Molecular Complexes of Crown Ethers, Part 5: Complexes of DB18C6 with Some Acceptors

The UV-Visible spectra of DB18C6 as a donor with TCNE (Tetracyanoethylene), and DDQ (2,3-dichloro-5,6-dicyano-1,4- benzoquinone) as acceptors were studied. Charge transfer spectra were obtained for these systems from which it was possible to calculate the formation constant, Kc. The effects of potassium halides were studied. This study shows that in the presence of the anion there is an electron transfer from the anion to the acceptor. This process is enhanced by the presence of the crown ether, CE. The formation of the anion salt with the acceptor, in the presence of the CE, follows the trend F = I > Br > Cl. It is also indicated in this study that the interaction between DB18C6 and the acceptor follow the trend DDQ > TCNE.     

取代酚及其酚钾与四氰基乙烯的电荷转移络合作用

本文以八种取代酚及相应的取代酚钾作为电子给体,以四氰基乙烯(TCNE)作为电子受体,在乙腈溶液中用紫外可见分光光度法测定了取代酚及相应的取代酚钾与TCNE作用的电荷转移络合物的光谱,以其CT光谱用比较法计算了取代酚及相应的取代酚钾的电离势。结果表明,除4-氰基苯酚外,其余取代酚及取代酚钾均可与TCNE形成电荷转移络合物,取代酚钾比相应的取代酚有更好的给电子能力,取代酚的电子转移能与Hammet取代基常数有较好的线性相关关系。     

Charge transfer complexes between 4,4′-disubstituted diphenyldiacetylenes: experimental and theoretical study

Novel charge transfer (CT) complexes containing donor and acceptor derivatives of diphenyldiacetylene have been synthesised and characterised. The structure of CT complexes was modelled at the B3LYP/6-31G(d)//B3LYP/6-31G(d) level of theory. It was found that the complex formation is mainly due to dipole–dipole interaction between side groups of diacetylene molecules and there was no significant charge transfer between donor and acceptor in the ground state. On the other hand, optical excitation of CT complexes leads to strong charge transfer from donor to acceptor molecule as followed from the modelling using time-dependent density functional theory (DFT) method. Diacetylene molecules adopt strongly bent configuration in CT complexes which is prohibitive for solid-state topochemical polymerisation of diacetylenes     

吩噻嗪衍生物的分子内电荷转移激发态的特性

A series of N-bonded donor-acceptor derivatives of phenothiazine containing benzene (PHPZ), anisole (ANPZ), pyridine (PYPZ), naphthalene (NAPZ), acetophenone (PEPZ), and benzonitrile (BNPZ) as an electron acceptor was synthesized. Their photophysical properties were investigated in solvents of different polarities by absorption and emission techniques. These studies clearly reveals the existence of an intramolecular charge transfer (ICT) excited state in the latter four compounds. The solvent dependent Stokes shift values were analyzed by the modified Lippert-Mataga equation to obtain the excited state dipole moment values. The large excited state dipole moment suggests that the full electron transfer takes place in the A-D systems. The obtained values of redox potentials indicate that both subunits of all the A-D molecules studied interact very weakly in the ground states. The results obtained from the analysis of the CT fluorescence spectra confirm that the small conformational changes accompanying excited state charge transfer, the twist angle between the donor and acceptor moieties in the excited 1CT state seems to be similar to that in the ground state.     

Study on the fluorescence resonance energy transfer between CdTe QDs and butyl-rhodamine B in the presence of CTMAB and its application on the detection of Hg(II)

