Volume 54, Number 8, 2000 APPLIED SPECTROSCOPY 1151 0003-7028 / 00 / 5408-1151$2.00 / 0 q 2000 Society for Applied Spectroscopy Spectroscopic Investigations of Polyamido Amine Starburst Dendrimers with Reichardt’s ET-30 Dye DANA L. RICHTER-EGGER, HONG LI, and SHERYL A. TUCKER * Department of Chemistry, University of Missouri, Columbia, Missouri 65211-7600 The association between Reichardt’s ET-30 dye and amine- and carboxylate-terminated polyamido amine (PAMAM) dendrimers was investigated and shown to be different from results reported for micelles and traditional polymers. The absorption spectra illus- trated that the solvatochromic, microenvironmental polarity probe associated with PAMAM dendrimers in less polar regions with a polarity comparable to 1-decanol. Model compounds that mimic PAMAMs’ surface groups and branching moieties were used to bet- ter dene the associated dye’s location and demonstrated that ET- 30 penetrated beyond the dendrimer’s surface groups, into the in- terfacial region. Additionally, changing sample preparation meth- ods controlled the nature and extent of the dye/dendrimer associa- tion. Under certain experimental conditions, the dye/dendrimer complexes formed large aggregates. Index Headings: Dendrimers; Reichardt’s ET-30 dye; Spectroscopy. INTRODUCTION Dendrimers are a highly diverse class of polymers that, unlike traditional polymers, have well-dened architec- ture. They are constructed from a core unit by iteratively adding branching units called generations, G (Fig. 1). Po- lyamido amine (PAMAM) dendrimers are commercially available with carboxylate terminal groups (PAMAM- CT), designated as half generations (e.g., G3.5), and with amine terminal groups (PAMAM-AT), designated as whole generations (e.g., G4). Many dendrimers are de- scribed as unimolecular micelles because of similar struc- tural features (Fig. 1). 1–4 Micelles, three-dimensional aggregates of surfactant monomers, typically have nonpolar interiors due to the surfactant’s hydrocarbon tails, and ionic head groups on their surface. Similarly, PAMAM dendrimers have rela- tively nonpolar interiors 3 and ionic (PAMAM-CT) or moderately polar (PAMAM-AT) surfaces. Micelles are dynamic systems, in which free surfactant monomers continuously migrate into and out of the micelle. They form only at concentrations above the critical micelle concentration (cmc), which is specic to each surfactant. For these reasons, micelles vary signicantly in size and shape. On the other hand, dendritic polymers are static because their ‘‘monomers’’ or repeat units are covalently bonded, eliminating any artifacts related to a cmc. They are advantageous because they are monodisperse, stable over a large pH range, 5 soluble and stable in many or- ganic solvents, and can be functionalized easily. 6,7 The unique architecture of dendritic polymers produces equally unique interior and exterior microenvironments. The purpose of this research is to advance the knowledge of these novel microenvironments and the way in which Received 8 January 2000; accepted 21 March 2000. * Author to whom correspondence should be sent. they are affected by generation—information that is vital to future applications. Previous spectroscopic studies of dendrimers 2,8–19 have frequently used uorescent probes, such as pyrene and 1,6-diphenyl-1,3,5-hexatriene. 20–22 In many of these studies, authors note a uorescent back- ground signal attributed to the PAMAM dendrimers, which is rather unexpected. Instead of a uorescent probe, an absorption probe was chosen. ET-30’s (see be- low) solvatochromic band is in the visible region of the spectrum rather than the UV region where the dendrimers absorb. This allows for additional information about den- dritic polymers to be obtained, without the hindrance of a uorescent or matrix background signal or formation of excited-state complexes. In this study 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridi- nio) phenoxide, better known as Reichardt’s ET-30 dye (Fig. 2), is used to probe aqueous dendrimer solutions. 23 This solvatochromic, UV-visible absorption probe was specically chosen for this study because it has been ex- tensively used to study structurally similar systems—mi- celles and polymers. 24–30 Its low water solubility (2 3 10 26 M) 31 results in an excellent driving force for the dye to interact with PAMAM dendrimers and other types of organized media in aqueous solution. Reichardt’s dye 31 also has the highest sensitivity to microenvironmental po- larity of any UV-visible absorption probe available. For example, the wavelength maximum ( l max ) of the solva- tochromic band shifts from 453 nm in water to 843 nm in toluene. 31 Though stable in solution, ET-30’s intermolecular charge transfer, which is responsible for its solvatochrom- ic property, is pH sensitive. In acidic microenvironments, ET-30 becomes protonated, which blocks the intermolec- ular charge transfer and results in the loss of the solva- tochromic band. 31 This phenomenon is not an issue in the presence of the PAMAM dendrimers, because they are basic in aqueous solution. In the generations of PAMAM- AT and PAMAM-CT used in these studies, the measured pH was between 8 and 10. Additionally, an in-house pH study of ET-30 showed no signicant change in its ab- sorbance spectrum between pHs 7 and 11. In addition to the use of ET-30 to study PAMAM-AT and PAMAM-CT dendrimers, mimics of the dendrimers’ surface groups and branching moieties are also examined to better establish the location of associated ET-30. EXPERIMENTAL Amine- and carboxylate-terminated PAMAM dendri- mers in methanol were obtained from Dendritech Inc. (Ann Arbor, MI) and stored at 0 8 C. Stock solutions of ET-30 were prepared by dissolving the dye in methanol and were stored in the dark at room temperature. Mate-