Fatigue resistance of photochromic 2,2-diaryl-[2H]heteroannellated chromenes in solution

The photodegradation in toluene of photochromic compounds in the [2H]-chromenes series has been studied under continuous irradiation. It is shown that the resistance to fatigue depends on the nature of the heteroatoms, the size of the ring and the annellation position. Sixmembered aza-heterocycles, as in the spiro[indoline-hetero-annellated benzoxazine] series, improve the fatigue resistance to UV-light; in contrast, the five-membered heteroannellated chromenes (containing benzimidazole, benzothiazole, benzofurane, indole or pyrrole moieties) show a significant increase in the photodegradation process, but a broadening of the visible spectra for the photomerocyanines.