Theoretical studies on the structures and absorption spectra of –Ph and t-Bu substituted 5-(2-pyridyl) pyrazolate boron complexes

Journal of Molecular Structure: THEOCHEM 809 (2007) 39–43 www.elsevier.com/locate/theochem

Theoretical studies on the structures and absorption spectra of –Ph and t-Bu substituted 5-(2-pyridyl) pyrazolate boron complexes W. Zheng a, X.M. Pan b

a,b,*

, L.L. Cui a, Z.M. Su a, R.S. Wang

a

a Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024, PR China State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, PR China

Received 11 October 2006; received in revised form 10 January 2007; accepted 15 January 2007 Available online 28 January 2007

Abstract We present a theoretical study using density functional theory (DFT) on molecular structures, electronic structures and absorption characters of –Ph and t-Bu substituted 5-(2-pyridyl) pyrazole boron complexes, namely, BPh2(2-(5-Phenyl-4H-pyrazole-3-yl)-pyridine) (2d), BPh2(2-[5-(2,2,2-Trifluoro-1,1-bis-trifluoromethyl-ethyl)-4H-pyrazole-3-yl]-pyridine) (2e), BPh2(4-Phenyl-2-(4H-pyrazole-3-yl)-pyridine) (2d-1) and BPh2(2-(4H-Pyrazole-3-yl)-4-(2,2,2-trifluoro-1,1-bis-trifluoromethyl-ethyl)-pyridine) (2e-1). The ground state structures of the title complexes are optimized at B3LYP/6-31G level of theory. In addition, time dependent density functional theory (TD-DFT) method is applied to investigate the properties of absorption spectra and electronic transition mechanism based on the ground state geometries. The results show that the chemical bond formed between nitrogen on the pyridyl ring and boron can be attributed to coordination effect and the coordinate bond in 2d-1 is the strongest among the four compounds. The calculated absorption wavelengths for 2d and 2e are in good agreement with the experimental ones. It can be detected that the main transitions of 2d, 2e and 2d-1 correspond to the intraligand p fi p* character. As the case of 2e-1, the main transition can be assigned as a mixed ligand-to-ligand/interligand charge transfer.  2007 Elsevier B.V. All rights reserved. Keywords: Boron compounds; Density functional theory; OLED; Electronic absorption spectra; TD-DFT

1. Introduction Organic electroluminescent (EL) devices are of growing interest in display applications because they can emit colors with high luminous efficiency throughout the visible spectrum. Extensive studies have been performed on the use of small molecular compounds to make organic EL devices with high brightness, multicolor emission, and desirable efficiency [1–5]. In particular, blue luminescent materials have remained a challenge in which most of them are Al and Ir compounds of 8-hydroxyquinolline derivatives [6,7]. Recently, various boron compounds have been introduced as blue emitting materials [8–11]. The boron-ligand *

Corresponding author. Tel.: +86 431 85099291; fax: +86 431 85099511. E-mail address: [email protected] (X.M. Pan). 0166-1280/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2007.01.012

bonds are fairly covalent comparing with the aluminumligand bonds which have ionic character. So boron compounds are found to be more stable than corresponding aluminum compounds. Moreover, boron compounds have been demonstrated previously to be good electrontransport materials due to the availability of empty pp orbital on boron center, which makes them an attractive and potentially useful class of compounds for EL applications [12]. Cheng et al. [13] had experimentally reported a new kind of –Ph and t-Bu substituted 5-(2-pyridyl) pyrazolate boron compounds which exhibit highly efficient blue electroluminescent. The 5-(2-pyridyl) pyrazole was selected because it can function as a chelating ligand upon treatment with a boron compound such BPh3, giving a nearly planar, conjugated p-system that was perfect for studying the photoluminescent properties. Furthermore, the absorption peak

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for the compounds at 340 nm is significantly blue shifted with respect to the Alq3. Being attracted by such new efficient blue emitting materials, we are interested in exploring the structure and absorption spectra properties of this kind of boron compounds with the help of quantum-chemical calculations. 2. Computational method The ground state calculations of –Ph and t-Bu substituted 5-(2-pyridyl) pyrazolate boron compounds were performed using Becke’s three-parameter hybrid exchange functional and Lee-Yang Parr correlation functional (B3LYP) with 6-31G basis set. It is reported that B3LYP function is widely employed on several kinds of boron compounds [14–17], the calculated results are in excellent agreement with experimental ones. In particular, Teng

et al. [18] had systemically compared the performance of various functionals – SVWN, BLYP, BP86, BPW91, B3LYP, B3P86, B3PW91, B1LYP, and MPW1PW91 on the absorption kmax of boron compounds. It is found that the performance of B3LYP is the best one. On the basis of the ground state geometries, time dependent density functional (TD-DFT) method using the same functional was performed to obtain the electronic absorption spectra. All the calculations were carried out with the Gaussian 03 program package [19]. 3. Results and discussion 3.1. Structure properties in the ground state Fig. 1 shows the ground state geometries of –Ph and t-Bu substituted 5-(2-pyridyl) pyrazolate boron compounds

