Compared with inorganic light-emitting materials, organic light-emitting materials have many advantages, such as variety, good tunability, rich color and flexible molecular design, and have large-scale application prospects in anti-counterfeiting, sensors, data recording and storage, light-emitting devices and so on. One of its main applications is Organic light-emitting diodes (OLED). OLED has many advantages over traditional display and lighting technologies, such as self-luminescence, flexibility, large area, high color gamut and high contrast, and is widely used in display and lighting^[1,2]. Luminescence of organic materials refers to the process of radiative transition from the excited state to the ground state and the generation of photons. Due to its wide range of luminescence colors, rich chemical tailoring and mature preparation process, it has great potential in practical applications. According to the luminescent mechanism of organic luminescent materials, it can be divided into fluorescence, phosphorescence and delayed fluorescence, as shown in Figure 1A. After the ground state molecule is excited to form an exciton, the exciton is excited from the ground state S₀ to the singlet state S_n. The exciton is vibrationally relaxed back to the lowest excited singlet state (Lowest excited singlet state,S₁). Subsequently, the exciton fluoresces upon transition from S₁ to S₀. Phosphorescence occurs when the exciton on the S₁ transitions from the T₁ to the S₀ after it reaches the lowest excited triplet state (Lowest excited triplet state,T₁) through Intersystem crossing (ISC). When the energy absorbed by the exciton on the Reverse intersystem crossing returns to the S₁ state through Reverse intersystem crossing (RISC) and then transitions back to the S₀ state, delayed fluorescence occurs. In order to achieve full-color display, organic light-emitting materials mainly have three primary colors of red, green and blue. The luminescent color of materials can affect the energy level structure of molecules by changing the stacking mode, chemical structure or intermolecular interaction of materials.

Benzophenone (1) is composed of a carbonyl group and two benzene rings and can be obtained either by direct oxidation of diphenylmethane with potassium permanganate or by benzoyl chloride and bromobenzene as reported by Stack et al. In 1991 as shown in fig. 3^[3][4]. As shown in Table 1 and Figure 1b, the tetrahydrofuran solution of benzophenone at room temperature fluoresces weakly at 310 and 380 nm, while the tetrahydrofuran solution of benzophenone at 77 K fluoresces at 310 nm and has three phosphorescence emission peaks at 410, 440, and 472 nm. Benzophenone crystals exhibit no fluorescence emission at room temperature and phosphorescence at 420, 449, and 483 nm^[5,6]. According to the theoretical calculation, the S₁ energy level of benzophenone is the 3.6 eV,T₁ energy level of 2.9 eV^[7]; The Highest occupied molecular orbital (Highest occupied mole cular orbital, HOMO) is − 4.04 eV and the Lowest unoccupied molecular orbital (Lowest unoccupied mole cular orbital, LUMO) is 0.56 eV^[8]; The 5% mass heat loss temperature is 190 ° C and the glass transition temperature is − 56 ° C^[9][10]. The carbonyl group of benzophenone contains a lone pair of electrons, which can promote the ISC process through spin-orbit coupling. Through reasonable molecular structure design and chemical modification, the excited state properties of benzophenone derivatives can be effectively controlled to achieve different channels of luminescence. There are three main chemical modification strategies commonly used to regulate its excited state properties. One is the direct introduction of multi-substitution mode, which can obtain modified compounds such as 1,4-dibenzoylbenzene (2) (2 powder phosphorescence peaks at 540 and 570 nm at 77 K)^[11]. The other is to use O, S, NH and other heteroatoms as bridging groups to modify benzophenone. Based on this, a series of modified compounds such as dibenzo [B, d] furan-2-yl (phenyl) methanone (3), dibenzo [B, d] thiophen-2-yl (phenyl) methanone (4), xanthone (5), acridone (6) and 9-thioxanthone (7) were obtained (for example, 3 crystal at 77 K has a phosphorescence peak at 450 ~ 500 nm; 4 crystal at 300 K has a fluorescence peak at 470 nm and a phosphorescence peak at 567 nm)^{[12][12][13][14][15]}. In addition to this, there are coupled modification strategies as shown in Fig. 2. For example, benzophenone is modified by coupling reaction with C = C double bond to expand its conjugation degree and enhance its structural rigidity, resulting in modified compounds such as tetraphenylethylene (8), 9,9-biacenaphthylene (9) and dibenzocycloheptene (10) (for example, the fluorescence peak of tetrahydrofuran at room temperature is 450 nm)^{[16][17][18,19]}. The 6 and 6 'positions of benzophenone can also be directly coupled to obtain 9-fluorenone (11) and 2-bromo-4-methyl-10,11-dihydro-5H-dibenzo [a, d] cyclohepten-5-one (12) after dehydrocyclization (e.g., room temperature 11 toluene fluorescence peak at 490 nm)^[20][21]. On the basis of 11, 4,5-diazafluoren-9-one (13) can be obtained by substitution with heteroatomic nitrogen, and fluorene (14) can also be obtained by hydrodeoxygenation^[22][23]. Similarly, 14 can be modified by multiple substitutions. It is possible to obtain structurally similar fragments such as 6,12-dihydroindeno [1,2-b] fluorene (15), 11,12-dihydroindeno [2,1-a] fluorene (16), 5,8-dihydro-indeno [2,1-c] fluoren (17) and trimeric indene (18) with an increased degree of conjugation, as well as spirofluorenes such as 9,9 '-spirofluorene (19) and spiro [fluorene-9,7' -indene [1,2-a] pyrene] (20) containing^{[24⇓~26][24⇓~26][24,26⇓ ~28][29][30][31]}. Among them, heteroatoms such as O, S, NH, SO₂ and CO can also be used as bridging groups on the basis of 19, thereby obtaining spiro [fluorene-9,9 '-xanthene] (21), spiro [fluorene-9,9' -thianthene] (22), 10H-spiro [acridine-9,9 ′ -fluorene] (23), spiro- [fluorene-9,9 ′ -thioxanthene] 10 ′, 10 ′ -dioxide (24), 10H-spiro-^{[32][33][34][35][36][37]}. Base on that collected photophysical property parameter of benzophenone framework-based derivatives, a variety of derivatization methods are effective in modifying the luminescence property. Except for modified compound 19, the fluorescence or phosphorescence peaks of other modified compounds were significantly red-shifted compared with benzophenone. Compared with modified compound 26, some modification strategies can significantly improve the thermal stability. Therefore, the above chemical modification strategies can finely control the photophysical properties and thermal stability of benzophenone derivatives, such as reducing the band gap of materials by constructing strong intramolecular charge transfer, prolonging electron delocalization by substituent effects such as cyano groups,The reorganization energy is reduced, so that the carrier mobility is improved, and the thermal stability of the benzophenone can be improved by fixing the benzophenone in a rigid framework, so that a plurality of multifunctional organic luminescent materials based on the benzphenone framework are developed^{[38][39,40][41]}.

