Pure Organic Luminogens with Room Temperature Phosphorescence

Chapter 1

Pure Organic Luminogens with Room Temperature Phosphorescence Downloaded by 203.64.11.45 on October 21, 2016 | http://pubs.acs.org Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1227.ch001

Shuqin Wang,1,2 Wang Zhang Yuan,*,1 and Yongming Zhang*,1 1School

of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China 2Office of Research Management, Shanghai Jiao Tong University, Shanghai 200240, China *E-mails: [email protected] (W.Z.Y.); [email protected] (Y.Z.)

Recent progress in the fields of pure organic luminogens with room temperature phosphorescence (RTP) is reviewed. Besides basic molecular design considerations, varying strategies adopted to rigidify molecular conformations of the luminogens and to isolate oxygen, such as chelation, (co)crystallization, doping/trapping in rigid matrix, crosslinking, and creation of host-guest interactions, are summarized. Some new phenomena, concepts, and strategies like crystallizationinduced phosphorescence (CIP), directed heavy atom effect (DHAE), and cocrystallization utilizing halogen bonding are emphasized. Moreover, exciting advancements in persistent RTP and efficient RTP from solutions have also been highlighted. Meanwhile, their promising applications are also briefly mentioned.

© 2016 American Chemical Society Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Downloaded by 203.64.11.45 on October 21, 2016 | http://pubs.acs.org Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1227.ch001

Introduction Luminogens with room temperature phosphorescence (RTP) enjoy much broader applications compared to their fluorescent counterparts due to the involvement of longlived triplet manifold. Besides the well known applications in high efficiency electroluminescent devices (1, 2), RTP luminogens offer new opportunities in bioimaging, which allows better monitoring of cellular phenomena through excluding the interference from shortlived cellular auto-fluorescence background (3, 4). Additionally, emerging new applications, such as cellular hypoxia imaging, photodynamic therapy, temperature monitoring, oxygen/ozone-sensing, solvent detection, and security inks, have also been demonstrated (5–10). RTP phosphors, however, are essentially confined to inorganics or organometallic complexes (4, 11, 12), pure organic luminogens are difficult to receive efficient RTP (13–15), despite they take the advantages of low cost, versatile molecular design, facile functionalization and good processability. This is because the rate of phosphorescence is slow owing to the occurrence of spin flip, which is quantum mechanically forbidden. During such durable time, triplet excitons could easily lose their energies through thermally vibrational and collisional processes and exposure to such quenchers as oxygen and moisture. Therefore, phosphorescence from pure organic luminogens has typically been limited to cryogenic (e.g. 77 K) and inert conditions for a long time (15). To suppress vibrational relaxations of the triplet manifold, several approaches have been developed, including the use of special solvents, adsorption onto solid substrate or embedded into silica glass, and inclusion into surfactants or cyclodextrins, however, normally special and complex fabrication techniques are required and only instrument detectable signals are obtained (16–19). In 2007, Zhang and Fraser reported the interesting persistent RTP from the pure organic materials of difluoroboron dibenzoylmethane polylactide (20). In 2010, Tang and Yuan discovered that some pure organic luminogens such as benzophenone and its derivatives, methyl 4-bromobenzoate, and 4,4′-dibromobiphenyl exhibit no emission in solution, in polymeric films, or on thin-layer chromatography (TLC) plates, but become highly phosphorescent in the crystalline state at room temperature and ambient conditions, exhibiting crystallization-induced phosphorescence (CIP) characteristics (13). Restriction of intramolecular motions (RIR) by effective intermolecular interactions in the crystals and isolation from oxygen and moisture are ascribed for the boosted phosphorescence emission. Discovery of the CIP phenomenon paves the way for the fabrication of new efficient pure organic RTP luminogens through crystal engineering. In 2011, Bolton and Kim reported similar phenomenon of enhanced RTP from pure organic luminogens utilizing mixed crystals and halogen bonding (14). Since then, more and more reports concerning on the RTP from pure organic chromophores are demonstrated, by the utilization of (co)crystallization, doping or trapping into rigid matrix, intermolecular interaction, and singlet fission (15, 21–42). In this chapter, we summarized the recent progress of these exciting works, aiming to give a brief but clear picture on this renewed area.

