TAUTOMERIC AND PROTOLYTIC PROPERTIES OF O-AMINOBENZOIC ACIDS
887
Tautomeric and Protolytic Properties of o-Aminobenzoic Acids in Their Lowest Singlet and Triplet States by A. Tramerl Inatitute of Physics, Polish Academy of Science, Waraaw, Poland
(Received June 9, 1060)
Absorption, fluorescence (in water at 293”K), and phosphorescence (in ethanol rigid glasses at 77°K) spectra of bications, cations, neutral molecules, and anions of N-methylanthranilic and N,N-dimethylanthranilic acids have been recorded. pK values in the excited singlet SI and triplet TI states have been evaluated by means of the Forster cycle. The constants K t of the tautomeric equilibria between the molecular (M) Ar(NH2)(COOH and quasizwitterionic (Z) Ar(NHa+)(COO-) forms of amino acids in both states have been determined. The solvent effects on the absorption and fluorescence spectra in nondissociating solvents and the kinetics of the Z w + M transition in excited states are discussed. I n the first part of the present work,2the structures of the tautomeric forms of the o-aminobenzoic acid derivative N-methylanthranilic acid (MA) and N,N-dimethyl-
anthranilic acid (DMA) were investigated by infrared and ultraviolet spectroscopy in their ground electronic state. It was shown that in the molecular (M) form, an intramolecular hydrogen bond between the carboxyl and amino groups is not formed, while in the quasizwitterionic form (Z) a strong intramolecular hydrogen bond (-COO. .He NRz) changes the conformation of the amino group and removes the conjugation between the nitrogen lone electron pair and the ring nelectron system.
The equilibrium constant of the M
+ Z reaction
was evaluated and shown to be strongly solvent dependent. In all solvents, the K t values are much higher in the case of DMA. I n this work, the problem of the tautomeric equi2 in the lowest excited singlet and triplet libria M states is discussed. The acidity constants K. of the amino and carboxyl groups conjugated to the aromatic systems are known to change by many orders of magnitude and in opposite directions in the lowest excited singlet state (SI).aa,b Similar, though less pronounced, changes occur on excitation of a molecule to the lowest excited triplet state TI.^ It may, therefore,
be expected that Kt, related to the K, values of interacting groups by Mason’s equation^,^ will be drastically changed in S1 and TI states. If the direct determination of excited state Kt values (pK? and pKtT) is impossible, they can be evaluated by combining the equilibrium constants pK; or pKaT of protolytic reaction~.~~~ In this purpose absorption, fluorescence, and phosphorescence spectra were recorded for bicationic (B), cationic (C), molecular (M), and zwitterionic (Z) forms of neutral molecules and anions (A) of MA and DMA. The difference of the acidity constants in the excited (pKae) and ground (pKao)state of the AH acid was estimated from the spectral shift of the electronic transition in the acid AH and its conjugated base A-.aaq7
pKae - pK2
hc
= - (YAH
kT
-
YA-)
When it was possible, ApK, was evaluated from the shift of the (0,O) band, determined as the intersection of normalized absorption and fluorescence bands. In other cases (nonfluorescent forms, S-T transitions) a similar reasoning may be based exclusively on the shift of the absorption or luminescence band, but the errors in the pKaevalues are then certainly much greater. From ApK, and from the ground-state pK,O values evaluated2 for M and Z forms, the pK? and pKaT constants and hence the pKF and pKtT have been estimated. The conclusions are confirmed by a study of (1) Laboratoire de Photophysique Molbulaire, Facult6 des Sciences, 91,Orsay, France. (2) A. Tramer, J . Mol. Structure, 4,313 (1969). (3) (a) A. Weller, “Progress in Reaction Kinetics,” Vol. I, Pergamon Press, Oxford, 1961,p 188; (b) H.Beens, H. Grellmann, M. Gurr, and A. H. Weller, Disc. Faraday Soc., 39, 183 (1965). (4) G. Jackson and G. Porter, Proc. Roy. SOC.,A260, 15 (1961). (5) 8.F. Mason, J. Chem. SOC.,675 (1958). (6) R.E.Ballard and J. W. Edwards, ibid., 4868 (1964). (7) T.Farster, 2.Elektrochem., 54,42, 531 (1950). Volume 74, Number 4 February 19,1970
A. TRAMER
888 -
~~
~
~
Table I : Absorption Spectra of Anthranilic Acids in Ethanol Solutions. Anthranilic acid Form and solvent
N-Methylanthranilic acid e
Y
Neutral (M or Z) ethanol
i""'
5,200
28,100
5,900
40 ,350
8,800
39 ,050
9,800
44 ,750
18,000
44,150
23 ,000
35,650 36 ,550 44,250
900 1,000 13,000
35,600 36 ,500 43 ,850
35 ,700 41,950
1,300 9,800
31 ,150 {40, 750
4,000 9 ,800
1
Bication (B) concd H2SOI Anion (A) 0.1 N NaOH in ethanol
...
