Extent Of Delocalization Research Articles

A deep insight into the structure-solubility relationship and molecular interaction mechanism of diverse flavonoids in molecular solvents, ionic liquids, and molecular solvent/ionic liquid mixtures

The dissolution behavior and structure-solubility relationships of diverse flavonoids in various solvent systems, including the pure molecular solvents, pure ionic liquids (ILs), and molecular solvent/IL mixtures, are investigated in conjunction with experiments as well as quantum chemical and COSMO-RS calculations. The dissolution of flavonoids is a complex process. The dissolution difference of various flavonoids is ascribed to the synergistic influence of multiple factors, including the structure and planarity of the diverse flavonoid molecules, the extent of delocalization, the type, number, and position of functional groups, properties of the various solvents, and inter/intramolecular interactions. The planarity of flavonoid molecules is usually the most critical of these factors. The degree of nonplanarity of flavonoids follows the order flavanone > isoflavone > flavone > flavonol, which basically coincides with the solubility ranking of diverse flavonoids. Additionally, the type, number, and position of functional groups capable of forming hydrogen bonds also significantly affect flavonoid dissolution in solvents. The dissolution sequence of different flavonoids in various solvents varies with the structure and configuration of flavonoids. Of all the investigated solvents, flavonoid compounds are the least soluble in water because of their hydrophobicity, and they are most easily soluble in ILs with higher HBA basicity due to solute–solvent hydrogen-bond interactions, such as [Bmim]Ac and [Bmim]NO3. Adding ILs into molecular solvents or water has a significantly enhanced solubilization effect for the poorly soluble flavonoid compounds. In addition, water can act as an effective antisolvent, allowing for efficient recovery of flavonoids and ILs.

Syntheses and Electrochemical and EPR Studies of Porphyrins Functionalized with Bulky Aromatic Amine Donors.

A series of nickel(II) porphyrins bearing one or two bulky nitrogen donors at the meso positions were prepared by using Ullmann methodology or more classical Buchwald-Hartwig amination reactions to create the new C-N bonds. For several new compounds, single crystals were obtained, and the X-ray structures were solved. The electrochemical data of these compounds are reported. For a few representative examples, spectroelectrochemical measurements were used to clarify the electron exchange process. In addition, a detailed electron paramagnetic resonance (EPR) study was performed to estimate the extent of delocalization of the generated radical cations. In particular, electron nuclear double resonance spectroscopy (ENDOR) was used to determine the coupling constants. DFT calculations were conducted to corroborate the EPR spectroscopic data.

Exciton transport in molecular organic semiconductors boosted by transient quantum delocalization

Designing molecular materials with very large exciton diffusion lengths would remove some of the intrinsic limitations of present-day organic optoelectronic devices. Yet, the nature of excitons in these materials is still not sufficiently well understood. Here we present Frenkel exciton surface hopping, an efficient method to propagate excitons through truly nano-scale materials by solving the time-dependent Schrödinger equation coupled to nuclear motion. We find a clear correlation between diffusion constant and quantum delocalization of the exciton. In materials featuring some of the highest diffusion lengths to date, e.g. the non-fullerene acceptor Y6, the exciton propagates via a transient delocalization mechanism, reminiscent to what was recently proposed for charge transport. Yet, the extent of delocalization is rather modest, even in Y6, and found to be limited by the relatively large exciton reorganization energy. On this basis we chart out a path for rationally improving exciton transport in organic optoelectronic materials.

