New erbium complexes emitting in infrared region based on oligothiophene and thiophenefluorene carboxylate

doi:10.1016/j.jlumin.2007.03.018

Journal of Luminescence

Volume 127, Issue 2, December 2007, Pages 601-610

https://doi.org/10.1016/j.jlumin.2007.03.018 Get rights and content

Abstract

Lanthanide ions emitting in the near-infrared (NIR) region possess an intrinsically small molar absorption coefficient in the ultraviolet (UV)–vis–NIR spectrum, which is unfavourable for pumping efficiency. On the contrary, using organic lanthanide complexes it is possible to populate the excited state levels of the emitting ion through an efficient intramolecular energy transfer from the optically excited ligands, which act as light-harvesting antennae.

With the aim of studying and maximizing the transfer to lanthanide metals, we have synthesized oligothiophene and thiophenefluorene ligands bearing carboxylate clamps able to complex erbium and other lanthanide 3⁺ ions. The complexes of {4′-(hydroxycarbonyl)methyl-[2,2′;5′,2″]terthiophen-3′-yl}acetic acid and 9-(hydroxycarbonyl)-methyl-2,7-dithien-2-yl-[fluoren-9-yl-]acetic acid with Er³⁺ and different ancillary ligands have been prepared and their optical properties were carefully studied. Moreover, relaxation dynamics measurements have been carried out on all complexes in order to determine emission lifetimes, which result to be of the order of magnitude 2 μs. Quantum chemical calculations have been performed to explain optical absorption data in terms of different coordination types. The complexes containing phenanthroline/pyridine are modelled by adding to the dianion of the ligand one univalent/divalent counterion. The absorption spectra computed in this way are in close agreement with experiment, and the univalent→divalent theoretical wavelength shift goes in the right direction. The addition of a counterion has an even bigger effect on the triplet states, and hence on their matching with the emitting states of the ion.

Introduction

The possibility of exploiting the erbium (III) emission in planar optical amplifiers operating at 1.5 μm has recently attracted much attention on the study of materials containing this ion [1], [2]. Unfortunately, Er³⁺ has an intrinsically low miscibility in silica and, due to the forbidden character of its intra-4f transitions, has a small molar absorption coefficient in the ultraviolet (UV)–vis–near infrared (NIR) spectrum, which makes it unsuitable for pumping efficiency. On the contrary, organic chromophores usually have molar absorption coefficients 3–5 orders of magnitude larger [3], [4]. Hence, population of emitting levels of Er³⁺ is best achieved by employing these chromophores as light-harvesting ligands that can sensitize the lanthanide ion via an intramolecular energy transfer process from their triplet states to metal excited states. Moreover, the complexes obtained in this way show good miscibility with plastic optical fibers (POF) enabling their use in amplification devices.

To achieve an efficient energy transfer, a matching is required between a triplet state of the organic sensitizer and the erbium levels corresponding to low-energy transitions such as ⁴F_7/2 (490 nm), ²H_11/2 (520 nm), ⁴S_3/2 (545 nm), ⁴F_9/2 (650 nm) or ⁴I_9/2 (800 nm). In this way, excitation wavelengths in the UV–vis spectrum may be reached, thus allowing use of relatively low-cost pumping sources. The energy transfer process can be realized either through direct coordination of a sensitizer multidentate molecule with a number of donor atoms or through coordination of a simple ligand covalently attached to a sensitizer molecule. Recently, also NIR emission originating from sensitizer functionalized ligand-based lanthanide (Er³⁺, Nd³⁺, and Yb³⁺) complexes has been reported [5], [6], [7], [8].

It is well known, however, that the NIR emission of Ln³⁺ ions can be strongly lowered by vibrational deactivation. The antenna, the ancillary ligands, the coordinated solvent (as well as moisture) usually contain high-energy oscillators such as O–H and C–H bonds (especially aromatic ones), which are able to quench the metal excited states nonradiatively, leading to decreased luminescence intensities and shorter excited-state lifetimes [9].

