Chinese Chemical Letters  2019, Vol. 30 Issue (10): 1809-1814   PDF    
Recent progress on pure organic room temperature phosphorescence materials based on host-guest interactions
Guojuan Qua, Yaopeng Zhangb, Xiang Maa,*     
a Key Laboratory for Advanced Materials and Feringa Nobel Prize Scientist Joint Research Center, Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science & Technology, Shanghai 200237, China;
b State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 201620, China
Abstract: Pure organic room temperature phosphorescence (RTP) has been attracting a lot interest recently. So far, many strategies have succeeded in achieving efficient organic RTP materials by increasing the rate of intersystem crossing (ISC) and suppressing non-radiative transitions. In supramolecular chemistry, the control and regulation of molecular recognition based on the role of the host and guest in supramolecular polymers matrix, has attracted much attention. Recently, researchers have successfully achieved room temperature phosphorescence of pure organic complexes through host-guest interactions. The host molecule specifically includes the phosphorescent guest to reduce non-radiative transitions and enhance room temperature phosphorescence emission. This review aims to describe the developments and achievements of pure organic room temperature phosphorescence systems through the mechanism of host-guest interactions in recent years, and demonstrates the exploration and pursuit of phosphorescent materials of researchers in different fields.
Keywords: Pure organic room temperature phosphorescence     Host-guest interactions     Supramolecule     Macrocyclic structure     Film matrix     Crystal matrix    
1. Introduction

Room temperature phosphorescence (RTP) is the result of radiation transitions between states of different multiplicities at room temperature. T1→S0 is the typical transition and Tn→S0 is rare. This process is spin-forbidden, so its rate constant is much smaller than fluorescence. Currently, it is widely used in photodynamic therapy [1], optical storage [2], molecular sensing [3], bioimaging [4], and organic light-emitting diodes (OLEDs) [5]. At present, most of the room temperature phosphorescent materials are inorganic or noble metal-containing organic complexes, which are expensive, highly toxic and brittle to process [6-10]. Thus, developing novel pure organic room temperature phosphorescent materials is indispensable. Pure organic room temperature phosphorescent materials are expected to be widely used in many areas due to their long luminescent lifetime, larger Stokes shift, high signal-to-noise ratio, simple detection, diversified design and convenient preparation [11, 12]. For pure organic compounds, the triplet excited state is easily quenched by vibration, high temperature or oxygen molecules [13], so phosphorescence emission of pure organic compounds is normally observed at low temperature or in inert gas condition [14-17]. Several specific strategies have been adopted to enhance room temperature phosphorescence by increasing the rate of intersystem crossing (ISC) from singlet to triplet states and suppressing the nonradiative relaxation of the triplet states [18], such as introducing an illuminant into a polymer [19, 20], host-guest interaction, halogen bonding [21-23], hydrogen bonding [24, 25] aromatic carbonyl groups [26-30], as well as forming radical anion pairs [31], charge transfer states [32]. Our group has recently developed many pure organic room temperature phosphorescent systems.

Host-guest interaction is one of the noncovalent interactions in the field of supramolecular chemistry [33]. The host molecule usually contains a cavity that specifically recognizes the guest molecule and provides a similar structure in the binding site. The host-guest interactions are selective because multiple factors limit the host's inclusion behavior on the guest suchassize, shape, charge, polarity[34-39].Therefore, it is very necessary to choose the suitable host molecule and guest molecule. The engineering of molecular recognition based on the action of host and guest in supramolecular polymers has attracted a lot of attention. Herein, the research progress of pure organic room temperature phosphorescence systemsbasedonhost-guest interactions in recent years is reviewed.

2. Host-guest interaction

In supramolecular system, the host molecules have a molecular size hydrophobic cavity. Due to the size, shape and polarity of the cavity, the host molecules can specifically encapsulate the guest molecules [40]. Most of the host molecules are macrocyclic molecules such as cucurbituril (CB[n]), cyclodextrin (CD), calixarene, crown ether. The rigid structure of cucurbituril makes it capable of forming stable complexes with molecules and ions. Cyclodextrin is a cyclic structure with a hydrophobic interior, so it can include hydrophobic molecules. Once the small molecule forms a host-guest inclusion with the cyclodextrin, the physicochemical properties of the guest, especially the water solubility, will improve a lot. The host molecules can also provide a rigid environment for the guest molecules to limit the vibration of the guest molecules, suppressing the nonradiative relaxation of the triplet states and effectively protect the phosphorescence from the triplet oxygen which will quench the luminance of guest molecules, thereby generating strong room temperature phosphorescence [41, 42]. In addition, heavy atoms in the molecule (such as Br, I) can increase the rate of intersystem crossing (S1→T1) [43] (Fig. 1). However, in addition to the macrocyclic molecules in the traditional supramolecular chemistry, forming a film or doping into a crystalline matrix is also a form of host matrix. Recently, researchers have successfully developed room temperature phosphorescence of pure organic complexes through the hostguest interactions.

