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First reported evidence that solvent polarity induces an 1Lb↔1La inversion in the indole chromophore
Chemical Physics Letters 368 (2003) 717–723 www.elsevier.com/locate/cplett
First reported evidence that solvent polarity induces an 1Lb $ 1La inversion in the indole chromophore J. Catal an *, C. Dıaz Departamento de Quımica Fısica Aplicada, Universidad Aut onoma de Madrid, Cantoblanco, E-28049 Madrid, Spain Received 21 October 2002; in final form 2 December 2002
Abstract The solvatochromism of indole in 26 pure solvents, 21 binary solvent mixtures, and the gas phase was determined. The description of the solvatochromism of indole in the light of the SPP, SB, and SA scales allowed us to derive the first reported evidence that the inversion in its first two excited electronic states is caused by an increase in the general solvent effect. Based on the results, indole only emits from its 1 La state in solvents with SPP > 0:8 and from its 1 Lb state in solvents with SPP < 0:8. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction The indolyl chromophore in tryptophan is especially important in UV–Vis spectroscopic studies on proteins as it governs the near-ultraviolet circular dichroism and absorption spectra of many proteins [1]. The spectral features of this chromophore differ among proteins by the influence of the environment of tryptophanyl side chains varying from completely hydrophobic to hydrophilic. The most salient changes induced by perturbations of the molecular environment in the spectroscopic features of this chromophore are marked destructuring of its UV–Vis absorption spectrum and an anomalously large Stokes shift.
*
Corresponding author. Fax: +34-91-3974-187. E-mail address: [email protected] (J. Catalan).
The poor solubility of tryptophan in non-polar solvents have propitiated its replacement by indole as a spectroscopic model for systematizing the spectroscopic behaviour. Ever since Weber [2] and Zimmermann and Joop [3] reported excitation polarization spectra of fluorescence in the 1950s, the photophysical interpretation of this chromophore has revolved around two electronic states designated 1 Lb and 1 La , which have been assumed to undergo inversion during emission in polar solvents. The anomalously large fluorescence Stokes shift of indole and its derivatives in polar solvents has received much attention and led to the proposal of various mechanisms to account for it. Such mechanisms include those based on (a) dual fluorescence [4], (b) 1 Lb $ 1 La level inversion [5], (c) exciplex formation [6], (d) solvent rearrangement [7], (e) thermal isomerization to 2H-indole [8], (f) an increase in the dipole moment of the
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(02)01959-0
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J. Catalan, C. Dıaz / Chemical Physics Letters 368 (2003) 717–723
excited electronic state 1 La [5a,9], (g) emission from a solvated Rydberg state [10], and (h) specific interactions with the solvent [11]. Meech et al. [12] discussed a number of these mechanisms, focusing on the nature of the fluorescent state in terms of its potential relationship to an internal charge-transfer (ICT) mechanism and on the influence of the nature of such a state on the chromophore–solvent interaction. Notwithstanding, despite of the long time elapsed and the vast amount of research conducted, a number of aspects of the photophysics of these major chromophores continue to be controversial, even though the presence of the 1 Lb and 1 La states is still widely endorsed. Many available models assume the presence in these chromophores of two excited electronic states of rather different polarity and hence that their stability can be influenced by the polarity of the environment. This justifies the continual attempts at establishing linear relations between the indolyl chromophore and so-called Ôpolarity functionsÕ based on the dielectric constant and refractive in-
dex of the medium. It should be noted that, while Lumry et al. [6b] and Tatischeff et al. [13] found no significant relationship between the Stokes shift and polarity functions, several other authors have reported good correlations [5b,9a,9c,12,14]. Worth special note here is the work of Lami and Glasser [15], who, using the solvatochromic model of Amos and Burrows [16], studied the variation of the frequency of the emission maximum for indole in 11 solvents (n-heptane, di-isopentyl ether, di-n-butyl ether, diethyl ether, ethyl acetate, ethyl formate, acetonitrile, n-butanol, methanol, water, and glycerol) against the polarity function f ðD; nÞ and, somewhat to their surprise, observed that the 1 Lb $ 1 La level inversion proposed by several authors [5,10,12] was apparent from the plot. As can be seen from Fig. 1, the first seven solvents, which are non-protic, exhibit a linear trend from which the other four deviate; these latter are hydroxyl compounds, which raises the question as to whether their deviation may be a result of their acidity because indole is known to interact with H-bond donors [17].
