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Theoretical predication on optical and electronic properties of multifunctional molecules for blue-light multifunctional materials
Journal of Molecular Structure: THEOCHEM 945 (2010) 71–77
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Theoretical predication on optical and electronic properties of multifunctional molecules for blue-light multifunctional materials Xinhua Ouyang, Heping Zeng * School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, PR China
a r t i c l e
i n f o
Article history: Received 23 July 2009 Received in revised form 8 January 2010 Accepted 9 January 2010 Available online 20 January 2010 Keywords: Multifunctional molecules Reorganization energies Emission and electronic spectra
a b s t r a c t 2-(Dimesitylboryl)-9-ethyl-9H-carbazole (CzB), 2-(4-(dimesitylboryl)phenyl)-9-ethyl-9H-carbazole (CzPhB), 2-(5-(dimesityl-boryl)thiophen-2-yl)-9-ethyl-9H-carbazole (CzThB), and (E)-2-(4-(dimesitylboryl)styryl)-9-ethyl-9H-carbazole (CzSB) have been studied by theoretical measurements with GAUSSIAN software. To reveal the relationship between the structures and properties of these multifunctional electroluminescent materials, their geometrical structures of ground and excited-states were optimized by B3LYP/6-31G(d), HF/6-31G(d), and CIS/6-31G(d) methods, respectively. The lowest excitation energies (Eg) and the maximum absorption and emission wavelengths of these compounds were calculated by time-dependent density functional theory methods. The important parameters for luminescent materials were also predicated including the reorganization energies, the ionization potentials and electron affinities. As a result of calculations, these molecules are considered as candidates for excellent OLEDs with good charge-transfer abilities, high blue-light emission and low energy barriers for charge injection, and the phene-based molecule has higher electron mobility and better equilibrium properties as compared to the other compounds. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Excellent organic light-emitting diodes (OLEDs) are considered as an important issue due to their potential applications such as highresolution full-color displays and solid state lighting applications [1–3]. OLED performance depends not only on the luminescent efficiency of the emissive materials but equally importantly also on the optical and semiconductor characteristics of charge-transporting materials [4,5]. Effective design and synthesis of multifunctional molecules are fairly complicated duo to the multiplicities of factors such as excited-states energy levels, film forming behavior, thermal stability and suitable emission wavelength. Therefore, theoretical methodology will be an important instrument which can provides some useful information for the experimentalist. Usually, highly efficient OLEDs are obtained though multilayer structures of OLEDs and dopant–host systems [5,6]. However, their processes and costs of fabrication are very high and complicated in these devices. Furthermore, the electroluminescence (EL) properties will be affected seriously by the concentration of dopants and hosts. In this regard, it will be necessary to design and synthesize some kinds of multifunctional organic molecules for single-layer OLEDs, which can be acted as charge-transporting material, highly efficient emitters with excellent performance and charge-injecting materials.
* Corresponding author. E-mail address: [email protected] (H. Zeng). 0166-1280/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2010.01.013
Such multifunctional materials are still relatively rare and frequently do not yield the highest-efficiency devices. Recently, Lin et al. [7] reported a series of carbazole-p-dimesitylborane bipolar molecules for multifunctional blue-light materials, and most of them displayed excellently photophysical properties. In order to reveal the relationship of structure and property, we choose these synthetic molecules as our target molecules. In these molecules containing a B–N bond, the response of these molecules under the electric field is mainly affected by the polarization of its p-electron. The amounts of p-electrons, the delocalization degree, and the property of polarization have direct influence on the energy structure and optoelectronic properties. Moreover, we also discuss the role of pconjugated bridge in these multifunctional molecules. The structures of the studied molecules are outlined in Fig. 1, geometrical structures and optical properties of these compounds are given by density functional theory (DFT), ab initio HF and CIS, and time-dependent density functional theory (TD-DFT) methods in this paper. 2. Computational methods All of calculations about the four molecules have been performed on the huge computer origin 2000 server center using the GAUSSIAN 03 program package [8]. Geometric and electronic structures of ground, excited-states, their cationic and anionic structures were studied by B3LYP/6-31G(d). The lowest singlet
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Fig. 1. The structures of the studied molecules.
excited-states were computed with ab initio CIS/6-31G(d) on the basis of the optimized geometries obtained from HF/6-31G(d) calculations [9]. And the absorption and emission wavelengths of these compounds were predicated by TD-DFT and TD-HF methods in the gas and solvent phase. The other important parameters, such as reorganization energy (k), ionization potentials, and electron affinities were also derived from the computed results. The compositions of molecular orbits were analyzed using the GaussView 3.0 program.
3. Results and discussion 3.1. Geometry optimization of ground-states and the first excited-state Fig. 2 plotted the optimized structures of CzB, CzPhB, CzThB and CzSB by B3LYP/6-31G(d), and the first excited-states of these molecules are optimized by CIS/6-31G(d) on the base of ground structures by HF/6-31G(d). Some important bond lengths and bond angles of these molecule in the neutral, cationic, anionic states
and excited-states obtained by B3LYP/6-31G(d) and CIS/6-31G(d) [9] calculations are summarized in Tables 1 and 2. In comparison of the structures of these molecules, CzB, CzPhB, CzThB and CzSB have been found with the same sectors on both sides of the molecules and different p-conjugated bridges in the middle. As shown in Table 1, the angles around B atom in CzB, CzPhB, CzThB and CzSB are very close to each other with the neutral, cationic, and anionic states, respectively. However, in CzSB, all of the bonds are shorter than the others, which indicate some charge transfer on excitation probably in CzSB due to the remarkably strengthened. The bond lengths in these states are analyzed and find, the bonds r(1, 2) and r(1, 3) in the neutral state are longer than in the cationic state but shorter than in the anionic state. On the other hand, the bonds r(1, 4) of these molecules in the neutral state are longer than those in the anionic but shorter than those in the cationic state. The bonds r(7, 9) of these molecules in the neutral state are longer than those in the cationic and anionic states. The results showed removing an electron will decrease the r(1, 2) and r(1, 3) anti-bonding interaction and lead to shorter distances, whereas, accepting an electron will increase the r(1, 4) anti-bonding interaction and result into much longer distance. However, whether accepting or removing an electron will both decrease the r(7, 9) anti-bonding interaction and change the bonds to much shorter distance. As shown the geometries of excited-states of these molecules in Table 2, some of the bond lengths are lengthened or shortened according to the results by comparing with the ground-states. The differences between the ground (S0) and first singlet excitedstate (S1) may be attributed to molecular orbits (MO) nodal patterns. It is well-known that the lowest singlet state corresponds to an excitation from the highest occupied molecular orbit (HOMO) to the lowest unoccupied molecular orbit (LUMO) in all organic molecules. In Table 2 and Fig. 3, we can see that the HOMO has no nodes across the r(1, 4) bonds in molecule CzB, the electronic density concentrates in N-ethylcarbazole sector. However, the HOMO has no nodes across r(1, 4), r(4, 5), r(5, 6), r(6, 7), r(7, 8), r(7, 9), r(8, 10), and r(4, 10) bonds in molecule CzPhB, r(1, 4), r(4, 5), r(5, 6), r(6, 7), r(7, 8), r(7, 9), and r(4, 8) bonds in molecule
Fig. 2. The stereograph of optimized molecules CzB, CzPhB, CzThB and CzSB by DFT//B3LYP/6-31G(d).
