J. Phys. Chem. 1082, 86, 4037-4040
4037
Excited-State Redox Properties of Ruthenium( I I ) Phthalocyanine from Electron-Transfer Quenching D. R. Prasad and 0. Ferraudl' Radlatbn Laboratory, UniversHy of Notre Dame, Notre Dame, Indlana 46556 (Recelved:Aprll21, 1982; In Flnal Form: June 24, 1982)
Electron-transferreactions between the lowest-lying triplet state, 3m*,of ruthenium (phthalocyanine)(pyridine)2 and various nitroaromatic compounds have been studied by laser and conventional flash photolysis. Quenching rate constants determined for the oxidation of the excited state have been treated according to the Marcus-Hush theory. A self-exchange rate constant k lo7 M-' s-l was determined for the self-exchangereaction between the 3m*and the radical cation, Ru(ph)(py)2+. Such a value indicates that the major component to the Franck-Condon reorganizational energy is the outer-sphere contribution. The photochemical properties of the phthalocyanines are discussed in terms of the redox potentials estimate for various excited states.
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Introduction The photochemical properties of transition-metal phthalocyanines, I, have been the subject of several in-
I vestigations in recent years.'-' The excitation in the region of the ultraviolet bands induces photoredox transformations which can be attributed to the population of reactive na* excited states, eq l.1-5Moreover, the photoredox hu
SH
M(ph) Z na* M(ph-H) SH = hydrogen donor
+ S.
(1)
dissociation of dimeric phthalocyanines in one-electronoxidized and -reduced species, eq 2, can be assigned to reactions originated in either na* or charge-transfer
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[M(ph)12 *[M(ph)Iz M(Ph)+ + M(Ph)- (2) The lowest-lying air* excited states, populated by excitation in the Q band, e.g., Aexci, 1 560 nm, does not show the type of reactivity indicated above. However, it has been reported that the excited states can be quenched by electron transfer to suitable quenchers.7-ll Although these (1) G. Ferraudi and E. V. Srisankar, Znorg. Chem., 17, 3164 (1978). (2) D. R. Prasad and G. Ferraudi, Zmrg. Chim. Acta, 51, L231(1981). (3) D. R. Praaad and G. Ferraudi, Znorg. Chem., in press. (4) K. C. Schmatz, K. Madden, S. Muralidharan, R. Fessenden, and G. Ferraudi, Inorg. Chim. Acta, 51, L23 (1981). (5) K. C. Schmatz, S.Muralidharan, and G. Ferraudi, Inorg. Chem., in press. (6) G. Ferraudi, Znorg. Chem., 18, 1005 (1979). (7) A. B. P. Lever. S. Licoccia. B. S. Ramaewamv. - . S.A. Kandil, and D. V; Stynes, Znorg. h i m . Acta,' 51, 169 (1981). (8) J. R. Darwent, I. McCubbin, and D. Phillips, J. Chem. SOC.,Faradav Trans. 2. 78. 347 (1982). 6)A. H&, M-C. Richoux, and G. Porter, J. Chem. SOC., Faraday Trans. 2, 77, 1175 (1981). (10) T. Tanno, D. WBhrle, M. Kaneko, and A. Yamada, Ber. Bunsenges. Phys. Chem., 84,1032 (1980). 0022-3654/82/2086-4037$01.25/0
studies have clearly established that the lowest triplet and singlet a~p*excited states can be quenched by electron acceptors, the mechanism of the electron donation has not been critically investigated. Therefore no meaningful information is available on redox properties such as selfexchange rate constants and redox potentials of various excited states.12 Recent work in our laboratory has demonstrated that ruthenium(I1) phthalocyanine obeys the photochemical behavior described above.13 Indeed, photoredox transformations, eq 1,were induced in ultraviolet excitations while excitations in the near-infrared region did not produce photochemical transformations of the phthalocyanine complex. Both the photochemical properties and the thermal stability of the ruthenium(I1) phthalocyanine make this compound a very convenient substrate for the study of the electron-transfer processes in the excited state. We report in this work our results on the quenching of the 3aa* by electron acceptors and the analysis of the results by Marcus and Hush electron-transfer theory.'*J5
Experimental Section Photochemical Procedures. The flash photolysis and laser flash photolysis apparatus have been previously de~ c r i b e d ; ~however, J~ some of their features are described below. Flash photolyses were carried out by firing two xenon flash lamps in series at energies between 50 and 250 J/pulse. The flash duration limits the resolution time of the apparatus to times longer than 50 ps. The laser flash photolysis unit, used for observations at times shorter than 30 e, is based on a Quanta Ray NdYAG pumped dye laser for excitations at 640 nm. The power of the laser was reduced to values where neither ground-state depletion nor biphotonic processes were detected. Moreover solutions of the photolytes were refreshed after each flash irradiation in either flash or laser flash experiments. Materials. R ~ ( p h ) ( p ywas ) ~ prepared and purified according to reported procedures. The solvents used for photochemical experiments (Aldrich gold label acetonitrile and dichloromethane) were (11) P. Maillard, S. Gaapard, P. Krausz, and C. Giannotti, J. Organometal. Chem., 212,185 (1981). (12) Some estimates on the redox potentials of the lowest-lyingexcited states have been reported by Lever et al? (13) D. R. Prasad and G. Ferraudi, Znorg. Chem., in press. (14) R. A. Marcus, Discuss. Faraday SOC.29, 21 (1960). (15) N. S. Hush, Trans. Faraday SOC.,57, 557 (1961). (16) S.Muralidharan and G. Ferraudi, Inorg. Chem., 20,2306 (1981).
