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The Progress of Room Temperature Phosphorescent Gel

Jinfeng Wang , Aisen Li , Zhen Li

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (3) : 487 -498.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (3) : 487-498 DOI: 10.7536/PC211124
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The Progress of Room Temperature Phosphorescent Gel
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Abstract

Room temperature phosphorescence (RTP) has arouse much interest due to their unique luminescence properties and wide potential applications in optoelectronics, sensing, bio-imaging and security devices. In recent years, various methods to promote phosphorescence emission at room temperature have been explored. At present, the commonly used methods for constructing room temperature phosphorescent materials with long lifetime and high quantum yield mainly center on the design of phosphorescent molecular structure and the construction of phosphorescent protective matrix. Supramolecular gel, as a new matrix for inducing room temperature phosphorescence, has attracted much attention owing to the advantages of three-dimensional network structure, thermal reversibility and stimulus responsiveness. This review focuses on metal-free room temperature phosphorescent gel and metal-containing room temperature phosphorescent gel, and summarizes current research status in recent years. In addition, a brief prospect for the future development of room temperature phosphorescent gel research is provided.

Contents

1 Introduction

2 Metal-free room temperature phosphorescent gel

3 Metal-containing room temperature phosphorescent gel

4 Conclusion and outlook

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Keywords

room temperature phosphorescence / gel / network structure

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Jinfeng Wang, Aisen Li, Zhen Li. The Progress of Room Temperature Phosphorescent Gel. Progress in Chemistry, 2022, 34(3): 487-498 DOI:10.7536/PC211124

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1 Introduction

Room temperature phosphorescence (RTP) is a luminescent phenomenon different from fluorescence. RTP materials have attracted much attention because of their potential applications in biological imaging, anti-counterfeiting encryption and optoelectronic devices. Early phosphors mostly concentrated on metal complexes, such as iridium, platinum and other complexes. A metal complex organic phosphor can generate metal-ligand charge transfer, and that strong spin orbit couple of the metal complex organic phosphor can greatly improve the intersystem crossing (ISC) rate, thereby realize RTP emission with high efficiency and long service life. However, the high cost and biological toxicity of precious metals limit their application and development to a certain extent. Compared with the metal-containing complex phosphor, the metal-free pure organic phosphor has the advantages of convenient design and adjustment of structure, low toxicity, low cost and the like. However, because the triplet exciton in organic compounds is easily quenched by oxygen or inactivated by non-radiative dissipation at room temperature, resulting in energy loss, the luminescence efficiency of organic RTP materials is often very low. Therefore, how to construct efficient RTP materials is a research difficulty in this field.
At present, there are two main ways to construct efficient RTP materials: the structural design of phosphorescent molecules and the construction of phosphorescent protective hosts. In the process of molecular design, the introduction of halogen heavy atoms, aromatic carbonyl and other heteroatoms (S, P, etc.) promotes the ISC from singlet to triplet state, thus enhancing RTP emission[1,2][3⇓~5][6,7]. Kim et al. Synthesized a series of brominated aromatic aldehydes with high RTP emission efficiency through the synergistic effect of aromatic aldehyde group, heavy atom effect and halogen bond[8⇓⇓⇓~12]. Li Zhen et al. achieved RTP performance regulation through fine adjustment of molecular structure (change of alkyl chain, change of electronic characteristics and steric hindrance of substituent groups, etc.)[13][14,15]. They introduced alkyl chains of different lengths on phenothiazine oxide and found that the lifetime of room temperature phosphorescence showed an obvious odd-even effect: the lifetime of room temperature phosphorescence of carbon molecules with odd chain length was significantly longer than that of carbon molecules with even chain length[13]. Another strategy to improve the efficiency of RTP is to construct a room temperature phosphorescence protective host, which is a relatively rigid environment to reduce the non-radiative decay of triplet state caused by the movement and collision of phosphorescent molecules, and at the same time, to reduce the quenching inactivation of triplet state excitons by oxygen. Common room temperature phosphorescence protection matrices include solid matrices, micelles, macrocyclic host compounds, as well as supramolecular gels and polymers developed in recent years[16][17][18⇓~20][21⇓⇓~24]. Among them, gel, as a kind of soft material with excellent performance, has adjustable fluidity and elasticity, while gel materials with different luminescent characteristics are gradually applied in sensors, drug control and release, self-healing materials, 3D printing and electronic skin due to their unique physical and chemical properties and easy processing[25,26]. Therefore, in the past few decades, the study of luminescent gel materials has attracted great attention from scholars in many disciplines, such as chemistry, physics, biology, environment and materials.
Gel is a three-dimensional network structure constructed by gelators through non-covalent bonds such as hydrogen bonds, π-π stacking and metal coordination bonds. The intrinsic network structure of the gel may be due to physical or chemical bonds, or it may be due to crystallites or other linkages used to maintain integrity in extended fluids. The stretched fluid may be any fluid such as water, organic solvents, air, and the like. Therefore, gel plays an important role in the composition of organisms, and it is a kind of material with good biological adaptability and is conducive to the RTP phenomenon. In recent years, pure organic small molecule RTP materials have been developed vigorously, and some RTP materials have been reported, but the review of RTP gel materials is relatively rare[27⇓⇓~30][31,32]. In this paper, different gel materials with RTP performance in recent years are reviewed, hoping to provide some help for the further construction of RTP gel materials and promote the application of RTP gel materials.

