Charge Transfer Compounds Research Articles

A quinoxaline-based charge-transfer compound for efficient deep-red organic light emitting diodes

Deep-red (DR) emitting materials with narrow band gap have important applications in optoelectronic field. However, limited by energy gap rules, the development of DR emitting materials with high luminous efficiency face great challenges. Herein, we present a new Quinoxaline-based compound 6,7-bis(4-(diphenylamino)phenyl)quinoxaline-2,3-dicarbonitrile (o-QxDC-DTPA) with D-A-D structure for deep-red organic light emitting diodes. Such a material features distinctive DR emitting property with strong charge transfer character. Highly efficient devices were achieved based on o-QxDC-DTPA with different doped concentrations. The doped device exhibited excellent electroluminescence efficiency, with a maximum EQE of 5.21% and DR emission with the emission peak at 680 nm.

Oxidization of aromatic heterocyclic molecules with superhalogens

The ability of superhalogens to oxidize aromatic heterocyclic molecules has been examined on the basis of ab initio and density functional theory calculations. It is found that superhalogens AlF4, MgF3, LiF2, and LiCl2 are able to form stable charge-transfer compounds with pyrrole, furan, and thiophene. As for the pyridine molecule, it can only be ionized and form strongly bound compound when combined with superhalogen SbF6. Results show that the competition between electron detachment energies of the superhalogen anions and ionization potentials of the aromatic heterocyclic molecules is a key factor for determining the stability of the resulting compounds. And this explains the ineffectiveness of ordinary superhalogens in oxidizing C5H5N which has a large ionization potential. These findings may provide new insights into organic aromatic molecule ionization and motivate further research in this direction.

The low energy phonon modes of the hydrogenated and deuterated π-conjugated system 7,7,8,8-tetracyanoquinodimethane: an inelastic neutron scattering study.

The organic acceptor, 7,7,8,8-tetracyanoquinodimethane (TCNQ), has been the subject of much research over the past few decades. Due to the π-conjugation of the quinodal structure, TCNQ as well as TCNQ based charge transfer compounds, exhibit some remarkable conducting properties. We present a study of the neutral TCNQ using inelastic neutron spectroscopy (INS) and show the first temperature dependent INS data collected on TCNQ allowing us to probe the temperature dependence of the low energy vibrational states, which have been shown to have an effect on charge delocalisation. Computational calculations have been used to help understand the data and the combination with the INS allows us to understand the phonon states. A complimentary experiment on deuterated TCNQ was also performed that allows a brief comparison of the isotopic substitution. This work utilises novel techniques to aid the understanding of the bulk properties of TCNQ in its neutral state that can be fed into work on other TCNQ based materials, including the exploration of deuteration as a technique to tune the properties of the parent compound.

Self‐assembly and tunable optical properties of intramolecular charge transfer molecules

AbstractCrystalline aggregates formed by self‐assembly of organic small molecules have aroused intensive attention in many fields due to their controllable morphologies and functionalization. The advantages of intramolecular charge transfer (ICT) compounds endow them with the ability to self‐assemble into diverse dimensional crystalline microstructures, as potential candidates for advanced materials and smart miniatures in practical applications. In this review, the recent progresses of self‐assembled microstructures of functional ICT molecules in our group for a decade are summarized. The factors influencing the fabrication of the microstructures in solvent‐involved processes are fully discussed. Distinct optical properties induced by self‐assembly behaviors are also depicted. We try to give an outlook of structure‐property relationships in functional microstructure engineering and clarify the crucial issues to be addressed in the future scientific research.

Understanding Intermolecular Interactions in a Tetracene–F4TCNQ Cocrystal via Its Electron Density Distribution and Topology

High-quality single crystals of the charge transfer compound tetracene–F4TCNQ (1:1) were grown by physical vapor transport and characterized by IR and UV/vis spectroscopy as well as by accurate sin...

Ab initio investigations on the interaction of CO2 and non-metallic superalkalis: structure, stability and electronic properties

Superalkalis are hypervalent species, having lower ionisation energy than alkalis and its strong reducing capabilities can be used in the design of a variety of charge-transfer compounds. The interaction of non-metallic superalkalis such as F2H3, O2H5, and N2H7 with CO2 has been systematically explored using ab initio MP2/6-311++G(d,p) method. We have studied the structural isomers of the resulting superalkali-CO2 complexes and their relative stabilities. We have analyzed their partial charge, spin density, and singly-occupied molecular orbitals’ energy, which reveal the relative strength and charge transfer due to the interaction of CO2 with superalkalis. We notice that this interaction leads to the reduction of CO2, as the structure of CO2 moiety in superalkali-CO2 complexes is bent due to electron transfer from superalkalis. Thus, this study should provide some interesting results on the single-electron reduction of CO2 by non-metallic compounds.

