The application of atomic force microscopy (AFM) to solid-state photodimerizations revealed previously
unexpected long-range molecular movements in the initial stages (phase rebuilding) and in the final
stages (phase transformation and disintegration) of reaction. The consequences for the new understanding
of solid-state photochemistry are discussed. The 4.2 Å criterion of organic topochemistry lacks a real basis
and is not applicable to regular photolyses, even under tail irradiation conditions for instance of α-cinnamic acid or in E/Z-isomerizations in the crystal bulk. The experimental observation of molecular movements in
reacting crystals requires more elaborate use of X-ray structural data by invoking the molecular packing. If a
crystal keeps its outer form upon photolysis this does not necessarily indicate a topotactic transformation,
and submicroscopically resolved AFM investigations are in order. The applications of molecular movements
or non-photoreactivities due to the prevention of movements by 3D-interlocked packing have numerous
applications. Thus, amorphous solids or inclusion compounds may enable the movements in these cases.
Hitherto puzzling E/Z-photoisomerizations in the crystalline state can now be mechanistically understood.
In some cases even rotational mechanisms can be modelled in combination with the movements. In others
the space saving twist mechanism is the only choice. The benefits of the new solid-state mechanisms for
crystal engineering, photochromism, mixed crystals, absolute asymmetric syntheses, and preparative photochemistry
derive from its experimental basis. Numerous presumed puzzles from the postulate of minimal
atomic and molecular movement vanish in a straightforward manner.
