Multiresponsive Tetradentate Phosphorescent Metal Complexes as

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Functional Inorganic Materials and Devices

Multi-responsive Tetradentate Phosphorescent Metal Complexes as Highly Sensitive and Robust Luminescent Oxygen Sensors: Pd(II) versus Pt(II) and 1,2,3-Triazolyl versus 1,2,4-Triazolyl Lijie Liu, Xiang Wang, Faraz Hussain, Chao Zeng, Bowen Wang, Zechen Li, Igor Kozin, and Suning Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02023 • Publication Date (Web): 11 Mar 2019 Downloaded from http://pubs.acs.org on March 16, 2019

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ACS Applied Materials & Interfaces

Multi-responsive Tetradentate Phosphorescent Metal Complexes as Highly Sensitive and Robust Luminescent Oxygen Sensors: Pd(II) versus Pt(II) and 1,2,3-Triazolyl versus 1,2,4-Triazolyl Lijie Liu,† Xiang Wang,‡ Faraz Hussain,‡ Chao Zeng, † Bowen Wang, † Zechen Li, † Igor Kozin,‡ and Suning Wang*†‡ †Beijing

Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of

Chemistry, Beijing Institute of Technology, Beijing 100081, People’s Republic of China ‡Department

of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada

KEYWORDS: Oxygen Sensor, blue phosphorescence, Pd and Pt complex, excimer emission, tetradentate ligand

ACS Paragon Plus Environment

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ABSTRACT

Two Pd(II) complexes based on tetradentate chelate ligands with either 1,2,4-triazolyl (Pd1) or 1,2,3-triazolyl (Pd2) unit were synthesized and their structure-property relationships were studied. Both Pd1 and Pd2 are rare bright deep blue Pd(II) phosphors with contrasting properties. Pd1 displays stimuli-responsive luminescence in response to UV irradiation, concentration or temperature change, which is ascribed to the facile switching of monomer to excimer emission. In contrast, similar stimuli-responsive luminescence was not observed for Pd2. Crystal structures and TD-DFT computational studies established that the excimer formation of Pd1 is caused by electronically favored intermolecular π-π interactions and less steric protection of the Pd core due to the position of its alky chains, compared to Pd2. In solution, the excimer emission of Pd1 shows a much greater sensitivity toward oxygen than the monomer emission with a very large SternVolmer constant (Ksv) that is more than twice of that of the monomer emission. Both Pd(II) complexes are found to be outstanding oxygen sensors in ethyl cellulous (EC) films with superior 𝑎𝑝𝑝 -1 sensitivity (𝐾𝑎𝑝𝑝 𝑠𝑣 = 0.228-0.346 Torr ) over their Pt(II) equivalents (𝐾𝑠𝑣 = 0.00674 - 0.0110 Torr 1),

owing to their long phosphorescence decay lifetimes. Furthermore, Pd1 shows an excellent

photostability, comparable to the Pt(II) analogue, making it one of the best and highly robust oxygen sensors based on cyclometalated metal complexes.


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INTRODUCTION Phosphorescent molecules are ideal luminescent oxygen sensor probes (OSPs) as their emission from the triplet state can be readily quenched by the ground state oxygen. Thus, OSPs based on Ru(II),1 - 4 Ir(III),5-7 Os(II),8, 9 Pt(II)10-12 and Pd(II)13, 14 complexes are of particular interests and have been extensively studied in the last few decades. Among them, cyclometalated phosphorescent Pt(II) and Ir(III) complexes received considerable attention due to their high emission quantum yields (ΦP), tunable emission color and good thermal and chemical stability.57,15-18

