Theoretical investigation of the conducting properties of substituted phosphole oligomers

Computational and Theoretical Chemistry 980 (2012) 68–72

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Theoretical investigation of the conducting properties of substituted phosphole oligomers Ahmed M. El-Nahas ⇑, Ahmed H. Mangood, Tamer S. El-Shazly Chemistry Department, Faculty of Science, El-Menoufia University, Shebin El-Kom, Egypt

a r t i c l e

i n f o

Article history: Received 7 July 2011 Received in revised form 1 October 2011 Accepted 8 November 2011 Available online 20 November 2011 Keywords: Phosphole oligomers Substituent effects Frontier molecular orbitals Charge carrier injection rate DFT

a b s t r a c t The conducting properties of phosphole oligomers have been investigated using density functional theory (DFT) at B3LYP/6-31G(d). Effects of electron-donor substituents (OCH3 and NHCH3) on the electrical conductivity of phosphole oligomers and polymers were studied. A significant reduction of the HOMO–LUMO energy gap (Eg), ionization energy (IE), and enhancing electron affinity (EA) has been obtained. Moreover, NHCH3 substituted oligomers show p-type conductivity regardless of the position of substitution where the hole injection rate oscillates between 104 and 1018. However, the OCH3 substituted oligomers at Cb indicates p-type conductivity with hole injection rate of 1010–1014 and n-type conductivity when substituted at the P-center with the electron injection rates range between 102 and 106. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Over the last decades, organic conductors or semiconductors based on oligomers/polymers of heterocyclic pentadienes attracted much attention due to their promising electronic properties [1–19]. Electrical conductivity of thiophene, pyrrole and furan has been intensively investigated [1,3,5–7,10–17,20–22,26– 30,32–36]. Oligomers of phospholes have also been studied [1–8,11,17–19,23–25,29,31,35–37]. Phosphole oligomers are considered as good building blocks with p-conjugated systems that cause some interesting properties [1,4,8]. One of the most important strategies to fine-tune the optical or electronic properties of oligomers is their grafting by proper substituents [5,8,11,17,18, 20,21,23,24,29]. This also may enhance other properties such as thermal stability and self assembly [16,18]. Substitution at P-center activates hyperconjugation between the exocyclic P–R r-bond with the endocyclic p-system [18,23]. This extension of the conjugation facilitates the mobility of electrons along the molecular backbone. The spatial distribution of HOMO and LUMO gives qualitative overview about the conductivity properties [27,32]. Substitution at C or P atoms was found to affect HOMO–LUMO energy gap (Eg), ionization energy (IE), and electron affinity (EA) [18,23,24]. The HOMO–LUMO energy gap represents an important parameter for predicting conductivity [20]. Both IE and EA are used for estimating the energy barrier for injection of holes and electrons to the investigated species ⇑ Corresponding author. E-mail address: [email protected] (A.M. El-Nahas). 2210-271X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.comptc.2011.11.020

reflecting its p- or n-type conductivity, respectively [27]. Design of molecular electronic devices such as organic light emitting diodes (OLED) depends on their efficiency for hole and electron injection [2]. The planarity and bond alternation parameters, d, of the oligomers represent two important criteria which affect extension of the p-conjugation [30]. Planarizing phosphorus atom in phosphole increases overlap between phosphorous lone pairs of electrons and the p-electrons of the pentadiene moiety and, therefore, enhances electron mobility across the oligomer/polymer backbone [23,24]. Very recently, the synthetic chemistry of polyphospholes has witnessed a great progress [19]. Here, we will consider oligomers of phosphole substituted at Cb and P atoms with OCH3 and NHCH3 groups as electron-donors. Phosphole monomers were sequentially connected via a– carbons in a head to tail (HT) fashion in order to manipulate the geometrical and electronic properties, and consequently the electrical properties of the corresponding polymers. The aim of this work was to investigate the electrical conductivity of phosphole oligomers (X-phosph)n (X = NHCH3, OCH3, n = 1–7) as depicted in Fig. 1. Extrapolation approach was used to predict the conduction properties of corresponding polymers. Density functional theory (DFT) at the B3LYP/6-31G(d) level was employed to conduct this project. 2. Computational details Electronic structure calculations have been carried out using the Gaussian 03 program package [38]. Geometries of the neutral

A.M. El-Nahas et al. / Computational and Theoretical Chemistry 980 (2012) 68–72

69.28 68.52 54.49 67.31 58.42

av. P7 P6

71.78 68.34 57.45 69.10 62.22 71.98 69.76 57.63 69.41 61.82

P5 P4

72.02 68.85 57.60 69.56 62.93 71.98 70.10 57.59 69.68 62.20

P3 P2

71.79 68.69 56.97 69.65 61.21 69.29 69.46 54.80 69.48 58.46 179.30 170.57 178.77 175.20 173.01

av.

