Charge-transfer absorption band electron acceptor, ionization

Fluorescence from the excited state complexes of t-1 and electron poor alkenes has been observed only with dimethylfuma-rate and fumaronitrile, both of which form weak ground state complexes with t-1 (76). Fluorescence of the same wavelength and lifetime is observed upon quenching of t or excitation in the charge-transfer absorption band of the complexes of t-1 with these acceptors. Some properties of these excited complexes and other exciplexes of t-1 are summarized in Table 7. Fluorescence maxima, like the absorption maxima, of related charge-transfer complexes, can be correlated with the donor ionization potentials (eq. 16). As shown in Fig. 3, the point for t-1 falls well below the line obtained by Shirota and co-workers (87) for the com-... 

For complexes of donors with low ionization potential or acceptors of high electron affinity (or both) the charge-transfer band may be found in the near infrared. For example, the p-phenylenediamine-chloranil complex shows a new absorption at 942 m in acetonitrile. In polar solvents the complex of )S-carotene and iodine shows a new absorption at 1000 m/z. This complex is a 1 2 complex, characterized as [(C4oH56)I ]l3". The charge-transfer absorption is attributed to the moiety (C4oH56-+I" ) in which the I is acting as a very powerful electron acceptor . A band at 900 mju has been assigned to a donor-acceptor complex of riboflavin and dihydroflavin . 

According to Mulliken theory [14-16], the energy gap ( ct) of the charge-transfer transition from the ground state to the excited ion-pair state determines the wavelength position of the CT absorption band (/Ict), i.e. E ct = hvci = hc/lci-This energy gap directly depends on the ionization potential IP) of the donor and the electron affinity [EA] of the acceptor, (Eq. 8) ... 

It is generally assumed that closed-shell molecules do not interact strongly with each other. However, as early as 1909, it was observed that new intense absorption bands were observed when I2 was dissolved in an aromatic hydrocarbon. By the mid-twentieth century the concept of nonbonded charge transfer complexes was postulated to explain the intense new absorption spectrum that arose when a closed-shell donor was added to a closed-shell acceptor. The theory of such complexes was formulated in terms of the electron affinity of the acceptor and the ionization potential of the donor and led to the development of new techniques for the determination of properties of such complexes. 

The considerable bandwidth and the high intensity are characteristic for absorption bands due to intermolecular charge transfer without ionization. They appear when an electron is transferred, under the effect of radiation energy, from a donor molecule to a free orbital of another molecule (acceptor). The charge transfer bands occur in the near ultraviolet and are broad in general. 

The preference of this conformation (the barrier hindering the rotation of the p-XCgH — at X = Cl, CHj, CFj amounts to 8.5 kcal/mol., cf. also ° 0 is assumed to be due, at least partially, to the donor-acceptor interaction between the K-system of the aryl residue and the carbenium centre at Cjg in the electronic absorption spectra of the 9-p-X-phenyl-9,10-dimethylphenanthrenium ions the charge transfer bands have been revealed whose position correlates well with the ionization potentials of respective X-substituted benzenes. Judging by the NMR- C spectra, however, the extent of this interaction in the main state of ions is insignificant — the chemical shift of the Cjo atom nearly remains unchanged as the X substituent... 

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