Thermochromism and solvatochromism of non-ionic polar polysilanes

Polar polysilanes bearing ethereal side groups [{CH3Si(CH2)mO(CH2CH2O)nCH3}x, m=3–5 and n=0–3}] were prepared and were found to be soluble in a wide range of polar solvents. UV spectroscopic behavior is highly dependent on the nature of the solvents used but no simple relationship between λmax and specific solvent parameter is found. Small molar absorbance (ε<10 000/SiSi bond) suggests that water-soluble polysilanes take highly entangled form in water. Solvatochromic redshift is induced by THF in aqueous solution of water-soluble polysilanes. This solvatochromism is ascribed to disentanglement the non-polar main chain by non-polar solvent. Another type of solvatochromic shift is also caused by (CF3)2CHOH (HFIP) in non-polar solvent (benzene, CH2Cl2) or even in (CH3)2CHOH solution, where higher HFIP concentration brings about longer wavelength absorption. This type of solvatochromism is originated by strong hydrogen bond formation between the ethereal side groups and HFIP. Because non-dissociative nature of HFIP-concerned hydrogen bonding, increased bulkiness of the side moieties brings about disentanglement. Polysilanes 1–5, though having long side chains, show thermochromism of continuous spectral shift on cooling. The same λmax value in thermochromism and solvatochromism indicates that nearly the same degree of disentanglement is caused by HFIP and thermally.