Novel molecular weight and solvatochromisms in poly(methyl-3,3,3- trifluoropropylsilane) induced by cooperative through-space Si⋯F-C interactions

Novel helical poly(methyl-3,3,3-trifluoropropylsilane) (1) has been synthesized via Wurtz coupling by taking advantage of noncovalent Si⋯F-C intramolecular interactions. Such weak Si⋯F-C interactions make the conformation of the polysilane unique and controllable by proper choice of solvent and molecular weight. We have studied the conformation of polysilane by means of UV absorption, photoluminescence, NMR spectra, and viscosity measurements. Polysilane displayed a 7₈ helical rodlike conformation in noncoordinating solvents (e.g., toluene and decane) but a globule-like conformation in coordinating solvents (with N, O, or F as donor) at room temperature. The coordinating nature of solvents played an important role for solvatochromic transition rather than the solvent polarity, as such interactions are competitive in the presence of solvents with donor atoms. On the other hand, an analogous nonfluorinated polysilane, polyfo-propylmethylsilane) (2), revealed a disordered conformation in coordinating as well as in noncoordinating solvents. The unusual molecular weight dependent UV spectra of 1 showed an isosbestic point, suggesting the equilibrium between globule- and rodlike conformations at room temperature, which was further evident from the viscosity measurements. The proposed weak intramolecular interaction in 1 was supported by the ²⁹Si NMR and ¹⁹F NMR in noncoordinating and coordinating solvents. The IR spectrum of cast film from a toluene solution of 1 showed an improper blue shift of 12 cm^-1 in C-F stretching frequency compared to the IR spectrum cast from THF, suggesting very weak Si⋯F-C interaction in analogy with blue-shifted hydrogen bonds.