Azobenzenes are widely used as light-responsive molecules in creating functional materials for applications ranging from non-linear optics to biotechnology. The light-sensitivity arises from the fully reversible light driven trans-cis-trans isomerization upon which the molecules exhibit large spectral and geometrical changes. Azobenzenes are often used as building blocks in self-assembling supramolecular materials. The self-assembly may strongly affect the isomerization dynamics of azobenzenes, especially the thermal half-life of the metastable cis-isomer. We show that the isomerization dynamics can be utilized to create a highly sensitive, optically readable relative humidity sensor.
The optically active molecules, i.e. hydroxyazobenzene derivatives, are embedded into a poly(4‑vinylpyridine) matrix, where they are supramolecularly bound to the polymer chains. Environmental humidity causes intrinsic changes in the thermal isomerization mechanism of the hydroxyazobenzene molecules, and this leads to a large change in the cis-isomer lifetime. The cis-lifetime decreases exponentially up to 3 orders of magnitude with the change from 0 to 100 %RH. The lifetimes are stable and highly reproducible, which allows a high accuracy of the sensor. The supramolecular concept allows to embed a high concentration of the probe molecules into the polymer, while retaining amorphous structure within the thin film. This allows a thin sensing layer and a fast response to changes of relative humidity. Our new humidity sensing concept is fully integrable with optical fibres and by optimising the materials, it may be extended to sensing of also other hydrogen-bonding gases.
Subwavelength-sized, periodically arranged holes in an opaque metal film have gained much attention since 1998, when Ebbesen et al. first reported the phenomenon of enhanced transmission of light through such a hole-array structure. Certain wavelengths show distinctly higher transmission than what would be expected based simply on the number of holes and the transmission of a single subwavelength hole, a phenomenon commonly attributed to different plasmonic modes in nanohole arrays. Traditionally, nanoscale holes and slits in metal films have been fabricated via electron-beam lithography or focused ion beam milling. Typically, finite hole arrays up to 50 μm in size with high control over hole size, shape, periodicity and resolution can be created with these methods. However, EBL and FIB become very costly and time-consuming to make larger-sized hole arrays and are not suitable for low-cost mass production. Herein, we exploit surface patterns on azopolymer films for making highly ordered and uniform arrays of nanoholes and nanoislands in thin gold films. The nanostructures can be created by employing azopolymer surface patterns as a template for metal deposition, after which the metal surface is subjected to large-area ion milling. Azopolymer-based surface patterning provides an easy way to vary the size and periodicity of the structures, which are manufactured homogeneously over large areas. The largest possible size of the structures depends merely on the size of the optical inscription beam and the used ion milling apparatus.
This contribution focuses on a relatively old topic of azobenzene photomechanics, namely the photoinduced surface patterning. The phenomenon was demonstrated alreay in 1995, yet it has not redeemed its promise as a simple, one-step patterning method that could challenge the more conventional microfabrication techniques. However, inspired by recent advances in fabrication techniques, materials development, and theoretical modelling, the field is going through a revival from both fundamental and applied perspectives. (i) How much (or how little) azobenzene needed in order to create the surface patterns? (ii) What is the maximum size of objects that can be moved with light? (iii) Can one pattern crystalline materials? (iv) Under what conditions ss the patterning process light-reversible? These questions will herein be addressed via four case studies, all employing supramolecular materials where non-covalent intermolecular interactions are used to attach the azobenzenes into a passive host matrix. All azobenzene-based material movements are triggered by photoisomerization and are therefore inherently related to one another, and therefore we believe our observations to provide useful insights also for photomobile materials and photomechanical actuation.
This presentation comprises two topics, both relating to the interplay of light and order in liquid-crystalline (LC) materials. Firstly, we will present a novel series of azobenzene-based, halogen-bonded supramolecular LCs, with rich photochemical phenomena and exhibiting both reversible photochemical crystal-to-isotropic and LC-to-isotropic phase transitions. Simultaneous analysis of light-induced changes in birefringence, absorption, and optical scattering allowed us to conclude that less than 4 % of the mesogenic units in the cis-form suffices to trigger the LC-to-isotropic phase transition. To the best of our knowledge, this is the first quantitative analysis of the phase transition process in supramolecular liquid crystals, demonstrating the versatility of these materials as functional liquid-crystalline assemblies and pinpointing their potential towards building supramolecular actuators. Secondly, we propose a conceptually novel approach to enhance the orientational optical nonlinearity of dye-doped LCs, based on polymer stabilization. Compared to azobenzene-triggered photochemical systems, photophysical systems, where the alignment change is caused by light-induced torques due to absorbing moieties, may provide some benefits, allowing for molecular reorientation only above certain threshold intensity, and reducing reorientation instabilities and fluctuations in the photostationary state. In addition to decreasing the light intensity at which self-phase modulation takes place, the polymer stabilization approach may open up a pathway towards all-optical realization of temporally stable photonic elements and guided-wave structures based on LC systems.
