Dynamic internal cavities of dendrimers as constrained media. A study of photochemical isomerizations of stilbene and azobenzene using poly(alkyl aryl ether) dendrimers.
Dendritic microenvironments defined by dynamic internal cavities of a dendrimer were probed through geometric isomerization of stilbene and azobenzene. A third-generation poly(alkyl aryl ether) dendrimer with hydrophilic exterior and hydrophobic interior was used as a reaction cavity in aqueous medium. The dynamic inner cavity sizes were varied by utilizing alkyl linkers that connect the branch junctures from ethyl to n-pentyl moiety (C(2)G3-C(5)G3). Dendrimers constituted with n-pentyl linker were found to afford higher solubilities of stilbene and azobenzene. Direct irradiation of trans-stilbene showed that C(5)G3 and C(4)G3 dendrimers afforded considerable phenanthrene formation, in addition to cis-stilbene, whereas C(3)G3 and C(2)G3 gave only cis-stilbene. An electron-transfer sensitized trans-cis isomerization, using cresyl violet perchlorate as the sensitizer, also led to similar results. Thermal isomerization of cis-azobenzene to trans-azobenzene within dendritic microenvironments revealed that the activation energy of the cis- to trans-isomer was increasing in the series C(5)G3 < C(4)G3 <C(3)G3 <C(2)G3. With the change in the alkyl linker from C(5) to C(2), significant changes in the activation parameters were observed.
Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012, India.
Baskar
B
Natarajan
Shipra
S
Gupta
Nithyanandhan
N
Jayaraj
V
V
Ramamurthy
Narayanaswamy
N
Jayaraman
D001391:Azo Compounds / Q000737:chemistry*; D050091:Dendrimers / Q000737:chemistry*; D015394:Molecular Structure; D010777:Photochemistry; D011108:Polymers / Q000737:chemistry*; D013237:Stereoisomerism; D013267:Stilbenes / Q000737:chemistry*; D013816:Thermodynamics
D001391:Azo Compounds; D050091:Dendrimers; D011108:Polymers; D013267:Stilbenes; C009850:azobenzene