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Article

AIPE-Active Neutral Ir(III) Complexes as Bi-Responsive Luminescent Chemosensors for Sensing Picric Acid and Fe3+ in Aqueous Media

by
Qinglong Zhang
1,†,
Jiangchao Xu
1,†,
Qiang Xu
2,* and
Chun Liu
1,*
1
State Key Laboratory of Fine Chemicals, Frontier Science Center for Smart Materials, School of Chemical Engineering, Dalian University of Technology, Linggong Road 2, Dalian 116024, China
2
Instrumental Analysis Center, Dalian University of Technology, Dalian 116024, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Chemosensors 2025, 13(1), 10; https://doi.org/10.3390/chemosensors13010010
Submission received: 8 December 2024 / Revised: 3 January 2025 / Accepted: 5 January 2025 / Published: 8 January 2025
(This article belongs to the Special Issue Advanced Real-Time On-Site Sensing Technologies for Food and Environment Analysis: 2nd Edition)
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Abstract

:
Three neutral iridium complexes Ir1–Ir3 were synthesized using diphenylphosphoryl-substituted 2-phenylpyridine derivatives as the cyclometalating ligand and picolinic acid as the auxiliary ligand. They exhibited significant aggregation-induced phosphorescent emission (AIPE) properties in H2O/THF and were successfully used as bi-responsive luminescent sensors for the detection of picric acid (PA) and Fe3+ in aqueous media. Ir1–Ir3 possesses high efficiency and high selectivity for detecting PA and Fe3+, with the lowest limit of detection at 59 nM for PA and 390 nM for Fe3+. Additionally, the complexes can achieve naked-eye detection of Fe3+ in aqueous media. Ir1–Ir3 exhibit excellent potential for practical applications in complicated environments. The detection mechanism for PA is attributed to photo-induced electron transfer (PET) and Förster resonance energy transfer (FRET), and the detection mechanism for Fe3+ may be explained by PET and the strong interactions between Fe3+ and the complexes.

Graphical Abstract

1. Introduction

The detection of trace explosives and heavy metal ions in the environment has attracted widespread attention from researchers out of concern for social security, human health, and environmental protection [1,2,3]. Compared with other nitro explosives, picric acid (PA) possesses stronger explosive properties and is a crucial component of military explosives [4]. In addition to its military applications, PA is also commonly used in the production of dyes, leather, and fireworks [5]. However, PA can cause serious environmental damage and human diseases such as skin irritation, liver function abnormalities, and cancer when it leaks into the environment [4,6]. Additionally, iron is a critical element in the body that plays a vital role in biochemical processes. The balance of Fe3+ is crucial for human health because its deficiency or over-accumulation can lead to various disorders of living system, including anemia, liver and kidney damage, organ failure, Parkinson’s, Alzheimer’s, and cancer [7,8,9,10]. Moreover, excessive Fe3+ can also cause severe environmental problems [11]. Therefore, the development of simple and efficient methods for the detection of PA and Fe3+ is highly urgent.
Luminescent sensing methods for detecting pollutants have garnered considerable attention due to their cost-effectiveness, simplicity, rapid response, and non-destructive nature compared to other techniques [12,13,14]. In recent years, iridium complexes have been widely applied in the luminescence sensing of various analytes due to their rich tunable photophysical properties and multifunctionality [15,16,17,18]. Furthermore, the concept of aggregation-induced phosphorescent emission (AIPE) has provided new approaches for iridium complexes to be used as chemosensors in aqueous media or solid-state, promoting their practical application in the environment [19,20,21,22,23].
A series of AIPE-active Ir(III) complexes have been successfully utilized for the detection of PA in aqueous media, exhibiting satisfactory sensing outcomes [24,25,26,27]. The luminescence quenching of most Ir(III) complexes is attributed to photo-induced electron transfer (PET) during the detection. However, Förster resonance energy transfer (FRET) has also been established as a significant quenching mechanism, and the presence of FRET can significantly improve the selectivity and quenching efficiency of Ir(III) complexes toward PA [28,29]. Based on the detection mechanism, the development of Ir(III) complexes that can achieve the synergistic effects of PET and FRET in detection is of great importance for the efficient and selective detection of PA. In addition, organic small molecules [11], metal–organic frameworks [30], coordination polymers [31], and nanoparticles [32] have been employed for detecting Fe3+ in previous studies. However, reports on the detection of Fe3+ by Ir(III) complexes in aqueous media are scarce to date [33]. To the best of our knowledge, there have been no reports of Ir(III) complexes performing bi-responsive detection of PA and Fe3+ in aqueous media.
Our group has long been committed to studying the structure-function relationship of cyclometalated metal complexes [34,35,36,37,38,39,40]. Recently, we have developed a series of cyclometalated Pt(II) and Ir(III) complexes for detecting PA in aqueous media by the detection mechanism of PET [34,35,36,37,38]. However, the impact of the structure of neutral Ir(III) complexes on their detection performance remains to be investigated. The diphenylphosphoryl group is a strong electron-withdrawing group, and its introduction at the corresponding positions of Ir(III) complexes can lower the highest occupied molecular orbital (HOMO), which leads to blue light emission from the Ir(III) complexes and enhances the FRET with the analyte [41,42]. Moreover, the oxygen atom in the diphenylphosphoryl group contributes to the specific recognition of metal ions [11,43,44]. Therefore, three neutral Ir(III) complexes Ir1–Ir3 have been synthesized using diphenylphosphoryl-substituted 2-phenylpyridine derivatives as the cyclometalating ligand and picolinic acid as the auxiliary ligand. They exhibit significant AIPE properties in H2O/THF. We successfully achieved efficient detection of PA and specific recognition of Fe3+ in aqueous media using their AIPE properties and discuss the possible detection mechanisms in detail. Moreover, Ir1–Ir3 show promising practical applications in environmental water samples. The structures of Ir1–Ir3 are shown in Scheme 1.

