Coordination environment driven luminescence in mononuclear and dinuclear dysprosium(iii) β-diketonate complexes

Abstract

A series of Dy(iii) complexes with 4-bromobenzoylacetone (BBA) as a sensitizing β-diketonate ligand and pyrazine as an auxiliary N-donor ligand was synthesized to study the influence of coordination environment and nuclearity on optical properties. Three complexes – aqua (DyA), mononuclear pyrazine-coordinated (DyM) and pyrazine-bridged dinuclear (DyD) have been studied spectroscopically and photophysically. Electronic absorption and excitation analysis indicate that the ligand-centered π → π* transitions dominate the excitation process, highlighting the antenna role of the coordinating ligand framework in sensitizing the Dy(iii) center. The complexes emit the typical Dy(iii) emission peaks from the ⁴F_9/2 excited state upon excitation, producing blue and yellow emission peaks along with a peak at ∼669 nm. The variations in emission intensity, lifetime, quantum yield and branching ratios show that there is a strong dependence of optical properties on the coordinating environment. The chromaticity analysis indicates tunable emission towards the white region and the optical band gap estimation indicates that the complexes are in the semiconducting range, which indicates their potential application in the field of photonic and optoelectronic applications.

A series of Dy(iii) complexes with BBA as a sensitizing β-diketonate ligand and pyrazine as an auxiliary ligand was synthesized to study influence of coordination environment and nuclearity on optical properties.

1. Introduction

The luminescent properties of lanthanide-based materials have been extensively investigated because of their distinctive features like narrow emission peaks, long-lived excited states and significant Stokes shifts.^1,2 Among the trivalent lanthanide ions (Ln³⁺), dysprosium(iii) (Dy(iii)) exhibits characteristic yellowish-blue emission arising from 4f–4f electronic transitions, mainly the hypersensitive ⁴F_9/2 → ⁶H_J (J = 15/2, 13/2, 11/2, 9/2) transitions.^3,4 Although these transitions are Laporte-forbidden, they can be greatly enhanced through appropriate ligand design, making Dy(iii) complexes promising materials for organic light emitting devices (OLEDs), temperature and magnetic field sensing, bioimaging and multifunctional optomagnetic devices.^5–7

The photoluminescent behaviour of Dy(iii) ions is greatly influenced by the coordination environment provided by organic ligands. As discussed above, a primary challenge in designing efficient Dy(iii) emitters lies in the weak direct absorption of f-electrons due to parity – forbidden transitions. This limitation is overcome through the antenna effect, wherein organic chromophores efficiently capture UV light and sensitize the Dy(iii) center via ligand-to-metal energy transfer (LMET) processes (Fig. 1).^8,9 β-Diketonate ligands are among the most efficient sensitizers of lanthanide luminescence owing to their high molar absorptivity, efficient intersystem crossing and favorable triplet-state (T₁) energies that enable efficient population of the excited states of Ln(III) ions.^10,11 Variations in the β-diketonate framework strongly influence the antenna efficiency and overall photophysical properties of the resulting complexes^12,13 and this can be modified by varying the degree of fluorination and aromatic substitution.

Fig. 1. Ligand to lanthanide energy transfer (antenna effect) in Ln³⁺ complexes.

Over the last few years, considerable research has been directed towards the optimization of Ln(iii) coordination architectures by using auxiliary ligands which tailor the coordination geometry, symmetry and electronic properties of the metal center. The N-heterocyclic ligands like 1,10-phenanthroline, pyrazine and their derivatives, have proven to be versatile building blocks in lanthanide coordination chemistry.^14,15 Depending on their molecular structure and the positioning of their donor atoms, these ligands can act as terminal monodentate ligand or as a bridging ligand that links two or more metal centres.^16,17 Furthermore, N-heterocyclic ligands not only sensitize the metal ion but their incorporation also enhances the structural rigidity of coordination complexes, shields the metal center from coordinated solvent molecules that can quench luminescence through O–H vibrations and thereby suppresses nonradiative decay and improving emission lifetimes.^18,19