2 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Fundamental Considerations To achieve efficient RTP from pure organic luminogens, several internal and external requirements should be fulfilled. Firstly, intersystem crossing (ISC) from the lowest excited singlet (S1) state to the triplet manifold (Tn) should be highly efficient. According to El-Sayed’s rule, ISC can be greatly promoted through efficient spin–orbit coupling (SOC) by the effective mixing of the singlet and triplet states of different molecular orbital (MO) configurations (43–45). Normally, carbonyls, nitrogen heteroatoms, heavy halogens, and/or small singlet–triplet splitting energy (ΔEST) are favorable to enhance the ISC process (14, 43–48). Meanwhile, radiationless relaxation from the lowest excited triplet (T1) state to ground (S0) state must be substantially impeded. Adequate suppression of vibrational and collisional dissipations is perhaps the most important and challenging aspect. What’s more, as triplet excitons are highly susceptible to triplet oxygen (5, 20, 49), which can facilitate the triplet-triplet quenching process, isolation of the luminogens from oxygen is a must. In short, considerable ISC, restricted molecular motions (rigidified conformations), and free of quenchers (particularly oxygen) are essential to receive efficient RTP from pure organic luminogens. Below, we summarized the recent endeavors in obtaining efficient pure organic RTP luminogens based on such fundamental aspects.

Difluoroboron Chelates Chelation of the dyes is an effective strategy to enforce conformation rigidity and charge redistribution in molecular systems (9, 20, 50–52). Difluoroboron β-diketonate (BF2bdk) luminogens are of great interest due to their intriguing properties in both solution and solid states (20, 50–54). Such skeleton provides the luminogens with rigid conformation as well as large Stokes shift, making them highly emissive in both solution and solid states (20, 50). Moreover, BF2 chelates are capable to impede intramolecular twisting of the aromatic-carbonyl moiety, thus offering phosphorescence in rigid environments (5, 9, 20, 50–54). Fraser and Zhang fabricated a series of BF2bdk dyes combined with biocompatible and biodegradable poly(lactic acid) (PLA) (Figure 1), which exhibit intense fluorescence, thermally activated delayed fluorescence (TADF) and oxygen-sensitive RTP (5, 9, 20, 51–54). The photophysical properties of BF2bdkPLA molecules can be well modulated by molecular structure, polymer molecular weight, halide substituents and their placement, as well as external temperature and oxygen. As the polymer molecular weight increases, dye−dye interactions and medium polarity are decreased, resulting in blue-shifted fluorescence with much shorter lifetime, longer phosphorescence lifetime, and larger singlet to triplet energy gap. While heavy atom normally promotes phosphorescent emission, systems without heavy atoms enjoy much longer phosphorescence lifetimes, which is favorable for highly sensitive, concentration independent time-resolved oxygen sensing. Specifically, with BF2dbmPLA (dbm = dibenzoylmethane) and its halogenated, heavy atom congener, BF2dbm(I)PLA, they demonstrated that emission wavelength, relative F/P intensity, and oxygen 3 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


sensitivity are tunable with heavy atom substitution and polymer molecular weight. They also showed that these materials can be exploited for cellular, tissue, and in vivo imaging studies (e.g., tumor hypoxia). Compared to dbm systems, the naphthyl materials with extended conjugation display redshifted absorbance, fluorescence, and phosphorescence. Through systematic variation of the chain length of PLA combined with heavy atom substitution, a new method for quantifying tumour hypoxia is demonstrated. For the iodine-substituted derivative, BF2dnm(I)PLA, clearly distinguishable fluorescence (green) and phosphorescence (orange) peaks are present, making it ideal for ratiometric oxygen-sensing and imaging. These dual emissive polymers are also ready to be fabricated as nanoparticles, which hold intense fluorescence and long-lived RTP, making them suitable for bioimaging and sensing (51).

Figure 1. Chemical structures of dual-emissive boron diketonates and β-hydroxyvinylimineboron compounds. (A) Foluorescence (F) and phosphorescence (P) emissions of the BF2dbmPLA film. Reprinted with permission from ref (20). Copyright 2007 American Chemical Society. (B) TEM and emission images of BF2dbmPLA nanoparticles (NPs). Reprinted with permission from ref (51). Copyright 2008 American Chemical Society. (C) Emission images of BF2dnm(I)PLA NPs. Reprinted with permission from ref (10). Copyright 2015 American Chemical Society. Besides BF2bdk chelates, Koch and coworkers reported a novel family of β-hydroxyvinylimineboron compounds (Figure 1, B-β-HOVI-1−7) that show high efficiency and color tunable RTP emission arisen from singlet fission (29). Variation of the molecular scaffold and fine-tuning of the electronic nature of the substituents were adopted to tailor the photophysical properties in solutions, neat solids, and doped PMMA films. Impressively, an absolute quantum yield over 100% is obtained for B-β-HOVI-3 and B-β-HOVI-4 in both solution and PMMA, 4 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


and B-β-HOVI-7 in PMMA. These remarkably high efficiencies are believed to the singlet fission, which is favored by such collective factors as strong π–π stackings, permanent molecular dipole moment, and the presence of a functional group that aids to promote radical character in the excited state.

