The Strategy Used for Controlling the Photostability of Tridentate Pt(II

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

The Strategy Used for Controlling the Photostability of Tridentate Pt(II) Complexes to Enhance the Device Lifetimes of Blue PhOLEDs: The Role of Pt-C*(NHC) Bond and Auxiliary Ligand Yafei Luo, Dianyong Tang, Zhong-Zhu Chen, Chunping Fu, Zhigang Xu, and Jiangping Meng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02768 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018

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The Journal of Physical Chemistry

1

The strategy used for controlling the photostability of tridentate

2

Pt(II) complexes to enhance the device lifetimes of blue

3

PhOLEDs: the role of Pt-C*(NHC) bond and auxiliary ligand

4 5

Yafei Luo a, Dianyong Tang a*, Zhongzhu Chen a, Chunping Fu a, Zhigang Xu a,

6

Jiangping Meng a

7 8

a

Collaborative Innovation Center of Targeted Therapeutics and Innovation,

9

Chongqing Key Laboratory of Kinase Modulators as Innovative Medicine, Chongqing

10

Engineering Laboratory of Targeted and Innovative Therapeutics, International

11

Academy of Targeted Therapeutics and Innovation, Chongqing University of Arts and

12

Sciences, Chongqing, 402160, P. R. China

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The Journal of Physical Chemistry 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

Abstract

1 2

Controlling the photostability of the organometallic complex is vital for obtaining

3

the long-lasting phosphorescent organic light-emitting diode (PhOLED). In this article,

4

the density functional theory (DFT) was employed to investigate the photostability of

5

a series of tridentate Pt(II) complexes in the triplet-triplet annihilation (TTA)

6

processes. The calculated results indicate that the existence of Pt-C*(NHC) bond can

7

increase the Gibbs free main ligand dissociation energy to avoid the main ligand

8

dissociation during the TTA process. According to the wiberg bond order and electron

9

localization function (ELF), the positive effect of Pt-C*(NHC) bond originates from

10

the strong d and p orbitals interaction and more electron localization between C*(NHC)

11

and Pt atoms. In addition, the study in this article also manifests that the auxiliary

12

ligands with strong electron-withdrawing/donating properties can facilitate the

13

electron localization, leading to the increase of photostability for the tridentate Pt(II)

14

complexes. These computed results are useful and valuable for controlling the

15

photostability of tridentate Pt(II) complexes and designing the stable blue Pt(II)

16

complexes.

17 18

Keywords: photostability, tridentate Pt(II) complexes, Pt-C*(NHC) bond, auxiliary

19

ligand

20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 2


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1

1. Introduction

2

Due to the low power consumption and high external quantum efficiencies, the

3

phosphorescent-based organic light-emitting diodes (PhOLEDs) have attracted

4

extensively attentions and become the most widespread electroluminescent devices.1

5

Electroluminescence in the PhOLEDs comes from the triplet excited states of the

6

organometallic complexes, for instance, Pt(II) and Ir(III) complexes, which enables

7

the conversion of nearly 100% of exciton created into photon due to the strong

8

spin-orbital coupling (SOC).1-4 Apart from the high efficiencies, the long-lived

9

operational stabilities of devices are essential to the practical application. In contrast

10

to red and green PhOLEDs, the fabrication of a long-lasting blue PhOLED is difficult

11

and challenging, which consequently hampers the large-scale commercialization of

12

PhOLEDs. In the course of device operations, the operational lifetimes of OLEDs are

13

closely related to the intrinsic operational degradation of the device materials,5 caused

14

by the destruction of functional molecules and accumulation of defects in the device

15

layers.6-9 The previous reports show that there are some various defects, including the

16

nonradiative recombination centers,10 luminescence quenchers and deep charge traps

17

causing formation of a barrier layer and raising the operational voltage.11 Additionally,

18

the highly reactive species stemmed from molecules’ destruction can react with the

19

neighboring molecules to result in the formation of defects.12,13

20

Accompanied by the development of fabrication methods of OLEDs, the devices

21

can be effectively encapsulated.14 In this circumstance, external contributors, i.e.,

22

water and oxygen, cannot affect the accumulation of defects and nonemissive

23

species.15 Therefore, it is reasonable to explore the operational lifetime of the device

24

on the basis of exploring the intrinsic process, that is, the functional materials

25

decompose into highly reactive species, including the ions, radicals, or ion-radicals.

