J. phys. Chem. 1983, 87, 4558-4560
4558
Two-step Laser Excitation Fluorescence Study of the Ground- and Excited-State Proton Transfer in 3-Hydroxyflavone and 3-Hydroxychromone Mlchlya Itoh' and Yoshlhlra Fubwara Faculty of Pharmaceutical Sciences, Kanazawa Unhwslty, Takara-machi, Kanazawa 920, Japan (Received: Ju& 27, 1983)
The f i t laser excitation of benzene solutions of 3-hydroxyflavone and 3-hydroxychromone induces the formation of an unstable tautomer (T)generated by excited-state proton transfer and relaxation. The second laser excitation, delayed from the f i t , of the absorption band of T affords the fluorescence spectrum of the excited-state tautomer (T*). This two-step laser excitation fluorescence including the variable delay and excitation spectra demonstrates the intervention of the long-lived tautomer in the relaxation of T* and the unexpectedly slow reverse proton transfer in the ground state which shows the extraordinary large deuterium substitution effect of the 3-hydroxyl hydrogen atom.
Several nano- and picosecond fluorescence studies on excited-state proton transfer in 3-hydroxyflavone (3-HF) have been since the mechanism was proposed by Sengupta and K a ~ h a . ~Recently, Itoh et alS6have reported a time-resolved fluorescence study of intramolecular excited-state proton transfer in 2-methyltetrahydrofuran and 3-methylpentane solutions of 3-HF and 3-hydroxychromone (3-HC) which lacks a phenyl group in 3-HF. They demonstrated the unusually slower proton transfer in 3-HF than in 3-HC and other intramolecular proton transfer systems. The unusually slow proton transfer is attributable to the concerted rotation of the 2-phenyl group in 3-HF with excited-state proton transfer. On the other hand, a transient absorption study that may provide us with valuable information on the reaction mechanism of proton transfer, both in the excited state and in the ground state, has never been reported except for a few recent Very recently, Itoh et al.' have demonstrated the existence of a rather long-lived tautomer in the ground state of 3-HF and 3-HC by transient absorption spectra. Further, they have reported transient absorption and two-step laser excitation (TSLE) fluorescence studies on the intermolecular proton transfer in the ground and excited states of methanol solution of 7hydroxyq~inoline.~~~ This paper reports the two-step laser excitation fluorescence study of intramolecular proton transfer of 3-HF and 3-HC in the ground state as well as in the excited state. The first N2laser excitation of the benzene solutions of 3-HF and 3-HC affords the rather long-lived groundstate tautomer (T) generated by excited-state proton transfer and the relaxation processes, and the second laser 300 ns-50 ps from the first (dye) excitation (delayed laser pulse) of the T absorption band within its lifetime provides us with the direct fluorescence of T*. Further,
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(1)G. J. Woolfe and P. J. Thistlethwaite, J.Am. Chem. Soc., 103,6916 (1981). (2)A. J. G. Strandjord, S. H. Courtney, D. M. Friedrich, and P. F. Barbara, J. Phys. Chem., 87,1125 (1983). (3)M.Itoh and H. Kurokawa, Chem. Phys. Lett. 91,487(1982). (4)P.K. Sengupta and M. Kasha, Chem. Phys. Lett., 68,382(1979). (5)M. Itoh, K. Tokumura, Y. Tanimoto, Y. Okada, H. Takeuchi, K. Obi, and I. Tanaka, J. Am. Chem. SOC.,104,4146 (1982). (6)A. L.Huston, G. W. Scott, and A. Gupta, J. Chem. Phys., 76,4978 (1982). (7)M.Itoh, Y.Tanimoto, and K. Tokumura, J.Am. Chem. SOC., 105, 3339 (1983). (8)M. Itoh, T. Adachi, and K. Tokumura, J. Am. Chem. Soc., 105, 4828 (1983). (9)M.Itoh, T. Adachi, and K. Tokumura, J. Am. Chem. SOC.,to be submitted.
the TSLE fluorescence intensities determined at various delay times of the second pulse from the first one afford the lifetime of T. The excitation spectra of the TSLE fluorescence of these solutions were also determined by changing the wavelength of the second dye laser. The TSLE fluorescence excitation spectra are more sensitive and reliable than the transient absorption spectra without interference from the triplet-triplet and/or S, S1 absorption bands. Therefore, this paper demonstrates the intervention of the intermediate (T) in the excited-state proton transfer and the relaxation process, and a strong confirmation of the transient absorption band of T. Further, an extraordinary deuterium substitution effect of the 3-hydroxyl hydrogen atom in 3-HF and 3-HC on the lifetimes of T was observed in benzene solutions.
