Effect of Covalently Incorporated Azobenzenes on Dendrimers
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ABSTRACT Several new azobenzene-containing dendritic molecules have been prepared and characterized. The photoresponsive characterization of these dendrimers illustrates the insensitivity of the photochromic behavior of the azobenzene with respect to the nature of the attached dendritic fragment. INTRODUCTION The placement of the photochromic azobenzene moiety in macromolecular systems has resulted in reversible influence of a wide spectrum of polymer and polymer solution properties.' This influence is the result of photochemical trans-cis isomerization of the azo units (Figure 1) and subsequent associated changes in macromolecular conformation. 2 Conformational changes are believed to result from both the geometric change in the azobenzene chromophore as well as the associated change in polarity. The conformation of polymers governs their various physicochemical properties in solution such as viscosity and solubility. Azobenzene-containing polymers have also been prepared and optimized for applications in liquid crystal displays and devices, reversible optical storage snstems, nonlinear optical waveguides, photorefractive switches, and holographic gratings. Dendritic macromolecules, 4 with the processibility of more traditional organic polymers and the inherent precision in their synthesis, represent an intriguing scaffold for photoresponsive materials. Precise placement of photochromic moieties within a dendrimer interior could lead to more predictable control of photoinduced configuration changes, 5
allowing reversible6 alteration of previously demonstrated behavior such as liquid crystallinity and encapsulation. 9.0 A
5.5 A N=N
hv dark or hv
~ 60°
1800
Figure 1. Geometrical changes in azobenzene upon trans/cis isomerization. We 7 and others8 have prepared dendrimers with photochromic azobenzene units at either 8~ the exterior, 8a,e interior, 7,8b or throughouticd the dendritic architecture with various intentions. In 7 prior work we prepared examples of dendrimers with an azobenzene moiety as the central linker, and demonstrated that the photoresponsive behavior of the azobenzene is not perturbed by incorporation into the dendrimer structure. This is an important prerequisite for the use of photochromic units in polymers for optical switching and information storage applications.3 In 319 Mat. Res. Soc. Symp. Proc. Vol. 543 ©1999 Materials Research Society
Chart 1
"0O0 3
= TO
CH 2
Key:
A.
li
the present contribution, we describe new photochromic dendritic building blocks. Specifically, we have prepared benzyl aryl ether dendrons 1-3 with azobenzene focal units that have a benzyl alcohol functional group (Chart 1). This functional group can be used in a nucleophilic capacity or converted to a benzyl bromide and used in an electrophilic capacity to incorporate photoresponsive dendritic fragments onto a wide variety of small and macromolecules. Accordingly, incorporation of 1 and 2 into the next larger generation dendrons 4 and 5 is reported. EXPERIMENT General Methods All procedures were carried out
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