Despite these advances, systematic investigations into how auxiliary ligands can be employed to tune the coordination environment, nuclearity and photophysical behaviour of Dy(iii) β-diketonate complexes remain limited. Pyrazine (Py) was therefore selected as the neutral co-ligand owing to its versatile coordination behaviour, allowing it to function either as a terminal donor or as a bridging linker, thereby providing an effective platform for modulating the coordination architecture. The β-diketone 1-(4-bromophenyl)butane-1,3-dione (BBA) was selected as the sensitizing ligand because its aromatic framework provides an extended π-conjugated system for ultraviolet light absorption and ligand-to-metal energy transfer (ET). Importantly, the para-bromo substituent introduces a heavy-atom centre that can enhance spin–orbit coupling and facilitate intersystem crossing, thereby influencing the population and dynamics of the ligand triplet state involved in lanthanide sensitization. These ligands are therefore used collectively to rationalize structure–property relationships in Dy(iii) complexes. Based on these considerations, three Dy(iii) complexes, [Dy(BBA)₃(H₂O)] (DyA), [Dy(BBA)₃(Py)₂] (DyM) and [Dy₂(BBA)₆(Py)] (DyD) were synthesized and comprehensively characterized to investigate effect of Py coordination and nuclearity on their structural, thermal and photophysical characteristics. The study investigated the correlation between the coordination environment and antenna-mediated sensitization, aqua-ligand quenching and ligand-field modulation to understand their combined contribution to Dy(iii) luminescence. These results will be helpful towards a rational development of effective Dy(iii) based luminescent complexes for optoelectronic as well as sensing purposes.

2. Experimental section

2.1. Materials and characterization methods

Hydrated dysprosium chloride (DyCl₃·6H₂O), pyrazine and all solvents required for synthesis and characterization were procured from Sigma-Aldrich. A 25% NH₄OH, together with ethyl alcohol and hexane, was employed as received in analytical or spectroscopic grade (AG/SG). The synthesized complexes were characterized and their photoluminescent properties evaluated using the following instrumentation: elemental (CHN) analysis was carried out using a PerkinElmer 2400 Analyzer; high resolution mass spectra of the complexes were recorded utilizing Maldi-TOF, Synapt XS HD mass spectrometer. Thermal behaviour was investigated using a Hitachi STA 7300 at a heating rate of 10 °C min⁻¹ under a nitrogen atmosphere; infrared spectra (4000–400 cm⁻¹) collected using KBr pellet method, were obtained using a PerkinElmer 5700 spectrometer; ¹H NMR spectra, referenced to TMS, were acquired on a Bruker Avance II 400 MHz spectrometer; UV-vis absorption spectra of dichloromethane solutions (200–800 nm) were recorded on a Shimadzu 2450 spectrophotometer; solid state photoluminescence spectral profiles were measured using a Horiba Jobin Yvon Fluorolog FL-3-11 spectrophotometer. Additionally, excited-state decay profiles corresponding to dominant emission peak were determined for the solid samples using an F-7000 FL spectrophotometer.

2.2. Synthesis of BBA ligand

A modified Claisen condensation route was employed for the synthesis of BBA (Fig. 2). Sodium ethoxide was first combined with ethyl acetate and p-bromoacetophenone was subsequently introduced in benzene. After stirring, the reaction mixture was acidified and extracted with ethyl acetate. The organic layer was then dried, concentrated and the resulting residue was purified to give the β-diketone ligand for subsequent complex synthesis.²⁰

Fig. 2. Synthetic pathway for preparing 1,3 β-diketone BBA.

2.3. Synthetic procedure for Dy(iii) complexes

Fig. 3 presents the Dy(iii) complexes synthesized in this study. A general procedure involved dissolving the BBA ligand (6.48 mmol) in ethanol (5 mL) and deprotonating it with ammonium hydroxide (6.48 mmol). After ammonia evolution stopped, the addition of 5 mL ethanolic solution of DyCl₃·6H₂O (2.16 mmol) to the resulting solution was done dropwise. Stirring was continued for 12 h before allowing solvent to evaporate gradually over 2–3 days.

Fig. 3. Stepwise synthetic route for the preparation of DyA, DyM and DyD complexes.

2.3.1. Aqua complex (DyA)

Following the general procedure, the resulting precipitate was filtered and washed with distilled water to yield DyA.²¹

2.3.2. Mononuclear pyrazine complex (DyM)

In addition to the metal salt solution, an ethanolic pyrazine solution (4.32 mmol) was added during complexation as per general procedure. The precipitates obtained after slow evaporation were washed with ethanol and hexane to obtain DyM.^21,22

2.3.3. Dinuclear complex (DyD)

The synthesized DyA complex was dissolved in ethanol and reacted with pyrazine in a 2 : 1 molar ratio (DyA : Py). After stirring at 50 °C for 12 h, the solid was isolated following the same workup procedure as DyM to yield DyD.²³

