R. D. HOLM, W. R. CARPER, AND J. A. BLANCHER
3960
Polarographic and Spectral Studies of Charge=TransferComplexes
by Roger D. Holm, W. R. Carper,' and James A. Blancher Chemistry Department, California State College at
Lo8
Angeles, Loa Angelea, California (Received May 16, 1967)
Formation constants for some charge-transfer complexes have been evaluated polarographically. Complexation was found to perturb the energy levels of the acceptor (7,7,8,8tetracyanoquinodimethane) causing an energy decrease in the lowest unfilled level and, consequently, reduction at more positive potentials for hexamethylbenzene and pentamethylbenzene. For durene and mesitylene, on the other hand, the lowest empty acceptor level was shifted to higher energy, causing reduction at more negative potentials. Corrections for these energy level perturbations were obtained from spectral shifts of the first acceptor transition and were applied to the observed half-wave potentials in computing formation constants. The relative order of donor character was found to be hexamethylbenzene > pentamethylbenzene > durene > mesitylene in chloroform solution using tetrabutylammonium perchlorate as the supporting electrolyte. This relative donor order difiers from that indicated by Benesi-Hildebrand investigations in chloroform, but the polarographic values do follow the relative order of the donors in many other solvents and probably reflect the true sequence in chloroform as well.
Introduction Several studies of charge-transfer complexes have been conducted in which shifts in polarographic halfwave potentials are utilized in obtaining formation constants.2 In these investigations, it has been assumed that the potential shift is entirely due to the formation of the ~ o m p l e x . ~I n several cases very poor agreement has been obtained between formation constants obtained polarographically and those obtained spectrally. It is the purpose of this paper to show that acceptor half-wave potentials depend not only upon the value of the complex formation constant, but also on the energy of the vacant molecular orbital of the acceptor which changes upon complexation. Experimental Section Solvents. Spectroquality carbon tetrachloride was used as received. Preliminary addition of tetracyanoethylene to the solvent yielded no observed donoracceptor interaction. Hexamethylbenzene, pentamethylbenzene, and durenewere obtained fromEastman and pyrene was from K & K Laboratories, all as reagent grade and used without purification. Mesitylene from Eastman was distilled through a spinning-band column at a temperature of 164". Tetrabutylammonium perchlorate (Southwestern Analytical Chemicals) was The Journal of Physical Chmistra,
recrystallized from methanol and dried in vacuo over anhydrous magnesium perchlorate at 100" for at least 5 hr. Tetrabutylammonium chloride (Southwestern Analytical Chemicals) was recovered from aqueous solution by evaporation under reduced pressure, followed by drying in vacuo at 50" over magnesium perchlorate. Chloride solutions in chloroform were prepared from this salt with great rapidity and it was experimentally verified that the amount of moisture acquired in dissolving the tetrabutylammonium chloride had no significant effect on the stability of the observed potentials. 7,7,8,8-Tetracyanoquinodimethane(TCNQ) was recrystallized from specifically purified acetonitrile three times and dried in vacuo over P205 at 80". Spectral Studies. Spectral measurements were made on a Cary 14 spectrophotometer at 25 f 0.1". The first absorption band position of the acceptor could be located to *3 A and the charge-transfer band maxima to =k5 mp. All solutions contained 0.1 M tetrabutylammonium perchlorate as an inert electrolyte in (1) Addresa correspondence to this author at Wichita State University, Wichita, Ran. 67208. (21 H. Irving, "Advances in Polarography," Vol. 3, Pergamon Press Inc., New York, N. Y., 1960, p 42. (3) (a) M.E. Poever, Trans. Faraday Soc., 58, 2370 (1962); (b) J. C h m . Soc., 4640 (1962).
