Emission Enhancement by Intramolecular Stacking between

Mar 14, 2019 - Synopsis. Intramolecular stacking between iridium(III) complex core and flexibly bridged aromatic pendant group results in a unique res...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Emission Enhancement by Intramolecular Stacking between Heteroleptic Iridium(III) Complex and Flexibly Bridged Aromatic Pendant Group Kaspars Traskovskis,*,† Valdis Kokars,† Sergey Belyakov,‡ Natalija Lesina,§ Igors Mihailovs,†,§ and Aivars Vembris§ †

Institute of Applied Chemistry, Riga Technical University, 3/7 Paula Valdena Street, Riga LV-1048, Latvia Latvian Institute of Organic Synthesis, 21 Aizkraukles Street, Riga LV-1006, Latvia § Institute of Solid State Physics, University of Latvia, 8 Kengaraga Street, Riga LV-1063, Latvia

Inorg. Chem. Downloaded from pubs.acs.org by EAST CAROLINA UNIV on 03/15/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: Phosphorescent iridium(III) complexes suffer from a strong aggregation quenching, limiting their use in solution-processed or crystalline organic light-emitting diodes. Here we report how an intramolecular stacking between a flexibly bridged bulky aromatic pendant group and the core of nonionic heteroleptic complex can be exploited to minimize the negative effects of this drawback. The stacked conformation provides a rigid sterical shielding of the polar molecular surface, improving photoluminescence quantum yield of the complex both in solution and crystalline state.

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quenching. The highest ΦPL for crystalline forms of nonionic iridium complexes is reported for one of the polymorphs of factris(2-phenylpyridine)iridium (Ir(ppy)3, ΦPL = 0.34).18 Ongoing efforts to reduce solid-state emission quenching have stimulated the intensive exploration of iridium complexes with aggregation-induced phosphorescence enhancement (AIPE),19 allowing in some cases to obtain materials with ΦPL values that surpass the 0.2 mark. The unshielded molecular surface of the aforementioned compounds determines that intermolecular π−π stacking between the cyclometalating ligands of the neighboring emitters takes place. The close contacts between the complex molecules lead to inevitable transformations of molecular orbital energy levels and, consequently, to the experimentally observed solid-state emission redshift in comparison to the emission wavelength in dilute solutions. Despite the relatively large solid-state ΦPL values of some of these iridium complexes, their direct incorporation possibility in OLEDs is limited, as strong efficiency roll-off occurs in the devices with the current density increase due to triplet−triplet annihilation. Such process takes place if the distance between individual emitter molecules is lower than the critical distance for concentration quenching for iridium complexes, estimated in the range of 0.8−1.4 nm.20 To attain a sufficient spatial isolation, the mass fraction of the active compounds in the emissive layers of OLEDs is usually limited to ∼10 wt %, as the phosphorescent complexes are dispersed into charge-trans-

INTRODUCTION Cyclometalated iridium(III) complexes are the most researched class of phosphorescent luminophores considering their extensive use as luminescent probes1,2 and, in particular, as the emitters in organic light-emitting diodes (OLEDs).3−7 The large photoluminescence (PL) quantum yield (ΦPL) values and triplet harvesting ability of the compounds allow manufacturing OLEDs with external quantum efficiencies (EQE) that are almost reaching the fundamentally attainable limit.8 Lately the research efforts involving iridium(III)-based materials have partly shifted toward the development of solution-processable compounds, to reduce the production cost of large-area emissive devices like OLED televisions (TVs) and lighting panels.9−11 Another emerging research direction involves acquisition of emitting layers with anisotropic alignment of molecules, where the fraction of the photons that can leave the device is increased.12−14 In such a way the theoretically attainable EQE of OLEDs can be increased from 30% to ∼40%.15 This approach has allowed to acquire vacuum-deposited iridium(III)-based OLEDs with EQE of 38%.16 Similar concept was exploited in the case of platinum(II) bis(3-(trifluoromethyl)-5-(2-pyridyl)-pyrazolate) [Pt(fppz)2], which forms crystals with unusually large ΦPL of 0.96. Owning to highly distinct spatial orientation of the molecules in crystalline state, a Pt(fppz)2-based OLED with record EQE of 39% was obtained,17 illustrating the great potential of crystalline emissive materials. In contrast to platinum-based compounds, iridium(III) complexes suffer greatly from aggregation-induced emission © XXXX American Chemical Society

Received: November 23, 2018

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DOI: 10.1021/acs.inorgchem.8b03273 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Chemical Structures of the Investigated Compounds and Synthesis of (trppy)2Ir(pic)

