Self-assembled Supramolecular Complexes Research Articles

Photoinduced Energy and Electron Transfer in a 'Two-Point' Bound Panchromatic, Near-Infrared-Absorbing Bis-styrylBODIPY(Zinc Porphyrin)2 - Fullerene Self-Assembled Supramolecular Conjugate.

Structurally well-defined self-assembled supramolecular multi-modular donor-acceptor conjugates play a significant role in furthering our understanding of photoinduced energy and electron transfer events occurring in nature, e. g., in the antenna-reaction centers of photosynthesis and their applications in light energy harvesting. However, building such multi-modular systems capable of mimicking the early events of photosynthesis has been synthetically challenging, causing a major hurdle for its growth. Often, multi-modularity is brought in by combining both covalent and noncovalent approaches. In the present study, we have developed such an approach wherein a π-extended conjugated molecular cleft, two zinc(II)porphyrin bearing bisstyrylBODIPY (dyad, 1), has been synthesized. The binding of 1 via a 'two-point' metal-ligand coordination of a bis-pyridyl fulleropyrrolidine (2), forming a stable self-assembled supramolecular complex (1 : 2), has been established. The self-assembled supramolecular complex has been fully characterized by a suite of physico-chemical methods, including TD-DFT studies. From the established energy diagram, both energy and electron transfer events were envisioned. In dyad 1, selective excitation of zinc(II)porphyrin leads to efficient singlet-singlet excitation transfer to (bisstyrly)BODIPY with an energy transfer rate constant, kEnT of 2.56×1012 s-1. In complex 1 : 2, photoexcitation of zinc(II)porphyrin results in ultrafast photoinduced electron transfer with a charge separation rate constant, kCS of 2.83×1011 s-1, and a charge recombination rate constant, kCR of 2.51×109 s-1. For excitation at 730 nm corresponding to bisstyrylBODIPY, similar results are obtained, where a biexponential decay yielded estimated values of kCS 3.44×1011 s-1 and 2.97×1010 s-1, and a kCR value of 2.10×1010 s-1. The newly built self-assembled supramolecular complex has been shown to successfully mimic the early events of the photosynthetic antenna-reaction center events.

Computational and data driven molecular material design assisted by low scaling quantum mechanics calculations and machine learning.

Electronic structure methods based on quantum mechanics (QM) are widely employed in the computational predictions of the molecular properties and optoelectronic properties of molecular materials. The computational costs of these QM methods, ranging from density functional theory (DFT) or time-dependent DFT (TDDFT) to wave-function theory (WFT), usually increase sharply with the system size, causing the curse of dimensionality and hindering the QM calculations for large sized systems such as long polymer oligomers and complex molecular aggregates. In such cases, in recent years low scaling QM methods and machine learning (ML) techniques have been adopted to reduce the computational costs and thus assist computational and data driven molecular material design. In this review, we illustrated low scaling ground-state and excited-state QM approaches and their applications to long oligomers, self-assembled supramolecular complexes, stimuli-responsive materials, mechanically interlocked molecules, and excited state processes in molecular aggregates. Variable electrostatic parameters were also introduced in the modified force fields with the polarization model. On the basis of QM computational or experimental datasets, several ML algorithms, including explainable models, deep learning, and on-line learning methods, have been employed to predict the molecular energies, forces, electronic structure properties, and optical or electrical properties of materials. It can be conceived that low scaling algorithms with periodic boundary conditions are expected to be further applicable to functional materials, perhaps in combination with machine learning to fast predict the lattice energy, crystal structures, and spectroscopic properties of periodic functional materials.

Controlling Complex Stability in Photoresponsive Macromolecular Host-Guest Systems: Toward Reversible Capture of DNA by Cyclodextrin Vesicles.

An effective and universal method for delivering structurally diverse biomolecules in vivo would greatly benefit modern drug therapy, but has yet to be discovered. Self-assembled supramolecular complexes containing vesicles of amphiphilic cyclodextrin and linker molecules with an azobenzene guest unit and a charged functionality have been established as nanoscale carriers for proteins and DNA, making use of multivalent electrostatic attraction. However, light-induced cargo release is only feasible up to a maximum net charge of the biomacromolecules. Herein, it is shown that it is possible to fine-tune macromolecular complex stability and size by addition of a competitive guest molecule that acts as a stopper, partly blocking the vesicle surface. The superior performance of arylazopyrazoles in photoisomerization compared to azobenzenes, which enables a lower surface charge density of the vesicles in the photostationary state, is also demonstrated. Both strategies allow reversible supramolecular aggregation of high molecular weight DNA (2 and 4.8 kbp).

