Computational Study on the Effect of Substituents on the Structural

Article pubs.acs.org/Macromolecules

Computational Study on the Effect of Substituents on the Structural and Electronic Properties of Thiophene−Pyrrole-Based π‑Conjugated Oligomers Harikrishna Sahu and Aditya N. Panda* Department of Chemistry, Indian Institute of Technology Guwahati, India, 781039 ABSTRACT: Theoretical calculations are carried out to study the effect of electron donating and withdrawing groups on the electronic structure and properties of a class of conjugated polymers where the basic unit consists of a thiophene and a pyrrole ring linked by a −CHN− group. Density functional theory (DFT) and time-dependent DFT (TDDFT) were employed for this series of oligomers with different substituents and different positions of those substituents. Ground state structural parameters, HOMO−LUMO gaps, excitation energies, and the oscillator strengths for the first three allowed electronic transitions were computed. The results show that sterically demanding substituents induce twisting in the conjugated backbone of the oligomers and increase both the HOMO−LUMO gaps and the excitation energies, compared to the basic compound. Properties of the oligomers are observed to be dependent on the positions of the substituents, for both electron donating and withdrawing groups. Presence of an intramolecular S··O interaction results in planar structure for two oligomers. Reduction in dihedral angles (leading to planarity) is associated with a reduction in excitation energies. S0 → S1 transitions are found to have the largest oscillator strengths for all the compounds. gap between the filled valence band and unfilled conduction band, of the polymer can be studied by different theoretical approaches. The simplest way is to find the difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)(denoted as ΔH−L).32−34 Although it is estimated crudely, the band gap obtained is closely related to the actual band gap. It is also represented by the lowest vertical excitation energy for the first dipole-allowed transition.34−37 In this regard, the time-dependent density functional theory (TDDFT) method has been very helpful in computing the excitation energies and oscillator strengths of polymers due to its low computational cost. There are several parameters those can assist in controlling the band gap of conducting polymers. Some of the important parameters are degree of the bond length alternation in the conjugated path, interring torsion angle responsible for effective overlap of the π-orbital of repeated unit, resonance stabilization energy of the repeated unit to compete with π-electron delocalization, intermolecular and intramolecular interactions, and inductive or mesomeric effect of the substituents over the energy levels of the HOMO and LUMO. A large number of studies have been performed, both experimentally and theoretically, in search of small band gap

I. INTRODUCTION π-conjugated polymers have been widely studied for their interesting optical and electrical conduction properties.1−14 They are employed in numerous organic devices such as organic field-effect transistor,1−8,15−18 organic light-emitting diodes,9−11,19−22 and photovoltaic cells.12−14,23,24 Although inorganic materials are more popular in the above devices, organic electronics are more attractive as these are lightweight, low cost manufacturable, easily synthesizable, and mechanically flexible.25−27 The delocalization of π molecular orbitals helps in improving conducting properties of these polymers. The extent of delocalization depends on the size of the overlap between the π orbitals of the neighboring carbon atoms and any hindrance to this conjugation affects the properties. In this respect, polypyrrole, polythiophene, and their derivatives have received a great deal of attention because of their highly conjugated π bonding systems, chemical stability and tunable electronic properties.28 Band gap of a polymer is one of the most important property to increase the efficiency of optoelectronic devices. Polymers with low band gap can harvest more photons with longer wavelengths in the visible and near-infrared regions.29,30 These polymers also enhance the probability of thermal population of the conduction band and thus increase the number of intrinsic charge carriers.31 Thus, control of the band gap of π-conjugated polymer is an important factor in the development of the electronic and optoelectronic devices. The band gap, the energy © 2013 American Chemical Society

Received: November 26, 2012 Revised: January 15, 2013 Published: January 31, 2013 844

dx.doi.org/10.1021/ma3024409 | Macromolecules 2013, 46, 844−855

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Figure 1. Sketch maps of the structures of thiophene- and pyrrole-based oligomers.

D−A polymer, the HOMO of the donor is destabilized and LUMO of the acceptor is stabilized leading to a low band gap. However, the presence of strong π electron withdrawing or donating groups reduces the bandwidths and oscillator strength of the lowest electronic transition. In these cases, the electronic transition is dominated by excitations to a higher excited states and this results in a larger energy gap.53,54 The presence of sterically demanding bulky groups in the conjugated polymers induces twisting of units against each other which decreases the degree of overlap of the frontier orbitals leading to a blue shift.33,35,55,56 Building of planar π-

polymers. One type of small band gap polymer is based on a single monomer unit which after polymerization results in a small energy difference between aromatic and quinoid resonance structures and bond length alternation (BLA) values are either decreased or inverted. Poly(isothianapthene) and poly(thienopyrazine) are good examples of this kind of polymer which have very low band gap of ∼1 eV.38−44 The other suggested strategy for the obtainment of narrow band gap of the conjugated polymer is to incorporate segments with alternating electron rich donor (D) and electron deficient acceptor (A) units in the backbone of the polymer.45−52 In a 845


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II. COMPUTATIONAL METHODS All the calculations were performed with ORCA 2.9 package.105 The DFT method has been very popular for the study of ΔH−L gaps in conjugated polymers.33,35,37,45,52,54,106−108 Despite the fact that the calculated results are not exactly equal to the experimental results, the trend has been found to be the same. The ground state geometries of the monomers were fully optimized using different functionals such as B3LYP, PBE0, PW6B95, and B2PLYP with 6-31G(d) basis set. Single point calculations using PWPB95 were performed for the optimized geometries obtained at PW6B95/6-31G(d) level. To check the effect of basis sets, the monomers were also optimized using the 6-311++G(d,p) basis set. Optimization of all the oligomers were carried out at B3LYP/6-31G(d) level. No symmetry constraints were enforced in the calculations. HOMO and LUMO levels were examined and ΔH−L gaps were evaluated. Excited state calculations were carried out using the TDDFT method at the ground state optimized geometries and the transition energies and oscillator strengths were computed. While TDDFT is widely used in the studies of excited states and produces satisfactory results for many cases,35,37,45,52,106−109 it has been much less successful in cases like charge transfer excitations.110−113 The ΔH−L gaps and the lowest excitation energies were linearly extrapolated to infinite chain length in order to get polymer value. As it is wellknown46,114,115 that the linear extrapolation is not best suited for computing the excitation energy for conjugated polymers, we have also carried out extrapolations of the HOMO−LUMO gaps and excitation energies for the S0 → S1 transition using the Kuhn’s formula116 based on coupled oscillators π EN = E0 1 − 2α cos (1) N+1

