Theoretical study on photophysical property of cuprous bis-phenanthroline coordination complexes

Organic Electronics 13 (2012) 2627–2638

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Theoretical study on photophysical property of cuprous bis-phenanthroline coordination complexes Lu-Yi Zou a,b, Ming-Shuo Ma a, Zi-Long Zhang b, Hui Li b, Yan-Xiang Cheng b,⇑, Ai-Min Ren a,⇑ a b

State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, PR China State Key Laboratory of Polymer Physics and Chemistry, Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China

a r t i c l e

i n f o

Article history: Received 1 June 2012 Received in revised form 19 July 2012 Accepted 20 July 2012 Available online 14 August 2012 Keywords: Density functional theory Electronic structure Cuprous bis-phenanthroline coordination complexes Counteranion effects Phosphorescence quantum yields

a b s t r a c t The electronic structures and photophysical properties of phenathroline ligands coordinated to Cu(I), which are substituted in the 2,9-positions with methyl, phenyl, trifluoromethyl and tert-butyl groups, has been studied by density functional theory (DFT) and time-dependent DFT (TDDFT). To investigate the role played by counteranion in these complexes, the highest occupied orbital energies (HOMO), the lowest virtual orbital energies (LUMO), DH–L (the energies difference between the HOMO and LUMO), the lowest excitation energies (ES1), ionization potentials (IPs), electron affinities (EAs) and reorganization energies (k) were computed. And through the study of the geometric relaxations, d-orbital splitting and spin-orbit couplings (SOC) at their optimized S0 and T1 geometries, non-radiative and radiative decay rate constants (knr and kr) were determined, for comparing and analyzing the different size and push/pull substituents effect on the luminescence quantum yield. Considering these factors, the dtbpdmp complex with tert-butyl group in the 2,9-positions has faster kr and singlet-triplet intersystem crossing (ISC) but slower knr, which leads to its higher photoluminescent quantum efficiency as compared to the methyl-, phenyl- and trifluoromethyl-based complexes. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Phosphorescent metal complexes have become the research core in the field of optoelectronic materials since the Pt-porphyrin complexes have been successfully applied in organic light emitting diodes (OLEDs) in 1998 [1]. It is because that OLEDs emerge as a preeminent technology in flat panel displays with superior viewing quality, efficiency and durability. Although the second and third row transition metal complexes (such as Ru(II), Pd(II), Os(II), Re(I) and Ir(III)) have been most studied in this area, the high costs and environmental concerns result in inherent limitations [2–4]. Therefore, an attractive alternative is emerging, Cu(I) complexes, which are developed to solve the problems. The Cu(I) complexes consist of non-toxic, ⇑ Corresponding authors. Fax: +86 431 85262106, +86 431 88945942. E-mail addresses: [email protected] (Y.-X. Cheng), aimin.ren@gmail. com (A.-M. Ren). 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.07.027

inexpensive, and environmentally friendly copper ion, but present the same photophysical characteristics compare to other metal complexes [5–7]. Here, we investigate the microcosmic mechanism of the photophysical properties of phenathroline ligands coordinated to Cu(I), which are substituted in the 2,9-positions with methyl, phenyl, trifluoromethyl or tert-butyl groups. Although studies on the metal-to-ligand charge transfer (MLCT) state of [Cu(NN)2]+ using quantum mechanical calculations and fluorescence lifetime measurements were carried out [8–13], to our knowledge, no detailed studies of the optimization with counterion to the chemical nature have been performed. Also, a systematic comparison and analysis of the different size and push/pull substituents effect on the luminescence quantum yield in one work is rare. The aim of the present work is to provide an in-depth understanding of the optical and electronic properties of these complexes, and to offer some directions for future research.

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2. Computation methods All of calculations of these studied complexes in this work have been performed with Gaussian 09 program package [14]. Geometric and electronic structures of the considered complexes, as well as their cationic and anionic structures, were fully optimized by DFT using the B3PW91 hybrid functional combined with the 6-31G(d) basis set except Cu. The quasi-relativistic pseudopotentials of Cu proposed by Hay and Wadt with 19 valence electrons were employed, and a ‘‘double-n’’ quality basis set LANL2DZ was adopted as the basis set [15–17]. The lowest triplet excited geometries were computed by DFT with the unrestricted B3PW91 (UB3PW91) and the configuration interaction with single excitations (CIS). Full geometry optimizations without symmetry constraints were carried out in the gas phase for both the singlet ground state and the lowest triplet excited state. Following these states optimization, the vibrational frequencies were calculated and the results showed that all optimized structures are stable geometric structures. The absorption and emission wavelengths of these complexes were systematically investigated by employing TDDFT method.

