4752
J. Phys. Chem. 1991, 95, 4752-4761
modified optically transparent thin-layer electrode cell was incorporated into a luminescence spectrometer as described previously2' and solutions of AQX (X = Br, H, or I) in acetonitrile/O.l M TBAP were flowed into the cell. The flow was then stopped and the solution electrolyzed for about 30 s to form the radical anion AQX' which was excited at wavelengths between 200 and 700 nm and any luminescence observed. AQBr was examined in addition to AQI since its radical anion shows a well-resolved UV/visible absorption spectrum, having clear bonds centered at 360,390, and 410 nm.7 It was found that, whereas excitation (of AQH- and AQHaP-) at 560 nm leads to fluroescence at 575 nm22the absorption between 360 and 420 nm shows emission at 480 nm. Evidently these radical anions luminesce from both the first excited state and an additional, higher, excited state, a phenomenon that is not commonly observed in molecules with closed-shell ground states.23 The dissociation of AQI'- into A Q ' and I- is symmetry forbidden in terms of orbital angular momentum, involving a ?r state to u state conversion, and is an activated p n x x s ~ . ~ The a question arises why the excited state of AQI'- reached by irradiation at 565 nm should expel iodide more than an order of magnitude faster than that reached by 417 nm. A hint to the explanation may lie in the interpretation of UV/visible and luminescence data. The transition corresponding to the 565-nm absorption is a simple r* to r* electronic transition.26 The excited state is a doublet and its dissociation (21) Compton, R. G.;Fisher, A. C.; Wellington, R. G. Elecrrwnalysis 1991, 3, 27-29. (22) Erikscn, J.; Jorgenscn, K. A.; Linderberg, J.; Lund. H. J . Am. Chem. SOC.1984.106.3083-3092. (23) Barltrop, J. A,; Coylc, J. D. Excired Stares in Organic Chemistry, Wiley: New York, 1975. (24) Clarke, D. D.; Coulson, C. A. J . Chem. Soc. ( A ) 1969, 169-172. 125) Konovalov. V. V.: Reitrimring. A. M.: Tsvetkov. Yu. D.: Bilkis. I. 1. Chem..Phys. Left. '1989, '157. 257-2gO. (26) Fulton, A. Ausr. J . Chem. I=, 21, 2847-2852.
-
+
2(AQI'-)* 2AQ' Ialthough symmetry forbidden, is nevertheless spin allowed. However, the transition at 417 nm, and those at 360 and 390 nm, result from the excitation of a different electron and involve (*,**I to (r*,r*)transition^.^^ The luminescence data indicate that the two excitations of higher energy (360 and 390 nm) relax to form the lowest 2(r*,r*) energy state which may then emit (giving rise to the 480-nm radiation) or also possibly rapidly intersystem state. The latter may cross into a lower energy quartet '(r*,r*) revert to ground-state AQI' (thermally, or by emission of 480-nm radiation) or else dissociate ~(AQI*-)* *AQ' + IThis is now a spin-forbidden process and this may explain why excitation at 417 nm, although giving rise to a more energetic excited state, leads to slower iodide expulsion than with excitation at 565 nm, since no quartet state can be associated with the latter.
-
Conclusions Channel electrode studies have been shown that the photoreduction of AQI proceeds via a mixed ECE/DISPl mechanism and the kinetics of iodide expulsion has been quantified. Thus it has been found that the different excited states of the AQI radical anion, obtained through excitation at either 417 or 565 nm, show different dynamic behavior. In particular, expulsion is some 7.4 times faster per photon absorbed with 565-nm radiation than with 417 nm and spectrofluorimetric electrochemical experiments have revealed luminescence from rwo excited states of the radical anion. It is suggested that the slower iodide loss may result from the formation of a quartet state, at the higher wavelength, from which reaction is spin forbidden. Further work, quantifying excited-state lifetimes and spin densities, will examine this possibility. (27) Almlof, J. E.; Feyenisen, M. W.; Jozefiak, T. H.; Miller, L. A. J. Am. Chem. Soc. 1990, 112, 12061212.
Electron Transfer Photoinduced from Naphtholate Anions: Anion Oxidation Potentials and Use of Marcus Free Energy Relathships B. Legros, P. Vadereecken, and J. Pb. Soumillion* Laboratory of Physical Organic Chemistry, Department of Chemistry, Catholic University of Louvain. Place Louis Pasteur, 1, 1348-Louvain-la-Neuve, Belgium (Received: November 2, 1990)
The fluorescing excited states of four naphtholate type anions are quenched by various acceptors according to an electron-transfer mechanism. By use of the Marcus free energy relationship, the electron-transfer rate constants were correlated with the reduction potentials of the acceptors and a first set of oxidation potentials (around 0.5 V) was estimated for the anions. The fluorescence of perylene was quenched by the naphtholate anions but a second set of oxidation potentials, different from the preceeding one was obtained for the anions (around 1 V). The difference was rationalized by considering that a hydrogen bond bctwccn the anion and the solvent is broken during the excitation when the anions are in their excited states. The hydrogen bond is broken during the electron-transfer quenching when the naphtholate anions are in their ground states. This implies, in the first case, an intrinsic activation barrier, taking the H bond rupture into account. In the second case, the fraction of the excitation energy available for electron transfer must be evaluated. The oxidation potentials after correction were finally estimated around 0.8 V. In DMF, the oxidation potential of the 2-naphtholate anion is found to be decreased (around 0.1 V) and this is probably related to the lack of any hydrogen bonding in this solvent. The use of the Marcus model and the possibility of measuring in the inverted Marcus region are discussed.
