Thermally reversible dual-state luminescent organic donor-acceptor conjugates and method thereof

Abstract

The present invention disclose a novel thermally reversible dual-state luminescent organic donor-acceptor conjugate of Formula 1. It includes dual emissive states comprising a local emission LE state and a charge transfer CT state, which are mutually interconvertible at different temperatures due to conformational isomerism at high and low temperatures. The present invention comprises of a light-emitting material with TRDSL that provides ratiometric emission response and detect thermal energy at low (0-300K) and high (300-350K), thereby the present invention also includes a process of preparation of thermally reversible dual-state luminescent organic donor-acceptor conjugate.

Description

[0001] THERMALLY REVERSIBLE DUAL-STATE LUMINESCENT ORGANIC DONOR-ACCEPTOR CONJUGATES AND METHOD THEREOF

[0002] FIELD OF INVENTION

[0003] The present invention generally relates to the field of thermodynamics. Specifically, the present invention relates to organic donor- acceptor based light emitting materials for optical thermal sensor. More particularly, the present invention relates to organic donoracceptor conjugates having thermally reversible dual-state luminescence and that find an application in optical thermal sensing.

[0004] BACKGROUND OF THE INVENTION

[0005] Over the past few years, thermally reversible dual state luminescence and optical research on chromatic materials or phosphors based thermometry systems has been becoming increasingly important in different fields as industrial engineering, nanosciences. It was basically performed by thermometry systems, the thermometry systems are divided between reversible and irreversible systems. Reversible thermos chromic modifications require in-situ measurements are generally based on their luminescence temporal modification since the incandescence phenomena prohibit at high temperature the characterization of absorption spectrum or emission spectrum with the thermal history is largely investigated from chromatic or luminescent properties modification vs thermal history.

[0006] The reason behind the thermally reversible optical sensor is they are mostly involved in ratio metric optical thermometer for temperature sensing and high efficiency, and they are purely organic light-emitting materials have been actively studied to enhance the light emission efficiency thereof. Most investigations into the role of organic light emitters in efficient energy harvesting have focused on their use in organic electroluminescence device applications. There are few of the reports available in the state of art about the existence of thermally reversible dual state optical sensor.

[0007] US20170186976A1 discloses a fluorescent organic light emitting elements having high efficiency, the invention discloses an organic light emitting elements, comprising thermally activated delayed fluorescence (TADF) emitters and hosts on basics of benzotriazoles, which have a sufficiently small energy gap between SI and T1 to enable up-conversion of the triplet exciton from T1 to SI. The organic light emitting elements show high electroluminescent efficiency.

[0008] US4859079A discloses an optical system using a luminescent material sensor for measuring very high temperatures, the invention discloses a technique of measuring very high temperatures by positioning a blackbody sensor in thermal communication with an environment or object whose temperature is to be measured, communicating infrared emissions having an energy level proportional to such temperature from the sensor to an infrared absorber positioned a distance away from the sensor where the ambient temperature is significantly reduced, and then optically measuring the temperature of the absorber by a technique that uses visible or near visible optical radiation, such as one using a luminescent sensor. The measured temperature of the absorber is proportional to that of the blackbody sensor.

[0009] EP2851408A1 discloses an organic light emitting element, and light emitting material and compound used in the same, the invention discloses an organic light-emitting device having a light-emitting layer containing a compound represented by the general formula below has a high light emission efficiency. In the general formula, at least one of R1to R5represents a cyano group, at least one of R1to R5represents a 9-carbazolyl group, a 1, 2, 3, 4-tetrahydro-9-carbazolyl group, a 1-indolyl group or a diary amino group, and the balance of R1to R3represents a hydrogen atom or a substituent. Over the past decades, attention to organic solid-state light emitters, in particular, donoracceptor (D-A) conjugate-based delayed fluorescent materials with a high quantum yield at ambient conditions is increasing. However, light emissions of existing organic emitters are not only limited to a non-equilibrium well- separated emissive states, but they suffer from aggregate caused quenching effect and thermal quenching at elevated temperatures caused by predominant non-radiative channels (knr ) and tunable luminescence for use across a wide-range of temperatures.

[0010] Surprisingly, the light emission efficiency thereof is limited to room-temperature (RT). Moreover, despite their potential role in display devices, the existing materials experience dominant single broadband emission and luminescence quenching above RT due to predominant non-radiative ( knr) channels at elevated temperatures and thermal instability of the materials. The challenge of thermally reversible dual state luminescence (TRDSL) upon heating and cooling, and luminescence enhancement with increasing temperature (LEIT), ratio metric emission response for a wide range of temperature (low to high), and thermal stability requirements, constitute the major obstacles faced by the thermal sensing technologies. Moreover, TRDSL with such features using organic light emitting materials has not been studied in the solid state. This invention has now found a principle that allows TRDSL to provide corresponding donor-acceptor (D-A) conjugates that can be used as optical thermal sensor (low to high temperature) and thermal imaging. According to the invention, the electron donor (D) and acceptor (A) components at the separation distance of a C-N single bond are held in twisted orientation, enabling intense emission from singlet locally excited (1LE) state of A unit and weak emission from excited singlet charge transfer (1CT) state caused by D-A electronic coupling at the excited state at low temperature (10 K). On heating (10-350 K), the emission intensity of the lowest1CT state is gradually enhanced. Surprisingly, increased temperature (10- 350 K) led to an increase in the population of the CT emission band that involved delayed fluorescence (DF) (at 150 K) due to the small singlet-triplet (1CT-3CT) gap caused by spatial separation of HOMO (on the donor primarily) and LUMO (mainly on the acceptor). On cooling (350-10 K), the D-A conjugates can reclaim their local emission1LE ), showing the thermal reversibility of the emissive centers.