3. Results and discussion 3.1. Geometry optimization The sketch maps of the studied complexes are depicted in Fig. 1, and the optimized S0 and T1 structures of these complexes are plotted in Fig. 2. It can be clearly seen from Fig. 2 that the [Cu(NN)2]+ complexes in S0 state are nearly orthogonal orientation of the two phenanthroline planes except the 2 and 9 positions with phenyl group. It seems that the p–p interactions cause considerable distortion from 90°. The dihedral angle between the phenanthroline planes (DHA) of the [Cu(NN)2](PF6) complexes in S0 state is significantly different from their [Cu(NN)2]+ complexes except bfp. It can be deduced that the structural distortion in S0 state strongly correlates with the presence of the counteranion in the 2 and 9 positions by introducing electron-donating

R1 R2

N1

N3

Cu

N4 N2

R2 R1

R1 = R2 = H, [Cu(Phen)2]+ R1 = R2 = CH3, [Cu(dmp)2]+ R1 = R2 = Ph, [Cu(dpp)2]+ R1 = R2 = CF3, [Cu(bfp)2]+ R1 = CH3, R2 = t-Bu, [Cu(dtbp)(dmp)]+ Fig. 1. Sketch map of the studied structures.

group. However, in T1 state, there is nearly no effect on the DHAs of the [Cu(NN)2]+ complexes relative to their [Cu(NN)2](PF6) complexes except dtbpdmp. The selected important bond lengths and angles, and dihedral angle DHA of these complexes in both the S0 and T1 states are listed in Tables 1a and 1b, associated with data from the X-ray structure. The results show that the adopted basis set and functional can reflect the change trend of the optical and electronic properties. In the all S0 optimized geometries, the calculated Cu–N bond lengths are in general longer than the data from experimental X-ray structures [18–21], which are carried out in isolated gas phase lacking of crystal packing forces. For the [Cu(NN)2]+ complexes, the average bond length (ABL) of Cu–N in dmp, dpp, bfp and dtbpdmp are 2.0970, 2.1337, 2.1049 and 2.1488 Å, respectively. There are all lengthened by the steric bulk of the 2 and 9 substituents compared with phen (2.0943 Å). And the ABL of Cu–N increases as the steric bulk of the 2 and 9 substituents become larger. However, the ABL of Cu–N in [Cu(NN)2](PF6) complexes for dmp, dpp, bfp and dtbpdmp are 2.0967, 2.1226, 2.1000 and 2.1446 Å, respectively. The ABL of Cu– N in [Cu(NN)2](PF6) complexes are all shorter than the one in [Cu(NN)2]+ complexes, respectively. The structures are properly tightened by the presence of the counteranion, resulting in increase of the probability charge transfer. To observe the dihedral angle between the phenanthroline planes of these complexes, we find that the calculated DHAs are almost 90° in [Cu(NN)2]+ complexes. However, the calculated DHAs in [Cu(NN)2](PF6) complexes have deviation significantly from 90°, which range from 71.49° to 89.95°, smaller than that expected for regular tetrahedral coordination. Especially in dpp complexes, the DHAs are smaller than others regardless of with or without the presence of counterion, it will be effect on the transition nature. To gain insight into the geometric relaxations, the geometric parameters at T1 state are also collected in Tables 1a and 1b. Compared with S0 state, their T1 state geometry structures change greatly. Especially the Cu–N1 and Cu–N2 bond lengths in [Cu(dtbp)(dmp)]+, their shortened 0.176 Å from S0 to T1. It is benefit to charge transfer from metal to ligand, meaning that the LUMO are mainly localized on the phenanthroline plane with methyl. This point is confirmed by Fig. 3 in the following. On excitation from the S0 to the T1 state, the ABL of Cu–N in [Cu(NN)2]+ and [Cu(NN)2](PF6) complexes for dmp, dpp, bfp and dtbpdmp decrease by 0.0837, 0.0817, 0.0672 and 0.0719, and 0.0751, 0.0420, 0.0508 and 0.0119 Å, accompanied by change in the DHA 22.62°, 14.70°, 11.44° and 0.10°, and 16.48°, 5.89°, 11.84° and 10.71°, respectively. The ABL of Cu–N in [Cu(NN)2](PF6) complexes changes smaller than their in [Cu(NN)2]+ complexes due to the presence of the counteranion constraint. However, the marked structural distortions in dmp support a conclusion that geometric relaxation is therefore more pronounced than in others. It can be concluded that addition of the highly electron-withdrawing trifluoromethyl group and largely steric bulk electron-donating phenyl and tertbutyl groups dramatically prevent the flattening distortion of the copper complex relative to the 2 and 9 positions with methyl group. It can enhance their quantum yields possibly.

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S0-side view

T1-side view

S0-front view

T1-front view

[Cu(Phen)2]+

[Cu(dmp)2]+

[Cu(dmp)2](PF6)

[Cu(dpp)2]+

[Cu(dpp)2](PF6)

[Cu(bfp)2]+

[Cu(bfp)2](PF6)

[Cu(dtbp)(dmp)]+

[Cu(dtbp)(dmp)](PF6)

the code of colors Cu

P

F

N

C

H

Fig. 2. The optimized S0 and T1 structures of these complexes.