conjugated polymers is an effective way to enhance this overlap of the frontier orbitals and to reduce the band gap of these polymers. A large amount of experimental and theoretical structural studies have been done on the planar π-conjugated polymers.57−65 There are many ways to planarize the neighboring units. One of the approaches is to prepare ladder type polymer by the formation of intramolecular hydrogen bond or a noncovalent (i.e., S··N or S··O) bond between the neighboring units.57−60,63 Because of strong intermolecular overlap, planar π-conjugated backbones also promote close solid-state π−π stacking and have efficient charge carrier transport properties.66,67 In the present report, we explore the electronic structure and properties of thiophene−pyrrole-based azomethines using DFT studies. An azomethine, −CHN−, linkage is isoelectronic to a CC, vinyl, type linkage. Preparation of the conventional vinyl linked polymers releases a lot of harmful byproducts and also, a great effort is needed for their purification.68−70 In contrast, polyazomethines are easy to synthesize with high purity and yield,71−74 liberating exclusively water as byproduct.72,75,76 But the associated problem of being nonfluorescent and insoluble in organic solvents73,77−79 has been an hindrance to their application. The problem with solubility has recently been overcome76,80,81 and fluorescent polymers are prepared.82−84 Their excellent thermal, semiconducting, optical, electrical and fiber forming properties78,85−89 have provided impetus for further research and development. In addition, these compounds are easily dopable because of the presence of nitrogen atom in the backbone82,90 and a number of compounds are being prepared having mutual hole and electron charge carrier properties.78,91,92 Easy control over their electronic structure by inter and intramolecular hydrogen bonding, complex formation with Lewis acid,88,93−96 in addition to conventional modification by backbone substitution and side group changes,76,78,86,97,98 make them interesting materials for studies.92 Many of the recent experimental studies99−101 have focused on heterocyclic-based azomethines as the heterocyclics help in planarizing the system leading to greater conjugation. Also, conjugated polymers containing pyrrole rings have high HOMO and LUMO energy levels. This results in a low ionization potential and electron affinity and this improves the redox properties of the polymer.102−104 Keeping in mind the recent interest and progress on the experimental side, we have carried out an ab initio investigation of these type of azomethines. Relaxation calculations are performed for oligomers and the effect of various basis sets and functionals are examined. The effect of substitution by various electron donating and withdrawing groups at different positions of the monomers and their effect on the HOMO, LUMO and ΔH−L gap are also studied. To keep computational time under check, oligomers of maximum three repeat units are considered. The characterization of excited states and electronic transition have also been carried out by using TDDFT method on the corresponding optimized ground state geometries. Both the vertical excitation energies and the HOMO−LUMO gaps are extrapolated to infinite chain length. The relationship of the oscillator strengths of first vertical electronic transitions with the geometry of the oligomers is discussed. Ladder like polymers obtained through intramolecular S··O interactions resulting in planar geometries with low band gap are proposed.

where N is the number of linearly conjugated double bonds. EN, E0, and α refer to the energy at infinite chain length, energy of an isolated oscillator and the coupling strength between neighboring oscillators, respectively.

III. RESULTS AND DISCUSSION The sketch maps of the 15 thiophene- and pyrrole-based copolymers considered in this work are shown in Figure 1. Compound 1 is the basic unit, where thiophene and pyrrole rings are connected through a −CHN linkage. Carbon− carbon bond numbering scheme of the basic monomer unit is shown in Figure 2. Various groups like −CH3, −CF3, −OCH3,

Figure 2. Numbering scheme of carbon−carbon bonds in the monomer 1.

tert-butyl (t-Bu), −NH2, −NO2, and −OC2H4O− are attached in different positions of the basic unit, in order to understand their effect on the geometry and optoelectronic properties of the compounds. For compounds 2−5, substituents are attached only at the pyrrole nitrogen positions. In compounds 6 and 7, in addition to the substitution at the pyrrole nitrogen position, 3 and/or 4 positions of the pyrrole and thiophene rings are also substituted. For compounds 8−14, substituents are attached 846


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Table 1. Calculated Bond Lengths for the B3LYP/6-31G(d) Optimized Geometries of Monomers. All Bond Lengths are in Å carbon−carbon bonds monomers

1

2

3

4

5

6

7

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

1.367 53 1.367 40 1.368 44 1.367 14 1.367 73 1.369 32 1.369 30 1.367 52 1.368 05 1.368 30 1.368 46 1.367 49 1.367 52 1.367 63 1.363 96

1.422 79 1.422 89 1.421 70 1.423 16 1.422 63 1.429 40 1.429 36 1.422 79 1.422 11 1.421 74 1.421 52 1.422 79 1.422 78 1.422 66 1.433 18

1.378 88 1.379 02 1.379 64 1.378 92 1.379 05 1.376 93 1.376 99 1.378 94 1.379 03 1.379 22 1.379 37 1.379 18 1.378 94 1.378 98 1.382 29

1.436 98 1.437 15 1.444 45 1.441 71 1.437 21 1.437 32 1.436 82 1.436 41 1.441 09 1.442 30 1.442 97 1.434 31 1.436 58 1.436 89 1.431 29

1.394 52 1.396 34 1.385 91 1.396 26 1.396 32 1.396 27 1.395 41 1.393 42 1.389 57 1.387 07 1.387 10 1.396 00 1.392 40 1.393 39 1.399 50

1.411 59 1.408 03 1.416 65 1.403 08 1.410 87 1.408 17 1.412 66 1.416 39 1.415 06 1.422 63 1.412 05 1.414 08 1.419 09 1.411 85 1.412 95

1.387 32 1.386 96 1.375 40 1.384 75 1.388 81 1.386 90 1.389 39 1.389 81 1.386 26 1.394 68 1.388 28 1.394 44 1.391 82 1.393 99 1.384 34

Figure 3. Representative structure showing dihedral angle between monomers where DNCCC = N1−C2−C3−C4.

Table 2. Calculated Dihedral Angles (in deg) for All the Studied Oligomers at the B3LYP/6-31G(d) Level of Theorya compound

type

D1NCCC

1

D T D T D T D T D T D T D T D T

13.5 11.528 35.58 35.037 33.485 33.185 127.893 53.791 17.638 17.432 55.441 55.255 62.618 62.827 15.577 14.974

2 3 4 5 6 7 8 a

D2NCCC

compounds

type

D1NCCC

9

D T D T D T D T D T D T D T

26.586 25.326 10.435 9.429 17.307 16.843 22.211 17.183 42.776 41.6 0.5 0.227 −0.221 0.51

−11.19 10 −35.344 11 −33.584 12 −54.648 13 16.828 14 −55.742 15 −62.762

D2NCCC −25.171 −8.717 −17.023 −21.413 −43.192 −0.245 0.289

−14.861

“D” and “T” denote dimers and trimers, respectively.

only at the 3 positions of the pyrrole rings and in the compound 15, both 3 and 4 positions of the pyrrole and thiophene rings are substituted. We have considered oligomers up to trimers for all the compounds. A. Ground State Electronic Structure and Effect of Functionals and Basis Sets. The optimized geometries of all the monomers are perfectly planar. All the carbon−carbon bond lengths calculated at B3LYP/6-31G(d) level are listed in Table 1. It is observed that all the carbon−carbon single bonds

are shorter than those in ethane (1.54 Å) at the same level of theory. On the contrary, all the double bond(CC) lengths are longer than that of the CC double bond in ethylene (1.33 Å). This shows that the bond distances are between CC and C− C bond lengths. A comparison with bare pyrrole molecule shows that the CC bond close to the −CHN linkage is slightly longer than the other CC bond. The arrangement of monomer units in oligomers can modify their conformational features. A representative structure for the 847


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Figure 4. Structures of the dimers 14 and 15. These two structures are planar owing to the noncovalent O···S interactions.