Active hydrogenation Rh nanocatalysts protected by new self-assembled supramolecular complexes of cyclodextrins and surfactants in water

New supramolecular assemblies as protective agents of rhodium(0) nanoparticles for hydrogenation of various substrates in water.

Self-Assembled Supramolecular Complexes with “Rods-in-Belt” Architecture in the Light of Soft X-rays

One of the most important properties of the recently discovered “rods-in-belt” supramolecular complexes containing Au–Cu or Au–Ag cluster cores is the possibility of tuning their electronic and photophysical properties through modification of the ligand environment. This opens great perspectives for their applications in light-emitting devices and bioimaging. The high structural ordering and self-assembly properties of these unique objects may be used to design artificial nanostructures with complex topologies that could become ideal building blocks for next-generation electronics. Here we present a detailed experimental study of the electronic structure of “rods-in-belt” supramolecular complexes. Applying X-ray photoemission and absorption spectroscopy, we systematically unraveled the structure of their occupied and unoccupied electronic states near the Fermi level. The major contribution to the highest occupied molecular orbitals is made by the triple-bonded carbons hosted in the dialkynylgold “rods” and the copper (silver) atoms from the central cluster core of the heterometallic Au–Cu (Au–Ag) molecules. The lowest unoccupied molecular orbitals are located on the carbon skeleton of the complexes, including −C≡C– and −C═C– aromatic orbitals. The onset of the valence band in the Au–Ag systems is ∼0.3 eV lower than that in the Au–Cu complexes, implying a slightly larger energy gap for the silver-based molecules. With increasing size, the complexes become more and more sensitive to X-ray damage.

Equilibrium Isotope Effects on Noncovalent Interactions in a Supramolecular Host–Guest System

The self-assembled supramolecular complex [Ga(4)L(6)](12-) (1; L = 1,5-bis[2,3-dihydroxybenzamido]naphthalene) can act as a molecular host in aqueous solution and bind cationic guest molecules to its highly charged exterior surface or within its hydrophobic interior cavity. The distinct internal cavity of host 1 modifies the physical properties and reactivity of bound guest molecules and can be used to catalyze a variety of chemical transformations. Noncovalent host-guest interactions in large part control guest binding, molecular recognition and the chemical reactivity of bound guests. Herein we examine equilibrium isotope effects (EIEs) on both exterior and interior guest binding to host 1 and use these effects to probe the details of noncovalent host-guest interactions. For both interior and exterior binding of a benzylphosphonium guest in aqueous solution, protiated guests are found to bind more strongly to host 1 (K(H)/K(D) > 1) and the preferred association of protiated guests is driven by enthalpy and opposed by entropy. Deuteration of guest methyl and benzyl C-H bonds results in a larger EIE than deuteration of guest aromatic C-H bonds. The observed EIEs can be well explained by considering changes in guest vibrational force constants and zero-point energies. DFT calculations further confirm the origins of these EIEs and suggest that changes in low-frequency guest C-H/D vibrational motions (bends, wags, etc.) are primarily responsible for the observed EIEs.

Co-facial magnesium porphyrin dimer complexed with fullerene: photosynthetic reaction center model of 'special pair' self-assembled to electron acceptor

Using a self-assembled supramolecular approach, a closer model for the photosynthetic reaction center 'special pair' assembled with an electron acceptor conjugate, is reported. As the 'special pair' donor, magnesium meso-(benzo-15-crown-5)porphyrin was self-assembled with K + to form a highly stable Mg porphyrin dimer with a co-facial geometry arrangement, which is a better electron donor since it is nearly 130 mV easier to oxidize compared to the earlier reported Zn porphyrin dimer analog. Further, pyridine and alkyl ammonium cation-functionalized fullerene was attached via axial coordination and crown ether-alkyl ammonium cation complexation to form the four-fold, donor-acceptor complex. The self-assembled supramolecular complex, constructed using this approach, possesses defined geometry and orientation. Efficient charge separation from the singlet excited state of the Mg porphyrin dimer to the fullerene entity and a relatively slow charge recombination, were revealed by time-resolved emission and nanosecond transient absorption spectral studies, supporting the impending exploitation of these novel biomimetic model systems for light energy harvesting applications.

