Remote Ligand Substitution in Imidazo[4,5-f][1,10]phenanthroline as a Strategy to Modulate Thermally and Aggregation-Driven Emission in Cu(I) Complexes

Abstract

Three new heteroleptic copper(I) complexes of the form [Cu(N,N)(XantPhos)]PF₆ were synthesized and characterized, where N,N refers to phenyl-substituted imidazo[4,5-f][1,10]phenanthroline. All complexes were obtained as yellow powders in yields ranging 82–95% and were fully characterized by NMR spectroscopy, FT-IR, and mass spectrometry. The complexes were also redox-optically characterized. Their absorption profiles display a lower-energy metal-to-ligand charge-transfer (MLCT) band at approximately 412 nm. In solution, weak dual emission is observed, combining ligand-centered and MLCT contributions, with oxygen-dependent quenching supporting the presence of triplet character in the latter. Temperature- and solvent-dependent studies reveal thermally coupled emissive states, in which a relaxed ³MLCT state dominates at low temperatures. In the solid state, intense orange-to-red emission arises from restricted molecular motion and stabilized ³MLCT states, with C3 showing the highest efficiency. Additionally, aggregation-induced emission (AIE) is observed in solvent mixtures. These results suggest that remote substitution can influence the excited-state dynamics and aggregation-driven emission in Cu(I) complexes.

1. Introduction

Light-emitting materials play a central role in modern optoelectronic technologies, underpinning applications ranging from solid-state lighting and display technologies to sensing and bioimaging [1,2]. In this context, the development of efficient and stable luminescent systems that can be processed by simple, cost-effective fabrication methods remains a major scientific and technological challenge [3,4]. While purely organic emitters have achieved remarkable performance, their operational stability and efficiency are often limited, motivating the exploration of alternative emissive platforms [5,6].

Transition-metal complexes have emerged as particularly versatile luminophores due to their rich excited-state landscapes, which enable phenomena such as phosphorescence and thermally activated delayed fluorescence (TADF) [7,8,9]. These features allow efficient harvesting of both singlet and triplet excitons, offering intrinsic advantages over conventional fluorescent emitters [10]. As a result, metal-based emitters have been extensively investigated for optoelectronic applications, especially in electroluminescent devices [7,11,12,13].

Within this broad family, copper(I) complexes have attracted growing attention as sustainable, earth-abundant alternatives to noble-metal systems such as iridium and ruthenium [7,14,15]. In recent years, [Cu(N,N)(P,P)]⁺ type complexes (where P,P refers to diphosphine ligands, and the N,N part indicates diimine ligands) have become a benchmark architecture for emissive and photoactive applications [15,16,17,18,19,20,21]. Different studies have shown that electron-rich biphosphines (P,P) stabilize the metal center, reduce flattening distortions in the excited state, and increase emission quantum yields [16,22]. Among the most-studied commercial phosphines, DPEPhos and XantPhos stand out for their ability to form highly emissive systems [23]. The latter, XantPhos, exhibits higher luminance, efficiency, and quantum yields in iTMCs-based LEECs [24,25,26] due to its structural rigidity [27].

On the other hand, phenanthroline-based N,N ligands have played a key role in the development of iTMC-based LEECs, particularly in complexes with noble metals [28,29,30] enabling a broad range of color emissions. When used with copper, these ligands provide structural rigidity and extended aromaticity, thereby modulating visible-range emission [16]. However, solution-phase emission efficiencies of Cu(I) complexes remain limited by structural reorganization and non-radiative decay pathways [21,31,32,33,34]. Therefore, strategies to restrict molecular motion and enhance lattice rigidity are essential for improving solid-state performance [32,34,35,36,37,38,39,40]. Functionalizing the periphery of the diimine ligand has been especially effective, as substituents far from the coordination center can subtly modulate intermolecular interactions, π–π stacking, and local packing environments without drastically altering the Cu-centered electronic structure [16,33,41,42]. Among these, imidazo[4,5-f][1,10]phenanthroline (imphen) ligands have shown promise because of their strong chelating ability, increased planarity, and favorable luminescent properties [19,39,41,43].