...
e
Y
29 ,500 35 ,000 36 ,000
20 900 1,000
42,100
9,000
750 800 14,000
35,200 36,100 42 I 700
800 900 11,800
35 ,700 41 ,750
1,600 10,750
35 ,500 42 ,200
1,800 12,600
29 ,750 39,150
1,900 8,600
32,700 37,900 43 ,850
1,750 7,000 10,000
500
I{
Cation (C) 1 N HzSOd in ethanol
N,N-Dimethylanthranilic acid
€
Y
...
...
Table 11: Characteristics of the First (SI u SOand T14 SO)Electronic Transitions for Differently Protonated Forms of N-Methylanthranilic (MA) and N,N-Dimethylanthranilic (DMA) Acids in Water
B Yabs
...
YO0
...
MA
36 ,600
35 I 800
Yfl
W
C
... ...
Z
A
28,400 22 ,500
37 ,000
30 ,250 23,900 26,4000 27,200b 21,900
25 ,OOOa
25,450b 21 ,600
24 ,400
24,100
M
... ...
....
DMA 35 ,700 27,200 YO0 32 ,500" 31, 450b b h 23 ,900 (00) frequencies evaluated as:
31 ,000 37 ,000 33,100 22 ,700 ... 23 ,700 25,600" 27, 300a ... 26 ,850a ... 28, 4OOb 24 ,400 20,100 24 ,700 21,600 frequencies of intersection points of absorption and fluorescence bands and as YOO = l / 2 ( vaba 36,400
Vaba
...
VI1
.
a
.
I
the solvent effect on the absorption and fluorescence spectra of aminobenzoic acids and of their hydrogenbonded complexes in nonaqueous solutions.
Experimental Section The methods of purification of chemical compounds and the apparatus used for absorption measurements have been described.2 The fluorescence spectra were recorded by means of a spectrofluorimeter composed of an SPM-1 Zeiss-Jena monochromator, an EM1 6256B photomultiplier, a UNIPAN 202 and 203 narrow-band amplifier and phase-sensitive detector, and an MAV recorder. The system was equipped with a 176-Hz chopper which could work also as a one-disk phosphoroscope. The spectral sensitivity distribution was determined by recording the fluorescence spectra of standard compounds.* The fluorescence was excite; by the groups of Hg lines at 3650, 3130, and 2750 A from an HBO 500 high-pressure mercury lamp isolated by means of Schott glass filters or a Zeiss 2750 A interference filter. The Journal of Physical Chemistry
...
+ vn).
The phosphorescence was excited either with the entire Hg spectrum or with selected wavelengths. The fluorescence spectra were measured at the room temperature. The spectra of deaerated and nondeaerated solutions were found to be identical. The phosphorescence spectra were studied in ethanol rigid glasses at 77'K with silica cells immersed in liquid nitrogen in a silica dewar vessel equipped with optical windows. All solvents were nonfluorescent or very weakly fluorescent; when necessary, the background of the solvent fluorescence was taken into account. Acid-Base Equilibria. To evaluate the pKaS values, absorption and fluorescence spectra of anthranilic acid, MA, and DMA were studied in water and in aqueous HzS04 and NaOH solutions. The principal absorption bands of differently protonated forms are listed in Table I. More detailed data concerning the first SI
-
(8) E. Lippert, W. Nagele, I. Seibold-Blankenstein, H. Steiger, and W. Voss, 2.Anal. Chem., 170, 1 (1959).
