Langmuir-Blodgett Film of a Processable Polydiacetylene with Tricyanovinyldialkylaniline Chromophore
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INTRODUCTION Polydiacetylenes (PDAs) have attracted much attention over the past two decades due to their large third order nonlinear susceptibility and ultrafast response time [1-3]. Since macroscopic single crystals of PDAs can be prepared via a topochemical polymerization of diacetylene (DA) monomers, PDAs also offer better defined structures and properties than other conjugated polymers. However, most PDA crystals are largely intractable, thus many efforts were undertaken to improve their solubility and processability as well as to control properties such as chemical reactivity [4], electronic structure [5], spectroscopy [6], and second-order NLO properties [7-10]. Some of these efforts have included functionalization with different side groups, the urethane 4-BCMU [11] or the ester ClCin22 [12]. Recently, our research group reported on a soluble and processable PDA system that is the most versatile reported to date, in terms of chemical richness and diverse optical properties [13,14]. In addition to a conjugated backbone, these types of PDAs have unique structural features in that one side group is a chromophore and the other side group is an aliphatic and urethane functionalized group to promote solubility and processability. Since Tieke et al. reported LB multilayers of PDA with amphiphilic structure [15,16], a tremendous amount of research has been carried out on the molecular organization of PDA films by the LB technique. The
majority of these films were made from either assembling amphiphilic DA monomers on a substrate and subsequent polymerization or instantaneous polymerization of DA monomers at the air-water interface followed by film deposition. However, only a few examples of LB films of preformed PDA have been described [17-19]. Preformed PDA with a hydrophobic PDA backbone and two side groups with different hydrophobicities can be also used to achieve LB multilayers [19]. Herein, we report a new PDA system with N,N-dialkylaniline and alkylurethane glutamate ester groups as side groups. In order to introduce a chromophore with useful second-order NLO properties, the PDA was tricyanovinylated using postpolymerization functionalization. Furthermore, LB multilayers of functionalized preformed polymer were assembled using a horizontal dipping method. These multilayers showed both main chain ordering and asymmetric orientation of the TCV-aniline chromophore. The film formation and characterization of the structural and optical properties, including second-order NLO properties of this PDA, are described.
EXPERIMENTAL The detailed synthesis and postpolymerization functionalization (Scheme 1) of PDA-1 are reported elsewhere [13]. NC
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Scheme 1 . Tricyanovinylation of PDA-1 via postpolymerization functionalization. Surface pressure versus area isotherms were obtained by using a Langmuir film balance (MGW Lauda). Hydrophobic glass substrates were prepared by cleaning glass slides ultraso
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