Synthetic Metals 205 (2015) 153–163

Spectral and electronic properties of p-conjugated oligomers andpolymers of Poly (o-chloroaniline-co-o-toluidine) calculated withdensity functional theory

Shah Masood Ahmad a, Salma Bibi b, Salma Bilal b, Anwar-ul-Haq Ali Shah a,*,Khurshid Ayub c

a Institute of Chemical Sciences, University of Peshawar, 25120, PakistanbNational Centre of Excellence in Physical Chemistry University of Peshawar, 25120, PakistancComsates Institute of Information Technology, Abbottabad, Pakistan

A R T I C L E I N F O

Article history:Received 12 January 2015Received in revised form 16 March 2015Accepted 18 April 2015Available online xxx

Keywords:Poly (o-chloroaniline-co-o-toluidine)DFTTD-DFTHOMO–LUMOUV–vis spectra

A B S T R A C T

Density functional theory (DFT) and time dependent DFT (TD-DFT) calculations have been carried outat the oligomers of poly(o-chloroaniline-co-o-toluidine)(POTOC), poly(o-chloroaniline)(POC) andpoly(o-toluidine) (POT). This work discusses conductivity, structural parameters, spectral properties,electronic properties like IPs, EAs, HOMOs, LUMOs and band gaps of POTOC, POT and POC by theapplication of density functional theory to their oligomers up to eight repeating units. The simulatedvibrational frequencies at B3LYP/6-31G (d) along with their assignments are correlated withexperimental frequencies. The UV–vis spectra are simulated with TD-DFT 6-31G (+d p) level of theory.For polymeric studies, the electronic properties of oligomers were extrapolated through second degreepolynomial fit equation. The calculated band gap of POTOC is 3.836 eV while those of POC and POT are3.456 eV and 3.641 eV, respectively. The larger band gap, low delocalization of HOMO–LUMO orbital overentire frame work, lower extent of conjugation and hypsoochromic shift in UV–vis spectra of POTOC ascompared to POT and POC showed that donor-acceptor concept is not playing any role in POTOC and itsconductivity after doping might not be increased as compared to the respective homo-polymers.

ã 2015 Elsevier B.V. All rights reserved.

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1. Introduction

Scientists have taken keen interest in the field of intrinsicconducting polymers since 2000s, when Shirakawa, Heeger andMac Diarmid were awarded with Noble Prize in this field. It isbecause of their wide range of technological applications in opticaland electronic devices [1], biosensor [2], corrosion protection [3],microelectronic devices [4], and electromagnetic shielding [5]. Thebroad spectrum of their applications is based somehow on theirband structure and there have been a number of attempts tominimize band gaps in conducting and redox active polymers.Among all conducting polymers, polyaniline (PANI) is of greatinterest mainly due to its ease of synthesis, good doping de-dopingchemistry, environmental stability and low cost of its monomers.In order to improve the solubility and the existing properties ofPANI, it has been substituted by various electron with drawing like��CN, F, Cl and ��COOH, and electron donating groups like

* Corresponding author. Tel.: +92 91 9216652; fax: +92 91 9216652.E-mail address: anwhq_pk@yahoo.com (A.-u.-H.A. Shah).

http://dx.doi.org/10.1016/j.synthmet.2015.04.0050379-6779/ã 2015 Elsevier B.V. All rights reserved.

��CH3,��OH and ��NH2. This has been achieved either bypolymerization of substituted monomers of aniline or bycopolymerization of aniline with substituted aniline.

Poly(o-chloroaniline-co-o-toluidine) is a copolymer ofo-chloroaniline and o-toluidine having structure chemistryidentical to polyaniline with a difference of only electronwithdrawing and donating substituent (chloro and mehyl group)on phenyl rings alternatively as shown in Scheme 1. It has a highsolubility in common organic solvents, electrochemical stabilityand electroactive in solution of high PH values. A controversialphenomenon has been found between two published literaturesabout the conductivity of copolymer poly(o-chloroaniline-co-o-toluidine)(POTOC) [6,7].

Savitha et al. [6] reported that conductivity of copolymer(POTOC) is 2–5 orders of magnitude higher as compared tohomo-polymer POT and POC. It is considered due to donor acceptorconcept which motivates charge along p conjugation system. Inthis case, the homo-polymer and copolymer were synthesizedchemically from aniline derivatives toluidine and chloroaniline,using ammonium persulphate as oxidizing agent, an aqueoussolution of sodium laurayl sulphate as an emulsifier, HCl as a

NH NH NH* NH *

Cl CH3 Cl CH3

NH N N* NH *

Cl CH3 Cl CH3

N N N* N *

Cl CH3 Cl CH3

LB

EB

PnB

OxidationReduction

Reduction Oxidation

Scheme 1. Different redox forms of poly(o-toluidine-co-o-chloroaniline): LB(Leucoemaraldine base), EB(Emeraldine base) and PNB(pernigraniline base).

