Poly(alkyl thiophene-3-carboxylates). Synthesis, properties and electroluminescence studies of polythiophenes containing a carbonyl group directly attached to the ring

Abstract

Improved synthetic methodology for poly(3-hexyl- and 3-octyloxycarbonylthiophene-2,5-diyl) (1a and 1b) is reported. _n =6700 and 9400 ( _w / _n =2.5 and 3.2), λ _max for fluorescence emission=600 and 610 nm and λ _max for electroluminescence=600 and 615 nm, for 1a and 1b respectively. The ¹ H NMR spectra required that pentads be considered to explain the spectra. That is the four nearest neighbours to a given ring influence the ¹ H NMR spectrum. Electroluminescence efficiencies of 0.016% and 0.018% were observed for devices made from 1a and 1b, respectively. A bilayer device of ITO/poly(3-octylthiophene)/1b/Al emitted at 646 nm, the same wavelength where poly(3-octylthiophene) itself emits. The efficiency was low but was an order of magnitude greater than for poly(3-octylthiophene) itself. Regioregular (HH-TT) poly(4,4′-bis(hexyl- and octyloxycarbonyl)[2,2′-bithiophene]-5,5′-diyl) (3a and 3b) were also prepared via the Ullmann reaction and _n =7900 and 11000 respectively. Films of 3a and 3b were yellow in color and showed λ _max =377 and 381 nm respectively, about 55-80 nm blue shifted compared with 1a and 1b. This is due to the large rotational barrier in the HH dyads which reduces the effective conjugation length in 3a and 3b. 3a and 3b showed bright fluorescence and electroluminescence with emission of yellow light. Electroluminescence efficiencies were 8.5×10 ^–3 % and 4.7×10 ^–3 %, respectively.