Stilbazolium Merocyanine Research Articles

Self-Aggregation and Optical Absorption of Stilbazolium Merocyanine in Chloroform

Dipolar aggregation is in many cases detrimental for the functioning of optical materials. In this study we investigate self-aggregation and optical absorption of stilbazolium merocyanine (SM) in chloroform solution by performing classical Molecular Dynamics (MD) simulations under ambient conditions. The reversal solvatochromic shift, the large bathochromic shift, and the structured absorption band presented by SM in chloroform solution are all aspects of its optical absorption behavior for which the existence of self-aggregation is yet not completely understood. Moreover, the spectroscopic properties of SM oligomers and their occurrence in solvent of low polarity remain a relevant topic that deserves to be investigated. Our analysis of the aggregation behavior of SM in chloroform verified that the majority of the chromophores are involved in the formation of oligomers in solution, where the whole dimer and part of the trimer populations present a stable π-stacking structure. The optical properties of the monomers and oligomers in solution were evaluated by means of a discrete polarizable embedding quantum mechanical/molecular mechanical (PE-QM/MM) response scheme where the quantum part is described at the level of density functional theory. The visible absorption spectrum of SM in chloroform is simulated using time average values obtained for the monomeric and oligomeric forms of SM from the PE-QM/MM calculations performed on uncorrelated configurations extracted from the classical MD simulations. This study shows that the self-aggregation of SM in chloroform may exist, but it is not essential for reproducing the reversal solvatochromic shift in chloroform and that the process does not contribute to enhance the bathochromic shift nor explain the structure observed in its absorption band. Moreover, it is verified that since the electronic transitions of the monomer and oligomers are close together, changes in the interplane separation between the monomeric units of the stacked oligomers substantially affect the spectral resolution of their contribution to the optical absorption spectrum.

Effect of substituents on the thermochromism of stilbazolium merocyanines incorporated in epoxy resin

The thermochromic effect (in the range 283–333K) on the spectroscopic properties of stilbazolium merocyanine derivatives are substituted by groups of different donor–acceptor qualities (–H, –C(CH3)3, –NO2 and –OCH3) located in the ortho position to the carbonyl group of the quinonoid ring was examined. The samples were embedded in a polymer matrix from the epoxy resin.Results show that the size of thermochromic effect of stilbazolium merocyanines can be influenced by attaching substituents of different donor–acceptor nature. The smallest value of this effect was recorded for dye substituted by large tert-butyl groups, and the largest for derivative with –NO2 group substituent. The thermosensitvity investigated dyes was dependent on: (1) an equilibrium between quinonoid form and benzenoid form (zwitterion), (2) the polarization of molecules that facilitate the formation of H-type aggregates.

Demystifying the solvatochromic reversal in Brooker’s merocyanine dye

Based on hybrid QM/MM simulation techniques, we rationalize the spectacular solvatochromic reversal behavior observed for a stilbazolium merocyanine (SM) called Brooker's merocyanine dye. This solvatochromic reversal is attributed to a change in the solute π-electron distribution from zwitterionic to neutral following the change in solvents from polar to non polar. Based on our calculations, we suggest that a polar solvent, like water, with larger relative permittivity is influential enough to bring the change in molecular structure from neutral to zwitterionic. Our results clearly indicate that SM exists in a neutral molecular structure in non polar solvents like trichloromethane and thereby we suggest that self-aggregation of SM may not occur in this solvent.

Modeling the Structure and Absorption Spectra of Stilbazolium Merocyanine in Polar and Nonpolar Solvents Using Hybrid QM/MM Techniques

We have performed Car-Parrinello mixed quantum mechanics/molecular mechanics (CP-QM/MM) calculations for stilbazolium merocyanine (SM) in polar and nonpolar solvents in order to explore the role of solute molecular geometry, solvation shell structure, and different interaction mechanisms on the absorption spectra and its dependence on solvent polarity. On the basis of the average bond length values and group charge distributions, we find that the SM molecule remains in a neutral quinonoid form in chloroform (a nonpolar solvent) while it transforms to a charge-separated benzenoid form in water (a polar solvent). Based on a quantum mechanical/molecular mechanical response technique, with different MM descriptions for the water environment, absorption spectra were obtained as averages over configurations derived from the CP-QM/MM simulations. We show that for SM in water the solute polarization plays a major role in predictions of the λ(max) and solvatochromic shift and that once this effect is included the contributions from solvent polarization and intermolecular charge transfer become less important. For SM in chloroform and water solvents, we have also performed absorption spectra calculations using a polarizable continuum model in order to address its relative performance compared to the QM/MM response technique. In the case of SM in water, our study supports the notion that, in order to predict accurate absorption spectra and solvatochromic shifts, it is important to use a discrete description of the solvent when it, as in water, is involved in site-specific interaction with the solute molecule. The technique is thus shown to outperform the more conventional polarizable continuum model in predicting the solvatochromic shift.

