Identification of TCNQF4 redox levels using spectroscopic and electrochemical fingerprints (TCNQF4=2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane)

Unlocking high-performance near-infrared photodetection: polaron-assisted organic integer charge transfer hybrids

Room temperature femtowatt sensitivity remains a sought-after attribute, even among commercial inorganic infrared (IR) photodetectors (PDs). While organic IR PDs are poised to emerge as a pivotal sensor technology in the forthcoming Fourth-Generation Industrial Era, their performance lags behind that of their inorganic counterparts. This discrepancy primarily stems from poor external quantum efficiencies (EQE), driven by inadequate exciton dissociation (high exciton binding energy) within organic IR materials, exacerbated by pronounced non-radiative recombination at narrow bandgaps. Here, we unveil a high-performance organic Near-IR (NIR) PD via integer charge transfer between Poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (C-14PBTTT) donor (D) and Tetrafluorotetracyanoquinodimethane (TCNQF4) acceptor (A) molecules, showcasing strong low-energy subgap absorptions up to 2.5 µm. We observe that specifically, polaron excitation in these radical and neutral D-A blended molecules enables bound charges to exceed the Coulombic attraction to their counterions, leading to an elevated EQE (polaron absorption region) compared to Frenkel excitons. As a result, our devices achieve a high EQE of ∼107%, femtowatt sensitivity (NEP) of ~0.12 fW Hz-1/2 along a response time of ~81 ms, at room temperature for a wavelength of 1.0 µm. Our innovative utilization of polarons highlights their potential as alternatives to Frenkel excitons in high-performance organic IR PDs.

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.

[FeII(L•)2][TCNQF4•−]2: A Redox-Active Double Radical Salt

The reaction of [FeII(L•)2][BF4]2 with LiTCNQF4 results in the formation of [FeII(L•)2][TCNQF4•−]2·2CH3CN (1) (L• is the neutral aminoxyl radical ligand 4,4-dimethyl-2,2-di(2-pyridyl)oxazolidine-N-oxide; TCNQF4 is 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane). Single-crystal X-ray diffraction; Raman, Fourier-transform infrared (FTIR) and ultraviolet–visible spectroscopies; and electrochemical studies are all consistent with the presence of a low-spin FeII ion, the neutral radical form (L•) of the ligand, and the radical anion TCNQF4•−. 1 is largely diamagnetic and the electrochemistry shows five well-resolved, diffusion-controlled, reversible one-electron processes.

Use of the TCNQF4 2- Dianion in the Spontaneous Redox Formation of [FeIII (L- )2 ][TCNQF4 ⋅- ]

The reaction of [Fe^II (L^. )₂ ](BF₄ )₂ with Li₂ TCNQF₄ results in the formation of [Fe^III (L^- )₂ ][TCNQF₄ ^{. -} ] (1) where L^. is the radical ligand, 4,4-dimethyl-2,2-di(2-pyridyl)oxazolidine-N-oxide and TCNQF₄ is 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane. This has been characterised by X-ray diffraction, Raman and Fourier transform infrared (FTIR) spectroscopy, variable-temperature magnetic susceptibility, Mössbauer spectroscopy and electrochemistry. X-ray diffraction studies, magnetic susceptibility measurements and Raman and FTIR spectroscopy suggest the presence of low-spin Fe^III ions, the anionic form (L^- ) of the ligand and the anionic radical form of TCNQF₄ ; viz. TCNQF₄ ^{. -} . Li₂ TCNQF₄ reduces the [Fe^II (L^. )₂ ]²⁺ dication, which undergoes a reductively induced oxidation to form the [Fe^III (L^- )₂ ]⁺ monocation resulting in the formation of [Fe^III (L^- )₂ ][TCNQF₄ ^{. -} ] (1), the electrochemistry of which revealed four well-separated, diffusion-controlled, one-electron, reversible processes. Mössbauer spectroscopy and electrochemical measurements suggest the presence of a minor second species, likely to be [Fe^II (L^. )₂ ][TCNQF₄ ^2- ].

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.

Charge transfer from and to manganese phthalocyanine: bulk materials and interfaces

Manganese phthalocyanine (MnPc) is a member of the family of transition-metal phthalocyanines, which combines interesting electronic behavior in the fields of organic and molecular electronics with local magnetic moments. MnPc is characterized by hybrid states between the Mn 3d orbitals and the π orbitals of the ligand very close to the Fermi level. This causes particular physical properties, different from those of the other phthalocyanines, such as a rather small ionization potential, a small band gap and a large electron affinity. These can be exploited to prepare particular compounds and interfaces with appropriate partners, which are characterized by a charge transfer from or to MnPc. We summarize recent spectroscopic and theoretical results that have been achieved in this regard.

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.