The thioglycolic acid-functionalized CdTe quantum dots (QDs) were synthesized in aqueous solution using safe and low-cost inorganic salts as precursors. Fluorescence resonance energy transfer (FRET) system was constructed between CdTe QDs (donor) and butyl-rhodamine B (BRB) (acceptor) in the presence of cetyltrimethylammonium bromide (CTMAB). CTMAB micelles formed in water reduced the distance between the donor and the acceptor significantly and thus improved the FRET efficiency, which resulted in an obvious fluorescence enhancement of the acceptor. Several factors which impacted the fluorescence spectra of the FRET system were studied. The energy transfer efficiency (E) and the distance (r) between CdTe and BRB were obtained. The feasibility of the prepared FRET system as fluorescence probe for detecting Hg(II) in aqueous solution was demonstrated. At pH 6.60, a linear relationship could be established between the quenched fluorescence intensity of BRB and the concentration of Hg(II) in the range of 0.0625-2.5mumolL(-1). The limit of detection was 20.3nmolL(-1). The developed method was proved to be sensitive and repeatable to detect Hg(II) in a wide range in aqueous solutions.     

Peculiarity of Hexamethylenetetratellurafulvalene (HMTTeF) Charge Transfer Complexes of Donor-Acceptor (D-A) Type

The complex formation of hexamethylenetetratellurafulvalene (HMTTeF) with 28 kinds of organic electron acceptors yielded 31 charge transfer (CT) complexes. The infrared and ultra-violet-visible-near-infrared spectra of the complexes were examined to study the ionicity of their ground states in solid. A plot of CT transition energies and the difference of redox potentials; ΔE(DA) of donor (D) and acceptor (A) molecules indicated that four complexes have a neutral ground state. Four other complexes exhibit characteristic features of a fully ionic ground state based on the vibrational spectra. Notably, the HCBD, F₄TCNQ and DDQ complexes indicate both a relatively low first CT band and high conductivity in a solid in spite of the fully ionic character being very plausible. Twenty-three complexes having a partially ionic ground state have a CT band below 4×10 cm⁻¹ and are highly conductive. The preparation of good single crystals of the HMTTeF complexes for structural analysis was only successful with Et₂TCNQ and BTDA-TCNQ, which have the structure of DA alternately stacking. These two complexes indicate high conductivities in spite of their disadvantageous packing manner. The intermolecular interactions are found to be strongly enhanced by both the bulky molecular orbital of HMTTeF and the decreased on-site Coulomb repulsion in the HMTTeF complexes. These two factors in particular seem to prevent both the fully ionic and the DA alternating HMTTeF complexes from becoming insulators, even though the redox parameters and the crystal structures predict them to be insulating.     

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.     

Spectroscopic,thermal studies and molecular modelling of charge transfer complexation between 5-amino-2-methoxypyridine with chloranilic acid in different polar solvents

Charge transfer (CT) interaction between 5-amino-2-methoxypyridine (5AMPy), as electron donor (proton acceptor), with 3,6-dichloro-2,5-dihydroxy-p-benzoquinone (chloranilic acid, H₂CA), as electron acceptor (proton donor), has been investigated spectrophotometrically in the polar protic solvents ethanol (EtOH) and methanol (MeOH) and the aprotic one acetonitrile (AN). Pink-coloured solution is formed instantaneously upon mixing 5AMPy with H₂CA solutions in all solvents, which is the hallmark evidence of CT complex formation. Based on Job’s method of continuous variations, as well as spectrophotometric titrations, the stoichiometry of the complex was found to be 1:1 [(5AMPy) (H₂CA)] in all solvents. Benesi–Hildebrand equation has been applied to estimate the formation constant of the produced CT complex (K_CT) and its molar absorptivity (ε), they reached high values, confirming the complex high stability. Solid CT complex has been synthesised and analysed by elemental analyses and FTIR, ¹H NMR spectroscopies, where 2:1 [(5AMPy)₂ (H₂CA)] CT complex was obtained.     

Enhanced Circularly Polarized Luminescence in Emissive Charge‐Transfer Complexes

Achieving a large dissymmetry factor (g_lum) is a challenge in the field of circularly polarized luminescence (CPL). A chiral charge‐transfer (CT) system consisting of chiral electron donor and achiral electron acceptor shows bright circularly polarized emission with large g_lum value. The chiral emissive CT complexes could be fabricated through various approaches, such as grinding, crystallization, spin coating, and gelatinization, by simply blending chiral donor and achiral acceptor. The structural synergy originating from π–π stacking and strong CT interactions resulted in the long‐range ordered self‐assembly, enabling the formation of supramolecular gels. Benefiting from the large magnetic dipole transition moment in the CT state, the CPL activity of CT complexes exhibited large circular polarization. Our design strategy of the chiral emissive CT complexes is expected to help the development of new molecular engineering strategies for designing highly efficient CPL‐active materials.     