889
TAUTOMERIC AND PROTOLYTIC PROPERTIES OF 0-AMINOBENZOIC ACIDS
I
Arbitrary
units
Arbitrary
units
f0 . 8-
4
6-
5: 6\
4 -4
_--2
2-
-3
30
33
Iui
I
units
32
34
35
37
36
38
39
xlO~cm.~
Figure 3. Absorption spectra of N,N-dimethylanthranilic acid: 1, M and Z forms in water; 2, cation (C) in 2 N H2SOlaq; 3, bication (B) in 35.7 N HiSO4; 4, 17.8 N HsSO4; 5, 28.4 N HzSO4; 6,33.0 N HzSO4.
Figure 1. Absorption and fluorescence spectra of N-methylanthranilic acid: solid line, neutral molecule in water; broken line, anion (A) in 0.1 N NaOH,,.
Arbitrary
31
if i
//I
08
0'41 0.2
Figure 4. Relative opt.ica1densities (0)and fluorescence intensities ( X ) of the bication (B) of N,N-dimethylanthranilic acid vs. the Ho acidity function of &SO&solutions. Figure 2. Absorption and fluorescence spectra of N,N-dimethylanthranilic acid: 1, neutral molecules (M water; 2, anion (A) in 0.1 N NaOH,,; 3, bication (B)in 35.7 N HzSO4.
+ 2) in
So and TI + So transitions in MA and DMA are given in Table 11; absorption and fluorescence spectra are presented in Figures 1 and 2. The absorption spectra of neutral M and Z forms of MA and DMA were reproduced and discussed in ref 2. The C forms (cations) of both acids are not fluorescent in 1-5 M HzS04solutions. In view of the close analogy of their absorption spectra with those of nonfluorescent o-alkylbenzoic acids, this fact is not astonishing. No fluorescence which could be ascribed as corresponding to the Z form of MA or DMA (close analogs of o-alkylbenzoate anions) was detected; the DMA fluorescence spectrum is always the mirror image of the absorption band of the M form, even when excited in the spectral region where all quanta are absorbed by the Z form. A characteristic feature of both acids is a very small (as compared to the benzoic acid) red shift of the order of a few hundred cm-' on protonation of the carboxyl
group in concentrated sulfuric acid solutions. The process B S C H + was studied in more detail in the case of DMA (Figure 3). The absorption spectrum is continuously red-shifted without any change in the band shape when the H2S04concentration is increased from 1N to 20 N . This shift may be considered due to the solvent dielectric effects. A pronounced change in the band shape suggesting the formation of a new species (bication) occurs only in 25-35 N Hi304 and is accompanied by the appearance of a violet fluorescence which is assigned to the B form. The optical densities D at 34,250 cm-l (where the intensity changes are the most distinct) are taken as a measure of the bication concentration. In Figure 4, DIDO(where DOis the optical density a t 34,250 em-' in 35.7 N HzS04)is plotted against Ho, the acidity function of HzS04 solutions (H+ would be more correct in this case, but we do not have reliable H+ values for this concentration range9). In the same plot relative intensities of the bication fluorescence excited with X 2750 8, I l l 0 (where IO is its intensity in 35.7 N HzSOa), us. HOare given. More con-
+
(9) M. A. Paul and F. A. Long, Chem. Rev., 57, 1 (1957).