In addition, the charge distribution in S0 and T1 states for metal is one of factors to rough estimate the quantum efficiency of the transition metal complex. The differences

between the charges on Cu from S0 to T1 in these complexes are given in Table 2. According to the natural bond orbital (NBO) population analysis, the dtbpdmp has larger charge

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Table 1a Selected bond lengths (Å), bond angles (°), and dihedral angle (°) at both optimized S0 and T1 geometries for these complexes. [Cu(Phen)2]+

Cu–N1 Cu–N2 Cu–N3 Cu–N4 N1–Cu–C2 N3–Cu–C4 N1–Cu–C3 N1–Cu–C4 N2–Cu–C3 N2–Cu–C4 DHA

[Cu(dmp)2]+

[Cu(dpp)2]+

[Cu(bfp)2]+

[Cu(dtbp)(dmp)]+

S0

T1

S0

T1

S0

T1

S0

T1

S0

T1

2.0942 2.0944 2.0942 2.0944 80.65 80.65 125.59 125.53 125.53 125.48 90.00

2.0026 2.0028 2.0026 2.0028 83.11 83.10 155.91 101.96 101.96 155.89 41.72

2.0968 2.0971 2.0968 2.0971 80.85 80.85 125.45 125.42 125.42 125.38 90.00

2.0131 2.0132 2.0133 2.0134 83.47 83.47 140.05 110.33 110.33 140.02 67.38

2.0837 2.1851 2.0856 2.1805 80.32 80.40 129.34 126.30 125.13 121.77 87.55

1.9523 2.0262 2.0695 2.1600 84.33 80.80 145.14 113.25 114.70 124.73 72.85

2.1047 2.1050 2.1050 2.1048 80.77 80.77 125.47 125.49 125.44 125.47 90.00

1.9863 2.1322 1.9700 2.0624 81.98 83.57 148.17 111.30 114.94 121.67 78.56

2.1438 2.1437 2.1540 2.1536 79.29 82.25 119.51 119.59 130.61 130.65 89.87

1.9678 1.9677 2.1860 2.1862 84.69 81.29 124.08 124.10 124.15 124.12 89.97

Table 1b Selected bond lengths (Å), bond angles (°), and dihedral angle (°) at both optimized S0 and T1 geometries for these complexes, together with the experimental data.

Cu–N1 Cu–N2 Cu–N3 Cu–N4 Cu–P N1–Cu–C2 N3–Cu–C4 N1–Cu–C3 N1–Cu–C4 N2–Cu–C3 N2–Cu–C4 DHA a b c

[Cu(dmp)2](PF6)

[Cu(dpp)2](PF6)

S0

T1

S0

Expt.a

T1

[Cu(bfp)2](PF6) S0

Expt.b

T1

[Cu(dtbp)(dmp)](PF6) S0

Expt.c

T1

2.0913 2.1012 2.0996 2.0945 5.5397 80.77 80.70 126.60 118.53 129.43 126.80 81.66

2.0360 2.0669 1.9772 2.0063 6.2790 82.06 84.30 141.88 109.34 111.55 138.59 65.18

2.1554 2.1043 2.1509 2.0797 7.0172 80.52 81.18 121.12 107.82 135.55 132.31 71.49

2.112 2.019 2.082 2.032 8.815 82.5 82.8 98.9 121.4 124.7 142.4 71.24

2.0522 1.9459 2.3293 1.9950 7.3367 84.52 78.19 111.05 108.36 118.33 154.67 77.38

2.0798 2.0962 2.1148 2.1092 8.5840 81.68 80.39 124.86 125.36 125.25 125.73 89.95

2.038 2.053 2.064 2.026 9.077 83.3 83.2 118.4 134.6 117.0 124.2 89.42

2.0323 2.1675 1.9352 2.0617 8.4199 81.06 83.85 148.30 115.42 112.05 120.06 78.11

2.1280 2.1515 2.1701 2.1287 5.7767 79.70 82.23 118.51 117.84 130.96 131.96 84.26

2.080 2.063 2.097 2.085 6.616 81.2 84.4 115.0 115.7 131.0 132.1 89.77

2.0460 2.0341 2.3787 2.0721 3.8233 83.68 79.32 123.16 156.67 102.61 98.24 73.55

Reference [19]. Reference [20]. Reference [21].

transfer than others, no matter whether the counteranion exists or not. It suggests that the larger charge transfer in Cu for dtbpdmp will lead to higher quantum efficiency. 3.2. Frontier molecular orbitals It will be meaningful to examine the HOMO and the LUMO of these complexes. Because a reasonable qualitative indication of the excitation properties and of the ability of electron or hole transportation is close related to the relative ordering of the occupied and virtual orbitals. The hole transport material with the smaller negative value of HOMO should lose their electrons more easily, while the electron transport material with larger negative value of LUMO should accept electrons more easily. The HOMO and LUMO energies were calculated by DFT in this study. The HOMO and LUMO energies, the HOMO–LUMO gaps (DH–L), and the lowest singlet excited energies (ES1) of these complexes are listed in Table 3. It is apparent from Table 3 that the HOMO and LUMO values in [Cu(NN)2]PF6 complexes are all higher than each of [Cu(NN)2]+, respectively. It indicates that the hole transport ability are greatly enhanced by the presence of counteranion. To evaluate further information, the HOMO energy

