Zero‐field‐splitting and π‐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 π‐electron spin density have been performed and compared with previous experimental EPR results. The calculations are based on a simple π‐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 π‐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.