Stimuli-responsive emission of quinoxalinone-based compounds. From experimental findings to theoretical insight by means of multiscale computational spectroscopy approaches

The drastic differences in absorption spectra of structurally close quinoxalinone derivatives, containing electron-donor and -acceptor units, are rationalized, by means of theoretical tools. They are ascribed to the difference in the intramolecular charge-transfer character of the electronic transition corresponding to the experimental longest-wavelength absorption bands of these systems. Charge-transfer compounds are not luminescent in solutions, in contrast to their analogs with weaker charge-separation and longer π-conjugated backbone, which suggests that the main factor “switching-off” emission is related to their “push-pull” properties and the balance between the length of the π-conjugated backbone and the donor-acceptor strength. Furthermore, the protonation of the studied quinoxalinones is shown to result in a weakening of the donor-acceptor strengths and conjugation efficiency, thus limiting charge-transfer and producing the emission response in the acidified solutions.

Assessment of the Density-Fitted Second-Order Quasidegenerate Perturbation Theory for Transition Energies: Accurate Computations of Singlet-Triplet Gaps for Charge-Transfer Compounds.

Organic light-emitting diodes (OLEDs) have been of significant interest because of their superior performance and low cost of production. Thermally activated delayed fluorescence (TADF) has attracted significant interest in the OLED technology because it improves the efficiency of OLEDs by harvesting triplet excitons. Therefore, the accurate computation of singlet-triplet transition energies (ΔES1-T1) of charge-transfer molecules is very important. However, the accurate computation of the ΔES1-T1 values is a challenging problem for single-reference methods because of the multireference character of excited states. In this research, an assessment of density-fitted second-order quasidegenerate perturbation theory (DF-QDPT2) [Bozkaya, U.; J. Chem. Theory Comput. 2019, 15, 4415-4429] for singlet-triplet transition energies (ΔES1-T1) of charge-transfer compounds is presented. The performance of the DF-QDPT2 method has been compared with those of several density-functional theory functionals, such as B3LYP, PBE0, M06-2X, ωB97X-D, and MN15; density-fitted state-averaged CASSCF (DF-SA-CASSCF); and single-state single-reference second-order perturbation theory (SS-SR-CASPT2) methods. For the TADF molecules considered, the DF-QDPT2 method provides a mean absolute error (MAE) of 0.13 eV, while the MAE values of DF-SA-CASSCF and SS-SR-CASPT2 are 0.65 and 0.74 eV, respectively. The performances of B3LYP and PBE0 are slightly better than that of DF-QDPT2, while M06-2X and ωB97X-D provide noticeably higher errors compared with DF-QDPT2. Furthermore, the standard CASSCF without state-averaging yields dramatic errors with an MAE value of 3.0 eV. Our results demonstrate that eigenvalues of the DF-QDPT2-effective Hamiltonian can be reliably used for the prediction of singlet-triplet transition energies, while eigenvalues of DF-CASSCF/DF-SA-CASSCF fail to provide accurate predictions. Overall, we conclude that the DF-QDPT2 method emerges as a very useful tool for the computation of excited-state properties.

Intramolecular charge transfer-based spirobifluorene-coupled heteroaromatic moieties as efficient hole transport layer and host in phosphorescent organic light-emitting diodes

In this study, we report two efficient intramolecular charge transfer (ICT) compounds with versatile functionality as hole transporting layers (HTL) and hosts in emission layer (EML) of phosphorescent organic light emitting diodes (Ph-OLEDs) with donor-acceptor-donor structures. In both molecules, 9,9′-spirobifluorene (SBF) was employed as a common electron acceptor, while phenoxazine (PXZ) or phenothiazine (PTZ) were used as donors. The molecules 2,7-di(10H-phenoxazin-10-yl)-9,9′-spirobi[fluorene] (SBF-PXZ) and 2,7-di(10H-phenothiazin-10-yl)-9,9′-spirobi[fluorene] (SBF-PTZ) were synthesized via a Buckwald-Hartwig cross-coupling reaction in a single step that exhibited typical ICT phenomena. The extremely twisted geometrical configurations of SBF-PXZ and SBF-PTZ may allow for effective intramolecular charge-transfer. Both molecules exhibited strong absorption and emission characteristics in the visible region in solution as well as in the thin film form; significant positive solvatochromism was also observed for both compounds. The replacement of PTZ donor with the PXZ donor caused a bandgap reduction together with an improved ionization potential (IP). The deeper IP (>5.1 eV), greater glass transition temperature, and higher hole mobility extracted from the hole only devices of both molecules revealed them as superior candidates for HTL applications (with/without exciton blocking layers) in Ph-OLEDs. It is further corroborated by the enhanced half-life time of Ph-OLEDs with SBF-PXZ/SBF-PTZ HTLs. Moreover, the OLED performances with SBF-PTZ and SBF-PXZ as EMLs and hosts for phosphorescent dopant OLEDs have exemplified the versatility of the proposed ICT compounds.

