The Art of Conjugation and Rotation - American Chemical Society

Chapter 5

Aggregation-Induced Emission Materials: The Art of Conjugation and Rotation Downloaded by CORNELL UNIV on October 7, 2016 | http://pubs.acs.org Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1226.ch005

Jie Yang, Jing Huang, QianQian Li, and Zhen Li* Department of Chemistry, Hubei Key Lab on Organic and Polymeric Opto-Electronic Materials, Wuhan University, Wuhan 430072, China *E-mail: [email protected] or [email protected]

Since the AIE phenomenon was first named by Tang’s group in 2001, more and more attention has been attracted for its great promising applications in the opto-electronic fields. In order to fully excavate the potential of AIE characteristic, the mechanism and applications should be both explored in details. In this chapter, we mainly focus on the restriction of intramolecular rotation (RIR) mechanism through the combination of experimental results and theoretical calculations, and the applications in OLEDs including some strategies to develop blue AIE luminogens, AIE host , and AIE PLEDs.

1. Introduction Organic luminogens with strong solid-state emission have attracted much attention due to their huge applications in the fundamental fields as biological probes, chemical sensors and particularly organic light-emitting diodes (OLEDs) (1, 2). However, the traditional organic luminogens with planar conformations, often suffer from the notorious aggregation-caused quenching (ACQ) effect or aggregation-induced red-shifted emission by the strong intermolecular π-π stacking which has been documented for more than half a century since Förster’s discovery of the concentration quenching effect in 1954, badly impeding their practical applications (3). In complete contrast to ACQ effect, the luminogens with aggregation-induced emission (AIE) characteristic might enjoy many advantages for the high performance of opto-electronic devices.

© 2016 American Chemical Society Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Downloaded by CORNELL UNIV on October 7, 2016 | http://pubs.acs.org Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1226.ch005

The AIE phenomenon was first reported by Tang’s group in 2001: some luminogens exhibit weak or even non-emission in solution, but much enhanced luminescence in the aggregate state, such as in solid state or nanoparticles, which was termed as aggregation-induced emission (AIE) or aggregation-enhanced emission (AEE) (4). As shown in Figure 1, 1-methyl-1,2,3,4,5-pentaphenylsilole (MPPS), the first reported AIE luminogen, shows non-emission in dilute solution but turns to be highly-emissive in aggregate state (5). When acting as the emitter layer in OLED, it exhibits an excellent performance with the current efficiency and external quantum efficiency up to 20.0 cd A-1 and 8%, respectively, much higher than that of traditional light-emitting materials and showing great advantages for its AIE property. Inspired by the promising application in highly efficient electroluminescent devices and other related fields, more and more attention has been paid to the AIE research, especially the inherent mechanism and practical applications (6).

Figure 1. The propeller-shaped luminogen of 1-methyl-1,2,3,4,5pentaphenylsilole (MPPS) is non-emissive in dilute solution but turns to be highly-emissive for the restricted intramolecular rotation (RIR) effect in aggregate state.

2. Mechanism 2.1. Evidences: RIR Is Main Mechanism for AIE Effect Systematic studies have suggested that the restriction of intramolecular motion (RIM) is the main cause of the AIE effect including the restriction of intramolecular rotation (RIR) and the restriction of intramolecular vibration (RIV) (7). In this chapter, we mainly focus on the RIR mechanism. Fundamental physics teaches us that any molecular motions consume energy. As shown in Figure 2, tetraphenylethene (TPE) is constructed by four phenyl rings surrounding an ethylene group with a propeller-shaped conformation, which allows the four phenyl rings to rotate freely in the solution state, and provides a relaxation channel for the excited state to decay with non-emission. Once aggregated, the intramolecular rotation is restricted due to physical constraints, thus, the energy is consumed from the radiative transition, namely fluorescence. To check the validity of RIR mechanism, a series of external control tests have been conducted. 62 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 2 shows the piezochromism, viscochromism and thermochromism in detail. We may find that aggregation, high pressure, high viscosity, low temperature all could restrict the intramolecular rotation and realize the enhanced emission, which powerfully proves the accuracy of RIR as the main mechanism for AIE effect (3).

