Photoswitching dendrimers

Vogtle et al. reported the first example of a photoswitching dendrimer [94] with six azobenzene moieties attached to a derivative of 1,3,5-trisubstituted benzene as the central core. Irradiation of the all (F)-isomer at 313 nm led to a photostationary equilibrium where most of the azobenzene units were switched to the (Z)-configuration. Conversely, irradiation of this species again at a lower energy frequency (436 nm) led to a second photostationary equilibrium where the (F)-form was dominant however, it was not proven as to how many azobenzene units isomerized after irradiation. 

Vogtle and co-workers first reported a photoswitchable dendrimer [33] with six peripheral azobenzene groups, which took advantage of the efficient and fully reversible photoisomerization reaction of azobenzene-type compounds (Scheme 7). In a follow-up study [34], polypropylene imine) dendrimers bearing azobenzene moieties (p-Im-Gn, n = 1-4) on the periphery were synthesized. These dendrimers displayed similar photoisomerization properties as the azobenzene monomers. Irradiation of the all-E azobenzene dendrimers at 313 nm led to the Z-form dendrimers, while irradiation at 254 nm or heating could convert the Z-form dendrimers back to the E-form dendrimers. The observation that the... 

Fig. 8.25 The first photoswitchable dendrimer (according to Vogtle et a/.) intermediates with partly isomerised azo groups are not shown...

McGrath and Junge [36] reported a photoresponsive poly(aryl ether) dendrimer with azobenzene as the dendrimer core. These dendrimers exhibited reversible trans to cis photoisomerization by irradiation at 350 nm. The authors proposed the use of this type of dendrimer as novel photoswitchable transport vectors. This is based on the expected ability of dendrimers to encapsulate or eject small molecules reversibly upon light perturbation. 

Functionalisation of PO PAM-dendrimers with the push/pull azobenzene derivative methyl orange (cf. Fig. 5.21) [27] offers further photoswitching opportunities. The change in colour of this pH indicator from red (pH 3.1) to yellow (pH 4.4) can be explained by protonahon of the azo function to form the me-somerically stabilised azonium ion (Fig. 5.22). 

POPAM dendrimers bearing up to 32 photoswitchable azobenzene groups at the periphery were used as host compounds for eosin Y (2, 4, 5, 7 -tetrabromo-fluorescein dianion) (see also Section 5.1.2). One reason for the choice of this dye was that it shows strong fluorescence which should be influenced by being enveloped by the dendrimer. The other reason was that the energy of its lowest triplet state is higher than that of the lowest triplet state of azobenzene, leading... 

Starting from l,3,5-tris(bromomefhyl)benzene as core unit, Vogtle et al. constructed the first dendrimer with intramolecularly accumulated azobenzene entities in the periphery by convergent synthesis (Fig. 8.25) it expectedly exhibits complex photoswitchable properties owing to the possibility of intramolecular E/Z isomerisation [79, 80]. 

The variety of the examples listed above documents that dendrimer chemistry has attained increasing interest. In this compilation of recent results the trend towards functional and application-oriented molecules including biochemically active, photoswitchable, and polymerlike dendrimers is particularly apparent. Dendrimers cross the boundaries of classical organic chemistry and as new materials will penetrate deeper into the topical fields of nanostructures , supramolecules and polymers in the future. Increasing industrial research on dendrimers and the commercial availability of PAMAM and polyamine dendrimers should stimulate further investigations in this field. 

Archut A, Azzellini GC, Balzani V, Cola LD, Vogtle F. 1998. Toward photoswitchable dendritic hosts interation between azobenzene functionalized dendrimers and eosin. J Am Chem Soc 120 12187 12191. 

Photoswitchable supramolecular dendrimer-like structures were constructed starting from azobenzene-based and CD-based building blocks. For example a bis-azobenzene with a dipyridyl linker trans-Azo dimer) and a p-CD trimer formed an hyperbranched structure whose shape could be controlled by light (Fig. 11). AFM evidenced branched structures of several microns turning to disordered particles upon UV irradiation. Moreover a supramolecular dendrimer was recently built up in water starting from an hydrophilic hyperbranched polyglycerol with a-CD apex (CD-g-HPG) and an hydrophobic hyperbranched poly(3-ethyl-3-oxetane-methanol) with azobenzene apex (AZO-g-HPBO) (Fig. 12). The two components in 1 1 molar ratio selfassembled into a Janus-like dendrimer (JHBP, HPBO-6-HPG) of diameter ca. 5.1 nm, due to inclusion complexation of the azobenzene apex into the CD cavity of the partner... 

Liao, L-X., Stellacci, F., and McGrath, D.V. (2004) Photoswitchable flexible and shape-persistent dendrimers comparison of the interplay between a photochromic azobenzene core and dendrimer structure. J. Am. Chem. Soc., 126, 2181-2185. 

Puntoriero, E, Ceroni, P, Balzani, V., Bergamini, G., and Vogtle, F. 2007. Photoswitchable dendritic hosts A dendrimer with peripheral azobenzene groups./Am Chem Soc 129(35), 10,714-10,719. 

Currently, dendrimer research is developing swiftly in the direction of highly functional materials. Also in the field of photoactive dendrimers the complexity of the systems has increased enormously. The investigation of dendritic structures functionalized with luminescent groups [14], photoswitchable units [15], energy and/or electron donor-acceptor components and the implementation of such functionalized dendrimers in devices [16], provide insight in the fundamental processes occurring in such complex systems and in their fumre applications. 

Archut, A., AzzelHni, G.C., Balzani, V., Cola.L. D., and Vogtle, F., Toward photoswitchable dendritic hosts. Interaction between azobenzene-functionalized dendrimers and eosin, /. Am. Chem. Soc., 120, 12187,1998. 

---