Electronic properties of a novel class of conjugated systems: transition metal substituted oligothiophenes

We report on a spectroscopical study of a novel class of push-pull molecules containing a Chromium or Tungsten atom connected to an oligothiophene through a carbenic bond. The electronic coupling between the pi electrons of the conjugated system and the d electrons of the transition metal has been monitored through absorption and Raman spectra. This interaction between d and pi electrons leads to a red shift of the thienylene pi-pi* absorption band and to the appearance of a new strong metal to ligand charge transfer band at lower energies. In this latter transition the electrical dipole reverses its direction upon photoexcitation from the ground to the first excited state, thus accounting for the enhanced non linear optical response of these molecules. Solvatochromic effect has been used to estimate the second order molecular hyperpolarizability