When one sees spectra such as those above, is it possible to identify the electronic excited states associated with the different features?
This question is receiving considerable attention because complexes such as these are the basis for organic LED's and photovoltaic cells.
In March I gave a talk at the thursday COPE science meeting, based on the classic paper by Kober and Meyer, concerning complexes with three-fold symmetry.
A few take home points:
The group theory analysis helps define the quantum numbers of the different states, including the spin-orbit interaction which mixes singlet and triplet states. Furthermore, polarised light is sensitive to the symmetry of states (A, E). Note how the two spectra above are different.
It is possible to describe the spectra in terms of just a few parameter.
The magnitude of the exchange interaction (singlet-triplet splitting) is 1600 cm-1, comparable to other energy scalings, showing the importance of electronic correlations.
The spin-orbit interaction is comparable to the other energy scales as well.
The paper only treats metal-to-ligand charge transfer (MLCT) states. For understanding the emissive states of OLED's ligand-centred transitions may be just as important. A nice discussion of these issues is discussed in a recent review by Yersin and Finkenzeller .