Breakdown of the first hyperpolarizability/bond-length alternation parameter relationship

We have investigated the dependence of the static first hyperpolarizability on the bond-length alternation (BLA) parameter. Our analysis indicates that the validity of the first hyperpolarizability/BLA parameter relationship is restricted to the no-field, vacuum, limit, while it successively breaks down along with increasing polarity of a surrounding medium, becoming invalid, for instance, in an aqueous solution. This contention is based on a series of TD-DFT, TD-DFT/PCM and hybrid TD-DFT/MM calculations of the first hyperpolarizability for a set of molecular configurations generated from Car-Parrinello hybrid QM/MM simulations of the stilbazolium merocyanine chromophore in chloroform and water solvents, and on a rationalization by means of the two-state model for the first hyperpolarizability. The BLA dependence of the three parameters entering the two-state model is shown to be qualitatively different in vacuum and in solvents. Particularly, in the vacuum case, the difference between ground and excited state dipole moments goes to zero for a vanishing BLA, which is not true in the presence of an aqueous medium. In the aqueous medium, an opposing behavior of the parameters involved in the two-state model results in an almost constant first hyperpolarizability with varying BLA parameter.

Stilbazolium merocyanine systems in Langmuir and Langmuir–Blodgett molecular layers

Stilbazolium merocyanine (1-(12-hydroxydodecyl)-4-[(3-hydroxy-4-oxocyclohexa-2,5-dienylidene)-ethylidene]-1,4-dihydropyridine) was investigated in form of Langmuir and Langmuir–Blodgett films for potential sensor of environment pH and presence of metal ions applications. Two forms of the merocyanine, free-base and protonated were observed in Langmuir films depending on the pH of subphase. The complex comprised of merocyanine and Cu 2+ or Co 2+ metal ions are also able to form stable Langmuir films which compression isotherms are different from non-complex merocyanine. Also films composed of a mixture of merocyanine and fatty acid (eicosanoic acid) were prepared. Attractive and repulsive interactions between molecules in mixture films were observed. All types of the films were successfully transferred onto hydrophobic quartz plates for the purpose of producing solid samples of ultrathin sensors. Multilayers of free-base merocyanine films remain sensitive to the pH of the environment. From the changes in absorbance spectra of the merocyanine film it is possible to evaluate the pH of the microenvironment.

Orientation and spectral properties of two stilbazolium merocyanine dyes in stretched and unstretched polyvinyl alcohol films.

Spectral properties (anisotropy coefficients calculated for absorption, emission and fluorescence decay time) of two stilbazolium merocyanine dyes have been determined to evaluate the applicability of these dyes as sensitizers in photodynamic therapy. The dyes were embedded in an anisotropic polymer matrix. Analysis of the emission decay components measured in polarized light provides information on the interactions of the dye molecules with the polymer matrix being a model of an anisotropic biological system. Different values of the emission anisotropies obtained from various polarized components of fluorescence decays have shown that the orientations of the dye molecules influence their interactions with the polymer. This means that differently oriented dye molecules located in biological systems should exhibit different interactions with membranes. The chain length and type of side groups attached as well as the salt form of the dye molecule were shown to influence the dye-polymer interactions and should be taken into account before the application of merocyanine dyes in medicine. These dyes seem to be promising optical sensors with spectral properties, including the calculated anisotropy coefficients, sensitive to the molecular environment, useful to study orientation and interaction with neighbouring molecules in biological membranes.

