Search

This review is devoted to the features of isomerization reactions of indoline spiropyrans and the possibilities for their applications as building blocks in the design of molecular machines and ensembles capable of self-organization. Nanoscale systems functioning on the base of changes in geometric parameters and molecular dipole moment when switching between the spirocyclic and merocyanine forms have been considered. The factors that alter the system morphology may be electromagnetic irradiation or the acidity of the medium.

New indoline spiropyran was synthesized by the cyclocondensation of 5-chlorine-substituted Fischer base with 2,7-dihydroxy-1-naphthaldehyde. This spiropyran was used to prepare a new derivative of α-lipoic acid with potential biological activity, which can be considered as a promising photopharmacological agent. The structures of the synthesized compounds were determined by NMR and IR spectroscopy and mass spectrometry and were confirmed by X-ray diffraction. Biological assays using HeLa cells showed that the spirocyclic compounds possess low cytotoxicity even at relatively high concentrations.

Abstract Indoline spiropyran containing an σ-acceptor chlorine atom in 6′ position of the 2H-chromene part of the molecule was synthesized and studied. The use of 1,2,3,3-tetramethyl-3H-indolium perchlorate as a starting compound made it possible to achieve higher product yields as compared to previous studies. The molecular structure of the compound was established by single crystal X-ray diffraction analysis. The features of the crystal structure and intermolecular interactions were investigated using CrystalExplorer17 software package. The photochromic behavior in acetonitrile solution was studied for the first time. It was found that the merocyanine form of spiropyran is characterized by an absorption maximum at 592 nm, which is 37 nm closer to the range of the “biological window” in comparison with the nitro-substituted analog.

This work deals with the study of two derivatives of spiropyrans of indoline series and α-lipoic acid, which are considered as potential photopharmacological agents with controllable biological activity. The structure of the compounds has been studied in detail by X-ray diffraction analysis that revealed high lability of lipoic acid fragments. To assess potential biological activity, molecular docking for the possibility of spiropyran molecule to bind to amylin protein in order to inhibit its aggregation has been performed. The results of molecular docking showed a potential increase in the affinity of spirocyclic derivatives toward this protein as compared with α-lipoic acid.

New spiropyrans with chromene and indoline cores bearing alkoxycarbonyl group and cationic π-acceptor in the chromene moiety were synthesized in one step from 1,2,3,3-tetramethylindolinium and isophthalic dialdehyde derivatives. The appending of a 8′-positioned conjugated cationic fragment causes a significant red-shift of the photoinduced isomers absorption maxima and provides them with extremely long lifetimes up to 75 min. The obtained spiropyrans exhibit both positive and negative photochromism in solutions, demonstrating the properties of ‘photochromic balance’.

The review is devoted to mechanically sensitive indoline spiropyrans and their structural features. Recent advances in the study of their isomerization processes are considered, special attention is paid to the effect of pressure on the behavior of spiropyrans in the composition of various photochromic materials and dyes, as well as in the process of the formation of J-aggregates in Langmuir–Blodgett films.

The paper describes the preparation of a new salt spiropyran of indoline series containing vinyl-3H-indolium fragment in the 8' position of the 2H-chromene moiety and a chlorine atom in the benzene ring of the indoline fragment. The structure of the reaction product was studied by IR, 1H, and 13C NMR spectroscopy. The data of X-ray diffraction study indicate that the reaction leads to the partial exchange of the indoline hetarene fragment to provide previously described salt spiropyran containing no chlorine atom in the indoline portion of the molecule.

