Rigidochromic effect

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. 

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. 

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. 

An early report by Wrighton and co-workers revealed that complex ClRe(CO)3(3-benzoylpyri-dine)2 exhibits typical MLCT emission in room-temperature benzene solution. However, in a 77 K EPA glass, the rigidochromic effect shifts the MLCT state to higher energy and, thus, multiple emissions from both MLCT and IL (u-tt ) excited states can be observed (see Figure 4). Here, the MLCT and n-7r states are clearly not thermally equilibrated in this glassy environment at low temperature. ... 

Hypotheses to explain this observation are the more hydrophobic character of the nucleus with respect to the cytoplasm but also the formation of some solid phase within the nucleus, due to concentration of the probe in the restricted nuclear region. Actually, it is well loiown that in this class of compounds the luminescence is environmentally sensitive, being blueshifted (rigidochromic effect) [73,74] and often also more brilliant, in rigid phases and apolar media [75-77]. 

Recently, the effect of pressure-induced luminescence rigidochromism on the emission behavior of 4a has also been reported [58], In ambient pressure, the emission of the complex occurs at 695 nm in benzene. However, at a pressure higher than that required to induce the solvent to undergo the room temperature phase transition from fluid to solid, the emission band shifts to 575 nm, which is similar to the solid state emission wavelength (580 nm) of the complex. 

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