Charge-transfer Complexes Research Articles

Assessment of Charge Transfer Energies of Noncovalently Bounded Ar-TCNE Complexes Using Range-separated Density Functionals and Double Hybrid Density Functionals.

Charge Transfer (CT) molecular complexes have recently received much attention in a broad variety of fields. The time-dependent density functional theory (TDDFT), which is essential for studying CT complexes, is a well-established tool to study the excited states of relatively large molecular systems. However, when dealing with donor-acceptor molecules with CT characteristics, TDDFT calculations based on standard functionals can severely underestimate the excitation energies. Here, we demonstrate that TDDFT can reliably be used for the calculations of the excitation energies of charge transfer molecular complexes, such as, Ar-TCNE (TCNE = tetracyanoethylene; Ar= benzene, naphthalene, anthracene, etc.) when using range-separated DFT and range-separated double-hybrid DFT functionals. The interactions between the donor-acceptor moieties of these molecular complexes are also studied and the relationship between the interaction and the charge transfer energies are shown here.

Design of Clickable Aromatic Donor-Acceptor Pairs for Easy Installation onto Polymers.

New intrinsically unsymmetric aromatic donors (D) and acceptors (A) were designed to simplify the incorporation of a single reactive group. Naphthalene diimide (NDI) was desymmetrized by replacing one of the imide units with two nitro groups to yield 3,6-dinitronaphthalene monoimide (NMI(NO2)2); likewise, 3,6-dimethoxycarbazole (CBZ(OMe)2) was designed as a 3-ring aromatic donor. Different derivatives of naphthalene monoimide (NMI) and carbazole (CBZ) were prepared, and charge-transfer complex formation between them was studied using NMR and UV-visible titrations; the estimated association constants (Ka) were compared with the common pair, NDI, and 1,5-dialkoxy naphthalene (DAN). To rationalize the variation of Ka values, the electrostatic potential maps and the energies of highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) of different derivatives were computed by using density functional theory. Some of the new D-A pairs outperform the DAN-NDI pair; as expected, the best pair was NMI(NO2)2 and CBZ(OMe)2. The Ka for this pair was 50 M-1, which is slightly higher than that of NDI-DAN but with the advantage that a single reactive handle, like azide, can be easily incorporated and used to click them onto supramolecular motifs or polymers. To illustrate this feature, a low Tg copolymer of n-butyl acrylate and propargyl acrylate (10 mol %) was clicked with the azido-derivatives of the new donors and acceptors; the influence of the charge-transfer interaction on the mechanical properties of blends of these copolymers carrying D and A units was examined using nanoindentation studies. Additionally, the propargyl acrylate copolymers were also coclicked with both D and A units, and their properties were examined as a function of the density of interacting sites. In summary, this study illustrates an alternate design strategy for aromatic donors and acceptors that are easier to synthesize, scale-up, and derivatize; importantly, they can be readily installed onto different motifs for the efficient design of supramolecular materials and polymers.

A TEMPO-N3 Complex Enables the Electrochemical C-H Azidation of N-Heterocycles through the Cleavage of Alkoxyamines.

A TEMPO-N3 charge-transfer complex enables the electrochemical C-H azidation of various N-heterocycles. The TEMPO+ ion, generated from TEMPO, assists in producing N3• by forming a TEMPO-N3 complex with N3-. The formation of this complex is supported by UV-vis absorption spectra, cyclic voltammetry studies, and ESI-HRMS studies. The reaction likely proceeds by forming a highly labile azidooxygenation adduct, which undergoes oxidative alkoxyamine mesolytic cleavage. Subsequent deprotonation of the resulting carbocation exclusively produces the azidation product. It is important to note that substituted olefins generally yield azidooxygenation or diazidation as the final product. Our study demonstrates that N-heterocycles deliver a selective monoazidation product, possibly due to steric reasons. ESI-HRMS studies provide evidence for forming azidooxygenation and alkoxyamine radical cation adducts. The regio- and chemoselectivity of this azidation reaction using the TEMPO-N3 complex have been discussed.

Lewis Acidic Ionic-Liquid-Catalyzed Radical-Cascade Alkylation/Cyclization of N-Alkyl-N-methacryloyl Benzamides with Alkanes via Visible-Light-Induced Ligand-to-Metal Charge Transfer: Access to Alkylated Isoquinoline-1,3(2H,4H)-diones.

