Linearly Acenaphthylene-Fused Pentacene Exhibiting Efficient Singlet Fission

Abstract

Acenes are poised to be highly sought after for singlet fission, which is among the most promising strategies for scaling solar power conversion efficiency beyond Shockley–Queisser limit. One critical problem associated with acenes is their low stability. Incorporation of a 5-membered ring into acenes is reported to stabilize acenes. However, cyclopentannulation on acenes at the peri-positions also transforms the electronic structure of acenes and endows them with nontypical photophysical properties, thus preventing them from undergoing singlet fission. Here, we report a new strategy to stabilize and simultaneously retain the photophysical properties of the acenes through linearly fusing acenaphthylene containing a 5-membered ring. The acenaphthylene-fused pentacenes exhibited UV-Vis absorption and fluorescence spectra with distinctively red-shifted absorption and emission bands, notably along with a substantially longer fluorescence lifetime, while still retaining the characteristic spectral features of pentacene. Despite possessing narrower HOMO-LUMO energy gaps and elevated HOMO energy levels, the elongated acenes exhibited stability comparable to that of their pentacene analogues. X-ray crystallography revealed a slip-stack columnar array in these molecules, a packing motif that differs significantly from the herringbone packing typically seen in pentacene. Strong vibronic coupling—the interrelation of electronic and vibrational motions—and favorable excited-state energetics drive an efficient, ultrafast singlet fission in thin films, resulting in a high triplet quantum yield of ~158%. TREPR spectroscopy confirmed the singlet fission mechanism—specifically the formation of triplet pairs—by resolving the sequence of spin-state changes and revealing the elusive quintet (⁵TT) state.