Figure 2. The propeller-shaped luminogen of tetraphenylethene (TPE) is non-emissive in dilute solution but turns to be highly-emissive for the restricted intramolecular rotation (RIR) effect through aggregation, high pressure, high viscosity, low temperature and structural modification.

Except for the external control tests, the structural modification as internal control tests also was conducted. Hexaphenylsilole (HPS), an analog of the first example of AIE molecules, is considered to be a typical AIE luminogen (Figure 3). It is non-emissive in dilute solution as the free rotation of the six peripheral phenyl rings. However, when the stereo-hindrance groups of isopropyl ones were introduced to yield HPS3,4, it shows strong green emission in solution. For example, in its dilute acetone solution, the quantum yield of HPS3,4 was found to be as high as 83%, 2-3 orders of magnitude higher than those of the ‘normal’ siloles (0.031-0.51%, with ~0.1% being most typical) (8). Thus, the minor structural change results in big differences from AIE to AEE behavior, as a result of the steric-hindrance effect from isopropyl groups, confirming the RIR mechanism of the AIE effect. Similar phenomenon was also observed in TPE and its derivatives (Figure 3): TPE is nearly non-emissive in solution, while TPE-TM gives a strong blue emission in THF solution with quantum yield up to 64% (9). The only difference is the absence or presence of methyl groups, which could make the rotation of the phenyl rotors much more difficult for TMTPE even in solution. Hence, the effective restriction of the rotation for the phenyl rotors would lead to the strong emission in solution, as powerfully proved by the cases of HPS3,4 and TMTPE. 63 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 3. HPS and TPE are typical AIE luminogens showing non-emission in solution. However, when stereo-hindrance groups such as isopropyl and methyl groups were introduced to get HPS3,4 and TMTPE, they show strong emissions in solutions as well as in aggregates.

2.2. Relationship between Rotors and AIE Effect The aforementioned studies have clearly demonstrated RIR as the main mechanism for the AIE effect. Then, one question arose, if once the bonded rotors exist in a propeller-shaped luminogen, could the AIE characteristic be obtained? The TPE system is simple, but can we develop even simpler AIE systems? Taking these into consideration, a pretty simple system was developed based on polyphenylbenzenes, in which one or two or three phenyl rings as rotors were linked to the central benzene core to yield compounds 1, 2 and 3 (Figure 4) (10). The results answer the questions perfectly: all of them are AIE-active. In dilute acetone solutions, compounds 1–3 are non-emissive. However, when large amounts of water were added to their acetone solutions, they tended to aggregate and gave strong emissions with the enhancement higher than 150 times from solution to aggregation states. What’s more, when increasing solvent viscosity and/or decreasing solution temperature, their PL emission could also be enhanced as a result of the restricted rotation of the peripheral phenyl rings, further proving that the RIR process is indeed involved in the AIE system. Then another propeller-shaped system was developed also based on benzene core, in comparison to compound 1-3, the only difference is that the peripheral phenyl rotors were replaced by carbazolyl units (Figure 5). The different rotors also led to different luminescent properties. For compounds 4-6, they are all emissive in the solution state and exhibited enhanced emissions in aggregation, with the typical AEE effect (9). However, their fluorescent quantum yields (ФF) in solutions decrease with the increasing number of the carbazolyl units from 4 (47.4%) to 5 (30.1%) to 6 (16.2%), indicating that with more carbazolyl units, more energy would be consumed from the intramolecular rotation, leading to lower ФF values. This phenomenon suggests that the RIR effect also exists in AEE system. 64 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


The changes of the ФF values are easy to be understood and confirm the RIR mechanism in another viewpoint. In order to repeat the above phenomenon and further confirm the RIR mechanism, we synthesized another system based on the carbazolyl core in which mono-rotor or dual-rotors were introduced (Figure 6) (11). For compounds 7, 9, and 11 with mono-rotor including the phenyl or biphenyl or carbazolyl rotor, they all show relatively higher ФF values in solution compared to those of compounds 8, 10, 12 with dual-rotors. These results also proved that more rotors would consume more energy from the intramolecular rotation and lead to lower ФF values. So this kind of phenomenon could be repeated and the RIR effect is widely existent in these propeller-shaped luminogens, confirming the RIR effect as the main mechanism for AIE and AEE characteristics.

