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.

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 NH 3 , CO 2 , and CO. The sensitivity and selectivity of nPANI ES among NH 3 , CO 2 , and CO are studied at UB3LYP/ 6-31G(d) level of theory. Interaction of nPANI 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 nPANI ES-NH 3 complexes are observed compared to nPANI ES−CO 2 , nPANI ES-CO(1), and nPANI ES-CO(2) complexes. The inertness of + CO − in nPANI ES-CO(1) and nPANI ES-CO(2) complexes are also discussed. Frontier molecular orbitals and energies indicate that NH 3 changes the orbital energy of nPANI ES to a greater extent compared to CO 2 , CO(1), and CO(2). Peaks in UV−vis and UV−vis−near-IR spectra of nPANI ES are blue-shifted upon doping with NH 3 , CO 2 , 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 NH 3 compared to CO 2 and CO and it is consistent with the experimental observations.

Carbon nanotubes (CNTs) are known as nano-architectured allotropes of carbon, having graphene sheets that are wrapped forming a cylindrical shape. Rolling of graphene sheets in different ways makes CNTs either metals or narrow-band semiconductors. Over the years, researchers have devoted much attention to understanding the intriguing properties CNTs. They exhibit some unusual properties like a high degree of stiffness, a large length-to-diameter ratio, and exceptional resilience, and for this reason, they are used in a variety of applications. These properties can be manipulated by controlling the diameter, chirality, wall nature, and length of CNTs which are in turn, synthesis procedure-dependent. In this review article, various synthesis methods for the production of CNTs are thoroughly elaborated. Several characterization methods are also described in the paper. The applications of CNTs in various technologically important fields are discussed in detail. Finally, future prospects of CNTs are outlined in view of their commercial applications.

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Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations have been performed to gain insight into the structure of poly(o-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.