0 1982 American Chemical Society
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The Journal of Physical Chemistv, Vol. 86, No. 20, 1982
Prasad and Ferraudi
TABLE I: Quenching Rate Constants for Ru(ph)(py), and Redox Potentials for Nitroaromatics 10-8k,P -E,,,, M-ls-' V vs. SCE
quencher
p-dinitrobenzene 22.10 N,N'-dimethyl-p-nitrosoaniline 11.80 p-nitrobenzaldehyde 2.59 0.87 o-dinitrobenzene methyl p-nitrobenzoate 0.86 methyl m-nitrobenzoate 0.68 0.38 m-dinitrobenzene 0.15 o-nitrobenzaldehyde 0.13 m-nitrobenzaldehyde 0.10 1-iodo-4-nitrobenzene
0.690b 0.41d 0.863' O.81Oc 0.947b
-'fn -
'I
h -
P \
1.040b 0.898b 0.99d 1.016b 1.050b
x
0
a Rate constants corrected for the solvent diffusional effects by eq 3. A. H. Maki and D. H. Geske, J. A m . Chem. SOC., 83, 1852 (1961). A. H. Maki and D. H. This work. Geske,J. Chem. Phys., 33, 825 (1960).
"
0.0
0.05
0.10
0.15
0.20
0.25
[Quencher] ,M
Flgure 2. Dependence of the quenching rate constant on quencher concentration. Solutions of Ru(ph)(py), and (a) N,Ndimethyl-pnitrosoaniline, (b) p-nitrobenzaldehyde, or (c) odinitrobenzene in deaerated CH,Ci,.
Scheme I
Time (ns) Flgure 1. Typical traces obtained for the decay of (3mr')Ru(ph)(py)2 in (a) deaerated CH,C12 and (b) deaearated 0.2 M methyl p-nitrobenzoate in CH,Ci2. Monitoring wavelength, ,A,, = 500 nm. Each bace is the average of three experiments.
distilled in an all-glass distillation apparatus fitted with a 40 theoretical plate fractionating column.
Results and Discussion The near-infrared excitation, Xexcit 640 nm, of deaerated solutions of Ru(ph)(py), in CH2C12produces the characteristic transient spectrum, A500 nm, that has been previously assigned to the lowest triplet excited state, 37r7r*.13Such a state has also been tentatively assigned as the photoemissive state which is observed in emission experiment^.'^ The 37r7r* excited state is formed within the time resolution of our apparatus, t 5 ns. Hence no information on the singlet state could be derived from these experiments. The disappearance of the 37r~*excited state obeys a first-order rate law with a rate constant ko 6.0 X lo6 s-'. Nitroaromatic compounds (Table I) are able to quench the 37r7r* excited state and its lifetime decreases with increasing concentrations of the quencher. However, the independence of the 37r7r* yield on quencher concentration demonstrates that the singlet state is not quenched in these experiments (Figure 1). Rate constants for the quenching process were obtained from the slope of the plots of the observed rate constant vs. quencher concentration (Figure 2). In general, such plots were linear over the range of quencher concentrations used in our study, Figure 2. Diffusional contributions to the quenching rate constant were corrected with eq 3.14J8J9 Equation 3 gives the
-
-
-
-
-1= - - 1 -
1
IZQ
IZD
IZq
(3)
(17) T.-H. Huang, K.E. Rieckhoff, and E, M. Voigt, J.Phys. Chem., 85,3322 (1981).
