N-HETEROCYCLIC THIONES AS LIGHT-EMITTING COMPOUNDS AND METHOD OF PREPARATION THEREOF

Abstract

N-HETEROCYCLIC THIONES AS LIGHT-EMITTING COMPOUNDS AND METHOD OF PREPARATION THEREOF The present disclosure relates to an organic light emitting compound of Formula I (I) wherein R is selected from the group consisting of -CHPh2 or –CH3. The present disclosure also relates to a method of preparing the same.

Specification

Description:FIELD OF THE INVENTION:
[0001] The present disclosure relates to light-emitting compounds. The present disclosure particularly relates to fluorescent N-heterocyclic thiones as light-emitting compounds for light-emitting applications in OLED devices. The present disclosure also relates to a process of preparing an organic light-emitting compound.

BACKGROUND OF THE INVENTION
[0002] Organic light-emitting device (OLED) development especially the emitting material has come a long way ever since the pioneering work of Tang et al. in 1987. The success of OLEDs is attributed to their self-emissive property, simple/inexpensive synthesis processes, and efficient charge transfer characteristics, along with OLEDs' most optoelectronics devices are made of π-conjugated organic material. OLED technology has evolved from fluorescent emitters to phosphorescent, TADF luminophores, and hyper fluorescence. Sulfur-fused heterocyclic derivatives are performing very well in OLEDs, commonly as host materials or emissive layers delivering superior efficiency and brightness. Due to their abundance (0.042%) in the Earth's crust, less toxicity, stability, and ease of synthesis, sulfur compounds are potential candidates for future use in OLEDs showing very good performance.
[0003] The organic light-emitting device has a structure in which an organic material layer interposed between an anode and a cathode is disposed. As a voltage is applied to the organic light-emitting device, electrons and holes injected from the electrode are combined in the organic material layer to form a pair and then dissipate to emit light. At this time, the organic material layer may be configured as a single layer or multiple layers as necessary. For example, as the material of the organic material layer, a compound capable of constituting an emission layer by itself may be used, or a mixture of compounds capable of serving as a host or a dopant of the emission layer may be used.
[0004] For applications in OLEDs, sulphur-fused heterocyclic derivatives represent one of the most investigated heterocyclic compounds. Sulfoxide or sulfone with electron-deficient characteristics can be easily formed by oxidation of sulphur, which is beneficial to electron injection and transportation. These sulphur-fused heterocyclic derivatives are commonly used as host materials or emissive layers in OLEDs. However, the method used for synthesising the light emitting material containing S-fused heterocyclic with low thermal stability uses expensive catalysts, biohazardous precursors, and sophisticated methodology. Further they have less chemical stability with broad spectrum and produces less intense emission. Eg:- phenothiazine derivative.
[0005] Accordingly, the development of a material by a simple and cost-effective method for an organic material layer to improve the performance, life, or efficiency of the organic light-emitting device is still required. In particular, there is an urgent need to develop a compound that can serve as a host for a light-emitting layer and a compound that can be used for an electron transport layer.

OBJECTS OF THE INVENTION
[0006] Some of the objectives of the present disclosure, with at least one embodiment herein satisfied, are listed herein below:
[0007] It is the primary objective of the present disclosure to provide the light-emitting compounds that could be used for fabricating OLED devices.
[0008] It is another objective of the present disclosure to provide a compound by a simple and cost-effective method for an organic material layer to improve the performance, life, or efficiency of the organic light-emitting device.
[0009] It is yet another objective of the present disclosure to provide novel N-heterocyclic thiones, as an organic light-emitting compound.
[0010] It is yet another objective of the present disclosure to provide a process for preparing the light-emitting compounds using cheap precursors and easy methodology.