Figure 2. Chemical structures of BP and its derivatives, MBP, and DBBP′. Photographs of (A) DFBP in solutions (from left to right: n-hexane, THF, DCM, acetonitrile, ethanol), in PMMA films, and on TLC plates at room temperature and 77 K, (B) crystals of different compounds taken under 365 nm UV light. (C) Fragmental molecular packing and intermolecular interactions in DFBP crystals. Reprinted with permission from ref (13). Copyright 2010 American Chemical Society.

(Co)Crystallization-Induced Phosphorescence Conventional luminogens normally suffer from ACQ problems, which is even serious for triplet emitters. In sharp contrast to ACQ, in 2010, Tang and Yuan discovered the CIP phenomenon, which offers a new strategy to efficient pure organic RTP luminogens through crystal engineering (13). A series of organic luminogens, including BP and its derivatives, MBB, and DBBP′ are induced to emit RTP upon crystallization (Figure 2) in high efficiencies (Φp up to ~40%), with lifetimes ranging from 19.2 μs to 4.8 ms (13). These luminogens are practically nonemissive when they are dissolved in solvents, doped into polymer films, and dotted on TLC plates, because active intramolecular motions under these conditions effectively annihilate their triplet excitons via nonradiative rotational and vibrational relaxation channels. In the crystalline state, intramolecular motions are suppressed by the crystal lattices and effective intermolecular interactions. For example, in DFBP crystals, numbers of ·C−H···O (2.724 Å) and C−H···F (2.777, 2.796 Å) short contacts form a firmly 3D interaction network (Figure 2C), which strikingly restricts the molecular motions. The physical constraints and multiple intermolecular interactions synergistically rigidify the molecular conformations of the luminogens, thus making them highly phosphorescent in the crystalline state even at room temperature. Such 5 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


rigidification effect induced by crystallization is comparable and somewhat even better than that of the conventional cryogenic cooling (Figure 2A). Generally, carbonyl group and nonplanar conformation are favorable for effective spin-orbit coupling. BZL with more carbonyl groups and even more twisted conformation compared to BP is thus expected to be CIP-active. Exactly, similar to BP, BZL and its derivatives demonstrate CIP characteristics with high RTP efficiencies at crystalline states (Figure 3A). For example, while BZL is nonluminescent in solution and when dotted on TLC plates, its crystals emit distinct green light at 521 nm with a lifetime of 142.02 μs. Furthermore, we found a unique phenomenon of crystallization-induced dual emission (CIDE), namely, simultaneously boosted fluorescence and phosphorescence upon crystallization, in a group of pure organic aromatic acids and esters even without any metal- or heavy atoms (25). Notably, two triplet-involved relaxations of delayed fluorescence and RTP are activated. Specifically, long afterglow from TPA and IPA after the stop of UV irradiation is observed (Figure 3B), which is rarely found for pure organic luminogens. To further explore luminogens with persistent RTP, we designed CZBP combined typical CIP compound BP with carbazole, and heavy bromine atom was also introduced for comparison (Figure 3C). It is found that persistent RTP can be rationally achieved based on CIP and severe crystallization, and heavy atom effect is not the predominant factor for the relatively short lifetimes of BCZBP and DBCZBP (26). Perfect crystal with dense molecular packing and effective intermolecular interactions isolates the triplet excitons from quenching sites, and moreover significantly blocks the high energy vibrational dissipations, thus yielding persistent RTP.

Figure 3. Chemical structures of BZL and its derivatives, some aromatic acids and esters, as well as CZBP and its derivatives. (A-C) Photographs of the crystals taken under UV irradiation. (A) Reprinted with permission from ref (24). Copyright 2013 Science China Press and Springer-Verlag Berlin Heidelberg. (B) Reprinted with permission from ref (25). 2015, published by the Royal Society of Chemistry. (C) Reprinted with permission from ref (26). Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. 6 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Hetero sulfur (S) and tellurium (Te) containing pure organic luminogens with CIP or aggregation-induced phosphorescence characteristics were also developed (9, 28, 55, 56). As shown in Figure 4, a series of persulfurated benzene molecules are nonemissive in both air-equilibrated and oxygen-free solutions at room temperature, but in a striking contrast, strong green RTP can be obtained in the solid state with efficiencies up to unity (28). This is a consequence of decreased intramolecular motions, however, conformational and rotamer factors along with substituents might also effect. He et al. demonstrated the first examples of tellurophenes capped with pinacolboronates (BPin) exhibiting RTP in the solid state under ambient conditions (Figure 4) (55). Furthermore, these luminogens readily form emissive host-free films that can be directly cast from THF solution. They revealed that both TeII and proximal BPin units are required for the RTP emission. Subsequently, new phosphorescent BPin-appended benzo[b]tellurophenes two phenyl/BPin substituted tellurophene isomers, with tunable emission colors have been achieved (56).

