Modern zethrene chemistry

Abstract

This paper gives a brief summary of zethrene chemistry after the pioneering works by Clar, Mitchell, Sondheimer, and Staab et al. In the past a few years, various synthetic methods and stabilizing strategies have been developed, which have made the synthesis of stable, vertically and laterally extended zethrenes practically achievable. One of the most important discoveries for this type of molecule is their open-shell diradical character, which is the origin of their unique optical, electronic, and magnetic properties. The fundamental structure-diradical character-physical property relationships were investigated, and these studies form the basis for the tailored design of more general open-shell singlet diradicaloids with controllable diradical character and physical properties.Key words: zethrene, polycyclic aromatic hydrocarbons, diradicaloid, polyradical.Résumé : Cet article présente un résumé de la chimie du zéthrène fondé sur les premiers travaux exploratoires de Clar, Mitchell, Sondheimer et Staab, ainsi que leurs collaborateurs. Au cours des dernières années, diverses méthodes de synthèse et stratégies de stabilisations ont été mises au point, rendant possible du point de vue pratique la synthèse de zéthrènes stables étendus sur les plans vertical et latéral. L'une des découvertes les plus importantes concernant ce type de molécules consiste en leur caractère de diradical à couches ouvertes, qui est à l'origine de leurs propriétés optiques, électroniques et magnétiques particulières. Nous avons étudié les relations entre la structure fondamentale, le caractère diradicalaire et les propriétés physiques. Ces études forment la base d'une conception généralisée aux diradicaloïdes singulets à couches ouvertes dont le caractère diradicalaire et les propriétés physiques seraient réglables. [Traduit par la Rédaction]

Learning how to fine‐tune diradical properties by structure refinement

“…As you increase the size of the acene core (point (i) above; computational numbers shown here) from benzene ( 7 , y = 0.24, Δ E ST = −19.4 kcal mol −1 ) to naphthalene ( 8 , y = 0.49, Δ E ST = −10.3 kcal mol −1 ) up to anthracene ( 9 , y = 0.62, Δ E ST = −4.9 kcal mol −1 ), the diradical character of the molecule increases to the point of being experimentally observable and the singlet‐triplet energy gap decreases to where it can be determined by SQUID. Notably, these results match other groups studies on altering the conjugation distance between the two distinct radical centers in other PCH diradicals such as zethrenes, [ 12a ] bisphenalenyls, [ 4a ] and the indeno[2,1‐ b ]fluorene analogues. [ 17,18b ] While this trend of increased core size leads to enhanced diradical character is a fundamentally important steppingstone, it only provides the ability to perform a “course shim” on the overall properties of the diradicals.…”

“…Fueled by a fundamental interest in their unique ground and magnetic states and a desire to better understand these fickle compounds, there have been many classes of carbon‐based diradicaloids (Figure 2) developed/explored in the last 12–15 years, including but not limited to zethrenes ( 2 ), [ 7a,12 ] anthenes ( 3 ), [ 13 ] bisphenalenyls ( 4 ), [ 14 ] extended quinodimethanes ( 5 ), [ 15 ] higher‐order acenes ( 6 ), [ 16 ] indenofluorenes ( 7 ), [ 10a,17 ] and related diindenoacenes ( 8 – 10 ), [ 1,11,18 ] the last of which are the focus of this mini‐review (see Figure 3). The diagnostic properties of diradicals, which arise from the unique combination of closed‐ and open‐shell resonance forms, can include narrow HOMO‐LUMO energy gaps, [ 7c,19 ] low‐energy electronic absorptions due a low‐lying doubly excited electronic configuration, [ 20 ] redox amphoterism, [ 14c,19a ] electron spin resonance (ESR) signals and/or a magnetic response in a superconducting quantum interference device (SQUID) magnetometer, [ 18d ] multicenter bonding, [ 14a,21 ] substantial two‐photon absorption cross‐sections, [ 22 ] and peak broadening or sharpening in the proton NMR spectrum due to the thermal equilibrium of the spin states.…”

Computationally aided design of a high-performance organic semiconductor: the development of a universal crystal engineering core

“…Nearly twenty years ago, we introduced silylethyne substitution as a simple method to increase stability, improve solubility, and induce p-stacking in aromatic molecules. 8,9 This functionalization strategy has been applied to acenes, [10][11][12][13][14][15][16] heteroacenes, [17][18][19][20][21][22][23][24][25] indeno-uorenes, 26 benzo-thiadiazoles, 27 zethrene, 28 and others, [29][30][31][32] typically via a simple alkynylation sequence, 33 to yield new materials for transistor, [34][35][36][37][38] photovoltaic, [39][40][41][42][43][44] imaging, 45 lightemitting, 46,47 and other applications. [48][49][50] While this approach is relatively general in scope, it cannot be applied to a number of promising chromophores due to difficulties in preparing the necessary precursors, or to the delicacy of certain chromophores that preclude ethynylation.…”

Synthesis and Characterization of Dibenzo[<i>hi,st</i>]ovalene as a Highly Fluorescent Polycyclic Aromatic Hydrocarbon and Its π-Extension to Circumpyrene

“…For example, Kubo and his coworkers reported the syntheses of teranthene 20 and quarteranthene, 21 revealing their open shell ground state (Figure 3). On the other hand, Wu s group pioneered the syntheses of zethrenes with open shell characters ( Figure 2) 22 and demonstrated a synthesis of a laterally extended heptazethrene in 2016. 23 In 2018, the long awaited peritetracene was achieved independently by Feng et al 24 and Wu et al, 25 exhibiting a moderate biradical character and a small energy gap (1.1 eV).…”