M. Muccini et al., Effect of wave-function delocalization on the exciton splitting in organicconjugated materials, PHYS REV B, 62(10), 2000, pp. 6296-6300
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 know
n 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 systems. We have therefore
measured the Davydov splitting of the lowest 1 B-1(mu) exciton in the model
systems of alpha-quaterthienyl (T-4) and alpha-sexithienyl (T-6) from the
low-temperature polarized absorption of oriented single crystals. Due to th
e very similar crystal structure of the two materials, the change in excito
n splitting can be directly related to the different molecular wave-functio
n 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, i
s confirmed by correlated quantum-chemical calculations. In addition, we pr
edict the size of the interchain interactions in the polymer where a direct
optical measurement is precluded by the intrinsic molecular disorder.