Merocyanine Form Research Articles

Dynamic Fluorescence Materials Based on Naphthalimide-Functionalized Silica Aerogels and Applications in Advanced Information Encryption.

With the progress of forgery and decryption, the traditional encryption technology is apparent not enough, which strongly requires the development of advanced multidimensional encryption strategies and technologies. Photo-stimuli responsive fluorescent materials are promising as candidate materials for advanced information encryption. Here, we have reported new photo-stimuli responsive materials by encapsulating photochromic molecules spiropyrans (SPs) into naphthalimide-functionalized silica aerogels. By introducing different modification groups (dimethylamino) into 1,8-naphthalimide, we obtained two kinds of silica aerogels that emit blue and green colors. The naphthalimide-functionalized silica aerogels/dye composite exhibits a blue (dimethylamino-modified naphthalimide-functionalized silica aerogel showing green) emission from naphthalimide of silica aerogels at 450 nm (520 nm) and a red emission around 650 nm of SP. Under exposure to ultraviolet light, SP gradually transformed into the merocyanine (MC) form, and a strong absorption band appeared near 540 nm. At that time, the fluorescence resonance energy-transfer (FRET) process occurred between naphthalimide and the MC isomer. As the irradiation time is extended, the fluorescence color changes continuously from blue (green) to red through the FRET process. Using the time dependence of fluorescence, dynamic encryption patterns and multiple codes were successfully developed based on these functionalized silica aerogels. This work has provided important guidance for designing advanced information encryption materials.

Time‐Dependent Information Encryption in Liquid Crystalline Polymer with Programmable Glass Transition Temperature

AbstractTime‐dependent dynamic information encryption technology is a promising approach to enhancing the security and complexity of information transmission. Herein, a time‐dependent information encryption model from a liquid crystalline polymer (LCP) film with a programmable glass transition temperature (Tg) and gradually adjustable fluorescence is demonstrated. The programmable Tg is achieved by adjusting the degree of order of the LC molecules via a configuration interconversion of spiropyran‐based materials (SPBMs), which can convert between a V‐shaped colorless spiro (SP) and a rodlike dark‐colored merocyanine (MC) form. An LCP film obtained by visible light polymerization exhibits a lower Tg than UV light, because the SPBM molecules keep different configurations in the two films. By adjusting the ratio of two isomerization forms of SPBM molecules during the polymerization process, the Tg values of LCP films can change from 11.6 °C to 31.1 °C. Based on the isomerization rate of SPBM in the LCP films with different Tg, time‐dependent information encryption is successfully achieved.

Slow magnetic relaxation in a complex of photochromic spiropyran in a merocyanine form and cobalt(II) hexafluoroacetylacetonate.

The interaction between photochromic 8-methoxy-1',3',3'-trimethyl-6-nitro-spiro[chromene-2,2'-indole] (MNSP) and cobalt(II) hexafluoroacetylacetonate yields a deep red-violet solution due to the coordination-induced transition of MNSP from a colorless closed state to a colored open merocyanine (MC) form. The resulting complex {CoII(hfac)2·MNSP} (1) is obtained as crystals, and its structure at 250 K shows the coordination of two oxygen atoms from MNSP with CoII, forming a distorted octahedral surrounding around CoII. The unit cell of 1 contracts below 170 K by 5.5%, which is accompanied by shortening O-Co bonds and altering O-Co-O angles in the structure solved at 150 K. The χMT value of 3.06 emu K mol-1 at 300 K indicates the formation of high-spin CoII (S = 3/2) with a large orbital contribution characteristic of the octahedral surrounding. The contraction of the unit cell of 1 below 170 K provides a reversible 2.2% change of χMT in the 170-160 K range during heating and cooling regimes without hysteresis. The electron paramagnetic resonance signal of 1 was simulated with g-values of gx = 2.342, gy = 2.364, and gz = 2.084, and an isotropic g-factor of 2.267. The temperature-dependent χMT and field-dependent magnetization of 1 allow us to determine a positive zero-field splitting parameter of +20.1 cm-1. A study of the dynamic properties of 1 shows slow magnetic relaxation for CoII in a static field of 1000 Oe. Magnetic hysteresis loops were observed for 1 at 0.5 and 2 K; these loops are closed in the zero field region but opened at fields higher than ±100 and 700 Oe, respectively. The collapse of the loop is observed at 5 K. Excitation of the solution of 1 with green light decreases partially the intensity of the bands of the complex, whereas subsequent exposure of this solution to UV light partially restores these intensities.