[0011] According to the invention, the torsional motion of the donor attached to the acceptor via a C-N single bond provides conformational change (rotational conformers), such that the two emissive centers (LE, CT) caused by conformational isomerism are in out- of-equilibrium, and luminescence enhancement of the lowest energy CT center with increasing temperature can be achieved due to thermal induced interconversion of the conformer. The chemical substituents at the respective D and A units provide perturbation of ground and excited energy levels of D-A molecules in such a way that required TRDSL is restored.

[0012] Thus, the organic light-emitting materials that show thermally reversible dual-state luminescence (TRDSL) of the present invention is conducive for ratiometric emission response (RER) for a wide range of temperatures and luminescence enhancement with increasing temperature (LEIT), have not been developed due to a lack of control in dominant knr at elevated temperatures. Therefore, it is needed to investigate new economically viable organic materials that would address these limitations.

[0013] From the above cited documents it is clearly inferred that the existing luminescence based optical sensors are cumbersome for analysis, lack of accuracy and lack of temporal resolution of emission signal which leads to troublesome in temperature change and also highly expensive.

[0014] Hence, there exists an urgent need in the state of art to develop a novel thermally reversible dual-state luminescent organic donor-acceptor conjugates for optical sensors that are devoid of the above mentioned drawbacks. OBJECTIVES OF THE INVENTION

[0015] The principle objective of the present invention is to develop organic thermally reversible light emitters.

[0016] Another objective of the present invention is to develop novel thermally reversible dual state luminescent compounds that are light emitters and find application as optical sensor.

[0017] Yet another objective of the present invention is to develop light-emitting materials that are easily processable as they are highly soluble in many organic solvents (for example, toluene, xylene, chlorobenzene, dichloromethane, tetrahydrofuran).

[0018] Another objective of the present invention is to produce light emitting material that includes the application of thermal imaging devices.

[0019] Further objective of the present invention is to utilize the developed thermally reversible dual-state luminescence (TRDSL) for ratiometric optical thermometer for temperature sensing and high efficiency OLEDs.

[0020] Yet another objective of the present invention is to provide thermally reversible duallight emitting donor acceptor conjugate compound that act via interconverting the excited emissive centers in solid-state.

[0021] It is another objective of the present invention to provide phenoxazine-quinoline structure or phenothiazine-quinoline compounds comprising electron-donating and withdrawing groups and peripheral aryl substituents as a thermally reversible dual-state luminescent (TRDSL) materials with luminescence enhancement with increasing temperature (LEIT) in solid-state.

[0022] It is another objective of the present invention to provide a method of preparation of thermally reversible dual-state luminescent organic donor-acceptor conjugates as light emitters. It is another objective of the present invention to provide phenothiazine-quinoline conjugate with a phenothiazine structure, a halogen, and naphthalene substituted quinoline structure and the like, as a thermally reversible dual- state luminescence (TRDSL) material for ratiometric optical-thermal sensing application. Further objective of the present invention is to utilize the developed thermally reversible dual - state luminescent organic donor- acceptor conjugates for optical thermal sensing.

[0023] SUMMARY OF THE INVENTION

[0024] Accordingly, In an aspect, the present invention discloses a novel thermally reversible dual-state luminescent organic donor-acceptor conjugate of Formula 1, comprising:

[0025] Formula - 1 wherein donor (D) comprises phenothiazine,

[0026] A is acceptor comprising naphthalene substituted quinolinyl acceptor;

[0027] R1is electron- withdrawing group selected from a halogens F, Cl, and Br, and the compound of Formula 1 is a light-emitting material with TRDSL that provides ratiometric emission response and detect thermal energy at low (0-

[0028] 300K) and high (300-350K); R2is selected from the group comprising of phenyl, p-tolyl, 4-tertbutyltolyl, 1- naphthyl, 1-mthoxyphenyl, 1, 2-dimethoxyphenyl, 3, 5-dimethoxyphenyl, thiophenyl, and R3is hydrogen (H) or CH3.

[0029] In an another aspect, the present invention also discloses a process of preparation of a thermally reversible dual-state luminescent organic donor-acceptor conjugate of Formula 1 as claimed in claim 1, comprising the steps:

[0030] Preparation of Donor D i. dissolving l-fluoro-2nitrobenzene and 2-(10H-phenothiazin-

[0031] 1 Oylfani line in solvent, sovent 1 selected from dimethylformamide (DMF), dimethyl sulphoxide (DMSO), dimethylacetamide (DMAc) under N2 gas, in the presence of base selected from sodium hydride or K2CO3, CS2CO3, potassium tert-butoxide at a temperature in a range of 80°C - 140 °C for 12 - 36 hours; ii. cooling of the resultant solution and pouring on ice cold water; iii. separating the precipitate; iv. adding Phenothiazine-NO2of Formula 2 to an acid to obtain a solution; v. adding stannous chloride (SnC12) or Hydrazine hydrate with Pd-C or H2gas with Pd-C to solution obtained in step (iv) and stirring for 30- 45 also (30 mins - 24 hrs.) at a temperature of 60-80 degrees,; vi. evaporating of solvent to yield Intermediate 1;

[0032] b. Preparation of Donor (D) - Acceptor (A) Conjugate i. adding Intermediate 1, a halogen substituted benzaldehyde and 1- ethynylnaphthalene in the presence of a catalyst in a high boiling aromatic solvent at 70-110°C for 6-12 hours under molecular oxygen; ii. cooling to room temperature followed by vacuum evaporation to yield the compound of Formula 1.

[0033] In an another aspect of the present invention, wherein in step (b) the catalyst is selected from ferric chloride, Iron(III) trifluoromethanesulfonate, Copper(II) trifluoromethanesulfonate, Silver(I) trifluoromethanesulfonate, or Iodine. In an another aspect of the present invention, wherein in step (b) the solvent is selected from nitromethane, benzene, toluene, xylene, chlorobenzene, 1 ,2-dichlorobenzene and the like.

