Photoinduced electron transfer for complexes of methyl viologen: Wavelength effects on the yield of charge carriers

Volume

119. number

2.3

CHEhfICAL

PHOTOINDUCED WAVELENGTH

ELECXRON TRANSFER EFFECTS ON THE YIELD

Guilford

II and Vincent

Received

JONES

PHYSICS

LETTERS

FOR COMPLEXES OF METHYL OF CHARGE CARRIERS

30 August

1985

VIOLOGEN:

MALBA

2S May 1985

I. Introduction The photochemistry of donor-acceptor or chargetransfer (CT) complexes (or ion-pairs) of methyl viologen (MV*+) has been the subject of several recent papers [l-6] Systems capable of viologen reduction are of special interest due to the well established role of Me as an electron relay in the photofomlat~on of hydrogen from water [7] _It has been pointed out recently [S-IO] that ground-state complex&ion with familiar “sacnticial” electron donors (e.g., EDTA and triethanolamine) and photoinduced electron transfer ym complexes can in principle comphcate an analysis of typical water photoreduction systems. Complexation or ion-pairing of MV2+ or derivatives has also been implicated in the operation of photoelectrochemicaf cells 14.1 I] =and in electron transfer at the interface of polymer-stabil~ed colloids, derivatized electrodes [ 121, or sen~t~er-bound polyelec~olytes [ 13]_ The origmal survey by \Vhite [14] of MV2* complexes with typical organic electron donors revealed that many donor-acceptor combinations led to strong visible absorption. The appearance of broad CT bands extending from the near UV into the visible provides an opportunity to investigate photoeffects over a wide range of wavelengths_ We have now examined a number of viologen complexes having welldefmed bands which permit selective excitation usmg a Nd - YAG la0 009-2614/85/S (North-Holland

03.30 0 Elsevier Science Publishers B V. Physics Publishing Division)

ser (355 and 532 nm). An important general finding concerns the pronounced increase in yield of the viologen radical, Me, on photolysrs at the shorter wavelength_ This behavior of CT complexes engaged in photoinduced net electron transfer has been documented in a number of cases mvolvmg combination of two neutral partners [15-171. In this paper the observation is affirmed for combinations of the ion-molecule and ion-pair type. Of particular interest also is an assessment of the nature of the wavelength dependence on free radical pair productron for CT compIexes as a function of the charge-type of photogenerated radicals. In addition, probes were conducted for the intervention of triplet states in photoinduced electron transfer_

2. Experimental Aniline and ~~-diniethylan~~e were dist~ed under ~gon_p-bromoan~~e,p-lodoan~ine, naphthalene, and diphenylamine were purified by repeated recrystallirations. Diphenylaminelt-sulfonate (Na+ salt) was Aldrich ACS reagent grade, and was used as received. 1 ,l’-dimethyl4,4’-bipyridmium dichloride @IV”+) was synthesized according to a procedure by Michaelis and Hill [ 181, and was repeatedly recrystallized from methanol. Baker HPLC grade acetonitrile was used as received_ 105

Volume 119, number 2,3

CHEMICAL PHYSICS LEXTERS

The transient absorption of Mw was observed using a laser fIash photolysis apparatus conststing of a Quantel YG-581 Nd I YAG laser, a LeCroy TR8818 transient recorder, a Kinetic Systems CAMAC dataway with PDP I l/23 microprocessor, a 150 W xenon lamp, an Instruments SA H-20 monochromator and an RCA 4840 PMT [19] _ Transient absorbances were measured at 395 and 605 nm with molar absorptrvlties assumed to be 4.2 X IO4 M-l cm-l and 1.4 X IO4 M-1 cm-l respectively [ZO] . For relative quantum efficiency measurements. donor concentratrons were adlusted to achreve values of OD = 0 3 at the irradiation wavelengths. All solutions were sparged with argon prior to irradiatron. A Scientech, Inc- power meter and absorbing disk calorimeter was used to measure the energy per pulse, which was maintained at 90 mJ/pulse for both 355 and 532 nm, the respective frequency-tripled and -doubled Nd : YAG frequencies_ The laser beams were expanded to fuIIy illuminate a 2 cm sample cell. The relative number of photons at these energies was 532/355 = 1.5.. The calculated quantum yields for electron transfer were the result of at least four detenninations, for each of which 100 shots were averaged. The yields of transient radicals on 355 nm photolysis were reproducible to within 10%. Relative quantum effciencies (355 nm) of viologen reduction were converted to absolute quantum yields using the appearance of the benzophenone triplet m benzene (hmax = 535 nm; = 7630 M-l cm-r) as an actinometer [3-l] _DeES35 gassed benzophenone solutions were 5.3 mM, CID,,, = 0_3_

