Singlet fission in anthracene in concentrated solution: excimer formation as a competing pathway

The photophysics of anthracene has extensively been studied, and thus, it serves as a classic standard for understanding the 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), which involves the generation of two triplet excitons from the absorption of one photon, was initially reported in an anthracene crystal. Here, we report the first observation of SF in anthracene in a concentrated solution. We employ time-resolved emission and absorption spectroscopy to reveal that photoexcitation leads to the formation of an emissive excimer that competes with triplet generation via singlet fission. Increasing the concentration quenches the photoluminescence lifetime, yielding a bimolecular quenching constant of 6.5 × 10⁹ M^-1 s^-1 and an interaction-distance of 1.75 nm. In the concentrated solution, the singlet state (S₁) decays at 1.1 ns. Simultaneous emergence (1.1 ns) and subsequent emission decay (2.6 ns) of a lower-energy excimer are observed in the time-resolved emission spectra. Transient absorption measurements show that the singlet excited-state absorption (385-400 nm) decays with a lifetime of 1.1 ns in the concentrated solution, while the triplet excited-state absorption increases simultaneously with the same 1.1 ns time persisting for >40 µs, corroborating the presence of free triplets. The triplet quantum yield for the concentrated solution that was greater than that obtained via ISC in the dilute solution, determined from ns transient absorption measurements, establishes SF as the underlying mechanism. These findings reveal that excimer formation competes with triplet generation via singlet fission in concentrated anthracene solutions.