Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Highly Selective Aluminum(III) Ion Sensing with Luminescent Iridium(III) Complexes Bearing a Distorted 2,2′-Bipyridine-3,3′-diol Moiety Utilizing a Rigidified Seven-Membered Chelate Ring Yota Suzuki,† Ibuki Mizuno,† Yui Tabei,† Yuri Fujioka,† Kazuteru Shinozaki,‡ Tomoaki Sugaya,*,§ and Koji Ishihara*,†
Downloaded via GUILFORD COLG on July 23, 2019 at 02:53:40 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda University, Okubo, Shinjuku-ku, Tokyo 169-8555, Japan ‡ Department of Materials System Science, Graduate School of Nanobioscience, Yokohama City University, Seto, Kanazawa-ku, Kanagawa 236-0027, Japan § Education Center, Faculty of Engineering, Chiba Institute of Technology, Narashino, Chiba 275-0023, Japan S Supporting Information *
ABSTRACT: To create an ion sensor utilizing a rigidified seven-membered chelate ring, we developed two Ir(III) complexes with 2,2′-bipyridine-3,3′-diol (bpy(OH)2, bpydL) ligands as reaction centers, namely Ir1 ([Ir(ppy)2{bpy(O−)(OH)}], ppy = 2-phenylpyridine) and Ir2 ([Ir(bzq)2{bpy(O−)(OH)}], bzq = benzo[h]quinoline), and evaluated their reactivities toward metal ions by spectrophotometry. When they are reacted with Al3+, these complexes exhibit dramatic enhancements in emission intensity (775-fold for Ir1 and 51.0-fold for Ir2) and distinct orange to green changes in emission color. The reactions of Ir1 and Ir2 with Al3+ were found to barely be affected by nearly all common metal ions. We conclude that these high selectivities arise from the high affinities of the (O,O) atoms in bpydL for hard metal ions and the increased strain of the seven-membered chelate ring due to the coordination of bpydL to the Ir(III) center in each complex, which excludes large metal ions out of the chelate ring.
■
INTRODUCTION Aluminum is one of the most abundant metallic elements in the Earth’s crust (∼8%). This common metal is widely used in our daily lives; however, releasing excess Al3+ into the environment inhibits plant growth, and the accumulation of excess of Al3+ in the body has harmful effects on its nervous system.1−3 Therefore, some regulations that limit the intake of Al3+ exist; indeed, the WHO has limited the weekly tolerable average intake of Al3+ to 7 mg/kg of human bodyweight.4 The development of a chemical sensing method that allows an analyte to be quantified in a sample solution without the need for expensive equipment, such as an inductively coupled plasma mass spectrometer (ICP-MS) or an atomic absorption spectrometer, is therefore an important objective. Various types of chemosensors for Al3+, such as fluorescent organic compounds,5−7 aggregation-induced emission sensors,8 and phosphorescent metal complexes, have been developed.9 However, many of these sometimes exhibit low selective recognition abilities toward Al3+ (e.g., they also react with other metal ions), do not vary significantly in luminescence during their reactions, or are difficult to synthesize. Transition metal complex based chemosensors have attracted much attention because the electrochemical, © XXXX American Chemical Society
colorimetric, and luminescence properties of metal complexes can be easily tuned by the choice of metal ions and ligands.10,11 In particular, cyclometalated iridium(III) complexes are highly promising as luminescent chemosensors because of their (1) high stabilities against heat and light, (2) high emission quantum yields, and (3) emission color tunabilities.12−14 Most Al 3+ chemosensors are based on Schiff base compounds such as salen15 and o-phenolsalicylimine,16 where Al3+ ions are caught by N,O atoms. In contrast, in the Ir(III) complex based chemosensors for Al3+ synthesized in this study, Al3+ ions are captured through O,O atoms and incorporated into seven-membered chelate rings, similar to those formed by [1,1′-binaphthalene]-2,2′-diol (BINOL).17 To the best of our knowledge, such a metal complex based chemosensor for Al3+ that uses a rigidified seven-membered chelate ring has never been reported. To develop a highly selective ion sensor molecule, we synthesized two novel luminescent cyclometalated Ir(III) complexes with 2,2′-bipyridine-3,3′-diol (bpy(OH)2, bpydL) ligands as reaction centers, namely [Ir(ppy)2{bpy(O−)(OH)}] Received: February 9, 2019
