Charge-Transfer Complexes: Fundamentals and Advances in Catalysis, Sensing, and Optoelectronic Applications

Supramolecular assemblies, formed through electronic charge transfer between two or more entities, represent a rich class of compounds dubbed as charge-transfer complexes (CTCs). Their distinctive formation pathway, rooted in charge-transfer processes at the interface of CTC-forming components, results in the delocalization of electronic charge along molecular stacks, rendering CTCs intrinsic molecular conductors. Since the discovery of CTCs, intensive research has explored their unique properties including magnetism, conductivity, and superconductivity. Their more recently recognized semiconducting functionality has inspired recent developments in applications requiring organic semiconductors. In this context, CTCs offer a tuneable energy gap, unique charge-transport properties, tailorable physicochemical interactions, photoresponsiveness, and the potential for scalable manufacturing. Here, an updated viewpoint on CTCs is provided, presenting them as emerging organic semiconductors. To this end, their electronic and chemical properties alongside their synthesis methods are reviewed. The unique properties of CTCs that benefit various related applications in the realms of organic optoelectronics, catalysts, and gas sensors are discussed. Insights for future developments and existing limitations are described.

Fluorine Substitution of TCNQ Alters the Redox-Driven Catalytic Pathway for the Ferricyanide-Thiosulfate Reaction

Mechanistic variation in catalysis through substituent-based redox tuning is well established. Fluorination of TCNQ (TCNQ=tetracyanoquinodimethane) provides ~850 mV variation in the redox potentials of theTCNQF n 0 / 1 - ${{{\rm {TCNQF}}}_{{\rm {n}}}^{{\rm {0/1-}}}}$ andTCNQF n 1 - / 2 - ${{{\rm {TCNQF}}}_{{\rm {n}}}^{{\rm {1-/2-}}}}$ (n=0, 2, 4) processes. WithTCNQF 4 1 - ${{{\rm {TCNQF}}}_{{\rm {4}}}^{{\rm {1-}}}}$ , catalysis of the kinetically very slow ferrocyanide-thiosulfate redox reaction in aqueous solution occurs via a mechanism in which the catalystTCNQF 4 1 - ${{{\rm {TCNQF}}}_{{\rm {4}}}^{{\rm {1-}}}}$ is reduced toTCNQF 4 2 - ${{{\rm {TCNQF}}}_{{\rm {4}}}^{{\rm {2-}}}}$ when reacting withS 2 O 3 2 - ${{{\rm {S}}}_{{\rm {2}}}{{\rm {O}}}_{{\rm {3}}}^{{\rm {2-}}}}$ which is oxidised toS 4 O 6 2 - ${{{\rm {S}}}_{{\rm {4}}}{{\rm {O}}}_{{\rm {6}}}^{{\rm {2-}}}}$ . Subsequently,TCNQF 4 2 - ${{{\rm {TCNQF}}}_{{\rm {4}}}^{{\rm {2-}}}}$ reacts with[ Fe ( CN ) 6 ]​ 3 - ${{{\rm {[Fe(CN)}}}_{{\rm {6}}}{{\rm {]}}}^{{\rm {3-}}}}$ to form[ Fe ( CN ) 6 ]​ 4 - ${{{\rm {[Fe(CN)}}}_{{\rm {6}}}{{\rm {]}}}^{{\rm {4-}}}}$ and reform theTCNQF 4 1 - ${{{\rm {TCNQF}}}_{{\rm {4}}}^{{\rm {1-}}}}$ catalyst, in another thermodynamically favoured process. An analogous mechanism applies withTCNQF 2 1 - ${{{\rm {TCNQF}}}_{{\rm {2}}}^{{\rm {1-}}}}$ as a catalyst. In contrast, since the reaction ofS 2 O 3 2 - ${{{\rm {S}}}_{{\rm {2}}}{{\rm {O}}}_{{\rm {3}}}^{{\rm {2-}}}}$ withTCNQ 1 - ${{{\rm {TCNQ}}}^{{\rm {1-}}}}$ is thermodynamically unfavourable, an alternative mechanism is required to explain the catalytic activity observed in this non-fluorinated system. Here, upon addition ofTCNQ 1 - ${{{\rm {TCNQ}}}^{{\rm {1-}}}}$ , reduction of[ Fe ( CN ) 6 ]​ 3 - ${{{\rm {[Fe(CN)}}}_{{\rm {6}}}{{\rm {]}}}^{{\rm {3-}}}}$ to[ Fe ( CN ) 6 ]​ 4 - ${{{\rm {[Fe(CN)}}}_{{\rm {6}}}{{\rm {]}}}^{{\rm {4-}}}}$ occurs with concomitant oxidation ofTCNQ 1 - ${{{\rm {TCNQ}}}^{{\rm {1-}}}}$ toTCNQ 0 ${{{\rm {TCNQ}}}^{{\rm {0}}}}$ , which then acts as the catalyst forS 2 O 3 2 - ${{{\rm {S}}}_{{\rm {2}}}{{\rm {O}}}_{{\rm {3}}}^{{\rm {2-}}}}$ oxidation. Thermodynamic data explain the observed differences in the catalytic mechanisms.CuTCNQF n ${{{\rm {CuTCNQF}}}_{{\rm {n}}}}$ (n=0, 4) also act as catalysts for the ferricyanide-thiosulfate reaction in aqueous solution. The present study shows that homogeneous pathways are available following addition of these dissolved materials. Previously, theseCuTCNQF n ${{{\rm {CuTCNQF}}}_{{\rm {n}}}}$ (n=0, 4) coordination polymers have been regarded as insoluble in water and proposed as heterogeneous catalysts for the ferricyanide-thiosulfate reaction. Details and mechanistic differences were established using UV-visible spectrophotometry and cyclic voltammetry.