In contrast, cyclometalated Pd(II) compounds are rarely reported as OSPs, owing to their

substantially smaller emission quantum yields and poor stabilities compared to their Pt(II) analogues.19 However, metal complexes containing the lighter Pd(II) core usually have longer phosphorescent decay lifetimes and thus higher oxygen sensitivity than Pt(II) complexes due to reduced spin-orbit coupling and phosphorescence decay rates. It is therefore advantageous if cyclometalated Pd(II) complexes that have a high ΦP and a high stability can be achieved. One way to simultaneously enhance ΦP and stability of Pd(II) complexes is the incorporation of tetradentate ligands, because of the high structural rigidity imposed by the chelate effect. Che et al. and Li et al. reported strongly luminescent Pd(II) complexes with tetradentate ligands and used them as catalysts in photochemical oxidation reactions and as emitters in organic light emitting diodes.20-22 However, the use of the tetradentate Pd(II) compounds as OSPs was not explored. We have recently reported a series of blue phosphorescent Pt(II) complexes with tetradentate cyclometalating ligands based on 1,2,4-triazolyl and 1,2,3-triazolyl, respectively, which are highly efficient emitters in solution or in doped polymer films and display impressive thermal, chemical and ultraviolet stabilities (Figure 1).23, 24 Despite their structural similarities, Pt1 and Pt2 show distinctly different photophysical properties in solution and the solid state. For example, while no


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excimer emission was observed for Pt2, Pt1 shows facile monomer to excimer phosphorescence switching in response to multiple external stimuli. Furthermore, the excimer emission of Pt1 is much more sensitive towards oxygen quenching, compared to the monomer emission, which could be useful as excimer-based OSPs. However, due to their short phosphorescence lifetimes, Pt1 and Pt2 are not ideal for applications as luminescent oxygen sensors. To develop efficient excimerbased OSPs, we initiated the investigation on tetradentate Pd(II) compounds shown in Figure 1 and compared their photophysical properties and oxygen sensing abilities with their analouges Pt1 and Pt2. Our study established that the Pd(II) compounds based on tetradentate ligands have a much better performance as OSPs, compared to the Pt(II) analogues, and the nature of the tetradentate ligands has a significant impact on excimer emission and the stability of the Pd(II) compounds. The results of our comprehensive inviestigation are presented herein. RESULTS AND DISCUSSION O

O Pt n-Bu N N

Pt N

N

N n-Bu N

Pt1

N N N n-Hex

O

O

n-Bu N N N

Pd N

Pd1

N N N n-Hex Pt2

N n-Bu N

N N N n-Hex

Pd N N N n-Hex Pd2

Figure 1. The structures of Pt1, Pt2, Pd1 and Pd2 examined in this work. Syntheses and Crystal Structures. The new Pd(II) complexes Pd1 and Pd2 were prepared via a one-step reaction similar to that reported for the related Pt(II) compounds, with isolated yields of 73% and 19%, respectively.23, 24 They are fully characterized by 1H NMR, 13C NMR and HRMS


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(see Figures S1-5). The crystal structures of Pd1 and Pd2 were determined by single-crystal Xray diffraction analyses and shown in Figure 2.

Figure 2. The crystal structures of Pd1 (right) and Pd2 (left). Pt: purple, oxygen: red, nitrogen: light blue. The aliphatic chains are shown as capped sticks for clarity. Like their Pt(II) analogues, the Pd(II) ions in both molecules adopt a distorted square-planar geometry with Pd-C bonds of 1.95-1.97 Å and Pd-N bonds of 2.08-2.11 Å, similar to the corresponding Pt-C and Pt-N bonds in the Pt(II) analogues. The molecules of Pd1 are packed in a parallel manner with partial π-stacking of the phenyl-triazolyl units between the adjacent molecules, similar to those of Pt1. The shortest atomic separation distance in Pd1 is 3.313(4) Å between a C atom and an N atom of the -stacked triazolyl rings, and the distance between Pd atoms of neighboring molecules in Pd1 are 6.7973(7) Å, respectively, both of which are similar to those of Pt1 (3.355(8) Å and 6.7425(4) Å, respectively), suggesting that Pd1 could also be highly prone to excimer formation like Pt1. In contrast, there are no -stacking interactions in the crystal lattice of Pd2, similar to that observed for Pt2. These observations support that a simple position switching of the C and N atoms on the triazolyl ring has a great impact on the intermolecular interactions of the resulting Pd or Pt complexes. DFT calculation (Figure S11) reveals that the Mulliken charges of the two carbon atoms in the 1,2,4-triazole ring of Pd1 are positively charged while the three N atoms are negatively charged. This alternating charge distribution is likely


