Substituent effects on the Su-Schrieffer-Heeger electron-phonon coupling in conjugated polyenes

The Su-Schrieffer-Heeger (SSH) model was introduced to describe the electron-phonon interactions leading to Peierls distortion in polyacetylene (PA). The same model was recently predicted to give rise to unique polaronic effects, including sharp transitions in polaron properties, quasi-self-trapping due to polaron interactions, and the formation of strongly bound yet light bipolarons, at strong electron-phonon coupling in the single and two-polaron limit. This suggests that organic polyenes with strong electron-phonon coupling may exhibit qualitatively different conduction properties from those with weak coupling. In order to observe this, it is necessary to design materials with a range of SSH electron-phonon coupling. Here, we use gradient-corrected density-functional-theory calculations to predict the SSH model parameters for a variety of polyenes, derived from PA by the substitution of the hydrogen atoms with molecular groups. We show that even though the calculations do not accurately reproduce the band gaps, the derivatives of the band structure parameters giving the phonon-induced couplings can be computed with good accuracy. We show that the electron-phonon coupling in such polyenes correlates with the Hammett constant of the substituted molecular group and that some substitutions enhance the coupling strength by as much as a factor of 2. We also find that the electron-phonon coupling in conjugated systems with heavier atoms (such as Si) is much weaker than in PA despite the lower phonon frequencies.