Tubular Au-TTF solid contact layer synthesized in a microfluidic device improving electrochemical behaviors of paper-based potassium potentiometric sensors

Tetrathiafulvalene (TTF) based charge-transfer compounds are ideal as a transducer layer for a solid-contact ion-selective electrode (SC-ISE), because of their excellent ionic conductivity and high capacitance. Since nanomaterials with tailored geometries exhibit unique shape-dependent properties and subsequent utilization of them for the fabrication of electrochemical devices is of significant interest. Gold-TTF (Au-TTF) with different morphologies such as flower-like, sheet-like, long wire and tubular with different length, were synthesized by microfluidics-dictated synthesis in a fast, simple and continuous procedure. The electrochemical performance of synthesized Au-TTF hybrid crystals was characterized by cyclic voltammetric, electrochemical impedance and constant-current chronopotentiometric measurement. The results demonstrate that long-tubular-like Au-TTF crystal is best suitable for ISE due to its great stability, high capacitance and co-existence of TTF/TTF+ providing a stable interphase potential. Then paper-based potentiometric potassium sensors were fabricated based on flower-like and long-tubular-like Au-TTF hybrid crystal as transducer layer of ISE and reference electrode. Compared to paper-based sensor with flower-like Au-TTF as transducer layer, paper-based sensor with long-tubular-like Au-TTF as transducer layer exhibits a much better Nernstian response with a slop of 57.64 ± 0.09 mV/dec and a lower detection limit of 2.75 × 10−6 M. Furthermore, the fabricated potentiometric sensor requires only 5 μL samples to test and displays little water-layer, good anti-interference, and great reproducibility and selectivity.

Mechanisms of nucleation and growth in the formation of charge transfer nanocrystals

Nanocrystals of the charge transfer compound perylene:7,7′,8,8′-tetracyanoquinodimethane (TCNQ) were created using the reprecipitation method, a simple technique for inducing crystallization in solution by the rapid addition of a miscible non-solvent. The nanocrystal size was followed in situ using dynamic light scattering, showing rapid formation of crystals with sizes on the order of 100 nm followed by much slower growth. The complexation of the electron donor perylene and electron acceptor TCNQ was monitored in situ through the appearance of the charge transfer band in the infrared. Both 1:1 and 3:1 cocrystal stoichiometries could be obtained, but the ratio of the components did not always result in the same stoichiometry in the nanocrystals. In particular, a sample prepared with equal molar amounts of the constituents contained both 1:1 and 3:1 forms, as well as pure perylene and TCNQ nanocrystals. This sample also demonstrated non-classical nucleation, with solid particles forming immediately, but longer range order appearing later.

A structural comparative study of charge transfer compounds: Synthesis, crystal structure, IR, Raman-spectroscopy, DFT computation and hirshfeld surface analysis

The present work focuses on the crystal structure analysis, vibrational spectroscopy investigation and DFT calculation. Two new charge transfer compounds; bis (creatininium) fumarate fumaric acid (I) and creatininium 3,5-dicarboxybenzoate monohydrate (II), have been synthesized, their Raman and IR modes of vibrations have been assigned and their crystal structures have been studied by means of single crystal X-ray diffraction. Complementary Hirshfeld surface analysis were carried out to investigate and quantify the contributions of the different intermolecular interactions within the crystal. This analysis revels that the main contributions in both compounds are provided by the O⋯H and H⋯H interactions that represent ∼70 (for I) and ∼75% (for II) of the total contributions to the Hirshfeld surface. The results of the theoretically predicted structural parameters and vibrational frequencies are in good agreements with the experimental investigations. These results show that both compounds exhibit similar features, however the energy gap between EHOMO and ELUMO obtained from the molecular orbital analysis indicates that compound (I) is characterized by a molecular structural more favourable for charge transfer.

Variation of Donor and Acceptor Strength in Analogues of Brooker's Merocyanine and Generalization to Various Classes of Charge Transfer Compounds.

