Triplet Excited-State Engineering of Phosphorescent Pt(II) Complexes

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Triplet Excited-State Engineering of Phosphorescent Pt(II) Complexes Yipei Wu, Xiao Tan, Anqi Lv, Feiling Yu, Huili Ma, Kang Shen, Zhengyi Sun, Fei Chen, Zhi-Kuan Chen, and Xiao-Chun Hang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b01685 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 17, 2019

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The Journal of Physical Chemistry Letters

Triplet Excited-State Engineering of Phosphorescent Pt(II) Complexes Yipei Wu, Xiao Tan, Anqi Lv, Feiling Yu, Huili Ma,* Kang Shen, Zhengyi Sun, Fei Chen, ZhiKuan Chen and Xiao-Chun Hang* †Key

Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM)

Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China.

Supporting Information Placeholder

AUTHOR INFORMATION Corresponding Authors Huili Ma, E-mail: [email protected] Xiao-Chun Hang, E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Three Pt(II) complexes, Pt(czpyOczpy), Pt(czpyOczpy-Me) and Pt(czpyOczpyOMe), are designed to elucidate the inherent relationship between electronically excited-state and photo- and electro-luminescent properties. These complexes showed a blue-shifted phosphorescence with narrowing spectral profile, which are interrelated with the variation of T1 states from 3MLCT, hybridized 3(MLCT/LC) to 3LC transition. This change is ascribed to the destabilization of LUMO energy levels on the pyridinyl site, leading more electron distribution on the carbazolide unit in T1. Moreover, the solution-processed device of Pt(czpyOczpy-OMe), featuring with 3LC transition, shows the best color purity of blue light. Compare to the device of Pt(czpyOczpy) in 3(MLCT/LC)

3MLCT

character, the device of Pt(czpyOczpy-Me) with hybridized

exhibits improved color purity and external quantum efficiency (10.2%) at a

luminance of 1000 cd/m2. Therefore, this work gives a mechanistic interpretation to the phosphorescent properties of tetradentate Pt(II) complexes derived from the manageable lowest triplet excited states.

TOC GRAPHICS


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Organic light-emitting diodes (OLEDs) have attracted worldwide attention for the successful utilization in electronic displays. Since the first generation of OLEDs using fluorescent emitters,[1] diligent exploration for the suitable luminescent materials has been continuous over 30 years.[2,3] Regarding high-efficiency OLEDs, phosphorescent[4,5] and thermally activated delayed fluorescent (TADF)[6] are two alternative decaying mechanisms in utilization of both singlet and triplet excitons generated in device systems.[1,7] Phosphorescent materials, normally incorporated with heavy-metal complexes,[8-11] such as Ir(III), Pt(II), Cu(I) and Au(I, III), are extensively exploited. To satisfy the demands in device application, correlation between photophysical properties and molecule structure is a critical issue in developing the high-performance OLEDs.[1215]

Generally, in the excited states of transition-metal complex, metal-to-ligand charge transfer

(MLCT) and ligand-centered (LC) are two main transition characters to emit photons.[16] Nevertheless, the excited states are normally not “pure” in MLCT or LC transition modes, but instead with admixed characters, [17,18] resulting in complicated and indistinguishable emission properties. On the current research tendency, materials featuring with charge transfer (CT) characteristics, such as intramolecular charge transfer (ICT)[19,20] and MLCT[21,22], are denoted as key ingredient for efficient photon generation with high quantum efficiency (QE). Nonetheless, deactivation transition with CT excited states afford broad emission spectra,[23,24] which hampers the optical applications in terms of low color purity and outcoupling efficiency by employing microcavities.[25,26] To overcome this dilemma, efficient emitters with narrow emission spectra were achieved by tetradentate Pt(II) complexes[27,28]. These complexes, fabricated into pure blue OLEDs achieving peak external quantum efficiency (EQE) at 24.8 % with Commission Internationale de l’Eclairage (CIE) coordinates of (0.148, 0.079), was developed by Li’s group.


