Donor-acceptor Ensembles Research Articles

Photoinduced asymmetric charge trapping in a symmetric tetraazapyrene-fused bis(tetrathiafulvalene) conjugate.

In fused donor-acceptor (D-A) ensembles, rapid charge recombination often occurs because the D and A units are spatially close and strongly coupled. To the best of our knowledge, a long-lived charge separated (CS) state is still elusive in such systems. The results presented here show that symmetric annulation of two tetrathiafulvalene (TTF) donors to a central tetraazapyrene (TAP) acceptor via two quinoxaline units leads to a CS state lifetime of a few ns. A detailed study of the electronic interactions between TTF and TAP units in the ground and excited states was performed and compared with the asymmetric counterpart by cyclic voltammetry, optical absorption and ultrafast transient absorption spectroscopy. The results demonstrate that the photoinduced asymmetric charge trapping between two TTFs significantly stabilizes the CS state, which is also verified theoretically.

Donor-Acceptor Couples of Metal and Metal Oxides with Enriched Ni3+ Active Sites for Oxygen Evolution.

Exploiting precious-metal-free and high-activity oxygen evolution reaction (OER) electrocatalysts has been in great demands toward many energy storage and conversion processes, for example, carbon dioxide reduction, metal-air batteries, and water splitting. In this study, the simple solid-state method is employed for coupling Ni (electron donors) with lower-Fermi-level MoO2 or WOx (electron acceptors) into donor-acceptor ensembles with well-designed interfaces as robust electrocatalysts for OER. The resulting Ni/MoO2 and Ni/WOx electrocatalysts exhibit smaller overpotentials of 287 and 333 mV at 10 mA cm-2 as well as smaller Tafel slopes of 51 and 65 mV/dec, respectively, with respect to the single Ni, MoO2, WOx, and even the benchmark RuO2 in 1 M KOH. Specially, on account of a higher Fermi level of Ni in comparison with MoO2 and WOx, their strong electronic interaction results in directional interfacial electron transfer and increases the hole density over Ni, dramatically enriching the population of high-valence Ni3+ active sites and decreasing the Fermi level of Ni. The existence of Ni3+ can strengthen the chemisorption of OH-, and the downshift of the Ni Fermi level can significantly expedite migration of electrons toward the surface of catalysts during OER, thus synergistically boosting the OER catalytic performance. Furthermore, the inner Ni/MoO2 and Ni/WOx heterostructures and the electrochemically induced surface layers of oxides/hydroxides collectively boost the OER kinetics. This study highlights the importance of designing highly efficient OER electrocatalysts with high-valence active species (Ni3+) and better matched energy levels induced by the work function difference through interfacial engineering.

Bimetallic-based food sensors for meat spoilage: Effects of the accepting metallic unit in Fe(II)[sbnd]C[tbnd]N[sbnd]MA (MA = Pt(II) or Au(I)) on device selectivity and sensitivity

Technologies for monitoring meat spoilage are important to ensuring consumer safety. As dimethyl sulfide (DMS) is a reliable marker for meat freshness, sensitive and selective DMS sensors are of great interest. Herein, two trinuclear cyano-bridged bimetallic donor–acceptor ensembles, FeII(bpy)2(CN)2–[PtII(DMSO)Cl2]2 (1) and FeII(bpy)2(CN)2–[AuICl]2, were synthesized, and corresponding solid-supported sensors were fabricated to determine the effect of the acceptor metal (MA) on DMS detection. Changing MA from AuI to PtII improved the sensitivity and selectivity owing to changes in the relative thermodynamic stabilities of the complex and MA–DMS adduct. When applied to real meat samples, 1 exhibited a linear spectroscopic response to DMS, even in the presence of interfering compounds, with a method detection limit of 1.0 ppm. The total bacteria count and gas chromatography–mass spectrometry results revealed that the spectroscopic signal generated by 1 correlated with the microbial growth level and DMS concentration during meat spoilage.

Mimicry and functions of photosynthetic reaction centers.

The structure and function of photosynthetic reaction centers (PRCs) have been modeled by designing and synthesizing electron donor-acceptor ensembles including electron mediators, which can mimic multi-step photoinduced charge separation occurring in PRCs to obtain long-lived charge-separated states. PRCs in photosystem I (PSI) or/and photosystem II (PSII) have been utilized as components of solar cells to convert solar energy to electric energy. Biohybrid photoelectrochemical cells composed of PSII have also been developed for solar-driven water splitting into H2 and O2 Such a strategy to bridge natural photosynthesis with artificial photosynthesis is discussed in this minireview.

