A. R. MONAHAN, J. A. BRADO, AND A. F. DELUCA
446 served in liquid n-hexane arid neopentane mixtures (and summarized by eq 3) allow one to conclude that the trapping process giving rise to the activation energy of the mobility is a collective effect. If this were not true, Le., if the activation process were a single molecule process (e.g., short-lived negative ion state, rotational resonance, etc., of a single molecule), then the activation energy a t low neopentane concentration should be approximately equal to the n-hexane activation energy in contradiction to our observation that the activation energy is proportional to the mole fraction of nhexane. I n fact, eq 3 requires that the activation energy E* of the mixture be of the form
E*
= EhXh
-k
EnpXnp
(4)
where Eh and E,, are the activation energies of pure hexane and neopentane, respectively. Thus, as in water, ammonia and amines, the electron trap in hydrocarbons involves many molecules acting collectively. However, unlike the polar liquids, the long-range Landau potential, of the form (5)
nearly equal for the hydrocarbons studied here. The traps must be due to local configurations of groups of molecules whose group-electron potential energy is favorable for trapping the electron. We have previously suggested the following mechanism of electron transport in hydrocarbons.2* The electron moves as a quasi-free particle ( i e . , in the conduction band) until it is trapped by a group of molecules in a configuration favorable for trapping. The electron then remains in the relatively immobile trap until thermally promoted back to the conduction band. The activation energy of the mobility then arises from the thermal promotion step. Another possible transport mechanism is electron tunnelling from trap to trap. In this picture the electron remains trapped until a thermal fluctuation (the activation step) provides a neighboring trap into which the electron may move by tunnelling. Theoretical investigation of these and other possible mechanisms suggested by recent work on amorphous solids will be the subject of a future pub1ication.O (9) NOTE ADDEDIN PROOF.I n a note to appear in Chem. Phys.
Lett., the authors have shown that a polyatomic version, the Cohen-
is not important in forming the trap since D,and Do,, the static and optical dielectric constants, are very
Lekner quasi-free electron theory, accounts for the preexponential factor in eq 1 and for the observed field independence of electron mobilities in hydrocarbons.a
The Dimerization of a Copper (11)-Phthalocyanine Dye in Carbon Tetrachloride and Benzene by Alan R. Monahan,* James A. Brado, and Allen F. DeLuca Xerox Rochester Research Center, Rochester, New York Publication costs assisted by Xerox Corporation
14603
(Received .July 19, 1971)
Analyses of the absorption spectra of 4,4',4",4'''-tetraoctadecylsulfo~~amidophthalocyaninecopper(II) in carbon tetrachloride and benzene solutions demonstrate the existence of monomer-dimer equilibria in the 10-6-10-4 M concentration range. The dimerization constants, K,, = Cd/Cmz,are (2.97 f 0.02) X lo6M-I and (1.58 f 0.09) x lo* M-1 at 22 f 2' in carbon tetrachloride and benzene, respectively. The absorption spectra of the pure monomer and pure dimer were calculated and found to be nearly identical in both solvent systems. A close correspondence was found to exist between the spectra of the resolved solution dimer and the solid state absorption spectrum of the phthalocyanine dye. The nature of the intermolecular interactions between phthalocyanine dye molecules in solution and the solid state is discussed.
The electronic spectroscopy of phthalocyanines and porphyrins has received a great deal of attention due to the similarity in structure between these molecules and chlorophyll and In addition' the semiconduction and photoconduction properties Of The Journal of Physical Chemistry, Vol. 76, N o . 3, 1979
phthalocyanines as well as their importance as cyans in colored reprographic systems has added to the intensity of research on this class of compounds.l (1) F. H.Maser and A. L. Thomas, "Phthalocyanine Compound&" Reinhold, New York, N. Y., 1963.
COPPER(II)-PHTHALOCYANINE DYE Since the dimeric and aggregated states of phthalocyanines,2 chlorophylls,g etc., have been found to be important in both fluid and solid-state media, many investigations have focused on the mechanistic details of intermolecular interactions in phthalocyanine-type molecules. In general, it has been found that spectroscopic investigations in solution are more amendable to well defined studies since complications arising from polymorphism can be eliminated.2,4 Several water-soluble tetrasulfonated phthalocyanines have been chosen by other authors5 as model compounds for equilibrium studies between monomer, dimer and higher order aggregates. The total dye concentration ranges studied have generally been of the order of 10-7-10-4 M in aqueous media. The resolved pure component monomer and aggregate spectra have not been reported in the past. In this paper we report a study on the monomer-dimer equilibria of a phthalocyanine dye molecule of the structure
where R is SO2?TH(CH2)&H8. The equilibria studies were carried out in benzene and CC14 using total dye concentrations in the range 10--6-10-4 M . Using this type of phthalocyanine molecule the dimerization process could be studied in solvents of low dielectric constant. Thus, the dye-dye interaction is the driving force for dimerization in these solvent systems since the screening of the dye-dye interaction is minimized in nonpolar solvents.6 In the studies involving water as a solvent, the dye-dye interaction is in all probability not the major force causing the molecules to associate; instead the strong solvent-solvent interaction excludes the dye molecules from solution and causes them to aggregatc7 The effects of phthalocyanine-dye dimerization were evaluated in CCI, and benzene by determining equilibrium constants for dimerization using previously reported7 spectroscopic-computer techniques. In addition, pure monomer and dimer spectra were measured in each system and compared to the solid state.
