Theoretical study on the electronic structures and phosphorescence properties of five osmium(II) complexes with different P^P ancillary ligands

Chemical Physics Letters 573 (2013) 29–34

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Theoretical study on the electronic structures and phosphorescence properties of five osmium(II) complexes with different P^P ancillary ligands Deming Han a, Gang Zhang b, Tian Li a, Hongguang Li a, Hongxing Cai c, Xihe Zhang c, Lihui Zhao a,⇑ a School of Life Science and Technology, International Joint Research Center for Nanophotonics and Biophotonics, Changchun University of Science and Technology, Changchun 130022, PR China b State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, PR China c International Joint Research Center for Nanophotonics and Biophotonics, School of Science, Changchun University of Science and Technology, Changchun 130022, PR China

a r t i c l e

i n f o

Article history: Received 26 March 2013 In final form 24 April 2013 Available online 2 May 2013

a b s t r a c t The geometry structures, electronic structures, absorption, and phosphorescence properties of five heteroleptic cyclometalated osmium(II) complexes have been theoretically investigated. The lowest absorption of these complexes are located at 442, 441, 445, 439, and 446 nm, respectively, and the HOMO ? LUMO or HOMO ? LUMO + 1 is the predominant transitions. The lowest energy emissions of these complexes are localized at 620, 615, 616, 609 and 638 nm, respectively. Ionization potential (IP) and electron affinity (EA) have been calculated to evaluate the injection abilities of holes and electrons into these complexes. The reorganization energies indicate complex 5 has the best electron injection ability and electron-transporting performance. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction In recent years, phosphorescent dopants with transition metal ions Ru(II), Os(II), Pt(II), and Ir(III) as core atom have attracted much attention in the fabrication of organic light emitting diodes (OLEDs), which is mainly due to their high phosphorescence emission quantum yields, short excited triplet state lifetime, and photochemical stability [1–7]. Theoretically, the devices prepared by using these phosphorescent heavy metal complexes would display efficiency 3–4 times better than that of devices based on fluorescent materials because the majority of excitons occurring in electron-hole recombination are triplets induced by heavy metalbased spin–orbit coupling effect [8,9]. As is known, the phosphorescence wavelength, quantum yields, and electroluminescent efficiency of phosphorescent complexes are closely related to the p-conjugation length of the cyclometalated ligand, substitution positions, and substituent inductive effect. To obtain ideal emitters in OLEDs, the modification of cyclometalated and/or ancillary ligands is used to tune the emission colors over the entire visible spectra [10–12]. On the whole, relative to the main ligands, the study on the ancillary ligands is limited because of the difficult effect of the color tuning [13,14]. However, many researchers have investigated the effect of the ancillary ligands of some complexes on the electrochemical and photophysical properties. For example, the study by Thompson and coworkers indicates that an pyrazolyl ⇑ Corresponding author. E-mail address: [email protected] (L. Zhao). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.04.054

ancillary ligand of cyclometalated Ir(III) complexes results in the blue-shifted emissions and affects the photophysical properties by changing the characters of excited states [15,11,16]. Indeed, there is few study on the photophysical properties of osmium complexes by adjusting the ancillary ligands with respect to the Ir(III) complexes. Recently, a series of heteroleptic cyclometalated Os(II) complexes functionalized with 2-pyridyl (or 2-isoquinolyl) pyrazole chelates have been synthesized by Lin and co-worker [17]. To gain an insight into the relationship between photophysical properties and the ancillary ligands, a series of Os(II) complexes have been designed to search the ideal emitting materials. In this Letter, five heteroleptic Os(II) complexes (N^N)2Os(P^P) [where N^N = 2-pyridyl pyrazole, P^P = 1,2-bis(phospholano)methylene (1); 1,2bis(phospholano)ethene (2); 1,2-bis(phospholano)benzene (3); 1,2-bis(phospholano)-4-trifluoromethyl-benzene (4); 1,2-bis(phos pholano)naphthalene (5)] have been investigated by using the density functional theory (DFT) methods. The electronic structures, charge injection, and transport, and spectral properties of these complexes have been calculated and compared to the available experimental data. It is anticipated that the theoretical results could provide useful information for these experimentalists in synthesizing new phosphors in OLED. 2. Computational details The ground state geometry for each molecule was optimized by the density functional theory (DFT) [18] method with Becke’s three