3. Experimental

3.1. Initial evaluation

The synthesized Dy(iii) complexes, labelled DyA–DyD were subjected to CHN elemental analysis to confirm their composition. The observed carbon, hydrogen and nitrogen percentages are in close approximation with theoretical values based on the formulated compositions, showing that synthesized complexes are of correct stoichiometry. The complexes are readily soluble in DMSO, DCM and DMF. A detailed summary of the analytical results, along with color for DyA–DyD, is included in Table 1. The synthesized complexes were further characterized by mass spectrometry (Fig. S1–S3) and the experimental mass spectra were consistent with the proposed molecular formulations. Table 1 comprises the m/z peak corresponding to the synthesized complexes. The close agreement between the observed and calculated molecular-mass regions supports the formation of the proposed complexes. In addition, the characteristic isotope envelopes observed in the mass spectra are consistent with the presence of bromine atoms in the complexes. The isotope distributions, together with the molecular-ion-region signals, provide further evidence supporting the proposed compositions of DyA, DyM and DyD.

Table 1. CHN analytical data, molecular composition and color of DyA–DyD complexes^a.

Complex	Color	C% E(T)	H% E(T)	N% E(T)	m/z
DyA	Creamish white	39.22 (39.37)	3.07 (3.04)	—	918.85
DyM	Creamish yellow	43.76 (43.82)	3.09 (3.13)	5.37 (5.34)	1042.95
DyD	Creamish yellow	41.65 (41.70)	2.84 (2.86)	1.52 (1.59)	1845.72

^aValues outside parentheses are experimentally found and inside parentheses are theoretical.

3.2. Thermogravimetric analysis

Thermogravimetric analysis (TGA) was employed to examine the thermal behaviour of the synthesized complexes and the thermograms of DyM and DyD are presented in Fig. 4. Complexes DyM and DyD do not show any mass loss in the low-temperature region, confirming that neither coordinated nor lattice water molecules are present in these complexes. DyM does not decompose below ∼221 °C, after which decomposition of the coordinated BBA and pyrazine ligands commences, with an overall mass loss of 85.27% (calcd. 84.86%). DyD exhibits enhanced thermal stability compared to DyM, with the onset of decomposition shifted to a higher temperature and stability maintained up to ∼253 °C. The experimentally observed mass loss for DyD (83.83%) closely matches the calculated value (82.76%), confirming the proposed composition of the complex. Compared with DyM, DyD exhibits greater thermal stability, indicating that the dinuclear framework and the presence of bridging ligands enhance the rigidity of the coordination structure and improve its resistance to thermal decomposition. This enhanced thermal stability makes these complexes a promising candidate for applications that require operation at elevated temperatures, such as optical and solid-state devices.^24,25

Fig. 4. Thermograms of DyM and DyD complexes.

3.3. FT-IR spectroscopy characterization

FT-IR data of DyA–DyD was collected to study binding modes of organic ligands to Dy(iii) center and to confirm the formation of the complexes. The tentative assignments of the characteristic vibrational frequencies are listed in a Table 2. The free BBA ligand possesses characteristic peaks at 1604 cm⁻¹ and 1507 cm⁻¹, assigned to the carbonyl stretching and enolic skeletal ν_{(C C)} vibration, respectively.²⁶ Upon complexation, these peaks shift to 1583 cm⁻¹ and 1511 cm⁻¹ in DyA, 1590 cm⁻¹ and 1513 cm⁻¹ in DyM and 1592 cm⁻¹ and 1512 cm⁻¹ in DyD, conforming bonding of deprotonated BBA to Dy(iii) ion via oxygen atoms. This is further supported by observation of new peaks in 453–459 cm⁻¹ range, attributable to ν_(Dy–O) stretching.²⁷ In Py containing complexes DyM and DyD, the ν_{(C N)} band of free pyrazine observed at 1605 cm⁻¹ (ref. 28) shifts to 1540 cm⁻¹ in DyM and 1534 cm⁻¹ in DyD, consistent with coordination through the Py nitrogen atoms. The coordination of the nitrogen atoms to Dy(iii) ion is further evidenced by the peaks appearing at 562 cm⁻¹ for DyM and 561 cm⁻¹ for DyD.²⁹ Overall, the IR data indicate that BBA acts as a bidentate O,O-donor ligand in all three complexes, while Py ligand participates as an N-donor ligand in DyM and DyD. Moreover, a band near 944 cm⁻¹ in Dy(BBA)₃(Py)₂ suggests terminally coordinated pyrazine, whereas its absence in Dy₂(BBA)₆(Py) is consistent with bridging pyrazine coordination in the dinuclear complex.³⁰ Lack of broad ν_(O–H) stretching of water molecules in ternary and dinuclear complexes confirms the successful replacement of coordinated water by the Py ligand.

