Doping and Dedoping Processes of Polypyrrole: DFT Study with

Article pubs.acs.org/JPCC

Doping and Dedoping Processes of Polypyrrole: DFT Study with Hybrid Functionals Habib 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-dependent DFT (TD-DFT) calculations at the UB3LYP/6-31G(d) level have been performed to investigate the tunable nature, i.e., doping and dedoping processes, of polypyrrole (PPy). The calculated theoretical data show strong correlation with the recent experimental reports, which validates our computational protocol. The calculated properties are extrapolated to the polymer (PPy) through a second-order polynomial fit. Changes in band gap, conductivity, and resistance of nPy and nPy-X (where n = 1−9 and X = +, NH3, and Cl) were studied and correlated with the calculated vibrational spectra (IR) and electronic properties. Upon doping, bridging bond distance and internal bond angles decrease (decrease in resistance over polymer backbone), whereas dedoping results in increases in these geometric parameters. In the vibrational spectrum, doping is characterized 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 rise to some new absorbances corresponding to localized and delocalized polaron bands. Specifically, the peaks in the near-IR region at 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 the polymeric backbone. However, a reversal of properties is obtained in n-doping or reduction of nPy+. In the case of oxidation and Cl dopant, the IP and EA increase, and consequently, there is a decrease in the band gap. NBO and Mulliken charges analyses indicate charge transferring from the polymer in the case of p-type dopants, while this phenomenon is reversed with n-type dopants.

1. INTRODUCTION

conduct electricity in the presence of a dopant (iodine). PPy is physically insoluble, amorphous, and infusible. PPy has been extensively studied both experimentally and theoretically for applications as sensors, actuators, and corrosion inhibitors. Recently, we have also carried out a theoretical study to investigate its ability as a sensor for NH3 gas in its undoped form.5 A large number of papers have been published on experimental studies of PPy as sensor for methanol, ethanol, NO2, NH3, CO2, and CO and other toxic gases.15−18 For improved performance (high crystallinity and conductance), PPy is modified with some coating material such as TiO2 or ZnO.19 Moreover, to enhance the performance of PPy, different approaches have been reported, such as minimizing the band gap, composites,20 and nano studies.21

Conjugated organic polymers (COPs) are technologically important,1 due to their tunable nature,2 free availability of πelectrons on the backbone of the polymeric chain, high stability,1 low cost,3 and ease of preparation.4 COPs have a wide range of applications in the fields of sensors,5,6 actuators,7 rechargeable batteries,8 solar cells,9 electrochromic display materials,1 anticorrosion protection,10 and electromagnetic shielding technology.11 For conduction, COPs can be doped either p-type (oxidation) or n-type (reduction), depending on the nature of the polymer.12 Polyaniline (PANI), polyacetylene (PA), polythiopene (PT), polyparaphenylene (PPP), polyparaphenylenevenylene (PPV), and poly(o-phenylenediamine) (POPD) are prominent examples of COPs.13 The COP family received another important member when, in the 1960s, Weiss et al. prepared polypyrrole (PPy) by the pyrolysis of tetraiodopyrrole.14 The authors concluded that PPy can © 2014 American Chemical Society