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take the difference between their total energies of equilibrium geometry, the evaluation of the first EAs and IPs can be given by:
Table 1 Some important bond lengths (Å) and bond angles (°) of molecules CzB, CzPhB, CzThB and CzSB obtained by B3LYP/6-31G(d) calculations. CzB
r(1, 2) r(1, 3) r(1, 4) r(7, 9) h(2, 1, 4) h(2, 1, 3) h(3, 1, 4)
EEA ¼ E0 E
ð1Þ
EIP ¼ Eþ E0
ð2Þ
Neutral
Cationic
Anionic
Neutral
Cationic
Anionic
1.5883 1.5866 1.5663 – 118.7 122.1 119.2
1.5739 1.5731 1.5868 – 117.4 124.9 117.7
1.6033 1.6032 1.5423 – 119.9 119.8 120.3
1.5867 1.5863 1.5676 1.4834 118.9 122.3 118.8
1.5769 1.5768 1.5788 1.452 119.1 122.0 118.9
1.6047 1.6043 1.5344 1.47 120.2 119.8 120.1
Neutral
Cationic
Anionic
Neutral
Cationic
Anionic
1.5875 1.5882 1.5441 1.4671 119.3 123.3 117.4
1.5723 1.5686 1.5749 1.4326 117.7 124.9 117.5
1.605 1.6068 1.5095 1.4361 121.4 120.4 118.9
1.5874 1.5869 1.5665 1.4618 118.7 122.4 118.9
1.5731 1.5727 1.5882 1.4332 118.0 123.6 118.3
1.606 1.6055 1.5285 1.4296 120.1 119.6 120.3
CzThB
r(1, 2) r(1, 3) r(1, 4) r(7, 9) h(2, 1, 4) h(2, 1, 4) h(2, 1, 4)
CzPhB
According the above formula, the EAs and IPs of these molecules are listed in Table 3. For OLED materials, the low values of IP will be easy to inject the hole to hole-transporting layer; and the higher the EA of the electron-transporting layer, the easier the injection of electrons from cathode. As shown in Table 3, the values of IP and EA of these molecules are 6.66 and 0.57 eV, 6.54 and 0.75 eV, 6.43 and 0.87 eV, and 6.30 and 1.01 eV, respectively. The variation trends of IPs and EAs are opposite when the values of IPs decrease and EAs increase. On the base of the rules of IP and EA for the OLEDs materials, the orders of charge transport abilities of these molecules are CzSB > CzThB > CzPhB > CzB, which are similar with the results of reorganization energy and energy gaps. Therefore, this demonstrates these materials can be used as charge injecting and transporting materials.
CzSB
3.3. The reorganization energies of studies molecules CzThB, and r(1, 4), r(4, 5), r(5, 6), r(6, 7), r(7, 8), r(7, 9), r (8, 10), r(4, 10), r(9, 11), and r(11, 12) bonds in molecule CzSB, and the LUMO is bonding in these above-mentioned regions. Therefore, one can be expected contraction of these bonds. The dipole moments are also considered as an important properties for organic OLEDs materials, the dipole moments of CzB, CzPhB, CzThB and CzSB are 1.96 D (1.51 D), 1.97 D (1.56 D), 1.98 D (1.53 D), and 2.21 D (1.53 D) by B3LYP/6-31G(d) (HF/6-31G(d)), respectively, which suggests that compound CzSB with still benezene-base bridge has a better electron push–pull ability on excitation than others. The results are good agreement with the experimental results and the following analysis of reorganization energies.
As far as organic luminescent materials are concerned, the carrier transfer balance of both injected electrons and holes is an important issue to obtain highly efficient OLEDs, which depends largely the reorganization energy (k), ionization potentials and electron affinities of these molecules. The internal reorganization energies of CzB, CzPhB, CzThB and CzSB are calculated by DFT with a thermally activated hopping-type mechanism at room temperature [10–13]. Viewing each hopping event as a nonadiabatic electron-transfer reaction, standard Marcus theory was using to express the rate of charge motion between neighboring molecules. The expressions are listed for electron transfer as following, and that of hole transfer is similar with formula (3).
V2 p 1=2 k exp 4kB T h kkB T
3.2. Electron affinities and ionization potentials of CzB, CzPhB, CzThB and CzSB
K electron ¼
Electron affinities (EA) and ionization potentials (IP) are corresponding with the energy barriers for electron and hole injection, which are very important properties to obtain highly efficient OLEDs. In this case, EA will decide the energy barrier for the electronic injection. The molecules are calculated by adding one electron to the neutral molecules in order to achieve the effect of electronic injection. IP is counterpart to the energy barrier of hole injection, we remove one electron from the neutral molecule, and
where k is the reorganization energy, V is the coupling matrix element, kB is the Boltzmann constant and T is the temperature. According to the Eq. (3), there are two factors k and V to determine the Kelectron. Usually, V value is very limited in the amorphous organic materials. Therefore, the mobilities of electrons and holes are expected to be only dominated by the reorganization energies k [14,15]. The reorganization energy k for electron transfer can be expressed as follows:
ð3Þ
Table 2 Optimized important interring distances (Å) and dihedral angles (°) of molecules CzB, CzPhB, CzThB and CzSB with HF/6-31G(d) and CIS/6-31G(d). HF
r(1, 2) r(1, 3) r(1, 4) r(4, 5) r(4, 8) r(4, 10) r(5, 6) r(6, 7) r(7, 8) r(7, 9) r(8, 10) r(9, 11) r(11, 12) U(2, 1, 4, 8) U(2, 1, 4, 10) U(8, 7, 9, 11)
CIS
CzB
CzPhB
CzTHB
CzSB
CzB
CzPhB
CzTHB
CzSB
1.6003 1.5991 1.5816 – – – – – – – – – – – – –
1.5996 1.5993 1.5805 1.3987 – 1.3984 1.3827 1.3935 1.3934 1.4904 1.3828 – – – 25.20 –
1.5992 1.5998 1.5630 1.3623 1.7439 – 1.4227 1.3579 1.7287 1.4786 – – – 17.32 – –
1.5995 1.5995 1.58 1.4006 – 1.3976 1.3805 1.3959 1.3935 1.4761 1.3828 1.3284 1.4773 – 25.29 159.03
1.6036 1.6028 1.5549 – – – – – – – – – – – – –
1.6036 1.6033 1.56 1.4428 – – 1.4201 1.3616 1.4354 1.4361 1.4201 1.3628 – – 19.08 –
1.602 1.6021 1.5446 1.4118 1.7479 – 1.3702 1.4204 1.7654 1.4021 – – – 14.56 – –
1.6032 1.6031 1.5642 1.4145 – 1.4191 1.367 1.4275 1.4255 1.4094 1.3639 1.3949 1.4069 – 19.48 179.87
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Fig. 3. Electronic density contours of the Frontier orbits for molecules CzB, CzPhB, CzThB and CzSB in ground-states and the first excited-states.