Is this how bromine spiropyran salt is converted to merocyanine under UV irradiation? A look through the prism of quantum chemical calculations.

The stability of merocyanine forms formed under UV irradiation of a solution of a spiropyran salt, in which an organic part acts as a cation and a compact bromide ion as an anion, their photophysical properties, and the formation mechanism are studied in this work using time-dependent density functional theory. Theoretical calculations show that TTC and CTT are the most stable open forms (the difference in stability energies is 10.5 and 12.0 kcal mol-1 relative to the formation energy of spiropyran, respectively). The simulated absorption bands in the UV spectrum of the merocyanine forms are observed both in the UV region at 308-366 nm and in the visible region at 544-757 nm due to n → π* and π → π* type transitions. We found that the isomerisation mechanism of spiropyran into merocyanine forms includes two key stages: the ring opening to form cisoid merocyanine forms (except unstable TCC) through conical intersection and their subsequent isomerisation to form stable transoid isomers. The length of the Cspiro-O bond is 1.97 Å and the C1'-C2'-C3'-C4' angle is 70° in the structure close to conical intersection. The stage that determines the rate of this process is the isomerisation between transoid forms, as in the case of transformation of open merocyanine forms into spiropyran.

Transparent Organogels as a Medium for the Light-Induced Conversion from Spiropyran to Merocyanine.

Low-molecular-weight peptide gelators are a versatile class of compounds able to form gels under a variety of conditions, even via simple ultrasound sonication. In this paper, the ability of Boc-L-Phe-D-Oxd-L-Phe-OBn to gelate three organic solvents (toluene, tert-butyl methyl ether, and ethanol) was evaluated. The rheological behaviour of the materials was assessed via strain sweep analysis, while the fibrous network was analysed via optical microscopy on the wet gels. The gel obtained from toluene is a highly transparent material, and the one from ethanol appears translucent, while the one from tert-butyl methyl ether is opaque. These gels were used to study the reversible light-induced transformation from spyropiran (SP) to merocyanine (MC) and back, as a model system to check the effect of the gel medium onto the rection kinetic. We observed that the solvent used to form the organogels has a crucial effect on the reaction, as gels from aprotic solvents stabilize the SP form, while the ones from protic solvents stabilize the MC form. We thus obtained a solid support to stabilize the two photochromic species just by changing the solvent polarity. Moreover, we could demonstrate that the self-assembled gels do not interfere with the light-driven conversion process, either starting from SP or MC, thus representing a valid and economical photochromic material.

90% Yield Production of Spiropyran Covalently Grafted MXene‐Based RRAM Devices for Optoelectronic Dual‐Response Switching

AbstractOver the past decade, great efforts have been devoted to both intellectual property protection and information security protection. As a promising electroactive material, MXene is covalently or non‐covalently functionalized with organic species/polymers to produce a large amount of novel functional materials for biomedicine, optoelectronics, and energy storage. By using spiropyran covalently functionalized MXene (MXene‐SP) as an active material, an optoelectronic dual‐response resistive random access memory (RRAM) device is successfully fabricated. Upon UV illumination, the active layer is changed from MXene‐SP (SP: ring‐closed spiropyran form) observed under the illumination of blue light to MXene‐MC (MC: ring‐opened merocyanine form) due to photo‐induced “close‐to‐open” isomerization. The as‐fabricated ITO/MXene‐SP/ITO device exhibits typical nonvolatile optoelectronic dual‐response RRAM performance, with a production yield exceeding 90%. The achieved switch‐on/off voltages are −1.25/2.07 V under the illumination of blue light and −0.58/0.91 V under UV illumination. The switching bias window (Δ|VON − VOFF|) and the ON/OFF current ratio of MXene‐MC are 44.9% and 14.7% of these of MXene‐SP, respectively. By utilizing the window difference between the RRAM performance achieved under the illumination of different lights, one can easily encrypt the quick response code.

