J . Phys. Chem. 1988, 92, 5282-5287
5282
eliminated. Fourth, the theoretical model for obtaining Dappis applicable to both the semiinfinite and finite diffusion cases. Thus, unlike many other method^,'^-^^ it is not necessary to adjust the experimental conditions such that the semiinfinite case is obtained. Finally, it should be noted that if the film to be studied is Nernstian, the method would be simplified in that an E , vs concentration of diffusing species calibration curve (Le., Figure 5) would not have to be obtained experimentally. Because of the versatility and power of this new method, we are currently using
this technique to evaluate charge-transport rates in ion-exchange and other polymer films on electrode surfaces.
Acknowledgment. This work was supported by the Air Force Office of Scientific Research, the Office of Naval Research, and the NASA Johnson Space Center. We acknowledge valuable discussions with Professor Dan Buttry. Registry No. Pt, 7440-06-4; Et4N*BFc, 429-06-1; BFC, 14874-70-5; pyrrole, 109-97-7; polypyrrole, 30604-81-0; acetonitrile, 75-05-8.
Acidity Constants in the Excited States: Absence of an Excited-State Prototropic Equilibrium for the Monocation-Neutral Pair of 2,3-Diaminonaphthalene R. Manoharan and Sneh K. Dogra* Department of Chemistry, I.I.T. Kanpur. Kanpur-208 016, India (Received: August 26, 1987; In Final Form: February 2, 1988)
The effects of solvents and pH on the absorption and fluorescence spectra of 2,3-diaminonaphthalene indicate that the two amino groups are twisted with respect to the naphthalene moiety. A similar conclusion is drawn for the case of the monocation, formed by the protonation of one of the amino groups. The ground-state pKa value obtained from the fluorimetric titration between the monocation and neutral species indicates that prototropic equilibrium is not established in the excited state. This is mainly caused by the extremely slow proton dissociation rate of the monocation compared to its decay rate ( 5 X 10' s-l). No proton-induced fluorescence quenching is observed for the neutral species prior to formation of the monocation, whereas proton-induced fluorescence quenching is noticed for the monocation, prior to the formation of the dication. This proton-induced fluorescence quenching is static in nature rather than dynamic. pK, values for various prototropic reactions have been determined in the So and SI states and discussed.
Introduction Since Forster' and Weller's2 study of the effect of pH on the electronic absorption and fluorescence spectra of 1-naphthylamine-Csulfonate, considerable study has been carried out on the electronic spectra of aromatic acids and base^,^-^ which are either monofunctional or bifunctional, with one functional group having electron-withdrawing and the other electron-donating properties. It is a well-known fact that in monofunctional molecules, electron-withdrawing groups, e.g., -COOH group or tertiary nitrogen atom, become more basic in the excited singlet state and thus lead R* is the lowest energy to a red shift in the spectra if R transition. Similarly in monofunctional molecules, electron-donating groups such as -OH and -NH2 become stronger acids in the SI state and lead to a red shift or blue shift in the electronic spectra when these groups become deprotonated or protonated. In bifunctional molecules, having electron-donating and electron-withdrawing functional groups, the spectral characteristics of the individual groups are retained. But in some molecules the increase in basicity of the electron-withdrawing group or in acidity of the electron-donating group is so large that the site of protonation is different in the ground and excited states. For example, at pH -2, 5-aminoinda~ole'~ exists as a monocation, formed by protonating the amino group in the ground state. However, in
-
( I ) Forster, Th. 2.Elektrochem. Angew. Phys. Chem. 1950, 54, 42. (2) Weller, A. Ber. Bunsen-Ges. Phys. Chem. 1952, 56, 662; 1956, 66, 1144.