A
DOI: 10.1021/acs.inorgchem.9b00373 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry (Ir1, ppy = 2-phenylpyridine) and [Ir(bzq)2{bpy(O−)(OH)}] (Ir2, bzq = benzo[h]quinoline) (Figure 1). We found that
to a mixed solvent of 2-ethoxyethanol (30 mL) and water (10 mL). The resulting suspension was irradiated in a 400 W microwave reactor for 45 min. The obtained clear solution was concentrated to dryness in vacuo and then purified by silica gel column chromatography (CH2Cl2/acetone = 2/1) to yield a yellow solid. The NMR spectra of the products are shown in Figures S1−S10. Ir1: yield 96 mg (70%). 1H NMR (400 MHz, CDCl3), δ (ppm): 17.69 (s, 1H), 7.87 (d, 2H, J = 8.60 Hz), 7.70 (t, 2H, J = 8.58 Hz), 7.64 (d, 2H, J = 7.80 Hz), 7.61 (d, 2H, J = 5.84 Hz) 7.24 (dd, 2H, J = 1.42, 4.94 Hz), 7.18 (dd, 2H, J = 1.40, 8.40 Hz), 7.00−6.82 (m, 8H), 6.27 (d, 2H, J = 7.48 Hz). 13C NMR (100 MHz, CDCl3), δ (ppm): 168.2, 162.4, 154.0, 148.7, 146.7, 143.6, 138.5, 137.1, 131.7, 130.3, 129.7, 126.3, 124.4, 122.6, 121.7, 119.1. ESI-MS (pos.)/MeCN: m/z 689.2 (M + H+, 689.2), Anal. Calcd for C32H23IrN4O2·0.1CH2Cl2: C, 55.38; H, 3.34; N, 8.05. Found: C, 55.12; H, 3.22; N, 7.78. Ir2: yield 81 mg (60%). 1H NMR (400 MHz, CDCl3), δ (ppm): 17.75 (s, 1H), 8.23 (dd, 2H, J = 1.12, 8.00 Hz), 7.96 (dd, 2H, J = 1.12, 5.44 Hz), 7.85 (d, 2H, J = 8.80 Hz), 7.66 (d, 2H, J = 8.80 Hz), 7.45−7.38 (m, 4H), 7.27 (dd, 2H, J = 1.48, 5.00 Hz), 7.21 (dd, 2H, J = 1.44, 8.36 Hz), 7.18 (t, 2H, J = 7.44 Hz), 6.84 (m, 2H), 6.29 (d, 2H, J = 7.12 Hz). 13C NMR (100 MHz, CDCl3), δ (ppm): 162.4, 157.9, 150.6, 147.5, 147.1, 140.8, 139.0, 136.2, 134.1, 129.9, 129.7, 129.7, 129.0, 126.9, 126.2, 123.4, 121.5, 119.7. ESI-MS (pos.)/ MeCN: m/z 737.2 (M + H+, 737.2). Anal. Calcd for C36H23IrN4O2· 0.5CH2Cl2: C, 56.36; H, 3.05; N, 7.20. Found: C, 56.70; H, 2.90; N, 7.40. Instrumentation. Absorption spectra were acquired on a Shimadzu UV-2400 spectrometer, and luminescence spectra were recorded using a JASCO FP-8300 instrument. Electrospray ionization (ESI) mass spectra were acquired on a JEOL Accu-TOF JMS T100CS instrument, while C, H, and N elemental analyses were performed on a PerkinElmer Series II CHNS/O Analyzer 2400. 1H and 13C NMR spectra were recorded at room temperature on a Bruker Avance 400 spectrometer. Emission quantum yields were measured using a Hamamatsu C9920-02 absolute photoluminescence quantum yield measurement system equipped with an integrating sphere apparatus and a 150 W CW xenon light source. The emission lifetimes were measured on a TSP-1000-M-PL (Unisoku) instrument using the THG (355 nm) of a Nd:YAG laser, Minilite II (Continuum), as the excitation source. The signals were monitored with an R2949 photomultiplier. Unless otherwise noted, luminescence properties (luminescence spectra, emission quantum yield, and emission lifetime) were examined under Ar atmosphere. Single Crystal X-ray Structure Analysis. X-ray diffraction data for complexes Ir1 and Ir2 were acquired using a Bruker SMART APEX II Ultra diffractometer equipped with CCD area detector with Mo Kα radiation (λ = 0.71073 Å). Indexing was carried out using APEX2.21 Data integration and reduction were conducted by
Figure 1. Structures of Ir1 and Ir2.