154 S.M. Ahmad et al. / Synthetic Metals 205 (2015) 153–163

dopant and the whole contents were stirred in chloroform for 24 h.The conductivities were measured by four probe method on presspellets.

The same homo-polymers and copolymers were synthesizedelectrochemically in our group [7] and in situ conductivities of thecopolymer were found to be lower by 1–3 orders of magnitude ascompared to POT and POC. The argument given was that of theinappropriate donor-acceptor concept or the difference betweentwo methods used for their synthesis i.e., chemical and electro-chemical.

Havinga et al. proposed donor acceptor concept that the regularalternation of donor and acceptor moieties in a conjugated chainlead to widening of valance and conduction band with inducingsmall band gap [8]. The prediction that band gap decreases andbandwidth increases becomes prominent when the electronega-tivity difference increases between donor and acceptor moieties.The small band gaps are the necessary condition for intrinsicconductivity and wide bandwidth are needed for high on chainmobility.

However, Tol and Brocks analyzed the work of Havinga et al.theoretically [9–11] concluding that band gap will be small if thereis weak interaction between the two units while in the case ofstrong interaction charge transfer increases the band gap. But weakinteraction leads to narrowing bandwidth thereby decreasingmobility of charge carriers. The authors specified that there isspecial effect playing role in Havinga et.al system which isnot applicable to all systems in general. Salzner and co-workers[12–14] carried out study on donor-acceptor phenomenon oncopolymers of thiophene based polymers and concluded thatdonor-acceptor concept is not at all apply generally to the systemhaving donor-acceptor moieties.

We became interested to study theoretical aspects ofpoly(o-chloroaniline), poly(o-toluidine) and their copolymerpoly(o-chloroaniline-co-o-toluidine) to get insight into theproperties of this copolymer and to support or otherwisethe two published [6,7] reports on this material with respect tothe conductivity issue.

2. Computational methodology

All calculations were carried out with GAUSSIAN O9 program[15] and visualization of results was done with Gauss View [16],Gabedit [17]. The structures of nPOC, n POT and nPOTOC oligomerswere optimized using DFT at B3LYP (Becke three parameter(exchange), Lee Yang, and Parr) method using 6-31 G (d) basis setwithout any symmetry constrains with both local and non-localcorrelation [18], where n represents number of phenyl rings(n = 2,4,6,8). The structures were considered completely optimizedas stationary point was located and were confirmed by absence ofimaginary frequencies. Taking the optimized geometric structuresthe I.R, Raman, I.E, E.A, HOMO, LUMO and band gap calculationswere performed with same level of theory at ground state, whileUV–vis spectra were simulated with TD-SCF using B3LYP/6-31+G(d,p). The oligomers properties such as IP, EA, HOMO, LUMO, andband gap and band width were extrapolated for polymeric studythrough second-degree polynomial fit equation [19]. For bettercorrelation of computed value with the experimental one, a scalingfactor of 0.9613 was multiplied to the calculated frequencies[20,21]. Frequencies are assigned mainly with Gabedit and origin.The band gaps (or the p–p* lowest electron transition) and bandwidths of valance and conduction bands were estimated from thedifference between HOMO–LUMO energies [22] and band

Fig. 1. Optimized geometric structures of 2–8POC, POT and POTOC oligomers as indicated.

S.M. Ahmad et al. / Synthetic Metals 205 (2015) 153–163 155

156 S.M. Ahmad et al. / Synthetic Metals 205 (2015) 153–163

structure calculation. IP and EA were determined from the negativeof HOMO and LUMO [21,23] orbital energies, respectively. Allcalculations were performed in the gas phase.

3. Result and discussion

3.1. Optimized geometric structures

Since the spectral and electronic properties like HOMO–LUMO,I.P, E.A, and band gap greatly depend on the structure of polymericmaterials, it is highly desirable to optimize the geometricstructures of nPOC, nPOT and their co-polymer (nPOTOC).To predict the effect of substitution on various geometricalparameters geometrical optimization of nPOC, nPOT and (nPOTOC)(n = 2, 4, 6 and 8) oligomers were carried out on reduced from(Leucoemeraldine) at DFT-B3LYP/ 6-31 G (d) level of theory. Theoptimized geometric structures of the oligomers are shown inFig. 1. All these geometries are in twisted manner and achievezigzag geometry as the chain length increase. This behavior is verymuch expected to minimize steric hindrance of substituent groupsand phenyl rings along a chain. The three important parametersbridging angle, dihedral angle and inter-substituents distance atB3LYP/6-31G (d) level of theory are considered here for analysisand discussion. These simulated parameters of bridging angle anddihedral angle are given in Tables 1 and 2, respectively. Thebridging angle in each type of homo-oligomers and co-oligomersincreases as we move from lower repeating units toward higherones. However, bridging angle along a chain in higher oligomers ofhomo-polymers POC and POT decreases as the steric effectdecreases while in their copolymer alternatively increase anddecrease were observed because of two different substituentsattached at phenyl rings. The bridging angles of <C��N��C ofoligomers of POC,POT and POTOC are in a range of 128.3�–128.8�,127.2�–128.0� and 127.44�–128.53�, respectively. Similarly,the values of dihedral angle of <C��N��C��C of oligomers of POC,POT and POTOC increase as chain length increases fromn = 2,4,6,8 while along a chain it randomly increases and decreasesto minimize the steric influence.

