1,2,4-Triazines in the Synthesis of Bipyridine Bisphenolate ONNO Ligands and Their Highly Luminescent Tetradentate Pt(II) Complexes for Solution-Processable OLEDs

: This article describes a convenient method for the synthesis of ONNO-type tetradentate 6,6 ′ -bis(2-phenoxy)-2,2 ′ -bipyr-idine (bipyridine bisphenolate, BpyBph) ligands and their platinum(II) complexes. The methodology includes the synthesis of 1,2,4-triazine precursors followed by their transformation to functionalized pyridines by the Boger reaction. Two complementary routes employing 3,3 ′ - and 5,5 ′ -bis-triazines allow a modi ﬁ cation of the central pyridine rings in di ﬀ erent positions, which was exempli ﬁ ed by the introduction of cyclopentene rings. The new ligands were used to prepare highly luminescent ONNO-type Pt(II) complexes. The position of the cyclopentene rings signi ﬁ cantly in ﬂ uences the solubility and photophysical properties of these complexes. Derivatives with closely positioned cyclopentene rings are soluble in organic solvents and proved to be the best candidate for solution-processable organic light-emitting devices (OLEDs), showing e ﬃ cient single-dopant candlelight electroluminescence.


■ INTRODUCTION
Tetradentate dianionic salen ONNO ligands are well known in coordination chemistry, and their metal complexes have found applications in a variety of scientific fields ranging from catalysis to bioinorganic and materials chemistry. For example, highly luminescent Pt(II) complexes of salen have been used as dopants in organic light-emitting diodes (OLEDs). 1 The tetradentate nature of the ligand reduces the possibility of ligand-exchange reactions and benefits both the stability and the luminescence properties of these complexes. Classical salen ligands are easily prepared by the condensation of salicylic aldehyde derivatives with the corresponding diamines. However, the resulting CN imine bonds are prone to hydrolysis and other degradation pathways. To solve this problem, 6,6′-bis(2-phenoxy)-2,2′-bipyridine (bipyridine bisphenolate, BpyBph) were developed as a more robust system. Indeed, ONNO-type Pt(II) complexes of BpyBph and their phenanthroline analogues (PhenBph) showed strong luminescence in solution at room-temperature, good performance in OLEDs, and excellent thermal stability 2 ( Figure 1). It should be noted that the performance of the complexes strongly depends on the rigidity and the substitution pattern of the ligand, with values of the photoluminescent quantum yield in solution varying from as low as 5% in BpyBph systems to as high as 58% in PhenBph-based complexes. Clearly, for BpyBph platinum(II) complexes there is room for the improvement of luminescence properties.
The synthesis of BpyBph ligands is more challenging than that of salen ligands, and there are only a few examples of such ligands described in the literature. The synthesis involves the reaction of 2,2′-bipyridine with 2-methoxyphenyllithium to give 6,6′-bis(2-methoxyphenyl)-2,2′-bipyridine, which is then demethylated to give the target ligand. 3 Because highly nucleophilic reagents are used (n-BuLi), only a limited number of functional groups can be tolerated in the ligand. In this article, we demonstrate a convenient method for the synthesis of BpyBph ligands based on the chemistry of 1,2,4-triazines. This methodology was previously employed to prepare a variety of polypyridine ligands 4 as well as their cyclometalated analogues. 5 The key aspect of the methodology is that 1,2,4-triazines can be easily prepared from readily available starting materials and then transformed to their corresponding pyridines by the Boger reaction with enamines. In this way, a variety of substituents can be introduced into the pyridine ring. In this article, we report two synthesis routes employing 3,3′-and 5,5′-bis-triazines as key building blocks in preparing four novel ligands. We then show that the ligands readily form Pt(II) complexes, which are highly luminescent and can successfully be used as dopants in solution-processed OLEDs. In this study, we concentrate exclusively on solution-processed OLEDs and demonstrate that the position of cyclopenteno units plays a key role in increasing the solubility of the complexes.

