Aggregation-Induced Emission E-Book

Ben Zhong Tang

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 486

Veröffentlichungsjahr: 2013

Beschreibung

Edited by the academic who first discovered this important phenomenon, Aggregation-Induced Emission is the first book to cover the applications of Aggregation-Induced Emission. This groundbreaking text explores the high-tech applications of AIE materials in optoelectronic devices, chemical sensors, and biological probes. A valuable resource for scientists, physicists, and biological chemists, topics covered include: AIE materials for LEDs and lasers; mechanochromic AIE materials; new chemo- and biosensors based on AIE fluorophores; AIE dye-encapsulated nanoparticles for optical bioimaging; and chiral recognition and enantiomeric excess determination based on AIE.

Leseprobe

Contents

Cover

Title Page

Copyright

Preface

List of Contributors

Chapter 1: AIE or AIEE Materials for Electroluminescence Applications

1.1 Introduction

1.2 EL Background, EL Efficiency, Color Chromaticity, and Fabrication Issues of OLEDs

1.3 AIE or AIEE Silole Derivatives for OLEDs

1.4 AIE or AIEE Maleimide and Pyrrole Derivatives for OLEDs

1.5 AIE or AIEE Cyano-Substituted Stilbenoid and Distyrylbenzene Derivatives for OLEDs

1.6 AIE or AIEE Triarylamine Derivatives for OLEDs

1.7 AIE or AIEE Triphenylethene and Tetraphenylethene Derivatives for OLEDs

1.8 White OLEDs Containing AIE or AIEE Materials

1.9 Perspectives

References

Chapter 2: Crystallization-Induced Phosphorescence for Purely Organic Phosphors at Room Temperature and Liquid Crystals with Aggregation-Induced Emission Characteristics

2.1 Crystallization-Induced Phosphorescence for Purely Organic Phosphors at Room Temperature

2.2 Liquid Crystals with Aggregation-Induced Emission Characteristics

2.3 Conclusions and Perspectives

References

Chapter 3: Mechanochromic Aggregation-Induced Emission Materials

3.1 Introduction

3.2 Mechanochromic Non-AIE Compounds

3.3 Mechanochromic AIE Compounds

3.4 Conclusion

References

Chapter 4: Chiral Recognition and Enantiomeric Excess Determination Based on Aggregation-Induced Emission

4.1 Introduction to Chiral Recognition

4.2 Chiral Recognition and Enantiomeric Excess Determination of Chiral Amines

4.3 Chiral Recognition and Enantiomeric Excess Determination of Chiral Acids

4.4 Mechanism of Chiral Recognition Based on AIE

4.5 Prospects for Chiral Recognition Based on AIE

References

Chapter 5: AIE Materials Towards Efficient Circularly Polarized Luminescence, Organic Lasing, and Superamplified Detection of Explosives

5.1 Introduction

5.2 AIE Materials with Efficient Circularly Polarized Luminescence and Large Dissymmetry Factor

5.3 AIE Materials for Organic Lasing

5.4 AIE Materials for Superamplified Detection of Explosives

5.5 Conclusion

References

Chapter 6: Aggregation-Induced Emission and Applications of Aryl-Substituted Pyrrole Derivatives

6.1 Introduction

6.2 Luminescence Properties of Triphenylpyrrole Derivatives in the Aggregated State

6.3 Applications

6.4 Aggregation-Induced Emission of Pentaphenylpyrrole

6.5 AIEE Mechanism of Pentaphenylpyrrole

6.6 Conclusion

References

Chapter 7: Biogenic Amine Sensing with Aggregation-Induced Emission-Active Tetraphenylethenes

7.1 Introduction

7.2 Fluorimetric Sensing of Biogenic Amines with AIE-Active TPEs

7.3 Summary and Outlook

References

Chapter 8: New Chemo-/Biosensors with Silole and Tetraphenylethene Molecules Based on the Aggregation and Deaggregation Mechanism

8.1 Introduction

8.2 Cation and Anion Sensors

8.3 Fluorimetric Biosensors for Biomacromolecules

8.4 Fluorimetric Assays for Enzymes

8.5 Fluorimetric Detection of Physiologically Important Small Molecules

8.6 Miscellaneous Sensors

8.7 Conclusion and Outlook

References

Chapter 9: Carbohydrate-Functionalized AIE-Active Molecules as Luminescent Probes for Biosensing

9.1 Introduction

9.2 Carbohydrate-Bearing AIE-Active Molecules

9.3 Luminescent Probes for Lectins

9.4 Luminescent Probes for Enzymes

9.5 Luminescent Probes for Viruses and Toxins

9.6 Conclusion

Acknowledgments

References

Chapter 10: Aggregation-Induced Emission Dyes for In Vivo Functional Bioimaging

10.1 Introduction

10.2 AIE Dyes for Macro In Vivo Functional Bioimaging

10.3 Multiphoton-Induced Fluorescence from AIE Dyes and Applications in In Vivo Functional Microscopic Imaging [36]

10.4 Summary and Perspectives

Acknowledgments

References

Chapter 11: Specific Light-Up Bioprobes with Aggregation-Induced Emission Characteristics for Protein Sensing

11.1 Introduction

11.2 In Vitro Detection of Integrin αvβ3 Using a TPS-Based Probe

11.3 Real-Time Monitoring of Cell Apoptosis and Drug Screening with a TPE-Based Probe

11.4 In Vivo Monitoring of Cell Apoptosis and Drug Screening with PyTPE-Based Probe

11.5 Conclusion

Acknowledgments

References

Chapter 12: Applications of Aggregation-Induced Emission Materials in Biotechnology

12.1 Introduction

12.2 AIE Materials for Nucleic Acid Studies

12.3 AIE Materials for Protein Studies

12.4 AIE Materials for Live Cell Imaging

12.5 Conclusion

References

Index

This edition first published 2013

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Preface

The discovery of new natural phenomena, the unveiling of new physical laws, the development of new methodologies, and the generation of new knowledge are at the core of scientific research. From this viewpoint, the study of light-emitting behaviors of luminogens in an aggregate state is a challenging yet important topic because it may lead to the creation of new photophysical knowledge.

Since the 1950s, studies have shown that the fluorescence of a number of luminophores became weaker or even completely quenched in concentrated solutions or in the solid state. This common photophysical phenomenon is widely known as ‘concentration quenching’ or ‘aggregation-caused quenching’ (ACQ) of light emission. The ACQ process has been studied in great detail, and mature theories have been established. The ACQ effect, however, is harmful in practice, because luminophores are usually used as solid films or aggregates in real-world applications, which hinders them from realizing their full potential. Numerous processes have been employed and many approaches have been developed to prevent the luminophores from aggregating, but these efforts have met with only limited success. The difficulty lies in the fact that chromophore aggregation is an intrinsic natural process when luminophore molecules are located in close vicinity in the condensed phase.

Exactly opposite to the ACQ effect, in 2001 we observed a unique luminogen system in which aggregation played a constructive, instead of destructive, role in the luminescence process: a molecule named 1-methyl-1,2,3,4,5-pentaphenylsilole was found to be almost nonemissive in dilute acetonitrile solution but became highly fluorescent when a large amount of water was admixed with acetonitrile. Because water is a poor solvent of the hydrophobic silole luminogen, addition of water to acetonitrile causes the silole molecules to aggregate in aqueous media. As the light emission is induced by aggregate formation, we coined the term aggregation-induced emission (AIE) for the phenomenon. In the past decade, a large variety of molecules with propeller shapes have been found to show the AIE effect, indicating that AIE is a general, rather than special, photophysical phenomenon.