In oligomers of POC, POT and POTOC, all the values of dihedralangle are positive and are in the range of 143.4�–155.2�, 134.3�–142.6� and 136.7�–146.9�, respectively except in 4POT where somevalue are negative as shown in Table 2. The inter separationdistance of substituent group Cl��Cl, methyl-Cl and methyl–methyl on oligomer of POC, POT and POTOC are in range 7.75–7.86,7.41–7.55 and 7.70–7.73 Å, respectively (given in Supportinginformation in Table 1).

The Vander Waals radii of methyl group and Cl are very close(2.0 Å and 1.8 Å, respectively) and considered generally similar

Table 1Optimized bridging angle of nPOC, nPOT and nPOTOC at B3LYP/6-31G (d) level of theo

S.NO u1 u2 u3

2POC 128.422974POC 128.63215 128.51770 128.302146POC 128.74164 128.63316 128.59788

8POC 128.79518 128.56410 128.43828

2POT 127.203174POT 127.94516 126.91480 126.686536POT 127.74961 127.72735 127.46135

8POT 128.01073 127.65840 127.47067

2POTOC 127.927854POTOC 128.52198 127.51442 128.152796POTOC 128.52478 127.86922 128.45909

8POTOC 128.53186 127.56702 128.41402

steric bulk [24]. The theoretically simulated parameters likebridging angle, dihedral angle and inter substituent distance in ourcase show that oligomeric geometries of nPOC minimize moresteric strain by twisting the substituent groups, increasing thebridging angle and dihedral angle as compared to nPOT while innPOTOC, the effect is nearly average of nPOC and nPOT. It might beattributed to higher electro negativity of Cl groups. The order ofincrease this of effect in oligomers is POC > POTOC > POT. In order tosee the effect of substituent on bond length of phenyl ring andbridging length we have tabulated the value of 8POC, 8POT and8POTOC (close to polymeric material) in Supporting information inTable (S1–S3). The C��C bond length of 8POC is found to be in therange of 1.389–1.41 Å, 1.39–1.417 Å of 8POT and 1.384–1.41 Å of8POTOC. The C��N bond length is 1.387–1.404 Å of 8POC,1.401–1.408 Å of 8POT and 1.384–1.41 Å of POTOC, respectively.The optimized bond lengths of C��N and C��C of 8POC, 8POT and8POTOC oligomers are given in Fig. 2. Generally, the C��N and C��Cbond length extension of 8POT is higher as compared to 8POC and8POTOC which may be due to bulky substituent group. However,from the figure it is also observed that the C��C and C��N bondchanges in phenyl ring of 8POTOC are higher as compared to 8POCand 8POT which leads to its unfavorable conductivity.

The values calculated for the mentioned parameters may beslightly different from experimental one because of the condensedphase nature of solid polymers [25].

3.2. Infra-red spectroscopy

The theoretically simulated I.R spectra of POC, POT and POTOColigomers of 8 repeating units are given in Fig. 3.The systematicerrors in calculations are removed by applying a scaling factor of0.9613 [15,26]. The major peak’s values of 8POC along with theirexperimental bands and approximate assignments are given inTable 3. The spectrum of 8POC is mainly composed of 17 bandpeaks at 3473, 3414, 3092, 3086, 1630, 1599–1613, 1569, 1552,1460–1515, 1213–1240, 1071, 1061–1027, 1015, 878–919, 414–802,398 cm�1. The calculated 3473 and 3414 cm�1 bands are assignedto N��H stretching, 3092 and 3086 cm�1 are to C��H stretching,1630 and 1599–1613 cm�1 bands are related to amine H��N��Hscissoring and ring deformation. The bands at 1569 and1552 cm�1

corresponding to bridging N��H bending and C¼C stretching ofbenzoid ring correlate with experimental 1500–1590 cm�1.Simi-larly, the bands 1213–1240 and 1015 cm�1 are assigned to combineC��N stretching, C��H, N��H bending, ring deformation andsubstituted C��Cl stretching vibration showing correlation withexperimental values 1298 and1006 cm�1, respectively. Someother important vibrations are 1460–1515 cm�1 (C��H andN��H bending), 1071 and 398 cm�1 (H��N��H rocking),

ry (Fig. 1).

u4 u5 u6 u7

128.50517 128.36910128.57282 128.48001 128.33780 128.34174

127.34845 126.84130127.56107 127.14077 127.16000 126.69484

127.44198 128.21112127.52415 128.45053 127.52068 127.91441

Table 2Optimized dihedral angles of nPOC, nPOT and nPOTOCat B3LYP/6- 31G (d) level of theory (Fig. 1).

