Artemis M. Ceschin

Improving the Description of the Optical Properties of Carotenoids by Tuning the Long-Range Corrected Functionals

In this work we use gap-fitting procedure to tune the long-range corrected functionals and accurately investigate the electronic and optical properties of the five main molecules composing Buriti oil (extracted from Mauritia flexuosa L.) in the framework of density functional theory (DFT) and time-dependent (TD) DFT. The characteristic length (1/ω) was observed to be entirely system dependent, though we concluded that its determination is of fundamental importance to rescue geometrical, electronic, and optical properties with accuracy. We demonstrate that our approach of tuning characteristic length for each system resulted in an absorbance spectra in better experimental agreement when compared to the traditional methodology. Therefore, this study indicates that the tuning of the range-separation parameter is crucial to improve the description of the optical properties of conjugated molecules when TDDFT is used. For example, the wavelength of maximum absorption, λmax, for the phytofluene, obtained using B3LYP, is 381 nm, while using the gap-fitting procedure for the tuned-LC-BLYP the estimated λmax changed to 358 nm. The latter estimate is in better agreement with the experimental value of 350 nm.

Observation of negative differential resistance and hysteretic effect on buriti oil:polystyrene organic devices

The authors have observed hysteretic behavior and negative differential resistance in single-layer devices made from buriti oil mixed with polystyrene and sandwiched between poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) and silver (Ag) electrodes. The hysteresis curve shows not only two different values of current but different behaviors as the increasing voltage curve shows negative differential resistance and the decreasing voltage curve does not. The fact that a simple fabrication technique yields reproducible conductance switching and hysteresis is relevant for the development of theories of organic electronic device behavior.

Humin based ammonia sensor: characterization and performance analysis

In this work we propose the use of peat as an alternative material to be used for the development of ammonia sensors. A thorough characterization of the material is carried out in order to ensure its feasibility as a source of an efficient sensor. Humin, which is a subproduct of peat, consists basically of carbon and is extracted through the centrifugation of KOH or NaOH solutions. Initially, the material was characterized by determining its physical and chemical properties such as carboxylic groups, exchangeable hydrogen, organic matter, PH and moisture content. The surface area, pore volume and mean pore radio was also evaluated. A topologic study was also performed by means of AFM experiment in order to access the desired morphology uniformity of the film to be used as a sensor. Finally the sensitivity of the samples to ammonia vapor was measured using the standard two-temperature method. Our results show that peat is an interesting substance to be used in the development of inexpensive ammonia sensors and suggests that it is also a promising material to the development of other gas sensors.

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.

Modeling optical properties of polymer–solvent complexes: the chloroform influence on the P3HT and N2200 absorption spectra

AbstractThe optical properties of polymer/solvent systems composed by the polymers P3HT and PolyeraActivInk N2200 under the present of chloroform as solvent are experimentally and theoretically investigated using UV-Vis spectroscopy, molecular dynamics (MD), and density functional theory (DFT) calculations. The study is focused on obtaining the theoretical methodologies that properly describes the experimentally obtained absorption spectra of polymer–solvent complexes. In order to investigate the solvent influence, two different approaches are taken into account: the solvation shell method (SSM) and the polarizable continuum model (PCM). Our findings shown that SSM simulations, which combine MD and DFT calculations, are in good agreement with the experimental data. Moreover, it is obtained that simulations in the framework of PCM do not provide a fair description of the real system. Importantly, these results may pave the way for better descriptions of some optoelectronic properties of interest in polymer/solvent systems. Graphical Abstractᅟ