Solvatochromic Studies on Reversed-Phase Liquid Chromatographic

Apr 15, 1996 - A new merocyanine dye with a stilbazolium betaine structure has been synthesized and characterized by its UV−visible absorption. This...
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Anal. Chem. 1996, 68, 1381-1386

Solvatochromic Studies on Reversed-Phase Liquid Chromatographic Phases. 1. Synthesis and Solvent Effects of a New Merocyanine Dye, 4,6-Dichloro-2-[2-(1-methyl-4-pyridinio)vinyl]phenolate Huiyan Lu and Sarah C. Rutan*

Department of Chemistry, Box 842006, Virginia Commonwealth University, Richmond, Virginia 23284-2006

A new merocyanine dye with a stilbazolium betaine structure has been synthesized and characterized by its UV-visible absorption. This compound, 4,6-dichloro-2[2-(1-methyl-4-pyridinio)vinyl]phenolate (DCMPVP), has an absorption wavelength shift of 166 nm across 13 polar solvents, with peak positions ranging from 454.8 nm in water to 620.9 nm in ethyl acetate. Correlation of the absorption energy of DCMPVP by least-squares fitting with the Kamlet-Taft dipolarity/polarizability, π*, and hydrogenbond-donating ability, r, yields a regression equation of ∆Emax (kcal/mol) ) (9.0 ( 1.1)π* + (10.1 ( 0.5)r + (41.8 ( 0.8). The pKa value of 6.26 is lower than those for other non-chlorinated derivatives of stilbazolium dyes. This dye is a suitable indicator for a variety of neutral and weak acidic media, including chromatographic phases. Solvent-sensitive dyes have been demonstrated to be useful for studying chromatographic phases.1-12 Since this approach allows the dissection of the dipolarity and hydrogen-bonding contributions to the solvation energy, studies of the solvatochromic properties of the mobile phase can reveal some important information about the mechanism of retention in liquid chromatography. A merocyanine dye, 2,6-diphenyl-4-(2,4,6-triphenyl-1pyridino)phenoxide (ET-30), has been most commonly used to detect the hydrogen-bond-donating ability of mobile phases because of its high solvatochromic sensitivity. Other R dyes, as developed by Kamlet and Taft for testing the hydrogen-bonddonating ability of pure solvents,13 worked well for studying (1) Johnson, B. P.; Khaledi, M. G.; Dorsey, J. G. Anal. Chem. 1986, 58, 2354. (2) Johnson, B. P.; Khaledi, M. G.; Dorsey, J. G. J. Chromatogr. 1987, 384, 221. (3) Dorsey, J. G.; Johnson, B. P. J. Liq. Chromatogr. 1987, 10, 2695. (4) Sadek, P. C.; Carr, P. W.; Doherty, R. M.; Kamlet, M. J.; Taft, R. W.; Abraham, M. H. Anal. Chem. 1985, 57, 2971. (5) Carr, P. W.; Doherty, R. M.; Kamlet, M. J.; Taft, R. W.; Melander, W.; Horva´th, Cs. Anal. Chem. 1986, 58, 2674. (6) Cheong, W. J.; Carr, P. W. Anal. Chem. 1989, 61, 1524. (7) Cheong, W. J.; Carr, P. W. Anal. Chem. 1988, 60, 820. (8) Park, J. H.; Jang, M. D.; Kim, D. S.; Carr, P. W. J. Chromatogr. 1990, 513, 107. (9) Jones, J. L.; Rutan, S. C. Anal. Chem. 1991, 63, 1318. (10) Helburn, R. S.; Rutan, S. C.; Pompano, J.; Mitchem, D.; Patterson, W. T. Anal. Chem. 1994, 66, 610. (11) Hayashi, Y.; Helburn, R. S.; Rutan, S. C. In Proceedings of the 4th Symposium on Computer-Enhanced Analytical Spectroscopy; Wilkins, C. L., Ed.; Plenum: New York, 1992. (12) Dallas, A. J. Ph.D. Dissertation, University of Minnesota, Minneapolis, MN, 1993. 0003-2700/96/0368-1381$12.00/0