In this context, an increasingly recognized phenomenon in Cu(I) photophysics is aggregation-induced emission (AIE), in which spin–orbit coupling predominantly drives singlet emission [44]. Unlike traditional aggregation-caused quenching (ACQ), AIE occurs when the aggregation process restricts intramolecular motions, reduces non-radiative relaxation, and thus increases radiative decay from triplet or mixed MLCT/LLCT states [44]. Several Cu(I) complexes with phenanthroline, imidazo-phenanthroline, or Schiff-base ligands have exhibited this effect, often with red-shifted and stronger emission as aggregation increases or when forming microcrystalline phases [45,46,47]. The phenomenon is closely linked to the rigidification-induced stabilization of ³MLCT or ³LLCT states, providing a mechanistic link between TADF and solid-state phosphorescence. Understanding and controlling AIE is therefore vital for the rational development of highly emissive Cu(I)-based LEEC materials and thin-film devices.

The present study examines this interaction through a series of three new heteroleptic complexes of the type [Cu(N,N)(XantPhos)]PF₆, where N,N represents 4-phenyl-substituted imidazo[4,5-f][1,10]phenanthroline ligands with hydrogen (C1), methyl (C2), or fluoro (C3) groups. These modifications were designed to introduce only subtle perturbations at the ligand periphery while maintaining the Cu(I) coordination environment. These complexes were studied electrochemically, spectroscopically, and photophysically to evaluate how weak electronic and intermolecular effects arising from remote functionalization influence excited-state properties in both solution and solid state.

2. Results and Discussion

2.1. Synthesis of Heteroleptic Cu(I) Complexes

Complexes C1–3, with the general formula [Cu(N,N)(XantPhos)]PF₆, were synthesized using a previously reported methodology [40,42,48]. This process involved reacting equimolar amounts of the copper(I) precursor [Cu(CH₃CN)₄]PF₆ with XantPhos under inert conditions to form the intermediate [Cu(XantPhos)(CH₃CN)₂]PF₆, followed by coordination with the corresponding phenanthroline-based N,N ligands L1–3 (Scheme 1). These ligands differ in the substituent at the distant phenyl ring on the imidazo[4,5-f][1,10]phenanthroline core: hydrogen (L1), methyl (L2), or fluoro (L3). All complexes were obtained as yellow powders in yields of 82–95%.

Scheme 1. Synthetic route to obtain complexes C1–3.

Complexes C1–3 were structurally characterized using spectroscopic and spectrometric methods. The ¹H NMR spectra exhibit characteristic aromatic signals for the phenanthroline/imidazo framework in the range of δ ≈ 6.8–9.5 ppm, which aligns with the highly conjugated ligand environment. The presence of the XantPhos moiety is supported by resonances in the aromatic region (ca. 6.6–8.1 ppm) and the distinct singlet at δ ≈ 2.0 ppm for the methyl groups on the xanthene scaffold. In the ³¹P NMR spectra, a single broad resonance appears around −11 to −12 ppm, typical of heteroleptic Cu(I) complexes with chelating phosphines, while the PF_6⁻ anion produces a heptet at approximately −143 to −144 ppm (J^P–F ≈ 713 Hz) in the same spectrum. Moreover, the ¹⁹F NMR resonance near −72 ppm confirms the presence of PF_6⁻. When L3 (containing a fluorophenyl group) is utilized, an additional signal at −110.30 ppm is observed, attributed to the fluorine substituent on the ligand. High-resolution mass spectrometry (HRMS) analyses validate the composition of the mononuclear Cu(I) complexes, with the general formula [Cu(N,N)(XantPhos)]⁺, and the [M]⁺ peaks match well with the calculated isotopic distributions. For more details on the NMR assignments, see the Supplementary Materials.