Introduction
Research in the field of photoindud e l m o n transfer has stimulated in a pionering work on electron-transfer fluorescence quenching' by Rehm and Weller. The MARCUS modelz4 or (1) Rehm, D.; Weller, A. fsr. J . Chem. 1970, 8, 259.
0022-3654191/2095-4752$02.50/0
the Rehm-Weller empirical equation have often been used with a view to correlating the kinetics and thermodynamics of these An inverted region in which the rate of reaction (2) Marcus, R. A. J . Chem. Phys. 1965,43, 679. (3) Marcus, R. A. J. Chem. Phys. 1%6,24,966, 979. (4) Marcus, R. A. Ann. Reo. Phys. Chem. 1964, 15, 155.
0 1991 American Chemical Society
Electron Transfer Photoinduced from Naphtholate Anions
The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 4153 SCHEME 111
SCHEME I
D +
A-
.+ D +
A'
A-
D'
+
A'
D +
.+ D +
-
0 -
CI
-
2N0
A-
D + A
RX
D-
1NO
9PO
4CINO
TABLE I: Absorption and Emission Data for Napbtbolnte Type Anions
SCHEME I1 D*
0 -
anion 1N O 2N0 4C1NO
RX
9PO
haha 334 349 342 382
&,a
464 420 470 477
9
xZc
D-W
5.2 6.2 7.7 4.8
1.07 1.06 0.96 1.00
1.87 1.74 1.81 1.72
OAbsorption or emission maximum in nm. *Lifetime of excited-state anions in ns. Reduced x 2 parameter. "Durbin-Watson parameter.
D
D-
RX kr
C
RX
kr > kb
kb
decreases with increasing exothermicity is predicted by the Marcus theory. In solution, this inverted Marcus region has been confirmed in several cases: in rigid systems with donor and acceptor held at a fixed distance"-" or in electron back transfer consisting of char e recombinations within the initially formed geminate ion pair.'*- With freely diffusing species, the inverted region still demands confirmation, and this has lead to many discussions.2e27 When we consider the charges, electron transfer may be classified in three categories: charge separation when starting from neutral precursors, charge shift between a neutral molecule and an ion, or charge recombination (Scheme I). Among these processes our interest has concentrated on the charge shift because in at least three aspects, this is an interesting reaction: (1) The products of the electron transfer, being free of electrostatic attraction are expected to diffuse more freely into
!
a.2
(5) Fukurumi, S.;Kochi, J. K. J . m. Chem. Soc. 1980,102, 2141. (6) Schlessner, C. J.; Amatore, t,A .L o,chi, J. K. J . Am. Chem. Soc. 1984, 106, 3567. (7) Schmidt, J. A.; Liu, J. Y.; Bolton, J. R.; Archer, M. D.; Gadzekpo, V.P.Y. J. Chem. Soc., Faraday Trans. I 1989,85, 1027. (8) Eberson, L. Chem. Scr. 1982, 20, 29. (9) Fery-Forgues, S.;Lavabre, D.; Paillous, N. J . Org. Chem. 1987, 52, 338 1. (IO) Eriksen. J.; Lund. M.; Nyvad, A. 1. Acra Chem. Scand. 1983,837, 459. (11) Miller, J. R.; Beitz, J. V. J . Chem. fhys. 1981, 74, 6746. (12) Miller, J. R.; Calcaterra, L. T.;Closs, G. L. J. Am. Chem. Soc. 1984, 106.3047, 5057. (13) Closs, G. L.; Calcaterra, L. T.; Green, N. J.; Penfield, K. W.; Miller, J. R. J . fhys. Chem. 1986,90, 3673. (14) Imine, M. P.; Harrison, R. J.; Beddard, G. S.;Leighton, P.; Sanders, J. K. Chem. fhys. 1986,104, 315. (151 McLendon. G.: Miller. J. R. J . Am. Chem. Soc. 1985. 107. 7811. (16) Joran, A. D.; Leland, B. A.; Felker, P. M.; Zewail, A. H.;Hopfield, J. J.; Dcrvan, P. B. Nature 1987, 327, 508. (17) Hill, R. H. J . Chem. Soc.. Chem. Commun. 1989, 293. (18) Vauthey, E.; Suppan, P.; Haselbach, E. Helu. Chim. Acra 1988,71, 93. (19) Baker, H. G. 0.;Pfeifer, D.; Urban, K. J . Chem. Soc., Faraday Trans. 2 1989,85, 1765. (20) Levin, P. P.; Luzhnikov, P. F.; Kuzmin, V. A. Chem. fhys. Lerr. 1988. 147. 283. (21) Gould, 1. R.; Moser, J. E.; Ege, D.; Farid, S.J. Am. Chem. Soc. 1988. 110, 1991. (22) Gould, 1. R.; Moody, R.; Farid, S.J . Am. Chem. Soc. 1988, 110, 7242, (23) Gould, I. R.; M a r , J. E.;Annitage. B.; Farid. S.J . Am. Chem. Soc. 1989, 111. 1917. (24) Wellcr, A. 2.fhys. Chem. 1982, 93, 133. (25) Efrima, S.; Bixon, M. Chem. fhys. Lcfr. 1974, 25, 34. (26) Boletta. F.; Bonafede, S.Pure Appl. Chem. 1986, 58, 1229. (27) Rau. H.; Frank, R.; Greiner. G. J . fhys. Chem. 1986, 90, 2476.
