J. Phys. Chem. 1985,89, 1945-1947 their consequences for solar energy conversion is being carried out in this laboratory.
Acknowledgment. This research was supported by the Israel National Research Council, The Balfour Foundation, and the
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Schrieber Hebrew University Center for Hydrogen. Registry No. Po-XV, 31531-96-1; Pm-XV, 32218-99-8; PPrV, 69860-51-1; PBuV, 37584-31-9; P2,4-V, 95864-72-5; P2,4-MeV, 95864-73-6; Ru(bpy),(CN),, 20506-36-9; Ru(bpy),*+, 15158-62-0; Fe(CN);, 13408-62-3.
Correct Assignment of the Low-Temperature Lumlnescence from 9-Nltroanthracene Satoshi Hirayama,* Faculty of Textile Science, Kyoto Technical University, Matsugasaki, Sakyo-ku, Kyoto 606, Japan
Ybichi Kajiwara, Toshihiro Nakayama, Kumao Hammoue,* and Hiroshi Teranishi Department of Chemistry, Faculty of Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo- ku. Kyoto 606, Japan (Received: May 14, 1984; In Final Form: December 26, 1984)
9-Nitroanthracene (9-NA) is found to exhibit low-temperature luminescencewith band maxima at 685 and 760 nm in EPA (ether/isopentane/ethanol = 5 5 2 in volume ratio) at 77 K. This luminescence is assigned as the intrinsic phosphorescence of 9-NA on the basis of the spectral shape of the luminescence, the excitation spectrum, the emission lifetime, and the photochemical reactivity of 9-NA. Thus the previous assertion made by Snyder and Testa that 9-NA does not phosphoresce is refuted (Snyder, R.; Testa, A. C. J . Phys. Chem. 1981, 85, 1871).
Introduction The nitro group is well-known as a fluorescence quenching substituent.’ For instance, fluorescence in 9-nitroanthracene (9-NA) is totally absent even at 77 K. In this molecule, the nitro group appears to enhance singlet to triplet intersystem crossing significanty by providing a triplet n?r* intermediate state as has previously been proposed by us.* Recently, Snyder and Testa3 disputed our proposal, since they were unable to observe any phosphorescence or triplet state for 9-NA which should be generated as a result of such an efficient intersystem crossing from the lowest excited singlet TA* to higher triplet n?r* states. From their study on the photolysis of 9-NA at 77 K, they were led to the conclusion that we did not measure the T’ TI absorption and lifetime inherent to 9-NA but rather those of anthraquinone formed via an efficient photochemical reaction. If their conclusion is correct, a photochemical reaction must be responsible for the lack of fluorescence of 9-NA. Unfortunately, however, their conclusion is based on insufficient experimental results and erroneous interpretation. Therefore, it is the purpose of the present work to show that 9-NA itself actually phosphoresces in the same wavelength region in which the phosphorescence of anthracene is observed.
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Experimental Section Materials. 9-NA was synthesized according to the literature4 and purified by recrystallization from ethanol. When necessary, it was further purified by sublimation or thin-layer chromatography. Anthracene of a scintillation grade was used as received. Isopentane of a guaranteed grade was purified by passing through a column of silica gel (200 mesh). Spectroscopic grade ethanol, diethyl ether, and methylcyclohexane were used as received. (1) Wehry, E. L.; Rogers, L. B. ‘Fluorescence and Phosphoresccnce Analysis”; Hercules, D. M., Ed.; University of Tokyo Press: Tokyo, 1966; p 89. (2) Hamanoue, K.; Hirayama, S.;Nakayama, T.; Teranishi, H. J . Phys. Chem. 1980.84, 2074. ( 3 ) Synder, R.;Testa, A. C. J . Phys. Chem. 1981, 85, 1871. (4) Braun, C. E.; Cook, C. D.; Merritt, C. Jr.; Roasseau, J. E. Org. Synth. 1951, 31, 17.
