Charge Transfer Transition Energies Research Articles

π-electronic charge-transfer interactions in supramolecular complex formation between fullerenes and 5,10,15,20-tetrahexylporphyrin

UV–Vis spectral investigations on fullerenes/5,10,15,20-tetrahexylporphyrin (TP) supramolecular complexes are reported in CCl 4 and CHCl 3. An absorption band due to charge transfer (CT) transition is observed in the visible region. Vertical ionization potential of TP was determined from the CT transition energy. CT is very weak in these complexes as reflected from very low values of oscillator strengths and transition dipole strengths. All of the complexes are found to be stable with 1:1 stoichiometry. Thermodynamic parameters for the supramolecular complexes of TP with [60]- and [70]fullerenes are reported. The high formation constant value for the [60]fullerene/TP complex as compared to [70]fullerene/TP complex indicates selectivity of TP molecule towards [60]fullerene.

Spectrophotometric Studies of Complexation of Safranine T and Safranine O with [60]- and [70]Fullerenes and Other Electron Acceptors

The electron donor-acceptor (EDA) interactions of the dye, namely, Safranine T (ST) and Safranine O (SO) with [60]- and [70]fullerenes and some other electron acceptors (e.g., 2,3-dichloro-5,6-dicyano-p-benzoquinone, o-chloranil, and p-chloranil) were studied in 1,4-dioxan medium using the absorption spectroscopic technique. An absorption band due to a charge transfer (CT) transition is observed in the visible region for all of the complexes studied. The experimental CT transition energies (hv C T 's) are well-correlated (through Mulliken's equation) with the vertical electron affinities of the acceptors. From an analysis of this correlation, vertical ionization potentials of ST and SO have been determined. The influences of ST and SO on the absorption spectrum of [60]- and [70]fullerenes are studied. The experimental results were explained using a theoretical model that takes into account the interaction between electronic subsystems of ST and SO with [60]- and [70]fullerenes. Oscillator strengths, transition dipole strengths, and resonance energies of the EDA complexes have been estimated. Formation constants of the complexes were determined in terms of the Benesi-Hildebrand equation. The magnitudes of the formation constants indicate that SO binds more strongly than ST with [60]-and [70]fullerenes.

Study of charge transfer complexes of [70]fullerene with phenol and substituted phenols

To improve the understanding of the charge transfer (CT) interaction of [70]fullerene with electron donors, interaction of [70]fullerene with a series of phenols, e.g., phenol, resorcinol and p-quinol were studied in 1,4-dioxan medium using absorption spectroscopy. An absorption band due to CT transition was observed in the visible region. The experimental CT transition energies ( hν CT) are well correlated (through Mulliken's equation) with the vertical ionisation potentials ( I D v ) of the series of phenols studied. From an analysis of this correlation degrees of charge transfer for the [70]fullerene-phenol complexes were estimated. The degrees of charge transfer in the ground state of the complexes have been found to be very low (<2%). The hν CT values change systematically as the number and position of the OH groups change on the aromatic ring of the phenol moiety. From the trends in the hν CT values, the Hückel parameters ( h Ö and k C Ö ) for the OH group were obtained in a straightforward way and the values so obtained, viz., 1.91 and 1.0, respectively, are close to the ones (1.8 and 0.8) recommended by Streitwieser on the basis of other evidence. Oscillator strengths, transition dipole strengths and resonance energies of the [70]fullerene-phenol complexes were determined. Formation constants of the CT complexes were determined at four different temperatures from which enthalpies and entropies of formation of the complexes were estimated.

