Design of two-photon absorbing fluorophores for FRET antenna-core oxygen probes

Four two-photon absorbing fluorophores A1–A4 are reported and their spectroscopic properties are analyzed for use, in combination with palladium–porphyrinato complexes C1 and C2, as two-photon absorbing antennas and energy donors for FRET-based antenna-core oxygen sensitive phosphorescent probes. The best antenna A3 displaying the highest biphotonic brightness Φ × σTPA = 210 GM at 800 nm was then directly grafted to the phosphorescent Pd–porphyrin cores via click chemistry to give new two-photon oxygen sensitive phosphorescent probes P1 and P2, in which energy transfer from the antenna to the core was demonstrated following two-photon excitation.