Solvent-Dependent Photophysical Properties of Eu(TTA)₃Phen: A Comprehensive Study

Abstract

The solvent dependence of the optical properties of tris(thenoyltrifluoroacetonate)(1,10-phenanthroline)europium(III) (Eu(TTA)₃Phen) solution was studied in chloroform, dichloromethane, toluene, acetonitrile, acetone, N,N-dimethylformamide, dimethyl sulfoxide, and methanol. The optical properties of solid Eu(TTA)₃Phen were carefully studied for comparison. The absorption spectra are dominated by ligand-centered bands, with only minor solvent-induced changes in the absorption maxima. Upon excitation into the ligand band, the emission spectra containe the characteristic narrow transition bands of europium(III), indicating efficient ligand-to-metal energy transfer. The photochemical properties of Eu(TTA)₃Phen significantly depend on the solvent. Nonpolar and weakly coordinating solvents preserve longer lifetimes and higher solution quantum yields, with maxima in chloroform (14.7%) and toluene (13.7%). In contrast, methanol produces the strongest perturbation, yielding the shortest lifetime (0.364 ms), highest nonradiative rate (1427 s⁻¹), lowest Eu(III) internal quantum yield (48.0%), and lowest photoluminescence quantum yield (4.6%). Analysis of radiative/nonradiative rates and ⁵D₀-level formation quantum yield shows that solvent-dependent luminescence arises from changes in both the Eu³⁺-radiative transition efficiency and ligand-to-metal sensitization, providing design guidelines for solution-processable Eu³⁺ emitters.

1. Introduction

Luminescent europium(III) complexes have attracted considerable attention as functional materials for light-emitting devices, sensors, bioimaging, and optical encoding applications [1,2,3,4]. Lanthanide-based coordination compounds are also gaining increasing attention as components of X-ray scintillators, where high-Z metal centers enhance X-ray absorption and organic ligands can act as antenna units that promote efficient energy harvesting and radiative emission [5]. Their interest arises from the characteristic photophysical features of Eu³⁺ ions, including narrow red emission bands, long excited-state lifetimes, ligand-sensitized emission after UV excitation, and high color purity [6,7]. However, direct excitation of Eu³⁺ is inefficient, because the 4f–4f transitions are Laporte-forbidden and therefore exhibit very low molar absorption coefficients [8,9]. This limitation is effectively addressed through the so-called antenna effect, where organic chromophores with strong UV absorption act as light-harvesting units [10]. Upon excitation, these ligands transfer energy to Eu³⁺, enabling population of its emitting excited states and significantly enhancing Eu³⁺ luminescence [7,11].

Among Eu³⁺ coordination compounds, β-diketonate complexes containing neutral ancillary ligands such as 1,10-phenanthroline (Phen) are well-established antenna systems that convert UV excitation into intense red emission [12,13]. Eu(TTA)₃Phen (Scheme 1), Tris(thenoyltrifluoroacetonate)(1,10-phenanthroline)europium(III), is a representative example of this family and has been widely investigated in crystalline form, polymer matrices, mesoporous hosts, and optoelectronic devices [12,14,15]. In this complex, the TTA and Phen ligands act as light-harvesting chromophores. After ligand-centered excitation, intersystem crossing populates ligand triplet states, followed by energy transfer to Eu³⁺ and emission from the ⁵D₀ level through the characteristic ⁵D₀ → ⁷F_J transitions (J = 0–4). Recent time-resolved studies of Eu³⁺ β-diketonate complexes indicate that ligand-to-metal energy transfer can be highly efficient once the ligand triplet state is populated, making ligand excited-state dynamics, particularly intersystem crossing, a key factor governing the overall sensitization efficiency [6,10,16].

Scheme 1. Chemical structure of Eu(TTA)₃Phen.

Whereas the photophysical properties and device performance of Eu(TTA)₃Phen in the crystalline state and in solid hosts such as mesoporous structures or polymeric films have been extensively characterized [15,17,18,19,20], the studies on the optical properties of Eu(TTA)₃Phen solutions in organic solvents are rather limited and comparatively less systematic. Solvent polarity, coordinating ability, and proticity can influence several key processes that determine Eu³⁺ luminescence, including the local symmetry around the metal center, the energy and population of ligand excited states, and the efficiency of nonradiative deactivation pathways [21,22]. Moreover, even subtle changes in the Eu³⁺ coordination environment or in the accessibility of high-energy vibrational oscillators (O–H, N–H, and C–H) can strongly influence nonradiative decay pathways, leading to substantial variations in luminescence lifetimes and quantum yields. These effects are particularly relevant in Eu-based sensors and photonic materials, where emission efficiency directly determines detection sensitivity and device performance [23,24]. Beyond displays, sensing, bioimaging, and optical encoding, rare-earth coordination materials and MOF-based scintillators have shown that scintillation performance can be improved through heavy-atom effects, ligand-centered energy transfer, restriction of intramolecular motion, and control of self-absorption [25,26,27].

Understanding these solvent-dependent effects is therefore critical for optimizing Eu(TTA)₃Phen and related complexes for solution-based sensing (e.g., detection of organophosphorus agents), bioimaging, and photonic applications, where the microenvironment around the luminophore can range from nonpolar polymer domains to highly polar or protic media [28,29]. In particular, correlating solvent-controlled changes in local symmetry (as probed by hypersensitive ⁵D₀ → ⁷F₂ intensity and the asymmetry ratio R₂₁, Eu(III) internal quantum yield, and the quantum yield of ⁵D₀-level formation with established energy-transfer models (Dexter vs. Förster, triplet-mediated vs. charge-transfer pathways [30,31]) provides a route to bridge detailed mechanistic insight from ultrafast spectroscopy with practical design rules for applied systems [29,32].

