The activation wavelength of a photochromic oxazine can be shifted bathochromically with the introduction of a methoxy substituent on the chromophore responsible for initiating the photochemical transformation. This structural modification permits switching under mild illumination conditions, enhances the photoisomerization quantum yield and ensures outstanding fatigue resistance. Thus, these results can guide the design of new members of this family of photoresponsive molecular switches with improved photochemical and photophysical properties.

The photoinduced decarbonylation of an α-diketone adduct generates 2,6-dimethoxyanthracene in the ground state. The photochemical product can also absorb the incoming photons and transfer its excitation energy to another molecule of the reactant to establish an autocatalytic loop. In the presence of silver nanoparticles, the efficiency of energy transfer increases significantly with a concomitant acceleration of the autocatalytic transformation. In principle, this mechanism to establish kinetic amplification can be extended to any photoactivatable fluorophore, if reactant and product can be engineered to satisfy the spectral requirements necessary for energy transfer. Furthermore, the resulting photochemical replicators can be exploited to monitor the influence of metallic nanostructures on photochemical reactions with convenient fluorescence measurements. In turn, the associated plasmonic effects offer the opportunity to accelerate autocatalysis with nanoscale resolution.

The covalent attachment of a carbazole fluorophore to an oxazine photochrome permits the reversible activation of fluorescence under optical control. Ultraviolet irradiation with a pulsed laser opens the oxazine ring to shift bathochromically the absorption of the carbazole component. Concomitant visible illumination excites selectively the carbazole fluorophore of the photochemical product to produce fluorescence. The photogenerated and fluorescent species reverts spontaneously on a submicrosecond timescale to the initial nonemissive state of the carbazole–oxazine dyad. The photochemical and photophysical properties engineered into this particular molecular switch allow the convenient monitoring of plasmonic effects on photochemical reactions with fluorescence measurements. In close proximity to silver nanoparticles, visible illumination with a continuous-wave laser also results in fluorescence activation. The metallic nanostructures enable the two-photon excitation of the oxazine component to induce the photochromic transformation and then facilitate the one-photon excitation of the photochemical product to generate fluorescence. Thus, these operating principles offer the opportunity to avoid altogether the need of pulsed ultraviolet irradiation to trigger the photochromic transformation and, instead, allow fluorescence activation with a single visible source operating at low illumination power.

The inherent reversibility of photochromic transformations can be exploited to switch on and off the fluorescence of appropriate organic chromophores under optical control. In turn, the photoactivation of fluorescence permits the monitoring of dynamic processes in real time as well as the reconstruction of images with spatial resolution at the nanometer level. Thus, the identification of viable structural designs to construct and operate photoactivatable fluorophores on the basis of photochromic processes can translate into the realization of valuable analytical tools for biomedical research. In this context, a strategy was designed to connect a simple photochromic oxazine to essentially any fluorescent chromophore with a pendant formyl group. In the resulting fluorophorephotochrome dyads, the photoinduced interconversion of the photochromic component between its two states controls that ability of the fluorescent component to absorb exciting radiation and emit as a result. Under these conditions, the interplay of two illuminating beams, designed to operate the photochromic component and excite the fluorescent one, respectively, offers the opportunity to switch fluorescence reversibly for multiple cycles. Furthermore, the fluorophorephotochrome dyads described herein can be entrapped within the hydrophobic core of polymer micelles and operated under these conditions in aqueous solutions and within the intracellular environment.

The spatial resolution of fluorescence microscopes is limited by diffraction to about half of the light wavelength, hampering the observation of many important intracellular processes. Recent emerging techniques have overcome that diffraction barrier using the temporal discrimination of close objects that are otherwise unresolved or blurred within the spatial resolution of the microscope. The key of these techniques is to switch the signal of fluorescence markers on and off exploiting their distinct molecular states, and detect and localize these markers at the single-molecule level. This underlying principle highlights the critical role of the photophysical properties of the probes, and the importance of finding adequate switching mechanisms. Here, we present strategies to achieve fluorescence modulation based on novel molecular assemblies containing a [1,3]oxazine as the two states, building block responsible for the transformation. Two different triggering events, based on the photochromic and halochromic properties of the oxazine, induce a large absorption and emission bathochromic shift of a pendant fluorophore, as the ultimate fluorescence switching event. The implementation of these approaches to achieve spatial resolution beyond the diffraction limit is also discussed.

A viable strategy to encapsulate a fluorophore/photochrome dyad and a nitric oxide photodonor within supramolecular assemblies of a cyclodextrin-based polymer in water was developed. The two photoresponsive guests do not interact with each other within their supramolecular container and can be operated in parallel under optical control. Specifically, the dyad permits the reversible switching of fluorescence on a microsecond timescale for hundreds of cycles, and the photodonor enables the irreversible release of nitric oxide. Furthermore, these supramolecular assemblies cross the membrane of human melanoma cancer cells and transport their cargo in the cytosol. The fluorescence of one component allows the visualization of the labeled cells, and its switchable character could, in principle, be used to acquire super-resolution images, while the release of nitric oxide from the other induces significant cell mortality. Thus, our design logic for the construction of biocompatible nanoparticles with dual functionality might evolve into the realization of valuable photoresponsive probes for imaging and therapeutic applications.

