Description:[001] The present disclosure pertains to the field of optoelectronics. In particular, the present disclosure pertains to organic compounds which serve as thermally activated delayed fluorescence (TADF) emitters. The compounds of the present disclosure result in a high external quantum efficiency (EQE) and low roll-off resulting in efficient light emission across a wide range of operating conditions. This is critical for practical organic light emitting diode (OLED) applications where both brightness and longevity are essential.

BACKGROUND OF THE DISCLOSURE
[002] OLEDs have emerged as a transformative technology in the fields of display and solid-state lighting. An OLED is a multilayer electroluminescent device typically composed of organic thin films sandwiched between two electrodes. When a voltage is applied, electrons and holes are injected from the cathode and anode, respectively, into the organic layers, where they recombine to form excitons that decay radiatively to emit light.

[003] A typical OLED device consists of several functional layers, including a hole injection layer (HIL), hole transport layer (HTL), emissive layer (EML), electron transport layer (ETL), and electron injection layer (EIL), among others. Among these, the emission layer (EML) plays a central role, as it is the region where exciton recombination and subsequent light emission occurs. The materials used in the EML largely determine the efficiency, color, and stability of the device.

[004] The development of organic light-emitting diodes (OLEDs) has been marked by continuous innovation in the design of emissive materials, driven by the need to enhance efficiency, color tunability, and material sustainability. In the early stages, fluorescent dyes were employed as emissive materials. These compounds emit light via prompt fluorescence, utilizing only singlet excitons, which constitute approximately 25% of the total exciton population generated during electrical excitation. The remaining 75% triplet excitons are typically lost through non-radiative decay processes, resulting in limited internal quantum efficiency (IQE).

[005] To overcome this intrinsic limitation, the field progressed toward phosphorescent emitters, which can harvest both singlet and triplet excitons via strong spin–orbit coupling, typically achieved through the incorporation of heavy metal atoms such as iridium (Ir) or platinum (Pt). These materials demonstrated near-unity internal quantum efficiency (~100%), significantly enhancing OLED performance. However, their dependence on rare and costly noble metals, along with environmental and supply chain concerns, has motivated the search for purely organic alternatives.

[006] Therefore, in recent years, attention has shifted to thermally activated delayed fluorescence (TADF) materials, particularly purely organic TADF emitters, as a promising class of emitters capable of harvesting both singlet and triplet excitons without the use of heavy metals. TADF materials utilize singlet-triplet energy gap (ΔEST), which allows triplet excitons to be thermally upconverted to the singlet state via reverse intersystem crossing (RISC), thereby contributing to fluorescence. The development of purely organic TADF emitters represents a significant advancement, combining high efficiency, metal-free composition, and color tunability, making them ideal candidates for sustainable and cost-effective OLED applications.

[007] Thermally activated delayed fluorescent (TADF) emitters are the 3rd generation of organic emitters which can show a maximum internal quantum efficiency (IQEmax) of 100% without using any heavy metal in the molecular structure. Due to the high IQE, the device performance of TADF OLEDs reached similar to that of phosphorescent OLEDs. However, the strong efficiency roll-off, which refers to the drastic drop of device performance at higher brightness, is still a challenging issue in TADF OLEDs. At higher luminance, the efficiency of converting electrical energy into light reduces and therefore, more input energy is needed to maintain a particular luminance, making TADF OLEDs power inefficient.

[008] TADF emitters are cost-effective alternatives to rare-earth-metal-based phosphorescent emitters, offering high exciton utilization for advanced optoelectronic applications. However, organic light-emitting diodes (OLEDs) employing TADF emitters usually face significant efficiency roll-off at higher brightness. Despite their promise, current organic TADF emitters face several limitations that hinder their commercial deployment, particularly in high-performance OLED devices

[009] There could be multiple reasons behind the efficiency roll-off, such as poor light extraction or outcoupling efficiency at higher brightness, materials degradation, inappropriate device architecture, and so on. In the OLED structure, emissive layer (EML) plays a crucial role as light generation happens in this part of the device. Therefore, the photophysical behaviour of EML also equally affects the OLED efficiency roll-off. A long triplet excited state lifetime, leading to slow rISC and a larger delayed fluorescence (DF) contribution in the overall radiative decay process, are the major factors of efficiency roll-off in TADF OLEDs. Long-excited state lifetime enhances the chance of bimolecular exciton quenching processes like triplet-triplet annihilation (TTA), singlet-triplet annihilation (STA), and so on. To reduce the efficiency roll-off, TADF emitters should have rapid exciton upconversion rate (krISC) along with a fast singlet radiative decay rate (kr). The larger contribution of DF is beneficial for high OLED efficiency; However, it also increases the overall decay lifetime, leading to efficiency quenching.

[010] The rISC process in TADF emitters heavily depends on the states character and energy barrier (ΔEST). In donor-acceptor type organic TADF emitters, singlet excited predominantly acquires charge transfer (CT) character; for better exciton upconversion, triplet excited state should have locally excited (LE) nature and should be situated within proximity of singlet state, such that ΔEST will be ≤ 0.25eV.

[011] The present disclosure relates to improvements in the design, synthesis, and application of such purely organic TADF emitters, particularly in achieving high external quantum efficiency (EQE) with minimal efficiency roll-off at high brightness levels, thereby addressing key challenges in the commercialization of next-generation OLED devices.

OBJECT OF THE DISCLOSURE
[012] An object of the present disclosure is to develop organic compounds which serve as thermally activated delayed fluorescence (TADF) emitters that exhibit low efficiency roll-off, and maintain high performance across a wide range of luminance and current densities, suitable for practical OLED applications.

[013] To arrive at molecular design strategy for purely organic thermally activated delayed fluorescence emitters that have a shorter excited state lifetime, leading to faster rISC rate enabling the development of high-performance OLEDs.

[014] Another aspect of the present disclosure is to arrive at compounds which achieve high external quantum efficiency (EQE) through efficient harvesting of both singlet and triplet excitons without the use of heavy metal complexes, ensuring energy-efficient light emission.

[015] To arrive at a sustainable process for the preparation of donor-acceptor type organic compounds which serve as purely organic thermally activated delayed fluorescence (TADF) emitters.

[016] Yet another object of the present disclosure is to provide OLEDs with extended operational lifetime, color tunability, and environmental sustainability, making them suitable for next-generation display and lighting technologies.

SUMMARY OF THE DISCLOSURE
[017] This summary is intended to introduce, in simplified form, a selection of concepts that are further described in the detailed description. This summary is merely presented as a brief overview of the subject matter described and claimed herein and does not aid in determining the scope of the claimed subject matter.

In one aspect, the present disclosure provides a compound having a structural Formula (I),
D-(E)n-(A)m Formula (I)
wherein n = 1 to 3; m = 1 to 3;
wherein D is a benzene ring

E is a group fused to benzene ring D at one or more positions selected from C1-C2, C3-C4, and C5-C6, to result in symmetric or asymmetric structures.
E is a heterocyclic ring selected from

and ;
wherein E is substituted or fused with Y group;
Y is selected from


, ,

and ;

A is independently selected from
or ;
wherein C is
or ; and
B1 and B2 are

, , ,
, , or ; and

wherein the group A is attached to E at one or more of the N atoms present in E via a C-N covalent bond;
and when the N atom is not substituted with A, it is substituted with Y.

[018] In another aspect, the present disclosure provides a process for synthesizing the compound of Formula (I) comprising reacting a precursor of the donor D-(E)n with a precursor of acceptor A in the presence of a catalyst and a base to result in C-N coupling reaction.