[0034] In an another aspect of the present invention, wherein the reaction employed in the oxygenated environment and the halogen is selected from Cl, F, or Bromine.

[0035] BRIEF DESCRIPTION OF DRAWINGS:

[0036] Figure 1. illustrates (a) Energy diagram for thermally reversible dual-state luminescence.1LE represents singlet locally excited state;1CT represents excited singlet charge transfer state;3CT represents excited triplet charge transfer state. ISC represent intersystem crossing; rISC represent reverse intersystem crossing. SO represents ground state, (b) Potential energy surface scan along the torsion angle when viewed along the C11-N2- C8-C9 atoms (Example 2). AE represents energy difference between the two energy minimized structures (conformers) as shown in the inset of the graph.

[0037] Figure 2. Illustrates the thermal stability of the (a) Example conjugates 1. (b) Example conjugate 2, and (c) Example conjugate 3.

[0038] Figure 3. Illustrates (a) Absorption and (b) emission of blend film (Example 2:mCP = 1:99 by wt), donor (PTz:mCP = 1:99 by wt) and acceptor (QCL: mCP = 1:99 by wt) at ambient conditions. (For Example 2)

[0039] Figure 4. (a) Illustrates the thermally reversible dual state luminescence (TRDSL) upon heating and cooling, (b) Ratio metric change in emission intensity upon heating and cooling. LE represents locally excited state; CT represents excited charge transfer state. (For Example 2)

[0040] Figure 5. illustrates the fluorescence decay of (a) local emission (LE) and (b) charge transfer emission (CT) at 150 and 350 K. TLE represents lifetime of LE; TPF and TDF represent lifetimes of the prompt and delayed fluorescence components of the CT emission band. (For Example 2)

[0041] Figure 6. Illustrates the steady state emission and phosphorescence spectra of Example 2, donor (PTz), acceptor (QC1) at RT and 77 K. (For Example 2)

[0042] Figure 7. Illustrates the switching cycles of (a) intensity of the local emission (LE) and (b) lifetimes of LE emission band upon heating (350 K) and cooling (150 K). (For Example 2). Switching cycles of (c) intensity, (d) lifetime of the prompt fluorescence (PF), and (e) delayed fluorescence (DF) components of the charge transfer emission (CT) upon heating (350 K) and cooling (150 K). (For Example 2)Figure 8. (a) illustrates the quantum yields and (b) rates upon heating (150-350 K). ΦE represents total quantum yield, (|)F(CT) represents fluorescence quantum yields of local emission (LE), (|)PF(CT) represents prompt fluorescence quantum yield of charge transfer emission (CT), (|)DF(CT) represents delayed fluorescence quantum yield of charge transfer emission (CT), Φisc represents quantum yield of intersystem crossing, kr(LE) represents radiative rate of local emission (LE), kr(CT) represents radiative rate of charge transfer emission (CT), km(CT) represents non-radiative rate of charge transfer emission (CT), kisc represents rate of intersystem crossing (ISC), krisc represents rate of reverse intersystem crossing (rISC). (For Example 2)

[0043] Figure 9 illustrates the thermodynamic equilibrium of the local emission (LE) and charge transfer emission (CT) of the conjugate. AG represents energy gap between the LE and CT centers. R represents regression coefficient. (For Example 2)

[0044] Figure 10 illustrates the optical-thermal sensing characteristics of the conjugate. Sa represents maximum absolute sensitivity (Sa = 0.50701). Sr represents relative sensitivity (Sr = 7.78%). (for Example 2) DETAILED DESCRIPTION OF THE INVENTION

[0045] The present invention as embodied by “-THERMALLY REVERSIBLE DUALSTATE LUMINESCENT ORGANIC DONOR-ACCEPTOR CONJUGATES AND METHOD THEREOF — ”, succinctly fulfils the above mentioned need(s) in the art. The present invention has objective(s) arising as a result of the above-mentioned need(s), said objective(s) being enumerated below. In as much as the objective(s) of the present invention are enumerated, it will be obvious to a person skilled in the art that the enumerated objective(s) are not exhaustive of the present invention in its entirety, and are enclosed solely for the purpose of illustration. Further, the present invention encloses within its scope and purview, any structural altemative(s) and / or any functional equivalent(s) even though such structural altemative(s) and / or any functional equivalent(s) are not mentioned explicitly herein or elsewhere, in the present disclosure. The present invention therefore encompasses also, any improvisation(s) / modification(s) applied to the structural altemative(s) / functional alternative / s) within its scope and purview. The present invention may be embodied in other specific form(s) without departing from the spirit or essential attributes thereof.

[0046] Throughout this specification, the use of the word "comprise" and variations such as "comprises" and "comprising" may imply the inclusion of an element or elements not specifically recited.

[0047] The invention in its various embodiments discloses thermally reversible dual-state luminescent organic donor-acceptor conjugates as Dual light-emitting organic materials. The invention in one embodiment discloses organic materials having thermally reversible dual-state luminescence, more particularly, to organic donor-acceptor based light emitting materials for optical thermal sensor. The invention discloses organic thermally reversible dual-state luminescent (TRDSL) materials that show ratiometric emission response with heating and cooling (10-350 K). The organic light emitting compounds of the present invention may be applied to an optical thermal sensor that can detect thermal energy from a wide range of temperatures (10-350). In a principal embodiment is disclosed the Donor (D) and Acceptor (A) based light emitting of the present invention is a thermally reversible dual- state luminescent organic donor- acceptor conjugate compound of Formula I:

[0048] FORMULA (I) The compound of Formula I, wherein donor (D) comprises pheno thiazine,

[0049] A is acceptor comprising naphthalene substituted quinolinyl acceptor;

[0050] R1is electron- withdrawing group selected from a halogens F, Cl, and Br;

[0051] R2is selected from the group comprising of phenyl, p-tolyl, 4-tertbutyltolyl, 1 -naphthyl, 1-mthoxyphenyl, 1, 2-dimethoxyphenyl, 3, 5-dimethoxyphenyl, thiophenyl, and R3is hydrogen (H) or CH3, wherein the compound of Formula I is a light-emitting material with TRDSL that provides ratiometric emission response and detect thermal energy at low (10-300K) and high (300-350K) temperatures.

