Donor-acceptor transition

When the absorption spectra of 2, 26 and 27 in the region around 300 nm are compared 26 shows a band at 297 nm, whereas 2 has a less intense shoulder at 302 nm. 27 lacks an absorption band in this region, which suggests that the 302-nm band of 26 is associated with an electron donor-acceptor transition, i.e. a tz-tz transannular interaction in an electron-withdrawing 7t-acidic 39> tetrafluorophenylene ring. 

Luminescence can also arise from a trapped exciton or from a transition between two centers (donor-acceptor transitions). 

The probability of the donor-acceptor transition decreases with increasing intrapair separation ... 

Donor Donor Acceptor Transition of energy transfer Energy Energy... 

The experimental challenge is determining the I2 and I2—Lewis base adduct concentrations these are often obtained via UV-Visible spectroscopy by examining charge transfer and donor-acceptor transitions (Section 6.4.2). 

Section 6.4.2 discussed that the donor-acceptor transition in I2—Lewis base complexes is modified depending on the extent of the donor interaction with the I2 LUMO. The blue shift (to higher energy) in the d-n/ 9o- transition upon I2 complexation has also been correlated to Lewis base strength. As shown in Figure 6.10, this transition increases in energy as the base strength increases. Table 6.11 lists blue shifts induced by selected bases, used to assess Lewis basicity. 

This equation Is a kinetic simplification for proton transfers of the "normal" type (7,31), In which rate processes defining proton transfer within the donor-acceptor transition complex are not considered (not rate-llmltlng). It provides a determination of the exchange site pK (pK or pKp) If the diffusion rate constant for formation of the transition complex (k ) Is known In addition to the pK of the acceptor (pKj ). Ftoai Brdnsted plot (Figure 5) we can guess k to be approximately 10° sec, because It Is safe to assume that water, whose kp value Is 2 log units above a line of unit slope defined by the other catalysts, acts as a proton acceptor with a k of lO IT sec (31) For the U mechanism k Is the only unknown In eq. 2 and Is equal to 10 sec" as well. The result Is a pK calculation showing that the... 

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