The potential of three-dimensional (3D) metal-halide perovskites to sensitize organic triplets is unveiled. Nanocrystals of surface-modified inorganic cesium lead halide perovskites (CsPbX, X = Br/I) are found to work as efficient triplet sensitizers for photon upconversion based on triplet-triplet annihilation (TTA-UC) at low excitation intensity.

A new family of surface-functionalized CdSe/ZnS core-shell quantum dots (csQD) has been developed, which work as triplet sensitizers for triplet-triplet annihilation-based photon upconversion (TTA-UC). The surface modification of csQD with acceptor molecules plays a key role in the efficient relay of the excited energy of csQD to emitter molecules in the bulk solution, where the generated emitter triplets undergo triplet-triplet annihilation that leads to photon upconversion. Interestingly, improved UC properties were achieved with the core-shell QDs compared with core-only CdSe QDs (cQD). The threshold excitation intensity, which is defined as the necessary irradiance to achieve efficient TTA process, decreases by more than a factor of four. Furthermore, the total UC quantum yield is enhanced more than 50-fold. These enhancements should be derived from better optical properties of csQD, in which the non-radiative surface recombination sites are passivated by the shell layer with wider bandgap.

Reversible emission color switching of triplet-triplet annihilation-based photon upconversion (TTA-UC) is achieved by employing an Os complex sensitizer with singlet-to-triplet (S-T) absorption and an asymmetric luminescent cyclophane with switchable emission characteristics. The cyclophane contains the 9,10-bis(phenylethynyl)anthracene unit as an emitter and can assemble into two different structures, a stable crystalline phase and a metastable supercooled nematic phase. The two structures exhibit green and yellow fluorescence, respectively, and can be accessed by distinct heating/cooling sequences. The hybridization of the cyclophane with the Os complex allows near-infrared-to-visible TTA-UC. The large anti-Stokes shift is possible by the direct S-T excitation, which dispenses with the use of a conventional sequence of singlet-singlet absorption and intersystem crossing. The TTA-UC emission color is successfully switched between green and yellow by thermal stimulation.

A blue dopant (BD) emitting pure blue fluorescence was created on new design strategies that combined specific partial molecular structures to reduce peak shoulder of the fluorescence spectrum. The top emission organic light emitting diode using the new BD achieved L/J/CIEy=201, which was about 10% higher than the conventional BD.

Reversible emission color switching of triplet-triplet annihilation-based photon upconversion (TTA-UC) is achieved by employing an Os complex sensitizer with singlet-totriplet (S-T) absorption and an asymmetric luminescent cyclophane with switchable emission characteristics.T he cyclophane contains the 9,10-bis(phenylethynyl)anthracene unit as an emitter and can assemble into two different structures, astable crystalline phase and ametastable supercooled nematic phase.T he two structures exhibit green and yellow fluorescence,r espectively,a nd can be accessed by distinct heating/ cooling sequences.T he hybridization of the cyclophane with the Os complex allows near-infrared-to-visible TTA-UC.T he large anti-Stokes shift is possible by the direct S-T excitation, which dispenses with the use of ac onventional sequence of singlet-singlet absorption and intersystem crossing. The TTA-UC emission color is successfully switched between green and yellowb ythermal stimulation.Photon upconversion (UC) processes allow the conversion of lower-energy photons to higher-energy photons.A mong various UC mechanisms,t riplet-triplet annihilation-based UC (TTA-UC) is particularly useful, as it allows UC under low excitation power density. [1] Recently,t he possibility to utilize an external stimulus to control the TTA-UC process has been studied for several advanced applications,s uch as spatial and temporal high-resolution fluorescence microscopy,m ulticolor barcoding,a nd remote control of molecular photoswitching reactions. [2] However,s uccessful examples have been limited to ON/OFF switching of UC emission by inhibiting TTAo rq uenching of the excited states, [2] while there have been no reports on UC emission color switching for condensed molecular assemblies.T here are two major challenges that have to be overcome to achieve color switching of TTA-UC in the solid state.F irst, it must be possible to assemble the luminophores in different molecular assemblies,w ith inter-luminophore distances that are close enough for triplet energy migration (TEM), and the luminophores should have suitable excited state energy levels for TTA-UC.S econd, while as ignificant red-shift of the lower energy state is required to provide ar ecognizable emission color change,t he emission should not overlap with the excitation wavelength of the sensitizer,a so therwise no upconverted emission would occur.Thefirst challenge can be overcome by utilizing luminescent molecular assembled materials that show achange of the molecular assembled structure in response to an external stimulus such as at emperature change or exposure to mechanical stimuli. As the photophysical properties of molecular materials significantly depend on the molecular assembled structure,e xternal stimuli-induced changes in molecular assembled structures can lead to pronounced alterations of the photoluminescent properties. [3] Indeed, many organic or organometallic compounds have been found to show thermally induced photoluminescent color changes in condensed states. [4] Thes eco...