Charge transfer interactions in polyphenylacetylene-electron acceptor systems

Charge transfer (CT) interaction of polyphenylacetylene (PPA) with iodine, arsenic pentafluoride and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) in solution is associated with the formation of broad CT bands extending beyond the absorption edge of the polymer into the near infra-red and with a substantial loss of the polymer's effective conjugation. For PPA-I₂ and PPA-DDQ in dilute solutions and at low doping levels, the 1:1 CT complex is susceptible to a Benesi-Hildebrand analysis. The microstructure of the polymer has a pronounced effect on the observed interaction rates and equilibrium constants. At high acceptor loadings, there are complicated time-dependent equilibria involving several complexes of different stoichiometry. The role of the CT state in this electroactive polymer is discussed in the context of a band-like model.     

ESCA studies of charge transfer interactions in electroactive polymers

X-ray photoelectron spectroscopy (XPS) data suggest that proton modifications of nitrogens in polypyrrole (PPY) give rise to a number of intrinsic redox states analogous to those observed in polyaniline (PAN). The behavior of the corresponding oxidation states in both polymers towards oxidation/reduction, deprotonation/reprotonation or charge transfer (CT) interactions are grossly similar. For the thiophene polymers, such as the poly(2,2′-bithiophene) (PBT) complexes and poly(3-methylthiophene) (P3MT) complexes, XPS results reveals the simultaneous presence of neutral and polarized (or partially charged) species in both carbon and sulfur. The relative amounts of the neutral and polarized species vary in accordance with the oxidation level of the polymer. These results suggest that each dopant anion is associated with a thiophenium ion in the polymer chain. Substantially lower extent of CT is observed in the complexes involving photoconductive substituted polyacetylenes, such as polyphenylacetylene (PPA) and poly[[o-(trimethylsilyl)phenyl] acetylene] or poly(o-Me₃SiPA).     

Positron annihilation studies of some charge transfer molecular complexes

Positron annihilation lifetimes were measured for some solid charge transfer (CT) molecular complexes of quinoline compounds (2,6-dimethylquinoline, 6-methoxyquinoline, quinoline, 6-methylquinoline, 3-bromoquinoline and 2-chloro-4-methylquinoline) as electron donor and picric acid as an electron acceptor. The infrared spectra (IR) of the solid complexes clearly indicated the formation of the hydrogen-bonding CT-complexes.

The annihilation spectra were analyzed into two lifetime components using PATFIT program. The values of the average and bulk lifetimes divide the complexes into two groups according to the non-bonding ionization potential of the donor (electron donating power) and the molecular weight of the complexes. Also, it is found that the ionization potential of the donors and molecular weight of the complexes have a conspicuous effect on the average and bulk lifetime values. The bulk lifetime values of the complexes are consistent with the formation of stable hydrogen-bonding CT-complexes as inferred from the IR-spectral data.     

X-ray photoelectron spectroscopic studies of charge transfer interactions in electroactive polyaniline

Four modes of charge transfer interactions in polyaniline (PAN), viz. acid protonation, self-doping, charge transfer interactions with organic acceptors, and charge transfer interactions with surface grafted functional polymers have been studied by X-ray photoelectron spectroscopy (XPS). In the case of acid protonation, the protonation behavior of volatile and non-volatile acid differs. The structures of sulfonated leucomeraldine (LM) and nigraniline (NA) are similar to those of sulfonated and self-protonated emeraldine (EM). A substantially higher degree of charge transfer interaction with the organic acceptors is observed for EM film that has been subjected to one cycle of acid/base treatment. The charge transfer interactions with the organic acceptors have proceeded further than the pure formation of molecular complexes. Both pristine and Ar plasma or O₃ pretreated EM films are susceptible to surface modifications by graft copolymerization. The protonic acid functional groups of the graft readily give rise to a self-protonated EM surface.     

