Donor-acceptor subunits

It is possible to assume that the absorption intensity originates mainly from the locally excited zero-order state since the AB+ zero-order state should barely contribute at all owing to the small overlap density of donor and acceptor orbitals. Therefore, the S, or T( should be reached at planar geometries by vertical excitation into the dominantly locally excited states. Only for the best donor-acceptor subunit combinations and high-lying locally excited states, a motion from planar minimum toward twisted minimum in S, or T, will be barrierless. Otherwise, a thermal activation is needed to overcome the barrier caused by crossings of zero-order states. 

In summary, if the good combination of donor-acceptor subunits can be found so that the charge-separated state does not lie substantially higher than the locally excited states and if the polar solvent is adequately chosen to lower the energy of the charge-separated state, TICT state will be responsible for the anomalous fluorescence emission. 

The formation of the donor acceptor complexes of Figure 1 occurs by self-assembly , which is defined as the noncovalent interaction of two or more molecular subunits to form an aggregate whose novel structure and properties are determined by the natures and positioning of the components [27 32]. The construction of complex, multicomponent aggregates requires intermolecular interactions that are directional and/or selective. The selectivity, particularly in asymmetric and multiply-bonded systems, allows for considerable influence over the association process by careful use of complementary components. With directionality, it is possible to control the separation and, in some cases, the relative orientation between donor-acceptor subunits. 

For example, in 1963 the photochemistry of magnesium phthalocyanine with coordinated uranium cations was studied in pyridine and ethanol and indicated the occurrence of PET to the uranium complex . A rapid photoinduced electron transfer (2-20 ps) followed by an ultrafast charge recombination was shown for various zinc and magnesium porphyrins linked to a platinum terpyridine acetylide complex . The results indicated the electronic interactions between the porphyrin subunit and the platinum complex, and underscored the potential of the linking para-phenylene bisacetylene bridge to mediate a rapid electron transfer over a long donor-acceptor distance. 

After electrochemical reduction electron is placed on the lowest unoccupied molecular orbital (LUMO) of the acceptor subunits of A-D molecule. In the electrochemical oxidation, an electron is correspondingly removed from the highest occupied molecular orbital (HOMO) of the donor moiety. In the diffusion-controlled reaction electrochemically generated ions A -D and A-D+ form an activated complex A-D + A -D for which the following reaction pathways are possible ... 

Since in this work the most studied molecular systems involve the 7r-donor and 7r-acceptor subunits, we will consider the relation between a nonbonding donor-acceptor combination, on the one hand, and the dative bond, on the other hand. 

The monophthalocyanines 106 and 107 show a weak aggregation tendency in chloroform. The latter has a self-dimerization constant of 1,175 M-1. By contrast, the donor-acceptor bis(phthalocyanine) 99 exhibits a much stronger aggregation tendency with a dimerization constant of 1.1 x 106 M-1 in chloroform. It is believed that in addition to the hydrophobic effect, the two phthalocyanine halves of compound 99 may be considered as donor and acceptor subunits that interact with each other. As revealed by electron microscopy, 99 forms one-dimensional nanoaggregates through intermolecular interactions between its complementary donor and acceptor phthalocyanine units as shown in Fig. 8. The dimerization constant of 99 is about one order of magnitude lower than that observed for the hetero-dimerization of 106 and 107, which may be due to the cyclophane step that hinders the formation of columnar aggregates of double phthalocyanine dimer. 

Assuming the translation process uses the same principles as RNA, where sequences of three subunits code for start , stop or the addition of a particular amino acid, a replicator composed of a single donor-acceptor motif could only generate a replica with the same repeat pattern. This could succeed if translation was based on a different chemical function to transcription as shown in Fig. 2.11. Subunits incorporating a range of secondary chemical functions would... 

Four donor acceptor [2]catenanes possessing cyclobis(paraquat-p-phenylene), as the 7i-electron acceptor, and 1,5-dioxynaphthalene-containing macrocyclic polyethers, as the 7i-electron donor moieties, have been constructed employing Cu+-catalyzed Huisgen 1,3-dipolar cycloaddition and Cu+2-mediated Eglinton coupling condition in the final step <07JA8236>. Desymmetrized [2]catenanes were synthesized by means of a template synthesis of pseudorotaxanes between 7i-rich crown ethers and a 7i-deficient pyromellitic subunit,... 

In a cluster formed by a donor molecule D with an acceptor A, partial charge transfer may occur between these units, but the derivation of q from quadrupole splitting measurements of one atom requires a detailed analysis. Considering the acceptor subunit ClBr, for example ... 

Similarly, the electronic absorption indicates only a weak electronic interaction between A and D [139-142]. Typically the UV-VIS absorption spectra of the molecules studied are a superposition of the bands attributed to the absorption of donor and acceptor subunits alone. In some cases a new band with a charge-transfer character appears [139,140]. This band can be separated if the parent bands (of both A and D subunits) are subtracted from the spectrum [143]. With the appropriate values of the absorption and emission maxima, the energies of the emitting species can be re-evaluated. 

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