Photochromic polypeptides

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Abstract
Two photochromic polypeptides were synthesized by reaction of 1‐(4‐iodobutyl)‐3,3‐ dimethylindolindolino‐6′‐nitrobenzospiropyran with poly‐L‐tyrosine; their molar contents on photochromic units were 27.3 and 44.7%. The spectra of the photo‐induced merocyanines and their decoloration kinetics were compared with these of the monomeric model compound, obtained by reaction of the same N‐(4‐iodobutyl)‐indolinospiropyran derivative with N‐acetyltyrosine methyl ester. Different types of solvents have been examined, mainly dimethylformamide and pyridine, acetone and tetrahydrofuran, and methanol and ethylene glycol. The polypeptides showed a much less pronounced solvatochromism than their model; on the other hand, their absorption spectra presented two absorption maxima instead of one for the model. These differences in photochromic behavior were interpreted on the basis of the solvatation of the polymeric chain. Inverse photochromism was observed for polypeptide P2 as well as for the model in ethylene glycol solution; this effect is due to a higher merocyanine content at the thermal equilibrium spiropyran ⇌ merocyanine in high polar solvent.

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