Effect of High Pressure on Vibrational Modes of Polyiodides in Poly(vinyl alcohol) Films

Abstract

We have used resonance-Raman scattering to study the effects of hydrostatic pressure (to 80 kbar) on polyiodide chain vibrations in iodine-doped poly(vinyl alcohol) films. Two strong fundamental bands are observed along with overtone and combination bands. The low-energy band at 107 cm-1 is attributed to symmetric stretching of I3 -. The higher lying band at 154 cm-1 is from symmetric stretching of the two end I2 units in I5 -. The I3 - and I5 - are intermixed within the polyiodide chains. The combination band arises from exciton coupling between molecules within the chains. Three primary effects are observed with increasing pressure. First, the Raman band at 107 cm-1 blue shifts under pressure, while the band at 154 cm-1 shifts weakly. This latter is attributed to the competing effects of compression, enhanced interaction of the outer I2 with the central I- (which weakens the outer I−I bond), and attractive host−guest interactions which increase in strength with compression. The second effect observed in the Raman bands is dramatically increasing line widths. This signifies locally nonhydrostatic conditions due to the variations in local microstructure within the polymer. Third, the relative intensities reverse in dominance as the pressure exceeds ≈18 kbar. We suggest that this is due to breaking of the I5 - chain elements into I2 and I3 -, which is caused by the locally inhomogeneous strain conditions. The intensities reflect the changing populations in I3 - and I5 -. These three effects are also observed in the overtone and combination bands of the Raman spectra. High-pressure absorption measurements in the visible support our interpretation and confirm the important role played by attractive host−guest interactions.

High-pressure behavior of polyiodides confined into single-walled carbon nanotubes: A Raman study

“…11 ͑iodine intercalated nanotubes͒ but similar to those of Ref. 24 ͑iodine intercalated polyvinyl alcohol͒. We note that the position of the A and B bands, as well as their pressure evolution, clearly differ from the case of pristine iodine, as an additional proof of the intercalation process.…”

“…24 However, this scenario is not fully satisfactory. Indeed, the symmetric I 3 − chain is expected to have covalent bonding, 17 so that the peak at 110 cm −1 should remain intense back to the ambient conditions, unlike our Raman results.…”

Molecular iodine/polymer complexes

“…Sengupta et al [178] studied the effect of pressure (up to 80 kBar) on the Raman vibrational modes of poly iodides in the I 2 PVA complex. In general, higher pressure was found to broaden the Raman bands, arising from distortion of the iodine chains.…”

An insight into the disorder properties of the α-cyclodextrin polyiodide inclusion complex with Sr2+ ion: dielectric, DSC and FT-Raman spectroscopy studies

“…Moreover, the continuous removal of the water molecules from the crystal lattice gradually minimizes the contribution of the strontium ions that act as localized charges. The remarkable increment of the ac-conductivity at T > 412.5 K, is caused by the sublimation of iodine which provides conduction paths along the polyiodide chains resulting in an enhanced connectivity between the neighboring building blocks and important electron delocalization (long-range Coulomb attraction of excited electron and hole) (34,35,64). …”