both complexes exhibit highly selective responses to Al3+, with dramatic enhancements in emission intensity (775-fold for Ir1 and 51.0-fold for Ir2) as well as distinct orange to green color changes observed. These high selectivities and dramatic emission changes observed for the Ir1 and Ir2 complexes are similar to or exceed those of existing chemosensors for Al3+.
■
EXPERIMENTAL SECTION
Materials. 2-Phenylpyridine (Kanto Chemical Industry), benzo[h]quinoline (Tokyo Chemical Industries), IrCl3·nH2O (Tanaka Kikinzoku Kogyo K. K.), 2,2′-bipyridine-3,3′-diol (Sigma-Aldrich), Al(ClO4)3·9H2O, In(ClO4)3·8H2O, Cd(ClO4)2·6H2O, Fe(ClO4)2· 6H2O (Fujifilm Wako Chemicals), Fe(ClO4)3·6H2O, LiClO4, Cu(ClO4)2·6H2O, AgClO4 (Kanto Chemical Industry), Zn(ClO4)2· 6H2O, Ga(ClO4)2·nH2O, Co(ClO4)2·6H2O, NaClO4, Mn(ClO4)2· 6H2O (Sigma-Aldrich), Ni(ClO4)2·6H2O, and Pb(ClO4)2·3H2O (Nacalai Tesque) were used as received. Mg(ClO4)2·6H2O, Ca(ClO4)2·4H2O, Cr(ClO4)3·9H2O, and KClO4 were synthesized according to literature methods.18−20 Spectroscopic-grade solvents were used to acquire UV−vis absorption and emission spectra. Unless otherwise noted, spectroscopic properties were examined in mixed solvents (methanol (MeOH)/2-ethoxyethanol/water = 90/5/5 v/v/ v). Silica gel C-60 (Kanto Chemical Industry) was used for silica gel column chromatography. Caution! The perchlorate salts are potentially explosive. They should be prepared in small amounts and handled with care. Syntheses of Ir1 and Ir2. Ir1 and Ir2 were synthesized from their precursor cyclometalated Ir(III) dimer complexes, under Ar. The Ir(III) dimer (0.1 mmol) and bpydL (38 mg, 0.2 mmol) were added
Scheme 1. Synthesis Routes for Ir1 and Ir2
B
DOI: 10.1021/acs.inorgchem.9b00373 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry SaintPlus 6.01.22 Absorption corrections were performed with a multiscan method implemented in SADABS or TWINABS.23 Space groups were given by XPREP implemented in APEX2.24 All structures were solved and determined using the direct method. All nonhydrogen atoms in both complexes were solved anisotropically, whereas a riding model was used to refine the hydrogen atoms of both complexes. Structures were refined with SHELXL-97 (full-matrix least squares on F2) in APEX2. CCDC 1884540 and 1884541 include the crystallographic data for the Ir1 and Ir2 complexes; these data can be obtained from the Cambridge Crystallographic Data Centre. Molecular graphics (ORTEP diagrams) were generated with the Ortep-3 program for Windows.25 The crystallographic data are given in Table S1, and selected bond lengths and angles are given in Tables S2 and S3. Ir1 crystals were found to contain no solvent molecules; in contrast, those of Ir2 contained two CHCl3 molecules per formula unit. The carbon and chlorine atoms of two CHCl3 molecules (first CHCl3 molecule, part A C41A, part B C41B; second CHCl3 molecule, part A C42A and Cl5A, part B C42B and Cl5B) were each disordered over two positions. The C−Cl bond lengths were constrained, and the anisotropic displacement parameters of these atoms were restrained. Computational Chemistry. All density functional theory (DFT) calculations were carried out using the GAUSSIAN09 program package.26 The ground-state structures described in this paper were optimized using the PBE1PBE27 functional with a mixed basis set that included 6-31G for C, H, O, N, B, and Al and the Stuttgart−Dresden pseudopotential (sdd)28 basis set for the valence and effective core potential functions of Ir, Ga, and In. Vibrational frequency calculations were carried out to confirm the absence of imaginary frequencies. TD-DFT calculations were performed on the optimized geometries of Ir1 and Ir2 using the B3LYP functional with the ccpVDZ correlation-consistent polarized double-ζ basis set29 for all atoms, except Ir, which was treated with the sdd basis set. The polarized continuum model (PCM)30 was applied to account for the solvent effect (MeOH, ε = 32.63).
