JAMESH. SHARP AND MARCEL LARDON
3230
Spectroscopic Characterization of a New Polymorph
of Metal-Free Phthalocyanine by James H. Sharp and Marcel Lardon Xerox Research Laboratories, Rochester, New York (Receiued March 81, 1968)
The infrared, visible, and near-infrared absorption spectra of thin films of a new polymorph of metal-free phthalocyanine (HaPc), designated as z-H~Pc,have been compared with those of the well-known QI and p-HzPc polymorphs. Distinct differences among the three polymorphs in the 700-800-cm-l infrared region and in the visible and near-infrared electronic spectra (10,000-20,000 cm-l) were observed. The electronic spectrum of Z-HZPCcan be interpreted as that of a dimer. I n accordance with this assumption, the correct number of electronic energy levels, their relative oscillator strengths, the resonance splittings, and the relative orientation of the molecules of the dimer have been obtained from an analysis of the electronic spectra. A parallel plane model for the dimer is considered and the intermolecular separation is computed from the exciton dipoledipole approximation. A comparison of the proposed structure for z-HaPc with that of P-HZPc, where the crystal structure is known, accounts satisfactorily for observed differences in the NH stretching frequencies.
I. Introduction Metal-free phthalocyanine (HzPc) has been reported to exist in a t least three polymorphic forms. The p form is the most stable polymorph and single crystals can be prepared by sublimation a t 550". The detailed structure of the p crystal has been reported by Robertson,' who found that it is a monoclinic crystal belonging to the P21/a space group and that there are two phthalocyanine molecules per unit cell. The X-ray powder diffraction pattern for another polymorph of phthalocyanine, designated as the a form, has been reported by several workers.2-6 Assour' and Sidorov and Kotlyars have found that the a form is completely converted to the p phase when heattreated above 300". A third crystalline modification of phthalocyanine, called the y form, has been reported by Eastes. Assour,' however, has presented experimental evidence that the a and y forms differ only in particle size. Since it has not been possible to grow single crystals of the a and y polymorphs, no detailed X-ray analyses have been reported. Infrared spectroscopy has been effectively employed to distinguish , * t no ~ ~infrared spectra between the a and p f ~ r m s , ~ but of the y polymorph have been reported. Another crystal form of phthalocyanine, designated x-phthalocyanine (z-HzPc) because of its unique X-ray diffraction pattern, has recently been reported.'l The purpose of this work is to characterize x-HzPc with respect to its infrared, visible, and near-infrared absorption spectra. For comparison purposes, similar spectra for the well-known a and p polymorphs are also presented.
11. Experimental Section and Results 1. Sample Preparation. Metal-free phthalocyanine is commercially available and was purified by repeated The Journal of Physical Chemistry
solvent extraction techniques. Thin films of a-HzPc were prepared by vacuum evaporation (lo+ torr) of the purified material onto rectangular KBr flats and Pyrex cover glass slides held at ambient temperatures, The evaporations were carried out in a Bendix Balzers Model BA-3 evaporator. Thin films of p-HzPc were obtained by heat treating the evaporated a films for several hours a t 300". A kinetic study of the a +,p thermal conversion has been reported. l 2 The new x polymorph can be prepared by neat milling a-HzPc for approximately 1 week.l1 Thin films of the resulting x material were prepared by depositing a suspension of x-H2Pc in heptane onto KBr or Pyrex windows and allowing the heptane to evaporate. The films were then gently heated a t 80" in an oven for several hours. Several samples for spectroscopic analyses were also prepared using the Nujol mull technique in which a suspension of the x polymorph in Nujol was pressed between KBr or Pyrex plates. For spectroscopic characterization in the visible and nearinfrared absorption regions, a thin film of x-HzPc in an epoxy-phenolic copolymer (1 part x-H~Pc:12 parts, (1) J. M. Robertson, J . Chem. Soc., 615 (1935); 1195 (1936); 219 (1937). (2) G. Susich, Anal. Chem., 2 2 , 425 (1950). (3) F. R. Tarantino, D. H. Stubbs, T. F. Cooke, and L. A. Melsheimer, Am. Ink Maker, 29, 35,425 (1950). (4) A. A. Ebert, Jr., and H. B. Gottlieb, J. Amer. Chem. Soc., 74, 2806 (1952). (5) F. W.Karasek and J. C . Decius, ibid., 74, 4716 (1952). (6) M. Shigemitsu, Bull. Chem. Soc. Jap., 3 2 , 607 (1959). (7) J. -M.Assour, J. Phys. Chem., 69, 2295 (1965). (8) A. N. Sidorov and I. P. Kotlyar, Opt. Spectry., 11, 92 (1961). (9) J. W.Eastes, U.8. Patent 2,770,620(1956). (10) D.N. Kendall, Anal. Chem., 2 5 , 382 (1953). (11) J. F. Byrne and P. F. Kun, U. 8. Patent 3,357,989(1967). (12) J. H.Sharp and R. L. Miller. J . ~ h y sChem., . to be published.