the lowest excited singlet state, the stable species is also a monocation but is formed by the protonation of the tertiary nitrogen atom. The change in the formation of species in the ground and first excited singlet states is termed phototautomerism because this involves only migration of the proton from one functional group to another. This kind of isomerization is called biprotonic phototautomerism, because the electron-donating and electronwithdrawing groups are widely separated. In the molecules where the electron-donating and electron-withdrawing groups are close to each other, e.g., 2-(2'-hydroxyphenyl)benzimidazole,~~the phototautomerism can occur by intramolecular hydrogen bonding. This is called monoprotonic or intramolecular phototautomerism. Many examples of molecules fall into either of the above categories. However, little systematic study has been undertaken for excited-state protonation and fluorescence phenomena of bifunctional molecules in which both of the functional groups are electron donating.I2-l6 Our recent study on a few system^'^-'^ of these kinds have revealed very interesting features. For example, all the phenylenediaminesI8 (ortho, meta, and para) give rise to a similar sequence of spectral changes when protonation is carried out stepwise, and this agrees with the normal behavior of aromatic amines. However, a small red shift is observed on the protonation of the first amino group (which is an anomalous behavior), followed by a large blue shift in the fluorescence spectrum when second amino group of 2,7-diarninofl~orene'~ is protonated. On
( 3 ) Weller, A. Prog. React. Kine?. 1951, 1, 189. (4) Vander Donckt, E. Prog. React. Kine?. 1970, 5 , 273. (5) Wehry, E. L.; Rogers, L. B. In Fluorescence and Phosphorescence Analysis; Hercules, D. M., Ed.; Wiley-Interscience: New York, 1966; p 125. (6) Schulman, S. G. In Modern Fluorescence Spectroscopy; Wehry, E. L., Ed.; Plenum: New York, 1976; Vol. 2. (7) Schulman. S. G. In Fluorescence and Phosphorescence Spectroscopy; Pergamon: Oxford, 1977. (8) Ireland, J. F.; Wyatt, P. A. H. Adu. Phys. Org. Chem. 1976, 12, 131,
Sarpal,
and a number of references mentioned therein. (9) Klopffer, W. Adu. Photochem. 1977, 10, 31 I . (10) Swaminathan, M.; Dogra, S. K. J . A m . Chem. SOC.1983,105,6223.
(17) (18) (19)
(11) (12) (13) (14) (15) (16)
Sinha, H. K.; Dogra, S. K. Chem. Phys. 1986, 102, 337. Cowgill, R. W. Photochem. Photobiol. 1971, 13, 183. Bridges, J. W.; Williams, R. T. Biochem. J . 1968, 107, 225. Ellis, D. W.; Rogers, L. B. Spectrochim. Acta 1964, 20, 1720. Ellis, D. W.; Rogers, L. B. Spectrochim. Acta 1964, 20, 1709. (a) Sarpal, R. S.; Dogra, S. K. J . Photochem. 1987, 38, 263. (b) R. S.; Dogra, S. K., unpublished results. Manoharan, R.; Dogra, S. K. Can. J. Chem. 1987, 65, 2013. Manoharan, R.; Dogra, S. K. Bull. Chem. SOC.Jpn. 1987,60,4409. Manoharan, R.; Dogra, S. K., unpublished results.
0022-3654/88/2092-5282$0l.50/00 1988 American Chemical Society
Acidity Constants in the Excited States
The Journal of Physical Chemistry, Vol. 92, No. 18, 1988 5283
the other hand, similar reactions carried out in a nonpolar medium (cyclohexane) give rise to the normal behavior of the aromatic amine. The first protonation leads to formation of a species that is very polar, and large solvent relaxation is observed in the polar medium. Another interesting feature of prototropic reactions is that equilibrium is not established in the first protonation reaction in the S1 state, and thus no proton-induced fluorescence quenching is observed, whereas the second prototropic reaction of the amino group follows the normal behavior. Shizuka et aLZ0were the first to observe that no proton-induced fluorescence quenching of 4-(9-anthryl)-N,N’-dimethylaniline is noticed prior to the formation of its monocation, even though the charge-transfer state is the lowest emitting state, a prime condition for the abovementioned fluorescence quenching, and also a ground-state pK, value is obtained from the fluorimetric titrations. This has been attributed to the larger rate constant for the radiative process as compared to the rate of protonation reaction. The present study is an extension of our earlier work. The molecule 2,3-diaminonaphthalene (DAN) has been selected, because the detailed studies on monoamino derivatives of naphthalene are a ~ a i l a b l e . ~ ’ -Second, ~~ we would like to find out whether this molecule is closer to the phenylenediaminesl* or to 2,7-diaminofluorene17 in its spectral behavior.
