First principles calculations of optical and electronical properties for 2,7-carbazole derivatives as solar cells materials

Journal of Molecular Structure: THEOCHEM 908 (2009) 102–106

Contents lists available at ScienceDirect

Journal of Molecular Structure: THEOCHEM journal homepage: www.elsevier.com/locate/theochem

First principles calculations of optical and electronical properties for 2,7-carbazole derivatives as solar cells materials Dadong Liang a,*, Shanshan Tang b, Junbo Liu a, Jinghua Liu a,*, Lijuan Kang a a b

College of Resource and Environmental Science, Jilin Agricultural University, Changchun 130118, China Department of Chemistry, Northeast Normal University, Changchun 130024, China

a r t i c l e

i n f o

Article history: Received 16 April 2009 Received in revised form 12 May 2009 Accepted 12 May 2009 Available online 18 May 2009 Keywords: First principles calculations Optical properties Electronical properties 2,7-Carbazole derivatives Solar cells

a b s t r a c t A quantum chemical investigation has been performed to explore the optical and electronical properties of a series of different cores (BT, PT, and BX) molecules with oligoethylene–thiophene branches, including considered symmetry and no symmetry (for branch), for solar cell materials. The frontier molecular orbital (FMO) and band gap energy calculations for all complexes were performed at the PBE0/6-31G(d) level. The values of Eg change less than 0.1 eV depending on the different cores. On the basis of the optimized geometries, the effect of the different cores on the absorption spectra was evaluated using the TD-PBE0/ 6-31+G(d,p) level. BT core can make the absorption spectra have a red shift in comparison with others (PT and BX). The ionization potential (IP), electron affinity (EA), and reorganization energy (k) were also computed. As a result of these calculations, different cores play key roles in the change the IP, EA, and k. Moreover, the molecules with BT core have lower ke, kh, and k than those of others. Additionally, the optical and electronical properties are similar for the molecules with symmetry (for branch) or not. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction The ever-increasing demands for energy, along with the increasingly pronounced effects of global warming, have led to intense research on new energy production possibilities [1,2]. Affluent solar energy has a widely recognized capacity to become a major sustainable energy supply [3]. Incontrovertibly, the price/performance ratio will play an important role in the eventual selecting of all kinds of photovoltaic devices [4,5]. As the growing of demand for cost-competitive renewable energy options, considerable emphasis is being placed on new technologies for photovoltaic energy conversion. One approach that shows promise involves the use of low-cost organic molecular as the active layers in photovoltaic devices. The introduction of new materials that tune molecular electronic properties improving the photoconversion efficiency is important to advance in the application of organic solar cells [6–9]. However, the photocurrent is limited in these devices, due to the overlap between the spectra of the sun and the absorption spectra of the polymer is not good [10]. This poor spectral overlap causes the loss of the energy held in the sunlight, for photons with energies below about 2.0 eV are not absorbed by the devices. So increasing the spectral overlap is necessary, namely, reducing the band gap of p-conjugated polymers. To the best of our knowledge, a lot of conjugated polymers with reduced band gaps have been prepared * Corresponding authors. Tel.: +86 431 84533521. E-mail addresses: [email protected] (D. Liang), [email protected]. edu.cn (J. Liu). 0166-1280/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2009.05.011

[11–13]. Until now, experimental studies have been focused on the electrochemical and optical properties of short-chain oligomers with small band gap systems [14]. Among these oligomers, ethylene–thiophene related oligomers and polymers with small band gap have attracted much interest in recent years, owing to their applications in organic electronic devices, such as solar cells [15,16]. On the other hand, charge transport in organic materials is one of the most important properties in the performance of OLEDs [17,18], organic field effect transistors [19,20], and organic solar cells [21,22]. Recently, Leclerc and co-workers reported a new polycarbazole derivative (PCDTBT) [23] bearing a secondary alkyl side chain on the nitrogen atom of the carbazole unit that shows high solubility and some organization, resulting in a very good PCE (3.6%). It was concluded that, by improving the electronic properties of the polymeric materials, much higher efficiencies could be reached [24]. In this contribution, we report a theoretical investigation of the electronic properties of oligoethylene–thiophene molecules with different cores (Fig. 1). The only difference between 1 and 2 is that, the two ethylene–thiophene branches are symmetry in 2 (the same for 3 and 4, and 5 and 6). Cation and anion radicals are referred to as + and  super indexes, respectively. The density function theory (DFT) [25] have been remarkable successful to accurately evaluate a variety of ground state properties of large systems. The TimeDependent DFT approach (TD-DFT) [26–29] calculations have been used for the study of these molecules. The frontier molecular orbitals (FMOs) (the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)) energies, the HOMO–LUMO gaps as well as the UV–Vis spectra were also