■ RESULTS AND DISCUSSION
Synthesis. The synthesis of the novel salen-type proligands is depicted in Scheme 1. Because the N1 and N2 atoms of the triazine ring are lost during the Boger reaction as dinitrogen, we are limited to using 3,3′-and 5,5′-bis-triazines as key building  (1). In order to prepare 5,5′-bis-triazine (9a-Et), we started with commercially available 2-ethoxybenzamidine (6), which was treated with 1 equiv of hydrazine hydrate to give the corresponding amidrazone. 6,7 The amidrazone was not isolated but was used in situ in the reaction with glyoxal to give 8a-Et in 66% yield. tert-Butyl derivative 8b-Me was synthesized by the desulfurative Suzuki reaction of sulfide 7 with commercially available 3-tertbutyl-5-methoxyphenyl boronic acid. 8 The next step involved the dimerization of 1,2,4-triazines 8a-Et and 8b-Me by the action of potassium cyanide. This high-yielding reaction was first reported in 1973 9 and has already been applied to the synthesis of bipyridines. 10 3,3′-Bis-triazines 3a and 3b and 5,5′-bis-triazines 9a-Et and 9b-Me were then converted to 2,2′-bipyridines 4a and 4b and 10a-Et and 10b-Me, respectively, by the Boger reaction with 1-morpholinocyclopentene. Finally, O-demethylation with pyridinium chloride or boron tribromide under standard conditions gave the desired salen-type pro-ligands 5a and 5b, and 11a and 11b. Complexes 12a and 12b, and 13a and 13b were then prepared by the reaction of the corresponding proligands with potassium tetrachloroplatinate in refluxing acetic acid. It was immediately apparent that 12a is far more soluble than 13a. One explanation for this might be that the cyclopentene units in 12a cause a slight twist of the pyridine rings, reducing the planarity of the molecule and thus minimizing the intermolecular interaction. Indeed, computational data supports this argument and shows that 12a is less planar than 13a ( Figure 2).
Interestingly, the introduction of the tert-butyl groups did not significantly improve the solubility in both series of complexes in toluene. Only 12a dissolves well in toluene, while other complexes have very limited solubility. For this reason, all of the solution-processed OLEDs in this work were made using chlorobenzene as the solvent.
Luminescent Properties. The small changes in the ligand structure between 12 and 13 cause noticeable differences in the photophysical properties of the complexes. For example, in toluene solution, 13a, which is an isomer of 12a differing only by the position of the cyclopentene ring, has a more blue-shifted emission spectrum relative to that of 12a. A similar trend is observed in 12b and 13b.
The photoluminescence spectra in solution ( Figure 3) are nearly featureless and relatively broad, but when dispersed in a rigid and nonpolar polymer (Zeonex), the spectra show a more resolved vibronic structure. Interestingly, the phosphorescence spectra of each pair of analogues in Zeonex (12a and 12b, 13a and 13b) are nearly identical, although they differ in solution. This indicates that the difference in the emission color of tertbutylated and nonsubstituted analogues is mostly a result of additional vibrational/rotational modes in tert-butylated molecules and the electron-donating effect of this group is less important.
At 77 K, all four complexes display well-resolved, blue-shifted emission. In all cases, the 0−0 transition is the strongest in the spectrum. Within each group of complexes (12a and 12b, 13a and 13b), the low-temperature phosphorescence spectra show a very similar shape with identical vibronic structure, but the analogues with tert-butyl groups (12b, 13b) give more red-shifted spectra than do their counterparts (12a and 13a). This is, in fact, an indication that the electronic effect of the tert-butyl group is significant. In general, notable rigidochromism is observed in these complexes, showing that the molecules are susceptible to intramolecular motion and interactions at room temperature. In both series, the introduction of the tert-butyl group reduces the luminescence yield, Φ PL , which is probably due to the larger number of vibrational/rotational modes. Molecules with cyclopentene rings on the outer periphery of the bipyridine moiety (13a, 13b) have a larger Φ PL than do the corresponding isomers (12a, 12b).
This argument is supported by the phosphorescence lifetime measurements. In toluene solution, 13a shows the longest phosphorescence lifetime, whereas 12b shows the shortest. Shorter phosphorescence time constants in 12b and 13b are also found compared to their corresponding analogues (12a and 13a). We attribute the shorter phosphorescence lifetime of 12b and 13b relative to their analogues (12a and 13a) to the increased nonradiative decay. All decay profiles recorded in toluene are perfectly monoexponential as shown in Figure 3. The photophysical properties of complexes 12a, 12b, 13a, and 13b are summarized in Table 1.
Electrochemistry. Cyclic voltammetry studies on complexes 12a, 12b, 13a, and 13b show that all compounds behave similarly and show a reversible reduction that involves the NN (bipyridine) ligands. The oxidation of the complexes is, however, irreversible. In this case, the behavior of 12a and 13a is nearly identical, and this is also observed for their tert-butylated analogues, 12b and 13b ( Figure 4, Table 2). This indicates that the oxidation of these complexes involves the phenyl rings of the OO ligand, as alkyl substitution of this moiety changes the electrooxidation signal pattern. This means that the alkyl-and nonalkyl-substituted complexes undergo different chemical reactions upon oxidation. The complexes have very similar oxidation and reduction potentials, but 13a and 13b have a larger electrochemical energy gap (0.05− 0.07 eV), which explains the observed blue shift of their emission in comparison to that of the other two molecules.
