Review pubs.acs.org/CR
Tetrathiafulvalene- (TTF‑) Derived Oligopyrrolic Macrocycles Atanu Jana,†,‡ Masatoshi Ishida,§ Jung Su Park,∥ Steffen Baḧ ring,⊥ Jan O. Jeppesen,*,⊥ and Jonathan L. Sessler*,‡,# †
Department of Chemistry, University of Sheffield, Sheffield S10 2TN, United Kingdom Department of Chemistry and Biochemistry, Graduate School of Engineering and Center for Molecular Systems, Kyushu University, Fukuoka 819-0395, Japan ∥ Department of Chemistry, Sookmyung Womens’s University, Seoul 140-742, South Korea ⊥ Department of Physics, Chemistry, and Pharmacy, University of Southern Denmark, Campusvej 55, 5230, Odense M, Denmark # Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712-1224, United States ‡ Institute for Supramolecular Chemistry and Catalysis, Shanghai University, Shanghai, 200444, China §
ABSTRACT: After the epochal discovery of the “organic metal”, namely, tetrathiafulvalene (TTF)−7,7,8,8-tetracyano-p-quinodimethane (TCNQ) dyad in 1973, scientists have made efforts to derivatize TTF for constructing various supramolecular architectures to control the charge-transfer processes by adjusting the donor−acceptor strength of the dyads for numerous applications. The interesting inherent electronic donor properties of TTFs control the overall electrochemical properties of the supramolecular structures, leading to the construction of highly efficient optoelectronic materials, photovoltaic solar cells, organic field-effect transistors, and optical sensors. Modified TTF structures thus constitute promising candidates for the development of so-called “functional materials” that could see use in modern technological applications. The versatility of the TTF unit and the pioneering synthetic strategies that have been developed over the past few decades provide opportunities to tune the architecture and function for specific purposes. This review covers the “state of the art” associated with TTF-annulated oligopyrrolic macrocyclic compounds. Points of emphasis include synthesis, properties, and potential applications. 2.2.1. “Pacman”-Type Schiff-Base TTF-Calix[4]pyrrole 2.2.2. TTF-Annulated Calix[2]pyrrole[2]thiophene 2.3. TTF-Annulated Expanded Calix[n]pyrroles 2.4. TTF-Annulated Porphyrins 2.4.1. TTF-Porphyrins with Different Numbers of Fused TTF Units 2.4.2. Quinoxaline-Fused TTF-Porphyrins 2.4.3. TTF-Annulated Expanded Porphyrins 2.5. ExTTF Porphyrins 2.6. Other Porphyrin−TTF-Based Donor−Acceptor Systems 2.6.1. Donor−Acceptor Systems Based on Porphyrins with Peripheral TTF Substituents Connected through Flexible Organic Spacers 2.6.2. Donor−Acceptor Systems Consisting of Porphyrins with Axially Coordinated TTF Derivatives
CONTENTS 1. Introduction 2. TTF-Annulated Oligopyrrolic Macrocyclic Compounds 2.1. TTF-Annulated Calix[4]pyrroles 2.1.1. TTF-Annulated Calix[4]pyrroles for Anion Sensing 2.1.2. TTF-Annulated Calix[4]pyrroles for Sensing Neutral Aromatic Electron-Deficient Guests 2.1.3. TTF-Annulated Calix[4]pyrroles for Sensing Spherical Guests 2.1.4. TTF-Annulated Calix[4]pyrroles and Surface Studies 2.1.5. TTF-Annulated Calix[4]pyrroles for Chemoresponsive Supramolecular Copolymerization with Heterocomplementary Calix[4]pyrroles 2.1.6. TTF-Annulated Calix[4]pyrroles for IonMediated Reversible Electron-Transfer (ET) Process in Supramolecular Ensembles Formed with Electron-Deficient Guests 2.2. TTF-Annulated Hybrid Calix[n]pyrrole Systems © XXXX American Chemical Society
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Chemical Reviews 2.6.3. Supramolecular Nonbonded Donor−Acceptor Ensembles of Porphyrin and TTFs 2.7. TTF-Functionalized Porphyrazines 2.7.1. TTF-Annulated Unsymmetrical and Symmetrical Porphyrazines 2.7.2. Porphyrazines Peripherally Modified with TTFs through Saturated Spacers 2.7.3. Pyrazinoporphyrazine with Peripheral TTF Units Tethered by Flexible Spacers 2.8. TTF-Functionalized Phthalocyanines 2.8.1. TTF-Annulated Symmetrical Phthalocyanines 2.8.2. TTF-Crown-Ether-Functionalized Phthalocyanines 2.8.3. TTF-Annulated Unsymmetrical Phthalocyanines 2.8.4. Phthalocyanines Containing Peripheral TTF Units Connected through Saturated Spacers 2.8.5. Norphthalocyanines Attached to TTFs through Saturated Spacers 2.9. TTF-Annulated Subphthalocyanines 3. Conclusions and Future Prospects Author Information Corresponding Authors Notes Biographies Acknowledgments Abbreviations References