On the basis of our experimental results, we have rationalized that the restriction of intramolecular rotation (RIR) is the main cause of the AIE phenomenon. In the solution state, intramolecular rotation of the aromatic rotors of the AIE luminogens is active, which serves as a relaxation channel for the excited states to decay nonradiatively. In the aggregate state, however, the intramolecular rotation is restricted owing to the physical constraint involved, which blocks the nonradiative pathway and opens the radiative channel.

The novel AIE phenomenon offers a new platform for researchers to look into the light-emitting processes from luminogen aggregates, from which useful information on structure–property relationships may be collected and mechanistic insights may be gained. Such information and insights will be instructive to the structural design for the development of new efficient AIE luminogens. Furthermore, the discovery of the AIE effect overturns the general belief of ‘concentration quenching’ or ACQ of luminescence processes, opens a new avenue for the development of new luminogen materials in the aggregate or solid state and may spawn new models or theorems for photophysical processes in solution and aggregate states.

As AIE is a photophysical effect concerning light emission in the practically useful solid state, AIE studies may also lead to hitherto impossible technological innovations. In AIE systems, one can take great advantage of aggregate formation, instead of fighting against it. The AIE effect permits the use of highly concentrated solutions of luminogens and their aggregates in aqueous media for sensing and imaging applications, which may lead to the development of fluorescence turn-on or light-up nanoprobes. A probe based on AIE luminogen nanoaggregates is in some sense the organic version of inorganic semiconductor quantum dots, but are superior to the latter in terms of wider molecular diversity, readier structural tunability, and better biological compatibility.

Attracted by this intriguing phenomenon and its promising applications, a number of research groups throughout the world have enthusiastically engaged in AIE studies, and exciting progress has already been made. In response to an invitation from the Wiley editors, we embarked on the preparation of two volumes dedicated to the study of AIE – this volume, Aggregation-Induced Emission: Applications and the related volume, Aggregation-Induced Emission: Fundamentals.

In this volume, we invited a group of active researchers in the area to contribute on the exploration of high-tech applications of AIE luminogens. The technological utilization of AIE materials in the areas of electroluminescence, mechanochromism, chiral recognition, ionic sensing, biomolecule detection, and cell imaging is covered. Their potential applications in room-temperature phosphorescence, liquid crystals, circularly polarized luminescence, organic lasing, and so on are also introduced in this volume.

This book is expected to be a valuable reference to readers who are now working or planning to be involved in the areas of research on organic optoelectronic materials and biomedical sensors. Although we have tried our best to make this book comprehensive, some important work may have inadvertently been omitted, owing to the limitations on the size of the book and the rapid developments in this area of research. The book may contain some overlapping contents in different chapters and possibly even some errors. We hope the readers will provide us with constructive comments, so that we may modify and improve the book in its next edition.

We would like to thank all the authors who have contributed to this book. Without their enthusiastic support, the foundation of this book could not have been be laid. We also thank the Wiley in-house editors, Sarah Hall, Sarah Tilley, and Rebecca Ralf, for their enthusiastic encouragement and technical support. We hope that this book will serve as a ‘catalyst’ to stimulate new efforts, to trigger new ideas, and to accelerate the pace in the research endeavors on the design of new AIE luminogen systems, the establishment of new theoretical models, and the exploration of innovative applications.

Anjun Qin

Department of Polymer Science and Engineering Zhejiang University, China

Ben Zhong Tang

Department of Chemistry, Division of Biomedical Engineering

The Hong Kong University of Science and Technology China

List of Contributors

Chin-Ti Chen Institute of Chemistry, Academia Sinica, Taiwan

Qi Chen National Center for Nanoscience and Technology, China

Zhenguo Chi PCFM Laboratory and DSAPM Laboratory, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen University, China

Yuping Dong College of Materials Science and Engineering, Beijing Institute of Technology, China

Bao-Hang Han National Center for Nanoscience and Technology, China

Sailing He Center for Optical and Electromagnetic Research, Zhejiang Provincial Key Laboratory for Sensing Technologies, State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, China

Yuning Hong Department of Chemistry, The Hong Kong University of Science and Technology, China

Jacky W.Y. Lam Department of Chemistry, The Hong Kong University of Science and Technology, China

Jing Liang Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore

Chiao-Wen Lin Institute of Chemistry, Academia Sinica, Taiwan

Bin Liu Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore

Jianzhao Liu Department of Chemistry, The Hong Kong University of Science and Technology, China

Jun Qian Center for Optical and Electromagnetic Research, Zhejiang Provincial Key Laboratory for Sensing Technologies, State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, China

Takanobu Sanji Chemical Resources Laboratory, Tokyo Institute of Technology, Japan

Haibin Shi Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore

Masato Tanaka Chemical Resources Laboratory, Tokyo Institute of Technology, Japan

Ben Zhong Tang Department of Chemistry, Institute of Molecular Functional Materials, The Hong Kong University of Science and Technology, Hong Kong, and Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, China

Bin Tong College of Materials Science and Engineering, Beijing Institute of Technology, China

Dan Wang Center for Optical and Electromagnetic Research, Zhejiang Provincial Key Laboratory for Sensing Technologies, State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, China

Ming Wang Beijing National Laboratory, Organic Solids Laboratory, Institute of Chemistry, Chinese Academy of Sciences, China

Jiarui Xu PCFM Laboratory and DSAPM Laboratory, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen University, China

Wang Zhang Yuan School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, China

Deqing Zhang Beijing National Laboratory, Organic Solids Laboratory, Institute of Chemistry, Chinese Academy of Sciences, China

Guanxin Zhang Beijing National Laboratory, Organic Solids Laboratory, Institute of Chemistry, Chinese Academy of Sciences, China

Yongming Zhang School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, China

Yan-Song Zheng School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, China

1

AIE or AIEE Materials for Electroluminescence Applications

Chiao-Wen Lin and Chin-Ti Chen

Institute of Chemistry, Academia Sinica, Taiwan

1.1 Introduction

The science and technology of organic light-emitting diodes (OLEDs) have been developing and progressing for more than 30 years since a small team led by Tang at Kodak invented the first thin-film-based high-efficiency OLED [1]. Nowadays, OLEDs have reached a stage where they are ready to be one of the main types of display in the marketplace, as is evident from the market demand for smartphones and tablets along with Samsung's Galaxy production line of AMOLED mobile devices. Several breakthroughs and discoveries, either intentionally or simply by serendipity, have brought OLEDs beyond being just a research niche in the laboratory. In this chapter, we illustrate one such serendipity, namely the aggregation-induced emission (AIE) or aggregation-induced enhanced emission (AIEE) found for a certain kind of fluorescent materials that leads to an electroluminescence (EL) efficiency of nondopant devices comparable to that of dopant-based OLEDs, the fabrication of which process is more complicated and less easy to control. AIE or AIEE is an inverse effect (see NPAFN and NPAMLI shown in Figure 1.1a) with respect to the more common aggregation-caused quenching (ACQ) or concentration-quenching effect (see Nile Red, DCM1, and TPP shown in Figure 1.1b) that takes place for most fluorophores in the solid state [2]. The difference between AIE and AIEE effects is the relative intensity of the fluorescence [or more generally photoluminescence (PL)] in solution, which is very much solvent dependent. If the chosen solvent enables no or nearly no PL of the material, any PL observed in the solid state is called AIE effect. If solution PL is observed but it is less intense than that in the solid state, the material is sad to show an AIEE effect. Since OLED devices are fabricated in a thin solid film structure, the common ACQ effect impairs the solid-state material PL or EL of OLEDs, the PL quantum yield or the brightness (electroluminance, L) of OLEDs, and hence the EL efficiency of OLEDs. Accordingly, materials that display PL having AIE or AIEE characteristics, instead of ACQ, are very desirable and valuable for high-performance OLEDs fabricated by a simpler fabrication process.

