doping and dedoping process of polypyrrole - dft study with hybrid functionals

Doping and Dedoping Processes of Polypyrrole: DFT Study withHybrid FunctionalsHabib Ullah,† Anwar-ul-Haq Ali Shah,*,† Salma Bilal,‡ and Khurshid Ayub*,§,∥

†Institute of Chemical Sciences and ‡National Centre of Excellence in Physical Chemistry, University of Peshawar, 25120 Peshawar,Pakistan§Department of Chemistry, COMSATS Institute of Information Technology, University Road, Tobe Camp, 22060 Abbottabad,Pakistan∥Department of Chemistry, College of Science, King Faisal University, Al-Hafouf 31982, Saudi Arabia

*S Supporting Information

ABSTRACT: Density functional theory (DFT) and time-dependentDFT (TD-DFT) calculations at the UB3LYP/6-31G(d) level have beenperformed to investigate the tunable nature, i.e., doping and dedopingprocesses, of polypyrrole (PPy). The calculated theoretical data showstrong correlation with the recent experimental reports, which validatesour computational protocol. The calculated properties are extrapolatedto the polymer (PPy) through a second-order polynomial fit. Changes inband gap, conductivity, and resistance of nPy and nPy-X (where n = 1−9and X = +, NH3, and Cl) were studied and correlated with the calculatedvibrational spectra (IR) and electronic properties. Upon doping, bridgingbond distance and internal bond angles decrease (decrease in resistanceover polymer backbone), whereas dedoping results in increases in thesegeometric parameters. In the vibrational spectrum, doping ischaracterized by an increase in the band peaks in the fingerprint region and/or red shifting of the spectral bands. Dedoping(9Py+ with NH3), on the other hand, results in decreases in the number of vibrational spectral bands. In the UV−vis and UV−vis−near-IR spectra, the addition of different analytes (dopant) to 9Py results in the disappearance of certain bands and gives riseto some new absorbances corresponding to localized and delocalized polaron bands. Specifically, the peaks in the near-IR regionat 1907 nm for Py+ and 1242 nm for 9Py-Cl are due to delocalized and localized polaron structures, respectively. Upon p-doping,the band gaps and resistance of nPy decrease, while its conductivity and π-electron density of conjugation increase over thepolymeric backbone. However, a reversal of properties is obtained in n-doping or reduction of nPy+. In the case of oxidation andCl dopant, the IP and EA increase, and consequently, there is a decrease in the band gap. NBO and Mulliken charges analysesindicate charge transferring from the polymer in the case of p-type dopants, while this phenomenon is reversed with n-typedopants.

1. INTRODUCTION

Conjugated organic polymers (COPs) are technologicallyimportant,1 due to their tunable nature,2 free availability of π-electrons on the backbone of the polymeric chain, highstability,1 low cost,3 and ease of preparation.4 COPs have awide range of applications in the fields of sensors,5,6 actuators,7

rechargeable batteries,8 solar cells,9 electrochromic displaymaterials,1 anticorrosion protection,10 and electromagneticshielding technology.11 For conduction, COPs can be dopedeither p-type (oxidation) or n-type (reduction), depending onthe nature of the polymer.12 Polyaniline (PANI), polyacetylene(PA), polythiopene (PT), polyparaphenylene (PPP), polypar-aphenylenevenylene (PPV), and poly(o-phenylenediamine)(POPD) are prominent examples of COPs.13 The COP familyreceived another important member when, in the 1960s, Weisset al. prepared polypyrrole (PPy) by the pyrolysis oftetraiodopyrrole.14 The authors concluded that PPy can

conduct electricity in the presence of a dopant (iodine). PPyis physically insoluble, amorphous, and infusible.PPy has been extensively studied both experimentally and

theoretically for applications as sensors, actuators, andcorrosion inhibitors. Recently, we have also carried out atheoretical study to investigate its ability as a sensor for NH3gas in its undoped form.5 A large number of papers have beenpublished on experimental studies of PPy as sensor formethanol, ethanol, NO2, NH3, CO2, and CO and other toxicgases.15−18 For improved performance (high crystallinity andconductance), PPy is modified with some coating material suchas TiO2 or ZnO.

19 Moreover, to enhance the performance ofPPy, different approaches have been reported, such asminimizing the band gap, composites,20 and nano studies.21

Received: June 6, 2014Revised: July 15, 2014Published: July 18, 2014

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© 2014 American Chemical Society 17819 dx.doi.org/10.1021/jp505626d | J. Phys. Chem. C 2014, 118, 17819−17830

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The conducting properties of COPs mainly depend on thearrangement and number of their repeating unit and can bereversibly tailored from insulator to semiconductor and then tometal by doping, with insertion of p-type or n-type carriers.22

One of the major applications of COPs is in organicphotovoltaic cells, wherein free charge generation is becauseof electron transfer from dopant (donor) to polymer(acceptor). The device performance depends on the chargeinjection, transfer, balance, and exciton confinement.2 Fur-thermore, photoelectrochemical properties of a photocatalystcan be enhanced by structural doping and substitutional andinterstitial doping.23 Selection of a proper doping agent forCOPs reduces the energy gap, enhances the visible lightabsorption, facilitates charge carrier mobility, and favors theseparation of photogenerated electron−hole pairs.23 Thepresence of counterions (dopants) in COPs is theoreticallyinvestigated, and these counterions can modify chargedistribution and affect the extent of charge delocalization.24,25