Table 2. Selected FT-IR vibrational frequencies (cm⁻¹) of DyA, DyM and DyD complexes.

Complex	Dy–O	Dy–N	C C	C N	C O	CH3	C–H
DyA	459	—	1511	—	1583	2924	3052
DyM	453	562	1513	1540	1590	2926	3051
DyD	454	561	1512	1534	1592	2923	3054

3.4. ¹H-NMR spectral study

Proton NMR spectral profiles of synthesized BBA ligand and Dy(iii) complexes were acquired in CDCl₃ to examine their behaviour in solution and are presented in Fig. S4–S7. The corresponding chemical shift values, δ (ppm), are compiled in Table 3. In trivalent lanthanide systems, paramagnetic shifts are predominantly dipolar (pseudo-contact) in origin because the 4f orbitals are efficiently screened by the outer 5s and 5p electrons.³¹ Consequently, coordination results in significant displacement of ligand resonances from their positions in the free state. Free pyrazine displays a single sharp resonance at δ 8.50 ppm, whereas BBA exhibits characteristic signals at δ 15.96 ppm corresponding to the enolic proton, along with aromatic resonances at δ 7.75, 7.73, 7.60 and 7.58 ppm and a methine signal at δ 6.09 ppm. Upon coordination in DyM, significant changes in the chemical shift values of both ligands are observed, indicating their interaction with the Dy(iii) center. The pyrazine resonance undergoes a substantial upfield shift, while the methine and aromatic protons of BBA are shifted downfield relative to their free ligand positions. The presence of two equivalent pyrazine resonances supports its monodentate coordination mode in DyM. A comparable trend in proton shift variations is observed for the DyD complex however, only a single pyrazine resonance is detected, consistent with a symmetric bridging coordination mode involving two Dy(iii) ions and indicative of preservation of the dinuclear framework in solution. The direction of the shifts differs for BBA and pyrazine protons, reflecting dependence of the dipolar interaction on geometric factor (3 cos² θ −1), which governs whether shielding or deshielding occurs.³¹ Line broadening is more significant for DyD than DyM owing to the cumulative magnetic influence of two paramagnetic Dy(iii) centres. The extent of broadening follows a strong distance dependence (∝ 1/r⁶), with protons closer to the metal centers exhibiting greater peak widening.³²

Table 3. PMR chemical shifts (δ, ppm) of uncoordinated ligands and Dy(iii) complexes in CDCl₃.

Complex	Peaks due to BBA	Peaks due to Py
Uncoordinated ligand	2.28 (–CH3); 6.09 (–CH); 7.58–7.75 (–CH aromatic); 15.96 (–OH)	8.50 (4H)
DyA	12.58 (9H), 158.93 (3H), 18.26 (6H), 10.29 (6H)	—
DyM	13.20 (9H), 159.22 (3H), 17.98 (6H), 10.67 (6H)	−68.47 (4H), −12.70 (4H)
DyD	13.08 (9H), 155.42 (3H), 17.51 (6H), 10.57 (6H)	−63.63 (4H)

3.5. UV-visible analysis

UV-visible spectra of free BBA along with DyA–DyD complexes (Fig. 5) were recorded in 1 × 10⁻⁵ mol L⁻¹ dichloromethane (DCM) solution at room temperature to evaluate the effect of coordination on their electronic structure. The free BBA ligand showed a strong absorption band at 310 nm attributed to π → π* transitions in its conjugated aromatic framework. After complexation with Dy(iii) ion, the ligand-centered absorption band undergoes a noticeable bathochromic (red) shift with maxima at 317 nm, 322 nm and 325 nm as we move along the series from DyA–DyD. This progressive red shift can be attributed to stabilization of the ligand π* orbitals on coordination.

Fig. 5. Normalized UV-visible spectra of BBA and DyA–DyD in DCM.

Binding of the carbonyl oxygen atoms of BBA to the hard Lewis acidic Dy(iii) center reduces electron density on the ligand and lowers the energy difference among π and π* orbitals, thereby producing a bathochromic shift in the absorption band.³³ In addition, coordination enhances effective π-delocalization by increasing the rigidity of the ligand framework, which further contributes to excited-state stabilization.³⁴ In the visible region, no distinct metal-centred absorption bands attributable to Dy(iii) ion were observed.

