1473
J. Phys. Chem. 1982, 86, 1473-7478
Photoelectrochemical Cell Based on the Ion Pair 1,I‘-Dimethyl-4,4’-bipyridinium Bis(tetrapheny1borate). Photocurrents from the Direct Irradiation of a Donor-Acceptor Complex B. Patrick Sullivan, Walter J. Dresslck, and Thomas J. Meyer’ LMpartment of Chemlsby, Unlversiiy of North Carollna, Chapel HM, North Carollna 27514 (Received: August 21, 1981)
An electron-transfer photoelectrochemical cell has been designed which utilizes the intermolecularcharge-transfer
(CT) transition of the donor-acceptor ion pair l,l’-dimethyl-4,4’-bipyridiniumbis(tetrapheny1borate) ((PQ)(BPh,),; PQz+= l,l’-dimethyL4,4’-bipyridinium(paraquat)) to produce photocurrents. The cell is based on the fact that photolysis (436 5 X 5 366 nm) into the intermolecular CT band of the ion pair in acetonitrile solution leads to photodecomposition giving PQ+,BPh3,and biphenyl. The PQ+ produced in the photoanode can act as a reductant toward an electron acceptor such as tetracyanoquinodimethane (TCNQ) in the dark, cathode compartment of a net electrochemicalcell. Evaluation of the total charge generated in 0.1 M LiC10, allows an estimate of 0.01 to be made for the separation yield (#J=Jfor the production of PQ+ and BPh,. following photolysis of the ion pair.
Photoelectrochemical cells based on electron-transfer quenching of molecular excited states have been shown to be capable of simultaneously giving appreciable photocurrents and driving potentially useful chemical reaction~.’-~One type of cell based on the electron-transfer quenching of the charge-transfer (CT) excited state of Ru(bpy)gP+ (bpy = 2,2’-bipyridyl) is outlined in eq 1-6,, hu
R u ( ~ P Y ) ~ ~ +[ R U ( ~ P Y )*~ ~ + I [ R u ( ~ P Y ) ~ ~++PQ2 ]* Ru(bpy)S3++ PQ’
-
+
Ru(~PY)~’+ +D D+ PQ+
fast
Ru(bpy),’+
+ D+
hu
[D+,A-]
(1)
(2)
(3)
irreversible products
(4)
PQ2+
t5 )
(photoanode)
-
(7) [D,AI [D+,A-I In polar solvents, following the light-induced chargetransfer step in eq 7, ion separation can occur, followed by recombination and back electron transfer (eq 8a,b).
H+5 ‘/,HZ (cathode) (6) where D is, for example, EDTA or triethanolamine. This scheme involves initial oxidative quenching of [Ru(bpy):+]* by paraquat (PQ2+)followed by a “scavenging”
D+
+ A-
5 D+ + A-
kr
[D+,A-]
D
+ A & [D,A]
[D,AI
hv
[D+,A-I
[D+,A-] & D+
(1)B.Durham and T. J. Meyer, J. Am. Chem. SOC.,100,6286(1978). (2) B.Durham, W. J. Drwick, and T. J. Meyer, J.Chem. Soc., Chem. Commun., 381 (1978);D. Paul Rillema, W. J. Dreseick, and T. J. Meyer, ibid., 247 (1980);381 (1978). (3)M. Neumann-Spallart, K.Kalyanasundaram, C. GrHtzel, and M. Gratzel, Helu. Chim. Acta, 63, 1112 (1980). 0022-3654/82/2086-1473$01.25/0
[D,A]
(8b)
These molecular events have been observed in a number of ground-state EDA complexes such as the tetrahydrofuran-tetracyanoethylene complex4or in pyromellitic dianhydride-aromatic hydrocarbon c~mplexes.~ If, following the charge-transfer step, one of the redox components is unstable, a basis is provided for a photoelectrochemical cell as outlined in eq 9-14.
k
PQ” reaction in which R ~ ( b p y ) , ~is+captured by oxidation of a third component, D, before back electron transfer can occur between Ru(bpy),3+ and PQ’. The irreversible decomposition of D+ leads to the permanent buildup of PQ’, and ultimately to the production of H,(g) at a platinum electrode. In principle, it should be possible to exploit a variety of excited states resulting from different types of optical transitions in photoelectrochemical applications. One of the most appealing classes of chemical systems is the electron donor-acceptor (EDA) complex which is notable both for the extent of the known examples and for the simplicity of the light-induced charge-separation step (eq 7).
