1488
J . Phys. Chem. 1986, 90. 1488-1491
conductivity) of the electrolyte, or from a modification of the ZnO surface by adsorption or injection of cations under the influence of the electric field at the interface. The crystallographic radius of monovalent alkali ions increases from Li' to NH4+, but the corresponding hydrated radius dec r e a s e ~in ' ~the same order (see Table I). This explains the higher conductivity of the electrolyte containing NH4' instead of Li' ions as shown by the variation of the specific equivalent conductance of the M' ions. Such a cation effect on the photocurrent performances and the stability of CdSe electrodes in polysulfide solutions has been reportedI4 already. Nevertheless, this analysis seems insufficient to account for the luminescence enhancement when we change NH4' to Li'. The more intense current observed with NH4' should inject more positive charges in ZnO and, consequently, increase the number of electron-hole recombinations as well as the EL intensity.6 Figure 4 shows that the EL intensity is stronger with Li' than with NH,' even before the Zn2+ reduction potential (-1.2 V) has been reached. Furthermore, the reduction wave of the S208*-ions at -0.6 V is twice as intense with NH4' (14 mA/cm2) as with Li+ (7 mA/cm2), predicting an EL emission more intense for NH4+ than for Li' in the -0.6 to -1.2 V potential range. As this is not the case, it appears necessary to put forward a modification of the electrode itself to account for all the experimental results. Alkali ions, mainly Li' and Na', when introduced inside the lattice of 11-VI semiconductors, are known to increase considerably their resistivity. In ZnO for instance, Li" creates a deep acceptor center which lies about 0.8 eV above the valence band.* On the other hand, the Li and Na' ions induce donor-acceptor pairs in the ZnO lattice and play a determinant role in its UV9 and visible*~1° luminescence. These studies on solid-state ZnO doped with alkali ions are consistent with our observations and we propose that M+ cations from the solution are injected into the ZnO electrode under the effect of the applied potential. Occupancy of preferential sites in the host lattice would modify its luminescence and electrical properties near the surface. The strong accumulation layer built up in ZnO at potentials cathodic to the flat-band potential implies (13) Harned, H. S.; Owen, B. B. In The Physical Chemistry of Elecfrolytic Solufions; 3rd ed.; Rheinhold: New York, 1958; p 537. (14) Licht, S.; Tenne, R.; Flaisher, H.; Manassen, J. J . Electrochem. So?. 1984, 131, 950.
a narrow space charge layer and impedes the cations to penetrate deeply inside the semiconductor. The cations injected during the excitation are then expected to be expelled when the potential is turned off, Le., at the end of each pulse. This is verified by using the electrode successively with various cations without surface pretreatment between each experiment; the same cationic dependence on EL and current is then observed. Besides, due to their large hydration shell, the hydrated cations may be trapped only at superficial sites from where they relax when the excitation is turned off. Since the EL spectral distribution is not strongly influenced by the cation, the radiative centers inside ZnO would be simply activated (or deactivated) by the metal ions M+. Such an EL activation of ZnO by small alkali ions probably reflects a compensation process in the lattice, decreasing the concentration of the nonradiative centers. However, in the case of Li+, the EL band peaking at 560 nm seems to correspond to the so-called "yellow luminescence" of ZnO:Li, which involves a hole trapped next to a Li' acceptor state.la This similarity constitutes an argument to favor a mechanism of countercation injection into the ZnO lattice; nevertheless, it is difficult, on the basis of these only results, to exclude a mechanism involving adsorption of the ions at the surface of ZnO. Then, the NH4' ions would be expected to show easier adsorption properties than Li', as a result of a weaker solvation than K+, Na', and Li'. It seems interesting to verify that a similar cation effect is operating with other 11-VI semiconductors in liquid junction,I5 specially with Li- or Na-doped ZnSe, for which the solid-state luminescence and conductivity have been reported7J6recently.
Conclusion Pulsed EL of the n-ZnO/aqueous electrolyte interface during persulfate reduction, combined with cyclic voltammetry, demonstrates the strong influence of the alkali cation upon the interfacial luminescence and electrical properties of ZnO. This cationic effect seems to result from both a variation of the conductivity of the electrolyte and a modification of the ZnO surface by cation injection in lattice sites or adsorption at the surface. ( 1 5) Further studies in this field are in progress. (16) Neumark, G. F.; Herko, S.P.; McGee, T. F.; Fitzpatrick, B. J. Phys. Reti. Lett. 1984, 53, 604.
Time-Resolved Emission Spectra of Ru(bpy),CI, and cis-Ru(bpy),(CN), at Low Temperature Noboru Kitamura, Haeng-Boo Kim, Yuji Kawanishi, Ritsuko Obata, and Shigeo Tazuke* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori- k u , Yokohama 227, Japan (Received: October 16, 1985; In Final Form: February 3, 1986)
Time-resolved emission spectroscopy of Ru(bpy)&12 and ci~-Ru(bpy)~(CN)~, where bpy is 2,2'-bipyridine, was conducted in an ethanol-methanol mixture above 80 K. The emission spectra of both complexes shifted to the red with increasing temperatures (>80 K) as well as with the delay time after excitation (110-150 K). The apparent activation energies for the time-dependent red shift of the emission (1 10-130 K) were calculated to be 570 and 1360 cm-' for Ru(bpy),C12 and cis-Ru(bpy)*(CN)*,respectively. It was concluded that charge localization in the excited state was not an intrinsic property of the complex but was induced by solvent relaxation processes.
