Investigation of Molecular Structures for Soft Matters with Mechanoresponsive Colors

Materials with mechnoresponses are a new discipline in organic soft matter synthesis. One mechanoresponse, thixotropy, is the tendency of a material to become less viscous upon the addition of mechanical stress such as shaking. Separately in other gels, a visible color change during the gel-to-solution transition has been observed. While gels with either thixotropic or color-change qualities have been reported, the molecular structure of a thixotropic gel with mechanoresponsive colors has not yet been confirmed. In this work, we investigate molecular structures of gels. Synthesized platinum-terpyridine (Ptterpy) complexes bearing an amino acid moiety were manipulated by changing solvent, counter anion and molecular structure. Pt-terpy gels and corresponding solution samples were analyzed through stable-byinversion tests and emission spectroscopy. A thixotropic Pt-terpy gel without (fluoro-9-yl)methoxylcarbonyl (Fmoc) moiety exhibited a 648 nm emission peak, which indicated the presence of Pt-Pt interactions. Thixotropic gels that display mechanoresponsive colors may be applicable as oscillation sensors for buildings and earthquakes. Reticular Materials Group has previously synthesized a Ptterpy thixotropic gel. The monomer for the thixotropic gel incorporates the Pt-terpy moiety and a functionalized tyrosine, shown in Scheme 1. The hydrogen bonding between the amino acid units are thought to cause the gelation capabilities of this complex. In addition, Pt-terpy possesses many spectral properties; specifically, emission at approximately 650 nm indicates Pt-Pt interactions [4]. However, the previously synthesized Pt-terpy thixotropic gel showed no absorption or emission that indicated the presence of Pt-Pt interactions. A gel that exhibits both properties of thixotropy and color change upon phase transition has yet to be reported. In this study, we employed traditional organic synthesis procedures to manipulate the Pt-terpy complex structure in order to create a thixotropic gel with mechanoresponsive colors. Experimental Procedure: A series of Pt-terpy complexes of analogous forms were synthesized and tested for their gelation potential and color change ability. The basic structure is shown in Scheme 2. Counter anion salts in 10x molar excess were dissolved in Pt-terpy solutions for anion exchange. Alteration to the Scheme 1: Molecular structure of Pt-terpy thixotropic gel. Scheme 2: Basic molecular structure for synthesized Pt-terpy complex analogs.