We designed a supramolecular strategy to modulate fluorescence in water under optical control. It is based on the entrapment of fluorophore–photochrome dyads within the hydrophobic interior of an amphiphilic polymer. The polymeric envelope around the dyads protects them from the aqueous environment, while imposing hydrophilic character on the overall supramolecular construct. In the resulting assemblies, the photochromic component can be operated reversibly on a microsecond timescale under the influence of ultraviolet stimulations. In turn, the reversible transformations control the emission intensity of the adjacent fluorophore. As a result, the fluorescence of such nanostructured constructs can be photomodulated for hundreds of cycles in water with microsecond switching speeds. Thus, our protocol for fast fluorescence switching in aqueous solutions can eventually lead to the realization of functional probes for the investigation of biological samples.

Two series of CdSe quantum dots (QDs) with different diameters are prepared, according to frequently used protocols of the same synthetic procedure. For each sample the photophysical properties and the potentials for the first reduction and oxidation processes in organic solution are determined. The band gap obtained from electrochemical experiments is compared with that determined from the absorption and luminescence spectra. While the optical band gap decreases upon increasing the nanocrystal diameter, as expected on the basis of quantum confinement, the redox potentials and the electrochemical band gap are not monotonously related to the QD size. For both series, the smallest and largest QDs are both easier to oxidize and reduce than mid-sized QDs. In fact, the latter samples exhibit very broad voltammetric profiles, which suggests that the heterogeneous electron-transfer processes from/to the electrode are kinetically hindered. Conversely, the electrochemical band gap for the smallest and largest particles of each series is somewhat smaller than the optical band gap. These results indicate that, while the optical band gap depends on the actual electron–hole recombination within the nanocrystal, and therefore follows the size dependence expected from the particle-in-a-box model, the electrochemical processes of these QDs are strongly affected by other factors, such as the presence of surface defects. The investigations suggest that the influence of these defects on the potential values is more important for the smallest and largest QDs of each series, as confirmed by the respective luminescence bands and quantum yields. An interpretation for the size-dependent evolution of the surface defects in these nanocrystals is proposed based on the mechanism of their formation and growth.

Two macromolecular constructs incorporating a single polymer backbone with multiple photochromic side chains are developed. Both systems are prepared from preformed photochromic [1,3]oxazines after the ring-opening polymerization of their norbornene appendages. In solution, UV illumination of these polymers opens the [1,3]oxazine rings in their side chains in less than 6 ns and with a quantum yield of 0.09 in both instances. The photogenerated species incorporate a 4-nitrophenolate chromophore, and hence, their formation is accompanied by the appearance of an intense band in the visible region of the absorption spectrum. The photoproducts revert spontaneously to the original state with first-order kinetics in microseconds. Furthermore, both photochromic polymers tolerate hundreds of switching cycles with no sign of degradation, even in the presence of molecular oxygen. Thus, this design logic and choice of functional building blocks can translate into the realization of innovative photoresponsive materials with excellent photochromic performance.

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 thestereoelectronic 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 modulation can be extended from relatively small molecular and supramolecular assemblies to large macromolecular, nanostructured and biomolecular constructs. In fact, a diversity of chemical systems with photoswitchable luminescence have already emerged on the basis of these operating principles and choice of functional components. Ultimately, these fundamental studies on the photochemical and photophysical signature of multicomponent assemblies might well lead to a new generation of photonic materials with unique properties for possible applications in biomedical imaging and sensing as well as in information processing and storage.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

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)

Molecular logic gates process physical or chemical “inputs” to generate “outputs” based on a set of logical operators. We report the design and operation of a chemical ensemble in solution that behaves as integrated AND, OR, and XNOR gates with optical input and output signals. The ensemble is composed of a reversible merocyanine-type photoacid and a ruthenium polypyridine complex that functions as a pH-controlled three-state luminescent switch. The light-triggered release of protons from the photoacid is used to control the state of the transition-metal complex. Therefore, the two molecular switching devices communicate with one another through the exchange of ionic signals. By means of such a double (optical–chemical–optical) signal-transduction mechanism, inputs of violet light modulate a luminescence output in the red/far-red region of the visible spectrum. Nondestructive reading is guaranteed because the green light used for excitation in the photoluminescence experiments does not affect the state of the gate. The reset is thermally driven and, thus, does not involve the addition of chemicals and accumulation of byproducts. Owing to its reversibility and stability, this molecular device can afford many cycles of digital operation.