Figure 4. Chemical structures of persulfurated benzene molecules and tellurophenes and emission characteristics of B-Te-6-B in solution and in the solid state. The photographs are reprinted with permission from ref (55). Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Besides above systems, very recently, there are increasing reports on pure organic CIP compounds. People try to evaluate the effects of substituents (22), heavy atom (46, 47), and mechanical stimuli on the photophysical properties of the phosphors, and endeavored to explore their potential applications. Some examples are shown in Figure 5. Shimizu and coworkers reported a new class 7 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


of pure organic CIP luminogens of 1,4-bis(aroyl)-2,5-dibromobenzenes. The bis(aroyl)-benzene derivatives are nonemissive either in solution or in doped polymeric films. On the contrary, aided by effective intermolecular interactions as C=O···H, Br···Br, C=O···Br, F···F, S···H, and MeO···H, their crystals exhibit intense RTP under ambient conditions, with tunable emission color from blue to green and luminescent quantum yields of 5~18% (22). For another example, Shi and Zhao reported a concise approach to obtain pure organic CIP luminogens with efficiency up to 21.9% by manipulating heavy-atom interaction based on a class of dibromobenzene derivatives. It is found that PhBr2C6Br2 and PhBr2C8Br2with two more bromine atoms show much higher luminescence efficiency than their PhBr2C6 and PhBr2C8 counterparts, owing to the increased intermolecular heavy-atom interaction in crystals (46). Maity and coworkers reported the bright green RTP from BaA crystals, despite its planar structure (27).

Figure 5. Chemical structures of some CIP pure organic luminones developed by Shimizu (22), Shi (46), and Maity (27) and the corresponding photographs of the crystals taken under UV irradiation. Upper graph: Reprinted with permission from ref (22). Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Lower first 4 graphs: Reprinted with permission from ref (46). Copyright 2016 American Chemical Society. Lower last graph: Reprinted with permission from ref (27). Copyright 2016 Elsevier B.V.

Apart from conventional conjugated compounds, efficient RTP was also observed by us in aggregated natural compounds and polymers without classic chromophores, such as rice, starch, cellulose, bovine serum albumin (BSA), and some other carbohydrates (23). We ascribed such unusual emission to the clustering of electron rich groups, whose lone pair electrons can be overlapped in clusters. Electron overlapping and sharing extend the effective conjugation and rigidify the molecular conformations, thus making the luminogens easier to be excited with higher efficiency compared to their dilute solutions. Soon after our report on the CIP phenomenon of pure organic luminogens, Kim and coworkers reported efficient purely organic phosphors generated through a ‘directed heavy atom effect’ (DHAE) and crystal engineering in 2011 (14). Planar chromophores with triplet-producing aromatic aldehydes and triplet-promoting bromine (Figure 6) were utilized. These phosphors demonstrate CIP characteristics. However, their planar conformations (e.g. Br6A) render the homocrystals with low (2.9% for Br6A) RTP efficiency due to the excimer-induced self-quenching. Brighter RTP is achieved by a mixed crystal design strategy. Analogous compounds in which the aldehyde group is replaced 8 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


by bromine are adopted as the hosts, which isolate the aldehyde molecules and prevent their self-quenching in the resulting intermixed crystals, thus affording higher RTP efficiencies. For example, with 1 wt% Br6A, Br6A–Br6 mixed crystals give a remarkably enhanced RTP efficiency of 55%. Importantly, variation of the aromatic building block and/or bridging units finely tunes the RTP emission color from blue to orange for the mixed crystals, with lifetimes of 0.1~6.4 ms and efficiencies of 0.5~28% (Figure 6).

Figure 6. Structures of various brominated aromatic aldehydes and corresponding dibromo compounds and the photographs of their cocrystals taken under UV irradiation. Reprinted with permission from ref (14). Copyright 2011 Rights Managed by Nature Publishing Group.

Jin et al. (31–34) and d’Agostino et al. (57) successfully fabricated cocrystals between 1,4-diiodotetrafluorobenzene (DITFB) and polycyclic aromatic hydrocarbons [PAHs, like naphthalene (Nap), phenanthrene (phe), pyrene (Pyr), carbazole (Cz), fluorine (Flu), dibenzofuran (Dbf), and dibenzothiophene (Dbt)], diphenylacetyl (DPA), and trans-stilbene (tStb) with RTP emissions (Figure 7). Halogen bonding between DITFB and other compounds is crucial to the formation of the cocrystals. Herein, the conformer DITFB plays vital roles as follows: (1) serves as halogen bonding donor to link the chromophores; (2) heavy iodine atoms promote spin–orbit coupling of the compounds, thus inducing their phosphorescence emission; (3) behaves as a ‘solid diluent’, reducing self-quenching of the luminogens. Therefore, halogen bonding combined with multiple intermolecular interactions provides a new strategy to obtain RTP cocrystals with modulated emissions. As depicted in Figure 7B, efficient green and orange RTP emissions are observed in the cocrystal of Nap-DITFB and Phe-DITFB, respectively (34). Specifically, d’Agostino et al. prepared the cocrystals of DPA-DITFB, DPA-2DITFB, tStb-DITFB, and tStb-2DITFB by mechanochemical methods. As a result of the external heavy atom effect and effective intermolecular interactions, and depending on the stoichiometry, DPA-DITFB and tStb-DITFB cocrystals exhibit both fluorescence and RTP, whereas the other two emit exclusive RTP (57). Such facile cocrystal strategy utilizing halogen bonding can well expand to other systems. 9 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 7. (A) Chemical structures of the conformer DITFB and other luminogens. (B) Phosphorescent excitation (blue lines) and emission spectra of Nap-DITFB (green) and Phe-DITFB (orange) cocrystals. Inset panels are the photographs of the cocrystals taken under UV irradiation. Reprinted with permission from ref (34). Copyright 2012 Royal Society of Chemistry.