A novel fluorescent sensor for Sn4+ detection: Dark resonance energy transfer from silole to rhodamine

A novel dark resonance energy transfer (DRET) off–on cassette SR1 was constructed by coupling a silole donor with a rhodamine acceptor. Due to the intramolecular rotations of the phenyl rings, the silole fluorophore served as a dark donor in solution state and fluorescence leakage from the donor emission could be avoided. Binding with Sn⁴⁺ ion induced the ring‐opening of the rhodamine acceptor, thus increase the overlapping between the emission spectra of the donor and absorption spectra of the acceptor. DRET was turned on and energy was transferred from the silole donor to the rhodamine acceptor. Emission from the rhodamine acceptor was achieved with a large Stokes shift up to 198 nm. The sensor showed good sensitivity and selectivity towards Sn⁴⁺ to other metal ions in methanol aqueous solution through the formation of a 1:1 complex between SR1 and Sn⁴⁺. This research provides a new approach for the development of rhodamine‐based sensors towards metal ions with large Stokes shifts.     

Anion-Coordination-Assisted Assembly of Supramolecular Charge-Transfer Complexes Based on Tris(urea) Ligands

Charge-transfer (CT) complexes, formed by noncovalent bonding between electron-rich (donor, D) and electron-deficient (acceptor, A) molecules (or moieties) have attracted considerable attention due to their fascinating structures and potential applications. Herein, we demonstrate that anion coordination is a promising strategy to promote CT complex formation between anion-binding, electron-rich tris(urea) donor ligands (D) and electron-deficient viologen cation acceptors (A), which form co-crystals featuring infinite ⋅⋅⋅DADA⋅⋅⋅ or discrete (circular DADA or three-decker DAD) π-stacking interactions. These CT complexes were studied by X-ray diffraction, UV/Vis spectroscopy, electric conductivity measurements, charge displacement curve (CDC) calculations, and DFT computations.     

Energy barriers of proton transfer reactions between amino acid side chain analogs and water from ab initio calculations

Proton transfer reactions were studied in all titratable pairs of amino acid side chains where, under physiologically reasonable conditions, one amino acid may function as a donor and the other one as an acceptor. Energy barriers for shifting the proton from donor to acceptor atom were calculated by electronic structure methods at the MP2/6-31++G(d,p) level, and the well-known double-well potentials were characterized. The energy difference between both minima can be expressed by a parabola using as argument the donor-acceptor distance R(DA). In this work, the fit parameters of the quadratic expression are determined for each donor-acceptor pair. Moreover, it was found previously that the energy barriers of the reactions can be expressed by an analytical expression depending on the distance between donor and acceptor and the energy difference between donor and acceptor bound states. The validity of this approach is supported by the extensive new data set. This new parameterization of proton transfer barriers between titratable amino acid side chains allows us to very efficiently estimate proton transfer probabilities in molecular modelling studies or during classical molecular dynamics simulation of biomolecular systems.     

Synthesis, characterization, spectrophotometric, structural and antimicrobial studies of the newly charge transfer complex of p-phenylenediamine with π acceptor picric acid

