English

• 0.   Introduction

  It is well known that an imidazole unit can coordinate to a metal ion through its neutral -N= donor or deprotonated -N^-- donor. In this regard, some cyclometalated Ir(Ⅲ) complexes incorporate imidazole units, leading to various structures and the related switching of photophysical properties. For example, Williams group firstly reported the proton-switchable behavior from a pair of benzoimidazole-based cyclometalated Ir(Ⅲ) complexes [Ir(ppy)₂(pybzH)]PF₆ and [Ir(ppy)₂(pybz)] (Scheme S1)^[1], in which ligands pybzH and pybz^- coordinate with Ir(Ⅲ) ions through -N=donor and deprotonated -N^-- donor, respectively. It was found that the two complexes in CH₂Cl₂ revealed structural interconversion upon addition of acid/base (i.e. proton-switchable behavior, Scheme S1), resulting in luminescence switching between the emission at 521 nm from [Ir(ppy)₂(pybzH)]PF₆ and the emission at 496 nm from [Ir(ppy)₂(pybz)]. After this report, the other imidazole/benzoimidazole-based [Ir(C^N)₂(N^N)]⁺ complexes were designed and synthesized^[2-6], in which the acid/base-induced transition of coordination mode between -N= mode and -N^-- mode resulted in the significant variations in electronic absorption spectra^[3], luminescence intensity^[4-5], and emission color^[6].

  Recently, our group reported complexes [Ir(dfppy)(qbiH)]PF₆ (1F·PF₆) and [Ir(dfppy)(qbi)] (2F) (Scheme 1)^[6], in which an {Ir(dfppy)₂}⁺ unit is chelated by a benzoimidazole-based neutral ligand qbiH using the coordination mode N^N in 1F·PF₆, while anion ligand qbi^- using the coordination mode N^N^- in 2F. Their distinct structures result in the significant differences in luminescence, strong emission at 558 nm for 1F·PF₆ in CH₂Cl₂ while weak emission at 546 nm for 2F. Upon addition of NEt₃/TFA, the two complexes can switch their luminescence between strong emission at 558 nm and weak emission at 546 nm, due to their acid-/base-induced structural interconversion between the protonation state and the deprotonation state of qbiH ligand. We synthesized complexes [Ir(ppy)(qbiH)]NO₃ (1·NO₃) and [Ir(ppy)(qbi)] (2), which incorporate cyclometalated ligand ppy^- instead of the dfppy^- ligands in the reported complexes 1F·PF₆ and 2F (Scheme 1). The aims of this study mainly include the below two aspects. (ⅰ) We explore the influence of ligands ppy^- on the structures and associated lumine-scence of complexes 1·NO₃ and 2. (ⅱ)The neigh-boring molecules in both 1F·PF₆ and 2F stack through π…π interactions (Fig.S1 and S2), due to the incorporation of some fluorine substituents^[7]. In contrast, complexes 1·NO₃ and 2 without any fluorine substit-uent are expected to be lack of inter-molecular π…π stacking interactions. The resultant loose packing structures of both 1·NO₃ and 2 would facilitate their possible solid-state luminescence switching. Herein, we discuss the structures and luminescence modulation/switching of 1·NO₃ and 2.

  Scheme 1

  

  Scheme 1.  Molecular structures of cations 1F⁺ and 1⁺, and complexes 2F and 2

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  1.   Experimental

  1.1   Materials and physical measurements

  Compounds qbiH and [Ir(ppy)₂Cl]₂ were prepared according to the literature methods^{[6, 8]}. All other reagents were commercially available and used without further purification. Elemental analyses were performed on a Perkin Elmer 240C elemental analyzer. The IR spectra were obtained as KBr disks on a VECTOR 22 spectrometer. The ¹H NMR spectra were recorded at room temperature with a 400 MHz BRUKER spectro-meter. UV-Vis absorption spectra were measured on a Cary 100 spectrophotometer. The luminescence spectra at room temperature and at 77 K were measured on a Hitachi F-4600 fluorescence spectrometer. The lumin-escence lifetimes of 1·NO₃ and 2 both in CH₂Cl₂ and in solid state were measured at room temperature on a HORIBA FL-3 Spectrofluorometer with a 374 nm LED pulsed from a NanoLED resource. The photolumines-cence quantum yields of 1·NO₃ and 2 in CH₂Cl₂ were measured by using a relative method by comparing with a standard, a solution of quinine sulfate in 0.5 mol·L^-1 H₂SO₄ (Φ=54.6%, λ_ex=366 nm)^[9]. The quantum yields of these complexes in the solid state were measured at room temperature on a HORIBA FL-3 spectrofluorometer.

