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Triboluminescence and Metal Phosphor for Organic Light-Emitting Diodes: Functional Pt(II) Complexes with Both 2‑Pyridylimidazol-2ylidene and Bipyrazolate Chelates Che-Wei Hsu,† Kiet Tuong Ly,† Wei-Kai Lee,‡ Chung-Chih Wu,*,‡ Lai-Chin Wu,§ Jey-Jau Lee,§ Tzu-Chieh Lin,∥ Shih-Hung Liu,∥ Pi-Tai Chou,*,∥ Gene-Hsiang Lee,∥ and Yun Chi*,† †

Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan Graduate Institute of Electronics Engineering and Department of Electrical Engineering, and ∥Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan § Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan ‡

S Supporting Information *

ABSTRACT: We report the utilization of both pyrid-2-yl-imidazolylidene and dianionic bipz chelates as constituents in syntheses of a new series of charge-neutral Pt(II) complexes 1−4, among which complex 4 revealed remarkable triboluminescence, i.e., generation of photoemission upon grinding or cracking of the solid sample. The triboluminescence is found to be sensitive to the subtle changes of the associated substituents of pyrid-2-yl-imidazolylidene chelate, as verified by the disappearance of the triboluminescence for complexes 1−3. Alternatively, the wellordered solid packing of 3, as indicated by the grazing incidence X-ray scattering experiment, serves as an ideal emitter for the fabrication of highly efficient OLEDs, rendering high external quantum efficienciy (25.9%) and luminesce efficiency (90 cd A−1) at the practical brightness of 100 cd m−2. The rather low roll-off in efficiency (24.4%, 85 cd A−1 at high brightness of 1000 cd m−2) is attributed to the short excited-state lifetime of 3 (∼800 ns) in the solid state, which in turn is associated with the MMLCT transition character. KEYWORDS: platinum, organic light-emitting diode, triboluminescence, mechanoluminescence, carbene, imidazolium, N-donor, pyrazolate

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INTRODUCTION

are the two most employed designs in assembly of squareplanar Pt(II) metal complexes (cf. Chart 1). Notably, the charge-neutral Pt(II) complexes bearing either bpy or dicarbene chelate will be produced only if the complexes incorporated two anionic ligands such as halides, 20,21 cyanides,22,23 or acetylides.24−26 However, the progress in

There has been an extensive interest in developing transitionmetal-based materials for the organic light emitting diodes (OLEDs).1−6 These transition metal complexes are distinct from the organic fluorophores due to the high efficiency and relatively long emission lifetime; the latter is a key characteristic of the so-called phosphorescence derived from the triplet excited states. The bright phosphorescence at room temperature is attributed to the heavy atom effect that induces strong spin−orbit coupling, facilitating the fast intersystem crossing between the singlet and triplet states.7−10 As a result, a large number of emissive Ru(II), Os(II), Ir(III) and Pt(II) metal complexes have been synthesized and applied to the fabrication of OLEDs and other optoelectronic devices.11−13 It has been well established that their fundamental properties are strongly influenced by the constituent chelates; particularly the donor ability, π-conjugation and steric encumbrance have proved to be crucial as part of the complex design strategy. Moreover, the choice of ligand can also permit fine-tuning of the emission color, as well as improving solubility, stability, and luminance efficiency, all of which are exceedingly important for the material processing.2,13 As for the charge-neutral bidentate ligands, to date, 2,2′-bipyridine (bpy)14−16 and dicarbene17−19 © 2016 American Chemical Society

Chart 1. Chelating Ligands with Varying Electronic Character

Received: October 6, 2016 Accepted: November 21, 2016 Published: November 21, 2016 33888

DOI: 10.1021/acsami.6b12707 ACS Appl. Mater. Interfaces 2016, 8, 33888−33898

Research Article

ACS Applied Materials & Interfaces

the regime δ 9.88−9.35 assigned to the acidic imidazolylium C−H fragment, which disappeared upon conversion to the imidazolylidene chelate after formation of the demanded Pt(II) metal complexes.46 Pt(II) complexes with formula [Pt(N^C) (bipz)], N^C = pyridyl imidazolylidene chelates such as mpyim, ipyim, mtpyim and itpyim, were typically obtained by treatment of imidazolium derivative with Pt(DMSO)2Cl2 in the presence of NaHCO3 at 100 °C for 12 h. Without isolation, the dianionic chelate (bipz)H2 were added and heated at 100 °C for another 6 h to afford the Pt(II) metal complexes, with typical synthetic yield spanning 52 −68%. This synthetic approach is different from that adopted for the analogous Pt(II)-bisacetylene derivatives [(pyim)Pt(CCAr)2] Ar = aryl group, which were alternatively obtained by replacement of the 1,5-cyclooctadiene (COD) chelate in [(COD)Pt(CCAr)2] with pyridyl imidazolylidene Ag(I) iodide in CH2Cl2 solution at RT.47

preparation of respective Pt(II) complexes using dianionic chelates has been hampered with limited success because of the lack of suitable designs. Only recently, the dianionic chelates such as 2,2′-biphenyl,27 benzene-1,2-dithiolate,28 5,5′-di(trifluoromethyl)-3,3′-bipyrazolate (bipz), 2 9 5,5′-di(trifluoromethyl)-3,3′bitriazolate (bitz), 2 9 5,5′-(1methylethylidene)bis(3-trifluoromethyl-1H-pyrazolate) (mepz),30 and 3-trifluoromethyl5(4-(trifluoromethylphenyl)1H-pyrazolate (phpz)31 were successfully utilized in the syntheses of d8-Pt(II) metal complexes and the Ru(II), Os(II) and Ir(III) based octahedral complexes with a d6-electronic configuration. In these metal complexes, computational analyses suggested that the occupied frontier orbitals primarily contain contributions from both central metal atom and dianionic chelate, whereas the unoccupied frontier orbitals reside on the neutral chelates.27 Accordingly, the dominant transitions are best described as metal-ligand-to-ligand charge transfers (MLLCT),5 which is distinctive from the typical transition character of combined ligand-centered (LC) ππ* and metal-to-ligand charge transfer (MLCT) transitions in typical transition-metal complexes.

