Studies on the synthesis and photochemical properties of chain linked bisheteroarylethenes

1, n-Bis[trans-4-[2-[2-(5-methylbenzoxazolyl)]ethenyl]phenoxy]alkanes[n = 3 (I a), 4 ( I b), 6 (I c)] and 2,2'-bis[trans-4-[2-[2-(5-methylbenzoxazolyl )]ethenyl]phenoxy]ethyl ether(I d) were synthesized. Their structures were identified by elemental analysis, IR, UV, H-1 NMR, C-13 NMR and MS. The UV spectra were used to illustrate the change of compounds I a- I d under the irradiation of high-pressure-mercury-lamp. It was found that these compound s undergo two kinds of reactions, the trans-cis isomerization and intramole cular photodimerization. The former is fast and reversible, and the latter is slow and irreversible. The speed of intramolecular photodimerization wil l increase with increasing the length of the carbon chain between the two 2 -[2-(5-methylbenzoxazolyl)]ethenyl]phenyl group. The fact that the photodim erization is not affected by oxygen shows that the reaction proceeds throug h an excited singlet state. It is also revealed that compounds I a- I d und ergo [2+2] photolysis easily when irradiated under short UV light. That the intramolecular photodimerizarion and photolysis of these compounds can rep eat many times indicate that these compounds have a high photostability.