Received: June 6, 2014 Revised: July 15, 2014 Published: July 18, 2014 17819

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

The Journal of Physical Chemistry C

Article

reliably predicts the excitation energies and oscillator strengths for a wide range of molecules.35−37 These molecules may be from small to large ones, including higher fullerenes, organic and inorganic molecules, biologically important systems, and transition-metal complexes.35,36 TD-DFT calculations can incorporates environmental effects and quickly give the best quantitative fit to UV−vis spectra (excitation energy) of these molecules, especially using hybrid functionals (B3LYP).37−39 DFT is the only approach that can handle long π-conjugated radicals without spin contamination in the unrestricted openshell formalism. Hence, we used the UB3LYP level of theory for our target species. In the case of approximate DFT, negative orbital energies (HOMO and LUMO) do not give accurate ionization potentials (IP) and electron affinities (EA), but the deviation is about 1 eV. Since the error is method-dependent and consistent for all oligomers, orbital energies can be still used to examine trends consistently.40 DFT at the UB3LYP/631G(d)41−48 level of theory was employed, as discussed in our previous work.5,6,11 Geometries of the neutral, cationic, and dopant−nPy oligomers were optimized at the above-mentioned level of theory. NH3 acts as a Lewis base in the gas phase, while neutral Cl is a radical that can neither accept nor donate an electron pair. Whether neutral Cl donates or accepts an electron pair is also investigated here. Neutral Cl and NH3 were reacted with 9Py (Cl-9Py and NH3-9Py, respectively); NH3 was also treated with 9Py+ (NH3-9Py+) for the investigation of its dedoping process. The selected species were confirmed to be true minima on the potential energy surface using frequency calculation (no imaginary frequency). According to the literature, the equilibrium structure of pyrrole oligomers is nonplanar.33,49 Thus, this minimum was used for all species (complexes). The nonplanar structure was found to be a minimum in all cases (except for 9Py+, which is planar). The geometric, vibrational, and electronic properties of the nPy oligomers with up to nine repeating units were evaluated theoretically, and the calculated properties were extrapolated to those of polymeric PPy through a second-order polynomial fit. A uniform scaling factor of 0.961350 is used for the vibrational wavenumber, obtained from the DFT calculations. The uniform scaling factor is very appropriate for our system and can be applied to a system with partial bonding, as reported by Halls et al.51 Moreover, some interesting literature52,53 and our previous experience11 also confirm that this scaling factor is suitable for conjugated systems. However, dual scaling factors were also used in the literature to improve the agreement between simulated and observed frequencies. In a dual scaling procedure, fingerprint and functional group regions should be scaled with two different scaling factors.51,54,55 The changes in band gap, conductivity, and resistance of nPy and nPy-X [where X = +, NH3 and Cl (radical)16,40,47] are related to and correlated with the perturbation in the vibrational spectra and electronic properties. The latter include IP, EA, highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), band gap, UV−vis (especially λmax), natural bond orbitals (NBO),56 and Mulliken charge analysis.57,58 All calculations were performed in the gas phase.

The conducting properties of COPs mainly depend on the arrangement and number of their repeating unit and can be reversibly tailored from insulator to semiconductor and then to metal by doping, with insertion of p-type or n-type carriers.22 One of the major applications of COPs is in organic photovoltaic cells, wherein free charge generation is because of electron transfer from dopant (donor) to polymer (acceptor). The device performance depends on the charge injection, transfer, balance, and exciton confinement.2 Furthermore, photoelectrochemical properties of a photocatalyst can be enhanced by structural doping and substitutional and interstitial doping.23 Selection of a proper doping agent for COPs reduces the energy gap, enhances the visible light absorption, facilitates charge carrier mobility, and favors the separation of photogenerated electron−hole pairs.23 The presence of counterions (dopants) in COPs is theoretically investigated, and these counterions can modify charge distribution and affect the extent of charge delocalization.24,25 In 1984, Bredas et al.25 reported the first theoretical study on the doping of PPy using ab initio methods (Hartree−Fock/ STO3-21G); however, their study was restricted to optimized geometric structures and orbital analyses. Alkan and Salzner studied the doping process of thiophene oligomers, using density functional theory (DFT).26 They reported that lightly doped chains contain electron polarons in oligothiophene in the presence of dopants (counterions), but these polarons are delocalized over the entire backbone in the absence of the counterions. Efficient nonoxidative doping and dedoping phenomena 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 conjugation lengths, color changes, and switch of states of COPs.27,28 Studies on the oxidative and nonoxidative doping and dedoping of polythiophene and PANI have been reported to some extent both theoretically and experimentally; however, a comparative investigation of the doping and dedoping process of PPy has not yet been performed. In the present work, we present a study of the doping and dedoping process of PPy oligomers with up to nine repeating units using hybrid DFT methods and its comparison with earlier theoretical and experimental work. In the oligomeric studies of systems with six to eight repeating units, convergence of the various physical properties toward those of the polymers can be assumed, as has been proven by several theoretical studies, including those from our group.4,5,14,29