Since the fluorescence is the exciton transition from S₁ to the ISC between S₀,S₁ and T₁ is radiationless transition, therefore, only about 25% of the S₁ excitons can emit light. Among the three primary colors of red, green and blue, red and green have achieved higher External quantum efficiency (EQE) and better color purity, but blue OLED still has the problem of lifetime and poor stability^[42,43]. The main reason for the aging of OLED devices is the cleavage of chemical bonds induced by excitons or charge carriers (radical anions or cations) in organic light-emitting materials in OLEDs, accompanied by complex intermolecular reactions^[44⇓~46]. Due to the high strength intramolecular weak C(sp²)-C(sp²) chemical bond between carbonyl and benzene ring, benzophenone is the main electron-withdrawing group for the synthesis of electron transport materials or bipolar hosts, which is expected to improve the stability of blue OLED devices^[46].

The benzophenone derivative-fluorene 14 has the advantages of high fluorescence quantum yield, easy synthesis and chemical modification, good solubility and high carrier mobility, and is an important blue-light fragment^[47⇓~49]. However, both photoluminescence and electroluminescence of fluorene derivatives often produce a low-energy "green" band covering a wide spectral range of 500-600 nm, which not only reduces the color purity and stability of blue light, but also reduces the luminous efficiency of the material^[50]. Therefore, different organic light-emitting materials are often synthesized by introducing different groups into fluorene to change its conjugation length and electron-donating/withdrawing ability. The benzophenone-based cyclized 9-fluorenone (11) and its derivatives are usually characterized by a Photoluminescence quantum yield n→π^*, which results in a forbidden radiative transition and a low Photoluminescence quantum yield (PLQY). The π→π^*state of the modified 11 may become a S₁ below the n→π^* state, and the n→π^* state is generally unfavorable for luminescence, which makes the luminescence of the derivatives of 11 weaker^[51]. Zhou et al. Synthesized a series of dark blue fluorenyl fluorescent materials 27 and 28 (fig. 6) with large Stokes shift by changing the conjugation length of the fluorenyl group, and the fluorescence quantum yields were 57.2% and 43.6%, respectively^[52]. Similarly, Chen et al. Synthesized a series of fluorene-anthracene copolymers 29 and fluorene-naphthalene copolymers 30 by solid-phase oxidative coupling polymerization using ferric chloride as an oxidant (Fig. 6)^[53]. X-ray diffraction analysis showed that the crystallinity of the copolymer increased with the increase of the number of fluorene units in the backbone. Moreover, the optical properties of the copolymers with different ratios of fluorene, anthracene and naphthalene can be tuned from red light in powder to red light in concentrated solution and blue light in dilute solution. In 2019, Shen et al. Used the Pd (0) -catalyzed Bell reaction to modify fluorenone with 4-cyanobenzene and triphenylamine, thereby reducing the π→π^*state as the radiation state to achieve strong luminescence of the material (31 and 32)^[38]. The introduction of electron-withdrawing 4-cyanobenzene changed the electronic transition configuration, effectively reduced the band gap, and enhanced the intramolecular charge transfer characteristics, so that 32 achieved a maximum deep red luminescence of 668 nm in solid state.

Rakstys et al. Successfully prepared 2,2 ', 7,7' -tetrakis (N, N-di-p-methoxyaniline) -9,9 '-biazulene (33) by replacing 9,9' -biazulene (9) with 4,4 '-dimethoxydiphenylamine^[54]. The double bond in 33 strengthens conjugation and promotes π electron delocalization, the four propeller-shaped diphenylamine units are twisted to varying degrees, and its fluorenyl moiety promotes carrier migration, transferring electrons from the donor to the whole acceptor. The room-temperature zero-field hole drift mobility of 33 is 4.6×10^-5cm²·V^-1s^-1, while the Gaussian-shaped hole transport is also observed in Fig. 7 B, C, and the transport time is clear, which indicates that 33 is a high-performance hole transport material.

In order to prepare high-performance blue fluorescent materials, Wei et al. Connected fluorene at the 5th position of 10. The butterfly structure of spiral connection can not only avoid the problem that the conformation of the central seven-membered ring of 10 is easy to flip, but also prevent the tight π-π stacking between chromophore molecules^[55][56]. 34 was prepared by adding diarylamine groups at positions 3 and 7 to increase hole transport. As the best blue fluorescent material at that time, the maximum EQE of undoped OLED prepared with 34 was as high as 7.9%, which was comparable to some phosphorescent doped OLEDs^[57]. Wang et al. First iodinated triindene (18), then coupled it with 2-thiophene boron, and on this basis, coupled it with trimethylsilyl acetylene by Sonogashira, and finally connected it with 18 unit to synthesize 35^[58,59]. The long π-conjugated linear molecule has a maximum emission wavelength of 460 nm and a fluorescence quantum efficiency of 25% in tetrahydrofuran solution. Li et al. Developed a new fluorescent dye 36 by combining 5,8-dihydroindeno [2,1-c] fluorene (17) with an electron-withdrawing imide group^[27]. The HOMO is distributed over the peripheral phenyl ring and the 17 nucleus, while the LUMO is distributed over the 17 nucleus and the imide substituent. The 17 core, which contains both HOMO and LUMO orbitals, has a large electron density, suggesting that the indene unit plays an important role in promoting the charge transfer of the π-π^* transition from HOMO to LUMO orbitals. With the increase of solvent polarity, the maximum emission wavelength of 36 increased from 460 nm to 503 nm, and the fluorescence lifetime also increased from 2. 55 ns to 6. 41 ns.

