Shift of the Acceptor Absorption Spectrum in n–π Charge Transfer Complexes

Abstract

IT has been well established1–4 that in n–π charge transfer complexes with iodine as the acceptor the absorption band of the free iodine at about 520 nm is shifted to shorter wave-lengths in the complexes. The magnitude of the shift can be roughly correlated with the heats of formation of the different iodine complexes4. It has also been shown that in n–π complexes with chloranil as the acceptor the absorption band of free chloranil at about 285 nm is shifted to longer wave-lengths in the complexes5,6, n–π complexes with chloranil in highly polar solvents exhibit the spectrum of the chloranil negative ion7. It has been suggested8 that in these cases charge transfer is complete in the ground-state. The principal absorption bands of the chloranil negative ion lie to the long wave-length side of the absorption bands of the chloranil, while the absorption bands of the I2 negative ion lie to the short wavelength side of the band of the I2 molecule. It has been shown that the I2 negative ion has an absorption band at 400 nm9,10. This appears to be the limit to which the ‘Blue shift’ of iodine in n–π complexes goes4. The spectra of some iodine complexes could well be interpreted as due to the presence of the I2 negative ion stabilized by complete charge transfer in the ground state.