■
RESULTS AND DISCUSSION Synthesis and Characterization. Ir1 and Ir2 were readily synthesized using microwave irradiation, as shown in Scheme 1. The precursors, namely the cyclometalated Ir(III) chlorobridged dimers [Ir(C∧N)2Cl]2, were synthesized according to well-established procedures.31 The 1H NMR spectra of Ir1 and Ir2 show single sharp signals at about 17 ppm, indicative of the formation of an intramolecular hydrogen bond between the two oxygen atoms of the coordinated bpydL ligand in each case (Figures S1 and S2). Single crystals of Ir1 and Ir2 were obtained by vapor diffusion of hexane into a mixed solvent (1/1 (v/v) CH2Cl2/ CH3OH) for Ir1 or CHCl3 for Ir2. As shown in Figure 2, the pyridyl nitrogen atoms of bpydL coordinate to the Ir(III) center in preference to the hydroxyl oxygen atoms because of their high affinities for Ir(III). An intramolecular hydrogen bond (O···H−O) is observed in the bpydL moieties of each complex, in which the hydrogen-bonded O···O distances are 2.401(3) Å for Ir1 and 2.356(14) Å for Ir2. These values are shorter than those of typical hydrogen-bonded O···O distances reported thus far (>2.5 Å).32 Interestingly, the two pyridine rings of the bpydL moieties of both complexes twist around the C(5)−C(6) bond (Ir1, 14.16(9)°; Ir2, 15.17(45)°) due to the ring strain caused by the formation of the seven-membered ring that includes the O···H−O unit. General Photophysical Properties of Ir1 and Ir2. The UV−vis absorption spectra of Ir1 and Ir2 exhibit spin-allowed strong absorption bands up to 500 nm (Ir1, 390 nm (ε = 1.65 × 104 M−1 cm−1), 345 nm (ε = 1.29 × 104 M−1 cm−1); Ir2, 390 nm (ε = 1.41 × 104 M−1 cm−1), 335 nm (ε = 1.88 × 104
Figure 2. ORTEP diagrams for (A) Ir1 and (B) Ir2. Thermal ellipsoids are presented at the 50% probability level, and solvent molecules have been omitted for clarity.
M−1 cm−1); Figure S11A,B). The absorption bands were assigned using time-dependent density functional theory (TDDFT) calculations, as described in the next section. Timecourse experiments revealed that Ir1 and Ir2 were stable for at least 16 h in the mixed solvent (Figure S11C,D). As shown in Figure S12A, Ir1 and Ir2 (10 μM each) show almost identical emission spectra (ΦIr1 = 0.05 and λmaxIr1 = 560 nm; ΦIr2 = 0.04 and λmaxIr2 = 560 nm) with large Stokes shifts (8100 cm−1). Table 1 summarizes the emission lifetimes, the calculated radiative rate constants (kr), and nonradiative rate constants (knr) for both complexes. The values of kr for Ir1 (7.31 × 103 s−1) and Ir2 (3.34 × 103 s−1) are smaller than that of a typical iridium(III) cyclometalated complex (∼105 s−1).12 In order to elucidate the excited states of Ir1 and Ir2, we acquired the emission spectra of these complexes at 77 K in a C
DOI: 10.1021/acs.inorgchem.9b00373 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Table 1. Emission Properties of Ir1, Ir2, Ir1−Al3+, and Ir2−Al3+ a complex Ir1 Ir2 Ir1−Al3+ Ir2−Al3+
λmax/nm (room temp) 560 560 485 520
Φ 0.05 0.04 0.40 0.49
τ/μs
kr/s−1
6.84 15.8 11.8 14.0
× × × ×
7.31 3.34 2.53 3.50
knr/s−1 3
10 103 104 104
1.39 8.14 3.80 3.64
× × × ×
105 104 104 104
λmax/nm (77 K) 540, 540, 480, 500,
570 570 500 540
a
The spectroscopic data were acquired in mixed solvents (90/5/5 (v/v/v) MeOH/2-ethoxyethanol/water at room temperature; 1/4 (v/v) MeOH/EtOH at 77 K).