kelectron ¼ k0 þ k ¼ E0 E0 þ ðE E Þ
ð4Þ
where E0 and E represent the energies of the neutral and anion species in their lowest energy geometries, respectively, while E0 and E represent the energies of the neutral and anion molecules with the geometries of the anion and neutral molecules, respectively. Likewise, k for hole transfer can be expressed as follows:
khole ¼ k0 þ kþ ¼ E0 E0 þ ðEþ Eþ Þ
ð5Þ
The calculated khole and kelectron values are summarized in Table 3. According to the Eq. (3) with the limited V, it can be seen that the lower the k values, the bigger the charge-transport rate, which is in agreement with the research by Nakanishi [16] and co-workers. In Table 3, the values of khole for molecules CzB, CzPhB, CzThB and CzSB are all smaller than their counterpart kelectron, this suggests that the hole transfer rates of these molecules are higher than
that of the electron transfer rates. By comparing the values of khole with kelectron of CzB, CzPhB, CzThB and CzSB, it can be seen the khole of CzB is lower than those but the kelectron is much higher than those of CzPhB, CzThB and CzSB, it indicates that CzB has a higher hole-transport rate than the other molecules but weaker electrontransport rate. In the meanwhile, we compare the difference between the khole and kelectron of all molecules and find CzB, CzPhB, CzThB and CzSB is 1.11, 0.12, 0.11 and 0.08 eV, respectively. The difference between the khole and kelectron of CzSB is about 10 times larger than the other molecules, this implies that CzB is not good for simultaneous hole-transporting and electron-transporting materials. On the other hand, the others have low difference between the khole and kelectron about 0.1 eV, which can be considered as the candidates for hole-transporting and electrontransporting materials. As the results shown in Table 3, the order of the charge-transfer ability of these molecules will be CzSB > CzPhB > CzThB > CzB. Moreover, the values k of hole and
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Fig. 3 (continued) Table 3 Reorganization energies ionization potentials (IP) and electron affinities (EA) for each molecule.
khole (eV) kelecton (eV) EA (eV) IP (eV)
CzB
CzPhB
CzThB
CzSB
0.17 1.28 0.57 6.66
0.27 0.39 0.75 6.54
0.42 0.53 0.88 6.43
0.38 0.44 1.01 6.30
electron are increased with the order from CzSB to CzThB except CzB, which only displays hole-transporting ability by above-said.
3.4. Frontier molecular orbits The electronic density contours of the Frontier orbits of considered molecules have been plotted by GaussView software in Fig. 3.
It is shown that the electronic clouds of HOMO in CzB, CzPhB, CzThB and CzSB localize on the side of N-ethyl carbazole sector, while the one of LUMO decentralizes at the whole molecule. For the hole-transporting materials, the smaller negative value of HOMO is, the more easily the molecules lose its electrons, on the other hand, the electron-transporting material with larger negative value of LUMO will accept electrons more easily. In this study, the energies of HOMO and LUMO are predicated by DFT and summarized in Table 4 of compounds CzB, CzPhB, CzThB and CzSB. In Table 4, it is noted that there are some changes in energies of HOMO and LUMO with the experiments. This may be induced by the interaction between molecules and solvents. In the results of calculation, we only consider the molecules with ideal conditions. And the gradual decrease of values in LUMO is good agreement with the experimental results. It is well-known that the higher the value of HOMO is, the easier creating a hole is, the lower the value of LUMO is, the easier creating an electron is. All of these
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Table 4 Value of the HOMO and LUMO energies, HOMO–LUMO gaps calculated by DFT, and the lowest excitation energies calculated by TD-DFT and experiment in eV for molecules CzB, CzPhB, CzThB and CzSB.
eHOMO (calculation) eLUMO (calculation) DL–H (calculation) Eg(TD) (calculation) DL–H (experiment) eHOMO (experiment) eLUMO (experiment)
CzB
CzPhB
CzThB
CzSB
5.36 1.63 3.73 3.64 3.25 6.15 3.25
5.39 1.71 3.68 3.37 3.01 5.92 3.01
5.41 1.82 3.59 3.35 2.73 5.8 2.73
5.35 1.93 3.42 3.07 2.75 5.75 2.75
molecules show that the energy of HOMO is about 5.35 eV according to the results of calculations. Comparing them with N,N0 (NPB), bis(naphthyl)-N,N0 -diphenyl-1,10 -biphenyl-4,40 -diamine which is a well-known hole-transporting material with eHOMO about 5.2 eV [17–20], it can be found values of HOMO in the molecules are very close that of NPB, so hole carrier can be injected easily into these molecules. Meanwhile, we also compare these molecules with TPBI and find, the values of LUMO in these molecules are slightly higher than that of TPBI (eLUMO, 2.7 eV). However, these molecules are still good electron-transporting materials owing to the empty p–p orbits on the boron element center, which is proved in the Lin’s paper. The energy gaps of molecules CzB, CzPhB, CzThB and CzSB are also predicated from the orbital energy differences between the HOMO and LUMO, named the HOMO–LUMO gaps (DL–H) in Table 4 [20,21]. Actually, there are three types of band gaps in experiment such as optical band gap, electrochemical band gap, and the band gap from photoelectron spectrum. The most common one is the optical band gap, which is deduced from the lowest transition energy from the ground-state to the first excited-state. In this paper, the optical band gaps (Eg) of these molecules are calculated by TD-DFT method. In Table 4, it can be seen there are discrepancies between the values of DL–H and Eg by calculation
with experimental results. The experimental results are obtained by cyclic voltammetry with solvents, which exists interaction including polarization of solvent, heating effect and so on. Moreover, the energy gaps of these molecules are also decided from onsets of absorptive spectra in the experiments, which also depend largely on the interaction of solvents. On the other hand, the theoretical predication is carried on in ideal states. Therefore, the differences between the theoretic and experimental results are necessary. In fact, the chance trends of DL–H and Eg are consentaneous with the experimental results, the orders are CzB > CzPhB > CzThB > CzSB. By analyzing the results of the DL–H and Eg, we can find the values of DL–H and Eg decrease with increasing length of p-conjugated bridges. And the electronic hopping from the ground-states to excited-states will be easy with enlarging the conjugation of these molecules. On the other hand, the values of energy gaps of these molecules are more than 3 eV, so they can be used as blue emitter in the OLEDs materials. 3.5. Electronic spectra and emission spectra The electronic spectra of these molecules have been studied by TD-DFT//B3LYP/6-31G(d) in the solvent of THF, and the emission spectra are also calculated by TD-DFT method on the basis of optimized excited-state geometries by B3LYP/6-31G(d). TD-DFT is considered as a relatively precise method to analyze the UV–visible absorption spectrum and luminescent spectrum in the organic molecules [22,23]. Table 5 summarizes the transition energies of electronic spectra, oscillator strength, configurations of the orbits and experimental results. As shown in Table 5, all of the electronic transitions are of the p ? p* type and in agreement with the experimental results. In this calculation, we use the PCM model for the THF solvent and give good agreement with observed band wavelength in the visible region. On the other hand, the oscillator strengths (f) of S0 ? S2 electronic transition are the largest in all these molecules except CzSB, which show the main contributions are from this hopping.