© 1996 American Chemical Society

Scheme 1

relatively homogeneous systems, such as the mobile phases used in liquid chromatography. But the retention of a solute in liquid chromatography is affected not only by the properties of the mobile phase but also by those of the stationary phase. Therefore, methods for probing complex, heterogeneous systems are becoming increasingly important. We have found for heterogeneous systems, such as solvated chromatographic stationary phases, that the requirements for a suitable R probe are much more rigorous. Many typical dyes are protonated when adsorbed to silica stationary phase materials and are no longer useful as polarity probes. The goal of this work is to design and characterize an R probe for this kind of system; the application of this dye for this type of study is described in the following paper.14 Solvatochromic dyes are compounds that have significant shifts in absorption wavelength when dissolved in different solvents. Among these compounds, the zwitterionic merocyanine dyes containing heteroatoms of O and N are of particular interest. In this system, an electron-donating group and an electron-accepting group are linked by a conjugated system which can be vinylogous, aromatic, or both. As demonstrated by the structures in Scheme 1, intramolecular charge transfer occurs between a donor and an acceptor within the molecule. Therefore, an electronic transition induced by light excitation can cause a substantial change in the dipole moment. The dipole moment of the excited state, µe, can be either smaller than or greater than the dipole moment of the ground state, µg. In the case of µe > µg, a more polar solvent stabilizes the excited state more than the ground state, where the excited state is better represented by the resonance structure on the right in Scheme 1 and the ground state by the structure on the left. Thus, solventsolute interactions stabilize the excited state more than the ground state and cause the excitation energy to shift to a lower energy, i.e., longer wavelength, as shown in Figure 1a. With an increase in solvent polarity, this bathochromic shift (to lower excitation energy) of the absorption band is called positive solvatochromism. The opposite shift is called a hypsochromic shift, or negative (13) Taft, R. W.; Kamlet, M. J. J. Am. Chem. Soc. 1976, 98, 2886. (14) Lu, H.; Rutan, S. C. Anal. Chem. 1996, 68, 1387.

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N-methyl or oxygen substituents in the ortho position with respect to the vinyl group, as shown by MPVP219 and MPVP3.20 All of

Figure 1. Schematic representation of solvent effects on the electronic transition energy of dipolar solutes. (a) µe > µg. (b) µe < µg. Adapted from ref 15.

solvatochromism, and is depicted in Figure 1b. For some strongly solvatochromic compounds, their solvatochromism is affected not only by the change in the permanent dipole moment between the excited and ground states but also by the dipole moment induced by the solvent dipoles in ground state. The magnitude of the induced dipole moment, µg′, depends on both the solute molecular structure and the surrounding solvent conditions. Scheme 1 shows two extreme resonance forms. The actual molecular structure is a resonance hybrid consisting of contributions from each form; that is, the π-electrons are delocalized across the whole molecule.16 This electron delocalization causes gradual shifts in the absorption band with a change in solvent. As the surroundings change from a nonpolar to a polar solvent, merocyanines change their solvatochromic behavior from bathochromic shifts (positive solvatochromism) to hypsochromic shifts (negative solvatochromism), because the total dipole moment (permanent dipole moment and induced dipole moment) of the ground state can change with the surrounding polarity. This inversion of solvatochromism was explained by Fo¨rster in 1939 on the basis of valence bond theory.17 The stilbazolium betaines are one class of merocyanine dyes. Because of their strong solvatochromism, they have attracted more attention than other merocyanines. The first dye of the [(Nmethylpyridino)vinyl]phenolate (MPVP) series was studied by Brooker in 1951, and has the structure MPVP1.18 Other similar

MPVP compounds have also been studied, which have the (15) Gough, T. E.; Irish, D. E.; Lantzke, I. R. Spectroscopic Measurements (Electron Absorption, Infrared and Raman, ESR and NMR Spectroscopy. In Physical Chemistry of Organic Solvent System; Convington, A. K., Dickson, T., Eds.; Plenum Press: London, New York, 1973; Chapter 4, p 405. (16) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry, 2nd ed.; VCH: Weinheim, Germany, 1988; pp 297-299. (17) Fo ¨rster, Th. Z. Elektrochem., Angew. Phys. Chem. 1939, 45, 548. (18) Brooker, L. G. S.; Keyes, G. H.; Heseltine, D. W. J. Am. Chem. Soc. 1951, 73, 5350.