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responsible for the partial  stacking of the triazolyl units in the crystal lattice of Pd1. In contrast, in the triazolyl unit of Pd2, only the carbon atom bonded to the phenyl ring has a positive charge, while all other atoms have a negative charge, which probably disfavors the formation of π-stacked structure for Pd2. UV-Vis absorption and emission spectra. The UV-Vis absorption spectra and normalized phosphorescence spectra of Pd1 and Pd2 in THF are shown in Figure 3 and the details are listed in Table 1, along with the data of their Pt analogues. Compared to their Pt counterparts, the low energy absorption bands (ε = 2000 – 4000 M-1 cm-1) of Pd1 (350-380 nm) and Pd2 (340-370 nm) are blue shifted by ~ 50 nm and can be assigned to admixture of ligand-centered (LC) transitions and metal-to-ligand charge transfer (MLCT) transitions (from the PhOPh moiety and Pd d orbital to π* orbital of the triazyl-ph moiety), according to the TD-DFT calculation results (see Table S5S8). The intense absorption bands (ε = 16000 – 18000 M-1 cm-1) at 336 nm for Pd1 and 314 nm for Pd2 also display 30 nm blue shift relative to those of Pt1 (366 nm) and Pt2 (344 nm), respectively, and are attributed to transitions mainly from the two phenyl rings of the chelating ligand and Pd d orbital to π* orbital of the triazole-ph moiety. The blue shift of the absorption bands of the Pd complexes relative to that of the Pt complexes can be explained by the higher ionization energy of Pd 4d electrons, compared to that of Pt 5d electrons.25


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Figure 3. UV–vis absorption spectra and normalized emission spectra (recorded in THF at 1 × 10-5 M) of Pd1, Pt1 (A), Pd2 and Pt2 (B). As shown in Figure 3, Pd1 and Pd2 display blue emission with λmax= 431 nm and 417 nm, and ΦP of 0.18 and 0.03, respectively, in THF at 10-5 M at ambient temperature. The decay lifetimes of 2.61 μs (298 K) and 140.9 μs (77 K) for Pd1, and 12.27 μs (298 K) and 260.4 μs (77 K) for Pd2 confirm that the luminescence of Pd1 and Pd2 is phosphorescence in nature. The smaller phosphorescence quantum yields and longer emission lifetimes of Pd1 and Pd2 relative to their Pt equivalents can be rationalized by the smaller spin-orbit coupling constant of Pd relative to that of Pt, as well as the smaller ligand field splitting of the d orbitals in Pd(II).26 For Pd2, its low quantum yield relative to that of Pd1 can be ascribed to more severe thermal quenching via the metal d-d state due to its higher emission energy, together with lower structural rigidity as indicated by its bigger Huang-Rhys factor (0.84 for Pd1 and 0.94 for Pd2).27 It is thus not surprising that at


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77K, the ΦP of Pd1 and Pd2 both reach unity (Table 1). Like those of their Pt counterparts, the emission spectra of Pd1 and Pd2 show vibronic progressions of 1200-1450 cm-1, indicating that their phosphorescence originates primarily from LC states, which is supported by TD-DFT calculation results.

Figure 4. A) Emission spectra and emission colors of Pd1 (RT, under N2) at different concentrations in MeTHF. B) Emission spectra of Pd2 at different concentrations in THF. Concentration and temperature dependent emission properties. Similar to Pt1, Pd1 displays concentration dependent emission, as shown by the spectra recorded in 2methyltetrahydrofuran (MeTHF) in Figure 4. With the increase of the concentration of Pd1 from 5×10-6 M to 10-3 M, the intensity of the monomer emission at 431 nm first increases until 5×10-5


