Synthesis, photophysical, photochemical, and redox properties of nitrospiropyrans substituted with Ru or Os tris(bipyridine) complexes

Photochromic nitrospiropyrans substituted with 2,2'-bipyridine (bpy), [Ru(bpy)3]2+, and [Os(bpy)3]2+ groups were synthesized, and their photophysical, photochemical, and redox properties investigated. Substitution of the spiropyran with the metal complex moiety results in strongly decreased efficiency of the ring-opening process as a result of energy transfer from the excited spiropyran to the metal center. The lowest excited triplet state of the spiropyran in its open merocyanine form is lower in energy than the excited triplet MLCT level of the [Ru(bpy)3]2+ moiety but higher in energy than for [Os(bpy)3]2+, resulting in energy transfer from the excited ruthenium center to the spiropyran but inversely in the osmium case. The open merocyanine form reduces and oxidizes electrochemically more easily than the closed nitrospiropyran. Like photoexcitation, electrochemical activation also causes opening of the spiropyran ring by first reducing the closed form and subsequently reoxidizing the corresponding radical anion in two well-resolved anodic steps. Interestingly, the substitution of the spiropyran with a Ru or Os metal center does not affect the efficiency of this electrochemically induced ring-opening process, different from the photochemical path.     

Spiropyran Photoswitches in the Context of DNA: Synthesis and Photochromic Properties

A new design is presented for the incorporation of spiropyran photoswitches into nucleic acids by oligonucleotide solid phase synthesis. This design enables interaction of the 6‐nitrospirobenzopyran (NitroBIPS) photoswitch with the adjacent nucleobases and leaves the photochemical properties of NitroBIPS intact. UV/Vis spectroscopy and HPLC revealed that NitroBIPS incorporated into DNA consists of up to 40 % merocyanine in its thermal equilibrium and undergoes reversible switching between the photoisomeric spiropyran (SP) and merocyanin (MC) state by alternating excitation using visible light or heat for at least fifteen switching cycles. Exchanging the chromene part of NitroBIPS on the DNA level gives access to differently substituted spiropyran derivatives allowing the screening for spiropyrans with suitable properties in a straightforward manner. Thus, by incorporating the highly hydrolysis‐stable pyrido‐spiropyran derivative PyBIPS pure light‐triggered reversible switching of a spiropyran in DNA has been realized for the first time. Therefore, this design represents a new useful platform for investigating the photochromic behavior of different spiropyran photoswitches in a nucleic acid environment and for using spiropyrans to induce light‐ or heat‐triggered changes in conformations or in fluorescence quenching properties of oligonucleotides.     

Spiropyran‐Conjugated Pluronic as a Dual Responsive Colorimetric Detector

Novel spiropyran‐conjugated Pluronic [polyethylene oxide (PEO)‐b‐polypropylene oxide (PPO)‐b‐polyethylene oxide (PEO)] micelles are developed as a new colorimetric detector showing photo‐ or thermo‐switchable behavior. Facile conjugation of spiropyran to Pluronic was confirmed by ¹H NMR, UV–Vis, and Fluorescence spectroscopy. A switchable photoluminescence is found depending on the irradiation with either UV or visible light, and temperature resulting from structural isomerization of spiropyran between spiropyran (SP) and merocyanine (MC) form. Cytotoxicity of the spiropyran‐conjugated Pluronic (SP‐PL) was evaluated following an MTT assay, whereas photo responsiveness of spiropyran within the micelles was determined by confocal laser scanning microscopy.     

Electrochemical investigation of a photochromic spiropyran containing a pyrrolidinofullerene moiety

Reversible transformations of a hybrid molecule based on [60]fullerene and a spiropyran moiety under conditions of combined photo- and electrochemical exposure were found. It has been established that the electrochemical oxidation of the UV-photoinduced colored form of the hybrid spiropyran in solution significantly accelerates the discoloration process and does not affect the fullerene part of the hybrid molecule. The research results are promising for designing a photo-electrochromic device that modulates electromagnetic radiation in the visible range of the spectrum.     

