J. Phys. Chem. 1989, 93, 7542-7544
7542
increasing excitation energy and then Saturates when the excitation energy becomes about 1 eV higher than the ionization threshold.' The rather short initial separation we have obtained may suggest that the energy of the excited state where the ionization is taking place is very close to the threshold. Transient absorption spectra of BDATP in cyclohexane are similar to those in Figure 1. Only at quite short delay times can the BDATP cation radical be observed. Figure 4 shows the time dependence of the transient absorbance of BDATP in cyclohexane excited with 295-nm laser pulse. At 981 nm, where the BDATP cation radical has a large extinction coefficient, we can see the short-lived component, while at 893 nm, where the S, S1 absorption of BDATP is dominant, no short lived component can be observed. The short-lived component should correspond to the geminate electron-cation recombination. The lifetime, which is too short to determine by our laser photolysis system, is much shorter than that in n-hexane. This can be explained by the difference of electronic mobility in n-hexane (0.08 cm2/(V s)) and cyclohexane (0.23 cm2/(V s)). The lifetime of the geminate electron-cation pair is shorter in the solvents of higher electronic mobility; therefore, in order to measure the geminate recombination in such solvents such as neopentane (70 cm2/(V s)16) and tetramethylsilane (90 cm2/(V s)"), femtosecond time resolution will be required.
893 nm
0 0
98 1 .&&o.*oo.o
.....
- 0 --
0
~
o
Q
o
~
0
o
~
I
-
0
8
d
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o
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0
0
00 0
e
4 70 ~ o o e o o o
0
0
250 f
0
5 00
( p s i
Figure 4. Time dependence of the transient absorbance of BDATP in
cyclohexane. we can expect that the electron ejected from BDATP should have higher kinetic energy. These results again strongly suggest that the lower the excess energy, the shorter the thermalization length. According to the external electric field dependence of the photoionization yield, the thermalization length first increases with
Acknowledgment. We thank Professor Y. Sakata and S. Misumi of this university for the sample of BDATP. The present work is supported partly by a Grant-in-Aid (No. 62065006) from the Japanese Ministry of Education, Science and Culture to N.M. (16) Bakale, G.;Schmidt, W. F. Chem. Phys. Lerr 1973, 22, 164. (17)Schmidt, W.F.;Allen, A. 0. J . Phys. Chem. 1968, 72, 3730.
Pyrolysis Jet Spectroscopy: Rotatlonaiiy Resolved Electronic Spectrum of Dichiorocarbene Dennis J. Clouthier* Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506
and Jerzy Karolczak Quantum Electronics Laboratory, Institute of Physics, A. Mickiewicz University, Crunwaldzka 6, 60- 780 Poznan, Poland (Received: June 8, 1989)
The rotationally resolved, gas-phase vibronic spectrum of dichlorocarbene has been recorded by using the technique of pyrolysis jet spectroscopy. Well-resolved rotational lines of the various isotopomers are observed, corresponding to a rotational temperature of 3-5 K. The transition is shown to be IB, 'Al with a ground-state bond length of 1.71, A and a bond angle of 109.,'.
-
Introduction Recently, we reported the use of pyrolysis jet spectroscopy to study the laser-induced fluorescence (LIF) spectra of jet-cooled transient molecules and free radicals.' The technique uses high-temperature pyrolysis to create reactive species in the throat of a supersonic expansion nozzle, from which they are expelled before secondary reactions predominate. This technique has not been widely exploited in the past, although the papers of Chen et aLZ3 have recently been brought to our attention. These authors (1) Dunlop, J. R.;Karolczak, J.; Clouthier, D. J. Chem. Phys. Lerr. 1988, 151, 362: 1989, 154, 613. (2) Chen, P.;Colson, S. D.; Chupka, W. A,: Berson, J. A. J. Phys. Chem. 1986, 90, 2319.
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report a pulsed pyrolysis jet which they have ingeniously applied to the mass-resolved multiphoton ionization spectrum of methyl radicals. We have successfully recorded spectra of the transient molecules thioformaldehyde,l*4 thi~acetaldehyde,~ thioacetone,6 thiocyclobutanone,' thiocyclopentanone,7 thiocyclohexanone,' and formyl cyanide* and the benzyl' and thioethynyl (HCCS)'q9 free (3) Chen, P.; Colson, S.D.; Chupka, W.A. Chem. Phys. Lett. 1988,147, 466. (4) Dunlop, J. R.; Karolczak, J.; Clouthier, D. J. Manuscript in preparation. ( 5 ) Karolczak, J.; Clouthier, D. J.; Smyers, Y . G.; Moule, D. C. Manuscript in preparation. (6)Karolczak, J.; Clouthier, D. J.; Smyers, Y. G.; Moule, D. C. Manuscript in preparation. (7) Karolczak, J.; Moule, D. C.; Clouthier, D. J. Unpublished results.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 22, 1989 7543
Letters
TABLE I: Rotational Constantsa for the
A
B
C AK
TO
17560.0
17563.5 FREQUENCY ( c m - l )
-
Figure 1. A portion of the high-resolution pyrolysis jet spectrum of dichlorocarbene showing rotational assignments for the K,' = 1 K/ = 0 subband of 2; band of C35C12.
