J. Phys. Chem. 1987, 91, 514-511
574
-
7"
C P
'O-
-0
-
5
60-
+ ul
-
8
.
z
0
0
0
0 0 0 O
0
5 0 C E
-
> 4: u
. -
-
-
W
40-
this number with the total number of vibrational levels at the congested region. Using the Whitten-Rabino~itch~~ approximation, we get at an excess energy of 2000 cm-I about 12000 states.@ Thus, only 0.5% of all vibrational levels of SI is coupled on the average with an S2 level. In summary, we have obtained the fluorescence excitation and emission spectra of BaP in a supersonic jet. Analysis of the spectrum has led to the assignment of the 0,O band of the SI So transition. The highly congested structure in the excitation spectrum observed at about 1500 cm-' above that origin is taken to indicate strong coupling between the SI and S2 states. This interaction is also responsible for the similarity between the fluorescence spectrum obtained upon excitation at the origin of the SI So transition and in the congested region. A further indication is the abnormally long decay times observed upon excitation into nominally S2 levels. These observations suggest that BaP belongs to the strong coupling limit of the intermediate case in radiationless transition theory.
0
0
Figure 6. Plot of the emission decay rate constant of jet-cooled BaP as a function of excess energy above SI.Numerical data are given in Table 11.
of S2levels will be "diluted" and as n approaches infinity will be equal to that of SI levels. The experimentally observed decay curves are exponential over three lifetime periods. Classifying BaP as belonging to the intermediate strong coupling case, we can estimate the number of SI levels interacting with an S2level. The change in rate constant is large at about 1000-cm-l excess energy, as shown in Figure 6. In the congested region of Figure 1, it reaches a fairly constant value of 6.7 X lo6 s-I. This value, together with eq 2 and the calculated decay times of SI and S2,leads to n 60, as expected for the intermediate coupling case.38 It is instructive to compare (38) This calculation neglects any interaction with triplet states. Should intersystem crossing be important in this case, n = 60 would be only a lower limit. However, there is yet no evidence for the occurrence of intersystem crossing in isolated BaP. We thank a referee for pointing out this possibility.
Acknowledgment. We thank 0.Anner for his help in various stages of this work and for useful discussions. We are indebted to Prof. G. J. Small for sending us a copy of ref 21 prior to publication. This work was supported in part by the Robert Szold Institute for Applied Research. The Fritz Haber Research Center for Molecular Dynamics is supported by the Minerva Gesellschaft fur die Forschung, mbH, Munich, FRG. Registry No. Benzo[a]pyrene, 50-32-8.
(39) Whitten, G.Z.;Rabinovitch, B. S. J . Chem. Phys. 1963,38, 2466. (40) The vibrational frequencies, in cm-', used in this calculation were as follows (degeneraciesin parentheses): 3000 (12), 1600 (6), 1400 (12), 1100 (lo), 900 (12), 700 (14), 400 (lo), 200 (10). They were estimated by comparison with fluorescence (ref 5 ) and IR spectra (Utkina, L. F. Zh. Prikl. Spektrosk. 1968, 9, 466).
Fluorescence Quenchlng of 9-Cyanoanthracene by 2-Naphthol and 2-Methoxynaphthalene Koichi Kikuchi, Masato Hoshi, Yasushi Shiraishi, and Hiroshi Kokubun* Department of Chemistry, Faculty of Science, Tohoku University, Aoba, Aramaki. Sendai 980, Japan (Received: July 9, 1986)
Fluorescence quenching of excited 9-cyanoanthracene (CA) by 2-naphthol (N) and 6-bromo-2-naphthol (BN) as well as their methyl ethers is investigated in cyclohexane. The rate of dynamic fluorescence quenching of CA by these quenchers is diffusion controlled. The photobleaching of CA is accompanied with quenching. The apparent triplet yield of CA decreases on addition of N, whereas it increases on addition of BN and the ethers. A weak exciplex fluorescence is observed when the ethers are used as the quencher. Quenching by the ethers is due to exciplex formation. Quenching by N and BN may be due to the competitive formation of an exciplex and a hydrogen-bonded species. Photobleachingand the enhanced intersystem crossing may occur in the exciplex.
Introduction In two conjugate n-electronic systems such as 2-naphtholpyridine, carbazolepyridine, and 7,8-benzoquinoline-phenol,the fluorescence is strongly quenched with hydrogen bond formation:'
The internal conversion of the hydrogen-bonded species is very rapid. When the quenching occurs dynamically, hydrogen atom transfer and hydrogen bond formation occur competitively in the encounter ~ t a t e . ~ -In~ a previous study4 on the fluorescence
(1) Mataga, N.; Kubota, T. Molecular Interactions and Electronic Spectra; Dekker: New York, 1970; p 351.
