Synthesis and photophysical studies of self-assembled multicomponent supramolecular coordination prisms bearing porphyrin faces

Abstract

Multicomponent self-assembly, wherein two unique donor precursors are combined with a single metal acceptor instead of the more common two-component assembly, can be achieved by selecting Lewis-basic sites and metal nodes that select for heteroligated coordination spheres. Platinum(II) ions show a thermodynamic preference for mixed pyridyl/carboxylate coordination environments and are thus suitable for such designs. The use of three or more unique building blocks increases the structural complexity of supramolecules. Herein, we describe the synthesis and characterization of rectangular prismatic supramolecular coordination complexes (SCCs) with two faces occupied by porphyrin molecules, motivated by the search for new multichromophore complexes with promising light-harvesting properties. These prisms are obtained from the self-assembly of a 90°Pt(II) acceptor with a meso-substituted tetrapyridylporphyrin (TPyP) and dicarboxylate ligands. The generality of this self-assembly reaction is demonstrated using five dicarboxylate ligands, two based on a rigid central phenyl ring and three alkyl-spaced variants, to form a total of five free-base and five Zn-metallated porphyrin prisms. All 10 SCCs are characterized by 31 P and 1 H multinuclear NMR spectroscopy and electrospray ionization mass spectrometry, confirming the structure of each self-assembly and the stoichiometry of formation. The photophysical properties of the resulting SCCs were investigated revealing that the absorption and emission properties of the free-base and metallated porphyrin prisms preserve the spectral features associated with free TPyP. supramolecular chemistry | metallacages T he use of sunlight is ubiquitous as the input for carbonneutral, renewable energy schemes (1). Every strategy that relies on solar energy conversion, ranging from direct conversion to electricity in photovoltaics (2) to the generation of fuels via electrocatalysis (3), photoanode (4, 5), or photocathode devices (6), or photocatalysis (7) requires that photons be absorbed by a molecule or material as the first step in providing the driving force for subsequent transformation. Natural systems have evolved light-harvesting complexes, comprising multiprotein ensembles embedded with pigment molecules to enhance photon absorption for photosynthesis (8). These pigment-rich sites are arranged such that excitation of a distal chromophore will ultimately result in energy transfer to a reaction center via a series of migration and transfer processes (9). Strategies to replicate natural light-harvesting complexes necessarily demand the organization of multiple chromophores (10), a requirement that makes self-assembly and supramolecular chemistry particularly well suited for such efforts (11-13). As such, a variety of approaches have been applied toward the development of new materials that exhibit broadband absorption and efficient energy transfer (14-16). The subsequent studies of such materials span investigations of the fundamental science behind energy migration and transfer, t...

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