Spin-Dependent Exciton Formation Rates in Π-Conjugated Oligomers and Polymers

We have measured the ratio, r = σ S /σ T of the formation cross section, σ of singlet (S) and triplet (T) excitons from polarons in π-conjugated oligomer and polymer films, using a spectroscopic magnetic resonance technique. We show that the exciton formation process from polarons in π-conjugated compounds is spin dependent, specifically r > 1. Moreover we discovered a universal relation between r and the conjugation length, CL: r -1 a (CL) -1 .