Two‐Photon Absorption Cross‐Sections of Reference Dyes: A Critical Examination

The electronic structure and one‐ and two‐photon absorption spectra of four fluorophores, p‐bis(o‐methoxystyryl)benzene (Bis‐MSB), coumarin 307, fluorescein and rhodamine B, commonly used as reference compounds for two‐photon absorption spectra, have been theoretically calculated and compared with available experimental data. The possible reasons for the wide discrepancies in two‐photon absorption cross‐sections reported in the literature are discussed on the basis of the theoretical findings. The role of a solvent environment on the electronic one‐ and two‐photon absorption spectra is also studied. We highlight some necessary precautions that one needs to take when comparing literature results of two‐photon absorption cross‐sections.