0022-3654/85/2089-1945$01 .50/0
Apparatus and Procedures. Irradiations were carried out at 77 K using an USH-SOOD super-high-pressure mercury lamp. Light of 366-nm monochromatic wavelength was selected by the combination of Toshiba UV-35 and UV-D35 glass filters and a filter solution (CuSO4.5Hz0, 100 g dm-’, pathlength 3 cm). The absorption spectrum was taken at 77 K with a Hitachi 200-20 spectrometer. Phosphorescence spectra were taken on either a Hitachi M P F 4 spectrophosphorometer or a home-built phosphorometer which consists of a Ritsu N20 monochromator with a Toshiba Y-46 glass filter to cut off the light below 460 nm, a rotating sector, a transparent Dewar vessel, a 500-W high-pressure mercury arc with a Toshiba UV-DIB glass filter to isolate the 366-nm light, and an N F Model LI-574A lock-in amplifier. In either case, an HTV R928 photomultiplier whose spectral response well extends to 900 nm was employed. The excitation spectra of the phosphorescence were measured m the M P F 4 spectrophosphorometer. The phosphorescence and excitation spectra were not corrected for the spectral response of the equipment. Both commercial and home-built spectrophosphorometers gave almost the same phosphorescence spectra, but because of the higher exciting light intensity, weak phosphorescences could be taken on the latter with less noise. Thus, the phosphorescence lifetime was measured on the latter. The procedure to measure the phosphorescencelifetime was the same as that previously r e p ~ r t e d . ~ The sample solutions were degassed by several freeze-pumpthaw cycles.
Results When weak luminescence is measured, great care must be taken if highly luminescent photoproducts are produced. This is true of 9-NA which readily yields anthraquinone upon irradiation. In fact, Synder and Testa’ observed that the phospohrescence intensity due to anthraquinone increased during irradiation of 9-NA in ethanol at 77 K, but they failed to observe any luminescence inherent to 9-NA. We obtained the same result during the repeated spectral measurements in aerated ethanol and EPA (ether/isopentane/ethanol = 5:5:2 in volume ratio) at 77 K. On ( 5 ) Hirayama, S . J. Chem. SOC.,Faraday Trans. 1 , 1982, 78, 2411.
0 1985 American Chemical Society
1946 The Journal of Physical Chemistry, Vol. 89, No. 10, 1985
Hirayama et al.
ti
1
Figure 1. Absorption spectral change of 9-NA (1.2 X loJ M) upon 366-nm photolysis in EPA at 77 K: (a) initial absorption spectrum of 9-NA; (b) after 2-h photolysis.
.
303
__ Loo Mbidergtt- / nm
350
.4%
Figure 3. Phosphorescence excitation spectra of 9-NA (ca. 1.4 X lo4 M) in EPA at 77 K, monitored at 685 (broken line) and 760 nm (solid
line). The phosphorescenceexcitation spectrum of anthraquinone (dotted line) monitored at 685 nm is also given. TABLE I: Comparison of the Triplet-StateProperties of 9-NA and Anthraquinone
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T' TI absorption band max, lifetime, nm ms
8
C I
9-NA anthraquinone a
Wavelength I nm
Figure 2. Phosphorescence spectra (I) and phosphorescence decay curve (1'1) of 9-NA (ca. 1.4 X lo4 M)in EPA at 77 K, (a), (b), and (c), taken at the first, second, and fourth scan, respectively. The phosphorescence spectrum of anthracene (d) is also shown for the sake of comparison.
