Comparative Evaluation of Substituent Effect on the Photochromic Properties of Spiropyrans and Spirooxazines

Scalable, Green, Flexible Photochromic Bacterial Cellulose for Multicolor Switching, Photo-patterning, and Daily Sunlight UV Monitoring

Sustainable, durable, and diverse photochromic smart textiles based on bacterial cellulose (BC) have emerged as attractive candidates in UV-sensing applications due to the green and easy functionalization of BC. However, existing BC-based photochromic textiles lack photochromic efficiency and combining fastness. In this study, a green strategy for in situ fermentation is developed to achieve the directional distribution of functional particles and remarkable photochromism in photochromic bacterial cellulose (PBC). The unique functional design obtained by regulating the photochromic dye distribution in 3D nanonetworks of PBCs during in situ growth affords a more uniform distribution and high fastness. Benefiting from the uniform distribution of photochromic dyes and adequate utilization of the 3D network structure, more surface area is provided to receive and utilize the photon energy from the UV rays, making the photochromic process more effective. The as-prepared PBCs exhibited rapid (within 1 min) and stable (30 cycles) discoloration and multicolor selectivity. Their simple preparation process and exceptional wearability, e.g., their flexibility, lightweight, and air permeability, make them suitable for various applications, including tunable color switching systems, photopatterning, and daily sunlight UV monitoring. This study provides empirical value for the biofabrication of photochromic textiles and wearable flexible UV sensors.

Recent advances in photoresponsive printing inks for security encoding applications

Counterfeiting of banknotes, important documents, and branded goods continues to be a major worldwide problem for governments, businesses, and consumers. This problem has serious financial, security, and health implications. Due to their stability for printing on various substrates, the photochromic anticounterfeiting inks have received important interest. There have been various photochromic agents, such as polymer nanoparticles, quantum and carbon dots, and organic and inorganic fluorophores and luminophores, which have been broadly used for antiforging applications. In comparison to organic agents, inorganic photochromic materials have better stability under reversible/long-term light illumination. Recently, the remarkable optical characteristics and chemical stability of photoluminescent and photochromic agents have led to their extensive usage anticounterfeiting products. There have been also several strategies to tackle the rising problem of counterfeiting. Both of solvent-based and water-based inks have been developed for security encoding purposes. Additionally, the printing methods, including screen printing, labeling, stamping, inkjet printing, and handwriting, that have been used to apply anticounterfeiting inks onto various surfaces are discussed. The limitations of photoluminescent and photochromic agents and the potential for their future preparation to combat counterfeiting were discussed. This review would benefit academic researchers and industrial developers who are interested in the area of security printing.

TD-DFT analysis of excitation of the closed-form spironaphthopyran in methanol solution: The contribution of vibronic transitions and intermolecular hydrogen bonds

A set of forty hybrid functionals, combined with a 6-31++G(d,p) basis set and an IEFPCM solvent description, was applied to calculate the S0→S1 excitation of the spironaphthopyran (SNP) photochromic compound. None of the hybrid functionals resulted in Cspiro-O bond cleavage upon optimization of the S1 excited state of the SNP, in contrast to the spiropyran BIPS which we studied earlier. At the same time, the deformations of the SNP during excitation turned out to be very small (in contrast to those for BIPS) which made it possible to calculate its vibronic absorption spectrum. The best agreement between the calculated and experimental absorption bands was demonstrated by the SOGGA11X functional. The main result of this work is confirmation of the vibration participation in the breaking of the Cspiro-O bond in the S1 excited state of the SNP. This assumption based on experimental data was previously put forward by other authors. Strong H-bonds between the SNP and two methanol molecules enhance the photoinduced distortions of the latter and are slightly weakened upon excitation.