solution than in the case of a charge separation. In photoinduced electron transfer leading to high-energy ions or radicals (Scheme II), a photoreaction leading to isolable products (k,) is often severely limited by the back electron transfer to the ground state (kb). In processes where an ion is photoexcited, a reduced yield of electron back transfer to the ground state may be expected with the consequence that these photoprocesses may be more efficient than the corresponding photoreaction initiated by a charge s e p aration. (2) When we try to work with photoexcited anions or cations, a very efficient electron transfer must be induced, since a negatively or positively charged species may be expected to be a much better donor or acceptor than the corresponding neutral molecule. (3) In their new theory of electron Mataga and Kakitani claimed that in the case of a charge shift, the inverted Marcus region may exist?' and this prediction needs to be tested in the case of freely diffusing species. In a study devoted to the possible use of an anion as an excited-state electron donor, the properties of naphtholate anions (NO-)have been examined. In the first part of the work, the photophysics of the 2-naphtholate ion was investigated in various s o l ~ e n t s ? 2 ~In~the second part of the work, some photochemistry based on photoinduced electron transfer from excited naphtholate was performed34and is still under study. In the present paper, the fluoresence quenching of this type of anion by a series of aromatic acceptors is studied and the correlation of the quenching rate constants with the reduction potentials of the acceptors is discussed.
Results and Discussion Photophysical Properties of the Anions. Four naphtholate type anions (Scheme 111) have been studied as excited electron donors. Their UV spectra in methanol show structureless absorptions with maxima in the near-UV region. These bands around 350 nm are due to m*transitions in the anion^?^ The fluorescence maxima (28) Kakitani, T.; Mataga, N. Chem. Phys. Len. 1986, 124, 437. (29) Kakitani, T.; Mataga, N. J. Phys. Chem. 1985, 89, 4752. (30) Kakitani, T.; Mataga, N. J . Phys. Chem. 1985, 89, 8. (31) Kakitani. T.; Mataga, N. J . Phys. Chem. 1986, 90, 993. (32) Vandereecken, P.; Soumillion, J.Ph.; Van Der Auweraer, M.; De Schryver, F. C. Chem. Phys. Lerr. 1987, 136, 441. (33) Soumillion, J.Ph.; Vandereecken, P.; Van Der Auweraer, M.; De Schryver. F. C.; Schanck, A. J . Am. Chem. Soc. 1989, 1 1 1 , 2217. (34) Soumillion,J.Ph.; Vandereecken, P.; De Schryver, F. C. Tetrahedron Lett. 1989, 30, 697. (35) Jortner, J.; Ottolenghi, M.; Stein, G. J . Am. Chem. Soc. 1963. 85, 2712. -
(36) Bard, A. J.; Lund, H. Encyclopedia of Electrochemistry of rhe Elements; Marcel Dekker: New York, 1984. (37) Connors, T. F.; Ruling, J. F.; Owlia, A. Anal. Chem. 1985,57, 170. (38) Rieger, P. H.; Bcrnal, I.; Reinmuth, W. H. J . Am. Chem. Soc. 1963, 85, 683. (39) Seas, J. W.; Burton, F. G.; Nichol, S.L. J . Am. Chem. Soc. 1968, 90, 2595. (40)Andrieux, C. P.; Saveant, J. M.; a n n , D. Nouu. J . Chim. 1984,8, 107. (41) Andrieux, C. P.; Blocman, C.; Dumas-Bouchiat, J. M.; Saveant, J. M. J . Am. Chem. Soc. 1979, 101, 3431. (42) Shukla, S.S.;Rusling, J. F. J . Phys. Chem. 1985, 89, 3353.
Legros et al.
4154 The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 TABLE II: Rate Constants for Qwnching Naphtholate Type Anions by Electron Acceptors in Methanol
'.k
N ' 1
2
3 4
5 6 7
8
9 IO 11
12
13 14 15
16 17 18
19 20 21 22
23 24 25 26 27
28 29 30 31 32 33
acceptor hexachlorobenzene p-dicyanobenzene o-dicyanobenzene m-dicyanobenzene 4-cyanopyridine iodobenzene acetophenone 3-cyanopyridine m-chlorobromobenzene 1 -bromonaphthalene benzonitrile 2- bromonaphthalene 1 -chloronaphthalene m-tolunitrile 2-chloronaphthalene 2-bromopyridine p-tolunitrile p-chlorobiphenyl m-dichlorobenzene 2-chloropyridine bromobenzene p-cyanoanisole naphthalene 2-methylnaphthalene p-dichlorobenzene 1-methylnaphthalene biphenyl 2-methoxynaphthalene 2,6-dimethylnaphthalene 1 -methoxynaphthalene acenaphthene pyridine chlorobenzene
-E1,2b
1.41 1.52 1.57 1.62
ref 36 37 37
38
1.72 1.81
39
1.96 2.03
37 10
2.13
2.19 2.24 2.23
2.26 2.27
2.30 2.30 2.34 2.37 2.38 2.40 2.44 2.47 2.53 2.58 2.45 2.56 2.54 2.60 2.61 2.65 2.67 2.76 2.78
37
39 40 41 40 40 41 40 41 41 42
1NO
13.5 13.4 12.8 12.9
45
44 45 43 36 41
9PO
16.3 15.6 15.5 15.0
12.9 13.0 11.5 11.8
15.0 14.6 14.1
11.4
13.8
5.32 13.2
0.82
11.3 11.4
12.1
10.6 9.52 9.68
9.5 10.7 9.1
7.90 9.52 11.2
6.05 0.23 10.6 C0.16 7.74 3.55 2.42 0.24 3.45 0.84 2.26 1.82 1.29 0.79 7.42 C0.16
10.5
41
38 43 44 39 43 41
4CINO
15.2
37
41
2N0
6.19 2.86 2.29 0.96 1.67 0.69 6.75
7.38
8.96 11.0
10.0
7.71 9.19 8.73 6.95 1.74
11.2
7.29 0.77
8.47
9.62
4.54 0.74 0.53
0.35 0.29
3.17
0.49
4.39
'Entry number. bReduction potential in volts, versus SCE. CQuenchingrate constant in IO+' M-l s-I obtained from the slope of Stern-Volmer plot (eq 1) divided by lifetime.
