Quadruple Switching of Pleated Foldamers of Tetrathiafulvalene–Bipyridinium Alternating Dynamic Covalent Polymers

Two dynamic covalent polymers P1 and P2 were prepared by alternately linking electron‐rich tetrathiafulvalene (TTF) and electron‐deficient bipyridinium (BIPY²⁺) through hydrazone bonds. In acetonitrile, the polymers were induced by intramolecular donor–acceptor interactions to form pleated foldamers, which unfolded upon oxidation of the TTF units to the radical cation TTF^.+. Reduction of the BIPY²⁺ units to BIPY^.+ led to the formation of another kind of pleated secondary structures, which are stabilized by intramolecular dimerization of the BIPY^.+ units. The diradical dicationic cyclophane cyclobis(paraquat‐p‐phenylene) (CBPQT^2(.+)) could further force the folded structures to unfold by including the BIPY^.+ units of the polymers. Upon oxidation of the BIPY^.+ units of the cyclophane and polymers to BIPY²⁺, the first folded state was regenerated. Switching or conversion between the four conformational states was confirmed by UV/Vis spectroscopic experiments.     

Quantum Mechanical and Experimental Validation that Cyclobis(paraquat‐p‐phenylene) Forms a 1:1 Inclusion Complex with Tetrathiafulvalene

The promiscuous encapsulation of π‐electron‐rich guests by the π‐electron‐deficient host, cyclobis(paraquat‐p‐phenylene) (CBPQT⁴⁺), involves the formation of 1:1 inclusion complexes. One of the most intensely investigated charge‐transfer (CT) bands, assumed to result from inclusion of a guest molecule inside the cavity of CBPQT⁴⁺, is an emerald‐green band associated with the complexation of tetrathiafulvalene (TTF) and its derivatives. This interpretation was called into question recently in this journal based on theoretical gas‐phase calculations that reinterpreted this CT band in terms of an intermolecular side‐on interaction of TTF with one of the bipyridinium (BIPY²⁺) units of CBPQT⁴⁺, rather than the encapsulation of TTF inside the cavity of CBPQT⁴⁺. We carried out DFT calculations, including solvation, that reveal conclusively that the CT band emerging upon mixing TTF with CBPQT⁴⁺ arises from the formation of a 1:1 inclusion complex. In support of this conclusion, we have performed additional experiments on a [2]rotaxane in which a TTF unit, located in the middle of its short dumbbell, is prevented sterically from interacting with either one of the two BIPY²⁺ units of a CBPQT⁴⁺ ring residing on a separate [2]rotaxane in a side‐on fashion. This [2]rotaxane has similar UV/Vis and ¹H NMR spectroscopic properties with those of 1:1 inclusion complexes of TTF and its derivatives with CBPQT⁴⁺. The [2]rotaxane exists as an equimolar mixture of cis‐ and trans‐isomers associated with the disubstituted TTF unit in its dumbbell component. Solid‐state structures were obtained for both isomers, validating the conclusion that the TTF unit, which gives rise to the CT band, resides inside CBPQT⁴⁺.     

Tetrathiafulvalene‐Based Mixed‐Valence Acceptor–Donor–Acceptor Triads: A Joint Theoretical and Experimental Approach

This work presents a joint theoretical and experimental characterisation of the structural and electronic properties of two tetrathiafulvalene (TTF)‐based acceptor–donor–acceptor triads (BQ–TTF–BQ and BTCNQ–TTF—BTCNQ; BQ is naphthoquinone and BTCNQ is benzotetracyano‐p‐quinodimethane) in their neutral and reduced states. The study is performed with the use of electrochemical, electron paramagnetic resonance (EPR), and UV/Vis/NIR spectroelectrochemical techniques guided by quantum‐chemical calculations. Emphasis is placed on the mixed‐valence properties of both triads in their radical anion states. The electrochemical and EPR results reveal that both BQ–TTF–BQ and BTCNQ–TTF–BTCNQ triads in their radical anion states behave as class‐II mixed‐valence compounds with significant electronic communication between the acceptor moieties. Density functional theory calculations (BLYP35/cc‐pVTZ), taking into account the solvent effects, predict charge‐localised species (BQ ^{. ?}–TTF–BQ and BTCNQ ^{. ?}–TTF–BTCNQ) as the most stable structures for the radical anion states of both triads. A stronger localisation is found both experimentally and theoretically for the BTCNQ–TTF–BTCNQ anion, in accordance with the more electron‐withdrawing character of the BTCNQ acceptor. CASSCF/CASPT2 calculations suggest that the low‐energy, broad absorption bands observed experimentally for the BQ–TTF–BQ and BTCNQ–TTF–BTCNQ radical anions are associated with the intervalence charge transfer (IV‐CT) electronic transition and two nearby donor‐to‐acceptor CT excitations. The study highlights the molecular efficiency of the electron‐donor TTF unit as a molecular wire connecting two acceptor redox centres.     

