Conventionally, the triplet state generation via S1T1is thought to be the slowest molecular photophysical process in organic conjugated molecules. However, recent advanced experimental spectroscopic investigations by Yushchenko et al. [J. Phys.Chem. Lett., 2012, 6, 2096-2100] revealed an ultrafast triplet formation of bromo core substituted naphthalene diimide (rNDI). Here, we estimated various stationary points of the low-lying singlet-triplet manifold of prototype NDIs using model vibronic Hamiltonian to gain insights into the mechanistic details of intersystem crossing (ISC) pathways of rNDI. Several ISC pathways involving S1 and S2 are identified where the higher triplet states, Tn (n > 2) are found to be energetically more accessible than T2 and T1. Our findings of NDIs are correlated to interpret the experimental ISC findings of rNDI.
Intersystem crossing, ultrafast triplet formation, energy gap law, linear vibronic coupling, stationary points