Donor-acceptor system

Several organofullerene donor-acceptor molecular material hybrid systems have been synthesized via 1,3-dipolar cycloaddition reactions of azomethine ylides, via Bingel cyclopropanation and methanofullerene formation intermediates as well as via cycloaddition reactions, that have already been discussed in previous sections. The majority of such hybrid systems possess always as acceptor unit the fullerene core and as donor moieties porphyrins, tetrathiafulvalenes, ferrocenes, quinones, or electron-rich aromatic compounds that absorb visible light [190-193]. The most active research topic in this particularly technological field relies (i) on the arrangement of several redox-active building blocks in 

In Chap. 8, we treated organic crystals which are composed of a single type of molecules. As we expect of organic substances, these crystals are semiconductors or insulators. The LUMOs of the molecules form the conduction band, the HO-MOs form the valence band, and the energy gap is large compared to ksT. At room temperature, typical values of the conductivity are less than about lO (S2 cm) , and values of the mobilities are less than around 1 cm /Vs. The lowest-lying optical excitation states of these substances are Frenkel excitons. 

Copyright 2007 WILEY-VCH Verlag GmbH Go. KGaA, Weinheim ISBN 978-3-527-40540-4 

Organic crystals with high dark conductivities have become an active and extensive area of research in recent decades. We refer the reader to the review articles [Ml, M4, M5] and [4]. 

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The aim of this chapter is to give a state-of-the-art report on the plastic solar cells based on conjugated polymers. Results from other organic solar cells like pristine fullerene cells [7, 8], dye-sensitized liquid electrolyte [9], or solid state polymer electrolyte cells [10], pure dye cells [11, 12], or small molecule cells [13], mostly based on heterojunctions between phthaocyanines and perylenes [14], will not be discussed. Extensive literature exists on the fabrication of solar cells based on small molecular dyes with donor-acceptor systems (see for example [2, 3] and references therein). 

Comparison of the spectral response and of the power efficiency of these first conjugated polymer/fullerene bilayer devices with single layer pure conjugated polymer devices showed that the large potential of the photoinduced charge transfer of a donor-acceptor system was not fully exploited in the bilayers. The devices still suffer from antibatic behavior as well as from a low power conversion efficiency. However, the diode behavior, i.e. the rectification of these devices, was excellent. 

General Considerations of Separated Donor-Acceptor Systems 83... 

The microphase structure of amphiphilic polyelectrolytes in aqueous solution provides photoinduced ET with an interesting microenvironment, where a photoactive chromophore and a donor or acceptor can be held apart at different locations. Photoinduced ET in such separated donor-acceptor systems allows an efficient charge separation to be achieved. 

Although the electrostatic potential on the surface of the polyelectrolyte effectively prevents the diffusional back electron transfer, it is unable to retard the very fast charge recombination of a geminate ion pair formed in the primary process within the photochemical cage. Compartmentalization of a photoactive chromophore in the microphase structure of the amphiphilic polyelectrolyte provides a separated donor-acceptor system, in which the charge recombination is effectively suppressed. Thus, with a compartmentalized system, it is possible to achieve efficient charge separation. 

However, these observations are not proof of the role of a donor-acceptor complex in the copolymcrization mechanism. Even with the availability of sequence information it is often not possible to discriminate between the complex model, the penultimate model (Section 7.3.1.2) and other, higher order, models.28 A further problem in analyzing the kinetics of these copolyincrizations is that many donor-acceptor systems also give spontaneous initiation (Section 3.3.6.3). 

The effects of spacer length on the fluorescence quantum yields of the benzofurazan compounds bearing a donor-acceptor system were reported <2002MI11>. 

Based on the fundamental dipole moment concepts of mesomeric moment and interaction moment, models to explain the enhanced optical nonlinearities of polarized conjugated molecules have been devised. The equivalent internal field (EIF) model of Oudar and Chemla relates the j8 of a molecule to an equivalent electric field ER due to substituent R which biases the hyperpolarizabilities (28). In the case of donor-acceptor systems anomalously large nonlinearities result as a consequence of contributions from intramolecular charge-transfer interaction (related to /xjnt) and expressions to quantify this contribution have been obtained (29). Related treatments dealing with this problem have appeared one due to Levine and Bethea bearing directly on the EIF model (30), another due to Levine using spectroscopically derived substituent perturbations rather than dipole moment based data (31.) and yet another more empirical treatment by Dulcic and Sauteret involving reinforcement of substituent effects (32). 