2. Materials and Methods

2.1. Materials and Instruments

Further details of the materials and instruments utilized for this study are in the Supplementary Materials.

2.2. Characterization of Complexes

The cyclometalating ligands and cyclometalated Ir(III) complexes Ir1–Ir3 have been synthesized following the previously reported methods, and the detailed characterization results of Ir1–Ir3 are in the Supplementary Materials Figures S1–S8 [45]. Scheme 1 shows the synthetic routes of Ir1–Ir3, and the detailed synthetic steps are illustrated as follows.
IrCl3·3H2O (0.2 mmol, 70.5 mg) and cyclometalating ligand (0.5 mmol) were added to 8 mL of 2-ethoxyethanol/water (3:1, v/v), which was heated at 120 °C for 24 h under a nitrogen atmosphere. After the reaction, the mixture was added to water, the resulting precipitate was filtered and washed with ethanol and n-hexane to obtain the chloro-bridged dimer. Without further purification, the dimer, picolinic acid (0.6 mmol, 73.9 mg), and Na2CO3 (1.0 mmol, 106.0 mg) were added to 5 mL of CH2Cl2/ethanol (4:1, v/v). The mixture was stirred at 40 °C for 12 h under N2. After the reaction, the mixture was added to brine and extracted with CH2Cl2. With CH2Cl2/CH3OH (10:1, v/v) as the eluent, the mixture was purified by silica gel column chromatography to obtain Ir1–Ir3.

2.3. Methods for Test of AIPE and Detection of PA and Fe3+

Stock solutions of Ir1–Ir3 in THF (100 μM) were prepared. Subsequently, the suspensions of Ir1–Ir3 (3 mL, 10 μM) at different water fractions were prepared by mixing 300 μL of the stock solution with THF and deionized water of appropriate volume, and their emission spectra were recorded. The suspensions of Ir1–Ir3 (10 μM) in H2O/THF with a 70% water fraction were prepared in a 200 mL volumetric flask, respectively, and 3 mL of the suspension was used for the measurement of emission and absorption spectra each time. PA solutions with concentrations from 0.1 to 40 mM and Fe(NO3)3 solutions at concentrations from 0 to 100 mM were prepared in H2O/THF (v/v = 7:3), respectively. For the detection of PA or Fe3+ by Ir1–Ir3, PA or Fe(NO3)3 solutions (30 μL) with different concentrations were added to 3 mL of the suspensions of the complexes each time, and their emission and absorption spectra were recorded. For the selectivity experiments with Ir1–Ir3, various analytes (20 mM, nitromethane (NM), m-dinitrobenzene (1,3-DNB), 4-methoxyphenol (MEHQ), nitrobenzene (NB), phenol, p-cresol, and m-cresol) and ionic compounds (20 mM, KF, KBr, NaHCO3, CH3COONa, FeCl2, ZnCl2, MgSO4, CuSO4, CaCl2, MnCl2, NiCl2, and CoCO3) were added to the suspensions of the complexes, and their emission spectra were tested. For anti-interference experiments with Ir1–Ir3 for detecting PA, PA solutions (20 mM) were added to suspensions of complexes with different analytes and ionic compounds, respectively, and their emission spectra were tested. In addition, Fe(NO3)3 solutions (80 mM for Ir1 and Ir2 and 100 mM for Ir3) were added to suspensions of the complexes with different ionic compounds for the measurement of their emission spectra to investigate the anti-interference ability of Ir1–Ir3 for the detection of Fe3+. In order to study the ability of Ir1–Ir3 when applied in the environment, several environmental water samples (tap water, lake water, and rainwater from Dalian University of Technology and seawater from Qixianling in Dalian) were selected instead of deionized water to prepare suspensions of the complexes. Subsequently, PA solutions (20 mM) or Fe(NO3)3 solutions (80 mM for Ir1 and Ir2, and 100 mM for Ir3) were added to the suspensions of Ir1–Ir3, respectively, and their emission spectra were recorded.
WARNING! The nitroaromatic compounds used in optical measurement are highly explosive and should be handled safely and in small quantities.