Self‐Assembly of Three‐Dimensional Supramolecular Polymers through Cooperative Tetrathiafulvalene Radical Cation Dimerization

The self‐assembly of a new type of three‐dimensional (3D) supramolecular polymers from tetrahedral monomers in both organic and aqueous media is described. We have designed and synthesized two tetraphenylmethane derivatives T1 and T2 , both of which bear four tetrathiafulvalene (TTF) units. When the TTF units were oxidized to the radical cation TTF^.+, their pre‐organized tetrahedral arrangement remarkably enhanced their intermolecular dimerization, leading to the formation of new 3D spherical supramolecular polymers. The structure of the supramolecular polymers has been inferred on the basis of UV/Vis absorption, electron paramagnetic resonance, cyclic voltammetry, and dynamic light scattering (DLS) analysis, as well as by comparing these properties with those of the self‐assembled structures of mono‐, di‐, and tritopic control compounds. DLS experiments revealed that the spherical supramolecular polymers had hydrodynamic diameters of 68 nm for T1 (75 μM ) in acetonitrile and 105 nm for T2 (75 μM ) in water/acetonitrile (1:1). The 3D spherical structures of the supramolecular polymers formed in different solvents were also supported by SEM and AFM experiments.     

Molecular Tethering or Aggregation: Is the Existence of Charge‐Transfer Bands Indicative of the Formation of Blue‐Box/Tetrathiafulvalene Inclusion Complexes?

The interaction between tetrathiafulvalene and tetracation cyclobis(paraquat‐p‐phenylene) fragments—the key elements of many rotaxane systems—was investigated theoretically by using ab‐initio second‐order perturbation methods. In addition to the inclusion complex observed in the solid state, a thermodynamically stable “exterior” complex was identified. Calculation of the UV/Vis spectra for the inclusion and the exterior complexes indicated that the charge‐transfer band that is often used to predict the formation of the inclusion complexes in solution is, in reality, due to the exterior mode of complexation. These results suggest that UV/Vis spectroscopy is not a reliable method for assigning the complexation modes in TTF:BB⁴⁺ rotaxanes and related systems.     

Design and Sensing Properties of a Self‐Assembled Supramolecular Oligomer

Supramolecular polymers are a class of macromolecules stabilized by weak non‐covalent interactions. These self‐assembled aggregates typically undergo stimuli‐induced reversible assembly and disassembly. They thus hold great promise as so‐called functional materials. In this work, we present the design, synthesis, and responsive behavior of a short supramolecular oligomeric system based on two hetero‐complementary subunits. These “monomers” consist of a tetrathiafulvalene‐functionalized calix[4]pyrrole (TTF‐C[4]P) and a glycol diester‐linked bis‐2,5,7‐trinitrodicyanomethylenefluorene‐4‐carboxylate (TNDCF), respectively. We show that when mixed in organic solvents, such as CHCl₃, CH₂ClCH₂Cl, and methylcyclohexane, supramolecular aggregation takes place to produce short oligomers stabilized by hydrogen bonding and donor–acceptor charge‐transfer (CT) interactions. The self‐associated materials were characterized by ¹H NMR and UV/Vis/NIR absorption spectroscopy, as well as by concentration‐ and temperature‐dependent absorption spectroscopy and dynamic light scattering (DLS) analyses of both the monomeric and oligomerized species. The self‐associated system produced from TTF‐C[4]P and TNDCF exhibits a concentration‐dependent aggregation behavior typical of supramolecular polymers. Further support for the proposed self‐assembly came from theoretical calculations. The fluorescence emitting properties of TNDCF are quenched under conditions that promote the formation of supramolecular aggregates containing TTF‐C[4]P and TNDCF. This quenching effect has been utilized as a probe for the detection of substrates in the form of anions (i.e., chloride) and nitroaromatic explosives (i.e., 1,3,5‐trinitrobenzene). Specifically, the addition of these substrates to mixtures of TTF‐C[4]P and TNDCF produced a fluorescence “turn‐on” response.     