Furthermore, several single crystals of complex C3 were measured on multiple occasions, but none yielded diffraction data with acceptable statistical parameters R and wR2. Nevertheless, the structural model shown in Figure S16 is consistent with the experimental NMR, IR, and HRMS data, confirming that the studied species corresponds to a mononuclear heteroleptic copper(I) complex.

2.2. Electrochemical Measurements

Electrochemical measurements of complexes (C1–3) were performed using cyclic voltammetry at a scan rate of 0.1 V s⁻¹ in dry dichloromethane at 1 mmol L⁻¹, with 0.1 mol L⁻¹ TBAPF₆ as the supporting electrolyte. The selected data from these measurements are summarized in Table 1, and the voltammograms are shown in Figure S17. All three complexes, C1–3, exhibit an irreversible oxidation process occurring between E_pa values of 1.48 and 1.52 V versus SCE (see Table 1). The first oxidation is attributed to the Cu⁺/Cu²⁺ redox process. It is generally irreversible for heteroleptic copper diphosphine-diimine complexes. The trend in E_pa values is C2 > C1 > C3, aligning with the electronic effects of the substituents, where the lower potential corresponds to the electron-donor (-CH₃) and the higher potential to the electron-withdrawing substituents (-F). During oxidation, the complex geometry shifts from tetrahedral to square-planar. A higher E_pa for the complexes indicates a more stabilized tetrahedral geometry compared to [Cu(dpq)(Xantphos)]PF₆. Additionally, the reduction wave, ranging from −0.97 to −1.05 V, is most likely associated with the N,N ligand [31,49,50].

Table 1. Summary of electrochemical ^a and photophysical properties of complexes C1–3.

Complex	Epa [V vs. SCE]	Epc [V vs. SCE]	ΔE [V]	${\mathit{\lambda }}_{\mathit{M}\mathit{a}\mathit{x}}^{\mathit{A}\mathit{b}\mathit{s}}$ c (ε[Lmol−1cm−1]) [nm]	CH2Cl2 at 298 K			Solid-State
					${\mathit{\lambda }}_{\mathit{M}\mathit{a}\mathit{x}}^{\mathit{E}\mathit{m}\mathit{i}}$[nm]	Φ e [%]	τ [ns]	λ f [nm]
C1	1.51 (1.08)	–1.05 (–1.51)	2.59	412 (5700)	468, 615	0.4	16.5	586
C2	1.48 (1.03)	–0.98 (–1.42)	2.45	413 (4900)	475, 626	0.4	134.3	595
C3	1.52 (1.09)	–0.97 (–1.40)	2.49	412 (4000)	473, 624	0.4	19.1	564
[Cu(dpq)(Xantphos)]PF6 b	–(0.77)	–(–1.82)	2.59					576

^a Ep values were referenced to SCE and (Fc⁺/Fc). ^b Values from ref [31]. ^c UV-Vis absorption spectra measured in anhydrous dichloromethane at concentrations of 2.93 × 10⁻⁵ M (C1), 2.71 × 10⁻⁵ M (C2), and 3.41 × 10⁻⁵ M (C3). ^d Emission spectra were recorded using solutions adjusted to an absorbance of 0.1 at λ^Exc = 412 nm for C1–3. ^e Estimated using [Cu(dmp)(DPEphos)]BF₄ in CH₂Cl₂ as a standard (ϕPLQY = 0.15) [20,51]. ^f λ^Exc = 412 nm for C1–3.

Comparing the redox potentials of complexes C1–3 with the reported [Cu(dpq)(Xantphos)]⁺ complex (see Table 1) [31], where dpq stands for pyrazino[2,3-f][1,10]phenanthroline, indicates that the Highest Occupied Molecular Orbital (HOMO) is significantly more stabilized in C1–3, while the Lowest Unoccupied Molecular Orbital (LUMO) is at a relatively lower energy level in the imidazo–phenanthroline series. However, C1–3 exhibited a similar electrochemical gap (2.45–2.59 V) compared to that of [Cu(dqp)(XantPhos)]PF₆ (≈2.6 V).

