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Spectroscopy and Photochemistry; General Theory

Probing Triplet Excited States and Managing Blue Light Emission of Neutral Tetradentate Platinum (II) Complexes Cong You, Fang Xia, Yue Zhao, Yin Zhang, Yongjian Sheng, Yipei Wu, XiaoChun Hang, Fei Chen, Huili Ma, Kang Shen, Zhengyi Sun, Takahiro Ueba, Satoshi Kera, Cong Zhang, Hong-Hai Zhang, Zhi-Kuan Chen, and Wei Huang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00797 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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The Journal of Physical Chemistry Letters

Probing Triplet Excited States and Managing Blue Light Emission of Neutral Tetradentate Platinum (II) Complexes ‡

‡

Cong You,† Fang Xia,† Yue Zhao,† Yin Zhang,† Yongjian Sheng,† Yipei Wu,† Xiao-Chun Hang,*,† Fei Chen,† Huili Ma,† ¶

¶

Kang Shen,† Zhengyi Sun,† Takahiro Ueba, Satoshi Kera, Cong Zhang,† Honghai Zhang,† Zhi-Kuan Chen*,† and Wei Huang*,†§ †

Key Laboratory of Flexible Electronics (KLOFE) and Institute of Advanced Materials (IAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211800, China

¶

Department of Photo-Molecular Science, Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan

§

Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi’an 710072, China

KEYWORDS (Triplet excited states, blue emission, tetradentated Platinum(II) complexes, delocalized triplets, delocalized spin transition ) ABSTRACT: The structural and photophysical properties of tetradentate Pt(ppzOppz), Pt(ppzOpopy), Pt(ppzOczpy) and Pt(czpyOczpy) have been experimentally and theoretically explored. Single crystal diffraction measurements provided accurate structural information. Electrochemical and photophysical characterizations revealed internal electronic energy levels in ground and excited states. (Time dependent) density functional theory calculation uncovered electron distributions in transition processes of S0→S1 and S1→T1→S0. Electronic transition study indicated that Pt(ppzOppz) demonstrated mixed MLCT/LC states and Pt(czpyOczpy) showed MLCT dominated states in S1 and T1. Both Pt(ppzOpopy) and Pt(ppzOczpy) presented strong delocalized spin transition (DST) during intersystem crossing. Upon frame modification of Pt(ppzOczpy), we found their S1 and T1 can be independently manipulated. These blue emitters showed tunable and narrow emission band (the narrowest FWHM is 19 nm) with luminescence efficiency as high as 86%. The findings of DST transition mode in the neutral Pt(II) complexes provide a guidance for rational design of new phosphorescent materials. Light-emitting materials have shown a wide range of potential applications in the fields of photocatalysts,1,2 optoelectronic devices3,4 and chemosensors.5,6 For particular interest in appliance,7 organic light-emitting diodes (OLEDs)8,9 upon phosphorescence10,11 are possible to internally convert all the injected charges into photons.12 To develop 100% quantum efficiency materials,13 transitional metal complexes tabbed with the strong spin orbital coupling (SOC) effects allow unrelated spin transitions decaying to ground state.14 Among all the transitional metals, cyclometallated iridium and platinum complexes15 have garnered particular attention in the past two decades,16,17 and research on inherent relations between their structure and electronic properties are essential perspective for providing guidance to achieve high quality phosphorescent materials.

ionic tridentate and dianionic tridentate ligands have been explored in the past few years.18 However, chelating ligands over trihapto cyclometallating didn’t match with octahedral Ir(III) structure. On the other hand, platinum(II) complexes with planar structure assorts well with various frames of ligands, including bidentate, tridentate and tetradentate ligands.5,19 Indeed, tetradentate ligands fit well with their planar Pt(II) coordinating structure and afforded corresponding complexes in high synthetic yield and good device performance.17,20 Recently, novel planar Pt(II) complexes consisting of tetradentate ligands have been developed as phosphorescent dopants and demonstrated prominent luminescent properties.2125 These phosphorescent emitters can cover whole red to deep blue emission region,22 and demonstrate high radiative decay rate23,24 and good color purity with narrow emission peaks.25