46, 149.
(2) Kikuchi,
K.; Watarai, H.; Koizumi, M. Bull. Chem. SOC.Jpn. 1973,
0022-3654/87/2091-0574$01.50/00 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 3, 1987 575
Fluorescence Quenching of 9-Cyanoanthracene WAVELENGTH/nm
600
,
400
500
A
300
+I .o
Figure 2. Stern-Volmer plots for fluorescence quenching.
Figure 1. Absorption, T-T absorption, and fluorescence spectra of CA in cyclohexane at 298 K: -, absorption spectrum (1 X M CA); 0-0, T-T absorption spectrum measured for a solution containing 20 mM BM; ---, monomer fluorescence spectrum; - - - -, exciplex fluorescence spectrum measured for a solution containing 100 mM M; _ - -, difference absorption spectrum for solutions containing 0 M and 10 mM N.
quenching of carbazole with benzonitrile, we found that the fluorescence is also quenched with hydrogen bond formation of the type
and that the hydrogen atom transfer reaction does not occur in this system. Since aromatic nitriles may act as efficient electron acceptors as well as hydrogen bond acceptors, there is the possibility that a photochemical reaction other than the hydrogen atom transfer reaction takes place in the encounter state. In order to confirm such a possibility it is desirable to adopt an aromatic nitrile as a fluorescer. In the present work we have studied the fluorescence quenching of 9-cyanoanthracene (CA) by 2-naphthol (N) and 2-methoxynaphthalene (MN) in cyclohexane. Since CA can act as an efficient electron acceptor for the excited-state electron-transfer reactions, one may expect that the primary quenching process is charge transfer from quencher to CA. If charge transfer is followed by an irreversible reaction: CA will be photobleached. For example, 9-cyanophenanthrene reacts with 2,3-dimethyl-2butene to form a (2 2) adduct via an exciplex. Furthermore, if charge transfer in an exciplex is a prerequisite for quenching, intersystem crossing from the exciplex to the triplet manifold will be strongly enhanced by a heavy atom contained in the quencher, and the yield of photoreaction may be reduced. When a hydrogen bond donor such as N is used as the quencher, quenching due to hydrogen bond formation is also possible. Hydrogen bond formation in an encounter state and/or an exciplex may diminish both the photoreaction and the intersystem crossing whether or not the quencher contains a heavy atom. Under these considerations the heavy atom effects on the quenching are studied with 6-bromo-2-naphthol (BN) and 2-bromo-6-methoxynaphthalene (BM) as quenchers.
+
Experimental Section Cyanoanthracene (CA; Aldrich) was recrystallized twice from toluene. 2-Naphthol (N; Guaranteed Reagent grade, Wako), 2-methoxynaphthalene (M; Aldrich), 6-bromo-2-naphthol (BN; (3) Kikuchi, K.; Yamamoto, S.; Kokubun, H. J . Photochem. 1984, 24, 271. (4) Kikuchi, K.; Hoshi, M.; Kokubun, H. Z . Phys. Chem. (Frankfurt am Main) 1985, 146, 43. ( 5 ) Hoshi, M.; Kikuchi, K.; Yamamoto, S.; Kokubun, H. J . Photochem. 1986, 34, 63. (6) Fox, M. A. Adv. Photochem. 1986, 13, 237.
N M BN
220 87 180
15 6 12
BM
130
9
0 0.029 1.6 10
0.15 2.8 0.12 0.27
0.01 0.47 0.01 0.03
250 53 450 110
Aldrich), and 2-bromo-6-methoxynaphthalene(BM; Aldrich) were recrystallized from a ethanol-water mixture and sublimated under vacuum. Purification of cyclohexane was reported e l ~ e w h e r e . ~ Sample solutions were degassed by freeze-pump-thaw cycles. Absorption spectra were recorded on a Hitachi 330 spectrophotometer. Fluorescence spectra were measured with a spectrophotometer built in our laboratory. The fluorescence lifetime was measured with an N, laser as the exciting light source. The method of measuring the transient absorption and time-integrated fluorescence intensity during a flash was the same as reported elsewhere.' A xenon flash lamp (32-200 J; full width at halfmaximum, 10-20 w s ) was used for excitation with a Toshiba VB-46 band-pass filter which transmits light in the range 360-580 nm. Light (365 nm) from a Toshiba SHL-100UV2 high-pressure mercury lamp with Toshiba UV-35 and UVD-2 filters was used for the steady light photolysis. The quantum yield of the photoreaction was determined by using an aerated solution of acridine in ethanol as an actinometer.* All measurements were made at 298 K.