Figure 4. Chemical structures of polyphenylbenzenes 1-3 (AIE).

Figure 5. Chemical structures of compound 4-6 (AEE).

65 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 6. Examples of carbazole-based luminogens 7-12 and their fluorescent quantum yields (ФF) in solutions.

2.3. Relationship between Rotational Energy Barrier and AIE Effect In the above-mentioned systems, we could see that different kinds of rotors would lead to different luminescent properties from AIE to AEE effect. In order to further study the correlation between the structure and AIE or AEE properties, we designed another system based on the pyrene and TPE moieties: Py-4MethylTPE and Py-4mTPE, in which the pyrene unit act as the core and meta-tetraphenylethene or methyl-tetraphenylethene groups as rotors (Figure 7) (12). As shown in Figure 7b, the small structural changes result in big differences from AIE to AEE behavior. In order to reveal the correlation in theory, DFT/TD-DFT calculations were carried out on Py-4MethylTPE and Py-4mTPE by using the B3LYP/6-31G* basis set. We calculated the rotational energy barriers in the ground state for isolated Py-4MethylTPE and Py-4mTPE, and found that Py-4mTPE is more rigid than Py-4MethylTPE owing to its better conjugation, which leads to the transition from AIE to AEE and confirms the RIR as the main mechanism for AIE effect again.



Figure 7. (a) The structures of Py-4MethylTPE and Py-4mTPE; (b) Plots of fluorescence quantum yields of Py-4MethylTPE and Py-4mTPE determined in THF/H2O solutions using 9,10-diphenylanthracene (Ф = 90 % in cyclohexane) as standard versus the fraction of water (fw). Insets: Photographs of Py-4MethylTPE and Py-4mTPE in THF/water mixtures (fw = 0 and 90 or 99 %) taken under the illumination of a 365 nm UV lamp; (C) the rotational energy barriers for isolated Py-4MethylTPE and Py-4mTPE in the ground state.

3. Application in OLEDs OLEDs have attracted increasing attention because of their huge potential in the applications as new display devices and solid state lighting. At the same time, AIE materials with enhanced solid state emissions might be good candidates as OLED emitters. Because of its grand AIE characteristic and simple structure, TPE unit was widely used to construct AIE materials ranging from deep blue to near-IR emissions (13). Particularly, through decorating traditional ACQ luminogens with TPE units, lots of highly efficient AIE molecules were obtained. Triphenylamine (TPA) and its derivatives are well-known for their high hole mobility, but showing typical ACQ effect, which much impedes their device 67 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


performances. Tang et. al decorated TPA dimer (DTPA) with four TPE units to yield 4TPEDTPA (Figure 8) (14). The resultant compound shows splendid AIE effect and excellent OLED performance. The device based on 4TPEDTPA gives a maximum current efficiency up to 8.0 cd A-1 even without the hole-transporting layer. Pyrene, another famous ACQ unit, was also decorated by four TPE moieties to obtain an efficient AEE molecule of TTPEPy, which shows a better performance with the current efficiency and external quantum efficiency up to 12.3 cd A-1 and 4.95%, respectively, closely approaching the theoretical limit for a singlet OLED (5%) (15). However, for these luminogens, their EL emissions are nearly out of the blue region owing to their extended conjugation. For three primary colors, green and red materials have been well developed, while efficient and stable blue materials are still scarce as a result of their intrinsic wide bandgap.

Figure 8. Examples of transition from the ACQ to AIE (AEE)—4TPEDTPA and TTPEPy and their OLED performances (OLED configurations-ITO/4TPEDTPA (30 nm)/TPBi (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al and ITO/NPB (60 nm)/TTPEPy (26 nm)/TPBi (20 nm)/LiF (1 nm)/Al).