U6 U5 U4 U3 U2

178.03 169.50 178.92 175.73 169.53

P1

Schematic diagram of phosphole oligomers is displayed Fig. 1. Pyramidalization at phosphorus (x), as emerged from the difference between 360° and sum of bond angles around the P atom [31], is reduced by 10.3–14.6° for substitution at P, Table 1. The co-planarity of phosphole oligomers was found to be unaffected by substitution, Table 1. Bond alternation parameter, d, is defined as difference in bond length between two adjacent single and double bonds. The smaller the value of d, the better is the p-conjugation. Two types of bond alternation were considered; bond alternation within the ring (dintra, rbb  rab) and bond alternation between rings (dinter, raa  rab). The dintra, and dinter bonds alternations are given in Table 1. The calculations showed that dintra and dinter decrease with increasing number of units of substituted oligomers. Both dintra and dinter decrease (ca. 0.001–0.004 Å) and (ca. 0.012–0.022 Å), respectively, upon substitution at Cb. The dintra increases (by 0.005–0.009 Å) with substitution at P. These results

U1

3.1. Geometric properties

0.062 0.058 0.067 0.059 0.070

3. Results and discussions

H/. . . NHCH3/Cb NHCH3/P OCH3/Cb OCH3/P

where msub and munsub are the electron injection rate for the substie e tuted and un-substituted oligomers, while msub and munsub are the h h hole injection rate for substituted and unsubstituted oligomers. For the same oligomer, the difference between EAs (Eq. (1)) or IEs (Eq. (2)) equals zero producing carrier injection rate of 1.00 [29].

P-Atoms Pyramidality x/°

ð2Þ

Torsion angel U/°

!

(av.dinter)/Å

ð1Þ

(av. dintra)/Å

msub IEsub  IEunsub h ¼ exp unsub kT mh

!

(X-phosph)7

msub EAunsub  EAsub e ¼ exp unsub kT me

Table 1 Bond alternation parameters averages (dintra and dinter), torsion angles U and P-atoms Pyramidality (x) for the studied Heptamer.

and singly charged monomers and oligomers were fully optimized at the B3LYP/6-31G(d) level [39–41]. Frequency calculations have been performed at the same level to ensure that the located stationary points are minima and to correct energies for zero-point vibrational energies. Vertical ionization energies and electron affinities were calculated from the energy difference between charged and neutral species at the optimized geometries of the latter [20,25,28]. For predicting p- or n-type conductivity, charge carrier injection rate [29] was used. The HOMO and LUMO energies are used qualitatively for calculating the charge carrier injection rate [29]. The Schottky-type injection barrier formula used for the charge carrier injection rate calculations.  This  can  be simply described by Boltzmann-like distribution m /  DkTE , where m is the carrier of injection rate, DE is the magnitude of the injection barrier, k is the Boltzmann constant and T is the absolute temperature. The relationships of the carrier injection rate as a function of IE and EA can be described as follows:

179.97 170.52 179.60 175.30 172.28

Fig. 1. Molecular structure of phosphole oligomers showing the bond alternation parameters (raa, rab and rbb) and Torsion angle U, X = NHCH3, OCH3.

179.98 170.89 178.56 175.37 171.00

rαα X

179.93 170.80 179.33 175.51 172.83

P H

179.95 170.66 177.88 175.26 173.07

rαβ

φ

177.98 171.07 178.36 174.02 179.33

P H

rββ

0.059 0.037 0.059 0.046 0.057

H P

71.16 69.10 56.65 69.17 61.04

X

X

69

70

A.M. El-Nahas et al. / Computational and Theoretical Chemistry 980 (2012) 68–72

Table 2 IE, EA, hole and electron injection rates of (NCH3-phosph)n and (OCH3-phosph)n. (NHCH3-phosph)n

(OCH3-phosph)n

n/position

IE (eV)