Study of Host−Guest Interaction of [70]Fullerene with Substituted Calix[6]arenes by Absorption Spectrometric Method

The host−guest interaction of various mono O-substituted calix[6]arenes, namely, (i) 37-benzyl-38,39,40, 41,42-pentahydroxy-5,11,17,23,29,35-hexa(4-tert-butyl)calix[6]arene (1), (ii) 37-ethylester-38,39,40,41,42-pentahydroxy-5,11,17,23,29,35-hexa(4-tert-butyl)calix[6]arene (2), and (iii) 37-allyl-38,39,40,41,42-pentahydroxy-5,11,17,23,29,35-hexa(4-tert-butyl)calix[6]arene (3) with [70]fullerene has been studied in a CCl4 medium by the absorption spectroscopic method. All of the complexes are found to be stable with 1:1 stoichiometry. An absorption band due to a charge transfer (CT) transition is observed in the visible region; with increasing ionization potential of the mono O-substituted calix[6]arenes, the peak energy shifts to the higher energy side and the oscillator strength of CT transition increases. Degrees of CT have been estimated by analyzing the CT transition energy as a function of ionization potential. Thermodynamic parameters for the supramolecular complex formation of [70]fullerene with 1, 2, and 3 receptors are reported. The high formation constant value for the [70]fullerene·2 complex indicates that 2 forms an inclusion complex with [70]fullerene.

Spectroscopic and computational studies of the azide-adduct of manganese superoxide dismutase: definitive assignment of the ligand responsible for the low-temperature thermochromism.

A variety of spectroscopic and computational techniques have been used to examine the thermochromic transition previously reported for the oxidized state of Mn-dependent superoxide dismutase from E. coli in the presence of substrate analog azide (N(3)-Mn(3+)SOD).[Whittaker, M. M.; Whittaker, J. W. Biochemistry 1996, 35, 6762-6770.] Although previous spectroscopic studies had shown that this thermochromic event corresponds to a change in coordination number of the active-site Mn(3+) ion from 6 to 5 as temperature is increased, the ligand that dissociates in this conversion had yet to be identified. Through the use of electronic absorption, circular dichroism (CD), and magnetic CD (MCD) spectroscopies, both d-->d and ligand-to-metal charge-transfer (LMCT) transition energies have been determined for native Mn(3+)SOD (possessing a five-coordinate Mn(3+) center) and Y34F N(3)-Mn(3+)SOD (forming a six-coordinate N(3)-Mn(3+) adduct at all temperatures). These two systems provide well-defined reference points from which to analyze the absorption and CD data obtained for N(3)-Mn(3+)SOD at room temperature (RT). Comparison of excited-state spectroscopic data reveals that Mn(3+)SOD and RT N(3)-Mn(3+)SOD exhibit virtually identical d-->d transition energies, suggesting that these two species possess similar geometric and electronic structures and, thus, that azide does not actually coordinate to the active-site Mn(3+) ion at RT. However, resonance Raman spectra of both N(3)-Mn(3+)SOD and Y34F N(3)-Mn(3+)SOD at 0 degrees C exhibit azide-related vibrations, indicating that azide does interact with the active site of the native enzyme at this temperature. To gain further insight into the nature of the azide/Mn(3+) interaction in RT N(3)-Mn(3+)SOD, several viable active-site models designed to promote either dissociation of coordinated solvent, Asp167, or azide were generated using DFT computations. By utilizing the time-dependent DFT method to predict absorption spectra for these models of RT N(3)-Mn(3+)SOD, we demonstrate that only azide dissociation is consistent with experimental data. Collectively, our spectroscopic and computational data provide evidence that the active site of N(3)-Mn(3+)SOD at RT exists in a dynamic equilibrium, with the azide molecule either hydrogen-bonded to the second-sphere Tyr34 residue or coordinated to the Mn(3+) ion. These results further highlight the role that second-sphere residues, especially Tyr34, play in tuning substrate (analog)/metal ion interactions.

