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

12 December 2023, Version 2
This content is a preprint and has not undergone peer review at the time of posting.


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).


Excited state aromaticity
Mandado's rule
npi* states

Supplementary materials

Supporting information of The lowest nπ* states of heteroaromatics: ...
Descriptions of the computational methods used, tabulated aromaticity data from MCI, HOMA, EDDB and MICD calculations, and data from further analyses.


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