Comparative Electrochemical and Photophysical Studies of Tetrathiafulvalene‐Annulated Porphyrins and Their ZnII Complexes: The Effect of Metalation and Structural Variation

A series of tetrathiafulvalene (TTF)‐annulated porphyrins, and their corresponding Zn^II complexes, have been synthesized. Detailed electrochemical, photophysical, and theoretical studies reveal the effects of intramolecular charge‐transfer transitions that originate from the TTF fragments to the macrocyclic core. The incremental synthetic addition of TTF moieties to the porphyrin core makes the species more susceptible to these charge‐transfer (CT) effects as evidenced by spectroscopic studies. On the other hand, regular positive shifts in the reduction signals are seen in the square‐wave voltammograms as the number of TTF subunits increases. Structural studies that involve the tetrakis‐substituted TTF–porphyrin (both free‐base and Zn^II complex) reveal only modest deviations from planarity. The effect of TTF substitution is thus ascribed to electronic overlap between annulated TTF subunits rather than steric effects. The directly linked thiafulvalene subunits function as both π acceptors as well as σ donors. Whereas σ donation accounts for the substituent‐dependent charge‐transfer transitions, it is the π‐acceptor nature of the appended tetrathiafulvalene groups that dominates the redox chemistry. Interactions between the subunits are also reflected in the square‐wave voltammograms. In the case of the free‐base derivatives that bear multiple TTF subunits, the neighboring TTF units, as well as the TTF ^{? +} generated through one‐electron oxidation, can interact with each other; this gives rise to multiple signals in the square‐wave voltammograms. On the other hand, after metalation, the electronic communication between the separate TTF moieties becomes restricted and they act as separate redox centers under conditions of oxidation. Thus only two signals, which correspond to TTF ^{. +} and TTF²⁺, are observed. The reduction potentials are also seen to shift towards more negative values after metalation, a finding that is considered to reflect an increased HOMO–LUMO gap. To probe the excited‐state dynamics and internal CT character, transient absorption spectral studies were performed. These analyses revealed that all the TTF–porphyrins of this study display relatively short excited‐state lifetimes, which range from 1 to 20 ps. This reflects a very fast decay to the ground state and is consistent with the proposed intramolecular charge‐transfer effects inferred from the ground‐state studies. Complementary DFT calculations provide a mechanistic rationale for the electron flow within the TTF–porphyrins and support the proposed intramolecular charge‐transfer interactions and π‐acceptor effects.     

A Tetrathiafulvalene (TTF)‐Conjugated Microporous Polymer Network

Conjugated microporous polymer networks have been prepared from the strong electron donor tetrathiafulvalene (TTF) and 1,3,5‐triethynylbenzene (TEB) by using the Sonogashira–Hagihara cross‐coupling reaction. Optimization of reaction conditions yields polymers with surface areas of up to 434 m² g^?1. The strong electron‐donating properties of the network can be proven by iodine exposure. Structural and electronic changes due to formation of the charge‐transfer salt from TTFs in the porous network and iodine within the pores are investigated.     

Tetrathiafulvalene‐Fused Porphyrins via Quinoxaline Linkers: Symmetric and Asymmetric Donor–Acceptor Systems

A tetrathiafulvalene (TTF) donor is annulated to porphyrins (P) via quinoxaline linkers to form novel symmetric P–TTF–P triads 1 a – c and asymmetric P–TTF dyads 2 a , b in good yields. These planar and extended π‐conjugated molecules absorb light over a wide region of the UV/Vis spectrum as a result of additional charge‐transfer excitations within the donor–acceptor assemblies. Quantum‐chemical calculations elucidate the nature of the electronically excited states. The compounds are electrochemically amphoteric and primarily exhibit low oxidation potentials. Cyclic voltammetric and spectroelectrochemical studies allow differentiation between the TTF and porphyrin sites with respect to the multiple redox processes occurring within these molecular assemblies. Transient absorption measurements give insight into the excited‐state events and deliver corresponding kinetic data. Femtosecond transient absorption spectra in benzonitrile may suggest the occurrence of fast charge separation from TTF to porphyrin in dyads 2 a , b but not in triads 1 a – c . Clear evidence for a photoinduced and relatively long lived charge‐separated state (385 ps lifetime) is obtained for a supramolecular coordination compound built from the ZnP–TTF dyad and a pyridine‐functionalized C₆₀ acceptor unit. This specific excited state results in a (ZnP–TTF)^?+ ??? (C₆₀py)^?? state. The binding constant of Zn^II ??? py is evaluated by constructing a Benesi–Hildebrand plot based on fluorescence data. This plot yields a binding constant K of 7.20×10⁴ M ^?1, which is remarkably high for bonding of pyridine to ZnP.     