Theory and Computational Chemistry. To support the interpretation of the experimental results, we have performed density functional theory (DFT) and time-dependent density functional theory (TDDFT) simulations on 12a and 13a, with the primary focus of rationalizing the structural and emission properties. The structure of the lowest triplet states was optimized using DFT(B3LYP) 12 and a def2-SVP basis set as implemented within the ORCA quantum chemistry package. 13 The excited-state energies were computed using linear responsetime-dependent density functional theory (LR-TDDFT) using the Tamm−Damcoff approximation and the B3LYP exchange and correlation functional as implemented within the Amsterdam Density Functional (ADF) code. 14 The SOC matrix elements were computed with the perturbative approach developed by Wang and Ziegler. 15 A DZP basis set was used for all atoms, except Pt (TZP), and scalar relativistic effects were accounted for using the zeroth-order relativistic approximation (ZORA). 16 Figure 5 shows HOMO and LUMO orbitals, which constitute the dominant transitions responsible for both the S 1 and T 1 excited states. These correspond to charge-transfer states, from  The large difference between the experimental and calculated lifetimes is that the calculated lifetime does not include nonradiative effects, which are relatively significant in solution, as indicated by the lower quantum yields. For 12a and 13a, the SOC between S 1 and T 1 states is about ∼350 cm −1 in both cases. OLED Fabrication and Characterization. Solution-processed devices were fabricated from complexes 12a, 12b, 13a, and 13b using chlorobenzene solutions of mixed host (PVK + PBD) and dopant. (see SI for more details about fabrication of devices.) The dopant concentration was limited in some cases due to the poor solubility of some of the complexes in chlorobenzene. For this reason, toluene could also not be used as a solvent, except for 12a, which was dissolved in toluene to produce a device with a 5% dopant concentration. Devices of 12a in toluene had performances that were similar to those of the presented chlorobenzene-based OLEDs. The poor solubility of complexes 13a and 13b resulted in a limited dopant concentration (1 and 3%, respectively). This is likely to decrease the efficiency of host to dopant energy transfer (due to a limited Forster and Dexter radii) and cause problems with direct charge injection into the dopant, which in turn decreases the efficiency of the device. This is clearly  the problem in devices containing 13a. This is also shown in the photophysical study where, in the case of PVK:PBD films with low doping concentrations (complexes 13a and 13b), another, slower mechanism for energy transfer is observed (i.e., limited by hole−electron pair recombination 17 ). When the photophysical data is compared with electroluminescence spectra it becomes clear that in OLEDs direct charge injection to the dopant is the dominant process, while energy transfer from host to guest is less dominant. On the other hand, energy transfer governs the photoluminescence in a PVK:PBD mixed host. For this reason, it is important that the efficiency of the OLED devices is slightly larger than it would be expected to be from photophysics. The compounds perform well as dopants in OLEDs (Figure 6), giving electroluminescence from yellow-green to orange that is in full agreement with the photophysical results. All dopants except 12b give efficient devices with an EQE of 8.4−10.4% and a satisfactory current efficiency of 22.3−24.9 cd A −1 . The brightness of these devices reached 7000 cd m −2 in the case of 12a, while the device derived from 12b shows the lowest brightness of 2500 cd m −2 . The turn-on voltage is in the typical range for the OLED structure used with a PVK:PBD host. 18 Because of the higher photoluminescence quantum yield both in doped film and in solution, the 13a-based OLED was expected to have a higher maximum EQE and in general to outperform the 12a-based OLED. However, the low solubility of the complex imposed limitations in doping concentration, and the device with 13a as the dopant (1%) is therefore less efficient than the device incorporating 12a (5%). The yellowish electroluminescence of devices 1 and 3 is close to that of candlelight, while device 2 has a more green and device 4 a more red electroluminescence. The colorimetric coordinates in the CIE 1931 color space of a typical candle (0.52, 0.42) 19 and those of presented devices 1 and 3 are close to this value (see Table 3). Interestingly, the candlelight color has been achieved with the use of only one emitter whereas other solutions use 2 to 3 emitters. 19−21

■ CONCLUSIONS
The synthesis methodology based on the chemistry of 1,2, 4-triazines proved to be efficient and versatile in the synthesis of tetradentate dianionic bipyridine bisphenolate ONNO ligands. The scope of the methodology goes beyond luminescent complexes and is likely to attract attention from scientists in other fields. Many other functional groups can be introduced by the Boger reaction, opening up additional pathways for optimizing and tuning ONNO ligands for different applications. In Pt(II) complexes formed by the new ligands, the introduction of the tert-butyl groups was found not to improve solubility in toluene but reduced the quantum efficiency of the complexes and is thus not a good strategy for optimizing the dopants. However, the 1,2,4-triazine methodology illustrated herein does allow for the synthesis of more soluble isomers by positioning the cyclopentene rings closer together, leading to less-planar structures. This significantly improves the solubility of the complex, but the detrimental effects on the photophysical properties are less profound. This allowed us to obtain the required doping concentration to achieve efficient single-dopant candlelight OLED devices.