  1.2   Synthesis of [Ir(ppy)(qbiH)]NO3 (1·NO₃)

  A mixture of [Ir(ppy)₂Cl]₂ (0.15 mmol, 0.161 1 g), qbiH (0.3 mmol, 0.073 5 g), CH₂Cl₂ (12 mL) and CH₃OH (12 mL) was heated in an oil bath (50 ℃) under argon for one day, and then was evaporated under vacuum. To a solution of the obtained residue in CH₂Cl₂ (30 mL) was added a solution of AgNO₃ (0.6 mmol, 0.102 0 g) in CH₃CN (10 mL), and this mixture was stirred at room temperature overnight. After removing the resultant AgCl, the filtrate was evaporated. The residue was dissolved in a mixture of CH₂Cl₂ (40 mL) and H₂O (15 mL). The CH₂Cl₂ layer was separated and evaporated. The resultant solid was purified through silica column chromatography using CH₃OH-CH₂Cl₂ (V_CH3OH/V_CH2Cl2=0~0.01) solution, obtai-ning orange-red solid (Fig.S3) with a yield of 194.4 mg (80% based on [Ir(ppy)₂Cl]₂). Anal. Calcd. for C₃₈H₂₇ N₆O₃Ir(%): C, 56.50; H, 3.37; N, 10.40. Found(%): C, 56.57; H, 3.63; N, 10.61. IR (KBr, cm^-1): 3 444(w), 3 043(w), 2 923(w), 2 853(w), 1 604(s), 1 583(m), 1 562(w), 1 520(m), 1 477(s), 1 420(m), 1 384(s), 1 338(m), 1 321(s), 1 267(m), 1 227(w), 1 161(w), 1 116(w), 1 062(w), 1 030(w), 993(w), 873(w), 842(w), 793(w), 756(s), 731(s), 630(w), 438(w). ¹H NMR (400 MHz, CDCl₃): δ 5.91 (d, J=8.4 Hz, 1H), 6.23 (d, J=7.6 Hz, 1H), 6.48 (d, J=7.2 Hz, 1H), 6.83~6.91 (m, 4H), 6.96 ~7.05 (m, 2H), 7.10 (t, J=7.4 Hz, 1H), 7.19~7.22 (m, 2H), 7.44~7.48 (m, 2H), 7.58~7.89 (m, 9H), 8.16 (d, J=7.6 Hz, 1H), 8.51 (d, J=6.4 Hz, 1H) and 9.12 (broad, 1H) (5.91~7.22 and 7.44~9.12: total 26H from two ppy^- units and one qbiH ligand).

  1.3   Synthesis of [Ir(ppy)(qbi)] (2)

  A mixture of [Ir(ppy)₂Cl]₂ (0.15 mmol, 0.161 1 g), qbiH (0.3 mmol, 0.073 5 g), K₂CO₃ (0.6 mmol, 0.082 8 g), CH₂Cl₂ (12 mL) and CH₃OH (12 mL) was heated in an oil bath (50 ℃) under argon for one day. After evaporation under vacuum, the resultant residue was dissolved in a mixture of CH₂Cl₂ (30 mL) and H₂O (10 mL). The CH₂Cl₂ layer was separated, dried with MgSO₄, and filtered. The filtrate was evaporated under vacuum. The resultant solid was purified through silica column chromatography using CH₃OH-CH₂Cl₂ (V_CH3OH/V_CH2Cl2=0~0.005) solution, obtaining an orange-yellow solid (Fig.S3) with a yield of 181 mg (81% based on [Ir(ppy)₂Cl]₂). Anal. Calcd. for C₃₈H₂₆N₅Ir(%): C, 61.27; H, 3.52; N, 9.40. Found(%): C, 61.42; H, 3.84; N, 9.40. IR (KBr, cm^-1): 3 420(w), 3 047(w), 2 924(w), 1 604(m), 1 583(w), 1 561(w), 1 521(w), 1 477(s), 1 338(w), 1 321(w), 1 267(w), 1 161(w), 1 116(w), 1 062(w), 1 030(w), 866(w) and 755(w). ¹H NMR (400 MHz, CDCl₃): δ 5.86 (d, J=8.4 Hz, 1H), 6.26 (d, J=8.4 Hz, 1H), 6.57 (d, J=8.4 Hz, 1H), 6.67~6.73 (m, 3H), 6.85 (t, J=8.2 Hz, 1H), 6.93~7.00 (m, 2H), 7.05 (t, J=8.0 Hz, 2H), 7.13 (t, J=8.0 Hz, 1H), 7.34~7.40 (m, 2H), 7.49 (t, J=8.0 Hz, 1H), 7.56 (t, J=8.0 Hz, 1H), 7.61~7.67 (m, 2H), 7.72~7.75 (m, 2H), 7.80 (d, J=8.0 Hz, 1H), 7.84 (d, J=8.0 Hz, 1H), 7.91 (d, J=6.4 Hz, 1H), 8.12 (d, J=8.8 Hz, 1H), 8.28 (d, J=8.4 Hz, 1H) and 8.80 (d, J=8.8 Hz, 1H) (5.86~7.13 and 7.34~8.88: total 26H from two ppy^- units and one qbi^- ligand).

