Donor-acceptor Charge-transfer Complexes Research Articles

Permittivity Threshold and Thermodynamics of Integer Charge-Transfer Complexation for an Organic Donor–Acceptor Pair

Molecular charge doping involves the formation of donor-acceptor charge-transfer complexes (CTCs) through integer or partial electron transfer; understanding how local chemical environment impacts complexation is important for controlling the properties of organic materials. We present steady-state and temperature-dependent spectroscopic investigations of the p-dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) complexed with the electron donor and hole transport material N,N'-diphenyl-N,N'-di-p-tolylbenzene-1,4-diamine (MPDA). Equilibrium formation constants (KCT) were determined for donor-acceptor pairs dissolved in a series of solvents covering a range of values of permittivity. A threshold for highly favorable complex formation was observed to occur at ϵ ∼ 8-9, with large (>104) and small (<103) values of KCT obtained in solvents of higher and lower permittivity, respectively, but with chloroform (ϵ = 4.81) exhibiting an anomalously high formation constant. Temperature-dependent formation constants were determined in order to evaluate the thermodynamics of complex formation. In 1,2-dichloroethane (ϵ = 10.36) and chlorobenzene (ϵ = 5.62), complex formation is both enthalpically and entropically favorable, with higher enthalpic and entropic stabilization in the solvent with higher permittivity. Complexation in chloroform is exothermic and entropically disfavored, indicating that specific, inner-shell solvent-solute interactions stabilize the charge-separated complex and result in a net increase in local solution structure. Our results provide insight into how modification to the chemical environment may be utilized to support stable integer charge transfer for molecular doping applications and requiring only modest changes in local permittivity.

Mildly-doped polythiophene with triflates for molecular recognition

Organic semiconductors have enough molecular versatility to feature chemo-specific electrical sensitivity to large families of chemical substituents via different intermolecular bonding modes. This study demonstrates that one single conducting polymer can be tuned to either discriminate water-, ethanol- or acetone-vapors, on demand, by changing the nature of its dopant. Seven triflate salts differ from mild to strong p-dopant on poly(3-hexylthiophene) sensing micro-arrays. Each material shows a pattern of conductance modulation for the polymer which is reversible, reproducible, and distinctive of other gas exposures. Based on principal component analysis, an array doped with only two different triflates can be trained to reliably discriminate gases, which re-motivates using conducting polymers as a class of materials for integrated electronic noses. More importantly, this method points out the existence of tripartite donor-acceptor charge-transfer complexes responsible for chemospecific molecular sensing. By showing that molecular acceptors can have duality to p-dope semiconductors and to coordinate donor gases, such behavior can be used to understand the role of frontier orbital overlapping in organic semiconductors, the formation of charge-transfer complexes via Lewis acid-base adducts in molecular semiconductors.