Construction of photo-responsive lignin as a broad-spectrum sunscreen agent

Lignin has potential to serve as promising sunscreen agents as it has good ultraviolet (UV) absorption and antioxidant properties. However, the weak absorption capacity of lignin in the long-wave UV region (UVA, 320–400 nm) limits its further development. In this work, a spiropyran-modified lignin (DLSP) with photo-responsive property was prepared by in-situ construction of spiropyran (SP) structure in the demethylated lignin (DL). Due to the presence of SP moiety, the absorption of DLSP in the UVA region was significantly improved. Under UV irradiation, its absorption peak was redshifted as unconjugated SP form isomerized to conjugated merocyanine (MC) form, and the UVA/UVB ratio increased from 0.62 to 0.74. The free-radical scavenging ability of lignin could protect SP from photodegradation, which provided DLSP excellent fatigue resistance. DLSP were blended into creams to investigate its sunscreen performance. Results indicated that DLSP exhibited radiation-enhanced sunscreen performance, the sun protection factor (SPF) of cream containing 10 wt% of DLSP improved from 20 to 67 after 8 h of UV irradiation. Moreover, DLSP showed low skin penetration and good biocompatibility. These results provide a useful guideline for the rational design of sunscreens with special functionalities.

Dynamic anticounterfeiting polymeric inks for multipurpose time-dependent encryption with a high level of security

Development of dynamic anticounterfeiting inks with time-dependent photochromism and photoluminescence characteristics has become highly interesting in advanced encryption systems. These inks are applicable in different fields, such as tracking UV rays and gas leakage and also quality monitoring. In this study, semi-continuous copolymerization of butyl acrylate and methyl methacrylate in the heterogeneous miniemulsion medium was used to synthesize colloidal nanoparticles with different flexibilities and polarities. Then, hydroxyl-functional spiropyran (SPOH) was physically incorporated into the nanoparticles to prepare photochromic and photoluminescent inks. The nanoparticles showed low light reflectance due to their small size, which makes them applicable in photoresponsive and sensing applications. In addition, the nanoparticles exhibited different glass transition temperature (Tg) that affects their photoresponsivity. Microscopic analysis revealed that all the latex samples have spherical nanoparticles with a mean size of about 70 nm and a low polydispersity index. UV–vis absorption and fluorescence emission spectra, isomerization kinetics, and photofatigue resistance of the photochromic latex nanoparticles were studied, which depend on chain polarity and flexibility of the latex nanoparticles. The optical properties of the photochromic materials are closely related to the structure and composition of the latex samples. Time-dependent features of the inks originate from their spiropyran molecules which gradually turns into the merocyanine form upon visible light induction. Photochromic inks with advanced encryption and dynamic characteristics were developed from the SPOH-doped colloidal latex nanoparticles. Different polarities and Tg of the inks result in their different dynamic chromism, which show their high level of information encryption technology. By virtue of these unique time-dependent properties, the encryption capabilities of these inks are significantly improved. These inks achieved unprecedented advances in the field of information security and encryption because of their innovative features.

Photoresponsive Spiropyran and DEGMA-Based Copolymers with Photo-Switchable Glass Transition Temperatures.

Herein, novel photoresponsive spiropyran (SP)-based P(DEGMA-co-SpMA) copolymers with variable percentages of SP fractions are synthesized. The SP group present in these polymers exhibited the abilities of reversible photoisomerism. Their photoresponsive, structural, and thermal properties have been investigated and compared using various characterization techniques. These light-responsive copolymers are found to exhibit photoswitchable glass transition temperature (Tg ), high thermal stability (Td > 250°C), instant photochromism as well as fluorescence upon exposure to UV light. It is demonstrated that the Tg of these synthesized polymers increased when irradiated with UV light (λ = 365nm), as a consequence of the photoisomerization of incorporated SP groups into their merocyanine form. This increase in Tg is attributed to an increase in polarity and a decrease in the overall entropy of the polymeric system when it switches from the ring-closed SP form (less-ordered state) to the ring-opened merocyanine form (more-ordered state). Therefore, such polymers with a unique feature of phototunable glass transition temperatures provide the possibility to be integrated into functional materials for various photoresponsive applications.

Light-driven self-assembly of spiropyran-functionalized covalent organic framework

Controlling the number of molecular switches and their relative positioning within porous materials is critical to their functionality and properties. The proximity of many molecular switches to one another can hinder or completely suppress their response. Herein, a synthetic strategy involving mixed linkers is used to control the distribution of spiropyran-functionalized linkers in a covalent organic framework (COF). The COF contains a spiropyran in each pore which exhibits excellent reversible photoswitching behavior to its merocyanine form in the solid state in response to UV/Vis light. The spiro-COF possesses an urchin-shaped morphology and exhibits a morphological transition to 2D nanosheets and vesicles in solution upon UV light irradiation. The merocyanine-equipped COFs are extremely stable and possess a more ordered structure with enhanced photoluminescence. This approach to modulating structural isomerization in the solid state is used to develop inkless printing media, while the photomediated polarity change is used for water harvesting applications.

