The light absorption processes are only allowed for transitions in which the ground
and the excited state have the same spin value. These transitions can be observed as intense
bands in the absorption spectra of the molecules. On the other hand, transitions from the
ground state to excited states with different spin values are considered forbidden and can
rarely be observed in absorption spectra. The MC, MLCT and LC transitions of an octahedral
transition-metal complex are related to the ligand field strength, the redox potential of the
metal complex and the intrinsic properties of the ligands, respectively [8]. For this reason,
changes in the molecular structure of the ligands attached to the ruthenium metal ion can
dramatically vary the relative energy positions of the excited states, with the consequent
change in their photophysical properties