Introduction The excited state of tris(2,2/-bipyridine)ruthenium(II), R ~ (bpy),", has been widely employed as a potential photocatalyst for chemical conversion of solar energy in fluid media including micellar and polymeric systems and also on electrodes or semi0022-3654/86/2090-1488$01.50/0
conductor partic1es.l The excited-state properties of R ~ ( b p y ) ~ * + , however, are known to be very sensitive to environmental conditions such as solvents and temperature,2 and thus, evaluation of pho(1) Kaiyanasundararn, K. Coord. Chem. Reo. 1982, 46, 159.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 8, 1986 1489
Letters
.-
in
e
Wavelength
(nm)
Figure 1. Time-resolved (b and d) and temperature-dependent (a and c) emission speectra of Ru(bpy),Clz and cis-Ru(bpy),(CN), in an ethanol-methanol mixture. Time-resolved emission spectra were measured at 130 K. Temperture-dependent emission spectra were recorded immediately after excitation ( t = 0). (a) Ru(bpy),Clz: 80 K (-), 120 K(---),130 K (---), 140 K and 200 K (b) Ru(bpy),CI,: 0 ns (-), 80 ns 160 ns (---),and 4 ps (-.). (c) cis-Ru(bpy),(CN),: 80 K (-), 120 K (-.-), 130 K (---), 140 K and 150 K (-). (d) cis-Ru(bpy),(CN),: 0 ns (-), I O ns (-*-), 50 ns (---), 250 (-.e-),
(-e).
(-e-),
(-.e-),
ns (- 1 ws. The present AE and k , thus correspond to the activation energy and the rate of solvent relaxation process, respectively. Since the excited-state dipole moment (De)of cis-Ru(bpy),(CN), is larger than that of Ru(bpy),C12,15the larger LIE for the former complex
-
~~~~~
~
~
~
~~
~
(11) (a) Morris, D. E.; Hanck, K. W.; DeArmond, M. K. J . Am. Chem. SOC.1983, 105, 3032. (b) Motten, A. G.; Hanck, K. W.; DeArmond, M. K . Chem. Phys. Letr. 1981, 79, 541. (12) Krausz, E. Chem. Phys. Lett. 1985, 116, 501. (13) Recently, Kober et al. reported that the excited electron of Ru(bpy),2+ was localized on a single bpy ligand in the initially populated excited stare as demonstrated by the solvent dependence of the metal-to-ligand chargetransfer absorption of Ru(bpy)32+(Kober, E. M.; Sullivan, B. P.; Meyer, T. J. Iriorg. Chem. 1984, 23, 2098). The results are apparently inconsistent with the present observations. At the present time, we do not have any reasonable explanation on these two contradictory results and further studies are necessary to fully understand these results. (14) Yersin, H.; Gallhuber, E. J . A m . Chem. SOC.1984, 106, 6582.
1491
J. Phys. Chem. 1986, 90, 1491-1493 rather than that for the latter will support the present solvent relaxation mechanism. Namely, the larger De requires more solvent reorganization around the complex which brings about the relatively large AE for ci~-Ru(bpy),(CN)~. Below 100 K, on the other hand, an ethanol-methanol mixture is rigid enough to prevent solvent relaxation and therefore to retain charge delocalization over bpy ligands, rendering no time-dependent spectral shift. Furthermore, solvent or solvent dipole relaxation is completed within 10-9-10-’2 s-l l6 depending on temperature above 130 K, so that no time-dependent shift is observable by nanosecond time-resolved spectroscopy. The break point of linearity in Figure 4 at 130 K corresponds to the rigid-fluid transition. Finally, we may suggest that the activation energy and the rate of the interligand electron hopping mentioned above include the contributions from the solvent reorganization processes, since the (15) Kitamura, N.; Obata, R.; Tazuke, S., unpublished results. (16) (a) Mataga, N.; Ottolenghi, M. In Molecular Association; Foster, R., Ed.; Academic: New York, 1979; Vol. 2, Chapter 1. (b) Werner, T. C. In Modern Fluorescence Spectroscopy; Wehry, E. L., Ed.; Plenum: New York, 1976; Vol. 2, Chapter 7. (c) Lippert, E. In Organic Molecular Phofophysics; Birks, J. B., Ed.; Wiley: New York, 1975; Vol. 2, Chapter 1.
