A model of a 'molecular' 2D crystal of quantum dots is considered. Each elementary cell contains two dots connected by tunnelling. Such an array of two-level quantum systems with strongly non-linear polarizability demonstrates the ferroelectric (antiferroelectric) phase transitions due to Coulomb interaction. We find the energy of the ground state, spontaneous moment and phase transition temperature in the self-consistent field approximation. Optical manifestations of the phase transitions in the far-infrared absorption are also investigated.