Review
on TTF that are attractive in the quest to make electronic devices,2−11 organic conductors,12 light-harvesting antennas,13−15 optically active materials,16,17 and pigments.18 To frame the seminal insights provided by the iconic R. S. Mulliken and his charge-transfer theory and R. A. Marcus and his electron-transfer mechanism in the context of TTF chemistry, a brief theoretical background is provided here. In an effort to create efficient donor−acceptor systems (e.g., a TTF−TCNQ complex), the choice of the electron-donating and -accepting molecular units in the ensembles is critical.19 Relative to the acceptor (A), an electron-donor molecule (D) has, in general, a lower ionization energy (Ip), thereby being oxidized more readily. In contrast, an electron-accepting molecule (A) has a larger electron affinity (Ea) and can be reduced at relatively low potentials. Electronic coupling between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor molecules typically results in a degree of partial charge transfer ρ
AP AR AR AU AV AV AV AY BA
BB
diffuse
ET
diffuse
D + A HoooooI [D···A] HooI [D ρ +···Aρ −] HoooooI D+ + A−
BE BH BI BJ BJ BJ BJ BK BK BL
(1)
Mulliken conceived that a diffusional encounter between an electron donor, D, and an electron acceptor, A, would form a reversible encounter complex (D···A) and worked to correlate the extent of the interspecies interaction with the observed optical transition(s).20 In the case where the value of ρ is close to 1, a radical ion pair (D•+ and A•−) can be obtained through an electron-transfer (ET) event. ET is a mechanistic description of the thermodynamic concept of a redox process, wherein the oxidation states of both reaction partners change. The accompanying electrontransfer state, [D•+ A•−] is often produced under the conditions of reversible equilibrium. Therefore, electron back-transfer generally occurs as well. By far, the most accessible contribution to the development of ET theory came from Marcus, whose seminal contributions from 1956 onward were recognized by the Nobel Prize in Chemistry in 1992.21,22 Marcus’ electron-transfer formulation provided a major framework for understanding electron-transfer dynamics. The charge transfer can be either thermally or optically induced. It can be described by transitions between, or motions on, free energy surfaces (FESs).23 The free energy of the whole electron donor−acceptor system, including the solvent environment, is given as a function of the reaction coordinate (Figure 2). Following the Franck−Condon principle, electron motion is much faster than nuclear motion. As a consequence, optically induced charge transfer occurs from the minimum of the reactant
1. INTRODUCTION Tetrathiafulvalene (TTF) is an effective π-electron-donating molecule that displays unique electrochemical behavior. These key attributes have resulted in its extensive use in the construction of organic conductors. A characteristic feature of TTF, mirrored in a number of its derivatives, is that, upon stepwise oxidation, it gives rise to two sequentially oxidized species, namely, the TTF radical cation (TTF•+) and the TTF dication (TTF2+). The oxidation is thermodynamically reversible (cf. Figure 1) within the accessible electrode potential range (viz.,
Figure 1. Reversible redox processes involving the neutral (TTF), radical cation (TTF•+), and dication (TTF2+) species, respectively. These redox changes endow TTF with unique photophysical and electrochemical properties.
