J. Del Nero

Theoretical and experimental investigation of the second hyperpolarizabilities of methyl orange

We describe experimental and theoretical studies of the third-order nonlinear optical coefficients of methyl orange solutions under different pH conditions. A combination of semiempirical and ab initio methods was adopted to investigate the most stable geometrical structures possible for this molecule. The experimental data obtained using the Z-scan technique for the third-order nonlinear optical properties of this compound has allowed the determination of the nonlinear refractive index and nonlinear absorption coefficient under picosecond excitation in the visible (532 nm) spectral region. From those experimental results, the second hyperpolarizability of methyl orange was inferred both for acidic and alkaline solutions. Comparison of these values to the results predicted by semiempirical methods suggests that even at low pH, when the probability of cis-trans isomerization is increased, the trans conformation of the methyl orange molecule should dominate the nonlinear spectra of this compound. The theoretical results were used as an auxiliary tool to identify possible trends on the nonlinear properties changes as a function of the distinct molecular conformations adopted by the methyl orange molecule under different pH conditions.

The UV-vis absorption spectrum of the flavonol quercetin in methanolic solution: A theoretical investigation

The UV-vis absorption spectrum of the solvated quercetin molecule in methanol was investigated theoretically by means of an elegant type of QM/MM scheme better known as sequential Monte Carlo/quantum mechanics (S-MC/QM) methodology. A set of 125 uncorrelated Monte Carlo molecular liquid structures were properly selected through the autocorrelation function of the energy in order to be used in the quantum mechanical calculations. These molecular liquid structures were obtained by means of the radial and minimum distance distribution functions. A detailed account of the pattern of hydrogen bond structures obtained in this study is also available. The computed results obtained here were directly compared with the available experimental data in order to validate our theoretical model and through this comparison a very good conformity between theoretical and available experimental results was found.

Semiempirical and ab initio investigation of defects in PPV oligomers

We report a theoretical study of the excited states and other electronic properties of para-phenylenevinylene oligomers and related compounds which present conformational defects. Our results reveal the existence of different electronic delocalization patterns for the lowest singlet and triplet structures of these molecules A similar behavior is also observed for the corresponding bond lengths.

Nonlinear optical properties of aromatic amino acids in the femtosecond regime

We report experimental and theoretical investigations of the third-order optical nonlinearities of aromatic amino acids (Phenylalanine, Histidine, Tryptophan, and Tyrosine) in aqueous solutions. The Z-scan technique with femtosecond laser pulses at 800 nm was explored for the determination of the nonlinear refractive index, nonlinear absorption coefficient, and the second-order hyperpolarizability of each amino acid. Experimental results were compared with theoretical analysis based on post-Hartree Fock MP2/6-311+G**.

Polyphosphate gel/methyl orange supramolecular composites

The aims of this work were to investigate theoretically the optical properties of methyl orange (MO) and the synthesis of new supramolecular composites based on the incorporation of this dye in an aluminum polyphosphate gel network. The theoretical methodology was based in semiempirical (AM1 and INDO/S-CI) and ab initio (3-21G*) methods. Our results reveal the existence of different electronic patterns for the acidic and basic forms of these molecules. Also, we present a theoretical spectroscopic study for the molecules including interactions with water molecules. MO was successfully incorporated in its acidic form within the host matrix, leading to pink–red transparent selfstanding films. The dye could be converted to its basic form upon exposure to ammonia vapor. The spectrum of MO basic form within the gel network differs from its behavior in aqueous solution. 2003 Elsevier B.V. All rights reserved.

Electronic structure investigation of biosensor polymer

We report a theoretical study of the ground, excited and ionic states of 3-methyl pyrrole-4-carboxilic acid (MPC) oligomers and related compounds which present conformational defects. Our results reveal the existence of differentiated electronic behavior for MPC with relation to oligopyrrole derivatives. These electronic features might explain why MPC works properly as a biosensor for cytochrome C while no voltametric response is observed for unsubstituted poly(pyrrole).

Malachite Green/Polyphosphate Gel Hybrid Materials: Synthesis and Optical Properties

In this work were describe the synthesis of a new supramolecular hybrid material based on the incorporation of the dye malachite green in an aluminum polyphosphate gel network and theoretical calculations on the optical properties of the organic guest molecule based in semi-empirical (PM3, PM5 and INDOIS-CI) methods. The dye is incorporated in its unprotonated form within the host matrix, leading to greenish-blue transparent free-standing films. The optical properties of the entrapped dye are sensitive to chemical changes within the matrix caused either by gel aging or an external stimulus such as exposition to ammonia vapors that can percolate through the gel.