Figure 1.1 From left to right: fluorescence image of (a) NPAFN and NPAMLI and (b) Nile Red, DCM1, and TPP in solution (CH2Cl2) and in the solid state.Reproduced with permission from [2], © 2004 American Chemical Society.

In a survey of the literature, we found many PL materials showing an AIE or AIEE effect but only a few of them have been reported with EL data, that is, their OLEDs were not fabricated and tested. For those have been applied in OLEDs, according to their chemical structure, we classify AIE or AIEE materials into five main categories and an extra category. The first two are five-membered heterocyclic compounds, namely silicon-containing silole derivatives (Section 1.3), imide-containing maleimide derivatives and nitrogen-containing pyrrole derivatives (Section 1.4), the third type is cyano-substituted stilbenoid and distyrylbenzene derivatives (Section 1.5), the fourth type is triarylamine-based derivatives (Section 1.6), and the fifth type is tri- or tetraphenylethene derivatives (Section 1.7). Finally, we group the white OLEDs containing AIE or AIEE materials into an extra category (Section 1.8). Fluorescent materials showing the AIE or AIEE effect are advantageous with respect to the solid-state PL quantum yield, which is one of four key factors that are decisive for achieving high EL efficiency, the external quantum efficiency (EQE or ηEXT), of OLEDs. Therefore, after this introductory section, the chapter begin with the background to EL, EL efficiency, color chromaticity, and fabrication issues of OLEDs. The rest of the chapter considers the six categories of AIE or AIEE molecular materials outlined above.

1.2 EL Background, EL Efficiency, Color Chromaticity, and Fabrication Issues of OLEDs

A typical architecture of an OLED device is illustrated in Figure 1.2. The diode device is composed of two electrodes, anode and cathode, sandwiching a hole-transporting layer (HTL), light-emitting layer (EML), and electron-transporting layer (ETL) at the center.

Figure 1.2 Typical architecture of an OLED device. ITO, indium tin oxide.

The anode is usually transparent, enabling EL from the device, and it is usually an indium tin oxide (ITO)-coated glass substrate. For the cathode, low work function metals, such as Al and Ca, or a metal alloy, such as Mg–Ag, are common choices. To facilitate charge injection and reduction of the driving voltage, injection layers are sometimes inserted adjacent to the electrodes. For electron injection, inorganic ionic substance such as LiF, CsF, or Cs2CO3 and low work function metal such as Ba or Cs are commonly adopted as the electron injection layer (EIL) in OLED fabrication. Owing to the usually <5.0 eV ionization potential or work function of ITO, materials having a shallow highest occupied molecular orbital (HOMO) energy level or work function are necessary for the hole injection layer (HIL) in OLED fabrication. Scheme 1.1 summarizes HIL materials mentioned in this chapter.

Scheme 1.1 HIL materials mentioned in this chapter.

For charge transport and hence charge balance in OLEDs, materials for the hole-transporting layer (HTL) and electron-transporting layer (ETL) are needed in OLED fabrication. HTL materials usually have a high-lying HOMO energy level and a relatively high hole mobility, such as the commonly used TPD and NPB shown in Scheme 1.2.

Scheme 1.2 HTL materials mentioned in the chapter.

If the stability of OLED operation is the primary concern, TFTPA HTL will be a better choice than TPB or NPB because of its high glass transition temperature (Tg) of more than 185 °C [3]. If wide bandgap HTL materials are necessary, compounds such as CDBP and TCTA are used for phosphorescence-based OLEDs. For ETL of OLEDs, an electron-deficient nature of the molecular structure and relatively low-lying lowest unoccupied molecular orbital (LUMO) level are a common feature of the materials used, such as Alq3, PyPySPyPy, TPBI, TAZ, BCP, and BPhen shown in Scheme 1.3. If the ETL material has a particularly low-lying HOMO energy level, such as >6.5 eV as for BCP and BPhen, it is useful for the hole-blocking layer (HBL) between the light-emitting layer (EML) and ETL to confine or enhance the charge recombination on EML.

Scheme 1.3 ETL materials mentioned in the chapter.

Using the charge balance factor (γ), the partition ratio of emissive states (rST), that is, exciton in singlet or triplet state (25 or 75%), PL quantum yield (ηPL), and the light out-coupling efficiency (ξ), the basic equation for EQE or ηEXT of the OLED can be written as [4]

(1.1)

Figure 1.3 depicts schematically each factor or component in Equation 1.1.

Figure 1.3 Schematic depiction of each term (γ, rST, ηPL, ξ) in Equation 1.1. A photograph of a turn-on red light-emitting OLED is included for illustration purposes.

Whereas the charge balance (γ) depends on the charge carrier mobility and energy level alignment of each material in an OLED device, the solid-state PL quantum yield (ηPL) is directly related to the AIE or AIEE effect of the material. The theoretical maximum ηEXT that one OLED can achieve depends on the light out-coupling (ξ) of the device and the nature of the emitting light (rST), either fluorescence or phosphorescence or both, of the materials used in OLEDs. For the first approximation, ξ is proportional to 1/(2n2), where n is the refractive index of the light-emitting layer and is commonly in the range 1.5–1.7 for most PL and EL materials. Accordingly, ξ ≈ 0.17–0.22 and ηEXT∼20% is the theoretical prediction of the maximum ηEXT. In fact, by utilizing both the phosphorescence and fluorescence energy in OLEDs, ηEXT values beyond theoretical limit and approaching 30% have been realized [5–7.] Even for OLEDs showing only fluorescence-based EL, ηEXT ≈ 5%, the top limit predicted by theory, has also been exceeded. One of the highest ηEXT values of ∼8% for a fluorescence-based OLED was reported with EML using silole compounds [8], the fluorescent materials showing AIE. Incidentally, two other commonly used units for EL efficiency are cd m−2 for the current efficiency (ηC) and lm W−1 for power efficiency (ηP).

As mentioned earlier, particularly in the solid state, light-emitting materials showing the AIE or AIEE effect can directly contribute to the high ηPL, which is beneficial for promoting the brightness (L, electroluminance) and ηEXT of the OLED. Whereas it is irrelevant to rST or ξ in Equation 1.1, light-emitting materials showing the AIE or AIEE effect do not necessarily have a high γ. Therefore, there are numerous OLEDs that show decent to exceptionally good ηEXT values, but there are even more OLEDs that show poor ηEXT, even though the device contains AIE or AIEE materials as the EML.

Before moving on to the next section, the PL or EL color specification is worth noting here. The RGB color specification is one of the quality checks for full-color OLED displays. Figure 1.4 shows a typical standardized 1931 CIE (Commission Internationale de l'Eclairage) color chromaticity diagram [9].

