Ion-Controlled On–Off Switch of Electron Transfer from Tetrathiafulvalene Calix[4]pyrroles to Li+@C60

Switchable molecular tweezers: design and applications

Switchable molecular tweezers are a unique class of molecular switches that, like their macroscopic analogs, exhibit mechanical motion between an open and closed conformation in response to stimuli. Such systems constitute an essential component of artificial molecular machines. This review will present selected examples of switchable molecular tweezers and their potential applications. The first part will be devoted to chemically responsive tweezers, including stimuli such as pH, metal coordination, and anion binding. Then, redox-active and photochemical tweezers will be presented.

Rational Design of Stimuli-Responsive Inorganic 2D Materials via Molecular Engineering: Toward Molecule-Programmable Nanoelectronics

The ability of electronic devices to act as switches makes digital information processing possible. Succeeding graphene, emerging inorganic 2D materials (i2DMs) have been identified as alternative 2D materials to harbor a variety of active molecular components to move the current silicon-based semiconductor technology forward to a post-Moore era focused on molecule-based information processing components. In this regard, i2DMs benefits are not only for their prominent physiochemical properties (e.g., the existence of bandgap), but also for their high surface-to-volume ratio rich in reactive sites. Nonetheless, since this field is still in an early stage, having knowledge of both i) the different strategies for molecularly functionalizing the current library of i2DMs, and ii) the different types of active molecular components is a sine qua non condition for a rational design of stimuli-responsive i2DMs capable of performing logical operations at the molecular level. Consequently, this Review provides a comprehensive tutorial for covalently anchoring ad hoc molecular components-as active units triggered by different external inputs-onto pivotal i2DMs to assess their role in the expanding field of molecule-programmable nanoelectronics for electrically monitoring bistable molecular switches. Limitations, challenges, and future perspectives of this emerging field which crosses materials chemistry with computation are critically discussed.

Recent update on the electroactive oligopyrrolic macrocyclic hosts with a Bucky-ball heart

Supramolecular chemistry is a multidisciplinary research area mostly associated with the investigation of host-guest interactions within intricate three-dimensional (3D) molecular architectures held together reversibly by various non-covalent interactions. Continuous efforts to develop such kinds of complex host-guest systems with designer oligopyrrolic macrocyclic receptors are a rapidly growing research domain, which is deeply involved in applied supramolecular chemistry research. These host-guest supramolecular complexes can be constructed by combining suitable electron-rich oligopyrrolic donors (as a host) with complementary electron-poor guests (as acceptors), held together by the ionic force of attraction triggered by intermolecular charge/electron transfer (CT/ET) transitions. Some of these resulting CT/ET ensembles are potential candidates for the construction of efficient optoelectronic materials, optical sensors, molecular switches, etc. In this Feature Article we aim to focus on these supramolecular ensembles composed by size and shape complementary electroactive oligopyrrolic molecular containers, which are suitable for spherical guest (e.g., buckminsterfullerene) complexation. We also provide a "state-of-the-art" overview on plausible applications of these particular host-guest systems. Our aim is to cover only specific electron-rich tetrathiafulvalene (TTF)-based oligopyrrolic receptors, e.g., TTF-calix[4]pyrroles, TTF-cryptands, TTF-porphyrins and exTTF-porphyrin-based molecular motifs reported to date, along with a brief outlining of their "functional behaviour" in materials chemistry research.

Supramolecular Recognition within a Nanosized "Buckytrap" That Exhibits Substantial Photoconductivity

We report here a nanosized "buckytrap", 1, constructed from two bis-zinc(II) expanded-TTF (exTTF) porphyrin subunits. Two forms, 1a and 1b, differing in the axial ligands, H₂O vs tetrahydrofuran (THF), were isolated and characterized. Discrete host-guest inclusion complexes are formed upon treatment with fullerenes as inferred from a single-crystal X-ray structural analyses of 1a with C₇₀. The fullerene is found to be encapsulated within the inner pseudohexagonal cavity of 1a. In contrast, the corresponding free-base derivative (2) was found to form infinite ball-and-socket type supramolecular organic frameworks (3D-SOFs) with fullerenes, (2•C₆₀)_n or (2•C₇₀)_n. This difference is ascribed to the fact that in 1a and 1b the axial positions are blocked by a H₂O or THF ligand. Emission spectroscopic studies supported a 1:1 host-guest binding stoichiometry, allowing association constants of (2.0 ± 0.5) × 10⁴ M^-1 and (4.3 ± 0.9) × 10⁴ M^-1 to be calculated for C₆₀ and C₇₀, respectively. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) studies of solid films of the Zn-complex 1a revealed that the intrinsic charge carrier transport, i.e., pseudo-photoconductivity (ϕ∑μ), increases upon fullerene inclusion (e.g., ϕ∑μ = 1.53 × 10^-4 cm² V^-1 s^-1 for C₆₀⊂(1a)₂ and ϕ∑μ = 1.45 × 10^-4 cm² V^-1 s^-1 for C₇₀⊂(1a)₂ vs ϕ∑μ = 2.49 × 10^-5 cm² V^-1 s^-1 for 1a) at 298 K. These findings provide support for the notion that controlling the nature of self-assembly supramolecular constructs formed from exTTF-porphyrin dimers through metalation or choice of fullerene can be used to regulate key functional features, including photoconductivity.