3. Results and Discussion

3.1. Photophysical Properties

The UV–vis absorption spectra and normalized emission spectra of Ir1–Ir3 in THF are shown in Figure 1a,b, and the detailed photophysical data are listed in Table S1. As shown in Figure 1a, the UV–vis absorption spectra of Ir1–Ir3 are similar to those of previously reported Ir(III) complexes, with intense absorptions at 200–350 nm, which are mainly attributed to spin-allowed ligand-centered (1π-π*) transitions. The weaker absorptions within 350 to 500 nm can be assigned to the combination of metal-to-ligand charge transfer (1MLCT and 3MLCT) with ligand-centered 3π-π* transitions [21,38]. The normalized emission spectra of Ir1–Ir3 in THF are shown in Figure 1b. The results show that the introduction of a methyl group onto the cyclometalating ligand leads to a slight redshift of 3 nm for Ir2 compared to non-substituted Ir1, while the introduction of a trifluoromethyl group for Ir3 leads to a larger redshift of 28 nm relative to Ir1. In addition, Ir1 and Ir2 exhibit vibronic fine structure, suggesting a large ligand-centered (CˆN) character (3LC). Unlike Ir1 and Ir2, the featureless emission spectrum of Ir3 may be attributed to more 3MLCT/3LLCT features contained in Ir3 [20,24,26]. Ir1–Ir3 exhibit high phosphorescence quantum yields (ΦPL) in deoxygenated CH2Cl2, which are estimated to be 0.69, 0.70, and 0.72, respectively (Table S1). The lifetimes (τ) of Ir1–Ir3 in degassed CH2Cl2 are estimated to be 1.32, 3.32, and 1.01 μs, respectively, at room temperature (Figure S9 and Table S1). This indicates that substituents on the cyclometalating ligand can significantly affect the excited-state properties of Ir2 and Ir3, leading to variations in their lifetimes. The data demonstrate that substituents on the cyclometalating ligand play an essential role in the modification of the photophysical properties of the Ir(III) complexes.

3.2. AIPE Properties

Multiple rotatable phenyl groups are present in Ir1–Ir3 due to the introduction of the diphenylphosphoryl group, which motivates us to investigate their AIPE properties. The emission spectra of Ir1–Ir3 in H2O/THF are shown in Figure 2a–c. Below a 70% water fraction, the emission intensities of Ir1–Ir3 increase continuously with increasing water fraction and reach the maximum at 70% water fraction, showing obvious AIPE properties. Dynamic light scattering (DLS) experiments were carried out in the case of Ir1 (Figure S10). The results indicate that Ir1 forms aggregates at 70%, 80%, and 90% water fractions, with hydrodynamic diameters of 208, 259, and 222 nm, respectively. In the aggregated state, the motion of the freely rotatable phenyl groups in the cyclometalating ligand of the complex is restricted, which inhibits the non-radiative pathway, thus allowing the excitons to return to the ground state by radiative pathway and leading to significant AIPE activity.

