Response Equation Based Thermochemical Analysis of Singlet Bipolaron Structures in Oligo(3-Methyl-Thiophenes)

A thermochemical investigation of singlet dication (bipolaron) structures in a set of 3-methyl-thiophene oligomers (up to the length of 24-mers) was carried out. Equlibrium structures and heats of formation were calculated with the semiempirical quantum chemical method RHF/PM3 for the doubly oxidized forms. All-cisoid planar structural variants of the form An1Qn2An3 were considered, where A and Q are thiophene monomer units in aromatic and quinoidal chain segments, respectively. The types of segments were determined from their bond-length pattern. At this level of theory, diverse structures combining both types of segments were found stable. The so-called “response equation based quantitative structure-property relationship” (REQ-QSPR) method, recently proposed by Fishtik et al. was used to perform and interpret regression data analysis. The REQ-based approach rationalizes mathematical treatment of stoichiometric equations. It provides a simple concept for breaking down a dataset for heat of formation into linear combination of terms that are easy to interpret in the framework of chemical groups. REQ-QSPR includes arbitrary structural descriptors rather than being restricted to using only the number of chemical constituents. Applied to our system, it is demonstrated that modelling nonlinear effects becomes possible as well. Supra-linear dependence of the energy with respect to the bipolaron localization length is indicated by REQ-QSPR, and detailed analysis suggests specific further calculations to carry out for improving the model.