In this work, we report a comprehensive study of the solvent-dependent photophysical properties of Eu(TTA)₃Phen in organic solvents spanning a wide range of polarity and hydrogen-bonding ability, such as chloroform, dichloromethane, toluene, acetonitrile, acetone, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and methanol. The absorption spectra, emission spectra, luminescence lifetimes, photoluminescence quantum yields (PLQYs), asymmetry ratios, radiative and nonradiative decay rates, Eu(III) internal quantum yields, and quantum yield of ⁵D₀-level formation values are evaluated and correlated with solvent properties. By introducing the quantum yield of ⁵D₀-level formation, Φ_form(⁵D₀), as a semi-quantitative measure of antenna efficiency, we demonstrate that both the Eu(III) internal quantum yield and the ligand-to-metal energy transfer are strongly solvent-dependent. These findings provide mechanistic insights into the photophysics of Eu(TTA)₃Phen in solution and establish design principles for tuning luminescence performance via solvent or microenvironment engineering.

2. Materials and Methods

2.1. Materials

2-Thenoyltrifluoroacetone (HTTA, >98%) and 1,10-phenanthroline (Phen) were purchased from Alfa Chemistry (Zhengzhou city, China). Europium(III) nitrate pentahydrate was purchased from Chemcraft (Kaliningrad, Russia). Sodium hydroxide (>99%), chloroform (HPLC grade), dichloromethane (HPLC grade), acetone (HPLC grade), dimethyl sulfoxide (DMSO, ACS reagent grade), N,N-dimethylformamide (DMF, ACS reagent grade), toluene (ACS reagent grade), acetonitrile (HPLC grade), methanol (HPLC grade), and ethanol (96%), sulfuric acid (98%), quinine sulfate (United States Pharmacopeia (USP) Reference Standard) were purchased from Sigma-Aldrich (Saint Louis, MO, USA). All reagents were used as received without further purification.

2.2. Synthesis of Crystalline Eu(TTA)₃Phen

Eu(TTA)₃Phen was synthesized using a modified literature method [33]. Briefly, HTTA (15 mmol) and Phen (5 mmol) were dissolved in 100 mL of a water/ethanol mixture (1:1, v/v). Solid NaOH (15 mmol) was then added under vigorous stirring. Separately, europium(III) nitrate pentahydrate (5 mmol) was dissolved in 50 mL of water and added to the alkaline ligand solution under vigorous stirring, resulting in the formation of a white precipitate. The suspension was heated at 60 °C and stirred for 1 h. The precipitate was collected by filtration, washed repeatedly with a water/ethanol mixture (1:1, v/v), and dried under vacuum at room temperature for one week. The final product was obtained as a white solid with a yield of 1.15 g (23%). The purity and crystalline nature of the complex were confirmed by powder X-ray diffraction (PXRD), Fourier-transform infrared spectroscopy (FTIR), and Raman spectroscopy (Figures S1 and S2, Supplementary Materials). The PXRD pattern was compared with a previously reported Eu(TTA)₃Phen-containing complex [34] (Figure S3, Supplementary Materials).

2.3. Preparation of Solutions

Eu(TTA)₃Phen solutions were prepared in chloroform, dichloromethane, toluene, acetonitrile, acetone, DMF, DMSO, and methanol. Two series of quinine sulphate solutions were prepared in 0.5 M H₂SO₄ to have an absorbance less than 0.2 at the excitation wavelength (365 or 405 nm). The concentration range was adjusted according to the solubility of the complex and the requirements of each optical measurement. For UV–Vis absorption measurements, Eu(TTA)₃Phen solutions with a concentration in the range from 1 μM to 0.2–3.0 mM were prepared, depending on the solvent. For photoluminescence quantum yield measurements, Eu(TTA)₃Phen solutions with concentrations of 1–5 μM were used for excitation at 365 nm, whereas concentrations of 0.20–3.0 mM were used for excitation at 405 nm. When required, the absorbance at the excitation wavelength was kept below approximately 0.2 to minimize inner-filter and reabsorption effects.

2.4. Instrumentation

UV–Vis absorption spectra of Eu(TTA)₃Phen solutions were recorded using a Jenway Genova Nano micro-volume spectrophotometer (Cole-Parmer Ltd., Staffordshire, UK). Measurements were performed using quartz cuvettes with optical path lengths of 1 and 10 mm, depending on the absorbance of the solutions. Spectra were collected in the 200–500 nm wavelength range.

Solid-state UV–Vis–NIR diffuse reflectance spectra of crystalline Eu(TTA)₃Phen were acquired using a Lambda 1050 UV/Vis/NIR spectrometer (PerkinElmer Inc., Waltham, MA, USA) equipped with a Praying Mantis diffuse reflectance accessory (DRP-SAP, Harrick Scientific Products, Inc., Mt Kisco, NY, USA). Reflectance spectra were recorded in the 200–700 nm wavelength range and subsequently transformed using the Kubelka–Munk function:

$$F\left(R\right)={\displaystyle \frac{(1-R{)}^2}{2R}},$$

(1)

for comparison with solution absorption spectra.

Steady-state photoluminescence spectra were recorded using a laboratory-built spectrofluorometer based on a Little Garden spectrometer (Lao Kang, Beijing, China). A 100 mW, 405 nm laser (405-100-14.5 × 45-XL, Q-Line, Dongguan, Guangdong Province, China) and a 365 nm LED (UV-3W High Power LED, HViero Shenzhen, China) were used as excitation sources. The 365 nm excitation wavelength was selected because it overlaps more strongly with the lowest-energy ligand-centered absorption band of Eu(TTA)₃Phen. The 405 nm excitation wavelength lies on the weak long-wavelength tail of the ligand-centered absorption band, where the absorption is much weaker than at the absorption maximum. Nevertheless, measurable Eu³⁺ emission was obtained under 405 nm excitation because of the relatively high excitation power of the laser and the sample concentrations used for these measurements. The spectral calibration of the spectrometer was verified by comparing the emission spectrum of Eu(TTA)₃Phen in chloroform with a previously reported spectrum of the same complex [35] (Figure S4, Supplementary Materials). The linear response of the spectrometer was verified from the linear dependence of the recorded signal intensity on the excitation intensity, which was attenuated using a set of color filters (Figure S5, Supplementary Materials). The emission spectra were corrected for the spectral sensitivity of the detector and grating.

