2012
The Journal of Physical ChemMty, Vol. 82, No. 18, 1978
E.
of similar complexes can provide a measure for the relative stabilities of two possible isomers. Acknowledgment. We thank Dr. Y. L. Chow of the Chemistry Department a t Simon Fraser University for allowing us to use his liquid N2 cryostat, which we adapted with a special sample holder for polymer films for these studies. The research reported here was supported by grants from the National Research Council of Canada to E.M.V. and K.E.R.
M. Kosower and H. Dodiuk
(6) J. N. Murreii, M. Randic, and D. R. Williams, Proc. R. SOC.London, Ser. A , 284, 566 (1963). (7) W. C.Herndon and J. Feuer, J. Am. Chem. SOC.,90,5914(1968). (8) D.D. Holder and C. C. Thompson, J. Chem. Soc.,Chem. Commun., 227 (1972). (9) H. Sakurai and M. Kira, J. Am. Chem. Soc., 97, 4879 (1975). (10) J. Prochorow, Bull. Acad. Polon. Scl., XV, 37 (1967). (11)G. Briegieb, "Elektronen-Donator-Acceptor-Kompiexe",SpringerVeriaa. Berlin. 1961. (12) K. H. kichaelian, K. E. Rieckhoff, and E. M. Voigt, Proc. Natl. Acad. Scl. U.S.A., 4196 (1975). (13) M. J. Mobiey, Master's Thesis, Simon Fraser University, B.C., Canada, 1977. (14) M. S. Sambhi, J. Phys. Chem., 77,2290 (1973). (15) R. S.Muiiiken, J. Am. Chem. SOC.,74, 811 (1952). (16) S.H. Hastings, J. L. Franklin, J. C. Schiller, and F. A. Matsen, J. Am. Chem. Soc., 75,2900 (1953). (17)T. Matsuo and H. Aiga, Bull. Chem. SOC.Jpn., 41, 271 (1968). (18) H. Kuroda, T. Amano, I. Ikemoto, and H. Akamotu, J. Am. Chem. SOC.,89,6056 (1967). (19) J. Feuer, Ph.D. Thesis, Texas Tech University, 1970. (20) B. B. Bhowmik and P. K. Srimani, Spectrochim. Acta, Part A , 29, 935 (1973).
References and Notes (1) M. W. Hanna and J. L. Lippert, "Molecular Complexes", Voi. 1, R. Foster, Ed., Eiek Science, London, 1973,Chapter 1. (2) M. J. Mobiey, K. E. Rieckhoff, and E.-M. Voigt, J. Phys. Chem., 81, 809 (1977). (3) E.-M. Voigt, J. Am. Chem. Soc., 86,3611 (1964). (4) A. Zweig, Tetrahedron Lett., 89 (1964).
(5) J. L. Lippert, M. W. Hanna, and P. J. Trotter, J. Am. Chem. Soc., 91,4035 (1969).
Intramolecular Donor-Acceptor Systems. 5. Heavy Atom Effects on Excited States of 6-N-Arylamino-2-naphthalenesulfonate Derivatives Edward M. Kosower*la,b and Hanna Dodiuk'' Department of Chemistty, Tel-Aviv University, Ramat Aviv, Tel Aviv, Israel, and the Department of Chemistry, State University of New York, Stony Brook, New York 11794 (Received March 3 1, 1978) Publication costs assisted by the U.S.-Israel Binational Science Foundation
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Heavy atoms substituted into the aryl group of 6-N-arylamino-2-naphthalenesulfonate (6,2-ANS)derivatives diminish the fluorescence quantum yield (+F) of the naphthalene-centered, Sl,np SO,np emission but do not affect @F from the charge-transfer state, the SI,& So emission. The low substituent sensitivities of fluorescence emission energies of 6,2-ANS derivatives in EPA glass at 77 K characterize them as Sl,np SO,np emissions. The phosphorescence quantum yield, +p, is augmented little and the phosphorescence lifetimes of 6,2-ANS derivatives are unaffected by such heavy atom substitutions except in the cases of 4-bromo derivatives.
-
Introduction Heavy atom effects on photophysical properties are mainly displayed through intersystem crossing rates, but also through electronic effects, as succinctly summarized by Miller et ala2In the course of our investigations on the excited state behavior of 6-N-arylamino-2-naphthalenesulfonate derivatives3-11 ( l ) ,we have utilized various inD
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tramolecular and extramolecular variations to establish the chief pathways followed by these molecules after the absorption of light. These are summarized briefly in eq 1. It was natural to utilize halogen substitution on the Ti,np
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Of the two types of fluorescence emission indicated in eq 1,the emission from the state should be weakly affected by halogen substitution, while emission from the SI,&state should be strongly affected. (np emission is naphthalene-centered and only mildly sensitive to solvent polarity; ct (charge transfer) emission involves both the N-aryl group and the naphthalene ring and is quite sensitive to solvent polarity.) Indeed, emission energies from each of the two states were altered in the expected manner. In addition, quantum yields of emission (&) were substantially changed for the np emission, but little, or not at all, for the ct emission. These divergent effects provide strong support for the scheme, including multiple fluorescences, outlined in eq 1. We describe in the present article some of the phenomenology associated with the effects of heavy atoms on the fluorescence and phosphorescence of 6,2-ANS derivatives, showing how a useful new tool is provided for probing the photophysical behavior of intramolecular donor-acceptor systems.
