A Review of Benzophenone-Based Derivatives for Organic Light-Emitting Diodes

Organic light-emitting diodes (OLEDs) have garnered considerable attention in academic and industrial circles due to their potential applications in flat-panel displays and solid-state lighting technologies, leveraging the advantages offered by organic electroactive derivatives over their inorganic counterparts. The thin and flexible design of OLEDs enables the development of innovative lighting solutions, facilitating the creation of customizable and contoured lighting panels. Among the diverse electroactive components employed in the molecular design of OLED materials, the benzophenone core has attracted much attention as a fragment for the synthesis of organic semiconductors. On the other hand, benzophenone also functions as a classical phosphor with high intersystem crossing efficiency. This characteristic makes it a compelling candidate for effective reverse intersystem crossing, with potential in leading to the development of thermally activated delayed fluorescent (TADF) emitters. These emitting materials witnessed a pronounced interest in recent years due to their incorporation in metal-free electroactive frameworks and the capability to convert triplet excitons into emissive singlet excitons through reverse intersystem crossing (RISC), consequently achieving exceptionally high external quantum efficiencies (EQEs). This review article comprehensively overviews the synthetic pathways, thermal characteristics, electrochemical behaviour, and photophysical properties of derivatives based on benzophenone. Furthermore, we explore their applications in OLED devices, both as host materials and emitters, shedding light on the promising opportunities that benzophenone-based compounds present in advancing OLED technology.

Originally, OLEDs were simple devices with a single emissive layer placed between a cathode and an anode, deposited on a transparent substrate-like glass.As interest in the technology grew, and the pursuit of higher efficiency intensified, additional layers were introduced to optimize the flow of electrons and holes to the emissive layer.This is where excitons form, leading to photon generation during the recombination process.In Figure 1, we illustrate a modern multi-layer OLED structure.Beginning with a transparent substrate, an anode is deposited, typically made of the low-resistance transparent material indium tin oxide (ITO).Subsequently, hole injection (HIL) and hole transporting (HTL) layers are added.For these layers, materials with high hole mobility and robust electron-donating properties are chosen.Following that, the emissive layer (EML) is introduced, composed either of the emitting material alone or doped within a host matrix.Materials in this layer commonly feature both electron-donating and electron-accepting groups, essential for an efficient recombination process.The primary purpose of the host material is to effectively transfer charge to the emitter while avoiding various emission-quenching mechanisms.OLED emitters are categorized into three generations: 1st-generation fluorescent, 2ndgeneration phosphorescent, and 3rd-generation thermally activated delayed fluorescence (TADF) phenomena featuring emitters.Moving forward, electron transporting (ETL) and electron injection (EIL) layers are applied, performing the opposite functions to HTL and HIL.These layers utilize materials with high electron mobility and robust electron-accepting properties.Finally, a cathode, often made of aluminium, is deposited.Presently, there is a diverse range of OLED structures, some incorporating even more layers (electron/hole blocking, multiple charge-transporting layers, etc.) than the example provided.This complexity aims to achieve well-aligned energy levels for each layer with its adjacent layers, thereby minimizing energy barriers for charge carrier transport and achieving more efficient OLED devices.
1, we illustrate a modern multi-layer OLED structure.Beginning with a transparent strate, an anode is deposited, typically made of the low-resistance transparent mat indium tin oxide (ITO).Subsequently, hole injection (HIL) and hole transporting (H layers are added.For these layers, materials with high hole mobility and robust elect donating properties are chosen.Following that, the emissive layer (EML) is introdu composed either of the emitting material alone or doped within a host matrix.Mate in this layer commonly feature both electron-donating and electron-accepting groups sential for an efficient recombination process.The primary purpose of the host mater to effectively transfer charge to the emitter while avoiding various emission-quenc mechanisms.OLED emitters are categorized into three generations: 1st-generation f rescent, 2nd-generation phosphorescent, and 3rd-generation thermally activated dela fluorescence (TADF) phenomena featuring emitters.Moving forward, electron transp ing (ETL) and electron injection (EIL) layers are applied, performing the opposite f tions to HTL and HIL.These layers utilize materials with high electron mobility and bust electron-accepting properties.Finally, a cathode, often made of aluminium, is de ited.Presently, there is a diverse range of OLED structures, some incorporating even m layers (electron/hole blocking, multiple charge-transporting layers, etc.) than the exam provided.This complexity aims to achieve well-aligned energy levels for each layer its adjacent layers, thereby minimizing energy barriers for charge carrier transport achieving more efficient OLED devices.In past decades, there has been a progressive transition from fluorescence-to p phorescence-based devices in the pursuit of higher efficiencies [23][24][25][26][27]. OLEDs utili conventional fluorescent emitters typically exhibit a maximum internal quantum ciency (IQE) of 25% [1].IQE can be elevated from 25% to 100% by harnessing triplet e tons through the use of intersystem singlet-to-triplet crossing in phosphorescent emi [28,29].Despite the high internal quantum efficiency and operational stability of emis iridium or platinum complexes [30][31][32][33], phosphorescent OLEDs (PhOLEDs) encou significant efficiency roll-off due to issues like aggregation-caused quenching and trip triplet and triplet-polaron annihilation [34][35][36][37].As a result, host-guest systems are c monly employed to disperse phosphorescent emitters into host matrices.To achieve h performance PhOLEDs, host materials must be ingeniously designed to adhere to fu mental principles such as good thermal stability and film formation quality [38][39][40][41][42][43] a sufficiently high triplet energy to prevent reverse energy transfer from emitter to host 46].Nevertheless, the persistent issue of poor stability of blue phosphorescent OLED mains a significant constraint [47,48].The presence of noble metals in phosphorescent terials raises an undeniable challenge to the future costs of devices and environme considerations [49,50].In recent years, there has been a notable focus on TADF emit In past decades, there has been a progressive transition from fluorescence-to phosphorescence-based devices in the pursuit of higher efficiencies [23][24][25][26][27]. OLEDs utilizing conventional fluorescent emitters typically exhibit a maximum internal quantum efficiency (IQE) of 25% [1].IQE can be elevated from 25% to 100% by harnessing triplet excitons through the use of intersystem singlet-to-triplet crossing in phosphorescent emitters [28,29].Despite the high internal quantum efficiency and operational stability of emissive iridium or platinum complexes [30][31][32][33], phosphorescent OLEDs (PhOLEDs) encounter significant efficiency rolloff due to issues like aggregation-caused quenching and triplet-triplet and triplet-polaron annihilation [34][35][36][37].As a result, host-guest systems are commonly employed to disperse phosphorescent emitters into host matrices.To achieve high-performance PhOLEDs, host materials must be ingeniously designed to adhere to fundamental principles such as good thermal stability and film formation quality [38][39][40][41][42][43] and a sufficiently high triplet energy to prevent reverse energy transfer from emitter to host [44][45][46].Nevertheless, the persistent issue of poor stability of blue phosphorescent OLEDs remains a significant constraint [47,48].The presence of noble metals in phosphorescent materials raises an undeniable challenge to the future costs of devices and environmental considerations [49,50].In recent years, there has been a notable focus on TADF emitters.This interest arises from their utilization in metal-free electroactive frameworks and their ability to upconvert full triplet excitons into emissive singlet excitons through reverse intersystem crossing (RISC), thereby achieving ultrahigh external quantum efficiencies (EQEs) [51][52][53][54][55][56][57].Despite the promise of TADF materials, many TADF OLEDs encounter same quenching and triplet-triplet (or singlet-triplet) annihilation problems as PhOLEDs, due to a long exciton lifetime [58][59][60].Consequently, to address the concentration quenching problem, many TADF molecules also need to be incorporated into suitable host matrices.However, because of a limited selection of TADF-specific host materials [61][62][63], researchers often utilize conventional host materials common to phosphorescent OLEDs [64].Therefore, one approach to overcoming efficiency challenges is the design of new host materials specifically tailored for TADF OLEDs.Another approach involves the development of new TADF molecules, especially for non-doped OLEDs based on materials exhibiting aggregation-induced emission (AIE) characteristics [65][66][67][68][69][70][71].
Units derived from benzophenone, shown in Figure 2, are renowned for their efficient intersystem crossing (ISC) capability, attributed to robust spin-orbit coupling [72].This characteristic renders them highly appealing as acceptor blocks for the fabrication of TADF emitters.Notably, benzophenone is recognized for its stability as an acceptor [73].Furthermore, benzophenone functions as a classical phosphor with a high intersystem crossing efficiency, potentially leading to effective reverse intersystem crossing with a small ∆E ST [74].The benzophenone framework not only serves as an electron-deficient core by integrating various donor units to create molecules with small ∆E ST and intramolecular charge transfer (CT) states [75], but it also features a highly twisted geometry, reducing intermolecular interactions and the self-quenching effect [76,77].Owing to the properties of benzophenone, most of its derivatives have found applications in emissive layers of OLEDs both as a host or as an emitter.
Units derived from benzophenone, shown in Figure 2, are renowned for th cient intersystem crossing (ISC) capability, attributed to robust spin-orbit coupli This characteristic renders them highly appealing as acceptor blocks for the fabric TADF emitters.Notably, benzophenone is recognized for its stability as an accep Furthermore, benzophenone functions as a classical phosphor with a high inte crossing efficiency, potentially leading to effective reverse intersystem crossing small ΔEST [74].The benzophenone framework not only serves as an electron-d core by integrating various donor units to create molecules with small ΔEST and i lecular charge transfer (CT) states [75], but it also features a highly twisted geom ducing intermolecular interactions and the self-quenching effect [76,77].Owing properties of benzophenone, most of its derivatives have found applications in e layers of OLEDs both as a host or as an emitter.In this review article, we examine the synthetic pathways, thermal charact electrochemical behaviour, photoelectrical, and photophysical properties of der based on benzophenone.Additionally, we explore their applications in OLED both as host materials as well as emitters.This article will systematically review device structures and their corresponding performances.This review is structu several sections based on the application and structure of benzophenone derivat cluding the following: host materials for phosphorescent emitters, host mater TADF emitters, donor-acceptor (D-A)-type emitters, donor-acceptor-donor (D type symmetric structure emitters, D-A-D-type asymmetric structure emitters, as emitters having dendritic structures.