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these compounds show strong solvatochromic effects.19,21-25 Furthermore, Gibson and Bailey employed monosubstituted derivatives of the dye MPVP2 to study how a substituent on the phenolate ring affects the solvatochromism.19 Recently, Catala´n et al. studied the influence of steric factors on solvatochromism using mono- and di-tert-butyl derivatives of MPVP1 substituted at the ortho position with respect to the O atom of the phenolate ring.26 In this paper, a new dichloro-substituted stilbazolium betaine dye is reported. The solvatochromic properties are characterized by UV-visible spectroscopy. The pKa value of this dye has also been determined. Its low basicity makes it an appropriate probe for the study of chromatographic stationary phases, an area of current interest in this research group.14,27 EXPERIMENTAL SECTION Reagents. 1,4-Dimethylpyridinium iodide (white crystals) was synthesized and purified according to a literature method.28 Piperidine and 3,5-dichlorosalicylaldehyde were purchased from Aldrich, Inc. The buffers were purchased from the Fisher Co. All chemicals and solvents were used without further purification. Synthesis. The white solid, 3,5-dichlorosalicylaldehyde (11.4 g, 0.06 mol), was added to ∼120 mL of absolute ethanol and heated until all the solid dissolved. 1,4-Dimethylpyridinium iodide (14.1 g, 0.06 mol) and piperidine (10 mL) were then added to the above solution. The mixture was refluxed for 15 h. A red precipitate appeared immediately after addition of 100 mL of a 0.4 M KOH aqueous solution. The solid was filtered and washed with 1% NaOH several times and then rinsed with water. Finally, the product was recrystallized twice from ethanol-water and dried (19) Gibson, H. W.; Bailey, F. C. Tetrahedron 1974, 30, 2043. (20) Abdel-Halim, S. T.; Awad, M. K. J. Phys. Chem. 1993, 97, 3160. (21) Abdel-Halim, S. T. J. Chem. Soc., Faraday Trans. 1993, 89, 55. (22) Jacques, P. J. Phys. Chem. 1986, 90, 5535. (23) Botrel, A.; Beuze, A. L.; Jacques, P.; Strub, H. J. Chem. Soc., Faraday Trans. 1984, 80, 1235. (24) Abdel-Halim, S. T.; Abdel-Kader, M. H.; Steiner, U. E. J. Phys. Chem. 1988, 92, 4324. (25) Benson, H. G.; Murrell, J. N. J. Chem. Soc., Faraday Trans. 2 1972, 68, 137. (26) Catala´n, J.; Pe´rez, P.; Elguero, J.; Meutermans, W. Chem. Ber. 1993, 126, 2445. (27) Li, Z.; Rutan, S. C. Anal. Chim. Acta 1995, 312, 127. (28) Minch, M. J.; Shah, S. S. J. Chem. Educ. 1977, 54, 709.

Table 1.

1H-NMR

Data for DCMPVP

δ

multiplicity

JH-H (Hz)

integration

position assignmenta

4.15 7.15 7.29 7.30 8.15 7.87 8.42

singlet doublet doublet doublet doublet doublet doublet

2.37 2.05 16.13 16.15 6.68 6.70

3H 1H 1H 1H 1H 2H 2H

13 12 10 5 6 3 and 3′ 2 and 2′

a

Molecular framework with positions assigned as follows:

