There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Aggregation-induced emission of 1-methyl-1,2,3,4,5-pentaphenylsilole

Ju Mei,†,‡,∥ Nelson L. C. Leung,†,‡,∥ Ryan T. K. Kwok,†,‡ Jacky W. Y. Lam,†,‡ and Ben Zhong Tang*,†,‡,§ †HKUST-Shenzhen Research Institute, Hi-Tech Park, Nanshan, Shenzhen 518057, China ‡Department of Chemistry, HKUST Jockey Club Institute for Advanced Study, Institute of Molecular Functional Materials, Division of Biomedical Engineering, State Key Laboratory of Molecular Neuroscience, Division of Life Science, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Guangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China

Aggregation-Induced Emission: Together We Shine, United We Soar!

Luminogenic materials with aggregation-induced emission (AIE) attributes have attracted much interest since the debut of the AIE concept in 2001. In this critical review, recent progress in the area of AIE research is summarized. Typical examples of AIE systems are discussed, from which their structure–property relationships are derived. Through mechanistic decipherment of the photophysical processes, structural design strategies for generating new AIE luminogens are developed. Technological, especially optoelectronic and biological, applications of the AIE systems are exemplified to illustrate how the novel AIE effect can be utilized for high-tech innovations (183 references).

Aggregation-induced emission

The field of chemical sensing is becoming ever more dependent upon novel materials. Polymers, crystals, glasses, particles, and nanostructures have made a profound impact and have endowed modern sensory systems with superior performance. Electronic polymers have emerged as one of the most important classes of transduction materials; they readily transform a chemical signal into an easily measured electrical or optical event. Although our group reviewed this field in 2000,1 the high levels of activity and the impact of these methods now justify a subsequent review as part of this special issue. In this review we restrict our discussions to purely fluorescence-based methods using conjugated polymers (CPs). We further confine our detailed coverage to articles published since our previous review and will only detail earlier research in this Introduction to illustrate fundamental concepts and terminology that underpin the recent literature.

Chemical sensors based on amplifying fluorescent conjugated polymers.

Phosphorescence is among the many functional features that, in practice, divide pure organic compounds from organometallics and inorganics. Considered to be practically non-phosphorescent, purely organic compounds (metal-free) are very rarely explored as emitters in phosphor applications, despite the emerging demand in this field. To defy this paradigm, we describe novel design principles to create purely organic materials demonstrating phosphorescence that can be turned on by incorporating halogen bonding into their crystals. By designing chromophores to contain triplet-producing aromatic aldehydes and triplet-promoting bromine, crystal-state halogen bonding can be made to direct the heavy atom effect to produce surprisingly efficient solid-state phosphorescence. When this chromophore is diluted into the crystal of a bi-halogenated, non-carbonyl analogue, ambient phosphorescent quantum yields reach 55%. Here, using this design, a series of pure organic phosphors are colour-tuned to emit blue, green, yellow and orange. From this initial discovery, a directed heavy atom design principle is demonstrated that will allow for the development of bright and practical purely organic phosphors.

Activating efficient phosphorescence from purely organic materials by crystal design

Cost effective and highly efficient renewable energy is becoming ever more important in our age of rising energy prices and global climate change. Solar energy is a nonexhaustible and green energy. Organic solar cells (OSC) have the merits of low cost and simplistic fabrication in addition to compatibility with flexible plastic substrates over large areas. They have therefore been considered a promising energy conversion platform for clean and carbon-neutral energy production. In recent years, the power conversion efficiency of OSCs based on conjugated polymers has steadily increased through improved energy harvesting, enhanced exciton separation in improved device structures, and optimization of processing parameters, e.g., solvent evaporation time, and annealing conditions. Most OSCs are built on indium tin oxide (ITO) coated substrates because ITO offers transparency in the visible range of the electromagnetic spectrum as well as good electrical conductivity. However, ITO is not the optimum electrode for solar cell applications as it has been reported that the band structure of ITO hinders efficient photocurrent generation. Moreover, the poor mechanical stability of ITO can cause device failure when an ITO-coated flexible substrate is bent. In addition, the limited supply of indium and the increasing demand from the rapidly expanding display market have increased the cost of ITO drastically, which potentially prevents the realization of low cost and large scale OSC fabrication. Therefore, there is a strong need to find alternative materials that can replace ITO as high transparency electrode. Some examples that have been investigated recently are nanotube networks, and Ag wire grids. In this communication, we report on high transparency metal wire grid electrodes for organic solar cell applications. The high transparency metal electrodes are fabricated by nanoimprint lithography (NIL), and have several advan-