3231
SPECTROSCOPIC CHARACTERIZATION OF METAL-FREE PHTHALOCYANINE
A d ) n 10-3 0.0 O
w
0.10
-
0.20
-
0.10
0,lO
0.20
0.20
0.30
0.30
0.40-
0.40
0.40
0.50-
0.50
0.50
0.60C
0.60
0.60
0.70 0.80
0.70 0.80
-
0.30
I
7 I
8
9 1 0
I
I
1
r
5 (em-1 ) n 10-3 Figure 2. Visible and near-infrared electronic spectra of thin films of a-,p-, and x - H ~ P ca t 77°K.
0
$
I
i
z a
O
6
I
0
2
’
I)
Q
1I:;:
and Z-HZPCPolymorphs
1.0
1.0-
1.5
Table I : Absorption Frequencies“ in the 700-800- and 3300-cm-* Infrared Regions for the a-,P-,
-
1.5
1.5
-
2.0Z L J .
2.0
800
700
V
U-HZPC
2.0 800
700
714 (s-m)
(cm-1)
a
733 (s) 738 (s) 743sh (m)
(b) 3100 -3500 c m - l
B
a
X
767 (w-m) 3302 (w)
0
0.3
X-HZPC
720 (9) 724 ( s ) 733 (s-m) 739 (s-m)
734 (s-m) 739 (9-m)
755 ( 9 ) 772 (s) 782 (m) 3284 (m)
755 (s) 772 (m-w ) 784 (w) 3302 (w-m)
a Intensities of the absorption are qualitatively described as weak (w), medium (m), or strong (s). A subscript sh indicates a shoulder.
m
a
@-HaPo
0.4
m 0.5
3500
3100
3500
3100 0‘33500
3100
P (cm-1) b Figure 1. (a) Infrared spectra of the CY-, 0-, and 2-HZPC polymorphic films in the 700-800-cm-l region. (b) Infrared spectra of the 01-, p-, and z-H2Pc polymorphic films near 3300 cm-’.
by weight, of resin) cast on a Pyrex substrate was also prepared. 2. Spectroscopy. The infrared absorption spectra of W, p-, or x-H2Pc on KBr substrates were recorded a t room temperature with either a Perkin-Elmer Model 337 spectrometer or a Beckman Model IR-10 spectrometer. The resulting spectra in the 700-800-cm-1 region, where the most significant differences among the three polymorphs are observed, are shown in Figure l a . Differences near 3300 cm-l were also noted and these are shown in Figure lb. A summary of the main absorption frequencies for the three polymorphs in these two regions is given in Table I. The visible and near-infrared absorption spectra of
the three polymorphic films were recorded on a Gary Model 14R automatic spectrophotometer. All spectra were recorded a t 77°K and a comparison of the cy-, 6-, and x-HzPc films is given in Figure 2. The absorption spectra of the x-HnPc films were essentially identical regardless of the method of preparation of the film.