differences between the [Cu(NN)2]PF6 and [Cu(NN)2]+ are obtained, which are 2.50, 2.18, 2.01 and 2.39 eV for dmp, dpp, bfp and dtbpdmp, respectively. The HOMO energy decreases in the following order: dmp, dtbpdmp, dpp and bfp. Although the electron-donating group results in more effective hole transport ability than the electron-withdrawing group, the influence on the hole injection ability increases with decreasing the electron-donating ability (the group charge of methyl, phenyl and tert-butyl is 0.18876, 0.44806 and 0.31814, respectively) if the substituents are at the 2 and 9 positions. As we know that there are three types of band gaps in experiment, namely, optical band gap, electrochemical band gap, and the band gap from photoelectron spectrum. Among these band gaps, the common one is the optical band gap. The optical band gap is corresponding to the lowest transition (or excitation) energy theoretically going from the ground-state to the first dipole-allowed excited state. Under the implicit assumption that the lowest excited singlet state can be described by only one singly excited configuration, the first dipole-allowed excited state means the lowest excited singlet state (S1) that an electron is promoted from the HOMO to the LUMO. In fact, the optical band gap is the energy difference between the S0

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HOMO-1

HOMO

LUMO

LUMO+1

LUMO+2

LUMO+3

[Cu(Phen)2]+

[Cu(dmp)2]+

[Cu(dmp)2](PF6)

[Cu(dpp)2]+

[Cu(dpp)2](PF6)

[Cu(bfp)2]+

[Cu(bfp)2](PF6)

[Cu(dtbp)(dmp)]+

[Cu(dtbp)(dmp)](PF6)

Fig. 3. Electronic density contours of the frontier orbitals for these complexes.

state and the S1 state. In this work, the optical band gaps of these complexes from the absorption spectra are computed at the TDDFT level and abbreviated as ES1. The DH–L obtained by DFT is larger than the ES1 values from TDDFT calculations [22]. It could be ascribed to the neglect of interelectronic interaction upon the single oneelectron excitation in estimating DH–L. Although there are discrepancies between the computed DH–L and ES1, the variation trend is similar. The energy gaps for DH–L and ES1

in [Cu(NN)2]+ complexes decrease in the order of dtbpdmp, dmp, bfp and dpp. We predict that the maximum absorption spectra of [Cu(NN)2]+ complexes are expected to be gradually red-shifted with the order. To provide the influence of the different substitutions in these complexes directly, the maps of the frontier orbitals of these complexes by GaussView are plotted in Fig. 3. As shown in Fig. 3, it is noted that the electronic cloud distribution of HOMOs in these complexes are mainly

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Table 2 NBO analysis in both S0 and T1 states for Cu of these complexes calculated by DFT.

Cu

[Cu(dmp)2]+ [Cu(dpp)2]+ [Cu(bfp)2]+ [Cu(dtbp)(dmp)]+ [Cu(dmp)2](PF6) [Cu(dpp)2](PF6) [Cu(bfp)2](PF6) [Cu(dtbp)(dmp)](PF6)

S0

T1

Dq(T1  S0)

0.87589 0.91492 0.87637 0.57939 0.56285 0.63521 0.53100 0.58218

0.92390 0.98836 0.90565 0.97055 0.91759 0.99413 0.90391 0.97524

0.04801 0.07344 0.02928 0.39116 0.35474 0.35892 0.37291 0.39306

Table 3 Negative value of the HOMO(eHOMO) and LUMO(eLUMO) energies, HOMO– LUMO gaps calculated by DFT, and the lowest singlet excited energies (ES1) calculated by TDDFT in eV for these complexes.

[Cu(Phen)2]+ [Cu(dmp)2]+ [Cu(dmp)2](PF6) [Cu(dpp)2]+ [Cu(dpp)2](PF6) [Cu(bfp)2]+ [Cu(bfp)2](PF6) [Cu(dtbp)(dmp)]+ [Cu(dtbp)(dmp)](PF6)