2.3. Photophysical Characterization

The photophysical properties of complexes C1–3 were measured both in solution and in the solid state. The first analysis presents the UV–Vis absorption spectra of C1–3, measured in dry dichloromethane at room temperature and shown in Figure S18; the data are summarized in Table 1. The spectra of the obtained complexes show intense high-energy bands (below ~350 nm), typically attributed to intra-ligand (π–π^*) transitions of both the phenanthroline-derived ligand and the XantPhos framework [16,26,31,52]. At lower energies, a relatively weaker but distinct absorption band appears at ~410–415 nm for all complexes. This is commonly assigned to metal-to-ligand charge-transfer (MLCT) transitions (d→π^*) [16,26,31,52]. The minor differences in absorption maxima among C1–3 (412–413 nm) suggest that the introduction of electron-donating or electron-withdrawing substituents on the distal phenyl ring of the imphen moiety exerts only a modest influence on the MLCT transitions. This is reasonable since the substituents on C2 (CH₃) or C3 (F) are spatially distant from the metal center, causing relatively small shifts in the overall ligand π^* energy levels compared to the unsubstituted C1.

To evaluate the nature of the observed electronic transitions, the UV–Vis spectra of C3 were recorded in different solvents (Figure 1). The spectra show similar profiles across solvents, with an intense absorption band centered at approximately 280 nm that is essentially independent of the solvent. In contrast, a weaker secondary band around 410 nm is observed, showing greater sensitivity to the solvent environment. This behavior can be explained by the different electronic nature of the two transitions. The intense band at 280 nm is essentially solvent-independent because it arises from a ligand-centered (π–π^*) transition, which involves minimal change in dipole moment and therefore weak interaction with the surrounding medium. In contrast, the weaker band at 410 nm shows greater solvent sensitivity, suggesting partial charge-transfer character (e.g., MLCT or LLCT), in which excitation involves a larger redistribution of electron density that is more strongly stabilized by the solvent [53,54].

Figure 1. UV-vis absorption spectra of C3 in different solvents at room temperature.

On the other hand, the emission properties of the Cu(I) complexes C1–3 were investigated in N₂-saturated dichloromethane and in the solid state (Table 1). In deaerated CH₂Cl₂ at room temperature, the three complexes show the same general emissive pattern: a weak high-energy band in the blue region (468–475 nm) and a much broader, dominant low-energy band centered in the orange-red region (615–626 nm), as shown in Figure 2A. The excited-state lifetimes were determined to be 16.5 ns for C1, 134.3 ns for C2, and 19.1 ns for C3, indicating that C2 exhibits a notably longer lifetime, although still within the nanosecond regime. All complexes show low photoluminescence quantum yields (F ≈ 0.4%). To elucidate the origin of the emission, the spectra of the free ligands L1–3 were recorded in acetonitrile, showing maxima at 429, 429, and 423 nm, respectively. The similarity between the ligand and complex emission profiles suggests that the higher-energy band arises from a ligand-centered (LC) transition associated with the N,N ligand.

Figure 2. Emission spectra of C1–3 in (A) anhydrous N₂-saturated CH₂Cl₂ and (B) solid-state.

Conversely, the broad band observed above 600 nm is absent in the spectra of the free ligands, supporting its assignment to a metal-to-ligand charge transfer (MLCT) emission. This interpretation is consistent with previous reports on heteroleptic Cu(I) complexes featuring diimine and bisphosphine ligands [16,21,26,55].

Furthermore, both emission bands show a significant decrease in intensity under air-equilibrated conditions (Figures S19–S21), indicating that both the high-energy (LC/ILCT) and low-energy (MLCT) emissions involve triplet-character states (³LC/³ILCT and ³MLCT). Notably, complex C1 exhibits greater oxygen sensitivity in the high-energy band, suggesting differences in excited-state dynamics across the series. These results support that the observed dual emission arises from a manifold of coupled excited states rather than from purely singlet transitions.