Experimental Section
Preparation of Dye. The dye, 4,4',4",4'''-tetraoctadecylsulfonamidophthalocyaninecopper(II), was pre-
447 pared by the method of Zickendraht.8 Purification was achieved by solvation of the dye (5 wt %) in chloroform and precipitation in methanol. The purification procedure was repeated five times. The material was then vacuum dried at 50" for 12 hr. (Anal. Calcd for dye: C, 65.6; H, 8.7; N, 8.8; Cu, 3.3. Found: C, 65.9; €7, 8.4; N, 9.1; Cu, 3.6.) Preparation of Solutions and Absorption Spectra. For each experiment, fresh solutions were prepared by weighing a sample of phthalocyanine into a known volume of either Matheson Coleman and Bell Spectro quality carbon tetrachloride or benzene. The amount of water in each freshly prepared "master solution" was found by the Karl Fisher method to be 0.27 mg/ml (CeHe) and 0.21 mg/ml (eel,). Less concentrated solutions were then prepared by parallel dilution and run on a Cary Model 14R automatic spectrophotometer using 0.1-, 0.5-, 1-, 2-, 5-, and 10-cm matched quartz cells.
Results and Discussion The unsubstituted copper phthalocyanine pigment and the substituted dye under consideration in this study both have D4hsymmetry. The lowest electronic transition in both compounds is observed at ca. 680 mp in l-chloronaphthalene. The extinction coefficients a t peak maxima are ca. 1.1 X lo51. mol-' cm-1 for the dye and ca. 2.1 X lo5 1. mol-l cm-l for the pigment. Vibrational satellites are noted at ca. 610 mp and 650 mp. I n general, the phthalocyanine pigments are difficult to study in solution due to their negligible or limited solubility in organic solvents. In marked contrast to the phthalocyanine pigment, the dye analog has a solubility greater than M in CCb, benzene, toluene, etc., and better than 10-2 M solubility in THF, DhIF, and dioxane. Further work in the two solvent systems, i.e., CC14and benzene, indicated that dye concentrations of the order of M in benzene produced spectra very similar to the dye spectrum in l-chloronaphthalene, whereas in CC14 at 10-6 M there was enhanced absorption in the 16,000-cm-' region. The concentration dependences of the dye molecule in cc14 and benzene are shown in Figure 1. With increasing concentration both dye-solvent systems show a decreasing apparent extinction coefficient at 14,700 (2) J. H. Sharp and M. Lardon, J . Phys. Chem.,72,3230 (1968). (3) K. Sauer, Proc. Nut. Aead. Sci. U. S., 53,716 (1965). (4) E. A . Lucia and F . D. Verderame, J . Chem. Phys., 48, 2674 (1968). (5) S. E. Sheppard and A. L. Geddes, J . Amer. Chem. Soc., 66, 1995 (1944); H. Kobayashi, Y. Torrii, and N . Fukuda, J. Chem. SOC. Jap., 81, 694 (1960); K . Bernauer and S. 'Fallub, Helv. Chim. Acta, 44, 1287 (1961). (6) K . Sauer, J. R. Lindsay Smith, and A. J. Schultz, J . Amer. Chem. SOC., 88,2681 (1966). (7) A. R. Monahan and D. F. Blossey, J. Phys. Chem., 74, 4014 (1970); A. R . Monahan, N. J. Germano, and D. F. Blossey, ibid., 75, 1227 (1971). (8) C. Zickendraht and E. J. Koller, U. S. Patent 2,897,207. The Journal of Physical Chemistry, Vol. 76,No. 8,1978
448
A. R. MONAHAN, J. A. BRADO, AND A. F. DELUCA
Table I : Monomer-Dimer Equilibrium of Phthalocyanine Dye at 22' Benzene: K,, = (1.58 & 0.09) X 10' 1. mol-.' Total dye concentrations, mol 1. - 1 ( x 108) Monomer concentrations, mol 1. -1 ( x 106) Dimer concentrations, mol 1.-1 ( x 108) Equilibrium constants, 1. mol-' ( x 10-4)
4.89
9.78
24.5
48.9
97.8
489.0
4.32
7.84
15.8
26.8
43.0
107.1
0.284
0.971
4.33
11.0
27.4
191.0
1.52
1.58
1.73
CC14: K,, Total dye concentrations, mol 1. ( ~ 1 0 ~ ) Monomer concentrations, mol 1. ( X 106) Dimer concentrations, mol 1. (x108) Equilibrium constants, 1. mol-1 (x10-4)
1.53
1.67
= (2.97 i . 0.02) X 106 1. mol-'
1.07
2.14
4.28
0.349
0.520
0.772
1.81
0.360
0,810
1.75
9.78
296.7
1.48
299.3
21.4
294.0
1.2c
298.7
--
I
I
-
DYE IN CCI4 C
D
PER~MONOMER ~
-
%ONOMER
0.8C
n
~
I
I
~
0.40 Y)
P
-
c
-k 0.w I
a
I
e 1.20
W
-
c, -1
DYE IN BENZENE
w
V
0.80
, / A\'.
