CHARGE-TRANSFER ABSORPTION AND LUMINESCENCE SPECTRA OF PYRIDINE SALTS
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Charge-Transfer Absorption and Luminescence Spectra of Alkyl Halide Salts of Pyridine'"
by J. S. Brinen, J. G. Koren, H. D. Olmstead,lb and R. C. Hirt American C?.jaamid Company, Central Research Division, Stamfmd, Connecticut
(Received May 4 , 1966)
The long wave length absorption bands of alkyl halide salts of pyridine have been studied &s a function of solvent and temperature. These systems luminesce brightly in the visible region at - 196'. The large separation between the absorption and luminescence bands is discussed in terms of solvent effects and the Franck-Condon principle.
Introduction The electronic absorption spectra of N-alkyl iodide21a salts of nitrogen heterocyclic molecules exhibit long wave length absorption bands which are not present in the parent molecule. These bands are very sensitive to solvent and temperature changes and have been attributed, by Masonla as being due to the transition of an electron from the anion to the cation within a solvated ion pair, ie., (N-methylpyridinium) + iodidesolvent. Further support for the assignment of these bands to charge-transfer transitions is that for pyridine methiodide in CHCl,; the maxima of the two long wave length bands are separated by 7950 cm.-'. This agrees with the separation between the 2P,/* and 2P1/1 states of the iodine atom (7600 ~ m . - ' ) . ~ Masons concentrated on varying the N-heterocyclic nucleus while using CHaI for salt formation and CHCla as a solvent and concluded that the transition energy followed closely a delocalized model for the excited state. I n the present investigation, the heterocycle is kept constant (pyridine) while the solvent and the alkyl halide are varied. Considerable attention is focused on the luminescence properties of these systems and on the behavior of the absorption spectrum as solvent and temperature are varied. Experimental Section Absorption measurements were performed on a Cary Model 14 spectrophotometer using spectrograde alcohol, CH2C12, and distilled water as solvents. Variable temperature-absorption experiments were obtained using a variable temperature cell specifically designed for use with the in~trument.~
Total luminescence spectra have been measured for the methiodide, methbromide, and ethiodide salts of pyridine at -196" in various solvents. The samples were excited by ultraviolet radiation from an H-3 mercury arc (quartz envelope) through a 5-mm. thick Corning 9863 filter. A complementary filter, 10 mm. of dilute aqueous NaN02 solution, was placed before the entrance slit of a modified Warren Electronics Spectracord used to record the luminescence spectra. The spectra are uncorrected for the response of the RCA 1P28 photomultiplier. Luminescence measurements were performed under higher resolution conditions for some of these systems, but no new features were observed in the spectra obtained. The alkyl halide salts were prepared by standard methods described in the literature.5
Spectroscopic Observations The effect of solvent polarity on the absorption spectrum of pyridine methiodide may be seen in Figure 1. I n CHzCl2 two low energy absorption bands are observed (as in CHC13)with maxima at 27,100 and 34,400 cm.-l, a separation of -7500 cm.-l. (1) (a) Presented in part by J. 9. Brinen at the 144th National Meeting of the American Chemical Society, Los Angeles, Calif., April 1963; (b) Summer Careers Program, 1964. (2) (a) A. Hantzch, Ber., 44, 1776, 1783 (1911); 52, 1535, 1544 (1919); (b) E. M. Kosower, J . Am. Chem. SOC.,80, 3253, 3261, 3267 (1958). (3) 9. F. Mason, J . Chem. Soc., 2437 (1960). (4) J. G. Koren, J. S. Brinen, and R. C. Hirt, AppE. Opt.,3, 1431 (1964). (5) M. Smith and M. C. R. Symons, Discuasions Faraduy Soc., 24, 206 (1957).
Volume 69, Number 1 1 Novaber 1.966
J. BRINEN,J. KOREN,H. OLMSTEAD, AND R. HIRT
3792
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Figure 3. Temperature effects on the C.T. absorption bands of pyridine methiodide in CHzC12.
Figure 1. Absorption spectra of pyridine methiodide in: - - - CH2C4, -alcohol, and - - - water, and of pyridine methbromide in - CHzC12. The 6 values for the C.T. bands are only apparent because of deviations from Beer's law.
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Figure 4. C.T. absorption spectrum of pyridine methiodide in alcohol as a function of temperature (left side). The right side shows the luminescence spectrum of a similar solution a t - 196".
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Figure 2. C.T. absorption spectrum of pyridine methiodide in CH2C12upon successive addition of alcohol.
In alcohol, there is only one low energy absorption band, and in water this band disappears entirely. These bands will be referred to as charge-transfer (C.T.) absorption bands. Two additional absorption bands are observed in the region between 37,000 and 50,000 cm.-l in alcohol and water solutions. The band centered at about 40,000 ern.-' is due to absorption from the pyridinium ion and agrees well with an absorption spectrum of pyridinium perchlor~ite.~In CHzCL this absorption is partially hidden under a stronger absorpkion band due to the transition between the iodide ion and the solThe Journal of Physdcal Chemistry
ventZbe5and causes an apparent increase in the molar absorptivity, e, of the pyridinium absorption. The absorption band involving the iodide ion and the solvent is shifted to higher energy in alcohol and water (43,000cm. -l), and the characteristic pyridinium absorption may be readily observed at 38,000 crn.-l. For the CH3Br salt in CHzClzthe transition between Br- and the solvent is shifted to higher energies, and the pyridinium absorption is readily observed. The gradual shifting of the two low energy absorption bands with solvent polarity may be seen from Figure 2. Successive amounts of alcohol are added to a CHzClz solution of pyridine methiodide. The absorption is shifted to higher energies as the proportion of alcohol is increased, and the band originally at 34,400 cm. -l gradually disappears under the higher energy pyridinium absorption band.