[019] In yet another aspect, the present disclosure provides an organic light emitting diode (OLED) comprising the compound of formula (I).

BRIEF DESCRIPTION OF FIGURES
[020] The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations and are not intended to limit the scope of the present disclosure. Further objectives and advantages of this invention will be more apparent from the ensuing description when read in conjunction with the accompanying drawing and wherein:

Figure 1 illustrates HOMO-LUMO distribution of representative emitters (a) TAT-(4BPy), (b) TAT-2(4BPy), and (c) TAT-3(4BPy).

Figure 2 illustrates HOMO-LUMO distribution of representative emitters (a) 4BPy-pPXZ and 3BPy-pPXZ.

Figure 3 illustrates photoluminescence (PL) properties of (a) TAT-(4BPy), (b) TAT-2(4BPy), and (c) TAT-3(4BPy) in their 0.01 mM toluene solution.

Figure 4 illustrates excited state lifetimes of the emitters in their mCBP host doped films; Transient PL decay under ambient and vacuum for (a) TAT-(4BPy), (b) TAT-2(4BPy), and (c) TAT-3(4BPy).

Figure 5 illustrates external quantum efficiency Vs Luminance plot of (a) TAT-(4BPy), (b) TAT-2(4BPy), and (c) TAT-3(4BPy) based OLEDs [Inserted EL spectra recorded at 7V and digital photograph of the devices]

Figure 6 illustrates UV-Visible absorption, room temperature fluorescence, and low temperature (77K) phosphorescence spectra of (a) 3BPy-pPXZ and (b) 4BPy-pPXZ in CBP host doped film excited @ 340 nm [Inserted molecular structure of the emitters].

Figure 7 illustrates excited state lifetimes of 3BPy-pPXZ and 4BPy-pPXZ in their CBP host doped films [Inserted prompt fluorescence lifetimes of the emitters].

Figure 8 illustrates external quantum efficiency Vs Luminance plot of (a) 3BPy-pPXZ and (b) 4BPy-pPXZ based OLEDs [Inserted EL spectra and digital photograph of the devices].

Figure 9 illustrates optimized OLED device structure; (b) molecular structure of the materials used in device fabrication.

DETAILED DESCRIPTION
[021] The present disclosure provides purely organic thermally activated delayed fluorescence (TADF) emitters which are cost-effective alternatives to rare-earth-metal-based phosphorescent emitters, offering high exciton utilization for advanced optoelectronic applications.

[022] The present disclosure can be understood more readily by reference to the following description, taken in conjunction with the accompanying Figures and Examples, all of which form a part of this disclosure.

[023] At the very outset of the detailed description, it may be understood that the ensuing description only illustrates a particular form of this invention. However, such a particular form is only an exemplary embodiment, and without intending to imply any limitation on the scope of this invention. Accordingly, the description is to be understood as an exemplary embodiment and teaching of invention and not intended to be taken restrictively.

[024] Before the present disclosure or methods of the present disclosure are described in greater detail, it is to be understood that the specific products, methods, processes, conditions or parameters, are not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting.

[025] Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is encompassed within the methods. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the methods. Certain ranges are presented herein with numerical values being preceded by the term “about.” The term “about” is used herein to provide literal support for the exact number that it precedes, as well as a number that is near to or approximately the number that the term precedes. For example, "about" can mean within one or more standard deviations, or within ± 30%, 25%, 20%, 15%, 10% or 5% of the stated value.

[026] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, preferred methods and materials are described. For the purposes of the present invention, the following terms are defined below.

[027] It is appreciated that certain features of the methods, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the methods, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. All combinations of the embodiments are specifically embraced by the present invention and are disclosed herein just as if each and every combination was individually and explicitly disclosed, to the extent that such combinations embrace operable processes and/or composites/scaffolds.

[028] The articles "a" and "an" are used herein to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article. By way of example, "an element" means one element or more than one element.

[029] As used herein, the term "comprises", "comprising", or “comprising of” is generally used in the sense of include, that is to say permitting the presence of one or more features or components. The term "comprises", "comprising", or “comprising of” when placed before the recitation of steps in a process or method means that the process or method encompasses one or more steps that are additional to those expressly recited, and that the additional one or more steps may be performed before, between, and/or after the recited steps.

[030] Reference throughout this specification to “certain embodiments”, “further embodiments”, “specific embodiments”, “further specific embodiment”, “one embodiment”, “a non-limiting embodiment”, “an exemplary embodiment”, “some instances”, or “further instances”, means that a particular feature, structure or characteristic described in connection with the embodiment may be included in at least one embodiment of the present disclosure.

[031] As used herein, the terms ‘include’, ‘have’, ‘comprise’, ‘contain’ etc. or any form of said terms such as ‘having’, ‘including’, ‘containing’, ‘comprising’ or ‘comprises’ are inclusive and will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

[032] The use of any and all examples, or exemplary language (e.g., “such as”) provided with respect to certain embodiments herein is intended merely to better illustrate the disclosure and does not pose a limitation on the scope of the disclosure otherwise claimed.

[033] As used herein, the term “invention”, “present invention”, “disclosure” or “present disclosure” as used herein is a non-limiting term and is not intended to refer to any single embodiment of the particular invention but encompasses all possible embodiments as described in the specification.

[034] The terms “process(es)” and “method(s)” are considered interchangeable within this disclosure.

[035] The terms “thermally activated delayed fluorescence”, and “TADF” are used interchangeably within this disclosure.

[036] The terms “organic light-emitting diodes”, and “OLEDs” are used interchangeably within this disclosure.

[037] The terms “reverse intersystem crossing”, and “rISC” are used interchangeably within this disclosure.

[038] In one aspect, the present disclosure provides a compound having a structural Formula (I),
D-(E)n-(A)m Formula (I)
wherein n = 1 to 3; m = 1 to 3;
wherein D is a benzene ring

E is a group fused to benzene ring D at one or more positions selected from C1-C2, C3-C4, and C5-C6 to result in symmetric or asymmetric structures;
E is a heterocyclic ring selected from

and ;
wherein E is substituted or fused with Y group;

Y is selected from


, ,

and ;
A is independently selected from
or ;
wherein C is
or ; and
B1 and B2 are

, , ,
, , or ; and

wherein the group A is attached to the E at one or more of the N atoms present in E via a C-N covalent bond;
and when the N atom is not substituted with A, it is substituted with Y.

[039] The moiety D(E)n is an electron rich moiety and is capable of donating electrons to an acceptor A. The primary function of D(E)n is to localize the highest occupied molecular orbital (HOMO), facilitating charge separation from the acceptor and enabling a small singlet-triplet energy gap (ΔEST), which is essential for efficient reverse intersystem crossing (rISC).

[040] Acceptor A stabilizes the electron density, serving as the region where the lowest unoccupied molecular orbital (LUMO) is localized. This spatial separation from the donor’s HOMO facilitates intramolecular charge transfer (ICT) upon excitation, a key requirement for achieving a small singlet–triplet energy gap (ΔEST). The information on donor–acceptor architecture for a representative compound of the present disclose has been provided in Figures 1 and 2. The compounds of the present disclosure have spatially separated HOMO and LUMO indicating strong donor acceptor interaction and charge transfer leading to low ΔEST.

[041] The donor-acceptor D-(E)n-(A)m interaction induces a charge transfer excited state, which is essential for promoting reverse intersystem crossing (rISC) from the triplet state (T₁) to the singlet state (S₁). Efficient rISC enables the utilization of triplet excitons via thermally activated processes. A smaller ΔEST is critical for efficient TADF, as it reduces the energy barrier for rISC, allowing triplet excitons to convert to singlets at ambient temperatures.