In 1984, Bredas et al.25 reported the first theoretical study onthe doping of PPy using ab initio methods (Hartree−Fock/STO3-21G); however, their study was restricted to optimizedgeometric structures and orbital analyses. Alkan and Salznerstudied the doping process of thiophene oligomers, usingdensity functional theory (DFT).26 They reported that lightlydoped chains contain electron polarons in oligothiophene inthe presence of dopants (counterions), but these polarons aredelocalized over the entire backbone in the absence of thecounterions. Efficient nonoxidative doping and dedopingphenomena are also observed in COPs, especially, in PANI.In this process Lewis acids and bases are reacted with polymer,which result in conductivity changes, control of conjugationlengths, color changes, and switch of states of COPs.27,28

Studies on the oxidative and nonoxidative doping anddedoping of polythiophene and PANI have been reported tosome extent both theoretically and experimentally; however, acomparative investigation of the doping and dedoping processof PPy has not yet been performed. In the present work, wepresent a study of the doping and dedoping process of PPyoligomers with up to nine repeating units using hybrid DFTmethods and its comparison with earlier theoretical andexperimental work. In the oligomeric studies of systems withsix to eight repeating units, convergence of the various physicalproperties toward those of the polymers can be assumed, as hasbeen proven by several theoretical studies, including those fromour group.4,5,14,29

2. COMPUTATIONAL METHODSAll calculations were performed with Gaussian 09.30 Thevisualization of results was achieved through Gabedit31 andGaussView 5.0.9.32 DFT and time-dependent DFT (TD-DFT)calculations were performed to investigate the doping processof nPy oligomers (where n = 1−9) and PPy. It was previouslyobserved in a number of reports5,33 that 9Py can accuratelyrepresent the characteristics of the polymer. Hartree−Fock(HF) at the TD-HF level is very accurate to determine theexcitation energies of neutral π-conjugated systems, but it failsfor the open shell systems because of spin contamination;14 fordetails, see ref 34. HF underestimates excitation energies forcharged π-conjugated systems, while TDDFT with a hybridfunctional does not and even does not suffer from spincontamination for nPy oligomers. From a computational costand accuracy point of view, TD-DFT is an intermediate theorybetween semiempirical and wave function approaches that

reliably predicts the excitation energies and oscillator strengthsfor a wide range of molecules.35−37 These molecules may befrom small to large ones, including higher fullerenes, organicand inorganic molecules, biologically important systems, andtransition-metal complexes.35,36 TD-DFT calculations canincorporates environmental effects and quickly give the bestquantitative fit to UV−vis spectra (excitation energy) of thesemolecules, especially using hybrid functionals (B3LYP).37−39

DFT is the only approach that can handle long π-conjugatedradicals without spin contamination in the unrestricted open-shell formalism. Hence, we used the UB3LYP level of theoryfor our target species. In the case of approximate DFT, negativeorbital energies (HOMO and LUMO) do not give accurateionization potentials (IP) and electron affinities (EA), but thedeviation is about 1 eV. Since the error is method-dependentand consistent for all oligomers, orbital energies can be stillused to examine trends consistently.40 DFT at the UB3LYP/6-31G(d)41−48 level of theory was employed, as discussed in ourprevious work.5,6,11 Geometries of the neutral, cationic, anddopant−nPy oligomers were optimized at the above-mentionedlevel of theory. NH3 acts as a Lewis base in the gas phase, whileneutral Cl is a radical that can neither accept nor donate anelectron pair. Whether neutral Cl donates or accepts anelectron pair is also investigated here. Neutral Cl and NH3 werereacted with 9Py (Cl-9Py and NH3-9Py, respectively); NH3 wasalso treated with 9Py+ (NH3-9Py

+) for the investigation of itsdedoping process. The selected species were confirmed to betrue minima on the potential energy surface using frequencycalculation (no imaginary frequency). According to theliterature, the equilibrium structure of pyrrole oligomers isnonplanar.33,49 Thus, this minimum was used for all species(complexes). The nonplanar structure was found to be aminimum in all cases (except for 9Py+, which is planar). Thegeometric, vibrational, and electronic properties of the nPyoligomers with up to nine repeating units were evaluatedtheoretically, and the calculated properties were extrapolated tothose of polymeric PPy through a second-order polynomial fit.A uniform scaling factor of 0.961350 is used for the vibrationalwavenumber, obtained from the DFT calculations. The uniformscaling factor is very appropriate for our system and can beapplied to a system with partial bonding, as reported by Halls etal.51 Moreover, some interesting literature52,53 and our previousexperience11 also confirm that this scaling factor is suitable forconjugated systems. However, dual scaling factors were alsoused in the literature to improve the agreement betweensimulated and observed frequencies. In a dual scalingprocedure, fingerprint and functional group regions should bescaled with two different scaling factors.51,54,55 The changes inband gap, conductivity, and resistance of nPy and nPy-X [whereX = +, NH3 and Cl (radical)16,40,47] are related to andcorrelated with the perturbation in the vibrational spectra andelectronic properties. The latter include IP, EA, highestoccupied molecular orbital (HOMO), lowest unoccupiedmolecular orbital (LUMO), band gap, UV−vis (especiallyλmax), natural bond orbitals (NBO),56 and Mulliken chargeanalysis.57,58 All calculations were performed in the gas phase.

