Novel asymmetric liquid crystalline ZnPc thin films for optical surface plasmon resonance sensor applications to dichloromethane vapor: Design, synthesis, preparation, characterization, and the influence of the substituent group on the sensor response

Abstract

This study aims to synthesize two newly designed, soluble asymmetrically substituted Zn(II) phthalocyanines (ZnPc1-A3B and ZnPc2-AB3) exhibiting liquid crystalline behavior, aimed at improving their chemiresistive sensing performance towards dichloromethane vapor. They were characterized using Nuclear Magnetic Resonance, Ultraviolet-visible, and Fourier Transform Infrared spectroscopies, together with mass spectrometry. Polarizing Optical Microscopy, X-ray Diffraction, and Differential Scanning Calorimetry techniques were employed to determine phase transition temperatures. The formation of a columnar hexagonal (Colh) mesophase for ZnPc1-A3B and a rectangular columnar (Colr) mesophase for ZnPc2-AB3 was confirmed over a wide temperature range by complementary methods. The observed phase transition temperatures were found to be strongly dependent on the nature of the substituents. Spin coating and Surface Plasmon Resonance techniques were employed to fabricate thin film sensors and to investigate the influence of phthalocyanine molecular architecture and the controlled incorporation of polyoxyethylene and alkylthio substituents on sensor behavior. For the detection of dichloromethane vapor, the ZnPc1-A3B thin film sensor produced the highest sensitivity, the lowest limit of detection, and limit of quantification values compared with the ZnPc2-AB3 thin film sensor. It is shown that the polyoxoethylene side chains having oxygen atoms play an important role in enhancing sensor performance before and after heating processes. The Elovich model was employed to determine the adsorption rate constants. This study improved gas sensor performance using Zn(II) phthalocyanine derivatives, optimizing the number of the substituents on the sensor response, and showed that the ZnPc1-A3B thin film sensor is a good candidate for an optical-based sensor in the field of environmental monitoring or dichloromethane vapor detection.