Phase rebuilding

Keywords Gas-solid reaction Solid-solid reaction IntracrystaUine reaction Phase rebuilding mechanism Solid-state cascade reaction 100% yield... 

ElZ isomerizations are usually not expected in the solid state. They have been widely studied in solutions or in liquids. This includes thermal, catalytic, and photolytic processes and ElZ isomerization was also observed in competition with biphotonic excimer laser photodecompositions [47]. Most of the ElZ isomerizations in the solid state have been photo chemically observed [48], but mostly not as uniform quantitative reactions. If these isomerizations cannot be performed under selective conditions of irradiation (an exception is 83/84) [49], the only chance to have these reactions uniform with 100% yield is a very efficient isomerization (according to the phase rebuilding mechanism) that leads to an isomeric product with heavily interlocked crystal lattice. Under such circumstances side reactions of the substrate and photoconversions of the product are prohibited (including the back reaction, of course). Four favorable cases... 

Scheme 2.1.2, Fig. 2.1.7) [27]. It can be nicely seen that there is an initially gradual growth of random volcanoes (phase rebuilding) that are suddenly replaced by the sharp edged rhombic pool basins with depths of 60-100 nm (phase transformation) (Fig. 2.1.7). Shortly thereafter, the reacted zone disintegrates from the crystal with formation of a fresh surface for a new reaction cycle etc. so that a complete reaction with 100% yield becomes possible in preparative runs. 

Sharp regular edges are not always obtained. The diazotization of 4-amino-benzoic acid (7) on (101) with nitrogen dioxide (Scheme 2.1.3) is a typical example for a very large increase in the surface features upon the sudden phase transition that follows gradual phase rebuilding (Fig. 2.1.9). Preparatively, this is a waste-free quantitative synthesis of the solid diazonium nitrate 8 in a complicated multistep cascade reaction [9, 28]. 

In still further cases the phase transformation can flatten out the features from the phase rebuilding stage. A typical example is the solid-state reaction of thiourea (monolayer structure) on (001) with phenacylbromide [(100)-cleavage plane] giving... 

If, as will become evident with the dimerizations discussed in Section III.C, phase rebuildings are essential for the success of solid state photodimerizations, the nonreactivity of some topochemically allowed systems can be looked at in a different way. First, if there is no reaction, the crystal surface will stay unchanged upon irradiation and this can be probed with AFM very sensitively. Figure 2 shows a particularly highly structured natural surface of photostable tetraphenylethylene 1 prior and after 10 min irradiation in air with a Hg high-pressure lamp through Solidex from a distance... 

P-Cinnamic Acid on (100). The weakly arched, saber-shaped crystal aggregates of monoclinic [1-4 can be measured both on the larger (100) face and on the side face (010). This allows for a test, if indeed the bulk crystal structure determines the transport phenomena which occur on phase rebuilding, because different features must occur on different faces in this case. 

Cinnamic Acids Discussion. It is evident from the difference of these results on (010) with those on (100), that the bulk crystal structure does indeed govern the transport phenomena and the recorded AFM features are connected to the phase rebuilding, which had hitherto not been considered. This conclusion can be reached because the AFM images are easy to comprehend and are extraordinarily rich in detail on the submicroscopic level, even though the important features are not on the atomic scale here. 

It is of some interest to compare these results with those of 9-methylan-thracene 7b, whose photodimerization has been termed topochemically allowed (d = 3.87 A [22])]. If the main surface of 7b (100) is irradiated, AFM scans floes initially and upon continuation of the irradiation these increase [8]. This shows that there are also long-range molecular transports and that phase rebuilding again governs the process. In any case, there is no special mechanism if a reaction may be termed topochemically allowed by previous convention (see Section C). 

Chloroanthracene. Single crystals of 7c provide particularly beautiful nanostructures if their main and side faces are irradiated to give 8c and if this reaction is probed with AFM [6, 8, 23]. Figure 16 (see color plates) shows the pronounced differences of the phase-rebuilding mechanisms on different faces, even though those could not be crystallographically specified because of lack of X-ray structural data. 

Photochemists are well aware that solid state photodimerizations may be highly stereoselective, but they are not frequently stereospecific. Long-range molecular transports connected to phase rebuildings provide an obvious explanation. However, there are also solid state photoprocesses which yield... 

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