CHARQE-TRANSFER ABSORPTION AND LUMINESCENCE SPECTRA OF PYRIDINE SALTS
I
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Figure 5. Absorption spectrum of pyridine methbromide in CHpClt aa a function of temperature.
The effect of temperature on the absorption spectrum of pyridine methiodide in CH2C1, and in alcohol is shown in Figure 3 and the left side of Figure 4. As the temperature is lower, the C.T. absorption bands in CH2C12are shifted to higher energies, a shift for both bands of -975 cm.-' at -32". Continued cooling results in a larger shift, and at - 196" (sample is opaque) the original yellow solution appears colorless. Cooling an alcohol solution of pyridine methiodide does not produce a large shift in the absorption energy but,does result in considerable enhancement of C.T. absorption of the lower temperatures. Pyridine ethiodide behaves in a similar manner with respect to solvent and temperature variations. Pyridinium methbromide exhibits a similar low energy band only in CHzCl2 (or CHCla) solution. This appears as a long wave length shoulder on the stronger pyridinium transition (at -38,000 cm.+). In the more polar solvents, this C.T. band is hidden under the pyridine transition. Figure 5 shows the spectrum of pyridinium methbromide in CHzClz as a function of temperature. As the temperature is decreased, the long wave length shoulder (C.T. band) disappears under the stronger pyridine band. At +40", the C.T. absorption is slightly more pronounced and is at lower energy than at room temperature. The charge-transfer bands examined in this study do
3793
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Figure 6. Luminescence spectra of some alkyl halide The intensity salte of pyridine in water at - 196'. scale is not the same for the three molecules.
not obey Beer's law in the concentration range studied (to 10-4 MI. When these solutions are frozen to -196" and exposed to ultraviolet radiation, they were found to luminesce strongly; the iodide salts appear to be yellowish orange and the methbromide a greenish yellow. The emissions from these materials are broad and featureless. For alcohol solutions, the intensity maximum for the CHJ salt luminescence is at -18,200 cm.-l; for the C2H61salt the maximum is -18,000 cm.-' while the CH3Br salt is at somewhat higher energies of -19,400 cm. -I. Luminescence spectra in CH2Clzare quite similar. Lifetime measurements using a mechanical phosphoroscope capable of fractional millisecond resolution indicate the lifetime of the luminescence is 6 sec. Although for water solutions of these salts the C.T. absorption is hidden under the pyridine absorption and is not observed in the spectrum, these solutions when carefully frozen to -196" luminesce when exposed to ultraviolet radiation. The luminescence of these salts in H2O is shown in Figure 6 and is not shifted appreciably (-100 cm.-I) from that in alcohol or CH2C12solution. Figure 3 shows the absorption and luminescence spectrum of pyridinium methiodide in alcoholic soluvolume 69. Number 11 November 1966
3794
tions at comparable temperatures. There is a large energy gap (-2500 cm.-I) between the onset of absorption and the onset of luminescence. The separation between absorption and emission is estimated as being somewhat smaller for CH2C12 solutions and considerably larger for water solutions. The nature of the last two solvents prevents direct measurements at -196". A search for additional absorption bands below 33,000 em.-' which might correspond to the luminescence was made in a 10 M aqueous solution of the CH31salt. No evidence of absorption was found. Discussion Solvent and temperature variation studies on the absorption spectra of pyridine methiodide agree with Mason's3 proposal that the species giving rise to the C.T. absorption is a solvated ion pair and that the transition involves a charged ground state and a neutral excited state. A charged ground state would be more stabilized by solvation in polar solvents than in solvents of lower polarity, and the charge-transfer absorption would be shifted to higher energies as the polarity increased. Increasing the alkyl chain by one carbon does not produce any appreciable difference in the absorption properties of the salt. Substituting Br- for I- shifts the charge-transfer absorption band to higher energies by about 8500 em.-' but does not seem to alter the behavior with respect to temperature and solvent variations. The shift agrees with the difference in absorption maxima of CHaBr and CH3I in the vapor phase of -9500 cm.-J.6 Since neither of the components of the salt luminesces independently and since no evidence for multiplicityforbidden absorption bands was observed, it is thought
The Journal of Physicid Chemistry
J. BRINEN,J. KOREN, H. OLMSTEAD, AND R. HIRT
that the emission observed in the experiments described here must be attributed to transitions between the same levels of the solvated ion pair which give rise to the C.T. absorption, that is, a charge-transfer fluorescence. Accepting this, the relative insensitivity of the luminescence with solvent and the large separation between the C.T. absorption and luminescence may be explained on the basis of solvent effects and the FranckCondon principle. As previously stated, the charged ground state of the solvated ion pair is very sensitive to solvent polarity. Energy differences in the transition from the ground state to the neutral excited state in different solvents primarily reflect changes in ground-state stabilization since, by the Franck-Condon principle, the solvent environment of the excited state during the transition must remain the same as that for the ground state. For the emission process, however, the molecule in the excited state has time to come to equilibrium with the solvent environment. The luminescence transition is now between states which have the solvent environment of the neutral excited state and thus might be expected to be fairly insensitive to solvent polarity. The observed energy gap reflects differences between the ground and excited states in a charged and neutral solvation environment. The role of the FranckCondon principle in solvent effects as applied to luminescence phenomena has been examined by other authors.' ( 6 ) G . Hersberg and G. Scheibe, 2.physik. Chem. (Leipsig), 87,397 (1930). (7) V. G. Krishna and L. Goodman, J . Chem. Phy8.. 33,381 (le@), and references therein.