  1.4   X-ray crystallographic studies

  The single crystals of 1·NO₃ and 2 were grown from the corresponding CH₂Cl₂-CH₃OH solution. Single crystals of dimensions 0.21 mm×0.13 mm×0.10 mm for 1·NO₃, and 0.18 mm×0.15 mm×0.11 mm for 2 were used for structural determination on a Bruker SMART APEX CCD diffractometer using graphite-monochromated Mo Kα radiation (λ=0.071 073 nm) at room temperature. A hemisphere of data were collected in the θ range of 1.50°~26.00° for 1·NO₃, and 1.70°~28.24° for 2 using a narrow-frame method with scan widths of 0.30° in and an exposure time of 10 s per frame. Numbers of observed and unique reflections are 48 567 and 6 096 (R_int=0.037 1) for 1·NO₃, and 21 076 and 7 412 (R_int=0.0430) for 2, respectively. The data were integrated using the Siemens SAINT program^[10], with the intensities corrected for Lorentz factor, polarization, air absorption, and absorption due to variation in the path length through the detector faceplate. Multi-scan absorption corrections were applied. The structures were solved by direct methods and refined on F² by full matrix least squares using SHELXTL^[11-12]. All the non-hydrogen atoms were located from the Fourier maps, and were refined anisotro-pically. All H atoms were refined isotropically, with the isotropic vibration parameters related to the non-H atom to which they are bonded. The crystallographic data for complexes 1·NO₃ and 2 are listed in Table 1, and selected bond lengths and bond angles are given in Table 2 and 3.

  Table 1

  

  Table 1.  Crystallographic data for 1·NO₃ and 2

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  Complex	1·NO3	2
  Formula	C38H27N6O3Ir	C38H26N5Ir
  Formula weight	807.85	744.84
  Crystal system	Orthorhombic	Monoclinic
  Space group	Pbca	P21/n
  T / K	296	296
  a / nm	1.650 56(8)	1.036 46(7)
  b / nm	1.390 24(7)	1.804 17(12)
  c / nm	2.711 91(13)	1.606 89(11)
  β / (°)		93.916 0(10)
  V / nm3	6.223 0(5)	2.997 8(4)
  Z	8	4
  Dc / (g·cm-3)	1.725	1.650
  F(000)	3 184	1 464
  GOF on F2	1.035	1.015
  R1, wR2 [I>2σ(I)]*	0.021 9, 0.072 6	0.031 6, 0.057 4
  R1, wR2 (all data)	0.028 8, 0.077 4	0.063 4, 0.065 3
  (Δρ)max, (Δρ)min / (e·nm-3)	1 118, -609	836, -617
  *R1=∑||Fo|-|Fc||/∑|Fo|; wR2=[∑w(Fo2-Fc2)2/∑w(Fo2)2]1/2.

  Table 2

  