Table 5 Electronic spectra obtained by TD-DFT with solvent THF for molecules CzB, CzPhB, CzThB and CzSB at the B3LYP/6-31G(d) optimized geometries.
CzB
CzPhB
CzThB
CzSB
Electronic transitions
k6abs max (nm)
Exp (nm)
f
Excitation energies (eV)
Main configurations
S0 ? S1 S0 ? S2
340.3
353
0.04 0.52
3.23 3.64
HOMO ? LUMO HOMO-1 ? LUMO HOMO-2 ? LUMO
0.68 0.62 0.12
S0 ? S1 S0 ? S2
383.6 368.2
358
0.01 0.88
3.23 3.37
HOMO ? LUMO HOMO-1 ? LUMO HOMO-3 ? LUMO
0.69 0.66 0.16
S0 ? S1 S0 ? S2
378.3 369.6
390
0.04 0.84
3.27 3.35
HOMO ? LUMO HOMO-1 ? LUMO HOMO-3 ? LUMO
0.69 0.65 0.10
S0 ? S1 S0 ? S2
404.1 394.1
390
1.53 0.01
3.07 3.14
HOMO-4 ? LUMO HOMO ? LUMO HOMO-1 ? LUMO
0.11 0.66 0.68
Table 6 Calculated emission data in gas and solvent THF for molecules CzB, CzPhB, CzThB and CzSB.
CzB gas phase THF CzPhB gas phase THF CzThB gas phase THF CzSB gas phase THF
Electronic transitions
kex (nm)
S1 ? S0 S1 ? S0 S1 ? S0 S1 ? S0 S1 ? S0 S1 ? S0 S1 ? S0 S1 ? S0
388.1 394.4 408.3 421.3 436.6 452.7 454.3 475.9
Exp (nm) 398 436 455 482
f
Excitation energies (eV)
Main configurations
0.12 0.17 1.18 1.38 1.09 1.27 1.71 1.88
3.19 3.14 3.04 2.94 2.84 2.74 2.73 2.61
HOMO ? LUMO HOMO ? LUMO HOMO ? LUMO HOMO ? LUMO HOMO ? LUMO HOMO ? LUMO HOMO ? LUMO HOMO ? LUMO
0.65 0.67 0.63 0.65 0.62 0.64 0.62 0.64
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The emission spectra of CzB, CzPhB, CzThB and CzSB are listed in Table 6, we investigate these molecules in gas and solvent phases with a chemical PCM model for THF. By comparison of the results in gas phase, it can be seen the results are good agreement with the observed peaks by using PCM model. The calculated values of the emission wavelength in the gas and solution phase are located at 388.1 and 394.4 nm, 408.3 and 421.3 nm, 436.6 and 452.7 nm, and 454.3 and 475.9 nm, respectively. On the basis of the special emission spectra, CzB, CzPhB, CzThB and CzSB can be used as bluelight emitting materials with high brightness, which are also in good agreement with the analysis of energy gaps. The emission wavelengths for these molecules exhibit gradual red-shifts of about 80 nm in the gas phase. It may be explained that these compounds have large change of the structures in the ground and excited-states. 5. Conclusions We have theoretically demonstrated the photophysical properties of CzB, CzPhB, CzThB, and CzSB by DFT, TD-DFT and CIS methods with GAUSSIAN software. According to the above research we conclude these molecules are excellent candidates for multifunctional OLED materials. The calculated values of IP and EA show that these molecules can be used as electron-transporting and holetransporting materials simultaneously. Among these molecules, it can be seen that the phene-based molecule displays higher intraand intermolecular charge-transfer ability and better equilibrium properties than that of others. Replacing the middle group with thiophene in these molecules has no positive effect on chargetransport rate but enhances the balance between the hole and electron transfer. With the gaps reducing of HOMO and LUMO, the absorption and emission wavelengths for CzB, CzPhB, CzThB, and CzSB exhibit gradual red-shifts. The Stokes shifts of these gas phase molecules are attributed to the large change of the structures in the ground and excited-states. The emission wavelengths of these molecules are located in the blue region, implying that they can be used as blue-light materials. Acknowledgments We also thank Prof. Paul. W. Ayers and Dr. Gongchang Zeng for their help in calculation. Financial support from the National Natural Science Foundation of China (Nos. 20671036, 2007A010500008
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and 2008B010800030) are gratefully acknowledged. Ouyang X.H. would like to acknowledge the receipt of the Doctorate Foundation from South China University of Technology (SCUT), Guangzhou and the China Scholarship Council. References [1] C.J. Tonzola, A.P. Kullkarni, A.P. Gifford, W. Kaminsky, S.A. Jenekhe, Adv. Funct. Mater. 17 (2007) 863. [2] M.Y. Lo, C. Zhen, M. Lauters, G.E. Jabbour, A. Sellinger, J. Am. Chem. Soc. 129 (2007) 5808. [3] M.Y. Lai, C.H. Chen, W.S. Huang, J.T. Lin, T.H. Ke, L.Y. Chen, M.H. Tsai, C.C. Wu, Angew. Chem. Int. Ed. 47 (2008) 581. [4] T. Matsushima, H. Murata, J. Appl. Phys. 104 (2008) 034507. [5] S.L. Lin, L.H. Chan, R.H. Lee, M.Y. Yen, W.J. Kuo, C.T. Chen, R.J. Jeng, Adv. Mater. 20 (2008) 3947. [6] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery, R.E. Stratmann, J.C. Burant, S. Dapprich, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Peterson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malik, A.D. Rabuk, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Chalacombe, P.M.W. Gill, B.G. Johnson, W. Chen, M.W. Wong, J.L. Andres, M. Head-Gordon, E.S. Replogle, J.A. Pople, GAUSSIAN 2003, Gaussian Inc., Pittsburgh, PA, 2003. [7] L.Y. Zou, A.M. Ren, J.K. Feng, Y.L. Liu, X.Q. Ran, C.C. Sun, J. Phys. Chem. A 112 (2008) 12172. [8] W.Q. Deng, W.A. Goddard III, J. Phys. Chem. B 108 (2004) 8614. [9] H.P. Zeng, X.H. Ouyang, T.T. Wang, G.Z. Yuan, G.H. Zhang, X. Zhang, Cryst. Growth Des. 6 (2006) 1697. [10] S. Arulmozhiraja, T. Ohno, J. Phys. Chem. C 112 (2008) 16567. [11] S.W. Yin, Y.P. Yi, Q.X. Li, G. Yu, Y.Q. Liu, Z.G. Shuai, J. Phys. Chem. A 110 (2006) 7138. [12] S.F. Nelsen, F. Blomgren, J. Org. Chem. 66 (2001) 6551. [13] I. Litani-Barzilai, V. Bulatov, V.V. Gridin, I. Schechter, Anal. Chim. Acta 501 (2004) 151. [14] T. Nakanishi, K. Ohkubo, T. Kojima, S. Fukuzumi, J. Am. Chem. Soc. 131 (2009) 577. [15] M.T. Li, W.L. Li, W.M. Su, F.X. Zang, B. Chu, Q. Xin, D.F. Bi, B. Li, T.Z. Yu, Solid State Electron. 52 (2008) 121. [16] Q.X. Tong, S.L. Lai, M.Y. Chan, K.H. Lai, J.X. Tang, H.L. Kwong, C.S. Lee, S.T. Lee, Chem. Mater. 19 (2007) 5851. [17] B.C. Wang, H.R. Liao, J.C. Chang, L. Chen, J.T. Yeh, J. Lumin. 124 (2007) 333. [18] S.W. Tsang, S.C. Tse, K.L. Tong, S.K. So, Org. Electron. 7 (2006) 474. [19] A.F. Jalbout, S. Fernandez, J. Mol. Struct. (THEOCHEM) 584 (2002) 169. [20] C. Peltier, P.P. Laine, G. Scalmani, M.J. Frisch, C. Adarno, I. Ciofini, J. Mol. Struct. (THEOCHEM) 914 (2009) 94. [21] M. Bruszynska, E. Sikorska, A. Komasa, I. Khmelinskii, L.F.V. Ferreira, J. Hernando, M. Kubicki, J.L. Bourdelande, M. Sikorski, Chem. Phys. 361 (2009) 83. [22] A. Irfan, R.H. Cui, J.P. Zhang, J. Mol. Struct. (THEOCHEM) 850 (2008) 79. [23] S. Fujita, T. Nakazawa, M. Asano, Jpn. J. Appl. Phys. 39 (2000) 5301.