Experimental Section 2,3-Diaminonaphthalene (DAN) was obtained from Aldrich Chemical Co. and was recrystallized from ethanol. The purity of the compound was checked by noting the melting point, electronic spectra, and similar fluorescence spectra when excited with different wavelengths. Spectrograde methanol (BDH), analytical grade sulfuric acid, orthophosphoric acid, sodium hydroxide, and trifluoroacetic acid (TFA) were used without further purification. Cyclohexane, ether, and acetonitrile were further purified according to a procedure suggested in the literature.z6 Triply distilled water was used for aqueous solutions. Solutions in the pH range 3-10 were prepared by adding appropriate amounts of N a O H and H3P04. A modified Hammett’s acidity scalez7 for HzS04-H20mixtures and Yagil’s basicity scalez8for NaOH-HzO mixtures were used below pH 1 and above pH 13, respectively. serves specifically as a measure Hammett’s acidity function (Ho) of the tendency for the solution in question to transfer a proton to an uncharged or charged basic molecule, increasingly negative values corresponding to higher acidity. Absorption spectra were recorded on a Shimadzu 190 UV spectrophotometer, equipped with a U135 recorder. Fluorescence measurements were carried out on a recording spectrofluorimeter, fabricated in our laboratory, and the details are available elsewhere.z9 Both of the monochromators were calibrated by using a low-pressure mercury lamp from time to time. The bandwidth of the excitation source was 8 nm. The fluorescence spectra were corrected according to the procedure suggested by Parker.30 Fluorescence lifetime measurements of the monocation and neutral species of DAN were done with a picosecond time-correlated single-photon-counting detection system. It is a Spectra Physics Model that uses a C W mode-locked Nd:YAG synchronously pumped, cavity-dumped dye laser as the excitation source. The (20) Shizuka, H.; Ogiwara, T.; Kimura, E. J. Phys. Chem. 1985,89,4302. (21) Forster, Th. Z . Elektrochem. 1950, 54, 531. (22) Boaz, H.; Roltefrorn, G. K. J . A m . Chem. SOC.1950, 72, 3435. (23) Schulman, S. G.; Capornacchia, A. C. Spectrochim. Acta, Part A 1972, 28A, 1. (24) Tsutsumi, K.; Shizuka, H. Chem. Phys. Lett. 1977, 5 2 , 485. (25) Mishra, A. K.; Swarninathan, M.; Dogra, S . K. J . Photochem. 1985, 28, 87. (26) Riddick, J. A.; Bunger, W. B. In Techniques of Chemistry: Organic Soloents; Weisberger, A,, Ed.; Wiley-Interscience: New York, 1970; Vol. 11, p 595. (27) Jorgenson, M. J.; Hartter, D. A. J. Am. Chem. SOC.1963, 85, 878. (28) Yagil, G. J . Phys. Chem. 1967, 71, 1034. (29) Swaminathan, M.; Dogra, S . K. Indian J . Chem., Sect. A 1983, ZZA, 853. (30) Parker, C. A. In Photoluminescence of Solutions with Applications to Photochemistry and Analytical Chemistry; Elsevier: Amsterdam, 1968; pp 261.
-Cyclohexane -0-0 -.-a-
----
1
1
250
I
I
Ether Acetonitrile Methanol Water (pH 6 . 6 )
I
275 300 325 WAVELENGTH ( n m )
350
Figure 1. Absorption spectra of D A N in different solvents at 298 K; the concentration is 2 x M.
---
-Cyclohexane
16
-
Ether Acetonitrile
12
VI
c I-
3 >I
i 8
d
.-
--e Q
r
4
340
360
380 400 420 440 WAVELENGTH ( n m )
460
Figure 2. Fluorescence spectra of D A N in different solvents a t 298 K; the concentration is 1 X M.
data were acquired in a multichannel analyzer (Tracer Northern Model T N 7200) and fed into a computer to reconvolute to extract the true fluorescence function. The quantum yields were calculated with quinine sulfate in 0.1 N H2SO4as a standard,31and the wavelength used for excitation for DAN was 340 nm. The M containing concentration used was in the vicinity of 3.8 X 1% (v/v) methanol. The solutions for absorptiometric and fluorometric titrations were prepared just before taking measurements. The isosbestic wavelengths were used for measuring the fluorescence intensities at any analytical wavelength, and these excitation wavelengths for dication-monocation, monocationneutral, and neutral-monoanion equilibria are 290, 337, and 342 nm, respectively. The pH of the solutions in the range 1-13 was measured on a Toshniwal pH meter, Model CL 44A.
and Discussion The absorption and fluorescence spectra of DAN have been recorded in solvents of different polarity and hydrogen-bondforming tendency. The absorption and fluorescence spectra are shown in Figures 1 and 2, respectively, and the relevant data are compiled in Table I. The absorption band maxima for DAN Results
(31) Guilbault, G. G. In Practical Fluorescence; Marcel Dekker: New York, 1971; pp 13.
