For the lower energy nπ * band, the monomer peak position is at 2.65 eV, which again is in reasonable aggrement with TD-lc-DFT results (2.5-2.8 eV). The experimental gasphase peak position is 2.82 eV. The nπ * band experiences a much smaller blue shifts when going to the tetrameric models: 0.016 eV for models with a = 5.5 Å, 0.024 eV for tetramer 3.5 Å, and 0.063 eV for SAM 3.5 Å (Figure ). The intensity of the nπ * band is considerably enhanced for loosely packed models (a = 5.5 Å), in 3.4 and 4.0 times for tetramer 5.5 Å and SAM 5.5 Å, respectively. However, the enhancement is much smaller for tetramer 3.5 Å (factor of 1.7) and, notably, diminution in absorbance is observed for tightly packed SAM 3.5 Å (factor of 0.4), see inset in Figure . This may be explained by the fact that the transition dipole moment of planar trans azobenzene is zero, and larger conformational changes, accessible for the free molecule or not strongly constrained aggregates, tend to increase it. We also note that the spectra of free and constrained (SAM) tetramers with a = 5.5 Å are very similar to each other, reflecting similarities between the conformationally disorded structural ensembles shown in Figure . For a = 3.5 Å, spectral and geometrical differences are much larger (see Figure and Figure ).