  Table 2.  Selected bond lengths (nm) and bond angles (°) for 1·NO₃

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  Ir1-C1	0.200 8(3)	Ir1-N1	0.204 8(2)	N3-C32	0.133 0(4)
  Ir1-C12	0.201 9(3)	Ir1-N3	0.214 2(2)	N4-C32	0.135 7(4)
  Ir1-N2	0.203 9(2)	Ir1-N5	0.224 9(2)
  C1-Ir1-C12	84.05(12)	N1-Ir1-N3	89.19(9)	C18-N2-Ir1	116.3(2)
  N2-Ir1-C1	95.19(10)	C1-Ir1-N5	167.73(10)	C32-N3-Ir1	114.8(2)
  N2-Ir1-C12	80.42(11)	C12-Ir1-N5	105.82(11)	C33-N3-Ir1	138.3(2)
  C1-Ir1-N1	81.10(10)	N2-Ir1-N5	93.67(9)	C31-N5-Ir1	113.3(2)
  C12-Ir1-N1	94.34(11)	N1-Ir1-N5	90.74(9)	C23-N5-Ir1	128.4(2)
  N2-Ir1-N1	173.91(10)	N3-Ir1-N5	75.65(9)	C2-C1-Ir1	128.8(2)
  C1-Ir1-N3	94.98(11)	C11-N1-Ir1	126.2(2)	C6-C1-Ir1	113.7(2)
  C12-Ir1-N3	176.14(11)	C7-N1-Ir1	115.1(2)
  N2-Ir1-N3	95.97(10)	C22-N2-Ir1	125.0(2)

  Table 3

  

  Table 3.  Selected bond lengths (nm) and bond angles (°) for 2

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  Ir1-N1	0.204 1(3)	Ir1-N5	0.226 0(3)	N3-C32	0.134 1(5)
  Ir1-N2	0.204 3(3)	Ir1-C1	0.201 8(4)	N4-C32	0.135 4(5)
  Ir1-N4	0.212 3(3)	Ir1-C12	0.201 4(4)
  C12-Ir1-C1	84.55(15)	N2-Ir1-N4	89.15(12)	C18-N2-Ir1	115.2(3)
  C12-Ir1-N1	95.00(16)	C12-Ir1-N5	170.39(14)	C32-N4-Ir1	115.9(3)
  C1-Ir1-N1	80.63(15)	C1-Ir1-N5	104.31(13)	C38-N4-Ir1	140.0(3)
  C12-Ir1-N2	80.83(16)	N1-Ir1-N5	90.18(12)	C31-N5-Ir1	112.8(2)
  C1-Ir1-N2	95.05(15)	N2-Ir1-N5	94.51(12)	C23-N5-Ir1	129.7(3)
  N1-Ir1-N2	174.30(13)	N4-Ir1-N5	75.93(12)	C2-C1-Ir1	129.8(3)
  C12-Ir1-N4	95.51(14)	C11-N1-Ir1	125.0(3)	C6-C1-Ir1	114.2(3)
  C1-Ir1-N4	175.75(14)	C7-N1-Ir1	115.9(3)
  N1-Ir1-N4	95.14(13)	C22-N2-Ir1	125.4(3)

  CCDC: 1907283, 1·NO₃; 1907284, 2.

  2.   Results and discussion

  2.1   Syntheses and structural transformation

  Complex 1·NO₃ were synthesized through the reaction of [Ir(ppy)₂Cl]₂ and qbiH in a CH₂Cl₂-CH₃OH solution at 50 ℃ for 24 hours, and followed by the anion exchange of Cl^- with NO₃^- for 1·NO₃. In contrast, complex 2 was obtained by the reaction of [Ir(ppy)₂Cl]₂ and qbiH in the presence of K₂CO₃. Thus, an [Ir(ppy)₂]⁺ unit is chelated by a qbi^- anion in complex 2, while a neutral ligand qbiH in complex 1·NO₃, which was confirmed by the crystal structures of these complexes.

  Complexes 1·NO₃ and 2 can interconvert in solution upon addition of an acid or a base, due to the structural transformation between ligand qbiH and ligand qbi^- (Scheme 1), which was supported by ¹H NMR spectra. After the CDCl₃ solution of 1·NO₃ was fully mixed with a D₂O solution of NaOH, its ¹H NMR spectrum clearly changed to that of 2 (Fig. 1). After adding some DCl in the CDCl₃ solution of 2, although the measured ¹H NMR spectrum is different from that of 1·NO₃ probably due to the influence of DCl, it is in agreement with the ¹H NMR spectrum of 1·NO₃ in CDCl₃ containing DCl (Fig. 2).

  Figure 1

  

  Figure 1.  ¹H NMR spectra of 1·NO₃ in CDCl₃ after adding NaOH and 2 in CDCl₃

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  Figure 2

  

  Figure 2.  ¹H NMR spectra of 2 in CDCl₃ after adding DCl and 1·NO₃ in CDCl₃ containing DCl

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  2.2   Crystal structures of 1·NO₃ and 2