Volume 74, Number 4 February 19, 1970
890
A. TRAMER
centrated acids would be needed for exact pK determiand PKBC'are very nations but it seems that ~KBCO close to each other and amount to 8 or less. The phosphorescence of differently protonated forms was studied in ethanol rigid glass solutions: in pure ethanol (neutral molecules), in 1 N ethanolic Hi304 (cations), and in 0.1 N ethanolic NaOH (anions). The bication spectrum was studied in concentrated HzS04 rigid solution. For the first three forms the solvent shift need not be taken into account but the shift from B to C may be due to the solvent as well as to the protonation effect. In the case of MA the phosphorescence spectra of B, C, If,and A forms may be easily identified. As might be expected, no Z-form phosphorescence was observed, the Z form being virtually nonexistent in ethanolic solutions. The phosphorescence of the B, C, and A forms of DMA resemble closely those of the analogous forms of MA, but quite specific effects are found in the case of neutral molecules. Here the spectrum when excited with unfiltered mercury light, consists of two bands: a weak band in the 25,000-cm-l region, characteristic of B and C (protonated in the amino group) forms of both acids and a stronger band at 20,100 cm-l which is red-shifted with respect to the Mform emission of MA (Figures 5, 6). When the phosphorescence is excited with Hg 3650-8 lines (absorbed only by the M form of DMA) the first band disagpears. When the excitation is carried out with 2750-A lines (ie., mainly the Z form is excited), the relative intensity of the first band increases. It seems reasonable to assign the first band to the phosphorescent transition in the Z form and the second to that in the nlr form of the neutral molecule. The dependence of the emission spectra on the wavelength of the exciting light shows that the Z .-* &f transitions may take place during the lifetime of the excited species and that this process is an. irreversible one. From the spectra of different protonated forms, the spectral shifts V A H - V A - may be determined but the accuracy of the Av measurement varies a great deal from one case to another. The most reliable data may be obtained from the SI cs SO transition in the fluorescent compounds, i.e., for M and A forms of both acids and the B form of DMA. The (0-0) transition is defined either as the average of the absorption and fluorescence band maxima or as the intersection point of normalized absorption and fluorescence band contours. The errors are somewhat greater in the case of DMA, where the SI SOabsorption bands appear as shoulders of stronger absorption bands. The Z and C forms are not fluorescent but since their absorption bands possess a similar vibrational structure, the shifts of the corresponding maxima may be taken as very closely to that of the (0-0) transition. vc - V M must be approximated also by the shift absorption maxima and, since the shapes of the bands
--
The Journal of Physical Chemistry
Arblfrary units
I
//
/
XlO'Crn+ L
18
19
20
21
22
23
24
25
26
27
28
Figure 5. Phosphorescence spectra of N-methylanthranilic acid at 77°K: 1, B in 35.7 N HzSOl; 2, C in 1 N HzSO, in ethanol; 3, M in ethanol; 4, A in 0.1 N NaOH in ethanol.
I
Arbitrary
units
Figure 6. Phosphorescence spectra of N,N-dimethylanthraniic acid at 77°K: 1, B in 35.7 N H2S04; 2, C in 2 N HzS04in ethanol; 3, M and 2 in ethanol excited with ( a ) 3650-1 Hg lines (b) unfiltered Hg spectrum (e) 2750-A Hg lines; 4, A in 0.1 N NaOH in ethanol.
are different, the errors in this case may be much greater. I n evaluating the VH - vc value the solvent shift from water to concentrated HzS04 must be taken into account: Av is considered as being equal to the frequency difference between the absorption maxima of B and C in concentrated sulfuric acid solutions. The data concerning the triplet state are deduced from the shifts in the phosphorescence spectra of ethanol rigid glass solutions. This procedure may be subjected to strong criticism. First of all, the shift of the band maximum may be not equal to that of the (0-0) transition, especially when the band shapes are different as it is for the M and C forms. A more serious objection is that the frequency shifts measured in ethanol rigid medium are applied to estimate the pK values in liquid water solutions where the energy of the phosphorescing species may be strongly influenced by the relaxation of solvent molecules.lo (10) A. Grabowska and B. Pakula, Photochem. Photobbl., 9, 339 (1989).
