Photochromism of Pyridine Derivatives

Although the crystal structures of 3-a and 3-b after the photo-irradiation have not been determined yet, the structures of trans-keto forms were calculated from the cis-enol form before UV irradiation. The calculated structures with the trans-keto form for 3-a and 3-b are shown in Fig. 7.15. It is clear that any hydrogen bond cannot be formed between the metastable structures with the trans-keto form in 3-a, whereas NIB-HIB—02A hydrogen bond stabilizes the metastable trans-keto form in 3-b. The difference in lifetime of photochromism between the two crystals is well explained by the calculated structures. 

Another intriguing photochemical reaction known for over 70 years is the photo-induced and thermally activated proton transfer between the colorless CH and blue NH forms of 2-(2, 4 -dinitrobenzyl)pyridine (DNBP). The reaction process was 

The UV-vis spectra of the colorless CH and photo-induced blue NH forms are shown in Fig. 7.16. When the crystal was irradiated with UV light of the wavelength of 254 run, which is the maximum absorption band of the CH form, only the surface of the crystal was colored. Then the crystal was irradiated with the laser light of 502 mn, since the wavelength of 502 nm is approximately twice of 254 nm and is apart from the maximum absorption band of the NH form ( 600 nm). 

The molecular structure of the CH form in the blue crystal after photo-irradiation resembles that before the irradiation. When the atomic positions of the CH form was refined, additional peaks of the photoproduct and the transferred proton, except for the o-nitro ifitrogen atom, were observed in the difference electron density map. Both of the closely overlapped CH and NH molecules were refined with the isotropic model. The occupancy factor of the photo-produced NH form became 32.8 %. The conversion to the NH form up to 36.4 % was confirmed from the UV-vis spectrum. Under these conditions, the OH form had accumulated to less than 0.5 %. 

The structural change between the CH and NH forms clearly suggests the following reaction mechanism (1) The blue coloration of 2-(2, 4 -dinitrobenzyl) pyridine exposed to light can be ascribed to the NH tautomer. (2) The proton transfer 

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