J. Phys. Chem. 1992,96, 10093-10096
10093
Nanosecond Time Scale Kinetics in the Flavin Mononucleotide on an Electrode Surface Using Time-Resolved Surface-Enhanced Raman Spectroscopy Chongtie Shi, Wei Zhang, John R. Lombard&*and Ronald L. Birke* Department of Chemistry, The City College of The City University of New York,Convent Ave. at 138th St., New York,New York 10031 (Received: August 31, 1992; In Final Form: October 26, 1992)
Using timeresolved surface-enhanced Raman scattering spectroscopy, in a puleprobe mode, we have detected two short-lived (nanosecond time scale) photoproducts in the flavin mononucleotide excited with 337-nm radiation from a nitrogen laser, The first product is obtained within the laser pulse duration plus an instrumental delay of 75 ns and decays to the second photoproduct in the space of several hundred nanoseconds. Both photoproducts are shown to be cation radicals formed on the silver electrode surface by photoinitiated charge transfer.
Introduction Since the first observation of a giant enhancement of Raman signal due to scattering from molecules in proximity to an electrode surface, the technique known as surface-enhanced Raman spectroscopy (SERS) has become an invaluable source of high-resolution spectra of numerous molecular species adsorbed on metal surfaces. The large enhancements observed have compensated for the rather low number of surface molecules, enabling highquality Raman spectra to be obtained under a variety of conditions. This technique has proven useful in identifying and characterizing surface species as well as providing insight as to the nature of the molecuhurface interactions.'J However, surface enhancement has been used surprisingly infrequently in kinetic studies for charackrizhg short-lived intermediates during chemical reactions on surfaces. The vast majority of studies have been carried out on relatively stable species, even though in principle one of the most promising advantages of the technique, namely, the ability to detect small quantities of molecules on surfaces with concurrent quenching of fluorescence interference, makes it ideal for transient studies. Time-resolved SERS studies of the electroreduction of nitro compounds illustrated this possibility on the time scale of second^^.^ and millisecond^.^ In the latter study? we identified the Raman spectrum of a transient intermediate, most likely a radical anion of the nitroso derivative of the pnitrobenzoate anion, with a lifetime on the surface of approximately 70 ms. Until recently, it was thought that photoinitiated reactions of adsorbates on metal surfaces would be inefficient due to the quenching of electronic excited states by the metal. However, charge transfer can be faster than electronic quenching! leading to the possibility of photoproducts on metal surfaces. In this paper we extend the technique of timeresolved SERS to the nanosecond regime in a study of the photoinitiated charge-transfer reaction of the flavin mononucleotide (FMN) on a roughened silver electrode. In order to carry out studies at such short times, it is necesary to take advantage of the optical multiplexing advantages provided by multichannel detectors. Further, such instruments can be gated, and with signal averaging, we can obtain sufficient signal/noise to obtain intense Raman spectra. Care must also be taken in photoinitiation of the reaction not to burn the molecule from the surface.' We have therefore utilized a lower power, but h i g h repetitionrate,N2laser to start our reactions. More careful optical alignment coupled with averaging for longer times has enabled us to develop usable signals despite the lower power. Our results indicate two photoproducts in FMN adsorbed on Ag surfaces, after illumination with a 10-ns laser pulse (337-nm N2 laser). The first product is obtained within the laser pulse duration plus an instrumental delay of 75 ns and decays to the second photoproduct in the space of several hundred nanoseconds. Over still longer times (microseconds) the second photoproduct reverts to the initial species FMN, allowing continuous repetition of the experiment.