LCICCI (expt), deg LClCCl (ab initio)," deg r C C l (expt), A reel (ab initio)," A no. of fitted transitions = 177c SD of fit = 0.005 cm-'
Band of CWl2
upper state (Vi = 1) 3.74546, (9) 0.1064860 (16) 0.1033533 (16) 0.0013688 (6) 17558.380 (3) 132.4 128.9 1.644 1.692
lower state (v2/1 = 0) 1.676333 (9) 0.1232520 (16) 0.114784, (15) 0.00009' 109.4 110.0 1.714 1.736
a In cm-I. The error limits quoted are 30 and are right justified to the last digit on the line; sufficient additional digits are quoted to reproduce the data with full accuracy. bNot determinable in the leastsquares fit. Fixed at the CF2 ground-state value, ref 21. 'Transitions arising from rotational levels up to K, = 3, J = 10 in the ground state to levels as high as K, = 4, J = 10 in the excited state were observed. dReference 19.
radicals. In this report, we describe high-resolution pyrolysis jet spectra of dichlorocarbene, CC12, whose analysis establishes the spin multiplicities and geometries in both electronic states and the direction of the transition moment. CJ5% Carbenes are a very important class of transient reactive intermediates whose chemistry has been extensively studied in recent years.I0 Dichlorocarbene is of considerable interest from a number of points of view. The role of freons in upper atmospheric reactions which deplete the ozone layer enhances the importance of their radical fragments. CC12 has been reported as a product of the vacuum-UV photolysisll and pulse radiolysis12of chloromethanes. The reactions of CCl2 suggest that it is a ground-state singlet,I0 although this has never been proven spectroscopically. Dichlorocarbene exhibits a long series of absorption bands in the 400-600-nm region. The vibrational frequencies in the ground and excited states have been established from IR a b ~ o r p t i o n , ~ ~ , ' ~ 17557.8 17563.3 fluorescence excitation,15and fluorescence emission16 spectra. The FREQUENCY (cm-' ) available data on gas-phase CC12 are very limited." Recently, Figure 2. A portion of the high-resolution pyrolysis jet spectrum of Predmore et al." reported that gas-phase pyrolysis of (CH3)3dichlorocarbene showing the chlorine isotope effect at natural abundance. SiCC13 vapor served as an excellent source of CC12 for LIF spectroscopy. Crystalline trichloromethyltrimethylsilane(Alfa Chemical) was used as received from the manufacturer. Experimental Section Our experiments were performed by seeding the room-temResults and Discussion perature vapor of solid (CH3)3SiCC13in 3 atm of Ar, pyrolysis Figure 1 shows a small portion of the spectrum recorded for of the gas mixture a t 500 OC, and expansion through a 150-pm a band in the 17550-17600-~m-~region, which we assign as 2;. quartz nozzle. The cooled, continuous expansion was probed 5 The rotational line structure is clearly resolved and is found to mm downstream with a tunable C W ring laser (Coherent 699-29) correspond to a rotational temperature of 3-5 K. The lines in operating in the 565-600-nm region. The resulting fluorescence Figure 1 are readily assigned as the K,' = 1 K/ = 0 subband was imaged on the photocathode of an EM1 9816QB photoof the C3sC135C1species, from which several conclusions can be multiplier through a spatial filter which limited the viewing zone drawn. The combination defect is positive, establishing that the to the cold, central core of the expansion. The spectra were transition moment lies along the c axis (out-of-plane, giving type calibrated with simultaneously recorded I2 LIF spectra. The C bands). The clear intensity alternation in J"estab1ishes that fluorescence excitation spectrum, I2 calibration spectrum, and the C2 axis in the ground state is perpendicular to the top axis etalon interpolation fringes were digitized simultaneously, and that the ground-state vibronic symmetry is AI or AS. Since transferred from the laser computer to high-density disk storage, there is no evidence of spin splittings in the spectrum, the transition and plotted on a graphics printer during acquisition, using an must be 'B1 'Al or 'B2 lA2. By analogy to the spectra of extensively modified version of the Coherent Autoscan software. other halocarbenes'* and in agreement with the results of ab initio prediction^,'^ the ground state is assigned as 'Al and the transition is IB, 'Al. (8) Karolczak, J.; Rae, J.; Clouthier, D. J. Manuscript in preparation. These conclusions are in accord with a detailed rotational (9) Karolczak, J.; Joseph, M.; Clouthier, D. J. Manuscript in preparation. analysis of the band. Both the ground- and excited-state energy (10) Carbenes; Moss, R. A., Jones Jr., M., Eds.; Wiley-Interscience: New levels can be readily fit by using Watson's A reduction of the York, 1975. (1 1) Ibuki, T.; Takahashi, N.; Hiraya, A.; Shobatake, K. J . Chem. Phys. asymmetric top Hamiltonian in the P representation.20 The 1986.85, 5717. resulting rotational constants are given in Table I. In both states, (12) Ha, T. K.; Gremlich, H. U.; Buhler, R. E. Chem. Phys. Letf. 1979, dichlorocarbene is a near-prolate, near-symmetric top. The large 65, 16.