Absorption spectrometric and thermodynamic study of charge transfer complexes of menadione (Vitamin K 3) with a series of phenols

The electron donor–acceptor (EDA) interactions between menadione (i.e., 2-methyl-1,4-naphthoquinone, which is also called ‘Vitamin K 3’) and a series of phenols (viz., phenol, resorcinol and p-quinol) have been studied in CCl 4 medium. In all the cases, charge transfer (CT) bands have been located. The CT transition energies ( hν CT) of the complexes are found to change systematically with change in the number and position of the –OH groups in the aromatic ring of the phenol moiety. From the trends in the hν CT values, the Hückel parameters ( h Ö and k C–Ö) for the –OH group have been obtained. The CT transition energies are well correlated with the ionisation potentials of the phenols. From an analysis of this variation the electron affinity of Vitamin K 3 has been found to be 2.28 eV. The stoichiometry of the complexes in each case has been found to be 1(menadione):2 (phenol). Formation constants of the complexes have been determined at four different temperatures from which the enthalpies and entropies of formation of the complexes have been estimated.

Energies of charge transfer and supramolecular interactions of some mono O-substituted calix[6]arenes with [60]fullerene by absorption spectrometric method

Equilibria for the formation of supramolecular complexes of [60]fullerene with a series of mono O-substituted calix[6]arenes, namely: (i) 37-methoxy-38,39,40,41,42-pentahydroxy-5,11,17,23,29,35-hexa(4- tert-butyl)calix[6]arene ( 1), (ii) 37-allyl-38,39,40,41,42-pentahydroxy-5,11,17,23,29,35-hexa(4- tert-butyl)calix[6]arene ( 2), (iii) 37-phenacyl-38,39,40,41,42-pentahydroxy-5,11,17,23,29,35-hexa(4- tert-butyl)calix[6]arene ( 3), (iv) 37-ethylester-38,39,40,41,42-pentahydroxy-5,11,17,23,29,35-hexa(4- tert-butyl)calix[6]arene ( 4) and (v) 37-benzyl-38,39,40,41,42-pentahydroxy-5,11,17,23,29,35-hexa(4- tert-butyl)calix[6]arene ( 5) have been studied in CCl 4 medium by absorption spectroscopic technique. The stoichiometry has been found to be 1:1 ([60]fullerene:calix[6]arene) in each case. An absorption band due to charge transfer (CT) transition is observed in each case in the visible region. The vertical ionisation potentials ( I D v ) of all the calix[6]arenes under study have been estimated utilising CT transition energy. The experimental I D v values also yield a good estimate of the electron affinity of [60]fullerene. The degrees of CT in the ground state of the complexes have been found to be very low (about 0.15%). Resonance energy of the complexes have been estimated. Thermodynamic parameters for the supramolecular complex formation of [60]fullerene with mono O-substituted calix[6]arene receptors are reported. It is observed that among the calix[6]arenes under the present study, only 1 and 4 form inclusion complexes with [60]fullerene. This has also been substantiated by theoretical calculation using PM3 method. Thus presence of one substituent group (of different types) on the lower rim of the calix[6]arene molecule has been shown to govern the host–guest complexation process.

Spectrophotometric study of complexation of benzoyl acetone with [60]- and [70]fullerenes and some other electron acceptors

To improve the understanding of the formation of electron donor–acceptor complexes of [60]- and [70]fullerenes, EDA interaction of [60]- and [70]fullerenes and some other electron acceptors with benzoyl acetone were studied in CHCl 3 medium using absorption spectrophotometry. An absorption band due to charge transfer (CT) transition is observed in the visible region. The vertical ionization potential ( I B v ) of the benzoyl acetone was determined from the CT transition energy. Oscillator and transition dipole strengths have also been calculated. Trends in the formation constant for the complexes [60]- and [70]fullerenes with benzoyl acetone were discussed in terms of enthalpies and entropies of formation.

