Brooker's Merocyanine Research Articles

Photophysical Characterization of a Benzo-Fused Analogue of Brooker’s Merocyanine: Solvent Polarity and pH Effects

The photophysical properties of 4-[2-(6-hydroxy-2-naphthalenyl)-ethenyl]-1-methyl-pyridinium (HNEP(+)) and its deprotonated form (NEP), a benzofused derivative of Brooker's merocyanine (BM), were investigated through a combined spectroscopic and computational approach. Despite their structural similarities and similar pK(a) values, HNEP(+)/NEP and BMH(+)/BM differ in the extent of charge delocalization in the ground and excited states. NEP exhibits the spectral characteristics of a charge transfer species in solvents in which BM exists in a charge-delocalized quinoid; however, quantum chemical calculations show that the CT absorption of NEP is not necessarily a consequence of the zwitterionic character. HNEP(+) displays larger Stokes shifts than BMH(+), and NEP demonstrates enhanced solvatochromism relative to BM as a consequence of benzofusion.

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.

Chromogenic anionic chemosensors based on protonated merocyanine solvatochromic dyes in trichloromethane and in trichloromethane–water biphasic system

Three merocyanine dyes (two pyridiniophenolates and Brooker's merocyanine) were dissolved in trichloromethane in their protonated form, and their potential as anionic chromogenic chemosensors was explored by adding various anions. The experimental data collected were treated considering a model based on the proton transfer from the protonated dye to the anion. One of the dyes was used in the development of an anionic colorimetric assay based on a trichloromethane–water biphasic system, which was able to selectively detect cyanide among other anionic species.

Modulation of pKa of Brooker’s Merocyanine by DNA Hybridization

Brooker's merocyanine (BM), which changes its emission and absorption maxima upon protonation, was introduced into oligodeoxyribonucleotide (ODN) via d-threoninol by postsynthetic modification on a CPG (controlled-pore glass) support. The pK(a) of BM in the modified ODN increased from 9.5 to 10.1 upon hybridization. As a result, absorption maxima shifted from 492 to 432 nm at pH 10.0 by the presence of its complementary strand. This spectral shift was sufficiently large so that DNA hybridization could easily be discriminated even by the naked eye; the color of the solution changed from orange to yellow upon hybridization. In addition, the fluorescence emission was strongly quenched upon hybridization, demonstrating that this probe can also detect the target DNA by the fluorescence change. Ratiometric detection of hybridization was also possible by simultaneous excitation of both protonated and deprotonated BMs. Furthermore, we could also modulate its pK(a) by the antiparallel stacking of two BM molecules in the duplex; the pK(a) of BM decreased from 10.1 to 9.7 by the stacking of two BMs in an antiparallel manner. Thus, control of the microenvironment around the BM molecule allowed modulation of its pK(a), which is applicable to the sequence-specific recognition of target DNA.

Chromogenic anionic chemosensors based on protonated merocyanine solvatochromic dyes: Influence of the medium on the quantitative and naked-eye selective detection of anionic species

Three merocyanine solvatochromic dyes, Brooker's merocyanine, Reichardt's dye and 4-(2,4,6-triphenylpyridinium-1-yl)phenolate were solubilized in acetonitrile. Although solutions of each of the compounds are colored, the color disappears after protonation. Addition of various anions to solutions of the protonated dyes revealed that only CN −, F −, and H 2PO 4 − led to the reappearance of the original color of protonated Reichardt's dye, and only CN − and F − ions caused the original color of the solutions containing the other two protonated dyes to reappear. Addition of small amounts of water to solutions containing the protonated compounds made them useful only for detecting CN − among all the other anions. A model was used to explain the experimental results, which considers proton transfer from the protonated dye to the anion through three dye:anion stoichiometries, namely 1:1, 1:2, and 1:3.