S.NO u1 u2 u3 u4 u5 u6 u7

2POC 143.434804POC 147.68805 147.98023 148.513366POC 148.51391 152.67593 151.55272 152.45193 148.696468POC 149.79355 153.09190 156.14763 155.19945 153.08647 155.22360 146.2398

2POT 134.306164POT �139.72564 �137.64741 135.328846POT 148.51391 142.61931 141.49651 140.80249 137.745558POT 139.56671 142.11194 141.79690 142.37215 140.43798 141.09661 137.02588

2POTOC 136.723584POTOC 143.15105 140.45918 141.268976POTOC 143.33888 142.99699 146.90834 141.93478 142.440408POTOC 142.96646 142.13245 147.74877 143.82557 146.56940 142.88766 138.83148

Fig. 2. Bond length of C��C and C��N of 8POC, 8POT, and 8POTOC oligomers.

S.M. Ahmad et al. / Synthetic Metals 205 (2015) 153–163 157

878–919 cm�1(H–C wagging, C¼C stretching, C��H bending) and414–802 cm�1 concerned to (H��C, H��N wagging).

The major calculated scaled frequencies of I.R spectrum of 8POTalong with their experimental bands and approximate assign-ments are given in Table 4.The bands at 3425–3436 cm�1,

Fig. 3. Scaled I.R spectra of 8 POC, 8POT and 8POTOC oligomer.

3384–3308 cm�1and 3043–3055 cm�1 are associated with H��N,amine H��N��H and C��H stretching while experimentalH��N stretching band is related to 3200 cm�1. The bands at2898–3009 cm�1 which are assigned for C��H stretching of methylsubstituent show correlation with experimental values 2920,2850 cm�1. The peaks at 1612–1631 cm�1 and 1611 cm�1 areassigned to H��N��H scissoring and C¼C stretching of phenyl ring.Some other major bands are 1507 cm�1 (H��N bending),1443–1482 cm�1 (H��C��H scissoring, N��H and C��H bending),1369–1389 cm�1 (H��C of methyl group wagging),1248–1262 cm�1

(C¼C,C��C stretching and H��N bending), 1278–1295 cm�1 (H��Cbending and C��N stretching), 991–1081 cm�1 (H��C��H rocking,H��N and H��C bending), 386–567 cm�1 (H��N and H��C wagging).

The calculated spectral values of 8POTOC oligomer along withexperimental values with their approximate assignment are given inTable 5. The region 3425–3497 cm�1 is corresponding to H��N andH��N��H stretching vibration correlated with experimental3215 cm�1. Similarly, the 2900–3010 cm�1 range assigned toH��C��H of methyl group stretching correlated to empirical2850–2920 cm�1,the 1603 cm�1 related to C¼C stretching of phenylring show correlation with experimental 1589 cm�1 with slightdifference, the 1323 cm�1 assigned to C¼C and C��N stretching alsostrongly correlated with experimental 1325 cm�1. The othertheoretical major bands showing correlation are 1056–1078, 1018,877–905 and 770–864 cm�1 with experimental 1107, 1005,758–764 and 810–880 cm�1 assigned to H��C bending, H��C��Hrocking of methyl substituent, C��Cl stretching and out of planebending of H��C means wagging, respectively. The peak at1634 cm�1 is concerned with (H��N��H scissoring),1268–1291 cm�1 with (H��C bending, C¼C and C��N stretching),1225 cm�1 with (C��C methyl stretching), 1159–1224 cm�1 with(H��N��H rocking C��C and H��C bending) and 905 cm�1 with(H��C wagging). The difference between experimental and theoret-ical values in some cases may be due to polymeric and oligomericstudy because increasing chain length lead shifting phenomena ormay be due to oxidized and reduced form as Savitha et al. [6] haveused emeraldine form in their experimental work but we have takenneutral form for theoretical consideration. The 1015 cm�1 relatedto C��Cl stretching of POC shifted to 877–905 cm�1 in copolymerPOTOC, the 3425–3436 cm�1 H��N stretching in POT showedshifting to higher wave length 3425–3497 cm�1. The2898–3009 cm�1 related to C��H stretching of methyl substituentof POT shifted to 2900–3010 cm�1 in copolymer.

3.3. UV–vis spectral characteristics

The theoretically simulated UV–vis spectra of nPOC, nPOT andcopolymer nPOTOC at TD-DFT/6-31+G (d, p) level of theory are

Table 3Experimental I.R and calculated frequencies (in cm�1) of 8POC.

