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Slipped Structure of Covalent Organic Framework Facilitates Two-Photon Adsorption for Improving Near-Infrared Excited Fluorescence Imaging.

Fluorescent materials exhibiting the characteristics of strong two-photon absorption (TPA) are extensively used for nonlinear optics, bio-imaging and phototherapy. One practical approach to obtain fluorescent materials with high TPA performance is to polymerize molecular chromophores to form π-conjugated structure. This leads to the increase in TPA cross-section per chromophore, however, efforts to towards this direction was capped by the lack of long-range ordering in the structure and the strong π-π stacking between the chromophores. Here, we reported the rational design of benzothiadiazole-based covalent organic framework (COF) for promoting TPA performance and obtaining the efficient two-photon excited fluorescence. Structure characterizations and spectroscopic studies revealed that the enhancement in TPA performance was attributed to the donor-π-acceptor-π-donor (D-π-A-π-D) configuration of the chromophore, long-range order, and large π-conjugation domain of COF crystals. The structural slipping in TPA-COF not only attenuates the π-π stacking interaction between the layers, but more importantly, overcomes the aggregation-caused emission quenching of the chromophores for improving near-infrared two-photon excited fluorescence imaging.