3. Resuits

30 August 1985

and discussion

Absorption data for a series of MV2+ complexes of substituted aniIines and naphthalene are shown in table I along with spectra (fig 1) for the MV2+/amine systems which typify the pronounced visible absorption of the complexes Equilibrium constants for complexation and molar exttnction coefficients were obtained using the Benesi-Hildebrand, Scott, and Scatchard methods [22] _The linearity of plots according to all of these methods suggested 1 I I stolchiometry for complex formation A Jobs plot of absorbance data [23] for MV2+/DPA supported thus assignment. The situation was more complicated in one case (MV2~/DPAS-) where multiple complexation was apparent from the Jobs analysis *_ Quantum efficiencies for photoreduction of MV2* on u-radiation of complexes at 355 nm are shown in table 1. AI1 data were recorded at 200 ns foIlowing the laser pulse where the transient absorbance had reached a maximum. Decay of the vrologen radrcal occurred with half-lives of about 80 C,LS. The time resolved spectrum is shown in fig. 2 for MVZ+/DMA; an absorption at 465 nm due to the DMA radicalcation 1s in evidence along with the vlologen peaks at 395 and 605 m-n_ The second-order decay rate constant for the disappearance of Me was obtained for the MV?‘/DMA system: k = 2.9 X IO9 M-l s-l _ The production of * /. Jobs plot and a Scatchard analysis both showed ihe presence of a complex with 2 DPAS- to 1 MV2’ stoichiomatry. In order to assure that most of the I&ht was absorbed by the I : 1 complex. a Iarge excess of MV2+ was used. [hIV2+] = 25 mhl. and IDPAS-] = 4.7 mhf.

-I-able 1 Speckal,

camplexitmn

and flash photolysis

Donor ‘1

aniIine pbromoanilme p-iodoanilinc N,N-dimethykuLu~e djphenykunme diphenylaminc4sulfonate naphthalenc

datafor bIV2+ complexes Ahr. AN BAN IAN DMA DPA DPAS NAP

Act max (nm)

K &

400 420 430 47.5 480 462 365

0.34

)

Em;U (M-l cm-r)

@et

570

0.015 0.019 0.051 0.028 0.025 0.051 0.021

16 28

70 71

2.0

165

=) [Donor] = O-03 to 0.11 hi for hzrc = 355 nm except for DPAS, for which 4.7 mM was used [hIV2+J = 25 106

mM in aft cases.

CHEMICAL PHYSICS LETTERS

Volume 119, number 2.3

0.25

and a hi~er~ner~ transition (e.g., a local excitation of the complexed MV3*) (note variation in h,,, ax, table 1). Yields of radicals did not appear to depend on these subtle differences for excitation at 355 nm. Excitation of the complexes at the red edge of the charge-transfer bands (532 run) led to a contrasting result for radical production. Transient absorbances were no more than 0.002 compared to typical values of AOD605 = 0.02 for excitation at 355 run. For these comparative measurements samples were adjusted to equal absorbance for the complexes at the irradiation wavelength (OD = 03) and pulse energies were maintained at 90 mJlpulse_ Due to the weakness of transient srgnals at 605 nm on 532 run excitation, a more quantiiat~ve comparison among the different complexes was not possible_ Also, a determ~ation of the size of the waveIeng~ effect (355 versus 532 nm excitation) for each complex showed some scatter resuhing from the low signal-to-noise ratio at h,,, = 605 = 532 nm. Nevertheless, a conservative nm when he,, estimate of the relative yield for the amine complexes is @355/~531 >, 10 when adjustments were made for the number of photons absorbed. A number of factors influencing the yield of photochemical electron transfer between donors and acceptors have been identified, including solvent polarity and viscosity, charge-type, and the spin state of nascent radical pairs [24] _A dominant feature controlling the separation of photogenerated radicals or charge carriers in the case of photolysrs of ground-state charge-transfer (CT) complexes appears to involve the wavelength of excitatron f 15-l 7] . Excitation via the CT transition leads to a contact radical pair, after which pair separatron and an exceedrngIy raped return to the ground state compete,