2 Metal-free RTP gel material

Commonly used host materials are a class of compounds with macrocyclic cavities, which are amphiphilic molecules because of their different affinities for molecules in the cavity and outside. Common macrocyclic compounds, including cyclodextrins, cucurbiturils, crown ethers and calixarenes, can form stable rigid structures with guest molecules through host-guest interactions[33⇓⇓⇓⇓⇓~39]. For example, cyclodextrins can form inclusion complexes with guest molecules such as phosphorescent molecules, heavy atom perturbers and organic small molecules with space regulation, stabilize the triplet state of phosphorescent molecules, partially resist the quenching of dissolved oxygen, and thus produce RTP. In 1982, Turro et al. First reported cyclodextrin-induced RTP (CD-RTP)[40]. In 2014, Tian He et al. reported that the host-guest interaction between β-cyclodextrin host polymer (poly-β-CD) and α-bromonaphthalene polymer (poly-α-BrNp) formed a self-healing RTP hydrogel material (Fig. 1A)[41]. Firstly, RTP was induced by the host-guest interaction of poly-BrNp and β-CD in solution, but the intensity of RTP was low. The RTP intensity of the solution of poly-BrNp/poly-β-CD was significantly enhanced with the introduction of the polymer side chain. They further prepared supramolecular polymer hydrogels by changing the concentration of poly-BrNp/poly-β-CD host and guest components, and the phosphorescence emission of the gel system was further enhanced at room temperature, with a lifetime of 0.56 ms. The gel has certain self-healing property and can self-heal in 1 minute without any additive.It is an ideal soft material with application potential. In addition, the photo-induced cis-trans isomerization of azobenzene unit can affect the interaction between β-CD and α-BrNp, so the introduction of azo polymer can realize the reversible intensity change of room temperature phosphorescence under the condition of alternating UV irradiation at 365 nm and 254 nm. In 2016, they also reported the host-guest interaction between 4-bromo-1,8-naphthalic anhydride (BrNpA) and γ-cyclodextrin (γ-CD), prepared a poly-BrNpA/γ-CD copolymer capable of producing RTP in aqueous solution (Fig. 1b), and achieved reversible regulation of phosphorescence intensity when photoisomerized polyazobenzene was introduced into the system[42]. Meanwhile, by controlling the concentrations of BrNpA and γ-CD, the obtained poly-BrNpA/γ-CD hydrogels also have room temperature phosphorescence properties.
In 2019, Liu Yu et al. Of Nankai University used the brominated aromatic aldehyde polymer (G) as a guest to penetrate the polymer (G) into the α-cyclodextrin (α-CD) cavity through hydrogen bonding and hydrophobic interactions to construct a gel system (Fig. 1C)[43]. When α-CD and polymer G are mixed at appropriate concentrations in aqueous solution, they are able to form typical hydrogels. At this time, the presence of water can destroy the hydrogen bonding interaction between α-CDs, thus intensifying the radiationless movement of aromatic aldehydes, and the hydrogel only shows bright blue fluorescence (λ = 470 nm) without phosphorescence. When the hydrogel is lyophilized, the rigidity of the obtained xerogel is obviously enhanced, and the xerogel can show white fluorescence-phosphorescence dual luminescence (λ = 445 nm, 550 nm) under the excitation of a 365 nm ultraviolet lamp, wherein,The phosphorescence lifetime of the emission peak at 550 nm is 2. 79 ms, which is attributed to the hydrogen bonding and hydrophobic interactions that can provide a rigid protective environment to immobilize the luminescent molecules and suppress the non-radiative transitions.Finally, the obtained xerogel exhibits fluorescence-phosphorescence dual emission. In the absence of gelation, the supramolecular complex of α-CD and G can also emit phosphorescence, but the phosphorescence intensity is low due to the lack of protection of the rigid environment of the phosphorescent emitter. Based on the water stimulus response characteristics of the system, the potential application value is reflected in the aspects of information encryption, water sensing and the like.
