. Using Rehm-Weller calculations, the excited state redox potentials occur a t -0.71 and +1.05 V for oxidation and reduction respectively. Laser excitation of these monolayers in conjunction with high-speed cyclic voltammetry was utilised to attempt to directly measure the excited state redox potentials of this complex. This experiment has not been entirely successful and suggestions for improvements to the experiment are discussed.
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