Photoswitchable dendritic hosts: A dendrimer with peripheral azobenzene groups

We have studied the adducts formed by eosin (E) with a fourth generation dendrimer (D) that comprises 30 tertiary amine units in the interior and 32 naphthyl and 32 trans azobenzene units in the periphery. We have found that: (i) the all trans dendrimer D(32t) can be converted by irradiation with 365 nm light (Phi=0.12) into species containing, as an average, 4 trans and 28 cis azobenzene units, D(4t28c), that at 313 K undergoes a D(4t28c) -> D(32t) thermal back reaction (k = 7.0 x 10(-5) s(-1)); (ii) D(32t) and D(4t28c) extract 8 and, respectively, 6 eosin molecules from water at pH 7, yielding the species D(32t)D8E and D(4t28c)superset of 6E; (iii) eosin uptake is significantly faster for D(32t) than for D(4t28c); (iv) irradiation at 365 nm of the D(32t)D8E species at 298 K leads to the release of two eosin molecules with formation of a photostable D(15t17c)superset of 6E species ((Phi = 0.15) that is also obtained from the back thermal reaction of D(4t28c)superset of 6E at 313 K (k = 2.7 x 10(-5) s(-1)); (v) thermal release of E from D(32t)superset of 6E is much faster than from D(4t28c)superset of 6E; and (vi) excitation of E in the adducts sensitizes the cis -> trans but not the trans -> cis) isomerization. The results obtained show that the isomerization of the 32 peripheral azobenzene units controls to some extent the hosting capacity of the dendrimer and, viceversa, eosin molecules hosted in the dendrimer affect the isomerization process of its azobenzene units.