[0052] In one embodiment of the present invention is disclosed thermally reversible dual-state luminescent organic donor- acceptor conjugate compounds as Dual light-emitting organic materials comprising organic donor- acceptor (D-A) based TRDSL materials containing a phenothiazine donor, a halogen (F, Cl, Br), and naphthalene substituted quinolinyl acceptor (A).

[0053] According to the first embodiment, the thermally reversible dual- state luminescent organic donor- acceptor conjugate compound of Formula I, comprises a Donor D, Acceptor A and the organic donor-acceptor (D-A) conjugates with three substituents, which are located at the acceptor A. The donor and acceptor segments of the formula are connected via C-N single bond, where other R1, R2and R3substituents are connected via C-C single bond. The steric constraints of both D and A segments provide a twist, such that a dual-state emission is achieved due to conformational isomerism (CI) caused by the donor segment. Organic donor-acceptor (D-A) conjugates of the invention comprising a heteroaromatic donor unit bonded via a C-N single bond to a heteroaromatic acceptor unit (A). By suitable substituents, it was possible to achieve local emission (LE) and charge transfer (CT) emission bands due to conformational heterogeneity and electronic coupling between the D and A components separated at a distance of a covalent bond at the excited state. TRDSL compound (Formula I) of the present invention impart light emission via radiative decay of the locally excited (LE) and charge transfer (CT) states which are in thermodynamic equilibrium caused by the conformational isomerism (CI) at the ground state. Heating pushes the LE to the CT center, where delayed fluorescence emerged (at 150 K) due to the up-conversion of closely lying triplet CT (3CT) state to the singlet CT (1CT) state and cooling induced a reversal back to its LE emission, and has clarified that these materials with TRDSL at 10-350 K may be provided inexpensively. The chemical substituents at the respective D and A units may provide thermodynamic equilibrium between the LE and CT states. The heating and cooling can push the two centers out of equilibrium due to the conformational effect. The increased population of the CT center comprising1CT and3CT states shows delayed fluorescence with increasing temperature to achieve high ICT / ILE ratio. On cooling, the compounds can re-claim their LE emission with high ratio of ILE / ICT.

[0054] The donor and acceptor segments of the formula I are connected via C-N single bond, where other Rl, R2and R3 substituents are connected via C-C single bond. The steric constraints of both D and A segments provide a twist, such that a dual-state emission is achieved due to conformational isomerism (CI) caused by the donor segment. Upon heating, one of the conformers can interconvert to other, and cooling induces a reversal back to its original structure. To be effective in a specific D and A, the energy difference between the conformers must be low, so that thermodynamic equilibrium can be restored. The HOMO and LUMO must represent electron-rich and electron-poor regions and be primarily localized on the D and A components. Thus, HOMO (based mainly on D) and LUMO (largely based on A) can be spatially separated to achieve a low singlet (SI,1CT)- triplet (Tl,3CT) gap conducive to harvesting triplet energy via delayed fluorescence (DF). Photoexcitation may lead to a substantial shift of the electron density from the D to the A component due to D-A electronic coupling at the excited states. According to the skilled person, such condition is called charge transfer (CT) at the excited state, which results in a singlet excited state,1CT (corresponding to the above-defined SI; state) as well as an excited triplet state,3CT (corresponding to the above-defined Tl; state). Photoexcitation may also lead to local emission (LE) form the donor or acceptor segment due to week D-A coupling.

[0055] By suitable selection of the acceptor substituents as well as D unit, dual-state luminescence (DSL) via radiative decay of D or A component (singlet locally excited,1LE) and singlet charge transfer (1CT) state which is associated with DF that occurs via upconversion of the triplet state (3CT), can be achieved. Further, these selections may be useful for special requirements of intense LE at low temperatures and intense CT emission at elevated temperatures, which allow the TRDSL and LEET features. According to a D-A conjugate is preferred in which a ratiometric emission response (intensity or lifetime) of the LE and CT bands is inherently preserved. Depending on the D and the substituents located at the acceptor A, an efficient ratiometric emission response can be modulated.

[0056] According to the invention, the electron donor (D) and acceptor (A) components at the separation distance of a C-N single bond are held in twisted orientation, enabling intense emission from singlet locally excited ^LE) state of A unit and weak emission from excited singlet charge transfer (1CT) state caused by D-A electronic coupling at the excited state at low temperature (10 K). On heating (10-350 K), the emission intensity of the lowest1CT state is gradually enhanced. Surprisingly, increased temperature (10- 350 K) led to an increase in the population of the CT emission band that involved delayed fluorescence (DF) (at 150 K) due to the small singlet-triplet (1CT —3CT) gap caused by spatial separation of HOMO (on the donor primarily) and LUMO (mainly on the acceptor). On cooling (350-10 K), the D-A conjugates can re-claim their local emission ^LE), showing the thermal reversibility of the emissive centers. The conformational isomerism (CI) caused by D-A torsional freedom of the conjugates brings two emissive centers (LE, CT) out of equilibrium, such that the population of the DF center is increased with increasing temperatures to show TRDSL, and the emission loss caused by predominant knrat elevated temperatures is compensated by the CI (Figure 1). The TRDSE characteristics of the conjugates can be realized at variable temperatures ranging from 10-350 K. To realize a molecule with those shown in Figure 1 energy characteristics, the HOMO energy of the D component must be higher than that of the A component, while the EUMO energy of the A component is lower than the D component.