-
(84
D+
-
A-
Q
Q-
+ A-
irreversible products
A
+e-
k-i
(9) (10) (11) (12)
(photoanode)
(13)
(in cathode compartment)
(14)
Although the irreversible step in eq 12 limits the application of such a cell for use in energy conversion, such cells are potentially valuable surrogates for understanding the microscopic processes occurring in reversible photogalvanic systems. They also serve as models for photoelectrochemical synthesis cells and for measuring fundamental parameters pertaining to the excitation-charge separation step in eq 10 and 11. The scheme in eq 9-14 provides the focus for this paper, where the point is to demonstrate that the charge-transfer transition of a properly chosen donor-acceptor complex can function as the light-gathering chromophore and a source of reducing ~
(4) D. F. Ilkn and M. Calvin, J. Chem. Phys., 42,3760 (1965). (5)J. Hinatu, F. Yoshida, H. Masuhara, and N. Mataga, Chem. Phys. Lett., 59,80 (1978).
@ 1982 American Chemical Society
1474
Sullivan et al.
The Journal of Physical Chemistry, Vol. 86, No. 8, 1982
equivalents in a photoelectrochemical cell.
Experimental Section Materials and Preparations. All experiments were performed by using Omnisolv MCB acetonitrile (MeCN) which was distilled over P205and then CaH2 immediately before use. Anhydrous LiC10, and NaC10, (both G. Fredrick Smith) were used as received. (PQ)ClZ,NaBPh,, 1,4-benzoquinone, and 2,3-dichloro-5,6-dicyano-1,4benzoquinone (DDQ) were all purchased from Aldrich and were used without further purification. Tetracyanoquinodimethane (TCNQ) was obtained from Aldrich and purified by recrystallization from acetonitrile under N2. Water was distilled from alkaline KMn0,. The preparation of the tetraphenylborate salt of PQ2+ was carried out by dissolving the C1- salt in a minimum of hot water and adding a saturated solution of NaBPh,. The orange salt which precipitated was collected by filtration, washed with boiling water, and washed with a minimum amount of acetone. After a final washing with diethyl ether, the salt was dried in a vacuum oven (15 mmHg at 65 "C) overnight, yielding a yellow powder. An alternate preparation involved mixing 580 mg of [CH3(CH2)3]4NBPh., in 20 mL of hot CH3CN with 330 mg of (PQ)(PF,), in 20 mL of hot CH3CN. After the solution was cooled to room temperature, orange rodlike crystals of (PQ)(BPh,), formed over a 2-h period. These were filtered, washed copiously with diethyl ether, and air dried overnight. A satisfactory elemental analysis was obtained for this material. Anal. Calcd: C, 87.42; H, 6.55; N, 3.40. Found: C, 87.33; H, 6.67; N, 3.29. Photoelectrochemical Cell Operation. The photoelectrochemical cell in which the photocurrent experiments were performed was constructed from Pyrex glass and is of the same design as that previously reported., The photoanode consisted of a (1 X 3) cm2 52 mesh Pt gauze electrode mounted to the front face of the cell. The anode solution was stirred by magnetic stirring. The cathode consisted of a (1 X 2) cm252 mesh Pt gauze electrode. The cathode solutions were not stirred during the experiments. The output of the cell was passed through a 1000 f 5 Q resistor, and the photocurrents were monitored by using a Hewlett-Packard Model 7004B X-Y recorder with 17172-A time base and 17171-A amplifier connected across this resistor. The cell was placed in an aluminum block which was machined to hold the square anode. The block contained a light shutter and was mounted above a Tri-R Model S-7 magnetic stirrer. The photolysis system used consisted of a Schoeffel Model LH-151N 1000-W Xe lamp mounted to an optical bench. The lamp was connected to a Bausch & Lomb high-intensity monochromator (catalog no. 33-86-79) through a 10-in. aluminum sleeve (3-in. diameter) which contained a high-intensity silica lens (focal length, 15 cm). The exit slit of the monochromator was attached directly to the aluminum block via a second aluminum sleeve. The intensity of the light impinging on the photoanode was 1.14 X (5%)einstein s-l at 436 nm with both the entrance and exit slits of the monochromator set at 6 mm as determined by Reinecke's salt actinometry. In a typical experiment, a solution of (PQ)(BPh,), in either 0.1 M NaC10, or 0.1 M LiC10, in MeCN was prepared by dissolving the desired amount of salt before use. The photoanode was charged with 15 mL of this solution, and the cathode was filled with the appropriate quinone solution in 0.1 M LiClO, or 0.1 M NaClO, in MeCN. The ( 6 ) W. J. Dressick, D. A. Rillema, B. Durham, and T. J. Meyer, Zmrg.