E1/21 = +0.34 V and E1/22 = +0.73 V vs Ag/AgCl in acetonitrile).1 TTF is also thermodynamically quite stable in a variety of different chemical environments (except in the presence of strong acids and oxidants). Thus, it is widely used as a building block in the formation of supramolecular structures and organic charge-transfer (CT) complexes. Functionalization of the TTF core is often easily achieved and allows for synthetic integration of the TTF unit into various D−A or D−π−A systems (where D stands for donor and A stands for acceptor). This has allowed the creation of both inter- and intramolecular CT constructs based
Figure 2. Diabatic (dashed line) and adiabatic (solid line) free energy surfaces of an electron donor−acceptor system. The optically induced (green) and thermally induced (blue) charge transfers are shown. B
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state generated by an increase in the energy along the vertical direction to the product FESs, whereas thermally induced charge transfer can occur only in the transition-state (TS) region. In the case of thermally induced charge transfer, the system has to move along the reaction coordinate to the TS region, resulting in an activation barrier. The barrier height can be estimated for optically induced charge transfer using geometric considerations. The free energy of activation, ΔG‡ET, can be calculated from the point where the two parabolas intersect ‡ ΔG ET =
0 (ΔG ET + λtotal)2 4λtotal
(2)
The free energy difference between the minima of the initial and final states is represented by ΔG0ET, and λtotal is the total reorganization energy, which is composed of the inner-sphere reorganization energies (λv) and outer-sphere (λo) reorganization energies. Therefore, λv and λo represent the intramolecular and solvent coordinate changes, respectively, in the optically induced charge-transfer process, and the sum ΔG0ET + λtotal is the total energy needed to induce optically driven charge transfer. Whereas λv cannot be calculated in a simple manner, λo can commonly be estimated according to the classic Marcus expression, where the donors and acceptors are assumed to be imbedded in a dielectric continuum λ0 =
e2 ⎛ 1 1 ⎞⎛ 1 1 1⎞ ⎜ − ⎟⎜ + − ⎟ 2 4πε0 ⎝ n D ⎠⎝ 2R+ 2R − d⎠
(3) Figure 3. (a) Schematic diagrams showing different regions described by the Marcus theory of electron transfer as applied to a D−A system. (b) Driving force dependence of log kET on different λ values.
and the electron-donor and electron-acceptor redox centers R± are assumed to be spherical. The distance between these redox centers is represented by d. The solvent continuum is described by its refractive index n, permittivity D, and dielectric constant ε0. In the adiabatic case, the free energy of activation, ΔG‡ET, is reduced in comparison to the diabatic limit. This is a result of the FES splitting in the TS region. The rate constant of charge transfer, k, is governed by the ‡ barrier height given by ΔGET . The probability P can be rationalized by the time the system needs to cross the TS region versus the time the system needs to change from the reactant to the product state. In the diabatic regime, the change of states is slow as the result of motion required through the TS region. Thus, the probability for the transition from the reactant to the product state is low. As a consequence, the overall charge transfer is limited by P, which, in turn, is proportional to the square of the electronic coupling element V2. In the diabatic limit, the rate constant of charge transfer, kna (na = nonadiabatic), can be described by the Arrhenius-type equation k na = 4π 2ℏc 2
Barrierless (i.e., optimal) electron transfer can occur if λtotal = ΔG0ET. In the normal region associated with Marcus theory, an increase in the free energy driving force, ΔG0ET, results in a decrease in the activation energy, ΔG‡ET. A further increase in ΔG0ET (≈ λtotal) yields the maximum rate of electron transfer. Eventually, an increase in the driving force leads to an increase in the activation barrier, ΔG‡ET, and the rate of electron transfer decreases. This portion of the FES is generally referred to as the Marcus inverted region (Figure 3b). A number of electron-donor−acceptor systems containing porphyrin and phthalocyanine analogues have been prepared in an effort to control electron-transfer processes. These frameworks are