SUMMARY OF INVENTION
[0011] The present disclosure relates to an organic light-emitting compound of Formula I

(I)

wherein R is selected from the group consisting of CHPh2 or -CH3.
[0012] The present disclosure also relates to a process of preparing an organic light emitting compound of Formula I, comprising the steps of:
1) dissolving the compound of formula (II) in methanol;
(II)
wherein R is selected from CHPh2 or -CH3, and
2) adding potassium carbonate and sulphur powder followed by stirring at a temperature ranging from 50 to 75°C for 36 hours to 48 hours to obtain the compound of formula (I).

BRIEF DESCRIPTION OF DRAWINGS
[0013] The present disclosure contains the following drawings that simply illustrates certain selected embodiments of the nanocarrier composition and processes that are consistent with the subject matter as claimed herein, wherein:
[0014] Figure 1 depicts optimized structure of compound 1 and 2 calculated using level B3LYP and basis set 6-31G (d,p) are shown as 1 and 2; The chemdraw structure is given to depicted the relevance of 1 and 2 followed by the reaction yield, photoluminescence quantum yield and life time in nanosecond scale; 1 and 2 were prepared by easy and affordable methodology as mentioned for potential use in OLED devices.
[0015] Figure 2 depicts 1H NMR spectrum of compounds.
[0016] Figure 3 depicts the HRMS of compounds 1 and 2
[0017] Figure 4 depicts (a) TGA-DTA profile of compound 1 recorded from 40 C to 600 C with a heated rate of 5 C/min under air atmosphere; (b) TGA-DTA profile of compound 2 recorded from 40 C to 600 C with a heated rate of 5 C/min under air atmosphere.
[0018] Figure 5 Fig. UV-Vis spectrum 1 and 2 w.r.t L1 and L2 in CH2Cl2 at room temperature (M = 2.3 x 10-6 mol/L);
[0019] Figure 6 depicts the Emission spectra of compound 1 and 2 in (b) CH2Cl2 solution (excited at 310 nm) and (c) solid-state (excited at 310 nm);
[0020] Figure 7 depicts Fluorescence spectrum CH3CN solution of 1 and 2 w.r.t L1 and L2 at Room Temperature (RT).
[0021] Figure 8 depicts the emission spectra of LED bulbs (410nm, 3V) and LED coating of 1 and 2 and bare LED when switched on and off.
[0022] Figure 9 depicts the commission Internationale de L'Eclairage (CIE) 1931 coordinates of compound 1 and 2 in solid, solution and LED coated form.
[0023] Figure 10 depicts Frontiers Molecular orbitals through DFT Calculation of 1 and 2.

DESCRIPTION OF THE INVENTION:
[0024] A detailed description of various exemplary embodiments of the disclosure is described herein. It should be noted that the embodiments are described herein in such detail as to communicate the disclosure. However, the amount of details provided herein is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure.
[0025] The terminology used herein is to describe particular embodiments only and is not intended to be limiting to the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising", or "includes" and/or "including" or "has" and/or "having" when used in this specification specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof.
[0026] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, e.g., those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
[0027] The present disclosure relates to a organic light emitting compound of Formula I
(I)

wherein R is selected from the group consisting of CHPh2 or -CH3.
[0028] In another embodiment of the present disclosure, the organic light-emitting compound of formula (I), wherein the compound of formula (I) is selected from the group consisting of



and .
[0029] In an embodiment of the present disclosure, structure 1 and 2 as shown in figure 1 are calculated through Gaussian16 package (DFT calculation) with level B3LYP and basis set 6-31G (d,p).

