On the microstructures of the bulk of P3HT amorphous films obtained from two protocols: Insights from molecular dynamics simulations.

An influential factor for the microstructure of semiconductor polymer films is the solvent chosen for their processing and subsequent performance as an active layer in electronic and optical devices. In this paper, we address the solvent-polymer interplays by reporting a comparison between the microstructure of the bulk of poly(3-hexylthiophene) (P3HT) films formed from two different simulation methodologies: using the o-dichlorobenzene (ODCB) as an aggregation chain parameter and a solvent-free protocol. In comparison to the length of an ideal planar chain, the solvent caused an average reduction of 12% of oligomer chains. Whereas, in the other film, the reduction was around 30%. Furthermore, the solvent allowed pathways of π-interactions due to the cohesional-like distribution of the chains, indicating that the solvent also induces chain orientation affecting its microstructure.

Structural dependence of MEH-PPV chromism in solution.

The chromism observed in the MEH-PPV polymer in tetrahydrofuran (THF) solution is discussed as a function of the structural morphology of the backbone chains. To evaluate this phenomenon, we carried out simulations employing a hybrid methodology using molecular dynamics and quantum mechanical approaches. Our results support the hypothesis that the morphological order-disorder transition is related to the change from red to blue phase observed experimentally. The morphological disorder is associated with total or partial twisted arrangements in the polymer backbone, which induces an electronic conjugation length more confined to shorter segments. In addition, the main band of the MEH-PPV UV-Vis spectrum at the lower wavelength is related to the blue phase, in contrast to the red phase found for the more planar backbone chains.

Effect of Blue Light on the Electronic and Structural Properties of Bilirubin Isomers: Insights into the Photoisomerization and Photooxidation Processes.

The central process of neonatal phototherapy by employing blue light has been attributed to the configurational conversion of (4Z,15Z)-bilirubin to (4Z,15E). Indeed, photoisomerization is the early photochemical event during this procedure. However, in this paper, we show that the bilirubin solutions under continuous blue light exposure undergo a photooxidation process. To ascertain the role of this photodegradation in the phototherapy, we evaluated UV­visible absorption spectra obtained from bilirubin solutions in CHCl3, milli-Q water, and physiological saline, as well as FTIR spectroscopy for bilirubin in CHCl3. These analyses also showed that the first 2 h of phototherapy are the most relevant period. In addition, quantum molecular modeling using B3LYP/6-31G(d,p) and ZINDO/S-CIS was performed to evaluate the electronic and structural properties of four bilirubin isomers, showing that the (4Z,15E)-bilirubin isomer is the most polar configuration. Therefore, it can be more soluble in aqueous environments than the other configurations. This clarifies why this is the faster isomer excreted during the phototherapy.

Polarized emission from stretched PPV films viewed at the molecular level.

We present a study on the photoluminescence (PL) of thin films of poly-(p-phenylene vinylene) (PPV), non-stretched and uniaxially stretched. The experimental study was carried out using linear polarized light as the excitation beam, oriented either parallel or perpendicular to the stretch axis (S). The results showed that when the excitation light source has polarization perpendicularly oriented to the stretch direction, the emitted PL presents maximum intensity in the orientation S, and a minimum in the direction orthogonal to S. In order to understand this interesting phenomenon, we employ theoretical simulations at the atomistic level. We use classical molecular dynamics to simulate amorphous PPV films, non-stretched and stretched, from which we find a tendency of overall alignment of PV units to S, and of local clustering in herring-bone and π-stacking partial symmetries. Our study of optical activity of these kinds of clusters, performed through a quantum semi-empirical method, allows us to explain this polarization conversion behavior, and indicates the possibility of using underivatized PPV as the active layer for polarized electroluminescent devices.