Charge-transfer complexes. Part III. Solvent shifts of the charge-transfer band of complexes formed between halide ions and organic electron acceptors

Abstract

The energy of the charge-transfer band maximum, E_C.T., of the complexes iodide–1,3,5-trinitrobenzene and bromide–chloranil, has been measured in a number of solvents having a wide range of polarity. Contrary to previous reports, a good linear correlation was found between E_C.T. and the empirical measures of solvent polarity, Z and E_T, for solvents of high polarity. Departures from this linear correlation were found for solvents of low polarity. These have been attributed to the raising of the energy of the charge-transfer transition owing to stabilisation of the ground state of the halide ion by the cation in an ion pair. Anomalous results are also obtained for those solvents known to form a specific complex with one of the components.