3.5.1. Optical band gap analysis

The optical band gaps (E_g) of DyA–DyD were estimated by using the electronic absorption spectra via Tauc's relation (eqn (1)), which correlates the absorption coefficient with the photon energy.^35,36

(αhν)² = A(hν − E_g) 1

In eqn (1), α represents the absorption coefficient, h is Planck's constant, ν is the frequency of the incident photon, hν corresponds to the photon energy, A is proportionality constant related to the transition probability and E_g is the optical band gap energy. The E_g values were obtained determined by extending the linear section of (αhν)²versus hν plots to the photon energy axis and the corresponding Tauc's plots are presented in Fig. 6. The calculated E_g values of complexes DyA–DyD are 3.54 eV, 3.48 eV and 3.41 eV, respectively.

Fig. 6. Tauc's plots of DyA–DyD complexes for optical band gap determination.

The band gap decreases gradually from DyA to DyD, potentially reflecting changes in the electronic structure and ligand–metal interactions upon pyrazine incorporation. The slightly lower band gap of DyM may be associated with coordination of both BBA and pyrazine ligands, while the lowest value for DyD may reflect changes arising from its pyrazine-bridged dinuclear framework. The estimated band gap values place these complexes within the characteristic range of semiconductor materials, suggesting that they are promising candidates for optoelectronic and electroluminescent technologies, including OLEDs and other photonic materials.³⁷

3.6. Excitation spectra

The excitation properties of solid DyA–DyD were investigated at room temperature and the corresponding spectra are presented in Fig. 7. Excitation spectra were recorded at 485 and 575 nm, corresponding to the ⁴F_9/2 to ⁶H_15/2 and ⁶H_13/2 emission transitions, respectively. Both spectra exhibit similar profiles, but the higher intensity at 575 nm indicates more efficient population of the corresponding emitting level. The excitation profiles of all the complexes exhibit broad ligand-centered bands between 220 nm and 400 nm centered at 288 nm, 337 nm (DyA), 289 nm, 339 nm (DyM) and 278 nm, 340 nm (DyD) which are resulting from π → π* transitions of the coordinated BBA ligand and associated ligand framework.

Fig. 7. Excitation profiles of solid DyA–DyD monitored at 575 nm.

The characteristic Dy(iii) f–f excitation transitions are not distinctly resolved³⁸ indicating negligible direct excitation. Instead, ligand-centred bands dominate, suggesting efficient ligand-to-metal energy transfer, with BBA acting as the sensitizing chromophore. Moreover, introduction of pyrazine as a co-ligand in DyM and DyD results in slight changes in the position and intensity of the ligand-centered bands reflecting changes in the coordination environment around the Dy(iii) center.

3.7. Emission spectra

Fig. 8 displays the solid-state emission profiles of DyA–DyD obtained under their respective excitation wavelengths. Excitation of the complexes leads to Dy(iii) centered luminescence, which is attributed to transitions from ⁴F_9/2 excited state down to lower lying ⁶H_J manifolds. The ⁴F_9/2 → ⁶H_15/2 transition (∼483 nm) in blue region is a magnetic dipole transition following selection rules ΔJ = 0, ±1 and is relatively insensitive to the coordination environment. Nevertheless, the resolved Stark components of this band indicate that the Dy(iii) ion resides in a non-centrosymmetric ligand field. The ⁴F_9/2 → ⁶H_13/2 transition (∼576 nm) in the yellow region possesses an electric dipole character following selection rule ΔJ = 0, ±2 and is highly sensitive to the local symmetry and ligand arrangement around the Dy(iii) ion, along with a ⁴F_9/2 → ⁶H_11/2 weaker transition (∼669 nm) in the red region.^39,40 A broad blue-region band is also observed and is tentatively attributed to residual ligand-centred emission.

Fig. 8. Emission spectral profiles of solid DyA–DyD complexes.

The emission intensities of the three complexes differ significantly depending on their coordination environment with DyM being most intense and DyA being least intense. The relatively lower emission intensity of the aqua complex DyA is associated with non-radiative quenching by O–H vibrational oscillators of coordinated water molecules.⁴¹ The stronger emission of the pyrazine-containing complexes is consistent with a modified coordination environment and reduced vibrational quenching. Emission intensities further reveal that DyM's environment promotes the most efficient ligand-to-metal energy transfer. Although DyD lacks coordinated water molecules, its weaker emission may arise from additional non-radiative pathways associated with its dinuclear structure. Overall, the emission differences highlight the influence of ligand structure and coordination environment on the photophysical properties of Dy(iii) complexes.

