Toward a Systematics for the Lowest Excited States of Heteroaromatics Enabled via Cyclic π-Conjugated Carbenes and Heteroelement Analogues.

Abstract

The photochemistry of heteroaromatic compounds depends on the character of their lowest electronically excited states, which are of either n,π* or π,π* type. For species with 4n + 2 π-electrons, the latter type of states can be antiaromatic to various extents according to Baird's rule and, thus, highly reactive. The n,π* type of states, on the other hand, will have an odd number of π-electrons leading to an unclear character, spanning from aromatic to antiaromatic. Six-membered ring (6-MR) heteroaromatics with in-plane lone-pair orbitals (nσ, herein n) have either n,π* or π,π* states as their lowest vertically excited states, but regular five-membered ring (5-MR) heteroaromatics with one or two N, O, and/or S atoms never have n,π* states as these states. However, this is different for cyclic π-conjugated (potentially aromatic) 5-MR carbenes that have the n orbitals at the divalent C atom. Also, 3-MR species have n,π* states as their lowest vertically excited states. Herein, we reveal which factors determine which type of vertical excited state is the lowest in energy for various heteroaromatics. The important factors are (i) the electronegativity of the heteroatom(s), (ii) the valence angle at the heteroatom impacting the lone-pair orbital energy, (iii) the number of π-orbitals and π-electrons, (iv) the degree of (anti)aromatic character of the n,π* state, (v) the electronegativity of atoms adjacent to the heteroatom, and (vi) the spatial extent of the n orbital affecting the intraorbital electron repulsion. Our findings point toward the development of a rational systematics for prediction of which heteroaromatics have n,π* as the first vertical excited states and which ones have π,π* states as these.