Conductivity titrations of charge-transfer complexes in solution—II

Electmchimica Acta. 1966, Vol. 11. pp. 1163 to 1169. Pergamon Preaa Ltd. Printed in Northern Ireland

CONDUCTIVITY TITRATIONS OF CHARGE-TRANSFER COMPLEXES IN SOLUTION-II* F. GUTMANN and H. KEYZER Department of Physical Chemistry, University of New South Wales, Sydney, N.S.W., Australia Abstract-The method described in Part I has been further developed and applied to a number of complexes. Experimental techniques are discussed and the value of the conductivity peak is expressed quantitatively in terms of a molar conductivity coefficient a, + (l/aM)(o, - uO)/uOwhere a, is the measured peak conductance, and a, the linearly interpolated background conductance at the concentration ratio of the peak, ie that conductance which would be obtained in the absence of any interaction, from merely mixing the donor and acceptor solutions. M stands for the molar concentration of the titrant at the stoichiometry of the complex, ie at the conductance peak. a is the dissociation constant of the complex. a, values are listed for 11 complexes, together with their stoichiometries. A number of solvent interactions, liable to affect the conductivity titrations, are discussed. Preliminary experiments indicate that some correlation exists between a, values and esr results. R&at&--La mtthode d&rite dans un precedent article a ete developpee et appliqued il un nombre de complexes. On discute les techniques exp&imentales et la valeur du pit de conductivite est exprimee quantitativement en fonction d’un coefficient de conductivim molaire a, = (l/aM) . (a, - u&r, oh a,, est le pit de conductance mesure et a, est la conductance de base interpoRe lin6airement au rapport de concentration du pit, c’est a dire la conductance que l’on obtiendrait en l’absence d’interaction par suite du simple melange de donneur et d’accepteur dans leurs solutions respectives. M reprt%ente.la concentration molaire du titrant a la stoechiometrie du complexe, c’est a dire au pit de conductance. a est la constante de dissociation du complexe. On presente les valeurs de uH et les stoechiometries de 11 complexes. On discute un certain nombre d’interactions dues au solvant qui sont susceptibles d’affecter les titrations de conductivite. Des experiences pr&ninaires indiquent qu’une certaine correlation existe entre les valeurs de a, et les don&es res (resonance &ctronique de spin). Zusammenf&saung---Die in Teil I beschriebene Methode wurde weiter entwickelt und bei einer Anzahl von Komplexen angewandt. Man diskutiert die experimentelle Methodik und gibt den Wert des Leitf%higkeitsmaximums quantitativ in Abhangigkeit des molaren Leitftigkeitenten 0, + (l/aM)(c, - uO/u,,),wobei Us die gemessene maximale Leitftiigkeit und u0 die linear extra polierte Grundleitftiigkeit beim Konxentrationsverh<nis des Maximums, u%diejenige LeitflUligkeit ist, welche in Abwesenheit jeglicher Wechselwirkung, ausschliesslich bei der Vermischung der Donator- und Akzeptorliisung erhalten wird. M ist die molare Konzentration des Titranden beim stijchiometrischen Verhlltnis des Komplexes, dh beim Leitfiihigkeitsmaximum. a ist die Dissoziationskonstante des Komplexes. uarWerte werden ftlr 11 Komplexe angegeben, zusammen mit ihrer Stiichiometrie. Eine Anrahl von Lijsungsmittel-Wechselwirkungen, welche miiglicherweise die Leitfahigkeitstitration beeintlussen kiinnten, werden diskutiert. Vorl%ige Untersuchungen zeigen, dass eine gewisse Bexiehung xwischen den uvWerten und den ESR-Daten existiert.

Cr-rAncE-transfer complexes form when the ionization potential of the donor is sufficiently low and the electron affinity of the acceptor sufficiently high to permit transfer of an electron from donor to acceptor. Such complexes have been widely studied,la2 in the solid state as well as in solution. In a previous communication3 we have shown that the formation of chargetransfer complexes can be followed by measuring changes in the electrical conductivity of a solution of, say, donor in an inert solvent of suitably high permittivity, consequent upon addition of a solution of acceptor in the same solvent, or vice versa. This, in effect, amounts to a conductivity titration. The stoichiometry of the complex may be deduced from the titration volumes at the end-point, viz the conductivity peak, and from the known concentrations of the stock solutions. The value of the conductivity * Manuscript received 12 November 1965. 1163

1164

F. GUTMANNand H.

[RELATIVE

KEYZER

mmToR1

1.0

I STOlCHlOtiETRY

PURE DONOR SOLUTION

PURE ACCEPTOR SOLUTION

FIG. 1. Idealized plot of a conductivity titration. The molar conductivity coefficient a, is defined in (1) and may be evaluated from the titration plot as shown.