3.3. Sensing of PA

The AIPE phenomenon exhibited by Ir1–Ir3 in H2O/THF prompted us to use them for the detection of PA in aqueous media. The good photostability of Ir1–Ir3 in H2O/THF (v/v = 7:3) was determined first (Figure S11). Subsequently, luminescence response experiments of the complexes to PA were conducted by adding PA solutions at different concentrations to suspensions of Ir1–Ir3. The emission intensities of the complexes decrease continuously with increasing PA concentration (Figure 3a–c). The quenching efficiencies of Ir1–Ir3 at a PA concentration of 10 μM (1 equiv.) are 23.3%, 23.0%, and 16.8%, respectively. With increasing PA concentrations up to 200 μM (20 equiv.), the quenching efficiencies of Ir1–Ir3 reach 97.4%, 96.0%, and 93.1%, respectively, and their emission intensities are negligible (Figure S12).
The phosphorescence response effect of the complex to PA can be studied utilizing the Stern–Volmer (SV) plot, which is constructed from the emission intensity ratio I0/I (I0 is the emission intensity in the absence of PA, and I is the emission intensity in the presence of PA) versus the concentration of PA, as shown in Figure 3d–f. The SV plots of Ir1–Ir3 show a good linear relationship at PA concentration from 0 to 10 μM, whereas the plots gradually deviate from the linearity and the luminescence quenching effects become stronger with the increase of PA concentration. At PA concentration from 0 to 10 μM, the quenching constant (KSV) can be determined by the SV equation: I0/I = KSV [Q] + 1 [46], which represents the sensitivity of the complex to detect PA. The KSV of Ir1–Ir3 is estimated to be 3.1 × 104, 3.0 × 104, and 2.0 × 104 M−1, respectively, indicating the high sensitivity of the complexes for the detection of PA. Additionally, the LODs of Ir1–Ir3 are estimated at 59, 84, and 95 nM, respectively, according to the limit of detection (LOD) equation: LOD = 3σ/K (Table S2 and Figure S13) [47]. The results indicate that Ir1–Ir3 are promising for the efficient detection of PA in aqueous media.
Considering the complexity of the probe in practical application, the selectivity, anti-interference properties, and applicability in environmental water samples of Ir1–Ir3 were explored.
Firstly, a variety of analytes (NM, 1,3-DNB, MEHQ, NB, phenol, p-cresol, and m-cresol) and ionic compounds (KF, KBr, NaHCO3, CH3COONa, FeCl2, ZnCl2, MgSO4, CuSO4, CaCl2, MnCl2, NiCl2, and CoCO3) were utilized to perform selectivity and anti-interference experiments. As shown in Figure 4a,b, Figures S14a,b, and S15a,b, the addition of analytes and ionic compounds (20 equiv.) has minimal effect on the emission intensities of Ir1–Ir3, and the quenching efficiencies are all less than 20%, which are much lower than the quenching efficiencies of the complexes for PA. Thus, the results suggest that the luminescence of Ir1–Ir3 can be selectively quenched by PA. Subsequently, PA solutions (20 equiv.) were added to the suspensions of Ir1–Ir3 in the presence of various analytes and ionic compounds for anti-interference experiments. The results indicate that the luminescence of Ir1–Ir3 in the presence of other analytes and ionic compounds can still be quenched effectively by PA, and the quenching efficiencies are almost the same as those in the presence of PA only (Figure 4c,d, Figures S14c,d, and S15c,d). The presence of various analytes and ionic compounds has almost no effect on the performance in detecting PA by the complexes. Therefore, Ir1–Ir3 exhibit excellent selectivity and anti-interference ability to detect PA in aqueous media.
To investigate the applicability of Ir1–Ir3 for the detection of PA in real environments, luminescence quenching experiments of these complexes for PA were conducted in H2O/THF, utilizing tap water, lake water, seawater, and rainwater instead of deionized water. As shown in Figure 5a–c, the emission spectra of Ir1–Ir3 in different water samples are almost the same as those in deionized water, indicating that the complexes have good stability in various environmental water samples. Moreover, the luminescence of Ir1–Ir3 in different water samples can be effectively quenched by PA with almost the same quenching efficiencies (Figure 5d). The results indicate that the complexes are promising for efficient and selective detection of PA in real environments.

3.4. Sensing of Fe3+

To achieve naked-eye detection of Fe3+ by the complexes, Fe3+ solutions were added to the suspensions of Ir1–Ir3 to observe the color change. As shown in Figures S16, S17a, and S18a, the colors of suspensions of Ir1–Ir3 gradually change from colorless to light yellow within 10 min after adding Fe3+ at different concentrations, which indicates that the possible interaction between Fe3+ and complexes leads to the color change. Subsequently, the selectivity of the complexes for the detection of Fe3+ was investigated by adding various ionic compounds to the suspensions of Ir1–Ir3 and monitoring the resulting color changes (Figure 6, Figures S17b, and S18b). The results show that only the color of the suspension of Ir1–Ir3 in the presence of Fe3+ changes to light yellow, while the color of the suspensions with other ions does not change. Therefore, the complexes are specific for the naked-eye detection of Fe3+.
Subsequently, luminescence quenching experiments were conducted by adding Fe3+ solutions of different concentrations to suspensions of Ir1–Ir3. Similar to the luminescence quenching phenomenon in the detection of PA by the complexes, the emission intensities of Ir1–Ir3 decrease with increasing Fe3+ concentration (Figure 7a–c). When the concentration of Fe3+ is 70 μM (7 equiv.), the quenching efficiencies of Ir1–Ir3 are 24.5%, 27.2%, and 22.7%, respectively. As the Fe3+ concentration reaches 800 μM (80 equiv.), the quenching efficiencies of Ir1 and Ir2 are 94.8% and 94.4%, respectively, while at a Fe3+ concentration of 1000 μM (100 equiv.), the quenching efficiency of Ir3 reaches 90.3%, and their luminescence is almost completely quenched (Figure S19).
The SV plots are constructed using the emission intensity ratio I0/I versus Fe3+ concentration, as shown in Figure 7d–f. At a Fe3+ concentration of 0–70 μM, the SV plots of Ir1–Ir3 show good linear relationships, whereas the plots gradually deviate from linearity and the luminescence quenching effects gradually become stronger with the increase of Fe3+ concentration. In the range of a Fe3+ concentration of 0–70 μM, the values of KSV for Ir1–Ir3 estimated by the SV equation are 4740, 5400, and 4370 M−1, respectively. These values suggest that the complexes exhibit high sensitivity for the detection of Fe3+. In addition, the LODs of Ir1–Ir3 for Fe3+ are estimated at 390, 510, and 450 nM, respectively, according to the LOD equation (Table S2 and Figure S20). Therefore, Ir1–Ir3 are promising for detecting Fe3+ efficiently in aqueous media.
Similarly, the selectivity, anti-interference properties, and environmental applicability of Ir1–Ir3 in the detection of Fe3+ were explored. It was demonstrated in the section on sensing of PA that various ionic compounds have minor effects on the emission spectra of Ir1–Ir3, suggesting that Ir1–Ir3 possess the ability to specifically recognize Fe3+ (Figure 4b, Figures S14b, and S15b). Subsequently, Fe3+ solutions were added to the suspensions of Ir1–Ir3 with various ions present, and their emission spectra were measured, as shown in Figure 8a–c. The luminescence of the complexes can still be effectively quenched by Fe3+ in the presence of various ions, and the quenching efficiencies are almost the same as those with only Fe3+ present (Figure 8d). The results demonstrate that Ir1–Ir3 exhibit good selectivity and anti-interference in the detection of Fe3+.
To study the applicability of Ir1–Ir3 for the detection of Fe3+ in real environments, luminescence quenching experiments of the complexes for Fe3+ were performed in different environmental water samples. As shown in Figure 9a–c, the luminescence of Ir1–Ir3 in different water samples is effectively quenched by Fe3+, with nearly identical quenching efficiencies (Figure 9d), suggesting that these complexes possess the potential to detect Fe3+ efficiently and highly selectively in the environment.