Charge Transport in Zirconium-Based Metal–Organic Frameworks

ConspectusMetal-organic frameworks (MOFs) are a class of crystalline porous materials characterized by inorganic nodes and multitopic organic linkers. Because of their molecular-scale porosity and periodic intraframework chemical functionality, MOFs are attractive scaffolds for supporting and/or organizing catalysts, photocatalysts, chemical-sensing elements, small enzymes, and numerous other functional-property-imparting, nanometer-scale objects. Notably, these objects can be installed after the synthesis of the MOF, eliminating the need for chemical and thermal compatibility of the objects with the synthesis milieu. Thus, postsynthetically functionalized MOFs can present three-dimensional arrays of high-density, yet well-separated, active sites. Depending on the application and corresponding morphological requirements, MOF materials can be prepared in thin-film form, pelletized form, isolated single-crystal form, polycrystalline powder form, mixed-matrix membrane form, or other forms. For certain applications, most obviously catalytic hydrolysis and electro- or photocatalytic water splitting, but also many others, an additional requirement is water stability. MOFs featuring hexa-zirconium(IV)-oxy nodes satisfy this requirement. For applications involving electrocatalysis, charge storage, photoelectrochemical energy conversion, and chemiresistive sensing, a further requirement is electrical conductivity, as embodied in electron or hole transport. As most MOFs, under most conditions, are electrically insulating, imparting controllable charge-transport behavior is both a chemically intriguing and chemically compelling challenge.Herein, we describe three strategies to render zirconium-based metal-organic frameworks (MOFs) tunably electrically conductive and, therefore, capable of transporting charge on the few nanometers (i.e., several molecular units) to few micrometers (i.e., typical dimensions for MOF microcrystallites) scale. The first strategy centers on redox-hopping between periodically arranged, chemically equivalent sites, essentially repetitive electron (or hole) self-exchange. Zirconium nodes are electrically insulating, but they can function as grafting sites for (a) redox-active inorganic clusters or (b) molecular redox couples. Alternatively, charge hopping based on linker redox properties can be exploited. Marcus's theory of electron transfer has proven useful for understanding/predicting trends in redox-hopping based conductivity, most notably, in accounting for variations as great as 3000-fold depending on the direction of charge propagation through structurally anisotropic MOFs. In MOF environments, propagation of electronic charge via redox hopping is necessarily accompanied by movement of charge-compensating ions. Consequently, rates of redox hopping can depend on both the identity and concentration of ions permeating the MOF. In the context of electrocatalysis, an important goal is to transport electronic charge fast enough to match or exceed the inherent activity of MOF-based or MOF-immobilized catalysts.Bandlike electronic conductivity is the focus of an alternative strategy: one based on the introduction of molecular guests capable of forming donor-acceptor charge transfer complexes with the host framework. Theory again can be applied predictively to alter conductivity. A third strategy similarly emphasizes electronic conductivity, but it makes use of added bridges in the form of molecular oligomers or inorganic clusters that can then be linked to span the length of a MOF crystallite. For all strategies, retention of molecular-scale porosity is emphasized, as this property is key to many applications. Finally, while our focus is on Zr-MOFs, the described approaches clearly are extendable to other MOF compositions, as has already been demonstrated, in part, in studies by others.

Photophysical Studies on Covalently-linked Naphthalene and TEMPO Free Radical Systems: Observation of a Charge Transfer State in the Ground State.

A series of molecules containing a naphthalene chromophore and a stable free radical 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) covalently linked by a spacer group of different lengths have been synthesized. In n-hexane solution, their photophysical behavior was studied and compared with a system of freely moving naphthalene and the free radical TEMPO. The linked molecules showed strong quenching of the singlet and triplet states of the naphthalene moiety, compared to when naphthalene and TEMPO were not linked. The quenching efficiency decreased with increasing the length of the spacer group. In addition, new electronic absorption and emission bands, along with the usual bands of the individual moieties, were also seen. These news bands have been attributed to the formation of electron donor-acceptor charge-transfer complexes in the ground state, arising from the interaction between the two moieties in close proximity. The photophysical dynamics of the linked molecules has been rationalized by assuming the existence of two types of population of the linked molecules: folded and extended. The ground state complex formation is proposed to occur only in the folded conformation of the linked molecules. To our knowledge, this is possibly the first example of a ground state charge-transfer complex formation involving a TEMPO free radical and naphthalene.

Charge-Transfer States and Upper Limit of the Open-Circuit Voltage in Polymer:Fullerene Organic Solar Cells

The power conversion efficiency of polymer:fullerene bulk heterojunction solar cells depends on the generated photocurrent and photovoltage. Here we show, using the thermodynamic theory of detailed balance, that the photovoltage in particular is limited by the presence of polymer:fullerene material interaction, resulting in the formation of a weak donor-acceptor charge transfer complex (CTC). Excited CTCs, or charge transfer (CT) states, are visible in highly sensitive measurements of the absorption and photovoltaic action spectrum, or in photoluminescence and electroluminescence measurements. It is shown that photovoltaic and electroluminescent actions of the polymer: fullerene CTC are related by a reciprocity relation. This relation reproduces the measured open-circuit voltage (Voc) of the photovoltaic device under solar conditions. Also, the temperature and illumination intensity dependence of Voc is reproduced by the theory. Assuming perfect conditions for charge generation and recombination, a maximum obtainable Voc value in function of polymer:fullerene CTC properties is derived.

