To verify the underlying mechanism for Eu³⁺ PL enhancement, an efficient ligand, named Chloro-phenylmethyl imidazo-phenanthroline (Cl-MIP), was used in this study and subjected to chemical modification. Mondel et al. and other previous studies have indicated that strong π–π interactions between neighboring imidazo-phenanthroline (IP) and aromatic units in aryl-σ-IP (AIP) chromophores resulted in a rigid structure and high electron mobility [31]. Nevertheless, the random arrangement of AIP molecules, usually with a strong aggregation tendency, largely affects the optical extraction performance of AIP; such as with charge dynamics, luminescence yield, and absorption profiles; similar results were shown in our related compounds [26]. An effective molecular design can obtain dynamic advantages of AIP and control its desired aggregation features. Cl-MIP served as an antenna ligand of the europium complex. This ligand was expected to have greater triplet energy transport away from its singlet pathway. The efficiency of triplet electronic energy transfer (EET) was clearly confirmed according to its parameters of molar absorption coefficient, molecular interaction and QE value.

Crystal types and molecular packing of EC systems are critical in understanding the molecular interactive effects, including single crystals, packing diagrams, and hydrogen bond analyses; related data are summarized in Tables S1–S3 (electronic supplemental information (ESI^†)). In individual molecular analyses, the free ligand of Cl-MIP consisted of 1-methyl-1H-imidazo[4,5,f] [1,10]phenanthroline and a 2-chlorophenyl ring, as shown in Figure S1 (ESI^†). The dihedral angle between the IP unit and the phenyl ring was 64.15(4)°. The solvent molecule of CH₂Cl₂ was co-crystallized with the Cl-MIP molecule. The crystal structure was stabilized by a series of hydrogen bonds between the CH₂Cl₂ and Cl-MIP molecules. The packing diagram of Cl-MIP is shown in Figure S2. The molecular structure of Eu(DBM)₃Cl-MIP is shown in Chart 1a. The central ion of Eu(III) is eight-coordinated by six oxygen atoms of three bidentate DBM anions and two nitrogen atoms from a bidentate Cl-MIP ligand, forming a distorted square antiprism. The average Eu-O bond distance was 2.318(2) Å, and the mean Eu-N bond distance was 2.596(2) Å. The dihedral angle between the imidazo-phenanthroline unit and the phenyl ring was 51.51(8)°, and was smaller than the free ligand at 64.15(4)°. Two phenyl rings from a DBM anion between the adjacent molecules were nearly parallel, with inter-centroid distances of 3.731(2) and 3.731(2) Å; and the shortest inter-ring distances were 3.447(1) and 3.402(1) Å. The results revealed π–π interactions. An additional π–π stacking was seen between two phenyl rings from adjacent Cl-MIP ligands where the inter-centroid distance was 3.908(2) Å, and the shortest inter-ring distance was 3.689(2) Å. These π–π interactions are shown in Chart 1b.

Cl-MIP and Eu(DBM)₃Cl-MIP with strong π–π stacking between the aromatic units, were shown to have good electron transport and high triplet energy as found by performing spectral analyses [25]. The aggregation behaviors of these two molecules not only led to a red shift of their emission spectra, but also resulted in deterioration of the solid-state optical performance. In view of this, the PBMA selected to act as an isolative and dispersive matrix was studied [18,22]. Aggregate suppression between the aromatic ligands was caused by increased steric congestion [25]. Where a heavier halogen atom was replaced by a hydrogen atom in the 2-position of the phenyl ring, the europium complex obtained extra carrier dynamics benefits, allowing a greater amount of excited energy from intersystem crossing (ISC) relax to the triplet state by the heavy ion effect [32]. Molecular studies indicated that, effective energy-transfer distance and polar aggregation can be reduced when the IP unit bears a methyl substitute [22,33].

