Amidinium-carboxylate Salt Bridges Research Articles

Remote Sensitized Photoisomerization via Through‐Bond Triplet–Triplet Energy Transfer Mediated by a Salt Bridge in a Supramolecular Dyad

A supramolecular dyad, BP-(amidinium-carboxylate)-NBD is constructed, in which benzophenone (BP) and norbornadiene (NBD) are connected via an amidinium-carboxylate salt bridge. The photophysical and photochemical properties of the assembled BP-(amidinium-carboxylate)-NBD dyad are examined. The phosphorescence of the BP chromophore is efficiently quenched by the NBD group in BP-(amidinium-carboxylate)-NBD via the salt bridge. Time-resolved spectroscopy measurements indicate that the lifetime of the BP triplet state in BP-(amidinium-carboxylate)-NBD is shortened due to the quenching by the NBD group. Selective excitation of the BP chromophore results in isomerization of the NBD group to quadricyclane (QC). All of these observations suggest that the triplet-triplet energy transfer occurs efficiently in the BP-(amidinium-carboxylate)-NBD salt bridge system. The triplet-triplet energy transfer process proceeds with efficiencies of approximately 0.87, 0.98 and the rate constants 1.8x10(3) s(-1), and 1.3x10(7) s(-1) at 77 K and room temperature, respectively. The mechanism for the triplet-triplet energy transfer is proposed to proceed via a "through-bond" electron exchange process, and the non-covalent bonds amidinium-carboxylate salt bridge can mediate the triplet-triplet energy transfer process effectively for photochemical conversion.

A hetero-stranded double helix composed of m-diethynylbenzene-based complementary molecular strands stabilized by amidinium-carboxylate salt bridges

A hetero-stranded double helix with a controlled helix sense was designed and synthesized from an optically active dimeric amidine and its complementary achiral dicarboxylic acid strand with conjugated m-diethynylbenzene backbones, being stabilized by the salt bridges.

Double-Stranded Supramolecular Assembly through Salt Bridge Formation between Rigid and Flexible Amidine and Carboxylic Acid Strands

A series of monomeric strands consisting of m-terphenyl backbones with chiral rigid C-linked (3) and flexible N-linked (5) formamidines and achiral carboxylic acid (4) and flexible carboxymethyl (6) residues were synthesized, and their duplex formations through amidinium-carboxylate salt bridges were investigated by NMR, circular dichroism (CD), and UV-visible spectroscopies. The salt bridge-derived duplex formation was largely dependent on the structures of the formamidine and carboxylic acid strands, and the C-linked formamidine strand 3 formed a more stable duplex with the complementary carboxylic acid strands (4 and 6) than did the flexible N-linked formamidine strand 5. The single crystal X-ray analysis revealed that the duplex 5.4 has a skewed right-handed double helical structure. A complementary duplex dimer was also synthesized from the dimers of 5 and 4 joined by diacetylene linkers. Variable-temperature CD measurements indicated that the duplex possesses a dynamic double helical structure resulting from the flexible N-linked formamidine units.

Energy transfer in non-covalent porphyrin assemblies: through-space or through-bond?

Photosynthetic antenna arrays found in nature funnel photoexcited energy into the reaction center. Attempts have been made to mimic the antenna function by using artificial chromophores, porphyrins in particular, not only to better understand the energy-transfer processes but also to create light-harvesting devices. This review covers non-covalent porphyrin assemblies, for which intra-ensemble energy-transfer processes were characterized. The essence of the mechanisms of energy transfer is summarized and specific examples are reviewed with an emphasis put on the rate and mechanism of singlet-singlet energy transfer. As these examples demonstrate, non-covalent intra-ensemble energy-transfer processes have been ascribed to the Förster-type through-space mechanism in almost all cases. The exception is porphyrin dyad and pentad from our group based on amidinium-carboxylate salt bridges. Through-bond superexchange mechanism is proposed to account for the fast excited energy-transfer processes for these unique assemblies. The importance of intermolecular interactions not only in terms of the structural aspects but also in terms of the electronic aspects is highlighted for the design of supramolecular systems in which efficient energy transfer is desired.