111. Discussion 1. Infrared Xpectra. The infrared absorption spectra of the three polymorphic HzPc films, shown in Figure 1, show subtle yet distinct differences in both intensity and frequency of the vibrational modes. We have found that the 700-800-cm-l region is particularly sensitive for differentiating between the three polymorphs. The out-of-plane CH deformation frequencies of the four adjacent hydrogen atoms of ortho-disubstituted aromaticals and the NH deformation f r e q u e n c i e ~ 8 ~ ~ ~ ~ ~ both occur in this region. Since phthalocyanine is a (13) L. J. Bellamy, “The Infrared Spectra of Complex Molecules,” John Wiley and Sons, Inc., New York, N. Y.,1959,p 77. (14) H.F. Shurvell and L. Pinzuti, Can. J. Chem., 44, 125 (1966). (15) S. F. Mason, J . Chem. Soc., 976 (1958). Volume 78,Number 9 September 1968
3232 planar molecule, these bending modes will be influenced by the orientation of adjacent molecules, which also determines the polymorphic phase. Other authors’* have assigned the 720-cm-’ absorption occurring in the a and p polymorphs to an out-of-plane NH deformation in analogy to a band observed a t 719 cm-l in porphins’5 whose ring structures are similar to that of phthalocyanine. However, since several metal phthalocyanine derivatives, where NH absorptions are absent, also show an absorption band a t 720 cm-l, this absorption frequency is probably a CH deformation mode. In the NH stretching absorption region ( ~ 3 3 0 0 cm-l) , the a and x polymorphs show absorption at 3302 cm-’ whereas the absorption in the p polymorph is redshifted by 18 cm-l to 3284 cm-’. In order to account for the observed NH stretching frequencies of the three polymorphs it is necessary to consider both intra- and intermolecular hydrogen bonding. I n pyrrole, l 6 for example, where intramolecular hydrogen bonding is negligible, the red shift of 97 cm-l in going from solution to the solid is attributed to intermolecular hydrogen bonding. However, for the porphins, where considerable intramolecular hydrogen bonding is present,l5 the NH stretching frequency in solution is appreciably redshifted (5200 cm-l) from that of pyrrole in solution. An additional red shift of only 4 cm-l is found in going from solution to solid. I n recent work, Chenle has proposed a “shared hydrogen” model for phthalocyanine where each imino hydrogen is shared by two neighboring nitrogen atoms as shown in Figure 3. Since the intramolecular hydrogen bonding is appreciable in each of the three phthalocyanine polymorphs, a substantial but identical red shift from the the “free” NH stretching frequency (-3497 cm-l) is predicted. The a and x polymorphs have the same X” stretching frequency but the additional red shift of 18 cm-l observed for theppolymorph indicates a weakening of the H-N-H bonding due to intermolecular bonding. This is in accord with the crystal structure1 of P-phthalocyanine where the molecules are arranged in parallel stacks as shown in Figure 4. 2. Visible and Near-Infrayed Spectra. The phthalocyanine molecule has D2h symmetry and this is evident from solution absorption spectra. For example, the lowest observed electronic transition of copper phthalocyanine, which has Dlh symmetry, is a t 6780 b.17 In a solutio? of metal-free phthalocyanine, however, the 6780 A transition is split into two peaks which are located a t 6990 d (14,306 cm-l) and 6650 A (15,038 cm-!) as shown in Figure 5. The polarized absorption spectra of single crystals of p-HzPc, reported by Lyons, et aZ.,lS also clearly show the splitting of the lowest electronic transition. The absorption spectra of thin films of a and P-HzPc, shown in Figure 2, also reflect the splitting of the lowest electronic transition. I n the case of a-HzPc the two resulting electronic transitions are located a t 14,430 and The Journal of Physical Chemistry
JAMES H. SHARPAND MARCEL LARDON
Figure 3. The ‘‘shared hydrogen” model for the phthalocyanine molecule.16
\ I & dN ...H =
Figure 4. Intermolecular hydrogen bonding in p-HtPc, p = 45.9’.
16,320 em-’, whereas the film of /3-H2Pcshows electronic transitions at 13,500 and 15,400 cm-’. In contrast to the electronic spectra of the a- and p-HzPc films, the x-HzPc film shows four electronic transitions located at 12,500,15,490,16,380,and 17,150 cm-l. The transition located at 12,500 em-’ (8000 b) in the z-H~Pcfilm (16) I. Chen, J . Mal. Spectrosc., 23, 131 (1967). (17) J. M.Assour and S. E. Harrison, J. Amer. Chem. Sac., 87, 661 (1965). (18) L. E. Lyons, J. R. Walsh, and J. W. White, J . Chem. Sac., 167 (1960).
SPECTROSCOPIC CHARACTERIZATION OF METAL-FREE PHTHALOCYANINE
2+or+
3233
x ( A ) x 10-3 ,5
6
I '
I
r I
8
B
IO
I
l
l
I
1.4 I'T 1.2
0.6
0.4
v' (cm-1) x10-3 Figure 6. The quantitatively resolved spectrum of z-H2Pc in the 9000-20,000-~m-~absorption region.