porting host materials.20,21 While this approach is highly effective for vacuum-deposited OLEDs, solution-processed devices still strongly suffer from emitter aggregation-induced negative effects, even in the presence of a host material. Encapsulation with massive covalently attached isolating groups is often employed to overcome the emitter aggregation for nonionic iridium(III) complexes designated for solutionbased processing techniques,11 but the resulting materials often suffer from reduced charge-transfer parameters.22 Generally, the highest emission efficiency among crystalline forms can be observed for ionic iridium(III) complexes. The presence of a sterically large counterion and more pronounced electrostatic interactions allow acquisition of self-assembled solid structures,23 where complete spatial isolation of cyclometalating ligands can be achieved by hydrogen bondstabilized isolating networks of solvent molecules24 or large substituents.25 In such a way ΦPL values up to 0.48 can be attained in addition to no observed emission band redshift in the crystalline state.25 Unfortunately, the practical use of the charged iridium complexes is limited to light-emitting electrochemical cells (LECs)26 or anionic-cationic ion pair (soft salt) OLEDs,27 whose efficiency, stability, and response time parameters are notably lower than those for the devices based on nonionic compounds. Intramolecular stacking is a powerful tool in materials science, and this phenomenon has been used to improve emissive properties of ionic iridium complexes in LECs.28−30 The stacking between aromatic rings of cyclometalating ligands blocks nucleophilic attacks on the complex cores and stabilizes Ir−ligand bonds, increasing the lifetime of devices. No notable attempts have been reported that further expand the use of aromatic interactions within this class of compounds. Encapsulation of an iridium complex molecule with intramolecularly stackable and bulky substituents can be proposed as an obvious structural approach toward the reduction of concentration quenching. In this paper we present a practical implementation of the aforementioned concept. An aromatic interaction between a

nonemissive aromatic pendant group and the polar surface of a nonionic iridium(III) complex stabilizes molecular conformation that prevents close contacts between complex cores in crystalline state thus limiting concentration quenching. Additionally, the unique electronic orbital configuration of the complex in combination with a solvent-structure-sensitive process of intramolecular stacking creates a unique PL enhancement mechanism in solvents with specific polarity characteristics.

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RESULTS AND DISCUSSION

Structural Characterization. On the basis of a known phosphorescent heteroleptic iridium(III) complex (ppy)2Ir(pic),31,32 composed of an ancillary picolinic acid (pic) and two main 2-phenylpyridine (ppy) ligands, the target compound (trppy)2Ir(pic) was synthesized (Scheme 1). Bulky triphenylmethyl (TR) groups were attached to the complex core through a flexible, four-atoms-long bridge. TR was chosen as the isolating group because of its ability to minimize interactions between highly polar chromophores,33,34 to shield functional fragments in crystal lattices,35,36 and to induce selfassembly in gel-like structures.37 The tendency to form an intramolecularly stacked conformation (TR-to-pic-oxygen) was confirmed for (trppy)2Ir(pic) by employing single-crystal X-ray diffraction analysis for a MeCN-grown single crystal (Figure 1a). The origin of such a molecular geometry is revealed by the calculated electrostatic potential (ESP) surfaces (Figure 1b). The stacking is driven by an attractive electrostatic force between TR phenyl ring and a complementary pocket formed by cyclometalating ligands. The positive edge of the phenyl ring strongly interacts with negatively charged oxygen of pic (C−H···O, 2.515 Å). The conformation is additionally stabilized by two edge-to-face interactions: one between the positive edge of TR phenyl ring and the negative face of ppyphenyl, and another between the negative face of TR phenyl and the positive edge of ppy-pyridine. B

DOI: 10.1021/acs.inorgchem.8b03273 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) X-ray structure of Δ-(trppy)2Ir(pic) molecule with thermal ellipsoids shown at the 50% probability level. Green dashed line marks close contact between TR group (purple) and oxygen of pic. (b) Calculated ESP surfaces of free (left) and on X-ray structurebased stacked conformation (right).

It is worth mentioning that the X-ray-examined MeCNgrown crystal contains homochiral molecules, thus indicating formation of enantiopure conglomerates. Presence of a mechanical mixture of enantiopure crystals is demonstrated by polarization microscopy (Figure S1). To the best of our knowledge, this is the first reported example of a spontaneous enantiopure conglomerate formation for cyclometalated iridium(III) complexes, as the racemic mixtures of Δ/Λ iridium(III) cyclometalating reaction products are usually resolved either chromatographically by employing chiral stationary phase38 or by subsequent synthesis of diastereomeric products with the use of chiral cyclometalating ligands.39 Photophysical Properties in Solution. (trppy)2Ir(pic) shows green phosphorescence in deoxygenated solvents with emission band almost completely overlapping with that of the base compound (ppy)2Ir(pic), but at the same time much higher emission quantum yield can be attained for the TRfunctionalized compound (Table 1, Figure 2). Generally, the solvatochromic response of organic luminophores follows a linear trend with the changes of the medium’s dielectric constant (ε).40,41 Instead, (trppy)2Ir(pic) shows the most intense emission in deoxygenated tetrahydrofuran (THF) (ΦPL = 0.90; ε = 7.58), while PL brightness significantly drops in other solvents, both less and more polar (Figures 2a and S3; Table S1). To interpret this solvent-dependent PL behavior and its possible relation to occurrence of intramolecular stacking, a series of quantum-chemical calculations was performed. Previous computational studies of the base compound

Figure 2. (a) Emission of UV (360 nm) irradiated solutions of (trppy)2Ir(pic) (c = 1 × 10−3 mol L−1) in different polarity solvents. Note the most intense PL in deoxygenated THF. The emission once again turns on in previously aerated THF solution after 24 h storage due to a precipitation of microcrystallites. (b) Absorption and emission bands of (trppy)2Ir(pic) in solutions. Emission of the base compound (ppy)2Ir(pic) given for the comparison.

