Nonionic water-soluble polysilanes: Global conformation in solution and the occurrence of preferential solvation

In aqueous solution UV-absorption and fluorescence emission spectroscopy re veal that the nonionic water-soluble polysilanes poly(4,7,10-trioxaundecylm ethylsilane) (1) and poly(4,7,10,13-tetraoxatetradecylmethylsilane) (2) are present in a highly folded random coil. In contrast, in organic solvents t he backbone adopts a more extended conformation. These changes in backbone conformation are accompanied by changes in the average backbone silicon seg mental length L from 15 to 21-24. Concomitantly, the exciton coherence leng th also exhibits a 2-fold increase. The results obtained for 1 and 2 are co mpared to those of poly(4,7,10-trioxahexadecylmethylsilane) (3) and poly(4, 7,10,13-tetraoxanonadecylmethylsilane) (4), which contain instead of a CH3, a C6H13 moiety as the side chain end group. A detailed analysis of aqueous /organic binary solvent mixtures of 2 reveals that already a low mole fract ion of the organic solvent induces large shifts in the photophysical proper ties; i.e., preferential solvation occurs at a mole fraction of ca. 0.15, H ence upon addition of small amounts of organic solvents, the solute-solvent interactions, which in H2O prevent the silicon backbone from adopting a mo re extended conformation, an reduced, causing the backbone to be less disto rted. This is supported by the increase in the quantum yield Phi in going f rom polar aqueous to apolar organic solvents.