Metal–organic frameworks (MOFs) display a wide range of luminescent behaviors resulting from the multifaceted nature of their structure. In this critical review we discuss the origins of MOF luminosity, which include the linker, the coordinated metal ions, antenna effects, excimer and exciplex formation, and guest molecules. The literature describing these effects is comprehensively surveyed, including a categorization of each report according to the type of luminescence observed. Finally, we discuss potential applications of luminescent MOFs. This review will be of interest to researchers and synthetic chemists attempting to design luminescent MOFs, and those engaged in the extension of MOFs to applications such as chemical, biological, and radiation detection, medical imaging, and electro-optical devices (141 references).

Luminescent Metal–Organic Frameworks

The development of white organic light-emitting diodes (OLEDs) holds great promise for the production of highly efficient large-area light sources. High internal quantum efficiencies for the conversion of electrical energy to light have been realized. Nevertheless, the overall device power efficiencies are still considerably below the 60-70 lumens per watt of fluorescent tubes, which is the current benchmark for novel light sources. Although some reports about highly power-efficient white OLEDs exist, details about structure and the measurement conditions of these structures have not been fully disclosed: the highest power efficiency reported in the scientific literature is 44 lm W(-1) (ref. 7). Here we report an improved OLED structure which reaches fluorescent tube efficiency. By combining a carefully chosen emitter layer with high-refractive-index substrates, and using a periodic outcoupling structure, we achieve a device power efficiency of 90 lm W(-1) at 1,000 candelas per square metre. This efficiency has the potential to be raised to 124 lm W(-1) if the light outcoupling can be further improved. Besides approaching internal quantum efficiency values of one, we have also focused on reducing energetic and ohmic losses that occur during electron-photon conversion. We anticipate that our results will be a starting point for further research, leading to white OLEDs having efficiencies beyond 100 lm W(-1). This could make white-light OLEDs, with their soft area light and high colour-rendering qualities, the light sources of choice for the future.

White organic light-emitting diodes with fluorescent tube efficiency

Organic materials that exhibit thermally activated delayed fluorescence (TADF) are an attractive class of functional materials that have witnessed a booming development in recent years. Since Adachi et al. reported high-performance TADF-OLED devices in 2012, there have been many reports regarding the design and synthesis of new TADF luminogens, which have various molecular structures and are used for different applications. In this review, we summarize and discuss the latest progress concerning this rapidly developing research field, in which the majority of the reported TADF systems are discussed, along with their derived structure–property relationships, TADF mechanisms and applications. We hope that such a review provides a clear outlook of these novel functional materials for a broad range of scientists within different disciplinary areas and attracts more researchers to devote themselves to this interesting research field.

Recent advances in organic thermally activated delayed fluorescence materials.

The design and characterization of thermally activated delayed fluorescence (TADF) materials for optoelectronic applications represents an active area of recent research in organoelectronics. Noble metal-free TADF molecules offer unique optical and electronic properties arising from the efficient transition and interconversion between the lowest singlet (S1) and triplet (T1) excited states. Their ability to harvest triplet excitons for fluorescence through facilitated reverse intersystem crossing (T1→S1) could directly impact their properties and performances, which is attractive for a wide variety of low-cost optoelectronic devices. TADF-based organic light-emitting diodes, oxygen, and temperature sensors show significantly upgraded device performances that are comparable to the ones of traditional rare-metal complexes. Here we present an overview of the quick development in TADF mechanisms, materials, and applications. Fundamental principles on design strategies of TADF materials and the common relationship between the molecular structures and optoelectronic properties for diverse research topics and a survey of recent progress in the development of TADF materials, with a particular emphasis on their different types of metal-organic complexes, D-A molecules, and fullerenes, are highlighted. The success in the breakthrough of the theoretical and technical challenges that arise in developing high-performance TADF materials may pave the way to shape the future of organoelectronics.

