Ion-pairing assemblies of π-extended anion-responsive organoplatinum complexes

ABSTRACT

Pt^II complexes of π-extended dipyrrolyldiketones were synthesized as anion-responsive π-electronic molecules. The dipyrrolyldiketone Pt^II complexes exhibited red-shifted absorption and photoluminescence properties. In the solid state, [1 + 1]-type anion complexes formed charge-by-charge ion-pairing assemblies when combined with countercations. Detailed theoretical studies of the packing structures revealed favorable interactions between the planar anion complexes and π-electronic cations.

GRAPHIC ABSTRACT

IMPACT STATEMENT

Pt^II complexes of π-extended dipyrrolyldiketones, introducing arylethynyl substituents, in the form of anion complexes exhibited the formation of charge-by-charge assemblies with π-electronic cations via ⁱπ–ⁱπ interactions.

KEYWORDS:

• Pt^II complexes
• anion-responsive π-electronic systems
• ion-pairing assemblies

1. Introduction

π-Electronic systems exhibit fascinating electronic and electrooptical properties based on the tunable electronic states that affect their absorption and luminescence properties. The assembly of π-electronic systems in an ordered arrangement results in a variety of supramolecular nanoarchitectonics [1,2] to show unique electrooptical properties that cannot be observed in single molecules [3–7]. In particular, the solid-state photophysical properties of π-electronic systems depend on the stacking arrangement of their components. Among the various solid-state luminescent materials, organoplatinum(II) complexes exhibit various photoluminescence properties such as intense phosphorescence from triplet excited states [8–13]. Assemblies with an ordered arrangement of organoplatinum(II) complexes are suitable for the applications in light-emitting devices. Recent studies on the micro- and nanocrystals of organoplatinum(II) complexes have also exhibited triplet energy transfer and phosphorescence anisotropy amplification [14,15]. π-Extended ligands in organoplatinum(II) complexes modulate the photophysical properties of single molecules as well as those in assembled states. Stacking of organoplatinum(II) complexes in the assembled state often interferes with photoluminescence [16]. Strategies for maintaining photoluminescence by isolating π-electronic molecules in the solid state have been reported [17,18]. The introduction of bulky substituents to the peripheral ligands prevents the Pt^II complexes from stacking. Furthermore, ion-pairing assemblies [19] of charged emissive species and appropriate counterions can yield photoluminescent materials (Figure 1(a)) [20,21]. For example, solid-state ion-pairing assemblies of non-emissive receptor–anion complexes and emissive countercations exhibit enhanced emission [22,23]. Controlling the ordered arrangement of emissive π-electronic molecules in the solid state is important for the fabrication of photoluminescent materials.

Figure 1. (a) Conceptual diagram for emissive charge-by-charge assembly comprising emissive π-electronic anion and bulky cation and (b) [1 + 1]-type anion-binding mode of dipyrrolyldiketone Pt^II complex 2a.

Among the various phosphorescent organoplatinum(II) complexes (e.g. 2a, Figure 1(b)), dipyrrolyldiketone Pt^II complexes bearing arylpyridine ligands exhibit efficient anion-binding behavior via hydrogen bonding at the pyrrole NH and bridging CH (Figure 1(b)) [24]. Modification of arylpyridine ligands results in red-shifted phosphorescence properties [25]. The square planar dipyrrolyldiketone Pt^II complexes exhibit non-emissive solid states owing to the efficient π–π stacking. In contrast, ion-pairing assemblies of the anion complexes and counter alkylammonium cations show enhanced phosphorescence behavior owing to their charge-by-charge assembly mode (Figure 1(a)). The emissive anion complexes are spatially isolated by counter alkylammonium cations, resulting in enhanced phosphorescence derived from the monomeric anion complexes. The solid-state phosphorescence properties, such as emission wavelength, can be tuned by the π-extension of ligands. Thus far, modifications at the pyrrole α-positions of dipyrrolyldiketone Pt^II complexes have been limited to the substitution with 2,6-dimethylphenyl moieties. In this study, Pt^II complexes of π-extended dipyrrolyldiketones were synthesized to evaluate ion-pairing assemblies of Pt^II complexes in their anion-binding forms with π-electronic cations.

2. Experimental section

2.1. Synthesis and characterization

2.1.1. General procedures

Starting materials were purchased from FUJIFILM Wako Pure Chemical Corp., Nacalai Tesque Inc., Tokyo Chemical Industry Co., Ltd., Sigma-Aldrich Co., and Tanaka Kikinzoku Kogyo K.K. and were used without further purification unless otherwise stated. Nuclear magnetic resonance (NMR) spectra used in the characterization of products were recorded on a JEOL ECA-600 600 MHz spectrometer. All NMR spectra were referenced to solvent. UV-visible absorption spectra were recorded on a Hitachi U-3500 spectrometer. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was recorded on a Shimadzu Axima-CFRplus. TLC analyses were carried out on aluminum sheets coated with silica gel 60 (Merck 5554). Column chromatography was performed on Wakogel C-300.

