Photochemical Reactions in the Solid State

Photochemical reactions proceed in the gas phase, in solution, and even in the solid state. Photochemical reactions in the solid state are especially important when considering the applications of photofunctional molecules in materials, where photophysical processes and photochemical reactions are used to trigger some change in a given physical property of a system, such as its color, solubility, or electroconductivity. 

When considering reactions in the solid state, it is important to draw distinctions between reactions in crystals and reactions in amorphous solids, since the controlling factors are quite different in these two situations. Reactions in the crystalline state are controlled primarily by topological factors, such as the distances and the relative orientation of potentially reactive groups. Trans-cinnamic acid undergoes photochemical dimerization when the distance between adjacent double bonds in the crystal is shorter than 4 A (Table 2.5 ). Irradiation of 2,5-distyrylpyrazine (DSP) which has two double bonds per molecule leads to a linear high 

Factors other t— excitation energy transfer and migration 

C PVCm solution in dichloroethane Insert histogram for the site reactivity distribution corresponding to curve B 

This reaction, called a four-center photopolymerization, is a typical example of topochemical reactions used to prepare polymer crystals.5 The changes in higher-order structure during the reaction are shown in Table 2.5 . Various polydiacetylene crystals have also been prepared by solid-state photopolymerization of diacetylene monomer crystals, such as 1,6-dicarbazoyl-2,4-hexadiene. These syntheses have attracted considerable interest, since they can lead to organic materials of high conductivity or of nonlinear optical properties. 

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Thermal and photochemical reactions in the solid state which were studied before 1999 are summarized in reviews [1] and books [2]. 

Toda F (2005) Thermal and Photochemical Reactions in the Solid-State. 254 1-40 Tour JM, see James DK (2005) 257 33-62... 

By chance rather than by design, the third chapter in this volume also emanates from Israel. Bernard S. Green, Rina Arad-Yellin, and Mendel D. Cohen have surveyed organic reactions in the solid state from the standpoint of the stereochemist. In the first part of the chapter, the authors discuss the stereochemical consequences of the crystallization of conformationally mobile systems. Conformational, crystal-field, and hydrogen-bonding effects, among others, are responsible for the selective crystallization of stereoisomers that may not be dominant in solution. The second part of the chapter is concerned with the stereochemical consequences of chemical, and especially photochemical, reactions in the solid state. 

Toda F (2005) Thermal and Photochemical Reactions in the Solid-State. 254 1-40 Trask AV, Jones W (2005) Crystal Engineering of Organic Cocrystals by the Solid-State Grinding Approach. 254 41-70 Uccelli L, see Boschi A (2005) 252 85-115 Vezin H,see Dias N (2005) 253 89-108... 

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