(ppy)2Ir(pic) have shown that the poor PL efficiency of this complex (ΦPL = 0.03) can be largely attributed to the spatial distribution of its lowest unoccupied molecular orbital (LUMO), as it resides on pic moiety.32 Such electronic configuration partly shifts the emissive excited states away from the main ppy ligands, lowering attainable ΦPL. Accordingly, molecular orbitals of (trppy)2Ir(pic) in nonstacked and TR-topic aligned conformations were calculated in vacuum, toluene (ε = 2.38), and dichloromethane (DCM, ε = 8.93). The results reveal that the slight chemical modification of the main ligand

Table 1. Photophysical and Electronic Properties of (trppy)2Ir(pic) and (ppy)2Ir(pic) compound

absorbance,a λmax, nm (lg ε)

emission,b λmax, nm

ΦPLc

τ,d μs

kr,e × 105 s−1

knr,f × 105 s−1

Im,g eV

AE,h eV

(trppy)2Ir(pic) (ppy)2Ir(pic)

481(2.9), 424(3.6), 404(3.7) 475(2.9), 421(3.6), 399(3.6)

516 (514) 513 (511)

0.90/0.03 0.10/0.03

1.95

1.56

1.44

5.42

3.11

Measured in THF solution. bValues in THF solution (and toluene). cValues in deoxygenated THF/toluene solution. dMeasured in PMMA film at 1% emitter concentration. eRadiative decay rate kr = ΦPL/τ. fNonradiative decay rate knr = (1 − ΦPL)/τ. gIm−molecular ionization energy level in thin amorphous film. hAE−electron affinity energy level in thin amorphous film. a

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DOI: 10.1021/acs.inorgchem.8b03273 Inorg. Chem. XXXX, XXX, XXX−XXX

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band, the value 2.40 eV can be obtained.5 Regarding the medium influence, NTO analysis shows that, in the case of nonstacked conformation, the previously discussed solvent polarity-induced convergence of LUMO and LUMO+1 levels causes notable transformations of S0 → S1 transition. Particularly, the excited electron becomes delocalized on both ppy ligands instead of the single-ligand configuration in toluene. As a result, ΦPL is expected to drop under such conditions, because, to emit from triplet state, an initial ligandto-ligand charge transfer must take place, in addition to a larger probability of vibrational relaxations.45,46 While no notable changes can be seen in the case of the lowest triplet excitation, it can be predicted that the exposure of pic to a strongly polar or coordinating medium would eventually lead to a complete overlap or switch of the two lowest LUMO levels, resulting in a poorly emissive (ppy)2Ir(pic)-like electronic configuration. In an attempt to determine the preferential conformations of (trppy)2Ir(pic) a computational/statistical analysis was performed based on the optimized geometries for a set of 105 differently seeded conformers. While the results reveal a clear tendency of the molecule to form a TR-to-ppy-stacked conformation in the stabilized edge-to-face configuration (Figure 4),47 the used polarizable continuum solvation

has shifted LUMO to ppy, while pic is hosting LUMO+1 level (Figure S4). At the same time the energetic difference between these orbitals is small, in the range of 0.1−0.2 eV, and, as the levels approach each other, their constituting ligand-centered orbitals increasingly mix together, causing the topology of LUMO and LUMO+1 to resemble each other more closely.42 A stark difference in the response to altering medium polarity can be seen between free and TR-to-pic stacked conformations. Because the electrostatic extremes of the exposed complex surface are located on the pic fragment, LUMO+1 state is becoming relatively more stabilized with solvent polarity increase, driving it closer to LUMO (Figure 3a). For the TR-to-pic stacked conformation the opposite can be observed, as solvation of pic is obstructed by TR moiety.

Figure 3. (a) LUMO and LUMO+1 energy level shifts in relation to different polarity medium for nonstacked and TR-to-pic-stacked conformations of (trppy)2Ir(pic). (b) NTO difference plots of singlet−singlet (S0 → S1) and singlet−triplet (S0 → T1) electron transitions (excitations proceed from green to red orbitals) for the nonstacked conformation.

Figure 4. Computational/statistical analysis of TR1/TR2 and ppy binding tendency in edge-to-face mode, showing the distribution of HF-3c-optimized conformers (black dots) in toluene and DCM in relation to their DFT energy and TR-ppy distance.

The results of natural transition orbital (NTO) analysis (Figure 3b) show that the singlet−triplet (T1 → S0) electron transition, mainly responsible for the phosphorescence of (trppy)2Ir(pic), is contained within just one of the ppy ligands, with an additional involvement of iridium d-orbitals. This is in accordance with the experimental observations, as the emission band of the complex at room-temperature conditions is wide and featureless, indicating a strong metal-to-ligand charge transfer (MLCT) character.43 The PL measurements at 77 K (Figure S5) show a rigidochromic blueshift typical for cyclometalated complexes together with an appearance of distinctive vibrionic features that can be associated with the increasing dominance of ligand-centered (LC) π−π transitions during the emission process.44 By estimating T1 → S0 transition energy from the highest-energy vibrational sub-

model under-represents the experimentally detected TR-topic geometry. This can be related to the fact that the model does not account for structure-specific solvent−solute interactions that obviously take place judging by the experimentally observed nonlinear PL response to medium’s dielectric constant. On the basis of the previous discussion the following solvent−solute interaction model can be proposed. As the calculated ESP surfaces show, the exposed parts of (trppy)2Ir(pic) consist of positively charged edges of ppy and pic pyridine rings and strongly negative pic oxygens. The extent of solvent attraction to these surfaces can be simplistically characterized with their basicity and acidity parameters, expressed through a variety of empirical scales (Table D

