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A theoretical investigation on the metal–metal interaction in a series of pyrazolate bridged platinum(II) complexes
Synthetic Metals 205 (2015) 222–227
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
A theoretical investigation on the metal–metal interaction in a series of pyrazolate bridged platinum(II) complexes Shuang Huang a , Baozhu Yang b, * , Jing Zhong b , Hongxing Zhang c a
School of Mathematics & Physics, Changzhou University, Changzhou 213164, China Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China c State Key Laboratory of Theoretical and Computational Chemistry Institute of Theoretical Chemistry, Jilin University, Changchun 130023, China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 31 March 2015 Received in revised form 22 April 2015 Accepted 27 April 2015
Five pyrazolate bridged platinum(II) complexes have been investigated with density functional theory (DFT) and time-dependent density functional theory (TDDFT). In contrast to common pyrazolate bridged platinum(II) complexes, these complexes have same pyrazolate bridged ligand while different cyclometalated C^N ligands. To explore the effects of Pt–Pt interaction on spectra and excited state properties, the ground state, singlet state and triplet state structures have been optimized. The Pt–Pt interaction is assessed by the distance between the two Pt atoms, the Mayer bond order and orbital composition analysis. On the lowest triplet excited state, the intermediate size complex shows PtPt Pt bond contraction is much larger than many reported bridged single bond formation and the Pt platinum complexes. With the Pt–Pt interaction strengthened, the quantum yield and HOMO–LUMO energy gap will decrease. The complex with the biggest cyclometalated C^N ligand shows great potential applications in the design and synthesis of OLED materials since the complex shows the best performance in the injection and transport of hole and electron. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Excited states Metal–metal interaction Pyrazolate bridged platinum(II) complex OLED
1. Introduction Dinuclear transition-metal complexes, such as Pt(II) [1], Pd(II) [2], Rh(II) [3], Ir(III) [4] complexes, have received considerable attention because of their intriguing photophysical properties and the promising applications in many fields [5]. The short metal– metal distance in these complexes will lead to strong metal–metal interaction by dz2–dz2 overlap, and give rise to ds and ds* orbitals, then a metal–metal-to-ligand charge transfer [6] (MMLCT) from the filled ds* orbital to the empty p* orbitals of aromatic ligand will be introduced. Previous studies on these dinuclear metal complexes have shown the energy levels of frontier molecular orbitals are directly correlated to metal–metal interaction, which provided an easily and predicted way to tune the absorptions and photoluminescence properties [7]. During the past few decades, pyrazolate bridged platinum dimers have generated great interest [8] since Minghetti and Banditelli performed the investigations first [9]. The Pt–Pt interaction had been proved to play an important role in controlling charge transfer,
* Corresponding author. Tel.: +86 51986330253. E-mail address: [email protected] (B. Yang). http://dx.doi.org/10.1016/j.synthmet.2015.04.013 0379-6779/ ã 2015 Elsevier B.V. All rights reserved.
emission color, excited-state lifetime, and photoluminescence quantum yield by affecting the frontier molecular orbitals. Usually, the Pt–Pt interaction is tuned by steric bulk at the 3,5-positions of the pyrazolate bridging ligands, which looks like scaffolding to alter the metal–metal distance. The more steric hindrance provided by the pyrazolate substituents the closer the two Pt atoms. This type of platinum system exhibited potential applications in many fields, such as chemosensors, photocatalysts, anticancer drugs delivery, and organic light emitting devices (OLEDs) [1c]. In contrast to the common method of changing 3,5-positions of pyrazolate ligand, there has been little research about changing cyclometalating ligand to affect the Pt–Pt distance. Recently, Han et al. reported a pyrazolate bridged platinum(II) binuclear complex with fluorin substituent on the cyclometalating ligand [10]. This complex displays a photoinduced molecular structure change in the excited state, then leading to interesting dual emission in certain circumstances. In this work, we calculated the photophysical and excited state properties of five “butterfly type” pyrazolate bridged platinum dimers which are shown in Fig. 1. The work focus on two points: (1) will the Pt–Pt interaction be affected by different cyclometalating C^N ligand? (2) What are the relations between Pt–Pt interaction and spectra, electron excitation, and quantum yield. Since the excited-state distortion occurs so fast that
S. Huang et al. / Synthetic Metals 205 (2015) 222–227
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Fig. 1. Sketch map of the pyrazolate bridged platinum(II) complexes.
