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J . Phys. Chem. 1991, 95, 2310-2317
various vibrational modes of the fragment. The vibrational temperature is estimated to be close to the room temperature. This vibrational excitation of the monomer fragment may be explained by the dissociation of the excimer in the ground state. Subsequent to the dissociation, each fragment will obtain an excess energy of ~ 2 8 0 cm-l. 0 The excess energy is expected to be distributed in part over the vibrational modes of the fragment. Without calculating the vibrational density of state of the fluorene monomer, it is somewhat dangerous to reach a definitive conclusion regarding the vibrational distribution of the fragments. Nevertheless, the result suggests that the excimer state relaxes to the ground state mainly via the spontaneous emission process to dissociate, leading to the vibrational excitation of the monomer fragments. This explains the Occurrence of the broad excitation spectrum of the fragment following the excimer dissociation. In conclusion, the excimer formation and dissociation dynamics upon excitation of the fluorene dimer into the SI state have been investigated by using pump-probe techniques with fluorescence detection. In analogy to the photochemical hole-burning experiments in condensed phase, the technique has been developed to probe the population depletion of the dimer undergoing the excimer formation and dissociation. This technique enabled the separation and identification of the congested spectra in the SI state of the dimer. The spectral features have been assigned as arising from homogeneous sources, i.e., exciton splittings and intermolecular vibrational structures. The pump-probe experiment has been also applied to monitor the monomer fragments produced by the excimer dissociation. The photofragments build up as the excimer state decays into the ground state. A vibrational excitation of
the monomer fragment is observed that indicates that the available energy from the dissociation process is distributed among vibrational modes. The vibrational distribution is similar to that of the monomer in the static vapor phase at room temperature. Future experiment will probe the excimer formation dynamics of the fluorene dimer by using a pumpprobe photoionization technique. One of the most important consideration in the photoionization dynamics is the question of whether ionization occurs efficiently from the excimer state to its ionic state. Very recently, a two-color ionization technique has been applied to probe an excimer state of benzene produced by the S2excitation of the benzene dimer.20 The ionization efficiency from the excimer state (two-color ionization) was found to be more efficient with respect to the ionization from the dimer configuration (one-color ionization). In addition, the ionization energy from the excimer state should be quite different from that of the excited state of the vdW dimer. Although some ionization experiments were reported for fluorene vdW clusters with rare-gas atoms and small molecules:l*z no attempt has been made for the fluorene dimer. It would therefore be extremely interesting to probe the difference in ionization behavior between the dimer and excimer states.
Acknowledgment. This work was supported in part by the National Science Foundation. (20) Shinohara, H.; Nishi, N. J . Chem. Phys. 1989, 91, 6743. (21) Leutwyler, S.;Even, U.; Jortner, J. J. Chem. Phys. 1983, 79, 5769. (22) Im,H. S.;Grassian, V. H.; Bernstein, E. R. J. Phys. Chem. 1990, 94,
222.
OxIdatien of 10-Methyiacridan, a Synthetic Analogue of NADH, and Deprotonation of Its Cation Radical. Convergent Application of Laser Flash Photolysis and Direct and Redox Catalyzed Electrochemistry to the Kinetics of Deprotonation of the Cation Radical Agnes Anne,lb Philippe Hapiot,'a.c Jacques Moiroux,lb Pedatsur Neta,**laand Jean-Michel Sav6ant*Ilb Chemical Kinetics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, and Laboratoire d'Electrochimie Moldculaire de I'Universitd Paris VU, Unitt de Recherche AssociEe No. 438, 2 Place Jussieu, 75251 Paris Cedex 05, France (Received: June 22, 1990; In Final Form: September 27, 1990)
Photolysis of 10-methylacridan (AH) in acetonitrile solutions containing CC14 led to quantitative oxidation of AH to A+. Laser flash photolysis of the solution with 248- or 308-nm light yielded the cation radical A H " which decays by deprotonation to give the neutral radical A'. AH'+ could be also generated by means of homogeneous redox catalysis using variously substituted ferrocenes and the rate of deprotonation kHby various bases was derived from both techniques. There is a good agreement between the values determined for kH (here and, previously, by direct electrochemistry using ultramicroelectrodes). The vast majority of the data points in a plot of log kH vs the pK, of the bases, covering 18 pK, units and rate constants up to the diffusion limit, fall on the same Bransted line in spite of the fact that they involve bases of quite different structure: pyridines, aliphatic nitrogen bases, and carboxylates. y-Radiolysisof acetonitrile/CC14/AH solutions also resulted in quantitative oxidation to A+. However, pulse radiolysis of this system indicated relatively slow oxidation of AH by Cl3C0O' radicals. Pulse radiolysis experiments in acidic aqueous solutions showed that AH is oxidized by C12'- to give AH'+, which decays with a first-order rate constant of 9 X lo2 s-'. Reduction of A+ gave A' which did not undergo protonation even at 2 M HC104, but reaction of A+ with H atoms resulted in partial addition of H to the 9-position to give AH".