  In order to clarify the structures of complexes 1·NO₃ and 2, their crystal structures were measured. In complex 1·NO₃, an [Ir(ppy)₂]⁺ unit is coordinated by a neutral qbiH ligand (Fig. 3). The resultant [Ir(ppy)₂(qbiH)]⁺ cation connects a NO₃- anion through hydrogen bond N4-H…O1-NO₂ (N4…O1 0.270 8(1) nm). In this cation, the Ir(Ⅲ) ion shows a distorted octahedral coordination geometry. Four of the six coordination sites around the Ir(Ⅲ) ion are occupied by two pyridine nitrogen atoms (N1, N2) and two carbon atoms (C1, C12) from two nonequivalent cyclometalated ppy^- ligands. The remaining two coordination sites are filled with atoms N3 and N5 from a qbiH ligand. In the molecular structure of 1·NO₃, two cyclometalated ppy^- ligands adopt the C, C-cis and N, N-trans arrange-ment as those in [Ir(ppy)₂Cl]₂^[13]. Ligand qbiH is arranged with their nitrogen atoms N3 and N5 lying in the trans-position for σ-bond carbon atoms (C12 and C1) in the ppy^- ligands (i.e. N, C-trans arrangement), leading to much longer distances for Ir1-N3 bond (0.214 2(2) nm) and Ir1-N5 bond (0.224 9(2) nm) compared to Ir-C(N)_ppy bonds (0.200 8(3)~0.204 8(2) nm)^[14].

  Figure 3

  

  Figure 3.  Molecular structure of 1·NO₃

  Orange dotted line indicating N-H…ONO₂ hydrogen bond; All H atoms attached to carbon atoms are omitted for clarity

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  Compared to 1·NO₃, complex 2 is a neutral complex (Fig. 4), in which a qbi^- anion uses its N4 and N5 atoms to coordinate with an [Ir(ppy)₂]⁺ unit. In the molecular structure of 2, ligands ppy^- and qbi^- adopt the same structural arrangements as those in 1·NO₃, i.e. C, C-cis, N, N-trans and N, C-trans arrangements. The Ir-C(N) distances in 2 (0.2014(4)~0.2260(3) nm) are comparable to those in 1·NO₃ (0.200 8(3)~0.224 9(2) nm).

  Figure 4

  

  Figure 4.  Molecular structure of 2

  All H atoms attached to carbon atoms are omitted for clarity

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  Clearly, complexes 1·NO₃ and 2 have different structures, a neutral ligand qbiH in the former while anion ligand qbi^- in the latter, which would lead to their distinct photophysical properties. Additionally, in the packing structures of both 1·NO₃ and 2, neigh-boring Ir(Ⅲ) fragments are held together by van der Waals interactions (Fig.S7 and S8), and there is no inter-molecular π…π stacking interactions as those in the reported complexes 1F·PF₆ and 2F. This indicates that the cyclometalated ligands ppy^- in both 1·NO₃ and 2 can significantly affect the molecular arrange-ments of these complexes.

  2.3   Electronic absorption spectra

  The UV-Vis spectra of complexes 1·NO₃ and 2 were measured in CH₂Cl₂ at room temperature (Fig. 5, Table 4). The complexes showed similar high-energy absorption bands at ~252 and 297 nm, which could be due to their ligand-centered (¹LC) transitions (ppy^- and qbiH/qbi^- ligands). However, the low-energy absorption bands of 2(368 and 390 nm) show significant red shift compared to that of 1·NO₃ (368 nm). These low-energy absorption bands in the complexes are likely to be a combination of metal-to-ligand charge transfer (¹MLCT) and ligand-centered (¹LC) transition, because of their high extinction coefficients in a range of 1.5×10⁴~2.0×10⁴ L·mol^-1·cm^{-1 [5, 14]}. In addition, both 1·NO₃ and 2 exhibited weaker absorption tails towards 490 nm, which are mainly attributed to ³MLCT absorptions in the complexes^[15-16].

  Figure 5

  

  Figure 5.  UV-Vis absorption spectra of 1·NO₃ and 2 in CH₂Cl₂

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  Table 4

  

  Table 4.  Photophysical data of 1·NO₃ and 2

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  Complex	Medium	λabs / nm	λem / nm	Lifetime / ns	Quantum yield / %
  1·NO3	CH2Cl2 (298 K)	252, 297, 368, 390 and a tail to 490	581	1 535	27.3
  	EtOH-MeOH (77 K)	—	545 and 580	—	—
  	Solid (298 K)	—	616	669 and 277	10.6
  2	CH2Cl2 (298 K)	250, 297, 368, 390 and a tail to 490	574	1 716	23.3
  	EtOH-MeOH (77 K)	—	543 and 577	—	—
  	Solid (298 K)	—	598	3 129 and 664	11.5

