Interchain coupling effects on absorption spectrum in nondegenerate conjugated polymers

• More than one metastable states are found in neutral nondegenerate polymer.
• Two kinds of origins are explained for the red shift in absorption spectrum.
• Low frequency absorption from far infrared to near microwave are obtained.

Based on an extended Su-Schrieffer-Heeger (SSH) model combined with a dynamical evolution method, a numerical study was presented about interchain coupling effects upon the absorption spectrum in nondegenerate conjugated polymers. In particular, I studied a neutral system composed of two coupling chains with different length. It was shown that the absorption edge of low frequency generally has a red shift by increasing the interchain coupling strength, in agreement with experiments. For neutral systems, the red shift in absorption has two kinds of origins. In the first kind, the red shift is related to the reduction of the energy gap. In the second kind, the red shift is resulted from the electronic transition between the subgap states and the band states. It was also shown that for the neutral interchain coupling systems there exist more than one metastable states which can be achieved by dynamical evolutions from various initial lattice configurations. Especially, for the system evolving from an initial configuration with uniform bond lengths, the strong interchain coupling can readily result in subgap absorptions with marked red shift compared with the interband absorption, indicating the creation of some subgap states. In addition, these subgap absorption peaks tend to shift to the red side if the interchain coupling region is enlarged. For the cases of the coupling region covering more than 70 lattice sites on each chain, a microwave absorption peak can be found for large enough interchain coupling strength.