POLAROGRAPHIC AND SPECTRAL STUDIESOF CHARGE-TRANSFER COMPLEXES
order to preserve the same solution environment for both spectral and polarographic studies. M The TCNQ concentration was generally about and the donor concentrations were varied from 0.10 to 1.0 M depending on solubility. All solutions were thoroughly deoxygenated with purified nitrogen which was presaturated with chloroform before being transferred to degassed absorption cells. For spectrophotometric measurements of the formation constants, dilutions of the most concentrated solutions were taken and constants were obtained at several wavelengths from the expression4
in which (D) and (A) are the concentrations of donor and acceptor, respectively, K is the formation constant, b is the path length, and di and ~i are the absorbance and molar absorptivit,y, respectively, for wavelength Xi. If one complex species exists in solution, the K is generally reasonably constant over the charge-transfer band. Polarographic Studies. A three-compartment cell was used with a central working compartment which was isolated from the auxiliary and reference compartments with fine frits. The working solution level was kept higher in this compartment than in the other compartments to ensure a net outflow through the frits and preclude contamination of the working solution. Provision was made to degas the solutions in all three cells to prevent entry of oxygen which not only is electroactive but also complexes with the TCNQ. Salt bridge solutions in the auxiliary and reference compartments were of freshly prepared chloroform solutions of 0.1 M tetrabutylammonium perchlorate. The working solutions contained 0.1 Jl tetrabutylammonium perchlorate, 0.1-1.0 M donor, and typically lo-* M TCNQ. These solutions were thoroughly degassed in a fritted cell, transferred under nitrogen to the polarographic cell, and degassed further. During all measurements, chloroform-saturated nitrogen was passed over the working solutions. A Sargeant XV polarograph was employed in conjunction with a Sargeant infrared compensator and a three-electrode cell system. The chart paper divisions were calibrated by measuring the potentials of the bridge windings with a Leeds and Korthrup Model K-3 potentiometer. The infrared compensator was periodically balanced so that the potential of the dropping mercury electrode (dme) was within A0.5 mv of the nominal bridge potential. Plots of applied potential vs. log (id - i)/i were
3961
made and the potential at which the log term became zero was taken as the half-wave potential. The plots consistently exhibited slopes within 10% of 59.1 mv. Values for the half-wave potential for the same solution were reproducible to A 1 mv. Preliminary trials using a silver-silver iodide electrode5were unsuccessful owing to erratic drifting of the electrode. More stable potentials were obtained with a silver-silver chloride electrode composed of a helix of silver wire anodized in a solution of tetrabutylammonium chloride and immersed in a chloroform solution containing 0.10 M tetrabutylammonium perchlorate and approximately 0.01 M tetrabutylammonium chloride. The electrode contained in a l-cm fritted tube was found to vary only 4-5 m during a 12-hr period when compared to a similarly prepared electrode. Although the concentretion of chloride was only approximately known, the reference potential was stable enough so that alternate polarograms could be run repeatedly for the acceptor and for the complex. The differences in half-wave potential were obtained over a time period short enough so that the reference electrode was essentially constant within experimental error. The shifts in half-wave potential are felt to be reliable to 1.5 mv.
Results and Discussion Spectral investigations revealed that the first transition of the TCNQ was shifted slightly to higher energies (from 4015 to as much as 4000 A) upon complexation with durene and mesitylene and to slightly lower energies (from 4015 to 4025 A) with hexamethylbenzene and pentamethylbenzene. At the higher concentration of donors, the absorption is believed to be predominantly due to the complexed TCNQ as is indicated by an apparent limiting amount of shift at the higher donor concentrations of each system. Since the complexes are rather weak, the spectral shifts and corrections may be considered minimum values. The charge-transfer bands for TCNQ-hexamethylbenzene and pentamethylbenzene are sufficiently far removed from the TCNQ band so that no overlapping occurs. Although the charge-transfer bands of durene and mesitylene do overlap considerably with the TCNQ band at 4015 A, little error is involved in measuring the peak intensity of the acceptor band since the absorbance of this band is roughly 100 times as great as the chargetransfer bands. I n addition, the latter bands are very broad with little change in absorption over a 20- or 30-A region. (4)H.A. Benesi and J. H. Hildebrand, J . Am. Chem. SOC.,71, 2703 (1949). (5) M.E. Poever, Trans. Faraday Soc., 60,417 (1964).
Volume 71, Number 12 November 1967
R. D. HOLM,W. R. CARPER,AND J. A. BLANCHER
3962
A 4 d (Rad Shift)
-\
“T
Figure 1. Perturbation of acceptor levels upon complexation. Filled levels of the donor, D, and acceptor, A, are shown as well &s the small perturbations, A‘, and the first unfilled acceptor level when the complex, C, is formed. The ECT represents the first charge-transfer band of the molecular complex. Upon complexation the donor level and the first unfilled acceptor level are shifted slightly toward each other.
