Polymethine Research Articles

Highly conjugated streptocyanine-ESIPT dyes via Vilsmeier-Haack reagent

Polymethine and ESIPT dyes are known to act as fascinating compounds for several applications. However, a combination of both fluorophores as a single molecule is poorly investigated. In this work, we report for the first time the successful synthesis of streptocyanine-ESIPT derivatives through a simple three-step route. Our synthetic route involves the coupling between amino ESIPT dyes and an iminium salt formed through the reaction between Vilsmeier-Haack reagent and cyclohexanone. Remarkably, the presence of a polymethine chain between two ESIPT nuclei changed the photophysical properties of the fluorophores individually. The new streptocyanine-ESIPT dyes contains electron acceptors groups able to increase of the conjugation of the π-system. In addition, the outstanding combination of these two different class of dye molecules increased significantly the Stokes shift of polymethine dyes, which enable the future application of these derivatives as efficient biological probes.

Corrigendum to “Characterization of Triphenylamino-Based Polymethine Dyes”

Corrigendum to “Characterization of Triphenylamino-Based Polymethine Dyes”

On-the-Fly Femtosecond Action Spectroscopy of Charged Cyanine Dyes: Electronic Structure versus Geometry.

Understanding optical properties of molecular dyes is required to drive progress in molecular photonics. This requires a fundamental comprehension of the role of electronic structure, geometry, and interactions with the environment in order to guide molecular engineering strategies. In this context, we studied charged cyanine dye molecules in the gas phase with a controlled microenvironment to unravel the origin of the spectral tuning of this class of molecules. This was performed using a new approach combining femtosecond multiple-photon action spectroscopy of on-the-fly mass-selected molecular ions and high-level quantum calculations. While arguments based on molecular geometry are often used to design new polymethine dyes, we provide experimental evidence that electronic structure is of primary importance and hence the decisive criterion as suggested by recent theoretical investigations.

Quantum-classical mechanics: Luminescence spectra in polymethine dyes and J-aggregates. Nature of the small Stokes shift

Abstract It can be shown that the joint and full-fledged (beyond the adiabatic approximation) motion of light electrons and very heavy nuclei in molecular and chemical physics in the framework of quantum mechanics can be only singular. This singularity is damped in a new theoretical approach called dozy-chaos mechanics or quantum-classical mechanics. Earlier, the effectiveness of the damping procedure for the above singularity was demonstrated in the simplest case of quantum-classical mechanics (of quantum-classical transitions), namely, by quantum-classical mechanics of elementary electron-charge transfers in condensed media and its applications to the shape of optical absorption bands in polymethine dyes and their aggregates and to other problems. In this article, on the basis of the quantum-classical mechanics of elementary electron-charge transfers, we find out the nature of the luminescence band shape and the small Stokes shift in polymethine dyes and J-aggregates. The very small experimental value of the Stokes shift (1 nm in the J-band) is explained by two factors, both of which are associated with a high degree of self-organization of molecular quantum transitions in J-aggregates. The first of these is related to the stimulation of the electronic transition in the J-chromophore by the chaotic dynamics (weak dozy chaos) of the reorganization of the nuclear environment, which leads to the well-known narrow and red-shifted absorption J-band. The second factor is associated with the spontaneous loosening of the excited electronic state immediately before the act of the photon’s creation by the electron in spontaneous emission. For polymethine dyes and J-aggregates, the universal effect of spontaneous dynamic loosening turns out to be abnormally strong, it must be supposed, due to the very extended π -electron systems undergoing the quantum-classical transitions there. A quantitative explanation is also given for the position and shape of the optical bands in J-aggregates for luminescence and optical absorption in the case of an admixture of monomers of a polymethine dye in an aqueous solution. The experimental fact of the absence of the optical band of the monomers on the shortwave wing of the J-band of luminescence with its apparent presence on the shortwave wing of the J-band of optical absorption is explained by a significant increase in the dynamic self-organization of quantum-classical transitions in J-aggregates as compared to just a small increase in the dynamic self-organization of these transitions in monomers upon the change from optical absorption to luminescence.

Interaction of Polymethine Dyes with Detonation Nanodiamonds.