Chem., in press.
bridge compartment was filled with 0.1 M LiC10, or 0.1 M NaC10, in MeCN. The photoanode solution was then Ar bubble degassed for 20 min. The Ar was passed through columns of KOH pellets and BASF BES R3-11 catalyst before use to remove traces of H 2 0 and 02. When the dark current stabilized after connecting the resistor bebween the two compartments ("2 min), the light shutter was opened, the solution was illuminated, and the observed Photocurrents were recorded. After irradiation, the absorption spectra of both the anode and cathode solutions were obtained by using a Bausch & Lomb Model 210 UV spectrophotometer and compared to spectra obtained before irradiation. The total amount of charge passed during each experiment was determined from the area under the current-time curves. The open-circuit voltage of the TCNQ/(PQ)(BP$), cell was determined by using a Radiometer Model pHM62 pH/mV meter. The internal resistance of the cell was determined by using a Model RC16B2 (Industrial Instruments, Inc.) conductivity bridge for the TCNQ/(PQ)(BPh,), cell with 0.1 M LiC104 in MeCN.
Results and Discussion Characterization of the Ion Pair. When saturated acetonitrile solutions of tetra-n-butylammonium tetraphenylborate were mixed with paraquat hexafluorophosphate, a deep orange color developed instantaneously, and after several minutes orange rodlike crystals separated from the supersaturated solution. An alternate method of preparation involved metathesis of NaBPh, and (PQ)C12 in aqueous solution, in which case the yellow orange flocculent precipitate of the ion pair so obtained was recrystallized from hot acetonitrile to yield small orange needles of the salt. Elemental-analysis data for the crystalline solid obtained by both methods were consistent with ita formulation as the simple salt, (PQ)(BPh,), (see Experimental Section). Figure 1shows the electronic spectrum of (P&)(BPh4), in CH3CN solution. The long tail of the PQ2+ BPh4CT transition extends from 500 nm into the ultraviolet with a slight shoulder at 330-335 nm. The edge absorption is typical of other viologen and pyridinium charge-transfer salts such as (PQ)Ip7p8 Figure l b also shows the calculated CT spectrum of the ion pair obtained by subtracting the absorbance contributions due to the isolated components. For a solution 014.44 X M (P&)(PF,),,the spectra of 4.44 X M (PQ)(PF,), and 8.88 X M [(CH3(CH,),),N]BPh, solutions in acetonitrile were used. The derived CT band has ,A, at 322 nm (3.85 eV) and a bandwidth at half-maximum for the transition manifold of 1.32 eV which was obtained by doubling the low-energy side of the absorption band. The observed absorption band at low concentrations of complex is undoubtedly a composite of the CT transitions from both the single and double ion pairs in solution (eq 15 and 16). In Figure 2 is shown the concentration de+-
K
(PQ)(BPh,),
& [PQ,BPh4]++ BPh,-
2PQ2++ BPh,
[PQ,BPh4J+
(15) (16)
pendence of the absorbance of (PQ)(BPh,), dissolved in CH3CN solution; at high concentrations the complex is seen to follow Beer's law behavior. Attempts to measure both Kl and Kz by spectrophotometricand electrochemical (7) R. Foster, "Organic Charge Transfer Complexes", Academic Press, New York, 1969, and references therein. (8) B. P. Sullivan, unpublished results.
The Journal of Physical Chemistry, Vol. 86, No. 8, 1982 1475 08-
a
- ABSORPTION SPECTRUM OF
PQ2+(BPh;),
IN ACETONITRILE 06366 nm
00 10
08
06
04
06 0
z
04
LL
55 9
0 - 0 2 -04 -06 -08
Figure 3. Cyclic vottammetry: (A) 8.0 X lo4 M (PQHBPh,),; (B) DDQ; (C) TCNQ; (D) l+Benzoqulnone, with 1 vol % H,O added. All voltammograms were recorded In CH,CN at a Pt button working electrode using 0.1 M LICIO, as electrolyte at a scan rate of 200 mV
W
2
02
VOLTS (SSCE)
S-1.