3.1. Some Strategies To Realize Blue Emission of AIE Luminogens TPE itself can emit deep blue emission (445 nm), but the corresponding OLED efficiency is not so good, just 0.45 cd A-1. Once some aromatic rings are linked to TPE moieties, the resultant AIE luminogens could possess higher device efficiencies, but the emissions would be red-shifted to outside of the blue region. So on the one hand, the introduction of additional aromatic rings could improve the OLED efficiency, meanwhile, on the other hand, the bonded aromatic moieties would extend the π-conjugation, leading to a red-shifted emission. Thus, to achieve pure blue or even deep blue emission, it is required to restrict the π-conjugation between TPE and the introduced aromatic rings. Thanks to the great effort of scientists, five main strategies are developed to control the intramolecular 68 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


conjugation and realize the blue emission: the crystallization-induced blue-shifted emission, the modification of the linkage mode, making a twisted conformation, the interruption of the conjugation, and the reduction of the conjugation unit. Figure 9 shows the basic models for these five strategies, by applying these, some good blue AIE luminogens have been designed (16).

Figure 9. Some models to control the conjugation and these would make much contribution to realize the blue emission of AIE materials.

3.1.1. Crystallization-Induced Blue-Shifted Emission For traditional luminogens, they often suffer from crystallization-induced quenched or red-shifted emission due to the close π-π stacking in their crystalline states. On the contrary, AIE materials were found to possess bluer and enhanced emission in the crystalline states compared to their amorphous types for their twisted conformations. Dong et al. have done a lot of work on this topic, however, seldom use this unique property to construct blue OLEDs (17, 18). In 2012, Li et al developed a blue AIE system based on a benzene core and TPE as peripheral rotors by utilizing crystallization-induced blue-shifted emission. As shown in Figure 10, Ph2TPE and Ph3TPE with peripheral TPE units show bluer emissions than PhTPE (19). It seems strange that Ph-2TPE and Ph-3TPE possess extended conjugation moieties and bluer emissions simultaneously, but it is actually reasonable. With the introduction of TPE moieties, the substituted benzene cores become more and more crowded and liable to form crystalline states, leading to the blue-shifted emission. Among them, Ph2TPE shows the bluest EL emission at 457 nm with a current efficiency up to 2.3 cd A-1 at CIE coordinates (0.16, 0.15). 69 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 10. Chemical structures of PhTPE, Ph2TPE and Ph3TPE and the EL performances of the corresponding blue OLEDs (OLED configurations-ITO/MoO3 (10 nm)/NPB (80 nm)/PhTPE or Ph2TPE or Ph3TPE (20 or 30 nm)/TPBi (30 nm)/LiF (1 nm)/Al).

3.1.2. To Modify the Linkage Mode Among the AIE luminogens, TPE can emit deep blue emission (445 nm) with the maximum current efficiency of only 0.45 cd A-1. When two TPE units were linked together to give BTPE, its efficiency could increase to 7.26 cd A-1, however the EL emission was red-shifted to sky blue (488 nm) owing to the extended conjugation. Basic organic chemistry tells us that ortho, meta, and para positions possess different conjugation effects. Thus, is it possible to develop blue AIE system by utilizing this strategy to control the intramolecular conjugation? In 2013, Li et al synthesized four BTPE derivatives, mTPE–pTPE, oTPE–pTPE, mTPE–mTPE, and oTPE–mTPE, by modifying the linkage mode (Figure 11) (20). Unlike their analogous BTPE (488 nm), these luminogens all show deep blue emissions from 435 nm to 459 nm. Among them, mTPE-pTPE exhibits the best performance with a current efficiency of 2.8 cd A-1 at CIE coordinates of (0.16, 0.16). As for oTPE-mTPE with bluest emission (435 nm), its OLED performance is inferior (1.8 cd A-1) owing to the much twisted conformation. So a balance should be adjusted between high efficiency and effective conjugation length.



Figure 11. Chemical structures of BTPE, mTPE–pTPE, mTPE–mTPE, oTPE–mTPE, and oTPE–pTPE and the EL performances of the corresponding blue OLEDs (OLED configurations-ITO/MoO3 (10 nm)/NPB (60 nm)/mTPE–pTPE or oTPE–pTPE or mTPE–mTPE or oTPE–mTPE (15 nm)/TPBi (35 nm)/LiF (1 nm)/Al).

