Intermolecular Interactions in Conjugated Oligothiophenes. 2. Quantum Chemical Calculations Performed on Crystalline Structures of Terthiophene and Substituted Terthiophenes

Quantum-chemical calculations are used to investigate the influence of intermolecular interactions on the absorption spectra of unsubstituted terthiophene (TT) as well as 3,3‘ ‘-dimethyl-2,2‘:5‘,2‘ ‘-terthiophene (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 subcrystalline forms of various sizes (2 and 4 molecules). For all molecules investigated, intermolecular interactions lead to a splitting (Davydov's splitting) of the lowest optical singlet transition compared to that calculated for an isolated molecule. These results are interpreted through the use of the excitonic model. The splitting of the first electronic transition is very sensitive to the different intermolecular distances and orientations found in the crystalline structures of each molecule. TT shows an important excitonic effect on the first allowed transition whereas the splitting is less important for substituted terthiophenes. The spectral shifts caused by intermolecular interactions are compared with those induced by conformational changes toward planarity for the isolated molecules in the crystals (packing effects). The results clearly show that the excitonic effect is mainly responsible for the optical properties of TT in its aggregated form whereas, for substituted terthiophenes, the conformational change suggested in part 1 of this series of papers is the major cause of the red shifts observed in their absorption bands following aggregation.