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Article

Design, Synthesis, and Characterization of Novel Phosphorescent Iridium Complexes with Pyrone Auxiliary Ligands and ppy/dfppy/piq Cyclometalating Ligands

by
Wen Jiang
1,*,
Leyuan Wang
1,
Xiangguang Li
1,
Caixian Yan
2 and
Qiaowen Chang
2
1
Yunnan Key Laboratory of Metal-Organic Molecular Materials and Device, School of Chemistry and Chemical Engineering, Kunming University, Kunming 650214, China
2
Yunnan Precious Metals Laboratory Co., Ltd., Kunming 650106, China
*
Author to whom correspondence should be addressed.
Inorganics 2026, 14(4), 95; https://doi.org/10.3390/inorganics14040095
Submission received: 19 November 2025 / Revised: 24 March 2026 / Accepted: 26 March 2026 / Published: 27 March 2026
(This article belongs to the Section Coordination Chemistry)
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Abstract

To develop high-performance iridium phosphorescent complexes, we designed and synthesized a series of iridium phosphorescent complexes (G-1, G-2, B-1, B-2, R-1, R-2) using 3-hydroxy-2-methyl-4-pyrone (maltol, short for mal) and 3-hydroxy-2-ethyl-4-pyrone (ethyl maltol, short for emal) as auxiliary ligands, in combination with 2-phenylpyridine (ppy), 2-(2,4-difluorophenyl)pyridine (dfppy), and 1-phenylisoquinoline (piq) as cyclometalating ligands. We systematically investigated their crystal structures, photophysical behavior, electrochemical properties, and electroluminescent performance. The results revealed that the combination of a pyranone auxiliary ligand with the highly conjugated piq ligand leads to the formation of R-1 and R-2, which possess high molecular symmetry and display favorable photophysical performance. These complexes exhibit solution-phase phosphorescence quantum yields of 64% and 55%, and electroluminescent devices incorporating them reach a maximum external quantum efficiency of 13.4%, with brightness exceeding 13,000 cd/m2 and minimal efficiency roll-off. In contrast, complexes incorporating pyridine-based cyclometalating ligands (ppy, dfppy)—G-1, G-2, B-1, and B-2—display weak emission in solution but show enhanced solid-state emission through π–π stacking, with a maximum quantum yield of 25.8%. Density functional theory calculations and electrochemical analysis indicate that the presence of both the pyranone auxiliary ligand and the piq ligand results in optimized frontier orbital energy alignment, enhanced metal-to-ligand charge transfer, and reduced non-radiative transitions, thereby improving emission efficiency. This study provides a theoretical framework and molecular design strategy for the application of pyranone auxiliary ligands in high-performance iridium phosphorescent materials.

1. Introduction

Phosphorescent iridium complexes have demonstrated promising application potential in fields such as organic light-emitting diode (OLED) technology, bioimaging, and photocatalysis, owing to their high quantum yields, long excited-state lifetimes, and tunable emission wavelengths [1,2,3,4]. Their luminescent behavior is largely determined by the electronic structure and spatial arrangement of the central metal ion and surrounding ligands, with the design of macrocyclic metal (primary) and auxiliary ligands playing a crucial role in modulating photophysical characteristics [5,6,7,8]. Notably, among iridium complexes, neutral derivatives often exhibit distinct advantages over ionic counterparts in terms of photophysical performance and device compatibility. For instance, neutral iridium(III) complexes typically show higher emission quantum yields in both solution and solid states, with more tunable emission energies spanning from sky-blue to near-infrared, compared to ionic iridium(III) complexes which may suffer from aggregation-induced quenching or limited charge transport efficiency [2,9]. Additionally, when compared to gold(III) complexes—another class of phosphorescent metal complexes with emission in similar wavelength ranges—neutral iridium(III) complexes often display longer excited-state lifetimes and better stability under operational conditions, making them more suitable for long-term device applications [10]. These advantages motivate our focus on neutral iridium complexes in this study, as we aim to further optimize their performance through rational ligand design.
Recent studies have aimed to enhance luminescent efficiency by modifying the conjugation systems of primary ligands, such as 2-phenylpyridine and quinoline derivatives, or by introducing various substituents [11,12,13,14]. For instance, incorporating electron-withdrawing groups such as fluorine into the primary ligands can adjust molecular orbital energy levels, promote π–π interactions between ligands, and thereby improve luminescence [15,16]. In addition to their electronic effects, the spatial positioning of substituents notably impacts both the configuration and photophysical properties of the complexes, ultimately modifying their molecular symmetry and luminescent efficiency [17,18]. However, the synergistic influence of auxiliary ligands on overall complex performance remains insufficiently explored.
Pyrone compounds, a class of heterocyclic molecules featuring extended conjugation and strong coordination ability, can form stable chelating structures with metal ions through their carbonyl oxygen atoms. Additionally, intramolecular π–π stacking may suppress non-radiative transitions, thereby enhancing luminescence efficiency. Recent studies indicate that pairing highly conjugated auxiliary ligands with primary ligands can improve molecular-orbital-energy-level alignment, minimize non-radiative decay pathways in the excited state, and enhance quantum yields [19,20,21]. For instance, combining a pyranone-type auxiliary ligand with a quinoline-type main ligand improves both their electronic and spatial compatibility. This alignment facilitates energy transfer and promotes luminescence, enabling the complex to achieve a high phosphorescence quantum yield in solution. However, the compatibility of pyranone auxiliary ligands with primary ligands of varying conjugation remains underexplored, and the mechanisms by which they influence molecular symmetry, frontier orbital distribution, and solid-state luminescent behavior remain unclear. Furthermore, numerous iridium complexes based on pyridine-type primary ligands frequently suffer from aggregation-induced quenching in the solid state, resulting in diminished luminescent performance [22,23,24]. Moreover, whether pyranone auxiliary ligands can enhance solid-state emission performance (EPESS) by modulating molecular packing remains to be determined.
Against this background, in the present study, a series of phosphorescent iridium complexes were designed and synthesized using pyranones as auxiliary ligands and primary ligands with different conjugation degrees—2-phenylpyridine (ppy), 2-(2,4-difluorophenyl)pyridine (dfppy), and 1-phenylisoquinoline (piq). The effects of ligand conjugation, molecular symmetry, and electronic properties on the crystal structure, photophysical behavior, electrochemical characteristics, and electroluminescent performance of the complexes were systematically examined. Using X-ray single-crystal diffraction, density functional theory (DFT) calculations, steady-state and transient spectroscopy, and OLED device measurements, the study elucidated the synergistic interaction between pyranone auxiliary ligands and the auxiliary ligands. These resulting insights provide a design foundation for developing high-performance phosphorescent iridium materials. Our research team previously reported the preliminary device performance of red phosphorescent iridium complexes with pyranone auxiliary ligand and piq main ligand [25], and the basic photophysical properties of green phosphorescent iridium complexes with pyranone auxiliary ligand and ppy main ligand [26]. To systematically reveal the regulation law of main ligand structure on the performance of pyranone-based iridium complexes, this work synthesized a total of 6 complexes containing three types of main ligands: ppy, dfppy, and piq. Through single crystal structure analysis, theoretical calculations, photoelectric performance testing and other methods, this work elucidates for the first time the intrinsic correlation between the conjugation degree of the main ligand, molecular symmetry and device performance, providing general guidance for the design of high-efficiency phosphorescent iridium complexes.

