J . Phys. Chem. 1985, 89, 4302-4306
4302
and Ar, complexes. Here is a definite case of vibrational predissociation limited by the finite rate of internal energy transfer. (ii) (CH,),-perylene is an intermediate case. Here, substantial internal randomization of the vibrational energy has occurred even below the dissociation threshold (see Figure 12). No resonance fluorescence is observed at 705 cm-I, and the dissociation event happens rapidly and completely when the excitation energy exceeds the binding energy of the complex. (iii) C3H6-perylene is again an intermediate case, but of a more complex variety. Here, the size of the guest species, the binding energy and the number of relevant internal degrees of freedom have increased so that, above the dissociation threshold, partial dissociation is observed. As the system becomes even larger, it is expected that internal statistical considerations will begin to become important, so that the dissociation rate becomes a function of excess energy above the vibrational predissociation threshold
and of the complexity of the system. With appropriate development, this promises to become a valuable test bed for studying unimolecular decomposition processes, especially since the complexity, the binding energy, and the available energy in the system may be varied over a wide range, while the central chromophore remains the same. Acknqwledgment. We are grateful to Ashish Bhattacharya and Stephen Cohen for valuable technical assistance. We also thank Bruce Boczar and Steven Schwartz for material from their Ph.D. theses, and Dr. Igor Gulis for valuable assistance in the early stages of this work. This work was supported by the National Science Foundation both directly (CHE-82-07889) and through the Materials Research Program (DMR-82-167 18). Registry No. Ar, 7440-37-1; CH4, 74-82-8; C2H6, 74-84-0; c-C,H,, 75-19-4; n-C,H,, 74-98-6; perylene, 198-55-0.
Apparent Acidity Constant In the Exclted State: Evidence for the Absence of Excited-State Prototroplc Equllfbrium in 4-( 9-Anthry1)-N,N-dlmethylanlllne' Haruo Shizuka,* Toshiaki Ogiwara, and Ehichi Kimura Department of Chemistry, Gunma University, Kiryu, Gunma 376, Japan (Received: April 4, 1985)
The excited-state proton-transfer reactions of 4-(9-anthryl)-N,N-dimethylaniline(A) in EtOH-H,O (4: 1 by weight) mixtures at 300 K have been studied by fluorimetry and the single-photon-countingmethod. It is found that there is no excited-state prototropic equilibrium of A. The excited acid and base forms of A decay independently with single exponential functions. The pK,* value (2.4 EL: 0.2) of A obtained from the usual fluorescencetitration curves is, therefore, a p appqrent value, reflecting the pKa value (2.5 f 0.2) in the ground state. This is mainly caused by the extremely slow proton-dissociation rate of 'A+H* compared to its decay rate (2 X lo8 s-l). No proton-induced quenching of A is observed, though the intramolecular CT state lACT*is produced very rapidly via 'A* under the experimental conditions.