Tetrathiafulvalene‐Based Macrocycles Formed by Radical Cation Dimerization: The Role of Intramolecular Hydrogen Bonding and Solvent

Compounds 1 a and 1 b were prepared by appending two tetrathiafulvalene (TTF) units to an aromatic amide segment that is driven by six or two intramolecular N? H???O hydrogen bonds to adopt a folded conformation. UV/Vis absorption experiments revealed that if the TTF units were oxidized to TTF^.+ radical cations, the two compounds could form a stable single molecular noncovalent macrocycle in less polar dichloromethane or dichloroethane or a bimolecular noncovalent macrocycle in a binary mixture of dichloromethane with a more polar solvent owing to remarkably enhanced dimerization of the TTF^.+ units. The stability of the (TTF^.+)₂ dimer was evaluated through UV/Vis absorption, electron paramagnetic resonance, and cyclic voltammetry experiments and also by comparing the results with those of control compound 2 . The results showed that introduction of the intramolecular hydrogen bonds played a crucial role in promoting the stability of the (TTF^.+)₂ dimer and thus the noncovalent macrocyclization of the two backbones in both uni‐ and bimolecular manners.     

Self‐Assembled Architectures with Segregated Donor and Acceptor Units of a Dyad Based on a Monopyrrolo‐Annulated TTF–PTM Radical

An electron donor–acceptor dyad based on a polychlorotriphenylmethyl (PTM) radical subunit linked to a tetrathiafulvalene (TTF) unit through a π‐conjugated N‐phenyl–pyrrole–vinylene bridge has been synthesized and characterized. The intramolecular electron transfer process and magnetic properties of the radical dyad have been evaluated by cyclic voltammetry, UV/Vis spectroscopy, vibrational spectroscopy, and ESR spectroscopy in solution and in the solid state. The self‐assembling abilities of the radical dyad and of its protonated non‐radical analogue have been investigated by X‐ray crystallographic analysis, which revealed that the radical dyad produced a supramolecular architecture with segregated donor and acceptor units in which the TTF subunits were arranged in 1D herringbone‐type stacks. Analysis of the X‐ray data at different temperatures suggests that the two inequivalent molecules that form the asymmetric unit of the crystal of the radical dyad evolve into an opposite degree of electronic delocalization as the temperature decreases.     

Recognition‐Mediated Hydrogel Swelling Controlled by Interaction with a Negative Thermoresponsive LCST Polymer

Most polymeric thermoresponsive hydrogels contract upon heating beyond the lower critical solution temperature (LCST) of the polymers used. Herein, we report a supramolecular hydrogel system that shows the opposite temperature dependence. When the non‐thermosesponsive hydrogel NaphtGel, containing dialkoxynaphthalene guest molecules, becomes complexed with the tetra cationic macrocyclic host CBPQT⁴⁺, swelling occurred as a result of host–guest complex formation leading to charge repulsion between the host units, as well as an osmotic contribution of chloride counter‐ions embedded in the network. The immersion of NaphtGel in a solution of poly(N‐isopropylacrylamide) with tetrathiafulvalene (TTF) end groups complexed with CBPQT⁴⁺ induced positive thermoresponsive behaviour. The LCST‐induced dethreading of the polymer‐based pseudorotaxane upon heating led to transfer of the CBPQT⁴⁺ host and a concomitant swelling of NaphtGel. Subsequent cooling led to reformation of the TTF‐based host–guest complexes in solution and contraction of the hydrogel.     

