The interplay of vibronic and spin-orbit coupling in the fluorescence quenching in trans-dithionated PDI

Organic chromophores such as the thionated derivatives of perylene diimides (PDIs) show prolonged triplet-excited state lifetimes in contrast to their pristine parent PDI molecule, which shows near unity fluorescence quantum yield. The excited state dynamics in the trans-dithionated PDI (S2-PDI) are studied here. Unlike PDI, the photo absorbing ππ* state of S2-PDI is in close proximity to quasi-degenerate nπ* states. The latter exhibits an interesting vibronic problem leading to the breaking of orbital symmetry mediated through non-totally symmetric vibrations. The time-dependent quantum dynamics are studied with a diabatic model Hamiltonian involving three singlet and three triplet states coupled via 22 vibrational modes. A combined effect of multiple internal-conversion and inter-system crossing (ISC) pathways leads to population transfer from the 1ππ* state to the 3ππ* state via the nπ* states, with an overall ISC rate of 0.70 ps that compares well with the experimental value. The calculated absorption spectra for PDI and S2-PDI reproduce the essential vibronic features in the observed experimental spectra. The dominant vibronic progressions are found to have significant contributions from the vinyl stretching modes of the PDI core.