At 776 nm lanthanide–ANTHR systems are excited by simultaneous absorption of three photons, whereas fluorescein is excited by two photons. Top: Emission intensity of fluorescein and ANTHR–DTPA–Eu measured under the same experimental conditions (excitation, 776 nm). Fluorescein, 1 nM, pH 11; Eu, 7.5 mM; DTPA–ANTHR, 6 mM; MOPS; 1 mM; DMF, 7.5%; D2O, 75%. Bottom: Emission intensity of fluorescein and ANTHR–DTPA–Tb measured under the same experimental conditions (excitation, 776 nm). Fluorescein, 10 nM, pH 11; Tb, 7.5 mM; DTPA–ANTHR, 6 mM; MOPS, 1 mM; DMF, 7.5 %.