Figure 1a,b, as well as the inset of 1b, shows the absorption, PL, and photoluminescent excitation (PLE) spectra of Eu³⁺ transition levels. The FWHM value of the Eu³⁺ emission spectrum in the solid state was approximately 4 nm, and the intense emission line at 611 nm was from ⁵D₀→⁷F₂. The molar absorption coefficient (ɛ) values of the absorption spectrum exhibited by this new europium complex reached λ_max (ɛ) = 352 (974,000) and 294 nm (72,400) in dichloromethane; the high absorption values of Cl-MIP can be competitive to similar AIPs [23,24]. This also demonstrated that sensitization of the europium complex through its singlet pathway can easily transport ligand-absorption energy levels to triplet states [34]. The optical absorption, photoluminescence (PL), quantum yield, electrochemical data, and thermal properties are summarized in Table 1. The photoluminescent excited spectrum showed a broad band at approximately 375–420 nm, which was attributed to the ligand’s π–π overlap, inducing a charge-transfer of the Eu³⁺ complex. The monitored diluted-state PL was excitation at 357 nm (Figure 1b). Figure 1c shows a broad band at approximately 417 nm, attributed to a ligand emission at the 1.0 wt% level of Eu³⁺ and based on the weight of PBMA. At lower europium content there was a large number of excited carriers trapped in the PBMA matrix, which conducted a poor Eu³⁺ PL signal. In this work, we adjusted the concentration of Eu³⁺ and the light-irradiation time to determine the maximal fluorescent response. Figure 2a shows the PL spectra of Eu³⁺/PBMA composites with various Eu³⁺ contents. The spectrum was the result of excitation by 365 nm radiation from a 400-W TP UV-lamp for 360 s. When the europium content was in the range of 1.0–2.0 wt%, it exhibited weak PL signals (611 nm) and a relatively strong defect emission that belongs to ligands detected at approximately 417 nm. When the Eu³⁺ content achieved a value of 2.0 wt%, a large enhancement of the Eu³⁺ emissions (611 nm, ⁵D₀→⁷F₂) was recorded. Simultaneously, the defect emission was reduced. Figure 2a demonstrates a threshold concentration level of the EC carrier that was irreversibly transported at its emissive levels. Here, the PL intensity ratio between the Eu³⁺ narrow-band and defect contents exceeded > 2.0 wt%, the Eu³⁺ emissions dramatically decreased, and the defect emission ratio exhibited a rapid increase, while a fluorescence-quenching mechanism rapidly increased. The UV-curing duration was simultaneously controlled to detect PL signals at 2 wt% Eu³⁺. This further purified the optical signals of PL intensity that appeared in the Eu³⁺/defect emission ratio. Thus, the PL intensity ratio of Eu³⁺ and the defect emission was amplified by up to 104-fold, as shown in Figure 3a,b.

By contrast, we attempted to understand how the fluorescence efficiency is influenced by structural properties; therefore, we tuned the adjacent guest/host microstructures. Compared to different UV-irradiation times, we clearly identified that the maximal ligand emission appeared at approximately 417 nm, implying that PL responses of the carrier pathways or microstructures of the EC complex had changed. The triplet energy transfer of ECs is highly dependent on the Eu-ligand distance and outer microstructure of the PBMA matrix [36–39]. Controlling the coordinated environments and luminescence mechanism of the EC complex by conducting certain procedures can achieve high performance of EC emissions. The maximal PL intensity of EC composites appeared at approximately 300 s when comparing different UV-curing times. The microstructure enhancement seemed to be improved, because most excited carriers were trapped at the ligand level. Spectral reflectance and transmittance were measured to examine the optical profiles of this EC composite with a scan range of 0.4–2.0 μm. Figure 4a,b shows the optical properties of each constituent component. PBMA and the europium/PBMA composite displayed excellent optical transmittance (>88%) and lower reflectance (<8.2% for PBMA and <7.9% for the europium/PBMA composite in the visible region with maximal values of 7.6% for PBMA and 5.6% for the europium/PBMA composite), indicating that the europium/PBMA composite system can be regarded as a high-transmittance material in the range of visible to near-IR radiation. This also suggests that the EC composite is capable of acting as a fluorescent signal for detection in this wavelength region [11,14]. As expected, the pristine Eu³⁺ complex showed a red-shifted absorption band at 420–600 nm and exhibited poorer transmittance in the overall range than that of neat PBMA and the europium/PBMA composites. Figure 4b shows the absorbance of the constituent components in the same wavelength range, where R values were calculated by the equation of A % (absorbance) = (1 − R − T) × 100%.

