Tailoring the thermotropic behavior of tetra-substituted phthalocyanines via the lateral chains architecture

Four newly synthesized alkoxy tetrasubstituted phthalocyanines (Pcs) were characterized by polarized optical microscopy, differential scanning calorimetry, and X-ray diffraction on oriented samples. It was found that the introduction of lateral alkyl chains branched in the second position renders the material liquid crystalline at room temperature. The studied Pcs exhibit a transition between a columnar hexagonal and a columnar rectangular phase. The tilt of the molecules with respect to the columnar axis in the rectangular phase, as measured by X-ray fiber diffraction, was found to be 15-18 degrees. The columnar structure, and defects therein, in relatively thick spin-coated mesomorphic Pc films were for the first time characterized by tapping mode atomic force microscopy, employing ultra-sharp tips with nanometer-thick whiskers. Spin-coated films reveal layerlike structure with domain boundaries that can induce significant columnar curvature without breaking of the columns. In some instances, the boundaries delimiting different LC domains completely disrupt the columnar structure making the columns in neighboring regions uncorrelated.