Benzophenone-Based Bipolar Host Materials Used for TADF Emitters
Scheme 2 illustrates the configurations of benzophenone-based derivatives, employed as host materials in TADF OLED devices.Objective compounds HB1 and HB2 [88] were synthesized via a straightforward one-step Friedel-Crafts reaction utilizing readily available, cost-effective triphenylamine and isophthaloyl or terephthaloyl dichloride as initial reagents.All the remaining host materials HB3, HB4 [89], HB5 [90], HB6, HB7 [91], and HB8 [92] were obtained by simple nucleophilic substitution reactions between fluorinated benzophenone and corresponding amines.For application as TADF host materials, benzophenone was combined either with carbazole or phenyl-/naphtyl-amino fragments as electron donors.4,4 ′ -Difluorobenzophenone reactions with N-phenyl-1-naphtylamine, N-phenyl-2-naphtylamine, 3-naphthyl-9H-carbazole, 3-phenyl-9H-carbazole, and 3-(4-(9Hcarbazol-9-yl)phenyl)-9H-carbazole yielded compounds HB3, HB4, HB5, HB6, and HB7, respectively.Material HB8 was obtained during reaction between 2,3,4,5,6-pentafluorobenzophenone and 9H-carbazole.Table 4 presents the thermal, electrochemical, photoelectrical, and photoph properties of materials HB1 to HB8.During the investigation of thermal properties, noticed that most of the benzophenone-based compounds were resistant to heat with mal decomposition temperatures ranging from 277 °C to as high as 497 °C.Most same materials also possessed the ability to form stable amorphous films with glass sition temperatures ranging from 90 °C to 187 °C.At the same time, HB series benz none derivatives had optical bandgap energies of 2.70-4.10eV, singlet state energ levels of 2.37-3.07eV, and triplet state energies ranging from 2.32 eV to 2.64 eV.No carbazole-substituted derivatives HB5-HB8 demonstrated higher ET levels.Table 4 presents the thermal, electrochemical, photoelectrical, and photophysical properties of materials HB1 to HB8.During the investigation of thermal properties, it was noticed that most of the benzophenone-based compounds were resistant to heat with thermal decomposition temperatures ranging from 277 • C to as high as 497 • C. Most of the same materials also possessed the ability to form stable amorphous films with glass transition temperatures ranging from 90 • C to 187 • C. At the same time, HB series benzophenone derivatives had optical bandgap energies of 2.70-4.10eV, singlet state energy (E S ) levels of 2.37-3.07eV, and triplet state energies ranging from 2.32 eV to 2.64 eV.Notably, carbazole-substituted derivatives HB5-HB8 demonstrated higher E T levels.The configurations of devices employing host materials HB1 to HB8 are illustrated in Table 5.All the manufactured devices used an ITO anode.To lower the hole injection barrier, molybdenum (VI) oxide (MoO 3 ) was used as a hole injection layer (HIL) for devices DHB3 and DHB4.The most popular material of choice for the formation of hole-transporting layers was PEDOT:PSS.NPB, TAPC, or a cross-linkable molecule 3,6-bis(4-vinylphenyl)-9-ethylcarbazole (VPEC), were also employed for HTLs in some cases.Each device utilized one or two hole-transporting layers (HTLs).The chosen TADF emitters included orange 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzTPN), green 2,3,5,6-tetracarbazole-4-cyano-pyridine (4CzCNPy), and blue 10,10 ′ -(perfluoro-[1,10biphenyl]-4,4 ′ -diyl)bis(2,7-ditert-butyl-9,9-dimethyl-9,10-dihydro-acridine) (PFBP-2b).The electron-transporting layers utilized compounds such as TmPyPB, TPBi, and 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T).Finally, for all devices under investigation, a combination of LiF as the electron-injecting layer and an Al cathode was employed.Measured characteristics of TADF OLED devices employing host materials HB1-HB8 are shown in Table 6.Most of the HB materials were used to dope green TADF emitters, except benzophenone derivatives HB3 and HB5, which were used in combination with orange and sky blue dopants to achieve white emission.Hu and co-authors investigated electron-donating triphenylamino and electron-accepting phthaloyl moieties having compounds HB1 and HB2.When these materials were used as hosts for the green TADF emitter, compound HB1 prevailed as a more effective choice that could be explained by higher singlet and triplet energy levels which enhance energy transfer efficiency and effectively prevent reverse energy transfer from the green TADF emitter to the TADF host, resulting in a CE, PE, and EQE of 43.5 cd/A, 33.3 lm/W, and 13.0% for device DHB1, respectively.Materials HB3 and HB4 were investigated by Mahmoudi et al.They found that the best use of the synthesized materials would be as exciton modulators between two TADF emitters.By combining orange and blue TADF materials with the new benzophenone-based hosts, efficient white TADF OLEDs were introduced with the EQE for devices DHB3 and DHB4 reaching 9.5% and 7.1%, respectively.Device-utilizing compound HB3 showed higherquality white electroluminescence, which is defined by CIE coordinates of (0.32, 0.31), a colour temperature of 4490 K, and a colour-rendering index of 80.A team of researchers led by Swyamprabha synthesized and characterized 3-naphtyl-9H-carbazole-substituted derivative HB5.The green TADF OLED device showed a maximum CE, PE, and EQE of 9.5 cd/A, 8.4 lm/W and 2.8%, respectively.The same researchers continued their work and synthesized new materials HB6 and HB7 with different substituents at the carbazole core.At last, green solution-processable OLEDs were also fabricated with a cross-linkable hole transport material VPEC and realized PE of 63.6 lm/W with an EQE of 25.3% for device D2HB7, which was more effective than an analogical device with a lower E T having host material HB6.Wang et al. introduced a new penta-carbazole-substituted benzophenone derivative HB8, which has also been tested as a host material for the green TADF emitter.Device DHB8 displayed a yellowish-green emission, possessing CIE coordinates of (0.34, 0.58), aligning with the emission characteristics of 4CzCNPy [93].The OLED achieved a maximum current and external quantum efficiency, reaching 38.3 cd/A and 12.5%, respectively.
FOR PEER REVIEW 11 of 34 thioxanthene) was employed in Ullmann reactions with 4-bromobenzophenone and (6bromopyridin-3-yl)(phenyl)methanone, respectively.The phosphorus-containing emitter EA20 [109] was synthesized through a palladium-catalysed reaction between the intermediate compound 4-iodo-4′-phenothiazin-10yl-benzophenone and diphenylphosphine oxide in the presence of triethylamine.The thermal, electrochemical, photoelectrical, and photophysical properties of materials EA1-EA22 are provided in Table 7.All the tested materials exhibited good thermal stability, as verified by the TGA measurements conducted on the samples.The thermal decomposition temperatures for these materials ranged from 278 °C to 497 °C.DSC experiments showed that most of the tested materials were capable of forming stable amorphous layers with glass transition temperatures of 80-194 °C.By analysing the HOMO-LUMO gap (energy bandgap Eg), it was seen that most of the EA series derivatives demonstrated Eg levels of around 3 eV or slightly lower.Only materials EA4 and EA5 showed significantly higher Eg levels of around 4 eV, which might lead to less efficient TADF processes compared to emitters with smaller bandgaps [59].A crucial characteristic for achievement of the TADF effect is a small energy difference between the singlet and triplet The thermal, electrochemical, photoelectrical, and photophysical properties of materials EA1-EA22 are provided in Table 7.All the tested materials exhibited good thermal stability, as verified by the TGA measurements conducted on the samples.The thermal decomposition temperatures for these materials ranged from 