In the solid state, complexes C1–3 display a single, significantly more intense band centered at 564–595 nm (Figure 2B). The close alignment with the benchmark ³MLCT emitter [Cu(Phen)(DPEPhos)]PF₆ strongly suggests that the emissive state is a ³MLCT. The increase in intensity is typical of a rigidochromic effect: crystal packing limits the flattening of the Cu(I) coordination sphere and reduces thermally activated non-radiative decay. Extra π–π interactions between neighboring N,N ligands, observed in related Cu(I)–phenanthroline lattices, likely help contribute to this suppression [46].

Among the series, C3 is the brightest solid-state emitter, with a maximum at 564 nm, slightly blue-shifted relative to C1 (586 nm) and C2 (595 nm). This trend is attributed to the para-fluorine substituent on the phenyl ring of the imidazo[4,5-f][1,10]phenanthroline ligand. Weak C–H···F hydrogen bonds and dipolar interactions with adjacent H-N moieties may tighten lattice packing, damp vibrational relaxation, and further amplify emission [56,57,58].

To gain deeper insight into the emissive properties, the emission spectra of C3 were recorded in acetonitrile under both air-equilibrated and N₂ atmospheres at 293 K, as well as in acetonitrile at 218 K under N₂ and in an EtOH/MeOH (4:1) glassy matrix at 77 K, as shown in Figure 3 and Figure S22. The comparison of spectra recorded in CH₂Cl₂ and CH₃CN (Figure 3) confirms the dual emission of C3, with a higher-energy, relatively structured band and a broader, lower-energy contribution. Here, the solvent modulates the energy balance between these two emissive states, which differ in polarity and degree of structural relaxation. In CH₂Cl₂, the lower-energy emission band remains clearly observable, whereas in CH₃CN, its contribution is significantly reduced, resulting in a spectrum dominated by the higher-energy emission band. This behavior is consistent with the coexistence of a higher-energy state, likely with greater ligand-centered (LC) character, and a lower-energy, more polarized and structurally relaxed state, characteristic of MLCT emission. Additionally, the spectra in acetonitrile under N₂ and air show the same spectral shape (Figure S22), with a significant decrease in emission intensity under oxygen. This observation is consistent with the results obtained using CH₂Cl₂, supporting the conclusion that the lower-energy emission band has a triplet component.

Figure 3. Normalized emission spectra of C3 recorded under different conditions.

Furthermore, upon cooling either the acetonitrile solution from 293 to 218 K or using EtOH/MeOH (4:1) glassy matrix at 77 K (Figure 3), the emission profile undergoes a pronounced transformation, evolving from a spectrum with a significant high-energy contribution to one dominated by a broad, strongly red-shifted band at 590 nm. This behavior is consistent with a thermally controlled redistribution of the excited-state population, in which emission from a lower-energy, more relaxed MLCT state becomes predominant at low temperature. These temperature-dependent spectral changes may indicate thermally activated processes involving emissive triplet states, potentially consistent with TADF-like behavior in C3. At 218 K and 77 K, the available thermal energy is insufficient to enable T₁→S₁ up-conversion, resulting instead in a more bathochromically shifted ³MLCT emission.

2.4. Aggregation-Induced Emission Evaluation

To verify that the enhanced solid-state emission is not solely due to MLCT stabilization, aggregate formation was investigated using solvent-mixture experiments. Emission spectra of several solutions with varying ratios of a good solvent (dichloromethane) and a poor solvent (hexane) were measured for complex C3 at a concentration of 9.1 × 10⁻⁵ M. As shown in Figure 4A, the systematic increase in the hexane proportion leaves the ~450 nm band essentially unchanged, while the low-energy band (~630 nm) grows in intensity. However, at hexane fractions above 60%, emission decreases. This behavior is better illustrated in Figure 4B, which shows the relationship between the maximum emission intensity and the proportions of the solvent mixture. The tendency toward increased maximum emission intensity in C3 supports an aggregation-induced emission (AIE) mechanism. The increase in intensity is not merely a refractive-index effect; it correlates with aggregate formation that restricts intramolecular motion, explaining why emission is much stronger in films or powders than in solution. The latter decrease in emission intensity is attributable to excessive aggregation, which leads to self-quenching and precipitation of the complex. This behavior aligns with previous studies on analogous Cu(I) complexes [39,45,46,47,59].