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Manoharan and Dogra
The Journal of Physical Chemistry, Vol. 92, No. 18, 1988
TABLE I: Absorption and Fluorescence Maxima, log e-, and 6,of 2,3-Diaminonaphthalene in Different Solvents and at Various Acid Concentrations
cyclohexane
ether
acetonitrile
methanol
neutral (pH 7)
monocation (pH 2.0)
dication ( H , -7)
monoanion (H-
dianion (H- 16.5)
l a b (log crnax) 337.5 330 327.5 300 288 276.5 241.0 341.4 (3.77) 328.5 (3.61) 290 (3.59) 280 (3.62) 245.8 (4.72) 214.6 (4.24) 343.4 (3.76) 329.3 (3.63) 292.5 (3.59) 281.0 (3.62) 246.8 (4.71) 214.2 (4.24) 340.5 (3.71) 326.0 (3.62) 288.5 (3.59) 277.5 (3.60) 245.0 (4.70) 214.6 (4.18) 335 (3.60) 324 (s) (3.47) 287.5 (3.53) 273.5 (3.58) 235.0 (4.66) 332.5 (3.34) 288 (3.53) 272.5 (3.69) 267.5 (3.63) 231.2 (4.62) 315 (2.73) 301 (3.02) 288.5 (s) (3.43) 279 (3.59) 266.5 (3.57) 256.0 (s) (3.43) 221 (4.7) 345 (3.53) 315 (3.64) 247.5 (4.06) 228 (4.37)
n
-2.3 D A N -2.w
1
(d'f) 362 (0.42) 375
Figure 3. Absorption and fluorescence spectra of neutral species of 2-naphthylamine (2AN) and 2,3-DAN.
371 (0.57)
that observed for 2AN (414 nrn)25 in the same solvent. The molecular model of DAN is depicted in structure I, and the molecular axes are also labeled. It is clear that both of the 374 (0.56)
La
377 (0.54) J
- -
384 (0.64) 370" 400 (0.91) 390" 395'
a 358 (0.037) 344 329 318 314
360 (s) 354 34 1 328 (s) 325 314
486 (0.27) 400 (-)
'Low-temperature measurement at 77 K. *In cyclohexane +O.Ol% TFA medium; X is in nanometers, e is in dm3 mol-' cm-I. CGroundstate equilibrium is not complete at this basic condition, because pK, > 16. Absorption data are for the absorption spectrum of DAN at this basic condition. resemble those of 2-aminonaphthalene (2AN) in any one particular The only difference is that the molecular extinction coefficient of the 'Lbband of DAN is twice that of 2AN, whereas the molecular extinction coefficients for the other bands of DAN are not very different from those of 2AN. Unlike the broad, long wavelength band of 2AN, structure is observed, and it is nearly retained in most of the polar solvents used. The magnitude of the vibrational frequency is 960 f 40 cm-I. Red shifts are observed in all the absorption bands with increasing solvent polarity, while for an increase in proton donor capacity of solvents, blue shifts are noticed in the absorption maxima. Contrary to the absorption spectra, the fluorescence spectrum of DAN with the vibrational structures -950 cm-' is observed only in cyclohexane, and a regular red shift is noticed in the fluorescence spectrum in the above environments. Further, the fluorescence maximum of DAN in water (384 nm) is at lower wavelength than (32) Jaffe, H. H.; Orchin, M. In Theory and Applications of Ultraviolet Spectroscopy; Wiley: New York, 1970; p 304.