Reactivity of the superhalogen/superalkali ion encapsulating C60 fullerenes

The Diels-Alder cycloaddition reaction between 1,3-cyclohexadiene and a series of C₆₀ fullerenes with encapsulated (super)alkali/(super)halogen species (Li⁺@C₆₀, Li₂F⁺@C₆₀, Cl^-@C₆₀, and LiF₂^-@C₆₀) was explored by means of DFT calculations. The reactivity of the ion encapsulating systems was compared to that of the parent C₆₀ fullerene. Significant enhancement in reactivity was found for cation-encapsulating Li⁺/Li₂F⁺@C₆₀ complexes. The cycloadduct formed by LiF₂^-@C₆₀ was found to be the most thermodynamically favorable among the studied ones. In contrast, encapsulation of Cl^- anions disfavors the cycloaddition reaction both kinetically and thermodynamically. Higher activation energy barrier and less stability of the reaction product in the case of Cl^-@C₆₀ were associated with the higher deformation energies of the fullerene cage and the lower interaction energy between the reactants in comparison with the other studied complexes.

Benzo-Tetrathiafulvalene- (BTTF-) Annulated Expanded Porphyrins: Potential Next-Generation Multielectron Reservoirs

Two sterically crowded benzo-tetrathiafulvalene (BTTF)-annulated expanded porphyrins (BTTF7-F and BTTF8) are synthesized. Detailed photophysical investigations reveal their intrinsic intramolecular charge transfer (CT) character, originated from partial electron transfer from electron-rich TTF units to the relatively electron-deficient macrocyclic core. This finding stands in contrast to what was observed in the previously reported Figure-of-eight conformer of BTTF-annulated [28]hexaphyrin (BTTF6), in which a typical π-π* electronic transition from HOMO to LUMO was observed. However, core expansion in BTTF7-F and BTTF8 makes the oligopyrrole macrocyclic cores relatively more electron-deficient, facilitating the effective intramolecular CT process. Comparative electrochemical investigations reveal that the current generated at the oxidative region is directly proportional to the number of TTF units attached to the macrocyclic core. This work demonstrates the control of the intramolecular CT process through incremental addition of TTF units to the macrocyclic core. Facile multielectron electrochemical oxidations of these expanded porphyrins suggest that they behave like potential multielectron reservoirs.

Anionic Fluorinated Zn-porphyrin Combined with Cationic Endohedral Li-fullerene for Long-Lived Photoinduced Charge Separation with Low Energy Loss

Here we report an anionic meso-tetrakis(4-carboxymethylthio-2,3,5,6-tetrafluorophenyl) zinc porphyrin (ZnTF₄PPTC^4-) to form a supramolecular complex with a cationic lithium endohedral [60]fullerene (Li⁺@C₆₀). The supramolecular ZnTF₄PPTC^4-/Li⁺@C₆₀ complex formed by strong electrostatic attraction with a large binding constant generates a long-lived charge-separated (CS) state with low energy loss by photoinduced electron transfer from ZnTF₄PPTC^4- to Li⁺@C₆₀. The anionic fluorinated zinc porphyrin with high oxidation potential reduces the energy loss associated with the charge separation and enhances the energy level of the CS state. The energy level of the CS state determined by electrochemical measurements is at 0.94 eV, which is much higher than that of a similar supramolecular complex using an anionic meso-tetrakis(sulfonatophenyl) zinc porphyrin (ZnTPPS^4-) at 0.55 eV. Time-resolved transient absorption spectroscopy demonstrates that ZnTF₄PPTC^4-/Li⁺@C₆₀ generates a long-lived CS state with a lifetime of 0.29 ms in a binary solvent of acetonitrile and chlorobenzene. The lifetime of the CS state is comparable to that of ZnTPPS^4-/Li⁺@C₆₀ in benzonitrile.

Bingel-Hirsch Addition of Diethyl Bromomalonate to Ion-Encapsulated Fullerenes M@C60 (M=Ø, Li+, Na+, K+, Mg2+, Ca2+, and Cl-)

In the last 30 years, fullerene-based materials have become popular building blocks for devices with a broad range of applications. Among fullerene derivatives, endohedral metallofullerenes (EMFs, M@C_x) have been widely studied owing to their unique properties and reactivity. For real applications, fullerenes and EMFs must be exohedrally functionalized. It has been shown that encapsulated metal cations facilitate the Diels-Alder reaction in fullerenes. Herein, the Bingel-Hirsch (BH) addition of ethyl bromomalonate over a series of ion-encapsulated M@C₆₀ (M=Ø, Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, and Cl^-; Ø@C₆₀ stands for C₆₀ without any endohedral metal) is quantum mechanically explored to analyze the effect of these ions on the BH addition. The results show that the incarcerated ion has a very important effect on the kinetics and thermodynamics of this reaction. Among the systems studied, K⁺@C₆₀ is the one that leads to the fastest BH reaction, whereas the slowest reaction is given by Cl^-@C₆₀.

