| Summary: | In this thesis, absorption, magnetic circular dichroism (MCD) and hole burning spectroscopies have been used to investigate a range of metalloporphyrins derivatives, in particular, the metallophthalocyanines (MPcs). The aim of these investigations was the elucidation of a number of excited-state properties, such as spinorbit splittings, orbital angular momenta and vibronic effects. Use has been made of quantum mechanics, group theory and moment analysis in relating theoretical expressions to the experimentally observed spectra. The main method of sample preparation was matrix isolation. The combination of this technique with the measurement of spectra at low, accurately known temperatures was achieved using a matrix-injection procedure, the refinement of which formed a significant part of the work in this thesis.
The Q band of MPcs and metalloporphyrins arises from the lowest ligand π → π* transition. Weak MCD temperature dependence is observed for this band in CuPc, CoPc, and Cu tetrabenzoporphyrin. This effect can be attributed to a zero-field splitting of the excited-state levels, which has been shown to arise from interference between second-order exchange and spin-orbit coupling between the so-called singdoublet and tripdoublet excited states. The spin-orbit coupling is proposed to arise from a (small) metal contribution to the ligand π orbital.
MCD temperature dependence has also been observed for lutetium bisphthalocyanine (LuPc₂). For the Q band, this effect is weak There are three transitions in the Q region, but it has been shown theoretically that the overall temperature dependence of two of these, as measured by moment analysis, will cancel The red vibronic (RV) band shows much stronger temperature dependence, with the sign of the MCD changing from positive to negative between 65 and 1.4 K. The strength of this effect is ascribed to a greater metal contribution to the excited-state orbital.
Hole burning and MCD spectroscopies have been combined to measure the MCD of holes for ZnPc, CuPc and LuPc₂. This is the first time this combination of techniques has been used. A qualitative analysis of the resulting spectra is given, including a proposed hole-burning mechanism involving intermolecular charge transfer.
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