TRIPLETTRIPLET ENERGY TRANSFER TO RAREEARTH CHELATES
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Studies on the Triplet -Triplet Energy Transfer to Rare Earth Chelates'"
by M. L. Bhaumik Electro-Optical Systems, Incorporated, Pasadena, California
and M. A. El-Sayedlb University of California, Los Angeles, Californblc (Received July 81, 1964)
Interniolecular triplet-triplet energy transfer between aromatic carbonyls and aromatic hydrocarbons as donors and rare earth chelates as acceptors is demonstrated. This is accomplished by observing the phosphorescence from the triplet level of gadolinium chelate, as well as the intramolecularly sensitized emission of the Eu3+ ion in europium hexafluoroacetylacetonate. The transfer is shown to be diffusion controlled. The possibility of the transfer taking place via actual chemical reaction is discussed, but evidence indicates it to be highly improbable for the chelate-benzophenone system. The transfer fro n the benzophenone triplet level to the chelate triplet level is not found to be niore efficient than that between benzophenone and naphthalene. This indicates that the heavy ion has little effect on the transfer probability. At concentrations of mole/l. the transfer between the ketone and the europium chelate triplet levels is found to be more efficient than the transfer between ketone and the bare E u + ion. ~ At higher concentrations, however, a rather efficient transfer of energy froin the ketone to the ion is observed. Application of the last class of material for laser purposes is discussed.
1. Introduction Energy transfer between excited and unexcited molecules has been observed spectroscopically as well as chemically. Two types are generally distinguishable, depending upon the multiplicity of the excited state of the donor, which should be the same as that of the acceptor in order to conserve spin selection rules in the transfer process. The first type is the singlet-singlet energy transfer, and the second one is the triplet-triplet energy transfer. Both the coulombic and the exchange terms are involved in the mechanism of the first type of transfer, whereas only a n exchange type mechanism can account for the triplet-triplet transfer. The coulonibic term can be expanded to give rise to a dipole-dipole type of interaction between the transition moments of the donor and the acceptor. This term usually dominates the exchange terni for allowed transitions and gives rise to the observed fact that singletsinglet transfer probability is orders of magnitude larger than triplet-triplet transfer. The actual efficiency of the triplet-triplet transfer can, however, be quite high because of the long lifetinie of the triplet state.
The main interest of this work was triplet-triplet energy transfer. This was first observed3 in glasses at concentrations of -lod2 M between aromatic ketones as donors and aromatic hydrocarbons as acceptors. Recently, the geometry of the donor-acceptor pair in these systems, which possess the niaxiniuni transfer probability, has been established using polarization ~neasurenients.~ Triplet-triplet energy transfer in fluid media was detected using flash techn i q u e ~by ~ observing the triplet-triplet absorption of the acceptor when sensitized by transfer froin the donor. In rare earth chelates, the phenomena of intramolecular energy transfer, Sllgsnd Tligand rare earth ion levels, has been illustrated by observing the red lines +
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(1) (a) Work supported in part by Rome Air Development Center, Rome, N. T.,under Contract AF 30(602)-3440: (b) consultant t o Electre-Optical Systems, Inc. ; ( c ) contribution No. 1738. (2) D. L. Dexter, J . Chem. Phys., 21, 836 (1951). (3) A. N. Terenin and V. Ermolaev, Trans. Faraday SOC.,52, 1042 (1956). (4) J. K. Roy and M. A. El-Sayed, J . Chem. Phys., 40, 3462 (1964). (5) G. Porter and M. R. Wright, J . chim. phys., 55, 705 (1958).
Volume 69,Number 1
January 1965
11. L. BHAUMIK A N D 11. A. EL-SAYED
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of Eu3+ when the ligand was excited.6-12 The intramolecular energy transfer is shown to proceed via the triplet state. This is shown688 by chemically perturbing the relative positions of the lowest triplet level and the rare earth levels. Only those chelates having a triplet level above the rare earth levels are found to transfer the energy by an iritraniolecular process. l Z a By adding a donor, which possesses a triplet level above the triplet state of the chelate, the rare earth ion emission is expected to appear following the process Tdonor Tchelate rare earth ion levels .-t red emission. This has been recently d e n i ~ n s t r a t e d 'in ~ the sysleni having benzophenone as donor and europium hexafluoroacetylacetonate as acceptor. In this paper a general discussion in terms of new, extended data is given. General conclusions concerning the effect of' temperature, viscosity, and the heavy ion on the transfer probability are given. Because of their unique properties, laser applications of some of these materials are examined.
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Experimental Arrangement
The measurement of absorption spectra was done in a Carey 14 spectrophotometer. The luminescence measurements were carried out in an Aminco-IGers spectrophosphoriineter modified for front surface excitation and v i e ~ i n g . 1 ~The excitation spectra are not corrected for either the source intensity distribution or the photomultiplier sensitivity. The temperature-dependence results were obtained in a specially designed cold cell in which the geometry could be kept constant during changes of either the temperature or the sample. A detailed description of the apparatus is given elsewhere. l5 The temperature was changed by first cooling the cell with liquid nitrogen and then gradually heating with hot, dry nitrogen gas. The excitation in this experinient was done with a Bausch and Loiiib nionochronxttor and a stabilized mercury arc. The quantum yields were nieasured by comparison with an EI'A solution of europium thenoyltrifluoroacetorlate whose eficirncy was nieasured earlier and found16to be 22% at 3OO0I
