Brief overview of photophysical properties of ruthenium(II) complexes
Variation of ligands has been demonstrated to have considerable effect on the efficiency of sensitisation by ruthenium(II) dyes adsorbed on semiconductor surfaces in photovoltaic cells. The ligand combination in a sensitiser can tune the energy of the excited state, absorption and excited-state lifetime of the complex. The effectiveness of a photoelectric transducer is reliant upon the sensitiser having an excited state energy level conducive to quenching by the conduction band of the semiconductor substrate and the excited state being sufficiently long lived to allow electron transfer to this band. These parameters need to be optimised in order to maximise cell efficiency .
For the application of ruthenium complexes, the combination of ligands comprising the Ru(II) primary coordination sphere needs to be capable of facilitating the absorption/emission of available light, giving rise to appropriate excited state. .
In simple terms, if we assume that [Ru(diimine)3]2 + complexes approximate an octahedral ligand field (although more correctly [Ru(bpy)3]2 + possesses D3 symmetry), then we might regard the full t2g orbitals as the highest occupied molecular orbitals (HOMO). Due to a strong crystal field, the lowest unoccupied molecular orbital (LUMO) in these complexes is a vacant p-antibonding orbital. Therefore, the lowest energy electronic transition is a metal to ligand charge transfer which can be considered as metal centre oxidation and ligand reduction. Stabilization of the excited state is enhanced by delocalisation of the electron over p-bonding orbitals of the acceptor ligand. An important feature of the heteroleptic tris(diimine) Ru II complexes, which differentiates them from the homoleptic species, is the increased number of p* levels into which an excited electron may be promoted. For example, incorporation of three inequivalent ligands with p* levels that allow the overlap of absorptions corresponding to t2g - p* transitions to each ligand would cause the complex to absorb all visible light, which could be termed a "black absorber" .