PtII Phosphors with Click-Derived 1,2,3-Triazole-Containing Tridentate

Dec 26, 2017 - N-type cyclometalated PtII complex, the high energy of the π* orbitals of the 1,2,3-triazole units gave rise to deep-blue phosphoresce...
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Article Cite This: Organometallics XXXX, XXX, XXX−XXX

PtII Phosphors with Click-Derived 1,2,3-Triazole-Containing Tridentate Chelates B. Schulze,†,‡ C. Friebe,†,‡ M. Jag̈ er,†,‡ H. Görls,§ E. Birckner,∥ A. Winter,†,‡ and U. S. Schubert*,†,‡ †

Laboratory of Organic and Macromolecular Chemistry (IOMC), ‡Center for Energy and Environmental Chemistry Jena (CEEC Jena), §Laboratory of Inorganic and Analytical Chemistry, and ∥Institute of Physical Chemistry, Friedrich Schiller University Jena, 07743 Jena, Germany S Supporting Information *

ABSTRACT: A series of PtII complexes featuring 1,2,3triazole-derived N∧N∧N-, N∧C∧N- and C∧N∧C-coordinating ligands were studied both experimentally and computationally aiming at the design of new PtII phosphors. By virtue of click chemistry, the new complexes were readily functionalized, e.g., with bulky groups in order to suppress aggregation of the complexes. For a N∧C∧N-type cyclometalated PtII complex, the high energy of the π* orbitals of the 1,2,3-triazole units gave rise to deep-blue phosphorescence; the poor luminescence quantum yield was attributed to an inadequate energy separation between the emissive state and the d−d states. However, when the 1,2,3-triazole donor moiety acted as a spectator/ancillary ligand only, an intense green emission could be achieved (ΦPL = 0.57, τ = 4.6 μs).

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ligands afforded blue-emitting IrIII complexes.34−38 The coordination chemistry of 2,6-bis(1,2,3-triazol-4-yl)pyridines was thus widely studied,23 and PtII complexes thereof have been reported previously, including small molecule39−41 as well as polymeric ones.42 Also, the PtII complexes of related bidentate 2-(1,2,3-triazol-4-yl)pyridine ligands have been in the focus of interest.43−45 In the case of N∧C∧N-cyclometalating ligands, namely, 1,3-bis(1,2,3-triazol-4-yl)benzenes, the resulting RuII complexes have been investigated in detail.28,46−50 Recently, bis(bidentate) PtII complexes featuring C∧N-cyclometalating 1,2,3-triazole-based ligands have also been presented and their potential for the development of efficiently blue-emitting OLEDs was revealed.51,52 In this study, the viability of several tridentate ligand platforms featuring 1,2,3-triazoles or 1,2,3-triazolylidenes regarding the design of PtII-based emitters is investigated (Figure 1). Besides a series of complexes with N∧C∧N-type cyclometalating ligands, a PtII complex bearing the anionic N ∧ N ∧ N-type 1,8-bis(1,2,3-triazol-4-yl)-9H-carbazolide ligand,53,54 which allows for a six-membered ring chelation,18,19,55,56 is presented. In addition, the complexs with 2,6bis(1,2,3-triazol-4-yl)pyridine16,41 and a C∧N∧C-coordinating carbene ligand16 are included as reference systems (the latter one was published while this study was already in progress). In all cases, the mesityl moiety was chosen as the N-substituent on the 1,2,3-triazol moieties in order to warrant structural comparability and to repress the well-known tendency of PtII

INTRODUCTION The complexes of platinum-group metal ions represent promising candidates for applications in organic light-emitting diodes (OLEDs) since they enable the design of phosphors with very high luminescence quantum yields.1 Due to a high spin−orbit coupling (SOC), such complexes rapidly undergo intersystem crossing (ISC) in the excited state, which allows the harvesting of both singlet and triplet excitons and, thereby, theoretical quantum efficiencies of up to 100% can be reached.2−4 Among others, cyclometalated PtII complexes featuring N∧C∧N-type tridentate ligands, in particular with the 1,3-di(2-pyridyl)benzene (Hdpb) motif, are well-known triplet emitters with photoluminescence quantum yields approaching unity.5−11 Moreover, complexes of this kind have been used as agents for microsecond lifetime emission imaging.12 Besides these prominent examples, the photophysical properties of PtII complexes bearing carbenecontaining C∧N∧C-type13−16 and of N∧N∧N-coordinating ligands featuring a central anionic nitrogen donor17−20 have been in the focus of several studies. Owing to their facile and modular synthesis21,22 as well as to their versatile coordination modes, including mesoionic carbenes, 1,2,3-triazole-containing ligands were studied extensively during the past decade.23−29 Notably, the electronic properties of 1,2,3-triazoles and their derivatives could be exploited for the design of transition metal complexes with beneficial photophysical characteristics. For instance, the notoriously short excited-state lifetimes of bis(tridentate) RuII complexes were overcome with the help of 1,2,3-triazolylidenebased ligands,28,30−33 and the employment of 1,2,3-triazole © XXXX American Chemical Society