it is hard to be observed on experiment. We hope our research will help us to design new chemosensors and phosphorescent materials. 2. Computational details and theory Density functional theory [11] (DFT) were used to optimized the ground state (S0) and the lowest triplet excited state (T1) structures of complexes 1–5. Based on the optimized structures, the spectroscopic properties related to the absorption and emission in tetrahydrofuran solution were calculated by time-dependent density functional theory (TDDFT) [12] with polarized continuum model (PCM) [13]. The Stuttgart effective core potentials (ECPs) and the associated basis set were applied to describe Pt [14] atom and 6-31G(d) basis set [15] were used to nonmetal atoms. All the calculations were accomplished by Gaussian 09 software package [16]. As we know, the functional is crucial to the TDDFT calculation results. According to our previous experience [17], we tested the hybrid functionals PBE0 [18] and M062X [19], and two long range corrected functionals CAM-B3LYP [20] and vB97XD [21]. As we expected, M062X functional are more suitable to calculate metal– metal interaction in N-heteroleptic aromatic ligand, since the weak interaction has also been considered in this functional. As mentioned above, the metal–metal interaction will affect the frontier molecular orbitals and then alter the charge transfer characters. To get an in-depth understanding about metal–metal interaction, we calculated the Mayer bond order of Pt Pt bond proposed by Mayer and Salvador [22] and analysed the molecular orbital composition with C-squared population analysis (SCPA) method proposed by Ros and Schuit [23], which divided orbital cross term according the proportion of the square of the basis function coefficients. The electron excitation analyses were performed with Multiwfn software [24]. 3. Results and discussion 3.1. Structures and photophysical properties
sum of the covalent radii of two platinum atoms is 2.72 Å [26]. With the “wings” C^N ligands extended, the conjugation effect enhaced, while there is no obvious change about the Pt–Pt distance on the ground state. On the lowest singlet state (S1), the Pt–Pt distances and u angles have a little increase compared to those in ground state. On the lowest triplet state (T1), the Pt–Pt distances of the complexes other than 3 have a little increase relative to the ground state. While the Pt–Pt distance of complex 3 is 2.6752 Å and u angle is 113.6 , which are decreased by 0.8 Å and 18.5 than those of ground state. The Pt Pt bond contraction of complex 3 is much larger than many bridged Pt complexes [1b,27], which are about 0.2 Å. To give a visualized image, the structure of complex 3 on T1 state is depicted in Fig. 2. It seems like the molecular “butterfly” holds its wings up on the triplet state. The Pt–Pt distance of 3 (2.6752 Å) is shorter than the sum of covalent radii of two platinum atoms (2.72 Å), which indicates that there may be a Pt Pt bond. To verify the Pt–Pt interaction, the Mayer bond order of Pt Pt bond was calculated. The calculation result implies there is almost a single bond between the two platinum atoms since the Mayer bond order is 0.82. The highest occupied molecular orbital (HOMO) shown in Fig. 2 indicates the d orbitals of the two platinum atoms were overlapped, and gave rise to ds orbital. Why only complex 3 shows Pt Pt bond formation on the triplet excited state? From the perspective of molecule structure, complex 3 is intermediate in size and the interaction between the cyclometalated C^N ligands is appropriate. In another word, the “wings” are strong enough and not too heavy. On the basis of the optimized structures, the absorption spectra, fluorescence and phosphorescence data have been calculated with TDDFT method in PCM model. The results in terms of the excitation energies, excitations with maximum CI coefficients, oscillator strengths and the experimental values of complexes 2 and 3 are listed in Table 2. For the absorptions, only the S1 state and the state Table 1 The Pt–Pt distances of complexes 1–5 in the ground state (G0), the lowest singlet state (S1), and the lowest triplet state (T1). Complex
The optimized structure parameters of 1–5 are shown in Table 1. The u angle is defined as the angle between PtN bond and N N bond when all atoms are projected onto a flat surface as shown in Fig. 1. On the ground state (G0), the Pt–Pt distances are about 3.49 Å which are agreement with Lai’s et al. reporting [25]. This indicates that there is no metal–metal interaction since the
1 2 3 4 5
Pt–Pt (Å)/u (deg) G0
S1
T1
3.4934/132.1 3.4914/135.0 3.4931/132.1 3.4758/132.8 3.4892/134.9
3.5271/134.3 3.5096/135.3 3.4992/133.5 3.4863/134.1 3.5280/135.8
3.5118/134.6 3.4955/134.8 2.6752/113.6 3.4924/134.9 3.4887/134.6
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Fig. 3. The energy levels of partial molecular orbitals in the ground state. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
Fig. 2. The frontier molecular orbital diagrams of complex 3 on T1 state.
with the biggest oscillator were shown and all the absorption spectra were plotted in Figs. S1–S5 by Multiwfn [24] software together with the contribution from individual transitions. The lowest-energy absorptions of these complexes show a red shift in the order 1 < 2 < 3 < 4 < 5. The same order also occurs in the phosphorescence and fluorescence emission data. To explore the nature of the red shift, we analyzed the composition of the frontier molecular orbitals in ground and T1 state inTables S1 and S2. To give a visualized image, the energy levels of the partial frontier molecular orbitals in ground and T1 state were plotted in Fig. 3 and Fig. S7, respectively. In Fig. 3, we can easily find that the energy levels of the LUMOs (red line) in ground state decline from complex 1 to complex 5, which indicates that the LUMOs are gradually stabilized with the conjugation effect enhanced from 1 to 5. The LUMOs are mainly localized on C^N ligand as shown in Tables S1 while the HOMOs (blue line in Fig. 3) are mainly constituted by Pt and C^N ligand Table 2 The absorption, phosphorescence and fluorescence data of complexes 1–5. Excitation (coefficient) Absorption 1 S1 H ! L (0.55) S18 H 5 ! L (0.43) 2 S1 H ! L (0.58) S9 H 4 ! L (0.46) 3 S1 H ! L (0.47) S5 H 3 ! L + 1 (0.46) 4 S1 H ! L + 1 (0.56) S5 H 3 ! L (0.59) 5 S1 H ! L (0.66) S7 H 4 ! L (0.46)
Oscillator Enm (eV)
0.1669 0.4345 0.1471 0.3745 0.1906 0.1528 0.1132 0.2084 0.5145 0.2494
Phosphorescence 1 T1 H ! L (0.67) 2 T1 H ! L (0.61) 3 T1 H ! L (0.68) 4 T1 H ! L (0.69) 5 T1 H ! L (0.71) Fluorescence 1 S1 H ! L 2 S1 H ! L 3 S1 H ! L 4 S1 H ! L 5 S1 H ! L a
(0.69) (0.66) (0.68) (0.68) (0.69)
From Ref. [7a].