Among cationic, anionic, and merocyanine polymethine dyes, the binding to detonation nanodiamond (DND) colloid particles in hydrosol occurs only for negatively charged dye species. This, in view of the positive ζ-potential of the DND used in this study, suggests the predominance of electrostatic interactions over other intermolecular forces in such systems. Indeed, after decorating the merocyanine and the cationic dye by one and two negatively charged sulfopropyl groups, respectively, so that the net charge of their colored species becomes negative, the compounds also demonstrate affinity to the DND particles. In all cases, the binding of dyes to DND is accompanied by a decrease in fluorescence intensity and a bathochromic shift of their absorption and fluorescence bands. A quantitative study of the dyes adsorption on the DND nanoparticles as performed using the Küster-Freundlich and Langmuir equations reveals some peculiarities of their attaching to the DND aggregates and allows estimating the specific area of the DND particles at a concentration of 0.0212 wt/vol %.

Isomerization and Properties of Isomers of Carbocyanine Dyes

One of the important features of polymethine (cyanine) dyes is isomerization about one of C–C bonds of the polymethine chain. In this review, spectral properties of the isomers, photoisomer-ization and thermal back isomerization of carbocyanine dyes, mostly meso-substituted carbocy-anine dyes, are considered. meso-Alkyl-substituted thiacarbocyanine dyes are present in polar solvents mainly as cis isomers and, hence, exhibit no photoisomerization, whereas in nonpolar solvents, in which the dyes are in the trans form, photoisomerization takes place. In contrast, the meso-substituted dyes 3,3′-dimethyl-9-phenylthiacarbocyanine and 3,3′-diethyl-9-(2-hydroxy-4-methoxyphenyl)thiacarbocyanine occur as trans isomers and exhibit photoisomerization in both polar and nonpolar solvents. The behavior of these dyes may be ex-plained by the fact that the phenyl ring of the substituent in their molecules can be twisted at some angle, removing the substituent from the plane of the molecule and reducing its steric ef-fect on the conformation of the trans isomer. In some cases, photoisomerization of cis isomers of meso-substituted carbocyanine dyes is also observed (for some meso-alkyl-substituted dyes com-plexed with DNA and chondroitin-4-sulfate; for 3,3′-diethyl-9-methoxythiacarbocyanine in moderate polarity solvents). The cycle photoisomerization–thermal back isomerization of cya-nine dyes can be used in various systems of information storage and deserves further investiga-tion using modern research methods.

Spectral-fluorescent study of the interaction of polymethine dye probes with biological surfactants - bile salts.

Spectral-fluorescent properties of polymethine dye probes anionic 3,3'-di(sulfopropyl)-4,5,4',5'-dibenzo-9-ethylthiacarbocyanine-betaine (DEC) and cationic 3,3',9-trimethylthiacarbocyanine iodide (Cyan 2) in the presence of biological surfactants, bile salts sodium cholate (NaC), sodium deoxycholate (NaDC) and sodium taurocholate (NaTC), as well as sodium dodecyl sulfate (SDS), have been studied in a wide range of surfactant concentrations. When a surfactant is introduced into a solution of DEC, changes of the spectral-fluorescent properties are observed due to decomposition of dye dimers into cis-monomers and cis-trans conversion of the resulting monomers. In the presence of SDS, both processes occur in parallel, caused by noncovalent interaction of dye monomers with micelles, and mainly occur near the critical micelle concentration (CMC). In contrast, upon the introduction of increasing concentrations of bile salts, decomposition of dye dimers into the monomers begins at lower concentrations than cis-trans conversion. The former process is almost completed at concentrations close to CMC of secondary micelles (CMC2), while the latter process occurs even at concentrations of bile salts much higher than CMC2. Hence, DEC can serve as a probe that permits estimating the value of CMC2 and is indicative of reorganization of secondary micelles upon an increase in bile salt concentration. Aggregation of DEC and Cyan 2 on bile salts is also observed. Since it is observed at relatively low concentrations of bile salts (<CMC2), the aggregation probably occurs on monomeric molecules of bile salts and their small associates and primary micelles. Decomposition of the aggregates formed begins at concentrations of bile salts above CMC2 (that is, upon the interaction with secondary micelles).