Fig. 1. Jablonski diagram for the fundamental photophysical process in organic phosphorescence materials.

3. Recent progresses in the role of host and guest in producing room temperature phosphorescence 3.1. Pure organic room temperature phosphorescence based on macrocyclic structure

Most of the host molecules in host-guest chemistry are macrocyclic compounds such as cyclodextrin, cucurbituril. In 1982, Turro et al. [44] first reported cyclodextrin-induced room temperature phosphorescence (CD-RTP). The phosphorescence of 1-bromonaphthalene and 1-chloronaphthalene was obviously observed in nitrogen purged aqueous solutions containing β-cyclodextrin. In 2007, Tao et al. [45] reported cucurbituril[7, 8]-induced quinoline derivatives emitted room temperature phosphorescence (CB-RTP) in solutions firstly. Considering with the performance of phosphorescence and photochromism, our group reported a RTP addressing pseudorotaxane induced by the inclusion of β-CD and α-BrNp. We constructed an optically driven rotaxane binary system composed of azobenzene-modified β-CD (β-CD-Azo) and α-bromonaphthalene (α-BrNp) in aqueous solution (Fig. 2a). In this β-CD-Azo-α-BrNp system, photoinitiation on β-CDAzo reversibly controlled its isomerization and complexation with α-BrNp. This method used the room temperature phosphorescence emission signal as an output [46]. After that, our group constructed a supramolecular hydrogel system capable of rapidly self-healing and room temperature phosphorescence based on the action of β-CD and α-BrNp [47]. Based on the host-guest interaction, the hydrogel material could be simply prepared by the mechanical mixing of the β-cyclodextrin host polymer and the α-bromonaphthalene guest polymer. The hydrogel material had obvious phosphorescence emission at room temperatuire and was able to self-heal within one minute (Fig. 2b). In 2016, our group [48] successfully synthesized a polymer based on 4-bromo-1, 8-naphthalene anhydride derivative (poly-BrNpA), and the polyBrNpA/γ-CD system based on the polymer, which could emit a room temperature phosphorescence signal (Fig. 2c). The BrNpA phosphor as the guest molecular could be immobilized by the inclusion of host and guest, and oxygen was isolated to some extent, thereby enhancing the room temperature phosphorescence emission of the whole system. And the phosphorescence signal had very good reversibility and repeatability. The room temperature phosphorescence signal of the system could respond to the azobenzene unit of poly-Azo and be regulated reversibly according to its photo-isomerization. This realized the transition of the hostguest system from the solution state to the polymer state. In 2018, based on previous research, we constructed amorphous metal-free RTP small molecular compounds by modifying phosphors into β-CD [49]. These amorphous β-CD derivatives emitted strong RTP luminescence with decent quantum yields in solid states. Furthermore, one of such cyclodextrin derivatives, BrNp-β-CD, was utilized to construct a host-guest system incorporating a fluorescent guest molecule AC, exhibiting excellent RTP-fluorescence dual-emission properties and multicolor emission with a wide range from yellow to purple including the white-light emission (Fig. 2d).

Fig. 2. (a) Synthetic routes of pseudorotaxane β-CD-Azo and its inclusion phenomenon with α-BrNP in water. Copied with permission [46]. Copyright 2014, Royal Society of Chemistry. (b) Construction of the supramolecular polymeric hydrogel by host-guest interaction between host/guest polymers and its rapidly self-healing property. Reproduced with permission [47]. Copyright 2014, Wiley Publishing Group. (c) Schematic representation of the CD-RTP system via host-guest interaction between polyBrNpA and γ-CD. Copied with permission [48]. Copyright 2016, Royal Society of Chemistry. (d) RTP emissive cyclodextrin derivatives, the fluorescent guest molecule AC and schematic illustration of the phosphorescence emission. Copied with permission [49]. Copyright 2018, American Chemical Society.