amino groups are present on the axis, which may affect the long 'Lbmore than the short axis poaxis polarized transition ' A ]La. It appears from' the data of Table larized transition 'A I that the two amino groups are twisted with respect to the naphthalene ring. This conclusion has been based on the following facts: (1) Had the two amino groups been in the plane of the naphthalene moiety, one would have observed the absorption band maxima at longer wavelengths than those for 2AN. This has been further manifested from the full band width at half-maximum (fwhm) of absorption and fluorescence spectra (shown in Figure 3). The band shape for the absorption and fluorescence spectra of 2AN and DAN are similar except that the structural features are observed in the latter. The fwhm for the absorption and fluorescence spectra (3.0 X lo3, 3.5 X lo3 cm-'; 3.2 X lo3, 3.0 X lo3 cm-I, respectively) are nearly equal. (2) Similar behavior is also observed in case of 2-hydroxy- and 2-methoxy-3naphthylamines, Le., absorption and fluorescence band maxima of these compound are at shorter wavelengths than that of 2AN, indicating that the hydroxy or methoxy and amino groups are twisted with respect to the naphthalene ring.'6b (3) The vibrational structure present in the absorption spectrum would have been absent, because in 2AN the long wavelength band is really a broad one. (4) The solvent interaction is observed with DAN, but the presence of vibrational structure even in water proves that the lone pairs are not perturbing the n-electron cloud of the naphthalene ring very much. (5) The vibrational frequency (-950 cm-I) observed in the fluorescence spectrum of DAN in cyclohexane further proves the points that a mirror image relationship is observed and that both the emitting and absorbing states are the same, Le., L,. (6) The fluorescence spectral band maximum of DAN in water is at higher frequency than that of 2AN. The above points indicate that the net effect of two amino groups on the absorption spectrum of naphthalene is of about equal degree to that of one amino group in 2AN, whereas the similar effect on the fluorescence spectrum is even less than that of one amino group. The additive effect of two amino groups have been noticed in o-phenylenediarnines.l* The increase in the molecular extinction coefficient of only the 'Lb band results from an increase in the dipolar length of DAN along the long axis, that is, the value of the dipole moment integral increases appreciably. The solvent molecules can have dispersion interactions with the solute and also can act as proton acceptor and proton donor to the solute molecule. If n x* is the lowest energy transition, the first two interactions lead to a red shift and the third leads to a blue shift in the absorption and fluorescence spectra. The red shift observed in the absorption spectrum in going from
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Acidity Constants in the Excited States
The Journal of Physical Chemistry, Vol. 92, No. 18, 1988 5285 I
-
M
i
Neutral (pH 7) Monoanion (H- 15) Monocation (pH 2) Dication (Ho-7)
- Neutral (pH 7 )
-
15L
I
-
Monoanion (H.15)
._.__ Dianion (H.16
..
1
5)
Monocation (pH 2) Dication (H0-7)
I
O0 8. 1
320
340
360
380
400
420 440 460 WAVELENGTH (nrn)
480
500
520
540
Figure 5. Fluorescence spectra of the various prototropic forms of DAN at 298 K.
spectrum of the monocation of DAN, recorded in cyclohexane containing 1 X (v/v) TFA as well as at 77 K (aqueous H2S04solvent) is also red shifted compared to that of the neutral DAN (375 to 395 nm in the former case and 370 to 390 nm in WAVELENGTH (nm) the latter case). The fluorescence spectrum of the monocation of 2,7-diaminofl~orene~~ was also red shifted as compared to that Figure 4. Absorption spectra of the various prototropic forms of D A N of the neutral 2,7-diaminofluorene (DAF) molecule, but under at 298 K. the above two conditions, it was blue shifted and thus followed the normal trend. The results of DAF were explained on the basis nonpolar to polar but aprotic solvents is because of dispersion that the solvent relaxation at 298 K was very much larger for the interactions, while the blue shift observed in methanol and water monocation than the neutral and thus the electronic excited state as solvents indicates that these solvents act as proton donors. The of the former was stabilized more than that for the latter. A regular red shift in the fluorescence spectra of DAN shows that similar explanation cannot be offered for the monocation of DAN, DAN is acting as proton donor in the excited singlet state. This because under all conditions, the fluorescence spectrum of the behavior is similar to that normally observed in the aromatic monocation is red shifted. Comparing the fluorescence spectrum amines.I6*l8This is because the charge migration from the amino of neutral DAN with that of neutral 2AN in any one particular group to the naphthalene