10323
J. Phys. Chem. 1992,96, 10323-10326
Photoinduced Electron Transfer within Metal Dithioiene-Vioiogen Ion Pairs Studied by Flash Horst Kiscb,*9+ Walter Dlimler? Claudio ChiorboIi,**tFranco Scandola,$Josef Salbeck,l and Jzirg Daub* Institut f i r Anorganische Chemie der Universitiit Erlangen-Niirnberg, 0-8520 Erlangen. Germany, Dipartimento di Chimica, Centro di Fotochimica CNR, Universitci di Ferrara, I-441 00 Ferrara, Italy, and Institut f i r Organische Chemie der Universitiit Regensburg, 0-8400 Regensburg, Germany (Received: July I O , 1992; In Final Form September 14, 1992)
Laser flash excitation of 1/1 ion pairs consisting of the reversible redox systems [M(n~nt)~]~(mntz- = maleonitriledithiolate) and V"+ (V = 2,2/- or 4,4'-bipyridinium derivative, n = 2,l) affords the electron-transferproducts [M(mnt)Jand V("')+ in the case of M = R,while no transients are observed when M = Ni. These primary redox products recombine by a fast second-order back-reaction. The cage escape efficiencies are very sensitive to the charge of the quenchers (1.0 for n = 1, 0.1 for n = 2). The back-electron-transfer rate constants are close to the diffusion-controlled values and seem to depend slightly on the driving force.
Apparatus uwl Procedures. Cyclovoltammetry was conducted in MeCN using ferrocene as internal standard and converting the obtained values relative to SCE (0.39 V in MeCN). Molar absorptivities of the onaelectron-reduced acceptors were measured by reduction of the corresponding PF{ salts in acetonitrile, using a spectroelectrochemical cell as described previously.12 Laser flash photolysis was conducted as reported" using a frequency-doubled ruby laser (A = 347 nm, pulse width of 25 ns). The concentration of the absorbed laser photons was obtained with an actinometric method based on benzophenone triplet absorption. DMSO was used instead of MeCN in laser flash photolysis experiments since the solubility of (TBA)z[M(mnt)2] is higher in this solvent. For this reason, viologen bromides were converted into their perchlorate salts. The initial concentrations in the laser flash photolysis experiments were (TBA),[M(mnt),] = 1.33 X lo4 and [viologens] = 3.33 X M. Solutions were nitrogen saturated before being transferred into 1.oCmcuvettes; absorbance at 347 nm was always higher than 2. In each experiment, the transient spectrum and its decay at 385 and 860 nm were measured. The estimated error limit of the rate constant is &lo%, based on repeated experiments. No decomposition was observed after repeated flash experiments. Association constants (KA) between [Pt(mnt)z]2- and the viologens in DMSO were calculated using the Eigen-Fuoss eq~ation.'~J~ Difbion-controlled rate constants (kd)and ion-pair dissociation constants (k4) were calculated for the same systems using the Debye16and Eigen17 equations, respectively, with nuExperimental Seetion merical integration over the interreactant distance.'5 The following Materirla (TBA)2[M(mnt)2J(TBA = tetrabutylammonium; physical parameters were used for the calculations: dielectric PQBrZ,Io M = Pt, Ni) were prepared according to literat~re;~ constant (DMSO) = 46.7, viscosity (DMSO) = 2.24 CP(23 "C), EPSPBr,I1and EPSDMPBr were converted to the mesponding r( [ P t ( m ~ ~ t ) ~=] ~230 - ) pm, r(PQ2+)= 330 pm,r(EPSP+) = 340 PFC and C104- salts by adding to their aqueous solutions a pm, r(EPSDMP+) = 430 pm, r(TBA+) = 530 pm, r(C104-) = saturated aqueous solution of NH4PF6 or NH,C104. I-EthyE1 ' - [ ( 3 ) - ~ ' - ~ ~ ~ ~ ~ 4 230 ' pm. - b The ~ radii ~ ~ of the reactants were obtained as r = l/2(d~f,d~)l'~, where d,, and d, are the dimensions along the Bromide (EFSDMPBr). l-Ethyl-4,4'-bipyridinium