In the Laboratory
Influence of Dielectric Constant on the Spectral Behavior of Pinacyanol
W
A Spectrophotometric Experiment for Physical Chemistry Raimon Sabaté, Llúcia Freire, and Joan Estelrich* Departament de Fisicoquímica, Universitat de Barcelona, Avda. Joan XXIII s/n, E-08028 Barcelona, Spain; *
[email protected] UV–vis spectrophotometry is used for the quantitative analysis of a large number of substances in solution. The macroscopic description of optical absorption is known as the Beer–Lambert law. This law states that, at reasonably low concentrations, the absorbance is proportional to the concentration of species absorbing radiation. Some approaches discussed in this Journal have been used to explore the variables inherent in this law (1–8). However, UV–vis spectrophotometry is also used to analyze qualitative aspects of molecular species, such as electronic transitions and the influence of the solvent environment on the absorption. We describe a laboratory experiment that reveals the effect of the polarity of the solvent (mixtures of methanol and water) on the visible spectrum of pinacyanol. The dielectric constant is used as a macroscopic solvent parameter for the evaluation of medium effects. Mixing of methanol with water affords solutions of different dielectric constant—that is, of different polarity. Pinacyanol (quinaldine blue) (1) belongs to the class of symmetric polymethine dyes and has been used to determine the critical micellar concentration (cmc) of surfactants spectrophotometrically (9), since its spectrum below the cmc differs from that above the cmc. Owing to its solvatochromic behavior, it can be used as solvent polarity indicator. The presence of nitrogen atoms in this molecule implies the existence of unbonded electrons (n-electrons), as well as π-electrons. +
N H3C
N
1
[Cl−]
results students can determine whether the transition that pinacyanol undergoes in the visible spectrum is n → π* or π → π*. Experimental Procedures Pinacyanol chloride (1,1′-diethyl-2,2′-carbocyanine chloride) obtained from Sigma was dissolved in doubledistilled water at 0.5 mM. An exact volume of methanol was added to a 5-mL volumetric flask, followed by 100 µL of the pinacyanol solution, and the volume was completed with water. Visible absorption spectra were collected using a UV2401 PC Shimadzu spectrophotometer at 25 ± 1 °C. Dielectric constants as a function of volume percentages were calculated using data in the Handbook of Chemistry and Physics (10). Hazards Pinacyanol is an irritant, and methanol is flammable and toxic. Ingestion or contact with skin and eyes should be avoided. Discussion of Results Figure 1 presents the absorption spectrum of pinacyanol in mixtures of methanol and water. In water, three peaks can be located at 600 (λ1), 550 (λ2), and ≈ 520 nm. The first is interpreted as the S0 → S1 electronic transition of monomer dye molecules. The peak at 550 nm is due to absorbance of the dimeric form of pinacyanol. The third component, usually
CH3
In the design of this experiment we had several pedagogical goals in mind. First, we aim to introduce students to the concepts of bathochromic and hypsochromic shift, as well as hyperchromic and hypochromic effects. Second, we wish to show an example of solvatochromism. Finally, we want to discuss how the kind of electronic transition can be determined from the influence of polarity on the UV–vis spectrum. Comparison of wavelengths corresponding to the maximum absorption (λmax) of each solution with that determined in water (maximal polarity) allows students to quantify the shift of λmax as a function of the dielectric constant and to establish whether the change of polarity produces a bathochromic or a hypsochromic shift. Moreover, comparison of the absorbance values at λ max reveals a hyperchromic or a hypochromic effect. So, students see clearly how the solvent can modify the spectrum in a qualitative (shift of λmax) or a quantitative (chromic effect) way. Further, on the basis of the
Figure 1. Absorption spectra of pinacyanol (10 µM) obtained at 25 °C in the following mixtures of methanol (MeOH) and water expressed as vol % of MeOH: (1) 0, (2) 2.5, (3) 5, (4) 10; (5) 15; (6) 20, (7) 40, and (8) 80.
