A. R. MONAHAN, J. A. BRADO, AND A. F. DELUCA
1994
The Association of Copper( 11), Vanadyl, and Zinc(I1) 4,4’,4”,4’~-Tetraalkylphthalocyanine Dyes in Ben-zene by Alan R. Monahan,* James A. Brado, and Allen F. DeLuca Xerox Rochester Research Center, Rochester, N e w York 14603 (Received January 28, 1972) Publication costs assisted by the Xerox Corporation
Analyses of the absorption spectra of three phthalocyanine dyes in the 12,000-32,000-cm-1region demonstrate the existence of monomer-dimer equilibrium in the concentration range from 10-6 to 10-4 M , The dimerization constants in benzene, K,, = Cd/Cm2,are (1.58 f 0.09) X lo4M-l, (1.09 f 0.23) x 106 M-l, and (2.01 f 0.49) X lo6A W - for ~ the Cu(11)-, Zn(l1)-, and vanadyl- 4,4’,4’’,4”’-tetraoctadecylsulfonan~idophthalocyanines, respectively. The absorption spectra of pure monomer and pure dimer in benzene are calculated for each dye. The relative dye-dye interaction as measured from the Davydov splitting in the dimer and the equilibrium constant for dimerization were found to be reasonably consistent for the three metallophthalocyanine dye molecules.
Introduction Metal-free 4,4’,4”,4”’-tetrasulfophthalocyanine(HzTSP) and its Cu(II), Co(II), Zn(II), Fe(III), and VO(I1) analogs are known to dimerize in water.l The stability of the dimers decreases in the order CuTSP > HzTSP > FeTSP 2 (V0)TSP N ZnTSP > CoTSP and the stability as measured by the equilibrium constant for dye association varies by roughly two orders of magnitude within the series. The lower stability of the Co and Zn phthalocyanines was tentatively explained on the basis of axially coordinated water molecules inhibiting dye association. However, it has recently been found that both FeIIITSP and VOIITSP, which have coordination numbers greater than 4,are more stable than the Zn and Co analogs. Hence it may be concluded that other factors which have not been previously taken into accountIb are operative in controlling phthalocyanine dye aggregation processes. The major difficulty in evaluating dye association processes in water is that the strong solvent-solvent interaction is the dominant force causing the molecules to associate rather than the dye-dye interaction. I n a previous paper2 we reported a study on the monomer-dimer equilibria of a phthalocyanine dye molecule of the structure R,
R The Journal of Physical Chemistry, Vol. 76, No. 14, 19’79
where R is SOzNH(CH2)17CH3and M is Cu(I1). The equilibrium studies were carried out in benzene and CC14 in the concentration range 10-6 to M . Using this alkylated phthalocyanine dye molecule the dimerization process could be studied in solvents of low dielectric constant where the dye-dye interaction is maximized. The resolved solution dimer spectrum in both solvent systems was virtually identical with that of the solid-state spectrum. This indicated that interactions operative in the solid state can be simulated in solution if the solvent system and structure-solubility characteristics of the dye are matched properly. I n this paper we report a preliminary attempt to evaluate the effect of metal atom on the dimerization of a phthalocyanine dye molecule in benzene using previously reported spectroscopic computer technique^.^ I n this study PI is Cu(II), (CuPc), Zn(II), (ZnPc), and VO(II), (VOPc). I n all three molecules the association constants were measured spectrophotometrically at 25” in the to M concentration region. I n addition, the resolved monomer and dimer spectra were obtained. Thus, the relative interaction energy could be obtained from the size of the Davydov splitting in the dimer. The approximate dye-dye interaction as obtained from Davydov theory and the magnitude of the dye-dye interaction as measured from the equilibrium constant for dimerization were found t o be reasonably consistent for the three metallophthalocyanine dye molecules.