[0030] In an embodiment of the present disclosure, the light-emitting compound of formula (I) shows an emission spectrum at a wavelength range of 420 nm to 600 nm. Compound 1 emits a greenish-blue color light of 495 nm on excitation at 310 nm in dichloromethane solution. Compound 2 emits a greenish-blue color light of 493 nm on excitation at 310 nm in dichloromethane solution.
[0031] In another embodiment of the present disclosure, the crystalline state of Compound 2 shows narrow spectrum emission at 470 nm with a shoulder at 495 nm and crystalline state of compound 1 shows narrow spectrum emission with at 475 nm with a shoulder at 508 nm.
[0032] In yet another embodiment of the present disclosure, the emission spectra of LED bulbs (410 nm, 3V) coated with compound 1 were observed in the range 440-600 nm and the emission spectra of LED bulbs (410 nm, 3V) coated with compound 2 were observed in the range 450-610 nm.
[0033] In an embodiment of the present disclosure, the bulky substitution affects the corresponding steric hindrance, rigidity and aggregation state of the molecule via various non-covalent interactions which is considered an important factor in enhancing the luminescence of the molecule.
[0034] In an embodiment of the present disclosure, the UV spectrum of N-heterocyclic thione molecule 1 and 2 w.r.t 2-(2,6-diphenyl methyl-4-methyl phenyl) imidazopyridine chloride (L1) or 2-(2-diphenyl methyl-4,6-dimethyl phenyl) imidazopyridine chloride (L2) in acetonitrile solution shows that upon substitution of sulfur there is an increase in the transition of π →π* and n →π* transitions at 305, 320 nm and 385 nm which is due to the effective charge transfer by the sulphur atom.
[0035] The present disclosure also relates to a process of preparing an organic light emitting compound of Formula I, comprising the steps of:
1) dissolving a compound of formula (II) in methanol;
(II)
wherein R is selected from CHPh2 or -CH3, and
2) adding potassium carbonate and sulphur powder followed by stirring to obtain the compound of formula (I).
[0036] In an embodiment of the present disclosure, wherein addition of potassium carbonate and sulphur powder is followed by stirring at a temperature ranging from 50 to 75°C for 36 hours to 48 hours to obtain the compound of formula I.
[0037] In an embodiment of the present disclosure, the compound of formula (II) is selected from the group consisting of 2-(2,6-diphenyl methyl-4-methyl phenyl) imidazopyridine chloride or 2-(2-diphenyl methyl-4,6-dimethyl phenyl) imidazopyridine chloride.
[0038] In another embodiment of the present disclosure, amount of the formula (II) is in the range of 0.4 to 0.6 g.
[0039] In another embodiment of the present disclosure, amount of the potassium carbonate is in the range of 0.25 g to 0.5 g.
[0040] In another embodiment of the present disclosure, amount of sulphur powder is in the range of 0.05 g to 0.08 g.

ADVANTAGES OF THE PRESENT INVENTION

[0041] In accordance with the present disclosure, the advantages of the present invention are
- excellent properties of the group of compounds as a light-emitting material,
- high light emission efficiency,
- ease of preparation,
- stability,
- ability to adjust the emission color
- less hazardous and
- less expensive.

EXAMPLE
[0001] The present disclosure will be explained using the following examples:

Example 1: Preparation of compound 1:
• The L1 [2-(2,6-diphenyl methyl-4-methyl phenyl) imidazopyridine chloride] (0.92 mmol, 1 eq.) was dissolved in methanol (MeOH).
• After dissolution potassium carbonate (K2CO3) (3.86 mmol, 1 eq.) and sulphur powder (2.73 mmol, 3 eq.) were added and then stirred at 75 oC for 48 hours.
• The product was extracted from a well-dried ethyl acetate workup, which was dissolved in acetonitrile and kept for crystallisation. Yield: 80%
Example 2: Preparation of compound 2:
• The L2 [2-(2-diphenyl methyl-4,6-dimethyl phenyl) imidazopyridine chloride] (0.92 mmol, 1 eq.) was dissolved in MeOH.
• After dissolution K2CO3 (3.86 mmol, 1 eq.) and sulphur powder (2.73 mmol, 3 eq.) were added and stirred at 75 oC for 48 hours.
• The product was extracted using ethyl acetate workup, which was dissolved in acetonitrile and kept for crystallization. Yield: 84%

Example 3: Characterization and Photophysical Studies of Compound 1 and Compound 2:
A. NMR Analysis
[0042] The 1H and 13C NMR were recorded using JOEL ECS-400 NMR Spectrometer (400 MHz) in CDCl3 solvent. The chemical shifts were reported in parts per million (ppm) relative to the residual proton solvent resonance.
[0002] The purity of the compounds 1 and 2 were confirmed by recording 1H NMR, and all the characteristic peaks of the compound were assigned, as shown in Figure 2.

