Charge-Transfer Complexes of Linear and Ladder-Type Conjugated Polymers as Promising Organic Narrow-Gap Semiconductors

Abstract

Charge-transfer complexes (CTCs) of small π-conjugated organic molecules have been studied for a long time as organic semiconductors and conductors. However, the formation of such complexes between two π-conjugated macromolecules, namely, conjugated polymers has only recently been discovered. In this work, the formation of such CTCs is theoretically considered and their specific features in comparison with low-molecular CTCs are revealed. In particular, it is shown that the energy gap (optical gap) of the CTC of conjugated polymers can be significantly narrower and the charge transfer can be higher than those in their low-molecular analogs (even at the same energies of the highest occupied molecular orbital of a donor and the lowest unoccupied molecular orbital of an acceptor), which is explained by charge delocalization along a polymer chain. This finding is most pronounced in the case of ladder-type polymers, the charge delocalization in which has a quasi-two-dimensional character. Density functional theory calculations of CTC fragments of two ladder-type conjugated polymers confirm the theoretical assumptions, emphasizing the prospects of using such complexes in organic electronic devices. Based on the results obtained, we propose to create and study a new type of CTC using higher-dimension structures, e.g., two-dimensional sheets or three-dimensional conductive frameworks.