Optimizing white light emission in Dy(III) complexes: impact of energy transfer from mono and bidentate ligands on luminescence

Abstract

Complexes of dysprosium(III) ions with 1,1,1,5,5,5-hexafluoro-2,4-pentanedione featuring various mono and bi-dentate neutral ligands have been prepared and thoroughly investigated. The synthesized complexes exhibit an octa-coordinated environment, achieved by stoichiometrically combining organic ligands and Dy(III) ions. This octa-coordination environment of Dy(III) ion was confirmed by FT-IR spectroscopy, thermogravimetry and elemental analysis. Near-white light (NWL) is emitted when complexes were exposed to UV radiation, indicating a significant flow of energy from the sensitizing moieties towards the Dy(III) ion. This NWL emission might have resulted due to a balance between the intensities corresponding to emission peaks at 480 nm (blue) and 575 nm (yellow) in Dy1–Dy3. Emission spectra recorded at different excitation wavelength were utilized to study the tunability of CIE color coordinates. In addition to their high thermal stability, the complexes display bipolar paramagnetic shifts in their NMR spectra. The ⁴F_9/2 → ⁶H_13/2 transition, contributing ∼62% of the total emission, stands out as a promising candidate for laser amplification due to its dominance in the emission spectra. Additionally, NWL emission observed in a solid Dy(III) complex opens intriguing possibilities for its application in next-generation white-light emitting devices.

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1 Introduction

There has been a strong push towards the development and wide spread use of energy efficient SSL (solid state lighting) technologies and smart devices in response to increased global demand for energy. Solid-state white-light-emitting materials offer remarkable benefits, like energy conservation and extended lifespan, leading to their widespread adoption across various applications, ranging from large-panel displays to lasers.^1–4 It is important to note that SSL sources can be classified based on the material used in their fabrication. For instance, LEDs employ inorganic phosphors, while OLEDs make use of organic semiconductors. Furthermore, SSLs can be categorized based on the method by which they emit light, with LEDs utilizing UV excitation and OLEDs relying on electrical excitation.^5,6 Considerable interest surrounds the realm of white light emission (WLE) which can be obtained using two different approaches: dual color emitters [blue either with red (B/R) or with yellow (B/Y)] or simply employing three-colour emitters that mix primary colors (RGB).^7–9 To generate white light, a variety of methods are employed, including the incorporation of different dopants or multiple phase matrices. These matrices can be composed of various materials like nanomaterials, organic compounds, hybrid organic–inorganic substances, metal complexes or inorganic phosphors. These materials have unique properties for achieving white light emission and sometimes combining these different materials is necessary to reach the desired outcome.

Additionally, WLE from single molecules is also attainable, covering entire visible spectrum.^10–12 Such systems comprises of a single-phase-single-emitter which eases the manufacturing process, resulting in materials which are light weight and thin, making them particularly well-suited for use in optical devices.¹³ WLE from single molecules can be generated by utilizing trivalent lanthanide (Ln) complexes. The fascinating aspect of lanthanide luminescence is that it relies on indirect excitation of lanthanide ions, unlike other conventional photo-excitation methods. This is on account of parity restricted behavior of their f–f transitions which results in low absorptivity (0.5–3 dm³ mol¹ cm¹).^14,15 To overcome this limitation, researchers have turned to suitable sensitizers, which possess the unique ability to indirectly transfer energy to lanthanide ions and function as antennas.^16,17 They absorb incident light and sensitize metal ion by efficiently transferring this energy to their excited states. Subsequently, excited Ln(III) ion engages in radiative relaxations to the lower electronic states. Narrow peaks are obtained in the emission spectra of these complexes due to the well-shielded 4f electrons of the lanthanide ions, which result in sharp and well-defined electronic transitions that are relatively unaffected by the ligand field and surrounding environment.

