Anion Photoelectron Spectroscopy of Rubrene: Molecular Insights into Singlet Fission Energetics

Rubrene (C₄₂H₂₈, RUB) has been seen to be attractive as a promising building block for organic semiconductors. By means of gas-phase anion photoelectron spectroscopy, the adiabatic electron affinity for RUB molecules is determined to be 1.48 ± 0.03 eV, and the S₀–T₁ and S₀–S₁ transition energies of RUB are evaluated to be 1.16 ± 0.05 and 2.42 ± 0.05 eV, showing the possibility of singlet fission in terms of energy. The photoelectron spectra indicate that the vibronic coupling in RUB is similar in the neutral electronic states of S₀, T₁, and S₁. Quantum chemistry calculation results demonstrate that the vibronic coupling in these states originates from their similarly restricted structural displacement upon photoexcitation. Molecular insights into energetics suggest the important role of a charge transfer state in singlet fission.