[042] The compounds of the present disclosure have an energy barrier ΔEST of ≤ 0.25eV, and an efficiency roll off at 1000 or 5000 cd/m2 luminescence of less than 40.

[043] The proper selection and design of the donor-acceptor moiety are essential for achieving high-performance TADF emitters with low efficiency roll-off, making them suitable for practical OLED display and lighting applications.

[044] In a specific embodiment, E is selected from

and ;

[045] E is fused to benzene ring D at C1-C2 position, or at C1-C2, and C3-C4 positions, or at C1-C2, C3-C4 and C5-C6 positions, either to result in symmetric or asymmetric structures.

[046] The benzene ring D can be fused with the same or different E groups, resulting in symmetric or asymmetric compounds.

[047] In an embodiment, the donor moiety D-(E)n has a symmetric structure and the compounds can be represented using the following structural formulas.

, or .

[048] In an embodiment, the donor moiety D-(E)n has an asymmetric structure and the compounds can be represented using the following structural formulas.


, and .

[049] In a specific embodiment, the D(E)n-(A)m compound of formula (I) is a triazatruxene moiety represented by the following structural formula:
, or
wherein A1, A2, and A3 is a substituent , wherein C is , B1 is , and B2 is , and Y is as defined in claim 1.

[050] In a further specific embodiment, the compound of formula (I) is represented by the following structural formula:
, , or .

[051] As a part of the molecular design strategy, in the triazatruxene (TAT) donor-based TADF emitters, the number of acceptors m were optimized to reduce the efficiency roll-off.

[052] Compounds can be prepared using a rigid triazatruxene (TAT) as donor core and 4-benzoyl pyridine (4BPy) as acceptor unit with the value of “m” ranging from 1, 2, and 3, represented as TAT-(4BPy), TAT-2(4BPy), and TAT-3(4BPy) respectively. The thermal degradation temperatures of TAT-(4BPy), TAT-2(4BPy), and TAT-3(4BPy) are recorded to be 483, 515, and 521°C, respectively. The high thermal stability of the materials ultimately improves the operational lifetime of OLED devices. These compounds showed sub-microsecond of excited state lifetime in the range of 0.25-0.85 μs, making the reverse intersystem crossing (rISC) in the order of 6x105 s-1. The vacuum-deposited OLED devices of TAT-(4BPy), TAT-2(4BPy), and TAT-3(4BPy) showed EQEmax of 29, 25, and 20%, and their efficiency roll-off at 1000 luminance were recorded to be 33, 6, and 12%, respectively. Among the three emitters, TAT-2(4BPy) having two acceptor units shows simultaneously high EQE and low roll-off. The 6% efficiency roll-off is one of the lowest values from a purely organic TADF emitter-based OLED.

[053] The number of acceptors attached to the donor has a significant impact on the photophysical properties and performance of thermally activated delayed fluorescence (TADF) emitters. The acceptors are attached such that there is a greater spatial separation between the HOMO (on the donor) and LUMO (on the acceptors).

[054] In another specific embodiment, the compound is a phenoxazine moiety represented by the structural formula.

[055] In a further specific embodiment, the donor D-(E)n is a phenoxazine (PXZ) moiety and the acceptor A is selected from 3-benzoylpyridine (3BPy) and 4-benzoyl pyridine (4BPy).
, .

[056] In a further specific embodiment, the compound is represented by the following structural formula:
or .
[057] In the said compounds, the effect of the acceptor pyridine “N” position on roll-off was evident. Two organic isomers 3BPy-pPXZ and 4BPy-pPXZ having 3-pyridine and 4-pyridine group in the acceptor units are studied. The EQEmax of 3BPy-pPXZ and 4BPy-pPXZ devices were 17.3% and 23.7%, respectively. Out of these two, 3BPy-pPXZ based OLED device experience a lower efficiency roll-off of 10.9% at 1000 cd/m2, which was 1/3rd the value of 4BPy-pPXZ device. The high-efficiency roll-off in the second device was due to a higher triplet/ DF contribution. Hence, a proper number of acceptor and position of hetero atoms play a significant role in deciding the rISC rate and delayed fluorescence contribution.

In an embodiment along with the covalently bonded D-(E)n compounds, separate donor D-(E)n compounds substituted with Y and acceptor compounds A (substituted with B1 and B2) mixtures can also be used as emissive materials, as their mixture form exciplex.

[058] In another aspect, the present invention pertains to a process for synthesizing the compound of Formula (I) as claimed in claim 1, comprising reacting a precursor of the donor D-(E)n with a precursor of acceptor A in the presence of a catalyst and a base to form C-N coupling reaction.

[059] In an embodiment, the present disclosure employs a Buchwald–Hartwig amination reaction to construct a carbon–nitrogen (C–N) bond between an aryl halide and an amine. The reaction is facilitated by a palladium-based catalyst system in the presence of a suitable ligand and a base, under an inert atmosphere.

[060] The donor D-(E)n having one or more amine groups is reacted with a halide precursor of acceptor A in the presence of catalyst and a base to replace one or more of the H attached to nitrogen with A.

[061] In one of the embodiments, when the “N-H” which is not replaced with A is replaced with the group Y. In some embodiments, Y is selected from

,

[062] In a preferred embodiment, the Y is selected from phenyl .

[063] In an embodiment, the precursor of donor D-(E)n and the precursor of the acceptor A are present in an equimolar ratio.

[064] Catalyst which can be employed for the purposes of the present disclosure is selected from palladium catalyst such as Pd(OAc)₂, Pd₂(dba)₃, or Pd(PPh₃)₄; copper powder or copper iodide.

[065] Base which can be employed for the purposes of the present disclosure include but are not limited to potassium tert-butoxide (KOtBu), sodium tert-butoxide (NaOtBu), cesium carbonate (Cs₂CO₃), potassium phosphate (K₃PO₄), potassium tert-pentoxide, sodium methoxide or potassium hydroxide. In a preferred embodiment, base is selected from sodium tertiary butoxide or potassium phosphate.

[066] Suitably, the reaction is conducted in the presence of a ligand, preferably a bulky, electron-rich phosphine ligand such as tBu3P, XPhos, SPhos, or BINAP.

[067] Reaction of the precursor of the donor D-(E)n with a precursor of acceptor A is carried out in the presence of a solvent. The solvent is selected from, but not limited to toluene, xylene, 1,4-dioxane, hexane, tert-butyl hydroxide, tetrahydrofuran dimethyl ether, dimethylformamide (DMF), and dimethoxyethane (DME).

[068] The reaction mixture can be heated to a temperature in the range of 80°C to 130°C, preferably between 100°C and 120°C, for a period ranging from 4 to 24 hours, depending on the reactivity of the substrates.

[069] After completion of the reaction, the reaction mixture is cooled to room temperature and diluted with an organic solvent such as ethyl acetate or dichloromethane. The mixture can be washed with water and brine, dried over anhydrous sodium sulfate, and filtered. The crude product is concentrated under reduced pressure.

[070] In a specific embodiment, the precursor of donor D-(E)n is selected from
, or ;
[071] In a further specific embodiment, the precursor of the acceptor A is selected from
3-benzoylpyridine or 4-benzoyl pyridine
or ;
Wherein “X” is Cl, Br, F, or I.
and the base is selected from sodium tertiary butoxide, or potassium phosphate.

[072] In yet another specific embodiment, the “N-H” groups which is not replaced with A is reacted with the group Y, selected from phenyl group.

