Creation of Ternary Multicomponent Crystals by Exploitation of Charge‐Transfer Interactions

Four new ternary crystalline molecular complexes have been synthesised from a common 3,5‐dinitrobenzoic acid (3,5‐dnda) and 4,4′‐bipyridine (bipy) pairing with a series of amino‐substituted aromatic compounds (4‐aminobenzoic acid (4‐aba), 4‐(N,N‐dimethylamino)benzoic acid (4‐dmaba), 4‐aminosalicylic acid (4‐asa) and sulfanilamide (saa)). The ternary crystals were created through the application of complementary charge transfer and hydrogen‐bonding interactions. For these systems a dimer was created through a charge‐transfer interaction between two of the components, while hydrogen bonding between the third molecule and this dimer completed the construction of the ternary co‐crystal. All resulting structures display the same acid ??? pyridine interaction between 3,5‐dnba and bipy. However, changing the third component causes the proton of this bond to shift from neutral OH ??? N to a salt form, O^? ??? HN⁺, as the nature of the group hydrogen bonding to the carboxylic acid was changed. This highlights the role of the crystal environment on the level of proton transfer and the utility of ternary systems for the study of this process.     

Thermochromism of CuI Tetrakisguanidine Complexes: Reversible Activation of Metal‐to‐Ligand Charge‐Transfer Bands

Tetranuclear, intensely blue‐coloured Cu^I complexes were synthesised in which two Cu₂X₃^? units (X=Br or I) are bridged by a dicationic GFA (guanidino‐functionalised aromatic) ligand. The UV/Vis spectra show a large metal‐to‐ligand charge‐transfer (MLCT) band around 638 nm. The tetranuclear “low‐temperature” complexes are in a temperature‐dependent equilibrium with dinuclear Cu^I “high‐temperature” complexes, which result from the reversible elimination of two CuX groups. A massive thermochromism effect results from the extinction of the strong MLCT band upon CuX elimination with increasing temperature. For all complexes, quantum chemical calculations predict a small and method‐dependent energy difference between the possible electronic structures, namely Cu^I and dicationic GFA ligand (closed‐shell singlet) versus Cu^II and neutral GFA ligand (triplet or broken‐symmetry state). The closed‐shell singlet state is disfavoured by hybrid‐DFT functionals, which mix in exact Hartree–Fock exchange, and is favoured by larger basis sets and consideration of a polar medium.     

Novel Method for the Fabrication of a Charge‐Transfer Complex Crystal by Photoirradiation

A novel method for the fabrication of a charge‐transfer complex crystal was developed. Photoirradiation of a solution of TPP[Co(tbp)(CN)₂] and TPP[Co(Pc)(CN)₂] (tbp=tetrabenzoporphyrin, Pc=phthalocyanine, TPP=tetraphenylphosphonium) gave a molecular conducting crystal of a charge‐transfer complex TPP[Co(tbp)(CN)₂]₂, which was produced by the process in which the photoexcited electron in tbp was transferred from the LUMO of tbp to that of Pc.     

Symmetry‐Breaking Charge‐Transfer Chromophore Interactions Supported by Carbon Nanodots

Carbon dots (CDs) and their derivatives are useful platforms for studying electron‐donor/acceptor interactions and dynamics therein. Herein, we couple amorphous CDs with phthalocyanines (Pcs) that act as electron donors with a large extended π‐surface and intense absorption across the visible range of the solar spectrum. Investigations of the intercomponent interactions by means of steady‐state and pump‐probe transient absorption spectroscopy reveal symmetry‐breaking charge transfer/separation and recombination dynamics within pairs of phthalocyanines. The CDs facilitate the electronic interactions between the phthalocyanines. Thus, our findings suggest that CDs could be used to support electronic couplings in multichromophoric systems and further increase their applicability in organic electronics, photonics, and artificial photosynthesis.     

