Absorption spectra of the tetrameric models as well as that of the monomer are shown in Figure . The spectra of all systems show a strong ππ * band in ultraviolet region and a weak nπ * band in the visible region. The peak positions of the nπ * and ππ * bands are plotted in Figure and, respectively. The ππ * band of the monomer is located at 4.57 eV. This value is blue-shifted by 0.45 eV with respect to the available gas-phase experimental value of 4.12 eV 63 but is in reasonable agreement with results obtained using time-dependent long-range corrected density functional theory (TD-lc-DFT) in combination with groundstate DFT MD, 4.2-4.6 eV. For the tetrameric models, the ππ * band is considerably blue-shifted because of exciton coupling (Figure ). For the models with a = 5.5 Å, the blue shift is 0.16 eV, for tetramer 3.5 Å 0.40 eV, and for SAM 3.5 Å 0.47 eV. Moreover, the tetramer absorbance at band maxima is larger than the monomer absorbance (as expected based on the molecular exciton model 21 ). Specifically, enhancement factors are 3.9, 3.8, 5.0, 4.8 for tetramer 5.5 Å, SAM 5.5 Å, tetramer 3.5 Å, SAM 3.5 Å, respectively.