Photochromic Dyes for Smart Textiles

*Virendra Kumar Gupta*

#### **Abstract**

Photochromism is a light induced reversible color change phenomenon in photochromic molecule due to light and heat effect and molecular species exist in two forms which have different absorption spectra. The fascinating color change by photochromic molecules in response to specific wavelength of light produces number of applications such as U.V. protective fabrics, ophthalmic photochromic lenses, optical data storing, optical switch, sensors and display. This chapter provides a brief and conclusive review of photochromism their mechanism and application in Textiles. Although photochromic materials are in use since 1960 in lenses and sunglasses, but the development is slow due to technical difficulties and poor commercial application. Now there is renewed interest in photochromic materials which are used in nanofibers in smart textiles and in allied items.

**Keywords:** photochromic colorants, thermochromic colorants, U.V. radiation

### **1. Introduction**

The wide and increasing application of photochromic and thermochromic colorants in different fields initiates new interest in dyes and pigments. The photochromic compounds got excited when irradiated at particular wavelength in range of 200–400 nm and few compounds in 430–455 nm range. But researchers are more interested to develop organic rather than inorganic photochromic materials because their response in 400–700 nm visible region. The use of photochromic and thermochromic colorants in making smart materials such as medical thermography, photochromic lenses [ 1], food packaging materials, liquid crystal alignment [2, 3], optical data storage [4, 5], non linear optics [6, 7], photo switching, molecular photonic devices and in photochromic polymers [8] are well known. There is demand for application of photochromic and thermochromic colorants in making smart textiles,which are designed to sense and respond to external environmental conditions and stimuli. Photochromic and thermochromic colorants are prone to change their colors temporarily and reversibly in presence of UV light, visible light, acids [9], alkalis, water, mechanical strain, temperature and in electric field. These dyes became colored when exposed to these environmental conditions temporarily and revert back upon disappearance of external environment. The photochromic dyes [ 10] are categorized as inorganic and organic molecules. In inorganic types the important are metal oxide, alkaline earth metals, sulphides, copper compounds and mercury compounds. The organic types are effective and environment friendly and they belong to the families of spiropyrans, spirooxazines, chromomenes, fulgides,

fulgimides and diarylethenes. Spiropyrans, spirooxazines and chromomenes are sensitive to thermal effect and reverse to colorless state under heat or visible light however fulgides, fulgimides and diarylethenes are thermally stable. Out of these spiropyrans are having more scientific interest than any other class.

**2. Types of photochromism**

*Photochromic Dyes for Smart Textiles*

*DOI: http://dx.doi.org/10.5772/intechopen.96055*

In this photochromism photochromic molecule absorb UV light whose ʎmax falls

It is opposite to the positive photochromism instead of coloration discoloration observed on exposure to UV light i.e. the original molecule is colored and after

In photochromic reactions, there is conformational modifications in the struc-

Photochromic reaction leads to change in electronic absorption spectra of molecules. The formation of new absorption band due to transition of electrons from various vibrational levels in the excitation of colorless molecules from S1A level to excited state S1B after absorption of energy photon in UV region [17] and then after the colored molecules deactivated to ground state S0B. Subsequently there is spontaneous energy release process and molecules come to original S0A ground state. Thermodynamically the molecules in B state due to higher energy are less stable and after releasing energy became colorless and more stable. The transition from state B to A takes place via a transition state X, whose energy is higher than the triplet state of colored form S0B and it is thermally activated. There are six mechanism which

The transition of molecules after absorption of light in different energy levels is

ture of photochromic molecules and due to that there are change in physicochemical properties of two form of the photochromic molecules and the change observed in physico- chemical properties of both molecules (colored and colorless)

in UV region and colorless molecule became colored and on reversal during

bleaching process in visible wavelength it become colorless.

**2.1 Positive photochromism**

**2.2 Negative photochromism**

exposure to UV light it loss their color.

**2.3 (c) Photo responsible materials**

are called photo responsible materials [16].

responsible for the photochromic effect and they are,

**3. Mechanism of Phtochromism**

• Triplet triplet photochromism.

• Hetrolytic cleavage

• Hemolytic cleavage

• Tautomerism

shown in **Figure 2**.

**83**

• Photodimerisation

• Trans-cis isomerismation

Spiro compounds have pyran ring and linked to another heterocyclic ring through spiro group. Spirooxazines molecules contain nitrogen atom at the place of carbon in spiro group. These molecules (colorless) have non planer structure and that inhibit delocalization of Л electrons in the molecules. In presence of UV light, molecules absorb photon energy and breaking of –C-O- bond in pyran ring takes place and there is formation of colored planar structure molecule. The planarity of molecule allow delocalization of Л electrons and molecule become colored. This is short term phenomenon and after absorption of heat or visible light molecules convert into original structure (colorless) as shown in **Figure 1**.

The photochromism [11, 12] may be defined as a reversible light –induced color change or reversible transformation between two different molecular structures with different absorption spectrum in reversible manner due to electromagnetic radiations. Photochromic materials are kind of chromic materials in which photochromic and thermochromic materials are of paramount importance. In photochromism the colorless molecule became colored in presence of UV Portion of light however in the thermochromic molecules heat is responsible for change of color [4].

The general physicochemical reaction of photochromic molecules are as given in equation no. 1 [13, 14].

$$\begin{array}{c} \mathsf{X} \{\mathsf{A}\_{1}\} \text{ (colorless)} \begin{array}{c} \mathsf{hy}\_{1} \ \mathsf{Y} \{\mathsf{h}\_{2}\} \text{ (colorred)} \end{array} \tag{1} \\\hline \\ \mathsf{\mathsf{A}h\mathsf{Y}\_{2}} \end{array} \tag{1}$$

ʎ<sup>1</sup> and ʎ<sup>2</sup> are the wavelength of maximum absorption by corresponding molecules and hY1 and hY2 are the energy absorbed by the molecules during transformation. Y1 is the frequency of wave in U.V. region and Y2 is the frequency of wave in either U.V. or visible region and Δ is heat requirement. The factors which influence reaction 1 are [15].


*Conversion of colorless spriopyran molecule into colored molecule.*