Photoluminescence decay curves were measured using a laboratory-built time-resolved spectrofluorometer. Excitation pulses at 405 nm, with a pulse duration of 100 μs, pulse energy of 25 μJ per pulse, and beam diameter of 1 mm, were generated using a 500 mW laser (Yilubao, Shanghai, China) modulated by TTL square pulses from an FNIRSI DSO-TC3 signal generator (FNIRSI, Shenzhen, China). The emission signal was detected using a high-speed TZT 76084 photodiode (Shenzhen Tianzhongtian Trading Co., Ltd., Shenzhen, China) and recorded with an FNIRSI 1014D digital oscilloscope (FNIRSI, Shenzhen,, China). Scattered excitation light was removed using an optical color filter.

Fourier-transform infrared (FTIR) spectra were obtained using an Agilent Cary 630 FTIR spectrometer (Agilent Technologies, Santa Clara, CA, USA) equipped with a diamond attenuated total reflectance (ATR) accessory. Spectra were collected in the 4000–650 cm⁻¹ range with a spectral resolution of 1 cm⁻¹.

Raman spectra of crystalline Eu(TTA)₃Phen were collected using a LabRAM HR Evolution Raman spectrometer (Horiba Jobin Yvon, Kyoto, Japan). Measurements were performed using a 785 nm excitation laser, with a nominal laser power of 40 mW and an accumulation time of 5 s.

Powder X-ray diffraction (PXRD) patterns were recorded using a MiniFlex-600 diffractometer (Rigaku Corporation, Akishima, Tokyo, Japan) equipped with a sealed-tube Cu Kα₁,₂ non-monochromatized radiation source and a D/tex Ultra2 detector operating in one-dimensional scan mode. The X-ray generator was operated at 40 kV and 15 mA. Diffraction data were collected over the 5–60° 2θ range using a step size of 0.01° and a scanning speed of 10° min⁻¹.

OriginPro 2018 (64 bit, SR1, b.9.5.1.195) software were used to analyze the experimental data.

2.5. Photophysical Data Analysis

Photoluminescence quantum yields (PLQYs) were determined by the relative method, following procedures commonly applied to luminescent materials [36], using quinine sulfate in 0.5 M H₂SO₄ as the reference standard (PLQY_R = 0.546 at 25 °C) [37,38]. Eu(TTA)₃Phen solutions and the reference were measured under identical instrumental conditions, including excitation wavelength, integration time, optical geometry, and detector settings. The integrated emission intensity was obtained by numerical integration of the Eu³⁺ emission bands over the 550–750 nm range. For each solvent, the integrated emission intensity was plotted as a function of the absorbed fraction (1 − 10^−A), where A is the absorbance at the excitation wavelength;and the slopes of the linear plots were used to calculate the PLQY according to the equation:

$$PLQY={PLQY}_R\left({\displaystyle \frac{{Grad}_S}{{Grad}_R}}\right){\left({\displaystyle \frac{n_S}{n_R}}\right)}^2,$$

(2)

where PLQY and PLQY_R are the photoluminescence quantum yields of the sample and reference, respectively; Grad_S and Grad_R are the slopes obtained for the sample and reference; and n_S and n_R are the refractive indices of the solvent and reference medium, respectively. Solvent refractive indices were taken from literature values [39]. For crystalline Eu(TTA)₃Phen, an approximate refractive index of n = 1.5 was used for the radiative-rate calculations [40]. PLQYs were measured at two wavelengths (365 and 405 nm), and the resulting values were averaged and given in Table 1.

Table 1. Photophysical properties of crystalline Eu(TTA)₃Phen and Eu(TTA)₃Phen solution in different solvents: photoluminescence decay time constants (τ); radiative (A_r), nonradiative (A_nr), and total (A_tot) decay rates; Eu(III) internal quantum yields (η_Eu); photoluminescence quantum yields (PLQY); formation quantum yields of the ⁵D₀ level; and asymmetry ratios (R₂₁).

Solvent	τ, ms	Atot, s−1	Ar, s−1	Anr, s−1	ηEu, %	PLQY, %	Φform(5D0), %	R21
solid	0.805 ± 0.007	1242	748	494	60.2 ± 1.5	38.15 [41]	63.4 ± 1.6	12.8 ± 1.3
chloroform	0.658 ± 0.007	1520	982	538	64.6 ± 1.5	14.7 ± 0.5	22.8 ± 0.8	19.4 ± 1.9
dichloromethane	0.717 ± 0.007	1395	966	428	69.3 ± 1.6	10.8 ± 0.5	15.6 ± 0.7	20.0 ± 2.0
toluene	0.637 ± 0.007	1570	1087	483	69.2 ± 1.6	13.7 ± 0.5	19.8 ± 0.7	19.4 ± 1.9
acetonitrile	0.632 ± 0.007	1582	980	603	61.9 ± 1.5	10.0 ± 0.5	16.2 ± 0.8	24.5 ± 2.5
acetone	0.695 ± 0.007	1439	1009	430	70.1 ± 1.7	10.9 ± 0.8	15.5 ± 1.1	24.6 ± 2.5
DMF	0.560 ± 0.007	1786	1129	657	63.2 ± 1.5	9.3 ± 0.5	14.6 ± 0.8	23.0 ± 2.3
DMSO	0.578 ± 0.007	1730	1179	551	68.2 ± 1.6	8.4 ± 0.5	12.3 ± 0.7	21.6 ± 2.2
methanol	0.364 ± 0.007	2747	1320	1427	48.0 ± 1.3	4.6 ± 0.5	9.5 ± 1.0	34.9 ± 3.5

Note: PLQY value for crystalline Eu(TTA)₃Phen was taken from ref. [41]. The other values were obtained by our group.

