2102
E. L. ONGAND MANJITS. SAMBHI
A Spectrophotometric and Thermodynamic Study of
Donor-Acceptor Complexes of Carbazole by E. L. Ong and Manjit S. Sambhi*l Department of Chemistry, University of Malayat Pantai Vdley, Kuala Lumpur, MaZayaia (Received October 12, 1971) Publication costs borne completely by The Journal of Physical Chemistry
The spectrophotometric and thermodynamic properties of donor-acceptor complexes of carbazole with 2,3dichloro-5,6-dicyanobenaoquinone (DDQ), tetracyanoethylene (TCNE), 9-dicyanomethylene-2,4,7-trinitrofluorenone (DTF), and 2,4,7-trinitrofluorenone (TNF) have been evaluated. The reliability of formation constants ( K O )as obtained by the Benesi-Hildebrand and Rose-Drago methods is discussed. The trends in K, values, the standard enthalpies of formation (AHfO), and the oscillator strengths (f)of the charge-transfer (CT) bands suggest that the stabilitiesof these complexes cannot be interpreted solely in terms of CT forces.
Introduction The nature of intermolecular bonding in donoracceptor complexes has been the subject of extensive investigation from both the theoretical and the experimental points of view. Briegleb2s3had postulated an electrostatic model to account for the stabilities of certain T-T complexes. Mulliken4 in 1952 proposed a charge-transfer theory which has been extensively used to explain and interpret the properties associated with donor-acceptor complexes. Donor-acceptor complexes have been commonly described as C T complexes and it has been widely accepted that C T forces play a dominant role in determining the ground-state properties of these complexes even though 1Clulliken4had clearly pointed out that classical intermolecular forces may play a significant role in determining the stabilities of such complexes. It has been shown recently636 that normal van der Waals forces are responsible for the ground-state properties of certain ?r complexes and in certain aromatic-halogen and aromatic-TCNE complexes,7 electrostatic, CT, and exchange repulsion interactions make important contributions to the stabilities of these complexes. I n complexes held together mainly by CT forces, one would expect that K O , AH*’, and f would increase steadily in a series of complexes between a common donor and a number of acceptors of increasing electron affinity.Sr9 We have measured the spectral and thermodynamic characteristics of carbazole complexes of DDQ, TCKE, DTF, and T N F in order to determine whether CT forces are mainly responsible for the stabilities of these complexes in the ground state. The reliability of K , as obtained by the Benesi-Hildebrand’O and R o s e Drago’l methods is discussed.
Experimental Section Materials. DDQ mas recrystallized twice from dry methylene chloride; mp 212-213”. The Journal of Physical Chemistry, Vol. 76, No. 15, 197.8
TCNE was recrystallized twice from dry methylene chloride and sublimed under high vacuum. The white solid obtained melted at 198-200”. D T F was recrystallized twice from dry methyl cyanide; mp 267”. Anal. Calcd for CleHaNsOe: C, 52.89; H, 1.38; N, 18.25. Found: C, 52.49; H, 1.37; N, 19.20. T N F was recrystallized from a 3 : 1 nitric acid-water mixture and dried under vacuum over phosphorus pentoxide and silica gel; mp 175-176”. 1,2-Dichloroethane was partially dried over calcium chloride, refluxed over calcium hydride, and fractionally distilled through a 35-cm Vigreux column. Spectrophotometric Measurements. The spectra were recorded by a Perkin-Elmer 137 UV spectrophotometer. Spectrophotometric data required to evaluate complex stoichiometry and thermodynamic parameters were obtained by measuring the absorbances of donor-acceptor solutions prepared by weighing appropriate quantities of donor and acceptor in standard volumetric flasks and filling the flasks to the mark with 1,2-dichloroethane. The absorbance of each solution was measured (1) Author to whom correspondence should be addressed. (2) G. Briegleb, Z. Phys. Chem., Abt. B , 31, 58 (1935). (3) G.Briegleb, “ZwischernmoleculareKrafte und Molekulstruktur,” Enke Verlag, Stuggart, 1937. (4) R. S.Mulliken, J. Amer. Chem. Soc., 74,811 (1952). (5) M. J. 9. Dewar and C. C. Thompson, Jr., Tetrahedron, Suppl., 7, 97 (1066). (6) R. J. W.Le Fevre, D V. Radford, and P. J. Stiles, J . Chem. SOC.B , 1297 (1968). (7) J. L. Lippert, M . W. Hmna, and P. J. Trotter, J . Amer. Chem. Soc., 91,4035 (1969). (8) R. S. Mulliken and W. B . Person, Ann. Rev. Phys. Chem., 13, 107 (1962). (9) R. Foster, “Organic Charge-Transfer Complexes,’’ Academic Press, London, 1969. (10) H . A. Benesi and J. H. Hildebrand, J . Amer. Chem. SOC., 71,2703 (1949). (11) M.J. Rose and R. S . Drago, ibid., 81,6138 (1959).