3. RESULTS AND DISCUSSION

Optimized Geometric Structures. The largest oligomerwith nine repeating units (9Py) best represents the structuralproperties of the polymer (PPy), and hence, we restrict thediscussion to 9Py and its derivatives.

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9Py. Optimized geometric structures of 9Py, 9Py+, 9Py-NH3,9Py+-NH3, and 9Py-Cl are given in Figure 1. The optimizedgeometric parameters, such as bond lengths, angles, anddihedral angles of the neutral and doped Py oligomers, arecompared in Table 1 to the earlier theoretical results of Bredaset al.25 and with the X-ray data on small Py oligomers such asbipyrrole and terpyrrole.25,59,60 Selected optimized geometricparameters of these five species are given in Table 1. (SeeFigure 2 for a definition of the various geometry parameters.)

The rC−C, rC−N (internal ring bond distances) and bridgingbond distances (bC−C) at the UB3LYP/6-31G(d) level oftheory are found to be 1.40, 1.37, and 1.43 Å, respectively, for9Py. The internal ring angle (aC−N−C) in all Py repeatingunits is found to be about 110.82°. All these parameters of theneutral species are consistent with the earlier computationaland experimental data.25,59,60

Figure 1. Optimized geometric structure of 9Py, 9Py+, 9Py-Cl, 9Py-NH3, and 9Py+-NH3.

Table 1. Optimized Geometric Parameters of 9Py, 9Py+,9Py-Cl, 9Py-NH3, and 9Py+-NH3 with Reference to Figure 2

species rC−C (Å) bC−C (Å) rC−N (Å) aC−N−C (deg)

9Py 1.40 1.44 1.37 110.829Py+ 1.40 1.42 1.37 110.659Py-Cl 1.40 1.41 1.36 109.809Py-NH3 1.40 1.44 1.37 109.939Py+-NH3 1.40 1.42 1.37 109.75

Figure 2. Structure of PPy.



9Py+. Upon removal of one electron from 9Py, the bridgingbond distances (bC−C) and internal ring angle (aC−N−C)decrease to 1.42 Å and 110.65°, respectively. However, the restof the geometric parameters remain essentially the same,although a planar geometry results due to the generation of theconducting form.9Py-Cl. Doping of 9Py with Cl, resulting in 9Py-Cl, has a

rather severe effect on its ground-state geometry. The angleaC−N−C also decreases to 109.80°, which is a consequence ofthe decrease in resistance over the polymeric backbone.5

9Py-NH3. From an analysis of the results in Table 1 it can beeasily concluded that NH3 has very little effect on the polymericchain of 9Py; only the internal rings are found to be affected.NH3 decreases the aC−N−C bond angle by about 0.31°.9Py+-NH3. The interaction of ammonia with 9Py+ is also

evaluated for its reducing power in dedoping process. Theoptimized geometric parameter of 9Py+ and 9Py+-NH3 arenearly the same, except for the internal angle, which increasesfrom 110.65° to 111.13°. This increase in the internal angleincreases the resistance over the polymeric backbone.Comparative bond lengths of C−N and C−C along thebackbone of 9Py, 9Py+, 9Py-Cl, 9Py+-NH3, and 9Py-NH3 aregiven in Figure 3.Infrared Spectral Characteristics. Computed infrared

spectra of 9Py, 9Py+, 9Py-Cl, 9Py-NH3 and 9Py+-NH3 are given

in Figure 4. (See Figures S1−S3 of the Supporting Informationfor details.) Comparisons of the important band peaks of 9Py,9Py+, 9Py-Cl, 9Py-NH3, and 9Py+-NH3 along with theirapproximate assignments are collected in Table 2, where theyare also compared to the available experimental data.61−64 (SeeTables S1−S6 of the Supporting Information for details.)Generalized gradient approximation (GGA)37,46,65 is also anappropriate method for simulating the vibration spectra of afinite and infinite number of atoms. Clavaguera-Sarrio et al.66

reported that GGA can successfully predict the structural andvibrational properties of closed and open-shell systems foroxides of actinide compounds. Furthermore, they correlated thereliability of GGA with CASPT2, which is a highly computa-tionally demanding method. Adjokatse et al.67 had alsosystematically studied the dielectric and piezoelectric responseof odd-numbered nylons with the help of the DFT methodwith GGA and found nice correlation of the theoreticallysimulated vibrational spectrum with that of available exper-imental data. For conducting polymers, literature reveals that

pure GGA and hybrid B3LYP are quite effective at simulatingthe vibrational spectra; however, the latter is more abundantlyused in the literature.37,52,65 Moreover, the B3LYP method hasproduced the experimental data quite well (see Table 2 of thetext). Because of the nice correlation between theory (usinguniform scaling factor) and experimental data, we choseB3LYP.

