Efficient Energy Transfer under Two-Photon Excitation in a 3D, Supramolecular, Zn(II)-Coordinated, Self-Assembled Organic Network

Multiphoton excited fluorescence of organic molecules is promising in the applications of efficient nonlinear optical devices and bioimaging. However, they usually have disadvantages of poor photostability and serious fluorescence quenching in aqueous media or solid state, which seriously limit their related applications. In this work, for the first time, the two-photon excited Forster resonance energy transfer (FRET) process is used to enhance the solid-state fluorescence of the supramolecular centre (acceptor) in an artificial 3D metal-organic complex (MLC), in which a 3D Zn (II)-coordinated tetrahedral core is utilized as the donor. More interestingly, the two-photon light harvesting system, which can be pumped with an optical intensity as low as 1 MW/cm(2), exhibits an ultrafast energy transfer rate (similar to 6.9 x 10(8) s(-1)) and ultrahigh photostability. The underlying physical mechanisms are revealed through comprehensive steady-state and time-resolved spectroscopic analysis. This work demonstrates that the 3D MLC can be directly used in two-photon bioimaging and also sheds light on developing other multiphoton harvesting systems, such as metal-organic frameworks.