Chainlength dependence of interchain splitting in electroluminescent organic conjugated materials

Interchain interactions in organic conjugated solids cause a splitting of t he lowest excited electronic states which depends on the relative packing o f the chromophores and on the conjugation length. The splitting (also known as Davydov splitting) was measured for the first time in a model compound (alpha -sexithienyl), but the question remained open on how the Davydov spl itting depends on the conjugation length. This question is particularly imp ortant because the relative location of the excitonic components plays a ma jor role in determining the photophysics of the organic systems. We have, t herefore, measured the Davydov splitting of the 1(1)B(u) exciton in the mod el systems of alpha -quaterthienyl (T-4) and alpha -sexithienyl (T-6) from the low-temperature polarized absorption of oriented single crystals. Due t o very similar crystal structures of the two materials, the change in inter chain interactions can be directly related to the different molecular wave function delocalization over the chain. The Davydov splitting is measured t o be 0.36 eV in T-4 and 0.32 eV in Tg These results are an experimental evi dence of chainlength dependence of Davydov splitting in organic conjugated model compounds. (C) 2001 Elsevier Science B.V. All rights reserved.