Inductive effects in alternants hydrocarbons: the two-photon spectra of biphenylene and 2,3,6,7-tetra-ethyl-biphenylene

The two-photon spectra of biphenylene (BP) and 2,3,6,7-tetra-ethyl-biphenylene (TEBP) have been measured in the region of the lowest two electronic transitions (310–440 nm). The S0 → S1 transition of BP, although group-theoretically allowed, is only weakly observed. The TEBP spectrum is, for both transitions, stronger than that of BP. The experimental data are related to the pseudoparity symmetry of altemant hydrocarbons and to the inductive perturbation of pseudoparity selection rules. Due to the enhancement, the TEBP spectrum shows additional spectral structures with respect to BP, among which is the weak onset of the S0 → S1 transition around 23100 cm−1.