02
Figure 3 shows a cyclic voltammogram of an 8.0 X
lo4
M (PQ)(BPh,), solution which shows the expected rever00 500
450 400 350 WAVELENGTH, nm
300
Figure 1. (a) Spectrum of 4.44 X lo-, M (PQXBPh,), In CH&N solution. (b) Spectral resolution of (PQXBPh,), band maxlmum. Curve A Is 4.44 X lo4 M (WXBPh,), in CH,CN, obtained by difference and by using curves B and C; curve B 1s 4.44 X lo-, M (PQ)(PF,), In CH,CN; curve C Is 8.86 X lo3 M tetra-n-butylammonium tetraphenylborate In CH,CN.
sible reduction of PQ2+ (eq 17) and the chemically irrePQz+ e- == PQ+ (17) versible oxidation wave for BPh4- (eq 18). (18) BPh4- BPh, l/zPh-Ph eThe process represented by eq 18 has been shown to occur both by controlled potential electrolysis at a Pt working electrode* and by chemical oxidation by Cu(II).S Photolysis of the Ion Pairs. Steady-state photolysis directly into the CT bands of (P&)(BPh,), in deaerated CH3CN at 436,406, or 366 nm results in the permanent buildup of PQ' (ew. = lo600 M-' cm-'). From the spectral and redox properties described above, the sequence of events which occurs upon photolysis can be summarized by eq 19-22.
+
-
+
+
[PQ2+,BPh4-]+A [PQ+,BPh4.]+
z
(19a)
[PQ2+,(SPh4-),] [PQ+,BPh4.,BPh4-] (19b) 'k-,b k 2 PQ' + BPh4. k-2
[PQ+,BPh,*]+
(20)
k
[PQ+,BPh4.,BPh4-] A [PQ+,BPh4-] + BPh4.
k-a
Flgure 2. Concentration dependence of (PQ)(BPh4)2absorbance in CH,CN solutlon (2.0cm path length). Curved soli line Is extrapolation of experimental data to infinite dilution. Dashed line shows the completely Ion-palred (Beer's law region) behavior at high complex concentrations.
methods7 were unsatisfactory either because of the low solubility of the complex in the presence of excess BPh4or PQ2+or because of electrode adsorption phenomena at high complex concentrations. From Figure 2 it is seen that complete ion pairiig occurs at high concentrations. Since the highest concentration used in Figure 2 is the maximum solubility of (PQ)(BPh4), in CH3CN solution (rodlike needles separate from the solution at higher concentrations), it is likely that the Beer's law behavior in this region is due to the ion triple (PQ)(BPh4),. In any case, for the purposes of the analysis used here, the molar extinction coefficient eapp = 151 f 20 M-' cm-l at 366 nm can be calculated for the concentration region where ion pairing is complete.
BPh4.
k4
BPh3
(21)
+ '/,Ph-Ph
(22) Direct irradiation of the PQ2+T-T* transition (at 313 nm, see Figure lb) also gave permanent buildup of PQ'. The W-induced reaction presumably occurs by reductive quenching of the PQ2+m*singlet state by BPh4- like the well-known photooxidation of methanol and other organic substrates by excited PQz+ (eq 23-25)."-13
- +
PQ2+,BPh4--!% PQ2+*,BPh4PQZf*,BPh4- PQ' BPh,. BPh4-
BPh3
+
'/zPh-Ph
(23) (24)
(25)
(9)D.H. Geske, J.Phys. Chem., 66, 1743 (1962). (10)G. E. Coates, M. L. H. Green, and K. Wade, "Organometallic Compounds", Methuen, London, 1967. (11)A. Ledwith, Acc. Chem. Res., 7 , 335 (1972). (12)A. S.Hopkins, A. Ledwith, and A. Stram, Chem. Commun., 494 (1970). (13) N.M. D. Brown, D. J. Cowley, and W. J. Murphy, Chem. Commun., 592 (1973).