Characterization of micelles of polyoxyethylene nonylphenol (Igepal) and its complexation with 3,7-diamino-2,8-dimethyl- 5-phenylphenazinium chloride

This paper explores the association of a nonionic surfactant, Igepal (polyoxyethylene nonylphenol), in aqueous media by means of absorption and fluorescence spectroscopy. The critical micellar concentration (CMC) of the aggregate formed in the aqueous medium has been determined, using three different methods: UV–visible spectroscopy, fluorescence spectroscopy, and Stokes shift. The correlation of CMC with hydrophile–lipophile balance (HLB) indicates that the CMC decreases with decreased HLB. The obtained CMC values from different methods are close to each other and have allowed the determination of Δ G values associated with the micellization. The association constant of the dye molecule Safranine T (ST) with the nonionic micelle of Igepal, aggregation number of the surfactant monomer, and location of the fluorophore in the micellar environment have been determined. The vertical ionization potential of Igepal, electron affinity of the dye, and degree of charge transfer from the micellar aggregate to the dye molecule have been determined by AM1 calculation. The experimental charge transfer transition energies are well correlated with the determined ionization potential values ( I D) of Igepal. The degree of charge transfer (ground state complexes) has been found to be low. The polarity of the micelle solubilization sites has been estimated from the solvatochromic shift, Kosowar Z value, and E T(30), and E T N values.

Study of Charge-Transfer Complexes of [60]- and [70]Fullerenes with a Series of Phosphine Oxides

Equilibria for the formation of molecular electron donor−acceptor (EDA) complexes of a series of phosphine oxides with [60]- and [70]fullerenes have been studied in CCl4 medium. An absorption band due to a charge transfer (CT) transition is observed in the visible region. The vertical ionization potentials (IDv) of the phosphine oxides determined from the CT transition energies using Mulliken's equation correlate well with those calculated by the semiempirical PM3 method. The experimental IDv values also yield good estimates of the electron affinities of [60]- and [70]fullerenes. Among the phosphine oxides studied, triphenyl phosphine oxide, trioctyl phosphine oxide, and tri-n-butyl phosphine oxide form stable EDA complexes with [60]- and [70]fullerenes while the complexes of bis-2-ethylhexyl phosphonate with [60]- and [70]fullerenes decay slowly into secondary products. The kinetics of these reactions have been studied by UV−vis absorption spectrophotometric methods, and the rate constants of the decay p...

Application of time-resolved linear dichroism spectroscopy: Intensity borrowing in charge transfer complex absorption spectra

Time-resolved linear dichroism spectroscopy has been used to study the influence of solvent on the charge transfer complex formed between hexamethylbenzene and 1,2,4,5-tetracyanobenzene. It was shown that cyano-substituted solvents induce a 1500 cm–1 increase in the charge transfer transition energies relative to those observed in chlorinated solvents. Furthermore, the angle between the charge transfer absorption transition moments and the photochemically produced radical anion absorption transition moment, after relaxation, has been measured for this complex in several solvents. A simple model was used to correlate the angles measured using time-resolved linear dichroism spectroscopy with the extent of localized excitation mixed into the charge transfer transitions. These measurements reveal that different charge transfer transitions borrow intensity from the localized excitation to different extents. By using different excitation wavelengths, the partitioning of the borrowed intensity among the charge transfer transitions of this complex could be evaluated for the first time.Key words: 1,2,4,5-tetracyanobenzene, hexamethylbenzene, donor–acceptor complex, photoinduced electron transfer, photoselection.

Spectrophotometric and thermodynamic studies of [60]fullerene/methylbenzene charge transfer complexes

The absorption spectra of charge-transfer (CT) complexes of [60]fullerene with liquid methylbenzenes, viz. toluene, o-xylene, m-xylene, p-xylene and mesitylene have been investigated in CCl 4 medium. An absorption band due to CT transition is observed in each case in the visible region. The experimental CT transition energies are well correlated (through Mulliken's equation) with the ionisation potentials ( I D) of the series of methylbenzenes studied. From an analysis of this variation the electron affinity of [60]fullerene has been found to be 2.32 eV. The degrees of charge transfer in the ground state of the complexes have been found to be very low (0.66–0.775%). It has been found that these methylbenzenes form stable 1:1 complexes with [60]fullerene. Formation constants of the complexes have been determined at four different temperatures from which the enthalpies and entropies of formation of the complexes have been obtained. The experimentally determined formation constants of the complexes of [60]fullerene with methylbenzenes exhibit a very good linear free energy relationship (Chem. Rev. 53 (1953) 191).