phenomenon is regarded as intervalence electron transfer which requires solvent reorganization as has been demonstrated theoretically and experimentally.” If that is the case, the close similarities of the present AE and k, with the values relevant to the interligand electron hopping are well explainable. In conclusion, a transition from the charge-delocalized excited state to the charge-localized one in the present ruthenium complexes is not an intrinsic property of the excited state but is strongly governed by solvent relaxation processes. Such a conclusion is consistent with the results derived from the polarized emission studies on single-crystal Ru(bpy),(PF6),l4 as well as from timeresolved resonance Raman spectro~copy.’~ The present observations clearly demonstrate that reorganization of solvent molecules or dipoles plays an essential role to trap an excited electron on a single bpy ligand and to relax to the charge-localized excited state. Acknowledgment. S.T. and N.K. are grateful to Tokyo Instruments Inc. for permitting us to use their SMA system. (17) (a) Meyer, T. J. Acc. Chem. Res. 1978, 1 1 , 94. (b) Creutz, C. In Progress in Inorganic Chemistry; Lippard, S . J . , Ed.; Wiley: New York, 1983; Vol. 30, Chapter I .
Internal Energy State Distribution of NH(A3n) from the Chemiluminescent Reaction of CH(X*n,v”=O) with NO Nobuaki Nishiyama, Hiroshi Sekiya,* Sumio Yamaguchi, Masaharu Tsuji, and Yukio Nishimura Department of Molecular Science and Technology, Graduate School of Engineering Sciences, and Research Institute of Industrial Science, Kyushu University, Kasuga-shi. Fukuoka 816, Japan (Received: November 13, 1985; In Final Form: January 3, 1986)
-
The CH(X211,v”=O)radicals were prepared from the C(ID) + H2 reaction by using a flowing afterglow apparatus. Strong NH(A311-X32-) chemiluminescence was observed from the reaction CH(X*II,v”=O) + NO NH(A311,v’=0,1) + CO. The vibrational and rotational distributions of ”(A) are more excited than the prior ones, suggesting that the reaction proceeds via a direct four-center mechanism.
Introduction The reactions of C H radicals are important processes in hydrocarbon combustion and flames. Although the, kinetic studies of the reactions of C H radicals with molecules have been performed exten~ively,l-~ investigations on the internal energy state distributions of products are limited.4 The observation of chemiluminescence gives information on the internal energy state distributions and reaction dynamics. Lichtin et al. have observed NH(A311-X32-) chemiluminescence arised from the reaction of C H radicals with NO.5 CH(X2n)
+ NO
NH(A311)
-
-
NH(A311)
NH(X3Z-)
+ CO
+ hu
(1)
The CH(X) radicals were produced from the photolysis of CHBr, by using a Nd:YAG laser (266 nm). The rate constant for reaction cm3 molecule-’ 1, has been measured to be (2.5 f 0.5) X s d . They have discussed the dynamics of reaction 1 on the basis of the kinetic data.ss6 However, the internal energy state dis( I ) Wagal, S. S.; Carrington, T.; Filseth, S. V.; Sadowski, c . M . Chem. Phys. 1982, 69, 61. (2) Duncanson, J. A., Jr.; Guillory, W. A. J . Chem. Phys. 1983, 78,4958. ( 3 ) Berman, M. R.; Lin, M. C. J . Phys. Chem. 1983, 87, 3933. (4) Lin, M. C. J . Phys. Chem. 1973, 77, 2726. (5) Lichtin, D. A,; Berman, M. R.; Lin, M. C. Chem. Phys. Lett. 1984, 108, 18.
0022-3654/86/2090-1491.$01.50/,0
tributions of products have not been measured. Recently we have reported that the He(2,S) C O reaction is a simple, convenient method for producing the metastable state atomic carbon.’ In pursuing the reactions of C(’D) with molecules, we have found the CH(X) radicals to be produced in high concentration from the C(’D) H2reaction.* The C(ID) H2 reaction produces CH(X) predominantly in the vibrational ground state.’ This method of producing the CH(X) radicals was applied to study the reaction of CH(X) with NO. Strong “(A-X) chemilumineScence has been observed from the reaction of CH(X,v”=O) with NO. The vibrational and rotational distributions within “(A) have been determined and compared with those predicted from the information-theoretic approach.
+
+
+
Experimental Section The experimental arrangement is illustrated in Figure 1. The flow reactor was continuously evacuated by a 10000 L/min mechanical booster pump. The CH(X) radicals were produced from the C(’D) + H, reaction C(’D) + H2 CH(X,v”=O,l) + H (2)
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( 6 ) Berman, M. R.; Fleming, J. W.; Harvey, A. B.; Lin, M. C. Symp. (Inr.) Combust. [Proc.],19fh 1982, 73. (7) Sekiya, H.; Tsuji, M.; Nishimura, Y. J . Chem. Phys. 1985, 83, 2857. (8) Braun, W.; Bass, A. M.; Davis, D. D.; Simmons, J. D. Proc. R. Sor. London, A 1969, 312, 41 7. (9) Jursich, G. M.; Wiesenfeld, J . R. Chem. Phys. Lett. 1984, 110, 14.
0 1986 American Chemical Society
