Nonlinear absorption spectra of ethidium and ethidium homodimer

Highlights: ► Two-photon absorption (TPA) spectra of ethidium bromide monomer and homodimer were measured. ► For EB TPA maxima are located at 950 nm and ∼600 nm. ► For EB-dimer maxima are located at 1100 nm and ∼600 nm. ► When normalized by molecular weights, TPA cross sections of both dyes are similar. ► Both dyes can be applied in DNA staining in TP fluorescence microscopy. -- Abstract: The Z-scan technique was used to determine the spectral dependence of the nonlinear absorption in well-known DNA intercalators: ethidium bromide and its homodimer. It is found that the compounds show essentially the same features of their nonlinear absorption spectra with the magnitudes of the relevant cross sections scaling with molecular weight of chromophore compound.