Templated Assembly of Biomembranes on Silica Microspheres Using Bacteriorhodopsin Conjugates as Structural Anchors

Membrane proteins are some of the most sophisticated molecules found in nature. These molecules have extraordinary recognition properties; hence, they represent a vast source of specialized materials with potential uses in sensing and screening applications. However, the strict requirement of the native lipid environment to preserve their structure and functionality presents an impediment in building biofunctional materials from these molecules. In general, the purification protocols remove the native lipid support structures found in the cellular environment that stabilize the membrane proteins. Furthermore, the membrane protein structure is often highly complex, typified by large, multisubunit complexes that not only span the lipid bilayer but also contain large (>2 nm) cytoplasmic and extracellular domains that protrude from the membrane. The present study is focused on using a biomimetic approach to build a stable, fluid microenvironment to be used to incorporate larger membrane proteins of interest into a tether-supported lipid bilayer membrane adequately spaced above a substrate passivated to liposome fusion and nonspecific adsorption. Our aim is to reintroduce the supporting structures of the native cell membrane using self-assembled supramolecular complexes constructed on microspheres in an artificial cytoskeleton motif. Central to our architecture is to utilize bacteriorhodopsin (bR), a transmembrane protein, as a biomembrane anchoring molecule to be tethered to surfaces of interest as a sparse structural element in the design. Compared to a typical lipid tether, which inserts into one leaflet of the lipid bilayer, bR anchoring provides an over 8-fold greater hydrophobic surface area in contact with the bilayer. In the work presented here, the silica microsphere surface was biofunctionalized with streptavidin to make it a suitable supporting interface. This was achieved by self-assembly of (p-aminophenyl)trimethoxysilane on the silica surface followed by subsequent conjugation of biotin-PEG3400 (PEG = poly(ethylene glycol) and PEG2000 for further passivation and the binding of streptavidin. We have conjugated bR with biotin-PEG3400 through amine-based coupling to use it as a tether. The biotin-PEG-bR conjugate was further labeled with Texas Red to facilitate localization via fluorescence imaging. Confocal microscopy was utilized to analyze the microsphere surface at different stages of surface modification by employing fluorescent staining techniques. Sparely tethered supported lipid bilayer membranes were constructed successfully on streptavidin-functionalized silica particles (5 mum) using a detergent-based method in which tethered bR nucleates self-assembly of the bilayer membrane. The fluidity of the supported membranes was analyzed using fluorescence recovery after photobleaching in confocal imaging detection mode. The phospholipid diffusion coefficients obtained from these studies indicated that nativelike fluidity was achieved in the tether-supported membranes, thus providing a prospective microenvironment for insertion of membrane proteins of interest.

Predicting the site of electron transfer using DFT frontier orbitals: Studies on porphyrin attached either to quinone or hydroquinone, and quinhydrone self-assembled supramolecular complexes

DFT calculations were performed on covalently linked porphyrin–quinone and porphyrin–hydroquinone dyads, and quinhydrone (1:1 quinone–hydroquinone) paired self-assembled supramolecular complexes. The frontier HOMO's and LUMO's of the B3LYP/3-21G(*) and PBEPBE/6-31G(d) optimized structures in the ground state were utilized to probe the site of electron transfer (porphyrin or quinone centered redox reactions) and the results were compared to the experimentally determined site of electron transfer. These studies revealed good agreement between the experimental results and computational predictions. The computed HOMO–LUMO energy gap also followed the trends of the electrochemical results. In the optimized structure of quinhydrone supramolecules, the quinone–hydroquinone entities were stacked on one another with an inter-planar distance of ∼3.1 Å. The calculated O–H–O distances suggested the presence of hydrogen bonds. The prediction of the structure and site of electron transfer suggests the usefulness of DFT calculations for probing the self-assembly process and redox behavior of structurally complex molecular and supramolecular systems. The B3LYP/3-21G(*) calculated association energy (the energy of the quinhydrone complex relative to the sum of the energies of the individual entities) for the quinhydrone formation was found to range between −7 and −15 kcal/mol, suggesting quinhydrone as a viable self-assembly mechanism to form supramolecular complexes. The B3LYP/3-21G(*) results were further improved by the PBEPBE/6-31G* method with solvation. Calculations performed at larger basis sets including 6-31G*, 6-31+G*, 6-31++G*, 6-311G*, 6-311G (d,p), and 6-311G (2df, dp) either at HF, BLYP, B3LYP, B3PW91, PBEPBE and MPW1PW91 methods also predicted negative association energies for the simple quinone:hydroquinone (quinhydrone) formation, with smaller association energies at the higher basis sets presumably due to reduced BSSE. Adding the basis set superposition error (BSSE), computed by counterpoise method, over-corrected the B3LYP/3-21G(*) association energy, but adding 50% of BSSE converged rapidly to the large basis set result. Larger association energies were obtained for the porphyrin–quinhydrone assemblies using PBEPBE/6-31G* calculations, presumably due to larger intermolecular ‘dispersion’ forces predicted with the PBEPBE method.

