Computational Insights on the Isomerization of Photochromic Oxazines

We investigated the isomerization of the simplest member of a family of photochromic oxazines with the aid of density functional theory, using three different functionals. Specifically, we simulated the thermal interconversion of the two enantiomers, associated with this compound, and established that the opening of the oxazine ring dictates the rate of the overall degenerate process. The M062X functional provides the best match to experimental data, whereas B3LYP calculations fail to model accurately the ground-state potential-energy surface of this system. In addition, we also modeled the absorption spectra of this compound and its photogenerated isomer with time-dependent calculations. The resulting data support the original assignment of the experimental spectra and confirm that the oxazine ring opens upon excitation. The MPW1PW91 functional provides the best match to experimental data, whereas M062X calculations fail to model accurately the spectroscopic parameters of this particular system. Furthermore, the MPW1PW91 calculations demonstrate that the photoinduced opening of the oxazine ring occurs along the potential-energy surface of the first triplet excited state. Indeed, the photoinduced isomerization appears to involve: (1) the initial excitation of one isomer to the second singlet excited state, (2) its thermal relaxation to the first triplet excited state, (3) its ring opening to produce the other isomer, and (4) the thermal relaxation of the product to the ground state. Thus, our calculations provide valuable information on the elementary steps governing the isomerization of this particular photochromic compound in the ground state and upon excitation. These useful mechanistic insights can guide the design of novel members of this family of photoresponsive compounds with specific properties

Computational Insights on the Isomerization of Photochromic Oxazines

We investigated the isomerization of the simplest member of a family of photochromic oxazines with the aid of density functional theory, using three different functionals. Specifically, we simulated the thermal interconversion of the two enantiomers, associated with this compound, and established that the opening of the oxazine ring dictates the rate of the overall degenerate process. The M062X functional provides the best match to experimental data, whereas B3LYP calculations fail to model accurately the ground-state potential-energy surface of this system. In addition, we also modeled the absorption spectra of this compound and its photogenerated isomer with time-dependent calculations. The resulting data support the original assignment of the experimental spectra and confirm that the oxazine ring opens upon excitation. The MPW1PW91 functional provides the best match to experimental data, whereas M062X calculations fail to model accurately the spectroscopic parameters of this particular system. Furthermore, the MPW1PW91 calculations demonstrate that the photoinduced opening of the oxazine ring occurs along the potential-energy surface of the first triplet excited state. Indeed, the photoinduced isomerization appears to involve: (1) the initial excitation of one isomer to the second singlet excited state, (2) its thermal relaxation to the first triplet excited state, (3) its ring opening to produce the other isomer, and (4) the thermal relaxation of the product to the ground state. Thus, our calculations provide valuable information on the elementary steps governing the isomerization of this particular photochromic compound in the ground state and upon excitation. These useful mechanistic insights can guide the design of novel members of this family of photoresponsive compounds with specific properties

Computational Insights on the Isomerization of Photochromic Oxazines

We investigated the isomerization of the simplest member of a family of photochromic oxazines with the aid of density functional theory, using three different functionals. Specifically, we simulated the thermal interconversion of the two enantiomers, associated with this compound, and established that the opening of the oxazine ring dictates the rate of the overall degenerate process. The M062X functional provides the best match to experimental data, whereas B3LYP calculations fail to model accurately the ground-state potential-energy surface of this system. In addition, we also modeled the absorption spectra of this compound and its photogenerated isomer with time-dependent calculations. The resulting data support the original assignment of the experimental spectra and confirm that the oxazine ring opens upon excitation. The MPW1PW91 functional provides the best match to experimental data, whereas M062X calculations fail to model accurately the spectroscopic parameters of this particular system. Furthermore, the MPW1PW91 calculations demonstrate that the photoinduced opening of the oxazine ring occurs along the potential-energy surface of the first triplet excited state. Indeed, the photoinduced isomerization appears to involve: (1) the initial excitation of one isomer to the second singlet excited state, (2) its thermal relaxation to the first triplet excited state, (3) its ring opening to produce the other isomer, and (4) the thermal relaxation of the product to the ground state. Thus, our calculations provide valuable information on the elementary steps governing the isomerization of this particular photochromic compound in the ground state and upon excitation. These useful mechanistic insights can guide the design of novel members of this family of photoresponsive compounds with specific properties