DFT, Electronic absorption spectra and Non-linear optical properties of some 4H-benzo[h]chromene Derivatives. Solvent effect and TD-DFT Approach

The electronic structure of some 4H-benzo[h]chromene derivatives are investigated theoretically at the B3LYP/6-311++G (d,p) level of theory. The extent of delocalization and intramolecular charge transfer are estimated and discussed in terms of natural bond orbital analysis (NBO) and second order perturbation interactions between donor and acceptor MOs. The calculated EHOMO and ELUMO energies of the studied compounds can be used to calculate the global properties; chemical hardness (η), softness (S) and electronegativity (χ). The equilibrium geometries of the studied compounds are determined, and it was found that these geometries are non-planar. The calculated nonlinear optical parameters (NLO); polarizability (α), anisotropy of the polarizibility (Δα) and first order hyperpolarizibility (β) of the studied compounds show promising optical properties. The molecular electrostatic potential (MEP) and Local reactivity descriptors (fukui function) have been studied for the title compound and its derivatives at the same level of theory. Electronic spectra of 4H-benzo[h]chromene derivatives in different organic solvents such as 1,4-Dioxane and DiChloroEthane as non-polar solvents and Ethanol, Methanol and Acetonitrile as polar solvents, are investigated experimentally and theoretically using the time dependent density functional theory (TD-DFT) method at the B3LYP/6-311++G(d,p) level of the theory. The origin of the spectrum of the parent compound is found to be an additive one. The solvent effect is investigated experimentally and theoretically and a blue shift is discerned

Superradiance of bacteriochlorophyll c aggregates in chlorosomes of green photosynthetic bacteria

Chlorosomes are the main light-harvesting complexes of green photosynthetic bacteria that are adapted to a phototrophic life at low-light conditions. They contain a large number of bacteriochlorophyll c, d, or e molecules organized in self-assembling aggregates. Tight packing of the pigments results in strong excitonic interactions between the monomers, which leads to a redshift of the absorption spectra and excitation delocalization. Due to the large amount of disorder present in chlorosomes, the extent of delocalization is limited and further decreases in time after excitation. In this work we address the question whether the excitonic interactions between the bacteriochlorophyll c molecules are strong enough to maintain some extent of delocalization even after exciton relaxation. That would manifest itself by collective spontaneous emission, so-called superradiance. We show that despite a very low fluorescence quantum yield and short excited state lifetime, both caused by the aggregation, chlorosomes indeed exhibit superradiance. The emission occurs from states delocalized over at least two molecules. In other words, the dipole strength of the emissive states is larger than for a bacteriochlorophyll c monomer. This represents an important functional mechanism increasing the probability of excitation energy transfer that is vital at low-light conditions. Similar behaviour was observed also in one type of artificial aggregates, and this may be beneficial for their potential use in artificial photosynthesis.

Quantitatively Switchable pH-Sensitive Photoluminescence of Carbon Nanodots.

pH sensing plays a key role in the life sciences as well as the environmental, industrial, and agricultural fields. Carbon nanodots (C-dots) with small size, low toxicity, and excellent stability hold great potential in pH sensing as nanoprobes due to their intrinsic pH-sensitive photoluminescence (PL). Nonetheless, the undesirable sensitivity and response range of C-dot PL toward pH cannot meet the requirements of practical applications, and the unclear pH-sensitive PL mechanism makes it difficult to control their pH sensitivity. Herein, the quantitative correlation of pH-sensitive PL with specific surface structures of C-dots is uncovered for the first time, to our best knowledge. The association of carboxylate and H+ increases the ratio of nonradiation to radiation decay of C-dots through excited-state proton transfer, resulting in the decrease of PL intensity. Meanwhile, the dissociation of α-H in β-dicarbonyl forming enolate increases the extent of delocalization of the C-dots conjugated system, which induces the PL broadening to the red region and a decreasing intensity. Based on the understanding of the pH-sensitive PL mechanism, the pH-sensitive PL of C-dots can be switched by quantitative modulation of carboxyl and β-dicarbonyl groups to achieve a desirable pH response range with high sensitivity. This work contributes to a better understanding of the pH-sensitive PL of C-dots and therefore presents an effective strategy for controllably tuning their pH sensitivity, facilitating the rational design of C-dot-based pH sensors.