are found between 420 and 480 nm. The fluorescence decays as measured by the time-correlated single-photon counting technique were found to be monoexponential in methanol. The obtained lifetimes together with spectral data are given in Table I. Fluorescence Quenching Measurements. In order to determine electron-transfer rate constants, the fluorescence of the naphtholate anions was quenched, in methanol, by various acceptors. The quenching rate constants were obtained according to the SternVolmer equation: I f o / I f = 1 + k,.[Q] (1) where I: and 1, = fluorescence intensities, at a given wavelength, in the absence and presence of quencher Q, respectively, [Q] = quencher concentration in M-I, k, = quenching rate constant in M-'s-l, and I = excited naphtholate lifetime in s. Quenchings were carried out by exciting the anion selectively at a wavelength longer than that of the absorption band of the quenchers. This means that energy transfer from excited naphtholate anion to the quenchers is endothermic. In this context, the most probable quenching mechanism is electron transfer. The values are reported in Table I1 together with the reduction potentials of the acceptors used. The observed quenching constants are found to increase with the accepting power of the quencher: values starting from lo8 and reaching 1Olo M-I s-I are observed when going from a weak acceptor like dimethoxynaphthalene to a good one like dicyanobenzene. When the acceptor reduction potential reaches -2 V, the quenching rate constants become diffusion limited. According to these observations and since, as previously stated, energy transfer (43) Pointcau, R. Ann. Chim. (Paris) 1962, 7, 669. (44) Streitwieser, A.; Schwager, I. J . Phys. Chem. 1962, 66, 2318. (45) Zweig, A.: Maurer, A. H.;Roberts, B. G. J . Org. Chem. 1967, 32,
1322.
(46) Ebersan, L. Electron Transfer Springer-Vcrlag,New York, 1987.
Reactions in Organic Chemistry;
is unlikely, the electron-transfer mechanism may be accepted as the quenching process. Kinetics of Quenching. To correlate the quenching rate constants with the reduction potentials of the quenchers, we used the following kinetic scheme. E q w t k ~ t 2corresponds to the formation
NO-*
+
NO NO-*
...Q]
(2)
(3) of an encounter complex within which the electron transfer will take place (eq 3). A steady state approximation applied to this scheme gives the following expression for the observed k,: (4) where Kd = kd/k4 corresponds to the diffusional equilibrium. The Eyring equation (assuming a unity transmission coefficient, K = 1) for the electron-transfer rate constant may be introduced into this expression giving k, =
kd + (kd/KdY) ~ ~ P ( A G C I ' / ~ T )
(5)
with Y = frequency factor of the unimolecular rate constant (k,) in the case of a barrierless transfer and AGcl*= free energy of activation of the electron-transfer step. In eq 5 , k, is shown to reach a maximum value when the activation free energy of the electron transfer is zero and in that case, k, differs from the diffusion rate constant (kd) by a factor related to the kd(&Y)-' product. The kd value is expected to be between 10 and 25.109 M-I s-I, depenging on the solvent and on the diffusion coefficients of the reacting species. The diffusive equilibrium constant Kd is given by the Fuoss-Eigen eq~ation:",~
The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 4755
Electron Transfer Photoinduced from Naphtholate Anions
Kd = k,j/kd =
4.lrr~~~NC 3000
(6)
where 'AB is the encounter distance and Cis a factor equal to 1 for unlike charged species. This equation leads to values between 0.5 and 1.3 depending on the considered distance rAB (between 6 and 8 A). Usually a mean distance of 7 A is considered in photoinduced electron transfer between aromatic molecules in polar nonviscous solvent^.^^.^^ The Y frequency factor could be set equal to k T / h , the prefactor in transition state theory expressions, but a better way is to consider the fastest vibrational frequency that destroys the activation complex configuration: it has-often been suggested that the frequency factor should reflect the dynamical properties of the s o l ~ e n t . ~According ~ . ~ ~ to S ~ t i n ? ~ Y ranges from 1012to lOI4 s-I, depending on the concerned vibration mode (solvent or intramolecular). A mean value of lOI3 s-' has been proposed by Marcus.54 A value of 6 X 10l2s-' for the first-order rate equation is also proposed by Eberson.& This means that the kd(Kdv)-'product has a maximum value around 0.04, and as a consequence the electron-transfer quenching rate constant, when measured in the region of maximum reactivity, may be considered as an experimental measure of the diffusion rate constant. Thermodynamics of Electron Transfer. The standard free energy difference for an electron-transfer reaction between an excited donor and a ground-state acceptor may be calculated according to the well-known equation:'Sa
TABLE III: Parameters Used in Marcus Correlations and Calculated Intrinsic Barriers AGOZ 10-I0kd f Em" anion 2N0 1.55 3.7 1 5.79 3.24 4CINO 3.12 1.23 3.83 5.60 1 NO 3.18 1.32 3.71 5.79 9PO 2,91 1.45 4.02 5.33 "Excitation energy in volts. bDiffusion rate constant in M-l s-l, taken as the plateau value in each anion quenching series. 'Anion radii
~ ~ ~ u ~ ~~a ~~ ~. nm~
TABLE 1V: Oxidation Potentials 01 Naphtholate Anions E,,, V' anion anion E,,, Va 2N0 0.54 & 0.01 1NO 0.49 f 0.01 9PO 0.34 0.02 4CINO 0.52 f 0.02
*
"As obtained from the correlations presented in Figure 1 .
