Multiaddressable molecular rectangles with reversible host–guest interactions: Modulation of pH-controlled guest release and capture

Abstract

A series of multiaddressable platinum(II) molecular rectangles with different rigidities and cavity sizes has been synthesized by endcapping the U-shaped diplatinum(II) terpyridine moiety with various bisalkynyl ligands. The studies of the host-guest association with various square planar platinum(II), palladium(II), and gold(III) complexes and the related low-dimensional gold(I) complexes, most of which are potential anticancer therapeutics, have been performed. Excellent guest confinement and selectivity of the rectangular architecture have been shown. Introduction of pH-responsive functionalities to the ligand backbone generates multifunctional molecular rectangles that exhibit reversible guest release and capture on the addition of acids and bases, indicating their potential in controlled therapeutics delivery on pH modulation. The reversible host-guest interactions are found to be strongly perturbed by metal-metal and π-π interactions and to a certain extent, electrostatic interactions, giving rise to various spectroscopic changes depending on the nature of the guest molecules. Their binding mode and thermodynamic parameters have been determined by 2D NMR and van't Hoff analysis and supported by computational study. noncovalent interactions | platinum complex | luminescence T he study of metal-metal interactions has drawn enormous attention since the past two decades because of the intriguing spectroscopic and photophysical properties arising from the close proximity of the metal centers (1, 2). Square planar d 8 platinum(II) complexes with coordination unsaturation are one of the important classes of metal complexes that have been extensively explored because of their capability to exhibit metalmetal interactions and display rich photophysical properties (3-26). Platinum(II) terpyridine complexes have been found to exhibit rich polymorphism in the solid state (16)(17)(18)(19)(20) owing to their square planar coordination geometry, which permits facile access to Pt(II)···Pt(II) interactions as well as π-π interactions between the chromophores. It was not until 2001 that the first successful synthesis of platinum(II) terpyridine alkynyl complexes, which possess enhanced solubility and luminescence compared with the chloro counterpart, was reported (16). Additional efforts have been devoted to the use of the system to respond to external stimuli, such as variation in solvent composition (17, 18), pH (19, 20), temperature (21, 22), addition of ionic (24-26), and polymeric species (27,28), in which spectral changes induced by strong Pt(II)···Pt(II) and π−π interactions have been displayed.In the past few decades, enormous efforts have been devoted to the construction of molecular architectures by fusing the organic framework to the transition metal centers through selfassembly processes (29-57). There has been continuous interest in the construction of stimuli-responsive metallosupramolecular architectures with diverse sizes, shapes, and symmetries to rationalize the criteria for molecular recognition and impa...

Self-assembly of alkynylplatinum(II) terpyridine amphiphiles into nanostructures via steric control and metal–metal interactions

“…Earlier work by our group showed the first example, to our knowledge, of an alkynylplatinum(II) terpyridine system [Pt(tpy)(C ≡ CR)] + that incorporates σ-donating and solubilizing alkynyl ligands together with the formation of Pt···Pt interactions to exhibit notable color changes and luminescence enhancements on solvent composition change (25) and polyelectrolyte addition (26). This approach has provided access to the alkynylplatinum(II) terpyridine and other related cyclometalated platinum(II) complexes, with functionalities that can self-assemble into metallogels (27)(28)(29)(30)(31), liquid crystals (32,33), and other different molecular architectures, such as hairpin conformation (34), helices (35)(36)(37)(38), nanostructures (39)(40)(41)(42)(43)(44)(45), and molecular tweezers (46,47), as well as having a wide range of applications in molecular recognition (48)(49)(50)(51)(52), biomolecular labeling (48)(49)(50)(51)(52), and materials science (53,54). Recently, metalcontaining amphiphiles have also emerged as a building block for supramolecular architectures (42)(43)(44)(55)(56)(57)…”

Encapsulation and Enhanced Luminescence Properties of IrIII Complexes within a Hexameric Self‐Assembled Capsule

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Encapsulation and luminescences tudies of [Ir-(ppy) 2 (bpy)]Cl (ppy = 2-phenylpyridinate, bpy = 2,2'-bipyridine) within ah exameric resorcinarenec apsulea re reported. [10] To obtain af undamental understanding of host-guest interactions between encapsulatedm etal complexes and selfassembled cages, the hydrogen-bonded capsulei so ne of the most suitable host systems.Self-assembly of resorcinarene 1 with eight water molecules forms hexameric self-assembled capsule 2 with al arge cavity (1375 ;S cheme 1), which is stabilized by an etwork of sixty hydrogen bonds in wet apolar solvents. The emission color of the Ir III complex was drastically changed from orange to yellow by encapsulation, in contrast with the lack of significant changes in the absorption spectrum.T he hexameric capsule effectively hampers the non-radiativep athway to increaseb oth the luminescence quantumy ield and the exited state lifetime.

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“…[7] The emissive state of Pt II complexes in the solid state are very sensitive to slight structural changes, which leads to interesting phenomena such as vapochromism, mechanochromism, and thermochromism. [10] To obtain af undamental understanding of host-guest interactions between encapsulatedm etal complexes and selfassembled cages, the hydrogen-bonded capsulei so ne of the most suitable host systems. [4d, 9] Yama nd co-workers recently demonstrated the encapsulationo fs quarep lanar metal complexes within self-assembled coordination rectangles, which exhibited guest-specific UV/Vis absorption and emissionc hanges by strong metal-metal and p-p interactions.…”

Using Complementary Ligand Denticity to Direct Metallosupramolecular Structure about Metal Ions with Square‐Planar Geometry

“…In contrast, Nabeshima and coworkers used a divergent bis ‐terpy ligand, a bis ‐monodentate ligand with pyridyl donors arranged in parallel, and Pt II to form a tetraplatinum(II) [Pt 4 L20 2 L21 2 ] 8+ prism (Figure c, bottom left , also Figure a) . Lastly, Yam and coworkers combined a ligand family comprised of parallel bis ‐terpy ligands and parallel bis ‐alkynyl monodentate ligands with Pt II for the formation of dinuclear complexes such as [Pt 2 ( L19 )( L22 )] 2+ (Figure c, bottom right ) . Therefore, using these approaches, a series of architectures with different cavity character and number have been generated.…”

“… a) The heteroleptic [Pt 2 ( L19 )( L22 )] 2+ complex ( L18 is bis‐ tridentate, left, L22 is bis ‐monodentate, right) capable of reversibly binding b) a planar neutral aromatic palatinate guest, reported by Yam and coworkers . Some solubilising elements on the host architecture omitted for clarity.…”