Single component white-OLEDs derived from tris(β-diketonato) europium(III) complexes bearing the large bite angle N^N 2-(4-thiazolyl)benzimidazole ligand

Abstract

Two new organo-europium complexes (OEuCs) [Eu(tfac)₃(TB-Im)] (Eu1) [Eu(hfac)₃(TB-Im)] (Eu2) incorporating fluorinated (hexafluoroacetylacetone; Hhfaa) or hemi-fluorinated (trifluoroacetylacetone; Htfac) β-diketones together with the large bite angle N^N ligand (2-(4-thiazolyl)benzimidazole; TB-Im) have been synthesized and characterized. The structure of the complexes has been established by single crystal X-ray diffraction (SC-XRD) analysis and shows that the coordination sphere is composed of a EuO₆N₂ core (octacoordinated). Continuous shape measures (CShMs) revealed that the geometry around Eu(III) is trigonal dodecahedral with approximate D_2d-symmetry. Efficient red emission is observed for both the complexes in solution with a fairly large photoluminescence quantum yield (PLQY (Q^L_Eu) = 39.00–47.00%). Furthermore, by utilizing the experimental photoluminescence (PL) data and theoretical modelling employing density functional theory (DFT) in conjunction with LUMPAC, energy transfer (ET) and back energy transfer rates were calculated, and an ET mechanism for the sensitized PL is proposed and discussed in detail. Finally, the complexes were used as an emitting layer (EML) to fabricate 20 organic light emitting diodes (OLEDs) by varying the doping concentration. Interestingly, both the complex-based OLEDs at 4 wt% doping concentration display white electroluminescence (EL) with the brightness (B) = 100.5–364.1 cd m⁻² at very low turn-on voltage (V_turn-on) = 3.9–4.6 V. The overall electroluminescence performance of Eu1 and Eu2 is higher than that of the reported europium based single component white-OLEDs.

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1. Introduction

Research into the design and development of coordination complexes capable of panchromatic emission either in their molecular forms or in devices has received considerable attention.¹ This is because of their wide application in full-colour flat-panel displays and solid-state lighting for the benefit of energy conservation.² One of the promising classes of complexes for further development is the class of efficient organo-europium complexes (OEuCs) because of their exemplary photophysical properties leading to many fascinating applications such as sensors³ and thermometers,⁴ and in anti-counterfeiting applications.⁵ To develop efficient OEuCs, the organic ligand(s) acting either as the primary antenna or as the ancillary ligands must have strong light absorption capabilities between 250 and 450 nm with a compatible triplet state (³ππ*) leading to sufficiently separated ³ππ* and Eu(III) emitting levels.⁶ This allows efficient harvesting of the absorbed energy by the organic ligand(s) through the well-known antenna effect.⁷ Besides, the intrinsic monochromatic red emission due to the electric-dipole (ED) ⁵D₀ → ⁷F₂ transition of Eu(III)⁸ between 608 nm and 620 nm makes these complexes potential candidates as the red component⁹ to fabricate tricolour RGB-based white organic light emitting diodes (white-OLEDs) given that the OEuCs exhibit efficient PL properties.

As simple as it sounds, the generation of white light i.e., panchromatic emission in any phase (crystal/solution/film) remains challenging if a three-component approach is used and has some serious drawbacks e.g., colour impurity as the device ages, manufacturing cost as well as labour to fabricate them, thereby impeding their real-life and industrial applications.^1b,10 To overcome these issues, the development of energy-efficient white-OLEDs from a single molecular platform could provide an elegant solution.^1b Among the different strategies, an appropriate way to achieve white light emission from a single compound is the generation of blue and reddish-orange emissions concomitantly.¹⁰ Interestingly, OEuCs could be employed as EMLs to fabricate white-OLEDs particularly because they display three emission transitions in the reddish-orange region also because they have microsecond to millisecond excited state lifetimes. It is well known that as the current density increases, the longer excited lifetime decays non-radiatively due to triplet–triplet annihilation¹¹ leading to emissions in the region between 400 and 500 nm.

Keeping this idea in mind, in the present work, we synthesized two new OEuCs complexes (Chart 1) by employing hemi-fluorinated (Htfac) and fluorinated (Hhfac) acetylacetones as the primary antenna ligands in conjunction with TB-Im as an ancillary ligand. It is well established that both the β-diketones act as efficient antenna ligands to generate highly luminescent OEuCs,^1b,d,12 which is attributed to the well-placed ³ππ* (³ππ* = 22 720 cm⁻¹ for tfac and 21 930 cm⁻¹ for hfac) transitions fulfilling Latva's empirical rule.^6b The bidentate ancillary TB-Im ligand was chosen because of the asymmetrical N^N-chelating coordination mode via the five-membered imidazolyl ring fused with phenyl ring (benzoimidazole ring C–N = 1.393 Å) and five-membered thiazolyl ring (C–N = 1.295 Å). As noted by us and others, large bite angle coordinating ligands have a profound effect in distorting the coordination geometry around the Ln(IIII) centre, which is beneficial for enhancing the PL properties.^9a,c,13 Besides, the NH proton of the imidazolyl ring is often engaged in the formation of intermolecular hydrogen bonds leading to supramolecular self-assembly, that result in fascinating structural arrangements in the solid-state.^1a We now present the details of the synthesis of Eu1 and Eu2, (Chart 1), their structural characterisation, and their photophysical properties. The results are supported by theoretical studies and an ET mechanism of the sensitized emission for the OEuCs is proposed. Finally, we have employed Eu1 and Eu2 as an emitter to fabricate single- and double-EML OLEDs.

Chart 1 Chemical structures of new OEuCs and organic compounds employed in the OLEDs fabrication.

2. Experimental

2.1. Synthesis of the complexes

Eu(III) chloride was purchased from Strem Chemicals, Inc. Htfac, Hhfac and TB-Im were obtained from Tokyo Chemical Industry (TCI) and were used as received. Solvents were pre-dried and distilled before use according to standard procedures. All organic compounds employed in OLED fabrication were procured from commercial sources and used without further purification unless otherwise specified (Chart 1). Elemental analysis was performed on an Euro EA – CHN in the Department of Chemistry, Sultan Qaboos University. Attenuated total-reflectance (ATR) infrared (IR) spectra were recorded on pure samples on diamond using a Cary 630 F T-IR spectrometer. Mass spectra were obtained using LCMS-8040, Shimazdu-Japan coupled to a triple quadruple tandem mass spectrometer equipped with electrospray ionization (ESI). The thermal stability of the complexes was determined by thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTA) in the temperature range between 50 and 700 °C under a dinitrogen (N₂) atmosphere and recorded on TA instrument model SDTQ600.