I II
I
-
-
-
-
(13) Milligan, D. E.; Jacox, M. E. J . Chem. Phys. 1967,47, 703; 1970,
53, 2688.
(14) Andrews, L. J . Chem. Phys. 1968, 48, 979. (1 5 ) Bondybey, V. E.J . Mol. Spectrosc. 1977, 64, 180. (16) Trevault, D. E.;Andrews, L. J . Mol. Spectrosc. 1975, 54, 110. (17) Predmore, D. A.; Murray, A. M.; Harmony, M. D. Chem. Phys. Lett. 1984, 110, 173, and references therein.
(18) Herzberg, G. Electronic Spectra of Polyatomic Molecules; Van Nostrand: Princeton, NJ, 1967. (19) Nguyen, M. T.; Kerins, M. C.; Hegarty, A. F.; Fitzpatrick, N. J. Chem. Phys. Lert. 1985, 117, 295. (20) Watson, J. K. G. J . Chem. Phys. 1967, 46, 1935.
J . Phys. Chem. 1989, 93, 1544-1541
7544
change in the A rotational constant on excitation leads to a nearly complete separation of the subbands, even at low resolution (1 cm-I), in the jet-cooled spectra. The derived geometries in the combining states (Table I) are also in accord with the ab initio res~1ts.I~It is interesting to note that the bond length of dichlorocarbene contracts substantially on excitation whereas the bond length of CF, elongates very slightly on excitation.21 In both cases the bond angle increases going from the ground to the excited state. Transitions involving the other abundant isotopomer, C35C137C1, are clearly resolved in the spectra as shown in Figure 2. The absence of an intensity alternation in the higher frequency subband identifies it as due to the isotopomer without identical nuclei. The approximately 2:3 intensity ratio between the two subbands is (21) Mathews. C. W. Can. J . Phys. 1967, 45, 2355.
consistent with a species having two chlorine atoms. The results of a more extensive analysis of the rovibronic structure in the spectrum will be reported at a later date. As pointed out in our recent paper on pyrolysis jet spectroscopy,’ the ability to produce continuous jets of transient species has advantages for high-resolution spectroscopy. This is particularly evident in the present work, where the narrow line width and precise wavelength of the C W ring laser were utilized without the necessity of pulse amplification or a photolysis source. Acknowledgment. This work has been supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the US Department of Energy, under Contract No. DE-FG05-86ER13544. We express our appreciation to Prof. Asit Ray for the generous use of his ring dye laser and Prof. R. H. Judge for the data acquisition software used in this work.
Photoinduced Charge Separation across the Sotid-Liquid Interface of Porous Sol-Gel Glasses: Catalyzed Hydrogen Generation from Water Anny Slama-Schwok,+David Avnir,*vt,l and Michael Ottolengbi*yt Institute of Chemistry and The Fritz Haber Research Center for Molecular Dynamics, The Hebrew University of Jerusalem, Jerusalem 91 904, Israel (Received: June 19, 1989)
Photoinduced electron transfer between [Ir(bpy)2(C3,N’)bpy]3+,Ir(III), trapped in a porous silica glass generated by the sol-gel process, and 1,4-dimethoxybenzene(DMB) dissolved in a water phase in the pores of the glass, was observed. The kinetics of the redox reactions associated with the Ir(I1) + DMB+ radical ion pair were studied at different pH and ionic strength values. A retardation of 4 orders of magnitude of the back-electron-transfer process, with respect to homogeneous solutions, was observed at neutral pH for -25% of the generated Ir(I1). The effect is attributed to the immobilization of Ir(I1) by trapping in the glass matrix and of DMB+ by adsorption on the pores’ surface. At acidic pH, retardation of the back-reaction leads to catalyzed hydrogen generation from water.
