The phenomenon of polymorphism is ubiquitous in biological systems and has also been observed in various types of self-assembled materials in solution and in the solid state. In the field of supramolecular polymers, different kinetic vs thermodynamic self-assembled species may exist in competition, a phenomenon termed as pathway complexity. In these examples, the transient kinetic species often has a very short lifetime and rapidly converts into the thermodynamic product. In this work, we report a π-conjugated Pt­(II) complex 1 that self-assembles in nonpolar medium into two competing supramolecular polymers with distinct molecular packing (slipped (A) vs pseudoparallel (B)) that do not interconvert over time in a period of at least six months at room temperature. Precise control of temperature, concentration, and cooling rate enabled us to ascertain the stability conditions of both species through a phase diagram. Extensive experimental studies and theoretical calculations allowed us to elucidate the packing modes of both supramolecular polymorphs A and B, which are stabilized by unconventional N–H···Cl–Pt and N–H···O-alkyl interactions, respectively. Under a controlled set of conditions of cooling rate and concentration, both polymorphs can be isolated concomitantly in the same solution without interconversion. Only if A is annealed at high temperature for prolonged time, does a slow transformation into B then take place via monomer formation. Our system, which in many respects bears close resemblance to concomitant packing polymorphism in crystals, should help bridge the gap between crystal engineering and supramolecular polymerization.

The self-assembly features of hydrophobic bispyridyldichlorido Pd(II) complexes, equipped with an extended aromatic surface derived from oligophenyleneethynylene (OPE) and polarizable amide functional groups, are reported. The cooperative supramolecular polymerization of these complexes results in bundles of thin fibers in which the monomer units are arranged in a translationally displaced or slipped fashion. Spectroscopic and microscopy studies reveal that these assemblies are held together by simultaneous π-stacking of the OPE moieties and NH···ClPd hydrogen bonds. These unconventional forces are often observed in crystal engineering but remain largely unexploited in supramolecular polymers. Both steric and electronic effects (the presence of bulky and polarizable metal-bound Cl ligands as well as hydrogen bonding donor NH units) prevent the establishment of short Pd-Pd contacts and strongly condition the aggregation mode of the reported complexes, in close analogy to the previously reported amphiphilic Pd(II) complex 4. The results presented herein shed light on the subtle interplay between different noncovalent interactions and their impact on the self-assembly of metallosupramolecular systems.