Conjugation-length dependence of spin-dependent exciton formation rates in pi-conjugated oligomers and polymers

We have measured the ratio, r=sS/sT of the formation cross section, s of singlet and triplet excitons from polarons in p-conjugated oligomer and polymer films, using a spectroscopic technique we developed recently. We discovered a universal relation between r and the conjugation length (CL): r-1 depends linearly on CL-1, irresp. of the chain structure. Since r is directly related to the max. possible electroluminescence quantum efficiency in org. light emitting diodes (OLED), our results indicate that polymers have an advantage over small mols. in OLED application