attractive because of their inherent highly delocalized electronic structures and the relatively rigid π-scaffolds they provide. For instance, porphyrins typically display relatively small reorganization energies and undergo minimal structural and solvation changes when used as components in ET couples.24−27 The electronic features of TTF and its derivatives also make them attractive for ET studies. Synthetic modifications can be used to modulate their intrinsic donor−acceptor characteristics, making TTF-based systems attractive as strong electron donors in both static and dynamic supramolecular motifs. Pyrrolo-TTF systems have had a particularly important role to play in the latter context. In recent years, considerable effort has been devoted to the construction of various supramolecular architectures displaying controlled molecular movement triggered by external stimuli such as specific acids, anions, and a variety of guest molecules. For the most part, the designs have been based on the use of receptors with strong affinity toward guest molecules and have
1 4π ℏcλtotalkBT
0 2⎤ ⎡ ℏc(λ total + ΔG ) ⎥ V 2 exp⎢ − 4λkBT ⎣ ⎦
(4)
where kB and ℏ are the Boltzmann and Plank constants, respectively; c is the speed of light in a vacuum; T is the temperature; and ΔG0 is the free energy difference between the reactant and product states. In this case, the logarithm of the ET rate constant (log kna) is related parabolically to the ET driving force (negative ET free energy change) between the electron donor and acceptor and the ET reorganization energy, λtotal, that is, the energy required to structurally reorganize the donor, acceptor, and their solvation spheres upon ET (Figure 3a). C
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Chart 1. Representative TTF-Annulated Oligopyrrolic Macrocycles
relied on a variety of noncovalent interactions (e.g., hydrogenbonding, electrostatics, and π−π D−A-based effects). TTF has proved to be a near-ideal molecular component in such systems. It has been exploited in the design of many different supramolecular architectures, including molecular machines,28−32 molecular flasks,33,34 molecular tweezers,35−37 and molecular switches.38−46 Some of the resulting constructs have potential applications as sensors for organic neutral guests and hazardous nitroaromatic compounds (vide infra). Generally, the observed sensing behavior arises from the formation of supramolecular ensembles by specifically designed guests with TTF-annulated hosts, which can be detected spectroscopically or electrochemically at the micro- to nanomolar levels. These hybrid systems are particularly important because they contain both a redox-active site and a chromophoric unit that jointly or separately provide for an efficient spectroelectrochemical or photophysical response to the targeted analyte. In many instances, TTF functionalization of a receptor serves to enhance the overall electron-donor strength of the resulting supramolecular construct compared to non-TTFbased systems. This modification often provides for enhanced hydrogen-bonding or π−π interactions with anions and electrondeficient guests. Moreover, as detailed further below, fusion of a TTF subunit to a calix[4]pyrrole core can give rise to molecular containers with concave openings suitable for encapsulating spherical guest molecules. In addition, TTF annulation to a
highly conjugated macrocyclic core (e.g., porphyrin, porphyrazine, and phthalocyanine) allows TTF chemistry to be combined with porphyrin chemistry to produce hybrid systems with many interesting photophysical and electrochemical properties. Therefore, these systems are a primary point of focus in this review. TTF-derived compounds have been widely described in book chapters,12,47 accounts,48 feature articles,27,49,50 and review articles.1,51−72 A few of the recent review articles60,73 discuss TTF oligomers and fused TTF derivatives. Some of the reviews74−79 emphasize the synthesis and properties of various TTF derivatives with different functionalities. There are also a couple of review articles on TTF-based cyclophanes and cagemotif systems,80 as well as those detailing D−A assemblies and their potential use in the areas of molecular materials and devices.81−85 Other reviews focus on the charge-transfer and energy-transfer properties of TTF systems and their potential in the areas of organic electronic materials86−89 and magnetic materials.90 However, to the best of our knowledge, a detailed review focused on TTF-annulated oligopyrrolic macrocyclic systems has yet to appear in the literature. Given the utility of these systems in the areas of ET study, molecular recognition, supramolecular materials chemistry, and sensor development, we believe that a comprehensive review is warranted. Therefore, our vision is to highlight the most recent developments involving various TTF-annulated oligopyrrolic macrocyclic compounds, D