In addition to cyclodextrins, cucurbiturils are also commonly used as host molecules for macrocyclic compounds. In 2020, Ma Xiang and Tian He realized the visible light excitation of pure organic RTP in aqueous phase based on host-guest assembly by using the interaction between supramolecular macrocyclic host molecule cucurbit [8] uril (CB [8]) and triazine derivative (TBP) guest molecules[44]. Among them, two TBP molecules can be accommodated in the cavity of CB [8] to form a confined dimer, and this structure leads to the formation of a new charge transfer triplet, which is beneficial to the promotion of yellow RTP emission in water under visible light excitation. Multicolor photoluminescence emission from blue to white and yellow can be achieved by adding different molar ratios of CB [8] to the blue fluorescent TBP solution (Figure 2a). In addition, they also introduced agarose gel into the system, and obtained blue, white and yellow luminescent hydrogels by adding different amounts of CB in TBP solution [8], in which the yellow luminescent hydrogel was pure organic RTP hydrogel (Fig. 2b). This special host-guest assembly of visible light excited aqueous phase pure organic room temperature phosphorescence was also used to achieve HeLa cell imaging.Under the excitation of visible light at λ = 405 nm, bright yellow phosphorescence emission can be observed in HeLa cells, and there is no obvious interference of fluorescence signal (Fig. 2b). Therefore, the hydrogel RTP material with good biocompatibility has potential biomedical application value. This strategy of realizing pure organic RTP in aqueous phase by Assembling-Induced Emission breaks the limitation of phosphorescence quenching in aqueous solution in previous work, and is expected to achieve a new breakthrough in the field of time-resolved imaging combined with the long-life characteristics of phosphorescence, which provides a new idea for the construction of aqueous room-temperature phosphorescence. In 2021, Zhao Bangtun et al. Also used the supramolecular complex formed by cucurbituril CB [8] and pyridine derivative (ABP) as a phosphorescent emitter and crosslinker to obtain RTP supramolecular hydrogel materials with excellent stretching and toughness[45]. Compared with the CB [8]/ABP complex, the hydrogel exhibited a longer phosphorescence lifetime (5.1 ms) and a higher phosphorescence quantum efficiency (9.05%).
1, 3 (R): 2, 4 (S) -Dibenzylidene-D-sorbitol (DBS) as a low molecular weight gelator, that hydrophobic aromatic re facilitates the dissolution of DBS in a variety of organic solvent, and the hydroxyl and acetal oxygen endow DBS with the ability of self-assembly, so DBS is soluble in a variety of organic solvents and polymers at high temperature, and can form a supramolecular gel at a lower concentration after cool. Using this characteristic of DBS, in 2015, Wang Hong of Huazhong University of Science and Technology reported that DBS supramolecular gel as a matrix effectively limited the vibration of 3-bromoquinoline (3-BrQ), thus inducing 3-BrQ to produce room temperature phosphorescence (Figure 3A)[46]. At the same time, the gel system has a temperature response characteristic, when the temperature reaches above the gel-sol transition temperature (TGS), such as 80 ℃, because the system has been converted to a solution state, no RTP is generated, when the temperature is rapidly reduced to 10 ℃, the system is converted to a gel state, and a stronger RTP can be generated. Thus, this system can achieve reversible phosphorescence "on-off" behavior between 10 ° C and 80 ° C (fig. 3B). In 2020, they also investigated the preparation of a pure organic luminescent supramolecular gel by simply heating and cooling a mixed solution containing the phosphor BrQ and the gelator DBC (Figure 3C)[47]. At room temperature, intense room-temperature fluorescence and phosphorescence emissions were observed at 367 and 550 nm, respectively, under UV irradiation (λ = 315 nm). Based on the structural reversibility of supramolecular gels, i.e., the gel-sol phase transition under thermal, pH, or redox stimuli, gel systems exhibit thermal, pH, and redox responsive properties. Kimizuka et al. Of Kyushu University in Japan reported that the self-assembled nanofibers of glutamic acid derivative (LBG) supramolecular gel could effectively block the quenching of oxygen on the triplet state of phosphors. Therefore, the two-component gel system PtOEP/LBG formed by donor molecules PtOEP and LBG showed RTP performance, with the maximum phosphorescence emission peak at 645 nm (Fig. 3e, f)[48].
