Effect of high pressure on vibrational modes of polyiodides in poly(vinyl alcohol) films

Archita Sengupta, Edward L. Quitevis, Mark W. Holtz

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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.

Original languageEnglish
Pages (from-to)11092-11098
Number of pages7
JournalJournal of Physical Chemistry B
Issue number51
StatePublished - Dec 18 1997


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