DOI: 10.1021/acs.inorgchem.8b03273 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry S1).48−51 After such a categorization the outlier behavior of THF becomes clear owing to a unique combination of its low acidity and high basicity. This determines solvent’s preferential interaction with positive pyridine rings (in such a way blocking possible TR-to-ppy stacking) and repulsion from negative pic oxygens, consequently promoting the formation of TR-to-pic conformation. At the same time the relatively high polarity of THF increases the energetic gap between LUMO and LUMO +1 levels, localizing the excited states strictly on ppy ligands (Figure 3a). In other solvents, where TR-to-pic binding is contested by weakly polar aromatic rings (toluene), or, particularly, by polar, high-acidity solvents that attach to pic oxygens, the two lowest LUMO levels are expected to converge, lowering ΦPL. This is illustrated by the most severe ΦPL drop and PL band transformations in MeCN, the solvent with the most pronounced hydrogen bond−donating ability.49 Characteristics and Photophysical Properties of Solid-State Polymorphs. The ability of (trppy)2Ir(pic) to undergo intramolecular stacking results in a pronounced solidstate polymorphism. The previously proposed formation of a stable-stacked conformation of (trppy)2Ir(pic) in THF is further supported by the observed precipitation of a strongly emissive solid after a prolonged storage of the corresponding solution (Figure 2a). While almost unlimited amount of the amorphous starting material can be initially dissolved, it is assumed that gradually progressing TR-to-pic stacking in the solution reduces the conformational freedom and surface polarity of the molecule, lowering its solubility in this solvent. Differential scanning calorimetry (DSC) and powder X-ray diffraction data (Figures S6 and S7) reveal that this solid is a crystalline polymorph of the complex, together with its other crystalline forms, obtained by slow evaporation method from MeCN and 2-ethoxyethanol (EET). All the polymorphs assume identical amorphous structure after one melting− cooling cycle. The same amorphous form can be obtained by a fast evaporation of the solutions of the compound. All crystalline forms show almost overlapping emission spectra and unusually large ΦPL values, up to 0.36 in the case of EETgrown sample (Figure 5). At the same time, the amorphous form of the compound is relatively weakly emissive, and PL band shows a strong batochromic shift (Figure 5a). On the basis of the MeCN-grown crystal structure, TR-topic−stacked conformation is assumed as the origin of the reduced emission quenching in crystalline state. Sterical shielding of the strongly electronegative center in pic ligand and the buffer provided by the massive TR substituents determine that only one close contact between the cyclometalating ligands of the neighboring complex molecules occurs (3.568 Å, Figure 6). Coincidently, the corresponding molecular conformation completely shields the ppy ligand that hosts T1 and S1 excited states. In this way the occurring close contact affects the ppy ligand that is not involved in the emissive process (both ppy ligands are electronically distinct). As a result, the powder emission band of the most efficient EET-grown crystal almost completely overlaps with that of THF solution (Figure S8). The deviation in the left shoulder of the band is attributed to a strong self-absorption that occurs due to the overlap between emission and absorption regions, in conjunction with the trapping of the emitted light in the plateand rodlike microcrystallites (Figure S9). This lets us assume that the inherent emission efficiency in the acquired crystals is even higher than the measured values. In cases where no TRto-pic intramolecular stacking takes place, the solid-state

Figure 5. (a) Emission bands of (trppy)2Ir(pic) solid-state polymorphs. Photograph (right) shows emission of powdered crystalline and amorphous samples under UV (360 nm) irradiation. (b) ΦPL values of different solid-state forms and guest−host systems bearing (trppy)2Ir(pic).

Figure 6. Representative projections of (trppy)2Ir(pic) crystal lattice in MeCN-grown crystal. Red line shows the only close contact between cyclometalating ligands. Blue lines mark distance between Ir cores of the neighboring complexes. Solvent molecules are omitted, and TR groups are purple for clarity.

packing arrangement is most likely determined by strong electrostatic intermolecular interactions between the cyclometalating ligands with direct involvement of the unshielded pic fragment, as it now hosts the electrostatic extremes of the complex surface. Such molecular packing is assumed for the weakly emissive amorphous form of (trppy)2Ir(pic). Electroluminescence Characteristics. The potential use of (trppy)2Ir(pic) as the emitter in solution-processed OLEDs was assessed. Taking into account the ability of the compound to form intramolecularly stacked conformation under certain conditions, it was theorized that a combination of suitable E

DOI: 10.1021/acs.inorgchem.8b03273 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. (a) Structures of charge transporting layers, device composition, and energy diagrams (in eV) for prepared OLEDs with PVK and CBP as hole-transporting host materials (emitter content 20 wt %). (b) Voltage−current density (filled circle)−luminance (empty circle) plots, (c) current density (filled circle) and power efficiency (empty circle) vs luminance, (d) EQE vs luminance, and (e) EL spectra.