Fig. 1. Plot of the emission maximum for indole vs. the polarity function f ðD; nÞ (11 solvents).
J. Catalan, C. Dıaz / Chemical Physics Letters 368 (2003) 717–723
If the aromatic system of indole can act as a H-bond acceptor, preferentially at positions 3 and 4, [18] and its N–H group is a H-bond donor to such an extent that it can yield dimers from these interactions [19] or complexes with other H-bond donors or acceptors [14a,17,20], then the solvatochromism of this substance in solvents bearing acid or basic groups cannot be analysed in the exclusive light of a polarity function f ðD; nÞ criterion, which is based on the assumption that such specific contributions must be negligible relative to the effect of solvent polarity. In this Letter, we examine the solvatochromism of the indolyl chromophore in order to assess the specific solvatochromic contributions involved and subtract them with a view to determining the general contribution to the solvatochromism, which will allow us to confirm a potential level inversion.
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The corresponding SPP, SB, and SA values for the 48 media studied are gathered in Table 1. To obtain the fits as a function of SA, SB, and SPP of the different mixtures we use a multiparametric analysis software. It provides an equation that has SA, SB, and SPP as variables. The program used is MI N I T A B program.
3. Results and discussion The maximum of the fluorescence band for indole in the gas phase and the 26 pure and 21 mixed solvents studied in this work, see Table 1 can be described in terms of the pure solvent scales SPP [21], SB [22], and SA [23], using the following expression: m~em =kK ¼ ð2:566 0:399ÞSPP ð2:656 0:217ÞSA
2. Experimental section All solvents used were of the highest available purity and purchased from Merck in Uvasol or similar grade. Solvent mixtures were prepared from freshly opened bottles, using Brand II 25.00 mL burettes to transfer the liquids. UV–Vis measurements were made on a Shimadzu 2100 spectrophotometer the monochromator of which was calibrated by using the 486.0 and 656.1 nm lines from a deuterium lamp. The instrument was routinely checked for wavelength accuracy by using holmium oxide and didymium filters. All spectral measurements were made at 25 °C, using a matched pair of quartz cells of 1 cm light path. Emission spectra corrected for instrument sensitivity were obtained on an Aminco-Bowman AB2 spectrofluorimeter using a continuous (CW) 150 W xenon lamp for steady-state spectra. Polarity (SPP), basicity (SB), and acidity (SA) values were obtained from the wavenumbers of the absorption maxima for the following probe/homomorph couples: 2-dimethylamino-7-nitrofluorene/ 2-fluoro-7-nitrofluorene, 5-nitroindoline/1-methyl5-nitroindoline, and o-tert-butylstilbazolium betaine dye/o,o0 -di-tert-butylstilbazolium betaine dye.
ð0:465 0:299ÞSB þ ð34:593 0:188Þ
ð1Þ
with n ¼ 48, r ¼ 0:973, sd ¼ 0:348 kK, and F ¼ 262. Based on this equation, the bathochromic shift in the emission of indole will increase with increasing solvent acidity and, to a lesser extent, with increasing basicity. From Eq. (1) it also follows that the contribution of solvent acidity to the solvatochromism of indole is quite substantial: )1.6 kK in methanol and )2.8 kK in water, for example. By subtracting the specific contributions, the contribution of the general solvent effect can be estimated to be m~em =kKðgeneralÞ ¼ m~em þ 2:656SA þ 0:465SB: ð2Þ Fig. 2 shows the general contribution of each solvent to the solvatochromism of indole, m~em ðgeneralÞ, as a function of the solvent polarity (expressed as SPP). The plot provides the first clear evidence of two linear trends, namely; one of very gentle slope that encompasses the solvents with SPP < 0:8 and the other, with a much steeper slope, which includes the solvents with SPP > 0:8. This clearly indicates that indole emits from two different
