636 89 (1970) RECUElL

541.49: 541.132

SOLVOLYTIC IONISATION OF ORGANIC CHARGETRANSFER COMPLEXES

(Short Communication)

BY

R. FOSTER and J. W. MORRIS (Chemistry Department, University of Dundee, Dundee DD 1 4HN, Scotland, U.K.)

Most organic charge-transfer complexes, when dissolved in a solvent of low ionising-power, appear to exist almost entirely as neutral electron- donor-acceptor pairs. Such complexes involve very little transfer of charge in the ground state. This type of behaviour is even observed for com- plexes formed by the combination of electron donors of low ionisation potential with electron acceptors of high electron affinity which in the solid phase have a significant degree of ionic character 1. However, when such strongly interacting systems (or mixtures of their components) are dissolved in solvents of high ionising power, ionic species are formed. It is suggested that the driving force for this ionisation is simply the energy of solvation of the ions produced 2 . This may be represented as:

(AD)cT + solvent + A-so~v + D+solv [I1

where (AD)cT is the charge-transfer complex, and A- and D+ are the two ions formed by single electron transfer from the electron donor (D) to the electron acceptor (A).

If the ionic products in solution are the result of solvation, it should be possible reversibly to change the position of equilibrium between the ionic and the neutral charge-transfer molecular species by altering the nature of the solvent.

The absorption spectrum of an equimolecular mixture of N , N , N ’ , N’- tetramethyl-p-phenylenediamine (TMPD) and chloranil in acetonitrile is shown in Fig. l(a). The absorptions at 428,448 nm are assigned to the dian- ion of the hydroquinone derived from chloranil, and the bands with maxima at 568, 618 nm to TMPD+ 3. The broad, low-intensity band at 843 nm is the intermolecular charge-transfer band of the molecular complex 3 . On

R. Foster and T. J . Thomson, Trans. Faraday SOC. 58, 860 (1962). K . M . C. Davis and M . C. R. Symons, J. Chem. SOC. 1965, 2079. G. T. Port and J. Kommandeur, Mol. Phys. 13, 373 (1967).

Solvolytic ionisation of organic charge-transfer complexes 89 (1970) RECUEIL 637

addition of an equal volume of carbon tetrachloride to this acetonitrile solution, the intensities of the absorptions of the ionic species decrease relative to the charge-transfer absorption (Fig. l(b)). The effect is accen- tuated by further addition of carbon tetrachloride (Fig. I(c)). However, when the ionising power of the solvent is increased again by now adding more acetonitrile, these trends are reversed and there is a relative intensifica- tion of the ionic species (Fig. l(d)). By further addition of carbon tetra- chloride, a second reversal, favouring the molecular species, can be demon- strated.

-.

400 500 600 700 800 900 1000 Wavelength (nm)

Fig. I. Spectra of a 1 : 1 molar mixture of chloranil and N,N,N’,N’-tetramethyl- p-phenylenediamine in (a ) acetonitrile; ( b ) addition of carbon tetrachloride to (a ) to make the solvent ratio CH3CN : CC14 = 1 : 1 v/v; (c) addition of CC14 to (b) to make the solvent ratio CH3CN : CCl4 = 1 : 6 v/v; ( d ) addition of CH3CN to (c) to make the solvent ratio 2 : I v/v.

638 R. Foster arid co., Solvolytic ionisation of organic charge-trarisfer complexes

Throughout this sequence the solution is, of course, progressively diluted. Furthermore the absorptions assigned to A- steadily decrease with time due to further irreversible reaction of A-. The instability of A- causes a gradual shift in the position of the equilibrium represented by equation 1, and makes true reversibility impossible.

TMPD+C104- (Wursters salt) and the sodium salt of the chloranil anion in acetonitrile solution.

This type of reversibility has also demonstrated for the complex of N , N , N ’ , N’-tetraethyl-p-phenylenediamine chloranil. Attempts to study the behaviour of complexes of TMPD with bromanil, fluoranil or 2,3- dichloro-5,6-dicyano-p-benzoquinone (DDQ) failed because of the high rate of hurther irreversible reactions.

Similar results have been obtained by starting with a mixture of

Acknowledgements This work was supported by a grant (to J . W. M.) from the Science

Research Council. Acknowledgement is also made to the Royal Society for the provision of a pair of variable path-length cells.

(Received February 9th, 1970).