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The Journal of Physical Chemistry, Vol. 83, No. 24, 1979
Ibemesi and El-Bayoumi
Interaction of Amine-Hydrocarbon Exciplexes with Polar Molecules J. A. Ibemeslt and M. Ashraf El-Bayoumi* Chemistry and Biophysics Departments, Michigan State University, East Lansing, Michigan 48824 (Received June 14, 1979) Publication costs assisted by Michigan State University
The fluorescence properties of 2,6-bis(N,N-dimethylaminomethyl)naphthalene(NMA) have been studied in media of different polarities in order to examine the nature of the interaction of intramolecular aminehydrocarbon exciplexes with polar molecules. Fluorescence spectra were obtained at room temperature in pure polar and nonpolar solvents and also in methylcyclohexane and diethyl ether solutions containing small amounts of the following polar species: methyl cyanide (MeCN),N,N-dimethylformamide (DMF),dimethyl sulfoxide (Me2SO), and hexamethylphosphoramide (HMPA). NMA shows intramolecular exciplex fluorescence only in media of moderate to high bulk dielectric constant and/or donicity (nucleophilic strength). Exciplex fluorescence is observable in hydrocarbon solvents only in the presence of small amounts of polar species, with intensity that appears dependent on the donicity of the latter. Red shifts in the exciplex fluorescence emission maximum observed in ether solutions containing small amounts of polar species follow the order HMPA = MezSO > DMF > MeCN (at equal amounts) and the order HMPA > MezSO = DMF > MeCN (at constant bulk dielectric constant), the former also corresponds to the order of donicity. Our observations are believed to arise from the interaction of the exciplex with polar molecules in the following manner: short-range specific interaction that appears dependent on the donicity of the polar species, intermediate-range interaction (“localized”orientation polarization) probably due to aggregation of polar species, and long-range interaction (“bulk” orientation polarization) governed by the bulk dielectric constant of the medium.
Introduction The fluorescence energies, intensities, and decay times of amine-hydrocarbon exciplexes are known to decrease with increase in solvent polaritie~.l-~The decrease in intensity and lifetime has been attributed to the formation of solvated ion pairs and/or dissociated radical ions. The red shift in the exciplex emission band with increasing solvent polarity is generally rationalized on the basis of orientation polarization interaction (bulk dielectric effect). This consideration excludes any specific interaction between the dipolar exciplex and the surrounding polar solvent molecules. In contrast to the above view, the formation of stoichiometric multicomponent complexes containing one exciplex moeity and one or more polar molecules has been proposed.44 The specificity of the interaction is based on the observed red shift and change in shape of exciplex bands on addition of small amounts of polar molecules to hydrocarbon solutions of the excited dipolar species. These observations were criticized7 as merely arising from the change in the dielectric constant of the nonpolar media on addition of polar molecules. However, it seems reasonable that highly dipolar excited species like amine-hydrocarbon exciplexes (with a localized positive charge on the amine nitrogen and a diffuse negative charge on the aromatic hydrocarbon) could interact in a specific manner with polar molecules, particularly if the latter are nucleophiles with moderate to high donicities. Donicity is a measure of the nucleophilic properties of a molecular species and is defined as the negative AH value in kcal mol-l for the 1:l adduct formation between a reference acid (SbC15) and the donor in a quasi-inert solvent, 1,Z-dichloroethane.s It has been shown that the dissociation constants of some ionic compounds in polar solvents of similar bulk dielectric constants (6) are dependent on the donor number (DN) of the polar solvent. For e ~ a m p l e , ~ the dissociation constants of quinuolidinium chloride (QHC1) are much lower in acetonitrile (e = 37.5, DN = 14.1) and nitromethane ( e = 36.67, DN = 2.7) than in t Midland Macromolecular Institute, Midland, MI 48640.
