The dimerization study of some cationic monomethine cyanine dyes by chemometrics method

The thermodynamics of the monomer-dimer equilibrium of five cationic monomethine cyanine dyes has been studied by means of UV-vis spectroscopy and advanced chemometrics methods. The dimerization constants were determined by studying the dependence of their absorption spectra on temperature in the range 20?C75°C and were found to be: 1.91 × 10³, 1.53 × 10³, 6.81 × 10³, 5.18 × 10³, and 8.28 × 10³ dm³ mol^?1 for 1, 2, 3, 4, and 5 respectively. The absorption spectra of the monomer and dimer forms of the dyes were also determined, as well as the enthalpy and entropy of the dimerization.     

Spectrophotometric investigation on the aggregation in water of some benzothiazole cationic dyes

The aggregation ability in water of some practically important benzothiazole diazahemicyanine dyes in the range, where only the monomer-dimer equilibrium exist, is investigated. The equilibrium constants K_D and thermodynamic parameters of dimerization are determined and their influence on the dye aggregation is discussed. The “sandwich” structure of the dimer is suggested from the pure monomer and dimer absorption spectra. The distance R between the molecules in the dimer and the angle α between the two transition dipole moments are determined.     

Synthesis,Crystal Structure,Spectral and Thermodynamic Properties of One Benzoindole Pentamethine Cyanine Dye

One benzoindole pentamethine cyanine dye was synthesized and characterized by ¹H NMR, IR, MS and UV‐Vis spectra. The UV‐Vis absorption and fluorescence spectra of the dye in chloroform, dimethyl sulfoxide, acetone, ethanol and methanol were investigated, and the λ_max of the dye was in the region 682.0–689.0 nm with large molar extinction coefficients (? > 10⁵ M^?1cm^?1) in different solvents. The structure of the dye was also characterized and analyzed by X‐ray diffraction. Crystallographic data revealed that the dye belonged to orthorhombic, with space group P2₁2₁2₁, a = 10.059(2) Å, b = 15.098(4) Å, c = 24.989(6) Å, V = 3794.8(15) ų, Z = 4. The C‐H···F intermolecular hydrogen bonds were displayed in the molecular system, which were effective in the molecular packing. The aggregation behavior and thermodynamic properties of the dye in aqueous methanol solution were also studied by means of UV‐Vis spectroscopy methods. The results indicated that the dye existed monomer‐dimer equilibrium in aqueous methanol solutions. The fundamental properties of the dye, such as the dimeric association constant K_D, the dimeric free energy ΔG_D, the dimeric entropy ΔS_D, and the dimeric enthalpy ΔH_D were determined. The ΔH_D of the dye was –46.0 kJ mol^?1.     

Thermodynamic characterization of the dimerization equilibrium of an asymmetric dye by spectral titration and chemometric analysis

The monomer-dimer equilibrium of an asymmetric cyanine dye has been investigated by means of UV-Vis spectroscopy. The data have been processed by a recently developed chemometric method for quantitative analysis of undefined mixtures, that is based on simultaneous resolution of the overlapping bands in the whole set of absorption. In this work the dimerization constant of 1-carboxydecyl-4-{3-[3-methyl-3H-benzothiazol-2-ylidene]-propenyl}-quinolinium (TO-3) has been determined by studying the dependence of absorption spectrum on temperature in the range 25-72.5 °C at different total concentrations of dye (8.5×10⁻⁶ to 2.87×10⁻⁵ M). Utilizing the van’t Hoff relation, which describes the dependence of the equilibrium constant on temperature, as constraint we determine the spectral responses of the monomer and dimer species as well as the enthalpy and entropy of the dimerization equilibrium.     

Kinetics of dehydrohalogenation of N-chloro-3-azabicyclo[3,3,0]octane in alkaline medium. NMR and ES/MS evidence of the dimerization of 3-azabicyclo[3,3,0]oct-2-ene

The formation of 3-azabicyclo[3,3,0]oct-2-ene in the course of the synthesis of N-amino-3-azabicyclo[3,3,0]octane using the Raschig process results from the following two consecutive reactions: chlorine transfer between the monochloramine and the 3-azabicyclo[3,3,0]octane followed by a dehydrohalogenation of the substituted haloamine. The kinetics of the reaction were studied by HPLC and UV as a function of temperature (15 to 44°C), and the concentrations of NaOH (0.1 to 1 M) and the chlorinated derivative (1 to 4×10⁻³ M). The reaction is bimolecular (k=103×10⁻⁶ M⁻¹ s⁻¹; ΔH^0#=89 kJ mol⁻¹; and ΔS^0#=−33.6 J mol⁻¹ K⁻¹) and has an E2 mechanism. The spectral data of 3-azabicyclo[3,3,0]oct-2-ene were determined. IR, NMR, and ES/MS analysis show dimerization of the water-soluble monomer into a white insoluble dimer. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 129–136, 1998.     

