Influence of Conformation on Two-Photon Spectra of Polyalkylthiophene

We compare the optical properties of polyalkyl-thiophenes and -bithiophene having different conformations. We measure in thin layer the modulus and phase of the second order coefficient 2(3ω;ω,ω,ω) from 0.6 eV to 1.5 eV. We show that Third Harmonic Generation measurements together with optical absorption allow the determination of two-photon energy levels even in the case of inhomogeneous broadening. These are located 0.5–0.6 eV below the one-photon states. Experimental results compare well with quantum chemical calculations.