Interaction of Cyclodextrins with Brooker's Merocyanine in Aqueous Solution

ABSTRACT The UV-Vis spectroscopic behavior of the solvatochromic dye 4-[(1-methyl-4(1H)-pyridinylidene)-ethylidene]-2,5-cyclohexadien-1-one (Brooker's merocyanine) was investigated in aqueous solution (pH 10.55) in the presence of increasing amounts of native (α- and β-) and modified (methyl-β- and hydroxypropyl-β-) cyclodextrins. The data obtained showed that a bathochromic shift occurs in the dye solution after the addition of the cyclodextrins, indicating that the probe is transferred to a low-polarity microenvironment with the addition of the cyclodextrin. Experimental data also suggest the occurrence of a 1:1 dye:cyclodextrin inclusion complex.

The Fluorosolvatochromism of Brooker's Merocyanine in Pure and in Mixed Solvents

The fluorescence-based solvatochromism (fluorosolvatochromism) of 4-[(1-methyl-4(1H)-pyridinylidene)-ethylidene]-2,5-cyclohexadien-1-one (Brooker's merocyanine) was studied. The results revealed that the fluorescence emission band of the dye was dependent on the medium (lambda(fl)(max)= 573 nm in water and lambda(fl)(max)=622 nm in DMF). The fluorescence quantum yields (phi (f)) were calculated for the dye in the solvents investigated. Low phi (f) values ( < 10%) were obtained for the dye and in order to better comprehend the radiative and nonradiative decay processes of this dye, its fluorescence lifetime in methanol was measured and was found to be very short (230 ps). The results suggest that the dye in the excited state decays rapidly through nonradiative processes. The behavior of the probe in binary mixtures including a hydrogen-bond accepting solvent (acetonitrile, N,N-dimethylformamide, and dimethylsulfoxide) and a hydroxylic solvent (water, methanol, ethanol, propan-2-ol, and butan-1-ol) was also investigated. All data were successfully fitted to a model based on solvent exchange equilibria, which allowed the separation of the different contributions of the solvent species in the solvation shell of the dye. The data obtained for the mixed solvents were explained based on solute-solvent and solvent-solvent interactions.

Absorption, fluorescence, and semiempirical ASED-MO studies on a typical Brooker's merocyanine dye

Solvatochromism and Solvatofluorchromism of Brooker's merocyanine 1-methyl-4- (4′-hydroxystyryl) pyridinium betaine, M, were studied in twelve polar protic and aprotic solvents. Moderate hypsochromic fluorescence energy shifts are 4.57 kcal mole −1 while strong hypsochromic absorption energy shifts are 16.63 kcal mole −1. Decreasing of the dipole moment of M upon excitation is the factor, which is responsible for the difference between the two energy shifts. The change of both energies rectilinearly with solvent acidity scale shows the importance of oxygen atom of M as a strong basic center. The application of the atom superposition and electron delocalization molecular orbital (ASED-MO) theory reproduces geometrical and electronic structures for M, which agree well with the experimental observations. The calculations suggest strongly that the dye has a benzenoid valence structure in the ground state and shifts towards a quinonoid one upon excitation with an observed decreasing of the dipole moment. The changing of the dipole moment is explained clearly depending upon the calculated charge distribution over the whole skeleton of the molecule. The formation of a H-bond between the water molecule and the highly negative oxycyclic oxygen atom of M has slightly effect on its dipole moment in the ground state. This leads to suggest that this kind of interaction could be represented as attacking of water with acidic character on the basic site of M. Also, the calculations predict that the formation of monohydrated complex is an exothermic, down hill reaction, which is confirmed from the stabilization of the frontier molecular orbitals, oxygen lone-pair and the HOMO levels.