0.0592 0.0491 0.0965 0.1094 0.0874
Expa (nm)
Assignment
315 243 325 268 356 302 396 311 441 324
(3.94) (5.11) (3.81) (4.62) (3.48) (4.10) (3.13) (3.98) (2.81) (3.83)
518 530 614 828 997
(2.39) (2.33) 575 (2.02) 604 (1.50) (1.24)
3
379 378 433 478 494
(3.27) (3.28) (2.86) (2.59) (2.51)
MLCT/ILCT MLCT/ILCT MLCT ILCT ILCT
337 314 355 310
MLCT/ILCT MLCT/LLCT MLCT/ILCT MLCT/LLCT MLCT/ILCT MLCT/LLCT MLCT/ILCT MLCT/LLCT ILCT MLCT
MLCT/3ILCT MLCT/3ILCT 3 MLCT 3 ILCT 3 ILCT 3
orbitals. A conclusion can be drawn from Table S1, the more the Pt compositions are in HOMOs the lower the energy levels get. From complex 1 to complex 5, with the conjugation effect enhanced, the energy levels of LUMOs decreased accordingly. At last, the decreasingly HOMO–LUMO energy gaps occurred. The energy levels of metal orbitals (purple line) both in occupied orbital and unoccupied orbital are stable with the conjugation effect enhanced and the energy difference between the two metal orbitals is about 7.6 eV for the complexes. The energy levels of the complexes on the triplet excited state are shown in Fig. S7 and the orbitals are analysed in Table S2. The energy levels of the complexes other than 3 show the same decreasing trend as that in the ground state. For complex 3, the HOMO–LUMO energy gap is about 4.2598 eV which decreased by 1.3 eV compared to that of ground state. The energy level of HOMO was obviously elevated since the HOMO is mainly composed by Pt orbitals as shown in Fig. 2. The Pt–Pt interaction elevated the energy level of the HOMO and lead to a smaller HOMO–LUMO energy gap on the triplet excited state. 3.2. Electron excitation properties To explore the nature of the electron excitation, a new index, Dr proposed by Guido et al. [28] was introduced to measure the charge-transfer length during electron excitation process. This index measures the average hole–electron distance based on the charge centroids of the orbitals involved in the transition. Usually, the threshold is set as 2.0 Å and the transition with Dr below the threshold is defined as local excitation (LE), and the transition which has bigger than 2.0 Å Dr is called charge-transfer excitation (CT). To get the accurate description, we also calculated the hole– electron distance with Mulfiwfn software [24]. The characters of the electron excitations are listed in Table 3. For the triplet excited state, only the transition of complex 3 is charge-transfer (CT) since the Dr and hole–electron distance are all over 2.0 Å. For the singlet excited state, complex 4 has a charge-transfer transition since it has a much bigger Dr (6.72 Å) and hole–electron distance (5.24 Å). To clear see the distribution of hole and electron, we also list them in Fig. 4. The electron and hole are composed by antibonding orbital and bonding orbital, respectively. For complex 2, the electron and hole localized on the same part of C^N ligand, so the Dr and hole–electron distance are small and the transition is local excitation. The electron of complex 3 is localized on C^N ligand and the hole is made of Pt ds orbital. Complex 4 has the biggest Dr and hole–electron distance since they are distributed on the two “wings” of the molecular “butterfly”.
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Table 3 The character of electron excitation, LE and CT represents local excitation and charge transition, respectively.
Dr (Å) 1 2 3 4 5 a b c d e
Dh–e (Å)a
DES1–T1b
Character
T1
S1
T1
S1
T1
S1
(eV)
0.72 2.02 2.92 0.42 0.66
1.61 1.74 1.20 6.72 1.43
0.72 1.48 2.92 0.43 0.68
1.60 1.72 1.19 5.24 1.43
LE LE CT LE LE
LE LE LE CT LE
0.59 0.57 0.88 0.71 0.63
Dd–d*(G0)c
Dd–d*(T1)d
DET1–S0e
7.58 7.62 7.60 7.63 7.73
7.72 7.68 6.06 7.69 7.71
2.97 2.25 0.11 2.14 1.78
The distance between hole and electron calculated with Multiwfn software. The energy gap between S1 and T1 state. The energy difference of d–d* on G0 state. The energy difference of d–d* on T1 state. The energy gap between T1 and S0 state.
3.3. The energy differences of S0, S1 and T1 states The energy differences between S0, S1 and T1 states are relevant with quantum yield. A large splitting between the occupied d orbital and unoccupied d* orbital is desirable, so that the metal center (MC) excited states are thermally inaccessible for emission
quenching [29]. A small energy difference between S1 and T1 state (DES1 T1 ) will lead to a big intersystem crossing (ISC) rate of S1 ! T1. While a big energy difference between T1 and S0 state (DET1 S0 ) will result in a small nonradiative decay rate constant [30]. From Table 3, we can conclude that there may be a smaller intersystem crossing (ISC) rate of complex 3 since the DES1 T1 of 3 is the biggest
Fig. 4. The distribution of electron and hole, (a–d) for complex 2 and 3 on the triplet excited state, (e and f) for complex 4 on the singlet excited state.
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Table 4 Ionization potentials (IPs), electron affinities (EAs), extraction potentials (HEP and EEP), internal reorganization energies (l) (eV) and Pt–Pt distance (Å) for the investigated complexes.
1 2 3 4 5 a b
IPv
IPa
HEP
EAv
EAa
EEP
lhole
lelectron
Da (Å)
Db (Å)
7.2364 7.2291 7.2525 6.9326 6.7662
6.5866 6.7153 6.9562 6.7590 6.7145
5.8839 6.1259 5.7169 6.5785 6.6642
0.1988 0.3795 0.7594 0.8759 1.1607
0.2810 0.4578 0.8564 0.9873 1.2184
0.3618 0.5358 1.0004 1.1561 1.2761
1.3525 1.1032 1.5356 0.3541 0.1020
0.1630 0.1563 0.2410 0.2802 0.1154
2.6966 2.7864 3.0456 3.3171 3.4668
3.5138 3.5168 3.5148 3.4931 3.4868
Pt–Pt distance on the optimized cation structure. Pt–Pt distance on the optimized anion structure.
