Intramolecular Charge Transfer Complexes Research Articles

Intramolecular fluorescence quenching in luminescent copolymers containing fluorenone and fluorene units: A direct measurement of intrachain exciton hopping rate

The quenching process of fluorescence emission in polyfluorene (PF) due to the presence of intramolecular 9-fluorenone (9 FL) moieties is studied in dilute toluene solution as a function of 9 FL content in eight copolymers containing both fluorene and fluorenone units (PF/FL(x)). The absorption spectrum of PF/FL(x) copolymers clearly shows a new absorption band, redshifted relatively to the PF and 9-fluorenone absorption, which increases in intensity when the fluorenone fraction increases and also decreases with solvent polarity. Fluorescence emission spectra of PF/FL(x) show that this redshifted and unstructured emission does not coincide with the 9-fluorenone emission and, with increasing solvent polarity, it further redshifts and decreases in intensity. An isoemissive point is clearly observed on the fluorescence emission spectra of PF/FL(x) as a function of fluorenone content, showing that the new emission band is formed at the expense of PF. We propose the formation of an intramolecular charge transfer complex (ICTC) between PF units and 9-fluorenone to explain the appearance of the new emission band. Global analysis of time resolved fluorescence decays collected at 415 nm (PF emission) and 580 nm (the ICTC emission) show that three exponentials are generally needed to achieve excellent fits. Two of the components (420 ps and 6.5 ns) are independent of 9-fluorenone fraction. A further fast component is strongly dependent on fluorenone fraction and ranges between 280 and 70 ps. This component appears as a decay time at 415 nm and as a rise time at 580 nm and is ascribed to the migration of exciton to quenching sites (formation of intramolecular CT complex or exciton ionization at CT complex). A kinetic mechanism involving three different kinetic species, quenched PF units kinetically coupled with the ICTC complex, and unquenched PF units is proposed to explain the experimental data and the quenching rate constant is obtained, k(1) congruent with 10(11) s(-1). This is an experimental measurement of the intrachain exciton hopping rate.

Comparison of Electron Transfer Dynamics in Molecule-to-Nanoparticle and Intramolecular Charge Transfer Complexes

Ultrafast electron-transfer dynamics of a catechol−TiO2 nanoparticle charge transfer (CT) complex and an intramolecular CT complex, [Ti(cat)3]2- were compared. Both complexes show similar CT bands, suggesting similar donor (catechol) and acceptor(Ti) orbitals are involved in CT transition. However, the electron is localized on one Ti center in the [Ti(cat)3] complex, but can delocalize into other Ti centers in TiO2 nanoparticles. The effect of charge delocalization on charge recombination dynamics was examined by comparing these complexes. In both cases, 400 nm excitation of the CT bands led to instantaneous promotion of an electron from a catechol ligand to a Ti(IV) center(s). In the molecular complex, the back electron-transfer dynamics from the Ti center to catechol ligand was observed on a 200 fs time scale. In the adsorbate-to-nanoparticle complex, back ET was much more complex, consisting of a 0.4 ps fast component and multiple slower components. The fast component was attributed to electrons trappe...

Perturbation of Charge-Transfer Complexes in Aqueous Solutions of PEG-DNB by Adding N-Ethylcarbazole and Li+ Cations

Intramolecular charge-transfer (CT) complexation occurring in aqueous solutions of poly(ethylene glycol) monomethyl ether (PEG) capped at one end with a 3,5-dinitrobenzoyl (DNB) group was perturbed by compounds that interact with either the donor PEG chains or the acceptor DNB group. Hydrophobic N-ethylcarbazole (NEK) can be solubilized in aqueous solutions by the PEG-DNB complex surfactant. CT interactions between the NEK solute and the hydrophobic part of the surfactant, the DNB group, favor the solubilization process. The symmetric coplanar sandwich geometry of the NEK/DNB complex does not interfere with the PEG/DNB complex because of the sidewise orientation of the polyether chain. Thus, DNB forms complexes simultaneously with a π-electron donor, NEK, and a p-electron donor, PEG. An interesting consequence of the dual complex is the substantial increase in molecular weight resolution in the NMR spectra. Dissolving LiClO4 in aqueous solutions of PEG-DNB produces decomplexation. Li+ cations form ion−dip...

