Molecular design of novel luminescent complexes based on hybrid phosphine ligands for chemo- and biosensing applications

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This joint project aims to provide access to novel luminescent systems based on a new class of multidonor hybrid phosphorus-containing heterocycles and their transition metal complexes for applications in chemo- and biosensing and bioimaging. The main synthetic efforts will be directed towards cyclic phosphine ligands containing two endocyclic phosphorus(III) atoms, two exocyclic chromophoric groups, and two tunable functions located on the endocyclic nitrogen atoms to allow maximal synthetic flexibility within a low-cost framework. These compounds will be synthesized by condensation reactions of primary heteroaryl phosphines with formaldehyde and subsequent ring formation through addition of primary amines or amino acids. Chromophoric heteroaromatic substituents at phosphorus and nitrogen provide luminescence properties and additional binding sites for supramolecular assemblies. Differing substituents on the endocyclic nitrogen atoms will also be used to modify the steric and electronic properties of the donor centers, and will simultaneously provide the means to fine-tune physical properties such as ligand lipo- and hydrophilicity and luminescence. The optical properties of metal complexes depend on the type of chromophore and metal ion. A variety of luminescence parameters will be achieved by employing d6, d8, and d10 metal ions (Ir(III), Pt(II), Au(I) und Cu(I)) with different coordination numbers and coordination spheres. For effective charge transfer and high luminescence, chromophoric groups should be coordinated to the metal center or be located in near proximity. Moreover, a number of chromophoric coligands will be coordinated at the metal center to modify the photophysical properties of the complexes. The unique structure of the targeted complexes will provide a basis for their substrate-induced luminescence response, which in turn results from redox transformations or host–guest complex formation. Quantum chemical investigations of the electronic structure will be performed for the synthesized complexes with the aim of revealing the orbitals involved in the transitions responsible for electronic absorption and luminescence. On the basis of an understanding of the spectroscopic origin and their structure–property relationships, the potential for the development of luminescent metal complexes for a variety of functions and applications, especially in the field of bioimaging and biosensing, will be evaluated. As insolubility of the complexes in water might restrict their suitability for bioimaging and biosensing, these complexes will also be converted to water-soluble hydrophilic nanoparticles for these studies.