Dopping/Trapping in Rigid Matrix For pure organic luminogens, to suppress vibrational dissipations and to isolate oxygen and/or moisture, doping or trapping in rigid environment is another choice. Hirata and coworkers explored organic host-guest materials with efficient persistent RTP (35, 58, 59). As depicted in Figure 8, through doping the highly deuterated aromatic compounds in amorphous steroidal matrix (such as β-estradiol) with rigidity and oxygen barrier properties, the nonradiative relaxation of triplet excitons is minimized. Firstly, deuteration greatly reduces the nonradiative decay rate (knr) of the guest molecules. Moreover, substitution with a secondary amino-group does not increase knr of the guest, but promotes the ISC process. Consequently, the host-guest systems afford both a nonradiative relaxation from the T1 state at less than 10-1 s-1 at room temperature and effective ISC process. Therefore efficient RGB RTP emissions with efficiencies > 10% are obtained and moreover with the lifetimes > 1 s in air. This approach allows for the fabrication of both amorphous and persistent pure organic RTP luminogens at the same time, which will surely find diverse applications in OLEDs, thermal sensing, recording, and security inks. 10 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 8. Material design for efficient persistent RTP from pure organic luminogens in air. Reprinted with permission from ref (35). Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Suitable polymeric matrix may also be used to create rigid and oxygen barrier environment. Lee and Kim reported bright RTP by embedding a pure organic phosphor, namely Br6A (see Figure 6) into an amorphous glassy isotactic PMMA (iPMMA) matrix (60). Compared to atactic and syndiotactic PMMA (aPMMA and sPMMA), the reduced beta (β)-relaxation of iPMMA most efficiently suppresses vibrational triplet decay of the embedded luminogens and allows them to achieve bright RTP with efficiency of 7.5%. They also fabricated a microfluidic device integrated with a novel temperature sensor based on this composite system (60). Following their success of physical blending, they further utilized chemical bonding to construct bright RTP systems. Figure 9 illustrates the involved pure organic phosphors and the design concept. Covalent linking between phosphors and a polymer matrix not only increases the matrix rigidity, and moreover effectively reduces nonradiative relaxations of embedded phosphors. Consequently, collision frequency and the Dexter-type triplet energy transfer processes and vibronic mixing between zero-order electronic states of T1 and S0 are decreased, thus yielding efficient RTP from pure organic luminogens in a variety of amorphous polymer matrices (36). As demonstrated in their work, Diels–Alder click chemistry results in cross-linked luminogen doped polymer system with efficient RTP (up to ~28% quantum yield), which is around 2~5 times higher than that of physically blended Br6A/polymer system without covalent linkages. 11 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 9. Chemical structures of the designed phosphor and polymer and description of covalent crosslinking strategy. Reprinted with permission from ref (36). Copyright 2015 Rights Managed by Nature Publishing Group.