1476
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The Journal of phvsical Chemistty, Vol. 86, No. 8, 1982
TABLE I: Limiting Photocurrents for t h e (PQ)(BPh,), Cellu
acceptorb
ip-Berzoquinatx- 0 028 M
I
, 0 /
0
0 0 M NaCIO,/MeCN "2
l
solver'
StlrVlPq
I
200
I
l
I
400 TIME, seconds
conditions
1,4-benzoquinone DDQ TCNQ TCNQ TCNQ
J
0
1 600
1
1
1
photocurrent, PA
0.1 M 0.1 M 0.1 M 0.1 M 0.1 M
NaClO,, unstirred 4.3t 0.5 NaClO,, unstirred 4 . 0 t 0.6 NaClO,, unstirred 3.9 t 0.3 NaClO, , stirredC 18 t 3 LiClO,, stirredC 30 t 4
All in CH,CN solution, 5.02 x M in complex. 0.015 M in acceptor in the cathode cell compartment. Stirring rate measured stroboscopically as 500 t 50 rpm.
800 3c
Figure 4. Typical cell current-time response arising from irradiation at 366 nm.
Conventional flash photolysis of the ion pairs in deaerated CH&N using a 400-nm cutoff filter resulted in no detectable signal for a back-reaction between PQ' and BPh,.. In this experiment, the monitoring wavelength was for PQ') and the pulse lifetime was -25 ps. 605 nm (A, Each flash resulted in a permanent base-line change of 26 Ti, which corresponded to the permanent buildup of PQ'. In an experiment using 3.4 X lo-, M (PQ)(BPh,),, the absorbance change after the first flash showed the production of 1.5 X M PQ'. Using the limiting assumption of complete excitation and neglecting possible pumping effects during the flash, one obtains an upper limit for the quantum yield for separation of PQ' and BPh, ) of -0.04. Photoerectrochemica1 Cell Based o n the Photolysis of (PQ)(BPh& Using the photodecomposition reaction of the ion-pair system (eq 19-22), one can construct a photoelectrochemical cell which operates according to the scheme outlined in eq 9-14. A schematic representation of the apparatus used in the cell reactions has been detailed previously.6 In our experiments three different one-electron acceptors were chosen for the dark (cathode) compartment: 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), tetracyanoquinodimethane (TCNQ), and 1,4benzoquinone (BQ). DDQ and TCNQ provide a convenient method for measuring quantum efficiencies of charge transport through the cell because of their stable and highly colored radical anions which are amenable to spectrophotometric analysis. 1,4-Benzoquinone provides a model for the use of such cells in net chemical applications since reduction in the presence of a proton source is irreversible, giving the corresponding hydroquinone. In CH,CN solution with 0.1 M LiC10, as supporting electrolyte, both DDQ and TCNQ exhibit charge-transfer-reversible, one-electron cyclic voltammetric waves at Eljz= -0.49 V and El = +0.13 V, respectively (see Figure 3). In the presence o/ 1vol Ti of water, 1,4-benzoquinone is chemically irreversible under the same conditions. The reduction of 1,4-benzoquinone is initially a one-electron process followed by protonation of the semiquinone, as shown in eq 26,14 followed by a further one-electron re-
c 80
--1
1
A B S O R B A N C E O F THE CATHODE SOLUTION
Ti \
1 '
-HE TCNO-
GENERATED HAS A
OF l l 9 9 f O 0 7 ) T H E AMOUNT E X P E C T E D
CONCENTRATION
I
x %01
FROM THE I N T E G R A T E D CURRENT IS
1199f0041x10~5M
a
32
m
c
N
000
I
I
I
900
800
700
600
WAVELENGTH, nm
Figure 5. Absorbance of the cathode solution from a (PQ)(BPh,)2/ TCNQ photocel. The recoTded spectrum Is identical with that of TCNQunder the same conditions.
the overall reduction under our experimental conditions is -0.17 V at a scan rate of 100 mV s-l. Photocurrent and Photovoltage Characteristics. Figure 4 shows a plot of photocurrent vs. time for a typical cell involving 1,4-benzoquinone as the electron acceptor. All of the cells studied exhibited the same exponential rise in photocurrent and, as shown in Table I, the responses of cells using different electron acceptors over a range in concentrations in the cathode compartment were all the same in terms of limiting photocurrents showing that the response of the cell is dictated by the photoanode. Table I also shows the expected increase in limiting photocurrent that is achieved by mechanical stirring. A somewhat surprising observation is the cation effect that is observed by changing the cell electrolyte from 0.1 M NaC10, to 0.1 M LiC104. The nearly twofold increase in limiting current for LiClO, vs. NaC10, may have as its origin specific ion effecta in steps like the charge-separation step in eq 20 and 21. Open-circuit photovoltages were measured (using a 10M Q internal resistance Radiometer pH meter) for the cells involving the electrochemically reversible couples, DDQ and TCNQ, in 0.1 M NaC104. For TCNQ the observed photovoltage was 0.60 V compared with 0.64 V for the difference between the PQ2+J+couple and the TCNQ0/-' couple. For DDQ the experimental value was 0.98 V compared with the calculated value of 1.00 V. Figure 5 shows the absorbance spectrum of a TCNQ solution in the cathode compartment after 15 min of cell operation. The electronic spectrum recorded is that characteristic of TCNQ-. Integration of the photocurrent-time curve (note Figure 4) after this period showed that quantitative charge transfer from PQ' to TCNQ had occurred through the cell. Figure 6 shows the steady-state absorbance of the PQ' produced during the experiment. From the absorbance measurement, the steady-state con-
-
duction which gives the hydroq~in0ne.l~ The E, value for (14) J. Q. Chambers in 'The Chemistry of the Quinoid Group", Academic Press, New York, 1975.