[073] Yet another aspect of the present invention provides an OLED device comprising compound of Formula I of the present invention present in the emission layer.

[074] The OLED device suitably comprises a substrate selected from glass, plastic substrates such as polyethylene terephthalate (PET) or polyimide (PI), or Metal foils. The anode may be selected from indium tin oxide (ITO), metal oxides, graphene or PEDOT:PSS, or metal nanowires. The cathode can be made of aluminum (Al), silver (Ag), calcium (Ca), or Mg:Au metal alloys.

[075] Apart from the substrate, anode and cathode, it comprises multiple layers which can be one or more of a hole injection layer (HIL) which can comprises compounds selected from poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), molybdenum trioxide (MoO₃), and/or NiOₓ (nickel oxide), 1,4,5,8,9,11-Hexaazatriphenylenehexacarbonitrile (HATCN); a hole transport layer (HTL) which can comprise compounds selected from 1,1-bis[4-[N,N-di(p-tolyl)amino]phenyl]cyclohexane (TAPC), N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine) (NPB) or (tris(4-carbazoyl-9-ylphenyl)amine (TCTA); an emission layer (EML) which comprises the compound of formula (I) along with a host material; an electron transport layer (ETL) which can comprise compounds selected from TPBi (2,2',2''-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)), 1,3,5-Tri(m-pyridin-3-ylphenyl)benzene (TmPyPB), BPhen (4,7-diphenyl-1,10-phenanthroline) and T2T (2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine), 2,8-Bis(diphenyl-phosphoryl)-dibenzo[b,d]thiophene (PPT) ; an electron Injection Layer (EIL) which can comprise compounds selected from LiF (lithium fluoride, used in very thin layers), Lithium-8-hydroxyquinolinolate (Liq), Cs₂CO₃ (cesium carbonate), Ba, Li, or other low-work-function materials.

[076] Optionally the OLED may comprise excitation blocking layer, or interfacial modifiers.

[077] The emissive layer (EML) comprises one or more host materials in combination with the compound of formula (I). The host material serves to support exciton formation and energy transfer to the dopant emitter, thereby facilitating electroluminescence at the desired emission wavelength.

[078] The host material can be selected from but not limited to mCBP (3,3'-di(9H-carbazol-9-yl)-1,1'-biphenyl), CBP (4,4'-N,N'-dicarbazole-biphenyl), mCP (1,3-bis(N-carbazolyl)benzene) and DPEPO (bis[2-(diphenylphosphino)phenyl]ether oxide) and the like. These materials are selected based on the photophysical compatibility with the selected emitter. In an embodiment, the device comprises 5 to 10 wt% of the compound of formula (I) along with 3,3-di(9H-carbazol-9-yl)biphenyl (mCBP) as host material in the emissive layer.

[079] In a specific embodiment, the OLED device, comprises:
• an aluminium cathode;
• an indium tin oxide anode;
• a hole injection layer comprising N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB);
• a hole transportation layer comprising 1,1-Bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC);
• an emissive layer comprising a compound of formula (I) as the emitter along with a host material selected from mCBP (3,3'-di(9H-carbazol-9-yl)-1,1'-biphenyl);
• an exciton blocking layer comprising 2,8-bis(diphenyl-phosphoryl)-dibenzo[b,d]thiophene (PPT); and
• an electron transport layer comprising 2,2′,2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole (TPBi);
wherein the emitter has a shorter excited state lifetime of less than 1.0 μs, faster reverse intersystem crossing rate (rISC), and smaller delayed fluorescence contribution.

[080] The OLEDs of the present disclosure have, color tunability, and environmental sustainability, making them suitable for next-generation display and lighting technologies.

[081] The foregoing outlines features of several embodiments so that those skilled in the art better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other fluorescence emitters. Those skilled in the art should also realize that such equivalent modifications do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein.

EXAMPLES:
[082] The following examples are given by way of illustration of the working of the invention in actual practice and therefore should not be construed to limit the scope of the present invention in any way.

[083] Example 1: Preparation of TAT-(4BPy), TAT-2(4BPy), and TAT-3(4BPy):

Preparation of N1,N3,N5-tris(2-bromophenyl)benzene-1,3,5-triamine (TAT-intermediate):
[084] In a cleaned and dried two-neck round bottom flask, phloroglucinol (5g, 36.9 mmol) and 2-bromo-aniline (25g, 158.5 mmol) were taken. After evacuating the air, RB was purged with nitrogen gas for three times. Under inert atmosphere, 100 ml of anhydrous toluene was added and the reaction mixture was allowed to stir at 115 °C for 3 hours. After 3 hours, the mixture was brought to room temperature and 2.71 ml of 36% HCl was added to the reaction mixture. The reaction mixture was then allowed to stir at 115 °C for next 48 hours. After completion of the reaction, the mixture was dried under reduced pressure. The crude solid was washed with methanol for multiple times to get the white colored N1,N3,N5-tris(2-bromophenyl)benzene-1,3,5-triamine (TAT-intermediate) compound.

Preparation of 10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole (Triazatruxene/ TAT compound):
[085] In a cleaned and dried sealed tube N1,N3,N5-tris(2-bromophenyl)benzene-1,3,5-triamine (5g, 8.50 mmol), Tricyclohexyl-phosphine tetrafluoroborate (0.18g, 0.51 mmol), palladium acetate (0.09g, 5 mol%), and potassium carbonate (4.7g, 34 mmol) were taken. After evacuating the air, sealed tube was purged with nitrogen gas for three times. Under inert atmosphere, 40 ml of anhydrous dimethyl acetamide (DMA) was added and the reaction mixture was allowed to stir at 130 °C for 24 hours. After completion of the reaction, the mixture was dried under reduced pressure. The dark black colored crude solid was washed with different organic solvents for multiple times to get the light brown colored 10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole (Triazatruxene/ TAT) compound.

Preparation of 5-phenyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole/ TAT-Ph and 5,10-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole/ TAT-2Ph:
[086] In a cleaned and dried sealed tube triazatruxene/ TAT (1g, 2.89 mmol), iodo benzene (0.88g, 4.3 mmol), palladium acetate (0.065g, 0.29 mmol), tri-tertbutyl phosphine tetrafluoroborate (0.42g, 1.45 mmol), and sodium tert-butoxide (1.39g, 14.5 mmol) were taken. After evacuating the sealed tube nitrogen was purged and under nitrogen condition the reaction mixture was dissolved in 20 ml of anhydrous 1,4-dioxane. The reaction was allowed to stir at a temperature of 100 °C for 24 hours. After completion of the reaction, the mixture was dried under reduced pressure, then water-ethyl acetate workup was performed with the crude. The filtrate was concentrated under reduced pressure and the residue was purified by a silica gel column chromatography using ethyl acetate/n-hexane as the eluent to afford white colored 5-phenyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole/ TAT-Ph and 5,10-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole/ TAT-2Ph.

