Photochromic Oxazines with Extended Conjugation

We synthesized four compounds with indole and benzooxazine fragments fused in their molecular skeleton and differing in the substituent in the para position, relative to the oxygen atom, of their phenoxy chromophore. This particular substituent extends the conjugation of the phenoxy chromophore and shifts its absorption bathochromically by up to 60 nm, relative to a parent compound with a 4‐nitrophenoxy group. The 1,3‐oxazine ring of all compounds opens upon addition of base to generate a hemiaminal incorporating a phenolate chromophore. Once again, the substituents on this fragment shift its absorption bathochromically by up to 60 nm, relative to the parent compound. Upon laser excitation at a wavelength within the absorption range of the phenoxy chromophore, the 1,3‐oxazine ring of the compound incorporating a 4‐nitrophenyl substituent opens in less than 6 ns to generate a zwitterionic isomer with a quantum yield of 0.11 in acetonitrile. Under these conditions, the photogenerated isomer has a lifetime of 29 ns and reverts spontaneously to the original species with first‐order kinetics. Furthermore, this photochromic system tolerates hundreds of switching cycles with no sign of degradation even in the presence of molecular oxygen. However, the excitation dynamics of the other three compounds, incorporating a 4‐nitrobiphenyl, 4‐nitrostyryl or 4‐nitrophenylethynyl substituent, are dominated by intersystem crossing. Consistently, the corresponding transient spectra reveal predominantly triplet–triplet absorptions. Thus, our studies demonstrate that the excitation wavelength and color of this class of photochromic compounds can be regulated by extending the conjugation of their phenoxy fragment with negligible influence on the photochromic performance only if the structural modification does not encourage intersystem crossing. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)