High in-plane anisotropy in phthalocyanine LB films

Phthalocyanines substituted by paraffinic chains are known to form liquid crystals with the macrocycle stacked in columns. The Langmuir-Blodgett (LB) technique is used to prepare thin films of three types oftetrasubstituted phthalocyanines that differ by the nature of the substituent. The LB films are investigated by linear dichroism in the infrared range, which reveals a moderate in-plane anisotropy that strongly improves upon annealing a few hours at 100 degrees C or higher. The highest in-plane anisotropy is obtained when using an aliphatic substituent (C-6) linked to the macrocycle through an ester link. This substituent seems to be a good compromise between a purely aromatic one and an aliphatic-rich one (with two C-8 chains), which ensures fluid Langmuir films and allows the high in-plane anisotropy after transfer. The texture of these high in-plane anisotropic films is further investigated by atomic force microscopy in tapping mode. We find that the films are lamellar and have defects one monolayer or a few monolayers deep. It is shown that the variations of the texture of the film can be explained assuming a transfer ratio slightly below 1. Investigations of the film after annealing reveal that the number and size of the defects of the layers are greatly reduced. Their shape becomes elongated, and they become aligned with the dipping direction.