Effect of Metal Cations on Polydiacetylene Langmuir Films Alexander Upcher, †,‡ Yevgeniy Lifshitz, †,‡ Leila Zeiri, ‡,§ Yuval Golan, †,‡ and Amir Berman* ,‡,∥,⊥ † Department of Materials Engineering, ‡ Ilse Katz Institute for Nanoscale Science and Technology, § Chemistry Department, ∥ Department of Biotechnology Engineering, and ⊥ National Institute for Biotechnology, Negev (NIBN), Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel ABSTRACT: Polydiacetylene (PDA) Langmuir films (LFs) are a unique class of materials that couple a highly aligned conjugated backbone with tailorable pendant side groups and terminal functionalities. The films exhibit chromatic transitions from monomer to blue polymer and finally to a red phase that can be activated optically, thermally, chemically, and mecha- nically. The properties of PDA LFs are strongly affected by the presence of metal cations in the aqueous subphase of the film due to their interaction with the carboxylic head groups of the polymer. In the present study the influence of divalent cadmium, barium, copper, and lead cations on the structural, mor- phological, and optical properties of PDA LFs was investigated by means of surface pressure−molecular area (π−A) isotherms, atomic force microscopy, optical absorbance, and Raman spectroscopy. The threshold concentrations for the influence of metal cations on the film structure, stability, and phase transformation were determined by π−A analyses. It was found that each of the investigated cations has a unique influence on the properties of PDA LFs. Cadmium cations induce moderate phase transition kinetics with reduced domain size and fragmented morphology. Barium cations contribute to stabilization of the PDA blue phase and enhanced linear strand morphology. On the other hand, copper cations enhance rapid formation of the PDA red phase and cause fragmented morphology of the film, while the presence of lead cations results in severe perturbation of the film with only a small area of the film able to be effectively polymerized. The influence of the metal cations is correlated with the solubility product (K sp ), association strength, and ionic−covalent bond nature between the metal cations and the PDA carboxylic head groups. ■ INTRODUCTION Polydiacetylene Langmuir films (PDA LFs) are ultrathin organic layers produced on the air−liquid interface. These linear con- jugated backbone polymers consist of alternating triple and double bonds in an “yne−ene” motif. PDA forms robust, crystalline polymer films with well-defined linear strand mor- phology, strong optical absorbance in the visible range, and good mechanical stability. 1 There is growing interest and in- tense investigations of polyconjugated organic films which can be incorporated into novel multifunctional “molecular” devices. 2−4 Long chain, amphiphilic PDA films can be chemically modified, a property that renders them attractive due to their optical 5−8 and sensing 9−12 properties, as well as their use as effective templates for oriented nucleation of calcite 13,14 and semicon- ductor nanocrystals. 15−18 Polydiacetylene LFs undergo topotactic polymerization reaction under UV irradiation, from the colorless monomer film to the metastable blue phase of the polymer. Upon further irradiation, transition from the transient blue to stable red phase of the polymer occurs, followed by the degradation stage of the polymeric film at high doses of radiation. 19 Each chro- matic phase has two absorbance peaks, vibronic and excitonic, positioned at 590 and 640 nm for the blue phase and 500 and 550 nm for the red phase, respectively. 8 The conjugated back- bone of PDA shows a strong Raman scattering signal: the PDA blue phase Raman shifts are positioned at 1455 and 2085 cm −1 and the red PDA shifts are at 1515 and 2120 cm −1 for the double and triple bonds, respectively. 20,21 The polymerization process of PDA LFs was monitored in situ using synchrotron grazing incidence X-ray diffraction (GIXD). 22 The monomer to blue to red chromatic phase transitions were shown to be ac- companied by variations in crystal structure and molecular alignment within the LF. Transition from monomer to the blue phase involves in-plane shear motion of the molecules, while the blue to red transition involves a decrease in the planar unit cell area, manifested in a shorter d-spacing between the poly- mer backbones, and accompanied with simultaneous movement of the alkyl residues to a near-upright position. To describe the rate of phase transitions in polydiacetylene films, reaction ki- netic models were extracted from changes in the optical absorbance in the films. 19 It was found that the reaction rate was strongly dependent on the film substrate and thickness. Polymerization directly at the air−water interface was found to be 2−3 orders of magnitude faster compared to that of solid- supported films of the same material. Gaining control over structural, morphological, and optical properties of PDA LFs bears both fundamental and potential technological interest. Possible routes to these objectives can be Received: December 1, 2011 Revised: January 26, 2012 Published: January 30, 2012 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 4248 dx.doi.org/10.1021/la204735t | Langmuir 2012, 28, 4248−4258