J. Phys. Chem. 1988, 92, 4636-4640
4636
threshold to changes in the level of excitation. These differences are attributable to the nature of the radicals involved in the particular isocyanide reaction. Further work on the ethyl isocyanide system involves a more extensive characterization of the radical channel. Experiments include the addition of a known ethyl radical generator, as well as the monitoring of the reaction in real time to obtain kinetic information on product formation. The addition of appropriate radical scavengers to the system is also being undertaken.
there is an increase in the ratio of ethylene to ethane with reactant pressure (Figure 6). Ethylene and ethane may also be produced and consumed in secondary reactions. Therefore until the radical channel is further characterized, the production of ethylene and ethane, as well as of methane and HCN, cannot be solely attributed to any particular reaction( s).
Summary The laser-induced reactions of ethyl isocyanide and methyl isocyanide are very similar. In both reactions, there is a pressure threshold above which nearly complete isomerization occurs in a single pulse. Furthermore, in neither case is this threshold the result of a thermal explosion. Both reactions contain a radical component that increases with reactant pressure and with incident fluence. The main differences between the two isocyanide reactions are the lower threshold at a given level of excitation for ethyl isocyanide and the greater sensitivity of the ethyl isocyanide
Acknowledgment for partial support of this work is made to the National Science Foundation (Grant No. Rll-8503811, M.J.S. and L.M.Y.), to the Research Corp., and to the Brachman Hoffman fund, Wellesley College (S.B.). We also thank Mike Crimmins for valuable assistance and discussions. Registry No. C2H5NC,624-79-3; C2H,CN, 107-12-0;C2H5*,202556-1.
Luminescence Decays and Spectra of Ru(bpy):+ the Presence of Water Vapor
Adsorbed on TiO, in Vacuo and in
K. Hashirnoto,t M. Hirarnoto,t T. Kajiwara,f§ and T. Sakata*>' Institute for Molecular Science, Myodaiji, Okazaki 444, Japan, and Department of Chemistry, Faculty of Science, Toho University, Miyama, Funabashi 274, Japan (Received: June 15, 1987)
The luminescence of Ru(bpy)?+ (bpy = bipyridyl) was quenched strongly on Ti02,and its decay rate was increased remarkably due to electron transfer from the excited Ru(bpy)?+ to Ti02. The decay curve was nonexponential and was fitted well by using the sum of four exponentials. The time-resolved spectra of the faster decay components were shifted to shorter wavelength by about 500 cm-' compared to those of the slower ones. On introduction of water vapor into the system, the decay rate of the fastest component became slower, indicating that the electron-transfer interaction of the Ru complex interacting most strongly with Ti02 was weakened by the solvation effect of water. When the vapor pressure exceeded the saturated value, the blue shift of the faster decay components disappeared and the time-resolved spectra of the faster components became almost the same as those of the slower ones.
was purchased from Furuuchi Chemical Corp. and was used as received. The particles were allowed to stand in contact with a water solution of Ru(bpy),2+, where the ratio of Ti02 particle to Ru complex weights was controlled, and then dried by evacuation ( Torr) at 50 O C for a few days. The surface coverage was calculated approximately by assuming that Ru(bpy)$+ is a sphere with a radius of 5 A and is adsorbed uniformly on the Ti02surface. The dried surface modified particles were set in an optical cryostat (Oxford Instruments Ltd., type CF1104) at room temperature in vacuo (lo+ Torr). Water vapor was introduced into the cryostat from a glass vacuum line in which degassed liquid water was trapped. The pressure was controlled with a needle valve, and pressures higher than the saturated water vapor pressure were obtained by heating the liquid water trap. R ~ ( b p y ) adsorbed ~~+ on T i 0 2 in liquid water was prepared as follows. The Ru-
Introduction Electron-transfer processes between molecules in homogeneous solution have been widely studied, and remarkable advances have recently been achieved both theoretically' and experimentally.2 On the other hand, the electron transfer between molecules and semiconductors has been less well studied, although it is an essential process in photoelectrochemistry, photocatalytic reactions, photography, etc. To get information on its dynamic process, we have measured the decays and spectra of the luminescence from photoexcited semiconductor3 or photoexcited adsorbed We previously reported that the luminescence of Ru(bpy)$+ and its derivatives adsorbed on various oxide semiconductors is quenched by electron transfer from the excited state of Ru(bpy)? to Ti02 both in vacuo4 and in various solvent^,^ and the electron-transfer rate in solvents is much slower than in vacuo. In the present study, detailed analyses of decay curves of R ~ ( b p y ) , ~ + in vacuo and effects of water vapor on the luminescence decay curves and time-resolved spectra were studied.
