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By contrast, in the optical-wavelength range, birefringence observed with the oriented bundle of fibers indicates an effective microordering. The color variation is symptomatic of a compatibility of the visible light wavelength range with a spatial periodicity in the structure of the oriented solid sample. Previous structural studies6J1J5have emphasized that chirality which is an intrinsic property of the steroid “monomers” is also present 5 nm in both the filament structure of the native gel (pitch (ref 6)) and the xerogel fibers (pitch 60 nm (ref 15)). In this situation, the transmitted light beam has crossed nematic layers of oriented chiral filaments which behave like a cholesteric-nematic system. The swelling experiment described using a dried oriented bundle of fibers shows that, in the absence of any orientational external
-
-
(19) Skoulios, A. Adu. Colloid Interface Sci. 1967, I , 79.
perturbation, a random gel network sets up where only occasional long-range orientation effects are observed in domains with typical optical textures. Magnetic field orientation experiments have underlined some analogies with polymeric systems. Complementary structural studies of orientation in these low molecular weight gelling compounds are now in progress using stronger magnetic fields but also mechanical shear stresses. Acknowledgment. We are grateful to Drs. F. Volino and R. Ramasseul for very helpful discussions. Dr. R. Ramasseul is particularly thanked for having provided us with the stqoid derivative. Mr. F. Tasset’s contribution to the experimental work is also acknowledged. We are grateful to Dr. B. Jacrot, Director of the EMBL Grenoble outstation, for the use of the X-ray facilities of his laboratory.
Temperature- Independent Electron Transfer: Rhodamine B/Oxide Semiconductor Dye-Sensitization System K. Hashimoto, M. Hiramoto, and T. Sakata* Institute for Molecular Science, Myodaiji, Okazaki 444, Japan (Received: April 11, 1988)
The fluorescence spectrum and decay of rhodamine B (Rh B) adsorbed on insulator (SO2) and oxide semiconductors (Zr02, TiOt (anatase)) were measured in vacuo at temperatures in the range 4-300 K. The effect of temperature on both the intensity and decay rates of the fluorescence is very weak, indicating that the electron transfer (ET) from Rh B in the excited state to those semiconductors (SC) is almost an activationless process. The result leads to the conclusion that continuous levels in the conduction band of the SC serve as the electron acceptor state.
Introduction Photoinduced electron-transfer (ET) process from an adsorbed molecule in the excited state to substrate semiconductors (SC) in vacuo has been studied.I4 The characteristics of the ET between adsorbate and solid may be quite different from that of the ET between molecules. When the solid serves as the electron acceptor, there are two possible candidates for the electron acceptor level. One is the conduction band composed of continuous energy levels, and the other is surface states with localized discrete levels. The ET rate dependence on substrate oxide SC for Ru(I1) complexes’ and Rh B has recently been r e p ~ r t e d .Based ~ on the results, it was suggested that continuous levels in the conduction band serve as the electron acceptor levels. In this paper studies are presented of the intensity and decay rate of the fluorescence from rhodamine B (Kh B) adsorbed on SC surfaces in vacuo over a wide temperature range, 4-300 K. The fluorescence measurement of such an adsorbate-substrate system in vacuo has advantages for studying the temperature dependence of the ET process because it is a solvent-free system and the distance between donor and acceptor is fixed. The fluorescence of Rh B adsorbed on Si02 (Rh B/Si02) as a reference in the absence of an ET process was also measured. The ET rate was found to be almost temperature independent. Experimental Section Commerical Rh B was recrystallized three times from water. The oxide powders were purchased from Kojundo Kagaku Corp. (1) Kajiwara, T.; Hashimoto, K.; Kawai, T.; Sakata, T. J . Phys. Chem. 1982,86,4516. (2) Hashimoto, K.; Hiramoto, M.; Lever, A. B. P.; Sakata, T. J . Phys. Chem. 1988, 92, 1016. (3) Hashimoto, K.; Hiramoto, M.; Kajiwara, T.; Sakata, T. J . Phys.
Chem., in press. (4) Hashimoto, K.; Hiramoto, M.; Sakata, T. Chem. Phys. Lett., in press.