Photoluminescence decay curves were fitted using a mono-exponential function:

$$I\left(t\right)=I_{offset}+I_1{e}^{-{\displaystyle \frac{t}{\tau }}}$$

(3)

where τ is the observed lifetime of the Eu^{3+ 5}D₀ emitting level. The total decay rate was calculated as A_tot = 1/τ. Luminescence decay is affected by the combination of radiative and nonradiative processes. The radiative decay rate is determined by dipole transition strength and local-field correction. Nonradiative processes include multiphonon relaxation, quenching on impurities (e.g., O-H group of water molecules), and cooperative processes (cross-relaxation, energy migration). Radiative and nonradiative decay rates of Eu³⁺-doped phosphors can be calculated from the emission spectrum using the 4f–4f intensity theory [11]. Magnetic dipole ⁵D₀ – ⁷F₁ transition probability A_0–1 = A_MD,0∙n₀³ = 14.65∙n₀³. A_MD,0 is the spontaneous emission probability of the magnetic dipole ⁵D₀ − ⁷F₁, 14.65 s⁻¹, and n₀ is the refractive index of the solid EuTTA₃Phen or the solvent. Radiative decay rates A_0–λ (λ = 2, 4) of the ⁵D₀ − ⁷F_λ emission transition can be obtained from formula:

$$A_{0–\mathsf{\lambda }}=A_{0–1}{\displaystyle \frac{{\mathsf{\nu }}_{0–1}}{{\mathsf{\nu }}_{0–\mathsf{\lambda }}}}{\displaystyle \frac{{\mathrm{I}}_{0–\mathsf{\lambda }}}{{\mathrm{I}}_{0–1}}}$$

where I_0–λ and ν_0–λ are integral intensity and frequency of the ⁵D₀ − ⁷F_λ emission transition. The total radiative decay rate, A_r, could be calculated by summing all the A_0–λ radiative decay rates (λ = 1, 2, 4). The total decay rate is the reciprocal of the observed lifetime of the ⁵D₀ level $A_{tot}={\displaystyle \frac{1}{{\tau }_{av}}}$, while the nonradiative probability can be calculated as $A_{nr}=A_{tot}-A_r$. The values of all parameters are given in Table S2, Supplementary Materials. The Eu(III) internal quantum yield was determined as η_Eu = A_r/A_tot. Here, Φ_form(⁵D₀) = PLQY/η_Eu denotes the apparent quantum yield of population of the Eu(III) ⁵D₀ emitting state following photon absorption. This derived quantity describes the net efficiency of all processes preceding ⁵D₀ emission, including ligand absorption, intersystem crossing, ligand excited-state deactivation, and ligand-to-Eu energy transfer. It should therefore not be interpreted as a direct measurement of an individual energy-transfer rate or mechanism.

3. Results and Discussion

3.1. Crystalline Eu(TTA)₃Phen Characterization

The FTIR spectrum of crystalline Eu(TTA)₃Phen confirms the presence of both TTA and Phen ligands in the synthesized complex (Figure S1 and Table S3, Supplementary Materials). The main experimental bands are in good agreement with previously reported FTIR data for Eu(TTA)₃Phen [42]. In particular, the band at 1624 cm⁻¹ is assigned mainly to the carbonyl-related C=O stretching vibration of coordinated TTA, whereas the band at 1598 cm⁻¹ is attributed to C=N/C=C vibrations of the Phen ligand and the conjugated ligand framework. These assignments are supported by previous reports showing that the carbonyl stretching bands of free HTTA shift from the 1660–1640 cm⁻¹ region to lower wavenumbers after coordination to Eu³⁺, and that Phen-related C=N/C–H vibrational bands also shift upon complex formation [15,42]. Additional bands at 1537 and 1505 cm⁻¹ are assigned to coupled C=C/C=N stretching modes of the TTA/Phen framework. The bands at 1355 and 1305 cm⁻¹ are associated with β-diketonate/CF₃-related vibrations of the TTA ligand, while the bands at 1184 and 1132 cm⁻¹ are attributed to coupled C–F/C–C/C=C vibrations within the TTA framework. The bands at 844, 723, and 715 cm⁻¹ are assigned mainly to out-of-plane C–H vibrations and ring deformation modes of the Phen ligand. Overall, the FTIR spectrum agrees well with reported vibrational features of Eu(TTA)₃Phen and supports the successful formation of the ternary Eu³⁺ β-diketonate–phenanthroline complex.

The Raman spectrum of crystalline Eu(TTA)₃Phen provides complementary vibrational evidence for the coordinated TTA/Phen ligand framework (Figure S2, Supplementary Materials). The spectrum exhibits well-defined bands that agree closely with Raman features reported for Eu(TTA)₃ complexes containing phenanthroline-derived ligands [43,44,45]. In particular, the bands observed at 1625, 1608, 1594, and 1523 cm⁻¹ are assigned to coupled C=O/C=C/C=N stretching modes of the coordinated β-diketonate and phenanthroline framework. These bands fall in the coordination-sensitive 1530–1660 cm⁻¹ region, where β-diketonate C=O/C=C vibrations have been reported to shift upon coordination to Eu³⁺ [43]. The intense Raman band at 1415 cm⁻¹, together with bands at 1355 and 1307 cm⁻¹, is attributed to β-diketonate chelate-ring and CF₃/C–C-related vibrations of the TTA ligand. Additional bands at 1084, 1055, 936, 862, 770, 753, 725, and 685 cm⁻¹ are assigned to coupled C–F/C–C vibrations, aromatic ring modes, and out-of-plane C–H/ring deformation modes of the TTA/Phen framework. The low-wavenumber bands below 650 cm⁻¹ are assigned tentatively to ligand-framework, chelate-ring, and lattice-related modes. Overall, the Raman spectrum supports the FTIR results by confirming the characteristic vibrational fingerprint of the coordinated Eu³⁺ β-diketonate–phenanthroline complex.

