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

Gerd Kaupp

doi:10.1155/s1110662x01000071

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.

Highlights

Scientific organic solid-state photochemistry started in the 19th century, for instance with the dimerization of cinnamic acid [1], thymoquinone [2], and anthracene [3]
The experimental proof of longrange molecular movements instead of minimal atomic and molecular movements restricts organic topochemistry to the uncommon phenomenon called topotaxy [9]
The submicroscopic investigation of solid-state reactions leads to totally new mechanistic insights on a purely experimental basis, that are eminently reasonable

Summary

INTRODUCTION

Scientific organic solid-state photochemistry started in the 19th century, for instance with the dimerization of cinnamic acid [1], thymoquinone [2], and anthracene [3]. The overwhelming number of these reactions proceeded non-topotactical, i.e. the crystals disintegrated and the space groups of the product crystallites differed from those of the starting monomers Despite these findings and the knowledge that the distance of the reacting centers in the crystals varied considerably from around 3.5 to more than 6 Å the meaning of Kohlschlütter’s term “topochemistry” [6] was changed by G. Minimal atomic and molecular movements in solid-state reactions and a limiting distance of 4.2 Å were repeatedly claimed to be the essential features for reactivity in organic crystals [7] Such claims were invoked to “explain” the stereoselectivities in the photodimerizations of α- and β-trans-cinnamic acid and the non-reactivity of trans-stilbene. They were highly acclaimed in organic chemistry and pushed the measurement of double-bond distances in reactive crystals in numerous research groups.

MOLECULAR MOVEMENTS IN THE CRYSTAL LATTICE

THE CHANGE IN PARADIGM

AMORPHOUS SOLIDS AND INCLUSION COMPOUNDS

Findings

CONCLUSIONS