B. ESI-MS analysis
[0043] High-resolution mass spectra (HRMS) were acquired using an Agilent 6538 UHD Q-TOF mass spectrometer in both electron spray ionization (ESI) and atmospheric pressure chemical ionization (APCI) modes.
[0044] The ESI-MS analysis of the compound was carried out to achieve the desired mass peak at 573.2362 (m/z) assigned for 1 and mass peak at 421.1768 (m/z) for 2 as shown in figure 3.

C. Thermogravimetric (TGA) Analysis
[0045] The thermogravimetric analysis/differential thermal (TGA/DTA) analysis was carried out using DTG-60AH analyser at a heating rate of 5 °C /min.
[0046] The thermal stability of the complex was investigated by Thermogravimetric Analysis (TGA), shown in Figure 4. It was observed that single step 98% weight loss occur due to nearly complete decomposition of the compounds 1 and 2.
[0047] The thermal stability of the molecule is defined by TGA and DTA analysis. The TGA analysis of both molecules reveal the higher decomposition temperature of molecule 1 and 2 (Td = 450 oC) due to their stronger bonds and stable molecular arrangement which gives more operational lifetime and longevity to the OLED material.

D. Differential Thermal Analysis (DTA) analysis
[0048] The thermogravimetric analysis/differential thermal (TGA/DTA) analysis was carried out using DTG-60AH analyser at a heating rate of 5 oC /min.
[0049] The Differential Thermal Analysis (DTA) analysis were performed for compound 1 and 2. The DTA analysis predicted melting temperature (Tm) at 184 C for 1 and 247 C for 2 as evident from Figure 4.

E. UV absorbance analysis
[0050] The JASCO-V-500 spectrometer was used to measure the UV-Vis absorption properties of the molecules.
[0051] The newly synthesized compound 1 and 2 depicts a UV absorbance peak at 382 nm and 322 nm attributed to the n* transition and 309 nm and 232 nm due to * transition, shown in Figure 5.
[0052] UV-Vis spectrum for the N-heterocyclic thione molecule 1 and 2 w.r.t L1 and L2 in CH2Cl2 solution were analysed. It is evident upon substitution of sulphur there is an increase in the transition of π →π* and n →π* transitions at 305, 320 nm and 385 nm which is due to the effective charge transfer by the sulphur atom.
[0053] The UV-Vis absorption spectra of 1 and 2 revealed the narrow band at 231 nm due to p→π*. In addition, two less intense peaks are observed at 309 nm and 322 nm for molecule 1, and 307 nm and 320 nm for molecule 2 due to π→π*. One shoulder peak is also observed at 388 nm for 1 and 2.

Example 4: Details of the LED bulb and the experiment which showed distinguishing peaks at 410 nm and 487 nm.
[0054] Lighting emitting property is demonstrated using LED Bulb coating. The commercially available LED bulb uses 3V having an emission maximum of 410 nm. A solution is prepared of PMMA and compound in a weight/weight ratio of 3:1. The coated bulb is provided with the external supply and an emission spectrum is recorded using a fluorescence spectrophotometer