2. Results and Discussion

2.1. Synthesis of the Target Products

A series of neutral iridium(III) complexes were synthesized via a two-step procedure:
  • Dimer Formation: Iridium(III) chloride was reacted with cyclometalating ligands (2-phenylpyridine, 2-(2,4-difluorophenyl)pyridine, or 1-phenylisoquinoline) in a 1:2.2 molar ratio under argon atmosphere to form chloro-bridged dimers. The specific synthetic route is illustrated in Figure 1.
  • Complexation with Pyranone Ligands: The resulting dimers were further reacted with 3-hydroxy-2-methyl-4-pyrone or 3-hydroxy-2-ethyl-4-pyrone in the presence of sodium carbonate, yielding six target complexes (G-1, G-2, B-1, B-2, R-1, R-2). The synthesis methods of G-series and R-series complexes refer to the optimized steps we reported previously [25,26]. The B-series complexes are designed and synthesized for the first time, and the specific synthesis steps are shown in the experimental section. The molecular structures and corresponding sample labels are presented in Figure 2.
All reactions were performed under reflux conditions using ethylene glycol monoethyl ether as solvent. Detailed synthetic procedures, including reagent quantities and purification methods, are provided in Section 3.1.

2.2. Crystal Structures

Figure 3 displays the crystal structures of the target complexes G-1, B-1, and R-1, which are phosphorescent iridium complexes containing 3-hydroxy-2-methyl-4-pyrone as the auxiliary ligand. As depicted in the figure, the single-crystal structures of all three complexes align with the target complex molecular configuration. The central iridium atom (Ir(III)) forms a neutral octahedral complex in a six-coordinate environment. Two five-membered chelate rings are formed by the N and C atoms of the pyridine and quinoline moieties. These are further coordinated to the O atoms of the two carbonyl groups in the pyranone, forming an additional stable five-membered chelate ring. The coordination angles range from approximately 76–79° (e.g., O–Ir–O angles: 76.9(9)° for G-1, 78.0(5)° for B-1, 78.4(3)° for R-1).
Crystallographic data for these complexes are provided in Table 1 and Table 2. The average Ir–N and Ir–C bond lengths associated with the macrocyclic ligands are shorter than the Ir–O bond lengths involving the auxiliary ligand, indicating stronger coordination between the macrocyclic ligands and the central Ir atom. Specifically:
  • G-1: Ir–C bond lengths are 2.036(13) Å and1.980(16) Å, Ir–N lengths are2.012(16) Å and1.910(4) Å, while Ir–O lengths are2.180(2) Å and2.249(2) Å;
  • B-1: Ir–C (1.995(11) Å), Ir–N (2.049(9) Å) are shorter than Ir–O (2.163(8) Å);
  • R-1: Ir–C (1.972(8) Å), Ir–N (2.040(6) Å) are shorter than Ir–O (2.148(6) Å).
The structural data indicate that the Ir–C, Ir–N, and Ir–O bond lengths in B-1 and R-1 are nearly identical, whereas G-1 exhibits noticeable variations in the corresponding bond lengths. For instance:
  • G-1 displays varying lengths: Ir–C (2.036(13) vs. 1.980(16) Å), Ir–N (2.012(16) vs. 1.910(4) Å), Ir–O (2.180(2) vs. 2.249(2) Å);
  • B-1 shows consistent bond lengths across both ligands: Ir–C (1.995(11) Å), Ir–N (2.049(9) Å), Ir–O (2.163(8) Å);
  • R-1 also has uniform values: Ir–C (1.972(8) Å), Ir–N (2.040(6) Å), Ir–O (2.148(6) Å).
Similarly, the C–Ir–O, N–Ir–O, and C–Ir–N bond angles remain consistent in B-1 and R-1, while G-1 displays deviations in the C–Ir–O and N–Ir–O angles.
  • G-1 exhibits a range of angles: C–Ir–O from 88.7(8)° to 171.6(10)°, N–Ir–O from 90.4(13)° to 169.7(9)°, C–Ir–N from 80.0(16)° to 98.6(12)°;
  • B-1 has consistent angles: C–Ir–O (173.0(4)°), N–Ir–O (93.9(4)°), C–Ir–N (96.4(4)°);
  • R-1 shows uniform angles: C–Ir–O (174.67(17)°), N–Ir–O (96.48(19)°), C–Ir–N (80.1(2)°).
These observations suggest that B-1 and R-1 possess higher molecular symmetry, whereas G-1 exhibits some degree of geometric distortion. In phosphorescent iridium complexes, such symmetry is predominantly dictated by the ligand architecture. Differences in the primary ligands or their substituents can influence the spatial configuration of the complexes, thereby altering molecular symmetry [27,28,29,30]. Moreover, the extent of conjugation and the steric hindrance of the ligands can impose geometric constraints on the coordination environment. Compared with ppy, the primary ligand in G-1, both dfppy and piq, employed in B-1 and R-1, respectively, exhibit greater conjugation. When combined with similarly conjugated auxiliary ligands, these ligands promote more symmetric molecular arrangements. Such enhanced symmetry may reduce non-radiative decay in the excited state, thereby improving quantum yield. Additionally, a more symmetric iridium complex may lead to greater luminescence efficiency owing to a more stable coordination environment [1,31]. The crystal structures of the target complexes G-2, B-2, and R-2 with the similar crystallographic data and rules are shown in Supplementary Materials.
Further analysis of the crystal structures reveals key insights into structural disparities and stability. The non-uniform Ir-C/Ir-N/Ir-O bonds in G-1 stem from the limited conjugation of its cyclometalating ligand ppy, poor spatial compatibility with the pyranone auxiliary ligand, and absence of electron-withdrawing groups, leading to geometric distortion and bond length fluctuations. By contrast, B-1’ s dfppy (with fluorine atoms enhancing conjugation) and R-1’ s piq (with an extended conjugated structure) exhibit higher conjugation degrees and better spatial matching with the auxiliary ligand. Additionally, the fluorine atoms in dfppy optimize electron distribution to stabilize coordination bonds, forming a symmetric coordination environment with more uniform bond lengths. Quantitative data confirm that B-1/R-1 have identical paired bonds/angles (e.g., both Ir-C bonds in B-1 are 1.995 Å) and high-symmetry space groups (Pccn for B-1, C2/c for R-1), whereas G-1 uses the lower-symmetry P21/c space group with distorted angles. Furthermore, intermolecular π–π stacking between aromatic ligands, C-H…O/F hydrogen bonds (e.g., those involving fluorine in B-1), and van der Waals forces stabilize crystal packing, enhancing lattice rigidity and solid-state structural integrity. These findings integrate the mechanisms of bond length differences, quantitative evidence of symmetry patterns, and contributions of weak interactions to crystal stability, refining the description of structural features of the target complexes.

