Optical spectra in polydiacetylene crystals substituted with fluorobenzenes.

Optical properties of new polydiacetylene (PDA) crystals substituted with fluorobenzene derivatives have been investigated by reflectance, Raman, and electroreflectance (ER) measurements. The results indicate that these fluorobenzene-substituted PDAs have an intermediate optical character between those of the acetylene-type and butatriene-type PDA chains. One of these PDA crystals, DFMDP, has been found to show an exceptionally high crystalline quality, which has enabled us to make a detailed spectroscopic study. ER studies on this DFMDP crystal have revealed the charge–transfer(CT)-type exciton lying at ∼2.7 eV on the higher-energy side of the lowest 1Bu exciton at ∼2.2 eV. On the basis of experimental results, it is conjectured that two-photon resonance with the CT exciton plays a major role in the third-order nonlinear optical process in PDA chains. [ABSTRACT FROM AUTHOR]