Functional doping of acrylic PMMA with photochromic dyes

Abstract

Work from Poland has studied the thermodynamic and kinetic stability of PMMA when doped with a photochromic dye molecule. Visible light absorption is induced in fabricated photochromic PMMA fibres for optoelectronic device applications. PMMA fibres doped in local regions with Bis-MSB under UV irradiation. Molecular switches are molecules that display a binary response when exposed to specific conditions or stimuli. A common example of this is a structural change in molecular switch materials when exposed to UV radiation. This means that meta-stable, higher-energy structural orientations of a molecule can appear under UV irradiation. A switching phenomenon between low- and high-energy geometries is often fully reversible making these systems suitable for use in several applications including optical switches, attenuators, holograms, optical memory, and smart textiles. The chemical structure of Photochromic Bis-MSB incorporated into PMMA fibres. PMMA, poly(methyl methacrylate), is a transparent thermoplastic material that is versatile, strong and recyclable. PMMA optical fibres offer numerous benefits, including high flexibility and numerical aperture, low production cost, and good biocompatibility. A current limitation of PMMA fibres compared to optically active silica fibres is relatively high optical absorption in the visible and near infrared region of the electromagnetic spectrum. The incorporation of photosensitive dyes introduces additional absorption bands and increases the frequencies of light that are absorbed by the material. A common photochromic dye, 1,4-bis(2-methylstyryl)benzene (Bis-MSB), displays high transmittance in the visible region of the electromagnetic spectrum. Bis-MSB also behaves as a molecular switch when exposed to light, where its geometry is changed from linear (cis) to bent (trans) configurations following bond rotation. Thus, incorporating Bis-MSB into PMMA creates a coloured material with controllable and reversible broad-range light absorption. Coloured photoresponsive materials that are transparent, such as Bis-MSB doped PMMA, are favourable polymer materials for organic electronics as they efficiently absorb light within a useful region of the electromagnetic spectrum. In this issue of Electronics Letters, Dr Piotr Miluski and colleagues at Bialystok University of Technology, Poland, fabricate Bis-MSB-doped PMMA and report the optical transmission properties and kinetic relaxation lifetimes of this unique material. Following fabrication of the Bis-MSB PMMA fibres, the samples were exposed to high-energy UV radiation with a 1 MW peak power at 355 nm laser. A colour change was immediately observed, which visibly demonstrates the molecular structural switch of Bis-MSB under UV radiation. A comparison of light transmission between the undoped fibres and the Bis-MSB-doped PMMA fibres is made in the paper. This comparison shows the frequency change of light absorption due to the presence of the dopant molecules and demonstrates that the dopant induces wideband visible light absorption between 400–670 nm. This range of wavelengths is of particular interest for solar cell applications. Thermal relaxation was shown to lead to the decolouration of the fabricated polymer fibres, which was further supported by measured relaxation times in an analysis of the kinetic properties of the system. Long relaxation times can be useful for UV indicators and optical memory systems. The “Main advantage of (Bis-MSB) doped PMMA is that it offers high transmittance of its lower energy form in the VIS spectrum range” said Miluski. The ability to harness and modulate the optical properties of a molecule and control its free-volume as it rotates from linear to bent is well demonstrated within this paper. The development of organic-based functional materials for optical fibre technology introduces some limitations, including organic breakdown at high temperatures, solvent evaporation and low thermal conductivity of polymers. Moreover, the dopant distribution and polymer host homogeneity have to be carefully controlled to ensure a continuous optical response throughout the material. Recent research developments in the field of smart organic materials offer new functional properties that do not suffer from the described disadvantages. Low-energy consumption of organic devices offers significant advantages over well-developed silicon-based electronics. In the field of photochromic polymers, one of the main challenges is the ability to modulate high-frequency optical properties. Dr Miluski predicts that photochromic PMMA fibres will be appropriate for use in real-time holograms and for optical radiation modulation in telecommunication applications. Ultra-stable cis/trans isomerisation is also strongly desired in the field of logical gates and optical memory, where the use of photo-responsive PMMA fibres could offer both lower power consumption and higher storage density.