278 • C to 497 • C. DSC experiments showed that most of the tested materials were capable of forming stable amorphous layers with glass transition temperatures of 80-194 • C. By analysing the HOMO-LUMO gap (energy bandgap E g ), it was seen that most of the EA series derivatives demonstrated E g levels of around 3 eV or slightly lower.Only materials EA4 and EA5 showed significantly higher E g levels of around 4 eV, which might lead to less efficient TADF processes compared to emitters with smaller bandgaps [59].A crucial characteristic for achievement of the TADF effect is a small energy difference between the singlet and triplet states (∆E ST ), which facilitates effective reverse intersystem crossing, allowing the conversion of triplet excitons to singlet excitons, which is essential for delayed fluorescence [59].This metric was measured for all benzophenone-based D-A emitters.From 22 derivatives described in this section, 17 of them demonstrated ∆E ST levels of 0.10 eV or lower.Higher singlet-triplet energy difference-having derivatives could be suffering from an ineffective TADF process, thus lowering the overall performance of the devices.A small ∆E ST is associated with a higher photoluminescence quantum yield (Φ PL ), as it facilitates efficient radiative decay from the triplet state to the ground state.We can compare materials EA1, EA2, and EA3 since experiments with them were executed under the same conditions.The biggest ∆E ST of 0.37 eV having derivative EA2 showed the lowest quantum yield (Φ PL ) of 20% in thin film.On the other hand, the narrowest singlet-triplet energy gap of just 0.02 eV was measured for compound EA3, which exhibited the highest Φ PL of 50%.This trend could be observed in other pairings of similarly structured materials that were characterized under the same conditions such as EA4 and EA5 as well as EA15 and EA21.Oxygen molecules can quench the triplet states involved in the TADF process [110].This quenching effect can lead to a decrease in the efficiency of delayed fluorescence and, consequently, reduced performance of TADF-based devices.Triplet state involvement in overall emission is proven by this way for benzophenone derivatives EA5, EA6, EA8, and EA11.In the cases of materials EA6, EA13, EA15, EA16, and EA21, prompt and delayed components of emissions were detected with the ratio of the delayed component in overall emission (R D ) ranging from 37.9% to 81.3%.All emitters described in this section, except fluorescent EA10, exhibited TADF properties.Architectures of devices utilizing emitters EA1-EA22 are displayed in Table 8.As it was mentioned in earlier sections, all of the devices formed with the D-A-type benzophenone emitters used an ITO anode and Al cathode except for EA20-based devices, which used cathodes made of a blend of magnesium and silver.To lower the hole injection barrier, hole-injecting material MoO 3 was used in some devices.Hole-transporting layers were made utilizing PEDOT:PSS, NPB, TAPC, HAT-CN, TCTA, 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi), mCP, 3,3 ′ -di(9H-carbazol-9-yl)-1,1 ′ -biphenyl (mCBP), 4,4 ′ -bis(9-carbazolyl)-1,1 ′ -biphenyl (CBP), or N,N ′ -bis(naphthalen-1-yl)-N,N ′bis(phenyl)-2,2 ′ -dimethylbenzidine (α-NPD).Doped and non-doped emissive layers (EMLs) were used during studies of the devices.In the case of doped devices, host materials bis [ 4′ -(9Hcarbazol-9-yl)-[1,10-biphenyl]-4-yl)ethene-1,1-diyl)bis(4,1-phenylene))bis(9H-carbazole) (2CzTPEPCz), was used in tandem with emissive material EA20.To balance the flow of electrons, TBPi, TmPyPB, DPEPO, and B3PyPB were applied for electron-transporting layers.In order to make a lower electron injection barrier, LiF, Cs 2 CO 3 , or Liq were used for EILs in some cases as it can be seen in Table 8.Various metrics of the OLEDs using EA derivatives are shown in Table 9. Green light emission of the D-A-type benzophenone derivatives was the most common result for 19 of the 28 single-emitter devices described in this section.There also were reported six blue, three yellow, and one white OLED prototype using materials of this group.The EQE exhibited by the devices ranged widely from 1.8% to 26.7%.The most effective yellow OLED device was achieved by Chen and colleagues and reached a maximum CE, PE, and EQE of 73.1 cd/A, 38.2 lm/W, and 26.7%, respectively [111].For all the emitters used in yellow OLED devices, the TADF effect was confirmed.Zhao et al. successfully applied the same emitter EA20 in the white TADF OLED prototype D2EA20 and achieved a high CE of 45.9 cd/A, PE of 18.0 lm/W, and EQE of 20.8% [112].Ho and co-workers synthesized and characterized triphenylethene-carbazole-substituted benzophenone derivative EA10.A big ∆E ST gap of 1.09 eV could not enable the TADF process in this case.The green OLED device with DEA10 achieved an EQE of 1.7% and high luminance of 11,802 cd/m 2 .Ma et al. tested three different benzophenone-carbazole materials EA1, EA2, and EA3 with different positions of triphenylamine moiety in the structures.The most efficient TADF emitter was compound EA3, which in a green device DEA3, reached an external quantum efficiency of 7.6%.Kreiza and colleagues also achieved good results utilizing a benzophenone-based D-A TADF emitter EA12.Non-doped green OLED device D1EA12 achieved a CE, PE, and EQE of 19.0 cd/A, 14.9 lm/W, and 10.3%.By doping the mentioned emitter in a DPEPO host, characteristics were enhanced by the authors with 34.9 cd/A, 27.3 lm/W, and 12.5% for device D2EA12.Ma and co-workers presented another very efficient green light-emitting sulfone group-enriched benzophenone TADF emitter EA18.Luminance of the doped device D2EA18 surpassed 30,000 cd/m 2 with an EQE reaching 20.6%.Other characteristics were also impressive with a CE reaching 64.6 cd/A and PE being 75.1 lm/W.Meanwhile, the similarly structured green TADF emitter EA17 obtained by the same researchers was less efficient in doped devices, which could be attributed to the absence of a sulfone group.However, it was impressive when applied in non-doped EMLs: device D1EA17 achieved a maximum PE, CE, and EQE of 53.7 cd/A, 52.7 lm/W, and 17.3%, respectively.In the realm of green light-emitting devices, the TADF emitter EA21, which had one of its benzophenone phenyl rings replaced with pyridine and was characterized by Wang and co-workers, demonstrated superior efficiency.Luminance of the doped device D2E21 surpassed 11,000 cd/m 2 , while its EQE was 25.6%.Other characteristics were also impressive with a CE and PE reaching 69.8 cd/A and 58.9 lm/W.The same green TADF emitter was also unrivalled when applied in non-doped EML.Device D1EA21 achieved a maximum PE, CE, and EQE of 56.4 cd/A, 43.5 lm/W, and 18.7%.These characteristics were considerably higher than those of devices D1EA15 and D2EA15, which used pyridine-unmodified green TADF emitter EA15.The benzophenone fragment was also successfully applied in the synthesis of blue TADF emitters.A group of researchers led by J. Wang successfully combined a benzophenone electron acceptor with the 11-phenyldihydroindolo[2,3-a]carbazole electron donor and obtained material EA6.When applied as a dopant in blue OLED DEA6, a maximum EQE of 17.7% and luminance of over 14,000 cd/m 2 were obtained.Other efficiencies such as the CE and PE were 44.8 cd/A and 45.6 lm/W, respectively.However, the most efficient benzophenone derivative used as a blue TADF emitter was EA19, characterized by J. Zhang and his group of scientists.A relatively simple structure of a benzophenone-acridine derivative was applied for a blue DEA19 device and reached a CE of 47.7 cd/A, PE of 29.9 lm/W, and EQE of 20.6%
