Strong nonlinear optical phosphorescence from water-soluble polymer dots: Towards the application of two-photon bioimaging

Multiphoton fluorescence imaging of tissues offers an essential tool for studying biological systems. However, autofluorescence becomes a serious issue in complicated tissue imaging where biological molecules compete with the fluorophore. Such a critical issue can be resolved through the design of phosphorescent molecules with long emission lifetime. In this work, the nonlinear optical properties of small phosphorescent polymer dots (Pdots) with strong red-emitting phosphorescence (620 nm) have been disclosed. An analysis of decay data from fs pump-probe experiment yields a value of 164 ps for the singlet excited state lifetime of the Pdots. Notably, thanks to efficient two-photon excited energy transfer and moderate phosphorescence quantum yield (η = 1.36%), the Pdots exhibit large two-photon absorption action cross-section (ηδ_max = 15 GM) in aqueous solution. Results of living cell imaging experiments show the potential of these Pdots in two-photon time-resolved phosphorescent microscopy.