Two new photochromic spiropyrans containing carboxyl groups in the hetarene part of the molecule and being of interest as modifiers of organometallic scaffolds to give them photo-controlled functions are obtained by the cyclocondensation reaction of 5-carboxy-1,2,3,3-tetramethyl-3H-indolium iodide with corresponding 2,4-diformylphenols. The structure of the compounds obtained is confirmed by infrared (IR), 1H and 13C NMR spectroscopy (using 1H–1H COSY, 1H–13C HSQC, 1H–13C HMBC, and 1H–15N HMBC 2D NMR techniques); the chemical composition is determined by elemental analysis. The diffraction patterns of powdered samples of the compounds are measured, which make it possible to determine the symmetry groups and unit-cell parameters. Photochemical studies reveal the photochromic properties of the compounds at room temperature, and measurement of the rate constants kBA of the thermal discoloration of merocyanine forms at different temperatures allowed us to calculate the activation energy of these processes.

Three new photochromic spiropyrans containing a carboxylic group in the indoline fragment and an aldehyde substituent in the [2H]-chromene moiety were synthesized. The structure of the compounds obtained was studied using 1 H and 13 C NMR, IR and elemental analysis. Single crystal X-ray analysis was used to refine the molecular structure of 8′-formyl-1,3,3,6′-tetramethyl-spiro[indoline-2,2′-2H-chromene]-5-carboxylic acid. It was found out that in the solid state the molecules of that compound are arranged in dimeric associates due to the formation of intermolecular hydrogen bonds between carboxylic groups of two adjacent molecules. All the spiropyrans were shown to be photochromic with the lifetimes of the open forms in the range of 1.5–46.0 s.

A one-pot synthesis and structure study of a new salt spiropyran of the indoline series containing 2H-chromene vinyl-3H-indolium moiety as a substituent at the 8'-position are described. The structure was confirmed by 1H and 13C NMR and IR spectroscopy and high-resolution mass spectrometry. The single crystals of the compound were investigated by X-ray diffraction analysis. The structure was compared with the previously known isostructural analog.

New indoline spiropyrans containing conjugated cationic fragments and bulky substituents in the 2H-chromene moiety were synthesized by a simple one-pot method. The structures of the obtained compounds were confirmed by 1H and 13C NMR (including 2D methods) and IR spectroscopy, as well as by element analysis and HRMS. Molecular structure of 1,3,3-trimethyl-6′-formyl-8′-benzyl-spiro[indoline-2,2′-2H-chromene] was refined by single crystal X-ray. A comparative analysis of the spectral and photochromic properties of salt spiropyrans and similar structures without a cationic fragment was carried out. It was found that the introduction of the cationic substituent at the 6′-position of the 2H-chromene moiety leads to a significant increase in the photoinduced isomer's lifetime.

Synthesis, structure study, and photochromic properties of new salt spiropyrans of indoline series containing chlorine and bromine atoms in the 6'-position of the 2H-chromene moiety have been reported. The structure of the obtained compounds has been confirmed by 1H NMR spectroscopy and X-ray diffraction analysis. The compounds are photochromic; the long-wavelength absorption maximum for photoinduced isomer shows a considerable hypsochromic shift, while the lifetime increases considerably as compared with the fluorine-containing analog.

Organic photochromic compounds are attracting great interest as photoswitchable components of various bioconjugates for using in photopharmacology, targeted drug delivery and bio-imaging. Here we report on the synthesis of two novel molecular hybrids of indoline spiropyrans and alpha-lipoic acid via an esterification reaction. Preliminary photochemical studies revealed photochromic activity of 5-methoxy-substituted spirocompounds in their acetonitrile solutions. Both hybrid spiropyrans along with their parent substances in the hybrids were tested for the short-term cytotoxicity on HeLa cell cultures. The results of cytotoxicity studies showed unpredictable biocompatibility of the hybrids in comparison with the parent hydroxyl-substituted spiropyrans and α-lipoic acid, especially at the relatively high concentration of 2 mM. Using flow cytometry, we demonstrated that the both hybrids induced antioxidant response in the model cells. After the 24 h treatment, the hybrids administered at lower (500 µM) concentration caused suppressed cytosolic ROS and/or induced cellular thiols. At higher concentration, one of the hybrids demonstrated properties qualitatively similar to alpha-lipoic acid, yet far more strong. Together, flow cytometry results suggested that both hybrids of spiropyrans possess emergent biochemical and signaling antioxidant properties, exceeding those of alpha-lipoic acid.