the other hand, in the deaerated system, the phosphorescence intensity due to anthraquinone did not show any increment corresponding to the disappearance of 9-NA during the above spectral measurements, unless the solvent glasses were softened by raising the temperature. The recooled glasses exhibited a very strong phosphorescence from anthraquinone. In accordance with these findings, we observed that a new absorption, which is given in Figure 1, builds up upon the 366-nm steady-state photolysis of 9-NA in EPA at 77 K. Since this spectrum is very similar to that of the 9-benzoyl- 10-anthryloxy radical which was produced upon the photolysis of 9-benzoyl10-nitroanthracene in poly(methy1 methacrylate) at room temperature6 or EPA at 77 K,' it is reasonable to assign the spectrum to that of the 9-anthryloxy radical produced from 9-NA. These studies indicate that anthraquinone may be present as an impurity and that the primary photoproduct is not anthraquinone but the 9-anthryloxy radical which gives anthraquinone as the major poduct in the presence of oxygen. All the results obtained so far are consistent with the findings of Chapman et al.* who observed that the photolysis of 9-NA in deaerated acetone at room temperature gave 10,lO'-bianthrone, dnthraquinone, and anthraquinone monoxime. In the presence of oxygen, anthraquinone was the major photoproduct. On the basis of these results, they proposed that photolysis of 9-NA gave I ise to the nitro to nitrite rearrangement, followed by cleavage of 9-anthryl nitrite to the 9-anthryloxy radical and nitrogen(I1) uxide. The rather complicated photochemistry of 9-NA may ~eadilylead one to an erroneous conclusion with respect to the luminescent property of 9-NA. By employing a red-sensitive R928 photomultiplier, we were able to observe the phosphorescence from 9-NA not only in EPA and ethanol but also in nonpolar methylcyclohexane at 77 K. The ~
~
~~
~~~
(6) Hamanoue, K.; Hirayama, S.;Hidaka, T.; Ohya, H.; Nakayama, T.; 'reranishi, H. Polym. Photochem. 1981, I , 57. (7) Hamanoue, K.; Amano, M.; Kimoto, M.; Kajiwara, Y.; Nakayama, 'T.;Teranishi, H. J. Am. Chem. SOC.1984, 106, 5993. (8) Chapman, 0. L.; Heckert, D. C.; Reasoner, J. W.; Thackerberry, S. P.J . Am. Chem. Soe. 1960, 88, 5550.
430" 365'
17.7"
3.4b
phosphorescence lifetime, bandmax.nm ms 685, 760
14.3 f 1.3 490 (strongest)b 3.2'
Reference 2. Reference 1 1.
observed spectra in EPA at 77 K are displayed in Figure 2 (I) together with that of anthracene which shows a reasonably good agreement with the published s p e c t r ~ m ~(the - ' ~ emission was measured with a band-pass of 4 nm by exciting at 366 nm). As described before, 9-NA undergoes photochemical reaction(s) even in frozen matrixes at 77 K, so that the phosphorescence intensity decreased gradually during the spectral measurements. This is indicated by spectra a, b, and c which were taken at the first, second, and fourth scan, respectively. Each scan took 15 min, and such a decrease in the intensity of the phosphorescence was noted irrespective of whether the solution was degassed. Compared with anthracene (spectrum d in Figure 2 (I)), the phosphorescence spectrum of 9-NA is less structured, red-shifted by 15 nm, and only about one-half as intense. In 9-NA, the first three clearly resolved peaks of anthracene fuse into a single broad band. (The phosphorescence spectra for other anthracene derivatives, including various carbonyl substituted ones,have already been noted to look very similar to that of a n t h r a ~ e n e . ~Such ,~) an unresolved band is also observed for the phosphorescence We therefore conspectrum of 9-ben~oyl-lO-nitroanthracene.~~~ clude that this broad band is characteristic of the phosphorescence spectrum of 9-NA. The phosphorescence excitation spectra of 9-NA are shown in Figure 3. The emissions were monitored at 685 (broken line) and 760 nm (solid line) with a band-pass of 4 nm. These two spectra differ significantly in intensity at around 330 nm. By comparison with the excitation spectrum of anthraquinone monitored at 685 nm (dotted line), the band peak at 330 nm in the broken line is ascribed to anthraquinone which may be present as an impurity or a photoproduct. When the emission was monitored at 685 nm, it was difficult to eliminate completely the phosphorescence from anthraquinone because the phosphorescence from 9-NA was very weak. However, in the excitation spectrum observed at 760 nm where the phosphorescence of anthraquinone no longer interferes, the emission intensity at 330 nm decreased, and hence the excitation spectrum looks more like the absorption spectrum of 9-NA whose main peaks are located at 333,349, 367, and 387 nm in EPA at 77 K (cf. Figure 1). For anthracene, the observed excitation spectrum of phosphorescence is highly structured and agrees very well with that of the fluorescence at 77 K. These serve to confirm that the observed low-temperature luminescences under the present experimental conditions are (9) Padhye, M.R.; McGlynn, S. P.; Kasha, M. J . Chem. Phys. 1956, 24, 588. (10) Ferguson, J.; Mau, A. W.-H. Mol. Phys. 1974, 28, 469.