Density Functional Vertical Self-Consistent Reaction Field Theory for Solvatochromism Studies of Solvent-Sensitive Dyes

On the basis of the Franck−Condon principle, a density functional vertical self-consistent reaction field (VSCRF) solvation model for vertical excitation and emission processes is established. The principles and implementation of the VSCRF model are presented. The predicted blue shifts of the vertical excitation energies of diazines in different solvents from n-heptane to water solutions are compared with the corresponding time dependent density functional calculations and are in very good agreement with experiment. We have also applied this method to predict the blue shifts and the vertical excitation and emission energies of Brooker's merocyanine dye with increasing solvent polarities from CHCl3 to H2O solutions. Overall, our calculations predicted the relative excitation and emission energy orderings for Brooker's merocyanine in different solvents with different polarities. Also, the calculated Stokes shift is fairly well represented for different solvents, and the calculations correctly show that the absorption energies have a much stronger solvent dependence than the emission energies. The importance of both relaxation of the molecular structures and consideration of explicit H-bonding H2O and CH3OH molecules in water and methanol solvents in predicting the solvatochromic shifts is also discussed.

Solute–solvent and solvent–solvent interactions in the preferential solvation of Brooker's merocyanine in binary solvent mixtures

AbstractThe ET polarity values of 4‐[(1‐methyl‐4(1H)‐pyridinylidene)ethylidene]‐2,5‐cyclohexadien‐1‐one, better known as Brooker's merocyanine, were collected in binary mixtures comprising a non‐protic [acetonitrile, dimethyl sulfoxide (DMSO) and acetone] and a protic (water, methanol, ethanol, propan‐2‐ol and butan‐1‐ol) solvent. Data referring to mixed systems involving water and alcohols (methanol, ethanol, propan‐2‐ol and 2‐methylpropan‐2‐ol) were also collected. These data were used in the investigation of the preferential solvation of the dye. Each solvent system was analyzed in terms of both solute–solvent and solvent–solvent interactions. These latter interactions are responsible for the synergistic effects observed for the ET parameter in many binary mixtures. All data were successfully fitted to a model based on solvent exchange equilibria, which allowed the separation of the different contributions of the solvent species in the solvation shell of the dye. This model was then successfully applied to explain the kinetic behavior of the methylation of sodium 4‐nitrophenoxide in acetone–water mixtures. This result suggests that this solvatochromic strategy can be successful in the analysis of many other solvent‐dependent kinetic and thermodynamic processes occurring in mixed solvents. Copyright © 2002 John Wiley &amp; Sons, Ltd.

An Easy and Versatile Experiment to Demonstrate Solvent Polarity Using Solvatochromic Dyes

In this easy and versatile experiment, undergraduate students of experimental physical chemistry can discuss the concept of media polarity on the basis of the solvatochromic approach. Two solvatochromic dyes, Reichardt's betaine 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)-1-phenoxide and Brooker's merocyanine 4-[(1-methyl-4(1H)-pyridinylidene)ethylidene]-2,5-cyclohexadien-1-one were used to construct polarity scales for various solvents by means of visual observation and UV-vis spectroscopy. Changes in media polarity arising from mixed solvents or added electrolytes are also easily demonstrated. The data are used to justify kinetic and spectroscopic parameters.

Spectroscopic Studies on Brooker's Merocyanine

The origin of the multiple absorption bands of Brooker's merocyanine, 1-(4-aza-4-methylphenyl)-2-trans-(4-oxyphenyl)ethene, found in the visible region of the spectrum in solvents such as chloroform and tetrahydrofuran, has been explored experimentally using 1H and 13C NMR spectroscopy and theoretically using molecular orbital methods. While NMR analysis rules out the presence of a cis conformer, AM1/COSMO calculations in various solvents strongly suggest that there are a number of stable nonplanar conformations of the merocyanine as well as a dimer which are predicted to absorb at longer and shorter wavelengths, respectively, than the favored planar conformer. Spectroscopic analysis at variable temperature is supportive with the intensity of the longer wavelength absorption initially reducing with decreasing temperature, followed by an increase in the concentration of dimers and possibly aggregates. Further support is provided by synthesis of the corresponding nonplanar merocyanine containing methyl grou...