Luminescent signalling MLCT

These conclusions were supported by transient absorption spectroscopy, which revealed signals corresponding to the formation of the diimine radical anion, with lifetimes in close agreement with the luminescence lifetimes. Time-resolved infrared spectroscopy of the acetylide C = C bonds provides further conclusive evidence for the MLCT assignment. Thus, in the ground state IR spectrum of 4, there are two v(C=C) bands at 2115 and 2124 cm-1, whilst the step-scan FTIR difference spectrum obtained 50 ns after irradiation at 355 nm reveals bleaching of the parent bands, and the formation... 

The luminescent emission of Ru (bpy)3 is also very useful for signalling anion binding. In the emission spectra of 37-40 both a blue shift (of up to 16 nm for 40) and an increase in intensity of the Amax of the MLCT emission band was observed on addition of dihydrogenphosphate. It has been proposed that the conformational flexibility of the receptors is decreased by complexation of the anion guest thus reducing the rate of non-radiative decay through vibrational and rotational relaxation. Similarly, macrocyclic complexes 41-44 and 77-79 were also found to sense chloride by luminescence enhancement. 

The photophysical properties of 20 are particularly interesting, in that the triplet energy of the porphyrin chromophore is comparable to the emissive state of and can be thermally repopulated at room temperature [52], This leads to a dependence of the emission intensity and lifetimes upon the degree of sample aeration. In most cases, energy transfer from the chromophore to the lanthanide occurs via the MLCT state of the chromophore, and normal behaviour is observed. However, the broad nature of the emission from luminescent MLCT states means that the tail of this emission is often superimposed upon the relatively weak luminescence from the lanthanide ion. In such situations the two signals can be resolved readily provided that their lifetimes differ significantly. This is not the case with many complexes related to [Rulbpyls] ", which have luminescence lifetimes comparable with those of neodymium and ytterbium complexes. 

---