eHOMO

eLUMO

DH–L

ES1

8.28 8.17 5.67 7.86 5.68 8.66 6.65 8.15 5.76

4.70 4.58 2.23 4.42 2.49 5.14 3.57 4.54 2.22

3.58 3.59 3.44 3.44 3.19 3.53 3.08 3.61 3.54

2.60 2.52 2.51 2.38 2.52 2.26 2.67 2.63

localized on 3d of Cu (>69%, see Table S1) with small contributions from phenanthroline ligands, while the LUMOs are largely localized on one or two phenanthroline plane. The absorptions of these complexes may primarily arise from the mixing of MLCT, intraligand and ligand-to-ligand charge transfer (ILCT and LLCT). 3.3. Ionization potentials and electron affinities The adequate and balanced transport of both injected electrons and holes are important in optimizing the performance of OLED devices. The IP and EA are well-defined properties that can be calculated by DFT to estimate the energy barrier for the injection of both holes and electrons into the complexes. The optimized structures for both the cationic and anionic states are shown in Fig. 4. From Fig. 4, all the distance between Cu and P in cationic state is markedly shorter than that in anionic state. It means that the hole injection ability will be significantly affected by the presence of counteranion. To confirm the estimate’s validity, the calculated IPs, EAs, both vertical (v; at the geometry of the neutral complex) and adiabatic (a; optimized structure for both the neutral and charged complex), and the extraction potentials (HEP and EEP for the hole and electron, respectively) that refer to the geometry of the ions are listed in Table 4. All the IP(a) and EA(a) values in [Cu(NN)2]PF6 are lower than that of [Cu(NN)2]+, respectively. The ability of the hole injection is dramatically improved by the presence of counteranion, which is agreement with the prediction made by Fig. 4. Measurements of the IP(a) energy differences between the [Cu(NN)2]PF6 and [Cu(NN)2]+ are consistent with the change trend of their HOMO energy, decreasing in the order: dmp (3.24 eV),

dtbpdmp (3.07 eV), dpp (2.36 eV) and bfp (2.12 eV). It also demonstrates the prediction in the HOMO energies discussion earlier. At the microscopic level, the charge transport mechanism in thin film can be described as a self-exchange transfer process, in which an electron or hole transfer occurs from one charged molecule to an adjacent neutral molecule. The rate of intermolecular charge transfer (Ket) can be estimated from Marcus theory [23] given in

 K et ¼ A exp

k 4kb T

 ð1Þ

where A is a prefactor related to the electronic coupling between adjacent molecules, k is the reorganization energy, and kb is the Boltzmann constant, T is the temperature. It will be seen that variations in the reorganization energies, which are exponential component (Eq. (1)), dominate the changes in overall carrier transfer rates as the molecular structures are varied. For efficient charge transport, the reorganization energy requires to be small. At this stage, our discussion focuses on the reorganization energy. The k value is generally determined by fast changes in molecular geometry (the inner reorganization energy ki) and by slow variations in solvent polarization of the surrounding medium (the external contribution ke). In the case of LEDs (there are condensed-state systems), however, the latter contribution is much smaller than the former, so that the former becomes the dominant factor. The calculation method of the inner reorganization energy ki for hole and electron transfer is same as our previous used one in reference [24]. The calculated khole and kelectron are also listed in Table 4. As emitting-layer materials, they need to achieve balance between hole injection and electron acceptance. Furthermore, the lower the k value, the higher the charge transport rate. Without the presence of counteranion, the difference between the khole and kelectron for dtbpdmp is 0.33 eV, implying that dtbpdmp has better transport equilibrium property than others. Therefore, dtbpdmp is a better emitter with high quantum efficiency. The order of the transport equilibrium property of these [Cu(NN)2]+ complexes has coincided with the hole transport rate, which is dtbpdmp > dmp > bfp > dpp. However, we are surprised to find that the differences between the khole and kelectron for dmp and dtbpdmp are 1.42 and 0.88 eV with the presence of counteranion. It maybe ascribe that the type of counteranion improves the hole injection ability greater, resulting in more imbalance between the hole and electron injection. The order of the transport equilibrium property of these [Cu(NN)2]PF6 complexes is also in accord with the hole transport rate, which is dpp > bfp > dtbpdmp > dmp. It can be deduced that the hole transport rate is condition of the transport equilibrium property in these complexes, regardless of with or without the presence of counterion. 3.4. Absorption spectra TDDFT has been used to study the nature and the energy of absorption spectra of these complexes on the basis of the optimized S0 geometries. Simulated absorption spectra of

L.-Y. Zou et al. / Organic Electronics 13 (2012) 2627–2638

Cationic

2633

Anionic

[Cu(dmp)2]+

[Cu(dmp)2](PF6)

[Cu(dpp)2]+

[Cu(dpp)2](PF6)

[Cu(bfp)2]+

[Cu(bfp)2](PF6)

[Cu(dtbp)(dmp)]+

[Cu(dtbp)(dmp)](PF6)

Fig. 4. Geometries of both the cationic and anionic states optimized by DFT calculations.

these [Cu(NN)2]PF6 complexes are sketched in Fig. 5. The transition energies from absorption spectra, oscillator strengths and configurations for the most relevant singlet excited states in each complex are listed in Table 5 accompa-

nying with the experimental results. As shown in Table 5, the oscillator strengths of the lowest S0 ? S1 electronic transition are nearly equal to zero in these complexes, which is a forbidden transition. Whereas, the S0 ? S3 electronic

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Table 4 IPs, EAs, extraction potentials, and reorganization energies for each complex (eV).