To evaluate this new EC on the energy transfer effects in the PBMA matrix, we further probed the donor: DBM, Cl-MIP; and the acceptor: the europium ion to detect their triplet state energy levels; the results are shown in Figure 5. These phosphorescence spectra use 375 nm as the excitation source. By comparison, Cl-MIP has a broad emission at 525 nm; DBM in a PBMA system (line-2) shows a broad phosphorescence band (λ_max = 500 nm) corresponding to singlet state emission. The phosphorescent results are consistent with an earlier report [18]. In this system, we also discovered a weak band, possibly attributed to a PBMA emission of the triplet state, which appeared at approximately 520 nm. Emission at this wavelength has also been observed in PBMA-DBM mixtures doped with Eu³⁺ (lines 3 and 4). The highest Eu³⁺ phosphorescence emission is observed in the Eu(DBM)₃Cl-MIP-PBMA (line-5) and herein there is no indication of DBM, Cl-MIP, or PBMA triplet state emission. As is known, the excited Cl-MIP and DBM photons undergo a nonradiative transition to their longer lived triplet state.

We further probed the interaction between the EC and adjacent microstructures by monitoring the emission decay, using a 375 nm laser pulse as the excitation source. This decay curve fitting showed a monoexponetial kinetic at ⁵D₀→⁷F₂ transition, and exhibits characteristic decay of 220 μs. For the Eu(Phen)₃.H₂O/PMMA system, the decay curve of the Eu³⁺ ion in the presence of DBM can be fitted to a multiexponential function to yield a longer decay of 670 μs [18]. An increase in the decay flourescence of Eu³⁺ appeared in the range of 670 to 740 μs. Here, these EC decay curves are effectively fitted by biexponential decay kinetics, which can be expressed in Equation 1, and we determined their average lifetime ( t ) using Equation 2.

By substituting the values of a₁, a₂, τ₁, and τ₂ in Equation 2, we obtained the average lifetime of each EC, as summarized in Table 2. The emission lifetime among these three EC compositions, doped in PBMA, in ascending order is: EC doped in PBMA with argon purging and a freeze-thaw cycle (730 μs) > EC doped in PBMA composite (651 μs) > neat EC system (565 μs). The observed emission lifetime indicates that the ligands interacting with the PBMA matrix results in energy transfer enhanced by the ligand’s effective shielding. Thus, the sequence of the energy transfer among the ligands is in solely PL intensity order of their emission lifetime. Figure 6 illustrates the carrier charge-transfer process used to interpret the Eu³⁺ photo-induced emission mechanism. After photoexcitation and generation of the carrier, the excited electrons were non-radiatively transferred to L1; and the lowest energy of Eu³⁺ surrounded the ligands, which subsequently relaxed to 1 the triplet state of T1 before beginning the intersystem cross-transfer to Eu³⁺ emission levels. Under lower carrier concentrations of the europium complex or an inadequate UV-irradiation dose, the carriers were trapped in the ligand molecules and were unable to penetrate and diffuse to the center of the Eu³⁺-ion energy levels. The photoexcited electrons, herein confined to the ligand state, created another transition pathway from the lowest level of the singlet state.