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Further theoretically spectroscopic investigation revealed an MLCT-LC composited luminescent mechanisms with a delocalized spin transition (DST) process of configuration transformation in excited states from the lowest singlet state (S1) to the lowest triplet state (T1), [18,29] resulting in ligand-centered (LC) dominated emission, and thereby giving efficiently narrow blue emission. Specifically, those narrow-spectrum phosphorescent Pt(II) emitters are featured with local 3ππ* transition in carbazolide (cz) center, which was also recently found to contribute for the structured emission in copper carbene complexes in Thompson’s group.[3,20]

Figure 1. Chemical structures of Pt(II) complexes Currently, OLEDs doped with neutral Pt(II) complexes Pt(czpyOczpy), embodying Npyridinylcarbazolide (czpy) moiety, was reported with high efficiency as well as long operational lifetime.[30,31] Pt(czpyOczpy) could experience DST transition from S1→T1, and capture the energy upon 1MLCT (singlet) state but extrude photons in the way still dominated via 3MLCT (triplet) transition, which is apparently distinct to Pt(ppzOczpy) dominated by 3LC (3ππ*) T1→S0 deactivation process.[18] In this study, manipulation of the excited states as well as the luminescent spectra upon structural modification was elucidated. By destabilizing the lowest unoccupied


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molecule orbital (LUMO), T1 characters of Pt(czpyOczpy)-skeleton molecules could be altered from initial 3MLCT to 3LC. Differing from Pt(czpyOczpy) with dominant 3MLCT character, Pt(czpyOczpy-Me) exhibits 3(MLCT/LC) hybridized emission properties, and Pt(czpyOczpyOMe) has a 3LC dominated T1 state. The significant difference in photophysical characteristics, even though with minor structure modulation, shows superb feats in the excited-state engineering of Pt(II) molecules. Pt(czpyOczpy), Pt(czpyOczpy-Me) and Pt(czpyOczpy-OMe), fabricated into the devices via solution process for parallel verification, give emission color in blue region with CIE coordinates of (0.29, 0.48), (0.24, 0.41) and (0.18, 0.23), respectively. These results are in accordance with the conventional understanding that transition-metal complexes dominated with 3LC

transition give structured emission spectra but with lower efficiency in both photo- and

electroluminescence. Therefore, the findings here underlines the complex Pt(czpyOczpy-Me) with a hybridized 3(MLCT/LC) transition state, which refer to not only with improved blue color purity but also with the higher EQE compared to Pt(czpyOczpy) dominated in 3MLCT character. Absorption and emission spectra are comparatively shown in Figure 2, and their energy levels correlated physical data are summarized in Table 1. The compounds show two similar absorption bands around 265 and 330 nm with high extinction coefficients above 5×104 M-1cm-1, and each can be assigned to the 1ππ* transitions of localized aromatic rings[18] (Figure 2a). The absorption bands below 350 nm are corresponding to the 1MLCT transitions from Pt(Ⅱ) center to py moieties (dPt→πpy*). The intensity of 1MLCT transitions decrease in the sequence of Pt(czpyOczpy), Pt(czpyOczpy-Me) and Pt(czpyOczpy-OMe), showing the noticeable variation in electron donating substituents on py. The energy of S1, which can be estimated from the terminal edge of 1MLCT

bands in absorption spectra, increasing in the value of 2.88 eV, 2.91 eV and 3.08 eV to

the complexes in the aforementioned sequence (Table 1). The changes in concerted to the energy


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gaps, measured in the value of 2.86 eV, 2.97 eV and 3.10 eV, respectively, are largely in dependency on their LUMO energy levels (also see Figure S1). Figure 2b-2c show the emission spectra of the complexes in DCM solution and polymethyl methacrylate (PMMA) solid matrix at ambient condition. Pt(czpyOczpy) exhibits broad and structureless emission in both solution and PMMA film, indicating a firm decay process dominated with

3MLCT

characteristic.