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Table 1 Wavenumbers of the 1 Lb (0–0) absorption ð~ mabs Þ and the emission maximum ð~ mem Þ of the indole, as well as the SPP, SB, and SA values for the 48 media studied Solvent
m~abs =kK
m~em =kK
SPP
SB
SA
Gas Perfluoro-n-hexane Perfluoro-n-hexane/n-hexane Xperfluoro-n-hexane ¼ 0.95 0.90 0.80 0.70 0.60 0.55 2-Methylbutane n-Pentane n-Heptane Cyclohexane Decaline Diisopentyl ether Di-n-butyl ether Diethyl ether Ethyl acetate 1,4-Dioxane/water Xwater ¼ 0.4 0.5 0.6 0.7 0.8 0.9 Ethyl formate tert-Butanol Formamide 1-Butanol Tetrahydrofuran Ethanol Methanol Acetonitrile Trifluoroethanol N-Methyl formamide 1,2-Ethanediol Glycerol N,N-Dimethylformamide N,N-Dimethylacetamide Ethanol/water Xwater ¼ 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Water Dimethyl sulfoxide
35.174 35.092
33.842 33.722
0.000 0.214
0.000 0.057
0.000 0.000
35.080 35.061 35.029 34.996 34.960 34.943 34.843 34.841 34.821 34.795 34.755 34.715 34.716 34.749 34.772
33.761 33.677 33.572 33.618 33.450 33.468 33.552 33.552 33.348 33.396 33.433 33.186 33.062 32.927 32.889
0.255 0.292 0.342 0.389 0.424 0.436 0.479 0.507 0.526 0.557 0.574 0.626 0.652 0.694 0.795
0.056 0.056 0.056 0.056 0.056 0.056 0.053 0.073 0.083 0.073 0.056 0.626 0.637 0.562 0.542
0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
34.725 34.723 34.735 34.741 34.772 34.826 34.791 34.732 34.745 34.719 34.692 34.743 34.772 34.821 35.073 34.686 34.714 34.708 34.663 34.647
31.611 31.097 31.054 30.689 30.637 29.683 32.526 31.519 30.055 30.849 32.554 30.915 30.769 31.728 29.936 30.849 30.416 30.116 31.094 31.011
0.806 0.844 0.876 0.882 0.900 0.914 0.812 0.829 0.833 0.837 0.838 0.853 0.857 0.895 0.908 0.920 0.932 0.948 0.954 0.970
0.463 0.449 0.451 0.456 0.442 0.324 0.490 0.928 0.414 0.809 0.591 0.658 0.545 0.286 0.107 0.590 0.534 0.309 0.613 0.650
0.499 0.501 0.514 0.562 0.686 0.829 0.000 0.145 0.674 0.341 0.000 0.400 0.605 0.044 0.893 0.444 0.565 0.618 0.031 0.028
34.752 34.761 34.768 34.776 34.784 34.793 34.810 34.841 34.923 34.994 34.583
30.709 30.644 30.661 30.521 30.501 30.464 30.270 29.912 29.472 29.127 30.630
0.845 0.851 0.862 0.872 0.882 0.888 0.899 0.907 0.938 0.962 1.000
0.649 0.634 0.615 0.596 0.543 0.512 0.459 0.407 0.236 0.025 0.647
0.497 0.521 0.548 0.568 0.593 0.659 0.727 0.798 0.963 1.062 0.072
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Fig. 2. Plot of the ð~ mem ÞðgeneralÞ (d) and ð~ mabs Þ(0–0, 1 Lb ÞðgeneralÞ ðsÞ for indole vs. the SPP parameters of the 48 media studied.
electronic states and that it possesses a rather different polarity depending on whether the polarity of the medium is greater or smaller than 0.8SPP units. The 0–0 component of the 1 A ! 1 Lb transition of indole in the 48 media studied, see Table 1, is described in terms of the pure solvents scales by the following equation: ð~ mabs Þ=kKð0–0; 1 Lb Þ ¼ ð0:448 0:071ÞSPP þ ð0:197 0:038ÞSA ð0:167 0:053ÞSB þ ð35:147 0:033Þ
ð3Þ
with n ¼ 48, r ¼ 0:892, sd ¼ 0:062 kK, and F ¼ 58. This equation allows one to rule out specific contributions to the solvatochromism of indole and hence to determine the general contribution to the solvatochromism of this component of indole absorption: ð~ mabs Þð0–0; 1 Lb ÞðgeneralÞ ¼ ð~ mabs Þð0–0; 1 Lb Þ 0:197SA þ 0:167SB:
ð4Þ
