Davydov splitting in the sexithiophene crystal

The apparent discrepancy is analysed between the literature value of the le vel splitting calculated by ZINDO method for sexithiophene clusters and the Davydov splitting calculated for the three-dimensional crystal of alpha -s exithiophene in the dipole approximation with the transition moment distrib uted over the six rings. Consistency is restored by identifying the relevan t interactions that are not taken into account in the cluster approach. The results suggest that the band in the alpha -sexithiophene absorption spect rum, previously assigned to the upper Davydov component, might be due to ch arge-transfer (CT) states; other evidence pointing to the same conclusion i s reviewed. (C) 2001 Elsevier Science B.V. All rights reserved.