Pt(czpyOczpy-Me) shows broad emission in DCM but structured spectrum in PMMA, suggesting an excited-state configuration change under the influence of the surrounding environments. Pt(czpyOczpy-OMe) emits in well-resolved spectrum both in solution and PMMA film, indicating an 3LC dominated transition mode. The spectral variation exemplarily demonstrates the color tuning methodology via regionally manipulating LUMO energy levels on py site. As we previously disclosed,[18] destabilized LUMO results significant electronic depopulation from the 1MLCT to local 3LC state. Similarly, DST process could also be inferred for Pt(czpyOczpy) framed molecules. That is, the 3ππ* transition on cz parts (3cz) is enhanced after DST process, the emission peaks are tuned from 518 nm to 447 nm, and the full width at half maximums (FWHMs) shrink from 98 nm to 56 nm in DCM. The comparative emission spectra enlighten the T1 configuration in dominant from 3MLCT to 3LC. In addition, Pt(czpyOczpy) and Pt(czpyOczpy-Me) show the negative solvatochromic shifts, suggesting the 3MLCT nature in their T1. The solvatochromic shifts between the solution in acetonitrile and toluene decreased for Pt(czpyOczpy), Pt(czpyOczpy-Me) and Pt(czpyOczpy-OMe) in the value of 14.0 nm, 4.6 nm and 2.6 nm, respectively (Figure S2-S4). The emissions of all three complexes at 77 K in 2-methyl tetrahydrofuran (2-MeTHF) converge with similar shoulder bands and primary ν0-0 peaks at 438 nm, indicating sufficiently confined triplet energy of 3cz with 3πcz*→πcz dominated transition. This can be explained by the destabilization of the 3MLCT states under rigid environment.[20]


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Figure 2 Absorption and emission spectra of Pt(czpyOczpy), Pt(czpyOczpy-Me) and Pt(czpyOczpy-OMe). Absorption spectra were obtained in DCM solutions under ambient condition (a). Emission spectra were measured in DCM solution (b), in PMMA under ambient condition (c) and in frozen 2-MeTHF (d) at 77 K. Table 1 Photophysical properties of Pt(czpyoczpy), Pt(czpyoczpy-Me) and Pt(czpyoczpy-OMe). Compounds

abs a

PLmax a/b [nm]

FWHM a/b [nm]

ES1c [eV]

ET1d [eV]

HOMO/LUMO/Ege [eV]

Pt(czpyoczpy)

264, 277, 338

518, 482

98, 82

2.88

2.80

−5.19/ −2.33/ 2.86

Pt(czpyoczpy-Me)

265, 277, 337

491, 451

96, 64

2.91

2.81

−5.18/ −2.21/ 2.97

Pt(czpyoczpyOMe)

265, 276, 328

447, 446

56, 52

3.08

2.81

−5.14/ −2.04/ 3.10

aAbsorption

[nm]

peaks (abs) and PL peaks (PLmax) were measured in DCM solution at room

temperature; bEmission peaks (PLmax) were measured in PMMA with 5 wt % doping; cS1 energy (ES1) were calculated from the absorption edge of UV-vis absorption spectra; dT1 energy (ET1) were calculated from the onset of phosphorescence spectra measured in 2-MeTHF at 77 K;


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eHOMO

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and LUMO were calculated from CV measurements ref to Fc+/Fc separately in DCM and

DMF, and Eg was counted by |ELUMO - EHOMO|. To gain more insights into the excited-state change, the temperature- and time-dependent luminescent characteristics were investigated in vacuum (Figure 3, S5-S6). As the temperature increases, Pt(czpyOczpy) and Pt(czpyOczpy-OMe) show tiny bathochromic peak shifting accompanied with little spectral broadening (Figure S5-S6). This can be identified as homogenous interaction of the excited states with increased thermal vibrations in the heating environment.[16] Pt(czpyOczpy) experiences a 3MLCT dominated deactivating process, while Pt(czpyOczpy-OMe) undergoes an

3LC

dominated

3ππ*

transition. Pt(czpyOczpy-Me) is observed with the

enhancement on the satellite peaks at high temperatures. This can be attributed to the amount increase of 3MLCT coupling with ground-state vibrations due to its admixed 3(MLCT/LC) excited state[3] (Figure 3a, b). As a result, emission of Pt(czpyOczpy-Me) turned to be structureless at high temperature. Transient photoluminescence spectra of the complexes were measured in situ within several intervals from few to tens of microseconds (Figure 3, S7-S8). Normally, triplet excited state in 3LC character takes longer time for extruding photons than in 3MLCT character within few microseconds. Pt(czpyOczpy-Me), designated with admixed 3(MLCT/LC) character in T1, explicitly specifies the deactivation process of shifting from broad 3MLCT to narrowly 3LC featured emission (Figure 3c, 3d). With respects to Pt(czpyOczpy) and Pt(czpyOczpy-OMe), both show with almost unchanged spectral profiles in normalized pattern during the emissive time intervals (Figure S7-S8). The former are broad, while the latter show vibronic fine structures.