Sequence- and Chain-Length-Specific Complementary Double-Helix Formation

The artificial sequential strands consisting of two, three, or four m-terphenyl groups joined by diacetylene linkers with complementary binding sites, either the chiral amidine (A) or achiral carboxyl (C) group, were synthesized in a stepwise manner. Using circular dichroism and (1)H NMR spectroscopies along with liquid chromatography, we showed that, when three dimeric molecular strands (AA, CC, and AC) or six trimeric molecular strands (AAA, CCC, AAC, CCA, ACA, and CAC) were mixed in solution, the complementary strands were sequence-specifically hybridized to form one-handed double-helical dimers AA.CC and (AC) 2 or trimers AAA.CCC, AAC.CCA, and ACA.CAC, respectively, through complementary amidinium-carboxylate salt bridges. Upon the addition of CCA to a mixture of AAA, AAC, and ACA, the AAC.CCA double helix was selectively formed and then isolated from the mixture by chromatography. Moreover, the homo-oligomer mixtures of amidine or carboxylic acid from the monomers to tetramers (A, AA, AAAA, C, CC, and CCCC) assembled with a precise chain length specificity to form A.C, AA.CC, and AAAA.CCCC, which were separated by chromatography.

Single- and Double-Stranded Helical Polymers: Synthesis, Structures, and Functions

Biological macromolecules, such as DNA and proteins, possess a unique and specific ordered structure, such as a right-handed double helix or a single alpha-helix. Those structures direct the sophisticated functions of these molecules in living systems. Inspired by biological helices, chemists have worked to synthesize polymers with controlled helicity, not only to mimic the biological helices but also to realize their functions. Although numerous synthetic polymers that fold into a single-handed helix have been reported, double-stranded helical polymers are almost unavailable except for a few oligomers. In addition, the exact structures of most helical polymers remain obscure. Therefore, the development of a conceptually new method for constructing double-stranded helical polymers and a reliable method for unambiguously determining the helical structures are important and urgent challenges in this area. In this Account, we describe the recent advances in the synthesis, structures, and functions of single- and double-stranded helical polymers from our group and others and provide a brief historical overview of synthetic helical polymers. We found unique macromolecules that fold into a preferred-handed helix through noncovalent bonding interactions with specific chiral guests. During the noncovalent helicity induction process, these guest molecules significantly amplified chirality in a dynamic helical polymer. During the intensive exploration of the helicity induction mechanism, we observed an unusual macromolecular helical memory in dynamic helical polymers. Furthermore, we found that rigid-rod helical poly(phenylacetylene)s and poly(phenyl isocyanide)s showing a cholesteric or smectic liquid crystal self-assemble to form two-dimensional crystals with a controlled helical conformation on solid substrates upon exposure to solvent vapors. We visualized their helical structures including the helical pitch and handedness by atomic force microscopy (AFM). We propose a modular strategy to construct complementary double helices by employing chiral amidinium-carboxylate salt bridges with m-terphenyl backbones. The double-stranded helical structures were characterized by circular dichroism in solution and X-ray diffraction of the crystals or the direct AFM observations. Serendipitously, we found that oligoresorcinols self-assemble into well-defined double helices resulting from interstrand aromatic stacking in water. These oligoresorcinols bound cyclic and linear oligosaccharides in water to form rotaxanes and hetero-double helices, respectively. The examples presented in this Account demonstrate the notable progress in the synthesis and structural determination of helical polymers including single- and double-stranded helices. Not only do we better understand the principle underlying the generation of helical conformations, but we have also used the knowledge of these unique helical structures to develop novel helical polymers with specific functions.