Experimental Section The apparatus for the experiments is diagramed in Figure 1. Pulsed light from a N2 laser at 337 nm is reflected from a dichroic mirror and focused onto a silver electrode which has been pretreated by an oxidation-reduction cycle (ORC) for surface enhancement. The laser pulse duration is 10 ns and initiates a reaction in FMN which is adsorbed on the electrode surface. The potential on the Ag electrode is controlled by a waveform generator/potentiostat ( E G t G PARC 1791173) system. A Pt counter electrode and a saturated calomel electrode (SCE) reference are also included in the electrochemical cell. The Ag electrode with adsorbed FMN was prepared by a single 1s ORC from -0.4 to +0.5 V and back to -0.4 V all vs SCE, in 1 X l C 5 M FMN and 0.1 M Na2S04. The Ag electrode was washed and placed in an electrolyte solution containing only 0.1 M Na2S04, pH 7.6. Steady-state SERS spectra of FMN were found to be exactly the same with both ex situ and in situ pretreatment of Ag electrodes; however, we used in situ pretreatment for the pulseprobe studies because they give more intense spectra. CW laser light from an Ar+ laser (Spectra Physics Model 164 or in some experiments, dye laser, Model 375) at selected wavelengths passes through the dichroic filter and is focused in the same spot as the N2 laser beam. This light is used as a probe to provide Raman intensity. Light scattered from the electrode is collected by a lens system and focused on to the slit of a SPEX triplemate monochromator (resolution 1 cm-l) and detected by a gated diode array detector (EG&G PAR Model 1455). The detector gate width and delay time may be controlled by an OMA ( E G t G PAR Model 1461) and pulse amplifier ( E G t G PAR Model 1304) with software on a MacIntosh computer, which also records and processes the spectral data. Up to 10000 pulses are typically needed to develop sufficient intensity when gate widths are 200 ns. Time dependence is obtained by varying the gate delay, and many spectra are recorded as a difference between the delayed spectrum and the spectrum taken without delay from the initiating pulse when the pulsed laser is blocked. Auxiliary measurements are made by determining the photovoltage produced in an open circuit varying the laser wavelength and comparing that with the dark voltage. Further, photocurrent measurements are made by chopping the CW laser light at 30 Hz and detecting the current utilizing a lock-in amplifier and scanning the applied potential. Results Spectra were taken as displayed in Figure 2, which show the surface-enhanced Raman spectrum of FMN at -0.4 V in the region 800-1640 cm-I, using the 488-nm Ar' laser line as probe beam. The gate width is 200 ns, and each spectrum represents an average over 10000 pulses. The lowest curve (a) is the spectrum with the 337-nm light blocked and, since it is identical to the SERS spectrum of FMN itself, indicates that under these
0022-365419212096-10093$03.00/0 0 1992 American Chemical Society
charge transfer can be optically induced in a SERS environment. One technique for the testing of this phenomenon, and determination of the direction of charge transfer, is to measure the
The Journal of Physical Chemistry, Yo]. 96, No. 25, 1992 10095
Letters
TABLE I: obsened R . " Frequcacics (cm-') of Flavin Mownuckotide, the Two ShOrt-Time Photoproducts, rad Semiquinone, Along with Vibrational Adgmeuta FMN photoproduct I photoproduct I1 semiquinone mode no. ring description 496 482 486 34 I, 11, 111 skel str skel str 532 532 535 33 I, 11, 111 567 skel str 32 I, I1 555 566 667
658
662
29
I11
740 803 833 863 994 1089
739 805 831 863 998 1093
144 809 834 863
28 27 26 25 24 22
I I I I I1 I, I1
1257 1269 1303 1344
1257 1269 1303 1358
111 111
1303 1359
18 17 16 15 14
1459 1532 1572
1459 1528 1576 1600
1459 1528 1576 1600
1096
C=O
bend skel str skel str skel str skel str skel str CH in-plane
1242 1252
1396 1532
skel str skel str skel str ring brth
I I1
skel str
11
111 I
9 8
I I1
I
1, I1
skel str skel str CN str skel str
6
I
skel str
1616 1626
1626
1626
once again indicating a threshold near 2.4 eV. Since the photocurrent increases as the potential is increased in a positive direction, the charge-transfer process is oxidative; i.e., a cationic species is produced. In order to understand these results, we must first discuss the Raman spectrum in terms of the normal modes14 each line represents. FMN is composed of an isoalloxazine to which a triglycerol-phosphate linkage is attached. All of the Raman frequencies observed may be assigned to the isoalloxazine, which itself is composed of three rings labeled I, 11, and 1II.shown below.
0.2
0
-0.2
CHOH-CHOH-CHOH-CH2
II
y
I
9 oH--P=O
2
I
-0.4
I
OH -0.6
I
I
I
2
2.5
3
Photon Energy, ev
Figure 4. Dependence of the potential threshold on laser excitation wavelength for FMN photoproduct.