[Cu(Phen)2]+ [Cu(dmp)2]+ [Cu(dmp)2](PF6) [Cu(dpp)2]+ [Cu(dpp)2](PF6) [Cu(bfp)2]+ [Cu(bfp)2](PF6) [Cu(dtbp)(dmp)]+ [Cu(dtbp)(dmp)](PF6)

IP(v)

IP(a)

HEP

EA(v)

EA(a)

EEP

khole

kelectron

9.85 9.63 7.14 10.55 6.92 10.55 8.07 9.59 7.19

9.19 9.27 6.03 8.97 6.61 9.84 7.72 9.36 6.29

8.71 8.96 5.26 8.75 6.33 9.36 7.26 9.02 5.35

3.47 3.37 1.03 3.34 1.36 3.90 2.15 3.31 1.05

3.55 3.46 1.25 3.44 1.53 4.01 2.35 3.43 1.55

3.63 3.55 1.49 3.52 1.65 4.10 2.57 3.55 2.01

1.14 0.67 1.88 1.80 0.59 1.19 0.81 0.57 1.84

0.16 0.18 0.46 0.18 0.29 0.20 0.42 0.24 0.96

0.14

dmp dpp bfp dtbpdmp

oscillator strength

0.12 0.10 0.08 0.06 0.04 0.02 0.00 400

450

500

550

600

wavelength (nm) Fig. 5. Simulated absorption spectra of these [Cu(NN)2]PF6 complexes.

transition has the largest oscillator strengths in these complexes, except for [Cu(dpp)2]PF6, [Cu(bfp)2]+ and [Cu(bfp)2]PF6 which are assigned to the S0 ? S6, S0 ? S5 and S0 ? S4 electronic transition, respectively. It means that the HOMOs and LUMOs of dpp and bfp are received affect the presence of counteranion. The oscillator strengths of the absorption maximum in [Cu(NN)2]PF6 are all smaller than that in [Cu(NN)2]+. Also, the HOMO1 to LUMO or HOMO to LUMO + 1 excitation plays a dominant role in these complexes, except dpp. Whether or not the counterion is existed, the complex of dpp has the smaller oscillator strength than others. These results confirm the prediction from the DHAs discussion above. All the electronic transitions are composed by mixed MLCT/ILCT/LLCT. The maximum absorption spectra in [Cu(NN)2]+ complexes for dtbpdmp, dmp, bfp and dpp (433.7, 439.1, 450.9 and 465.8 nm) is gradually red-shifted, which is in accord with the decrease in their DH–L and ES1. However, depend on the presence of counteranion, the calculated and the experimental kabsmax for dmp, dpp, bfp and dtbpdmp are 451.8, 443.6, 490.9 and 442.4 nm, and 454, 448, 462 and 440 nm, respectively [18–21]. Obviously, it is very consistent with the experimental values, and the values of both calculated and experimental have the same sequence in red-shifted. It because that all complexes had

a PF6 counterion and were measured in this electrolyte in experiment. 3.5. Phosphorescence The TDDFT/DFT and TDDFT/CIS methods were used for measuring the emission spectra of the [Cu(NN)2]+ complexes. It can be seen from the results by TDDFT/DFT and TDDFT/CIS that the emission spectra peaks are red and blue shift with respect to the experimental data by 51.1– 131.5 nm and 54.3–130.9 nm, respectively [18–21]. Owing to CIS method does not consider the electronic correlation energy, it always overestimate the excitation energy, thus, the combination of CIS and TDDFT methods are not suitable for quantitive evaluation of kem in here (Table S2). Although the two methods deviate away from the experimental values, the TDDFT/DFT method correctly reflects the change trend of the optical and electronic properties. Table 6 shows that all the calculated emissions originate mainly from HOMO to LUMO (>69%), which is mainly composed by mixed MLCT/ILCT/LLCT. Furthermore, the 0–0 and T1–S0 vertical transition energies of these complexes are obtained on the basis of the present DFT results. The 0–0 transition takes into account the zero-point energies (zpe) of both the optimized S0 and

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L.-Y. Zou et al. / Organic Electronics 13 (2012) 2627–2638 Table 5 Absorption spectra obtained by TDDFT methods for these complexes, together with experimental values.

[Cu(Phen)2]+

[Cu(dmp)2]+

[Cu(dmp)2](PF6)

[Cu(dpp)2]+

[Cu(dpp)2](PF6)

[Cu(bfp)2]+

[Cu(bfp)2](PF6)

[Cu(dtbp)(dmp)]+

[Cu(dtbp)(dmp)](PF6)

a

Electronic transitions

kabsmax (nm)

f

Excitation energies (eV)

Main configurations

S0 ? S1

479.8

0.0000

2.58

S0 ? S3

438.4/458a

0.1906

2.83

HOMO  1 ? LUMO HOMO ? LUMO + 1 HOMO  1 ? LUMO HOMO ? LUMO + 1

0.49 0.49 0.49 0.49

S0 ? S1

477.4

0.0000

2.60

S0 ? S3

439.1/454a

0.1492

2.82

HOMO  1 ? LUMO HOMO ? LUMO + 1 HOMO ? LUMO HOMO  1 ? LUMO + 1 HOMO  1 ? LUMO HOMO ? LUMO + 1 HOMO ? LUMO HOMO  1 ? LUMO + 1