2.1.1. [Eu(tfac)₃(TB-Im)] (Eu1). Eu-1 was synthesized by reacting equimolar quantities of [Eu(tfac)₃(H₂O)₂]^1b (0.5 g; 0.771 mmol) and TB-Im (0.155 g; 0.771 mmol) in 1 : 1 mixture of dichloromethane (DCM) and ethanol (EtOH). The reaction mixture was stirred overnight at room temperature and left for slow solvent evaporation. After a week crystals suitable for single-crystal X-ray analysis were obtained. Colour: white; yield (70%); microanalysis calculated for C₂₅H₁₉EuF₉N₃O₆S: C, 36.96; H, 2.36; N, 5.17; found: C, 36.98; H, 2.34; N, 5.16; FTIR (solid; cm⁻¹)- ν(N–H) 3251 cm⁻¹; ν(C=O)_st 1617 cm⁻¹; ν(C=N)_st 1524 cm⁻¹; out-of plane asymmetric ν(C–F)_st 1177 cm⁻¹; in-plane ν(C–H)_bend. 1124 cm⁻¹, (Fig. S1, ESI†); ESI-MS (m/z) = 812.1 [M + H] (Fig. S2, ESI†) melting temperature (T_m) = 228.9 °C decomposition temperature (T_d) with 5% weight loss = 241 °C.

2.1.2. [Eu(hfac)₃(TB-Im)] (Eu2). Eu2 was synthesized using a one-pot method reported earlier for the synthesis of related complexes.^1d Colour: white; yield (68%); microanalysis calculated for C₂₅H₁₀EuF₁₈N₃O₆S: C, 30.82; H, 1.03; N, 4.31; found: C, 30.84; H, 0.99; N, 4.29; FT-IR (solid; cm⁻¹)- ν(N–H) 3297 cm⁻¹; ν(C–O)_st 1642 cm⁻¹; ν(C=N)_st 1531 cm⁻¹; out-of plane asymmetric ν(C–F)_st 1194 cm⁻¹; in-plane ν(C–H)_bend. 1137 cm⁻¹; (Fig. S3, ESI†); ESI-MS (m/z) = 1013.20 [M + K − 2H] (Fig. S4, ESI†); T_m = 202 °C, T_d with 5% weight loss = 239 °C.

2.2. Single crystal X-ray structure determination

Single crystals of Eu1 and Eu2 suitable for single-crystal X-ray analysis were grown by the slow solvent (EtOH) evaporation method. The structure determination was performed at room temperature on a Stoe IPS II diffractometer using monochromatic Mo-Kα radiation (λ = 0.71073 Å). A multi-scan absorption correction was applied. The data reduction, including an empirical absorption correction using spherical harmonics, was implemented in LANA. The crystal structure was solved by direct methods using the online version of WinGX¹⁴ and then refined by full-matrix least-squares (SHELXL-2018) on F².¹⁵ The non-hydrogen atoms were refined anisotropically. All of the hydrogen atoms were positioned geometrically in idealized positions and refined with the riding model approximation, with U_iso(H) = 1.2U_eq(C). The displacement ellipsoids particularly on the fluorine atoms of the CF₃ groups were elongated because of disorder however they could be modelled appropriately using a single atom position for each fluorine atom. Crystal data and structure refinement for europium complexes are detailed in Table S2, ESI.† The molecular graphics were produced using the program MERCURY from the CSD package.¹⁶

2.3. Spectroscopic measurements and OLED fabrication process

Spectroscopic measurements of the complexes that include optical absorption, excitation, emission spectra, decay profiles and absolute PLQY values were performed at room temperature; the methodology for the measurements have been reported previously.^9c Optical absorption spectra were obtained using Varian Cary 5000 UV-Visible-NIR spectrophotometer while excitation, emission spectra and decay profiles were recorded on an Edinburgh FS5 fluorimeter. The absolute PLQY were determined using a calibrated integrating sphere on a C-9920-02 from the Hamamatsu Photonic instrument. Theoretical methodology details (section 1, ESI†) involving the calculation of ground-state geometry, ET rates, radiative emission rate (A_Rad) and theoretical PLQY are detailed in the ESI.† Important experimental photophysical parameters were calculated by the following equations as detailed in our previous reports:^1d,17

ITO coated glass with the sheet resistance of 10 Ω sq⁻¹ was used as the anode substrate. Prior to film deposition, patterned ITO substrates were cleaned with detergent, rinsed in de-ionized water, and finally dried in an oven. All organic layers were deposited at a rate of 0.1 nm s⁻¹ under a high vacuum (≤3.0 × 10⁻⁵ Pa). The doped EMLs were prepared by co-evaporating dopant and host material from two sources, and the doping concentration was modulated by controlling the evaporation rate of the dopant. LiF and Al were deposited in another vacuum chamber (≤8.0 × 10⁻⁵ Pa) at rates of 0.01 and 1.0 nm s⁻¹, respectively, without being exposed to the atmosphere. The thicknesses of these deposited layers and the evaporation rate of individual materials were monitored in vacuum with quartz crystal monitors. A shadow mask was used to define the cathode and make eight emitting dots with an active area of 9 mm² on each substrate. Current density (J)–brightness (B)–voltage (V) characteristics were measured by using a programmable brightness light distribution characteristics measurement system C9920-11 from the Hamamatsu Photonic instrument. PL and EL spectra were measured with a calibrated Hitachi F-7000 fluorescence spectrophotometer and an Ocean Optics spectrophotometer.

