Demétrio Antônio da Silva Filho

Interplay of alternative conjugated pathways and steric interactions on the electronic and optical properties of donor–acceptor conjugated polymers

Donor–acceptor π-conjugated copolymers are of interest for a wide range of electronic applications, including field-effect transistors and solar cells. Here, we present a density functional theory (DFT) study of the impact of varying the conjugation pathway on the geometric, electronic, and optical properties of donor–acceptor systems. We consider both linear (“in series”), traditional conjugation among the donor–acceptor moieties versus structures where the acceptor units are appended orthogonally to the linear, donor-only conjugated backbone. Long-range-corrected hybrid functionals are used in the investigation with the values of the tuned long-range separation parameters providing an estimate of the extent of conjugation as a function of the oligomer architecture. Considerable differences in the electronic and optical properties are determined as a function of the nature of the conjugation pathway, features that should be taken into account in the design of donor–acceptor copolymers.

Optimally tuned functionals improving the description of optical and electronic properties of the phthalocyanine molecule

By means of Density functional theory and time-dependent density functional theory calculations, we present a comprehensive investigation on the influence of different functional schemes on electronic and optical properties of the phthalocyanine molecule. By carrying out our own tuning on the OT-LC-BLYP/6-31G(d,p) functional, we show that such a procedure is fundamental to accurately match experimental results. We compare our results to several others available in the literature, including the B3LYP/6-31+G(d,p) set, which is commonly portrayed as the best combination in order to obtain a good description of the band gap. The results obtained here present not only significant improvement of the optical properties from the conventional BLYP, but we can also objectively report an improvement of our tuned functional when compared to the current benchmark of the literature as far as optical properties are concerned. Particularly, by means of this approach, it was possible to achieve a good agreement between the theoretical and experimental optical gap as well as of the positioning of the main peaks in the absorption spectrum. Our results thus suggest that correcting the long-range term on exchange term of the Coulomb operator, by means of a tuning procedure, is a good option to accurately describe properties of the phthalocyanine molecule.

Efficient Exciton Diffusion and Resonance-Energy Transfer in Multilayered Organic Epitaxial Nanofibers

Multilayered epitaxial nanofibers are exemplary model systems for the study of exciton dynamics and lasing in organic materials because of their well-defined morphology, high luminescence efficiencies, and color tunability. We use temperature-dependent continuous wave and picosecond photoluminescence (PL) spectroscopy to quantify exciton diffusion and resonance-energy transfer (RET) processes in multilayered nanofibers consisting of alternating layers of para-hexaphenyl (p6P) and α-sexithiophene (6T) serving as exciton donor and acceptor material, respectively. The high probability for RET processes is confirmed by quantum chemical calculations. The activation energy for exciton diffusion in p6P is determined to be as low as 19 meV, proving p6P epitaxial layers also as a very suitable donor material system. The small activation energy for exciton diffusion of the p6P donor material, the inferred high p6P-to-6T resonance-energy-transfer efficiency, and the observed weak PL temperature dependence of the 6T acceptor material together result in an exceptionally high optical emission performance of this all-organic material system, thus making it well suited, for example, for organic light-emitting devices.

A DFT study of a set of natural dyes for organic electronics

We systematically investigate, at density functional theory level, the electronic properties of a set of ten carotenoid molecules with different conjugation length. Ground state geometries were fully optimized using both B3LYP and its long-range corrected version, i.e., the CAM-B3LYP functional. The time-dependent DFT approach (TD-DFT) was also performed for the calculation of the excited states of the optimized geometries and the results were compared to the experimental ones, when available. Our findings indicate a dependence of the transition vertical energies, oscillator strengths, and transition dipole moments on the extension of conjugation, as expected. We also investigate the impact of the intra-molecular vibrations on the absorption spectrum by means of the Franck–Condon (FC) and nuclear ensemble (NE) approach to spectra simulation. Our simulations suggest that the Franck–Condon approximation may not be suitable to appropriately characterize the vibronic progression of these molecules, whereas the NE approach provides a contribution that vary from negligible to meaningful depending on which molecule and energy region is under analysis.

Synthesis, Structure, Properties, and Bioimaging of a Fluorescent Nitrogen-Linked Bisbenzothiadiazole

This paper describes the synthesis, structure, photophysical properties, and bioimaging application of a novel 2,1,3-benzothiadiazole (BTD)-based rationally designed fluorophore. The capability of undergoing efficient stabilizing processes from the excited state allowed the novel BTD derivative to be used as a stable probe for bioimaging applications. No notable photobleaching effect or degradation could be observed during the experimental time period. Before the synthesis, the molecular architecture of the novel BTD derivative was evaluated by means of DFT calculations to validate the chosen design. Single-crystal X-ray analysis revealed the nearly flat characteristics of the structure in a syn conformation. The fluorophore was successfully tested as a live-cell-imaging probe and efficiently stained MCF-7 breast cancer cell lineages.