We adjusted the Eu³⁺ concentration and the light-irradiation duration to increase the efficiency of the EC fluorescence. This decreased the probability of photoexcited electrons being trapped, and led to an enhancement of europium emissions. In our proposed mechanism for PL enhancement, the existence of a carrier confined to the ligand-state performs a vital function. Usually, the carrier coupling from the ligand state can easily transfer to Eu³⁺ energy levels because of the non-trapped photoexcited electrons in the europium complex. Under this condition, pure Eu³⁺ PL signals were amplified by adjusting the concentration of europium and the UV-curing duration. To confirm this prediction, a europium/PBMA sample was optimized. The measured PL spectra showed considerable enhancement of europium emissions, but substantial ligand emissions still remained. Dissolved impurities in the precursor mixture trapped carriers in the defect state, leading to the ligand’s emission. Through argon purging and a freeze-thaw cycling technique to remove impurities from the precursor, the pure europium PL signal was enhanced. Figure 7 shows the europium/PBMA composite at 2.0 wt% of Eu³⁺, after UV-lamp curing for 300 s, for which the strongest peak of the pure Eu³⁺ PL signal (⁵D₀→⁷F₂, 611 nm) was enhanced by approximately 2.1-fold of that of the pristine Eu(DBM)₃Cl-MIP, as shown in the inset figure. The ratio between pure Eu³⁺ and the defective emissions were more than two orders of magnitude higher, implying that this optical alignment successfully enhanced the Eu³⁺ PL signal.

High performance of polymer-based europium composite greatly depends on the synthesis methods structural nature, and morphological result. In this article, the optimized chemical, micro-structural and optical controlled europium composites were extensively demonstrated. These approaches provided more efficient enhancement of the energy transfer between the Eu³⁺ emissive levels. The charge/energy transfer interactions between europium complexes and metal ions were investigated using fluorescence quenching spectroscopy [10,40]. In a feasible approach, the photostability of a europium composite accessing metal ion can be used to evaluate the europium material’s optical sensitivity by metal ion-chelating in the matrix, altering electron transfer from the ligands triplet states into the Eu³⁺ emissive levels.

We used a series of simple metal ions, including Fe³⁺, Zn²⁺, Ca²⁺, and Mg²⁺ chelated to the carboxylate group on the BMA unit, which provided useful insight into the behavior of these photoinduced carriers at trace levels. Figure 8 shows that pristine europium fluorescence was synthesized following the optimized conditions, so that the europium composite with the highest fluorescent intensity at ⁵D₀→⁷F₂ transition (611 nm) was the referenced sample; other europium samples were prepared corresponding to europium/PBMA contents of 0~0.04 wt%. Results showed that PL signals occurred at different directions in species of four metal ions. The presence of Zn²⁺ was inefficient in quenching europium emission in the specific range, which may be attributed to highly diminished triplet carriers between the intercrossing and europium energy levels. Next, small PL increases that appeared in the Ca²⁺ and Mg²⁺ ions corresponded to contents of a certain wt%, which showed no significant change on europium emission and PLE spectra. The presence of Ca²⁺ and Mg²⁺ ions seem to give weak interactions with europium complex in this region, therefore, the optical feature was not investigated further. As for the contents at 0–0.04 wt% for Fe³⁺, the PL signals were significantly increased in this region. The PL increased as the Fe³⁺ proportion by weight of europium composite increased to a maximum of ~60% level, the percentage increase was relative to the referenced sample. This may be attributed to stronger attachments linking the europium and polymer pathways by Fe³⁺, allowing more carrier populations into the Eu³⁺ emissive levels [41]. Consequently, we investigated its absorption and PLE spectra demonstrated that no new charge-transport or ligand bands occurred, indicating that Fe³⁺ can largely enhance the PL signal of europium-chelated at the triplet state. This europium material will be helpful to achieve highly fluorescent efficiency in optical detections.