2.1.2. 1,3-Bis(3,4-diethyl-5-phenylethynylpyrrol-2-yl)-1,3-propanedione, 1b´

In a round-bottomed flask, according to the procedure that has been modified from the previous one [26], BF₂ complex of 1,3-bis(3,4-diethyl-5-phenylethynylpyrrol-2-yl)-1,3-propanedione [27,28] 1b (240 mg, 0.427 mmol) and LiOH·H₂O (1.31 g, 31.2 mmol) were dissolved in 1,4-dioxane (90 mL) and water (90 mL) under N₂ atmosphere. To the solution was added acetic acid (AcOH) (66 μL, 1.17 mmol). The reaction mixture was stirred at 90°C for 2.5 h. The solution was acidified dropwise with 1 M HCl aq at pH 7. After the removal of the solvent under vacuum, the residue was then partitioned between water and CH₂Cl₂. The organic phase was dried over anhydrous Na₂SO₄, and was evaporated under vacuum. The residue was then purified by chromatography over silica gel column (Wakogel C-300; eluent: CHCl₃) and was recrystallized from CH₂Cl₂/n-hexane to give 1b´ (182 mg, 0.354 mmol, 83%) as a yellow solid. R_f = 0.75 (CH₂Cl₂). ¹H NMR (600 MHz, DMSO-d₆, 20°C; diketone 1b´ was obtained as a mixture of keto and enol tautomers in the ratio of 0.59:1): δ (ppm) keto from 12.06 (br, 2 H, NH), 7.53 (d, J = 7.2 Hz, 4 H, Ar-H), 7.43 (t, J = 6.9 Hz, 6 H, Ar-H), 4.33 (s, 2 H, bridging CH₂), 2.69 (q, J = 7.8 Hz, 4 H, CH₂), 2.57–2.54 (m, 4 H, CH₂), 1.18–1.13 (m, 12 H, CH₃); enol from 11.74 (br, 2 H, NH), 7.53 (d, J = 7.2 Hz, 4 H, Ar-H), 7.43 (t, J = 6.9 Hz, 6 H, Ar-H), 6.49 (s, 1 H, bridging CH), 2.77 (q, J = 7.8 Hz, 4 H, CH₂), 2.57–2.54 (m, 4 H, CH₂), 1.18–1.13 (m, 12 H, CH₃). ¹³C{¹H} NMR (151 MHz, DMSO-d₆, 20°C): δ (ppm) 184.56, 176.04, 132.22, 132.09, 131.47, 131.06, 131.10, 130.84, 128.98, 128.96, 128.92, 128.52, 125.29, 122.26, 122.10, 115.05, 114.75, 94.43, 93.68, 92.29, 81.60, 81.40, 50.85, 18.08, 18.02, 17.49,17.30, 15.92, 15.75, 15.73, 15.38. MALDI-TOF-MS (% intensity): m/z: 513.3 (100), 514.3 (80), 515.3 (20). Calcd for C₃₅H₃₃N₂O₂ ([M – H]^–): 513.25.

2.1.3. 1,3-Bis(3,4-diethyl-5-(4-fluorophenyl)ethynylpyrrol-2-yl)-1,3-propanedione, 1c´