Proton-transfer reactions in the excited state of aromatic compounds are elementary processes in chemistry. The acid-base properties in the excited state of aromatic compounds are closely related to the corresponding electronic structure, which is significantly different from that in the ground state. Since the original work of Forster2 (1950) and Weller3 (1952) showing that the acidity constant pKa* in the excited state is markedly different from that in the ground state, a large number of studies of the pK,* have been r e p ~ r t e d . ~ - It ' ~ is well-known that pKa* values can be estimated f r o g the Forster ~ycle:-~J~the fluorescence T, absorbance titration curve.I2 titration c ~ r v e ,and ~ . ~the T, These methods involve the aSsumption that very fast proton transfer in the excited state leads to the establishment of an acid-base equilibrium during the lifetime of the excited state. Establishment of prototropic equilibrium has been reported in the
-
(l).This work was supported by a Scientific Research Grant-in-Aid from the Ministry of Education of Japan (No. 58470001). (2) FGrster, Th. Z. Elektrochem. Angew. Phys. Chem. 1950, 54, 42, 531. (3) Wellex, A. Eer. Eunsenges.Phys. Chem. 1%2,56,662. 1956,66, 1144. (4) Weller, A. Prog. React. Kinet. 1961, I , 189. (5) Beens, H.; Grellmann, K. H.; Gurr, M.; Weller, A. Discuss. Faraday SOC.1965, 39, 183. (6) Vander Doncket, E. Prog. React. Kiner. 1970, 5, 273. (7) W;hry, E. L.; Rogers, L. B. In "Fluorescence and Phosphorescence Analyses , Hercules, D. M.,Ed.;Wiley-Interscience: New York, 1966; p 125. (8) (a) Schulman, S. G. In "Modern Fluorescence Spectroscopy", Vol. 2, Wehry, E. L., Ed.; Plenum: New York, 1976. (b) Schluman, S. G. In 'Fluortsccncc and Phosphorescence Spectroscopy"; Pergamon: Oxford, 1977. (9) Ireland, J. F.; Wyatt, P. A. H. Adu. Phys. Org. Chem. 1976, 12, 131, and a number of references therein. (IO) KMpffer, W. Ado. Photochem. 1977, I O , 31 1. (1 1) (a) Grabowski, 2.R.; Grabowska, A. Z. Phys. Chem. (Wiesbaden) 1976, 101, 197. (b) Grabowski, 2. R.; Rubaszewska, W. J. Chem. SOC. Faraday Trans. 1 1977, 73, 11. (12) Jackson, G.; Porter, G. Proc. R . SOC.London, Ser. A 1961, 260, 13.
excited singlet state of 2-naphth01-6,8-~isulfonate.~~ A laser study of the prototropic equilibrium of triplet benzophenone has been reported by Rayner and Wyatt.I4 Similar equilibria in the triplet state of phenyl alkyl ketones have been recently demonstrated, since the triplet lifetime is long enough to allow acid-base equilibrium. However, it has been shown by Tsutsumi and Shizuka16 (1977) that proton-induced fluorescence quenching competitive with proton-transfer reactions is present in the excited singlet state of neutral naphthylamines (Le., simple acid-base equilibrium cannot occur in the excited state of aromatic amines) and dynamic analysis accounting for the proton-induced quenching is, therefore, needed in order to obtain accurate pKa* values. Dynamic analysis with fluorimetry was applied to 1-aminopyrene," 1-aminoanthracene,I8 phenanthrylamine~,'~and naphthols.20 The Stuttgart grou8' has supported our method to determine the pKa* values of naphthylamines. Similar experiments for excited naphthols have been reported by Harris and SelingerS2,
'
(13) Schulman, S.G.; Rosenberg, L. S.; Vincent, Jr. W. R. J. Am. Chem. SOC.1979, 101, 139. (14) Rayner, D.M.; Wyatt, P. A. H. J. Chem. SOC.,Faraday Trans. 2, 1974, 70,945. (15) Shizuka, H.; Kimura, E. Can. J. Chem. 1984, 62, 2041. (16) Tsutsumi, K.; Shizuka, H. Chem. Phys. Lett. 1977,52,485. Z . Phys. Chem. (Wiesbaden) 1978, 1 1 1 , 129. (17) Shizuka, H.; Tsutsumi, K.;Takeuchi, H.; Tanaka, I. Chem. Phys. Lett. 1979, 62, 408. Chem. Phys. 1981,59, 183. (18) Shizuka, H.; Tsutsumi, K . J. Phorochem. 1978, 9, 334. (19) Tsutsumi, K.;Sekiguchi, S.; Shizuka, H. J. Chem. SOC.,Faraday Trans. I , 1982, 78, 1087. (20) Tsutsumi, K.; Shizuka, H. Z . Phys. Chem. (Wiesbaden) 1980, 122, 179 .
(21) Hafner, F.; Worner, J.; Steiner, U.; Hauser, M. Chem. Phys. Lett. 1980, 72, 139.