Rigid spirofluorene materials are very promising optoelectronic materials due to their unique physical properties, such as higher glass transition temperature, good solubility and amorphous nature, high quantum efficiency, and the tendency to reduce aggregation quenching. In addition, the introduction of electron-withdrawing fluorine atoms into organic materials is beneficial to the adjustment of molecular frontier orbital energy levels, emission wavelength and fluorescence quantum yield, and promotes the crystallization of molecules due to the interaction of C-F and H-C, which is helpful to obtain efficient electroluminescence^[60,61]. Li et al. Synthesized a novel fluorine-containing 9,9-spirofluorene derivative 37 for organic electroluminescent devices by combining fluorine atoms with spirofluorene (fig. 9)^[62]. 37 exhibited a higher electron mobility (~10^-3cm²·V^-1·s^-1) under an electric field of 4×10⁵V·cm^-1 as well as a comparable hole mobility, which is an order of magnitude higher than that of a conventional hole transport material (N, N ′ -di-1-naphthyl-N, N ′ -diphenylbenzidine). The maximum emission peak of undoped dark blue OLED with 37 as the light-emitting layer is 408 nm. In addition, the pure blue emission CIE coordinates of the OLED doped with 4,4 ′ -bis (9-ethyl-3-carbazolevinyl) -1,1 ′ -biphenyl as the guest and 37 as the host are (0. 149,0.187), the turn-on voltage is 3. 4 V, the current efficiency is 6.66 cd·A^-1, and the brightness is more than 10000 cd·m^-2. Thirion and Poriel et al. Have proposed the successful preparation of isomers 38 and 39 using 6,12-dihydroindeno [1,2-b] fluorene (15) and 11,12-dihydroindeno [2,1-a] fluorene (16) as the core^[63][24]. Among them, 38 exhibited purple luminescence with a maximum emission peak at 360 nm and a fluorescence quantum yield of 77%. Differently, the excited state 39 leads to quasi-unimolecular luminescence through intramolecular interaction, with a maximum luminescence peak at 450 nm and a quantum yield of blue fluorescence of 30%.

Spiro [fluoren-9,9 '-xanthene] (21) with spiro structure has also attracted great attention due to its orthogonal interconnection of carbon 9, steric hindrance effect and good thermal stability. The molecular configuration with 21 as the core can effectively suppress the intermolecular π-π stacking, and the oxygen atom can broaden the band gap of the material to a certain extent, so that the material has a higher T₁ energy level, so 21 is a star molecule for building blue-emitting materials. By introducing different functional groups, 21 can be applied to the synthesis of wide band gap blue fluorescent materials, phosphorescent hosts, and hole transport materials^{[64⇓⇓~67]}. Li et al. Prepared two novel imidazole/spiro [fluorene-9,9 ′ -xanthene] hybrid materials 40 and 41 by attaching the imidazole group to the No.2 position of 21 (Fig. 10)^[68]. The two materials show strong deep blue luminescence in dichloromethane solution, with 40 emission peak at 430 nm and 41 emission peaks at 381 and 399 nm, and the PL QY is as high as 73. 5% and 98. 5%, respectively^[68]. Similarly, Li et al. Introduced thiogen into spirofluorene to break the π-conjugated and planar structure of spirofluorene, and obtained a new spiro structure, spiro [fluorene-9,9 '-thioxanthene] and spiro [fluorene-9,9' -thioxane-S, S-dioxide], followed by the introduction of electron-rich N-phenylcarbazole and triphenylamine.Then, the excessive red shift of luminescence caused by intramolecular charge transfer is avoided by using the moderate electron-withdrawing property of the sulfone group, and the high-efficiency local excited state deep blue light material and the charge transfer state luminescent materials 42 and 43 (fig. 10) are obtained. In addition, the sulfone group improves the electron injection and transmission properties of the material^[69].

In order to study the effect of phenothiazine on the photoelectric properties of spirofluorene with stereo-vertical configuration, Jiang et al. Synthesized a derivative 26 based on spirofluorene and phenothiazine (Fig. 11)^[37]. The solid powder of 26 has a 5% mass heat loss temperature of 386 ° C and a melting point of 221 ° C, showing high thermal stability. The 26 solid film has a band gap of 2.63 eV and emits green-blue light. Compared with 2,7-dibromo-9,9-diethylfluorene, the introduction of phenothiazine into the spirofluorene framework can reduce the LUMO energy level to − 3.05 eV and increase the HOMO energy level to − 5.68 eV. The results show that the introduction of rigid phenothiazine into spirofluorene skeleton can improve its thermal stability and effectively adjust the photophysical properties and energy level structure of the obtained derivatives.

Phosphorescent OLED is an important milestone in the history of multifunctional luminescent materials^[70⇓~72]. Organic complexes of noble metals such as Ir, Os, and Pt can utilize S₁ and T₁ excitons, facilitating spin-orbit coupling to achieve a maximum internal quantum efficiency of 100%^[73]. At the same time, organometallic phosphorescent complexes with Ir, Os and Pt as cores are often used as guests doped into the host to prevent triplet-triplet annihilation and efficiency roll-off caused by non-radiative deactivation^[74⇓~76]. The first condition for an efficient host is that the T₁ energy level of the host is higher than the T₁ energy level of the dopant, which prevents the back transfer of energy from the dopant to the host and limits the T₁ exciton of the dopant molecule. In phosphorescent OLEDs, energy transfer from the host to the guest occurs by: first, Forster or Dexter energy is transferred from the electrically excited S₁ of the host to the S₁ of the guest, followed by ISC in the guest to produce a T₁ exciton; Second, the Dexter energy is transferred from the T₁ exciton in the host to the T₁ exciton of the guest. As far as we know, although there are many guests of organometallic complexes with Ir, Os and Pt as cores, there are few reports on benzophenone and its derivatives as noble metal ligands. However, the host based on benzophenone skeleton not only retains a high T₁ energy level, but also has good electron transport ability and thermal stability. Neogi et al. Prepared butterfly-shaped materials 44 and 45 by inlaying benzophenone on both sides of a rigid, non-planar Tr Tröger base skeleton (Fig. 12)^[41]. The combination of benzophenone with non-conjugated butterfly Tr Tröger base, which is not easy to crystallize, retains the higher T₁ energy level of benzophenone and suppresses the non-radiative transition^[77]. The 5% mass heat loss temperature of these two non-planar molecular solids exceeds 320 ° C, and the non-planar geometry of 44 and 45 improves the thermal stability compared to benzophenone. Using Tris (2-phenylpyridine) iridium as the green dopant, 44 and 45 as the host, the maximum emission wavelength of the OLED is 516 nm, and the corresponding EQE is 6. 1% and 6. 9%, respectively.

Ma et al. Coupled 2-bromo-4-methyl-10,11-dihydro-5H-dibenzo [a, d] cyclohepten-5-one (12) with carbazole to synthesize a bipolar host material 46 (Fig. 13)^[21]. 46 exhibits a highly distorted "butterfly-like" conformation that can either suppress tight intermolecular π-π interactions, to reduce quenching of excitons, or suppress larger intermolecular flips, to reduce nonradiative decay. In addition, the methyl group on the heptagonal skeleton leads to significant steric hindrance, shielding the electron cloud around the oxygen atom and thus disturbing the intermolecular hydrogen bonding interaction. 46 has good morphological stability and hole-electron transport characteristics, and has a triplet energy as high as 3. 0 eV, which indicates that it is very suitable for the preparation of host materials for blue, green and red phosphorescent OLEDs, with the corresponding maximum EQE of 17. 8%, 27.0% and 28. 1%, respectively, and a low efficiency roll-off at high brightness.

