Stereoselective Photodimerization of (E)-Stilbenes in Crystalline γ-Cyclodextrin Inclusion Complexes

Abstract

Solid-state irradiation of the crystalline inclusion complex of (E)-stilbene in gamma-cyclodextrin (gamma-CD) yields a single isomer of syn-tetraphenylcyclobutane stereoselectively in high yield. In contrast, the photodimerization of stilbene in solution is very inefficient and unselective, and no photodimer is observed even upon prolonged irradiation of pure crystals. The monosubstituted stilbenes form a pair of photodimers stereoselectively, viz. the syn head-to-head and syn head-to-tail isomers, in comparable yields. The photodimer yields of about 70% and the biphasic decay kinetics of the excited stilbene (as established by picosecond time-resolved diffuse-reflectance spectroscopy) indicate that the stilbene guests are located in at least two distinct sites in the gamma-CD crystal lattice, i.e., a dimerization site where excited stilbene is in close reach of another stilbene guest molecule and an isomerization site where excited stilbene does not find a close neighbor for dimerization and thus undergoes trans --> cis isomerization only.

Phototransformation of Stilbene in van der Waals Nanocapsules

“…After a variable duration of photolysis, the samples were dissolved in deuterated acetone and analyzed by 1 H NMR spectroscopy. The results indicated the www.chemeurj.org presence of at least one of the following products (Scheme 1): cis-stilbene 1 (olefinic protons at 6.65 ppm), trans-stilbene 2 (p-and m-aromatic protons at 7.62 and 7.38 ppm, respectively), phenanthrene 3 (H-9,10 at 8.82 ppm), and the two photodimers of 1,2,3,4-tetraphenylcyclobutane, 4 and 5 (cyclobutane protons at 4.60 and 3.71 ppm, [27,41] respectively). All three photochemical reactions depicted in Scheme 1 occurred upon irradiation of complex A (Figure 8).…”

“…The cis/ trans isomerization is the fastest process with the highest quantum yield, [18] and this agrees with literature data for the photolysis of stilbenes in solution [18][19][20][21][22] and in some organized media. [23][24][25][26][27] In addition, slow accumulation of the dimeric products is observed after longer periods of photolysis. An increase in light intensity (Figure 8 b) increases dimer formation, although the quality of the material worsens and the solid turns brown.…”

“…Hence, they are good candidates for demonstrating the modulation of photochemical reactivity by solid-state host-guest interactions. So far, stilbenes have been utilized to study their phototransformation on flat surfaces, [23a] in zeolites, [23b] cyclodextrins [24][25][26][27] in solution and in the solid state, as well as in micelles in aqueous solutions. [25] The essential feature of unsubstituted stilbenes is that neither liquid cis-nor crystalline trans-isomers undergo photodimerization.…”

Photo‐Cross‐Linking of Polymethacrylates with Stilbene Chromophores in the Side Chains

“…The regio-and stereoselectivity of the dimerization depends strongly on the special system and on the reaction conditions. Among the investigated examples, 4-methoxystilbene [40] [41] represents the only 4-alkoxy compound -comparable to 5a -5f. However, the irradiation of 4-methoxystilbene was performed for an inclusion complex in g-cyclodextrin, a constrained medium [40].…”

Triplet state dynamics within cyclodextrin solid complexes

“…2 Despite numerous studies on photophysical and photochemical properties of organic guest species complexed with cyclodextrins in the solid state, 3 there is only a handful of publications on the kinetics of photoinduced transient species within cyclodextrin solid complexes. [4][5][6][7][8][9][10][11][12][13] The work reported here focuses on the application of time-resolved diffuse-reflectance laser-flash photolysis techniques to characterize the triplet state dynamics of several aromatic compounds within cyclodextrin solid complexes. The aromatic guest substrates employed in this study are summarized in Chart 1; all of these substrates are known to form excited triplet states in essentially quantitative yield.…”