0.40
/
0.00
1.20
/
I.40
I .60
1-80
F ( cm-1 I x 10-4
Figure 2. Calculated absorption spectra of pure phthalocyanine monomer and dimer in carbon tetrachloride and benzene.
t ( c m - 4 ) x10-4
Figure 1. Concentration dependence of the phthalocyanine dye in benzene and carbon tetrachloride. The Journal of Physical Chemistry,Vol. 76,No.3, 1972
& 100 em-' with an increase in L. in the 16,000-cm-' region. Isosbestic points occur at 14,200 rfs 100 and 15,100 100 cm-1 in each dye-solvent system. Based on the observations of monomcr-dimer equilibria in-
*
COMMUNICATIONS TO THE EDITOR
449
volving sulfonated phthalocyanines in water,6 a similar mechanism was assumed to be operative in thesc systems. Monomer-Dimer Equilibrium. From each set of data, the pure monomer spectrum, pure dimer spectrum and the equilibrium constant can be calculated using a previously reported computer procedure.’ For each total concentration ct, the monomer conccntration cm, dimer concentration c d , and equilibrium constant K e , was found. The best fit was obtaincd at equilibrium constants of Keq = (1.58 0.09) X lo41. mol-‘ for the dye-benzene system and K,, = (2.97 0.02) X lo6 1. mol-’ for the dye-CC14 system. The best fit monomer and dimer spectra are shown in r’‘1 gure 2. The results of the analyses are tabulated in Table I. The equilibrium constants for the phthalocyanine in CC14 and benzene are in qualitative agreement with equilibrium constants reported for sulfonated phthalocyanines in ~ a t e r and ~ . ~the chlorophylls in Cc14.‘ The equilibrium constants obtained for these molecules were also realized by using the observed changes in the long wavelength electronic absorption bands as a function of concentration in the 10-6-10-4 M range. For example, the monomer-dimer equilibrium of the tetrasodium salt of 4,4”4”,4”’-tetrasulfophthalocyaninecobalt(I1) has been investigated by equilibrium spectrophotometric measurements5 and by stopped-flow relaxation techniquesg in aqueous solution. The equilibrium constant was found by both techniques to be (2.05 f 0.05) X lo5 1. mol-’. Sauer and Lindsaya have reported the dimerization of three chlorophylls in CC14 and found the association constants to be ca. 1 X lo‘ 1. mol-’. Dimer formation in our system was observed and compared in solvents other than CC1, and benzene. This was accomplished by measuring the ratio of the solution optical density a t 14,700 cm-’ relative to the 16,000 cm-1 density a t nearly equivalent total dye concentrations (ca. 2 X M ) . The order of decreasing phthalocyanine-dye dimerization in several solvent
*
*
WAVELENGTH Irng I
Figure 3. Comparison of melt cast solid-state absorption spectrum of dye with the benzene solution dimer spectrum.
systems is CC14 > benzene > tjoluenc > chloroform > dioxane > D H F > THF, etc. It is probably pertinent to note that the aggregation tendency of the dye is diminished in the solvents yielding the greatest dye solubility and, in general, the largest diclectric constant. To form the dimer, the dyc-dye interaction must be strong enough to overcome any other forces which would favor solvation of the monomer. Thus, the lower the diclectric constant of the solvent, the loss the screening of the dye-dye interaction by the solvent. Relative to benzene, the dye-solvent interaction is probably weaker in carbon tetrachloride. Thorefore, the conditions for maximum dye association or intcraction are maximized in the cc14 system, viz., low diclcctric constant and low solvation of the dye. By inspection of I’igure 3, it can be concluded that the solid state is also composed of Phthalocyanine molecules acting via similarly structured dimeric pairs. Thus, molecular interactions operative in the solid state can be simulated in solution if the solvent properties and structure-solubility characteristies of the dye are matched properly.
Acknowledgments. Stimulating discussions with Drs. D. F. Blossey and 1’1. S. Walker are acknowledged with pleasure. (9) 2. A. Schelly, R. D. Farina, and E. M. Eyring, J. Phys. Chem., 74,617 (1970).
C O M M U N I C A T I O N S TO THE E D I T O R
Near-Infrared Spectroscopic Study of the Interactions between Water and Acetone Publication wsts borne completely
by The
Journal of
Physical Chemiatry
Sir: McCabe, Subramanian, and Fisher have recently published “A Near-Infrared Spectroscopic Investigation
of the Effect of Temperature on the Structure of Water.” In order to support their interpretation, they have cxamined the near-infrared spcctra of water-acetone mixtures. We had studied absorption spcctra, betxvecn 1000 and 11000 cm-I, of H20, DsO, and HOD a8 frcr (1) W. C. McCabe, S. Subramanian, and H.1.‘. Fisher, J. PhUs. Chem., 74,4360 (1970). The Journal of Physical Chemistry, Vol. 76, N o . 8, 1972
