On the role of intramolecular vibrations in the nonradiative decay of excited charge-transfer states of molecular complexes

An attempt for a theoretical treatment of radiationless transitions from excited charge-transfer states in molecular complexes is made within the framework of the statistical limit of radiationless transitions theory. This work deals with the S₁ → S₀ internal conversion in charge-transfer complexes of tetracyanoethylene (an electron acceptor) with benzene and toluene and their perdeuterated analogues. A dominant role of the high-frequency totally symmetric intramolecular vibrational modes in the nonradiative decay of excited charge-transfer states is assumed (this was inferred from the experimentally observed deuterium isotope effect on radiationless S₁ → S₀ transitions). Calculated absolute rate constants for internal conversion are found to be in good agreement with experimental ones. The results of our calculations reflect very well the observed moderate deuterium isotope effect.     

Charge Transfer Complexes of 1-Naphthylmethyl Cotton Cellulose

A cotton cellulose derivative that forms charge-transfer complexes was prepared in yarn form. A charge-transfer complex is composed of two partners, one referred to as the donor and the other as an acceptor of electrical charge. In the present investigation, a donor was chemically bound to the cotton. It was shown that the modified donor cellulose adsorbs various acceptors from solution. Celluloses possessing donor or acceptor moieties are potentially useful as chromatographic agents and also in medicinal chemistry where a controlled release of a chemical agent is desired.     

Molecular complexes of aromatic N-oxides with tetracyanoethylene

Donor—acceptor interaction of tetracyanoethylene with aromatic N-oxides leads to the formation of two kinds of molecular complexes of the ,-type: -complexes and charge-transfer complexes. A correlation is observed between the reactivity of the N-oxides and the electronic properties of the substituents in the N-oxides.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 488–492, April, 1991.     

Spectrophotometric assay of retinol via charge-transfer complexes

Simple and sensitive spectrophotometric methods for the assay of retinol have been presented. The first method was based on the reaction of retinol with iodine to give a molecular charge-transfer complex, the retinol acting as n-donor and iodine as -electron acceptor. The second method depends on the formation of a highly coloured stable radical anion between retinol and 7,7,8,8-tetracyanoquinodimethane (TCNQ as a -electron acceptor. The molecular ratios of the reactants in the complexes have been established and the experimental conditions leading to maximum charge-transfer bands were also studied. Beer's law is obeyed over the retinol concentration range 2.5–26 µg/ml. The proposed procedures have been applied successfully to the analysis of drug formulation. The average recovery and average standard deviation was 99.99 ±1.13% with retinol-iodine and 100.001 ± 1.31% with retinol-TCNQ. A kinetic study was performed by heating retinol at 50°C for different periods of time, the result obtained by plotting log c against time indicates that thermal decomposition of retinol is of first order. The results obtained by both methods were in good agreement with those obtained by the official method. The developed procedures were found to be simple, accurate and precise and can be used for the determination of retinol in presence of its degradation products.     

Studies on Charge-Transfer Complexes of 2,3-Dicyano-1,4-naphthoquinone with Some Ring and <Emphasis Type="Italic">N</Emphasis>-Substituted Anilines

Summary. The charge-transfer complexes (CT-complexes) between 2,3-dicyano-1,4-naphthoquinone (DCNQ) and some aromatic anilines, both ring and N-substituted, were studied spectrophotometrically in three chlorinated solvents, viz. chloroform, dichloromethane, and 1,2-dichloroethane at different temperatures. All the donors are known to form stable 1:1 stoichiometric complexes with DCNQ and their stoichiometry was unaffected by the variation of temperature over the studied range. The change in polarity of the solvent also doesnt affect the stoichiometry of the complexes. The determined formation constant values are in the range of 0.49–10.8dm³mol^–1, the highest is for the N-benzylaniline and the lowest is for o-chloroaniline. The effect of functional groups on the aniline moiety towards the strength and its electron releasing property has been discussed. The H, S, and G values are all negative, so the studied complexes are reasonably stable and exothermic in nature. The ionization potentials of the donors were determined using the charge-transfer (CT) absorption bands of the complexes. The dissociation energies (W) of the charge-transfer excited state for the CT-complexes were also determined and are found to be constant.Present address: Materials Research Centre, Indian Institute of Science, Bangalore-12, India     

