Sensitizer-acceptor system

Figure 4.8 Simplified energy level scheme of a sensitizer-acceptor system used for photon energy upconversion (sensitized delayed fluorescence). The first step is absorption (Abs) of a photon by the sensitizer followed by efficient intersystem crossing (ISC) In the sensitizer molecules. The triplet Is then...

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). 

In a penalty test, a property cf the system is modified to reduce the probability of the desired result. For example, to predict safety, a particular expl train interface may be tested with a standard donor and a more sensitive acceptor conversely, to predict reliability, a less sensitive acceptor material is used. If this probability is reduced sufficiently, it is possible to obtain mixed responses (that is, some fires and some no-fires) with samples of reasonable size, and to develop data from which the mean value of the penalty and its standard deviation (as well as confidence limits) can be established. These estimates can be used iri statistical extrapolation to estimate safety or reliability under the original design conditions. The term VARICOMP (VARIation of explosive COMPosition) was coined by J.N. Ayres for a method developed at the Naval Ordnance Lab, White Oak, in the 1950 s and early 1960 s (Ref 1)... 

The ability of the maleimide unit to switch off emission is also exemplified by 86, due to Verhoeven s coworkers [161] at the University of Amsterdam and Akzo Nobel in The Netherlands. Again the Michael reaction of the maleimide with thiols produces nicely emissive material. Solvent-sensitive emission, characteristic of these donor-acceptor systems with strongly coupling bridges, is a special feature of 86 after thiolation. An added interest of 86 stems from the occurrence of PET to the maleimide unit from the through-bond charge-transfer excited state [162], an unusual combination of photophenomena. 

In this context it is useful to remember that the concept of the possible recombination of triplet radical ion pairs is not an ad hoc assumption to rationalize certain Z - E isomerizations, although the CIDNP effects observed during an isomerization reaction played a key role in understanding this mechanism. Triplet recombination has been accepted in several donor-acceptor systems as the mechanism for the generation of fast (optically detected) triplets [169-171], and invoked for several other reaction types [172]. The CIDNP technique is a sensitive tool for the identification of this mechanism, for example, in the geometric isomerization of Z- and E-1,2-diphenylcyclopropane and in the valence isomerization of norbornadiene (vide infra). Most of these systems have in common that the triplet state can decay to more than one minimum on the potential surface of the parent molecule. 

The electronic coupling between an initial (reactant) and a final (product) state plays a key role in many interesting chemical and biochemical photoinduced energy and electron transfer reactions. In excitation (or resonance) energy transfers (EET or RET) [1,2], the excitation energy from a donor system in an electronic excited state (D ) is transferred to a sensitizer (or acceptor) system (A). Alternatively, in photoinduced electron transfers (ET) [3,4], a donor (D) transfers an electron to an acceptor (A) after photoexcitation of one of the components (see Figure 3.50). 

Holley, A. and Doving, K.B. (1977). Receptor sensitivity, acceptor distribution, convergence and neural coding in the olfactory system. In J. LeMagnen and P. MacLeod (Eds), Olfaction and Taste, VI. pp. 113-133. Information Retrieval Ltd., London. 

A combination of sensitizer dye, N-diethylaminoketobiscoumarine (DEKB), and N-dimethylaminoethylbenzoate (DMAEB) gives a donor acceptor system (see Figure 1.2). 

In this paper we want to concentrate on the electronic couplings. Assuming that the energetics and the intramolecular as well as the solvent effects vary less sensitively than the electronic coupling term for different donor acceptor systems, we find a good correlation between the electronic couplings and the observed rate constants. 

Representative chemical shifts from the large amount of available data on isothiazoles are included in Table 4. The chemical shifts of the ring hydrogens depend on electron density, ring currents and substituent anisotropies, and substituent effects can usually be predicted, at least qualitatively, by comparison with other aromatic systems. The resonance of H(5) is usually at a lower field than that of H(3) but in some cases this order is reversed. As is discussed later (Section 4.17.3.4) the chemical shift of H(5) is more sensitive to substitution in the 4-position than is that of H(3), and it is also worth noting that the resonance of H(5) is shifted downfield (typically 0.5 p.p.m.) when DMSO is used as solvent, a reflection of the ability of this hydrogen atom to interact with proton acceptors. This matter is discussed again in Section 4.17.3.7. 

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