[087] Preparation of (4-(10,15-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazol-5-yl)phenyl)(pyridin-4-yl)methanone (TAT-(4BPy):

[088] In a cleaned and dried sealed tube TAT-2Ph (0.5g, 1.0 mmol), 4-benzoyl pyridine (0.39g, 1.5 mmol), tris(dibenzylideneacetone)dipalladium (0.046g, 0.05 mmol), tri-tertbutyl phosphine tetrafluoroborate (0.087g, 0.3 mmol), and sodium tert-butoxide (0.289g, 3.1 mmol) were taken. After evacuating the sealed tube nitrogen was purged and under nitrogen condition the reaction mixture was dissolved in 20 ml of anhydrous toluene. The reaction was allowed to stir at a temperature of 115 °C for 30 hours. After completion of the reaction, the mixture was dried under reduced pressure, then water-ethyl acetate workup was performed with the crude. The filtrate was concentrated under reduced pressure and the residue was purified by a silica gel column chromatography using ethyl acetate/n-hexane as the eluent to afford yellow colored (4-(10,15-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazol-5-yl)phenyl)(pyridin-4-yl)methanone/ TAT-BPy. 1H NMR (400 MHz, Chloroform-d) δ 8.79 (d, J = 4.9 Hz, 2H), 8.00 (d, J = 8.5 Hz, 2H), 7.73 (d, J = 8.5 Hz, 2H), 7.66 – 7.53 (m, 12H), 7.43 (d, J = 8.2 Hz, 1H), 7.29 (dd, J = 15.5, 8.0 Hz, 2H), 7.17 – 7.10 (m, 3H), 6.77 (dd, J = 13.9, 7.0 Hz, 2H), 6.71 (d, J = 7.0 Hz, 1H), 6.19 (d, J = 7.9 Hz, 1H), 5.98 (d, J = 5.1 Hz, 2H). 13C NMR (101 MHz, Chloroform-d) δ 194.26, 150.58, 145.54, 144.08, 141.82, 141.68, 140.97, 140.76, 140.70, 137.77, 137.52, 136.91, 134.68, 131.90, 130.07, 130.05, 129.03, 129.01, 128.47, 128.42, 128.28, 123.54, 123.49, 123.46, 123.28, 123.00, 122.88, 122.67, 122.27, 122.18, 122.05, 120.83, 120.15, 119.77, 110.26, 109.98, 109.61, 105.21, 105.03, 104.34. HRMS (ESI+) calcd for C48H31N4O [M + H]+, 679.2498 and found 679.2497.

[089] Preparation of ((15-phenyl-5H-diindolo[3,2-a:3',2'-c]carbazole-5,10(15H)-diyl)bis(4,1-phenylene))bis(pyridin-4-ylmethanone) (TAT-2(4BPy)):

[090] In a cleaned and dried sealed tube TAT-Ph (0.5g, 1.2 mmol), 4-benzoyl pyridine (1.09g, 3.5 mmol), tris(dibenzylideneacetone)dipalladium (0.054g, 0.06 mmol), tri-tertbutyl phosphine tetrafluoroborate (0.11g, 0.35 mmol), and sodium tert-butoxide (0.34g, 3.5 mmol) were taken. After evacuating the sealed tube nitrogen was purged and under inert condition the reaction mixture was dissolved in 20 ml of anhydrous toluene. The reaction was allowed to stir at a temperature of 115 °C for 30 hours. After completion of the reaction, the mixture was dried under reduced pressure, then water-ethyl acetate workup was performed with the crude. The filtrate was concentrated under reduced pressure and the residue was purified by a silica gel column chromatography using ethyl acetate/n-hexane as the eluent to afford yellow colored ((15-phenyl-5H-diindolo[3,2-a:3',2'-c]carbazole-5,10(15H)-diyl)bis(4,1-phenylene))bis(pyridin-4-ylmethanone)/ TAT-2BPy. 1H NMR (400 MHz, Chloroform-d) δ 8.85 (s, 4H), 8.05 (s, 4H), 7.79 (t, J = 7.7 Hz, 4H), 7.72 (s, 4H), 7.64 (s, 5H), 7.50 (dd, J = 14.6, 8.1 Hz, 2H), 7.36 (d, J = 8.2 Hz, 1H), 7.27 (d, J = 9.2 Hz, 2H), 7.19 (d, J = 6.4 Hz, 1H), 6.94 – 6.77 (m, 3H), 6.24 (d, J = 8.0 Hz, 2H), 6.03 (d, J = 8.1 Hz, 1H). 13C NMR (101 MHz, Chloroform-d) δ 194.20, 150.59, 145.33, 145.24, 143.99, 141.88, 141.18, 141.05, 140.52, 137.60, 137.01, 136.76, 134.91, 134.84, 131.92, 130.13, 129.02, 128.63, 128.34, 128.29, 123.91, 123.83, 123.04, 122.97, 122.36, 122.18, 121.96, 120.97, 120.57, 119.92, 110.37, 110.00, 109.71, 105.83, 105.12, 104.96. HRMS (ESI+) calcd for C54H34N5O2 [M + H]+, 784.2712 and found 784.2710.

[091] Preparation of ((5H-diindolo[3,2-a:3',2'-c]carbazole-5,10,15-triyl)tris(benzene-4,1-diyl))tris(pyridin-4-ylmethanone) (TAT-3(4BPy):

[092] In a cleaned and dried sealed tube, triazatruxene/ TAT (1g, 2.89 mmol), 4-benzoyl pyridine (3.0g, 11.5 mmol), Pd2(dba)3 (0.13g, 5 mol%), tBu3PHBF4 (0.4g, 1.4 mmol), and NaOtBu (1.4g, 14.5 mmol) were taken. After completely removing the air, the sealed tube was purged with nitrogen gas three times. Under inert atmosphere the solid mixture was dissolved in 40 ml of anhydrous toluene. The reaction mixture was then allowed to stir at 115 °C for next 30 hours. After completion of the reaction, the mixture was passed through celite/ silica gel filtration to remove unreacted catalyst and inorganic salts. The crude solution was concentrated under reduced pressure. The solid crude was purified by silica gel column chromatography using methanol-dichloromethane solvent mixture as eluent to afford yellow colored ((5H-diindolo[3,2-a:3',2'-c]carbazole-5,10,15-triyl)tris(benzene-4,1-diyl))tris(pyridin-4-ylmethanone)/ TAT-3(4BPy). 1H NMR (400 MHz, Chloroform-d) δ 8.83 (d, J = 5.1 Hz, 6H), 8.07 (d, J = 8.5 Hz, 6H), 7.80 (d, J = 8.4 Hz, 6H), 7.67 (d, J = 6.0 Hz, 6H), 7.52 (d, J = 8.2 Hz, 3H), 7.28 (t, J = 7.7 Hz, 3H), 6.89 (t, J = 7.1 Hz, 3H), 6.25 (d, J = 8.1 Hz, 3H). HRMS (ESI+) calcd for C60H37N6O3 [M + H]+, 889.2927 and found 889.2922.

[093] Example 2: Preparation of 3BPy-pPXZ, and 4BPy-pPXZ:

[094] Preparation of (4-bromophenyl)(pyridin-3-yl)methanone (3BPy-pBr) or (4-bromophenyl)(pyridin-4-yl)methanone (4BPy-pBr):
As both intermediates are isomers of each other their synthesis procedure is same. In a 500 ml clean and oven dried two necks round bottom flask, 10 gm of 1,4-dibromo benzene (42.4 mmol) was taken. Under nitrogen atmosphere compound was dissolved with dry diethyl ether. The reaction was kept at -78°C and the temperature was maintained. After ½ an hour 25 ml of n-BuLi was added to the reaction mixture. In another oven dried RB, 4.41 gm of 4-cyanopyridine or 3-cyanopyridine (42.3 mmol) was dissolved with dry diethyl ether and after one hour this solution was added to the reaction mixture and kept it for stirring. After 3 hours 100 ml of 2N HCl was added to the reaction mixture and left the reaction for overnight stirring. After 24 hours the reaction mixture was neutralized using 2N Na2CO3 solution. After workup with DCM/water, the organic layer was dried over reduced pressure. Pure (4-bromophenyl)(pyridin-3-yl)methanone/ 3BPy-pBr or (4-bromophenyl)(pyridin-4-yl)methanone/ 4BPy-pBr materials were obtained by washing the crude solid with different solvent (n-pentane, hexane, methanol).