[0057] Figure 1 of the present invention discloses the torsional motion of the donor attached to the acceptor via a C-N single bond provides conformational change (rotational conformers), such that the two emissive centers (EE, CT) caused by conformational isomerism are in out-of-equilibrium, and luminescence enhancement of the lowest energy CT center with increasing temperature can be achieved due to thermal induced interconversion of the conformer. The chemical substituents at the respective D and A units provide perturbation of ground and excited energy levels of D-A molecules in such a way that required TRDSE is restored.

[0058] The light emitting materials have thermally reversible dual-state luminescence (TRDSL) and luminescence enhancement with increasing temperature (LEIT) and high photoluminescence quantum efficiency. The conjugates with TRDSL characteristics of the invention have such a feature that when these materials are heated (10-350 K) under photolight excitation (D or A absorption band), the conjugates show reversible ratiometric emission response of both emission centers, where emission loss due to predominant non- radiative pathways at elevated temperatures is compensated by interconverting the higher energy locally excited (LE) state to the lower energy charge transfer (CT) state caused by conformational isomerism (CI) of the compounds.

[0059] COMPOUNDS OF FORMULA I - DONOR AND ACCEPTOR

[0060] According to one embodiment of the present invention, the TRDSL compound with donor- acceptor (D-A) structural motifs, according to general formulas (I), have one covalently linked donor to the acceptor via one single bond segment. The D component is made of a heteroaromatic ring unit with two aromatic fused groups, while the element of A consists of one aromatic group connected with one heteroaromatic ring containing an aromatic group that can have further substitutions. The donor is a segment on which the HOMO is localized, and the A is on which the LUMO is localized. The chemical structures of these components are illustrated by the formulas II to V.

[0061] The compound of Formula I has donor D and Acceptor A,

[0062] Wherein the acceptor has substitutions R1, R2and R3.

[0063] Formula I represents the donor (D). Formula II represents acceptor (A) with R1substitution at para position of the phenyl ring. Formula IV represents acceptor (A) with

[0064] R1substitution at meta position of the phenyl ring, and Formual V represents the acceptor (A) with R1substitution at meta position of the phenyl ring.

[0065] In a preferred embodiment X=N, Y=S, R1is a halogen, R2is selected from the group comprising of phenyl, p-tolyl, 4-tertbutyltolyl, 1 -naphthyl, 1-mthoxyphenyl, 1, 2- dimethoxyphenyl, 3, 5-dimethoxyphenyl, thiophenyl, and R3is hydrogen (H) or CH3.

[0066] Variation of the TRDSL compound of Formula I

[0067] These groups can adjust optimal conditions for TRDSL. Introducing additional hetero atoms (N, O, S, Se) in the fused hetero ring system of the D component, an electron-rich or electron-deficient and twisting motion of the donor can be realized. Electron-deficient or electron-rich acceptor components can be realized by the substitution of EWGs (R1 = F, Cl, Br, I, CO2CH3, CO2CH2CH3, CF3, CN, SO2CH3, SO2Ph) or electron donating group (EDG) (R3 = Me). Further, ratiometric emission-modulation can be realized using an aromatic hydrocarbon (R2= phenyl, naphthalene) or other aromatic substituents (R2= phenyl, 4-methyphenyl, 4-tertbutylphenyl, 1 -naphthyl, 4- methoxyphenyl, 1,2-dimethoxyphenyl, 3, 5 -dimethoxyphenyl, and 2-thiophene) as peripheral groups. The energies of HOMO and EUMO according to the formulas (III) to (VII) may be adjusted by the nature of the heteroatoms and R2inserted in the fused heteroaromatic ring or EDGs (Electron donating groups) (R3) and EWGs (Electron withdrawing groups) (Rl) of the A components.

[0068] LIST OF COMPOUNDS REPRESENTATIVE OF FORMULA I

[0069] The formula 1 of the present invention comprises of X, Y, RI,R2,R3. when,

[0070] X = N, Y = S,

[0071] R1= F, Cl, Br, I, CF3, CN, R2= phenyl, p-tolyl, 4-tertbutyltolyl, 1 -naphthyl, 1-mthoxyphenyl, 1, 2- dimethoxyphenyl, 3, 5-dimethoxyphenyl, thiophenyl. R3= a hydrogen atom or a CH3 substituent.

[0072]

[0073]

[0074]

[0075]

[0076]

[0077]

[0078]

[0079]

[0080]

[0081] CH

[0082] O

[0083]

[0084]

[0085]

[0086] When,

[0087] X = N, Y = O,

[0088] R1= F, Cl, Br, I, CF3, CN, R2= phenyl, p-tolyl, 4-tertbutyltolyl, 1 -naphthyl, 1-mthoxyphenyl, 1, 2- dimethoxyphenyl, 3, 5-dimethoxyphenyl, thiophenyl, R3= a hydrogen atom or a CH3 substituent, following are the conjugates

[0089]

[0090]

[0091]

[0092] When,

[0093] X = N,

[0094] Y = Se,

[0095] R1= F, Cl, Br, I, CF3, CN, R2= phenyl, p-tolyl, 4-tertbutyltolyl, 1 -naphthyl, 1-mthoxyphenyl, 1, 2- dimethoxyphenyl, 3, 5-dimethoxyphenyl, thiophenyl, R3= a hydrogen atom or a CH3 substituent, following are the conjugates

[0096]

[0097]

[0098]

