Structural and Spectroscopic Properties of Homo-and Co-Oligomers of

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Structural and Spectroscopic Properties of Homo- and Co-oligomers of o-Phenylenediamine and o-Toluidine: Theoretical In-sights Compared with Experimental Data Salma Bilal, Sania Bibi, Rudolf Holze, and Anwar-ul-Haq Ali Shah J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09393 • Publication Date (Web): 17 Nov 2016 Downloaded from http://pubs.acs.org on November 20, 2016

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The Journal of Physical Chemistry

Structural and Spectroscopic Properties of Homo- and Cooligomers of o-Phenylenediamine and o-Toluidine: Theoretical Insights Compared with Experimental Data Salma Bilala, Sania Bibia, Rudolf Holzeb, Anwar-ul-Haq Ali Shahc* a

National Centre of Excellence in Physical Chemistry, University of Peshawar, 25120 Peshawar, Pakistan b

Institute für Chemie,AG Elektrochemie,Technische Universität Chemnitz,09107 Chemnitz, Germany c

*

Institute of Chemical Sciences, University of Peshawar, 25120 Peshawar, Pakistan

To whom correspondence should be addressed: E-mail: anwarulhaqal-

[email protected] Tel: +92(091) 9216652, 9216701-20, Fax: +92(091) 9216652

1 ACS Paragon Plus Environment


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ABSTRACT Density Functional Theory (DFT) and Time Dependent Density Functional Theory (TD-DFT) calculations have been performed to get insights into the structural, optical and electronic properties of homo- and co-oligomers of o-phenylenediamine (OPD) and o- toluidine (OT). UV-Vis spectral bands assigned to various neutral, cationic and dicationic homo- and co-oligomers of OPD and OT have been analyzed at TD-DFT UB3LYP /6-31G (d, p) level and complete assignments/correlation with experimental results are reported. The calculated vibrational bands of both homo- and co-oligomers of OPD and OT at B3LYP/6-31G (d) level along with their assignments are compared with experimental frequencies. Electronic properties such as ionization potentials (IP), electron affinities (EA) and HOMO-LUMO bandgap energies of both homo- and co-oligomers of OPD and OT have been calculated and are compared in the present work. DFT calculations with the 6-31 G (d) basis set predict very accurately experimentally observed vibrational modes as well as energy bandgap values.


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INTRODUCTION An important way to synthesize polyanilines (PANIs) with tailored properties is to copolymerize different aniline derivatives via chemical or electrochemical methods. Copolymerization is considered as one of the most convenient ways to obtain PANI derivatives with desirable properties1. It is an accessible way that utilizes and combines the structural and chemical properties of monomeric species involved in copolymer formation. The basic advantage of copolymerization is that the properties of copolymers can be fine-tuned variably by changing the monomer concentrations. Wei et al.2 carried out inventory efforts to get copolymers of aniline with alkyl substituted anilines having modified properties when compared with respective homopolymers. Savitha et al.3 reported a copolymer of aniline with m-aminoacetophenone having enhanced conductivity, good solubility as well as better crystallinity. A copolymer of aniline and o-aminophenol with good electrochemical activity has been reported.4 Copolymers of aniline and diphenylamine sulphonic acid (DPASA) have been reported by Wen et al.5 It is often assumed that electro-copolymerization may proceed along a number of different routes. Formation of several highly reactive intermediate species is reported during copolymerization.6, 7 These intermediates can be neutral species and/or unstable cationic/dicationic radicals which are either present in solution or at the interface of solution and electrode. These entities combine in one way or the other to form co-oligomer and finally copolymers. Wen et al.5 identified various dimeric and oligomeric species formed as intermediates in the form of cations and dications which proceed to form homo- and copolymers. Shah and Holze8 copolymerized o-aminophenol (OAP) and aniline and proposed the



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formation of head-to-tail-coupled dimers and oligomers during the early stages of copolymerization. Various techniques are employed to detect and identify these short lived intermediates. A rotating disc electrode,9 electrochemical thermospray mass spectroscopy10 and fast scan cyclic voltammetry11 have been used to analyze the intermediates formed during the polymerization of aniline. We have reported on the polymerization of aniline derivatives yielding different copolymers having various desirable properties.8, 12, 15 As part of these investigations we also tried to elucidate the mechanism of the formation of these copolymers via different experimental techniques including in situ UV-Vis and in situ Raman spectroelectrochemical studies.12,16 Among these studies was the synthesis of the novel copolymers of ophenylenediamine (OPD) and o-toluidine (OT).13 These copolymers were found to have a redox activity in a wide range of electrode potentials, very high electrochemical activity even at high pH values, and good electrochemical stability. Recently, a report on in situ Ultraviolet-Visible (UV-Vis) spectroelectrochemical studies of homo- and co-oligomers/copolymers of o-phenylenediamine (OPD) and o-toluidine (OT) discussed the formation of copolymers with modified optical properties.12 Based on spectroelectrochemical studies12 a mechanism of the finally proceeding formation of copolymers was proposed assuming participation of different short-lived cationic and dicationic species, which may react either with neutral species or with other intermediates in the solution to form homo- and co-oligomers/copolymers. Different new spectral bands were observed in the optical spectra recorded during homo- and copolymerization of OPD and OT. These bands were suggested as being due to the presence of cationic and dicationic intermediate species. The proposed structures of these species and