2. COMPUTATIONAL METHODS All calculations were performed with Gaussian 09.30 The visualization of results was achieved through Gabedit31 and GaussView 5.0.9.32 DFT and time-dependent DFT (TD-DFT) calculations were performed to investigate the doping process of nPy oligomers (where n = 1−9) and PPy. It was previously observed in a number of reports5,33 that 9Py can accurately represent the characteristics of the polymer. Hartree−Fock (HF) at the TD-HF level is very accurate to determine the excitation energies of neutral π-conjugated systems, but it fails for the open shell systems because of spin contamination;14 for details, see ref 34. HF underestimates excitation energies for charged π-conjugated systems, while TDDFT with a hybrid functional does not and even does not suffer from spin contamination for nPy oligomers. From a computational cost and accuracy point of view, TD-DFT is an intermediate theory between semiempirical and wave function approaches that

3. RESULTS AND DISCUSSION Optimized Geometric Structures. The largest oligomer with nine repeating units (9Py) best represents the structural properties of the polymer (PPy), and hence, we restrict the discussion to 9Py and its derivatives. 17820



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

9Py. Optimized geometric structures of 9Py, 9Py+, 9Py-NH3, 9Py+-NH3, and 9Py-Cl are given in Figure 1. The optimized geometric parameters, such as bond lengths, angles, and dihedral angles of the neutral and doped Py oligomers, are compared in Table 1 to the earlier theoretical results of Brédas et al.25 and with the X-ray data on small Py oligomers such as bipyrrole and terpyrrole.25,59,60 Selected optimized geometric parameters of these five species are given in Table 1. (See Figure 2 for a definition of the various geometry parameters.) Figure 2. Structure of PPy.

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 9Py+ 9Py-Cl 9Py-NH3 9Py+-NH3

1.40 1.40 1.40 1.40 1.40

1.44 1.42 1.41 1.44 1.42

1.37 1.37 1.36 1.37 1.37

110.82 110.65 109.80 109.93 109.75

The rC−C, rC−N (internal ring bond distances) and bridging bond distances (bC−C) at the UB3LYP/6-31G(d) level of theory are found to be 1.40, 1.37, and 1.43 Å, respectively, for 9Py. The internal ring angle (aC−N−C) in all Py repeating units is found to be about 110.82°. All these parameters of the neutral species are consistent with the earlier computational and experimental data.25,59,60 17821



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

9Py+. Upon removal of one electron from 9Py, the bridging bond distances (bC−C) and internal ring angle (aC−N−C) decrease to 1.42 Å and 110.65°, respectively. However, the rest of the geometric parameters remain essentially the same, although a planar geometry results due to the generation of the conducting form. 9Py-Cl. Doping of 9Py with Cl, resulting in 9Py-Cl, has a rather severe effect on its ground-state geometry. The angle aC−N−C also decreases to 109.80°, which is a consequence of the decrease in resistance over the polymeric backbone.5 9Py-NH3. From an analysis of the results in Table 1 it can be easily concluded that NH3 has very little effect on the polymeric chain 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. The optimized geometric parameter of 9Py+ and 9Py+-NH3 are nearly the same, except for the internal angle, which increases from 110.65° to 111.13°. This increase in the internal angle increases the resistance over the polymeric backbone. Comparative bond lengths of C−N and C−C along the backbone of 9Py, 9Py+, 9Py-Cl, 9Py+-NH3, and 9Py-NH3 are given 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 Information for details.) Comparisons of the important band peaks of 9Py, 9Py+, 9Py-Cl, 9Py-NH3, and 9Py+-NH3 along with their approximate assignments are collected in Table 2, where they are also compared to the available experimental data.61−64 (See Tables S1−S6 of the Supporting Information for details.) Generalized gradient approximation (GGA)37,46,65 is also an appropriate method for simulating the vibration spectra of a finite and infinite number of atoms. Clavaguéra-Sarrio et al.66 reported that GGA can successfully predict the structural and vibrational properties of closed and open-shell systems for oxides of actinide compounds. Furthermore, they correlated the reliability of GGA with CASPT2, which is a highly computationally demanding method. Adjokatse et al.67 had also systematically studied the dielectric and piezoelectric response of odd-numbered nylons with the help of the DFT method with GGA and found nice correlation of the theoretically simulated vibrational spectrum with that of available experimental data. For conducting polymers, literature reveals that

