Impact of ligand-centered excited states on luminescence sensitization in Pr3+ complexes with β-diketones

https://doi.org/10.1016/j.saa.2021.119863Get rights and content

Highlights

  • Novel Pr3+ complexes with 1,3-diketonate ligands were synthesized.

  • Two ligand-to-ligand charge transfer states of different types were found.

  • LLCT states can transfer energy to the 3P0 and 3P1 levels of Pr3+.

Abstract

In this study, two novel Pr3+ complexes with different 1,3-diketonate ligands were synthesized and investigated. To study the effect of the ancillary ligand on the energy transfer mechanisms in the complexes, a phenanthroline ligand was introduced. To take into account the influence of the ligand environment composed of different ligands on the energy transfer and relaxation processes, we compared the synthesized compounds with a similar complex containing the phenanthroline ligand. The spectroscopic studies in the visible and near-infrared spectral regions were supplemented with DFT and TD-DFT calculations. We found two ligand-to-ligand charge transfer (LLCT) states, with one state corresponding to energy transfer between 1,3-diketones and the other – to energy transfer from the 1,3-diketone to the phenanthroline motif. It was demonstrated that optical excitation via the latter channel leads to a fourfold increase in the luminescence quantum yield as compared with excitation via the π-π transitions in 1,3-diketones. Moreover, both LLCT states provide sensitization of the Pr3+ luminescence involving the 3P0 and 3P1 levels.

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 Ln3+ 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 Ln3+ ions are almost independent of the ligand environment [1], [7]. At the same time, optical absorption cross-sections of free Ln3+ 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 Ln3+ coordination compounds, complexes of highly luminescent Eu3+,Tb3+, and Sm3+ are most studied [1], [14], [15], [16]. Also, much attention has been paid to magnetic ions, e.g. Dy3+,Er3+, and Ho3+ [17], [18]. Coordination compounds of Pr3+ are much less studied [1], [19]. Meanwhile, the Pr3+ ion is also luminescent; moreover, its emission bands fall within both visible and near-infrared (NIR) regions. Since complexes of Pr3+ 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 Pr3+ 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 Pr3+ 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 PrCl3·7H2O (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 [Pr(L2)3(Phen)] was obtained by the standard synthetic procedure based on the interaction of sodium salt of HL2 with PrCl3 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 [Pr(L1)3(EtOH)]; however, no Phen ligand was added in this case.

The structure of complex [Pr(L1)3(EtOH)] was elucidated based on the elemental analysis, as well as FTIR

Conclusion

We investigated two novel Pr3+ 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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