Langmuir and Grafted Monolayers of Photochromic Amphiphilic Monodendrons of Low Generations Kirsten L. Genson, ² Jason Holzmuller, ² Ovette F. Villacencio, ‡ Dominic V. McGrath, ‡ David Vaknin, § and Vladimir V. Tsukruk* ,² Department of Materials Science and Engineering and Ames Laboratory and Department of Physics and Astronomy, Iowa State UniVersity, Ames, Iowa 50011, and Department of Chemistry, UniVersity of Arizona, P.O. Box 210041, Tucson, Arizona 85721 ReceiVed: May 11, 2005; In Final Form: August 31, 2005 Four generations of monodendrons with multiple dodecyl alkyl tails (AA-N, N representing number of alkyl tails from 1 to 8), an azobenzene spacer group, and a carboxylic acid polar head have been studied at the air-water and air-solid interface using AFM, GIXD, X-ray reflectivity, and UV-vis spectrometry. The one and two tail molecules formed orthorhombic lateral packing with long-range intramonolayer ordering. Good agreement between molecular models and thickness measurements indicated that the one and two tail molecules orient along the surface normal. The increase in the cross-sectional mismatch caused by the presence of the multiple chains for the higher generations disrupted the long-range ordering and forced the alkyl tails to adopt quasi-hexagonal structure. The higher generations (AA-4 and AA-8) formed a kinked structure with the alkyl tails oriented perpendicular to the surface with the azobenzene group tilted at a large degree toward the surface. The photoisomerization behavior in dilute solutions, at the air-water interface, and for grafted layers demonstrated that lower generation monodendrons maintained the photochromic behavior after chemical grafting to the silicon substrates, although the confinement of the molecules in monolayers significantly increased the reorganization time. Introduction One of the challenges in the field of nanotechnology is controlling the selective response of thin films, especially organized surface monolayers. 1 Recent investigations in this area have concentrated on tailoring the response of a thin film to environmental stimuli at the nanometer scale. 2 In this regard, molecules with azobenzene fragments have been considered as potential components of films with selective light-triggered responses. Indeed, a multitude of studies focused on various molecules with azobenzene fragments such as holographic media 3 for optical storage, 4 as reversible optical waveguides, 5 for photoalignment of liquid crystal systems, 6 and for drug delivery. 7 The azobenzene group is attractive due to the two stable isomers it assumes upon selective wavelength stimuli. The main complication of the inclusion of photoresponsive molecules in selective response monolayers is the preservation of the full and fast isomerization response. 8 Deposition of thin surface films onto solid supports often restricts the mobility of the photochromic fragments within the molecular architecture and thin films, thereby reducing the overall response and the reversibility. The tilted orientation of azobenzene containing molecules in self-assembled monolayers reduced the surface coverage (50-60%) and lowered the monolayer thickness, thereby resulting in a slight increase in thickness during photoisomerization. 9 Similarly, the tailoring of material’s properties, especially the packing structure, by tailoring the molecular architecture has been discussed in detail. 10 Since the discovery of dendrimers, a multitude of applications has been considered, but the overall theme of the studies has been the understanding of the exponential increase in end groups on the overall material’s behavior and properties. 11 Many studies focused on higher generation monodendrons and dendrimers with little attention paid to the lower generations. 12,13 The inclusion of hydrophobic and hydrophilic fragments creating an amphiphilic balance facilitates the study of the molecules in ordered monolayers at the air-water interface. 14 Combining experimental techniques at both the air-water and the air-solid interface provides in- depth understanding of the molecular ordering and rearrange- ment of fragments at both interfaces. To facilitate the grafting of the photochromic monodendrons onto solid substrates while preserving the photoisomerization of the molecules, we have investigated several chemically disparate polar heads. Previ- ously, we showed that the attachment of a bulky crown ether polar head to photochromic monodendrons created a kinked structure at the air-water and air-solid interface and preserved photochromic properties under a balance of cross-sectional areas of the bulky polar groups and dendritic alkyl shells. 15 In this paper, we report on the interfacial behavior of four generations of photochromic monodendrons, their molecular ordering, and its contribution to reversible photoisomerization at interfaces. Replacement of the bulky polar group in previously studied monodendrons 15 with a smaller functional group (i.e., a traditional polar group) is expected to promote chemical grafting to solid surfaces while preserving the cross-sectional area necessary for complete and reversible photoisomerization within the surface monolayers. The attachment of a traditional * Corresponding author: vladimir@iastate.edu. ² Department of Materials Science and Engineering, Iowa State Univer- sity. ‡ University of Arizona. § Ames Laboratory and Department of Physics and Astronomy, Iowa State University. 20393 J. Phys. Chem. B 2005, 109, 20393-20402 10.1021/jp0524678 CCC: $30.25 © 2005 American Chemical Society Published on Web 10/12/2005