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

pure GGA and hybrid B3LYP are quite effective at simulating the vibrational spectra; however, the latter is more abundantly used in the literature.37,52,65 Moreover, the B3LYP method has produced the experimental data quite well (see Table 2 of the text). Because of the nice correlation between theory (using uniform scaling factor) and experimental data, we chose B3LYP. 9Py. The simulated scaled IR spectrum of 9Py has two prominent band peaks in the functional group region at ca. 3521 (expt 3404 cm−1, N−H stretching) and 3118 cm−1 (expt 2920 cm−1, C−H stretching).61−64 Some characteristic band peaks in the fingerprint region for conjugation in the polymeric backbone of 9Py (as discussed by Omastova et al. and Zerbi et al.61,64) are 1396 (expt 1400, N−H wagging), 1297 (expt 1312, 17822



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Table 2. Experimental IR and Calculated Frequencies (in cm−1) of 9Py, 9Py+, 9Py-Cl, 9Py-NH3, and 9Py+-NH3a calcd frequency no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 a

expl16,68−73 IR 3403

9Py

9Py+

3521

3519

3521

3118

3145

3145

2920

9Py-Cl

9Py-NH3 3521 3440 3160 3117

9Py+-NH3

approx assignment

3519

ν N−H ν N−H of NH3 ν N−H ν C−H ν N−H (near NH3) ν C−H (near Cl) ν N−H (near Cl) cis of NH3 ν CC; wag C−H, H−N ν CC; wag C−H, N−H ν N−C; wag C−H, N−H ν N−C, CC; wag C−H, N−H ν N−C, CC; wag N−H, C−H ν N−C, CC; wag N−H, C−H ν CC; β C−H; cis C−N−C ν CC; wag N−H wag C−H, N−H; ν N−C cis C−H, N−H wag C−H, N−H Cis C−H rings breathing def(ring)//γ C−H, N−H γ C−H, N−H γ C−H, N−H γ C−H, N−H γ N−H

3144 3083

3072 2540 1653

1550−1460 1514 1484 1440 1400 1312 1250 − 1220 1081 1050 990 930

1396 1297 1218 1095 1051

856 734 680 426

1575 1537 1500 1463 1439

1542

1208 1099

1474 1453 1386 1304 1268 1202 1096

1027 994 874 743

1030 971 865 769/737

594 457

457

1313

1517

1579 1544

1440 1396 1287

1470 1440 1377 1312

1217 1094 1052 1036 874 731 679 462/447

1213 1100 1026 965 874 742 595 456

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

Moreover, some new bands appear in the functional group and fingerprint regions. The two new peaks in the functional group region at ca. 3072 and 2540 cm−1 are due to the presence of a counter species, such as Cl in this case, and have assignments of C−H and N−H stretching, respectively. Compared to neutral 9Py, five new bands appear at ca. 1474 (CC stretching), 1453 (C−H bending), 1268 (N−H bending), 1030 (C−H scissoring), and 971 cm−1 (ring breathing). The increased number of band peaks in the 1600−900 cm−1 region means that Cl has caused longer π-electron conjugation in the polymeric backbone of 9Py. 9Py+-NH3. To investigate the dedoping process of 9Py+ through IR spectral analysis, an ammonia molecule was placed near 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 3083 cm−1 (N−H stretching ammonia). The band peak at ca. 1500 cm−1 (C−H, N−H wagging and minor N−C stretching) in the 9Py+ spectrum disappeared upon reaction with NH3. The diminishing of this band in the 9Py+-NH3 complex is evidence of the lower delocalized π-electron conjugation, which means that this analyte creates localization in the polymeric backbone. Comparison of the various band frequencies of the 9Py+ and 9Py+-NH3 (Figure 4 and Table 2) led us to conclude that NH3 causes a blue-shift in the frequencies of 9Py+, a consequence of the dedoping phenomena. UV−Visible and UV−Vis−Near-IR Spectroscopic Study. Polaron states are generally formed in π-conjugated systems such as oligomers of PPy.4,74−78 These polarons may be localized or delocalized depending on the amount and nature of dopant. When the number of polarons increase, then