The spectral shift of the TCNQ band may be interpreted as reflecting a perturbation of the acceptor orbitals upon complexation as is shown in Figure 1. Though all levels may be somewhat affected, it is the lowest unfilled acceptor level which participates most in complex formation and which probably undergoes the greatest perturbation effect. The filled level of the acceptor is likely to be relatively unaffected and the extent of the spectral energy shift of the first transition (A --t A’) is therefore probably close to the true change in energy of the lowest unfilled level. The reference energy level of the calomel electrode may be thought of as lying above the acceptor levels. I n the red shift case in Figure 1, complexation perturbs the acceptor level to a lower value (closer to the donor level), causing a greater energy separation between the reference electrode and the acceptor level and requiring a more negative potential for electron transfer, -4 slightly more negative half-wave potential is observed for this reduction process in addition to the conventional negative potential shift from complexation, giving rise to an apparent formation constant which is too large. The observed half-wave potential is therefore a function of the energy level perturbation from complexation, as well as of concentration of donor and formation constant of the complex. The energy correction calculated in this manner from the perturbation of both filled and unfilled orbitals represents a maximum correction for the half-wave potential of the complex. Since the filled level is probably little affected, however, the correction is probably close to the true correction. The Journal of Physical Chemistry
It has been shown that molecular complexation is accompanied by a shift of the reversible polarographic half-wave potential to more negative potentials according to the relation
for a 1:1 complex of small formation constant K and donor concentration (D). It is reasonable to assume that a t the prevailing concentrations only 1:1 complexes are involved and the treatment in this study has ignored any higher complexes. It is assumed that the activity coefficients of acceptor and complex are identical and that the diffusion current constants for acceptor and complex are also identical. The first condition is reasonable and the latter condition appears to be true. Reports5 of a reversible and diffusioncontrolled reduction wave for TCKQ were confirmed. The donors were all reduced at potentials roughly 2 v more negative than the TCNQ. The perchlorate ion is probably of little complexing effect as its donor strength for TCNQ is less than that of the ?r donors used which are present in equal or greater concentration. The observed shift in half-wave potential for durene is greater (more negative) than that produced by the equilibrium effect by the amount of perturbation shift and the formation constant can be calculated once the spectral potential component in the potential shift is deducted. This component may be evaluated by measuring the wavelength shift in the band maximum of the acceptor. Since 1 ev equals 1.60 X erg, one may express the energy difference in millielectron volts
AE,/, = hcA ( l / X )
I‘
=
MI/*
=
ADA
(0.124) A ;[
-
-h;Al
where h and c are Planck’s constant and velocity of light, respectively, and XA and ADA are the band maxima expressed in cm-’ for the uncomplexed and complexed acceptor. I n a similar manner, TCNQ complexation with hexamethylbenzene and pentamethylbenzene results in a lowering of empty acceptor level as is seen from the diminution in the energy of t,he TCNQ transition. The energy level shift will partially compensate the equilibrium effect and the observed half-wave potential shift to negative values will be too small. It was found in
POLAROGRAPHIC AND SPECTRAL STUDIES OF CHARGE-TRANSFER COMPLEXES
Table I :
Formation Constants of Complexes
concn, M
AEiizl
Cor term,
K"IlC0r.
KC,,,
rnv
mv
l./mole
]./mole
0.10 0.20 0.30 0.40 0.50
7 11 17 16 20
TCNQ-Hexamethylbenzene 3.93 $1.5 3.12 3.61 $3.0 2.68 4.83" $6.0 3.14 3.62 +7.0 2.16 3.72 $7.0 2.36 Av 2 . 6 9 i 0 . 4 4 3 . 7 2 & 0 . 1 5
0.10 0.20 0.30 0.40 0.50
4 8 12 14 16
TCNQ-Pentamethylbenzene 0.0 1.68 1.68 2.10 $1.0 1.83 2.30 $1.5 1.99 2.36 +3.1 1.82 2.33 +3.8 1.73 Av 1 . 8 1 1 0 . 1 2 2.15 =k 0 . 2 8
0.20 0.40 0.60 0.80 1.00
5 8 18 24 24
TCNQ-Durene 0.750 -1.4 1.08 0.608 -2.4 0.91 0.822 -7.7 1.69 0.825 -11.0 1.93 0.991 -6.3 1.55 Av 1 . 4 3 1 0 . 4 3 0.799 3 t 0 . 1 4
0.20 0.40 0.60 0.80 1.00
3 8 23 14 20
TCNQ-Mesitylene 0.0 0.36 0.62 0.92 0.0 0.91 1.96" -3.1 2.42 -4.5 0.91 0.561 0.77 -5.4 1.18 Av 1 . 1 5 i 0 . 7 7 0 . 7 2 & 0 . 5 1
Donor
* These values were rejected with better than 95y0 confidence before computing average values.