dimers is presented in Figure 3 showing the interunit dihedral angle DNCCC. Table 2 shows the calculated dihedral angles of all the oligomers and it is observed that dimer 4 has the largest dihedral angle of 127.9° among the studied compounds. The presence of the bulky t-Bu group distorts the geometry and increases the torsional angle. The −CH3 and −CF3 groups are smaller than the t-Bu group. This results in smaller dihedral angles for the dimers 2 and 3 than that in the dimer 4. The −CF3 group is bigger than the −CH3 group, but the DNCCC of dimer 3 is 2.1° less than in the dimer 2 as shown in Table 2. This anomalous behavior can be explained in terms of the C−F and C−H bond distances. The C−F bond distance in the −CF3 group is 1.337 Å which is much larger than the C−H bond distance (1.090 Å) of the −CH3 group and this results in less repulsion between the −CF3 group and the thiophene ring of the adjacent monomer unit. In the dimers 6 and 7, the presence of large number of −CH3 groups results in more electronic repulsion between the monomer units and hence, large dihedral angles are found. The dihedral angle of dimer 9 is larger than in dimer 8. This is due to the presence of the −CF3 group in dimer 9, which is bulkier than the −CH3 group in dimer 8. These results are found to be different in comparison to the geometries of dimers 2 and 3. In the present case, the C−F bond distance does not affect the dihedral angle as the substitution occurs at different position of the monomer unit. Even though the dimers of compounds 2 and 8 bear the same −CH3 group, the dihedral angle of dimer 8 is 17.9° smaller than in dimer 2. Furthermore, dimer 13 shows a dihedral angle of 42.8°, but this is much smaller (by 85.1°) than the angle in dimer 4. The above results show that the presence of substituent in the pyrrole nitrogen

position causes more repulsion between the monomer units than at the 3 position of the pyrrole ring. Dimers 14 and 15 are shown in Figure 4. Both the oligomers are planar due to the presence of intramolecular O··S interaction. The intramolecular O··S distances are 3.009 and 3.167 Å in dimers 14 and 15, respectively. These distances are shorter than the sum of van der Waals radii of sulfur and oxygen.117 This indicates a noncovalent interaction between the sulfur and oxygen atoms.34,51,57,58,118−120 A comparison of various bond lengths between adjacent monomer units shows very small differences. The main structural changes happen at the junctions of adjacent monomer units. We have calculated the bond lengths between two adjacent monomer units for all the oligomers. Bonds are denoted as J for dimers and J1 and J2 for trimers. The results are plotted in Figure 5. The variation in these junction bond lengths are related to the dihedral angles between the monomer units. Dimer 4 shows the largest J value because of its largest dihedral angle. A comparison of dimers 2 and 8 shows that dimer 8 has a smaller J value than dimer 2, owing to the larger junction dihedral angle in dimer 2 than in dimer 8. As discussed earlier, dimer 2 has a larger dihedral angle than in dimer 3 whereas the J values of these dimers are in the opposite order. This is because of electron withdrawing nature of the −CF3 group in dimer 3. Although dimer 10 contains electron withdrawing −CN groups, the J value is small due to a small junction dihedral angle. Presence of strong electron donating −NH2 group in dimer 12 results small junction bond distance. The interring torsion angles of the dimers 14 and 15 are found to be 0°, which makes these dimers to exhibit shorter junction distances than in other dimers. Overall, the dimers having large dihedral angle or bearing electronic withdrawing groups have 848


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length. Both the Δr and δr are plotted in Figure 7. For all the compounds except monomers 3 and 10, Δr values are smaller

Figure 5. Comparison of calculated junction bond distances of all the studied oligomers. J stands for the junction bond distances of the dimers, and J1 and J2 are two junction bond distances of the trimers.

large junction bond distances whereas comparatively smaller J values are found for those dimers which are planar or have electron donating groups. In trimers, there are two junction dihedral angles, D1NCCC and D2NCCC. So two types of conformations are possible for each trimer: (1) both dihedral angles are positive (denoted as d+d+) and (2) one angle is positive and other is negative (denoted as d+d−). From the optimized structures, it is found that these two types of conformations for all the trimers have similar energies. Figure 6 shows both these conformations for trimer 7

Figure 7. BLA values, Δr and δr, of the studied compounds. See text for the definitions of Δr and δr.

than the Δr values of the thiophene and pyrrole rings (for thiophene and pyrrole, Δr values are 0.062 and 0.047 Å, respectively at B3LYP/6-31G(d) level of theory). It indicates that the monomers 3 and 10 are less aromatic than the individual thiophene and pyrrole rings. As shown in Figure 7, compound 3 has the highest Δr value. For all the compounds, Δr decreases from the monomer to the corresponding trimer except in the compound 4. Δr value of the dimer 4 is larger than in monomer 4, and this result shows that the presence of t-Bu groups in the pyrrole nitrogen positions decrease the conjugation length upon dimerization of the compound 4. It is also observed that the magnitude of Δr for the trimers 14 and 15 are smaller than for other trimers indicating more delocalization over the oligomer chains. Overall, small dihedral angles or the presence of electron donating groups in a compound results a small Δr value whereas the opposite happens for the compounds having large dihedral angles or electron withdrawing groups. The δr is less affected by the dihedral angle as compared to the Δr, and in most of the cases, electronic effects of the substituents are predominant over the steric effect. Among all the monomers and oligomers, compound 11 has the highest δr value because of the presence of a strong electron withdrawing −NO2 group. The δr value of the compounds 3, 9, 10 are also large because of the electronwithdrawing effect of the substituents. The presence of two electron-donating −OC2H4O− groups in the repeated unit of compound 15 causes the lowest δr values. All the compounds (i.e., compounds 2, 5, 6, 7, 8, 12, 14) containing electrondonating groups have small δr values except in compounds 4 and 13. This is due to the presence of a bulkier t-Bu group. For all compounds, the δr value decreases from the monomer to the corresponding trimer. To check the effect of different DFT functionals on the electronic structures, extra calculations were performed using B2PLYP, PBE0, PW6B95, and PWPB95 functionals, for all the monomers. The energies of the monomers calculated by different functionals are plotted in Figure 8, with respect to the energies obtained by B3LYP functional. The energies obtained

Figure 6. Optimized geometries of trimer 7: (a) d+d+ configuration and (b) d+d− configuration.

and the energy difference between these two is 0.003 kcal/mol. In trimers, the trends of dihedral angle are found to be similar to those in the dimers except that in trimer 4. The dihedral angles of trimer 4 are found to be smaller than in trimers 6 and 7. With increase in repeated units, small changes take place in the bond lengths of the trimers. As shown in Figure 5, trimer 7 has the highest J1 and J2 values unlike the case of dimers where compound 4 has the highest interunit bond length. Two different types of BLA values are calculated for all the compounds. First one is the difference between the average carbon−carbon single and double bond lengths along the π conjugated path60,121 and it is denoted as Δr. Other one is the junction BLA (δr) of the repeated unit, i.e., the average difference between the C−C bond lengths, located between the −CHN− linkage and pyrrole ring, and the CN bond 849


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Figure 8. Comparisons of energies of monomers calculated by different functionals with respect to B3LYP functional.

follow the order, PBE0 > B2PLYP > PWPB95 > B3LYP > PW6B95. It is also observed that the energies of the monomers follow the same order, for all the functionals. The C−C bond lengths and Δr values obtained by different functionals and basis sets are listed in Table 3 and the bond Table 3. Bond Lengths and BLA (Δr) Values (in Å) of the Basic Monomer Unit Obtained Using Different Functionals and Basis Sets bond lengths(Å) B3LYP

6-31G(d)

bonds

6-31G(d)

6-311G++(d,p)