5′-Substituted Indoline Spiropyrans: Synthesis and Applications

Methods for preparation of 5′-substituted spiropyrans, their chemical properties, and the effects of various factors on the relative stabilities of the spiropyrans and their isomeric merocyanine forms are examined, reviewed, and discussed.

Light-Dependent RAFT Polymerization of Spiropyran Acrylates.

A light-assisted RAFT copolymerization protocol is established and investigated for the synthesis of 2-hydroxyethyl acrylate (HEA) / spiropyran acrylate (SPA) copolymers with enhanced SPA-contents. Radiation with visible light prevents the spiropyran (SP) motif from isomerizing into the open merocyanine (MC) form which can interfere with the polymerization process by abstracting a hydrogen atom from an active radical via its phenolic oxygen. This article is protected by copyright. All rights reserved.

The guest CH3CN molecule triggers solid-state photochromism in a Cu2I2-based MOF for advanced time-dependent encryption and inkless erasable printing

It remains a big challenge to develop solid-state stimuli-responsive materials for time-dependent information encryption and inkless erasable printing with long retention times. Herein, a 2D Cu2I2-based MOF with photoresponsive spiropyran (SP) groups orderly installed on its skeleton is developed. The structural isomerization from SP to colored merocyanine (MC) form can be triggered by removing the CH3CN guests. Besides, the degree of structural isomerization and the retention time can be adjusted by controlling the amount of CH3CN guests, exhibiting dynamic photochromic behavior with multicolor states and tunable retention time. Based on these advantages, time-dependent information encryption is successfully achieved. Furthermore, the long retention time (>72 h) of the MC form under daylight conditions in the CH3CN-removed Cu2I2-based MOF and good repeatability make it promising in various applications, such as temporary calendars, price-cards, billboards, and reusable identity cards. This work provides a novel design strategy to fabricate multi-functional MOF-based smart materials for challenging applications of time-dependent information encryption and inkless erasable printing.

New cationic spiropyrans with photoswitchable NIR fluorescence

Visible-light-mediated photochromic compounds with NIR absorption and fluorescence are of great interest for use in different biomedical applications. In this work, new representatives of spiropyrans with conjugated cationic 3H-indolium substituents in different positions of 2H-chromene moiety were synthesized. The electron-donating methoxy groups were introduced in the uncharged indoline and charged indolium cycles to form the effective conjugation chain between the hetarene moiety and the cationic fragment for reaching NIR absorption and fluorescence. The molecular structure and the effects of cationic fragment position on the mutual stability of the spirocyclic and merocyanine forms of compounds were carefully studied in the solutions and solid state by NMR, IR, HRMS, single-crystal XRD, and quantum chemical calculations. It was found that the obtained spiropyrans demonstrate positive or negative photochromism depending on the cationic fragment’s position. One of spiropyrans has shown bidirectional photochromic properties induced exclusively by visible light of different wavelengths in both directions. The photoinduced merocyanine forms of compounds possessed far-red shifted absorption maxima and NIR fluorescence, which makes them prospective fluorescent probes for bioimaging.

Reversible Photocontrol of Microtubule Stability by Spiropyran-Conjugated Tau-Derived Peptides.

Spatiotemporal modulation of microtubules by light has become an important aspect of the biological and nanotechnological applications of microtubules. We previously developed a Tau-derived peptide as a binding unit to the inside of microtubules. Here, we conjugated the Tau-derived peptide to spiropyran, which is reversibly converted to merocyanine by light, as a reversible photocontrol system to stabilize microtubules. Among the synthesized peptides with spiropyran/merocyanine at different positions, several peptides were bound to the inside of microtubules and stabilized the structures of microtubules. The peptide with spiropyran at the N-terminus induced polymerization and stabilization of microtubules, whereas the same peptide with the merocyanine form did not exert these effects. Reversible formation of microtubules/tubulin aggregates was achieved using the peptide with spiropyran conjugated at the N-terminus and irradiation with UV and visible light. Spiropyran-conjugated Tau-derived peptides would be useful for spatiotemporal modulation of microtubule stability through reversible photocontrol of binding.