D. Liang et al. / Journal of Molecular Structure: THEOCHEM 908 (2009) 102–106

N

S

N

S

N

reorganization energy (k) was predicted from the single point energy at the B3LYP/6-31G(d,p) level based on the PBE0/6-31G(d,p) optimized neutral, cationic, and anionic geometries. Here, the reorganization energy was just the internal reorganization energy of the isolated active organic p-conjugated systems due to ignoring any environmental relaxation and changes. Hence, the reorganization energy (k) for hole and electron transfer can be defined as [43]

N

S S

BT

S

S

1 S

N

2 N

N

S

k ¼ ½E0  E  þ ½E0  E0 ;

N

S

S

where is the energy of the cation (anion) calculated with the optimized structure of the neutral molecule. Similarly, E± is the energy of the cation (anion) calculated with the optimized cation (anion) structure, E0 is the energy of the neutral molecule calculated at the cationic (anionic) state, and finally E0 is the energy of the neutral molecule at ground state.

N

N

3 N

O

S

4 N

N

O

3. Results and discussion N

3.1. Frontier molecular orbitals

S

PX

ð3Þ

E 0

S

PT

103

N

5

S

S

N

S

6

Fig. 1. Chemical structures of the molecules we investigated.

predicted. Moreover, some other electronic properties including ionization potential (IP), electron affinity (EA), and reorganization energy (k) were investigated. In addition, the correlation between these properties and the molecular structure is also discussed. This should be helpful in other further theoretical works to be done on such kind of systems and also could be helpful to the experimental study for solar cell.

The sketches of the FMOs for the molecules we studied are plotted in Fig. 2. The calculated HOMO, LUMO energies, and HOMO–LUMO energy gaps are listed in Table 1. As shown in Fig. 2, one can find that, the FMOs are of p characters and spread over the conjugated molecules for 1–6. To supplement these studies, on the way to a detailed comparative study of the effect of the core on the electronic structure for the molecules under investigation, the TDOS and PDOS on four fragments were calculated based on the current level of theory.

2. Computation details All the calculations were carried out with the aid of Gaussian 03 package [30]. The PBE0/6-31G(d) [21–35] method was used in the all geometry optimization including neutral, cation, and anion molecules. The molecules were without any symmetry constrains (C1). Frequency calculations using the same methods as those for the geometry optimizations were performed for the obtained structures. All real frequencies have confirmed the presence of a local minimum. This observation is consistent with the report by Jacquemin [36,37] in which PBE0 appears notably adapted to sulfur-bearing molecules. And the density of states (DOS) calculated and convoluted using Pymolyze 2.0 [38]. The UV–Vis spectra of all the compounds were investigated with the TD-PBE0/6-31+G(d,p) [33– 35] based on the optimized geometries obtained by the PBE0/631G(d,p) method in gas. Recently, the B3LYP/6-31G(d,p) [39] functional was successfully used to calculate charge transport parameters for thiophene oligomers [40]. And in order to compare with the results reported previously [41,42], the B3LYP/6-31G(d,p) single point calculations were performed and all the parameters necessary for the calculation of electronic properties recomputed, such as single point energies of the neutral, cation, and anion molecules in S0. The ionization potential (IP) and electron affinity (EA) of the molecules were calculated as described in the formulas below

IP ¼ EðM þ Þ  EðMneutral Þ; EA ¼ EðMneutral Þ  EðM  Þ;

ð1Þ ð2Þ

where E (Mneutral) is the total energy of the neutral form at the optimized geometry, E (M+) is the total energy of the cationic and E (M) is that of the anionic forms of the molecule. In addition, the

Fig. 2. The FMOs of 1–6 computed at the PBE0/6-31G(d,p) level.