azolium-catalysed reaction of 4,4′-dibromobenzophenone with 9,9-dimethyl-9,10-dihy droacridine.Two compounds EB17 and EB18 [88] were obtained through a simple one step Friedel-Crafts reaction by using commercially available cheap starting materials tri phenylamine (TPA) and, correspondingly, isophthaloyl dichloride for compound EB17 o terephthaloyl dichloride for emitter EB18.Table 10 depicts thermal, electrochemical, and photophysical properties of the mentioned light-emitting symmetrical benzophenone materials EB1-EB27.The destruction temperatures were reported for eighteen compounds described in this section.Even though EB13 had a lowest T D of 218 • C owing to two phenoxazine fragments, it was still high enough to cope with conditions of the device forming and operating, especially bearing in mind a high melting temperature of 118 • C for the compound.All the other presented emitters here were characterized by a T D of 270 • C or higher.Glass transition temperatures for derivatives EB1, EB2, EB7, EB17, EB18, EB19, EB26, and EB27 were registered at 90 • C or higher, so these emitters could form stable amorphous layers.Crystalline benzophenonebased materials EB4, EB9, and EB22 demonstrated melting temperatures of 300 • C or higher as it was confirmed by DSC.Examining the HOMO-LUMO gap (energy bandgap E g ), it becomes evident that the majority of the symmetrical D-A-D benzophenone derivatives exhibit measured or calculated E g levels in the region of 2.47-3.37 eV.Only EB9 presented a distinctly elevated E g level of 3.66 eV.This could be the reason of decreased quantum yield and less effective TADF processes, when compared to emitters possessing smaller bandgaps.Achieving a low ∆E ST is essential in the development of an emitter to enhance the efficiency of the TADF process, as it is evident from the data of Table 10, where materials EB11, EB14, EB23, EB24, and EB25 with ∆E ST values of 0.03 eV or lower demonstrate a Φ PL exceeding 70% in a nitrogen atmosphere.The participation of the triplet state in the emission process was also demonstrated for some compounds.For example, this was observed in EB6, where the overall Φ PL substantially decreased upon exposure to oxygen.In addition, in derivatives EB1, EB12, EB23, EB24, and EB25, substantial R D levels were identified.All emitters described in this section, except fluorescent EB2 and EB9, exhibited TADF properties.The majority of the D-A-D benzophenone-based derivatives were tested as emitters in OLEDs, whose structures are illustrated in Table 11.The dominant choice for anode in these devices was also ITO.To ease the energy barrier, such hole-injecting materials as Mo 2 O 3 , rhenium (VI) oxide (ReO 3 ), and MoO 3 were used.NPB, α-NPD, mCP, PEDOT:PSS, TAPC, HAT-CN, or TCTA were chosen for the formation of holetransporting layers.To further elevate the efficiency of the devices, well-known host materials diphenyl [4-(triphenylsilyl)phenyl]phosphine oxide (TSPO1), DPEPO, TCzl, 9-(3-(9Hcarbazol-9-yl)phenyl)-9H-carbazole-3-carbonitrile (mCPCN), 2,8-bis(diphenyl-phosphoryl)dibenzo[b,d]furan (PPF), mCP, mCBP, CBP, and some others were used in emissive layers.To achieve desired white-light OLEDs, some phosphorescent emitters such as Ir(ppy) 2 (acac) or Ir(bt) 2 (dipba) were used in conjunction with benzophenone derivative EB4.Electron transport layers were made from DPEPO, TPBi, B3PYMPM, TmPyPB, BPhen, PPF, or TSPO1.Also, LiF, Liq, and rubidium carbonate (Rb 2 CO 3 ) were used as electron-injecting materials.For all the devices described in this section, aluminium cathodes were used.This section reviews all devices constructed by researchers utilizing benzophenonebased emitters EB1-EB27 with a D-A-D structure, and the characteristics of these devices are detailed in Table 12.The combination of a central benzophenone electron-accepting group with various electron donors yielded red, yellow, green, and blue emitters.Utilization of the described compounds with phosphorescent emitters or other benzophenone-based materials also resulted in white light emission.Lee et al. reported the red-emitting material EB15.Device DEB15, which utilized the mentioned TADF emitter, exhibited a low turn-on voltage of 2.8 V, CE of 11.1 cd/A, and EQE of 4.2%.Additionally, maximum luminance of the device exceeded 50,000 cd/m 2 .The same authors introduced blue TADF emitters EB3 and EB5, which in devices achieved EQE values of 8.1% and 14.3%, respectively, with the latter one being the most efficient blue OLED in this D-A-D group.They also presented a green TADF emitter EB14, which attained an EQE of 10.7% in its device, and a yellow TADF emitter EB16, which reached an EQE of 6.9%.Liang and co-workers presented material EB4 which can act as a host and as an emitter at the same time.For example, device D1EB4 with non-doped emitter EB4 demonstrated a PE and EQE of 6.9 lm/W and 4.0%, respectively.By combining phosphorescent emitters Ir(ppy) 2 (acac) and Ir(bt) 2 (dipba) with the compound EB4, device D2EB4 achieved white emission and demonstrated an exceptionally high current efficiency of 48.6 cd/A and external quantum efficiency of 25.6%.By utilizing the symmetrical D-A-D structure, researchers developed nine green TADF emitters EB6, EB10, EB11, EB14, EB20, EB21, EB23, EB24, and EB25, which achieved over 10% external quantum efficiency.EB6, designed by Tani and colleagues, was used in an emissive layer of the TADF device DEB6 and achieved an EQE of 10.4%.Zhang and collaborators employed a straightforward structural derivative EB10 as a non-doped TADF emitter.The resultant device exhibited excellent PE and EQE values of 59.0 lm/W and 18.0%, respectively.Liu and colleagues created a structurally similar derivative EB11, where they replaced the acridine electron donor with a biacridine fragment.This modification led to a notable improvement in the EQE, reaching 22.5%, along with an impressive CE of 69.3 cd/A.Derivatives EB20 and EB21, which were characterized by Sharif and co-workers, also were highly efficient when applied in green TADF OLED devices.By combining phenoselenazine electron donors with benzophenone, researchers synthesized EB20 and fabricated the DEB20 device, which exhibited an exceptionally high EQE of 30.8% and a CE of 64.0 cd/A.In comparison, the DEB21 device, which employed derivative EB21 with an additional ketone group as the emitter, achieved a lower EQE of 18.8% but with a higher CE of 73.5 cd/A.The highest overall efficiencies of green TADF OLEDs by utilizing symmetrical D-A-D benzophenone-based emitters were achieved by Huang et al., who synthesized and characterized derivatives EB23, EB24, and EB25.These materials underwent testing in both non-doped and doped emissive layers.The most efficient nondoped device D1EB23 demonstrated exceptional performance with a maximal CE, PE, and EQE reaching 53.9 cd/A, 48.9 lm/W and 18.6%, respectively.In the doped device D2EB23, efficiencies were further elevated, achieving an impressive CE of 90.9 cd/A, PE of 91.2 lm/W, and EQE of 30.3%.If the device D2EB23 attained the highest CE and PE values among the green devices described in this section, the OLED D2EB24 with emitter EB24 doped in a host material remained unparalleled with an extraordinary high EQE level of 32.2%.Within the realm of blue TADF emitters described in this section, aside from the previously mentioned derivatives EB3 and EB5, notable efficiencies were also observed by using EB8 and EB22 emitters in TADF-based OLEDs.The compound EB8, presented by Cai and colleagues, achieved an EQE of 8.90% in the DEB8 device.Another team led by Sun developed and utilized the TADF emitter EB22 as crucial element in the construction of the DEB22 device, displaying a V ON of 3.6 V, an L MAX of 2021 cd/m 2 , and an EQE of 11.4%.The previously mentioned EB5 emitter proved to be the most effective blue TADF emitter in this section.When integrated into a device, it exhibited a CE of 25.5 cd/A, an EQE of 14.3%, and a maximum luminance of 3900 cd/m 2 .