Spiropyran-Based Photoisomerizable α-Amino Acid for Membrane-Active Peptide Modification

Photoisomerizable peptides are promising drug candidates in photopharmacology. While azobenzene- and diarylethene-containing photoisomerizable peptides have already demonstrated their potential in this regard, reports on the use of spiropyrans to photoregulate bioactive peptides are still scarce. This work focuses on the design and synthesis of a spiropyran-derived amino acid, (S)-2-amino-3-(6'-methoxy-1',3',3'-trimethylspiro-[2H-1-benzopyran-2,2'-indolin-6-yl])propanoic acid, which is suitable for the preparation of photoisomerizable peptides. The utility of this amino acid is demonstrated by incorporating it into the backbone of BP100, a known membrane-active peptide, and by examining the photoregulation of the membrane perturbation by the spiropyran-containing peptides. The toxicity of the peptides (against the plant cell line BY-2), their bacteriotoxicity (E. coli), and actin-auxin oscillator modulation ability were shown to be significantly dependent on the photoisomeric state of the spiropyran unit.

Wavelength Selective Photocontrol of Hybrid Azobenzene-Spiropyran Photoswitches with Overlapping Chromophores

Compounds with multiple photoswitching units are appealing for complex photochemical control of molecular materials and nanostructures. Herein, we synthesized novel meta- and para- connected (related to the nitrogen of the indoline) azobenzene-spiropyran dyads, in which the central benzene unit is shared by both switches. We investigated their photochemistry using static and time-resolved transient absorption spectroscopy as well as quantum chemical calculations. In the meta-compound, the individual components are photochemically decoupled due to the meta-pattern. In the para-compound the spiro-connectivity leads to a bifunctional photoswitchable system with a red-shifted absorption. The azobenzene and the spiropyran can thus be addressed and switched independently by light of appropriate wavelength. Through the different connectivity patterns two different orthogonally photoswitchable systems have been obtained which are promising candidates for complex applications of light control.

Photoswitchable Photochromic Chelating Spironaphthoxazines: Synthesis, Photophysical Properties, Quantum-Chemical Calculations, and Complexation Ability

The stable and efficient photochromic and photoswitchable molecular systems designed from spirooxazines are of increasing scientific and practical interest because of their present and future applications in advanced technologies. Among these compounds, chelating spironaphthoxazines have received widespread attention due to their efficient optical response after complexation with some metal ions being of biomedical interest and environmental importance, as well as their good cycle performance and high reliability, especially by metal ion sensing. In this mini-review, we summarize our results in the design of novel photoswitchable chelating spironaphthoxazines with specific substituents in their naphthoxazine or indoline ring systems in view of recent progress in the development of such molecular systems and their applications as metal ion sensors. The design, synthesis methods, and photoresponse of such spirooxazine derivatives relevant to their applications, as well as quantum-chemical calculations for these compounds, are presented. Examples of various design concepts are discussed, such as sulfobutyl, hydroxyl, benzothiazolyl, or ester and carboxylic acid as substituents in the chelating spironaphthoxazine molecules. Further developments and improvements of this interesting and promising kind of molecular photoswitches are outlined.

Naphtho[1,8-de][1,2]Oxazin-4-ol: Precursor to 1,2,8-Trisubstituted Naphthalenes and 1-Unsubstituted Naphtho[1,2-d]isoxazole 2-Oxide: A Novel Isomerization of the N-Oxide to Nitrile Oxide en Route to Isoxazol(in)es

Naphtho[1,8-de][1,2]oxazin-4-ol and its acyl or benzyl derivatives ring open to various 2,8-dihydroxy-1-naphthonitriles, which, through (de)protection protocols and reduction, afford the target (E)-2-hydroxy-8-methoxy-1-naphthaldehyde. This was converted to its corresponding oxime, which was oxidatively o-cyclized with phenyliodine(III) diacetate (PIDA) to 9-methoxynaphtho[1,2-d]isoxazole 2-oxide. The latter, in deuterated DMSO at room temperature, was rearranged to its isomer 2-hydroxy-8-methoxy(naphthalen-1-yl)nitrile oxide. The isomerization was detected by time-course plot 1H NMR spectroscopy and further identified from its 13C NMR and HRMS spectra. The nitrile oxide was stable in (non)deuterated DMSO for at least 18 h. A 3,4-bis(2-hydroxy-8-methoxynaphthalen-1-yl)-1,2,5-oxadiazole 2-oxide, as a dimerization product or an isocyanate as a rearrangement isomer, was ruled out, the former by its HRMS spectrum and the latter by its 1,3-dipolar cycloaddition reactions to substituted isoxazoles.