˚ and degree) of –Ph and t-Bu substituted 5-(2-pyridyl) pyrazolate boron compounds. Fig. 1. The molecular structures and geometry parameters (A

W. Zheng et al. / Journal of Molecular Structure: THEOCHEM 809 (2007) 39–43

2d, 2e, 2d-1 and 2e-1 along with the numbering of some key atoms. As to the four compounds, the optimized results show that they are four-coordinate organic compounds with C1 symmetry. The 5-(2-pyridyl) pyrazole ligand is chelated to the boron atom and exhibits a nearly planar structure (see Fig. 1). Moreover, the optimized dihedral angles for C(13)–B–N(11)–C(6) and C(12)–B–N(1)–C(5) are 63 and 68, respectively. It is obviously that the boron atom has a typical tetrahedral geometry with the adjacent atoms. The values of \NBN, \NBC, \CBC and \CBN change from 93.3 to 116.7. Furthermore, the bond lengths of N(11)–B(3) in the four compounds are found to be distinctively longer than that of the N(1)–B(3) which can be attributed to the coordination effect. The calculated coordination bond lengths for 2e, 2e-1, 2d and 2d-1 are 1.661, ˚ , respectively. It indicates that 1.656, 1.656 and 1.653 A the dative bond in 2d-1 is the strongest among the four compounds and the coordination effect of 2e-1 is nearly the same as 2d. The assignment of dative bond is in agreement with that pointed by Cheng et al. on 2d and 2e [13]. As a whole, the calculated results indicate that altering the position of –Ph and t-Bu mainly leads to geometry changes on 5-(2-pyridyl) pyrazole ligand.

3.2. Frontier molecular orbital (FMO) analyses In order to study the FMO distribution in the ground state, the electronic density contours of the frontier orbitals (HOMO and LUMO) of 2d, 2e, 2d-1 and 2e-1 by B3LYP/ 6-31G are given in Fig. 2. Table 1 shows the contributions of each segment to the FMO. It can be seen that the frontier molecular orbitals mainly concentrate on the three ligands, with little contribution from the boron. This indicates that the boron main exerts supporting and stabilization effect on the structure of the four compounds. For 2d and 2e, the HOMO are mostly p orbitals with orbital contributions of 43.9% and 49.7% (Table 1) from the pyrazole ring, whereas LUMO are mostly

41

p* orbitals with orbital contribution of 81.7% and 81.5% from the pyridyl ring. The results show that the pyrazole ring is functioned as a hole transport group and the pyridyl ring as an electron transport group. In 2d, the –Ph on pyrazole ring exhibits a strong localization effect to electron on HOMO. The contributions from –Ph and pyrazole ring are nearly 85.4%, which indicate that –Ph substituted pyrazole will function as a better hole transport group than a single pyrazole ring. On the contrary, it can be seen from Table 1 that the HOMO of 2d-1 and 2e-1 mainly locate on phenyl ring (1) with the component of 71.1% and 70.8%, whereas contribution from the pyrazolate rings decrease to 8.6% and 9.6%. Meanwhile, the LUMO are contributed mainly by the pyridyl ring. It suggests that, for the four compounds, the electron transport groups are pyridyl rings while the hole transport group has changed from pyrazole ring (in 2d and 2e) to phenyl ring (1) (in 2d-1 and 2e-1) after altering the position of –Ph and t-Bu. In addition, comparing the distribution of electrons on the four HOMO orbitals (Fig. 2), it can be detected that the localization effect of HOMO on phenyl ring (1) (in 2d-1 and 2e-1) is more obviously than that on pyrazole ring (in 2d and 2e), suggesting that the hole transport capability of phenyl ring (1) (in 2d-1 and 2e-1) is better than that of pyrazole group. Thus, it is clearly that altering the position of –Ph and t-Bu can efficiently improve the distribution of the FMO. The energies of the ten highest occupied and ten lowest unoccupied molecular orbitals for the four compounds are shown in Fig. 3. From 2d to 2d-1, the energy of LUMO decreases from 2.20 to 2.30 eV. The same trend is also found on the energy of HOMO which decreases from 5.64 to 6.05 eV. According to the Koopman theorem, the energies of LUMO and HOMO can be described to a good approximation as: ELUMO ¼ jEAj;

EHOMO ¼ jIPj

where EA is the electron affinity, IP is the ionization potential. It suggests that after changing the position of –Ph from pyrazole ring to pyridyl ring, the EA and IP all increase, indicating that 2d-1 is a better electron accepted compound and will more facilitate the electron injection from metal anode than 2d. As to 2e and 2e-1, removing the position of t-Bu from pyrazole ring to pyridyl ring mainly leads to the increase of LUMO (2.11 fi 1.97 eV) and the slightly decrease of HOMO (6.06 fi 6.08 eV), which finally leads to the increase of energy gap between HOMO and LUMO. 3.3. Absorption spectrum

Fig. 2. Frontier molecular orbitals (FMOs) for –Ph and t-Bu substituted 5-(2-pyridyl) pyrazolate boron compounds in the ground states.