(1) For example: (a) Kestner, N. R.; Logan, J.; Jortner, J. J. Phys. Chem. 1974, 78,2148. (b) Marcus, R. A.; Siders, P.Ibid. 1982,86,622. (c) Miller, J. R.; Beitz, J. V.; Huddleston, K. J. Am. Chem. SOC.1984, 106, 5057. (d) Kakitani, T.; Mataga, N. Chem. Phys. 1985, 93, 381, and references therein. (2) For example: (a) Miller, J. R.; Calcaterra, L. T.; Closs, G. L. J. Am. Chem. SOC.1984, 106, 3047. (b) Closs, G. L.; Calcaterra, L. T.; Green, N. J.; Penfield, K. W.; Miller, J. R. J. Phys. Chem. 1986,90, 3673, and references
Experimental Section R ~ ( b p y ) , ~was + purchased from Strem Chemicals Inc. and was sometimes purified by several recrystallizations from water, but no difference in the results was observed between purified and nonpurified samples. Ti02powder (rutile, ca.0.5 fim, ca. 10 m2/g)
therein. (3) Hiramoto, M.; Hashimoto, K.; Sakata, T. Chem. Phys. Lett. 1987,133, 440. (4) Kajiwara, T.; Hashimoto, K.; Kawai, T.; Sakata, T. J. Phys. Chem. 1982, 86, 4516. ( 5 ) (a) Takemura, H.; Saji, T.; Fujihira, M.; Aoyagui, S.; Hashimoto, K.; Sakata, T. Chem. Phys. Letf. 1985, 122, 496. (b) Hashimoto, K.; Hiramoto, M.; Sakata, T.; Muraki, H.; Takemura, H.; Fujihira, M. J . Phys. Chem. 1987, 91, 6198.
Institute for Molecular Science. *Toho University. f Deceased.
0022-3654188 , ,12092-4636$01.50/0 0 1988 American Chemical Societv I
-
Ru(bpy),2+ Adsorbed on Ti02
The Journal of Physical Chemistry, Vol. 92, No. 16. 1988 4631
r l
0
0.4
0.8
1.2
0
04
Time/ipsl
08
12
TI me/lps)
Figure 1. Bottom: Experimental luminescence decays of R ~ ( b p y ) ~ ~ * adsorbed on Ti02 in vacuo (points) and best-fit convolution with a sum of four exponentials (lines through experimental points). Top: distribution of weighted residuals for the best-fit convolution. (a) Observed at 580 nm. (b) Observed at 660 nm. TABLE I: Dependence of the Lifetimes (nanoseconds) and Relative Weights of Preexponential Factors (in Parentheses) of the Luminescence from Ru(bpy),*+ Adsorbed on TiOz on Observed Waveleneth
wavelength, nm 580 600 620 640 660
TI
1.8 2.1 2.0 2.1 2.0
(11)
(0.76) (0.70) (0.65) (0.62) (0.61)
7 2 (12)
36 32 30 30
(0.15) (0.20) (0.20) (0.20) 35 (0.20)
73
(4)
120 (0.06) 110 (0.06) 130 (0.11) 110 (0.12) 144 (0.12)
74
490 450 500 480 500
(14) (0.03)
(0.05) (0.05) (0.06)
(0.07)
(bpy),Z+/Ti02 powder was sandwiched between two glass plates and then deaerated liquid water was poured into the gap. The sample was then set in the nitrogen atmosphere. In this case, a very thin bulk water layer exists between the plates. A Molectron DL I1 dye laser pumped by a Molectron UV-22 N 2 laser or a Lambda Physik FL2002 dye laser pumped by a Lambda Physik 103 MSC excimer laser were used as excitation sources (455 nm, pulse width 6 ns). The energy of the exciting light was controlled to be less than 1 hJ/(cm2 pulse). The luminescence decay and time-resolved spectra were measured by using a photon-counting type of nanosecond time-resolved spectrometer at the Instrument Center of I M S 6 The spectrometer consists of a Nikon P-25 monochromator, a Biomation 6500 transient digitizer, a homemade discriminator, and an N E C 9801 personal computer. All the measurements were done at room temperature. The decay curve were analyzed by an interactive nonlinear least-squares deconvolution of the instrument response function. The time resolution of the system was about 2 ns. Luminescence spectra were measured on a Spex Fluorolog 2 spectrometer.