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Scheme 1. Synthetic Routes Leading to Various TTF-Annulated Pyrroles
including TTF-calix[n]pyrroles (where n = 4, 5, and 6), TTFporphyrins, so-called exTTF-porphyrins, TTF-porphyrazines, TTF-phthalocyanines, and TTF-subphthalocyanines, as well as several closely related structures (Chart 1). In particular, we have made efforts to discuss in detail the supramolecular recognition and charge-transfer properties of nonaromatic TTF-pyrrole systems, with an emphasis being placed on the detection and sensing behavior of these
macrocycles as reported over the past two decades. We also summarize work focused on TTF-fused porphyrins, as well as several extended TTF analogues that contain formal porphyrin bridges (termed exTTF porphyrins). All of these oligopyrrolic macrocyclic compounds containing TTF(s) are potential electron donors capable of complexing a wide range of electron-poor guests. Their sensing behavior mainly reflects their ability to form supramolecular D−A-type E
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Chart 2. Chemical Structures of Various TTF-Annulated Calix[4]pyrrole Receptors and the Reference Compound C[4]P
The resulting TTF-annulated pyrrole derivative represents a useful new building block that can bring both rigidity and electron-donating character to various types of oligopyrrolic macrocycles (e.g., porphyrins, calixpyrroles). When it is incorporated into a calix[n]pyrrole (n = 4, 5, and 6) framework, higher binding affinities toward anions as the result of NH− anion hydrogen-bonding interactions are generally observed. The peripheral TTF unit can also be utilized as secondary interaction sites by providing a set of electron-rich π-surfaces. From the viewpoint of the pyrrolic TTF building block and systems created from it, modification of the terminal side of the TTF unit (i.e., that farthest from the pyrrole NH proton) can allow for further fine-tuning of the electronic structure. Common functionalized TTF pyrrole derivatives are provided by benzo annulation and thioalkyl substitution. These two substitution patterns have received particular attention because they provide a reasonable balance between solubility in organic solvents and chemical stability of the derivatives. However, other substituted analogues are known. A key step in making a TTF-annulated oligopyrrolic macrocyclic system is the synthesis of a TTF-fused pyrrole precursor specifically designed for the particular macrocyclic target in mind. Approaches used to prepare a few well-studied TTF-annulated pyrroles (e.g., PnS-TTF-pyrrole,91 MeS-TTFpyrrole,91,92 PrS-TTF-pyrrole,93,94 BTTF-pyrrole,95 TTTFpyrrole,95 TTF-dipyrrole91,92,94) are outlined in Scheme 1. Typically, the synthesis of TTF-pyrroles involves a CC bond-forming reaction carried out in triethylphosphite, P(OEt)3, involving the coupling between an appropriately chosen 1,3dithiole-2-thione and N-tosyl-(1,3)-dithiolo[4,5-c]pyrrol-2-one (OX) derivatives. This is an approach that has proved useful in obtaining unsymmetrical cross-coupling products (e.g., PrSTTF-pyrrole, BTTF-pyrrole, and TTTF-pyrrole). The requisite heterocoupling often proceeds well because the corresponding symmetrical products require a homocoupling of either the 1,3dithiole-2-thione or OX components. Although these latter homocouplings can be exploited to obtain species, such as TTFbridged bis-pyrroles (TTF-dipyrrole), to date, the products in question have not been exploited extensively in the preparation of more elaborated products. This stands in contrast to what is true for the pyrrolic heterocoupling products. Generally, the pyrrole nitrogen is tosylated. Detosylation using sodium