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Figure 3 Environment-dependent emission spectra of 5 wt% Pt(czpyOczpy-Me) doped in PMMA. Both Temperature-dependent spectra (a,b) and time-dependent transient photoluminescence spectra (c,d) were measured in vacuum. Transient spectra were recorded at specified time intervals after stopping excitation (340 nm) with bandwidth < 1 ns at 296 K. Figure 4 schematically illustrate the excited-state transitions accompanied with experimentally measured luminescent quantum efficiency (ϕ), decay lifetime (τ) and radiative decay rate (kr). The 1,3MLCT

states are destabilized accompanied with the energy levels of πpy orbitals corresponding

to the electron donating substituents on py moiety, while the local 3LC excited states 3cz are almost in the same level. Pt(czpyOczpy), dominated by 3MLCT in T1, gives short lifetimes (τ = 2.1 μs in DCM and 3.4 μs in PMMA), and it was observed with kr of 2.0×105 s-1 in both DCM and PMMA. On the other hand, Pt(czpyOczpy-OMe) decays with lifetime in couples of microseconds (τ = 15.6 μs in DCM and 23.3 μs in PMMA) and kr (0.2×105 s-1 in DCM and 0.13×105 s-1 in DCM) in one


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order of magnitude smaller than that of Pt(czpyOczpy), indicating a firm emission with 3LC character. Pt(czpyOczpy-Me), featuring with changeable configuration in T1, has a kr of 1.7×105 s-1 in DCM, which is twice as that in PMMA. Those luminescent parameters give clear evidence to support the excited-state engineering for the Pt(II) complexes.

Figure 4 Diagrams depicting emission on the different excited states.


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Figure 5 The hole and electron distributions of S1 and T1 for Pt(II) complexes. For a better understanding, the frontier orbitals of the complexes were obtained from the optimized S0 geometries based on density functional theory (DFT) calculations (Figure S10 and S11). The LUMOs are primarily localized on the py sites, with small portions of dz2 orbital residing on the Pt center. The HOMOs are predominantly on the Pt(II) oxybiscarbazolide circles with d orbital of Pt, p-orbital of oxygen and conjugated π-orbitals residing on the phenyl rings. Natural transition orbitals (NTO) of S1 and T1 states were evaluated at the level of TDDFT/B3LYP/631G(d) based on optimized S0 geometries (Figure 5 and S12). Complying with intensity variation of 1MLCT bands in absorption spectra, the electron distribution of S1 on the πpy* of Pt(czpyOczpy) Pt(czpyOczpy-Me) and Pt(czpyOczpy-OMe) successively decreased in the ratio of 88%, 86% and 77%, respectively. During ISC for S1→T1, the complexes experience DST process. Higher ratio of 1πpy*→3πcz* electron delocalization was observed for the complexes with more destabilized πpy* orbitals. As a result, electron density on 3πcz* of Pt(czpyOczpy), Pt(czpyOczpy-Me) and


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Pt(czpyOczpy-OMe) in T1 was calculated in the ratio of 20%, 27 % and 51%. This supports the fine-structured emission at 77K in frozen 2-MeTHF, which were dominated by 3cz for all three complexes, whereas the 3MLCT state of 3πpy*→dptπczOcz is destabilized in rigid environment.[20] In addition, the ratio variation also backs up the transformation of T1 character for the complexes. Pt(czpyOczpy-Me) with 27 % 3(MLCT/LC)

3π cz

plus 71%

3π * py

mixed triplet configuration exhibits

hybridized photophysical properties. Pt(czpyOczpy-OMe) equally with electron

distribution on 3πcz (51%) and 3πpy* (46%) exhibits almost purely cz centered local emission properties, implying 3cz has larger oscillator strength than 3MLCT (3πpy*→dptπczOcz) states in T1→S0 transition. In response with the temperature increase, 3MLCT could prevail the T1→S0 transition. This can be explained by the faster enhancement on the oscillator strength of 3MLCT via electron-phonon coupling to the ground state.[3] To investigate the electroluminescent properties, we fabricated the OLED devices using 9-(4tert-Butylphenyl)-3,6- bis(triphenylsilyl)-9H-Carbazole (CzSi) as host with 30 wt% doping in solution process due to their efficiency enhancements and reductions in efficiency roll-off at higher doping concentration.[27] Electroluminescence (EL) performances are then shown in Figure 6 and Table

2.

