We
theoretically show that diaza (N2)-substitution to s-indacene with 4n π-electrons, by
which the number of π-electrons in N2-s-indacene amounts to 4n+2, is a new strategy to
design efficient singlet fission (SF) molecules. By N2-substitution,
the diradical character and the exchange integral are found to be
tuned moderately, leading to satisfying the excitation energy level
matching condition for SF with a large triplet excitation energy.
On the basis of the effective electronic coupling related to the SF
rate, we explore the optimal slip-stack dimer packings for fast SF.
Their underlying mechanisms are well understood from the odd-electron
density, resonance structure, and frontier orbital distribution, as
the functions of the N2-substituted positions. Furthermore,
aromaticities of N2-s-indacenes are evaluated
explicitly on the basis of the magnetically induced current. Although
N2-s-indacenes display strengths of aromaticities
similar to that of anthracene, a local decrease in aromaticity is
found to correlate to the spatial feature of diradical character,
i.e., odd-electron density. The present findings not only newly propose
N2-s-indacenes as feasible SF molecules
but also contribute to comprehending the interplay between aromaticity
and diradical electronic structures contributing to SF.
The effect of applying a static electric field on the singlet fission dynamics was investigated using the quantum master equation method. The singlet fission dynamics of pentacene dimer models was greatly accelerated by applying a static electric field.
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