Fig. 2 also shows the variation of the general solvatochromic contributions with the polarity (SPP) of the studied solvents. As can be seen, the behaviour of the general solvatochromism of the 0–0 component of the 1 A ! 1 Lb transition towards SPP is clearly parallel to that of the general solvatochromism of indole emission in the group of solvents with SPP < 0:8; this clearly indicates that indole emits via the 1 Lb ! 1 A transition in these solvents. The behaviour of both solvatochromisms is scarcely dependent on solvent polarity and this suggests that the dipole moment must change very little in this transition. This is quite consistent with the small value of Dl (0.14D) obtained by Lombardi and his coworkers [24] from the Stark effect for indole in the gas phase. It should be noted that Kawski [25] using the thermochromic method, also obtained a small change in dipole moment (1.04D) for indole in ethyl acetate. Clearly, in solvents with SPP > 0:8, indole emits from a different state that is much more polar than its ground state [in cyclohexane, lðS0 Þ ¼ 1:93
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0:04D ; [26] the emission must thus be assigned to the 1 La ! 1 A transition. Because the solvatochromic scales were developed using the gas phase as reference, by extrapolating to SPP ¼ 0 one can locate the emission maxima for the gas phase in the 1 Lb and 1 La states. From Fig. 2 it follows that 1 La must lie about 3500 600 cm1 above 1 Lb . That 1 La should lie so well above 1 Lb is consistent with the experimental evidence obtained by Jortner and his coworkers [27] using the free-jet technique: they found a vibrational excess of 2200 cm1 above the 0–0 component of 1 Lb to be assignable to no peak for 1 La . Also, theoretical calculations have shown the gap to be about 2500 cm1 wide [28]. The results of Fig. 2 clearly show, for the first time, that an increased solvent polarity (SPP) can induce the inversion of the 1 Lb and 1 La electronic states in indole and, also, that the pure solvent scales are highly powerful tools for studying the solvatochromism of chromophores.
Acknowledgements The authors are grateful to SpainÕs DGICYT for funding this research within the framework of Project BQU2002-02106.
References [1] (a) D.B. Wetlaufer, Adv. Protein Chem. 17 (1962) 303; (b) E.H. Strickland, J. Horwitz, C. Billups, Biochemistry 8 (1968) 3205; (c) S.V. Konev, in: S. Udenfried (Ed.), Fluorescence and Phosphorescence of Proteins and Nucleic Acids, Plenum Press, New York, 1967. [2] (a) F.W.J. Teale, G. Weber, Biochem. J. 8 (1969) 3205; (b) G. Weber, Biochem. J. 75 (1960) 335. [3] H. Zimmermann, N.Z. Joop, Elektrochemische 65 (1961) 61. [4] (a) H. Zimmermann, H.Z. Geisenfelder, Elektrochemische 65 (1961) 368; (b) H.V. Schutt, H. Zimmermann, Ber. Bunsenges Phys. Chem. 67 (1963) 54; (c) P.S. Song, W.E. Kurtin, J. Am. Chem. Soc. 91 (1969) 4892; (d) L.J. Andrews, L.S. Forster, Photochem. Photobiol. 19 (1974) 353.
[5] (a) N. Mataga, Bull. Chem. Soc. Jpn. 36 (1963) 654; (b) N. Mataga, Y. Torihashi, K. Ezumi, Theoret. Chim. Acta 2 (1964) 158; (c) S. Suzuki, T. Fujii, A. Imai, H. Akanori, J. Phys. Chem. 81 (1977) 1952; (d) M. Martinand, A. Kadiri, Chem. Phys. 28 (1978) 473. [6] (a) R. Lumry, S. Yanari, F.A. Bovey, Am. Chem. Soc., Div. Org. Coating Plastics Chem. Preprint (1962) 142; (b) M.S. Walker, T.W. Bednar, R. Lumry, J. Chem. Phys. 45 (1966) 3455; J. Chem. Phys. 47 (1967) 1020; (c) M.S. Walker, T.W. Bednard, in: E.C. Lim (Ed.), Molecular Luminescence, Benjamin, New York, 1969, p. 135; (d) M.V. Herbergger, R. Lumry, Photochem. Photobiol. 23 (1976) 391. [7] J. Eisinger, G. Navon, J. Chem. Phys. 50 (1969) 2069. [8] C.M. Chopin, J.H. Wharton, Chem. Phys. Lett. 3 (1969) 552. [9] (a) A. Kawski, G. Karcz-Jacyno, Bull. Acad. Pol. Sci. XXI (1973) 289; (b) J. Gryczynski, A.Z. Kawski, Z. Naturforch 30A (1975) 287; (c) M. Sun, P.S. Song, Photochem. Photobiol. 