We synthesized a series of photochromic [1,3]oxazines and investigated their photochemical properties in solution and within rigid polymer matrices. These compounds share a common molecular skeleton, consisting of fused 3H-indole and nitrobenzooxazine heterocycles. They differ in the groups in the para (R 1) and/or ortho (R 2) positions, relative to the nitrogen atom, of the 3H-indole fragment and/or that (R 3) attached to the chiral center of the [1,3]oxazine ring. Specifically, R 1 can be a hydrogen atom, a methoxy group, a nitro group, or a fluorine atom, R 2 can be a hydrogen or fluorine atom, and R 3 can be a phenyl, 4-methoxyphenyl, 4-dimethylaminophenyl, or 2-(4-dimethylaminophenyl)ethylene group. When R 1 and R 2 are hydrogen atoms, the excitation of the photochrome opens the [1,3]oxazine ring in less than 6 ns, with quantum yields of 0.01-0.11 in acetonitrile at 20 °C. This process generates a zwitterionic isomer, incorporating a 3H-indolium cation and a 4-nitrophenolate anion. Consistently, the characteristic ground-state absorption of the a 4-nitrophenolate appears at 440 nm in the transient spectrum. When R 3 is a 2-(4-dimethylamminophenyl)- ethylene group, the spectrum reveals also an additional band at 550 nm for the extended π-system associated with the photogenerated 3H-indolium cation. When R 3 is a hydrogen atom, the nature of R 1 controls the photochemical behavior of these compounds. In particular, the presence of a methoxy group at R 1 prevents the photoinduced ring-opening, while the introduction of a fluorine atom increases the quantum yield of the photochemical transformation to 0.29. In all instances, the transient absorptions decay monoexponentially with the reisomerization of the zwitterionic species back to the original state. Interestingly, R 1 and R 2 have negligible influence on the lifetime of the photogenerated isomer, which instead changes from 21 ns to 10 μs with the nature of R 3. Indeed, this group dictates the stability of the 3H-indolium cation of the zwitterionic isomer and, hence, the reisomerization kinetics. Furthermore, our photochromic compounds tolerate hundreds of switching cycles with no sign of decomposition, even in the presence of molecular oxygen, and can be operated effectively within rigid poly(methyl methacrylate) matrices. In summary, our investigations demonstrate that the color, efficiency, and speed of our photochromic [1,3]oxazines can be manipulated with the careful selection of their substituents without compromising their excellent fatigue resistances. Thus, photoresponsive materials with tunable properties can eventually emerge from our insights on the stereoelectronic factors regulating the photochromism of this particular family of heterocyclic compounds.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films