Photochemistry

Stilbenes belong to the best investigated compounds in photochemistry. Together with the photophysical properties, E/Z isomerization reactions, electrocyclic ring closures (with subsequent oxidation to phenanthrenes or higher polycyclic aromatics), cyclodimerizations, and polymerizations/cross-linking processes render the stilbenoid compounds suitable for various applications in materials science and biomedical chemistry [50, 54].

Dendrimers consisting of stilbene building blocks can principally exhibit the above mentioned photoreactions, unless a fast photophysical deactivation (Fig. 6) prevents them. We performed detailed studies in the series 1a-e and 2a-c. 1,3,5-tristyrylbenzene has, at room temperature in apolar solvents, a relatively long fluorescence lifetime of 8.313.6 ns [55]. Thus, photocyclodimerizations work even in diluted solutions, where (by diffusion) an encounter complex S0S1 has to be formed, which reacts regio- and stereoselec-tively via an excimer to a cyclophane [50, 56, 57]. In more concentrated solutions, aggregation may facilitate the dimer-ization; however, the photopolymerization by statistical CC bond formations is then favored, too. Scheme 3 shows the regioselective head-to-head cyclodimerization of 1a (R1 = R2 = OCH3). The process is stereoselective concerning the trans configuration, which is preserved in the products; it is not selective concerning the arrangement of the three four-membered rings. The ratio of 18 : 19 depends on the individual system. In the 3,4,5-trimethoxy compound, it is 1 : 2; the statistical distribution of the relevant conformers of the starting compound is 1 : 3.

Scheme 3. Selective photocydodimerization of 1a (R1 = R2 = OCH3) to the cyclophanes 18 and 19.

Concerning the selective and statistical CC bond formation, the other systems 1a with longer alkoxy chains behave in a similar way. Moreover, the UV/Vis spectroscopic investigation of the photochemistry of the higher generations 1b-e reveals that prolonged irradiation leads to a complete or almost complete disappearance of the typical stilbene absorption, that is, that virtually all olefinic double bonds (Table 1) are finally involved in the process. Figure 7 shows the time-dependent photodegradation of 1b (R1 = R2 = OC12H25) in cyclohexane with monochromatic light (À = 366 nm). The process is even much faster when UV light with À < 300 nm is used. On the contrary, the process is slower, as depicted in Figure 7, when a better (and more polar) solvent like THF is used because the aggregation is less efficient. The generation of four-membered rings plays a minor role in the polymerization reactions. The CC bond formation of the olefinic centers occurs in different directions to different neighbor molecules (20). The MALDI-TOF mass spectrum, measured after the irradiation, shows the peaks of the oligomer series from the monomer to the pentamer [39].

Figure 6. Scheme of the primary and secondary photoprocesses of stilbenoid dendrimers with special core-chromophores. A: absorption, F: fluorescence, IC: internal conversion, ISC: intersystem crossing, CT: charge transfer, PR: photoreaction.
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