A novel indolino-spironaphthooxazine as the highly sensitive and selective probe for colorimetric detection of Au3+

A novel colorimetric probe in a spironaphthooxazine class, SPNO, was successfully synthesized. The single crystal and spectroscopic analyses were utilized to confirm its molecular structure and photo properties. The results showed that SPNO in a 1,4-dioxane-H2O solution displayed remarkable selectivity and sensitivity for colorimetric detection of Au3+ over other competing cations. Based on the UV–vis absorption study, the mixtures of SPNO and Au3+ exhibited a new absorption band at 580 nm due to the structural isomerization of SPNO into its merocyanine form which acts as a ligand to form an SPNO–Au3+ complex. The stoichiometric ratio of SPNO to gold(III) cations in the complexes was determined using Job's plot and the results indicated that the complexes were formed at a 1:2 mol ratio. The detection limit of Au3+ in 1,4-dioxane-H2O was investigated to be 335 nM. Moreover, the SPNO-coated test strips were also prepared to demonstrate that this probe can be used practically as a tool for highly sensitive detection of Au3+ in mixed 1,4-dioxane:water solutions. Finally, the sensitivity of SPNO toward Au3+ in drinking water was determined to verify its applicability in real samples.

Photo‐ and pH‐Responsive Hybrid Colloidal Particles

AbstractMaterials that respond to environmental changes in a predictable manner are called “smart”. Here, smart dual‐responsive hybrid colloidal particles were prepared using the interface‐mediated reversible addition–fragmentation chain‐transfer (RAFT) polymerization method. The structure and morphology of the prepared poly(methacrylated‐spiropyran) (p(MMASP)) colloids were characterized by FTIR, 1H‐NMR and GPC spectroscopies, and SEM. The molecular weight and polydispersity were determined as 7877 g mol−1 and 1.16, respectively. Heterocyclic ring cleavage of the spiropyran (SP) pendant group enables the pH‐ and light‐responsive behavior with a reversible conversion between the hydrophobic and hydrophilic states. The isomerization from SP to merocyanine (MC) form proceeds in 7 min, while the reverse action takes 9 min. The pronounced photochromic effect and high fatigue resistance make these colloids suitable for smart applications.

Polarity-Driven Isomerization of a Hydroxynaphthalimide-Containing Spiropyran at Room Temperature.

Design of spiropyrans showing spontaneous isomerization driven by the polarity of solvents is an important consideration for the synthesis of optical sensory materials. Although some spiropyrans undergo polarity-driven isomerization, they must be heated owing to the high activation energy required for isomerization. In this study, we describe that a spiropyran containing a hydroxynaphthalimide unit (1) exhibits a polarity-driven isomerization at room temperature. It exists as a colorless spirocyclic (SP) form in less polar solvents but is isomerized to a colored merocyanine (MC) form in polar solvents. The equilibrium amount of the MC form increases with an increase in the polarity of solvents. The MC form involves two resonance structures-the quinoidal and zwitterionic forms. In polar media, the zwitterionic form dominates mainly owing to solvation by polar molecules. Solvation stabilizes the negative charge of the zwitterionic form and decreases its ground state energy, thereby enhancing SP → MC isomerization. The SP ⇌ MC isomerization terminates within barely 30 s even at room temperature because the naphthol moiety with high π-electron density lowers the activation energy for the rate-determining rotational step.

Methyl 5′-Chloro-8-formyl-5-hydroxy-1′,3′,3′-trimethyl-spiro-[chromene-2,2′-indoline]-6-carboxylate

Spiropyrans modified with reactive polyfunctional substituents are of great interest as building blocks for the creation of various smart systems with controllable properties for materials science and biomedicine. In this study, a new highly modified spiropyran of the indoline series, methyl 5′-chloro-8-formyl-5-hydroxy-1′,3′,3′-trimethyl-spiro[chromene-2,2′-indoline]-6-carboxylate, was obtained via the cyclocondensation reaction from 5-chloro-1,2,3,3-tetramethyl-3H-indolium perchlorate and methyl 3,5-diformyl-2,4-dihydroxy-benzoate. The molecular structure of the target compound was confirmed by 1H, 13C NMR, and IR spectroscopy, as well as LC/MS and elemental analysis. Photochemical studies revealed photochromic activity for the obtained spiropyran at room temperature. The photoinduced merocyanine form demonstrated an enhanced lifetime and fluorescent properties in the red region of the spectrum.

Binuclear coordination complex of open merocyanine form of photochromic spiropyran with MnII(hfac)2 having high spin (S = 5) ground state

The interaction of 1,3,3-trimethylindolino-β-naphthopyrylospiran (TMI-NPS) with manganese(ii) hexafluoroacetylacetonate yields crystals of {MnII(hfac)2 (μ2-TMI-NPS)}2 (1) in which Mn(ii) atoms are bridged by oxygen atoms of ring-opened TMS-BPI into the dimer.