Addition or removal of one electron to or from organic molecules often yields intermediates that undergo a very rapid transformation leading to a secondary intermediate whose chemistry controls the distribution of products. The mechanistic analysis of such systems and the determination of the rate constant of the transformation ( I ) ,(a) National Institute of Standards and Technology. (b) Universit€ de Paris 7. (c) Visiting scientist at NIST. Permanent address: Laboratoire d'Electrochimie Mol6culaire de I'UniversitE de Paris 7.
of the primary intermediate thus require the development and mutual comparison of fast kinetic techniques. Direet electrochemical techniques, such as cyclic voltammetry or potential-step methods, appear particularly attractive in this respect since the same experimental setup that allows the injection or removal of the electron at any desired potential of the working electrode also permits the investigation of the reaction mechanism and the kinetic characterization of the rate-determining step through the current response of the electrode." These techniques, however, do not
0022-365419 112095-2310%02.50/0 0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 2371
Oxidation of IO-Methylacridan allow the kinetic characterization of intermediates having lifetimes below a few tenths of a microsecond, even if one takes advantage of the improvements brought about by the use of ultramicroelectrodes (disk electrodes with diameters in the micrometer range).2b Redox catalysis of the electrochemical reaction, whereby the addition or the removal of one electron from the substrate is no longer carried out at the electrode but by the reduced or oxidized form of a redox couple reversibly generated a t the electrode, is a powerful means of increasing the range of attainable rate constankh Intermediates having lifetimes down to a few nanoseconds can be characterized by this method. Therefore, redox catalysis appears as complementary to the direct electrochemical techniques in the sense that its domain of application ranges from about 5 X lo4 to 5 X IO8 s-l in terms of first-order or pseudo-first-order rate constants whereas that of the direct electrochemical techniques ranges from ca. 5 X IO-* to 5 X IO6 s-l. In spite of the wide range of rate constants covered by the combination of the above two techniques, it is desirable to have additional independent methods allowing a further extension toward high rate constants and a verification of their results, particularly at the upper edge of their range of applicability where their precision may be altered. Laser flash photolysisgaand pulse radiolysisgbboth offer such possibilities, although the chemistry required to produce the ion radicals of interest is rarely straightforward. There is thus a converse need of checking their results by independent methods such as direct or redox catalyzed electrochemical techniques. The system we investigated in this connection is the oxidation of 1O-methylacridan in acetonitrile. Fast cyclic voltammetry and potential step techniques utilizing ultramicroelectrodes (diameter < IO pm) allowed recently4 a detailed mechanistic and kinetic analysis of the oxidation of 10-methylacridan (AH) and the reduction of IO-methylacridinium (A+). This system is of particular interest as synthetic analogue of the NADH/NAD+ couple, both in terms of the one-electron redox steps and of the acid-base reactivities of the radicals derived from AH and A+, i.e., the cation radical AH'+ and the neutral radical A*.5 The electrochemical oxidation of A H proceeds by the following
reaction^.^
+ eAH" + B -% A' + BH+ A' + AH'+ A+ + A H AH
FL
AH'+
-
(2)
(3) The rate constant kH for deprotonation of AH'+ by various pyridine bases, B, was derived from the electrochemical measurements and found to depend on the pK, of the bases6 The resulting B r h t e d plot log kH vs pKa starting a t pK, = 4.2 and log kH = 3.3 exhibited a rising linear section with a slope of 0.39 clearly indicating that the forward and backward reactions are under activation control except for a few data points at large pK, that seemed to plateau at a value 3-4 orders of magnitude below the diffusion limit? The aim of the work reported hereafter was to confirm the determinations made by the direct electrochemical techniques and extend them to other bases' in addition to the (2) (a) Andrieux, C. P.; Saviant, J-M. Investigation of Rates and Mechanisms of Reactions. In Techniques of Chemistry; Bernasconi, C. F., Ed.; Wiley-lnterscience: New York, 1986; Vol. 6, Part 2, pp 305-390. (b) Andrieux, C. P.; Hapiot, P.; SavCnt, J-M. Chem. Rev. 1990, 90, 723. (3) (a) Porter, G.; West, M. A. Techniques of Chemistry, 3rd ed.; Hammes. G. G., Ed.; Wiley-lnterscience: New York, 1974; Vol. 6, Part 2, pp 367-462. (b) Dorfman, L. M. Techniques of Chemistry, 3rd 4.;Hammes, G. G.. Ed.; Wiley-Interscience: New York, 1974; Vol. 6, Part 2. pp 463-519. (4) Hapiot, P.; Moiroux, J.; Savbant, J-M. J. Am. Chem. Soc. 1990, 112, 1337. ( 5 ) (a) Carlson, B. N.; Miller, L. L. J . Am. Chem. SOC.1985, 107, 479. (b) Fukuzumi, S.;Koumitsu, S.;Hironaka, K.; Tanaka, T. J . Am. Chem. Soc. 1987, 109, 305. (c) Kreevoy, M. M.; Ostovic, D.; Lee, I. S.;Blinder, D. A.; King, G. W. J . Am. Chem. Soc. 1988,110,524. (d) Miller, L. L.; Valentine, J. R. J . Am. Chem. Soc. 1988, 110, 3982. (e) Lee, L. H.; Ostovic, D.; Kreevoy, M. M. J . Am. Chem. Soc. 1988,110,3989. (f) Powell, M. F.;Wu, J. C.; Bruice, T. C. J . Am. Chem. Soc. 1984, 106. 3850. (6) Unless otherwise specified all value given in the present paper are those of pK,'s in acetonitrile.
I
E(V(SCE
0.5
1.0
Figure 1. Redox catalysis of the oxidation of 10-methylacridan (1 mM) by ferrocene(/ferrocenium)carboxylic acid (2 mM) couple in buffered 10 mM 3.5-dimethylacetonitrile (10 mM 3,Sdimethylpyridine pyridinium): (a) ferrocenecarboxylic acid alone; (b) 10-methylacridan alone; (c) both compounds present. Scan rate 1 Vas-'. Temperature 20
+
OC.
pyridine series that give rise to faster deprotonation reactions. The two main techniques that were used in this connection were homogeneous redox catalysis electrochemistry and laser flash photolysis. Pulse radiolysis was used in additional experiments mostly aiming at investigating the role of the oxidative quenchers used in the flash photolysis experiments. Kinetic studies of the oxidation of NADH and synthetic analogues by several one-electron acceptors such as ferrocenium and substituted ferrocenium cation^^^.^ and ferrocyanideg in waters or in mixed aqueous solvent^^^,^ have been reported previously. In some of these the redox catalytic approach was used; i.e., the substrate was oxidized by an electrochemically generated one-electron acceptor. However, these studies led to the determination of the rate constant of the electron transfer between the substrate and the one-electron acceptor rather than to the determination of the rate constant of the proton transfer from the AH'+ to the solvent or to an added base as was the purpose of the present work. Photoxidation of 10-methylcridan has been studied under various conditions and the intermediate cation radical has been observed.1° However, the properties of the cation radical have not been studied in detail and the final products contained only partial yields of IO-methylacridinium. The rate constants of the deprotonation of the AH" radical, the main goal of the present laser flash photoxidation study, have not been determined so far. Out of a series of attempts, carbon tetrachloride in aerated acetonitrile was found to be the best oxidative quencher allowing the quantitative oxidation of AH to A+ upon 3 13-nm photolysis. We also report the results of a radiolysis investigation of the oxidation of IO-methylacridan that helped to validate the laser flash photolysis procedure for determining the AH'+ deprotonation rate constants. The radiolytic reduction of IO-methylacridinium was also investigated. (7) Including negatively charged base such as carboxylate ions. In these cases reactions 2 should read AH'+
+ B- s A' + BH
(2)
(8) (a) Carlson, B. W.; Miller, L. L. J . Am. Chem. SOC.1983,105,7453. (b) Carlson, B. W.; Miller, L. L.; Neta, P.; Grcdkowski, J. J . Am. Chem. Soc. 1984, 106, 7233. (c) Matsue, T.; Suda, M.; Uchida, I.; Kato, T.; Akiba, U.; Osa, T. J. Electroanal. Chem. 1987, 234, 163. (9) (a) Sinha, A.; Bruice, T. C. J . Am. Chem. Soc. 1984,106,7291. (b)
Brewster, M. E.; Kaminski, J. J.; Gabanyi, Z.; Czako, K.;Sinray. A,; Bodor, N. Tetrahedron 1989, 45,4395. (10) (a) Peters, K.S.;Pang, E.; Rudzki, J. J. Am. Chem. Soc. 1982,104, 5535. (b) Zanker, V.; Erhardt, E.; Mantsch, H. Z . Naturforsch. B 1967.22, 795. (c) Kano, K.; Zhou, B.; Hashimoto, S.Bull. Chem. Soc. Jpn. 1987.60, 1041.