B2PLYP

PBE0

PW6B95

1 2 3 4 5 6 7 Δr

1.368 1.423 1.379 1.437 1.395 1.412 1.387 0.0418

1.365 1.421 1.376 1.437 1.392 1.411 1.385 0.0435

1.369 1.42 1.379 1.437 1.394 1.411 1.387 0.0404

1.366 1.418 1.376 1.434 1.39 1.407 1.384 0.0407

1.362 1.416 1.372 1.433 1.386 1.405 1.38 0.0430

Figure 9. Bond lengths of basic monomer unit calculated by different functionals and basis sets.

pyrrole ring, i.e., −CH3 in monomer 2 and t-Bu in monomer 4. The change in the position of substitution also affects the band gaps as seen in cases of monomers 3 and 9. Both have the same −CF3 group, but the ΔH−L gap of monomer 3 is smaller than that of monomer 9. This is because of the presence of −CF3 group in the pyrrole nitrogen position in monomer 3. In monomers 6 and 7, the increase in the number of −CH3 substituents and their positions in the pyrrole and thiophene rings decrease the magnitude of the ΔH−L gaps. The HOMOs of the monomers 11 and 15 are the most and the least stable, respectively. The presence of an electron withdrawing nitro group in the monomer 11 and two electron-donating −OC2H4O− groups in monomer 15 affect these changes. Both monomers 12 and 15 have small ΔH−L gaps. While the presence of an amino group in monomer 12 helps in decreasing the band gap by 0.216 eV compared to monomer 1, the presence of two −OC2H4O− groups in monomer 15 results in a value smaller by 0.192 eV than in monomer 1. Monomer 9 shows the maximum ΔH−L of 3.67 eV, and monomer 12 exhibits the lowest ΔH−L of about 3.402 eV. The band gaps of oligomers are directly proportional to the torsional angles at the junctions.32,33,45,52,55,122 A large torsional deviation from the coplanarity leads to a large band gap. Thus,

lengths are plotted in Figure 9. The C−C single and double bond lengths derived using the PBE0 and PW6B95 functionals are slightly smaller than those obtained using the B3LYP functional. While the calculated Δr values using B2PLYP and PBE0 functionals are smaller than the one obtained using B3LYP, the result calculated using PW6B95 functional is larger. Extra calculations were also carried out to see the effect of basis sets and the results are shown in Figure 9. Bond lengths obtained at the B3LYP/6-311++G(d,p) level are slightly smaller than those at the B3LYP/6-31G(d) level. Also the BLA obtained at the B3LYP/6-311++G(d,p) level is larger than the result at B3LYP/6-31G(d) level. B. HOMO−LUMO Gaps, Excitation Energies, and Oscillator Strengths. Energies of HOMOs, LUMOs, and the corresponding ΔH−L values of all the compounds are listed in Table 4. From the table, it can be observed that the HOMO and LUMO energies of the monomers 2 and 4 are smaller than those of monomer 1. The observed red shift in the bandgap for both the monomers compared to that of monomer 1 is due to the electron donating nature of the group attached to the 850


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Table 4. Calculated Energies of HOMOs and LUMOs and HOMO−LUMO Gaps (ΔH−L) at B3LYP/6-31G(d) Level for All the Studied Compoundsa compounds

type

HOMO

LUMO

ΔH−L

compounds

type

HOMO

LUMO

ΔH−L

1

M D T M D T M D T M D T M D T M D T M D T M D T

−5.142 −4.714 −4.564 −5.065 −4.702 −4.575 −5.43 −5.057 −4.929 −4.978 −4.822 −4.642 −5.177 −4.734 −4.582 −5.016 −4.763 −4.679 −4.963 −4.758 −4.691 −5.084 −4.633 −4.47

−1.524 −2.033 −2.245 −1.506 −1.96 −2.151 −1.844 −2.334 −2.535 −1.447 −1.758 −1.946 −1.558 −2.047 −2.247 −1.458 −1.812 −1.963 −1.43 −1.743 −1.875 −1.493 −1.981 −2.174

3.618(3.558) 2.681 2.319 3.558(3.475) 2.742 2.424 3.586(3.518) 2.722 2.394 3.531(3.478) 3.065 2.697 3.619(3.542) 2.687 2.335 3.559(3.469) 2.951 2.716 3.533 3.015 2.816 3.591(3.523) 2.652 2.296

9

M D T M D T M D T M D T M D T M D T M D T

−5.53 −5.192 −5.072 −5.687 −5.338 −5.222 −5.778 −5.49 −5.4 −4.844 −4.424 −4.252 −5.094 −4.807 −4.696 −5.064 −4.441 −4.266 −4.649 −4.091 −3.881

−1.86 −2.354 −2.554 −2.023 −2.602 −2.813 −2.168 −2.722 −2.89 −1.442 −1.926 −2.115 −1.5 −1.891 −2.045 −1.523 −1.973 −2.154 −1.223 −1.654 −1.821

3.670(3.616) 2.838 2.518 3.664(3.605) 2.736 2.408 3.610(3.492) 2.768 2.51 3.402(3.340) 2.498 2.137 3.594 2.916 2.651 3.541(3.483) 2.468 2.111 3.426 2.437 2.060

2

3

4

5

6

7

8

10

11

12

13

14

15

For the monomers, the values inside the brackets indicate the band gaps at B3LYP/6-311++G(d,p) level. All the energies are in eV and “M”, “D”, and “T” stand for monomer, dimer, and trimer, respectively.

a

the neighboring units. As listed in Table 4, the calculated band gaps for these trimers are smaller than in the other trimers. The energies of HOMOs, LUMOs, and ΔH−L gaps of the basic monomer unit obtained by different functionals are shown in Figure 10. Compared to the B3LYP functional, all the functionals produce larger ΔH−L gaps. The ΔH−L calculated by B2PLYP and PWPB95 functionals are very large compared to the other functionals. This is due to more stabilization of the HOMO orbitals and destabilization of the LUMO orbitals at the B2PLYP/6-31G(d) and PWPB95/6-31G(d) levels. The ΔH−L gaps of all the studied monomers obtained by different

the arrangement of monomers in the backbone of oligomers plays a vital role in determining the optoelectronic properties of oligomers. Analysis of the results obtained for the dimers shows that compounds 4 and 15 exhibit the highest and the lowest ΔH−L gaps, respectively. As already discussed, the DNCCC is the largest for dimer 4. This results in the reduction in conjugation length and makes the dimer 4 to exhibit the highest ΔH−L gap. For the compounds 1−7, the order of ΔH−L is consistent with the order of the dihedral angle. By dimerization of the monomers 2 and 8, the ΔH−L gap increases and decreases respectively, with respect to dimer 1. As discussed above, −CH3 groups in dimer 2 produce a larger dihedral angle than that in dimer 8. In compound 2, the steric influence of the substituent is more effective than the electronic effect whereas the reverse happens in the dimer 8. Both the dimers 14 and 15 are planar and have small ΔH−L gaps, i.e., 2.47 and 2.44 eV, respectively. In case of trimers, the trimer of compound 7 is shown to have the highest band gap of about 2.82 eV unlike the case of dimers where the dimer of compound 4 is found to have the highest band gap. As listed in Table 2, the dihedral angles in the trimer of compound 4 are 53.8° and −54.6°, smaller than in the dimer. The effect of change in the dihedral angle on the band gap is seen in Table 4. The dihedral angles in the trimer 7 are larger than the ones in the trimer of compound 4. Larger dihedral angles decrease the degree of π−π conjugation between the adjacent units resulting in a larger band gap in compound 7.123−127 On the other hand, highly conjugated πbonding systems have smaller band gaps, like in the cases of trimers of compounds 14 and 15. Planar structures of these compounds help in increasing the HOMO energies while stabilizing the LUMOs, thus increasing the coupling between

Figure 10. HOMOs, LUMOs, and ΔH−L gaps of the basic monomer unit calculated by different functionals. 851


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functionals are plotted in Figure 11. As seen in the figure, for all the monomers, the ΔH−L gaps are overestimated by B2PLYP

the inverse number of double bonds are plotted in Figure 12, for compounds 7, 14, and 15. The properties of polymers are

Figure 11. Comparison of HOMO−LUMO gaps (ΔH−L) calculated by using five different functionals as mentioned in the figure.

