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

Since the discovery of highly conducting polyacetylene by Shirakawa, MacDiarmid, and Heeger in 1977, π-conjugated systems have attracted much attention as futuristic materials for the development and production of the next generation of electronics, that is, organic electronics. Conceptually, organic electronics are quite different from conventional inorganic solid state electronics because the structural versatility of organic semiconductors allows for the incorporation of functionality by molecular design. This versatility leads to a new era in the design of electronic devices. To date, the great number of π-conjugated semiconducting materials that have either been discovered or synthesized generate an exciting library of π-conjugated systems for use in organic electronics. 11 However, some key challenges for further advancement remain: the low mobility and stability of organic semiconductors, the lack of knowledge regarding structure property relationships for understanding the fundamental chemical aspects behind the structural design, and realization of desired properties. Organic field-effect transistors (OFETs) are a kind of device consisting of an organic semiconducting layer, a gate insulator layer, and three terminals (drain, source, and gate electrodes). OFETs are not only essential building blocks for the next generation of cheap and flexible organic circuits, but they also provide an important insight into the charge transport of πconjugated systems. Therefore, they act as strong tools for the exploration of the structure property relationships of πconjugated systems, such as parameters of field-effect mobility (μ, the drift velocity of carriers under unit electric field), current on/off ratio (the ratio of the maximum on-state current to the minimum off-state current), and threshold voltage (the minimum gate voltage that is required to turn on the transistor). 17 Since the discovery of OFETs in the 1980s, they have attracted much attention. Research onOFETs includes the discovery, design, and synthesis of π-conjugated systems for OFETs, device optimization, development of applications in radio frequency identification (RFID) tags, flexible displays, electronic papers, sensors, and so forth. It is beyond the scope of this review to cover all aspects of π-conjugated systems; hence, our focus will be on the performance analysis of π-conjugated systems in OFETs. This should make it possible to extract information regarding the fundamental merit of semiconducting π-conjugated materials and capture what is needed for newmaterials and what is the synthesis orientation of newπ-conjugated systems. In fact, for a new science with many practical applications, the field of organic electronics is progressing extremely rapidly. For example, using “organic field effect transistor” or “organic field effect transistors” as the query keywords to search the Web of Science citation database, it is possible to show the distribution of papers over recent years as shown in Figure 1A. It is very clear

Semiconducting π-conjugated systems in field-effect transistors: a material odyssey of organic electronics.

A survey is provided of recent progress in the understanding of singlet fission, a spin-allowed process in which a singlet excited molecule shares its energy with a ground-state neighbor to produce two triplet excited molecules. It has been observed to occur in single-crystal, polycrystalline, and amorphous solids, on timescales from 80 fs to 25 ps, producing triplet yields as high as 200%. Photovoltaic devices using the effect have shown external quantum efficiencies in excess of 100%. Almost all the efficient materials are alternant hydrocarbons of the acene series or their simple derivatives, and it is argued that a wider structural variety would be desirable. The current state of the development of molecular structure design rules, based on first-principles theoretical considerations, is described along with initial examples of implementation.

Recent Advances in Singlet Fission

We reevaluate the absolute fluorescence and phosphorescence quantum yields of standard solutions by using a novel instrument developed for measuring the absolute emission quantum yields of solutions. The instrument consists of an integrating sphere equipped with a monochromatized Xe arc lamp as the light source and a multichannel spectrometer. By using a back-thinned CCD (BT-CCD) as the detector, the sensitivity for spectral detection in both the short and long wavelength regions is greatly improved compared with that of an optical detection system that uses a conventional photodetector. Using this instrument, we reevaluate the absolute fluorescence quantum yields (Φf) of some commonly used fluorescence standard solutions by taking into account the effect of reabsorption/reemission. The value of Φf for 5 × 10−3 M quinine bisulfate in 1 N H2SO4 is measured to be 0.52, which is in good agreement with the value (0.508) obtained by Melhuish by using a modified Vavilov method. In contrast, the value of Φf for 1.0 × 10−5 M quinine bisulfate in 1 N H2SO4, which is one of the most commonly used standards in quantum yield measurements based on the relative method, is measured to be 0.60. This value is significantly larger than Melhuish’s value (0.546), which was estimated by extrapolating the value of Φf for 5 × 10−3 M quinine bisulfate solution to infinite dilution using the self-quenching constant. The fluorescence quantum yield of 9,10-diphenylanthracene in cyclohexane is measured to be 0.97. This system can also be used to determine the phosphorescence quantum yields (Φp) of metal complexes that emit phosphorescence in the near-infrared region: the values of Φp for [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) are estimated to be 0.063 in water and 0.095 in acetonitrile under deaerated conditions at 298 K, while that in aerated water, which is frequently used as a luminescent reference in biological studies, is reevaluated to be 0.040.

Reevaluation of absolute luminescence quantum yields of standard solutions using a spectrometer with an integrating sphere and a back-thinned CCD detector.

Remarkable progress has been made in developing high performance organic field-effect transistors (OFETs) and the mobility of OFETs has been approaching the values of polycrystalline silicon, meeting the requirements of various electronic applications from electronic papers to integrated circuits. In this review, the key points for development of high mobility OFETs are highlighted from aspects of molecular engineering, process engineering and interface engineering. The importance of other factors, such as impurities and testing conditions is also addressed. Finally, the current challenges in this field for practical applications of OFETs are further discussed.

25th anniversary article: key points for high-mobility organic field-effect transistors.

We measured the fluorescence quantum yield (Φf) of several aromatic hydrocarbon crystals: p-terphenyl, trans-stilbene, anthracene, pyrene, and α-perylene. The Φf is reduced by chemical impurities, ...

Fluorescence Quantum Yield of Aromatic Hydrocarbon Crystals

The absorption spectrum of excimers in fluid media has been measured with 1-methylnaphthalene, anthracene, pyrene, and perylene. In all compounds studied an absorption band characteristic of an excimer has been observed in the near-IR. It is shown that the IR band can be understood by assuming that an excimer is essentially a charge-transfer complex:  The near-IR band is to be assigned to a transition from the lowest excimer state to a higher state which has an ion-pair character.

Near-IR Absorption Spectrum of Aromatic Excimers

Electroabsorption has been measured with thin films of several (phthalocyaninato)galliums. It has been observed that the lowest energy absorption peak of most of the compounds studied is sensitive ...

Gallium Phthalocyanines: Structure Analysis and Electroabsorption Study

Charge carrier transport in high purity perylene single crystal studied by time-of-flight measurements and through field effect transistor characteristics

Masahiro Kotani

Papers

Fluorescence Quantum Yield of Aromatic Hydrocarbon Crystals

Near-IR Absorption Spectrum of Aromatic Excimers

Gallium Phthalocyanines: Structure Analysis and Electroabsorption Study

Charge carrier transport in high purity perylene single crystal studied by time-of-flight measurements and through field effect transistor characteristics

Triplet exciton formation in a benzophenone single crystal studied by picosecond time-resolved absorption spectroscopy