An energy decomposition analysis approach to the rotational barriers of amides and thioamides

The most popular interpretation for the origin of rotational barrier in amides is the electron delocalization of the nitrogen lone pair over the NCO π system. Many attempts have been made to quantify the extent of delocalization in amide and amide like systems. An alternative approach to understand rotational barriers is the energy decomposition analysis (EDA) class of methods. In this work, the rotational barrier of a series of amides is investigated using the Canonical Molecular Orbital Energy Decomposition Analysis (CMOEDA) method. Based on CMOEDA, it was possible to explain i) the origin of the rotational barriers, ii) the energy difference between the two transition states (TS1/TS2) and iii) the variation in the rotational barrier as structural modifications are introduced. Regarding the origin of the rotational barrier, for instance, it was verified that electrostatic and polarization interactions are responsible for building the barrier, while exchange-repulsion contributes to decrease its value.

A local perspective on the conjugation of double bonds in acyclic polyenes

The study is devoted to the elaboration of an alternative image of conjugation in acyclic polyenes as a weak and essentially local delocalization of initially-localized pairs of $$\pi$$ -electrons of individual formally-double (C=C) bonds (instead of formation of a completely delocalized $$\pi$$ -electron system as usual). Polyenes are modelled as sets of weakly-interacting C=C bonds, where the single (C–C) bonds represent the interaction. Expressions for the relevant non-canonical (localized) molecular orbitals (NCMOs) are derived directly without any reference to usual (canonical) MOs in the form of power series with respect to the small resonance parameter of C–C bonds. An explicit interdependence is established between the conjugation energy (CE) of polyene and the shapes of its NCMOs. As a result, a local relation follows between the constitution of the nearest environment of a certain C=C bond, delocalization pattern of the respective single pair of $$\pi$$ -electrons and contribution of just this pair to the total CE. A simple additive scheme is deductively suggested to evaluate the CEs of polyenes in terms of essentially local increments easily found by inspection. The actual order of a C=C bond also is related to the extent of delocalization of the only pair of its $$\pi$$ -electrons. Within a certain rough approximation, the approach yields a simplified image of acyclic conjugation in terms of shifts of separate pairs of $$\pi$$ -electrons from C=C bonds to the nearest C–C bonds in accordance with the picture offered by the curly arrow formalism. The results are illustrated by specific examples.

Revisiting the Hole Size in Double Helical DNA with Localized Orbital Scaling Corrections.

The extent of electronic wave function delocalization for the charge carrier (electron or hole) in double helical DNA plays an important role in determining the DNA charge transfer mechanism and kinetics. The size of the charge carrier's wave function delocalization is regulated by the solvation induced localization and the quantum delocalization among the π stacked base pairs at any instant of time. Using a newly developed localized orbital scaling correction (LOSC) density functional theory method, we accurately characterized the quantum delocalization of the hole wave function in double helical B-DNA. This approach can be used to diagnose the extent of delocalization in fluctuating DNA structures. Our studies indicate that the hole state tends to delocalize among 4 guanine-cytosine (GC) base pairs and among 3 adenine-thymine (AT) base pairs when these adjacent bases fluctuate into degeneracy. The relatively small delocalization in AT base pairs is caused by the weaker π-π interaction. This extent of delocalization has significant implications for assessing the role of coherent, incoherent, or flickering coherent carrier transport in DNA.

Effects of Mixed Valency in an Fe-Based Framework: Coexistence of Slow Magnetic Relaxation, Semiconductivity, and Redox Activity.

A 2-D coordination framework, (NEt4)2[Fe2(fan)3] (1·5(acetone); H2fan = 3,6-difluoro-2,5-dihydroxy-1,4-benzoquinone), was synthesized and structurally characterized. The compound is structurally analogous to a formerly elucidated framework, (NEt4)2[Fe2(can)3] (H2can = 3,6-dichloro-2,5-dihydroxy-1,4-benzoquinone), and adopts a 2-D (6,3) topology with the symmetrical stacking of [Fe2(fan)3]2- sheets that are held in position by the NEt4+ cations between the sheets. The investigation of the dc and ac magnetic properties of 1·5(acetone) revealed ferromagnetic ordering behavior and slow magnetization relaxation, as evinced from ac susceptibility measurements. Furthermore, the exposure of 1·5(acetone) to air led to the formation of a heptahydrate 1·7H2O which displayed distinct magnetic properties. The study of the redox state and extent of delocalization in 1·5(acetone) was undertaken via crystallography, in combination with Mössbauer and vis-NIR spectroscopy, to reveal the mixed-valence and delocalized nature of the as-synthesized material. As a result, the conductivity studies conducted on a pressed pellet showed a relatively high conductivity of 1.8 × 10-2 S cm-1 (300 K). In order to compare structurally related anilate-based structures, a relationship among the redox state, spectroscopic properties, and electronic properties was elucidated in this work. A preliminary investigation of 1·5(acetone) as a candidate anode material in lithium ion batteries revealed a high reversible capacity of 676.6 mAh g-1 and high capacity retention.