Figure 3. (A) Photographic images of Ir1 (10 μM) solutions in the presence of 80 μM Li+, Na+, Mg2+, Al3+, K+, Ca2+, Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Ga3+, Ag+, Cd2+, In3+, and Pb2+ as well as B(OH)3 under ambient atmosphere with 365 nm UV light at room temperature. (B) Relative emission intensities of Ir1 (10 μM) at 483 nm in the presence of each additive (80 μM) under Ar atmosphere at room temperature (λex = 380 nm). Inset: emission spectra of 10 μM Ir1 and mixtures of 10 μM Ir1 and 80 μM of each additive. The counterion for each cation was perchlorate.
the addition of Al3+ to the Ir1 and Ir2 solutions, the emission spectra of both complexes showed blue shifts with approximately 10-fold enhancement in emission quantum yields (Figure 4; ΦIr1−Al = 0.40 and λmaxIr1−Al = 485 nm, ΦIr2−Al = 0.49 and λmaxIr2−Al = 520 nm). At 77 K, the emission spectra of Ir1−Al3+ and Ir2−Al3+ in a mixed solvent (1:4 (v/v) MeOH/EtOH) showed slight hypsochromic shifts compared with the spectra acquired at room temperature (Figure S17). The spectra of Ir1−Al3+ at 77 K and room temperature are similar and have a moderate vibronic structures that are slightly broadened (λmaxIr1−Al = 480, 520 nm), while an obvious vibronic structure was observed for Ir2−Al3+ (λmaxIr2−Al = 500, 540 nm) at 77 K. The emission spectra of Ir1−Al3+ and Ir2−Al3+ are similar to those of fac-[Ir I I I (ppy) 3 ] 3 4 (λ m a x = 492, 530 nm) and [IrIII(bzq)2(bpy)]+ 35 (λmax = 500, 534 nm), respectively. The emissions from fac-[IrIII(ppy)3] and [IrIII(bzq)2(bpy)]+ are assigned predominantly to triplet metal to ligand charge transfer (3MLCT: Ir to C∧N) transitions, which suggest that the emissions from Ir1−Al3+ and Ir2−Al3+ are mainly due to mixed 3MLCTs (Ir to C∧N). The radiative rate constants are larger while the nonradiative rate constants are smaller for both
mixed solvent (1/4 (v/v) MeOH/ethanol (EtOH)), as shown in Figure S13. Both complexes exhibit small hypsochromic shifts, and vibronic-structured emission spectra were observed with small rigidochromic effects. These photophysical properties (large Stokes shifts, small radiative rate constants, and small rigidochromic shifts) for Ir1 and Ir2 show that the emissions from both complexes are mainly due to 3π−π* transitions rather than charge transfer. The phosphorescence spectra of Ir1 and Ir2 were acquired in the presence of the perchlorate salts of various metal ions (Li+, Na+, Mg2+, Al3+, K+, Ca2+, Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni 2+ , Cu 2+ , Zn 2+ , Ga 3+ , Ag + , Cd 2+ , In 3+ , and Pb 2+ ). Interestingly, both complexes exhibit very high selectivities toward Al3+ (Figure 3 and Figure S14). All the other cations except for Ga3+, In3+, and Fe3+ (80 mM) result in little to no emission spectral changes when they are reacted with Ir1 or Ir2 (Figures S15 and S16). Ga3+ and In3+ show slight emission enhancements; however, their intensities are very small in comparison to that of Al3+. Luminescence was almost completely quenched when Fe3+ was added to the Ir1 or Ir2 solution, which is ascribable to the coordination of the bpydL moiety of each complex to the paramagnetic Fe3+ ion.7,33 Upon D
DOI: 10.1021/acs.inorgchem.9b00373 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 4. Emission spectra of (A) Ir1 and (C) Ir (10 μM) at various Al(ClO4)3 concentrations (0, 10, 20, 30, 40, 50, 60, 80, and 100 μM) in degassed 90/5/5 (v/v/v) methanol/2-ethoxyethanol/water at room temperature (λex = 380 nm). Emission colors of 10 μM (B) Ir1 and (D) Ir2 upon addition of Al3+ under 365 nm UV light.