  2.4   Luminescence properties

  We measured the luminescence spectra of both 1·NO₃ and 2 in CH₂Cl₂ at room temperature under the excitation with 398 nm (Fig. 6). Compared to 1·NO₃ with an emission at 581 nm, complex 2 revealed a slightly blue-shifted emission at 574 nm, due to the fact that the deprotonated ligand qbi^- leads to higher energy of MLCT (from Ir(Ⅲ) ion to ligand qbi^-). At 77 K, the complexes showed blue-shifted emissions with respect to their emissions at room temperature, occurring at 545 and 580 nm for 1·NO₃, and 543 and 577 nm for 2(Fig.S9, Table 4). This rigidochromism is characteristic of a CT character for the luminescence of these complexes^[17-18]. In addition, the luminescence quantum yields (Φ) and the emission lifetimes (τ) of both 1·NO₃ and 2 were measured in degassed CH₂Cl₂ at room temperature, Φ=27.3% and τ=1 535 ns for 1·NO₃, and Φ=23.3% and τ=1 716 ns for 2.

  Figure 6

  

  Figure 6.  Luminescence spectra of 1·NO₃ and 2 in CH₂Cl₂

  c=0.1 mmol·L^-1, λ_ex=398 nm

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  Clearly, complexes 1·NO₃ and 2 in CH₂Cl₂ have different luminescence, due to their different ancillary ligands (qbiH and qbi^-, respectively). On the other hand, compared to their analogous complexes 1F·PF₆ and 2F (Scheme 1), both 1·NO₃ and 2 incorporate cyclometalated ligands ppy^-, which leads to their significantly different luminescence behaviors, mainly including the below three aspects. (ⅰ) The emission wavelengths of 1·NO₃ (581 nm) and 2 (574 nm) were longer than those of 1F·PF₆ (558 nm) and 2F (546 nm), because the molecular structures of 1·NO₃ and 2 have no electron-drawing fluorine group (Scheme 1). (ⅱ)Complexes 1·NO₃ and 2 revealed higher lumines-cence quantum yields (27.3% and 23.3%, respectively) than both 1F·PF₆ (14%) and 2F (3.2%). (ⅲ)The quantum yield of 2F (3.2%) was significantly lower than that of 1F·PF₆ (14%), but the similar quantum yields for 1·NO₃ (27.3%) and 2 (23.3%). This suggests that ligand qbi- in complex 2 has less contri-bution to the excited state of this complex.

  Although complexes 1·NO₃ and 2 revealed the similar emission color in CH₂Cl₂ (Fig. 6), they exhibited significantly different solid-state luminescence (Fig. 7). A red emission at 611 nm was observed for 1·NO₃, while an orange emission at 598 nm for complex 2. The emissions of both 1·NO₃ and 2 in solid state reveal significantly red shift compared to the corres-ponding emission in CH₂Cl₂, with Δλ=35 nm and 24 nm, respectively, which could be due to molecular aggregation in solid state^[19]. For the solid-state samples of both 1·NO₃ and 2, we further measured their luminescence quantum yields and lifetimes. The complexes revealed similar solid-state luminescence quantum yield, Φ=10.6% for 1·NO₃ and 11.5% for 2. However, the solid-state emission lifetimes of 1·NO₃ (τ₁=669 ns, 79% contribution and τ₂=277 ns, 21% contribution) are significantly shorter than those of 2 (τ₁=3 129 ns, 77% contribution and τ₂=664 ns, 23% contribution). The solid-state luminescence behaviors of 1·NO₃ and 2 are clearly different from those of complexes 1F·PF₆ (emissions at 542, 572 and 611 nm) and 2F (emissions at 595 and 633 nm). This is in agreement with their distinct molecular structures and stacking structures (Scheme 1).

  Figure 7

  

  Figure 7.  Luminescence spectra of 1·NO₃ and 2 in solid state at room temperature

