Infra-red spectra of phthalocyanines with different central metal atoms

Colloy~~ium

Syecttoscopicum

I&a-red

Iirternlrtlonnle

VI (Amsterdam,

1060). Petgnmon

Press Ltd., T.ondon

spectra of phthalocyanines with different central metal atoms A. N. Department

TEREPI’IN of Physics,

and A. N. Leningrad

SIDOROV University (USSR)

Abstract--In a series of investigations of absorption spectra, of the fluorescence and photoelectric phenomena of the phthalocyanines in the visible range, a marked sensitivity haa been observed towards the presence of small concentrations of some 0- and N-containing molecules, which are capable of forming a co-ordinative bond with the central metal atom in these pigments. In order to explore the details of such a molecular interaction, we investigated the vibration spect.ra of phthalocyanines in the infra-red (4000-700 cm-‘) and especially the influence on these different compounds in the gaseous state. Thin (0.01 mm thick) layers of the pigments have been used, obtained by sublimation in VOCUO. The layers were highly porous. Molecules of different behaviour (electron donors and bases) were brought in contact as vapoura with the phthalocyanine layers in vacua, viz. H,O, D,O, H,S, hydrazine, phenyl hydrazine, ammonia, benzylamine, pyridine, aniline, diphenylamine, indole, oxygen and others. The addition of these molecules to the phthalocyanines was spectrally revealed by a shift of the infra-red bands of the added molecules, in specified cases, as compared with their position in Moreover in these cases the addend the free gaseous or dissolved molecule in a neutral solvent. could not be removed by simple evacuation and freezing out without heating, LW could be judged from the spectrum. On addition the whole infra-red spectrum of the phthalocyanine structure was also changed in a characteristic manner, depending on the nature of the added molecule, and as well on the central metal atom. Phthalocyanine without metal does not exhibit any change on contact with the vapoura of all the compounds indicated above. This means that the interaction is exerted through the intermediary of the central metal atom. From these spectral data the most active centres for addition in the phthalocyanines are Mg, Zn, Fe. The central Cu atom was far less able to add the same molecules, as could be judged from the spectra. Iutroductory

THE structure of the phthalocyanines [l] includes as a main pattern 4 pyrrole rings, equally typical for the porphyrines and chlorophylls with the difference that these rings are interconnected by nitrogen bridges, instead of methine ones. We have also here: in contrast to those pigments, benzene rings at the top of the pyrrole ones. In spite of these marked differences the narrow absorption maximum of Mg-pht.halocyanine in the visible is situated very close to that of chlorophyll. It has been found by a research group associated with us [2] that the absorption coefficient in this maximum and the intensity of fluorescence are strikingly increased by the presence in the hydrocarbon solvent of traces of H,O, pyridine and of other 0- and N-containing molecules, which are capable to .form a co-or&native bondlwith the central metal atom in these pigments. We used instead of a sclution thin (about 10 ,u thick) layers of these pigments, sublimated at 400-450°C in vacua on to a KC1 window. A spectrogram of this layer being recorded in vacua, vapours of various compounds were introduced in the cell under pressures depending on their volatility at room temperature (Table I). The lager stayed in contact with the vapour some 15-20 hours before another 673

A. N. TERENIN and A. N. SIDOROV

spectrogram was taken. We had at our disposal two recording infra-red spectrophotometers with NaCl and LiF prisms: a one-beam apparatus and a two-beams one. The former was preferred on account of its greater resolving power. The samples of the phthalocyanines with Mg, Zn, Gun, Fen Con and the metal free pigment have been synthesized according to LINSTEAD [A] and purified by means of a fractionated sublimation in vacua (lo-” mm Hg). The purity of the samples could be checked with the help of their very characteristic visible absorption spectra. Moreover the infra-red spectra of the layers of Cu- and metal-freephthalocyanines recorded by us were found to be practically identical with those previously recorded by other authors [3, 41. On the schematic diagram (Fig. 1) the positions of the i&a-red maxima in the range 700 to 1800 cm-r for the various phthalocyanines are given. The lower part of the figure is for the layers in vacua. As can be seen all the metal-containing pigments possessfrequencies common to each other and to those of the metal-free one. However, shifts of frequencies are exhibited peculiar to each metal atom, and especially conspicuous around 900 and.1500 cm-l. The shift is systematically to the higher frequencies in the sequence of the phthalocyanines: vnIg < vzn < vcU < vre < vco . The spectrum of the metal-free pigment differs in some respects from that of the metal containing ones. This shows that the introduction of a

Pig.