Development of a New Kinetic Spectrophotometric Method for the Determination of Chromium with an Optical Probe.

The aim of this work is the development, optimization, and validation of a new spectrophotometric kinetic method for the determination of dissolved chromium species in water samples with the use of the polymethine dye Astra Phloxine FF. The progress of the chemical reaction was simple, effective, and precisely monitored from the start of the reaction using an optical probe. The method is based on the impact of Cr(VI) concentration on the rate of decrease in the Astra Phloxine FF concentration. The experimental data were evaluated using four experimental data analysis methods, namely with the initial rate method, the average rate method, the fixed time method, and the absorption peak volume change method. Under the optimal reaction conditions, the best results were achieved using the method of the average rate constant for evaluating the experimental data. Using this data evaluation method for the determination of Cr(VI), the LoD was found to be 1.87 µg L-1 and RSD ( n = 6; 0.2 mg L-1 Cr) 3.59%. The presented work was used for the determination of chromium in model samples-CRM material and tap and waste water-and with the calibration line method and the standard additions method.

Dark states of symmetric polymethine dyes revealed by pump–dump–probe spectroscopy

We have applied three-pulse transient absorption spectroscopy to investigate the ultrafast photoisomerization dynamics in two symmetric polymethine dyes. Pump–deplete–probe experiments have revealed that the excited state manifold of these molecules contains two closely lying excited states in the dynamic equilibrium. One of these states is emissive, while the other is largely dark. It is the dark state that ultimately results in the photoisomer formation and is the main channel of excited state decay in these dyes. We have shown that excited state populations do flow between these two states and therefore it can be inferred that the photoisomerization pathway is not predetermined by ground state distortions of the molecular structure. The decision whether the molecule will isomerize or not seems to be taken in the excited state. Global analysis of three-pulse transient data allowed us to determine the spectra of the excited and ground intermediate states and build a comprehensive picture of photoinduced dynamics in symmetric polymethine dyes.

Photocatalytic reduction of methylene blue by the heterostructure of Titanium (IV) oxide and symmetric cationic polymethine dye

The results of the spectral, electrochemical and energy characteristics of symmetric cationic polymethine dye 2 - [(1E) -1,3,5-hexatrienyl -] - 1- (phenylmethyl)-benzindolium boron fluoride (D) indicate the possibility of its use as an effective titanium(IV) sensitizer oxide. Comparison of the absorption spectra of the dye solution and the reflectance spectra of the D/TiO2 heterostructures showed that the deposition of the dye on the TiO2 surface leads to bathochrome and hypochromic shifts of the absorption bands, the value of which depends on the content of this component in the heterostructure (HS). The obtained changes in the spectra of heterostructures indicate the possibility of formation of both J- and H-aggregates. Cyclic voltammetry determines the oxidation and reduction potentials of the investigated polymethine dye and calculates their redox potentials in the excited state. They have been found to be sufficient for sensitization by electron transfer to the TiO2 conduction band. The photocatalytic activity of HS D/TiO2 in the model reaction of methylene blue reduction depending on irradiation conditions and dye concentration has been determined. It is shown that, when irradiated with D/TiO2 heterostructures with light with λ &lt;400 nm, their photocatalytic activity decreases with increasing dye content in the whole concentration range of the studied concentrations. When irradiated with heterostructures by light absorbed by the dye (λ = 872 nm), the photocatalytic activity increases to a maximum value at a concentration of the dye equal to 0,20 mg/g, and then decreases with increasing concentration. The energy of possible electronic processes caused by the action of light absorbed by a semiconductor and a dye-sensitizer is considered. The possibility of using created heterostructures as functional materials with a given level of photocatalytic activity and extended range of light sensitivity is shown.