26

Up to today, according to the previous reports,8,9,16 the possible degradation

27

mechanisms of functional materials in OLEDs can be classified as degradation of

28

molecules in charged excited states or highly excited states, degradation of excited

29

molecules or charged molecules. For the PhOLED, the degradation of molecules in 3



1

highly excited state is considered as the most possible degradation mechanism due to

2

the high density of the long-lived triplet exciton. This process is also named as TTA

3

which occurs via a diffusion-based Dexter transfer mechanism17 and the

4

corresponding annihilation process can result in the formation of hot excited states.18

5

On the basis of the spin statistics, the TTA process can generate 1/9 of hot singlet

6

excited states per 3/9 hot quintet (Qn) states and 5/9 of hot triplet (Tn) states,

7

respectively. Compared with the hot singlet excited states (Sn) and quintet (Qn) states,

8

the hot triplet (Tn) excited states seem more important for the degradation of the

9

emissive complexes and the corresponding energy requirement is E(Tn) ≤ 2E(T1).19

10

Once the hot triplet (Tn) excited states are formed, they will rapidly decay back to the

11

lowest-lying triple (T1) excited state via the internal conversion. In addition, those in

12

the reactive excited vibrational state may enter into the dissociation states, leading to

13

the degradation of the emissive complexes. Generally speaking, in the phosphorescent

14

complex, the typical bond dissociation energies of weak bonds are lower than the hot

15

triplet (Tn) excited states and higher than the T1. Therefore, the TTA process is the

16

major source for the emissive complexes degradation in the PhOLEDs.

17

So far, the efficient ways applied to reduce the TTA process are challenging

18

because it is difficult to obtain the organometallic complexes with triplet lifetime

19

shorter than a few µs and control the aggregation on the vacuum deposited emitting

20

layer. Therefore, by reasonably and reliably designing the organometallic complexes

21

should be a feasible way to prevent the degradation of complexes and increase the

22

operation lifetime of the PhOLEDs. Recently, Escudero and co-workers unveiled the

23

degradation mechanisms of state-of-the-art blue Ir(III) complexes upon PhOLED

24

operation.19 They found the kinetic and thermodynamic criteria do play important role

25

in the degradation processes. Moreover, they provided a very promising method to

26

prevent ligand dissociation reactions by combining bis(tridentate) architectures with

27

NHC ligands. Compared with the other ligands, the NHC ligands possess some

28

distinctive advantages. Firstly, the NHC ligands have exceptionally strong σ-bonding

29

and readily tunable steric and electronic properties. Secondly, the strong ligand field

30

of the carbene can raise the energy of metal-centered d−d excited states, thus leading 4


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1

to prevent the formation of metal-centered d−d excited states, which is harmful for

2

meeting the kinetic criteria for the ligand dissociation reactions. Thirdly, the strong

3

metal−carbene bonds are beneficial for suppressing the degradation of phosphorescent

4

materials in PhOLEDs. Although the NHC ligands possess those excellent properties,

5

so far, a systemically studies is scarce about the degradation mechanisms of the

6

complexes coordinated with NHC ligands. In this article, a series of Pt(II) complexes

7

coordinated with NHC ligands are investigated to illustrate following questions: (1).

8

unveiling the role of the Pt-C*(NHC) bond in the main ligand dissociation; (2).

9

inspecting the influence of the substituent groups and π conjugation on the ligand

10

dissociation reactions; (3). controlling the degradation processes by means of

11

ancillary ligands. By solving these questions, it can provide some meaningful and

12

valuable information for designing the stable blue Pt(II) complexes and increasing the

13

operation lifetimes of blue PhOLEDs.