3. Results and discussion

3.1. Synthesis, characterization and X-ray diffraction studies

The new complexes were synthesized by the method reported earlier^1b,d and characterized by elemental analysis, mass spectrometry and FT-IR spectroscopy. The results suggested the formation of the complexes with the formulae [Eu(tfac)₃(TB-Im)] (Eu1) [Eu(hfac)₃(TB-Im)] (Eu2). These results were confirmed by the single-crystal X-ray diffraction studies. The X-ray crystallographic data for Eu1 and Eu2 are given in Table S2, ESI.† The complex Eu1 crystallizes in the triclinic space group P1̄ while Eu2 in monoclinic space group P2₁/n. The asymmetric unit in both cases consists of a single mononuclear complex in which the Eu(III) ion is eight coordinate comprising six oxygen (O) atoms from the primary β-diketone ligands and two nitrogen (N) atoms of ancillary ligands i.e., N₂O₆, Fig. 1. The bond distances (Table 1) are comparable to the analogous OEuCs reported by us.^1d,9b The Eu–N bond distances [(2.560(3) Å)_avg. for Eu1 and (2.554(4) Å)_avg. for Eu2] are, in both cases, longer than Eu–O bond distances [(2.365(3) Å)_avg for Eu1 and (2.382(3) Å)_avg. for Eu2] (Table 1). The coordination geometry and assignment of symmetry around the Eu(III) centres of the complexes were determined by the SHAPE software package which calculates continuous shape measures (CShMs) of a set of atomic positions relative to the vertices of ideal reference polyhedra.¹⁸ The EuN₂O₆ coordination polyhedron can be assigned as a distorted triangular dodecahedron, with idealized D_2d symmetry around the metal centre (Fig. 1c). Comparison between the two structures revealed that the distortion is similar with CShM values 0.629 and 0.695 for Eu1 and Eu2 (Table S3, ESI†), respectively, despite the primary antenna ligand (hfac) in the latter being symmetrical.

Fig. 1 Single-crystal X-ray structures of (a) Eu1 and (b) Eu2. The displacement ellipsoids have been drawn with 50% probability (c) trigonal dodecahedral coordination polyhedron. Hydrogen atoms are omitted for clarity.

Table 1 Selected bond lengths [Å] and angles [°] for Eu1 and Eu2

Eu1
Eu(1)–O(6)	2.353(3)	Eu(1)–O(3)	2.386(3)
Eu(1)–O(4)	2.354(3)	Eu(1)–O(1)	2.392(3)
Eu(1)–O(5)	2.357(3)	Eu(1)–N(1)	2.542(3)
Eu(1)–O(2)	2.385(3)	Eu(1)–N(3)	2.606(3)
O(6)–Eu(1)–O(4)	76.76(14)	O(3)–Eu(1)–O(1)	124.98(13)
O(6)–Eu(1)–O(5)	72.10(12)	O(6)–Eu(1)–N(1)	80.85(12)
O(4)–Eu(1)–O(5)	90.55(12)	O(4)–Eu(1)–N(1)	146.82(12)
O(6)–Eu(1)–O(2)	149.60(12)	O(5)–Eu(1)–N(1)	105.55(11)
O(4)–Eu(1)–O(2)	101.95(13)	O(1)–Eu(1)–N(3)	127.79(12)
O(5)–Eu(1)–O(2)	138.07(12)	N(1)–Eu(1)–N(3)	64.26(11)

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Eu2
Eu(1)–O(3)	2.320(3)	Eu(1)–O(2)	2.413(3)
Eu(1)–O(6)	2.345(4)	Eu(1)–O(1)	2.444(3)
Eu(1)–O(4)	2.381(3)	Eu(1)–N(1)	2.508(4)
Eu(1)–O(5)	2.394(4)	Eu(1)–N(2)	2.601(4)
O(3)–Eu(1)–O(6)	93.87(14)	O(2)–Eu(1)–O(1)	69.50(11)
O(3)–Eu(1)–O(4)	71.04(11)	O(3)–Eu(1)–N(1)	100.59(12)
O(6)–Eu(1)–O(4)	77.08(15)	O(6)–Eu(1)–N(1)	145.19(13)
O(3)–Eu(1)–O(5)	147.45(12)	O(2)–Eu(1)–N(2)	75.86(13)
O(4)–Eu(1)–O(2)	130.60(13)	O(1)–Eu(1)–N(2)	81.27(12)
O(5)–Eu(1)–O(2)	128.52(13)	N(1)–Eu(1)–N(2)	64.98(11)

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To underpin the analysis of the photophysical properties through theoretical calculations, it is necessary to determine the ground state geometry of the complex in question. The ground state geometry of the complexes (Eu1 and Eu2) was optimized from the crystallographic coordinates (details are included in the ESI† and the optimized structure is shown in Fig. S5, ESI†). The comparison involving the crystallographic geometry and the geometries calculated by different DFT methods in terms of root mean square deviations (RMSD) is shown in Table S4, ESI.† Contrary to our previous work,^9a,b the TZVPPD basis set (PBE1PBE/TZVPPD/MWB52) provided the best results for all the atoms in the complexes (Table S4, ESI†).

It is important to establish the solid-state packing in a given complex since it helps to understand the thermal and optical properties. The packing diagram of Eu2 exhibits an extensive set of π⋯π stacking interactions of 3.411 Å and 3.781 Å between the aromatic rings of adjacent ligands to generate a one-dimensional (I-D) chain structure along the a-axis (Fig. 2b). Interestingly, Eu1 does not show any sign of π⋯π interactions; however, a π⋯F (2.417 Å) interaction is observed between the aromatic ring of the TB-Im ligand and tfac ligand within the crystal lattice that generates a long chain of stacked molecules along the crystallographic a-axis (Fig. 2a). For Eu1 there is an intermolecular N–H⋯O hydrogen bond involving the N–H unit of the TB-Im ligand (N(2)–H(2)⋯O(2ⁱ), H(2)⋯O(2ⁱ) 2.24 Å, N(2)⋯O(2ⁱ) 3.028(5) Å, N(2)–H(2)⋯O(2ⁱ) 153°; symmetry code (i) ½ − x, −½ + y, ½ − z). There is also a relatively short C–H⋯F intermolecular interaction (C(11)–H(11b)⋯F(6ⁱⁱ), H(11b)⋯F(6ⁱⁱ) 2.54 Å, C(11)⋯F(6ⁱⁱ), 3.461(10) Å, C(11)–H(11b)⋯F(6ⁱⁱ) 161°; symmetry code (ii) ½ + x, 3/2 − y, −½ + z). The N–H unit in Eu2 also participates in a hydrogen bond (N(3)–H(3)⋯O(1ⁱⁱⁱ), H(3)⋯O(1ⁱⁱⁱ) 2.16 Å, N(3)⋯O(1ⁱⁱⁱ) 2.915(6) Å, N(3)–H(3)⋯O(1iii) 147°; symmetry code (iii) 1 − x, 1 − y, 1 − z), there is also a NH⋯F (2.586 Å) interaction with one of the hfac ligands, and F⋯F interactions (2.938 Å) between the molecules.

Fig. 2 Packing diagram of the complexes displaying different interactions (a) Eu1 and (b) Eu2.