0.44 0.44 0.21 0.21 0.43 0.43 0.24 0.24

S0 ? S1

492.4

0.0022

2.52

S0 ? S3

451.8/454a

0.1258

2.74

HOMO ? LUMO HOMO  1 ? LUMO HOMO  1 ? LUMO + 1 HOMO ? LUMO + 1 HOMO  1 ? LUMO + 1 HOMO ? LUMO HOMO  1 ? LUMO HOMO ? LUMO + 1

0.51 0.33 0.26 0.22 0.44 0.37 0.34 0.21

S0 ? S1

494.8

0.0001

2.51

S0 ? S3

465.8/448a

0.0349

2.66

HOMO  1 ? LUMO + 1 HOMO ? LUMO HOMO ? LUMO + 3 HOMO ? LUMO + 2 HOMO  1 ? LUMO + 2 HOMO  1 ? LUMO + 3 HOMO ? LUMO HOMO  1 ? LUMO + 1 HOMO  1 ? LUMO+3 HOMO ? LUMO+2 HOMO  1 ? LUMO+2

0.46 0.33 0.24 0.23 0.15 0.10 0.49 0.38 0.23 0.15 0.14

S0 ? S1

521.5

0.0037

2.38

S0 ? S6

443.6/448a

0.0138

2.79

HOMO ? LUMO HOMO  1 ? LUMO HOMO ? LUMO+1 HOMO  1 ? LUMO+1 HOMO  1 ? LUMO+2 HOMO ? LUMO+2 HOMO ? LUMO+3 HOMO  1 ? LUMO+1 HOMO  1 ? LUMO+4 HOMO ? LUMO

0.48 0.40 0.25 0.20 0.46 0.31 0.30 0.21 0.14 0.10

S0 ? S1

491.2

0.0000

2.52

0.1404

2.75

HOMO ? LUMO+1 HOMO  1 ? LUMO+2 HOMO ? LUMO+1 HOMO  1 ? LUMO+2

0.48 0.48 0.49 0.49

HOMO  1 ? LUMO+1 HOMO  2 ? LUMO+1 HOMO ? LUMO+1 HOMO  1 ? LUMO+2

0.69 0.14 0.69 0.12

HOMO ? LUMO HOMO  1 ? LUMO+1 HOMO  1 ? LUMO HOMO ? LUMO+1

0.66 0.19 0.52 0.46

HOMO ? LUMO HOMO  1 ? LUMO+1 HOMO ? LUMO+1 HOMO  1 ? LUMO

0.63 0.29 0.53 0.45

a

S0 ? S5

450.9/462

S0 ? S1

548.3

0.0000

2.26

S0 ? S4

490.9/462a

0.0983

2.53

S0 ? S1

464.6

0.0001

2.67

0.0887

2.86

a

S0 ? S3

433.7/440

S0 ? S1

471.6

0.0000

2.63

S0 ? S3

442.4/440a

0.0840

2.80

All compounds had a PF6 counterion and were measured in this electrolyte.

T1 states geometries. The T1–S0 vertical transition energies is the difference between the T1 and S0 states at the T1 optimized geometries, as shown in Fig. 6. In general, the 0–0 transition energies are higher than the experimental data,

whereas the T1–S0 vertical transition energies are in well agreement with the experimental data (Table 7). It is because that the T1–S0 vertical transitions consistent with the Kasha rule. In addition, compared with our studied

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L.-Y. Zou et al. / Organic Electronics 13 (2012) 2627–2638

Table 6 Emission Spectra Obtained by TDDFT Method for [Cu(NN)2]+ Complexes at DFT method Optimized Geometries.

[Cu(dmp)2]+ [Cu(dpp)2]+ [Cu(bfp)2]+ [Cu(dtbp)(dmp)]+ a

kem (nm)

a

823.3 809.8 796.5 697.1

730 715 665 646

Exp.

Excitation energies (eV)

Main configurations

1.51 1.53 1.56 1.78

HOMO ? LUMO HOMO ? LUMO HOMO ? LUMO HOMO ? LUMO

0.76 0.69 0.69 0.70

All compounds had a PF6 counterion and were measured in this electrolyte.

E

where kr and knr are the radiative and nonradiative rate constants, respectively. The knr from the Tm to the S0 states is usually expressed in the form of the energy law Eq. (3) [26], and the kr is given by Eq. (4) [27]:

T1

knr ¼ a exp fb½EðTm ! S0 Þg ( X hTm jHsoc jSn i

Evert

S0

kr ¼ c

0-0

n

v1=2 n ðvn  1Þ

vn ¼ EðSn Þ=EðTm Þ

Fig. 6. Potential-energy surfaces of the S0 and the T1 states. The solid line indicates the vertical transition and the dotted line corresponds to the 0–0 transition.

Table 7 0–0 And vertical (T1 ? S0) transition energies of these complexes obtained from DFT, together with experimental values.

[Cu(dmp)2]+ [Cu(dpp)2]+ [Cu(bfp)2]+ [Cu(dtbp)(dmp)]+

0–0

Evert (eV)

Emission maximum (eV) aexp

2.25 2.14 2.24 2.45

1.74 1.83 1.83 2.09

1.68 1.73 1.87 1.92

a All compounds had a PF6 counterion and were measured in this electrolyte.