However, it is essential to highlight that selection of chromophoric sensitizer should be done carefully so that it functions as antenna along with producing its own emission in blue region which is basic necessity to produce white light.^18–20 This dual functionality is crucial for the production of white light when it is coordinated with the yellow light-emitting Dy³⁺ ion. Frequently employed antenna ligands include β-diketones and aromatic carboxylic acids, among others.^21–23 Along with these ligands, various neutral ligands like triphenylphosphine oxide, indazole, bipyridine, pyrazine etc. are also utilized in ternary complexes. These ligands apart from sensitizing the central metal ion, forms a protective coordination sphere to keep the high vibration solvent molecules away from the complex.^24,25 In this article, one [Dy(hfpd)₃(M₂)] and three [Dy(hfpd)₃(L)] (hfpd: 1,1,1,5,5,5-hexafluoro-pentane-2,4-dione), M: tri-n-octylphosphineoxide (TOPO), L: 1,10-phenanthroline (phen), neocuproine (neoc) and bathocuproine (bathoc) type of complexes were synthesized. The ligands used in the synthesis have high probability to coordinate with Ln³⁺ ions due to the strong affinity of oxygen and nitrogen atoms towards lanthanide ions and their π–π* transitions result in strong UV absorption and efficiently sensitize Ln³⁺ ions. The [Dy(hfaa)₃phen] complex²⁶ has been previously documented in the literature. The potential applications of these complexes span a broad spectrum, covering diverse fields such as solid-state lighting, chemical and biological sensing, temperature monitoring and telecommunication.^27–29

2 Experimental section

2.1 Materials

DyCl₃·6H₂O, hfpd, TOPO, phen, neoc and bathoc of high purity were sourced from Sigma Aldrich. AR or spectroscopic grade hexane and ethanol along with reagent 25% ammonia solution were sourced from Sigma-Aldrich. All the procured chemicals were employed as received and the reactions were performed under ambient conditions.

2.2 Method

Quantification of carbon, hydrogen and nitrogen contents was achieved by employing Series II PerkinElmer Elemental Analyser (2400). While, FT-IR spectra (4000–400 cm⁻¹) were collected employing a Nicolet iS-50 FTIR spectrometer. The KBr pellets were formed by incorporating a small quantity of the complex into KBr and then thoroughly grounding it. ¹H NMR spectra were recorded using a FT-NMR spectrometer in CDCl₃. Shimadzu 2450 UV-visible spectrophotometer was employed to observe electronic spectra ranging 200–800 nm. Photoluminescent (PL) properties were analyzed through Horiba Jobin YVON Fluorolog Model FL-3-11 spectrofluorometer equipped with xenon lamp as light source and 0.5 mm excitation and emission slit width. A Fluorescence Spectrophotometer was utilized to investigate the lifetimes of the prepared samples. The current–voltage relationship was characterized at a scan rate of 0.1 V s⁻¹ using the PARSTAT 4000 instrument. The complexes were subjected to thermal analyses under a N₂ at temperature rise of 10 °C per minute, utilizing Hitachi STA thermal analyzer 7300 and the findings are presented in the form of a thermogram.

2.3 Synthesis of ternary Dy³⁺ complexes

Synthesis of dysprosium(III) complexes was carried out using an in situ approach (Fig. 1).^30,31 25% NH₄OH solution was added to 5 mL ethanolic solution of 6.48 mmol hfpd. The contents were kept in a 50 mL container and sealed until the complete dissolution of ammonia vapours which led to the formation of ammonium salt of hexafluoroacetylacetone, [NH₄(hfpd)]. Subsequently, this mixture was combined with 5 mL solution of 2.16 mmol DyCl₃·6H₂O solution in ethanol and further ethanolic solution of neutral ligand was added. The stoichiometric relationship between DyCl₃·6H₂O, hfpd and neutral ligands was 1 : 3 : 1 in Dy2–Dy4 complex and 1 : 3 : 2 in Dy1. This mixture was kept within a pH range of 6–7 and underwent stirring for 12 h at room temperature. Completion of synthetic reaction was marked by formation of precipitates of Dy1, Dy2, Dy3 and Dy4 with TOPO, phen, neoc and bathoc, respectively. The mixtures were left for slow evaporation at room temperature and the formed precipitates underwent repeated washing with both ethanol and hexane.

Fig. 1 Schematic pathway for preparation of Dy1–Dy4.

3 Findings and interpretation

3.1 Preliminary assessment

CHN analysis was conducted for precise establishment of elemental composition of synthesized trivalent dysprosium compounds. The elemental analysis findings showed close approximation with calculated elemental content, indicating that the complexes were prepared in the correct stoichiometry. Moreover, all the compounds displayed solubility in dichloromethane, chloroform and dimethyl sulphoxide under standard laboratory conditions. Table 1 presents the color and elemental composition for Dy1 to Dy4.