In round-bottomed flask, according to the procedure that has been modified from the previous one [26], BF₂ complex of 1,3-bis(3,4-diethyl-5-(4-fluorophenyl)ethynylpyrrol-2-yl)-1,3-propanedione [27,28] 1c (7.4 mg, 0.012 mmol) and LiOH.H₂O (20.1 mg, 0.479 mmol) were dissolved in 1,4-dioxane (30 mL) and water (3 mL) under N₂ atmosphere. To the solution was added acetic acid (3.0 μL, 0.053 mmol). The reaction mixture was stirred at 90°C for 12 h. The solution was acidified dropwise with 1 M HCl aq at pH 7. After the removal of the solvent under vacuum, the residue was then partitioned between water and CH₂Cl₂. The organic phase was dried over anhydrous Na₂SO₄, and was evaporated under vacuum. The residue was then purified by chromatography over silica gel column (Wakogel C-300; eluent: CHCl₃) and was recrystallized from CH₂Cl₂/n-hexane to give 1c´ (5.94 mg, 0.011 mmol, 87%) as a yellow solid. R_f = 0.80 (CH₂Cl₂). ¹H NMR (600 MHz, DMSO-d₆, 20°C; diketone 1c´ was obtained as a mixture of keto and enol tautomers in the ratio of 0.51:1): δ (ppm) keto from 12.05 (br, 2 H, NH), 7.58–7.56 (m, 4 H, Ar-H), 7.29 (t, J = 8.4 Hz, 4 H, Ar-H), 4.31 (s, 2 H, bridging CH₂), 2.68 (q, J = 7.8 Hz, 4 H, CH₂), 2.55–2.51 (m, 4 H, CH₂), 1.16–1.11 (m, 12 H, CH₃); enol from 11.74 (br, 2 H, NH), 7.58–7.56 (m, 4 H, Ar-H), 7.29 (t, J = 8.4 Hz, 4 H, Ar-H), 6.46 (s, 1 H, bridging CH), 2.76 (q, J = 7.8 Hz, 4 H, CH₂), 2.55–2.51 (m, 4 H, CH₂), 1.16–1.11 (m, 12 H, CH₃). ¹³C{¹H} NMR (151 MHz, DMSO-d₆ 20°C): δ (ppm) 184.43, 175.96, 162.85, 162.81, 161.21, 161.16, 133.38, 133.32, 133.31, 133.25, 132.12, 131.99, 131.39, 130.73, 128.49, 125.25, 118.70, 118.68, 118.55, 118.53, 116.24, 116.21, 116.09, 116.07, 114.85, 114.56, 93.25, 92.50, 92.24, 91.30, 81.09, 70.76, 17.99, 17.91, 14.40, 17.21, 15.79 15.61, 15.29. MALDI-TOF-MS (% intensity): m/z: 549.24 (100), 550.2 (55), 551.2 (15). Calcd for C₃₅H₃₁F₂N₂O₂ ([M – H]^–): 549.24. This compound was further characterized by single-crystal X-ray analysis.

2.1.4. (1,3-Bis(3,4-diethyl-5-phenylethynylpyrrol-2-yl)-1,3-propanedionato-κ²O,O´)[2-(2-pyridinyl-κN)phenyl-κC]platinum, 2b

According to the literature procedure [24,25], in a dried round-bottomed flask, [(PtMe₂)₂(µ-SMe₂)₂] [29] (50 mg, 0.086 mmol) was dissolved in tetrahydrofuran (THF) (1.5 mL) under N₂ atmosphere. To the solution was added 2-phenylpyridine (ppy) (25 μL, 0.16 mmol). The resulting mixture was stirred at room temperature (r.t.) for 1 h, and trifluoromethanesulfonic acid (TfOH) (15 μL, 0.17 mmol) was added dropwise. The reaction mixture was stirred for 1 h, and then a solution of 1b´ (41.2 mg, 0.080 mmol) and K₂CO₃ (50.6 mg, 0.361 mmol) in THF (5 mL) was added. The mixture was stirred for 3 h. After the removal of the solvent under vacuum, the residue was purified with column chromatography over silica gel (Wakogel C-300; eluent: CH₂Cl₂/n-hexane = 3/2) to give 2b (12.8 mg, 14.8 µmol, 15%) as an orange solid. Silica gel column chromatography and recrystallization processes were conducted under dark condition by covering with aluminum foil. R_f = 0.67 (CH₂Cl₂/n-hexane = 3/2 (v/v)). m.p.: 146°C. ¹H NMR (600 MHz, DMSO-d₆, 20°C): δ (ppm) 11.58 (br, 1 H, NH), 11.45 (br, 1 H, NH), 9.04 (d, J = 5.4 Hz, 1 H, Ar-H), 8.07 (t, J = 7.5 Hz, 1 H, Ar-H), 8.02 (d, J = 7.8 Hz, 1 H, Ar-H), 7.68 (d, J = 7.8 Hz, 1 H, Ar-H), 7.59 (d, J = 7.2 Hz, 1 H, Ar-H), 7.55–7.53 (m, 4 H, Ar-H), 7.45–7.40 (m, 7 H, Ar-H), 7.13 (t, J = 7.2 Hz, 1 H, Ar-H), 7.08 (t, J = 6.6 Hz, 1 H, Ar-H), 6.31 (s, 1 H, bridging CH), 2.94–2.86 (m, 4 H, CH₂), 2.60–2.58 (m, 4 H, CH₂), 1.26–1.18 (m, 12 H, CH₃). UV/vis (CH₂Cl₂, λ_max[nm] (ε, 10⁴ M⁻¹cm⁻¹)): 462 (6.6). MALDI-TOF-MS (% intensity): m/z: 860.3 (70), 861.4 (100), 862.3 (80). Calcd for C₄₆H₄₀N₃O₂Pt ([M – H]^–): 860.28. This compound was further characterized as the Cl^– complex (ion pairs) by single-crystal X-ray analysis.