in these complexes. Meanwhile, complex 3 has the smallest Dd–d* and DET1 S0 , which will lead to a bigger nonradiative decay rate constant. Therefore, we can deduce that the quantum yield of complex 3 will be the smallest in these complexes. This means the quantum yield will decrease with the Pt–Pt interaction strengthened. Our theoretical conclusions agree with Castellano’s experimental results [7a]. In his paper, complex 3 has a smaller quantum yield, a bigger nonradiative decay rate constant and a smaller radiative rate constant than those of complex 2. 3.4. Electroluminescent (EL) properties In this section, we calculated the IPv (vertical ionization potential), IPa (adiabatic ionization potential), EAv (vertical electron affinities), EAa (adiabatic electron affinities), and l (reorganization energy), together with HEP (hole extraction potential) and EEP (electron extraction potential). The results were listed in Table 4 and the computation details could be obtained from our previous reports [31]. It has been proved that the lower l is crucial to improve the performance of charge transport for organic light-emitting device (OLED) material [32]. A smaller IP value will make the hole injecting easier and larger EA value will facilitate electron injection [33]. In Table 4, we find that complex 5 shows great potential applications in design and synthesis of OLED materials. Since complex 5 has the smallest IP values, the biggest EA values, and the smallest lhole and lelectron values. These values imply that complex 5 has the best abilities for the injection and transport of hole and electron in these five complexes. To explore the reason for this phenomenon, we checked the optimized structures of cation and anion and listed the Pt–Pt distances in Table 4. For the cation structures, the Pt–Pt distance of complex 5 is only slightly shortened compared to that of neutral structure. However, the Pt–Pt distances of other complexes are obviously shortened. This means if the molecular “butterfly” holds the “wings” up on cation structure, then the hole injection and transport will be getting harder. For the anion structure, the Pt–Pt distance of complex 5 has no obvious change while those of other complexes have a little increase. This lead to better electron transport ability for complex 5, while the advantage of electron transport is not so obvious than the hole transport. To further explore the Pt–Pt interactions in neutral, cation and anion structures, we calculated the natural charges of the two Pt atoms. The charge differences were listed in Table 5. When the neutral molecule lost an electron, the natural
charges of Pt atoms were increased and the Pt–Pt distance decreased compared to neutral molecule. With the conjugation effect of C^N ligands enhanced from complex 1 to complex 5, the charge differences decreased from 0.39 to 0.11, which resulted in increasingly weaker Pt–Pt interactions and longer Pt–Pt distance. For the anion structures, the natural charges of Pt atoms only decreased a little compared to neutral structures, and then the Pt–Pt distances only have a little increase. This means the minimum energy points of anion structures are closed to those of neutral structures, which can also be verified by the small electron affinities in Table 4. The electron injection ability of complex 5 is far beyond than the other complexes since the EA values are much larger than the others. The reason may be that this molecular “butterfly” has the biggest “wings” and the strongest conjugation effect, and then the structures of cation and anion are the most stable in the series. 4. Conclusions In this paper, we reported the detailed investigation of structures, spectra, excited states and electroluminescent properties for five bridged Pt(II) complexes. In our calculation results, the cyclometalated C^N ligand have little effect on Pt–Pt interaction on the ground state. The lowest-energy absorptions show red shifts from complex 1 to 5. The reason is that the energy levels of HOMOs increased with the Pt composition decreased, while the energy levels of LUMOs decreased with the ligand conjugation effect enhanced. At last, the decreasingly HOMO–LUMO energy gaps occurred. Complex 3 shows great promise in chemosensors design since only complex 3 has a PtPt single bond on the lowest triplet excited state. The reason may be that complex 3 has the proper size and the interaction between the C^N ligands is strong. The quantum yield of complex 3 will be the smallest in these complexes since complex 3 has the smallest Dd–d*,DET1 S0 and the biggest DES1 T1 energy gaps in these five complexes. The Pt–Pt interaction leads to smaller HOMO–LUMO energy gaps and smaller quantum yield, while the injection and transport of electron and hole are getting harder. Complex 5 with the biggest cyclometalated C^N ligand shows great potential applications in the design and synthesis of OLED materials. The reason is that complex 5 has the most stable cation structure and anion structure and the Pt–Pt distance have no obvious change compared to that of neutral structure.
Table 5 The natural charge differences of Pt atoms between neutral and cation, anion structures. Charge differencea
1
2
3
4
5
DCharge (cation–neutral) DCharge (anion–neutral)
0.39 0.09
0.39 0.05
0.30 0.05
0.12 0.06
0.11 0.01
a
The natural charge differences of the two Pt atoms.
