Kinetics of photochemical reactions of biphotochromic compounds based on spironaphthopyran and enamine--conjugation effect.

A novel biphotochromic compound (BPC) with two photochromic fragments, namely spironaphthopyran and hydroxyazomethine, was synthesized and studied by nanosecond laser flash photolysis using the excitation wavelengths of 337, 430, 470 and 500 nm in methanol and toluene. The photoexcitation of BPC results in the formation of at least two colored transients. The first one is the merocyanine form B (the maximum in the absorption spectrum is near 600 nm and the lifetime is 0.1 and 0.05 s in methanol and toluene, respectively) due to the spiro-bond break followed by isomerization. The second one is the trans-keto form A(Kt) (the maximum in the absorption spectra is near 480 nm and the lifetime is 0.1 and 5 ms in methanol and toluene, respectively) as a result of cis-enol or cis-keto tautomer transformations. The relative yields of B and A(Kt) depends essentially on the wavelength of excitation. The form A(Kt) is the key transient formed under excitation with the visible light, whereas its yield under excitation with UV light is comparable with that of B. The specific solvation by methanol molecules favors the spirocycle opening even under visible light excitation. The results obtained for novel BPC were compared with those for other BPC where the same fragments are combined in such a way that the form B is the major one under excitation with UV light whereas it virtually is not observed under visible light excitation. The difference in both BPC is discussed in terms of conjugation (π-coupling) between photochromic fragments.