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M, then declines, while a broad excimer peak at 510 nm increases steadily. The ~140 nm blue shift of emission energy of the excimer of Pd1 relative to that of its Pt analogue is consistent with TDDFT calculated data (Table S9). Compared to Pt1, the Iexcimer/Imonomer ratios of Pd1 at the same concentrations are much larger, an indication that the Pd1 is more prone to excimer emission. This could be ascribed to the longer decay lifetime of the Pd1 molecules and the fact that excimer emission is diffusion controlled. As shown in Table S1 and the photographs in Figure 4A, the ΦP of Pd1 in MeTHF increases drastically with increasing concentration, (e.g. ΦP = 0.08 at 5×10-6 M, 0.46 at10-3 M) and the solution becomes brighter with a distinct color change from deep blue to blue-green, which can be explained by high concentration facilitated aggregation of the Pd1 monomer which reduces quenching from solvent molecules and enhances excimer formation. The excimer decay lifetimes of Pd1 is longer than its monomer decay lifetime (Table 1), indicating that the excimer emission of Pd1 is more likely from intermolecular π-π interactions, as that of Pt1.23, 24 In deoxygenated MeTHF at 1×10-3 M, Pd1 shows reversible thermochromic behavior with emission color changing from deep-blue (77 K) to green (298 K) (Figure 5). Below 157 K, Pd1 shows the monomer emission peak at 431 nm, which decreases as temperature increases. A structureless excimer peak at 510 nm appears between 137 K and 157 K and grows as temperature increases. Above 217 K, the excimer peak gradually blue shifts somewhat from 526 nm (217 K) to 510 nm (298 K), which is responsible for the change of the CIE coordinates in the high T region shown in Figure 5C. This emission spectrum blue shift could be explained by the increased thermal dissociation of the excimer at high temperature, which is well documented in the literature.28


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Figure 5. A): Emission spectra of Pd1 at various T in MeTHF. B): A diagram showing the reversibility of the emission spectral change of Pd1 with T. (c = 1×10-3 M, λex = 330 nm, under N2). C): A photograph showing the gradient emission color change in a quartz tube with T, and CIE coordinates (CIE 1931, right) change of Pd1 with T in MeTHF (1 × 10-3 M, λex = 330 nm, under N2).


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In sharp contrast to Pd1, no excimer emission was observed in the emission spectra of Pd2 in MeTHF and the emission quantum yield is approximately the same in the concentration range of 10-5 M to 10-3 M (Figure 4B and Table S2). This demonstrates that the Pd(II) complex with a phenyl-1,2,3-triazolyl backbone has a less tendency to form excimers than the that with a phenyl1,2,4-triazolyl backbone, which is consistent with what we previously observed for the corresponding Pt(II) complexes. In addition to the different Mulliken charge distributions of the 1,2,4-triazolyl ring vs. the 1,2,3-triazolyl ring, it is possible that the alkyl chains in Pd2/Pt2 are more effective in protecting the Pd/Pt core from intermolecular interactions than those in Pd1/Pt1, a trend similar to that observed for the para-/meta-BMes2 substituted Pt(II) complexes previously.29, 30 Table 1. Photophysical data of the Pd(II) and Pt(II) complexes.

Complex

a

Emission,b 77 K

Emission, 298 K

Absorptiona

λmax (nm), ε (104 cm-1M-1) λmax (nm), τ (μs)

Pd1

333 (1.65), 307 (2.34), 275 (1.92)

Pd2

314 (1.78), 302(1.34), 263 (2.52)

Pt1 24

366 (3.17), 349(1.17), 305 (1.60)

Pt2 23

386 (0.29), 344 (2.35), 333 (1.40)

431 (2.61), 503 (3.233) 417 (12.27) 477 (0.509), 510 (1.57) 450(2.348)

ΦP

λmax (nm), τ (μs) ΦP

0.46 b

431 (140.9)

0.99

0.03 b

417 (260.4)

0.99

0.45 b

471 (3.68)

0.99

0.46 c

443 (3.237)

0.99

Measured in MeTHF at 5.0 × 10−5 M, RT. b Recorded in MeTHF at 1.0 × 10−3 M, under N2. c Recorded in CH2Cl2 at 2.0 × 10−5 M.