S. Huang et al. / Synthetic Metals 205 (2015) 222–227
Acknowledgements This work is supported by the National Natural Science Foundation of China (11404041), the Natural Science Foundation of Jiangsu Provincial Department of Education (Grant SCZ1212400003) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Thanks the Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology (BM2012110). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. synthmet.2015.04.013. References [1] (a) B. Ma, J. Li, P.I. Djurovich, M. Yousufuddin, R. Bau, M.E. Thompson, J. Am. Chem. Soc. 127 (2005) 28–29; (b) J.V. Lockard, A.A. Rachford, G. Smolentsev, A.B. Stickrath, X. Wang, X. Zhang, K. Atenkoffer, G. Jennings, A. Soldatov, A.L. Rheingold, F.N. Castellano, L.X. Chen, J. Phys. Chem. A 114 (2010) 12780–12787; (c) E.J. Rivera, C. Barbosa, R. Torres, H. Rivera, E.R. Fachini, T.W. Green, W.B. Connick, J.L. Colon, Inorg. Chem. 51 (2012) 2777–2784; (d) A.A. Rachford, F.N. Castellano, Inorg. Chem. 48 (2009) 10865–10867. [2] (a) S.-W. Lai, T.-C. Cheung, M.C.W. Chan, K.-K. Cheung, S.-M. Peng, C.-M. Che, Inorg. Chem. 39 (2000) 255–262; (b) J.E. Bercaw, A.C. Durrell, H.B. Gray, J.C. Green, N. Hazari, J.A. Labinger, J.R. Winkler, Inorg. Chem. 49 (2010) 1801–1810. [3] (a) S.F. Rice, H.B. Gray, J. Am. Chem. Soc. 103 (1981) 1593–1595; (b) S.F. Rice, V.M. Miskowski, H.B. Gray, Inorg. Chem. 27 (1988) 4704–4708. [4] (a) H.-Y. Li, T.-Y. Li, M.-Y. Teng, Q.-L. Xu, S. Zhang, Y.-M. Jin, X. Liu, Y.-X. Zheng, J.L. Zuo, J. Mater. Chem. C 2 (2014) 1116–1124; (b) I.V. Kurnikov, L.D. Zusman, M.G. Kurnikova, R.S. Farid, D.N. Beratan, J. Am. Chem. Soc. 119 (1997) 5690–5700. [5] (a) B. Ma, P. Djurovich, S. Garon, B. Alleyne, M.E. Thompson, Adv. Funct. Mater. 16 (2006) 2438–2446; (b) A.J. Esswein, A.S. Veige, D.G. Nocera, J. Am. Chem. Soc. 127 (2005) 16641– 16651; (c) A.F. Heyduk, D.G. Nocera, Science 293 (2001) 1639–1641. [6] V.M. Miskowski, V.H. Houlding, Inorg. Chem. 30 (1991) 4446–4452. [7] (a) A. Chakraborty, J.C. Deaton, A. Haefele, F.N. Castellano, Organometallics 32 (2013) 3819–3829; (b) K. Saito, Y. Nakao, S. Sakaki, Inorg. Chem. 47 (2008) 4329–4337.
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Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
A theoretical investigation on the metal–metal interaction in a series of pyrazolate bridged platinum(II) complexes Shuang Huang a , Baozhu Yang b, * , Jing Zhong b , Hongxing Zhang c a
School of Mathematics & Physics, Changzhou University, Changzhou 213164, China Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China c State Key Laboratory of Theoretical and Computational Chemistry Institute of Theoretical Chemistry, Jilin University, Changchun 130023, China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 31 March 2015 Received in revised form 22 April 2015 Accepted 27 April 2015
Five pyrazolate bridged platinum(II) complexes have been investigated with density functional theory (DFT) and time-dependent density functional theory (TDDFT). In contrast to common pyrazolate bridged platinum(II) complexes, these complexes have same pyrazolate bridged ligand while different cyclometalated C^N ligands. To explore the effects of Pt–Pt interaction on spectra and excited state properties, the ground state, singlet state and triplet state structures have been optimized. The Pt–Pt interaction is assessed by the distance between the two Pt atoms, the Mayer bond order and orbital composition analysis. On the lowest triplet excited state, the intermediate size complex shows PtPt Pt bond contraction is much larger than many reported bridged single bond formation and the Pt platinum complexes. With the Pt–Pt interaction strengthened, the quantum yield and HOMO–LUMO energy gap will decrease. The complex with the biggest cyclometalated C^N ligand shows great potential applications in the design and synthesis of OLED materials since the complex shows the best performance in the injection and transport of hole and electron. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Excited states Metal–metal interaction Pyrazolate bridged platinum(II) complex OLED
1. Introduction Dinuclear transition-metal complexes, such as Pt(II) [1], Pd(II) [2], Rh(II) [3], Ir(III) [4] complexes, have received considerable attention because of their intriguing photophysical properties and the promising applications in many fields [5]. The short metal– metal distance in these complexes will lead to strong metal–metal interaction by dz2–dz2 overlap, and give rise to ds and ds* orbitals, then a metal–metal-to-ligand charge transfer [6] (MMLCT) from the filled ds* orbital to the empty p* orbitals of aromatic ligand will be introduced. Previous studies on these dinuclear metal complexes have shown the energy levels of frontier molecular orbitals are directly correlated to metal–metal interaction, which provided an easily and predicted way to tune the absorptions and photoluminescence properties [7]. During the past few decades, pyrazolate bridged platinum dimers have generated great interest [8] since Minghetti and Banditelli performed the investigations first [9]. The Pt–Pt interaction had been proved to play an important role in controlling charge transfer,
* Corresponding author. Tel.: +86 51986330253. E-mail address: [email protected] (B. Yang). http://dx.doi.org/10.1016/j.synthmet.2015.04.013 0379-6779/ ã 2015 Elsevier B.V. All rights reserved.
emission color, excited-state lifetime, and photoluminescence quantum yield by affecting the frontier molecular orbitals. Usually, the Pt–Pt interaction is tuned by steric bulk at the 3,5-positions of the pyrazolate bridging ligands, which looks like scaffolding to alter the metal–metal distance. The more steric hindrance provided by the pyrazolate substituents the closer the two Pt atoms. This type of platinum system exhibited potential applications in many fields, such as chemosensors, photocatalysts, anticancer drugs delivery, and organic light emitting devices (OLEDs) [1c]. In contrast to the common method of changing 3,5-positions of pyrazolate ligand, there has been little research about changing cyclometalating ligand to affect the Pt–Pt distance. Recently, Han et al. reported a pyrazolate bridged platinum(II) binuclear complex with fluorin substituent on the cyclometalating ligand [10]. This complex displays a photoinduced molecular structure change in the excited state, then leading to interesting dual emission in certain circumstances. In this work, we calculated the photophysical and excited state properties of five “butterfly type” pyrazolate bridged platinum dimers which are shown in Fig. 1. The work focus on two points: (1) will the Pt–Pt interaction be affected by different cyclometalating C^N ligand? (2) What are the relations between Pt–Pt interaction and spectra, electron excitation, and quantum yield. Since the excited-state distortion occurs so fast that
S. Huang et al. / Synthetic Metals 205 (2015) 222–227
223
Fig. 1. Sketch map of the pyrazolate bridged platinum(II) complexes.
