Excited states ligand centered

Colombo, M. G., Hauser, A., and Giidel, H. U. Competition Between Ligand Centered and Charge Transfer Lowest Excited States in bis Cyclometalated Rh3+ and Ir3+ Complexes. 171, 143-172(1994). 

BCPDA is a tetradentate ligand, and forms 1 1 complexes in these conditions. The metal ion is thus coordinatively unsaturated, and also binds a number of water molecules, which can lead to non-radiative quenching of the metal-centered excited state (vide supra). These must be removed by drying to achieve optimal luminescence intensity. This form of assay is, in fact, slightly less sensitive than DELFIA. 

In brief, irradiation leads to the sub-picosecond oxidation of the Re(I) center and the production of a mixture of two reduction sites i.e. either the MQ+ or dmb ligand can be reduced. On the time-scale of several picoseconds inter-ligand electron transfer occurs, converting the mixture of excited states to [Ren(MQ )(CO)3(dmb)]2+. 

The relaxation of Fe(II) protoporphyrin dimethyl ester in benzene is seen in Figure 1. It can be seen tnat in this complex which has no axial ligands and a high spin state, S=2 (14), there is an excited state absorption band which extends from 520 nm into the blue. The peak is not seen because of experimental cut-off in the spectrum at 450 nm. This difference spectrum also indicates an absorption band extending to the blue from 620 nm and interupted by the strong ground state bleaching at 570 nm. The peak of the excited state absorption band seems to be centered at about 585-590 nm. 

---