The electrostatic interaction between anions and cations is another way to form the network gel structure. In 2020, Liu Yu et al. first formed SCD SCDPYCl by host-guest interaction between PYCl and SCD, and then immobilized SCD SCDPYCl on amino clay (AC) by electrostatic interaction to form a mixed xerogel (Fig. 4A)[49]. The xerogel has a relatively rigid network structure that effectively immobilizes the phosphorescent emitter, limiting its vibrational and nonradiative relaxation, resulting in green RTP emission (λ = 500 nm) and good fluorescence-phosphorescence dual emission with a phosphorescence lifetime of 1.24 ms at 550 nm (Figure 4B). It is worth noting that the emission intensity of the xerogel RTP changes with humidity, and the humidity-responsive phosphorescent switch can be turned on and off repeatedly, indicating that the xerogel has potential applications in the field of organic light-emitting humidity-sensing materials.
In 2018, George and others in India adopted the strategy of organic-inorganic supramolecular scaffolds to self-assemble gel materials through electrostatic interaction[50]. They used clay with complementary charges on the surface and edges as anchoring molecules to anchor phosphors (naphthalimide derivatives) to form supramolecular gel materials (Fig. 4C). Due to the heavy atom effect of Br, BrNDI-LP shows red RTP in both aqueous solution and thin film. In 2021, they again adopted the organic – inorganic supramolecular scaffold strategy to report efficient RTP emission in organic – inorganic supramolecular aqueous solutions and hydrogels (Figure 4D)[51]. First, they used the high complexation constant of the ammonium group and the cyclic carbonyl group in CPthBr and CB [7] to perform host-guest complexation through ion-dipole interaction to obtain a stable CPthBr-CB [7] complex, which showed RTP performance in solution, with a maximum phosphorescence emission peak at 475 nm, a phosphorescence lifetime of 104 μs, and a maximum phosphorescence quantum yield of 41.8%. Subsequently, they used electrostatic interaction to anchor cationic phosphor CPthBr on the surface of negatively charged clay LP, and obtained a gel material with cyan phosphorescence emission by adjusting the concentration and content, with a lifetime of 1.34 ms and a phosphorescence quantum yield of 34.2%.The negatively charged acceptor dye molecule was anchored at the edge of the clay LP, and the delayed fluorescence of the acceptor material in the gel state was realized through the efficient triplet-to-singlet F Förster resonance energy transfer (TS-FRET) between the donor molecule CPthBr and the acceptor dyes (SRG and SR101).
The above studies are mainly based on the formation of RTP gels through non-covalent bonds (including host-guest interaction, hydrogen bonding, electrostatic interaction, etc.). In addition, the formation of covalent bonds is also a way to construct RTP gel materials.
In 2020, Tian He and Ma Xiang et al. copolymerized iodine substituted boron dipyrrole (Bodipy) with acrylamide free radical to obtain an amorphous polymer material p-1IBDP/p-2IBDP with near-infrared room temperature phosphorescence emission excited by visible light (Fig. 5)[52]. In the system, iodine atoms are introduced to promote intersystem crossing, and meanwhile, the rigid structure of the acrylamide polymer can not only weaken the vibration of luminescent molecules, but also reduce the interference of external quenching factors such as oxygen and the like, thereby reducing non-radiative transitions and enhancing RTP emission. In addition, by introducing UPy and MBAA as crosslinking agents, they obtained near-infrared RTP gels g-1 IBDP and G-2 IBDP with rapid self-healing function, with lifetimes of 63 μs and 83 μs, respectively. The gel shows good self-healing ability, which lays a foundation for designing multifunctional RTP materials and potential applications of the materials in electronic devices in the future.