Single-molecular diodes based on opioid derivatives

We propose an efficient single-molecule rectifier based on a derivative of opioid. Electron transport properties are investigated within the non-equilibrium Green’s function formalism combined with density functional theory. The analysis of the current–voltage characteristics indicates obvious diode-like behavior. While heroin presents rectification coefficient R>1, indicating preferential electronic current from electron-donating to electron-withdrawing, 3 and 6-acetylmorphine and morphine exhibit contrary behavior, R<1. Our calculations indicate that the simple inclusion of acetyl groups modulate a range of devices, which varies from simple rectifying to resonant-tunneling diodes. In particular, the rectification rations for heroin diodes show microampere electron current with a maximum of rectification (R=9.1) at very low bias voltage of ∼0.6 V and (R=14.3)∼1.8 V with resistance varying between 0.4 and 1.5 M Ω. Once most of the current single-molecule diodes usually rectifies in nanoampere, are not stable over 1.0 V and present electrical resistance around 10 M. Molecular devices based on opioid derivatives are promising in molecular electronics.

Isomeric effects tuning the electron transport in carotenoid derivatives: from ohmic to rectifier behavior

Single-molecules have been widely investigated in the last decades due to their promises as devices in molecular electronics. One of the advantages in the use of natural compounds in molecular electronics is the economy of material and molecular synthesis, which makes the process both cheaper and self-sustaining. Although many studies have considered electronic transport in single molecules, there are few studies associated with isomeric effects in biologically appealing systems. In the present work, we have studied ballistic electron transport in two isomeric forms of a retinol molecule: 11-cis and all-trans-retinol. The molecules were connected between two Au(111) electrodes and calculations were performed with the NEGF-DFT methodology. Current–voltage, differential conductance, and rectification curves were obtained and compared for two structures. While 11-cis-retinol shows a more symmetrical current–voltage curve, all-trans-retinol acts as molecular diode for low applied voltages. Our results suggest that a simple isomeric effect modulates the molecular device from nanowires to diodes with potential applications as field-effect transistors.

Topological insulator-metal transition and molecular electronics device based on zigzag phagraphene nanoribbon

In this work, we investigate the electronic transport properties of a graphene allotrope composed of 5–6-7 carbon aromatic rings called phagraphene and compare with the results of the transition-voltage spectroscopy (TVS) and propose the behavior at low voltage characteristic of a topological insulator. Phagraphene properties were compared to those of graphene in a zigzag nanoribbon configuration, zigzag graphene vs zigzag phagraphene nanoribbon (zzGNR and zzPGNR). The molecular geometry and the electronic properties were calculated by density functional theory (DFT) without spin, and the electronic transport and TVS were obtained by means of DFT combined with non-equilibrium Green´s function when we couple the optimized geometry of zzGNR and zzPGNR to the leads (left and right), forming the molecular junction that will be subjected to the action of an external bias voltage (Ve) to generate the molecular device. The results exhibit (i) a metal-insulator transition when Ve is increased until Ve = 1.4 V which corresponds to the nonlinear region (resonance), showing the field effect transistor behaviour for zzGNR junctions; and (ii) two nonlinear regions (two negative differential resistances), showing a resonant tunnel diode behaviour with two operation windows (Ve = 0.5 V and Ve = 1.7 V) for the zzPGNR junction. In addition, the zzPGNR junction exhibits topological insulator characteristics upon introducing topological defects such as pentagons and heptagons in the hexagonal lattice of graphene, and when Ve = 1.7 V, there occurs a topological insulator-metal transition that can be seen in the behaviour of the density of states, transmittance, and frontier molecular orbitals with Ve.In this work, we investigate the electronic transport properties of a graphene allotrope composed of 5–6-7 carbon aromatic rings called phagraphene and compare with the results of the transition-voltage spectroscopy (TVS) and propose the behavior at low voltage characteristic of a topological insulator. Phagraphene properties were compared to those of graphene in a zigzag nanoribbon configuration, zigzag graphene vs zigzag phagraphene nanoribbon (zzGNR and zzPGNR). The molecular geometry and the electronic properties were calculated by density functional theory (DFT) without spin, and the electronic transport and TVS were obtained by means of DFT combined with non-equilibrium Green´s function when we couple the optimized geometry of zzGNR and zzPGNR to the leads (left and right), forming the molecular junction that will be subjected to the action of an external bias voltage (Ve) to generate the molecular device. The results exhibit (i) a metal-insulator transition when Ve is increased until Ve = 1.4 V whi...