Studies on Charge-Transfer Complexes of 2,3-Dicyano-1,4-naphthoquinone with Some Ring and N-Substituted Anilines

The charge-transfer complexes (CT-complexes) between 2,3-dicyano-1,4-naphthoquinone (DCNQ) and some aromatic anilines, both ring and N-substituted, were studied spectrophotometrically in three chlorinated solvents, viz. chloroform, dichloromethane, and 1,2-dichloroethane at different temperatures. All the donors are known to form stable 1:1 stoichiometric complexes with DCNQ and their stoichiometry was unaffected by the variation of temperature over the studied range. The change in polarity of the solvent also doesnt affect the stoichiometry of the complexes. The determined formation constant values are in the range of 0.49–10.8dm³mol^–1, the highest is for the N-benzylaniline and the lowest is for o-chloroaniline. The effect of functional groups on the aniline moiety towards the strength and its electron releasing property has been discussed. The H, S, and G values are all negative, so the studied complexes are reasonably stable and exothermic in nature. The ionization potentials of the donors were determined using the charge-transfer (CT) absorption bands of the complexes. The dissociation energies (W) of the charge-transfer excited state for the CT-complexes were also determined and are found to be constant.     

Hydrogen bonded С–H···Y (Y = O,S, Hal) molecular complexes: A natural bond orbital analysis

Hydrogen bonded C–H···Y complexes formed by H₂O, H₂S molecules, hydrogen halides, and halogen-ions with methane, halogen substituted methane as well as with the C₂H₂ and NCH molecules were studied at the MP2/aug-cc-pVDZ level. The structure of NBOs corresponding to lone pair of acceptor Y, n_Y, and vacant anti-σ-bond C–H of proton donor was analyzed and estimates of second order perturbation energy Е⁽²⁾ characterizing donor–acceptor n_Y → σ_C-H^* charge-transfer interaction were obtained. Computational results for complexes of methane and its halogen substituted derivatives show that for each set of analogous structures, the Е_nY→σ*C-H⁽²⁾ energy tends to grow with an increase in the s-component percentage in the lone pair NBO of acceptor Y. Calculations for different C···Y distances show that the equilibrium geometries of complexes lie in the region where the E⁽²⁾ energy is highest and it changes symbatically with the length of the covalent С–H bond when the R(C···Y) distance is varied. The performed analysis allows us to divide the hydrogen bonded complexes into two groups, depending on the pattern of overlapping for NBOs of the hydrogen bridge.     

Chemical stability and reactivity of the 1∶1 and 2:1 halobis(N,N-dialkyldithiocarbamato)iron(III): Molecular halogen loose associations

EHMO-SCCC calculations have been used to probe the molecular orbital interactions of 1:1 and 21 charge-transfer complexes of halobis(N,N-dialkyldithiocarbamato)iron(III) complexes with molecular halogens, treated as super molecules. The molecular orbital structure of the compounds is being described in essentially the same way to that successfully applied in the cases of-, -gs, and n- CT complexes. It is found that all types of orbital interactions (- and/or type) have a destabilizing effect on the complexes; hence, the forces of attachment have to be of the charge-transfer and van der Waals type, to offset this molecular orbital-based repulsion. The relative stability of the compounds is broadly discussed in terms of the calculated binding energies, which are found to depend on the nature of both constituent molecules. Moreover, orbital populations, two-center energy terms, and computed atomic charges of the super molecules, as compared to those of the constituent molecules, provide adequate information on the mechanism of the charge-transfer processes occurring upon complexation. It is important that weak bonding interactions between the constituent molecules do exist, but surprisingly directly, between the molecular halogens and the iron(III) central atoms, and not via the apical halogeno ligands. These bonding interactions, although not responsible for the stability of the compounds, account well for the observed magnetic behavior of the dimeric (21) complexes and are of key importance in promoting intramolecular electron-transfer reactions responsible for the oxidation of half of the dithio ligands into the thiuram disulfides observed in some of the reactions of the iron(III) halobisdithiocarbamates with molecular halogens.     