A Switch between Antioxidant and Prooxidant Properties of the Phenolic Compounds Myricetin, Morin, 3',4'-Dihydroxyflavone, Taxifolin and 4-Hydroxy-Coumarin in the Presence of Copper(II) Ions: A Spectroscopic, Absorption Titration and DNA Damage Study.

The beneficial effects of polyphenols, predominantly in the context of oxidative stress-related diseases such as cancer, cardiovascular diseases and neurological conditions including Alzheimer’s and Parkinson’s diseases, have been documented by a number of papers and reviews. The antioxidant/prooxidant properties of phenolic compounds are related mainly to the number and positions of hydroxyl groups and to their redox metal (Cu, Fe) chelating capacity. In this work we studied structurally distinct phenolic molecules such as myricetin, morin, 3′,4′-dihydroxy-flavone, taxifolin and 4-hydroxycoumarin, either alone or as interacting with Cu2+ ions. EPR and UV-Vis spectroscopy confirmed that the effective binding of cupric ions to phenolic compounds requires the presence of the 3-OH and 4-CO groups on the flavonoid C ring and unsaturated C2-C3 bond of the C-ring, which permits through-conjugation with the B-ring. An ABTS assay revealed that radical scavenging activities of phenolic compounds are related to their number of hydroxyl groups, planarity of the molecular skeleton, extent of delocalization and they decrease in the order: myricetin > morin > 3′,4′-dihydroxyflavone ~ 4-hydroxy coumarin > taxifolin. Absorption titrations indicate that copper ions can modulate the DNA binding affinity of flavonoids via the formation of their Cu-chelates. Gel electrophoresis measurements indicated that the protective effect of the phenolic compounds decreases in the order: 3′,4′-dihydroxyflavone > 4-OH coumarin > morin > taxifolin ~ myricetin. This can be explained by the fact that myricetin, taxifolin and morin form stable Cu(II) complexes capable of causing DNA damage via interaction with DNA and ROS formation via the Fenton reaction. Application of ROS scavengers revealed the formation of singlet oxygen, superoxide and hydroxyl radicals and their concerted synergistic effect on the DNA. The overall results suggest that the most pronounced DNA damage has been observed for flavonoids containing higher number of hydroxyl groups (including 3-OH group of the C ring), such as myricetin (six hydroxyl groups), morin and taxifolin (five hydroxyl groups) in the presence of Cu(II) ions. The proposed mechanism of action by which Cu(II) complexes of myricetin, morin and taxifolin interact with DNA predispose these substances to act as potential anticancer agents. The anticancer activity of phenolic compounds can be explained by their moderate prooxidant properties, which can boost ROS formation and kill cancer cells. Alternatively, slight prooxidant properties may activate antioxidant systems, including antioxidant enzymes and low molecular antioxidants such as glutathione and thus act as preventive anticancer agents.