25 (1977) 3. [10] H. Lami, Chem. Phys. Lett. 48 (1977) 447; J. Chem. Phys. 67 (1977) 3274; J. Mol. Struct. 45 (1978) 437. [11] I. Tatischeff, R. Klein, T. Zemb, M. Duguesne, Chem. Phys. Lett. 54 (1978) 394; B. Skalski, D.M. Rayner, A.G. Szabo, Chem. Phys. Lett. 70 (1980) 587. [12] S.R. Meech, D. Phillips, A.G. Lee, Chem. Phys. 80 (1983) 317. [13] I. Tatischeff, R. Klein, Photochem. Photobiol. 22 (1975) 221. [14] (a) D.A. Chignell, W.B. Gratzer, J. Phys. Chem. 72 (1968) 2934; (b) E. Vander Donckt, Bull. Soc. Chim. Belg. 78 (1969) 69; (c) A. Chamma, P. Viallet, CR Acad. Sci. Ser. C 270 (1970) 1901; (d) A. Kawski, J. Sepiol, Bull. Acad. Pol. Sci. Cl. 20 (3) (1970) 707; (e) B. Koutek, Collect. Czech. Chem. Commun. 43 (1978) 2368. [15] H. Lami, N.J. Glasser, J. Chem. Phys. 84 (1986) 597. [16] A.T. Amos, B.L. Burrows, Adv. Quantum Chem. 7 (1973) 289. [17] (a) W. Hager, D.R. Demmer, S.C. Wallace, J. Phys. Chem. 91 (1987) 1375; (b) S. Arnold, M. Sulkes, J. Phys. Chem. 96 (1992) 4768. ~ez, Tetrahedron 38 (1982) 3693. [18] J. Catalan, P. Perez, M. Yan [19] (a) A. Lantie, M.F. Lantie, A. Gruger, S.A. Fakhri, Spectrochim. Acta 36A (1980) 85; (b) L. Pejov, Chem. Phys. Lett. 339 (2001) 269. [20] P. Dubin, R. Jordan, Biopolymers 14 (1975) 2435. [21] (a) J. Catalan, V. L opez, P. Perez, R. Martın-Villamil, J.G. Rodrıguez, Liebigs Ann. (1995) 241; (b) J. Catalan, V. L opez, P. Perez, Liebigs Ann. (1995) 793.
J. Catalan, C. Dıaz / Chemical Physics Letters 368 (2003) 717–723 [22] J. Catal an, C. Dıaz, V. L opez, P. Perez, J.L.G. de Paz, J.G. Rodrıguez, Liebigs Ann. (1996) 1785. [23] J. Catal an, C. Dıaz, Liebigs Ann./Recueil (1997) 1941. [24] (a) C.T. Chang, C.Y. Wu, A.R. Muirhead, J.R. Lombardi, Photochem. Photobiol. 19 (1974) 347; (b) J.R. Lombardi, J. Phys. Chem. A 103 (1999) 6335.
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[25] A. Kawski, Asian J. Spectrosc. 1 (1997) 27 (and references therein). [26] J.J.M. Ramos, Rev. Port. Chim. 22 (1980) 64. [27] R. Bersohn, U. Even, J. Jortner, J. Chem. Phys. 80 (1984) 1050. [28] J. Catalan, P. Perez, A.U. Acu~ na, J. Mol. Struct. 142 (1986) 179.
First reported evidence that solvent polarity induces an 1Lb $ 1La inversion in the indole chromophore J. Catal an *, C. Dıaz Departamento de Quımica Fısica Aplicada, Universidad Aut onoma de Madrid, Cantoblanco, E-28049 Madrid, Spain Received 21 October 2002; in final form 2 December 2002
Abstract The solvatochromism of indole in 26 pure solvents, 21 binary solvent mixtures, and the gas phase was determined. The description of the solvatochromism of indole in the light of the SPP, SB, and SA scales allowed us to derive the first reported evidence that the inversion in its first two excited electronic states is caused by an increase in the general solvent effect. Based on the results, indole only emits from its 1 La state in solvents with SPP > 0:8 and from its 1 Lb state in solvents with SPP < 0:8. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction The indolyl chromophore in tryptophan is especially important in UV–Vis spectroscopic studies on proteins as it governs the near-ultraviolet circular dichroism and absorption spectra of many proteins [1]. The spectral features of this chromophore differ among proteins by the influence of the environment of tryptophanyl side chains varying from completely hydrophobic to hydrophilic. The most salient changes induced by perturbations of the molecular environment in the spectroscopic features of this chromophore are marked destructuring of its UV–Vis absorption spectrum and an anomalously large Stokes shift.
*
Corresponding author. Fax: +34-91-3974-187. E-mail address: [email protected] (J. Catalan).
