Azobenzene moiety variation directing self-assembly and photoresponsive behavior of azo-surfactants

The effect of varying the position of the azobenzene group within two comparable photoresponsive amphiphiles on their capability to form lyotropic liquid crystals (LLCs) was investigated in detail in this study. Two photoresponsive amphiphiles having comparable structures were designed and synthesized consisting of hydrophilic oligooxyethylene units, a hydrophobic alkyl chain and a light-sensitive azobenzene moiety. When the azobenzene group was located in the middle of the hydrophobic alkyl chain, multiple LLC phases were observed at various water contents in the azo-surfactant–water binary system. In contrast, when the azobenzene group was directly attached to the hydrophilic domain, the azo-surfactant–water binary system exhibited only lamellar phases. The temperature dependence of these self-organised nanostructures was also investigated by the combination of small angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and rheology. Under alternating UV and visible light irradiation, reversible trans–cis photoisomerization of the azobenzene group occurred efficiently in dilute solution for both azo-surfactants. However, only photoisomerization of the surfactant possessing the azobenzene group localized in the middle of the alkyl chain induced significant changes in the self-assembled structure and its bulk properties. This study demonstrates that self-assembly and photoresponsive behaviour of photosensitive amphiphiles is extremely sensitive to the position of the photoactive moiety within the surfactant molecular architecture.

Plasmon-enhanced two-photon-induced isomerization for highly-localized light-based actuation of inorganic/organic interfaces

Two-photon initiated photo-isomerization of an azobenzene moiety adsorbed on silver nanoparticles (Ag NPs) is demonstrated. The azobenzene is linked to a materials-binding peptide that brings it into intimate contact with the Ag NP surface, producing a dramatic enhancement of its two-photon absorbance. An integrated modeling approach, combining advanced conformational sampling with Quantum Mechanics/Capacitance Molecular Mechanics and response theory, shows that charge transfer and image charges in the Ag NP generate local fields that enhance two-photon absorption of the cis isomer, but not the trans isomer, of adsorbed molecules. Moreover, dramatic local field enhancement is expected near the localized surface plasmon resonance (LSPR) wavelength, and the LSPR band of the Ag NPs overlaps the azobenzene absorbance that triggers cis to trans switching. As a result, the Ag NPs enable two-photon initiated cis to trans isomerization, but not trans to cis isomerization. Confocal anti-Stokes fluorescence imaging shows that this effect is not due to local heating, while the quadratic dependence of switching rate on laser intensity is consistent with a two-photon process. Highly localized two-photon initiated switching could allow local manipulation near the focal point of a laser within a 3D nanoparticle assembly, which cannot be achieved using linear optical processes.