Apart from CD, CB is another excellent host macrocyclic molecule which can provide a rigid matrix for luminogens to prevent quenchers such as water and oxygen. In 2016, we successfully constructed the host-guest system of CB[7] and bromo-substituted isoquinoline in aqueous solution, which could be adjusted by managing the solution[50].Forinstance, pHregulates the intensity of the phosphorescent emission signal. A carboxylic acid is attached to the end of the alkyl chain of the guest molecule. After deprotonation, the carboxylic acid can strongly repel the electron cloud on both sides of the cucurbit. Therefore, by adjusting the pH, the position of the cucurbiturone on the guest molecule can be controlled, thereby changing the phosphorescence signal strength of the system (Fig. 3a). Subsequently, we found that CB [7] and a naphthalimide derivative 2-(4-aminobutyl)-6-bromo-1H-benzo[de]isoquinoline-1, 3(2H)-dione(ANBrNpA) could also emit room-temperature phosphorescence through host-guest interactions [51] (Fig. 3b). CB[7] could provide a rigid matrix for ANBrNpA to protect it from oxygen and water. This CB[7] featured supramolecular system could emit room-temperature phosphorescence signal in aqueous solution, as well as the 4-bromo-1, 8-naphthalic anhydride polymer/CB[7]system.Recently, Liu et al.[52] chose CB[6] as the host molecule which could specifically identify and envelop the guest molecule, organic salts 4-(4-bromophenyl)-N-methylpyridinium with different counterions (PYX, X = Cl, Br, I and PF6). Interestingly, the phosphorescence of the complex had indeed been greatly enhanced to 81.2%, which was the highest record of pure organic room temperature phosphorescence quantum yield. The host molecule CB[6] was ellipsoidal, which could tightly include the guest molecule PYX through the host and guest interactions. Thus it greatly suppressed the nonradiative relaxation.Inaddition, the carbonylgroup on CB[6]wasverycloseto PY, which promoted the rate of intersystem crossing (ISC). Therefore, efficient room-temperature phosphorescence of solidstate supramolecule was enhanced by CB[6].

Fig. 3. (a) Structures of the guests and the reversible inclusion phenomenon of the binary IQC[5]/CB[7] system and corresponding photograph of the aqueous solution upon illuminationwith a handheld UV lamp (254 nm). Copied with permission [50]. Copyright 2016, Wiley Publishing Group. (b) Schematic representation of the CB-RTP system via host-guest interaction between ANBrNpA and CB[7]. Copied with permission [51]. Copyright 2017, Elsevier Ltd.

3.2. Pure organic room temperature phosphorescence based on film matrix

In recent years, some researchers have used film matrix to achieve pure organic room temperature phosphorescence. The film matrix effectively provides a rigid environment for the host and guest molecules, allowing the mixture to produce room temperature phosphorescence through the host-guest interactions. Besides macrocyclic compound, amorphous hydroxyl steroid is also a commonly used host molecule. Hirata et al. have in-depth research in this compound. It is worth noting that the organic host-guest materials they used here are not the conventional host-guest concept in supramolecular chemistry. In 2013, Hirata et al. [53] proposed to use aromatic without heavy metals and halogens (secondary amino-substituted deuterated carbon) as the guest, amorphous hydroxy steroid as the host molecule (Fig. 4a). The use of amorphous rigid steroid compounds as a host can not only reduce non-radiative transitions but also reduce the quenching of long-lifetime RT triplet excitons of the guest by interaction with the host matrix and oxygen. The guest molecule in Fig. 4a has no heavy atoms such as halogens, and it is surrounded by the host, which can significantly reduce non-radiative transitions. This hostguest amorphous film will successfully achieve compatibility of the aromatic carbon with a significant long excited state lifetime and high photoluminescence efficiency. In 2015, Hiratia et al. [54] found a photoreversible switch record of red long-lasting RTP. The host-guest material consisting of a deuterated aromatic compound as the red phosphorescent guest, a diarylethene as a photochromic guest, and an amorphous hydroxyl steroidal compound (A) could have a lifetime of up to 1.01 s. The red persistent RTP was erased by ultraviolet radiation and recovered by green light (Fig. 4b). The photocyclization and decyclization of reversible A allowed or prevented double dipole energy transfer, respectively. Recent progresses on steroidal compounds as the host molecules were reported in 2017, they chose β-estradiol and 2, 5, 8, 11, 14, 17-hexa(4-(2-ethylhexyl))-hexa-peri-hexabenzocoronene (6EhHBC) as the host-guest materials respectively [55]. Doping the rigid guest molecule of 0.3 wt% 6EhHBC into the β-estradiol, the material emitted blue-green thermally activated delayed fluorescence (TADF), as well as red phosphorescence with a lifetime of 3.9 s, at room temperature in air (Fig. 4c). In 2017, Adachi et al. [56] obtained long-lived room temperature phosphorescence by doping two deuterated organic phosphors into various aromatic host molecules (Fig. 4d). The effect of the host molecule on the non-radiative decay process was demonstrated by the decay curve of the RTP after mixing. In addition, the research team also studied the relationship between room temperature phosphorescence lifetime and glass transition temperature, and proved the dependence of the triplet energy of the host on temperature.