Results Luminescence in Vacuo. The luminescence of Ru(bpy),2+ adsorbed on TiOl powder in vacuo decayed nonexponentially and was dependent on observation wavelength. For example, decays observed at 580 and 660 nm are shown in Figure 1. The decay profile a t each wavelength hardly changed by controlling the surface coverage from on average monolayer (one tenth of monolayer. For the data reported here the a monolayer) to l/lw surface coverage was fixed to be 1/40 monolayer. Figure 2 represents the spectra gated at different times after excitation. The spectra are not corrected for the sensitivity of the detecting system. The spectra in the time domain of 0-100 ns shift to a shorter wavelength compared to those of the slower decay part. These data indicate that there are several types of excited-state Ru(bpy)?+ molecules with different natures on the T i 0 2 surface. When the decay curves were analyzed by using a sum of exponentials, all the decay curves observed at five different wavelengths, 580,600,620,640, and 660 nm, could be fitted by a sum of four exponentials in the time domain of about 3 orders of decay. The results are listed in Table I. The lines through points in Figure ( 6 ) Yamanaka, 1984, 137.
r.; Yoshida, H.; Yamazaki, Y. Annu. Rev. Inst. Mol. Sci.
550
600
650
Wavelength /(nrn) Figure 2. Time-resolved spectra of luminescence from Ru(bpy),*+ adsorbed on TiOz in vacuo. The spectra were not corrected for the sensitivity of the detecting system.
1 represent the simulated decay curves. The distributions of weighted residuals are also shown in the figure. In the literature, the maximum number of components that were determined from one decay curve is three: three lifetimes and three preexponentials.' In the present work, four lifetimes and four preexponential factors were determined. The procedures we have used are as follows. We measured decay curves with different time scales. In the case the time scale was sufficiently long enough, a single-exponential decay was observed in the slow decay time region. Thus the slowest decay component was determined first. Then the other components were successively determined by substracting the already determined components from the original decay curve. By this procedure, the number of components and rough values of their lifetimes and ratio of preexponential factors could be determined. When the number of decay components became three or four as in the present case, the decay curves with the longer time scale were divided into two parts, and the slower decay part was analyzed with two exponentials by a least-squares best-fit deconvolution method, using the initial values of lifetimes and ratios of preexponential factors that were determined above. The two exponentials determined in this way were then substracted from the fast decay part and also from the decay curve measured with the short time scale. Then the remaining decay curves were analyzed with one or two exponentials by the deconvolution method. In this way, the decay curves could be analyzed reproducibly with a very good fit as shown by solid line in Figure 1. Each lifetime determined by this method differs by a factor of between 4 and 15. The errors in lifetimes and preexponential factors were estimated to be less than f10% and f20%, respectively. One of the important results obtained from Table I is that the lifetimes of each decay component are almost the same within the experimental error regardless of the observed wavelength, and only the ratio of preexponential factors of each component varies. This result indicates that the fit with four exponentials is a rea(7) Kemnitz, K.; Tamai, N.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. J . Phys. Chem. 1987, 91, 1423.
