Singlet Fission in Anthracene in Solution

Photophysics of anthracene has extensively been studied, such that it serves as a classic standard for understanding photophysics of polycyclic aromatic hydrocarbons. Anthracene undergoes efficient intersystem crossing to populate the triplet state. Alternative pathways to generate triplets in solutions are unknown for unsubstituted anthracene. Singlet fission (SF), generation of two triplets from absorption of one photon, was initially reported in anthracene crystal. Here, we report the first observation of SF in anthracene in solution. We employ time-resolved emission and absorption spectroscopy to reveal that SF in anthracene involves an emissive intermediate state. Increasing concentration quenches photoluminescence lifetime, yields bimolecular quenching constant (6.5 × 10⁹ M⁻¹ s⁻¹) and interaction-distance (1.75 nm). In a concentrated solution, the singlet state (S₁) decays at 1.1 ns. Simultaneous emergence (1.1 ns) and subsequent emission decay (2.6 ns) from another lower energy, emissive state (I) is observed in time-resolved emission spectra. An iso-emissive point in time-resolved area-normalized emission spectra displays a single step population transfer from S₁ to I. Transient absorption spectra shows the singlet excited-state absorption at 385-400 nm decays with 1.1 ns. A narrow, long-lived excited-state absorption band rises with 2.6 ns, concomitant with the emission decay of I, ascertaining I to be an intermediate. Subsequently, this band decays with >40 μs, corroborating free triplets. A triplet quantum yield of ~119±14% for concentrated solution substantiates the process to be SF.