**3. Mechanism of Phtochromism**

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 responsible for the photochromic effect and they are,


The transition of molecules after absorption of light in different energy levels is shown in **Figure 2**.

• Smaller concentration should produce intense color with effective color

• The conversion from colorless to colored form upon exposure to U.V. light

absorbance half i.e., photo chromic effect should be lost as soon as removal of

• The photochromic molecules should have quick response in presence of U.V.

• The life time of photochromic molecules should be longer (Approx. 2 years) in both colored and colorless form. It should also have good resistance to fatigue.

It is first T type photochromic molecules and widely used in the industry. It have application in memory disks, optical switches, sensors and as a photochromic dyes in textiles and plastics. The general structure contain a second ring structure, which attach to pyron core at position two [24–29]. The coloring of the molecules take place under exposure to U.V. light. The ring opening in molecular structure occur under U.V.light and merocyanine form is created and that exist as cic-cis/transtrans mixture. The absorption of either U.V. or visible wavelength takes place after ring opening and after absorption, new wavelength in visible region is produced. It has low thermal bleaching rate and at high temperature the color became weaker. In **Figure 3** the ring opening of spiropyrans and formation of conjugated structure is

The chemical structure is similar to spiropyrans, the difference exit that instead of pyron core, it has oxazine group [ 30, 31] (**Figure 4**). The spirooxazines has very good resistance to photo degradation. The spirooxazines produces fast fading blue

*Photochromic reaction of spiropyrans (from closed ring structure to open ring structure).*

• The photochromic molecules should have minimum time to loss their

• The photochromic molecules should be less vulnerable to variation in

**5. Types of photochromic materials (T types)**

change.

radiation.

temperature.

**5.1 Spiropyrans**

shown.

**Figure 3.**

**85**

**5.2 Spirooxazines**

must be very quick.

*Photochromic Dyes for Smart Textiles*

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

activating light takes place.

**Figure 2.** *Representations of electronic, radiative and non radiative transitions in photochromic materials.*

### **4. Classification of photochromic materials**

Classification of photochromic materials are based on back reaction i.e., if it from colored state to colorless state is brought in the presence of light then it is called P type photochromic materials (**Figure 1**), where as if back reaction occurred due to heat energy, it is called T type of photochromic materials.

The p type photochromic materials exist in two reversible forms upon irradiation and have good thermal stability with better fatigue resistance. In p type photochromic materials [18, 19] during exposure to UV light ring opening takes place. After ring opening, molecules absorb visible wavelength light and became colored. This state of molecule is temporary and again it became closed ring system (colorless). Most of the p type photochromic materials do not follow trans- cis isomerism and which follow they have they have open ring structure in colorless form after exposure to UV light they became closed ring structure (colored). The reverse reaction i.e. ring opening is promoted by visible wavelength [20, 21].

In T type photochromic materials photochromic reactions takes place due to thermal irradiation or by photo irradiation with visible light [ 22, 23]. It shows reversible equilibrium between trans and cis isomerism of different stability. In T type photochromism there is no breaking of bonds occur however there is rearrangement of electrons between energy levels and alteration in geometric arrangement of the molecules. The thermal reversal of molecule takes place in dark. The important T type photochromic materials are perimidinespirocyclohexadienes, spirodihydroindolizines and anils. There are some requirement in T type photochromic materials which are,