Received: October 18, 2017

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DOI: 10.1021/acs.organomet.7b00777 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Scheme 2. Schematic Representation of the Synthesis of Complexes 1−7a

Figure 1. Overview of the PtII complexes presented in this study (the numbering schemes in the structures refer to the respective NMR assignment).

complexes to aggregate.57−63 Hence, self-quenching of the emission might be suppressed57,64 and an improved color purity might be achieved.60

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RESULTS AND DISCUSSION Synthesis. The ligands used in this study (Scheme 2) were prepared by following (or adopting) previously reported procedures,32,47,54 except for asymmetric ligand L2, which features both a pyridine and a 1,2,3-triazole ring (Scheme 1). In

(a) K2PtCl4, HOAc, 130 °C, 1 to 36 h, 50−84%. (b) Pt(COD)Cl2, NEt3, MeOH, 80 °C, 12 h, 84%. a

Scheme 1. Schematic Representation of the Synthesis of L2a

literature, the more acute bite angle of tridentate chelates with five-membered rings (vide inf ra) appears to provoke this alternative coordination mode.65,68 Although this tendency is much less pronounced for 1,3-dipyridylbenzene ligands,69 the methyl-substituted reference complex (1) was prepared to warrant comparability within the series of cyclometalated complexes (1−3). Literature-known complex 5 was prepared starting from the Pt(COD)Cl2 (COD: 1,5-cyclooctadiene) precursor complex in ethanol. This protocol was adopted for the synthesis of 7;41 however, the solvent was changed to methanol and triethylamine was added to achieve Ndeprotonation of the carbazole subunit (Scheme 2). Finally, heating of L6, Ag2O, and K2PtCl4 in DMSO gave 6 in modest yields (Scheme 2).16 Notably, the yields were comparable when using Pt(DMSO)2Cl2 and a preformed AgI1,2,3-triazolylidene precursor as starting materials and dry dichloromethane as solvent,32 whereas Pt(COD)Cl2 did not afford the desired product under these conditions. Presumably, K2PtCl4 and DMSO in situ form Pt(DMSO)2Cl2 and the liberated KCl facilitates the formation of a AgI-1,2,3triazolylidene complex from L6 and Ag2O,33,70 i.e., both the PtII and the AgI species that are required for the transmetalation are generated in situ; thus, the synthesis is simplified compared to the protocol given by Naziruddin et al.16 Solid-State Structures. For all complexes included in this work, single crystals suitable for X-ray diffraction analysis could be grown, in most cases by vapor diffusion of diethyl ether into a concentrated dichloromethane and acetonitrile solution of the charge-neutral and the cationic complexes, respectively (Figure

(a) 2-Methylbut-3-yn-2-ol, Pd(PPh3)4, CuI, NEt3, THF, 70 °C, 12 h, 48%. (b) 2-(Tributylstannyl)pyridine, Pd(PPh3)4, LiCl, DMF, 120 °C, 24 h, 41%. (c) Bu4NOH, toluene, 70 °C, 3 h, 99%. (d) CuSO4, sodium ascorbate, EtOH/CH2Cl2/H2O (2:1:1), 50 °C, 2 h, 95%. a

this case, a sequence of Sonogashira- and Stille-type crosscoupling reactions,65 followed by the cleavage of the 2methylbut-3-yn-2-ol moiety66 yielded the pyridine-functionalized alkyne building block, which was readily converted into the N∧C∧N-ligand via a CuI-catalyzed azide−alkyne cycloaddition (CuAAC) reaction, i.e., a facile and modular ligand functionalization during the last step of the ligand formation is given. The series of N∧C∧N-cyclometalated PtII complexes (1−4) was synthesized in moderate to good yields by refluxing the ligand and K2PtCl4 in glacial acetic acid (Scheme 2).6,65,67 Notably, the cyclometalating phenyl ring was equipped with methyl groups in its 4- and 6-position in order to circumvent an alternative bidentate cyclometalation. As mentioned in the B

DOI: 10.1021/acs.organomet.7b00777 Organometallics XXXX, XXX, XXX−XXX

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Figure 2. Summary of the solid-state structures of the PtII complexes (ellipsoids drawn at 50% probability level; the hydrogen atoms, the counterions, the solvent molecules and the disordering of the tert-butyl groups of 7 were omitted for clarity).

