This tutorial review illustrates the structural design, photochemical and photophysical properties of nanostructured constructs incorporating luminescent and photochromic components. In these systems, the pronounced structural and electronic modifications that accompany the transformations of the photochromic components can be exploited to modulate the emission intensity of the luminescent components on the basis of electron and energy transfer processes. These photoresponsive systems can be assembled by: (1) integrating fluorescent and photochromic components within the main chain of the same polymer; (2) attaching multiple photochromes to a fluorescent organic polymer or luminescent inorganic nanoparticle; (3) appending either independent fluorophores and photochromes or fluorophore-photochrome dyads to a common polymer scaffold; (4) trapping distinct fluorophores and photochromes within the hydrophobic interior of the same cross-linked polymer. In all instances, the changes in absorbance and/or redox potentials associated with the reversible interconversion of the two states of each photochromic component regulate the radiative deactivation of the luminescent components. As a result, the emission intensity of these nanoscaled assemblies can reversibly be switched between high and low values under the influence of optical stimulations. Thus, these clever operating principles for fluorescence modulation can lead to the development of innovative functional and nanostructured materials with photoresponsive character. In particular, protocols for the optical writing and reading of data as well as luminescent probes for bioimaging applications might ultimately emerge from these fundamental studies on photoresponsive molecular switches.

[reaction, structure: see text] We report a unimolecular system functioning as a combinatorial logic circuit for half-subtractor. The emission characteristics can be modulated by chemical inputs, and when followed at two different wavelengths, two functionally integrated logic gates XOR and INHIBIT are obtained. Both logic gates function in the emission mode, and with very large differences in the signal intensity allowing unequivocal assignment of logic-0 and logic-1.

We synthesized five fluorophore–photochrome dyads designed to switch reversibly between nonfluorescent and fluorescent isomers under optical control. These compounds pair an oxazine photochrome to a biphenyl, fluorene, pyrene, coumarin, or cyanine fluorophore in their molecular skeleton and can be prepared in a single step from known precursors in yields ranging from 30 to 63%. Nuclear magnetic resonance spectroscopy indicates that the oxazine ring of these compounds opens and closes spontaneously on a millisecond time scale in acetonitrile at ambient temperature. Under these conditions, the fraction of ring-open isomer at equilibrium is negligible in all instances with the exception of the cyanine derivative, which instead is almost exclusively in this form. Absorption and emission spectroscopies demonstrate, however, that the fraction of ring-open isomer is sensitive to solvent polarity and increases with a transition from acetonitrile to methanol. Alternatively, the ring-open isomer can be populated photochemically or trapped with the addition of acid. In both instances, the characteristic absorption and emission bands of the 3H-indolium chromophores, embedded within the ring-open species, can clearly be observed in the visible region. In the case of the coumarin derivative, the brightness of this chromophoric fragment is sufficiently high to permit the imaging of individual molecules with excellent signal-to-noise ratios. In fact, the fluorescence of single fluorophore–photochrome dyads can be activated under the influence of ultraviolet inputs and the resulting species can be localized with nanoscale precision under visible illumination. Indeed, subdiffraction images of polymer nanoparticles, doped with this particular dyad, can be reconstructed with nanoscale resolution. Thus, our operating principles for fluorescence switching at the single-molecule level can offer the opportunity to overcome diffraction and, eventually, lead to the development of an entire family of probes for super-resolution fluorescence imaging.

We designed and synthesized an amphiphilic copolymer with pendant hydrophobic decyl and hydrophilic poly(ethylene glycol) chains along a common poly(methacrylate) backbone. This macromolecular construct captures hydrophobic boron dipyrromethene fluorophores and hydrophobic spiropyran photochromes and transfers mixtures of both components in aqueous environments. Within the resulting hydrophilic supramolecular assemblies, the spiropyran components retain their photochemical properties and switch reversibly to the corresponding merocyanine isomers upon ultraviolet illumination. Their photoinduced transformations activate intermolecular electron and energy transfer pathways, which culminate in the quenching of the boron dipyrromethene fluorescence. As a result, the emission intensity of these supramolecular constructs can be modulated in aqueous environments under optical control. Furthermore, the macromolecular envelope around the fluorescent and photochromic components can cross the membrane of Chinese hamster ovarian cells and transport its cargo unaffected into the cytosol. Indeed, the fluorescence of these supramolecular constructs can be modulated also intracellularly by operating the photochromic component with optical inputs. In addition, cytotoxicity tests demonstrate that these supramolecular assemblies and the illumination conditions required for their operation have essentially no influence on cell viability. Thus, supramolecular events can be invoked to construct fluorescent and photoswitchable systems from separate components, while imposing aqueous solubility and biocompatibility on the resulting assemblies. In principle, this simple protocol can evolve into a general strategy to deliver and operate intracellularly functional molecular components under optical control.

The photochemical transformations associated with photochromic compounds can be exploited to switch the emission of complementary fluorophores under the influence of optical stimulations. Specifically, fluorescent and photochromic components can be integrated within the same molecular or supramolecular assembly and the significant changes in the stereoelectronic properties associated with the photoinduced interconversion of one component can be designed to modulate the emission intensity and/or wavelength of the other. In particular, the modifications in absorption properties, conjugation, dipole moment, redox potentials and shape of a photochrome can all be transduced effectively into reversible alterations of the emissive behavior of a fluorophore. Furthermore, some of these ingenious mechanisms for fluorescence 1. Photochromism History and DefinitionsThe term "photochromism" was introduced in the early 1950s to indicate the photoinduced and reversible change in color of certain compounds.[1] Since then, the number of publications on photochromism has increased exponentially [a]