In addition, the branching ratios (β_i) were estimated from the integrated emission intensities of the observed transitions using eqn (2). This parameter is more important for laser and photonic applications because it determines the probability of stimulated emission from a transition.^42,43

2

where, I_i represents the integrated emission intensity of the ith transition and ∑I_j corresponds to the sum of the integrated intensities of all observed Dy(iii) emission peaks. The calculated branching ratios for the observed transitions are summarized in Table 4. Among the observed transitions, β_i follows the trend ⁴F_9/2 → ⁶H_13/2 > ⁴F_9/2 → ⁶H_15/2 > ⁴F_9/2 → ⁶H_11/2. The dominant ⁴F_9/2 → ⁶H_13/2 transition contributes the largest fraction of the total emission, accounting for nearly 65% of the overall luminescence intensity. Moreover, the yellow-to-blue intensity ratio (Y/B), corresponding to the ⁴F_9/2 → ⁶H_13/2 (yellow) and ⁴F_9/2 → ⁶H_15/2 (blue) transitions, acts as a sensitive probe for local symmetry of Dy(iii) metal site. In present complexes, relatively high Y/B values suggest that the Dy(iii) ions occupy coordination sites of low symmetry in the solid state, consistent with the structural features of complexes.^44,45

Table 4. β _i values and Y/B ratios of Dy(iii) complexes.

Complex	Branching ratio (4F9/2 → 6HJ/2)			Y/B ratio
	J = 15	J = 13	J = 11
DyA	26.95	63.58	9.47	2.35
DyM	27.27	65.02	7.71	2.38
DyD	24.00	67.40	8.59	2.81

3.8. Energy transfer mechanism

Luminescence in lanthanide complexes is predominantly generated through ligand-mediated sensitization, in which the coordinated organic chromophore absorbs the excitation energy and transfers it efficiently to the lanthanide ion.^46,47 A schematic representation of proposed LMET pathway in these complexes is illustrated in Fig. 9. In lanthanide complexes, intramolecular energy transfer generally occurs through a dexter exchange pathway, where ligand's T₁ directly populates Ln³⁺ ion's excited levels. As per Latva et. al., T₁ excited-state energies of the sensitizing ligands has an important role in efficient antenna effect.⁴⁸ In the present system, the triplet state of the BBA ligand determined from the 77 K phosphoresce spectrum of the corresponding Gd(iii) complex (Fig. S8) is located at 21 325 cm⁻¹, while the lowest relevant emitting level of Dy(iii) ion (⁴F_9/2) lies at 20 833 cm⁻¹ (ref. 49) resulting in an energy gap (ΔE_{{BBA–Dy(III)}}) of 492 cm⁻¹. In contrast, the T₁ state of the Py ligand occurs at a much higher energy (26 820 cm⁻¹),⁵⁰ corresponding to a larger gap (ΔE_{{Py–Dy(III)}}: 5987 cm⁻¹).

Fig. 9. Energy-level diagram showing the ligand-sensitized emission in Dy(iii) complexes.

In DyA, BBA sensitizes Dy(iii) emission through intersystem crossing and energy transfer from its triplet state. However, presence of coordinated water introduces high-frequency O–H vibrations. These vibrations enhance non-radiative relaxation through multiphonon processes and act as highly efficient quenching centers, thus reducing the luminescence and quantum yield of DyA complex. In DyM and DyD complexes, the coordinated water molecules are substituted by the Py ligands. This substitution increases the rigidity of the coordination environment around Dy(iii) ion. The O–H oscillators are replaced by the conjugated pyrazine ligand, which also reduces the vibrational quenching pathways and thus, LMET is enhanced through organic antennas. Although pyrazine has a high T₁ and does not sensitize Dy(iii) emission directly, it helps increase the rigidity in the coordination and the complex's electronic environment.

Since the energy gap between the ligand BBA's T₁ and emissive state of Dy(iii) ion is very small, a reverse energy transfer process from the Dy(iii) center to the ligand is energetically allowed. This process explains observation of a residual, ligand-centered emission band in blue region, which appeared in emission spectra of DyA–DyD complex (Fig. 8).

