Melanins represent an important class of natural pigments present in plants and animals that are currently considered to be promising materials for applications in optic and electronic devices. Despite their interesting properties, some of the basic features of melanins are not satisfactorily understood, including the origin of their intrinsic paramagnetism. A number of experiments have been performed to investigate the electron spin resonance (ESR) response of melanin derivatives, but until now, there has been no consensus regarding the real structure of the paramagnetic centers involved. In this work, we have employed electronic structure calculations to evaluate the ESR parameters of distinct melanin monomers and dimers in order to identify the possible structures associated with unpaired spins in this biopolymer. The g-factors and hyperfine constants of the cationic, anionic and radicalar structures were investigated. The results confirm the existence of at least two distinct paramagnetic centers in melanin structure, identifying the chemical species associated with them and their roles in electrical conductivity.
Melanins are an essential natural pigment found throughout nature. Due to their inherent biocompatibility, optical and electronic properties, the melanins are attracting attention as models for sustainable organic optoelectronic and...
Melanins are natural pigments with promising bioelectronic applications. Among their unique properties, the existence of a persistent paramagnetic signal can be considered one of the most intriguing and controversial features. Additionally, the possible influence of such centers on charge transport accentuates the need for a better understanding of their origin and specific characteristics. In this report, electron paramagnetic resonance spectra of melanin samples obtained at different experimental conditions were systematically studied. From the fitting procedure, three distinct resonant lines are proposed, two associated with carbon-centered radicals and one with semiquinone free radicals.
Melanin, an important class of natural pigment found in the human body, has stood out as a promising bioelectronic material due to its rather unique collection of electrical properties and biocompatibility. Among the available melanin derivatives, the sulfonated forms has proven to not only be able to produce homogeneous device quality thin-films with excellent adhesion even on hydrophobic surfaces, but also to act as an ion to electron transducing element. It has recently been shown that the transport physics (and dominat carrier generation) may be related to a semiquinone free radical species in these materials.Hence, a better understanding of the paramagnetic properties of sulfonated derivatives could shed light on their charge transport behavior, and thus enable improvement in regard use in bioelectronics. Motivated by this question, in this work, different sulfonated melanin derivatives were investigated by hydration-controlled continuous-wave X-band electron paramagnetic resonance spectroscopy and electronic structure calculations. Our results show that sulfonated melanin behaves similarly to non-functionalized melanin, but demonstrates a less pronounced response to humidity vis-à-vis standard melanin. We thus speculate on the structural and charge transport behavior in light of these differences with a view to further engineering structure-property relationships.
In this paper we investigate the causes of the chromatic alteration observed in chloroform solutions of poly (2-methoxy-5-(2 0 -ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) after gamma ray irradiation. Structural and chemical changes were analyzed by gel permeation chromatography, fourier transform infrared spectroscopy, and proton nuclear magnetic resonance techniques and complemented by electronic structure calculations. The results indicate chlorine incorporation in the polymer structure and main chain scission after irradiation. Based on our findings we propose that the main mechanism for the blue-shifts, observed in the UV-Vis absorption spectra of MEH-PPV after irradiation, is the result of a radical attack on the polymer main chain. Gamma rays generate radicals,
Cl andCHCl 2 from chloroform radiolysis that attack preferentially the vinyl double bonds of the polymer backbone, breaking the electronic conjugation and eventually the chain. Our results indicate that oxygen does not play a major role in the effect. Electronic spectra simulations were performed based on these assumptions reproducing the UV-Vis experimental results.
DFT calculations were used to evaluate the effect of side groups on the structural, optical and electronic properties of 3-substituted PPy derivatives. Simple relationships between the optoelectronic properties and Hammet parameters are proposed.
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