Methylation and the system-size effect over the structural, electronic, magnetic (NICS) and reactive properties of pentalene derivatives

► We study the electronic and molecular properties of polycyclic π systems. ► Optical properties are related to methylation and system size in these systems. ► Electronic excitations are assigned mainly to π → π∗ transitions. ► Antiaromaticity character decreasing with the increment of the system-size.