JChemEd.chem.wisc.edu • Vol. 78 No. 2 February 2001 • Journal of Chemical Education
243
In the Laboratory
observed as a shoulder to the dimer peak at about 520 nm, is thought to be due to the polyaggregate form of the chromophore. When pinacyanol is dissolved in water–methanol mixtures, an increase in the absorbance at λ1 is observed in comparison with the value obtained when pinacyanol is in water. Moreover, λmax shifts toward longer wavelengths (lower energy). Chromophores can be perturbed by the general local environment. The environmental effects can include the pH, dielectric constant, rigidity of the medium, and the presence of nearby groups capable of specific chemical interaction (such as proton or charge transfer, or metal-ion binding [11]). In our case, pH and dielectric constant are apparently the only factors that could explain the spectral changes. However, since pinacyanol has a pKa of 3.5, the slight differences in pH in the various solutions cannot account for the variations in the spectra. So, such variations must to be due to changes in the polarity of the solvent. Thus, a decrease of the polarity involves a hyperchromic effect (increase of intensity of absorption) and a bathochromic shift (the maximal absorbance appears at longer wavelengths). Table 1 summarizes the changes in the λmax of both bands (λ1 and λ2) as a function of the dielectric constants of the methanol–water mixtures. From this table it can be inferred that the bathochromic shift is greater in λ2 (10.8 nm) than in λ1 (4 nm). However, the hyperchromic effect is only associated with λ1. In this way, the molar absorption coefficient of pinacyanol at λ1 moves from 64,000 M᎑1 cm᎑1 in water to 122,000 M᎑1 cm᎑1 in methanol 80 vol %. Finally, the influence of the solvent on the λ max shift allows us to distinguish the kind of electronic transition. When a bathochromic shift is observed on reducing the dielectric constant of the solvent, the transition is n → π*; but if a hypsochromic shift were produced (12) the transition would be π → π*. The bathochromic shift apparently arises from the reduced solvation of the unbonded electron pair, which increases the energy of the n orbital. Hence, the energy difference between excited and ground state is lower in a less polar environment than in a more polar, and in consequence, the wavelength associated with such a transition is longer. Conclusion The measurement and interpretation of electronic spectra of molecules is an interesting and important topic in physical chemistry. The solvatochromism of pinacyanol is suitable to demonstrate the effect of the polarity of the solubilizing medium on the spectra, since a high correlation is found between the shift of the wavelength of maximal absorbance and the dielectric constant of the solvent.
244
Table 1. Data for Methanol–Water Mixtures at 25 °C Vol %
ε
0
λ1/ nm
∆λ1/ nm
A1
∆A1
λ2 nm
∆λ2/ nm
A2
∆A2
78.0
599.6
—
0.64
—
549.4
—
0.50
—
2.5 76.9
600.0
0.4
0.66
0.02
550.0
0.6
0.48
᎑0.02
5
75.8
600.4
0.8
0.70
0.06
551.2
1.8
0.48
᎑0.02
10
73.5
600.8
1.2
0.78
0.14
553.0
3.6
0.49
᎑0.01
15
71.3
601.2
1.6
0.86
0.22
555.0
5.6
0.50
0.00
20
69.0
602.2
2.6
1.12
0.48
556.6
7.2
0.50
0.00
40
60.0
603.2
3.6
1.20
0.56
559.6 10.2
0.52
0.02
80
42.0
603.6
4.0
1.22
0.58
560.2 10.8
0.54
0.04
The experiment reported here was designed to be completed by students in approximately one 2-hour laboratory period, as follows: preparation of solutions (one laboratory hour) spectrophotometric run and discussion (one hour)
The experiment is simple and easy to perform, and the concepts learned in the lecture are reinforced by practical application. W
Supplemental Material
A handout for students and background notes for the instructor are available in this issue of JCE Online. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.
Wentworth, W. E. J. Chem. Educ. 1967, 44, 699. Castellan, G. W. J. Chem. Educ. 1983, 60, 912. Tan, H. S.; Jones, W. W. J. Chem. Educ. 1989, 66, 650. Berberan-Santos, M. N. J. Chem. Educ. 1990, 67, 757. Lykos, P. J. Chem. Educ. 1992, 69, 730. Ricci, R. W. J. Chem. Educ. 1994, 71, 983. Calloway, D. J. Chem. Educ. 1997, 74, 744. Steward, S. A.; Sommer, A. J. J. Chem. Educ. 1999, 76, 399. James, A. M.; Prichard, F. E. Practical Physical Chemistry; Longman: Harlow, UK, 1974. 10. CRC Handbook of Chemistry and Physics; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1999–2000. 11. Cantor, C. R.; Schimmel, P. R. Biophysical Chemistry; Part II: Techniques for the Study of Biological Structure and Function; Freeman: San Francisco, 1980. 12. Skoog, D. A.; Leary, J. J. Principles of Instrumental Analysis; Harcourt Brace Jovanovich: Orlando, FL, 1992.
Journal of Chemical Education • Vol. 78 No. 2 February 2001 • JChemEd.chem.wisc.edu