Fluorescence photoluminescence quantum yield
[0055] The Fluorescence photoluminescence quantum yield (Φ) was measured using a Hitachi F-7000 fluorescence spectrophotometer using a calibrated integration sphere system (Hitachi, Tokyo, Japan) using alumina oxide as the standard reference.
[0056] Fluorescence spectrum in the solution also shows the increase in the emission intensity of the molecule upon the substitution as plotted in figure 6a. And for more clarity, the CIE plot has been drawn.
[0057] Compound 1 and 2 possess a greenish blue colour emission of 493 nm and 495 nm in dichloromethane solution when irradiated at a wavelength of 310 nm, shown in Figure 6b. However, the emission band of 1 and 2 shifts to lower wavelength of 475 nm and 470 nm in solid state and appears as cyan colour when irradiated at 320 nm (Figure 6c).
[0058] The emission spectra of 1 and 2 w.r.t. L1 and L2 is 495 as shown in Figure 7.The crystal images of 1 and 2 under UV-light (365 nm) and normal light, shown in Figure 8, clearly demonstrate the observed colour changes of the compound.
[0059] Additionally, the compounds 1 & 2 (in THF solution with added PMMA) coated on LED bulbs (410 nm emitting; 3V) also illuminated as green colour on applying forward bias 3V to the bulb, shown in the inset of Figure 8. The dotted black line here represents the bare LED emission at 410 nm which should be as minimum as possible for a properly coated LED. We have recorded negligible emission at this point 410nm in coated LED and more emission of compound compared with solid emission. This experiment justifies the light-emitting properties of the material for potential application in light-emitting devices.
[0060] The peak wavelengths, i.e., excitation wavelength (λex) emission wavelength (λem), life time and quantum yield were summarized in Table 1.
[0061] It is evident from the CIE diagram (figure 8) that solid sample of 1 and 2 emits cyan colour, solution phase emits green colour and LED coated bulbs also emits colour similar to the solution phase.
[0062] The solution state photoluminescence emission for 1 and 2 gave emission at λem = 495 nm (λex = 306 nm) and λem = 493 nm (λex = 309 nm) respectively, which corresponds to a bluish-green emission. The crystalline state photoluminescence spectra of 1 and 2 exhibited λem = 475 nm and 508 nm for 1 (λex = 342 nm) and λem = 470 nm and 495 nm for 2 (λex = 395 nm). The photoluminescence quantum yield study was carried out using crystalline molecules 1 and 2 at room temperature. The quantum yield observed for 1 and 2 is 54.7% and 50.3% respectively mentioned in table 1. The lifetime measurements were also performed using the time-correlated single photon counting (TCSPC) technique. The solution
phase nanosecond decay profile was recorded for 1 (τavg = 17.729 ns) at 495 nm and 2 (τavg = 17.155 ns) at 493 nm. In the emission spectra of crystalline molecules, a shoulder peak has been observed for molecules 1 at 508 nm and 2 at 495 nm. The nanosecond decay profile also indicates the lifetime of the shoulder peak. The nanosecond decay profile for 1 (τavg = 12.178 ns and 16.356 ns) at 475 nm and 508 nm, for 2 nano-second decay profile was observed (τavg = 16.680 ns and 17.330 ns) at 470 nm and 495 nm. The decay profile of molecules is fitted by a biexponential decay curve and the nano-second decay curve corresponds to fluorescence.

[0063] Table 1: Photophysical properties of compound 1 and 2 in solid and solution (in CH2Cl2) at room temperature.
λabs
(nm) λex
(nm)
λem
(nm) Ꚍ
(ns)
kr
Φf
(%)

Solution
1 232, 309, 322 and 387 306,318, 395 495 17.7295 - -
2 231,307, 320, 386 309, 320, 395 493 17.1559 - -
Crystalline
1 - 342, 395, 418, 450 475, 508 12.18 at 475nm

16.35 at 508nm 4.49
3.34 54.7
2 - 395, 418, 450 470, 495 16.68 at 470nm
17.33 at 495nm 3.01
2.90 50.3
#  concentration of the sample is 2.3 x 10-6.