in air. Elemental analysis indicated that the resulting compound contained some water. The red crystals were then oven-dried at temperatures below 120 °C and then kept in the oven at ∼80 °C. The product turned to a black-green color with mp ) 224.5-225.0 °C and yielded 14.1 g, for an overall yield of 84%. Elemental analysis showed that this compound was anhydrous. Anal. Calcd for C14H11NOCl2: C, 60.02; H, 3.96; N, 5.00; Cl, 25.31. Found: C, 59.74; H, 4.01; N, 4.97. Total halogen calculated as Cl, 25.14. The 13C-NMR spectra in CD OD yielded peaks at the following 3 chemical shift values: δ 46, 116, 119, 123, 126, 126.5, 127.5, 131, 141, 144, 161. The 1H-NMR (CD3OD) results are summarized in Table 1. This product is not stable in some solutions, especially tetrahydrofuran (THF). The newly prepared THF solution has a blue color but turns colorless after ∼15 h. Procedures. The purity of the product was examined by thinlayer chromatography on alumina with a CH2Cl2-ACN-MeOH (10/85/5) mobile phase. NMR spectra were obtained on a GE 300 MHz NMR spectrometer using TMS as a standard. Elemental analyses were performed by Atlantic Microlabs (Atlanta, GA). All absorption spectra were obtained using a Shimadzu UV-265 spectrophotometer with a quartz cell with a 1.0 cm path length and a 2 nm slit width. Data were collected and transferred to an IBM PC AT computer through an IEEE-488 interface using software supplied by Shimadzu. All data treatment was done with programs written with Turbo Pascal (Borland) and Matlab (Mathworks) on IBM-compatible PCs. To reduce the noise and to determine peak positions, the spectral curves were smoothed using a quadratic polynomial first derivative smoothing filter with a 21-point window width.29 The peak positions were found at the zero point of a linear regression fit to the first derivative of the smoothed curves near the peak region. RESULTS AND DISCUSSION Structure. Due to their high basicity,20 the MPVP dyes discussed in the introduction of this paper are not satisfactory probes for chromatographic stationary phases such as silica or ODS slurries in organic-aqueous mixtures. To probe solvated stationary phases with moderate proton acidities, a new merocyanine dye in the stilbazolium betaine family was synthesized, (29) Savitzky, A.; Golay, M. J. E. Anal. Chem. 1964, 36, 1627.

Figure 2. Absorbance spectra of DCMPVP aqueous solutions with pH 5-7. The numbers in the graph indicate pH values.

4,6-dichloro-2-[2-(1-methyl-4-pyridinio)vinyl]phenolate (DCMPVP).

Addition of the two chlorine substituents on the phenolate ring significantly decreases the basicity of the dye. The inductive effect of chlorine reduces the electron density on the phenolate ring and decreases the pKa value to 6.28, as compared to a pKa of 8.40 for MPVP3.20 It is this property of the new dye that makes it suitable for studying the solvated surfaces of chromatographic stationary phases. For ∼10-4 M aqueous solutions with pH near 6, the UV-visible spectra show an obvious absorbance at 454 nm, which is the characteristic peak of the unprotonated form of DCMPVP. Figure 2 shows the spectra of DCMPVP in pH 5.0, 5.8, 6.0, 6.8, and 7.0 buffer solutions. From this figure, it is clear that the pH value should be no lower than 5.8, if one wants to see the absorbance peak corresponding to the unprotonated form. But for the non-chlorine-substituted compound MPVP3, even in a solution with a pH of 8, it is still difficult to detect a peak corresponding to the unprotonated form.20 The spectra of MPVP3 in MeOH-H2O silica slurries show only the protonated peak over the entire composition range with a dye solution concentration of ∼10-4 M.30 Basic Considerations for pKa Determination. The unprotonated form of the dye is of interest in this solvatochromic study, since the protonated form is much less sensitive to the solvent environment.19 Therefore, when a dye is used to test the solvent polarity in protic environments, its pKa value becomes a very (30) Lu, H.; Rutan, S. C. Unpublished results, Virginia Commonwealth University, 1994.

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Table 2. pKa Calculation value

parameter pH [dye]a [dyeH+]a pKa

5.80 0.250 0.756 6.282

6.00 0.349 0.658 6.276

average 6.80 0.761 0.216 6.256

7.00 0.760 0.123 6.211

6.26

a Concentration relative to the pH 2 and pH 12 solutions, where each solution has a concentration of 3.5 × 10-5 M.