complexes on reaction with Al3+ (Table 1), which is reflected in the emission intensity enhancements of Ir1 and Ir2 after reaction with Al3+. These emission enhancements are explained as follows: the bpydL moiety in Ir1 or Ir2, which has reduced flexibility due to the hydrogen bond (O···H−O), is more rigid as a result of coordination to Al3+. Computational Chemistry. We turned to TD-DFT calculations to assign the excited states in these systems. Electron orbital maps for selected states and selected electronic excitations are summarized in Figures S18 and S19 and Table S6, which show that the lowest-energy emissions from Ir1 and Ir2 are mainly due to a HOMO → LUMO transition (81.5%) of the 3π−π* (bpydL) with triplet ligand to ligand charge transfer (3LLCT: bpydL to C∧N) in the former and a HOMO → LUMO+2 transition (79.2%) of the 3π−π* (bpydL) in the latter. In contrast, the emissions from the Al3+-coordinated species are assigned to mixed 3MLCT (Ir to C∧N) transitions (Ir1−Al3+, 3MLCT (Ir to ppy) with 3LLCT (ppy to bpydL); Ir2−Al3+, 3MLCT (Ir to bzq) with 3LC (bzq)). These calculations also show that the 3π−π* (bpydL−Al3+) energy gap is greater than that of the mixed 3MLCT (Ir to C∧N) emissive transition, indicating that the lowest triplet excited states of both complexes switched from 3π−π* (bpydL) to the mixed 3MLCT (Ir to C∧N) through coordination to Al3+ (Figure 5). Since the electronic orbitals of the C∧N ligands mainly contribute to the mixed 3MLCT (Ir to C∧N) excited states, the structure of the C∧N ligand directly reflects the shapes of the emission spectra of Ir1−Al3+ and Ir2−Al3+, which suggests that excited-state switching in Ir1 and Ir2 facilitates control of the emission color of [IrIII(C∧N)2{bpy(O−)(O−)}−Al3+] through selection of the C∧N ligand. Selected singlet electron transitions for Ir1 and Ir2 are given in Table S7. The absorption band at 390 nm for each complex
is attributable to spin-allowed transitions with 1LC (bpydL) and 1LLCT (bpydL → C∧N) character. The band at 345 nm observed for Ir1 is ascribable to spin-allowed 1MLCT (Ir → ppy) transition mixing with some 1LLCT (ppy → bpydL) character, whereas the band at 335 nm observed for Ir2 mainly corresponds to a spin-allowed 1LLCT (bzq → bpydL) transition. Sensing Properties of Ir1 and Ir2. The calibration curves shown in Figures S20 and S22 reveal that Ir1 and Ir2 can be used to quantify the Al3+ concentrations in the 0−50 μM range. The limits of quantification (LOQs) and the limits of detection (LODs) were also determined: LOQ = 0.653 ± 0.047 μM and LOD = 0.592 ± 0.043 μM for Ir1 and LOQ = 1.82 ± 0.15 μM and LOD = 1.31 ± 0.11 μM for Ir2 (see the Supporting Information). These results enable these complexes to be used in practical situations, with reference to the WHO limits mentioned earlier.4 The pH dependences of phosphorescence were investigated for Ir1 and Ir2 in the presence of Al3+. Figures S24 and S25 show the emission spectra of Ir1 and Ir2 at various pH values in a mixed solvent (90/5/5 (v/v/v) MeOH/2-ethoxyethanol/ water). The emission intensity increases with increasing pH to a maximum at pH 3, after which it gradually decreases to almost zero at pH ≥ 6. Therefore, the optimum pH range for the phosphorescence detection of Al3+ with the Ir1 and Ir2 complexes lies between 3 and 5. The acid dissociation constants of Ir1 and Ir2 could not be determined because the UV−vis spectra did not change in the pH 2−13 range (Figure S26); however, spectral changes corresponding to the protonation equilibria of Ir1 and Ir2 were observed at pH