Device

structures

comprise

of

an

indium

tin

oxide

(ITO)/poly(3,4-

ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)(35 nm)/ CzSi:Pt complex (30 wt%, 40 nm) / Bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO) (10 nm)/1,3,5-tri(mpyridin-3-ylphenyl)benzene (TmPyPB) (35 nm)/LiF (1.2 nm)/Al (100 nm) (Figure 6a). PEDOT:PSS is the hole-injecting layer. DPEPO, TmPyPB and LiF, fabricated via evaporation technique, are the exciton-blocking layer, electron- transporting layer and the electron-injecting layer, respectively. Although the fully-evaporation device of Pt(czpyOczpy) was reported with blue color (peak at 476 nm),[32] we got the emission peak at 507 nm in blue-green region. Similar


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to the spectral profiles in DCM, Pt(czpyOczpy-Me) and Pt(czpyOczpy-OMe) peak at 495 and 450 nm in blue region with CIE coordinates of (0.24, 0.41) and (0.18, 0.23) (Figure 6a and S13). Figure 6b-6c show the current density-voltage-brightness (J-V-L) curves. Device of Pt(czpyOczpy-OMe), with higher band gap than Pt(czpyOczpy) and Pt(czpyOczpy-Me), has higher driving voltage to the same luminance. The electroluminescent efficiencies agree well with their photoluminescent yield measured in PMMA matrix (Figure 6d), while the enhanced maximum EQE (9.8 %) of Pt(czpyOczpy-OMe) certainly signify a level of dopant alignment base on the planar coordination structure.[33] The higher roll-off in Pt(czpyOczpy-OMe) OLEDs is due to triplet-triplet annihilation enhanced by the longer lifetime at high brightness. Pt(czpyOczpy-Me) gets the highest maximum CE, PE and EQE at 28.4 cd/A, 10.3 lm/W and 10.8%, and achieved best EQE (10.2 %) at 1000 cd/m2 (Figure 6d). Compare to Pt(czpyOczpy) with pure 3MLCT character, Pt(czpyOczpy-Me) with hybridized 3(MLCT/LC) shows the advantage in high performance OLEDs by improving color purity as well as keeping efficiency. These results give the answer to previous reported Pt(II) complexes with narrow emissions[27,28,32] are in 3(MLCT/LC) hybridized character featuring local 3ππ*

dominated emission as well as highly efficient properties imparted by strong 3MLCT

perturbation.


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Figure 6 OLED performances. (a) EL spectra and Device structure: ITO/PEDOT:PSS (35 nm)/CzSi:Pt-complex (30 wt% ,40 nm)/DPEPO (10 nm)/TmPyPB (35 nm)/LiF (1.2 nm)/Al (100 nm), (b) luminance vs n voltage J-V curves, (c) current density (J) vs voltage L-V curves, (d) current efficiency (CE) / power efficiency (PE) / EQE vs current density curves. Table 2 Summary of the device performances.

FWHM [nm]

Vona [V]

Dopants

CIE(x, y)

Peak [nm]

Pt(czpyOczpy) Pt(czpyOczpyMe) Pt(czpyOczpyOMe)

(0.29, 0.48)

507

112

(0.24, 0.41)

495

(0.18, 0.23)

450

aVoltage

@1000 cd/m2

@Max CEb [cd/A]

PEc [lm/W]

EQEd [%]

CEb [cd/A]

PEc [lm/W]

EQEd [%]

4.0

28.4

11.2

10.1

28.2

10.4

10.1

102

4.0

27.5

13.3

10.8

25.9

9.6

10.2

72

4.5

16.7

10.5

9.8

7.3

2.3

4.5

at 1 cd/m2, bCurrent efficiency (CE), cPower efficiency (PE), dExternal quantum

efficiency (EQE).