Organic Solar Cells Using Nanoimprinted Transparent Metal Electrodes

The role of conjugated polymers in emerging electronic, sensor and display technologies is rapidly expanding. In spite of extensive investigations, the intrinsic spectroscopic properties of conjugated polymers in precise conformational and spatial arrangements have remained elusive. The difficulties of obtaining such information are endemic to polymers, which often resist assembly into single crystals or organized structures owing to entropic and polydispersity considerations. Here we show that the conformation of individual polymers and interpolymer interactions in conjugated polymers can be controlled through the use of designed surfactant poly(p-phenylene-ethynylene) Langmuir films. We show that by mechanically inducing reversible conformational changes of these Langmuir monolayers, we can obtain the precise interrelationship of the intrinsic optical properties of a conjugated polymer and a single chain's conformation and/or interpolymer interactions. This method for controlling the structure of conjugated polymers and establishing their intrinsic spectroscopic properties should permit a more comprehensive understanding of fluorescent conjugated materials.

Control of conformational and interpolymer effects in conjugated polymers.

A high density of strong hydrogen bonds connecting two polymers that are homogeneously mixed in a thin film is shown to enhance the intrachain thermal conductance, boosting thermal transport in fully organic layers. Thermal conductivity is an important property for polymers, as it often affects product reliability (for example, electronics packaging), functionality (for example, thermal interface materials) and/or manufacturing cost1. However, polymer thermal conductivities primarily fall within a relatively narrow range (0.1–0.5 W m−1 K−1) and are largely unexplored. Here, we show that a blend of two polymers with high miscibility and appropriately chosen linker structure can yield a dense and homogeneously distributed thermal network. A sharp increase in cross-plane thermal conductivity is observed under these conditions, reaching over 1.5 W m−1 K−1 in typical spin-cast polymer blend films of nanoscale thickness, which is approximately an order of magnitude larger than that of other amorphous polymers.

High thermal conductivity in amorphous polymer blends by engineered interchain interactions

Developing metal-free organic phosphorescent materials is promising but challenging because achieving emissive triplet relaxation that outcompetes the vibrational loss of triplets, a key process to achieving phosphorescence, is difficult without heavy metal atoms. While recent studies reveal that bright room temperature phosphorescence can be realized in purely organic crystalline materials through directed halogen bonding, these organic phosphors still have limitations to practical applications due to the stringent requirement of high quality crystal formation. Here we report bright room temperature phosphorescence by embedding a purely organic phosphor into an amorphous glassy polymer matrix. Our study implies that the reduced beta (β)-relaxation of isotactic PMMA most efficiently suppresses vibrational triplet decay and allows the embedded organic phosphors to achieve a bright 7.5% phosphorescence quantum yield. We also demonstrate a microfluidic device integrated with a novel temperature sensor based on the metal-free purely organic phosphors in the temperature-sensitive polymer matrix. This unique system has many advantages: (i) simple device structures without feeding additional temperature sensing agents, (ii) bright phosphorescence emission, (iii) a reversible thermal response, and (iv) tunable temperature sensing ranges by using different polymers.

Jinsang Kim

Papers

Activating efficient phosphorescence from purely organic materials by crystal design

Organic Solar Cells Using Nanoimprinted Transparent Metal Electrodes

Control of conformational and interpolymer effects in conjugated polymers.

High thermal conductivity in amorphous polymer blends by engineered interchain interactions

Room Temperature Phosphorescence of Metal-Free Organic Materials in Amorphous Polymer Matrices