TD-DFT//B3LYP/6-31G have been used on the basis of the optimized geometry to obtain the nature and the energy of the singlet–singlet electronic transitions of the title compounds under study. The calculated results involving compositions, oscillator strengths (f), transition energies and

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Table 1 The contributions of each segment to the FMO (%) Compounds

Orbitals

Pyridyl

Pyrazole

Phenyl (1)

2d

L H H-5

81.7 6.0 7.9

10.9 43.9 25.6

3.0 2.5 4.3

3.8 4.6 2.9

0.4 1.5 12.1

2e

L H H-5

81.5 10.5 4.0

10.8 49.7 61.4

3.2 1.9 15.7

3.8 29.2 12.5

0.4 1.7 2.6

2d-1

L H H-3

67.4 2.0 21.6

6.3 8.6 53.3

3.0 71.1 18.3

2.4 17.3 4.7

0.4 1.0 1.2

2e-1

L H H-3 H-4 H-5

72.8 2.4 22.1 3.6 1.5

11.7 9.6 52.2 7.6 63.1

3.6 70.8 22.2 19.4 14.1

2.9 16.1 1.8 69.1 17.9

0.4 1.1 1.4 0.3 3.4

Fig. 3. Diagrams of energy levels of some frontier molecular orbitals in –Ph and t-Bu substituted 5-(2-pyridyl) pyrazolate boron compounds.

wavelengths are listed in Table 2 comparing with the available experimental wavelengths. For 2d and 2e, the calculated dominant peaks locate at 325.3 and 325.0 nm (expt. 350 nm, 340 nm), which indicates that the B3LYP excitation energies are in good agreement with the experimental ones. The results show that the excitation corresponds almost exclusively to the promotion of an electron from the HOMO-5 to the LUMO (72%, 88%). Since the HOMO-5 mostly comes from the pyrazolate ring with the components of 72.8% and 65.2% and the LUMO is constituted importantly by pyridyl ring with the components of 81.7% and 81.5%, the transition can be

Phenyl (2)

Boron

t-Bu

–Ph 0.2 41.5 47.2

0.4 7.0 3.8 20.5 0.0 0.9 8.6 0.0 0.3 0.0 0.0

designated a IL (intraligand) transition. For 2d-1, the calculated absorption peak (358.0 nm) exhibited 33 nm red shift relative to that of 2d. As is shown in Table 2, altering the position of –Ph (from C(3) to C(8)) mainly leads to the change of transition composition. The excitation is mainly from HOMO-3 to LUMO and the transition type is also designated as an IL charge transfer. Compared with 2e, the dominant peak of 2e-1 does not change obviously, which ranges around 327 nm. Particularly, there are two new transition types included in the transition which are HOMO-3 fi LUMO (32%) and HOMO-4 fi LUMO (47%). The HOMO-3 mostly comes from the pyrazolate ring with the component of 52.2% while the HOMO-4 is constituted mainly by phenyl ring (2) with the component of 69.1% and the LUMO mainly contributed by the pyridyl ring. So the transition can be assigned as a mixed ligand-toligand/intraligand charge transfer.

4. Summary We have applied DFT method to study some important trends on electronic structures and ground state properties of –Ph and t-Bu substituted 5-(2-pyridyl) pyrazole boron compounds. For the four serious compounds, the dative bond in 2d-1 is the strongest among the four compounds and the coordination effect of 2e-1 is nearly the same as 2d. The boron mainly exerts supporting and stabilization effect on the structure of the four compounds. The pyridyl

Table 2 The character of absorption spectra for –Ph and t-Bu substituted 5-(2-pyridyl) pyrazolate boron compounds calculated by TD-DFT with 6-31G basis set Compounds

Compositions

2d 2e 2d-1 2e-1

H-5 fi L H-5 fi L H-3 fi L H-3 fi L H-4 fi L H-5 fi L

(%) 72 88 84 32 47 6

f

DE (eV)/k (nm)

Expt. [13]

Character

0.1705 0.0822 0.0614 0.0742

3.81/325.3 3.82/325.0 3.46/358.0 3.79/326.9

350 340

p fi p* p fi p* p fi p* p fi p*

W. Zheng et al. / Journal of Molecular Structure: THEOCHEM 809 (2007) 39–43

ring is functioned as an electron transport group. The hole transport group changes from pyrazole ring to phenyl ring (1) when the positions of –Ph and t-Bu are removed from C(3) to C(8). For 2d and 2e, the main transition corresponds to the intraligand p fi p* character. As to 2d-1 and 2e-1, the main transition can be designated a mixed ligand-to-ligand/intraligand charge transfer.

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