A series of azobenzene functionalized poly(aminoamine) dendrimers (Gn-azo(n=1～5))and the corresponding model compounds were synthesized and characterized by 1H-NMR

13C-NMR

FTIR and elemental analysis

and the photo and H+ responsive behaviors of these dendrimers were studied through UV-Vis and H-NMR spectroscopy.It was found that the graft ratios of these azobenzene-shelled dendrimers were in the range of 70%～90%.A structural transition occurred between the third (G3-azo)and the fourth (G4-azo)generation

indicating that a relative open and loose structure of lower generation dendrimers became a dense spherical structure at higher generations.Therefore

the fourth (G4-azo)and the fifth (G5-azo)generation modified PAMAM dendrimers are ideal nano-containers that can encapsulate guest molecules.The modified dendrimers(Gn azo(n=1～5))exhibited the similar photo-responsive behaviors to azobenzene itself.Under 365 nm UV-light irradiation

the intensity of the π-π* band at 336～348 nm decreased

and the intensity of the n-π* band at 440～443 nm increased.The case was reverse under visible-light irradiation.1H-NMR results indicated that Gn-azo dendrimers could be protonized when adding trifluoroacetic acid (TFA) into the chloroform solution of Gn azo.It was also found that the protonated and nonprotonated dendrimers showed different behaviors in response to UV-light.The ratio of trans cis isomerization of the azobenzene chromophores was lower in protonated form when compared to that in non-protonated form.It might be due to that the NN group of azobenzene on the periphery was also protonized under the TFA environment.Moreover

the ratio of trans cis isomerization decreased with the generation increasing

which might be owing to the structure and size of dendrimers.The encapsulation ability of azobenzene-shelled G4 dendrimer decreased relative to the unmodified G4 dendrimer

but the release of salicylic acid was indeed slowed down because of the shielding effect of azobenzene chromophores on the periphery.More interestingly

the release of guest molecules was further slowed down with irradiating the modified dendrimers by UV-light.Under UV-light

the trans cis isomerization of the azobenzene chromophores occured

which made it more difficult for the salicylic acid to pass through the dendrimer periphery.