Zero-field splitting of the lowest excited triplet state in thiophene oligomers. An experimental and theoretical investigation

The lowest photoexcited triplet state of thiophene oligomers (nT) with n = 2 – 8 has been investigated by pulsed EPR spectroscopy. The characteristic EPR lineshape is analyzed by performing spectral simulations. The determined D values decrease with increasing chain length showing a linear dependence on the inverse chain length. Furthermore conformational effects have been observed. We compare the results with theoretical calculations of the zero-field splitting parameters. The calculations are based on a semiempirical ’one electron per site’ model including electron-electron interactions. The computed D values are in excellent agreement with the EPR data. The chain length dependence of D could be rationalized by computing electron spin densities and bond length distortions, which clearly indicate that the triplet excitation reaches a finite extention over about four thiophene rings.