The crystalline nature of the synthesized Eu(TTA)₃Phen was further evaluated by PXRD (Figure S3, Supplementary Materials). The molecular structure of Eu(TTA)₃Phen has previously been determined by single-crystal X-ray diffraction; the analysis showed that Eu(TTA)₃Phen crystallizes in the triclinic system and contains an eight-coordinate Eu(III) center with a slightly distorted square-antiprismatic coordination polyhedron [46]. Accordingly, the PXRD experiment in the present work is used only to verify the crystallinity and phase consistency of the obtained bulk sample. The experimental diffraction pattern obtained in this work shows a series of sharp reflections, confirming that the obtained solid is crystalline. The main reflections of the synthesized complex are in good agreement with the reported PXRD pattern of Eu(TTA)₃Phen-containing rare-earth complexes, supporting the successful formation of the expected crystalline phase [34]. The presence of intense low-angle and mid-angle diffraction peaks is characteristic of the ordered packing of molecular Eu³⁺ β-diketonate–phenanthroline complexes. Similar sharp diffraction features have also been reported for Eu(TTA)₃Phen-containing polymer films, with reflections at 2θ values around 15.0°, 20.6°, 21.9°, 26.6°, and 30.6°, confirming the crystalline contribution of the Eu(TTA)₃Phen phase [20]. Minor differences in relative peak intensities between the experimental and reference patterns can be attributed to preferred orientation, crystallite-size effects, sample preparation, or differences in measurement conditions. Overall, the PXRD results, together with the FTIR analysis, confirm the formation of crystalline Eu(TTA)₃Phen.

3.2. Absorption Spectra and Solvent Dependence

The UV–Vis absorption spectra of Eu(TTA)₃Phen in all studied solvents were measured over a concentration range between 1 μM and 0.2–3.0 mM, depending on the solubility of Eu(TTA)₃Phen in each solvent. The linearity of the absorption response was verified within the investigated concentration range, confirming that the Beer–Lambert law was obeyed under the experimental conditions used. Due to self-absorption of the solvents, for several solvents, the UV-vis spectrum is not reported at short wavelengths.

The absorption spectra are dominated by intense ligand-centered π–π* bands in the near-UV region (Figure 1 and Table S1, Supplementary Materials), with molar extinction coefficients in the range of 5.77 × 10⁴ to 6.24 × 10⁴ M⁻¹ cm⁻¹. The lowest-energy absorption band maximum, λ₁, lies between 340 and 346 nm and shifts only slightly with solvent, from 340 nm in acetone to 346 nm in DMSO. This minor solvatochromic shift, Δλ = 6 nm, indicates that the lowest ligand-centered singlet excited state of the TTA/Phen chromophores is only weakly perturbed by changes in solvent polarity and coordinating ability. Higher-energy bands appear at λ₃ = 266–274 nm in chloroform, dichloromethane, acetonitrile, and methanol. For the chloroform, dichloromethane, toluene, acetone, and acetonitrile solutions, the shoulder appears at λ₂ = 292 nm, whereas this shoulder is absent in DMF, DMSO, and methanol. This shoulder may arise from specific solvent–complex interactions, such as weak coordination of donor solvents close to the Eu³⁺ center, because DMF, DMSO, and methanol solvents possess more pronounced donor properties relative to other solvents and, in principle, can weakly coordinate to the europium(III) ion, increasing its coordination number to 9, which has been observed for other Eu complexes [47]. However, this assumption should be further investigated. The absorption spectra of Eu(TTA)₃Phen in chloroform, dichloromethane, acetonitrile, and methanol have an absorption band at λ₄ = 228–230 nm in acetonitrile and methanol, corresponding to higher-lying π–π* transitions of the conjugated TTA and Phen chromophores.

Figure 1. Absorption spectra of (a) crystalline Eu(TTA)₃Phen (Kubelka-Munk function) and (b) Eu(TTA)₃Phen solution in (1) chloroform, (2) dichloromethane, (3) toluene, (4) acetonitrile, (5) acetone, (6) N,N-dimethylformamide, (7) dimethyl sulfoxide, and (8) methanol.

The observed molar extinction coefficients are in good agreement with previously reported values for Eu(TTA)₃Phen in solution, confirming the strong light-harvesting ability of the organic antenna ligands. The solid-state absorption spectrum of crystalline Eu(TTA)₃Phen, represented using the Kubelka–Munk function, shows absorption bands centered at approximately 350, 270, and 230 nm (Figure 1a). The similarity between the spectral profile of the crystalline material and those of the solutions (Figure 1b) indicates that the main ligand-centered absorption features are preserved across the solvent series. No clear evidence of significant aggregation, decomposition, or ligand dissociation is observed from the absorption spectra under the experimental conditions used.

3.3. Photoluminescence Spectra and Local Symmetry

Upon excitation into the ligand-centered absorption bands at 365 or into its weak long-wavelength absorption tail at 405 nm, Eu(TTA)₃Phen exhibits the characteristic emission of Eu³⁺ in all studied solvents and in the crystalline state (Figure 2). The emission spectra consist of narrow ⁵D₀ → ⁷F_J transitions: ⁵D₀ → ⁷F₀ at 579–580 nm, ⁵D₀ → ⁷F₁ at 590–592 nm, ⁵D₀ → ⁷F₂ at 610–616 nm, ⁵D₀ → ⁷F₃ at approximately 650 nm, and ⁵D₀ → ⁷F₄ at 699–702 nm. The normalized spectra show that the positions of the ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₃, and ⁵D₀ → ⁷F₄ band maxima remain nearly unchanged across the solvent series, with variations not exceeding 2–3 nm. This behavior is consistent with the shielding of the Eu³⁺ 4f orbitals by the filled 5s² and 5p⁶ shells, which limits the direct influence of the external solvent field on the 4f–4f transition energies [6,48].

Figure 2. Normalized emission spectra of (1) crystalline Eu(TTA)₃Phen and Eu(TTA)₃Phen solution in (2) chloroform, (3) dichloromethane, (4) toluene, (5) acetonitrile, (6) acetone, (7) N,N-dimethylformamide, (8) dimethyl sulfoxide, and (9) methanol (a) upon 365 nm excitation (b) upon 405 nm excitation.