Results and Discussion CA forms a hydrogen bond with N and BN in the ground state, but it does not with M and BM. The hydrogen bond equilibrium constants K8 in the ground state were determined by measuring the difference absorption spectra at various concentrations of N and BN to be about 2 and 3 M-' at 298 K, respectively. The difference spectrum is shown in Figure 1. The fluorescence lifetime T~ was determined to be 14.5 ns. With the addition of N and BN the fluorescence of CA is quenched without any change in the spectral shape. This is a general feature for two conjugate .Ir-electronic systems.' When M and BM were used as quenchers, the fluorescence was quenched and the exciplex fluorescence appeared near 21 000 cm-' as shown in Figure 1. The quantum yield of exciplex fluorescence for M is about 4 times higher than that for BM. The bimolecular quenching rate constants k, for monomer fluorescence were determined from the Stern-Volmer plots shown in Figure 2. At the quencher concentration employed for the plots neither hydrogen bond formation in the ground state nor exciplex fluorescence is noticeable. The values for k, are listed in Table I. On steady light irradiation, photobleaching of CA occurs in the presence of quenchers. Since the plots of In (e2303fcd - 1) vs. time were linear, the quantum yields $r of photobleaching were (7) Kikuchi, K.; Kokubun, H.; Koizumi, M. Bull. Chem. SOC.Jpn. 1968, 41, 1545. ( 8 ) Niizuma, S . ; Koizumi, M. Bull. Chem. SOC.Jpn. 1963, 36, 1629.
Kikuchi et al.
576 The Journal of Physical Chemistry, Vol. 91, No. 3, 1987
0\
0
20
10
Figure 4. Plots of Do(X’)/JZ,(X)
uO
50
only at high quencher concentrations. Exciplex formation is not described explicitly, because it is an intermediate state to the photoproducts, 3CA, and CA. According to Scheme I the concentration effects of quenchers on the quantum yields of photobleaching and intersystem crossing can be analyzed. The yield 9 , of photobleaching is related to the quencher concentration as follows:
100
M/101 Figure 3. Plots of
1/arvs. l/[Q].
SCHEME I
CA ’CA*
-- + hv
ICA* CA hu
’CA* 3CA ‘CA* + Q photoproducts
Iabs
1/@r = (kq/kr)(l + l / k q ~ ~ [ Q l )
kf
kist
--.
+ Q 3CA + Q ICA* + Q CA
’CA*
determined from their slopes. Plots of 1/arvs. 1/ [Q]are linear as shown in Figure 3: 1/@r = a + b/[Q1
30
IQl/mM dt vs. [Q].
Equation 2 is in the same form as empirical eq 1, so that the ratio a/b of the intercept to the slope of the plots shown in Figure 3 gives kqTO. a/b is close to kqTOexcept for BN. The reaction efficiency of photobleaching is defined as k , / k , (=l/a). The reaction efficiency for M is extremely high. k, was calculated from k, and l / a and listed is in Table I. The ratio of to Qf is related to the quencher concentration as follows: @isc/@f
(1)
From the intercepts and slopes we obtain the values for a and a/b listed in Table I. The reliability of a and a/b obtained for B N and BM is low because these quenchers absorb at 365 nm. Flashing of the deaerated solution of CA gives only a weak transient absorption shown in Figure 1. It was assigned to the triplet-triplet (T-T) absorption of CA by the energy transfer from triplet fluoranthene to CA. The molar extinction coefficient eT of the T-T absorption was determined to be 2.9 X lo4 M-’ cm-’ at 427 nm by triplet energy transfer using anthracene (eT = 6.47 X lo4 M-’ cm-I at 422 nm9) as a reference. The quantum yield ais,of intersystem crossing for CA was determined to be 0.025 f 0.005 by an emission-absorption flash technique’ using anthracene in cyclohexane (af= 0.301° and @,, = 0.71”) as a reference. Triplet decay was a superposition of first- and second-order processes: k l = 2 X lo3 s-l and k2 = 3.2 X lo9 M-I s-I. The triplet decay was not affected with the addition of quenchers. This implies that the reactive state for photobleaching is the excited singlet state alone. The absorbance of the T-T absorption immediately after flashing decreased upon addition of N, whereas it increased upon addition of M, BN, and BM. The transient absorption other than the T-T absorption was not observed. Since the 2-naphthoxyl radical absorbs around 470 nm,2 it was confirmed that hydrogen atom transfer from N and BN to CA does not occur. On the basis of the above results, we summarize the reactions as shown in Scheme I. In Scheme I hydrogen bond formation in the ground state is not taken into account, because it participates (9) Bensasson, R.; Land, E. J. Trans. Furuday SOC.1971, 67, 1904. (10) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: London, 1970; p 123. (1 I ) Amand, B.; Bensasson, R. Chem. Phys. Lett. 1975, 34, 44.
