Zero-field-splitting and pi-electron spin densities in the lowest excited triplet state of oligothiophenes

The electronic properties of thiophene oligomers (nT, n = 2-8) have been investigated in the lowest excited triplet state. Theoretical calculations of the zero field splitting parameters and of the pi-electron spin density have been performed and compared with previous experimental EPR results. The calculations are based on a simple pi-electron (one-electron-per-site) model including electron-electron interaction at the extended Hubbard level. Optimized bond lengths result from making them self-consistent to the corresponding bond orders via Coulson's relationship, The calculated D values decrease from D = 0.0959 cm(-1) for n = 2 to D = 0.0597 cm(-1) for n = 8, in agreement with EPR data. The measured as well as the calculated E values are rather small. Furthermore, we found that ZFS parameters are affected by the torsion angles between the thiophene rings. The chain length dependence of D can be rationalized comparing pi-electron spin density calculations and computed bond length distortions. These clearly indicate that the triplet excitation reaches a finite extension over about four thiophene rings. (C) 1996 American Institute of Physics.