F. Molecular orbital calculation through TD-DFT Calculation of 1 and 2.

[0064] All the experiment we have performed is supported by the theoretical calculation also, in which the more concentrated the sulphur atom and the charge to the core of the ring in both of the cases. Both of the calculations are performed using the Gaussian 16 package, on the level B3LYP and basis set 6-31G (d,p).
[0065] The electron-donating sulfur and imidazopyridine ring unit are the main contributors to the distribution at the highest occupied molecular orbital (HOMO). In the case of the Lowest occupied molecular orbital (LUMO), the main contributor is the imidazopyridine ring in both the organosulfur derivatives. The orbital contributions are more in 1 compared to 2.

INDUSTRIAL APPLICABILITY
[0066] The compound of the invention is useful as a light emitting material. Accordingly, the compound of the invention may be effectively used as a light emitting material of an organic light emitting device, such as an organic electroluminescent device. The compound of the invention includes a compound that emits delayed fluorescent light, and thus may be capable of providing an organic light emitting device having a high light emission efficiency. Thus, the invention has high industrial applicability.

SPECIFIC EMBODIMENTS OF THE PRESENT INVENTION
[0067] The present disclosure relates to an organic light emitting compound of Formula I

(I)

wherein R is selected from the group consisting of CHPh2 or -CH3.
[0068] Such a light emitting compound of formula (I) shows an emission spectrum at a wavelength range of 420 nm to 600 nm.
[0069] Such a light emitting compound of formula (I), wherein the compound of formula (I) is selected from the group consisting of

(1)
and (2)
[0070] The present disclosure also relates to a process of preparing an organic light emitting compound of Formula I as claimed in claim 1, comprising the steps of:
1) dissolving the compound of formula (II) in methanol; and
(II)
wherein R is selected from CHPh2 or -CH3, and
2) adding potassium carbonate and Sulphur powder followed by stirring at a temperature ranging from 50 to 75°C for 36 hours to 48 hours to obtain the compound of formula (I).
[0071] Such a method wherein the compound of formula (II) is selected from the group consisting of 2-(2,6-diphenyl methyl-4-methyl phenyl) imidazopyridine chloride or 2-(2-diphenyl methyl-4,6-dimethyl phenyl) imidazopyridine chloride.
[0072] Such a method, wherein amount of the formula (II) is in the range of 0.4 to 0.6 g.
[0073] Such a method, wherein amount of the potassium carbonate is in the range of 0.25 g to 0.5 g.
[0074] Such a method, wherein amount of sulphur powder is in the range of 0.05 g to 0.08 g.
, C , C , Claims:We Claim
1. An organic light emitting compound of Formula I

(I)

wherein R is selected from the group consisting of CHPh2 or -CH3.
2. The light emitting compound of formula (I) as claimed in claim 1 shows an emission spectrum at a wavelength range of 420 nm to 600 nm.
2. The compound of formula (I) as claimed in claim 1, wherein the compound of formula (I) is selected from the group consisting of

(1)




and (2)
3. A process of preparing an organic light emitting compound of Formula I as claimed in claim 1, comprising the steps of:
1) dissolving the compound of formula (II) in methanol; and
(II)
wherein R is selected from CHPh2 or -CH3, and
2) adding potassium carbonate and Sulphur powder followed by stirring at a temperature ranging from 50 to 75°C for 36 hours to 48 hours to obtain the compound of formula (I).
4. The method as claimed in claim 3, wherein the compound of formula (II) is selected from the group consisting of 2-(2,6-diphenyl methyl-4-methyl phenyl) imidazopyridine chloride or 2-(2-diphenyl methyl-4,6-dimethyl phenyl) imidazopyridine chloride.
5. The method as claimed in claim 3, wherein amount of the formula (II) is in the range of 0.4 to 0.6 g.

6. The method as claimed in claim 3, wherein amount of the potassium carbonate is in the range of 0.25 g to 0.5 g.
7. The method as claimed in claim 3, wherein amount of sulphur powder is in the range of 0.05 g to 0.08 g.

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