3 Metal-containing RTP gel material

Palladium-porphyrin (Pd-TCPP), as one of the few probes with RTP properties, has been developed rapidly. The results show that the interaction of hydrogen bond and electrostatic interaction can provide a rigid environment for palladium porphyrin to reduce its phosphorescence quenching. In 2014, Lovell et al. Reacted palladium porphyrin Pd-mTCPP with polyethylene glycol-diamine to obtain oxygen-sensitive RTP Pd-mTCPP hydrogel material (Fig. 6a)[53]. The hydrogel exhibits near-infrared phosphorescence emission with a maximum phosphorescence emission peak λmax=705 nm. They implanted the hydrogel into the back of BALB/C mice, and after 10 days, using imaging technology, they could observe the red light image of the hydrogel on the back of mice (Fig. 6B). In 2019, Wang Hong et al. prepared a hybrid supramolecular hydrogel with a double ordered structure as a new matrix material. When phosphor (Pd-TCPP) was introduced into the hydrogel with a double ordered structure, efficient RTP emission could be induced[54].
A possible mechanism for the room-temperature phosphorescence of Pd-TCPP induced by the Gemini surfactant G12-8-12 hybrid N, N-diformyl-L-cysteine (DBC) supramolecular gel was proposed through the limitation of the hydrophobicity and double-order structure of the mixed system (as shown in Figure 6 C). In the solution state, the Pd-TCPP molecule is in a free state and cannot emit phosphorescence at room temperature. When the surfactant G12-8-12 and the gelator DBC are self-assembled to form nano fibers in the mixed solvent of the DMSO/H2O and then are mutually wound to form a three-dimensional network structure,The Pd-TCPP molecules are encapsulated in the hydrophobic cavities of the G12-8-12 micelles on the three-dimensional network fibers through hydrophobic and electrostatic interactions, and the solvent molecules are confined in the network structure through capillary interaction and surface tension, thus forming the G12-8-12 hybrid DBC supramolecular gel similar to a solid. The probability of nonradiative deactivation of the excited triplet state of Pd-TCPP is greatly reduced, resulting in strong and stable room temperature phosphorescence. In addition, they found that the RTP intensity could be adjusted by changing the concentration of Gemini surfactant. The hybrid gel material shows potential application prospects in the fields of biological imaging and sensors.
Among various metal ions, Al3+ is considered to be a hard acid due to its small ionic radius, and has a strong chelating effect on hard bases containing N and O, which plays an important role in fluorescence and phosphorescence sensitization. In 2013, Tian et al added Al3+ to conjugated polymer nanoparticles (PCzDCN/PST-SO3H NPs), which hindered the intermolecular charge transfer (ICT) and enhanced the fluorescence[55]. In 2015, after adding Al3+ on the surface of ZnS quantum dots, Yan et al. Complexed with Al3+ due to the carboxyl group on the surface of quantum dots, which enhanced the phosphorescence intensity[56]. Therefore, metal ions have potential applications in enhancing the sensitivity of room temperature phosphorescence. In 2019, Wang Hong et Al. again constructed a Gemini/Al/DBC ternary supramolecular gel that induced Pd-TCPP to produce high intensity and controllable room temperature phosphorescence (Fig. 6d)[57]. In the G/DBC gel state, Pd-TCPP is in the nanofiber hybrid gel state formed by the micelle assembled by the surfactant and the self-assembly of the gelator DBC, so it can emit weak phosphorescence at room temperature. When Al3+ was added into the hybrid gel, the room temperature phosphorescence was significantly enhanced, and the RTP of Pd-TCPP could be effectively enhanced and adjusted by the concentration of Al3+. This is mainly attributed to the interaction between Al3+ and DBC and porphyrin, and the gel system forms a more compact 3D network structure after adding Al3+. In addition, the measurement of critical micelle concentration showed that Al3+ increased the surface activity of G12-8-12 and promoted the formation of micelles, thereby increasing the dispersion of Pd-TCPP in the mixed gel. Stemming from the synergistic effect of these results, the non-radiative transitions of Pd-TCPP are effectively suppressed, resulting in a highly efficient RTP.