assemblies. As a general rule, the efficiency whereby such D−A supramolecular ensembles are formed depends strongly on the electronic nature of the components, the three-dimensional arrangement of the planar TTF units within the macrocyclic receptor and guest, and the environment surrounding the selfassembled complex. In the first part of this review, we discuss the design and synthesis of TTF-annulated pyrrolic precursors before turning to calix[4]pyrroles containing TTF subunits, as well as their higher analogues. TTF-calix[n]pyrroles are structurally flexible systems wherein a number of TTF units are fused to the periphery of the core macrocycle. As detailed further below, they are often capable of sensing anions, detecting nitroaromatic explosives, and recognizing a variety of guest molecules. The electrochemical response and photophysical behavior of these systems are also discussed thoroughly. In some cases, the reversible switching of an electron-transfer process within a supramolecular ensemble as a function of guest molecules is considered. The effects of metalation and various guest inputs and their effects on the structure and properties of a few highly flexible hybrid calixpyrrole systems are also highlighted. In the second part, we focus on TTF-fused porphyrins, as well as higher-order TTFderived porphyrins and their artificial photosynthetic applications. We also discuss the use of specific guest inputs to control charge-transfer (CT) phenomena in the case of TTF-fused porphyrins. The interesting ground-state and excited-state dynamics, as well as electrochemical behavior, that originate from the presence of a chromophoric porphyrin subunit and one or more redox-active TTF groups are also detailed. The electrochemical and optical properties of various TTF-fused porphyrazines, phthalocyanines, and subphthalocyanines are discussed thoroughly in the last part of this review. Every effort has been made to cover the literature through early 2016.
2. TTF-ANNULATED OLIGOPYRROLIC MACROCYCLIC COMPOUNDS 2.1. TTF-Annulated Calix[4]pyrroles
Annulation of a TTF unit onto a pyrrole scaffold through the β,β′-positions has a significant effect of the acidity of the pyrrolic NH proton because of the inductive effect of the sp2-hybridized S atoms and the intrinsic electron-rich nature of the TTF subunit. F
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Scheme 2. Synthesis of Various TTF-Calix[4]pyrroles
tetramerization of PrS-TTF-pyrrole in acetone; after purification, this product was isolated in 18% yield. The anion-recognition properties of receptor 1 were investigated by means of NMR spectroscopic titrations and electrochemical analyses. The Job’s plots obtained upon the titration of 1 with a number of anion salts exhibited maxima at a mole fraction of 0.5, as would be expected for the formation of 1:1 complexes. An electrochemical response toward different anions was seen for 1 and was considered to be enhanced by the presence of a relatively more acidic −NH proton present in the PnS-TTF-pyrrole subunit because of the presence of a TTF fragment that is electron-rich relative to a normal pyrrole (C[4]P; Chart 2). The binding constants corresponding to the interaction of 1 with halide anions F−, Cl−, and Br− (as the tetran-butylammonium, TBA+, salts) were determined from 1H NMR spectroscopic titration data obtained at 300 K; the results are summarized in Table 1. The underlying experiments involved
methoxide (NaOMe) proceeds well, with the result that several key TTF-pyrrole precursors are now routinely synthesized in the authors’ laboratories on gram scales. 2.1.1. TTF-Annulated Calix[4]pyrroles for Anion Sensing. In 2003, Becher and co-workers reported the first calix[4]pyrrole system fused with TTF, the mono-PnS-TTFcalix[4]pyrrole receptor 1 (Chart 2), which was produced as a possible electrochemical sensor for anions.96 Compound 1 was produced through a straightforward synthetic procedure that involved the BF3·OEt2-catalyzed macrocyclization of a bishydroxymethyltripyrrane with pentylthio-TTF-pyrrole (PnSTTF-pyrrole) in a 1:1 stoichiometric ratio, as outlined in Scheme 2A. Representative target-oriented synthetic routes used to prepare other TTF-annulated calix[4]pyrrole receptors are shown in Scheme 2B. For instance, the symmetrical TTF-fused calix[4]pyrrole 4 was synthesized by the acid-catalyzed cycloG
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result of the strong hydrogen-bonding interactions between the pyrrolic protons and the bound anionic guest (Figure 5).
