Optical Properties and Photoexcitations In Regularly Alternating Conjugated Copolymers
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in which M is the number of molecules or unit cells per unit volume, Ep is the transition energy, IF is the damping constant and op is the absorption coefficient at Ep. The reduction of the dimensionality from "1" (conjugated polymer or quantum wire) to "0" (conjugated block copolymer or quantum dot) changes the sawtooth behaviour of the density of states g(E) into a singular behaviour [3] which can be broadened by inhomogeneous distribution of energies or by finite lifetimes effects. With this in mind we have designed and prepared a series of regularly alternating copolymers whose translational unit consists of a sequence of thiophene rings of various length (T2,T3 and T6) connected to a sequence of benzene rings (B1 and B2) by an azomethine link. The resulting copolymers correspond to the general formula
\ T 4/1 "Is. S XCH
N)N N-C
-- CH
673 Mat. Res. Soc. Symp. Proc. Vol. 328. ©1994 Materials Research Society
The regular alternation of low-energy gap (thienylenic moiety) and high-energy gap yields therefore a structure that can be considered, in principle, a model for a one dimensional superlattice. An attempt to synthesize polymeric superlattices has been done by Jenekhe [4,5] by alternating oligothienylenic segments and their quinoid counterparts. These polymers exhibit large third-order optical non linearities measured by time-resolved degenerate four-wave mixing [6]. EXPERIMENTAL Polymer preparation Two different solution reactions have been used to prepare the polyazomethine copolymers. For T 2 B1 and T 6 B2 synthesis we have chosen an oxidative coupling starting from monomers already containing azomethine linkage and ending with a thiophene ring. To polymerize the thienylenic residue CuClO4-H 2 0 in chloroform/acetonitrile solution has been used as catalyst. The copolymers were recovered as black doped powders which were washed with a large amount of hydrochloric acid-methanol solution and dried. The doped materials were neutralized by stirring the powders overnight in methanol or DMSO with 15% NH 3 . The neutral polymers are red-brownish. T3 B2 copolymer was synthesized using the polycondensation reaction of equimolecular amounts of biphenylene diamine and of terthienylene dialdehyde. The reaction was carried out in DMSO at 130'C in order to obtain essentially quantitative yields of higher molecular weight polymers. Paratoluensulphonic acid was used as catalyst. The red neutral polymer was purified in a watersodium carbonate solution to remove the catalyst, washed with water, extracted with refluxing methanol and dried. As the other aromatic polyazomethines, these polymers are insoluble materials, but they can dissolve by complexation of protonic strong acids, such as sulfuric, metanesulfonic and trifluoroacetic acids, or Lewis acids at the imine nitrogen. Deep blue solutions of T3 B2 and blueviolet solutions of T 6 B2 have been prepared by complexing the powders with concentrated H2 SO4 and GaC13 or FeC13 . The preliminary characterizations using TGA, DSC, FTIR, XRD analyses were performed on p
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