Cyclocondensation of gossypol, the pigment of Gossypium (cotton), and Fischer’s bases produced two new spirocyclic compounds with potentially combined pharmacological and photoswitching activities. The structures and photochemical properties of the synthesized compounds were studied using PMR, IR, and UV spectroscopy.

A series of new spiropyrans of indoline series with ortho-hydroxyaldimine substituent has been synthesized, their structure and photochromic properties have been studied. The structure of the obtained compounds was confirmed by the data of 1H NMR and IR spectroscopy and elemental analysis. Photochromic properties of the prepared compounds have been studied. The introduction of ortho-hydroxy group into aldimine fragment of molecule (in the 6 position of the 2H-chromene fragment) has increased the lifetime of merocyanine isomer by 40 times as compared with amino-substituted spiropyran.

A new salt spiropyran of indoline series containing benzyl substituent in the 8'-position of the [2H]-chromene moiety of the molecule has been synthesized and studied. The structure of the obtained compound has been confirmed by methods 1H NMR and IR spectroscopy. The study of spectral characteristics and photochromic behavior of the target compound has shown that growing of its single crystal resulted in destruction on account of hydrolysis in acetonitrile. The 1H NMR spectra of the initial salt spiropyran and product of hydrolysis have been compared.

Based on a new polyfunctional derivative of dihydroxyisophtalic aldehyde (1) spiropyrans of 1,3-benzoxazin-4-one series with two carbonyl and a hydroxygroup in the [2H]-chromene fragment were synthesized. The results of single crystal X-ray analysis of 6′-carbomethoxy-3,5-dimethyl-8′-formyl-5′-hydroxy-4-oxospiro[1,3-benzoxazine-2,2′-chromene] (7a) helped to establish that condensation of 1,3-benzoxazine-4-onium perchlorates with (1) leading to the formation of spiropyrans occurs on the formyl group located at the position 3 of the starting aldehyde. The obtained spropyrans demonstrate photochromic activity at room temperature. Introduction of an electron donating methoxy group into the heteroarene part of the spiropyran molecule enhances its photocolorability.

The computational simulation of structure, energy and spectral characteristics of spiropyrans and their complexes with the cluster containing ten silver atoms was carried out using the density functional theory method (B3LYP/6-31G(d,p)/SDD). A competitive coordination of oxygen-containing substituents in the indoline and benzopyran moieties in spirocyclic molecules to the metal atoms was studied and an effect of position of donor groups on the activity of CO vibrational bands was established in the calculated Raman spectra. We revealed compounds, whose trans-trans-trans isomers upon a complexation with the silver cluster are characterized by the more than 100 times enhanced activity of band associated with the valence vibrations of CO bond in the Raman spectrum.

The structure and photochromic transformations of composite organometallic nanosystems composed of Ag nanoparticles covered with a layer of indoline-spiropyran (ISP) molecules from solutions were studied by spectrophotometry, surface Raman scattering spectroscopy (SERS) and quantum chemistry. Analysis of experimental data showed that studied nanostructured systems have photochromic properties, manifested in the reversible photo-induced changes both of the electronic-absorption spectra and of the relative intensity of the SERS bands. The open form of ISP, formed in the organometallic nanostructured film as a result of photochromic transformations, has an adsorption geometry, and, possibly, a conformation different from that formed as a result of chemisorption of ISP molecules on silver colloids.

Synthesis, study of structure and photochromic properties of new salt spiropyran of indoline series containing a fluorine substituent in the 6′-position of the 2H-chromene moiety have been described in the work. The structure of the compound has been confirmed by 1H NMR spectroscopy and X-ray diffraction analysis. Comparison of the structure of the compound with the previously studied analogue containing a methyl group instead of a fluorine atom in the 6′-position of the [2H]-chromene moiety has been made. This compound has been found to be photochromic, long-wavelength absorption maximum of its open form shows a bathochromic shift, and lifetime is increased three times as compared with the methyl analogue.

