Tetrasubstituted copper phthalocyanines: Correlation between liquid crystalline properties, films alignment and sensing properties

•Sensing properties of tetrasubstituted copper phthalocyanines were investigated.•Sensing performance was studied against ammonia by measuring conductivity changes.•Effect of substituents type and the heteroatoms on sensor performance was revealed.•Phthalocyanines liquid crystalline at room temperature exhibit maximal sensor response. Copper phthalocyanines (CuPc) containing alkylthio (-S(CH2)nCH3, n=7 and 15), alkyloxy- (-O(CH2)nCH3, n=7 and 15) and polyoxo [-O(CH2CH2O)3CH3 and −S(CH2CH2O)3CH3] substituents were synthesized and investigated to reveal the effects of substituents type (alkylthio, alkyloxy and polyoxo) and the type of the connecting heteroatom (oxygen or sulphur) on the mesogenic properties, films alignment and sensing behaviour. The liquid crystalline properties of these phthalocyanines were investigated by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction techniques. The structure and morphology of spun thin films of copper phthalocyanine derivatives were studied by the UV–vis and Raman spectroscopies as well atomic force microscopy. The sensing properties of CuPc films were studied by the measurement of conductivity change upon interaction with ammonia in the range 10–50ppm. All investigated films of CuPc derivatives display thermotropic columnar mesomorphism. It was shown that the films with polyoxo- [-O(CH2CH2O)3CH3 and −S(CH2CH2O)3CH3] substituents as well as with alkylthio −S(CH2)nCH3 (n=7) substituents, which are liquid crystalline at room temperature, form ordered films with a random planar alignment of columns. Their films exhibit the better sensor performance with the maximal sensor response for the films of CuPc containing (-S(CH2CH2O)3CH3) substituents.