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 photovoltaics. We analyze the n ,π* excited states of 6π-electron heteroaromatics with in-plane lone pairs ( n σ , herein n ) and use qualitative theory and quantum chemical computations, starting at Mandado's 2 n + 1 rule for aromaticity of separate spins. After excitation of an electron from n to π*, a (4 n + 2)π-electron species has 2 n + 2 π α -electrons and 2 n + 1 π β -electrons (or vice versa ) and becomes π α -antiaromatic and π β -aromatic. Yet, the antiaromatic π α - and aromatic π β -components seldom cancel, leading to residuals with aromatic or antiaromatic character. We explore vertically excited triplet n ,π* states ( 3 n ,π*), which are most readily analyzed, but also singlet n ,π* states ( 1 n ,π*), and explain which compounds have n ,π* states with aromatic residuals as their lowest excited states (e.g., pyrazine and the phenyl anion). If the π β -electron population becomes more (less) uniformly distributed upon excitation, the system will have an (anti)aromatic residual. Among isomers, the one that has the most aromatic residual in 3 n ,π* is often of the 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 (T 1 and S 1 ), while this is nearly always the case for six-membered ring heteroaromatics with electropositive heteroatoms and/or highly symmetric ( D 2 h ) diheteroaromatics. For the complete compound set, there is a modest correlation between the (anti)aromatic character of the n ,π* state and the energy gap between the lowest n ,π* and π,π* states ( R 2 = 0.42), while it is stronger for monosubstituted pyrazines ( R 2 = 0.84).