The lowest nπ* states of heteroaromatics: When and in what way are they aromatic or antiaromatic?

Heteroaromatic molecules are ubiquitous and found in many areas of chemistry, ranging from biochemistry to organic electronics. Herein we analyse the nπ* excited states of (4n+2)π-electron heteroaromatic compounds that have in-plane lone-pair orbitals, using both qualitative theory and quantum chemical computations. The starting point of our analysis is 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 will have 2n+2 π(alpha)-electrons and 2n+1 π(beta)-electrons (or vice versa), and thus would be π(alpha)-antiaromatic and π(beta)-aromatic. We ask, does this lead to a nonaromatic nπ* state? We show that the situation is complex as the antiaromatic π(alpha)- and the aromatic π(beta)-components often do not cancel, leading to residuals which either lean towards aromaticity or antiaromaticity. Focus is placed on the vertically excited nπ* states with triplet multiplicity as they are most readily analysed, yet we also explore singlet nπ* states. Pyrazine and the phenyl anion are examples of molecules with residuals in their nπ* states which are markedly aromatic. We seek and provide qualitative explanations as to which compounds have nπ* states with residuals which are aromatic in character, and which ones are antiaromatic. Our results show that if the π(beta)-electron population becomes more uniformly distributed in the excitation, the system will have an aromatic residual and vice versa. For isomeric species, the isomer with the most aromatic residual in triplet nπ* is often of lowest relative energy in this state. Finally, we connect our findings to the recently observed adaptive aromaticity phenomenon, especially found in some metallaaromatics, and show that it can be understood with the general theoretical framework described herein.