S.No Experimental [27,28] Calculated frequency Approximate assignment

I.R Scaled Unscaled

1 – 3473 3613 nH��N2 – 3414 3552 nH��N3 – 3092 3217 nH��C(sym)4 – 3086 3211 nH��C(Anti sym)5 – 1630 1696 cis H��N��H6 – 1613 �1599 1678–1664 Def(ring)7 – 1569 1633 bN��H8 – 1552 1615 nC¼C(B)9 1500–1590 1515–1460 1576–1519 b N��H, bC��H10 – 1241–1213 1292–1262 bC��H; N��H, Def nC��N11 1298 1071 1115 H��N��H(Rocking)12 – 1027–1016 1069–1057 C��H(Twisting)13 – 1015 1056 nC��Cl14 1006 919–878 957–914 Wag H��C,nC¼C(B), bC��H15 – 802–414 835–431 Wag H��C; H��N16 – 398 415 H��N��H(Rocking)

Note: n: stretching; Wagg: Wagging; cis: scissoring; b: bending; B: benzoid; Def: deformation mode; Sym: symmetric stretching.

Table 4Experimental and calculated I.R frequencies (cm�1) of 8POT.

S.No Experimental [6] Calculated frequency Approximate assignment

I.R Scaled Unscaled

1 3200 – – nH��N2 – 3425 3563 nH��N3 – 3436 3575 nH��N4 – 3384–3408 3521–3546 nH��N��H5 – 3066–3076 3190–3200 nH��C6 – 3043–3055 3166–3178 nH��C7 2920,2850 2898–3009 3015–3131 nH��C(methyl)8 – 1612–1631 1677–1697 cis H��N��H9 – 1611 1676 nC¼C(B), Def(ring)10 – 1507 1568 bN��H11 – 1443–1482 1502–1542 cisH��C��H, bN��H, bC��H12 – 1424 1482 nH��C, nC¼C(B), bN��H13 – 1369–1389 1425–1445 Wagg C��H(methyl)14 – 1323 1377 nC¼C, nC��N15 – 1278–1295 1339–1348 bC��H, nC��N16 – 1248–1262 1299–1313 nC¼C, bN��H, nC��C17 – 1230 1280 bN��H, bC��H18 – 1212–1221 1261–1271 n C��C,bN��H,bC��H, nC��N19 - 1093–1204 1138–1253 bN��H, bC��H, nC��N20 – 991–1081 1031–1125 H��C��H(R), bN-H,bC-H21 – 928–942 966–980 Def(r), WaggC H,nC��C(m)22 – 691–796 719–829 Wagg C��H,C��C(M)23 – 599–674 624–702 bH��N��H, Wagg C��H24 – 386–567 402–590 Wagg N��H;C��H

Note: n: stretching, Wagg: Wagging, cis: scissoring, b: bending B: benzoid; Def: deformation mode, R: Rocking, M;methyl.

158 S.M. Ahmad et al. / Synthetic Metals 205 (2015) 153–163

given in Figs. 4–6, respectively. We have simulated the UV–visspectra for different oligomers of the same polymer together inorder to see the substituent effect on spectrum. The UV–visspectrum of oligomer 2POC consists mainly of three peaks at 328,290 and 197 nm. The peak at 328 nm is assigned for p–p* transitionor HOMO–LUMO transition, 290 nm is for HOMO to LUMO+1 transition and 197 nm is for inter-band transition. The peakconcerned to p–p* transition showed bathochromic shift towardhigher wavelength 353, 358 and 378 nm, while moving from 2 to8 oligomer repeating units. It is because of increase in the extent ofconjugation.

The others peaks in spectra are related to inter-band transition.The p–p* transition in experimental UV–vis spectrum ofo-chloroaniline is reported at 301 nm [6]. Similarly, in UV–visspectrum of 2POT, the peak at 316 nm is assigned for HOMO–LUMOtransition, 285.3 nm is for HOMO to LUMO + 3 transitions and thatof 186 nm is for inter-band gap transition. With increasing chain

length of oligomers the p–p* transition band is shifted towardshigher wavelengths of 328 nm, 344 nm and 346 nm in spectrum of4, 6, 8POT. This shift is comparatively less than in the case of nPOC.The reason might be greater steric factor of methyl group in thecase of nPOT. The experimental value reported for p–p* in UV–visspectrum is 311 nm [6] showing correlation with theoretical result.In case of copolymer nPOTOC, the peaks at 326 nm, 337 nm,368 nm and 370 nm in spectrum of 2–8 repeating units areassigned to p–p* transition while the other peaks are concernedwith inter-band transition. The p–p* transition in the spectra ofnPOC showed more bathochromic shift as compared to nPOT andits copolymer with toluidine. This shifting toward higherwavelength show that delocalization in aromatic rings of nPOCis higher as compared to nPOT and its copolymer, providing a cluethat conductivity of the copolymer will be lower as compared to itshomo-polymer POC. The experimental p–p* transition valuereported for the copolymer POTOC is 298–322 nm [6] correlated

Table 5Experimental and calculated I.R frequencies (cm�1) of 8POTOC.

