Quinoidal Azaacenes: 99 % Diradical Character

Abstract

Quinoidal azaacenes with almost pure diradical character (y=0.95 to y=0.99) were synthesized. All compounds exhibit paramagnetic behavior investigated by EPR and NMR spectroscopy, and SQUID measurements, revealing thermally populated triplet states with an extremely low‐energy gap ΔEST′ of 0.58 to 1.0 kcal mol−1. The species are persistent in solution (half‐life≈14–21 h) and in the solid state they are stable for weeks.

Synthetic tuning of the quantum properties of open-shell radicaloids

“…This indicates that this site should be a prime target for substitution with bromine or chlorine atoms: substitution would decrease the hyperfine contribution and hinder the mesithyl rotation. This result also confirms the previous assignments on different p-delocalized paramagnetic structures with unresolved electron spin dipolar interactions, [26][27][28][29][30]34,[66][67][68][69] and is consistent with the previous observation of dipolar coupling solely in small radicaloids. 19,35 Trends reported for linear acenes show that the value of the dipolar coupling strongly decreases when increasing the length of the molecule, 70,71 and this agrees perfectly with our observations and the lack of a half-field line.…”

Visualisation of Chemical Shielding Tensors (VIST) to Elucidate Aromaticity and Antiaromaticity**

“…Moreover, the application of Clar's sextet theory [8] to control local aromaticity provides a powerful way of tuning biradical character, thus, opening the route to a range of optoelectronic applications. [9][10][11][12] A new and exciting frontier is opened in terms of macrocycles and larger π-conjugated systems, in which local and global (anti)aromaticity can both play a role. Here, (anti) aromaticity is being studied in systems as diverse as nanographenes, [13,14] porphyrin nanorings, [15,16] carbon nanobelts, [17] cyclocarbon, [18] cycloparaphenylenes, [19][20][21] cycloparaphenylmethine, [22] paracyclophanetetraene, [23,24] norcorrole [25,26] and other porphyrinoids.…”

A Trapezoidal Octacyanoquinoid Acceptor Forms Solution and Surface Products by Antiparallel Shape Fitting with Conformational Dipole Momentum Switch

“…The new 2D π‐conjugated 1 is formed by coupling of two thienoquinoidal cores in antiparallel fashion: a tetracyano quinoidal thiophene [23] ( MQ ) and a tetracyano quinoidal bithiophene [24] ( BQ ) in Figure 1, the three thiophenes surrounding a benzenoid ring (i.e., generating a trapezoidal‐like form) [25, 26] . Tetracyanoquinoidal or tetracyanoquinodimethane (TCNQ)‐based π‐extended molecules are important electroactive units in the field of organic conductors [18–24] and organic electronics [27] .…”