Rigidochromic

The low temperature luminescence spectra of the title compounds in poly(methyl methacrylate) (PMMA) are shown in Fig. 4. In addition to the solvatochromic effect the MLCT luminescence bands show a pronounced rigidochromic effect. However, not only the emission energies, but also the band shapes change when the low viscosity solvents are replaced by a rigid glassy matrix. 

The increases in both emission intensity and emission lifetime of fac-ClRe(CO)3(4,7-Ph2-phen) are again related to a reduction in the nonradiative decay pathways as the epoxy material polymerizes and forms a rigid network. Significantly, though, the observed rigidochromic shifts in the epoxy-based net-... 

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. 

Table 3. With reference to Eq. 1, comparison of results for the luminescence measurements at 77 K and room temperature (r.t.) allows some assessment of the predominant nature of the emission. Thus, the extent of the rigi-dochromic effect, i.e., the blue shifting of the emission peak on going from fluid to the frozen solvent, can be used to probe the CT nature of the emission [35]. For instance, the rigidochromic effect is relevant for Ir(ppy)3, and a minor one for Ir(btp)3. This is consistent with the evaluated kr values, Table 4, that are 2 x 105 and 0.3 x 105 s 1, respectively, for the two complexes.

These bands show negative solvatochromism as revealed by band shifts to lower energy in less polar solvents [5, 7, 8, 12], The direction of the solvent dependence is associated with a reduced (and reversed) molecular dipole in their MLCT excited states. Emissions from these complexes are typically broad and structureless, and they also often exhibit a rigidochromic effect [7-12], Tables 1 and 2 summarize the luminescence characteristics and environmental effects on absorption and emission maxima for rhenium(I) tricarbonyl diimine complexes. 

Rhenium carbonyl-diimine complexes were discovered in the mid 1970s [44, 45] as strongly emissive complexes. The long emission lifetime (tens-hundreds of ns) and structureless and broad shapes of emission bands and a prominent rigidochromism... 

The most intriguing aspect of the emission response is the blue shift that occurs upon the transition from fluid to rigid media, as in the transformation of a sol to a xerogel (115, 136-140). Formerly attributed to rigidochromism this phenomenon is actually a result of inhibition of solvent reordering around the dipole, which is created by an asymmetric excited-state charge distribution (136, 140). The solvent molecules that reorient themselves around the dipole lower... 

NN = polypyridyl ligands and L = stilbene-like ligands, in acetonitrile solution and in poly(methyl methacrylate) (PMMA) polymer film exhibited hypsochromic shifts as the medium rigidity increases due to the luminescence rigidochromic effect. Time-resolved IR (TRIR) spectroscopy, in combination with other techniques, characterized the excited-state electronic properties of the fac-[Re(CO)3(phen) (bpe)]PFa complex, where bpe is l,2-bis(4-pyridyl)ethylene. 

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