S.No Experimental [30] Calculated frequency Approximate assignment

I.R Scaled Unscaled

1 3215 3425–3474 3563–3614 nH��N2 3396–3403 3533–3541 nH��N��H(sym)3 3045–3105 3168–3231 n H��C(sym)4 2850–2920 2900–3010 3017–3132 nH��C��H(methyl)5 1634 1700 cis H��N��H6 1611 1676 cis H��N��H, nC¼C(B), bH��N7 1610 1675 nC¼C(B), n C��N, cis H��N��H8 1589,1480 1603 1668 nC¼C9 1480–1515 1540–1577 bN��H, bC��H10 1448–1454 1507–1513 Wagg H��C��H(methyl)11 1444 1503 bN��H,bC��H, Wagg H��C��H12 1400 1457 bN��H, bC��H, nC��C13 1325 1323 1377 nC¼C(B), nC��N14 1268–1291 1320–1343 bC��H, nC¼C(B), nC��N15 1225 1275 nC��C(methyl)16 1170-1190 1159–1224 1206–1274 bC��H, nC��N, nC��Cl17 1107 1056–1078 1099–1122 H��N��H(R), bC��C, bC��H18 1005 1018 1059 H��C��H(Rocking)19 922–945 960–984 H��C��H(R), nC��C, nC��N20 905–928 942–966 Wagg C��H21 758-764 877–905 913–942 nC��Cl22 810-880 770–864 802–899 Wagg C��H23 727–755 757–786 NC��C(methyl)24 665–708 692–737 Wagg C��H;H��N��H,25 630 656 nC��Cl26 325–556 339–579 Def (ring) Wagg C��H

Note: n: stretching, Wagg: Wagging, cis: scissoring, b: bending B: benzoid; Def: deformation mode; Sym:symmetric, R: rocking.

Fig. 4. UV–vis spectra of POC from 2 to 8 repeating units.

S.M. Ahmad et al. / Synthetic Metals 205 (2015) 153–163 159

well with theoretical value. The excitation energies of HOMO–LUMO transition with oscillator strength of nPOC, nPOT andnPOTOC oligomers are quoted in Table 6. The difference betweenexperimental and theoretical is because of the gas phase andoligomers study in our work.

3.4. HOMO, LUMO, band gaps, I.P and E.A

The HOMO and LUMO of nPOC, nPOT and nPOTOC elevated atB3LYP/6-31G (d) of (n = 2 and 8 repeating units are shown in Fig. 7.The HOMO and LUMO orbitals are allowed to extend throughoutthe optimized geometry structures of an oligomer. Generally, inhomopolymers (nPOC, nPOT) and their copolymer (nPOTOC), theextent of conjugation increases with chain length elongation ofoligomers. But the planarity of polymer decreases due to sterichindrance of substituent groups. Similarly, increase in the numberof atoms decreases the electron density of HOMO and LUMOthereby preventing delocalization over the entire frame. The

conductivity depends on planarity of the system and band gap andpolymers having smaller band gap and planner geometry areconsidered to show good conductivities. In our system, thedelocalization of HOMO and LUMO is slightly prominent inoligomers of nPOC as compared to nPOT and copolymer indicatingslightly more planarity in nPOC as compared tonPOT and nPOTOC.

The HOMO, LUMO and band gap energies in eV of POC, POT andits copolymer POTOC from monomer up to infinite repeating unitsare given in Table 7. For getting polymeric data, we haveextrapolated oligomers data through second degree polynomialfit equation. In all three cases, the energy of HOMO increases as thechain length of oligomer increases while that of LUMO energydecreases because of increase in the extent of conjugation. Thisleads to decrease in band gap which is one of the requirements forbetter conductivity. The band gap, also considered to be theHOMO–LUMO energy difference, as estimated through polynomialfitting from oligomers data of POC, POT and POTOC are 3.456 eV,3.632 eV and 3.837 eV, respectively.

Fig. 5. UV–vis spectra of POT from 2 to 8 repeating units.

Fig. 6. UV–visible spectra of copolymer POTOC from 2 to 8 repeating units.

160 S.M. Ahmad et al. / Synthetic Metals 205 (2015) 153–163

The band gap of homo-polymer POC is smaller as compared tohomo-polymer POT and copolymer POTOC. The band gap values ofoligomers of nPOT are found to be slightly higher than those ofcopolymer POTOC, however, elevated band gap for polymerthrough second degree polynomial fit equation from oligomersdata show lower band gap for POT. The development of conductionand valance bands from HOMOs and LUMOs energies of oligomersare shown in Figs. 8–10, respectively. About 0.381 eV band gaplowering is found in POC and 0.205 eV in POT as compared to

Table 6Calculated excitation energies, oscillator strength, and molecular orbital of the first sin

Species Energy(eV) Wavelength(nm)

2POC 3.7771 328.25

4POC 3.5032 353.92

6POC 3.4546 358.90

8POC 3.2790 378.12

2POT 3.9208 316.22

4POT 3.7738 328.5

6POT 3.6001 344.39

8P0T 3.5746 346.85

2POTOC 3.8027 326.04

4POTOC 3.6718 337.67

6POTOC 3.3611 368.88

8POTOC 3.3443 370.73

copolymer POTOC showing one of the indication that donor-acceptor substituent in that copolymer does not lead to decreasethe band gap.