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Experimental Section The method of purification of 3-HF and 3-HC was described in the previous papers5 The deuterium substitution of the 3-hydroxyl hydrogen atom was performed by CHBOD(CEA, deuterium grade 99.0%) in a vacuum system. After CH30D was removed, benzene pretreated by potassium mirror was added through a vacuum system. The isotope purity was checked by proton NMR. Fluorescence lifetimes and transient absorption spectra were measured by the methods described previously. A home-made N2laser (fwhm 7 ns, a peak power 500 kW, and repetition rate 1Hz) was used for the first laser excitation, and a N2 laser pumped dye laser (Molectron UV 12 and DL 14) was used as the second laser. The TSLE fluorescence was determined by a monochromator-photomultiplier-oscilloscope system, where the oscilloscope was triggered by the second laser pulse detected by a biplanar phototube (HTV R617-02). The lifetimes of T were determined from the fluorescence intensities at various delay times between the two pulses. The TSLE fluorescence excitation spectra were obtained at various wavelengths of the second excitation laser whose intensity was taken into account.
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Results and Discussion The transient absorption spectra of 3-methylpentane (MP) solutions of 3-HF and 3-HC were reported in the previous comm~nication.~ In aerated MP solution of 3-HF, the long-lived absorption band due to the ground-state tautomer (T)was observed at A, = 440 nm in addition S1absorption band (Am, = 460 nm) of the to the S, excited-state tautomer (T*). Further, a triplet-triplet absorption band was observed at 390-420 nm in the aer-
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0 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 23, 1983 4559
Letters
3-HF in benzene
A Inm
1:.
3-HC in benzene
1
600
500
Figwe 1. Transient absorption spectra of an aerated benzene sdutlon of 3-HF at 100 (--0--0-) and at 250 ns (-0-0-) laser excitation. TSLE fluorescence (-0-0-) and excitation (-0-0-) spectra of a deaerated benzene solution of 3-HF (excitation spectrum, 3 4 F ) at 1.2 ps after the first laser pulse. An ordinary fluorescence spectrum (- - -) was measured with excitation at 340 nm.
ated solution. Upon deaeration, the weak T absorption band was covered by a triplet-triplet absorption so strong that the T absorption band cannot be detected as reported previ~usly.~The transient absorption spectrum of the benzene solution of 3-HF was measured as shown in Figure 1. However, the strong triplet-triplet absorption band at 390-420 nm prevents us from obtaining the accurate absorption spectrum and decay time for T. On analogy with the MP solution, the absorption band due to T in the benzene solution seems to be situated in the 400-450-nm region. The decay time of the long-lived transient was determined to be 5.9 ps at room temperature (-22 "C). However, no significant bleaching and recovery of the ground-state absorption spectra (320-360 nm) were observed because of the strong triplet-triplet absorption superimposed in this region. The first laser excitation of the aerated benzene solution of 3-HF induces the formation of the ground-state tautomer (T)generated by excited-state proton transfer folT* T). The lowed by fluorescent relaxation (N* second dye laser excitation of the T absorption band at a delay time of -1.2 ps from the first one exhibits a fluorescence spectrum as shown in Figure 1, which is almost identical with the ordinary T* fluoresence spectrum obtained upon excitation of the normal form (N) of 3-HF. Further, the decay time of this TSLE fluorescence was measured to be 3.1 ns at room temperature, which is identical with that (3.0 ns) determined upon excitation at 340 nm within experimental error. Therefore, the TSLE fluorescence is ascribed to the direct fluorescence of T* by the excitation of T. In a benzene solution of 3-HC, the transient absorption spectra are more obscure than that of 3-HF. It is considerably difficult to distinguish the absorption band of T from triplet-triplet absorption which seems to be in the shorter wavelength region, as shown in Figure 2. However, the transient absorption maximum due to T of 3-HC seems to be in 400-450-nm region with a lifetime of 0.45 ps. The TSLE fluorescence spectrum of this benzene solution of 3-HC was determined as shown in Figure 2, where the delay time between the two laser pulses was -300 ns. The TSLE fluorescence spectrum and lifetime are also identical with the ordinary fluorescence spectrum and lifetime, respectively, though the too short lifetime has less accuracy. Therefore, the direct observation of T* fluorescence by the TSLE technique demonstrates the real existence of the absorption band of the long-lived transient T in the excited-state proton transfer and relaxation processes. The TSLE fluorescence intensity (relative) of these compounds was determined at variable delay times be-
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400
A Inm
l 600
500
Flgure 2. Transient absorption spectrum of an aerated benzene solution of 3-HC (-0- -0-) at 200-ns delay. TSLE fluorescence (- 00-1 and excitation (-0-0spectra 1 of a deaerated solution of 3-HC (excitation spectrum, 3 0 2 ) under the same conditions as Figure 1. All spectra were determined at room temperature. TABLE I: Lifetimes ( w s ) of the Ground-State Tautomer ( T ) in Benzene Solutions of 3-HF ( 3 - D F ) and 3-HC ( 3 - D C ) Determined b y Transient Absorption and TSLE Fluorescence at R o o m Temperature ( 2 3 "C) trans absorp. aerated 3-HF ( 3 - D F ) 3-HC (3-DC)
5.9 0.45
TSLE fluores aerated
deaerated
a
14.5 (34.2)b 8.9 (18.0)
-0.3
No data obtained because of a very weak signal. Deuterium substituted samples ( 3 - D F and 3-DC) of 3-
a
OH hydrogen. The lifetimes in 3-DF and 3-DC can be obtained only under deaerated conditions because of D-H exchange reaction o n aeration.
