Date of Award
Master of Science in Chemical Sciences (MSCB)
Photoluminescent copper(I) complexes have received tremendous attention in recent years as highly efficient and cheap alternatives to traditional 4d- and 5d-transition metals which represent key components in OLEDs (Organic Light-Emitting Diodes). OLEDs have revolutionized energy- saving light applications, ranging from lighting to flat screens and foldable displays for smartphones. Multinuclear CuI clusters are especially interesting for the tunability of their luminescence properties. In particular, controlled cluster formation methods such as creating linear arrangements of cuprous ions allow for a systematic design of new photoluminescent materials, with a particular focus on highly demanded blue light emitters. However, to date only a few of such copper(I) array complexes with usually three to four metal centers have been reported in the literature. Our group has previously demonstrated that bis(amidines) can undergo with mesitylcopper a clean conversion into neutral CuI cluster arrays with up to eight metal centers. Herein, new propylene-bridged bis(amidines) capable of providing six to eight donor sites each, are reported. The coordination behavior of two of these ligands was explored.
A series of polydentate bis(amidine) ligands (L1-4H2) featuring highly flexible propylene bridges and several multinuclear copper(I) complexes (I-III) was synthesized and characterized. L1-4H2 were prepared by a three step protocol: amidation of propylene diamine by either pivaloyl chloride or benzoyl chloride gave the corresponding bis(amide) precursor, which was chlorinated using PCl5 to form a bis(imidoyl chloride) intermediate; this intermediate was then aminated with 2-amino-6-methylpyridine or 5,7-dimethyl-1,8-naphthyridin-2-amine to obtain the final bis(amidine) product. L1-4H2 were prepared in yields between 24 and 72%.
Ligands L1-2H2 were demonstrated to undergo clean reactions with mesitylcopper to form luminescent multinuclear copper(I) complexes in yields between 48% and 71%. X-ray crystallography revealed that complex I is a homoleptic tetracopper(I) complex with a unique intertwined arrangement of the polydentate bis(amidinate) framework that exhibits ZZ(syn/syn) configuration and has two binding pockets with two CuI centers, each constituted by two 6-methyl- pyridyl amido side groups, one in parallel and the other in antiparallel orientation. Complex II is a heteroleptic tricopper(I) complex with only one bis(amidinate) ligand adopting ZZ(syn/syn) configuration and a retained mesityl fragment, in which a triangular core is formed by the copper(I) centers. Similar to I, complex III is a homoleptic intertwined complex, but with significantly decreased steric restrictions resulting in a EE(anti/anti) bis(amidinate) configuration that allows for two antiparallel orientations of the terminal N-donor side groups, which accompanied by the formation of two bis(amidinate) binding pockets that leave the pyridyl substituents uncoordinated.
The photoluminescent properties of I, II, and III were explored by excitation and steady state emissions spectra. These data reveal a notable blue-shifted emission from solution to solid state for complex II and a large Stokes shift for complex III in solution (6147 cm-1). X-ray crystallographic data confirms close CuI⋯CuI contacts (2.45-3.01 Å) in all three clusters, which likely plays a substantial role in the luminescence properties of these complexes.
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