3.1.3. To Make Twisted Conformation It is noteworthy that to make twisted conformation is another efficient approach to restrict the conjugation in basic organic chemistry. So also from BTPE, we designed another blue AIE system: Methyl-BTPE, Isopro-BTPE, Ph-BTPE and Cz-BTPE by the introduction of additional resistance groups between two TPE units (21). In order to investigate the structure-property relationship in theory, DFT/TD-DFT calculations were carried out on these four BTPE derivatives by using the B3LYP/6-31G* basis set. In Figure 12, the dihedral angles between the adjacent phenyl blades of the two TPE units are 88.6°, 84.5°, 57.4° and 50.0° for Methyl-BTPE, Isopro-BTPE, Ph-BTPE and Cz-BTPE respectively, much more twisted than that of BTPE (35.5°). Their EL emissions also follow the change trend of dihedral angles: with the increase of dihedral angles, their emissions were blue-shifted from 488 nm (BTPE) to 479 nm (Cz-BTPE) to 467 nm (Ph-BTPE) to 451 nm (Methyl-BTPE and Isopro-BTPE). Among them, Cz-BTPE exhibits the highest current efficiency of 3.7 cd A-1 at CIE coordinates of (0.17, 0.26). 71 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 12. Chemical structures of Methyl-BTPE, Isopro-BTPE, Ph-BTPE, and Cz-BTPE and the EL performances of the corresponding blue OLEDs (OLED configurations-ITO/MoO3 (10 nm)/NPB (60 nm)/Methyl-BTPE or Isopro-BTPE or Ph-BTPE or Cz-BTPE (15 nm)/TPBi (35 nm)/LiF (1 nm)/Al).

3.1.4. Combination of Modifying the Linkage Mode and Making Twisted Conformation In the abovementioned examples, we have gotten some blue AIE materials through modifying the linkage mode or making twisted conformation. However, the highest current efficiency is only 3.7 cd A-1, leaving much room for the further improvement. So how about the combination of these two strategies? Could we develop some blue AIE materials with higher efficiency and realize the “one plus one larger than two” effect? In 2014, our group designed a blue AIE system based on TPA and TPE derivatives: TPA–3mTPE, TPA–3MethylTPE, MethylTPA–3pTPE, MethylTPA–3mTPE and MethylTPA–3MethylTPE (Figure 13), which were derived from the AIE molecule of 3TPETPA, reported by Tang et al. in 2010 (22). In the system, methyl groups were introduced to make a more twisted conformation and the linkage modes were tuned from para to meta to control the intramolecular conjugation. Their EL emissions are all in the blue region ranging from 459 to 480 nm for the restricted conjugation totally different from that of 3TPETPA (493, 511 nm). Among them, MethylTPA-3pTPE gives the best performance with the current efficiency and external quantum efficiency up to 8.03 cd A-1 and 3.99% respectively at CIE coordinates of (0.17, 0.28). MethylTPA-3mTPE shows the bluest emission (459 nm) while 72 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


retaining a comparable efficiency of 2.58 cd A-1 at CIE coordinates of (0.17, 0.19) owing to the combination of modifying the linkage mode and making twisted conformation. What’s more, they all show good hole-transporting ability inherited from the constructing block of TPA groups. When the hole-transporting layer was eliminated with the OLED configurations of ITO/MoO3 (10 nm)/ emitters (75 nm)/TPBi (35 nm)/LiF (1 nm)/Al, they could still retain similar performances. For example, the device based on MethylTPA-3pTPE exhibits better EL performance with the current efficiency and external quantum efficiency up to 6.51 cd A-1 and 3.39% respectively at CIE coordinates of (0.18, 0.25), which is comparable with those obtained from the standard device. Thus, through the combination of modifying the linkage mode and making twisted conformation, efficient blue AIE materials were obtained and “one plus one larger than two” effect was successfully realized.

Figure 13. Chemical structures of 3TPETPA, TPA–3mTPE, TPA–3MethylTPE, MethylTPA–3pTPE, MethylTPA–3mTPE and MethylTPA–3MethylTPE and the EL performances of the corresponding blue OLEDs (OLED configurations-ITO/NPB (60 nm)/3TPETPA (20 nm)/TPBi (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al and ITO/MoO3(10 nm)/NPB (60 nm)/TPA–3mTPE or TPA–3MethylTPA or MethylTPA–3pTPE or MethylTPA–3mTPE or MethylTPA–3MethylTPE (15 nm)/TPBi (35 nm)/LiF (1 nm)/Al). 73 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