Figure 4. (A) Emission spectra of C3 in various CH₂Cl₂/hexane mixture proportions. (B) Maximum emission intensity for each solvent mixture proportion.

3. Materials and Methods

3.1. General Information

Tetrakis(acetonitrile)copper(I) hexafluorophosphate, 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (XantPhos), and 1,10-phenanthroline (phen) were purchased from commercial sources and were used without further purification. Solvents (dichloromethane, diethyl ether, acetonitrile, hexanes, and toluene) were obtained from commercial sources and dried with activated molecular sieves. Ligands L1–3 were synthesized according to the reported procedure [60,61,62]. NMR spectra were recorded on a Bruker AVANCE 400 instrument (Bruker, Billerica, MA, USA). Chemical shifts are given in parts per million relative to TMS [¹H and ¹³C, δ(SiMe₄) = 0.0] or an external standard [δ(CFCl₃) = 0.0 for ¹⁹F NMR and δ(H₃PO₄) = 0.0 for ³¹P NMR]. Additional 2D experiments supported most NMR assignments. FT-IR spectra were recorded on a Shimadzu IRTracer-100 Spectrophotometer (Shimadzu, Kyoto, Japan), using potassium bromide (KBr) as the solid matrix. High-resolution mass spectra were obtained using a ThermoFisher Scientific Exactive™ Plus Orbitrap Spectrometer (ThermoFisher Scientific, Bremen, Germany). UV-Vis absorption spectra were recorded using a Shimadzu UV-1900i spectrophotometer (Shimadzu, Kyoto, Japan). Electrochemical measurements were performed using a PalmSens 3 Instrument Potentiostat (PalmSens, Utrecht, The Netherlands) in a typical three-electrode electrochemical cell, with a Pt wire as the counter electrode, a saturated Ag/AgCl reference electrode, and a glassy carbon working electrode (0.196 cm²). The supporting electrolyte was 0.1 mol L⁻¹ tetrabutylammonium hexafluorophosphate (TBAPF₆) in anhydrous CH₂Cl₂. The complex concentration was 1 × 10⁻³ mol L⁻¹ [40,48]. Emission spectra were recorded on a Shimadzu RF-6000 Spectrophotometer (Shimadzu, Kyoto, Japan). The 77 K luminescence measurements were performed by freezing alcoholic solutions (methanol/ethanol, 4:1) and recording the spectra on a Jasco FP-8550 spectrofluorometer (Jasco, Tokyo, Japan).

3.2. General Synthesis of Heteroleptic Cu(I) Complexes

In a sealed, nitrogen-purged vial, one equiv. of [Cu(CH₃CN)₄]PF₆ was dissolved in anhydrous acetonitrile. In another nitrogen-purged vial, one equiv. of XantPhos was dissolved in anhydrous dichloromethane. Once both solutions were homogeneous, the XantPhos solution was added dropwise to the [Cu(CH₃CN)₄]PF₆ precursor solution and stirred for 10 min under a nitrogen atmosphere at room temperature to form the [Cu(XantPhos)(CH₃CN)₂]PF₆ intermediate. In a separate, sealed, and nitrogen-purged vial, one equiv. of the respective ligand (L1–3), and 5 mL of anhydrous dichloromethane were added, forming a suspension of the ligand. Afterward, the intermediate [Cu(XantPhos)(CH₃CN)₂]PF₆ in dichloromethane/acetonitrile solution was added to the ligand suspension and kept under a nitrogen atmosphere with constant stirring for 1 h to form the corresponding complex C1–3. Then, the volatiles were removed under reduced pressure, and the crude product was recrystallized using a mixture of dichloromethane and toluene at a low temperature (−20 °C).

