Density Functional Theory Study of Poly(<i>o</i>‑phenylenediamine) Oligomers

Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations have been performed to gain insight into the structure of poly­(<i>o</i>-phenylenediamine) (POPD). Both reported structures of POPD, ladder (L)- and polyaniline (P)-like, are investigated theoretically through the oligomers approach. The simulated vibrational properties of 5POPD­(L) and 5POPD­(P) at B3LYP/6-31G (d) along with their assignments are correlated with experimental frequencies. Vibrational spectra show characteristic peaks for both POPD­(L) and POPD­(P) structures and do not provide any conclusive evidence. Excited-state properties such as band gap, ionization potential, electron affinities, and HOMO–LUMO gaps of POPD­(L) and POPD­(P) from monomers to five repeating units are simulated. UV–vis spectra are simulated at the TD-B3LYP/6-31+G (d, p) level of theory, supportive to the ladder-like structure as the major contributor. Comparison of the calculated data with the experimental one strongly suggests that the ladder-like structure is the predominant contributor to the molecular structure of POPD; however, a small amount of POPD­(P) is also believed to be present

Structural and Spectroscopic Properties of Homo- and Co-Oligomers of <i>o</i>‑Phenylenediamine and <i>o</i>‑Toluidine: Theoretical Insights Compared with Experimental Data

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 <i>o</i>-phenylenediamine (OPD) and <i>o</i>- 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 (<i>I</i>_P), electron affinities (<i>E</i>_A) 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

Systematic Analysis of Poly(<i>o</i>‑aminophenol) Humidity Sensors

A thin film of poly­(<i>o</i>-aminophenol), POAP, has been used as a sensor for various types of toxic and nontoxic gases: a gateway between the digital and physical worlds. We have carried out a systematic mechanistic investigation of POAP as a humidity sensor; how does it sense different gases? POAP has several convenient features such as flexibility, transparency, and suitability for large-scale manufacturing. With an appropriate theoretical method, molecular oligomers of POAP, NH and O functional groups and the perpendicular side of the polymeric body, are considered as attacking sites for humidity sensing. It is found that the NH position of the polymer acts as an electrophilic center: able to accept electronic cloud density and energetically more favorable compared to the O site. The O site acts as a nucleophilic center and donates electronic cloud density toward H₂Ovap. In conclusion, only these two sites are involved in the sensing process which leads to strong intermolecular hydrogen bonding, having a 1.96 Å bond distance and Δ<i>E</i> ∼ −35 kcal mol^–1. The results suggest that the sensitivity of the sensor improved with the oxidization state of POAP

DFT Study of Polyaniline NH₃, CO₂, and CO Gas Sensors: Comparison with Recent Experimental Data

Density functional theory studies (DFT) have been carried out to evaluate the ability of polyaniline emeraldine salt (PANI ES) from 2 to 8 phenyl rings as sensor for NH₃, CO₂, and CO. The sensitivity and selectivity of <i>n</i>PANI ES among NH₃, CO₂, and CO are studied at UB3LYP/6-31G­(d) level of theory. Interaction of <i>n</i>PANI ES with CO is studied from both O (CO(1)) and C (CO(2)) sides of CO. Interaction energy, NBO, and Mulliken charge analysis were used to evaluate the sensing ability of PANI ES for different analytes. Interaction energies are calculated and corrected for BSSE. Large forces of attraction in <i>n</i>PANI ES-NH₃ complexes are observed compared to <i>n</i>PANI ES–CO₂, <i>n</i>PANI ES-CO(1), and <i>n</i>PANI ES-CO(2) complexes. The inertness of ⁺CO^– in <i>n</i>PANI ES-CO(1) and <i>n</i>PANI ES-CO(2) complexes are also discussed. Frontier molecular orbitals and energies indicate that NH₃ changes the orbital energy of <i>n</i>PANI ES to a greater extent compared to CO₂, CO(1), and CO(2). Peaks in UV–vis and UV–vis–near-IR spectra of <i>n</i>PANI ES are blue-shifted upon doping with NH₃, CO₂, CO(1), and CO(2) which illustrates dedoping of PANI ES to PANI emeraldine base (PANI EB). Finally, it is concluded that PANI ES has greater response selectivity toward NH₃ compared to CO₂ and CO and it is consistent with the experimental observations

Doping and Dedoping Processes of Polypyrrole: DFT Study with Hybrid Functionals

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 <i>n</i>Py and <i>n</i>Py-X (where <i>n</i> = 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 <i>n</i>Py 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 <i>n</i>Py⁺. 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

Molecular and Electronic Structure Elucidation of Polypyrrole Gas Sensors

Sensitivity and selectivity of polypyrrole (PPy) toward NH₃, CO₂, and CO have been studied at density functional theory (DFT). PPy oligomers are used both in the doped (PPy⁺) and neutral (PPy) form for their sensing abilities to realize the best state for gas sensing. DFT calculations are performed at the hybrid functional, B3LYP/6-31G­(d), level of theory. Detection/interaction of CO is investigated from carbon [CO(1)] and oxygen termini of CO [CO(2)]. Interaction energies and charge transfer are simulated which reveal the sensing ability of PPy toward these gases. Furthermore, these results are supported by frontier molecular orbital energies and band gap calculations. PPy, in both the doped and neutral state, is more sensitive to NH₃ compared to CO₂ and CO. More interestingly, NH₃ causes doping of PPy and dedoping of PPy⁺, providing evidence that PPy/PPy⁺ is an excellent sensor for NH₃ gas. UV–vis and UV–vis–near-IR spectra of <i>n</i>Py, <i>n</i>Py⁺, and <i>n</i>Py/<i>n</i>Py⁺–X complexes demonstrate strong interaction of PPy/PPy⁺ with these atmospheric gases. The better response of PPy/PPy⁺ toward NH₃ is also consistent with the experimental observations