0022-3654/85/2089-4302SOl .50/0 0 1985 American Chemical Society
Apparent Acidity Constant in the Excited State For the proton-induced quenching mechanism, it has been established that proton-induced quenching m u r s via electrophilic protonation at one of the carbon atoms of the aromatic ring, leading to proton exchange (or isotope exchange) and that the intramolecular charge-transfer structure in the excited state is responsible for the q u e n ~ h i n g . ~It~has ~ ~ been ~ reported that proton-induced quenching occurs in the excited state of aromatic ~-~~ compounds such as l - m e t h o ~ y n a p h t h a l e n e , ~cyanonaphthalene^,^^ 9,9’-bianthr~l,~~ and 1-(p-aminophenyl)pyrene2’ having intramolecular C T character in the excited state. On the other hand, it is known that 4-(9-anthryl)-N,N-dimethylaniline (A) gives rise to dual emissions from a strongly polar
p,
CH3, N
The Journal of Physical Chemistry, Vol. 89, No. 20, 1985 4303 r
aJ
0.4 (d
9 L
0
In
n
a 0.2
0 A C T and a less polar excited state in polar media.28-35 It is considered that the excited CT state of A results from a solvent-induced rapid relaxation of the excited state of A*. In low-viscosity polar solvents the equilibrium between these two states is attained immediately after picosecond pulsed (-30 ps) e~citation.~~ In a course of a study on proton-transfer reactions in the excited state containing proton-induced quenching,36we became interested in the acid-base properties in the excited state of A. This paper reports evidence for the absence of excited-state prototropic equilibrium of A having an apparent acidity constant in the excited state obtained by nanosecond single-photon-counting method with fluorimetry. Experimental Section Materials. 4-(9-Anthryl)-N,N-dimethylanilinewas synthesized according to the usual m e t h ~ d . ~The ’ crude product was chromatographed on silica gel and recrystallized from benzene. Ethanol (Spectrosol, Wako) and distilled water were used as a mixed solvent (EtOH:H20 = 4:l by weight). H$04 (97% Wako) was used without further purification. The actual acid content was determined by titration. The mean activity coefficient (r*) of sulfuric acid in EtOH-H,O (4:l by weight) mixtures3’ was used. (22) Harris, C. M.; Selinger, B. K. J . Phys. Chem. 1980, 84, 891, 1366. (23) Shizuka, H.; Tobita, S.J. Am. Chem. SOC.1982,104,6919. Tobita, S.;Shizuka, H. Chem. Phys. Left. 1980, 75, 140. (24) (a).Shizuka, H.; Tsutsumi, K. Bull. Chem. SOC.Jpn. 1983, 56, 629. (b) Weller predicted this quenching mechanism in an early stage, which was quoted in F6rster, Th. Chem. Phys. Lett. 1972, 17, 309. (25) Shizuka, H.; Fukushima, M.; Fuju, T.; Kobayashi, T.; Ohtani, H.; Hoshino, M. Bull. Chem. Soc. Jpn. 1985,58,2107. A nanosecond laser flash photolysis study of methoxynaphthalens has been carried out. (26) Shizuka, H.; Ishii, Y.; Morita, T. Chem. Phys. Lett. 1977, 51, 40. (27) Hagopian, S.;Singer, L. A. J . Am. Chem. SOC.1983, 105, 6760. (28) Okada, T.; Fujita, T.; Kubota, M.; Masaki, S.;Mataga, N.; Ide, R.; Sakata, Y.; Misumi, S.Chem. Phys. Letr. 1972, 14, 563. (29) Chandross, E. A. In “The Exciplex”,Gordon, M., Ware, W. R., Eds.; Academic Press: New York, 1975; p 187. (30) Okada, T.; Fujita, T.; Mataga, N. Z . Phys. Chem. (Wiesbaden) 1976, 101, 75. (31) Migita, M.; Okada, T.; Mataga, N.; Sakata, Y.; Misumi, S.;Nakashima, N.; Yoshihara, K. Bull. Chem. SOC.Jpn. 1981, 54, 3304. (32) Siemiarczuk, A,; Grabowski, 2. R.; Kr6wczyfiski, A,; Asher, M.; Ottolenghi, M. Chem. Phys. Left. 1977, 51, 315. (33) Siemiarczuk, A.; Koput, J.; Pohrille, A. Z . Nuzurforsch A 1982,37A, 598. (34) Siemiarczuk, A. Chem. Phys. Lerf. 1984, 110,437. (35) Okada, T.; Kawai, M.; Ikemachi, T.; Mataga, N.; Sakata, Y.; Misumi, S.;Shionoya, S.J . Phys. Chem. 1984.88, 1976. (36) Shizuka, H. Acc. Chem. Res. 1985, 18, 141. (37) Parsons, R. ‘Handbook of Electrochemical Constants”; Butterworths: London, 1959; p 33.