Contents lists available at ScienceDirect
Journal of Molecular Structure: THEOCHEM journal homepage: www.elsevier.com/locate/theochem
Theoretical predication on optical and electronic properties of multifunctional molecules for blue-light multifunctional materials Xinhua Ouyang, Heping Zeng * School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, PR China
a r t i c l e
i n f o
Article history: Received 23 July 2009 Received in revised form 8 January 2010 Accepted 9 January 2010 Available online 20 January 2010 Keywords: Multifunctional molecules Reorganization energies Emission and electronic spectra
a b s t r a c t 2-(Dimesitylboryl)-9-ethyl-9H-carbazole (CzB), 2-(4-(dimesitylboryl)phenyl)-9-ethyl-9H-carbazole (CzPhB), 2-(5-(dimesityl-boryl)thiophen-2-yl)-9-ethyl-9H-carbazole (CzThB), and (E)-2-(4-(dimesitylboryl)styryl)-9-ethyl-9H-carbazole (CzSB) have been studied by theoretical measurements with GAUSSIAN software. To reveal the relationship between the structures and properties of these multifunctional electroluminescent materials, their geometrical structures of ground and excited-states were optimized by B3LYP/6-31G(d), HF/6-31G(d), and CIS/6-31G(d) methods, respectively. The lowest excitation energies (Eg) and the maximum absorption and emission wavelengths of these compounds were calculated by time-dependent density functional theory methods. The important parameters for luminescent materials were also predicated including the reorganization energies, the ionization potentials and electron affinities. As a result of calculations, these molecules are considered as candidates for excellent OLEDs with good charge-transfer abilities, high blue-light emission and low energy barriers for charge injection, and the phene-based molecule has higher electron mobility and better equilibrium properties as compared to the other compounds. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Excellent organic light-emitting diodes (OLEDs) are considered as an important issue due to their potential applications such as highresolution full-color displays and solid state lighting applications [1–3]. OLED performance depends not only on the luminescent efficiency of the emissive materials but equally importantly also on the optical and semiconductor characteristics of charge-transporting materials [4,5]. Effective design and synthesis of multifunctional molecules are fairly complicated duo to the multiplicities of factors such as excited-states energy levels, film forming behavior, thermal stability and suitable emission wavelength. Therefore, theoretical methodology will be an important instrument which can provides some useful information for the experimentalist. Usually, highly efficient OLEDs are obtained though multilayer structures of OLEDs and dopant–host systems [5,6]. However, their processes and costs of fabrication are very high and complicated in these devices. Furthermore, the electroluminescence (EL) properties will be affected seriously by the concentration of dopants and hosts. In this regard, it will be necessary to design and synthesize some kinds of multifunctional organic molecules for single-layer OLEDs, which can be acted as charge-transporting material, highly efficient emitters with excellent performance and charge-injecting materials.
* Corresponding author. E-mail address: [email protected] (H. Zeng). 0166-1280/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2010.01.013
Such multifunctional materials are still relatively rare and frequently do not yield the highest-efficiency devices. Recently, Lin et al. [7] reported a series of carbazole-p-dimesitylborane bipolar molecules for multifunctional blue-light materials, and most of them displayed excellently photophysical properties. In order to reveal the relationship of structure and property, we choose these synthetic molecules as our target molecules. In these molecules containing a B–N bond, the response of these molecules under the electric field is mainly affected by the polarization of its p-electron. The amounts of p-electrons, the delocalization degree, and the property of polarization have direct influence on the energy structure and optoelectronic properties. Moreover, we also discuss the role of pconjugated bridge in these multifunctional molecules. The structures of the studied molecules are outlined in Fig. 1, geometrical structures and optical properties of these compounds are given by density functional theory (DFT), ab initio HF and CIS, and time-dependent density functional theory (TD-DFT) methods in this paper. 2. Computational methods All of calculations about the four molecules have been performed on the huge computer origin 2000 server center using the GAUSSIAN 03 program package [8]. Geometric and electronic structures of ground, excited-states, their cationic and anionic structures were studied by B3LYP/6-31G(d). The lowest singlet
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Fig. 1. The structures of the studied molecules.
excited-states were computed with ab initio CIS/6-31G(d) on the basis of the optimized geometries obtained from HF/6-31G(d) calculations [9]. And the absorption and emission wavelengths of these compounds were predicated by TD-DFT and TD-HF methods in the gas and solvent phase. The other important parameters, such as reorganization energy (k), ionization potentials, and electron affinities were also derived from the computed results. The compositions of molecular orbits were analyzed using the GaussView 3.0 program.