quenching rate constant corrected for diffusional effects, k,, in terms of the experimentally determined rate constant, k,, and the rate constant for a diffusion-controlled quenching of the excited state, kD,19 Values of k , are reported in Table I. The nitroaromatic series has considerably large triplet energies, namely, 200-250 kJ/moL21 Hence energy transfer from the 37r7r* of Ru(ph)(py), with an energy -130 kJ/ molN will result in a highly endoergonic process. This is in agreement with our observation that the quenching process produces a one-electron-oxidized ruthenium(I1) phthalocyanine ligand radical which then undergoes back-electron-transfer reactions in a microsecond to millisecond time scale (Figure l). In this regard the quenching can be described in terms of the Rehm-Weller mechanism as indicated in Scheme I.% According to this mechanism one arrives to an expression for the quenching rate constant in terms of the rate constants for various elemental processes, eq 4. The replacement of the ratio k,/kji, by k.,
k.
k30 K 2 3
the equilibrium constant, Kij,is justified under the as(18) R.M. Noyes, Prog. React. Kinet., 1, 129 (1961). (19) Values for k~ were calculated according to M. von Smoluchowaki, 2.Phys. Chem., 92, 129 (1917). The molecular radii required in this treatment were obtained from the interatomic distance and crystallographic properties reported for each species.%** (20) A. B. P.Lever, Ado. Znorg. Chem. Radiochem., 7, 27 (1965). (21) L. J. Boucher in 'Coordination Chemistry of Macrocyclic Compounds", G. A. Melson, Ed., Plenum, New York, 1979, Chapter 7. (22) L. E. Sutton, Ed.,'Interatomic Distances",The Chemical Society, London, 1958, Supplement, 1965. (23) G.N.Lewis and M. Kasha, J.Am. Chem. SOC.,66,2108 (1964). (24) Estimated from the 0-0 transition of the phosphorescence." (25) D. Rehm and D. Weller, Ber. Bunsenges. Phys. Chem., 73,834 (1969).
The Journal of phvsical Chemlsfry, Vol. 86, No. 20, 1982 4030
Excited-State Redox Properties of Ru(phNpy), 57.91
E, Volt
I
Ru(;hl(py);te-**RU
(phl(py$
I'
Ru (ph)(py12t e-* -1
Ru (p'h)(pyI;
nn
-3n9r
-'n 'n
*
1 -19.3 0 AGq KJ/mol
19.3
-3nr'
Flgure 3. Logarithmic dependence of the quenching rate constant on
-
the free energy, AGO, of the quenching reaction. The theoretical line uses an intercept RT In k:
-ground
stote-
43.4 kJ/moi.
sumption that the electron-transfer, back-electron-transfer, and diffusive processes are elementary steps of the mechanism. If this is the case, one can also express K23 by means of the free energy change in the electron-transfer equilibrium that involves the encounter complex, namely, AG' in eq 5. Moreover, the rate constant k23 can be re-
placed by an Arrhenius equation, eq 5, where AG* is the activation energy and ko the frequency factor for the redox reaction. The expression of k, can be further reduced to a more convenient form by using given expressions from the electron-transfer theory for the expansion of AG* and AG'in terms of the standard free-energy change, AGO, of the redox equilibrium with both reactants and both products separated at a distance where there is no interaction between them. The values of the free energies AGO expanded with the nitroaromatic series are small, namely, !AGO1 5 20 kJ/mol, and quenching rate constants are still far from the inverted r e g i ~ n . ' Therefore ~ ~ ~ ~ ~we ~ have considered that the theoretical treatment of Marcus and Hush is very appropriate for our calculations, see below. According to Marcus theory,I4 the free-energy AG'can be obtained from the redox potentials of the two couples, and EQ' namely, t 0 p p for R~(ph)(py)~+/(~rr*)Ru(ph)(py)Z for and work terms Wp and W, for bringing the reactants and products to the distances in the encounter complex, eq 6. Also AG* can be expanded in terms which AG' = 23.16(toPp- ~ O Q ) + Wp - W, (6)
0.0 -9round
3 r r*-
state-
$0
Flgure 4. Diagramatic representation of the redox potentials for the oxidation and reduction of selected electronic states in Ru(phXpy),. The n r ' states are those corresponding to transitions from e, and b,, orbitals centered in bridging nitrogens, e.g., those which are not coordinated to the metal center and the lowest unoccupied MO. For a description of the electronic levels in various phthalocyanines see ref 38 and references therein.