Similar to Kim’s work, Reineke and Baldo reported efficient, simultaneous fluorescence and RTP (74% yield) from a single molecule ensemble of N,N′-bis(4-benzoyl-phenyl)-N,N′-diphenyl-benzidine [(BzP)PB], which was diluted into PMMA. Such RTP from (BzP)PB is efficient (50% yield) and long lasting (208 ms lifetime) with extremely low knr for the triplet state (2.4 × 10 s−1). Interstingly, in contrast to the general trend–an increase of phosphorescence for (BzP)PB/PMMA going from 77 to 293 K is observed (40). Later, they proposed a new more general method to observe RTP from pure organics (15). Through controlling nonradiative rates by engineering a polymeric and energetically inert host matrix embedded with the target molecule, RTP is readily observable for a wide variety of molecules with functionalities spanning multi-exciton generation (singlet exciton fission), OLED host materials, and TADF emitters. And very recently, Zhou and Zhang reported rather an impressive work on the fabrication of fluorescent and phosphorescent single-component dual emissive materials (SDMs) (61). On the basis of a general design principle, N,N-hydroxyethylamino benzophenone (K1) is covalently incorporated into waterborne polyurethanes (WPU) and results in SDMs. The luminescent properties of SDMs are dependent on the chromophores concentration: increased concentrations generate progressively narrowed singlet-triplet energy gaps which can be well explained by the polymerization-enhanced intersystem crossing (PEX) model (62). Namely, polymerization of luminophores results in exciton coupling and subsequently enhanced forward and reverse ISC processes. Theoretical calculation for the WPU system suggests that the presence of K1 aggregates indeed enhances the crossover from excited singlets to triplets. This work sheds lights on the fabrication of diverse pure organic polymeric RTP luminogens. 12 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Unlike above rigid steroidal and polymeric matrices, Dong reported the utilization of inorganic host to receive persistent RTP from carbon dots (CDs) (37). Upon dispersing the CDs into a potash alum KAl(SO4)2·x(H2O) host, the resulting composite powders exhibit green RTP (500 nm) with a lifetime of 707 ms and an average lifetime of 655 ms (37). Such RTP emission is originated from triplet excited states of aromatic carbonyls (C=O) on the surface of CDs (7, 37), however, without KAl(SO4)2·x(H2O), no RTP is observed. Therefore, the matrix must have effectively rigidified the surface structure of CDs, thus suppressing the energy loss of C=O bonds from rotational or vibrational dissipations. It is also noticed that both crystal water and KAl(SO4)2 molecules play crucial roles in rigidifying the surface C=O bonds of CDs. Meanwhile, the KAl(SO4)2·x(H2O) matrix also shows good oxygen barrier performance, diminishing the oxygen contact and quenching of the triplet excitons.

Creating of Intermolecular Interactions between Luminogens and Hosts Doping in rigid matrices is effective to yield RTP. Specifically, if proper intermolecular interactions between the luminogens and hosts were constructed, it would be even better to create rigid surroundings. Our story may start from Al-Attar and Monkman’s work on the RTP of water soluble conjugated polymers (WSCP). In 2012, they discovered and described a simple but useful method to fabricate near perfect isolation of dense luminescent WSCP chains (see Figure 10 for examples) using caging within such polymeric surfactants as poly(vinyl alcohol) (PVA) and poly(vinylpyrrolidone) (PVP) (63). The PVA or PVP surfactant is ready to breaks up WSCP chain aggregates in solution, moreover, the in-situ formation of hydrogen bonds between PVA or PVP molecules upon drying firmly locks in the isolation of the WSCP, thus preventing aggregation and yielding long term stability to the resultant systems. Such perfect isolation and rigid locking by hydrogen bonds of the matrix render the WSCP luminogens highly emissive, and moreover, for the first time, unprecedented RTP from conjugated polymers is observed, thus offering an ideal system to study the triplet dynamic and energy transfer in conjugated polymers. Despite there seems no typical intermolecular interactions such as hydrogen bonding between the guest and host, this work inspires the researchers to further develop novel RTP systems through building specific interactions between luminogens and matrices.

Figure 10. Chemical structure of some water soluble conjugated polymers and naphatic acids. 13 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Gahlaut studied the photophysical properties of 1-naphthoic acid (1-NpA) and 2-naphthoic acid (2-NpA) (Figure 10) in three polymeric hosts of PVA, PMMA, and cellulose acetate (CA) (42). In case of PVA, besides fluorescence, strong RTP of 1-NpA is detected, which can be remarkably quenched by oxygen. In PMMA, weak phosphorescence is observable only under inert atmosphere and no RTP is noticed in CA. For 2NA, except weak RTP in PVA, no phosphorescence signal is recorded in the other two matrices. The presence of strong and weak hydrogen bonded conformers and polymer heterogeneity are invoked to explain above emission behaviors. While both species are present in PVA, possibly only weakly bonded one exists in PMMA and CA (42). The effective hydrogen bonding between the surface aromatic C=O groups and PVA chains also generates dual emissive CDs, which was first reported by Deng and coworkers (7). Dispersing CDs into a PVA matrix, besides well documented and studied fluorescence, persistent green RTP (~500 nm) with a lifetime of around 380 ms is observed. As illustrated in Figure 11, For CDs in solution, triplet emission from C=O groups is not optimal owing to remarkable nonradiative quenching processes, only fluorescence is hence observed. Upon dispersing into PVA, besides the general rigid polymeric matrix and oxygen isolation effects, abundant hydrogen bonds between PVA molecules and C=O units play an essential role, which further effectively rigidify C=O bonds, impeding their intramolecular motions and thus promoting the RTP emission. Subsequently, in 2014, Kwon et al. reported a design strategy for tailoring intermolecular interactions to enhance RTP from pure organic luminogens in PVA at ambient conditions (41). Figure 12 shows the involved compounds and schematic illustration of the designed principle. Halogen bonding between adjacent G1 luminogens and hydrogen bonding between the phosphor G1 and the PVA matrix are present in the resulting composite film. While the former facilitates ISC processes as well as suppresses the vibration of the phosphor, the latter effectively restricts the vibrational dissipations, thus enabling intense RTP with efficiency up to 24% (41). Furthermore, water can be used to modulate the strength of halogen and hydrogen bonding in the G1–PVA system, yielding unique reversible switch of phosphorescence to fluorescence. This property renders the system utilizable as a ratiometric water sensor.