it is hard to be observed on experiment. We hope our research will help us to design new chemosensors and phosphorescent materials. 2. Computational details and theory Density functional theory [11] (DFT) were used to optimized the ground state (S0) and the lowest triplet excited state (T1) structures of complexes 1–5. Based on the optimized structures, the spectroscopic properties related to the absorption and emission in tetrahydrofuran solution were calculated by time-dependent density functional theory (TDDFT) [12] with polarized continuum model (PCM) [13]. The Stuttgart effective core potentials (ECPs) and the associated basis set were applied to describe Pt [14] atom and 6-31G(d) basis set [15] were used to nonmetal atoms. All the calculations were accomplished by Gaussian 09 software package [16]. As we know, the functional is crucial to the TDDFT calculation results. According to our previous experience [17], we tested the hybrid functionals PBE0 [18] and M062X [19], and two long range corrected functionals CAM-B3LYP [20] and vB97XD [21]. As we expected, M062X functional are more suitable to calculate metal– metal interaction in N-heteroleptic aromatic ligand, since the weak interaction has also been considered in this functional. As mentioned above, the metal–metal interaction will affect the frontier molecular orbitals and then alter the charge transfer characters. To get an in-depth understanding about metal–metal interaction, we calculated the Mayer bond order of Pt Pt bond proposed by Mayer and Salvador [22] and analysed the molecular orbital composition with C-squared population analysis (SCPA) method proposed by Ros and Schuit [23], which divided orbital cross term according the proportion of the square of the basis function coefficients. The electron excitation analyses were performed with Multiwfn software [24]. 3. Results and discussion 3.1. Structures and photophysical properties
sum of the covalent radii of two platinum atoms is 2.72 Å [26]. With the “wings” C^N ligands extended, the conjugation effect enhaced, while there is no obvious change about the Pt–Pt distance on the ground state. On the lowest singlet state (S1), the Pt–Pt distances and u angles have a little increase compared to those in ground state. On the lowest triplet state (T1), the Pt–Pt distances of the complexes other than 3 have a little increase relative to the ground state. While the Pt–Pt distance of complex 3 is 2.6752 Å and u angle is 113.6 , which are decreased by 0.8 Å and 18.5 than those of ground state. The Pt Pt bond contraction of complex 3 is much larger than many bridged Pt complexes [1b,27], which are about 0.2 Å. To give a visualized image, the structure of complex 3 on T1 state is depicted in Fig. 2. It seems like the molecular “butterfly” holds its wings up on the triplet state. The Pt–Pt distance of 3 (2.6752 Å) is shorter than the sum of covalent radii of two platinum atoms (2.72 Å), which indicates that there may be a Pt Pt bond. To verify the Pt–Pt interaction, the Mayer bond order of Pt Pt bond was calculated. The calculation result implies there is almost a single bond between the two platinum atoms since the Mayer bond order is 0.82. The highest occupied molecular orbital (HOMO) shown in Fig. 2 indicates the d orbitals of the two platinum atoms were overlapped, and gave rise to ds orbital. Why only complex 3 shows Pt Pt bond formation on the triplet excited state? From the perspective of molecule structure, complex 3 is intermediate in size and the interaction between the cyclometalated C^N ligands is appropriate. In another word, the “wings” are strong enough and not too heavy. On the basis of the optimized structures, the absorption spectra, fluorescence and phosphorescence data have been calculated with TDDFT method in PCM model. The results in terms of the excitation energies, excitations with maximum CI coefficients, oscillator strengths and the experimental values of complexes 2 and 3 are listed in Table 2. For the absorptions, only the S1 state and the state Table 1 The Pt–Pt distances of complexes 1–5 in the ground state (G0), the lowest singlet state (S1), and the lowest triplet state (T1). Complex
The optimized structure parameters of 1–5 are shown in Table 1. The u angle is defined as the angle between PtN bond and N N bond when all atoms are projected onto a flat surface as shown in Fig. 1. On the ground state (G0), the Pt–Pt distances are about 3.49 Å which are agreement with Lai’s et al. reporting [25]. This indicates that there is no metal–metal interaction since the
1 2 3 4 5
Pt–Pt (Å)/u (deg) G0
S1
T1
3.4934/132.1 3.4914/135.0 3.4931/132.1 3.4758/132.8 3.4892/134.9
3.5271/134.3 3.5096/135.3 3.4992/133.5 3.4863/134.1 3.5280/135.8
3.5118/134.6 3.4955/134.8 2.6752/113.6 3.4924/134.9 3.4887/134.6
224
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Fig. 3. The energy levels of partial molecular orbitals in the ground state. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
Fig. 2. The frontier molecular orbital diagrams of complex 3 on T1 state.
with the biggest oscillator were shown and all the absorption spectra were plotted in Figs. S1–S5 by Multiwfn [24] software together with the contribution from individual transitions. The lowest-energy absorptions of these complexes show a red shift in the order 1 < 2 < 3 < 4 < 5. The same order also occurs in the phosphorescence and fluorescence emission data. To explore the nature of the red shift, we analyzed the composition of the frontier molecular orbitals in ground and T1 state inTables S1 and S2. To give a visualized image, the energy levels of the partial frontier molecular orbitals in ground and T1 state were plotted in Fig. 3 and Fig. S7, respectively. In Fig. 3, we can easily find that the energy levels of the LUMOs (red line) in ground state decline from complex 1 to complex 5, which indicates that the LUMOs are gradually stabilized with the conjugation effect enhanced from 1 to 5. The LUMOs are mainly localized on C^N ligand as shown in Tables S1 while the HOMOs (blue line in Fig. 3) are mainly constituted by Pt and C^N ligand Table 2 The absorption, phosphorescence and fluorescence data of complexes 1–5. Excitation (coefficient) Absorption 1 S1 H ! L (0.55) S18 H 5 ! L (0.43) 2 S1 H ! L (0.58) S9 H 4 ! L (0.46) 3 S1 H ! L (0.47) S5 H 3 ! L + 1 (0.46) 4 S1 H ! L + 1 (0.56) S5 H 3 ! L (0.59) 5 S1 H ! L (0.66) S7 H 4 ! L (0.46)
Oscillator Enm (eV)
0.1669 0.4345 0.1471 0.3745 0.1906 0.1528 0.1132 0.2084 0.5145 0.2494
Phosphorescence 1 T1 H ! L (0.67) 2 T1 H ! L (0.61) 3 T1 H ! L (0.68) 4 T1 H ! L (0.69) 5 T1 H ! L (0.71) Fluorescence 1 S1 H ! L 2 S1 H ! L 3 S1 H ! L 4 S1 H ! L 5 S1 H ! L a
(0.69) (0.66) (0.68) (0.68) (0.69)
From Ref. [7a].