TAUTOMERIC AND PROTOLYTIC PROPERTIES OF O-AMINOBENZOIC ACIDS
891
Table 111: pK Values of N-hlethylanthranilic (MA) and N,N-Dimethylanthranilic (DMA) Acids in So (pKO), SI (pKs), and TI(pKT)Electronic States MA pKS
7 -
PKQ
PKBG- PKBCO PKGM pKcz ~KMA ~KZA PKt
-
1.7 - 15 2.9 8.5 -6.7 17.5
.,.
2.61 2.03 4.66 5.48 -0.58
DMA------.------,
c
PK~
pKT
0.65 -3.4
... 5.3
... ...
... 4 1.4 6 8.58 -2.6
pKS
Pfl
1.5 -11.5 2.7 10 0.3 9.8
1.1 -7.5 2 .o 9 2 6
pKaT values must be, therefore, considered only as rough approximations. From the frequency differences v, - vu deduced in this way the ApK values pKZuS- pKzUoand pKZuTpK,,O may be calculated by means of the Weller 0.625
pKeXc - pKO
~
T
Av
if the entropy of the protolytic reaction is assumed to be equal in the ground and in the excited state and pK,, characterize different protolytic processes listed in Figure 7. pK,,O values (except for ~KBoO) were evaluated previously.' The values of pKs and pKT are given in Table 111; for the B C H+ equilibrium only ApK values are available. Since the constant of the tautomeric equilibrium, Kt, is related to the acidity constants6
+
Kt in SI state (K?) for both acids and in TI state (KtT) for DMA only (in MA the phosphorescence ~ KzAT of the Z form was not detected and K C Zand constants could not be determined) may be evaluated in two independent ways. The results differ by 2-3 pK units in all cases, and this may be taken as evidence that the errors in pKeXcestimations do not greatly exceed ordinary error limits in pK evaluations based on the Forster cycle. From the data given in Table 111, important conclusions concerning the structure of excited molecules may be deduced. (a) The M Z equilibrium is strongly displaced to the left in the S1 and TI states. This would be expected on general considerations of the acid-base properties of aromatic amines and acids in the excited states. That the excitation of the Z form of DMA leads, in a variety of solvents, to the fluorescence emission of the RI form is direct evidence of the stabilization of the M form in the SI state. This problem will be discussed in a more detailed way in the last section of this paper. (b) The acid properties of the -NR2H+ group are strongly increased in both excited states. As in the case of aromatic amines''
+
wc
'0-H
Figure 7. Scheme of protolytic reactions.
the change in pK is very high for the SI SOtransition but is different for two processes involving the acid dissociation of the -NR2H+ group: C S &I H + and Z A H+. The corresponding ApK values are -17.6 and -11.5 in the former case and -12-2 and -8.3 in the latter for MA and DMA, respectively. This difference probably exceeds the error limits and may be explained by the influence of the intramolecular hydrogen bond. The proton of the ammonium group is more strongly bonded to the -COO- group in Z than in the case of C which contains a more acidic -COOH group.12 In the excited state, increased basic properties of the carboxyl cancel to some extent the enhancement of the acid strength of the amino group. It should be pointed out that the pK increase in the T1 state, although considerably weaker than that in the S1 state, is relatively large compared to aniline and naphthylamine^.^ (c) Similar but more pronounced effects due to the presence of intramolecular hydrogen bonds are observed in the cafie of protolytic processes involving the carboxyl group. The acidity of the M form is lowered in the SI state (ApK = 4) as for other aromatic a c i d ~ . l ~In ~ ' ~contrast, the +
+
+
+
(11) T. Forster in "Photochemistry in Liquid and Solid States," F. Daniels, Ed., John Wiley and Sons, New York, N. Y., 1960. (12) E. Czarnecka and A. Tramer, submitted for publication. (13) E. L. Wehry and L. B. Rogers, J. Amer. Chem. Hoc., 88, 351
(1966). (14) E. van der Donckt and G. Porter, Trans. Faraday 3216 (1968). Volume 74, Number 4
Soc., 64,
February l o 3 1970
A. TRAMER
892 -~
~~~~
~~~
