Computational note on the chemical reactivity of pyrrole derivatives

Abstract

The nanosciences and their companion nanotechnologies are a hot topic all around the world. They promise the development of revolutionary new materials. These materials are based on nanostructures so they could be called Molecular Nanostructured Materials (MNMs). These MNMs have a lot of potential applications, and among them we could mention getting energy and a cleaner environment, creating new sources of energy by generating hydrogen and utilizing our current sources of energy more efficiently at the microscopic level. Organic heterocycles are systems of growing interest in materials science in view of the potential technological applications in fields such as electronics, photonics, sensors, or corrosion protection. The pyrrole-derived MNMs could be the starting materials for Organic Photovoltaics and for High-Performance OLEDs (Organic Light-Emitting Devices). The objective of this work is to perform a detailed calculation of the molecular structure of some pyrrole derivatives as well as of their dimers, and to predict the chemical reactivity of them using concepts derived from Density Functional Theory (or Conceptual DFT) [1] like the electronegativity, hardness, global electrophilicity and the condensed Fukui functions. These compounds have several desirable characteristics related to their use in Organic Photovoltaics and OLEDS: (i) they contain the pyrrole group, with molecular parameters similar to the thiophene derivatives that have been shown to be useful as MNMs; (ii) the p-conjugated derivatives are generally efficient fluorophores, and as such, useful for the fabrication of nanobiosensors; (iii) they can be used as an attractive building block for Organic Molecular Materials. The analysis of the chemical reactivity parameters will help to identify which of the pyrrole derivatives will be more prone to react with functionalized fullerenes in order to be able to design an Organic Photovoltaics device. The studied molecules are phenyl-(4-phenyl-1H-pyrrole-3-yl)methanone (Molecule 1) [2–4], (4-methoxy-phenyl)-(4-phenyl1H-pyrrole-3-yl)-methanone (Molecule 2) [2–4], [4-(4-methoxyphenyl)-1H-pyrrole-3-yl]-phenylmethanone (Molecule 3) [2–4], and their dimers called Molecule 1-1, Molecule 2-2 and Molecule 3-3, respectively. For this reason, we have calculated the molecular structure of the parent molecules pyrrole and anisole through the use of two different theory levels, PBE/DND and PBE/TNP. The molecular structures of the parent molecules obtained from each calculation were compared with the experimental results by aligning both structures and calculating the Root Mean-Square Deviation (RMSD). The agreement is very good: the standard error of the differences between the experimental and the calculated bond lengths and bond angles being very low for both model chemistries (an average value of 0.03). Moreover, the results for both model chemistries are the same. The conclusion is that with the PBE/ DND model chemistry it is possible to obtain accurate values for the molecular structures without the need to resort to other model chemistry with a higher computational cost. The next step in our study was to perform a conformational analysis on every one of the studied molecules in order to obtain the most stable structure at the semiempirical AM1 level. This was performed by modifying the distances angles of the moieties connected to the pyrrole unit. The resulting structures were used as the starting point for geometry optimizations with the PBE/DND model chemistry. The results for the optimized molecular structures of the six molecules are presented in Figs. 1 and 2 of the Supplementary Materials. Listings showing the values of the interatomic bond lengths and bond angles for the six studied molecules are provided in the form of Tables in the Supplementary Materials section accompanying this work.