Comparisons of optical properties on meta- and para-linkages of copolymers for poly(phenylene vinylene) and poly(phenylene azomethine) with alkylthio group at the side chain

Abstract

Three series of soluble copolymers were synthesized: I, poly[m (or p)-phenylene (diacetonitrile) vinylene-co-methoxy-5-(2'-ethylhexylthio)-p-phynylene vinylene] (m-C=C(CN), p-C=C(CN)); II, poly[m (or p)-phenylene azomethine-co-2-methoxy-5-(2'-ethylhexylthio)-p-phenylene azomethine] (m-C=N, p-C=N); III, poly[m (or p)-phenylene vinylene-co-2-methoxy-5-(2'-ethylhexythio)-p-phenyle vinylene] (m-C=C, p-C=C). Of all these copolymers, the copolymers containing meta-linkage showed glass transition temperatures (T-g's) approximately 10 degrees C to 30 degrees C lower than those of the corresponding para-linkage copolymers because of lower crystallinity. 5% weight loss of all the copolymers occurred at around 381 to 344 degrees C. Copolymers of the meta-linkage also caused a relative asymmetry and irregularity of the copolymer main chain, thus leading to a decrease of effective conjugation length and a reduction of the molecular aggregation: this subsequently led to less excimer formation. These phenomena can be evidenced from the blue-shift of PL emission, 15 to 49 nm for the meta-linkage copolymers compared with the para-linkage copolymers. In addition, due to the lesser electron-donating capability of the sulfur atom, m-C=C, p-C=C and p-C=N containing the alkylthio group indicated a blue shift of 8 to 24 nm, when compared with those copolymers of similar structures containing the alkyloxy group as reported in the literature. Since series H contains strong electron-withdrawing -C=N- at the main chain, and series I contains even stronger electron-withdrawing cyano group, thus, the observed lambda(max) are in the order: series I [m-C=C(CN), p-C=C(CN)] > series II [mC=N, p-C=N] > series III [m-C=C, p-C=C]. Furthermore, since in series II, the lone pair electrons on the -C=N- main chain would overlap partially with the six pi electrons in the phenyl ring, and decrease the coplanar nature of the main chain, thus their PL emission intensities were less than those of series I and III. Finally, a device of the structure (ITO/PEDOT: PSS/m-CC(CN)/LiF/Al) was constructed, and a yellow light of lambda(max) = 570 nm was observed.