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In conclusion, we have demonstrated tetradentate Pt (II) complexes can emit light with 3MLCT, 3(MLCT/LC)

hybridized and 3LC character. Those characters are identified unambiguous

with their photophysical properties, and can be tuned through excited-state management. Upon simple structure modification for T1 engineering, photoluminescent peaks in PMMA were tuned from 482 nm to 446 nm as well as changes of FWHMs from 82 nm to 52 nm. With the materials fabricated into devices by solution process, EL peaks shift from 507 nm to 450 nm in blue light region, and FWHMs were tuned from 112 nm to 72 nm. Pt(czpyOczpy-OMe), featuring with 3LCcharacter emission, has the best blue color in PL/EL emission, but relatively small kr (104 s-1 order) and low efficiency (CE= 7.3 cd/A, PE= 2.3 lm/W and EQE = 4.5 % at 1000 cd/A) in device. Compared to the 3(MLCT/LC)

3MLCT-character

complex Pt(czpyOczpy), Pt(czpyOczpy-Me) with

hybridized character has compatible kr in the order of 105 s-1 and much better PL

color in blue region. Device of Pt(czpyOczpy-Me) among the three complexes achieves highest EQE (10.2 %) at 1000 cd/A, and also has better EL color in blue region than the device of Pt(czpyOczpy). Presented with excellent device operational lifetimes estimated by other groups for those tetradentate Pt (II) complexes,[27,32,34] of which the luminescent mechanism became a critical issue to make further improvements. The results in this work showcase methodologically to design the stable and efficient blue phosphorescent emitters. ASSOCIATED CONTENT Supporting Information Experimental section, computational details, synthesis and additional photophysical measurements.

AUTHOR INFORMATION


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*Huili Ma. E-mail: [email protected] * Xiao-Chun Hang. E-mail: [email protected] Notes The authors declare no competing financial interests. Acknowledgements The authors acknowledge the financial support from the National Key Research and Development Program of China (No. 2017YFB0404501), the National Natural Science Foundation of China (No. 91833302), Major Program of Natural Science Research of Jiangsu Higher Education Institutions of China (No. 18KJA150005), the Natural Science Foundation of Jiangsu Province (No. BK20181373). We are grateful to the High Performance Computing Center of Nanjing Tech University for supporting the computational resources.

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TOC 50x50mm (300 x 300 DPI)


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Figure 1. Chemical structures of Pt(II) complexes 85x50mm (300 x 300 DPI)



Figure 2 Absorption and emission spectra of Pt(czpyOczpy), Pt(czpyOczpy-Me) and Pt(czpyOczpy-OMe). Absorption spectra were obtained in DCM solutions under ambient condition (a). Emission spectra were measured in DCM solution (b), in PMMA under ambient condition (c) and in frozen 2-MeTHF (d) at 77 K.


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Figure 3 Environment-dependent emission spectra of 5 wt% Pt(czpyOczpy-Me) doped in PMMA. Both Temperature-dependent spectra (a,b) and time-dependent transient photoluminescence spectra (c,d) were measured in vacuum. Transient spectra were recorded at specified time intervals after stopping excitation (340 nm) with bandwidth < 1 ns at 296 K. 93x75mm (300 x 300 DPI)



Figure 4 Diagrams depicting emission on the different excited states. 97x76mm (300 x 300 DPI)


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Figure 5. The hole and electron distributions of S1 and T1 for Pt(II) complexes. 85x56mm (300 x 300 DPI)



Figure 6 OLED performances. (a) EL spectra and Device structure: ITO/PEDOT:PSS (35 nm)/CzSi:Ptcomplex (30 wt% ,40 nm)/DPEPO (10 nm)/TmPyPB (35 nm)/LiF (1.2 nm)/Al (100 nm), (b) luminance vs n voltage J-V curves, (c) current density (CD) vs voltage L-V curves, (d) current efficiency (CE) / power efficiency (PE) / EQE vs current density curves. 90x67mm (300 x 300 DPI)


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Triplet Excited-State Engineering of Phosphorescent Pt(II) Complexes

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