In order to reduce the turn-on voltage of phosphorescent OLEDs, the mainstream strategy is to use doped transport layers, design better transport materials, and use mixed hosts in the light-emitting layer^[78][79][80]. The acridone (6) containing an electron-withdrawing carbonyl group has better electron injection as well as transport ability. The bipolar material 47 developed by Dileep et al. Is obtained by carbon-nitrogen coupling of 4,4 '-bis (9-carbazol-9-yl) biphenyl with acridone (fig. 14)^[81][81]. The large steric hindrance between biphenyl and acridone in the middle of the molecule leads to the distortion of the geometric conjugation structure of the molecule. Therefore, the 47 molecule not only has higher carrier transport properties, but also retains the desired larger band gap. When 47 was used as the host of green phosphorescent dopant, the turn-on voltage of OLED devices was significantly reduced, and the maximum EQE was as high as 16.9%.

It is reported that fluorene has higher ambipolar carrier mobility, while carbazole has lower hole injection barrier^[82]. Wong et al. Synthesized a phosphorescent OLED host 48 by linking fluorene and carbazole in a non-conjugated hybrid manner (fig. 15)^[83]. 48 has a high T₁ energy level, excellent thermal stability, and membrane morphology stability. The two phosphorescent OLEDs with 48 as the host have very low turn-on voltage of 2. 0 ~ 2.5 V. The green device has a maximum emission peak at 520 nm with a maximum EQE of 10. 5%, and the red device has a maximum emission peak at 630 nm with a maximum EQE of 9%.

Spirofluorene also has bipolar carrier transport characteristics, high thermal stability and fluorescence quantum efficiency^[84]. Jiang et al. Reported an ortho-linked spirofluorene host material 49 (Fig. 16), and first proposed a more convenient strategy for the synthesis of the 49 intermediate 2-bromo-9,9 ′ -spirofluorene^[84]. From the old synthetic route as shown in Fig. 16, it can be found that 4-bromo-9,9 '-spirofluorene is prepared by 4-bromo-9-fluorenone and 2-bromobiphenyl, while the synthetic route of the key intermediate 4-bromo-9-fluorenone is longer^[84,85].

In the new synthetic route, 2,2 '-dibromobiphenyl was substituted by n-butyllithium and then reacted with fluorenone to give 4-bromo-9,9' -spirofluorene through intramolecular ring-closure by Friedel-Crafts reaction. However, the T₁ energy level of 49 is as high as 2.55 eV, which is very suitable for the host of green and red phosphorescent dyes. They developed a green phosphorescent OLED with a maximum EQE of 12.6% and a red phosphorescent OLED with a maximum EQE of 10.5% using Tris (2-phenylpyridine) iridium and bis (2- (3,5-dimethylphenyl) quinoline-C2, N) (acetylacetonate) iridium (III) as dopants, respectively. Spiro [fluorene-9,9 '-xanthene] (21) has good carrier transport properties and is often used as a host for highly efficient blue, green, and red phosphorescent OLEDs^[86,87]. Zhao et al. Studied in detail the effect of diphenylphosphine oxygen at different substitution positions of 21 on its chemical and photophysical properties^[88]. The electron-withdrawing diphenylphosphine oxide can not only separate the host 50 HOMO and LUMO, but also improve the T₁ energy level and the carrier injection and transport ability^[88,89]. The green OLED fabricated with 50 host has smaller turn-on voltage and efficiency roll-off than its blue counterpart. Chi et al. First reported a highly efficient host 51 with 6,12-dihydroindeno [1,2-b] fluorene (15) as a core for phosphorescent OLEDs in 2009^[90]. The rigid planar 15 not only helps to expand the conjugation degree of the material and improve the fluorescence quantum efficiency, but also 51 shows high T₁ energy level, thermal stability and hole mobility, which makes it suitable for the host of green phosphorescent OLEDs. The device achieves ultra-high brightness of 10000 cd/m² at 5 V, the maximum brightness reaches 255000 cd/m² at 9.5 V, and the maximum EQE of the device is 15.8%.

Thermally activated delayed fluorescence (TADF) materials are the third generation of luminescent materials after the first generation of fluorescent materials and the second generation of noble metal complex phosphorescent materials. In the luminescence process of TADF materials, the Thermally activated delayed fluorescence exciton transitions to the S₁ energy level through RISC, and nearly 100% internal quantum efficiency fluorescence emission is achieved^[91⇓~93]. So far, the EQE of OLED luminescence of blue and green TADF materials exceeds 37%, and the EQE of red light is close to 30%^{[94⇓⇓~97]}. In order to ensure the high RISC efficiency of TADF materials in the luminescence process, the molecules are usually required to have a small enough difference between the S₁ and T₁ levels (Energy gap between S₁and T₁,ΔE_ST), and the effective separation of HOMO and LUMO levels is the key to achieve a small ΔE_ST and an efficient RISC process. Therefore, the design of effective TADF materials usually requires the construction of highly distorted charge transfer structures to achieve an effective separation of energy levels^[98]. According to the El-Sayed rule, some aromatic compounds have weak spin-orbit coupling and lower ISC rates due to the slow conversion between ¹(π,π^*) and ³(π,π^*). Through chemical modification, carbonyl or other fragments containing O, N, S, P and other heteroatoms can be introduced, and the abundant lone pair electrons endow the material with the characteristics of S₁ (n,π^*) and maintain the characteristics of T₁ (π,π^*). The enhancement of the n-π^* transition by lone-pair electron-π interaction can significantly enhance the spin-orbit coupling between S₁ and T₁ and promote the RISC process of TADF. This helps to effectively reduce the efficiency roll-off of TADF OLEDs while guaranteeing high EQE. In addition, pure organic TADF materials can avoid the use of rare and precious metals necessary for traditional metal organic phosphorescent complexes, thus reducing the cost. Therefore, heteroatoms containing lone pairs of electrons have great advantages in the construction of high-performance TADF materials. Benzophenone is often used as an important fragment for the synthesis of TADF materials because of its strong electron-withdrawing ability^[99,100]. The lone pair electron of carbonyl oxygen provides the n orbital to trigger the ISC from S₁ to T_n, which can realize the n-π^* transition, and at the same time, it can enhance the spin-orbit coupling of benzophenone, which is stronger than that of organic materials containing only hydrogen, carbon and nitrogen, and is beneficial to promote the RISC rate, even to the same order of magnitude as the corresponding ISC rate^[101,102]. Carbazole is a commonly used electron donor with high efficiency due to its good hole transport properties and high T₁ level. By introducing benzophenone into carbazole, the RISC rate is increased, and the materials usually show low ΔE_ST^[103]. In 2019, Kreiza et al. Introduced five carbazole groups on the benzophenone framework to obtain blue TADF material 52 (Fig. 17)^[104]. Due to the non-radiative quenching of the T₁ level in benzophenone, the non-radiative decay promoted by intramolecular rotation or vibration cannot be completely suppressed even in the solid state, and the effective RISC rate of TADF materials must exceed the non-radiative decay rate of T₁^{[103,105,106]}. Therefore, Kreiza et al. Proposed an effective molecular design method to suppress the excitonic quenching of benzophenone derivatives by nonradiative T₁ based on 52. 53 was synthesized by the reaction of methyl 2,3,4,5,6-pentafluorobenzoate with 3,6-di-tert-butyl-9H-carbazole. The replacement of benzophenone in 52 with methyl benzoate successfully reduced the nonradiative quenching rate of T₁ by nearly half, while the RISC efficiency remained essentially unchanged. The PLQY of the 53 doped bis (2-diphenylphosphinophenyl) ether film is close to 100%, while having a ΔE_ST as small as 0.08 eV. While only 28% of the corresponding 52 have a PLQY,ΔE_ST of 0.1 eV. The maximum EQE of the doped and non-doped devices of the 53 based sky blue OLED is 24.6% and 13.4%, respectively.