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D. Han et al. / Chemical Physics Letters 573 (2013) 29–34

parameter hybrid method combined with the Lee–Yang–Parr correlation functional (denoted as B3LYP) [19,20]. The geometry optimizations of the lowest triplet states (T1) were performed by unrestricted B3LYP approach. On the basis of the ground- and excited-state equilibrium geometries, the time-dependent DFT (TDDFT) approach associated with the SCRF (self-consistent reaction field) theory using the polarized continuum model (PCM) in dichloromethane (CH2Cl2) media was applied to investigate the absorption and emission spectral properties from the experimental results by Lin et al. [17]. The ‘double-n’ quality basis set LANL2DZ [21,22] associated with the pseudopotential was employed on atom Os. The 6-31+G(d) basis set was used for nonmetal atoms in the gradient optimizations. Furthermore, the stable configurations of these complexes can be confirmed by frequency analysis, in which no imaginary frequency was found for all configurations at the energy minima. In addition, the positive and negative ions with regard to the ‘electron-hole’ creation are relevant to their use as OLED materials. Thus, ionization potentials (IP), electron affinities (EA), and reorganization energy (k) were obtained by comparing the energy levels of neutral molecule with positive ions and negative ions, respectively. All calculations were performed with the GAUSSIAN 09 software package [23].

3. Results and discussion

ground state geometric structure for 3 is shown in Figure 1b along with the numbering of some key atoms. The optimized geometry parameters of 1–5 in the ground and lowest triplet states (T1) are shown in Table S1. It can be seen that the calculated geometrical parameters are in close agreement with the measured values [17]. The bond length deviation between the calculated and experimental ones changes in the range 0.07–1.7%. The discrepancy between our calculations and experiments is not only due to the fact that the crystal packing forces appeared in the experiment is not considered in the current calculations (performed on the single molecule), but it is well known that B3LYP overestimates the structural parameters (particularly bond lengths) in transition metal complexes. It can be also seen that at ground state (singlet state S0), the selected bond distances for complexes 2, 3 and 4 are nearly the same, while the bond distances of complexes 1 and 5 are slightly different from the other three. On the whole, for the triplet excited state T1, the bond distances are slightly changed for Os–N bonds and longer for Os–P bonds compared with those in the singlet state (S0). Meanwhile, the bond angle P1–Os–P2 is slightly smaller compared with the ground state one. In addition, the dihedral angles of P1–Os–N1–N2 in T1 state are obviously larger than those in S0 state for all the five complexes. In contrast, the dihedral angles of P1–N1–N3–N4 and P2– N1–N2–N4 in T1 state for all these complexes are obviously smaller than those in S0 state.

3.1. Geometries in the ground state 3.2. Molecular orbital properties The sketch map of the five heteroleptic cyclometalated osmium(II) complexes 1–5 is presented in Figure 1a, and the optimized

(a)

(1) R = CH2

CF3 N

(2) R = CH=CH

N

(3) R = P

N Os

R

N

P N

CF3 (4)

R=

N CF3

(5) R =

(b)