Unprecedented host-induced intramolecular charge-transfer complex formation.

For the first time, host-induced intramolecular charge-transfer complex formation in a guest containing both an electron donor and an electron acceptor is demonstrated in the cucurbit[8]uril cavity, leading to unusual back-folding of the guest molecule.

Intramolecular Complexation in Aqueous Solutions of an End-Capped Poly(Ethylene Glycol)

1H NMR, 2D NOESY, and spin−lattice relaxation time (T1) measurements were used to investigate the aqueous solution behavior of poly(ethylene glycol) monomethyl ether 3,5-dinitrobenzoate (PEG-DNB). A cloud point diagram revealed a phase behavior consisting of two phase separation processes, one, at high temperature, common for solutes containing oxyethylene groups whose solubility in water decreases with temperature, and the other, at low temperature, driven by the hydrophobicity of the DNB group. The intramolecular charge-transfer complexation between the ether oxygens from the PEG chains, acting as p-electron donors, and the dinitrobenzoyl group, which is a π-electron acceptor, was found to be the cause of the unusual phase behavior and 1H NMR multiplet patterns. The molecular weight selectivity of the chemical shift was correlated to the different conformations adopted by complexes having different PEG lengths. These conformations are the result of the competition between hydration forces on one hand, a...

Conformationally controlled intramolecular charge transfer complexes.

Trans-1-acceptor-2-donor-substituted cyclohexanes (1), as well as their 4- (or 5-)methyl-substituted homologues (2), have been prepared and are shown to form intramolecular charge-transfer (donor-acceptor) complexes. These weak complexes are turned on and off by the chair-chair interconversion of the cyclohexane ring. The CT absorptions have been measured and the equilibrium constants for the ring reversal have been determined by UV/vis spectroscopy at 298 K, as well as by NMR spectroscopy at two temperatures: at 183 K, by direct comparison of signals due to the two chair conformations, and at 300 K, by comparison of calculated and measured widths of the alpha-proton signals. The Gibbs free energies assigned to the donor-acceptor interactions range between 0 and -1 kcal mol(-1). A crystal structure of one of the complexes (1b) confirms the intramolecular donor-acceptor alignment and interaction. The regioisomers of the methyl-substituted complexes were characterized by NOE interaction between the methyl and an alpha-proton cis to it.

Intramolecular [2+2] cycloaddition of some 1, n-diene-1,1-dicarbonitriles : Difference between singlet and triplet reactivity

Intramolecular [2+2] cycloadditions on direct irradiation have been observed with four 1-alkene-1,1-dicarbonitriles bearing an additional CC bond positioned in Δ 5, Δ 6, or Δ 7within a flexible chain. The cycloadducts formed are the “parallel” regioisomers. These cycloadditions occur by excitation of an intramolecular charge transfer complex formed by the two double bonds. Triplet sensitisation has also been investigated in one case and found also to lead to intramolecular [2+2] cycloaddition, however furnishing the “crossed” regioisomer. It appears that the triplet cycloaddition path, other than the singlet one, does not involve an exciplex but rather proceeds in two steps via the most stable triplet 1,4-diradical.

Differently coloured crystalline modifications of organic autocomplexes of the nitroaromatic type

The improved methods for preparation of the differently coloured crystalline modifications of the organic intramolecular charge transfer complexes ( autocomplexes ), having electron acceptor fragments of the nitroaromatic type, have been developed. The preparation of the coloured modifications - yellow, orange and red for N-( p-tolyl ) -2,4,6-trinitroaniline - potential nonlinear optical material are carried out under carefully controlled conditions in different organic solvents or their mixtures. The crystalline modifications have been characterized by means IR-spectroscopic date; fluorescence properties of these are described.

Ultrafast third-order non-linear optical response of Diels–Alder adduct of fullerene C 60 with a metallophthalocyanine

The third-order optical non-linearity of a Diels–Alder adduct of fullerene C 60 with a copper phthalocyanine was investigated using the femtosecond time-resolved optical Kerr gate method at 830 nm wavelength. Its non-resonant second-order hyperpolarizability was estimated to be 5.4×10 −31 esu. The non-linearity enhancement from the formation of the intramolecular charge-transfer complex is discussed.

Generation-Dependent Intramolecular CT Complexation in a Dendrimer Electron Sponge Consisting of a Viologen Skeleton.