Complex C1. Yellow powder, 95% yield (343 mg, 316 mmol). ¹H NMR (400 MHz, CDCl₃, 298 K): δ/ppm = 9.17 (d, J = 8.2 Hz, 2H), 8.37 (d, J = 7.7 Hz, 2H), 8.27 (br., 2H), 7.69 (d, J = 7.7 Hz, 2H), 7.55 (dd, J = 8.3, 4.7 Hz, 2H), 7.37 (t, J = 7.7 Hz, 2H), 7.28–7.24 (m, 1H), 7.17 (dd, J = 8.2, 5.7 Hz, 8H), 7.03 (t, J = 7.5 Hz, 8H), 6.92 (q, J = 6.1 Hz, 8H), 6.60 (dt, J = 7.8, 3.8 Hz, 2H), 1.80 (s, 6H). H₂₄ not observed.¹³C{¹H} NMR (100 MHz, CDCl₃, 298 K): δ/ppm = 155.10 (t, J^C–P = 6.4 Hz), 153.01, 146.70, 141.17, 137.98, 134.06 (t, J^C–P = 1.9 Hz), 132.78 (t, J^C–P = 8.0 Hz), 132.09, 131.55 (t, J^C–P = 17.1 Hz), 131.28, 130.13, 130.01, 129.08, 129.01, 128.80 (t, J^C–P = 4.8 Hz), 127.32, 127.05, 125.27 (t, J^C–P = 2.7 Hz), 125.00, 123.12 (br.), 119.96 (t, J^C–P = 13.6 Hz), 36.31, 28.31. ¹⁹F NMR (400 MHz, CDCl₃, 298 K): δ/ppm = –72.04 (d, J^F-P = 713.3 Hz). ³¹P{¹H} NMR (160 MHz, CDCl₃, 298 K): δ/ppm = −11.75 (s), −143.66 (hept, J^P-F = 713.6 Hz). FT-IR (KBr, selected characteristic bands): ν/cm⁻¹ = 3421.01, 1477.47, 1404.17, 1230.20, 840.98. HRMS(ESI): (C₅₈H₄₄CuN₄OP₂ [M]⁺) = Calculated 937.2286, Found 937.2283.

Complex C2. Yellow powder, 82% yield (306 mg, 278 mmol). ¹H NMR (400 MHz, CDCl₃, 298 K): δ/ppm = 11.74 (br., 1H), 9.18–9.07 (m, 2H), 8.42 (br., 1H), 8.24 (d, J = 7.6 Hz, 2H), 8.15 (br., 1H), 7.69 (dd, J = 7.8, 1.5 Hz, 2H), 7.60 (br., 2H), 7.31 (d, J = 7.4 Hz, 2H), 7.18–7.14 (m, 6H), 7.02 (br., 8H), 6.96 (br., 4H), 6.88 (br., 4H), 6.59 (qd, J = 4.5, 1.3 Hz, 2H), 2.37 (s, 3H), 1.83 (s, 3H), 1.78 (s, 3H). ¹³C{¹H} NMR (100 MHz, CDCl₃, 298 K): δ/ppm = 155.11 (t, J^C-P = 6.3 H), 153.44, 146.76, 146.40, 141.37, 140.82, 140.53, 136.47, 134.06, 132.78, 132.03, 131.78, 131.56 (t, J^C-P = 17.2 Hz), 131.30, 130.02, 129.94, 128.80 (t, J^C-P = 4.7 Hz), 127.30, 126.84, 126.44, 125.27, 125.13, 124.73, 121.01, 120.04 (t, J^C-P = 13.4 Hz), 36.32, 28.80, 27.82, 21.58. ¹⁹F NMR (400 MHz, CDCl₃, 298 K): δ/ppm = −71.67 (d, J^F-P = 714.0 Hz). ³¹P{¹H} NMR (160 MHz, CDCl₃, 298 K): δ/ppm = −11.75 (s), 144.70j (hept, J^P-F = 713.7 Hz). FT-IR (KBr, selected characteristic bands): ν/cm⁻¹ = 3421.72, 1509.21, 1403.29, 1226.60, 846.01. HRMS(ESI): (C₅₉H₄₆CuN₄OP₂ [M]⁺) = Calculated 951.2443, Found 951.2438.

