**5.1 Spiropyrans**

**4. Classification of photochromic materials**

*Dyes and Pigments - Novel Applications and Waste Treatment*

**Figure 2.**

due to heat energy, it is called T type of photochromic materials.

reaction i.e. ring opening is promoted by visible wavelength [20, 21].

spirodihydroindolizines and anils. There are some requirement in T type

photochromic materials which are,

**84**

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

*Representations of electronic, radiative and non radiative transitions in 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

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,

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 shown.