Figure 12. Variation of HOMO−LUMO gaps (a) and S0 → S1 excitation energies (b) against the reciprocal of N, where N is the number of linearly conjugated double bonds. The figure shows the results for compounds 7, 14, and 15. Both the ΔH−L and the Eg values are extrapolated to infinite chain length. Linear extrapolated results are shown as dotted lines and the results obtained using the Kuhn’s formula (see eq 1) are shown as solid lines.

and PWPB95 functionals. But all the functionals have the same trend in our studied systems. The ΔH−L gaps of the monomers by different basis sets are listed in Table 4. Compared to the 631G(d) basis set, the 6-311++G(d,p) basis set results in slightly smaller ΔH−L gaps but the trend is same for both the basis sets. Excitation energies, oscillator strengths ( fosc), and the configurations involved for first three singlet−singlet electronic transitions are listed in Table 5. For trimers of compounds 14 and 15, the ΔH−L values (see Table 4) are almost the same as the excitation energies. The relationships between the calculated ΔH−L gaps and lowest excitation energies (Eg) with

determined by extrapolation to infinite chain length and in the present studies, we have used linear extrapolation and the Kuhn’s formula (see eq 1) for computing the energies for a polymer. Figure 12 shows that the trend in the variation of Δ H−L and Eg values remain the same, i.e. the linear

Table 5. Electronic Transition Data Obtained by the TDDFT Method for Trimers at B3LYP/6-31G(d) Level trimer 1

3

5

7

9

11

13

15

transitions S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 →

S1 S2 S3 S1 S2 S3 S1 S2 S3 S1 S2 S3 S1 S2 S3 S1 S2 S3 S1 S2 S3 S1 S2 S3

Eg (eV)

fosc

2.237 2.550 2.996 2.269 2.583 2.958 2.240 2.565 2.994 2.543 2.854 2.892 2.358 2.693 3.041 2.311 2.559 2.798 2.429 2.757 2.964 2.039 2.332 2.818

3.58 0.17 0.05 3.10 0.41 0.01 3.45 0.29 0.03 1.44 0.82 0.16 2.97 0.32 0.05 2.41 0.32 0.21 2.24 0.51 0.11 3.76 0.42 0.03

configuration H H H H H H H H H H H H H H H H H H H H H H H H

trimers

→ L (98%) → L + 1 (53%) → L + 2 (62%) → L (98%) → L + 1 (49%) → L + 2 (41%) → L (98%) → L+1(53%) → L+2(54%) → L(97%) → L + 1 (63%) − 1 → L (49%) → L (98%) → L + 1 (52%) → L + 2 (38%) → L (98%) → L + 1 (82%) − 1 → L (60%) → L (98%) → L + 1 (54%) − 1 → L (30%) → L (97%) → L + 1 (51%) → L + 2 (50%)

2

4

6

8

10

12

14

852

transitions S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 → S0 →

S1 S2 S3 S1 S2 S3 S1 S2 S3 S1 S2 S3 S1 S2 S3 S1 S2 S3 S1 S2 S3

Eg (eV)

fosc

2.286 2.611 2.958 2.460 2.754 2.966 2.481 2.796 2.949 2.217 2.525 2.983 2.269 2.598 2.971 2.085 2.363 2.832 2.100 2.391 2.889

3.03 0.41 0.03 2.50 0.24 0.01 2.01 0.62 0.07 3.69 0.06 0.06 3.12 0.30 0.07 3.64 0.00 0.10 3.98 0.16 0.09

configuration H H H H H H H H H H H H H H H H H H H H H

→ L (98%) → L + 1 (52%) → L + 2 (45%) → L (98%) → L + 1 (48%) → L + 2 (28%) → L (98%) → L + 1 (52%) − 1 → L (30%) → L(98%) → L + 1 (53%) → L + 2 (67%) → L (98%) → L + 1 (53%) → L + 2 (35%) → L (98%) → L + 1 (53%) → L + 2 (66%) → L (97%) → L + 1 (54%) → L + 2 (64%)


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Figure 13. Oscillator strengths ( fosc) for the first vertical transition, S0 → S1, of all the studied compounds.

extrapolation underestimates the band gaps. Linear extrapolated values for 7 and 15 polymers are the highest (2.42 eV) and the lowest (1.31 eV) as shown in Figure 12a, which is due to the largest and the smallest dihedral angles, respectively. The corresponding results obtained using the eq 1 are 2.71 and 1.78 eV. Excitation energies plotted in Figure 12b show that while the extrapolated Eg values of the compounds 7 and 15 are 2.26 and 1.71 by using the Kuhn’s formula, values of 2.09 and 1.02 are obtained by linear extrapolation. The small band gaps in compounds 14 and 15 signify that those can be interesting materials with good conduction properties. Excitation energies of 14 and 15 seem to approach each other as they near the polymer limit. In the case of trimers, the orders of the ΔH−L values and Eg energies are similar except that of trimer 12. Table 5 shows that while the ΔH−L for trimer 14 is smaller than the ΔH−L in trimer 12, the order reverses for Eg. Table 5 also shows that the lowest optically allowed electronic transition (i.e., S0 → S1) has the largest intensity for all cases. The first absorption bands can be assigned to HOMO → LUMO transitions predominantly. The oscillator strengths for the transitions to higher excited states are much smaller than that for the lowest energy transition. As shown in Figure 13, the oscillator strengths of the S0 → S1 transitions of all the compounds are found to increase with an increase in the conjugation length except that in compounds 4 and 7. For these two, the oscillator strengths for the lowest excitation in monomer is larger than that in dimer. The oscillator strengths are found to decrease with increase in the dihedral angles. In accordance with the excitation energies, the oscillator strength is the smallest in case of trimer 7 while it is the largest in cases of trimers 14 and 15, for the S0 → S1 transition. Presence of a strong π electron-withdrawing group, as in the case of trimer 11, results in a small oscillator strength of 2.42. It is instructive to look at the frontier molecular orbitals to get a reasonable qualitative indication of the excitation properties. The electron densities of these orbitals are shown in Figure 14, for compounds 7, 11, 14, and 15. For 14 and 15, π electrons are delocalized over the entire molecule, for all the four orbitals. Maximal absorption wavelengths of 590.5 and 607.9 nm with large oscillator strengths of 3.98 and 3.76 are found for the trimers 14 and 15, respectively. On the other hand, for the trimers of 7 and 11, the conjugation does not extend over the entire backbone of the oligomers. In 7, this is again due to the large interunit dihedral angles which distorts

Figure 14. Electron density plots of the HOMO and LUMO orbitals of the trimers 7, 11, 14, and 15.

the structure. In trimer 11, the electron density in the LUMO is not homogeneously distributed due to the presence of strong electron withdrawing −NO2 group.