Coordination polymers of 5-substituted 1,2,3,4-tetracyanocyclopentadienides: structural and electrochemical properties of complex compounds of 5-amino- and 5-nitro-tetracyanocyclopentadienide

Compounds of the 5 amino (ATCC) and 5 nitro 1,2,3,4 tetracyanocyclopentadienide (NTCC) ligand with iron(II) and iron(III), silver(I) and potassium(I) were prepared and characterized by electrochemical methods using EPR, cyclic voltammetry and Mößbauer spectroscopy as well as UV-Vis spectroscopy. The investigation sheds light on the free radical activity in non- as well as transition metal compounds, and shows signs of photoinduced electron transfer in transition metal compounds of the ligands. We characterized silver(I) and potassium compounds of the compounds via X-ray diffraction, with one of the structures exhibiting a porous coordination polymer.

Increased Crystallization of CuTCNQ in Water/DMSO Bisolvent for Enhanced Redox Catalysis

Controlling the kinetics of CuTCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane) crystallization has been a major challenge, as CuTCNQ crystallizing on Cu foil during synthesis in conventional solvents such as acetonitrile simultaneously dissolves into the reaction medium. In this work, we address this challenge by using water as a universal co-solvent to control the kinetics of crystallization and growth of phase I CuTCNQ. Water increases the dielectric constant of the reaction medium, shifting the equilibrium toward CuTCNQ crystallization while concomitantly decreasing the dissolution of CuTCNQ. This allows more CuTCNQ to be controllably crystallized on the surface of the Cu foil. Different sizes of CuTCNQ crystals formed on Cu foil under different water/DMSO admixtures influence the solvophilicity of these materials. This has important implications in their catalytic performance, as water-induced changes in the surface properties of these materials can make them highly hydrophilic, which allows the CuTCNQ to act as an efficient catalyst as it brings the aqueous reactants in close vicinity of the catalyst. Evidently, the CuTCNQ synthesized in 30% (v/v) water/DMSO showed superior catalytic activity for ferricyanide reduction with 95% completion achieved within a few minutes in contrast to CuTCNQ synthesized in DMSO that took over 92 min.

