Excited‐state aromaticity and antiaromaticity special issue

About 50 years ago, in 1972, the seminal paper by Professor N. Colin Baird on the aromaticity and antiaromaticity of the lowest lying ππ* triplet states of annulenes appeared in J. Am. Chem. Soc.[1] According to the theoretical work laid forth by him, annulenes with 4n π-electrons are aromatic and those with 4n + 2 are antiaromatic in their lowest lying ππ* triplet states. A few years earlier, in 1966, Professors Michael J. S. Dewar and Howard E. Zimmerman independently investigated pericyclic reactions,[2] both thermal and photochemical ones, and concluded that allowed (forbidden) reactions of this type proceed via aromatic (antiaromatic) transition states. They reasoned that the electron counts for aromaticity and antiaromaticity were opposite in the lowest excited state when compared to the ground state. Even though Baird in 1972 placed the concept on a more solid theoretical foundation and applied it to excited-state minima, for many years, the electron counting rules for aromaticity and antiaromaticity in the lowest excited states were exclusively applied to pericyclic photoreactions. Indeed, it was not until the mid-1980s that the concept, through the seminal experimental work of Professor Peter Wan,[3] escaped from the "pericyclic confinement" and started to become applied more broadly to organic photochemistry. Then, in the late 1990s the concept received growing attention within the theoretical and computational chemistry community. Today, there is no doubt that many photophysical and photochemical processes are driven by the gain of aromaticity or release of antiaromaticity in the lowest lying ππ* triplet (or singlet) excited states. Together with the Editors of the Journal of Physical Organic Chemistry, we decided to celebrate the progress in the last 50 years of Baird's rule and the beginning of the next 50 and to publish a special issue devoted to excited-state aromaticity and antiaromaticity. In the next 50 years, we hope that the chemistry community will pay close attention to Baird's rule and that it will find applications in a wide range of areas. For this reason, and to provide an overview of the present status of this field, we invited renowned and emerging scientists within the community to contribute, and we gathered 14 articles that illustrate various aspects of the concepts, both fundamental and applied. Five papers in this issue are devoted to the excited-state (anti)aromaticity of polycyclic conjugated hydrocarbons. The honoree of this issue, Professor Colin Baird,[4] proves that the spin densities of the triplet states of polybenzenoid hydrocarbons closely mimic those expected for the free radicals that combined produce a given polybenzenoid hydrocarbon. Also addressing triplet-state polybenzenoid hydrocarbons, Jorner[5] uses graph theory to develop the magnetically based superaromatic stabilization energy as a local aromaticity index. Govorov et al.[6] explore the triplet states of corannulene and coronene and make observations on their antiaromaticity and potentials for photochemical applications. Looking at the dication and dianion of corannulene, Aleksić et al.[7] find that the dianion in its triplet state has a Baird-aromatic rim. Fite et al.[8] report on a text-based machine-learning approach for polybenzenoid hydrocarbons and reveal new interesting structure–property relationships. Two papers address intermolecular/through-space interactions where (anti)aromaticity plays important roles. In their minireview, Krishnan et al.[9] discuss the aromatic character of singlet-state excimers and through-space aromaticity of triplet-state excimers of [n]acenes. Nishiuchi et al.[10] discuss the concept of stacked antiaromaticity created in the π-congested space between the two central aromatic rings in an anthracene dimer (a benzannelated anthracenophane) during the dimerization process. Two of the papers are focused on computational explorations of organic photoreactivity. Halder et al.[11] study the photoenolization of aromatic ketones and aldehydes with an ortho-alkyl group which proceeds on the triplet-state surface. In a mechanistic study, Nitu and Crespi[12] explore two competing triplet-state pathways for the reduction and solvolysis of variously substituted chlorobenzenes, where the substituents help alleviate the excited-state antiaromaticity. Photophysical applications of excited-state aromaticity are the topic of two contributions. In their perspective paper, Martin et al.[13] provide a comprehensive overview of recent progress in utilizing Baird-aromatic cyclooctatetraene side-groups as triplet-state quenchers to improve the performance of fluorophores for microscopy and imaging applications. Furthermore, in a minireview, Zeng et al.[14] describe photostable Cibalackrot-type singlet fission chromophores that benefit from Hückel aromaticity in both ground and excited states, whereby the aromatic units ensure high photostability. Towards the end of the special issue, three papers with a focus on fundamental aromaticity aspects in excited states are presented. You et al.[15] report on osmapentalene and osmapyridinium complexes with carbone ligands which are aromatic in both the lowest lying singlet and triplet states, a phenomenon labeled as adaptive aromaticity. The paper by Escayola et al.[16] describes the double Hückel and Baird aromaticity of the triplet state of C7Br7+3, which features an inner Hückel-aromatic tropylium ring and an outer weak Baird-aromatic Br7 ring. Finally, Preethalayam et al.[17] analyze the extent of Baird aromaticity in the T1 states of BN/CC cyclooctatetraene isosteres, revealing large differences between the magnetic aspect of aromaticity (response aromaticity) and the electronic and energetic aspects (intrinsic aromaticity). As guest editors, we are very grateful to all the authors who have contributed to this special issue and we consider that the resulting collection of articles provides a good account of the current state-of-the-art within research on excited-state aromaticity and antiaromaticity. We hope that the papers published in this issue will be a source of inspiration for many and a start of the next 50 years of excited state aromaticity and antiaromaticity research.