104

D. Liang et al. / Journal of Molecular Structure: THEOCHEM 908 (2009) 102–106

Table 1 A comparative study of the predicted energies for the FMOs, HOMO–LUMO gaps, and the longest wavelength of maximal absorption wavelength for compounds obtained at the TD-PBE0/6-31+G(d,p)//PBE0/6-31G(d,p) level.a Species

HOMO

LUMO

Eg

Assignment

E(TD)

kabs (nm)

f

1

5.27

2.54

2.73

2.26

547.90

0.72

2

5.28

2.55

2.73

2.28

543.84

0.78

3

5.43

2.80

2.63

2.17

570.07

0.67

4

5.40

2.77

2.63

2.18

568.34

0.68

5

5.57

2.88

2.69

2.30

538.71

0.99

6

5.54

2.88

2.67

H ? L (0.62) H ? L + 1 (0.15) H ? L (0.62) H ? L + 1 (0.14) H ? L (0.63) H ? L + 1 (0.14) H ? L (0.63) H ? L + 1 (0.14) H ? L (0.62) H ? L + 1 (0.11) H ? L (0.62) H ? L + 1 (0.10)

2.29

542.22

0.99

a f = oscillator strength; kabs = maximum absorption wavelength; Eg = HOMO– LUMO gap; E(TD) = the values obtained from the TD calculation; the values are the energies in eV for HOMO, LUMO, Eg, and E(TD).

The results for derivatives are plotted in Fig. 3. As clearly shown in this figure, the PDOS reveals that the HOMOs are fairly localized on the core with only minor but nonzero contributions from its atoms, the main contributions come from the ethylene–thiophene fragments. In the same figure one can find that the LUMOs are highly delocalized throughout the ethylene–thiophene with the main contributions coming from the core. It is also can be found that, the DOS and PDOS of 1 and 2 are nearly the same (similar situation for 3 and 4, and 5 and 6). Information regarding the results can also be obtained from Fig. 2. As shown in Table 1, the HOMO and LUMO energies of 1–6 change significantly, which are 5.27 and 2.54 eV, 5.28 and 2.55 eV, 5.43 and 2.80 eV, 5.40 and 2.77 eV, 5.57 and 2.88 eV, and 5.54 and 2.88 eV, respectively. It can also be found that, the HOMO and LUMO energies of 1 and 2 are nearly the same (similar situation for 3 and 4, and 5 and 6). This implies that different cores play key roles in weakening the hole-creating properties and enhancing the electron-accepting ability, and the effect of the symmetry for ethylene–thiophene branch on the HOMO and LUMO energies can be neglected. Moreover, the Eg energies of 1–6 vary slightly from 2.63 eV to 2.73 eV depending on the different cores. They are investigated in the following order 1 = 2 > 5 > 6 > 3 = 4, and the effect of the symmetry for ethylene–thiophene branch on the Eg energies is also small. 3.2. Absorption spectra The longest wavelength of maximal absorption wavelength and corresponding oscillator strength of the molecules are listed in Table 1. According to calculations, the longest wavelength of maximal absorption wavelength is observed for 3, whereas the smallest for 5. The max-absorption wavelength for 1 is 547.90 nm, and 2 (543.84 nm) is similar with it. Moreover, 3 has a red-shift of about 23 nm compared with 1, and 4 has a 21 nm blue-shift in comparison with 1. Furthermore, the longest wavelength of maximal absorption wavelength of 5 and 6 are 538.71 and 542.22 nm, respectively. In the case of oscillator strength for 1–6, the difference does not exceed 0.22. From these results, it is noteworthy that, the symmetry of ethylene–thiophene branch does not lead to any significant effect on the absorption spectra, and the PT core can make the absorption spectra have a red shift (vs. BT and PX). 3.3. Electronic properties According to our calculations with the aim to get an in-depth explanation of the relationship between the structure and the