isolated and aggregated molecules (Figure S10). The results suggest that the intramolecular stacking process is hindered for (trppy)2Ir(pic) in the examined solution-processed guest−host systems, and a large population of the complex molecules assumes a solid-state packing arrangement that is comparable to the one previously discussed for the poorly emissive amorphous polymorph. Further optimization of the materials and processing methods is needed to find optimal conditions for the exploitation of intramolecular stacking process to achieve a complete spatial isolation of iridium complex molecules in the presence of a host material.

solvent and host material would allow acquisition of emissive layers with a greatly reduced fraction of aggregated complex molecules. Initially, guest−host-type mixtures were prepared to assess the emissive behavior of (trppy)2Ir(pic) with the addition of conventional hole-transporting materials: polymeric poly(9-vinylcarbazole) (PVK) and molecular 4,4′-bis(Ncarbazolyl)-1,1′-biphenyl (CBP) (Figure 5b). The emitter content in these samples is 20 wt % (wt %). Large fluctuations in ΦPL for the prepared spin-coated films were observed in regard to both the used host material and solvent. Generally, the best results were observed when CHCl3 was used as solvent and CBP was the host. The combination of these prerequisites resulted in the most efficient sample (ΦPL = 0.67). Still, this marks a substantial drop in ΦPL in comparison to THF solution, indicating a large population of aggregated complex molecules. The use of THF as the processing solvent resulted in a steep ΦPL drop with the use of CBP host, despite the fact that the most pronounced intramolecular stacking is assumed for (trppy)2Ir(pic) in this medium. We attribute the poor ΦPL of this sample to the relatively low solubility of CBP in THF that promotes phase aggregation between host and guest molecules. Two different OLEDs were prepared with spin-coated emissive layers using PVK and CBP as host materials and CHCl3 as the processing solvent (Figure 7, Table S2). As expected from PL measurements in films and better energy-level alignment between the layers, the measured performance is better for the OLED system, where CBP is used as host, with the best attained EQE of 6.1%. Additionally, the CBP-based OLED has lower turn-on voltage and lesser leaking current, which results in higher overall efficiency parameters. Emitter aggregation is assumed as the main efficiency-reducing factor for these OLEDs. This is supported by the observed dual-emission character in the devices with the increase in driving voltage, sequentially originating from the

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CONCLUSIONS In summary, on the basis of (trppy)2Ir(pic), we have presented the first structural example where intramolecular stacking between a nonionic heteroleptic iridium(III) complex core and a flexibly bridged bulky aromatic substituent takes place. The main application direction of this effect can be proposed toward the development of efficient solid-state emissive materials. A potential acquisition of phosphorescent crystals with limited emission quenching for OLED applications can be named as the most promising perspective, as, to the best of our knowledge, (trppy)2Ir(pic) exhibits the highest reported ΦPL value among nonionic iridium(III) complexes in their crystalline state. Additionally, minimal PL band transformations between solid and dissolved states are observed. Further investigations are being performed to modify the existing molecule and to apply the principle to other structurally similar iridium(III)-based emitters.

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EXPERIMENTAL SECTION

General. If unspecified further, the used materials, starting compounds, or solvents were purchased from commercial suppliers. Optical measurements were performed in spectrophotometric grade F

DOI: 10.1021/acs.inorgchem.8b03273 Inorg. Chem. XXXX, XXX, XXX−XXX

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ration of DCM solution). In the case of THF a precipitate formed after short storage and subsequently was filtered off and washed with THF. For MeCN and 2-ethoxyethanol, the solutions were slowly evaporated, and the resulting solid was filtered off and washed with a small amount of the corresponding solvent. Emission spectra, lifetimes, and ΦPL for solutions, thin films, and powders were measured using a QuantaMaster 40 steady-state spectrofluorometer (Photon Technology International, Inc.) equipped with 6 in. integrating sphere by LabSphere, using the software package provided by the manufacturer. Excitation wavelength of 410 nm was used for photoluminescence measurements, except for PVK and CBP guest− host systems, where films were excited at 325 nm. Spectral resolution was set to 1 nm. The molecular ionization energy in thin film (Im) and photoconductivity measurements (Eth) were performed on a self-made experimental system using a method described in our previous work.53 The values of electron affinity in thin film (AE) were calculated using the difference of experimentally determined values of Eth and Im. X-ray Analysis. Powder X-ray data were collected at room temperature using a Rigaku Ultima IV diffractometer in Bragg− Brentano reflectance geometry, employing Ni-filtered Cu Kα line focused radiation at 1125 W (45 kV, 25 mA) power. Radiation was focused using parallel focusing Gobel mirrors. The diffraction data were collected from 2θmin = 3° up to 2θmax = 40°. Sufficiently large crystals of (trppy)2Ir(pic) for single-crystal X-ray analysis were obtained by the slow evaporation method from MeCN solution. A small amount of the compound was dissolved in MeCN, slightly above the saturation point. The solution was filtered through 0.45 μm Teflon filter and left to slowly evaporate over 7 d in a glass container, covered with a plastic cap with a needle-drilled hole. The analyzed crystals were stored in mother liquor until the analysis procedure. Diffraction data were collected on a Bruker-Nonius KappaCCD diffractometer using graphite-monochromated Mo Kα radiation at −100 °C. The crystal structure of (trppy)2Ir(pic) was solved by direct methods and refined by full-matrix least-squares. All nonhydrogen atoms were refined in anisotropical approximation. H atoms were refined by riding model with Uiso(H) = 1.2Ueq(C). Crystal data for (trppy)2Ir(pic): monoclinic; a = 9.6757(2), b = 17.3524(4), c = 18.8367(4) Å, β = 100.971(1)°; V = 3104.8(1) Å3, Z = 2, μ = 2.208 mm−1, Dcalc = 1.404 g·cm−1; space group is P21 (No. 4). A total of 14 401 reflection intensities were collected up to 2θmax = 58°; for structure refinement 12 657 independent reflections and their related Friedel’s pairs with I > 2σ(I) were used. The final R-factor is 0.0435. Half of the crystal unit cells contain two solvent molecules, while the other half contained one, resulting in an overall complex/MeCN ratio of 1.5. Structural uncertainty persists for three carbon atoms of TR group, as the corresponding phenyl ring is located at the edge of one of the crystal voids, and the atomic motion is not suppressed by the crystal lattice. For further details, see crystallographic data for (trppy)2Ir(pic), deposited at the Cambridge Crystallographic Data Centre as Supplementary Publication No. CCDC 1845861. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK. OLED Preparation and Characterization. Sandwich-type samples with pixel size of 16 mm2 were prepared for the electroluminescence measurements with the following structure: ITO/PEDOT:PSS(40 nm)/emitting layer (60 nm)/TPBi(20 nm)/ LiF(1 nm)/Al(100 nm), where poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBi), and LiF were used as the hole-injection, the electron-transport, and the electron-injection layers, respectively. Indium tin oxide (ITO) glass (Präzisions Glas & Optik GmbH) with a sheet resistivity of 15 Ω/square was used as a substrate. A 12 mm wide ITO strip in the middle of the substrate was made by wet etching. Afterward, ITO substrates were cleaned by the following method: sonicated in CHCl3; sonicated in acetone; rinsed with deionized (DI) water twice; sonicated in water with 3 vol % of Hellmanex II detergent; rinsed with DI water; sonicated in DI water and then in isopropyl alcohol. Each sonication lasted for 15 min.