14

2. Computational details

15

In this article, the geometries of ground states (S0) and triplet excited states are

16

optimized using the restricted and unrestricted density functional theory (DFT and

17

UDFT) methods without consideration of the symmetry constrains. The dispersion

18

corrected hybrid functional B3LYP-D320,21 was employed in the optimization of

19

ground states and the lowest triplet excited states (T1) for all investigated complexes

20

because it has been confirmed as the suitable functional for assessing the

21

photostability of organometallic complexes.19 In order to explore the equilibrium

22

between the T1 states and metal center d-d excited states (3MC), the states involved in

23

the potential energy surface curves, including the 3MC states, transition states (TS)

24

between T1 and 3MC states and the minimal energy crossing points (MECP), were

25

constructed adopting the B3LYP-D3 method. Therein, the methodology in Persson’s

26

work22,23 was referenced to optimize the electron configurations of 3MC d-d states.

27

The MECPs were calculated in the sobMECP program, which is a modified version of

28

Harvey’s MECP program24,25 by Tian Lu. The sobMECP is a wrapper of the Harvey’s

29

MECP program to simplify the use of the MECP program.26 On the basis of optimized 5



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1

geometries, the frequency calculations were also implemented to confirm the natures

2

of the stationary points. In addition, to obtain accurate thermodynamic properties of

3

the studied complexes, the singlet-point computations were performed with the

4

ORCA software27 with the help of the dispersion-corrected double-hybrid

5

PWPB95-D3BJ

6

pseudopotential for Pt).

functional28

and

the

def2-SVP

basis

set

(ECP-60-mwb

7

Besides, in order to accurately compute the emission wavelengths, the Huang-Rhys

8

factors for the normal modes were obtained using Dushin package.29 When the

9

maximum Huang-Rhys factor is smaller than 1, the maximum absorption or emission

10

peaks are corresponding to 0-0 transition due to the significant vibrational overlaps

11

between the two states. While, when the maximum Huang-Rhys factor is larger than 1,

12

the maximum absorption or emission peaks are corresponding to vertical transition

13

because vibrational overlap between the ground vibrational state of the T1 would be

14

with a higher vibrational state of the ground state. For the Complexes 3 and 3-1-8, the

15

Huang-Rhys factors versus the normal mode wavenumber were calculated (see

16

Supporting Information, Figure S1-2). As Figure S1-2 shown, the maximum

17

Huang-Rhys factors are smaller than 1, implying that the 0-0 transitions used to

18

elaborate the experiment maximum emission wavelengths are reasonable.30,31

19

All calculations apart from the singlet-point PWPB95-D3BJ computations were

20

carried out with the Gaussian 09 program package.32 The 6-31G(d, p) basis set was

21

applied for the all light atoms and the transition metal Pt was described by the SDD

22

basis set.

23

3. Results and discussion

24

In here, both homolytic and heterolytic reactions, for the degradation of complexes,

25

were considered. The heterolytic reactions exhibit much larger Gibbs Free energies

26

(see Supporting Information, Figure S3-5) as compared to those of the homolytic

27

reactions shown in the main text. In the case of the heterolytic reactions, the ∆Greact

28

values are well above the energy of the hot states, therefore, the ligand dissociation

29

reaction are in principle energetically unfeasible. According to these computed results, 6


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1

only the preferred homolytic pathways are discussed in the main text.

2

3.1 Investigation of the main ligand dissociation for the Pt(II) complexes

3 4

Scheme 1. The chemical geometries of the Pt-complexes.

5

In order to test the reliability of B3LYP-D3 method for geometric optimization, the

6

root-mean-square deviation (RMSD) between X-ray structure of Complex 3 and the

7

computational structure was calculated. As shown in the Figure S6, the two structures

8

are nearly overlapped, indicating the B3LYP-D3 method is reliable for geometric

9

optimization. Therefore, the method was also applied to optimize the other tridentate

10

Pt(II) complexes (see Scheme 1). By investigating these chosen complexes can clearly

11

elaborate the influence of the number of Pt-C* bonds or Pt-N bonds on main ligand

12

dissociation in the excited-state reactivities. The thermodynamic criterion,19 i.e.