Supramolecular Assemblies by Charge‐Transfer Interactions between Donor and Acceptor Chromophores

We have collated various supramolecular designs that utilize organic donor–acceptor CT complexation to generate noncovalently co‐assembled structures including fibrillar gels, micelles, vesicles, nanotubes, foldamers, conformationally restricted macromolecules, and liquid crystalline phases. Possibly inspired by nature, chemists have extensively used hydrogen bonding as a tool for supramolecular assemblies of a diverse range of abiotic building blocks. As a structural motif, CT complexes can be compared to hydrogen‐bonded complexes in its directional nature and complementarities. Additional advantages of CT interactions include wider solvent tolerance and easy spectroscopic probing. Nevertheless the major limitation is their low association constant. This article shows different strategies have evolved over the years to overcome this drawback by reinforcing the CT interactions with auxiliary noncovalent forces without hampering the alternate stacking mode. Emerging reports on promising CT complexes in organic electronics are intimately related to various supramolecular designs that one can postulate based on donor–acceptor CT interactions.     

Photomagnetic Switching of Heterometallic Complexes [M(dmf)4(H2O)3(μ‐CN)Fe(CN)5]⋅H2O (M=Nd,La, Gd,Y) Analyzed by Single‐Crystal X‐ray Diffraction and Ab Initio Theory

Single‐crystal X‐ray diffraction measurements have been carried out on [Nd(dmf)₄(H₂O)₃(μ‐CN)Fe(CN)₅]?H₂O ( 1 ; dmf=dimethylformamide), [Nd(dmf)₄(H₂O)₃(μ‐CN)Co(CN)₅]?H₂O ( 2 ), [La(dmf)₄(H₂O)₃(μ‐CN)Fe(CN)₅]?H₂O ( 3 ), [Gd(dmf)₄(H₂O)₃(μ‐CN)Fe(CN)₅]?H₂O ( 4 ), and [Y(dmf)₄(H₂O)₃(μ‐CN)Fe(CN)₅]?H₂O ( 5 ), at 15(2) K with and without UV illumination of the crystals. Significant changes in unit‐cell parameters were observed for all the iron‐containing complexes, whereas 2 showed no response to UV illumination. Photoexcited crystal structures have been determined for 1 , 3 , and 4 based on refinements of two‐conformer models, and excited‐state occupancies of 78.6(1), 84(6), and 86.6(7) % were reached, respectively. Significant bond‐length changes were observed for the Fe–ligand bonds (up to 0.19 Å), the cyano bonds (up to 0.09 Å), and the lanthanide–ligand bonds (up to 0.10 Å). Ab initio theoretical calculations were carried out for the experimental ground‐state geometry of 1 to understand the electronic structure changes upon UV illumination. The calculations suggest that UV illumination gives a charge transfer from the cyano groups on the iron atom to the lanthanide ion moiety, {Nd(dmf)₄(H₂O)₃}, with a distance of approximately 6 Å from the iron atom. The charge transfer is accompanied by a reorganization of the spin state on the {Fe(CN)₆} complex, and a change in geometry that produces a metastable charge‐transfer state with an increased number of unpaired electrons, thus accounting for the observed photomagnetic effect.     

Charge‐Transfer Phase Transition of a Cyanide‐Bridged FeII/FeIII Coordination Polymer

Heterometallic Prussian blue analogues are known to exhibit thermally induced charge transfer, resulting in switching of optical and magnetic properties. However, charge‐transfer phase transitions have not been reported for the simplest FeFe cyanide‐bridged systems. A mixed‐valence Fe^II/Fe^III cyanide‐bridged coordination polymer, {[Fe(Tp)(CN)₃]₂Fe(bpe)?5 H₂O}_n, which demonstrates a thermally induced charge‐transfer phase transition, is described. As a result of the charge transfer during this phase transition, the high‐spin state of the whole system does not change to a low‐spin state. This result is in contrast to FeCo cyanide‐bridged systems that exhibit charge‐transfer‐induced spin transitions.     

Trapping of a Thiolate→Dibromine Charge‐Transfer Adduct by a Macrocyclic Dinickel Complex and Its Conversion into an Arenesulfenyl Bromide Derivative

Stuck on sulfur : The first transition‐metal complexes with S? Br units are surprisingly stable. Solid 3 is stable for at least six months and under vacuum solid 2 does not lose Br₂. The formation of the first structurally characterized transition‐metal arenesulfenyl bromide complex 3 occurs with a change of the spin ground state from S=2 to S=0.

