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

Abstract

Singlet fission (SF) materials hold the potential to raise power conversion efficiency of solar cell by reducing thermalization of high-energy excited-states. The major hurdles 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 dual aromaticity character. The embedded pyrrole ring with 4n+2 π-electron features aromaticity in the ground state, while the dipole resonance of amide bonds promotes a 4n π-electron Baird’s aromaticity in the triplet state. Such a dual aromaticity renders the molecule efficient SF process [E(S₁) > 2×E(T₁)] without compromise of its stability. Up to 173% triplet yield, strong blue-green light absorption, suitable triplet energy of 1.2 eV, as well as excellent stability, make DPND a promising SF sensitizer towards practical application.