From Molecules to Devices: A DFT/TD-DFT Study of Dipole Moment and Internal Reorganization Energies in Optoelectronically Active Aryl Azo Chromophores

The present paper elicits a very useful, computational exploration of molecular architectonics of benzothiazole scaffold. The investigation elucidates hopping transport phenomenon in donor–acceptor ensembles lying within a same molecule, comprising an azo group as the core entity. In depth electronic and charge transfer behavior of certain substituted aryl azo benzothiazoles (organic π conjugated) considering notions of density functional theory (DFT) and time-dependent density functional theory (TD-DFT) has been investigated. Moreover, the effect of structural variation in aryl azo moiety (−CF3, −SCH3) in presence/absence of solvent has been examined. Also, the impact of disparate solvents namely, polar protic, polar aprotic, and nonpolar solvents has been deduced. Interestingly, results indicate that (E)-2-((4-(trifluoromethyl)phenyl)diazenyl)benzo[d]thiazole (BAF) and (E)-2-(phenyldiazenyl)benzo[d]thiazole (BAB) have affirmed to be the promising candidates for the organic charge transfer material in or...

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

Great progress has been made in combining a TTF moiety with a porphyrin unit by covalent bonds, but only a few examples were reported in which TTF and porphyrin assembled by noncovalent interactions. In contrast to the energy- and time-consuming synthetic procedures for the covalent system, the assembly of a non-covalent ionic system would be a cost-effective way to construct donor-acceptor ensembles. Herein a new type of ionic TTF-porphyrin dyad is obtained. A methylated tetra(4-pyridyl) porphyrin (5,10,15,20-tetrakis-(N-methyl-4-pyridyl)-porphyrin, TMPyP) is selected as the cation, and TTF-bicarboxylate (L(1)) or TTF-tetracarboxylate (L(2)) is used as the anion. Crystal structures of two TTF-TMPyP ionic D-A compounds, TMPyP-(HL(1))4·3H2O (1) and TMPyP-(H2L(2))2·5H2O (2), were characterized by single-crystal X-ray diffraction. The strong ionic interaction enhances the charge-transfer between the regular mixed-stacking donors and acceptors, which are investigated comprehensively by spectral, electrochemical and theoretical studies. The variation in properties between L(1) and L(2) is of great advantage to understand the influence factors for charge-transfer. The charge-transfer properties can be modulated not only by the nature of the donor or the acceptor, but also the cation-anion ratio in the salt, which shows great flexibility of the D-A ionic dyad in the design and preparation of new charge-transfer systems.

Bidirectional Electron Transfer Capability in Phthalocyanine-Sc3N@I(h)-C80 Complexes.

To activate oxidative and/or reductive electron transfer reactions, N-pyridyl-substituted Sc3N@I(h)-C80 (4) and C60 (3) fulleropyrrolidines have been prepared and axially coordinated to electron-rich (1) or electron-deficient (2) Zn(II)phthalocyanines (Zn(II)Pcs) through zinc-pyridyl, metal-ligand coordination affording a full-fledged family of electron donor-acceptor ensembles. An arsenal of photophysical assays as they were carried out with, for example, 1/4 and 2/4 show unambiguously that a Zn(II)Pc-to-Sc3N@I(h)-C80 photoinduced electron transfer takes place in the former ensemble, whereas a Sc3N@I(h)-C80-to-Zn(II)Pc electron transfer occurs in the latter ensemble. To the best of our knowledge, this is the first time that a fullerene-based molecular building block shows an electron transfer dichotomy, namely acting both as electron-acceptor or electron-donor, and its outcome is simply governed by the electronic nature of its counterpart. In light of the latter, the present work, which involves the use of Sc3N@I(h)-C80, one of the most abundant and easy-to-purify endohedral metallofullerenes, is, on one hand, a paradigmatic change and, on the other hand, an important milestone en-route toward the construction of easy-to-prepare molecular materials featuring switchable electron transfer reactivity.

Phthalocyanine-nanocarbon ensembles: from discrete molecular and supramolecular systems to hybrid nanomaterials.

