Observation of the electric-dipole-forbidden states in polydiacetylene by means of resonance second-harmonic generation spectroscopy

We have observed resonance second-harmonic generation (SHG) spectra in polydiacetylene (poly [5,7-dodecadiyne-1,12-diyl-bis-ethylurethane]. 4U2 for short) films. The resonant peak appears on the high-energy side of the linear absorption peak. The peak observed in SHG spectra is assigned to an Ag exciton state, and the linear absorption peak to the lowest B-u exciton state. The observation of the dipole-forbidden A(g) state originates from the resonance enhancement of SHG due to an electric quadrupole transition at the SH frequency region. Thus, the resonance SHG technique serves as a powerful tool for investigating dipole-forbidden states in centrosymmetric media such as pi-conjugated polymers. The advantages of this method compared with other methods such as two-photon absorption and electroreflectance will be discussed.