Donor-sensitizer-acceptor system

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). 

Synthetic polymers are best known for their insulating dielectric properties which have been exploited for numerous applications in both the electrical and electronic industries. It was found recently that some polymers can also be rendered conductive by an appropriate treatment, thus opening the way to a new field of applications of these materials (2, 3). Usually, electrical conductivity is obtained by doping a neutral polymer, rich in unsaturation, with donor or acceptor molecules. These polymers are rather difficult to synthesize, which makes them very expensive besides they are often sensitive to environmental agents, like oxygen or humidity, thus restricting their practical use to oxygen-free systems. 

Some bichromophoric systems, whose structure is based on the donor-bridge-acceptor principle, can undergo complete charge transfer, i.e. electron transfer. The resulting huge dipole moment in the excited state explains the very high sensitivity to solvent polarity of such molecules. An example is FP (l-phenyl-4-[(4-cyano-l- ... 

It is interesting to note that energy transfer in the rapid diffusion limit is sensitive to the distance of closest approach of the donor and acceptor. Based on this observation, some interesting applications in biology have been described, such as the measurement of the distance at which an acceptor is buried in biological macromolecules and membrane systems. 

The success of the energy transfer theory in correlating data for a number of systems with sensitizers of widely varying structure, particularly aromatic hydrocarbons for which bond formation between donor and acceptor does not appear to be a reasonable process, led to its acceptance as the general mechanism for photosensitized olefin isomerization. However, there are special cases in which another mechanism of sensitization is operative. 

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)... 

Those photosensitive systems mentioned above consist of at least one vinyl compound which has an electron donating or accepting property. When both acceptor and donor are non-polymerizable, the system is not photosensitive. Photopolymerization of styrene is not sensitized by the ECZ-CH3CN pair. The definition of donor and acceptor is a matter of relativity. Styrene is by no means neutral, but there should be no objection to considering it as a weaker donor than VCZ or ECZ and a weaker acceptor than AN or CH8CN. Photoirradiation of AN, VCZ or styrene alone in a neutral solvent, such as benzene, or in bulk does not bring about any appreciable rate of polymerization. 

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). 

The last four columns of Table 4 contain numerical data that largely support this contention. The entries represent the changes in the various quantities that arise when either the proton donor or acceptor molecule is rotated by 40° from the equilibrium geometry. For example, a 40° distortion of the donor molecule, when R=3.25 A, raises the energy of the system by 4.43 kcal/mol, hence diminishing the H-bond energy by this amount. The same distortion reduces the H-bond energy by 6.24 kcal/mol when the two waters are separated by 2.75 A. The last two columns indicate that the interaction is somewhat less sensitive to 40° rotations of the proton acceptor molecule. 

The majority of the research on the photochemistry of porphyrins linked to other moieties has been in the area of photoinduced electron transfer, and the systems studied are all in some sense mimics of the photosynthetic process described above. The simplest way to prepare a system in which porphyrin excited states can act as electron donors or acceptors is to mix a porphyrin with an electron acceptor or donor in a suitable solvent. Experiments of this type have been done for years, and a good deal about porphyrin photophysics and photochemistry has been learned from them. Although these systems are easy to construct, they have serious problems for the study of photoinduced electron transfer. In solution, donor-acceptor separation and relative orientation cannot be controlled. As indicated above, electron transfer is a sensitive function of these variables. In addition, because electron transfer requires electronic orbital overlap, the donor and acceptor must collide in order for transfer to occur. As this happens via diffusion, electron transfer rates and yields are often affected or controlled by diffusion. As mentioned above, porphyrin excited singlet states typically have lifetimes of a few nanoseconds. Therefore, efficient photoinduced electron transfer must occur on a time scale shorter that this. This is difficult or impossible to achieve via diffusion. Thus, photoinduced electron transfer between freely diffusing partners is confined mainly to electron transfer from excited triplet states, which have the required long lifetimes (on the micro to the millisecond time scale). 

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