Double-Stranded Helical Polymers Consisting of Complementary Homopolymers

Two complementary homopolymers of chiral amidines and achiral carboxylic acids with m-terphenyl-based backbones were synthesized by the copolymerization of a p-diiodobenzene derivative with the diethynyl monomers bearing a chiral amidine group and a carboxyl group using the Sonogashira reaction, respectively. Upon mixing in THF, the homopolymer strands assembled into a preferred-handed double helix through interstrand amidinium-carboxylate salt bridges, as evidenced by its absorption, circular dichroism, and IR spectra. In contrast, when mixed in less polar solvents, such as chloroform, the complementary strands kinetically formed an interpolymer complex with an imperfect double helical structure containing a randomly hybridized cross-linked structure, probably because of strong salt bridge formations. This primary complex was rearranged into the fully double helical structure by treatment with a strong acid followed by neutralization with an amine. High-resolution atomic force microscopy revealed the double-stranded helical structure and enabled the determination of the helical sense.

Through‐Bond Excited Energy Transfer Mediated by an Amidinium–Carboxylate Salt Bridge in Zn–Porphyrin Free‐Base Porphyrin Dyads

Excited energy-transfer processes were investigated for a supramolecular Zn-porphyrin free-base porphyrin dyad, ZnPA-2 x FbPC-2, in which beta-octaalkylated meso-diarylporphyrins are connected through an amidinium-carboxylate salt bridge. The rate of energy transfer in the dyad (1.3 x 10(9) s(-1)) is substantially slower than that in the previously reported dyad, ZnPA-1FbPC-1 (4.0 x 10(9) s(-1)), in which meso-tetraarylporphyrins are connected through the same amidinium-carboxylate salt bridge. The Förster-type mechanism can explain only minor parts of these rates (3.3 x 10(8) and 5.1 x 10(8) s(-1), respectively). Thus, Dexter-type through-bond energy transfer may be invoked. Indeed, bridge-mediated electronic processes would be favored in ZnPA-1 x FbPC-1 over ZnPA-2 x FbPC-2 on the basis of steric and electronic factors. Sterically, the phenyl groups in ZnPA-2 and FbPC-2 are more closely perpendicular to the porphyrin planes than those in ZnPA-1 and FbPC-1. Electronically, the energy and symmetry of the occupied frontier orbitals should favor ZnPA-1 x FbPC-1 over ZnPA-2 x FbPC-2 in terms of electronic interactions through the bridge. Therefore, the observed trend (ZnPA-1 x FbPC-1>ZnPA-2 x FbPC-2), consistent with these considerations, lends further support to the through-bond mechanism. Thus, the amidinium-carboxylate salt bridge is effective in mediating through-bond energy transfer even though the bond is noncovalent.

Molecular design and synthesis of artificial double helices

This account describes novel artificial double helices recently developed by our group. We have designed and synthesized the double helices consisting of two complementary, m-terphenyl-based strands that are intertwined through chiral amidinium-carboxylate salt bridges. Due to the chiral substituents on the amidine groups, the double helices adopted an excess one-handed helical conformation in solution as well as in the solid state. By extending the modular strategy, we have synthesized double helices bearing Pt(II) linkers, which underwent the double helix-to-double helix transformations through the chemical reactions of the Pt(II) complex moieties. In addition, artificial double-stranded metallosupramolecular helical polymers were constructed by combining the salt bridges and metal coordination. In contrast to the design-oriented double helices based on salt bridges, we have serendipitously developed a spiroborate-based double helicate bearing oligophenol strands. The optical resolution of the helicate was successfully attained by a diastereomeric salt formation. We have also unexpectedly found that oligoresorcinols consisting of a very simple repeating unit self-assemble into double helices with the aid of aromatic interactions in water. Furthermore, a bias in the twist sense of the double helices can be achieved by incorporating chiral substituents at both ends of the strands.