(2)
= (kisc + k’isc[Ql)/kf
(3)
According to the emission-absorption flash photolysis method eq 3 is modified as follows:
sIf(X)
where Dois the T-T absorbance at the end of the flash, dt is the time-integrated fluorescence intensity during the flash, a(X) is a constant depending upon the experimental conditions, and d = 10 cm is the optical path length of the sample cell. From the intercepts and slopes in Figure 4 we obtain the ratio k’,sc/k,s. Putting k,,, (=@,,,/T~) = 1.7 X lo6 s-I, we obtain the k’,sclisted in Table I. It is noteworthy that fluorescence quenching by BM is almost due to enhancement of intersystem crossing: k‘,,,/k, zz 1. This is consistent with the fact that the yield of exciplex fluorescence is lower for BM than for M. When M and BM are the quenchers, it is obvious that the quenching is due to exciplex formation. The rate of enhanced intersystem crossing is much larger for BM than M, and vice versa for the reaction rate. Since the reactive state for photobleaching is only the excited singlet state, enhanced intersystem crossing and photobleaching occur competitively in the exciplex. When N and B N were used as quenchers, fluorescence other than monomer fluorescence was not found. Although the concentration of N and BN might not be high enough to detect exciplex fluorescence, this result suggests that the quenching is almost due to hydrogen bond formation in the excited singlet state. Both the reaction efficiency and the rate of enhanced intersystem crossing are smaller for N and BN than for their corresponding ethers. These results can also be attributed to hydrogen bond formation in the excited singlet state, because in hydrogen-bonded species of two conjugate n-electronic systems neither reaction nor intersystem crossing O C C U ~ S . ~ *It~is~ ~remarkable that intersystem
J. Phys. Chem. 1987, 91, 577-581 crossing is enhanced by quenching even when BN is used as a quencher. Since intersystem crossing is not enhanced by hydrogen bond formation but by exciplex formation,14 it is supposed that exciplex formation occurs simultaneously with hydrogen bond formation. Charge transfer from quencher to fluorescer is a prerequisite for both exciplex formation and hydrogen atom transfer. Therefore, such a supposition is consistent with the (12) Mataga, N.; Tanaka, F.; Kato, M. Acta Phys. Polon. 1968,34,733. (13) Yamamoto, S.; Kikuchi, K.; Kokubun, H. J. Photochem. 1976,5,469. (14) Hamai, S. Bull. Chem. SOC.Jpn. 1984, 57, 2700.
577
previous conclusion obtained for the two conjugate a-electronic systems
that hydrogen atom transfer and hydrogen bond formation are simultaneous proces~ess.~-~ Consequently, it may be concluded even for N and BN that photobleaching as well as enhanced intersystem crossing occur in.the exciplex. Registry No. CA, 1210-12-4;.N, 135-19-3; BN, 15231-91-1; M, 9304-9; BM, 51 11-65-9.
New Photocathode Materlals for Hydrogen Evolution: CaFe,O, and Sr,Fe,,O,, Yasumichi Matsumoto,* Masaru Omae, Kazuyoshi Sugiyama, and Ei-ichi Sato Department of Industrial Chemistry, Faculty of Engineering, Utsunomiya University, Ishi-icho 2753, Utsunomiya 321, Japan (Received: January 6, 1986; In Final Form: July 21, 1986)
The photoelectrochemicalproperties of p-type CaFe204and Sr7Fei0OZ2 are studied in Nz-saturated 0.25 M KzSO4 (pH 6.0). The differences between the Fermi level and the top of the valence band are determined to be 0.14 and 0.4 eV for Sr7Fei0022 and CaFe204,respectively, from the activation energies of the conductivities and the Seebeck coefficients. The cathodic photocurrents of the hydrogen evolution are observed in the potential region more positive than RHE by 0.7-0.8 V. The band gaps are 1.8 and 1.9 eV for Sr7Fe10022 and CaFe2O4,respectively. The flatband potential of Sr,Felo02, is 0.1 V vs. SCE, but that of CaFe204cannot be determined because of the Fermi level pinning. Pt deposited on the surfaces of both electrodes accelerates the electrochemical process of the hydrogen evolution reaction. CaFe204is more stable than Sr7Fe10022 in the long-term test. The short circuit photocurrent of 0.3-0.4 bA/cm2 is observed in a Pt-deposited CaFez04 (p type)/Znl,zFel,80,(n type) assembly under a xenon lamp illumination. The band structures of Sr7Fe,,02, and CaFezOp and their photoelectrochemical processes are also discussed.