These Pt(II) complexes 1−4 were initially characterized using 1H NMR spectroscopies. The key data were the doublet observed at δ 10.67 (1), 10.75 (2), 10.52 (3) and 10.82 (4), which are assigned to the ortho-CH group of the pyridyl fragment and are much downfield compared to the same signal of the free imidazolylium derivatives (δ 8.66, 8.62, 8.41 and 8.41). Moreover, the N-isopropyl derivatives 2 and 4 also exhibited a notable downfield septet at δ 6.87 (2), and 7.13 (4), which are assigned to the methine group of isopropyl substituent, whereas the same fragment of the uncomplexed imidazolylium ligands appeared at δ 4.77 and 4.92, respectively. These downfield shifted 1H NMR signals are due to the interligand C−H···N hydrogen bonding coupled to the bipz chelate, which can be utilized as a proof to the formation of Pt(II) metal complexes 1−4. The single-crystal X-ray diffraction study on 4 was executed; in fact, it is the only complex that can grow the suitable single crystals. The molecules crystallize in a trigonal space group P3121, and the lack of a center of symmetry is demonstrated by the dimeric molecular motif along the c axis. The only crystallographically imposed symmetry is a 3-fold translational screw axis. The perspective view of single Pt(II) unit, packing in the crystal lattices, selected bond lengths, and angles are shown in Figure 1a, b. As can be seen, all Pt(II) complexes adopt the distorted square planar arrangement because of the geometrical constraint imposed by both neutral pyridyl imidazolylidene itpyim and dianionic bipz chelates. The metal to itpyim chelate distances in 4, cf. Pt−N(1) = 2.032(6) Å and Pt−C(6) = 1.988(8) Å, are notably shorter than those of Pt(II) metal complex bearing both pyridyl imidazolylidene chelate and two cis-arranged methyl substituents (i.e., Pt−N = 2.128(5) Å and Pt−C = 2.006(5) Å), showing the diminished trans-influence of the bipz chelate versus the alkyl groups.34 Moreover, the interligand H-bond observed in the 1H NMR studies is also evidenced by the shortened nonbonding contacts of N(7)···H(13) = 2.102(6) and N(4)···H(1) = 2.294(5) Å between the nitrogen atom of bipz chelate and the pyridyl and isopropyl fragment of the pyridyl imidazolylidene chelate. The

In an aim to extend the synthetic scope of functional Pt(II) metal complexes, our attention was drawn to the pro-chelates, namely, 3-alkyl-1-(pyrid-2-yl)1H-imidazolylium, which can be converted to the neutral chelates bearing both pyridyl and imidazolylidene donors, displaying a subtle balance between the electronic and steric properties. In fact, relevant Ru(II), Ir(III) and Pt(II) metal complexes comprising similar carbene chelate have been extensively studied for catalysis applications32−34 and in studying luminescent properties;35−38 the latter may show high potential in organic optoelectronics due to the much destabilized π*-orbitals. In this study, we report the utilization of both pyrid-2-yl-imidazolylidene and dianionic bipz chelates as constituents in syntheses of the corresponding Pt(II) complexes, among which one has showed novel triboluminescence, i.e., generation of photoemission upon gentle grinding or cracking of the solid sample.39−42 Furthermore, upon changing the associated substituents, the triboluminescence vanished; alternatively, they serve as the ideal emitters for the fabrication of highly efficient OLEDs.

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RESULTS AND DISCUSSION Synthesis and Structural Characterization. Four 3-alkyl1-(pyrid-2-yl)-1H-imidazolylium derivatives, i.e., (mpyim)HPF6, R′ = H, R″ = Me; (ipyim)HPF6, R′ = H, R″ = iPr; (mtpyim)HPF6, R′ = tBu, R″ = Me; and (itpyim)HPF6, R′ = t Bu, R″ = iPr, were prepared and isolated as hexafluorophosphate salts according to the literature method.43,44 The 5,5′di(trifluoromethyl)-3,3′-bipyrazole chelate (bipzH2) was alternatively prepared using Claisen condensation employing ethyl trifluoroacetate and 2,3-butanedione, followed by heating with hydrazine in refluxing ethanol solution. 45 The parent imidazolylium derivatives, i.e., (mpyim)HPF6 and (ipyim)HPF6, are relatively insoluble in nonpolar solvents, such that their 1H NMR spectra were recorded in d6-acetone, in contrast to the t-butyl substituted derivatives (mtpyim)HPF6 and (itpyim)HPF6, for which the NMR spectra are instead recorded in CDCl3 solution. All these prechelates exhibited a signal in 33889


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Figure 2. UV−vis absorption spectra in CH2Cl2 and solid-state emission spectra of Pt(II) complexes 1−4 measured at RT.

coefficient, all observed peak profiles are very close to each other, supporting the existence of similar structure for these Pt(II) complexes in solution state. As for the emission spectra, these Pt(II) complexes were all nonemissive in degassed CH2Cl2 solution (vide infra), but showed intense emission for powders that were obtiend from vacuum sublimation (using a temperature-gradient tube furnace). Their higher emission quantum yield (Q.Y.), on the one hand, is caused by the rigid environment, which is expected to reduce the external quenching as well as to have a more destabilized MC d−d excited state because of the hindered stretching motion of metal−ligand bonding that destabilized the dσ* orbitals. On the other hand, Pt(II) complex 1, bearing the N-methyl substituted imidazolylidene, showed a broadened and structureless profile with peak max. located at 510 nm. The exceedingly large Stokes shift (cf. absorption in CH2Cl2, see Figure 2) and high emission quantum yield Q.Y. = 91% with short phosphorescence lifetime τobs = 1077 ns are in good agreement with the metal−metal-to-ligand charge transfer (MMLCT) character observed in many square planar Pt(II) complexes that exhibited extensive solid-state aggregation.48−50 For the respective N-isopropyl substituted Pt(II) complex 2, the increased steric encumbrance increases the spatial separation between the Pt(II) metal complexes and reduces the MMLCT character, giving a rather broad emission profile with multiple peak wavelengths at 455, 480, 524 nm, which probably originate from the mixture of monomeric and aggregated species. The concomitant increase of average emission lifetime (τobs = 3074 ns) and reduction of Q.Y. (53%) are also consistent with the reduction of MMLCT character. Similarly, the corresponding mtpyim Pt(II) complex 3 showed the largest red-shifted emission with λmax = 552 nm, Q.Y. = 95%, and τobs = 804 ns. Though lacking the X-ray structural data, it is reasonable to assume that complex 3 adopts similar crystal structure with that of 4, for which the Pt2 dimer units are stacked in the pyim-to-pyim and bipz-to-bipz arrangement. Accordingly the balance between van der Waal attraction and steric interaction for both alkyl substituents becomes crucial. For example, the t-butyl substituent on the pyridyl fragment (cf. complexes 3 and 4) would have improved, rather than prohibited, the intermolecular packing interaction by rotation and sliding of the adjacent Pt(II) metal complex against one another, so that the unfavorable steric interaction