4638 The Journal of Physical Chemistry, Vol. 92, No. 16, 1988
Hashimoto et al.
\
22.5 orr
550
0
550
600
650
50 100 Time/[nsl
Figure 3. Initial decays of luminescence from Ru(bpy),2t adsorbed on Ti02in the presence of water vapor. Observed at 580 nm. The excitation
pulse shape is also shown. TABLE II: Lifetimes (nanoseconds) and Relative Weights of Preexponential Factors (in Parentheses) of the Luminescence from Ru(bpy),*+ Adsorbed on TiO, in the Presence of Water Vapor Observed at 580 nm 10-5 1.4 5.0 10.4 17.5 22.5 in water'
650
I/
" t
pressure, Torr
600
7,
2.0 3.0 3.0 4.4 5.4
(I,)
7,
(I,)
(0.74) 36 (019) (0.67) 36 (0.18) (0.62) 37 (0.20) (0.53) 39 (0.30) (0.60) 39 (0.19) 22 (0.27) 32 (0.26)
T,
(I,)
130 (0.05) 120 (0.12) 130 (0.I4) 140 (0.13) 130 (0.17) 230 (0.38) 290 (0.17)
7 4 (14)
500 (0.02) 480 (0.04) 470 (0.04) 450 (0.05) 440 (0.04) 440 (0.35) 590 (0.57)
Ru(bpy),2t/Ti02 in deaerated water. See text, sonable approximation for the present decay curve analysis. From these observations it can be demonstrated that the blue shift observed at short times in the time-resolved spectra can definitely be ascribed to the fast decay component and hence shows the differences in the spectra of the decay components. Luminescence in the Presence of Water Vapor. It was found that the luminescence decay of Ru(bpy)3Z+adsorbed on TiOz changes when water vapor is introduced into the system. Even in the absence of water vapor, the decay profile was sensitive to the back pressure of the system and the evacuation time. After evacuation at below Torr for about 40 h at room temperature, the decay profile did not change further upon longer evacuation. Therefore, all the measurements were done after more than 40 h of evacuation. Figure 3 shows the initial decay curves observed at 580 nm in the presence of differing water vapor pressures. On increase of the pressure of water vapor, the luminescence decay became slower. Its effect was especially clear on the initial decay process. The results of curve analyses are listed in Table I1 and can be summarized as follows: (1) All the decay curves could be fitted with four exponentials except for those at 22.5 Torr and in water. (2) The fastest decay component was most greatly affected, its lifetime increased and the ratio of preexponential factor decreased. (3) The lifetimes of intermediate decay components were hardly affected, although the relative weights of their preexponential factors were increased. (4)The lifetimes of the slowest decay component decreased somewhat. When the pressure was higher than the saturated value (17.5 Torr at 20 "C),the decay profile changed drastically. For example, in the presence of 22.5 Torr of water vapor, the decay could be analyzed with a sum of three exponentials. Moreover, the lifetime of the fastest decay component became much longer, and its ratio decreased drastically. This decay profile was rather similar ~ ~TiOZ + in deaerated liquid water as shown to that of R ~ ( b p y ) on in the last line of the table. The effect of water vapor was observed clearly in the timeresolved luminescence spectra as shown in Figure 4. On increase of the vapor pressure, the spectra of the fast and slow decay components became a little broader and shifted to a shorter
Wavelength/ lnm)
Wavelength/l nml
Figure 4. Time-resolved spectra of luminescence from Ru(bpy),,+ adsorbed on TiO, in the presence of water vapor. Gated at the time range 0-20 ns (solid lines) and 5OC-1000 ns (dotted lines) after excitation. The spectra are not corrected for the sensitivity of the detecting system.
1 n
VI .-c C
?
e
.
9 0.5 >r .-VI Y
C aJ c C
-
0 1
*-rr*.
,.y
fl
n
VI c
.-c
3
e
.
/y 22.5
f l,$-
'.>,
'..*>, 'b
Torr
...h '