Heterostructures based on nanodispersed TiO2 and binary systems TiO2-CdTe, TiO2-Cu(In,Ga)Se2 sensitized by polymethyn dye, as photocatalysts of redox processes

New light-sensitive heterostructures of semiconductors (TiO2, TiO2-CdTe, TiO2-Cu(In,Ga)Se2) sensitized with an anionic polymethine dye with three conjugated chromophores have been developed. Based on the analysis of the results of spectral, electrochemical and energy characteristics of the studied dye, the possibility of its use as an effective sensitizer of these semiconductor materials is shown. It was found that the deposition of the dye on TiO2 leads to both batochromic and hypsochromic shifts of long wavelength absorption bands, the values ​​of which depend on the dye content in the heterostructure. This leads to a redistribution of the intensities of the absorption bands, they expand and cover almost the entire part of the visible and near-IR regions of the spectrum. This feature of the created heterostructure allows to increase the efficiency of their photocatalytic action through the use of light quanta of a wide energy range. The results of the analysis of the absorption spectra of heterostructures testify to the significant influence of semiconductors on the electronic system of dye molecules. As a result, the angle between the directions of the chromophores changes. The antibate dependence of the degree of conformational changes on the amount of dye in the heterostructure is revealed. This indicates a decrease in the interaction with the semiconductor with increasing concentration and explains the decrease in the photocatalytic activity of heterostructures at a significant content of sensitizer. Using the method of cyclic voltammetry, for the first time the oxidation and reduction potentials of the investigated polymethine dye were determined. The values ​​of HOMO and LUMO energy levels are calculated, a forecast is made regarding the possibility of their use as sensitizers of the studied semiconductors and the creation of highly efficient redox systems. The photocatalytic activity of heterostructures in the reactions of reduction of methylene blue and oxidation of iodide ions under irradiation with light absorbed by a semiconductor and a sensitizer has been revealed. Based on the analysis of energy parameters of electronic processes, the scheme of this photocatalytic transformation is proposed and substantiated.

Photochemical Processes in Molecular Polymethine Dye Probes in the Presence of Bile Salts

Primary photochemical processes in polymethine dye probes 3,3′-di(γ-sulfopropyl)-4,5,4′,5′-dibenzo-9-ethylthiacarbocyanine betaine (DEC) and 3,3′,9-trimethylthiacarbocyanine iodide (Cyan 2) in micellar systems of bile-acid salts (BASs) sodium cholate, deoxycholate, and taurocholate and sodium dodecyl sulfate (SDS) as a reference are studied by flash photolysis. Signals due to photoisomerization of dye trans-isomers and dark reverse isomerization of the resulting cis-photoisomers are observed during pulse photolysis of air-saturated aqueous dye solutions in the presence of BAS and SDS micelles. The lifetimes of the photoisomers are 60–190 μs. Pulse photolysis of Cyan 2 and DEC solutions without oxygen and with BAS and SDS micelles induced photoisomerization and transition of the dyes into an excited triplet state followed by the reverse transition (intersystem crossing) into the initial singlet state. Triplet–triplet absorption spectra of these dyes isomers in polar (EtOH, i-PrOH) and nonpolar (dioxane) solvents were obtained for comparison using triplet–triplet energy transfer from anthracene. The conclusion was drawn that the photochemical behavior of the dyes in BAS and SDS micellar systems were similar.

Polymethine Thiopyrylium Fluorophores with Absorption beyond 1000 nm for Biological Imaging in the Second Near-Infrared Subwindow.

Small-molecule fluorescence imaging in the second near-infrared (NIR-II, 1000-1700 nm) window has gained increasing interest in clinical application. Till now, very few studies have been exploited in the small-molecule fluorophores with both excitation and emission in the NIR-II window. Inspired by the indocyanine green structure, a series of polymethine dyes with both absorption and emission in the NIR-II window have been developed for NIR-II imaging, providing the feasibility to directly compare optical imaging in the NIR-IIa (1300-1400 nm) subwindow under 1064 nm excitation with that in the NIR-II window under 808 nm excitation. The signal-background ratio and the tumor-normal tissue ratio achieved great improvement under 1064 nm excitation in the imaging of mouse blood pool and U87 glioma tumors. Our study not only introduces a broadband emission fluorophore for both NIR-II and NIR-IIa imaging, but also reveals the advantages of NIR-II excitation over NIR-I in in vivo imaging.