2.3. Photophysical Properties

Figure 4 presents the ultraviolet-visible (UV–Vis) absorption and photoluminescence (PL) spectra of the six phosphorescent iridium complexes in dichloromethane at room temperature, and the corresponding photophysical parameters are listed in Table 3. These complexes exhibit similar absorption profiles, comprising singlet and triplet π–π* and metal-to-ligand charge transfer (MLCT) transitions. The intense absorption band below 360 nm is attributed to spin-allowed ligand-centered 1(π–π*) transitions, which closely resemble those of the main ligand. In contrast, the weaker and broader absorption band in the 360–450 nm region arises from singlet 1MLCT and triplet 3MLCT and 3(π–π*) transitions. Owing to the heavy atom effect of the iridium center, strong spin–orbit coupling facilitates the occurrence of otherwise spin-forbidden 3MLCT transitions. The UV-Vis measurements were performed in dichloromethane (CH2Cl2), a moderately polar solvent widely used for photophysical characterization of iridium complexes. Solvent polarity primarily modulates the metal-to-ligand charge transfer (MLCT) bands: polar solvents stabilize the charge-separated excited state of MLCT, leading to a slight red shift compared to non-polar solvents [32,33]. In our study, the intense bands below 360 nm (ligand-centered π–π* transitions) are less solvent-dependent, while the broader bands at 360–450 nm (MLCT transitions) show moderate sensitivity to CH2Cl2 polarity. However, the spectral features and transition assignments in CH2Cl2 are consistent with standard reports for similar complexes [25], ensuring the validity of our structure-property analysis. The choice of CH2Cl2 does not alter the essential spectral characteristics or the conclusion of this work.
The solution luminescence of G-series complexes is weak, which is consistent with the previous report [26]. G-1 and G-2 both exhibit a maximum emission wavelength of 492 nm in dichloromethane solution, corresponding to green emission. B-1 and B-2 display blue emission, with a maximum emission wavelength of 484 nm. The solution luminescence wavelengths of R-1 and R-2 are consistent with the results we reported previously [25]. R-1 and R-2 both emit deep red light, with a maximum emission wavelength of 640 nm. To quantify the effect of conjugation length on emission redshift, the piq ligand (1-phenylisoquinoline) has an extended conjugation system (10 π-electrons) and 6 π-electrons in ppy/dfppy. Each additional benzene ring in the conjugated backbone causes a 50 nm redshift—consistent with our observation of R complexes 150 nm redshift relative to G/B. These observations suggest that the two pyranone auxiliary ligands do not influence the emission wavelengths of the three series of target products. Additionally, compared to phosphorescent iridium complexes featuring acetylacetone as the auxiliary ligand, as reported in our group’s previous study, shifts in emission wavelengths are observed in the six current complexes. Specifically, G-1 and G-2 exhibit a 27 nm blue shift relative to the green-emitting Ir(ppy)2(acac) complex [34]. Likewise, B-1 and B-2 display an 8 nm blue shift relative to the blue-emitting Ir(dfppy)2(acac) complex (λ = 484 nm) [35]. Further, compared to the red iridium complex Ir(piq)2(acac) (λ = 622 nm) [36] and the widely used commercial complex Ir(pq)2(acac) (λ = 600 nm) [37], R-1 and R-2 exhibit approximately 20 nm and 40 nm red shifts in their emission wavelengths, respectively. Among the six target products, G-1, G-2, B-1, and B-2 exhibit detectable emission; however, owing to their weak luminescence, they display emission with the PLQY less than 1%. In contrast, the iridium phosphorescent complexes R-1 and R-2 demonstrate high luminescence efficiency, with absolute phosphorescent quantum yields in solution of 64% and 55%, respectively. These values represent a considerable improvement over Ir(piq)2(acac). The high solution quantum yields of R-series complexes are consistent with the long conjugation of piq, which enhances metal-to-ligand charge transfer (MLCT) efficiency and reduces non-radiative decay; in contrast, G/B-series complexes with ppy/dfppy show low solution quantum yields due to shorter conjugation but enhanced solid-state emission via π–π stacking. The ancillary ligand steric effect is evident in R1 (methyl-substituted pyranone) exhibiting higher quantum yield than R2 (ethyl-substituted) because the smaller methyl group reduces steric hindrance, increasing molecular rigidity and suppressing non-radiative decay. Additionally, the solid-state quantum yield enhancement of G/B-series is expected as π–π stacking in solids stabilizes excited states and reduces intramolecular motion. These trends confirm the observed quantum yield variations are consistent with ligand type expectations. The above test results indicate that the pyranone-type auxiliary ligands effectively improve both luminescence purity and efficiency in the quinoline-based iridium phosphorescent complexes R-1 and R-2. However, these ligands do not notably enhance the luminescence performance of the pyridine-based complexes G-1, G-2, B-1, and B-2. This difference may arise from the larger conjugated system of the quinoline main ligands. When the lowest unoccupied molecular orbital (LUMO) energy levels of the main and auxiliary ligands are well aligned, MLCT can regulate the excited-state energy, suppress non-radiative decay, and enhance emission efficiency. In comparison, the smaller conjugated system of the pyridine ligands leads to a LUMO mismatch with the pyranone ligands, which may promote energy quenching [38,39].
These experimental photophysical trends are consistent with the DFT calculations in Section 2.3. The band gap order (B > G > R) derived from theory matches the experimental emission wavelength order (B:484 nm < G:492 nm < R:640 nm), as smaller band gaps lead to red-shifted emissions. For the R series, R-1’ s higher quantum yield (64%) than R-2 (55%) correlates with its marginally smaller calculated band gap (2.423 eV vs. 2.432 eV), driven by a higher HOMO level. Additionally, the red shift of R-1/R-2 relative to Ir(piq)2(acac) is theoretically supported by their elevated HOMO levels, which reduce band gaps and extend emission wavelengths. All six complexes exhibit similar single-exponential decay profiles, with excited-state lifetimes in the range of 0.13–0.25 μs, confirming the phosphorescent nature of their emission originating from triplet excited states. The relatively short lifetimes are particularly favorable for OLED applications, as they reduce the probability of non-radiative losses via triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA) under electrical excitation. Comparison of the lifetime values across the series shows that complexes bearing the 1-phenylisoquinoline (piq) cyclometalating ligand (R-1, R-2) exhibit slightly shorter lifetimes (0.13 μs) than those with 2-phenylpyridine (G-1, G-2: 0.21, 0.15 μs) and 2-(2,4-difluorophenyl)pyridine (dfppy) ligands (B-1, B-2: 0.16, 0.25 μs), which can be attributed to the stronger spin–orbit coupling induced by the more extended π-conjugation of the piq ligand, facilitating faster intersystem crossing and radiative decay from the triplet state.
Notably, most previously reported phosphorescent iridium complexes containing ppy, dfppy, or their derivatives as macrocyclic ligands exhibit luminescence in solution but not in the solid state [40,41,42,43] In this study, the four target complexes—G-1, G-2, B-1, and B-2—bearing ppy and dfppy as macrocyclic ligands displayed solid-state luminescence under UV light exposure. The corresponding solid-state PL emission spectra are presented in Figure 5. Specifically, the solid powders of G-1 and G-2 appear dark red and orange-red, respectively. Both exhibit maximum PL emission at 620 nm, corresponding to orange-red emission, with phosphorescence quantum yields of 3.4% and 5.5%, respectively. The solid powders of B-1 and B-2 are yellow and emit orange light, with maximum PL emission at 600 nm and phosphorescence quantum yields of 12.7% and 25.8%, respectively. The above experimental results indicate that the solid-state luminescence wavelengths of the four target products containing pyridine-based macrocyclic ligands all exceed 600 nm, displaying a clear red shift relative to their solution-phase emission. This shift may arise from pronounced π–π stacking interactions in the solid state, facilitated by the pyridine-based ligands, which stabilize the excited states and enhance the luminescence process, ultimately resulting in EPESS [8,44]. Consistent with the crystallographic analysis (Section 2.1): G-series (ppy ligands) exhibit weak π–π stacking due to their P21/c low-symmetry space group and geometric distortion, resulting in low solid-state quantum yields (3.4% for G-1). B-series (dfppy ligands) adopt the Pccn high-symmetry space group with strong C-H…F hydrogen bonds and π–π stacking, which stabilize excited states and enhance solid-state emission (up to 25.8% for B-2). The fluorine substituents in dfppy optimize intermolecular interactions, contributing to higher efficiency than G-series. R-series (piq ligands) have a C2/c high-symmetry space group with dense π–π stacking, but their high solution quantum yields (64%/55%) may lead to solid-state quenching due to excessive aggregation—this trend aligns with their crystallographic packing characteristics. These results confirm that molecular interactions (π–π stacking, hydrogen bonds) and space group symmetry directly regulate solid-state phosphorescence efficiency, as supported by crystallographic evidence.
Moreover, the solid-state quantum yields of B-1 and B-2, which feature dfppy as the main ligand, are substantially higher than those of G-1 and G-2, which are based on ppy. This disparity is likely attributed to the electron-withdrawing fluorine atoms in dfppy, which may optimize the distribution of molecular orbital energy levels and strengthen π–π interactions between ligands, thereby enhancing stacking and stabilizing the excited states [45,46,47]. In contrast, ppy may lack similar electronic effects or exhibit spatial configuration disparities, leading to less pronounced solid-state stacking and lower luminescence enhancement. R-1 and R-2 do not exhibit solid-state emission. The piq ligand in R-complexes has an extended conjugated system and larger steric bulk compared to ppy/dfppy ligands in G/B-series. This steric hindrance may suppress intermolecular π–π stacking interactions in the solid state, which are critical for stabilizing excited states and enhancing solid-state emission. The absence of π–π stacking may lead to non-radiative decay pathways (e.g., vibrational relaxation) dominating over radiative emission, resulting in quenching.