Photo-, thermally, and pH-responsive microgels

Microgels with photo-, thermally, and pH-responsive properties in aqueous suspension have been synthesized and characterized using dynamic light scattering and UV-visible spectroscopy. The new route involved first preparing poly(N-isopropylacrylamide) (PNIPAM)-allylamine copolymer microgels and a spiropyran photochrome (SP) bearing a carboxylic acid group. Then the functionalized spiropyran was coupled to the microgel via an amide bond. The dark-equilibrated gel particles feature spiropyran molecules in the polar, merocyanine form. After irradiation of visible light, the particle size becomes smaller because spiropyran changes to the relatively nonpolar, closed spiro form. The PNIPAM-SP microgels undergo a volume phase transition in water from a swollen state to a collapsed state with increasing temperature under all light conditions. However, the transition temperature range of the PNIPAM-SP is much broader than that for the PNIPAM without SP. The PNIPAM-SP microgels are monodisperse and self-assemble into a crystalline lattice while in suspension. The UV-visible spectra of an aqueous suspension of PNIPAM-SP microgel in the dark-adapted, merocyanine form showed both an absorption peak around 512 nm due to the merocyanine (giving a reddish color to the suspension) and two sharp peaks from Bragg diffraction of colloidal crystallites. Upon visible irradiation, the 512-nm band bleached significantly due to spiropyran photoisomerization. The spiropyran photoisomerization and accompanying color changes of the suspension were reversible upon alternating dark, UV, and visible light irradiation. Due to the residues of amine groups, the swelling capability of PNIPAM-SP microgels reduces as the pH value is changed from 7 to 10.     

光致变色冠醚部花菁染料的合成及其逆光致变色性

作者用含冠醚结构单元的2-甲基-苯并噻唑季铵盐与取代水物醛缩合得到了五个光致变色冠醚部化菁化合物。这些新的冠醚化合物具有逆光致变色性质, 较稳定的结构是开环部花菁式。     

Opening a spiropyran ring by way of an exciplex intermediate

A molecular dyad has been synthesized in which the main chromophore is a 1,4-diethynylated benzene residue terminated with pyrene moieties, this latter unit acting as a single chromophore. A spiropyran group has been condensed to the central phenylene ring so as to position a weak electron donor close to the pyrene unit. Illumination of the pyrene-based chromophore leads to formation of a fluorescent exciplex in polar solvents but pyrene-like fluorescence is observed in nonpolar solvents. The exciplex has a lifetime of a few nanoseconds and undergoes intersystem crossing to the pyrene-like triplet state with low efficiency. Attaching a 4-nitrobenzene group to the open end of the spiropyran unit creates a new route for decay of the exciplex whereby the triplet state of the spiropyran is formed. Nonradiative decay of this latter species results in ring opening to form the corresponding merocyanine species. Rate constants for the various steps have been obtained from time-resolved fluorescence spectroscopy carried out over a modest temperature range. Under visible light illumination, the merocyanine form reverts to the original spiropyran geometry so that the cycle is closed. Energy transfer from the pyrene chromophore to the merocyanine unit leads to an increased rate of ring closure and serves to push the steady-state composition in favor of the spiropyran form.     

New Sol-Gel Photochromic Thin Films Made by Super-Fine Particles of Organic Photochromic Compounds

New photochromic thin gel films have been prepared by the dipping method from a sol-gel coating solution containing Malachite Green Leucocyanide soluble in sol and super-fine dispersed solution of spiropyran or spironaphthooxazine, which was effective to increase the concentration of photochromic compounds in gel film. Photochromism of these gel films is compared with that in sol under the conditions of acid catalysis. Though in sols using spiropyran and Malachite Green Leucocyanide normal photochromism was not observed, photochromism in thin gel films prepared from the same sol was confirmed. Normal photochromism of spironaphthooxazine in thin gel films was similar to that in solution. The reaction rate from the colored form to the spiro form of spiropyran and spironaphthooxazine in thin gel film is dependent on the gel matrix, which is influenced by the reaction time of the sol.     

一种逆向光致变色螺吡喃化合物的光消色关环反应机理研究

研究了逆向光致变色化合物１′－十八烷基－３′，３′－二甲基－６，８二硝基螺［２Ｈ－１－苯并吡喃－２，２′－吲哚啉］的可见光消色机理。通过吸收光谱测定其在不同溶剂及含不同添加剂时，可见光照射后有色体［ＭＣ］关环的光量子产率。结果表明Ｏ２、三苯基胺等三线态猝灭剂对关环反应光量子产率几乎没有影响。据此提出关环反应主要是通过单线态的途径进行的结论。     

Induction of Columnar and Smectic Phases for Spiropyran Derivatives: Effects of Acidichromism and Photochromism

Liquid‐crystalline (LC) properties have been induced in a number of spiropyran derivatives by the addition of methanesulfonic acid. Spiropyran derivatives containing one or two gallic acid moieties with one, two, or three long alkyl chains were prepared. Acid‐induced spiro–protonated‐merocyanine isomerization induced mesomorphism for these materials. Equimolar mixtures of methanesulfonic acid and the spiropyran derivatives with one or two dodecyloxy chains exhibited smectic A phases, whereas the spiropyran derivatives containing the gallic acid moiety with three dodecyloxy chains showed hexagonal columnar phases. On the contrary, photoirradiation of the spiropyran compounds in the bulk liquid state did not lead to the induction of mesomorphism, although the merocyanine form was induced. These results suggest that these merocyanine derivatives with ionic and nonionic moieties cannot simply form nanosegregated LC structures. Complex formation of the merocyanine form with methanesulfonic acid may play a key role in the formation of LC molecular assemblies.     

