*8.1.1 Method 1*

**6.2 Diarylethenes**

*Photochromic reaction of fulgides.*

**Figure 7.**

**Figure 8.**

**88**

*Photochromic reactions of diarylethenes.*

**7. Effect of temperature**

increasing temperature.

complexity of photochemical reactions.

It has hetrocyclic five membered rings such as thiophene or benzothiophene rings and undergo thermally irreversible and fatigue resistant photochromic reactions as shown in **Figure 8**. The thermal stability of both isomers of diarylethens are due to presence of aryl groups and when aryl groups are furan or thiophene, the closed form is thermally stable. However diarylethenes exhibit thermally reversible

Surrounding temperature influence the performance of photochromic dyes. The bleaching effect of photochromic dyes are accelerated by the temperature of sample [38–40]. Dulic et al. find that ring opening process of diarylethene is temperature dependent whereas ring closing process shows only slight dependence. Ortica [41] reviewed the effect of temperature on the characteristics of various photochromiv materials such as spirooxazines, chromenes and arylethenes, which are as follows,

• In thermo reversible photochromic materials, thermal bleaching increase with

• Specific temperature can induce spontaneous coloration in thermochromic materials however decreasing temperature will not help in reducing the

reactions when the aryl group is phenyl or indole.

*Dyes and Pigments - Novel Applications and Waste Treatment*

In this method the photochromic dyes are dispersed with dispersing agent [42] and dissolved in water keeping the M:L ratio of dyeing 1:50. The dyebath pH is maintained between 4.5–5.5. The dyebath temperature raised from 40 <sup>0</sup> C to 60 °C with 20 C/min gradient and then after it is reduced 1<sup>0</sup> C/min and final dyeing temperature is maintained at 90 <sup>0</sup> C and dyeing is continued at this temperature for 60 min. After completion of dyeing, soaping, rinsing and washing are done to improve fastness properties.
