Different Photochromic Properties in Polymorphic Crystals

In order to examine the lifetime of the colored species of 2a, 2p, and 1 more quantitatively, KBr pellets (0.5 wt %) of the three crystals were prepared and were irradiated with UV light at room temperature. A new absorption band appeared around 3,400 cm , which was assigned to the N-H stretching vibration mode of the produced trans-keto form. From the intensity change of the band, the rate constant K and lifetime t were calculated for the three crystals. The observed lifetime T values for 2a, 2p, and 1 are 17,780, and 1,200 min, respectively. Although it was reported that 1 has a lifetime of 40 d from the change of the UV reflectance spectra [29], the lifetime from the IR spectra is only 20 h. Probably the lifetime seems to depend strongly on the measurement methods. 

The UV-vis spectra of 2a, 2p, and 2y before and after 40 min irradiation are shown in Fig. 7.8a, b, and c, respectively. The crystal of 2a shows no absorption at wavelengths more than 450 nm before UV irradiation but a new band appears between 420 and 600 nm. This causes the color change from colorless to red. The absorption band of 2p before irradiation has a tail from 450 to 530 nm. This causes the pale yellow color of 2P before irradiation. After irradiation, a similar band to that of 2a appears between 420 and 600 nm. This causes the color change from the pale 

In order to examine the relation between the lifetime and the crystal structure, the crystal structures of the three forms before the photo-irradiation were analyzed. In each crystal, the carboxyl group makes two hydrogen bonds with the carboxyl group of a neighboring molecule to form a dimer structure. The molecular structures are very similar to each other as shown in Fig. 7.9, except the torsion angle of aniline ring in 2a is different from the other two. 

Since a very thin crystal of 2a (0.03 mm) was obtained, the crystal was irradiated with UV light (365 nm) using the high-pressure Hg lamp through the glass filter at 267 K for 40 min. Then the intensity data were collected at 93 K. On the difference electron density map, there appeared new peaks which were assigned to the atoms 

The final problem remains why the crystal of 2a showed the color change faster than that of 2p. In the transformation from the enol to trans-keto form, the central -N=CH- plane becomes upside down in the pedal motion, keeping the two phenyl rings in both sides almost unchanged. This means that the rate of the transformation should be affected by the size of reaction cavity for the central -N=CH- group. 

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