Table 1 Primary findings of C, H and N component in % of Dy1–Dy4

Complex	Colour shown	C calcd. (Expt.)	H calcd. (Expt.)	N calcd. (Expt.)
Dy1	White	48.60 (48.65)	6.80 (6.78)	—
Dy2	White	33.65 (33.61)	1.15 (1.18)	2.91 (2.93)
Dy3	Cream	35.12 (35.17)	1.52 (1.50)	2.82 (2.85)
Dy4	Light yellow	43.04 (43.09)	2.03 (2.01)	2.45 (2.49)

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3.2 IR spectroscopy study

The application of IR spectroscopy enables the determination of bonding of ligand with emissive ion. hfpd chelates with Dy³⁺ ion were formed through a covalent bond with enolic oxygen(=C–O) and a coordinate bond with carbonyl oxygen (–C=O), resulting in stable complexes. Table 2 provides information of the peaks associated with the synthesized complexes. The unbound ligand (hfpd) exhibits a band ∼3400 cm⁻¹, in relation to =C–OH stretch. Absence of this stretching vibration band in complexes indicates the chelation of Dy(III) via deprotonated ligand in its enol form (C=C–O). Additionally, peaks around 1690 and 1636 cm⁻¹ relative to carbonyl vibration of enol-isomer and C=C stretching, respectively in free hfpd³² are displaced to 1660 cm⁻¹ and 1434 cm⁻¹ in Dy1–Dy4, highlighting the potential participation of C=O in coordination processes. Complexes exhibit multiple peaks corresponding to CF₃ groups in the range 1144–1154 cm⁻¹, which are observed in the range 1368–1090 cm⁻¹ in free hfpd ligand.³³

Table 2 Key findings from FT-IR spectroscopic analysis of Dy1–Dy4 (cm⁻¹)

Complex	Dy–O	Dy–N	C–H	C–F	P=O	C–N	C=C	C=N	C=O	CH3	=C–H
Dy1	425	—	844	1146	1204	—	1427	—	1663	2858	3105
Dy2	428	554	842	1144	—	1349	1428	1556	1653	—	3058
Dy3	426	546	851	1146	—	1355	1434	1553	1651	2834	3062
Dy4	426	541	854	1154	—	1357	1441	1542	1662	2841	3055

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The free phen ligand displays the C–H bending vibration at 857 cm⁻¹ as well as C=C and C=N ring vibrations in 1650–1400 cm⁻¹ range.³⁴ Discussed peaks undergo significant downward shifts in wavenumber to 842 cm⁻¹ in mononuclear [Dy(hfpd)₃phen] complex as a result of perturbation brought on by coordination of ligand to metal ion. Similarly, free TOPO exhibits a peak at 1210 cm⁻¹ ascribed to P=O that is shifted towards 1204 cm⁻¹ in Dy1.³⁵ Moreover, the complexes exhibit low-intensity peaks around 425–428 cm⁻¹, associated with Dy–O modes and at ∼546 cm⁻¹, attributed to Dy–N coordination in Dy2–Dy4 complexes.³⁶ Such changes in spectrum suggest that Dy(III) ion not only coordinates with carbonyl oxygen of hfpd but also directly interacts with two nitrogen atoms of phen and its derivatives in Dy2–Dy4 and two oxygen atoms of TOPO in Dy1 complex. The lack of vibrational band between 3600 and 3200 cm⁻¹ indicates that water molecules are absent in Dy1–Dy4.³⁷

3.3 ¹H NMR analysis

Proton NMR spectra of the synthesized complexes (Fig. S1–S4†) were recorded in CDCl₃ at a bandwidth of 500 MHz and the obtained chemical shifts are outlined in Table 3. Salient features of the spectra are as follows: (i) in comparison to uncoordinated ligands, the complexes exhibit significant upfield and downfield shifts³⁸ (ii) instead of the anticipated 4 peaks, complex Dy2 displayed only 3 peaks corresponding to phen. Notably, one proton signal was absent within the range of +200 to −200 ppm, owing to the higher magnetic moment of dysprosium ion and resulting in paramagnetically enhanced relaxation of this nucleus (iii) opposite shift directions are observed for methine protons (hfpd) and ancillary ligand protons in Dy1–Dy4 (iv) the absence of a signal relative to enolic OH of hfpd in the spectral profiles of complex implies the binding of the Dy³⁺ ion to the oxygen atom of hfpd.^26,39