Since the plot of RT In k, vs. tl12 for the quencher gives a straight line with a slope -0.5 as predicted by eq 8, our approximation of using the Marcus theory for our calculations is then justified (Figure 3). The Marcus theory shows that the quenching rate constant is given by eq 9.
&/&-e,
contain the free energy for the electron transfer within the encounter complex AG ' and the Frank-Condon reorganization parameter A, eq 7. Combining the expressions for AG* = (A/4)[1 + (AG'/A)I2 (7) AG*, eq 7, AG', eq 6, and k,, eq 5, one can obtain a relationship which is reduced to eq 8 for small values of the difference in the redox potentials, Coph* - e o Q eoph*
€'Q
RT In k, = RT In k,(O) - - + 2
2
(26) R. P. Van Duyne and S. F. Fisher, J. Chem. Phys., 5,183 (1974). (27) J. Ulstrup and J. Jortner, J. Chem. Phys., 63,4358(1975). (28) N.Kestner, J. Logan, and J. Jortner, J.Phys. Chem., 78, 2148 (1974). (29) S. Efrima and M. Bixon, Chem. Phys. Lett., 25, 34 (1974). (30)S.Efrima and M. Bixon, J . Chem. Phys., 13,447 (1976). (31)W.Schmickler, J. Chem. SOC.,Faraday Trans. 2,72,307(1976). (32)R. R. Dogonadze, A. M. Kustenov, and A. Vorotyntaev, 2.Phys. Chem. (Wwsbaden), 100,1 (1976). (33)P.Siders and R. A. Marcus, J . Am. Chem. SOC.,103,748(1981).
The rate constants kji and kj, are the self-exchange rate constants for the quencher and excited state, respectively, and Kij is the equilibrium constant for the cross reaction. The average self-exchange rate constant for the nitroaromatic series, kii 4.0 X lo8 M-' s - ' , ~ and ~ the values obtained for k, (Table I) give a value, kjj 1.9 X lo' M-' s-l for the self-exchange rate constant of the excited Ru(ph)(py)z,eq Such a value of the self-exchange rate
-
-
*
+~R) U (3rr*)Ru(~h)(~ z (~~)(PY)~+ Ru(P~)(PY)z+ + ( 3 a r * ) R u ( ~ h ) ( ~(10) ~)z constant compares very well with one, Itjj
-
5.9 X lo6 M-'
(34)B. A. Kowert, L. Marcoux, and A. J. Bard, J.Am. Chem. SOC.,94, 5538 (1972). (35)The reduction potential, s'ph., of the couple, Ru(ph)(py),*/ (Smr*)Ru(ph)(py)2is required for a solution of eq 9, namely, K;, = exp(-nFAco;j/RT);with Aco;j= coph* - c ' ~ . An approximate value of c)p cm, be obtained from the expresslons of eq 8 under two different con itions ~ 0 and iteration. namely, for RT In k, = 0 or c o =
4040
J. Phys. Chem. 1982, 86. 4040-4045
s-', obtained from the reaction between Fe3+and (3?r?r*)-
as a reductant with enough strength for reducing substrates as paraquat. However, attempts to induce the reductive quenching of the 37r7r* state by various donors as triRu(ph)(py)z+,Fe2+ (11) ethanolamine were unsuccessful and the lifetime of the excited state remains constant even at high quencher K ,= 7.4 x 109 M-' s-1 concentrations. These results cast some doubts on the The self-exchange rate constant, obtained in these expossibility of a direct reductive quenching of the 3a7r*state periments, corresponds to a Franck-Condon reorganizaby weak reducing agents. Nevertheless states placed at tional energy, A, + Xi 41.7 kJ/mo1.14J5 It is feasible that higher energies, namely, the lowest lmr* or the reactive the main contribution to the reorganizational energy is n?r*states, can still behave as powerful oxidants. One can mostly related to the outer-sphere reorganizational term, get an idea of the redox potential of the reactive n?r* and A,. Indeed, the eq 12, proposed for expressing A, in terms lan* states by using some of the energies reported for the electronic levels of various transition-metal phthaloc y a n i n e ~and ~ . ~the ~ energy of the 0-0 transition from the ground to 'mr*state. These values suggest that the 'n?r* state, namely, one that involves the electronic density of of the dielectric properties of the medium and the size of the bridging nitrogens, must be 0.56 V more oxidant than the various species, gives A, 54 kJ/m01.'