With the flourishing progress of ligand-to-metal charge transfer (LMCT) photocatalysis, various metals were developed as catalysts to activate abundant alkane feedstocks for the synthesis of functionalized organic compounds. However, to the best of our knowledge, most of the LMCT catalysts are difficult to recover and reuse for the next cycles. Herein, we report a reusable Lewis acidic ionic liquid (LAIL)-catalyzed radical-cascade alkylation/cyclization of N-alkyl-N-methacryloyl benzamides with unactivated alkanes for the synthesis of alkylated isoquinoline-1,3-(2H,4H)-diketones. The protocol features mild reaction conditions, high atom utilization efficiency, scale-up synthesis, simple operation, and recycling of catalysts.

Chiral Self-Assembly of a Pyrene-Appended Glutamylalanine Dipeptide and Its Charge Transfer Complex: Fabrication of Magneto-Responsive Hydrogels and Human Cell Imaging.

The formation of a robust, self-healing hydrogel of a novel pyrene-appended dipeptide, Py-E-A (L-Glutamic acid short as E; L-Alanine short as A) is demonstrated. Detailed studies suggest that nanoscopic fibers with a length of several micrometers have formed by chiral self-organization of Py-E-A gelators. Additionally, live human PBMCs imaging is shown using the Py-E-A fluorophore. Interestingly, electron-rich Py-E-A couples with electron-deficient NDI-β-A (β-Alanine short as β-A) by charge transfer (CT) complexation and forms stable deep violet-colored CT super-hydrogel. X-ray diffraction, DFT, and 2D ROESY NMR studies suggest lamellar packing of both Py-E-A and the alternating CT stack in its hydrogel matrixes. Supramolecular chirality of the Py-E-A donor can be altered by adding an achiral acceptor NDI-β-A. Notably, the fibers of the CT hydrogel are found to be even thinner than the Py-E-A fibers, which, in turn, makes the CT hydrogel more tolerant to the applied strain. Further, the self-healing and injectable properties of the hydrogels are shown. Finally, the magneto-responsive behavior of the Py-E-A and CT hydrogels loaded with spin-canted Cu-ferrite (Cu0.6Zn0.4Fe2O4) nanoparticles (NPs) is demonstrated. The presence of magnetic NPs within the hydrogels has changed the fibrous morphology to rod-like nanoclusters.

Isoreticular Squaraine-Linked Titanium-Organic Frameworks for Photocatalytic Water Splitting to Hydrogen Under Visible Light.

Inspired by the excellent photocatalytic activity of TiO2, titanium metal-organic frameworks (Ti-MOFs) with broad absorption of visible light are regarded as promising photocatalysts, but carboxylate-linkers used in them are mainly limited to the large extended π-electron systems. Developing Ti-MOFs using organic linkers with a donor-acceptor-donor (D-A-D) structure is expected to improve their charge separation but is still challenging. Herein the design of two new isoreticular Ti-MOFs, Ti6-SQ1 and Ti6-SQ2 are reported, by using squaraines bearing different electron donors as organic linkers. Discrete fourier transform (DFT) calculations demonstrate that ligand-to-metal charge transfer (LMCT) from the acceptor units of squaraines to the Ti6-oxo secondary building units (SBUs) drives the photocatalytic water splitting to hydrogen reaction. Compared with Ti6-SQ2, the shorter distance between the squaraine centers and the Ti6-oxo SBUs in Ti6-SQ1 makes stronger LMCT, showing higher photocatalytic hydrogen evolution efficiency of 11.5mmol g-1h-1 under visible light (λ > 420nm), which is ≈8 times that of Ti-based MOF photocatalysts reported so far. This work provides a new strategy to design Ti-MOF photocatalysts and understand their structure-property relationship.