Table 3 Proton δ (ppm) data of Dy1–Dy4

Complex	Peaks relative to hfpd	Peaks relative to ancillary moiety
hfpd	10.85 (-OH), 6.48 (-C-H)	—
Dy1	101.47 (3H)	−1.48 (18H), −3.42 & −3.81 (60H), −5.89 (12H), −7.72 (12H)
Dy2	117.36 (3H)	−28.72 (2H), −42.64 (2H), −60.90 (2H)
Dy3	134.10 (3H)	−1.34 (6H), −32.85 (2H), −48.11 (2H), −62.86 (2H)
Dy4	128.08 (3H)	−2.19 (6H), −33.22 (2H), −48.18 (2H), −61.39 (10H)

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3.4 UV-vis absorption spectroscopy

Fig. 2 illustrates room temperature electronic spectra of free hfpd and Dy1–Dy4, captured in 1 × 10⁻⁵ mol L⁻¹ dichloromethane solution. The uncoordinated hfpd displays a prominent absorption band at 279 nm, predominantly associated with spin-allowed π → π* transitions. In contrast, the UV-vis absorption spectra of Dy1–Dy4, besides the UV absorption band at 243–277 nm, reveal a broad maxima at 287–314 nm owing to corresponding transitions of π → π* and n → π*. The initial band in each spectrum (shorter-wavelength band), corresponds to the ancillary ligand.⁴⁰ Complexation leads to a red-shift in the absorption bands of β-diketone ligands, revealing stabilization of their orbitals (nephelauxetic effect).⁴¹ Table 4 comprises of absorption maxima corresponding to Dy1–Dy4.

Fig. 2 Absorption spectra of free hfpd and Dy1–Dy4 complexes.

Table 4 Optical parameters for Dy1–Dy4

Complex	^aλ^maxabs	^aEg	^aλex	^aλem
a λ and E in nm and eV, respectively.
Dy1	286	3.87	279, 371	485, 575, 661
Dy2	295	3.79	278, 370	486, 574, 662
Dy3	308	3.70	279, 372	486, 574, 660
Dy4	314	3.55	302, 374	574, 662

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For Dy1–Dy4 complexes, the determined molar absorption coefficients (ε) are 1.4 × 10⁴, 2.91 × 10⁴, 6.24 × 10⁴, and 1.07 × 10⁵, respectively (Fig. S5†). Compared to the ε of free hfpd, which is 6.7 × 10³,⁴² these values are approximately threefold higher. This result shows that inclusion of ancillary ligands not only meets the coordination needs of the Dy(III) ion but also enhances the absorption intensity. The lowest ε value observed for complex Dy1 can be attributed to the lack of an extended π-system, resulting in less efficient light absorption. In contrast, Dy4 exhibits the highest ε value due to its more extended π-system, which significantly enhances its ability to absorb light. Band gap (E_g) of aforementioned compounds is also measured utilizing eqn (1) i.e. Tauc's equation, on data obtained from electronic spectra:⁴³

(αhυ)² = A(hυ − E_g)ⁿ (1)

Here, α denotes absorptivity, hυ denotes energy of photon, A is used to signify the band edge parameter, while E_g symbolizes the optical band gap. The parameter n is a transition-dependent factor with a value of 1/2 for indirect and 2 for direct excitation.⁴⁴ E_g values of Dy1–Dy4 were measured by the help of Tauc plot between energy and (αhυ)² as shown in Fig. 3. This was accomplished by extrapolation of straight line to (αhυ)² = 0. E_g of Dy1–Dy4 complexes in eV are detected as 3.87, 3.79, 3.70 and 3.55, respectively. This decreasing order in the band gap is attributed to the increasing conjugation in synthesized complexes. The effectiveness of the considered complexes in display devices is closely tied to E_g values falling in semiconductor range, contributing to enhanced performance in electronic technologies.⁴⁵

Fig. 3 Tauc's plots and band gap values of Dy1–Dy4 complexes.

3.5 Photoluminescence spectroscopy investigations

3.5.1 Excitation spectra. To analyze photoluminescent behaviour exhibited by Dy1–Dy4, solid samples were excited using emission wavelengths (λ_em) of 480 nm (⁴F_9/2 → ⁶H_15/2) and 575 nm (⁴F_9/2 → ⁶H_13/2). However, the spectra recorded at 575 nm was more intense in comparison to the one recorded at 480 nm. Normalized excitation spectrum (λ_em = 575 nm) (Fig. 4) of each complex revealed ligand based broad bands ∼280 and ∼370 nm (Table 4) relative to energy required for an electron to jump from π to π*.^46,47 Since, no metal centered peaks were observed in the excitation spectra and the spectral profiles are dominated by ligand's absorption, it suggests that sensitization in Dy1–Dy4 happened via indirect energy transfer [ligand → Dy(III)]. The excitation spectra of Dy1–Dy4 exhibit different intensities with Dy3 being most intense and Dy4 being least intense.