Computational Insights on the Isomerization of Photochromic Oxazines

We investigated the isomerization of the simplest member of a family of photochromic oxazines with the aid of density functional theory, using three different functionals. Specifically, we simulated the thermal interconversion of the two enantiomers, associated with this compound, and established that the opening of the oxazine ring dictates the rate of the overall degenerate process. The M062X functional provides the best match to experimental data, whereas B3LYP calculations fail to model accurately the ground-state potential-energy surface of this system. In addition, we also modeled the absorption spectra of this compound and its photogenerated isomer with time-dependent calculations. The resulting data support the original assignment of the experimental spectra and confirm that the oxazine ring opens upon excitation. The MPW1PW91 functional provides the best match to experimental data, whereas M062X calculations fail to model accurately the spectroscopic parameters of this particular system. Furthermore, the MPW1PW91 calculations demonstrate that the photoinduced opening of the oxazine ring occurs along the potential-energy surface of the first triplet excited state. Indeed, the photoinduced isomerization appears to involve: (1) the initial excitation of one isomer to the second singlet excited state, (2) its thermal relaxation to the first triplet excited state, (3) its ring opening to produce the other isomer, and (4) the thermal relaxation of the product to the ground state. Thus, our calculations provide valuable information on the elementary steps governing the isomerization of this particular photochromic compound in the ground state and upon excitation. These useful mechanistic insights can guide the design of novel members of this family of photoresponsive compounds with specific properties

Reversible Disassembly–Assembly of Octa Acid–Guest Capsule in Water Triggered by a Photochromic Process

Octa acid (OA), a calixarene-based cavitand, forms a 1:2 capsular assembly with neutral 1,3,3-trimethyl-6′-nitrospiro­[2<i>H</i>-1]­benzopyran-2,2′-indoline and 1:1 cavitandplex with its open zwitterionic merocyanine form. Photochromic interconversion between the spiropyran and merocyanine leads to unprecedented reversible capsular disassembly and assembly. OA provides stability to the merocyanine in both the ground and excited states. The photochemically controlled disassembly and assembly process established here points toward the opportunity of using the OA capsule in delivering small molecules at the desired locations

Fluorescence Switching with a Photochromic Auxochrome

We synthesized a photoswitchable fluorescent probe incorporating a coumarin fluorophore and an oxazine photochrome within the same molecular skeleton. The visible illumination of this fluorophore−photochrome dyad results in the excitation of the fluorescent component only if the photochromic element is activated with ultraviolet irradiation. Indeed, the photoinduced opening of the oxazine ring bathochromically shifts the absorption of the coumarin fragment sufficiently to encourage its visible excitation with concomitant fluorescence. These operating principles translate into fluorescence photoactivation with good contrast ratio and brightness as well as short fluorescence lifetime. The modular character and relative simplicity of this synthetic strategy for the assembly of photoswitchable constructs might evolve into a general design logic for the photoregulation of the electronic structure of a given chromophore with a photochromic auxochrome