Isomerization and Properties of Isomers of Carbocyanine Dyes

One of the important features of polymethine (cyanine) dyes is isomerization around one of C–C bonds of the polymethine chain. In this review, spectral properties of the isomers, photoisomerization and thermal back isomerization of carbocyanine dyes, mostly meso-substituted carbocyanine dyes, are considered. meso-Alkyl-substituted thiacarbocyanine dyes are present in polar solvents mainly as cis isomers and, hence, exhibit no photoisomerization, whereas in nonpolar solvents, in which the dyes are in the trans form, photoisomerization takes place. In contrast, the meso-substituted dyes 3,3′-dimethyl-9-phenylthiacarbocyanine and 3,3′-diethyl-9-(2-hydroxy-4-methoxyphenyl)thiacarbocyanine occur as trans isomers and exhibit photoisomerization in both polar and nonpolar solvents. The behavior of these dyes may be explained by the fact that the phenyl ring of the substituent in their molecules can be twisted at some angle, removing the substituent from the plane of the molecule and reducing its steric effect on the conformation of the trans isomer. In some cases, photoisomerization of cis isomers of meso-substituted carbocyanine dyes is also observed (for some meso-alkyl-substituted dyes complexed with DNA and chondroitin-4-sulfate; for 3,3′-diethyl-9-methoxythiacarbocyanine in moderate polarity solvents). The cycle photoisomerization–thermal back isomerization of cyanine dyes can be used in various systems of information storage and deserves further investigation using modern research methods.

Isomerization and Properties of Isomers of Carbocyanine Dyes

One of the important features of polymethine (cyanine) dyes is isomerization around one of C–C bonds of the polymethine chain. In this review, spectral properties of the isomers, photoisomerization and thermal back isomerization of carbocyanine dyes, mostly meso-substituted carbocyanine dyes, are considered. meso-Alkyl-substituted thiacarbocyanine dyes are present in polar solvents mainly as cis isomers and, hence, exhibit no photoisomerization, whereas in nonpolar solvents, in which the dyes are in the trans form, photoisomerization takes place. In contrast, the meso-substituted dyes 3,3′-dimethyl-9-phenylthiacarbocyanine and 3,3′-diethyl-9-(2-hydroxy-4-methoxyphenyl)thiacarbocyanine occur as trans isomers and exhibit photoisomerization in both polar and nonpolar solvents. The behavior of these dyes may be explained by the fact that the phenyl ring of the substituent in their molecules can be twisted at some angle, removing the substituent from the plane of the molecule and reducing its steric effect on the conformation of the trans isomer. In some cases, photoisomerization of cis isomers of meso-substituted carbocyanine dyes is also observed (for some meso-alkyl-substituted dyes complexed with DNA and chondroitin-4-sulfate; for 3,3′-diethyl-9-methoxythiacarbocyanine in moderate polarity solvents). The cycle photoisomerization–thermal back isomerization of cyanine dyes can be used in various systems of information storage and deserves further investigation using modern research methods.

Estimation of maximum absorption wavelength of polymethine dyes in visible and near-infrared region based on time-dependent density functional theory

Polymethine dyes have been utilized in various applications, including optical functional materials. The maximum absorption wavelength (λmax) of polymethine dyes is the key for designing optical functional materials. However, there have been few theoretical studies on λmax, especially in the near-infrared region. Therefore, using time-dependent density functional theory (TD-DFT), we calculated the excited states of 70 polymethine dyes with λmax ranging in the visible-to-near infrared region. We also compared these calculated λmax values with experimental ones, using linear regression analysis, and confirmed a strong correlation between the experimental and theoretical λmax values when the portion of the Hartree-Fock exchange in exchange–correlation functionals is relatively high. Linear regression analysis revealed that the experimental λmax values of the polymethine dyes can be quantitatively estimated by combining the TD-DFT calculations with the regression curves. Although several polymethine dyes may exhibit different behaviors at λmax, we could categorize these dyes into a few groups and estimate their excitation energies using regression curves suitable for the respective groups.