9Py. The simulated scaled IR spectrum of 9Py has twoprominent band peaks in the functional group region at ca.3521 (expt 3404 cm−1, N−H stretching) and 3118 cm−1 (expt2920 cm−1, C−H stretching).61−64 Some characteristic bandpeaks in the fingerprint region for conjugation in the polymericbackbone of 9Py (as discussed by Omastova et al. and Zerbi etal.61,64) are 1396 (expt 1400, N−H wagging), 1297 (expt 1312,

Figure 3. Bond length changes along the polymeric backbone of 9Py, 9Py+, 9Py-Cl, 9Py-NH3, and 9Py+-NH3 with reference to Figure 2.

Figure 4. Scaled IR spectra of 9Py, 9Py-NH3, 9Py+, 9Py+-NH3, and

9Py-Cl.



N−C stretching), 1218 (expt 1220, C−H wagging), and 1095cm−1 (expt 1081, C−H wagging). Differences betweenexperimental and simulated frequencies are primarily due tothe comparison of condensed phase (experimental) and gas-phase simulation. This has been discussed in the literature infairly good detail.16,68−73

9Py-NH3. Compared to 9Py, 9Py-NH3 has three additionalband peaks in the functional group region (Figure 4), 3440(N−H stretching, ammonia), 3160 (N−H stretching, 9Py), and1653 cm−1 (H−N−H scissoring, ammonia). Two band peaks atca. 1440 and 1036 cm−1 in the IR spectrum of 9Py-NH3provide evidence of the presence of increased π-electrondensity in the polymeric backbone, compared to isolated 9Py.As seen in Table 2, the other peaks in the fingerprint region arenot comparable for 9Py and 9Py-NH3.9Py+. Removal of an electron from the backbone of 9Py

results in doping to form 9Py+, which causes red shifts in the IRfrequencies compared to those of the neutral 9Py. The higherfrequency bands in the functional group region, such as 3519and 3145 cm−1, have low intensity in 9Py+ but similarassignments as those of the 9Py band peaks. Red shifts in thefrequencies of the fingerprint region are also observed andsome new band peaks appear at ca. 1537, 1500, 1439, 1027, and994 cm−1. Examining the IR spectrum of 9Py+ (Figure 4) andits prominent band peaks as listed in Table 2 leads us toconclude that the conductivity is increased, based on thepresence of strong band peaks in the 1600−900 cm−1 region(vide infra). This statement also corroborates well with theearlier reported work.16,68−73

9Py-Cl. On comparison of 9Py-Cl with the neutral 9Py, wenotice that Cl causes red-shifting in the IR spectrum of 9Py.

Moreover, some new bands appear in the functional group andfingerprint regions. The two new peaks in the functional groupregion at ca. 3072 and 2540 cm−1 are due to the presence of acounter species, such as Cl in this case, and have assignments ofC−H and N−H stretching, respectively. Compared to neutral9Py, five new bands appear at ca. 1474 (CC stretching),1453 (C−H bending), 1268 (N−H bending), 1030 (C−Hscissoring), and 971 cm−1 (ring breathing). The increasednumber of band peaks in the 1600−900 cm−1 region meansthat Cl has caused longer π-electron conjugation in thepolymeric backbone of 9Py.

9Py+-NH3. To investigate the dedoping process of 9Py+

through IR spectral analysis, an ammonia molecule was placednear the backbone with a suitable (optimized) distance.Compared to the 9Py and 9Py+, the IR spectrum of 9Py+-NH3 has an extra peak in the functional group region at 3083cm−1 (N−H stretching ammonia). The band peak at ca. 1500cm−1 (C−H, N−H wagging and minor N−C stretching) in the9Py+ spectrum disappeared upon reaction with NH3. Thediminishing of this band in the 9Py+-NH3 complex is evidenceof the lower delocalized π-electron conjugation, which meansthat this analyte creates localization in the polymeric backbone.Comparison of the various band frequencies of the 9Py+ and9Py+-NH3 (Figure 4 and Table 2) led us to conclude that NH3causes a blue-shift in the frequencies of 9Py+, a consequence ofthe dedoping phenomena.