tween the two laser pulses. From plots of the TSLE fluorescence intensity vs. delay time, the decay times of T in 3-HF and 3-HC were determined in aerated and deaerated benzene solutions. The obtained decay times are summarized in Table I and compared with those of the transient absorption, where the TSLE fluorescence may be more reliable. As shown in Table I, the decay time of T remarkably increases upon deaeration.l0 The decay rate of T means the reaction rate of the ground state reverses proton'transfer from T to N and this does not seem to depend on oxygen molecules. Therefore, the unusual differences in the decay time of T between the aerated and deaerated solutions may be attributable to the oxygenation reaction of T. Matsuura et a1."J2 reported the photooxygenation reaction of 3-HF and its related compounds giving the corresponding depsides. Since the lifetimes of the excited state of 3-HF (N*) and of T* are in the subnanosecond region and independent of deaeration, the photooxygenation reaction seems to take place from the ground-state tautomer T. Further, the decay times of T were measured in benzene solutions of the deuteriumsubstituted compounds (3-DF and 3-DC) of the 3-hydroxyl hydrogen of 3-HF and 3-HC. The lifetimes of T in 3-DF and 3-DC were determined to be 34 and 18pt~,respectively. These decay rates mean the reaction rates of the ground state reverse proton transfer from T to N in the absence of oxygen. The extraordinary deuterium isotope effect of 3-OH on the decay time of T which implies a decrease in the reverse proton transfer rate may be attributable to a (10) The lifetime of the transient absorption band of 3-HFin MP solution was reported to be invariant upon deaeration of the solution in the previous paper (ref 7). In benzene solution, however,the lifetime is dependent on aeration and deaeration. (11) T. Matauura, H.Mataushima, and R. Nakashima, Tetrahedron, 26, 435 (1970). (12) T. Matauura, T. Takemoto, and R. Nakashima, Tetrahedon, 29, 3337 (1973).
J. Phys. Chem. 1083, 87,4560-4561
4580
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difference in the zero-point energy of the initial state (T) of the reaction (T N).I3J4 Since the transient absorption spectra are considerably obscure as mentioned above, the TSLE fluorescence excitation spectra of benzene solutions (deaerated) of 3-DF and 3-DC were measured by changing the wavelength of the second ex~itation.'~The excitation spectra are shown in Figures 1 and 2 in comparison with the transient absorption spectra. In the intramolecular excited-state proton transfer of o-hydroxybenzophenone in ethanol solution, Hou et a1.16 reported a very rapid recovery of the ground-state absorption bleaching, which implies no stable intermediate is involved in the excited-state proton transfer and relaxation process. Recently, Huston et aL6 have reported the ground-state absorption recovery kinetics and the fluorescence decay kinetics of 2-(2'-hydroxy-5'-methylphenyl)-W-benzotriale in several solvents and suggested the intervention of intermediate formed during the excited-state relaxation from the controversy that the fluorescence lifetimes are shorter than the ground-state
recovery times. The lifetime of this intermediate seems to be extremely short, if there is an intermediate as predicted. In this paper, it is noteworthy that the first observation of a long-lived transient of T by TSLE fluorescence and transient absorption provides us with complete evidence for the existence of a long-lived tautomer T in the excited-state proton transfer and its relaxation, and with a relationship between the ground-state (T) and the excited-state tautomer (T*). The transient absorption band which is confirmed by the TSLE fluorescence and excitation spectra is a mirror image of the T* fluorescence. Since the decay times of T in the deaerated solution reflect the reaction rate of the reverse proton transfer in the ground state, the smaller decay time in 3-HC compared with that in 3-HF indicates that a more rapid reverse proton transfer takes place in 3-HC than 3-HF. The fact is interpreted in terms of a discussion analogous to that for the excited-state proton transfer, as reported previ0us1y.~The deuterium isotope effect of the 3-OH group on the decay time which corresponds to the reaction rate from T to N is unusually large. The differences in the decay times of T may imply an important contribution of the 0-H (0-D) stretching vibration to proton transfer in the ground state.