13

∆Great > 2E(T1), is firstly carried out to demonstrate if these complexes will meet to

14

completely avoid ligand dissociation from the hot excited states. Therefore, the Gibbs

15

free energies and the adiabatic triplet energies were calculated and the results are

16

shown in the Figure 1. For all studied complexes, the ∆Great values for the

17

photodechelation reactions are well high the energies of the hot excited states and thus,

18

their main ligand dissociation reaction in principle energetically unfeasible. Compared

19

with Complex 3-1, 3-2 and 3-3, the Complex 3, 3-4 and 3-5 possess larger ∆Great -

20

2E(T1) values, indicating that the Pt-C*(NHC) bond is beneficial for enhancing the

21

photostability of tridentate Pt(II) complexes.

7



1 2

Figure 1. The calculated PWPB95-D3BJ Gibbs Free ligands dissociation energies

3

(eV), the double adiabatic triplet energies (eV) for Complexes 3 and 3-1-5. ∆GreactM:

4

Gibbs Free main ligands dissociation energies, ∆GreactA: Gibbs Free auxiliary ligands

5

dissociation energies.

6

Moreover, the Pt-Cl dissociation reactions were also considered to explore the

7

possibility of the auxiliary ligand degradation during the TTA process. The

8

corresponding Gibbs free energies are pictured in the Figure 1. The Gibbs free

9

energies, for the Complexes 3, 3-1-5, are almost identical (4.56 to 4.79 eV),

10

indicating that the Pt-Cl dissociations are insensitive to the different main ligands. An

11

analysis of the relationship between the Gibbs Free auxiliary ligands dissociation

12

energies and doubled adiabatic triplet energies can illustrate the Pt-Cl dissociations

13

rely on the doubled adiabatic triplet energies of the complexes.

14 15 16 17 8


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1

Table 1. The wiberg bond order of coordinate bond for all complexes. Complex 3 Complex 3-1 Complex 3-2 Complex 3-3 Complex 3-4 Complex 3-5

B1 0.70 0.50 0.50 0.50 0.82 0.84

B2 0.78 0.85 0.85 0.85 0.81 0.80

B3 0.70 0.50 0.50 0.50 0.41 0.41

2 3

For the sake of unveiling the role of Pt-C*(NHC) bond in main ligand dissociation,

4

the wiberg bond orders of coordinate bonds were computed using Multiwfn 3.5

5

software33 (Table 1). Seen from the results in Table 1, the wiberg bond orders of B2

6

(Pt-C(phenyl) bonds) are 0.78, 0.85, 0.85, 0.85, 0.81 and 0.80, respectively,

7

suggesting that the Pt-C(phenyl) bonds can cause minor effect for the main ligand

8

dissociations. So, the different strengths of Pt-C*(NHC), Pt-N*(NHC) and

9

Pt-N(pyridyl) bonds are the key factor for the stability of studied complexes. Based on

10

the values of the wiberg bond orders of B1 and B3, they can obviously reflect that the

11

Pt-C*(NHC) bond exhibits more strong strength than Pt-N*(NHC) and Pt-N(pyridyl)

12

bonds, enhancing the photostability of tridentate Pt(II) complex. Subsequently, the

13

electron localization function (ELF) analyses for the four representative complexes

14

were carried out with the Multiwfn 3.5 software.33 The color-filled ELF maps are

15

presented in Figure S7 and the detailed discussions are performed in the Supporting

16

information (SI). The ELF analysis can suggest that the electron between the Pt and

17

C(phenyl)/C* (NHC) atoms show more localized feature. Hence, one can speculate

18

that the electron localized feature between C* and Pt atoms could be internal factor

19

leading to the strong photostability of tridentate Pt(II) complexes.

20

In consideration of the 0-0 transitions, for the Complexes 3 and 3-1-5, the emission

21

wavelengths are 479, 543,497, 559, 547 and 503 nm, respectively. By comparing the

22

calculated value (479 nm) with the experimental one (476 nm),34 it suggests that the

23

0-0 transition is very well suited to obtain the maximum emission wavelength in this

24

system. The computed emission wavelengths imply the emission wavelengths can be

25

effectively controlled adopting the various coordinate forms. Especially, the blue-shift 9



1

of emission wavelength will occur when the C*(NHC) coordinate with the Pt atom.