Neutral octahedral iridium(III) metal complexes comprising of bidentate monoanionic cyclometallate ligands have drawn much attention for efficiently harnessing both singlet and triplet excitons in solid state device.7 These complexes are catalogued in two groups: homoleptic Ir(C^N)3 and heteroleptic Ir(C^N)2(LX), which have been practically applied in fullcolor display. Due to the bidentate chelating model, there is limited freedom of structural design for Ir(III) complexes. Charge-neutral Ir(III) complexes composed of mixed monoan-

Spectroscopic properties of OLED emitters are greatly important for device performance.26,27 High efficiency devices can be achieved by using purely organic emitters, which have thermally activated delay fluorescence (TADF) effects with small singlet-triplet gap (⊿Est).28,29 In regard of phosphorescent emitters, typically 5d orbitals Ir and Pt complexes with large SOC parameter,30 small ⊿Est is beneficial to reduce the energy difference between excited singlet and triplet states, enhance the singlet-triplet intersystem crossing rate and in-

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crease the luminescent efficiency.31 However, very few works have been devoted to manage the ⊿Est for transitional metal complexes due to lack of available frames.32-35 Currently, a number of planar neutral Pt(II) complexes have been developed for efficient and stable OLED devices.36 However, study of the correlation between the structure and photophysical properties of the Pt(II) complexes, especially the excited state properties, is missing.37-39 In this study, we selected four nonsubstituted typical frame structures of tetradentate Pt(II) complexes, to systematically investigate their photophysical properties. Within this work, density functional theory (DFT) and time dependent DFT (TD-DFT) simulations were utilized to understand the intramolecular charge transition as well as their bonding properties. We found good agreements between the simulations and our experimental findings. Basically, three types of electronic transitions dominated in the four complexes during emission and radiation. They were ligand-centered with metal-to-ligand charge transfer 1,3(LC/MLCT) transitions, 1 MLCT/1→3DST (delocalized spin transition) /3MTT (mixed triplet transition) and 1,3MLCT. (Superscripts indicate the spin state) We also demonstrated peculiarity of modification on Pt(ppzOczpy) skeletons with DST model, and achieved appropriate triplet energy gap, color purity and high photon luminescent quantum efficiency (PLQE) emitter in blue emission spectra region.

Scheme 1. Skeletons of Pt(II) complexes Scheme 1 shows the cyclometalated Pt(II) complexes with different ligand segments: pyrazolebenzenide, pyridinyloxybenzenide and pyridinylcarbazolide. In order to reveal the correlation of photophysical properties to the molecular scaffolds explicitly, the four complexes are constructed without any extra substituents. We name the four pro-ligand moieties in short, which are N-phenylpyrazole (ppz), phenoxypyridine (popy), N-pyridinylcarbazole (czpy). The capital letter O stands for the tendon oxygen atom. The four complexes are abbreviated as Pt(ppzOppz), Pt(ppzOpopy), Pt(ppzOczpy) and Pt(czpyOczpy). According to the metallocycle arrangement, there exist three types of consecutive intramolecular metallocycles in the complexes, as shown in Scheme 1, which are 56-5, 5-6-6 and 6-6-6. The planar molecular structure of Pt(II) complexes deform through coordinating with various ligands. To visualize authentic internal valence bonds of the compounds, none solvent mingled crystals were prepared according to the conventional methods. The crystal structures were determined by Xray crystallography diffraction measurement. Table 1 summa-

