Singlet Fission in a Pyrrole-Fused Cross-Conjugated Skeleton with Adaptive Aromaticity

Singlet fission (SF) materials hold
the potential to increase the
power conversion efficiency of solar cells by reducing the thermalization
of high-energy excited states. The major hurdle in realizing this
potential is the limited scope of SF-active materials with high fission
efficiency, suitable energy levels, and sufficient chemical stability.
Herein, using theoretical calculation and time-resolved spectroscopy,
we developed a highly stable SF material based on dipyrrolo­naphthyridine­dione
(DPND), a pyrrole-fused cross-conjugated skeleton with a distinctive
adaptive aromaticity (dual aromaticity) character. The embedded pyrrole
ring with 4n+2 π-electron features aromaticity
in the ground state, while the dipole resonance of the amide bonds
promotes a 4n π-electron Baird’s aromaticity
in the triplet state. Such an adaptive aromaticity renders the molecule
efficient for the SF process [E(S1) ≥
2E(T1)] without compromising its stability.
Up to 173% triplet yield, strong blue-green light absorption, and
suitable triplet energy of 1.2 eV, as well as excellent stability,
make DPND a promising SF sensitizer toward practical applications.