The homologous dendrimers G0 to G3, which consist of 3, 9, 21, and 45 viologens, behave as electron sponges and exhibit an electrochemically switchable, generation-dependent, intramolecular charge transfer (CT) complexation (see schematic diagram of G3).

A new class of poly(1,6-heptadiyne)-based photorefractive materials by metathesis polymerization

We utilized the first metathesis reaction to synthesize a new type of photorefractive polymers that contain both a carbazole moiety as a hole transporter and NLO chromophores. These polymers have a long tail band at above 700 nm, indicating the formation of intramolecular charge transfer complexes between the carbazole groups and the conjugated double bonds in the polymer backbone. Also, these polymers show two maximum values of photocurrent around 350 and 700 nm. The electro-optic coefficient (r 33 ) at the wavelength of 1.3 μm for polymer thin films poled around the 85°C were in the range of 1.6 ∼6.3 pm/V The hyperpolarizabilities, , of the resulting polymers were ca. 2.9 x 10 -32 esu. at the incident wavelength of 1.907 μm.

Linkage Length Dependence of Intramolecular Photoinduced Electron Transfer Reactions in Aromatic Donor−Viologen Acceptor Molecules Linked by Polymethylene Bridges

The intramolecualr charge transfer (CT) complex formation and photoinduced electron-transfer reactions in aromatic donor−viologen acceptor dyad systems linked by polymethylene linkage (-(CH2)n-) have been studied. The donors and the numbers of methylene unit in the linkages are 1-naphthoxyl with n = 3, 6, 8, and 10, 2-naphthoxyl with n = 3−10 and 12, and 2-dibenzofuranoyl with n = 3, 6, 8, and 10. The formation constants of the intramolecular CT complexes (Kint) were determined from the absorbance of CT absorption by using the absorptivities of the complexes determined from the intermolecular complexation between the model donor compounds, the 1-aryloxy-3-aminopropanes, and dimethyl viologen. The Kint values depend little on the length of the linkage and are about 0.2 for 1-naphthol and 2-naphthol derivatives, and 0.6 for dibenzofuranoyl derivatives. The addition of β-CD disrupts the formation of the intramolecular CT complexes. The 1:1 association constants of the dyad molecules with β-CD (KCD) were esti...

Electrochemical study of intramolecular charge transfer complexes derived from 1,4-naphthoquinone. Part 2. Electro-oxidation of 2-substituted 3-arylamino-1,4-naphthoquinone autocomplexes

Electrochemical oxidation of intramolecular charge transfer complexes (ICTs) derived from 1,4-naphthoquinone has been studied in acetonitrile using cyclic voltammetry and RRDE. The anodic oxidation of 2-substituted 3-arylamino-1,4-naphthoquinone autocomplexes AC I proceeds in one quasi-reversible stage. The ICT conformation is preserved not only during the electron transfer, but also in the cation radicals formed. The results, obtained from electro-oxidation of AC, confirm the marked electron acceptor properties of die arylamino moiety, at the same time acting as an electron donor in the intramolecular donor-acceptor (D-A) bond. A relationship between redox potentials and ICT electron transfer energy has been found in the case of AC I reversible electrochemical redox reactions.

Electrochemical study of intramolecular charge transfer complexes derived from 1,4-naphthoquinone.Part 2. Electro-oxidation of 2-substituted 3-arylamino-l,4-naphthoquinone autocomplexes1

Electrochemical study of intramolecular charge transfer complexes derived from 1,4-naphthoquinone.Part 2. Electro-oxidation of 2-substituted 3-arylamino-l,4-naphthoquinone autocomplexes1

Absorption, emission, and chiroptical spectra of neurokinin 1 tachykinin receptor antagonists: the role of charge-transfer states on the biological activity.