Sensitization of singlet oxygen. In addition to thermochromism, Pd1 also shows reversible emission color change between deep blue and green upon UV irradiation under air in MeTHF at


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10-4 M, with a drastic increase of ΦP (0.002 to 0.34) with increasing irradiation time. As illustrated in Figure 6B, under continuous irradiation of 365 nm UV light, the monomer emission band at 431 nm increases intensity gradually while the excimer peak at 501 nm grows at a much faster rate, a phenomenon similar to that observed previously for the analogous Pt compounds. After 12 min irradiation, the final emission spectrum resembles that recorded in degassed MeTHF and the luminescence can be brought back to the original state (irradiation time = 0 min) by re-saturating the solution with air (Figure 6c). This unusual phosphorescence enhancement and emission color switching of Pd1 with UV irradiation and air can be explained by sensitization of singlet oxygen and the consumption of singlet oxygen by the solvent molecule MeTHF, as illustrated in Figure 7. Similar singlet oxygen sensitization by transition metal complexes are well-documented in the literature.1-19, 31,32 Upon UV irradiation, the Pd1 acts as an 3O2 sensitizer, converting it in airsaturated MeTHF to singlet oxygen 1O2, which readily reacts with solvent molecule to produce MeTHF hydroperoxide (MeTHF-OOH) that was confirmed by the iodine test (Figure S9). The 1O2 molecules generated can be detected by adding 1,5-dihydroxynaphthalene (Figure S7) or 1,3diphenyl-isobenzofuran (Figure S8) to the solution and monitoring the change in absorption spectra.33-35 The reduction of the triplet oxygen level in solution led to the great increase of phosphorescence, especially the excimer emission peak of Pd1. Replenishing the solution with air quenches the phosphorescence, allowing the photocatalytic oxidation of MeTHF and the emission color switching to be repeated.


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Figure 6. A) Photographs showing the emission color change of Pd1 in MeTHF (1 × 10-3 M, λ ex = 365 nm) with irradiation and oxygen. B) Emission spectral change of Pd1 in MeTHF (1× 10-4 M, air-saturated) with irradiation time at room temperature at 365 nm. C): Reversible change of Iexcimer / Imonomer ratio of Pd1 in MeTHF (1 × 10-4 M) with alternating 365 nm irradiation and air exposure, also see Video S1.


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Figure 7. The proposed mechanism of the “photochromism” of Pd1 in MeTHF at room temperature. Oxygen sensing properties. The drastic change of ΦP of Pd1 in the presence and absence of oxygen motivated us to investigate its oxygen sensing capability in solution and in a solid substrate. As shown in Figure 8A, the addition of air into CH2Cl2 solution of Pd1 (5 × 10-5 M, under N2) results in intensity decrease of both the monomer emission peak at 431 nm and the excimer emission peak at 494 nm. For Pd1, compared to the monomer peak, the excimer peak declines at a much faster rate, as supported by the Stern-Volmer plots in Figure 8C. The oxygensensing data was fitted using the Stern-Volmer (SV) equation shown below and the results are given in Table 2. I0/I = τ0/τ = 1 + Ksv  PO2 The Ksv values of the monomer and the excimer were determined to be 1.047 Torr-1 and 2.878 Torr-1, respectively, assuming that the inner pressure of the cuvette containing Pd1 solution is about 1 atm. The Ksv of the excimer of Pd1 is nearly three times that of the monomer emission, supporting that the excimer has a higher sensitivity towards oxygen, which explains why only monomer emission can be observed in the MeTHF solution under air. For comparison, we also


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tested the oxygen sensing ability of Pd2 under the same conditions and the results are shown in Figure 8B, 8D and Table 2. The Ksv of Pd2 is 2.637 Torr-1, which is between those obtained for Pd1 monomer and excimer emission peak.

Figure 8. A) and B): Emission intensity change of Pd1 (λ monomer = 431 nm, λ excimer = 494 nm) and Pd2 (λem = 442 nm) in CH2Cl2 with the addition of air. C) and D): Stern-Volmer plots and fittings of I0/I versus O2 for Pd1 and Pd2 in CH2Cl2 (5 × 10-5 M).


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Table 2. The O2-sensing properties of the Pd (II) complexes in CH2Cl2 (5 × 10−5 M) solution.