Benzophenone-Based TADF Emitters Employing an Asymmetric D-A-D Structure
Structures of benzophenone-based TADF emitters having asymmetric D-A-D molecular configurations are shown in Scheme 5. Except for one material, all others utilized carbazol-9-yl as one of the electron-donating fragments in conjunction with a benzophenone acceptor, along with other donating moieties like 3-substituted carbazole, 9,10-dihydro-9,9dimethylacridine, spiro[acridine-9,9 ′ -fluorene], or phenoxazine.Only the derivative EC11 did not incorporate carbazole as one of the electron donors, but employed 9,10-dihydro-9,9dimethylacridine and thianthrene fragments.To obtain the derivatives presented in this section, at least two synthetic steps were required.Derivative EC1 [94] was synthesized through a two-step Buchwald-Hartwig amination procedure, involving the replacement of one bromine atom in 4,4 ′ -dibromobenzophenone with carbazole by following reaction with 4-(9H-carbazol-1-yl)-N,N ′ -diphenylaniline.Material EC2 [66] was obtained by reaction between (3,5-bis-carbazol-9-yl-phenyl)-(4-bromophenyl)-methanone and 9,10-dihydro-9,9dimethylacridine under Buchwald-Hartwig reaction conditions.Derivatives EC3 and EC4 [124] were created through the two-step synthesis process.Initially, the fluorine atom of 4-bromo-4 ′ -fluorobenzophenone was reacted with spiro[acridine-9,9 ′ -fluorene] using a nucleophilic substitution reaction.Subsequently, the bromine atom was replaced with carbazole or 3,6-di-tert-butylcarbazole during Buchwald-Hartwig reaction.Materials EC5-EC10 were synthesized using very similar procedures [125].In the initial step, 3,6-ditert-butyl-9H-carbazole underwent Buchwald-Hartwig reaction with (4-bromophenyl)(4fluorophenyl)methanone, (6-bromopyridin-3-yl)(4-fluorophenyl)methanone, or (5-bromopyridin-2-yl)(4-fluorophenyl)methanone, resulting in intermediate compounds.Subsequently, nucleophilic substitution reactions of 9,10-dihydro-9,9-dimethylacridine were employed with, correspondingly, (4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)(4-fluorophenyl)methanone, (6-(3,6-di-tert-butyl-9H-carbazol-9-yl)pyridin-3-yl)(4-fluorophenyl)methanone, and (5-(3,6-di-tert-butyl-9H-carbazol-9-yl)pyridin-2-yl)(4-fluorophenyl)methanone, to yield compounds EC5, EC6, and EC7, respectively.Utilizing the same reactions with phenoxazine instead of 9,10-dihydro-9,9-dimethylacridine resulted in compounds EC8, EC9, and EC10, respectively.The synthesis of derivative EC11 also involved a two-step process [126].Initially, one bromine atom of 4,4 ′ -dibromobenzophenone was replaced with a thianthrene moiety through a Suzuki reaction.Subsequently, the second bromine atom was substituted with 9,10-dihydro-9,9-dimethylacridine in a Buchwald-Hartwig cross-coupling reaction.Table 13 presents the thermal, electrochemical, and photophysical properties of materials EC1-EC11.The asymmetric D-A-D-type benzophenone derivatives exhibited exceptional thermal stability, with a TD ranging from 309 to 451 °C, as verified through TGA measurements.It is reported that for materials EC2 and EC11, during DSC experiments, Table 13 presents the thermal, electrochemical, and photophysical properties of materials EC1-EC11.The asymmetric D-A-D-type benzophenone derivatives exhibited exceptional thermal stability, with a T D ranging from 309 to 451 • C, as verified through TGA measurements.It is reported that for materials EC2 and EC11, during DSC experiments, T G values were registered at 72 • C and 104 • C, respectively.The HOMO levels of materials EC1-EC11 ranged from −5.92 to −5.23, while the LUMO levels varied between −3.09 and −2.61.The energy difference between HOMO and LUMO levels for all the materials discussed in this section was 2.88 eV or lower.This small bandgap facilitates a minimal energy difference between the lowest singlet and triplet states, enabling efficient reverse intersystem crossing.Experimental results support this, with the ∆E ST being 0.10 eV or lower, resulting in a high Φ PL ranging from 33.2% to 90.0%.EC3 to EC11 derivatives exhibited aggregation-induced emission properties with lower Φ PL values detected in solutions compared to film states.Additionally, materials EC1 and EC3 to EC10 demonstrated significant involvement of triplet states in photon generation, as evidenced by a R D ranging from 33.0% to 89.7% with one of the potential light-generating mechanisms being TADF.Every benzophenone derivative presented in this section underwent testing as an emitter in OLED devices, with their structures depicted in Table 14.Consistent with earlier sections, ITO was the only selection for the anode in these devices.To reduce the energy barrier for holes, the device DEC11 utilized MoO 3 as the hole-injecting material.For this device or others, a stack of one to three layers of hole-transporting materials was employed, featuring layers composed of PEDOT:PSS, HAT-CN, TAPC, TCTA, NPB, or mCP.While most devices using EC1-EC11 emitters employed non-doped configurations, some of them also utilized the host-guest approach.Specifically, the host material CBP was applied in device DEC2, and PPF was employed for D2EC3 and D2EC4.TPBi, TmPyPb, and PPF were employed as electron-transporting layers, while Cs 2 CO 3 and LiF served as electroninjecting materials.For all the devices discussed in this section, the Al cathode was the only option.All the recently presented derivatives EC1-EC11 underwent testing as emitters in devices and the characteristics of these OLEDs are detailed in Table 15.Although most of these devices displayed green emission, there were exceptions such as DEC8, which emitted yellow light, and DEC9 as well as DEC10, which emitted orange light.Ma and co-workers successfully synthesized and characterized an EC1 derivative, revealing noteworthy characteristics.The mentioned emitter was integrated into the non-doped emissive layer of device DEC1, which demonstrated a CE of 35.5 cd/A, PE of 22.3 lm/W, and EQE of 13.3%.Another derivative EC2, synthesized and characterized by Zhao et al., showcased impressive performance with CE, PE, and EQE values of 61.8 cd/A, 40.4 lm/W, and 19.7%, respectively, as well as with an impressive L MAX of 116,000 cd/m 2 .Huang's team developed derivatives EC3 and EC4.Among them, TADF emitter EC3-based devices exhibited the highest efficiencies between devices described in this section, achieving a CE, PE, and EQE of 76.9 cd/A, 71.0 lm/W and 29.0%, respectively, in the non-doped device D1EC3.Introducing host material PPF in the emissive layer of the D2EC3 further elevated efficiencies to 82.9 cd/A for CE, 70.1 lm/W for PE, and a peak EQE reaching 33.3%.Although EC4 using OLEDs were slightly less efficient, D1EC4 exhibited a remarkably low V ON of 3.0 V and EQE of 21.6%.D2EC4 demonstrated an impressive EQE of 32.9%, with CE and PE values of 77.2 cd/A and 65.0 lm/W, respectively.The emitting materials EC5-EC10 were meticulously designed, synthesized, and tested by Ma and colleagues in non-doped OLED prototypes.These materials combined a 3,6-di-tert-butylcarbazole donor with various other donors such as 9,10-dihydro-9,9-dimethylacridine (EC5, EC6, EC7) or phenoxazine (EC8, EC9, EC10) with acceptors like benzophenone (EC5, EC8), phenyl(3-pyridyl)methanone (EC6, EC9), or phenyl(2-pyridyl)methanone (EC7, EC10).For instance, the incorporation of 9,10-dihydro-9,9-dimethylacridine moiety in benzophenone-based TADF emitter EC5 led to the device DEC5 having a PE, CE, and EQE of 14.3 cd/A, 6.4 lm/W, and 6.70%, respectively.The incorporation of a pyridinyl fragment in TADF emitter EC6 significantly improved efficiencies in its corresponding device DEC6, reaching a CE of 35.4 cd/A, PE of 15.9 lm/W, and EQE of 11.4%.A phenoxazine fragment in benzophenone derivative EC8 resulted in a yellow TADF OLED device that achieved an EQE of 4.8%.An introduction of the pyridinyl fragment in compound EC9 elevated the efficiency of the orange TADF device DEC9, demonstrating a CE, PE, and EQE values of table21.6 cd/A, 6.80 lm/W, and 9.40%, respectively.Finally, the last TADF emitter of the group EC11, designed and synthesized by Tomkeviciene and colleagues, featured two electron-donating fragments of thianthrene and 9,10-dihydro-9,9-dimethylacridine.When incorporated into the emissive layer of device DEC11, CE, PE, and EQE values of 57.8 cd/A, 38.8 lm/W, and 22.2% with an L MAX exceeding 15,000 cd/m 2 were achieved.