Structure and redox properties of polymethine dyes: Electrochemical and DFT/TD-DFT study

The electrochemical properties of 48 cationic, anionic and merocyanine polymethine dyes have been studied using cyclic voltammetry. The effect of structural parameters, viz. the chromophore charge, the donor-acceptor properties of the terminal groups, the polymethine chain length, and the molecular symmetry on the redox-potentials are analyzed. The DFT calculations of all the studied dyes have been performed with three different exchange-correlation functionals (B3LYP, CAM-B3LYP, and B97D3) in both vacuum and acetonitrile using the PCM solvation model; additionally, for selected species the BP86, PBE0 and M06-2X functionals were employed. The HOMO and LUMO energies of the dyes obtained from voltammetry data are compared with those from the DFT calculations to select the best performing theoretical approach for evaluation of the frontier orbital energy levels and energy gap of these π-conjugated compounds. Linear correlation equations obtained from the data for 48 dyes may be employed for credible prediction of the HOMO and LUMO energies in the search for designed molecules with required redox properties. TD-DFT calculations have demonstrated that the pure (non-hybrid) BP86 and B97D3 functionals give a relatively low mean absolute error for excitation energies, proving to perform better than the hybrid functionals applied, especially CAM-B3LYP.

Near Length-Independent Conductance in Polymethine Molecular Wires

Polymethine dyes are linear π-conjugated compounds with an odd number of carbons that display a much greater delocalization in comparison to polyenes that have an even number of carbon atoms in their main chain. Herein, we perform scanning tunneling microscope based break-junction measurements on a series of three cyanine dyes of increasing length. We demonstrate, at the single molecule level, that these short chain polymethine systems exhibit a substantially smaller decay in conductance with length (attenuation factor β = 0.04 Å-1) compared to traditional polyenes (β ≈ 0.2 Å-1). Furthermore, we show that by changing solvent we are able to shift the β value, demonstrating a remarkable negative β value, with conductance increasing with molecular length. First principle calculations provide support for the experimentally observed near-uniform length dependent conductance and further suggest that the variations in β with solvent are due to solvent-induced changes in the alignment of the frontier molecular orbitals relative to the Fermi energy of the leads. A simplified Hückel model suggests that the smaller decay in conductance correlates with the smaller degree of bond order alternation present in polymethine compounds compared to polyenes. These findings may enable the design of molecular wires without a length-dependent decay for efficient electron transport at the nanoscale.

Suppression of Kasha’s rule in higher excited states of 4-pyrilocyanines

Spectral properties of the series of cationic polymethine dyes containing conjugated chain of different length and 4-pyrylium terminal groups were studied in details. Besides, the steady-state long-wavelength fluorescence from the lowest excited state and the fluorescence from higher excited states were experimentally revealed. The steady-state fluorescence band from the lowest excited state is shifted bathochromically with lengthening of the polymethine chain, which is also reflected in the absorption shift. According to quantum-chemical modeling, the appearance of unconventional fluorescence, which suppresses Kasha’s rule, is associated with the different nature of the lowest excited states: the first and forth transitions are typical polymethine electron transitions involved only totally delocalized molecular orbitals (MOs), whereas the second and third transitions correspond to the electron jumping from the degenerated MOs localized only within the terminal groups (so called quasi-local MOs and hence quasi-local transitions). The forbidden transitions between the excited states of different types stimulate the fluorescence from the higher excited states.

Electrochemical Synthesis of Photovoltaic Layers of Metal Complexes of J-Aggregates of Polymethine Dyes on ITO-Glass Electrodes

Photosensitive layers of metal complexes of anionic polymethine dyes are prepared on ITO-coated glass electrodes using an electrosynthetic route in which a thin metal film (zinc or cadmium) electrodeposited on an ITO electrode is converted electrochemically into a respective dye–metal complex. Optical absorption measurements suggest that the prepared metal complexes of dyes exist onto ITO electrodes in the form of J-aggregates. We investigate the spectral and photoelectric characteristics of these dye–metal complexes. For the dye–metal complexes prepared using the electrosynthetic route, the quantum efficiency of photoconductivity is found to be around 5–7%. Our results show that the technique for the electrosynthesis of photosensitive layers comprised of metal complexes of anionic polymethine dyes on ITO electrodes is promising for the nanotechnology fabrication of thin-film photoelectric converters.