Experimental and theoretical electronic absorption spectra of 2,4‐diphenyl‐1,5‐benzothiazepine and its derivatives: Solvatochromic effect and time dependent density functional theory approach

AbstractElectronic spectra of 2,4‐diphenyl‐1,5‐benzothiazepine and some of its derivatives in 1,2‐dichloromethane and ethanol are investigated experimentally and theoretically using the time dependent density functional theory (TD‐DFT) method at the B3LYP/6‐311G** level of the theory. The origin of the spectrum of the parent compound is found to be an additive one. The observed ultra violet (UV) spectra in both solvents show two bands S1 in the range between 312–334 nm and S2 in the range between 248–272 nm. The solvent effect is investigated experimentally and theoretically and a blue shift is observed, which is explained in terms of a hydrogen bond model between the solvent and the most negative site of the solute (N atom). This theoretical model is robust in reproducing the experimental blue shift and calculating the hydrogen bond energy and hydrogen bond length. The extent of delocalization and charge transfer processes of the studied compounds is estimated and discussed in terms of natural bond orbital (NBO) analysis and second order perturbation interactions (E2) between donors and acceptors. The effect of substituents of the studied compounds in both solvents shows a noticeable red shift attributed to hyperconjugation effects of the π electron systems of the different moieties.

Facile synthesis of simple arylamine-substituted naphthalene derivatives as hole-transporting materials for efficient and stable perovskite solar cells

Massive efforts are devoted to exploring alternative hole-transporting materials for efficient and stable perovskite solar cells. As one typical polycyclic aromatic hydrocarbon, naphthalene group endows a favorable planar conformation and further extended π-conjugation, both of which accelerate the process of hole extraction and transport. In this work, two small molecule hole-transporting materials based on naphthalene core and arylamine terminal groups, termed NH-2,6 and NH-2,7, are prepared via facile synthesis from commercial precursors with a low synthetic cost. NH-2,6, which incorporates particular 2,6-substituted naphthalene core and arylamine terminal, exhibits a deeper highest occupied molecular orbital and a higher hole mobility ascribing to the discriminating structural substitution position effect and better electronic delocalization extent between naphthalene core and arylamine terminal. Thus with NH-2,6, impressive efficiency of 18.7%, which is associated with excellent reproducibility and reasonable stability, is afforded based on the optimized mixed-cation perovskite solar cells. These results not only highlights that the properties of hole-transporting materials can be tuned to some extent through the substitution position effect, but also provides the significative insights into the molecule design for facile synthesis, low-cost and efficient hole-transporting materials.

Structural and electronic properties of chemically modified fullerenes

ABSTRACTChemically modified fullerenes are emerging as an important class of technological materials. This study deals with one such category of modified fullerenes, viz., heterofullerenes. Structure-property relationships of Pd, Ni and N-substituted heterofullerenes were probed using density functional theory calculations. Distortion in spheroid of C cage during formation of three different heterofullerenes, viz., CM, CNM and CNM (), was analyzed from DFT optimised geometries and13C NMR characterisation. NBO analysis was done to identify the strength of bonds based on their bond orders and coordination of metal M with C and N atoms. Extent of delocalisation of π electrons in terms of aromaticity of C after foreign atoms substitution was examined by NICS(0) characterisation. HOMO-LUMO gap, ionisation potential, electron affinity and chemical potential were calculated to obtain insights into the reactivities and stabilities of the heterofullerenes with reference to C. Amongst CM, CNM and CNM heterofullerenes, CNM and CNM were concluded to be the most stable and reactive heterofullerenes, respectively.

TD-DFT Calculations, NBO, NLO Analysis and Electronic Absorption Spectra of Some Novel Thiazolo[3,2-a]Pyridine Derivatives Bearing Anthracenyl Moiety

