On the second-order polarizability of conjugated pi-electron molecules with octupolar symmetry: the case of triaminotrinitrobenzene

The sum-over-states approach to the determination of the relationship between quadratic nonlinear optical properties and molecular structure provides for the identification of the pi-electron excited states that greatly influence the off-resonant quadratic linear response. Three double-degenerate pi-electron excited states, which include states located at 4.64, 5.26 and 6.64 eV above the ground state, are primary contributors.