X-ray Structures and Electronic Spectra of the Halogen Complexes between Halogen Donors and Acceptors with-Receptors - Halogen Bonding: Fundamentals and Applications - page 144

the series of complexes of the same donor, the linear dependence of the tran-
sition energy will be observed as a linear function of the reduction potential
of the acceptor.
Indeed, the charge-transfer absorption energies of dibromine complexes
with various arenes [65] and alkenes [45] both show clear correlations with
the ionization potentials of various donors (Fig. 5).
Bromocarbons are weak acceptors, as typically revealed by cyclic voltam-
metric measurements of tetrabromomethane and bromoform with reduction
waves at - 0.96 and - 1.5 V vs. SCE, respectively, in dichloromethane solu-
tion [5, 53]. As such, the electronic absorptions of their complexes with most
arene and alkene donors are expected to lie in the far-UV region, where
they are overshadowed by strong donor absorptions. Therefore among aro-
matic donors, the charge-transfer bands with the CBr 4 acceptor have been
unambiguously identified only with TMPD and DMA, since these are char-
acterized by strong donor strengths with E ox =0.12and1.11 V, respectively,
in combination with their rather high-energy absorptions. By comparison,
the absorption band of the CBr 4 complexes with halides are relatively easy
to identify owing to the spectral transparency of these anions at > 250 nm,

combined with the good donor properties of iodide, bromide and chloride
that show CV oxidation waves at 0.42, 0.96, and 1.5 V, respectively [53]. Most
important, however, is that the clear Mulliken correlations were unambigu-
ously demonstrated in the series of CBr 4 associates with different donors,
as well as in the series of bromide and iodide complexes with various ac-
ceptors [53]. Such correlations confirm the charge-transfer character of the
bromocarbon complexes with various donors and their close relationship to
theassociateswithotherorganicacceptors,aswellasthesamecharacteras
halide complexes.

λ

Fig. 5 Mulliken dependence of the charge-transfer energy in the series of dibromine com-
plexes with alkyl and chloro-substituted arenes and alkene donors (data from [45, 65])