The organization of phthalocyanines into controlled structures is essential for their applications in electrical and optical molecular devices. Highly homogeneous bar-shaped phthalocyanine aggregates with large aspect ratios are obtained by tuning of alkyloxy substitutions. The layer-by-layer growth mechanism for both one-dimensional nanometer scale bar-shaped structures and two-dimensional films are revealed by atomic force microscope and X-ray diffraction. Density functional computations show that the different interactions of in-plane and displaced parallel stacking for phthalocyanines are responsible for the different self-assembly structures. (c) 2007 Elsevier B.V. All rights reserved.