Distance dependence of electron transfer across peptides with different secondary structures: the role of Peptide energetics and electronic coupling.

The charge-transfer transition energies and the electronic-coupling matrix element, |H(DA)|, for electron transfer from aminopyridine (ap) to the 4-carbonyl-2,2'-bipyridine (cbpy) in cbpy-(gly)(n)-ap (gly = glycine, n = 0-6) molecules were calculated using the Zerner's INDO/S, together with the Cave and Newton methods. The oligopeptide linkages used were those of the idealized protein secondary structures, the alpha-helix, 3(10)-helix, beta-strand, and polyproline I- and II-helices. The charge-transfer transition energies are influenced by the magnitude and direction of the dipole generated by the peptide secondary structure. The electronic coupling |H(DA)| between (cbpy) and (ap) is also dependent on the nature of the secondary structure of the peptide. A plot of 2.ln|H(DA)| versus the charge-transfer distance (assumed to be the dipole moment change between the ground state and the charge-transfer states) showed that the polyproline II structure is a more efficient bridge for long-distance electron-transfer reactions (beta = 0.7 A(-1)) than the other secondary structures (beta approximately 1.3 A(-1)). Similar calculations on charged dipeptide derivatives, [CH(3)CONHCH(2)CONHCH(3)](+/)(-), showed that peptide-peptide interaction is more dependent on conformation in the cationic than in the anionic dipeptides. The alpha-helix and polyproline II-helix both have large peptide-peptide interactions (|H(DA)| > 800 cm(-1)) which arise from the angular dependence of their pi-orbitals. Such an interaction is much weaker than in the beta-strand peptides. These combined results were found to be consistent with electron-transfer rates experimentally observed across short peptide bridges in polyproline II (n = 1-3). These results can also account for directional electron transfer observed in an alpha-helical structure (different ET rates versus the direction of the molecular dipole).

Second-order optical nonlinearity of surface-capped CdS nanoparticles and effect of surface modification

Surface-capped CdS nanoparticles (5 nm mean diameter) with hexametaphosphate and CTAB (hexadecyltrimethyl-ammonium bromide) in chloroform were prepared by colloidal chemistry and phase transfer method. The second-order nonlinear optical response was measured by incoherent hyper-Rayleigh scattering (HRS) technique. Results show that the surface-capped CdS nanoparticles have a large first-order hyperpolarizability β value of 1.30×10 −26 esu per particle. Surface capping molecule modification was performed by adding pyridine to a chloroform sol of the surface-capped CdS nanoparticles where pyridine can be bound to the CdS nanoparticle surface through Cd–N chemical bonds. A reduction in HRS signal intensity was observed when increasing pyridine concentration. This is explained in terms of a two-level model approximation derived from molecular chromophores, when considering the influence of the formation of Cd–N coordinating bonds on the charge-transfer transition energy of surface molecule-like scatterers of Cd–S polar bonds.

Charge Transfer Molecular Complexes of Some Pyrazoles with σ‐ and π‐Electron Acceptors

Charge‐transfer (CT) complexes of some pyrazoles with iodine as a σ‐electron acceptor and with 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ), tetracyanoethylene (TCNE), chloranil (CHL) as a π‐electron acceptors were investigated spectroscopically. The spectral characteristics and stability constants of the formed 1: 1 CT complexes were examined and discussed in terms of the donor molecular structure, the nature of the electron acceptor as well as the solvent polarity. It is deduced that the complexes with iodine exist in the ionic structure (pyrazole)2 I+ · I3 − whereas the complex with DDQ is mainly in the dissociated state (radical cation and anion; pyrazole· +, DDQ· −). On the other hand pyrazoles‐TCNE, –CHL complexes are essentially of the nonbonding type (pyrazole‐TCNE, –CHL). Moreover, it is concluded that the solvent polarity plays an important role in determining the stability of these kinds of CT complex. The ionization potentials of the donors were calculated based on the CT transition energies of their complexes.

