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Synthesis and Photo-Isomerization of Stilbene Crown Ether Calix[4]arenes

Stilbene crown ether calix[4]arenes synthesized via McMurry coupling from bisaldehydes exhibit photoisomerization properties. Study shows cis/trans geometry deduced from NMR & UV-VIS data, comparing well with model compounds. Unlike diazobenzene derivatives, stilbene derivatives do not undergo thermal isomerization. Unique isomerization properties include distinct UV absorption and photostationary states.

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Synthesis and Photo-Isomerization of Stilbene Crown Ether Calix[4]arenes

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  1. Abstract Five stilbene crown ether calix[4]arene derivatives were prepared in one step from the corresponding bisaldehyde calix[4]arenes be McMurry coupling. The coupling reaction yielded both cis and trans stilbenes from the o- and m-bisaldehydes. Only cis isomer however produced from the reaction of the p-isomer. The cis and trans geometry deduced from NMR and UV-VIS data ingood agreement with the spectra of model compounds, stilbene and dimethoxy stilbene. Unlike the diazobenzene crown ether calix[4]arene derivatives, the stilbene crown ether calix[4]arene derivatives did not undergo thermal isomerization. All synthesized stilbene crown ether calix[4]arenes can photoisomerize and a photostationary state was observed. 1

  2. Objective Synthesize stilbene crown ether calix[4]arenes Study the photo-isomerization properties of the synthesized calix[4]arenes 2

  3. Diazobenzene Azobenzene crown ether calix[4]arenes can isomerize between 2 geometrical isomers in the dark, at room temperature and can be catalyzed by light. 3

  4. Isomerization For azobenzene, there are two possible isomerization coordinates; 1. Twisting about the N=N double bond, and 2. in-plane inversion at one of the two nitrogen atoms. Stilbene can not isomerize in the dark but the UV light is required to control the isomerization 4

  5. K2CO3, CH3CN, Bu4N+Br- Strategy McMurry is the shortest way to synthesized stilbene from bisaldehyde using TiCl4 and Zn as reducing agent. 5

  6. Products Obtained The m-bisaldehydeyielded 28% of the stilbene (m-1) and 20% of the corresponding pinacol by-product. The o- and p-bisbenzaldehydes gave higher yields of stilbenes (o-1 and p-1), 67% and 51%, respectively. Both cis and trans stilbenes were obtained from the coupling o- and m-bisbenzaldehydes but only the cis stilbene was obtained from the p-isomer. 6

  7. Orientation Erythro Threo The predominant of the cis- over trans-products suggests that the pre-organized structure of bisbenzaldehydes controls the geometry of products. The short ethylene glycol linkages are less likely to allow the threo orientation resulting in less trans product. This becomes more obvious for the coupling of the p-bisbenzaldehyde in which the threo orientation would be most difficult to form and no trans-isomer was observed. 7

  8. UV Absorption The shorter lmax and lower extinction coefficients of all cis-isomers comparable to the model compounds was observed. 8

  9. Photostationary Stilbene crown ether p-tert-butylcalix[4]arenes did not isomerize under room light but readily isomerized under UV light. The photo-stationary states were observed for all isomers. 9

  10. This work was supported by: Thailand Research Fund (TRF) (PDF/13/2541) Ministry of University Affairs (UDC39/2543) National Science and Technology Development Agency (NSTDA) (CO-B-11-11-09-202D). Acknowledgement 10

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