3.2. Photophysical studies

Optical absorption spectroscopy was used to evaluate the light absorbing capability of the complexes. The complexes displayed broad spectra in the region between 250 and 350 nm with λ^max_abs at 287 nm (ε = 8041 M⁻¹ cm⁻¹) and 293 nm (ε = 11 375 M⁻¹ cm⁻¹) for Eu1 and Eu2, respectively, attributed to a composite of π–π* transitions of both primary antenna and ancillary ligands (Fig. 3a). Moreover, the spectrum of Eu2 displayed a minor (6 nm) but meaningful redshift in λ^max_abs compared to that of Eu1. This could be attributed to the greater number of fluorine (F) atoms on the ligands in Eu2 which is favourable for inducing increased intramolecular charge transfer (ICT).¹⁹

Fig. 3 (a) Optical absorption spectra of Eu1 and Eu2 in DCM solution (1 × 10⁻⁵ M). (b) Overlapped experimental and theoretical absorption spectra calculated by the TD-DFT (with and without the effect of solvent) and INDO/S-CIS methods for Eu1 and Eu2 using the geometry optimized at the PBE1PBE/TZVPPD/MWB52 level of theory.

To rationalize the experimental optical absorption spectroscopy results, we calculated the theoretical absorption spectra by the TD-DFT and INDO/S-CIS methods (Fig. 3b) employing the optimized geometry with the PBE1PBE/TZVPPD/MWB52 level of theory. As can be seen from Fig. 3b, the λ^abs_max value is marginally under calculated which is similar to our recent study, where different TD-DFT methods (CAM-B3LYP, M06-2X, PBE1PBE and ωB97X-D3BJ) were used.^9b However, the band shape and redshift (ca. 9 nm) of spectra obtained by the TD-DFT methods are in qualitative agreement with the experimental spectra of Eu1 and Eu2 suggesting that the TD-DFT approach could be an important tool to provide the most relevant electronic transitions to the most intense band.

The electronic transitions calculated at the CAM-B3LYP/TZVPPD/MWB52 level of theory, considering the effect of the DCM solvent, are presented in Table 2. It is possible to observe that the most important molecular orbitals (MOs) involved in the most intense band range from HOMO−4 to LUMO+4. The most intense bands of both complexes are due to electronic transitions involving MOs centred both in the primary β-diketonate and ancillary ligands (Fig. 4). The natural transition orbitals (NTOs) analysis performed at the same level of theory (Fig. S6, ESI†) further corroborated this observation. This analysis provides a simple representation of the transition density between the ground and the excited state²⁰ and reveals additionally that the longest wavelength band of Eu1 is due to the electronic transitions involving MOs centred on the ancillary ligand. It is important to mention that the ligand containing a greater number of F-atoms provides a larger electron density resulting in a larger contribution to the band. Similar conclusions are obtained from the analysis of the TD-DFT results without considering the implicit effect of the DCM solvent (Table S5 and Fig. S7, ESI†).

Fig. 4 Most relevant MOs calculated at the TD-DFT CAM-BLYP level of theory that explains the main electronic transitions. The implicit effect of the DCM solvent was considered in the calculation.

Table 2 Electronic transitions calculated at the CAM-B3LYP/TZVPPD/MWB52 level of theory for the most intense bands of Eu1 and Eu2 considering the effect of the DCM solvent

	λ, nm/oscillator strength	Major contribution	Total
MOs centred on the ancillary ligand are highlighted in bold.
Eu1	246.3/0.7912	HOMO−1 → LUMO+3 (17.88%)	68.90%
		HOMO−4 → LUMO+1 (13.62%)
		HOMO−3 → LUMO (13.43%)
		HOMO−2 → LUMO+2 (11.59%)
		HOMO−1 → LUMO+1 (6.31%)
		HOMO−2 → LUMO (6.07%)
	269.0/0.5553	HOMO → LUMO (60.81%)	83.78%
		HOMO → LUMO+1 (17.46%)
		HOMO → LUMO+4 (5.50%)
Eu2	254.4/0.9220	HOMO → LUMO+1 (28.41%)	72.19%
		HOMO → LUMO (21.12%)
		HOMO → LUMO+2 (10.90%)
		HOMO−2 → LUMO (6.64%)
		HOMO−4 → LUMO+1 (5.12%)
	270.7/0.4056	HOMO → LUMO+3 (60.77%)	76.92%
		HOMO−2 → LUMO (10.59%)
		HOMO → LUMO+4 (5.55%)

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Exciting the complexes at their λ^max_abs exhibited five typical well-resolved emission transitions a–e (Table 3) in the region between 550–750 nm (Fig. 5) without any residual ligand fluorescence (RFL) between 380–500 nm (Fig. S8 and S9, ESI†) implying good energy transfer ET. Table 3 shows the barycentre of the emission transitions (a–e) and % contribution of the transition intensities relative to the magnetic dipole (MD) ⁵D₀ → ⁷F₁. The intensity of the emission spectra follow a similar trend as the molar absorptivity values i.e., Eu2 > Eu1. The spectra of both complexes are dominated by the narrow (FWHM < 4 nm) electric dipole (ED) ⁵D₀ → ⁷F₂ transition with a contribution greater than 78% of the total emission intensity. Moreover, the superiority of the ED transition over the MD transition indicates dynamic coupling (DC) as the dominant mechanism in the emission process.²¹ From the emission spectra of the complexes, we further calculated the CIE coordinates of the emitted colour (Table 3). As can be seen from the inset of Fig. 5 (Fig. S10 and S11, ESI†) both the complexes displayed pure red emission with CIE coordinates very close to recommended CIE by NTSC (x, 0.67; y, 0.33) for red emission.

Fig. 5 PL spectra of (a) Eu1 and (b) Eu2 in DCM solution. Inset showing the magnified view of the CIE colour with their coordinates calculated from the emission spectra.

