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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]