bromide (1.2 three molecular axes, which were estimated for the dithiolenes g, 4 "01) and 1,3-propanesultone (3.5 mL, 4 m o l ) are heated from X-ray structure data,18J9 and with van der Waals radii r(H) in 110 mL of acetone for 72 h at 60 'C. The white powder = 80 pm, r(N) = 120 pm, r(S) = 150 pm for the bipyridinium precipitated was washed three times with 30 mL of acetone, dried, cations from CPK models; the calculated values are reported in and crystdhd from MdlH/et&er to give 0.420 g (25%) of white Table I.2o In Table I are also reported the calculated fractions crystalline EPSDMPBr. IH NMR (D20/MeOD): 1.1 (t, 3 H, of metal dithiolenes present as contact ion pairs (Flp). J = 7 Hz, CHa-CHZ), 1.8 (s, 3 H, CH3), 2.0 (m, 2 H, CH2CH2CH$03), 2.5 (t, 2 H, CH2CHzCH$03), 4.3 (m, 4 H, Results CH2CH2CH2SO3, CH$HZ), 7.4 (d, 4 H, J = 6 Hz), 8.5 (d, 4 Redox PotdUrrd U V - h SpeCtR oftbeR&dAcoepQra H, J = 6 Hz), 8.6 (s, 1 H). Elemental Analysis The reduction potential of the dicationic acceptor PQ2+is known ( C ~ ~ H U N ~ B ~ S O ~ C, * H47.13 ~ O )(47.17), : H, 5.75 (5.81), N, 6.15 from the literature (-0.58 V in MeCN vs SCE);2' in MeCN the (6.46). two zwitterionicviologens E P W and EPSDMP+ have reversible reduction potentials of -0.47 and -0.88 V vs SCE, respectively. Inrtitut far Anorganische Chemie der Universitat Erlangen-NLlmbcrg. The oxidized forms of all the viologens absorb at wavelengths * Univenita di Ferrara. 8 Inrtitut far Organiscbe Chemie der Univenitat Regensburg. below 300 nm (vide infra) while the one-electron reduced com-
IIltrodUCti0n Contact ion pairs, which can be viewed as limiting cases of supramolecular systems: offer the possibility to study fundamental We have rccently features of intramolecular electron transfer.4~~ isolated a series of ion-pair complexes of the type {V2+[MLZI2-) ([ML2I2-: M = Pt, Ni, L = an ethylene-1,2-dithiolate;V2+ = 2,2'- or 4,4'-bipyridinium compounds), consisting of a a n n h t i o n of a reversible inorganic with a reversible organic redox system.6 These compoundsusually exhibit ion-pair charge transfer (IPCT) bands in the vis-near IR. The dianionic metal dithiolene donor ([MLA2-) and the diationic viologen acoeptor (Vz+)can be easily tuned in their redox properties by proper selection of M, L, and substituents, respectively. By application of the Hush theory' is was possible to obtain informations on the relation between optical and thermal electron transfer from the spectral properties of the IPCT bands. It was found that with donors like [M(mnt)J2-, (M = Ni, pt, mnt2- = maleonitrile-l,2dithiolate)the reorganization energy of the e l e c t r o n - d e r reaction [ M ( ~ I I ~ ) ~+]V2+ > [M(mnt),]- + V+ stays constant upon variation of the central metal or viologen redox potential! In this article, we report a study of photoinduced electron transfer and thermal back-reaction in ion-pair complexes between [pt(mnt)d* and various mone and diationic viologens in DMSO solution (Figure 1). Preliminary results on some of these systems appeared
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,
Q 1992 American Chemical Society
Kisch et al.
10324 The Journal of Physical Chemistry, Vol. 96, No. 25, 1992
m +U+ k (nm) Figure 3. UV-vis spectra of PQ(PF&, 2.8 X lo-' M in acetonitrile: (-), no added salt; (- - -), in the presence of (TBA)I; and (---) in the presence of (TBA)Br. SCHEME I
EPSP'
M = Ni , Pt N ' ~ ) ( ~ c N NC CN
Figure 1. 1.5
I'
0.5
0 2 0 0 3 0 0 4 0 0 s o a e O 0 7 0 0 e O 0 wavdongth (nm)
Figure 2. Spcctroelectrwhemistry of EPSP(PF,) in acetonitrile; changes in U V 4 spectrum upon application of an increasingly negative potential (a, initial spectrum; b, final spectrum, see text).