Interaction of polyphenylacetylene with electron-acceptor molecules

ESR spectra show that the number of paramagnetic centers in polyphenylacetylene (PPA) is linearly related to the oxygen pressure; iodine has an even more marked effect, the limiting concentration (7×10¹⁸ spins per g) being reached for a molar ratio of PPA to iodine of 16. The central part of the ESR line is lorentzian, with gaussian wings. A Lorenz curve fits a large fraction of the spectrum for PPA-iodine. The delocalization region for the unpaired electron corresponds to two units of PPA for oxygen and one for iodine. The results are interpreted in terms of charge-transfer complexes (CTC) between the PPA and electron acceptor (oxygen or iodine). Microwave saturation occurs in pumped PPA; loss of saturation is related to the formation of CTC.     

Electron Transfer and Charge Transfer: Twin Themes in Unifying the Mechanisms of Organic and Organometallic Reactions

The broad varieties of organic and organometallic reactions merge into a common unifying mechanism by considering all nucleophiles and electrophiles as electron donors (D) and electron acceptors (A), respectively. Comparison of outer-sphere and inner-sphere electron transfers with the aid of Marcus theory provides the thermochemical basis for the generalized free energy relationship for electron transfer (FERET) in Equation (37) and its corollaries in Equations (43) and (44) that have wide predictive applicability to electrophilic aromatic substitutions, olefin additions, organometallic cleavages, etc. The FERET is based on the conversion of the weak nucleophile–electrophile interactions extant in the ubiquitous electron donor—acceptor (EDA) precursor complex [D, A] to the radical ion pair [D^⊕, A^?], for which the free energy change can be evaluated from the charge-transfer absorption spectra according to Mulliken theory. FERET analysis thus indicates that the charge-transfer ion pairs [D^⊕, A^?] are energetically equivalent to the transition states for nucleophile/electrophile transformations. The behavior of such ion pairs can be directly observed immediately following the irradiation of the charge-transfer bands of various EDA complexes with a 25-ps laser pulse. Such studies confirm the radical ion pair [Arene^⊕, NO₂] as a viable intermediate in electrophilic aromatic nitration, as presented in the electron-transfer mechanism between arenes and the nitryl cation (NO) electrophile.     

Synthesis,molecular structure,and spectral properties of new metal chelates based on organic autocomplexes and charge transfer therein

A number of new chelate compounds based on 3d-transition metals and autocomplexes of the dinitroquinoline series were synthesized. The crystalline structure of the new complexes was determined by X-ray analysis, and their spectral properties were studied. The complexes have a conformation which gives rise to two modes of contact intramolecular charge transfer: intraligand, which is intrinsic to the initial ligand molecules, and interligand (as in binary charge-transfer complexes). The latter involves the corresponding pairs of donor and acceptor fragments of the initial ligand molecules, which appear spatially close as a result of complex formation.     

Spectral and photophysical studies of thiazine dyes in triton X-100

The absorption spectra of several thiazine dyes such as thionine, azure A, azure B, azure C and methylene blue in aqueous solution of Triton X-100 show that dyes as electron acceptors form 11 charge-transfer (CT) or electron-donor-acceptor (EDA) complexes with Triton X-100, which acts as an electron donor. From the thermodynamic and spectrophotometric properties of these complexes, the abilities of dyes to accept an electron are in the order: azure C > thionine > azure A > azure B > methylene blue. The photogalvanic effect in the aqueous solution dye-surfactant has been studied. Generation of photovoltage supports a CT or EDA interaction between thiazine dyes and Triton X-100. There is a good correlation among the photophysical (photovoltage), spectral and thermodynamic properties of these complexes.     

Two forms of complexes of heteroonium salts with amines

Heteroonium salts form molecular compounds of two forms with amines: true complexes (charge-transfer complexes) and complexes with a localized bond between the components.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 663–667, May, 1973.     