Table 3 Experimental and theoretical photophysical properties of Eu1 and Eu2 in DCM

Photophysical parameters	Eu1	Eu2
Values in the bracket are % contribution relative to MD transition; values in square parentheses are theoretically calculated; Ω2 and Ω4 were calculated by applying eqn (1) and (2); ARad and ANRad were calculated by applying eqn (2) and (4); Q^EuEu and ηSen were calculated by applying eqn (6) and (7).
^5D0 → ^7F0 (a)	17 261.32 cm^−1 (1.09%)	17 247.68 cm^−1 (0.86%)
^5D0 → ^7F1 (b)	16 880.18 cm^−1	16 863.62 cm^−1
^5D0 → ^7F2 (c)	16 237.29 cm^−1 (78.63%)	16 236.99 cm^−1 (80.36%)
^5D0 → ^7F3 (d)	15 298.74 cm^−1 (2.73%)	15 324.53 cm^−1 [2.61%]
^5D0 → ^7F4 (e)	14 286.42 cm^−1 (11.35%)	14 308.93 cm^−1 (10.33%)
FWHM of ^5D0 → ^7F2	3.76 nm	3.36 nm
Intensity ratio (R21)	12.88	14.02
CIE color coordinates	x = 0.663; y = 0.331	x = 0.668; y = 0.330
τ obs	885 ± 2.37 μs (χ^2 = 1.002)	977± 1.82 μs (χ^2 = 1.021)
Ω 2 (× 10^−20 cm^2)	22.66 × 10^−20 cm^2	24.57 × 10^−20 cm^2
	[22.61 × 10^−20 cm^2]	[24.57 × 10^−20 cm^2]
Ω 4 (× 10^−20 cm^2)	7.62 × 10^−20 cm^2	7.25 × 10^−20 cm^2
	[7.81 × 10^−20 cm^2]	[7.24 × 10^−20 cm^2]
A Rad	744.01 s^−1 [721.90 s^−1]	786.18 s^−1 [764.37 s^−1]
A NRad	385.93 s^−1 [408.04 s^−1]	236.52 s^−1 [259.17 s^−1]
Q Eu ^Eu	65.84% [63.89%]	76.87% [74.68%]
Q Eu ^L	39.33% [38.32%]	47.00% [45.97%]
η Sen	59.73% [59.98%]	61.14% [61.56%]

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Apart from the steady-state emission studies, it is of importance to establish other potential experimental photophysical parameters to fully understand the emission phenomena in a given complex before their potential use in optoelectronic devices such as OLEDs can be established. In this regard, we first determined the excited lifetime (τ_obs) of the ⁵D₀ emitting state of Eu1 and Eu2 in solution. The τ_obs was calculated by the fitting of the PL decay curve (Fig. S12, ESI†) as shown in Fig. S13 and S14, ESI.† The PL decay profiles for both the complexes reveal monoexponential behaviour and confirm the presence of single emitting species. This is supported by the crystal structure determinations of the complexes and further corroborates the steady-state emission spectra where a single well-resolved emission peak is observed for ⁵D₀ → ⁷F₀. The τ_obs values in the microsecond timescale regime (Eu1 = 885 ± 2.37 μs and Eu2 = 977± 1.82 μs, Table 3) are well within the range of typical europium β-diketonate N^N complexes. The absolute PLQY (Q^L_Eu) of the complexes follows a similar trend as noted for the steady-state emissions and the excited lifetimes. The complexes displayed fairly large values of Q^L_Eu1 = 39.33% and Q^L_Eu2 = 47.00% in solution. To understand it better, we have calculated A_Rad and A_NRad rates for the complexes by applying a set of eqn (2)–(4), which are presented in Table 3. It is clear from Table 3 that the large Q^L_Eu2 value of Eu2 compared to Eu1 is due to the lower A_NRad = 236.52 s⁻¹ of Eu2, which is further reflected in its Q^Eu_Eu = 76.87% leading to sensitization efficiency of η_Sen = 61.14%. Finally, J–O intensity parameter (Ω₂ and Ω₄) was calculated (Table 3). In each case Ω₂ displayed a large value implying that the symmetry around the Eu(III) is distorted which indeed is the case as determined by the single-crystal structure determinations (distorted triangular dodecahedron). As expected, Ω₂ value of Eu2 (24.57 × 10⁻²⁰ cm²) is larger than that of Eu1 (22.66 × 10⁻²⁰ cm²), which is due to more distorted coordination geometry at the Eu(III) centre; in Eu2 (CShM = 0.695) compared to Eu1 (CShM = 0.629). The Ω₄ parameter is less sensitive to the coordination sphere; however, it is related to long-range effects (hydrogen bonding and π–π stacking).²¹ The significantly large value of the Ω₄ = 7.62 × 10⁻²⁰ cm² and 7.25 × 10⁻²⁰ cm² for Eu1 and Eu2, respectively, points to the presence of these effects. This is further supported by the single crystal X-ray structure where these long-range effects are encountered. The theoretically calculated Ω₂ and Ω₄ (Table 3) using the QDC model²² (Table S6, ESI†) and A_Rad compare well with the experimental data (Table 3). Moreover, we further calculated the contribution of the forced electric dipole intensity parameters Ω^FED_λ. The low values of Ω^FED_λ (Table S6, ESI†) attest that the emission for the present complexes is dominated by the dynamic coupling (DC) mechanism, a commonly observed phenomenon in these types of complexes. Furthermore, a low value of Ω^FED_λ indicates that Ω_λ is strongly dependent on the polarizabilites of the ligand atoms of the complexes. It is worth mentioning that Ω^FED_λ plays an important role in estimating the ET rates from the direct coulombic interaction (CI) mechanism which is operative when the J quantum number of the states of the Eu(III) ion involved in the electronic excitations satisfy the |ΔJ| = 2, 4, and 6 selection rule.

3.3. Intermolecular energy transfer (IET)

TD-DFT calculations were also performed to evaluate the impact of the geometry on the lowest energy singlet (S₁) and triplet states (T₁). The TD-DFT CAM-B3LYP/TZVP/MWB52 approach was applied considering all the optimized geometries by the different DFT methods (Table S7, ESI†). It is important to emphasize that R_L parameter and energy of S₁ and T₁ varied modestly which could be attributed to the similar RMSD values in each structure (Table S4, ESI†). Moreover, a careful analysis of Table S6, ESI,† further revealed that the effect of solvent on the T₁ of Eu2 is more prominent than that on Eu1. The solvent caused an average stabilization of 285 cm⁻¹ in the T₁ state of Eu2 while this is only 25 cm⁻¹ for Eu1. An explanation for this is that the electron density due to the various peripheral F atoms of hfac in Eu2 interacts more strongly with the electric field produced by the solvent compared to tfac of Eu1. Using the default setting of LUMPAC²³ for INDO/S-CIS model, we found a much lower value for T₁ for both the complexes (14 550.3 and 14 424.1 cm⁻¹ for Eu1 and Eu2, respectively, Table S7, ESI†) which is in contrast to our recent work.^9a,c The analysis of the electronic transitions involved in T₁ calculated by the INDO/S-CIS model showed that the most important MOs are situated on the TB-Im ligand. To confirm this behaviour, we further replaced the ancillary ligand by two water molecules and found the T₁ was around 20 000 cm⁻¹ thus confirming that the lowering is due to the TB-Im ligand.

