Two-photon spectra of stiff stilbenes: A contribution to the assignment of the low lying electronically excited states of the stilbene system

The one- and two-photon spectra of trans-stilbene are reinvestigated and compared to the spectra of two stilbene derivatives in which rotation around the vinyl-phenyl bonds is prevented by methylene or ethano bridges. The spectra are compared to recent theoretical results that did not agree with some of the earlier assignments. The new experimental data lead to a consistent assignment of the low energy part of the singlet spectrum of trans-stilbene up to an excitation energy of about 50 000 cm(-1), which is in excellent agreement with the theoretical findings. Special consideration is given to the weaker of the first two allowed singlet transitions whose location has been under discussion for quite some time.