Results Emission energies for most of the compounds exhibited two types of behavior as a function of solvent polarity, a low slope in the low-to-moderate polarity solvents and a high slope in the high polarity solvents. We illustrate these solvent polarity effects for a series of 2-N-(6bromophenyl)
0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No, 18, 1978 2013
Intramolecular Donor-Acceptor Systems
TABLE I: Emission Data for 6-N-(X-phenyl)amino-2-naphthalenesulfonate Derivatives in Ether:Pentane:Ethanol Glasses at 77 Ka X
PIP
$P/$Fc
h a ~ , F nm , ~
hID.XJJ, e
-~
m
nm
NI1 Compound (Na' Salt) 3,5-C1, 3.4-C1, 3:F * 341 3-Br 4-Br 441 4-F H 4-CH3 4-OCH, 2,6-(CH3)*
1.97 2.56 1.00 1.83 4.70 27.0 2.34 1.10 1.00 1.03 1.17 1.63
0.059 0.077 0.030 0.055 0.14 0.81 0.070 0.033 0.030 0.031 0.035f 0.049
371 377 380 380 380 386 386 391 390 406 418 387
4-Br 4-C1 4-F H 4-CH3 4-OCH,
22.4 3.70 1.64 2.78 2.66 2.34
4-Br H 4-CH3 4-OCH,
6.35 0.18 0.18 0.18
NH Compound (N,N-Dimethylamide) 0.19 398 500, 530, 570 sh 0.0054g 400 510, 540, 570 sh 0.0054g 408 510, 540, 570 sh 0.0 0 54g 422 520, 545, 570 sh
0.24 1.6 1.5 1.4
H
1.13
N-Methyl Compound (N,N-Dimethylamide) 0.034 400 510, 540, 570 sh
1.8
N-Methg. -2rivative (Na' Sa 0.67 388 0.11 388 0.049 390 0.083 389 0.080 395 0.070 410
1
496, 525, 570 sh 496, 526, 570 sh 496, 526, 570 sh 496, 526, 570 sh 496, 526, 570 sh 496, 526, 570 sh 496, 526, 570 sh 502, 532, 570 sh 502, 532, 570 sh 502, 532, 570 sh 502, 532, 570 sh 500,520, 570 sh
1.5 1.3 1.7 1.5 0.9 0.4 1.3 1.7 1.7 1.7 1.8 1.8
500, 525 sh, 570 sh 500, 525 sh, 570 sh 500, 525 sh, 570 sh 500, 525 sh, 570 sh 500, 525 sh, 570 sh 500, 525 sh, 570 sh
0.8 1.6 1.9 1.9 1.9 1.8
for a particular compound to @ p / Ratio of a Ether:pentane:ethanol, 2:2: 5, nitrogen-purged before freezing. i.1 nm, for sodium 6-N-phenylamino-2-naphthalenesulfonate. * 576, peak height ratio, using excitation at 320 nm. Fluorescence and phosphorescence fluorescence emission maximum. e ?r 1 nm, phosphorescence emission maximum, not well resolved. g Estimated by comparison with another compound because of low phosphorescence intensity.
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TABLE 11: $ P / @ F Ratios in Various Glasses at 77 K for BN-(4-Bromophenylamino )-2-naphthalenesulfonate
75
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Flgure 1. A plot of emission maxima (in kcal/mol) vs. the solvent polarity parameter, ET(30) (in kcal/mol) for three 6- N-(4.-brornophenyl)amino-2-naphthalenesulfonate derivatives (lH, lM, lHA, X = 4-Br) in a series of dioxane-water mixtures.