To propose the ET mechanism of the complexes, the results obtained using the TD-DFT CAM-B3LYP/TZVPPD/MWB52 method (DCM solvent) with the geometry optimized at the PBE1PBE/TZVPPD/MWB52 level of theory were utilized. The ligand–metal ET rate for a given pathway depends on the energy and distance R_L of the excited states involved in the ET process (see eqn (S2) and (S3), ESI†). The energy of the lowest S₁ and T₁ states and their corresponding R_L value states are shown in Table 5. An analysis of the electronic transitions for S₁ and T₁ (Table 5) in conjunction with the MOs (Fig. 4) suggests that S₁ and T₁ involved MOs centred on the β-diketonate and ancillary ligands. In case of Eu2, 93.4% of the electronic transitions that compose T₁ strongly depend on MOs centred only on the hfac ligands without any role of the TB-Im ligand. The analysis of MOs showed additionally that the lowest three triplet states of Eu2 had almost similar energies (T₁ = 22 252.4 cm⁻¹, T₂ = 22 385.9 cm⁻¹ and 22 622.0 cm⁻¹) and involved only MOs centred on the different β-diketones ligands. A slightly different situation was observed for the degenerate T₁ (24 326.3 cm⁻¹), T₂ (24 379.8 cm⁻¹) and T₃ (22 622.0 cm⁻¹) states of Eu1, since these states had contribution from TB-Im mainly through HOMO−3, HOMO−1, LUMO, and LUMO. The T₄ state (27 129.5 cm⁻¹) of Eu2 is composed of MOs centred mainly on the TB-Im. The contribution of MOs centred on the TB-Im explains why the energy of T₁ for Eu1 is higher than the corresponding state of Eu2.

Table 4 Energy of the S and T excited states, distance from energy donor to acceptor centre (R_L), and electronic transitions for the corresponding excited states of the complexes calculated with the TD-DFT CAM-B3LYP/TZVPPD/MWB52 approach considering the effect of DCM

Compound	State	Energy/cm^−1	R L/Å	Major contribution	Total
MOs highlighted in bold are those centred in the ancillary ligand.
Eu1	S1	35 475.1	3.4485	HOMO−6 → LUMO+3 (32.83%)	89.18%
				HOMO−6 → LUMO+1 (14.25%)
				HOMO−6 → LUMO+2 (9.80%)
				HOMO−5 → LUMO+3 (9.64%)
				HOMO−6 → LUMO (9.21%)
				HOMO−5 → LUMO+2 (7.15%)
				HOMO−5 → LUMO+1 (6.29%)
	T1	24 326.3	3.6026	HOMO−1 → LUMO+1 (27.66%)	84.14%
				HOMO−4 → LUMO+3 (17.40%)
				HOMO−4 → LUMO+2 (11.48%)
				HOMO−1 → LUMO (9.89%)
				HOMO−4 → LUMO+1 (9.52%)
				HOMO−3 → LUMO+1 (8.19%)
Eu2	S1	34 505.5	3.2805	HOMO−6 → LUMO+2 (26.04%)	88.04%
				HOMO−6 → LUMO (17.21%)
				HOMO−7 → LUMO+2 (15.53%)
				HOMO−5 → LUMO+2 (9.19%)
				HOMO−8 → LUMO+2 (8.07%)
				HOMO−7 → LUMO (6.30%)
				HOMO−5 → LUMO (5.70%)
	T1	22 252.4	3.2957	HOMO−2 → LUMO (38.87%)	93.40%
				HOMO−4 → LUMO (19.17%)
				HOMO−2 → LUMO+2 (16.89%)
				HOMO−4 → LUMO+2 (12.68%)
				HOMO−4 → LUMO+1 (5.79%)
	T4	27 129.5	3.1828	HOMO → LUMO+3 (78.57%)	85.22%
				HOMO−5 → LUMO+3 (6.66%)

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Table 5 Key electroluminescent properties of the single-EML devices of Eu1 and Eu2 operating at J = 10 mA cm⁻²

Doping concentration (device)	V turn-on (V)	B (cd m^−2)	η c ^b (cd A^−1)	η p ^c (lm W^−1)	EQE	CIEx,y ^d
a The data for maximum brightness (B). b Maximum current efficiency (ηc). c Maximum current efficiency (ηp). d CIE at J = 10 mA cm^−2.
Eu1
2 wt% (Device 1)	5.0	32.56	0.068	0.034	0.044%	(0.284, 0.264)
3 wt% (Device 2)	4.6	55.11	0.079	0.046	0.049%	(0.354, 0.288)
4 wt% (Device 3)	4.6	100.5	0.109	0.055	0.068%	(0.329, 0.285)
5 wt% (Device 4)	4.6	29.47	0.049	0.027	0.035%	(0.294, 0.243)
6 wt% (Device 5)	5.4	21.52	0.048	0.025	0.030%	(0.285, 0.282)
Eu2
2 wt% (Device 1)	3.5	710.4	0.441	0.396	0.304%	(0.283, 0.199)
3 wt% (Device 2)	3.5	736.1	0.524	0.461	0.327%	(0.305, 0.220)
4 wt% (Device 3)	3.9	364.1	0.276	0.223	0.186%	(0.333, 0.228)
6 wt% (Device 4)	4.1	238.8	0.229	0.176	0.150%	(0.386, 0.259)
8 wt% (Device 5)	4.1	186.2	0.205	0.157	0.135%	(0.386, 0.256)

