Self-assembly properties of solution processable, electroactive alkoxy and alkyl-thienylene derivatives of fused benzoacridines - an STM study.

Self-organization in mono- and bilayers on HOPG of two groups of benz[5,6]acridino[2,1,9,8-klmna]acridines derivatives, namely: 8,16-dialkoxybenzo[h] benz[5,6]acridino[2,1,9,8-klmna]acridines with increasing alkoxy substituents length and 8,16-bis(3 or 4 or 5-octylthiophene-2-yl)benzo[h]benz[5,6]acridino[2,1,9,8-klmna]acridines i.e. three positional isomers of the same benzoacridine is investigated by STM. The layers were deposited from a solution of the adsorbate (in hexane or dichloromethane) and ex-situ imaged at molecular resolution. In all cases the resulting 2D supramolecular organization is governed by the interactions between large, fused heteroaromatic cores which form densely packed rows separated by areas covered by substituents. In 8,16-dialkoxybenzo[h] benz[5,6]acridino[2,1,9,8-klmna]acridines the alkoxy substituents, separating the rows of densely packed cores, are interdigitated. Increasing substituent length leads to intuitively expected increase of this 2D unit cell parameter which corresponds to the orientation of the substituent in the monolayer. In the case of 8,16-bis(3 or 4 or 5-octylthiophene-2-yl)benzo[h]benz[5,6]acridino[2,1,9,8-klmna]acridine positional isomers the self-assembly processes are more complex. Although the determined 2D unit cell is in all the cases essentially the same, the role of alkylthienylene substituents in the layer formation is distinctly different. Thus, the formation of monolayers and bilayers is very sensitive to isomerism. 8,16-bis(5-octylthiophene-2-yl)benzo[h]benz[5,6]acridino[2,1,9,8-klmna]acridine is capable of forming the most stable monolayer and the most labile bilayer. In the case of 8,16-bis(3-octylthiophene-2-yl)benzo[h]benz[5,6]acridino[2,1,9,8-klmna]acridine an inverse phenomenon is observed leading the most labile monolayer and the most stable bilayer. These differences are rationalized in terms of dissimilar molecular geometries of the studied isomers and different interdigitation patterns in their 2D supramolecular structures.