1. Diagram

of infra-red absorption spectra of sublimated layers in vacua and in the presence of vapours.

phthalocyanine

central metal atom affects the whole array of the bonds in the organic part of the molecule. The spectrum of the metal-free phthalocyanine possesses a narrow absorption maximum at 3298 cm-l belonging to the NH frequency of two such bonds at the N atoms of the pyrrole rings. The position of this maximum is a normal one, as is the case for an imino or amino-group and does not suggest the presence of two protons shared in common by all the four nitrogen atoms of the central ring. The upper part of Fig. 1 shows that the sorption by the phthalocyanine layers 574

In&-red

spectra

of phthelocyanines

with

different

central

metal

atoms

of some gaseous molecules produces conspicuous changes in the spectra. The changes occurring are specific for the kind of the added gaseous molecule as can be clearly seen for the group of frequencies near 1100 cm-l. We ascribe these changes primarily to the interaction with the phthalo6yanine molecule and to the formation of an intermolecular compound. However, it is possible that some spectral changes would also occur if the phthalocyanine layers modify their crystalline structure on addition. Some spectral changes due to different crystalline modifications of Cu-phthalocyanine have been noticed [5]. From the fact that near the shifted peak no trace of the initial one remained, it may be inferred that a sorption of the vapour throughout the porous layer has taken place. The wide peaks on Fig. 1 are due to the added vapour molecules. Their characteristic frequencies are also somewhat different from those usually found (cf. Table 2). Table

1. Influence

of different

vapours

on the

infra-red

spectra

of phthalocyanines

Phthalocyanines “itH,O

and

1

D,O

Mg-

1

Zn-

-I-

I , I

+

0

I

+

I

i-

i

+

H,S

0 Ol

Hydrazine

0

Phenylhydrazine

0

-1.

0

0

T\‘H,

I

i

Cu-

1

0

Fe-

1

0

CO0

1

itztg 10-15 200

0

+

0

5-10
0

0

0

+

0

+

+

+

0

0

0

0

0

200

-Benzylamine

0

Aniline

0

Diphenylamine

0

Pyridine

0

+ ’

I

+

+

0

0

+

+

0

O

0

0

0

/

?

0.3

+

?

0.2

0

0


+

0

lo-15

+

0


0

0

-Indole 0,

I

i

’

250-500

In Table 1 we have summarized the results so far obtained. The + sign indicates that a change in the phthalocyanine spectrum has occurred, the sign 0 that no change has been noticed. It must be stressed upon that we did not notice ‘in the rpresence of vapours any change in the spectrum of the metal-free phthalocyanine. This clearly shows that the addition of the vapour molecules is taking place at the central metal atoms which retain a co-ordinating power in spite of being saturated by complexing with the N atoms of the pyrrole rings. This circumstance while entirely admissible for such atoms as Zn, Fe or Co is somewhat puzzling for the Mg atom, a quite poor complexing agent. 575

A. N. TERENIN

and A. N. SIDOROV

It seems rather strange that although water ie conspicuously bound by the Irgand Znpigments;the Cu-, Fe- and Co-ones do not add water aa can be judged not only from the absence of any change in the spectra of pigments, but also from the absence of a spectrum of sorbed H,O Another example of anomalous behaviour moleculee, which latter is a very sensitive criterion. is that ammonia, very apt to strong complexing, did not show any effect on the phthaIocyanine layers, whereas the more bulky molecules of hydrazine, phenylhydrazine, pyridine and benzylamine did it very conspicuously. A part of the accessibility and the co-ordinating power of the central metal atom, a sufficient sorption, or solvation of the vaponr is evidently needed in order The phenylated compounds perhaps owed to detect it with the help of the i&a-red spectra. their better sorption in the pigment layers on account of the stronger v. d. Waals’ attraction to of the $hthaloc&ine molecules. the flat ring stkcture

3300

2900

2x)0

Fig.

2. Infre-red spectra of a sublimated layer of Mg-phthalooyanine: l-in H,O vapour; 241 D,O vapour; 34iquid D,O (5 p thick).

Fig. 3. NH-band of indole: l-dissolved in Ccl,; 2-solid layer; I-sorbed in Mg-pht.; 4-in Zn-pht.; &in Fe-pht.

For most of the cases studied these changes in the infra-red duced by vapour sorption are reversible in that sense, that the initial be restored

by

evacuation

of the

vapour

during

2-3

hours,

keeping

the

spectra spectrum layer

procan

at 20°C

H,O vapour and at 200”-250” for the other compounds*. The fact that H,O molecules are being strongly bound by Mg-phthalocyanine has been found by LMSTEAD [l]. The specific state of H,O and D,O molecules bound to Mg-phthalocyanine is shown by their spectra in Fig. 2. It can be seen that the OH absorption peak in this state is quite narrow and assumes another position than for liquid H,O (and also D,O). In Table 2 shifts 0~ the NH frequency for various nitrogenous molecules are given, when they are sorbed by the phthalocyanine layers as compared with their solution in CCI, and in the liquid state. The corresponding spectra are given for the case of indole and various phthalocyanine layers in Fig. 3. for

* Only Zn-phthalocyanine exhibited e different behaviour. At 2O’C it was impossible to free its Iayers from the sorbed molecules of aniline, benzylamine and phenylhydrazine. A temperature increase brings about a chemical reaction between the partners, &s can be inferred from further irreversible changes qbserved in the spectrum. A similar chemical reaction is also taking place for aniline and benzylamme with Co-phthalocyanine already at 2O’C.