and acceptor, respectively, rAD = reaction distance, e = dielectric constant of the solvent, and n = refractive index of the solvent. The quenching constants of Table I1 were correlated with the reduction potentials of the acceptors, according to the eqs 5-8. In these correlations, the diffusion constant was always chosen, for the above explained reasons, as the plateau value for each of the anion quenching rate constants (see Table 111). A Y value of 6 X 10l2s-' was chosen according to the suggestions of WeaverSs for methanol as a solvent and in agreement with the arguments e2 AGO 23.06(EOx- Elcd - EM) (ZI - 2 2 - 1)already given. A Kd value of 0.86 was used, as calculated with (7) AD the reasonable 7 8, distance of collisional electron transfer in s o l ~ t i o n .The ~ ~ excitation ~~~ energies of the anions (Table 111) where AGO is in kcal mol-', E , and Erd = redox potentials, in were the mean values of the energies associated with absorption volts, of the donor and acceptor, Em = excitation energy, in volts, and emission maxima (Table I), of the donor, ZI and Z2 are the charge numbers of donor and In a first series of calculations, the intrinsic barrier and the acceptor, respectively, e = charge of an electron, t = dielectric oxidation potentials of the anions were adjusted to fit the results. constant of the medium, and rDA = reaction distance. The values obtained in that way were very uncertain: in the case In our case of photoinitiated electron transfer from an excited of 1 N 0 , for instance, a correlation obtained with E,, = 0.36 V anion to neutral acceptors, no electrostatic interaction has to be and AGOS= 7 kcal mol-' is equivalent to another one with E,, taken into account, since one of the involved species is uncharged = 0.55 V and AGO' = 5 kcal mol-'. In no case, however, was it in the reactants as well as in the products. The reduction potentials reasonable to accept values of AGO' in the range of 2.5 kcal mol-' are found in Table 11, while the excitation energies of the used as used by many authors since the proposal of Weller:1*57an anions were given in Table I. important discrepancy between calculated and observed kq values Free Energy Relationship. A correlation between the elecwas observed in that case. On the contrary, the best-fit calculations tron-transfer rate constant and the thermodynamics of the reaction showed a decrease in the residuals when an increased intrinsic has been proposed by Marcus, who derived the following general activation barrier was used. expression: Equation 9 was then used in order to estimate the intrinsic barriers. This equation gave the values shown in Table 111. AG,,' AGo'(1 AG0/4AGo')2 (8) Taking a mean value of 5.62 kcal mol-' for AGOS,the oxidation where AGO*= intrinsic activation barrier of the reaction and AG potentials of the anions were adjusted to fit the results, and the = free energy of the transfer, as calculated by eq 7. The intrinsic values obtained are shown in Table IV. Figure 1 shows the activation barrier is the energy associated with bond length changes correlation diagrams. and solvent reorganization. In the majority of cases, in electron In the correlations of Figure 1, the reduction potentials used transfer between organic molecules diffusing freely in solution, are literature values obtained in DMF as solvent. The question the reorganization energy associated with the solvation shell has of applicability of these potentials to our experiments in methanol been considered as the most important term. This reflects changes is thus to be addressed. This problem was examined by Wellers6 in the polarization of solvent molecules during the r e a c t i ~ n . ~ ? ~ by considering the solvation energies of the ions. It was shown According to the Marcus theory, this can be estimated by the Born that the (E, - Erd) difference used for one solvent can be caltreatment, leading to the following equation: culated from the same difference measured for another solvent by applying eq 10.
+
+
where e = charge of the electron, rDand rA = radii of the donor (47) Fuoss, R. M. J . Am. Chem. SOC.1958,80, 509. (48) Eigen, M. Z . fhys. Chem. (Munich) 1954. I , 176. (49) Weller, A. Z . Phys. Chem. (Munich) 1982, 130, 129. (50) Costa, S.M. B.; Macanita, A. L.; Formoshino, S.J. J . fhys. Chem. 1984.88,4089. (51) Nadler, W.; Marcus, R. A. Chem. fhys. Lett. 1988, 144, 24. (52) Hynes,J. T. J . Phys. Chem. 1986, 90, 3701. (53) Sutin, N . Acc. Chem. Res. 1982, 15, 275. (54) Marcus, R.A. Int. J . Chem. K i m . 1981, 13, 865
where t l and c2 = dielectric constants of the two solvents and r+ and r- are the radii of the considered ions. With radii around 3 and 4 A and with the values of dielectric constants for D M F and methanol (37 and 32) a negligible cor(55) McManis, G. E.;Golovin, M. N.; Weaver, M. J. J . Phys. Chem. 1986,
90, 6563.
(56) Weller, A. Z . fhys. Chem. (Munich) 1982, 133, 93. (57) Rehm,D.; Weller. A. Ber. Bunsen-Ges.fhys. Chem. 1%9,73,834.
4756 The Journal of Physical Chemistry, Vol. 95, No. 12, 1991
Legros et al. 1 NAPHTHOLATE
2 NAPHTHOLATE "
I
a
11 M -0
\
23?31
J 8
/
1.4
,
1.8
.