The frequencies of the observed Raman spectra of FMN and its two photoproducts, dong with those of the semiquinone radical? are listed in Table I. Also included in the table are normal-mode a~signments,'~ discussed in more detail below. Note that when the potential is set at -0.4 V but an additional beam at 364 nm (from another argon ion laser) is focused on the sample, the resulting spectrum may be explained in terms of a mixture of FMN and both photoproducts. At 364 nm (as at 337 nm pulsed) there is sufficient energy to produce sufficient steady-state concentration of photoproduct I for spectral observation. Further evidence of the interrelationship of the photo- and electrochemical nature of the observations is provided by phot* voltage and photocurrent measurements. The open-circuit photovoltage observed as a function of excitation wavelength shows a decided increase starting at wavelengths shorter than 5 14 nm, indicating the need for an energetic threshold near about 2.4 eV. Similar results are evident in the photocurrmt "mats. The dark photocurrent is near zero throughout the potential range, while with 488-nmlaser light impinging on the electrode, a sizable photocurrent develops for potentials more positive than -0.2 V,
0
According to normal-coordinate analysis,14 the normal vibrations may be viewed as arising from one or another of the rings, and we have indicated these in Table I. Note fmtly that all the spectral lines associated exclusively with ring I are essentially unshifted thtoughout the reaction. These include lines at 740 (vZ), 803 ( u ~ ~ ) , 833 (v26), 863 ( u ~ s ) ,1303 (v16), 1459 (ull), 1532 (v9), and 1626 cm-'(Pa). We take this to mean that most of the changes which take place arc in rings I1 and 111. All the lines which involve these two rings show some degree of change during the course of the experiment. Further analysis shows that those lines which change in forming photoproduct I, namely, 555 ( Y ~ ~ )994 , (u~),1089 (vu), 1344 (ulS), and 1572 c m - I (v8), almost all (except 667 cm-'(& involve ring 11, while those changing from photoproduct I to photopduct 11, namely, 667 (vB), 1257 (v18),and 1269 cm-'(v17), tend to involve ring 111,although some slight changes in the ring I1 lines may also be observed. Note that none of these changes match those obtained during the formation of the semiquinone for which we have previously obtained spectra.8 Therefore, the semiquinone is ruled out as either photoproduct in this experiment, although it had been proposed as a photoproduct in a preliminary report of our work.15 This observation is consistent with the abovediscussed photocurrent measurements. Since the spectral changes are slight, and the negative slope of the potential dependence (Figure 4) as well as the photocurrent-potential curve indicates formation of a cation, we might expect at least one of the photoproducts is FMN'. Since all
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J. Phys. Chem. 1992,96, 10096-10098
experiments were carried out at pH 7, proton transfer from the solvent is unlikely. We thus tentatively interpret these results as stemming from a photoinduced molecule to metal charge transfer, forming a flavin radical cation. Such a species has been identified in the one-electron oxidation of a flavin triplet ~pecies.’~J’The energy needed for ionization is considerably below that for the liquid-phase ionization potential if the excited electron tunnels to the low-lying empty Fermi levels of the metal, and indeed we can measure a threshold in the visible region for this process as a function of Fermi level. On the other hand, UV irradiation at 253.7 nm has been required to produce flavin radical cations in solution.18 Thus, for an adsorbed molecule on a metal surface where the metal substrate and absorbate wave function overlap, an excitation to an electronic level above the Fermi level can lead to electron tunneling to the metal by resonant charge transfers6 It must be assumed that the reverse process is quenched by some relaxation mechanism, either solvent reorganization or possibly intramolecular enolization, leading to a possible candidate for photoproduct I. We are currently conducting further work to characterize these photoproducts and to determine the kinetics more precisely. To our knowledge, this is the first report of SERS observation of nanosecond time scale kinetics on an electrode surface.
Acknowledgment. This work has been supported by the National Science Foundation (CHE-8711638 and CHE-9122257) with supplementary support from the PSC-BHE award program of the City University of New York (666367,667261, and 66875)
and the National Institutes of Health MBRS program (RR08 168).