IV. CONCLUSION Theoretical studies on the geometric, electronic and optical properties of a large number of novel thiophene- and pyrrolebased azomethines were successfully performed using density functional theory. Different functionals and basis sets were used to check the variation of the results. We find that the polymer geometric structure is very much dependent on the steric influence of the bulky substituents and their positions. Torsional angle plays a major role in determining the localization and delocalization of wave functions and hence, any deviation from planar structure affects the optoelectronic properties of the π-conjugated polymer. All the compounds having large torsional angles have high band gap values and small oscillator strengths for S0 → S1 transition. The largest band gap was found for compound 7. For this system, the oscillator strength for the lowest electronic transition is also the smallest. Ladder-like oligomers of compounds 14 and 15 are found to be planar due to intramolecular S··O interactions and have significantly lower band gaps and higher oscillator strengths for the lowest electronic transition than the other compounds. 853


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(26) Forrest, S. R. Nature 2004, 428, 911−918. (27) Gustafsson, G.; Cao, Y.; Treacy, G. M.; Klavetter, F.; Colaneri, N.; Heeger, A. J. Nature 1992, 357, 477−479. (28) Chehimi, M. M.; Abdeljalil, E. Synth. Met. 2004, 145, 15−22. (29) Bundgaard, E.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2007, 91, 954−985. (30) Wienk, M. M.; Turbiez, M. G. R.; Struijk, M. P.; Fonrodona, M.; Janssen, R. A. J. Appl. Phys. Lett. 2006, 88, 153511. (31) Roncali, J. Chem. Rev. 1997, 97, 173−206. (32) Koepnick, B. D.; Lipscomb, J. S.; Taylor, D. K. J. Phys. Chem. A 2010, 114, 13228−13233. (33) Radhakrishnan, S.; Parthasarathi, R.; Subramanian, V.; Somanathan, N. J. Chem. Phys. 2005, 123, 164905. (34) Ö zen, A. S. J. Phys. Chem. C 2011, 115, 25007−25018. (35) Chen, R.-F.; Liu, L.-Y.; Fu, H.; Zheng, C.; Xu, H.; Fan, Q.-L.; Huang, W. J. Phys. Chem. B 2011, 115, 242−248. (36) Ren, X.-F.; Ren, A.-M.; Zou, L.-Y.; Feng, J.-K. Theor. Chem. Acc. 2010, 126, 305−314. (37) Granadino-Roldán, J. M.; Garzón, A.; García, G.; Moral, M.; Navarro, A.; Fernández-Liencres, M. P.; Peña Ruiz, T.; FernándezGómez, M. J. Phys. Chem. C 2011, 115, 2865−2873. (38) Hoogmartens, I.; Adriaensens, P.; Vanderzande, D.; Gelan, J.; Quattrocchi, C.; Lazzaroni, R.; Brédas, J. L. Macromolecules 1992, 25, 7347−7356. (39) Wudl, F.; Kobayashi, M.; Heeger, A. J. J. Org. Chem. 1984, 49, 3382−3384. (40) Kobayashi, M.; Colaneri, N.; Boysel, M.; Wudl, F.; Heeger, A. J. J. Chem. Phys. 1985, 82, 5717−5723. (41) Wen, L.; Duck, B. C.; Dastoor, P. C.; Rasmussen, S. C. Macromolecules 2008, 41, 4576−4578. (42) Nietfeld, J. P.; Schwiderski, R. L.; Gonnella, T. P.; Rasmussen, S. C. J. Org. Chem. 2011, 76, 6383−6388. (43) Rasmussen, S. C.; Schwiderski, R. L.; Mulholland, M. E. Chem. Commun. 2011, 47, 11394−11410. (44) Sánchez, C. O.; Bernede, J. C.; Cattin, L.; Daz, F. R. Synth. Met. 2011, 161, 2335−2338. (45) Yang, L.; Liao, Y.; Feng, J.-K.; Ren, A.-M. J. Phys. Chem. A 2005, 109, 7764−7774. (46) Karsten, B. P.; Viani, L.; Gierschner, J.; Cornil, J.; Janssen, R. A. J. J. Phys. Chem. A 2008, 112, 10764−10773. (47) Hou, J.; Chen, T. L.; Zhang, S.; Chen, H.-Y.; Yang, Y. J. Phys. Chem. C 2009, 113, 1601−1605. (48) Banerji, N.; Gagnon, E.; Morgantini, P.-Y.; Valouch, S.; Mohebbi, A. R.; Seo, J.-H.; Leclerc, M.; Heeger, A. J. J. Phys. Chem. C 2012, 116, 11456−11469. (49) Pappenfus, T. M.; Schmidt, J. A.; Koehn, R. E.; Alia, J. D. Macromolecules 2011, 44, 2354−2357. (50) Das, S.; Pati, P. B.; Zade, S. S. Macromolecules 2012, 45, 5410− 5417. (51) Guo, X.; Kim, F. S.; Jenekhe, S. A.; Watson, M. D. J. Am. Chem. Soc. 2009, 131, 7206−7207. (52) Pandey, L.; Risko, C.; Norton, J. E.; Brédas, J.-L. Macromolecules 2012, 45, 6405−6414. (53) Salzner, U.; Köse, M. E. J. Phys. Chem. B 2002, 106, 9221−9226. (54) Hung, Y.-C.; Jiang, J.-C.; Chao, C.-Y.; Su, W.-F.; Lin, S.-T. J. Phys. Chem. B 2009, 113, 8268−8277. (55) Oberhumer, P. M.; Huang, Y.-S.; Massip, S.; James, D. T.; Tu, G.; Albert-Seifried, S.; Beljonne, D.; Cornil, J.; Kim, J.-S.; Huck, W. T. S.; Greenham, N. C.; Hodgkiss, J. M.; Friend, R. H. J. Chem. Phys. 2011, 134, 114901. (56) Fu, Y.; Li, M.; Shen, W. Mol. Simul. 2009, 35, 1279−1287. (57) Huang, H.; Chen, Z.; Ortiz, R. P.; Newman, C.; Usta, H.; Lou, S.; Youn, J.; Noh, Y.-Y.; Baeg, K.-J.; Chen, L. X.; Facchetti, A.; Marks, T. J. Am. Chem. Soc. 2012, 134, 10966−10973. (58) Vangheluwe, M.; Verbiest, T.; Koeckelberghs, G. Macromolecules 2008, 41, 1041−1044. (59) Tian, Y.-H.; Kertesz, M. Macromolecules 2009, 42, 2309−2312. (60) Tian, Y.-H.; Kertesz, M. Macromolecules 2009, 42, 6123−6127.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.S. acknowledges the University Grants Commission (UGC), New Delhi, for a junior research fellowship. REFERENCES