Table IV : Absorption ( v S b s ) and Fluorescence ( v f l ) Band Maxima Frequencies, Frequencies of the (00) Transitions ( y D 0 ) , Stokes Shifts ( a u ) , and Equilibrium Constants of Complex Formation in the Ground (Kg) and Excited SI (K,) States of N-Methylanthranilic Acid (MA)
Cyclohexane (monomer) (dimer) Cyclohexane Cyclohexane Cyclohexane Dioxane Ethyl acetate Acetonitrile Ethanol Water
None Dioxane
TEA Ethanol Dioxane
27 , 750 27,300 28,050 28,300 28,100 28,000 27,950 27,800 28,100 28,400
24 ,400 23,850 24 ,300 24 ,650 24 000 24 ,000 23 ,680 23 ,350 23 300 22 ,500
equilibrium constants KCE and KBC are less sensitive to the electronic excitation (ApK is 0.9 and 1.3 in the former and 1.7 and 1.5 in the latter case for MA and DMA, respectively) than the pK of the protonation H+ reaction) of benzoic acid (an analog of the B C which is increased in the S1 state by +7 pK The low values of A ~ K B cand ApKcz as compared to A ~ K M may A be explained by the effect of the NR2+-H. .O=C hydrogen bond in Z (whose existence was shown by infrared spectroscopy*) and in C. As the acidity of the ammonium group increases in the S1 state, the hydrogen bond is strengthened and this may counterbalance the enhancement of basic properties of the carboxyl group. Very poor proton-acceptor properties of the C form of DMA, stabilized probably by the intramolecular hydrogen bond, are directly evidenced by direct measurements of the absorption and fluorescence as a, function of the acid concentration (Figure 4). For DMA, PKBCO= ~ K B c '= -8 while, for the benzoic acid, pKo = -7.4l6 and pKs = - l.2.3b The results may be summarized by saying the effects of the electronic excitation on the acid-base properties of amino and carboxyl groups in the ortho position are qualitatively the same as in the case of isolated groups but they may be attenuated by the influence of the intramolecular hydrogen bond. Solvent Effects in Nondissociating Solvents. hbsorption and fluorescence of MA and D N A were studied in a series of solvents (cyclohexane, dioxane, ethyl acetate, aceto-nitrile, dimethylformamide, and ethanol) as well as in mixed solvents (cyclohexane-dioxane, cyclohexane-ethanol, cyclohexane-triethylamine The solvent (TEA) , dimethylformamide-TEA). effects are due to a superposition of the dielectric solvent shift and of the spectrum changes resulting from the formation of hydrogen-bonded complexes with basic compounds. The effects of the second kind were studied in more detail for MA in cyclohexane solutions; the frequencies of absorption and fluorescence bands and the equilibrium constants of complex
26,100 25,550 26,250 26,450 26,000 26,000 25,800 26 100 25 700 25,450
3350 3450 3750 3680 4100 4000 4300 4450 4800 5900
7 . 5 x 108 30 900
... .,.
...
,..
...
x
8.5
104
15 100
... *.. ... *
.
I
...
Arbitrary
units
+
The Journal of Physical Chemistry
Figure 8. Absorption and fluorescence spectra of N-methylanthranilic acid: 1, 5 X mol/l. in cyclohexane (dimer); 2, 5 X 10-6 mol/l. in cyclohexane (monomer); 3, 10-6 mol/. in mol/l. TEA in cyclohexane (TEA complex).
formation in the ground state (K,) and in the excited Si state of the amino acid molecule ( K J are given in Table IV. K , values were evaluated from the dependence of optical densities on the base concentration by means of the simplified Benesi-Hildebrand equation;16 K , from the shift of the (0-0) band on complex formation by a procedure analogous to the Forster cycle.17 Some examples of the spectra are given in Figures 8 and 9. As was previously shown,2 MA is present in cyclohexane solutions solely in the M form. The dimer formation in more concentrated solutions is accompanied by a red shift of absorption and fluorescence bands. This effect seems to be quite general and L.ts explanation was given by Hochstrasser in his study, of l-naphthoic acid solutions. The dimer composed (15) L. A. Flexsner, L. P. Hammett, and A. Dingwall, J. Amer. Chem. Soc., 57,2103 (1935). (16) K.Szcaepaniak, M. Golinska, and J. Mikolajcayk, Acta Phys. Pol., 34,431 (1968). (17) N. Mataga and Y. Kaifu, Mol. Phys., 7,137 (1964).