Introduction
A key issue in the design of artificial photosynthetic systems is the retardation of the back-electron-transfer stage.’ A wide variety of organized microenvironments have been employed for this purpose.14 The desired charge separation is usually achieved by means of compartmentization (Le., spacial separation of the photoinduced radical pairs) or through electrostatic and hydrophobic effects. Retardations of up to 2 orders of magnitude, with respect to analogous reactions in homogeneous solutions, have been reported for two-component donor-acceptor s y s t e m ~ .Larger ~ factors are known in multicomponent system^.^ Here we wish to report the observation of a 4-order retardation factor for a part (-25%) of the radical pair population. This was achieved by employing the recently developed technique of trapping of photoactive organic molecules in transparent, inert porous oxide glassesS by the so-called “sol-gel” process.6 The method has initiated intensive studies of spectroscopic, photophysical, and photochemical properties of trapped organic molecules in inorganic g l a s ~ e s . ~In * ~the present work it is extended toward its potential applications to the photocatalytic utilization of light (e.g., to solar energy conversion). Specifically, we report that photoexcited [(Ir(bpy)2(C3,N’)bpy)]3+, Ir(II1) trapped in a SiOz matrix (prepared by polymerization of tetramethyl orthosilicate6), is capable of reacting via (3) with 1,4-dimethoxybenzene (DMB) dissolved in the water-filled pores of the glass. (Ir(I1) represents the reduced form of the Ir complex, with no precise assignment Institute of Chemistry.
*The Fritz Haber Research Center for Molecular Dynamics.
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of the reduction (ligand or metal) site.) The back-reaction (4) is slowed down to such a degree that under appropriate (low) pH (1) For reviews set: (a) Graetzel, M. Acc. Chem. Res. 1981,14, 376. (b) Matsuo, T. J. Photochem. 1985, 29,41. (c) Rabani, J. Phoroinduced Elecrron Transfer, Pari A ; Fox, M. A., Channon, M., Eds.; 1988; pp 642-696. (d) Baral, S.; Fendler, J. H. Ibid. pp 541-598. (2) (a) Nagamura, T.; Takeyama, N.; Tanaka, K.; Matsuo, T. J . Phys. Chem. 1986, 90, 2247. (b) Persaud, L.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.; Webber, S.E.; White, J. M. J. Am. Chem. Soc. 1987, 109, 7309. (c) Milosavljevic, B. H.; Thomas, J. K. J. Phys. Chem. 1985,89, 1830. (d) Wheeler, J.; Thomas, J. K. J. Phys. Chem. 1982,86,4540. (e) Barmasov, A. V.; Kholmogorov, V. Y.; Korotkov, V. I. Book of Absrracrs, XI1 IUPAC Symposium on Photochemistry, Bologna, 1988; PH 24. (f‘) Shi, W.; Gafney, H. D. J. Am. Chem. Soc. 1987,109,1582. (g) Surridge,N. A.; McClanahan, S. F.; Hupp, J. T.; Danielson, E.; Gould, S.;Meyer, T. J. J. Phys. Chem. 1989,
93, 294.
(3) (a) Lerenbours, B.; Chevalier, Y.;Pileni, M. P. Chem. Phys. Letr. 1985, 117, 89. (b) Kurlhara, K.; Tundo, P.; Fendler, J. H. J. Phys. Chem. 1983, 87,3777. (c) Brugger, P. A.; Graetzel, M.; Guarr, T.; McLcndon, G. J. Phys. Chem. 1982,86,944. (d) Willner, I.; Yang, J. M.; Otvos, J. W.; Calvin, M. J. Phys. Chem. 1981,85,3277. (e) Adar, E.; Degani, Y.;Goren, Z.; Willner, I. J . Am. Chem. SOC.1986, 108,4696. (4) (a) Parmon, V. N.; Zamaraev, K. I. Phoiochemical Energy Conuersion; Norris, J. R., Meisel, D., Eds.; 1989; pp 316-342. (b) Krueger, J. S.; Mayer, J. E.; Mallouk, T. E. J . Am. Chem. Soc. 1988, 110, 8232. (c) Rabani, J.; Sassoon, R. E. J. Photochem. 1985,29, 1. (d) Sassoon, R. E. J . Am. Chem. SOC.1985, 107, 6134. (e) Morishima, Y.; Furui, T.; Nozakura, S.; Okada, T.; Mataga, N. J . Phys. Chem. 1989, 93, 1643, and references cited therein. (f) Kamat, P. V. J. Phys. Chem. 1989, 93, 859. ( 5 ) Avnir, D.; Levy, D.; Reisfeld, R. J . Phys. Chem. 1984, 88, 5956. (6) Brinker, C. J.; Scherer, G. W. J . Non-Crysr. Solids 1985, 70, 301. Sakka, S. Bull. Inst. Chem. Res. (Kyoio Uniu.) 1983, 61, 376. Dislich, H. Angew. Chem., Inr. Ed. Engl. 1971, IO, 363. Chen, K. C.; Tsuchiya, T.; Mackenzie, J. D. J . Non-Cryst. Solids 1986, 81, 227.
0 1989 American Chemical Society
