Tuning the architectures and luminescence properties of Cu(I) compounds of phenyl and carboranyl pyrazoles: the impact of 2D versus 3D aromatic moieties in the ligand backbone

Abstract

Incorporation of one or two o-carborane moieties at the backbone of the pyrazole ring was achieved by lithiation and nucleophilic addition onto the corresponding 3,5-dimethyl-1-(2-toluene-p-sulfonyloxyethyl)pyrazole. Two monosubstituted carboranyl pyrazoles (L2 and L3) and one disubstituted carboranyl pyrazole (L4) were synthesized and fully characterized. All new compounds, and the corresponding monosubstituted phenylderivative (L1) behave as N-type ligands upon coordination with CuI to afford different polynuclear Cu(I) compounds 1-4. Compounds 1-4 were fully characterized and their molecular structures were determined by X-ray diffraction. It is noteworthy that whereas the pyrazolylphenyl ligand L1, without o-carborane, provides a 1D coordination polymer (1), ligands containing carborane, L2-L3, affords 0D coordination compounds 2 and 3, and disubstituted carboranyl pyrazole ligand L4 gives rise to a 3D coordination polymer. The photoluminescence behaviour of compounds 1-4 has been investigated in the solid state and by TDDFT calculations for molecular compounds 2 and 3. Complex 2 exhibits blue emission with a maximum at 483 nm and a high fluorescence quantum yield of 66.5%. According to TDDFT calculations the emission occurs from LUMO to HOMO-1 and HOMO-2 and deexcitation could be described as cluster-centred excited state of d-s transition in origin. This result contradicts previous studies of scarce tri-coordinated rhombohedral Cu(I) clusters, where it was assumed the origin of their emissions is (X + M)LCT in nature by analogy with tetra-coordinated rhombohedral Cu(I) clusters. Complex 3 exhibits very weak emission (Phi(F) of 5%) in the green region with a maximum at 517 nm, which according to TDDFT is through a (CC)-C-3 state. Calculations also show that, upon excitation, 3 suffers a notable distortion resulting in the total cleavage of the Cu4I4 framework. This cleavage could be the cause of the relatively large Stokes shift observed for 3. To the best of our knowledge, this is the first time that such behaviour is observed for this type of octahedral compounds. Additionally, the 1D polymer 1 exhibits weak fluorescence emission in the orange range with a maximum at 609 nm and a remarkable Stokes shift, whereas the 3D polymer 4 exhibits a similar emission to compound 2, with a moderate quantum yield (FF of 13.7%).