2

3.2 The influence of structural modification on the main ligand dissociation

3 4 5

Scheme 2. The geometries of Complex 3-7-8.

6

To illustrate the influence of π conjugation and the substituent group on the main

7

ligand dissociation, Complex 3-6, 3-7 and 3-8 were designed (Scheme 2). Since the

8

thermodynamic criterion should always be met to completely avoid ligand

9

dissociation, thus, the thermodynamic criterion, firstly, was taken into account. The

10

∆Greact for main ligand dissociation and the E(T1) were computed. For the Complex 3

11

and Complex 3-6, they have the nearly identical ∆Greact (see Figure 2), implying the

12

substituent group located on NHC ligand can cause the insignificant effect on the

13

main ligand dissociation. As well, the π conjugation can result in the slight change of

14

the photostability for the main ligand since the ∆Greact values are similar. In summary,

15

the Gibbs free energy for main ligand dissociation is closely related to the bond order

16

of coordinate bonds.

17

In here, the emission wavelengths for the Complexes 3-6-8 were obtained via

18

considering the 0-0 transitions and the values are 494, 443 and 455 nm, respectively.

19

Since the π conjugation and substituent group can result in the slight change of the

20

photostability for the main ligand, thus, by reasonably decorating the main ligand of

21

tridentate Pt(II)-NHC complexes is a sagacious strategy to obtain the stable blue Pt(II)

22

complex.

10


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1 2

Figure 2. The calculated PWPB95-D3BJ Gibbs Free main ligand dissociation

3

energies (eV) and the double adiabatic triplet energies (eV) for Complexes 3-6-8.

4

3.3 The role of auxiliary ligand in the photostability of tridentate Pt(II)

5

complexes

N

N N

N

N

N

Pt

N N

Pt

N

N Pt

N

N C

Cl

6 7

Complex 3-8

N

Complex 3-8-1

Complex 3-8-2

Scheme 3. The geometries of the Complex 3-8, 3-8-1 and 3-8-2, respectively

8

Finally, the exploration was also carried out to illustrate the role of auxiliary ligand

9

in the photostability, and thus, the Complex 3-8-1 and 3-8-2 were designed based on

10

the Complex 3-8 (Scheme 3). Similar to above-mentioned discussions, the

11

thermodynamic criterion is first considered. Hence, the Gibbs free energies and

12

adiabatic triplet energies were calculated. As shown in the Figure 3, the relationship

13

between Gibbs free energies and adiabatic triplet energies shows that the

14

photostability of tridentate Pt(II) complex will be controlled by means of utilizing 11



1

diverse auxiliary ligands. For the auxiliary ligand dissociation does not meet the

2

thermodynamic criterion, suggesting that the auxiliary ligand dissociation reaction is

3

energetically feasible. Nevertheless, for Complex 3-8-1 and 3-8-2, the auxiliary

4

ligand dissociation becomes more difficult as compare to that of Complex 3-8.

5

Therefore, one can anticipate that the auxiliary ligand dissociation can be prohibited

6

when the auxiliary ligand with strong electron-donating and electron-withdrawing are

7

employed. Although the auxiliary ligand dissociation of these complexes might

8

happen for practical PhOLEDs applications, this general design approach should help

9

improving complex stability. Moreover, we also calculated ∆Greact values for the main

10

ligand dissociation reactions of Complex 3-8, Complex 3-8-1 and 3-8-2, respectively.

11

All main ligand dissociations are highly endergonic and they are located significantly

12

above the energetic threshold of the hot states. Accordingly, it can estimate that the

13

main ligand dissociations cannot take place for the Complex 3-8, Complex 3-8-1 and

14

3-8-2 upon PhOLED operation.