N−C stretching), 1218 (expt 1220, C−H wagging), and 1095 cm−1 (expt 1081, C−H wagging). Differences between experimental and simulated frequencies are primarily due to the comparison of condensed phase (experimental) and gasphase simulation. This has been discussed in the literature in fairly good detail.16,68−73 9Py-NH3. Compared to 9Py, 9Py-NH3 has three additional band peaks in the functional group region (Figure 4), 3440 (N−H stretching, ammonia), 3160 (N−H stretching, 9Py), and 1653 cm−1 (H−N−H scissoring, ammonia). Two band peaks at ca. 1440 and 1036 cm−1 in the IR spectrum of 9Py-NH3 provide evidence of the presence of increased π-electron density in the polymeric backbone, compared to isolated 9Py. As seen in Table 2, the other peaks in the fingerprint region are not 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 IR frequencies compared to those of the neutral 9Py. The higher frequency bands in the functional group region, such as 3519 and 3145 cm−1, have low intensity in 9Py+ but similar assignments as those of the 9Py band peaks. Red shifts in the frequencies of the fingerprint region are also observed and some new band peaks appear at ca. 1537, 1500, 1439, 1027, and 994 cm−1. Examining the IR spectrum of 9Py+ (Figure 4) and its prominent band peaks as listed in Table 2 leads us to conclude that the conductivity is increased, based on the presence of strong band peaks in the 1600−900 cm−1 region (vide infra). This statement also corroborates well with the earlier reported work.16,68−73 9Py-Cl. On comparison of 9Py-Cl with the neutral 9Py, we notice that Cl causes red-shifting in the IR spectrum of 9Py. 17823



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For 9py+, π → π* (415 nm) and midgap transitions (560 nm) are considerably red-shifted compared to the neutral 9Py (vide supra). The 414 nm peak of 9Py is replaced by a delocalized polaron band (transition from the valence band to the lower polaron band) at 1907 nm in 9Py+. On reacting NH3 with 9Py+ (Figure 6), a blue-shift (1907 → 1829 nm) is observed in its λmax. This blue-shifting can also be assigned to the dedopoing process of 9Py+, which consequently results in decreased conductivity, delocalized π-conjugation length, and an increase in the band gap.4,15−18,74−76 Interaction of Cl with 9Py causes formation of a localized polarion (polaron formation in the presence of a counter species) 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 redshifted to 466 and 528 nm, respectively (Figure 7). The absorbance band at 528 nm for 9Py-Cl (and also 560 nm for 9Py+) 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 localized polaron structures, respectively. A polaron is localized in the presence of a counterion (9Py-Cl), whereas it is delocalized in the absence of any counterion (9Py+). In summary, the ionization process converts polarons into localized (in the presence of a counter radical) and delocalized (absence of a counterion such as in the case of 9Py+) polarons (extended πconjugation length). The extension of the excitation energies beyond 1800 nm under the polaron and bipolaron regime is the result of the new transition energy levels between the conduction and valence bands. The presence of localized and delocalized polarons allows one to draw conclusions regarding the doping level and conductivity, optical, and electronic properties of CPs, especially in the case of PPy and PANI.4,15−18,74−76 Natural Bonding Orbital and Mulliken Charge Analysis. Charge transfer phenomena between the nPy and dopants (NH3, Cl) are simulated by Mulliken (QMULLIKEN) and NBO (QNBO) charge analysis at the UB3LYP/6-31G(d) level of theory. These properties are basis set dependent; however, if the same level of theory is used for different structures [such as UB3LYP/6-31G(d) or UB3LYP/6-311++G(d,p)], then the results will provide trends and therefore be meaningful. The basis set dependence of these charge analysis tools has been discussed by Fonseca Guerra et al.57 and Martin et al.58 The net charge transfer in 9Py-NH3 from ammonia to 9Py is 0.047 e− and 0.046 e−, based on QNBO and QMULLIKEN, respectively. The NH3 transfers about 0.057 e− based on QNBO and 0.065 e− based on QMULLIKEN to 9Py+. In the case of the Cl dopant, Cl receives about −0.801 e− charge based on QNBO and −0.705 e− based on QMULLIKEN from 9Py. From this charge analysis it can be easily concluded that, in the 9Py-Cl complex, Cl has caused oxidation in the 9Py (doping). However, in the case of the 9Py+-NH3 complex, reduction in the 9Py+ is observed (dedoping). HOMO and LUMO Energy. The HOMO and LUMO of 9Py, 9Py+, 9Py-Cl, 9Py-NH3, and 9Py+-NH3 calculated at UB3LYP/6-31G(d) are shown in Figure 8, and their corresponding energies from monomer up to infinity are listed in 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 provides comparisons of the HOMO and LUMO of the neutral 9Py and cationic species (9Py+). The HOMO of 9Py+ extends over