New spiropyran salts in the row of 1,3,3-trimethylspiro-indoline-2,2'-[2Н]-chromene are prepared and their structures are investigated. Molecular structures of obtained compounds are confirmed by X-ray diffraction and NMR spectroscopy. An open merocyanine isomer is found to be the most stable structure for one of these compounds.

A polyfunctional spiropyran of 1,3-benzoxazin-4-one series containing one hydroxy and two carbonyl substituents in the [2H]-chromene moiety was synthesized on the basis of a new analogue of 2,4-dihydroxyisophthalic aldehyde, methyl 3,5-diformyl-2,4-dihydroxybenzoate. The structure of resulting 6'-carbomethoxy-3,7-dimethyl-8'-formyl-5'-hydroxy-4-oxo-spiro-(1,3-benzoxazine-2,2'-[2H]-chromene) was unambiguously established by single crystal X-ray diffraction analysis. These data allowed us to reveal the route for the reaction of the new polyfunctional aldehyde with 1,3-benzoxazin-4-onium perchlorate.

The new spiropyran systems with an azomethine bridge were synthesized from a spiropyran containing aminogroup at 6′ position of the benzopyran fragment and substituted aromatic aldehydes. The chemical structure of compounds is confirmed by elemental analysis data, NMR (1H and 13C) and IR spectroscopy. Photochemical studies revealed the presence of photochromic properties at room temperature for one of the obtained spiropyrans.

Computer modeling (DFT B3LYP/6-31G(d,p)/SDD) of the structure and spectral characteristics of photochromic 1,3,3,8′-tetramethyl-6′-formylspiro[indoline-2,2′-[2H]-chromene] and 1,3,3-trimethyl-6′-methoxy-8′-formyl[spiroindoline-2,2′-[2H]-chromene] complexes adsorbed onto the surface of a 10-atom silver cluster has been performed. It has been demonstrated that, depending on the position of the formyl group in the quinoline ring and the isomeric form of the spirocyclic compound, the change in the band intensity in Raman spectra caused by complexation with the silver cluster can reach three orders of magnitude.

Photochromic molecules which can absorb and emit light within the “biological window” (650–1450 nm) are of great interest for using in various important biomedical applications such as bio-imaging, photopharmacology, targeted drug delivery, etc. Here we present three new indoline spiropyrans containing conjugated cationic fragments and halogen substituents in the 2H-chromene moiety which were synthesized by a simple one-pot method. The molecular structure of the obtained compounds was confirmed by FT-IR, 1H and 13C NMR spectroscopy (including 2D methods), HRMS, elemental and single crystal X-ray analysis. Photochemical studies revealed the photochromic activity of spiropyrans at room temperature which caused photoswitchable fluorescence in the near-IR region after UV-irradiation. While the spirocyclic forms of compounds demonstrated absorption bands in the UV–Vis spectra with maxima in the visible region at about 445 nm and were not fluorescent, the photogenerated merocyanine isomers absorbed in the near-IR range at 708–738 nm and emitted at 768–791 nm. It was found that compound 1a with fluorine substituent possesses the most red-shifted absorption and emission bands of merocyanine form among all the known photochromic spiropyrans with maxima at 738 and 791 nm correspondingly. TD DFT calculations have shown that the longest wavelength absorption maxima of the merocyanine forms correspond to S0-S1 transitions of the isomers with at least one trans-trans-trans-configured vinylindolium fragment which brings them closer to cyanine-like structure and causes an appearance of the absorption and emission bands in the near-IR region.