(1) (a) K . Bernauer and S. Fallab, Helv. Chim. Acta, 44, 1287 (1961); (b) H. Sigel, P. Waldmeier, and B. Prijs, Inorg. Nucl. Chem. Lett., 7, 161 (1971). (2) A. R. Monahan, J. A. Brado, and A. F. DeLuca, J . Phgs. Chem., 76, 446 (1972). (3) A. R. Monahan and D. F. Blossey, ibid., 74, 4014 (1970); A . R. Monahan, N. J. Germano, and D. F. Blossey, ibid., 75, 1227 (1971).
1995
ASSOCIATION OF METAL-PHTHALOCYANINE DYES I
1
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I
1
I
I
Cu DYE I N C&
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'DIMER -'MONOMER
8.0
Zn DYE IN COHO e.0
-7-
-'MONOMER
7
I
T
7
0
-5I
-4I
-3
LOG Cm
Figure 2. Plot of log C d us. log C , for phthalocyanine dyes in benzene. Solid lines are drawn with theoretical slopes of 2.
6.0
Results and Discussion
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VO DYE IN C6Ha
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72 * c
-
-
-@MONOMER
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8
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2
20-
Y
0.0-
1.2
1.6
2.0
ii (em-') a
2.4
2.8
32
IO-'
Figure 1. Calculated absorption spectra of phthalocyanine monomers and dimers in benzene.
Experimental Section The prcparation and characterization of copper(I1)4,4',4'',4t'' - tetraoctadecylsulfonamidophthalocyanine was reported previously,2as well as solution preparation techniques and measurement of absorption spectra. The corresponding vanadyl and zinc compounds were prepared by a similar procedure. Anal. Calcd for ZnPc dye: C, 65.6; H, 8.7; N, 8.5; Zn, 3.4. Found: C, 65.6; H, 8.9; K, 8.5; Zn, 3.5. Calcd for VOPc dye: C, 65.6; H, 8.7; K, 8.8; V, 2.7. Found: C, 65.3; H, 8.8; S , 8.6; V, 2.5.
The concentration dependenct>of the copper phthalocyanine dye in benzene was published in a previous papera2 Similar spectral dependences were found as a function of total dye concentration for the VO and Zn molecules. For each set of spectral data the equilibrium constant and pure component monomer and dimcr spectrum can b r calculated using an iterative computer t e ~ h n i q u e . ~For oach total dye concentration, the monomer conrentration C,, dimer concentration C d , and equilibrium constant K,, can be found. The calculation is made assuming the monomer-dimer equilibria follow the law of mass action whfw K,, = C d / C,*. The best-fit monomer and dimer spectra for the three dyes are shotvn in Figure 1. The monomer and dimer spectra were used to calculate the equilibrium constants for each total dye concentration. Figure 2 shows the fit t o a simple monomer-dimer equilibrium by graphical display of the mass law equation. The best fits were obtained at equilibrium constants of K,, = (1.58 + 0.09) X l o 4 M - l for the CuPc dye, (1.09 0.23) X lo6A l - l for the ZnPc dye and (2.01 f 0.49) X lo6A1-l for the VOPc dye. According t o the theory of molecular excitons14the size of the splitting of an allowed transition is a nieasurc of the interaction energy between molecules in t h r dimer. The VO and Zn dimers are characterized by larger exciton splittings (ca. 1500 cm-I) than the Cu dimer (1100 cm-l). Thc splittings were obtained by means of a computer Curve fitting calculation. The absorption spwtra of the dyes at 77°K in hydrocarbon matrices (ca. 21.1) and as solid phase melt cast films2
*
(4) A. S. Davydov, "Theory of Molecular Escitons," Nauka Press, .Moscow, 1968, p 56.