Semiconducting Supramolecular Organic Frameworks Assembled from a Near-Infrared Fluorescent Macrocyclic Probe and Fullerenes

We report here a new extended tetrathiafulvalene (exTTF)-porphyrin scaffold, 2, that acts as a ball-and-socket receptor for C₆₀ and C₇₀. Supramolecular interactions between 2 and these fullerenes serve to stabilize 3D supramolecular organic frameworks (SOFs) in the solid state formally comprising peapod-like linear assemblies. The SOFs prepared via self-assembly in this way act as "tunable functional materials", wherein the complementary geometry of the components and the choice of fullerene play crucial roles in defining the conductance properties. The highest electrical conductivity (σ = 1.3 × 10^-8 S cm^-1 at 298 K) was observed in the case of the C₇₀-based SOF. In contrast, low conductivity was seen for the SOF based on pristine 2 (σ = 5.9 × 10^-11 S cm^-1 at 298 K). The conductivity seen for the C₇₀-based SOF approaches that seen for other TTF- and fullerene-based supramolecular materials despite the fact that the present systems are metal-free and constructed entirely from neutral building blocks. Transient absorption spectroscopic measurements corroborated the formation of charge-transfer states (i.e., 2^δ+/C₆₀^δ- and 2^δ+/C₇₀^δ-, respectively) rather than fully charge separated states (i.e., 2^•+/C₆₀^•- and 2^•+/C₇₀^•-, respectively) both in solution (toluene and benzonitrile) and in the solid state at 298 K. Such findings are considered consistent with an ability to transfer charges effectively over long distances within the present SOFs, rather than, for example, the formation of energetically trapped ionic species.

Caged-Electron States in Endohedral Li Fullerenes

By employing large-scale high-level EA-EOM-CCSD calculations, we have computed and analyzed the low-lying states of neutral Li@C₆₀. Apart from one state, all states are found to be charge-separated states of the type Li⁺@C₆₀^-. The new state is the first reported non-charge-separated state in endohedral alkali fullerenes. This caged-electron state is analyzed in detail. Arguments are given that in larger highly symmetric endohedral fullerenes the caged-electron state can be the electronic ground state of the system. HF and DFT calculations on Li@C₁₈₀ indeed find that the caged-electron state is the ground state and that in its equilibrium geometry Li sits at the center of the cage. Applications are mentioned.

Unified Mechanism of Oxygen Atom Transfer and Hydrogen Atom Transfer Reactions with a Triflic Acid-Bound Nonheme Manganese(IV)-Oxo Complex via Outer-Sphere Electron Transfer

Outer-sphere electron transfer from styrene, thioanisole, and toluene derivatives to a triflic acid (HOTf)-bound nonheme Mn(IV)-oxo complex, [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂ (N4Py = N, N-bis(2-pyridylmethyl)- N-bis(2-pyridyl)methylamine), has been shown to be the rate-determining step of different types of redox reactions such as epoxidation, sulfoxidation, and hydroxylation of styrene, thioanisole, and toluene derivatives, respectively, by [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂. The rate constants of HOTf-promoted epoxidation of all styrene derivatives with [(N4Py)Mn^IV(O)]²⁺ and electron transfer from electron donors to [(N4Py)Mn^V(O)]²⁺ exhibit a remarkably unified correlation with the driving force of outer-sphere electron transfer in light of the Marcus theory of electron transfer. The same electron-transfer driving force dependence is observed in the oxygen atom transfer from [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂ to thioanisole derivatives as well as in the hydrogen atom transfer from toluene derivatives to [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂. Thus, mechanisms of oxygen atom transfer (epoxidation and sulfoxidation) reactions of styrene and thioanisole derivatives and hydrogen atom transfer (hydroxylation) reactions of toluene derivatives by [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂ have been unified for the first time as the same reaction pathway via outer-sphere electron transfer, followed by the fast bond-forming step, which exhibits the singly unified electron-transfer driving force dependence of the rate constants as outer-sphere electron-transfer reactions. In the case of the epoxidation of cis-stilbene by [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂, the isomerization of cis-stilbene radical cation to trans-stilbene radical cation occurs after outer-sphere electron transfer from cis-stilbene to [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂ to yield trans-stilbene oxide selectively, which is also taken as evidence for the occurrence of electron transfer in the acid-catalyzed epoxidation.

Tetrathiafulvalene-calix[4]pyrrole: a versatile synthetic receptor for electron-deficient planar and spherical guests

The chemistry of tetrathiafulvalene-calix[4]pyrrole is reviewed with focus on conformational behavior, receptor properties and ionically controlled electron transfer processes.

New dimensions in calix[4]pyrrole: the land of opportunity in supramolecular chemistry

The quest for receptors endowed with the selective complexation and detection of negatively charged species continues to receive substantial consideration within the scientific community worldwide. This study is encouraged by the utilization of anions in nature in a plethora of biological systems such as chloride channels and proteins and as polyanions for genetic information. The molecular recognition of anionic species is greatly interesting in terms of their favourable interactions. In this comprehensive review, in addition to giving accounts of some selected syntheses, we illustrated diverse applications ranging from molecular containers to ion transporters and drug carriers of a supramolecular receptor named calix[4]pyrrole. We believe that the present review may act as a catalyst in enhancing the novel applications of calix[4]pyrrole and its congeners in the other dimensions of science and technology.