Photochemistry on surfaces: Fluorescence emission of monomers and dimers and triplet state absorption of acridine orange adsorbed on microcrystalline cellulose

Prompt fluorescence as well as delayed fluorescence emission of acridine orange was detected at room temperature from samples where this dye is adsorbed on microcrystalline cellulose. Ground state absorption studies provided evidence for dimer formation of the dye when adsorbed on cellulose, and the equilibrium constant for dimerisation was determined as 1.6±0.1 × 10⁶mol⁻¹g. At low loadings of acridine orange on cellulose (<1 μmol g⁻¹) the fluorescence emission is mainly due to the monomer and is similar to that observed in ethanolic solutions where little aggregation occurs, and peaks at 530 nm. A linear dependence of the fluorescence intensity on the amount of light absorbed by the dye was established for these “diluted” samples. However, at higher loadings (>20 μmol g⁻¹), the fluorescence intensity decreases, and the emission is broad with its maximum at 620 nm, and is mainly due to the dimer. By assuming that the excited monomer and dimer of acridine orange are the only emitting species, it was possible to determine the fluorescence quantum yields for these two species when adsorbed on microcrystalline cellulose as 0.95±0.05 and 0.40±0.10, respectively. Pulsed emission studies at room temperature in the millisecond time-range also revealed monomer and dimer emissions on this longer time-scale. These are shown to be due to thermally activated delayed fluorescence arising from the triplet states of monomer and dimer acridine orange as confirmed by diffuse reflectance transient absorption studies.     

Complexes of tetracyanobiimidazole—V. Self-association of Ir(I) complex anions

The self-association of the planar anion [Ir(CO)₂Tcbiim]⁻ where H₂Tcbiim is 4,4′,5,5′-tetracyano-2,2′-biimidazole has been studied in acetontrile solution by analysis of the charge-transfer spectra. At low concentrations (∼ 10⁻⁵ M) monomer units prevail. At intermediate concentrations (∼ 10⁻³ M) dimerization is evident and a very strong (ε = 18 500 M⁻¹ cm⁻¹) new absorption appears at 475 nm. A novel procedure was used to obtain this value and to estimate the equilibrium constant for dimerization, 13.2 M⁻¹ at 24.4°C. From the temperature dependence of the spectra, ΔH is calculated to be −28 to −30 kJ M⁻¹. At higher concentrations (∼ 10⁻² M) spectral changes consistent with further oligomerization are observed.     

Application of multivariate curve resolution analysis for studying the thermodynamics of methylene blue aggregations in aqueous solutions

In water, methylene blue (MB) is known to be aggregated to dimmer and trimmer at high concentration levels. We conducted a thermodynamic study on the aggregation of MB using multivariate curve resolution analysis of the visible absorbance spectra over a concentration range of 2.0 × 10^-6?4.0 × 10^-4 M. A hard modeling-based multivariate curve resolution method was applied to determine the dissociation constants of the MB aggregates at various temperatures ranging from 25 to 75 °C. This method was able to resolve the visible absorbance spectra as a function of MB concentration in individual temperature or simultaneously in all other temperature ranges. Utilizing the van??t Hoff relation, the enthalpy and entropy of the dissociation equilibriums were calculated. For the dissociation of both aggregates, the enthalpy and entropy changes were positive and negative, respectively.     