893
TAUTOMERIC AND PROTOLYTIC PROPERTIES OF 0-AMINOBENZOIC ACIDS Table V: Absorption and Fluorescence Maxima
(vabs
and
vfi),
Band Shifts (Avabs and Avfi), and Stokes Shifts ( 6 ~ )for
N,N-Dimethylanthranilic Acid (DMA) Solutions Solvent
Base
Vaba
Cyclohexane Acetonitrile Tetrahydrofuran Cyclohexane Dimethylformamide Dimethylformamide
None None
28,600 28,900 29,600 30,100 30,200 32,600
TEA TEA
Arbitrary units
Figure 9. Absorption and fluorescence spectra of lo-' mol/l. N,N-dimethylanthranilic acid in: 1, cyclohexane; 2, 0.7 mol/l. TEA in cyclohexane; 3, dimethylformamide; 4, 0.7 mol/l. TEA in dimethylformamide.
of one excited and one ground-state molecule is stabilized by the increased basic properties of the excited species and by the excitonic effects; hence, K , < K O and Av < 0. In contrast, in the hydrogen-bonded complexes with basic (proton-acceptor) compounds, a weakening of intermolecular interactions in the excited state should be expected. The spectra of MA in the presence of dioxane, TEA, etc., are blue-shifted as anticipated, the shift being stronger for the absorption than for the fluorescence band. This effect is more pronounced in DMA, which is a very weak acid in the SIstate (pK = 9.6). While the absorption spectrum of the M form is very sensitive to small quantities of TEA added to cyclohexane the fluorescence remains practically unchanged even a t the TEA concentration of 1 mol/l. I n strongly polar solvents (dimethylformamide, acetonitrile) where the absorption spectrum of the DMA-TEA complex, quite similar to that of the DMA anion, points to the formation of hydrogenbonded ion pairsj2 the emission spectrum is almost identical with that of free (bonded to solvent) DMA molecules (Figure 9). The acidity of DMA in the
Uf I
24,500 23,700 23,900 24,600 23,800 23,700
Ahbs
Avf 1
0
6U
0
+300 +1000 +1500
+100
+MOO f4000
- 800
4100 8200 5700 5500 6400 8900
- 800
- 600 - 700
excited state is so strongly reduced that hydrogen bonds are broken or extremely weakened. The dielectric solvent effects are thus obscured by those of specific intermolecular interactions. It seems, however, possible to estimate their magnitude (in the case of MA only) if the choice of solvents is limited to those of similar basicity (dioxane, ethyl acetate) and to the nearly inert solvents containing a small amount of the base (dioxane) sufficient to ensure the bonding of almost all MA molecules. When the values of the Stokes shift S v = Vabs - vfl are plotted against the values of the Lippert-Mataga function of the so1vent,1gv20the points corresponding to this group of solvents (marked by crosses in Figure 10) lie almost on a straight line, the slope of which is similar to that found by MatagaZ0for anthranilic acid. The difference between the ground-state ( p g ) and excited-state (p,) dipole moments of MA may be roughly estimated from the Lippert-Mataga equation
where
F(n,s) =
6 - 1
nz-1
-- ___ 2e + 1 2n2 + 1
as equal to p, - pg = 1.75 D, if the Onsager radius of the molecule is taken as a = 3.5 A. The Stokes shifts in other solvents are either smaller (monomer and dimer in cyclohexane) or much greater (ethanol, water). It seems that in the last case the strong solvent shifts may be due to the re-forming of the hydrogen-bonded system, solvent-solute, in the excited state (breaking of the ROH-e-NR2- and strengthening of the RO-H O=C bonds). I n view of specific effects due to intermolecular interactions, the estimation of dielectric shifts in the case of DMA was not possible. The Z ,-+ M Transitions in Excited States. As mentioned above, the excitation of the Z form of DMA results in a fluorescence emission corresponding to the (18) R.M. Hochstrasaer, Can.J . Chem., 39,1776 (1961). (19) E.Lippert, 2.Elektrochem., 61,962 (1957). (20) N.Mataga, Bull. Chem. Soc. Jup., 36,654 (1963). Volume 74, Number 4
Febrwzry 19, 1970
894
A. TRAMER
I 55
I t
45v 50
40 x
Of
02
03
Fhfj
Z
M
Figure 10. The Stokes shift in N-methylanthranilic acid spectra plotted 21s. the F(n,r) function of solvents.