carbazole resulted in ED7, and the reaction with 3,6-bis(3,6-di-tert-butylcarbazol-9-yl) bazole produced material ED8.Compounds ED9 and ED10 [108] were also synthesi through the Ullmann reaction procedure, when 3,6-bis(3,6-di-tert-butylcarbazol-9-yl) bazole reacted with the 4-bromobenzophenone to yield material ED9 and with (6mopyridin-3-yl)(phenyl)methanone to yield derivative ED10.The thermal, electrochemical, and photophysical characteristics of the emitting terials ED1-ED10 are presented in Table 16.The dendritic benzophenone derivatives played remarkable thermal stability with the measured TD for the tested materials exce ing 470 °C.In the cases of materials ED1, ED7, and ED8, DSC experiments confirmed o a TG of 218 °C, 283 °C, and 289 °C, respectively.The HOMO levels for materials EC1-E ranged from −5.87 to −5.04, while the LUMO levels varied between −3.09 and −2.03.energy gap between the HOMO and LUMO levels for all discussed materials in this tion was lower than 3.12 eV.This narrow gap facilitates a minimal energy difference tween the lowest singlet and triplet states, promoting efficient reverse intersystem cr ing.Experimental findings corroborated this, with a ΔEST of 0.15 eV or lower, and a h ΦPL of up to 77.0% was obtained.Specifically, derivatives ED1, ED4, ED5, and ED6 ex ited aggregation-induced emission properties with lower ΦPL values observed in soluti as compared to film states.Furthermore, materials ED1-ED6, ED9, and ED10 dem strated the involvement of triplet states in photon generation, as indicated by the RD ra ing from 9.0% to 64.5%.One of the potential light-generating mechanisms could be TA in the case.The thermal, electrochemical, and photophysical characteristics of the emitting materials ED1-ED10 are presented in Table 16.The dendritic benzophenone derivatives displayed remarkable thermal stability with the measured T D for the tested materials exceeding 470 • C. In the cases of materials ED1, ED7, and ED8, DSC experiments confirmed only a T G of 218 • C, 283 • C, and 289 • C, respectively.The HOMO levels for materials EC1-EC11 ranged from −5.87 to −5.04, while the LUMO levels varied between −3.09 and −2.03.The energy gap between the HOMO and LUMO levels for all discussed materials in this section was lower than 3.12 eV.This narrow gap facilitates a minimal energy difference between the lowest singlet and triplet states, promoting efficient reverse intersystem crossing.Experimental findings corroborated this, with a ∆E ST of 0.15 eV or lower, and a high Φ PL of up to 77.0% was obtained.Specifically, derivatives ED1, ED4, ED5, and ED6 exhibited aggregation-induced emission properties with lower Φ PL values observed in solutions as compared to film states.Furthermore, materials ED1-ED6, ED9, and ED10 demonstrated the involvement of triplet states in photon generation, as indicated by the R D ranging from 9.0% to 64.5%.One of the potential light-generating mechanisms could be TADF in the case.All in this section, the described dendritic benzophenone derivatives underwent testing as emitters in OLED devices, and their structures are illustrated in Table 17.As in previous sections, ITO was consistently chosen as the anode material for these devices.The devices utilized one or a stack of two layers of hole-transporting materials, incorporating layers made of PEDOT:PSS, poly(9-vinylcarbazole) (PVK), TAPC, or mCP.While most devices incorporating emitters ED1-ED10 adopted non-doped configurations, devices D2ED9 and D2ED10 employed the host-guest approach with the host material DPEPO.The electron-transporting layers featured TPBi, TmPyPb, 2,7-bis(diphenylphosphoryl)-9,9 ′spirobi[fluorene] (SPPO13), and DPEPO, while Cs 2 CO 3 , calcium (Ca), Liq, or LiF were employed as electron-injecting materials.In all the devices discussed in this section, the exclusive choice for the cathode was Al.Table 18 presents the characteristics of OLED devices utilizing emitters ED1-ED10.Consistent with preceding sections, most of these devices emitted a green light, except for the yellow-emitting OLED with methoxy-substituted derivative ED3 and the blue-emitting device with derivative ED9 employing a D-A structure.Material ED1 exhibited a CE and EQE of 9.2 cd/A and 4.3%, respectively, in device D1ED1.Matsuoka et al. further explored this TADF derivative, optimizing layer structures in device D2ED1 to achieve the highest CE, PE, and EQE of 46.6 cd/A, 40.7 lm/W, and 17%, respectively, between nondoped devices of this section.The same research team also synthesized and characterized derivatives ED2, ED3, and ED4. Green devices DED2 and DED4 were less efficient than D2ED2 with EQEs of 9.0% and 8.8%, respectively.This difference may be attributed to substituting groups of outermost carbazoles in the structures.The only yellow device, DED3 of this section, demonstrated a CE of 17.7 cd/A, PE of 19.0 lm/W, and EQE of 6.4%.Matsuoka and team investigated dendritic TADF materials ED5 and ED6 [130].When applied in emissive layers, emitter ED5 proved more effective than the additional carbazole fragments having derivative ED6.Device DED5 reached a maximum CE, PE, and EQE of 14.0 cd/A, 11.5 lm/W, and 5.7%, respectively.Li and co-workers developed and tested new benzophenone-acridine TADF cored dendritic materials ED7 and ED8.The derivative ED7, with fewer carbazole fragments, proved more efficient, reaching a maximum EQE of 12.0% compared to 5.20%, demonstrated by device DED8.Moreover, device DED7 exhibited an L MAX of over 10,000 cd/m 2 .Wang et al. designed and synthesized D-A-type carbazoledendronized TADF emitting materials ED9 and ED10.Non-doped emitter ED9, utilized in the blue device D1ED9, achieved a CE, PE, and EQE of 9.70 cd/A, 6.10 lm/W, and 4.2%, respectively.Doping ED9 in host material DPEPO significantly increased the efficiency of D2ED9, demonstrating a CE of 28.5 cd/A, PE of 24.9 lm/W, and EQE of 13.4%.Compound ED10, designed by changing the benzophenone electron acceptor with 3-benzoylpyridine, proved more efficient.In a non-doped configuration, device D1ED10 achieved a CE, PE, and EQE of 24.0 cd/A, 15.6 lm/W, and 8.50%, respectively.By introducing host material DPEPO in the emissive layer, the highest efficiency described in this section was achieved in D2ED10, demonstrating CE of 44.4 cd/A, PE of 42.8 lm/W, and EQE of 18.9% for the green-blue device.