3. Results and discussion 3.1. Geometry optimization of ground-states and the first excited-state Fig. 2 plotted the optimized structures of CzB, CzPhB, CzThB and CzSB by B3LYP/6-31G(d), and the first excited-states of these molecules are optimized by CIS/6-31G(d) on the base of ground structures by HF/6-31G(d). Some important bond lengths and bond angles of these molecule in the neutral, cationic, anionic states
and excited-states obtained by B3LYP/6-31G(d) and CIS/6-31G(d) [9] calculations are summarized in Tables 1 and 2. In comparison of the structures of these molecules, CzB, CzPhB, CzThB and CzSB have been found with the same sectors on both sides of the molecules and different p-conjugated bridges in the middle. As shown in Table 1, the angles around B atom in CzB, CzPhB, CzThB and CzSB are very close to each other with the neutral, cationic, and anionic states, respectively. However, in CzSB, all of the bonds are shorter than the others, which indicate some charge transfer on excitation probably in CzSB due to the remarkably strengthened. The bond lengths in these states are analyzed and find, the bonds r(1, 2) and r(1, 3) in the neutral state are longer than in the cationic state but shorter than in the anionic state. On the other hand, the bonds r(1, 4) of these molecules in the neutral state are longer than those in the anionic but shorter than those in the cationic state. The bonds r(7, 9) of these molecules in the neutral state are longer than those in the cationic and anionic states. The results showed removing an electron will decrease the r(1, 2) and r(1, 3) anti-bonding interaction and lead to shorter distances, whereas, accepting an electron will increase the r(1, 4) anti-bonding interaction and result into much longer distance. However, whether accepting or removing an electron will both decrease the r(7, 9) anti-bonding interaction and change the bonds to much shorter distance. As shown the geometries of excited-states of these molecules in Table 2, some of the bond lengths are lengthened or shortened according to the results by comparing with the ground-states. The differences between the ground (S0) and first singlet excitedstate (S1) may be attributed to molecular orbits (MO) nodal patterns. It is well-known that the lowest singlet state corresponds to an excitation from the highest occupied molecular orbit (HOMO) to the lowest unoccupied molecular orbit (LUMO) in all organic molecules. In Table 2 and Fig. 3, we can see that the HOMO has no nodes across the r(1, 4) bonds in molecule CzB, the electronic density concentrates in N-ethylcarbazole sector. However, the HOMO has no nodes across r(1, 4), r(4, 5), r(5, 6), r(6, 7), r(7, 8), r(7, 9), r(8, 10), and r(4, 10) bonds in molecule CzPhB, r(1, 4), r(4, 5), r(5, 6), r(6, 7), r(7, 8), r(7, 9), and r(4, 8) bonds in molecule
Fig. 2. The stereograph of optimized molecules CzB, CzPhB, CzThB and CzSB by DFT//B3LYP/6-31G(d).
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take the difference between their total energies of equilibrium geometry, the evaluation of the first EAs and IPs can be given by:
Table 1 Some important bond lengths (Å) and bond angles (°) of molecules CzB, CzPhB, CzThB and CzSB obtained by B3LYP/6-31G(d) calculations. CzB
r(1, 2) r(1, 3) r(1, 4) r(7, 9) h(2, 1, 4) h(2, 1, 3) h(3, 1, 4)
EEA ¼ E0 E
ð1Þ
EIP ¼ Eþ E0
ð2Þ
Neutral
Cationic
Anionic
Neutral
Cationic
Anionic
1.5883 1.5866 1.5663 – 118.7 122.1 119.2
1.5739 1.5731 1.5868 – 117.4 124.9 117.7
1.6033 1.6032 1.5423 – 119.9 119.8 120.3
1.5867 1.5863 1.5676 1.4834 118.9 122.3 118.8
1.5769 1.5768 1.5788 1.452 119.1 122.0 118.9
1.6047 1.6043 1.5344 1.47 120.2 119.8 120.1
Neutral
Cationic
Anionic
Neutral
Cationic
Anionic
1.5875 1.5882 1.5441 1.4671 119.3 123.3 117.4
1.5723 1.5686 1.5749 1.4326 117.7 124.9 117.5
1.605 1.6068 1.5095 1.4361 121.4 120.4 118.9
1.5874 1.5869 1.5665 1.4618 118.7 122.4 118.9
1.5731 1.5727 1.5882 1.4332 118.0 123.6 118.3
1.606 1.6055 1.5285 1.4296 120.1 119.6 120.3
CzThB
r(1, 2) r(1, 3) r(1, 4) r(7, 9) h(2, 1, 4) h(2, 1, 4) h(2, 1, 4)
CzPhB
According the above formula, the EAs and IPs of these molecules are listed in Table 3. For OLED materials, the low values of IP will be easy to inject the hole to hole-transporting layer; and the higher the EA of the electron-transporting layer, the easier the injection of electrons from cathode. As shown in Table 3, the values of IP and EA of these molecules are 6.66 and 0.57 eV, 6.54 and 0.75 eV, 6.43 and 0.87 eV, and 6.30 and 1.01 eV, respectively. The variation trends of IPs and EAs are opposite when the values of IPs decrease and EAs increase. On the base of the rules of IP and EA for the OLEDs materials, the orders of charge transport abilities of these molecules are CzSB > CzThB > CzPhB > CzB, which are similar with the results of reorganization energy and energy gaps. Therefore, this demonstrates these materials can be used as charge injecting and transporting materials.
CzSB
3.3. The reorganization energies of studies molecules CzThB, and r(1, 4), r(4, 5), r(5, 6), r(6, 7), r(7, 8), r(7, 9), r (8, 10), r(4, 10), r(9, 11), and r(11, 12) bonds in molecule CzSB, and the LUMO is bonding in these above-mentioned regions. Therefore, one can be expected contraction of these bonds. The dipole moments are also considered as an important properties for organic OLEDs materials, the dipole moments of CzB, CzPhB, CzThB and CzSB are 1.96 D (1.51 D), 1.97 D (1.56 D), 1.98 D (1.53 D), and 2.21 D (1.53 D) by B3LYP/6-31G(d) (HF/6-31G(d)), respectively, which suggests that compound CzSB with still benezene-base bridge has a better electron push–pull ability on excitation than others. The results are good agreement with the experimental results and the following analysis of reorganization energies.
As far as organic luminescent materials are concerned, the carrier transfer balance of both injected electrons and holes is an important issue to obtain highly efficient OLEDs, which depends largely the reorganization energy (k), ionization potentials and electron affinities of these molecules. The internal reorganization energies of CzB, CzPhB, CzThB and CzSB are calculated by DFT with a thermally activated hopping-type mechanism at room temperature [10–13]. Viewing each hopping event as a nonadiabatic electron-transfer reaction, standard Marcus theory was using to express the rate of charge motion between neighboring molecules. The expressions are listed for electron transfer as following, and that of hole transfer is similar with formula (3).