Fig. 4 Normalized excitation spectra of solid Dy1–Dy4 at room temperature at λ_em = 575 nm.

3.5.2 Emission spectra. Fig. 5 contains the normalized emission profiles of solid Dy1–Dy4 complexes. The spectral profiles exhibit emission peaks (Table 4) associated with ⁴F_9/2 → ⁶H_x/2 (x = 15, 13, 11) and π–π* transitions of organic moieties, upon excitation at ∼370 nm. These peaks correspond to three different electronic transitions: a blue, magnetic-dipole (MD) transition (⁴F_9/2 → ⁶H_15/2) at ∼486 nm; a yellow, electric-dipole (ED) transition (⁴F_9/2 → ⁶H_13/2) at ∼574 nm and a least intense peak at ∼661 nm.^48,49 The transition at ∼574 nm is most intense and sensitive to its environment. The greater intensity of transition centered at 574 nm indicates that complexes are situated in low molecular symmetry and lack inversion centre. This makes the yellow transition a valuable reference for assessing symmetry in such systems. While magnetically allowed blue emission (⁴F_9/2 → ⁶H_15/2) remains unaffected by the local environment surrounding Dy(III) ion.⁵⁰ A broad band at lower wavelength is exhibited by all the complexes due to transition from ligand's excited state to ground state indicating that the ligands do not transfer the absorbed energy effectively to the emissive state of metal ion which is discussed in detail in Section 3.5.7. Moreover, the emission spectrum of Dy4 exhibits only 2 metal based peaks (at 574 nm and 662 nm) along with a broad ligand based band which merged with the third peak (485 nm).

Fig. 5 Normalized emission spectra of solid Dy1–Dy4 obtained at room temperature with λ_ex = ∼370 nm.

3.5.3 Branching ratio. Calculating the branching ratio (β) involves the comparison between radiative transition probability for each peak and combined radiative transition probability of all peaks as expressed in eqn (2).⁵¹Here, A_ψJ–ψ′J denotes the total area under the observed peak in photoluminescence emission (PLE) spectrum of Dy1–Dy4 compounds. The importance of β in laser design lies in its role in determining the chances of stimulated emission from particular emission transitions. A greater β value indicates a higher suitability for achieving stimulated emission cross-section.⁵² Table 5 presents the calculated branching ratios for Dy1–Dy4. Analysis of β ratios reveals a predominance of ΔJ = 2 transition, accounting for ∼60% of the total emission in Dy1–Dy3. This suggests its suitability for laser amplification.⁵³ The yellow emission originating from ΔJ = 2 transition dominates the luminescence spectra, followed by blue light of ΔJ = 3 transition and lastly, red emission from ΔJ = 1 transition. Intensity ratio of yellow [Y] (ΔJ = 2) to blue [B] (ΔJ = 3) emission lines reflects neighbouring environment experienced by Dy(III) ion in Dy1–Dy4.⁵⁴ In this study, the Y/B ratio is pointing towards a highly asymmetric surrounding in the solid phase.

Table 5 Branching ratio of characteristic peaks for dysprosium and intensity ratio, lifetime and quantum yield across Dy1–Dy4

Complex	Branching ratio				Intensity ratio	Lifetime (ms)	Quantum yield (%)
	Ligand based band	ΔJ = 3	ΔJ = 2	ΔJ = 1
Dy1	11.894	23.441	54.992	9.671	2.34	0.46	1.9
Dy2	15.703	11.605	68.556	4.134	5.90	0.65	2.7
Dy3	15.069	16.997	61.525	6.407	3.61	0.76	3.4
Dy4	90.616		7.987	1.395	—	0.37	1.2

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3.5.4 Decay analysis and quantum yield. Fig. 6 depicts emission lifetime curves associated with Dy1–Dy4 acquired by examining emission at 574 nm with excitation at about 370 nm. These curves were properly fitted by a single exponential function (eqn (3)),⁵⁵ yielding lifetime of τ (in ms) = 0.46, 0.65, 0.76 and 0.37 for Dy1–Dy4, respectively. This result strongly supports the existence of a single emissive center within the complexes.⁵⁶

I = I₀e^(−t/^τ) (3)

Fig. 6 Luminescence decay profiles of Dy1–Dy4 complexes.

