C.P. de Melo

Bio: C.P. de Melo is an academic researcher from Federal University of Pernambuco. The author has contributed to research in topics: Polypyrrole & Ab initio. The author has an hindex of 17, co-authored 56 publications receiving 1011 citations. Previous affiliations of C.P. de Melo include Massachusetts Institute of Technology.

Variational-perturbational treatment for the polarizabilities of conjugated chains. II. Hyperpolarizabilities of polyenic chains

Abstract: We use a perturbative density matrix treatment to investigate the behavior of the first and second static hyperpolarizabilities of linear conjugated chains CNHN+2 described by a Pariser–Parr–Pople Hamiltonian. We examine the evolution of the hyperpolarizabilities with increasing chain lengths, and the effect of different conformations (such as those corresponding to soliton and polaron defects) and of different charge states on the polarizability response of the chains. It is shown that charged soliton chains have large first hyperpolarizabilities β, and that the behavior of the different components of the second hyperpolarizability tensor γiiii is highly dependent on the geometry and charge of the conjugated chain. In all cases, the contribution of the longitudinal component is shown to dominate the orientationally averaged hyperpolarizability of the chains.

Variational–perturbational treatment for the polarizabilities of conjugated chains. I. Theory and linear‐polarizabilities results for polyenes

Abstract: We use a perturbative density matrix treatment to investigate the behavior of the static polarizabilities α of linear conjugated chains CNHN+2, with respect to the chain length and to the presence of neutral and charged conjugation defects of soliton and polaron type. The molecules are described by the Pariser–Parr–Pople Hamiltonian; both closed and open shells are treated. It is shown that both the longitudinal component of α and the orientationally averaged linear polizability scale as different powers of N in each case. It is also shown how the present treatment can be considered as a generalization of standard variation–perturbation methods for the Hartree–Fock case.

Role of disorder in the conduction mechanism in polyanilines.

Abstract: We present the first theoretical calculations of the electronic structure of long (200 rings) linear chains of polyaniline, ranging in composition from leucoemeraldine to emeraldine, allowing for compositional disorder in that the sequence of quinoid-benzenoid groups is random. We show that random protonation of the disordered polymers may induce p-type conductivity: This process pulls the Fermi energy down into the valence band, past localized band tails, to extended states. The effect is only seen if disorder is taken into account

Use of conducting polypyrrole blends as gas sensors

Abstract: We discuss the use of polymeric blends formed by the controlled growth of doped polypyrrole chains in the interior of films of different dielectric polymers as sensors of the presence of representative examples of volatile organic compounds. The dielectric films (of either (poly(caprolactone), poly(ethylene oxide), poly(methyl-methacrylate), poly(vinyl alcohol), or poly(vinyl-acetate)) containing an appropriate oxidizing agent were prepared atop a conducting glass substrate and the polymerization of polypyrrole was induced through the vapor phase technique, after exposure of the desired matrix to pyrrole vapor. The UV–vis–NIR spectra of the films were used to monitor the increase of their conductive character through the assessment of the intensity of the polaron band characteristic of the doped form of polypyrrole. The change of conductivity upon exposure to the volatile compounds used (methanol, ethanol, carbon tetrachloride and benzene) was adopted as a measure of the corresponding sensitiveness of the films. Differently to the previous case in which doped films of polypyrrole were examined as sensors (when a similar low level of response was found towards apolar compounds), now we have found that some of the blends (PMMA/PPY and PVA/PPY, in special) exhibit a particular sensitiveness to the polarity of the molecule of the vapor tested. We therefore suggest that the simultaneous use of such polypyrrole blends in an arrangement pattern can improve the efficiency of electronic nose devices.

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

Abstract: A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Gas Sensors Based on Conducting Polymers

Abstract: The gas sensors fabricated by using conducting polymers such as polyaniline (PAni), polypyrrole (PPy) and poly (3,4-ethylenedioxythiophene) (PEDOT) as the active layers have been reviewed. This review discusses the sensing mechanism and configurations of the sensors. The factors that affect the performances of the gas sensors are also addressed. The disadvantages of the sensors and a brief prospect in this research field are discussed at the end of the review.

Local treatment of electron correlation in coupled cluster theory

Abstract: The closed‐shell coupled cluster theory restricted to single and double excitation operators (CCSD) is formulated in a basis of nonorthogonal local correlation functions. Excitations are made from localized molecular orbitals into subspaces (domains) of the local basis, which strongly reduces the number of amplitudes to be optimized. Furthermore, the correlation of distant electrons can be treated in a simplified way (e.g., by MP2) or entirely neglected. It is demonstrated for 20 molecules that the local correlation treatment recovers 98%–99% of the correlation energy obtained in the corresponding full CCSD calculation. Singles‐doubles configuration interac‐ tion (CISD), quadratic configuration interaction (QCISD), and Mo/ller–Plesset perturbation theory [MP2, MP3, MP4(SDQ)] are treated as special cases.

Low-order scaling local electron correlation methods. IV. Linear scaling local coupled-cluster (LCCSD)

Abstract: A new implementation of local coupled-cluster theory with single and double excitations (LCCSD) is presented for which asymptotically all computational resources (CPU, memory, and disk) scale only linearly with the molecular size. This is achieved by: (i) restricting the correlation space for each electron pair to domains that are independent of molecular size; (ii) classifying the pairs according to a distance criterion and treating only strong pairs at the highest level; (iii) using efficient prescreening algorithms in the integral transformation and other integral-direct procedures; and (iv) neglect of small couplings of electron pairs that are far apart from each other. The errors caused by the various approximations are negligible. LCCSD calculations on molecules including up to 300 correlated electrons and over 1000 basis functions in C1 symmetry are reported, all carried out on a workstation.

Recent development in the design of organic materials for optical power limiting

Abstract: During the past five years there has been considerable progress in the design of organic materials for optical power limiting. Among the more promising of the new material approaches are new reverse saturable absorbers (RSA) which derive their limiting capability on the photogeneration of highly absorbing charge states. Equally intriguing are the new approaches to designing chromophores with large two-photon cross-sections which give access to highly absorbing transient excited states. It now seems possible to combine these two new optical limiting paradigms in single structures which may therefore be considered as bimechanistic optical power limiters.