Negative Solvatochromism of the Intramolecular Charge Transfer Band in Two Structurally Related Pyridazinium—Ylids

Negative Solvatochromism of the Intramolecular Charge Transfer Band in Two Structurally Related Pyridazinium—Ylids

Evaluation of the optical and electrical properties of (C8BTBT)(F4TCNQ) charge-transfer complexes according to film formation temperatures during solution-coating

Abstract This study focused on (C8BTBT)(F4TCNQ) charge-transfer complexes, where C8BTBT is 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene and F4TCNQ represents fluorinated derivatives of 7,7,8,8,-tetracyanoquinodimethane. These complexes exhibit excellent thermal and atmospheric stability and near-infrared absorption, serving as transparent thin films for photoelectric conversion. Here, we sought to improve the photoelectric conversion efficiency (and thus the quantum efficiency, QE) of (C8BTBT)(F4TCNQ) thin films by altering the film formation temperatures during solution-coating; we evaluated optical and electrical properties of the films. We found that increased film coverage of a substrate surface (leading to continuous film formation) enhanced optical absorption and improved the photocurrent extraction efficiency when such films were incorporated into devices. C8BTBT and its components, when present alone, negatively affected both the device fabrication yield and photocurrent extraction. It is thus important to maximize (C8BTBT)(F4TCNQ) formation when seeking to improve the QE.

Photocatalytic Reactions over TiO2-Based Interfacial Charge Transfer Complexes

The present review is related to the novel approach for improvement of the optical properties of wide bandgap metal oxides, in particular TiO2, based on the formation of the inorganic–organic hybrids that display absorption in the visible spectral range due to the formation of interfacial charge transfer (ICT) complexes. We outlined the property requirements of TiO2-based ICT complexes for efficient photo-induced catalytic reactions, emphasizing the simplicity of the synthetic procedure, the possibility of the fine-tuning of the optical properties supported by the density functional theory (DFT) calculations, and the formation of a covalent linkage between the inorganic and organic components of hybrids, i.e., the nature of the interface. In addition, this study provides a comprehensive insight into the potential applications of TiO2-based ICT complexes in photo-driven catalytic reactions (water splitting and degradation of organic molecules), including the identification of the reactive species that participate in photocatalytic reactions by the spin-trapping electron paramagnetic resonance (EPR) technique. Considering the practically limitless number of combinations between the inorganic and organic components capable of forming oxide-based ICT complexes and with the knowledge that this research area is unexplored, we are confident it is worth studying, and we emphasized some further perspectives.

Unraveling the Mechanisms of the Formations and Transformations of Metal-Ligand Charge Transfer States in [Ru(tpy)2]2+*: Consequences of Jahn-Teller Conical Intersections and the Pseudo-Jahn-Teller Effect.

This work investigates Jahn-Teller conical intersections (CoIns) and the pseudo-Jahn-Teller effect on the formations and transformations of the low-lying singlet metal-ligand charge transfer (1MLCT) excited states during the ultrafast evolution process of photoexcited [Ru(tpy)2]2+* (tpy = 2,2':6',2″-terpyridine). Longuet-Higgins' geometric phase analyses indicate that the potential energy surface (PES) crossing between charge transfer states 1MLCT1 and 1MLCT2 is a CoIn, originating from the change in diabatic Hamiltonian matrix elements around the CoIn. Moreover, an E⊗(b1 + b2) Jahn-Teller distortion can occur around the Franck-Condon and minimal energy CoIn (MECI) configurations, causing the molecule to distort spontaneously from the high-symmetry D2d configuration to C2v symmetry configurations that are close to it. Furthermore, the pseudo-Jahn-Teller effect can cause the molecule to distort further from C2v to C1 geometries since the former is a second-order saddle point on the whole dimensional PES but the latter is a true minimum. Eight minima in total are symmetrically distributed around the MECI. These minima are connected by the interligand electron transfer, the charge transfer, and the butterfly-like conformational inversion reactions, all of which have extremely small energy barriers.

Origin of the intermolecular forces that produce donor–acceptor stacks in π-conjugated charge-transfer complexes

The attraction between π-conjugated planar electron donor and acceptor molecules that form many stable charge-transfer (CT) complexes has been explained by quantum chemical CT interactions, although the fundamental origin remains unclear. Here, we demonstrate the mechanism of CT complex formation by potential energy map analysis for TTF–CA and BTBT–TCNQ, using energy decomposition of intermolecular interaction by symmetry-adapted perturbation theory (SAPT) combined with coupled cluster calculation. We find that the source of attraction between donor and acceptor molecules is ascribed primarily to the dispersion force and also to the electrostatic force. In contrast, the contribution of CT interactions to the attractive forces is minimal. We demonstrate that the highly directional feature of the exchange repulsion force, coupled with the attractive dispersion and electrostatic forces, is crucial in determining the intermolecular arrangements of actual CT crystals. These findings are key for understanding the unique structural and electronic properties of π-conjugated CT complexes.