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rized the key information for their single-molecule ORTEP structures. Both Pt(ppzOpopy) and Pt(czpyOczpy) have a set of four deformed geometries in a crystal. Geometries listed in Table 1 are the most stable structural conformer for each structure (Table S2, S3). Lone pair electron on azacycle coordinated Pt-N bond lengths are within 2.05-2.12 Å, and phenide PtC bond are within the length of 1.95-2.07 Å. Compared with other similar compounds,19 the bond lengths bonded to platinum ion are in normal range from 1.92 Å to 2.12 Å. N1-Pt-N2 angles of the compounds decrease from obtuse angle to acute angle and are 107.09o, 99.41o, 98.88o and 89.89o corresponding to Pt(ppzOppz), Pt(ppzOpopy), Pt(ppzOczpy) and Pt(czpyOczpy) in sequence, which indicates that the intramolecular force trends from expansion to extrusion, since six membered fused metallocycles occupy more angular space of Pt centroid than five membered Pt-ppz cycle. X-ray diffraction analysis reveal that all the four complexes present big torsion angles (Table 1). At the edge of concave skeletons of the complexes, terminal N-heterocyclic rings marked with A and B are staggered to each other. The dihedral angles between face A and B of the four complexes are 4.581o, 39.749o, 38.415o and 67.345o, respectively, which intuitively reflects the extent of intramolecular distortion. The deviated N-heterocyclic rings (A and B) imply a platinum centroid helix chirality in each molecule. X-ray analysis on Pt(ppzOczpy) crystals affirmed platinum centroid helix chirality. It is found that the molecules piled up to form pairs of helical columns with right-hand and left-hand alternately (Fig. S1, right). The structure of Pt(ppzOppz) is close to coplanar, which piles up head to tail through weak ligand-to-ligand π…π interaction with a separation at 3.369 Å (Fig. S1, left). The closest Pt…Pt distance is 3.598 Å in Pt(ppzOppz), which is beyond the distance for metal…metal interaction in normal Pt(II) molecules.40,41 Pt…H interaction is also observed with a distance of 2.869 Å. Pt(ppzOppz) presents a single molecule emission peak at 437 nm and an excimer emission peak at 570 nm in dichloromethane (DCM) solution. The excimer emission intensity is enhanced with increasing the solution concentration (Fig. S3). Compared with its 5-6-5 homologues with other azacycle moiety at the same concentration,42 Pt(ppzOppz) shows larger excimer energy gap, and emits in yellow wavelength range, which is just fit as the complementary color of its own single molecule blue emission. Pt(ppzOpopy), Pt(ppzOczpy) and Pt(czpyOczpy) have much distorted structural frame, reflected by their dihedral angles between ring A and B, thus no excimer emission is observed. Ring B and C in Pt(II) compound with similar structure have been reported as the location centers for the lowest unoccupied molecule orbital (LUMO) and the highest occupied molecule orbital (HOMO) in their analogues,23 which is in accordance with our theoretical calculation results. The values of dihedral angles for ψBC are 1.327o, 18.451o, 28.866o and 32.113o for Pt(ppzOppz), Pt(ppzOpopy), Pt(ppzOczpy) and Pt(czpyOczpy) respectively, revealing that their intramolecular twisting is increased in sequence.43,44 UV-visible absorption, phosphorescent emission and excitation spectra were measured for the four complexes to investigate their photophysical properties. As shown in Fig. 1, the complexes exhibit strong absorption bands below 300 nm assigned to π-π* transitions corresponding to the LC states. The intense absorption bands in 300-420 nm region are assigned to d-π* transitions corresponding to MLCT states. Pt(ppzOczpy) and Pt(czpyOczpy) showed relatively larger