2.4. Theoretical Calculations

To examine the influence of pyranone auxiliary ligands on the frontier orbital characteristics of the target products, DFT calculations were performed to optimize the ground-state molecular structures. All calculations were carried out using Gaussian 16 software with the TPSSh functional and def2-TZVP basis set for all atoms, consistent with the methodology described in Section 3.2.3. The spatial distributions of the highest occupied molecular orbital (HOMO) and LUMO levels for the six optimized iridium phosphorescent complexes are depicted in Figure 6.
To validate the chosen theoretical level, we compared the calculated bond lengths and angles of G-1, B-1, and R-1 with their experimental crystal structure data (Section 2.1). For G-1, the calculated Ir-C bond lengths (2.02 ± 0.01 Å) align closely with the experimental values (2.036(13) Å and 1.980(16) Å), and the calculated O-Ir-O angle (77.0 ± 0.2°) matches the experimental 76.9(9)° with minimal deviation. Similar consistency was observed for B-1 (calculated Ir-C: 1.99 ± 0.01 Å vs. experimental 1.995(11) Å) and R-1 (calculated Ir-N: 2.04 ± 0.01 Å vs. experimental2.040(6) Å). These deviations (≤0.05 Å for bond lengths, ≤1° for angles) are within the acceptable range for iridium complex DFT calculations, confirming the reliability of the TPSSh/def2-TZVP framework—consistent with its application in similar systems and the validation approach [48].
The calculation results reveal that the LUMO levels of all six complexes are localized on the corresponding ring metal ligands and the central iridium atom. The high electron density on the iridium center indicates that the complexes exhibit effective MLCT, which is critical for achieving high photoluminescence quantum yield (PLQY) [49,50,51]. Notably, the introduction of pyranone auxiliary ligands does not alter the LUMO level distributions. Complexes containing the same ring metal ligands display nearly identical LUMO levels, except for R-1 and R-2, which exhibit lower LUMO levels when 1-phenylisoquinoline is used as the ring metal ligand. In contrast, the HOMO level distributions vary substantially among the six complexes. Some molecular orbitals are distributed across the ring metal ligands and the central iridium atom, while a considerable portion originates from the pyranone auxiliary ligands. This HOMO and LUMO energy level distribution suggests that the target products possess bipolar charge transport properties, allowing electrons and holes to migrate through separate pathways. In B-1 and B-2, which incorporate dfppy as the ring metal ligand, the HOMO energy level contribution from the ring metal ligands is lower than in the other four complexes, and the HOMO energy levels are also lower. This is primarily attributed to the electron-withdrawing effect of the fluorine atom on the phenyl ring of the ring metal ligand [32,52,53,54].
Based on the relationship Eg = ELUMO − EHOMO, the calculated band gaps follow the trend B > G > R, indicating that the two target products with piq as the cyclometalating ligand have the smallest band gaps. The band gap trend (B > G > R) aligns with photophysical observations indicates that R’s elevated HOMO (from pyranone auxiliary ligands) and lowered LUMO (from piq’s extended conjugation) reduce the gap, facilitating redshift. Compared with similar iridium complexes containing piq and its derivatives as cyclometalating ligands [33], the HOMO energy levels of R-1 and R-2 are notably elevated, resulting in distinctly smaller Eg values. This facilitates charge injection and inherently promotes redshifted emission, consistent with the observed photophysical properties of the target products. Because R-1 and R-2 share the same LUMO energy level and R-1 has a slightly higher HOMO energy level, its band gap (2.423 eV) is marginally smaller than that of R-2 (2.432 eV), which also accounts for R-1’s slightly higher PLQY.

2.5. Electrochemical Performance

The six target products were dissolved in dichloromethane, and their cyclic voltammograms were recorded using ferrocene as the reference, as depicted in Figure 7. Based on the onset oxidation potentials and the empirical electrochemical equation EHOMO = −e(Eonsetox + 4.4) eV [55], the HOMO energy levels were calculated. The optical band gaps (Eg) were determined from the absorption spectra using Eg = 1240/λ, and the LUMO energy levels were obtained from ELUMO = Eg + EHOMO, as indicated in Table 4. Only oxidation curves were recorded for the six compounds, while reduction curves were not obtained. All oxidation curves displayed similar trends, with reversible oxidation peaks observed between 0.7 and 1.1 V, corresponding to the Ir3+/Ir4+ redox couple at the complex center. These results indicate that the substituents on the two pyranone auxiliary ligands exert no appreciable effect on the oxidation potentials of the complexes. The HOMO energy levels across the three series follow the order R > G > B, and the band gaps follow the order B > G > R. These trends are consistent with the phosphorescence emission wavelengths and the theoretical calculations. Moreover, the variation in auxiliary ligand type has a negligible effect on the frontier orbital energy levels.
To validate the electrochemical trends, we compared our data with reported analogous iridium complexes. For G-1 (HOMO = −4.94 eV), its HOMO level is ~0.16 eV higher than that of Ir(ppy)2(acac) (−5.10 eV) [29], facilitating easier hole injection. B-1 (HOMO = −5.08 eV) shows a ~0.12 eV higher HOMO than Ir(dfppy)2(acac) (−5.20 eV) [30], consistent with the electron-withdrawing effect of fluorine atoms in dfppy. For R-1 (HOMO = −4.90 eV, band gap = 2.15 eV), its HOMO is ~0.10 eV higher and band gap ~0.15 eV smaller than Ir(piq)2(acac) (−5.00 eV, ~2.3 eV) [31], which explains its red-shifted emission and enhanced charge injection. These comparisons confirm that pyranone auxiliary ligands effectively tune frontier orbital levels, aligning with the design strategy of optimizing ligand conjugation and energy alignment.

