Intersystem crossing and intramolecular triplet excitation energy transfer in spiro[9,10-dihydro-9-oxoanthracene-10,2'-5',6'-benzindan] investigated by DFT/MRCI methods

Recent experimental studies of a spiro-linked anthracenone (A)-naphthalene (N) compound (AN) in hutyronitrile (BuCN) solution (Dohkowski et al., J. Phys. Chem. A 2019, 123, 6978) proposed an excited-state energy dissipation pathway [[.sup.1]nn* (N) + [.sup.1][pi][pi]* (A)][??][.sup.1]n[pi]* (A)[??][.sup.3]n[pi]* (A)[.sup.1][pi][pi]* (N). However, a detailed theoretical study employing comhined density functional theory and multireference configuration interaction methods, performed in the present work, suggests that the photoexcitation decay follows a different pathway. In BuCN solution, the intersystem crossing (ISC) follows the well-established El-Sayed rule and involves the [.sup.1][pi][pi]* (A) state which is found to be the lowest excited triplet state localized on the anthracenone moiety. Because the Dexter triplet excitation energy transfer (TEET) to the first excited triplet state of the naphthalene subunit is forbidden in [C.sub.2v] symmetry, it is mandatory to go beyond the Condon approximation in modeling this process. Nonadiabatic coupling matrix elements were computed to obtain a TEET rate different from zero. Our calculations yield time constants of 5 ps for the [.sup.1]n[pi]* (A)[??][.sup.3][pi][pi]* (A) ISC and of 3 ps for the subsequent [.sup.3][pi][pi]* (A)[??][.sup.3][pi][pi]* (N) TEET in BuCN whereas the energy dissipation involving the [.sup.3]n[pi]* (A) state as an intermediate occurs on a much longer time scale. Key words: multireference methods, intersystem crossing, nonadiabatic coupling, intramolecular triplet energy transfer, solvent effects
1. Introduction Excitation energy transfer (EET) between a triplet sensitizer and a triplet acceptor is a common chemical process used to photoinitiate radical reactions (1) or triplet-triplet annihilation upconversion. (2) [...]