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related bands are summarized in supporting information tables 11 and 12 (ST11 and ST12), where the structures in the tables represent cationic oligomers 2OPD+, 2OT+, cationic co-oligomers (OPD-co-OT) + and dicationic oligomers 2OPD2+, 2OT2+ and dicationic co-ologomers (OT-co-OPD)

2+

respectively. Computational methods are exten-

sively used for theoretical estimation of various properties of electroactive and conducting polymers17,18,19. Oligothiophene anions20, oligopyrrole

21

and oligomers of aniline 22

have investigated with various levels of Density Functional Theory (DFT). In the present work we have utilized DFT and Time Dependent Density Functional Theory (TD-DFT) calculations in order to get further insights into the copolymer structure and to further support the proposed mechanism12 and our assumption of formation of co-dimers and oligomers in the very beginning of the copolymerization process. With DFT we have examined structural, electronic and spectral properties like bond lengths, bond angles, dihedral angles, energies of highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals, band gap energies, UV-Vis, IR and Raman spectra of homo- and co-oligomers of OPD and OT, and conceivable structural differences between homo- and co-oligomers of OPD and OT. By comparing DFT-results and experimental observations we have attempted to verify specific reaction intermediates assumed to be possibly present during homo- and co-oligomerization of OPD and OT, and the way how they tend to proceed for copolymerization. The ionization potentials IP and electron affinities Ea of both homoand co-oligomers of OPD and OT were calculated.

COMPUTATIONAL METHODOLGY DFT and TD-DFT calculations were performed using GAUSSIAN 09 software, the 5 ACS Paragon Plus Environment


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results were analyzed with Gauss View software. The optimized geometries of neutral, cationic, dicationic dimers/homo-oligomers and co-dimers/co-oligomers of OPD and OT were obtained by gradient minimization at DFT with correlation functions B3LYP/UB3LYP [Becke 3-Parameter (Exchange), Lee, Yang and Parr (correlation; density functional theory)] at 6-31G (d) basis set by ignoring symmetries.23 The geometry optimizations were considered complete when a stationary point was located on the Potential Energy Surface (PES). The UV-Vis spectra of optimized geometric structures of neutral, cationic, dicationic dimers/homo-oligomers and co-dimers/co-oligomers of OPD and OT were simulated at TD-DFT/B3LY, UB3LYP-6-31+G (d, p) level.24 The IR and Raman vibrational frequencies were simulated with the same hybrid functional as used in geometry optimizations. A scaling factor of 0.9613 was applied to the calculated frequencies. 25Four repeat units of the OPD, OT and OPD-co-OT represent the polymeric nature quite well, therefore, calculations were restricted to four repeating units. The frontier molecular orbital simulations such as energies of highest occupied molecular orbitals (HOMO), energies of lowest unoccupied molecular orbitals (LUMO)26 and band gap calculations were performed using DFT/B3LY/6-31G level.27 Further electronic properties such as ionization potential IP28 and electron affinities EA29 were obtained from HOMO and LUMO energy values, respectively, using the same level of theory as stated above.

RESULTS AND DISCUSSIONS Optimized Geometric Parameters The neutral, cationic and dicationic co-oligomers of o-phenylenediamine and otoluidine (OPD-co-OT) from two up to four repeat units were optimized. The optimized 6 ACS Paragon Plus Environment

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neutral structure of co-oligomer (2OPD-co-2OT) is given in Figure 1. The optimized structures as well as the corresponding Cartesian coordinates of the cationic (2OPD-co2OT)+ and dicationic (2OPD-co-2OT)2+ species are collected in the Supporting Information (S1). Several important calculated geometric parameters (bond lengths, bond angles, and dihedral angles) at the B3LYP/6-31G (d) level of theory are given in Supporting Information Table 1 (ST1). The C-N bond length for neutral co-oligomer of (2OPD-co2OT) is found to be in the range of 1.40 - 1.41 Å. The C-N bond length for cationic cooligomer of (2OPD-co-2OT) + is in the range of 1.37-1.38 Å, while the C-N bond length of dicationic co-oligomer of (2OPD-co-2OT)2+ ranges from 1.35 to 1.37 Å. The

Structural and Spectroscopic Properties of Homo-and Co-Oligomers of

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