Figure 1.4 A typical 1931 CIE color chromaticity diagram [9].

Considering the wide color gamut range of a display, it is highly desirable that the materials used in an OLED display should exhibit a color purity of red, green, or blue that is as high as possible. This issue is a challenge to be overcome particularly for blue and red. Moreover, the problems associated with blue and red light-emitting material differ. It is relatively difficult to acquire pure or deep blue-emitting material because of the red-shifting emission, either PL or EL, caused by material aggregation in the solid state, which applies also to AIE or AIEE materials. For red light-emitting materials, red-shifted PL or EL is satisfactory in terms of red color purity. It is the emission intensity that is usually impaired due to the material aggregation in the solid state, which is most serious for red light-emitting materials [2]. However, the AIE or AIEE effect of red light-emitting materials can alleviate the problem of reduced emission intensity. Moreover, in order to reduce the adverse ACQ of light-emitting materials, OLED fabrication often utilizes a doping process. Unfortunately, this fabrication process is relatively problematic in terms of uniformity and reproducibility, thus hindering a high production yield in volume fabrication [2]. AIE or AIEE materials can take advantage of a ‘nondoping process’ in OLED fabrication and hence are more feasible for volume production of OLED devices.

Finally, for white OLEDs (WOLEDs) in lighting applications, the EL efficiency under lighting conditions (L ≥ 1000 cd m−2) should be examined, because most of the OLED devices exhibit efficiency ‘roll-off’ at high brightness, and most exhibit peak or maximum EL efficiency (ηEXT, ηC, or ηP) at low current density or low brightness. Such low brightness may be acceptable for small-sized displays (such as those on smartphones or the display panel of laptop computers), but is insufficient for lighting applications. In addition, for lighting applications, the color rendering index (CRI) is an important specification of WOLEDs. The CRI is a quantitative measure of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source, that is, sunlight. A CRI of 100 represents the maximum value, which is defined for sunlight. An incandescent lamp is a poor light source because of its low efficiency, but it has an excellent CRI of >95, almost as high as for sunlight. A light source with CRI >80 is usually required for general lighting applications. Normally, a two-color-white light source rarely has CRI >80 and a three-color white light system is necessary to achieve CRI >80 for practical lighting applications.

1.3 AIE or AIEE Silole Derivatives for OLEDs

The first literature report on OLEDs based on a series of silole-based small-molecule compounds, DMTPS, MPClTPS, MPTPS, and HPS (Table 1.1) as EML, by Tang et al., appeared in 2001 [10]. The performance of the OLEDs was rather poor (maximum brightness Lmax <5000 cd m−2 and ηEXT <1%). At about the same time, one of the silole compounds, MPTPS (known as MPS in the literature), was also reported separately with significantly improved OLEDs performance (Lmax >9200 cd m−2, ηEXT = 8%, ηC = 12, ηP = 12.6 or 20 lm W−1) [11]. In this paper [11], the specific term ‘aggregation-induced emission’ (AIE) was mentioned for the first time to manifest the extraordinary behavior of the solid-state fluorescence. The OLED data for MPTPS were elaborated further in a paper published in 2002 [8]. Basically, the super high ηEXT value (8%) beyond ∼5.5%, the theoretical limit of fluorescence-based EML, is attributed to the underestimated ξ and rST. Also in 2002, another silole-based AIE material, 2PSP, was employed as the EML in OLEDs by Kafafi and co-workers, with ηEXT as high as 4.8% and ηP as high as 12 lm W−1, which ranks the second highest in Table 1.1, next to MPTPS [12]. The high ηEXT of the reported MPTPS and 2PSP OLEDs may also be ascribed to the high electron mobility of the silole compound, such as PyPySPyPy shown in Scheme 1.3. Its electron mobility was determined 2 × 10−4 cm2 V−1 s−1, which is more than two orders of magnitude higher than that of the most widely used electron transport material, tris(8-hydroxyquinolinolato)aluminum(III) (Alq3) [13].

Table 1.1 Summary of reported OLEDs containing silole-based AIE or AIEE materials.

Not matching the needs of display applications, all silole-based AIE or AIEE materials listed in Table 1.1 show EL with less desirable colors: greenish blue, green, yellowish green, and even yellow (DPyASPTPS). There seems to be one plausible exception, HPS2,4, for which an OLED exhibited EL with and a reasonably good ηC of 5.86 cd A−1 has been reported (see Table 1.1) [16]. However, there is no information available regarding the 1931 CIEx,y color chromaticity of the device. Based on the EL spectrum displayed in the paper, the HPS2,4 OLED exhibits a fairly wide EL band [the full width-at-half-maximum (FWHM) is >100 nm] and such an EL band has substantial intensity (more than one-third of the peak intensity at ) extending far beyond 550 nm. Even though EL peaked at a relatively short wavelength of 464 nm, the color of the HPS2,4 OLED is unlikely to be authentic blue and more possibly green–blue or blue–green as for most silole-based AIE or AIEE materials. The silole HPS2,4 has built-in steric hindrance due to the isopropyl substituent at the ortho-position of two phenyl rings forcing a twist on the π-conjugation and shortening the EL wavelength. Based on fundamental intuition, the twisted conformation should be helpful in reducing the exciplex formation with the HTL (such as TPB and NPB), which usually results in a red-shifted emission wavelength. Therefore, the higher than normal EL intensity around 550 nm may be partly due to Alq3, the material used as the ETL in an HPS2,4 OLED.

1.4 AIE or AIEE Maleimide and Pyrrole Derivatives for OLEDs

In 2002, Chen and co-workers reported a red OLED based on NPAMLI, a maleimide fluorophore, as the EML in a nondopant device [20], namely a red OLED fabricated without the application of a doping process. This is one of the first long-wavelength (>600 nm) AIE or AIEE materials to be reported for OLED application and yet the OLED exhibited reasonably good performance. The NPAMLI OLED exhibited Lmax ≈ 8000 cd m−2 and ηEXT = 2.4% (Table 1.2), and such a performance is comparable to that of red OLEDs fabricated with a doping process.

Table 1.2 Summary of reported OLEDs containing maleimide- or pyrrole-based AIE or AIEE materials.

It is worth mentioning that the device was fabricated without an HTL because NPAMLI has the capability of transporting holes in an OLED. NPAMLI shares a common structural feature, namely 2-naphthylphenylamine, with NPB (Scheme 1.2), one of the most widely used materials for HTLs. In fact, when the paper was first published it was not realized that the maleimide NPAMLI is indeed one of the materials that show an AIE or AIEE effect. The image shown in Figure 1.1a demonstrating the AIEE effect (dichloromethane solution versus solid state) was taken two years later in 2004. To provide the missing evidence thus far, Figure 1.5 displays the AIE effect of NPAMLI in acetonitrile solution with increasing amount of water addition (from left to right).

Figure 1.5 Fluorescence images of NPAMLI in acetonitrile–water mixture with water fractions of 0, 20, 50, 60, 70, and 80% from left to right.

Recently, our laboratory synthesized an asymmetric NPAMLI, the red–orange AsNPAMLI (see Table 1.2 for its structure). Although its OLED application awaits exploration, we have clearly demonstrated its AIE effect in solution (Figure 1.6).