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.

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.

Spiropyran-based chromic hydrogels for CO2 absorption and detection

By enabling rapid, cost-effective, user-friendly and in situ detection of carbon dioxide, colorimetric CO₂ sensors are of relevance for a variety of fields. However, it still remains a challenge the development of optical chemosensors for CO₂ that combine high sensitivity, selectivity and reusability with facile integration into solid materials. Herein we pursued this goal by preparing hydrogels functionalized with spiropyrans, a well-known class of molecular switches that undergo different color changes upon application of light and acid stimuli. By varying the nature of the substituents of the spiropyran core, different acidochromic responses are obtained in aqueous media that allow discriminating CO₂ from other acid gases (e.g., HCl). Interestingly, this behavior can be transferred to functional solid materials by synthesizing polymerizable spiropyran derivatives, which are used to prepare hydrogels. These materials preserve the acidochromic properties of the incorporated spiropyrans, thus leading to selective, reversible and quantifiable color changes upon exposure to different CO₂ amounts. In addition, CO₂ desorption and, therefore, recovery of the initial state of the chemosensor is favored by irradiation with visible light. This makes spiropyran-based chromic hydrogels promising systems for the colorimetric monitorization of carbon dioxide in a diversity of applications.

A Modern Look at Spiropyrans: From Single Molecules to Smart Materials

Photochromic compounds of the spiropyran family have two main isomers capable of inter-switching with UV or visible light. In the current review, we discuss recent advances in the synthesis, investigation of properties, and applications of spiropyran derivatives. Spiropyrans of the indoline series are in focus as the most promising representatives of multi-sensitive spirocyclic compounds, which can be switched by a number of external stimuli, including light, temperature, pH, presence of metal ions, and mechanical stress. Particular attention is paid to the structural features of molecules, their influence on photochromic properties, and the reactions taking place during isomerization, as the understanding of the structure-property relationships will rationalize the synthesis of compounds with predetermined characteristics. The main prospects for applications of spiropyrans in such fields as smart material production, molecular electronics and nanomachinery, sensing of environmental and biological molecules, and photopharmacology are also discussed.

Photochromic spiro-indoline naphthoxazines and naphthopyrans in dye-sensitized solar cells

Photochromic dyes possess unique properties that can be exploited in different domains, including optics, biomedicine and optoelectronics. Herein, we explore the potential of photochromic spiro-indoline naphthoxazine (SINO) and naphthopyran (NIPS) for application in photovoltaics. We designed and synthesized four new photosensitizers with a donor-pi-acceptor structure embedding SINO and NIPS units as photochromic cores. Their optical, photochromic and acidochromic properties were thoroughly studied to establish structure-properties relationships. Then, after unravelling the possible forms adopted depending on the stimuli, their photovoltaic properties were evaluated in DSSCs. Although the photochromic behavior is not always preserved, we elucidate the interplay between photochromic, acidochromic and photovoltaic properties and we demonstrate that these dyes can act as photosensitizers in DSSCs. We report a maximum power conversion efficiency of 2.7% with a NIPS-based dye, a tenfold improvement in comparison to previous works on similar class of compounds. This work opens new perspectives of developments for SINO and NIPS in optical and photovoltaic devices, and it provides novel research directions to design photochromic materials with improved characteristics.