Free radical alternating 1 1 copolymerization of donor-acceptor monomer systems has been known for quite some time. Linear copolymers from donor-acceptor systems... 

In this paper, we report efforts to find donor/acceptor systems, comprised of at least one multifunctional monomer, capable of sustaining rapid free-radical polymerization without the need for external photoinitiators. Although we will include in this report comonomer systems which form ground state CT complexes, we stress that the primary mechanism for generating free-radical in each case may not be via excitation of ground state CT complexes. 

Figure 6.7 Helical columns based on a donor-acceptor system of atkoxytriphenylenes and menthol 3,5-dinitrobenzoates.

A. M. Oliver, D. C. Craig, M. N. Paddon-Row, J. Kroon, J. W. Verhoeven, Strong Effects of the Bridge Configuration on Photoinduced Chaige Separation in Rigidly Linked Donor-Acceptor Systems , Chem. Phys. Lett. 1988, 150, 366-373. 

The location of the hydrogen atoms in hydrogen bonded systems is often difficult to ascertain. When X-ray diffraction is used there is an experimental limitation to face, as it is usually difficult to locate the very light H-atom in Fourier maps and, even when this is possible, the technique can provide information on electron density centroids rather than on the position of the light nucleus. Neutron diffraction is required for an unambiguous location of the H-atom. In ionic hydrogen bonds the situation may occur where a knowledge of the proton position in a donor-acceptor system is necessary to know whether proton transfer, i.e. protonation of a suitable base, has occurred or not. 

Excitation energy transfer in dendritic host-guest donor-acceptor systems. Chem Phys Chem 3 1005-1013... 

Hermant R. M., Barker N. A. C., Scherer T., Krijnen B. and Verhoeven J. W. (1990) Systematic Studies of a Series of Highly Fluorescent Rod-Shaped Donor-Acceptor Systems, J. Am. Chem. Soc. 112, 1214-1221. 

X-ray crystal analysis indicates that the DABCO/CBiq complex consists of alternating planes of the diamine and carbon tetrabromide in which each acceptor is bound to two donor units. The quinuclidine/CBr4 complex consists of pairs of donor-acceptor systems in which every quinuclidine molecule is bound to only a single molecule of carbon tetrabromide. 

Recognition Based on Cation Control of Photoinduced Electron Transfer in Nonconjugated Donor-Acceptor Systems... 

Figure 2.2. Principle of cation recognition based on cation control of photoinduced electron transfer in nonconjugated donor-acceptor systems. (Example from Ref. 26.)...

Senesi and Testini [147,156] and Senesi et al. [150,153] showed by ESR the interaction of HA from different sources with a number of substituted urea herbicides by electron donor-acceptor processes involves organic free radicals which lead to the formation of charge-transfer complexes. The chemical structures and properties of the substituted urea herbicides influence the extent of formation of electron donor-acceptor systems with HA. Substituted ureas are, in fact, expected to act as electron donors from the nitrogen (or oxygen) atoms to electron acceptor sites on quinone or similar units in HA molecules. 

Two donor-acceptor systems were examined, with Coumarin 1 (Cl) and 9-aminoacridine (9AA) as donors and Rhodamine 6G (R6G) as the acceptor. Initial experiments were performed to compare the amount of transfer observed in bulk solution and in particles made of the same material. Glycerol was chosen as the solvent, mainly because of its low vapor pressure and high viscosity. The low vapor pressure was necessary so that particles would be relatively stable in size, and the high viscosity ensures that the excited donor is essentially stationary for the lifetime of the excited state. The concentrations used were chosen to minimize donor reabsorption and to make the extinction of the donor considerably larger than the extinction of the acceptor at the excitation wavelength. Excitation wavelengths of either 365 or 387 nm were used in the experiments. Concentration ratios, donor to acceptor, of 10 1 and... 

Horak, J., Maier, N.M., and Lindner, W., Investigations on the chromatographic behavior of hybrid reversed-phase materials containing electron donor-acceptor systems ii. Contribution of pi-pi aromatic interactions, J. Chromatogr. A, 1045, 43, 2004. 

In an electron donor-acceptor system (D-A) in which an electron donor (D) and an electron acceptor (A) are separated by a carbon chain, the exciplex state may be considered as a resonance hybrid of the electron transfer configuration (D A" ) mixed with the locally excited configuration (D A) or (DA ) [211]. 

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