Syntheses, crystal structures and phosphorescence properties of cyclometalated iridium(III) bis(pyridylbenzaldehyde) complexes with dithiolate ligands

1 Introduction

Recently, phosphorescent heavy-metal complexes have been attracting more and more attention due to their applications such as in organic light-emitting diodes, photovoltaic cells, biological labeling reagents, and hydrogen production via the photoreduction of water [1], [2]. Among these complexes, the luminescent cyclometalated iridium(III) complexes have been widely used due to their relatively short excited state lifetime, high photoluminescence efficiency, and excellent color tuning [3]. In particular, Thompson and coworkers have studied the influence of a variety of ancillary ligands on the excited-state properties of bis-cyclometalated iridium(III) complexes [4]. Dithiolates are well known as heavy metal chelating agents and as collector agents in the flotation process for sulfide minerals [5]. Their strong ligand field as ancillary ligands increases their electron transporting ability, and accordingly, transition metal complexes containing dithiolate ligands have been widely reported [6], [7]. Platinum(II), gold(I), and iridium(III) complexes with dithiolate ligands often exhibit interesting luminescent properties [8], [9], of which the cyclometalated iridium(III) complexes with the dithiocarbamate ligands can serve as highly selective turn-on chemodosimeters for Hg²⁺ by the dissociation of the dithiocarbamate ancillary ligands [10], [11]. The compound 4-(2-pyridyl)benzaldehyde (pbaH) has an aldehyde group, which can react with primary amine functions of biomolecules to form stable secondary amines after reductive amination [12], [13], [14]. As a result, several groups have investigated the possibility of the corresponding iridium(III) complexes as novel luminescent cross-linkers for biological substrates [15], [16] or as environmentally friendly novel materials for the preparation of oxygen-sensor films [17]. In this paper, we report a series of neutral luminescent bis-cyclometalated iridium(III) complexes with pba as a cyclometalated ligand and dithiolates as ancillary ligands with different ligand field strength, [Ir(pba)₂(S^S)] (pbaH=4-(2-pyridyl)benzaldehyde; S^S=Et₂NCS₂⁻ (1), ⁱPrOCS₂⁻ (2), (ⁿPrO)₂PS₂⁻ (3)).

2 Experimental section

3 Results and discussion

As shown in Scheme 1, direct reaction of IrCl₃·nH₂O and 2.5 equivalents of pbaH in refluxing ethoxyethanol-water overnight easily afforded the typical cyclometalated iridium(III) dimer [Ir(pba)₂(μ-Cl)]₂, which was then treated with Na[Et₂NCS₂], K[ⁱPrOCS₂], or K[(ⁿPrO)₂PS₂], respectively, to result in the isolation of the corresponding final products [Ir(pba)₂(S^S)] (1–3) in moderate yields. All the new complexes were characterized by ¹H and ¹³C NMR spectroscopy along with infrared spectroscopy and satisfactory micro-analyses. The characteristic aldehyde moieties of complexes 1–3 are observed as a singlet at ≈δ=9.6 ppm in the ¹H NMR spectra, at δ≈193 ppm in the ¹³C NMR spectra, and an intense IR absorption band at ≈1688 cm⁻¹. These data are comparable to those of other related complexes [Ir(pba)₂(N^N)]PF₆ (N^N=2,2′-bipyridine, 1,10-phenanthroline, 3,4,7,8-tetra- methyl-1,10-phenanthroline, 4,7-diphenyl-1,10-phenanthroline) [16], [17]. The ethyl, iso-propyl, and n-propyl groups of the dithiolate ligands in complexes 1–3 showed the expected signals in the high field region of the ¹³C NMR spectra (δ=10.3–43.8 ppm). The ³¹P NMR signal appeared at δ=100.96 ppm for complex 3, which compares well to that in the bis-cyclometalated iridium(III) complex {Ir(FOX)₂[(EtO)₂PS₂)]}(FOX=2,5-bis(4-fluorophenyl)-1,3,4-oxadiazole; δ=106.5 ppm) [22].

Scheme 1:

Syntheses of complexes 1, 2, and 3.

The structures of complexes 1 and 3 were unambiguously established by single-crystal X-ray diffraction analysis. The molecular structures of 1 and 3 along with selected bond lengths and bond angles are shown in Figs. 1 and 2, respectively. Complex 1 crystallizes in the orthorhombic space group Pccn with Z=4, and complex 3 crystallizes in the triclinic space group P1̅ with Z=2. Complex 1 has crystallographic two-fold (C₂) symmetry. Both complexes have distorted octahedral coordination geometry around the iridium atom by two cyclometalated pba ligands and one dithiolate ligand with cis-C–C and trans-N–N dispositions. In complex 1, the Ir–C distance is 2.012(5) Å, the Ir–N distance is 2.040(5), and the Ir–S bond distance is 2.4638(15) Å, all being normal by comparison with other related bis-cyclometalated iridium(III) complexes bearing dithiolate Et₂NCS₂⁻ ligands [23], [24], [25]. The average Ir–C, Ir–N and Ir–S distances in 3 are 2.012(5), 2.052(4), and 2.5233(14) Å, respectively, similar to those in 1 and other related iridium(III) complex with fluorinated 1,3,4-oxadiazole derivatives as cyclometalated ligands and dithiolate (EtO)₂PS₂⁻ as an ancillary ligand [22]. The bond angle of S–Ir–S in 1 is 71.46(6)°, more acute than that in 3 (79.44(5)°); accordingly, the N–Ir–N angle in 1 is 174.0(2)°, a little larger than that in 3 (170.82(13)°).