Comp. λex (nm) λem (nm) I0 /Iair Pd1

330

Pd2

310

R2

Ksv (Torr-1)

PO2 (Torr)

431

37.7

0.97

1.047

0.96

494

160.0

0.99

2.878

0.35

442

88.7

0.94

2.637

0.38

Table 3. O2-sensing properties of the Pd(II) and Pt(II) complexes in a EC matrix Complex

λex (nm)

λem (nm)

I0 /I100

f1

f2

KSV1 (Torr-1)

KSV2 (Torr-1)

R2

𝑲𝒂𝒑𝒑 𝒔𝒗 (Torr-1)a

PO2 (Torr)b

Pd1

330

431

142

0.999

0.001

0.228

1.1 × 10-10

0.986

0.228

4.38

Pd2

310

417

163

0.998

0.002

0.347

9.3× 10-14

0.996

0.346

2.89

Pt1

330

475

6.17

0.999

0.001

0.00674

1× 10-7

0.966

0.00673

149

Pt2

330

443

11.5

0.999

0.001

0.0110

1× 10-4

0.936

0.0110

91.0

𝑎𝑝𝑝

Weighted quenching constant, 𝐾𝑠𝑣 = f1·Ksv1+f2·Ksv2, b The oxygen partial pressure at which the initial emission intensity of the film is 𝑎𝑝𝑝 quenched by 50% and calculated as 1 / 𝐾𝑠𝑣 , in Torr. a

Table 4. Phosphorescence decay lifetimes of the O2-sensing EC films of the Pd(II) and Pt(II) complexes at room temperature

Complex

λem (nm)

condition

τ / μs

Std. Dev / μs

χ²

Pd1

431

Under N2

90.84

0.484

0.883

Under Air

3.845

0.034

1.129

Under N2

198.3

1.123

1.045

Under Air

2.947

0.024

1.206

Under N2

1.479

0.015

1.234

Under Air

0.909

0.006

1.230

Under N2

2.359

0.020

1.163

Under Air

0.790

0.086

1.112

Pd2

Pt1

Pt2

417

475

443

Inspired by the highly oxygen-sensitive luminescence of Pd1 and Pd2 in solution, they were further examined as oxygen sensors on a solid substrate. These two compounds were doped in


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ethyl cellulose (EC) films at 5 wt%, respectively, to evaluate their performance as oxygen sensors in polymer matrices, which is important for practical applications.36, 37 The EC films containing the corresponding Pt analogues were also prepared to compare the oxygen sensing abilities. The dynamic responses of the films towards oxygen were examined by varying the O2 partial pressure using a flowmeter. As shown in Figure 9, the EC films of all four compounds can be effectively quenched by the stepwise addition of O2. It is worth noting that no excimer emission was observed in the emission spectra of the 5 wt% EC films of Pd1 and Pt1. The oxygen sensing data were fitted with the Demas two-site model38 (see the equation in Table 2 footnote and Figure 9) and the results are summarized in Table 3. The EC films of Pd1 and Pd2 show excellent oxygen sensing −1 and 0.346 Torr−1, performance with the weighed quenching constants (𝐾𝑎𝑝𝑝 𝑠𝑣 ) of 0.228 Torr

respectively, which are much higher than their Pt(II) counterparts (0.00673 Torr−1 for Pt1 and 0.0110 Torr−1 for Pt2, Figure 9) and those of the recently reported Ir(III) and Pt(II) complexes ( −1 36,37 To elucidate the drastic oxygen sensitivity enhancement of Pd1 and Pd2 𝐾𝑎𝑝𝑝 𝑠𝑣 < 0.021 Torr ).

over their Pt analogues, the phosphorescence decay lifetimes (τ) of the EC films of Pd1, Pd2, Pt1, and Pt2 were determined both under air and N2 and the results are shown in Table 4. Under N2, the decay lifetimes of 5 wt% EC films of Pd1 and Pd2 are 90.8 μs and 198.3 μs, respectively, which are reduced significantly to 3.85 μs and 2.95 μs when measured under air. Such abrupt change of decay lifetimes is clearly caused by the increase of non-radiative decay rates induced by oxygen quenching. In contrast, the decay lifetimes of 5 wt% EC films of Pt1 and Pt2 show a much smaller change, from 1.48 μs (Pt1) and 2.36 μs (Pt2) under N2 to 0.91 μs and 0.79 μs, respectively, under air, demonstrating that the Pt(II) complexes are more resilient towards oxygen quenching than their Pd(II) analogues, hence less effective as OSPs. The 𝐾𝑎𝑝𝑝 𝑠𝑣 of the EC films of the four