Heterometallic Actinide-Containing Photoresponsive Metal-Organic Frameworks: Dynamic and Static Tuning of Electronic Properties

Acquiring fundamental knowledge of properties of actinide-based materials is a necessary step to create new possibilities for addressing the current challenges in the nuclear energy and nuclear waste sectors. In this report, we established a photophysics-electronics correlation for actinide-containing metal-organic frameworks (An-MOFs) as a function of excitation wavelength, for the first time. A stepwise approach for dynamically modulating electronic properties was applied for the first time towards actinide-based heterometallic MOFs through integration of photochromic linkers. Optical cycling, modeling of density of states near the Fermi edge, conductivity measurements, and photoisomerization kinetics were employed to shed light on the process of tailoring optoelectronic properties of An-MOFs. Furthermore, the first photochromic MOF-based field-effect transistor, in which the field-effect response could be changed through light exposure, was constructed. As a demonstration, the change in current upon light exposure was sufficient to operate a two-LED fail-safe indicator circuit.

FTIR Spectroelectrochemistry of F4TCNQ Reduction Products and Their Protonated Forms

The tetrafluorinated derivative of 7,7,8,8-tetracyanoquinodimethane (TCNQ), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), is of interest for charge transfer complex formation and as a p-dopant in organic electronic materials. Fourier transform infrared (FTIR) spectroscopy is commonly employed to understand the redox properties of F4TCNQ in the matrix of interest; specifically, the ν(C≡N) region of the F4TCNQ spectrum is exquisitely sensitive to the nature of the charge transfer between F4TCNQ and its matrix. However, little work has been done to understand how these vibrational modes change in the presence of possible acid/base chemistry. Here, FTIR spectroelectrochemistry is coupled with density functional theory spectral simulation for study of the electrochemically generated F4TCNQ radical anion and dianion species and their protonation products with acids. Vibrational modes of HF4TCNQ^-, formed by proton-coupled electron transfer, are identified, and we demonstrate that this species is readily formed by strong acids, such as trifluoroacetic acid, and to a lesser extent, by weak acids, such as water. The implications of this chemistry for use of F4TCNQ as a p-dopant in organic electronic materials is discussed.

Correlating growth mechanism and morphology in Cu-TCNQ organometallic complex: A microscopic study

The structure-property correlation in the Cu-TCNQ organometallic complex is very important for explaining its unusual electrical, optical and magnetic properties. Consequently several morphological studies and their correlation with the properties of these materials can be found in the literature, although no systematic study of various morphologies with growth conditions and their correlation has been reported to the best of our knowledge. Therefore in this manuscript the interconversion of various morphologies is reported using electron and probe microscopies. A conventional Cu TEM grid acted as the copper source to form a Cu-TCNQ complex and the complex, which formed at the surface of the TEM grid. The complex thus prepared was characterized by FTIR and Raman spectroscopic techniques. The shifting of ̵-CN from 2221 cm^-1 (TCNQ) to 2201 cm^-1 indicates formation of a complex and the identical nature of IR spectra in two phases indicates that they are polymorphs. The morphologies of Cu-TCNQ were followed through FE-SEM and TEM studies. Various morphologies such as needle, square tube, platelet etc. were observed as a function of time. A distinct transition from needle to platelet morphology was observed as the complex grew. The conductance of various morphologies in phase-I as well as phase-II were also measured and compared by Spreading Resistance Imaging (SRI) at different bias voltage i.e. 1 V, 3 V and 5 V.

Solvent-, Cation- and Anion-Induced Structure Variations in Manganese-Based TCNQF4 Complexes: Synthesis, Crystal Structures, Electrochemistry and Their Catalytic Properties

