Mesoscopic order and the dimentionality of long-range resonance energy transfer in supramolecular semiconductors

We present time-resolved photoluminescence measurements on two series of oligo-p-phenylenevinylene materials that self-assemble into supramolecular nanostructures with thermotropic reversibility in dodecane. One set of derivatives form chiral, helical stacks while the second set form less organised, frustrated stacks. Here we study the effects of supramolecular organisation on the resonance energy transfer rates. We measure these rates in nanoassemblies formed with mixed blends of oligomers and compare them with the rates predicted by Foerster theory. Our results and analysis show that control of supramolecular order in the nanometre lengthscale has a dominant effect on the efficiency and dimentionality of resonance energy transfer.Comment: 17 Pages, 5 Figures, Submitted to J. Chem. Phy

Dual light and temperature responsive micrometer‐sized structural color actuators

Externally induced color- and shape-changes in micrometer-sized objects are of great interest in novel application fields such as optofluidics and microrobotics. In this work, light and temperature responsive micrometer-sized structural color actuators based on cholesteric liquid-crystalline (CLC) polymer particles are presented. The particles are synthesized by suspension polymerization using a reactive CLC monomer mixture having a light responsive azobenzene dye. The particles exhibit anisotropic spot-like and arc-like reflective colored domains ranging from red to blue. Electron microscopy reveals a multidirectional asymmetric arrangement of the cholesteric layers in the particles and numerical simulations elucidate the anisotropic optical properties. Upon light exposure, the particles show reversible asymmetric shape deformations combined with structural color changes. When the temperature is increased above the liquid crystal-isotropic phase transition temperature of the particles, the deformation is followed by a reduction or disappearance of the reflection. Such dual light and temperature responsive structural color actuators are interesting for a variety of micrometer-sized devices

Structural characterization of new fluorinated mesogens obtained through halogen-bond driven self-assembly

We describe the synthesis and characterization of new trimeric complexes obtained upon halogen-bond driven self-assembly of 1,4-diiodotetrafluorobenzene or α,ω- diiodoperfluoroalkanes, acting as XB-donors, with an alkoxystilbazole derivative functionalized with a methacrylate terminal group, acting as XB-acceptor. Despite the fact that the starting materials are not mesomorphic in nature, the obtained halogen-bonded complexes exhibit monotropic LC behaviour with smectic A phases possibly resulting from segregation between fluorocarbon and hydrocarbon chains. The obtained supramolecular mesogens possess reactive groups suitable for incorporation into liquid crystalline elastomeric actuators

Tuning MEMS cantilever devices using photoresponsive polymers

Microelectromechanical systems (MEMS) energy harvesting devices have had limited commercial success partly due to the frequency mismatch between the device and the vibration source. Tuning the cantilever device is one possible solution but developing a tunable MEMS device is difficult. This paper demonstrates a novel method of tuning a MEMS cantilever device post-fabrication by using light-responsive azobenzene liquid crystal polymers (LCP). Light exposure causes the photoresponsive polymers to change their elastic modulus, thus affecting the resonant frequency of the device. The photoresponsive polymer was integrated with three different MEMS cantilever substrates including LCP, parylene, and silicon. The three cantilever beams all demonstrated changes in resonant frequency when exposed to UV light of 10.4%, 8.13%, and 4.86%, respectively. The change in resonant frequency is dependent on the stiffness of the substrate, the thickness of the azo-LCP, the intensity and duration of the light exposure, and the wavelength of the light. The results in this paper validate that light responsive polymers can be used to reduce the frequency of MEMS cantilevers post-fabrication, which could lead to developing devices that can be precisely tuned for specific applications

Exciton bimolecular annihilation dynamics in supramolecular nanostructures of conjugated oligomers

We present femtosecond transient absorption measurements on $\pi$-conjugated supramolecular assemblies in a high pump fluence regime. Oligo(\emph{p}-phenylenevinylene) monofunctionalized with ureido-\emph{s}-triazine (MOPV) self-assembles into chiral stacks in dodecane solution below 75$^{\circ}$C at a concentration of $4\times 10^{-4}$ M. We observe exciton bimolecular annihilation in MOPV stacks at high excitation fluence, indicated by the fluence-dependent decay of $1^1$B$_{u}$-exciton spectral signatures, and by the sub-linear fluence dependence of time- and wavelength-integrated photoluminescence (PL) intensity. These two characteristics are much less pronounced in MOPV solution where the phase equilibrium is shifted significantly away from supramolecular assembly, slightly below the transition temperature. A mesoscopic rate-equation model is applied to extract the bimolecular annihilation rate constant from the excitation fluence dependence of transient absorption and PL signals. The results demonstrate that the bimolecular annihilation rate is very high with a square-root dependence in time. The exciton annihilation results from a combination of fast exciton diffusion and resonance energy transfer. The supramolecular nanostructures studied here have electronic properties that are intermediate between molecular aggregates and polymeric semiconductors

Full Color Camouflage in a Printable Photonic Blue-Colored Polymer

A blue reflective photonic polymer coating which can be patterned in full color, from blue to red, by printing with an aqueous calcium nitrate solution has been fabricated. Color change in the cholesteric liquid-crystalline polymer network over the entire visible spectrum is obtained by the use of nonreactive mesogen. The pattern in the coating is hidden in the blue color dry state and appears upon exposure to water or by exhaling breath onto it due to different degrees of swelling of the polymer network. The degree of swelling depends on the printed amount of calcium which acts as a cross-linker. The printed full color pattern can also be hidden simply by using a circular polarizer. The responsive full color camouflage polymers are interesting for various applications ranging from responsive house and automobile decors to anticounterfeit labels and data encryption