solvents. NMR spectra were obtained on a Bruker Avance 300 MHz spectrometer using tetramethylsilane (TMS) as an internal reference. The elemental analysis was performed using a Costech Instruments ECS 4010 CHNS-O Elemental Combustion System. Differential scanning calorimetry (DSC) thermograms were acquired using a Mettler Toledo DSC-1/200W apparatus at a scanning rate of 10 °C min−1 while keeping samples under N2 atmosphere. Thermogravimetric (TG) curves were obtained using a PerkinElmer STA 6000 thermal analyzer. Scanning electron microscopy (SEM) images were taken on a Phenom Pro Desktop SEM. Optical images were taken on a high-resolution optical microscope Nikon Eclipse L150 with a rotating polarizer. Synthesis of Iridium(III) Bis(2-(4-(formyl)phenyl)pyridinatoN,C2′)picolinate (2). Chloro-bridged dimer complex (1) (1.00 g, 0.81 mmol),52 picolinic acid (0.22 g, 1.78 mmol), and K2CO3 (0.25 g, 1.78 mmol) were heated at 100 °C in 2-ethoxyetanol (5 mL) for 2 h. The reaction mixture was poured into water; the precipitate was filtered off and washed with ethanol. The product was purified by column chromatography using DCM/acetone (10:1, v/v) as the eluent to afford complex 2 (0.98 g, 85%) as orange powder. 1H NMR (300 MHz, CDCl3, δ, ppm): 9.73 (s, 1H), 9.66 (s, 1H), 8.91−8.84 (m, 1H), 8.37 (d, J = 7.8 Hz, 1H), 8.03 (t, J = 8.1 Hz, 2H), 7.95 (td, J = 7.7 Hz, J = 1.5 Hz, 1H), 7.90−7.82 (m, 2H), 7.80 (d, J = 8.0 Hz, 1H), 7.78−7.73 (m, 2H), 7.59−7.54 (m, 1H), 7.47 (dd, J = 8.0 Hz, J = 1.5 Hz, 1H), 7.43−7.36 (m, 2H), 7.35−7.29 (m, 1H), 7.14−7.06 (m, 1H), 6.88 (d, J = 1.5 Hz, 1H), 6.64 (d, J = 1.5 Hz, 1H). Synthesis of Iridium(III) Bis(2-(4-(((3,3,3triphenylpropionyl)oxo)methyl)phenyl)piridinato-N,C 2 ′)picolinate [(trppy)2Ir(pic)]. Complex 2 (0.98 g, 1.39 mmol) was suspended in methanol (10 mL) under cooling on ice bath. NaBH4 (0.19 g, 4.87 mmol) was added, and the mixture was stirred for 1 h. After the addition of water (10 mL) THF was removed under reduced pressure. The resulting yellow precipitate was collected by filtration, washed with water, and dried (0.77 g, 78%). Without additional purification, the reduction product (0.77 g, 1.09 mmol) was dissolved in DCM (10 mL), followed by addition of 3,3,3triphenylpropionic acid (0.76 g, 2.50 mmol), 4-dimethylaminopyridine (DMAP; 0.01 g, 0.08 mmol), and N,N′-dicyclohexylcarbodiimide (DCC; 0.57 g, 2.50 mmol) sequentially. The mixture was stirred for 24 h and then filtered. The filtrate was evaporated under reduced pressure, and the acquired solid was purified by column chromatography using DCM/ethyl acetate (3:1, v/v) as the eluent to afford (trppy)2Ir(pic) (0.65 g, 47%) as yellow glass. 1H NMR (300 MHz, CDCl3, δ, ppm): 8.75 (d, J = 5.7 Hz, 1H), 8.34 (d, J = 7.7 Hz,, 1H), 7.90 (td, J = 7.7 Hz, J = 1.4 Hz, 1H), 7.83 (d, J = 8.0 Hz, 1H), 7.77 (d, J = 8.0 Hz, 1H), 7.72 (d, J = 4.7 Hz, 1H), 7.64−7.55 (m, 2H), 7.55−7.48 (m, 2H), 7.46 (d, J = 5.6 Hz, 1H), 7.36−7.29 (m, 1H), 7.29−7.12 (m, 30H), 7.06 (t, J = 6.7 Hz, 1H), 6.87 (t, J = 6.1 Hz, 1H), 6.65 (d, J = 8.0 Hz, 1H), 6.60 (d, J = 8.0 Hz, 1H), 6.25 (s, 1H), 6.04 (s, 1H), 4.63 (s, 2H), 4.54 (s, 2H), 3.64 (s, 4H). 13C NMR (75 MHz, CDCl3, δ, ppm) 170.66, 167.24, 149.01, 148.95, 148.38, 148.08, 147.39, 146.59, 146.51, 143.98, 143.76, 137.67, 137.32, 137.30, 137.18, 136.54, 131.50, 131.20, 129.19, 128.29, 128.01, 127.94, 127.80, 126.27, 126.21, 124.28, 124.01, 122.27, 122.10, 121.31, 120.51, 119.20, 118.52, 66.18, 65.95, 46.58, 46.49. Anal. Calcd for C72H56IrN3O6: C, 69.10, H, 4.51, N, 3.36. Found: C, 69.31, H, 4.82, N, 3.46%. Photophysical Measurements. Optical measurements of solutions were performed at typical material concentration of 1 × 10−5 mol L−1. The UV−vis absorption spectra were recorded with a PerkinElmer Lambda 35 spectrometer. Solutions for ΦPL measurements were prepared in a glovebox under Ar atmosphere, using previously degassed solvents, and filled in sealed cuvettes for immediate use. Films for optical measurements were prepared using spin-coating technique with a Laurell WS-400B-6NPP/LITE spincoater on glass slides, using solutions with material concentration of 30 mg/mL. After the preparation all films were dried in an oven at 100 °C for 2 h. Powdered solid-phase polymorphs were obtained by initially preparing the corresponding solutions (100 mg/mL) of the amorphous polymorph of (trppy)2Ir(pic) (acquired by fast evapoG