It is known that the frontier molecular orbitals (FMO) of the ground states (S0) are very important because they are closely related to spectral properties, especially HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular obital), which are controlled by the basic skeletal arrangement of both the chromophoric and/or the ancillary ligands [24]. The HOMO and LUMO distribution, energy levels, and energy gaps between of LUMO and HOMO (DEL–H) of the five complexes 1–5 are plotted in Figure 2. The calculated FMO compositions for 1–5 were presented in Tables S2–S6 (Supporting information). Figure 2 and Table S2–S6 show that for 1–5, the HOMO has similar distribution with Os d-orbital and N^N main ligands p-orbital. For example, for complex 1, the HOMO mainly resides on the 67% Os 5d orbital and 29% p(N^N). For LUMO, there are similar distribution in 1–4 with N^N ligand p⁄-orbital, i.e., dominantly contributed by pyridyl segments of the N^N ligands (p⁄(N^N) orbital). However, it is different that the LUMO of the 5 has 70% p⁄(P^P) and 29% p⁄(N^N) compositions, which can also be seen from the Figure 2. For 1 and 2, although the energies of HOMO and LUMO are different due to the different ancillary P^P ligands, i.e., HOMO 5.12 eV and LUMO 1.30 eV in 1, HOMO 5.19 eV, LUMO 1.36 eV in 2, the HOMO–LUMO energy gaps (DEL–H) are nearly the same, i.e., 3.82 eV for 1, 3.83 eV for 2. This indicates that the change of ancillary P^P ligands from single bond (CH2) in 1 to double bond (CH@CH) in 2 alters the energy levels, but not the energy gap. Compared to 3, the 4 with the electron-withdrawing group – CF3 located at the phenyl of the ancillary P^P ligand has lower values of HOMO and LUMO energies and similar energy gap (3.83 eV). Especially, for 5, the DEL–H (3.73 eV) is the smallest among these complexes 1–5, due to the extended p-conjugation length on the ancillary P^P ligand. 3.3. Ionization potentials (IP) and electronic affinities (EA)

Figure 1. (a) The schematic structures for the studied complexes 1–5. (b) Representative optimized structure of 3 in the ground state at the B3LYP level (The hydrogen atoms are omitted for clarity).

It is well-known that the device performance of OLEDs depends on the charge injection, transfer, and balance as well as the exciton

D. Han et al. / Chemical Physics Letters 573 (2013) 29–34

31

Figure 2. Molecular orbital diagrams and HOMO and LUMO energies for complexes 1–5.

Table 1 Ionization potentials (IP) (eV), electron affinities (EA) (eV), and reorganization energies (eV) data for complexes 1–5.

1 2 3 4 5

IPv

IPa

EAv

EAa

HEP

EEP

khole

kelectron

6.005 6.093 6.030 6.174 5.989

5.754 5.844 5.781 5.912 5.741

0.177 0.241 0.271 0.400 0.446

0.286 0.354 0.271 0.503 0.512

5.531 5.607 5.545 5.675 5.504

0.391 0.467 0.382 0.613 0.575

0.473 0.485 0.484 0.499 0.485

0.214 0.225 0.111 0.212 0.129

confinement in a device. In this section, ionization potentials (IP), electron affinities (EA), and reorganization energy (k) have been calculated for complexes 1–5, together with hole extraction potential (HEP) and electron extraction potential (EEP). The IP and EA can be either for vertical excitations (v; at the geometry of neutral molecule) or adiabatic excitations (a; optimized structure for both the neutral and charged molecule). For complexes with similar structures, the higher HOMO and lower LUMO energy levels will facilitate the hole- and electron-transporting abilities, respectively. The IP and EA are used to evaluate the energy barrier for the injection of holes and electrons. A larger EA (smaller IP) indicates easier injection of electrons (holes) into the emitting materials from the electron (hole) transporting layer. In Table 1, the IP value of complex 5 is the smallest one, which means that the hole injection is much easier in complex 5 than others. When the electron-withdrawing group –CF3 located at the phenyl of the ancillary P^P ligands, the IP values increased obviously than 2 and other complexes. The EA value of complex 5 is the largest, which means the electron injection is much easier than the other complexes. According to the Marcus–Hush model [25–27], the charge (hole or electron) transfer rate Ket can be expressed by the following formula:

