Intermolecular interactions in conjugated oligothiophenes. 2. Quantum chemical calculations performed on crystalline structures of terthiophene and substituted terthiophenes
N. Dicesare et al., Intermolecular interactions in conjugated oligothiophenes. 2. Quantum chemical calculations performed on crystalline structures of terthiophene and substituted terthiophenes, J PHYS CH A, 103(7), 1999, pp. 803-811
Quantum-chemical calculations are used to investigate the influence of inte
rmolecular interactions on the absorption spectra of unsubstituted terthiop
hene (TT) as well as 3,3"-dimethyl-2,2':5',2"-terthiophen (DMTT) and 3',4'-
dibutyl-2,2':5',2"-terthiophene (DBTT). The semiempirical ZINDO/S method is
employed to calculate the energy of the electronic transitions of a single
molecule in this crystalline structure and of interacting molecules in sub
crystalline forms of various sizes (2 and 4 molecules). For all molecules i
nvestigated, intermolecular interactions lead to a splitting (Davydov's spl
itting) of the lowest optical singlet transition compared to that calculate
d for an isolated molecule. These results are interpreted through the use o
f the excitonic model. The splitting of the first electronic transition is
very sensitive to the different intermolecular distances and orientations f
ound in the crystalline structures of each molecule. TT shows an important
excitonic effect on the first allowed transition whereas the splitting is l
ess important for substituted terthiophenes. The spectral shifts caused by
intermolecular interactions are compared with those induced by conformation
al changes toward planarity for the isolated molecules in the crystals (pac
king effects). The results clearly show that the excitonic effect is mainly
responsible for the optical properties of TT in its aggregated form wherea
s, for substituted terthiophenes, the conformational change suggested in pa
rt 1 of this series of papers is the major cause of the red shifts observed
in their absorption bands following aggregation.