2.6. Electroluminescence Devices

To investigate the electroluminescent performance of the target compounds, single-color phosphorescent OLED (PhOLED) devices were fabricated using B-2, R-1, and R-2 at their optimal doping concentrations, as these complexes exhibit favorable photoluminescent properties. The architectures of the three devices and the energy levels of their functional layers are illustrated in Figure 8. The device configuration for B-2 is as follows: ITO/HAT-CN (3 nm)/NPB (40 nm)/TCTA (10 nm)/TCTA: (8% B-2) (15 nm)/TmPyPB (50 nm)/Liq (1 nm)/Al (100 nm). In this structure, indium tin oxide (ITO) serves as the anode; 2,3,6,7,10,11-hexacyanido-1,4,5,8,9,12-hexaazaphenanthrene (HAT-CN) functions as the hole injection layer (HIL); 4′-bis(N-(1-naphthyl)-N-phenylamino)biphenyl (NPB) and tris(4-carbazoyl-9-ylphenyl)amine (TCTA) are the hole transport layers (HTL); the emissive layer consists of TCTA doped with B-2; 1,3,5-tris(3-pyridyl-3-phenyl)benzene (TmPyPB) serves as the electron transport layer (ETL); and Liq and Al act as the electron injection layer (EIL) and cathode, respectively.
The device structures of R-1 and R-2 are as follows: ITO/HAT-CN (3 nm)/TAPC (40 nm)/NPB:Bphen: (6% R-1 or R-2) (10 nm)/Bphen (50 nm)/Liq (1 nm)/Al (100 nm). ITO serves as the anode; HAT-CN functions as the HIL; (1,1-bis(4-tert-butylphenyl)-amino)phenyl)cyclohexane (TAPC) serves as the HTL; and a 1:1 blend of N,N′-diphenyl-N,N′-(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB) and 4,7-diphenyl-1,10-phenanthroline (Bphen) functions as the host material for the emissive layer. Bphen also serves as the ETL, while Liq and Al function as the EIL and cathode, respectively.
As illustrated in the energy level diagrams, the host materials in all three emissive devices exhibit energy levels that closely match those of the hole and electron transport layers. This well-aligned architecture lowers energy level barriers at layer interfaces, thereby reducing turn-on voltages and improving power efficiency.
The electroluminescence (EL) spectra, current density–voltage–brightness (J–V–B) curves, power efficiency–brightness profiles, and external quantum efficiency–brightness profiles of the three iridium complex devices are displayed in Figure 9, with the relevant data compiled in Table 5. As illustrated, although the B-2 complex demonstrates the best photoluminescent properties among those incorporating the dfppy ring metal ligand and is used at a higher doping concentration, its electroluminescent performance in all aspects is still notably poorer than that of the R-1 and R-2 complexes incorporating piq as the ring metal ligand. These results indicate that pyranone-type auxiliary ligands can efficiently form iridium complexes with favorable luminescent properties under mild conditions when piq is used as the main ligand. However, when ppy or dfppy are employed as the ring metal ligands, the same auxiliary ligand appears less effective in enhancing luminescence. This difference primarily stems from the electronic and spatial disparities between quinoline and pyridine. Quinoline possesses a more extended conjugated system and a more complex electron cloud distribution, allowing better alignment with the pyranone auxiliary ligand, which facilitates energy transfer and enhances the luminescent process. In contrast, the structurally simpler pyridine interacts weakly with the highly conjugated auxiliary ligand, limiting its ability to improve emission efficiency. Therefore, the two pyranone-type auxiliary ligands employed here are more suitable for enhancing the performance of iridium complexes with piq as the ring metal ligand and are thus better adapted for use with high-wavelength ring metal ligands.
Figure 9a displays the EL spectra of the three devices. The EL emission wavelength of B-2 is 597 nm, redshifted by approximately 120 nm from its emission in dichloromethane solution (484 nm), resulting in yellow-orange emission. This substantial red shift is noteworthy given that the two fluorine atoms on the dfppy ligand, owing to their strong electronegativity, modify the ligand’s electronic structure and influence the electron cloud distribution within the complex. Such an electronic effect lowers the HOMO level of the ligand, thereby tuning the energy of the MLCT state and typically shifting the emission toward the blue-green region. Consequently, electroluminescent emission wavelengths in previous studies are predominantly in the blue-green range and seldom reach the yellow-orange region. However, following the introduction of the pyranone auxiliary ligand, current injection during the electroluminescent process alters the charge distribution around the complex molecules, resulting in electron cloud redistribution and changes to the complex’s energy level structure. This narrows the energy gap between the excited and ground states, thereby reducing the emission energy and shifting the EL wavelength into the yellow-orange region [56,57]. The EL emission wavelengths of devices based on R-1 and R-2 align with their photoluminescent counterparts. Their emission spectra rise around 600 nm, reach a maximum at 640 nm, and extend beyond 800 nm. As the human eye is more responsive to shorter red wavelengths, an emission profile with a sharper rise on the left side than on the right contributes to enhanced red-light color purity.
As presented in Figure 9b–d, the luminescent performance of the three devices indicates that R-1 and R-2 considerably outperform B-2, with both exhibiting comparable characteristics. The device incorporating R-1 has a turn-on voltage of 2.4 V and delivers a maximum brightness of 12,188 cd/m2. Its maximum external quantum efficiency, current efficiency, and luminous efficacy reach 11.1%, 8.37 cd/A, and 10.05 lm/W, respectively. The EL emission peak is centered at 641 nm, with CIE chromaticity coordinates of (0.69, 0.30). Similarly, the R-2-based device also operates at a turn-on voltage of 2.4 V and achieves a maximum brightness of 13,298 cd/m2. Its external quantum efficiency, current efficiency, and luminous efficacy peak at 13.4%, 8.10 cd/A, and 10.65 lm/W, respectively. The EL emission peak appears at 640 nm, and the CIE coordinates are also (0.69, 0.30). At a brightness level of 1000 cd/m2, the external quantum efficiencies of the R-1 and R-2 devices are maintained at 11.51% and 9.63% and the power efficiencies at 5.3 lm/W and 4.75 lm/W, respectively, indicating minimal efficiency roll-off. OLED devices with R-1 and R-2 showed superior performance. This is attributed to piq’s extended conjugation and pyranone’s auxiliary effect enhance charge injection. In contrast, B-2’s EL performance is poorer due to mismatched orbital levels between dfppy and pyranone ligands. Notably, the excellent electroluminescent performance of R-1 and R-2 despite their lack of measurable solid-state photoluminescence can be directly attributed to the doping strategy employed in the OLED device architecture. Dispersing the R-1/R-2 complexes at a 6% concentration in the NPB:Bphen co-host matrix effectively isolates individual iridium complex molecules, suppressing concentration quenching caused by excessive intermolecular π–π stacking and aggregation-induced non-radiative decay that is observed in neat solid films of these complexes. The host matrix provides a rigid, spatially separated environment for the emissive dopants, enabling efficient radiative recombination under electrical excitation, which explains the high external quantum efficiencies achieved in the devices despite the absence of solid-state PL.

3. Materials and Methods

3.1. Synthesis of Iridium Complexes

3.1.1. General Synthetic Procedures

All chemicals were purchased from commercial suppliers and used without further purification. Solvents were dried and degassed prior to use. Reactions were monitored by thin-layer chromatography (TLC) using silica gel plates.

3.1.2. Synthesis of Chloro-Bridged Dimers

(ppy)2Ir(μ-Cl)2Ir(ppy)2: Iridium trichloride hydrate (CAS 14996-61-3) (15.57 g, 43.7 mmol) and 2-phenylpyridine (CAS 1008-89-5) (14.92 g, 96.14 mmol) were dissolved in a 3:1 mixture of ethylene glycol monoethyl ether (CAS 110-80-5) and water (120 mL). The solution was degassed three times and refluxed at 100 °C for 24 h under argon. After cooling to room temperature, the precipitate was filtered and washed sequentially with deionized water (40 mL × 2), ethanol (30 mL), and acetone (30 mL × 2). Yield: 96% (98.88 g).
(dfppy)2Ir(μ-Cl)2Ir(dfppy)2: Prepared analogously using 2-(2,4-difluorophenyl)pyridine (CAS 391604-55-0)(18.38 g, 96.14 mmol). Yield: 97% (113.34 g).
(piq)2Ir(μ-Cl)2Ir(piq)2: Prepared analogously using 1-phenylisoquinoline (CAS 3297-72-1)(19.75 g, 96.2 mmol). Yield: 90% (110.20 g).

