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photoexcitation with a uranyl nitrate flash filter solution lo (which transmitted throughout the bromine absorption band and thus led to both 2P8/zand zPiiz atoms) and with a potassium chromate-dichromate flash filter solution which transmitted only above the 51 I-nm convergence limit for bromine41(1 at 516 nm, 4 at 520 nm, 55 at 540 nm), so that only groundstate bromine atoms were produced. Within the rather large experimental errors, the two quantum yields were identical (0.31 us. 0.38). Although some reactions involving only *PI/,halogen atoms have been observed with photolysis above the convergence limit,42the effect
z
z
(41) J. G. Calvert and J. N. Pitts, Jr., “Photochemistry,” Wiley, New York, N. Y., 1966, p 184. (42) A. B. Callear and J. F. Wilson, Trans. Faraday Soc., 63, 1358 (1967).
is strongly wavelength dependent and extends only a few kilojoules into the banded region. It is thus concluded that photobromination of benzene resulting in this work is not initiated solely by excited bromine atoms. Although the gas-phase formation constant is not known for either the molecular or atomic bromine complex with benzene, it is highly unlikely that either complex could be present in sufficiently high concentration to participate in the bromination reactions to the extent observed, again unless a very efficient chain process results. The fact that good second-order kinetic plots result from assuming that all of the bromine loss occurs during the flash ("
40-
200
300
400
500
WAVELENGTH nm
Figure 2. Emission and excitation spectra of a mixture of obenzoylbenzoate (0.0015 M ) and o-benzoylbenzoicacid (0.0015 M ) in a 1 : 1 ethanol-water solution.
Table I. Stability Constants ~~~
Solvent
Fluorescence data ---pH titration-Kl, 1. mol-' K,, 1. mol-' Kz, 1. mol-' ~~
50% HzO-5OP;: EtOH 33.3% Hz0-66.7% EtOH
143 i 10 334 i 20
147 i 10 336 + 9
80 i 15 119 + 1 3
E u ( C I O ~ )and ~ the o-benzoylbenzoate buffer are given in Figure 3. In this figure the excitation spectra is for the europium emission at 590 nm, while the emission spectra was measured with the excitation wavelength set at X 320 nm, the maximum in the excitation spectra. Figure 1 indicates that at this excitation wavelength the emission intensity of Eu3+ itself is small, while Figure 2 indicates that 320 nm is an effective wavelength for the excitation of o-benzoylbenzoate. The excitation spectrum of Eu3+ in the mixture is essentially the same as that of o-benzoylbenzoate except for the small peak at 390 nm which is for the direct excitation of Eu(II1). Thus, it can be concluded that energy transfer is taking place between the absorbing benzoylbenzoate and the emitting europium(II1). In Figure 3 there is no evidence of emission from the organic absorber itself. This cannot be taken as evidence for quenching by the metal ion, since even if quenching did not take place, the weak Journal of the American Chemical Society
1 96:3 1
WAVELENGTH nm Figure 3. Emission and excitation spectra of a mixture of 0benzoylbenzoate (0.0015 M ) and Eu(C104), (0.00625 M ) in a 1 :1 ethanol-water solution.
