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The application of atomic force microscopy (AFM) to solid-state photodimerizations revealed previouslyunexpected long-range molecular movements in the initial stages (phase rebuilding) and in the finalstages (phase transformation and disintegration) of reaction. The consequences for the new understandingof solid-state photochemistry are discussed. The 4.2 Å criterion of organic topochemistry lacks a real basisand 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 inreacting crystals requires more elaborate use of X-ray structural data by invoking the molecular packing. If acrystal 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 movementsor non-photoreactivities due to the prevention of movements by 3D-interlocked packing have numerousapplications. 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 othersthe space saving twist mechanism is the only choice. The benefits of the new solid-state mechanisms forcrystal engineering, photochromism, mixed crystals, absolute asymmetric syntheses, and preparative photochemistryderive from its experimental basis. Numerous presumed puzzles from the postulate of minimalatomic and molecular movement vanish in a straightforward manner

Solid-state photochemistry: new approaches based on new mechanistic insights