The electronic structure and spectra of the studied compounds 1–4 are investigated using TD-DFT/B3LYB/6-311G (d, p) level of theory. The results of calculations show that all the studied compounds 1–4 are non-planar, as indicated from the dihedral angles. The electronic absorption spectra of the studied compounds are recorded in the UV-VIS region, in both Acetone (as polar solvent) and Xylene (as non-polar solvent). The observed vertical electronic transitions assignments are facilitated via time-dependent density functional theory TD-DFT. Solvent dependence of the band maxima (λmax) and intensities of the observed spectra are explained in terms of blue and red shifts. Electronic configurations contributing to each excited state are identified and the relevant MOs are characterized. The natural bond orbital (NBO) analysis were discussed in terms of the extent of delocalization, intermolecular charge transfer and second order perturbation interactions between donor and acceptor MOs. The Coulomb-attenuating method (CAM-B3LYP) and Corrected Linear Response Polarizable Continuum Model (CLR) PCM studied for theoretically obtaining the electronic absorption spectra in gas phase, Acetone and Xylene, respectively, indicate a good agreement with the observed spectra. The calculated nonlinear optical parameters (NLO); polarizibilty (α), anisotropy of the polarizibility (Δα) and first order hyperpolarizibility (β) of the studied compounds show promising optical properties. The HOMO-LUMO energy gap helped in analyzing the chemical reactivity, hardness, softness, chemical potential and electro negativity. 3D-plots of the molecular electrostatic potential (MEP) for the studied compounds are investigated and analyzed showing the distribution of electronic density of orbital's describing the electrophilic and nucleophilic sites of the selected molecules.

Broadband 2D IR spectroscopy reveals dominant asymmetric H5O2+ proton hydration structures in acid solutions.

Given the critical role of the aqueous excess proton in redox chemistry, determining its structure and the mechanism of its transport in water are intense areas of experimental and theoretical research. The ultrafast dynamics of the proton's hydration structure has made it extremely challenging to study experimentally. Using ultrafast broadband two-dimensional infrared spectroscopy, we show that the vibrational spectrum of the aqueous proton is fully consistent with a protonated water complex broadly defined as a Zundel-like H5O2+ motif. Analysis of the inhomogeneously broadened proton stretch two-dimensional lineshape indicates an intrinsically asymmetric, low-barrier O-H+-O potential that exhibits surprisingly persistent distributions in both its asymmetry and O-O distance. This structural characterization has direct implications for the extent of delocalization exhibited by a proton's excess charge and for the possible mechanisms of proton transport in water.

Control of Electronic Structure and Conductivity in Two-Dimensional Metal-Semiquinoid Frameworks of Titanium, Vanadium, and Chromium.

The isostructural, two-dimensional metal-organic frameworks (H2NMe2)2M2(Cl2dhbq)3 (M = Ti, V; Cl2dhbqn- = deprotonated 2,5-dichloro-3,6-dihydroxybenzoquinone) and (H2NMe2)1.5Cr2(dhbq)3 (dhbqn- = deprotonated 2,5-dihydroxybenzoquinone) are synthesized and investigated by spectroscopic, magnetic, and electrochemical methods. The three frameworks exhibit substantial differences in their electronic structures, and the bulk electronic conductivities of these phases correlate with the extent of delocalization observed via UV-vis-NIR and IR spectroscopies. Notably, substantial metal-ligand covalency in the vanadium phase results in the quenching of ligand-based spins, the observation of simultaneous metal- and ligand-based redox processes, and a high electronic conductivity of 0.45 S/cm. A molecular orbital analysis of these materials and a previously reported iron congener suggests that the differences in conductivity can be explained by correlating the metal-ligand energy alignment with the energy of intervalence charge-transfer transitions, which should determine the barrier to charge hopping in the mixed-valence frameworks.

TD-DFT calculations, NBO analysis and electronic absorption spectra of some thiazolo[3,2-a]pyridine derivatives

The electronic structure of thiazolo[3,2-a]pyridine and some of its derivatives are investigated theoretically using B3LYB/6-311G (d, p) method. The calculations show that all the studied compounds 1–5 are non-planar, as indicated from the dihedral angles. The electronic absorption spectra of the studied compounds are recorded in the UV–VIS region, in both DMF and Dioxane solvents. Assignments of the observed electronic transitions are facilitated via time-dependent density functional theory (TD-DFT) computations. Band maxima (λmax) and intensities of the spectra are found to have solvent dependence reflected as blue and red shifts. Electronic configurations contributing to each excited state are identified and the relevant MOs are characterized. The extent of delocalization and intermolecular charge transfer are estimated and discussed in terms of natural bond orbital (NBO) analysis and second order perturbation interactions between donor and acceptor MOs. The theoretical spectra computed at “A new hybrid exchange-correlation functional using the Coulomb-attenuating method (CAM-B3LYP),” at the 6-311G (d, p) bases set in gas phase and with the Polarizable Continuum Model (PCM) in Dioxane and DMF indicate a good agreement with the observed spectra.