Referencea and Notes (1) Birke, R. L.; Lombardi, J. R. In Spectr~lectrochemfst~; Theory und Pructice; Gale, R. J., Ed.;Plenum Press: New York, 1988; p 263. (2) Birke, R. L.; Lu, T.;Lombardi, J. R. In Techniquesfor the Charucterizution of Electrodes and Electrochemical Processes; Varma, R.. Selman, J. R., Eds.; John Wiley & Sons: New York, 1991; p 211. (3) Gao, P.; Gasztola, D.; Weaver, M. J. J . Phys. Chem. 1988,92,7122. (4) Shi, C.; Zhang, W.; Birke, R. L.; Gosser, Jr., D. K.; Lombardi, J. R. J . Phys. Chem. 199L95.6276. ( 5 ) Shi, C.; Zhang, W.; Birke, R. L.; Lombardi, J. R. J. Phys. Chem. 1990, 94, 4766. (6) Avouris, P.; Persson, B. N. J. J. Phys. Chem. 1984, 88, 837. (7) Voss, D. F.; Paddock, C. A.; Miles, R. B. Appl. Phys. h t r . 1982,41, 51. (8) Xu, J.; Birke, R. L.; Lombardi, J. R. J . Am. Chem. Soc. 1987, 109, 5645. (9) Furtak, T. E.; Macombcr, S. H. Chem. Phys. Lett. 1983, 95, 38. (10) Furtak, T. E.; Roy, D. Phys. Rev. Lett. 1983, 50, 1301. (11) Billman, J.; Otto, A. Surf Sci. 1984, 138, 1 . (12) Lombardi, J. R.; Birke, R. L.; Sanchez, L. A.; Bernard, I.; Sun, S. C. Chem. Phys. I r r t . 1984,104,240. (13) Lombardi, J. R.; Birke, R. L.; Lu, T.;Xu, J. J. Chem. Phys. 1986, 84, 4174. (14) Abe, M.; Kyogoku, Y. Spectrochim. Acta 1987, 43.4, 1027. (15) Shi, C.; Zhang, W.; Birke, R. L.;Lombardi, J. R. Proc. SHE-lnr. Soc. Opr. Eng. 1992, 1637, 41. (16) Heelis, P. F.; Parsons, B. J.; Thomas, B.; Phillip, G. 0.J. Chem. Soc., Chem. Commun. 1985,954. (17) Heelis, P. F.; Parsons, B. J.; Phillips, G. 0.;Swallow, A. J. J . Phys. Chem. 1986, 90,6833. (18) Getoff, N.; Solar, S.; McCormick, D. B. Science 1978, 201, 616.
Electron Transfer Dynamlcs at p-GaAs/Liquid Interfaces Y. Rosenwaks, B. R. Thacker, R. K. Ahrenkiel, and A. J. Nozik* National Renewable Energy Laboratory lformerly the Solar Energy Research Institute), Golden, Colorado 80401 (Received:September 9, 1992; In Final Form: October 6, 1992)
The rates of photoinduced electron transfer from sulfide-passivated p-GaAs to outer-sphere redox acceptors (ferricenium and cobalticinium) in acetonitrile have been measured using time-correlated single-photon-counting of photoluminescence e sfor electron transfer were found to be very fast, manifested by electron transfer velocities decay. The characteristic time d ranging from 2 X lo5to lo6 cm/s at 1 m M concentrations. These rates are 4-5 orders of magnitude faster than predicted by other workers.
A critical issue in the field of photoelectrochemistry based on semiconductor/liquidjunctions is the rate at which photoinduced charge carriers can be transferred from the illuminated semiconductor to redox acceptors in the adjacent solution. This is vital not only for understanding the fundamental processes of charge transfer at semiconductor/liquid interfa- but also for clarification of the important issue of whether hot carrier injection’-’ into liquid solutions from illuminated semiconductor photoelectrodes can be an important process in photoelectrochemical (PEC) cells. Relatively few measurements have been reported of the chargetransfer rates from illuminated semiconductor electrodes to redox acceptors. All such previous experiments8-” have been conducted with n-type semiconductors; the reported time scales for hole transfer in these systems ranged from