Figure 5 . (a) Absorption spectrum of metal-free phthalocyanine in l-chloronaphthalene. (b) Normalized fluorescence spectrum of metal-free phthalocyanine in 1-chloronaphthalene a t 77°K (Assour and Harrison").
sitions. The relative oscillator strengths and the assigned electronic transitions are given in Table 111.
spectrum is particularly unique for identification purposes. The additional peaks in the x-HzPc film spectrum can be qualitatively accounted for by assuming that the x-HzPc modification has a dimeric structure. I n an attempt t o develop a model for x-HzPc, the absorption spectrum of the x-HzPc film has been compared to that of the a-HzPc film. Since a-HzPc is a metastable and less ordered polymorph, l 2 its absorption spectrumshould more closely approximate that of a monomer (solution spectrum), The lowest electronic transition observed 4 in the a film shows only a small red shift ( ~ 1 2 cm-l) from that observed in solution. The observed absorption frequencies of the monomer (a-HzPc) and the dimer (x-HzPc) and the assigned resonance splittings, A,(O), are given in Table 11. An estimate of the rela-
Table I1 : Observed Absorptions and Assigned Resonance Splittings Av observed resonance splittinge, Phthalocyanine form
(monomer) z (dimer) a (monomer) z (dimer)
--Absorption
-
frequency, om -10
+
Ar(O),
16,380
&1940
17,150
A830
om -1
14,430
a
12,500 16,320 15,490
tive orientation of the molecules of a dimer can be made from the intensities and frequencies of the observed electronic transitions. Figure 6 shows the absorption spectrum of the x-HzPc film which has been quantitatively resolvled into the four observed electronic tran-
Table 111: Relative Oscillator Strengths of the Assigned Electronic Transitions of z-HaPc Assigned electronic transition, cm -1
Relative oscillator strength
M - (12,500) M + (16,380)
55 28 29 14
'L- (15,490) L + (17,150)
3. The Parallel Plane Dimer. The simplest dimer model to account for the above oscillator strengths is one in which the planes of the two phthalocyanine molecules are parallel to one another. This situation is depicted in Figure 7 where the electric dipole vectors along the M axes of the two molecules form an angle e with the x axis of the planar coordinate system. Vectorial addition gives two components, M + and M - , whose squares are proportional t o their respective oscillator strengths (see Table 111) as expressed in
pw+p =
1 2 COS ~ e12
a
28
(1)
1M-I2 = l2M sin el2
a
55
(2)
From the above equations, 0 is found to be 55". Similarly, the electric dipole moments along the L axes of the two molecules each form an angle C$ with the x axis of the planar coordinate system and the resulting L + and L- components are related to their respective oscillator strengths by
lL+I2 = 12L sin +I2
0:
14
(3)
lL-I2 = )2L cos C$I2
a
29
(4)
Volume 78, Number 0 September 1068
JAMES H. SHARP AND MARCEL LARDON
3234
i
,
HY
where D,, is the dipole strength and R1, is the intermolecular separation. The x axis is the line joining the centers of the molecules and alg is the angle between the direction of the transition dipoles along the L, M , or N axes of molecule 1 and molecule g. In the case of a parallel plane dimer, eq 6 reduces to Dm,n
Ae(0) = -
R1za
(7)
‘Os
M’
\
Figure 7. Pictorial representation of the parallel plane dimer model of z-H2Pc.
Equations 3 and 4 give a value of 35” for 4. Since L and M are orthogonal to one another, 8 plus 4 must be 90”. It is interesting t o note that this model is in agreement with the apparent lack of intermolecular hydrogen bonding found in the infrared spectrum of the x-HzPc polymorph. Since the two molecules of the dimer form an angle of 110” (28 or 180 - 24) with respect to each other in the molecular plane, intermolecular hydrogen bonding is reduced as shown in Figures 8a and 8b. For example, if the distance between the moleculp planes in x-H2Pc is the same as in P-H2Pc, i.e., 3.4 A, then the distance between the bridge nitrogen and the imino hydrogen is increased from 3.7 A in p-H2Pc t o 3.9 A in x-H~Pc. 4 . T h e Exciton Dipole-Dipole Approximation. A physical picture of the exciton dipole-dipole splitting arising from the interaction of two molecules of a dimer can be made through the dipole-dipole approximation.l9 This approximation, derived from a classical pointmultiple expansion, considers only the dipole-dipole term and is valid only when the intermolecular separation, R, is significantly larger than the distance, ri, associated with the dipole strength, D,,, of the molecule defined by
D,,
= ( w )= ~ (J*,M9,d7)2
(5)
M is the dipole moment operator, Z,er,, where rl is the distance of the ith electron from the center of positive charge of the molecule, and m is the transition moment integral. Although this approximation is questionable in the case of the phthalocyanine molecule, a semiquantitative description of the observed resonance splittings and the intermolecular separation can be obtained. The dipole-dipole exciton splitting, Ae(O), is given by20 the equation The Journal of Physical Chemistry
M
U
t
\
I---7-
I
3.4i
,
2 i
,
~ N . ~ e3.9.i . H
b Figure 8. (a) Normal projection of the dimer of z - H ~ P cwith an angle, 01, of 110’ between the M and I” axes of the two molecules. (b) Cross section A-A through the dimer, showing the intermolecular hydrogen bonding between the imino (shared) hydrogen of the upper and the bridge nitrogen of the lower molecule. The interplanar distance is assumed to be 3.4 A as in P-HzPc.