2.1.5. (1,3-Bis(3,4-diethyl-5-(4-fluorophenyl)ethynylpyrrol-2-yl)-1,3-propanedionato-κ²O,O´)[2-(2-pyridinyl-κN)phenyl-κC]platinum, 2c

According to the literature procedure [24,25], in a dried round-bottomed flask, [(PtMe₂)₂(µ-SMe₂)₂] [29] (39.2 mg, 0.0679 mmol) was dissolved in THF (2 mL) under N₂ atmosphere. To the solution was added ppy (21.1 μL, 0.136 mmol). The resulting mixture was stirred at r.t. for 1 h, and TfOH (15 μL, 0.14 mmol) was added dropwise. The reaction mixture was stirred for 1 h, and then a solution of 1c´ (74.8 mg, 0.136 mmol) and NaOH (5.40 mg, 0.135 mmol) in THF (3.5 mL) and MeOH (0.5 mL) was added. The mixture was stirred for 14 h. After the removal of the solvent under vacuum, the residue was purified with column chromatography over silica gel (Wakogel C-300; eluent: CH₂Cl₂/n-hexane = 3/2) to give 2c (38.6 mg, 43.0 µmol, 32%) as an orange solid. Silica gel column chromatography and recrystallization processes were conducted under dark condition by covering with aluminum foil. R_f = 0.72 (CH₂Cl₂/n-hexane = 3/2 (v/v)). Decomposed at > 220°C without melting. ¹H NMR (600 MHz, CDCl₃, 20°C): δ (ppm) 9.14 (br, 1 H, NH), 9.01 (d, J = 6.0 Hz, 1 H, Ar-H), 8.93 (br, 1 H, NH), 7.86 (t, J = 8.4 Hz, 1 H, Ar-H), 7.66 (d, J = 7.8 Hz, 1 H), 7.62 (d, J = 7.8 Hz, 1 H, Ar-H), 7.54–7.49 (m, 5 H, Ar-H), 7.30 (t, J = 7.2 Hz, 1 H, Ar-H), 7.21 (t, J = 6.9 Hz, 1 H, Ar-H), 7.15 (t, J = 6.9 Hz, 1 H, Ar-H), 7.08 (t, J = 7.8 Hz, 4 H, Ar-H), 6.35 (s, 1 H, bridging CH), 2.86–2.80 (m, 4 H, CH₂), 2.66–2.62 (m, 4 H, CH₂), 1.31–1.23 (m, 12 H, CH₃). UV/vis (CH₂Cl₂, λ_max[nm] (ε, 10⁴ M⁻¹cm⁻¹)): 460 (7.9). MALDI-TOF-MS (% intensity): m/z: 896.2 (64), 897.3 (100), 898.2 (94). Calcd for C₄₆H₃₈F₂N₃O₂Pt ([M – H]^–): 897.26. This compound was further characterized as the Cl^– complex (ion pair) by single-crystal X-ray analysis.

2.2. Method for single-crystal X-ray analysis

Crystallographic data are summarized in Table 1. A single crystal of 1c´ was obtained by vapor diffusion of n-hexane into a CH₂Cl₂ solution. The data crystal was a yellow block of approximate dimensions 0.135 mm × 0.135 mm × 0.088 mm. A single crystal of 2b·Cl^–-TBA⁺ (TBA⁺ = tetrabutylammonium) was obtained by vapor diffusion of n-hexane into an acetone solution of the mixture of 2b and TBACl in the 1:1 ratio. The data crystal was a yellow needle of approximate dimensions 0.10 mm × 0.05 mm × 0.01 mm. The data crystal was an orange needle of approximate dimensions 0.182 mm × 0.021 mm × 0.014 mm. A single crystal of 2c·Cl^–-TBA⁺ was obtained by vapor diffusion of n-octane into a CHCl₃ solution of the mixture of 2c and TBACl in the 1:1 ratio. A single crystal of 2b·Cl^–-TPPAu⁺ was obtained by vapor diffusion of n-hexane into a THF solution of the mixture of 2b and tetraphenylporphyrin Au^III complex as a Cl^– salt (TPPAuCl) [30] in the 1:1 ratio. The data crystal was a yellow needle of approximate dimensions 0.10 mm × 0.05 mm × 0.02 mm. A single crystal of 2c·Cl^–-TPPAu⁺ was obtained by vapor diffusion of n-hexane into an acetone solution of the mixture of 2c and TPPAuCl [30] in the 1:1 ratio. The data crystal was an orange needle of approximate dimensions 0.30 mm × 0.30 mm × 0.05 mm. The data of 1c´ and 2b·Cl^–-TPPAu were collected at 90 K on a DECTRIS PILATUS3 CdTe 1 M diffractometer with Si (311) monochromated synchrotron radiation (λ = 0.4134 Å) at BL02B1 (SPring-8) [31], and those of 2b·Cl^–-TBA⁺, 2c·Cl^–-TBA⁺, and 2c·Cl^–-TPPAu⁺ were collected at 90, 100, and 90 K, respectively, on a DECTRIS EIGER X 1 M diffractometer with Si (111) monochromated synchrotron radiation (λ = 0.80977, 0.81070, and 0.81250 Å, respectively) at BL40XU (SPring-8) [32,33]. All the structures were solved by dual-space method. The structures were refined by a full-matrix least-squares method by using a SHELXL 2014 [34] (Yadokari-XG) [35,36]. In each structure, the non-hydrogen atoms were refined anisotropically. CIF files (CCDC-2326910–2326914) can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Table 1. Crystallographic details for 1c´, 2b·Cl^–-TBA⁺, 2c·Cl^–-TBA⁺, 2b·Cl^–-TPPAu⁺, and 2c·Cl^–-TPPAu⁺.