Biexciton cascade emission in multilayered organic nanofibers

The optical performance of multilayered organic nanofibers results from the dynamics of excited states in the system. Here, we show that the presence of biexcitons is crucial to correctly describe such dynamics. This may be the case even if the intensity of the light source is not high. The cascade emission mediated by biexcitons is mainly responsible for the behavior of the photoluminescence profile in the initial steps after light absorption. By using a combination of Kinetic Monte Carlo model and Genetic Algorithm, we simulate Time-Resolved Photoluminescence measurements of multilayered nanofibers. These simulations are compared with experimental results, thus revealing that the usual singlet exciton recombination is insufficient to reproduce the complete physical picture. Our results also include predictions for the behavior of the biexciton signal. These findings are observed to be valid for a wide temperature range, showing the importance of the biexciton cascade emission in several regimes for orga...

Modeling temperature dependent singlet exciton dynamics in multilayered organic nanofibers

Organic nanofibers have shown potential for application in optoelectronic devices because of the tunability of their optical properties. These properties are influenced by the electronic structure of the molecules that compose the nanofibers and also by the behavior of the excitons generated in the material. Exciton diffusion by means of Förster resonance energy transfer is responsible, for instance, for the change with temperature of colors in the light emitted by systems composed of different types of nanofibers. To study in detail this mechanism, we model temperature dependent singlet exciton dynamics in multilayered organic nanofibers. By simulating absorption and emission spectra, the possible Förster transitions are identified. Then, a kinetic Monte Carlo model is employed in combination with a genetic algorithm to theoretically reproduce time-resolved photoluminescence measurements for several temperatures. This procedure allows for the obtainment of different information regarding exciton diffusion in such a system, including temperature effects on the Förster transfer efficiency and the activation energy of the Förster mechanism. The method is general and may be employed for different systems where exciton diffusion plays a role.

Investigation of the torsional barrier of EDOT using molecular mechanics and DFT methods.

When heterocyclic monomers are polymerized by electrochemical or chemical methods, they form fully conjugated polymers which have a wide range of applications due to their outstanding electronic properties. Among this class of compounds, thiophene derivatives are widely used due to their chemical stability and synthesis flexibility. With the goal to investigate the torsion barrier of polymer chains, a few units of 3,4-ethylenedioxythiophene (EDOT) were chosen and submitted to molecular mechanics (MM), density functional theory (DFT) and coupled cluster CCSD(T) calculations. This study helps to understand the performance and transferability of force fields used in molecular mechanics and molecular dynamics simulations often used to describe structure–property relationships of those systems. Determination of inter-ring torsion angle was performed in a comparative study using both force field, DFT and CCSD(T) methods. A good agreement was noticed between MM and QC results and highlights the importance of the description of the interactions involving the oxygen atoms present in the structure of EDOT. These observations are related to the α,α-coupling that occurs between the monomer units and yields a linear polymer. DFT HOMO and LUMO orbitals were also presented. Finally, UV–vis spectra of EDOT units were obtained using several levels of theory by means of time-dependent DFT calculations (TD-DFT).

Optical properties of P3HT and N2200 polymers: a performance study of an optimally tuned DFT functional

The optical properties of systems composed of the polymers PolyeraActivInk™ N2200 and P3HT are experimentally and theoretically investigated using UV-Vis spectroscopy and time-dependent density functional theory calculations, respectively. From a theoretical point of view, we carried out an analysis considering several functionals and model oligomers of different sizes to mimic the polymers. As our studies were performed with and without solvents, a first important result regards the fact that, by considering solvent effects, a better agreement between theoretical and experimental results could be achieved. Our findings also show that an optimally tuned functional is better suited to describe the experimental absorption profile than a hybrid one for the flexible polymer (P3HT). For the almost rigid polymer considered here (N2200), on the other hand, hybrid functionals may perform better than tuned functionals.

Experimental and theoretical description of the optical properties of Myrcia sylvatica essential oil

We present an extensive study of the optical properties of Myrcia sylvatica essential oil with the goal of investigating the suitability of its material system for uses in organic photovoltaics. The methods of extraction, experimental analysis, and theoretical modeling are described in detail. The precise composition of the oil in our samples is determined via gas chromatography, mass spectrometry, and X-ray scattering techniques. The measurements indicate that, indeed, the material system of Myrcia sylvatica essential oil may be successfully employed for the design of organic photovoltaic devices. The optical absorption of the molecules that compose the oil are calculated using time-dependent density functional theory and used to explain the measured UV-Vis spectra of the oil. We show that it is sufficient to consider the α-bisabolol/cadalene pair, two of the main constituents of the oil, to obtain the main features of the UV-Vis spectra. This finding is of importance for future works that aim to use Myrcia sylvatica essential oil as a photovoltaic material.