The n,π* States of Heteroaromatics: When are They the Lowest Excited States and in What Way Can They Be Aromatic or Antiaromatic?

Heteroaromatic molecules are found in areas ranging from biochemistry to organic electronics. We analyze the n,π* excited states of 6π-electron heteroaromatic compounds with in-plane lone-pair orbitals (n(sigma), here labelled n), and use qualitative theory and quantum chemical computations, with a start at Mandado’s 2n+1 rule for aromaticity of separate spins. After excitation of an electron from n to π*, a (4n+2)π-electron species has 2n+2 π(alpha)-electrons and 2n+1 π(beta)-electrons (or vice versa), and becomes π(alpha)-antiaromatic and π(beta)-aromatic. Yet, the antiaromatic π(alpha)- and aromatic π(beta)-components often do not cancel, leading to either aromatic or antiaromatic residuals. We focus on vertically excited triplet n,π* states (3n,π*), which are most readily analyzed, but also explore singlet n,π* states (1n,π*), and explain which compounds have n,π* states with aromatic residuals as their lowest excited states (e.g., pyrazine and the phenyl anion). Our results show that if the π(beta)-electron population becomes more (less) uniformly distributed upon excitation, the system will have an (anti-)aromatic residual. Among isomeric heteroaromatics, the isomer which has the most aromatic residual in 3n,π* is often of lowest energy in this state. Five-membered ring heteroaromatics with one or two N, O and/or S atoms never have n,π* states as their first excited states (T1 and S1), while this is nearly always the case for six-membered ring heteroaromatics with electropositive heteroatoms and/or highly symmetric (D2h) diheteroaromatics. For the complete compound set, there is a visible yet modest correlation between the (anti)aromatic character of the n,π* state and the energy difference between the lowest n,π* and π,π* states (R2 = 0.42) while the correlation is significantly stronger for monosubstituted pyrazines (R2 = 0.84).