The Journal of Phusical Chernistru, Vol. 76, No. 14, 1972
STEPHEN G. SCHULMAN AND IRENE PACE
1996 confirmed the magnitudes of the calculated exciton splittings. The association constants also reflect the stronger intermolecular interactions in the VO and Zn dimers relative to the Cu dimer. The interaction energy in the Zn dimer is in all probability larger than in the Cu pair because the filled 3dZ2orbital in Zn contracts relative to Cu and allows for a stronger interplanar interaction.6 The VO dimer is the most stable probably because of the formation of a relatively strong interplanar OV. . - 0 V or OV . .K bond. Dickson and Petrakis6 have studied the bonding of a vanadyl mesoporphyrin using infrared spectroscopy and demonstrated the existence of specific types of coordinated VO bonds with relatively largc bonding energies (ea. 17 kcal mol-'). The equilibrium constants obtained in benzene solution seem to give a fairly accurate indication of the true dye-dye interaction since the relative intermolecular interactions of the three phthalocyanine dye molecules are reasonable in terms of previously proposed intermolecular bonding schemes. In marked contrast to our study in a nonpolar solvent, the previous studies of CuTSP and ZnTSP in water' demonstrated that the Cu dimer associates t o a greater extent than the Zn pair. I n water as a solvent, the dye-dye interaction is not the major driving force
for aggregation. Instead the strong solvent-solvent interaction tends to exclude the dye molecules from solution and forces them t o aggregate. I n addition, complications arising from metal-water complexation may be operative. Solid-state studies on unsubstituted Cu and Zn pigments by Day and Williams5 show that the Zn phthalocyanine is two orders of magnitude more conducting than the Cu' pigment. Also the Davydov splitting in the crystal is 2230 ernF1 for the Zn and 1890 cm-l for the Cu, indicating a larger interaction energy in the Zn crystal. Thus, the relative magnitudes of the interactions in the solid state agrees qualitatively with our studies in benzene. Therefore, molecular association processes in solvents of low dielectric constant perhaps reflect the ordering of the dye-dye interactions in the solid more accurately than in aqueous systems, where the solvent-solvent interaction is the major driving force for self-a~sociation.~
Acknowledgment. Informative discussions with Drs. 34. S. Walker and D. F. Blossey are acknowledged with pleasure. (5) P. Day and R. J. P. Williams, J . Chem. Phys., 37, 567 (1962). (6) F. E. Diokson and L. Petrakis, J. Phys. Chem., 74, 2850 (1970).
The Dissociation Constant of the 9-Anthroic Acidium Cation in the Lowest Excited Singlet State by Stephen G. Schulman* and Irene Pace College of Pharmacy, University of Florida, Gainesville, Florida
38601
(Received December 17, 1971)
Publication costs borne completely by The Journal of Physical Chemistry
The large red shift of the fluorescence of 9-anthroic acid in moderately concentrated acid solutions has been investigated and attributed to prototropic equilibrium in the ILa state between 9-anthroic acid and its conjugate cation. Comparison of pK,* values for the latter equilibrium, obtained by fluorometric titration and by Forster cycle calculations, is used to evaluate thermal relaxation processes in ground and electronically excited states. Conversion of the fluorescence of 9-anthroic acid to that of the conjugate anion is found to be static. The excited-state reaction apparently is too slow to compete with fluorescence.
The fluorescences of 9-mthroic acid and the 9anthroate anion have been studied recently.'S2 The large Stokes shift of the acid has been well characterized as the result of rotation of the carboxyl group from perpendicularity, with the anthracene ring2 in the anion, to coplanarity in the neutral acid. The previous studies also attempted to draw conclusions about the acidity of the acid in the lowest excited singlet state The Journal
of
Physical Chemistry, Vol. 76, No. 14, 1978
(the ILa state), relative to that of the ground state, by means of Forster cycle calculations.a Forster cycle calculations of the dissociation constant of 9(1) E. Vander Donckt and G. Porter, Trans. Faraday Soc., 64, 3218
(1968). (2) T. C. Werner and D. M. Hercules, J. Phys. Chem., 73, 2005 (1969). (3) T. Forster, 2.Elektrochem., 54, 42 (1950).