Pluri-dimensional hydrogen-bonded networks of novel thiophene-introduced oligo(imidazole)s and physical properties of their charge-transfer complexes with TCNQ

Novel π-extended oligo(imidazole)s composed of imidazole and thiophene ring systems, bis(imidazolyl)thiophenes 1-4, were synthesized as new building blocks for electron-donor molecules having diverse hydrogen-bonding directionalities in order to explore hydrogen-bonded charge-transfer complexes and supramolecular assemblies. The cyclic voltammetry of these compounds showed increase of electron-donating ability compared with those of 2,2′- and 4,4′-biimidazoles. In the crystal structure, 1, 2 and 3 exhibited multi-dimensional hydrogen-bonded networks via solvent molecules including the π-π interaction. Charge-transfer complexes of 1, 2 and 4 with TCNQ were characterized as partial charge-transfer complexes with segregated stacks. The compressed pellets of the TCNQ complex of 2 showed a high electrical conductivity (σ_rt=5.2×10⁻² S cm⁻¹) at room temperature with semiconducting behavior (activation energy, E_a=71 meV).     

Spectroscopic Study of Charge-Transfer Complexes of Some Dicyclohexano-Crown Ethers with Π-acceptors in Dichloromethane Solution

A spectrophotometric study was conducted on solutions of dicyclohexano-18-crown-6 and dicyclohexano-24-crown-8 with some -acceptors in methylene choride at 25 °C. The spectroscopic data indicate the formation of a charge-transfer complex. In contrast to previous results, our study shows that the Ph-O-CH2 structure is not essential for the formation of charge-transfer complexes. The formation constants (Kf) were calculated and the effect of KCl and NaCl salts on the formation and stability of the complexes is discussed.     

All-valence-electron and transition density matrix calculations of the electronic spectra of [2.2]paracyclophanequinones

The combined CNDO/S and transition density matrix methods reproduced well the UV-VIS spectra of the intramolecular charge-transfer complexes of double- and triple-layered [2.2]paracyclophanequinones in which benzene and p-benzoquinone represent the donor and acceptor layers: DA, DDA and DAD. Calculations pointed to the already known experimental bathochromic shifts of the longest wavelength absorption band for the DADDADAD transformations. The electronic transitions corresponding to this band are for DA and DDA the CT transitions of the ^* type; however, for DAD the band represents the n ^* transition localized on the acceptor ring.     

Transformations of 4,4′-dimethoxystilbene in early stages of oxidation

4,4-Dimethoxystilbene forms a dimer as a result of one-electron anodic oxidation and undergoescis/trans-isomerization when bound in a charge-transfer complex (chloranil as acceptor).A. N. Nesmeyanov Institute of Organoelemental Compounds, Russian Academy of Sciences, 117813 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 5, pp. 1219–1221, May, 1992.     

Charge-transfer salts of methylferrocenes with DCNQI derivatives (DCNQI = N,N′-dicyano-1,4-benzoquinonediimine): Crystal structures and magnetic properties

Organometallic charge-transfer salts composed of decamethyl-, octamethyl-, and dimethylferrocene with N,N′-dicyano-1,4-benzoquinonediimine (DCNQI) derivatives were prepared. [Fe(C₅Me₅)₂][(MeO)₂DCNQI] (1) and [Fe(C₅Me₄H)₂][Me₂DCNQI] (2) were 1:1 DA salts with mixed-stack structures, whereas [Fe(C₅Me₄H)₂][(MeO)₂DCNQI]₂ (3) and [Fe(C₅MeH₄)₂][DCNQI]₂ (4) were 1:2 DA salts. 3 exhibited a segregated-stack structure, in which charge separation was observed in the acceptor column. 1 became a metamagnet below T_N = 5.0 K, whereas the other salts were paramagnets. Valence states and charge-transfer (CT) transitions in these complexes are discussed based on the neutral-ionic (NI) phase diagram.     

Substituent Effects in I-Centered Radical Cations

The inductive, resonance, and polarization effects of substituents on the ionization potential of iodine n orbitals in IX, ISnR₃, and ICCX molecules, and also on the energy of the charge-transfer band in the UV spectra of the complexes of IX and ISnR₃ with tetracyanoethylene and iodine were studied. The radical cations generated by photoionization of individual molecules in the gas phase and occurring as components of contact radical ion pairs (excited state of charge-transfer complexes) in solution have similar electronic structure. The resonance parameters ⁺ _R of organosilicon, organogermanium, and organotin substituents bound to the radical cation centers I^{+ ·} and I^{+ ·}CC were calculated for the first time.