Enabling Visible Light Sensitization of YbIII, NdIII and ErIII in Dimeric LnIII/GaIII Metallacrowns through Functionalization with RuII Complexes for NIR‐II Multiplex Imaging

AbstractMultiplex imaging in the second near‐infrared window (NIR‐II, 1000–1700 nm) provides exciting opportunities for more precise understanding of biological processes and more accurate diagnosis of diseases by enabling real‐time acquisition of images with improved contrast and spatial resolution in deeper tissues. Today, the number of imaging agents suitable for this modality remains very scarce. In this work, we have synthesized and fully characterized, including theoretical calculations, a series of dimeric LnIII/GaIII metallacrowns bearing RuII polypyridyl complexes, LnRu‐3 (Ln=YIII, YbIII, NdIII, ErIII). Relaxed structures of YRu‐3 in the ground and the excited electronic states have been calculated using dispersion‐corrected density functional theory methods. Detailed photophysical studies of LnRu‐3 have demonstrated that characteristic emission signals of YbIII, NdIII and ErIII in the NIR‐II range can be sensitized upon excitation in the visible range through RuII‐centered metal‐to‐ligand charge transfer (MLCT) states. We have also showed that these NIR‐II signals are unambiguously detected in an imaging experiment using capillaries and biological tissue‐mimicking phantoms. This work opens unprecedented perspectives for NIR‐II multiplex imaging using LnIII‐based molecular compounds.

Enabling Visible Light Sensitization of YbIII, NdIII and ErIII in Dimeric LnIII/GaIII Metallacrowns through Functionalization with RuII Complexes for NIR-II Multiplex Imaging.

Multiplex imaging in the second near-infrared window (NIR-II, 1000-1700 nm) provides exciting opportunities for more precise understanding of biological processes and more accurate diagnosis of diseases by enabling real-time acquisition of images with improved contrast and spatial resolution in deeper tissues. Today, the number of imaging agents suitable for this modality remains very scarce. In this work, we have synthesized and fully characterized, including theoretical calculations, a series of dimeric LnIII/GaIII metallacrowns bearing RuII polypyridyl complexes, LnRu-3 (Ln=YIII, YbIII, NdIII, ErIII). Relaxed structures of YRu-3 in the ground and the excited electronic states have been calculated using dispersion-corrected density functional theory methods. Detailed photophysical studies of LnRu-3 have demonstrated that characteristic emission signals of YbIII, NdIII and ErIII in the NIR-II range can be sensitized upon excitation in the visible range through RuII-centered metal-to-ligand charge transfer (MLCT) states. We have also showed that these NIR-II signals are unambiguously detected in an imaging experiment using capillaries and biological tissue-mimicking phantoms. This work opens unprecedented perspectives for NIR-II multiplex imaging using LnIII-based molecular compounds.

Impact of electron acceptors on mechanochromism in carbazole-based charge transfer complexes

The synthesis of a dicarbazole derivative (C4C) with a flexible alkyl chain was conducted, and its charge transfer complexes (CTCs) were prepared by incorporating three acceptors (3,4,5,6-tetrafluorophthalonitrile (TFPN), 2,3,5,6-tetrafluoroterephthalonitrile (TFTPN), 1,2,4,5-tetracyanobenzene (TCNB)), aiming to investigate the influence of electron acceptors on their mechanofluorochromism (MFC). It was observed that CTC3 containing TCNB displayed the longest emission wavelength due to the lowest LUMO energy level of TCNB. Interestingly, while CTC1 with TFPN emitted green fluorescence, CTC2 incorporating TFTPN showed a shorter emission wavelength than that of CTC1 although TFTPN had a lower LUMO energy level than TFPN. Single-crystal structural analysis revealed that loose packing between donor and acceptor in CTC2 contributed to its shorter emission wavelength. Notably, CTC2 exhibited MFC because of enhanced charge transfer interaction after grinding. However, no MFC behaviors were observed for the other two complexes. These findings highlight that electron acceptors not only impact photoluminescent properties but also strongly influence response towards mechanical forces.