The quest for receptors endowed with the selective complexation and detection of negatively charged species continues to receive substantial consideration within the scientific community worldwide.

Programmed Hierarchical Hybrid Nanostructures from Fullerene-Dendrons and Pyrene-Dendrons

The construction of fullerene (C₆₀ ) hierarchical nanostructures with the help of amphiphilic molecules remains a challenging task in nanoscience and nanotechnology. Utilizing the host-guest complex concept, sub-10 nm layered superstructures are constructed from a monofunctionalized C₆₀ dendron (C₆₀ D, guest) and tweezer-like pyrene dendron (PD, host). Since C₆₀ D and PD are asymmetric shape amphiphiles having liquid crystal (LC) dendrons, both C₆₀ D and PD construct head-to-head bilayer superstructures by themselves. From fluorescence titration experiments, it is realized that the host-guest complex shows 1:1 stoichiometric binding with a binding constant (K_sv = 2.45 × 10⁵ m^-1 ). Based on the morphological observations and scattering analyses, it is found that buckle-like asymmetric building blocks (C₆₀ D·PD) are self-assembled by the host-guest complex and construct multilayer hybrid nanostructures. The hierarchical hybrid nanostructures consist of the self-assembled C₆₀ D·PD bilayer with a 2D C₆₀ ·P nanoarray sandwiched between LC dendrons. This advanced strategy is expected to be a practicable and rational guideline for the fabrication of programmed hierarchical hybrid nanostructures.

Tetrathiafulvalene (TTF)-Annulated Calix[4]pyrroles: Chemically Switchable Systems with Encodable Allosteric Recognition and Logic Gate Functions

Molecular and supramolecular systems capable of switching between two or more states as the result of an applied chemical stimulus are attracting ever-increasing attention. They have seen wide application in the development of functional materials including, but not limited to, molecular and supramolecular switches, chemosensors, electronics, optoelectronics, and logic gates. A wide range of chemical stimuli have been used to control the switching within bi- and multiple state systems made up from either singular molecular entities or supramolecular ensembles. In general, chemically triggered switching systems contain at least two major functional components that provide for molecular recognition and signal transduction, respectively. These components can be connected to one another via either covalent or noncovalent linkages. Of particular interest are switchable systems displaying cooperative or allosteric features. Such advanced control over function is ubiquitous in nature and, in the case of synthetic systems, may allow the capture and release of a targeted chemical entity or permit the transduction of binding information from one recognition site to another. Allosterically controlled complexation and decomplexation could also permit the amplification or deamplification of analyte-specific binding affinity, lead to nonlinear binding characteristics, or permit a magnification of output signals. Our own efforts to develop chemically driven supramolecular switches, advanced logic gates, and multifunction cascade systems have focused on the use of tetrathiafulvalene (TTF) annulated calix[4]pyrroles (C4Ps). These systems, TTF-C4Ps, combine several orthogonal binding motifs within what are conformationally switchable receptor frameworks. Their basic structure and host-guest recognition functions can be controlled via application of an appropriate chemical stimulus. Homotropic or heterotropic allosteric molecular recognition behavior is often seen. This has allowed us to (1) produce self-assembled structures, (2) control switching between bi- and multistate constructs, (3) generate chemical logic gates performing chemical-based Boolean logic operations, (4) create ionically controlled three-state logic systems that release different chemical messengers and activate disparate downstream reactions, and (5) encode a variety advanced functional operations into what are relatively simple molecular-scale devices. Looking to the future, we believe that exploiting allosteric control will expand opportunities for supramolecular chemists and allow some of the complexity seen in biology to be reproduced in simple constructs. Of particular appeal would be a capacity to release chemical messengers at will, perhaps after a prior capture and chemical modification step, that then encode for further downstream functions as seen in the case of the small molecules, such as neurotransmitters and pheromones, used by nature for the purpose of intraentity communication. Molecular scale logic devices with allosteric functions are thus the potential vanguard of a new area of study involving interactions between multiple discrete components with an emphasis on functional outcomes.

Penetrating probability and cross section of the Li+-C60 encapsulation process through an ab initio molecular dynamics investigation

The endohedral complex system of Li⁺-C₆₀ has been shown to possess interesting applications in photovoltaics, supramolecular chemistry, and functionalized materials. In this study, we perform a theoretical investigation of Li⁺ encapsulation within a C₆₀ cage by employing an ab initio molecular dynamics approach. The Li⁺ cation is positioned 9 Å away from the C₆₀ center of mass, and fired towards a randomized spot in a six-membered ring with a certain level of inletting energy, which is 7.5 eV, 9 eV, 12 eV, or 15 eV. In total, 2000 samples of MD trajectories are investigated. Our statistical results yielded a penetrating probability in the range of 0.8% to 15.6% with respect to the above inletting energy, while the cross section ranges from 0.006 Ų to 0.123 Ų. Moreover, we observed that the penetrating probability exhibited direct proportionality to the inletting energy. Hence, we can determine that the minimum required inletting energy for reaction occurrence is 6.6 eV. Overall, it seems difficult for Li⁺ to penetrate through the sp²-carbon wall, because a very high inletting energy is required to open the entrance. At the same time, Li⁺ must approach closely to the center of a six-membered ring to enhance the penetration probability.