  Inset: photographs of the complexes under 365 nm light

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  It is interesting that complexes 1·NO₃ and 2 in solid state show acid/base-induced emission switching (Fig. 8). Upon meeting the vapor of Et₃N, complex 1·NO₃ changed its emission color from red to orange, indicating Et₃N-induced structural transition from 1⁺ to 2 (i.e. from [Ir(ppy)(qbiH)]⁺ to [Ir(ppy)(qbi)]). On the other hand, complex 2 showed emission-color change from orange to red upon meeting TFA vapor, which could be due to structural transition from 2 to 1⁺. These experimental results indicate that solid-state complexes 1·NO₃ and 2 can undergo TFA-/NEt₃-induced structural interconversion, leading to their emission color switching between red and yellow. It should be noted that the similar solid-state emission switching behavior has not been observed for 1F·PF₆ and 2F, which could be due to their close molecular stacking through π…π interactions (Fig.S1 and S2). Moreover, we found that the anion NO₃^- in complex 1·NO₃ could be partly replaced by CF₃COO^- in TFA vapor, which was confirmed by the measurement of IR spectra (Fig.S14). After the treatment by TFA vapor, complex 1·NO₃ showed two new peaks at 1 672 and 1 202 cm^-1 from CF₃COO^- anion, indicating the replacement of NO₃^- by CF₃COO^-. Before and after meeting TFA vapor, complex 1·NO₃ always revealed the absorption peaks of NO_3- (1 425 and 842 cm^-1), suggesting that only a part of NO₃^- anions are replaced by CF₃COO^- anions.

  Figure 8

  

  Figure 8.  TFA-/Et₃N-induced emission switching for solidsate 1·NO₃ and 2 under 365 nm light at room temperature

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  3.   Conclusions

  In summary, we synthesized two new cyclo-metalated Ir(Ⅲ) complexes [Ir(ppy)(qbiH)]NO₃ (1·NO₃) and [Ir(ppy)(qbi)] (2). Their crystal structures indicate that an [Ir(ppy)₂]⁺ unit is chelated by a neutral benzoimidazole-based ligand qbiH in 1·NO₃, while anion ligand qbi^- in 2. Neighboring Ir(Ⅲ) fragments in both 1·NO₃ and 2 are held together only by van der Waals interaction. The distinct molecular structures and packing structures between 1·NO₃ and 2 lead to their different luminescence both in CH₂Cl₂ and in solid state. In CH₂Cl₂, an emission at 581 nm with Φ=27.3% was observed for 1·NO₃ and 574 nm with Φ=23.3% for 2. In solid state, complexes 1·NO₃ and 2 exhibit a red emission at 611 nm and an orange emission at 598 nm, respectively. Complexes 1·NO₃ and 2 revealed structural interconversion upon addition of acid/base (i.e. DCl/NaOD) in their CDCl₃ solution, which is assigned to acid/base-induced structural transformation between ligand qbiH and ligand qbi^-. This structural transformation can even occur in solid-state 1·NO₃ and 2, probably due to the loose inter-molecular stacking in the two complexes. Upon meeting Et₃N/TFA vapor, the solid-state samples of 1·NO₃ and 2 revealed luminescence switching between red emission and orange emission.

  Supporting information is available at http://www.wjhxxb.cn

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• 

  Scheme 1  Molecular structures of cations 1F⁺ and 1⁺, and complexes 2F and 2

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  Figure 1  ¹H NMR spectra of 1·NO₃ in CDCl₃ after adding NaOH and 2 in CDCl₃

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  Figure 2  ¹H NMR spectra of 2 in CDCl₃ after adding DCl and 1·NO₃ in CDCl₃ containing DCl

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  Figure 3  Molecular structure of 1·NO₃

  Orange dotted line indicating N-H…ONO₂ hydrogen bond; All H atoms attached to carbon atoms are omitted for clarity

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  Figure 4  Molecular structure of 2

  All H atoms attached to carbon atoms are omitted for clarity

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  Figure 5  UV-Vis absorption spectra of 1·NO₃ and 2 in CH₂Cl₂

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  Figure 6  Luminescence spectra of 1·NO₃ and 2 in CH₂Cl₂

  c=0.1 mmol·L^-1, λ_ex=398 nm

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  Figure 7  Luminescence spectra of 1·NO₃ and 2 in solid state at room temperature

  Inset: photographs of the complexes under 365 nm light

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  Figure 8  TFA-/Et₃N-induced emission switching for solidsate 1·NO₃ and 2 under 365 nm light at room temperature

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  Table 1.  Crystallographic data for 1·NO₃ and 2

  Complex	1·NO3	2
  Formula	C38H27N6O3Ir	C38H26N5Ir
  Formula weight	807.85	744.84
  Crystal system	Orthorhombic	Monoclinic
  Space group	Pbca	P21/n
  T / K	296	296
  a / nm	1.650 56(8)	1.036 46(7)
  b / nm	1.390 24(7)	1.804 17(12)
  c / nm	2.711 91(13)	1.606 89(11)
  β / (°)		93.916 0(10)
  V / nm3	6.223 0(5)	2.997 8(4)
  Z	8	4
  Dc / (g·cm-3)	1.725	1.650
  F(000)	3 184	1 464
  GOF on F2	1.035	1.015
  R1, wR2 [I>2σ(I)]*	0.021 9, 0.072 6	0.031 6, 0.057 4
  R1, wR2 (all data)	0.028 8, 0.077 4	0.063 4, 0.065 3
  (Δρ)max, (Δρ)min / (e·nm-3)	1 118, -609	836, -617
  *R1=∑||Fo|-|Fc||/∑|Fo|; wR2=[∑w(Fo2-Fc2)2/∑w(Fo2)2]1/2.