Homeotropic alignment through charge-transfer-induced columnar mesophase formation in an unsymmetrically substituted triphenylene derivative

An unsymmetrically substituted triphenylene, with two adjacent chloroethoxyethyl lateral flexible chains, was synthesized and characterized. Although this compound showed no mesomorphic behavior, it formed a donor–acceptor charge-transfer complex with 2,4,7-trinitrofluorenone (TNF). The resulting 1:1 complex has been investigated using UV–vis and IR spectroscopy, optical microscopy, thermal analysis, and X-ray diffraction. A columnar mesophase with hexagonal symmetry was found. More interestingly, this charge-transfer complex can be easily aligned on a glass surface in a homeotropic orientation, which is stable at room temperature (RT) and over a wide temperature range.

Charge-transfer complexes of conjugated polymers as intermediates in charge photogeneration for organic photovoltaics

Ultrafast visible-pump/IR-probe spectroscopy is applied to study the wavelength dependence of charge photogeneration in materials based on donor–acceptor charge-transfer complexes (CTCs) of the conjugated polymer MEH-PPV. In binary polymer–acceptor blends, photoexcitation in the absorption band of either CTC or polymer results in similar dynamics of the charge-associated transient absorption. Likewise, in polymer/CTC–acceptor/fullerene ternary blends, where charge separation occurs via a two-step pathway, the photophysics is also independent of excitation wavelength. These similarities in charge dynamics indicate that CTC excited states serve as an intermediate for charge photogeneration. The conclusions of the ultrafast study are supported by photocurrent spectroscopy.

Efficient two-step photogeneration of long-lived charges in ground-state charge-transfer complexes of conjugated polymer doped with fullerene

Polarization-sensitive time-resolved visible-infrared pump-probe experiments demonstrate that one can efficiently generate long-lived charges in donor-acceptor charge transfer complex (CTC) of conjugated polymer doped with fullerene, MEH-PPV/dinitroanthraquinone/C(60). In particular, a strong enhancement of the photoinduced charge generation is observed in the red part of the spectrum, i.e. inside the polymer band gap, which makes the current material attractive for photovoltaic applications. The spectroscopic results indicate that enhanced generation of charges is due to a consecutive photoinduced electron transfer from the polymer to the CTC-acceptor in the first step and then, in the second step, to the fullerene. The LUMO energy difference between the CTC-acceptor and fullerene appears to be a key parameter for efficient charge separation in these ternary systems. The results are also discussed in respect to the charge generation processes in widely used polymer-fullerene blends, where formation of weak CTCs has recently been discovered.

Raman spectroscopy of intermolecular charge transfer complex between a conjugated polymer and an organic acceptor molecule

Intermolecular donor-acceptor charge transfer complex (CTC) formed in the electronic ground state between poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and 2,4,7-trinitrofluorenone (TNF) has been investigated by Raman and optical absorption spectroscopies. Blending of MEH-PPV and TNF results in appearance of the CTC absorption band in the optical gap of the both components and in changes in the characteristic MEH-PPV Raman bands including shifts, change in bandwidth, and intensity. The experimental data are similar in films and solutions indicating the CTC formation in both. We associate the low-frequency shift of the strongest MEH-PPV Raman band at approximately 1580 cm(-1) reaching 5 cm(-1) with partial electron transfer from MEH-PPV to TNF amounting approximately 0.2e(-). We suggest that polymer conjugated segments can form the CTC of variable composition MEH-PPV:TNF=1:X, where X<or=0.5 is per MEH-PPV monomer unit. Our Raman data indicate that MEH-PPV conjugated segments involved in the CTC become more planar; however, their conjugation length seemingly does not change.

