Inclusion of quinoidal
cores in conjugated hydrocarbons is a common
strategy to modulate the properties of diradicaloids formed by aromaticity
recovery within the quinoidal unit. Here we describe an alternative
approach of tuning of diradical properties in indenoindenodibenzothiophenes
upon anti → syn isomerism
of the benzothiophene motif. This alters the relationship of the S
atom with the radical center from linear to cross conjugation yet
retains the same 2,6-naphtho conjugation pattern of the rearomatized
core. We conduct a full comparison between the anti and syn derivatives based on structural, spectroscopic,
theoretical, and magnetic measurements, showing that these systems
are stable open-shell singlet diradicaloids that only access their
triplet state at elevated temperatures.
Haley and co-workers report the synthesis and physical characterization of a family of diradicaloids on the basis of the diindenoanthracene framework. Computations suggested that altering the external fused rings on both sides of the pentacyclic core would yield systematic changes in their open-shell properties, results that have been corroborated experimentally. This structure refinement approach provides a deeper understanding on how to tune the intrinsic properties of diradicaloid compounds in a rational and predictable manner.
The
preparation of a series of dibenzo- and tetrabenzo-fused fluoreno[3,2-b]fluorenes is disclosed, and the diradicaloid properties
of these molecules are compared with those of a similar, previously
reported series of anthracene-based diradicaloids. Insights on the
diradical mode of delocalization tuning by constitutional isomerism
of the external naphthalenes has been explored by means of the physical
approach (dissection of the electronic properties in terms of electronic
repulsion and transfer integral) of diradicals. This study has also
been extended to the redox species of the two series of compounds
and found that the radical cations have the same stabilization mode
by delocalization that the neutral diradicals while the radical anions,
contrarily, are stabilized by aromatization of the central core. The
synthesis of the fluorenofluorene series and their characterization
by electronic absorption and vibrational Raman spectroscopies, X-ray
diffraction, SQUID measurements, electrochemistry, in situ UV–vis–NIR
absorption spectroelectrochemistry, and theoretical calculations are
presented. This work attempts to unify the properties of different
series of diradicaloids in a common argument as well as the properties
of the carbocations and carbanions derived from them.
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