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  Table 2.  Selected bond lengths (nm) and bond angles (°) for 1·NO₃

  Ir1-C1	0.200 8(3)	Ir1-N1	0.204 8(2)	N3-C32	0.133 0(4)
  Ir1-C12	0.201 9(3)	Ir1-N3	0.214 2(2)	N4-C32	0.135 7(4)
  Ir1-N2	0.203 9(2)	Ir1-N5	0.224 9(2)
  C1-Ir1-C12	84.05(12)	N1-Ir1-N3	89.19(9)	C18-N2-Ir1	116.3(2)
  N2-Ir1-C1	95.19(10)	C1-Ir1-N5	167.73(10)	C32-N3-Ir1	114.8(2)
  N2-Ir1-C12	80.42(11)	C12-Ir1-N5	105.82(11)	C33-N3-Ir1	138.3(2)
  C1-Ir1-N1	81.10(10)	N2-Ir1-N5	93.67(9)	C31-N5-Ir1	113.3(2)
  C12-Ir1-N1	94.34(11)	N1-Ir1-N5	90.74(9)	C23-N5-Ir1	128.4(2)
  N2-Ir1-N1	173.91(10)	N3-Ir1-N5	75.65(9)	C2-C1-Ir1	128.8(2)
  C1-Ir1-N3	94.98(11)	C11-N1-Ir1	126.2(2)	C6-C1-Ir1	113.7(2)
  C12-Ir1-N3	176.14(11)	C7-N1-Ir1	115.1(2)
  N2-Ir1-N3	95.97(10)	C22-N2-Ir1	125.0(2)

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  Table 3.  Selected bond lengths (nm) and bond angles (°) for 2

  Ir1-N1	0.204 1(3)	Ir1-N5	0.226 0(3)	N3-C32	0.134 1(5)
  Ir1-N2	0.204 3(3)	Ir1-C1	0.201 8(4)	N4-C32	0.135 4(5)
  Ir1-N4	0.212 3(3)	Ir1-C12	0.201 4(4)
  C12-Ir1-C1	84.55(15)	N2-Ir1-N4	89.15(12)	C18-N2-Ir1	115.2(3)
  C12-Ir1-N1	95.00(16)	C12-Ir1-N5	170.39(14)	C32-N4-Ir1	115.9(3)
  C1-Ir1-N1	80.63(15)	C1-Ir1-N5	104.31(13)	C38-N4-Ir1	140.0(3)
  C12-Ir1-N2	80.83(16)	N1-Ir1-N5	90.18(12)	C31-N5-Ir1	112.8(2)
  C1-Ir1-N2	95.05(15)	N2-Ir1-N5	94.51(12)	C23-N5-Ir1	129.7(3)
  N1-Ir1-N2	174.30(13)	N4-Ir1-N5	75.93(12)	C2-C1-Ir1	129.8(3)
  C12-Ir1-N4	95.51(14)	C11-N1-Ir1	125.0(3)	C6-C1-Ir1	114.2(3)
  C1-Ir1-N4	175.75(14)	C7-N1-Ir1	115.9(3)
  N1-Ir1-N4	95.14(13)	C22-N2-Ir1	125.4(3)

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  Table 4.  Photophysical data of 1·NO₃ and 2

  Complex	Medium	λabs / nm	λem / nm	Lifetime / ns	Quantum yield / %
  1·NO3	CH2Cl2 (298 K)	252, 297, 368, 390 and a tail to 490	581	1 535	27.3
  	EtOH-MeOH (77 K)	—	545 and 580	—	—
  	Solid (298 K)	—	616	669 and 277	10.6
  2	CH2Cl2 (298 K)	250, 297, 368, 390 and a tail to 490	574	1 716	23.3
  	EtOH-MeOH (77 K)	—	543 and 577	—	—
  	Solid (298 K)	—	598	3 129 and 664	11.5

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