Comment on “Designing potentially singlet fission materials with an anti-Kasha behaviour” by R. Pino-Rios, R. Báez-Grez, D. W. Szczepanik, and M. Solá, Phys. Chem. Chem. Phys., 2024, 26, 15386

Abstract

The article Phys. Chem. Chem. Phys., 2024, 26, 15386–15392, https://doi.org/10.1039/D4CP01284D, by Pino-Rios et al. explored the scope of applying the dipole allowed S₀ → S₂ transition in azulene and its substituted analogues for singlet-fission, following the scheme: S₂ + S₀ → 2T₁. The condition for this process to be exoergic is that the net energy, Δ = S₂ − 2T₁, is positive. While TD-DFT vertical excitation energies of azulene resulted in Δ > 0, this result is an outcome of favourable error cancellations. We show that rationalizing the experimentally noted Δ > 0 requires considering adiabatic effects, without which wavefunction methods more accurate than DFT yield Δ < 0. Further, as azulene is known to exhibit triplet instability, DFT modelling should be done within the TDA formalism.