For PLY-3N (Fig. ), as the electron-withdrawing nature of the nitrogen increases (i.e., with more negative ε N ), both bright and dark states move first to the blue (ε N going from 0 to -2 eV), and then red-shift for -6 eV < ε N < -3 eV, with the dark 2 A ′′ 2 doublet lying below the bright 2 E ′′ state for ε N = -6 eV. In panel b, the absolute value of the ground state expectation value of the particle-hole difference operator first decreases, reaching a minimum for ε N = -2 eV, and then increases for further stabilization of the aza nitrogen orbital. Indeed, when ε N = -2 eV the term in the square bracket in Eq. 10 is equal to 11.5 eV in the case of nitrogen atoms and to 11.26 eV for carbon atoms, thus making minimal the particle-hole symmetry breaking as well as the oscillator strength of the 2 E ′′ ← 2 A ′′ 1 transition (see panel c). Moving away from ε N = -2 eV, particle-hole symmetry is increasingly broken, |⟨ D⟩| increases, and the oscillator strength of the 2 E ′′ ← 2 A ′′ 1 transition blows up.