Figure 11. Proposed RTP mechanism of CDs dispersed in PVA. Reprinted with permission from ref (7). Copyright 2013 Royal Society of Chemistry. 14 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 12. (A) Chemical structures of Br6A, G1, and PVA. (B) Phosphorescence image of G1 in PVA100 under UV illumination and schematic illustration of phosphorescence processes in the G1–PVA composite film. Reprinted with permission from ref (41). Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 13. The steady-state photoluminescence (left) and ultralong phosphorescence (right) spectra of a series of reported molecules. The insets show the corresponding photographs taken under UV irradiation (left) and after the stop of irradiation (right). Reprinted with permission from ref (6). Copyright 2015, Nature Publishing Group. 15 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Persistent RTP Persistent RTP has significant fundamental importance and promising applications in high density data recording, high contrast background independent imaging, and anti-counterfeiting. Unfortunately, it is typically observed in inorganic materials, especially in the rare-earth element containing crystals. For pure organic luminogens, to achieve persistent RTP, it is the prerequisite that kr must be at a suitable level. Based on proper kr, nonradiative relaxations including vibrational dissipation and oxygen/moisture quenching should be strictly impeded. Recently, researches endeavored to explore general strategies to persistent RTP luminogens (64–70). So far, however, no universal mechanism is derived, possibly because it is still difficult to predict the kr values of designed luminogens. In this part, we summarized the currently reported long lasting RTP systems. As we have mentioned above, BF2bdkPLA molecules (5, 9, 20, 51–54), deuterated aromatic compounds (35, 58, 59), and CDs (7, 37) are promising to produce persistent RTP under proper conditions. Embedding suitable luminogens in rigid polymeric systems is also hopeful to generate persistent RTP (15, 40). Meanwhile, serve crystallization with dense molecular packing based on CIP luminogens is also helpful to achieve long lasting RTP (25, 26). What is more, in 2015, An et al. realized ultralong RTP (lifetime: 0.23~1.35 s, efficiency: 0.08~2.1%) through effective stabilization of triplet excitons by strong coupling in H-aggregates of a group of pure organics (Figure 13) (6). The longest lifetime of 1.35 s is observed in DECzT crystals under ambient conditions. Meanwhile, through tailoring the molecular structure of the luminogens, the emission color of persistent RTP can be readily tuned from green (515 nm) to red (644 nm) (Figure 13) (6). There are also other scattered examples of pure organic persistent RTP systems (Figures 14 and 15) (64–70). Zhang and coworkers reported the interesting dual-emissive property of BF2EMO molecules in the crystalline state, which exhibit green RTP after ceasing the irradiation (Figure 14A) (64). Single-crystal structure analysis reveals the formation of multiple hydrogen bonds between neighboring molecules in crystals, which may significantly enhance the conformation rigidity of the luminogens. Li et al. reported a carbazole-based benzophenone derivative DCZBP (Figure 14B), whose cocrystal with chloroform exhibits weak RTP lasting for > 1.7 s (65). Removal of chloroform will eliminate the RTP emission, which can be switched on upon fuming with chloroform vapors. Dual emission were also observed by Kuno (IPA, TPA, PMA, and TPM) (66), Yang (CZBBP, CZDPS, and CZBDPS) (68), and Xue (CBA) (64), in different systems (Figure 15). Despite their similar phenomena or even resembling structures, no general mechanism for persistent RTP is reached.



Figure 14. (A) Chemical structure of BF2EMO and its crystal emission photographs. (B) Steady-state and delayed emission spectra of BF2EMO at room temperature in air. A and B are reprinted with permission from ref (64). Copyright 2014 American Chemical Society. (C) RTP spectrum of DCZBP crystal obtained through recrystallization from chloroform solution. (D) RTP decay curve of DCZBP crystal at 553 and 600 nm and its crystal emission photographs. C and D are reprinted with permission from ref (65). Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Normally, above persistent RTP emissions are in low yields, because the ultralow radiative process cannot compete with the other fast transitions. Until very recently, Xue et al. proposed a new strategy to explore efficient persistent RTP from carbazole derivatives through an intermolecular moderate heavy atom effect (47). Isolation with flexible alkyl chains rather than direct connection between the heavy atom (Br) and carbazole moieties is adopted to avoid strong internal heavy atom affect (Figure 16). Seven among eight designed compounds exhibit persistent RTP in their crystals (lifetime: 20~340 ms, efficiency: 1.6~39.5%). Notably, CC6PhBr demonstrates persistent RTP (lifetime: 200 ms) with efficiency as high as 39.5%. Moreover, white light emission with efficiency of 72.6% is achieved from it owing to the presence of strong blue fluorescence and yellow-orange phosphorescence.



