| Summary: | This thesis is concerned with three distinct but interrelated problems in inorganic photophysics, namely (i) the photosensitisation of an important alkylcobalt(III) complex, (ii) the photophysics and photoelectron transfer reactions of two bis(bipyridine) ruthenium(II) complexes and (iii) a preliminary study of a luminescent binuclear complex of platinum. The first two problems comprise the major part of the work and the literature survey is focused on these. The principal methods of investigation have been 347 nm laser flash photolysis (in emission and absorption), fluorimetry and quantum yield determination. Photosensitisation of Alkylcobalt(III) Complexes: While alkyl- cobalamins and their model compounds, the alkylcobaloximes are known to photodissociate in high yield under visible light irradiation, the multiplicity and energy of the photoreactive state have not been identified. Now acidopentammine complexes of cobalt(III) are known to be photosensitised both by organic triplet states and by the 'solar energy' complex [Ru(bpy)3]2+, indicating the dissociating state to have triplet character and to be of lower energy than signified by the absorption maxima. Accordingly we investigated the possibility of such sensitisers functioning in the case of the organocobalt(III) compounds both by measuring the kinetics of interaction between the donor triplet states and the alkylcobalt(III) compound by laser flash photolysis, which indicated that while donors with ET > 170 kJ mol-1 were quenched very effectively, those with low values of ET (< 110 kJ mol-1), such as rubrene and 6-carotene, where virtually without effect (kq < 107 dm3 mol-1 s-1). The detailed picture, covering a comprehensive range of triplet donors, enables a Wilkinson plot to be compiled and hence a clear picture of the energies of the active states of the alkylcobalt(III) complex. This work was extended to a range of inorganic donors of widely varying reduction potential from which it became clear that the principal process of quenching is energy- rather than electron- transfer (although the latter might participate in favoured cases). A comparative study was carried out on tris-(acetylacetonato) cobalt(III). The laser kinetic studies were augmented by a series of quantum yield experiments using sensitisers with Sj absorption well to the red of the band maxima of the CT transitions of the R-Co(III) species, but with very high values of φT. Photophysics and Photoelectron Transfer Reactions of Bis(polypyridyl) Ruthenium(11) Complexes: We examined the temperature dependence of the luminescence lifetimes of the complexes [Ru(bpy)2acac]+ and [Ru(bpy)2en]2+ from 77 to 300 K with the view of elucidating whether their comparatively very short lifetimes at ambient temperature are due to small activation barriers to deactivation, as shown by two groups for the well-known complex [Ru(bpy)3]2+, or to large pre-exponential terms. In fact the activation barriers are very much lower than for [Ru(bpy)3]2+ and the frequency terms are also somewhat lower, the overall behaviour is thus energy-controlled. However, sizeable solvent-isotope effects (up to ca. 2) are found indicating a considerable amount of CTTS character in the electronic transition responsible for the deactivation step. Studies of the quenching of these same Ru(II) complexes by a wide variety of electron acceptors, combined with electrochemical and lifetime measurements, indicates that whilst the behaviour of [Ru(bpy)2acac]+ follows Weller theory for excited- state electron-transfer very closely, as does [Ru(bpy)3]2+, the result for [Ru(bpy)2en]2+ indicates a generally much faster quenching than predicted for full electron-transfer, indicating the possibility of either non-radiative exciplex formation or an initial one electron oxidation of the ligand, ethylenediamine. Luminescence from a Binuclear Platinum Complex: The complex [Pt(PPh3)2]2 luminesces not only at low-temperature (77 K), t 660 nm in glassy solvents (as recorded before), but also at ambient temperatures at 'v 460—520 nm. The room temperature emission is strongly quenched by dioxygen, which indicates it to be spin-forbidden. Temperature studies indicate that there is no gradual shift in the emission band as the temperature is raised, but rather are two coexistent and distinct emissions at 460-520 and 660 nm.
|
|---|
