Pseudo-Jahn–Teller Effect in Natural Compounds and Its Possible Role in Straintronics I: Hypericin and Its Analogs

The distortions and instability of high-symmetry configurations of polyatomic systems in nondegenerate states are usually ascribed to the pseudo-Jahn-Teller effect (PJTE). The geometries of hypericin, isohypericin, and fringelite D were optimized within various symmetry groups. Group-theoretical treatment and (TD-)DFT calculations were used to identify the corresponding electronic states during the symmetry descent. The symmetry descent paths (up to the stable structures without imaginary vibrations) were determined using the corresponding imaginary vibrations as their kernel subgroups starting from the highest possible symmetry group. The vibronic interaction between the ground and excited electronic states relates to an increasing energy difference of both states during the symmetry decrease. This criterion was used to identify possible PJTE. We have shown that the PJTE in these naturally occurring compounds could explain only the symmetry descent paths C_2v → C₂ and C_2v → C_s in hypericin, and the D_2h → C_2v, D_2h → C_2v → C₂, and D_2h → C_2h ones in fringelite D. The electric dipole moments of hypericin and its analogs were determined prevailingly by the mutual orientations of the hydroxyl groups. The same held for the energies of frontier orbitals in these systems, but their changes during the symmetry descent were less significant.