Charge transfer complex (CTC) of donor, p-phenylenediamine (PPD) and acceptor, 2,4,6-trinitrophenol (picric acid) has been studied in methanol at room temperature. The CT complex was synthesized and characterized by elemental analysis, FTIR spectra, 1H NMR spectroscopy and electronic absorption spectra which indicate the CT interaction associated with proton migration from the acceptor to the donor followed by hydrogen bonding via N+-H?O-. The thermal stability of CT complex was studied using TGA and DTA analyses techniques. The CT complex was screened for its antifungal activity against Aspergillus niger (Laboratory isolate), Candida albicans (IQA-109) and Penicillium sp. (Laboratory isolate) and antibacterial activity against two Gram-positive bacteria Staphylococcus aureus (MSSA 22) and Bacillus subtilis (ATCC 6051) and two Gram-negative bacteria Escherichia coli (K 12) and Pseudomonas aeruginosa (MTCC 2488). It gives good antimicrobial activity. The stoichiometry of the CT complex was found to be 1:1. The physical parameters of CT complex were evaluated by the Benesi-Hildebrand equation. On the basis of the studies, the structure of CT complex is [(PPDH)+(PA)-], and a general mechanism for its formation is proposed.     

Symmetry Effects in Photoinduced Electron Transfer in Chlorin-Quinone Dyads: Adiabatic Suppression in the Marcus Inverted Region

In donor–acceptor dyads undergoing photoinduced electron transfer (PET), a direction or pathway for electron movement is usually dictated by the redox properties and the separation distance between the donor and acceptor subunits, while the effect of symmetry is less recognized. We have designed and synthesized two isomeric donor–acceptor assemblies in which electronic coupling between donor and acceptor is altered by the orbital symmetry control with the reorganization energy and charge transfer exothermicity being kept unchanged. Analysis of the optical absorption and luminescence spectra, supported by the DFT and TD-DFT calculations, showed that PET in these assemblies corresponds to the Marcus inverted region (MIR) and has larger rate for isomer with weaker electronic coupling. This surprising observation provides the first experimental evidence for theoretically predicted adiabatic suppression of PET in MIR, which unambiguously controlled solely by symmetry.     

电化学方法用于分子内电荷转移特性的研究

设计合成一系列具有不同取代基的苯甲酰萘（苯）胺衍生和，测定它们在非极性溶剂环己烷中的荧光发射光谱，发现系列合成产物具有双重荧光，其长滤发射具有电荷转移特性，其长波发射态能量与不同取代基的苯甲酰萘（苯）胺衍生物的失（得）电子能力，即给（受）体的氧化还原电位之间符合Weller方程。     

A Periodic Mesoporous Organosilica‐Based Donor–Acceptor System for Photocatalytic Hydrogen Evolution

A new solid‐sate donor–acceptor system based on periodic mesoporous organosilica (PMO) has been constructed. Viologen (Vio) was covalently attached to the framework of a biphenyl (Bp)‐bridged PMO. The diffuse reflectance spectrum showed the formation of charge‐transfer (CT) complexes of Bp in the framework with Vio in the mesochannels. The transient absorption spectra upon excitation of the CT complexes displayed two absorption bands due to radical cations of Bp and Vio species, which indicated electron transfer from Bp to Vio. The absorption bands slowly decayed with a half‐decay period of approximately 10 μs but maintained the spectral shape, thereby suggesting persistent charge separation followed by recombination. To utilize the charge separation for photocatalysis, Vio–Bp–PMO was loaded with platinum and its photocatalytic performance was tested. The catalyst successfully evolved hydrogen with excitation of the CT complexes in the presence of a sacrificial agent. In contrast, reference catalysts without either Bp–PMO or Vio gave no or little hydrogen generation, respectively. In addition, a homogeneous solution system of Bp molecules, methylviologen, and colloidal platinum also evolved no hydrogen, possibly due to a weaker electron‐donating feature of molecular Bp than that of densely packed Bp in Bp–PMO. These results indicated that densely packed Bp and Vio are essential for hydrogen evolution in this system and demonstrated the potential of PMO as the basis for donor–acceptor systems suitable for photocatalysis.     