The poor solubility of tryptophan in non-polar solvents have propitiated its replacement by indole as a spectroscopic model for systematizing the spectroscopic behaviour. Ever since Weber [2] and Zimmermann and Joop [3] reported excitation polarization spectra of fluorescence in the 1950s, the photophysical interpretation of this chromophore has revolved around two electronic states designated 1 Lb and 1 La , which have been assumed to undergo inversion during emission in polar solvents. The anomalously large fluorescence Stokes shift of indole and its derivatives in polar solvents has received much attention and led to the proposal of various mechanisms to account for it. Such mechanisms include those based on (a) dual fluorescence [4], (b) 1 Lb $ 1 La level inversion [5], (c) exciplex formation [6], (d) solvent rearrangement [7], (e) thermal isomerization to 2H-indole [8], (f) an increase in the dipole moment of the
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(02)01959-0
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J. Catalan, C. Dıaz / Chemical Physics Letters 368 (2003) 717–723
excited electronic state 1 La [5a,9], (g) emission from a solvated Rydberg state [10], and (h) specific interactions with the solvent [11]. Meech et al. [12] discussed a number of these mechanisms, focusing on the nature of the fluorescent state in terms of its potential relationship to an internal charge-transfer (ICT) mechanism and on the influence of the nature of such a state on the chromophore–solvent interaction. Notwithstanding, despite of the long time elapsed and the vast amount of research conducted, a number of aspects of the photophysics of these major chromophores continue to be controversial, even though the presence of the 1 Lb and 1 La states is still widely endorsed. Many available models assume the presence in these chromophores of two excited electronic states of rather different polarity and hence that their stability can be influenced by the polarity of the environment. This justifies the continual attempts at establishing linear relations between the indolyl chromophore and so-called Ôpolarity functionsÕ based on the dielectric constant and refractive in-
dex of the medium. It should be noted that, while Lumry et al. [6b] and Tatischeff et al. [13] found no significant relationship between the Stokes shift and polarity functions, several other authors have reported good correlations [5b,9a,9c,12,14]. Worth special note here is the work of Lami and Glasser [15], who, using the solvatochromic model of Amos and Burrows [16], studied the variation of the frequency of the emission maximum for indole in 11 solvents (n-heptane, di-isopentyl ether, di-n-butyl ether, diethyl ether, ethyl acetate, ethyl formate, acetonitrile, n-butanol, methanol, water, and glycerol) against the polarity function f ðD; nÞ and, somewhat to their surprise, observed that the 1 Lb $ 1 La level inversion proposed by several authors [5,10,12] was apparent from the plot. As can be seen from Fig. 1, the first seven solvents, which are non-protic, exhibit a linear trend from which the other four deviate; these latter are hydroxyl compounds, which raises the question as to whether their deviation may be a result of their acidity because indole is known to interact with H-bond donors [17].
Fig. 1. Plot of the emission maximum for indole vs. the polarity function f ðD; nÞ (11 solvents).
J. Catalan, C. Dıaz / Chemical Physics Letters 368 (2003) 717–723
If the aromatic system of indole can act as a H-bond acceptor, preferentially at positions 3 and 4, [18] and its N–H group is a H-bond donor to such an extent that it can yield dimers from these interactions [19] or complexes with other H-bond donors or acceptors [14a,17,20], then the solvatochromism of this substance in solvents bearing acid or basic groups cannot be analysed in the exclusive light of a polarity function f ðD; nÞ criterion, which is based on the assumption that such specific contributions must be negligible relative to the effect of solvent polarity. In this Letter, we examine the solvatochromism of the indolyl chromophore in order to assess the specific solvatochromic contributions involved and subtract them with a view to determining the general contribution to the solvatochromism, which will allow us to confirm a potential level inversion.
719
The corresponding SPP, SB, and SA values for the 48 media studied are gathered in Table 1. To obtain the fits as a function of SA, SB, and SPP of the different mixtures we use a multiparametric analysis software. It provides an equation that has SA, SB, and SPP as variables. The program used is MI N I T A B program.