The Journal of Physical Chemistty, Vol. 86, No. 8, 1982 1477
PhotoelectrochemicalCell Based on (PQHBPh,),
+
irradiation wavelength (in our cell, AT = A, A,+, where A,,+ is the absorbance due to the PQ' generated during the photolysis); (viii) PA, the incident light intensity at the front face of the cell in einstein s-l under conditions where all of the light enters the cell; (ix) q, the charge generated in the cell in coulombs. Equation 28 provides the basis for understanding the dependence of the total charge generated in the cell on the light-absorption properties of the ion pair(s) (A,), the efficiency of redox product separation (4 = $,p), the irradiation time (t), the incident light intensity (Iox), and the physical characteristics of the cell (A,). The use of relationships like eq 28 in quantitative applications has been described previ~usly.'-~J~ The form of the current-time profile in Figure 4 is also rationalizable in terms of earlier observations. In eq 29
STEADY - S T A T E PHOTOANODE 0 2 1 SOLUTION A B S O R P T I O N S P E C T R U M A L L SOLUTIONS ARE IN 0 I M NaCIO,/MeCN T H E PO(TPB), ANODE SOLUTION H A S A N ABSORBANCE ( 3 6 6 n m )OF 0 4 8 2 v s B L A N K B E F O R E PHOTOLYSIS T H E CATHODE H A S [TCNO] = 0 015 M A L L SOLUTIONS ARE Ar B U B B L E DEGASSED
W
0
z Q
01
m
LL 0 v,
m
6
00; 750
I
650
1
I
I
550
I
I
450
I
350
W A V E L E N G T H , nm
1 =
Flguro 6. Steady-state photoanode absorbance spectrum trom a (W)(BPh4&lCNQ cell. The absorbance spectrum is identical wlth that of W'. The ckcles represent data points taken by spectrophotometry using a manually operated monochromator and a photomultiplier that was mounted at right angles to the photolysis beam.
centration of PQ' that builds up during the operation of the cell is -4.2 X lo+ M. Photoelectrochemical Kinetics. As noted above, under our concentration conditions for the photoelectrochemical M, comexperiment, typically [(PQ)(BPh,),] = 4.5 X plete ion pairing has occurred. The kinetics describing the cell are therefore determined by a rather simple set of molecular events occurring in the photoanode compartment. The efficiency of the appearance of PQ', $pQ+, is given by eq 27 based on the scheme in eq 9-14 (also note eq 19-22). 4PQ' = k ~ p / ( ~ a e + p kb) (27)
In eq 27, ,k is the separation rate constant of PQ' from the ion pair following excitation. kb is the thermal electron-transfer rate constants for back electron transfer within the association complex, PQ', BPh4-,before separation can occur. The current characteristics of the photoelectrochemical cell will be determined by the steady-state and concentration-time characteristics of [PQ'] = C. Recently, a model was proposed for the operation of such cells based on the physical characteristics of the cell and the dynamics controlling the PQ' concentrati~n.~~ As described in the Experimental Section, the experiments were carried out in an optical cell in which a platinum screen electrode was mounted to the front face. The platinum screen, which is the collector electrode for PQ', acts as a wavelengthindependent neutral density filter of absorbance, AS = 0.25, as determined experimentally. In the earlier work eq 28 was shown to be valid for the q = 10-AwFt$(l - 10-A~)(A,/AT)F'A
(28)
total charge arising from both light and dark currents in a cell similar to the one described here. In eq 28, the following terms appear: (i) n, the number of electrons passed during the photochemical reaction (n = 1,note eq 21); (ii) F, the Faraday constant (96496C/equiv); (iii) A,, the absorbance of the screen (A, = 0.25 for our cell); (iv) t, the time of irradiation in seconds; (v) 4, the quantum yield for the photochemical reaction; (vi) A,, the solution absorbance due to (PQ)(BPh,), at the irradiation wavelength; (vii) AT, the total absorbance of the solution at the (15) W. J. Dressick, B. Durham, and T. J. Meyer, Zsr. J. Chem., in press; W. J. Dressick, Ph.D. Dissertation,University of North Carolina, Chapel Hill, NC, 1981.