Chromium in Visible‐Light Photocatalysis: Unique Reactivity, Mechanisms and Future Directions

AbstractVisible‐light photocatalysis has emerged as a prominent research area in modern organic synthesis and environmental science. As an important transition metal, chromium has garnered widespread attention in the field of visible‐light photocatalysis in recent years, primarily manifested in the following aspects: a) the unique photochemical properties of chromium(III) complexes endow them with longer excited‐state lifetimes and higher reactivities under certain conditions; b) by harnessing visible light to induce single‐electron transfer or hydrogen atom transfer to generate radicals, which subsequently form alkyl‐chromium(III) intermediates with chromium(II) ligands, these intermediates can selectively attack electron‐deficient carbonyl compounds, enabling the construction of target products; c) following metals like cerium, copper, iron, nickel, cobalt, titanium, and bismuth, the ligand‐to‐metal charge transfer (LMCT) reaction pathways in chromium photocatalysis have also been extensively investigated. This review will provide a comprehensive summary of recent research on chromium‐mediated photocatalytic reactions, offering an in‐depth exploration of their unique reactivity, mechanisms, and future directions.

Photostimulated Anticancer Activity of Mitochondria Localized Rhenium(I) Tricarbonyl Complexes Bearing 1H-imidazo[4,5-f][1,10]phenanthroline Ligands Against MDA-MB-231 Cancer Cells.

We have introduced Re(I) tricarbonyl complexes (ReL1 - ReL6) [Re(CO)3(N^N)Cl] where N^N=extensive π conjugated imidazo-[4,5-f][1,10]-phenanthroline derivatives that helps in strong DNA intercalation, enhanced photophysical behavior, increase the 3π-π* character of T1 state for PDT and high value of lipophilicity for cell membrane penetration. These complexes exhibited prominent intraligand/ligand-centered (π-π*/1LC) absorption bands at λ 260-350 nm and relatively weak metal-to-ligand charge-transfer (1MLCT) bands within the λ 350-550 nm range. Among the six synthesized complexes, [(CO)3ReICl(K2-N,N-2-(4-(1-benzyl-1H-tetrazol-5-yl)phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline] (ReL6) exhibited outstanding potency (IC50~6 μM, PI>9) under yellow light irradiation compared to dark conditions. Importantly, extremely lipophilic complex ReL6 showed effective penetration through the cell membrane and localized primarily in mitochondria (Pearson's correlation coefficient, PCC=0.918) of MDA-MB-231 cells. Complex ReL6 exhibited more than 9 times higher photo-toxicity in normoxic and hypoxic environment of tumor by inducing 1O2 generation (type II PDT), radical generation triggered by NADH oxidation (type I PDT). This complex is a promising candidate for TNBC treatment in hypoxic tumors, with efficacy comparable to photofrin and have demonstrated CO release ability under UV light irradiation.

Dramatic enhancement in lithium-ion battery capacity through synergistic effects of electronic transitions in light-assisted organic coordination cathode material Co(bpy)(dhbq)2

Integration of photoactive and lithium storing units into a single cathode endows it with notable capacity in the presence of suitable light. However, the interfacial effect between the two materials causes significant loss of photogenerated electrons during their transfer which is one of the biggest obstacles in the development of current photo-assisted rechargeable batteries. In this paper, a bifunctional cobalt-coordinated organic cathode is fabricated by combining 2,2′-bpy and DHBQ via Co by adopting the spin evaporation technique. It optimizes the pristine interfaces of photoactive and lithium storage units into a photoactive unit-metal interface and a metal-lithium storage unit interface through application of Co. In the presence of light, Co causes a strong metal-ligand charge transfer. Meanwhile, ligand-ligand charge transfer also takes place between the multi-ligands. The synergistic effect of these two phenomena offers a discharge capacity of 387 mAh g -1 which is significantly higher than that of 316 mAh g -1 recorded in the absence of light. The demonstrated design of bifunctional metal-ligand cathode by incorporation of photoactive ligands into lithium storage ligands through applications of metal centers can open the pathways for establishing a new type of photo-assisted lithium-ion batteries with higher efficiency and lower cost.

