Stimuli-responsive dendritic structures: The case of light-driven azobenzene-containing dendrimers and dendrons

Azobenzene derivatives are very attractive for light-driven experiments, due to their easy trans/cis isomerization, which is one of the cleanest photochemical processes known to date. Azobenzenes have been linked in various places to the structure of dendrimers and dendrons. The azobenzene groups can constitute the terminal functions of the dendrimers (or dendrons): a single azobenzene can be located at the core of a dendron or a dendrimer; finally, several azobenzene groups can be incorporated at one or several layers of dendrimers or dendrons. The first part of this review is an exhaustive compilation of all these types of azobenzene-containing dendritic structures. Most dendritic macromolecules possessing azobenzenes in their structure were synthesized with the aim of studying their photo-responsive properties. The influence of the placement of azobenzene groups on the isomerization properties was determined in solution: generally, burying the azobenzene inside the structure induces reluctance to isomerization, especially when the generation increases, but has a larger influence on the size modification. Several dendritic azobenzenes were also studied as thin films, in particular monolayer Langmuir and Langmuir-Blodgett films. Photo-induced alignment was observed for thin films obtained by spin-coating, others were used for holographic experiments. Self-arrangements of photo-responsive dendrimers have resulted in liquid crystalline organizations, vesicles, nano-fibers, and organogels. In addition to these particular properties, some more practical uses were reported such as for molecular imprinting, to trap/release guest molecules, or for drug delivery by cleavage of the N=N bond. (C) 2010 Elsevier B.V. All rights reserved.