Figure 1.6 Fluorescence images of AsNPAMLI in acetonitrile–water mixture with water fractions of 0, 20, 50, 60, 70, and 80% from left to right. (See color figure in color plate section).

More maleimide compounds bearing indole substituents (Table 1.2), either symmetrical (INMLI series) or asymmetric (AsINMLI series) [22,24], were subsequently reported for OLED application. However, all of these indole-substituted maleimide OLEDs show shorter EL wavelengths in the orange to red–orange region and their performances are all inferior to that of the first reported NPAMLI OLED.

One non-maleimide-based material listed in Table 1.2 is a five-membered heterocylic pyrrole derivative, NPANPy [23]. Pyrroles are nitrogen-containing five-membered cyclic dienes similar to siloles except that the silicon is the heteroatom of the five-membered ring structure. Structure-wise, tetraaryl-substituted pyrrole derivatives have a propeller-like molecular conformation very similar to that of tetraaryl-substituted silole derivatives. Recent studies have revealed that a propeller-like molecular structure is vital for the AIE or AIEE effect caused by the restriction of intramolecular bond rotation in the solid state. It is not surprising that such pyrrole derivatives exhibit stronger fluorescence intensities than those in dichloromethane solution [23], a typical AIEE effect found for structurally similar silole derivatives. Unlike the electron-deficient nature of the silole derivatives, pyrrole derivatives are electron rich and seldom produce red-shifted exciplex emission with the HTL. Therefore, it is relatively easy for pyrrole derivatives to achieve blue EL when fabricated as the EML in OLEDs. Provided that low-lying HOMO TPBI is used as the ETL, authentic blue EL with 1931 CIEx,y (0.16, 0.14–0.17) can be readily obtained (Table 1.2). However, the EL efficiency is not very good (none of the ηEXT values of the blue emissions is more than 1.5%).

1.5 AIE or AIEE Cyano-Substituted Stilbenoid and Distyrylbenzene Derivatives for OLEDs

The observation of the enhanced fluorescence on cyano-substituted stilbenoid and distyrylbenzene derivatives (CN-DSB) in the solid state can be traced back as early as that of silole derivatives. In fact, one of the first reported CN-DSB OLEDs was observed in 2002 by Luo et al. [25]. However, the device was fabricated with CN-DSBx (Table 1.3) by a doping process and the reported OLED performance was far from satisfactory. Shortly after, in 2003, Chen and co-workers reported high performance (maximum ηEXT ≈ 2.4%) non-dopant red–orange OLEDs containing a dicyano-substituted, 2-naphthylphenylamine-appended stilbenoid, NPAFN (Table 1.3) [26]. Once again, similarly to the case of the NPAMLI OLED, NPAFN OLEDs performed better without including NPB as the HTL. As shown in Figure 1.1a, NPAFN exhibits a pronounced AIE or AIEE effect. The AIE or AIEE effect of NPAFN has recently been confirmed in solution, as shown in Figure 1.7.

Table 1.3 Summary of reported OLEDs containing cyano-substituted stilbenoid and distyrylbenzene AIE or AIEE materials.

Figure 1.7 Fluorescence images of NPAFN in acetonitrile–water mixture with water fractions of 0, 20, 50, 60, 70, and 80% from left to right.

Several years later, in 2008, Chen and co-workers developed the second generation of NPAFN, PhSPFN and FPhSPFN (Table 1.3) [27]. With longer π-conjugation between donor and acceptor moieties, PhSPFN and FPhSPFN OLEDs both display EL at longer wavelength, corresponding to authentic red color chromaticity, 1931 CIEx,y (0.67, 0.30) and (0.66, 0.34), respectively. The maximum ηEXT of the FPhSPFN OLED reaches 3.1%, one of the highest among AIE or AIEE nondopant red OLEDs, only exceeded by TTPEBTTD [maximum ηEXT = 3.7%, CIEx,y (0.67, 0.32)] [28]. In this case, AIE and AIEE take place for PhSPFN and FPhSPFN, respectively (Figure 1.8). The fluorine ortho-substituent of FPhSPFN enhances the restriction of intramolecular bond rotation and hence increases the fluorescence intensity not only in the solid state but also in solution.

Figure 1.8 Fluorescence images of PhSPFN (a) and FPhSPFN (b) in solution (dichloromethane) and in the solid state. (See color figure in color plate section).

Although EFPAFN in Table 1.3 has a longer EL wavelength and hence better chromaticity of red color, in terms of EL efficiency [29], FPhSPFN is by far one of the best red AIE or AIEE materials for OLED application. In Table 1.3, CN-DPASDB exhibits a fairly high ηC of 4.0 cd A−1 but it is a yellow OLED [31], mostly not a desirable color for display applications.

1.6 AIE or AIEE Triarylamine Derivatives for OLEDs

Triarylamines represent the smallest family of AIE or AIEE materials, although they are often present in the molecular structure of long wavelength-emitting AIE or AIEE materials (see examples in Section 1.4 and Section 1.5). A triarylamine has an electron-rich character and is therefore suitable as a strong electron donor in structural design, readily raising the HOMO energy level and narrowing the emission bandgap energy of the molecular compound. Basically, a triarylamine molecule possesses a nonplanar molecular structure, similar to the propeller-like structural feature of most AIE or AIEE materials. Two types of triarylamine-based AIE or AIEE materials have been demonstrated for OLED application (Table 1.4). Constructed with an appropriate electron acceptor segment, two of them (SBCN and M1) exhibit near-IR emission [32,33], although their OLED performance is rather poor, not very different from those of other near-IR OLEDs reported in the literature.

Table 1.4 Summary of reported OLEDs containing triarylamine-based AIE or AIEE materials.

1.7 AIE or AIEE Triphenylethene and Tetraphenylethene Derivatives for OLEDs

As summarized in this section, a wide variety of chemical structures have been reported for tri- and tetraphenylethene derivatives. This group of organic fluorophores is one of the latest discovered AIE or AIEE materials, although this family is the largest in number. However, their OLED application was documented long before the invention of AIE or AIEE terminology.

The first compound considered in this section is DPVBi (Table 1.5), which was first commercialized by Idemitsu Kosan Co. as one of the most widely used blue fluorophores in the mid-1990s [34]. Apart from being a derivative of triphenylethene, DPVBi can also be looked as upon as a diphenyl-substituted stilbenoid dimer. In fact, the tetraphenylethene moiety can also be considered as a cross-conjugated stilbenoid. The triphenylethene moiety exhibits a similar propeller shape of to the tetraphenylethene moiety. As illustrated by other examples shown in Section 1.5, stilbenoid compounds are well known for the free rotor effect on the radiationless transitions that cause serious fluorescence quenching in solution state [35,36].

Table 1.5 Summary of reported OLEDs containing DPVBI, DPVPA, or TPVAn AIE or AIEE materials.

The AIE or AIEE effect of DPVBi has never been shown until now (see Figure 1.9). Accordingly, in terms of the AIE or AIEE effect and structural similarity, triphenylethene derivatives can be considered as one member of the large tetraphenylethene family. According to our survey, the best nondopant DPVBi OLED was probably reported by Park and co-workers in 2007 (Table 1.5) [41]. The device was fabricated as a control OLED and it was an authentic blue device with an EL wavelength at 465 nm corresponding to 1931 CIEx,y (0.15, 0.16). The device has a maximum ηC of 3.92 cd A−1 and a maximum ηP of 1.61 lm W−1, which are good compared with other OLEDs in Table 1.6 having a similar blue color purity.

