ELECTRON-PHONON COUPLING IN CONJUGATED POLYMERS - REFERENCE FORCE-FIELD AND TRANSFERABLE COUPLING-CONSTANTS FOR POLYACETYLENE

A Herzberg-Teller expansion for pi electrons in the ground state of conjugated polymers identifies quadratic electron-phonon (e-ph) contributions and suggests a sigma + pi reference force field e based on butadiene. Linear response theory then fixes linear e-ph coupling constants for the Raman shifts in polyacetylene (PA) due to pi-electron fluctuations. The same coupling constants and the known isotopic dependences of the reference accurately give the Raman frequencies of (CD)x and ((CH)-C-13)x; with a different susceptibility, F0 and the coupling constants also account for Raman modes of polyenes and for photo- or dopant-induced infrared modes of PA and its isotopes. We develop a general phenomenological approach for identifying modes coupled to pi-electron fluctuations and show that both CCC and CCH bends are weakly active in PA. Linear and quadratic e-ph coupling constants and various susceptibilities are related to pi-electron Hamiltonians, primarily to Huckel models and to Coulomb interactions in the Pariser-Parr-Pople model. We also relate our approach to previous PA models with linear e-ph coupling or modified force fields.