Chromic Materials: Fundamentals, Measurements, and Applications PDF by Aravin Prince Periyasamy and Michal Vik

CONTENTS

PREFACE
We many times come across words like “smart materials” and “intelligent materials.” Chromic materials, we can understand, are an important part of smart materials. The expression “chromic materials” refers to materials that show color change depending upon an external stimulus. The most important chromic phenomena—photochromism, thermochromism, ionochromism, and electrochromism—are dealt with in individual sections in this volume, each providing a description of the physicochemical principles underlying the color changes and a discussion of the molecular structures of the most important colorant classes.

A wide range of materials that exhibit color change effects have been investigated in recent decades, and numerous products have been introduced commercially. Currently, chromic materials are most commonly used in high technology. Some of the applications have been already developed to the technological level and have been commercialized successfully, whereas some are in the developmental stage. One of most common examples is color changeable glasses that are transparent in the shade or inside and become dark colored in the presence of sunlight. Due to this color change such adaptive glasses protect eyes from intense sunlight, UV radiation, respectively.

Other examples of successful applications are temperature and humidity sensors that are broadly used in the food industry for evaluation of high quality and safe food products. Not least we can mention an electrochromic device (ECD) that controls optical properties such as optical transmission, absorption, reflectance, and/or emittance in a continual but reversible manner on application of a voltage (electrochromism).

The main goal of this book is to describe the phenomenon of color changeable materials from the point of application, spectrophotometry of color changeable materials, instrumentation, and their testing. We also discuss how to control quality of materials as well and also how to measure objectification of colorimetric parameters and absorption and remission spectra by using standardized colorimetric systems (CIE XYZ, CIE LAB, and CIE CAM).

Also in this book, we describe our experience with measurement of color changeable materials, their kinetic behavior in exposure, and decay phases from the point of colorimetric and spectrophotometric characteristics. And we try to show the way for quality control of these chromic materials. We hope that this book will be helpful for researchers and innovators in this area and will help to facilitate good experiments, which are comparable, exact, adequate, and precise with a small error of measurement.

INTRODUCTION
Color changeable materials refer to stimuli-induced reversible change in color. Such stimuli can be light, temperature, chemicals, electricity, mechanical impact, etc. Based on that we can speak about photochromic, thermochromic, electrochromic, and piezochromic materials. Color changeable materials, we can call as molecular devices, as the name implies, are composed of molecules that are designed to accomplish a specific function. The simplest device that can be imagined is a switch. The defining characteristic of a switch is that of bistability, means it has an “ON” and “OFF” state. Thus, in its “ON” state, the switch must either perform some functions or allow another device to perform its function. In the “OFF” state the system must totally impede the function.

The bistability might be based on various properties of molecules like electron transfer, isomerization, differences in complexation behavior, and photocyclization; whereas light, heat magnetic or electric fields, chemical reactions, etc., can be used to achieve the change in bistable state. Molecules which experience a color change upon exposure to external light, heat or electrical stimuli are termed photochromic, thermochromic, and electrochromic compounds, respectively.

Fritzsche first discovered the phenomenon of photochromism when he observed the photo bleaching of tetracene. Photochromism is defined as a light-induced reversible transformation of a chemical species between two forms that have different absorption spectra. Molecules that are capable to interconvert reversibly interconverting between two isomers that differ in color are termed as photochromic compounds. In the above-mentioned example, the two isomers A and B can reversibly interconvert between the two forms by irradiating with appropriate wavelength of light. Irradiation of isomer A with one wavelength of light (hν1) results in a photoisomerization reaction producing isomer B. The reverse reaction is carried out by irradiation of isomer B with a different wavelength of light (hν2).

The term ‘photochromism’ was originally suggested by Hirshberg in 1950. The study of the excited states derived from the photochromic response and the transient species involved in the photoreactivity of photochromic molecules was facilitated by the development of techniques such as flash spectroscopy and laser photophysical methods. The interest in the applications of photochromic systems increased in the 1980s when the obstacle of the low fatigue resistance of photochromic compounds was overcome by synthesizing stable organic photochromic compounds, such as spirooxazine, naphthopyran, and mainly by diarylethene derivatives. Since then, commercial applications of photochromism, such as the plastic photochromic ophthalmic lenses, have become widespread.

Similarly, thermochromic pigments are materials which change color as a function of temperature. Types of pigments are known that change color either reversibly or irreversibly. The materials which change color reversibly and may be used in textiles are leuco dye based and cholesteric liquid crystal thermochromic pigments. The leuco dye based thermochromic pigments generally change from colored to colorless or to another color with an increase in temperature. The cholesteric liquid crystals exhibit ‘color play’ by passing through the whole spectrum with an increase in temperature.

Thermochromic pigments have been commercialized since the late 1960s and used, for example, in thermographic recording materials. The thermochromic pigments are also used as temperature indicators such as measuring the body temperature, in food containers to determine the temperature or history of the food storage, in medical thermography for diagnosis purposes, in thermal mapping of engineering materials to diagnose faults in product design and in mechanical performance, in the cosmetic industry for moisturizing and as a carrier for vitamins, etc.

Coloration made by electroactive species that exhibits new optical absorption bands in accompaniment with an electron-transfer or redox reactions in which it either gains or loses an electron is termed as electrochromism. The most widely studied inorganic system is solid tungsten trioxide WO3, also called as tungsten oxide or tungstic oxide, comprising WVI, in which the introduction of small amount of WV allows intense optical absorption or, with particular conditions, reflection. The adjective ‘electrochromic’ is often applied to a widely differing variety of fenestrate and device applications. For example, a routine web search using the phrase ‘electrochromic window’ yielded many pages describing a suspended-particle-device (SPD) window. Some SPD windows are also termed ‘Smart Glass’ – a term that, until now, has related to genuine electrochromic systems.

Chemochromic materials, frequently also called as halochromic, are found in a range of forms, resulting in a variety of applications – the most common form it is found in is a dye. The smart material is used in litmus paper, which detects the acidity and alkalinity of chemicals. The chemochromic chemical in the dye has a low acidity, and will change color depending on its pH level. For example, methyl red is yellow if pH > 6 and red if pH < 4. For example, in pregnancy tests, the chemicals used detect and respond to human chorionic gonadotropin (HCG) traces found in a pregnant woman’s urine. Chemochromic dyes are also used to show the ripeness of fruit, as the chemical reacts with gases released by the fruit as it ripens. The more sensitive the chemical, the better the product is for the user. In this case understanding how to measure dynamic colors correctly should be useful tool.