A theoretical understanding of the energy difference between singlet and triplet states of oligoacene molecules

The origin of singlet–triplet energy difference, Δ E ST , in π‐conjugated molecules was elucidated by carrying out ab initio calculations of acene molecules at scaled opposite‐spin (SOS)‐CIS(D 0 ) level of theory using aug‐cc‐pVDZ basis sets. Both singlet and triplet excitation energies decrease monotonously as π‐conjugation expands. However, Δ E ST is found to evolve in rather complicated manner; it increases in going from benzene to anthracene and diminishes thereafter. To better understand this behavior, excitation energy decomposition at CIS level of theory was conducted, and the variations of Coulomb‐ and exchange‐type integrals between the excited and remnant electrons as a function of π‐conjugation length were discussed. The effects of dynamic electron correlation on both singlet and triplet excitation energies and Δ E ST were also discussed.