Fig. 4. (a) Material design for efficient persistent RTP from pure organic luminogens in air. Copied with permission [53]. Copyright 2013, Wiley Publishing Group. (b) Recording material design and photoreversible recording and erasing of persistent RTP. Reproduced with permission [54]. Copyright 2015, Wiley Publishing Group. (c) Structures of β-estradiol as an amorphous host and 6EhHBC as an aromatic guest. Reproduced with permission [55]. Copyright 2017, Wiley Publishing Group. (d) The molecular structures of organic semiconducting hosts and guests used in this study. Reproduced with permission [56]. Copyright 2017, Wiley Publishing Group.

3.3. Pure organic room temperature phosphorescence based on hostguest crystal matrix

Lattices containing host molecules can be connected by supramolecular interactions to provide a soft, powerful cavity to efficiently accommodate a variety of guest molecules. The host molecule is a rigid crystal matrix that can significantly limit the vibration of the molecule, thereby suppressing non-radiative transitions and enhancing the room temperature phosphorescence emission of the host-guest complex [57-59]. In 2011, Kim et al. [60] selected a chromophore with triplet-producing aromatic aldehydes and triplet-promoting bromine as guest molecule, and a bihalogenated, non-carbonyl similar to the chromophore as host molecule. More effective room temperature phosphorescence was obtained by doping the guest molecule into the host crystal matrix. In this situation, the host dispersed the chromophore and provided a rigid but non-quenching matrix for it. After that, Liu et al. [61] reported co-crystals of two-component host systems composed of 1, 4-diiodotetrafluorobenzene and 4-phenylpyridine N-oxide, which had the capacity to include functional guests, Nap, Car, BhQ, and Phe molecules, to further form the host-guest cocrystals. It was found that host frameworks containing heavy halogen atoms had different photoluminescence (fluorescence and phosphorescence) behavior at room temperature depending on the guest molecule. Indeed, pure organic RTP materials have attracted great interests in recent years. In 2017, Fu et al. [62] doped pure organic guest compounds (S-2CN and S-2I) into the crystalline matrix of halogenated aromatic compound (I-ph-NH2), the formation of S-2CN…I-ph-NH2 and S-2I…NH2-ph-I halogen bonds led to bright red RTP emissions from these two host/guest doped crystals (hgDCs). The halogen-bonding enhanced intermolecular heavy-atom effect promoted the spin-orbit coupling (SOC) for accelerating both the S1→T1 and subsequent T1→S0 phosphorescence processes. The rigid crystalline environment significantly minimizes the T1→S0 non-radiative decay and prohibits the oxygen quenching effect (Fig. 5a). In 2018, Hisaeda et al. [63] also used a rigid crystal matrix as the host molecule. They introduced aromatic guest in a supramolecular host and constructed a series of threecomponent crystals (C1-C8) that displayed guest-dependent photoluminescence switching. Among them, C8 exhibited roomtemperature fluorescence and phosphorescence dual-emission characteristics with vibrational structure (Fig. 5b). These results indicated that the confinement of heavy atom-bearing compounds like iodobenzene in the supramolecular host EBPDI-TPFB could enhance the spin-orbit coupling of the excited electrons of organic compounds, thereby increasing the rate of intersystem crossing (ISC) from singlet to triplet states to produce phosphorescence emission even at room temperature.

Fig. 5. (a) An optical image of I-ph-NH2 powder crystals under daylight, luminescence photographs of S-2CN/I-ph-NH2, S-2I/I-ph-NH2 hgDCs, S-2CN and S-2I powder crystals under 365 nm UV light. Copied with permission [62]. Copyright 2015, Wiley Publishing Group. (b) Chemical structure of the multicomponent molecular motif used in this study. Copied with permission [63]. Copyright 2015, Wiley Publishing Group.

4. Summarization and prospect

In summary, this review describes the developments and achievements of pure organic room temperature phosphorescence systems achieved through host-guest interactions in recent years. Pure organic room temperature phosphorescence can be achieved by host-guest interaction in macrocyclic host matrix, film matrix, and crystal matrix. It is both an opportunity and a challenge to realize pure organic room temperature phosphorescence through the strategy of host-guest interaction. Henceforth, researchers have attempted to construct a novel host structure and look for guest molecules with simple and tunable structures to achieve high quantum yields, long-lifetime room temperature phosphorescence. The development of pure organic room temperature phosphorescent materials based on host-guest interactions will supply facile solution and strategy to construct applicable RTP materials in future.


We gratefully acknowledge the financial support from the National Natural Science Foundation of China (NSFC) (Nos. 21788102, 21722603 and 21871083), Project supported by Shanghai Municipal Science and Technology Major Project (No. 2018SHZDZX03), the Innovation Program of Shanghai Municipal Education Commission (No. 2017-01-07-00-02-E00010), State Key Laboratory for Modification of Chemical Fibers and Polymer Materials (No. KF1803), Donghua University and the Fundamental Research Funds (No. KF1803) for the Central Universities.

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