J. Phys. Chem. 1985,89, 1947-1954 undoubtedly due to the phosphorescences from 9-NA and anthracene. The phosphorescence decay curve of 9-NA was monitored at 685 nm (Figure 2 (11)), and the lifetime was solvent insensitive. (Observation at 760 nm did not give any better decay curve because of the much weaker intensity at this wavelength.) In Table I, the lifetimes of the phosphorescence and the T’ TI absorptionz of 9-NA are listed together with those of anthraquinone.” For 9-NA the lifetime of phosphorescence is slightly shorter than that TI absorption. This may be due to the fact that the of T’ phosphorescence measured a t 685 nm with a wide band-pass includes some emission from anthraquinone, as is obvious from the excitation spectrum (broken line shown in Figure 3). On account of the short triplet lifetime of anthraquinone (3.2 ms), the apparent lifetime may become shorter than the true phosphorescence lifetime of 9-NA. Unfortunately, the S / N ratio was too poor to resolve the observed decay curve into two components, Le., the true triplet lifetimes cf 9-NA and anthraquinone.
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Discussion It is well established that the phosphorescences in anthracenes are observed in the red-wavelength region (- 14 700 cm-I) and that the energy level of the lowest triplet state is hardly influenced by substitution.lZ As has previously been reported by one of us (S.H.)? this is true even for such a substituent as a carbonyl group, which is expected to interact strongly with the anthracene moiety. Therefore, it is most reasonable to expect that 9-NA also exhibits phosphorescence in the same wavelength region as does anthracene. In fact, low-temperature luminescence from 9-NA was observed in the expected wavelength region. The assignment of this luminescence as phosphorescence from 9-NA is based on the following facts: (1) the excitation spectrum agrees well with the (1 1) Hamanoue, K.; Kajiwara, Y.; Miyake, T.; Nakayama, T.; Hirase, S.; Teranishi, H. Chem. Phys. Lett. 1983, 94, 276. (12) McGlynn, S.P.; Azumi, T.; Kinoshita, M. “Molecular Spectroscopy of the Triplet State”; Prentice-Hall: Englewocd Cliffs, NJ, 1969; p 286.
1947
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absorption spectrum of 9-NA observed at 77 K (2) the lifetimes determined from the decays of the phosphorescence and T’ TI absorption are in reasonable agreement; (3) upon continuous irradiation, the phosphorescence intensity decreases owing to the photochemical consumption of 9-NA, which rules out the possibility of the phosphorescence being due to photoproducts; and (4) the phosphorescence spectral shape of 9-NA is characteristic of substituted anthracenes, Le., the first three peaks are fused into a single broad band. Thus, it is unlikely that the observed phosphorescence is due to some anthracene derivative which might happen to be present as an impurity. Taking these things into account, it is very surprising to note that, in their search for the phosphorescence from 9-NA, Snyder and Testa3J3used an IP 21 photomultiplier whose spectral response suddenly drops above 650 nm and hence hardly spans the expected phosphorescence spectral range. Undoubtedly, therefore, their conclusion that no phosphorescence in 9-NA is observable is based on the insensitivity of their detection system and their failure to examine the proper spectral region. In addition, their assertion that the transient absorption assigned to the T’ TI absorption of 9-NA in our previous paperZ must be due to that of anthraquinone is utterly unfounded. While the lowest triplet state of 9-NA is expected to be of AT* character, the lowest triplet state of anthraquinone is of n r * character, and hence its lifetime cannot be as long as 17.7 ms. Actually, the measured phosphorescence lifetime of anthraquinone is only 3.2 ms, and the band maximum TI absorption of 9-NA is located at considerably of the T’ longer wavelength than that of anthraquinone, as shown in Table
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I. In conclusion, genuine phosphorescence of 9-NA has been observed and its lifetime has been measured. Previously reported failures to detect this luminescence are attributable to the use of an inappropriate photomultiplier and a failure to search in the proper spectral region. Registry No. 9-NA, 602-60-8. (13) Snyder, R.; Testa, A. C. J. Phys. Chem. 1979, 83, 3041.