(19) See, for example, E. 8. Emerson, M. A. Conlin, A. E. Rosenoff, K. S. Norland, H. Rodriguei, D. Chin, and G. R. Bird, J . Phys. Chern., 71, 2396 (1967). (20) R. M. Hochstrasser, “Molecular Aspects of Symmetry,” W. A. Benjamin, Ino.,New York, N. Y., 1966, p 311.
SPECTROSCOPIC CHARACTERIZATION OF METAGFREEPHTHALOCYANINE since mh and a2. are 90". The dipole strength is related to the oscillator strength, f,,,",of an electronic transition by
CRVSTALkAXIS.2-AXIS
I
where h is Planck's constant, e is the electronic charge, c is the velocity of light, me is the mass of the electron, g. is the degeneracy of the excited state, and v,, is the frequency of the transition in em-'. The oscillator strengths for M and L electronic transitions of metal-free phthalocyanine can be computed from the solution absorption spectrum and the normalized fluorescence spectrum," which are shown in Figure 5. The oscillator strength of an electronic transition is given by
f,"
=
4.32 X 10-o
1:
d u erg em
where 6 is the molar extinction coefficient in l./mol ern and Y is the frequency in em-'. Hence, fm." and fmnL are found to he 0.44 and 0.23 erg cm, respectively. The ratio of these oscillator strengths, fm."/fmnL, is 1.9, which is identical with the ratio of the relative oscillator strengths of the dimer given in Table 111. Using eq 8 the dipole strengths, D,," and D,.' are computed to be 6.5 X 1OWaaand 3.2 X erg ema, respectively. The separation distance between molecules of z-H~Pc, R122, is found from eq 7 hy using the observed resonance splitting of 1940 em-1 for the M transition, the correponding dipole strength, D,,", and a value of 110' (28) for aI2. Similarly, RI2*can be determined using the splitting of 830 em-' for the L transition, the corresponding dipole strength, DmnL,and the value of 110" (180 - 26) for aI2, I n the former case, RI2' is computed to be 3.9A, whereas the latter cme gives a value of 4.1 A, I n Figure 9 the stacking of HzPc molecules along the z axis in the proposed dimer model is compared to that along the b-crystal axis in @-H2Pc. It should be emphasized that the above model is necessarily a simple one and the close agreement be-
Z-AXIS 1 .,10.
I
I
B-HzR
(9)
3235
X-H&lDIMERI
Figure 9. Stacking of HaPc molecule8 along the b-cryatal axis in SHPc and along the z axis in the proposed model of rHnPc.
tween the model and the spectroscopic data may be somewhat fortuitous. The validity of the dipoledipole approximation, in the case of the phthalocyanine molecule, is certainly open to question, the interaction between the M + and L- and the M - and L+ electric dipole vectors has been neglected, and other nonparallel plane models may also he envisioned. In conclusion, however, the proposed model for x-H,Pc accounts satisfactorily for the observed electronic spectrum and for the reduced intermolecular hydrogen bonding o b served in the infrared spectrum. I n the case of the dimer model, where the molecules are 4.0 apart, the distance between the bridge nitrogen and the imino hydrogen is 4.5A compared to 3.7 A in @-H2Pc.
Acknourledgment. We wish to thank Mr. R. L. Miller and Mi-. H. Six for their assistance in sample prepars, tion and spectroscopic characterization. We are particularly indebted to Drs. D. L. Stockman, G. E. Johnson, P. K. Watson, and J. W. Weigl for stimulating discussions.
Volume 76,Number 9 Sep(smbet 1968
