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Covalent organic frameworks (COFs) are an emerging kind of crystalline porous polymers which present the precise integration of organic building blocks into extensible structures with regular pores and periodic skeletons. The diversity of organic units and covalent linkages makes COFs a rising materials platform for the design of structure and functionality. In this review, we summarize the recent research progress in developing COFs for photoluminescent materials. We highlight structural and functional design strategies, and identify fundamental problems that need to be solved in conjunction with potential applications from perspectives of photoluminescent materials.

Fluorescent materials exhibiting two-photon induction (TPI) are used for nonlinear optics, bioimaging, and phototherapy. Polymerizations of molecular chromophores to form π-conjugated structures were hindered by the lack of long-range ordering in the structure and strong π-π stacking between the chromophores. Reported here is the rational design of a benzothiadiazole-based covalent organic framework (COF) for promoting TPI and obtaining efficient two-photon induced fluorescence emissions. Characterization and spectroscopic data revealed that the enhancement in TPI performance is attributed to the donor-π-acceptor-π-donor configuration and regular intervals of the chromophores, the large π-conjugation domain, and the long-range order of COF crystals. The crystalline structure of TPI-COF attenuates the π-π stacking interactions between the layers, and overcomes aggregation-caused emission quenching of the chromophores for improving near-infrared two-photon induced fluorescence imaging.

Polycyclic aromatic derivatives can trap 1 O2 to form endoperoxides (EPOs) for O2 storage and as sources of reactive oxygen species. However, these materials suffer from structural amorphism, which limit both practical applications and fundamental studies on their structural optimization for O2 capture and release. Metal-organic frameworks (MOFs) offer advantages in O2 binding, such as clear structure-performance relationships and precise controllability. Herein, we report the reversible binding of O2 is realized via the chemical transformation between anthracene-based and the corresponding EPO-based MOF. It is shown that anthracene-based MOF, the framework featuring linkers with polycyclic aromatic structure, can rapidly trap 1 O2 to form EPOs and can be restored upon UV irradiation or heating to release O2 . Furthermore, we confirm that photosensitizer-incorporated anthracene-based MOF are promising candidates for reversible O2 carriers controlled by switching Vis/UV irradiation.

Conventional oxygen-dependent photodynamic therapy (PDT) has faced severe challenges because of the non-specificity of most available photosensitizers (PSs) and the hypoxic nature of tumor tissues. Here, an O2 self-sufficient cell-like biomimetic nanoplatform (CAT-PS-ZIF@Mem) consisting of the cancer cell membrane (Mem) and a cytoskeleton-like porous zeolitic imidazolate framework (ZIF-8) with the embedded catalase (CAT) protein molecules and Al(III) phthalocyanine chloride tetrasulfonic acid (AlPcS4, defined as PS) is developed. Because of the immunological response and homologous targeting abilities of the cancer cell membrane, CAT-PS-ZIF@Mem is selectively accumulated at the tumor site and taken up effectively by tumor cells after intravenous injection. After the intracellular H2O2 penetration into the framework, it is catalyzed by CAT to produce O2 at the hypoxic tumor site, facilitating the generation of toxic 1O2 for highly effective PDT in vivo under near-infrared irradiation. By integrating the immune escape, cell homologous recognition, and O2 self-sufficiency, this cell-like biomimetic nanoplatform demonstrates highly specific and efficient PDT against hypoxic tumor cells with much reduced side-effect on normal tissues.