3.5. Sensing Mechanism

The luminescence quenching process usually consists of dynamic quenching and static quenching, and the key to distinguishing between these two processes is whether the lifetime of the luminescence sensor is changed by the addition of analytes [48]. Therefore, the lifetime decay traces were measured after adding PA at different concentrations to the suspensions of Ir1 (Figure 10a) and were fitted with computer software (Fluoracle, version 2.17.2) to obtain the lifetimes of Ir1. As shown in Figure S21, the lifetime of Ir1 decreases with the increase in PA concentration, which indicates that there are dynamic quenching processes in the luminescence quenching of Ir1 for PA. Subsequently, the UV–vis absorption spectra of Ir1 with PA at different concentrations were tested, as shown in Figure 10b. The addition of PA only leads to the increase in the absorbance for Ir1 at 225 and 250 nm, whereas there is no significant change in the shape and position of the absorption peaks. The increasing absorption peak appearing at 360 nm is attributed to the increasing concentration of PA. Therefore, there is no static quenching process in the luminescence quenching of Ir1 for PA.
In order to better understand the dynamic quenching process of Ir1 for PA, density functional theory (DFT) calculations were performed for Ir1, PA, and their adduct (Ir1 + PA) to determine the presence or absence of PET in the detection process, as shown in Figure 11a. The LUMO energy of Ir1 is higher than that of PA, and thus the excited state electrons of Ir1 will be transferred from the LOMO of Ir1 to that of PA and will not return to the HOMO of Ir1, and thus the luminescence of Ir1 will be quenched. In addition, the adduct has the highest stability because of its lowest energy gap. The results indicate the existence of the PET process in the luminescence quenching of Ir1 for PA. Furthermore, there is a partial overlap between the emission spectrum of Ir1 and the absorption spectrum of PA, suggesting the presence of FRET during luminescence quenching (Figure 11b). Therefore, the luminescence quenching of Ir1 for PA is the result of the synergistic effect of PET and FRET, which leads to the high efficiency and selectivity of Ir1 for detecting PA. In addition, the Job’s plot was obtained by measuring the emission spectra of the mixed systems at the different molar fractions of Ir1 while keeping the total concentration of Ir1 and PA constant (10 μM). As shown in Figure 12, the intersection in the Job’s plot is observed at the molar fraction of Ir1 of 0.5, indicating that the stoichiometric ratio of Ir1 toward PA is 1:1 [49].
Similarly, the lifetime decay traces of Ir1 in the presence of Fe3+ at different concentrations were tested (Figure 13a). The results indicate that the presence of Fe3+ also significantly reduces the luminescence lifetime of Ir1, suggesting that the dynamic quenching process occurs during the detection of Fe3+ (Figure S22). The absorption spectrum of Fe3+ does not overlap with the emission spectrum of Ir1, indicating the absence of FRET in detecting Fe3+ (Figure 13b). Thus, PET may be a reason for the luminescence quenching of Ir1 in response to Fe3+.
In addition, the UV–vis absorption spectra of Ir1 with the addition of Fe3+ at different concentrations were tested, and the results are shown in Figure 14a. With the increase in Fe3+ concentration, the absorption peak of Ir1 at 245 nm gradually disappears, while the absorption peak of Ir1 at 225 nm gradually enhances and red-shifts to 245 nm. The absorption peaks of Ir1 within the range of 200–250 nm change significantly with the addition of Fe3+, suggesting the existence of a static quenching process in the luminescence quenching of Ir1 for Fe3+. This static quenching may arise from the interactions between Fe3+ and the oxygen atoms at the diphenylphosphoryl group of the cyclometalating ligand [11,44,50,51]. Thus, the luminescence quenching of Ir1 for Fe3+ may be the result of a joint action of PET and static quenching. Additionally, the Job’s plot of Ir1 and Fe3+ are constructed and the intersection is noted at the molar fraction of Ir1 of 0.5 (Figure 14b). Consequently, the results indicate that the stoichiometric ratio of Ir1 toward Fe3+ is 1:1.

