COULOMBIC SCREENING IN POLYDIACETYLENE CRYSTALS

The strong colour of polydiacetylene polymer crystals is due to the backbone exciton. The energy of the exciton is strongly affected by Coulombic interactions with the surrounding crystalline environment. X-ray diffraction and optical absorption measurements of the blue phase polymer crystals of 4BCMU show that a change in the energy of the polydiacetylene backbone exciton can be related to a change in the inter-backbone spacing within the crystals. Over the range of inter-backbone spacings measured, this variation is in very good agreement with Coulombic screening theory. It is calculated from these results that inter-backbone Coulombic screening lowers the value of the exciton energy in the blue phase polymer crystals of 4BCMU by -0.362eV, which is 15.4% of the total exciton energy. Coulombic screening is shown to have an important effect on the optical absorption properties of other polydiacetylene derivatives. Calculations and resonant Raman spectroscopy measurements show that a combination of a low level of Coulombic screening and strain in the crystal lattice accounts for the red colour of the polymer crystals of PTS-12 and TCDU.