     

Photocurrent Generation through Charge‐Transfer Processes in Noncovalent Perylenediimide/DNA Complexes

The charge‐transfer process in noncovalent perylenediimide (PDI)/DNA complexes has been investigated by using nanosecond laser flash photolysis (LFP) and photocurrent measurements. The PDI/DNA complexes were prepared by inclusion of cationic PDI molecules into the artificial cavities created inside DNA. The LFP experiments showed that placement of the PDI chromophore at a specific site and included within the base stack of DNA led to the efficient generation of a charge‐separated state with a long lifetime by photoexcitation. When two PDI chromophores were separately placed at different positions in DNA, the yield of the charge‐separated state with a long lifetime was dependent upon the number of A–T base pairs between the PDIs, which was explained by electron hopping from one PDI to another. Photocurrent generation of the DNA‐modified electrodes with the complex was also dependent upon the arrangement of the PDI chromophores. A good correlation was obtained between observed charge separation and photocurrent generation on the PDI/DNA‐modified electrodes, which demonstrated the importance of the defined arrangement and assembly of organic chromophores in DNA for efficient charge separation and transfer in multichromophore arrays.     

Fullerol–Titania Charge‐Transfer‐Mediated Photocatalysis Working under Visible Light

The development of visible‐light‐active photocatalysts is being investigated through various approaches. In this study, C₆₀‐based sensitized photocatalysis that works through the charge transfer (CT) mechanism is proposed and tested as a new approach. By employing the water‐soluble fullerol (C₆₀(OH)_x) instead of C₆₀, we demonstrate that the adsorbed fullerol activates TiO₂ under visible‐light irradiation through the “surface–complex CT” mechanism, which is largely absent in the C₆₀/TiO₂ system. Although fullerene and its derivatives have often been utilized in TiO₂‐based photochemical conversion systems as an electron transfer relay, their successful photocatalytic application as a visible‐light sensitizer of TiO₂ is not well established. Fullerol/TiO₂ exhibits marked visible photocatalytic activity not only for the redox conversion of 4‐chlorophenol, I^?, and Cr^VI, but also for H₂ production. The photoelectrode of fullerol/TiO₂ also generates an enhanced anodic photocurrent under visible light as compared with the electrodes of bare TiO₂ and C₆₀/TiO₂, which confirms that the visible‐light‐induced electron transfer from fullerol to TiO₂ is particularly enhanced. The surface complexation of fullerol/TiO₂ induced a visible absorption band around 400–500 nm, which was extinguished when the adsorption of fullerol was inhibited by fluorination of the surface of TiO₂. The transient absorption spectroscopic measurement gave an absorption spectrum ascribed to fullerol radical cations (fullerol^.+) the generation of which should be accompanied by the proposed CT. The theoretical calculation regarding the absorption spectra for the (TiO₂ cluster+fullerol) model also confirmed the proposed CT, which involves excitation from HOMO (fullerol) to LUMO (TiO₂ cluster) as the origin of the visible‐light absorption.     

Surface‐State‐Mediated Charge‐Transfer Dynamics in CdTe/CdSe Core–Shell Quantum Dots

Herein, we report the synthesis of aqueous CdTe/CdSe type‐II core–shell quantum dots (QDs) in which 3‐mercaptopropionic acid is used as the capping agent. The CdTe QDs and CdTe/CdSe core–shell QDs are characterized by X‐ray diffraction (XRD), high‐resolution transmission electron microscopy (HR‐TEM), steady‐state absorption, and emission spectroscopy. A red shift in the steady‐state absorption and emission bands is observed with increasing CdSe shell thickness over CdTe QDs. The XRD pattern indicates that the peaks are shifted to higher angles after growth of the CdSe shell on the CdTe QDs. HR‐TEM images of both CdTe and CdTe/CdSe QDs indicate that the particles are spherical, with a good shape homogeneity, and that the particle size increases by about 2 nm after shell formation. In the time‐resolved emission studies, we observe that the average emission lifetime (τ_av) increases to 23.5 ns for CdTe/CdSe (for the thickest shell) as compared to CdTe QDs (τ_av=12 ns). The twofold increment in the average emission lifetime indicates an efficient charge separation in type‐II CdTe/CdSe core–shell QDs. Transient absorption studies suggest that both the carrier cooling and the charge‐transfer dynamics are affected by the presence of traps in the CdTe QDs and CdTe/CdSe core–shell QDs. Carrier quenching experiments indicate that hole traps strongly affect the carrier cooling dynamics in CdTe/CdSe core–shell QDs.     

