Monomeric, Dimeric and Polymeric Re^I(CO)₃ Schiff Base Complexes: Synthetic, Spectroscopic, Electrochemical, and Computational Studies

The spectroscopy, photophysics, and photochemistry of organometallic complexes of the general type [Re(X)(CO)₃(diimine)]^0/+ (X= halogen, CN^-, alkyl, benzyl, pyridyl, phosphine, isonitrile, or metal fragment group) have received much attention since the 1970s, when their excited state properties were recognized and presented by Wrighton. These organometallic complexes occupy an important position in the photophysics and photochemistry of transition-metal complexes due to their potential uses in many applications such as emission sensitizers, photosensitizers, photooxidants, photocatalysts and electrocatalysts. Due to their promising photophysical properties as well as their excellent thermal and chemical stabilities, Re^I(CO)₃ complexes have been incorporated into metal containing polymers and macromolecules as emitters in light emitting diodes (LEDs), photosensitizers in photovoltaic devices (PVs) and luminescent probes in biological systems.

In the second and third chapters, the syntheses and characterization of monomeric and dimeric Re^I(CO)₃ complexes is described by using one-pot conditions via metal mediated Schiff base complexation. These compounds are fully characterized by X-ray crystallography, NMR, mass spectroscopy, UV-visible and cyclic voltammetry. Cyclic voltammetry experiments confirm that in dimeric complexes the bridging ligands unit do mediate coupling between the Re(CO)₃X coordination complexes. Two-electron redox behaviour allows to calculate a comproportionation constant (K_c), which corresponds to the degree of coupling as the presence of an apparent inter-valence charge transfer (IVCT) band between the Re^I(CO)₃ centers. DFT and TDDFT calculations indicate that the reductions are ligand-centered, which support intra-ligand transitions for the IVCT bands.

In the next chapter, the synthesis of series of main chain organometallic polymers (MCOPs) containing Re(CO)₃Cl(diimine) cores is described. Three different types of polymerization reactions, including Yamamoto coupling, Heck coupling, and a new metal mediated Schiff base formation/condensation reaction, allowed for the formation of metal polymers with 100% metal complex incorporation and average molecular weights (Mn) ranging from 25 to 850 kDa.

Finally, a study of the induction of E/Z isomerization in two Re(CO)₃ diimine complexes with a pendant azobenzene group is presented. Time-resolved photophysical characterization reveales that excitation of the MLCT bands of the Re(CO)₃ diimine fragments results in isomerization of the azo bonds in the pendant groups.