4. Conclusions

In conclusion, three AIPE-active neutral Ir(III) complexes Ir1–Ir3 were synthesized and successfully utilized as bi-responsive luminescent sensors for the detection of PA and Fe3+, respectively. All complexes provide efficient and selective detection of PA and Fe3+ in aqueous media. The quenching constants of Ir1–Ir3 for PA are 3.1 × 104, 3.0 × 104, and 2.0 × 104 M−1, respectively, and their LODs for PA are 59, 84, and 95 nM, respectively. The complexes also allow for naked-eye detection of Fe3+, which provides a more simplified method of Fe3+ detection. In addition, the quenching constants of Ir1–Ir3 for Fe3+ are 4740, 5400, and 4370 M−1, respectively, and their LODs for Fe3+ are 390, 510, and 450 nM, respectively. Ir1–Ir3 perform well in the detection of PA and Fe3+ in environmental water samples, thus promising to realize their applications in real environments. The detection mechanism of PA is attributed to the synergistic effect of PET and FRET, whereas the detection of Fe3+ may result from the joint action of PET and static quenching. These studies provide useful insights into the development and application of luminescent probes for multifunctional Ir(III) complexes.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/chemosensors13010010/s1, Figure S1: The 1H NMR spectrum of Ir1 in CDCl3; Figure S2: The HRMS of Ir1. Inset: Theoretical (top) and high-resolution mass spectra (bottom) of Ir1; Figure S3: The 1H NMR spectrum of Ir2 in CDCl3; Figure S4: The 13C NMR spectrum of Ir2 in CDCl3; Figure S5: The HRMS of Ir2. Inset: Theoretical (left) and high-resolution mass spectra (right) of Ir2; Figure S6: The 1H NMR spectrum of Ir3 in CDCl3; Figure S7: The 13C NMR spectrum of Ir3 in CDCl3; Figure S8: The HRMS of Ir3. Inset: Theoretical (top) and high-resolution mass spectra (bottom) of Ir3; Figure S9: Phosphorescence decay profiles of Ir1–Ir3 in deoxygenated CH2Cl2; Figure S10: DLS analysis of Ir1 at 70% (a), 80% (b), and 90% (c) water fractions (10 μM, H2O/THF); Figure S11: The emission spectra of Ir1 (a), Ir2 (b), and Ir3 (c) in H2O/THF (v/v = 7:3, 10 μM) at eleven time points (blank measurement). The excitation wavelength was 330 nm; Figure S12: Quenching percentages of Ir1 (a), Ir2 (b), and Ir3 (c) after adding PA at various concentrations. Insert: Photos of Ir1–Ir3 at PA concentrations of 0 and 200 μM under 365 nm UV light; Figure S13: The linear graphs of the emission intensities of Ir1 (a), Ir2 (b), and Ir3 (c) vs. the concentration of PA; Figure S14: The emission spectra of Ir2 in H2O/THF (v/v = 7:3, 10 μM) with different analytes (a) and ionic compounds (b) present. The excitation wavelength is 330 nm. Quenching percentages of Ir2 with different analytes (c) and ionic compounds (d) before (red) and after (gray) the addition of PA; Figure S15: The emission spectra of Ir3 in H2O/THF (v/v = 7:3, 10 μM) with different analytes (a) and ionic compounds (b) present. The excitation wavelength is 330 nm. Quenching percentages of Ir3 with different analytes (c) and ionic compounds (d) before (red) and after (gray) the addition of PA; Figure S16: The color change in Ir1 in H2O/THF in the presence of Fe3+ at different concentrations. The Fe3+ concentrations are 0, 50, 100, 200, 300, 400, 500, 600, 700, and 800 μM from left to right, respectively; Figure S17: (a) The color change in Ir2 in H2O/THF in the presence of Fe3+ at different concentrations. The Fe3+ concentrations are 0, 50, 100, 200, 300, 400, 500, 600, 700, and 800 μM from left to right, respectively. (b) The color of Ir2 in H2O/THF in the presence of various ionic compounds; Figure S18: (a) The color change in Ir3 in H2O/THF in the presence of Fe3+ at different concentrations. The Fe3+ concentrations are 0, 50, 100, 200, 300, 400, 500, 600, 700, and 800 μM from left to right, respectively. (b) The color of Ir3 in H2O/THF in the presence of various ionic compounds. Figure S19: Quenching percentages of Ir1 (a), Ir2 (b), and Ir3 (c) after adding Fe3+ at various concentrations; Figure S20: The linear graphs of the emission intensities of Ir1 (a), Ir2 (b), and Ir3 (c) vs. the concentration of Fe3+; Figure S21: Lifetimes of Ir1 in H2O/THF (v/v = 7:3, 10 μM) after the addition of PA at different concentrations; Figure S22: Lifetimes of Ir1 in H2O/THF (v/v = 7:3, 10 μM) after the addition of Fe3+ at different concentration; Table S1. Photophysical data of Ir1–Ir3; Table S2. The emission intensities of Ir1 at 456 nm, Ir2 at 461 nm, and Ir3 at 488 nm at eleven time points in H2O/THF (v/v = 7:3, 10 μM).