Building up 1‐D, 2‐D,and 3‐D Polyiodide Frameworks by Finely Tuning the Size of Aryls on Ar‐S‐TTF in the Charge‐Transfer (CT) Complexes of Ar‐S‐TTFs and Iodine

The arylthio‐substituted tetrathiafulvalenes (Ar‐S‐TTFs) are electron donors having three reversible states, neutral, cation radical, and dication. The charge‐transfer (CT) between Ar‐S‐TTFs ( TTF1 — TTF3 ) and iodine (I₂) is reported herein. TTF1 — TTF3 show the CT with I₂ in the CH₂Cl₂ solution, but they are not completely converted into cation radical state. In CT complexes of TTF1 — TTF3 with I₂, the charged states of Ar‐S‐TTFs are distinct from those in solution. TTF1 is at cation radical state, and TTF2 — TTF3 are oxidized to dication. The iodine components in complexes show various structures including 1‐D chain of V‐shaped (I₅)^–, and 2‐D and 3‐D iodine networks composed of I₂ and (I₃)^–.     

Long‐Lived Charge Separation in Novel Axial Donor–Porphyrin–Acceptor Triads Based on Tetrathiafulvalene,Aluminum(III) Porphyrin and Naphthalenediimide

Two self‐assembled supramolecular donor–acceptor triads consisting of Al^III porphyrin (AlPor) with axially bound naphthalenediimide (NDI) as an acceptor and tetrathiafulvalene (TTF) as a secondary donor are reported. In the triads, the NDI and TTF units are attached to Al^III on opposite faces of the porphyrin, through covalent and coordination bonds, respectively. Fluorescence studies show that the lowest excited singlet state of the porphyrin is quenched through electron transfer to NDI and hole transfer to TTF. In dichloromethane hole transfer to TTF dominates, whereas in benzonitrile (BN) electron transfer to NDI is the main quenching pathway. In the nematic phase of the liquid crystalline solvent 4‐(n‐pentyl)‐4′‐cyanobiphenyl (5CB), a spin‐polarized transient EPR spectrum that is readily assigned to the weakly coupled radical pair TTF^.+NDI^.? is obtained. The initial polarization pattern indicates that the charge separation occurs through the singlet channel and that singlet–triplet mixing occurs in the primary radical pair. At later time the polarization pattern inverts as a result of depopulation of the states with singlet character by recombination to the ground state. The singlet lifetime of TTF^.+NDI^.? is estimated to be 200–300 ns, whereas the triplet lifetime in the approximately 350 mT magnetic field of the X‐band EPR spectrometer is about 10 μs. In contrast, in dichloromethane and BN the lifetime of the charge separation is <10 ns.     

Metal‐Ion‐Promoted Electron Transfer between Tetrathiafulvalene and Quinone Units Within a Calix[4]arene Framework and Tuning through Coordination of the Neighboring Crown Ether with a Sodium Cation

A new calix[4]arene 1 with tetrathiafulvalene (TTF), quinone, and crown ether units in the lower rim was designed and synthesized with the aim to investigate the possibility to tune the metal‐ion promoted electron transfer by coordination of the crown ether unit with additional metal ions. Both absorption and electron spin resonance (ESR) spectroscopic studies clearly indicate that electron transfer occurs efficiently from TTF to the quinone units in the presence of Sc³⁺/Pb²⁺/Zn²⁺. Moreover, the intramolecular electron transfer within 1 induced by Zn²⁺ can be switched off by addition of Na⁺ and further turned on by addition of either Sc³⁺ or Pb²⁺.     

Synthesis of Poly(benzothiadiazole‐co‐dithienobenzodithiophenes) and Effect of Thiophene Insertion for High‐Performance Polymer Solar Cells

We describe herein the synthesis of novel donor–acceptor conjugated polymers with dithienobenzodithiophenes (DTBDT) as the electron donor and 2,1,3‐benzothiadiazole as the electron acceptor for high‐performance organic photovoltaics (OPVs). We studied the effects of strategically inserting thiophene into the DTBDT as a substituent on the skeletal structure on the opto‐electronic performances of fabricated devices. From UV/Vis absorption, electrochemical, and field‐effect transistor analyses, we found that the thiophene‐containing DTBDT derivative can substantially increase the orbital overlap area between adjacent conjugated chains and thus dramatically enhance charge‐carrier mobility up to 0.55 cm² V^?1 s^?1. The outstanding charge‐transport characteristics of this polymer allowed the realization of high‐performance organic solar cells with a power conversion efficiency (PCE) of 5.1 %. Detailed studies on the morphological factors that enable the maximum PCE of the polymer solar cells are discussed along with a hole/electron mobility analysis based on the space‐charge‐limited current model.     

