UNUSUAL REDOX BEHAVIOR OF ALPHA-OLIGOHETEROAROMATIC COMPOUNDS - AN INCREASING FIRST OXIDATION POTENTIAL WITH INCREASING CONJUGATION LENGTH

Phenyl end-capped alpha-oligoheteroaromatic compounds consisting of py rrole and thiophene units have been synthesized via Stille coupling re actions, Cyclic voltammetry studies on diphenyl-alpha-oligopyrroles (P hP(n)Ph) reveal two chemically reversible oxidation waves for n greate r than or equal to 2, with decreasing potentials for larger n. For bis (phenylpyrrolyl)-alpha-oligothiophenes (PhPT(n)PPh), in contrast, we o bserve an increase of the first and a decrease of the second oxidation potential going from n = 1 to n = 3. The bandgap in both series follo ws the usual decreasing behavior with increasing conjugation length. T he oxidation potentials of both PhP(n)Ph and PhPT(n)PPh are explained using a Huckel-type band model. This theoretical model indicates that in the oxidized form of PhPT(n)PPh, the positive charge tends to local ize on the pyrrole units.