J. Phys. Chem. 1982, 86, 4733-4737
strates. It can be concluded from Figure 4 that diffusion of adsorbed l19Sb5+into the second and deeper layers of a-Fe203requires 2-h heating at about 750 "C. By means of the conventional analyzing procedure49for one-dimensional bulk diffusion from a surface, the diffusion constant of l19Sb5+in the surface layers of a-Fe203is estimated to cm2 s-l at 750 "C, whereas that of be of the order of Fe3+in bulk a-Fez0350extrapolated to 750 "C is 5 X cm2s-l. Thus, it seems that the diffusion of %b5+ in the surface layers of a-Fe203along the normal direction is considerably faster than that of Fe3+in bulk crystals of the oxide. Presumably, this difference is due to very defective structure of the surface layers. Comparison of Figures 4 and 9 indicates that the diffusion of l19Sb5+into the bulk matrix of Cr203is considerably more difficult than that into the bulk of a-Fe203. In Cr203, the weight of region I11 never arrives at 100% and heat treatments at higher temperatures induce coagulation of diamagnetic impurites including ll9Sb. These experimental results indicate that the replacement of M3+ ions in the lattice by llsSb5+ions is much easier in a-Fe203 than that in Cr203,probably because the reduction of nearby M3+ions to M2+is much easier in the former matrix. The size of the ions is considered to be not essential (49) C.D.Thurmond in "Methods of Experimental Physics", Vol. 6, Part A, L. Marton, Ed., Academic Press, New York, 1959, p 39. (50) R.Lindner, Z.Naturforsch. A , 10, 1027 (1955).
4733
in the present case of a-FezO3 and Cr203,since the ionic radii of Sb5+,Fe3+,and Cr3+happen to be practically the same. Summary and Conclusion By emission Mossbauer measurement of supertransferred hyperfine (STHF) magnetic fields on l19Sn4+ions produced by the EC decay of l19Sb5+,it was shown that carrier-free pentavalent lI9Sbions hydrolytically adsorbed on surfaces of antiferromagnetic a-Fe203and Cr2O3 from aqueous solutions are predominantly bound with the metal ions of the substrates by the M3+-02--Sb5+bonds (M = Fe or Cr). It was also shown that there exist no "nonmagnetic layers" on the surfaces of the oxides. From variation of the hyperfine fields by heat treatment of the a-Fe203-119Sb5+ specimens, the diffusion rate of Sb5+ions into a few surface layers of the substrate was estimated with the aid of a simplified surface model. Emission Mossbauer spectroscopy of STHF interactions has thus been shown to be a valuable technique in studying the chemical state of very dilute diamagnetic metal ions adsorbed on surfaces of magnetic oxides. It also provides useful information on the magnetic state of the surfaces as well as on diffusion of adsorbed ions into a few layers of the surfaces. Acknowledgment. Cooperation of the staff of the RIKEN cyclotron in many irradiation runs is gratefully acknowledged.
Relaxation Mechanism of Excited Acridine in Methyl Methacrylate and Poly(methy1 methacrylate) Kunlhlko Kasama, Kolchl Klkuchl, Kojl Ujl-Ie, Sada-Akl Yamamoto, and Hlroshl Kokubun Department of Chemistty, Facuw of Science, Tohoku University, Aoba, Aramaki, Sendal980, Japan (Received: June 17, 1982)
Electronic relaxation processes of excited acridine have been investigated in methyl methacrylate (MMA) and poly(methy1 methacrylate) (PMMA)where photoreduction occurs with the irradiation of 365-nm light. In PMMA, and the fluorescence yield @F at 296 K and the fluorescence lifetime T F at 77 K were determined to be 14.2 ns, respectively. @F, 7F, and the yield @ST of the lowest triplet state Tl(a,n*) increase with decreasing temperature. In MMA, *ST = 0.62-0.70 and @F = 5 X lo-' at 296 K. @sT increases with decreasing temperature. The transient absorption of acridine C radical was observed in MMA but not in PMMA. The radical is formed from both Tl(x,r*)and the excited states higher than Tl(a,n*). The possible reactive higher excited states are S2(n,a*)and Tz(n,a*). It was concluded that both Sz(n,a*)and TS(a,a*)participate in the intersystem crossing.
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Introduction In previous w ~ r k , l the - ~ relaxation mechanisms of excited acridine in poly(viny1alcohol) film (PVA), water, and benzene were studied in detail. In these solvents the photoreaction does not occur with the irradiation of 365nm light. In PVA and water, it was found that the (1) Kikuchi,
K.;Uji-ie, K.; Miyashita, Y.; Kokubun, H. Bull. Chem.
SOC.Jpn. 1977, 50, 879.
(2) Kasama, K.; Kikuchi, K.; Yamamoto, S.;Uji-ie, K.; Nishida, Y.; Kokubun, H.J.Phys. Chem. 1981,85, 1291. (3) Kasama, K.; Kikuchi, K.; Nishida, Y.; Kokubun, H. J.Phys. Chem. 1981,85, 4148. OO22-3654182f 2086-4733$0 1.25fO
deactivation of Sl(n,r*) occurs through both temperature-dependent and -independent processes. The temperature-dependent process is practically the Sl(a,r*) (+U) T3(r,r*) Tl(a,r*) transition in PVA and the Sl(a,a*) * (+AE)S2(n,r*) T3(r,a*) Tl(a,r*)transition in water. The temperature-independent processes are the fluorescence radiation, internal conversion
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.
