Preparation of novel reversible thermochromic polyethylenimine dendrimer and tricyanofuran hydrazone chromophore

A reversible chromogenic thermometer composite of polyethylenimine (PEI) dendrimer immobilized with tricyanofuran hydrazone (TCFH) probe was prepared. Tricyanofuran hydrazone with an arylformyl group was synthesized by the azo-couple of TCF with the diazonium salt of 4-aminobenzaldehyde. Upon heating a mixture of the produced TCFH chromophore with polyethylenimine dendrimer, an imine bond is generated between the hydrazone aldehyde group and the polyethylenimine amino groups. Thus, the push-pull tricyanofuran hydrazone system is disrupted leading to color shift from pink to red. After cooling, the produced organogel showed a thermally reversible sol-gel transition in association with colorimetric changes. The origin of the detected thermochromic activity can be attributed to an equilibrium between two conformational changes of TCFH skeleton which reversibly stimulates the formation of polymer-dye complex (PEI-TCFH). The findings showed a novel mechanism for naked-eye detection of temperature changes. Upon heating from 25 to 65 °C, PEI-TCFH acted as a thermochromic sensor displaying colorful changes between pink (600 nm) and red (455 nm). Those colorful changes were attributed to the reversible hydrazone-imine phase transition. The self-assembly of PEI-TCFH molecules arises from van der Waals interactions leading to gelation. Additionally, acidic gases and liquid mediums demonstrated significant shifts in both of rheological and colorimetric properties of the gel to indicate potential applications in many fields like thermochromic protective textiles and gas sensors. Scan electron microscope (SEM) and transmission electron microscope (TEM) were applied to study the xerogels generated from n-propanol to indicate the configuration of fibrous nanostructures. The cytotoxic activity of the PEI-TCFH gel thermometer was also evaluated.