In 2003, the Nobel Assembly at the Karolinska Institute has awarded the Nobel Prize in Medicine or Physiology to Paul Lauterbur and Peter Mansfield for their seminal discoveries concerning the development of Magnetic Resonance Imaging (MRI), a non-invasive diagnosis technique notably used in medicine to visualize the human internal organs and to distinguish between healthy and pathological cells. For this purpose, contrast agents are valuable tools that enhance this distinction which is resulting from the difference in the water content of the different tissues. These drugs are used nowadays in more than 30% of the 60 millions MRI examinations performed each year. And both numbers are still growing fast. Most of the contrast agents contain a paramagnetic ion (GdIII) that accelerates the relaxation of the water protons by providing them a new relaxation pathway. Relaxivity is a way of quantifying the contrast agent efficiency and is governed by four key parameters: hydration number of the lanthanide ion, water exchange rate, electronic relaxation rate and rotational correlation time of the system. A judicious simultaneous optimization of these parameters is predicted to result in a 20-fold increase in relaxivity compared to the currently used contrast agents. In this work, with the aim of increasing the relaxivity and better understanding the involved mechanisms, the synthesis and physico-chemical characterization of new dinuclear GdIII complexes, based on thoroughly studied monomeric equivalents, were performed. We particularly stressed the outmost importance of introducing rigidity together with the increased size of the system, for a flexible assembly will hinder the relaxivity gain brought by the higher molecular weight. This rigidity is induced by using an aromatic linker between the chelating subunits or by means of the self-assembling properties of terpyridine units around FeII or RuII. We investigated the thermodynamic stability, which in crucial for toxicological reason, the spectrophotometric properties, and the rotational and water exchange dynamics of these complexes. As a result, a remarkable increase in relaxivity is obtained for all the studied compounds with respect to the clinically used actual contrast agents. Finally, insights into a promising novel class of MRI contrast agents acting simultaneously as luminescence probes are provided.