Hydrogen-Bond Interaction in Organic Conductors: Redox Activation, Molecular Recognition, Structural Regulation, and Proton Transfer in Donor−Acceptor Charge-Transfer Complexes of TTF-Imidazole

Hydrogen-bond interaction in donor-acceptor charge-transfer complexes of TTF-imidazole demonstrated the electronic effects in terms of control of component ratio and redox activation. These unprecedented effects of hydrogen bonds renewed the criteria giving "a high probability of being organic metals" and produced a number of highly conductive complexes with various acceptors having a wide range of electron-accepting ability. In p-chloranil complex, both molecules were linked by hydrogen bonds and formed a D-A-D triad, regulating the donor-acceptor composition to be 2:1. Theoretical calculations have revealed that the polarizability of hydrogen bonds controls the redox ability of the donor and p-benzoquinone-type acceptors and afforded different ionicity in complexes from those expected by the difference of redox potentials between donor and acceptors. In the p-chloranil complex, this electronic and structural regulation by hydrogen bond realized the first metallic donor-acceptor charge-transfer complex based on hydrogen bond functionalized TTF. Hydrogen bonds controlled also molecular arrangements in charge-transfer complexes, giving diverse and highly ordered assembled structures, D-A-D triad in the p-chloranil complex, one-dimensional zigzag chain in I(5) salt, alternating donor-acceptor chain in chloranilic acid complex, and D-A-D-A cyclic tetramer in nitranilic acid complex. Furthermore, TTF-imidazole acted as electron donor as well as proton acceptor in anilic acid complexes and realized the simultaneous charge- and proton-transfer complexes. These investigations demonstrated the new and intriguing potentials of the hydrogen bond in the development of organic conductors and multifunctional molecular materials.

An overview of the first half-century of molecular electronics.

The seminal ideas from which molecular electronics has developed were the theories of molecular conduction advanced in the late 1940s by Robert S. Mulliken and Albert Szent-Gyorgi. These were, respectively, the concept of donor-acceptor charge transfer complexes and the possibility that proteins might in fact not be insulators The next two decades saw a burgeoning of experimental and theoretical work on electron transfer systems, together with a lone effort by D.D. Eley on conduction in proteins. The call by Feynman in his famous 1959 lecture There's Plenty of Room at the Bottom for chemists, engineers and physicists to combine to build up structures from the molecular level was influential in turning attention to the possibility of engineering single molecules to function as elements in information-processing systems. This was made tangible by the proposal of Aviram and Ratner in 1974 to use a Mulliken-like electron donor-acceptor molecule as a molecular diode, generalizing molecular conduction into molecular electronics. In the early 1970s the remarkably visionary work of Forrest L. Carter of the U.S. Naval Research Laboratories began to appear: designs for molecular wires, switches, complex molecular logic elements, and a host of related ideas were advanced. Shortly after that, conferences on molecular electronics began to be held, and the interdisciplinary programs that Feynman envisaged. There was a surge in both experimental and theoretical work in molecular electronics, and the establishment of many research centres. The past five years or so have seen extraordinarily rapid progress in fabrication and theoretical understanding. The history of how separate lines of research emanating from fundamental insights of about 50 years ago have coalesced into a thriving international research program in what might be called the ultimate nanotechnology is the subject of this review; it concentrates on the lesser-appreciated early developments in the field.

Static nonlinear optical susceptibilities: Testing approximation schemes against exact results

The reliability of the approximations commonly adopted in the calculation of static optical (hyper) polarizabilities is tested against exact results obtained for an interesting toy-model. The model accounts for the principal features of typical nonlinear organic materials with mobile electrons strongly coupled to molecular vibrations. The approximations introduced in sum over states and finite field schemes are analyzed in detail. Both the Born–Oppenheimer and the clamped nucleus approximations turn out to be safe for molecules, whereas for donor–acceptor charge transfer complexes deviations from adiabaticity are expected. In the regime of low vibrational frequency, static susceptibilities are strongly dominated by the successive derivatives of the potential energy and large vibrational contributions to hyperpolarizabilities are found. In this regime anharmonic corrections to hyperpolarizabilities are very large, and the harmonic approximation, exact for the linear polarizability, turns out totally inadequate for nonlinear responses. With increasing phonon frequency the role of vibrations smoothly decreases, until, in the antiadiabatic (infinite vibrational frequency) regime, vibrations do not contribute anymore to static susceptibilities, and the purely electronic responses are regained.