Table 3. Absorption Maxima and Colors of Various Solutions of DCMPVP

Figure 3. Absorbance spectra of DCMPVP aqueous solutions with pH 2 and 12.

important parameter. DCMPVP can be protonated on the oxygen by a solvent proton, as shown below:

This protonation causes a change in the spectrum. Figure 3 shows two extreme cases for this compound. One is the dye solution in a strongly basic (pH 12) buffer, and the other is a strongly acidic solution with a pH of 2. The spectrum of the pH 12 solution has two bands in the λ > 300 nm range, at λmax ) 454.7 nm and near 330 nm. In the spectrum of the pH 2 buffer solution, there is a peak at about 330 nm and a shoulder at ∼355 nm, but no absorbance at 454 nm. Therefore, we assign the band at 454.7 nm as the characteristic absorbance for the unprotonated form and the shoulder at ∼355 nm to the protonated form of the dye. The pH 12 solution is assumed to have all dye molecules present in the unprotonated form, and the pH 2 solution is assumed to have all dye molecules present in the protonated form. (This assumption will be verified later.) The spectra of the other buffer solutions were assumed to be due to the combined absorption of the protonated form and the unprotonated form at different concentration ratios, depending on the pH. Calculation of pKa. The concentrations of both the protonated and the unprotonated forms were determined by matrix least-squares of the spectral data.31 The pKa value was then calculated using eq 1:

pKa ) pH - log [dye] + log [dyeH+]

(1)

where [dyeH+] is the concentration of the protonated form and (31) Massart, D. L.; Vandeginste, B. G. M.; Deming, S. N.; Michotte, Y.; Kaufman, L. Chemometrics: A Textbook; Elsevier: New York, 1988; Chapter 13.

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solventa

λmax (nm)

color

water methanol ethanol 1-butanol 2-propanol DMSO acetonitrile DMF HMPA acetone THF methylene chloride ethyl acetate

454.8 493.7 516.7 531.7 540.6 557.5 563.8 570.6 584.4 587.0 601.6 620.7 620.9

amber red red red-purple purple-red purple purple purple purple-blue blue blue blue blue

a DMSO, dimethyl sulfoxide. DMF, N,N-dimethylformamide. HMPA, hexamethylphosphoramide. THF, tetrahydrofuran. All solvents were dried over molecular sieves for ∼12 h.

[dye] is the concentration of the unprotonated form. Table 2 lists the calculated data for the pKa values for all the buffered dye solutions. The assumption that the dye is essentially totally protonated at pH 2 and unprotonated at pH 12 can be verified with eq 2:

log [dye]/[dyeH+] ) pH - 6.26

(2)

at pH 2, [dye]/[dyeH+] ) 5.5 × 10-5 at pH 12, [dye]/[dyeH+] ) 5.5 × 105 Solvatochromism. DCMPVP, similar to the other stilbazolium betaine dyes, shows strong solvatochromism. The solvatochromic absorption wavelength shift across 13 polar solvents is 166 nm, with peak positions ranging from 454.8 nm in water to 620.9 nm in ethyl acetate. The dye shows different colors in solutions of various solvents. For example, the dye is amber in water, red in alcohol, purple in DMSO, and blue in THF. The colors exhibited by the various solutions and the maximum absorbance wavelengths are listed in Table 3. Similar to the other MPVP dyes, the DCMPVP molecule also has two conjugated resonance structures. One is the benzenoid form, with a positive charge on the N atom and a negative charge on the O atom, and the other is a neutral quinonoid form, as shown below:

∆Emax (kcal/mol) ) hc/λmax ) (9.0 ( 1.1)π* + (10.1 ( 0.5)R + (41.8 ( 0.8) (3) n ) 12, sd ) 0.73, r ) 0.990

Figure 4. Excitation energy (kK) of DCMPVP versus that of N-methyl-2-nitroaniline. (1) DMSO, (2) DMF, (3) CHCON(CH), (4) THF, (5) CHCOOCH, (6) pyridine, (7) 4-picoline, and (8) 2-picoline.