Introduction The possibility of efficiently photoelectrolyzing water with illuminated semiconducting electrodes as a solar energy conversion technique has received considerable attention. Recent interest in this process has led to intensive studies of the photoelectrochemical properties of a variety of materials. The oxides having Fe in their lattice are promising as n-type photoanode materials because of their relatively narrow band gap (about 2.1 f 0.2 eV) and their stability in aqueous For the unbiased photoelectrolysis of water, a p-type semiconductor for photocathodic hydrogen evolution is necessary, because the n-type oxides have the flatband potentials or the onset potentials of anodic photocurrent of oxygen evolution reaction more positive than that of the reversible hydrogen electrode (RHE).'-I4 Recently, Turner (1) Hardee, K. L.; Bard, A. J. J . Electrochem. SOC.1976, 123, 1024. (2) Kung, H. H.; Tarett, H. S.;Sleight, A. W.; Fenetti, A. J. Appl. Phys. 1977, 48, 2463. (3) Butler, N. A.; Ginley, D. S.; Eibschutz, M. J . Appl. Phys. 1977, 48, 3070. (4) Ginley, D. S.; Butler, M. A. J. Appl. Phys. 1977, 48, 2019. ( 5 ) Butler, M. A.; Ginley, D. S.J. Electrochem. SOC.1978, 125, 228. (6) Scaife, D. E. Solar Energy 1980, 25, 41. (7) Kennedy, J. H.; Shinar, R.;Ziegler, J. P. J. Electrochem. SOC.1980, 127, 2307. (8) Koenitzer, J.; Khazai, B.; Hormadaly, J.; Kershaw, R.; Dwight, K.; Wold, A. J. Solid State Chem. 1980, 35, 128. (9) Hatanaka, N.; Kobayashi, T.; Yoneyama, H.; Tamura, H . Electrochim. Acta 1982, 27, 1129. (10) Pollert, E.; Hejtmaneck, J.; Doumerc, J. P.; Clawerid, J.; Hagenmulle, P. J. Phys. Chem. Solids 1983, 44, 237. (1 1) Kochev, K.; Tzvethova, K.; Gospodinov, M. Mater. Res. Bull. 1983, 8, 915. (12) Shami, A.; Wallace, W. E. Mater. Res. Bull. 1983, 18, 389. (13) Turner, J. E.; Hendewerk, M.; Parmeter, J.; Neiman, D.; Somorjai, G. A. J. Electrochem. SOC. 1984, 131, 177. (14) Matsumoto, Y.; Omae, M.; Watanabe, I.; Sato, E. J . Electrochem. Soc. 1986, 133, 711.
et al.13 reported the production of hydrogen from water by illumination of both electrodes in a Mg-doped (p type) and Si-doped (n type) iron oxide assembly. Thus, the investigation of p-type semiconducting materials for H2 evolution as well as n-type semiconducting materials will be very important for the unbiased photoelectrolysis of water by a p/n assembly. We have already discovered the p-type semiconducting oxides CaFez04 and Sr7FeloOz2with relatively narrow band gaps, that can act as the photocathode for H2evolution under visible light.l5 In this study, the details of the photoelectrochemical properties of the polycrystalline CaFe204and Sr7FeloOZ2 and the effect of surface deposited Pt on the H2evolution reaction are described.
Experimental Section Solutions of metal nitrates were used as the starting materials. The solutions were stoichiometrically mixed so that the metal cations are distributed uniformly throughout the oxide. The mixed solutions were evaporated to dryness and then heated at 900 OC. The materials consisting of Fe and S r or Ca were ground in an agate mortar and then pressed into a pellet at 100 kg cm-z. The pellet samples were sintered at 1200 OC in air followed by oxidation in O2 at 1000 O C to make them sufficiently conductive. The sintered disk samples consisted of a single phase of Sr7Fe100z2 or CaFe204which was determined by X-ray analysis. The conductivities and Seebeck coefficients were measured at temperatures from room to about 600 O C in 02.All samples were waterproofed with polystyrene in order to obtain good reproducible results in the electrochemical tests. The whole electrode was sealed with epoxy except for the front surface. The solution was N2saturated 0.25 M K2S04 (pH 6.0) in order to inhibit the photo(15) Omae, M.; Matsumoto, Y.; Sato, E. Abstract of Papers, Electrochemical Society of Japan Meeting, Yamanashi, April 1985. Abstract A31 1.
0022-3654/87/2091-0577.$01.50/0 0 1987 American Chemical Society