K et ¼ Aexpðk=4kB TÞ

ð1Þ

where T is the temperature, kB is the Boltzmann constant, k is the reorganization energy. Due to the limited intermolecular charge transfer range in the solid state, the mobility of charges has been

Figure 3. Schematic description of internal reorganization energy for hole transfer.

demonstrated to be predominantly related to the internal reorganization energy k for OLED materials [28–31]. Thus, at constant temperature, a low k value is required for an efficient charge transport process. Herein, we focus on the inner reorganization energy ki, which is caused by the change of the internal nuclear coordinates from the reactant A to the product B and vice versa (Figure 3). It can be expressed by the following formula:

ki ¼ k0 þ k1 ¼ ðEAB  EA Þ þ ðEBA  EB Þ

ð2Þ

where EA and EBA are the energies of A and B at the optimized geometry of A, respectively; EAB and EB are the energies of A and B at the optimized geometry of B, respectively. The hole and electron injection and transport balance is important to emitting layer materials. The low reorganization energy is necessary for an efficient charge

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D. Han et al. / Chemical Physics Letters 573 (2013) 29–34

transport process. Table 1 shows that reorganization energies for hole transport (khole) are obviously larger than electron transport (kelectron), which indicates that electron-transporting performance of these Os complexes is better than the hole-transporting performance. The highest EA is found for 5, suggesting the best electron injection ability and the best electron-transporting performance among these complexes. In addition, one can find that complex 1 has the best hole transfer ability with the smallest khole value with respect to other four complexes. Moreover, the energy difference between khole and kelectron for 1 (0.259 eV) is smaller than those of complexes 2–5, which can greatly improve the charge transfer balance of 1, thus further enhancing the device performance of OLEDs.

3.4. Absorption spectra

a

On the basis of the optimized ground state geometries, the PCM-TD-B3LYP method was used to calculate the absorption properties of complexes 1–5. The vertical electronic excitation energies, oscillator strengths (f), dominant configurations, and their assignments of singlet excited states have been listed in Table S7 (Supporting information). The stimulated absorption spectra of the studied complexes in CH2Cl2 media based on the TDDFT calculations is shown in Figure 4. The lowest allowed transitions are located at 442 nm (f = 0.0098) for 1, 441 nm (f = 0.0102) for 2, 445 nm (f = 0.0091) for 3, 439 nm (f = 0.0097) for 4, and 446 nm (f = 0.0084) for 5. It can be seen that the calculated 282 nm absorption for 3 corresponds to the 290 nm in experiment [17]. The lowest energy absorption wavelengths are slightly changed with the different ancillary P^P ligands from 1 to 5. For 1–4, the lowest lying transition are attributed to the HOMO ? LUMO transition with MLCT (metal to ligand charge transfer)/ILCT(intraligand charge transfer) [d(Os) + p(N^N) ? p⁄(N^N)]. With respect to 5, the calculated 446 nm absorption can be described as [d(Os) ? p⁄(P^P/N^N)] transition with the character of MLCT. The absorption spectra of complexes 1–5 have a very similar shape in the region of 200– 450 nm except the large absorbance at about 225 nm for 5.

3.5. Phosphorescence spectra On the basis of the optimized triplet excited state (T1) geometries, the emission properties of complexes 1–5 in CH2Cl2 solution obtained using the TDDFT method are shown in Table 2. The plots of the molecular orbitals related to emissions of 1–5 are also presented in Figure 5.

