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

Improved synthetic methodology for poly(3-hexyl- and 3-octyloxycarbonylthio phene-2,5-diyl) (1a and 1b) is reported. (M) over bar(n)=6700 and 9400 ((M) over bar(w)/(M) over bar(n)=2.5 and 3.2), lambda(max) for fluorescence emi ssion=600 and 610 nm and lambda(max) for electroluminescence=600 and 615 nm , for 1a and 1b respectively. The H-1 NMR spectra required that pentads be considered to explain the spectra. That is the four nearest neighbours to a given ring influence the H-1 NMR spectrum. Electroluminescence efficiencie s of 0.016% and 0.018% were observed for devices made from 1a and 1b, respe ctively. A bilayer device of ITO/poly(3-octylthiophene)/1b/Al emitted at 64 6 nm, the same wavelength where poly(3-octylthiophene) itself emits. The ef ficiency was low but was an order of magnitude greater than for poly(3-octy lthiophene) itself. Regioregular (HH-TT) poly(4,4'-bis(hexyl- and octyloxyc arbonyl)[2,2'-bithiophene]-5,5'-diyl) (3a and 3b) were also prepared via th e Ullmann reaction and (M) over bar(n)=7900 and 11000 respectively. Films o f 3a and 3b were yellow in color and showed lambda(max)=377 and 381 nm resp ectively, 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 effi ciencies were 8.5x10(-3)% and 4.7x10(-3)%, respectively.