3.9. Quantum yield analysis

The photoluminescence quantum yield (PLQY), defined as the ratio of the number of photons emitted to the number of photons absorbed, was determined for the synthesized complexes in dichloromethane (DCM). The relative quantum yield was calculated using eqn (3), with quinine bisulphate in dilute H₂SO₄ as the reference standard.^51,52

3

Here, s and r refer to the sample and reference, respectively and I represent the integrated emission intensity, A is the absorbance at the excitation wavelength and n denotes the refractive index. The PLQY values provide insight into the influence of the coordination environment on the photoluminescence properties of the metal centre. The observed quantum yields are comparable to those reported for related lanthanide complexes.^42,53,54 The increase in photoluminescence quantum yield from DyA (0.03%) to DyM (0.16%) suggests that the incorporation of pyrazine as an ancillary ligand favourably modifies the coordination environment of Dy(iii) ion, potentially reducing non-radiative deactivation of the emissive (⁴F_9/2) state. This enhancement may be associated with changes in the coordination sphere, including a possible reduction in the influence of high-energy O–H vibrational oscillators associated with coordinated water molecules. Although DyD also exhibits a substantially higher quantum yield (0.10%) than DyA, its efficiency remains slightly lower than that of DyM. This observation suggests that the formation of a dinuclear assembly does not necessarily lead to improved emission efficiency.

3.10. Luminescence lifetime analysis

Lifetime measurements of the solid complexes were performed by collecting emission decay at 574 nm, arising from ⁴F_9/2 → ⁶H_13/2 transition of Dy(iii) ion. The decay profiles are presented in Fig. 10 and the corresponding lifetimes are provided in Table 5. The decay profiles of all the complexes are well described by a monoexponential function (eqn (4)), consistent with the existence of a single emissive species in each complex without contributions from distinct coordination environments.⁵⁵

I = I₀e^(−t/τ) 4

Fig. 10. Luminescence decay profiles of Dy(iii) complexes monitored at 574 nm.

Table 5. CIE chromaticity coordinates, CCT and lifetime values of the DyA–DyD.

Complex	x, y	u′, v′	CCT (K)	Lifetime (µs)
DyA	0.2755, 0.2258	0.2137, 0.3939	>10 K	0.42
DyM	0.3505, 0.3383	0.2205, 0.4788	4736	1.93
DyD	0.2947, 0.2560	0.2150, 0.4203	>10 K	1.21

The luminescence lifetimes of the present DyA (0.42 µs), DyM (1.93 µs) and DyD (1.21 µs) complexes are relatively short compared to many reported Dy(iii) β-diketonate systems, reflecting the intrinsically weak and highly environment-sensitive emission of Dy(iii) complexes. DyA exhibits a lifetime markedly lower than those of fluorinated hydrated complexes such as [Dy(hfaa)₃(bpy)] (13–14 µs),⁵³, [Dy(hfaa)₃(indazole)₂] (13.9 µs),⁵⁶, [Dy(hfaa)₃(pyz)₂] (6.9 ± 0.10 µs)⁴² and [Dy(bfa)₃phen] (0.77 µs),⁵⁴ which may be attributed to the combined effect of coordinated water molecules and the less effective sensitization ability of the BBA ligand relative to fluorinated β-diketonates. Although, T₁ of BBA is reasonably close to Dy(iii) ion's emissive peak (∼20 833 cm⁻¹), fluorinated ligands such as bfa⁵⁴ and hfaa⁵⁶ provide a more favorable energy gap together with reduced vibrational losses due to CF₃ substitution, leading to longer lifetimes.

Upon replacement of coordinated water molecules by pyrazine in DyM, the lifetime increases substantially to 1.93 µs, confirming suppression of O–H vibrational quenching and improved excited-state stabilization. Nevertheless, DyM remains far lower than highly emissive systems such as [Dy(acac)₃(pyz)₂] (1029 µs)⁵⁷ and [Dy(TFPB)₃(pyz)₂] (0.260 ms),⁵⁸ indicating that although BBA provides moderate sensitization, it is less effective than ligands such as acac or TFPB in promoting long-lived Dy(iii) centered emission. Overall, the lifetime trend indicates that while replacement of coordinated water by pyrazine improves the Dy(iii) ion's emission behavior in the present BBA-based complexes, the absence of strongly fluorinated β-diketonates and the less optimal ligand field environment limit the excited-state lifetime relative to the best-performing Dy(iii) systems reported in the literature.

