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A bichromophoric molecule consisting of two coumarins linked by a pentakis(ethylene oxide) spacer can efficiently bind Pb2+ ions in acetonitrile and in propylene carbonate. The resulting changes in photophysical properties are reported with special attention to photoinduced electronic energy transfer. Steady-state data allow one to determine the transfer efficiency and stoichiometry of the complex, whereas subpicosecond time-resolved experiments by the excite-and-probe technique provide information on the transfer kinetics. The stoichiometry of the complex is 1:1 in acetonitrile and 1:3 (ligand:metal) in propylene carbonate. A significant increase in efficiency and rate of energy transfer for the complex is observed in acetonitrile, whereas there is almost no change in propylene carbonate. The results are discussed in terms of a possible structure of the complex which is different in the two solvents.

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6427the rate constant is probably a reflection of the methyl migration and not an accurate measure of the self-exchange rate constant between two molybdenum centers. The self-exchange rate must be slower than the rate of methyl migration to a CO. The reported value for self-exchange of methyl between two tungsten centers is also quite c~F e ( C 0 )~-+ M e M o c~( C 0 )~ -MeFeCp( CO) + CpMo( C0)3- (9) is also required. For this reaction there is no trace of the reactants by 'H N M R or IR spectroscopy after the reaction, indicating an equilibrium constant that is large, at least >lo4. Using the pK, values4 as a means to approximate the equilibrium constant gives a value of Kq = 106. Using this value for Kq and the self-exchange values of kl (for Fe, reaction 6) = 50 s-l M-I and k2 (for Mo, reaction 7) <5.3 X lo4 s-I M-l gives k12 = [(50 s-I M-')(5.3 X 200 s-I M-I This value is in moderate agreement with the observed value of 1100 s-l M-I. Thus the methyl transfer may obey relative The equilibrium constant for the cross reaction, s-I M-1)(106)J1/2 = (28) Butler, I. S.; Basolo, F.; Pearson, R. G. Inorg. Chem. 1967,6, 2074. Marcus-type relationships, but uncertainty in the value for the C~M O ( C O )~-/ C~M O ( C O )~M~self-exchange rate constant and in the equilibrium constant for reaction 9 prohibits a more detailed analysis. Similar analysis for CH3+ transfer between two organic nucleophilesz6 indicates reasonable utility of eq 5 for methyltransfer reactions. ConclusionThe results reported in this paper show that transfer of alkyl groups between metal carbonyl anions is closely related to organic nucleophilic displacement reactions in reactivity and mechanism. This represents the first comparison of transfer of different alkyl groups between transition metal centers. Acknowledgment.We are grateful to the Department of Energy, Office of Basic Energy Sciences (DE-FGO2-87ER13775.A004), for support of this research. The Varian VXR-400 N M R spectrometer was purchased with funds from the Department of Education (2-2-0101 1). Supplementary Material Available: Plots of the absorbance change at 1790 cm-I for reaction of MeMn(CO), with CpFe-(CO),-, [MeMn(CO),] versus kobsd, and line width of the Cp resonance of CpFe(CO),-versus [ MeFeCp(CO)2] for the selfexchange reaction (3 pages). Ordering information is given on any current masthead page.Abstract: The synthesis and photophysical properties of 0-cyclodextrins bearing seven 2-naphthoyloxy chromophores in specific positions, either on the primary face or the secondary face., or 14 2-naphthoyloxy chromophores, seven on each face, are reported. These multichromophoric cyclodextrins are good models for the study of excitation energy migration among chromophores in well-defined positions. The investigation was performed in dichloromethane and in a mixture of ethanol and methanol that can form a glass at low temperature. The absorption spectra show that the interactions between chromophores in the ground state are weak, whereas the fluorescence spectra reveal the existence. of ...