Distinct Pathways in "Thermally Bisignate Supramolecular Polymerization": Spectroscopic and Computational Studies

In general, supramolecular polymers are thermally labile in solution and easily depolymerized upon heating. This dynamic nature is beneficial in many aspects but limits certain applications. Recently, we developed "thermally bisignate supramolecular polymerization", through which a polymer is formed upon heating as well as cooling in a hydrocarbon solvent containing a small amount of alcohol. Here, we present a detailed mechanistic picture for this polymerization based on both spectroscopic and computational studies. For this particular type of polymerization, we mainly employed a copper porphyrin derivative (POR) as a monomer with eight hydrogen-bonding (H-bonding) amide units in its chiral side chains. Because of a strong multivalent interaction, the resulting supramolecular polymer displayed an extraordinarily high thermal stability in a hydrocarbon medium such as methylcyclohexane (MCH)/chloroform (CHCl₃) (98/2 v/v; denoted as MCH*). However, when a small volume (<2.0 vol %) of ethanol (EtOH) was added to this solution at ambient temperatures as a H-bond scavenger, the supramolecular polymer dissociated into its monomers. Here, it should be noted that, both upon cooling (clustering of EtOH) and heating (lower-critical-solution-temperature behavior, LCST), the monomer was liberated from the H-bond scavenger and underwent supramolecular polymerization. In this Article, we conducted detailed spectroscopic studies, analyzed the results using theoretical models, and eventually succeeded in supporting the pathways explaining why the monomer deactivated by the H-bond scavenger turns active upon both heating and cooling. We also investigated the thermally bisignate nature of the supramolecular polymerization of other monomers such as triphenylamine (TPA) and pyrene (Py) derivatives together with free-base (POR) and zinc porphyrin (POR) derivatives and rationalized the large potential for this multicomponent supramolecular polymerization.