Benzyl methacrylate (BMA) (Alfa Aesar, >99%) was passed through basic alumina to remove impurities, and the photoinitiator, (1-hydroxycyclohexyl) (phenyl) methanone (98%, Dupont Chemicals), was crystallized from ethyl acetate (EA) (3 mL/g). Dibenzoyl methane (DBM, >98%) was crystallized from toluene (2.5 mL/g). Most synthetic chemicals were purchased from Sigma-Aldrich or Acros Organics and were used as received. All solvents used in the experiments were purified following laboratory chemical procedures. Synthetic procedures and the yields of the ligand (Cl-MIP) and Eu(DBM)₃Cl-MIP are shown in Scheme 1.

¹H nuclear magnetic resonance (NMR) and ¹³C NMR spectra were recorded using a Bruker DMX-500 FT-NMR spectrometer. Chemical shifts were recorded in parts per million (ppm) downfield from that of the Me₄Si standard. High-resolution electrospray ionization (ESI) mass spectrometric (HRMS (ESI)) data were obtained by a JEOL JMS-700 spectrometer (Tokyo, Japan). Photoluminescence (PL) measurements were performed with a Hitachi F-4500 fluorescence spectrophotometer (Japan) equipped with a 150-W xenon lamp. A solid-state quantum yield was determined by an F-7000 Fluorescence Spectrophotometer (Hitachi, Japan) using an integrating sphere (part No. 5J0–0148) to observe the standard sample and the corresponding europium complex. Crystallographic analyses of the ligand and europium complex were carried out on a Nonius Kappa CCD single-crystal XRD-equipped Oxford Cryotream 700 (Oxford Cryosystems, UK). Thin–layer chromatography (TLC) films were visualized under UV light (254 nm) after treatment with iodine vapor, or heating treatment followed by exposure to 5% phosphor-molybdic acid in ethanol. Flash column chromatography was performed to purify all synthesized compounds using Merck (cat. no. 9385) 40–63 mm silica gel 60.

1,10-Phenanthroline-5,6-dione (phen-dione) was prepared as in a previous study [31]. 2-Chloroimidazophenanthroline (Cl-IP) was prepared by refluxing a mixture of phen-dione (0.53 g, 2.5 mmol), 2-chlorobenzaldehyde (0.50 g, 3.5 mmol), ammonium acetate (3.88 g, 50 mmol), and glacial acetic acid (7 mL) for 2 h in accordance with the Steck and Day method [42]. Chloro-phenylmethyl imidazophenanthroline (Cl-MIP) was synthesized through the methylation of Cl-IP, using sodium hydride in dry dimethyl-formamide (DMF) under N₂ with EA: n-hexane of 20:1 elution to provide a favorable yield (85%) of brown powder. The melting point was 254–255 °C. ¹H-NMR (500 MHz, CDCl3, 25 °C) δ: 9.21–9.15 (m, 2H), 9.05 (dd, 1H, J = 8.1, 8.2 Hz), 8.79 (dd, 1H, J = 8.2, 8.5 Hz), 7.75–7.66 (m, 3H), 7.60 (dd, 1H, J = 1.2, 8.0 Hz), 7.58–7.46 (m, 2H), 4.14 (s, 3H). ¹³C-NMR (125 MHz, CDCl₃, 6 25 °C) δ: 151.0, 149.0, 148.0, 145.0, 144.2, 136.2, 134.8, 132.9, 131.7, 130.4, 129.9, 129.5, 128.1, 127.4, 125.5, 124.1, 123.6, 122.5, 120.2, 34.9. HRMS (ESI) calcd. For C₂₀H₁₃ClN₄Na [(M+Na)⁺]: 367.0721, determined 367.0719.