Figure 15. Chemical structures of a series of pure organic luminogens with persistent RTP emission and photographs of their crystals. Left panel is reprinted with permission from ref (66). 2015, published by the PCCP Owner Societies. Right panel is reprinted with permission from ref (68). Copyright 2016 John Wiley and Sons.

Figure 16. Design principle, molecular stacking of carbazole crystals, molecular structures of reported CCnBr and CCnPhBr compounds and photographs of their crystals taken under 365 nm UV light with fluorescence and phosphorescence quantum yields and average phosphorescence lifetimes indicated. Reprinted with permission from ref (47). 2016, published by the Royal Society of Chemistry. 18 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


RTP from Solutions Due to strong vibrations and collisions in solvents, efficient RTP emission from solutions is difficult to achieve even in oxygen free solutions. However, there is also some exciting progress. The realization of efficient RTP in solution will further facilitate their applications in sensing and imaging. Herein, we list two impressive examples. The first example is a fluorene derivative, 7-bromo-9,9didodecylfluorene-2-carbaldehyde (Br–FL–CHO) reported by Xu and coworkers, which exhibits RTP in conventional oxygen free organic solvents (49). Its absolute RTP efficiency reaches 5.9% in chloroform. The directly-linked bromo and formyl substituents may promote the ISC from S1 state to T1 state, and the rigid fluorene framework may suppress the nonradiative process in solution. They also prepared other fluorene derivatives to compare the substituent effect on emission properties (Figure 17). None of these luminogens, however, can generate RTP in solution. Meanwhile, due to the rigidity of Br-FL-CHO, its doped PMMA film also shows bright RTP emission, which lasts for several days even upon exposure to oxygen.

Figure 17. Chemical structure of fluorenen derivatives and luminescent photographs of Br–FL–CHO taken under Ar (left) and air (right) in CHCl3 at 298 K. Reprinted with permission from ref (49). Copyright 2013 Royal Society of Chemistry.

Figure 18. Schematic illustration of the reversible inclusion of the binary IQC[5]/CB[7] system and corresponding photographs of the aqueous solutions. Reprinted with permission from ref (71). Copyright 2016 John Wiley and Sons. 19 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


The second example is from Gong and coworkers. They reported a cururbit[7]uril (CB[7]) based pH-controlling molecular shuttle encoded by a visible RTP signal without any deoxidant, which is generated by complexation of CB[7] and bromo-substituted isoquinolines, for example IQC[5] (Figure 18), in aqueous solution (71). In the acidic medium, the CB[7] host shuttles along the axial guest, and only weak RTP emission is observed, whereas deprotonation of IQC[5] makes the CB[7] wheel locate on the phosphor group, leading to increased RTP by more than six times. 1H NMR data indicate that the internal void space of CB[7] is mainly occupied by a nitrogen heterocycle and a heavy atom at high pH values. Moreover, the switching process along with the visible RTP signal is recyclable by adjusting the pH between ~4.0 and 7.0.

Conclusions and Perspectives There is growing interest in high-efficiency pure organic RTP luminogens owing to the fundament importance and promising applications, and significant advances have been achieved in recent years. Based on the concepts of spin-orbit coupling, new kinds of possible phosphorescent luminogens were designed by taking advantages of carbonyl groups, hetero atoms, and heavy atoms. To receive efficient RTP, varying strategies were adopted to rigidify the molecular conformations and to isolate oxygen. Novel phenomena and new strategies such as CIP, DHAE, chelation, and cocrystallization utilizing halogen bonding have been observed or proposed. Moreover, persistent RTP and efficient RTP from solutions have also been demonstrated. Due to the sensitivity of triplet excitons to oxygen and moisture, these pure organic luminogens have found their applications in biological hypoxia imaging, temperature, water and ion sensing, document security, and smart optical recording. Moreover, OLEDs utilizing these pure organic RTP luminogens were fabricated (72–74). Despite the relatively low performance, it paves the way for future optoelectronic applications. Further exploration in these renewed area will not only reveals more underlying fundamentals on the dynamics of excitons, but also offers new intriguing applications.

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Pure Organic Luminogens with Room Temperature Phosphorescence

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