Figure 1.9 Fluorescence images of DPVBi in THF–water mixture with water fraction 0, 20, 40, 60, and 80% from left to right. Its solid-state fluorescence image is shown in front of the solution samples. (See color figure in color plate section).

Table 1.6 Summary of reported OLEDs containing DPFv-NH2, DPFV-OMe, TPE, TPEPh, BTPE, or TDPVBi AIE or AIEE materials.

In addition to the early work on DPVBi, triphenylethene and tetraphenylethene have been attached to anthracene (Table 1.5), one of the first organic fluorophores studied for EL by Pope et al. in the 1960s [45]. DPVPA is bistriphenylethene-attached anthracene [43] and TPVAn, BTPPA, and BTBPPA are bistetraphenylethene-attached anthracene derivatives [41,44]. Among all the compounds listed in Table 1.6, the nondopant TPVAn OLED shows the highest fluorescence-based EL efficiency (ηEXT = 5.3%). In addition, its blue color purity is excellent, 1931 CIEx,y (0.14, 0.12), corresponding to an authentic blue color. This is not surprising because the solution PL quantum efficiency of 9,10-diphenylanthracene is very high and close to unity [46]. The bulky propeller-like tetraphenylethene substituent prevents the anthracene moiety from close contact and red shifting the emission wavelength. On the other hand, bistetraphenylethene substituents contribute to the strong AIE or AIEE effect that simply preserves the high PL quantum efficiency of the material in the solid state.

DPFvNH2 and DPFvNH2 are two-ring-fused versions of tetraphenylethene, although the propeller molecular shape remains and the AIE or AIEE effect is observed [47]. DPFvNH2 and DPFvNH2 OLEDs have been fabricated and tested, and the results were not very good (Table 1.6). Both devices have a significantly red-shifted EL at 485 and 520 nm, respectively, and weak EL has been observed for DPFvNH2 OLEDs, Considering the red-shifted and weak EL, it can be suspected that DPFvNH2 forms an excimer in the thin-film device. Interestingly, PhBAPN is a three-ring-fused version of tetrephenylethene and the propeller-like structure of DPFvNH2 and DPFvNH2 no longer exists in this compound (Table 1.6), so nor is the AIE or AIEE effect. A PhBAPN OLED was reported with an even weaker excimer EL (Lmax = 219 cd m−2) at 556 and 580 nm [47].

Whereas TPE is the parent structure of tetraphenylethene, TPEPh is probably the simplest tetraphenylethene derivative (Table 1.6) [48], being a single phenyl-substituted TPE. In the literature, the solid-state PL wavelength of TPEPh was demonstrated to be morphology dependent. It is 454 and 503 nm for crystalline and amorphous TPEPh, respectively, although its EL wavelength is at 476 nm and it is green–blue. The EL efficiency of green–blue TPEPh is just moderate (ηEXT = 2.56%, ηC = 5.15 cd A−1). The parent compound TPE has a very short EL wavelength at 445 nm (indicative of a deep blue color) and nearly overlaps with the PL spectrum of its crystal. However, the device's Lmax is only ∼1800 cd m−2 and none of the EL efficiency criteria is over 0.5 (%, cd A−1, or lm W−1), although a very low turn-on voltage (L of the device equal to 1 cd m−1) of 2.9 V was observed for a TPE OLED.

BTPE is a single covalent bond-connected dimer form of the parent TPE structure (Table 1.6) [49]. Like TPE or TPEPh mentioned above, BTPE is one of the tetraphenylethene derivatives having a relatively simple chemical structure. Its solid-state PL also behaves similarly to TPEPh, with the PL wavelength being morphology dependent, 445 and 499 nm for crystalline fibers and amorphous film, respectively. Similarly to a TPEPh OLED, the BTPE EL wavelength at 488 nm (green–blue) is between the PL wavelengths of crystalline and amorphous samples, although its OLED performance is better than that of TPEPh OLEDs. On the other hand, TDPVBi can be considered as a triphenylethene version of BTPE bearing two extra propeller-like moieties (Table 1.6) [42]. It also can be considered as an ‘upgraded’ version of DPVBi because of the two extra propeller-like moieties. Concerning the OLED performance, TDPVBi has a much shorter EL wavelength at 468 nm corresponding to a green–blue color, 1931 CIEx,y (0.16, 0.21), which is bluer than that of the TPEPh OLED. The EL efficiency of the TDPVBi OLEDs is also in general better than that of TPEPh OLEDs.

TPESiPh3, (TPE)3SiPh2, and (TPE)2SiPh are hybrid structures of TPE and tetraphenylsilane (Table 1.7) [50], which is known for the amorphous feature of the OLED material that can extend the morphological stability of the thin-film structure in OLEDs [51,52]. However, from the reported OLED data for these materials, as more TPE units are attached to the tetraphenylsilane, the performance of the OLEDs becomes worse, not only the EL efficiency and maximum brightness but also the EL wavelength (becoming longer with less blue color purity). Among the series, the least TPE-containing TPESiPh3 has the best-performing OLED. It is an authentic blue OLED with a short EL wavelength at 452 nm and Lmax = 5672 cd cm−2, but its EL efficiency is not poor (ηEXT = 1.6%, ηC = 2.1 cd A−1, ηP = 1.1 lm W−1).

Table 1.7 Summary of reported OLEDs containing silane-based TPESiPh3, (TPE)2SiPh2, (TPE)3SiPh AIE or AIEE materials.

In contrast to an earlier report on triarylamine-based AIE or AIEE materials by He and co-workers [42], Tang et al. clearly showed that triphenylamine (TPA) and N4,N4,N4′,N4′-tetraphenylbiphenyl-4,4′-diamine, a TPA dimer (DTPA), are ACQ materials instead of AIE or AIEE materials (Table 1.8) [53]. After attaching three TPE units to TPA or four TPE units to DTPA, 3TPETPA and 4TPEDTPA become typical AIE or AIEE materials. However, regarding their OLEDs, 3TPETPA peculiarly exhibits multiple EL bands at 493 and 511 nm (in a device containing NPB as HTL) or 499 and 513 nm (in a device without NPB HTL), which are both red shifted from PL wavelength in solution (THF–water mixture) of 484 nm. The OLED of 4TPEDTPA behaves normally with a single EL band at 488 nm, which is green–blue not much different from the PL wavelength in solution (THF–water mixture) of 486 nm. Without the red-shifted EL and anomalous emission band, 4TPEDTPA OLEDs outperform 3TPETPA OLEDs.

Table 1.8 Summary of reported OLEDs containing triphenylamine-based 3TPETPA or 4TPEDTPA AIE or AIEE materials.

Pyrene is another fluorophore known for notorious aggregation even in the solution state. Pyrene is a typical ACQ material because of its rigid and flat molecular structure. There have been two approaches to chemical modification of pyrene with a TPE moiety (Table 1.9). The first involves surrounding the pyrene structure with four propeller-like TPE moieties like that of TTPEpy [54]. The second approach is to incorporate pyrene moieties, either one or two, into TPE as part of the structure like those of TPPyE and DPDPyE [55]. Both approaches are successful in changing pyrene from an ACQ material to an AIE or AIEE material. However, considering the performance of the OLEDs, the former approach, namely TTPEpy OLEDs, can reach a high EL efficiency (ηEXT = 4.98%, ηC = 12.3 cd A−1, ηP = 7.0 lm W−1) and substantially outperforms the latter (TPPyE and DPDPyE). TPPyE and DPDPyE OLEDs are also worse in blue color purity and their EL wavelength is in the range 504–516 nm, which is longer than the 488–492 nm of green-blue TTPEpy OLEDs.