On the Ligand‐to‐Metal Charge‐Transfer Photochemistry of the Copper(II) Complexes of Quercetin and Rutin

In contrast to the UV‐photoinduced ligand photoionization of the flavonoid complexes of Fe^III, redox reactions initiated in ligand‐to‐metal charge‐transfer excited states were observed on irradiation of the quercetin ( 1 ) and rutin ( 2 ) complexes of Cu^II. Solutions of complexes with stoichiometries [Cu^IIL₂] (L=quercetin, rutin) and [Cu^II₂L_n] (n=1, L=quercetin; n=3, L=rutin) were flash‐irradiated at 351 nm. Transient spectra observed in these experiments showed the formation of radical ligands corresponding to the one‐electron oxidation of L and the reduction of Cu^II to Cu^I. The radical ligands remained coordinated to the Cu^I centers, and the substitution reactions replacing them by solvent occurred with lifetimes τ<350 ns. These are lifetimes shorter than the known lifetimes (τ>1 ms) of the quercetin and rutin radical's decay.     

Charge‐Transfer Interactions in Tris‐Donor–Tris‐Acceptor Hexaarylbenzene Redox Chromophores

Symmetric‐ and asymmetric hexaarylbenzenes (HABs), each substituted with three electron‐donor triarylamine redox centers and three electron‐acceptor triarylborane redox centers, were synthesized by cobalt‐catalyzed cyclotrimerization, thereby forming compounds with six‐ and four donor–acceptor interactions, respectively. The electrochemical‐ and photophysical properties of these systems were investigated by cyclovoltammetry (CV), as well as by absorption‐ and fluorescence spectroscopy, and compared to a HAB that only contained one neighboring donor–acceptor pair. CV measurements of the asymmetric HAB show three oxidation peaks and three reduction peaks, whose peak‐separation is greatly influenced by the conducting salt, owing to ion‐pairing and shielding effects. Consequently, the peak‐separations cannot be interpreted in terms of the electronic couplings in the generated mixed‐valence species. Transient‐absorption spectra, fluorescence‐solvatochromism, and absorption spectra show that charge‐transfer states from the amine‐ to the boron centers are generated after optical excitation. The electronic donor–acceptor interactions are weak because the charge transfer has to occur predominantly through space. Moreover, the excitation energy of the localized excited charge‐transfer states can be redistributed between the aryl substituents of these multidimensional chromophores within the fluorescence lifetime (about 60 ns). This result was confirmed by steady‐state fluorescence‐anisotropy measurements, which further indicated symmetry‐breaking in the superficially symmetric HAB. Adding fluoride ions causes the boron centers to lose their accepting ability owing to complexation. Consequently, the charge‐transfer character in the donor–acceptor chromophores vanishes, as observed in both the absorption‐ and fluorescence spectra. However, the ability of the boron center as a fluoride sensor is strongly influenced by the moisture content of the solvent, possibly owing to the formation of hydrogen‐bonding interactions between water molecules and the fluoride anions.     

Revealing the Charge‐Transfer Interactions in Self‐Assembled Organic Cocrystals: Two‐Dimensional Photonic Applications

A new crystal of a charge‐transfer (CT) complex was prepared through supramolecular assembly and it has unique two‐dimensional (2D) morphology. The CT nature of the ground and excited states of this new Bpe‐TCNB cocrystal (BTC) were confirmed by electron spin resonance measurements, spectroscopic studies, and theoretical calculations, thus providing a comprehensive understanding of the CT interactions in organic donor–acceptor systems. And the lowest CT₁ excitons are responsible for the efficient photoluminescence (Φ_PL=19 %), which can actively propagate in individual 2D BTCs without anisotropy, thus implying that the optical waveguide property of the crystal is not related to the molecular stacking structure. This unique 2D CT cocrystal exhibits potential for use in functional photonic devices in the next‐generation optoelectronic communications.     