Spiropyran Meets Guanine Quadruplexes: Isomerization Mechanism and DNA Binding Modes of Quinolizidine-Substituted Spiropyran Probes

The recent delivery of a fluorescent quinolizidine-substituted spiropyran, which is able to switch in vivo and bind to guanine quadruplexes (G4) at physiological pH values, urged us to elucidate its molecular switching and binding mechanism. Combining multiscale dynamical studies and accurate quantum chemical calculations, we show that, both in water and in the G4 environment, the switching of the spiropyran ring is not promoted by an initial protonation event—as expected by the effect of low pH solutions—but that the deprotonated merocyanine form is an intermediate of the reaction leading to the protonated open species. Additionally, we investigate the binding of both deprotonated and protonated open forms of merocyanine to c-MYC G4s. Both species bind to G4s albeit with different hydrogen-bond patterns and provide distinct rotamers around the exocyclic double bond of the merocyanine forms. Altogether, our study sheds light on the pharmacophoric points for the binding of these probes to DNA, and thereby, contributes to future developments of new G4 binders of the remarkable family of quinolizidine-substituted spiropyrans.     

7-trifluoromethylquinoline-functionalized luminescent photochromic spiropyran with the stable merocyanine species both in solution and in the solid state

A new spiropyran (SP2) with the stable merocyanine form (MC2) both in solution and in the solid state at room temperature was designed and synthesized. The stability of MC2 is believed to be due to the electron-withdrawing effect of both the quinoline and the trifluoromethyl groups. (1)H NMR spectra indicate that the ratio of the open form vs the closed form of SP2 is dependent on the polarity of solvents. Single crystals composed of only the open form (MC2) were successfully obtained. X-ray structural analysis indicates that except trifluoromethyl and two methyl groups MC2 is completely planar with an s-trans,s-cis conformation. It should be noted that this is the first report of the X-ray crystal structure of the pure open form of spiropyran. MC2 can be slowly transformed into SP2 at -30 degrees C or lower temperature, and the process is accelerated by visible light irradiation. This special photochromic behavior can be explained by the calculated thermodynamic data. The spectral properties of SP2/MC2 in the presence of different metal ions are also studied, and the results show the potential application of SP2/MC2 in sensing metal ions.     

Photon gated transport at the glass nanopore electrode

The interior surface of the glass nanopore electrode was modified with spiropyran moieties to impart photochemical control of molecular transport through the pore orifice (15-90 nm radius). In low ionic strength acetonitrile solutions, diffusion of a positively charged species (Fe(bpy)(3)(2+)) is electrostatically blocked with approximately 100% efficiency by UV light-induced conversion of the neutral surface-bound spiropyran to its protonated merocyanine form (MEH+). Transport through the pore orifice is restored by either irradiation of the electrode with visible light to convert MEH+ back to spiropyran or addition of a sufficient quantity of supporting electrolyte to screen the electrostatic field associated with MEH+. The transport of neutral redox species through spiropyran-modified glass nanopores is not affected by light, allowing photoselective transport of redox molecules to the electrode surface based on charge discrimination. The glass nanopore electrode can also be employed as a photochemical trap, by UV light conversion of surface-bound spiropyran to MEH+, preventing Fe(bpy)(3)(2+) initially in the pore from diffusing through the orifice.     

Photoisomerization of spiropyran for driving a molecular shuttle

A novel light-powered molecular shuttle was synthesized which can switch the movement of a macrocycle between two distinct stations-dipeptide and zwitterionic ME-by exploiting the photoisomerization of spiropyran. The macrocycle resides selectively in the dipeptide station in the SP form and moves to the ME station under the irradiation of UV light. This movement process of the macrocycle is accompanied by reversible absorptive output signals which can be detected by the naked eye.     

Spiropyran-azobenzene-DBU system as solvent indicator

A dyad bearing azobenzene and spiropyran units was synthesized and its applications in indicating the polarity and protic or aprotic properties of a solvent were explored. The spiropyran-azobenzene derivative (SPAB) can be induced to different forms in different miscellaneous solvents accompanied with different color changes and spectral characteristics at the presence of organic base DBU. In a nonpolar or low-polar solvent, SPAB exists in thermostable spiropyran form with yellow color output. While in an aprotic polar solvent, the spiropyran part isomerized to merocyanine form giving a blue color. When SPAB is subjected to a protic solvent, the alkylation reaction occurs at the oxygen generating the alkylated-SPAB with red color. This solvent-dependent property can be used for discriminating solvent type.     