In solutions of this dye with aprotic solvents, the predominant interactions between the dye molecule and the surrounding solvent molecules are through dipole-dipole and dipole-induced dipole forces, which are strongly affected by the dipolarity and polarizability of the solvent. Based on this consideration, the transition energies of maximum absorption at the longest wavelength band of this dye were plotted against those of N-methyl2-nitroaniline, which reflect only dipole-dipole and dipoleinduced dipole interactions with solvent molecules.32 Figure 4 shows that the absorption energy increases with increasing dipolarity of the solvent in “well-behaved” 33 solvents, such as DMSO (1), DMF (2), CH3CON(CH3)2 (3), THF (4), and CH3COOC2H5 (5). This observation confirms that the ground state is more like the zwitterionic benzenoid form, while the excited state is more like the quinonoid form in these solvents. This behavior is similar to that exhibited by MPVP2, which also has a negative solvatochromic shift in polar solvents.21 More polar surroundings can stabilize the charged species, which is mostly the benzenoid form in the ground state. Since DCMPVP is almost insoluble in nonpolar solvents, it is impossible to do any solvatochromic studies for solutions of nonpolar solvents such as hexane, cyclohexane, CCl4, toluene, benzene, and CCl2dCClH. The absorption data for some pyridine derivative solvents, such as pyridine (6), 4-picoline (7), and 2-picoline (8) (methylpyridine), are also shown in Figure 4. These solvent data have obvious negative deviations from the line derived for the other solvents. These deviations may be caused by other specific interactions in addition to the dipole-dipole interactions between the dye molecules and the pyridine derivatives. In addition to dipole-dipole forces, the dye molecule also has the ability to form a hydrogen bond through the phenolate oxygen. In polar, protic solvents, the relative stabilization of both excited and ground states can be attributed to both interactions. Here we use the π* parameter and the R parameter, the Kamlet-Taft polarity scales, to measure the solvent dipolarity/polarizability34 and the hydrogen-bond-donating ability of solvent, respectively.35 A regression equation can be made to correlate both parameters with the absorption energy. Equation 3 can be used to characterize (32) Yokoyama, T.; Taft, R. W.; Kamlet, M. J. J. Am. Chem. Soc. 1976, 98, 3233. (33) Linert, W.; Jameson, R. F. J. Chem. Soc., Perkin Trans. 1993, 2, 1415. (34) Kamlet, M. J.; Abboud, J. L.; Taft, R. W. J. Am. Chem. Soc. 1977, 99, 6027. (35) Taft, R. W.; Kamlet, M. J. J. Am. Chem. Soc. 1976, 98, 2886.

the interactions of the dye molecule with the environment. The positive coefficient of π* in eq 3 indicates that the excitation energy increases with solvent polarity, and that the excited state is less polar than the ground state, which is more stable in polar solvents. Also, solvents with stronger hydrogen-bond-donating ability increase the excitation energy, indicating that hydrogen bond donation from the solvent can stabilize the ground state more than the excited state, because the ground state has a stronger hydrogen-bonding ability than the excited state. The benzenoid and quinonoid forms are two extremely electron-localized structures. In reality, the π-electrons are delocalized across the whole molecule. This electron delocalization can provide a qualitative interpretation of the strong negative solvatochromic shifts of this dye. As the surrounding conditions change, the net electron density on each atom may vary. With more polar and protic solvents, the electron density on the phenolate ring, especially on the O atom, is higher than that in less polar or aprotic solvents. As the solvent changes from acetone, with π* and R values of 0.71 and 0.08, to water, with values of 1.09 and 1.17, respectively,36 increasing orientation interactions with the surrounding molecules can decrease the energy of ground state. The coefficient ratio of R and π* (close to 1) in the above equation indicates that hydrogen bonding makes a significant contribution to the stabilization of the ground state. In an aprotic solvent, the solvent molecules have no hydrogenbond-donating ability, and the dye molecule has predominantly dipole-dipole and dipole-induced dipole interactions with the solvent molecules. Another empirical parameter of solvent polarity is the ET(30) value.37 This scale is based on the position of the absorption peak at the longest wavelength in the spectrum of an ET-30 solution. This parameter is a measure of the overall solvent “polarity”, i.e., a combination of π* and R contributions. The absorption energies of the dye DCMPVP in different solutions are regressed in eq 4, which correlates the excitation energy for DCMPVP with the ET(30) value. A coefficient of 0.63 in eq 4 indicates that the sensitivity of DCMPVP to overall solvent polarity is less than that of ET-30.

