Alternating aromatic and transannular chromophores with and without linker: Effect of transannular π-π interaction on the optical property of dithiaparacyclophane-based copolymers

Abstract

A series of bithiophene-cyclophane (PPP-type), acetylene-fluorene- acetylene-cyclophane (PPE-type), and ethylene-fluorene-ethylene-cyclophane (PPV-type) copolymers, namely, 4-6, was synthesized via a nickel-catalyzed reaction, a palladium-catalyzed Sonagashira reaction, and a Heck coupling, respectively. Unlike in the fluorene-cyclophane copolymer 3, the transannular π-π interaction of the cylophane unit in copolymer 4 resulted in a significant blue shift in its emission spectrum compared to a reference polymer, giving it an essentially blue emitting light. By introducing an acetylene or ethylene linker in going from 3 to 5 or 6, the effective conjugation in the polymer backbone due to higher rigidity in coplanarity decreased the effect of the transannular π-π interaction. While the PPV-type copolymer 6 still exhibited an appreciable red shift in both absorption and emission with respect to a reference polymer, negligible shifts were observed for copolymer 5.