DONOR-ACCEPTOR COMPLEXES OF CARBAZOLE without delay with a Hilger H700 spectrophotometer fitted with a water-thermostated cell compartment. The temperature was maintained constant to within =kO.l". The CT absorption shows no change during the time required for the absorbance measurements. Matched silica rectangular cells with ground-glass stoppers were employed for all spectroscopic measurements. Complex stoichiometry was determined by Job's methodl2 of continuous variation. The Rose-Drago methodll was used to evaluate K , and the maximum molar absorptivity (emsx) for the complexes at 30, 40, 50, and 60". The Benesi-Hildebrand methodlo was used to determine K , and Emax at the above temperatures for the TCNE and T N F complexes while the data for the DDQ and D T F complexes were obtained at 30" only. (The least-squares method was used to calculate the slope and intercept of the Benesi-Hildebrand plot.) The standard enthalpies for complex formation were obtained from the slopes of the van? Hoff plots of log K , us. l / T . Again, the least-squares method was employed. The standard entropies (AS') and free energies (AGO) were evaluated in the normal manner. The oscillation strengths of the CT bands at 30" were evaluated experimentally by the equationS f
=
4.3
x
i0-9Emrtx~V1,,
where emax is the maximum molar absorptivity and AV'/~is the half-width of the band. f for the T N F CT band was not calculated as the band was not completely resolved.
Results Solutions of carbazole in 1,2-dichloroethane were colorless and the addition of +electron acceptors DDQ, TCNE, DTF, and T X F resulted in the immediate formation of intense colors. The spectra of these solutions were characterized by broad featureless bands at 625, 605, 540, and 425 mp, respecwhich had A, tively. The band due to the addition of T X F was not completely resolved and A,, was obtained by differential spectroscopic techniques. These broad bands can be assigned to the formation of intermolecular CT transition between the s-electron acceptors and the carbazole molecule. This assignment is supported by the observation that the electron affinities (EA) of DDQ, TCNE, DTF, and T N F are 1.95, 1.80, 1.45, and 1.0 eVIO while the C T absorption maxima frequencies (vmax) are 1.986, 2.048, 2.296, and 2.915 eV, respectively. This relationship between EA and vmax is to be expected from theoretical consideration~.~ The CT bands for the complexes are shown in Figure 1. Plots of absorbances us. mole fraction of donor had maxima occurring at 0.5 mole fraction, and according to Job's method of continuous variation12 this means that the complexes have 1: 1 stoichiometry in solution.
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Figure 1. The charge-transfer bands for carbazole complexes : (A) TNF, (B) DTF, (C) TCNE, ( D ) DDQ.
The successful use of Benesi-Hildebrand and RoseDrago equations for 1: 1 complexes readily confirmed this result. The donor and acceptor concentrations were varied generally from to lo-' M . The spectral and thermodynamic results are shown in Tables I and 11, respectively.
Discussion The spectrophotometric evaluation of K , and emax has been discussed critically by a number of authors."a-16 Person's has shown that for weak comTable I : The Spectral Characteristics of the Carbazole Complexes of TNF, DTF, TCNE, and DDQ in 1,2-Dichloroethane Amax, Acceptor
mr
TNF
425
DTF
540
TCNE
605
DDQ
625
Temp, 'C
30 40 50 60 30 40 50 60 30 40 50 60 30 40 50 60
me.x---
-----e
a
b
1420 1440 1420 1400 1510
1390 f 15c 1390 f 30 1395 f 15 1385 f 15 1540 f 20 1490 f 15 1450 f 20 1440 i: 15 1480 f 40 1510 f 10 1480 f 30 1550 f 20 1680 f 10 1700 i: 50 1690 f 10 1700 f 20
1480 1520 1500 1500 1690
AVI/Z. om-1
f
5880
0.039
6940
0.044
5120
0.038
Benesi-Hildebrand procedure. b Rose-Drago procedure. Uncertainties are expressed &s standard deviations.