Table 1. Crystallographic Data of the PtII Complexes complex

Pt−N/C (peripheral) (Å)

Pt−C/N (central) (Å)

Pt−Cl (Å)

∠N−Pt−N or ∠C−Pt−C (deg)

1 2 3 4 [Pt(tpy)Cl]OTf62 5 6 [Pt(CNC)Cl]Cl13 7

2.026(4), 2.022(4) 2.015(4) (1,2,3-triazole), 2.019(4) (pyridine) 2.013(5), 2.006(5) 2.008(2), 2.007(2) 2.030(5), 2.018(5) 1.993(6), 1.985(6) 2.024(5), 2.018(5) 1.978(9), 1.972(8) 2.028(3), 2.022(3)

1.915(4) 1.936(4) 1.945(7) 1.935(3) 1.930(4) 1.965(6) 2.006(5) 1.968(5) 1.997(4)

2.4195(11) 2.4198(12) 2.4163(17) 2.3777(6) 2.302(7) 2.290(2) 2.2818(15) 2.278(2) 2.3230(10)

162.33(14) 161.15(16) 159.6(2) 160.21(9) 161.8(2) 160.5(3) 159.8(2) 158.1(3) 177.93(14)

rings result in a more acute bite angle for 3 (159.6(2) °) compared to 1 (162.33(14) °). For the former, the bite angle is comparable to that of 1,3-bis(1-pyrazolyl)benzene PtII complexes.68 As a result of the well-known trans-influence, the Pt− Cl bond is particularly long in the case of the cyclometalated complexes.71,72 In line with that, the Pt−Cl distance is significantly shorter for 4 as the electron donation by the carbanion is lowered. The comparison between 5 and [Pt(tpy)Cl]OTf (tpy = 2,2′:6′,2″-terpyridine)62 again reveals shorter peripheral Pt−N bonds, a longer central Pt−N bond, and a more acute bite angle for the former 1,2,3-triazole complex (Table 1). The X-ray crystallographic analysis of 616 was performed and allowed a comparison of its structure to that of the related [Pt(CNC)Cl]Cl complex (CNC = 2,6-bis(1-butylimidazol-2-

2, Table 1). In all cases, the tridentate, (distorted) squareplanar coordination could be confirmed. For the cyclometalated complexes, the Pt−C distance successively increases in the order 1 (1.915(4) Å) < 2 (1.936(4) Å) < 3 (1.945(7) Å), i.e., upon replacement of the outer pyridine rings by 1,2,3-triazole ones. However, the Pt−N distances are essentially identical for pyridine and 1,2,3-triazole when they are part of the same complex, namely, for 2 (2.019(4) and 2.015(4) Å, respectively). In contrast, the Pt−N distances are shortened when replacing pyridines with 1,2,3triazoles (1 and 3); this has also been observed in related studies and is mainly attributed to the more pronounced scharacter of the triazole’s N3 lone pair.28 Beside electronic effects, structural constraints upon replacement of sixmembered pyridine rings with five-membered 1,2,3-triazole C