EA (eV)

m

n = 1/Cb n = 1/P n = 2/Cb n = 2/P n = 3/Cb n = 3/P n = 4/Cb n = 4/P n = 5/Cb n = 5/P n = 6/Cb n = 6/P n = 7/Cb n = 7/P

7.194 8.033 6.039 6.674 5.418 6.056 5.063 5.654 4.743 5.394 4.548 5.193 4.376 5.049

1.403 0.942 0.278 0.090 0.267 0.750 0.617 1.085 0.878 1.368 1.071 1.530 1.210 1.699

7.01E+22 4.48E+08 3.63E+18 6.81E+07 8.43E+17 1.38E+07 1.80E+17 1.78E+07 6.65E+15 6.70E+04 1.35E+18 1.67E+07 3.54E+18 1.45E+07

sub unsub h = h

m

IE (eV)

EA (eV)

unsub msub h =mh

unsub msub e =m e

1.11E04 6.88E+03 1.22E07 2.05E01 1.11E09 1.63E01 1.03E10 8.44E03 4.29E11 8.21E03 2.47E11 1.46E03 1.28E11 2.34E03

7.968 8.263 6.534 7.009 5.743 6.416 5.333 5.972 4.998 5.614 4.774 5.416 4.594 5.260

1.343 0.639 0.147 0.498 0.532 1.146 0.847 1.478 1.165 1.648 1.356 1.852 1.512 2.011

5.77E+09 5.75E+04 1.55E+10 1.48E+02 2.71E+12 1.12E+01 4.73E+12 7.38E+01 3.34E+11 1.23E+01 2.01E+14 2.83E+03 7.39E+14 3.93E+03

1.16E03 9.22E+08 1.97E05 1.60E+06 3.42E05 8.32E+05 7.95E07 3.67E+04 3.03E06 4.48E+02 1.62E06 4.01E+02 1.60E06 4.51E+02

sub unsub e = e

m

HOMO

LUMO

m

Monomer

Dimer

Trimer

Tetramer

Pentamer

Hexamer

Heptamer

Fig. 2. The special distribution of frontier molecular orbitals HOMO and LUMO for the unsubstituted phosphole oligomers.

of inter-ring bonds alternations and the chain co-planarity reinforce the p-conjugation system which agrees the findings reported by Pham-Tran and Nguyen [36] for the fluorinated and perfluoroarene-modified phosphole oligomers. Therefore, the current investigated oligomers are expected to be good candidates for n- and/or p-type semiconducting materials. 3.2. IE, EA and their effects on p-type or n-type conductivity Table 2 collects the calculated IE, EA, and relative hole and electron injection rates for the investigated oligomers. The IE/EA decreases/increases with enlarging oligomer size as a result of

-0.643

-1.0 -1.5

orbital Level (eV)

-2.0 -2.5 -3.0

-1.015 -1.82 -2.201 -2.423 -2.568 -2.668 -2.741

-3.5

-3.502 -3.609 -3.726 -3.917 -4.114 -4.469

-4.0 -4.5 -5.0 -5.5

-4.569 -4.630 -4.714 -4.838 -5.036 -5.135 -5.393

-6.0 -6.5

-0.5

LUMO

-1.097 -1.294 -1.585 -1.779 -1.676 -1.913 -2.076 -2.025 -2.242 -2.103 -2.39 -2.462 -2.543

-4.239 -4.347 -4.466 -4.691 -5.028

-4.184

HOMO

-5.932 -6.25

-1.0 -1.5 -2.0

orbital Level (eV)

-0.5

destabilizing/stabilizing HOMO/LUMO indicating high potential for losing/accepting electrons. The IE and EA can shed light on the degree of hole and electron injection to form p- and n-type semiconductors [36]. The NHCH3 substituted oligomers show p-type conductivity regardless of the position of substitution where the hole injection rate oscillates between 104 and 1018. However, the OCH3 substituted oligomers at Cb indicates p-type conductivity with hole injection rate of 1010 and 1014 and n-type conductivity when substituted at the P-center with the electron injection rates range between 102 and 106. The latter case is similar to the P–F substituted phospholes [36] which was attributed to LUMO stabilization. Comparing the injection rate

-2.5

Cβ

at P

(NHCH3-phosph) Fig. 3. The substitution effect on the HOMO and LUMO energy levels of NHCH3phosph.