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The calculated ET rates for the complexes employing Malta's model²⁴ for different excited levels of Eu(III) and the excited states of the ligands are shown in Table 4 and Table S8, ESI.† ET channels that are governed by the CI and Ex. mechanisms are distinguished in Table S8† and the back ET rate (W_BET) was estimated by using the sum where both mechanisms were operative. The largest value of ET rate (W_ET) observed for the S₁ state of Eu1 is of the order of 10⁵ s⁻¹ and is related to the ⁷F₁ → ⁵G_2,3 acceptor levels of Eu(III). Because of the resonance condition involving S₁ of Eu2 and the ⁵D₄, ⁵D₃ and ⁵G₃ states, rates of this order were observed for ⁷F₀ → ⁵D₄, ⁷F₁ → ⁵D₃ and ⁷F₁ → ⁵G₃ acceptors, where all these rates are governed by the CI mechanism. A rate of the order of 10⁶ was observed for the ⁷F₁ → ⁵G₂ acceptor for Eu2 and is dominated by the Ex. mechanism. Since S₁ of both complexes were overestimated, W_BET involving all the excited states considered for Eu(IIII) are practically negligible. When the ET pathways from T₁ is analysed, it is noted that the ⁷F₁ → ⁵D₀ state has W_ET value of 10⁸ and 10⁹ s⁻¹ for Eu1 and Eu2, respectively, revealing the importance of this channel in sensitizing the PL of the complexes. Table S8† shows large values of W_BET for some pathways involving T₁. In these cases, since the states of the Eu(III) ion are significantly above T₁, the energy returns to T₁ which can then be transferred to more resonant states of Eu(III) such as the ⁵D₀ and ⁵D₁ states. Moreover, ET from T₄ state in Eu2 provides a significant value of rate (10⁷ s⁻¹) for the pathways related to ⁷F₀ → ⁵D₁ (3.24 × 10⁷ s⁻¹), ⁷F₁ → ⁵D₀ (2.79 × 10⁷ s⁻¹) and ⁷F₁ → ⁵D₂ (1.54 × 10⁷ s⁻¹). The R_L value for T₄ is sufficiently small (3.1828 Å) to favour the ligand-Eu(III) ET. A rate of 10⁸ s⁻¹ is noted for the T₄ → ⁷G₂ channel; however, a considerable W_BET is obtained due to the good resonance condition between T₄ and ⁵G₂ (9.80 × 10⁶ s⁻¹; Table S8, ESI†).

The most important energy migration channels for the sensitized PL of the complexes are depicted as the Jablonski diagram (Fig. 6). Since the experimental η_Sen for both the complex is ≈60%, a plausible explanation for these experimental observations could be due to the presence of pathways that eventually depopulates to T₁. To reproduce the experimental results some rates involving ligand states were adjusted as already applied in previous work.^9b As can be seen in Fig. 6, decay rates for Eu1 involving the S₁ → T₁ and T₁ → S₁ pathways of the order of 10⁶ and 10⁷ s⁻¹, respectively, provided a theoretical η_Sen = 60.0%, which agrees with the experimental η_Sen = 59.73%. The energy-level diagram for Eu2 shows that rates of the order 10⁵, 10⁹ and 10⁹ s⁻¹ for S₁ → T₄, T₄ → T₁ and T₁ → S₀ provided a theoretical η_Sen of 61.6%. It is important to mention that the other experimental photophysical parameters were also reproduced well for both complexes.

Fig. 6 Schematic energy-level diagram for (a) Eu1 and (b) Eu2, showing the states considered in the modelling of the ET of both complexes.

3.4. Electroluminescence and electrophysical properties of Eu1 and Eu2 based OLEDs

The thermal stability of Eu1 and Eu2 is of critical importance since inferior thermal stability of the complex tends to reduce device stability, especially at the peak of its operation. This is due to the Joule heating when current flows through the organic layers.²¹ In view of this, the thermal stability of Eu1 and Eu2 in the temperature range between 50–800 °C under a dinitrogen (N₂) atmosphere was determined. The thermogram of the complexes (Fig. 7 and Fig. S15, ESI†) does not exhibit any weight loss in the 50–150 °C region, implying that complexes have no lattice held/coordinated solvent/water molecules as is evident from the FT-IR and crystallographic studies. The DTA of the complexes displayed an endothermic peak at 228.9 and 202 °C for Eu1 and Eu2, respectively, representing the melting temperature (T_m). It is noteworthy that T_m of Eu1 is higher than Eu2 which possibly could be attributed to π⋯F (2.417 Å) interactions within the crystal lattice. The decomposition temperature (T_d) with 5% weight loss of the complexes is 241 °C for Eu1 and 239 °C for Eu2, respectively. The high thermal stability of the complexes implies that they can easily be employed to fabricate OLEDs by the vacuum thermal evaporation method.

Fig. 7 TGA and DTA profile of Eu2 under N₂ atmosphere.

Realizing the good PL and thermal properties of Eu1 and Eu2 and to fully establish their potential as an active component in OLEDs, we finally fabricated OLEDs by the vacuum thermal evaporation method and evaluated their EL and electrophysical properties. To investigate EL performances of the complexes, multilayers OLEDs were fabricated (please see ESI† for device details). The doping concentrations of the active component (Eu1 and Eu2) were varied. Interestingly, as the doping concentration increased the temperature of evaporation (T_evp) increased simultaneously gradually [145–154 °C for Eu1 and 139–149 °C for Eu2]. However, it remained very low compared to T_m and T_d [T_m = (228.9 °C)_Eu-1 and (202 °C)_Eu2 and T_d = (241 °C)_Eu-1 and (239 °C)_Eu2] during the fabrication processes of the devices and implies that no decomposition and melting of the complexes occurred, thus EL arising in the OLEDs are due to complexes. The EL performance data obtained such as brightness (B), current efficiency (η_c), power efficiency (η_p), external quantum efficiency (EQE) and CIE colour coordinates for single-EML as well as double-EML OLEDs of Eu1 and Eu2 at J = 10 mA cm⁻² are summarized in Table 5 and Table S9, ESI.†

As can be seen the EL spectra of the single- and double-EML devices of both Eu1 and Eu2 (Fig. 8) displayed emission transitions originating from the Eu(III) ion implying that both carrier-trapping and Förster ET are taking part in the device luminescent processes.^9b–d,21 Moreover, EL spectra of Eu1 and single-EML Eu2 devices exhibited broad host emission in the region between 380–550 nm similar to the PL spectra of the doped films except the change in the intensity (Fig. S16–S19, ESI†). The presence of broad emission suggests that ET from host to Eu(III) ion is small. Moreover, the intensity of host emission for Eu1 based devices increased with the doping concentration except for the 6 wt% device, impling poor carrier-trapping on the Eu1 molecules and incomplete ET from the host to Eu(III) molecules. However, this trend is reversed for Eu2 based devices implying enhanced carrier-trapping and improved ET from host to Eu2 molecules.

Fig. 8 Normalized EL spectra of single- and double-EML devices of Eu1 and Eu2 at different doping concentrations operating at J = 10 mA cm⁻².

