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

Abstract

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 ofα-cinnamic acid or inE/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 puzzlingE/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.

Organic Solid-State Reactions with 100% Yield

“…Mechanistic investigations of gas-solid and solid-solid reactions as well as their proper engineering require identifiable crystal surfaces for atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM) [1,3,[13][14][15] in combination with X-ray diffraction data, which are the basis of crystal packing analyses [1,3,[16][17][18].…”

Organometallic Chemistry: Structural Isomerization Reactions in Confined Environments

“…[38] However, the more space and energy demanding E/Z isomerization reactions do not generally obey the topochemical principle, and often proceed unidirectionally, either from the Z to E or from the E to Z configuration. [39] Mechanisms of solid-state isomerizations have been the subject of intense deliberation in recent years, [39Ϫ41] particularly since the application of new techniques such as scanning near field optical microscopy (SNOM) and atomic force microscopy (AFM), which have suggested, for example ''phase-rebuilding'' mechanisms. [39] …”

“…[39] Mechanisms of solid-state isomerizations have been the subject of intense deliberation in recent years, [39Ϫ41] particularly since the application of new techniques such as scanning near field optical microscopy (SNOM) and atomic force microscopy (AFM), which have suggested, for example ''phase-rebuilding'' mechanisms. [39] …”

Kinetic analysis of solid‐state photodimerization reaction of photosensitive monomers and a polymer with cinnamoyl moieties

“…Trans ‐cinnamic acid and its derivatives could undergo both reversible E–Z photoisomerization and irreversible photodimerization in the crystalline states (Scheme ) 1. The crystalline‐state photodimerizations of cinnamic acid to truxillic acid have been studied in detail by X‐ray diffraction (XRD),2, 3 atomic force microscopy,4, 5 vibrational spectroscopy,6–8 and solid‐state NMR 2, 9. In 1964, to demonstrate the reaction behavior of cinnamic acid crystals, Schmidt and coworkers established the topochemical principle in which the reaction tends to occur with a minimum of atomic and molecular motion, and the solid‐state packing of reactants ultimately determines the resulting products including their stereochemistry 10…”