Figure 1. (a) Structural drawing of [Pt(itpyim) (bipz)] (4) with thermal ellipsoids shown at 50% probability level. Selected bond distances: Pt−C(6) = 1.988(8), Pt−N(6) = 2.019(5), Pt−N(1) = 2.032(6), Pt−N(5) = 2.052(6), N(7)···H(13) = 2.102(6), and N(4)··· H(1) = 2.294(5) Å, and bond angles: N(1)−Pt−C(6) = 79.0(3), N(1)−Pt−N(6) = 174.3(3), N(5)−Pt−C(6) = 171.4(2), and N(5)− Pt−N(6) = 77.8(2)°. (b) Molecular stacking diagram with Pt···Pt contacts of 3.480(4) and 4.3214(4) Å and Pt···Pt···Pt angle of 154.4°.

packing diagram also revealed a reduced Pt···Pt contact of 3.480(4) Å within the Pt2 dimer and significantly lengthened Pt···Pt contact of 4.321(4) Å between the Pt2 dimer units. The Pt···Pt···Pt angle is calculated to be 154.4°, showing a tilted packing arrangement. Moreover, within each dimer unit, the Pt(II) fragments are packed in the pyim-to-pyim and bipz-tobipz fashion, and are rotated by ∼12.6° along the Pt···Pt axis for reducing possible steric encumbrance between the peripheral bulky substituents. On the other hand, the Pt2 dimer units are stacked in similar pyim-to-pyim and bipz-to-bipz fashion, but with a much greater degree of rotation along the Pt···Pt vector by ∼45°. Photophysical Properties. The pertinent absorption spectra of Pt(II) complexes 1−4 are shown in Figure 2, whereas numerical data are listed in Table 1. The shoulder shown in the longer wavelength region is assigned to the metalto-ligand charge transfer (MLCT) transition, whereas the intense absorptions at ∼340 nm in CH2Cl2 solution are due to the intraligand π−π* transitions from the dianionic bipz chelate to the neutral N^C chelate. Albeit variation of the extinction 33890


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ACS Applied Materials & Interfaces Table 1. Selected Photophysical Properties of Pt(II) Complexes 1−4 abs λmax (nm)a (ε × 103 M−1cm−1) 344 343 340 340

1 2 3 4

(11.7) (7.5) (13.6) (13.2)

em λmaxb (nm) 510 455, 480, 524 552 501

Φb (%) 91 53 95 99

τb (ns)

krc (s−1)

1077 3074 804 1033

× × × ×

8.4 1.7 1.2 9.6

5

10 105 106 105

knrc (s−1) 8.4 1.5 6.2 9.7

× × × ×

104 105 104 103

a

UV−vis spectra were recorded in CH2Cl2 solution. bPL data measured in solid state as neat powders at RT. Solid-state powders of all complexes were collected from temperature-gradient vacuum sublimation. ckr and knr were calculated according to the equations, kr = Φ/τobs and knr = (1/τobs) − kr.

due to the strong spin−orbit coupling induced by Pt(II) metal atom the lowest lying states in the singlet and triplet manifolds, i.e. S1 and T1, should be strongly mixed. On this basis, the calculated energy gaps are in agreement with the threshold/ onset of their absorption spectra around 400−430 nm in CH2Cl2 (see Figure 2). For the S0 → S1 and S0 → T1 optical transition, their main assignments are all HOMO → LUMO processes, in which the electron density distribution of HOMO for Pt(II) complexes 1−4 are mainly localized at bipz fragments and partially at Pt(II) atoms. On the other hand, the electron density distribution of LUMO for Pt(II) complexes 1−4 are all mainly localized at mpyim (for 1), ipyim (for 2), mtpyim (for 3) and itpyim (for 4) fragments, respectively, with few percentages at Pt(II) atoms (see Figure 3). The computational results thus suggest that HOMO primarily contains contributions from both dianionic chelate and central Pt(II) atom, whereas LUMO reside on the neutral chelates. Accordingly, the lowest lying transitions are commonly described as metalligand-to-ligand charge transfers (MLLCT).5 We then moved one more step to probe the self-assembly of the titled Pt(II) complexes in an attempt to mimic the observed emission in the solid state. In this approach, we mainly focused on complex 4 due to its well resolved X-ray crystal structure (vide supra), from which the packing diagram revealed a Pt···Pt contact of 3.480(4) Å and was truncated as a Pt2 dimer to perform the computation. As a result, the energy gaps of the S0 → S1 and S0 → T1 optical transition for Pt(II) complex 4 dimer are calculated to be 463.5 and 482.4 nm, respectively (Table 2 and Table S5), in which the S0 → T1 optical transition closely matches the observed peak wavelength of the complex 4 emission in solid (see Figure 2). The assignments of the S0 → S1 and S0 → T1 optical transition for dimer of 4 are both solely HOMO → LUMO process. The electron density distribution of HOMO and LUMO for the dimer of 4 are largely localized at bipz and itpyim fragments, respectively (Figure 3 and Figure S5). However, interestingly, as for the truncated Pt2 dimer the electron density distributions have non-negligible contribution from both Pt atoms in HOMO and LUMO, which are located at nonbonding 5dπ and 5dσ* orbitals, respectively. Similar MC dd excited state contributed to the HOMO → LUMO process was also noticed in its monomeric form (vide supra). It is believed that the repulsive dd potential energy surface, in part, accounts for the nonemissive character of the titled monomeric Pt(II) complexes in solution. On this basis, the calculated lowest lying MC dd state in the Pt2 dimer cannot rationalize the high emission QY of 4 in the solid. In other words, the truncated Pt2 dimer is not sufficient to represent the origin of emission in solid. It is notable that our calculation was based on the Pt···Pt contact of 3.480(4) Å to simulate the corresponding photophysical properties, for which the employed Pt···Pt distance is longer than the known distance with substantial Pt···Pt interaction (i.e., ∼3.3 Å)51,52 and the GIXS result of

