Effect of the number and position of substituents on the orientation and sensor response of triethylene glycol-substituted silicon phthalocyanine films to ammonia

Abstract

Si(IV)Pc derivatives bearing two –O(CH₂CH₂O)₃CH₃ substituents in axial positions (SiPc-1) and four (SiPc-2) or eight (SiPc-3) –O(CH₂CH₂O)₃CH₃ substituents on the phthalocyanine ring were investigated to reveal the effect of position and number of substituents on the orientation and chemiresistive sensor response of their films to ammonia. The liquid crystalline behavior of the Si(IV)Pc derivatives was studied using differential scanning calorimetry and polarizing optical microscopy. Only the SiPc-3 derivative was found to exhibit a mesophase over a wide temperature range, identified as a discotic columnar hexagonal (Col_h) phase. The orientation of SiPc films prepared by spin coating was determined by polarized Raman spectroscopy. SiPc-1 formed crystalline films with a preferential orientation of phthalocyanine macrocycles relative to the substrate surface, with the inclination angle of approximately 65°. SiPc-2 films, on the other hand, were disordered. The introduction of eight triethylene glycol-substituents into SiPc-3 resulted in a change in the film orientation, with the macrocycles oriented parallel to the substrate surface. SiPc-3 films exhibited the best chemiresistive sensor response to ammonia, with a detection limit of 100 ppb and low response and recovery times.