3.1.5. To Interrupt the Conjugation It is easy to understand that the introduction of some appropriate less or even unconjugated moieties to interrupt the conjugation would be another effective approach to avoid the extension of conjugation and restrict strong intramolecular charge transfer, then lead to blue-shifted emission. In 2014, Li et al designed another blue AIE system based on TPA and TPE through a unconjugated fluorene core: TPE-pTPA and TPE-mTPA, while TPE-2pTPA and TPE-2mTPA are for extension (Figure 14) (23). Due to the presence of the sp3-hybridized carbon atom, the moieties on the 9,9’-positions are almost perpendicular, so the conjugations between TPE and TPA are effectively restricted. TPE-pTPA and TPE-mTPA both show deep blue emission at about 450 nm, totally different from that of TPATPE (492 nm) (24). Among the four blue AIE luminogens, TPE-pTPA gives the best performance with a current efficiency up to 3.37 cd A-1 at CIE coordinates of (0.16, 0.16), indicating that the introduction of fluorene groups could restrict the conjugation effectively as well as retain the EL efficiency for its unconjugated conformation and high carrier mobility.

Figure 14. Chemical structures of TPE–pTPA, TPE–mTPA, TPE–2pTPA, TPE–2mTPA, and the EL performances of the corresponding blue OLEDs (OLED configurations-ITO/PEDOT:PSS (30 nm)/NPB (30 nm)/TPE–pTPA or TPE–mTPA or TPE–2pTPA or TPE–2mTPA (10–30 nm)/TPBi (10 nm)/Alq3 (30 nm)/Ca:Ag). 74 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


3.1.6. To Reduce the Conjugation Unit

Figure 15. Chemical structures of DPA–PPB, Cz–PPB, DPA–TTP–CN, Cz–PPB–CN, mDPA–PPB–CN, mCz–PPB–CN, Cz–TPB–CN and BmPyPb (host) and the EL performances of the corresponding blue OLEDs (OLED configurations-ITO/MoO3 (10 nm)/NPB (60 nm)/DPA–PPB or Cz–PPB or Cz–PPB–CN or mDPA–PPB–CN or mCz–PPB–CN or Cz–TPB–CN or DPA–TTP–CN (nondoped 1) (30 nm)/TPBi (30 nm)/LiF (1 nm)/Al and ITO/MoO3 (10 nm)/NPB (60 nm)/mCP (10 nm)/DPA–TTP–CN (nondoped 2) (30 nm)/TPBi (30 nm)/LiF (1 nm)/Al and ITO/MoO3 (10 nm)/NPB (40 nm)/mCP (10 nm)/DPA–TTP–CN (nondoped 3) (15 nm)/TPBi (30 nm)/LiF (1 nm)/Al).

Because of the intrinsic conjugation length of TPE and silole, it is very difficult to utilize them as building blocks to develop deep blue AIE materials with CIE values (x≤0.15, y≤0.10) (25). Thus, some other AIE building blocks with efficient emission but low conjugation are in great demand. Pentaphenylbenzene (PPB), which has been proven to be AEE active, might be a good candidate. PPB shows weak conjugation and purple emission at about 355 nm, which violet-shifts by as much as 90 nm in comparison with TPE (445 nm) (26). So our group utilized pentaphenylbenzene as a platform to design a series of efficient deep blue emitters: DPA–PPB, Cz–PPB, DPA–TTP–CN, Cz–PPB–CN, mDPA–PPB–CN, mCz–PPB–CN, and Cz–TPB–CN (Figure 15) (27). Due to the AEE property of 75 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