[095] Preparation of (4-(10H-phenoxazin-10-yl)phenyl)(pyridin-3-yl)methanone (3BPy-pPXZ) or (4-(10H-phenoxazin-10-yl)phenyl)(pyridin-4-yl)methanone (4BPy-pPXZ):
Synthesis methodology of 3BPy-pPXZ and 3BPy-pPXZ (as both emitters are isomers of each other their synthesis procedure is same.
In a 100 ml cleaned and dried sealed tube, (3-bromophenyl)(pyridin-3-yl)methanone (3BPy-pBr) or (4-bromophenyl)(pyridin-4-yl)methanone (4BPy-pBr) (1g, 3.8 mmol), 10H-phenoxazine (PXZ) (0.74g, 4.0 mmol), palladium acetate (0.086g, 10 mol%), tri-tert-butyl phosphine tetrafluoro borate (0.17g, 0.57 mmol), and sodium tert-butoxide (0.55g, 5.7 mmol) were taken. After evacuating air, the sealed tube was purged with nitrogen and this process was repeated three times. Then 40 ml of anhydrous toluene was added, and the reaction mixture was allowed to stir at 115 °C for 24 hours. After completion of the reaction, toluene was removed under reduced pressure and product was extracted with ethyl acetate. The final organic part was concentrated under reduced pressure. The compound was purified by silica gel column chromatography using ethyl-acetate/ n-hexane as eluent to afford yellow colored (4-(10H-phenoxazin-10-yl)phenyl)(pyridin-3-yl)methanone/ 3BPy-pPXZ or (4-(10H-phenoxazin-10-yl)phenyl)(pyridin-4-yl)methanone/ 4BPy-pPXZ.

3BPy-pPXZ: 1H NMR (400 MHz, Chloroform-d) δ 9.03 (d, J = 1.3 Hz, 1H), 8.82 (d, J = 3.1 Hz, 1H), 8.16 (d, J = 8.4 Hz, 1H), 8.02 (d, J = 8.4 Hz, 2H), 7.50 (d, J = 8.4 Hz, 2H), 7.48 – 7.45 (m, 1H), 6.73 – 6.64 (m, 4H), 6.63 – 6.57 (m, 2H), 5.98 (d, J = 7.9 Hz, 2H). 13C NMR (101 MHz, Chloroform-d) δ 193.82, 153.18, 150.92, 144.10, 143.90, 137.13, 136.43, 133.60, 132.86, 132.72, 131.06, 123.53, 123.34, 122.03, 115.82, 113.44. HRMS (ESI+) calcd for C24H17N2O2 [M + H]+, 365.1290 and found 365.1288.

4BPy-pPXZ: 1H NMR (400 MHz, Chloroform-d) δ 8.83 (d, J = 6.0 Hz, 2H), 8.02 (d, J = 8.5 Hz, 2H), 7.62 (d, J = 6.0 Hz, 2H), 7.50 (d, J = 8.4 Hz, 2H), 6.77 – 6.64 (m, 4H), 6.63 – 6.56 (m, 4H), 5.98 (d, J = 7.9 Hz, 4H). 13C NMR (101 MHz, Chloroform-d) δ 194.05, 150.56, 144.34, 144.16, 143.98, 135.50, 133.53, 132.86, 131.01, 123.33, 122.77, 122.12, 115.88, 113.50. HRMS (ESI+) calcd for C24H17N2O2 [M + H]+, 365.1290 and found 365.1289.

[096] Analytical Data
TAT-(4BPy), TAT-3(4BPy), and TAT-3(4BPy) emitters
TD-DFT calculated energy levels
Theoretical calculation: Time-dependent density functional theory (TD-DFT) calculations were performed using B3LYP 6-31g (d, p) basis set (Table 1). In all three emitters, the highest occupied molecular orbitals (HOMOs) are primarily localized on the central TAT core, while the lowest unoccupied molecular orbitals (LUMOs) spread over the 4-benzoyl pyridine acceptor and slightly extended to the donor unit. This HOMO-LUMO separation gives rise to the charge transfer (CT) character. Both HOMO and LUMO energy levels are stabilizing with increasing the acceptor number, but the stabilization strength is more in HOMO. Also, there is a gradual increment in the HOMO-LUMO gap while moving from TAT-(4BPy) to TAT-3(4BPy).

[097] Table 1: TD-DFT calculated energy levels of the emitters
Emitter DFT calculated values
TAT-(4BPy) S1/ T1 (eV)=2.40/ 2.33
ΔEST (eV) = 0.07
HOMO (eV) =-4.98
LUMO (eV)=-2.18
f=0.0069
TAT-2(4BPy) S1/ T1 (eV)=2.49/ 2.39
ΔEST (eV) = 0.10
HOMO (eV) =-5.15
LUMO (eV)=-2.26
f=0.0184
TAT-3(4BPy) S1/ T1 (eV)=2.56/ 2.44
ΔEST (eV) = 0.12
HOMO (eV) =-5.32
LUMO (eV)=-2.35
f=0.0369

[098] Photophysical properties:
The toluene solutions (0.01 mM) of TAT-(4BPy), TAT-2(4BPy), and TAT-3(4BPy) showed major absorption peaks at 388, 383, and 378 nm, respectively, corresponding to the intramolecular charge transfer transition. The fluorescence spectra of TAT-(4BPy), TAT-2(4BPy), and TAT-3(4BPy) are structureless in appearance, and their PLmax are found at 530, 520, and 512 nm. With the increase in acceptor numbers, absorption and emission maxima are shifting to high-energy regions. From the onsets of the low temperature (@77K) fluorescence and phosphorescence spectra, singlet-triplet energy gaps (ΔEST) of TAT-(4BPy), TAT-2(4BPy), and TAT-3(4BPy) were calculated and are found to be 0.01, 0.04, and 0.07 eV, respectively. It can be noticed from Figure 3 that the phosphorescence spectra of the emitters are gradually getting more structure-ness while moving from TAT-(4BPy) to TAT-3(4BPy). Therefore, it can be stated that the charge transfer (CT) character in the triplet state of the emitters is gradually reducing with increasing the acceptor numbers. All of the three compounds showed improvement with respect to the singlet triplet energy gaps ΔEST.

[099] PL properties of the emitters are also explored in rigid host-doped conditions. In 5 wt.% emitter: mCBP doped films, the PLmax of TAT-(4BPy), TAT-2(4BPy), and TAT-3(4BPy) are recorded to be 531, 525, 518 nm, respectively. Both the doped film and toluene solution PL of the emitters match. Photoluminescence quantum yields (PLQYs) were measured for the emitters in their mCBP host-doped films. Under the aerated conditions, TAT-(4BPy), TAT-2(4BPy), and TAT-3(4BPy) showed PLQYs of 74.9, 72.1, and 58.7%, respectively. Under inert condition TAT-(4BPy) shows the highest quantum yield of ≃100%, followed by 91.5% in TAT-2(4BPy), and 82.8% in TAT-3(4BPy). The enhancement of PLQY under inert conditions confirms the involvement of triplet exciton in radiative relaxation processes, a characteristic feature of delayed fluorescent emitters.