derivatives in Figure 1. As noted in eq 1, a variety of evidence has been previously published demonstrating that the solvent-insensitive emission is np (naphthalenecentered) and the solvent-sensitive emission is charge transfer. Not illustrated is the previously documented point that the absorption bands of the 6,2-ANS derivatives a t 360,320-330, and 260-280 nm are almost insensitive to solvent or substitution on the N-aryl group, with the electron-withdrawingsubstituents used in the present work
glass EPA(2:2:5) Ethanol
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providing no exception to the rule.6-s Changing the 4-substituent in a 6-N-(4-halophenyl)amino-2-naphthalenesulfonate (1H) from fluoro- to chloroto bromo- markedly depresses the quantum yield of fluorescence ($F) of the np emission, without apparent effect on the $F for the ct emission (see Figure 3, ref 6). The same order of influence for the heavy atom is shown for substitution in the 3 position (lH), with $F changing in the order F > C1> Br 3 3,5-C12(Figure 2). In all cases, the $F increases markedly on proceeding from nonpolar to moderate polarity solvents (np emission) and, in almost all cases, decreases with increasing solvent polarity for the ct emission. The 3,5-dichloro derivative is an exception (lH, X = 3,5-C12),the $F remaining almost constant for the ct emission, although its overall behavior in emission resembles that of the other ANS derivatives (Figure 3). The general pattern indicated above is also found for N-methyl 6,2-ANS derivatives (1M) and for the sulfon dimethylamide derivatives (lHA), as illustrated by a plot of $F vs. solvent polarity in Figure 4A and 4B. An extensive survey was made of emissions at 77 K, mostly in EPA (ether-pentane-ethanol) glass, but also in a few glasses of different polarity. Both fluorescence and phosphorescence data are summarized in Table I for derivatives of all four types of 6,2-ANS (lH, lM, lHA, and IMA). The fluorescence maxima varied with substituent
The Journal of Physical Chemistry, Vol. 82, No. 18, 1978
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E. M. Kosower and H. Dodiuk
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in a manner characterized by a relatively small p value, from -3.0 to -4.3 depending upon the compound (Figure 5). The ratio of phosphorescence quantum yield to fluorescence quantum yield is listed in Table I as a
4a
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Flgure 5. A plot of €~/2.303RT(energiescorresponding to fluorescence maxima divided by a scaling factor for proper comparison with Hammett u constants) against the Hammett u constant. The upper line is plotted using the right-hand scale and the two lowest lines are plotted using the left-hand scale, as indicated by the arrows. To understand the numbers used, it should be remembered that the u constants characterize substituent effects at 298.16 K. We might have converted the u constants into free energy changes for direct comparison with the emission energies, but this is not the common way of presenting the comparison. The relatively low p values arise from {naphthalene-centered)emissions. The fact that the p value for the emissions from the N-methyl derivatives is appreciably lower than the p value for emissions from the NH compounds in the sulfonate series (1M vs. 1H) is in agreement with chemical expectations, with the N-methyl group diminishing electron demand upon the substituent in the N-aryl group. (Further discussion on the various states will be found in other articles in this series.)
function of substituent and in Table I1 as a function of polarity for 6-N-(4-bromophenyl)amino-2-naphthalene-
Intramolecular Donor-Acceptor Systems
sulfonate in various glasses. Only 4-bromo derivatives showed a marked enhancement in 4p for any of the compound types (three examples); the one 3-bromo derivative examined showed some augmentation of 4 p but much less than that noted for the corresponding 4-bromo compound. Similar statements could be made about phosphorescence lifetimes which are also listed in Table I. One further point about the effect of conversion of the sulfonate to the N,N-dimethylamide might be noted The 4p is lowered considerably.
Discussion We have established quite clearly that heavy atom substitution diminishes the quantum yield of fluorescence from np (Sl,np)states of 6,2-ANS derivatives but has little effect upon 4Ffrom charge-transfer (S1J states. We had previously found that intensity of excited state Tl-Tn absorption rose considerably upon substitution of a 4X= bromo for a hydrogen in a 6,2-ANS derivative (lH, 4-Br), using a laser pulse to generate the excited state. In that study, it had also been shown that the yield of triplet decreased rapidly as a function of solvent polarity.6 The pattern of ~p~ behavior as a function of solvent polarity for the np emission is in accord with these findings, since it increases with increasing solvent polarity. In other words, kiscdecreases with increasing solvent polarity, either due to increased participation of the charge-transfer state in the excited state structure or in equilibrium with the np state, or because of an increased energy gap (leading to a lower