	1c´	2b·Cl^–-TBA^+	2c·Cl^–-TBA^+	2b·Cl^–-TPPAu^+	2c·Cl^–-TPPAu^+
formula	C35H32F2N2O2	C46H41N3O2PtCl· C16H36N	C46H39N3O2F2PtCl· C16H36N	C46H41N3O2PtCl· C44H28AuN4	C46H39F2N3O2PtCl· C44H28AuN4·0.5C3H6O
fw	550.62	1140.81	1176.80	1708.02	1773.05
crystal size, mm	0.135 × 0.135 × 0.088	0.10 × 0.05 × 0.01	0.10 × 0.05 × 0.02	0.182 × 0.021 × 0.014	0.30 × 0.30 × 0.05
crystal system	triclinic	triclinic	triclinic	monoclinic	triclinic
space group	P-1 (no. 2)	P-1 (no. 2)	P-1 (no. 2)	P21/c (no. 14)	P-1 (no. 2)
a, Å	13.939(13)	8.3650(12)	8.6106(4)	21.253(11)	12.9662(9)
b, Å	15.653(15)	15.407(3)	15.5274(8)	13.828(7)	13.3402(9)
c, Å	22.30(2)	43.844(5)	21.8913(9)	26.237(13)	22.4086(18)
α, °	91.48(2)	81.485(8)	103.845(4)	90	105.8934(15)
β, °	90.724(14)	85.275(9)	93.341(4)	108.162(8)	105.4630(15)
γ, °	114.070(11)	86.419(13)	97.512(4)	90	94.294(2)
V, Å^3	4440(7)	5562.1(14)	2805.2(2)	7326(6)	3546.9(4)
ρcalcd, gcm^−3	1.235	1.362	1.393	1.549	1.660
Z	6	4	2	4	2
T, K	90(2)	90(2)	100(2)	90(2)	90(2)
μ, mm^−1	0.036^a	3.623^a	3.633^a	0.989^a	5.744^a
no. of reflns	91446	52768	33930	199572	38823
no. of unique reflns	19154	20112	12424	16266	12978
variables	1209	1253	535	917	958
λ, Å	0.4134^a	0.80977^a	0.81250^a	0.4134^a	0.81070^a
R1 (I > 2σ(I))	0.0954	0.1126	0.1351	0.0506	0.0483
wR2 (I > 2σ(I))	0.2691	0.2614	0.3408	0.1088	0.1522
GOF	0.992	1.048	1.045	1.149	1.205

^aSynchrotron radiation.

2.3. Computational method

Density functional theory (DFT) calculations of the geometrical optimizations were carried out using the Gaussian 16 program [37].

2.4. Method for emission spectra, quantum yields, and emission lifetimes

Emission spectra and quantum yields were recorded on a Hitachi F-4500 fluorescence spectrometer and a Hamamatsu Quantum Yields Measurements System for Organic LED Materials C9920-02, respectively. Emission lifetimes were measured using a C7990S system (Hamamatsu Photonics) equipped with a 403-nm excitation laser, producing 62-ps pulses with a repetition rate of 100 kHz.