UV−Visible and UV−Vis−Near-IR SpectroscopicStudy. Polaron states are generally formed in π-conjugatedsystems such as oligomers of PPy.4,74−78 These polarons maybe localized or delocalized depending on the amount andnature of dopant. When the number of polarons increase, then

Table 2. Experimental IR and Calculated Frequencies (in cm−1) of 9Py, 9Py+, 9Py-Cl, 9Py-NH3, and 9Py+-NH3a

calcd frequency

no. expl16,68−73 IR 9Py 9Py+ 9Py-Cl 9Py-NH3 9Py+-NH3 approx assignment

1 3403 3521 3519 3521 3521 3519 ν N−H2 3440 ν N−H of NH3

3 3160 ν N−H4 3118 3145 3145 3117 3144 ν C−H5 3083 ν N−H (near NH3)6 2920 3072 ν C−H (near Cl)7 2540 ν N−H (near Cl)8 1653 cis of NH3

9 1550−1460 1575 1579 ν CC; wag C−H, H−N10 1514 1537 1542 1517 1544 ν CC; wag C−H, N−H11 1484 1500 ν N−C; wag C−H, N−H12 1440 1463 1474 1470 ν N−C, CC; wag C−H, N−H13 1400 1439 1453 1440 1440 ν N−C, CC; wag N−H, C−H14 1312 1396 1386 1396 1377 ν N−C, CC; wag N−H, C−H15 1250 1297 1313 1304 1287 1312 ν CC; β C−H; cis C−N−C16 − 1268 ν CC; wag N−H17 1220 1218 1208 1202 1217 1213 wag C−H, N−H; ν N−C18 1081 1095 1099 1096 1094 1100 cis C−H, N−H19 1050 1051 1052 wag C−H, N−H20 1027 1030 1036 1026 Cis C−H21 990 994 971 965 rings breathing22 930 856 874 865 874 874 def(ring)//γ C−H, N−H23 734 743 769/737 731 742 γ C−H, N−H24 680 679 γ C−H, N−H25 594 595 γ C−H, N−H26 426 457 457 462/447 456 γ N−H

aν, stretching; wag, wagging; cis, scissoring; β, bending; def, deformation mode; γ, out-of-plane bending.



they are converted (through an ionization process) tobipolarons, which means the presence of two similar chargeson the same molecule, usually at the terminal of a polymer/oligomer backbone. UV−vis spectroscopy is a useful tool todifferentiate between polarons and bipolarons and has widelybeen examined theoretically and experimentally by theMacDiarmid and Bredas groups.4,74−78

The UV−vis and UV−vis−near-IR spectra of 9Py and 9Py-X(where X = +, Cl, and NH3) have been simulated in the gasphase at the TD-DFT/UB3LYP/6-31+G(d,p) level of theory(Figures 5−7). Our simulated UV−vis spectrum of the neutral

9Py is in close agreement with the reported experimental andtheoretical data.14,79 Three prominent peaks are observed in theUV−vis spectra of PPy: 274 nm (π → π*), 331 nm (midgaptransition), and at 414 nm (λmax, transition from the valenceband to the conduction band). Doping of 9Py oligomers withNH3 molecules (shown in Figure 5) causes slight red-shifting inits λmax (414 → 416 nm), while a slight blue-shift is observed inthe midgap transition (331→ 329 nm). The red-shifting in λmaxwith NH3 dopant illustrates its n-type doping nature (basicnature, dedoping). With a decreasing band gap, conduction anddelocalization are slightly enhanced in 9Py-NH3 compared to9Py (vide infra). The blue-shifting in the second peak (331 →329 nm) of 9Py-NH3 is due to distortion (quinoid form) in theregular pyrrole rings (vide supra), resulting in a decrease ofinterband transition.

For 9py+, π → π* (415 nm) and midgap transitions (560nm) are considerably red-shifted compared to the neutral 9Py(vide supra). The 414 nm peak of 9Py is replaced by adelocalized polaron band (transition from the valence band tothe lower polaron band) at 1907 nm in 9Py+. On reacting NH3with 9Py+ (Figure 6), a blue-shift (1907 → 1829 nm) isobserved in its λmax. This blue-shifting can also be assigned tothe dedopoing process of 9Py+, which consequently results indecreased conductivity, delocalized π-conjugation length, andan increase in the band gap.4,15−18,74−76

Interaction of Cl with 9Py causes formation of a localizedpolarion (polaron formation in the presence of a counterspecies) characterized by an absorption band at 1242 nm(transition from the valence band to the lower polaron band).69

The π → π* and midgap transitions are significantly red-shifted to 466 and 528 nm, respectively (Figure 7). Theabsorbance band at 528 nm for 9Py-Cl (and also 560 nm for9Py+) is indicative of extended π-conjugtion length.The peaks in the near-IR region at 1907 nm (in Py+) and

1242 nm (in 9Py-Cl) are due to delocalized and localizedpolaron structures, respectively. A polaron is localized in thepresence of a counterion (9Py-Cl), whereas it is delocalized inthe absence of any counterion (9Py+). In summary, theionization process converts polarons into localized (in thepresence of a counter radical) and delocalized (absence of acounterion such as in the case of 9Py+) polarons (extended π-conjugation length). The extension of the excitation energiesbeyond 1800 nm under the polaron and bipolaron regime is theresult of the new transition energy levels between theconduction and valence bands. The presence of localized anddelocalized polarons allows one to draw conclusions regardingthe doping level and conductivity, optical, and electronicproperties of CPs, especially in the case of PPy andPANI.4,15−18,74−76