Photoactivatable Anthracenes

Fifteen substituted maleimide cycloadducts of anthracene derivatives were synthesized in one or two steps from available precursors in yields ranging from 32 to 63%. They differ in the nature of the group on the maleimide nitrogen atom and of the substituents on the anthracene platform. In all instances, the introduction of a maleimide bridge across positions 9 and 10 of the anthracene skeleton isolates electronically its peripheral phenylene rings and suppresses its characteristic fluorescence. The cycloadducts with a 4-(dimethylamino)­phenyl group on the maleimide nitrogen atom undergo retro-cycloaddition upon ultraviolet illumination with quantum yields ranging from 0.001 to 0.01. This structural transformation restores the aromatic character of the central ring of the oligoacene chromophore and activates its emission with fluorescence quantum yields ranging from 0.07 to 0.85. Thus, this particular choice of building blocks for the construction of photoresponsive compounds can translate into viable operating principles for fluorescence activation and, ultimately, lead to the realization of valuable photoactivatable fluorophores for imaging applications

Fast Fluorescence Photoswitching in a BODIPY−Oxazine Dyad with Excellent Fatigue Resistance

We synthesized a photoswitchable fluorescent probe incorporating a BODIPY fluorophore and an oxazine photochrome within the same molecular skeleton. The selective excitation of the BODIPY component at visible wavelengths is accompanied by the emission of light in the form of fluorescence. However, the illumination of the sample at ultraviolet wavelengths opens reversibly the oxazine ring and activates the intramolecular transfer of energy from the fluorophore to the photochrome with concomitant fluorescence quenching. As a result, the emission of this fluorophore−photochrome dyad can be modulated on a microsecond time scale for hundreds of switching cycles, relying on the interplay of two exciting beams. Our operating principles for fast fluorescence photoswitching with excellent fatigue resistance can lead to the development of valuable probes for the super-resolution imaging of biological samples

Photoactivatable Fluorophores for Bioimaging Applications

Photoactivatable fluorophores provide the opportunity to switch fluorescence on exclusively in a selected area within a sample of interest at a precise interval of time. Such a level of spatiotemporal fluorescence control enables the implementation of imaging schemes to monitor dynamic events in real time and visualize structural features with nanometer resolution. These transformative imaging methods are contributing fundamental insights on diverse cellular processes with profound implications in biology and medicine. Current photoactivatable fluorophores, however, become emissive only after the activation event, preventing the acquisition of fluorescence images and, hence, the visualization of the sample prior to activation. We developed a family of photoactivatable fluorophores capable of interconverting between emissive states with spectrally resolved fluorescence, instead of switching from a nonemissive state to an emissive one. We demonstrated that our compounds allow the real-time monitoring of molecules diffusing across the cellular blastoderm of developing embryos as well as of polymer beads translocating along the intestinal tract of live nematodes. Additionally, they also permit the tracking of single molecules in the lysosomal compartments of live cells and the visualization of these organelles with nanometer resolution. Indeed, our photoactivatable fluorophores may evolve into invaluable analytical tools for the investigation of the fundamental factors regulating the functions and structures of cells at the molecular level

Photoswitchable Fluorescent Dyads Incorporating BODIPY and [1,3]Oxazine Components

We designed and synthesized three compounds incorporating a BODIPY fluorophore and an oxazine photochrome within the same molecular skeleton and differing in the nature of the linker bridging the two functional components. The [1,3]oxazine ring of the photochrome opens in less than 6 ns upon laser excitation in two of the three fluorophore−photochrome dyads. This process generates a 3H-indolium cation with a quantum yield of 0.02−0.05. The photogenerated isomer has a lifetime of 1−3 μs and reverts to the original species with first-order kinetics. Both photochromic systems tolerate hundreds of switching cycles with no sign of degradation. The visible excitation of the dyads is accompanied by the characteristic fluorescence of the BODIPY component. However, the cationic fragment of their photogenerated isomers can accept an electron or energy from the excited fluorophore. As a result, the photoinduced transformation of the photochromic component within each dyad results in the effective quenching of the BODIPY emission. Indeed, the fluorescence of these photoswitchable compounds can be modulated on a microsecond time scale with excellent fatigue resistance under optical control. Thus, our operating principles and choice of functional components can ultimately lead to the development of valuable photoswitchable fluorescent probes for the super-resolution imaging of biological samples