they are converted (through an ionization process) to bipolarons, which means the presence of two similar charges on the same molecule, usually at the terminal of a polymer/ oligomer backbone. UV−vis spectroscopy is a useful tool to differentiate between polarons and bipolarons and has widely been examined theoretically and experimentally by the MacDiarmid 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 gas phase at the TD-DFT/UB3LYP/6-31+G(d,p) level of theory (Figures 5−7). Our simulated UV−vis spectrum of the neutral

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).

9Py is in close agreement with the reported experimental and theoretical data.14,79 Three prominent peaks are observed in the UV−vis spectra of PPy: 274 nm (π → π*), 331 nm (midgap transition), and at 414 nm (λmax, transition from the valence band to the conduction band). Doping of 9Py oligomers with NH3 molecules (shown in Figure 5) causes slight red-shifting in its λmax (414 → 416 nm), while a slight blue-shift is observed in the midgap transition (331 → 329 nm). The red-shifting in λmax with NH3 dopant illustrates its n-type doping nature (basic nature, dedoping). With a decreasing band gap, conduction and delocalization are slightly enhanced in 9Py-NH3 compared to 9Py (vide infra). The blue-shifting in the second peak (331 → 329 nm) of 9Py-NH3 is due to distortion (quinoid form) in the regular pyrrole rings (vide supra), resulting in a decrease of interband transition. 17824



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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 Band Gap in eV of nPy

a

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

na

IP

EA

HOMO

LUMO

band gap

species

IP

EA

HOMO

LUMO

band gap

1 2 3 4 5 6 7 8 9 10 ∞

5.48 4.75 4.43 4.26 4.17 4.11 4.06 4.03 4.01 3.99 3.80

−1.38 −0.35 0.03 0.25 0.38 0.46 0.53 0.57 0.6 0.63 0.90

−5.48 −4.75 −4.43 −4.26 −4.17 −4.11 −4.06 −4.03 −4.01 −3.99 −3.80

1.38 0.35 −0.03 −0.25 −0.38 −0.46 −0.53 −0.57 −0.60 −0.63 −0.90

6.86 5.10 4.40 4.01 3.79 3.65 3.53 3.46 3.41 3.36 2.90

9Py 9Py-NH3 9py-Cl 9Py+ 9Py+-NH3

4.01 3.85 4.28 6.26 6.16

0.60 0.46 1.21 3.35 3.20

−4.01 −3.85 −4.28 −6.26 −6.16

−0.60 −0.46 −1.21 −3.35 −3.20

3.41 3.39 3.07 2.91 2.96

energies of 9Py are −4.01 and −0.60 eV, while for 9Py+ they are −6.26 and −3.35 eV, respectively. The higher magnitude of the HOMO energy (−6.26 eV) of the 9Py+ is a consequence of the existence of a longer conjugation length (vide infra), delocalization of π-electron density, and high conductivity, in addition to being a charge effect. Frontier Molecular Orbitals of 9Py vs 9Py-NH3. From Figure 8, frontier molecular orbitals of 9Py and 9Py-NH3 species can be comparatively analyzed. NH3 is a reducing agent, as has already been discussed in the analysis of 9Py and 9Py-NH3 molecular orbitals. Ammonia has reduced in magnitude 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). It also decreased the π-electron density over the 9Py polymeric backbone, resulting in an increase in resistance. Frontier Molecular Orbitals of 9Py+ vs 9Py+-NH3. Contours of the HOMO and LUMO of 9Py+ and 9Py+-NH3 are given in Figure 8. NH3 has caused a slight decrease in magnitude of the HOMO and LUMO energies of 9Py+-NH3 (compared to 9Py+). Figures 1 and 8 show clearly that NH3 has led to slight bending of the terminal rings of the polymer, compared to the other cases. Moreover, Figure 8 shows that NH3 decreases the delocalization of the π-electron density, resulting in lower conductivity. (See the discussion of the band gap, vide infra, and dedoping phenomena.) These results also support the earlier conclusions from the analysis of the UV−vis−near-IR spectra, optimized geometric parameters, and IR spectral characteristics. The estimated HOMO and LUMO energies

n is the number of repeating units.