0.20

D ..A 4

0.00 400

350

450 Wavelength

500

550

Mla

(am)

Fzg. 1. Absorption spectra for methyl viologen @IV23 compkxes in 70% a~to~~e/water: (a) [hfVzi] = 25 mM; (b) [MV2’] = 25 m&f, [DPAS] = 0.0047 h3; (cl [MV2’] = 25 mM, [DPA] = 0.051 M.
the vioIogen radical in bulk solution appeared to be highly reversible (efficient second order back reaction) since the CT bands of the complexes were not bleached after 400 pulses at 355 nm (90 mJ/pulse), and Me was not present after the flash experiment (no blue color; no 395 and 605 run peaks). The 35.5 nm excitation wavelength falls at the blue edge of the CT band for all the complexes (population of the highest CT vibronic levels assuming excitation of a single complexed species) and even at the junction of the CT

030

i

hv

3

30 August 1985

0.15

(D+, A-)*,

(ID’, AT)*

+ D + A,

(D+, A’)+

-+Dt+A-.

0.10 0.05 0.00

i,,(

,, , 450 400 ‘,,

,

,

,

,*

500

,,,,,,,,,,,,,,,I 550

800

850

Tnoelerr@h (pm1

Fig 2. Time-resolved absorption of transientsfollowing laser flash excitation of the complex of fifV2+ and DMA at 355 nm. [hWzi] = 25 m&f; [DMA] = 0 16 M in 70% aceto~~~e/~~ater.

For some CT complexes, the latter back electron transfer can occur in picosecond time, according to several recent studies [35-27]_ The wavelength effect might have been understood in terms of the necessrty of the excited CT complex to cross a specific barrier largely resulting from Coulombic forces holding the radical pair. The following types of to7

Volume 119, number 2,3

CHEMICAL

PHYSICS

excited complexes, differentiated according to charge type, are rnvestrgated in this study. (Me, DMA%)* and (Me, DPAS’)* _ The important feature to note is that within approximately a factor of three all of the complexes behave similarly m terms of the value of quantum yreld for pair separation on 355 run excitation. Thus, the deposition of excess energy which is responsible for enhanced pair separation must not be very sensitive to Coulombic intra-pair interaction. The situation contrasts with the result of bimolecular donor-acceptor quenching for which thermally equilibrated radical pair intermediates participate and where charge-type effects can dominate electron transfer efficiencies [28] . Another model for enhanced pair separation at high excitation energies would employ mtersystem crossing between upper vibronic levels of the singlet and triplet CT manifolds (scheme 1) [29] _If the singlet excited CT state were induced to intersystem cross with higher efficiency, higher radical yrelds would be expected due to the spin prohibition on back electron transfer. The results for the aniline donors show that enhanced intersystem crossmg rates (kisc) that might have been induced by Br and I substituents influence minimally the electron transfer yield. Larger heavy-atom enhancements of intersystem crossing have been observed for hexamethylbenzene/substituted phthalic anhydride complexes [30] _Values for kisc for exciplexcs can range widely (factors of lo4 for linked pyrene/aniline systems [3 l] ); an e~anc~m~nt of alO3 = 7 X lOlo s-r) has been observed for reverse (to k,,c intersystem crossing for the radical pair evolved from triplet thionine and p-iodoaniline [32] . We conclude that interna conversion, of singlet CT*) via back electron transfer, for the vrologenfaniline compIexes is so rapid (k > G X IO9 s-r for recombination of MC,

-_

-~--___

s1 (CT]

-f-

=) hV

I s, Ic.a-----

T1

(CT) , Tz b-E]