Adsorption of Methylene Blue from Aqueous Solution on High Lime Fly Ash: Kinetic,Equilibrium, and Thermodynamic Studies

Kinetic, equilibrium, and thermodynamic studies were performed for the batch adsorption of methylene blue (MB) on the high lime fly ash as a low cost adsorbent material. The studied operating variables were adsorbent amount, contact time, dye concentration, and temperature. The kinetic data were analyzed using the pseudo-first order and pseudo-second order kinetic models and the adsorption kinetic was followed well by the pseudo-second order kinetic model. The equilibrium data were fitted with the Freundlich, Langmuir, and Dubinin Radushkevich (D–R) isotherms and the equilibrium data were found to be well represented by the Freundlich and D–R isotherms. Based on these two isotherms MB is taken by chemical ion exchange and active sites on the high lime fly ash have different affinities to MB molecules. Various thermodynamic parameters such as enthalpy of adsorption (ΔH°), free energy change (ΔG°), and entropy change (ΔS°) were investigated. The positive value of ΔH° and negative value of ΔG° indicate that the adsorption is endothermic and spontaneous. The positive value of ΔS° shows the increased randomness at the solid–liquid interface during the adsorption. A single-stage batch adsorber was also designed based on the Freundlich isotherm for the removal of MB by the high lime fly ash.     

Graphical Determination of Monomer Shift,Dimer Shift and Dimerization Constant for Self-Association: 2-Isopropyl Phenol in C6D12 as an Example

We propose a graphical method to treat dilution shift in NMR spectra to determine the monomer shift, dimer shift and dimerization constant simultaneously and consistently for the system of monomer/dimer equilibrium. The salient features include avoiding error-prone manual extrapolation to zero concentration to obtain a monomer shift, and providing consistent assessment and error estimation for the determination. The dilution shifts of 2-isopropyl phenol in C₆D₁₂ serve to demonstrate this graphical method. The enthalpy and entropy of dimerization are determined via a van't Hoff plot from the temperature variation of the dimerization constant.     

Aggregation Behaviors of Tricarbocyanine Dye in Water and in AOT Reverse Micelles

The photophysical property of the tricarbocyanine dye IR144 has been extensively studied in non‐aqueous solvents. However, as a potential near‐infrared biomedical imaging probe, the photophysical property of IR144 in water is still little known. So, the aggregation behaviors of IR144 in water with steady‐state absorption spectroscopy and integrated polarization dependent femtosecond pump‐probe spectroscopy were investigated. Through comparing the absorption spectral bandshape of IR144 in water and in water pool of AOT reverse micelles, It is found that IR144 form dimer aggregates in water even at very low concentration (<1.0×10^?7 mol·L^?1). And the absorption spectrum of the IR144 aggregates always displays a bimodal feature, which is independent of the dye concentration ranging from 1.0×10^?7 to 1.0×10^?4 mol·L^?1. For better understanding the aggregation behaviors of IR144 in water, we measured the ground state recovery kinetics and the reorientation kinetics of IR144 in water and in water pool of AOT reverse micelles (W₀=[H₂O]/[AOT], W₀=40). It is found that the fluorescence quantum yield of IR144 in water is lower than that in water pool of AOT reverse micelles, and the reorientation time of IR144 in water is slower than that in water pool of AOT reverse micelles. Those kinetic measurements also verify that IR144 exists as dimer aggregates in water.     

Conformational Dynamics of Monomer‐ versus Dimer‐like Features in a Naphthalenediimide‐Based Conjugated Cyclophane

The design and synthesis of an enantiomeric pair of 1,8‐diethynylanthracene‐bridged naphthalenediimide (NDI)‐based cyclophanes ( Cyclo‐NDI s) are reported. Each enantiomer of Cyclo‐NDI exhibits a circularly polarized luminescence signal with a relatively large luminescence dissymmetry factor (g_lum=±8×10^?3). We have further investigated the modulation of through‐space electronic communication between co‐facially oriented NDIs in a discrete Cyclo‐NDI with changes in the temperature. Tuning of the electronic communication results from the conformational transformation of monomer‐ versus dimer‐like features of Cyclo‐NDI , as confirmed by UV/Vis, fluorescence, circular dichroic, and NMR spectroscopic analysis. The temperature‐dependent optical response in the Cyclo‐NDI through the conformational transformation could be utilized as a highly sensitive and reversible optical thermometer in a wide temperature range (100 to ?80 °C).     