Throughout the reported TADF materials, compared with blue and green TADF materials, there are few kinds of orange and red TADF materials^[107,108]. This is mainly because the strong intramolecular charge transfer of red materials greatly promotes the non-radiative transition, and usually has a low radiative rate constant, which seriously affects the RISC and radiative transition process, so it is difficult to achieve orange and red TADF^[109][110]. Gan et al. Synthesized two isomeric TADF orange materials 54 and 55 by introducing acridine at the 3, 6 and 2, 7 positions of 9-fluorenone (11) (fig. 18)^[111]. The ΔE_ST of 54 and 55 in toluene at 77 K are 0.19 and 0.09 eV, respectively. The 55 based orange OLED device has a maximum EQE of 8.9% and good stability, with an efficiency roll-off of 15% at 100 cd/m² luminance.

Tang et al. Synthesized two green and blue TADF materials 56 and 57 by modifying the 3 and 6 positions of rigid planar xanthone (5) with acridine (Fig. 19)^{[39][100,112]}. The orthogonal configuration of the acceptor and the bulky donor not only facilitates the separation of HOMO and LUMO, but also effectively inhibits the close packing of molecules and reduces the intermolecular interaction in the aggregation state. The pure films of the two materials have small ΔE_ST of 0.025 and 0.024 eV, and have PLQY as high as 96% and 94%. The maximum EQE of pure light-emitting layer devices and doped layer devices based on these two materials is between 20% and 25%. Acridone (6) has similar chemical structure and photophysical properties to 5, and can be easily modified by various groups^[113⇓~115]. In 2019, Siddiqui et al. Substituted carbazole at the 2 and 7 positions of acridone respectively to prepare a green TADF material 58 with donor-acceptor-donor structure^[116]. It has a ΔE_ST of 0.17 eV and a PLQY of 44% in dichloromethane solution. Due to the strong electron-donating ability of carbazole, the delayed fluorescence lifetime of 58 in acetonitrile solution reached 446 μs. The electroluminescent spectrum of the OLED device based on 58 exhibits a peak emission at 490 nm with a brightness up to 1250 cd/m².

9,9 ′ -Spirofluorene (19) is a cornerstone for the construction of efficient blue fluorescent materials due to its high PLQY. In order to achieve a small Δ {E_{\mathrm{S}\mathrm{T}}}^{[117~119]}, Liang et al. Introduced 19 to the No.3 position of 9- (4- (4,6-diphenyl-1,3,5-triazin-2-yl) phenyl) -9H-carbazole (59 without spirofluorene) in the construction of blue TADF materials, which enhanced the delocalization of HOMO orbital, extended the electron distribution of HOMO orbital to the fluorene unit, and effectively reduced the ΔE_ST^[120][121]. Compared with the material 9- (4- (4,6-diphenyl-1,3,5-triazin-2-yl) phenyl) -9H-carbazole used in Device 1 reported by Chen et al., spirofluorene improved the thermal stability of the material, and the ΔE_ST of 59 in toluene at 77 K was reduced from 0.43 eV to 0.38 eV, the PLQY of the film doped with bis- (2- (diphenylphosphino) phenyl) ether was improved from 58% to 73%, and the OLED based on the blue TADF material of 59 (Device 2^[121].

As described previously, the construction of TADF materials by introducing fluorene has gradually attracted the interest of researchers. In 2021, Yi et al. Synthesized a pure blue-light TADF material 60 from 4,5-dicyanoimidazole and 10H-spiro (acridine-9,9 ′ -fluorene) (Fig. 21)^[122]. Dicyanoimidazoles with short conjugation length have high rigidity, high electron mobility, high luminescence efficiency, suppress intermolecular π-π interactions, and provide relatively low LUMO levels. 10H-Spiro [acridine-9,9 '-fluorene] (23) as the electron donor and electron acceptor groups are nearly orthogonal, the ΔE_ST of the material is reduced to 0.19 eV, and the fluorescence lifetime of nearly microseconds also indicates the TADF property. The PLQY of the material reaches 73%, and the maximum EQE of the 60-based OLED device reaches 13.8%. According to the Dexter energy transfer model, the long-lived T₁ exciton involved in the intermolecular electron exchange interaction plays a dominant role in the whole quenching process of TADF in thin films. In order to solve the problem of efficiency roll-off caused by quenching associated with T₁ in OLED devices at high current density and high brightness, Wu et al. Developed a donor-acceptor green TADF material 61 (fig. 21) by combining 23 with 10H-spiro [anthracene-9,9 ′ -fluorene] -10-one (25) in the same year^[36]. The benzophenone in 2-bromo-10H-pyran (anthracene-9,9 '-fluorene) -10one can effectively separate the HOMO and LUMO by inserting a rigid fluorenyl structure, which reduces the intermolecular interaction and suppresses the intermolecular interaction, and has a small ΔE_ST of 0. 03 eV. The delayed fluorescence lifetime of 1.3 μs demonstrates the TADF property, and such a short delayed lifetime is beneficial to suppress the device efficiency drop by accelerating the conversion of T₁ excitons into delayed fluorescence. The PLQY of the material 61 film reaches 99%, corresponding to a maximum EQE of 21.3% for the OLED, and the EQE is still maintained at 20.9% at a luminance of 5000 cd/m². In 2019, Fan et al. used TADF material 10,10 '- (sulfonylbis (4,1-phenylene)) bis (10H-benzoxazine) as a reference (62 without fluorene group), and used 2', 7 '-dibromospiro (fluorene-9,9' -thioxanthene) -10 ', 10' -sulfone and 10H-benzoxazine to couple to form blue light TADF material 62. By introducing the fluorene group, the intermolecular steric hindrance was significantly improved, which helped to inhibit the strong π between^{[35][123][124,125]}. The ΔE_ST decreases from 0. 081 eV to 0. 013 eV, and the PLQY increases from 32. 3% to 60. 3%. Meanwhile, 62 exhibits a low doping concentration dependence in OLEDs, maintaining an EQE above 20.8% over a wide doping concentration range.

