Luminescence from Frozen Solutions

Colorless, non-luminescent solutions of [Au C(NHMe)2 2](PF6)-0.5(acetone) become intensely luminescent when they are frozen in a liquid N2 bath [48]. Strikingly, the colors of the emission vary in different solvents and appear only after the solvent has frozen. The frozen acetonitrile solution produces a green-yellow luminescence, with dimethyl sulfoxide and pyridine the emission is different shades of blue, with acetone it is orange, but with dimethyl-formamide no luminescence is observed. The process is entirely reversible  

The emission spectra obtained from these frozen solutions of [(C6HnNC)2 Au ](PF6) also vary as the solvent is changed [38]. Visually the effect is not as striking as it is in the case of frozen solutions of [Au C(NHMe)2 2](PF6)-0.5(acetone). Relevant spectra are shown in Fig. 31 for 6.0 mM solutions of [(C6HiiNC)2Au ](PF6). Dilution of the solutions of [(C6HnNC)2Au ](PF6) can also produce significant changes in the luminescence from some solutions as was the case with [Au C(NHMe)2 2](PF6) 0.5(acetone) as well. 

The causes of the variations seen in Figs. 29, 30, and 31 are likely to result from a number of factors including the number of gold(I) ions involved in specific aggregates (dimers, trimers, extended chains, etc.), the distance between the gold(I) ions within any particular aggregate, the relative orientation 

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James Dewar observed in 1894 phosphorescence from frozen solutions utilizing liquid air [5], Jean Becquerel discovers in 1907 that samples frozen at liquid air temperatures considerably narrow the spectral shape and increased information is obtained from the luminescence spectra [26],... 

Fig. 2. The isosbestic points at 446 and 556 nm in the absorption spectra are matched by an isoemissive point at 685 nm indicating only two species present in solution, both of which are emissive. The shift in emission maximum from 606 nm in neutral solutions to 728 nm upon addition of acid may have interesting sensor applications. The results for 4 stand in contrast with results from dppz-containing Ru(II) tris diimine complexes, where dppz = dipyrido-ipyridophenazine, in which reversible protonation of quinoxaline N atoms leads to quenching of emission. Luminescence in frozen solvent glasses for 4 at 77 K is much stronger ( = 0.044 for the qdt complex), but still broad and without resolved structure.

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