60000

Absorbance

50000 40000

20000 10000 0 300

350

400

Configuration

Assign

1

620/1.99

L ? H(89%)

2

615/2.01

L ? H(97%)

3

616/2.01

L ? H(96%)

4

609/2.03

L ? H(95%)

5

638/1.94

L ? H(75%)

LMCT/ ILCT[p⁄(N^N) ? d(Os) + p(N^N)] LMCT/ ILCT[p⁄(N^N) ? d(Os) + p(N^N)] LMCT/ ILCT[p⁄(N^N) ? d(Os) + p(N^N)] LMCT/ ILCT[p⁄(N^N) ? d(Os) + p(N^N)] LMCT/ILCT/LLCT[p⁄(N^N/ P^P) ? d(Os) + p(N^N)]

450

Wavelength (nm) Figure 4. Simulated absorption spectra in CH2Cl2 for complexes 1–5.

500

Exptl.a

618 nm

Ref. [17].

Table 2 shows that the calculated lowest energy emissions of the five complexes are localized at 620, 615, 616, 609, and 638 nm, respectively. The calculated lowest energy emission of 3 is good agreement with the experimental data 618 nm [17]. Compared to 3 and 4, the 5 with the naphthalene substitution on the ancillary P^P ligands exhibits a red-shifted emission at 638 nm, which could be rationalized by an increase of the p-conjugation length. The calculated Stokes shifts between the lowest-lying absorptions and emissions are 0.70, 0.70, 0.69, 0.70, and 0.54 eV for complexes 1–5, respectively, which indicates complexes 1–4 have the similar Stokes shifts values. The small shift of 5 is in agreement with the small geometry changes from the ground state to the excited state. For 5, its emission transition character is assigned to LMCT/ILCT/LLCT (ligand to ligand charge transfer) [p⁄(N^N/P^P) ? d(Os) + p(N^N)]. The difference can be seen from the Figure 5 that the LUMO of 5 are localized on the N^N and P^P fragments different from those of complexes 1–4. The calculated lowest energy phosphorescent emissions for 1–4 are mainly contributed by LUMO ? HOMO transition configurations characterized as LMCT/ILCT [p⁄(N^N) ? d(Os) + p(N^N)]. 3.6. The phosphorescence quantum efficiency The emission quantum yield (U) can be affected by the competition between kr (radiative decay rate) and knr (nonradiative decay rate), i.e. U = kr/(kr + knr). It can be seen, to increase the quantum yield, kr should be increased and knr should be decreased simultaneously or respectively [32,2]. In addition, kr is also theoretically related to the mixing between S1 and T1, which is proportional to the spin–orbit coupling (SOC) and inversely proportional to the energy gaps between the S1 and T1 states according to the following formula [33,34]: hwS jHS jwT i2 l2S 1

0

1

ðDES

1 T1

1

Þ

c ¼ 16p3 106 n3 E3em =3h0

30000

250

k/E (nm)/(eV)

kr  c

1 2 3 4 5

200

Table 2 Calculated emission wavelengths (nm)/energies (eV) and dominant orbital emissions from TDDFT results for complexes 1–5 (H Indicates HOMO, L Indicates LUMO).

ð3Þ

where HS0 is the Hamiltonian for the spin–orbit coupling, lS1 is the transition dipole moment in the S0 ? S1 transition, DES1 T 1 is the energy gaps between the S1 and T1 states, Eem represents the emission energy in cm1 and n, h, and e0 are the refractive index, Planck’s constant and the permittivity in a vacuum, respectively. Accordingly, the variation of quantum yield (U) can be qualitatively analyzed in theory from the above formula. A larger 3MLCT composition and thus the intersystem crossing (ISC) can increase the phosphorescence quantum efficiencies. The direct involvement of the d(Os) orbital enhances the first-order SOC in the T1 ? S0 transition, resulting in a drastic decrease of the radiative lifetime and an increased nonradiative rate constant. From the Table 3, it can be seen that the calculated 3MLCT contributions are 57.8%, 63.0%, 62.4%, 60.8%, and 49.5% for 1, 2, 3, 4, and

D. Han et al. / Chemical Physics Letters 573 (2013) 29–34

33

Figure 5. Transitions responsible for the emissions at 620, 615, 616, 609, and 638 nm for complexes 1–5, respectively, simulated in CH2Cl2 media.