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complexes follows the same order as their phosphorescent lifetimes (Pd2 > Pd1 > Pt2 > Pt1, Table 4), which agrees well with previous literature findings.39


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Figure 9. Top: Dynamic emissive intensity response of oxygen sensing films of Pd1 (λem = 431 nm), Pd2 (λem = 417 nm), Pt1 (λem = 475 nm) and Pt2 (λem = 443 nm) immobilized in EC under varying concentrations of O2 in N2, at 25 °C, using ex shown in Table 3. Bottom: Stern-Volmer plots (I0/I versus O2 partial pressure) and fittings for EC films of metal complexes.

Figure 10. Time trace curves of Pd1, Pd2, Pt1 and Pt2 immobilized in EC with O2 / N2 saturation cycles at 25 °C. The operational stability test of the oxygen sensing films was conducted by monitoring their emission intensity change with repeatedly switching of the atmosphere from pure O2 to pure N2 (10 cycles) under continuous irradiation at ex. The results show that for Pd1, Pt1 and Pt2 films


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no emission intensity change was observed (Figure 10) and the quenching and recovering cycles are fully reversible, an indication that these three compounds have an excellent operational stability as oxygen sensors in the EC matrices. The emission intensity of Pd2 film experiences decrease by 15% after 10 cycles, rendering it less useful as OSP due to lower photostability.

Figure 11. Photo-degradation histograms of the 5wt% EC films of the metal complexes under continuous illumination of a 254 nm UV lamp for 60 min. under air. The photostability of oxygen sensing films of the Pd(II) and Pt(II) complexes were further evaluated by continuous irradiation with a 4W 254 nm UV lamp under air.34 As shown in Figure 11, after 60 min. of irradiation, the emission intensity of the EC films of Pd1, Pd2, Pt1 and Pt2 decreases by 8%, 54%, 8% and 26%, respectively. This result shows that the 1,2,4-triazolyl based tetradentate ligand provides a greater photostability to the Pd(II) and Pt(II) complex toward oxidation in the excited state by oxygen, compared to the 1,2,3-triazolyl based ligand. For the 1,2,3-triazolyl system, Pd2 is much less stable than Pt2 while for the 1,2,4-triazolyl system, Pd1


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and Pt2 have a similar stability toward 254 UV irradiation under air. These data demonstrate that that both ligand and metal ions have a significant impact on photostability of the complexes. Based on its high O2 sensitivity and excellent photostability, Pd1 is an excellent candidate for oxygen sensing applications. CONCLUSIONS In summary, two new tetradentate blue phosphorescent Pd(II) complexes based on tetradentate 1,2,3- and 1,2,4-triazolyl ligands have been obtained. Comparative study of the new Pd(II) compounds and their Pt(II) analogues on the influence of the ligands and the metal ions on phosphorescence, photostability under air, and oxygen sensing capability has been accomplished. The 1,2,4-triazolyl based compound Pd1 shows multi-responsive luminescence due to efficient excimer emission, originating from intermolecular π-π interactions. The Pd(II) complexes have much superior performance as oxygen sensors in solution and in EC films, compared to their Pt(II) analogues, and many previously reported Pt(II) and Ir(III) complexes, owing to the greatly enhanced triplet lifetimes and the steric constraint imposed by the tetradentate ligands. The excimer emission of Pd1 is much more sensitive towards oxygen quenching than that of the monomer, which should be considered as a strategy in future design of highly sensitive phosphorescent oxygen sensors. EXPERIMENTAL SECTION General. All commercial chemicals were used without further purification. UV-visible absorption spectra were recorded on a Cary 50 UV-Vis spectrophotometer. Fluorescent quantum efficiencies were determined using a Hamamatsu C11347-11 Quantaurus-QY spectrometer. 1H, 11B

and 13C NMR spectra were recorded on a Bruker Avance 400 MHz or 700MHz spectrometer.


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Luminescent spectra were recorded on an Edinburgh Instruments FLS980 Photon Technologies International

QuantaMaster

Model

2

spectrometer,

or

Lengguang

Tech.