15 16

Figure 3. The calculated PWPB95-D3BJ Gibbs Free main and auxiliary ligand

17

dissociation energies (eV) and the double adiabatic triplet energies (eV) for

18

Complexes 3-8, 3-8-1 and 3-8-2, respectively. ∆GreactM: Gibbs Free main ligands

19

dissociation energies, ∆GreactA: Gibbs Free auxiliary ligands dissociation energies. 12


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1

While the ligand dissociation do not meet the thermodynamic criterion, i.e.,

2

∆Greact>2E(T1), the excited state kinetic does play an important role in modulating the

3

degree of degradation of complex.19 As above-mentioned, the vast majority of hot Tn

4

states rapidly decay back nonradiatively to the T1 geometry. In addition, the

5

spectroscopic35 and computational investigations36,37 have confirmed the active role of

6

metal centered triplet excited states, 3MC states, in the excited state dynamics of the

7

organometallic complexes. The 3MC states can be thermally populated from the T1

8

state surpassing the corresponding barriers, TS. Once formation of the 3MC states,

9

there are two processes might occur. One is the 3MC states reversible return to the T1

10

state well. Another is the 3MC states irreversible access to the ground state (1GS)

11

geometry via the 1GS/3MC minimum energy crossing point (MECP). At the room

12

temperature, the T1-3MC equilibration is controlled by the magnitude of the activation

13

barriers, including the ∆E(T1-TS), ∆E(TS-3MC) and ∆E(3MC-MECP), respectively. A

14

straightforward approach to prevent the thermal population of the 3MC state is to

15

maximize the ∆E(T1-TS) values. Figure 4 presents the schematic potential energy

16

profiles of the deactivation pathways and the ∆E(T1-TS), ∆E(TS-3MC) and

17

∆E(3MC-MECP) for the Complex 3-8 and 3-8-2, respectively. The geometries of

18

excited states, for Complex 3-8 and 3-8-2, are collected in the Figure S9. As shown in

19

the Figure S9, the displacement vectors of transition states can clearly reflect the

20

obtained results are reliable. Compare the emissive states (T1), the optimized 3MC and

21

MECP structures show distinct distortions caused by the auxiliary ligands, which

22

away from the plane. Without doubt, the distort geometries of 3MC and MECP origin

23

from the change of Pt-X bonds, enlarge or shorten. (X is the coordinated atoms.) The

24

geometrical features of the optimized 3MC and MECP structures are similar to those

25

in the article reported by Wai Han Lam and co-workers.38

26

The potential energy profile of Complex 3-8-1 is not shown because the 3MC state

27

is adiabatically located lower in energy than the emissive state, indicating that the

28

3

29

nonradiative pathway is the most prominent deactivation channel, causing a complete

30

quench of phosphorescence. Combing the kinetic considerations and under

MC state can be populated in a barrierless manner. So, for the Complex 3-8-1, this

13



1

steady-state conditions, TTA will predominantly take place in Complex 3-8-1

2

between two 3MC states. In comparison, the positive ∆E(T1-3MC) value and large

3

∆E(T1-TS) value for the Complex 3-8 greatly prevents the formation of the 3MC state.

4

Hence, at room temperature, the equilibration process between the T1 and 3MC states

5

is not plausible and the TTA processes mainly occur between two T1 states in the

6

Complex 3-8. Moreover, in view of the potential energy profiles of Complex 3-8-2,

7

at room temperature, the T1-3MC equilibration will be pushed towards the T1 state

8

under steady-state conditions. Consequently, these complexes exhibit three different

9

types of kinetic features, and one can estimate that the Complex 3-8-1 may

10

experience fast degradation upon PhOLED operation on the basis of article proposed

11

by Escudero and co-workers19. But, the larger tendency to form the 3MC states in the

12

Complex 3-8-1 (enhanced kinetic liability) cannot facilitate the ligand dissociation,

13

reducing the intrinsic photostability. The geometric properties in 3MC states account

14

for the different ultimate reason for intrinsic photostability of the Ir(III) and tridentate

15

Pt(II) complexes. One iridium-heteroatom single bond is, for the Ir(III) complexes,