Synthesis and Function of Artificial Double Helices

This account mainly describes the synthesis and functions of novel artificial double helices recently developed by our group. We have proposed the first design rationale for artificial double helices with a controlled helicity by employing chiral amidinium-carboxylate salt bridges with m-terphenyl ligands, and synthesized several artificial double helices together with triple- and quadruple-stranded helices. An artificial double helix bearing Pt (II) -acetylide complex moieties has proven effective as a catalyst for the asymmetric cyclopropanation, demonstrating that the chiral space provided by double helices can be effective for asymmetric catalytic reactions. Furthermore, we have serendipitously discovered a spiroborate-based double helicate bearing oligophenol strands and water-soluble double helices of oligoresorcinols. The oligoresorcinol double helices bound cyclic and linear oligosaccharides in water to form rotaxanes and hetero-double helices, respectively, with a remarkable selectivity for the glucosidic linkage patterns and chain lengths.

Double Helix-to-Double Helix Transformation, Using Platinum(II) Acetylide Complexes as Surrogate Linkers

We describe novel optically active double helices consisting of complementary strands stabilized by amidinium-carboxylate salt bridges. The m-terphenyl groups of each strand are joined by trans-Pt(II) acetylide complexes with pendant PPh(3) ligands as the surrogate linker, which converts to cis counterparts by a ligand exchange reaction with cis-1,2-bis(diphenylphosphino)ethylene, resulting in the formation of double helices with different structures. Subsequent iodine-promoted reductive elimination on the Pt(II) atoms generates the fully organic, enantiomerically pure double helices. [structure: see text]

Construction of Double-Stranded Metallosupramolecular Polymers with a Controlled Helicity by Combination of Salt Bridges and Metal Coordination

We describe the construction of the first double-stranded metallosupramolecular helical polymers. We designed and synthesized a supramolecular duplex comprised of complementary m-terphenyl-based strands bearing a chiral amidine or achiral carboxylic acid together with two pyridine groups at the four ends. Supramolecular polymerization of the duplex with cis-PtPh2(DMSO)2 in 1,1,2,2-tetrachloroethane produced the double-stranded metallosupramolecular polymer with a controlled helicity of which the two complementary metallostrands are intertwined through the amidinium-carboxylate salt bridges. The structures and hydrodynamic dimensions of the metallosupramolecular polymers were characterized by 1H NMR, diffusion-ordered NMR, dynamic light scattering, absorption, and CD measurements. The polymeric structure was also visualized by atomic force microscopy.

A Modular Strategy to Artificial Double Helices

No metal required: Double-helical assemblies can be constructed through hydrogen bonds upon formation of amidinium–carboxylate salt bridges (see schematic representation). The double-helical structures formed including the helix sense are predictable by this approach.

Theoretical Studies of Proton‐Coupled Electron Transfer Reactions

AbstractFor Abstract see ChemInform Abstract in Full Text.

Supramolecular porphyrin assemblies through amidinium-carboxylate salt bridges and fast intra-ensemble excited energy transfer.

Well-defined supramolecular assemblies of Zn and free-base porphyrins are constructed through the formation of amidinium-carboxylate salt bridges. A one-to-one donor-acceptor pair and a four-to-one antenna-type assembly are investigated. The steady-state and time-resolved fluorescence measurements unequivocally showed that efficient singlet-singlet excited energy transfer from the Zn-porphyrin complex to the free-base porphyrin takes place in these assemblies. Indeed, the observed energy-transfer rates in both types of assemblies are much faster than those the Förster mechanism would suggest, implying the involvement of an intermolecular through-bond mechanism.

