On the origin of optical activity in polythiophenes

Optically active substituents attached as sidechain to pi-conjugated polymers provide a particularly interesting and revealing tool to study the nanoscopic and mesoscopic supramolecular organization of pi-conjugated polymers in general. Chiral substituents allow the properties of these polymers to be studied with circularly polarized light in both absorption and emission. Here we describe a comprehensive investigation on the structure and optic al properties of poly{3,4-bis [(S)-2-methylbutoxyl]thiophene} (PBMBT) as an example to elucidate the molecular origin of the chiroptical effects in chiral conjugated polymers. The chiral side chains of PBMBT induce very strong bisignate circular dichroism (CD) effects in the pi-pi* absorption when the polymer is in an aggregated phase. The anisotropy factor in absorption (g(abs)) is as high as 8 x 10(-2). The photoluminescence of PBMBT aggregates is characterized by a very small Stokes shift (<90 meV) and exhibits circular polarization (CPL). The chiroptical properties in absorption and emission of PBMBT are associated with a high degree of intrachain and interchain order as they are present in aggregated phases but absent when the polymer chains are molecularly dissolved or in a melt. The observed CD and CPL effects are consistent with an exciton splitting between transition dipole moments in the excited state on different polymer chains. The strong optical activity associated with the pi-pi* transition of PBMBT is attributed to originate from a helical packing of predominantly planar chains, rather than from a helical intrachain conformation. As a result a detailed molecular description of the polythiophene morphology is obtained. (C) 2000 Elsevier Science B.V. All rights reserved.