(13) R. A. O'Ferrall in 'Proton Transfer Reactions",E. F. Caldin and V. Gold, Ed.,Chapman and Hall, London, 1975, p 201. (14) T.Kishi, J. Tanaka, and T.Kouyama, C h m . Phys. Lett., 41,497 (1976). (15) The deuterium-substituted compounds 3-DF and 3-DC were used for the determinationof the TSLE fluorescenceexcitation spectra, since the longer lifetimes of T have the advantage of easy determinationof the spectra. However, there is no spectral difference between these comp o ~ & with 3-HF and 3-HC. (16) S.-Y.Hou, W. M. Hetherington, G. M. Korenowski, and K. B. Eiaenthal, Chem. Phys. Lett., 68, 282 (1979).
AcknowZedgment. The authors are indebted to Drs. M. Sumitani and K. Hashimoto, Institute for Molecular Science, Okazaki, for supplying an amplifier circuit for the pin photodiode. Registry No. 3-HF, 577-85-5; 3-HC, 13400-26-5;D, 7782-39-0.
Selectivity and Activity in the Oxidation of Benzene, 1-Butene, and 1,3-Butadiene on Supported Vanadium Oxide Catalysts KenJlMarl,+ Makolo Inomata,t Aklra Mlyamoto,' and Yulchl Murakaml Depertmenfof SynWtlc Chemkrby, Facuw of Engheefing, Nagoya UnlversnY. Chkusa-ku, Nagoya 464, Japan: and Klnu-ura Research Depattmt, JC%
Co.. Sumki-cho, Hande. Aichl475,
Japan (Received: Ju& 22, 1983)
Selectivity with respect to partial oxidation producta in the oxidation of benzene, 1-butene,and 1,3-butadiene on V205/Ti02and V205/A1203 catalysts is determined by the number of Vz05layers on the support, while the activity is controlled by the number of surface V=O species on the catalyst.
Supported metal oxide catalysts exhibit interesting catalytic properties depending on the kind of support and
on the composition of the catalyst. However, the selectivity and activity have not been thoroughly clarified in terms of the catalyst structure. This seems to be due to the lack of a well-established method to characterize the structure of supported metal oxide catalysts. By using the rectangular pulse technique' coupled with the various physicochemical measurements, we have succeeded in determining the structures of V205/Ti02and V206/A1203catalysk2 The vanadium oxide forms a layer structure on the support. The number of V206layers on the support and the 'Kinu-ura Research Department, JGC Co. Q022-3654l83/2087-4560~01.5QlO
two-dimensional spread of the V205layers on the support vary greatly with the kind of support and the V205content. The purpose of this study is to reveal the selectivity and activity in the oxidation of benzene, 1-butene, and 1,3butadiene in terms of the structure of V205on the support. These reactions are of significance to the commercial (1) (a) A. Miyamoto,Y. Yamazaki, M. Inomata,and Y. Murakami, J. Phys. Chem., 85, 2366 (1981); (b) M. Inomata, A. Miyamoto, and Y. Murakami, ibid., 85, 2372 (1981). (2) (a) M. Inomata, K. Mori, A. Miyamoto, T. Ui, and Y. Murakami, J. Phys. Chem., 87,754 (1983); (b)M. Inomata, K. Mori, A. Miyamoto, and Y. Murakami, ibid., 87, 761 (1983); (c) Y. Murakami, M. Inomata, K. Mori, T. Ui, K. Suzuki, A. Miyamoto, and T. Hattori, "Preparation of Catalysta 111",G. Poncelet, P. Grange, and P. A. Jacobs, Ed., Elsevier, Amsterdam, 1983, p 531.
0 1983 American Chemical Society