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Organometallics ylidenyl)pyridine).13 The former features longer Pt−C bond lengths (Table 1) as well as larger bond angles at the carbene centers (∠C4−C5−N1, 103.2(5) and 102.7(5)°, vs ∠N1−C2− N3, 101.1(7) and 101.6(7)°). This finding might be rationalized by a more pronounced p-character of the PtII-coordinated carbene σ lone pair and a larger s-orbital contribution toward the C4−C5−N1 σ framework within 6;28,73 however, the comparison of the structural parameters between the aforementioned complexes might not be straightforward in view of the different substitution patterns of the corresponding ligands. Relative to the other complexes discussed herein, the bite angle of 6 is among the smallest (159.8(2) °), which is mainly attributed to the relatively long central Pt−N bond. In contrast to all other complexes within this study, carbazolebased complex 7 features six-membered ring chelators, which results in an almost ideal square-planar coordination with a bite angle close to 180°. Considering the aggregation tendency of the studied PtII complexes, only 1 and 2, having no or only a single mesityl moiety, undergo π−π stacking between two complex planes via the peripheral pyridine rings and the central phenyl rings, respectively (Figures S46 and S47).57,74 In contrast, for complexes 3−7, which feature two mesityl substituents, the central complex planes are well-separated from each other (Figures S48−S52). Even for the cationic complexes, which are prone to assemble via metallophilic PtII−PtII interactions,13,41 the PtII−PtII distances for 5 and 6 are at least 8 Å. Photophysical and Electrochemical Properties. The UV/vis absorption and emission spectra of the studied PtII complexes in dichloromethane are displayed in Figure 3.

higher energy of the excited state facilitating the thermally activated population of short-lived triplet metal-centered (3MC) excited states and/or due to the radiative decay becoming much slower.9,65,79−82 In line with a significantly accelerated radiationless deactivation, the excited-state lifetime was found to be in the ns region, while usually the excited states of N∧C∧N-cyclometalated PtII complexes decay within μs.65 The attempts to achieve an additional blue shift and to improve the quantum yield by employing a meta-difluoro-functionalized cyclometalating phenyl ring,1,6,11,83,84 however, resulted in the absence of detectable emission for 4, which could be attributed to an additional lowering of the 3MC energy (vide inf ra) and, thus, an even more facilitated radiationless deactivation than for 3. For complex 2, bearing both a pyridine and a 1,2,3-triazole donor, an intense green emission (CIEx,y = 0.25, 0.53) was observed, with the photoluminescence quantum yield being even higher than for reference complex 1. Furthermore, the methyl groups on the central phenyl ring apparently have a deleterious effect on the quantum yields, since higher values have been reported for the nonmethylated analog [Pt(dpb)Cl] (Table 2).6,65 The absorption spectrum of 2 apparently combines absorption features of 1 and 3 (Figure 3), i.e., electronic transitions involving the π* orbitals of the pyridine and the 1,2,3-triazole rings are expected to be present; however, the emission spectrum of 2 is essentially identical to that of 1 (Figure 3), suggesting that the LUMO is localized on the pyridine ring (vide inf ra).65 Apparently, as a result of the energetically low-lying π* orbitals of the pyridine moiety,28 the energetic separation between the emissive state and the 3MC is increased. Notably, in contrast to 1, no excimer emission was observed even at elevated concentrations for 2, i.e., owing to the presence of the bulky mesityl moieties, the aggregation tendency is lowered. Not only the photophysical but also the electrochemical properties of the cyclometalated complexes are strongly affected when the pyridine rings are replaced by 1,2,3triazole ones. In line with a significantly lower energy of the triazole’s σ lone pair,28 a progressive anodic shift for the oxidation process is observed when comparing 1−3 (Table 2). For comparison, for N∧N∧N-coordinated PtII-acetylides, the replacement of the parent 2,2′:6′,2″-terpyridine (tpy) ligand by a 2,6-bis(1,2,3-triazol-4-yl)pyridine ligand afforded an anodic shift for the oxidation of 300 mV,15,85 while this effect is much lower for RuII complexes.47,77,86 In the case of 4, the lowering of the electron donation from the carbanion by the fluorosubstituents is also manifest in an anodic shift of 260 mV for the oxidation process. Nonetheless, as commonly found for PtII complexes, the oxidation processes of the investigated complexes are irreversible and an interpretation of the observed differences has to be carried out with caution.83 Cationic complex 5 is not emissive at ambient temperatures, which can be rationalized by the low 3MC energy in such complexes87 and the anticipated excited-state destabilization brought about by the 1,2,3-triazole moieties.28 As for [Pt(tpy)Cl]+, 5 could not be oxidized within the available potential window; however, the cathodically shifted reduction40 clearly confirms the energetically higher π* system of the 1,2,3triazole-containing ligand.28,88 As also reported elsewhere, 1,2,3-triazolylidene-based complex 6 only showed a very weak green emission (Figure 3, Table 2).16 While analogous imidazol-2-ylidene complexes exhibit a bright emission of the dimeric species,13 they are reported to be nonemissive in diluted solution.15 Presumably,

Figure 3. UV−vis absorption and emission spectra in dichloromethane (solid and dashed lines, respectively).