Ni(II) Complexes of the Redox-Active Bis(2-aminophenyl)dipyrrin: Structural, Spectroscopic, and Theoretical Characterization of Three Members of an Electron Transfer Series.

The sterically hindered bis(2-aminophenyl)dipyrrin ligand H3NL was prepared. X-ray diffraction discloses a bifurcated hydrogen bonding network involving the dipyrrin and one aniline ring. The reaction of H3NL with one equivalent of nickel(II) in the air produces a paramagnetic neutral complex, which absorbs intensively in the Vis-NIR region. Its electron paramagnetic resonance spectrum displays resonances at g1 = 2.033, g2 = 2.008, and g3 = 1.962 that are reminiscent of an (S = 1/2) system having a predominant organic radical character. Both the structural investigation (X-ray diffraction) and density functional theory calculations on [NiII(NL•)] points to an unprecedented mixed "pyrrolyl-anilinyl" radical character. The neutral complex [NiII(NL•)] exhibits both a reversible oxidation wave at -0.28 V vs Fc+/Fc and a reversible reduction wave at -0.91 V. The anion was found to be highly air-sensitive, but could be prepared by reduction with cobaltocene and structurally characterized. It comprises a Ni(II) ion coordinated to a closed-shell trianionic ligand and hence can be formulated as [NiII(NL)]-. The cation was generated by reacting [NiII(NL•)] with one equivalent of silver hexafluoroantimonate. By X-ray diffraction we established that it contains an oxidized, closed-shell ligand coordinated to a nickel(II) ion. We found that a reliable hallmark for both the oxidation state of the ligand and the extent of delocalization within the series is the bond connecting the dipyrrin and the aniline, which ranges between 1.391 Å (cation) and 1.449 Å (anion). The cation and anion exhibit a rich Vis-NIR spectrum, despite their nonradical nature. The low energy bands correspond to ligand-based electronic excitations. Hence, the HOMO-LUMO gap is small, and the redox processes in the electron transfer series are exclusively ligand-centered.

Arrays of Ag and Au Nanoparticles with Terpyridine- and Thiophene-Based Ligands: Morphology and Optical Responses.

The assembly of Ag and Au nanoparticles (NPs) into nanoparticulate arrays mediated by terpyridine (tpy), 4'-(2-thienyl)terpyridine (T-tpy), and short α,ω-bis(tpy)oligothiophene ligands has been accomplished at the interface between the Ag or Au NP hydrosol and a solution of the molecular species in dichloromethane. The relationship between the morphology and the optical responses of the arrays has been investigated by advanced methods of TEM (transmission electron microscopy) image analysis and surface plasmon extinction (SPE) spectra. It has been established that the size of islands of closely spaced NPs rather than the average interparticle distance affects the extent of delocalization of the surface plasmon excitations and thus also the SPE spectra. Furthermore, the structure of surface-adsorbate complexes formed in these arrays has been investigated by SERS spectral measurements carried out as a function of the excitation wavelength. Photoinduced charge transfer (CT) transitions from the neutral Ags0 and Aus0 adsorption sites on metal NPs to antibonding orbitals of the adsorbates have been identified for Ag/tpy, Ag/T-tpy, Au/tpy, and Au/T-tpy nanoparticulate arrays. Although the surface-adsorbate complexes displaying a photoinduced CT are known for Ag NPs, the Aus0 surface complexes with this CT are newly reported. Bis(tpy)oligothiophenes were found to be attached to both Ag and Au NPs via the tpy group(s). The match between the interparticle distances within the NP islands and the lengths of the oligomers molecules indicates that the molecules act as interparticle linkers. In this case, unequivocal spectral marker band evidence of the Ags0 as well as Aus0 surface complex formation has not been obtained.