pounds are red (PQ+)to blue (EPSF"')and their abeorption spectra extend down to the near IR. Spectroelectrochemistry was the method of choice to determine the absorption spectra of these reduced forms, as demonstrated for EPSP/EPSP" in Figure 2. Upon application of increasingly negative potentials the formation of EPSP is indicated by the growing in of new absorption maxima at 393 and 600 nm. The molar absorptivities of EPSPO can therefore be calculated from the maximal absorbance obtained at -0.55 V vs SCE. The spectra of all the reduced acceptors were obtained by the same method. Pbotobduced Electron T d e r . The ion-pair complexes, previously isolated and characterized as 1:l adducts in the solid state: were "prepared" in situ by addition of the dithiolenes to viologens in DMSO. The initial concentration used in standard experiments were as follows: V(C104)1,2 3.33 X M, (TBA)2[M(mnt)2] 1.33 X lo-" M in DMSO. Under these experimental conditions the spectrum is always the sum of the spectra
of the two components since the IPCT bands cannot be observed in solution due to their low extinction coefficient (e.g. 42 M-' P I at 681 nm for PQ[Pt(n1nt)~]).6J~The use of halogenides instead of perchlorates or hexafluorophosphates as counter ions must be avoided since these acceptors exhibit CT transitions" in the range of 347 nm, the wavelength of exciting laser light. Figure 3 shows the appearance of these bands at 350 and 450 nm upon adding (TBA)Br and (TBA)I to PQ(PF,), dissolved in MeCN. When perchlorate salts of the viologens were used, the laser pulse at 347 nm was exclusively absorbed by the metal dithiolene, as a free species or as a part of the ion pair. With platinum as the central metal, laser flash excitation of the DMSO solutions led to the formation of long-lived, strongly absorbing transient species, formed within 25 ns from the flash. The intensity of the transient decreased by decreasing the viologen concentration or by increasing the ionic strength. No long-lived transient was observable in the case of the corresponding nickel complex, or in the absence of the v i o l ~ g e n s . ~ ~ . ~ ~ The transient spcctm obtained from [Pt(mnt)d' in the presmce of PQ2+,EPSP, and EPSDMP are given in the left part of Figure 4 (parts a-c). For comparison, the spectra of the corresponding reduced forms, obtained by spcctroelectrochemistry,arc depicted on the right part of the figure (parts d-f). Finally, the spectrum of the oxidized donor, TBA[Pt(n~nt)~l, prepared according to ref 24, is included for comparison (Figure 4g). The dccay of the transient was followed by measuring the decrease of absorbance at 385 and 860 nm. At both wavelengths the reaction followed second-order kinetics, as indicated by linear plots of 1/ AA (AA = observed absorbance change) as a function of time. Discwion
In the flash photolysis of the Pt(mnt)22-/viologensystems, the transient spectra (Figure 4,parts a-c) match very closely the sum of those of the reduced viologens (Figure 4,parts d-f) and the oxidized platinum malaonitrilcdithiolate complex (Figure 4,part 8). This clearly indicates that photoinduced electron transfer from the platinum complex to the viologen acceptor (A*) has occurred (es 1). [Pt(mnt)2]2-
+ V"+
hv
[Pt(mnt)J-
+ V(n-l)+
(1)
The experimental results can be discussed in terms of Scheme I.
Under the experimental conditions used, substantial ion pairing between the [Pt(mnt)Jz- complex and the viologen occurs (Table
The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 10325
Electron Transfer Studied by Flash Photolysis a
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Unm) Figure 4. Parts a-c show transient spectra obtained 30 11s after flashing (& = 347 nm) a DMSO solution containing 1.33 X 10-4 M (TBA),[Pt(mnt),] and 3.33 X lo-' M PQ2+(a), E P S P (b) or EPSDMP (c). Parts d-f show UV-vis spectra of the electrochemically generated reduced forms of F Q + (d), EPSP (e), and EFSDMP (0.Part g shows a UV-vis spectrum of the oxidized complex (TBA)[Pt(mnt),].
