Donor electronic absorption spectrum

We suggest that these results can be explained if the aggregation process in these solid TTF polymers proceeds by means of a two-step mechanism (Figure 7) in which the fast oxidative electron transfer step is followed by a slow process of ion clustering/reorganization which is favored by a low viscosity environment. This mechanism is consistent with the fact that the starting neutral homopolymer shows no spectroscopic evidence for site-site interaction between the pendant donors. The absorption spectrum of the polymer is... 

In CT complexes such as that between furan and tetracyanoethylene split maxima are often observed in the electronic absorption spectrum, since inner, as well as the outermost, occupied orbitals of the (donor) heterocycle are involved. However, the absorption maxima run parallel with the first ionization constants and can be used to predict them (75JCS(F1)2045, 68MI31000). Again, furan is thought to quench the fluorescence of 1-cyanonaphthalene because it forms an exciplex intermediate with a fluorescence of its own to the red side of the original one the exciplex has CT character and the frequency of the new fluorescence correlates with the first ionization potentials in a series of furans (75CC203). 

The occurrence of energy transfer requires electronic interactions and therefore its rate decreases with increasing distance. Depending on the interaction mechanism, the distance dependence may follow a 1/r (resonance (Forster) mechanism) or e (exchange (Dexter) mechanisms) [ 1 ]. In both cases, energy transfer is favored by overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor. 

Nonradiative transfer of excitation energy requires some interaction between donor and acceptor molecules and occurs if the emission spectrum of the donor overlaps the absorption spectrum of the acceptor, so that several vibronic transitions in the donor must have practically the same energy as the corresponding transitions in the acceptor. Such transitions are coupled, i.e., they are in resonance, and that is why the term resonance energy transfer (RET) or electronic energy transfer (EET) are often used. 

When the emissive state is a charge transfer state that is not attainable by direct excitation (e.g. which results from electron transfer in a donor-bridge-acceptor molecule see example at the end of the next section), the theories described above cannot be applied because the absorption spectrum of the charge transfer state is not known. Weller s theory for exciplexes is then more appropriate and only deals with the shift of the fluorescence spectrum, which is given by... 

As well as returning to the ground state by radiative or radiationless processes, excited states can be deactivated by electronic energy transfer. The principal mechanisms for this involve dipole-dipole interactions (Forster mechanism) or exchange interactions (Dexter mechanism). The former can take place over large distances (5 nm in favourable cases) and is expected for cases where there is good overlap between the absorption spectrum of the acceptor and the emission spectrum of the donor and where there is no change in the spin... 

A classic example of CT complex formation occurs in the solution of iodine (an acceptor) in cyclohexene (a donor), when the solution assumes a brown color due to a shift in its absorption spectrum. The brown is not a color in the physical sense, but rather the result of a very broad absorption band encompassing about 200 nm in the visible spectrum and evolving as a result of electronic changes in the CT complex. In contrast, a solution of iodine in CCI4—an inert solvent—is purple. 

No zirconium(III) complexes with oxygen donor ligands have been isolated. However, the electronic absorption spectra of aqueous solutions of Zrl3 have been interpreted in terms of the formation of aqua complexes (equation 4).29 The spectrum of a freshly prepared solution of Zrl3 exhibits a band at 24 400 cm-1, which decays over a period of 40 minutes, and a shoulder at 22000 cm-1, which decays more rapidly. The 24400 cm-1 band has been assigned to [Zr(H20)6]3+, and the 22000 cm-1 shoulder has been attributed to an unstable intermediate iodo-aqua complex. If it is assumed that the absorption band of [Zr(H20)6]3+ is due to the 2T 2Ee ligand-field transition, the value of A is 24 400 cm. This corresponds to a A value of 20 300 cm-1 for [Ti(H20)6]3+ 30 and 17 400 cm-1 for the octahedral ZrCl6 chromophore in zirconium(III) chloride.25... 

The combination of two redox reagents in the roles of donor and acceptor normally results in the formation of a charge transfer complex where the crystal structure of planar molecules show stacks of alternating donor and acceptor molecules (Figure la). The interplanar spacing between the molecules is usually shorter than the accepted norm of about 400 pm, a result interpreted as due to electronic interaction in agreement with the appearance of charge transfer bands in the absorption spectrum. 

Photosystem 1 is basically similar to the photosynthesizing system of bacteria just discussed. The difference between PSl and the photosystem of bacteria lies mainly in the fact that, instead of bacteriochlorophyll P890, the photochemical active centre of PSl contains chlorophyll a as a primary electron donor having the peak in the differential absorption spectrum at 700 nm and thus denoted as P700. In PS2 the primary donor of electrons is a chlorophyll molecule P680 with the peak in the differential optical spectrum at 680 nm. Photosystems 1 and 2 are located close to each other. Between them there is an electron transport chain containing molecules of plasto-quinones and cytochromes. 

It has been noted by Potasek [105] that electron tunneling in the donor-acceptor pair D-A may lead to the appearance of a charge transfer band in the absorption spectrum of this pair. The author obtained the following formula describing the dependence of the extinction coefficient, , of this band on the energy, E, of the absorbed light quantum... 

---