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The Journal of Physical Chemistry Letters extinction coefficient than Pt(ppzOppz) and Pt(ppzOpopy) between 300 nm to 360 nm, which are contributed by 1π-π* transitions of carbazole groups. Pt(ppzOczpy) and Pt(czpyOczpy) also demonstrate strong MLCT transitions at longer wavelengths (between 360-400 nm) mainly attributed to Pt-czpy chelating metallocycles, which is also featured on their largely distorted py plane in structure. The emission spectra of the complexes in solution at room-temperature and low-temperature (77 K), in polymethylmethacrylate (PMMA) matrix (5 wt %) and crystallite powder were also recorded in Fig. 1. Their photophysical characterization results are summarized in Table 1. All the four complexes emit in the blue wavelength region, indicating their high triplet energy. The quantum efficiencies of the complexes are in the range of 39%-72%. The luminescent lifetime of the complexes was measured in the range of 2.2-10.0 µs in DCM solutions at room temperature, and prolonged to 4.8-14.2 µs in PMMA matrices. The radiative decay rates (Kr) are relatively higher than the reported Pt(II) analogues with planar molecular structure.14,19 Excitation spectra are collected in response to their second vibrational satellite peaks (ν0-1) and measured at 77 K to minimize the influence from excitation light and thermalized reversible intersystem crossing (RISC) transitions. The excitation spectra of the complexes extended into emission spectra wavelength region measured under 77 K, indicating strong SOC effects via center platinum which allows spin-forbidden S0→T1 transitions. Emission spectra at room temperature are different from emission at 77 K, since the vibrational satellites would mix together, thus extend into longer wavelength region under thermalized effects, such as molecule fluctuation, bond stretch and rotations. Pt(ppzOppz), Pt(ppzOpopy) and Pt(ppzOczpy) show well resolved vibrational satellite peaks in their emission spectra at room temperature, and the parameters of Huang-Rahys factor (S) can be approximately worked out through the function (ν is the quantum number of sub-electronic emission peak, I is the peak intensity). The parameter S represents the geometric change in the T1-S0 electron transition process. The S values for Pt(ppzOppz) and Pt(ppzOpopy) obtained from their solutions and solid matrices at RT and 77 K, were found to be nearly the same. For the compound Pt(ppzOczpy), S factor varied within 0.3-0.9 under various conditions as seen from the large variation of the emission intensity of the vibrational peaks (Fig. 1). This phenomenon indicates that perturbations on T1 state of Pt(ppzOczpy) may be manipulated through structural modification or varied external conditions. In the case of Pt(czpyOczpy), the emission peak shifts from 443 nm to 512 nm in DCM when the temperature change from 77 K to RT. The satellite peaks are smeared out and result in a broad emission spectrum extending into green and red wavelength region. The emission peak wavelength shifts significantly in different solvents (Fig. S4), which indicates 3MLCT dominates the Pt(czpyOczpy) emission. The dependence of emission peak wavelength on the media is strongly correlated to the twist geometry of ground states, which is supported by the big dihedral angle of ψBC (32o) measured in the crystal structure. Nevertheless, though Pt(ppzOczpy) in crystal also has big twist dihedral angle (ψBC = 29o), it shows well resolved and relatively steady emission spectra as seen from Fig. 1. The anomalous but interesting photophysical behav-

iour inspires us to investigate the triplet transition dynamics in these complexes. The energy levels of transition states of the complexes are experimentally estimated in various means to understand their structural relations (Table 3). HOMOs/LUMOs are estimated from the onset values of the oxidation and reduction processes via cyclic voltammeter (CV) test in DMF. The HOMO orbitals of all the four complexes are comprised of platinum metal, the oxygen bridging and phenyl ring or carbazole moiety profiled as aryloxoplatinum circle (Fig. S8). The heteroatoms O and N in the popy and cz make less contribution to the HOMO. The HOMOs of Pt(ppzOppz) and Pt(ppzOpopy) possess the same orbital components and the two HOMO energy levels are close. One phenyl ring and one carbazole moiety in Pt(ppzOczpy) contribute to its HOMO orbital, while there are two carbazoles in Pt(czpyOczpy) correlated with HOMO. Compared to Pt(ppzOpopy), the HOMO energy levels increased by 0.13 eV for Pt(ppzOczpy) and) 0.30 eV for Pt(czpyOczpy). In our experiments, HOMO energy levels were also deduced from ionic potential (IP) measurement, which was determined via ultraviolet photoelectron spectroscopy (UPS) in solid state samples. IP values obtained from UPS measurements are in conformity to those estimated from CV with a maximum 0.18 eV deviation for Pt(czpyOczpy). The measurement deviation between CV and UPS is reasonable because condensed solid state may have the intermolecular, surface and deformation affections.45 Nevertheless, Pt(ppzOczpy) shows the same HOMO energy value of 5.29 eV estimated from oxidation and ionization potential energies, due to its highly twisted but rigid molecular scaffold, which determines unimolecular photophysical property. Pt(ppzOppz), whose LUMO orbital is located on ppz (Fig. S8), has higher LUMO energy level than the others. The LUMO of Pt(ppzOpopy), Pt(ppzOczpy) and Pt(czpyOczpy) are localized mainly on pyridine ring. We found the stabilized trend of LUMO is consistent with the extent of twisting between pyridine ring and metal centered plane C as well as their charge transfer (CT) effects. The lowest singlet excited states (ES1) are estimated from excitation and absorption spectra in solution, as the singlet emission is hardly detectable for the complexes with rapid ISC transition in 10-14 to 10-13 second. In general, the energy gap (Eg) between HOMO and LUMO is approximately equal to their lowest singlet excitation energy ES1. In the complexes Pt(ppzOpopy), Pt(ppzOczpy) and Pt(czpyOczpy), ES1 is very close to Eg, implying a high ratio of minimum energy of electron transition between the HOMO and LUMO, which is also in agreement with our simulated results (Table S5). Pt(ppzOppz) shows lower ES1 energy comparing to Eg, which is mainly due to its intermolecular interaction. Notably, the energy gap between the lowest singlet S1 and triplet states T1 (∆EST) are in the range of 0.12-0.28 eV. Small ∆EST means a large triplet radiative decay rate (Kr) as )2 is approximately proportional to Kr.46 The small ∆EST here manifested those tetradentate Pt(Ⅱ) complexes are good candidates as efficient phosphorescent luminescent materials. We investigated hole and electron distributions of S1, T1 and ground state S0 by DFT and TD-DFT calculations, which are depicted in Fig. 2. Due to strong SOC, S1 relaxes into T1 rapidly with electron spin flip through intersystem crossing. Electron distributions of S1 locate on ppz for Pt(ppzOppz) and