Derivatives of Brooker's merocyanine (BM) have been investigated, which possess different donors and acceptors and therefore vary their donor-acceptor strength SDA. The 00 energies have been extracted from the spectra and compared. Under basic conditions, where the neutral (merocyanine) form is present, the absorption energies for all compounds are similar, whereas there is a large difference for acidic conditions where the cationic (cyanine) form is present. This behavior could be explained by a simple theoretical model involving the dependence of the excitation energy Δ E01 on SDA. This model can be generalized to describe in a consistent way two different well-known classes of neutral chromophores with a certain degree of charge separation, namely merocyanine I (TICT) and merocyanine II (often betainic) compounds. Merocyanines I are characterized by a medium polar aromatic ground state and a zwitterionic quinoid excited state and hence positive solvatochromism, whereas merocyanines II are formally characterized by a zwitterionic aromatic ground state and a less polar quinoid excited state and, accordingly, by negative solvatochromism. On increasing the donor-acceptor strength SDA sufficiently, merocyanines II can, however, move to the so-called overcritical region with the excited state dominated by the zwitterionic valence bond structure. For many of the merocyanine II molecules investigated here, a weakly positive solvatochromic behavior is observed indicating that the ground state contains less of the zwitterionic valence bond wave function than the excited state and that these compounds belong to the overcritical region. The fluorescence spectra have been analyzed in terms of the Franck-Condon model and confirm these conclusions.

Solvent dependent linear and nonlinear optical properties of triphenylamine unit incorporated difluoroboron β-diketonate complexes

In this work, four intramolecular charge transfer (ICT) compounds, TBT, TVBVT, TSBST and TSVBVST, with difluoroboron β-diketonate as the electron acceptor, triphenylamine unit as the electron donor and different π-conjugated bridges have been synthesized. The linear absorption and emission spectra in different organic solvents indicate a positive solvato-chromism. The multilinear regression analyses of the linear optical spectra in different solvents indicate that the solvato-chromism is mainly caused by the polarity/dipolarity properties of the solvents and possibly the halogen-halide interaction. Time-dependent density functional theory (TD-DFT) calculations were used to investigate the charge density difference between the ground and excited states of the complex molecules of interest and the ICT processes of the excited molecules were verified. The nonlinear optical properties of the molecules were studied using a Z-scan technique. It is found that not only the structure of the molecules influences the nonlinear optical properties, but that also the solvent-solute interaction plays an important role.

Synthesis, crystal structure, vibrational spectra, nonlinear optical property of an organic charge-transfer compound―4-nitrobenzyl isoquinolinium picrate based on DFT calculations

Single crystals of an organic charge-transfer compound, 4-nitrobenzyl isoquinolinium picrate, [4NO2BzIQl][PIC](1), were grown by slow evaporation technique at room temperature. Single crystal XRD and powder XRD studies conform the crystal structure and crystalline nature. In the title compound, the [4NO2BzIQl]+ cations and the [PIC]- anions stack into a columnar structure through π···π and CH···π interactions. The FT-IR and Raman spectra were recorded and analyzed with the aid of density functional theory calculations in order to make a suitable assignment of the observed bands. The simulated spectrum satisfactorily coincides with the experimental UV–visible spectrum. Thermal stability and the major gaseous products of decomposition at 293 °C were examined by TG-DTA-MS technique. The material shows three main emission peaks about 385, 467 and 538 nm upon excitation at 241 nm in solid state at room temperature. The excited state energy from theoretical UV–vis spectra, the total energy of intramolecular interactions and nonlinear optical properties were studied using DFT calculations.

1, 3-Indanedione functionalized fluorene luminophores: Negative solvatochromism, nanostructure-morphology determined AIE and mechanoresponsive luminescence turn-on

Luminous efficiency in polar environment and aggregated state, as well as high-contrast luminescence on/off switching under external stimuli are significant issues to be addressed in developing functional fluorescent materials. Combining the advantages of solvatochromic effect in intramolecular charge transfer (ICT) compounds with their nanostructured morphological effects, we herein report a series of A–π–D–π–A fluorene derivatives, which simultaneously exhibit negative solvatochromism, aggregation-induced emission (AIE) and mechanoresponsive luminescence (MRL) turn-on. These molecules contain the same fluorene donor (D) and 1,3-indandione acceptor (A) but differ in alkyl substituents in the 9-position of the fluorene segment, including 9,9-dibutyl (b-DIPF), 9,9-dioctyl (o-DIPF) and 9,9-didodecyl (d-DIPF). The emission colors of these compounds were changed from blue to orange-red, and the fluorescence quantum yields increased upon increasing the solvent polarity from nonpolar hexane to polar dimethyl sulfoxide. Due to the formed random nanostructured aggregates, the compounds exhibited AIE characteristics in THF/H2O mixtures. Oppositely, their emission quenched 0D particles which were afforded by quick evaporation of their dichloromethane solvents under vacuum displayed remarkable mechanoresponsive luminescence turn-on behavior. Our results suggest that rationally design ICT molecules and control their nanostructures would be promising way for realizing multifunctional fluorescent materials.