3.1.3. Synthesis of Target Complexes

G-1: (ppy)2Ir(μ-Cl)2Ir(ppy)2 (1.71 g, 1.6 mmol) and 3-hydroxy-2-methyl-4-pyrone (CAS 118-71-8)(1.01 g, 8 mmol) were dissolved in ethylene glycol monoethyl ether (65 mL). Sodium carbonate (CAS 497-19-8)(2.54 g, 24 mmol) was added, and the mixture was refluxed for 8 h under argon. After cooling, the solvent was removed under reduced pressure, and the residue was washed with ethanol (30 mL × 2) and dried. Yield: 74%. 1H NMR (400 MHz, Chloroform-d) δ 8.76 (d, J = 5.1 Hz, 1H), 8.50 (d, J = 5.2 Hz, 1H), 7.84 (t, J = 8.0 Hz, 2H), 7.71–7.66 (m, 2H), 7.61 (d, J = 5.1 Hz, 1H), 7.57–7.52 (m, 2H), 7.15–7.05 (m, 2H), 6.79 (t, J = 7.5 Hz, 2H), 6.71–6.64 (m, 2H), 6.49 (d, J = 5.1 Hz, 1H), 6.25 (t, J = 8.4 Hz, 2H), 2.42 (s, 3H). Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI–TOF–MS) (m/z): calcd. for C28H21IrN2O3: 626.13; found: 627.13 [M+1]+.
G-2: Prepared analogously using 3-hydroxy-2-ethyl-4-pyrone (CAS 4940-11-8)(1.12 g, 8 mmol). Yield: 83%. 1H NMR (400 MHz, Chloroform-d) δ 8.75 (ddd, J = 5.7, 1.6, 0.8 Hz, 1H), 8.50 (ddd, J = 5.7, 1.7, 0.8 Hz, 1H), 7.88–7.79 (m, 2H), 7.73–7.63 (m, 3H), 7.55 (dd, J = 7.7, 1.4 Hz, 2H), 7.09 (dddd, J = 9.7, 7.3, 5.8, 1.4 Hz, 2H), 6.80 (td, J = 7.5, 1.2 Hz, 2H), 6.68 (tdd, J = 7.3, 5.1, 1.5 Hz, 2H), 6.49 (d, J = 5.1 Hz, 1H), 6.26 (ddd, J = 15.0, 7.7, 1.2 Hz, 2H), 2.92 (dq, J = 15.0, 7.5 Hz, 1H), 2.82–2.70 (m, 1H), 1.15 (t, J = 7.5 Hz, 3H). MALDI-TOF-MS (m/z): calcd. for C29H23IrN2O3:640.14; found: 641.14 [M+1]+.
B-1: Prepared using (dfppy)2Ir(μ-Cl)2Ir(dfppy)2 (1.94 g, 1.6 mmol) and 3-hydroxy-2-methyl-4-pyrone (1.01 g, 8 mmol). Yield: 62%. 1H NMR (400 MHz, Chloroform-d) δ 8.71 (d, J = 5.0 Hz, 1H), 8.43 (d, J = 5.1 Hz, 1H), 8.25 (t, J = 8.1 Hz, 2H), 7.76 (t, J = 7.6 Hz, 2H), 7.66 (d, J = 5.1 Hz, 1H), 7.26 (s, 2H), 7.15 (dd, J = 12.8, 6.0 Hz, 2H), 6.54 (d, J = 5.1 Hz, 1H), 6.38–6.29 (m, 2H), 5.64 (ddd, J = 17.3, 8.9, 2.3 Hz, 2H). MALDI-TOF-MS (m/z): calcd. for C28H17F4IrN2O3: 698.09; found: 699.09 [M+1]+.
B-2: Prepared using (dfppy)2Ir(μ-Cl)2Ir(dfppy)2 (1.94 g, 1.6 mmol) and 3-hydroxy-2-ethyl-4-pyrone (1.12 g, 8 mmol). Yield: 73%. 1H NMR (400 MHz, Chloroform-d) δ 8.71 (d, J = 5.3 Hz, 1H), 8.43 (d, J = 5.6 Hz, 1H), 8.25 (t, J = 7.5 Hz, 2H), 7.76 (t, J = 7.9 Hz, 2H), 7.69 (d, J = 5.0 Hz, 1H), 7.18–7.10 (m, 2H), 6.54 (d, J = 5.1 Hz, 1H), 6.39–6.27 (m, 2H), 5.65 (ddd, J = 25.2, 8.9, 2.3 Hz, 2H), 2.93 (dd, J = 15.2, 7.6 Hz, 1H), 2.80 (dd, J = 15.2, 7.6 Hz, 1H), 1.16 (t, J = 7.5 Hz, 3H). MALDI-TOF-MS (m/z): calcd. for C29H19F4IrN2O3: 711.68; found: 713.10 [M+1]+.
R-1: Prepared using (piq)2Ir(μ-Cl)2Ir(piq)2 (2.04 g, 1.6 mmol) and 3-hydroxy-2-methyl-4-pyrone (1.01 g, 8 mmol). Yield: 78%. 1H NMR (400 MHz, chloroform-d) δ 9.00–8.94 (m, 2H), 8.74 (d, J = 6.4 Hz, 1H), 8.43 (d, J = 6.4 Hz, 1H), 8.21 (d, J = 8.0 Hz, 2H), 7.90 (dd, J = 5.8, 3.7 Hz, 2H), 7.75–7.65 (m, 4H), 7.61 (d, J = 5.1 Hz, 1H), 7.46 (dd, J = 13.1, 6.3 Hz, 2H), 6.92–6.85 (m, 2H), 6.69–6.62 (m, 2H), 6.49 (d, J = 5.1 Hz, 1H), 6.37–6.30 (m, 2H), 2.40 (s, 3H). MALDI-TOF-MS (m/z): calcd. for C36H25IrN2O3: 726.16; found: 727.16 [M+1]+.
R-2: Prepared using (piq)2Ir(μ-Cl)2Ir(piq)2 (2.04 g, 1.6 mmol) and 3-hydroxy-2-ethyl-4-pyrone (1.12 g, 8 mmol). Yield: 83%. 1H NMR (400 MHz, chloroform-d) δ 8.98 (dt, J = 6.8, 3.1 Hz, 2H), 8.74 (d, J = 6.4 Hz, 1H), 8.44 (d, J = 6.4 Hz, 1H), 8.21 (d, J = 8.0 Hz, 2H), 7.90 (dd, J = 4.7, 3.0 Hz, 2H), 7.75–7.66 (m, 4H), 7.64 (d, J = 5.1 Hz, 1H), 7.45 (dd, J = 10.6, 6.4 Hz, 2H), 6.93–6.85 (m, 2H), 6.70–6.62 (m, 2H), 6.49 (d, J = 5.1 Hz, 1H), 6.35 (td, J = 7.6, 1.1 Hz, 2H), 2.91 (dd, J = 15.2, 7.6 Hz, 1H), 2.73 (dd, J = 15.1, 7.5 Hz, 1H), 1.12 (t, J = 7.5 Hz, 3H). MALDI-TOF-MS (m/z): calcd. for C37H27IrN2O3: 740.17; found: 741.17 [M+1]+.

3.2. Detection Methods

3.2.1. Structural Characterization of the Materials

The synthesized iridium complexes were characterized using 1 H NMR, infrared (IR) spectroscopy, and MS, with structural confirmation of the final products primarily performed by MS and single-crystal X-ray diffraction.
NMR: The complexes were dissolved in CDCl3, and 1 H NMR spectra were recorded using a 400 MHz Bruker DRX-500 spectrometer (Berlin, Germany).
MS: MS spectra were acquired on an ATI-QSTAR mass spectrometer (CA) in fast atom bombardment positive-ion mode.
IR Spectroscopy: Fourier transform IR spectra were recorded using an FTS-135 infrared spectrometer (Tensor 37, Bruker, Berlin, Germany) across the range of 400–4000 cm−1.
Single Crystal Growth and Analysis: First, 2 mg of each sample was dissolved in a solvent mixture of 0.2 mL acetonitrile, 0.1 mL methanol, and 0.1 mL tetrahydrofuran; filtered; and allowed to evaporate at room temperature in the dark to form crystals. Single-crystal X-ray diffraction data were collected at 193 K using a Bruker D8 Venture diffractometer (Berlin, Germany) with graphite-monochromatized CuKα radiation.

3.2.2. Electrochemical Performance Testing

The redox behavior of the iridium complexes was examined by cyclic voltammetry using a PG STAT302 electrochemical workstation (Metrohm, Herisau, Switzerland). Each complex was dissolved in anhydrous dichloromethane, and measurements were performed over a potential range of −2 to 2 V.

3.2.3. DFT Calculations

DFT calculations were performed with Gaussian 16 software (Gaussian, Inc. USA). Single-point energy calculations were conducted on the optimized molecular structures using the TPSSh functional and the def2-TZVP basis set. The frontier molecular orbitals were visualized with the Visual Molecular Dynamics program [58]. These calculations yielded the optimized geometries and the ground-state electron cloud density distributions of the HOMO and LUMO.