In addition to the enhancement of RTP intensity by the Al3+ of metal ions, the room temperature phosphorescence (RTP) of the gel materials can also be enhanced by the stimulation of ultraviolet light. In 2019, Wang Hong et al. reported a system that promoted the emission of high-intensity RTP from mixed supramolecular gels under UV irradiation (Fig. 6 e, f)[58]. After 10 min of UV irradiation, it was found that the RTP intensity of the gel system was significantly enhanced compared with the gel without UV irradiation, and the lifetime was extended from 0.14 ms to 0.74 ms (Fig. 6d). EPR experiments show that the dissolved oxygen in the gel system is transformed into 1O2 with strong oxidation ability under ultraviolet irradiation. In the gel system, the disulfide group (-S-S-) of DBC has reducibility and is oxidized by singlet oxygen to form sulfonic acid group. On the one hand, the formation of sulfonic acid group can form stronger hydrogen bonds between molecules, and these hydrogen bonding interactions make the distance between DBC aggregates closer, which is conducive to the formation of a dense three-dimensional network structure. On the other hand, there is a strong electrostatic interaction between the negatively charged part of the sulfonic acid band and the positively charged part of G12-12-12. Under the synergistic effect of hydrogen bonding and electrostatic interaction, the supramolecular 3D network structure of the gel system becomes more dense, and the non-radiative transition of Pd-TCPP is more restricted, resulting in RTP enhancement. In addition, by changing the duration and power of UV irradiation, the regulation of RTP intensity can be effectively achieved.
Although the room-temperature phosphorescence of the gel material of the palladium porphyrin complex shows long-wavelength emission (λmax=700 nm), it is theoretically a better bio-imaging material. But in fact, most of the reported palladium porphyrin gel materials are used for the detection of temperature, pH and so on, and very few are used in the field of biological imaging, which may be due to the short phosphorescence lifetime and low quantum yield, limiting their application in the field of biological imaging.
In addition to palladium porphyrin complexes, Pt-complexes can also self-assemble to form RTP gels. Terao et al. Synthesized the Pt-acetylene crosslinker PtCD-CX, and introduced the Pt-acetylene crosslinker into the polymer network to obtain a phosphorescent emission gel (Fig. 7A). The formation of the gel effectively inhibited the aggregation between the phosphorescent crosslinkers, and the formed hydrogel showed enhanced phosphorescence and mechanical properties, and the gel showed strong sensitivity to oxygen (Fig. 7B)[59]. Shinkai et al. Synthesized 8-quinoline platinum (II) complexes 1Pt and 2Pt, which can be used as gelators for various organic solvents. The formed gels are helpful to inhibit the quenching of triplet state by oxygen and promote the generation of phosphorescence[60]. And exhibited unique thermochromic and solvatochromic red phosphorescent emission at the sol-gel phase transition (Figure 7c and d), the RTP emission of the 1Pt compound gel-forming in p-xylene was more red-shifted than that in the tetrachloroethane solution state. In 2016, Xiang Haifeng et al. Of Sichuan University reported a new class of dinuclear cyclometalated Pt II enantiomers with propeller-like structures, which exhibit unusual aggregation/gel-induced phosphorescence through strong multiple intermolecular interactions such as H-C, H-Pt, H-O and H-H interactions[61]. And, the intermolecular π-π interaction between the two enantiomers can be tuned by adjusting the chemical structure to induce gel formation.
Gold and silver are also often introduced to construct RTP gel materials. In 2010, Omary et al. introduced a water-soluble phosphor Na8[Au(TPPTS)3] into the polymer network of PNIPAM (Fig. 8A) to obtain a stimulus-responsive phosphorescent microgel with significant sensitization[62]. The microgels and hydrogels exhibit asymmetric green phosphorescent emission (λmax=525 nm). The lyophilized xerogel exhibited the same phosphorescent emission characteristics of turquoise as the crystalline Na8[Au(TPPTS)3], with the phosphorescent emission peak blue-shifted to 490 nm (fig. 8 B). When the temperature is higher than the critical solution temperature (34 deg C) of the PNIPAM, the microenvironment of the microgel changes from hydrophilicity to hydrophobicity, so the hybrid microgel formed by the Na8[Au(TPPTS)3] of the phosphor shows remarkable irritant response characteristics and stability to the temperature. Thus, when the temperature is increased from room temperature to 37.d egree. C., the fluorescence (PL) spectrum undergoes a blue-shift enhancement and a reversible cycle of PL enhancement and reduction can be achieved by heating and cooling (fig. 8C). In the absence of PNIPAM microgels, when the aqueous solution of gold phosphor was heated from ambient temperature to 37 ℃, the PL was quenched due to the increase of non-radiative decay. Therefore, the enhanced PL signal of the hybrid microgel system at physiological temperature of 37 ℃ is helpful for the application of this kind of materials in the fields of biology and medicine.