Conformational Relaxation and Thermally Activated Delayed Fluorescence in Anthraquinone-Based Intramolecular Charge-Transfer Compound

A novel donor-π-acceptor-π-donor-type (D-π-A-π-D-type) chromophore, 2,6-bis[4-(diphenylamino)phenyl]-9,10-anthraquinone (AQ(PhDPA)2), has been reported as an efficient red thermally activated delayed fluorescence (TADF) emitter. Molecular structure and conformation, which directly determine the nature of excited states of a TADF emitter, are critical for obtaining efficient reverse intersystem crossing (rISC) and TADF. In this work, a series of excited-state deactivation processes of AQ(PhDPA)2, from the optical excitation to fluorescence and TADF emitting, have been investigated by theoretical calculations and ultrafast transient absorption (TA) spectroscopy. Theoretical calculations and steady-state spectra suggest that the TADF emitter appears to have conformational twisting in the excited state. Both the relaxed S0 and S1 conformations have a small energy difference between the lowest singlet and triplet excited states (ΔEST) in favor of rISC, whereas ΔEST increases at the relaxed T1 conformation. Ult...

Electronic structure of negative charge transfer CaFeO3 across the metal-insulator transition

We investigated the metal-insulator transition for epitaxial thin films of the perovskite ${\mathrm{CaFeO}}_{3}$, a material with a significant oxygen ligand hole contribution to its electronic structure. We find that biaxial tensile and compressive strain suppress the metal-insulator transition temperature. By combining hard x-ray photoelectron spectroscopy, soft x-ray absorption spectroscopy, and density functional calculations, we resolve the element-specific changes to the electronic structure across the metal-insulator transition. We demonstrate that the Fe sites undergo no observable spectroscopic change between the metallic and insulating states, whereas the O electronic configuration undergoes significant changes. This strongly supports the bond-disproportionation model of the metal-insulator transition for ${\mathrm{CaFeO}}_{3}$ and highlights the importance of ligand holes in its electronic structure. By sensitively measuring the ligand hole density, however, we find that it increases by $\ensuremath{\sim}5--10%$ in the insulating state, which we ascribe to a further localization of electron charge on the Fe sites. These results provide detailed insight into the metal-insulator transition of negative charge transfer compounds and should prove instructive for understanding metal-insulator transitions in other late transition metal compounds such as the nickelates.

Impact of C–H···X (X = F, N) and π–π Interactions on Tuning the Degree of Charge Transfer in F6TNAP-Based Organic Binary Compound Single Crystals

Single crystals of four new charge transfer compounds, triphenylene (TP)-1,3,4,5,7,8-hexafluorotetracyanonaphthoquinodimethane (F6TNAP), pyrene-F6TNAP, phenanthrene (PA)-F6TNAP, and naphtho[1,2-b:5,6-b’]dithiophene (NDT)-F6TNAP, with needle-like morphologies and dark color were grown by chemical vapor transport. The influence of C–H···X (X = F, N) interactions as well π–π interactions on the degree of charge transfer (DCT) in these compounds was investigated. The results indicate that the DCT is proportional to the interplanar D–A distance along the π–π interaction direction in TP-F6TNAP, pyrene-F6TNAP, and PA-F6TNAP single crystals. With the decreasing of the D–A interplanar distance from 3.3560 A of TP-F6TNAP to 3.3254 A of PA-F6TNAP, the DCT is significantly increased from 0.03 ± 0.02 of TP-F6TNAP to 0.26 ± 0.03 of PA-F6TNAP, showing the strong relationship between π–π interactions and the DCT. In addition to π–π interactions, a stronger C–H···F interaction (C–H···F bond length: 2.3689 A) is observed i...

A 3D antiferromagnetic ground state in a quasi-1D π-stacked charge-transfer system

With the rising interest in organic based materials for spintronic and multiferroic applications it is important to fully understand their electrical and magnetic properties and to identify correlations between their structural and physical attributes. In this work we look at a TCNQ based charge transfer compound to provide insight into the low temperature magnetism.