ring is increased on excitation. Further, solvent, it is clear that the band maximum of DAN is at lower the loss of vibrational structure in the fluorescence spectrum by wavelength than that of 2AN, suggesting that the two amino increasing both solvent polarity and proton donor capacity is due groups are not coplanar to the naphthalene ring even in the excited to their increased interaction in the SIstate. singlet state. Further, the fluorescence maximum of the monoEffect of p H . The effect of proton concentration on the abcation of DAN (400 nm) is also at lower wavelength than that sorption spectrum of DAN is similar to that observed for other of neutral 2AN (415 nm). In reality, the fluorescence spectra aromatic amines. The absorption spectra of various protropic of the former species should resemble the latter, because the lone species are depicted in Figure 4, and the relevant data are compiled pair of one of the amino groups is protonated. For example, the in Table I. At pH 2, the species is a monocation, formed by the fluorescence spectra of the monocations of various phenyleneprotonation of one of the amino groups. The long wavelength diaminesIs resembled that of aniline, and similar behavior was absorption band is slightly blue shifted, whereas the band at -288 also observed in the case of 2,7-diaminofluorene1’ in a nonpolar nm is not much affected. The data of Table I further prove our medium. This suggests that the amino group of the monocation earlier point that the two amino groups are not in the plane of of DAN tends to become coplanar with the naphthalene moiety, the naphthalene moiety. This is because the first protonation will but due to the presence of the -NH3+ group ortho to the amino block one of the amino group lone pair electrons, and thus the group sterically it cannot attain the same coplanarity as is observed absorption spectrum should resemble that of 2AN. This point for 2AN. Thus, the fluorescence spectrum of the monocation of has been substantiated by the data of Table I, Le., the band DAN is still blue shifted with respect to neutral 2AN but red maximum of monocation of DAN is at 332.5 nm and that of 2AN shifted with respect to neutral DAN. is at 334 nm. This type of behavior has been observed in the case Similar to the behavior of aromatic amines23-z5*33-39 and the of diamines of fluorenel’ and benzene.Is With a further increase monocations of phenylenediamines,I* the fluorescence intensity -6), the absorption spectrum of DAN of proton concentration (Ho is largely blue shifted and exactly resembles that of n a ~ h t h a l e n e . ~ ~ of the 400-nm band decreases with an increase of proton concentration without the appearance of any new fluorescence band, This clearly indicates that the dication is formed by protonating even though the dication of DAN is formed in the ground state. the second amino group, in other words, both the lone pairs of It is only at Ho-4 that a very large blue shifted but structured the amino groups are blocked by the protons. band appears. This exactly resembles that of naphthalene40 and After H- 15, the absorption spectrum of DAN starts getting thus can be assigned to the dication of DAN, formed by the red shifted, and the red shift is nearly 10 nm. This indicates that the formation of monoanion of DAN starts in the ground state but is not complete even at H- 17. (33) Shizuka, H.; Tsutsumi, K. J . Photochem. 1978, 9 , 334. The fluorescence spectra of various prototropic species of DAN, (34) Tsutsumi, K.; Sekiguchi, S.; Shizuka, H.J . Chem. SOC.,Faraday Trans. 1 1982, 78, 1087. formed in the H,/pH/H- range from -10 to 16 are recorded in (35) Tsutsumi, K.; Shizuka, H. Z . Phys. Chem. (Munich) 1978,111, 129. Figure 5, and the data are compiled in Table I. The formation (36) Swaminathan, M.; Dogra, S. K. Can. J . Chem. 1983, 61, 1064. of various prototropic species in the excited state are the same (37) Mishra, A. K.; Dogra, S . K. J . Chem. SOC.,Perkin Trans. 2 1984, as obtained in the ground state, but the fluorescence spectra of 943. (38) Mishra, A. K.; Dogra, S. K. J . Photochem. 1983, 23, 163. some of the species do not follow the same trend as observed in (39) Subba Rao, R. V.; Krishnamurthy, M.; Dogra, S . K. Indian J . Chem. the absorption spectra. With a decrease of pH, unlike the abSect. A 1986, 2SA, 517. sorption spectrum, the fluorescence spectrum of the monocation (40) Berlman, I. B. In Handbook of Fluorescence Spectra of Aromatic of DAN shifts to red, which is anomalous. The fluorescence Molecules; Academic: New York, 1965; p 104.
The Journal of Physical Chemistry, Vol. 92, No. 18, 1988
5286
Manoharan and Dogra TABLE 111: Lifetime of the Monocation at Different Acid Concentrations
TABLE 11: Acidity Constants of DAN in the Ground and First Excited Singlet States
eauilibrium
PK. (Sn)
PK. (S,)
PH
T
DANHZ2' ==DANH' + Ht DANH' DAN HC DAN DAN- H t
0.7 3.4 >16
-5.3
2.0 1.8
17.4 17.1
+
1 .8
+
,,-,
c
$
