Infrared and Resonance Raman Spectroscopic Study on the Photopolymerization Process of the Langmuir−Blodgett Films of a Diacetylene Monocarboxylic Acid, 10,12-Pentacosadiynoic Acid

Abstract

Absorption, infrared reflection absorption (IRA), and resonance Raman scattering (RRS) spectroscopies were applied to obtain direct information about the conformation change of a polydiacetylene (abbreviated to PDA) produced by UV irradiation of the Langmuir−Blodgett (LB) films of the cadmium salt of a diacetylene derivative, 10,12-pentacosadiynoic acid (DA), with the number of monolayers from 1 to 31. The rate and extent of the polymerization of DA monitored by the relative intensity change of the IRA bands due to C⋮C stretching vibrations were correlated with orientation changes of the alkyl groups of DA and changes in the conjugation length of the PDA backbone elucidated by RRS spectra in the C⋮C stretching vibration region. The results proved the following mechanism of the conversion from a blue (λmax ≈ 635 nm) to a red phase (λmax ≈ 540 nm) during the polymerization of DA. The conversion proceeds through two steps. The polymerization reaction itself is almost completed in the first step, although a residual reaction may take place in the second step. During the first step the plane formed by the all-trans zigzag backbone of the alkyl groups changes its arrangement in the following way: the orientation of the long axis of the alkyl backbone changes from a tilted state to a less tilted one, keeping the axis perpendicular to the long axis within the plane parallel to the substrate surface. This orientation change conforms to an increase in the interlayer spacing of the LB films of long-chain diacetylene monocarboxylic acids during their photopolymerization process observed through a small angle X-ray diffraction technique (Lieser, G.; et al. Thin Solid Films (1980, 68, 77). During the first step the PDA backbone keeps a fully extended conformation without any interruption of its conjugation, and the formation of the blue phase proceeds. When the polymerization proceeds to the second step, the regular plane of the all-trans alkyl group is converted to an irregular one containing gauche conformations. This conversion causes an interruption of the fully extended backbone structure and a reduction of the average conjugation length, resulting in the formation of the red phase.

Rational Design of Conjugated Polymer Supramolecules with Tunable Colorimetric Responses

“…Previously, it has been suggested that the red-phase PDA is in more disordered state than the blue-phase counterpart. [84] However, observations made with AFM images of the blueand red-phase PDAs confirm the alkyl chains in red-phase PDAs exist in a more ordered state. [85] The observations made with AFM images, 13 C NMR data reported by Tanaka et al, [86] and theoretical calculations from Tripathy et al, [78] have led to the suggestion that the shear forces imposed on the PDA film induce sidechain rotation about the conjugated backbone which reduce the effective conjugation length of the polymer.…”

Stable polydiacetylene/ZnO nanocomposites with two-steps reversible and irreversible thermochromism: The influence of strong surface anchoring

“…1) [1,2,4,5]. The unperturbed PDA vesicles, in which the side chains arrange in an ordered fashion, exhibit a deep blue color [1,2,30,31]. The increase of temperature promotes the dynamics of side chains, which eventually breaks the hydrogen bonds at the surface of PDA vesicles [1,5,27,32,33].…”

High Molecular Orientation in Mono- and Trilayer Polydiacetylene Films Imaged by Atomic Force Microscopy

“…This conformation is affected by the orientation and packing of the hydrocarbon side chains pendant to the backbone that can either alter the length or change the mode of n-conjugation (10). A variety of analytical techniques, such as electron diffraction (11), FT-IR (12), resonance Raman (13), and fluorescence polarization (14), have been employed to identify the molecular orientation of the side chains and backbones.…”