[0099] When,

[0100] X = N,

[0101] Y = N-phenyl,

[0102] R1= F, Cl, Br, I, CF3, CN, R2= phenyl, p-tolyl, 4-tertbutyltolyl, 1 -naphthyl, 1-mthoxyphenyl, 1, 2- dimethoxyphenyl, 3, 5-dimethoxyphenyl, thiophenyl,

[0103] R3= a hydrogen atom or a CH3 substituent, following are the conjugates

[0104]

[0105]

[0106]

[0107]

[0108]

[0109]

[0110]

[0111]

[0112]

[0113]

[0114]

[0115]

[0116]

[0117] The light-emitting D-A conjugates having TRDSL characteristic of the invention can be synthesized by simple multicomponent condensation reaction among the donor 5 substituted primary aromatic amine, respective aryl aldehyde, and aryl acetylene with subsequent aromatization of the ring systems newly formed. The donor substituted primary aromatic amine can be synthesized using a simple nucleophilic aromatic substitution reaction between heteroaromatic secondary amine and 1-fluoro-2- nitrobenzene derivative followed by reduction of the nitro function.

[0118] 10 (i)

[0119] 10-(2 -(4-chlorophenyl) -4-(nap hthalen- 1 -yl) quinolin-8-yl)- 10H-phenothiazine

[0120] 5

[0121] (iii)

[0122]

[0123] 10-(2-(4- bromophenyl)-4-(naphthalen - 1 -yl) quinol in - 8- yl)- 10H-phenothiazine

[0124] SYNTHESIS OF TRDSL COMPOUND OF FORMULA I

[0125] The light-emitting D-A conjugates having TRDSL characteristic of the invention can be synthesized by simple multicomponent condensation reaction among the donor substituted primary aromatic amine, respective aryl aldehyde, and aryl acetylene with subsequent aromatization of the ring systems newly formed. The donor substituted primary aromatic amine can be synthesized using a simple nucleophilic aromatic substitution reaction between heteroaromatic secondary amine and l-fluoro-2- nitrobenzene derivative followed by reduction of the nitro function.

[0126] In a second embodiment is disclosed a process of preparation of a thermally reversible dual-state luminescent organic donor-acceptor conjugate of Formula 1, comprising the steps: the process of preparation of a thermally reversible dual-state luminescent organic donor- acceptor conjugate of Formula 1, comprising the steps:

[0127] a. Preparation of Donor D i. dissolving l-fluoro-2nitrobenzene and 2-(10H-phenothiazin- 10yl)aniline in solvent, sovent 1 selected from dimethylformamide (DMF), dimethyl sulphoxide (DMSO), dimethylacetamide (DMAc) under N2 gas, in the presence of base selected from sodium hydride or K2CO3, CS2CO3, potassium tert-butoxide at a temperature in a range of 80°C - 140 °C for 12 - 36 hours; ii. cooling of the resultant solution and pouring on ice cold water; iii. separating the precipitate; iv. adding Phenothiazine-NO2of Formula 2 to an acid to obtain a solution; v. adding stannous chloride (SnC12) or Hydrazine hydrate with Pd-C or H2gas with Pd-C to solution obtained in step (iv) and stirring for 30- 45 also (30 mins - 24 hrs.) at a temperature of 60-80 degrees; vi. evaporating of solvent to yield Intermediate 1;

[0128] b. Preparation of Donor (D)-Acceptor(A) Conjugate vii. adding Intermediate 1, a halogen substituted benzaldehyde and 1- ethynylnaphthalene in the presence of a catalyst in a high boiling aromatic solvent at 70- 110°C for 6- 12 hours under molecular oxygen; viii. cooling to room temperature followed by vacuum evaporation to yield the compound of Formula 1.

[0129] A clear description of synthesis of conjugate includes the donor-substituted primary aromatic amine was synthesized by a nucleophilic aromatic substitution reaction.

[0130] 2-(10H-phenothiazin-10-yl)aniline (1.0 eq.) and l-fluoro-2-nitrobenzene (1.0 eq.) were dissolved in DMF. Anhydrous CS2CO3(1.0 eq.) was added to this mixture under nitrogen gas. This reaction mixture was heated at 80 °C for 15 h. Thereafter, it was cooled to room temperature and poured into ice water. The yellow precipitate formed was collected by vacuum filtration and recrystallized using ethyl acetate / hexane (90:10).

[0131] PTzNO2 (1.0 eq.) was added to 5N HC1 solution. SnCh●H2O (2.0 eq.) was added to this solution mixture. The reaction was stirred at 70 °C under N2 for 30 min. Thereafter, it was cooled to room-temperature, and 5M NaOH solution was added to the mixture. The mixture was extracted with ethyl acetate. The organic phase was dried with anhydrous Na2SO4, and the solvent was evaporated under a vacuum. The crude product (intermediate 1) (Yield, 90.5%) was used for the next step without further purification. As discussed below, all the example conjugates were synthesized following a general multicomponent condensation reaction.

[0132] 2-(10H-phenothiazine-10-yl)aniline (1.0 eq.), halogen-substituted benzaldehyde (1.0 eq), 1-ethynylnaphthalene (1.0 eq.), Iron(III) trifluoromethanesulfonate are dissolved in toluene.

[0133] PTzNO2: 90.5%. Rf (hexane / ethyl acetate, 10: 1): 0.60.1H NMR (DMSO-d6, 400.2 MHz): 5 8.30 (dd, J = 8, 1 Hz, 1H), 8.04 (td, J = 8, 2 Hz, 1H), 7.86 (ddd, J = 16, 8, 1 Hz, 2H), 7.07 (dd, J = 7, 2 Hz, 2H), 6.97 - 6.82 (m, 4H), 6.03 (dd, J = 8, 1 Hz, 2H).

