Universität zu Köln

Niazi, Maryam ORCID: 0009-0006-3486-3690 (2025). Molecular Rigidification for Efficient Transition Metal Complex-Based Triplet Emitters. PhD thesis, Universität zu Köln.

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Abstract

Transition metal complexes are promising candidates for optoelectronic applications due to their ability to emit from triplet states. However, non-radiative deactivation pathways often caused by structural distortions between the ground and excited states, quench emission. In this work, we explore molecular rigidification as a strategy to enhance luminescence efficiency in cyclometalated Pt(II), Pd(II), and Ni(II) complexes by tuning second sphere interactions and steric bulk. Square planar [M(N^C^N)Cl] (M = Pt, Pd, Ni; N^C^N = 1,3-dipyridyl-benzene) complexes with NH-functionalized ligands were synthesized to facilitate intramolecular hydrogen bonding. Structural analysis confirmed distorted square planar geometries and hydrogen bonding interactions, and DFT calculations revealed minimal reorganization between the ground and triplet excited states. While Pt(II) complexes exhibited intense phosphorescence in solution at 298 K, Pd(II) analogues remained non-emissive at 298 K, indicating that rigidification alone was insufficient to suppress non-radiative decay. In Ni(II) complexes, elongated Ni–N bonds in the excited state were observed, and emission studies confirmed structured phosphorescence. However, transient absorption spectroscopy suggested that the excited state absorptions were ligand-centered and short-lived, with potential decomposition pathways complicating the photophysical analysis. To optimize photoluminescence, chloride coligands were replaced with phenoxide (OPh–), strengthening hydrogen bonding (M–O···H−N), but further studies are needed to assess the impact on emission. To distinguish steric from hydrogen bonding effects, benzyl-substituted analogues of [M(N^C^N)Cl] (M = Pt, Pd, Ni), instead of NHR functionalities in the earlier complexes were synthesized. X-ray diffraction revealed steric-induced distortions, particularly in Ni(II). Pt(II) complex displayed room-temperature phosphorescence, forming emissive aggregates in polymer matrices, while Pd(II) and Ni(II) complexes were only emissive at 77 K. Further modifications introduced electron-donating (Me) and electron-withdrawing (F or Cl) substituents on different positions of the N^C^N ligand framework, revealing ··· interactions. Pt(II) complexes exhibited highly efficient triplet emission (quantum yields up to 0.97 at 298 K), whereas Pd(II) and Ni(II) complexes showed weak luminescence at 77 K due to metal-centered deactivation. Lastly, incorporating quinoline units into the NCN ligand backbone in [M(LQu)Cl] (M = Pt, Pd, Ni; LQu = bis(2-pyridyl)/2-quinolyl)benzene) complexes induced minimal structural reorganization in Pt(II) but increased reorganization in Pd(II) and Ni(II) derivatives. This modification subtly red shifted absorption bands, highlighting its influence on electronic and photophysical properties. Overall, this study demonstrates that both second sphere interactions and steric bulk influence structural rigidity and emission properties, with implications for designing next generation triplet emitters.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Niazi, Maryam mary.niazi91@gmail.com orcid.org/0009-0006-3486-3690 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-781699
Date:	2025
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Chemistry > Institute of Inorganic Chemistry
Subjects:	Natural sciences and mathematics
Uncontrolled Keywords:	Keywords Language Organometallic English cyclometalation English phosphorescence English
Date of oral exam:	25 April 2025
Referee:	Name Academic Title Klein, Axel Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/78169