Polarized two-photon studies of biphenyl and several derivatives

The two-photon absorption spectrum of biphenyl dissolved in CCl4 is reported, using both the absolute two-photon absorption technique and the two-photon excited fluorescence technique. Polarization methods were used which permit the strong two-photon absorptions to be assigned without resort to any subsidiary vibrational analysis. We find that the lowest singlet of biphenyl is a B state (in group D2, with twisted rings); its band stretches from about 33 000–36 000 cm−1, well to the red of the lowest strong one-photon transition. We also find clear evidence of an A state hidden to one-photon spectroscopy in the 42 000–45 000 cm−1 region. Other A states are also possibly present. Two-photon spectra of 4,4′ dichlorobiphenyl, 4,4′ difluorobiphenyl, 2,2′ bipyridine, 2,2′ difluorobiphenyl, and terphenyl are also given and discussed.