An intermolecular pyrene excimer in the pyrene-labeled N-thiophosphorylated thiourea and its nickel(ii) complex

Abstract

A new N-thiophosphorylated thiourea (1-pyrene)NHC(S)NHP(S)(OiPr)₂ (HL) has been synthesized. The molecular structure of HL was elucidated by X-ray diffraction revealing a linear intramolecular hydrogen bond. Additionally, its crystal structure is stabilized by two intermolecular hydrogen bonds, which in turn leads to a centrosymmetric R²₂(8) dimer formation. These dimers are packed into polymeric chains through π⋯π stacking interactions between the pyrene rings. The reaction of the deprotonated HL with Ni^II leads to the Ni^II complex ([NiL₂]). The crystal structure of [NiL₂] exhibits a centrosymmetric homoleptic structure, where the Ni^II ion is coordinated in a square-planar fashion with the ligands arranged in a trans-N₂S₂ configuration. The pyreneNH protons in [NiL₂] are involved in intramolecular hydrogen bonds of the pyreneN–H⋯SP type. Molecules of [NiL₂] form π⋯π stacking interactions between the pyrene rings, yielding 1D polymeric chains similar to those observed in the structure of HL. These π⋯π stacked 1D polymeric chains are linked to each other through C–H⋯S and anagostic C–H⋯Ni interactions, yielding 2D sheets. Hirshfeld surface analysis showed that the structures of both HL and [NiL₂] are highly dominated by H⋯H, H⋯C, H⋯S and C⋯C contacts and also characterized by H⋯O and H⋯N contacts. The molecular surfaces of HL and [NiL₂] also contain O⋯S and H⋯Ni contacts, respectively. Both HL and [NiL₂] were found to be emissive in CH₂Cl₂ solution, which is due to the concentration dependent emission of the pyrene monomer and excimer. It was established that the latter fluorescence is due to the intermolecular excimer formation. The DFT calculations allowed us to confirm the aggregation ability of the synthesized species in solution through the numerous non-covalent interactions C–H⋯S, C–H⋯Ni and C–H⋯π, which, in turn, might be responsible for the concentration dependent photophysical properties.