The above relation expresses exponential decay of intensity I over time t, characterized by the decay time constant τ and initial intensity I₀. It is noteworthy to emphasize that upon coordinating three hfpd units (possessing low-energy C–F oscillators) alongside auxiliary ligands enables the emissive ions in these complexes to attain coordinative saturation. Additionally, these ligands shield the metal ion from external disturbances. Table 5 presents the data that was obtained through this analysis.

Photoluminescence quantum yield quantifies the proportion of absorbed photons that are re-emitted, typically in the visible spectrum, through the process of photoluminescence. To understand the effect of ancillary ligands on the photoluminescent characteristics, the luminescence quantum yields of the synthesized complexes were calculated using eqn (4) and are tabulated in Table 5. These calculations were carried out in DCM against quinine bisulphate in 1 M H₂SO₄ (Φ_r = 0.546).^57,58

Here, Φ, A, I, n, r and s are quantum yield, absorbance at excitation wavelength, integrated emission intensity, refractive index, reference and sample, respectively.⁵⁹ Complex Dy4 exhibited lowest quantum yield which could be attributed to back energy transfer whereas highest quantum yield amongst the synthesized samples was exhibited by Dy3 suggesting better sensitizing nature of neoc unit.

3.5.5 Color parameters. Evaluation of emitted color and its quality from the solid Dy³⁺ complexes involved the use of CIE color coordinates and Correlated Color Temperature (CCT) values, as detailed in Table 6 and Fig. 7. The emission spectrum was employed to calculate x and y (CIE color coordinates), providing information on the color emitted by these ternary complexes. The conversion from x, y coordinates to 1976 CIE uniform chromaticity coordinates (u′, v′) (Fig. 8) was accomplished by employing eqn (5), utilizing the x, y values determined previously.^60,61

Table 6 Color Characteristics corresponding to Dy1 to Dy4

Complex	(x, y)	(u′, v′)	CCT
Dy1	(0.3191, 0.3143)	(0.2081, 0.4612)	6240.99
Dy2	(0.3330, 0.3305)	(0.2114, 0.4722)	5467.00
Dy3	(0.3545, 0.3714)	(0.2101, 0.4954)	4733.92
Dy4	(0.2206, 0.3086)	(0.1409, 0.4435)	>10 000

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Fig. 7 CIE 1931 color coordinates of Dy1–Dy4 complexes.

Fig. 8 CIE 1976 color coordinates for Dy1–Dy4 complexes.

CCT serves as a standard for assessing the properties of light imparted by a light source, playing a critical role in alleviating mental and visual stress. The careful selection of an appropriate CCT is crucial for ensuring effective lighting. Generally, a CCT value below 3200 K suggests warm white light, ideal for residential use, while a value exceeding 4000 K implies cold white light, often preferred for commercial lighting.^62,63 Calculation of these values was performed using McCamy's equation (eqn (6)):^64,65

CCT = −437n³ + 3601n² − 6861n + 5514.31 (6)

The variable n in the provided equation corresponds to the inverse slope of line and its calculation can be executed according to eqn (7).^66,67

Here, x_e (0.332) and y_e (0.186) denote color epicentre. Complexes with ligands, TOPO, phen and neoc, are emitting in the region of cold white light at λ_ex = 370 nm. The CCT values recorded for these complexes are between 4733 to 6240 K, indicating the complexes' potential for applications in cold white light, particularly in commercial lighting. CIE data shows Dy2 emits white light (CIE x, y ≈ 0.333), with CCT around 5467.004 K. Dy1 leans towards cold-white (bluish), while Dy3 exhibits cold-white (yellowish) emission. However, subtle differences emerge for Dy4 due to presence of a broad ligand based emission band in blue region. This color tunability is attained by modifying the coordination environment with various ancillary ligands.

3.5.6 Dependence of color coordinates on excitation wavelength. In an attempt to study the potential modulation of color in CIE graphs, the photoluminescence spectra of Dy3 complex was recorded at various excitation wavelengths (250 nm to 380 nm). The resulting emission data was used to generate the emission map (Fig. 9). It was observed that the intensity of the emission spectra varies with the excitation wavelength, indicating that the emissive color coordinates are also influenced by the excitation wavelength. The emission data was analyzed to determine the x and y color coordinates (Table S1†) which were subsequently plotted on a CIE 1931 diagram for color visualization (Fig. 10). Utilizing eqn (4), the u′, v′ chromaticity coordinates were calculated and plotted on CIE 1976 color triangle (Fig. S6†). The dependence of color coordinates on excitation wavelength is shown in Fig. 10 & S6.† This comprehensive analysis provides insights into the tunability of emission properties and the potential for modulating the color output paving the way for future applications in color-tunable photonic devices.