Indoline spiropyrans containing conjugated cationic vinyl-3H-indolium fragment and their analogues with formyl substituent in the 2H-chromene moiety were synthesized by a simple one-pot method. The structure of the obtained compounds was confirmed by 1H and 13C NMR (including 2D methods) and IR spectroscopy, as well as by element analysis and HRMS. Molecular structures of 1,3,3,6´-tetramethyl-8´-formyl-spiro[indoline-2,2´-2H-chromene] 8 and its cationic derivative 5 were refined by single crystal X-ray and compared. Notably, it was found that the crystals of formyl-substituted spiropyran contain both enantiomers (R- and S-) of the molecule in the unit cell. Photochromic properties of all the compounds were studied and compared using spectral-photokinetic methods. It was found out that electron-withdrawing substituents (vinyl-3H-indolium fragment and formyl group) located at the position 6´ of 2H-chromene moiety stabilize merocyanine isomer more effectively, than at the position 8´, leading even to the appearance of negative photochromism for the compound 3 with cationic substituent, which is quite unusual for this class of organic photochromes. Also, it was shown that chlorine atoms as σ-acceptors at the positions 5 and 5´´ of hetarene moiety and cationic fragment can suppress photochromic behavior.

Novel indoline spiropyrans containing bulky neopentyl substituents at the nitrogen atom of the indoline fragment were prepared. The structure of the obtained compounds was established by a set of physicochemical methods of analysis. Comparison was made for the spectral absorption and kinetic parameters of the obtained compounds and their analogs containing methyl and benzyl substituents instead of neopentyl ones.

New 3’-(4-chlorophenyl)-5’,5’-dimethyl-2’-oxospiro[(2H)-chromene-2,4’-1,3-oxazolidine] was synthesized to study the donor influence of the chlorine atom at the N-phenyl substituent on the structure and photochromic properties of 1,3-oxazolidine-containing spiropyrans. Photochemical properties of synthesized 1,3-oxazolidine derivatives were studied at room temperature in toluene. The X-ray diffraction parameters for the new compound were compared with the data obtained earlier for similar spiropyrans.

A new series of spiropyrans based on 1,3-benzoxazine-2-one were synthesized using acid catalysis. Contrary to expectations, 6′-bromo-6-chloro-3-methyl-8′-nitro-spiro[1,3-benzoxazine-4,2′-chromene]-2-one and 6,6′-dichloro-3-methyl-8′-nitro-spiro[1,3-benzoxazine-4,2′-chromene]-2-one did not exhibit photochromic behavior, which was explained by TD DFT calculations to be a consequence of the existence of an effective non-radiative decay pathway allowing the structural relaxation of the spiropyrans in the first singlet excited state.

The formation of complexes of nitrogenated molecules with a small copper cluster Cun (n=1-7, 13) is systematically studied through the calculations by the density functional theory method. It is shown that the molecules of R1N=Y (Y=CR2R3, NR2, O) are promising for searching agents for copper clusters, as they are synthetically-available, can exert reducing properties, are firmly bound to the copper atoms, and do not distort the original cluster structure. Using any bulky substituent R, it is possible to block access to a large surface area of the cluster for aggressive compounds. Oxygen complexes on the surface of the copper cluster drastically fall short of the strength of the structures formed by molecules R1N=Y (Y=CR2R3, NR2, O). Depending on the cluster size, the interaction force varies in a sinusoidal manner from minimum to maximum.

Asymmetric indoline-benzoxazine bis-spiropyran showing improved photochromic properties due to the electron-donating methoxy group has been prepared by multistage synthesis. The molecular structure of 8-methoxy-3-methyl-4-oxo-2H-1,3-benzoxazine-2-spiro-2′H,8′H-pyrano-[2,3-f]chromene-8′-spiro-2″-1″,3″,3″-trimethylindoline has been studied by X-ray diffraction. After exposure to UV light, the absorption spectra display several long-wavelength maxima, which can correspond to different merocyanine isomers of the bis-spiro compound.