Py for Pt(ppzOpopy), Pt(ppzOczpy) and Pt(czpyOczpy). Therefore, Pt(ppzOppz) shows a classic 1(LC/MLCT) excited transition admixed with πph→πppz*(1LC) and dxy→πppz* (1MLCT). The others present almost “pure” 1MLCT (dxy→πpy*) character of S0→S1 transition, agreeing well with their excitation characters and calculated transition orbitals coefficients (Table S5). T1 conformations tabbed with their electron distribution probability on ppz and cz fragments are highlighted with red/blue circles in Fig. 2. Pt(ppzOppz) has 3 (LC/MLCT) admixture of πppz*→πph (3LC) and πppz*→dxy (3MLCT) triplets radiative transitions (> 94%), and Pt(czpyOczpy) has large 3MLCT character of πpy*→dxy triplets radiative transition (> 96%). The electron distributions on excited states S1 and T1 of Pt(ppzOppz) and Pt(czpyOczpy) molecule are almost fixed to the region during the ISC. With regards to Pt(ppzOpopy) and Pt(ppzOczpy), electron density of T1 are delocalized to the other parts of the ligands in the molecule. 44.33% of electron distribution of T1 is on ppz for Pt(ppzOpopy) and 8.95% on ppz and 27.28% on cz fragment for Pt(ppzOczpy). Therefore, a delocalized spin transition (DST) mechanism through S1→T1 from the localized singlets and mixed triplets transition can be inferred. This gives us a new perspective to view the luminescent properties, especially in phosphorescent OLED device, since 75% energy is originated from host-to-guest triplet-triplet Dexter energy transfer process. Delocalization could occur with electron spanning through space resulted from spatial separated ppz and py. It is worth noting that the closest two atoms between py and ppz π frames are the two N atoms with a distance of 3.3 Å-3.5 Å away from each other according to single crystal diffraction data. As the electron distributions span from π* (py) to other adjacent orbitals in triplet excited states, 3LC components increase, which is in favor of giving environmentally stable intrinsic emission in comparison to CT state dominated emission. Thus, Pt(ppzOpopy) and Pt(ppzOczpy) could give the well resolved emission spectra at room temperature, whereas Pt(czpyOczpy) could not. Comparing with Pt(ppzOpopy), Pt(ppzOczpy) is more representative of DST model for its triplets electron distribution across the whole molecule region, therefore, its bond variation between S0 and T1 states is further investigated. Natural bond orbital (NBO) analysis of S0 state in Pt(ppzOczpy) indicates that the lone pair of N atom in pyrazole ring donates into the opposite σ* (Pt-C) orbital and associate with a large secondary energy ∆Ei→j*(2) of 64.85 kcal/mol. Meanwhile donation from the lone pair of N atom in pyridine ring to anti-bond of opposite Pt-C with secondary interaction is 68.41 kcal/mol. For the excited T1, the secondary interaction is greatly reduced. The secondary interaction between pyrazole N and Pt-C anti-bond is 32.80 kcal/mol and the interaction between pyridine N and Pt-C anti-bond is 37.22 kcal/mol. These results are coincident with the delocalized triplet distribution excited states model, whereas excited triplet has delocalized to both ppz and czpy fragments. Compared with S0, the positive natural population analysis (NPA) charge for T1 of pyridine decreases from 0.21 to 0.13, and increases on the other parts of the complex (Fig. S9). The NBO and NPA results give insights to understand the delocalized spin transitions, where the platinum coordinating bonds change instantaneously after Franck-Cotton excitation in the way of 1 MLCT transition. As a result, electron distribution of T1 disperse into the entire molecule space when electronic and vi-