A spectroscopic investigation, based on both electronic absorption and emission spectra as well as on chiroptical data, was performed on novel neurokinin 1 (NK1) tachykinin receptor antagonists, exhibiting interesting biological activity. These pseudopeptides have two fluorophores, i.e. indole (I) and naphthalene (N), and a central scaffold with different conformational mobility. Absorption spectra in methanol show the presence of a new band with respect to the sum spectrum of the isolated chromophores at around 285 nm, the intensity of which linearly increases as the bioactivity increases. This absorption disappears by using dioxane as solvent. It is ascribed to an intramolecular I-N charge-transfer (CT) complex that forms to different extent, depending on the flexibility of the scaffold. Under this condition, the molecules fold and apparently attain the correct conformation for competing substance P binding to the NK1 receptor, lending plausibility to the role of dipolar charged, spatially close aromatic moieties as topochemical elements in the mechanism of action of substance P antagonists. The excited-behavior parallels that in the ground state, as the quenching of the singlet state at 340 nm is found to be linearly dependent on the biological activity, too. Upon decreasing solvent polarity (methanol vs dioxane) the emissions of the dipolar state at around 370 nm disappears, while exciplex emission in the range of 400-500 nm occurs. This transition from charge-separated to exciplex-like states by lowering the dielectric constant of the medium very likely reflects a change in the structural features of the intramolecular I-N stacked complex, from a twisted or an asymmetrically overlapped conformation of the indolyl and naphthyl rings to a face-to-face geometry. Implications of the rigidity of the molecules, arising from the formation of the intramolecular CT complex, on the ellipticity are briefly discussed.

Allenyl(vinyl)methane Photochemistry. Photochemistry of 5-[2-(1,2-Propadienyl)-Substituted Alkylidene]-2,2-dimethyl-1,3-dioxane-4,6-diones

Abstract The Photochemistry of 5-[2-(1,2-propadienyl)-substituted alkylidene]-2,2-dimethyl-1,3-dioxane-4,6-diones has been studied. These acylals formed an intramolecular charge-transfer (CT) complex between the 1,2-propadienyl and vinyl moieties in acetonitrile and in hexane, the direct photolyses of which gave (1-alkenylidene)cyclopropanes as the main products, and the intramolecular [2+2] cycloadducts, 3-alkylidene-2′,2′,5,5-tetramethylspiro[bicyclo[2.1.0]pentane-2,5′-[1,3]dioxane]-4′,6′-diones as minor products. The (1-alkenylidene)cyclopropanes were produced from the enol intermediate via a preferred 1,5-hydrogen transfer of a diradical, which was formed via an intramolecular CT excited state. On the other hand, the acetone-sensitized photolyses afforded mainly the intramolecular [2+2] cycloadducts. It is considered that the lowest triplet energies of these acylals in acetonitrile were estimated to lie in the range of 250—290 kJ mol−1, and that these energies were near to the values of the reported triplet energies of α,β-unsaturated esters, because the allene underwent a [2+2] cycloaddition to the triplet vinyl moiety, excited by triplet sensitization.

Electrochemical study of intramolecular charge transfer complexes derived from 1,4-naphthoquinone Part 1. Electroreduction

Electrochemical investigations (polarography, cyclic voltammetry, rotating ring-disk electrode voltammetry) of intramolecular charge transfer complexes (AC) derived from 1,4-naphthoquinone were performed in acetonitrile. It is concluded that in the first one-electron stage of its electrochemical reduction the AC is in a complexed form. At the same time the amino moiety, which shows a marked inductive (−I) effect, acts as an electron donor in the donor-acceptor (D-A) interaction. Facilitation of the reduction of the quinone part by the aryl amine residue is compensated by the D-A interaction, which makes the electroreduction more difficult. By using ultramicroelectrodes it was shown that the primary reaction products (naphthosemiquinone anions) easily form ion pairs with the tetrabutylammonium ion (TBA + ).