3. Results and discussions

3.1. Synthesis and characterization

Dipyrrolyldiketone Pt^II complex 2a was synthesized via Pt^II complexation of dipyrrolyldiketone 1a´ as a precursor to dipyrrolyldiketone BF₂ complex 1a. Arylethynyl-substituted dipyrrolyldiketones as key building blocks for the Pt^II complexes were prepared by removing a BF₂ unit from the corresponding BF₂ complexes using a modified method [26]. Treatment of α-arylethynyl dipyrrolyldiketone BF₂ complexes 1b,c [27,28] with LiOH·H₂O in 1,4-dioxane/water followed by the addition of AcOH afforded dipyrrolyldiketones 1b´,c´ with yields of 83% and 87%, respectively. Pt^II complexes 2b,c were prepared with yields of 15% and 32%, respectively, by treating 1b´,c´ with a mixture of arylpyridine and [(PtMe₂)₂(SMe₂)₂] [29] at r.t. in the presence of TfOH and K₂CO₃ (Figure 2) [38]. The obtained Pt^II complexes 2b,c were characterized by ¹H NMR and MALDI-TOF-MS. The complexes 2b,c exhibited red-shifted absorption maxima (λ_max) at 462 and 460 nm, respectively, in CH₂Cl₂ compared to unsubstituted 2a (416 nm) (Figure 3 and S5) [24]. Time-dependent (TD)-DFT calculations [37] at PCM-B3LYP/6–31+G(d,p) with LanL2DZ for Pt (CH₂Cl₂) for the optimized structures revealed that the main absorption bands were mainly attributed to the highest occupied molecular orbital (HOMO)-to-lowest unoccupied molecular orbital (LUMO) transitions, with small contributions from the ligand-to-ligand charge transfer (LLCT) from the dipyrrolyldiketone unit to the arylpyridine ligand and the ligand-to-metal charge transfer (LMCT) (Figures S23 and S24). Furthermore, in deoxygenated CH₂Cl₂, 2b,c exhibited red-shifted phosphorescence emissions at 600 and 598 nm, respectively, which were red-shifted by ~80 nm compared to that of 2a, with quantum yields of 0.11 and 0.14, respectively (Figure S32). The emission lifetimes for 2b,c were 17.3 and 8.0 μs, respectively (Figure S34), suggesting similar magnitude for previously reported dipyrrolyldiketone Pt^II complexes including 2a [24,25]. Although dipyrrolyldiketone Pt^II complexes in the absence of pyrrole α-substituents, including 2a, are stable under ambient conditions, π-extended Pt^II complexes 2b,c showed degradation over 10 h when kept in solution, converting to the corresponding dipyrrolyldiketones 1b´,c´ by Pt^II demetallation. Therefore, a detailed evaluation of the solution-state electronic properties over a prolonged time could not be conducted.

Figure 2. Synthesis of π-extended dipyrrolyldiketone Pt^II complexes 2b,c.

Figure 3. UV/vis absorption spectra (solid lines, CH₂Cl₂) and normalized emission spectra (dashed lines, deoxygenated CH₂Cl₂) with excitations at 415 and 462 nm for 2a (gray) and 2b (black), respectively (inset: photographs of 2a,b under UV₃₆₅ (0.02 mM)).

3.2. Anion-binding behaviors

Despite slow degradation, the anion-binding behavior of the dipyrrolyldiketone Pt^II complexes in solution was preliminarily elucidated by anion-titration experiments via ¹H NMR. Upon the addition of 1.3 equivalent of TBACl to 2b in CD₂Cl₂ (1.0 mM) at −50°C, the ¹H NMR signals of the pyrrole NH and bridging CH at 9.29/9.23 and 6.38 ppm were shifted downfield to 12.62 and 6.61 ppm, respectively, suggesting the formation of [1 + 1]-type Cl^– complexes as also observed for the dipyrrolyldiketone Pt^II complex 2a (Figure S31) [24,25]. It should be noted that the [2 + 1]-type anion-binding mode, seen in arylethynyl-substituted BF₂ complexes including 1b [27,28], was not observed. This can be attributed to the smaller anion-binding cavity for 2b·Cl^– than 1b·Cl^–, as suggested by theoretically optimized structures [37]. The bridging ∠C–C–C angle in the diketone unit of the optimized 2b·Cl^– was 128.1°, which is larger by 9.1° than that of 1b·Cl^–, suggesting that the larger Pt^II induced the larger ∠ C–C–C angle and resulting smaller anion-binding cavity (Figure S19). The formation of the [1 + 1]-type Cl^– binding mode was also evaluated using UV/vis absorption spectral changes upon the addition of TBACl in CH₂Cl₂ (2.0 × 10⁻⁵ M) (Figure S30). The absorbance at the λ_max of 462 and 460 nm for 2a,b, respectively, decreased upon the addition of TBACl, suggesting the inversion of arylethynyl-substituted pyrrole units upon Cl^– binding. A significant decrease in the λ_max absorbances was observed in the corresponding BF₂ complexes 1b,c [27,39].