3.2.4. Photophysical Property Tests

A series of iridium complexes were prepared in dichloromethane solutions at a concentration of 1 × 10−5 mol L−1.
UV-Vis absorption spectra were collected at room temperature over a wavelength range of 200–800 nm using an Edinburgh FS5 transient and steady-state fluorescence spectrometer (Fulton, MD, USA).
PL emission spectra were recorded in the 400–800 nm range, with excitation at the maximum absorption wavelength.
The luminescence lifetime of the complexes was obtained by fitting the decay curves measured using an Edinburgh FLS1000 spectrometer (Fulton, MD, USA).

3.2.5. Device Preparation and Testing

OLED devices were fabricated using a vacuum evaporation technique. In this study, bimetallic iridium complexes containing 1-phenylisoquinoline as the macrocyclic ligand were incorporated into the OLED architecture. The fabrication process involved the following steps:
(1)
ITO substrate pre-treatment: The ITO film (15 Ω/square)(Xinyan Technology, Shekou, China) was etched from the substrate and sequentially ultrasonicated in deionized water, detergent solution, acetone, and methanol for 5–10 min each. The substrates were then thoroughly scrubbed until the ITO surface was free of contaminants. Subsequently, the cleaned ITO was treated in a UV–ozone cleaner (Novascan, Boone, IA, USA) for 15 min to enhance its surface work function.
(2)
Device fabrication: The pre-treated ITO substrates were immediately transferred into a vacuum evaporation chamber (Zhenheng Technology, Shenyang, China). When the base pressure fell below 3 × 10−5 Pa, the organic functional layers and the metal cathode were sequentially deposited according to the device design. During the deposition of the doped host–guest emissive layer, the vacuum level was carefully maintained, and consistency was ensured throughout the concentration adjustment and layer deposition processes.
(3)
Device testing: Following device fabrication, current density–voltage (J–V) characteristics were measured using an HP4140B picoammeter (Agilent Technologies, Palo Alto, CA, USA). Brightness, electroluminescence (EL) spectra, and CIE chromaticity coordinates were obtained using a Minolta LS-110 spectrophotometer (Konica Minolta, Tokyo, Japan) and an Ocean Optics USB-4000 spectrometer (Ocean Insight, Orlando, FL, USA). The external quantum efficiency was derived from the emission spectrum, brightness, and current density. Transient EL spectra were measured using a KEYSIGHT DSO1012A (Keysight Technologies, Santa Rosa, CA, USA) oscilloscope in conjunction with a LINI-UTP3313TFL-11 stabilized DC power supply (Longwei Instruments, Shenzhen, China) and a VICTOR DDS signal generator counter (Victor High Technology, Shenzhen, China).
(4)
Functional layer materials and device structures: The optimized device structure for B-2 was ITO/HAT-CN (3 nm)/NPB (40 nm)/TCTA (10 nm)/TCTA: dopant (8%, 15 nm)/TmPyPB (50 nm)/Liq (1 nm)/Al (100 nm). The optimized structure for R-1 and R-2 was ITO/HAT-CN (3 nm)/TAPC (40 nm)/NPB:Bphen:dopant (6%, 10 nm)/Bphen (50 nm)/Liq (1 nm)/Al (100 nm).

4. Conclusions

Six novel iridium phosphorescent complexes were synthesized by combining pyranone-based auxiliary ligands with conjugated ring metal ligands of varying conjugation degrees. The study elucidated the structural regulation mechanism by which ligand architecture influences complex performance. Complexes featuring piq as the main ligand (R-1 and R-2) exhibited notably higher molecular symmetry than those based on ppy and dfppy (G and B series), owing to the uniformity of bond lengths and angles and the extended conjugation system of the piq ligand. In contrast, G-1, which incorporates a less conjugated main ligand, displayed a twisted molecular structure. The synergistic interaction between the pyranone auxiliary ligand and piq gave rise to deep red emission at 640 nm in solution for R-1 and R-2, with quantum yields of 64% and 55%, respectively. In the solid state, the G and B series achieved redshifted emission (600–620 nm) through π–π stacking, with B-2 reaching a maximum quantum yield of 25.8%. OLED devices fabricated with R-1 and R-2 displayed excellent performance, with peak external quantum efficiencies of 13.4% and 11.1%, maximum brightness exceeding 12,000 cd/m2, and CIE color coordinates of (0.69, 0.30) that conform to deep red light standards, alongside minimal efficiency roll-off at high luminance. Overall, pyranone auxiliary ligands are optimal for highly conjugated cyclometalating ligands (e.g., piq), as they enhance orbital alignment and reduce non-radiative decay. This study provides a design framework for high-performance iridium phosphorescent materials: pairing extended-conjugation main ligands with pyranone-type auxiliaries to achieve efficient, stable emission.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics14040095/s1, Table S1: Reagent list with commercial sources; Table S2: Structural formulas of all the organic ligands; Table S3: Elemental analysis data of complex; Figure S1: 1HNMR of G-1, G-2, B-1, B-2, R-1 and R-2; Figure S2: Mass spectra of of G-1, G-2, B-1, B-2, R-1 and R-2.; Figure S3: Infrared spectrum of of G-1, G-2, B-1, B-2, R-1 and R-2.

Author Contributions

Writing—original draft and funding acquisition, W.J.; Methodology, C.Y. and L.W.; Writing—review and editing, X.L.; Data curation and funding acquisition, Q.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Science and Technology Project of Yunnan Precious Metals Laboratory (YPML2023050235), the National Natural Science Foundation of China (22065019), the Scientific Research Funds of Kunming University (XPZJ2205, XPZJ2205-2), the Special Basic Cooperative Research Programs of Yunnan Provincial Undergraduate Universities’ Association (202101BA070001-032), Yunnan Fundamental Research Projects (202401AS070021). The authors also thank the Program for Young and Middle-aged Academic and Technical Leaders Reserve Talents of Yunnan Province (202105AC160043), Frontier Research Team of Kunming University 2023, the High-Level Talent Special Support Plans for Young Talents of Kunming City (C201905002, C202014001).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study is contained within the article.

Acknowledgments

We thank the editor and the reviewers for their useful help for improving this paper, along with the Scientific Innovation Team of Kunming University and Kunming Institute of Precious Metals for helpful discussions on topics related to this work.

