Red lights, camera, photoactivation!

A photoswitchable bichromophoric oxazine with fast switching speeds and excellent fatigue resistance

In search of strategies to regulate the photochemical and photophysical properties of photochromic oxazines, we designed a multichromophoric compound incorporating 3H-indole, benzooxazine, and 2-(4-dimethylaminophenyl)ethynyl fragments. We synthesized this molecule in two steps in an overall yield of 51%, starting from commercial precursors. The ultraviolet irradiation of this photochrome opens a [1,3]oxazine ring in less than 6 ns to generate a zwitterionic isomer with a quantum yield of 0.10. In particular, the photoinduced ring opening generates a 4-nitrophenolate anion and a 3H-indolium cation. Additionally, this process brings the 2-(4-dimethylaminophenyl)ethynyl appendage into conjugation with the 3H-indolium cation. As a result, two distinct bands for the anionic and cationic fragments of the photogenerated zwitterion appear in the visible region of the absorption spectrum. The photogenerated isomer has a lifetime of 2 µs and switches back to the original form with first-order kinetics. Furthermore, this bichromophoric photochrome tolerates hundreds of switching cycles with no sign of degradation and can be operated within rigid polymer matrices. Thus, this particular structural design can lead to the development of a new family of bichromophoric photochromes and photoresponsive materials with microsecond switching times and excellent fatigue resistances.

Molecular strategies to read and write at the nanoscale with far-field optics

Diffraction prevents the focusing of ultraviolet and visible radiations within nanoscaled volumes and, as a result, the imaging and patterning of nanostructures with conventional far-field illumination. Specifically, the irradiation of a fluorescent or photosensitive material with focused light results in the simultaneous excitation of multiple chromophores distributed over a large area, relative to the dimensions of single molecules. It follows that the spatial control of fluorescence and photochemical reactions with molecular precision is impossible with conventional illumination configurations. However, the photochemical and photophysical properties of organic chromophores can be engineered to overcome diffraction in combination with patterned or reiterative illumination. These ingenious strategies offer the opportunity to confine excited chromophores within nanoscaled volumes and, therefore, restrict fluorescence or photochemical reactions within subdiffraction areas. Indeed, information can be "read" in the form of fluorescence and "written" in the form of photochemical products with resolution down to the nanometre level on the basis of these innovative approaches. In fact, these promising far-field optical methods permit the convenient imaging of biological samples and fabrication of miniaturized objects with unprecedented resolution and can have long-term and profound implications in biomedical research and information technology.

Fluorescence patterning in films of a photoswitchable BODIPY-spiropyran dyad

A BODIPY-spiropyran dyad was embedded within poly(methyl methacrylate) films spin-coated on glass slides. Visible illumination of the resulting materials excites selectively the BODIPY fragment, which then deactivates radiatively by emitting light in the form of fluorescence. Ultraviolet irradiation promotes the isomerization of the spiropyran component to the corresponding merocyanine. This photoinduced transformation activates electron and energy transfer pathways from the fluorescent to the photochromic fragment. Consistently, the BODIPY fluorescence is effectively suppressed within the photogenerated isomer. As a result, ultraviolet illumination with a laser, producing a doughnut-shaped spot on the sample, confines the fluorescent species within the doughnut hole. This behavior is an essential requisite for the implementation of super-resolution imaging schemes based on fluorescence photodeactivation. Thus, the operating principles governing the photochemical and photophysical response of this molecular switch can ultimately lead to the development of innovative probes for fluorescence nanoscopy.