3.11. Colorimetric characterization

Luminescence characteristics of the DyA–DyD complexes were further evaluated in terms of their chromaticity parameters, which provide quantitative information about the perceived emission color and its suitability for photonic applications. The chromaticity coordinates in the CIE 1931 color space [Fig. 11(a)] were determined according to Commission Internationale de l'Éclairage (CIE) standards using emission data of synthesized complexes. As shown in Fig. 11(a), the emission coordinates of DyA, DyM and DyD are located in the pinkish-blue to near white-light region of CIE 1931 chromaticity diagram. In addition to CIE 1931 chromaticity coordinates, the emission colors were also represented in the CIE 1976 uniform chromaticity scale system [Fig. 11(b)] using eqn (5).^59,60

5

Fig. 11. (a) CIE 1931 and (b) CIE 1976 chromaticity diagrams of the Dy(iii) complexes.

The chromaticity coordinates from both color spaces allow a more precise characterization of emission color and its perceived uniformity. The CIE 1976 plot [Fig. 11(b)] also depicts DyA and DyD remaining in the pinkish-blue region, whereas DyM is shown at a higher v′ value, thus, a comparatively warmer near white emission color. In an effort to assess the lighting properties of the complexes, correlated color temperature (CCT) was determined by the CIE 1931 chromaticity coordinates by using McCamy's empirical relationship (eqn. (6) and (7)).^61–63

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

7

wherein x, y are the chromatic coordinates in CIE 1931 diagram and the epicentral points (0.332, 0.186) are (x_e, y_e). The calculated CCT indicates warmth or coolness of emitted light.⁶³ CCT of synthesized complexes depicts cool light, thus, the complexes can be applied in office and commercial buildings to enhance focus and effectiveness.⁶⁴ The CIE 1931 and CIE 1976 chromaticity coordinates for DyA–DyD, along with the calculated CCT, are presented in Table 5.

The emission color arises from the combined contribution of characteristic trivalent Dy ion's blue and yellow emissions together with a broad ligand-centered blue band. Overlap of the ligand emission with the intrinsic Dy(iii) blue transition modifies relative intensity of blue and yellow peaks, thereby governing overall chromaticity of the complexes. The differences in emission color among DyA, DyM and DyD can therefore be ascribed to variations in local coordination environment of Dy(iii) centre, which affect ligand field, site symmetry and ligand-to-metal energy transfer efficiency. These results demonstrate that the tuning of emissive color of present Dy(iii) ion based complexes can be done through modulation of ligand environment and the relative contributions of ligand and metal-centered emission pathways.

4. Conclusion

A series of Dy(iii) complexes containing the β-diketonate ligand BBA and pyrazine as an auxiliary ligand have been synthesized and characterized to gain insight into effects of coordination environment on structural and photophysical characteristics. Spectroscopic analysis confirmed coordination of organic ligands with Dy(iii) center and paramagnetic character of Dy(iii) ion caused significant shifts and broadening of proton resonances in the ¹H NMR spectra. No metal-centered transitions were detected in the excitation spectra, while the ligand-based bands predominated, indicating that the organic ligands efficiently function as antennae responsible for sensitizing the Dy(iii) ion based emission. Band gap analysis performed with Tauc's method showed that the band gaps of the complexes are in the range of 3.41–3.54 eV suggesting their suitability for the use in optoelectronic devices.

Photoluminescence analysis confirmed that the emission is due to transfer of energy from BBA ligand to Dy(iii) center's excited state ⁴F_9/2, leading to emission of blue and yellow transitions typical of Dy(iii) ion in its excited state along with the ⁶H_11/2 transition. The emission behavior is very strongly dependent on the coordination environment. The coordinated water molecules in DyA cause partial quenching by O–H vibrational relaxation, while the addition of pyrazine in DyM and DyD hinders non-radiative channels and increases the luminescence. In addition, the nuclearity of the complexes affects the emission properties, as the dinuclear structure slightly affects the local symmetry around the Dy(iii) centers. The combined blue and yellow emissions result in tunable chromaticity coordinates from pinkish blue to near-white of the complexes. The decay profiles of DyA–DyD followed a monoexponential function. The introduction of pyrazine increased the excited-state lifetimes of DyM and DyD relative to DyA, reflecting reduced non-radiative decay due to the replacement of coordinated water molecules. Nevertheless, the observed lifetimes are lower than those reported for analogous Dy(iii) complexes. In general, the study shows that rational control of ligand environment and coordination architecture can be a valuable strategy to develop the optical properties of Dy(iii) complexes for new applications in photonic and optoelectronic fields.

Author contributions

Sofia Malik = data curation, writing – original draft; Komal Jakhar and Devender Singh = writing – review and editing and supervision; Swati Dalal = formal analysis; Sumit Kumar = methodology; Rajender Singh Malik = visualization; Parvin Kumar = resources and validation; Jayant Sindhu = 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.

Data availability

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

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d6ra06053f.