this study that the spectral potential shifts can amount to 30% of the observed shift in half-wave potential. I n Table I are listed the observed half-wave potential shifts a t different donor concentrat'ions, the corresponding correction term in millivolts computed from TCNQ spectral shifts, the formation constant computed from the observed half-wave potential alone, and the constant computed from the corrected half-wave potentials. As would be expected, the effect of the spectral shift correction is seen in an increase in the constant for hexamethylbenzene and pentamethylbenzene and a diminished constant for durene and mesitylene. Moreover, the standard deviations listed are generally smaller for the corrected constants. The donor strengths observed were in the order hexamethylbenzene > pentamethylbenzene > durene > mesitylene. Such a sequence is reasonable since the inductive effect of six methyl groups would be expected to make the r-cloud
3963
more available in a charge-transfer complex. hielby, et ~ l . have , ~ reported that the constant in methylene chloride for TCNQ-hexamethylbenzene exceeds that of durene. I n a polarographic study in chloroform, Poever obtained a value of 2.0 f 0.2 for TCNQ-hexamethylbenzene in ionic strength 0.5 M . This value essentially corresponds to the uncorrected constant of this study, though at a different ionic strength. For comparison with the polarographic results, spectral measurements were made of each solution and constants were calculated using the Benesi-Hildebrand equation. Comparison of the constants obtained from the two methods, presented in Table 11, shows that the velues not only disagree but the relative order of donor strength is different. A similar order of spectrally determined donor strength in chloroform has been obtained by Thompson and de Maine' for trinitrobenzene with the same donor molecules, indicating that the progression of values from the Benesi-Hildebrand equation is a t least reproducible with other acceptors in chloroform, although it may not produce real values. Spectrally determined donor-acceptor constants in other solvents yield relative donor strengths which parallel that progression observed polarographically in this study. Such is the case for complexes of tetracyanoethylene with the same donors in methylene chloride.8 The same relative progression of constants was found by Thompson and de Maine' for trinitrobenzene with the donors in carbon tetrachloride, n-heptane, n-hexane, and cyclohexane. Moreover, in the study reported in this paper the chargetransfer bands exhibited a regular trend toward higher energy from hexamethylbenzene to mesitylene, suggesting that this really is the true order of decreasing stability for these complexes in chloroform rather than as suggested by the Benesi-Hildebrand treatment. Maxima could be obtained only for TCNQ-hexamethylbenzene (6010 50 A) and pentamethylbenzene (5400 f 50 A) since the other two bands partially overlapped the TCNQ band. It would appear that the absorptions used in the Benesi-Hildebrand extrapolations in chloroform arise from some effect more complicated than merely the excitation of an isolated complex. The origin of this effect is unknown but may be related to interaction of the solvent dipole in the absorption by the complex. Since the polarographic constants (both corrected
*
(6) L. R. Melby, R. J. Harder, W. R. Hertler, W '. AMahler,R . E. Benson, and W. E. Mochel, J . Am. Chem. SOC.,84,3374 (1962). (7) C. C. Thompson, Jr., and P. A. D. de Maine, J . Phys. Chem., 69, 2766 (1966). (8) G . Briegleb, "Elektronen-Donator-Accep t or- K o m p l e x e , " Springer-Verlag, Berlin, 1961, p 129.
Volume 71, Aviklnber12 November 1967
R. D. HOLM, W. R. CARPER,AND J. A. BLANCHER
3964
potential resulting from complete charge-transfer,
Table I1 : Comparison of Polarographic and Spectral Constants in Chloroform
PDA = resonance integral between donor and acceptor
+
Donor
Polarographic constants in CHCla (cor)
Benesi-Hildebrand constants in CHCla (this study)
Hexametbylbenzene Pentamethylbenzene Durene Mesit vlene
3 . 7 f0 . 2 2 . 2 f0 . 3 0 . 8 =t 0.1 0 . 7 f 0..5
0 . 3 0 =t0 . 0 3 1.36 f 0.05 0 . 1 8 f 0.05 0.07 f 0 . 0 3
and uncorrected) are consistent with the usual donor strengths, it is probable that values obtained from the Benesi-Hildebrand treatment in chloroform are suspect and that the true values are close to those obtained polarographically. Calculations.