Table 1. Binding Constant Values Corresponding to the Interaction of Receptor 1 with Various Anions anion −
F Cl− Br−
Kab (M−1) 2.1 × 106 1.2 × 105 7.6 × 103
a
Underlying NMR spectroscopic titrations were carried out using tetra-n-butylammonium (TBA+) salts of the corresponding halides at 300 K. Binding constant values were obtained by the nonlinear curvefitting computer program EQNMR and is the average of two independent experiments using the chemical shifts of the three −NH protons as the probe signals. The estimated error was 4 was found (Table 3). Zhu et al. reported a TTF-calix[4]pyrrole 6 that displayed a colorimetric response toward polynitroaromatic explosive both in chloroform solution and when incorporated on a polymer microcantilever with an integrated deflection-sensing element.99 This work represents an approach to creating a practical device capable of stand-off detection of nitroaromatic explosive vapors. It was also expected that the microcantilever systems would display enhanced sensitivity and improved detection limits
Figure 8. (a) X-ray structure of the supramolecular complex (TNB)2⊂4. (b) UV−vis−NIR spectral changes seen for mixtures of 4 and TNB under various experimental conditions. (Reproduced with permission from ref 98. Copyright 2004 American Chemical Society.)
An intermolecular CT band at λmax = 677 nm was seen in the UV−vis−NIR absorption spectrum (Figure 8b) when 4 was treated with TNB. This band disappeared upon addition of Cl−. This was taken as evidence that an anion-induced conformational change leads to a guest-release process and a breakup of the initial CT complex. After removal of the anion by means of an aqueous wash, the initial sandwich structure was restored, along with the low-energy CT-visible absorption feature.
Scheme 3. Proposed Supramolecular Host−Guest Chemistry Involving Receptor 4 and TNB Guest Observed under Different Experimental Conditions
J
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Figure 9. Proposed origin of the positive homotropic allosteric effect observed in the case of 5 and TNP. (This figure was redrawn using data that were originally published in ref 95.)
Table 3. Microscopic Association Constants, and Cooperativity Parameters for the Receptors 4, 5, and 6 with Various Nitroaromatics (TNB, TNP, and TNT) compound
Ka (M−2)
4·2TNB 4·2TNP 4·2TNT 5·2TNB 5·2TNP 5·2TNT 6·2TNB 6·2TNP 6·2TNT
4.3 × 10 3.8 × 103 3.3 × 102 1.5 × 105 9.1 × 104 1.2 × 104 3.4 × 106 3.7 × 106 2.3 × 104 3
n
K1 (M−1)
K2 (M−1)
K2/K1
1.27 1.30 1.23 1.34 1.34 1.31 1.70 1.86 1.45
3.9 × 10 2.8 × 102 5.9 × 101 2.8 × 103 1.7 × 103 5.7 × 102 1.3 × 103 6.4 × 102 3.2 × 102
1.4 × 10 1.2 × 103 2.0 × 102 1.7 × 104 1.1 × 104 2.6 × 103 3.1 × 104 2.0 × 104 2.8 × 103
3.6 4.1 3.3 6.2 6.5 4.5 24 31 10
2
3
achieved. This represents an improvement in the detection limit of at least 30-fold compared to the colorimetric detection limit of 0.3 ppm for TNB measured in chloroform solution using receptor 6. This study provides an indication that stand-alone colorimetric receptors could potentially be translated into practical devices with improved sensitivities by incorporating them into cantilever sensor arrays. Self-Assembly Driven Sensing of TNB with a TTF-Calix[4]pyrrole Receptor. Nielsen and Stein reported the synthesis and binding properties of a TTF-calix[4]pyrrole receptor 7 appended with an electron-deficient 3,5-dinitrobenzoate guest moiety.100 Preliminary spectroscopic studies revealed that the receptor selfassociates into a dimer (7·7) at high concentration (Scheme 4). The self-association of the receptor leads to preorganization in its 1,3-alternate conformation. As a result, dimer 7·7 displays an approximately 2-fold higher binding affinity for nitroaromatic analytes (e.g., TNB) than the model receptor 4. This binding, in turn, leads to the formation of a supramolecular ensemble
a
Based on absorption spectroscopic titrations of the respective hosts (0.20 mM) in CHCl3 at 298 K. bEstimated error for calculated binding constants is ≤12%.