Optical and surface properties of Schiff base ligands and Cu(II) and Co(II) complexes

Objectives. To study the transition of electrons in 1,2-phenyl(4’-carboxy)benzylidene Schiff base ligand and transition metal ions, optical properties, as well as the surface chemistry of supported transition metals using diffuse reflectance spectroscopy (DRS); to study the roughness and morphology of the Schiff base ligand and its complexes using atomic force microscopy (AFM).Methods. DRS, AFM, and Fourier-transform infrared spectroscopy instruments were used to identify electron transitions, optical properties, and surface morphology in Schiff base ligands and their complexes.Results. The DRS revealed the d–d transitions and charge transfer shifts of all compounds, and helped identify the structure of the ligand. One of the optical properties studied was the energy gap calculation of the ligand and its complexes. The copper complex exhibited more semiconducting behavior with surface morphology properties such as surface roughness parameters lower than those of the ligand and the cobalt complex. This can be attributed to the smaller size of the copper atom, as well as lower electron transitions compared to the cobalt complex and the square planar bonding shape.Conclusions. In Schiff base ligands, the reflectance spectrum bands reveal three electron transitions: n→π*, π→π*, and σ→σ* transitions. In cobalt complexes, four transitions are indicated: 4A2(F)→4T1(F), 4A2(F)→4T1(P), charge transfer bands, and tetrahedral geometry. Copper complexes exhibit three transitions: 2B1g→2A1g, 2B1g→2Eg, and charge transfer bands, with a square planar geometry for their structure. The energy gap calculations were 2.42, 2.29, and 2.30 eV, respectively. In the case of the SH ligands, copper complexes, and cobalt complexes, all compounds exhibited semiconductor properties. However, the complexes displayed increased conductivity due to the influence of the metal and coordination structure.

An Increase in the Rigidity of the Environment Favors MLCT over the MC State in [Ru(bpy)2(Nicotine)2](Cl)2: A Case Study of Photolabile Ligands.

Ru(II)-complexes with photolabile ligands find a wide range of applications, e.g., in drug release and in the design of light-responsive interfaces. While light-driven ligand loss has been studied mechanistically in detail for complexes in solution, comparably few studies are present that investigate the process in a material context, i.e., in a rigid environment and in the absence of solvent. This paper adds to this underrepresented perspective by studying the excited-state dynamics of [Ru(bpy)2(nicotine)2] (Cl)2 (Ru-nico) as a model system in poly(methyl methacrylate) (PMMA) and polyacrylonitrile (PAN) matrices. Femtosecond transient absorption spectroscopy and time-resolved emission spectroscopy are employed to monitor the photodissociation of labile nicotine ligands in polymer environments. Photoexcitation within the metal-to-ligand charge transfer (MLCT) band leads to transient dissociation of the nicotine ligand when the complex is dissolved in water. However, optical excitation of the 1MLCT transition of the complexes embedded in polymer matrices does not result in photodissociation, likely due to the rigidity of the environment, which cannot solvate the undercoordinated complex after ligand dissociation and the dissociated ligand. These insights shed light on the role of the local environment when considering the photophysics of ligand loss from Ru(II)-polypyridyl complexes and, hence, their use in the light-activation of reactive molecular components in materials.

Chromogenic Chemosensor for Simultaneous Detection of PO4 3- and CO3 2- Anions in Organo-Aqueous Solutions: Application in Arduino Based Electronic Color Sensor Device and Logic Gate.

Three new chromogenic receptors have been synthesized with the primary objective of facilitating the selective recognition of PO4 3-and CO3 2- ions in an organo-aqueous medium. R1 and R2 exhibit an extraordinary detection limit aligning with both EPA and WHO guidelines. R1 shows LOD of 0.135 ppm for PO4 3- and 0.175 ppm for CO3 2-, while R2 sets forth a LOD of 0.427 ppm for PO4 3- and 0.729 ppm for CO3 2-. The binding mechanism involves intramolecular charge transfer (ICT) band are substantiated by comprehensive studies that include UV-Vis titration, 1H-NMR titration, DFT studies and electrochemical studies. Chemosensors were employed in the formulation of logic gate, the fabrication of a paper strip test kit and its application in RGB color sensor device.