In contrast, the hypersensitive ⁵D₀ → ⁷F₂ electric-dipole transition is more strongly affected by the solvent environment. This transition is highly sensitive to the local symmetry and polarizability around Eu³⁺ and becomes more intense when the Eu³⁺ ion occupies a non-centrosymmetric site [6,35]. The dominance of the ⁵D₀ → ⁷F₂ band in all media confirms the absence of inversion symmetry around the Eu³⁺ center. However, changes in its position, width, and intensity indicate that the local coordination environment is modified upon dissolution and depends strongly on solvent identity.

The full width at half maximum of the hypersensitive ⁵D₀ → ⁷F₂ transition, FWHM₀₋₂, shows a pronounced solvent dependence (Table S2, Supplementary Materials). FWHM₀₋₂ increases from 86 cm⁻¹ in the crystalline complex to approximately 100–108 cm⁻¹ in nonpolar and weakly coordinating solvents such as chloroform, dichloromethane, and toluene. This modest broadening can be attributed to weak inhomogeneous broadening associated with solvation and increased conformational freedom of the TTA and Phen ligands in solution. A much stronger broadening is observed in DMF, DMSO, and methanol, where FWHM₀₋₂ reaches 241–264 cm⁻¹. These values indicate a broader distribution of Eu³⁺ local environments in strongly coordinating or protic media.

The asymmetry ratio R₂₁, defined as the ratio between the integrated intensities of the ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ transitions, provides additional information about the local symmetry around Eu³⁺. The ⁵D₀ → ⁷F₂ transition is strongly affected by the coordination environment, whereas the ⁵D₀ → ⁷F₁ magnetic-dipole transition is much less sensitive to changes in the Eu³⁺ coordination sphere. Therefore, the asymmetry ratio is widely used as a spectroscopic parameter to track changes in local symmetry around Eu³⁺ ions [6,49,50,51,52,53].

In the crystalline state, Eu(TTA)₃Phen exhibits R₂₁ = 12.8 (Table 1), reflecting a moderately asymmetric Eu³⁺ environment. Upon dissolution in chloroform, dichloromethane, and toluene, the asymmetry ratio increases to 19.4–20.0, indicating a further lowering of local symmetry and increased electric-dipole character of the ⁵D₀ → ⁷F₂ transition. This enhancement can be attributed to the loss of long-range crystal-packing constraints and greater conformational flexibility of the ligands in solution.

Higher asymmetry ratio values are observed in polar aprotic solvents, reaching 24.5 in acetonitrile, 24.6 in acetone, 23.0 in DMF, and 21.6 in DMSO. These elevated values indicate stronger solvent-induced symmetry lowering. This effect may arise from specific solvent–complex interactions, such as weak coordination of donor solvents close to the Eu³⁺ center or hydrogen-bonding/electrostatic interactions with the β-diketonate carbonyl groups. Nevertheless, the preservation of the overall Eu³⁺ emission pattern suggests that the primary coordination structure of Eu(TTA)₃Phen remains largely intact in these aprotic solvents.

Methanol produces the largest asymmetry ratio value, 34.9, indicating the strongest perturbation of the Eu³⁺ local environment. This value is nearly three times larger than that of the crystalline complex and substantially higher than those obtained in aprotic solvents. The exceptionally high asymmetry ratio in methanol suggests a qualitatively different solvation regime, likely involving strong hydrogen bonding and/or direct interaction of methanol molecules with the coordination sphere. As discussed below, this strong structural perturbation is accompanied by the most pronounced lifetime shortening, the highest nonradiative decay rate, and the lowest photoluminescence quantum yield.

3.4. Luminescence Lifetimes and Radiative/Nonradiative Decay Rates

Photoluminescence decay curves of Eu(TTA)₃Phen were fitted using a mono-exponential function (Figures S6 and S7, Supplementary Materials), indicating that the ⁵D₀ excited-state decay can be described by a single lifetime component under the experimental conditions used. Lifetime analysis is widely used to evaluate emitting-site uniformity and radiative/nonradiative processes in Eu³⁺-based luminescent materials [54,55]. The mono-exponential behavior suggests that the dominant emitting Eu³⁺ species is uniform in each medium and that no significant contribution from multiple emissive species is detected within the sensitivity of the measurement.

The crystalline complex exhibits the longest lifetime, τ = 0.805 ± 0.007 ms, corresponding to a total decay rate of A_tot = 1242 s⁻¹ (Table 1). This relatively long lifetime is consistent with efficient shielding of the Eu³⁺ ion by the bulky TTA and Phen ligands and with a comparatively low nonradiative decay rate, A_nr = 494 s⁻¹. In solution, the lifetime decreases in all solvents. Among aprotic solvents, the longest lifetimes are observed in dichloromethane, τ = 0.717 ± 0.007 ms, and acetone, τ = 0.695 ± 0.007 ms. Shorter lifetimes are found in DMF, τ = 0.560 ± 0.007 ms, and DMSO, τ = 0.578 ± 0.007 ms. Methanol produces the strongest lifetime shortening, τ = 0.364 ± 0.007 ms, with A_tot = 2747 s⁻¹.

Radiative and nonradiative decay rates were estimated from the corrected emission spectra using a Judd–Ofelt-based approach [56,57], taking the magnetic-dipole ⁵D₀ → ⁷F₁ transition as a reference. This method is commonly used for Eu³⁺ complexes because the ⁵D₀ → ⁷F₁ transition has a relatively well-defined magnetic-dipole character, whereas the electric-dipole transitions are sensitive to the local ligand field [6,16]. The calculated radiative decay rate, A_r, varies from 748 s⁻¹ in the crystalline state to 1320 s⁻¹ in methanol. The highest A_r values are observed in methanol, DMSO, and DMF, whereas the lowest values are obtained for the crystalline complex and dichloromethane. This increase in A_r upon dissolution, especially in more strongly perturbed media, is consistent with the increase in R₂₁ and reflects enhanced electric-dipole character of the ⁵D₀ → ⁷F₂ transition in more asymmetric environments.