Light manipulation for fabrication of hydrogels and their biological applications

The development of biocompatible materials with desired functions is essential for tissue engineering and biomedical applications. Hydrogels prepared from these materials represent an important class of soft matter for mimicking extracellular environments. In particular, dynamic hydrogels with responsiveness to environments are quite appealing because they can match the dynamics of biological processes. Among the external stimuli that can trigger responsive hydrogels, light is considered as a clean stimulus with high spatiotemporal resolution, complete bioorthogonality, and fine tunability regarding its wavelength and intensity. Therefore, photoresponsiveness has been broadly encoded in hydrogels for biological applications. Moreover, light can be used to initiate gelation during the fabrication of biocompatible hydrogels. Here, we present a critical review of light manipulation tools for the fabrication of hydrogels and for the regulation of physicochemical properties and functions of photoresponsive hydrogels. The materials, photo-initiated chemical reactions, and new prospects for light-induced gelation are introduced in the former part, while mechanisms to render hydrogels photoresponsive and their biological applications are discussed in the latter part. Subsequently, the challenges and potential research directions in this area are discussed, followed by a brief conclusion. STATEMENT OF SIGNIFICANCE: Hydrogels play a vital role in the field of biomaterials owing to their water retention ability and biocompatibility. However, static hydrogels cannot meet the dynamic requirements of the biomedical field. As a stimulus with high spatiotemporal resolution, light is an ideal tool for both the fabrication and operation of hydrogels. In this review, light-induced hydrogelation and photoresponsive hydrogels are discussed in detail, and new prospects and emerging biological applications are described. To inspire more research studies in this promising area, the challenges and possible solutions are also presented.

Highly Sensitive and Selective Spiropyran-Based Sensor for Copper(II) Quantification

The metal-binding capabilities of the spiropyran family of molecular switches have been explored for several purposes from sensing to optical circuits. Metal-selective sensing has been of great interest for applications ranging from environmental assays to industrial quality control, but sensitive metal detection for field-based assays has been elusive. In this work, we demonstrate colorimetric copper sensing at low micromolar levels. Dimethylamine-functionalized spiropyran (SP1) was synthesized and its metal-sensing properties were investigated using UV-vis spectrophotometry. The formation of a metal complex between SP1 and Cu²⁺ was associated with a color change that can be observed by the naked eye as low as ≈6 μM and the limit of detection was found to be 0.11 μM via UV-vis spectrometry. Colorimetric data showed linearity of response in a physiologically relevant range (0-20 μM Cu²⁺) with high selectivity for Cu²⁺ ions over biologically and environmentally relevant metals such as Na⁺, K⁺, Mn²⁺, Ca²⁺, Zn²⁺, Co²⁺, Mg²⁺, Ni²⁺, Fe³⁺, Cd²⁺, and Pb²⁺. Since the color change accompanying SP1-Cu²⁺ complex formation could be detected at low micromolar concentrations, SP1 could be viable for field testing of trace Cu²⁺ ions.

Photoinduced reconfiguration to control the protein-binding affinity of azobenzene-cyclized peptides

The impact of next-generation biorecognition elements (ligands) will be determined by the ability to remotely control their binding activity for a target biomolecule in complex environments. Compared to conventional mechanisms for regulating binding affinity (pH, ionic strength, or chaotropic agents), light provides higher accuracy and rapidity, and is particularly suited for labile targets. In this study, we demonstrate a general method to develop azobenzene-cyclized peptide ligands with light-controlled affinity for target proteins. Light triggers a cis/trans isomerization of the azobenzene, which results in a major structural rearrangement of the cyclic peptide from a non-binding to a binding configuration. Critical to this goal are the ability to achieve efficient photo-isomerization under low light dosage and the temporal stability of both cis and trans isomers. We demonstrated our method by designing photo-switchable peptides targeting vascular cell adhesion marker 1 (VCAM1), a cell marker implicated in stem cell function. Starting from a known VCAM1-binding linear peptide, an ensemble of azobenzene-cyclized variants with selective light-controlled binding were identified by combining in silico design with experimental characterization via spectroscopy and surface plasmon resonance. Variant cycloAZOB[G-VHAKQHRN-K] featured rapid, light-controlled binding of VCAM1 (KD,trans/KD,cis ∼ 130). Biotin-cycloAZOB[G-VHAKQHRN-K] was utilized to label brain microvascular endothelial cells (BMECs), showing co-localization with anti-VCAM1 antibodies in cis configuration and negligible binding in trans configuration.

