Cluster-centered excited state

A hypsochromic shift in the emission band would be observed if the amount of excited-state distortion were lower in a rigid matrix than in fluid solution. Such an interpretation has been recently suggested for Cu4l4py4 and related copper clusters, and this constitutes an unusual case of molecules that exhibit rigidochromism but not solvatochromism in its emission from a cluster-centered excited state [115]. It is likely that both excited-state destabilization and distortion arise during the polymerization processes in the tungsten and rhenium organometallic systems. 

DFT calculations on the model compound Cu6I6(SPhCH3)6 in its triplet state predict that the SHOMO localizes the spin density mainly on one Cu atom and its three iodo neighbors with the rest on one -SPh fragment, whereas it is rather delocalized over the Cu and I atoms in the HOMO (Fig. 32). The SHOMO is located at -0.042 a.u. above the HOMO (-0.125 a.u.). The simple arithmetic difference between the two levels suggests that the radiative relaxation should occur in the vicinity of 550 nm, which fits the experimentally observed maxima (Fig. 31). So the first theoretical study confirms a cluster-centered excited state as responsible for the observed emission, similar to other Cu4I4-containing species.146... 

In addition to this luminescence from a I" —> pyridine LLCT state a second more intense emission occurs at longer wavelength (A ax = 690 nm) which comes from the cluster-centered excited state of I" —> Cu LMCT and MC ds parentage. 

The excited state absorption spectra of these luminescent tetranuclear cop-per(I) iodide complexes have also been reported [57]. The absorption spectral data of the cluster-centered triplet states of 4a, 4c, 4f, and are listed in Table 2. The energy of the absorption is in line with the ordering of the n orbital energy of the pyridine ligands. For die piperidine analog 4i, no excited state absorption is observed. It has been suggested that the excited state absorption bands are associated with the transition from a thermally relaxed [cluster-centered] state to Franck-Condon states of the [XLCT] manifold. 

The relatively broad and featureless absorption spectra of [Fe3S4l clusters belies their complex excited-state electronic structure. This is illustrated in Fig. 5 using P. furiosus 3Fe Fd as an example (42). In addition to the protein band centered at 280 nm, the UV-visible... 

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