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Transforming carbon dots (CDs) fluorescent materials into smart materials with complex functions is a topic of great interest to nanoscience. However, designing CDs with regulating fluorescence/phosphorescence that can be visually monitored with the environment changes in real‐time remains a challenge. Here, a very simple strategy, one‐step solvent‐free catalytic assistant strategy, which is low cost, facile, environment‐friendly, and high throughput, is put forward. Hydrogen bond is used to manipulate nanostructure of CDs, and the obtained carbon dots (M‐CDs) show a series of attractive properties including matrix‐free room‐temperature phosphorescence, time‐dependent fluorescence, and near‐infrared emissive characteristics. Different from the traditional aggregation caused quenching or aggregation‐induced emission fluorescent materials, M‐CDs exhibit unprecedented and unique dispersion induced redshift fluorescence phenomenon, promoting the studies of fluorescence from static to dynamic. The causes of this phenomenon are further analyzed in detail. As a kind of intelligent fluorescent materials, this new designed CDs greatly enrich the basic recognition of CDs by illustrating the relationship between redshift fluorescence behaviors and the dispersion states, and may provide with an opportunity for solid‐state fluorescent materials, anti‐counterfeiting, cellular imaging, and hopefully many others. [ABSTRACT FROM AUTHOR]

Efficient solid-state room-temperature phosphorescence (RTP) carbon dots (CDs) usually need to be embedded by a matrix to suppress the vibration dissipation between triplet and singlet states. However, selecting a suitable matrix is time-consuming, and the preparation process is often complicated. Here, we adopted a simple one-step hydrothermal method to obtain matrix-free solid-state RTP CDs with a long lifetime (1.22 s) and high fluorescence quantum yield (21.6%) by selecting suitable chain-like structure molecules for dehydration condensation and carbonization. The main source of RTP is the n-π* transition of C O/C N on the surface of CDs. The polymer chain structure effectively avoids the disordered collision of CDs during curing. The formed cross-linked network structure helps stabilize the excited triplet state and provides a rigid environment to prevent the triplet state excitons from being quenched by ambient oxygen. The facile preparation process and efficient RTP properties provide a new idea for researching and developing matrix-free CDs. A simple one-step hydrothermal method for preparing matrix-free solid-state carbon dots with ultralong-lifetime room-temperature phosphorescence (1.22 s) and high fluorescence quantum yield (21.6%). [Display omitted] • Efficient matrix-free solid-state RTP CDs were achieved by a simple one-step solvothermal method. • The FL-QY is up to 21.6%, which is a high level among matrix-free RTP CDs. • The CDs have an ultralong-lifetime RTP (1.22 s), and the green phosphorescence visible to the naked eye is up to 7 s. • The structures of CDs in aqueous solution and solid state were proposed by morphology and structure characterization. [ABSTRACT FROM AUTHOR]

[Display omitted] • R-CDs with excitation-independent red fluorescence were successfully prepared by hydrothermal treatment of p -phenylenediamine. • Y-CDs with aggregation-induced emission and strong solid-state emission were prepared by the reaction of R-CDs and salicylaldehyde. • The synthesized Y-CDs showed obvious aggregation-induced and solvent crystallization-induced fluorescence enhancement behaviors. As a novel carbon-based fluorescent nanomaterial, the optical properties of carbon dots (CDs) in dispersed state have been extensively studied. Nevertheless, carbon dots-based fluorescent materials with high luminescence efficiency in both aggregated and dispersed states are rarely reported. Herein, hydrothermal treatment of p -phenylenediamine (PPD), followed by column chromatography, the obtained R-CDs showing excitation-independent red fluorescence (FL) emission. Significantly, yellow fluorescent powder (Y-CDs) with strong solid-state emission has been prepared by the reaction of R-CDs and salicylaldehyde (SA). More interestingly, the synthesized Y-CDs showed obvious aggregation-induced and solvent crystallization-induced FL enhancement behaviors. These findings have a certain reference significance for the development of CDs-based materials with high luminescence efficiency in both aggregated and dispersed states. [ABSTRACT FROM AUTHOR]

• HB-CDs were successfully prepared by hydrogen bonding self-assembly. • The preparation process is environmentally friendly, easy to operate and low cost. • The fluorescence emission of HB-CDs shifts from yellow to blue when hydrogen-bonding breaking reagents are introduced. Carbon dots (CDs) can be wildly used as fluorescence (FL) sensing, bioimaging, drug delivery and optoelectronic devices because of their distinguished photo-stability against photobleaching, low toxicity and easy surface functionalization toward different applications. However, solid fluorescent CDs are rarely reported due to the notorious aggregation caused quenching (ACQ) effects. Here, we proposed a simple synthesis strategy to prepare CDs via hydrogen bonding self-assembly (HB-CDs). Most important of all, the FL emission of HB-CDs can be easily regulated based on hydrogen bond dynamic adjustability. When hydrogen-bonding break reagents were introduced, the emission shifted from yellow to blue. Hence, we put forward a novel solid-state photoluminescence mechanism, namely, the yellow emission can be excited by the formation of J -aggregates and the blue emission can be excited by dispersion. Based on this phenomenon, we developed a new CDs composites with dual modes FL emissions. These findings may provide a novel strategy to easily prepare CDs with solid-state fluorescence and tunable fluorescence. [ABSTRACT FROM AUTHOR]