Synthesis and spectral investigations of a new dyad with spiropyran and fluorescein units: toward information processing at the single molecular level

A new dyad 1 with two spiropyran units as the photochromic acceptors and one fluorescein unit as the fluorescent donor was synthesized and characterized. External inputs (ultraviolet light, visible light, and proton) induce the reversible changes of the structure and, concomitantly, the absorption spectrum of dyad 1 due to the presence of two spiropyran units. Only the absorption spectrum of the ME form of the spiropyran units in dyad 1 has large spectral overlap with the fluorescence spectrum of the fluorescein unit. Thus, the fluorescence intensity of dyad 1 is modulated by reversible conversion among the three states of the photochromic spiropyran units and the fluorescence resonance energy transfer (FRET) between the ME form and the fluorescein unit. Based on the fact that dyad 1 could "read out" three external input signals (ultraviolet light, visible ligh,t and proton) and "write" a compatible specific output signal (fluorescence intensity), dyad 1 described here can be considered to perform an integrated circuit function with one OR and one AND interconnected logic gates. The present results demonstrate an efficient strategy for elaborating and transmitting information at the single molecular level.     

Electronic tera-order stabilization of photoinduced metastable species: structure of the photochromic product of spiropyran determined with in situ single crystal X-ray photodiffraction

The extraordinary stability of the photoinduced red form of a cationic spiropyran ( k approximately 10 (-6) s (-1) in water and approximately 10 (-6) to less than 10 (-8) s (-1) in the solid state) was employed to obtain in situ X-ray diffraction evidence of its molecular structure. By UV excitation under selected experimental conditions, on average, approximately one third of the cations in a single crystal of spiropyran iodide salt was converted and retained as the red form during the experiment. According to the structure of the mixed crystal, the ring opening, which is due to increased distance between the spiro oxygen and carbon atoms, is associated with slight molecular flattening caused by concurrent out-of-plane shift (11.2(5) degrees ) of the pyranopyridinium half and in-plane shift (4.8(7) degrees ) of the indoline half. The overall geometry change of the cation fits the steric requirements imposed by the ion packing in the crystal and can be viewed as molecular flattening caused by breaking of the spiroconjugation. The structure of the cation confirms that (at least in the case of cationic spiropyrans) the product is confined in the crystal mainly as a zwitterionic resonance structure in cis configuration similar to the (early) transition state. Although the positive charge of the closed form facilitates the ring-opening reaction by moving the reactant closer to the transition state, neither the weakening of the spiropyran C-O bond nor the space provided by the iodide alone can account for the stability of the product. Instead, the density functional theory calculations indicate that the stabilization of the red form of the cationic relative to the neutral spiropyran is thermodynamically controlled, probably through compensation of the charge within the zwitterion by the methylpyridinium group.     

Disordered Low Molecular Weight Spiropyran Exhibiting Photoregulated Adhesion Ability

The functions of the materials composed of small molecules are highly dependent on their ordered molecular arrangements in both natural and artificial systems. Without ordered structure, small molecules hardly gain complicated functions, due to the absence of intermolecular covalent bond connection or strong network. Here, a low molecular weight spiropyran that could exhibit attractive photochromism and powerful adhesion property in disordered solid state is demonstrated. With maximum up to ∼8 MPa, the adhesion strength could be photoregulated in multiple levels, which also shows one-to-one correspondence to the specific color state. The working mechanism analysis on the photoregulated adhesion reveals that the isomer ratio of merocyanine form and the molecular packing density of spiropyran are the determining factors for the adhesion ability. The discovery of photoregulated adhesion from pure spiropyran provides a new strategy for developing functional materials by employing low molecular weight compounds.     

A semiclassical molecular dynamics of the photochromic ring-opening reaction of spiropyran

The photochromic ring-opening reaction of spiropyran(SP) has been investigated by a realistic semiclassical dynamics simulation,accompanied by SA3-CASSCF(12 10)/MS-CASPT2 potential energy curves(PECs) of S0–S2.The main simulation results show the dominate pathway corresponds to the ringopening process of trans-SP to form the most stable merocyanine(MC) product.These findings provide more important complementarity for interpreting experimental observations.     

Ultrafast dynamics of a spiropyran in water

The reversible switching of a water-soluble spiropyran compound is recorded over 1 ns by means of femtosecond vis-pump/vis- and IR-probe spectroscopy under aqueous conditions. Our investigations reveal that the photochemical conversion from the closed spiropyran to the open merocyanine takes 1.6 ps whereas the reversed photoreaction is accomplished within 25 ps. The combination of time-resolved and steady-state observations allows us to reveal central parts of the reaction pathway leading to either form. The enhanced water solubility, its fast and efficient switching behavior, and its stability against hydrolysis over a time range of several weeks make this compound an attractive and versatile tool for biological applications.