Additional photophysical and electrochemical data are presented in Table 2; moreover, absorption and emission spectra measured in acetonitrile as well as absorption spectra in dichloromethane highlighting the spin-forbidden singlet−triplet transitions are provided in the Supporting Information. In the case of the N∧C∧N-cyclometalated complexes, the complete replacement of the pyridine rings by 1,2,3-triazole ones, which is known to raise the energy of the π* orbitals,28,43,77 yields a substantial hypsochromic shift in both the absorption and emission spectra with the latter reaching the challenging deep-blue region (CIEx,y = 0.16, 0.17; CIE: Commission internationale de l’éclairage).65,78,79 However, this is accompanied by a significant drop in the photoluminescence quantum yield (ΦPL), presumably due to the D

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Organometallics Table 2. Electrochemical and Photophysical Data of the Pt(II) Complexes complex 65

[Pt(dpb)Cl] 1 2 3 4 [Pt(tpy)Cl]TFPBk,75 5 6 7

λmaxems (nm)a,b 491, 497, 491, 436,

524, 529, 524, 462,

562 (695) 567 (648) 561 500 c 500, 535, 590 c 489, 523, 561 c

ΦPL (%)a

τ (μs)a

60 49d,e 57d,e 0.3d,e c 0.04 c 0.03d,e c

7.2 3.4d 4.6d 0.0028f c c 0.0024f c

Eox vs Fc+/Fc (V)a,g h

0.43 0.26, 0.32, 0.82, 0.96 0.63, 0.86 0.94 1.20 i 1.31h 0.28 (68), 0.59 (67), 1.15

Ered vs Fc+/Fc (V)a,g −2.03h −2.10 −2.42 i i −1.24 (70), −1.82 (70)j −1.56 (75), −2.25 −1.64 (71), −2.29h i

a

Measured in CH2Cl2 unless stated otherwise. bExcimer emission given in parentheses. cNot observed. dDeaerated by four freeze−pump−thaw cycles. eQuinine bisulfate (ca. 10−5 M in 1 N H2SO4, λexc = 345 nm) was used as emission reference (ΦPL = 60%).76 fDeaerated by purging with N2. g Measurements were conducted with Bu4NBF4 as supporting electrolyte (0.1 M) and with a scan rate of 200 mV s−1; for irreversible processes, peak potentials are given; for reversible processes, E1/2 is given (peak splits are given in parentheses). hMeasured in acetonitrile. iNo signal observed within the potential window of the solvent. jMeasured in DMF. kTFPB = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate.

Figure 4. Ground-state energy levels and frontier molecular orbitals of the PtII complexes with relative contributions of molecular fragments according to a Mulliken population analysis.

Figure 5. Spin densities of the optimized triplet excited state of the PtII complexes.

in analogy to the related RuII complexes,30−32 the stronger electron donation by the 1,2,3-triazolylidenes provides a better 3 MC destabilization,26,28 thereby impeding radiationless deactivation. Consistently, 6 is more electron-rich than its imidazol-2-ylidene counterpart, for which no oxidation was observed in acetonitrile.15 Presumably, the luminescence quantum yield of complex 6 might be significantly enhanced by replacing the chloro-ligand by acetylides15 or as shown recently, by cyanide.16 Finally, the charge-neutral, carbazolebased complex 7 was found to be nonemissive at ambient temperatures, which might be ascribed to the high energy of the ligand’s π* orbitals. In contrast to the other complexes presented herein, a much narrower energy gap can be concluded for 7 on the basis of the bathochromically shifted

absorption spectrum. Furthermore, three oxidation processes were detected, with the first and second one being reversible, but no reduction is observable within the available potential window. Computational Results. In order to better understand the electronic properties of the above-mentioned PtII complexes, density functional theory (DFT) calculations have been performed for the ground as well as for the excited state. In Figure 4, the ground-state energy levels as well as the frontier molecular orbitals are given; additional molecular orbitals are depicted in the Supporting Information. The spin densities of the optimized triplet states are displayed in Figure 5. Additionally, the electronic transitions were computed via time-dependent density functional theory (TD-DFT) to E

DOI: 10.1021/acs.organomet.7b00777 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

changes, e.g., of the central Pt−C bond (+0.2 Å) and the peripheral Pt−N bonds (+0.3 Å), are significantly larger compared to the corresponding changes for 1−4 (