(12) P.Job, Ann. Chim. (Paris),10, 113 (1928). (13) S. Carter, J. N. Murrell, and E. J. Rosoh, J. Chem. Sac., 2048 (1965). (14) P. R. Hammond, ibid., 479 (1964). (15) W. B.Person, J.Amer. Chem. SOC.,87, 167 (1965). (16) D. A. Deranleau, ibid., 91, 4044 (1969).
The Journal of Physical Chemistry, Val. 76, hTo. 16,1979
E. L. ONGAND MANJITS. SAMBHI
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Table I1 : The Thermodynamic Characteristics of the Complexes -AGO,
Temp, ----KO, Acceptor "C a
TNF
DTF
TCNE
DDQ
c
30 40 50 60 30 40 50 60 30 40 50 60 30 40 50 60
5.9 5.1 4.5 4.1 13.7
2.9 2.5 2.1 1.9 28.1
1. mol-1--b
5.88 f 0.05c 5.04 f 0.05 4.52 f 0.05 4 . 1 f 0.10 13.6 f 0.25 11.6 f 0.21 10.2 f 0.25 8.5 f 0.10 2.89 f 0.09 2.45 f 0.04 2.16 f 0.05 1.87 f 0.05 28.2 f 0.2 21.4 f 0.2 16.5 f 0 . 1 13.1 f 0 . 1
kcal mol-1
-AHro, koa1 mol-1
-ASo, eu
1.06
2.9
5.9
1.6
3.1
4.9
0.64
2.9
7.4
2.0
5.1
10.2
Benesi-Hildebrand procedure. Rose-Drago Uncertainties are expressed as standard deviations.
procedure.
plexes the Benesi-Hildebrand treatment gives reliable results if the initial concentration of the donor [DIo is between 0.1 and 9.0 times the value of l/K,. This allows K , and Emax to be separately evaluated. This condition was met in all our Benesi-Hildebrand determinations. The differences in the K , values of the complexes are significant as AK is greater than 3 times the standard deviation in all cases.15 Deranleau16 has shown that for weal: molecular complexes an accurate evaluation of K , and tmaxcan best be obtained when the saturation fraction s is between 0.2 and 0.8 and measurement over 75% of the curve, e.g., the Scatchard" plot, is required before the proposed 1: 1 complex model can be considered t o be verified by the use of any single equilibrium equation. The saturation factor s is defined by Deranleau16as the ratio of the concentration of the complex to the initial concentration of the most dilute component. On this s scale Person's criterion of donor concentration between 0.1/KCand 9/K, corresponds to saturation fractions of 0.09 and 0.9, respectively. I n our experiments s values were in the region of 0.1 and 0.25 and since our
The Journal of Physical Chemistry, Vol. 76, N o . 16,1978
standard deviations in K , and emsx are small, it is reasonable to say that 1: 1 complexes do exist at least in the region of s covered in our experiments. The Rose-Drago plots were characterized by a large degree of sharpness18 as the intersection of the lines occurred at one point. This large degree of sharpness indicative of a reliable K , value was achieved by reducing the experimental errors to a minimum and choosing experimental conditions such that the concentration of the acceptor was held fairly constant while the donor concentrations were varied over a wide range. This gives lines with significantly different slopes. The K , values of carbazole complexes of TNF, DTF, TCNE, and DDQ at 30" are 5.88, 13.6,2.89,and 28.2 1. mol-', and the respective AH?" values are 2.9, 3.1,2.9, and 5.1 kcal mol-l. Clearly, order of acceptor strength based on K , values would be different from the order based on AH$" values and no linear relationship exists between AHf" and AS" (see Table 11). However, AHt" indicates that the TNF, DTF, and TCNE complexes are of comparable stability in spite of their divergent electron affinities of 1.0, 1.45, and 1.80 eV, respectively. The AH," for the DDQ complex a t 30" is significantly greater than AHfO of the TCNE complex while their electron affinities are similar being 1.95 eV for DDQ and 1.80 eV for TCNE. These observations suggest the CT forces are not solely responsible for the stabilities of these complexes. Perhaps an estimate of CT in the ground state of these complexes may be reflected in the f values of the absorption bands. Thef values for the DTF and DDQ complexes are similar while the K , and AH?"values are distinctly different, suggesting that other forces besides CT forces have to be considered when describing the stabilities of these complexes.
Conclusion The spectral and thermodynamic properties of these complexes do not subscribe to the view that these complexes in the ground state are held together mainly by CT forces. (17) G.Scatchard, Ann. A;. Y . Acad. Sci., 51, 660 (1949). (18) K. Conrow, G. D. Johnson, and R. E. Bowen, J. Amer. Chem. Soc., 86, 1025 (1964).