(purity, 99.999%;particle diameter, ca. 1 km; surface area, 5-10 m2/g) and used without further treatment. Rh B was adsorbed on the powders by the following procedure: 1 g of the powder was suspended in 10 mL of lo-’ M aqueous solution of Rh B for about 20 min; then it was filtered. The powder was dried by evacuation Torr) at 50 OC for at least 24 h. The surface coverage estimated was less than ca. 1/10000. Here the radius of the Rh B molecule was assumed to be 8 A and all the molecules in solution were adsorbed uniformly on the surface. The samples sat in an optical cryostat (Oxford Instruments Ltd, C F 1104) and were evacuated to ca. lod Torr. Fluorescence spectra were recorded on a Spex Fluorolog 2 spectrometer. Decay curves were measured with several time scales (5,25, and 50 ps/channel; 1024 channels) by using picosecond photon-counting apparatus in the instrument center of IMS (excitation wavelength, 548 nm; observation, 600 nm time resolution, ca. 10 P S ) . ~We analyzed the decay curves by the least-squares autofitting method reported previ~usly.~
Results and Discussion Fluorescence Spectra. The fluorescence spectra of Rh B adsorbed are almost the same regardless of the substrates studied here. It is reported that the peak wavelength shifts to longer wavelength as the surface coverage increase^.^^^ This is explained by a mixing of a dimer emission6and/or by a fluorescence spectral change induced by an interaction between an excited Rh B monomer and neighboring monomer^.^ The peak wavelength observed in the present study was 570 (&2) nm, which is coincident with the literature value of the peak of monomer Rh B fluorescence ( 5 ) Murao, T.; Yamazaki, I.; Yoshihara, K. Appl. Opt. 1982, 21, 2297. (6) Kemnitz, K.; Tamai, N.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. J . Phys. Chem. 1986, 90, 5094; 1987, 91, 1423. (7) Ito, K.; Chiyokawa, Y.; N a h o , M.; Honda, K. J . Am. Chem. SOC. 1984, 106, 1620.
0022-3654/88/2092-4272$01.50/0 0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 15, 1988 4213
Letters -2
L
Rhodamns B (anat&
TABLE I: Lifetimes (ps), heexponential Factors, and Integrated Intensities of the Fluorescence of Rhodamine B Adsorbed on Metal Oxide in Vacuo at Various Temperatures (K)
-Eox. RedOx pimila
w
lifetime" (weight)b
-E,
temp
71
(11)
72
(12)
intensitycsd
Si02
>
Figure 1. Energy level diagram of semiconductors (at p H 0) and Rh B in water. 10,
1
300 200 125 90 70 60 50 10 4
3700 (0.66) 3900 (0.57) 4000 (0.59) 4000 (0.61) 4100 (0.63) 4000 (0.63) 4100 (0.64) 4200 (0.66) 4200 (0.65)
7600 (0.34) 7700 (0.43) 7800 (0.41) 7800 (0.39) 7900 (0.37) 7900 (0.37) 7900 (0.36) 7900 (0.34) 7900 (0.35)
300 200 125 90 70 60 50 4
1500 (0.67) 1600 (0.67) 1700 (0.67) 1700 (0.70) 1700 (0.69) 1600 (0.64) 1700 (0.67) 1700 (0.68)
3900 (0.33) 4200 (0.33) 4300 (0.33) 4500 (0.30) 4500 (0.31) 4200 (0.36) 4300 (0.33) 4400 (0.32)
300 200 125
210 (0.90) 230 (0.90) 240 (0.89) 250 (0.90) 250 (0.90) 250 (0.89) 250 (0.90) 250 (0.91) 250 (0.91)
Ti02 880 (0.10) 1000 (0.10) 1000 (0.11) 1200 (0.10) 1100 (0.10) 1200 (0.11) 1100 (0.10) 1100 (0.09) 1200 (0.09)
1o o c 99 90 90 89 88 86
88
Zr02
1 550
600
650
WAVELENGTH/ nm
Figure 2. Fluorescence spectra of rhodamine B adsorbed on Zr02 in vacuo. Excitation wavelength, 524 nm.