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

a

na

IP

EA

HOMO

LUMO

band gap

1 3 5 7 9 11 ∞

13.09 8.6 7.32 6.68 6.26 5.97 4.91

6.21 4.75 4.09 3.67 3.35 3.07 2.41

−13.09 −8.60 −7.32 −6.68 −6.26 −5.97 −4.91

−6.21 −4.75 −4.09 −3.67 −3.35 −3.07 −2.41

6.28 3.85 3.23 3.01 2.91 2.90 2.46

n is the number of repeating units.

all carbons, hydrogens and nitrogens and forms a planar structure that involves delocalization of the π-electrons over the entire molecular backbone, contrary to its neutral counterpart 9Py. On the other hand, only the central atoms contribute to the LUMO; therefore, the LUMO is localized in the polymeric framework. The HOMO and LUMO energies of nPy and nPy+, from monomer up to infinite repeating units, are given in Tables 3 and 4 (vide infra). The estimated HOMO and LUMO 17825



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

of 9Py+-NH3 are −6.16 and −3.20 eV, respectively (Table 5). The HOMO and LUMO energies of the 9Py+-NH3 complex are 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 in Figure 8, and the corresponding estimated energies are listed in Table 5. Analysis of the optimized geometric structure (vide supra) and molecular orbitals led us to conclude that Cl planarizes the geometry of 9Py and extends the π-electron conjugation density over its polymeric backbone. This extended π-electron conjugation density increases the conductivity and decreases the band gap (vide infra) and resistance in the polymer. This statement confirms and extends the mentioned characterizations of the Cl doping for 9Py (or PPy). The presence of Cl radical affects the HOMO and LUMO energies of 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 IP and EA obtained from the negative values of the DFT orbital (HOMO and LUMO) energies (Koopman’s theorem) with typical approximate exchange correlation functionals is usually too small as compared with experimental values. However, hybrid functionals (such as UB3LYP), which account for the effects of self-interaction to some degree, result in a better correlation (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 values along with their valence, conduction, and polaron bands are given in Figures 9−11. Comparisons of the IP, EA, and band gap of these five different species, restricted to nine repeating units, are listed in Table 5. Increasing conjugation (as explained in the frontier molecular orbital analysis) over the polymeric backbone 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 its band gap (3.41 eV), along with valence and conduction band energies, is given in Figure 9a. The valence and conduction bands of 9Py are at −4.01 and −0.60 eV, respectively, while the interband or midgap transition is at about 2.99 eV above the valence band. Ammonia (donor) decreases the IP and EA values of 9Py by about 0.16 and 0.14 eV, respectively, as it has donated electrons to the 9Py orbitals (Table 5 and Figure 9b). Furthermore, we see from Figure 9b that the valence and conduction bands of 9Py-NH3 are at −3.85 and −0.46 eV, respectively. Its band gap is 3.39 eV, while the midgap transition 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 causes dedoping of PPy and decreases the IP and EA of Py+. The valence, conduction, and localized polaronic bands of Py+-NH3 are at −6.16, −3.20 and 0.67 eV, respectively. NH3 has increased 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 and 0.15 eV, respectively. The lower IP, lower EA, and increased band gap show that the polymer is reduced (dedoped). Moreover, it also demonstrates reduced delocalization of πelectron (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 from the orbitals of 9Py, consequently increasing its IP and EA values by about 0.27 and 0.60 eV, respectively. Simultaneously, with the increasing of these values, its band gap decreases from 3.41 to 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 2.25 and 2.75 eV, respectively. Its band gap (2.91 eV) also decreases by about 0.51 eV compared to that of neutral 9Py. The decrease in band gap of nPy+ from the monomer (n = 1) up to the infinite polymer (n = ∞) is given in Figure 13. The valence and conduction bands are situated at −6.26 and −3.35 eV, respectively. Another prominent band, which can be identified as a delocalized polaron, is located at 0.65 eV. This delocalized polaron band is responsible for the high conductivity and lower resistance and band gap value of 9Py+. An interband transition from the singly bonded polaron state to the antibonding polaron state (nonbonding polaron) has an energy of 2.21 eV. This delocalized polaron transition (bonding to antibonding) is responsible for the delocalized extended π-electron conjugation over the polymeric backbone of 9Py+.

