A Para-Substituted 2-Phenoxy-1,10-Phenanthroline Ligand for Lanthanide Sensitization: Asymmetric Coordination and Enhanced Emission from Eu3+, Tb3+, Sm3+ and Dy3+ Complexes

Zaharieva, Joana; Videva, Vladimira; Kolarski, Mihail; Lyapchev, Rumen; Morgenstern, Bernd; Tsvetkov, Martin

doi:10.3390/molecules30173548

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A Para-Substituted 2-Phenoxy-1,10-Phenanthroline Ligand for Lanthanide Sensitization: Asymmetric Coordination and Enhanced Emission from Eu³⁺, Tb³⁺, Sm³⁺ and Dy³⁺ Complexes

by

Joana Zaharieva

^1,*

,

Vladimira Videva

^2,3,

Mihail Kolarski

²

,

Rumen Lyapchev

²,

Bernd Morgenstern

⁴

and

Martin Tsvetkov

^1,*

¹

Laboratory of Chemistry of Rare-Earth Elements, Faculty of Chemistry and Pharmacy, Sofia University “St. Kliment Ohridski”, 1164 Sofia, Bulgaria

²

Faculty of Chemistry and Pharmacy, Sofia University “St. Kliment Ohridski”, 1164 Sofia, Bulgaria

³

Institute of Optical Materials and Technologies, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

⁴

Inorganic Solid State Chemistry, Saarland University, Campus Geb. C4 1, 66123 Saarbrücken, Germany

^*

Authors to whom correspondence should be addressed.

Molecules 2025, 30(17), 3548; https://doi.org/10.3390/molecules30173548

Submission received: 10 June 2025 / Revised: 4 August 2025 / Accepted: 27 August 2025 / Published: 29 August 2025

(This article belongs to the Special Issue Rare Earth Luminescent Materials and Complexes: Structure, Properties and Applications)

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Abstract

A para-substituted 1,10-phenanthroline ligand, 2-(4-methylphenoxy)-1,10-phenanthroline (L24), was synthesized and structurally characterized. Complexes with Eu³⁺, Tb³⁺, Sm³⁺, and Dy³⁺ were obtained in a 2:1 ligand-to-metal ratio and analyzed using single-crystal x-ray diffraction, photoluminescence spectroscopy, and TD-DFT calculations. Coordination via the phenanthroline nitrogen atoms, combined with steric asymmetry from the para-methylphenoxy group, induces low-symmetry environments favorable for electric-dipole transitions. Excited-state lifetimes reached 2.12 ms (Eu³⁺) and 1.12 ms (Tb³⁺), with quantum yields of 42% and 68%, respectively. The triplet-state energy of L24 (22 741 cm⁻¹) aligns well with emissive levels of Eu³⁺ and Tb³⁺, consistent with Latva’s criterion. Fluorescence titrations indicated positively cooperative complexation, with association constants ranging from 0.60 to 1.67. Stark splitting and high ⁵D₀→⁷F₂/⁷F₁ intensity ratios (R₂ = 6.25) confirm the asymmetric coordination field. The para-methylphenoxy substituent appears sufficient to lower coordination symmetry and strengthen electric-dipole transitions, offering a controlled route to enhance photoluminescence in Eu³⁺ and Tb³⁺ complexes.

Keywords: lanthanide complex; luminescence properties; crystal structure; 1,10-phenanthroline derivate

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MDPI and ACS Style

Zaharieva, J.; Videva, V.; Kolarski, M.; Lyapchev, R.; Morgenstern, B.; Tsvetkov, M. A Para-Substituted 2-Phenoxy-1,10-Phenanthroline Ligand for Lanthanide Sensitization: Asymmetric Coordination and Enhanced Emission from Eu³⁺, Tb³⁺, Sm³⁺ and Dy³⁺ Complexes. Molecules 2025, 30, 3548. https://doi.org/10.3390/molecules30173548

AMA Style

Zaharieva J, Videva V, Kolarski M, Lyapchev R, Morgenstern B, Tsvetkov M. A Para-Substituted 2-Phenoxy-1,10-Phenanthroline Ligand for Lanthanide Sensitization: Asymmetric Coordination and Enhanced Emission from Eu³⁺, Tb³⁺, Sm³⁺ and Dy³⁺ Complexes. Molecules. 2025; 30(17):3548. https://doi.org/10.3390/molecules30173548

Chicago/Turabian Style

Zaharieva, Joana, Vladimira Videva, Mihail Kolarski, Rumen Lyapchev, Bernd Morgenstern, and Martin Tsvetkov. 2025. "A Para-Substituted 2-Phenoxy-1,10-Phenanthroline Ligand for Lanthanide Sensitization: Asymmetric Coordination and Enhanced Emission from Eu³⁺, Tb³⁺, Sm³⁺ and Dy³⁺ Complexes" Molecules 30, no. 17: 3548. https://doi.org/10.3390/molecules30173548

APA Style

Zaharieva, J., Videva, V., Kolarski, M., Lyapchev, R., Morgenstern, B., & Tsvetkov, M. (2025). A Para-Substituted 2-Phenoxy-1,10-Phenanthroline Ligand for Lanthanide Sensitization: Asymmetric Coordination and Enhanced Emission from Eu³⁺, Tb³⁺, Sm³⁺ and Dy³⁺ Complexes. Molecules, 30(17), 3548. https://doi.org/10.3390/molecules30173548

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A Para-Substituted 2-Phenoxy-1,10-Phenanthroline Ligand for Lanthanide Sensitization: Asymmetric Coordination and Enhanced Emission from Eu³⁺, Tb³⁺, Sm³⁺ and Dy³⁺ Complexes

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