Table 1.9 Summary of reported OLEDs containing pyrene-based 3TPETPA or 4TPEDTPA AIE or AIEE materials.

The next group in the triphenylethene family is DPVP2Mst, DPVPP2Mst, and DPVP4Mst (Table 1.10) [56], which can be considered as an improved version of DPVBi in terms of blue color purity. With two central methyl substituents twisting the biphenyl structure, the effective π-conjugation length of the molecule is reduced, the bandgap energy is increased, and the emission wavelength is shortened. An authentic blue EL, 1931 CIEx,y (0.15, 0.17), was acquired for a DPVPP2Mst OLED. As the number of triphenylethene moieties attached increases, the AIE or AIEE effect intensifies the solid-state blue emission of these bimesitylene fluorophores. With Bim-DPAB (Scheme 1.2) as the HTL in the device, the EL efficiency of a DPVPP2Mst OLED was increased (ηEXT = 4.7%, ηC = 5.4 cd A−1, ηP = 2.02 lm W−1).

Table 1.10 Summary of reported OLEDs containing bimesitylene-based DPVP2Mst, DPVPP2Mst, or DPVP4Mst AIE or AIEE materials.

The molecular hybrid structure of TPE and silole, two well known AIE or AIEE moieties, seems to be interesting and worth OLED testing. Tang and co-workers prepared 2,5-BTPEMTPS and 3,4-BTPEMTPS after the first failed attempt at the preparation of TPE hybrid hexaphenyl-substituted siloles (Table 1.11) [49]. The silole and TPE hybrids 2,5-BTPEMTPS and 3,4-BTPEMTPS [57] are in fact a pair of structural isomers. Both of their OLEDs exhibit green–yellow EL, that of the 2,5-isomer having a longer wavelength at 552 nm than that of the 3,4-isomer at 520 nm. The not very good performance of nondopant OLEDs can be improved by making a dopant device with BTPE as the host material for 2,5-BTPEMTPS after optimizing the dopant concentration. A similar molecular hybrid approach was applied to TPE and triphenylethene by Chi and co-workers (Table 1.11). [58] One of such hybrid molecules is (VP)3-(TPE)3 and its OLED has been tested. However, (VP)3–(TPE)3 OLEDs perform poorly without EL efficiency data. The device requires a relatively high 6.0 V to be turned on and Lmax is only 1908 cd m−2 Its EL color is not blue but green–blue at 474 nm, corresponding to 1931 CIEx,y (0.18, 0.31).

Table 1.11 Summary of reported OLEDs containing TPE–silole hybrid 3,4-BTPEMTPS or 2,5-BTPEMTPS AIE or AIEE materials.

In order to increase the hole-transporting ability of TPE or triphenylethene AIE or AIEE materials, arylamine moieties are a potent structural feature to be incorporated in the structure [59]. We have seen that 3TPETPA and 4TPEDTPA mentioned earlier (Table 1.8) are two examples following the principle of structural design herein. More TPE or triphenylethene AIE or AIEE materials containing an arylamine moiety are known in the literature. Arylamine-containing C4 [60], TPATPE [61], 2TPATPE [61], TPE-2Cz [62], TPECa [63], and TPECaP [63] are all such materials. Their OLED data are all summarized in Table 1.12. Except for TPE-2Cz, most of the OLEDs show green–blue to blue–green EL with wavelengths in the range 484–514 nm. The TPE-2Cz OLED has an EL wavelength at 462 nm corresponding to 1931 CIEx,y (0.17, 0.21), approximately blue, but its EL efficiency is only moderate (ηC = 2.80 cd A−1, ηP = 2.51 lm W−1).

Table 1.12 Summary of reported OLEDs containing arylamine-based TPE derivative AIE or AIEE materials.

Fluorene (or the more rigid and bulky 9,9′-spirobifluorene) is the repeating unit of high-performance blue light-emitting polymers, polyfluorenes (PFs). The hybrid or fused structure of TPE and fluorene (or 9,9′-spirobifluorene) can take good advantage of the AIE or AIEE effect and high PL efficiency of blue emission. Three sets of such materials, BTPEBCF [64], SFTPE [62], and Fn-TPE (n = 1–5) [65], are known in the literature (Table 1.13). Whereas SFTPE, F1-TPE, and F2-TPE OLEDs exhibit EL with wavelengths shorter than 470 nm, an empirical benchmark of blue color, BTPEBCF, F3-TPE, F4-TPE, and F5-TPE OLEDs all exhibit EL at wavelengths 476–508 nm, indicative of green–blue color. Regardless of the unsatisfactory blue color purity, the EL efficiency of BTPEBCF is the best; ηC and ηP can reach 7.9 cd A−1 and 3.7 lm W−1, respectively.

Table 1.13 Summary of reported OLEDs containing fluorene- or 9,9′-spirobifluorene-based TPE derivative AIE or AIEE materials.

Organoboron compounds are particularly attractive and promising owing to their unique properties stemming from the pπ–π* conjugation between the vacant p orbital on the boron atom and the p* orbital of the π-conjugated framework. Organoboron compounds also have electron-transporting properties [66,67]. A very high blue EL efficiency (ηEXT >6%) has been reported for D–π–A-type blue fluorophores containing dimesitylboron as electron acceptor [68]. In the literature, two TPE-containing organoboron compounds have been reported for OLED applications, TPEMesB and BTPEPBN (Table 1.14) [69,70]. For TPEMesB OLEDs, it has been shown that the EL efficiency is in fact higher without using an ETL (TPBI) in the device, demonstrating the electron-transporting property in addition to the green–blue light-emitting function of TPEMesB AIE or AIEE materials. For BTPEPBN OLED, no EL wavelength was reported, but ‘sky blue’ color was mentioned for its PL, probably in the solid state. A BTPEPBN OLED was reported with low EL efficiency (ηEXT = 1.52%, ηC = 4.43 cd A−1, ηP = 1.64 lm W−1), although it is better than the OLED with the same organoboron material lacking two TPE substituents [70]. This is yet further experimental evidence supporting the AIE or AIEE effect of a TPE moiety that is beneficial for EL efficiency. Regarding electron transport of OLED materials, the other well-known electron transporting material is 2,5-diphenyl-1,3,4-oxadiazole structural moiety, and it has been attached with two TPE units to form OXa-p TPE and OXa-m TPE (Table 1.14) [71]. Although the AIE or AIEE effect has been demonstrated for both OXa-p TPE and OXa-m TPE compounds, high EL efficiency (ηEXT = 5.0%, ηC = 9.79 cd A−1, ηP = 9.92 lm W−1) has been achieved with the dopant OLEDs, of which OXa-p TPE or OXa-m TPE are used as the host material for BUBD-1 blue dopant. Compared with the EL spectra of BUBD-1 reported elsewhere [72], the EL spectra shown in the paper on OXa-p TPE and OXa-m TPE OLEDs [71] indicate that it is a co-emission EL from both host and dopant and it is green–blue in color, 1931 CIEx,y (0.15, 0.34).