I). The effects of ionic strength and viologen concentration indicate that photoinduced electron transfer occurs within the ion pairs and not between the solvent-separated ions. On the other hand, under the experimental conditions used, the calculated diffusion rate constants (Table I) arc of the order of 1O1O M-I s-' and [Vz+] = 3.33 X lW3 M,so that the pseudo-first-order rate constant for dynamic quenching is at most kd[V2+]= 3.33 X lo7 s-', which is too slow to compete with the fast excited-state
deactivation of *[F%(mnt)2]2-( 1 / ~2 1 X lo8 s-').~~ Therefore we can assume that the only effective light is that absorbed by the ion pair. Under this assumption, the quantum yield of product formation (e in Table 11) in the laser flash experiments is given by cq 2 where = FIP9ct9Cc (2) FIpis the fractional concentration of contact ion pairs, qM and rloc
10326 The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 TABLE I: Cdculrtcd M W i d p l m ~ t e r (ka ~ l kd), ASSOChtiOO Comtaots (K,,), and Frrctioa of Contact Ion Pairs (Fp)for tbe [M(mnt)$/Viobgen systems" 10+kd, lO-9k4, KA, viologen p, M M-l s-l M-1 s-l M-' Fpb 0.01 11.7 0.023 504 0.63 pQ2+ 0.0037 9.1 0.478 19.1 0.06 EPSP+ EPSDMP 0.0037 8.5 0.508 16.7 0.05
'For the calculation of kd, k4, and KA, see Experimental Section. bFraction of [M(mnt)$ prescnt as ion paired species, in solutions containing 1.33 X IO-' M [M(mnt)$- and 3.33 X IO-' M viologen. TABLE II: Rate Coostants of Back-Electron Transfer, k-% Initial C-t&im CC Qurntum Yieldq e, rad Efncimy O f Fo-tion, q ~ w Of ' Redox Rodueb in the QueneO f [Pt(mt)J by Different V i o b g e d 104k-2, viologens M-I s-I IO%,', M v.,v, -AG,e v e 10.0 1.0 6.8 0.07 0.11 0.79 pQ+ EPSP 10.0 & 1.0 6.1 0.06 1.0 0.71 EFSDMP 6.0 1.0 4.9 0.05 1.0 1.12
+
*
based on the calculated fraction of ion-paired species (see text). *DMSO, natural ionic strength, absorbed laser photons: 1 X IO-' M. cCalculated from the corresponding .Ell2values in MeCN, scc also ref 8. a Efficiency
are the efficiencies of electron transfer and cage escape respectively. Thus, although the product yields as such do not change much with the type of viologen, when the difference in ion pairing is taken into account, the qaqm product is seen to undergo drastic changes. Such changes can likely be ascribed to charge effects should be faster on qoe,whereby the cage escape reaction, in the case of the monopitive viologens (Z+Z- for reaction products q u a l to 0) than for the dipositive viologen (Z+Z- for reaction products equal to Formation of the redox products occurs within 25 ns from the laser pulse and the decay of the absorbance is on the micrcwxnd timescale. The fact that there is no difference between the a b sorption spectra measured before and after repeated flashing, and the observation of a second-order process for the disappearance of the transient (see Experimental Section), clearly prove that the decay is due to back-electron transfer (eq 3) and that it occurs
-
[Pt(mnt)J + V(rl)+ [Pt(mn&I2- + V"+ (3) between the solvent-separatedredox products (Scheme I). Thus, the intercept of the second-order plots correspond to l/(coAer) and the slope to k2/Ael, where co is the "initial" concentration of products, 1 is the optical path length (1 cm),and Ae = (tpl(,,,,,++ q ~ )-+(eR(mnt)zz) + cy.+). The values of k2are collected in Table 11. Since the back-reaction of solvent-separated redox products requires diffusion followed by electron transfer within the contact ion pair (Scheme I), k2can be e x p d by eq 4. The measured k-2 = k'd[k-ct/(kt + k'-d)1 (4) k2values (Table 11) are somewhat higher that the k',, values calculated using standard equations (5.6 X 109 M-I s-l for +1/-1 system, and 3 X lo9 M-I s-' for 0/-1 systems). Therefore, within the limitsof such calculations (in particular the zwitterionic nature of the monopitive quenchers and the unrealistic assumption of
Kisch et al. spherical reactants), the back-electron-transfer reaction appear to be nearly diffusion controlled, implying that k+ 1 k'+27 Thus, the values of the bimolecular rate constants should bear limited kinetic information. The experimental observation that no transients are obtained when Ni replaces FY as central metal is most likely due to very fast radiatiodes deactivation of excited *[Ni(mnt)#-. Picosecond flash photolysis experiments28point to an approximate lifetime of 0.2 ns for the excited state of the Ni maleonitriledithiolate complex (to be compared with 110 ns for the platinum analogue22). The lowest excited state of the nickel complex is most likely a d-d state (as indicated by the weak absorption at 850 nm in DMSO), which can function as an efficient funnel for radiationless decay to the ground state.
