Photoreactions And Applications Of Photoactive Molecules

Photo-induced reactions including the irreversible chemical transformations and the reversible structure or conformation changes are important topics in photochemistry.Photo-induced reactions via the excited states of the substrates usually proceed through different reaction pathways form those of thermal reactions,which may lead to the formation of complex framework that are difficult to obtain by conventional synthetic approaches based on thermal reactions.The reversible structure or conformation change of phot-active molecules upon irradiation with light of different wavelengths has greatly expanded the applications of photo-active molecules as "photo-switches" to realize dynamic modulation in biological system.Photoactive molecules that can undergo fast bond-cleavage reactions upon light irradiation have been used as photo-labile protecting groups in "caged compounds"which have found wide applications in biological studies.In this dissertation,we studied the irreversible photo-reactions of para-quinones and the reversible structure change of spiropyrans to explore their applications as "photo-switches".The main work in this dissertation was included in the following four parts:1.Photo-induced reactions of para-quinones with the strained alkene bicyclopropylidene.Photo-induced reactions of different para-quinones including benzoquinones,naphthoquinones,anthraquinones and naphtho[2,3-b]furan-4,9-dione with bicyclopropylidene(BCP)were investigated.Diverse polycyclic products bearing spiropropyl moiety were obtained via the photocycloaddition followed by various ring arrangements.The nature of the excited state of quinones(n?*or ??*triplet state)and the ring-opening reaction of cyclopropane ring during the reaction process led to the diversity of the products.Besides of the primary products oxetanes and cyclobutanes(depending on the nature of the excited states of para-quinones),secondary products from the ring-opening reactions of cyclopropane rings followed with rearrangement reactions were also obtained.In the reaction between 9,10-anthraquinone with BCP,an unprecedented reaction pathway was observed via the Paterno-Buchi 1,4-diradical intermediate leading to the formation of the novel addition-rearrangement product.Some of the photoreactions between para-quinones with BCP also hold potentials as useful synthetic protocol to prepare polycyclic compounds with the spiropolycyclic framework.2.Reversible isomerizations of spiropyrans indued by light/heat in aqueous solution.Four types of spiropyrans(SP?,SP?,SP? and SP?)were synthesized and studied with respect to their isomerization with the merocyanine(MC)form upon light irradiation or heating in aqueous solution at different pH.The isomerization procees was monitored by UV-vis spectra and HPLC analysis.Three of the four SPs(SP?,SP? and SP?)were found to undergo fast isomerization leading to the ring-open(MC?,MC? and MC?)in aqueous solution upon moderate heating.All the MC forms were able to be rapidly converted into their SP counterparts upon visible light irradiation.The reverse photochromism of SP?,SP? and SP? in aqueous solution makes it possible to use these moieties in the construction of self-asembling systems that were able to be reversibly modulated with light/heat.3.Application of spiropyrans in reversible modulation of self-assembly system based on short peptides.Short peptides with 6-8 amido acid residues were linked with SP?,SP? or SP? on the N terminal or C-termial to get photo-sensitive peptides.Upon reverse photochromism of the SP-peptides to form MC-short peptides,the self-assembling properties of the peptides were investigated in detail.Most of the short peptides linked with MC? or MC? were able to self-assemble into fibrous network to form supramolecular hydrogels.But none of the short peptides linked to MC? showed hydrogelation ability.The hydrogels formed by the MC? or MC?modified peptides underwent gel to solution phase transition upon visible light irradiation to induce the change from MC-peptides to SP-peptides.The resulted solution containing large portions of SP-peptides upon proper heating for "reverse"photochromism of the SP moiety was able to form supramolecular hydrogel again."Smart" hydrogel with multi-responses was also constructed through the proper combination of the MC moiety and short peptides with specific sequence.4.Spiropyran modified glycosyamines and PNA oligomers.Using SPI and SPII linked with carboxylic acid,we further modified several types of bioactive molecules including glycosaymine and PNA oligomers with this photo-active moiety.Upon reverse photochromism to transform the SP moiety to the planar MC moiety,the self-assembling properties of the modified glycosaymine and PNA oligomers were investigated.We found that glycosyamines linked with MCI were able to self-assemble to form supramolecular hydrogels at neutral pH.PNA oligomers A-A and T-T linked with MCI were also able to gel water but with relatively high concentration.None of the MCII linked molecules showed hydrogelations ability.The hydrogel formed by MCI-glycosyamine or MCI-PNA oligomer all showed photoresponse upon irradiation with visible light.