Author Contributions

Investigation, Q.Z. and J.X.; Data curation, Visualization, Writing—original draft, Q.Z.; Formal analysis, J.X.; Writing—review and editing, J.X., Q.X. and C.L.; Resources, Q.X. and C.L.; Funding acquisition, Supervision, C.L. All authors have read and agreed to the published version of the manuscript.

Funding

The authors thank the financial support from the National Natural Science Foundation of China (21978042) and the Fundamental Research Funds for the Central Universities (DUT22LAB610).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Scheme 1. Structures and the synthetic routes of Ir1–Ir3.
Scheme 1. Structures and the synthetic routes of Ir1–Ir3.
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Figure 1. The UV–vis absorption spectra (a) and normalized emission spectra (b) of Ir1–Ir3 at room temperature (10 μM in THF). The excitation wavelength was 330 nm.
Figure 1. The UV–vis absorption spectra (a) and normalized emission spectra (b) of Ir1–Ir3 at room temperature (10 μM in THF). The excitation wavelength was 330 nm.
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Figure 2. Emission spectra of Ir1 (a), Ir2 (b), and Ir3 (c) in H2O/THF with different water fractions (c = 10 μM, λex = 330 nm). Insert: The relationship between the relative emission intensity I/I0 (I is the maximum emission intensity in H2O/THF and I0 is the maximum emission intensity in THF) of Ir1–Ir3 and different water fractions.
Figure 2. Emission spectra of Ir1 (a), Ir2 (b), and Ir3 (c) in H2O/THF with different water fractions (c = 10 μM, λex = 330 nm). Insert: The relationship between the relative emission intensity I/I0 (I is the maximum emission intensity in H2O/THF and I0 is the maximum emission intensity in THF) of Ir1–Ir3 and different water fractions.
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Figure 3. The emission spectra of Ir1 (a), Ir2 (b), and Ir3 (c) in H2O/THF (v/v = 7:3, 10 μM) with PA at different concentrations. The excitation wavelength was 330 nm. The Stern–Volmer plots of Ir1 (d), Ir2 (e), and Ir3 (f) for PA. Insert: Linear SV plots of Ir1–Ir3 at PA concentrations from 0 to 10 μM.
Figure 3. The emission spectra of Ir1 (a), Ir2 (b), and Ir3 (c) in H2O/THF (v/v = 7:3, 10 μM) with PA at different concentrations. The excitation wavelength was 330 nm. The Stern–Volmer plots of Ir1 (d), Ir2 (e), and Ir3 (f) for PA. Insert: Linear SV plots of Ir1–Ir3 at PA concentrations from 0 to 10 μM.
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Figure 4. The emission spectra of Ir1 in H2O/THF (v/v = 7:3, 10 μM) with different analytes (a) and ionic compounds (b). The excitation wavelength was 330 nm. Quenching percentages of Ir1 with different analytes (c) and ionic compounds (d) before (red) and after (gray) the addition of PA.
Figure 4. The emission spectra of Ir1 in H2O/THF (v/v = 7:3, 10 μM) with different analytes (a) and ionic compounds (b). The excitation wavelength was 330 nm. Quenching percentages of Ir1 with different analytes (c) and ionic compounds (d) before (red) and after (gray) the addition of PA.
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Figure 5. The luminescent response of Ir1 (a), Ir2 (b), and Ir3 (c) toward PA in several environmental water samples (c = 10 μM, λex = 330 nm). (d) Quenching percentage of Ir1–Ir3 towards PA in environmental water samples.
Figure 5. The luminescent response of Ir1 (a), Ir2 (b), and Ir3 (c) toward PA in several environmental water samples (c = 10 μM, λex = 330 nm). (d) Quenching percentage of Ir1–Ir3 towards PA in environmental water samples.
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Figure 6. The color of Ir1 in H2O/THF in the presence of various ionic compounds.
Figure 6. The color of Ir1 in H2O/THF in the presence of various ionic compounds.
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Figure 7. The emission spectra of Ir1 (a), Ir2 (b), and Ir3 (c) in H2O/THF (v/v = 7:3, 10 μM) with Fe3+ at different concentrations. The excitation wavelength was 330 nm. The Stern–Volmer plots of Ir1 (d), Ir2 (e), and Ir3 (f) for Fe3+. Insert: Linear SV plots of Ir1–Ir3 at Fe3+ concentrations from 0 to 70 μM.
Figure 7. The emission spectra of Ir1 (a), Ir2 (b), and Ir3 (c) in H2O/THF (v/v = 7:3, 10 μM) with Fe3+ at different concentrations. The excitation wavelength was 330 nm. The Stern–Volmer plots of Ir1 (d), Ir2 (e), and Ir3 (f) for Fe3+. Insert: Linear SV plots of Ir1–Ir3 at Fe3+ concentrations from 0 to 70 μM.
