Formation of excited triplet states in naphthalene diimide and perylene diimide derivatives: A detailed theoretical study.

Mechanistic details of the excited triplet state formation upon photoexcitation to the low-lying singlet manifold in naphthalene diimide and perylene diimide derivatives are explored theoretically. Static and dynamic aspects of two singlets (S and S) and six triplets (T-T) of these molecules are investigated. Suitable vibronic Hamiltonians are constructed to investigate the internal conversion dynamics in both the singlet and triplet manifolds. Computed singlet-triplet energetics, spin-orbit coupling matrix elements, and intersystem crossing rates strongly suggest an efficient intersystem crossing process involving higher triplet states (T, T, and T). Separate full dimensional quantum wavepacket simulations of singlet and triplet manifolds in the approximate linear vibronic model by assuming initial Franck-Condon conditions are carried out to unravel the internal conversion decay dynamics in the respective manifolds. The obtained diabatic electronic populations and nuclear densities are analyzed to illustrate the triplet generation pathways involving higher triplet states in these molecules.