Investigation of the Spectra of Electronic Transitions in Small Clusters of the Pigment Yellow 3

Electronic absorption spectra were calculated in the visible region for clusters of the pigment Yellow 3 that comprise one, two, and four molecules. The geometry was optimized by the PBEh-3c and B3LYP-D4/def2-SVPD methods. The results obtained by the B3LYP-D4/def2-SVPD method correlate best with the experimental data. The spectral characteristics were calculated by the TD-DFT and sTD-DFT methods with the PBE0 functional and the def2-SVPD basis set. By analyzing the natural transition orbitals (NTOs) and changing the electron density during the formation of excited states of the studied clusters, it was shown that the main contribution to the spectral lines in the visible range is made by the density transfer from the aromatic rings to the nitro group and the conjugated bond system in the center of the molecule. In this case, for the crystalline state of matter, all excited states are delocalized, and the main contribution to the intermolecular transfer of the electron density is made by the formation of excitons.