Substitution effects on bipolarons in alkoxy derivatives of poly(1,4-phenylene-vinylene)

We have studied the vibrational and electronic structure and the elementary charged excitations of poly(1,4-phenylene-vinylene) and a series of its (2,5-alkoxy) derivatives including poly(2,5-methoxy-PV), poly(2,5-hexoxy-PV), poly(2,5-octoxy-PV), and poly[2-methoxy,5-(2'ethyl-hexoxy)-PV] by absorption, photoinduced (PI) absorption, and doping-induced (DI) absorption. For the pristine materials, we observe that the band gap decreases by as much as \ensuremath{\sim}0.3 eV with the introduction of alkoxy side chains. We assign the observed ir-active phonons to specific structural vibrations. Upon either PI or DI carrier injection, we observe a series of infrared-active vibrational (IRAV) modes and two subgap electronic absorptions that we associate with the formation of bipolarons. Assignment of the IRAV modes indicates that the charged bipolarons couple strongly to lattice modes of the polymer backbone, and the measured IRAV mode frequencies indicate that the pinning strength of bipolarons in a given system decreases as the length of the side chain increases. In addition, from the energy of the two subgap electronic absorptions, we determine that the confinement parameter \ensuremath{\gamma} increases, the Coulomb interaction energy ${\mathit{U}}_{\mathit{b}}$ remains unchanged, and the Coulomb binding energy ${\mathit{E}}_{\mathit{b}}$ decreases with side-chain length.