I

0

1

I

I

t

t

I

I

05

Relative

phenothiazine

I

I

I_

I-0

FIG. 2. Conductivity titration of the phenothiazine/I, in acetonitrile system. Points A ,refer to the addition of phenothiazine, and points 0 to the converse, viz addition of I,. The peak indicates a 1:2 complex; o;, = 2100.

peak above a base-line connecting the conductivities of pure donor and acceptor solutions is a measure of the excess conductivity caused by the formation and subsequent dissociation of the complex. We now define a molar conductivity coeficient a,,

M is the molar concentration of the t&rant, either donor or acceptor, at the conductivity peak where o = op. a0 is the linearly interpolated conductivity background,

Conductivity titrations of charge-transfer complexes in solution-II

1165

off a base-line joining the conductivities of pure donor and pure acceptor solutions as shown in Fig. 1. The complex may not be fully dissociated; from the theoryS it follows that the dissociation constant has to be allowed for, since the carrier concentration is proportional not to the concentration M of the reagents, but rather to NM, where a is the dissociation constant of the complex. A well developed conductivity maximum, such as that resulting from the phenothiazine-iodine in acetonitrile system, illustrated in Fig. 2, is not always obtained. In particular, a steeply sloping base-line caused by serious mismatch of the initial conductivities of the pure donor

read

Rdotive

nophtholene

FIG. 3. Conduetivity titration of the naphthalene/I, in acetonitrile system. The initial conductances are seen to be badly mismatched; a curved line rather than a

conductivitypeak results. and acceptor solutions tends to yield ill-defied, or completely obscured, conductivity peaks. This is illustrated in Fig. 3 which refers to the iodine-naphthalene complex formed in a&o&rile. The decidedly non-linear plot indicates complexing. The stoichiometry of this complex can be determined by correcting for the background conductivity obtained by linear interpolation, ie by plotting u - a,, vs relative concentration. This is shown in Fig. 4. A peak becomes evident at a stoichiometric ratio of 0.57, indicating a 1: 1 complex. Mismatch usually occurs when the initial conductivities differ by more than about 50 %. Thus, if the solution of iodine in acetonitrile, which is relatively well conducting, is diluted to match the donor solution to within about 30% a well defined peak corresponding to a 1: 1 complex is obtained, as shown in Fig. 5, in agreement with the result of the background correction, Fig. 4. Correct choice of solvent is important: if its conductivity is too high, conductance changes due to complex formation may become obscured; if its permittivity is too low, the complex may fail to dissociate. If the solubility of the complex is low, the complex might precipitate, which may even lead to a reduction of the number of charge carriers present in solution and cause the appearance of a conductivity minimum in the titration. The minimum exhibited in the titration of pyridine and

1166

F. GUTMANNand H. &YZER

0 Relative FIG. 4.

napthalene

Replot of the data in Fig. 3, deducting the background conductivity. A peak corresponding to a 1: 1 complex is now evident.

13

I

I 0.2

I I I '06 04 Relative naphthalene I

I

1 O-8

FIG. 5. Conductivity titration of the naphthalene/I, in acetonitrile system. The conductances of the component solutions have been matched to within about 30 % by dilution of the acceptor solution. The peak at a 1: 1 ratio, deduced in Fig. 4 by correcting for the background conductivity, is now immediately evident.

in carbon tetrachloride at a 1: 1 stoichiometry may be cited as an example, Fig. 6. Pyridine and iodine are known4 to complex 1: 1 in this solvent. When the pyridine/iodine titration is carried out in acetonitrile a well developed curve with a maximum indicating 1: 1 complexing is outlined, Fig. 7. If the solvent has decided donor or acceptor character, then it may compete with the reagents and actively enter into the reaction. Thus, eg Ccl4 is known5 to act as an electron acceptor. Space charges may form due to either blocking, injecting, or catalytically active electrodes, resulting in excessively long relaxation times and time constants, in drifts and poor reproducibility. In that case, a titration curve and its converse will fail to coincide. iodine

Conductivity titrations of charge-transfer complexes in solution-II

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Reletivspyridine FIG. 6. Conductivity titration of the pyridine/I, in CC& system. Carbontetrachloride tends to act as an acceptor; it is not an inert medium. The complex also tends to precipitate out in this medium; after some time it even deposits on the electrodes. There is some indication for a conductance minimum at a 1: 1 stoichiometry; the data are subject to drifts. Points 0 refer to the addition of Is and points A to the converse.

Relativepyridine FIG. 7. The pyridine/I, in a&o&rile system. Acetonitrile is an inert solvent for the complex formed; a maximum at a 1: 1 stoichiometry is evident.

On the present model, the conductivity is caused by the free ions produced by the dissociation of the complex ion pairs; a mechanism closely similar to that proposed by Riehl et UP in order to account for the conductivity of pure organic liquids. One would then expect that these ions be solvated,’ causing a relatively low ionic mobility and thus a lowering of the conductivity peak in strongly solvating media. Values of a, for a number of complexes are listed in Table 1. together with the stoichiometries as determined from the position of (T,. The dissociation constant a has been assumed to be unity.