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Figure 8. The emission spectra of Ir1 (a), Ir2 (b), and Ir3 (c) in H2O/THF (v/v = 7:3, 10 μM) in the simultaneous presence of other ionic compounds and Fe3+. The excitation wavelength was 330 nm. (d) Quenching percentages of Ir1–Ir3 in the simultaneous presence of other ionic compounds and Fe3+.
Figure 8. The emission spectra of Ir1 (a), Ir2 (b), and Ir3 (c) in H2O/THF (v/v = 7:3, 10 μM) in the simultaneous presence of other ionic compounds and Fe3+. The excitation wavelength was 330 nm. (d) Quenching percentages of Ir1–Ir3 in the simultaneous presence of other ionic compounds and Fe3+.
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Figure 9. The luminescent response of Ir1 (a), Ir2 (b), and Ir3 (c) toward Fe3+ in several environmental water samples (c = 10 μM, λex = 330 nm). (d) Quenching percentage of Ir1–Ir3 toward Fe3+ in environmental water samples.
Figure 9. The luminescent response of Ir1 (a), Ir2 (b), and Ir3 (c) toward Fe3+ in several environmental water samples (c = 10 μM, λex = 330 nm). (d) Quenching percentage of Ir1–Ir3 toward Fe3+ in environmental water samples.
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Figure 10. (a) Phosphorescence decay traces of Ir1 in H2O/THF (v/v = 7:3, 10 μM) with PA at various concentrations present. (b) UV–vis absorption spectra of Ir1 in H2O/THF with PA at different concentrations.
Figure 10. (a) Phosphorescence decay traces of Ir1 in H2O/THF (v/v = 7:3, 10 μM) with PA at various concentrations present. (b) UV–vis absorption spectra of Ir1 in H2O/THF with PA at different concentrations.
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Figure 11. (a) Calculated energy level diagram of Ir1, PA, and adduct (Ir1 + PA). (b) UV–vis absorption spectra of Ir1 (pink) and PA (black), and normalized emission spectrum of Ir1 (purple) (λex = 330 nm).
Figure 11. (a) Calculated energy level diagram of Ir1, PA, and adduct (Ir1 + PA). (b) UV–vis absorption spectra of Ir1 (pink) and PA (black), and normalized emission spectrum of Ir1 (purple) (λex = 330 nm).
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Figure 12. Job’s plot of Ir1 with PA obtained by emission spectra measurements.
Figure 12. Job’s plot of Ir1 with PA obtained by emission spectra measurements.
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Figure 13. (a) Phosphorescence decay traces of Ir1 in H2O/THF (v/v = 7:3, 10 μM) with Fe3+ at various concentrations present. (b) UV–vis absorption spectra of Ir1 (pink) and Fe3+ (black), and normalized emission spectrum of Ir1 (purple) (λex = 330 nm).
Figure 13. (a) Phosphorescence decay traces of Ir1 in H2O/THF (v/v = 7:3, 10 μM) with Fe3+ at various concentrations present. (b) UV–vis absorption spectra of Ir1 (pink) and Fe3+ (black), and normalized emission spectrum of Ir1 (purple) (λex = 330 nm).
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Figure 14. (a) UV–vis absorption spectra of Ir1 in H2O/THF with Fe3+ at different concentrations. (b) Job’s plot of Ir1 with Fe3+ obtained by emission spectra measurements.
Figure 14. (a) UV–vis absorption spectra of Ir1 in H2O/THF with Fe3+ at different concentrations. (b) Job’s plot of Ir1 with Fe3+ obtained by emission spectra measurements.
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Zhang, Q.; Xu, J.; Xu, Q.; Liu, C. AIPE-Active Neutral Ir(III) Complexes as Bi-Responsive Luminescent Chemosensors for Sensing Picric Acid and Fe3+ in Aqueous Media. Chemosensors 2025, 13, 10. https://doi.org/10.3390/chemosensors13010010

AMA Style

Zhang Q, Xu J, Xu Q, Liu C. AIPE-Active Neutral Ir(III) Complexes as Bi-Responsive Luminescent Chemosensors for Sensing Picric Acid and Fe3+ in Aqueous Media. Chemosensors. 2025; 13(1):10. https://doi.org/10.3390/chemosensors13010010

Chicago/Turabian Style

Zhang, Qinglong, Jiangchao Xu, Qiang Xu, and Chun Liu. 2025. "AIPE-Active Neutral Ir(III) Complexes as Bi-Responsive Luminescent Chemosensors for Sensing Picric Acid and Fe3+ in Aqueous Media" Chemosensors 13, no. 1: 10. https://doi.org/10.3390/chemosensors13010010

APA Style

Zhang, Q., Xu, J., Xu, Q., & Liu, C. (2025). AIPE-Active Neutral Ir(III) Complexes as Bi-Responsive Luminescent Chemosensors for Sensing Picric Acid and Fe3+ in Aqueous Media. Chemosensors, 13(1), 10. https://doi.org/10.3390/chemosensors13010010

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