F97Pro

spectrophotometer. Crystal structure were recorded on a Bruker D8 Venture X-ray single crystal diffractometer. HRMS spectra (EI) were recorded on a Micromass GC-TOF spectrometer. Column chromatography was carried out on silica gel (300–400 mesh). Analytical thin-layer chromatography was performed on glass plates of Silica Gel GF-254 by detection with UV. L1 and L2 were prepared according to literature procedures.23, 24 Synthesis of Pd1. Under nitrogen, L1 (98.2 mg, 0.24 mmol, 1.0 eq), Palladium(II) acetate (58.2 mg, 0.26 mmol, 1.1 eq), n-Bu4NBr (7.7 mg, 0.024 mmol, 0.1 eq) were added in a sealed Schlenk vessel. The vessel was evacuated and backfilled with nitrogen three times before acetic acid (30 ml) was added under the protection of nitrogen. The reaction mixture was stirred at ambient temperature for 12 hours and then heated at 140 oC for 72 h under a nitrogen atmosphere. Then the reaction was quenched with cold water, extracted with dichloromethane, then washed with saturated NaHCO3 solution and water. After the organic phase was dried over anhydrous MgSO4, the solvent was removed in vacuo and the residue was purified by column chromatography with 10:1 (v/v) CH2Cl2/EtOAc as the eluent to give the final compound as a white powder. (86.4 mg, 73 %). 1H NMR (700 MHz, CDCl3) δ 7.95 (s, 2H), 7.36 (dd, J = 5.4, 6.0 Hz, 2H), 7.31 – 7.27 (m, 4H), 4.57 (t, J = 7.5 Hz, 4H), 1.98 – 2.02 (m, J = 7.5 Hz, 4H), 1.53 – 1.46 (m, 4H), 1.01 (t, J = 7.4 Hz, 6H). 13C NMR (100 MHz, CDCl3) δ 159.39, 151.68, 148.03, 135.71, 135.13, 125.65, 118.61, 118.39, 50.75, 31.24, 19.95, 13.74. HRMS (EI), calcd for C24H26N6OPd [M]+: 520.1212, found: 520.1210. Synthesis of Pd2. Under nitrogen, L2(100 mg, 0.21 mmol, 1.0 eq), n-Bu4NBr (6.8mg, 0.021 mmol, 0.1 eq) and Na2PdCl4 (68 mg, 0.231 mmol, 1.1 eq) were added to a sealed Schlenk tube


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with 10 mL degassed acetic acid. The mixture was stirred at room temperature for 1 day, and then heated at 140 oC for 4 days. 10 mL of water was added to the resulting solution and the precipitate was collected via vacuum filtration. The solid was then dissolved in dichloromethane and washed with water and brine. The combined organic phase was dried over anhydrous MgSO4, filtered and purified on using flash chromatography through silica (dichloromethane as eluent) to give the final product as a white powder (23 mg, 19%).1H NMR (400 MHz, CDCl3) δ 7.70 (s, 2H), 7.23 – 7.18 (m, 6H), 4.45 (t, J = 7.5 Hz, 4H), 2.00 (p, J = 7.5 Hz, 4H), 1.44 – 1.27 (m, 12H), 0.90 (t, J = 6.9 Hz, 6H). 13C NMR (100 MHz, CDCl3) δ 155.02, 151.63, 138.65, 132.19, 125.59, 117.11, 116.22, 116.13, 51.49, 31.18, 30.10, 26.19, 22.41, 13.93. HRMS (EI), calcd for C28H34N6OPd [M]+: 576.1840, found: 576.1845. ASSOCIATED CONTENT SUPPORTING INFORMATION: NMR spectra, luminescent spectra, oxygen sensing data, TDDFT computational data, crystal structural data. The Supporting Information is available free of charge on the ACS Publications website at DOI: AUTHOR INFORMATION Corresponding Author * S. Wang, E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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We thank the National Nature Science thank the National Natural Science Foundation of China (Grant number 21571017, 21761132020) and the Natural Sciences and Engineering Research Council of Canada (RGPIN-2018-03866) for financial support. We also thank The Centre for Advanced Computing at Queen’s University for computational facilities.


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For Table of Contents Only


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Multiresponsive Tetradentate Phosphorescent Metal Complexes as

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