The nπ * trans → cis isomerization quantum yields are lower by a factor of about two for free / not strongly constrained tetramers than for the monomer, and no switching is observed for the most tightly packed model (SAM 3.5 Å). The nπ * quantum yields for the aggregates are found to be slightly larger than the ππ * quantum yields (obtained at the same level of theory in ref. 47).