Comparative analysis of the energy levels of planar and core-twisted perylene bisimides in solution and solid state by UV/VIS, CV, and UPS/IPES

The frontier orbital energies of four different functionalized perylene bisimide derivatives, PBI-Ph(iPr)₂, PBI-H₄, PBI-F₂ and PBI-Cl₄, were directly determined by UV-photo electron spectroscopy (UPS) and inverse photo electron spectroscopy (IPES) and are compared to the results from cyclic voltammetry (CV) and optical absorption spectroscopy (UV/VIS). The optical spectra reveal significant differences between monomeric species in solution and assembled molecules in the condensed state for the nearly planar PBI-H₄ and PBI-F₂, which are attributed to significant π–π stacking interactions in the condensed phase. In contrast, for PBIs with bulky substituents or twisted core, i.e. 2,6-isopropylphenyl substituents at the imide positions (PBI-Ph(iPr)₂) or four chlorine substituents at perylene bay positions (PBI-Cl₄), similar spectra are observed in solution and in the condensed state, which suggests the absence of strong intermolecular π–π stacking interactions. An entirely different result is obtained for the HOMO/LUMO energy values obtained from UPS/IPES and CV measurements which do not reveal a significant impact of intermolecular π–π stacking interactions. When comparing CV and UPS/IPES results, an accentuated deviation was observed for the perylene bisimide derivatives as compared to correlations found in the literature.