~
~
2.2
,
~
~
2d
-E RW
9 PHENANTHROLATE
4 CHLORO I NAPHTHOLATE 11---
1
9-
23. 24
+: 28
I I I
8t
-
7
1.8
14
I
28
22
3
-E RED
Figure 1. Fluorescence quenching rate constants of excited naphtholate anions correlated with reduction potentials of accepting quenchers. The curve corresponds to the calculated log (&J. The numbers refer to the entries of Table 11.
rection factor may be calculated (around 0.01 V). This renders the use of the D M F values for Erd quite reasonable. All the experimental results of Table I1 were not used in the correlations of Figure 1. Quenching constants for biphenyl, pyridine, and chlorobenzene derivatives including chlorobiphenyl were excluded from the calculations. In the case of pyridine, (entry 32) this seems to be related to a specific interaction between methanol and the heterocycle. When the same type of correlation is made in DMF between the quenching constants of the 2naphtholate fluorescence and the reduction potentials of acceptors (see below), the pyridine acceptor fits correctly in the correlation. The reason is probably that, in methanol, a hydrogen bond between the pyridine lone pair and the solvent renders the molecule a better acceptor than expected from the DMF reduction potential. This seems reasonable since hydrogen bonding to the solvent will decrease the charge density of the pyridine molecule. When Erd for pyridine is calculated in methanol from the observed quenching constants, values of -2.48 and -2.49 V (instead of -2.76 V in DMF) are obtained from measurements made with 1- and 2naphtholate anions, respectively. In the cases of the halogenated aromatics, the deviations are probably related to a particular intrinsic barrier. Electron transfer leading to the haloaromatic radical anions is normally followed by a dissociation step: D- + Ar-X D' + Ar-X'Ar' XExperimental evidence exists showing that before dissociation, the odd electron is transferred from a A* MO to a u* M0.58-60 On the other hand, molecular orbital studies show that in electrochemical reductions of organochlorine compounds the first
-
-
+
(58) Riederer, H.; Huttennann, J.; Symons, M.C.R. J . Chem. Soc., Chem. Commun. 1978. 313. (59) Ross!, R. A. J . Chem. Educ. 1982, 59, 310. (60) Rossi, R. A. Acc. Chem. Res. 1982, I S , 164.
electron enters a u antibonding orbitaL6' According to results obtained by Symons, in the cases of chlorobenzenes, u* radical anions are formed, while A* species arise from chloronaphthalene$z In our results, the chloronaphthalene quenchers behave like other aromatic quenchers, in agreement, as we shall see, with their expected A* radical anion character. On the other hand, it seems reasonable, in the case of electron transfer to molecules giving u* radical anions with elongated bonds, to use some inner reorganization energy related to this elongation. In a work devoted to dissociative electron transfer to alkyl halides by Saveant, an inner reorganization barrier, corresponding to one-fourth of the C-X bond energy was ~ s e d . 6 In ~ the cases of the 2-naphtholate fluorescence quenching by chlorobenzene derivatives, through the use of the optimized correlation of Figure 1, the intrinsic barrier necessary to fit the results was recalculated and found to be around 9 kcal mol-' in the cases of p- and m-dichlorobenzene. This corresponds after substracting the outer term of 5.62 kcal mol-' to an inner reorganization between 3 and 3.5 kcal mol-'. This value is certainly less than one-fourth of the C-CI bond energy of chloroaromatics. However, chloroaromatic radical anions are discrete intermediates and this means that the electron transfer in these cases is not a dissociative one. Accordingly, a lower inner barrier may be expected in the case of the formation of a u* radical anion than in the case of a dissociative electron transfer. In a similar calculation inner barriers of 5.1 and 6.2 kcal mol-' were estimated in the cases of m-chlorobenzene and iodobenzene, respectively. If these values are significant, it would indicate that the corresponding u* radical anions with more elongated C-Br and C-I bonds are formed. (61) Beland, F. A.; Farwell, S.0.; Callis, P. R.;Geer, R. D. J . E/ecrroanal. Chem. 1977. 78, 145. (62) Mishra, S.P.; Symons, M. C. R. J. Chem.Soc.,ferkin Trow. 2 1981, 185.
(63) Saveant, J. M. J . Am. Chem. Soc. 1987, 109, 6788.
1
1
The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 4757
Electron Transfer Photoinduced from Naphtholate Anions
SCHEME IV
11
I
kl
I
j
I
04
I\ I
6 :
-m
/ - so
-30
.\ I
-10
\ I
10
Pe absorption A-: 431 nm emission: ,A 438 nm Em = 2.84 V T = 5.4 ns
RESOabsorption A-: 573 nm emission: ,A 589 nm E a = 2.12 V T = 5.6 ns
TABLE V Fluorescence Quenching8 of Excited R e s o d o A d a by N ~ t Down d rad N8pbtbd.t~hb# ~
Figure 2. Rehm-Weller resultsl (fluorescencequenchings in acetonitrile) correlated by the Marcus model with kd = 1.6 X 1O'O M-' s-l and 6 X 10'2 s-I frequency factor.