between the pair of Pt2 dimers can be sequestered. As for R′ = t Bu and R″ = Me in complex 3, it is believed that the void space between two Pt2 dimers is large enough to accommodate tBu and Me groups such that the Pt···Pt stacking in solid state is shortened, resulting in the higher proportion of MMLCT character and hence lower emission energy gap. Changing R″ = methyl group (mtpyim) in 3 to bulky isopropyl substituent (itpyim) in 4 may increase the steric hindrance. As a result, the spatial separation between the Pt···Pt distance increases with corresponding reduction of MMLCT, which accounts for a blue-shift of MMLCT emission maximized at 501 nm for 4 (cf. 552 nm in 3). This phenomenon is virtually the same as the reduction in MMLCT character for isopropyl-to-methyl substitution detected in the parent Pt(II) metal complexes 1 and 2. Nevertheless, 4 exhibits the highest emission Q.Y. of 99% and comparable lifetime of τobs = 1033 ns as the neat powder. We then employed a computational time-dependent density functional theory (TD-DFT) approach to gain in-depth insight into these Pt(II) complexes at the single molecular level. The calculated singlet and triplet absorption characteristics for monomeric Pt(II) complexes 1−4 are displayed in Table 2, Figure 3 and Tables S1−S4 and Figures S1−S4. The calculated energy gaps for the S0 → S1 optical transition (in terms of wavelength) are 421.5 nm (1), 420.3 nm (2), 408.8 nm (3), 408.1 nm (4), and for the S0 → T1 optical transition are 446.2 nm (1), 445.2 nm (2), 435.5 nm (3), 434.5 nm (4). In reality, Table 2. Calculated Wavelengths, Transition Probabilities, and Charge Character of the Lowest Optical Transitions S1 and T1 (with mainly assignments) for Monomeric Pt(II) Complexes 1−4 and Dimer of 4 in CH2Cl2

1

2

3

4

4×2

state

λcal (nm)

T1

446.2

0

S1

421.5

0.0002

T1

445.2

0

S1

420.3

0.0259

T1

435.5

0

S1

408.8

0.0004

T1

434.5

0

S1

408.1

0.0005

T1

482.4

0

S1

463.5

0.0001

f

assignment HOMO → (86%) HOMO → (97%) HOMO → (86%) HOMO → (97%) HOMO → (84%) HOMO → (96%) HOMO → (84%) HOMO → (96%) HOMO → (86%) HOMO → (95%)

MLCT (%)

LUMO

−4.92

LUMO

−4.63

LUMO

−4.98

LUMO

−4.55

LUMO

−5.28

LUMO

−4.88

LUMO

−5.12

LUMO

−4.68

LUMO

−7.35

LUMO

−7.98

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Figure 3. Frontier molecular orbitals (HOMO [H] and LUMO [L]) involved in the lowest-lying transition for monomeric Pt(II) complexes 1−4 and dimer of 4 in CH2Cl2. The electron density distribution of Pt(II) atom(s) in each molecular orbital is also showed.

vacuum deposited thin film of 3 (vide infra). Further optimization is unfortunately prohibited due to the limited computation capacity. Triboluminescence. Triboluminescence is the phenomenon where the fracture of material gives production of the spontaneous emission.39,53 It has been well-established that dipolar structures and noncentrosymmetric arrangements of crystals are the two crucial criteria for achieving the triboluminescence.40−42,54 In the present system, the Pt(II) complexes 1−4 are assembled using both the charge neutral and dianionic chelates, which fulfill the requirement of having dipolar structures.55−57 Furthermore, in contrast to the Pt(II) complexes 1−3, which show no triboluminescence, grinding the powder of 4, for which the molecules crystallized in a noncentrosymmetric space group P3121, with a spatula or glass slide produces prominent bluish green flashes that are visible to naked eyes under the condition of indoor lighting at RT (see Video S1), confirming the second criterion. This triboluminescence is considered as an electric polarization phenomenon, with charge separation being the primary event and the recombination of separated charges during fracture resulting in the spontaneous emission. Hence, breaking the crystals of 4 by mechanical stresses electrifies the cracked crystal surfaces, which then electronically excites the molecules to afford the triboluminescence.58 This observation suggests that the existence of both t-butyl and i-propyl groups of chelates among the titled Pt(II) complexes are crucial for the formation of noncentrosymmetric crystals and hence the generation of triboluminescence. Nevertheless, what controlled the formation of noncentrosymmetric arrangements in the crystalline state is still pending resolution. For example, the crystal packing is similar between 3 and 4 whereas triboluminescence was not observed in 3. To gain further insight into the triboluminescent property, we show both photoluminescence and triboluminescence spectra of solid Pt(II) complex 4 in Figure 4. Comparing the photoluminescence spectra, which reveals typical broad MMLCT emission (vide supra) maximized at ∼506 nm, the triboluminescence shows a similar spectral feature but is slightly red-shifted to 509 nm by ∼3 nm. This spectral shift is authentic because an identical detecting system incorporating a charge coupled detector (CCD) was used throughout the measurement. For further comparison, photoluminescence of the sample after being ground is also shown in Figure 4. Clearly, the slight red shift of the emission was also observed. After dissolution of the ground sample in, for example, CH2Cl2 and