Influence of Silver Colloid Presence on Stilbazolium Merocyanine Emission

Absorption, fluorescence emission, and fluorescence excitation spectra of stilbazolium merocyanine (1-(n-butyl)-4[(3,5-dimethoxy-4-oxocyclohexa-2,5-dienylidene)ethylidene]-1,4-dihydropyridyne) dye in water solution without and with colloidal silver addition were measured. In the presence of the colloid, besides the absorption band assigned to the protonated species of the dye (at 391 nm), an absorption band related to the free-base species appears at 490 nm. From the absorption and emission spectra, measured at various dye concentrations, follows that the aggregates are not effectively formed. Therefore, the long-wavelength absorption and fluorescence bands have to be related to some dye forms created by the solvatochromic effects. The fluorescence bands of the protonated and the free-base species are located at 559 nm and at about 630 nm, respectively. The shape of the long-wavelength band suggests the occurrence of more than one free-base form of the dye. At some dye and colloid concentrations, the global emission of the sample is enhanced as a result of silver addition. The increase in the emission yield of the dye could be partially due to not only the change in the concentrations of dye forms exhibiting various emission yields but is also due to the resonance surface plasmon effect. Because of the superposition of several effects, before the practical application of merocyanine as an indicator of metal presence in biological samples, its spectral properties in the system investigated should be established.

Stilbazolium merocyanine dye determination in different solutions, concentrations and colloids using SERS

AbstractSurface‐enhanced Raman scattering (SERS) measurements were carried out on stilbazolium merocyanine dye in methanol and pyridine solvents. Both solutions were measured in a series of concentrations covering a range of 5 × 10−5 M to 5 × 10−8 M. In these measurements, Ag and Au colloids were used, and the results have shown that Ag colloids yield better enhancement in the Raman spectra of this dye. Moreover, the effect of adding NaCl solution to the SERS samples was also studied. All measurements were carried out using the state‐of‐the‐art ChiralRaman instrument, which utilizes a 532 nm laser source. We report here on the success of using SERS to obtain Raman spectra of merocyanine dye at very low concentrations in an attempt to find a new approach that can be used for further investigations of the dye. The SERS spectra are reported here, and the results from different solutions, colloids, concentrations and pH values are compared. Copyright © 2006 John Wiley & Sons, Ltd.

The effect of the microenvironment on spectroscopic properties of complexes comprised of stilbazolium merocyanine and Cu(II) and Co(II) ions in isotropic and anisotropic media

The effect of the microenvironment on the spectroscopic properties of the complexes comprised of the derivative of merocyanine and specific ions (either dissolved or embedded in the polymer matrix) was examined. The effect of water concentration and the concentration of various anions on complex-forming ability was also established in this paper. As shown here, the mero–Co 2+ complex undergoes dissociation in the presence of water. However, for mero–Cu 2+ complex only a hipsochromic shift in the absorption spectrum was observed. The presence of the examined anions exhibited a strong effect on the formation of the mero–Cu 2+ complex. In anisotropic polymer matrix, the examined complexes also exhibited discrepant features.

Analysis of spectral properties of stilbazolium merocyanine complexes with Fe(II) and Fe(III) ions

The potential use of merocyanine dye (1-(12-hydroxydodecyl)-4-[(3-hydroxy-4-oxocyclohexa-2,5-dienylidene)-ethylidene]-1,4-dihydropyridine) was investigated as an effective monitoring tool for the presence of free iron ions. We show that merocyanine forms stable complexes with both Fe2+ and Fe3+ ions provided that the pH level is maintained at 10. Only the basic form of merocyanine was capable of forming these complexes. Complexes between merocyanine and iron ions demonstrated some features of charge-transfer complexes and did not dissociate in the presence of water.

Interactions of stilbazolium merocyanine with transition metal ions

The visible absorption spectra of merocyanine 1-(12-hydroxydodecyl)-4-[(3-hydroxy-4-oxocyclohexa-2,5-dienylidene)-ethylidene]-1,4-dihydropyridine in the presence of Cu 2+, Co 2+ and Fe 3+ ions has been investigated. The metal ions impacted a long wave absorption band of merocyanine with a characteristic wavelength. Both the intensity of the maximum and its position are related to changes in the metal ion concentration. This observed behaviour was dependent on the pH of the solution. Since concentrations of metal ions below 10 −6 M l −1 can be detected, we concluded that the investigated dye demonstrated a high sensitivity for the presence of metal ions.