Functionalised tetrathiafulvalene- (TTF-) macrocycles: recent trends in applied supramolecular chemistry

Tetrathiafulvalene- (TTF-) based macrocyclic systems, cages and supramolecularly self-assembled 3D constructs have been extensively explored as functional materials for sensing and switching applications.

Synthesis and Crystal Structure of Li+@Fluoreno[60]fullerene: Effect of Encapsulated Lithium Ion on Electrochemistry of Spiroannelated Fullerene

The reaction of [Li⁺@C₆₀]TFSI^- (TFSI = bis(trifluoromethanesulfonyl)imide) with 9-diazofluorene directly produced a [6,6]-adduct of lithium-ion-containing fluoreno[60]fullerene, [6,6]-[Li⁺@C₆₀(fluoreno)]TFSI^-, which was crystallographically characterized. Cyclic voltammetry of the compound showed a reversible one-electron reduction wave at -0.51 V (vs Fc/Fc⁺) and an irreversible reduction wave for the second electron. The latter was attributed to opening of the three-membered ring due to strong stabilization of the resulting sp³-carbanion by the encapsulated Li⁺ and formation of a 14π-electron aromatic fluorenyl anion.

Bromide Photo-oxidation Sensitized to Visible Light in Consecutive Ion Pairs

The titration of bromide into a [Ru(deeb)(bpz)₂]²⁺ (Ru²⁺, deeb = 4,4'-diethylester-2,2'-bipyridine; bpz = 2,2'-bipyrazine) dichloromethane solution led to the formation of two consecutive ion-paired species, [Ru²⁺, Br^-]⁺ and [Ru²⁺, 2Br^-], each with distinct photophysical and electron-transfer properties. Formation of the first ion pair was stoichiometric, K_eq 1 > 10⁶ M^-1, and the second ion-pair equilibrium was estimated to be K_eq 2 = (2.4 ± 0.4) × 10⁵ M^-1. The ¹H NMR spectra recorded in deuterated dichloromethane indicated the presence of contact ion pairs and provided insights into their structures and were complimented by density functional theory calculations. Static quenching of the [Ru(deeb)(bpz)₂]^2+* photoluminescence intensity (PLI) by bromide was observed, and [Ru²⁺, Br^-]^+* was found to be nonluminescent, τ < 10 ns. Further addition of bromide resulted in partial recovery of the PLI, and [Ru²⁺, 2Br^-]* was found to be luminescent with an excited-state lifetime of τ = 65 ± 5 ns. Electron-transfer products were identified as the reduced complex, [Ru(deeb)(bpz)₂]⁺, and dibromide, Br₂^•-. The bromine atom, Br^•, was determined to be the primary excited-state electron-transfer product and was an intermediate in Br₂^•- formation, Br^• + Br^- → Br₂^•-, with a second-order rate constant, k = (5.4 ± 1) × 10⁸ M^-1 s^-1. The unusual enhancement in PLI for [Ru²⁺, 2Br^-]* relative to [Ru²⁺, Br^-]^+* was due to a less favorable Gibbs free energy change for electron transfer that resulted in a smaller rate constant, k_et = (1.5 ± 0.2) × 10⁷ s^-1, in the second ion pair. Natural atomic charge analysis provided estimates of the Coulombic work terms associated with ion pairing, ΔG_w, that were directly correlated with the measured change in rate constants.

Crystallographic Structure Determination of Both [5,6]- and [6,6]-Isomers of Lithium-Ion-Containing Diphenylmethano[60]fullerene

Organic functionalization of lithium-ion-containing [60]fullerene, Li⁺@C₆₀, was performed by using diphenyl(diazo)methane as a stable, readily available diazo compound to obtain lithium-ion-containing [5,6]- and [6,6]-diphenylmethano[60]fullerenes, Li⁺@C₆₁Ph₂. The bis(trifluoromethanesulfonyl)imide (TFSI) salts of [5,6]- and [6,6]-Li⁺@C₆₁Ph₂ were successfully separated by using a cation exchange column with eluent containing LiTFSI. Improved separation protocol and high crystallinity of ionic components in less polar solvents enabled separate crystallization of each isomer. Both [5,6]-open and [6,6]-closed structures of Li⁺@C₆₁Ph₂ were determined by synchrotron radiation X-ray crystallography. Elucidating the [5,6]-open methano[60]fullerene (fulleroid) structure will contribute to materials research on fulleroids.