Synthesis and Photochromism of Novel Pyridyl-Substituted Naphthopyrans

Multitarget synthetic strategies to access novel photochromic 3H-naphtho[2,1-b]pyrans decorated with pyridyl units are described. The new pyridyl-substituted 3H-naphtho[2,1-b]pyrans display good photochromic properties with reversible generation of photomerocyanines, which exhibit mainly orange/red hues. Photochromic parameters including photocolorability and persistence of color vary tremendously on structural modification of the naphthopyran core.

Synthesis, Photochromic Properties, and Crystal Structures of Salts Containing a Pyridinium-Fused Spiropyran: Positive and Negative Photochromism in the Solution and Solid State

Salts containing the merocyanine form of a pyridinium-fused spiropyran ([6'-MC]X; X = I and PF₆) were prepared, and their crystal structures were determined. In addition, the photochromic properties of the salts were spectroscopically and kinetically investigated. In the solution state, the salts exhibited negative photochromism. Theoretical calculations revealed that the negative photochromism of the salt originates from the drastic stabilization of the merocyanine structure by electron delocalization of the pyridinium ring. Furthermore, the salts containing the merocyanine and spiropyran forms ([6'-MC]I, [6'-MC]PF₆, and [6'-SP]PF₆) were obtained by recrystallization. The crystals of [6'-SP]PF₆ exhibited positive photochromism in the solid state; however, no photochromism was observed in the [6'-MC]X crystals.

Synthesis and Comparative Evaluation of Photoswitchable Magnetic Resonance Imaging Contrast Agents

A series of spiropyran (SP)-based magnetic resonance imaging (MRI) contrast agents have been synthesized and evaluated for changes in relaxivity resulting from irradiation with visible light. Both electron-donating and electron-withdrawing substituents were appended to the SP ring in order to study the electronic effects on the photochromic and relaxivity properties of these photoswitchable MRI contrast agents. Photoswitches lacking an electron-withdrawing substituent isomerize readily between the merocyanine and SP forms, while the addition of a nitro group prevents this process. Complexes capable of isomerizing were demonstrated to effect a change in the relaxivity of the appended gadolinium complex.

Cation-induced ring-opening and oxidation reaction of photoreluctant spirooxazine-quinolizinium conjugates

Two new spiroindolinonaphthoxazine derivatives with an electron-accepting styrylquinolizinium or styrylcoralyne unit, respectively, were synthesized, and the influence of such an arylvinyl substituent on the chemical and photochemical properties of the compounds was investigated. Specifically, these spirooxazines turned out to be resistant towards the photoinduced merocyanine formation, and the irradiation with light mainly led to photodegradation of the substrates. However, it was shown by colorimetric and fluorimetric screening assays as well as by detailed NMR spectroscopic and mass spectrometric studies that the addition of particular metal ions (Cu2+, Fe3+, and to a certain extent Hg2+) initially induced a ring-opening reaction that was irreversibly followed by a fast ring closure-deprotonation-oxidation sequence to give styryl-substituted naphthoxazole derivatives as the products quantitatively. For the quinolizinium-substituted spirooxazine derivative, the formation of the respective oxidation product caused the development of a broad absorption band between 425 nm and 500 nm and a new emission band at λfl = 628 nm, so that it may be employed as a selective chemosensor or chemodosimeter for the colorimetric and fluorimetric detection of Cu2+ and Fe3+.

Color-Switchable Polar Polymeric Materials

Spiropyran is an important mechanophore, which has rarely been incorporated as a cross-linker in polar polymer matrices, limiting its applications in innovative mechanochromic devices. Here, three spiropyrans with two- or three-attachment positions were synthesized and covalently bonded in polar poly(hydroxyethyl acrylate) (PHEA), to achieve color-switchable materials, triggered by light and when swollen in water. The negative photochromism in the dark and mechanical activation by swelling in water were investigated. Measurements of negative photochromism were conducted in solution and cross-linked PHEA bulk polymers, with both showing color reversibility when stored in the dark or on exposure to visible light. The force of swelling in water was sufficient to induce the ring-opening reaction of spiropyran. It was found that tri-substituted spiropyran (SP3) was less influenced by the polar matrix but showed the fastest color activation during swelling. SP3 also showed accelerated ring opening to the colored state during the swelling process. Bleaching rates and color switchability were investigated under swollen and dehydrated conditions. The effect of cross-link density on the swelling activation was explored to better understand the interaction between the mechanophore and the polar environment. The results demonstrated that influences from both the polar environment and the mechanochromic nature of spiropyran had an impact on the absorption intensity, rate of change, and the decoloration rate of the materials. This study provides the opportunity to manipulate the properties of spiropyrans to afford materials with a range of color-switching properties under different stimuli.