10-AvzF$(l- 10AT)(AC/AT)IOAB(1+ E o m ) [ l- exp(-Gt)] (294 i = $L',,(l
+ Eom)[l - exp(-Gt)]
(29b)
L'A = 10-A~nFPA(l- 10-AT)(A,/AT)F'AB
(29~) is given a current-time relationship which fits the cell response and which is derivable based on a series-of simplifying but not unreasonable assumptions. In eq 29 B, E, m, and G are all constants characteristic of the cell and o is the stirring rate in stirred-solution experiments. Rearrangement of eq 28 yields eq 30, where the fact 4 $sep
= q/[lO-As?ZFF'At(l- l o - A ~ ) ( A c / A ~ ) ](30)
= dSepfor the (PQ)(BPh,), donor-acceptor complex (note eq 27, where 4sep= $pQ+) has been used. Equation 30 suggests that $sep may be determined from total-charge measurements. M In a separate experiment, a solution of 6.03 X (P&)(BPh,), in 0.1 M NaC104/MeCN was irradiated for 250 s at 366 nm (IOsss= 1.26 X lod8einstein s-l (15%))in M TCNQ. A a cell. The cathode contained 1.88 X charge of q = 9.85 X 10"' C was passed. When eq 29 was used, a value of (0.1 M NaC104/MeCN) = (8 f 2) X was obtained. kepetition of the experiment in 0.1 M LiC104/MeCN gave $,p(O.l M LiC104/MeCN) = 0.017 f 0.03. The difference in results obtained for the two media is notable and points out clearly that the increase in generated charge (and limiting photocurrent) in LiC10, noted in Table I has its origin in specific ion effects on the separation efficiency c $ ~ ~ ~ . The fact that 4pe+is appreciably less than unity is also worth noting given the relative simplicity of the photochemical mechanism, eq 19-22. Following charge transfer and separation of PQ' and BPh,., there are no complications from back electron transfer because of the irreversible decomposition of BPh4. (eq 22). As a consequence, @pQ+ is determined by the relative rate constants for back electron transfer between PQ' and BPh4. before separation, PQ', BPh,. PQ2+,BPh4-, compared to separation of PQ' and BPh,., PQ', BPh4. PQ' + BPh,.. It is apparent that the back electron transfer rate constant is more rapid than the separation rate constant. The separation rate constant k,, can be estimated from the Eigen equation (eq 31).16 In eq 31 the separating
-
4
particles are assumed to be spheres, a is the distance between their centers in centimeters, D, is the diffusion (16)M. Eigen, 2.Phys. Chem. (Frankfurta m Main), 1, 176 (1954).
J. Phys. Chem. 1982, 86, 1478-1484
1478
coefficient in cm2s-l of particle n, and b is an electrostatic work term which vanishes here since the photolytic CT step produces PQ’ and BPh,.. Using the Stokes-Einstein equation for the diffusion coefficients (eq 32) ( q is the
Dn = k ~ T / ( h r n )
(32)
macroscopic viscosity, 3.45 X P, for CHBCNat 25 “C and r is the radius of component nl’) allows the calculation k , = 2 X lo9 s-l to be made. From the value of r$sep = 0.008 in 0.1 M NaC104and eq 27, the back electron transfer rate constant within the ion pair is kb 2.1 X 10l1 s-’. In the design of more efficient photoelectrochemical cells based on photolysis of donor-acceptor complexes, the critical factor is the maximization of r$8ep: Since the form of r$ssp in terms of kinetic rate constants is especially simple, it is possible to consider the factors involved. k , can be enhanced by creating an electrostatically unfavorable environment in a medium of low dielectric constant in the CT step (eq 33).