Figure 4. Emission spectra of Pt(II) complex 4 at RT. (a) Photoluminescence excited by 375 nm laser (emission maximum 506 nm), (b) triboluminescence (emission maximum 509 nm), and (c) photoluminescence of grinded complex 4 excited by 375 nm (emission maximum 508 nm).

recovering the solid sample, its photoluminescence regains the original spectrum (prior to grinding). The results demonstrate that upon imposing the mechanic force, the surface of origin crystal structure of 4, to a certain extent, has been altered, giving rise to the triboluminescence that is slightly different from the photoluminescence of the unperturbed one. In an aim to verify the viewpoint of structural difference before and after mechanic perturbation, Raman spectroscopy was then carried out to probe the possible surface structure transformation. In comparison to the Raman spectrum of the original complex 4, the results shown in Figure 5 clearly reveal that the ground sample causes the changes of several vibrational peaks in terms of frequencies or relative intensity. Careful analyses indicate that almost all vibration peaks were shifted by ∼2−4 cm−1 to the lower frequency. Especially, the lowfrequency Raman peak of 151 cm−1 for 4 (Figure 5a), which is assigned to the Pt−Pt stretching vibration of A1 symmetry,59 shifts to lower frequency of 149 cm−1 (Figure 5b). Also changed were the vibrational peaks ascribable to C=C and C=N stretching around 1600−1650 cm−1, which were shifted from 1623 and 1637 cm−1 to 1619 and 1633 cm−1, respectively. These variations lead us to conclude that during grinding, the mechanical stress energy weakens both Pt···Pt and π−π interactions at the crystal surfaces, in a way similar to that exerted by photoexcitation. The loosened molecule then 33892


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Figure 5. Raman spectra of Pt(II) complex 4 excited by 532 nm laser. (a) Before and (b) after grinding.

regains their ground state architecture and releases energy that induces the observed triboluminescence. Electrochemistry. Data of cyclic voltammetry were compiled in Figure S6. As can be seen, all Pt(II) complexes exhibit irreversible oxidation peak potentials in a narrow range of 1.12−1.17 V, which are consistent with the metal-centered oxidation. Therefore, the substituents on the pyridyl imidazolylidene can only impose minor influence to the oxidation peak potential. On the other hand, the reduction peak potentials occur at a slightly larger range between −2.19 V and −2.34 V, among which both of the 4-t-butylpyridine derivatives 3 and 4 (−2.30 V and −2.34 V) exhibit a notable negative shift versus that of the simple pyridine derivatives 1 and 2 (−2.23 V and −2.19 V). This difference may be rationalized in terms of the electron donating character of tbutyl substituent that destabilized the π*-orbital of the pyridyl imidazolylidene chelate. Electroluminescence. Among complexes 1−4, complex 3 retains a high emission QY of 96% when vacuum-deposited as a pure/nondoped thin film, with emission properties (i.e., emission spectrum and PLQY) mimicking those of the powder and is promising for use as the pure/nondoped emitting layer for OLEDs.60 Complex 3 was thus subjected to electroluminescent (EL) studies using architecture: ITO/MoO3 (1 nm)/TAPC (65 nm)/mCP (8 nm)/3 (pure/nondoped, 30 nm)/3TPYMB (50 nm)/LiF (1 nm)/Al (120 nm). TAPC, mCP and 3TPYMB stand for 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane, N,N-dicarbazolyl-3,5-benzene and tris-[3-(3pyridyl)mesityl]borane, respectively, and serve either as the hole- or electron-transport layers.61,62 Vacuum-deposited thin film of 3 constituted the emitting layer (EML). Representative EL characteristics of OLEDs using the nondoped emitting layer of 3 are shown in Figure 6a−c). Its EL spectrum (yellow emission) is similar to its PL spectra in both vacuum-deposited thin films and in powders. The slight difference between the thin-film PL spectrum and the EL spectrum is mainly due to the weak microcavity effect in the device environment. The device exhibits a turn-on voltage of ∼2.8−3 V and an operation voltage ∼4 V for a brightness of 100 cd m−2. Most importantly, the nondoped device of 3 shows very high maximal EL efficiencies of (26%, 91 cd A−1, 64.4 lm W−1). In addition, the device retains high external quantum

Figure 6. (a) EL spectrum together with PL spectrum of the vacuumdeposited thin film of 3, (b) current density−voltage-luminance (J− V−L) characteristics, and (c) efficiency characteristics of the OLED using Pt(II) complex 3 as the pure/nondoped emitting layer.

efficienciy (EQE) and luminesce efficiency of (25.9%, 90 cd A−1) at the practical brightness of 100 cd m−2 and (24.4%, 85 cd A−1) even at the high brightness of 1000 cd m−2, respectively. Such a low efficiency roll-off may be attributed to the short excited-state lifetimes of 3 (∼1000 ns in the vacuum-deposited thin film as shown in Figure 7, and ∼800 ns in the solid-state powder as discussed in the previous section), which in turn is associated with the MMLCT transition character of 3. To the best of our knowledge, the recorded EL

Figure 7. Transient PL decay curves of vacuum deposited films of 3 and 4. The decay curve of 3 can be fitted by a monoexponential function with a lifetime of 1050 ns. The curve of 4 is more complicated and cannot be fitted by a monoexponential decay kinetics. 33893