Acknowledgment. This work was supported by Deutsche Forschungsgemeinschaft, Fonds der Chemischen Industrie, and Consiglio Nationale delle Ricerche.
References and Notes (1) Charge-Transfer complexes of metal dithiolenes IX.For number VIII, see ref 2.
(2) Kisch, H.; "ler, W.; NWlein, F.; Zcnn, I.; Chiorboli, C.; Scandola, F.; Albrecht, W.; Meier, H. Z . Phys. Chem. 1991, 170, 117. (3) Balzani, V.;Scandola, F. Supramolecular Photochemistry; Horwood: Chichester, 1991. (4)Haim, A. Comments Inorg. Chem. 1985,4, 1985. (5) Billing, R.; Rehorek, D.; Hennig, H. Top. Curr. Chem. 1990,158,151. (6)Ntisslein, F.; Peter, R.; Kisch, H. Chem. Eer. 19%9, 122, 1023 and references cited therein. (7)Hush, N.S.Progr. Inorg. Chem. 1967,8,391. (8) Dtimler, W.; Kisch, H. Nouv. J . Chem. 1991, 15, 649. (9) Billig, E.; Williams, R.; Bernal, I.; Waters, J. H.;Gray, H. B. Inorg. Chem. 1964, 3, 663. (10)Homer, R. F.; Tomlinson, T. E. J. Chem. Soc. 1960, 2498. Hiinig, S.;Gross,J.; Schenk, W. Liebigs Ann. Chem. 1973,324. (1 1) Nbslein, F. Ph.D. Thesis, University of Erlangen-Ntirnberg, 1989. (12)Salbeck, J. Anal. Chem., submitted for publication. (13) Chiorboli, C.; Scandola, F.; Kisch, H. J. Phys. Chem. 1986,90,2211. (14)Fuoss, R. M. J. Am. Chem. Soc. 1958,88, 5059. (15) Chiorboli, C.; Indelli, M. T.; Rampi, M. A.; Scandola, F. J . Phys. Chem. 1988,92,156. (16)Debye, P. Trans. Electrochem. Soc. 1942,82,265. (17) Eigen, M. 2.Phys. Chem. (Munlch) 1954, I , 176. (18)Lahner, S.;Wakatsuki, Y.; Kisch, H. Chem. Err. 1987,120,1011. (19)Kisch, H.; Fernandez, A.; Wakatsuki, Y.;Yamazaki, H.Z . N o w forsch. 1985,4Ob, 292. (20)The reliability of the calculated values is demonstrated for example in the case of PQ[Pt(mnt),J by the good agreement between the measured value of KA = 328 A 39 M-' (p = 0.016[SI)and the calculated value of 342 M-1. (21) Farrington, J. A,; Ebert, M.; Land, E. J. J . Chem. SOC.,Faraday Trans. 1978,74,665. (22)A short-lived transient is observed by laser flash excitation of DMSO solutions of nickel and platinum dithi~lenes.~~ (23) Chiorboli, C.; Scandola, F.; Kisch, H. Manuscript in preparation. (24)Davidson, A.; Edelstein, N.; Holm, R.; Maki, A. H. Inorg. Chem. 1963,2, 1227. (25)The absorptivities of reactants and products at 385 and 860 nm as obtained from spectroelectrochemistry are the following (M-l/cm-l): [Pt( ~ n n t ) ~ ] 2400 ~ - , and 0; [Pt(mnt),]-, 850 and IOOOO; PQ2+, EPSP+, and EPSDMP, 0 and 0; FQ+, 16000and 2850;EPSPO, 32000 and 0; EPSDMPO, 16 000 and 7900. (26)The values of k h (M-l S-I) and kL (8-I) calculated for the ex rimental conditions employed are as follows: PQt/[Pt(mnt)2]-, 5.6 X 10 and 2.2 X lo9; E P S P O / [ P ~ ( I ~ ~ ~2.8 ) ~ X] - ,lo9 and 5.9 X IO9; EPSDMPO/[Pt(~nnt)~]-, 3.0X IO9 and 4.15 X IO9. (27)The fact that measurable cage escape efficiencies are obtained indicat= that k2 is not much smaller than .,k (28)Persaud, L.;Sharma, D. K.; Langford, C. H. Inorg. Chim. Acta 1986, 114, L5.
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