Interactions of Metal-Based and Ligand-Based Electronic Spins in Neutral Tripyrrindione π Dimers

The ability of tetrapyrrolic macrocycles to stabilize unpaired electrons and engage in π-π interactions is essential for many electron-transfer processes in biology and materials engineering. Herein, we demonstrate that the formation of π dimers is recapitulated in complexes of a linear tripyrrolic analogue of naturally occurring pigments derived from heme decomposition. Hexaethyltripyrrindione (H₃TD1) coordinates divalent transition metals (i.e., Pd, Cu, Ni) as a stable dianionic radical and was recently described as a robust redox-active ligand. The resulting planar complexes, which feature a delocalized ligand-based electronic spin, are stable at room temperature in air and support ligand-based one-electron processes. We detail the dimerization of neutral tripyrrindione complexes in solution through electron paramagnetic resonance (EPR) and visible absorption spectroscopic methods. Variable-temperature measurements using both EPR and absorption techniques allowed determination of the thermodynamic parameters of π dimerization, which resemble those previously reported for porphyrin radical cations. The inferred electronic structure, featuring coupling of ligand-based electronic spins in the π dimers, is supported by density functional theory (DFT) calculations.

The inner-induced effects of YCN in C76 on the structures and nonlinear optical properties

Very recently, an unprecedented novel monometallic cluster of fullerenes entrapping a yttrium cyanide (YCN) cluster inside a popular C82 cage YCN@Cs(6)-C82 was synthesized and characterized. Inspired by this investigation, four non-IPR YCN@C1(17459)-C76, YCN@C2v(19138)-C76, YCN@C2(17646)-C76, and YCN@C1(17894)-C76 (1, 2, 3, and 4) containing a pair of adjacent pentagons are designed to explore the encapsulated molecular effect on their interaction energies and nonlinear optical properties. The interaction energy (E int) values of 1, 2, 3, and 4 are -481.35 (1), -477.91 (2), -482.04 (3), -482.69 (4) kcal mol(-1), respectively, which shows that the E int value of 4 is the largest. Furthermore, the electron-transfer is mainly from the YCN to C76 cage. When YCN is encapsulated into C76 cage, we can find that the α0 values of the four molecules are very close, ranging from 6.50 × 10(2) to 6.65 × 10(2) au. Significantly, the first hyperpolarizabilities are in relation to the encapsulated molecular: 1.63 × 10(3) (1) > 8.03 × 10(2) (2) > 7.76 × 10(2) (4) > 4.86 × 10(2) au (3), the results show that the βtot value of 1 is the largest. Besides this, the encapsulation of the YCN to C76 cage brings some distinctive changes in its UV-vis spectra along with its other electronic properties that might be used by the experimentalists to develop the potential nonlinear optical nanomaterials based on endohedral metallofullerenes.

Electrochemical reduction of cationic Li+@C60 to neutral Li+@C60˙-: isolation and characterisation of endohedral [60]fulleride

Li@C₆₀ was synthesised by electrochemical reduction of ionic Li⁺@C₆₀ salt. This is the first report of isolation and unambiguous characterisation of endohedral metallo[60]fullerene.

Lithium-encapsulated [60]fullerene Li@C₆₀, namely, lithium-ion-encapsulated [60]fullerene radical anion Li⁺@C₆₀˙^–, was synthesised by electrochemical reduction of lithium-ion-encapsulated [60]fullerene trifluoromethanesulfonylimide salt [Li⁺@C₆₀](TFSI^–). The product was fully characterised by UV-vis-NIR absorption and ESR spectroscopy as well as single-crystal X-ray analysis for the co-crystal with nickel octaethylporphyrin. In solution Li@C₆₀ exists as a monomer form dominantly, while in the crystal state it forms a dimer (Li@C₆₀–Li@C₆₀) through coupling of the C₆₀ radical anion cage. These structural features were supported by DFT calculations at the M06-2X/6-31G(d) level of theory.

Evidence for an intrinsic binding force between dodecaborate dianions and receptors with hydrophobic binding pockets

A gas phase binding study revealed strong intrinsic intermolecular interactions between dianionic halogenated closo-dodecaborates [B12X12](2-) and several neutral organic receptors. Oxidation of a tetrathiafulvalene host allowed switching between two host-guest binding modes in a supramolecular complex. Complexes of β-cyclodextrin with [B12F12](2-) show remarkable stability in the gas phase and were successfully tested as carriers for the delivery of boron clusters into cancer cells.

Aryl-fused tetrathianaphthalene (TTN): synthesis, structures, properties, and cocrystals with fullerenes

A library of aryl-fused TTN has been synthesized to show shape complementary with fullerene molecules and form “TTN·fullerene” cocrystals.

An ionic charge-transfer dyad prepared cost-effectively from a tetrathiafulvalene carboxylate anion and a TMPyP cation

The assembly of a simple ionic supramolecular system would be a cost-effective way to construct donor–acceptor ensembles and the strong anion–cation interaction can enhance the charge-transfer between them.

Multiple photosynthetic reaction centres of porphyrinic polypeptide-Li(+)@C60 supramolecular complexes

Multiple photosynthetic reaction centres have been successfully constructed using strong supramolecular complexes of free base porphyrin polypeptides with lithium ion-encapsulated C60 (Li(+)@C60) as compared with those of C60. Efficient energy migration and electron transfer occur in the supramolecular complexes.

