Observation of the lowest Ag excited state lying in the optical gap in polydiacetylene by means of second-harmonic generation spectroscopy

We have observed second-harmonic generation (SHG) spectra in polydiacetylene (poly [5,7-dodecadiyne-1,12-diyl-bis-ethylurethane], PDA-4U2 for short) films of red and blue forms. In addition to resonant peaks located at the high-energy side of the main absorption peak, a new resonant peak was observed at the lower-energy side of the absorption peak in blue-form PDA-4U2. In contrast, in red-form PDA-4U2, the SHG resonance peak position in the lower-energy side coincides with the absorption peak position. The resonance enhancement of SHG originates from the electric-quadrupole transition to the dipole-forbidden state. The lowest resonance peak observed in the blue-form PDA-4U2 strongly suggests the existence of the lowest Ag excited state lying below the lowest Bu excited state.