Dynamics of twisted intramolecular charge transfer complexes in polar solvents

Abstract The excited-state electron transfer and time-dependent fluorescence (TDF) for twisted intramolecular charge transfer (TICT) molecules in polar solvents are studied theoretically. This reaction class involves critical solvent stabilization of the charge-separated state with a large dipole moment compared to the less polar, locally-excited state; it is also accompanied by a significant solute geometry distortion compared to vacuum, i.e., from a planar to a twisted configuration. A theoretical framework for TICT dynamics is constructed and illustrated throughout for a model dimethylaminobenzonitrile (DMABN) solute. By employing a model two valence-bond state description for the solute in a dielectric continuum solvent for illustration, we obtain a two-dimensional free energy surface in terms of the solute torsional angle θ and a solvent coordinate s that gauges the nonequilibrium solvent orientational polarization. The TICT rate constant and TDF are analyzed via the minimum free energy solution-phase reaction path (SRP), which is found to be strongly curved on the reactive free energy surface. For slow aprotic solvents, the SRP is mainly along the θ coordinate near the transition state. There is little motion in the solvent coordinate s; the solvent lags the solute twisting motion and there is nonadiabatic nonequilibrium solvation. Near the reactant and product states, however, the SRP is almost completely along s with little solute torsional motion. This indicates that the solvent orientational polarization fluctuation is important in initiating the TICT reaction. By contrast, for faster solvents (on the same surface), the SRP becomes nearly parallel to s at the transition state, while it is almost along θ near the reactant and product states. Thus the reactive mode relevant for electron transfer at the transition state changes markedly with the solvent time scale. The consequences of the SRP curvature on the TICT rate constant and its contrasts with traditional activated electron transfer are also discussed. The SRP analysis also shows that the dynamics relevant for TDF vary during the course of the reaction. In particular, for DMABN in acetonitrile solvent, about the first 70% of the dynamic Stokes shift upon photoexcitation occurs mainly via the solute torsional dynamics with a minor s participation, followed by the remaining relaxation along s with a minimal θ motion near the perpendicular geometry. Thus the TDF dynamics probed via DMABN involve the solute twisting dynamics as well as the solvation dynamics; the former dominates at the early and middle stages of TDF until the latter eventually takes over towards the end. As the solvent becomes faster, the solvent motions begin to participate in the TDF dynamics progressively earlier; as a result, the TDF probes the concomitant motions of both the solute and solvent. This picture provides a theoretical explanation for the recent experimental findings that the dynamic Stokes shift measured with DMABN is much faster than that with coumarin dyes (with no torsional degrees of freedom) for the same solvents. Finally, it is found that the variations of the activation free energy with solvent polarity and overall reaction free energetics correlate well with Hammond postulate behavior and experimentally observed trends for activated TICT reactions.

Photoelectrodes based on acceptor–donor assemblies in functionalized polypyrrole films

AbstractThe capability to act as molecular photoelectrodes under visible light irradiation of optically transparent electrodes (ITO) modified by thin films of polypyrrole containing several kind of electron acceptor–donor assemblies has been examined. Photolysis of electrodes coated by thin films of polypyrrole substituted by a reversible electron donor (phenothiazine) in the presence of an irreversible electron acceptor (tropylium cation) in acetonitrile gives weak photocurrents. In contrast, appreciable photocurrents can be obtained using a symmetrical arrangement; viologen as reversible electron acceptor, benzilate anion as irreversible donor. The photoresponses result from the photo‐induced charge separation of the charge transfer complexes created in the film. The measured photocurrents are markedly larger (up to five times), with films of polypyrrole substituted by a reversible electron acceptor (viologen) covalently linked with a donor (phenothiazine, triphenylamine or benzidine) than with the unimolecular immobilized system in similar experimental conditions. The greater efficiency of these materials is attributed to the formation of an intramolecular charge transfer complex occurring inside films between the two molecular entities. Markedly weaker photocurrents are obtained with polypyrrole films based on bilayers of the two independent components than those with the unimolecular design, while films based on copolymers arrangements give moderately weaker photoresponses.

モンタニン型ヒガンバナ科アルカロイド，（±）‐モンタニン，（±）‐コクシニン，（±）‐パンクラシンおよび（±）‐ブルンスビジンの全合成

The first stereoselective total syntheses of the montanine-type Amaryllidaceae alkaloids, (±) -montanine 1, (±) -coccinine 2, (±) -pancracine 4, (±) -O- acetylinontanine 5, and (±) -brunsvigine 6 are described. The key steps in the reaction sequences are as follows : (1) stereoselective hydroboration-oxidation of exomethylene by means of an intramolecular charge transfer complex to provide alcohol 44 as a single isomer, (2) cyclization of tosylamide alcohol 49 with vitride® [sodium bis (2-methoxyethoxy) aluminum hydride] to provide functionalized 5, 11-methanomorphanthridine 50, which possesses the basic skeleton of montanine-type alkaloids; and (3) conversion of 56 to allylic chloride 57 a by treatment with PhSeCl in MeOH under ultrasonication followed by NaIO4 oxidation. A formal total synthesis of (±) -manthine 3 was also achieved.