Structure and properties of all organic conductor (BEDT-TSeF) 2(X 1X 2TCNQ) (X 1,X 2=Cl,Br)

We synthesized donor-acceptor charge-transfer complexes composed of BEDT-TSeF ( or “BETS”) and TCNQ derivatives and obtained isosturctual complexes represented as (BETS) 2(X 1X 2TCNQ) (X 1, X 2=Cl, Br). The single crystals of (BETS) 2 (Cl 2TCNQ) preserve the metallic state down to 2K.

Dynamics of photoexcited donor-acceptor complexes between C60 andN,N-diethylaniline. Polarization picosecond spectroscopy study

The kinetics of the formation and decay of photoexcited radical ion pairs of donoracceptor charge-transfer complexes between C60 andN,N-diethylaniline (DEA) in chlorobenzene was studied by picosecond laser-induced diffraction gratings. It was established that the anisotropy of polarization of the diffraction signal decreases as the concentration of DEA increases. The radical ion states of the photoexcited C60−...DEA+ complex have zero anisotropy. This effect is likely due to the isotropic intracomplex transfer of an electron from the local excited state to the radical ion state. The rate constant of quenching of the singlet excited C60 byN,N-diethylaniline (1.4·1010 L mol−1 s−1) and the lifetimes of the solventseparated C60−...DEA+ and tight [C60−...DEA+] (95±7 and 31±4 ps, respectively) radical ion pairs were measured.

Synthesis, structure, and electronic properties of (BEDT-TTF)(F nTCNQ) (n=0,1,2,4)

We have synthesized donor-acceptor charge-transfer complexes composed of BEDT-TTF (ET) and TCNQ fluorinated derivatives, F nTCNQ (n=0,1,2,4). In (ET)(F nTCNQ) (n=1,2,4), nonmagnetic Peierls state was suppressed; (ET)(F 1TCNQ) preserves metallic state down to 2K, whereas (ET)(F 2TCNQ) and (ET)(F 4TCNQ) undergo antiferromagnetic transitions at T N=30K and T N=14K, respectively. Novel high conductive phase was also obtained in ternary compound, (ET) 1(F 2TCNQ) 0.5(TCNQ) 0.5, which shows metallic behavior down to 2K.

Mobile π Electrons From Donors and Acceptors

Abstract Concepts that evolved over the years for the design of organic metals and superconductors based on electron donor-acceptor charge transfer complexes are described. The preparation and complete characterization of three new acceptors; p-hexacyano divinylbenzene (HCDV), i-hexacyano divinylbenzene (i-HCDV) and 1,3,5-nonacyano trivinylbenzene (NCTV) are described. These were designed with different properties in mind. The latter for the preparation of organic ferromagnets based on the Breslow modification of the McConnell model.

On the enigma of old yellow enzyme's spectral properties.