An endor and ESR study of the radical cation of N,N′-bis-(4-fluorophenyl)-4,4′-bipyridylium dichloride

The ESR and ENDOR spectra of the radical cation of N,N′-bis-(4-fluoro-phenyl)-4-4′-bipyridylium dichloride (fluorophenylquat FPQ) in methanol was studied over a temperature range from +_40° to ?90°. The ENDOR technique was used to obtain accurately the splitting constans for a highly complicated ESR spectrum and computer simulation showed excellent agreement. Fluorine ENDOR resonance was clearly observed with a line width similar to that of the protons. On decreasing the temperature the concentration of the radical cation decreases until at ?90° the ESR intensity was very small. This process is reversible and concentration studies indicate that the radical cation is in equilibrium with a diamagnetic dimer species. The thermodynamic parameters ΔH°, ΔG° and ΔS° for the process are reported.     

Fourier transform infrared spectroscopy and theoretical study of dimethylamine dimer in the gas phase

Dimethylamine (DMA) has been studied by gas-phase Fourier transform infrared (FTIR) spectroscopy. We have identified a spectral transition that is assigned to the DMA dimer. The IR spectra of the dimer in the gas phase are obtained by spectral subtraction of spectra recorded at different pressures. The enthalpy of hydrogen bond formation was obtained for the DMA dimer by temperature-dependence measurements. We complement the experimental results with ab initio and anharmonic local mode model calculations of monomer and dimer. Compared to the monomer, our calculations show that in the dimer the N-H bond is elongated, and the NH-stretching fundamental shifts to a lower wavenumber. More importantly, the weak NH-stretching fundamental transition has a pronounced intensity increase upon complexation. However, the first NH-stretching overtone transition is not favored by the same intensity enhancement, and we do not observe the first NH-stretching overtone of the dimer. On the basis of the measured and calculated intensity of the NH-stretching transition of the dimer, the equilibrium constant for dimerization at room temperature was determined.     

Interaction of poly(vinylpyrrolidone) with methyl orange and its homologs in aqueous solution over the temperature range 60–90°c

The binding of methyl orange, ethyl orange, propyl orange, and butyl orange by poly(vinylpyrrolidone) has been examined by a technique of equilibrium dialysis over a high temperature range (60–90°C). The first binding constants and the thermodynamic parameters in the course of the binding were evaluated. The results obtained at these temperatures were compared to those at lower ones (5–35°C) described previously in order to estimate the contribution of hydrophobic bonds to the binding. It was found that at the 60–90°C range complex formation between the dye and the macromolecule is associated with an exothermic enthalpy change and a positive entropy change. The enthalpy and entropy changes of the binding are of the order of ?4.5 kcal/mole and 6 eu, respectively, for each dye measured. Thus the binding is mainly enthalpy-controlled. Furthermore the effect of the alkyl chain length of the dye on both the ΔH° and ΔS° values is not pronounced. Also temperature dependences of the ΔH° and ΔS° terms were not observed. All these observations in the higher temperature range can be explained as a result of the disruption of water structure in the binding environment and hence a decrease in hydrophobic bond formation between the dye and the polymer.     

Synchronous fluorescence and UV-vis spectroscopic studies of interactions between the tetracycline antibiotic, aluminium ions and DNA with the aid of the Methylene Blue dye probe

Synchronous fluorescence spectroscopy (SFS) was applied for the investigation of interactions of the antibiotic, tetracycline (TC), with DNA in the presence of aluminium ions (Al³⁺). The study was facilitated by the use of the Methylene Blue (MB) dye probe, and the interpretation of the spectral data with the aid of the chemometrics method, parallel factor analysis (PARAFAC). Three-way synchronous fluorescence analysis extracted the important optimum constant wavelength differences, Δλ, and showed that for the TC-Al³⁺-DNA, TC-Al³⁺ and MB dye systems, the associated Δλ values were different (Δλ = 80, 75 and 30 nm, respectively). Subsequent PARAFAC analysis demonstrated the extraction of the equilibrium concentration profiles for the TC-Al³⁺, TC-Al³⁺-DNA and MB probe systems. This information is unobtainable by conventional means of data interpretation. The results indicated that the MB dye interacted with the TC-Al³⁺-DNA surface complex, presumably via a reaction intermediate, TC-Al³⁺-DNA-MB, leading to the displacement of the TC-Al³⁺ by the incoming MB dye probe.     