Phthalocyanines (Pcs) are macrocyclic and aromatic compounds that present unique electronic features such as high molar absorption coefficients, rich redox chemistry, and photoinduced energy/electron transfer abilities that can be modulated as a function of the electronic character of their counterparts in donor-acceptor (D-A) ensembles. In this context, carbon nanostructures such as fullerenes, carbon nanotubes (CNTs), and, more recently, graphene are among the most suitable Pc "companions". Pc-C60 ensembles have been for a long time the main actors in this field, due to the commercial availability of C60 and the well-established synthetic methods for its functionalization. As a result, many Pc-C60 architectures have been prepared, featuring different connectivities (covalent or supramolecular), intermolecular interactions (self-organized or molecularly dispersed species), and Pc HOMO/LUMO levels. All these elements provide a versatile toolbox for tuning the photophysical properties in terms of the type of process (photoinduced energy/electron transfer), the nature of the interactions between the electroactive units (through bond or space), and the kinetics of the formation/decay of the photogenerated species. Some recent trends in this field include the preparation of stimuli-responsive multicomponent systems with tunable photophysical properties and highly ordered nanoarchitectures and surface-supported systems showing high charge mobilities. A breakthrough in the Pc-nanocarbon field was the appearance of CNTs and graphene, which opened a new avenue for the preparation of intriguing photoresponsive hybrid ensembles showing light-stimulated charge separation. The scarce solubility of these 1-D and 2-D nanocarbons, together with their lower reactivity with respect to C60 stemming from their less strained sp(2) carbon networks, has not meant an unsurmountable limitation for the preparation of variety of Pc-based hybrids. These systems, which show improved solubility and dispersibility features, bring together the unique electronic transport properties of CNTs and graphene with the excellent light-harvesting and tunable redox properties of Pcs. A singular and distinctive feature of these Pc-CNT/graphene (single- or few-layers) hybrid materials is the control of the direction of the photoinduced charge transfer as a result of the band-like electronic structure of these carbon nanoforms and the adjustable electronic levels of Pcs. Moreover, these conjugates present intensified light-harvesting capabilities resulting from the grafting of several chromophores on the same nanocarbon platform. In this Account, recent progress in the construction of covalent and supramolecular Pc-nanocarbon ensembles is summarized, with a particular emphasis on their photoinduced behavior. We believe that the high degree of control achieved in the preparation of Pc-carbon nanostructures, together with the increasing knowledge of the factors governing their photophysics, will allow for the design of next-generation light-fueled electroactive systems. Possible implementation of these Pc-nanocarbons in high performance devices is envisioned, finally turning into reality much of the expectations generated by these materials.

Oligothiophene/graphene supramolecular ensembles managing light induced processes: preparation, characterization, and femtosecond transient absorption studies leading to charge-separation.

Advances in organic synthetic chemistry combined with the exceptional electronic properties of carbon allotropes, particularly graphene, is the basis used to design and fabricate novel electron donor-acceptor ensembles with desired properties for technological applications. Thiophene-based materials, which are mainly thiophene-containing polymers, are known for their notable electronic properties. In this frame moving from polymer to oligomer forms, new fundamental information would help for a better understanding of their electrochemical and photophysical properties. Furthermore, a successful combination of their electronic properties with those of graphene is a challenging goal. In this study, two oligothiophene compounds, which consist of three and nine thiophene-rings and are abbreviated 3T and 9T, respectively, were synthesized and noncovalently associated with liquid phase exfoliated few-layered graphene sheets (abbreviated eG), thus forming donor-acceptor 3T/eG and 9T/eG nanoensembes. Markedly, intra-ensemble electronic interactions between the two components in the ground and excited states were evaluated with the aid of UV-Vis and photoluminescence spectroscopy. Furthermore, redox assays revealed the one-electron oxidation of 3T accompanied by one-electron reduction due to eG in 3T/eG, whereas there were two reversible one-electron oxidations of 9T accompanied by one-electron reduction of eG9T/eG. The electrochemical band gap for the 3T/eG and 9T/eG ensembles were calculated and verified, in which the negative free-energy change for the charge-separated state of 3T/eG and 9T/eGvia the singlet excited state of 3T and 9T, respectively, were thermodynamically favorable. Finally, the results of transient pump-probe spectroscopy studies at the femtosecond time scale were supportive of charge transfer type interactions in the 3T/eG and 9T/eG ensembles. The estimated rates for intra-ensemble charge separation were found to be 9.52 × 10(9) s(-1) and 2.2 × 10(11) s(-1), respectively, for 3T/eG and 9T/eG in THF, which reveal moderate to ultrafast photoinduced events in the oligothiophene/graphene supramolecular ensembles.