Antioxidant Sensing by Spiropyrans: Substituent Effects and NMR Spectroscopic Studies

The development of stimuli-responsive small molecules for probing biologically active antioxidants such as glutathione (GSH) has important ramifications in the detection of oxidative stress. An ideal sensor for biological applications should exhibit sufficient sensitivity and selectivity for detection at physiological concentrations and be reversible to allow continuous and dynamic monitoring of antioxidant levels. Designing a suitable sensor thus requires a detailed understanding of activation properties and mechanism of action. In this work, we report a new set of GSH-responsive spiropyrans and demonstrate how changes in the electronic structure of spiropyrans influence GSH sensing with high specificity versus other structurally similar and biologically relevant redox-active molecules. The sensitivity, selectivity, kinetics, binding constant, and reversibility of GSH-responsive-substituted spiropyrans were investigated using UV-vis spectroscopy and laser irradiation experiments. Detailed studies of the mechanism of interaction between spiropyrans with GSH were investigated using NMR spectroscopy. Understanding how electronic effects impact the sensing ability of spiropyrans toward antioxidants and elucidating the mechanism of the spiropyran-GSH interaction will facilitate the design of more effective sensors for detection of antioxidants in vivo.

Design and Applications of Photoresponsive Hydrogels

Hydrogels are the most relevant biochemical scaffold due to their tunable properties, inherent biocompatibility, and similarity with tissue and cell environments. Over the past decade, hydrogels have developed from static materials to "smart" responsive materials adapting to various stimuli, such as pH, temperature, chemical, electrical, or light. Light stimulation is particularly interesting for many applications because of the capability of contact-free remote manipulation of biomaterial properties and inherent spatial and temporal control. Moreover, light can be finely adjusted in its intrinsic properties, such as wavelength and intensity (i.e., the energy of an individual photon as well as the number of photons over time). Water is almost transparent for light in the photochemically relevant range (NIR-UV), thus hydrogels are well-suited scaffolds for light-responsive functionality. Hydrogels' chemical and physical variety combined with light responsiveness makes photoresponsive hydrogels ideal candidates for applications in several fields, ranging from biomaterials, medicine to soft robotics. Herein, the progress and new developments in the field of light-responsive hydrogels are elaborated by first introducing the relevant photochemistries before discussing selected applications in detail.

New mechanism, new chromophore: investigating the electrophilic behaviour of styrylindolium dyes

To inform the design of future merocyanine-based sensors for nucleophilic analytes, a range of model styrylindolium salts were synthesised, and their behaviour towards cyanide, methanethiolate and sulfide was examined using spectroscopic techniques. In the majority of cases, standard 1,2- and 1,4-nucleophilic additions predominated; however, 4-nitrostyrylindolium salts underwent an unexpected dearomatising 1,8-addition with sulfur-centred nucleophiles. The enamine triene products thus produced display useful optical properties and provide a platform for novel sensor design, and the unusual 1,8-reaction pathway enables synthesis of novel molecular architecture.

Synthesis and solution behaviour of dual light- and temperature-responsive poly(triethylene glycol-co-spiropyran) copolymers and block copolymers

We herein report on the synthesis and characterization of materials featuring a dual-responsive copolymer segment consisting of photo-responsive spiropyran (SPA) and temperature-responsive triethylene glycol acrylate (TEGA).

Insights into the mechanochromism of spiropyran elastomers

Colourless polymeric samples comprising mechanochromic spiropyrans (SPs) rapidly appear coloured under external pressure, due to their transition from ring closed SP to ring-opened merocyanine (MC).