The Effect of Pressure on the Solid State Structure and UV- Visible Absorption Spectra of Symmetrically Substituted Poly (di-n-alkysilylenes).

The effects of pressure on the solid state structures and electronic absorption properties of three symmetrically substituted poly (di-n-alkysilylenes) are determined by using X-ray diffraction, solid-state NMR, and UV spectroscopy. At a pressure of 250 MPa and a temperature of 46C the conformationally disordered high temperature crystalline phase of Poly(di-n-hexylsilylene) is almost entirely transformed into the all-trans crystalline form with a UV absorption at 374 nm. The polymer rapidly reverts to the disordered structure upon release of the pressure. Under pressure the crystalline 7/3 helical structure of both poly(di-n-pentysilylene) (PDPS) and poly(di-n-butylsilylene) (PDPS) is partially transformed to an all-trans crystalline form. Upon release of pressure at lower temperatures both crystalline phases remain. On heating a phase transformation occurs (at ca. 26.5C for PDPS and 53 - 60C for PDPS), and the all-trans form converts to a 7/3 helical polymorph.