Photoactivity and pH Sensitivity of Methyl Orange Functionalized Poly(Propyleneamine) Dendrimers A. Dirksen, E. Zuidema, R. M. Williams, and L. De Cola* Universiteit van Amsterdam, Institute of Molecular Chemistry, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands C. Kauffmann and F. Vo 1 gtle* Kekule ´ Institut fu ¨ r Organische Chemie und Biochemie der Universita ¨ t Bonn, Gerhard-Domagk Strasse 1, 53121 Bonn, Germany A. Roque and F. Pina Departamento de Quı ´mica, Centro de Quı ´mica-Fina e Biotecnologia, Faculdade de Cie ˆ ncias e Tecnologia, Universidade Nova Lisboa, Quinta da Torre, 2825 Monte de Caparica, Portugal Received July 30, 2001; Revised Manuscript Received December 10, 2001 ABSTRACT: For the first time a pH indicator that responds to two different external stimuli, i.e. pH and light, namely methyl orange, has been implemented in a dendrimer. Six generations (G0-G5) of methyl orange-functionalized poly(propyleneamine) dendrimers (“MO dendrimers”) have been synthesized and characterized using 1 H NMR, 13 C NMR, and MALDI-TOF mass spectrometry. Subsequently, the response of the MO dendrimers toward pH changes has been monitored using UV-vis spectroscopy and the photophysical properties have been investigated. Furthermore, the photoisomerization from E to Z and the thermal recovery from Z to E, have been studied using UV-vis and transient-absorption spectroscopy. Interestingly, the response of the MO units toward pH changes is found to be generation dependent. On the other hand, the photophysical properties are found to be similar for all generations, except for the ǫ value, which deviates significantly from the expected value in the case of G4 and G5. Also the isomerization processes show no generation dependence within the same solvent, but different rate constants for the Z/E isomerization have been observed for different solvents. Introduction The interest in dendrimers has grown enormously over the past 2 decades, since they present a new class of molecules with some unique properties, in particular those substituted with groups able to perform specific functions. 1 Of particular interest are dendrimers con- taining switchable units, where controlled interconver- sion between two stable states is possible using external stimuli such as light 2 , protons, 3 or electrons. 4 Less common are dendrimers containing functional groups at the periphery, which can respond to two dif- ferent external stimuli, i.e., light and pH. This class of compounds not only can provide new information about the interactions within dendritic molecules, but also can produce materials with unique characteristics. Very suitable for this purpose is methyl orange, which is widely used as a pH indicator for titration in aqueous solutions, having a pH transition interval in water be- tween pH 3.1 (red) and pH 4.4 (yellow) (pK a ) 3.46). 5 The clear color change observed at the transition point is the result of protonation at the azo moiety, leading to an azonium ion, which is stabilized by mesomery (Scheme 1). 6 Methyl orange is not only a pH-sensitive but also a light-responsive molecule, since it contains a photo- isomerizable azo moiety, which can be converted from the thermodynamically more stable E form into the Z form upon light excitation. Subsequently, the Z form will normally recover to the E form either thermally or photochemically (Scheme 1). 7 Therefore, the double functionality with respect to light and pH of methyl orange, once attached at the periphery of a dendrimer, is an interesting tool to study the possible interactions within dendritic molecules. Here we present the complete synthesis, including a full characterization with 1 H NMR, 13 C NMR, and MALDI-TOF mass spectrometry, and the photophysical properties of six generations (G0-G5) of methyl orange functionalized poly(propyleneamine)dendrimers (“MO dendrimers”). Furthermore, the solvent dependency of the photoisomerization and the thermal Z to E isomer- ization have been investigated, as well as the spectro- scopic behavior as a result of the protonation at the azo moieties. This resulted in the case of the protonation in the finding of different properties for the higher generations (dendritic effects) as compared to methyl orange itself. The isomerization processes were found to be generation independent. Six generations (G0-G5) of MO dendrimers have been synthesized through the reaction of the sulfonic acid chloride of methyl orange with amine-functional- ized poly(propyleneamine) dendrimers. 8 Methyl orange itself could not be used as a model compound for this study because of its very different solubility properties as compared to the MO dendrimers. For this purpose, G0 has been synthesized, by reacting propylamine with the sulfonic acid of methyl orange, to create a good model compound containing the same sulfonamide group as the higher generations. The number of MO units increases from 1 in G0, 3 in G1, 8 in G2, 16 in G3, and 32 in G4 to 64 in G5 (Figure 1). Photophysical Properties The absorption spectra of G0-G5 show ππ* bands at 305 nm and at 445 nm (ǫ ≈ 25 000 M -1 cm -1 ) charac- teristic of the E-MO units. The nπ* band typical of azobenzene units 9 is not visible, because the more intense ππ* band appears in the same spectral region 2743 Macromolecules 2002, 35, 2743-2747 10.1021/ma011350o CCC: $22.00 © 2002 American Chemical Society Published on Web 02/28/2002