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bronic energy levels of local parts match up. As the delocalized triplets still depend on the characters of local components, phosphorescent emission could be finely manipulated upon structure modification in certain region of the molecule. To deeply and systematically understand the effect of structural modification on the photophysical properties of the tetradentate Pt(II) complexes, we chose Pt(ppzOczpy) as the scanfold and synthesized four analogues to Pt(ppzOczpy), which are PtON1, Pt(ppzOczpy-m), Pt(ppzOczpy-mesi) and PtON2. PtON1 has been reported as good deep blue phosphorescent emitter with high photoluminescent quantum efficiency (89% in PMMA).22 Compared to Pt(ppzOczpy), it has two additional methyl groups substituted on pyrazole moiety and its main energy levels are almost the same as those for Pt(ppzOczpy) (Table 4). PtON1 showed better luminescent efficiency due to high degree of mixed vibrates from local rotations of two methyl groups. Compared to Pt(ppzOczpy), Pt(ppzOczpy-m) has one methyl group on the pyridinyl ring on the para-position to the N atom. Pt(ppzOczpy-m) also showed high luminescent efficiency and slightly drop of radiative decay rate. Interestingly, its vibronic satellite peaks are significantly depressed, thus very narrow emission with 19 nm of FWHM is achieved from Pt(ppzOczpy-m) in PMMA at room temperature. It can be ascribed to the increased molecular rigidity, which restricted the structural reorganization between T1 and S0 states. We found its electron delocalized extensively on T1 state, and only 34.19% distributed on the pyridine site (Fig. S10). This may be attributed either to destabilized LUMO orbitals (-2.15 eV) or indirect influence of reduced bond stretching (C-C bond vs C-H bond). PtON2 shows 13.90% distribution of T1 at phenylmethylimidazole (pmi) segment, and shifts the emission peak to 470 nm. Pt(ppzOczpy-mesi) presents triplet transition of lower energy with the emission peak at 455 nm, illustrating 3LC component on cz group also has great impact on its triplet energy level. In comparison with Pt(ppzOczpy), PtON2 presents the same LUMO energy level while Pt(ppzOczpy-m) shows the same triplet energy level. The above results confirmed that T1 or S1 of Pt(ppzOczpy) can be independently manipulated through structural modification at certain positions on the molecular skeleton to achieve enhanced luminescent efficiency and controllable color purity. Very recent, Son et. al. reported their latest findings of delocalized triplet features in the 5-6-6 metallocyclic Pt-complexes.47 Their work also confirmed phosphorescent emission from delocalized triplets is a rather general behavior in these well-designed tetradentate Pt(Ⅱ) complexes. In conclusion, we have studied the triplet decay process and spectra performance of tetradentate Pt(Ⅱ) complexes through combination of both experiments and computational simulations. Firstly, we investigated the photo-physical properties with structural changes of four fundamental Pt(Ⅱ) complex units. The 5-6-5 metallocycle scaffold was approximately in plane. 5-6-6 and 6-6-6 frames had larger distortions leading their terminal py ring twisted away from the C2-Pt-C1 plane (ring C in Table 1). 5-6-6 frame structure is an appropriate scaffold for high performance phosphorescent materials via controlling intermolecular interactions.22 (TD)DFT simulation and NBO analysis provided insights into electron transition process during the excitation and emission, and three types of electron transition modes have been identified for different


