Photochemistry Driven by Excited‐State Aromaticity Gain or Antiaromaticity Relief

Abstract

Gain of aromaticity or relief of antiaromaticity along a reaction path are important factors to consider in mechanism studies. Analysis of such changes along potential energy surfaces has historically focused on reactions in the electronic ground state (S 0 ), but can also be used for excited states. In the lowest ππ* states, the electron counts for aromaticity and antiaromaticity follow Baird's rule where 4n π-electrons indicate aromaticity and 4n + 2 π-electrons antiaromaticity. Yet, there are also cases where Hückel's rule plays a role in the excited state. The electron count reversals of Baird's rule compared to Hückel's rule explain many altered physicochemical properties upon excitation of (hetero)annulene derivatives. Here we illustrate how the gain of excited-state aromaticity (ESA) and relief of excited-state antiaromaticity (ESAA) have an impact on photoreactivity and photostability. Emphasis is placed on recent findings supported by the results of quantum chemical calculations, and photoreactions in a wide variety of areas are covered.

Excited-State Aromaticity Reversals in Naphthalene and Anthracene

“…A molecule that is aromatic or antiaromatic in its electronic ground state can experience a reversal of aromaticity upon transition to a ππ* excited electronic state; this reversal is accompanied by profound changes in its electronic structure and properties. Behavior of this type has been aptly named the molecular analogue of Robert Louis Stevenson’s “Dr Jekyll and Mr Hyde”, and it has numerous applications, which include designing molecules with light-controllable behavior, for example, molecular photoswitches, molecular motors, , “flapping” flurorophores, − and rationalizing experimental evidence about photochemical reactions such as excited-state intramolecular proton transfers. , Further applications of excited-state aromaticity reversals have been discussed in a recent review …”

“…8,9 Further applications of excited-state aromaticity reversals have been discussed in a recent review. 10 Excited-state aromaticity is usually associated with Baird's rule, 11 according to which the familiar Huckel 4n + 2 and 4n rules for electronic ground-state aromaticity in cyclic conjugated hydrocarbons are reversed in the lowest triplet state: rings with 4n π electrons switch from antiaromatic to aromatic, while those with 4n + 2 π electrons switch from aromatic to antiaromatic. Similar aromaticity reversals have been shown to take place in the lowest singlet excited state.…”

Remote Electrostatic Repulsion Trigged by Excited State Antiaromaticity Relief as Origin of Photoacidity in an Organic Dye

“…systems with 4n+2 π-electrons are antiaromatic in their lowest ππ* triplet and singlet excited states. [34][35][36] Excited-state aromaticity and antiaromaticity have been used to explain several photophysical and photochemical processes, [37][38][39][40][41][42][43][44] including photoacidity. For instance, the different 𝛥𝑝𝐾 𝑎 values of photoacids derived from naphthalenes could be correlated to their degree of ESAA relief when going from the acid to the conjugate base.…”

Concealed antiaromaticity

“…[1][2][3][4] These rules of ground-state aromaticity are reversed in the lowest singlet and triplet excited states, where molecules with a cyclic delocalized system of 4n π-electrons are expected to be aromatic and with [4n+2] π-electrons to be antiaromatic according to what is known as Baird's rule. [5][6][7][8][9][10] In all cases, the π-electron system has to be planar or close to planar for the π-electron delocalization to occur. While the π-electron delocalization results in energetic stabilization in the case of aromatic molecules (compared to an acyclic structural analogue), it leads to destabilization in the case of antiaromatic molecules.…”