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, F 4TCNQ 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 Et 2TCNQ 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.

Absorption Spectroscopic Study of the Complexation of C70 with Aniline and Substituted Anilines

Complexes of fullerene C 7 0 with aniline and various substituted anilines have been studied in CCl 4 medium by electronic absorption spectroscopy. An absorption band due to a charge transfer (CT) transition is observed in the visible region. The experimental CT transition energies are well correlated (through Mulliken's equation) with the ionization potentials (I D ) of the series of anilines studied. From an analysis of this correlation, the electron affinity of C 7 0 has been found to be 2.28 eV. The degree of charge transfer in the ground state of the complexes has been found to be very low (0.2-0.3%). The experimentally determined formation constants of the complexes of C 7 0 with N-substituted anilines exhibit a very good linear free-energy relationship (Hammett, L. P. Chem. Rev. 1953, 53, 191).

Effects of Hydrostatic and Chemical Pressures on Impurities Determined through Optical Parameters

A survey on the influence of hydrostatic and chemical pressures upon optical properties due to Transition-Metal impurities in insulators is offered in this article. Aside from recalling the theoretical framework where the properties of impurities in insulating materials are reasonably explained particular attention is paid to the dependence of crystal field and charge transfer transitions upon the impurity-ligand distance, R . By means of 10 Dq dependent d-.d transitions or the energy of charge transfer transitions it is pointed out that j R variations around 0.1 pm can be detected. The R dependence of 10 Dq is shown to determine also the corresponding dependence of Huang-Rhys factors and Stokes shift. The Huang-Rhys factor, S A , of the A 1 local mode is found to increase when R does a conclusion supported by recent experimental data on Cs 2 NaScCl 6 : Cr 3+ under hydrostatic pressure. The increase of the Stokes shift when R increases observed in the series of cubic fluoroperovskites doped with Mn 2+ is also discussed. It is pointed out that for the same j R variation, j y A changes produced by chemical pressures can be bigger than those produced through a hydrostatic pressure.

Structure and Ionicity of Intramolecular Charge Transfer Zwitterions, D δ+ − π − A δ−

Intramolecular charge transfer (CT) compounds (D δ+ -π-A δ− ) were synthesized by the reaction between enamine and TCNQ. The CT transition energy ( h ν CT ) showed the solvent dependence (solvatochromism). The ionicity of these compounds was deduced from the plot of h ν CT against the solvent polarity. The plot exhibited abnormal behavior.

Optical spectroscopy ofAl2O3:Ti3+ single crystal under hydrostaticpressure.The influence on the Jahn-Teller coupling

This work investigates the effect of hydrostatic pressure on theexcitation and emission spectra, as well as on the lifetime, ofAl2O3:Ti3+ at room temperature. The aim is toestablish correlations between the pressure-induced band shifts andthe corresponding local structural changes undergone by theTiO6 complex. A blue-shift of 8.52and 6.86 cm-1 kbar-1 was found for the lower (E1) andupper (E2) energy components of the excitation band at17760and 20500 cm-1, respectively, and blue-shifts of 5.93 and5.40 cm-1 kbar-1 for the two overlapping bands of theemission spectrum located at 12680 and 14210 cm-1. Weexplain these results on the basis of a reduction of the TiO6Jahn-Teller distortion upon increasing the pressure. In contrast,the increase of the overall Stokes shift, which is mainly associatedwith electron-vibration coupling to the totally symmetrica1g vibration, is explained by the increase of the excited-statestabilization energy, Sa1gℏωa1g,with increasing pressure.The luminescence lifetime is also found to be pressure dependent,varying from 2.6 µs at ambient conditions to 3.2 µs at80 kbar. This increase is caused by a diminution of the transitionoscillator strength that is related to the odd-vibration assistancemechanism. The softening of the transition mechanism is interpretedin terms of the blue-shift experienced by the O2- to Ti3+ charge-transfer transition energy upon increasing thepressure.