In oxygen-containing solution, the excited triplet state of long-lived phosphorescent compounds is easily quenched by molecular oxygen. The traditional methods of oxygen removal include low temperature freezing, vacuuming and replacing air with inert gas, which bring some difficulties to the practical application of phosphorescent compounds. Room temperature phosphorescence (RTP) induced by the consumption of oxygen in the photoactivation process is a common method for organic materials. In 2017, Lu Wei et al. Of Southern University of Science and Technology reported the generation of phosphorescence induced by ultraviolet activation[63]. They found that the phosphorescent gold (I) aryne compound (Figure 9 a ~ d) in saturated dimethyl sulfoxide (DMSO) solution gradually changed from blue to yellow and showed a process of illumination under the stimulation of continuous ultraviolet light, and the intensity of phosphorescence (λmax=537 nm) increased with the extension of illumination time, reaching the maximum at 60 min. In other solvents such as methanol, ethylene glycol and N, N-dimethylformamide (DMF), this phenomenon does not occur, which is mainly due to the oxidation of dimethyl sulfoxide by excited singlet oxygen generated during sensitization under light irradiation to form dimethyl sulfone, thus eliminating the existence of molecular oxygen in the local microenvironment and activating the triplet excited state of phosphorescence. Through the experimental comparison, it is found that the effect of light deoxygenation is better than that of inert gas bubbling deoxygenation. In addition, the gold (Ⅰ) compound can also remain relatively stable in DMSO solution after a long time of strong light irradiation, and the λmax=379 nm and 537 nm intensity of the gold (Ⅰ) compound in the same DMSO solution do not decrease significantly after 20 cycles. In addition, they prepared the dimethyl sulfoxide solution into a gel, and "photoetched" various patterns or characters in the gel system, and then the patterns or characters could be scrubbed off by heating and cooling. Repeating this process could easily realize the cycle of "writing, erasing and rewriting" on the gel (Fig. 9c).
In 2019, Xin Xia et al. from Shandong University prepared a new type of water-soluble complex Ag9NCs (Fig. 10a). With the addition of polar organic solvent ethanol, in the first 15 min, the emission peak with the maximum emission wavelength of 480 nm gradually increased with the change of dispersed aggregates to dense fibers, and after 15 min, the emission peak red shifted to 580 nm[64]. Compared with the lifetime of the Ag9NCs dilute solution (3 ns), it is increased by a factor of 114 (Figure 10B). The emission of Ag9NCs solution belongs to fluorescence emission, while the emission of Ag9NCs self-assembly and gradual gelation to form long-lived metal organogels (MOGs) belongs to phosphorescence emission. It was found that the gelation not only enhanced the photoluminescence, but also induced the conversion from fluorescence to phosphorescence. The optical properties and microstructure of MOGs were studied during the dynamic process of gelation, and the internal mechanism of the change of optical properties caused by gelation was revealed, that is, with the self-assembly of Ag9NCs into a fibrous network structure,The aggregated molecules were further rearranged to form a more compact molecular arrangement, resulting in a significant fluorescence enhancement and a transition from fluorescence to phosphorescence (Fig. 10c). Therefore, both the PL intensity and the fluorescence-phosphorescence switching are closely related to the order of the aggregated molecules. MOGs based on the organometallic gels have a wide temperature response range: with the increase of temperature, the PL intensity gradually decreases (Fig. 10d). In the temperature range of 83 ~ 383 K, the quantitative curve of PL intensity versus temperature takes 200 K as the temperature division point.Two linear relationships can be used for quantification (Fig. 10d). At the same time, the increase of temperature also causes the morphological change of MOGs from fibrous to spherical and the disorder of molecular arrangement. Therefore, the ordered properties of self-assembly are closely related to the PL properties of samples. The PL of MOGs increases at low temperature and decreases at high temperature, which can be dynamically and reversibly adjusted by heating and cooling multiple cycle experiments, and has good stability. Therefore, the wide temperature response and high stability of MOGs show potential application value in the field of luminous ratio thermometers.

