Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Impact of ligand-centered excited states on luminescence sensitization in complexes with -diketones
Graphical abstract
Introduction
Lanthanide coordination compounds show interesting physical and chemical properties promising for a large variety of important applications in multiple fields, including single-molecule magnets (SMM), non-linear optics, lasing, light-emitting diodes, and luminescent probes [1], [2], [3], [4], [5], [6].
Narrow, “atomic-like” absorption and emission bands of ions are due to transitions in the partially filled 4f electronic shell. These orbitals are shielded by the 5s and 5p shells; therefore, the spectral properties of ions are almost independent of the ligand environment [1], [7]. At the same time, optical absorption cross-sections of free ions are low, but the so-called “antenna effect” can be employed to overcome this problem [1], [8]. Here the point is that in such compounds, the optical excitation energy is absorbed by organic ligands and transferred to the central ion.
The proper choice of ligands is vital to obtain strong luminescence of complexes since the energy structure of the complex as a whole must be well-balanced. Recently, we have demonstrated that 1,3-diketones bearing a pyrazole moiety can be superior sensitizers of lanthanide emission [9], [10], [11], [12], [13]. Among all coordination compounds, complexes of highly luminescent , and are most studied [1], [14], [15], [16]. Also, much attention has been paid to magnetic ions, e.g. , and [17], [18]. Coordination compounds of are much less studied [1], [19]. Meanwhile, the ion is also luminescent; moreover, its emission bands fall within both visible and near-infrared (NIR) regions. Since complexes of reveal dual emission with relatively long lifetimes, these compounds may be of interest for sensing. Also, these materials can be involved in fluorescence microscopy, as well as act as active components in optoelectronic devices [20], [21].
Herein, we report the synthesis and photophysical properties of three novel luminescent (vis + NIR) complexes with pyrazole-substituted 1,3-diketones and 1,10-phenanthroline as an ancillary ligand. The energy transfer in the complexes was investigated in detail, and the role of the ancillary ligand in the sensitization of luminescence was discussed.
Section snippets
Materials and methods
Ligands were synthesized by the method described earlier in [22], [23]. Absolute ethanol was distilled over Mg tunings before use and then stored over molecular sieves 3A. All other reagents, including (99.99%), were purchased from Aldrich and Acros Organics and used without further purification. Elemental analysis was performed on the Elementar CHNO(S) analyzer. The praseodymium content in the complexes was determined by complexometric titration with a standard Trilon B solution in
Synthesis and determination of structure
Complex was obtained by the standard synthetic procedure based on the interaction of sodium salt of with in the presence of the Phen ligand. The pH value should not exceed 6.5–7.0 in order to prevent the retro-Claisen cleavage of the diketone. The same procedure was used for the preparation of complex ; however, no Phen ligand was added in this case.
The structure of complex was elucidated based on the elemental analysis, as well as FTIR
Conclusion
We investigated two novel complexes that exhibit luminescence in a wide region of the visible and NIR spectrum. We found that the emission occurs due to radiative transitions in the ion and the ligands. The measured quantum yields of the synthesized tris-(1,3-bis(1,3-dimethyl-1H-pyrazol-4-yl) propane-1,3-dionato)(monoethanolo) praseodymium (III) and tris-(1-(1,3-dimethyl-1H-pyrazol-4-yl)-4,4,4-trifluoro-1,3-butanedionato)(1,10-phenanthroline) praseodymium (III) compounds were 0.2% and
CRediT authorship contribution statement
V.M. Korshunov: Supervision, Investigation, Methodology, Writing - original draft, Writing - review & editing. M.T. Metlin: Investigation. S.A. Ambrozevich: Formal analysis, Writing - original draft. I.S. Golovanov: Methodology, Formal analysis. V.E. Gontcharenko: Investigation. A.S. Selyukov: Formal analysis, Writing - original draft. I.V. Taydakov: Conceptualization.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The synthetic part of the work was supported by the Russian Science Foundation under project #19–13-00272. Spectral measurements were funded by the Russian Foundation for Basic Research under project #20-33-70208. Luminescence spectra were collected using the equipment of the Center for molecular composition studies of INEOS RAS.
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