Calculation of UV–Vis Absorption Spectra of Linear Perylene Diimide Oligomers by Time‐Dependent Density Functional Theory Simulation

The vibronic structure of the UV–visible absorption spectra of linear perylene diimide oligomers is simulated using time‐dependent density functional theory and compared to experiment. The spectra were calculated using the B3LYP, B3LYP‐35, M05‐2X, and CAM‐B3LYP exchange–correlation (XC) functionals, while accounting for solvent effects with the integral equation formalism polarizable continuum model. Two computational approaches for vibronic structure simulations, the gradient and geometry methods, were evaluated for their ability to reproduce experimental spectral features of these aggregates. While both methods retrieve the expected J‐like aggregate behavior for the monomer and the dimer spectra, significant challenges emerge for the trimer. For the former, when combined with the CAM‐B3LYP XC functional, the gradient method consistently agrees with experiment, suggesting that it provides reliable extrapolations of the potential energy surface, leading to physically accurate minima. Further dimer calculations confirm that obtaining accurate vibronic spectra depends on correctly capturing whether the electronic excitation and geometry changes are spread across the entire aggregate or confined to individual units.