[Cu(NN)2]+ complexes, the dtbpdmp has the smallest stokes shift by using either the combination of TDDFT/DFT or the T1–S0 vertical transition energies to research the phosphorescence spectrum. 3.6. The phosphorescence quantum yields Generally, there are three processes to govern the phosphorescence quantum yields /p: (1) singlet–triplet intersystem crossing (ISC), (2) radiative decay from a triplet state to the singlet ground state, and (3) nonradiative decay from an excited state to the ground state. If one emitter has fast processes 1 and 2, and slow process 3, it will be a strong emitter [25]. The Up can be expressed as Eq. (2):

/p ¼

kr kr þ knr

ð2Þ

ð3Þ

)2 fn1=2

ð4Þ

ð5Þ

where a, b and c are constant, vn is the energy ratio between the nth singlet and mth triplet excited states, hTm jHsoc jSn i is the SOC matrix element, and fn is the oscillator strength. From Eq. (3), the knr of an excited state increases as the energy difference between the mth triplet excited state and the singlet ground state decreases [28]. The energy gap law E(T1–S0) of these [Cu(NN)2]+ complexes are listed in Table 6, which are 1.51, 1.53, 1.56 and 1.78 eV for dmp, dpp, bfp and dtbpdmp, respectively. The nonradiative decay rate constant decreases with increasing of E(T1–S0), which is following the order: dmp < dpp < bfp < dtbpdmp. Here, dtbpdmp shows slower nonradiative decay than others. In addition, the nonradiative geometry relaxation of dtbpdmp is also the smallest in the T1–S0 process, because of its smallest DHAs and the smallest stokes shift. Therefore, the dtbpdmp shows slower nonradiative decay than others again. Usually, the d-orbital splittings are critical in determining the phosphorescence efficiency of a transition metal complex [25,27,29]. The smaller the energy difference between two different occupied d orbitals (Dddocc), the larger the Hsoc matrix element and the faster the radiative decay rate constant kr. The large splitting between the highest occupied and the lowest unoccupied d-orbitals (Ddd⁄) may result in thermally inaccessible metal-centered d–d excited states. The calculated Dddocc and Ddd⁄ of these [Cu(NN)2]+ complexes at both the S0 and T1 optimized geometries are listed in Table S4. At the optimized S0 geometries, these complexes nearly have the same value of Dddocc, whereas the dtbpdmp has the largest Ddd⁄ (Table S1). On the basis of T1 optimization, the dtbpdmp has the smaller Dddocc and larger Ddd⁄ than others (S3). Thus, the dtbpdmp will be expected to have high phosphorescence efficiencies. This is consistent with the experimental results. On the other hand, strong SOC and fast ISC rate between the Sn and Tm excited states (named DEST) requires these two

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L.-Y. Zou et al. / Organic Electronics 13 (2012) 2627–2638

3.0

Excitation energy (eV)

S5

S3 S5

2.5

S4 S3

2.0

S3

S4

S2

S3

S2

S2

S1 T2

1.5

S2

S5

S4

S1

S1

S1

T1

T2

T1

T1

dmp

dpp

T1

bfp

dtbpdmp

þ

Fig. 7. Calculated excitation energy levels of these ½CuðNNÞ 2 complexes at their T1 optimized geometries.

states to be close in energy (shown in Fig. 7), leading to the increased kr. To have nonvanishing SOC matrix elements hTm jHsoc jSn i between the Sn and Tm excited states, the spinorbit-coupled singlet and triplet excited states should involve the same unoccupied p⁄ orbital but different occupied d orbitals under the direct SOC assumption [25]. For complexes dmp, dpp and bfp, the S5 excited state is in accord with this standard (Table S5–8). And the oscillator strength f5 is much larger than each of f1–f4, which makes the SOC contribution between the T1 and S5 dominant. However, the S3 excited state has the largest SOC matrix elements with the T1 excited state in dtbpdmp. The DEST decreases with the order of dmp > dpp > bfp > dtbpdmp. Among them, the dtbpdmp has the smallest DEST. From the above discussion, it can be concluded that dtbpdmp with stronger SOC, better ISC rate, and slower nonradiative decay leads to its higher photoluminescent quantum efficiency than others which our studied in this paper. 4. Conclusions In this study, the geometries, electronic structures, photophysical properties and phosphorescence efficiencies of these complexes were investigated by DFT, CIS and TDDFT methods. From the above research, the calculated values of HOMO, LUMO, IPs, EAs and k show that the ability of the hole injection is dramatically improved with the presence of counteranion. If the type of counteranion improves the hole injection ability greater, it will be resulting in more imbalance between the hole and electron injection. The hole transport rate is condition of the transport equilibrium property in these complexes, regardless of with or without the presence of counterion. We also conclude that the dtbpdmp complex has faster kr and ISC but slower knr, which leads to its highest photoluminescent quantum efficiency among the studied complexes. Moreover, the dtbpdmp has smaller DHAs and stokes shift than others, owing to the tert-butyl group dramatically prevent the flattening distortion of the copper complex.

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