The Eu(DBM)₃Cl-MIP was synthesized using the following method. Dibenzoyl methane (DBM) at (1.36 g, 6 mmol) and Cl-MIP (0.74 g, 2 mmol) were dissolved in 15 mL of hot anhydrous ethanol. NaOH (1 N, 6 mL) was added, and the pH was adjusted to 7.0. The mixture was stirred while 2 mmoles of europium salt (EuCl₃·6H₂O) solution was added drop-wise. The mixture was then stirred for 5 h at 50 °C. After the mixture had cooled, a brown crude product was collected. Flash chromatography with EA: n-hexane at 20:1 provided Eu(DBM)₃Cl-MIP (67% yield for crude) as a pale-yellow product. The melting point was 233–234 °C. ¹H-NMR and ¹³C-NMR data of this compound are not shown here because of its magnetic property. The structure of Eu(DBM)₃Cl-MIP confirmed by its single crystal data were shown in ESI^†. HRMS (ESI) calcd. for C₆₅H₄₆ClEuN₄NaO₆ [(M+Na)⁺]: 1189.2210, determined 1189.2196.

A mixture containing Eu(DBM)₃Cl-MIP, BMA monomer, and photoinitiator was dip-coated on a slide (2.54 cm × 4.0 cm × 1.01 mm), the obtained film thickness was between 22–24 μm (Scheme 1). The coated substrate was subsequently covered with polyethylene terephthalate (PET) film for UV light-irradiation under oxygen-free conditions. The coated composites were placed in a 400-W UV-light curing instrument (OPAS, TX-500, Taiwan) with a light source at 365 nm and an irradiation intensity of 50–70 mW/cm². The distance between the coated substrate and light source was 8 cm. The content of Eu(DBM)₃Cl-MIP ranged between 1.0–2.4 wt%, based on the monomer. The irradiation duration required to obtain the resultant products ranged from 250 to 400 s. The amount of UV-photoinitiator used was fixed at 3 wt% based on BMA. After the UV-irradiated sample had cooled to ambient temperature, the PET film was removed and immediately dried in a vacuum for 30 min. The film deposited on the substrate surface was transparent, colorless, and smooth with a thickness of 25–35 ± 5 μm. The Eu(DBM)₃Cl-MIP of various contents and cured for different times was analyzed using photoluminescence (PL) and PL excitation (PLE) spectroscopy, performed using an F-4500 (Hitachi, Japan) equipped with a solid-sample holder and a 450-W Xe lamp. Ion-binding tests were stoichiometrically added to form a mixture of metal ions, including anhydrous FeCl₃, MgCl₂, CaCl₂, and ZnCl₂ dissolved in BMA (0.0243 g/0.5 g BMA) before UV irradiation.

Differential scanning calorimetric (DSC) measurements were carried out using a PerkinElmer 7 series thermal analyzer (PE, USA) at a heating rate of 10 °C/min, from room temperature to 200 °C. Thermal gravity analysis (TGA) was performed on an SDT Q600 V20.5 Build 15 Universal V4.4A (TA Instruments, USA), at a heating rate of 10 °C/min, from room temperature to 450 °C. Molecular weight distributions were determined using the Waters Alliance System (Waters 2414 differential refractive index detector, MA, USA), using a series of three linear styragel columns (HT2, HT4, and HT5) at ambient temperature; THF and 1 g/L of LiBr were used as the eluent. UV-visible absorption spectra were recorded on a Varian UV-Vis spectrometer (USA), using a quartz cuvette with 1-cm double-beam wavelength scanning. GPC was performed to yield an average molecular weight <M_n> of 8775 and an average distributed molecular weight <M_w/M_n> of 2.38. The elution trace was approximately mono-modal and symmetrical, implying that the system created homogeneous photo-polymerization. Thermal stability tests revealed that the temperature of PBMA with a 5 wt% weight loss achieved 220 °C, and the maximum degradation temperature was 350 °C, which can be considered a suitable rank 1 used in the polymer matrix. In addition, this europium dopant had a melting point of 234 °C and T_g of 137 °C, indicating that the europium-composite was associated with favorable thermal stability for optical applications.