F. G~NN

1168

and H. KEYZER TABLE1

Donor

Acceptor

Anthracene Anthracene Benzene Benzene Benzene Naphthalene Phenothiazine Phenothiaxine Phthalocyanine (metal-free) Tetracene Pyridine

Chloranil I* chl0rani1 Chloranil :I ChloLil 1, 1, I* I*

Solvent

Apparent Stoichiometry

uy

Reference .l 138.9

Methanol Acetonitrile ccl, Methanol Acetonitrile A&o&rile Acetonitrile Acetonitrile Dimethylsulphoxide

1:l 1:3 1:l 1:l 1:2 1:l 1:l 1:2 I:2

1200 2200 150 390 480. 240 280 2100 870

Acetonitrile Acetonitrile

1:3 1:l

1540 3.50

: 1, 10 1

1

The titrations were carried out using spectral grade solvents and purified donors and acceptors. The solution was stirred after each addition and a constant time interval, of the order of three minutes, was permitted to elapse between the addition of titrant and the bridge adjustment. The solutions were thermostated to 20°C but not outgassed. The bridge was a Wayne-Kerr Bridge Type B-221. Iodine is a well-known o acceptor, so that I, complexes with certain donors to yield n-a complexes. If the donor has lone pair electrons, as in the case of eg chlorpromazine or phenothiazine, which readily form free radicals, then the interaction is expected to be of the n-u* type ie to involve an excited state.4 In addition a IIinteraction should also be expected4 in the case of very strong electron donors possessing extensive ILelectron systems. In fact, stoichiometries different from unity have been observed, together with evidence for the formation of several complexes having different stoichiometries. 1*11 It then may become difficult to resolve these complexes conductimetrically; the peak in such cases indicates a statistical average. Thus in the case of eg tetracene, there may be contributions also from 1: 4, 1: 2 and 1: 1 complexes which thus far have not been resolved; their statistical average then yields a 1: 3 stoichiometry from the conductivity plot. The presence of an ion pair should give rise to an esr signal,‘*12thus the formation of the phenothiazine-iodine charge transfer complex in acetonitrile was found to increase the spin concentration from a value of about 1014cm-a for the donor to about 101s cm-3 for the complex. A very large a, value has been obtained for this titration, see Table 1, similarly, the formation of a charge-transfer complex between chlorpromazine and iodine in acetonitrile causes the appearance of an esr signal associated with a spin concentration of about 1017cm*, while neither donor nor acceptor solutions separately give a signal, unless irradiated.12 Neither an esr signal nor a conductivity peak could be obtained for pyrene and iodine in acetonitrile. The same holds for the pyridine-iodine complex in carbontetrachloride as the solvent. In acetonitrile a very faint signal indicating a spin concentration of 1013 cm-a was observed and a small u, was obtained. Acknowledgements-We are indebted to the National Institute of Mental Health, Bethesda, Md, for the award of Grant MH-06361. which made this work nossible. We also wish to thank Mr. Ian-Neering for his assista& with the measurements and to Drs. J. Garnett and W. Sollich for the esr measurements.

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REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

G. BRIEGLD,Elektronen-Donator Acceptor-Komplexe. Springer, Berlin (1961). F. GUTMANNand L. E. LYONS,Organic Semiconductors. Wiley, New York (1965). F. GUTMANNand H. KEYZER,Electrochim. Actu 11, 555(1966). G. ALQJSIet al, Trans. Faraaby Sot. 61, 1406 (1965). R. F. WEIMER and J. M. PRAUSNITZ, J. them. Phys. 42,3643 (1965). H. BOER and N. Rnuu, Z. Nuturforsch. 19A, 1070(1964); 2OA,85 (1965); 2OA,394,X401, 587(1965). H. B&LER and N. RIEHL,Z. Naturforsch. XIA, 227 (1965). L. J. ANDREWSand R. M. KEEFER,J. Am. them. Sot. 74,450O (1952). R. BHAITACHARYA and S. BASU, Trans. Faruuby Sot. 54,1286 (1958). G. KORT~%end H. WALZ, Z. Elektrochem. 57,73 (1953). Y. MATSUNAGA, J. them. Phys. 42,1982 (1965). C. LAGERCRANTZ, Sixth International Symp. on Free Radicals. Cambridge, England (1963). C. LAGERCRANTZ and M. YELAND,Acta. them. Scamf. 16,1043 (1962). L. H. Prxrrx, P. LUDWIGand R. N. AD-, J. Am. them. Sot. 84,4212 (1965). N. S. Husn and J. R. RO~LANDS,Mol. Phys. 6, 317 (1963). L. S. SINGERand J. KOMMANDEUR, J. them. Phys. 34, 133 (1961).