Anne et al.
2372 The Journal of Physical Chemistry, Vol. 95, No. 6, 1991
TABLE I: Homogemous Redox Catalysis of the Oxidation of IO-Metbylacridan CB”, U,C (ip/2i:) x kineticd kH[(k +e)/ EP: V P,,V cpo,” catalyst (P) vs SCE mM base B P&BHtb mM v S-’ ( C p ” / c ~ ~ ’ ) control ( k -e)] kH/(k -e) VS SCE ferrocenecarboxylic acig benzoylferrocene
0.632 0.632 0.655
0.4-2 0.4-2 0.4-2
ferrocene
0.405 0.405 0.405 0.655 0.405 0.405 0.405 0.405 0.405 0.405
2-8 2-8 4 0.4-2 4 4 4 4 4 4
benzoylferrocene ferrocene
ovridine 3,5-dimethylpyridine 1 -ammonio-3-aminopropane salicylate benzylamine benzylamine 2,4,6-trimethylpyridine terr-butylamine piperidine 3-bromobenzoate pyrrolidine 1,3-diaminopropane benzoate
12.3 14.5 15
.I
16.7 16.8 16.8 16.8 18.1 18.9 19.5 19.6 19.7 20.7
10.0 1 10.0 1 10.2 1 20.0 20.6 100.0 10.0
100.0 40.0 20.0 20.1 20.5 20.0
0.05 0.05 0.063 1 0.063 0.063 0.063 0.063 0.063 0.063
1.48 1.67 1.96
X X X
1.05 1.17 1.26 1.96 1.24 1.28 1.28 1.24 1.25 1.29
X X
xx X
xx xx xx xx xx xx
+ + + + + + -
-
-
-
-
0.795 0.795 0.785
-
0.707 0.695
-
0.795
+ + + + + + +
a CAuo/Cpo .... , . = 0.5. From ref 15. 4.2) justifies the neglect of the backward rate constant of reaction 2 as compared to the rate constant of reaction 6. Maintaining the bulk concentration CBoof the base B at least 10 times larger than the bulk concentration of AH, CMo,allows one to consider that the deprotonation of AH’+ (forward reaction 2 ) obeys pseudo-first-order kinetics. As discussed previously,14 a convenient approach to the kinetics of reactions 5 and 2 is to investigate the variations of the observable (ip/2i:) (Cpo/CAHo)as a function of the scan rate (u) and of the concentration of both the catalyst, Cpo, and of the base CBo (ip is the catalytic oxidation peak current and i is the anodic peak current of the catalyst in the absence of A .)!-I Depending upon the nature and concentration of the catalyst and of the strength and concentration of the base, two limiting kinetic behaviors may be reached: (i) If k+HCBo >> k*Cp0, forward reaction 5 is the rate-determining step. The system then depends upon only two parameters, (RT/F)(k+,Ct/u)and CAHo/Cpo.This situation is readily recognized experimentally by the fact that the observable (i,/ 2i:)(C$/CAHo) varies with the concentration of the catalyst, while keeping CAHo/CpO constant. The rate of this variation is that predicted from previously computed working curves relating (ip/2i,0)(Cpo/CAHO) to ( R T / F ) ( ~ + , C , ~for / U various ) values of the “excess factor”, CAHO/Cpol4 (see particularly Figure 3 in ref 14c). The rate constant k+, can then be derived from the experimental data by using the same working curves. No information on the follow-up reaction ( 2 ) is available under these conditions. Another diagnostic criterion of the kinetic control by forward reaction 5 is that the observable (ip/2i:)(cpo/cAHo) is independent of CBo. (ii) If kHCBo
