ZERO-FIELD SPLITTING OF THE LOWEST EXCITED TRIPLET-STATE IN THIOPHENEOLIGOMERS - 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 chara cteristic EPR lineshape is analyzed by performing spectral simulations . The determined D values decrease with increasing chain length showin g a linear dependence on the inverse chain length. Furthermore conform ational We compare the results with theoretical calculations of the ze ro-field splitting parameters. The calculations are based on a semiemp irical 'one electron per site' model including electron-electron inter actions. The computed D values are in excellent agreement with the EPR data. The chain length dependence of D could he rationalized by compu ting electron spin densities and bond length distortions, which clearl y indicate that the triplet excitation reaches a finite extention over about four thiophene rings.