0022-3654/79/2083-3 142$0 1.OO/O
NJV-dimethylacetamide, DMA ( E = 37.78, DN = 27.8), and N,N-dimethylformamide, DMF ( E = 37, DN = 26.6), even though these solvents have similar bulk dielectric constants. Also, QHCl is equally strongly dissociated in DMF or DMA as in 1,2-propanediolcarbonate, PDC (6 = 65, DN = 15.1) even though the latter solvent has a much higher dielectric constant. However, QHCl exhibits a higher dissociation constant in PDC than in acetonitrile although both solvents have essentially the same nucleophilic properties. This was ascribed to the higher Born solvation energy contribution in PDC which has a considerably higher dielectric constant. It does seem that both forms of interaction (specific and orientation polarization) could be strongly coupled, thus making any sharp distinction very difficult. For example, in the case of an exciplex in low dielectric (hydrocarbon) medium, addition of small amounts of highly dipolar molecules could lead to aggregation of the latter.lOJ1Such aggregates will most probably be formed around the exciplex, and may lead to “local” dielectric effects. Consequently, a complete neglect of dielectric effects in exciplex interaction with polar molecules does not appear tenable, as some previous works have tended to This work deals with the interaction of the intramolecular exciplex of 2,6-bis(N,N-dimethylaminomethyl)naphthalene (designated as naphthylmethylamine, NMA)
NMA
with a number of polar species. The objective is to demonstrate the involvement of 1:n complex formation and/or “local” dielectric effects giving rise to red shifts of the emission maxima of the amine-hydrocarbon exciplexes as the polarity of the medium is increased. Experimental Section Synthesis. 2,6-Bis(N,N-dimethylaminomethyl)naphthalene (NMA) was prepared by treating 2,6-bis(bromomethy1)naphthalene with dimethylamine. The resulting
0 1979 American Chemical Society
The Journal of Physical Chemlstry, Vol. 83, No. 24, 7979 3143
Amine-Hydrocarbon Exciplexes
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Flgure 1. Room temperature fluorescence spectra of NMA in solvents of different polarities: (1) 3MP, (2) isopropyl alcohol, (3) methanol, (4) methylene chloride.
TABLE I: Exciplex Fluorescence Quantum Yields Emission Maxima in Different Solvents dielectric hh?max, hEmaj', solvent constant nm nm 346 3-methylpentane 2.020 458 2.209 dioxane 429 4.235 diethyl ether 455 methylene chloride 8.93 480 17.51 339 1-butanol 343 491 24.55 ethyl alcohol 339 498 methyl alcohol 32.70 -500 acetonitrile 38.80
and
WAVELENGTH (nrn)
Flgure 2. Onset and growth of fluorescence of Intramolecular exciplex in NMA (1 X lo4 M in methylcyclohexane) In the presence of varying amounts of DMF (moles/lker), - (01, (0.07),-e-.(0.14), (0.49), A,, 290 nm.
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0.020 0.015 0.017 0.011 0.007 0.004
0.010
amine salt was dissolved in dilute HC1, followed by addition of dilute NaOH solution to recover the free amine. The molecular formula of NMA was confirmed by elemental analysis. Its structure was elucidated by NMR and mass spectra. Solvents. Spectroquality methylene chloride, chloroform, methyl alcohol, methylcyclohexane, and 1-butanol were obtained from Mallinckrodt Chemical Works. Diethyl ether was dried over lithium aluminum hydride and distilled (when needed) through a 1-m vacuum-jacket column. Ethanol was fractionally distilled through a 1-m jacket column to remove benzene. 3-Methylpentane (3MP) was purified by the method of Potts." Polar Solutes. Spectroquality acetonitrile, p-dioxane, methyl sulfoxide, and N,N-dimethylformamide were obtained from Matheson Coleman and Bell. Hexamethylphosphoramide was purchased from Aldrich Chemical Co. and was used without further purification. Spectral Measurements. The absorption spectra were obtained by means of a Cary 17 spectrophotometer. An Aminco Bowman spectrophosphorimeter was used to obtain fluorescence spectra. Exciplex Fluorescence Quantum Yields. Quinine sulM in 0.1 N H2S04)was used as a standard fate (1 X to determine the relative quantum yields of intramolecular exciplex fluorescence of NMA in organic solvents of different polarities. A quantum yield of 0.55 was assumed for the standard.13 Uncorrected fluorescence spectra were used because of the close overlap of the spectrum of the standard with that of the exciplex. However, refractive index correction was made since different solvents were used.
Results Absorption and Emission Properties of 2,6-Bis(N,IV-dimethylaminomethy1)naphthalene i n Different Media. Room temperature absorption spectra of NMA in solvents of different polarities are similar; however, the fluorescence emission spectra are significantly different (Figure 1). The following trends are observed (see Table I): (i) No exciplex
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The Journal of Physical Chemistry, Vol. 83, No. 24, 1979
Ibemesi and El-Bayoumi
48 -
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Figure 4. Ratio of intensities of monomer and exciplex fluorescence as a function of bulk dielectric constant: ( 0 )DMF, (A)MeCN.
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Figure 6, Exciplex spectra shift as a function of concentration of polar molecule: ( 0 )DMF, (0)HMPA, (W) Me2S0, (A)MeCN. I
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Figure 7. Exciplex spectral shift as a function of dielectric constant of medium: (0)HMPA, ( 0 )DMF, (H) Me,SO, (A)MeCN.