The main solvent effect, however, is observed in the nonradiative decay rate A_nr, which varies over a much wider range, from 428 s⁻¹ in dichloromethane to 1427 s⁻¹ in methanol. In nonpolar and moderately polar aprotic solvents, the nonradiative decay rate remains relatively low, ranging from 428 to 657 s⁻¹, leading to high Eu(III) internal quantum yields, η_Eu, between 61.9% and 70.1%. The lowest nonradiative decay rate values are observed in dichloromethane and acetone, whereas the highest nonradiative decay rate values among aprotic solvents are obtained in DMF and acetonitrile. These differences suggest that solvent coordination ability, polarity, and vibrational coupling influence the nonradiative deactivation of the Eu³⁺ emitting state.

In methanol, the nonradiative decay rate increases sharply to 1427 s⁻¹, reducing Eu(III) internal quantum yield (η_Eu) to 48.0%, the lowest value among all studied media. This pronounced enhancement of nonradiative decay is consistent with multiphonon relaxation mediated by high-energy O–H vibrational oscillators [58], in this case from methanol molecules located near the Eu³⁺ center [59,60]. O–H, N–H, and C–H oscillators are well known to quench lanthanide excited states when they are close to the metal center, with O–H vibrations being particularly effective because of their high vibrational energy [23]. Thus, the short lifetime and high nonradiative decay rate observed in methanol provide direct evidence that solvent proticity plays a decisive role in controlling nonradiative decay in Eu(TTA)₃Phen solutions.

3.5. Photoluminescence Quantum Yields and Efficiency of Antenna Sensitization

The photoluminescence quantum yield of Eu(TTA)₃Phen shows a pronounced dependence on the solvent (Table 1). The crystalline complex exhibits a photoluminescence quantum yield (PLQY) of 38.15% [41]. In solution, the highest PLQY values are obtained in chloroform, 14.7 ± 0.5%, and toluene, 13.7 ± 0.5%, followed by dichloromethane, 10.8 ± 0.5%. Polar aprotic solvents give lower PLQY values: 10.0 ± 0.5% in acetonitrile, 10.9 ± 0.8% in acetone, 9.3 ± 0.5% in DMF, and 8.4 ± 0.5% in DMSO. The lowest PLQY is observed in methanol, with a value of 4.6 ± 0.5%.

The decrease in photoluminescence quantum yield upon dissolution cannot be explained solely by changes in the Eu(III) internal quantum yield. To distinguish between the Eu³⁺-radiative transition efficiency and the efficiency of ligand-to-metal sensitization, the ⁵D₀-state formation quantum yield, Φ_form(⁵D₀), was calculated as PLQY/η_Eu. This parameter represents the fraction of absorbed photons that successfully populate the Eu^{3+ 5}D₀ level before radiative or nonradiative decay occurs. It therefore provides a quantitative measure of antenna sensitization efficiency, including ligand absorption, intersystem crossing, triplet-state population, ligand-to-metal energy transfer, and competing deactivation pathways [61,62].

The crystalline complex shows the highest ⁵D₀-state formation quantum yield, 63.4%, indicating efficient formation of the Eu³⁺ emitting state in the solid. In solution, ⁵D₀-state formation quantum yield decreases substantially, reaching 22.8% in chloroform, 19.8% in toluene, and 15.6% in dichloromethane. In polar aprotic solvents, ⁵D₀-state formation quantum yield ranges from 12.3% to 16.2%, with the lowest value among these solvents observed in DMSO. Methanol shows the lowest ⁵D₀-state formation quantum yield, Φ_form(⁵D₀) = 9.5%.

These results demonstrate that antenna efficiency (described by ⁵D₀-state formation quantum yield) and the Eu³⁺-radiative transition efficiency (described by Eu(III) internal quantum yield) are partially independent factors that both control the overall luminescence output. For example, dichloromethane exhibits one of the highest Eu(III) internal quantum yields, η_Eu = 69.3%, but its PLQY is limited to 10.8% because Φ_form(⁵D₀) is only 15.6%. In contrast, chloroform provides a better balance between η_Eu = 64.6% and Φ_form(⁵D₀) = 22.8%, resulting in the highest solution PLQY. Therefore, a high Eu(III) internal quantum yield alone is not sufficient to guarantee high total emission efficiency; efficient ligand-to-metal sensitization is also required.

The lower Φ_form(⁵D₀) values observed in polar aprotic solvents suggest that solvent-induced perturbation affects the antenna pathway. In Eu³⁺ β-diketonate complexes, ligand-to-metal sensitization is generally associated with excitation of the organic antenna ligands, intersystem crossing to ligand triplet states, and subsequent energy transfer to Eu³⁺. The energy of the ligand triplet state and its energy gap relative to the Eu³⁺ accepting levels are key parameters controlling sensitization efficiency [63,64]. Recent ultrafast optical and X-ray spectroscopic studies of Eu³⁺ complexes have provided direct evidence that energy transfer from antenna ligands to Eu³⁺ can proceed efficiently from ligand triplet states through exchange-mediated mechanisms [61]. In the present system, specific solvent–complex interactions in DMF, DMSO, acetone, and acetonitrile may perturb the ligand triplet-state energy, modify intersystem crossing efficiency, or introduce additional ligand-centered deactivation pathways that compete with Eu³⁺ sensitization.

Methanol combines both detrimental effects: inefficient formation of the ⁵D₀ level and enhanced nonradiative deactivation from that level. Its Φ_form(⁵D₀) value is the lowest of the series, 9.5%, while its η_Eu is also the lowest, 48.0%. The low PLQY of methanol, 4.6%, therefore probably results from both reduced antenna sensitization and strong O–H-mediated nonradiative quenching. The strong deviation of R₂₁ in methanol further supports the conclusion that the protic solvent induces substantial perturbation of the Eu³⁺ coordination environment, which affects both energy transfer to Eu³⁺ and decay from the ⁵D₀ level.

3.6. Solvent Dependence of the Optical Response and Its Structural Implications

The combined dataset of absorption spectra, emission spectra, lifetimes, radiative and nonradiative decay rates, photoluminescence quantum yield, ⁵D₀-state formation quantum yield, and asymmetry ratio values reveals clear correlations between the photophysical response of Eu(TTA)₃Phen and solvent properties such as polarity, coordinating ability, and proticity.

