"The concept of Möbius aromaticity, a postulate which states that [4n]annulenes lying in a twisted Möbius strip are aromatic, is fascinating from both theoretical and experimental standpoints. Although this concept was proposed by Heilbronner in 1964, the synthesis of a stable aromatic Möbius system was a challenge during 40 years until 2003. Recently, the expanded porphyrins have emerged as a new class of heteroannulene variants that display a range of interesting electronic and conformational characteristics, including the possibility to switch between Möbius and Hückel topologies with different photophysical properties, applying only small changes in the external conditions (temperature, pH and solvent) or in the structure of the ring (meso-substituent, metal). Despite the numerous applications of such structures, a systematic study on expanded porphyrins has not been made so far. The main aim of this proposal is to design new viable Möbius aromatic compounds from annulenes and expanded porphyrins with applications as non-linear optical materials or molecular transporters. For that purpose, the role of the aromaticity, strain and metallation in the stability of Möbius aromatic compounds and their photophysical properties will be assessed.The comprehensive computational study will deal with the following issues:a) Performance of several density functionals to describe their geometrical, energetic and electrical properties;b) Conformational analysis, classifying the most stable conformers as Hückel, twisted Hückel or Möbius;c) Dynamic switch between Hückel and Möbius topologies, evaluating the solvent and temperature effects;d) Effect of the number and nature of substituents on the pyrrolic and meso position on the viability of Möbius aromatic systems;e) Study of the metal complexes of Möbius-type macrocyclic ligands;f) Molecular dynamics studies of the most appealing Möbius aromatic systems to evaluate their thermodynamic stability;g) NLO properties."