EFFECTS OF STRUCTURE ON REFRACTIVE-INDEX OF CONJUGATED POLYIMINES

The optical dispersion of the isotropic refractive index of 14 conjuga ted polyimines has been measured in the 700-2500-nm wavelength range a nd used to investigate the effects of molecular structure on the refra ctive index of pi-conjugated polymers. The measured refractive index d ispersion data were found to be described well by a three-term Sellmei er equation. A modified Abbe number based on data at 1064, 1319, and 2 500 nm is suggested as a numerical measure of the degree of refractive index dispersion in the nonabsorbing wavelength region of conjugated polymers. Off-resonance at 2500 nm, the refractive index was in the ra nge of 1.79 for the parent conjugated polyimine ethylidynenitrilo-1,4- phenylenenitrilomethylidyne) to 1.62 for the trimethoxy-substituted de rivative itrilo-2-methoxy-1,4-phenylenenitrilomethylidyne). The struct ure-refractive index relationships in these pi-conjugated polymers wer e accounted for by the competing effects of polarizability and molar v olume on the refractive index. A comparison between the present data a nd various chemical group-contribution models based on existing tabula ted group molar refractions at 589 nm shows that these tabulated value s are unsatisfactory for predicting the refractive indices of pi-conju gated polymers.