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

We have observed second-harmonic generation (SHG) and third-harmonic generation (THG) spectra in polydiacetylene (PDA), poly[5,7-dodecadiyne-1,12-diyl-bis-ethylurethane], thin films. According to linear absorption and THG spectra in PDA, two dipole-allowed states, the lowest Bu exciton and the band edge, exist around 2.0 eV and 2.6 eV, respectively. Therefore, a resonant peak appearing around 2.35 eV in SHG and THG spectra is due to the dipole-forbidden Ag exciton state. Thus, the SHG spectroscopy serves as a powerful tool for investigating dipole-forbidden states as much as THG spectroscopy.