Complex C3. Yellow powder, 91% yield (421 mg, 382 mmol). ¹H NMR (400 MHz, CDCl₃, 298 K): δ/ppm = 11.66 (br., 1H), 9.13 (dd, J = 31.4, 8.4 Hz, 2H), 8.64–8.02 (m, 4H), 7.72 (d, J = 7.7 Hz, 2H), 7.63 (dt, J = 7.9, 3.7 Hz, 2H), 7.30–7.13 (m, 8H), 7.10–6.85 (m, 16H), 6.67–6.55 (m, 2H), 1.85 (s, 3H), 1.81 (s, 3H). ¹³C{¹H} NMR (100 MHz, CDCl₃, 298 K): δ/ppm = 163.93 (d, J^C–F = 250.4 Hz), 155.02 (t, J^C–P = 6.3 Hz), 151.94, 147.05, 146.55, 141.31, 140.94, 136.43, 134.03, 132.69 (dd, J^C–P = 15.5, 7.9 Hz), 131.61, 131.34 (t, J^C–P = 17.2 Hz), 131.17, 129.95, 128.97 (d, J^C–F = 8.6 Hz), 128.74, 127.32, 126.47, 125.20, 125.02, 120.78, 119.80 (t, J^C–P = 13.7 Hz), 116.10 (d, J^C–F = 21.9 Hz), 36.24, 28.61, 27.81. ¹⁹F NMR (400 MHz, CDCl₃, 298 K): δ/ppm = –71.54 (d, J^F-P = 713.6 Hz), –110.30 (s). ³¹P{¹H} NMR (160 MHz, CDCl₃, 298 K): δ/ppm = −11.77 (s), −141.43 (hept, J^P-F = 713.8 Hz). FT-IR (KBr, selected characteristic bands): ν/cm⁻¹ = 3356.14, 1481.33, 1404.17, 1222.87, 844.82. HRMS(ESI): (C₅₈H₄₃CuFN₄OP₂ [M]⁺) = Calculated 955.2192, Found 955.2187.

4. Conclusions

Three new heteroleptic copper(I) complexes of the type [Cu(N,N)(XantPhos)]PF₆, containing phenyl-substituted imidazo[4,5-f][1,10]phenanthroline ligands, were successfully synthesized and characterized. Spectroscopic, electrochemical, and photophysical analyses show that subtle electronic changes induced by remote substituents (H, CH₃, F) on the phenyl ring have measurable effects on photoluminescence behavior without significantly perturbing the electronic structure at the metal center.

In solution at 293 K, a weak emission is observed, consisting of a higher-energy ligand-centered (LC) band and a lower-energy MLCT contribution, the latter of which is more quenched by oxygen, confirming the predominantly triplet character of the MLCT. Solvent-dependent measurements show that the relative contribution of these bands is modulated by the medium, reflecting differences in polarity and structural relaxation.

Temperature-dependent studies further support a redistribution of the excited-state population, in which a more relaxed, red-shifted ³MLCT state becomes dominant at low temperatures, whereas at room temperature a higher-energy contribution remains significant, consistent with thermally activated processes. In contrast, the solid state exhibits intense emission due to restricted molecular motion and suppressed non-radiative decay, thereby stabilizing the ³MLCT state. Additionally, solvent-mixture experiments confirm aggregation-induced emission (AIE), where controlled aggregation enhances emission by limiting intramolecular dynamics.

These results show that peripheral ligand functionalization is an effective means of tuning the emissive properties of Cu(I) complexes in the solid state, providing useful design guidance for the development of next-generation copper-based light-emitting materials.