0.0592 0.0491 0.0965 0.1094 0.0874
Expa (nm)
Assignment
315 243 325 268 356 302 396 311 441 324
(3.94) (5.11) (3.81) (4.62) (3.48) (4.10) (3.13) (3.98) (2.81) (3.83)
518 530 614 828 997
(2.39) (2.33) 575 (2.02) 604 (1.50) (1.24)
3
379 378 433 478 494
(3.27) (3.28) (2.86) (2.59) (2.51)
MLCT/ILCT MLCT/ILCT MLCT ILCT ILCT
337 314 355 310
MLCT/ILCT MLCT/LLCT MLCT/ILCT MLCT/LLCT MLCT/ILCT MLCT/LLCT MLCT/ILCT MLCT/LLCT ILCT MLCT
MLCT/3ILCT MLCT/3ILCT 3 MLCT 3 ILCT 3 ILCT 3
orbitals. A conclusion can be drawn from Table S1, the more the Pt compositions are in HOMOs the lower the energy levels get. From complex 1 to complex 5, with the conjugation effect enhanced, the energy levels of LUMOs decreased accordingly. At last, the decreasingly HOMO–LUMO energy gaps occurred. The energy levels of metal orbitals (purple line) both in occupied orbital and unoccupied orbital are stable with the conjugation effect enhanced and the energy difference between the two metal orbitals is about 7.6 eV for the complexes. The energy levels of the complexes on the triplet excited state are shown in Fig. S7 and the orbitals are analysed in Table S2. The energy levels of the complexes other than 3 show the same decreasing trend as that in the ground state. For complex 3, the HOMO–LUMO energy gap is about 4.2598 eV which decreased by 1.3 eV compared to that of ground state. The energy level of HOMO was obviously elevated since the HOMO is mainly composed by Pt orbitals as shown in Fig. 2. The Pt–Pt interaction elevated the energy level of the HOMO and lead to a smaller HOMO–LUMO energy gap on the triplet excited state. 3.2. Electron excitation properties To explore the nature of the electron excitation, a new index, Dr proposed by Guido et al. [28] was introduced to measure the charge-transfer length during electron excitation process. This index measures the average hole–electron distance based on the charge centroids of the orbitals involved in the transition. Usually, the threshold is set as 2.0 Å and the transition with Dr below the threshold is defined as local excitation (LE), and the transition which has bigger than 2.0 Å Dr is called charge-transfer excitation (CT). To get the accurate description, we also calculated the hole– electron distance with Mulfiwfn software [24]. The characters of the electron excitations are listed in Table 3. For the triplet excited state, only the transition of complex 3 is charge-transfer (CT) since the Dr and hole–electron distance are all over 2.0 Å. For the singlet excited state, complex 4 has a charge-transfer transition since it has a much bigger Dr (6.72 Å) and hole–electron distance (5.24 Å). To clear see the distribution of hole and electron, we also list them in Fig. 4. The electron and hole are composed by antibonding orbital and bonding orbital, respectively. For complex 2, the electron and hole localized on the same part of C^N ligand, so the Dr and hole–electron distance are small and the transition is local excitation. The electron of complex 3 is localized on C^N ligand and the hole is made of Pt ds orbital. Complex 4 has the biggest Dr and hole–electron distance since they are distributed on the two “wings” of the molecular “butterfly”.
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Table 3 The character of electron excitation, LE and CT represents local excitation and charge transition, respectively.
Dr (Å) 1 2 3 4 5 a b c d e
Dh–e (Å)a
DES1–T1b
Character
T1
S1
T1
S1
T1
S1
(eV)
0.72 2.02 2.92 0.42 0.66
1.61 1.74 1.20 6.72 1.43
0.72 1.48 2.92 0.43 0.68
1.60 1.72 1.19 5.24 1.43
LE LE CT LE LE
LE LE LE CT LE
0.59 0.57 0.88 0.71 0.63
Dd–d*(G0)c
Dd–d*(T1)d
DET1–S0e
7.58 7.62 7.60 7.63 7.73
7.72 7.68 6.06 7.69 7.71
2.97 2.25 0.11 2.14 1.78
The distance between hole and electron calculated with Multiwfn software. The energy gap between S1 and T1 state. The energy difference of d–d* on G0 state. The energy difference of d–d* on T1 state. The energy gap between T1 and S0 state.
3.3. The energy differences of S0, S1 and T1 states The energy differences between S0, S1 and T1 states are relevant with quantum yield. A large splitting between the occupied d orbital and unoccupied d* orbital is desirable, so that the metal center (MC) excited states are thermally inaccessible for emission
quenching [29]. A small energy difference between S1 and T1 state (DES1 T1 ) will lead to a big intersystem crossing (ISC) rate of S1 ! T1. While a big energy difference between T1 and S0 state (DET1 S0 ) will result in a small nonradiative decay rate constant [30]. From Table 3, we can conclude that there may be a smaller intersystem crossing (ISC) rate of complex 3 since the DES1 T1 of 3 is the biggest
Fig. 4. The distribution of electron and hole, (a–d) for complex 2 and 3 on the triplet excited state, (e and f) for complex 4 on the singlet excited state.