Fast and slow processes of thermal deactivation of excited stilbazolium merocyanine dyes

The long living triplet states play important role in sensitizing action in all photochemical reactions. The yield of generation of triplet states of dyes can be evaluated on the basis of measurements of their slow (microsecond) thermal deactivation (TD). All experiments were carried out in the oxygen presence, it means under quenching dye triplets. The pulse dye laser generates in the investigated solution pressure signal. The high of the amplitudes of first maximum of this pressure wave-form signal in the solution of the investigated dye and in the reference sample were measured. Reference sample exhibits only fast processes of TD. The comparison of the first maximum of wave-form photothermal signal of sample and of reference enable to calculate part of energy exchanged into heat in time longer than time resolution of arrangement. The fluorescence yields of investigated dyes were also established. On the basis of such data, using procedure described in literature, the yield of singlet–triplet intersystem crossing (ISC) was evaluated. It was shown that this yield depends on the length of stilbazolium merocyanine chain. The product of triplet state yield and energy was lower for merocyanines with longer chains. At lower temperatures the yield of fluorescence increases and amount of excitation exchanged in short time into heat decreases. The slow TD process increases in low temperature because of the decrease in the quenching of the dyes triplet states by oxygen. The amount of energy exchanged into heat in a time longer than time resolution of apparatus is due predominantly through TD of the dye triplet states.

Two forms of stilbazolium merocyanine on Langmuir-Blodgett monolayers

Monolayers of stilbazolium merocyanine were prepared at the air-water interface, and were transferred onto a hydrophilic quartz surface by the vertical dipping method. The molecular forms in the films were studied for two types of merocyanine: A) 1-(6′-hydroxyhexyl)-4[(3,5-di- t-butyl-4-oxocyclohexane-2,5-dienylidene)ethylidene]-1,4-dihydropyridine and B) 1-(hexadecyl)-4[(3,5-di- t-butyl-4-oxocyclohexane-2,5-dienylidene)ethylidene]-1,4-dihydropyridine both with reference to the pH of the buffer. The protonated form gave a more stable monolayer film than the free-base form. Also the molecules of protonated merocyanine did not show degradation, but the free-base was more fragile. Both forms could be transferred onto a hydrophilic substrate building a multilayer structure. Using UV-Vis absorption spectroscopy for one monolayer on quartz, the orientation of molecules relative to the film surface was found to depend on the form of merocyanine. The polarized components of the absorbance spectra indicate that the free-base form of merocyanine is preferentially oriented parallel to the quartz substrate, in contrast to the protonated form which has a more out-of-plane orientation.

Photocurrent kinetics (in the microsecond time range) of chlorophyll a, chlorophyll b and stilbazolium merocyanine solutions in a nematic liquid crystal located in an electrochemical cell

The photosynthetic pigments chlorophyll a and chlorophyll b solutions in a nematic liquid crystal were located between semiconducting and metallic transparent electrodes and illuminated by lamp or laser light. Cells with two semiconducting electrodes were also investigated. Such systems represent models of biological membranes simulating their fluid and oriented structure. The kinetics of photocurrent generation in the time range from microseconds to minutes were measured. The action spectrum of the photocurrent effect was established. The kinetics of the photocurrent generated by laser flash photolysis in the chlorophyll solutions are complex and to some extent similar to the kinetics reported for thylakoids. For comparison, the photocurrent kinetics of stilbazolium merocyanine, a highly protonated dye, were also measured. In this case, on flash illumination, only one photocurrent peak was observed. This peak is due to electron tunnelling to the semiconducting electrode. In the case of the chlorophylls, several effects, such as charge displacement and migration of the ions, are superimposed giving a complex photocurrent generation. The results suggest that, in the case of chloroplasts, the kinetics of photopotential generation may be partially dependent on the membrane structure and fluidity.

Incorporation of stilbazolium merocyanines into resting and stimulated mononuclear leukocytes.

Stimulated and resting mononuclear leukocytes were incubated with a stilbazolium merocyanine dye 1-(6'-hydroxyhexyl)-4-[(4-oxocyclohexa-2,5- dienylidene)ethylidene]-1,4-dihydropyridine and immobilized in isotropic and stretched polyvinyl alcohol film. Polarized absorption, fluorescence and fluorescence excitation spectra were collected and the anisotropy of absorption and emission were calculated. Analysis of the spectra pointed to: i. the occurrence of perturbation of the membrane structure by incubation with the dye, and ii. influence of the blood serum addition, during the process of incubation with the dye, on the efficiency of incorporation of merocyanine into the cells and the degree of the dye orientation in the membrane. A small fraction of the dye molecules introduced into resting cells was found oriented to a higher degree than a large fraction incorporated into stimulated cells. The incubation time longer than 15 min caused strong changes in the membrane structure both of the resting and stimulated cells.