Natural Bonding Orbital and Mulliken ChargeAnalysis. Charge transfer phenomena between the nPy anddopants (NH3, Cl) are simulated by Mulliken (QMULLIKEN) andNBO (QNBO) charge analysis at the UB3LYP/6-31G(d) level oftheory. These properties are basis set dependent; however, ifthe same level of theory is used for different structures [such asUB3LYP/6-31G(d) or UB3LYP/6-311++G(d,p)], then theresults will provide trends and therefore be meaningful. Thebasis set dependence of these charge analysis tools has beendiscussed by Fonseca Guerra et al.57 and Martin et al.58

The net charge transfer in 9Py-NH3 from ammonia to 9Py is0.047 e− and 0.046 e−, based on QNBO and QMULLIKEN,respectively. The NH3 transfers about 0.057 e− based on QNBOand 0.065 e− based on QMULLIKEN to 9Py+. In the case of the Cldopant, Cl receives about −0.801 e− charge based on QNBO and−0.705 e− based on QMULLIKEN from 9Py. From this chargeanalysis it can be easily concluded that, in the 9Py-Cl complex,Cl has caused oxidation in the 9Py (doping). However, in thecase of the 9Py+-NH3 complex, reduction in the 9Py+ isobserved (dedoping).

HOMO and LUMO Energy. The HOMO and LUMO of9Py, 9Py+, 9Py-Cl, 9Py-NH3, and 9Py+-NH3 calculated atUB3LYP/6-31G(d) are shown in Figure 8, and theircorresponding energies from monomer up to infinity are listedin Tables 3−5. (See the Supporting Information, Figures S8−S13, for pictures of individual HOMOs and LUMOs.)

Frontier Orbitals of 9Py vs 9Py+. Figure 8 providescomparisons of the HOMO and LUMO of the neutral 9Pyand cationic species (9Py+). The HOMO of 9Py+ extends over

Figure 5. UV−vis spectra of 9Py (red) and 9Py-NH3 (black).

Figure 6. UV−vis of 9Py+ (red) and 9Py+-NH3 (black).

Figure 7. UV−vis of 9Py (red) and 9Py-Cl (black).



all carbons, hydrogens and nitrogens and forms a planarstructure that involves delocalization of the π-electrons over theentire molecular backbone, contrary to its neutral counterpart9Py. On the other hand, only the central atoms contribute tothe LUMO; therefore, the LUMO is localized in the polymericframework. The HOMO and LUMO energies of nPy and nPy+,from monomer up to infinite repeating units, are given inTables 3 and 4 (vide infra). The estimated HOMO and LUMO

energies of 9Py are −4.01 and −0.60 eV, while for 9Py+ theyare −6.26 and −3.35 eV, respectively. The higher magnitude ofthe HOMO energy (−6.26 eV) of the 9Py+ is a consequence ofthe existence of a longer conjugation length (vide infra),delocalization of π-electron density, and high conductivity, inaddition to being a charge effect.

Frontier Molecular Orbitals of 9Py vs 9Py-NH3. FromFigure 8, frontier molecular orbitals of 9Py and 9Py-NH3species can be comparatively analyzed. NH3 is a reducingagent, as has already been discussed in the analysis of 9Py and9Py-NH3 molecular orbitals. Ammonia has reduced inmagnitude both the HOMO and LUMO energies of the 9Py,from −4.01 to −3.85 eV and −0.60 to −0.46 eV (Table 5). Italso decreased the π-electron density over the 9Py polymericbackbone, resulting in an increase in resistance.

Frontier Molecular Orbitals of 9Py+ vs 9Py+-NH3. Contoursof the HOMO and LUMO of 9Py+ and 9Py+-NH3 are given inFigure 8. NH3 has caused a slight decrease in magnitude of theHOMO and LUMO energies of 9Py+-NH3 (compared to9Py+). Figures 1 and 8 show clearly that NH3 has led to slightbending of the terminal rings of the polymer, compared to theother cases. Moreover, Figure 8 shows that NH3 decreases thedelocalization of the π-electron density, resulting in lowerconductivity. (See the discussion of the band gap, vide infra,and dedoping phenomena.) These results also support theearlier conclusions from the analysis of the UV−vis−near-IRspectra, optimized geometric parameters, and IR spectralcharacteristics. The estimated HOMO and LUMO energies

Figure 8. HOMO and LUMO of the 9Py, 9Py+, 9Py-Cl, 9Py+-NH3, and 9Py-NH3 complexes.

Table 3. IP, EA, HOMO Energy, LUMO Energy and BandGap in eV of nPy

na IP EA HOMO LUMO band gap

1 5.48 −1.38 −5.48 1.38 6.862 4.75 −0.35 −4.75 0.35 5.103 4.43 0.03 −4.43 −0.03 4.404 4.26 0.25 −4.26 −0.25 4.015 4.17 0.38 −4.17 −0.38 3.796 4.11 0.46 −4.11 −0.46 3.657 4.06 0.53 −4.06 −0.53 3.538 4.03 0.57 −4.03 −0.57 3.469 4.01 0.6 −4.01 −0.60 3.4110 3.99 0.63 −3.99 −0.63 3.36∞ 3.80 0.90 −3.80 −0.90 2.90

an is the number of repeating units.