emission of o-benzoylbenzoate would not be detected with instrumental settings employed in these measurements. Another feature of importance in Figure 3 is the emission spectrum of the europium. Compared to the emission spectrum of Figure 1, in the presence of the benzoylbenzoate the intensity of the emission peak of 615 nm is considerably increased relative to the intensity of peak at 590 nm. Electrical dipole transitions are more sensitive to changes in the symmetry of the ion than are magnetic dipole transition^,'^ and hence the change in relative intensity indicates that the symmetry of the environment of the Eu3+is affected by the presence of the ligand. As the concentration of the ligand is increased, the spectral changes become more pronounced. These efforts suggest that the o-benzoylbenzoate is complexing with the metal ion. If energy is transferred only when the europium is complexed, then it can be assumed that the intensity of emission is proportional to the concentration of the complex providing that the excitation wavelength is such that no direct excitation of europium takes place; i.e., I = complex]. Concentrations were adjusted so that [o-benzoylbenzoate] was always less than [Eu3+]and the concentration of EU(O-BB)~+ was negligible. Under these circumstances
where K is the equilibrium constant K =
[Eu(o-BB)'+] [Eu3+][o-BF]
[o-BB-1, is the stoichiometric concentration of the anionic ligand and [Eu3+] is the concentration of uncomplexed europium. The concentration of o-benzoylbenzoate was held constant while the europium concentration was varied. In each case the emission intensity I at X 590 nm was measured while the system was excited at X 320 nm. l/I was plotted against 1/[Eu3+Itotal(see Figure 4) and approximate values of a and k were determined. Corrections were then made t o account for the fraction
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of europium(II1) which was in the form of the complex and a new set of values was calculated. After three corrections, no further refinements were necessary and the final values were obtained. The whole series of measurements was repeated using new solutions and at the different solvent. compositions. The values quoted in Table I are an average of three separate determinations. The correlation between the two sets of values supports the suggestion that the energy transfer takes place through complex formation between the ligand and the metal ion. When a mixture of o-benzoylbenzoate and Eu3+ is acidified, energy transfer ceases, and the excitation and emission spectra of Eu3+ are identical with that of a E U ( C I O ~ )solution. ~ The protonation of the ligand reduces complex formation and at the same time reduces energy transfer. It must be pointed out that the fluorescence data are not inconsistent with eq 2 which is derived without consideration of complex f o r m a t i ~ n ~ ~ ~ ~ ~
20.0
LH 5.0
0.0 0
100
300 400 v ~ ~ 3 + 1
200
500
600
Figure 4. l/I os. 1/[Eu3*]t,t,l: [o-benzylbenzoate] = 0.0165 M ; [o-benzoylbenzoicacid] = 0.0135 M ; solvent = 1 :1 ethanol-water.
where k, is the collision rate, y o is the lifetime of the excited state of o-benzoylbenzoate, and G is a factor depending on geometry and fluorescence yield. However, even for systems for which a diffusioncontrolled mechanism has been suggested, the experimental value of k, is often less than the diffusion rate constant. Although the concentration dependence indicated by eq 2 is obeyed, a simple collision mechanism may be only an approximation and other factors may be important. For example, in acetone solution, energy transfer between Tb3+, Dy3+, Eu3+, and substituted organic molecules shows a dependence on the presence of a group with an unshared electron pair.lg A different study in acetone solution showed that efficiency of energy transfer from organic carbonyl compounds to Eu3+ depends on the electron density of the
carbonyl group. 2o Recently, Morina 2 1 demonstrated that the rate constant for energy transfer between benzophenone and derivatives and Eu3+in acetone and methanol depended on the overlap of orbitals of the donor and acceptor. In the transfer of energy from o-benzoylbenzoate to europium(II1) in ethanol-water solution, the important factor is the formation of a complex between the ligand donor and the metal ion acceptor. The ability of the o-benzoylbenzoate to transfer energy to Eu3+ contrasts with its failure to transfer energy to EuEDTA.I4 We feel that this is indicative of the inability of the obenzoyibenzoate to form a complex with EuEDTArather than its tendency to dissipate energy to the solution. Acknowledgment. The authors wish to thank Mr. Rodger N. Capps and Mrs. W. Faye Bowers who carried out preliminary experiments. Acknowledgment is made to Research Corporation and to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research.
(18) S. G. Cohen and A. Weinreb, Proc. Phys. SOC.,London, Sect. B, 69, 593 (1956). (19) V. L. Ermolaev and V. S. Tachin, Opt. Spektrosk., 29,93 (1970).
(20) V. F. Morina and Yu. K. Khudenskii, Monokrist., Stsintill. Org. Lyuminofory, 2, 132 (1967). (21) V. F.Morina, Izv. Akad. Nauk SSSR, Ser. Fiz., 36,988 (1972).
Tanner, Thomas
Europium(ZZ1)and o-Benzoylbenzoate