350
460
560
650
WAVELENGTH (nm) Figure 5. Fluorescence spectra of NMA in ether, in the presence of varying amounts of Me,SO (moles/liter): - 0.0 (a), 0.1 1 (b), 0.31 (c); 0.02 (a), 0.16 (b), 0.56 (c); 0.04 (a), 0.21 (b), 0.72 (c); 0.07 (a), 0.26 (b), 0.94 (c);...s 0.09 (a), 0.31 (b), 1.18 (c). A, 290 nm.
---
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case of MeCN; this trend is reversed for t >2.525. Approximately 0.08 M of Me2S0 produced the same effect as 0.07 M of DMF. Higher concentrations of Me2S0 remained immiscible in methylcyclohexane, and thus proratio. duced no further change in the IE/IM Interaction of NMA Exciplex with Polar Solutes in Ether. The absorption and fluorescence spectra of dilute M) of NMA in ether were taken in the solution (1 X presence of small amounts of the following polar molecules: dioxane, methyl cyanide (MeCN), N,N-dimethylformamide (DMF), dimethyl sulfoxide (Me2SO), and hexamethylphosphoramide (HMPA). Addition of the polar molecules caused no detectable change in the absorption spectra. However, with the exception of dioxane, the presence of the polar molecules caused significant red shifts in the exciplex emission maximum. This is illustrated in Figure 5 for the case of Me2S0. Isoemissive points are observed in all cases, but they do not appear to be real since they undergo a shift to longer wavelength with increasing concentration of the polar solute. The exciplex
band appears to undergo some broadening as the concentration of the polar solute is increased. Also, exciplex fluorescence intensity undergoes substantial decrease only a t fairly high concentrations (21.0 M) of the polar solute. Plots of exciplex spectral shifts, AP (cm-I), as a function of concentration of polar molecules (Figure 6) indicate varying degrees of red shifts in the order HMPA = MezSO > DMF > MeCN. Acetonitrile causes no spectral shifts for concentrations less than 0.02 M. The effect of dioxane is very slight, producing a red shift of -3.6 nm a t a concentration of 0.4 M (compared to 30 nm for DMF). Plots of exciplex spectral shifts, AP (cm-I), as a function of bulk dielectric constant, t, are shown in Figure 7. At any given e value, the magnitude of the red shift increases in the order HMPA > Me2S0 == DMF > MeCN. MeCN causes spectral shifts only above t -4.5. Also, plots of exciplex fluorescence energy vs. donor number of polar molecules (Figure 8) show that the extent of red shift is in the order HMPA = MezSO > DMF > MeCN.
Discussion The foregoing observations on the fluorescence behavior of 2,6-bis(N,N-dimethylaminomethyl)naphthalene in different media may be explained by considering the contributions of the following molecular properties: bulk dielectric constant, donicity, and “local” dielectric effect. The observance of an exciplex emission in NMA in moderate to highly polar solvents may be explained in the same way as for the case of /3-(dimethylamino),P-(methylamin0)na~hthalene.l~ Because of the asymmetric location of the donating orbital of the amino group with respect
The Journal of Physical Chemistry, Vol. 83,
Amine-Hydrocarbon Exciplexes
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16
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32
40
DONOR NUMBER (DN)
Figure 8. Exciplex spectral shift as a function of donor number of polar molecule. Concentration of polar molecule (moles/liter): ( 0 )0.05, (A)
0.29.
