On the origin of optical activity in polythiophenes

Optically active substituents attached as sidechain to pi-conjugated polyme rs provide a particularly interesting and revealing tool to study the nanos copic and mesoscopic supramolecular organization of pi-conjugated polymers in general. Chiral substituents allow the properties of these polymers to b e studied with circularly polarized light in both absorption and emission. Here we describe a comprehensive investigation on the structure and optic a l properties of poly{3,4-bis [(S)-2-methylbutoxyl]thiophene} (PBMBT) as an example to elucidate the molecular origin of the chiroptical effects in chi ral conjugated polymers. The chiral side chains of PBMBT induce very strong bisignate circular dichroism (CD) effects in the pi-pi* absorption when th e 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 circul ar polarization (CPL). The chiroptical properties in absorption and emissio n of PBMBT are associated with a high degree of intrachain and interchain o rder as they are present in aggregated phases but absent when the polymer c hains are molecularly dissolved or in a melt. The observed CD and CPL effec ts are consistent with an exciton splitting between transition dipole momen ts in the excited state on different polymer chains. The strong optical act ivity associated with the pi-pi* transition of PBMBT is attributed to origi nate from a helical packing of predominantly planar chains, rather than fro m a helical intrachain conformation. As a result a detailed molecular descr iption of the polythiophene morphology is obtained. (C) 2000 Elsevier Scien ce B.V. All rights reserved.