-
-
,k .& [D+,A+] kb
[D,A2+]
D+ + A+
(33)
At this point, a detailed discussion of the factors which determine the magnitude of kb is not warranted, but in a separate publication we will discuss the calculation of rate data, including quantities such as kb, from the spectral and electrochemical properties of organic EDA complexes.18 However, it should be noted that a great deal can be learned about such processes through a combination of experimental advances in the study of the spectral properties of donor-acceptor complexes and exciplexes, and use of available theoretical results on thermally activated (17) The average radii of [PQz+,BPh,-] and BPh,- were calculated by using van der Waals scaled molecular models and found to be 13 and 6 A,respectively. (18) B. P. Sullivan and T. J. Meyer, manuscript in preparation; also, for rate calculations as applied to donor-acceptor complexes, see J. C. Curtis, B. P. Sullivan, and T. J. Meyer, Inorg. Chem., 19,3833 (1980).
electron transfer and excited-state nonradiative decay. Conclusions and Final Comments. Photodecomposition of an electronically weakly coupled donor-acceptor complex, in this case an ion pair between PQ2+and BPh;, can lead to redox products by direct irradiation of the donor-acceptor charge-transfer transition. Following optical excitation, the relatively strong reductant PQ’ is built up in the solution, and its reducing equivalents can be utilized in an electrochemical cell to produce an observable photocurrent and drive a net redox reaction in a second cell compartment. The net cell reaction is sacrificial in nature and the production of separated redox products is inefficient, but it is notable for a number of reasons: (1) It may set the stage for related cells which are catalytic and energy storing. (2) If one of the components is sufficiently inexpensive, it could lead to a photochemical “synthesis” cell in which a useful component is built up in a second cell compartment. (3) Given the vast extent of the area of donor-acceptor chemistry, the use of a donor-acceptor CT band in an energy-conversion application is certainly worth noting. In addition, the agreement between experimental results and the kinetic model for the operation of the cell is also worth noting. It reinforces the application of the model in related cells and points out the value of the kinetic analysis. Included in the description of the operation of the cell is the use of the kinetic analysis in determining photochemical efficiencies for processes that might otherwise be difficult to obtain. In the particular case of the donor-acceptor photolysis, charge measurements allow for the determination of the redox product separation efficiency, 4w+, and the experiments are straightforward and easily carried out by using equipment commonly available in a photochemical laboratory.
Acknowledgment. Acknowledgment is made to the Department of Energy under Grant No. DE-A50578ER06034 for support of this research and to the National Science Foundation for a fellowship for W.J.D.
Local Mode Spectra of Inequlvalent C-H Osclllators in Cycloalkanes and Cycloalkenes James S. Wong,+ Richard A. MacPhall,* C. Bradley Moore,t and Herbert L. Slrauss’ Department of Chemlstty and the Materials and Mokcular Research Division of the Lawrence 6erkeky Laboratoty, University of California, Berkeley. California 94720 (Received September 1, 198 I; In Flnal Form: November 23, 198 1)
Local mode spectra of cycloalkanes and cycloalkenes are observed by using both gas-phase high overtone and isotopically isolated fundamental spectroscopy. The shapes of the bands reflect the changes in the C-H bonds during the conformational motion. Absorptions by axial- and equatorial-type C-H bonds are resolved in cyclobutane, cyclopentane, cyclopentene, and cyclohexane, and the isotopically isolated fundamentals are close to the frequencies extrapolated from the overtones except for liquid cyclohexane. The equatorial bands are consistently more intense than the axial ones.
I. Introduction The cycloalkanes and cycloalkenes exhibit considerable conformational diversity and thus a variety of C-H bonds in different environments. The conformations Department of Chemistry and Materials and Molecular Research Division. t Department of Chemistry. *Address correspondence to this author a t the following address: Department of Chemistry, University of California, Berkeley, CA 94720.
interconvert-some very rapidly-and thus this series of molecules provides a unique opportunity to examine the effect of well-defined motion on C-H overtone spectra. Of particular interest is the question of the effect of motion on the local mode (LM) picture1 of the overtones. Previous work2Bhas shown that, in the liquid phase, multiple peaks (1) For a recent review, see M. L. Sage and J. Jortner, Adu. Chem. Phys., 47,293 (1981). (2) B. R. Henry, 1.-F. Hung,R. A. MacPhail, and H. L. Strauss, J.Am. Chem. Soc., 102,515 (1980).
0 1982 American Chemical Society