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Table 4 Ionization potentials (IPs), electron affinities (EAs), extraction potentials (HEP and EEP), internal reorganization energies (l) (eV) and Pt–Pt distance (Å) for the investigated complexes.
1 2 3 4 5 a b
IPv
IPa
HEP
EAv
EAa
EEP
lhole
lelectron
Da (Å)
Db (Å)
7.2364 7.2291 7.2525 6.9326 6.7662
6.5866 6.7153 6.9562 6.7590 6.7145
5.8839 6.1259 5.7169 6.5785 6.6642
0.1988 0.3795 0.7594 0.8759 1.1607
0.2810 0.4578 0.8564 0.9873 1.2184
0.3618 0.5358 1.0004 1.1561 1.2761
1.3525 1.1032 1.5356 0.3541 0.1020
0.1630 0.1563 0.2410 0.2802 0.1154
2.6966 2.7864 3.0456 3.3171 3.4668
3.5138 3.5168 3.5148 3.4931 3.4868
Pt–Pt distance on the optimized cation structure. Pt–Pt distance on the optimized anion structure.
in these complexes. Meanwhile, complex 3 has the smallest Dd–d* and DET1 S0 , which will lead to a bigger nonradiative decay rate constant. Therefore, we can deduce that the quantum yield of complex 3 will be the smallest in these complexes. This means the quantum yield will decrease with the Pt–Pt interaction strengthened. Our theoretical conclusions agree with Castellano’s experimental results [7a]. In his paper, complex 3 has a smaller quantum yield, a bigger nonradiative decay rate constant and a smaller radiative rate constant than those of complex 2. 3.4. Electroluminescent (EL) properties In this section, we calculated the IPv (vertical ionization potential), IPa (adiabatic ionization potential), EAv (vertical electron affinities), EAa (adiabatic electron affinities), and l (reorganization energy), together with HEP (hole extraction potential) and EEP (electron extraction potential). The results were listed in Table 4 and the computation details could be obtained from our previous reports [31]. It has been proved that the lower l is crucial to improve the performance of charge transport for organic light-emitting device (OLED) material [32]. A smaller IP value will make the hole injecting easier and larger EA value will facilitate electron injection [33]. In Table 4, we find that complex 5 shows great potential applications in design and synthesis of OLED materials. Since complex 5 has the smallest IP values, the biggest EA values, and the smallest lhole and lelectron values. These values imply that complex 5 has the best abilities for the injection and transport of hole and electron in these five complexes. To explore the reason for this phenomenon, we checked the optimized structures of cation and anion and listed the Pt–Pt distances in Table 4. For the cation structures, the Pt–Pt distance of complex 5 is only slightly shortened compared to that of neutral structure. However, the Pt–Pt distances of other complexes are obviously shortened. This means if the molecular “butterfly” holds the “wings” up on cation structure, then the hole injection and transport will be getting harder. For the anion structure, the Pt–Pt distance of complex 5 has no obvious change while those of other complexes have a little increase. This lead to better electron transport ability for complex 5, while the advantage of electron transport is not so obvious than the hole transport. To further explore the Pt–Pt interactions in neutral, cation and anion structures, we calculated the natural charges of the two Pt atoms. The charge differences were listed in Table 5. When the neutral molecule lost an electron, the natural
charges of Pt atoms were increased and the Pt–Pt distance decreased compared to neutral molecule. With the conjugation effect of C^N ligands enhanced from complex 1 to complex 5, the charge differences decreased from 0.39 to 0.11, which resulted in increasingly weaker Pt–Pt interactions and longer Pt–Pt distance. For the anion structures, the natural charges of Pt atoms only decreased a little compared to neutral structures, and then the Pt–Pt distances only have a little increase. This means the minimum energy points of anion structures are closed to those of neutral structures, which can also be verified by the small electron affinities in Table 4. The electron injection ability of complex 5 is far beyond than the other complexes since the EA values are much larger than the others. The reason may be that this molecular “butterfly” has the biggest “wings” and the strongest conjugation effect, and then the structures of cation and anion are the most stable in the series. 4. Conclusions In this paper, we reported the detailed investigation of structures, spectra, excited states and electroluminescent properties for five bridged Pt(II) complexes. In our calculation results, the cyclometalated C^N ligand have little effect on Pt–Pt interaction on the ground state. The lowest-energy absorptions show red shifts from complex 1 to 5. The reason is that the energy levels of HOMOs increased with the Pt composition decreased, while the energy levels of LUMOs decreased with the ligand conjugation effect enhanced. At last, the decreasingly HOMO–LUMO energy gaps occurred. Complex 3 shows great promise in chemosensors design since only complex 3 has a PtPt single bond on the lowest triplet excited state. The reason may be that complex 3 has the proper size and the interaction between the C^N ligands is strong. The quantum yield of complex 3 will be the smallest in these complexes since complex 3 has the smallest Dd–d*,DET1 S0 and the biggest DES1 T1 energy gaps in these five complexes. The Pt–Pt interaction leads to smaller HOMO–LUMO energy gaps and smaller quantum yield, while the injection and transport of electron and hole are getting harder. Complex 5 with the biggest cyclometalated C^N ligand shows great potential applications in the design and synthesis of OLED materials. The reason is that complex 5 has the most stable cation structure and anion structure and the Pt–Pt distance have no obvious change compared to that of neutral structure.
Table 5 The natural charge differences of Pt atoms between neutral and cation, anion structures. Charge differencea
1
2
3
4
5
DCharge (cation–neutral) DCharge (anion–neutral)
0.39 0.09
0.39 0.05
0.30 0.05
0.12 0.06
0.11 0.01
a
The natural charge differences of the two Pt atoms.
S. Huang et al. / Synthetic Metals 205 (2015) 222–227
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