Organic diradicals
play an important role in many fields of chemistry,
biochemistry, and materials science. In this work, by means of high-level
theoretical calculations, we have investigated the effect of representative
chemical substituents inp-quinodimethane (pQDM) and Thiele’s hydrocarbons with respect to the
singlet–triplet energy gap, a feature characterizing their
diradical character. We show how the nature of the substituents has
a very important effect in controlling the singlet–triplet
energy gap so that several compounds show diradical features in their
ground electronic state. Importantly, steric effects appear to play
the most determinant role forpQDM analogues, with
minor effects of the substituents in the central ring. For Thiele
like compounds, we found that electron-withdrawing groups in the central
ring favor the quinoidal form with a low or almost null diradical
character, whereas electron-donating group substituents favor the
aromatic-diradical form if the electron donation does not exceed 6-π
electrons. In this case, if there is an excess of electron donation,
the diradical character is reduced. The electronic spectrum of these
compounds is also calculated, and we predict that the most intense
bands occur in the visible region, although in some cases characteristic
electronic transition in the near-IR region may appear.