Figure 11. Scheme of lowest electronic levels and transitions in the neutral N,N-dimethylanthranilic acid molecule,
M form. When the Z form is excited in rigid solutions,
energy barriers for Z I*w+ M processes very important in the TI, much lower in the SI state, although the energy differences between the 2 and M forms are similar in both excited states. This barrier could be considered as corresponding to the proton transfer in the hydrogen bond but this explanation seems to be highly improbable. The potential energy barrier for the proton transfer in the much weaker and more symmetric hydrogen bond in the excited molecule of salicylic ester was shown to be very 10w.~b In our case we are certainly concerned with a very strong, short (the N * * * Odistance may be estimated to be -2.5 A) hydrogen bond. The other explanation that can be proposed is based on the assumption (discussed in more detail in ref 1) that the conformation of the dimethylamino group is different in the M form (the symmetry plane of the NRB group perpendicular to the ring plane-the nitrogen lone-electron pair conjugated with the ring ?r-electron system) and in the Z form (the symmetry plane of NR2 in the ring plane). If this is the case, the Z ,.-* SI transition would necessitate the rotation of the dimethylamino group around the C,,-N bond, which would be the rate-determining step of the process. In any case, the difference between the rates of the (8) and (9) processes is puzzling. More detailed studies of the fluorescence and phosphorescence yields and lifetimes and of their solvent and temperature dependence are needed to elucidate this problem and they are now in progress in this laboratory.
the phosphorescence consists of two bands assigned to Z and lliI forms, while the excitation of the M forms yields only the 11 form phosphorescent emission. The 2 ,-+ 11 radiationless transition may thus take place during the lifetime of the excited species, but the process is irreversible as might be expected in view of the energy differences between corresponding SI and TI levels and of the vaIues of the Kts and KtT equilibrium constants. In Figure 11, possible ways of energy degradation in the case of the Z form excited to the SI state are listed. Relatively inefficient SI -+ So transitionsZ1 are omitted. The TI level of the R/1 form may be populated by two different ways, SI' A+ TI' % , TI" and SI' k+ SIM ,&+ TIM. The first way is certainly effective since the (8) process is responsible for the M-form fluorescence excited in the Z form absorption region. Since the intersystem crossing in 2 (7) and the Z w+ M transition are competitive, kg must be of a similar order of magnitude to k,. No data concerning k7 in aromatic acids are available, but values of lolo10l2sec-' seem to be reasonable (cf. ref 22). If k8 = IC,, the lifetime of the excited Z form would exceed by 2-3 orders of magnitude the period of 0-H. N stretching vibrations. The analogous process in the lowest triplet state, the T? ++,*TIM transition, if it occurs a t all, is an extremely slow one. Its rate k b which constant (lcg) must be comparable to kq is of the order of 1 sec-l (in the case of l c ~>> kq k6 the TIZ level would be rapidly depopulated and the Z-form phosphorescence would be quenched). These estimations point to the existence of potential
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The Journal of Physical Chemistry
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(21) V. L. Ermolaev and E. B. Sveshnikova, Acta Phz/s. Pol., 34, 771 (1968). (22) M. A. El-Sayed, ibid., 34,649 (19&38),