electronic behavior of the molecule, in particular the response of the molecule to the formation of a hole, or to the addition of an electron, additional information is derived. Ionization potentials (IP), electron affinities (EA), both vertical (v: at the geometry of the neutral molecule) and adiabatic (a: at the optimized structures of both the neutral and charged molecule) were computed at the B3LYP/6-31G(d,p) level on the basis of the optimized structures at the PBE0/6-31G(d) level. Calculation results are shown in Table 2. As we know that for the materials, the lower the IP of the hole-transport layer (HTL), the easier the entrance of holes from HTL to ITO; and the lower the EA of the electron-transport layer (ETL), the easier the entrance of electrons from ETL to cathode [44]. The results presented in Table 2 show that, the IPa in 1–6 are ranging from 5.92 eV for 6 to 6.27 eV for 3, they are predicted in decreasing order of 3 > 4 > 2 > 1 > 5 > 6. And the range of IPv for 1–6 is from 6.26 eV to 6.57 eV, the order for them is 3 > 4 > 2 > 1 > 5 = 6. On the other hand, the lowest EAa among 1–6 is observed for 1 (1.44 eV), and all the EAa are investigated in the following order 1 < 2 < 4 = 3 < 5 = 6. And the EAv for 1–6 are ranging from 1.29 eV to 1.59 eV, they increase in the order 1 < 2 < 4 < 3 < 5 = 6. From the results one can found that, the IPa, IPv, EAa, and EAv for 1 and 2 are nearly the same (similar situation for 3 and 4, and 5 and 6), respectively, and the molecules with PX core have higher IPv and EA than the ones with BT and PT cores. However, the values of IPa for molecules with PX core are smaller than the ones with BT and PT cores. It is noticeable that, the effect of the symmetry for ethylene–thiophene branch on the values of IP and EA is can be neglected. Fig. 4 shows the distribution of polaron cations and anions in 1+, 1, 3+, 3, 5+, and 5, respectively. As seen in Fig. 4, the polaron cations and anions are localized essentially over the entire molecules for 1+, 1, 3+, 3, 5+, and 5, respectively. Thus, the different cores (BT, PT, and PX) do not influence the distribution of the polaron cations and anions for the molecules, respectively. Our calculations of the reorganization energy associated with different geometries of two states (anion and cation) are based on the hopping model schematically illustrated in Fig. 5. For each molecule, the geometry was optimized for both the neutral and the cation (anion) states at the PBE0/6-31G(d,p) level. The single point energies corresponding to the neutral and the cationic/anionic electronic configurations were then computed for each of the two optimized geometrical structures at the B3LYP/6-31G(d,p) level in order to compare with the values available. Thus, for each derivative we obtained a set of four energy values, corresponding to the neutral molecule at the neutral geometry, the cation (anion) at the neutral geometry, the neutral molecule at the cation (anion) geometry, and the cation (anion) at the cation (anion) geometry (Fig. 5). The calculated reorganization energies for hole and electron according to Formula 3 are listed in Table 2. The reorganization energies of 1–6 computed for electron are between 0.30 eV and 0.82 eV, while for kh are in the range of 0.20–0.72 eV. And for electron ke in 1–4 (between 0.30 and 0.32 eV), they are slightly larger than that of Alq3 which is a typical electron transport materials (ke = 0.276 eV) [41], while the kh of 1– 4 are smaller than that of TPD which is a typical hole transport materials (kh = 0.29 eV) [42]. It is well-known that, the smaller the k values, the bigger the charge-transport rate. The smaller kh would suggest that the carrier mobilities of the hole are larger than that of the electron [44]. It indicates that the molecules (1–4) can be a series of good hole transport materials from the stand point of the smaller reorganization energy. As shown in Table 2, the ke of 1–6 are 0.30, 0.30, 0.32, 0.33, 0.81, and 0.82 eV, and the kh of them are 0.20, 0.21, 0.21, 0.21, 0.72, and 0.72 eV, respectively. Meanwhile, the smallest k (ke + kh) among 1–6 is observed for 1 (0.50 eV), whereas the greatest for 6 (1.54 eV), and they increase

D. Liang et al. / Journal of Molecular Structure: THEOCHEM 908 (2009) 102–106

105

Fig. 3. Total and partial density of states (TDOS and PDOS) around the HOMO–LUMO gap for 1–6 (dashed vertical lines indicate the HOMO and LUMO energies, respectively).