3. Results and discussion The maximum of the fluorescence band for indole in the gas phase and the 26 pure and 21 mixed solvents studied in this work, see Table 1 can be described in terms of the pure solvent scales SPP [21], SB [22], and SA [23], using the following expression: m~em =kK ¼ ð2:566 0:399ÞSPP ð2:656 0:217ÞSA
2. Experimental section All solvents used were of the highest available purity and purchased from Merck in Uvasol or similar grade. Solvent mixtures were prepared from freshly opened bottles, using Brand II 25.00 mL burettes to transfer the liquids. UV–Vis measurements were made on a Shimadzu 2100 spectrophotometer the monochromator of which was calibrated by using the 486.0 and 656.1 nm lines from a deuterium lamp. The instrument was routinely checked for wavelength accuracy by using holmium oxide and didymium filters. All spectral measurements were made at 25 °C, using a matched pair of quartz cells of 1 cm light path. Emission spectra corrected for instrument sensitivity were obtained on an Aminco-Bowman AB2 spectrofluorimeter using a continuous (CW) 150 W xenon lamp for steady-state spectra. Polarity (SPP), basicity (SB), and acidity (SA) values were obtained from the wavenumbers of the absorption maxima for the following probe/homomorph couples: 2-dimethylamino-7-nitrofluorene/ 2-fluoro-7-nitrofluorene, 5-nitroindoline/1-methyl5-nitroindoline, and o-tert-butylstilbazolium betaine dye/o,o0 -di-tert-butylstilbazolium betaine dye.
ð0:465 0:299ÞSB þ ð34:593 0:188Þ
ð1Þ
with n ¼ 48, r ¼ 0:973, sd ¼ 0:348 kK, and F ¼ 262. Based on this equation, the bathochromic shift in the emission of indole will increase with increasing solvent acidity and, to a lesser extent, with increasing basicity. From Eq. (1) it also follows that the contribution of solvent acidity to the solvatochromism of indole is quite substantial: )1.6 kK in methanol and )2.8 kK in water, for example. By subtracting the specific contributions, the contribution of the general solvent effect can be estimated to be m~em =kKðgeneralÞ ¼ m~em þ 2:656SA þ 0:465SB: ð2Þ Fig. 2 shows the general contribution of each solvent to the solvatochromism of indole, m~em ðgeneralÞ, as a function of the solvent polarity (expressed as SPP). The plot provides the first clear evidence of two linear trends, namely; one of very gentle slope that encompasses the solvents with SPP < 0:8 and the other, with a much steeper slope, which includes the solvents with SPP > 0:8. This clearly indicates that indole emits from two different
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Table 1 Wavenumbers of the 1 Lb (0–0) absorption ð~ mabs Þ and the emission maximum ð~ mem Þ of the indole, as well as the SPP, SB, and SA values for the 48 media studied Solvent
m~abs =kK
m~em =kK
SPP
SB
SA
Gas Perfluoro-n-hexane Perfluoro-n-hexane/n-hexane Xperfluoro-n-hexane ¼ 0.95 0.90 0.80 0.70 0.60 0.55 2-Methylbutane n-Pentane n-Heptane Cyclohexane Decaline Diisopentyl ether Di-n-butyl ether Diethyl ether Ethyl acetate 1,4-Dioxane/water Xwater ¼ 0.4 0.5 0.6 0.7 0.8 0.9 Ethyl formate tert-Butanol Formamide 1-Butanol Tetrahydrofuran Ethanol Methanol Acetonitrile Trifluoroethanol N-Methyl formamide 1,2-Ethanediol Glycerol N,N-Dimethylformamide N,N-Dimethylacetamide Ethanol/water Xwater ¼ 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Water Dimethyl sulfoxide
35.174 35.092
33.842 33.722
0.000 0.214
0.000 0.057
0.000 0.000
35.080 35.061 35.029 34.996 34.960 34.943 34.843 34.841 34.821 34.795 34.755 34.715 34.716 34.749 34.772
33.761 33.677 33.572 33.618 33.450 33.468 33.552 33.552 33.348 33.396 33.433 33.186 33.062 32.927 32.889
0.255 0.292 0.342 0.389 0.424 0.436 0.479 0.507 0.526 0.557 0.574 0.626 0.652 0.694 0.795
0.056 0.056 0.056 0.056 0.056 0.056 0.053 0.073 0.083 0.073 0.056 0.626 0.637 0.562 0.542
0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
34.725 34.723 34.735 34.741 34.772 34.826 34.791 34.732 34.745 34.719 34.692 34.743 34.772 34.821 35.073 34.686 34.714 34.708 34.663 34.647