a t infinite dilution of the surface concentration on quartz and SnOz-coated glass plates.' The intensity, however, depended strongly on the substrates. The intensities for Rh B/Ti02 and for Rh B / Z r 0 2 are ca. 1/20 and 1 / 2 of that for Rh B/Si02, respectively.E The formation of an oxidized xanthene dye on these oxide SC in vacuo is observed by time-resolved resonance Raman scattering? It is considered that the fluorescence quenching on the SC is caused mainly by E T from Rh B in the excited state to the ~ u b s t r a t e . ~ - ~ - 'In J~ Figure 1 are shown energy levels of Rh B and the SC used here. Because the data on the energy levels in vacuo are not available, we used the electrochemical data at pH 0 in water as an approximation. The redox potential of Rh B in the excited state (Eo,*) was estimated by using the values of the absorption edge and the ground-state oxidation potential (E,,)." We defined the energy gap, AE,by the difference of the Eo,* and the flat-band potential of the SC.12 The ET quenching proceeds efficiently for a SC with a larger AE.2,4 By decreasing the temperature, both the peak intensity and width of the spectra changed slightly. However, the peak wavelength did not change. The spectra of Rh B / Z r 0 2 at 300 and 4 K are shown in Figure 2. Therefore, we can compare the fluorescence quantum yield by the integrated intensity in the wavelength region 550-650 nm. The results are summarized in Table I. Here the integrated intensity for Rh B/SiOZ at 300 K is normalized to 100. The experimental error is estimated to be (8) In the present study powders were used as a substrate, so we could not determine the absolute concentration of Rh B. Therefore we corrected the relative integrated intensities (I) by using the fluorescence decay data at 300 K as follows.
Here P and T are the preexponential factors and the lifetime, respectively. The subscripts sc and i represent SC and insulator, respectively. The subscript j = 1,2 represents the fast 0' = 1) and slow 0' = 2) components of the fluorescence decay. (9) Hashimoto, K.;Carrolle, P.J.; Brus, L. E., manuscript in preparation. (10) The observation of oxidized dye does not completely exclude the possibility of some contribution of energy transfer in the fluorescence quenching involving surface states. However, the fluorescence decay rate of Rh B is related to the energy of conduction band of semiconductor: indicating that the main quenching process is the ET process. (1 1) Pettinger, B.; Schappel, H.-R.; Gerischer, H. Ber. Bunsen-Ces. Phys. Chem. 1974, 78, 450. (12) Scaife, D. E. Sol. Energy 1980, 25, 41.
90 70 60 50 10 4
49 45 45 46 45 44 44 49
5.6 5.5 5.8 5.6 5.7 5.8 6.0 6.8
" Experimental error ca. 5%. Experimental error ca. 10%. cExperimental error ca. 10%. "Integrated intensity of the spectrum from 550 to 650 nm. CIntensity on S O 2 at 300 K was normalized to 100. 'Relative intensities on ZrOl and Ti02 were calculated by using the decay data at 300 K (see ref 8). about 10%. The variation in the integrated intensity is very small, less than 20% over the wide (4-300 K) range of temperature. Luminescence Decay. All the fluorescence decay curves observed were nonexponential. The surface coverage was extremely low in the present system and the fluorescence decay profile did not depend on the surface concentration in such a low-concentration Moreover, the spectral peak is coincident with that in an infinite diluted case as described in the previous section. These results indicate that the nonexponential behavior is caused not by the energy transfer among Rh B molecules on the surface but by the existence of several kinds of adsorption sites on the s ~ r f a c e . ~Consequently .~ we analyzed the decay curves by multiexponential All the decay curves observed could be fitted with a sum of two exponentials over all the temperature region studied.13 The residuals, x2,are ca. 0.99-1.02, 1.1-1.2, and 1.5-1.6 for R h B/SiOZ, Rh B/ZrOz, and Rh B/Ti02, re~ p e c t i v e l y . ~The ~ lifetimes and the preexponential factors of the two decay components are summarized in Table I. The experimental errors of lifetimes and preexponential factors were estimated to be less than 5% and lo%, respectively. Such a double-exponential approximation applied well to the case of Rh B adsorbed on various oxide SC substrates. The lifetimes and the preexponential factors of the two components varied depending on the substrates4 It appears that the Rh B molecules adsorbed (13) For SC substrates, we observed a rise of the fluorescence very clearly which did not exist for insulator substrates at all. One explanation is that the rise is related to the energy-transfer process from the excited surface states to adsorbed dye. The other is that adsorbed molecules, besides being coupled to conduction electrons, are strongly coupled to localized surface electrons, which is responsible for the rise in some way. Anyhow, at the present stage the origin of the rise is not clear. (14) Because of the rise of the fluorescencedescribed above, the residuals for SC substrates are larger than that for Si02 substrate.
