Molecular photoswitches, of which azobenzene is a fundamental example, hold promise for developing novel functional materials controlled by light. In complex systems, however, numerous photoswitchable units may be present together in a relatively small space volume, and resulting intermolecular interactions between chromophores may affect their photophysics and photochemistry. Examples are azobenzene-functionalized self-assembled monolayers (SAMs), micelles, surfactant-polymer aggregates, and crystalline systems. The aggregation / congestion of azobenzene chromophores in such systems leads to changes in light absorption and emission properties in comparison to "isolated" switches (e.g., in a solution), and, moreover, may hinder or suppress photoisomerization, as has been observed for tightly packed SAMs. Assembling multiple chromophores together leads to a formation of collective electronic states, known as molecular excitons. These states dictate photophysical and photochemical processes in molecular aggregates. Moreover, tight molecular packing induces steric hindrance. Two effects, steric and excitonic, are intrinsically connected, since both become stronger with decreasing intermolecular distances and thus are enhanced at large packing densities. And both effects were proposed to be responsible for hindering azobenzene isomerization in SAMs. Furthermore, quantum chemical calculations on azobenzene dimers and larger aggregates yielded sizable exciton splittings (of several hundreds meV) for the bright, ππ * states, suggesting rapid excitation energy transfer between monomers. The simplest theoretical modelling of the steric effects on photodynamics of a photoswitch requires a quantum mechanical (QM) description of a single chromophore, whereas an environment can be described with classical molecular mechanics (MM). This approach has been applied to reveal the role of steric hindrance for azobenzene isomerization in various systems, such as azobenzene (or its derivatives) on a surface, in a SAM, attached to RNA, in a micelle, and in a lipid membrane. The account of the excitonic effects, on the other hand, requires the QM treatment of multiple chromophores, and therefore presents a challenge. Benassi and Corni calculated electronic couplings in azobenzene-containing SAMs and modelled exciton diffusion as well as isomerization based on a kinetic model. They concluded that the exciton transfer should not inhibit isomerization in SAMs. In our earlier works, we used a minimal QM azobenzene dimer model to study a possible role of the exciton coupling for azobenzene photoisomerization dynamics. These studies highlighted the dominance of the steric hindrance in reducing quantum yield of the trans→cis azobenzene isomerization in a tightly packed aggregate. However, the questions of exciton dynamics, localization and transfer have not been addressed at that stage and, moreover, the dimer model may seem too small for exploration of these questions.