Fig. 1:

Perspective view of Ir(pba)₂(Et₂NCS₂) (1). Displacement ellipsoids are shown at the 40% probability level. Selected distances (Å) and angles (deg): Ir(1)–S(1) 2.4638(15), Ir(1)–N(1) 2.040(5), Ir(1)–C(1) 2.012(5), S(1)–Ir(1)–S(1A) 71.46(6), N(1A)–Ir(1)–S(1) 96.69(13), N(1)–Ir(1)–S(1) 88.20(11), C(1)–Ir(1)–S(1A) 171.92(12), C(1)–Ir(1)–S(1) 100.98(14), N(1)–Ir(1)–N(1A) 174.0(2), N(1)–Ir(1)–C(1) 79.9(2), N(1)–Ir(1)–C(1A) 95.7(2), C(1)–Ir(1)–N(1A) 95.7(2), C(1)–Ir(1)–C(1A) 86.7(3).

Fig. 2:

Perspective view of Ir(pba)₂[(ⁿPrO)₂PS₂] (3). Displacement ellipsoids are shown at the 40% probability level. Selected distances (Å) and angles (deg): Ir(1)–S(1) 2.5191(12), Ir(1)–S(2) 2.5275(15), Ir(1)–N(1) 2.048(4), Ir(1)–N(2) 2.056(4), Ir(1)–C(7) 2.012(5), Ir(1)–C(19) 2.012(4); S(1)–Ir(1)–S(2) 79.44(5), N(1)–Ir(1)–S(1) 91.52(10), N(1)–Ir(1)–S(2) 96.97(12), N(2)–Ir(1)–S(1) 96.96(11), N(2)–Ir(1)–S(2) 88.07(11), C(7)–Ir(1)–S(1) 95.33(11), C(7)–Ir(1)–S(2) 173.90(11), C(19)–Ir(1)–S(1) 174.25(12), C(19)–Ir(1)–S(2) 95.32(13), N(1)–Ir(1)–N(2) 170.82(13), N(1)–Ir(1)–C(7) 79.93(17), N(2)–Ir(1)–C(7) 95.73(16), C(19)–Ir(1)–N(1) 91.44(15), C(19)–Ir(1)–N(2) 80.42(16), C(7)–Ir(1)–C(9) 90.03(16).

Figure 3 shows the UV/Vis absorption spectra of complexes 1–3 in dichloromethane solution. All the three complexes exhibit very strong absorption bands below 350 nm (ε>9×10⁴ L mol⁻¹ cm⁻¹) due to the spin-allowed π-π* transition of the ligands. The moderately intense bands between 350 and 450 nm (ε~2×10⁴ L mol⁻¹ cm⁻¹) probably correspond to spin-allowed singlet metal-to-ligand charge transfer (¹MLCT) [17]. Compared with complex 1, replacing the auxiliary ligand Et₂NCS₂⁻ with ⁱPrOCS₂⁻ in 2 and (ⁿPrO)₂PS₂⁻ in 3 leads to a hypsochromic shift in the ¹MLCT transition energy from 420 to 400 nm, confirming that the dithiolates (ⁿPrO)₂PS₂⁻ and ⁱPrOCS₂⁻ are stronger ligands than the Et₂NCS₂⁻ ligand. The lowest energy peaks (ε~1.4×10⁴ L mol⁻¹ cm⁻¹) can be assigned to a spin-forbidden triplet metal-to-ligand charge transfer (³MLCT) and ³π-π* transitions, which gain considerable intensity by mixing with the ¹MLCT transition spin-orbit coupling [22].

Fig. 3:

UV/Vis absorption spectra of complexes 1, 2, and 3 in dichloromethane solutions.

Complexes 1–3 are non-luminous in air-saturated dichloromethane solution due to the oxygen quenching. Thus, the room temperature photoluminescence spectra of the three complexes were tested on samples doped in poly(methylmethacrylate) (PMMA) with 1% mass concentration [26]. The photoluminescence spectra are shown in Fig. 4. Luminescence emission spectroscopy is more sensitive to structural changes than absorption spectroscopy in general. Complex 1 emits intense luminescence with an emission wavelength at 568 nm, while the wavelengths of complexes 2 and 3 are blue-shifted to 555 and 553 nm, respectively, which is consistent with the corresponding UV/Vis absorption wavelengths. The full width at half maximum of complexes 1, 2, and 3 are 64, 63 and 62 nm, respectively.

Fig. 4:

Photoluminescence spectra of complexes 1, 2, and 3 in PMMA (1 wt% doping concentration).

To evaluate the metal ion-reactive nature of these cyclometalated iridium(III) complexes, the influence of two equivalents of different metal cations (Hg²⁺, Ag⁺, Cu²⁺, Fe³⁺, Fe²⁺, Co²⁺, Ni²⁺, Zn²⁺, Mg²⁺, Mn²⁺, Cr³⁺, Pb²⁺, and Cd²⁺) was investigated with UV/Vis absorption spectroscopy. As is shown in Fig. 5a–c, all transition metal ions gave nearly no disturbance to the UV/Vis absorption spectra of complexes 1–3. These results indicate that the present cyclometalated iridium(III) complexes have adequate stability to different transition metal ions.

Fig. 5:

UV/Vis absorption spectra of complexes 1–3 in the presence of two equivalents of transition metal ions: (a) 1; (b) 2; and (c) 3.

In summary, three bis-cyclometalated iridium(III) complexes [Ir(pba)₂(S^S)] with 4-(2-pyridyl)benzaldehyde as a cyclometalated ligand and different dithiolates as ancillary ligands have been synthesized and characterized. The luminescence emission spectroscopy is more sensitive to structural changes than absorption spectroscopy in general. Moreover, the three neutral cyclometalated iridium(III) complexes have adequate stability to different transition metal ions.