Polymer-based emitters are a promising route to the production of low-cost, scalable solution-processable luminescent materials. Here we describe a series of acrylic oxadiazole-based donor–acceptor monomers with tunable emission from blue to orange, with quantum yields as high as 96%. By introducing structural constraints that limit donor–acceptor orbital overlap, thermally activated delayed fluorescence (TADF) was observed in these materials. Polymerization by Cu(0) reversible deactivation radical polymerization (RDRP) gave high-molecular-weight copolymers (M n > 20 kDa) with dispersities ranging from 1.10 to 1.45, using a room-temperature procedure with Cu wire as a catalyst. One of these materials, which had phenothiazine as donor moiety, exhibited conformationally dependent dual emission, giving a mixture of prompt fluorescence and delayed fluorescence peaks, whose relative ratios varied based on the amount of O2 present during measurement. We demonstrate that this material can combine prompt and delayed fluorescence to act as a single-component, all-organic, ratiometric oxygen sensor without external calibrant. Application to ratiometric oxygen sensing is demonstrated both using a polymer thin film and via incorporation of this material into water-soluble polymer dots (Pdots), with a ratiometric response to O2 throughout the range of partial pressures relevant to biological environments.

Fluorescence imaging in living cells is key to understanding many biological processes, yet autofluorescence from the sample can lower sensitivity and hinder high-resolution imaging. Time-gated measurements using phosphorescent metal complexes can improve imaging, at the cost of potential toxicity from the use of heavy metals. Here, we describe orange/red-emitting polymer dots (Pdots) exhibiting thermally activated delayed fluorescence (TADF) for time-gated imaging. Inspired by the cell invasion mechanism of the HIV TAT protein, the Pdots were formed from block copolymers composed of a hydrophilic guanidine-rich block as a cell-penetrating peptide mimic, and a rigid organic semiconductor block to provide efficient delayed fluorescence. These all-organic polymer nanoparticles were shown to efficiently enter HeLa, CHO, and HepG2 cells within 30 min, with cell viabilities remaining high for Pdot concentrations up to 25 mg mL–1. Pdot quantum yields were as high as 0.17 in aerated water, with the Pdot structure effectively shielding the TADF emitters from quenching by oxygen. Colocalization experiments revealed that the Pdots primarily accumulate outside of lysosomes, minimizing lysosomal degradation. When used for fixed cellular imaging, Pdot-incubated cells showed high signal-to-background ratios compared to control samples with no Pdot exposure. Using time-resolved spectroscopy, the delayed emission of the TADF materials was effectively separated from that of both a biological serum and a secondary fluorescent dye.

Near‐infrared‐emitting polymers were prepared using four boron‐difluoride‐curcuminoid‐based monomers using ring‐opening metathesis polymerization (ROMP). Well‐defined polymers with molecular weights of ≈20 kDa and dispersities <1.07 were produced and exhibited near‐infrared (NIR) emission in solution and in the solid state with photoluminescence quantum yields (ΦPL) as high as 0.72 and 0.18, respectively. Time‐resolved emission spectroscopy revealed thermally activated delayed fluorescence (TADF) in polymers containing highly planar dopants, whereas room‐temperature phosphorescence dominated with twisted species. Density functional theory demonstrated that rotation about the donor–acceptor linker can give rise to TADF, even where none would be expected based on calculations using ground‐state geometries. Incorporation of TADF‐active materials into water‐soluble polymer dots (Pdots) gave NIR‐emissive nanoparticles, and conjugation of these Pdots with antibodies enabled immunofluorescent labeling of SK‐BR3 human breast‐cancer cells.

A series of naphthalimide (NAI)-based red-emissive thermally activated delayed fluorescence (TADF) acrylic monomers has been designed and synthesized. When copolymerized with a host material by Cu(0)-reversible deactivation radical polymerization (Cu(0)-RDRP), polymers exhibiting orange to deep-red TADF were obtained with quantum yields of up to 58% in solution and 31% in the solid state. These emitters exhibit dual emission consisting of high-energy prompt fluorescence from the NAI acceptor (λ max = 340 nm in toluene) and red-delayed fluorescence from the charge-transfer process (λ max = 633−711 nm in toluene). This dual emissive property was utilized to create redto-blue temperature-responsive polymers by copolymerization of NAI−DMAC with N-isopropylacrylamide and a blue fluorescent dopant. These polymers exhibit red TADF at room temperature and blue fluorescence at 70 °C, with a high ratiometric fluorescent thermal response of 32 ± 4% K −1 . Such systems are anticipated to have utility in bioimaging, drug delivery, and temperature sensing, further expanding the range of applications for red TADF materials.

Though molecules exhibiting thermally activated delayed fluorescence (TADF) have seen extensive development in organic light-emitting diodes, their incorporation into polymer nanomaterials and thin films has led to a range of applications in sensing and imaging probes. Triplet quenching can be used to probe oxygen concentration, and the reverse intersystem crossing mechanism which gives rise to TADF can also be used to measure temperature. Moreover, the long emission lifetimes of TADF materials allows for noise reduction in time-gated microscopy, making these compounds ideal for time-resolved fluorescence imaging (TRFI). A polymer matrix enables control over energy transfer between molecules, and can be used to modulate TADF behavior, solubility, biocompatibility, or desirable mechanical properties. Additionally, a polymer's oxygen permeability can be tuned to suit imaging applications in a range of media. Here we review the applications of polymer nanoparticles and films exhibiting TADF in sensing and imaging, demonstrating that this class of materials has great potential beyond electroluminescent devices still waiting to be explored.