Table 1.14 Summary of reported OLEDs containing organoboron- or 1,3,4-oxadiazole-based TPE derivative AIE or AIEE materials.

One large family of TPE derivatives is derived from attaching to or being attached with various polycyclic aromatic hydrocarbons (PAHs). TPTPE (or thiophene-based TPTDPE) and BTPTPE are the fused TPE dimer and trimer (Table 1.15), respectively, via one common benzene ring, the fundamental repeating unit of PAHs [73]. Whereas the trimer BTPTPE OLED exhibits very short wavelength EL at 448 nm, indicative of an authentic blue color, the TPTPE (or thiophene-based TPTDPE) OLED has a green–blue EL at 488 nm (512 nm for thiophene-based TPTDPE). The TPTPE OLED shows better EL efficiency than the other two. However, the green–blue EL efficiency of TPTPE is only fair (ηEXT = 2.7%, ηC = 5.8 cd A−1, ηP = 3.5 lm W−1). PAHs such as pyrene, anthracene, phenanthrene, and naphthacene have been singly attached to TPE forming TPEPy, TPEAn, TPEPa, TPENp, and TPE-2-Np (Table 1.15) [74]. They are also doubly attached to TPE in forming TPEBPy, TPEBAn, TPEBPa, TPEBNp, and TPEB-2-Np (Table 1.15) [74]. Although the solid-state PL of TPENp has the shortest wavelength at 469 nm, TPEPa has the shortest EL wavelength at 464 nm, a possible blue color. The EL of TPENp is red shifted to 480 nm and becomes green–blue. A similar red shift, from the smallest +2 nm of TPEB-2-Np to the largest +30 nm of TEPBAn, are common for these two types of TPE derivatives, except one TPEPa, which has a blue-shifted rather than a red-shifted EL. All of the non-dopant OLEDs containing the materials mentioned here have EL efficiencies ranging from poor (ηEXT = 1.3%, ηC = 2.4 cd A−1, ηP = 1.1 lm W−1 for TPENp OLED) to above average (ηEXT = 3.0%, ηC = 7.3 cd A−1, ηP = 5.6 lm W−1 for TPEPy OLED). However, the performance of TPEPy OLED is not as good as that of another pyrene-containing TPE derivative OLED (TTPEPy) reported earlier, and both OLEDs have comparable green–blue color.

Table 1.15 Summary of reported OLEDs containing PAH–TPE derivative AIE or AIEE materials.

The latest member joining the family may be the mono-, di-, and triphenylethene n-hexyloxybenzene derivatives PhTPE, Ph2TPE, and Ph3TPE reported by Li and co-workers [75] (Table 1.15). It must be because of the steric hindrance twisting the coplanarity between the TPE unit and ortho-substituted n-hexyloxybenzene that the Ph2TPE OLED shows the shortest EL wavelength at 457 nm, an authentic blue corresponding to 1931 CIEx,y (0.16, 0.15). Similarly to several of the TPE derivatives shown earlier, Ph2TPE OLED has an unusually blue-shifted EL compared with its solid-state PL at 494–498 nm. Nevertheless, the EL efficiency of the Ph2TPE OLED is not good (ηC = 2.3 cd A−1, ηP = 1.7 lm W−1) and that of Ph3TPE OLED [also with a blue EL at 467 nm and 1931 CIEx,y (0.17, 0.20)] is better (ηC = 3.7 cd A−1, ηP = 2.5 lm W−1).

As we have seen with the many triphenylethene or TPE derivatives so far, they mostly exhibit EL with green–blue, blue–green, green, and even yellow–green color, and these are not the desirable colors in display applications. From this survey, it is evident that authentic blue AIE or AIEE materials containing a triphenylethene or TPE moiety are difficult to come by. The last group in the family of triphenylethene or TPE derivatives are the AIE or AIEE effects shown in the other extreme of visible spectra, namely the long wavelengths in the orange, red, and even near-IR region. In order to acquire a long emission wavelength, either PL or EL, the triphenylethene or TPE moiety has to be π-conjugated to a strong electron acceptor. From a survey of the literature, it was found that the benzo[c][1,2,5]lthiadiazole (BTZ) moiety seems to be the most potent electron acceptor for long-wavelength PL or EL (Table 1.16).

Table 1.16 Long-wavelength EL from OLEDs with TPE-containing AIE or AIEE materials.

For EL wavelengths longer than 650 nm, a benchmark for red color, thiophene-bridged triphenylethene and BTZ is necessary and it requires two thiophene bridges (like BTPEBTTD) instead of one thiophene (like BTPETTD) or two benzene bridges (BTPETD) (Table 1.16) [78]. This is because the thiophene ring has a lower bandgap energy than the benzene ring. Similarly, for V2BV2 and T2BT2 (Table 1.16) [79], a C=C double bond as the connecting π-conjugation bridge is not good enough to extend the EL wavelength beyond 650 nm, even though there is a strong electron donor triphenylamine between the triphenylethene and BTZ. Another approach is to increase the electron-deficient power of BTZ by changing to [1,2,5]thiadiazolo[3,4-g]quinoxaline (QTD) of TPEQTD or MTPEQTD, or benzo[1,2-c;4,5-c ′]bis[1,2,5]thiadiazole (BBTD) of TPEBBTD or MTPEBBTD (Table 1.16). [80] These four compounds (TPEQTD, MTPEQTD, TPEBBTD, and MTPEBBTD) have EL wavelengths in the range 706–864 nm, within the near-IR region. However, their EL Lmax are all low and none of their ηEXT values is over 1%. The most successful design for red EL is the branched version of BTPEBTTD, that is, TBTPEBTTD (Table 1.16) [28]. TBTPEBTTD has an EL wavelength at 650 nm, 1931 CIEx,y (0.67, 0.32), at the edge of red color on the chromaticity diagram. It was reported with ηEXT as high as 3.7%, superior to the 3.1% of the FPhSPFN OLED [27], one of the most efficient nondopant red OLEDs based on AIE or AIEE materials.

1.8 White OLEDs Containing AIE or AIEE Materials

Although its AIE or AIEE effect was demonstrated above, the blue fluorophore DPVBi was utilized in RGB multi-color OLED displays as early as 1997 [81]. For white EL used for lighting, there are a number of materials and device configurations for generating white EL from a DPVBi-containing OLED (Table 1.17) [38,82–86]. In terms of EL efficiency and Lmax, a DPVBi blue-emitting layer doped with yellow–orange rubrene is probably the best approach, as reported by Li and Shinar [83] Such a WOLED exhibits EL with 1931 CIEx,y (0.27, 0.31) near-white chromaticity, Lmax as high as 50100 cd m−2, ηEXT 4.0% and ηP 3.9 lm W−1. However, since it is a two-color-white WOLED, it is not possible for the CRI to be high (the original paper did not report the CRI value of the device).

Table 1.17 Summary of reported WOLEDs containing AIE or AIEE materials.

The second kind of AIE or AIEE material being used in WOLEDs is the orange–red fluorophore NPAFN (Table 1.3 and Table 1.17) [87]. Taking advantage of its strong AIE or AIEE effect reported by Chen and co-workers, NPAFN enabled the first all-nondopant three-color WOLED to be obtained. Because of its nondopant nature, the NPAFN WOLED showed almost constant white chromaticity with 1931 CIEx,y