Wavelength I nm Figure 1. The change in absorption spectra of A in EtOH-H,O (4:l) mixtures ([A],, = 7.12 X M) at various p H values at 300 K: (a) without acid, (b) pH 3.15, (c) p H 2.31,and (d) pH 1.13.
t
0’30
6
4
2
0
PH Figure 2. The absorbance titration curves measured at 374 (a), 365 (b), and 305 nm (c) at 300 K.
All samples were thoroughly degassed by freeze-pumpthaw cycles. Absorption and Emission Measurements. UV absorption spectra were taken with Hitachi 200-10 and 139 spectrophotometers. The fluorescence spectra were recorded with a Hitachi M850 spectrofluorimeter. Spectral corrections for the emission spectra were made. The fluorescence quantum yields were measured by comparison with a quinine bisulfate4 1 N H2S04 solution (aF = 0.54).38~39The fluorescence response functions were recorded with a Horiba NAES-1100 nanosecond time-resolved spectrofluorimeter equipped with a computer system. The functions were analyzed by the deconvolution method. Results and Discussion UV Absorption Spectra. Figure 1 shows the change in absorption spectra of 4-(9-anthryl)-N,N-dimethylaniline (A) in (38) Melhuish, M. H. J. Phys. Chem. 1961, 65, 229. (39) Demas, J. N.; Crosby, G. A. J . Phys. Chem. 1971, 75, 991.
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The Journal of Physical Chemistry, Vol. 89, No. 20, 1985
Shizuka et al.
Wavelength I n m
1
0. 4 IO U
L L
0.2
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50.5
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.-wcn
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.-m
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- 0.4
0.2
- 0.2
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20
.
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Figure 3. Absorption and fluorescence spectra of (a) neutral A, (b) A+H at pH l.13, and (c) the mixture of A and A+H at pH 3.1, in EtOH-H,O (4:l) at 300 K. For details, see the text.
EtOH-H,O (4:l) mixtures ([A], = 7.12 X M) at various pH values3' at 300 K. The profile of absorption spectra of pH 1. 1 3 is similar to that of 9-phenylanthracene, showing that the amino group of A is protonated. There is no change in the peak intensity at 385 nm (e = l.Os X lo4 M-l cm-' ) of A at various pH values. Isosbestic points a t 325, 349, 360, 370, and 385 nm were observed at various concentrations of protons, reflecting the equilibrium:
A+H
+ H,O
'k k
A -1
+ H,O+
(1)
where A+H denotes the corresponding anilinium ion and ko1and ko-, are the rate constants for proton dissociation and association, respectively. Figure 2 shows the absorbance titration curves measured at 374 (a), 365 (b), and 305 nm (c) at 300 K. From the midpoints of the titration curves, the acidity constant pKa in the ground state of A can be determined to be 2.5 f 0.2 at 300 K. Fluorescence Spectra and Fluorescence Titration Curves. Figure 3 shows the fluorescence spectra of neutral A (a), A+H at pH l.13 (b), and a mixture of A and A+H at pH 3.15 (c) in EtOH-H,O (4:l) at 300 K. For neutral A, an intramolecular charge-transfer emission with a broad and structureless band at 567 nm is observed. It is known that the C T emission of A arises from the complete CT structure of A ('Acr*) in polar media,28-35 and that C T state formation from the normal excited singlet state of A ('A*) is very rapid (