Spectroscopic studies of 5,5′-dimethoxy-3,3,′-disulfobutyl-9-ethylthiacarbocyanine (DDTC) in solutions and immobilized in sol-gel matrices

Absorption spectra of 5,5′-dimethoxy-3,3,′-disulfobutyl-9-ethylthiacarbocyanine (DDTC) in aqueous solutions and immobilized in xerogels prepared by the sol-gel method were obtained. Influence of pH, detergent (Triton X-100), ethanol addition and sol-gel preparation method on the dye aggregation equilibria and its photostability were investigated. In liquid solutions lowering of pH, as well as addition of the detergent, shift the aggregation equilibrium towards the DDTC monomeric form. However, while more acidic conditions result in a decrease of the dye stability, addition of the detergent has a slightly stabilizing effect on the dye. However, addition of ethanol drastically reduces the DDTC photodecomposition and shifts the aggregation equilibrium towards the dye monomer. Entrappment of the thiacarboxycyanine in sol-gel matrix does not prevent the bleaching although the entrapped dye displays significantly increased stability. Lowering of temperature and keeping the doped xerogels in the dark further retards the immobilized dye bleaching and dimer formation.     

Determination of the dimerization constant of pinacyanol: role of the thermochromic effect

Pinacyanol (PIN), as other cyanine dyes, has demonstrated a unique ability to form associates such as dimers, and H- and J-aggregates. This association is strongly favoured in water, and even at low dye concentrations, dimers and superior order aggregates are present. As a consequence, the determination of the dimerization constant involves sometimes a significant error when these aggregates are neglected. As an increase in temperature shifts the equilibrium among the different species towards the lowest order aggregates, we have obtained the spectra of PIN at several temperatures. By extrapolating some spectral characteristics at high temperatures, a spectrum of the dimer without any contribution of other aggregates was obtained. From this spectrum and that of the monomer, the dimerization constant was calculated, as well as the Gibbs energy change associated to the reaction. The enthalpy and entropy changes of the dimerization were determined from the dependence of the dimerization constants on the temperature. From these results it can be inferred that the driving force of the dimerization is of enthalpic origin.     

Dynamic proton magnetic resonance studies on complex spin systems. Non-mutual three-spin exchange in N-(trideuteriomethyl)-2-cyanoaziridine

At room temperature and below, the proton NMR spectrum of N-(trideuteriomethyl)-2-cyanoaziridine consists of two superimposed ABC patterns assignable to two N-invertomers; a single time-averaged ABC pattern is observed at 158.9°C. The static parameters extracted from the spectra in the temperature range from –40.3 to 23.2°C and from the high-temperature spectrum permit the calculation of the thermodynamic quantities ΔH⁰ = ?475±20 cal mol^?1 (?1.987 ± 0.084 kJ mol^?1) and ΔS⁰ = 0.43±0.08 cal mol^?1 K^?1 (1.80±0.33 J mol^?1 K^?1) for the cis ? trans equilibrium. Bandshape analysis of the spectra broadened by non-mutual three-spin exchange in the temperature range from 39.4–137.8°C yields the activation parameters ΔH_tc^≠ = 17.52±0.18 kcal mol^?1 (73.30±0.75 kJ mol^?1), ΔS_tc^≠ = ?2.08±0.50 cal mol^?1 K^?1 (?8.70±2.09 J mol^?1 K^?1) and ΔG_tc^≠ (300 K) = 18.14±0.03 kcal mol^?1 (75.90±0.13 kJ mol^?1) for the trans → cis isomerization. An attempt is made to rationalize the observed entropy data in terms of the principles of statistical thermodynamics.     

The Divergent Dimerization Behavior of N‐Substituted Dicyanomethyl Radicals: Dynamically Stabilized versus Stable Radicals

Herein, we demonstrate that the dimerization behavior of amine‐substituted dicyanomethyl radicals can be switched from σ‐ to π‐dimerization simply by varying the electron‐donating substituents. For dicyanomethyl radicals with a 4,4′‐ditolylamine ( DT^. ) or a phenothiazine ( PT^. ) substituent, the monomeric radical form and the corresponding dimer connected by a reversible C−C bond (σ‐dimer) are in equilibrium in solution. On the other hand, the radical with the julolidine skeleton ( JD^. ) does not undergo σ‐dimerization and was isolated as a stable radical in spite of the absence of bulky protecting groups. X‐ray single‐crystal analysis revealed that JD^. forms the π‐dimer in the crystalline state, and variable‐temperature spectroscopy showed that JD^. is in equilibrium with the π‐dimer in toluene solution. DFT calculations point to the importance of electrostatic interactions as a driving force for the π‐dimerization of JD^. because of its polarized structure.