[0134] Example 1 is the conjugate 1 which Yields in the percentage of 88% (1). Rf (hexane / ethyl acetate, 10: 1): 0.60.1H NMR (CDC13, 400.2 MHz) (for 1): 5 8.17 - 8.01 (m, 4H), 7.98 (s, 1H), 7.92 (dd, J = 7.1, 1.2 Hz, 1H), 7.73 - 7.67 (m, 1H), 7.64 - 7.56 (m, 3H), 7.52 (dd, J = 14.6, 7.6 Hz, 2H), 7.45 (dd, J = 11.2, 3.9 Hz, 1H), 7.17 - 6.98 (m, 4H), 6.85 - 6.71 (m, 4H), 6.19 (dd, J = 7.6, 1.8 Hz, 2H). Mass spectrometry MS: (HRMS-ESI) m / z: 547.1636 [M + H]+.

[0135] 1:

[0136] Example 2 is the conjugate 2 which Yields in the percentage of 78% (1). Rf (hexane / ethyl acetate, 10: 1): 0.65.1H NMR (CDCI3, 400.2 MHz) (for 2): δ 8.06 (m, J = 7.9 Hz, 4H), 7.99 (s, 1H), 7.93 (dd, J = 7.1, 1.4 Hz, 1H), 7.74 - 7.65 (m, 1H), 7.64 - 7.56 (m, 3H), 7.52 (dd, J = 8.4, 7.2 Hz, 2H), 7.49 - 7.42 (m, 1H), 7.39 - 7.32 (m, 2H), 7.13 - 7.05 (m, 2H), 6.85 - 6.73 (m, 4H), 6.23 - 6.13 (m, 2H). Mass spectrometry MS: (HRMS-ESI) m / z: 563.1341 [M + H]+.

[0137] Example 3 is the conjugate 3 which Yields in the percentage of 75% (1). Rf (hexane / ethyl acetate, 10: 1): 0.62.1H NMR (CD2CI2, 400 MHz) (for 1): δ 8.06 (dd, J = 16.4, 8.2 Hz, 2H), 8.02-7.96 (m, 3H), 7.92 (dd, J = 7.1, 1.4 Hz, 1H), 7.72-7.66 (m, 1H), 7.63-7.56 (m, 3H), 7.52 (dd, J = 12.0, 4.9 Hz, 4H), 7.47-7.42 (m, 1H), 7.12-7.06 (m, 2H), 6.85-6.72 (m, 4H), 6.22-6.12 (m, 2H). Mass spectrometry MS: (HRMS-ESI) m / z: 607.0837 [M + H]+. All the acceptor components were synthesized following the same general multicomponent condensation reaction as discussed above using aniline.

[0138] PREPARATION OF ACCEPTOR

[0139] QF: Yield 85%. Rf (hexane / ethyl acetate, 10: 1): 0.75 (for QF).1H NMR (CDC13, 400.2 MHz): δ 8.31-8.21 (m, 3H), 8.02 (dd, J = 15.1, 8.3 Hz, 2H), 7.90 (s, 1H), 7.75 (ddd, J = 8.4, 6.8, 1.4 Hz, 1H), 7.66 (dd, J = 8.1, 7.1 Hz, 1H), 7.59-7.51 (m, 2H), 7.46 (d, J = 8.3 Hz, 2H), 7.41-7.33 (m, 2H), 7.26-7.19 (m, 2H). Mass spectrometry MS: (HRMS-ESI) m / z: 350.1339 [M + H]+.

[0140] QC: Yield 85%. Rf (hexane / ethyl acetate, 10: 1): 0.75 (for QC).1H NMR (CDCI3, 400 MHz): δ 8.28 (d, J = 8.4 Hz, 1H), 8.23-8.16 (m, 2H), 8.02 (dd, J = 15.4, 8.2 Hz, 2H), 7.91 (s, 1H), 7.76 (ddd, J = 8.3, 6.8, 1.4 Hz, 1H), 7.69-7.62 (m, 1H), 7.58-7.48 (m, 4H), 7.45 (dd, J = 7.6, 5.0 Hz, 2H), 7.42-7.34 (m, 2H). Mass spectrometry MS: (HRMS-ESI) m / z: 366.1045 [M + H]+. QB: Yield 85%. Rf (hexane / ethyl acetate, 10: 1): 0.75 (for QB).1H NMR (CDCl3, 400.2 MHz): δ 8.29 (d, J = 8.4 Hz, 1H), 8.16-8.10 (m, 2H), 8.03 (dd, J = 15.3, 8.2 Hz, 2H), 7.91 (s, 1H), 7.76 (ddd, J = 8.4, 6.8, 1.4 Hz, 1H), 7.70-7.63 (m, 3H), 7.55 (dd, J = 5.3, 1.2 Hz, 2H), 7.49-7.42 (m, 2H), 7.41-7.33 (m, 2H). Mass spectrometry MS: (HRMS- ESI) m / z: 410.0537 [M + H]+.

[0141] The D-A conjugate, according to Examples 1-3, can be soluble in many organic solvents (for example, toluene, xylene, benzene, 1,2-dichlorobenzene, dichloromethane (CH2CI2), tetrahydrofuran (THF), dimethylformamide (DMF)). The excellent solubility in toluene also allows doping in 1,3-bis(N-carbazolyl)benzene (mCP). Photophysical measurements of 1.0 wt% mCP film (compound:mCP = 1:99 by wt) demonstrate the occurrence of TRDSL and LEIT features.

[0142] ADVANTAGES OF THE PRESENT INVENTION

[0143] The existing organic light emitting systems suffer from broad emissive bands with short luminescence lifetimes that cannot always be differentiated. The D-A conjugates of the invention are extremely useful as a TRDSL material for optical, thermal sensors, and thermal distribution in high-resolution microscopic electronic devices due to the following advantages.