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a poly(ethylene glycol) pendant or bioorthogonal reaction group as biological probes and photocytotoxic agents. Coord. Chem. Rev. 2018, 361, 138−163. (2) Lo, K. K. W. Luminescent Rhenium(I) and Iridium(III) Polypyridine Complexes as Biological Probes, Imaging Reagents, and Photocytotoxic Agents. Acc. Chem. Res. 2015, 48, 2985−2995. (3) Li, T. Y.; Wu, J.; Wu, Z. G.; Zheng, Y. X.; Zuo, J. L.; Pan, Y. Rational design of phosphorescent iridium(III) complexes for emission color tunability and their applications in OLEDs. Coord. Chem. Rev. 2018, 374, 55−92. (4) Tao, P.; Miao, Y.; Wang, H.; Xu, B.; Zhao, Q. High-Performance Organic Electroluminescence: Design from Organic Light-Emitting Materials to Devices. Chem. Rec. 2018, DOI: 10.1002/tcr.201800139. (5) Tao, P.; Li, W. L.; Zhang, J.; Guo, S.; Zhao, Q.; Wang, H.; Wei, B.; Liu, S. J.; Zhou, X. H.; Yu, Q.; Xu, B. S.; et al. Facile Synthesis of Highly Efficient Lepidine-Based Phosphorescent Iridium (III) Complexes for Yellow and White Organic Light-Emitting Diodes. Adv. Funct. Mater. 2016, 26, 881−894. (6) Choy, W. C.; Chan, W. K.; Yuan, Y. Recent Advances in Transition Metal Complexes and Light-Management Engineering in Organic Optoelectronic Devices. Adv. Mater. 2014, 26, 5368−5399. (7) Xu, H.; Chen, R.; Sun, Q.; Lai, W.; Su, Q.; Huang, W.; Liu, X. Recent progress in metal−organic complexes for optoelectronic applications. Chem. Soc. Rev. 2014, 43, 3259−3302. (8) Park, Y. S.; Lee, S.; Kim, K. H.; Kim, S. Y.; Lee, J. H.; Kim, J. J. Exciplex-Forming Co-host for Organic Light-Emitting Diodes with Ultimate Efficiency. Adv. Funct. Mater. 2013, 23, 4914−4920. (9) Zhang, M.; Höfle, S.; Czolk, J.; Mertens, A.; Colsmann, A. Allsolution processed transparent organic light emitting diodes. Nanoscale 2015, 7, 20009−20014. (10) Yook, K. S.; Lee, J. Y. Small molecule host materials for solution processed phosphorescent organic light-emitting diodes. Adv. Mater. 2014, 26, 4218−4233. (11) Xu, X.; Yang, X.; Zhao, J.; Zhou, G.; Wong, W. Y. Recent Advances in Solution-Processable Dendrimers for Highly Efficient Phosphorescent Organic Light-Emitting Diodes (PHOLEDs). Asian J. Org. Chem. 2015, 4, 394−429. (12) Schmidt, T. D.; Lampe, T.; Djurovich, P. I.; Thompson, M. E.; Brütting, W.; et al. Emitter Orientation as a Key Parameter in Organic Light-Emitting Diodes. Phys. Rev. Appl. 2017, 8, No. 037001. (13) Moon, C. K.; Kim, K. H.; Kim, J. J. Unraveling the orientation of phosphors doped in organic semiconducting layers. Nat. Commun. 2017, 8, 791. (14) Kim, K. H.; Lee, S.; Moon, C. K.; Kim, S. Y.; Park, Y. S.; Lee, J. H.; Huh, J.; You, Y.; Kim, J. J.; et al. Phosphorescent dye-based supramolecules for high-efficiency organic light-emitting diodes. Nat. Commun. 2014, 5, 4769. (15) Kim, S. Y.; Jeong, W. I.; Mayr, C.; Park, Y. S.; Kim, K. H.; Lee, J. H.; Moon, C. K.; Brütting, W.; Kim, J. J. Organic Light-Emitting diodes with 30% external quantum efficiency based on a horizontally oriented emitter. Adv. Funct. Mater. 2013, 23, 3896−3900. (16) Kim, K. H.; Ahn, E. S.; Huh, J. S.; Kim, Y. H.; Kim, J. J. Design of Heteroleptic Ir Complexes with Horizontal Emitting Dipoles for Highly Efficient Organic Light-Emitting Diodes with an External Quantum Efficiency of 38%. Chem. Mater. 2016, 28, 7505−7510. (17) Kim, K. H.; Liao, J. L.; Lee, S. W.; Sim, B.; Moon, C. K.; Lee, G. H.; Kim, H. J.; Chi, Y.; Kim, J. J. Crystal Organic Light-Emitting Diodes with Perfectly Oriented Non-Doped Pt-Based Emitting Layer. Adv. Mater. 2016, 28, 2526−2532. (18) Komiya, N.; Okada, M.; Fukumoto, K.; Jomori, D.; Naota, T. Highly Phosphorescent Crystals of Vaulted trans-Bis(salicylaldiminato)platinum(II) Complexes. J. Am. Chem. Soc. 2011, 133, 6493−6496. (19) Mauro, M.; Cebrián, C. Aggregation-induced Phosphorescence Enhancement in IrIII Complexes. Isr. J. Chem. 2018, 58, 901−914. (20) Kawamura, Y.; Brooks, J.; Brown, J. J.; Sasabe, H.; Adachi, C. Intermolecular interaction and a concentration-quenching mechanism of phosphorescent Ir (III) complexes in a solid film. Phys. Rev. Lett. 2006, 96, No. 017404.

