Energy provision is one of the major challenges for the Human Society, and it is increasingly clear that the current production/consumption model is not sustainable. The envisaged energy system is smarter, more decentralised and integrated. Energy conversion systems based on the organic Rankine thermodynamic cycle (ORC) have the potential to play a major role in this framework, being one of the most proven solutions for the exploitation of external thermal sources in the power-output range from, say, few kWe, up to tens of MWe. In ORC power converters, a phase-changing organic compound is adopted as the evolving fluid which, following the working principle defining the Rankine cycle, allows to exploit a given source in order to convert part of its energy content into useful outputs, such as, e.g., mechanical, electrical, and thermal energy. The ORC energy converters are extremely flexible in nature, and able to exploit a virtually infinite variety of thermal sources. At the same time, this poses great challenges from the design point of view. Innovative concepts can be devised drawing from the fundamentals of the working fluid behavior, passing to the component- and up to the system-level of detail, but the corresponding generalized design methodologies have to be concurrently developed and integrated. The work documented in this thesis aims at contributing to these topics, by presenting the original results of numerical and experimental research investigating the potential of molecularly heavy and complex organic compounds as working fluids for the ORC power systems of the future.