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J. Phys. Chem. 1994,98, 532-536
Energy-Transfer Processes in Lanthanide Dinuclear Complexes with p tert-Butylcalix[8]arene: An Example of Dipole-Dipolar Mechanism1,* Pascal Froidevaux' and Jean-Claude G. Biinzli Institut de chimie minCrale et analytique, UniversitC de Lausanne, 3, Place du Chateau, CH-1005 Luusanne, Switzerland Received: August 17, 1993; In Final Form: October 15, 1993'
The study of energy-transfer processes between Tb(II1) and various Ln(II1) ions (Ln = Ho, Nd, Eu) in dinuclear complexes ofp-tert-butylcalix[8]arene,[ (TbXLnz-J(LH~)(DMF)5]*4DMF, is reported. Easily available crystals allow an extended work on these transfers. The compounds contain isolated molecules, so that a simple model accounts for the observed luminescence decays. Only two types of interacting Tb-Ln pairs need to be taken into account, one at 3.7 A and the other at 11 A. A rate equation is developed to quantify the energy transfer, and the analyzed data give an interaction strength sequence proportional to the overlap integral, FT~.H.,> F-Nd > F T ~ E "Moreover, . the good fits obtained point unambiguously to a dipoledipolar character of the energytransfer processes. The low-lying ligand-to-metal charge-transfer state (LMCT) increases the absorption probabilities of Eu(II1) f-f transitions considerably, and the resulting quasi-allowed 5Do 'FO transition plays an important role in the energy-transfer process. This feature permits us to assign a dipolar character to the interaction responsible for the energy transfer. The low-lying LMCT state also induces a strong intramolecular Eu-Eu interaction. +
Introduction The absorption of light by a molecule or an ion in a dense medium leads to a number of different consequences. One of them is the transfer of the absorbed energy to a nearby molecule or ion.' Radiative and nonradiative energy-transfer processes involving trivalent lanthanide ions in inorganic solids have been widely studied in recent years because they are proving useful in the design of miniature laser devices." Moreover, it has been realized that trivalent lanthanide ions may be incorporated in supramolecular complexesacting as nanometricphotonic devices.s Theories have been developed which give formulas for the rate of energy transfer by electric dipols-dipole or quadrupoledipole interactions and by exchange intera~tion.~-~ These theories fail to provide a good fit of the real cases because they consider only the interaction between one donor in an excited state and one acceptor in the ground state without taking into account the multiple interactions between the donors and the acceptors randomly distributed around them. Inokuti and Hirayamalo have proposed a statistical treatment of all the n(&) expressions, n(&) being the rate constant for energy transfer from a donor D to an acceptor Ak at a distance Rk. The expressions derived from this treatment permit quantitative analysis of the luminescence decay of the donor in condensed materials. Therefore, it appears necessary to develop systems in which the donor and acceptor can be considered as an isolated pair. Horrocks and al.11 have reported a quantitative treatment of energy transfer by electric dipole-dipole interaction between different Ln(II1) ions as isolated pairs in metalloproteins. Banzli et a1.12 have found that dinuclear macrocyclic complexes give a good opportunity to isolate true heterodinuclear Tb-Eu pairs. In dinuclear Tb-Eu complexes with a Schiff base, they have been able to determinethe intermetallicdistanseby analyzing the luminescence decay of the donor. Recently, we have reported photophysical studies of triple-helical dinuclear lanthanide complexessb and of dinuclear Ln(II1) complexes with p-tertbutylcalix[8]arene (LH8).13 In the latter case, we have shown the isostructural nature of the [Ln1Ln2(LH2)(DMF)+4DMF complexes along the lanthanide series. This feature allows a *Abstract published in Aduance ACS Abstracts, December 15, 1993.
0022-3654/94/2098-0532$04.50/0
systematic study of the Lnl-LnZ interactions when these ions are encapsulated into a macrocycle at a relatively short distance (3.7 '41.14 R
R=H n = 1 : calixillarene 3 : colixi6larene
R = C(CH313
5 : calix[8]orene
n = 5 : LH8
In this work, we report the quantitative analysis of the energytransfer processes between the Tb(II1) ion as donor and the Eu(111), Nd(III), and Ho(II1) ions as acceptors, when encapsulated in p-tert-butylcalix[8]arene. Particular attention is devoted to the nonradiative decay of the 5Do(Eu) excited state promoted by a low-lying ligand-to-metal charge-transfer state (LMCT). Experimental Section The complexes used in this study were [TbXGd2-J(LH2)(DMF)+4DMF,x = 2,1.9,1.7,1.0,andO.l; [(TbXEu2-,)(LH2)(DMF)S]JDMF, x = 1.8, 1.6, and 1.0; [(TbXHo2J(LH2)(DMF)+4DMF, x = 1.9, 1.5, and 1.0; [(TbXNd2-&LH2)(DMF)S].4DMF, x = 1.5 and 1.0; [(EuxGd2-,)(LH2)(DMF)s]-4DMF,x = 2,1.8,1.0,0.4,andO.l; [(Eu,Tbd(LH2)(DMF)+4DMF, x = 1.Oand 0.4. LH2 stands for the hexaanion of thep-tert-butylcalix[8]arene.The synthesis14andpreparati~nl~ of samples for luminescencemeasurements have been previously described. Selective pulsed excitations of the sDo(Eu) level at 578.2 nm and of the sD4(Tb) level at 482 nm were obtained with 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 533
Energy-Transfer Processes
a Lambda Physik FL 3002 tunable dye laser (rhodamine 6G or coumarine 102) pumped with an Xe-Cl exciplex laser (Lambda Physik EMG 101 MSC).” The luminescencedecay curves were averages of 512 scans achieved with a W&W SMR memory recorder (20 MHz, 8 bits); data were transferred into an IBM PS/2 360 computer for mathematical treatment. The mathematical analysis was made using the nonlinear curvefitting programs MinsqlS*or Enzfitter.lsb The number of points used was 256 with Enzfitter and 600 with Minsq. A statistical weight was selected for all the calculations with the variance ui proportional to C y i ) l / 2 . Both programs adjust parameters of nonlinear functions with the least-squares method which minimizes the sum of squares of the differences between the calculated and the experimental data: A good fit requires a x2 value