Planarity, band gap and band width of valence and conductionband are the three important features of a polymeric system thatmust be considered while describing conductivity phenomenon.The conductivity of conducting polymers is classically related tothe number of charge carriers n and the mobility of these chargecarriers, which depends on the time t between collisions in the

glet transition involved in the excitation for nPOC, nPOT and nPOTOC.

Oscillator strength MOs(H–L) Coefficient

0.0177 69–70 0.696300.2276 133–134 0.689951.0120 197–198 �0.382881.0312 261–262 0.135350.0148 61–62 0.695941.0205 117–118 0.681271.3779 173–174 0.587570.9183 229–230 �0.249890.0272 65–66 0.695600.9453 125–126 0.594351.2256 185–186 0.626870.9543 245–246 0.50343

Fig. 7. Frontier orbitals at isovalue = 0.02 of nPOC, nPOT and nPOTOC of 2 and 8 repeating units.

Table 7IPs, EAs, HOMOs, LUMOs and band gaps of POC, POT and POTOC.

n, rep. units I.Ps(eV)

E.As(eV)

HOMOs(eV)

LUMOs(eV)

Band gaps (eV)

2POC 5.047 0.389 �5.047 �0.389 4.6584POC 4.697 0.597 �4.697 �0.597 4.1006POC 4.599 0.710 �4.599 �0.710 3.8898POC 4.555 0.766 �4.555 �0.766 3.8441 4.435 0.979 �4.435 �0.979 3.4562POT 4.624 �0.131 �4.696 0.131 4.8274POT 4.239 �0.017 �4.239 0.017 4.2566POT 4.114 0.078 �4.114 �0.078 4.0368POT 4.072 0.114 �4.072 �0.114 3.9581 3.941 0.300 �3.941 �0.300 3.6412POTOC 4.835 0.062 �4.835 �0.062 4.7734POTOC 4.517 0.476 �4.517 �0.476 4.0416POTOC 4.338 0.394 �4.338 �0.394 3.9448POTOC 4.305 0.439 �4.305 �0.439 3.8661 4.039 0.203 �4.039 �0.203 3.837

S.M. Ahmad et al. / Synthetic Metals 205 (2015) 153–163 161

Eq. (1)

s ¼ ne2tm

(1)

where e is the elementary charge and m is the mass of the chargecarriers. The charge carrier concentration increases as the energyseparation between the valence and conduction bands decreases

and the mobility of the charge carriers is influenced by thebandwidth of conduction and valence band [29]. The current studyindicates that the energy separation between the valence andconduction bands in homo-polymers POC and POT is smaller ascompared to their copolymer POTOC concluding that theconductivity of copolymer cannot be greater than those ofhomo-polymers. This theoretical finding supports our experimen-tal results [7].

The electronic properties like I.P and E.A are determined fromthe negative of HOMO and LUMO energies (Koopmans theorem) ofnPOC, nPOT and nPOTOC oligomers and extrapolated throughpolynomial fit equation. Although the I.P and E.A obtained from thenegative of the DFT orbitals (HOMO–LUMO) with typical exchangecorrelation functional is usually small as compared to experimen-tal results, implementation of hybrid functional (B3LYP) whichaccounts for the effect of self-interaction is used to achieve bettercorrelation. Generally, the I.P and E.A increases as we move from2 to 8 repeating units. The I.Ps and E.As of nPOC, nPOT andcopolymer nPOTOC up to infinity level are given in Table 7. All the I.P values from monomer up to infinity of POC, POT and theircopolymer (POTOC) are positive values while that of E.A values ofPOC and copolymer are positive but in case of POT some values ofE.A are negative. The negative E.A means that the anionic state isunbound and that is why the autocatalytic behavior of POT wasreported in electrochemical synthesis [7].

Fig. 8. Band structure of POC from energy levels of oligomers up to infinity.

Fig. 9. Band structure of POT from energy levels of oligomers up to infinity.

Fig.10. Band structure of energy levels of POTOC from oligomers up to infinity.

162 S.M. Ahmad et al. / Synthetic Metals 205 (2015) 153–163

The I.P and E.A values of POC are comparatively greater than forPOT while that of copolymer POTOC are approximately average ofhomo-polymer POC and POT. Since experimental I.P and E.A ofcopolymer is not reported so far, theoretical correlation is notachieved.