[100] Table 2. PL properties of the emitters
Emitter Abs (nm)
PLmax (nm) Tol/host doped
ΔEST (eV) HOMO/ LUMO (eV) PLQY (%) Air/ Vacuum τP (ns)/ τD (μs)

a b c d e f
TAT-(4BPy) 388 530/531 0.01 -5.18/ -2.43 74.9/ 100 21.8/2.2
TAT-2(4BPy) 383 520/525 0.04 -5.24/ -2.44 72.1/ 91.5 20.9/2.1
TAT-3(4BPy) 378 512/518 0.07 -5.38/ -2.54 58.7/ 82.8 19.1/ 1.5

[101] a. Absorption in toluene solution; b. Emission maxima in 0.01 mM toluene and in mCBP host doped film; c. Singlet-triplet energy gap in toluene solution; d. Highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels evaluated from cyclic voltammetry; e. Photoluminescence quantum yield of the mCBP host doped film under ambient conditions and under vacuum; f. Prompt and delayed fluorescence lifetime of the emitters in their mCBP host-doped film.

[102] Excited state lifetime: The prompt fluorescence lifetimes of mCBP host doped films of TAT-(4BPy), TAT-2(4BPy), and TAT-3(4BPy) were recorded to be 21.8, 20.9, and 19.1 ns, respectively, reflecting faster fluorescence radiative rate in the multi-acceptor system. All the emitters showed biexponentially decay in their delayed fluorescence curves. Interestingly, under the aerated condition, the emitters showed lifetimes in the sub-microsecond range of 0.85 μs in TAT-(4BPy), 0.54 μs in TAT-2(4BPy), and 0.26 μs in TAT-3(4BPy), suggesting ongoing faster triplet reverse intersystem crossing process. Under vacuum, the delayed lifetime of TAT-(4BPy), TAT-2(4BPy), and TAT-3(4BPy) increases to 2.2, 2.07, and 1.47 μs, respectively. It can also be noticed that the delayed lifetimes (τd) gradually decrease with the number of acceptor units (Table 2). The kinetic parameters such as intersystem crossing (kISC), reverse intersystem crossing rate constants (kISC) of the mCBP doped films of TAT-(4BPy), TAT-2(4BPy), and TAT-3(4BPy) are calculated to be 1.14 x 107 s-1 and 6.06 x 105 s-1, 1.01 x 107 s-1 and 6.14 x 105 s-1, and 1.52 x 107 s-1 and 9.58 x 105 s-1, respectively. With increasing acceptor numbers, the excited state lifetime is gradually shortening, and the rISC process gets faster (Figure 4).

[103] OLED device fabrication: Vacuum-deposited OLEDs (Device-A, B, and C) are fabricated using 5 wt.% of emitters in mCBP host doped film as an emissive layer. EML of device A= TAT-(4BPy): mCBP, device B= TAT-2(4BPy): mCBP, and Device C= TAT-3(4BPy): mCBP. The optimized device architecture was ITO/ NPB (30 nm)/ TAPC (20 nm)/ EML (15 nm)/ PPT (10 nm)/ TPBi (50 nm)/ Liq (2 nm)/ Al (100 nm). Indium tin oxide (ITO) and aluminium were used as anode and cathode, respectively. NPB was used as hole injection, TAPC was used for hole transportation, PPT was used for exciton blocking, and TPBi was used as an electron transportation layer. A 2 nm thin layer of Liq was used for the smooth injection of electrons.

[104] The electroluminance (EL) spectra of all three devices were in the green region (Figure 5 & Table 3); However, while moving from TAT-(4BPy) to TAT-3(4BPy), the ELmax was gradually blue shifting, following the similar trend of PL emission. The EQEmax of TAT-(4BPy), TAT-2(4BPy), and TAT-3(4BPy) were found to be 29.1, 24.6, and 20%, respectively. The gradual reduction trend of EQE is again similar to the doped film PLQY trend of the emitters (Table 2 and 3).

[105] The efficiency roll-off of TAT-(4BPy), TAT-2(4BPy), and TAT-3(4BPy) based OLED devices at 1000 cd/m2 are found to be 33.1, 6.5, and 11.9%, respectively. The latter two emitters showed the smallest roll-off compared to the first emitter due to their fast rISC rate. Among TAT-2(4BPy) and TAT-3(4BPy), the latter shows a slightly higher efficiency roll-off due to its greater delayed fluorescence contribution in the radiative decay process.

[106] Table 3. EL properties:
OLED device Emitter used in EML ELmax (nm) Von (V) PEmax (lm/W) CEmax (cd/A) EQEmax (%)/ @1000 cd/m2 Roll-off (%) @1000 cd/m2
a b c d e f
Device-A TAT-(4BPy) 534 4.5 65.9 83.9 29.1/ 19.4 33.1
Device-B TAT-2(4BPy) 524 3.5 37.8 78.8 24.6/ 23.0 6.5
Device-C TAT-3(4BPy) 505 3.9 32.8 52.2 20.0/17.6 11.9

[107] a. Electroluminescence maxima; b. Turn on voltage; c. Maximum power efficiency; d. Maximum current efficiency; e. Maximum external quantum efficiency and at 1000 luminance; f. Efficiency roll-off @ 1000 luminance.

3BPy-pPXZ and 4BPy-pPXZ emitters
TD-DFT calculated energy levels
[108] Theoretical calculations: Theoretical calculations for the emitters were performed using time-dependent density functional theory (TD-DFT) calculations (Table 4). Both 3BPy-pPXZ and 4BPy-pPXZ showed nearly similar ΔEST values and oscillator strengths, suggesting acceptor “N” has no role in the singlet-triplet energy barrier and transition probability. However, compared to 3BPy-pPXZ, 4BPy-pPXZ shows stabilized HOMO and LUMO energy levels (Table 4).

[109] Table 4. TD-DFT calculated energy levels of the emitters:
Emitter DFT calculated values
3BPy-Ppxz S1/ T1 (eV)=2.09/ 2.04
ΔEST (eV) = 0.05
HOMO (eV) =-4.79
LUMO (eV)=-2.21
f=0.0237
4BPy-pPXZ S1/ T1 (eV)=2.02/ 1.96
ΔEST (eV) = 0.05
HOMO (eV) =-4.85
LUMO (eV)=-2.35
f=0.0253

[110] Photophysical properties: UV-visible absorptions of the emitter were recorded in 0.01 mM toluene solution (Figure 6). The toluene solution of 4BPy-pPXZ shows a comparatively higher PLmax than 3BPy-pPXZ (Table 5). Both emitters are non-emissive in the solution state due to their strong CT nature; their PL were recorded in 7 wt.% emitter: CBP host doped condition using 340 nm excitation wavelength (Figure 6). The fluorescence spectra of both emitters are structureless in appearance, suggesting the CT nature of the excited singlet state. The PLmax of 3BPy-pPXZ and 4BPy-pPXZ are found at 538 and 547 nm, respectively. By changing the pyridine position from 3rd to 4th in the acceptor units, a bandgap reduction and lowering of excited state energy level was observed, resulting in a red shift of the spectra.

[111] The PLQYs of the emitters in their CBP host doped films were measured in the air and under inert conditions. Between the two emitters, 3BPy-pPXZ shows a higher PLQY of 73.8% under inert conditions (Table 5). However, the doped film of 4BPy-pPXZ shows the greater triplet/ DF contribution of 49%, while in 3BPy-pPXZ the value was 31%. This suggests that the acceptor “N” position has an important role towards the overall DF contribution in an emitter.