4 Conclusion and prospect

RTP materials, especially organic RTP materials, have been developed rapidly in recent years[65⇓⇓~68]. Usually, most organic RTP materials are based on solid systems such as crystals and polymers, while RTP gel materials are amorphous materials with special properties such as viscoelasticity, biocompatibility and moisture retention. Among them, RTP gel materials have potential applications in the field of biomedical imaging due to their good biocompatibility. In addition, some RTP gel materials have good self-healing properties and rapid self-healing ability, which have potential applications in the fields of optical soft actuators, optical switches, intelligent material development and so on. Meanwhile, the RTP gel material is a self-assembled network structure formed by weak non-covalent interactions such as host-guest interaction, hydrogen bonding, electrostatic interaction, and the like, and shows stimulus response characteristics to the external environment, so that the RTP gel material has application value in the sensing field.
As a new functional material, the exploration of RTP gel material is still in the initial stage. At present, it is generally believed that the mechanism of gel inducing/promoting RTP is that the formation of gel system can fix the phosphor, restrict its vibration and inhibit its non-radiative transition on the one hand; On the other hand, the network structure formed by the gel system can prevent oxygen from entering and reduce the non-radiative deactivation of the excited triplet state (T1) caused by the interaction between the phosphor and oxygen. Therefore, the gel system can achieve a good protection effect on the phosphor, reduce the non-radiative collision deactivation and effectively protect the excited triplet state, and promote the generation of RTP.
Although RTP gel materials have made some progress, there are still some problems and shortcomings that need to be improved in the future development: (1) At present, there are few kinds of RTP gel materials reported, whether based on metal-free or containing metal ions, they need to be further enriched and expanded. At present, RTP gels are mainly formed by non-covalent interactions (host-guest interaction, hydrogen bonding, electrostatic interaction, intermolecular multiple interactions, etc.), while there are few reports on RTP gels based on the formation of covalent bonds. (2) The RTP lifetime of gel materials is generally short and the quantum yield is low, which limits the application of RTP gel materials in the fields of biological imaging, sensors and smart materials to a certain extent. At present, RTP materials are developing in the direction of improving the phosphorescence quantum yield and prolonging the lifetime.Researchers have accumulated rich experience in promoting intersystem crossing from molecular singlet state to triplet state and inhibiting non-radiative decay from triplet state to ground state based on organic small molecule RTP materials, and have also developed new strategies for constructing efficient pure organic room temperature phosphorescent materials. In the development process of RTP gel material system, we can learn from the mature concept of organic small molecules, introduce efficient phosphors into the gel system, improve the phosphorescence efficiency and life of RTP gel system, and promote the further development of RTP gel material in the field of biological imaging. (3) Compared with the organic small molecule room temperature phosphorescence system, the construction of RTP gel system is still immature and in the exploratory stage, and the relationship between phosphorescence performance and internal interaction is still unclear, which requires further research on the internal mechanism of RTP gel materials.

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Funding

Starting Foundation of Tianjin University and the National Natural Science Foundation of China(22105143)

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