LUMO -1.434

-1.465 -1.82 -2.201 -2.423 -2.568 -2.668 -2.741

-1.875 -2.031 -2.211 -2.316 -2.4

-3.0 -3.5

-3.714 -3.821 -3.955 -4.157 -4.395

-4.0 -4.5 -5.0 -5.5

-4.569 -4.63 -4.714 -4.838 -5.036 -5.393

-6.0 -6.5

phosph

-0.750 -1.015

-4.911

-4.406 -4.475 -4.568 -4.785 -5.036 -5.342

HOMO

-5.828

-6.12

-6.25

phosph

-2.105 -2.489 -2.634 -2.669 -2.77-2.846

Cβ

at P

(OCH3-phosph) Fig. 4. The substitution effect on the HOMO and LUMO energy levels of OCH3phosph.

71

3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8

E =4.8901(1/n)+1.1609 g

E =4.6659(1/n)+0.9443

2 R =0.99 E =4.9079(1/n)+0.8033

R =0.99 E =5.8749(1/n)+0.54904

R =0.99 E =4.7735(1/n)+0.9936

R =0.99

g 2

Phosph NHCH3-Phosph-C β NHCH3-Phosph-P OCH3-Phosph-C β OCH3-Phosph-P

g 2

2.2

g 2

2.0 1.8

g 2

1.6

R =0.99

1.4 1.2

EA (eV)

Eg (eV)

A.M. El-Nahas et al. / Computational and Theoretical Chemistry 980 (2012) 68–72

Phosph NHCH3-Phosph-Cβ NHCH3-Phosph-P OCH3-Phosph-Cβ OCH3-Phosph-P

1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

-0.6

EA= -4.8006(1/n)+2.4711 2 R =0.99 EA= -4.1436(1/n)+1.7279 2 R =0.99 EA= -4.4313(1/n)+2.2637 2 R =0.99 EA= -4.5806(1/n)+2.0939 2 R =0.99

0.10

1/n

0.15

0.20

0.25

EA= -4.1155(1/n)+2.5317 2 R =0.99

0.30

0.35

0.40

0.45

0.50

0.55

1/n Fig. 5. Extrapolation of HOMO–LUMO gap (Eg) for (unsubstituted phosph), (NHCH3phosph), (OCH3-phosph) at Cb and P atoms vs 1/n.

of the substituted phosphole oligomers reported here with that given by Zhang et al. [29] for substituted heterocyclic pentadienes reveals enhanced rates of hole injection in the former oligomers. For example, CH3 substituted phosphole trimer gives hole injection rate lower than that recorded for the corresponding Cb-NHCH3 and –OCH3 ones, by 1014 and 109, respectively. For the hexamers, IEs of the investigated systems have low values (4.77 to 5.62 eV) compared to fluorine substituted phospholes (5.73 to 6.69 eV) [36]. This indicates an improved p-type conductivity for the former case.

3.3. Frontier molecular orbitals (FMO) The spatial distribution of the HOMO and LUMO of the unsubstituted oligomers are depicted in Fig. 2, while those of substituted oligomers are presented in the Supporting information. As shown in Fig. 2, frontier orbitals spread over the whole backbone of the substituted and unsubstituted p-conjugated oligomers. This indicates that all of investigated oligomers could serve as molecular wire [33]. Figs. 3 and 4 illustrate substituent effect on the energies of HOMO and LUMO. The NHCH3 substituent at Cb (P) destabilizes both HOMO and LUMO by 1 (0.4) and 0.5 (0.2) eV, respectively. Thus, the NHCH3 substituted phosphole oligomers can possess p-type characteristics. For OCH3 substitution, two behaviors were