The reaction of Mn(BF₄ )₂ ⋅x H₂ O with (Pr₄ N)₂ TCNQF₄ (TCNQF₄ =2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) in a mixture of CH₃ OH/CH₂ Cl₂ gives a 2:3 stoichiometric complex of (Pr₄ N)₂ [Mn₂ (TCNQF₄ )₃ (CH₃ OH)₂ ] (1). If the solvent system used for the crystallisation of 1 is changed to CH₃ OH/DMF, then a different product, [Mn(TCNQF₄ )(DMF)₂ ]⋅(CH₃ OH)₂ (2), is obtained. The use of Li₂ TCNQF₄ instead of (Pr₄ N)₂ TCNQF₄ leads to the generation of [Mn₂ (TCNQF₄ )₂ (DMF)₄ ]⋅3 DMF (3). An unexpected mixed oxidation state network with a composition of [Mn^II ₄ Mn^III ₁₆ O₁₀ (OH)₆ (OCH₃ )₂₄ (TCNQF₄ )₂ ](NO₃ )₂ ⋅24 CH₃ OH (4), is formed if Mn(NO₃ )₂ ⋅x H₂ O is used in place of Mn(BF₄ )₂ ⋅x H₂ O in the reaction that leads to the formation of 3. Compounds 1-3 have been characterised by X-ray crystallography; FTIR, Raman and UV/Vis spectroscopy; and electrochemistry. Compound 4 has only been analysed by X-ray crystallography and vibrational spectroscopy (Raman, FTIR), owing to rapid deterioration of the compound upon exposure to air. These results indicate that relatively minor changes in reaction conditions have the potential to yield products with vastly different structures. Compound 1 adopts an anionic 2D network with unusual π-stacked dimers of the TCNQF₄ ^2- dianion, whereas 2 and 3 are composed of similar neutral sheets of [Mn(TCNQF₄ )(DMF)₂ ]. Interestingly, the solvent has a significant influence on the stacking of the sheets in the structures of 2 and 3. In compound 4, clusters with a composition of [Mn^II ₄ Mn^III ₁₆ O₁₀ (OH)₆ (OCH₃ )₂₄ (CH₃ OH)₄ ]⁶⁺ serve as eight-connecting nodes, whereas TCNQF₄ ^2- ligands act as four-connecting nodes in a 3D network that has the same topology as fluorite. Compound 3 exhibits an exceptionally high super-catalytic activity for the electron-transfer reaction between ferricyanide and thiosulfate ions in aqueous media.

Structural, Spectroscopic, and Electrochemical Characterization of Semi-Conducting, Solvated [Pt(NH3)4](TCNQ)2·(DMF)2 and Non-Solvated [Pt(NH3)4](TCNQ)2

The demand for catalysts that are highly active and stable for electron-transfer reactions has been boosted by the discovery that [Pt(NH3)4](TCNQF4)2 (TCNQF4 = 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) is an efficient catalyst. In this work, we prepare and characterize the two related [Pt(NH3)4]2+ complexes, [Pt(NH3)4](TCNQ)2·(DMF)2 (1) and [Pt(NH3)4](TCNQ)2 (2). Reaction of [Pt(NH3)4](NO3)2 with LiTCNQ in a mixed solvent (methanol/dimethylformamide, 4 : 1 v/v) gives [Pt(NH3)4](TCNQ)2·(DMF)2 (1), whereas the same reaction in water affords [Pt(NH3)4](TCNQ)2 (2). 2 has been previously reported. Both 1 and 2 have now been characterized by single-crystal X-ray crystallography, Fourier-transform (FT)IR, Raman and UV-vis spectroscopy, and electrochemistry. Structurally, in 1, the TCNQ1− anions form infinite stacks with a separation between adjacent anions within the stack alternating between 3.12 and 3.42 Å. The solvated structure 1 differs from the non-solvated form 2 in that pairs of TCNQ1− anions are clearly displaced from each other. The conductivities of pressed pellets of 1 and 2 are both in the semi-conducting range at room temperature. 2 can be electrochemically synthesized by reduction of a TCNQ-modified electrode in contact with an aqueous solution of [Pt(NH3)4](NO3)2 via a nucleation growth mechanism. Interestingly, we discovered that 1 and 2 are not catalysts for the ferricyanide and thiosulfate reaction. Li+ and tetraalkylammonium salts of TCNQ1−/2− and TCNQF41−/2− were tested for potential catalytic activity towards ferricyanide and thiosulfate. Only TCNQF41−/2− salts were active, suggesting that the dianion redox level needs to be accessible for efficient catalytic activity and explaining why 1 and 2 are not good catalysts. Importantly, the origin of the catalytic activity of the highly active [Pt(NH3)4](TCNQF4)2 catalyst is now understood, enabling other families of catalysts to be developed for important electron-transfer reactions.

Nanostructured charge transfer complex of CuTCNQF4 for efficient photo-removal of hexavalent chromium

The fabrication of a nanostructured CuTCNQF₄ organic charge transfer complex on copper foil by employing a facile redox reaction in acetonitrile and its ability to promote catalytic reduction of toxic Cr⁶⁺ to its non-toxic Cr³⁺ counterpart.