Chiara Bertarelli

Photochromic Electret: A New Tool for Light Energy Harvesting.

In this paper, a photochromic electret for light energy harvesting is proposed and discussed. Such electret directly converts the photon energy into electric energy thanks to a polarization modulation caused by the photochromic reaction, which leads to a change in dipole moment. Theoretical concepts on which the photochromic electret is based are considered with an estimation of the effectiveness as a function of material properties. Finally, an electret based on a photochromic diarylethene is shown with the photoelectric characterization as a proof of concept device.

Kinetics of photochromic conversion at the solid state: quantum yield of dithienylethene-based films.

Quantum yield is one of the most important properties of photochromic systems. Unfortunately, a lack of data at the solid state exists, because measurements are intrinsically not straightforward. A kinetic model describing the conversion of the photoactive species is reported and both analytic and numeric solutions are provided according to relevant cases. The model is then applied to measure the quantum yield of dithienylethene-based polymers; the ring-opening quantum yield is measured for different laser beam profiles (i.e., Gaussian and uniform) and at different wavelengths, showing an increased value with increasing photon energy.

Diarylethene-based photochromic polyurethanes for multistate optical memories

Photochromic polyurethanes (PPUs) are synthesised by reaction between a diarylethene end-capped with hydroxyl groups and an aliphatic diisocyanate. In situpolymerisation provides amorphous films with remarkable optical properties. The possibility to vary the amount and the chemical structure of the photochromic monomer without affecting the reactivity of the polymerization allows one to tune in a wide range the optical properties of these films. In addition, different diarylethenes can be mixed together to give copolymers which may find application as photochromic layers for multistate optical memories. A setup for a non-destructive readout based on Raman signal is proposed.

Subwavelength nanopatterning of photochromic diarylethene films

The resolution of optical patterning is constrained by the far-field diffraction limit. In this letter, we describe an approach that exploits the unique photo- and electro-chemistry of diarylethene photochromic molecules to overcome this diffraction limit and achieve sub-wavelength nanopatterning.

Photochromic materials for holography and astronomy

Many of the optical functions of astronomical instrumentation and other disciplines can be performed by holographic optical elements (HOEs). These objects transmit and manipulate light analogously to more conventional lenses, mirrors, and diffraction gratings. Of particular interest are computer-generated holograms (CGHs), which can be designed and constructed purely from knowledge of their optical layout and intended function, and without requiring any physical reference object. In general, holograms fall into two classes: amplitude elements and phase elements. Amplitude elements are created by modulating the transparency of the active substrate, while phase elements are created by modulating the substrate’s refractive index and/or thickness. Although many materials are suited to holography, our research has mainly centered on photochromic materials, which undergo a reversible color change when illuminated by specific wavelengths of light: typically, they become colored when illuminated by UV light, while visible light renders them colorless again.1 As shown in Figure 1, this color change is basically a change in transparency in the visible light range. At near-IR wavelengths (typically 800–2000nm), both forms are transparent, and the transition is instead associated with a change in refractive index. These differences of transparency and refractive index can be exploited to create amplitude elements in the visible light range and phase elements in the near-IR.2 Among the different classes of photochromic materials, we have focused our attention on diarylethenes, whose thermal stability and fatigue resistance make them especially attractive candidates.3 Like all photochromic materials, they have the advantages of rewritability and self-development. Rewritability means that a hologram can be erased from the substrate, and a new one written over it, as often as desired. Self-development Figure 1. Ultraviolet–visible–near-IR spectrum of uncolored (black line) and colored (blue line) photochromic thin films. A, B: States of photochromic material. : Wavelength. T: Transmittance. VIS: Visible light.

Nanopatterning with optical saturable transitions of photochromic molecules

Here, we report on a method that resolves nanoscale patterns by exploiting the transitions of organic photochromic derivatives induced by their photoisomerization at low light intensities.

Photochromic Langmuir-Blodgett multilayers as photoactive coatings

The opportunity to exploit the light-induced switching of photochromic systems to develop photoactive coatings is considered. Using a bottom-up approach as Langmuir-Blodgett (LB) technique we realize a monomolecular up to tens layers coating. In order to effectively apply LB technique a proper class of diarylethene-based amphipilic materials are designed and synthesized through an optimized synthetic route. Photochromism both of the solution and of the active coating is characterized. The molecular design of these molecules is effective in promoting the formation of the Langmuir monolayer on water sub-phase, whose quality depends on the isomeric form the switching material is deposited. According to the character of the substrate and the number of multilayer deposited a tunable change in the final system wettability is obtained.

Light-triggered conducting properties of a random carbon nanotubes network in a photochromic polymer matrix

Photochromic materials reversibly change their colour due to a photochemical reaction that takes place when the material is irradiated with photons of suitable energy. This peculiar feature has been extensively exploited to develop smart sunglasses, filters and inks. With a proper molecular design it is possible to enable modulation not only of colour but also of other properties such as refractive index, dipole moment, nonlinear optical properties or conductivity by a photoswitching of the molecular structure. The approach herein developed consists in modifying, upon irradiation, the properties of a molecular component coupled with the photochromic molecule. In particular, the switching features of photochromic systems are matched with the intriguing peculiar properties of carbon nanotubes (CNTs). A photochromic polyester has been properly synthesised to be used as switching polymer matrix coupled with a network of CNTs. Irradiation of the polymer/CNTs blend results into a light-triggered conductance switching. The reversible electrocyclization of the polymer under UV-vis illumination results into a modification of the inter-tube charge mobility, and accordingly, of the overall resistance of the blend. Solution techniques allow us to obtain blended films with sheet resistance modulation larger than 150%, good thermal stability and fatigue resistance at room conditions, in an easier, faster and scalable way as respect to the single-molecule approach.ÿ