Structure-Spectroscopy\nCorrelation in the Self-Assembled\nPerylene Diimide-Based Dimers via Inter-Chromophore Coupling

The impact of conformational change on the ground and\nexcited states\nof seven perylene diimide (PDI)-based dimeric systems is examined\nby introducing longitudinal shift, transverse shift, and rotation\nof one monomer with respect to another. The minimum energy conformations\nare compared via an energy decomposition analysis. The heteroatom-substituted\ndimeric systems, such as B<sub>2</sub> N<sub>2</sub>-embedded PDI,\ntrans-thio-PDI (trans-S<sub>2</sub>-PDI), and N-PDI, show BN···π,\nCS···π, and N···H interactions\nthat survive over a longer range of longitudinal and transverse shifts.\nThe excitonic coupling analysis reveals that both Coulomb- and CT-mediated\ncouplings are crucial for understanding aggregate absorption spectra.\nWhile the Coulomb coupling exhibits a monotonic behavior with conformation\nchanges, the CT component changes significantly with minor geometrical\ndeviations. The interplay between the two couplings leads to J-type,\nH-type, and null aggregates, depending on the conformations of the\ndimers. The overall trend of both couplings is consistent across all\nsystems, although they differ in magnitude. The trans-S<sub>2</sub>-PDI shows the strongest Coulomb and CT couplings, while it is weak\nin perylene and B<sub>2</sub>N<sub>2</sub>-PDI dimers. The resonant\nmodel for strongly coupled Frenkel excitonic (FE) and CT states successfully\ncharacterizes the single- and double-band nature of absorption spectra\nin dimers. In strong coupling regions, the dimers show blue-shifted\nsingle-band excitation to the upper FE state. In contrast, excitation\nto the lower FE and upper CT states produces a red-shifted two-band\nspectrum in the weakly coupled regions. The intensity of the CT band\ndiminishes with the monomer separation. In most cases, the perpendicularly\nstacked structures show null-aggregate behavior with no spectral shift\ndue to the absence of Coulomb and CT couplings. The exciton relaxation\npathway of the heteroatom-substituted PDIs is found to be influenced\nby the presence of <i>n</i>π* states between the FE\nand CT states.