Theoretical formulation for electron transfer coupled to multiple protons: Application to amidinium–carboxylate interfaces

This paper presents a theoretical formulation for electron transfer coupled to the motion of multiple protons. This theory is applied to proton-coupled electron transfer (PCET) through amidinium–carboxylate salt bridges, where the electron transfer reaction is coupled to the motion of two protons at the proton transfer interface. The rate for the donor–(amidinium–carboxylate)–acceptor system is found to be substantially slower than the rate for the switched interface donor–(carboxylate–amidinium)–acceptor system. This trend is consistent with experimental data for photoinduced PCET in analogous systems. The calculations indicate that this difference in rates is due mainly to the opposite dipole moments at the proton transfer interfaces for the two systems, leading to an endothermic reaction for the donor–(amidinium–carboxylate)–acceptor system and an exothermic reaction for the donor–(carboxylate–amidinium)–acceptor system. The deuterium kinetic isotope effects are found to be moderate (i.e., kH/kD<3) for both types of systems. These moderate kinetic isotope effects are due to the dominance of vibrationally excited product states, leading to significant overlap between the reactant and product proton vibrational wave functions.

From Molecules to the Crystalline Solid: Secondary Hydrogen-Bonding Interactions of Salt Bridges and Their Role in Magnetic Exchange

Secondary hydrogen-bonding interactions have been exploited to construct layered magnetic solids based on amidinium–carboxylate salt bridges. The salt bridge is utilized as a supramolecular synthon to set the structural organization in a layered motif; it also serves as a spin communication element. The magnetic material shown in the accompanying figure is formed between 3-cyanobenzamidinium and 2,2,4,4-tetramethylpyrroline-N-oxyl-3-carboxylate.

Bis-amidinium calixarenes: Templates for self-assembled receptors

The synthesis of two bis-amidinium calixarenes 3 and 4 has been achieved. Compounds 3 and 4 self-assemble with carboxylate salts via amidinium-carboxylate salt bridges producing ditopic receptor species. Two new bis-amidinium calixarenes have been synthesised which self-assemble with carboxylic acids via amidinium-carboxylate salt bridges.

The Amidinium–Carboxylate Salt Bridge as a Proton‐Coupled Interface to Electron Transfer Pathways

The Amidinium–Carboxylate Salt Bridge as a Proton‐Coupled Interface to Electron Transfer Pathways

Electron transfer within ruthenium(II) polypyridyl-(salt bridge)-dimethylaniline acceptor-donor complexes

The kinetics of electron transfer from N, N′-dimethylaniline to electronically excited Ru(II) polypyridyl acceptor complexes through an amidinium-carboxylate salt bridge have been investigated. Acceptor-(salt bridge)-donor complexes are formed by the 1:1 association of [(4,4′-X-2,2′-bipyridine) 2Ru II(Mebpy-amH +)] 3+ and (4,4′-X-2,2′-bipyrdine) 2Ru II(Mebpy-COO −)] + (X = diethylcarboxy, X = H, Mebpy-amH + = 4-methyl-4′-amidinium-2,2′-bipyridine, Mebpy-COO − = 4-methyl-4′-carboxylate-2,2′-bipyridine) to N, N′-dimethylaminobenzoate (DMA-COO −) and N, N′-dimethylaminobenzamidinium (DMA-amH +), respectively. The design of the excited state structure of the Ru(II) polypyridyl complex is crucial to a proper kinetics study of the electron transfer reaction through the salt bridge of these complexes. The reaction is photoinitiated by the metal-to-ligand charge (MLCT) transfer excitation. Ester modification of the ancillary bipyridine ligands results in an MLCT excitation that places the electron on the ligands remote to the salt bridge, clearing the way for very fast electron transfer from DMA into the Ru(II) center via the salt bridge. We observe facile electron transfer when the Ru(II) complex is attached to the amidinium-side of the salt bridge. This behavior is opposite to that previously observed by us for the oxidative quenching of Ru(II) polypyridyl complexes joined to acceptors via the same salt bridge; in this case, electron transfer out of the Ru(II) complex is fast. These results suggest that the quenching mechanism of these types of Ru(II) polypyridyl assemblies is dominated by the orientation of the salt bridge relative to the direction of electron transport.