4. Conclusion

Quantum mechanical calculations were carried out for homo-polymers nPOC, nPOTand their copolymer (nPOTOC) at both TD-DFTand DFT-B3LYP/6-31G(d) level of theory where (n = 2,4,6,8 repeatingunits) in order to study structure parameters, spectral properties

and electronic properties like HOMO, LUMO, I.P, E.A and band gap.The optimized geometries of homo-polymers and copolymer arefound to be in zigzag and twisted; it is due to steric strain of donorand acceptor substituents attached at phenyl rings of conjugatedsystem. The quoted values of I.R spectrum which is elevated at samelevel of theory along with their assignment is correlated withexperimental reported values. The p–p* transition of UV–visspectrum of nPOC, nPOT and nPOTOC elevated at TD/DFT with basisset 6-31+G(d, p) also correlated with experimental reported valuesshowing over estimation which may be due to gas phase oroligomeric studies. Higher bathochromic shift is observed in nPOCas compared to homo-polymer nPOT and copolymer (nPOTOC)

S.M. Ahmad et al. / Synthetic Metals 205 (2015) 153–163 163

concluded that donor-acceptor substituent do not lead to increasein extent of conjugation. The delocalization of frontier orbitals overentire geometries showed that nPOC has to some extent plannergeometries as compared to nPOTand copolymer nPOTOC. The IP, EA,HOMO, LUMO and band gap values are extra plotted through seconddegree polynomial fit equation from oligomers data. The band gapsestimated from the HOMO–LUMO orbital energies decreases frommonomer up to infinity in homo-polymers as well in copolymer(nPOTOC). The extraplotted band gap value of copolymer is 3.837 eVwhile those of homo-polymers (POC, POT) are 3.456 and 3.641 eVreflecting that the conductivity of the copolymer cannot be greaterthan those of homo-polymers.

Acknowledgements

Financial support from Higher Education Commission Islam-abad under Project # 20-3111 is highly appreciated.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.synthmet.2015.04.005.

References

[1] H.J. Zhu, N.J. Tao, L.A. Nagahara, I. Amlani, R. Tsui, J. Am. Chem. Soc. 123 (2001)7730.

[2] G. Inzelt, M. Pineri, J.W. Schultze, M.A. Vorotyntsev, Electrochim. Acta 45(2000) 2403.

[3] J.M. Yeh, C.L. Chen, Y.C. Chen, C.Y. Ma, K.R. Lee, Y. Wei, S. Li, Polymer 43 (2002)2729.

[4] B. Wessling, P.K. Kahol, A. Raghunathan, B.J. McCormick, Synth. Met.119 (2001)197.

[5] W.K. Lu, R.L. Elsenbaumer, B. Wessling, Synth. Met. 71 (1995) 2163.[6] P. Savitha, D.N. Sthyanaraynana, Synth. Met. 145 (2004) 519.[7] A.A. Shah, S. Bilal, R. Holze, Synth. Met. 162 (2012) 356.[8] E.E. Havinga, W. Hoeve, H. Wynberg, Synth. Met. 55–57 (1993) 299.[9] A.J.W. Tol, J. Chem. Phys. 100 (1994) 8463–8470.

[10] G. Brocks, J. Chem. Phys. 102 (1995) 2522.[11] G. Brocks, A. Tol, J. Phys. Chem. 100 (1996) 1838.[12] U. Salzner, S. Okur, J. Phys. Chem. A 112 (2008) 11842.[13] U. Salzner, J. Phys. Chem. B 106 (2002) 9214.[14] U. Salzner, Synth. Met. 119 (2001) 215.[15] M. Frisch, G. Trucks, H. Schlegel, G. Scuseria, M. Robb, J. Cheeseman, G.

Scalmani, V. Barone, B. Mennucci, G. Petersson, Gaussian 09. Revision A. 02,Gaussian Inc., Wallingford, CT, 2009, pp. 115.

[16] Computer Program Gauss View, Ver. 2, Gaussian Inc., Pittsburgh, PA.[17] A.R. Allouche, Gabedit – a graphical user interface for computational

chemistry softwares, J. Comput. Chem. 32 (2011) 174–182.[18] A.D. Becke, Density-functional exchange-energy approximation with correct

asymptotic behavior, Phys. Rev. A 38 (1988) 3098.[19] U. Salzner, J. Phys. Chem. A 114 (2010) 5397.[20] J. Casado, V. Hernández, F. Ramirez, L. Navarrete, J. Mol. Struct. (THEOCHEM)

463 (1999) 211.[21] J.B. Foresman, E. Frisch, Exploring Chemistry with Electronic Structure

Methods: A Guide to Using Gausion, Gaussian. Inc., 1996.[22] U. Salzner, P. Pickup, R. Poirier, J.B. Lagowski, Phys. Chem. A 102 (1998)

2572.[23] U. Salzner, J. Phys. Chem. A 112 (2008) 5458.[24] A. Bondi, J. Phys. Chem. 68 (1964) 441.[25] A.U. Rani, N. Sundaraganesan, M. Kurt, M. Cinar, M. Karabacak, Spectrochim.

Acta Part A: Mol. Biomol. Spectrosc. 75 (2010) 1523.[26] P. Rosmus, H. Bock, M. Soluki, G. Maier, G. Mihm, Angew. Chem. Int. Ed. Engl. 20

(1981) 598.[27] S. Bilal, A.A. Shah, R. Holze, Electrochim. Acta 56 (2011) 3353.[28] H. Zagal, M.E. Vaschetto, B.A. Retamal, J. Polym. Matter 44 (1999) 225.[29] U. Salzner, T. Kiziltepe, J. Org. Chem. 64 (1999) 764.