~8~This result the lowest l?r?r* state. Moreover an excess potential, em suggests that the self-exchange proceeds with a minimum 2.0 V, with respect to ground state can be assigned to participation of the metal center as one can expect from the 'aa* from the 0-0 transition in the absorption specthe nature of the products. However, this may not be the and trum. If this is the case, the potentials of the 'm* general situation in transition-metal phthalocyanines and h a * states amount to topp 1.56 V vs. NHE and eoph, lo7 M-' s-l would eventually reflect departures from kah 2.10 V vs. NHE, respectively, for couples *Ru(ph)(py)2/ various contributions from the metal center. Ru(ph)(py)2-. In this way one can obtain the oxidation Substitution of kll, kZ2,and k values in eq 8 gives the potentials for various excited states of Ru(ph)(py)zas it reduction potential of the R~(ph)qpy)~+/(~?ra*)Ru(ph)(py)~ is summarized in Figure 4. It seems clear from the Figure couple, copha -1.00 V vs. NHE. This potential is in 4 that the n?r* states have potentials which will provide agreement with the potential that is obtained by adding a considerable driving force for the observed hydrogen the energy difference between the zero vibrational levels abstractions, eq 1, that are induced ultraviolet excitations of the 3?ra* and ground states, ew, to the ground-state of transition-metal phthalocyanines. oxidation potential, eoph -0.524 V vs. NHE. The asAcknowledgment. The research described herein was signment of the 917-nm emission of Ru(ph) to the 0-0 supported by the Office of Basic Energy Sciences of the tran~ition'~ shows that the excited state must have an Department of Energy. This is Document No. NDRL-2350 additional potential, e+,, 1.34 V, with respect to the from the Notre Dame Radiation Laboratory. ground state. According to this, the 3ir?r*state behaves Ru(ph)(py)2,eq 11.36 (3?r?r*)Ru(ph)(py)z+ Fe3+
-
-
-
-
-
-
-
-
-
-
(36) D.R. Prasad and G. Ferraudi, work in progress. (37) The solution of eq 12 was obtained under the approximations ra = rb and d 2r,. The radius of the reactants was assigned according to the structural properties reported for the
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(38) A. M.Schaffer, M. Gouteman, and E. R. Davidson, Theor. Chim. Acta, 30, 9 (1973). (39) A. Henribson, B. Roos, and M. Sundbom, Theor. Chim. Acta, 27, 303 (1972).
Enthalpies of Dilution of Aqueous Solutions of Cyclohexanol, Inositol, and Mannitol Ian R. tasker and Robert H. Wood' Department of Chemisby, Universiiy of Delaware, Newark, Delaware 1971 7 (Received: March 76, 7982; I n Final Form: May 20, 1982)
Enthalpies of dilution for binary and ternary aqueous systems of cyclohexanol, myo-inositol,and mannitol have been determined at 25 "C. Data have been treated in terms of the Savage-Wood additivity principle and show myo-inositol to exhibit anomalous behavior. Using literature data, we have determined a new set of group interaction enthalpies for methylene and carbinol groups. Explanations for the behavior of inositol me suggested, and the significance of the present group interaction enthalpies is considered.
Introduction This paper is part of a continuing study of the thermodynamic properties of dilute aqueous solutions of nonelectrolytes in order that the influence of various functional groups on solute-solute interactions may be determined. In the first paper of this series, Savage and Wood' measured the enthalpies of interaction in dilute (1) Savage, J. J.; Wood, R. H. J. Solution Chem. 1976, 5, 733. 0022-3654I82l2O86-4040$0 1.2510
aqueous solution of a large number of compounds containing amide and hydroxyl groups. A simple additivity principle was proposed allowing both the sign and the magnitude of the various functional group interactions to be determined from experimental data. Further work2 extended the principle to free energies of interaction. (2) Okamoto, B. Y.; Wood, R. H.; Thompson, P. T. J. Chem. Soc., Faraday Trans. I 1978, 74, 1990.
0 1982 American Chemical Society