kist) between Sl,npand Tl,np,or perhaps due to a combination of these. Although it is not evident why the heavy atom effect does not operate on the Sl,,t state, the lack of heavy atom effect on 4Ffor the charge-transfer state provides a further, useful differentiation in behavior from that seen for the Sl,npstate. The generality of this distinction remains to be established for other emitting systems, but should be tried since the examination of only two or three (F, C1, Br) well-chosen halogenated derivatives can lead to significant information about the nature of the emitting state. The phosphorescence quantum yields are, for the most part, low except in the cases of 4-bromo substituted derivatives. In glycerol, a wavelength dependence for 4p showed that vibrationally excited Sl,npstates contributed to the formation of triplet statesa4 No wavelength dependence was noted in EPA glasses, implying that relaxation was faster in less structured glasses. Assuming that the radiative rate for the 4-bromo compounds was not affected by temperature, we may estimate kiscI6.5 X lo7 s-l from a knowledge of the ratio of 4 p to 4F (0.81) and the radiative rate, k,, which we have previously measured as 8.1 X lo7 s-l for the 4-bromo 6,2-ANS derivative in di0xane.l’ (We make the reasonable assumption that kic from SI,, and ki, from Tl,npstates is small; in view of the high quantum yields of emission in viscous media (e.g., glycero15v6),this assumption cannot be far from correct.) The equilibrium form of both the Sl,npand Tl,npstates is thought to have the N-phenyl group almost perpendicular to the plane of the naphthalene ring. Some conjugation of the phenyl group with the naphthalene is required to permit the spin-orbit coupling from the bromine substituent to exert an effect on intersystem
The Journal of Physical Chemistty, Vol. 82, No. 18, 1978 2015
crossing. The distant 4-bromo substituent, by the criterion of the P/F ration (Table I), exerts an influence on ki, from Sl,nponly one-fourth as great as that of the 2-bromo substituent in naphthalene on k, from the T, state.’ Admittedly, the comparison with k, for the state is less dramatic, the influence of the bromine on the 4 position of the phenyl group leading to an increase in k, of only fourfold, in comparison to the more than 100-fold increase observed on k, for the 2-bromonaphthalene.’ We may conclude from these “distant” heavy atom effects in 6,2-ANS derivatives that libration even in glasses is sufficient to permit expression of the spin-orbit coupling for heavy atoms on the N-phenyl group. The difference between 3- and 4-halo substituents is a clear indication of the difference in overlap expected for these two positions. The lower for the amides results from the greater ease with which the Sl,ctstates are formed, because of the greater electron-withdrawing power of the sulfon dimethylamide group compared to the sulfonate group.8 The low p values found for the fluorescence emissions in glasses are characteristic of np emissions in low polarity solvents (ET(30)N 40);12emission at wavelengths much shorter than those for fluid dioxane-water solutions of comparable polarity implies that relaxation at 77 K leads to relatively unstabilized states. Only in the case of 4bromo 6,2-ANS is the kiscpromoted sufficiently to yield wavelength dependence for 4p/&.
Experimental Section 6-N-Phenylamino-2-naphthalenesulfonatesand N,Ndimethylamideswere prepared, purified, and characterized as previously described.@ Emission spectra were recorded with a Perkin-Elmer-Hitachi MPF-4 spectrofluorimeter with a corrected spectra attachment and a digital integrator. Quantum yields were referred to quinine sulfate in 0.1 M H2S04, 4F = 0.55. Fluorescence and phosphorescence spectra of glasses at 77 K were uncorrected. Phosphorescence lifetimes were measured with a Tektronix 564 storage oscilloscope and a camera. Dioxane-water solutions were prepared and data plotted (HewlettPackard 9810-9862 calculator-plotter) as previously described.6is EPA (ether:pentane:ethanol, 2:2:5) glasses were prepared from nitrogen-purged lo4 M, 6,2-ANS solutions by freezing in liquid nitrogen.
Acknowledgment. The United States-Israel Binational Science Foundation is thanked for providing a grant in support of this work. References and Notes (1) (a) TeCAvhr University. (b) State University of New York, Stony Brook. (2) J. C.Mlller, J. S. Meek, and S. J. Strickler, J. Am. Chem. Soc., 99, 8175 (1977). (3) E. M. Kosower and K. Tanizawa, Chem. Phys. Left., 16, 419 (1972). (4) E. M. Kosower and H. Dodiuk, Chem. fhys. Lett., 26, 545 (1974). (5) E. M. Kosower and H. Dodiuk, J . Am. Chem. SOC.,96,6195 (1974). (6) E. M. Kosower, H. Dcdiuk, K. Tanizawa, M. Ottolenghi, and N. Orbach, J . Am. Chem. SOC.,97, 2167 (1975). (7) H. Dodiuk and E. M. Kosower, J. Am. Chem. Soc., 99, 859 (1977). (8) H. Dodluk and E. M. Kosower, J . Phys. Chem., 81, 50 (1977). (9) H. Dcdiuk, E. M.Kosower,M. Ottolenghi, and N. Orbach, Chem. phys. Left., 49, 174 (1977). (10) E. M. Kosower and H. Dcdiuk, J. Am. Chem. Soc.,100, 4173 (1978). (11) E. M. Kosower, H. Dodiuk, and H. Kanety, J. Am. Chem. SOC.,100, 4179 (1978). (12) H. Dodiuk, H. Kanety, and E. M. Kosower, submitted (apomyoglobin).