Table 3 The contribution of 3MLCT (%) in the T1 state and the energy gaps between the S1 and T1 states (DES1 T1 ) (in eV), along with the transition dipole moment in the S0 ? S1 transition lS1 , the radiative decay rate kr (105 s1) and nonradiative decay rate knr (105 s1), together with the measured lifetime s [ls] and quantum yields U [%] for the studied complex 3 in CH2Cl2 solution. 3

DES1 T1

lS1

Ua

sa

kr

knr

57.8 63.0 62.4 60.8 49.5

0.2461 0.2304 0.2318 0.2404 0.3099

0.1433 0.1486 0.1338 0.1406 0.1227

0.73

1.39

5.25

1.90

MLCT

1 2 3 4 5 a

Ref. [17].

5, respectively. The 3MLCT contribution (49.5%) of 2 is the largest one among these complexes, whereas 5 has the smallest one. In addition, it is also known that the phosphorescence quantum efficiencies are inversely proportional to the DES1 T1 [35]. A minimal DES1 T1 is required for enhancing the ISC rate, leading to the increased kr. The DES1 T1 for these complexes are also listed in Table 3, along with the transition dipole moment in the S0 ? S1 transition lS1 values. It can be seen that the complex 2 has the smallest DES1 T1 value and largest lS1 value among these complexes. As mentioned above, a lower DES1 T1 and larger 3MLCT contributions and higher lS1 values may account for a larger kr according to Eq. (3). Therefore, the complex 2 has possibly the largest kr value among these complexes.

4. Conclusions We have investigated the geometry structures, electronic structures, absorption, and phosphorescence properties of five heteroleptic cyclometalated Os(II) complexes by using the density functional theory. The study shows that the HOMO of the five complexes are mainly composed of the Os d-orbital and N^N main ligands p-orbital. For LUMO of 1–4, there are dominantly distribution on N^N ligand p⁄-orbital, relative to the 71% p⁄(P^P) and 29% p⁄(N^N) compositions of the 5. The lowest energy absorptions of

1–4 have the transition configurations of HOMO ? LUMO, compared to the HOMO ? LUMO + 1 and HOMO ? LUMO transitions of 5. The calculated phosphorescence emission of 3 is in good agreement with the experimental result. Compared to complexes 1–4, the emission of 5 is obviously red-shifted to 638 nm with LMCT/ILCT/LLCT character. Complex 5 is the easiest for electron injection and has the best electron-transporting performance. Ionization potentials (IP) and electron affinities (EA) calculations show that the highest EA of 5 results in the the best electron injection ability and the best electron-transporting performance among these complexes. In addition, complex 1 with the smallest khole value and the energy difference between khole and kelectron can greatly improve the charge transfer balance, further enhancing the device performance of OLEDs. It is expected that these theoretical studies could provide some inspiration in the design of new phosphorescent materials. Acknowledgments The authors are grateful to the financial aid from the Program of Science and Technology Development Plan of Jilin Province (Grant No. 20110438) and the Funds for Doctoral Scientific Research Startup of Changchun University of Science and Technology (Grant No. 40301855). 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.cplett.2013.04. 054. References [1] M.S. Lowry, W.R. Hudson, R.A. Pascal, S. Bernhard, J. Am. Chem. Soc. 126 (2004) 14129. [2] A.B. Tamayo, S. Garon, T. Sajoto, P.I. Djurovich, I.M. Tsyba, R. Bau, M.E. Thompson, Inorg. Chem. 44 (2005) 8723. [3] S. Thomas, K. Venkatesan, P. Müller, T.M. Swager, J. Am. Chem. Soc. 128 (2006) 16641. [4] F.C. Hsu et al., Inorg. Chem. 45 (2006) 10188. [5] V.L. Whittle, J.A.G. Williams, Inorg. Chem. 47 (2008) 6596.

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