Not fully twisted, not fully planar, but intermediate torsions for ideal chromophore design: A computational study on p-phenylene bridged pyridinium phenolate betaines

This contribution reports extensive studies on structure–property correlations of two isoelectronic betaine metamers (positional isomers). These zwitterions differ from each other with respect to bonding modes of pyridinium acceptors (Reichardt's mode: through N-atom versus Brooker's mode: through C-atom) with the p-phenylene bridged phenolate donors. Various quantum chemical methodologies are used in this investigation, with time-dependent (TD) and coupled perturbed (CPHF) theories for computations of many molecular response properties. Analysis of first hyperpolarizabilities (β) indicates that Reichardt's metamer (ωB97xD: β = 349.5 × 10⁻³⁰ esu) is more efficient chromophore (~5-fold enhanced) than Brooker's metamer (ωB97xD: β = 69.4 × 10⁻³⁰ esu). The gyratory abilities of the bridge junctions resulted in a cohort of metastable conformations, in the rotational potential energy surfaces (PES) of the two metamers. Moreover, rotational PES establishes that intermediary torsions are suitable for optimal chromophore design strategies (Reichardt's metamer: β = 956.5 × 10⁻³⁰ esu, and Brooker's metamer: β = 1104.7 × 10⁻³⁰ esu), Thus suitable conformational manipulations, can be used to obtain more efficient zwitterionic molecular chromophores. Compared unbridged prototype molecules, p-phenylene bridged zwitterions showed ~6–9 times enhanced values of β. In addition, important aspects of suitable chromophore design strategies are suggested.