In the cases involving 4-chlorobiphenyl, the estimated inner barrier is found between 1.3 and 1.7 kcal, while in the case of biphenyl, an inner barrier between 0.4 and 1.2 kcal is obtained in the same way. In this last case, an extra inner barrier may be justified if some elongation of the C - C bond or if some modification of the twist angle between the two phenyl rings occurs in the radical anion. The quenching rate constants by 2-chloroand 2-bromopyridines are correctly correlated with Ed, which was not the case for pyridine itself. This may be explained in two different ways: a first explanation could be that, when an electron-withdrawing halogen is present near the heterocycle lone pair, the hydrogen bonding effect is no longer significant. Another possibility is a compensation effect: as pyridine derivatives these molecules are better accepting quenchers than expected, but this effect is counteracted by the C-X bond elongation barrier. Use of the Marcus Model. The oxidation potentials of Table IV were obtained after calculation of AGO' by the Marcus equations, and the differences between the values obtained for the four naphtholates are small. A question concerning the significance of these differences can be formulated. Hence, a method of confirmation of these values is necessary, since electrochemical measurements are not possible for irreversibility reasons. However, before looking far a possible confirmation of the results obtained, some check on the validity of the Marcus model as used here may be useful. Our results of course rely on the assumptions made: the limit of the quenching rate corresponds to the experimental diffusion rate, the diffusive equilibrium may be calculated by the Eigen equation, the frequency factor is assumed very high, and AGOc is calculated by using an oversimplified model with molecular radii whose estimation may be controversial. It seems however that the model works quite well and some supplementary argument may be given for this by reexamining some literature results and by trying to correlate them in the same way. Let us for instance look to the Rehm-Weller important series of fluorescence quenching measurements made in acetonitrile:' 65 rate constants were measured and an attempt was made by the authors to correlate them by using the Marcus model with a frequency factor of 10" 6' and an activation barrier of 2.5 kcal mol-'. No more than half of the values were found to fit the model. In retrying to correlate this series of results, we used the plateau rate constants as the diffusion value and the frequency factor suggested by Weaver et al.55 for acetonitrile (6 X 10l2s-l). In that way, 60 values were found to fit quite well with a mean intrinsic activation barrier of 5.48 kcal mol-'. This is shown in Figure 2 and gives credence to our assumptions. Moreover, this also is consistent with the AGO*already used in the naphtholate anion correlations. Only five quenching constants located in the region of extreme exothermicity do not fit the Weller correlation results, and since TCNE is involved in all those five cases, some explanation of the deviations may be given. As already proposed in the literat ~ r e , S * *it~may * ~ ~be that excited-state complexes are formed,
donors phenothiazine N,N-dimethylaniline 1,4-dimethoxynaphthalene
E,'
kp"
0.53c
11.5 11.2 8.0
0.78c 1.10"
anions 2NO 4CINO 1NO 9P0
kp" 5.11 7.05 6.89 8.32
'Oxidation potentials of the donors, in volts, versus SCE in acetonitrile. bQuenching rate constant in lo4 M-l s-l as obtained from Stern-Volmer plots and resorufin anion excited-state lifetime. cReference 67. "Reference 45. resulting in erroneous free energy estimations. Other explanations were also propo~ed,2~**' but as an alternative we suggest that an extra inner reorganization barrier might be needed in order to twist the ethylenic acceptor: the TCNE radical anion might well be perpendicular or distorted and a reasonable estimation of the global AGO' in our correlation of the Weller results gives a value of 11 kcal mol-'. After substracting the 5.4 kcal mol-' of outer reorganization, this value then gives a 5.6 kcal mol-' as the specific twisting barrier. The use of the Marcus model in the way we suggest results in an important improvement of the correlation and if this is correct makes the Weller empirical equation unuseful. Moreover, our approach may be instructive in the discussions about the inverse Marcus region: this shows that, in freely diffusing systems, the exothermicity needed to demonstrate the inverted area is much more difficult to reach than previously expected. When this region is approached, some new efficient acceptors (or donors) will be needed, and in general this will introduce new uncertainties as far as the quenching mechanism or the AGO' calculation is concerned. In the Weller series, for instance, the TCNE cases illustrate this difficulty. Oxidability of Naphtholate Anions. A confirmation of the oxidability sequence found in Table IV could be given by measuring the abilities of the naphtholate series anions to quench the fluorescence of a common excited acceptor. This molecule must absorb at longer wavelength relative to the anions in order to exclude energy-transfer quenching. After several assays two sensitizers, resorufin (RESOH) and perylene (Pe), were finally chosen. RESOH is rather acidic and the anion is quantitatively formed in the presence of the sodium hydroxide concentrations needed to form the naphtholate anions. The needed spectral characteristics together with the excited-state lifetimes of the accepting sensitizers are given in Scheme IV. The resorufin anion fluorescence is quenched by the naphtholate anions with quenching constants given in Table V. In the absence of energy-transfer quenching, the following electron transfer is thought to be responsible for the quenchings:
NO-
+ RESO-* NO- + Pe*
--
NO' NO'
+ RESW2+ Pee-
An electron transfer from donor quenchers to the excited resorufin anion giving a radical dianion is not unreasonable: this (64) Marcus, R . A. J. Phys. Chem. 1965,43, 2654. (65) P. Siders, R. A. Marcus, J . Am. Chem. Soc. 1981, 103, 748.
