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

In aqueous solution UV-absorption and fluorescence emission spectroscopy reveal that the nonionic water-soluble polysilanes poly(4,7,10-trioxaundecylmethylsilane) (1) and poly(4,7,10,13-tetraoxatetradecylmethylsilane) (2) are present in a highly folded random coil. In contrast, in organic solvents the backbone adopts a more extended conformation. These changes in backbone conformation are accompanied by changes in the average backbone silicon segmental length L from 15 to 21-24. Concomitantly, the exciton coherence length also exhibits a 2-fold increase. The results obtained for 1 and 2 are compared 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 fraction of the organic solvent induces large shifts in the photophysical properties; i.e., preferential solvation occurs at a mole fraction of ca. 0.15, Hence upon addition of small amounts of organic solvents, the solute-solvent interactions, which in H2O prevent the silicon backbone from adopting a more extended conformation, an reduced, causing the backbone to be less distorted. This is supported by the increase in the quantum yield Phi in going from polar aqueous to apolar organic solvents.