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separation detected by single-crystal XRD experiment on 4 (3.48 Å), and allowed the formation of MMLCT transitions.51,52 Although the detected signal for intermolecular stacking of Pt complex 3 in GIXS is not strong, it is however clearly visible. The absorption spectrum of the vacuum deposited thin film 3 (as shown in Figure 8a), when compared to absorption spectrum of 3 in solution, also firmly supports formation of notable ground-state aggregation. The weak GIXS signal for 3 is due to the poorer crystallinity exerted by vacuum deposition. Its intensity in thin film depends on many factors, such as the sizes of crystallites, the ratios and the orientation distribution of aggregates, and the excitation beam intensity, etc. The weakened stacking signal in GIXS analyses and its relatively broad angular distribution may indicate the formation of small (nanoscale) aggregates and somewhat scattered orientations of columnar aggregates. Meanwhile, the broadened diffraction peak along the out-of-plane axis (i.e., along Qz) and the smaller Qz = 0.38 Å−1 represents the intercolumnar assembly and corresponds to an intercolumnar d-spacing of 22.2 Å.65,66 The broadened and ringlike GIXS pattern, however, indicates a somewhat random distribution of the intercolumnar packings. The excitation spectrum of 3 in vacuum-deposited thin films is similar to the corresponding absorption spectra, indicating that the emission of vacuum deposited 3 films originates from the identical ground electronic state, i.e., the ground-state aggregation. The transient PL measurement on vacuumdeposited thin films of 3 (Figure 7) reveals monoexponential decay characteristics with an excited state lifetime of ∼1000 ns, similar to that in the solid-state powder of 3 as discussed in the previous section). These photophysical properties again support the MMLCT transition character of 3 from the ground-state aggregation in vacuum deposited thin films. Possible emitting dipole orientation in films of 3 were also characterized by the angle- and polarization-dependent photoluminescent experiment.67−69 The preferential horizontal (inplane) transition dipoles were attained in vacuum-deposited films of 3, with the intermolecular stacking being parallel to the substrate and the MMLCT transition along the stacking axis.70 The measured p-polarized PL intensities of 3 as a function of the emission angles are shown in Figure 9a. The measured data are compared to results simulated with different horizontal dipole ratios Θ// (the percentage of horizontal dipoles among all emitting dipoles) to extract Θ// in the emitting layer (cf. Θ// = 100% for fully horizontally arranged orientation and Θ// = 67% for the isotropic arranged transition dipole). The measured data match well with those of Θ// = 74% shown in Figure 9b, clearly revealing its favorable horizontal (in-plane) transition orientation. Recent reports have revealed the importance of having preferential transition dipoles along the in-plane (horizontal) orientation for higher optical outcoupling,67−69 because vertical emitting dipoles in OLEDs generally contribute little to the extracted emission and, hence, OLED with emitters showing preferential horizontal dipole ratio (Θ// > 67%) are desirable for improving OLED optical out-coupling and external quantum efficiencies.67−69 The high external quantum efficiency of 3-based OLED thus may result from both the higher PLQY and the preferential horizontal transition dipoles. As a comparison, although (crystalline) powder of 4 (obtained from vacuum sublimation) also show a very high PLQY, yet its vacuum-deposited nondoped thin film gives a much reduced PLQY of only 5−6%. As a consequence, the

efficiencies are among the highest of nondoped phosphorescent OLEDs (using pure/nondoped transition metal complex emitting layers) ever documented in literature.63,64 Although >30% EQEs had been reported for red-emitting nondoped phosphorescent OLEDs (with emission wavelength >600 nm) utilizing MMLCT based Pt(II) complexes,63,64 this work represents a first demonstration that similarly high EQEs can be achieved for shorter emission wavelengths (i.e., < 600 nm) based on the MMLCT transition mechanism of Pt(II) complexes. The results thus shed light to further development of high-efficiency nondoped phosphorescent OLEDs and MMLCT-based OLEDs spanning different visible colors. The formation of ground-state aggregates in vacuumdeposited thin film of 3 is verified by UV−vis spectroscopy and GIXS (grazing incidence X-ray scattering) analyses.65,66 Figure 8(a) exhibited the UV−vis absorption spectra of 3

Figure 8. (a) Absorption spectra of 3 in the dilute CH2Cl2 solution and in the vacuum-deposited thin film. (b) GIXS pattern of the vacuum-deposited nondoped thin film of 3.

recorded in both CH2Cl2 solution and as vacuum-deposited thin film; the latter clearly shows the emergence of a broad peak profile at ∼400 nm associated with ground-state aggregates. The GIXS data shown in Figure 8b also exhibited broadened diffraction peaks along the out-of-plane orientation (Qz), and weak diffractions along the in-plane orientation (i.e., along Qxy). Peaks around Qxy = 1.9 Å−1 of the in-plane orientation can be attributed to the self-assembly/stacking of 3 into the columnar stacks in the vacuum-deposited thin film.65 In addition, the intermolecular stacking signal in the in-plane direction (i.e., along Qxy) suggests a preferential columnar alignment parallel to the substrate surface.65 The stacking distance is calculated to be 3.33 Å, which is similar to the Pt···Pt 33894


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an excellent emitter for the fabrication of highly efficient OLEDs, achieving superb external quantum efficienciy (25.9%,) and luminesce efficiency (90 cd A−1) at the practical brightness of 100 cd m−2. The rather low roll-off in efficiency is attributed to the short excited-state lifetime of 3 (∼800 ns) in the solid state, which in turn is associated with the MMLCT transition character. The results broaden the horizon for further development of high-efficiency nondoped MMLCT phosphorescence based OLEDs.