Four novel bis-spiropyrans possessing two different photochromic units based on indoline and 1,3-benzoxazine heterocycles which are connected through the common 2H,8H-pyrano[2,3-f]chromene moiety and modified with electron donating substituents were obtained by multistep synthesis starting with 2,4-dihydroxy-iso-phthalic aldehyde. The structure of the compounds was confirmed by 1H and 13CNMR, FTIR and HRMS. All bis-spirocompounds exhibited photochromic activity at room temperature. Investigation of photochromic properties of bis-spiropyrans revealed that their once-opened merocyanine isomers were more stable than the double-opened ones under experimental conditions. It was also found that modification of the benzoxazine moiety of the molecule with electron donating substituents along with introduction of bulky benzyl group close to the oxazinespiro-center leads to an increase of the merocyanine lifetime up to 148 s.

An attempt was made to determine the influence of the nature of the lubricating medium on the qualitative composition of wear products during long-term frictional interaction of a brass-steel friction pair in model media. Copper nanoparticles deposited on the steel surface in the form of a tribo-film are formed in the lubricant composition. Using quantum chemical calculations, it was shown that carboxylic acids form complexes with copper ions and clusters. The concentration of iron ions decreases during the transition from formic to caproic acid in the homologous series of monobasic carboxylic acids. It is shown that particles of wear products agglomerate in a lubricating medium with an increase in the time of frictional interaction of friction pairs.

Indoline-derived spiropyrans, the precursors of nonsymmetric bis-spiropyrans, which are the promising prototypes of multistate molecular switches, were synthesized based on 4,6-diformyl-2-methylresorcinol. Molecular structure of one of these compounds was studied by X-ray diffraction analysis. Photochemical studies showed the presence of photochromic properties in the obtained spiropyrans at room temperature.

Here we applied photochromic spiropyran-based salts SP1 and SP2 as light-sensitive components of OFET-based non-volatile optical memory elements. Electrooptical programming by applying simultaneously light bias and gate (programming) voltage allowed us to demonstrate wide memory windows, high programming speeds (programming time of 0.5–20 ms), and good retention characteristics of the devices. It is remarkable that a minor difference in the molecular structures of the used spiropyran-based salts (the hydrogen atom in the structure of SP1 is replaced with the NO2 group in SP2) altered completely the behavior of the devices. Thus, OFETs comprising interlayers of the spiropyran-based salt SP1 showed a reversible photoelectrical switching which is characteristic for flash memory elements with good write–read–erase cycling stability. In contrast, devices based on the spiropyran-based salt SP2 demonstrated irreversible switching and operated as read-only memory (ROM). Both types of devices revealed the formation of multiple distinct electrical states thus resembling the behavior of multibit memory elements capable of high-density information storage.

Syntheses are reported for a series of N-substituted indolinospiropyrans containing a fused furan fragment in the benzopyran part of the molecule. The structure of these compounds was established by IR and multinuclear NMR spectroscopy. A study was carried out on the effect of the fused benzene ring and bulky N-benzyl substituent on the photochromic properties of these compounds in solution and in a polymer matrix. Dynamic NMR spectroscopy was used to study the thermally induced isomerization of the N-benzyl derivative.

Quantum-chemical modeling of the geometry and electronic structure of the ground state and the lowest excited states of various multiplicity was undertaken for the photochromic compound 1,3,3-tri-methyl-1′,2′-diphenylspiro[indoline-2,7′-furo[3,2-f]chromene], which belongs to the spiropyran class. After analysis of the localization of electronic excitation in the molecule, it was suggested that the photoinduced transformation of the colorless spiro form to the colored merocyanine photoproduct takes place with great probability in its lowest singlet state and not in the triplet excited state.

A new benzoxazinone spiropyran containing a crown ether fragment have been synthesized. This compound was studied by both experimental and theoretical methods. In contrast to previously studied analogs, photoinduced coloration is not observed. By irradiating this crown-containing spiropyran, only an E-Z isomerization reaction proceeds. Complexation of the closed spiropyran form runs in a solution containing a calcium salt. A hypsochromic shift of the long-wavelength maximum in the absorption spectrum is observed.