Table 2 Calculated molecular ionization potential (IPa and IPv), electron affinity (EAa and EAv), and reorganization energy (ke, kh, and k) for compounds.* Species

IPaa

IPvb

EAac

EAvd

kee

khf

kg

1 2 3 4 5 6

6.12 6.13 6.27 6.24 5.94 5.92

6.26 6.26 6.42 6.39 6.57 6.55

1.44 1.46 1.67 1.67 2.26 2.26

1.29 1.31 1.51 1.50 1.59 1.59

0.30 0.30 0.32 0.33 0.81 0.82

0.20 0.21 0.21 0.21 0.72 0.72

0.50 0.51 0.53 0.54 1.53 1.54

* a

Adiabatic IP, bvertical IP, cadiabatic EA, dvertical EA, eelectron reorganization, hole reorganization, gtotal reorganization. The values are the energies in eV.

f

in the following order 1, 2, 3, 4, 5, and 6. According to calculations, one can see that, the ke, kh, and k for 1 and 2 are similar (similar situation for 3 and 4, and 5 and 6), respectively, and the molecules with PX core have greater kh, ke, and k than the ones with BT and PT cores. Moreover, the values of kh, ke, and k for molecules with BT and PT cores are nearly the same, respectively. Obviously, the symmetry of ethylene–thiophene branch does not lead to any significant effect on the values for kh, ke, and k. These molecules 1–4 are predicted to show low reorganization energies, in those for already well known charge-transport materials with variable frontier molecular orbitals levels, which are sometimes needed to fit the work function of solar cell materials.

106

D. Liang et al. / Journal of Molecular Structure: THEOCHEM 908 (2009) 102–106

lower IPa and higher IPv, EA, kh, ke, and k than the ones with BT and PT cores, respectively. To summarize the above discussion, we notice that the molecules with PT core have the longest wavelength of maximal absorption and lower k among the models in this paper, which can be a better candidate for solar cell material. Hence the present results should provide useful information for prepare photovoltaic devices. References

Fig. 4. PBE0/6-31G(d,p) optimized geometries and unpaired spin density distribution in cation and anion radicals of 1+, 1, 3+, 3, 5+, and 5.

Fig. 5. Internal reorganization energy for hole and electron transfer.

4. Conclusion In the present work, our computational results predicted the electronic and optical properties for a series of newly designed derivatives. They were studied by means of quantum chemical method based on the DFT-PBE0, allowing reliable predictions and interpretations of the structural and electronic properties of organic molecules bearing sulfur atoms. The absorption spectra were evaluated at the TD-PBE0/6-31+G(d,p) level. The effect of the ethylene–thiophene branch symmetry on the absorption spectra was can be neglected, and the PT core could make the absorption spectra have a red shift (vs. BT and PX). The IP, EA, and reorganization energy of the derivatives were also investigated on the basis of the B3LYP/6-31G(d,p) single point energies. The IPa, IPv, EAa, EAv, ke, kh, and k for 1 and 2 are nearly the same (similar situation for 3 and 4, and 5 and 6), respectively. And the molecules with PX core had