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EXPERIMENTAL SECTION

General Procedures. All reactions were performed under nitrogen. Solvents were distilled from appropriate drying agents prior to use. Commercially available reagents were used without further purification. The corresponding 3-alkyl-1-(pyrid-2-yl)-1Himidazolium salts were synthesized according to literature procedures, alkyl = methyl and iPr.43,44 5,5′-di(trifluoromethyl)-2H,2′H-3,3′bipyrazole (bipzH2) was prepared from condensation of 2,3butanedione and ethyl trifluoroacetate, followed by treatment with hydrazine.45 All reactions were monitored by TLC with precoated silica gel plates (Merck, 0.20 mm with fluorescent indicator UV254). Flash column chromatography was carried out using silica gel obtained from Merck (230−400 mesh). Mass spectra were obtained on a JEOL SX-102A instrument operating in electron impact (EI) or fast atom bombardment (FAB) mode. 1H and 19F NMR spectra were recorded on a Bruker-400 or INOVA-500 instrument. Elemental analysis was carried out with a Heraeus CHN-O Rapid Elementary Analyzer. Preparation of 3-Alkyl-1-(pyrid-2-yl)-1H-imidazolium PF6− Salt. To a typical reaction, a DMSO solution of corresponding 2bromopyridine, imidazole, K2CO3, CuI, and L-proline was stirred at 60 °C for 24 h; after then, excess of ethyl acetate was added. The solution was washed with deionized water, dried over Na2SO4 and concentrated by rotary evaporation. Flash column chromatography afforded the corresponding 2-pyridylimidazole. It was then treated with methyl iodide in acetonitrile at 65 °C for 12 h. After being cooled to RT, the resulting mixture was filtered, and a saturated KPF6 water solution was added to the filtrate, giving white precipitate as the desired product upon concentration. Data of (mpyim)HPF6, yield: 91%. 1H NMR (400 MHz, d6acetone, 298 K): δ 9.88 (s, 1H), 8.66 (dd, J = 4.0, 1.2 Hz, 1H), 8.47 (t, J = 2.0 Hz, 1H), 8.21 (dt, J = 7.6, 2.0 Hz, 1H), 8.03 (d, J = 7.6 Hz, 1H), 7.97 (t, J = 2.0 Hz, 1H), 7.66 (ddd, J = 7.6, 4.0, 1.2 Hz, 1H), 4.23 (s, 3H). 19F NMR (376 MHz, d6-acetone, 298 K): δ − 71.64 (d, JFP = 717 Hz, 6F). Data of (ipyim)HPF6, yield: 54%. 1H NMR (400 MHz, d6-acetone, 298 K): δ 9.99 (s, 1H), 8.62 (d, J = 4.8 Hz, 1H), 8.48 (t, J = 1.6 Hz, 1H), 8.17 (dt, J = 7.6, 1.6 Hz, 1H), 8.10 (t, J = 1.6 Hz, 1H), 7.99 (d, J = 7.6 Hz, 1H), 7.61 (dd, J = 7.6, 4.8 Hz, 1H), 4.77 (septet, J = 6.8 Hz, 1H), 1.53 (d, J = 6.8 Hz, 6H). 19F NMR (376 MHz, d6-acetone, 298 K): δ − 70.19 (d, JFP = 711 Hz, 6F). Data of (mtpyim)HPF6, yield: 64%. 1H NMR (400 MHz, CDCl3, 298 K): δ 9.35 (s, 1H), 8.41 (d, J = 5.2 Hz, 1H), 8.16−8.14 (m, 1H), 7.78 (s, 1H), 7.44−7.42 (m, 2H), 4.09 (s, 3H), 1.36 (s, 9H). 19F NMR (376 MHz, CDCl3, 298 K): δ − 71.97 (d, JFP = 710 Hz, 6F). Data of (itpyim)HPF6, yield: 62%. 1H NMR (400 MHz, CDCl3, 298 K): δ 9.39 (s, 1H), 8.41 (d, J = 5.2 Hz, 1H), 8.21 (s, 1H), 7.83 (s, 1H), 7.51 (s, 1H), 7.44 (d, J = 5.2 Hz, 1H), 4.92 (septet, J = 6.8 Hz, 1H), 1.64 (d, J = 6.8 Hz, 6H), 1.38 (s, 9H). 19F NMR (376 MHz, CDCl3, 298 K): δ − 71.9 (d, JFP = 714 Hz, 6F). Preparation of Pt(II) Complexes. A mixture of Pt(DMSO)2Cl2 (703 mg, 1.7 mmol), (mpyim)HPF6 (560 mg, 1.7 mmol) and NaHCO3 (210 mg, 2.5 mmol) in 10 mL of anhydrous DMSO was heated at 100 °C for 12 h. After cooled to RT, bipzH2 (520 mg, 1.7 mmol) was added and the mixture was stirred at 100 °C for 6 h. After then, the reaction was quenched by addition of excess of water to induce the immediate precipitation. It was filtered and washed with water and ethyl ether in sequence, and sublimed in vacuo to afford light green colored solid of [Pt(mpyim) (bipz)] (1, 540 mg, 68%). Other Pt(II) analogues, i.e., colorless [Pt(ipyim) (bipz)] (2, 62%),

Figure 9. (a) Measured (symbols) p-polarized PL intensity (around PL peak wavelength) of the vacuum-deposited pure film of 3 as a function of the emission angle and wavelength. The measured data are compared with simulated ones with different horizontal dipole ratios Θ// (e.g., Θ// = 100% for fully horizontal dipoles and Θ// = 67% for the isotropic dipole orientation) to extract the horizontal emitting dipole ratios of the emitting layer. The measured data match well with those calculated with Θ// = 74% shown in (b).

OLED fabricated using vacuum-deposited thin layer of 4 exhibit much inferior EL efficiencies (3.4%, 10.7 cd A−1, 6.5 lm W−1) (see the Supporting Information) vs the OLED fabricated using vacuum deposited thin film of 3. Furthermore, in sharp contrast to the spectral behavior of 3, neither the UV−vis spectrum nor the GIXS analyses of 4 have shown the aggregate formation and stacked structure in the vacuum-deposited thin film (see Figure S9), whereas the transient PL exhibited the multiexponential decay kinetics (see Figure 7). The latter is attributed to the weakened ground-state aggregation, giving various trapping sites that exhibit slightly different excited-state relaxation dynamics. Hence, this result explains the lowered PLQY of vacuum-deposited thin film vs that of sublimed sample of 4.