The host emission in the region between 400–500 nm coupled (Fig. S20, ESI†) with the typical red emission of Eu(III) ion at 600–620 nm due to the electric dipole ⁵D₀ → ⁷F₂ transition prove beneficial in obtaining colour tunable OLEDs (Fig. 9, Fig. S21 and S22, ESI†). At the 4 wt% (Device 3) doping concentration, the emission of devices based on Eu1 and Eu2 fall in the white light emission region with CIE colour coordinates of (0.329, 0.285)_Eu1 and (0.333, 0.228)_Eu2 (Table 6), respectively. Furthermore, we have calculated the colour correlated temperature (CCT) of the emitted light that measures the coolness and warmness of the light by the McCamy method.²⁵ The CCT values of Device 3 for Eu1 (5719 K) and Eu2 (5356 K) fall in the cool white light category and thus could be a potential candidate for lighting that will represent the real colour of the objects i.e., kitchen, garage, workshop, product displays, industrial applications, etc.

Fig. 9 CIE 1931 chromaticity diagrams of single-EML Eu1 and Eu2 based devices at different doping concentrations operating at J = 10 mA cm⁻².

Table 6 A comparative electroluminescent characteristic of single component W-OLEDs

Complex	B (cd m^−2)	V turn-on (V)	η p (lm W^−1)	CIE	Ref.
Where TCPD = 1-[3,4,5-tris[4-(9H-carbazol-9-yl)butoxy]-phenyl]-3-phenylpropane-1,3-dione; Phen = 1,10-phenanthroline; tta = thenoyltrifluoroacetonate; L = 2-(3,5-dimethyl-2H-pyrrol-2-yl)-4-(3,5-dimethyl-3H-pyrrol-2-yl)-6-(4-(pentan-3-yl)phenyl)-1,3,5-triazine; bpm = 2,2′-bipyrimidine.a The data for maximum brightness (B) and be placed after the definitions of the ligands on a new line.
Eu1	55.11	4.6	0.034	0.354, 0.288	This work
Eu2	736.1	3.5	0.461	0.305, 0.220	This work
[Eu(TCPD)3Phen]	229	20.5	0.2 (10.5 V)	0.333; 0.348	26a
[Eu(tta)3L]	945.1	16	—	0.337; 0.362	27
[Eu2(tta)6]bpm	19.7	7.6	—	0.350; 0.330	26b

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The current density (J)–voltage (V)–brightness (B) curves are shown in Fig. 10 and Fig. S23 & S24, ESI† for Eu1 and Eu2, respectively. The detailed EL performances such as B, η_c, η_p and EQE of single- as well as double-EML devices based on Eu1 and Eu2 are summarized in Table 5 & Table S9, ESI.† The white-OLED of Eu1 displayed B = 100.5 cd m⁻², η_c = 0.109 cd A⁻¹, η_p = 0.055 lm W⁻¹, EQE = 0.068% with the turn-on voltage = 4.6 V, while Eu2 based W-OLED exhibited improved EL performance (B = 364.1 cd m⁻², η_c = 0.276 cd A⁻¹, η_p = 0.223 lm W⁻¹, EQE = 0.186% with the very low turn-on voltage = 3.9 V). It is important to mention that the electroluminescent properties of the present single component white-OLEDs are higher than the reported ternary europium complexes.²⁶

Fig. 10 Current density (J)–voltage (V)–brightness (B) curve of the single component OLEDs of (a) Eu1 and (b) Eu2.

4. Conclusion

Two new OEuCs were succesfully synthesized and structurally charcaterized. Analysis of the crystallographically determined molecular structures through the CShMs analysis revealed that the geometry around the Eu(III) centre is distorted triangular dodecahedral, with approximate D_2d symmetry’ in both structures. The complexes displayed fairly strong light absorbing capability and the most intense absorption transitions are a composite of MOs centred both on the primary β-diketonate and ancillary ligands. NTOs analysis also revealed that ligand with greater number of Fatoms provides a larger electron density and thus results in a larger contribution to the most intense transition. Exciting the complexes at their λ^max_abs exhibited typical well-resolved red emission with reasonable Q^L_Eu value. A close scrutiny of the TD-DFT results suggest that electronic transitions that compose T₁ in Eu2 (93.4%) exclusively rely on the MOs of hfac while for Eu1 both tfac and Tb-Im MOs are involved resulting in higher T₁ state in Eu1 compared to Eu2. The ⁷F₁ → ⁵D₀ state have large W_ET values of 10⁸ and 10⁹ s⁻¹ for Eu1 and Eu2, respectively, emphasising the significance of this channel in sensitizing the PL of the complexes. Finally, the complexes were successfully employed as EML to fabicate OLEDs. The emission of Device 3 of Eu1 and Eu2 falls in the white light emission region with superior EL performance to the few reported single component white-OLEDs of ternary europium complexes. Furthermore, calculated CCT values fall in the cool white light category and points to its potential for lighting that will represent the real colour of the objects i.e., kitchen, garage, workshop, product displays, industrial applications, etc.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

MSK acknowledges His Majesty's Trust Fund for Strategic Research (Grant No. SR/SQU/SCI/CHEM/21/01) for funding. RI thanks HM's Trust Fund for a postdoctoral fellowship. JDLD appreciates the financial support from the Brazilian funding agencies: CAPES, CNPq (421733/2018-7), FACEPE (APQ - 0675-1.06/14) and FAPITEC-SE (Process N. 019.203.01074/2011-1). WFO thanks PIBIC/CNPq/UFS for a scientific initiation fellowship. The computing for this project was performed on the LCAD-UFS (UFS High Performance Computing Laboratory). LZ is grateful for the financial aid from the National Natural Science Foundation of China (62174160), Youth Innovation Promotion Association of the Chinese Academy of Sciences (2013150). WYW thanks the Hong Kong Research Grants Council (PolyU 153058/19P), Guangdong-Hong Kong-Macao Joint Laboratory of Optoelectronic and Magnetic Functional Materials (2019B121205002), the CAS-Croucher Funding Scheme for Joint Laboratories (ZH4A), Hong Kong Polytechnic University (1-ZE1C), Research Institute for Smart Energy (CDAQ) and the Endowed Professorship in Energy from Miss Clarea Au (847S) for the financial support. PRR is grateful to the Engineering and Physical Sciences Research Council (EPSRC) for funding (Grant EP/K004956/1).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2178685 and 2178686. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2dt01873j

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