Theoretical Molecular Design of Heteroacenes for Singlet Fission: Tuning the Diradical Character by Modifying π‑Conjugation Length and Aromaticity

A theoretical molecular design for
efficient singlet fission (SF)
is performed for several heteroacene models involving nitrogen (N)
atoms based on the diradical character criterion of the energy level
matching conditions. This criterion is found to be closely related
to the relative contributions of diradical and zwitterionic resonance
structures of the heteroacenes, i.e., the aromaticity of the central
ring(s). From the analysis of the diradical characters of these heteroacene
models, the increase in the aromaticity of the central ring(s) is
found to prefer the diradical form to the zwitterionic form. From
the comparison of the excitation energies evaluated by multireference
second-order perturbation theory calculations, two promising candidates,
chosen based on the diradical character criterion, are found to satisfy
the energy level matching conditions and to possess high triplet energies
of ∼1.1 eV, which are suitable for an application in organic
photovoltaic cells. The proposed two candidates are shown to have
mutually different types of the first excited singlet states, which
are distinguished by the primary excitation configurations. These
results suggest that the proposed two candidates exhibit different
singlet fission dynamics due to the different amplitude of the electronic
coupling.