Photophysics of phenalenone: quantum-mechanical investigation of singlet–triplet intersystem crossing

Abstract

We have examined the electronic and molecular structure of 1H-phenalen-1-one (phenalenone) in the electronic ground state and in the lowest excited states, as well as intersystem crossing. The electronic structure was calculated using a combination of density functional theory and multi-reference configuration interaction. Intersystem crossing rates were determined using Fermi’s golden rule and taking direct and vibronic spin–orbit coupling into account. The required spin–orbit matrix elements were obtained applying a non-empirical spin–orbit mean-field approximation. Our calculated electronic energies are in good agreement with experimental data. We find the lowest excited singlet states to be of the nπ* (S₁) and ππ* (S₂) type. Energetically accessible from S₁ are two triplet states of the ππ* (T₁) and nπ* (T₂) type, the latter being nearly degenerate to S₁. This ordering of states is retained when the molecular structure in the electronically excited states is relaxed. We expect very efficient intersystem crossing between S₁ and T₁. Our calculated intersystem crossing rate is ≈2 × 10¹⁰ s⁻¹, which is in excellent agreement with the experimental value of 3.45 × 10¹⁰ s⁻¹. Our estimated phosphorescence and fluorescence rates are many orders of magnitude smaller. Our results are in agreement with the experimentally observed behavior of phenalenone, including the high efficiency of ¹O₂ production.