Linear response of the indirect J‐coupling alternation to the energy gap of increasing π‐conjugated oligomers

Abstract

We have investigated the dependence of the electronic energy gap of organic conjugated oligomers on the J‐coupling alternation (JCA) parameter, derived from the nuclear magnetic resonance (NMR) spectroscopy. We exploit the systematic and concerted oscillations observed for the indirect spin‐spin coupling constants (SSCCs), between two adjacent 13C nuclei (1JCC), in the π‐conjugated systems. Hence, we define a scalar JCA parameter in terms of the differences of 1JCC, which is further analyzed as a function of the HOMO‐LUMO gaps (the energy difference between the frontier molecular orbitals), as well as other properties of the studied oligomers. Using an adequate exchange‐correlation density functional approach, we show that, for all systems considered here, that is, polyacetylene (PA), poly(para‐phenylene) (PPP), polypyrrole (PPy), and polythiophene (PTh), their calculated electronic energy gaps may exhibit a linear dependence on the JCA. Our results indicate that a single‐valued NMR‐based parameter properly renders the electronic features of highly conjugated oligomers, allowing us to compare the strengths of J‐couplings in different systems. The present description yields a way to capture both the bonding features and the electronic absorption of these systems provided by the SSCCs. The JCA gives evidence for the differences in the electronic structure of distinct classes of conjugated systems, such as PA, and those that can exhibit a quinoid character, such as PPP, PPy, and PTh.