Fig. 9 Emission Map of Dy3 at different excitation wavelengths.

Fig. 10 CIE 1931 color coordinates for Dy3 complex at different λ_ex.

3.5.7 Energy transfer mechanism. Understanding the photoluminescent behaviour of Dy(III) complexes requires careful examination of energy transfer pathways between dysprosium ion and coordinated ligands. Intramolecular energy transfer (IET) mechanism described by Dexter and Sato et al. states that two processes are mainly responsible for the IET efficiency.^68,69 (i) Through resonant exchange interaction, energy migrates from T₁ of organic ligand to emissive level of Ln³⁺ ion. (ii) Through a process known as thermal deactivation, energy is migrated inversely from Ln ion back to organic moiety. Back energy transfer (BET) predominates when the energy difference (ΔE) is too small for coordinated ligands and metal ion. ΔE plays a very significant role in energy transfer via above mentioned processes. However inefficient ligand-to-metal sensitization due to BET can be overcome by vibrationally-assisted energy transfer.⁷⁰ This holds particular significance in Dy³⁺ coordination compounds, having main and long-lived emitting level, ⁴F_9/2, positioned at approximately 20 833 cm⁻¹ which is relatively higher when compared to the T₁ of the majority of aromatic organic ligands. However, Dy(III) complexes lacks a reported optimal energy gap as suggested by Latva that would support efficient IET.⁷¹

The energy levels of the ligands (employed in complexation in Dy1–Dy4) in their triplet states are 21 978 cm⁻¹ (TOPO),⁷² 22 100 cm⁻¹ (phen),⁷³ 22 624 cm⁻¹ (neoc),⁷⁴ 20 163 cm⁻¹ (bathoc)⁷⁵ and 21 930 cm⁻¹ (hfpd).⁷⁶ The ΔE between emissive level of Dy(III) ion and T₁ of ligands is 1145 cm⁻¹ (TOPO), 1267 cm⁻¹ (phen), 1791 cm⁻¹ (neoc), −670 cm⁻¹ (bathoc) and 1097 cm⁻¹ (hfpd). Energy transfer process in considered complexes is shown in Fig. 11. The relatively small ΔE is the contributing factor to BET from Dy³⁺ to organic ligands which is observed as a ligand based band in the emission spectrum of all the complexes. The most favourable ΔE has been observed for Dy3 complex which results in least BET and most intense emission (Fig. S7 and S8†). While, least favourable ΔE is found for Dy4 where the T₁ state of the ancillary ligand is positioned lower than the emissive state of the dysprosium ion, hindering efficient IET and promoting BET, resulting in a broad ligand based emission. Between Dy1 and Dy2 complex, better luminescence enhancement of Dy(III) ion is observed in Dy2 owing to higher energy of T₁ state of phen in comparison to T₁ state of TOPO. Above that, the overall intensites of these complexes is low in comparison to the other reported compounds with europium, terbium and samarium metal ion, owing to less ΔE between emissive state of Dy and T₁ state of hfpd.²⁶

Fig. 11 Schematic diagram showing energy levels and energy transfer pathway in Dy1–Dy4 (S₀: Singlet; T₁: Triplet; ISC: Inter System Crossing; ET: Energy Transfer; BET: Back Energy Transfer).

3.6 CV analysis

For the purpose of studying electrochemical characteristics of synthesized Dy1–Dy4 complexes, CV scans were recorded in DCM utilizing Fc/Fc⁺ (ferrocene/ferricenium) redox couple ion as internal standard. A three-electrode system comprising of working electrode (glassy carbon), reference electrode (Ag/AgCl) and a counter electrode (Pt. wire) were employed along with supporting electrolyte (Tetra-n-butylammonium perchlorate) to obtain the CV curves (Fig. 12).

Fig. 12 CV profiles of Dy1 and Dy4, with the inset displaying the CV profile of ferrocene.