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complexes. Predominated electronic transitions of 1,3 (LC/MLCT), 1MLCT/1→3DST/3MTT and 1,3MLCT were correlated with the frame structure of Pt(ppzOppz), Pt(ppzOpopy) or Pt(ppzOczpy), and Pt(czpyOczpy), respectively. Compared with 1,3(LC/MLCT) predominated photoluminescence process in common complexes, DST model was newly discovered. The molecules with DST transition could overcome the insufficiency in common complexes dominated by local 1,3(MLCT/LC) transition where the excited energy states S1 and T1 strongly interrelated. Pt(ppzOczpy) scaffold was modified to optimize luminescent efficiency, color purity and their emission peaks in blue spectra region. The new developed complex Pt(ppzOczpy-m) with the emission peak at 447 nm constricted the emission band to FWHM less than 20 nm. PtON1 demonstrated enhanced efficiency due to the DST mechanism. PtON2 and Pt(ppzOczpy-m) showed finely tuned blue emission bands. We believe more phosphorescent materials with high performance and new photophysical properties can be developed following the DST model.

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ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. General information and methods for common measurements, single crystal analyses data, photophysical properties, computational simulation data, CV and UPS spectra, 1H NMR and 13 C NMR spectra. (PDF)

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AUTHOR INFORMATION (18)

Corresponding Author Email: [email protected] (X.C.H). Email: [email protected] (Z.K.C). Email: [email protected] (W.H.).

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Author Contributions ‡These authors contributed equally. (20)

ACKNOWLEDGMENT This work was supported by the Nanjing Tech University (NJU) &AAC Technologies collaborated funds 20161117, National Natural Science Foundation of China (Nos. 61605075, 11504168, NSFC-PSF 51661145021, 2015CB932200).

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Pt-complexes with high colour purity Phys. Chem. Chem. Phys. 2017, 19, 32670–32677.


Page 7 of 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. X-ray molecular structures a, selected bond lengths (Å) and angles (deg) for Pt(ppzOppz), Pt(ppzOpopy), Pt(ppzOczpy) and Pt(czpyOczpy).

Pt(ppzOppz)

Pt(ppzOpopy)

Pt(ppzOczpy)

Pt(czpyOczpy)

bond length (Å) Pt-N1

2.0584(69)

2.0813(68)

2.0984(94)

2.1186(93)

Pt-N2

2.0877(52)

2.0828(73)

2.0817(99)

2.1045(130)

Pt-C2

1.9625(60)

1.9699(88)

1.9922(106)

1.9419(128)

Pt-C1

1.9482(76)

1.9208(87)

1.9619(122)

2.0665(63)

bond angle (deg) N1-Pt-N2

107.092(235)

99.409(267)

98.883(372)

89.893(437)

N2-Pt-C2

81.035(249)

89.964(322)

91.951(422)

90.699(510)

C2-Pt-C1

91.344(280)

91.960(368)

89.749(471)

91.798(429)

C1-Pt-N1

80.508(263)

79.773(324)

80.506(422)

91.124(339)

dihedral (deg)

a

A-B

4.581(319)

39.749(294)

38.415(467)

67.345(414)

B-C

1.327(270)

18.451(395)

28.866(297)

32.113(419)

ORTEPs at 30% thermal ellipsoid probability.

Table 2. Photophysical properties of the complexesa Compounds λmaxb [nm]

τb [µs]

Pt(ppzOppz)

437

Pt(ppzOpopy)

Φb [%]

krb [105 s-1]

λmaxc [nm]

τc [µs]

5.1

61

1.2

438

6.6

432

10.0

72

0.72

433

14.2

Pt(ppzOczpy)

449

2.4

39

1.6

450

9.8

Pt(czpyOczpy)

512

2.2

49

2.2

483

4.8

a

Emission peaks (λmax), luminescent lifetime (τ) and quantum efficiency (Φ) were experimentally measured, and radiative decay rate (kr) was calculated by . b Data were measured from DCM solution at room temperature. c Data were measured from PMMA films at room temperature.