Thermally Activated Delayed Fluorescence Materials Towards the Breakthrough of Organoelectronics

Most present-day semiconductor devices use inorganic crystalline materials, with single-crystalline silicon dominating other materials like GaAs by about a factor of 1000. Despite the advantages of single-crystalline inorganic semiconductors like high room-temperature mobility (up to 1000 cm2/(V s)) and high stability, these materials are less suitable for low-cost and large-area applications. Additionally, silicon is an indirect semiconductor and therefore is not well suited for optoelectronic applications like light-emitting diodes. Solar cells from silicon are expensive and require a large amount of energy for their fabrication, leading to a long energy payback time. As an alternative, organic semiconductors have recently gained much attention (for review articles, see refs 1 -3 (OLEDs), ref 4 (organic electronics in general), and refs 5 and 6 (organic solar cells)). Originally, much of the research concentrated on single crystals, which can have mobilities of a few cm2/(V s) at room temperature and even much higher values at low temperature, as shown in the pioneering work of Karl et al.7 However, for practical applications, thinfilm organic semiconductors with disordered morphology, such as evaporated small-molecule compounds or polymers processed from solution, are prevailing. Organic semiconductors are already broadly applied as photoconductors for copiers and laser printers. * Corresponding author. E-mail: leo@iapp.de. Web address: www.iapp.de. 1233 Chem. Rev. 2007, 107, 1233−1271

Highly efficient organic devices based on electrically doped transport layers.

Summary of the physical properties of the carbazole monomer, oligomers,and other carbazole-based molecules. Compound T d [a][°C] T g [b][°C] k max, abs. [c][nm] k max, fluo. [d][nm] k max, phos. [e][nm] E singlet [f][eV] E triplet [g][eV]Cz – – 343 351 409 3.53 3.03BCz1 299 67 344 376 416 3.30 2.98BCz2 334 104 343 373 413 3.32 3.00BCz3 322 77 358 414 448 3.00 2.77TCz1 384 88 344 389 424 3.19 2.92TCz2 413 150 343 388 422 3.20 2.94CBP – 62 341 355 485 3.49 2.56mCP – 60 340 348 427 3.56 2.90SimCP – 101 [h] – – 427 [h] – 2.90 [h] [a] Temperature corresponding to a 5% weight loss in thermogravimetric analysis.[b] Glass-transition temperature determined by differential scanning calorimetry.[c] Absorption maximum in solutions (1×10 –5 in toluene). [d] Fluorescence peak insolutions (1×10 –5 in toluene). [e] Phosphorescence peak in solutions (1×10 in tolu-ene, 77 K). [f] Singlet energy estimated from the highest-energy fluorescence peak.[g] Triplet energy estimated from the highest-energy phosphorescence peak. [h] Prop-erties of SimCP were obtained from reference [7].

3-(9-Carbazolyl)carbazoles and 3,6-Di(9-carbazolyl)carbazoles as Effective Host Materials for Efficient Blue Organic Electrophosphorescence**

