Two-photon spectra of substituted benzenes

A perturbation approach to the two-photon spectra of monosubstituted benzenes for the 1B2(Lb)←1A1 transition is developed. The theory predicts that inductive substituents are ineffective in making the benzene 1Lb←1A two-photon spectrum allowed (opposite to the effect on the one-photon spectrum), but weakly mesomeric substituents are much more effective. Intensity measurements have been made which show that the allowed part of the toluene two-photon spectrum is much stronger than in fluorobenzene, and that the halobenzene intensity ordering is Br≳Cl≳≳F, in direct contrast with results found for one-photon spectra and in agreement with the theory. The two-photon spectrum of aniline is reported and found to be identical with the one-photon spectrum where they can be compared. This result also comes out of the theory, where the Franck–Condon part of the one- and two-photon spectra for strongly mesomeric substituents are predicted to be identical.