V2 p 1=2 k exp 4kB T h kkB T
3.2. Electron affinities and ionization potentials of CzB, CzPhB, CzThB and CzSB
K electron ¼
Electron affinities (EA) and ionization potentials (IP) are corresponding with the energy barriers for electron and hole injection, which are very important properties to obtain highly efficient OLEDs. In this case, EA will decide the energy barrier for the electronic injection. The molecules are calculated by adding one electron to the neutral molecules in order to achieve the effect of electronic injection. IP is counterpart to the energy barrier of hole injection, we remove one electron from the neutral molecule, and
where k is the reorganization energy, V is the coupling matrix element, kB is the Boltzmann constant and T is the temperature. According to the Eq. (3), there are two factors k and V to determine the Kelectron. Usually, V value is very limited in the amorphous organic materials. Therefore, the mobilities of electrons and holes are expected to be only dominated by the reorganization energies k [14,15]. The reorganization energy k for electron transfer can be expressed as follows:
ð3Þ
Table 2 Optimized important interring distances (Å) and dihedral angles (°) of molecules CzB, CzPhB, CzThB and CzSB with HF/6-31G(d) and CIS/6-31G(d). HF
r(1, 2) r(1, 3) r(1, 4) r(4, 5) r(4, 8) r(4, 10) r(5, 6) r(6, 7) r(7, 8) r(7, 9) r(8, 10) r(9, 11) r(11, 12) U(2, 1, 4, 8) U(2, 1, 4, 10) U(8, 7, 9, 11)
CIS
CzB
CzPhB
CzTHB
CzSB
CzB
CzPhB
CzTHB
CzSB
1.6003 1.5991 1.5816 – – – – – – – – – – – – –
1.5996 1.5993 1.5805 1.3987 – 1.3984 1.3827 1.3935 1.3934 1.4904 1.3828 – – – 25.20 –
1.5992 1.5998 1.5630 1.3623 1.7439 – 1.4227 1.3579 1.7287 1.4786 – – – 17.32 – –
1.5995 1.5995 1.58 1.4006 – 1.3976 1.3805 1.3959 1.3935 1.4761 1.3828 1.3284 1.4773 – 25.29 159.03
1.6036 1.6028 1.5549 – – – – – – – – – – – – –
1.6036 1.6033 1.56 1.4428 – – 1.4201 1.3616 1.4354 1.4361 1.4201 1.3628 – – 19.08 –
1.602 1.6021 1.5446 1.4118 1.7479 – 1.3702 1.4204 1.7654 1.4021 – – – 14.56 – –
1.6032 1.6031 1.5642 1.4145 – 1.4191 1.367 1.4275 1.4255 1.4094 1.3639 1.3949 1.4069 – 19.48 179.87
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Fig. 3. Electronic density contours of the Frontier orbits for molecules CzB, CzPhB, CzThB and CzSB in ground-states and the first excited-states.
kelectron ¼ k0 þ k ¼ E0 E0 þ ðE E Þ
ð4Þ
where E0 and E represent the energies of the neutral and anion species in their lowest energy geometries, respectively, while E0 and E represent the energies of the neutral and anion molecules with the geometries of the anion and neutral molecules, respectively. Likewise, k for hole transfer can be expressed as follows:
khole ¼ k0 þ kþ ¼ E0 E0 þ ðEþ Eþ Þ
ð5Þ
The calculated khole and kelectron values are summarized in Table 3. According to the Eq. (3) with the limited V, it can be seen that the lower the k values, the bigger the charge-transport rate, which is in agreement with the research by Nakanishi [16] and co-workers. In Table 3, the values of khole for molecules CzB, CzPhB, CzThB and CzSB are all smaller than their counterpart kelectron, this suggests that the hole transfer rates of these molecules are higher than
that of the electron transfer rates. By comparing the values of khole with kelectron of CzB, CzPhB, CzThB and CzSB, it can be seen the khole of CzB is lower than those but the kelectron is much higher than those of CzPhB, CzThB and CzSB, it indicates that CzB has a higher hole-transport rate than the other molecules but weaker electrontransport rate. In the meanwhile, we compare the difference between the khole and kelectron of all molecules and find CzB, CzPhB, CzThB and CzSB is 1.11, 0.12, 0.11 and 0.08 eV, respectively. The difference between the khole and kelectron of CzSB is about 10 times larger than the other molecules, this implies that CzB is not good for simultaneous hole-transporting and electron-transporting materials. On the other hand, the others have low difference between the khole and kelectron about 0.1 eV, which can be considered as the candidates for hole-transporting and electrontransporting materials. As the results shown in Table 3, the order of the charge-transfer ability of these molecules will be CzSB > CzPhB > CzThB > CzB. Moreover, the values k of hole and
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Fig. 3 (continued) Table 3 Reorganization energies ionization potentials (IP) and electron affinities (EA) for each molecule.
khole (eV) kelecton (eV) EA (eV) IP (eV)
CzB
CzPhB
CzThB
CzSB
0.17 1.28 0.57 6.66
0.27 0.39 0.75 6.54
0.42 0.53 0.88 6.43
0.38 0.44 1.01 6.30
electron are increased with the order from CzSB to CzThB except CzB, which only displays hole-transporting ability by above-said.
3.4. Frontier molecular orbits The electronic density contours of the Frontier orbits of considered molecules have been plotted by GaussView software in Fig. 3.
It is shown that the electronic clouds of HOMO in CzB, CzPhB, CzThB and CzSB localize on the side of N-ethyl carbazole sector, while the one of LUMO decentralizes at the whole molecule. For the hole-transporting materials, the smaller negative value of HOMO is, the more easily the molecules lose its electrons, on the other hand, the electron-transporting material with larger negative value of LUMO will accept electrons more easily. In this study, the energies of HOMO and LUMO are predicated by DFT and summarized in Table 4 of compounds CzB, CzPhB, CzThB and CzSB. In Table 4, it is noted that there are some changes in energies of HOMO and LUMO with the experiments. This may be induced by the interaction between molecules and solvents. In the results of calculation, we only consider the molecules with ideal conditions. And the gradual decrease of values in LUMO is good agreement with the experimental results. It is well-known that the higher the value of HOMO is, the easier creating a hole is, the lower the value of LUMO is, the easier creating an electron is. All of these
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Table 4 Value of the HOMO and LUMO energies, HOMO–LUMO gaps calculated by DFT, and the lowest excitation energies calculated by TD-DFT and experiment in eV for molecules CzB, CzPhB, CzThB and CzSB.