Space charge transfer is also one of the effective strategies to develop TADF materials^[126⇓~128]. Song et al. Immobilized benzophenone on spirofluorene and introduced a variety of triphenylamine electron donors to fluorene to synthesize a series of sterically hindered TADF materials 63 ~ 67 (Fig. 22)^[128]. In their crystals, the dihedral angles of the fixed benzophenone and 1,3,6,8-tetramethyl-9H-carbazole, 9,9-dimethyl-9,10-dihydroacridine, diphenylamine, triphenylamine, benzo [5,6] [1,4] oxazine [4-yl] phenoxazine are in the range of 9.8 ° ~ 28.9 °, indicating that they are arranged face to face and have strong π-π stacking, which is beneficial to spin-orbit coupling and promotes charge transfer and local excited States. OLEDs based on 63 ~ 67 emit sky blue to yellow light at 494, 503, 507, 527 and 550 nm, respectively, with a maximum EQE of 20.9%.

In order to further improve the photoelectric performance of the small molecule material, the film forming quality can also be improved^{[129][130,131]}. Jing et al. First synthesized TADF small molecule 68 with benzophenone as the acceptor and 9,9 '-dimethylacridine as the donor, and then synthesized TADF small molecule 69 by introducing stereotriptycene to 68 (Fig. 23)^[132]. The unique three-dimensional rigid structure of triptycene makes it easy to form uniform, stable and dense films. 69 solid exhibits higher 5% mass heat loss temperature and glass transition temperature than 68, which indicates higher thermal and film morphology stability of 69^[133]. Interestingly, 68 did not show mechanical force-induced solid color change behavior, while 69 showed obvious mechanical force-induced solid color change behavior. The blue-green and green emission peaks of the doped OLEDs prepared with 68 and 69 as guests are 499 and 510 nm, respectively, and the doped OLEDs show high color purity in a wide operating voltage range. At the same time, the performance of the doped OLED with 69 as the guest is better than that of the doped OLED with 68 as the guest, and the former has a maximum EQE of 21. 2% and a lower efficiency roll-off. A similar trend is also exhibited in their undoped OLED devices.

The performance of OLED based on small molecule TADF materials has made great progress, but the practicality and high cost of vacuum evaporation of small molecules limit its further development^[134,135]. In contrast, TADF polymers with good film-forming properties can be used to fabricate low-cost and large-area devices by spin coating or inkjet printing. Polymeric TADF materials 71 ~ 73 reported by Philipps et al. Have monomers 70 of the acridine-benzophenone framework as the backbone TADF chromophore and are connected by conjugated biphenyl and non-conjugated alkyl chain bridging groups^[136]. The results show that the TADF properties of monomer 70 are perfectly retained on polymers 71 ~ 73 (Fig. 24), and the device based on alkyl chain-bridged TADF polymers 71 ~ 73 as the doping host exhibits an EQE of about 12%.

The luminescence of most TADF polymers showed significant concentration dependence including Aggregation-induced quenching or aggregation-induced emission (AIE). Aggregation-induced quenching TADF polymers can achieve better device performance only by physical doping. The luminescence of AIE TADF polymers increases with the increase of concentration, which makes their undoped device performance better^{[137][138,139]}. In addition, a few TADF polymers with weak concentration dependence of luminescence properties have a wide range of doping concentration control, which can be used in both doped and undoped devices. Li et al. Used a small molecule TADF material 74 as a comparison object, and coupled it with 1,2,4,5-tetramethylbenzene along the unilateral and bilateral directions of 74 acridine to obtain polymers 75 and 76, respectively (fig. 25)^[138]. The spin density distribution calculation shows that the exciton is distributed over the entire 74 molecule. For polymer 75, the excitons are distributed over the polymer branches, which leads to the presence of packing of segments 74 between adjacent polymers, severely quenching the excitons and thus producing a concentration dependence of luminescence. In contrast, the excitons of polymer 76 are distributed along the polymer backbone, which can suppress unnecessary exciton quenching, and the luminescence of polymer 76 is not concentration-dependent. The EQE of doped and undoped devices based on polymer 76 is as high as 22.7% and 19.2%, respectively.

Most organic luminescent materials have strong luminescence in dilute solution, but there is a significant decrease in luminescence in concentrated solution or in the aggregated state, which is due to the quenching effect caused by the strong π-π stacking in the aggregated state^[140⇓~142]. Tang et al. First discovered the AIE activity of polyphenyl-substituted silasilole derivatives^[143]. The AIE molecule with mononuclear polyphenyl substitution mode has weak luminescence in solution because multiple benzene rings can rotate freely around a single bond, resulting in the loss of most of the excited state energy in the form of non-radiative transition. In the aggregation state, due to the intermolecular and hydrophobic interactions, which limit the intramolecular rotation, more energy is emitted in the form of radiative transition^[143]. In addition to rotation as a way to dissipate excited state energy, some AIE materials without rotatable groups dissipate excited state energy in a non-radiative manner through vibration^[144]. At present, the widely accepted working mechanism of AIE effect is intramolecular motion restriction mechanism, including intramolecular rotation restriction and intramolecular vibration restriction, which has guided the design and synthesis of many new AIE materials^[144]. It can be seen from the above that the design of AIE materials with excellent performance requires consideration of the group rotatability in the material^[145]. For example, the two chromophores in the material are connected by a rotating C-C single bond, and the electron delocalization makes the C-C single bond have a certain degree of double bond characteristics, which makes the material more resistant to intramolecular rotation. The chromophore, while probably still rotating with a small amplitude, has a small low-frequency motion that is not sufficient to quench the fluorescence process. Therefore, the high-frequency vibration becomes the dominant mode of molecular motion, and the potential energy surface becomes steeper. Because of the conformational rigidity, the overall reorganization energy of the chromophore is small, which makes it have higher luminescence intensity in the aggregated state. In addition, intermolecular interactions are also worthy of attention. Intermolecular interactions such as hydrogen bonds, C — H … π bonds and halogen bonds can affect the molecular conformation, and changes in the external environment such as increasing viscosity, decreasing temperature and increasing pressure can lead to changes in the luminescent properties of materials^{[146][147][148][149,150]}.