∆Emax (kcal/mol) ) (0.63 ( 0.03)ET(30) (kcal/mol) + (22.6 ( 1.2) (4) n ) 12, sd ) 0.646, r ) 0.992 It is important to keep the solvent and solution dry. Table 4 shows the position of the maximum absorption of several solutions prepared with nondried solvents and the peak shift with respect to the dried solvents. The position of the absorbance maximum shifts to a shorter wavelength for the solution prepared with the solvents that have been open to air for a period of time. It was also found that, even when a solvent was dried carefully, the (36) Kamlet, M. J.; Abboud, J. L.; Abraham, M. H.; Taft, R. W. J. Org. Chem. 1983, 48, 2877. (37) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry, 2nd ed.; VCH: Weinheim, Germany, 1988; pp 364-371.

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Table 4. Absorption Maxima for Solutions with Nondried Solvents and Shifts Relative to Dried Solvents solvent

nondried solvent (nm)

peak shift (nm)a

methylene chloride DMF DMSO pyridine ethyl acetate THF acetone 1-butanol 2-propanol

617.5 567.9 552.6 563.9 608.8 586.5 580.5 527.4 538.8

-3.2 -2.7 -4.9 -16.9 -12.1 -15.1 -6.5 -4.3 -1.8

a

Compared to the data with dried solvents from Table 3.

reproducibility of the peak position became significantly reduced once the solution was open to air for a couple of minutes. This phenomenon occurred probably because the trace amount of water absorbed by the system increased the effective solvent polarity. The solvatochromic data for DCMPVP in acetonitrile- and methanol-water mixtures are shown in Figure 5; these are the most often used mobile phases in reversed-phase liquid chromatography. The excitation energy ET values are sensitive to solvent composition. As the water ratio in these solvent mixtures increases, the ET value increases because water has a higher ET value. This increase is not linear, especially for the acetonitrilewater mixtures. This is caused by the nonideality of the binary solvent mixtures.1,7,38 In acetonitrile-water mixtures, the ET value of DCMPVP increases rapidly from 100% to 90% (v/v) of acetonitrile, and then the ET value gradually increases as the water content increases throughout the rest of composition range. This kind of variation is similar to that observed with other R dyes, such as ET-301 and the Fe complex, bis[R-(2-pyridyl)benzylidene3,4-dimethylaniline]bis(cyano)iron(II).8 In methanol-water mixtures, the ET value increases gradually over the entire composition range (from 100% to 5% methanol). The polarity of methanolwater mixtures probed with this dye is more like that obtained with ET-301 than with the Fe complex.8 This dye is also suitable for making polarity measurements for silica-based stationary phases, as is explained in the following paper in this issue.14 These (38) Horvath, C.; Melander, W. J. Chromatogr. Sci. 1977, 15, 393. (39) Lu, H.; Rutan, S. C. Presented at Pittcon ’95, New Orleans, LA, March 9, 1995; Abstract No. 1260.

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Figure 5. Excitation energy of DCMPVP versus composition of solvent mixture in acetonitrile- and methanol-water.

data cannot be obtained with ET-30 and the other MPVP series dyes because of their high basicities. Like those of other merocyanines, the DCMPVP energy levels in the ground and excited states are strongly affected by changing the solvent polarity and hydrogen-bonding ability. This negative solvatochromism can be characterized by the longest wavelength absorption band in polar solvents. Based on the results obtained here, the dye DCMPVP is suitable as a probe for the determination of the dipolarity/polarizability and hydrogen-bond-donating ability of the medium. Compared to other stilbazolium betaines, this dye has the advantage of a low pKa value, which is highly desirable for probing neutral and weakly acidic media, such as solvated chromatographic stationary phase surfaces.14,27,39 ACKNOWLEDGMENT This work was supported by a grant (CHE-9318484) from the National Science Foundation. Received for review August 2, 1995. Accepted November 22, 1995.X AC9507808 X

Abstract published in Advance ACS Abstracts, March 1, 1996.