(1) Orgiu, E.; Masillamani, A. M.; Vogel, J.-O.; Treossi, E.; Kiersnowski, A.; Kastler, M.; Pisula, W.; Dötz, F.; Palermo, V.; Samorì, P. Chem. Commun. 2012, 48, 1562−1564. (2) Mushrush, M.; Facchetti, A.; Lefenfeld, M.; Katz, H. E.; Marks, T. J. J. Am. Chem. Soc. 2003, 125, 9414−9423. (3) Iino, H.; Kobori, T.; Hanna, J.-i. H. J. Non-Cryst. Solids 2012, 358, 2516−2519. (4) Geib, S.; Martens, S. C.; Zschieschang, U.; Lombeck, F.; Wadepohl, H.; Klauk, H.; Gade, L. H. J. Org. Chem. 2012, 77, 6107− 6116. (5) González, S. R.; et al. J. Phys. Chem. B 2011, 115, 10573−10585. (6) Aradilla, D.; Estrany, F.; Alemán, C. J. Phys. Chem. C 2011, 115, 8430−8438. (7) Liu, P.; Wu, Y.; Pan, H.; Ong, B. S.; Zhu, S. Macromolecules 2010, 43, 6368−6373. (8) Hammock, M. L.; Sokolov, A. N.; Stoltenberg, R. M.; Naab, B. D.; Bao, Z. ACS Nano 2012, 6, 3100−3108. (9) Grimsdale, A. C.; Leok Chan, K.; Martin, R. E.; Jokisz, P. G.; Holmes, A. B. Chem. Rev. 2009, 109, 897−1091. (10) Intemann, J. J.; Hellerich, E. S.; Tlach, B. C.; Ewan, M. D.; Barnes, C. A.; Bhuwalka, A.; Cai, M.; Shinar, J.; Shinar, R.; Jeffries-EL, M. Macromolecules 2012, 45, 6888−6897. (11) Heredia, D.; Fernandez, L.; Otero, L.; Ichikawa, M.; Lin, C.-Y.; Liao, Y.-L.; Wang, S.-A.; Wong, K.-T.; Fungo, F. J. Phys. Chem. C 2011, 115, 21907−21914. (12) Min, J.; Zhang, Z.-G.; Zhang, S.; Li, Y. Chem. Mater. 2012, 24, 3247−3254. (13) Huang, H.; Cao, Z.; Gu, Z.; Li, X.; Zhao, B.; Shen, P.; Tan, S. Eur. Polym. J. 2012, 48, 1805−1813. (14) Lecover, R.; Williams, N.; Markovic, N.; Reich, D. H.; Naiman, D. Q.; Katz, H. E. ACS Nano 2012, 6, 2865−2870. (15) Dutta, G. K.; Patil, S. Org. Electron. 2012, 13, 1266−1276. (16) Lehnherr, D.; Waterloo, A. R.; Goetz, K. P.; Payne, M. M.; Hampel, F.; Anthony, J. E.; Jurchescu, O. D.; Tykwinski, R. R. Org. Lett. 2012, 14, 3660−3663. (17) Duan, Z.; Hu, D.; Ohuchi, H.; Zhao, M.; Zhao, G.; Nishioka, Y. Synth. Met. 2012, 162, 1292−1298. (18) Duarte, D.; Sharma, D.; Cobb, B.; Dodabalapur, A. Appl. Phys. Lett. 2011, 98, 133302. (19) Goel, A.; Kumar, V.; Nag, P.; Bajpai, V.; Kumar, B.; Singh, C.; Prakash, S.; Anand, R. S. J. Org. Chem. 2011, 76, 7474−7481. (20) Evenson, S. J.; Mumm, M. J.; Pokhodnya, K. I.; Rasmussen, S. C. Macromolecules 2011, 44, 835−841. (21) Zuniga, C. A.; Barlow, S.; Marder, S. R. Chem. Mater. 2011, 23, 658−681. (22) Qian, L.; Bera, D.; Holloway, P. H. J. Chem. Phys. 2007, 127, 244707. (23) Sun, S.-S.; Zhang, C.; Ledbetter, A.; Choi, S.; Seo, K.; Carl E. Bonner, J.; Drees, M.; Sariciftci, N. S. Appl. Phys. Lett. 2007, 90, 043117. (24) Qin, R.; Li, W.; Li, C.; Du, C.; Veit, C.; Schleiermacher, H.-F.; Andersson, M.; Bo, Z.; Liu, Z.; Inganäs, O.; Wuerfel, U.; Zhang, F. J. Am. Chem. Soc. 2009, 131, 14612−14613. (25) Kelley, T. W.; Baude, P. F.; Gerlach, C.; Ender, D. E.; Muyres, D.; Haase, M. A.; Vogel, D. E.; Theiss, S. D. Chem. Mater. 2004, 16, 4413−4422. 854


Macromolecules

Article

(61) Pesant, S.; Boulanger, P.; Côté, M.; Ernzerhof, M. Chem. Phys. Lett. 2008, 450, 329−334. (62) Kertesz, M. Macromolecules 1995, 28, 1475−1480. (63) Delnoye, D. A. P.; Sijbesma, R. P.; Vekemans, J. A. J. M.; Meijer, E. W. J. Am. Chem. Soc. 1996, 118, 8717−8718. (64) Yang, S.; Kertesz, M. Macromolecules 2007, 40, 6740−6747. (65) Cheng, Y.-J.; Ho, Y.-J.; Chen, C.-H.; Kao, W.-S.; Wu, C.-E.; Hsu, S.-L.; Hsu, C.-S. Macromolecules 2012, 45, 2690−2698. (66) Street, R. A.; Northrup, J. E.; Salleo, A. Phys. Rev. B 2005, 71, 165202. (67) Salleo, A. Mater. Today 2007, 10, 38−45. (68) Turbiez, M.; Frère, P.; Roncali, J. Tetrahedron 2005, 61, 3045− 3053. (69) Frère, P.; Raimundo, J.-M.; Blanchard, P.; Delaunay, J.; Richomme, P.; Sauvajol, J.-L.; Orduna, J.; Garin, J.; Roncali, J. J. Org. Chem. 2003, 68, 7254−7265. (70) Jestin, I.; Frère, P.; Mercier, N.; Levillain, E.; Stievenard, D.; Roncali, J. J. Am. Chem. Soc. 1998, 120, 8150−8158. (71) Zabulica, A.; Balan, M.; Belei, D.; Sava, M.; Simionescu, B. C.; Marin, L. Dyes Pigm. 2013, 96, 686−698. (72) Işik, D.; Santato, C.; Barik, S.; Skene, W. G. Org. Electron. 2012, 13, 3022−3031. (73) Marin, L.; Perju, E.; Damaceanu, M. D. Eur. Polym. J. 2011, 47, 1284−1299. (74) Simionescu, C. I.; Grigoras, M.; Cianga, I.; Olaru, N. Eur. Polym. J. 1998, 34, 891−898. (75) Moussallem, C.; Gohier, F.; Mallet, C.; Allain, M.; Frère, P. Tetrahedron 2012, 68, 8617−8621. (76) Barik, S.; Skene, W. G. Macromolecules 2010, 43, 10435−10441. (77) Bruma, M.; Hamciuc, E.; Schulz, B.; Köpnick, T.; Kaminorz, Y.; Robison, J. Macromol. Symp. 2003, 199, 511−521. (78) Tsai, F.-C.; Chang, C.-C.; Liu, C.-L.; Chen, W.-C.; Jenekhe, S. A. Macromolecules 2005, 38, 1958−1966. (79) Liu, C.-L.; Chen, W.-C. Macromol. Chem. Phys. 2005, 206, 2212−2222. (80) Barik, S.; Bishop, S.; Skene, W. G. Mater. Chem. Phys. 2011, 129, 529−533. (81) Bolduc, A.; Lachapelle, V.; Skene, W. G. Macromol. Symp. 2010, 297, 87−93. (82) Barik, S.; Bletzacker, T.; Skene, W. G. Macromolecules 2012, 45, 1165−1173. (83) Barik, S.; Skene, W. G. Polym. Chem. 2011, 2, 1091−1097. (84) Knipping, É.; Roche, I. U.; Dufresne, S.; McGregor, N.; Skene, W. G. Tetrahedron Lett. 2011, 52, 4385−4387. (85) Iwan, A.; Sek, D. Prog. Polym. Sci. 2008, 33, 289−345. (86) Grigoras, M.; Catanescu, C. O. J. Macromol. Sci., Part C: Polym. Rev. 2004, 44, 131−173. (87) Hussein, M. A.; Abdel-Rahman, M. A.; Asiri, A. M.; Alamry, K. A.; Aly, K. I. Des. Monomers Polym. 2012, 15, 431−463. (88) Puértolas, J. A.; Carod, E.; Daz-Calleja, R.; Cerrada, P.; Oriol, L.; Piñol, M.; Serrano, J. L. Macromolecules 1997, 30, 773−779. (89) Sek, D.; Iwan, A.; Jarzabek, B.; Kaczmarczyk, B.; Kasperczyk, J.; Mazurak, Z.; Domanski, M.; Karon, K.; Lapkowski, M. Macromolecules 2008, 41, 6653−6663. (90) Farcas, A.; Ghosh, I.; Nau, W. M. Chem. Phys. Lett. 2012, 535, 120−125. (91) Bourgeaux, M.; Skene, W. G. Macromolecules 2007, 40, 1792− 1795. (92) Yang, C.-J.; Jenekhe, S. A. Macromolecules 1995, 28, 1180−1196. (93) Kumar, S.; Dhar, D. N.; Saxena, P. N. J. Sci. Ind. Res. 2009, 68, 181−187. (94) Khalid, M. A.; El-Shekeil, A. G.; Al-Yusufy, F. A. Eur. Polym. J. 2001, 37, 1423−1431. (95) Cerrada, P.; Oriol, L.; Piñol, M.; Serrano, J. L. J. Am. Chem. Soc. 1997, 119, 7581−7582. (96) Oriol, L.; Alonso, P. J.; Martínez, J. I.; P̃ inol, M.; Serrano, J. L. Macromolecules 1994, 27, 1869−1874. (97) Chen, J.-C.; Liu, Y.-C.; Ju, J.-J.; Chiang, C.-J.; Chern, Y.-T. Polymer 2011, 52, 954−964.