Reactivity of self-assembled supramolecular complexes in the gas phase: A supramolecular neighbor group effect

Infrared multi-photon dissociation (IRMPD) spectra of mass-selected, self-assembled supramolecular squares and their gas-phase fragments have been recorded in a Fourier-transform ion-cyclotron resonance (FT-ICR) mass spectrometer. The squares have been transferred into the gas phase by electrospray ionization (ESI) under very soft ionization conditions resulting in a series of signals for intact squares in different charge states by stripping off two or more counter ions. The fragmentation patterns of the squares strongly depend on the parent ion's charge state. For species with a lower number of charges, expulsions of edge ligands prevail, whereas charge separation pathways dominate the dissociation pathways of more highly charged species.

Self-Assembled Supramolecular Films Derived from Marine Deoxyribonucleic Acid (DNA)−Cationic Surfactant Complexes: Large-Scale Preparation and Optical and Thermal Properties

Series of polyelectrolyte−surfactant complexes, DNA−cationic surfactant complexes (cetyltrimethylammonium, cetylpyridinium, and cetylbenzyldimethylammonium), and their self-assembled bulk film materials were prepared on a large scale. Circular dichroism (CD) analysis indicated that the right-handed double helix structure of DNA was retained in these bulk film materials. TGA analysis suggested that 4 molecules of water were required to retain the B-type conformation of the DNA helix in the self-assembled bulk film materials. In addition, it revealed that DNA and the DNA−surfactant complex film materials were thermostable up to as high as 180 °C. Thermodynamical analysis indicated that these film materials were thermo-extensive over a temperature range from 100 to 148 °C. The DNA conformation in the supramolecular complex films can be reversibly tuned by changing the environmental humidity. Film formation was found to occur by self-assembly and self-organization with evaporation of solvent molecules. Various functional dyes such as laser dye, NLO dye, and photochromic dye could easily be incorporated in the self-assembled supramolecular complex films as adducts. Studies of the induced CD spectra demonstrated that 4[4-(dimethylamino)styryl]-1-dococylpyridinium (DMASDPB) could orient on the chiral nanotemplates of DNA in the self-assembled films. UV−vis analysis indicated that these film materials have high transparency from 300 to about 1000 nm. These self-assembled functional-dye-containing DNA−surfactant complex materials, with good processability for multilayer integration into large-area devices, will have promising applications in molecular optical and molecular optoelectronic fields.

Lanthanide Metal Ions as Cornerstones in Functional Self-Assembled Supramolecular Complexes

The peculiar spectroscopic, magnetic, and chemical properties of lanthanide ions (4f block, LnIII) are particularly attractive for the design of functional supramolecular devices if these ions can be selectively introduced into organized self-assembled architectures. The systematic investigation of a complete library of tridentate receptors leading to nine-coordinate tricapped trigonal prismatic sites upon coordination to LnIII allows the elucidation of the factors governing the structural, thermodynamic, electronic, magnetic, and spectroscopic properties of the final complexes. The simultaneous use of LnIII as cement between the molecular components of the supramolecular edifices and as functional vectors of the devices has been realized in self-assembled poly nuclear d-f and f-f complexes. Predetermined properties may result from a judicious molecular programming of the nanometric architecture leading to fascinating applications in luminescence, magnetism, template syntheses, and liquid crystals.