Redox- and pH-Responsive Orthogonal Supramolecular Self-Assembly: An Ensemble Displaying Molecular Switching Characteristics

Two heteroditopic monomers, namely a thiopropyl-functionalized tetrathiafulvalene-annulated calix[4]pyrrole (SPr-TTF-C[4]P 1) and phenyl C61 butyric acid (PCBA 2), have been used to assemble a chemically and electrochemically responsive supramolecular ensemble. Addition of an organic base initiates self-assembly of the monomers via a molecular switching event. This results in the formation of materials that may be disaggregated via the addition of an organic acid or electrolysis.

Charge transfer and first hyperpolarizability: cage-like radicals C59X and lithium encapsulated Li@C59X (X=B, N)

Very recently, two new cage-like radicals (C59B and C59N) formed by a boron or nitrogen atom substituting one carbon atom of C60 were synthesized and characterized. In order to explore the structure-property relationships of combination the cage-like radical and alkali metal, the endohedral Li@C59B and Li@C59N are designed by lithium (Li) atom encapsulated into the cage-like radicals C59B and C59N. Further, the structures, natural bond orbital (NBO) charges, and nonlinear optical (NLO) responses of C59B, C59N, Li@C59B, and Li@C59N were investigated by quantum chemical method. Three density functional methods (BHandHLYP, CAM-B3LYP, and M05-2X) were employed to estimate their first hyperpolarizabilities (β tot) and obtained the same trend in the β tot value. The β tot values by BHandHLYP functional of the pure cage-like radicals C59B (1.30 × 10(3) au) and C59N (1.70 × 10(3) au) are close to each other. Interestingly, when one Li atom encapsulated into the electron-rich radical C59N, the β tot value of the Li@C59N increases to 2.46 × 10(3) au. However, when one Li atom encapsulated into the electron-deficient radical C59B, the β tot value of the Li@C59B sharply decreases to 1.54 × 10(2) au. The natural bond orbital analysis indicates that the encapsulated Li atom leads to an obvious charge transfer and valence electrons distribution plays a significant role in the β tot value. Further, frontier molecular orbital explains that the interesting charge transfer between the encapsulated Li atom and cage-like radicals (C59B and C59N) leads to differences in the β tot value. It is our expectation that this work will provide useful information for the design of high-performance NLO materials.

Modulation of Energy Conversion Processes in Carbonaceous Molecular Bearings

The energetics and photodynamics of carbonaceous molecular bearings with discrete molecular structures were investigated. A series of supramolecular bearings comprising belt-persistent tubular cycloarylene and fullerene molecules accepted photonic stimuli to afford charge-separated species via a photoinduced electron transfer process. The energy conversion processes associated with the photoexcitation, however, differed depending on the molecular structure. A π-lengthened tubular molecule allowed for the emergence of an intermediary triplet excited state at the bearing, which should lead to an energy conversion to thermal energy. On the other hand, low-lying charge-separated species induced by an endohedral lithium ion in fullerene enabled back electron transfer processes to occur without involving triplet excited species. The structure-photodynamics relationship was analyzed in terms of the Marcus theory to reveal a large electronic coupling in this dynamic supramolecular system.

Catalytic two-electron reduction of dioxygen catalysed by metal-free [14]triphyrin(2.1.1)

The catalytic two-electron reduction of dioxygen (O₂) by octamethylferrocene (Me₈Fc) occurs with a metal-free triphyrin (HTrip) in the presence of perchloric acid (HClO₄) in benzonitrile (PhCN) at 298 K to yield Me₈Fc⁺ and H₂O₂. Detailed kinetic analysis has revealed that the catalytic two-electron reduction of O₂ by Me₈Fc with HTrip proceeds via proton-coupled electron transfer from Me₈Fc to HTrip to produce H₃Trip˙⁺, followed by a second electron transfer from Me₈Fc to H₃Trip˙⁺ to produce H₃Trip, which is oxidized by O₂via formation of the H₃Trip/O₂ complex to yield H₂O₂. The rate-determining step in the catalytic cycle is hydrogen atom transfer from H₃Trip to O₂ in the H₃Trip/O₂ complex to produce the radical pair (H₃Trip˙⁺ HO₂˙) as an intermediate, which was detected as a triplet EPR signal with fine-structure by the EPR measurements at low temperature. The distance between the two unpaired electrons in the radical pair was determined to be 4.9 Å from the zero-field splitting constant (D).

Formation of Ground State Triplet Diradicals from Annulated Rosarin Derivatives by Triprotonation

Annulated rosarins, β,β'-bridged hexaphyrin(1.0.1.0.1.0) derivatives 1-3, are formally 24 π-electron antiaromatic species. At low temperature, rosarins 2 and 3 are readily triprotonated in the presence of trifluoroacetic acid in dichloromethane to produce ground state triplet diradicals, as inferred from electron paramagnetic resonance (EPR) spectral studies. From an analysis of the fine structure in the EPR spectrum of triprotonated rosarin H₃3(3+), a distance of 3.6 Å between the two unpaired electrons was estimated. The temperature dependence of the singlet-triplet equilibrium was determined by means of an EPR titration. Support for these experimental findings came from calculations carried out at the (U)B3LYP/6-31G* level, which served to predict a very low-lying triplet state for the triprotonated form of a simplified model system 1.