(98) Cai, J.; Niu, H.; Wang, C.; Ma, L.; Bai, X.; Wang, W. Electrochim. Acta 2012, 76, 229−241. (99) Dufresne, S.; Skene, W. G. J. Phys. Org. Chem. 2012, 25, 211− 221. (100) Dufresne, S.; Skene, W. G. J. Org. Chem. 2008, 73, 3859−3866. (101) Al-Yusufy, F. A.; Bruckenstein, S.; Schlindwein, W. S. J. Solid State Electrochem. 2007, 11, 1263−1268. (102) Nishinaga, T.; Miyata, T.; Tateno, M.; Koizumi, M.; Takase, M.; Iyoda, M.; Kobayashi, N.; Kunugi, Y. J. Mater. Chem. 2011, 21, 14959−14966. (103) Oliva, M. M.; Pappenfus, T. M.; Melby, J. H.; Schwaderer, K. M.; Johnson, J. C.; McGee, K. A.; da Silva Filho, D. A.; Brédas, J.-L.; Casado, J.; López Navarrete, J. T. Chem. Eur. J. 2010, 16, 6866−6876. (104) Fujii, M.; Nishinaga, T.; Iyoda, M. Tetrahedron Lett. 2009, 50, 555−558. (105) Neese, F. ORCA, An ab initio, DFT and semiempirical SCF-MO package; Max-Planck Institute for Bioinorganic Chemistry: Mülheim a. d. Ruhr, Germany, 2010. (106) Jansson, E.; Jha, P. C.; Ågren, H. Chem. Phys. 2006, 330, 166− 171. (107) Meeto, W.; Suramitr, S.; Vannarat, S.; Hannongbua, S. Chem. Phys. 2008, 349, 1−8. (108) Li, Y.; Zou, L.-Y.; Ren, A.-M.; Feng, J.-K. Comput. Theor. Chem. 2012, 981, 14−24. (109) Fratiloiu, S.; Fonseca, S. M.; Grozema, F. C.; Burrows, H. D.; Costa, M. L.; Charas, A.; Morgado, J.; Siebbeles, L. D. A. J. Phys. Chem. C 2007, 111, 5812−5820. (110) Casida, M. E.; Huix-Rotllant, M. Annu. Rev. Phys. Chem. 2012, 63, 287−323. (111) Dreuw, A.; Head-Gordon, M. J. Am. Chem. Soc. 2004, 126, 4007−4016. (112) Hieringer, W.; Görling, A. Chem. Phys. Lett. 2006, 419, 557− 562. (113) Kuritz, N.; Stein, T.; Baer, R.; Kronik, L. J. Chem. Theory Comput. 2011, 7, 2408−2415. (114) Gierschner, J.; Cornil, J.; Egelhaaf, H.-J. Adv. Mater. 2007, 19, 173−191. (115) Torras, J.; Casanovas, J.; Alemán, C. J. Phys. Chem. A 2012, 116, 7571−7583. (116) Kuhn, W. Helv. Chim. Acta 1948, 31, 1780−1799. (117) Pauling, L. The Nature of the Chemical Bond, 4th ed.; Cornell University Press: Ithaca, NY, 1973. (118) Minkin, V. I.; Minyaev, R. M. Chem. Rev. 2001, 101, 1247− 1266. (119) Ö zen, A. S.; Atilgan, C.; Sonmez, G. J. Phys. Chem. C 2007, 111, 16362−16371. (120) Roncali, J.; Blanchard, P.; Frére, P. J. Mater. Chem. 2005, 15, 1589−1610. (121) Martínez de Baroja, N.; Garín, J.; Orduna, J.; Andreu, R.; Blesa, M. J.; Villacampa, B.; Alicante, R.; Franco, S. J. Org. Chem. 2012, 77, 4634−4644. (122) Xie, L.-H.; Yin, C.-R.; Lai, W.-Y.; Fan, Q.-L.; Huang, W. Prog. Polym. Sci. 2012, 37, 1192−1264. (123) Brédas, J. L.; Street, G. B.; Thémans, B.; André, J. M. J. Chem. Phys. 1985, 83, 1323−1329. (124) Quattrocchi, C.; Lazzaroni, R.; Brédas, J.-L.; Zamboni, R.; Taliani, C. Macromolecules 1993, 26, 1260−1264. (125) Coheur, P.-F.; Cornil, J.; dos Santos, D. A.; Birkett, P. R.; Liévin, J.; Brédas, J.-L.; Walton, D. R. M.; Taylor, R.; Kroto, H. W.; Colin, R. J. Chem. Phys. 2000, 112, 6371−6381. (126) dos Santos, D. A.; Kogej, T.; Brédas, J.-L.; Boutton, C.; Hendrickx, E.; Houbrechts, S.; Clays, K.; Persoons, A.; Xhang, J. X.; Dubois, P.; Jérôme, R. J. Mol. Struct. 2000, 521, 221−230. (127) Ko, S.; Mondal, R.; Risko, C.; Lee, J. K.; Hong, S.; McGehee, M. D.; Brédas, J.-L.; Bao, Z. Macromolecules 2010, 43, 6685−6698.

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