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The Journal of Physical Chemistry, Vol. 92, No. 15, 1988
l # y A y TEMPERATURE1K
Letters
can be calculated as k,, = 1 IT, - 1IT,.Since there are two decay were calculated components, two different ET rates (k,,' and ,= ~ 1, on each SC by using the equation kctJ = 1 / r S c J- 1 / ~(j 2). Here j = 1 and 2 represent the fast and slow decay components, respectively. The ET rates estimated are shown in Figure 3a versus the reciprocal of temperature. Relative values of the ET rate (k,J can be estimated from the quantum yields of fluorescence on the SC (& = k , / ( k , k,, k,J) and on the insulator (a, = k , / ( k , knf)). The integrated intensity of the fluorescence spectrum (ZBc for SC and Z, for intulator) in Table I is proportional to the quantum yield. Then ket can be ca_lculated as k,, = l/Zsc - l/Zi. Here it is important to note that k,, is proportional to the auerage value of the ET rates of the fast and slow decay components. The estimated k, is shown in Figure 3b, in which k, for Rh B/Zr02 at 300 K is normalized to unity. The ET rates estimated from pulsed light experiments and steady-state light experiments show essentially the same temperature dependence. The rates are almost constant with temperature. At high temperatures, they increase slightly. The decrease of the rate from 300 to 4 K is less than 20%. In other words, the E T from photoexcited Rh B to both Z r 0 2 and T i 0 2 is almost an activationless process. It should be noted again that the difference of the conduction band edge of Z r 0 2 and TiOz is ca. 1 eV. Therefore, if discrete surface levels, existing between valence and conduction bands, serve as electron acceptor levels, the standard free energy change (AGO) of the ET reaction for Rh B / Z r 0 2 may differ very much from that for Rh B/Ti02. This means that the temperature dependence of the ET rate should differ depending on the kind of SC. However, the experimental results show that the temperature dependence of the ET rates from Rh B to Z r 0 2 is almost the same as that to Ti02, although the magnitude of k,, is quite different. This suggests that the electron acceptor levels are the continuous levels of the conduction band in both cases. This conclusion is consistent with that obtained from the ET rate energy-gap d e p e n d e n ~ e . ~ , ~ The E T rate can be expressed by two terms. One is an electron-exchange matrix element, and the other is a Franck-Condon term.I5 The latter is responsible for the temperature dependence. According to the conventional ET which describes ET between discretdiscrete levels, a negative activation energy might appear when -AGO is equal to the reorganization energy.I7 Therefore, almost activationless ET in the present system cannot be explained with the discrete-discrete E T theory. The energy level density distribution of the SC must be taken into account to explain the temperature dependence.I8 Future work will attempt to explain the temperature dependence quantitatively.
+
ld 0
5
10
15
20
100 250
1000/ T TEMPERATURE 1 K
300
100
50
4
i
on Z r Q
0
5
10
15
20
250
10001 T
Figure 3. (a, top) Temperature dependence of the electron-transfer rates determined from the fluorescence decay rates. (b, bottom) Temperature dependence of the relative electron-transfer rates determined from the fluorescence integrated intensity. The relative ET rate of Rh B/Zr02 at 300 K is normalized to unity.
on those substrates can be classified into roughly two group^.^ Mainly the ET process shortens the lifetimes on SC as was described previously.I0 From the table, it is seen that the preexponential factors do not change within experimental error, in the present temperature range. The lifetimes also do not change much, but at high temperatures they shorten slightly. This change is outside the experimental error. Temperature Dependence of the Electron-Tramfer Rate. From the fluorescence decay rates, the E T rate can be estimated under the assumption that the contribution of the energy transfer to the surface state is small in the lifetime shortening on SC.14,'o The decay rate on SC (1/7=) is a sum of the ET rate (k,J and the other relaxation rates (radiative (k,) and nonradiative (k,J processes). On the other hand, the decay rate on insulators (1 /q) is thought to be a sum of k, and k,. Because q is almost constant regardless of the i n ~ u l a t o r , " we ~ ~ ,may ~ use the values of k , and k,, on S O 2as those on SC with a good approximation. Then k,,
+ +
Acknowledgment. We thank Dr. N. Tamai for technical assistance in the decay measurements. We also thank Dr. G. Black for careful reading of the manuscript. (15) For example: Marcus, R. A. Annu. Rev. Phys. Chem. 1964,15, 155. (16) Jortner, J. J . G e m . Phys. 1976, 64, 4860. (17) Sarai, A. Chem. Phys. Lett. 1979,63, 360. (18) Sakata, T.; Hiramoto, M.; Hashimoto, K. Proceedings of a Symposium on Photoelectrochemistry and Electrosynthesis on Semiconducting Materials; Electrochemical Society, in press.
