Received: May 31, 2007

In Final Form: August 27, 2007

Abstract:

This work discusses quantitative aspects of energy transfer occurring in sol-gel derived organic-inorganic di-ureasil hybrids incorporating either [Eu(btfa)₃(4,4'-bpy)(EtOH)] (btfa = benzoyltrifluoroacetonate, 4,4'-bpy = 4,4'-bipyridine) or Eu(CF₃SO₃)₃. Host-to-Eu³⁺ energy transfer occurs either via ligand singlet and triplet (T) excited states or directly from the hybrid emitting centers through the dipole-dipole, dipole-2 pole ( = 2, 4, and 6) and exchange mechanisms. This latter process is dominant for all discussed energy transfer pathways. The ligand-to-Eu³⁺ energy transfer rate is typically 1 order of magnitude larger than the value estimated for direct hybrid-to-Eu³⁺ transfer (3.75 × 10¹⁰ and 3.26 × 10⁹ s^-1, respectively, to the ⁵D₁ level). The most efficient luminescence channel is (S₀)_Hybrid (T)_Hybrid (T)_Ligand (⁵D₁, ⁵D₀) ⁷F_0-6. The predicted emission quantum yield of the di-ureasil incorporating [Eu(btfa)₃(4,4'-bpy)(EtOH)] is in excellent agreement with the corresponding experimental value (53 and 50 ± 5%, respectively), pointing out that the optimization of the ground state geometry by the Sparkle/AM1 model can, under certain conditions, be implemented in Eu³⁺-based organic-inorganic hybrids. For di-ureasils incorporating Eu(CF₃SO₃)₃, the energy transfer rates could not be quantitatively predicted because of the higher computational effort necessary for calculating the singlet and triplet excited states in complex structures, such as these di-ureasils. Instead, the classic Förster and Dexter approaches were applied. Although less efficient, as compared with the di-ureasil incorporating [Eu(btfa)₃(4,4'-bpy)(EtOH)], the hybrid-to-Eu³⁺ energy transfer is also dominated by the exchange (Dexter) interaction.

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