30 August 1985

SCN- 1261) that moderately efficient intersystem crossing is exchrded even with heavy-atom perturbation. Thus, unperturbed complexes are most unIikeIy to employ a mechanism for ionic dissociation involving triplets, and enhanced radical yieIds at short excitation wavelengths are not likely to result from intersystem crossrng of vibrationally excited rCT* _ Another test of the same principle is provided by the MV2+/naphthalene combination. In this case, the Franck-Condon CT state after 355 nm excitation lies at =80 kcal/mol above the ground configuration and well above the level of the local naphthalene trrplet (60 kcaljmol) (scheme 1). Intersystem crossing between CT and locally excited (LE) states is expected to be enhanced according to the results of low temperature studies of neutral organic complexes [33], yet we were unable to observe the naphthzdene trrplet on excrtation of MV~~/naph~~ene at 355 run (@ C O_OOS), nor was the yield of MV? appreciably higher for this system displaying the favorably poised LE triplet. Naphthalene triplet production was compared with the result of flash photolysis of a 4.5 mM benzophsnone solution with 1 0 mM naphthalene m benzene which gave the naphthalene triplet &, = 400,430, 465 nm) in quantrtative yield (with complete quenchmg of the benzophenone triplet) **_ Thus, internal conversion (back electron transfer) is the dommant mode of decay for viologen complexes, a result already noted for complexes of organic neutrals in fluid solution 1331. We suggest that several factors may be involved in the mechamsm for enhanced pan separation for CT* having excess vibrational energy (the wavelength effect)_ A classical (Noyes) model [35] would highlight the importance of converslon of excess vibrational energy in the Franck-Condon CT state into translational energy of the radical fragments The classic example is the wavelength dependence of the photodissociaA quantum metion yreld for molecular iodme [36] chanical feature involves the coupling of the CT electromc transitron to the low frequency intra-pair *’ Strictly speaking a high yield of radica.U would be anticipated under conditions in which the LE state is not low-lying in the tiplet manifold, and that intersystem crossing is induced at d&&e. upper vibrational levels of S1 and T(LE). For an example of intersystem crossing of radicalion pairs followed by decay to a low-lying tiplet, see ref.

1 Scheme

108

I

LEl-TERS

1.

r341-

CHEMiCAL

Volume 119, number 2,3

stretching mode which sends the pax along a separation trajectory (a longer mtra-pair distance is more probable at higher frequencies, above the dissociative hmit for mtra-pair

stretch)

[ 15]_

A third

30 August 1985

PHYSICS LETTERS

feature

which is likely to be important reff ects the fact that excitation over the wide range of wavelengths available for these broadly absorbing complexes may influence dramatically the type of molecular vibration which is ultimately excited in non-radiative decay. In the present case, excitation at low energies (e.g., 532 MI) prepares CT* in such a way as to efficiently promote conventional mtrachromophore vibrations (i.e. a C-H stretch, or the twist of the brphenyl-like rings of the viologen moiety are good accepting modes for vibronic relaxation)_ Franck-Condon factors for these conventions vibrations will dimbush at high excitation energies in favor of other nuclear motion, ~clud~g interchromophore (iow frequency, but large amplitude separation of the components of the excited complex [33,37]) (i.e couphng with the radical dissociation continuum) or the excitation of solvent phonons which in turn facilitate cavity formation and radicalpair separation. We are presently investigating the specral role that changes in medium may play on altering the wavelength effect on photodissoclatron of CT complexes. a process which we show here to have consrderable generality.

[4] BP. Suck, W-J. Dressick and TJ. Meyer, J Phys. Cbem 86 (1982) 1473. [5] A. Harriman, G. Porter and A. \Vilowska, J. Chem. Sot. Faraday Trans. II 80 (1984)

191.

[6] J.R. Barnett, A S. Hopkins and A. Ledwith. J. Chem. Sot. Perkin Trans II (1984) 80. [7] A. Harriman, Photogencration of hydrogen (Academic Press, New York, 1982). [8] MZ. Hoffman, D-R. Prand, G. Jones II and V. hlalba, J. Am. Chem. Sot. 105 (1983) 6360. [9] D.R. Pramd and hl.Z. Hoffman, J. Phys. Chem. 88 (1984)

5660.

