Conformational Locking of the Geometry in Photoluminescent Cyclometalated N^C^N Ni(II) Complexes

Niazi, Maryam; Maisuls, Iván; Mai, Lukas A.; Schäfer, Sascha A.; Oster, Alex; Diaz, Lukas Santiago; Guldi, Dirk M.; Doltsinis, Nikos L.; Strassert, Cristian A.; Klein, Axel

doi:10.3390/molecules30091901

Open AccessArticle

Conformational Locking of the Geometry in Photoluminescent Cyclometalated N^C^N Ni(II) Complexes

by

Maryam Niazi

¹

,

Iván Maisuls

^2,3

,

Lukas A. Mai

⁴

,

Sascha A. Schäfer

¹,

Alex Oster

⁵,

Lukas Santiago Diaz

⁴,

Dirk M. Guldi

⁴,

Nikos L. Doltsinis

^5,*

,

Cristian A. Strassert

^2,3,*

and

Axel Klein

^1,*

¹

Department of Chemistry and Biochemistry, Institute for Inorganic and Materials Chemistry, Faculty for Mathematics and Natural Sciences, University of Cologne, Greinstraße 6, D-50939 Köln, Germany

²

Institut für Anorganische und Analytische Chemie, Universität Münster, Corrensstraße 28/30, D-48149 Münster, Germany

³

CiMIC, CeNTech, Heisenbergstraße 11, D-48149 Münster, Germany

⁴

Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials (ICMM), Friedrich-Alexander-University Erlangen-Nuremberg, Egerlandstraße 3, D-91058 Erlangen, Germany

⁵

Institut für Festkörpertheorie and Center for Multiscale Theory and Computation, Universität Münster, Wilhelm-Klemm-Straße 10, D-48149 Münster, Germany

^*

Authors to whom correspondence should be addressed.

Molecules 2025, 30(9), 1901; https://doi.org/10.3390/molecules30091901

Submission received: 26 March 2025 / Revised: 18 April 2025 / Accepted: 22 April 2025 / Published: 24 April 2025

(This article belongs to the Special Issue 30th Anniversary of Molecules—Recent Advances in Inorganic Chemistry)

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Abstract

In our research aimed at replacing precious transition metals like platinum with abundant base metals such as nickel for efficient triplet emitters, we synthesized and studied Ni(II) complexes [Ni(L^NHR)Cl]. These complexes containing the N^C^N cyclometalating dipyridyl-phenide ligand, equipped with pending H-bonding amine groups (NH(C₆H₅) (L^NHPh) and NH(C₆H₅CH₂), ClL^NHBn). Molecular structures determined from experimental X-ray diffractometry and density functional theory (DFT) calculations in the ground state showed marked deviation of the Cl⁻ coligand (ancillary ligand) from the ideal planar coordination, with τ₄ values of 0.35 and 0.33, respectively, along with hydrogen bonding interactions of the ligand NH function with the Cl⁻ coligand. The complexes exhibit long-wavelength absorption bands at approximately 425 nm in solution, with the experimental spectra being accurately reproduced through time-dependent density functional theory (TD-DFT) calculations. Vibrationally structured emission profiles and steady-state photoluminescence quantum yields of 30% for [Ni(L^NHPh)Cl] and 40% for [Ni(L^NHBn)Cl] (along with dual excited state lifetimes in the ns and in the ms range) were found in frozen 2-methyl-tetrahydrofuran (2MeTHF) glassy matrices at 77 K. Furthermore, within a poly(methyl methacrylate) matrix, the complexes showed emission bands centered at around 550 nm within a temperature range from 6 K to 300 K with lifetimes similar to 77 K. Based on TD-DFT potential scans along the metal–ligand (Ni–N) coordinate, we found that in a rigid environment that restricts the geometry to the Franck-Condon region, either the triplet T₅ or the singlet S₄ state could contribute to the photoluminescence.

Keywords: nickel(II); cyclometalation; electrochemistry; photoluminescence; DFT calculations

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MDPI and ACS Style

Niazi, M.; Maisuls, I.; Mai, L.A.; Schäfer, S.A.; Oster, A.; Diaz, L.S.; Guldi, D.M.; Doltsinis, N.L.; Strassert, C.A.; Klein, A. Conformational Locking of the Geometry in Photoluminescent Cyclometalated N^C^N Ni(II) Complexes. Molecules 2025, 30, 1901. https://doi.org/10.3390/molecules30091901

AMA Style

Niazi M, Maisuls I, Mai LA, Schäfer SA, Oster A, Diaz LS, Guldi DM, Doltsinis NL, Strassert CA, Klein A. Conformational Locking of the Geometry in Photoluminescent Cyclometalated N^C^N Ni(II) Complexes. Molecules. 2025; 30(9):1901. https://doi.org/10.3390/molecules30091901

Chicago/Turabian Style

Niazi, Maryam, Iván Maisuls, Lukas A. Mai, Sascha A. Schäfer, Alex Oster, Lukas Santiago Diaz, Dirk M. Guldi, Nikos L. Doltsinis, Cristian A. Strassert, and Axel Klein. 2025. "Conformational Locking of the Geometry in Photoluminescent Cyclometalated N^C^N Ni(II) Complexes" Molecules 30, no. 9: 1901. https://doi.org/10.3390/molecules30091901

APA Style

Niazi, M., Maisuls, I., Mai, L. A., Schäfer, S. A., Oster, A., Diaz, L. S., Guldi, D. M., Doltsinis, N. L., Strassert, C. A., & Klein, A. (2025). Conformational Locking of the Geometry in Photoluminescent Cyclometalated N^C^N Ni(II) Complexes. Molecules, 30(9), 1901. https://doi.org/10.3390/molecules30091901

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Conformational Locking of the Geometry in Photoluminescent Cyclometalated N^C^N Ni(II) Complexes

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