[0144] Conformational isomerism of the conjugates brings the emissive centers out-of- equilibrium

[0145] Thermally reversible fluorescence (nanosecond) and delayed fluorescence (lifetime nanosecond to microsecond) emission response upon heating (350 K) and cooling (10 K)

[0146] Thermally reversible fluorescence (nanosecond) and fluorescence (nanosecond) emission response upon heating (140 K) and cooling (10 K)

[0147] Well-separated emission bands (366-381 nm, 564-580 nm)

[0148] Ratiometric emission response at variable temperatures ranges from 10-350 K Rapid response of the emission signals. High accuracy of the emission signals

[0149] High spatial and temporal resolution of the emission signal High thermal stability of the materials.

[0150] Thus, the present invention includes the utization of developed thermally reversible dual- state luminescence (TRDSL) for the application of thermal imaging devices, ratiometric optical thermometer for temperature sensing and high efficiency OLEDs. Hence, technical advancement properties of the present invention makes the invention more efficient and resourceful. Although the invention has now been described in terms of certain preferred embodiments and exemplified with respect thereto, one skilled in art can readily appreciate that various modifications, changes, omissions and substitutions may be made without departing from the spirit thereof. It is intended therefore that the present invention be limited solely by the scope of the following claims.

Claims

WE CLAIM:

1. A thermally reversible dual-state luminescent organic donor-acceptor conjugate of Formula 1, comprising:F omula - 1 wherein donor (D) comprises phenothiazine,A is acceptor comprising naphthalene substituted quinolinyl acceptor;R1is electron- withdrawing group selected from a halogens F, Cl, and Br, and the compound of Formula 1 is a light-emitting material with TRDSL that provides ratiometric emission response and detect thermal energy at low (0- 300K) and high (300-350K); R2is selected from the group comprising of phenyl, p-tolyl, 4-tertbutyltolyl, 1- naphthyl, 1-mthoxyphenyl, 1, 2-dimethoxyphenyl, 3, 5-dimethoxyphenyl, thiophenyl, and R3is hydrogen (H) or CH3.

2. The thermally reversible dual-state luminescent organic donor-acceptor conjugate of Formula 1 as claimed in claim 1, wherein the Donor is attached to acceptor by C-N bond, in orthogonal orientation.

3. The thermally reversible dual-state luminescent organic donor-acceptor conjugate of Formula 1 as claimed in claim 1, wherein the compound of formula 1 comprises dual emissive states comprising an local emission LE state and a charge transfer CT state, which are mutually interconvertible at different temperatures due to conformational isomerism at high and low temperatures.

4. The thermally reversible dual-state luminescent organic donor-acceptor conjugate of Formula 1 as claimed in claim 1, wherein at low temperature (10- 150K) D-A electronic coupling causes intense emission from singlet locally excited LE state and weak emission from excited single charge transfer CT state.

5. The thermally reversible dual-state luminescent organic donor-acceptor conjugate of Formula 1 as claimed in claim 1, wherein high temperature (150- 350K) causes conformational isomeric shift from LE state to Charge Transfer state and cooling to low temperature causes CT to LE shift.

6. The thermally reversible dual-state luminescent organic donor-acceptor conjugate of Formula 1 as claimed in claim 1, wherein the compound of formula 1 is selected from,(i)(ii)10-(2-(4-chlorophenyl)-4-(naphthalen-1-yl) quinol in - 8- yl)- 10H-phenothiazine(iii)10-(2-(4- bromophenyl)-4-(naphthalen - 1 -yl) quinol in - 8- yl)- 10H-phenothiazine7. A process of preparation of a thermally reversible dual- state luminescent organic donor- acceptor conjugate of Formula 1 as claimed in claim 1, comprising the steps:a. Preparation of Donor D i. dissolving l-fluoro-2nitrobenzene and 2-(10H-phenothiazin-10yl)aniline in solvent, sovent 1 selected from dimethylformamide (DMF), dimethyl sulphoxide (DMSO), dimethylacetamide (DMAc) under N2 gas, in the presence of base selected from sodium hydride or K2CO3, CS2CO3, potassium tert-butoxide at a temperature in a range of 80°C - 140 °C for 12 - 36 hours; ii. cooling of the resultant solution and pouring on ice cold water; iii. separating the precipitate; iv. adding Phenothiazine-NO2of Formula 2 to an acid to obtain a solution; v. adding stannous chloride (SnC12) or Hydrazine hydrate with Pd-C or H2gas with Pd-C to solution obtained in step (iv) and stirring for 30- 45 also (30 mins - 24 hrs.) at a temperature of 60-80 degrees,; vi. evaporating of solvent to yield Intermediate 1;b. Preparation of Donor (D) - Acceptor (A) Conjugatevii. adding Intermediate 1, a halogen substituted benzaldehyde and 1- ethynylnaphthalene in the presence of a catalyst in a high boiling aromatic solvent at 70- 110°C for 6- 12 hours under molecular oxygen; viii. cooling to room temperature followed by vacuum evaporation to yield the compound of Formula 1.

8. A process of preparation of a thermally reversible dual- state luminescent organic donor- acceptor conjugate of Formula 1 as claimed in claim 7, wherein in step (b) the catalyst is selected from ferric chloride, Iron(III) trifluoromethanesulfonate, Copper(II) trifluoromethanesulfonate, Silver(I) trifluoromethanesulfonate, or Iodine.

9. A process of preparation of a thermally reversible dual- state luminescent organic donor- acceptor conjugate of Formula 1 as claimed in claim 7, wherein in step (b) the solvent is selected from nitromethane, benzene, toluene, xylene, chlorobenzene, 1,2-dichlorobenzene and the like.

10. A process of preparation of a thermally reversible dual-state luminescent organic donor- acceptor conjugate of Formula 1 as claimed in claim 7, wherein thereaction employed in the oxygenated environment and the halogen is selected from Cl, F, or Bromine.

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