Benzophenone is an ideal fragment for designing AIE molecules because the benzene ring in benzophenone is easy to rotate freely. Xie et al. obtained AIE material 77 (Fig. 26) by asymmetric substitution of phenothiazine and carbazole at the 4 and 4 ′ positions of benzophenone, which showed typical AIE behavior in the blue light region of 456 nm and the yellow light region of 554 nm^[124]. And 77 achieved white light in the solid powder state, with yellow light from phenothiazine and blue light from carbazole. Similarly, in order to inhibit the fluorescence quenching of pyrene in the aggregate state, Guo et al. Introduced benzophenone at the 2 and 7 positions of pyrene through an oxygen bridge to obtain 78, which emitted weak dark blue fluorescence in tetrahydrofuran solvent with a maximum emission peak at 398 nm^[151]. With the addition of water, the emission peak at 398 nm decreased, and a new emission peak at 436 nm gradually increased.

The propeller-like configuration of tetraphenylethylene can effectively prevent π-π stacking, and multiple C-H … π hydrogen bonds can be formed between the hydrogen atoms of the benzene ring of the tetraphenylethylene molecule and the π electrons of the benzene ring of the adjacent molecule. These intermolecular hydrogen bonds can further stabilize the molecular conformation to suppress the non-radiative transition, making tetraphenylethylene AIE active^[152]. Chromophores with special luminescence characteristics are introduced to tetraphenylethylene. Due to the expansion of molecular conjugation, the electroluminescence spectrum of tetraphenylethylene derivatives with chromophores will shift from dark blue to green in most cases, so efficient blue AIE materials are very scarce. Huang et al. Successfully synthesized blue AIE material 79 (Fig. 27) by connecting tetraphenylethylene to 2,5-diphenyl-1,3,4-oxadiazole nucleus and properly adjusting the balance between π conjugation length and emission wavelength, using the characteristics of crystallization-induced AIE molecular luminescence blue shift^[153]. The luminescence intensity of 79 in a mixed solvent with 99% water by volume is 120 times that of pure tetrahydrofuran solution. Similarly, Miao et al. Synthesized AIE material 80 by using the characteristic that bromine atoms can form halogen bonds. 80 hardly emits light in pure tetrahydrofuran solution, but emits strong blue light in the crystal, and the fluorescence quantum yield of the powder is 93%^[148]. Duan et al. Synthesized fluorenone derivative 81 by coupling 2,7-diiodo-9-fluorenone with 4-biphenylboronic acid at both ends (fig. 28)^[154]. The carbonyl group can promote the formation of hydrogen bonds between molecules in a non-centrosymmetric zigzag C — H … π interaction.81 The rotation restriction of the self-assembly leads to a significant linear enhancement of the luminescence, and the maximum luminescence intensity of the mixed solution of chloroform and heptane is 6 times that of the pure chloroform solution, and the luminescence spectrum has a blue shift of 34 nm relative to the chloroform solution. The fluorescence quantum yield increased from 3.9% to 35.8% after precipitation from solution.

Spirofluorene is also a star fragment for the design and synthesis of AIE molecules because of its three-dimensional structure in space, which can inhibit non-radiative transitions in the aggregation state. Spiro [fluoren-9,9 '-xanthene] (21) is one of the best. The oxygen atom of 21 promotes the formation of intermolecular hydrogen bonds to suppress the non-radiative transition in its aggregation state. Wang et al. Linked two indole groups at the 2,7 positions of 21, then reacted with CH₃I, and finally condensed with 2-hydroxy-5-nitrobenzaldehyde to obtain the derivative 82 of 21 (Fig. 29), which successfully realized the reversible conversion of blue light and red light under light and heat stimulation^[155]. In the mixed solvent of dimethyl sulfoxide and water with 25% water content, the blue light at 461 nm is enhanced by 10 times.

In recent years, pure organic Room temperature phosphorescence (RTP) materials have attracted more and more attention in information display, data storage, information encryption and biological imaging due to their long luminescence lifetime, large Stokes shift, convenient synthesis and low cost^{[156,157][158][159][160][161]}. The T₁ exciton, which is largely required for long-lived RTP, can be supplemented by the S₁ exciton through the ISC process, which can be achieved by spin-orbit coupling^[162,163]. A small ΔE_ST favors ISC. In terms of electronic configuration, EI-Sayed's study shows that mixing S_n and T_n with different electronic configurations such as (π,π^*) and (n,π^*) can promote spin-orbit coupling^[164]. In addition, heavy atom effects such as bromine or iodine atoms are also used to promote S₁ to T₁ conversion. Secondly, the radiative transition rate of T₁ should be greater than the non-radiative transition rate. Non-radiative transitions can be divided into intramolecular losses and external losses due to interaction with the environment^[163]. Therefore, suppressing non-radiative transitions is also one of the important strategies to achieve efficient RTP. Up to now, strategies such as host-guest doping, heavy atom effect, crystallization, H-aggregation have been successfully developed in order to enhance the ISC rate and suppress non-radiative transitions^{[165][166][167][168]}. Kearns et al. And Hoshino et al. Have shown that benzophenone with carbonyl as the bridging group is a better RTP material^[169][156]. Subsequently, Tang et al. Found that benzophenone and its derivatives 83 ~ 85 have obvious crystal-induced phosphorescence emission^[5]. Benzophenone derivatives do not exhibit phosphorescence in dilute solution or in amorphous state at room temperature; But at low temperatures, their single crystals all exhibit strong RTP. In the crystal state, various types of hydrogen bonds are formed inside the molecule, and the rigidity of the molecular structure is enhanced, so the intramolecular motion is restricted, the non-radiative transition is inhibited, and the phosphorescence emission is enhanced. Shimizu et al. prepared a benzophenone-based green RTP material 86 (Fig. 30) by introducing silane oxygen groups without the help of heavy atoms such as noble metals and halogens. The PLQY of the crystal at 300 K is as high as 64%, and the lifetime is as long as 98.3 ms^{[170][171,172]}. The hydrogen bond network between oxygen and various hydrogens effectively reduces the intramolecular motion. Using density functional theory calculations, it is found that the σ orbital on Si-C and the non-bonding orbital on Si-O form σ-n conjugation, which stabilizes the exciton on T₁, which is essential for the generation of RTP. Zhao et al. Combined benzofuran and benzothiophene with benzophenone and reported dibenzo [B, d] furan-2-yl (phenyl) methanone (3) and 87^[173]. The phosphorescence lifetime of 3 crystal is 232 ms at room temperature, and the phosphorescence quantum efficiency is as high as 34. 5%, showing a green afterglow. The phosphorescence lifetime of 87 in cyclohexane solution at 77 K is 110 ms, and the phosphorescence quantum efficiency is 6.5%, so it emits orange-red afterglow, and higher temperature achieves faster ISC rate and longer phosphorescence lifetime. This indicates that molecules with (n,π^*) and (π,π^*) hybrid structures, can effectively tune the excited state of the T₁ exciton.

