Effect of wave-function delocalization on the exciton splitting in organic conjugated materials

Interchain interactions in organic conjugated solids causes 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 systems. We have therefore measured the Davydov splitting of the lowest $1{}^1{B}_u$ exciton in the model systems of α-quaterthienyl ${(T}_4)$ and α-sexithienyl ${(T}_6)$ from the low-temperature polarized absorption of oriented single crystals. Due to the very similar crystal structure of the two materials, the change in exciton splitting can be directly related to the different molecular wave-function delocalization over the chain. Here we show that the Davydov splitting of the lowest exciton decreases as the molecular wave-function delocalization increases. This result, which contradicts the dipole model expectations, is confirmed by correlated quantum-chemical calculations. In addition, we predict the size of the interchain interactions in the polymer where a direct optical measurement is precluded by the intrinsic molecular disorder.