Table shows the photoisomerization and thermal bleaching properties
Among the ten compounds, the AWD 131-138 10 exhibits the best photochromic properties. So we further investigated its thermal bleaching process at various temperatures. The inset in Fig. 4 shows the decay curves of the absorbance of 10 (the keto-form) at 415 nm in the range of 373–403 K. It takes nearly 900 s and 180 s to decolor from the yellow keto form to the white enol form at 373 K and 403 K, respectively. Moreover, the decoloring reaction is accelerated with the rise of temperature. In addition, it can be found that the relation of ln [(A∞ − A0)/(A∞ − At)] and t is approximately linear, which shows that the thermal bleaching reaction of the compound 10 (the keto-form) follows the quasi-first-order kinetics. The first-order rate constants (kk–e) of the thermal bleaching reaction for the compound 10 (the keto-form) are summarized in Table 3. The thermal bleaching first-order rate constants of 10 (the keto-form) is 6.5 × 10−3 s−1 at 383 K, which is much larger than that of 1,3-diphenyl-4-(3-bromobenzal)-5-hydroxypyrazole phenylsemicarbazone (3.9 × 10−4 s−1)  at the same temperature. The result shows that thermal bleaching reaction of 10 (the keto-form) is accelerated due to the substitution of the cyano. It can be seen clearly that the substituent on the meta-position of the phenyl group at 4-position of the pyrazolone ring has a significant effect on its photochromic rate.