Acid/base-regulated reversible electron transfer disproportionation of N-N linked bicarbazole and biacridine derivatives

Regulation of electron transfer on organic substances by external stimuli is a fundamental issue in science and technology, which affects organic materials, chemical synthesis, and biological metabolism. Nevertheless, acid/base-responsive organic materials that exhibit reversible electron transfer have not been well studied and developed, owing to the difficulty in inventing a mechanism to associate acid/base stimuli and electron transfer. We discovered a new phenomenon in which N-N linked bicarbazole (BC) and tetramethylbiacridine (TBA) derivatives undergo electron transfer disproportionation by acid stimulus, forming their stable radical cations and reduced species. The reaction occurs through a biradical intermediate generated by the acid-triggered N-N bond cleavage reaction of BC or TBA, which acts as a two electron acceptor to undergo electron transfer reactions with two equivalents of BC or TBA. In addition, in the case of TBA the disproportionation reaction is highly reversible through neutralization with NEt3, which recovers TBA through back electron transfer and N-N bond formation reactions. This highly reversible electron transfer reaction is possible due to the association between the acid stimulus and electron transfer via the acid-regulated N-N bond cleavage/formation reactions which provide an efficient switching mechanism, the ability of the organic molecules to act as multi-electron donors and acceptors, the extraordinary stability of the radical species, the highly selective reactivity, and the balance of the redox potentials. This discovery provides new design concepts for acid/base-regulated organic electron transfer systems, chemical reagents, or organic materials.

Donor-acceptor type co-crystals of arylthio-substituted tetrathiafulvalenes and fullerenes

A series of donor-acceptor type co-crystals of fullerene (as the acceptor) and arylthio-substituted tetrathiafulvalene derivatives (Ar-S-TTF, as the donor) were prepared and their structural features were thoroughly investigated. The formation of co-crystals relies on the flexibility of Ar-S-TTF and the size matches between Ar-S-TTF and fullerene. Regarding their compositions, the studied co-crystals can be divided into two types, where types I and II have donor:acceptor ratios of 1:1 and 1:2, respectively. Multiple intermolecular interactions are observed between the donor and acceptor, which act to stabilize the structures of the resulting co-crystals. In the type I co-crystals, the fullerene molecule is surrounded by four Ar-S-TTF molecules, that is, two Ar-S-TTF molecules form a sandwich structure with one fullerene molecule and the other two Ar-S-TTF molecules interact with the fullerene molecule along their lateral axes. In the type II co-crystals, one fullerene molecule has the donor-acceptor mode similar to that in type I, whereas the other fullerene molecule is substantially surrounded by the aryl groups on Ar-S-TTF molecules and the solvent molecules.

Supramolecular photovoltaic cells utilizing inclusion complexes composed of Li+@C60 and cyclic porphyrin dimer

We have newly constructed supramolecular photovoltaic cells using inclusion complexes of lithium-ion-encapsulated [60]fullerene ( Li +@ C 60) and cyclic porphyrin dimers (M-CPDPy, M = H 4 and Ni 2). First, supramolecular inclusion complexes of Li +@ C 60 and M-CPDPy were prepared in MeCN/PhCN (3/1, v/v) by rapid injection method. The molecular aggregates with spherical nanoparticles demonstrated a broad absorption property in the visible region. The macroscopic structures were also estimated to be ca. 200 nm in diameter by transmission electron microscope (TEM) and dynamic light scattering (DLS) measurements. The photoelectrochemical solar cells composed of these assemblies on nanostructured SnO 2 electrode were fabricated by electrophoretic deposition method. The photoelectrochemical behavior of the nanostructured SnO 2 film of supramolecular nanoassemblies of Li +@ C 60 and M-CPDPy is significantly higher than those of the single component films ( Li +@ C 60 or M-CPDPy) and supramolecular inclusion complexes of pristine C 60 and M-CPDPy.

Ultrafast photoinduced electron transfer in face-to-face charge-transfer π-complexes of planar porphyrins and hexaazatriphenylene derivatives

Charge-transfer (CT) π-complexes are formed between planar porphyrins and 1,4,5,8,9,12-hexaazatriphenylene (HAT) derivatives with large formation constants (e.g., 104 M-1), exhibiting broad CT absorption bands. The unusually large formation constants result from close face-to-face contact between two planar π-planes of porphyrins and HAT derivatives. The redox potentials of porphyrins and HAT derivatives measured by cyclic voltammetry indicate that porphyrins and HAT derivatives act as electron donors and acceptors, respectively. The formation of 1 : 1 CT complexes between porphyrins and HAT derivatives was examined by UV-vis, fluorescence and 1H NMR measurements in nonpolar solvents. The occurrence of unprecedented ultrafast photoinduced electron transfer from the porphyrin unit to the HAT unit in the CT π-complex was observed by femtosecond laser flash photolysis measurements. A highly linear aggregate composed of a planar porphyrin and an HAT derivative was observed by transmission electron microscopy (TEM) and atomic force microscopy (AFM).