We study photoinduced molecular reorientation of the azobenzene derivative Disperse Red 1 embedded in poly(4-vinylpyridine) polymer matrix. Photoinduced axial order leading to birefringence and polar order leading to
second-order nonlinear optical (NLO) response are induced by purely optical means. These two photoinduced
properties are found to exhibit markedly different dependences on the chromophore concentration: the photoinduced
second-order NLO response reaches its peak already at 23 wt. % concentration while the photoinduced
birefringence increases up to 51 wt. % concentration. The results show that chromophore-chromophore intermolecular
interactions work against polar order already at modest concentration. The axial order, on the other
hand, is not as easily affected by such interactions.
The all-optical poling technique allows writing non-centrosymmetric gratings that are automatically phase-matched for
second-harmonic generation by purely optical means. One drawback of all-optical poling in organic materials is the poor
stability of the recorded gratings due to thermal and/or photo-induced molecular randomization. Using a two beam
technique, we have compared the all-optical poling process in different kinds of polymers with Disperse Red 1 dye:
guest-hosts with hydrogen-bond interactions between the guest dye and the host polymer and a side-chain polymer in
which the dyes are attached through covalent bonds. We show that in the investigated polymer systems, hydrogen-bonded
guest-hosts are capable of surpassing the stability of side-chain polymers.
We show that hydrogen bonding between azo molecules and polymer host enhances the photoinduced optical
anisotropy in azo-containing polymers without sacrificing the ease of processing of conventional guest-host
systems. The primary mechanism behind the enhancement is the possibility to use high dye doping levels
compared to conventional guest-host systems due to reduced aggregation tendency of the dyes. For Disperse
Red 1, the saturated birefringence is enhanced by a factor of 8 due to hydrogen bonding at 30 wt % loading.
Moreover, hydrogen bonding reduces the mobility of the guest molecules in the polymer host which improves
the temporal stability of the induced birefringence to a level comparable to side-chain polymers.
We demonstrate that the aggregation tendency of dye molecules in a host polymer can be significantly reduced by exploiting non-covalent interactions between the host polymer and guest dye molecules. Such interactions occur spontaneously with no need for chemical synthesis, and could thus be utilized to combine the ease of processing of traditional guest-host systems with the high dye concentrations achievable in covalently linked systems. We studied the aggregation properties of the common azo-dye Disperse Red 1 in polymers with different functional groups. Compared to a nonpolar polymer (polystyrene), dye aggregation tendency is substantially reduced in polar polymer matrices containing hydrogen-bond donating [poly(vinylphenol)] or hydrogen-bond accepting [poly(4-vinylpyridine)] functional sites. Furthermore, by forming a polyelectrolyte-dye complex [Disperse Red 1/poly(styrenesulfonic acid)], a dye monomer can be attached to approximately each polymer unit, resulting in dye concentration of 63 wt. %. Complexation through proton transfer was further studied by using a fluorescent dye 5-phenyl-2-(4-pyridyl)oxazole. Our results indicate that polymer-dye complexes could provide a facile route for new type of optical materials, with potential applications in various fields of optics and photonics.
We demonstrate a simple method for monitoring all-optical poling in real time. The susceptibility pattern
created by two collinear beams, a writing beam at the fundamental frequency and a seed beam at the secondharmonic
frequency, is reconstructed by two-beam second-harmonic generation due to the writing beam and
a non-collinear probe beam at the fundamental frequency. This leads to spatial separation of the seeding
and signal beams, and allows easy detection of the signal during the seeding period. When the probe beam
is sufficiently weak, it does not distort the collinear poling process. Furthermore, the method provides a
significantly stronger signal than monitoring based on a probe beam alone.
We demonstrate that the aggregation tendency of dye molecules in a host polymer can be significantly reduced by exploiting non-covalent interactions between the host polymer and guest dye molecules. Such interactions occur spontaneously with no need for chemical synthesis, and could thus be utilized to combine the ease of processing of traditional guest-host systems with the high dye concentrations achievable in covalently linked systems. We studied the aggregation properties of the common azo-dye Disperse Red 1 in polymer matrices with different functional groups. The dye aggregation tendency is substantially reduced in a polar polymer matrix and, most importantly, by forming a polyelectrolyte-dye complex, a dye monomer can be attached to approximately each polymer unit. This indicates that polymer-dye complexes provide a facile route for new type of optical materials, which could lead to applications in various fields of optics and photonics.
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