4758 The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 TABLE-VI: Fluorescence Quenching Rate Constants of Excited Perylene by Accepting Quencbers
entry
auencher
".k 13.0 10.2
1
4CINO
2 3 4
2N0 4-chloronaphthol
8.8
1 -naphthol
0.63
5 6
1NO
15 16
2-naphthol N,N-dimethylaniline 1,4-dimethoxynaphthaIene 2-methoxynaphthalene 4-methoxyphenolate ion 4-chlorophenolate ion phenolate ion 4-cyanophenolate ion 4-methoxyphenol 4-chlorophenol phenol
17
4-cyanophenol
7 8
9
IO 11
12 13 14
1.01
EOzb 0.84 0.92 0.95
&,C
NO'
0.27 10.3 8.56
0.13 9.27
0.98 1.02 1.30
0.786s 1.1045
1.52"
5.65 5.66
1.82 1.68
0.14 0.1 1
0.04
1.20 1.34 1.32
hv
HOCH3
A
I
1.1668
1.29" 1.306*
"In lo4 M-' 8,as obtained from Stern-Volmer plots and perylene excited-state lifetime. "Oxidation potentials, in V, calculated in this work by using the Marcus equations. CLiteraturevalue of oxidation potential. is shown in Table V by the quenching rate constants observed with neutral electrondonating quenchers of known oxidation potentials. For those donors, the observed values are in the order expected from their oxidation potentials. A very similar electron transfer may be found in the literature in the case of fluoresceinamine. This molecule is formed by an anilino moiety and an anionic part resembling the resorufin anion. An intramolecular fluorescence self-quenching occurs in this particular case and this is probably related to the intramolecular electron transfer from the anilino to the anionic moiety of the molecule.66 In the cases of the anions and with the exception of the 4chloronaphtholate, the quenching constants of the resorufin anion fluorescence are found in the expected order according to the oxidation potentials, as shown in Table IV. This at least partially confirms the coherence of the results obtained from the correlations of Figure 1. However, quantitative oxidability estimations are impossible in the absence of a value for the reduction potential of the resorufin anion. As for the naphtholate oxidation, the value is not electrochemically accessible for irreversibility reasons. With perylene as excited acceptor, several fluorescence quenching rate constants were measured, including those for quenchers such as neutral phenols or naphthols as well as phenolates. The results are presented in Table VI. Unfortunately, the phenanthrolate anion was of no use in this case: diffusional quenching is expected and spectral overlap in the absorption spectra of perylene and phenanthrolate ion makes quenching measurements difficult. The quenching constants in the cases of 1- and 2-naphtholate (entries 2 and 3) are found in the sequence expected from the values of Table 1V. On the other hand, in all the series of quenching measurements the chlorine-substituted naphthol, naphtholate, phenol, and phenolate are more reducing than the corresponding unsubstituted molecule. Thus, the discrepancy between our sequence of oxidation potentials (Table IV) and the quenching measurements in the case of resorufin or perylene is not accidental. Another observation, well emphasized in Table VI, is the increase of the oxidability when going from the neutral phenol or naphthol to the corresponding phenolate or naphtholate: the reactivity is enhanced by > 1 order of magnitude. This shows that, as far as the electron-transfer free energy is concerned, the decrease in the oxidation potential in going from the neutral to the anionic (66) Munkholm. C.; Parkinson, D. R.; Walt, D. R., J. Am. Chem. Soc. 1990, 112, 2608. (67) Ballardini, R.; Varani, G.: Indelli, M. T.; Scandola, F.: Balzani, V. J . Am. Chem. Soc. 1978, 100, 7219. (68) Bard, A. J.; Lund, H.Encyclopedia ofElecfrochemisrryof rhe Elements; Marcel Dekker: New York, 1984: Vol. XI.
NO---- HOCH3
A
--.' . \ *-.',
*a
H bond broken
1
quencher largely overcomes the loss of the stabilizing Coulombic energy (last term of eq 9). At this stage of our measurements some tests on the validity of the Marcus calculations as used in the naphtholate correlations are necessary. In that view, some known oxidation potentials were recalculated starting from perylene excited-state fluorescence quenching constants (entries 7-9 and 14-16). The influence on the obtained oxidation potentials of variations of the parameters used in these calculations has been tested. Changes between reasonable limits of the frequency factor (from lo'* to I O l 3 ) or of the diffusion rate constant (from 1.3 X loioto 1.8 X lolo) led to negligible changes (less than 0.05 V) on the obtained E,. The results were more sensitive to changes in AGO*. Anyway, the discrepancies between calculated and experimental values never exceed 0.2 V for the neutral quenchers (entries 7-9 and 12-14 of Table VI). Thus, it may be considered that the agreement found is rather good since these values were obtained with the help of a mean AGO*(5.6 kcal mol-'), which may not be exactly optimized to the individual measured cases. Furthermore, it seems that the intrinsic barrier used is rather accurately adapted to the case of the phenolic quenchers, sin'ce the agreement between the recalculated E,, and the literature value is excellent (entries 14-16). When we consider the oxidation potentials of the naphtholate anions (entries 1-3), a larger discrepancy (0.3-0.45 V) is observed between the values in Table VI and the potentials obtained from the previous correlations (Table IV). In order to understand this discrepancy some reference to the calculation method is necessary. In the "cascade" operations leading from the experimental quenching rate constant to the oxidation potentials two steps are crucial: (1) The electron-transfer free energy AGO is accurately estimated only if a correct value of the intrinsic barrier is used. (2) The oxidation potential of the donor is correctly extracted from the AGO only if the excitation energy really available for the electron transfer is used. These two key values (AGO*and Em) must now be considered in the light of a particular feature of the electron transfers we are dealing with: we start from an anion in its excited or ground state and the reactions m u r in methanol as solvent. When the perylene fluorescence is quenched, the anion is solvated by hydrogen bonding in its ground state, but when used as before in their excited states, the anions are a t least partly desolvated. This results from the fact that a redistribution of the anion negative charge occurs on excitation.33 The thermodynamic cycle of Schemes V and VI must now be considered. When we consider the excited naphtholate anions quenchings (Scheme V), it can be seen that everything is correctly balanced: hydrogen bond breaking is implicitly included in the excitation energy and in the oxidation potential of naphtholate anion. In the case of perylene quenchings by naphtholate ground-state anions, the energy balance is still correct (Scheme VI) but the hydrogen bond is now broken during the course of the electron transfer, and this has not been taken into account
The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 4759
Electron Transfer Photoinduced from Naphtholate Anions
4CINO
AG Pe
NO'
TABLE VIII: Oxidation Potentiale of Anions before and after Correcting for a 5 kcal mol-' Hydrogen Bond
HOCH3
anion hv
Ered(Pe)
Pe
NO----HOCH~
.
48 1
363
2N0 4CINO IN0 -
,,'
EO/
E O :
EO:
Emd
0.54
0.76 0.74 0.71
0.95 0.84 0.92
0.87 0.75 0.84
0.52 . ._
0.49
a Oxidation potential in volts, as obtained from the correlations. bSame as footnote a, but after correcting Em by adding 0.22 V. Coxidationpotentials as obtained from perylene quenchings. dSame as footnote c, but after adding 1.25 kcal mol-' to AGO#.
.,