31.611 31.097 31.054 30.689 30.637 29.683 32.526 31.519 30.055 30.849 32.554 30.915 30.769 31.728 29.936 30.849 30.416 30.116 31.094 31.011
0.806 0.844 0.876 0.882 0.900 0.914 0.812 0.829 0.833 0.837 0.838 0.853 0.857 0.895 0.908 0.920 0.932 0.948 0.954 0.970
0.463 0.449 0.451 0.456 0.442 0.324 0.490 0.928 0.414 0.809 0.591 0.658 0.545 0.286 0.107 0.590 0.534 0.309 0.613 0.650
0.499 0.501 0.514 0.562 0.686 0.829 0.000 0.145 0.674 0.341 0.000 0.400 0.605 0.044 0.893 0.444 0.565 0.618 0.031 0.028
34.752 34.761 34.768 34.776 34.784 34.793 34.810 34.841 34.923 34.994 34.583
30.709 30.644 30.661 30.521 30.501 30.464 30.270 29.912 29.472 29.127 30.630
0.845 0.851 0.862 0.872 0.882 0.888 0.899 0.907 0.938 0.962 1.000
0.649 0.634 0.615 0.596 0.543 0.512 0.459 0.407 0.236 0.025 0.647
0.497 0.521 0.548 0.568 0.593 0.659 0.727 0.798 0.963 1.062 0.072
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Fig. 2. Plot of the ð~ mem ÞðgeneralÞ (d) and ð~ mabs Þ(0–0, 1 Lb ÞðgeneralÞ ðsÞ for indole vs. the SPP parameters of the 48 media studied.
electronic states and that it possesses a rather different polarity depending on whether the polarity of the medium is greater or smaller than 0.8SPP units. The 0–0 component of the 1 A ! 1 Lb transition of indole in the 48 media studied, see Table 1, is described in terms of the pure solvents scales by the following equation: ð~ mabs Þ=kKð0–0; 1 Lb Þ ¼ ð0:448 0:071ÞSPP þ ð0:197 0:038ÞSA ð0:167 0:053ÞSB þ ð35:147 0:033Þ
ð3Þ
with n ¼ 48, r ¼ 0:892, sd ¼ 0:062 kK, and F ¼ 58. This equation allows one to rule out specific contributions to the solvatochromism of indole and hence to determine the general contribution to the solvatochromism of this component of indole absorption: ð~ mabs Þð0–0; 1 Lb ÞðgeneralÞ ¼ ð~ mabs Þð0–0; 1 Lb Þ 0:197SA þ 0:167SB:
ð4Þ
Fig. 2 also shows the variation of the general solvatochromic contributions with the polarity (SPP) of the studied solvents. As can be seen, the behaviour of the general solvatochromism of the 0–0 component of the 1 A ! 1 Lb transition towards SPP is clearly parallel to that of the general solvatochromism of indole emission in the group of solvents with SPP < 0:8; this clearly indicates that indole emits via the 1 Lb ! 1 A transition in these solvents. The behaviour of both solvatochromisms is scarcely dependent on solvent polarity and this suggests that the dipole moment must change very little in this transition. This is quite consistent with the small value of Dl (0.14D) obtained by Lombardi and his coworkers [24] from the Stark effect for indole in the gas phase. It should be noted that Kawski [25] using the thermochromic method, also obtained a small change in dipole moment (1.04D) for indole in ethyl acetate. Clearly, in solvents with SPP > 0:8, indole emits from a different state that is much more polar than its ground state [in cyclohexane, lðS0 Þ ¼ 1:93
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0:04D ; [26] the emission must thus be assigned to the 1 La ! 1 A transition. Because the solvatochromic scales were developed using the gas phase as reference, by extrapolating to SPP ¼ 0 one can locate the emission maxima for the gas phase in the 1 Lb and 1 La states. From Fig. 2 it follows that 1 La must lie about 3500 600 cm1 above 1 Lb . That 1 La should lie so well above 1 Lb is consistent with the experimental evidence obtained by Jortner and his coworkers [27] using the free-jet technique: they found a vibrational excess of 2200 cm1 above the 0–0 component of 1 Lb to be assignable to no peak for 1 La . Also, theoretical calculations have shown the gap to be about 2500 cm1 wide [28]. The results of Fig. 2 clearly show, for the first time, that an increased solvent polarity (SPP) can induce the inversion of the 1 Lb and 1 La electronic states in indole and, also, that the pure solvent scales are highly powerful tools for studying the solvatochromism of chromophores.
Acknowledgements The authors are grateful to SpainÕs DGICYT for funding this research within the framework of Project BQU2002-02106.
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