Phosphorescent OLEDs are under intense investigation because of their potential in greatly improving device performances. Electrophosphorescence is easily generated from both singlet and triplet excited states so the internal quantum efficiency of phosphorescent organic light-emitting diodes (OLEDs) can reach a theoretical level of unity, rather than the inherent 25% upper limit imposed by the formation of singlet excitons for their fluorescent counterparts. Thus, a great deal of effort has been spent investigating the third-row transition-metal complexes to develop highly efficient phosphors that can emit all three primary colors. Despite elegant research on both red and green phosphors, there are only a few reports on room-temperature blue phosphors. Two wellunderstood examples, bis(4’,6’-difluorophenylpyridinato)iridium(III) picolinate (FIrpic) and bis(4’,6’-difluorophenylpyridinato)iridium(III) tetra(1-pyrazolyl)borate (FIr6), have proven to be excellent dopants for greenishblue and sky-blue phosphorescent OLEDs. Further improvements were made by substituting the picolinate ions with other ancillary ligands, such as triazolate or tetrazolate ions to give FIrtaz or FIrN4, respectively, and even by employing a combination of cyanide and phosphine ligands. These modifications have produced a hypsochromic shift of around 10 nm compared to the emission of FIrpic. However, significant lowering of emission quantum yield was noted in some cases, which has hampered the fabrication of true-blue phosphorescent OLEDs. In theory, some critical criteria have to be considered in obtaining highly efficient blue phosphorescence. Of primary importance is to increase the contribution of the metal-toligand charge transfer (MLCT) in the lowest-lying triplet manifold. The direct involvement of the metal dp orbital enhances the coupling of the orbital angular momenta to the electron spin, such that the T1!S0 transition would have a large first-order spin–orbit coupling term, which would result in a drastic decrease in the radiative lifetime and hence the possibility of increasing the quantum yield. Conversely, care has to be taken to avoid enhanced radiationless pathways arising from enlargement of the band gap. One familiar deactivation pathway lies in the d–d transition, which may weaken the metal–ligand bond, thus resulting in a shallow potential-energy surface. In an extreme case, the shallow d–d potential-energy surface may intercept surfaces of other states, which may result in radiationless deactivation. This process, however, should be minor for third-row transition metals owing to their high coordination strength that increases the energy of the ds* orbitals. Furthermore, upon increasing the energy gap towards blue, it becomes facile for the lowest-lying excited state—a state perhaps comprising both p–p* (or intraligand charge transfer, ILCT) and MLCT in character—to mix with a thermally accessible ligand-to-ligand charge-transfer (LLCT) state. Mixing with the LLCT state may increase the radiative lifetime owing to the largely charge-separated character of this state and hence the probability of a partially forbidden transition (versus the ground state). The net result is a reduction in the corresponding emission quantum yield if similar deactivation mechanisms operate. In our view, the above scenario may be avoided by selecting facially arranged homoleptic complexes or by using [*] C.-H. Yang, Dr. Y.-M. Cheng, Prof. Y. Chi Department of Chemistry National Tsing Hua University Hsinchu 300 (Taiwan) Fax: (+886)3-572-0864 E-mail: ychi@mx.nthu.edu.tw

Blue-emitting heteroleptic iridium(III) complexes suitable for high-efficiency phosphorescent OLEDs.

Highly Efficient Blue‐Emitting Iridium(III) Carbene Complexes and Phosphorescent OLEDs

This paper presents power loss comparison of single- and two-stage grid-connected photovoltaic (PV) systems based on the loss factors of double line-frequency voltage ripple (DLFVR), fast irradiance variation + DLFVR, fast dc load variation + DLFVR, limited operating voltage range + DLFVR, and overall loss factor combination. These loss factors will result in power deviation from the maximum power points. In this paper, both single-stage and two-stage grid-connected PV systems are considered. All of the effects on a two-stage system are insignificant due to an additional maximum power point tracker, but the tracker will reduce the system efficiency typically about 2.5%. The power loss caused by these loss factors in a single-stage grid-connected PV system is also around 2.5%; that is, a single-stage system has the merits of saving components and reducing cost, and does not penalize overall system efficiency under certain operating voltage ranges. Simulation results with the MATLAB software package and experimental results have confirmed the analysis.

Chih-Hao Chang

Papers

3-(9-Carbazolyl)carbazoles and 3,6-Di(9-carbazolyl)carbazoles as Effective Host Materials for Efficient Blue Organic Electrophosphorescence**

Blue-emitting heteroleptic iridium(III) complexes suitable for high-efficiency phosphorescent OLEDs.

Highly Efficient Blue‐Emitting Iridium(III) Carbene Complexes and Phosphorescent OLEDs

Power Loss Comparison of Single- and Two-Stage Grid-Connected Photovoltaic Systems

A robust and accurate approach to computing compressible multiphase flow: Stratified flow model and AUSM+-up scheme