Liu et al. Introduced two boronic acid units into the para position of tetraphenylethylene to obtain material 88, and studied its AIE activity, but did not notice its weak RTP characteristics^[174]. Later, Tian et al. First reported the RTP properties of 88, and used click chemistry to embed 88 into polyvinyl alcohol matrix^{[175][176,177]}. The RTP of 88 is further promoted by the suppression of 88 molecular rotation and non-radiative transitions by polyvinyl alcohol. With the increase of 88 content in the polymer, the RTP intensity of 89 increases continuously, and when the 88 content is 0. 08 mg, the RTP intensity reaches the strongest. Due to the advantages of simple operation, high efficiency and strong preparation scalability, this method of developing efficient RTP by chemical doping provides the possibility of constructing polymer RTP materials^[178].

Although many teams have studied the effect of solid condensed state structure on RTP performance from the perspective of controlling the arrangement of substituents in RTP solid or aiming at different crystal forms of materials,However, due to the complexity and diversity of the chemical and crystal structures of materials, there is no systematic study to reveal this relationship, and it is urgent to develop efficient RTP materials by forming aggregates with specific structures^{[179,180][181,182][183]}. In 2021, Hamzehpoor et al. Synthesized compounds 90 – 93 (Fig. 32) by using acridone (6) as the basic unit and introducing substituents with different molecular sizes and electronic effects on the nitrogen atom^[184]. The phosphorescence lifetime is as long as 81.5 ms for 90 solid powder, as low as 5.2 ms for 91 solid powder, 48.0 ms for 92 solid powder, and 15.7 ms for 93 solid powder. Depending on the size of the substituents, the solid-state packing mode gradually changes between co-parallel and anti-parallel π-π stacking, and changing the size of the substituents can suppress T₁ exciton quenching and thus achieve more efficient RTP^[179,185]. Wen et al. Synthesized a series of halogenated derivatives 94 ~ 97 based on 9-thioxanthone (7), all of which showed significant solid state aggregated RTP luminescence^[186]. In the solid powder state, 7 appears white, 94 is blue, 95 is orange, 96 is yellow, and 97 is dark blue. The orange RTP of 95 is very strong, and the PLQY is as high as 74.7%. The reason for this phenomenon is that the one-dimensional strong π-π stacking significantly enhances the ISC and phosphorescence radiation rate. It is worth noting that a monomolecular white luminescent film was prepared by doping the chlorinated derivative into the polymethyl methacrylate film and accurately controlling the doping ratio, which realized the precise control of cold white light to warm white light. This white luminescence contains blue fluorescence of the monomer, green RTP of the monomer, and orange RTP of the aggregate.

Although common organic light-emitting materials have unstable T₁ excited States, which are easily quenched by molecular vibrations or collisions with other media, there are still some literatures reporting efficient RTP in organic solvents or dilute aqueous solutions^{[187⇓⇓~190]}. In 2022, Zhang et al. Reported two brominated derivatives of fluorene, 98 and 99 (Fig. 33). The rigid distorted structure and large steric hindrance of the 9,9 ′ -diphenyl group can suppress π-π stacking^[191]. In addition, the heavy atom effect of bromine increases the spin-orbit coupling and promotes the ISC process. Both materials can exhibit RTP luminescence in deoxygenated organic solvent and polymer doped film States. Among them, 98 not only emits blue and green RTP in the deoxygenated organic solvent and thin film state, but also emits yellow RTP in the crystal state, and its RTP phosphorescence lifetime is between microseconds and milliseconds (fig. 33).

Benzophenone has been widely used in the synthesis of various multifunctional organic light-emitting materials due to its strong electron-withdrawing ability and chemical modification. In recent years, various chemical synthesis methods have been proposed to develop multifunctional organic light-emitting materials based on the benzophenone molecular framework, including the introduction of various groups using substitution, the modification of the benzene ring using heteroatoms or other groups as bridging groups, and the coupling using C = C double bonds or direct coupling at the 6 and 6 ′ positions of benzophenone. The direct introduction of various groups on the benzophenone skeleton, such as combining benzophenone with different electron donors, can adjust its band gap, suppress the exciton quenching of T₁, and thus produce luminescence at different wavelengths. The use of heteroatoms such as O, S, NH, SO₂, and CO as bridging groups can further enrich the molecular skeleton of benzophenone derivatives. In addition to the above two methods, other modification methods are centered on the benzophenone-coupled derivative, which is conducive to increasing its conjugation to inhibit intermolecular π-π stacking and other interactions, not only to adjust its molecular orbital energy level, improve PLQY, but also to achieve the purpose of improving the injection and transport capacity of carriers. Based on this, various multifunctional organic light-emitting materials based on benzophenone framework have been developed, which have been widely used in the fields of ordinary fluorescent materials, metal complex phosphorescent hosts, TADF materials, AIE materials and RTP materials. To sum up, benzophenone has been proved to be an important multifunctional group in both chemical structure and condensed state design. However, from the current research progress, its regularity is not strong, and more material systems need to be studied to summarize its regularity, so as to guide the synthesis of new multifunctional organic light-emitting materials. From the point of view of luminescence mechanism, there are still some deficiencies in the single molecule integration of fluorescence, metal organic complex phosphorescent host, TADF, AIE and RTP. In addition, the application of these multifunctional organic light-emitting materials in sensing, bioimaging and OLED devices is still in its infancy, and needs to be strengthened and expanded in the future. Finally, artificial intelligence can also be used to guide the screening of multifunctional organic light-emitting materials based on benzophenone skeleton to save research costs^[192][193]. It is believed that with the solution of the above problems, the multifunctional organic light-emitting materials based on benzophenone skeleton will have a brighter future.