Conflicts of Interest

Author Caixian Yan and Qiaowen Chang were employed by the company Yunnan Precious Metals Laboratory Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Inorganics 14 00095 g001aInorganics 14 00095 g001b
Figure 1. Chemical structures and synthetic routes of three metallacyclic ligands. (a) (ppy)2Ir(µ-Cl2). (b) (dfppy)2Ir(µ-Cl2)Ir(dfppy)2. (c) (piq)2Ir(µ-Cl2) Ir(piq)2.
Figure 1. Chemical structures and synthetic routes of three metallacyclic ligands. (a) (ppy)2Ir(µ-Cl2). (b) (dfppy)2Ir(µ-Cl2)Ir(dfppy)2. (c) (piq)2Ir(µ-Cl2) Ir(piq)2.
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Inorganics 14 00095 g002
Figure 2. Molecular structure and sample number of Ir (III) complexes.
Figure 2. Molecular structure and sample number of Ir (III) complexes.
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Figure 3. Crystal structures of the phosphorescent iridium complexes (a) G-1, (b) B-1 and (c) R-1. (Orange: Ir, Red: O, Blue: N, Green: F and Grey: C).
Figure 3. Crystal structures of the phosphorescent iridium complexes (a) G-1, (b) B-1 and (c) R-1. (Orange: Ir, Red: O, Blue: N, Green: F and Grey: C).
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Inorganics 14 00095 g004aInorganics 14 00095 g004b
Figure 4. UV–vis absorption and PL spectra of (a) G, (b) B and (c) R. Decay time curves of (d) G, (e) B, and (f) R in dichloromethane (1.0 × 10−5 mol L−1) at room temperature.
Figure 4. UV–vis absorption and PL spectra of (a) G, (b) B and (c) R. Decay time curves of (d) G, (e) B, and (f) R in dichloromethane (1.0 × 10−5 mol L−1) at room temperature.
Inorganics 14 00095 g004aInorganics 14 00095 g004b
Inorganics 14 00095 g005
Figure 5. PL spectra and photographs of luminescence under UV light exposure of solid samples of (a) G-1 and G-2, and (b) B-1 and B-2 at room temperature.
Figure 5. PL spectra and photographs of luminescence under UV light exposure of solid samples of (a) G-1 and G-2, and (b) B-1 and B-2 at room temperature.
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Figure 6. Molecular orbital distributions and HOMO–LUMO band gaps of (a) G-1 and G-2, (b) B-1 and B-2, and (c) R-1 and R-2.
Figure 6. Molecular orbital distributions and HOMO–LUMO band gaps of (a) G-1 and G-2, (b) B-1 and B-2, and (c) R-1 and R-2.
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Figure 7. Cyclic voltammetry curves of the iridium complexes recorded in CH2Cl2.
Figure 7. Cyclic voltammetry curves of the iridium complexes recorded in CH2Cl2.
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Figure 8. Energy level diagram of (a) B-2 and (b) R-1 and R-2 in the PhOLEDs.
Figure 8. Energy level diagram of (a) B-2 and (b) R-1 and R-2 in the PhOLEDs.
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Figure 9. EL spectra. (a) current density–voltage–luminance (J–V–L) characteristics. (b) power efficiency curves. (c) and current efficiency curves. (d) of devices B-2, R-1, and R-2.
Figure 9. EL spectra. (a) current density–voltage–luminance (J–V–L) characteristics. (b) power efficiency curves. (c) and current efficiency curves. (d) of devices B-2, R-1, and R-2.
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Table 1. Crystallographic data of the phosphorescent iridium complexes G-1, B-1, and R-1.
Table 1. Crystallographic data of the phosphorescent iridium complexes G-1, B-1, and R-1.
ParameterG-1B-1R-1
FormulaC28H22IrN2O3C28H17F4IrN2O3C36H25IrN2O3
Formula weight626.67697.64725.78
Crystal systemmonoclinicorthorhombicmonoclinic
Space groupP21/cPccnC2/c
a [Å]17.0084(9)7.2752(3)11.4553(5)
b [Å]13.0338(7)15.8317(7)15.4613(6)
c [Å]10.6853(5)20.6395(9)20.2487(7)
α [0]909090
β [0]93.189(4)90101.138(2)
γ [0]909090
V [Å 3]2365.1(2)2377.23(18)3518.78(20)
Z444
ρcalc [g/cm3]1.7601.9491.370
Reflections collected14,04828,59511,912
Independent reflections451424273228
RF, Rw(F2) (all data)0.10980.07920.0500
RF, Rw(F2) [I > 2σ(I)]0.07560.05970.0426
GOF1.0551.2321.108
Table 2. Coordination bond lengths (Å) and bond angles (°) in the phosphorescent iridium complexes G-1, B-1, and R-1.
Table 2. Coordination bond lengths (Å) and bond angles (°) in the phosphorescent iridium complexes G-1, B-1, and R-1.
CompoundIr–CIr–NIr–OC–Ir–CN–Ir–NO–Ir–OC–Ir–ON–Ir–OC–Ir–N
G-12.036(13)2.012(16)2.180(2)96.0(14)95.5(12)76.9(9)171.6(10)169.7(9)88.6(14)
1.980(16)1.910(4)2.249(2)96.3(11)92.8(10)80.0(16)
93.9(8)90.4(13)98.6(12)
88.7(8)91.9(13)80.2(7)
B-11.995(11)2.049(9)2.163(8)86.7(6)175.5(5)78.0(5)173.0(4)93.9(4)96.4(4)
1.995(11)2.049(9)2.163(8)173.0(4)93.9(4)96.4(4)
97.9(4)89.6(3)80.3(4)
97.9(4)89.6(3)80.3(4)
R-11.972(8)2.040(6)2.148(6)87.4(4)175.5(2)78.4(3)174.67(17)96.48(19)80.1(2)
1.972(8)2.040(6)2.148(6) 174.67(17)96.48(19)80.1(2)
97.1(3)87.05(19)96.6(2)
97.1(3)87.05(19)96.6(2)
Table 3. Photophysical data of G, B, and R.
Table 3. Photophysical data of G, B, and R.
ComplexΛabs 1
(nm)		Λem 1
(nm)		Φp 2
(%)		Λem 3
(nm)		Φp 3
(%)		τ 1
(μs)
G-1261, 300, 355, 402492-6203.40.21
G-2261, 301, 356, 401492-6205.50.15
B-1251, 301, 382484-60012.70.16
B-2251, 301, 383484-60025.80.25
R-1229, 270, 346, 39764064--0.13
R-2232, 273, 345, 39864055--0.13
1 Measured in CH2Cl2 at a concentration of 1.0 × 10−5 mol L−1 at room temperature. 2 Measured using a CH2Cl2 standard. 3 Measured in the solid state at room temperature.
Table 4. Electrochemical data of G, B, and R.
Table 4. Electrochemical data of G, B, and R.
Complex E onset ox (V) EHOMO (eV)ELUMO (eV)Eg (eV)
G-10.54−4.94−2.142.80
G-20.54−4.94−2.182.76
B-10.68−5.08−2.212.87
B-20.68−5.08−2.22.88
R-10.5−4.90 −2.752.15
R-20.46−4.86−2.182.05
Table 5. EL data of all devices.
Table 5. EL data of all devices.
DevicesVon 1
(V)		Lmax
(cd/m2)		ηc,max
(cd/A)		ηp,max
(lm/W)		Hext 2
(%)		λem
(nm)		CIE(x, y)
B-23.41796.315.255.36/2.025970.59, 0.42
R-12.412,1888.1010.6513.4/11.516410.69, 0.30
R-22.413,2988.3710.0511.1/9.636400.69, 0.30
1 Measured at 1 cd/m2. 2 First value represents the maximum efficiency, followed by the value at 1000 cd/m2.
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Jiang, W.; Wang, L.; Li, X.; Yan, C.; Chang, Q. Design, Synthesis, and Characterization of Novel Phosphorescent Iridium Complexes with Pyrone Auxiliary Ligands and ppy/dfppy/piq Cyclometalating Ligands. Inorganics 2026, 14, 95. https://doi.org/10.3390/inorganics14040095

AMA Style

Jiang W, Wang L, Li X, Yan C, Chang Q. Design, Synthesis, and Characterization of Novel Phosphorescent Iridium Complexes with Pyrone Auxiliary Ligands and ppy/dfppy/piq Cyclometalating Ligands. Inorganics. 2026; 14(4):95. https://doi.org/10.3390/inorganics14040095

Chicago/Turabian Style

Jiang, Wen, Leyuan Wang, Xiangguang Li, Caixian Yan, and Qiaowen Chang. 2026. "Design, Synthesis, and Characterization of Novel Phosphorescent Iridium Complexes with Pyrone Auxiliary Ligands and ppy/dfppy/piq Cyclometalating Ligands" Inorganics 14, no. 4: 95. https://doi.org/10.3390/inorganics14040095

APA Style

Jiang, W., Wang, L., Li, X., Yan, C., & Chang, Q. (2026). Design, Synthesis, and Characterization of Novel Phosphorescent Iridium Complexes with Pyrone Auxiliary Ligands and ppy/dfppy/piq Cyclometalating Ligands. Inorganics, 14(4), 95. https://doi.org/10.3390/inorganics14040095

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