Aryl-Bridged 1-Hydroxypyridin-2-one: Sensitizer Ligands for Eu(III)

Abstract

The synthesis, crystal structure, solution stability and photophysical properties of an aryl group bridging two 1-hydroxypyridin-2-one units complexed to Eu(III) are reported. The results show that this backbone unit increases the rigidity of the ensuing complex, and also the conjugation of the ligand. As a result of the latter, the singlet absorption energy is decreased, along with the energy of the lowest excited triplet state. The resulting efficiency of sensitization for the Eu(III) ion is influenced by these phenomena, yielding an overall quantum yield of 6.2% in aqueous solution. The kinetic parameters arising from the luminescence data reveal an enhanced nonradiative decay rate for this compound when compared to previously reported aliphatic bridges.Due to the Laporte forbidden character and intraconfigurational nature of the 4f transitions, 1 luminescence from lanthanide cations is typically highly monochromatic, exhibits long-lived excited state lifetimes when compared to organic compounds, and is usually insensitive to quenching by molecular oxygen, making these metal ions ideal for applications as fluorescent probes, 2 optical signal amplifiers 3 or light emitting diodes. 4 Unfortunately, for the same reason, the molar absorption coefficient of lanthanide transitions is very small (less than 10 M −1 .cm −1 ). 5 To obviate this problem, organic ligands having a large molar absorption coefficient can be coordinated to the lanthanide ion, resulting in sensitized emission through the so-called antenna effect. The mechanism of antenna sensitization is comprised of three differing steps: the initial excitation of the ligand, followed by intersystem crossing to give an excited triplet state, then subsequent energy transfer to yield the metal centered excited state which emits light. 1,[5][6][7] For this mechanism, the energy of the sensitizing triplet state relative to the 5 D 0 or 5 D 1 excited states of Eu(III) is one of the critical parameters which can be tuned in order to optimize the system. Furthermore, the rate of intersystem crossing, energy transfer and radiative vs. non-radiative decay of the metal must be considered, and these rates may all be influenced by the geometry and chemical structure of the sensitizing ligand. We report here the synthesis, crystal structure, solution stability and photophysical properties of a new 1-hydroxypyridin-2-one (1,2-HOPO) based chelator, o-Phen-1,2-HOPO (Chart 1), acting as a tetradentate ligand to form an ML 2 complex with Eu(III). This ligand bridges two 1,2-HOPO units via an aryl group, yielding a higher degree of conjugation when compared to previously reported alkyl linkages. [8,9] The resultant stability constants and optical properties of this new ligand architecture are compared to the parent 5LI-8 and 5LIO-1,2-HOPO 9 compounds.* To whom correspondence should be addressed. raymond@socrates.berkeley.edu. Fax: +1 (510) The o-Phen-1,2-HOPO ligand is readily prepared by reaction of the benzyl-protected 1,2-HOPO-6-carboxylic acid intermediate prepare...

A [Cyclentetrakis(methylene)]tetrakis[2‐hydroxybenzamide] Ligand That Complexes and Sensitizes Lanthanide(III) Ions

“…3). The deconvolution of this triplet-excited-state emission band into a vibronic progression [27–29] of several overlapping Gaussian functions with separations of ca . 1000–1100 cm –1 led to the determination of a value of 24690 cm –1 for the triplet-excited-state energy.…”

EuIII Complexes of Octadentate 1-Hydroxy-2-pyridinones: Stability and Improved Photophysical Performance

“…The 1,2-HOPO units can also be linked to form a tetradentate ligand using aliphatic[15, 16] or aromatic spacers[17] via the amide functional groups. For the octadentate topology, we recently reported on the N,N,N’,N’ -tetrakis-(2-aminoethyl)-ethane-1,2-diamine (H(2,2)-) backbone bearing four 1,2-HOPO units (Chart 1).…”

Using the Antenna Effect as a Spectroscopic Tool: Photophysics and Solution Thermodynamics of the Model Luminescent Hydroxypyridonate Complex [EuIII(3,4,3-LI(1,2-HOPO))]−

“…805 μs and ca . 1120 μs, respectively, which are slightly longer than the typical lifetimes determined for bis-tetradentate Eu III (1,2-HOPO) complexes 6,7. Using the improved Horrocks equation,8 the number of inner sphere water molecules on the [Eu III ( 1 )] - complex was determined as q = 0.1 ± 0.1, essentially zero.…”