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-H-4, PBI-F-2 and PBI-Cl-4, 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-H-4 and PBI-F-2, which are attributed to significant pi-pi 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-Cl-4), similar spectra are observed in solution and in the condensed state, which suggests the absence of strong intermolecular pi-pi 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 pi-pi 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.