The Lowest Excited State of Heptacenes Is Dark

Understanding the electronic structure of polycyclic aromatic compounds is of fundamental importance for their potential applications. The optoelectronic properties of shorter acenes such as tetracene and pentacene have been extensively studied with regard to excitation, emission, and nonlinear effects such as singlet fission. The longer homologues present a unique challenge due to their low stability both in the solid state and in solution. In this work, we synthesized persistent 6,8,15,17-tetrakis((triisopropylsilyl)ethynyl)heptacene and investigated its photophysical properties as well as those of the parent heptacene. Our steady-state electronic absorption and emission experiments combined with transient absorption spectroscopy show that the Franck-Condon electric-dipole-forbidden ("dark") transition to the 2¹A_g singlet state is the lowest-energy excited state of heptacene. This contrasts with the optical properties of the well-known shorter acenes. Transient absorption data further suggest singlet fission or intersystem crossing as potential pathways to rapid population of the triplet state facilitated by the dark singlet state.