Semiconducting Neutral Microstructures Fabricated by Coordinative Self‐Assembly of Intramolecular Charge‐Transfer Tetrathiafulvalene Derivatives

A new class of tetrathiafulvalene‐based microstructures fabricated by coordinative self‐assembly has been prepared by a solution process. Upon incorporation of Pb²⁺ and Zn²⁺ ions, 1D wirelike microstructures and spherical polymer particles were achieved, respectively (see picture). The neutral coordination polymers are conductive and magnetic at room temperature without external manipulation.

     

Magnesium(II) Complexes with High Emission: The Distinct Charge‐Transfer Process from Transition Metal

Generally, the first‐row transition‐metal complexes are notorious in luminescence materials because of their metal‐ligand charge transfer in emission process. Herein, we rationally used magnesium instead the first‐row transition metal to coordinate with 2‐(anthracen‐9‐yl)‐1H‐imidazo[4,5‐f][1,10]phenanthroline (AIP) in the construction of luminescent complexes. Further investigation revealed AIP could work as detector for quantitative determination of Mg²⁺ cation. Comparing to other divalent cations, this fluorescence sensor exhibited high selectivity for the quantitative determination of Mg²⁺ with the low limit of detection (5 × 10^–7 m ). Through X‐ray single crystal diffraction, the crystal structures of [Mg(AIP₂)(NO₃)₂ · (H₂O)₄] ( 1 ), [Mn(AIP)(NO₃) · EtOH] ( 2 ), and [Co₂(AIP)₂Cl₄ · (MeOH)₂] ( 3 ) were observed in various arrangements. The theory calculations based on crystal structures indicated the Mg^II complex undergoes distinct charge‐transfer process from other transition‐metals based compounds, in which charge‐transfer excited‐state lifetimes were deactivated rapidly through metal‐to‐ligand charge‐transfer (MLCT) process. This study provided insight into construction of luminescence compounds by using d⁰ metals in main groups instead of transition metals.     

Versatile Photophysical Properties of meso‐Aryl‐Substituted Subporphyrins: Dipolar and Octupolar Charge‐Transfer Interactions

Donor–acceptor systems based on subporphyrins with nitro and amino substituents at meta and para positions of the meso‐phenyl groups were synthesized and their photophysical properties have been systematically investigated. These molecules show two types of charge‐transfer interactions, that is, from center to periphery and periphery to center depending on the peripheral substitution, in which the subporphyrin moiety plays a dual role as both donor and acceptor. Based on the solvent‐polarity‐dependent photophysical properties, we have shown that the fluorescence emission of para isomers originates from the solvatochromic, dipolar, symmetry‐broken, and relaxed excited states, whereas the non‐solvatochromic fluorescence of meta isomers is of the octupolar type with false symmetry breaking. The restricted meso‐(4‐aminophenyl) rotation at low temperature prevents the intramolecular charge‐transfer (ICT)‐forming process. The two‐photon absorption (TPA) cross‐section values were determined by photoexcitation at 800 nm in nonpolar toluene and polar acetonitrile solvents to see the effect of ICT on the TPA processes. The large enhancement in the TPA cross‐section value of approximately 3200 GM (1 GM=10^?50 cm⁴ s photon^?1) with donor–acceptor substitution has been attributed to the octupolar effect and ICT interactions. A correlation was found between the electron‐donating/‐withdrawing abilities of the peripheral groups and the TPA cross‐section values, that is, p‐aminophenyl>m‐aminophenyl>nitrophenyl. The increased stability of octupolar ICT interactions in highly polar solvents enhances the TPA cross‐section value by a factor of approximately 2 and 4, respectively, for p‐amino‐ and m‐nitrophenyl‐substituted subporphyrins. On the other hand, the stabilization of the symmetry‐broken, dipolar ICT state gives rise to a negligible impact on the TPA processes.