Photoswitchable materials are important in broad applications. Recently appeared inorganic photoswitchable upconversion nanoparticles (PUCNPs) become a competitive candidate to surmount the widespread issue of the organic counterparts -photobleaching. However, current PUCNPs follow solely Yb/Nd cosensitizing mode, which results in complex multilayer doping patterns and imperfectness of switching in UV-blue region. In this work, we have adopted a new strategy to construct Nd free PUCNPs-NaErF@NaYF@NaYbF:0.5%Tm@NaYF. These PUCNPs demonstrate the superior property of photoswitching. A prominent UV-blue emission from Tm is turned on upon 980 nm excitation, which can be completely turned off by 800 nm light. The quasi-monochromatic red upconversion emission upon 800 nm excitation-a distinct feature of undoping NaErF upconversion system-endows the PUCNPs with promising image-guided photoinduced "off-on" therapy in biomedicine. As a proof-of-concept we have demonstrated the imaging-guided photodynamic therapy (PDT) of cancer, where 800 nm excitation turns off the UV-blue emission and leaves the emission at 660 nm for imaging. Once the tumor site is targeted, excitation switching to 980 nm results in UV-blue emission and the red emission. The former is used to induce PDT, whereas the latter is to monitor the therapeutic process. Our study implies that this upconversion photoswitching material is suitable for real-time imaging and image-guided therapy under temporal and spatial control.

Internal hydroxyl impurity is known as one of the main detrimental factors affecting the upconversion (UC) efficiency of upconversion luminescence (UCL) nanomaterials. Different from surface/ligand-related emission quenching which can be effectively diminished by, e.g., core/shell structure, internal hydroxyl is easy to be introduced in synthesis but difficult to be quantified and controlled. Therefore, it becomes an obstacle to fully understand the relevant UC mechanism and improve UC efficiency of nanomaterials. Here we report a progress in quantifying and large-range adjustment of the internal hydroxyl impurity in NaYF4 nanocrystals. By combining the spectroscopy study and model simulation, we have quantitatively unraveled the microscopic interactions underlying UCL quenching between internal hydroxyl and the sensitizers and activators, respectively. Furthermore, the internal hydroxyl-involved UC dynamical process is interpreted with a vivid concept of “Survivor effect,” i.e., the shorter the migration path of an excited state, the larger the possibility of its surviving from hydroxyl-induced quenching. Apart from the consistent experimental results, this concept can be further evidenced by Monte Carlo simulation, which monitors the variation of energy migration step distribution before and after the hydroxyl introduction. The new quantitative insights shall promote the construction of highly efficient UC materials.

losses due to a suitable Δ E T (≤0.3 eV) between the respective host and guest. Employing as few as possible components in the emitting system of WOLEDs is a means to reduce the energy losses through the simplifi ed exciton-formation and energy-transfer processes. For achieving this, appropriate multifunctional emitter molecules are needed combined with a smart device design strategy.A few effi cient phosphorescent OLEDs (PhOLEDs) have been reported based on phosphorescent hosts. [ 7a,d ] These nofl uorescent-host emitting systems are promising to simplify/optimize the electrophosphorescent process. [ 7 ] However, until now, integrating such an advanced doping model into the construction of the WOLEDs has not yet been achieved. In this work, three emitting complexes originating from our group: bis(2-(2-hydroxyphenyl)-pyridine)beryllium (Bepp 2 , λ max ≈ 450 nm), [ 8 ] bis(4,6-di-fl uorophenyl)pyridinato-N , C 2′ iridium (III) N , N ′-diisopropyl-carbazol-9-yl-amidine (FPPCA, λ max ≈ 500 nm), and bis (7,8-benzoquinolinato)[ 9 ] which could provide essential colors of blue (B), green (G), and orangered (OR), respectively, for white light, were well organized for realizing a simplifi ed WOLED composed of two adjacent G-OR (FPPCA:BZQPG) and B-G (Bepp 2 :FPPCA) emitting layers (EMLs). In this strategy, phosphorescent (P) molecule FPPCA was distributed through both EMLs and showed an unprecedented multifunctional property by playing four key roles: (i) the charge-transporting host, (ii) the green emitting host, (iii) the sensitizer for the dopant P molecule BZQPG in the G-OR layer, and (iv) the green dopant emitter in the B-G layer. This new method endowed the device with the advantage of a reduced number of constituent components and EMLs, which allows for a simplifi ed fabrication processes and effectively reduces structural heterogeneity. Furthermore, careful manipulation for the well-matched FPPCA:BZQPG combination utilizes all the electrically generated excitons in the phosphor-phosphor type (PPT) G-OR layer, where the bipolar character of FPPCA results in a wide emission zone to enhance carrier and exciton utilization, thereby dominating the high electrophosphorescent effi ciency. Meanwhile, the fl uorescent (F) molecule Bepp 2 is used to generate blue singlet emission in the B-G layer and served as a host for sensitizing green emission, thereby achieving the broad white EL spectrum. An optimal management of B-G and G-OR layers aiming at balanced charge injection together with simultaneous effi cient charge/exciton confi nement, ensured this three-color device possesses stable and high EL performance White organic light-emitting diodes (OLEDs) are a very promising technology for next-generation solid-state lighting. [ 1 ] High-quality illumination sources require white OLEDs (WOLEDs) with a high color-rendering index (CRI) of >80. [ 1c ] Although some two-color WOLEDs produced by an orange or yellow emitter complemented with a blue emitter, exhibit impressive electroluminescence (EL) effi ciency, they ha...

A perfect "off" to "on" switch of the therapeutic function is very important to minimize the phototoxicity of nanoplatforms assisted imaging-guided photodynamic therapy (PDT) of cancer. Current approaches rely on preloaded photosensitizers, where the off/on state of PDT is regulated by the sensitizing light of photosensitizers. However, the photoactivities inevitably occur when imaging/diagnosis or exposure to sunlight, etc. These preloading approaches will cause the damage to normal cells and the photosensitivity to the skin. Taking upconversion photodynamic therapy as an example we report here a biorthogonal chemistry solution to circumvent this problem. The luminescence upconversion nanoparticles (UCNPs) are anchored with one handle of click reaction and targeting entity, these nanoplatforms enable the imaging/labelling/tracking, especially for imaging-guided surgery. Once they are targeted, the photosensitizers armed with the other match handle will be injected in situ and click reaction will occur between the two handles to link the photosensitizers closely with the targeted nanoplatforms in a very short time, enabling the PDT function of the nanoplatforms. Proof of principle has been demonstrated in vitro and in vivo. This approach can be readily extended to chemotherapy, radiotherapy, etc. to overcome the side effect of these therapies of cancers.