Progress in charge transfer systems utilizing porphyrin donors and simple aromatic diimide acceptor units

An overview of the evolution of artificial photosynthetic charge transfer systems containing porphyrin donors and pyromellitic or naphthalene diimide acceptor units is presented. Progression in this area of research is highlighted by the complexity of the systems, the nature of the medium separating donor and acceptor as well as the progression in the lifetime of the charge-separated state upon photoexcitation. A number of supramolecular systems that utilize hydrogen bonding or axial ligation of zinc porphyrins as a means for spatial orientation are highlighted.     

Stepwise charge transfer complexation of some pyrimidines with sigma-acceptor iodine involving a new unconventional acceptor

Interactions of some pyrimidine derivatives, 4-amino-2,6-dimethylpyrimidine, kyanmethin, (4AP), 2-amino-4,6-dimethylpyrimidine (2AP), 2-aminopyrimidine (AP), 2-amino-4-methylpyrimidine (AMP), 2-amino-4-methoxy-6-methylpyrimidine (AMMP), and 4-amino-5-chloro-2,6-dimethylpyrimidine (ACDP) as electron donors, with iodine (I(2)), as a typical sigma-electron acceptor, have been studied. Electronic absorption spectra of these interactions in several organic solvents of different polarities have performed instant appearance of clear charge transfer (CT) bands. Formation constants (KCT), molar absorption coefficients (epsilonCT) and thermodynamic properties, DeltaH, DeltaS, and DeltaG, of these interactions have been determined and discussed. Electronic absorption spectra of the solutions of the synthesized pyrimidines-iodine, P-I2, CT complexes have shown the characteristic bands of the triiodide ion, I3*. UV/vis spectral tracking of these interactions have shown that by lapse of time the first formed CT complex, P-I2, is transformed to the corresponding triiodide complex, P(+)I.I3*, then, the later interacts as a new unconventional acceptor and it forms a CT complex of the form (P).(P+I.I3*). Elemental analyses of these solid complexes have indicated the stoichiometric ratio 2:2, or formally 1:1, P:I2.     

Spectroscopic studies of charge transfer complexes between colchicine and some pi acceptors

Charge transfer complexes between colchicine as donor and pi acceptors such as tetracyanoethylene (TCNE), 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), p-chloranil (p-CHL) have been studied spectrophotometrically in dichloromethane at 21 degrees C. The stoichiometry of the complexes was found to be 1:1 ratio by the Job method between donor and acceptors with the maximum absorption band at a wavelength of 535, 585 and 515 nm. The equilibrium constant and thermodynamic parameters of the complexes were determined by Benesi-Hildebrand and van't Hoff equations. Colchicine in pure form and in dosage form was applied in this study. The formation constants for the complexes were shown to be dependent on the structure of the electron acceptors used.     

Influence of acceptor functionality on charge transfer interactions in mixtures of poly(9-vinylcarbazole) with nitroaromatic compounds

Charge transfer interactions in mixtures of poly(9-vinylcarbazole) with three nitro compounds (4,4′dinitrodibenzyl, ethyl 3,5-dinitrobenzoate and 2,2′,4,4′-tetranitrodibenzyl) were examined.     

Resonance Raman studies of excited state structural displacements of conjugated polymers in donor/acceptor charge transfer complexes

Resonance Raman spectra of poly(2-methoxy-5-(3'-7'-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) and small molecule acceptor blend charge transfer (CT) complexes reveal long and detailed progressions of overtone and combination bands. These features are sensitive to the specific MDMO-PPV/acceptor interactions and enable quantitative calculations of vibrational mode specific displacements of the polymer CT complex.     

