Analysis of Excitons in Stacked Perylene Diimide Derivatives*

π-stacked organic molecules are tunable light-absorbing materials that are promising for many optoelectronic applications; thus, it is necessary to understand how both inter- and intra-molecular interactions influence optical excitations. We use first-principles time-dependent density functional theory (TDDFT) to study the nature of these interactions in functionalized peryelene diimide oligomers stacked on a DNA-like backbone. Taking a Franck-Condon Herzburg-Teller approach to vibronic coupling, and sampling of ground state vibrations via molecular dynamics, we obtain an ensemble of excited-states that may occur. By analyzing ~100 dimer and trimer structures, we elucidate the parameters that correlate with the nature of the excited-state. Charge transfer (CT) character analysis shows that the degree of CT-like character in a bright state correlates with the strength of electronic transiton and with its inter-molecular vibrations. Furthermore, we determine that displacement and rotation between monomers leads to reordering of direct excitations. Such a finding is important for technologies such as singlet fission photovoltaics where the CT excitation character is directly related to performance.