We have proposed {4′-(hydroxycarbonyl)methyl-[2,2′;5′,2″]terthiophen-3′-yl} acetic acid (hereafter referred to as Lig. 1, see Fig. 1) as a ligand that can satisfy the requirement of a low-energy excitation due mostly to the increased number of thiophene rings [10]. Afterwards, the possibility of synthesizing perfluorinated sexithiophene has been demonstrated [11]. The combination of these results allows to envisage the design and preparation of funzionalized oligomers absorbing at high wavelength and carrying the lower energy C–F oscillators in place of the C–H ones. Moreover, different ancillary ligands have been tested in order to overcome the well-known difficulty of crystallizing erbium complexes. We report here on the preparation and characterization of two complexes based on Lig. 1 with either pyridine or phenanthroline as ancillary ligands. In addition, aiming to a progressive shift towards lower energy pumping sources, the oligomer 9-(hydroxycarbonyl)methyl-2,7-dithien-2-yl-[fluoren-9-yl-]acetic acid (Lig. 2, see Fig. 1) has been prepared, in which a fluorene residue replaces a thiophene ring while maintaining the chelating functionality.

Electronic and Fourier transform infrared (FTIR) spectra strongly depend on the metal ion–carboxylate coordination. Hence quantum chemical calculations have been carried out in order to explain optical absorption data in terms of different coordinating types. We have studied the geometry, the electronic spectrum and the lowest triplet transitions of the neutral and doubly deprotonated forms of each isolated ligand. In the dianion case, the complexes containing phenanthroline/pyridine are modelled by adding one univalent/divalent counterion. The computed absorption spectra, as well as the univalent→divalent theoretical shift in the wavelength of the strongest absorption peak, are compared with experiment. The effect on triplet transitions of the addition of a counterion to the dianion has been also studied, and on these grounds predictions are made on the relative efficiency of the two ancillary ligands.

Section snippets

Experimental

All the procedures for complex preparation were carried out under nitrogen and using dry reagents to avoid the presence of water and oxygen, which can quench metal photoluminescence (PL).

Preparation of the complexes

The complexes are based on the coordination to Er³⁺ of two carboxyl (COO⁻) groups which are substituents of aromatic rings acting as sensitizers in the energy-transfer process from the visible to the IR region. we have considered two different aromatic moieties, namely a thiophene ring in which the carboxyl functionalities are attached to adjacent carbon atoms, and a fluorene one where the carboxyls are substituents on the same carbon atom. Two additional thiophene rings terminate the antenna

Conclusions

Three erbium complexes carrying thiophene (Lig. 1) and thiophene–fluorene (Lig. 2) moieties as antennae have been synthesized and characterized. Lig. 1 complexes result to be stable whatever the ancillary ligand, as proved by mass spectrometry determinations. Because of the poor stability of Lig. 2-Phen, MALDI could not provide a unique chemical structure for this complex. However, by combining MALDI and SEM-FTP techniques, a reliable stoichiometry for all materials has been obtained. Detailed

Acknowledgements

The research was supported by the Italian MIUR through the Fondo per gli Investimenti della Ricerca di Base (FIRB RBNE01P4JF “Nanostutture molecolari e ibride organiche inorganiche per fotonica” and FIRB RBNE019H9K “Manipolazione molecolare per macchine nanometriche”) projects.

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New erbium complexes emitting in infrared region based on oligothiophene and thiophenefluorene carboxylate

Abstract

Introduction

Section snippets

Experimental

Preparation of the complexes

Conclusions

Acknowledgements

Coord. Chem. Rev.

Opt. Mater.

Coord. Chem. Rev.

Tetrahedron

Chem. Phys.

Synth. Met.

Inorg. Chim. Acta

J. Appl. Phys.

J. Appl. Phys.

J. Chem. Soc., Dalton Trans.