XPS studies of octadecylphosphonic acid (OPA) monolayer interactions with some metal and mineral surfaces

Partial and complete self‐assembled monolayers (SAMs) of octadecylphosphonic acid (OPA) have been deposited onto air‐exposed surfaces of the metals copper, silver, gold, iron, silicon and aluminium, as well as onto freshly cleaved, air‐exposed surfaces of the minerals muscovite and biotite. The line width of the C(1s) signal in the XPS spectra of the surface narrowed, as the extent of coverage increased to 100%, to a half‐width of 0.9 eV. Moreover, the line widths associated with the insulating muscovite substrate also became substantially narrower as OPA coverage increased. Binding energy differences on this charge‐shifted surface were found to be more consistent when OPA was used as a charge reference, compared to using adventitious carbon as a reference. OPA coverage of the air‐exposed metals copper, silver, gold and iron also produced narrow C(1s) spectra whose binding energies were consistently close to 284.9 eV. The C(1s) binding energy positions on Al and Si samples were charge‐shifted by the insulating nature of the thin oxide formed on air exposure, or by the insulating nature of the substrate in the case of the minerals. Correction of the observed C(1s) energy position to 284.9 eV gave sets of elemental binding energies for the substrate materials that were reproducible. Thus, OPA coverage could be a possible alternative candidate for use in charge correction of binding energies of insulating materials. The OPA coverage cases were modelled using the software QUASES^? Analyse. For the substrates copper, silver, gold, iron and aluminium, analyses of the metal core line spectra gave OPA overlayer thicknesses close to those measured by AFM (1.6 nm). However, QUASES^? analyses of the C(1s) extrinsic backgrounds for the same surfaces required the use of an attenuation length of only 0.4 nm to derive a comparable thickness—much lower than literature values for carbon. This discrepancy is ascribed to the structured nature of the SAM. Copyright © 2005 John Wiley & Sons, Ltd.     

Computational studies on optoelectronic and charge transfer properties of some perylene‐based donor‐π‐acceptor systems for dye sensitized solar cell applications

We report DFT studies on some perylene‐based dyes for their electron transfer properties in solar cell applications. The study involves modeling of different donor‐π‐acceptor type sensitizers, with perylene as the donor, furan/pyrrole/thiophene as the π‐bridge and cyanoacrylic group as the acceptor. The effect of different π‐bridges and various substituents on the perylene donor was evaluated in terms of opto‐electronic and photovoltaic parameters such as HOMO‐LUMO energy gap, λ_max, light harvesting efficiency(LHE), electron injection efficiency (Ø_inject), excited state dye potential (E^dye*), reorganization energy(λ), and free energy of dye regeneration (). The effect of various substituents on the dye–I₂ interaction and hence recombination process was also evaluated. We found that the furan‐based dimethylamine derivative exhibits a better balance of the various optical and photovoltaic properties. Finally, we evaluated the overall opto‐electronic and transport parameters of the TiO₂‐dye assembly after anchoring the dyes on the model TiO₂ cluster assembly.     

Electronic polarization reversal and excited state intramolecular charge transfer in donor/acceptor ethynylpyrenes

An attempt to tune the electronic properties of pyrene (Py) by coupling it with a strong electron donor (-PhNMe2, DMA)/acceptor (anthronitrile, AN) through an ethynyl bridge has been undertaken. A moderate electron donor (iPrOPh-, IPP)/acceptor (2-quinolinyl, 2Q) has also been incorporated, and all four molecules were studied with reference to a neutral molecule, namely, 1-phenylethynylpyrene (PhEPy). All the arylethynylpyrenes (ArEPy's) have been thoroughly characterized, and their electronic properties were studied by absorption and emission spectral properties of these ArEPy's. The electrochemical characteristics were also studied for arriving at the electrochemical band gap which has been compared with the HOMO-LUMO energy gap derived from the photophysical measurements and theoretical calculations performed by density functional theory (DFT) using B3LYP/6-31G basis sets. The results obtained from experimental and theoretical studies are critically discussed.     

