On the role of high-lying correlated triplet pair states for singlet fission in TIPS-pentacene.

Electronic structure calculations at the combined density functional theory and multi-reference configuration interaction (DFT/MRCI) level of theory are reported for dimer and trimer pentacene complexes representative of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) in a brickwork polymorph. In addition to the low-lying correlated triplet pair (CTP) states, high-energetic CTP states are reported which lie above the bright exciton state and feature non-negligible oscillator strength. Using a vibronic coupling Hamiltonian in conjunction with multiconfigurational quantum dynamical calculations, it is shown that despite the ultrafast decay towards the low-lying CTP manifold, these high-lying CTP states exhibit residual occupancies on a picosecond time scale. These findings support the interpretation given in [Grieco et al., J. Phys. Chem. C 2018, 122, 2012] regarding both prompt and persistent mid-IR absorption features following transient absorption of the bright exciton in crystalline TIPS-pentacene films.