Imidazole‐Annulated Tetrathiafulvalenes Exhibiting pH‐Tuneable Intramolecular Charge Transfer and Redox Properties

In order to study the electronic interactions in donor-acceptor ensembles as a function of pH, an efficient synthetic route to three imidazole-annulated tetrathiafulvalene (TTF) derivatives 1-3 is reported. Their electronic absorption spectra, in view of photoinduced intramolecular charge transfer, and their electrochemical behavior were investigated, and pK(a) values for the two protonation processes on the acceptor unit were determined in organic solvents by photometric titration. The influence of the TTF moiety on these values is discussed.

Recent advances in the design, synthesis and study of covalent conjugated oligomer–C60 ensembles

This review presents an overview of the most recent results in the field of conjugated oligomer covalently attached to the C60 sphere focusing mainly on donor–conjugated oligomer–C60 triads and conjugated oligomer–multifullerene materials. Well-defined monodisperse oligomers as new materials that exhibit interesting optoelectronic properties have been the subject of intense study during the last decade. In this regard, a huge amount of work has been devoted to the development of new synthetic strategies toward the synthesis of conjugated oligomeric materials with precise length and constitution and to their chemical functionalization in order to incorporate them into more complex molecular and supramolecular architectures. An important area of research in the field of conjugated oligomers involves the design and synthesis of donor–acceptor ensembles by combination of monodisperse π-conjugated oligomeric systems with C60 fullerene. Such hybrid systems have shown excited-state interactions making them excellent candidates for fundamental photophysical studies. In addition, these materials have found applications in the field of photovoltaic devices. A review with 70 references.

A Pair of Coordination Donor–Acceptor Ensembles for the Detection of Tartrate in Aqueous Media

A chemosensing approach utilizing a pair of heterodimetallic chemodosimetric ensembles of different analyte selectivity for the specific detection of targeted analytes in aqueousmedia was developed. Two new trinuclear, heterodimetallic donor–acceptor complexes, {Ru(tBubpy)(CN)4[Cu(cyclen)]2}(ClO4)2 ([Ru-Cu]) and {Ru(tBubpy)(CN)4[Ni(cyclen)]2}(ClO4)2([Ru-Ni]), where tBubpy = 4,4′-di-tert-butyl-2,2′-bipyridine and cyclen = 1,4,7,10-tetraazacyclododecane, have been synthesized and characterized. Their formation constants from K2[Ru(tBubpy)(CN)4] and [Cu(cyclen)](ClO4)2 and [Ni(cyclen)](ClO4)2 were measured to be 5.54 × 106 ([Ru-Cu]) and 1.24 × 107 M–2 ([Ru-Ni]), respectively, at 298 K. Luminescent chemodosimetric responses of these two ensembles towards various anionic analytes in neat aqueous buffer at pH 7.0 were evaluated. While each of these two ensembles showed relatively low analyte specificity, they can be used together to manifest a chemosensing XOR logic output for the specific detection of tartrate. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Synthesis and Electronic Properties of a Conjugated TTFAQ Trimer and Donor-Acceptor Ensembles of TTFAQ and Anthraquinone

We report the first synthesis of a conjugated TTFAQ trimer as well as two TTFAQ-anthraquinone based donor-acceptor ensembles using Sonogashira and Horner-Wadsworth-Emmons reactions. Electronic absorption and electrochemical redox properties of these compounds are described.

Photofunctional nanomaterials composed of multiporphyrins and carbon-based π-electron acceptors

Recent development of artificial photosynthetic systems using photofunctional nanomaterials composed of multiporphyrins and carbon-based π-electron acceptors such as fullerenes, carbon nanotubes and a new type of nanocarbon is reviewed by focusing on their photoinduced electron-transfer properties. Electron donor–acceptor ensembles composed of porphyrins and fullerenes are described, covering small electron donor–acceptor dyads to larger multi-component systems including porphyrin-based photofunctional nanometerials. Such photofunctional nanomaterials composed of multiporphyrins and carbon-based π-electron acceptors (in particular C60) have been utilized to construct efficient light energy conversion systems such as photovoltaic devices.

Development of bioinspired artificial photosynthetic systems

The specific objective of this review is to describe recent development of bioinspired artificial photosynthetic systems and their applications. First, multi-step electron-transfer systems composed of electron donor-acceptor ensembles are presented, mimicking functions of the photosynthetic reaction center. However, a significant amount of energy is lost during the multi-step electron-transfer processes. Then, as an alternative to conventional charge-separation functional molecular models based on multi-step long-range electron transfer within redox cascades, simple electron donor-acceptor dyads linked by covalent or non-covalent bonding have been developed to attain a long-lived and high-energy charge-separated state without significant loss of excitation energy. Such simple molecular dyads, capable of fast charge separation but extremely slow charge recombination, have significant advantages with regard to synthetic feasibility, providing a variety of applications including construction of organic solar cells and development of efficient photocatalytic systems for the solar energy conversion.

