**2.1 Positive photochromism**

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

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

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

The general physicochemical reaction of photochromic molecules are as given in

ΔhY2

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

c. Long term stability of molecule to produce high number of cycles

ð1Þ

spiropyrans are having more scientific interest than any other class.

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convert into original structure (colorless) as shown in **Figure 1**.

color [4].

**Figure 1.**

**82**

equation no. 1 [13, 14].

influence reaction 1 are [15].

a. Wavelength of incident light

b. Speed of recovery or fatigue resistance

*Conversion of colorless spriopyran molecule into colored molecule.*

In this photochromism photochromic molecule absorb UV light whose ʎmax falls in UV region and colorless molecule became colored and on reversal during bleaching process in visible wavelength it become colorless.