Figure 10. Developments of band structure of PPy from energy levels of oligomers. (The monomer is used as a repeat unit.) 17826



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

as in 9Py+ and causes red-shifts in the IR frequencies. The doping process of 9Py is also achieved on reacting with Cl. This results in the emergence of five new bands in the 1600−900 cm−1 region. Dedoping of 9Py+ is achieved when ammonia is added; consequently, some bands in the 1600−900 cm−1 region disappear, and the result is lower delocalized π-electron conjugation. 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 rise to some new absorbances corresponding to localized and delocalized polaron bands. Specifically, the peaks in the near-IR region at 1907 nm for Py+ and 1242 nm for 9Py-Cl are due to delocalized and localized polaron structures, respectively. The presence of localized and delocalized polarons in the UV−vis near-IR spectra is correlated with the doping level, conductivity, optical, and electronic properties of CPs, especially in the case of PPy and PANI.4,74−76 It can also be concluded that the polarons/ bipolarons fall in the visible and near-IR region. Thus, they actually affect the vibrational, electrical, optical, and electronic properties of CPs. The net charge transfer in theses complexes is simulated with NBO and Mulliken charge analysis. NH3 transfers charge to 9Py and 9Py+, while Cl receives charge from 9Py, confirming the dedoping and doping phenomena, respectively. Frontier molecular orbitals (HOMO and LUMO), IP, EA, and band gap are also consistent with the vibrational and UV−vis−near-IR spectroscopy. The electron density in the MOs extends over the polymeric geometry in the case of 9Py+ and 9Py-Cl, while this is reversed in 9Py+-NH3 compared to neutral 9Py. In the case of oxidation and Cl dopant, the IP and EA increase, and consequently, there is a decrease in the band gap.

Figure 12. Energy level diagram of 9Py+.

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

4. CONCLUSION DFT calculations have been carried out on a number of molecules with different characteristics, notably, 9Py, 9Py+, 9Py-Cl, 9Py+-NH3, and 9Py-NH3, to investigate their doping and dedoping processes. In the vibrational analysis, the band peaks in the 1600−900 cm−1 region give information about the short and extended π-electron conjugation length. The number of peaks in this region is evidence of the presence of conjugation in the polymeric backbone. This is further confirmed from the high IP, high EA, low band gap, as well as the high electron density of the HOMO and LUMO. Slight differences are found between experimental and simulated frequencies; however, these are due to the condensed and gasphase IR spectra, respectively. Compared to neutral 9Py, the different additives (NH3, +, Cl, etc.) result in extra band peaks both in the fingerprint and functional group regions. Removal of an electron from the backbone of 9Py results in doping such

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ASSOCIATED CONTENT

S Supporting Information *

IR and UV−vis spectra, diagrams of HOMO and LOMO orbitals, and tables of selected IR band peaks along with approximate assignments. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*A.-u.-H.A.S. tel, +92-91-9216652; e-mail, anwarulhaqalishah@ upesh.edu.pk. *K.A. tel, +92-992-383591; e-mail, [email protected], [email protected]. 17827



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Author Contributions

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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully thank Prof. Ulrike Salzner and Prof. Georg Schreckenbach for his valuable comments, suggestions and discussion. We acknowledge the Institute of Chemical Sciences (ICS), University of Peshawar and Higher Education Commission, Islamabad.

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ABBREVIATIONS DFT, density functional theory; PPy, polypyrrole; B3LYP, Becke 3 parameter exchange functional combined with the Lee−Young−Parr correlation functional; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital; IP, ionization potential; EA, electron affinity; NBO, natural bonding orbital.

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