relative to what was previously been found in the solution state. In fact, a detection limit for TNB vapor of less than 10 ppb was
Scheme 4. Proposed Mechanism for the Self-Assembly of a TTF-Calix[4]pyrrole Derivative Driven by Complexation
K
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Review
TNB2⊂7·7 that is stabilized through CT and hydrogen-bonding interactions. UV−vis−NIR spectral titrations with various anions (as their TBA + cation salts) were carried out at two different concentrations of receptor 7.101 It was found that, at higher concentrations [(0.15−0.40) × 10−3 M], the proposed anionbinding events could be monitored spectroscopically in terms of the progressive disappearance of the CT band centered at 560 nm that occurs upon the incremental addition of coordinating anions, such as chloride. At lower concentrations of 7 [(6−7) × 10−6 M], the absorption band centered at 326 nm was used to determine the binding constants. The resulting values are given in Table 4. An inspection of the data in this table reveals that the
switch” consisting of the asymmetric TTF-calix[4]pyrrole derivative 8 that displayed responsive binding behavior in the case of TNB recognition.102 This receptor consists of three identical TTF-pyrrole units and a fourth TTF-pyrrole unit appended with a phenol moiety. It is worth noting that the asymmetric system 8 was prepared by simply condensing two different TTF-annulated pyrroles in accord with the normal onepot synthetic strategy used to produce analogue 4. The host− guest interactions between 8 and TNB were studied by means of absorption and 1H NMR spectroscopies. The underlying studies revealed that the molecular receptor could be switched between limiting locked and unlocked states by using base and acid as the inputs (Scheme 5). In the unlocked state, the receptor is able to accommodate two TNB guest molecules through strong hydrogen-bonding interactions with the NH protons, as well as interactions between the electron-donating TTF units and the electrondeficient TNB guest molecules. In contrast, TNB guests are not able to bind appreciably to the receptor in the locked state in which the phenolate anion strongly coordinate with the NH protons to construct a partial cone-like conformer. The addition of acid serves to protonate the phenolate anion and restores TNB binding. Coordination-Driven Switching of a Preorganized and Cooperative TTF-Calix[4]pyrrole Receptor. Nielsen and coworkers also reported a metal-ion-coordination-triggered switching system103 that consists of a preorganized asymmetric TTFcalix[4]pyrrole receptor, 9, bearing an appended pyridine moiety (Scheme 6). This system utilizes intramolecular hydrogen bonding between the pyrrole NH protons and the pyridine nitrogen atom to switch off TNB binding in the 1,3-alternate conformation and metal-based complexation of the pyridine moiety to turn it back on. Absorption and 1H NMR spectroscopic data analyses along with theoretical calculations revealed that the system has the ability to complex electron-deficient nitroaromatics, such as TNB, and that the binding events could be reversibly modulated by using Zn2+ as an external stimulus. In the free state (i.e., in the absence of Zn2+), the receptor system is locked into a relatively higher level of preorganization as a result of the intramolecular hydrogen-bonding interactions (self-complexation) between the pyrrole NH protons of the TTF-calix[4]pyrrole skeleton and the
Table 4. Binding Constants (Ka, M−1) Corresponding to the Interactions between the Receptor System 7 and the Parent System 4 with Different Anions as Determined by Absorption Spectroscopy and Isothermal Titration Calorimetry (ITC) in CH2Cl2 at 298 K anion −
Cl Br− CN− MeCO2−
7 (326 nm)d
7 (600 nm)d
4 (ITC)
2.55 × 10 5.55 × 104 4.65 × 105 5.35 × 106
8.5 × 10 3.5 × 104 2.4 × 105 nde
2.5 × 106 5.8 × 104 1.1 × 106 1.3 × 106
6
5
a
Estimated errors are