Before deposition of PEDOT:PSS the ITO glass was blown dry with nitrogen and treated by UV-ozone for 20 min. ITO layer was covered with PEDOT:PSS (from H.C. Starck, Al4083) using spin coater Laurell WS650. Rotation lasted for 1 min with speed 2000 rpm. The thickness of the layer was 40 nm. The sample was moved into a glovebox and heated at 200 °C for 10 min. The emitting layer consisted from the investigated compound (trppy)2Ir(pic) in poly(9vinylcarbazole) (PVK, Sigma-Aldrich 368350) or 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP). In all cases 20 wt % host/guest system was prepared in the corresponding solvent (CHCl3 for PVK; CHCl3 or THF for CBP). Solution with concentration of 5 mg/mL was spincoated on the PEDOT:PSS layer with 2000 rpm for 40 s and heated at 120 °C for 15 min afterward. Further, the sample was moved from the glovebox to a vacuum chamber for thermal evaporation of electron transport layer (TPBi, Sigma-Aldrich 806781), electron injection layer (LiF, Sigma-Aldrich 449903), and the electrode (Al) at the pressure 6 × 10−6 Torr. The deposition speed was 1, 0.1, and 5 Å s−1 for TPBi, LiF and Al, respectively. Finally, the samples were encapsulated using two-component epoxide. The current−voltage characteristics of the OLEDs were measured by the multimeter Keithley 2700, with the voltage source being Keithley 230 unit. The electroluminescence brightness and CIE 1931 coordinates were measured by Konica Minolta Luminance and Color Meter CS-150. The electroluminescence (EL) spectrum was recorded by the calibrated Ocean Optics HR4000 spectrometer. CIE 1931 coordinates and spectrum presented in this work was obtained at the highest achieved brightness.

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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/acs.inorgchem.8b03273. Additional tables and figures, NMR spectra of (trppy)2Ir(pic) thermogravimetric and DSC analysis for solid state polymorphs, computational methodology (PDF) Accession Codes

CCDC 1845861 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kaspars Traskovskis: 0000-0003-1416-7533 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the ERDF 1.1.1.1. activity Project No. 1.1.1.1/16/A/131. REFERENCES

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