to the accepting orbital of the aromatic group, the stabilizing energy of the charge transfer state due to Coulombic forces may be small. However, the large dipole moment of the CT state makes it effectively stabilized by solvation, leading to the observance of exciplex fluorescence in polar media. Such additional stabilization is absent in a hydrocarbon solvent. The question that remains is the exact nature of the electrostatic interaction between the excited dipolar species and the surrounding polar molecules. Our experimental data suggest that these interactions could be short range (dependent on donicity), “intermediate” range (due to localized dielectric effect), and long range (determined by bulk dielectric constant). The observed decrease in exciplex fluorescence quantum yields in NMA with increase in solvent bulk dielectric constant seems to reflect longrange (orientation polarization) interactions. However, the high quantum yield of exciplex fluorescence in dioxane, despite its low bulk dielectric constant (t = 2.209), clearly suggests the involvement of other molecular property such as nucleophilicity. Evidence for this is amply provided by the observed effects of small amounts of polar molecules on the excited state fluorescence properties of NMA in methylcyclohexane and ether. Thus, intramolecular exciplex fluorescence becomes observable in the former solvent at such low concentrations of added polar molecules that no significant changes in bulk dielectric constant of the medium would be expected. Also, the observation of a higher Z E / Z ratio ~ for DMF ( t = 37, DN = 26.6) than for MeCN (t = 37.5, DN = 14.1) is in accord with their donicities. On the basis of bulk dielectric effect both polar molecules will be expected to show similar intensity ratios. The interactions in ether again demonstrate the influence of donicity, and hence the specificity of exciplex interaction with the added polar species. Considering bulk dielectric effect, addition of equal amounts of MeCN, DMF, MezSO ( t = 46.68, DN = 29.8), and HMPA ( e = 30, DN = 38.8) would give red shifts in the order MezSO >> MeCN = DMF > HMPA. However, the observed order is HMPA = MezSO > DMF > MeCN, which essentially is the same order of their nucleophilic strength. Also, addition of different amounts of these polar molecules to obtain the same bulk dielectric constant produced red shifts in the order HMPA > MezSO = DMF > MeCN,
No. 24, 1979 3145
which is strictly in the increasing order of their donicity. This correlation of the red shift of exciplex emission maximum with donicity of the polar molecule appears to be a strong indication for the specific nature of interaction with polar species. The behavior of our system seems to lead to loose complex formation (with 1:n stoichiometry) since there is no real isoemissive point and no detectable change in exciplex band shape. However, the influence of dielectric constant does not appear altogether absent even at low concentrations (20.05 M) of added polar molecule. This seems to explain the closeness of the red shifts caused by MezSO to those of HMPA (at equal amounts of each). The much higher polarity of Me2S0 appears to have compensated for its much lower donicity. Furthermore, the observance in ether ( t = 4.3, DN = 19.2) of red shifts in exciplex fluorescence emission maximum on addition of small amounts of MeCN ( e = 37.5, DN = 14.1) points to some influence of dielectric constant even at low concentrations. If the red-shift occurs only by specific interaction which in turn appears dependent on the donicity of the polar molecule, then no additional red shifts would have been observed upon addition of small amounts of MeCN. The added polar molecules could form aggregates around the highly dipolar exciplex in the comparatively nonpolar solvent.lOJ1Aggregation of polar molecules will increase with increase in their concentration and this may lead to a localized dielectric effect on the central dipole (exciplex). Such “local” dielectric effect will gradually lead to a bulk dielectric effect at very high concentration of the polar molecules. This seems to explain the continuous lowering of the exciplex energy (with an increase in the concentration of polar molecules) to a final value corresponding to that of the pure polar solvent. In conclusion, it appears that the red shift in exciplex emission maximum observed in polar media involves both specific interaction and orientation polarization interaction (bulk dielectric effect). The contribution of the former will probably depend on the nucleophilic properties (donicity) of the solvent, and will be effective at short range, preceding the dielectric effect. Also, the observed red shift in exciplex emission maximum of NMA in the presence of small amounts of a polar molecule in a nonpolar medium appears explicable on the basis of both specific interaction and “local” dielectric effect. The latter is believed to result from aggregation of polar molecules in a nonpolar medium.
References and Notes (1) N. Mataga, T. Okada, and N. Yamamoto, Chem. Phys. Lett., 1, 119 (1967). (2) H. Knibbe, D. Rehm, and A. Weller, Z. Phys. Chem. (Frankfurtam Main), 56, 95 (1967). (3) H. Beens, H. Knibbe, and A. Weller, J. Chem. Phys., 47 1183 (1967). (4) E. A. Chandross and H. T. Thomas, Chem. Phys. Lett., 9, 397 (1971). (5) E. A. Chandross, “The Exciplex”, M. Gordon and W. R. Ware, Ed., Academic Press, New York, 1975, p 187. (6) G. S. Beddard, S. E. Carlin, and C. Lewis, J . Chern. SOC.,Faraday Trans. 2, 71, 1894 (1975). (7) B. K. Selinger and R. J. McDonald, Aus. J . Chem., 25, 897 (1971). (8) V. Gutman, Chem. Tech., 255 (1977). (9) U. Mayer, Coord. Chem. Rev., 21, 159 (1972). (10) R. M. Fuoss, J . Am. Chem. Soc., 56, 1031 (1934). (11) C. Treiner, J. F. Skinner, and R. M. Fuoss, J . Phys. Chem., 68, 3406 (1964). (12) N. S. Potts, J . Chem. Phys., 20, 809 (1952). (13) C. A. Parker and W. T. Rees, Analyst, 85, 587 (1960). (14) J. A. Ibernesi, M. Ashraf Ekyoumi, and J. B. Kinsinger, Chem. mys. Lett., 53, 270 (1978).