Old yellow enzyme (NADPH oxidoreductase) in the free and complexed state was thoroughly investigated by the following techniques: absorption, circular dichroism, fluorescence/phosphorescence and electron paramagnetic resonance spectroscopy and fluorescence/phosphorescence decay measurements, applied over a wide range of temperature (7-293K). The data obtained were interpreted by comparison with results from similar measurements on free FMN, existing spectral data on isoalloxazine model systems and theoretical data. The results clearly demonstrate the inadequacy of a simple phenolate-FMN donor-acceptor charge-transfer complex to explain the phenomena occurring upon the addition of phenols to old yellow enzyme. Instead it was found that the phenolate anion interferes strongly with an existing tight complex between FMN and the apoprotein, probably an H-bonded structure in which FMN is tautomerized and interacts with an L-chiral center. This is concluded from a separate electronic transition with an origin at 496 nm, thus far not recognized as such, and the circular dichroism observed. The emission is dominated by that of free FMN, although protein-bound FMN seems also to become luminescent in glassy solution at 143 K. A second fluorescence/phosphorescence emission appears upon excitation of both native and complexed old yellow enzyme in the ultraviolet. This emission is quenched by the addition of phenol to the enzyme, shows a large (3000-cm-1) blue shift on going to a low-temperature glass and is tentatively assigned to excimers of nucleic acids. Long-wavelength excitation with a synchronously pumped, mode-locked Rhodamine 6-G dye laser revealed a third, extremely weak emission in both native old yellow enzyme and its complexes. It decays with a lifetime of about 3 ns at 143 K. Electron paramagnetic resonance spectra revealed the presence of a low amount of an unpaired spin in old yellow enzyme. Owing to an unusual relaxational behaviour it could only be observed below 15 K and, again, the signal was measured in both the native enzyme and its complexes. Possible assignment and consequences of this observation are discussed. In frozen aqueous solutions of the enzyme-phenolate complex, a phase transition was discovered at which the colour of the complex reverted to that of the native enzyme. Subsequent melting restored the original colour. The observed phenomena and existing literature data lead to the conclusion that the only model from which no apparent inconsistencies emerge is that of a very complicated network of hydrogen-bonded structures in the protein. These involve several, partly unknown, chromophores. Phenols interfere with this network, leading to the formation of the long-wavelength absorption band in old yellow enzyme.

Localized Spins inp-Phenylene Diamine-Bromanil Complexes

Abstract In a number of studies of the electron magnetic resonance spectra of donoracceptor charge transfer complexes,1–3 the temperature dependence of the absorption approximately follows the Curie Law, x ∼ 1/T at temperatures near that of liquid nitrogen. It is generally assumed that in this temperature region most of the absorption is of extrinsic origin. Indeed, Pott and Kommandeur4 showed that such effects could be produced in their samples by varying the method of preparation. The magnitudes of the absorption are outside the limits expected for impurities. Hughes and Soos5 have proposed that the low temperature absorption in p-phenylene diamine-chloranil is due to molecular chains of only finite length with an odd number of radicals. The purpose of this note is to report that in some samples of p-phenylene diamine-bromanil the number of spins appears to be constant over a wide range of temperature, from 77°K to 300°K and that the spins appear to be localized on bromanil radical ions.

Electron spin resonance study of phosphorescent centres in biphenyl, biphenyl–tetracyanoethylene, and biphenyl–tetracyanoethylene–naphthalene molecular crystals

Phosphorescent centres in single crystals of zone-purified biphenyl have been examined by their triplet e.s.r. spectra. These centres are attributed to impurities, possibly naphthalene, trapped at crystal defect sites, rather than conformationally strained biphenyl molecules. The strong electron acceptor tetracyanoethylene is found to enter the lattice isomorphically, and to have negligible, if any, contribution to the triplet through its own triplet state or that of a donor–acceptor charge-transfer complex. Naphthalene in the presence of an excess of tetracyanoethylene forms a donor–acceptor charge-transfer complex which crystallises out of biphenyl solid solution. Insofar as a residue of this complex is retained in the lattice there is, within the limits of the experimental technique, no detectable charge-transfer contribution to the triplet of the phosphorescent centres of the host biphenyl.

Electron spin resonance of transient negative ions in the formation of donor–acceptor charge-transfer complexes of solvent–tetracyanoethylene systems

The first-order decay of negative ions from solvent–tetracyanoethylene donor–acceptor charge-transfer complexes has been observed by e.s. spectroscopy. A mechanism based upon simultaneous hydrogen-bonded proton and electron hole transfer is used to account for the decay.