[1] P. Szuromi, B. Jasny, D. Clery, J. Austin, B. Hanson, Science 315 (2007) 781. [2] D. Clery, Science 315 (2007) 782. [3] M.D. Archer, R. Hill, Clean Electricity from PhotoVoltaics, Imperical College Press, London, 2001. [4] D. Butler, Nature 454 (2008) 558. [5] A. Nantalaksakul, A. Mueller, A. Klaikherd, C.J. Bardeen, S. Thayumanavan, J. Am. Chem. Soc. 131 (2009) 2727. [6] J.G. Xue, S. Uchida, B.P. Rand, S.R. Forrest, Appl. Phys. Lett. 84 (2004) 3013. [7] K.L. Mutolo, E.I. Mayo, B.P. Rand, S.R. Forrest, M.E. Thompson, J. Am. Chem. Soc. 128 (2006) 8108. [8] R.F. Bailey-Salzman, B.P. Rand, S.R. Forrest, Appl. Phys. Lett. 91 (2007) 013508. [9] Y. Shao, Y. Yang, Adv. Mater. 17 (2005) 2841. [10] C. Winder, N.S. Sariciftci, J. Mater. Chem. 14 (2004) 1077. [11] L.M. Campos, A. Tontcheva, S. Gunes, G. Sonmez, H. Neugebauer, N.S. Sariciftci, F. Wudl, Chem. Mater. 17 (2005) 4031. [12] F. Zhang, W. Mammo, L.M. Andersson, S. Admassie, M.R. Andersson, O. Inganas, Adv. Mater. 18 (2006) 2169. [13] E. Perzon, F.L. Zhang, M. Andersson, W. Mammo, O. Inganas, M.R. Andersson, Adv. Mater. 19 (2007) 3308. [14] H.A.M. Van Mullekom, J.A.J.M. Vekemans, E.W. Meijer, Chem. Eur. J. 4 (1998) 1235. [15] Y. Wang, E.J. Zhou, Y.Q. Liu, H.X. Xi, S.H. Ye, W.P. Wu, Y.L. Guo, C. Di, Y.M. Sun, G. Yu, Y.F. Li, Chem. Mater. 19 (2007) 3361. [16] J.H. Hou, Z. Tan, Y.J. He, C.H. Yang, Y.F. Li, Macromolecules 39 (2006) 4657. [17] C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913. [18] M.A. Baldo, D.F. O’Berien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151. [19] F. Garnier, R. Hajlaoui, A. Yassar, P. Srivastava, Science 265 (1994) 1684. [20] H. Sirringhaus, P.J. Brown, R.H. Friend, M.N. Nielsen, K. Bechgaard, B.M.W. Langeveld-Voss, A.J.H. Spiering, R.A.J. Janssen, E.W. Meijer, P. Herwig, D.M. de Leeuw, Nature 401 (1999) 685. [21] N.S. Sariciftci, L. Smilowitz, A.J. Heeger, F. Wudl, Science 258 (1992) 1474. [22] J.J.M. Halls, C.A. Walsh, N.C. Greenham, E.A. Marseglia, R.H. Friend, S.C. Moratti, A.B. Holmes, Nature 376 (1995) 498. [23] N. Blouin, A. Michaud, M. Leclerc, Adv. Mater. 19 (2007) 2295. [24] N. Blouin, A. Michaud, D. Gendron, S. Wakim, E. Blair, R. Neagu-Plesu, M. Belletête, G. Durocher, Y. Tao, M. Leclerc, J. Am. Chem. Soc. 130 (2008) 732. [25] R.G. Parr, W. Yang, Density Functional Theory of Atoms and Molecules, Oxford University Press, Oxford, 1989. [26] R.E. Stratmann, G.E. Scuseria, M.J. Frisch, J. Chem. Phys. 109 (1998) 8218. [27] R. Bauernschmitt, R. Ahlrichs, Chem. Phys. Lett. 256 (1996) 454. [28] D. Jacquemin, E.A. Perpte, I. Ciofini, C. Adamo, Acc. Chem. Res. 42 (2009) 326. [29] V. Barone, A. Polimeno, Chem. Soc. Rev. 36 (2007) 1724. [30] M.J. Frisch et al., Gaussian 03, Revision B.03, Gaussian, Inc., Pittsburgh, PA, 2003. [31] C. Adamo, V. Barone, J. Chem. Phys. 110 (1999) 6158. [32] M. Ernzerhof, G.E. Scuseria, J. Chem. Phys. 110 (1999) 5029. [33] P.C. Hariharan, J.A. Pople, Mol. Phys. 27 (1974) 209. [34] M.S. Gordon, Chem. Phys. Lett. 76 (1980) 163. [35] M.J. Frisch, J.A. Pople, J.S. Binkley, J. Chem. Phys. 80 (1984) 3265. [36] D. Jacquemin, E.A. Perpete, Chem. Phys. Lett. 429 (2006) 147. [37] D. Jacquemin, J. Preat, V. Wathelet, M. Fontaine, E.A. Perpete, J. Am. Chem. Soc. 128 (2006) 2072. [38] A.L. Tenderholt, Pymolyze, Version 2.0, Stanford University, Stanford, CA, 2007. [39] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [40] E.G. Kim, V. Coropceanu, N.E. Gruhn, R.S. Sanchez-Carrera, R. Snoeberger, A.J. Matzger, J.L. Brédas, J. Am. Chem. Soc. 129 (2007) 13072. [41] B.C. Lin, C.P. Cheng, Z.Q. You, C.P. Hsu, J. Am. Chem. Soc. 127 (2005) 66. [42] N.E. Gruhn, D.A. da Silva Filho, T.G. Bill, M. Malagoli, V. Coropceanu, A. Kahn, J.L. Bredas, J. Am. Chem. Soc. 124 (2002) 7918. [43] M.E. Köse, W.J. Mitchell, N. Kopidakis, C.H. Chang, S.E. Shaheen, K. Kim, G. Rumbles, J. Am. Chem. Soc. 129 (2007) 14257. [44] 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.