Table 4. IP, EA, HOMO Energy, LUMO Energy and BandGap in eV of nPy+, where

na IP EA HOMO LUMO band gap

1 13.09 6.21 −13.09 −6.21 6.283 8.6 4.75 −8.60 −4.75 3.855 7.32 4.09 −7.32 −4.09 3.237 6.68 3.67 −6.68 −3.67 3.019 6.26 3.35 −6.26 −3.35 2.9111 5.97 3.07 −5.97 −3.07 2.90∞ 4.91 2.41 −4.91 −2.41 2.46

an is the number of repeating units.

Table 5. IP, EA, HOMO, LUMO and Band Gap in eV of9Py, 9Py-NH3, 9Py-Cl, and 9Py+-NH3

species IP EA HOMO LUMO band gap

9Py 4.01 0.60 −4.01 −0.60 3.419Py-NH3 3.85 0.46 −3.85 −0.46 3.399py-Cl 4.28 1.21 −4.28 −1.21 3.079Py+ 6.26 3.35 −6.26 −3.35 2.919Py+-NH3 6.16 3.20 −6.16 −3.20 2.96



of 9Py+-NH3 are −6.16 and −3.20 eV, respectively (Table 5).The HOMO and LUMO energies of the 9Py+-NH3 complexare 0.10 and 0.15 eV lower in magnitude than those of 9Py+.Frontier Molecular Orbitals of 9Py vs 9Py-Cl. Molecular

orbitals (HOMO and LUMO) of 9Py and 9Py-Cl are given inFigure 8, and the corresponding estimated energies are listed inTable 5. Analysis of the optimized geometric structure (videsupra) and molecular orbitals led us to conclude that Clplanarizes the geometry of 9Py and extends the π-electronconjugation density over its polymeric backbone. This extendedπ-electron conjugation density increases the conductivity anddecreases the band gap (vide infra) and resistance in thepolymer. This statement confirms and extends the mentionedcharacterizations of the Cl doping for 9Py (or PPy). Thepresence of Cl radical affects the HOMO and LUMO energiesof 9Py by about −0.27 and −0.61 eV, respectively.Electronic Properties like IP, EA, and Band Gap. It is

also very well-known from the literature5,6,34,42,80 that the IPand EA obtained from the negative values of the DFT orbital(HOMO and LUMO) energies (Koopman’s theorem) withtypical approximate exchange correlation functionals is usuallytoo small as compared with experimental values. However,hybrid functionals (such as UB3LYP), which account for theeffects of self-interaction to some degree, result in a bettercorrelation (vide supra).The IP, EA, and band gap of nPy, nPy+, 9Py-Cl, 9Py+-NH3,

and 9Py-NH3 are listed in Tables 3−5. The band gap valuesalong with their valence, conduction, and polaron bands aregiven in Figures 9−11. Comparisons of the IP, EA, and bandgap of these five different species, restricted to nine repeatingunits, are listed in Table 5. Increasing conjugation (as explainedin the frontier molecular orbital analysis) over the polymericbackbone causes higher IP and EA and decreased band gap.The band gap is estimated from the difference of the valence

and conduction band’s orbital energies (HOMO−LUMO).The IP and EA of 9Py are 4.01 and 0.60 eV, respectively, and itsband gap (3.41 eV), along with valence and conduction bandenergies, is given in Figure 9a. The valence and conductionbands of 9Py are at −4.01 and −0.60 eV, respectively, while theinterband or midgap transition is at about 2.99 eV above thevalence band. Ammonia (donor) decreases the IP and EAvalues of 9Py by about 0.16 and 0.14 eV, respectively, as it hasdonated electrons to the 9Py orbitals (Table 5 and Figure 9b).Furthermore, we see from Figure 9b that the valence andconduction bands of 9Py-NH3 are at −3.85 and −0.46 eV,respectively. Its band gap is 3.39 eV, while the midgaptransition is at 2.97 eV.As another attempt to confirm the dedoping process of PPy

with NH3, NH3 is reacted with 9Py+ and characterized with IP,EA, and band gap analysis, as shown in Figure 11a and Table 5.Analysis of Figure 11a leads us to conclude that NH3 causesdedoping of PPy and decreases the IP and EA of Py+. Thevalence, conduction, and localized polaronic bands of Py+-NH3

are at −6.16, −3.20 and 0.67 eV, respectively. NH3 hasincreased the band gap of Py+ (Py+-NH3) from 2.91 to 2.96 eV.The decrease in IP and EA of 9Py+ by NH3 is about 0.10 and0.15 eV, respectively. The lower IP, lower EA, and increasedband gap show that the polymer is reduced (dedoped).Moreover, it also demonstrates reduced delocalization of π-electron (vide supra).Cl has caused oxidation (doping) in 9Py, as can be seen from

the data of Table 5 and Figure 11b. Cl attracts electrons fromthe orbitals of 9Py, consequently increasing its IP and EA valuesby about 0.27 and 0.60 eV, respectively. Simultaneously, withthe increasing of these values, its band gap decreases from 3.41to 3.07 eV.Table 5 and Figure 12 show that, when 9Py is oxidized in the