7.2 6.9 6.6 6.3

recorded: destabilization of both HOMO and LUMO by 0.6 and 0.4 eV when substituted at Cb, whereas substitution at phosphorous destabilizes HOMO by 0.1 eV and stabilizes LUMO by 0.2 eV. Accordingly, p- and n-type conductivities are expected for oligophospholes substituted at Cb and P, respectively. This finding is consistent with Pham-Tran and Nguyen [36] where the conductivity can be fine-tuned via substituents at proper positions of the oligomer backbone. It was noticed that 1F-sexiphosphole (6-1F-P) and perfluoroarene-modified 1F-phosphole oligomers (1F-FPPPPF) possess n-type properties, while perfluoroarene-modified 1Hphosphole (1H-PFPPFP) has p-type conductivity [36]. In the current study, p- and n-type properties could be achieved for the same substituent via its substitution at different position as noticed in the case of -OCH3 substitution. Regardless of the substitution position, the HOMO–LUMO energy gap shows the following trend: (OCH3-phosph)n < (NHCH3phosph)n with the large reduction recorded for (OCH3-phosph)n oligomers. Comparing the current results with furan oligomers at the same level of theory [20] indicates lowering of Eg (by ca. 1 eV) in the former case which reflects the importance of replacing oxygen atom by phosphorous in modifying the electrical property of these oligomers. The (OCH3-phosph-Cb)6 oligomer has lower Eg compared to the fluorinated and perfluoroarene-modified thiophene and phosphole hexamers [36] by approximately 0.91–1.16 eV and 0.20–0.76 eV, respectively.

IE=4.7359(1/n)+4.8206 2 R =0.99

IE=4.9390(1/n)+4.6362 2 R =0.99

3.4. Electronic properties of infinite chains (polymer)

IE=4.6021(1/n)+3.8090 2 R =0.99

IE=5.3695(1/n)+3.9034 2 R =0.99

The calculated Eg, IE and EA for infinite chain lengths of some selected oligomers (polymers) are listed in Supporting information and plotted in Figs. 5–7. An insight in Fig. 5 reveals reduction of the Eg with substitution. The Eg of OCH3 substituted polymers is lower than that of NHCH3 regardless of the position of substitution. Experiments reported Eg of polypyrrole, polyfuran and polythiophene at 3.20, 2.35 and 2 eV, respectively [10,34]. The Eg of the unsubstituted polyphosphole of 1.2 eV agrees with previous theoretical studies of 1.1–1.5 eV using DFT and TDDFT with different basis sets [6,35]. The NHCH3 and OCH3 substitutions further reduce the Eg of their phosphole polymers (to ca. 0.55 eV). This result should motivate organic chemists to overcome difficulties in synthesizing polyphospholes [19] to tailor polymers with desired electrical properties. For NHCH3 and OCH3, the reduction in IE follows the order HT < P. The calculations show that the IE of the unsubstituted polyphosphole is slightly lower (by 0.2 eV) than the experimental

IE=4.5368(1/n)+4.4652 2 R =0.99

6.0

IE(eV)

Fig. 7. Extrapolation of electron affinity (EA) for for (unsubstituted phosph), (NHCH3-phosph), (OCH3-phosph) at Cb and P atoms vs 1/n.

5.7 5.4

Phosph NHCH3-Phosph-Cβ NHCH3-Phosph-P OCH3-Phosph-Cβ OCH3-Phosph-P

5.1 4.8 4.5 4.2 3.9 0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

1/n Fig. 6. Extrapolation of ionization energy (IE) for (unsubstituted phosph), (NHCH3phosph), (OCH3-phosph) at Cb and P atoms vs 1/n.

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IE of polythiophene of 5 eV [42]. Crassous and coworker [11] reported the positive EA of phosphole as one of its most important electronic properties, which not only confirmed by this work, but also enhanced by introducing a proper substituent at a particular position.

[9] [10] [11] [12]

4. Summary and conclusions

[14]

[13]

[15]

Using B3LYP/6-31G(d), the present work studied electronic properties of phosphole oligomers substituted with electron donors in an attempt to shed some light on the conducting properties of these phospholes. The results can be summarized as follows: 1. The investigated oligomers show lower ionization energy (IE) and HOMO–LUMO energy gap (Eg) compared to the unsubstituted ones. 2. Substituted phosphole polymers posses lower Eg and IE compared to the unsubstituted polymers. The electron affinity (EA) is enhanced only for OCH3 substitution at the P-center. 3. Calculation of hole and electron injection rates indicates p-type conductivity. However, n-type conductivity is predicted for OCH3 substitution at the P-center. 4. This work would urge experimentalist to devote more effort in synthesizing substituted polyphospholes of potential conductivities as reflected from lower Eg, IE, and more positive EA for the investigated systems.

[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]

Acknowledgements The authors would like to thank Professor El-Sayed E. El-Shereafy for his help and discussion during the progress of this project. TE also wants to thank Dr. Morad El-Hendawy for his valuable comments.

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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.comptc.2011.11.020. References [1] [2] [3] [4] [5] [6]

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