Hydrogen-bonding versus π-π stacking in the design of organic semiconductors: From dyes to oligomers

This review discusses some general aspects of the highly directional intermolecular interactions in organic solids, followed by an overview of pi-conjugated systems exhibiting directional preference in the intermolecular connection and demonstrating enhanced energy and/or charge carrier transfer. As examples, small dyes and pigments, intermediate-sized N-heteroacenes, and large hydrogen-bonded oligomers are considered. In all of these systems the intermolecular interactions between polarized peripheries of the pi-conjugated moieties ensure a preferential direction in the molecular packing. The most important among these interactions are hydrogen bonds (both strong and weak). As demonstrated recently, hydrogen bonding represents a dominant attractive force even for molecules with different polarization of the pi-conjugated aromatic or heterocyclic moieties, considered to be the most favorable condition for face-to-face orientation (pi-pi stacking). The solid-state anisotropy, most likely, results from the nematic liquid-crystalline ordering of both thermotropic and lyotropic origin. Moreover, the unique efficiency of the exciton diffusion in the J-aggregates of dyes indicates significant potential of the nematic liquid-crystalline state itself for the design of organic semiconductors. (C) 2014 Elsevier Ltd. All rights reserved.