Page 8 of 11

Table 3. Transitional state energy levels a Complex

HOMO/LUMO/ Egb

IPc

ES1d / ET1e/△ESTf

S1g /T1g /∆ESTg

Pt(ppzOppz)

-5.43/-2.01/3.42

5.26

2.98/2.86/0.12

3.16/2.90/0.26

Pt(ppzOpopy)

-5.42/-2.17/3.25

5.55

3.17/2.92/0.25

3.02/2.87/0.15

Pt(ppzOczpy)

-5.29/-2.25/3.04

5.29

3.08/2.80/0.28

2.88/2.68/0.20

Pt(czpyOczpy)

-5.12/-2.30/2.82

5.30

3.00/2.81/0.19

2.52/2.41/0.11

a

All data are recorded in the units of eV. b The HOMO and LUMO levels were estimated by using Cp2Fe0/+ values of 4.8 eV below the vacuum level, and Eg = ELUMO– EHOMO. c Ionic potential (IP). d Excitation energy of the lowest singlet excited state. e The lowest triplet energy. f Energy gap between the lowest singlet and triplet states. g Calculated energy levels of vertical excited states at TD-B3LYP/6-31G*//Lanl2dz level based on S0 geometry.

Table 4. Photophysical properties of modified compounds Complex

HOMO/LUMO/ Ega/ ES1b/ET1c(eV)

λ(nm)d /FWHM(nm)d /Φe/τe(µs)/Kr(105s-1)e

Pt(ppzOczpy)

-5.29/-2.25/3.04/3.08/2.80

449/42/46%/2.4/1.9

PtON1

-5.29/-2.26/3.03/3.01/2.80

449/51/86%/3.3/2.6

PtON2

-5.21/-2.24/2.97/2.94/2.67

470/56/71%/4.0/1.8

Pt(ppzOczpy-m)

-5.32/-2.15/3.17/3.12/2.80

447/18/86%/7.6/1.1

Pt(ppzOczpy-mesi)

-5.31/-2.25/3.06/3.06/2.77

455/40/48%/4.9/1.0

a

The HOMO and LUMO levels were estimated by using Cp2Fe 0/+ values of 4.8 eV below the vacuum level, and Eg = ELUMO– EHOMO. b Excitation energy of S1. c The lowest triplet energy. d Emission peaks (λ) and full width of half maximum (FWHM) in PMMA at room temperature. e Luminescent efficiency and lifetime are measured from DCM solution of the complexes, and radiative decay rate was calculated by .

8 ACS Paragon Plus Environment

Pt(ppzOppz) 2

0.6 0.4

1

0.2 0.0 800

0 300

400

500

600

700

Absortion 1.0 Excitation At 77K in MeTHF At RT in DCM 0.8 At RT in PMMA At RT in solid

4 3

0.6 Pt(ppzOpopy)

2

0.4

1

0.2

0 300

3

0.6 Pt(ppzOczpy)

2

0.4

1

0.2

0 300

400

500

600

Wavelength (nm)

700

0.0 800

4 -1 -1 Extinction Coefficiency (10 M cm )


4

500

600

700

0.0 800

Wavelength (nm)

Normalized Intensity (a.u.)

5

400


3

5

5


4 3

0.6

Pt(czpyOczpy) 2

0.4

1

0.2

0 300

400

500

600

700


4




5

Wavelength (nm)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 9 of 11

0.0 800

Wavelength (nm)

Figure 1. Absorption, emission and excitation spectra of Pt(ppzOppz), Pt(ppzOpopy), Pt(ppzOczpy) and Pt(czpyOczpy). Absorption spectra were obtained in DCM solutions under ambient conditions; excitation spectra were measurd at 77 K under N2 atmophere.

Figure 2. Hole and electron distribution at S0, S1 and T1 states.



Page 10 of 11

Figure 3. Molecule structures and emission spectra of Pt(ppzOczpy) analogues.


Page 11 of 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(II) Complexes - ACS Publications

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