Utilizing data obtained from a CV plot, the oxidation (E_ox) and reduction (E_red) potentials were determined, resulting in values of 3.12 V (Dy1), 2.89 V (Dy2), 2.78 V (Dy3), 2.80 V (Dy4) for oxidation and −0.79 V (Dy1), −0.88 V (Dy2), −0.86 V (Dy3), −0.79 V (Dy4) for reduction. All the complexes exhibited a reversible electrochemical behaviour. Estimation of the energy of frontier molecular orbitals (FMO) involves the incorporation of the E_ox and E_red values into eqn (8) and (9).^77,78

E_HOMO = −[(E_ox − E_{1/2(ferrocene)}) + 4.8] eV (8)

E_LUMO = −[(E_red − E_{1/2(ferrocene)}) + 4.8] eV (9)

The CV data of trivalent dysprosium complexes are tabulated in Table 7. E_1/2 of Fc as determined from their CV curve is equal to 1.03 V.⁷⁹ From values of FMOs, electronic band gap (E^b_g) of Dy1–Dy4 complexes was calculated which exhibited a decreasing order by virtue of extended pi-electron conjugation. The electronic band gap values ranging from 3 to 4 eV confirm their semiconducting properties, making them suitable for application in display devices.

Table 7 Comparative analysis of redox potential and electronic E^b_g for Dy1–Dy4

Complex	^aEox	^aEred	^aEHOMO	^aELUMO	^aE^bg
a Eox and Ered are in (V); EHOMO, ELUMO and E^bg are in (eV).
Dy1	3.12	−0.79	−6.89	−2.98	3.91
Dy2	2.89	−0.88	−6.66	−2.89	3.77
Dy3	2.78	−0.86	−6.55	−2.91	3.64
Dy4	2.80	−0.79	−6.57	−2.98	3.59

---

3.7 Thermal characterization

One of the key advantages of lanthanide complexes for use in optoelectronic devices is their remarkable thermal stability. This stability is clearly evident in Fig. 13, which shows the thermogravimetric (TG) plot over a wide range (25–650 °C). As the TG plot demonstrates, Dy4 remains stable up to a temperature of 208 °C. However, at 210 °C and 578 °C, inflection point was observed on the TG plot, corresponding to 88.43% (calcd.: 86.06%) weight loss. This sharp change in weight indicates decomposition of Dy4. This remarkable stability makes it a promising candidate for various applications in optoelectronic devices. Above 617 °C, dysprosium oxide (Dy₂O₃) is identified as the stable oxide residue. The absence of an inflection point in the thermogram before 200 °C implies the absence of water, whether crystalliferous or coordinated.^80,81

Fig. 13 Thermogravimetric curve of Dy4 complex.

4 Conclusions

Heteroleptic ternary anhydrous dysprosium fluorinated β-diketonates were synthesized in one-step using four different auxiliary ligands of varying denticities. The triplet (T₁) states of ligands show close alignment with the emissive level of Dy³⁺ ion (⁴F_9/2) which induced BET from this emissive state to the excited states of ligand. Consequently, this phenomenon gave rise to ligand based emission in blue region along with yellow-blue emission from Dy³⁺ ion. This characteristic was used to achieve near white-light emission. The observed white light emission (WLE) of Dy1 (x = 0.3191, y = 0.3143), Dy2 (0.3330, 0.3305) and Dy3 (0.3545, 0.3714) is in close proximity to ideal white light, with CCT values in cold white-light region. From the study, it was found that the yellow emission in synthesized complexes is significantly influenced by the choice of coordinating moieties. Furthermore, the color coordinates are also influenced by the excitation wavelength used while recording the emission spectra. These complexes have applications in lasers, WLEDs, photoluminescent films and optoelectronic devices due to their semiconducting nature and large thermal stability. Additionally, the potential for wider utilization in multifunctional molecular materials, such as lighting and molecular magnets, introduces an intriguing element for further design exploration.

Data availability

The authors affirm that the information/data of this research article is available inside the article.

Author contributions

Sofia Malik = data curation, writing – original draft; Komal Jakhar and Devender Singh = writing – review & editing and supervision; Anjli Hooda = formal analysis; Sonia Redhu = methodology; Swati Dalal = data curation; Vandana Aggarwal = investigation; Sumit Kumar = visualization; Rajender Singh Malik = resources and validation; Parvin Kumar = project administration.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Sofia Malik is thankful to UGC-New Delhi for providing JRF [221610212505].

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ra03897e

‡ The authors contributed equally to this work.

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