[112] Table 5. PL properties of the emitters
Emitter Abs (nm) PLmax (nm) ΔEST (eV) HOMO/ LUMO (eV) PLQY (%) Air/ Vacuum τP (ns)/ τD (μs)
a b c d e f
3BPy-pPXZ 410 538 0.14 -5.13/ -2.54 56.1/ 73.8 23.03/2.07
4BPy-pPXZ 422 547 0.11 -5.18/ -2.64 42.7/ 63.6 21.78/ 2.74

[113] a. Absorption in toluene solution; b. Emission maxima in CBP host doped film; c. Singlet-triplet energy gap in doped film; d. Highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels evaluated from cyclic voltammetry; e. Photoluminescence quantum yield of the CBP host doped film under ambient conditions and under vacuum; f. Prompt and delayed fluorescence lifetime of the emitters in their CBP host-doped film.

[114] Transient PL measurement: Transient PL measurements of the emitters were done in CBP host-doped film. The 7 wt.% 3BPy-pPXZ: CBP host doped film showed DF lifetime of 2.07 μs and PF lifetime of 23 ns (Figure 7 & Table 5). At the same time, the PF and DF lifetimes of the corresponding 4BPy-pPXZ film were 21.8 ns and 2.7 μs, respectively. Among the two emitters, 3BPy-pPXZ shows a comparatively faster rISC rate with krISC of 6.5 x 105 s-1; 4BPy-pPXZ value was 5.4 x 105 s-1. 3BPy-pPXZ (2.4 x 107 s-1) also shows a higher radiative rate (kr) compared to 4BPy-pPXZ (1.9 x 107 s-1).

[115] Electroluminescent properties: Two vacuum-deposited OLEDs were fabricated device-D and E using Bis(N-carbazolyl)-1,1′-biphenyl (CBP) host doped films of 3BPy-pPXZ and 4BPy-pPXZ, respectively, in the emissive layer. The optimized device architecture was ITO/ NPB (30 nm)/ TAPC (20 nm)/ EML (7 wt.% emitter: CBP) 15 nm/ PPT (10 nm)/ TPBi (50 nm)/ Liq (2 nm)/ Al (100 nm). The electroluminescence maxima (ELmax) of Device-D and E were found at 534 and 545 nm, respectively. Device-D showed an EQEmax of 17.3%, and efficiency roll-off at 1000 cd/m2 was found to be 10.9%. On the other hand, by changing the acceptor “N” position from 3rd to 4th the EQEmax of device-E increased to 23.7%, which was 1.4 times higher than the value of device-D (Figure 8). The efficiency roll-off of 4BPy-pPXZ based OLED at 1000 cd/m2 was calculated to be 36%, which is 3 times higher than the value of 3BPy-pPXZ-based device. The efficiency roll-off at 5000 cd/m2 of device-D was found to be 27%, which is one of the lowest values from a purely organic TADF OLED. In comparison to 3BPy-pPXZ, 4BPy-pPXZ possesses a comparatively slower rISC and radiative rate but exhibits a faster rISC rate, along with it has a greater delayed fluorescence (DF) contribution in its radiative decay process. We anticipate all of these factors contributing to a strong efficiency roll-off in 4BPy-pPXZ. , Claims:1. A compound having a structural Formula (I),
D-(E)n-(A)m Formula (I)
wherein n = 1 to 3; m = 1 to 3;
wherein D is a benzene ring

E is a group fused to benzene ring D at one or more positions selected from C1-C2, C3-C4, and C5-C6 to result in symmetric or asymmetric structures;
E is a heterocyclic ring selected from

and ;
wherein E is substituted or fused with Y group;
Y is selected from


, ,

and ;
A is independently selected from
or ;
wherein C is
or ; and
B1 and B2 are

, , ,
, , or ; and
wherein the group A is attached to the E at one or more of the N atoms present in E via a C-N covalent bond;
and when the N atom is not substituted with A, it is substituted with Y.

2. The compound as claimed in claim 1, wherein E is selected from

or ;
and group A is attached to the E at one or more of the N atoms present in E via a covalent bond;
and when the N atom is not substituted with A, it is substituted with Y; wherein, E is fused to benzene ring D at C1-C2 position, or at C1-C2, and C3-C4 positions, or at C1-C2, C3-C4 and C5-C6 positions.

3. The compound as claimed in claim 1 or 2, wherein D-(E)n is a symmetric group selected from

and the group A is attached to the E at one or more of the N atoms present in E via a covalent bond and when the N atom is not substituted with A, it is substituted with Y.

4. The compound as claimed in claim 1 or 2, wherein D-(E)n is an asymmetric group selected from


, and .
5. The compound as claimed in claims 1 to 3 represented by the following structural formula:
, or
wherein A1, A2, and A3 is a substituent , wherein C is , B1 is , and B2 is , and Y is as defined in claim 1.

6. The compound as claimed in claim 5, wherein Y is phenyl
.
7. The compound as claimed in claims 4 to 6 represented by the following structural formula:
, , or .
8. The compound as claimed in claim 1, wherein the compound is represented by the structural formula:
.

9. The compound as claimed in claim 8, wherein A is selected from 3-benzoylpyridine or 4-benzoyl pyridine
, .

10. The compound as claimed in claims 8 to 9 represented by the following structural formula:
or .
11. The compound as claimed in claim 1, wherein the D-(E)n is an electron donor, and A is an electron acceptor and the compound D-(E)n-(A)m is a thermally activated delayed fluorescence emitter having an energy barrier ΔEST of ≤ 0.25eV, and an efficiency roll off at 1000 or 5000 cd/m2 luminescence of less than 40%.

12. A process for synthesizing the compound of Formula (I) as claimed in claim 1, comprising reacting a precursor of the donor D-(E)n with a precursor of acceptor A in the presence of a catalyst and a base to form C-N coupling reaction.

13. The process as claimed in claim 12, wherein the precursor of donor D-(E)n is selected from , or ;
and the precursor of the acceptor A is or ;
wherein “X” is Cl, Br, F, or I;
and the base is selected from sodium tertiary butoxide, or potassium phosphate.

14. The process as claimed in claims 12 to 13, wherein the donor D-(E)n is reacted with the acceptor A in the presence of catalyst and a base to replace one or more of the N-H with A.

15. The process as claimed in claim 14, wherein the “N-H” group which is not replaced with A is replaced with the group Y, selected from phenyl group.

16. The process as claimed in claim 13, wherein the catalyst is selected from palladium, copper powder or copper iodide and base is selected from sodium tertiary butoxide or potassium phosphate A.

17. An organic light emitting diode (OLED) comprising the compound as claimed in claim 1.

18. The OLED as claimed in claim 17, comprising:
an aluminium cathode;
an indium tin oxide anode;
a hole injection layer comprising N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB);
a hole transportation layer comprising 1,1-Bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC);
an emissive layer comprising a compound of formula (I) as the emitter along with a host material selected from mCBP (3,3'-di(9H-carbazol-9-yl)-1,1'-biphenyl);
an exciton blocking layer comprising 2,8-bis(diphenyl-phosphoryl)-dibenzo[b,d]thiophene (PPT); and
an electron transport layer comprising 2,2′,2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole (TPBi);
wherein the emitter has a shorter excited state lifetime of less than 1.0 μs, faster reverse intersystem crossing rate (rISC), and smaller delayed fluorescence contribution.

19. The OLED as claimed in claims 17 to 18, wherein the emitter has a shorter excited state lifetime in the range of 0.25-0.85 μs.

20. The OLED as claimed in claims 17 to 18, wherein the diode comprises 5 to 10 wt% of the compound of formula (I) along with 3,3-di(9H-carbazol-9-yl)biphenyl (mCBP) as host material in the emissive layer.