eHOMO (calculation) eLUMO (calculation) DL–H (calculation) Eg(TD) (calculation) DL–H (experiment) eHOMO (experiment) eLUMO (experiment)
CzB
CzPhB
CzThB
CzSB
5.36 1.63 3.73 3.64 3.25 6.15 3.25
5.39 1.71 3.68 3.37 3.01 5.92 3.01
5.41 1.82 3.59 3.35 2.73 5.8 2.73
5.35 1.93 3.42 3.07 2.75 5.75 2.75
molecules show that the energy of HOMO is about 5.35 eV according to the results of calculations. Comparing them with N,N0 (NPB), bis(naphthyl)-N,N0 -diphenyl-1,10 -biphenyl-4,40 -diamine which is a well-known hole-transporting material with eHOMO about 5.2 eV [17–20], it can be found values of HOMO in the molecules are very close that of NPB, so hole carrier can be injected easily into these molecules. Meanwhile, we also compare these molecules with TPBI and find, the values of LUMO in these molecules are slightly higher than that of TPBI (eLUMO, 2.7 eV). However, these molecules are still good electron-transporting materials owing to the empty p–p orbits on the boron element center, which is proved in the Lin’s paper. The energy gaps of molecules CzB, CzPhB, CzThB and CzSB are also predicated from the orbital energy differences between the HOMO and LUMO, named the HOMO–LUMO gaps (DL–H) in Table 4 [20,21]. Actually, there are three types of band gaps in experiment such as optical band gap, electrochemical band gap, and the band gap from photoelectron spectrum. The most common one is the optical band gap, which is deduced from the lowest transition energy from the ground-state to the first excited-state. In this paper, the optical band gaps (Eg) of these molecules are calculated by TD-DFT method. In Table 4, it can be seen there are discrepancies between the values of DL–H and Eg by calculation
with experimental results. The experimental results are obtained by cyclic voltammetry with solvents, which exists interaction including polarization of solvent, heating effect and so on. Moreover, the energy gaps of these molecules are also decided from onsets of absorptive spectra in the experiments, which also depend largely on the interaction of solvents. On the other hand, the theoretical predication is carried on in ideal states. Therefore, the differences between the theoretic and experimental results are necessary. In fact, the chance trends of DL–H and Eg are consentaneous with the experimental results, the orders are CzB > CzPhB > CzThB > CzSB. By analyzing the results of the DL–H and Eg, we can find the values of DL–H and Eg decrease with increasing length of p-conjugated bridges. And the electronic hopping from the ground-states to excited-states will be easy with enlarging the conjugation of these molecules. On the other hand, the values of energy gaps of these molecules are more than 3 eV, so they can be used as blue emitter in the OLEDs materials. 3.5. Electronic spectra and emission spectra The electronic spectra of these molecules have been studied by TD-DFT//B3LYP/6-31G(d) in the solvent of THF, and the emission spectra are also calculated by TD-DFT method on the basis of optimized excited-state geometries by B3LYP/6-31G(d). TD-DFT is considered as a relatively precise method to analyze the UV–visible absorption spectrum and luminescent spectrum in the organic molecules [22,23]. Table 5 summarizes the transition energies of electronic spectra, oscillator strength, configurations of the orbits and experimental results. As shown in Table 5, all of the electronic transitions are of the p ? p* type and in agreement with the experimental results. In this calculation, we use the PCM model for the THF solvent and give good agreement with observed band wavelength in the visible region. On the other hand, the oscillator strengths (f) of S0 ? S2 electronic transition are the largest in all these molecules except CzSB, which show the main contributions are from this hopping.
Table 5 Electronic spectra obtained by TD-DFT with solvent THF for molecules CzB, CzPhB, CzThB and CzSB at the B3LYP/6-31G(d) optimized geometries.
CzB
CzPhB
CzThB
CzSB
Electronic transitions
k6abs max (nm)
Exp (nm)
f
Excitation energies (eV)
Main configurations
S0 ? S1 S0 ? S2
340.3
353
0.04 0.52
3.23 3.64
HOMO ? LUMO HOMO-1 ? LUMO HOMO-2 ? LUMO
0.68 0.62 0.12
S0 ? S1 S0 ? S2
383.6 368.2
358
0.01 0.88
3.23 3.37
HOMO ? LUMO HOMO-1 ? LUMO HOMO-3 ? LUMO
0.69 0.66 0.16
S0 ? S1 S0 ? S2
378.3 369.6
390
0.04 0.84
3.27 3.35
HOMO ? LUMO HOMO-1 ? LUMO HOMO-3 ? LUMO
0.69 0.65 0.10
S0 ? S1 S0 ? S2
404.1 394.1
390
1.53 0.01
3.07 3.14
HOMO-4 ? LUMO HOMO ? LUMO HOMO-1 ? LUMO
0.11 0.66 0.68
Table 6 Calculated emission data in gas and solvent THF for molecules CzB, CzPhB, CzThB and CzSB.
CzB gas phase THF CzPhB gas phase THF CzThB gas phase THF CzSB gas phase THF
Electronic transitions
kex (nm)
S1 ? S0 S1 ? S0 S1 ? S0 S1 ? S0 S1 ? S0 S1 ? S0 S1 ? S0 S1 ? S0
388.1 394.4 408.3 421.3 436.6 452.7 454.3 475.9
Exp (nm) 398 436 455 482
f
Excitation energies (eV)
Main configurations
0.12 0.17 1.18 1.38 1.09 1.27 1.71 1.88
3.19 3.14 3.04 2.94 2.84 2.74 2.73 2.61
HOMO ? LUMO HOMO ? LUMO HOMO ? LUMO HOMO ? LUMO HOMO ? LUMO HOMO ? LUMO HOMO ? LUMO HOMO ? LUMO
0.65 0.67 0.63 0.65 0.62 0.64 0.62 0.64
X. Ouyang, H. Zeng / Journal of Molecular Structure: THEOCHEM 945 (2010) 71–77
The emission spectra of CzB, CzPhB, CzThB and CzSB are listed in Table 6, we investigate these molecules in gas and solvent phases with a chemical PCM model for THF. By comparison of the results in gas phase, it can be seen the results are good agreement with the observed peaks by using PCM model. The calculated values of the emission wavelength in the gas and solution phase are located at 388.1 and 394.4 nm, 408.3 and 421.3 nm, 436.6 and 452.7 nm, and 454.3 and 475.9 nm, respectively. On the basis of the special emission spectra, CzB, CzPhB, CzThB and CzSB can be used as bluelight emitting materials with high brightness, which are also in good agreement with the analysis of energy gaps. The emission wavelengths for these molecules exhibit gradual red-shifts of about 80 nm in the gas phase. It may be explained that these compounds have large change of the structures in the ground and excited-states. 5. Conclusions We have theoretically demonstrated the photophysical properties of CzB, CzPhB, CzThB, and CzSB by DFT, TD-DFT and CIS methods with GAUSSIAN software. According to the above research we conclude these molecules are excellent candidates for multifunctional OLED materials. The calculated values of IP and EA show that these molecules can be used as electron-transporting and holetransporting materials simultaneously. Among these molecules, it can be seen that the phene-based molecule displays higher intraand intermolecular charge-transfer ability and better equilibrium properties than that of others. Replacing the middle group with thiophene in these molecules has no positive effect on chargetransport rate but enhances the balance between the hole and electron transfer. With the gaps reducing of HOMO and LUMO, the absorption and emission wavelengths for CzB, CzPhB, CzThB, and CzSB exhibit gradual red-shifts. The Stokes shifts of these gas phase molecules are attributed to the large change of the structures in the ground and excited-states. The emission wavelengths of these molecules are located in the blue region, implying that they can be used as blue-light materials. Acknowledgments We also thank Prof. Paul. W. Ayers and Dr. Gongchang Zeng for their help in calculation. Financial support from the National Natural Science Foundation of China (Nos. 20671036, 2007A010500008
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