16

broken because the corresponding geometries in 3MC states display a trigonal

17

bipyramid arrangement. Whereas the coordinate bonds of the tridentate Pt(II)-NHC

18

complexes are connected in the 3MC states, although the obviously geometric

19

deformations have occurred. The electron localization function (ELF) analyses were

20

investigated again to elucidate the strength of coordinate bonds and the related

21

color-filled maps are drawn in the Figure S8. On the basis of the ELF analysis in SI,

22

one can conclude that of the electron delocalized feature tridentate Pt(II) complex can

23

be efficiently controlled by reasonably choosing the auxiliary ligand.

14


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) C S/ 3 M P( 1 G EC M

Energy(kcal/mol)

M

EC

P( 1 G S/ 3 M

Energy(kcal/mol)

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


C )

Page 15 of 22

1

Figure 4. Schematic potential energy profiles of the deactivation pathway for the

2

Complex 3-8 and 3-8-2, respectively. 3ES refers to the T1 state in this system. (The

3

profiles are constructed via employing the B3LYP-D3 method.)

4 5

4. Conclusions

6

In this article, the dispersion corrected hybrid functional B3LYP-D3 and

7

double-hybrid PWPB95-D3BJ functional were employed to explore the role of

8

Pt-C*(NHC) bond and auxiliary ligand in the photostability of tridentate Pt(II)

9

complexes upon PhOLEDs operation. On the basis of both kinetic and thermodynamic

10

criteria, the photostability of tridentate Pt(II) complexes were unveiled in detail and

11

the conclusions can be drawn below:

12

(1). Pt-C*(NHC) bond plays an active role for the photostability of tridentate Pt(II)

13

complexes. Whilst the corresponding emission wavelength is found to be blue-shift, it

14

is certainly a promising approach to design the stable blue Pt(II) complex.

15

(2). The substituent group and π conjugation located on NHC ligand can cause

16

slight effect on the main ligand dissociation, that may potentially provide a strategy to

17

obtain the stable Pt(II) complex with various colors via the structure modification.

18

(3). In view of the calculated results of dissociations, we found the Gibbs free

19

energies of dissociation reactions are closely related to the auxiliary ligands. Thus,

20

tridentate Pt(II) complexes design should be with a careful choice of the auxiliary

21

ligands.

22

In summary, employing the Pt-C*(NHC) coordinate bonds and choosing the 15



Page 16 of 22

1

suitable auxiliary ligands look a very promising method to prevent the ligand

2

dissociation reactions. Moreover, the stable tridentate Pt(II) complexes with different

3

colors can be realized via using the various substituent groups and π conjugations,

4

because they act as an insignificant role for preventing the ligand dissociation.

5

The photostability of a series of tridentate Pt(II) complexes are predicted by the

6

means

of

PWPB95-D3BJ/def2-svp//B3LYP-D3/6-31G(d,p)

7

corresponding experimental verification of the theoretical predictions on a larger set

8

of OLED molecules might be the subject for future work.

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 16


protocol.

The

Page 17 of 22 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


1

ASSOCIATED CONTENT

2

Supporting Information

3

Huang-Rhys

4

PWPB95-D3BJ Gibbs Free ligand dissociation energies for heterolytic reactions, ELF

5

analysis, xyz coordinate. This material is available free of charge via the Internet at

6

http://pubs.acs.org.

7

AUTHOR INFORMATION

8

Corresponding Author

9

[email protected] (D. Tang)

factors,

X-ray

structure

versus

the

computational

structure,

10

Notes

11

The authors declare no competing financial interest.

12

ACKNOWLEDGMENTS

13

This work was supported by the National Natural Science Foundation of China (Grant

14

No. 21573030), and the program for Innovation Team Building at Institutions of

15

Higher Education in Chongqing (CXTDX201601036). The calculations were

16

performed at the National Supercomputing Center in Shenzhen (Shenzhen Cloud

17

Computing Centre).

18 19 20 21 22 23 17



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Table of Content

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The Strategy Used for Controlling the Photostability of Tridentate Pt(II

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