Photoresponsive Azobenzene Photonic Crystals

Abstract

We demonstrate azobenzene photochemically driven diffraction switching of a photonic crystal consisting of a crystalline colloidal array (CCA) polymerized within a hydrogel matrix. A novel azobenzene derivative that has a large ground-state activation barrier between the cis and trans forms in water is used. The system is actuated by excitation with UV light (wavelength of 365 nm), which photoisomerizes the azobenzene trans state to the cis ground state. The increased dipole moment of the cis state increases the free energy of mixing, causing a hydrogel swelling, which red-shifts the embedded CCA diffraction. Excitation with visible light photoisomerizes the cis state to the trans state, which resets the diffraction. This material acts as a memory storage material. Information is recorded and erased by exciting the photonic crystal in the UV or visible spectral region. The written information is read out completely and nondestructively by the wavelength of the Bragg diffraction (in this case, in the red).Many groups are developing fabrication methods to produce photonic crystals with band gaps in the visible, infrared (IR), and microwave spectral regions. [1][2][3][4][5][6] Photonic crystals are materials with periodic variations in their optical dielectric constants. The resulting periodic variations in the refractive index lead to the diffraction of light and to the occurrence of photonic band gaps. Light with frequencies within the band gaps cannot propagate within the photonic crystal materials. If the refractive index is purely real, then all of the light is diffracted. Photonic crystal materials provide the opportunity to control the flow of light. Photonic crystals may be the key elements of all-optical integrated circuits. 7 The earliest chemical approach to fabricating photonic crystals was through the self-assembly of highly charged monodisperse colloidal particles into crystalline colloidal arrays (CCAs). 8 These CCAs are complex liquids that self-assemble because of long-range electrostatic repulsions between particles into fluid, plastic, face-centered cubic (fcc) crystalline arrays. The CCAs diffract light (Bragg diffraction) in the ultraviolet (UV), visible, or near-IR range, depending on the colloidal particle array spacings. 8,9 Robust semisolid polymerized crystalline colloidal array (PCCA) materials were fabricated by imbedding the CCA into a hydrogel network (see Figure 1). 9 PCCAs can be fabricated from stimuli-responsive polymer networks, 10,11 where appropriate physical or chemical stimuli alter the PCCA volume, which alters the resulting CCA plane spacings and diffraction wavelengths. 12-14 Previously, we described photochemically controlled PCCA photonic crystals, which operated in organic solvents such as dimethylsulfoxide (DMSO), which utilized azobenzene and spirobenzopyran photochemistry. 15 In our azobenzene-functionalized PCCA, the photochemistry involved photoisomerization between the trans and cis isomers in an organic medium. In the spiropyran-modified PCCA, the photochromi...

Isomerization and electronic relaxation of azobenzene after being excited to higher electronic states

“…According to the previous calculations on the photoisomerization reaction of azobenzene, the active space including six electrons distributed among six orbitals, referred as to CAS (6,6), can reasonably map the PES of photoisomerization on the ground state and three excited states [45]. In this study, we chose an active space of ten electrons in eight orbitals, namely CAS (10,8), in order to describe more excited states and not heavily increase the calculation workload at the same time.…”

“…Organic or inorganic materials based on the azobenzene (AB) unit have attracted significant attention because of their photoresponsive properties that arise from the reversible cis-trans photoisomerization of AB [1][2][3][4][5][6][7][8][9][10]. Accordingly, the photochemistry and photophysics of AB is again becoming a research focus in recent years [11][12][13][14][15].…”

Optical and Physical Applications of Photocontrollable Materials: Azobenzene-Containing and Liquid Crystalline Polymers

“…This is an example of anisotropic changes in optical constants and physical form, which may find uses in advanced applications. On the other hand, Asher and coworkers developed photo-responsive colloidal crystals composed by hydrogel network possessing azobenzene moieties [48,49]. The diffraction peak switched upon irradiations with UV or visible light.…”

Poly(vinyl alcohol) Rehydratable Photonic Crystal Sensor Materials

“…Alterations to the hydrogel polymer that result in an increase in the polymer solubility make the free energy of mixing more favorable, thereby swelling the hydrogel and red-shifting the diffraction wavelength. This motif was previously demonstrated for photochemically controlled photonic crystal materials [82][83][84][85] and utilized with the sensing of creatinine. [68] This new PVA photonic crystal hydrogel material efficiently Bragg diffracts light and can be reversibly dried and rehydrated, without the use of fillers, while retaining the original diffraction and swelling properties.…”