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
((S)PORCu) 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 ((S)TPA) and pyrene ((S)Py) derivatives together with free-base
((R)POR2H) and zinc porphyrin ((S)PORZn) derivatives and rationalized the large potential
for this multicomponent supramolecular polymerization.