576

I&e-red Table

spectra

2. Frequency

/

Benzylamine

Aniline

of phthalocyanines

with

different

central

metal

decre=es of the NH infra-red maxima molecules added to the phthalocyanines

atoms

of different

Av cm-?

Solution in ccl,, 0.02 mole/l

liquid

state

/

in Mg-pht

1 in Zn-pht

90 63 67

/

in Fe-pht

48

I

66

37

’

34

3433

58

3344

50

3490

55

35

3396

35

39

Indole

* The maximum et 3490 disappears. t In a solid layer. The figures in brackets are for two

maxima

observed

instead

of the initial

one.

SInfra-red absorption spectra of sublimated layers in vacua of Mg-, Zn-, Cu-, Fe-, Co-phthalocyanines and of the metal-free pigments have been recorded and specific changes in t.heir spectra aa a result of sorption of various gaseous molecules found; as well shifts have been observed of the frequencies of the latter. It has been shown that the bonding of these molecules, if any, is taking place only in the presence of a central metal atom. Our thanks are due to Dr. V. ZELWSKY for the synthesis of some of the phthalocyanines and to Dr. G. BACDHIKIANZ for electron diffraction experiments with the layers.

References [l]

LINSTEAD

R.

P.

J. Chem.Soc.

1934

1016;

LINSTEAD

R.

P.

and Lowe

A.

R.

ibid.

1934

1022. [2]

Dokl. Akad. h’auk. SSSR EVSTIGNEEV V. B., GAVRILOVA V. A., and KRASSNOVSKY A. A. 1949 66 1133; 1950 70 261; 1950 74 315; TERENIN A. N. Photochemi&y of Chlorophyll and

Photosynthesis Moscow, 1951. Analyt. Chem. 1953 25 390. J. E. and EHRHARDT S. A. Spectrochim. Acta CANNON C. G. and STJTHERLAND G. B. B. M. KENDALL D. N. Adyt. Chem. 1953 25 382.

[3] TYLER [4] [5]

1951

4 373.

Discussion Prof. MEC~E: Haben Sie Doppelstrahlgerllte benutzt urn geringe Frequenzverschiebungen der Spektren festzustellen? Antwort: Die Frequenzverschiebungen waren geniigend gross und deshalb war in diesem Friiher haben +r diese letzt,eMethode Falle eine Doppelstrahldifferentialmethode iiberfliissig. im nahen UR angewandt um schwache zwischenmolekulare Verbindungen aufzudecken. M~~'SALOMON: What is the explanation that NH, does not react with the dye, contrary to expectations? Answer : This fact is very puzzling since NH, is known to enter into strong co-o&native bonding with various metal atoms in complexes even more strongly than H,O. AE was shown above much bigger, viz. amino-aromatic compounds enter into combination with the phthalocyanines. The only explanation may be that they are more easily sorbed in the pigment layer, owing to the stronger v. d. W&s forces, arising from their aromatic rings. 577

A. N.

TERENIN and A. N.

SIDOROV

&I&. VEFCRIJN STUART: Is there any evidence that the formation of the intermolecular complexes discussed is a reversible reaction? Answer: in most cases the i.r. spectrum of the pigment layer returns to ita initial structure We did not as yet analyse the composition of this latter, after the sorbed vapour is removed. and thus cannot be sure whether a catalytic reaction on the pigment has occurred. Prof. KETELAAR: What is the proportion of e.g. indole in the Mg-phthalocyanine layer? Answer: No trace of the initial i.r. peaks of the pigment layer are found after the sorption. So we presume that the whole layer is affected with the molecular compound formation which may be 1 : 1 or 1 : 2 in molecular ratio. Mr. CHAPMAN: Has the sequence of metals, (Mg, Zn, Cu, Fe, Co), which you state is the order of shifts in frequency in the 1800-700 cm-’ region of these metal phtalocyanines, been related to some function of the metal? 2. Has X-ray analysis of each of the metal phthalocyaninea after contact with the various vapouk been obtained? Is the crystal structure the same in each case? If the crystal structure of these “complexes” have been obtained and they are the same, has a clathrate structure been codidered? Answer: 1. We have not so yet made any detailed analysis of the structure of t,he spectrum, which is a dif&ult task; moreover we intend to enlarge the list of the metals and of their valency state. 2. We have made electron diffraction experiments and have found that in some of the cases studied the crystalline state of the pigment layer does change after vapour sorption. We have found however that differently ‘Lprocessed” layers which exhibited some differences in the crystalline structure on vapour sorption, had about the same i.r. spectrum, i.e. same frequency shifts. This was the main argument for stating that the change of the crystalline structure has a minor intluence in the i.r. spectrum of the layers with sorbed vapours. The structure may be a clathrate one, but the main point is that the sorbed molecule is definitely bound to the metal atoms of the pigment molecules.

578