absence of a counterion, its IP and EA increase by about 2.25and 2.75 eV, respectively. Its band gap (2.91 eV) also decreasesby about 0.51 eV compared to that of neutral 9Py. Thedecrease in band gap of nPy+ from the monomer (n = 1) up tothe infinite polymer (n = ∞) is given in Figure 13. The valenceand conduction bands are situated at −6.26 and −3.35 eV,respectively. Another prominent band, which can be identifiedas a delocalized polaron, is located at 0.65 eV. This delocalizedpolaron band is responsible for the high conductivity and lowerresistance and band gap value of 9Py+. An interband transitionfrom the singly bonded polaron state to the antibondingpolaron state (nonbonding polaron) has an energy of 2.21 eV.This delocalized polaron transition (bonding to antibonding) isresponsible for the delocalized extended π-electron conjugationover the polymeric backbone of 9Py+.

Figure 9. Energy level diagram of 9Py (a) and 9Py-NH3 (b).

Figure 10. Developments of band structure of PPy from energy levelsof oligomers. (The monomer is used as a repeat unit.)



4. CONCLUSIONDFT calculations have been carried out on a number ofmolecules with different characteristics, notably, 9Py, 9Py+,9Py-Cl, 9Py+-NH3, and 9Py-NH3, to investigate their dopingand dedoping processes. In the vibrational analysis, the bandpeaks in the 1600−900 cm−1 region give information about theshort and extended π-electron conjugation length. The numberof peaks in this region is evidence of the presence ofconjugation in the polymeric backbone. This is furtherconfirmed from the high IP, high EA, low band gap, as wellas the high electron density of the HOMO and LUMO. Slightdifferences are found between experimental and simulatedfrequencies; however, these are due to the condensed and gas-phase IR spectra, respectively. Compared to neutral 9Py, thedifferent additives (NH3, +, Cl, etc.) result in extra band peaksboth in the fingerprint and functional group regions. Removalof an electron from the backbone of 9Py results in doping such

as in 9Py+ and causes red-shifts in the IR frequencies. Thedoping process of 9Py is also achieved on reacting with Cl. Thisresults in the emergence of five new bands in the 1600−900cm−1 region. Dedoping of 9Py+ is achieved when ammonia isadded; consequently, some bands in the 1600−900 cm−1 regiondisappear, and the result is lower delocalized π-electronconjugation. In the UV−vis and UV−vis−near-IR spectra, theaddition of different analytes (dopant) to 9Py results in thedisappearance of certain bands and gives rise to some newabsorbances corresponding to localized and delocalized polaronbands. Specifically, the peaks in the near-IR region at 1907 nmfor Py+ and 1242 nm for 9Py-Cl are due to delocalized andlocalized polaron structures, respectively. The presence oflocalized and delocalized polarons in the UV−vis near-IRspectra is correlated with the doping level, conductivity, optical,and electronic properties of CPs, especially in the case of PPyand PANI.4,74−76 It can also be concluded that the polarons/bipolarons fall in the visible and near-IR region. Thus, theyactually affect the vibrational, electrical, optical, and electronicproperties of CPs. The net charge transfer in theses complexesis simulated with NBO and Mulliken charge analysis. NH3

transfers charge to 9Py and 9Py+, while Cl receives charge from9Py, confirming the dedoping and doping phenomena,respectively. Frontier molecular orbitals (HOMO andLUMO), IP, EA, and band gap are also consistent with thevibrational and UV−vis−near-IR spectroscopy. The electrondensity in the MOs extends over the polymeric geometry in thecase of 9Py+ and 9Py-Cl, while this is reversed in 9Py+-NH3

compared to neutral 9Py. In the case of oxidation and Cldopant, the IP and EA increase, and consequently, there is adecrease in the band gap.

■ ASSOCIATED CONTENT

*S Supporting InformationIR and UV−vis spectra, diagrams of HOMO and LOMOorbitals, and tables of selected IR band peaks along withapproximate assignments. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Authors*A.-u.-H.A.S. tel, +92-91-9216652; e-mail, [email protected].*K.A. tel, +92-992-383591; e-mail, [email protected],[email protected].

Figure 11. Energy level diagram of 9Py+-NH3 (a) and 9Py-Cl (b).

Figure 12. Energy level diagram of 9Py+.

Figure 13. Developments of band structure of PPy+ from energy levelof oligomers. (The monomer is used as a repeat unit.)



http://pubs.acs.org

mailto:[email protected]




Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe gratefully thank Prof. Ulrike Salzner and Prof. GeorgSchreckenbach for his valuable comments, suggestions anddiscussion. We acknowledge the Institute of Chemical Sciences(ICS), University of Peshawar and Higher EducationCommission, Islamabad.

■ ABBREVIATIONSDFT, density functional theory; PPy, polypyrrole; B3LYP,Becke 3 parameter exchange functional combined with theLee−Young−Parr correlation functional; HOMO, highestoccupied molecular orbital; LUMO, lowest unoccupiedmolecular orbital; IP, ionization potential; EA, electron affinity;NBO, natural bonding orbital.

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doping and dedoping process of polypyrrole - dft study with hybrid functionals

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