Theoretical Study on Singlet Fission in Aromatic Diaza -Indacene Dimers.

We theoretically show that diaza (N)-substitution to -indacene with 4 π-electrons, by which the number of π-electrons in N--indacene amounts to 4+2, is a new strategy to design efficient singlet fission (SF) molecules. By N-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 N-substituted positions. Furthermore, aromaticities of N--indacenes are evaluated explicitly on the basis of the magnetically induced current. Although N--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 N--indacenes as feasible SF molecules but also contribute to comprehending the interplay between aromaticity and diradical electronic structures contributing to SF.