PPB, these luminogens exhibit enhanced emission in the deep blue or blue-violet region, ranging from 420 to 451 nm. Among them, DPA-TTP-CN shows the best performance, and the nondoped OLED device with the configuration of ITO/MoO3 (10 nm)/NPB (60 nm)/mCP (10 nm)/DPA–TTP–CN (30 nm)/TPBi (30 nm)/LiF (1 nm)/Al gives a maximum current efficiency up to 2.0 cd A-1 at CIE coordinates of (0.15, 0.08). Furthermore, DPA–TTP–CN also displayed promising application as guest materials for its enhanced emission in the solid state. Its corresponding OLED device with a configuration of ITO/MoO3 (10 nm)/NPB (60 nm)/mCP (10 nm)/ BmPyPb:50%DPATTP-CN (20 nm)/BmPyPb (10 nm)/TPBi (30 nm)/LiF (1 nm)/ Al exhibits a better performance with current efficiency and external quantum efficiency up to 4.51 cd A-1 and 3.98% respectively at CIE coordinates of (0.16, 0.11). 3.2. AIE Host in OLEDs It is well known that for AIE luminogens, the restriction of the intramolecular rotation in solid state could block the nonradiative path and contribute much to the efficient emission, which is almost the same as one of the basic requirements for a good host materials in OLEDs. So could we utilize AIE materials as the host to achieve good device performance, not the general role of emitting layer? Because of the splendid AIE effect, easy functionalization and wide bandgap, we chose TPE as the build block to construct AIE host materials. However, our experimental results reveal that TPE is hole-dominated with the hole mobility about 10-4 cm2 V-1 s-1 and electron mobility about 10-5 cm2 V-1 s-1, which could not achieve the balance of the hole and electron injection and meet the demand of host materials. So oxadiazole, a well-known electron-transporting moiety, was chosen as the co-block to build new AIE host materials—Oxa-pTPE and Oxa-mTPE (Figure 16) (28). Oxa-pTPE and Oxa-mTPE both show bipolar transporting characteristics, owing to the hole-transporting ability of TPE and the electron-transporting ability of oxadiazole. Their carrier mobilities were measured by the time-of-flight (TOF) transient photocurrent technique and all in the same order of magnitude (about 10-4 cm2 V-1 s-1), showing the promising application as host materials. When fabricated in OLED devices as host materials, they both exhibited outstanding performances with a current efficiency up to 9.79 cd A-1 at CIE coordinates of (0.15, 0.34) for Oxa-pTPE and 9.82 cd A-1 at CIE coordinates of (0.15, 0.33) for Oxa-mTPE, respectively, by reasons of their bipolar transporting and specific AIE feature. Inspired by the abovementioned results, Wang et al. developed another AIE host system based on TPE and phosphine oxide (PO) of TPEDPO, TPEPO, DTPEPO and TTPEPO, in which TPE is responsible for the hole transporting property and phosphine oxide for the electron transporting property (Figure 17) (29). By adjusting the proportion of TPE and PO moieties, the electronic nature of these molecules could be successfully tuned from n-type (TPEDPO) to ambipolar (TPEPO) and then to p-type (TTPEPO). Among them, TPEPO gave the best performance as host material with a maximum current efficiency of 9.7 cd A-1 at CIE coordinates of (0.15, 0.35), owing to its more balanced carrier transporting 76 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


ability and specific AIE feature. Thus, this successful research confirms bipolar AIE luminogens as good candidate for host materials again.

Figure 16. Chemical structures of Oxa-pTPE, Oxa-mTPE and BUBD-1 (guest) and the performances of the corresponding blue OLEDs (OLED configurations-ITO/NPB (10 nm)/Oxa-pTPE:6% BUBD-1 or Oxa-mTPE:3% BUBD-1 (40 nm)/TPBi (10 nm)/Alq3 (20 nm)/Al) and the Electron and hole mobilities versus E1/2 for TPE, Oxa-pTPE and Oxa-mTPE.

Figure 17. Chemical structures of TPEDPO, TPEPO, DTPEPO and TTPEPO and the performances of the corresponding blue OLEDs (OLED configurations-ITO/NPB (10 nm)/TPEDPO or TPEPO or DTPEPO or TTPEPO: 5%BUBD-1 (40 nm)/TPBi (10 nm)/LiF (1 nm)/Al) and the Electron and hole mobilities versus E1/2 for TPEDPO, TPEPO, DTPEPO and TTPEPO. 77 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