3.3. Solid-state ion-pairing structures

Solid-state ion-pairing assemblies of receptor–anion complexes and countercations were revealed by X-ray analysis of single crystals. The single crystals of 2b·Cl^–-TBA⁺ and 2c·Cl^–-TBA⁺ were prepared by vapor diffusion of acetone/n-hexane and CHCl₃/n-octane, respectively, for the mixed solutions of the Pt^II complexes and TBACl. Single-crystal X-ray analysis of 2b·Cl^–-TBA⁺ and 2c·Cl^–-TBA⁺ revealed the [1 + 1]-type Cl^–-binding mode using pyrrole N–H···Cl^– and bridging C–H···Cl^– hydrogen-bonding interactions with the N/C(–H)···Cl^– distances of 3.18/3.19 and 3.60 Å (for an independent structure) and 3.28/3.17 and 3.60 Å, respectively (Figure 4, S11, and S12). The planar Cl^– complexes 2b·Cl^– and 2c·Cl^–, showing mean-plane deviations of 0.37/0.49 (two independent structures) and 0.35 Å, respectively, for the planes consisting of arylethynyl-substituted dipyrrolyldiketones and Cl^–, were alternately arranged with counter TBA⁺, forming charge-by-charge columnar structures. The proximally located Pt^II···Pt^II distance in 2b·Cl^–-TBA⁺ and 2c·Cl^–-TBA⁺ were 6.73 and 5.78 Å, respectively, indicating the absence of favorable Pt^II···Pt^II interactions. In both cases, TBA⁺ was positioned in close proximity to the dipyrrolyldiketone–Cl^– complex unit. Consequently, the phenylpyridine unit of the Pt^II complex was partially stacked with the distances measuring 3.45 and 3.58 Å, respectively. The nearly flat geometries of the square planar Pt^II complexes were also indicated by τ₄ values [40] of 0.08/0.09 and 0.09 for 2b·Cl^– and 2c·Cl^–, respectively.

Figure 4. Single-crystal X-ray analysis of (a) 2b·Cl^–-TBA⁺ and (b) 2c·Cl^–-TBA⁺ ((i) top and side views and (ii) packing diagrams). Atom color code in Figure 4 and the following figures: brown, pink, blue, red, yellow green, green (spherical), and gray refer to carbon, hydrogen, nitrogen, oxygen, fluorine, chlorine, and platinum, respectively.

Ion-pairing assemblies with π-electronic cations have also been investigated using single-crystal X-ray analysis. Single crystals of 2b·Cl^–-TPPAu⁺ and 2c·Cl^–-TPPAu⁺ (TPPAu⁺: meso-tetraphenylporphyrin Au^III complex [30]) were prepared by vapor diffusion of THF/n-hexane and acetone/n-hexane, respectively. TPPAu⁺ has been used as a planar π-electronic cation for ion-pairing assemblies with anion-responsive π-electronic molecules in their anion complex forms [19,24,41]. In the solid state, 2b·Cl^– and 2c·Cl^– formed planar [1 + 1]-type Cl^–-binding structures with hydrogen bonding with the pyrrole-N(–H)···Cl^– and bridging-C(–H)···Cl^– distances of 3.19/3.18 and 3.68 Å and 3.09/3.12 and 3.60 Å, respectively (Figure 5, S13, and S14). Mean-plane deviations for 2b·Cl^– and 2c·Cl^– are 0.20 and 0.28 Å, respectively, which are smaller than those in the ion-pairing assemblies with TBA⁺. The smaller mean-plane deviations are attributed to the stacking of the planar anion complexes and TPPAu⁺. In fact, the distances between the mean planes of the Cl^– complexes and TPPAu⁺ are 3.91 and 3.61 Å, respectively. Planar units of the phenylpyridine–Pt^II units are also stacked with TPPAu⁺ with the stacking distances of 3.65 and 3.26 Å, respectively, forming charge-by-charge stacking columnar structures. It should be noticed that the planes comprising the phenylpyridine and dipyrrolyldiketone–Cl^– complex units were independently stacked with the proximally located TPPAu⁺. The Hirshfeld surface analysis [42] of the stacking structures of phenylpyridine and TPPAu⁺ showed red and blue triangles arranged in bow-tie shapes on the shape-index surface and a flat region on the curvedness, suggesting characteristic mapping patterns for π–π stacking structures (Figure 6, S17, and S18).

