Multilayer Self-Assemblies as Electronic and Optical Materials
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Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
hold the film together, should have enough bonding affinity to overcome non-selective interactions. In this paper, we describe a multilayer molecular self-assembly with alternating macrocycles containing four charges and positively charged polymers. The macrocycles used are xoj3,y,8-tetrakis(5-sulfothienyl)-porphine (TSTP) and nickel phthalocyanine-tetrasulfonic acid (NiPc), tetrasodium salt; and the polymer is poly(diallyldimethylammonium chloride) (PDDA) (Fig. 1). soi
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Figure 1. Structures of a,3,'y,5-tetrakis(5-sulfothienyl)-porphine (TSTP) and nickel phthalocyanine-tetrasulfonate (NiPc), and their self-assembly of PDDA/NiPc/PDDAITSTP. The purpose of this study is to determine whether a macrocycle with four charges can be used as a building block to produce a self-assembled multilayer with optical properties. In order to gauge the success of thin-film formation, small-angle X-ray reflectometry is used to characterize the layered structure of this self-assembled system. In particular, X-ray reflectometry provides information about the electron density of the multilayer, its thickness, and the roughness of the growth interface.78 RESULTS AND DISCUSSION In the process of fabricating multilayer assemblies through layer-by-layer growth pattern, PDDA (polycation) will attract NiPc or TSTP (anions) molecules, which results in the growth of alternating PDDA/NiPc or PDDAITSTP multilayer. Multilayer growth is impossible with either component alone because polyions can not stack on top of themselves without multivalent counterions. The multilayer thin films are a result of sequential reactions between PDDA and NiPc or TSTP on the growth surface, which yields alternating layered structures of PDDA and NiPc or TSP. We do not expect that there is ordering or crystalline within each layer because of the nature of organic polymers. Considering long polymeric chains, a mixture formation through interpenetration and diffusion of the macrocycle molecules between layers is unfavorable while compared to layered structures. Figure 2 shows the optical spectra of multilayer thin-film growth from alternating dipping from negatively charged macrocycles and positively charged polymer PDDA. For PDDA/NiPc system, we observed strong Q bands at 630 nm and 665 nm. For the PDDAITSTP system, the strong phorphyrin Soret band at -430 nm was observed. These characteristic bands are ideal for monitoring the growth of multilayer thin films. Complex systems such as PDDA/NiPc/PDDA/TSTP can be obtained while replacing one of the macrocycle with another in
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every other deposition sequence. This flexibility at nanoscale of thin-film growth will allow incorporation of molecules with unusual properties such as charge transfer and nonlinear optical response. Figure 3 illustrates the successful growth of the multilayer in a layer-by-layer manner. Optical absorption spectrum shows that the amount of materials deposited
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