Fast intramolecular charge transfer with a planar rigidized electron donor/acceptor molecule

The planar rigidized molecule fluorazene (FPP) undergoes fast reversible intramolecular charge transfer (ICT) in the excited state, with a reaction time of 12 ps in the polar solvent ethyl cyanide at -45 degrees C. The ICT state of FPP has a dipole moment mu(e)(ICT) of 13 D, much larger than that of the locally excited state LE (1 D). The ICT behavior of FPP is similar to that of its flexible counterpart N-phenylpyrrole (PP), for which mu(e)(ICT) = 12 D. These results show that intramolecular charge transfer to a planar ICT state can occur efficiently. In designing ICT systems capable of rapid switching, it is therefore important to realize that large amplitude motions such as those necessary for the formation of a twisted intramolecular charge transfer (TICT) state are not required.     

Influence of donor:acceptor ratio on charge transfer dynamics in non-fullerene organic bulk heterojunctions

The donor:acceptor(D:A) blend ratio plays a very important role in affecting the progress of charge transfer and energy transfer in bulk heterojunction(BHJ) orga nic solar cells(OSCs).The proper D:A blend ratio can provide maximized D/A interfacial area for exciton dissociation and appro p riate domain size of the exciton diffusion length,which is beneficial to obtain high-performance OSCs.Here,we comprehensively investigated the relationship between various D:A blend ratios and the charge transfer and energy transfer mechanisms in OSCs based on PBDB-T and non-fullerene acceptor IT-M.Based on various D:A blend ratios,it was found that the ratio of components is a key factor to suppress the formation of triplet states and recombination energy losses.Rational D:A blend ratios can provide appropriate donor/accepter surface for charge transfer which has been powerfully verified by various detailed experimental results from the time-resolved fluorescence measurement and transient absorption(TA) spectroscopy.Optimized coherence length and crystallinity are verified by grazing incident wide-angle X-ray scattering(GIWAXS) measurements.The results are bene ficial to comprehend the effects of various D:A blend ratios on charge transfer and energy transfer dynamics and provides constructive suggestions for rationally designing new materials and feedback for photovoltaic performance optimization in non-fullerene OSCs.     

Deep insight into the charge transfer interactions in 1,2,4,5-tetracyanobenzene-phenazine cocrystal

A new charge transfer cocrystal of 1,2,4,5-tetracyanobenzene (TCNB)-phenazine (PTC) was prepared by solvent evaporation method. The donor and acceptor molecules of cocrystal are stacked face to face with a mixed-stacking, implying a strong charge transfer (CT) interactions in the cocrystal system. The spectroscopic studies, single-crystal X-ray diffraction structure, density functional theory (DFT) and Hirschfield surfaces calculations are carried out to explore the relationship between structure and properties of cocrystal system, which show that the intermolecular interactions in PTC are stronger than those of single components, leading to the stability and photophysical behaviors of cocrystal different from their constitute units. This study will be helpful for the design and preparation of multifunctional cocrystal materials.     

Dioxastilbenophanes—synthesis and charge transfer complexation studies

Intramolecular McMurry coupling of dialdehydes derived from xylenyl dibromide and 4-hydroxy benzaldehyde afforded cis-stilbenophanes along with cyclophane diols. Stilbenophanes with a large cavity were also synthesized. Charge transfer complexations of the stilbenophanes with TCNE, TCNQ and PQT were studied. Some stilbenophanes form a relatively stronger complex with PQT rather than with TCNE and TCNQ.     

Scaling laws for charge transfer in multiply bridged donor/acceptor molecules in a dissipative environment

The ability of multiple spatial pathways to sum coherently and facilitate charge transfer is examined theoretically. The role of multiple spatial pathways in mediating charge transfer has been invoked several times in the recent literature while discussing charge transfer in proteins, while multiple spatial pathways are known to contribute to charge transport in metal-molecule-metal junctions. We look at scaling laws for charge transfer in donor-bridge-acceptor (D-B-A) molecules and show that these scaling laws change significantly when environment-induced dephasing is included. In some cases, D-B-A systems are expected to show no enhancement in the rate of charge transfer with the addition of multiple degenerate pathways. The origins of these different scaling laws are investigated by looking at which Liouville space pathways are active in different dephasing regimes.