Concluding Remarks
This review delves into the recent advancements in electroactive materials derived from benzophenone, providing meticulous attention to their synthesis and physical properties and the performance of organic light-emitting diodes incorporating these derivatives.Benzophenone-based materials exhibit versatile roles, serving as host materials for phosphorescent or TADF emitters, as well as functioning as emitting materials, including those with a TADF effect.A number of derivatives based on benzophenone have proven highly efficient as host materials for phosphorescent emitters, significantly improving the quantum efficiency and reducing the driving voltage of the organic light-emitting devices.Notably, benzophenone materials featuring two 3,6-diphenylcarbazole fragments have demonstrated an exceptional effectiveness for red, orange, and green phosphorescent OLEDs (PhOLEDs), achieving respective EQEs of 22.1%, 23.1%, and 25.1%.For blue phosphorescent emitters, the most suitable host exhibited a donor-acceptor (D-A) structure, incorporating one dimethylcarbazole fragment and achieving a device with an EQE of 19.4%.In the realm of green TADF materials, benzophenone derivatives with two 3-(4-(9Hcarbazol-9-yl)phenyl)-9H-carbazole fragments emerged as highly effective hosts, resulting in a device with an EQE of 25.3%.Additionally, the substitution of benzophenone with Nphenyl-1-naphthylamine enabled the creation of a TADF host-emitter, successfully applied to a white OLED device with an EQE of 9.5%.Substituted benzophenones as light-emitting materials span the spectrum of light emission ranging from red to blue, which is achieved through the modification and incorporation of various moieties into the benzophenone backbone.Among the array of benzophenone-based TADF materials, derivatives employing donor-acceptor (D-A) structures proved highly effective as emitters for green and yellow TADF OLEDs, boasting EQE levels of 25.6% and 26.7%, respectively.This structural type demonstrated optimal suitability for application as blue TADF materials with acridinesubstituted benzophenone serving as an emitter for blue TADF OLEDs, achieving an impressive EQE of 20.6%.In the case of symmetrical D-A-D structure benzophenone-based TADF emitters, their EQE reached 14.3% for blue and a remarkable 32.2% for green devices.The efficiency of green devices was further enhanced with asymmetrical D-A-D structured benzophenones by incorporating carbazole and 10H-spiro[acridine-9,9'-fluorene] electron donors.A derivative with this emitter demonstrated a device EQE of 33.3%.Researchers also aimed to achieve stable, long-lasting, non-doped TADF OLED devices by designing carbazole-dendronized benzophenone derivatives.The most efficient green OLED of this type exhibited an EQE of 17.0%.Therefore, benzophenones substituted with various electron donors are promising as emitters and hosts for various configurations of OLED devices, and further research in the field of new benzophenone-based electroactive materials is actively ongoing in order to improve the characteristics of future OLED devices.

