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 butyronitrile solution [Dobkowski et al., J. Phys. Chem. A 2019, 123, 6978] proposed an excited-state energy dissipation pathway {1ππ*(N)+1ππ*(A)}→1nπ*(A)→3nπ*(A)→3ππ*(N). However, a detailed theoretical study employing combined density functional theory and multireference configuration interaction methods, performed in the present work, suggests that the photoexcitation decay follows a different pathway. In butyronitrile solution, the intersystem crossing (ISC) follows the well-established El-Sayed rule and involves the 3ππ*(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 C2v symmetry, it is mandatory to go beyond the Condon approximation in modeling this process. Non-adiabatic coupling matrix elementswere computed to obtain a TEET rate different from zero. Our calculations yield time constants of 5 ps for the 1nπ*(A)→3ππ*(A) ISC and of 3 ps for the subsequent 3ππ*(A)→3ππ*(N) TEET in butyronitrile whereas the energy dissipation involving the 3nπ*(A) state as an intermediate occurs on a much longer time scale.