Figure 5. Single-crystal X-ray analysis of (a) 2b·Cl^–-TPPAu⁺ and (b) 2c·Cl^–-TPPAu⁺ ((i) top and side views and (ii) packing diagrams).

Figure 6. Hirshfeld surface analysis of (a) 2b·Cl^–-TPPAu⁺ and (b) 2c·Cl^–-TPPAu⁺ mapped over (i) shape-index and (ii) curvedness properties for selected stacked structures.

The charge-by-charge assemblies of planar [1 + 1]-type anion complexes as pseudo-π-electronic anions and countercations were further analyzed to determine the interaction energies between the components. Energy decomposition analysis (EDA) based on an FMO2-MP2 [43–47] using mixed basis sets including NOSeC-V-TZP for Pt and NOSeC-V-DZP for the other atoms for stacked structure of 2b·Cl^– and TPPAu⁺ suggested total interaction energy (E_tot) of −203.1 kcal/mol with contributions of electrostatic (E_es) and dispersion (E_disp) interaction energies of −61.9 and −155.6 kcal/mol (for the larger E_tot ion pair), respectively, whereas the E_tot, E_es, and E_disp of the proximally located 2b·Cl^– and TBA⁺ were −170.5, −86.3, and −95.2 kcal/mol, respectively (Figure 7(a)). The larger absolute value of E_tot for 2b·Cl^–-TPPAu⁺ than 2b·Cl^–-TBA⁺ is mainly attributed to the larger E_disp value, suggesting the occurrence of effective ⁱπ–ⁱπ interactions between stacked 2b·Cl^– and TPPAu⁺ [48]. Similarly, the larger E_disp for 2c·Cl^–-TPPAu⁺ than 2c·Cl^–-TBA⁺ resulted in the larger E_tot value (Figure 7(b)). In contrast to dipyrrolyldiketone boron complexes [49], the charge-by-charge assemblies of anion-responsive π-electronic systems with π-electronic cations through ⁱπ–ⁱπ interactions were limited to the Pt^II complexes [24,25]. π-Extension of the dipyrrolyldiketone Pt^II complexes resulted in the formation of larger π-electronic anions, which can be used as the versatile building units in ion-pairing assemblies.

Figure 7. Energy decomposition analysis (EDA) of single-crystal X-ray structures: (a)(i) 2b·Cl^–-TBA⁺ and (ii) 2c·Cl^–-TPPAu⁺ and (b)(i) 2b·Cl^– TPPAu and (ii) 2c·Cl^–-TPPAu⁺ (left: packing structures and right: interaction energies for proximally located ion pairs). The anion and cation parts are represented in magenta and cyan colors, respectively.

4. Conclusions

Arylethynyl-substituted dipyrrolyldiketone Pt^II complexes, as anion-responsive π-electronic systems, exhibit red-shifted absorption and photoluminescence properties. Single-crystal X-ray analysis revealed charge-by-charge assemblies of anion complexes and countercations. In particular, ion-pairing assemblies with π-electronic cation form effective stacked structures via ⁱπ–ⁱπ interactions. The stacking of phenylpyridine as a π-electronic ligand for Pt^II complexes is also important in ion-pairing assemblies. Further modifications through π-extension and the incorporation of chiral units would lead to the development of phosphorescent ion-pairing materials with chiroptical properties.

Acknowledgments

This work was supported by JSPS KAKENHI Grant Numbers JP18H01968 and JP22H02067 for Scientific Research (B), JP19K05444 for Scientific Research (C), JP23K17951 for Challenging Research (Exploratory), and JP20H05863 for Transformative Research Areas (A) ‘Condensed Conjugation’, and Ritsumeikan Global Innovation Research Organization (R-GIRO) project (2017–22 and 2022–27). Theoretical calculations were partially performed at the Research Center for Computational Science, Okazaki, Japan (Projects: 21-IMS-C077, 22-IMS-C077, and 23-IMS-C069). Synchrotron-radiation analysis was performed at BL40XU (2022A1483 and 2023B1390) and BL02B1 (2021B1808) of SPring-8 with approval from the Japan Synchrotron Radiation Research Institute (JASRI). We thank Dr. Kouhei Ichiyanagi, Dr. Nobuhiro Yasuda, and Dr. Yuiga Nakamura JASRI/SPring-8, for the single-crystal synchrotron-X-ray analysis and Prof. Hitoshi Tamiaki, Ritsumeikan University, for the various measurements.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplemental data

Supplemental data for this article can be accessed online at https://doi.org/10.1080/14686996.2024.2313958.

Additional information

Funding

The work was supported by the Japan Society for the Promotion of Science [JP18H01968, JP22H02067, JP19K05444, JP23K17951, JP20H05863].