Electrochemical Investigation of Photocatalysis at CdS Suspensions in the Presence of Methylviologen James R. White and Allen J. Bard* Department of Chemistry, The University of Texas, Austin, Texas 78712 (Received: June 7. 1984)
Addition of electron acceptors, such as methylviologen (MVz+), :suited in enhanced photocurrents in photoelectrochemical (PEC) cells containing CdS powder suspensions. The variation of steady-state photocurrents with pH allowed the determination of band positions in aqueous electrolytes. The quasi-Fermi level for electrons (pH 14) was found to be at -0.81, -0.76, and -1.01 V vs. SCE for suspensions of CdS, CdS/Ru02 (5%), and CdS/Pt (5%), respectively. Photocoulometric experiments, in which the total photogenerated charge (Le., integrated photocurrent) was used to determine stability of semiconductor particles, are also reported. Turnover numbers for the CdS/catalyst suspensions were near 1 when sacrificial donors were not present; Le., they are only stable toward decomposition in the presence of substances such as tartrate.
Introduction The use of CdS colloids and suspensions as catalysts in several photosynthetic and photocatalytic reactions has been studied by a number of CdS is particularly interesting because (1) Frank, S. N.; Bard, A. J. J . Phys. Chem. 1977, 81, 1484. (2) Spikes, J. Phorochem. Phorobiol. 1981, 34, 549. (3) Darwent, J. R.; Porter, G. J . Chem. SOC.1981, 4, 145. (4) (a) Kalyanasundaram, K.; Borgarello, F.; Grltzel, M. Helu. Chim. Acro 1981,64, 362. (b) Kalyanasundaram, K.; Borgarello, E.; Duonghong, D.; Grltzel, M. Angew. Chem., In?. Ed. Engl. 1981, 20, 987. (c) Borgarello, E.; Erbs, W.; Grltzel, M. Noun J. Chim. 1983, 7, 195. (5) (a) Harbour, J.; Hair, M. J. Phys. Chem. 1977,81, 1791. (b) Harbour, J.; Wolhow, R.; Hair, M. Ibid. 1981, 85, 4026.
its band gap (2.4 eV) is such that it absorbs an appreciable fraction of light in the visible region. For photochemical applications of CdS, information about the particle energetics (Le., the band positions) for CdS in contact with a solution and about the stability of the catalysts under long-term irradiation is very useful. A (6) Matsumara, M.; Hiramoto, M.; Iehara, T.; Tsubomura, H. J . Phys. Chem. 1984,88, 248. (7) Rossetti, R.; Brus, L. J . Phys. Chem. 1982, 86, 4470. (8) Kuczynski, J.; Thomas, J. K. J . Phys. Chem. 1983, 87, 5498. (9) Meissner, D.; Memming, R.; Kastening, B. Chem. Phys. Lett. 1983, 96, 34. (10) (a) Henglein, A. Ber. Bunsenges. Phys. Chem. 1982, 86, 241. (b) Henglein, A. J . Phys. Chem. 1981, 86, 2291.
0022-3654/85/2089-1947$01.50/00 1985 American Chemical Society