Bipolar poly(p‐phenylene vinylene)s bearing electron‐donating triphenylamine or carbazole and electron‐accepting quinoxaline moieties

Two new poly(phenylene vinylene)s (PPVs) carrying electron‐donating triphenylamine or carbazole and electron‐deficient quinoxaline units were synthesized and characterized. Their properties were compared with those of PPV containing only quinoxaline unit. The two polymers showed PL maximum at 501–510 in solution and 533–540 in thin film. Because of the presence of electron donor and acceptor units they displayed strong intramolecular charge transfer (ICT) effects; hence, low‐photoluminescence quantum yields. The polymers showed reversible electrochemical reduction with electron affinity of 2.75 eV and irreversible oxidation with ionization potential of 5.10–5.24 eV. Single‐layer LED of configuration ITO/PEDOT/polymer/Al showed low turn‐on voltage at 5 V, but limited brightness of 50–60 cdm^?2. The electroluminescence maximum was voltage‐tunable varying from 500 to 542 nm. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2367–2378, 2008     

Selective Co‐Encapsulation Inside an M6L4 Cage

There is broad interest in molecular encapsulation as such systems can be utilized to stabilize guests, facilitate reactions inside a cavity, or give rise to energy‐transfer processes in a confined space. Detailed understanding of encapsulation events is required to facilitate functional molecular encapsulation. In this contribution, it is demonstrated that Ir and Rh‐Cp‐type metal complexes can be encapsulated inside a self‐assembled M₆L₄ metallocage only in the presence of an aromatic compound as a second guest. The individual guests are not encapsulated, suggesting that only the pair of guests can fill the void of the cage. Hence, selective co‐encapsulation is observed. This principle is demonstrated by co‐encapsulation of a variety of combinations of metal complexes and aromatic guests, leading to several ternary complexes. These experiments demonstrate that the efficiency of formation of the ternary complexes depends on the individual components. Moreover, selective exchange of the components is possible, leading to formation of the most favorable complex. Besides the obvious size effect, a charge‐transfer interaction may also contribute to this effect. Charge‐transfer bands are clearly observed by UV/Vis spectrophotometry. A change in the oxidation potential of the encapsulated electron donor also leads to a shift in the charge‐transfer energy bands. As expected, metal complexes with a higher oxidation potential give rise to a higher charge‐transfer energy and a larger hypsochromic shift in the UV/Vis spectrum. These subtle energy differences may potentially be used to control the binding and reactivity of the complexes bound in a confined space.     

Vectorial Electron Transfer in Donor–Photosensitizer–Acceptor Triads Based on Novel Bis‐tridentate Ruthenium Polypyridyl Complexes

The first examples of rodlike donor–photosensitizer–acceptor arrays based on bis‐2,6‐di(quinolin‐8‐yl)pyridine Ru^II complexes 1 a and 3 a for photoinduced electron transfer have been synthesized and investigated. The complexes are synthesized in a convergent manner and are isolated as linear, single isomers. Time‐resolved absorption spectroscopy reveals long‐lived, photoinduced charge‐separated states (τ_CSS ( 1 a )=140 ns, τ_CSS ( 3 a )=200 ns) formed by stepwise electron transfer. The overall yields of charge separation (≥50 % for complex 1 a and ≥95 % for complex 3 a ) are unprecedented for bis‐tridentate Ru^II polypyridyl complexes. This is attributed to the long‐lived excited state of the [Ru(dqp)₂]²⁺ complex combined with fast electron transfer from the donor moiety following the initial charge separation. The rodlike arrangement of donor and acceptor gives controlled, vectorial electron transfer, free from the complications of stereoisomeric diversity. Thus, such arrays provide an excellent system for the study of photoinduced electron transfer and, ultimately, the harvesting of solar energy.     

Charge‐transfer character of halogen bonding: Molecular structures and electronic spectroscopy of carbon tetrabromide and bromoform complexes with organic σ‐ and π‐donors

Carbon tetrabromide and bromoform are employed as prototypical electron acceptors to demonstrate the charge‐transfer nature of various intermolecular complexes with three different structural types of electron donors represented by (1) halide and pseudohalide anions, (2) aromatic (π‐bonding) hydrocarbons, and (3) aromatics with (n‐bonding) oxygen or nitrogen centers. UV–Vis spectroscopy identifies the electronic transition inherent to such [1:1] complexes; and their Mulliken correlation with the donor/acceptor strength verifies the relevant charge‐transfer character. X‐ray crystallography of CBr₄/HCBr₃ complexes with different types of donors establishes the principal structural features of halogen bonding. © 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:449–459, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20264