J9. Kuaynski, B-H. Milouvljevic, A.G. Lappin and J.K_ Thomas, Chem- Phys. Letters 104 (1984) 149. [ll] A. Deronizier, J. Chem. Sot Chem. Commun. (1982) 329 1121 D. Mersel, W-A. hiulac and MS. hiatheson, J. Phys. C&em 85 (1981) 179; R-N Domioey, T-J_ Lewis and MS. Wrghton, J. Phys. Chem. 87 (1983) 5345. [13] 1. Matnto, Pure AppI- Chem. 54 (1982) 1693. /14] B-G_ Wkite. Trans Faraday Sot. 65 (1969>200. il5j G. Jones Hand W. Becker, J. Am. C&m. ioc. 103 (1981) 4630. I161 G. Jones II and W. Becker, J. Am. Chcm. Sot. 105 [lo]

(1983)

1276.

[I71 G. Jones II and W. Becker, Chcm. Phys Letters 8.5 (1982)

271.

[W L. hirchaelis and E. Hill, J. Gcn. Physiol. 16 (1933) 859. IJQI V. hlalba, G_ Jones II and ED. Pobakoff, Photochcm. Phorobiol.,

to be publlshcd.

r201 T. Watanabe and K Honda, J. Phys. Chcm. 86 (19S2) 2617.

I211 J-K_ Huxley, N Sioai and II_ Linschitz, Pbctochem. Photobiol.

38 (1983)

9.

1721 D. Deratieau; J. Am. Chem. Sot. 91 (1969) 4044,4050_ 1231 R. Foster, Organic charge-transfer complexes (Academic

We thank the Office of Basic Energy Science of the US Department of Energy for support of this work. Funds were also provided by the US Department of Defense through Its University Research Instrumentation Program. We thank Professor M.Z. Hoffman and Dr. D. Prasad for helpful discussions during the course of this work.

References I11 T.W. Ebbesen and G. Ferraudl, 3.

Phys. Chem_ 87 (1983)

3717.

I21 A.T. Poulos. CK. Kelley and R_ Simone. J. Phys. Cbem.

85 (1981) 823. c31 T-W_ Ebbesen, G Levey and L-K. Patterson, (London) 298 (1982) 545

Nature

Press, New York, 1969). H. hlasuhara and N_ hiatags. Accounts Chem. Res 14 (1981) 312; N. Mataga and hf. OttoIenghi, hiolecular association, ed. R. Foster (Academic Press. New York, 1979). 1251 E.F. Hihnski, J.hi. Masnovi, J.K. Koch1 and P.M. Rentzcpis. J. Am. Chem. Sot. 106 (1984) 8071. I261 T.W. Ebbesen, L-E. _Manning and K S. Peters, J. Am. Chem. Sot. 106 (1954) 7400. r271 N. Mataga, A. Karen. T. Okada, S. Nishjtani, N. Kurxa, Y. Sakata and S. hlisumi. J. Am. Chem. Sot. 106 (1984) 2442. [=I hi C. Richous, Intern. J. Solar Energy l(1982) 161; K. Kalyanasundaram and hi. Ncumann-SpaUart, Cbem. Phys. Letters 88 (1982) 7. 2291 hi. Ottolcnghi. Accounts Chem. Res. 6 (1973) 153. {30] I. Deperasioska, J. Drcsner, B. Kozankiewica, K. Luc~ak [24]

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T. Okada, I. Karaki, E. Matsuzawa, N. Mataga, Y Sakata and S. hlisumi. J. Pbys_ Chem. B.5 (1981) 3957.

109

Volume

119, number 2,3

CHEMKAL

132) T. Wrich, UE. Steiner and R.E. FoeI& J. Phys. Chem. E)7 C1983) 1873. [33 ] B*T* Lim, S, Okajima. AK. Chan&a and’E.C. Lim, Chcm. Phyr. Letters 79 (1981) 22. (34) G. Jones 11, W. Schwartz and V. Mlrlba, J. Phys. Chem. xi (1982) 2286. f3S ] R.M. Noyes, J. Am. Cbem. Sot. 77 11956) 2042; 2. Elektroehcm. 54 (1P60f 153.

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PHYSICS LElTJTRS [36)

L.F. Meadows and R-M_ Noyes* .I. Am. C&em. Soc:80 (1960) 1872; M. Berg, A.L. Harris and C.B.,Warris, Phys. Rev. Letters. to be published. [37) S. Okajima and E.C. f&t, J, Phys. Chem. 86 (1982) 4120.