Chainlength dependence of interchain splitting in electroluminescent organic conjugated materials

Interchain interactions in organic conjugated solids cause a splitting of the lowest excited electronic states which depends on the relative packing of the chromophores and on the conjugation length. The splitting (also known as Davydov splitting) was measured for the first time in a model compound (α-sexithienyl), but the question remained open on how the Davydov splitting depends on the conjugation length. This question is particularly important because the relative location of the excitonic components plays a major role in determining the photophysics of the organic systems. We have, therefore, measured the Davydov splitting of the 11Bu exciton in the model systems of α-quaterthienyl (T4) and α-sexithienyl (T6) from the low-temperature polarized absorption of oriented single crystals. Due to very similar crystal structures of the two materials, the change in interchain interactions can be directly related to the different molecular wave function delocalization over the chain. The Davydov splitting is measured to be 0.36 eV in T4 and 0.32 eV in T6. These results are an experimental evidence of chainlength dependence of Davydov splitting in organic conjugated model compounds.