3.3. AIE PLED Apart from the small AIE molecules, AIE polymers also show great potential in the application of polymer OLEDs (PLED) for their good solubility, efficient solid emission, good film-forming ability and low-cost fabrication of spin coating (30). Among the AIE polymers, hyperbranched polymers are of special interest because their easy synthetic accessibility, typically by one-pot syntheses, which allows their production in large quantities and their application on an industrial scale. However, the conjugated hyperbranched polymers with AIE characteristics are still seldom reported, especially in the application of PLEDs. Carbazole, a well-known hole-transporting unit, has been widely used to build OLED materials (31). Li et al designed a hyperbranched polymer of HP-TPE-Cz through the combination of multiple carbazole and TPE and the integration of the hole-transporting ability and AIE effect in the hyperbranched polymer, thus achieved the good PLED performance (32). For comparison, its analog linear polymer, LP-TPE-Cz, was also prepared. Thanks to the AIE effect of TPE units, the two polymers showed AEE characteristic. Then the PLED devices were fabricated through spin coating in which HP-TPE-Cz and LP-TPE-Cz as emitting layers (Figure 18). The hyperbranched polymer HP-TPE-Cz showed much better EL performance with a current efficiency of 2.13 cd A-1 at 508 nm, while LP-TPE-Cz was just 1.04 cd A-1. This abnormal outstanding EL performance of HP-TPE-Cz might be derived from its specific AEE characteristics, as well as the excellent hole-transporting property of its multiple carbazole-based core. In addition, the PLED result based on HP-TPE-Cz represents one of the highest values reported so far for conjugated hyperbranched polymers. At the same time, another hyperbranched polymers system based on fluorene, carbazole and TPE was developed (HP-Flu and HP-Cz) through an ‘A2+B4’ approach using an one-pot Suzuki polycondensation reaction (Figure 19) (33). As expected, they both show AEE characteristic. However, their PL emission peaks are red-shifted to about 530 nm due to the good conjugation of TPE and fluorene or carbazole moieties. When fabricated as PLED, HP-Flu exhibited a better performance with a current efficiency of 1.15 cd A-1 at 508 nm. Although this efficiency is higher than most other normal conjugated hyperbranched polymers, its EL emission is outside the blue region, not so satisfactory. As mentioned above, unlike their green and red congeners, efficient and stable blue materials are still scarce as a result of their intrinsic wide bandgap. Thus, in order to get blue AIE conjugated hyperbranched polymers, we utilized the strategy of modifying the linkage mode, just as in the small AIE materials, to develop another AEE conjugated hyperbranched polymers system of HP-mFlu and HP-mCz, in which TPE and fluorene or carbazole moieties were linked through the meta-position. Because of the strong AIE effect of TPE, they both show AEE characteristic. More excitedly, these two conjugated hyperbranched polymers exhibited sky blue PL emission about 470 nm owing to the meta-linkage mode of TPE, showing promising application in the blue PLED. This again confirmed the powerful control of intramolecular conjugation by simply changing the linkage mode.



Figure 18. Chemical structures and PLED performances of HP-TPE-Cz and LP-TPE-Cz (PLED configurations-ITO/PEDOT:PSS (25 nm)/Poly-TPD (25 nm)/HP-TPE-Cz or LP-TPE-Cz (32 nm)/TPBi (35 nm)/Cs2CO3 (8 nm):Ag).



Figure 19. Chemical structures of HP-Flu, HP-Cz, HP-mFlu and HP-mCz and their PL emissions.

4. Conclusion For any research topic, the mechanism and application are probably the two most important parameters just as that the driving force of scientific research mainly originates from the pursuit of truth and the solving approaches to practical problems. It is easily to be understood that the important practical applications could apparently affect our daily life, while the inherent mechanism is liable to be ignored, although with the clear mechanism, all the potentials of the materials can be well optimized and fully utilized. Since AIE phenomenon was firstly reported by Tang’s group in 2001, great attention has been paid on its mechanism and applications. Experimental results and theoretical calculations have been incorporated to explore the AIE mechanism. Based on the RIR and other mechanisms, many basic AIE building blocks have been designed. Also, the AIE materials for OLEDs have been well developed ranging from deep blue to near-IR, especially with TPE as building blocks. Particularly, five main strategies are developed to control the intramolecular conjugation and realize the blue emission. It is believed that more and more other excellent AIE materials would be developed by applying these strategies, to achieve better performance. In conclusion, although AIE is a newborn research topic, it has exhibited attractive research significance and great applications in many fields. We are enthusiastically looking forward to new advancements in this exciting area. 80 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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