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CONCLUSION In summary a new series of charge-neutral Pt(II) metal complexes 1−4 bearing both pyrid-2-yl-imidazolylidene and dianionic bipz chelates have been strategically designed and synthesized. In serendipity, complex 4 revealed remarkable triboluminescence which is tentatively rationalized by the unique arrangement for the isopropyl and tert-butyl substituents on the pyrid-2-yl-imidazolylidene chelate. The triboluminescence is found to be sensitive to subtle changes of the associated substituents and disappears in 1−3 upon replacing either isopropyl or tert-butyl substituents by methyl or hydrogen atom, respectively. For complex 3, the well-ordered solid packing, supported by the grazing incidence X-ray scattering experiment, and thus the aligned dipole serves as 33895


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ACS Applied Materials & Interfaces *E-mail: [email protected]. *E-mail: [email protected].

yellow [Pt(mtpyim) (bipz)] (3, 68%), and light green [Pt(itpyim) (bipz)] (4, 52%), were prepared in an analogous manner. Spectral Data of [Pt(mpyim) (bipz)] (1). MS (FAB, 195Pt): m/z 622.7 [M+]; 1H NMR (400 MHz, d6-DMSO, 298 K): δ 10.67 (br, 1H), 8.35 (br, 2H), 8.08 (br, 1H), 7.64 (br, 1H), 7.57 (br, 1H), 6.65 (br, 1H), 6.59 (br, 1H), 4.45 (s, 3H). 19F NMR (376 MHz, d6-DMSO, 298 K): δ − 58.87 (s, 3F), − 59.34 (s, 3F). Anal. Calcd for C17H11F6N7Pt: C, 32.81; H, 1.78; N, 15.75. Found: C, 32.59; H, 1.62; N, 15.58. Spectral Data of [Pt(ipyim) (bipz)] (2). MS (FAB, 195Pt): m/z 650.4 [M+]; 1H NMR (400 MHz, d6-DMSO, 298 K): δ 10.75 (d, J = 4.8 Hz, 1H), 8.48 (d, J = 2.4 Hz, 1H), 8.37 (t, J = 6.8 Hz, 1H), 8.14 (d, J = 8.0 Hz, 1H), 7.97 (d, J = 2.4 Hz, 1H), 7.69 (t, J = 6.8 Hz, 1H), 6.87 (septet, J = 6.6 Hz, 1H), 6.72 (s, 1H), 6.66 (s, 1H), 1.44 (d, J = 6.6 Hz, 6H). 19F NMR (376 MHz, d6-DMSO, 298 K): δ − 58.90 (s, 3F), − 59.54 (s, 3F). Anal. Calcd for C19H15F6N7Pt: C, 35.08; H, 2.32; N, 15.07. Found: C, 34.85; H, 2.26; N, 15.19. Spectral Data of [Pt(mtpyim) (bipz)] (3). MS (FAB, 195Pt): m/z 678.6 [M+]; 1H NMR (400 MHz, d6-acetone, 298 K): δ 10.52 (d, J = 6.0 Hz, 1H), 8.47 (d, J = 2.0 Hz, 1H), 8.02 (s, 1H), 7.65 (d, J = 6.0 Hz, 1H), 7.56 (d, J = 2.0 Hz, 1H), 6.63 (s, 1H), 6.56 (s, 1H), 4.41 (s, 3H), 1.37 (s, 9H). 19F NMR (376 MHz, d6-acetone, 298 K): δ − 58.83 (s, 3F), − 59.31 (s, 3F). Anal. Calcd for C21H19F6N7Pt: C, 37.17; H, 2.82; N, 14.45. Found: C, 37.49; H, 2.78; N, 14.67. Spectral Data of [Pt(itpyim) (bipz)] (4). MS (FAB, 195Pt): m/z 706.8 [M+]; 1H NMR (400 MHz, d6-acetone, 298 K): δ 10.82 (d, J = 6.4 Hz, 1H), 8.30 (d, J = 2 Hz, 1H), 8.00 (s, 1H), 7.73 (d, J = 2 Hz, 1H), 7.59 (d, J = 6.4 Hz, 1H), 7.13 (septet, J = 6.6 Hz, 1H), 6.60 (s, 1H), 6.54 (s, 1H), 1.51 (d, J = 6.6 Hz, 6H), 1.46 (s, 9H). 19F NMR (376 MHz, d6-acetone, 298 K): δ − 60.66 (s, 3F), − 61.23 (s, 3F). Anal. Calcd for C23H23F6N7Pt: C, 39.10; H, 3.28; N, 13.88. Found: C, 39.22; H, 3.44; N, 13.92. Selected Crystal Data of 4. C23H23F6N7Pt; M = 706.57; trigonal; space group = P3121; a = 14.0948(2) Å, b = 14.0948(2) Å, c = 21.1510(3) Å; α = β = 90°, γ = 120°, V = 3638.98(9) Å3; Z = 6; ρcalcd = 1.935 Mg·m−3; F(000) = 2052; μ = 5.857 mm−1; crystal size =0.40 × 0.20 × 0.18 mm3; λ(Mo−Kα) = 0.71073 Å; T = 150(2) K; 16228 reflections collected, 5508 independent reflections (Rint = 0.0339), restraints/parameters =19/349, GOF = 1.160, final R1 [I > 2σ(I)] = 0.0296 and wR2 (all data) = 0.0817; largest diff. peak and hole =1.270 and −1.372 e Å−3. Single-Crystal X-ray Diffraction Studies. Single-crystal X-ray diffraction study was measured with a Bruker SMART Apex CCD diffractometer using (Mo−Kα) radiation (λ = 0.71073 Å). The data collection was executed using the SMART program. Cell refinement and data reduction were performed with the SAINT program. An empirical absorption was applied based on the symmetry-equivalent reflections and the SADABS program. The structures were solved using the SHELXS-97 program and refined using the SHELXL-97 program by full-matrix least-squares on F2 values. The structural analysis and molecular graphics were obtained using the SHELXTL program on a personal computer.

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ORCID

Pi-Tai Chou: 0000-0002-8925-7747 Yun Chi: 0000-0002-8441-3974 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of Taiwan (MOST) under Grants 104-2119-M007-001, 105-3113-E-002-008, and 104-2221-E-002-152-MY3.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12707. Details of experimental procedures and data, results of the TD-DFT calculation of all Pt(II) complexes, and OLED device characteristics (PDF) CIF data of Pt(II) complex 4 (with deposition number: CCDC 1499101) (CIF) Video showing the triboluminescence (MPG)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. 33896


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