-During the course of our investigation, several observations were made concerning the spectral shifts associated with the acceptor, TCKQ. First of all there is a shift, a fact which is hardly surprising in view of Mulliken’s early papersg and consideration of perturbation theory. In his paper concerning the molecular orbital treatment of chargetransfer complexes, Flurrylo provides a review of the various perturbation treatments and points out the difficulties of such calculations. However, he does provide a simple molecular orbital treatment (neglecting overlap) of the problem in question. Returning to the experimental results once again, it is seen that both red and blue shifts are observed for the acceptor first transition energy. This would suggest several interesting possibilities : (a) a simple energy scheme (as shown in Figure 1) wherein a red shift implies that the empty acceptor orbital is of higher energy than the filled donor orbital; (b) a more complicated picture wherein either one or both orbitals have been perturbed by the solvent molecules before (or during) complexation. Obviously, case b would not permit a simple correlation between shifts and the relative energies of the donor and acceptor orbitals except in rather fortuitous cases. Having pointed out the problems concerning the interpretation of the spectroscopic shifts, we would like to confine ourselves to an examination of a, or a special case of b, where the solvent effects are treated as a const ant factor. Utilizing the one-electron molecular orbital treatment,10 the energy of the charge-transfer transition (AEcT)is
AECT = (b2 - a2)(D- A
+ Ves) - 4abP~.4
where D = negative of donor ionization potential, A = negative of electron affinity of acceptor, V,, = The Journal of Physical Chemistry
and a and b = molecular orbital coefficients (a* b2 = 1). The value of A for TCIiQ was taken as -1.70 ev.11 D values were taken from Flurry’s calculations’o and Watanabe’s values.12 The charge-transfer band maxima were obtained from spectral results and, where necessary, from AECT us. electron affinity plots. The latter plots are seldom linear unless the acceptor molecules are similar (p-benzoquinone, chloranil, and TCXE gave linear relationships for the donors benzene, toluene, o-xylene, mesitylene, durene, pentamethylbenxene) and the donor molecules are of the same series. Special emphasis must be placed upon both geometric and orbital (atomic) similarities in acceptor molecules if any reliance is to be placed on such plots. Recently, zt reexamination of X-ray data13J4has revealed that bond distances in the
1
group of TCNQ are the same as those in TCNE. Furthermore, the bond distances in the ring of p-benzoquinone are similar to those found in the ring of TCNQ. X-Ray information concerning the methylben~ene‘~ indicates that the standard benzene ring distance is relatively unaltered by the addition of the methyl groups. While all of thc above information may not be conclusive, it gives us encouragement. Proceeding to the next problem, there is the choice of Ves. As Flurry pointed out,1othe choice of Ves is somewhat arbitrary in view of the limited information available.1° With this in mind, we have set the complex equilibrium distance equal to three different values (3.25, 3.35, and 3.65 A) and compiled three sets of data (Table 111). The values of a and b (a = 0.88 and b = 0.47) for the TCNQ
(9) R. S. Mulliken, J . Phys. Chem., 56, 801 (1952); J . Am. Chem. SOC.,74, 811 (1952),and other related papers.
(10) R. L. Flurry, Jr., J . Phys. Chem., 69, 1927 (1965). (11) G. Briegleb, Angew. Chem. Intern. Ed. Engl., 3 , 617 (1964). (12) K.Watanabe, J. Chem. Phys., 2 6 , 542 (1957). (13)C.J. Fritchie, Jr., Acta Cryst., 20, 107 (1966). (14) R. E. Long, R. A. Sparks, and K. N. Trueblook, ibid., 18, 932 (1965). (15) “Tables of Interatomic Distances and Configuration in Molecules and Ions,” Supplement, 1956-1959, The Chemical Society, London, 1965.
POLAROGRAPHIC AND
SPECTRAL STUDIES OF CHAROE-TRANSFER COMPLEXES
complexes are almost identical with those values (a = 0.89 and b = 0.45) obtained for the TCNE complexes.1° A distance of 3.25 A produces the only reasonable values of complex formation const>ants (see Tables I and 11). This may be compared with a value of 3.26 A, the TCNQ--TCNQ- distance found by Fritchie'" in the unit cell of N-methylphenazinium tetracyanoquinodimethanide. While it can be correctly argued1° that equilibrium constants calculated by this method are subject to doubt, there still remains the fact that the MO method Table 111: Variations in Potential ( V e mand ) Resonance ) Intermolecular Distance (r)O Integral ( ~ D A with r,
VS.,
-hA,
A
BV
BV
3.25 3.35 3.65 a
a2
+ b'
4.44 4.30 3.95
= 1 ( a = 0.88,b = 0.47).
0.78 0.73 0.61
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Table IV : Equilibrium Constants ( K ) Calculated from Molecular Orbital Theory with r = 3.25 A LO6
TCNQ complex
(K/Ko)
Hexamethylbenzene Pentamethylbenzene Durene Mesitylene
1.65 1.10 0.74 0.00
K'
43.7 12.6 5.5 1.0
' All K's normalized to the mesitylene-TCNQ complex.
should still correctly predict the relative order of constants. (See Table IV.) Aclcnowledgmpt. Financial support was provided by National Science Foundation Grant GP-6031 and an institutional grant. W. R. C. wishes to acknowledge helpful discussions with R. L. Flurry concerning the calculations. (16) C.
J. Fritchie, Jr., Acta Cr&.,
20, 892 (1966).
Volume 71, Number 18 November 1967