Nonpolar and weakly coordinating solvents, namely chloroform, dichloromethane, and toluene, preserve relatively narrow emission bands, long lifetimes, low nonradiative decay rates, high Eu(III) internal quantum yields, and the highest solution photoluminescence quantum yield values. In these solvents, FWHM₀₋₂ remains close to 100–108 cm⁻¹, τ ranges from 0.637 to 0.717 ms, A_nr remains between 428 and 538 s⁻¹, and Eu(III) internal quantum yield remains between 64.6% and 69.3%. The R₂₁ values of 19.4–20.0 indicate a moderately asymmetric Eu³⁺ environment, but the overall photophysical performance suggests that the complex remains well protected from strong solvent-induced perturbation. Chloroform and toluene provide the most favorable balance between antenna efficiency and the Eu³⁺-radiative transition efficiency, leading to the highest solution photoluminescence quantum yields.

Polar aprotic solvents, including acetonitrile, acetone, DMF, and DMSO, induce greater changes in local symmetry and antenna efficiency. Their values of asymmetry ratio increase to 21.6–24.6, and DMF and DMSO produce substantial broadening of the ⁵D₀ → ⁷F₂ band. Although Eu(III) internal quantum yield remains relatively high in these solvents, the photoluminescence quantum yield values decrease because the ⁵D₀-state formation quantum yield is reduced to 12.3–16.2%. This indicates that the overall luminescence output in polar aprotic solvents is limited primarily by less efficient population of the Eu^{3+ 5}D₀ level rather than by complete quenching of the emitting state. The trend is consistent with partial solvent-induced perturbation of the antenna pathway through donor interactions, electrostatic effects, or hydrogen bonding to ligand carbonyl groups.

Methanol represents the most unfavorable medium for Eu(TTA)₃Phen luminescence. The exceptionally high asymmetry ratio, 34.9, the shortest lifetime, 0.364 ms, the highest A_nr, 1427 s⁻¹, the lowest Eu(III) internal quantum yield, 48.0%, the lowest ⁵D₀-state formation quantum yield, 9.5%, and the lowest photoluminescence quantum yield, 4.6%, collectively demonstrate that methanol strongly perturbs the Eu³⁺ local environment, enhances O–H-mediated vibrational quenching, and reduces ligand-to-metal sensitization. These results show that protic solvents are particularly detrimental to Eu(TTA)₃Phen emission.

Overall, the solvent-dependent luminescence of Eu(TTA)₃Phen is governed by two coupled but distinguishable contributions: the Eu(III) internal quantum yield and the efficiency of ligand-to-metal sensitization. Nonpolar and weakly coordinating solvents preserve both factors more effectively, whereas strongly coordinating polar solvents mainly reduce antenna efficiency, and methanol reduces both antenna efficiency and Eu(III) internal quantum yield. This distinction is important for the rational design of Eu(TTA)₃Phen-based luminophores for solution-processable photonic systems, sensing platforms, and emissive polymeric or hybrid materials [15,24,42]. In particular, the combined analysis of R₂₁, FWHM₀₋₂, τ, A_r, A_nr, η_Eu, PLQY, and Φ_form(⁵D₀) provides a practical framework to determine whether a decrease in emission efficiency originates mainly from Eu³⁺-centered nonradiative quenching or from inefficient antenna sensitization.

The optical data demonstrate solvent-dependent perturbation of the Eu(III) emitting environment. However, they do not by themselves establish a unique coordination structure in solution. Accordingly, references to direct solvent coordination, ligand dissociation, or a specific structural rearrangement are treated as hypotheses rather than conclusions. Therefore, future work should correlate these photophysical parameters with quantitative solvent descriptors, including dielectric constant, donor number, acceptor number, and Kamlet–Taft parameters [65,66], to establish predictive models for solvent-dependent emission behavior. In addition, ligand design strategies such as increased steric shielding, fluorination, or reduced access of high-energy vibrational oscillators to the Eu³⁺ center may help improve the resistance of Eu(TTA)₃Phen-related complexes to solvent-induced quenching.

4. Conclusions

In this work, the solvent-dependent photophysical properties of Eu(TTA)₃Phen were systematically investigated in organic solvents with different polarity, coordinating ability, and proticity. The UV–Vis absorption spectra showed only minor solvent-induced shifts in the ligand-centered absorption bands, indicating that the primary light-harvesting properties of the TTA/Phen antenna system are largely preserved across the solvent series. In contrast, the Eu³⁺ emission spectra, asymmetry ratios, luminescence lifetimes, radiative and nonradiative decay rates, photoluminescence quantum yields, and ⁵D₀-level formation efficiencies showed pronounced solvent dependence.

Nonpolar and weakly coordinating solvents, particularly chloroform and toluene, provided the most favorable conditions for Eu(TTA)₃Phen luminescence in solution, yielding the highest photoluminescence quantum yield values and relatively efficient ⁵D₀-level formation. Polar aprotic solvents maintained relatively high Eu(III) internal quantum yields but reduced the overall luminescence output mainly through decreased antenna sensitization efficiency. Methanol produced the strongest perturbation, leading to the highest asymmetry ratio, shortest lifetime, highest nonradiative decay rate, lowest Eu(III) internal quantum yield, lowest ⁵D₀-state formation quantum yield, and lowest photoluminescence quantum yield.

These results demonstrate that the solvent-dependent emission of Eu(TTA)₃Phen is governed by both Eu³⁺-centered deactivation and ligand-to-metal sensitization efficiency. The combined analysis of the optical properties provides a practical framework for identifying whether luminescence losses originate primarily from nonradiative quenching of the Eu³⁺ emitting state or from inefficient antenna-mediated population of the ⁵D₀ level. This understanding is useful for the rational design of Eu(TTA)₃Phen-based luminophores for solution-processable photonic materials, sensing platforms, and emissive hybrid systems.