Charge transfer interactions in polyesters with a donor‐(σ‐bridge)‐acceptor moiety in the repeating unit

Two high molecular weight linear polyesters were investigated to gain insight in how the photophysics of electron donor‐(σ‐spacer)‐electron acceptor (DσA) compounds are affected by incorporation into a polymer. They were prepared by condensation of either adipoyl or sebacoyl chloride with a diol that was functionalized with an N,N‐dialkylaniline donor, a cyclohexyl type σ‐spacer, and a 1,1‐dicyanovinyl acceptor. The solubility, which is very low, and the thermal properties of the polyesters are dictated by physical crosslinking as a consequence of interchain donor‐acceptor interactions. Charge transfer (CT) absorption and emission are observed, which involve CT between DσA moieties of different chains rather than CT processes within a single DσA unit. As a result, the photophysics of the DσA units in the polyesters differs strongly from that of similar DσA compounds in solution. Upon swelling the polymers with THF, the CT fluorescence disappears partly. Analogous polymers containing only an N,N‐dialkylaniline donor display dual fluorescence; one band reflects local emission, while the other is attributed to excimer emission. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4775–4784, 2004     

Spectrophotometric and spectroscopic studies of complexation of 8-hydroxyquinoline with π acceptor metadinitrobenzene in different polar solvents

The complexation of electron donor–acceptor complexes of 8-hydroxyquinoline (8HQ) and metadinitrobenzene (MNB) have been studied spectrophotometrically and thermodynamically in different polar solvent at room temperature. A new absorption band due to charge transfer (CT) transition is observed in the visible region. A new theoretical model has been developed which take into account the interaction between electronic subsystem of 8HQ and MNB. The results indicate the extent of charge transfer complexes (CTCs) formation to be more in less polar solvents. Stoichiometry of the complex was found to be 1:1 by straight line method and ¹H NMR between donor and acceptor at the maximum absorption bands. Ionization potential (I_D) and resonance energy (R_N) were determined from the CT transition energy in different solvents. The formation constants of the complexes were determined in different polar solvents from which ΔG° formation of the complexes was estimated and also extinction coefficient of the charge transfer complex (CTC) was calculated. Oscillator strength, transition dipole strengths and maximum wavelength of the CTC (λ_CT) in various solvents and IR spectra of the CTC have also been discussed. It has been observed that all parameters described above changed with change in polarity and concentration of donor.     

The Charge Transfer Complex between 2, 3-diamino-5-bromopyridine and Chloranilic acid: Preparation,Spectroscopic Characterization,DNA binding,and DFT/PCM analysis

A charge transfer hydrogen bonded complex was prepared and experimentally explored in an acetonitrile (ACN) medium between the proton acceptor (electron donor) 2, 3-Diamino-5-bromopyridine and the proton donor (electron acceptor) chloranilic acid. The stoichiometry of the charge transfer complex is 1:1. The Benesi-Hildebrand equation is used to calculate the molar absorptivity (ε_CT), association constant (K_CT) and other spectroscopic physical characteristics. The solid compound was synthesized and studied using several spectroscopic methods. The presence of charge and proton transfers in the resultant complex was supported by ¹H NMR, FT-IR and SEM-EDX investigations. The complex DNA binding ability was investigated using electron absorption spectroscopy, and the CT complex binding mechanism is intercalative. The intrinsic binding constant (K_b) value is 5.2 × 10⁶M?1. The good binding affinity of the CT complex makes it potentially suitable for usage as a pharmaceutical in the future. Molecular docking calculations have been performed between CT complex and DNA (ID = 1BNA) to study the CT-DNA interaction theoretically. To corroborate the experimental findings, calculations based on DFT were carried out in the gas and PCM analysis where the existence of charge and hydrogen transfers. Finally, good agreement between experimental and theoretical computations was observed confirming that the basis set used is appropriate for the system under examination.     

苯酚钾及对位取代苯酚钾的电离势

在我们以前的研究工作中发现,酚类钾盐是很好的电荷给体,可以和顺丁烯二酸酐、三硝基苯以及醌类等电荷受体生成电荷转移(CT)络合物。表示分子的给电子能力的参数是分子的第一电离势(简称为电离势I_p)。对于苯酚的I_p值,近年来已有文献记载,但苯酚钾