+
+
+
S,(n,n*)-+ T,(n,n*) -+ T , ( n , n * )
intersystem crossing. In benzene, the yield as, of Tl(a,a*) is unity in the range of 278-336 K and the fluorescence yield aPF is less than 5 X at 296 K; the energy levels 0 1982 American Chemical Society
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The Journal of Physical Chemistry, Vol. 86, No. 24, 1982
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of S1(*,r*), S2(n,r*),and T3(r,7r*)are close to one another so that the Sl(r,r*) 2 S2(n,r*) T3(r,a*) T1(r,r*) transition is predominant in the deactivation of S1(r,r*). On the other hand, it was found that aST decreases with increasing temperature in less polar solvents such as methyl methacrylate (MMA) and poly(methy1 methacrylate) (PMMA) where the photoreduction occur^.^ It was suggested that the photoreduction occurs in S2(n,7r*) upon thermal activation of S1(*,n*),because acridine is known to be photoreduced in alcohols in the excited singlet state.5 In the present work, we study the temperature dependences of @F, the fluorescence lifetime TF, @ST, and the yield % of acridine semiquinone C radical R in MMA and PMMA in order to confirm the above suggestion and to clarify the relaxation mechanism of excited acridine in these solvents. It was concluded that both S2(n,7r*)and T3(r,7r*) participate in the intersystem crossing: ( + A E s ) S;! l n . r * )
S,(T,
7r*)
' (+AFT) T 3 ( r . T*)
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Kasame et al. I
1
Wavenumber (cm-') Figure 1. Absorption, fluorescence,and phosphorescence spectra of acridine in PMMA.
T , i u . Ti*i
Experimental Section Acridine (C.P. grade, Tokyo Kasei) was recrystallized from an ethanol-water mixture after pretreatment with activated charcoal in ethanol. Ferrocene (G.R. grade, Tokyo Kasei) was purified by recrystallization from benzene, zone refining, and sublimation. MMA (Kokusan Kagaku) was distilled under vacuum after pretreatment with 10% NaOH aqueous solution and was stored in a refrigerator. Azobis(isobutyronitri1e) which was used as the polymerization initiator, was kindly supplied by Dr. Osamu Ito, Tohoku University. The degassed MMA solution of acridine was polymerized in the dark at room temperature. The absorption spectra were recorded on a Hitachi EPS-3T spectrophotometer. The fluorescence and phosphorescence spectra were measured with a spectrophotometer built in this laboratory and an NF PC-545 A photon counter. The transient absorption spectra in the visible region were measured with a conventional flash apparatus. The T-T absorption spectrum in the near-infrared region was measured with a Hamamatsu R 406 photomultiplier and an Ushio JC-24-300 halogen lamp as a monitoring light source. The flash apparatus used for the determination of asTin MMA has been described.' Monochromatic light (365 nm) from a Toshiba SHL-100 UV mercury lamp with a Hoya U2 and a Toshiba L-1A filter was used for the steady light photolysis. The quantum yield of the photoreduction was determined by the use of the aerated ethanol solution of acridine as an actinometer.6 An N2 laser (fwhm, 7 ns) was used as an exciting light source for the measurement of TF. The fluorescence decay was measured with an RCA 1P 28 photomultiplier with a fast responce circuit' and a Tektronix 7904-7814 sampling oscilloscope, and the decay rate was determined by simulating the decay curve. The error limit is estimated to be 7-8%. aFwas determined by the use of a 1 N H2S04aqueous solution of quinine sulfate as standard.* Sample solutions were degassed unless otherwise noted. (4) Uji-ie, K.; Kaeama, K.; Kikuchi, K.; Kokubun, H. Chem. Lett. 1978, 247. (5) Koizumi, M.; Ikeda, Y.; Yamashita, H. Bull. Chem. SOC.Jpn. 1968, 41, 1056. (6)Niizuma, S.; Koizumi, M. Bull. Chem. SOC.Jpn. 1963, 36, 1629. (7)Beck, G. Reo. Sci. Instrum. 1976, 47, 537. (8)Melhuish, W. H. J. Phys. Chem. 1960, 64, 762.
Wavenumter(cm-1)
Figure 2. Triplet-triplet absorption spectrum of acridine in PMMA (a). Absorption spectra of acridine C radical in MMA (b) and in cyclohexane (C).
Results and Discussion Acridine in PMMA Matrix. Figure 1 shows the absorption and fluorescence spectra of acridine at 296 K and the phosphorescence spectrum at 77 K. The phosphorescence spectrum is the mirror image of the So T, absorption spectrum in chlor~form.~ The energy level of Sl(r,7r*) was evaluated to be 25 980-26 080 cm-I from the mirror-image relation of the absorption and fluorescence spectra at 77 K. The energy level of T1(r,r*) was determined to be 15800-16 000 cm-I from the 0-0 band of the phosphorescence. Figure 2a shows the T-T absorption spectrum. Since no transient absorption is observed below 9800 cm-', the absorption maximum at 10200 cm-' is assigned to the 0-0 band of the T1(r,r*) T3(r,r*)transition. The energy level of T3(7r,7r*)is evaluated to be 26000-26200 cm-'. The triplet decay is of first order below -230 K, but not above -230 K, though the decay rate varies from sample to sample. The lifetime of 25 ms at 180 K and the half-life of 0.25 ms at 296 K are typical values. This suggests that the microscopic viscosity of the PMMA matrix is not uniform. No transient absorption except for the T-T absorption was observed. Figure 3 shows the temperature dependence of rF,the relative fluorescence yield @F', and the absorbance DT of the T-T absorption immediately after flashing. @F was only roughly estimated to be at 296 K, because the optical polishing of PMMA matrix was difficult for us.
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(9)Evans, D.F.J. Chem. SOC.1957, 257, 1351.
The Journal of Physics1 Chemistty, Vol. 80, No. 24, 1982 4735
Relaxation Mechanism of Excited Acridine
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