An Experimental and Computational Study on Intramolecular Charge Transfer: A Tetrathiafulvalene‐Fused Dipyridophenazine Molecule

To study the electronic interactions in donor-acceptor (D-A) ensembles, D and A fragments are coupled in a single molecule. Specifically, a tetrathiafulvalene (TTF)-fused dipyrido[3,2-a:2',3'-c]phenazine (dppz) compound having inherent redox centers has been synthesized and structurally characterized. Its electronic absorption, fluorescence emission, photoinduced intramolecular charge transfer, and electrochemical behavior have been investigated. The observed electronic properties are explained on the basis of density functional theory.

ExTTF as a Building Block for Fullerene Receptors. Unexpected Solvent-Dependent Positive Homotropic Cooperativity

The first exTTF-based receptor for molecular recognition of fullerene is described. Unexpectedly, the receptor shows completely different binding modes in chlorobenzene and CHCl3/CS2 mixtures. In the aromatic solvent, the receptor binds C60 in a noncooperative fashion (nH = 1) with a Kassoc = (2.98 +/- 0.12) x 103 M-1, whereas in CHCl3/CS2 mixtures, it shows a marked positive homotropic cooperative effect (nH = 2.7) toward binding of C60, with an apparent binding constant of (3.56 +/- 0.16) x 103 M-1. The unique solvent-switchable behavior of our receptor might find use in the controlled self-assembly of exTTF-C60 donor-acceptor ensembles.

Bioinspired Electron-Transfer Systems and Applications

Abstract Bioinspired electron-transfer systems including artificial photosynthesis and respiration are presented herein together with some of their applications. First, multi-step electron-transfer systems composed of electron donor–acceptor ensembles have been developed, mimicking functions of the photosynthetic reaction center. However, a significant amount of energy is lost during the multi-step electron-transfer processes. Then, as an alternative to conventional charge-separation functional molecular models based on multi-step long-range electron transfer within redox cascades, simple donor–acceptor dyads have been developed to attain a long-lived and high energy charge-separated state without significant loss of excitation energy, by fine control of the redox potentials and of the geometry of donor–acceptor dyads that have small reorganization energies of electron transfer. Such simple molecular dyads, capable of fast charge separation but extremely slow charge recombination, have significant advantages with regard to synthetic feasibility, providing a variety of applications including construction of organic solar cells and development of efficient photocatalytic systems for the solar energy conversion. An efficient four-electron reduction of dioxygen to water by one-electron reductants such as ferrocene derivatives as well as by an NADH analog has also been achieved as a respiration model by using a cofacial dicobalt porphyrin that can form the μ-peroxo Co(III)–O2–Co(III) complex. The catalytic mechanism of the four-electron reduction of dioxygen has been clarified based on the detailed kinetic study and the detection of the intermediate.

The charge percolation mechanism and simulation of Ziegler-Natta polymerizations. Part III. Oxidation states of transition metals

The oxidation state of the transition metal (Mt) active centre is the most disputable question in the polymerization of olefins by Ziegler-Natta (ZN) and metallocene complexes. In this paper the importance and the changes of the Mt active centers are presented and discussed on the basis of a charge percolation mechanism (CPM) of olefin polymerization. Mt atoms can exist in different oxidation states and can be easily transformed from one to another state during activation. In all cases, the Mt atoms are present in several oxidation states, i.e., Mt+(n-1), Mt+(n) to Mt+(n+1), producing an irregular charge distribution over the support surface. There is a tendency to equalize the oxidation states by a charge transfer from Mt+(n-1) (donor) to Mt+(n+1) (acceptor). This cannot occur since the different oxidation states are highly separated on the support. However, monomer molecules are adsorbed on the support producing clusters with stacked ?-bonds, making a ?-bond bridge between a donor and an acceptor. Once a bridge is formed (percolation moment), charge transfer occurs. The donor and acceptor equalize their oxidation states simultaneously with the polymerization of the monomer. The polymer chain is desorbed from the support, freeing the surface for subsequent monomer adsorption. The whole process is repeated with the oxidation-reduction of other donor-acceptor ensembles.

Stabilisation of Emissive Charge-Separated States in π-Stacked Donor–acceptor Ensembles

pi-Stacking of an electron donor and acceptor minimises reorganisation energy and stabilises charge-separated state.