Scheme 1 .
Scheme 1. Structures of benzophenone-based materials used as hosts in PhOLEDs.

Scheme 1 .
Scheme 1. Structures of benzophenone-based materials used as hosts in PhOLEDs.

Scheme 2 .
Scheme 2. Structures of benzophenone-based materials used as hosts in TADF OLEDs.

Scheme 3 .
Scheme 3. Structures of benzophenone-based D-A materials used as emitters in OLEDs.

Scheme 4 .
Scheme 4. Structures of benzophenone-based symmetrical D-A-D materials used as emitters in OLEDs.

Scheme 5 .
Scheme 5. Structures of benzophenone-based asymmetrical D-A-D materials used as emitters inOLEDs.

Scheme 5 .
Scheme 5. Structures of benzophenone-based asymmetrical D-A-D materials used as emitters in OLEDs.

Scheme 6 .
Scheme 6. Structures of benzophenone-based dendritic materials used as emitters in OLEDs.

°C TG, °C TCr, °C TD, °C Eg, eV ES, eV ET, eV
Scheme 2. Structures of benzophenone-based materials used as hosts in TADF OLEDs.
Characteristics of best performing white and green devices are highlighted in bold.
Characteristics of best performing blue and green devices are highlighted in bold.

Table 11 .
Architectures of devices utilizing the D-A-D emitters EB1-EB27.
Characteristics of best performing device are highlighted in bold.
Scheme 6. Structures of benzophenone-based dendritic materials used as emitters in OLEDs.
Characteristics of best performing green and blue devices are highlighted in bold.