TRANSFORMATIONS OF

AND

ELECTRONIC

Fe-PHTHALOCYANINE

STRUCTURE

MONOANION

D

A.

N.

Sidorov

and

V.

G.

Maslov

UDC 539.2+541.65

Among the metal derivatives of the porphin s e r i e s the Fe derivatives occupy a special place since the central Fe atom plays an i m p o r t a n t role in the t r a n s f o r m a t i o n s of the molecules; like the t e t r a p y r r o l e ligand with which it is linked, it is capable of being either donor or acceptor of the electron in the r e a c tions of the m o l e c u l a r s y s t e m s . With the object of explaining the effect of the Fe atom on the p r o p e r t i e s and electronic s t r u c t u r e , w e have studied F e - P C * monoanions of one of the simplest iron containing t e t r a p y r r o l e pigments. The monoanions of Ni-PC and C o - P C were also studied under the same conditions for comparison. The negative ions of the pigments, like the anions of M g - P C [1, 2], were p r e p a r e d by the interaction of a solution of the original pigment with sodium in a 1 : 1 mixture of THF and diethyl ether. Under these conditions the neutral M - P C molecule is capable of adding up to four e l e c t r o n s [2, 3]. By discontinuing the contact of the solution with sodium in good time, the reaction is e a s i l y stopped at the stage of f o r m a tion of the monoanions or dianions.t The identification and investigation of the M - P C negative ions were c a r r i e d out on the absorption s p e c t r a in the 350-13,00 nm region by a method developed by us for the ions of other t e t r a p y r r o l e pigments; the sequence of spectral changes was r e c o r d e d in the c o u r s e of s u c c e s sive acts of s i n g l e - e l e c t r o n reduction of the pigment and also in the course of r e v e r s i b l e dark or photochemical discharge r e a c t i o n s of the negative ions which in the final count led to the r e g e n e r a t i o n of neutral m o l e c u l e s of the original pigment [1, 2, and 4]. The s p e c t r u m of N i - P C monoanlons has a band of average intensity with weak satellites in the IR r e gion near 910 nm, two intense bands in the 550-650 nm region, and a s h o r t - w a v e band near 368 nm (Fig. 1, curve 1). The s p e c t r a of the monoanions of the m e t a l - f r e e PC [5], M g - P C [2], and C u - P C [6, 7] have the same s t r u c t u r e with small differences. The r e s u l t s of the calculations and the m e a s u r e m e n t of the ESR s p e c t r a [3, 7-9], and also the s i m i l a r i t y of the absorption s p e c t r a of the monoanions of PC and M-PC (M = Ni, Cu, and Mg) indicate that on f o r m i n g the monoanion the electron attached to the pigment molecule is localized on the t e t r a p y r r o l e ligand and only undergoes c o m p a r a t i v e l y little perturbation from the side of the central atom M. In the monoanion of Co-PC, as assumed in [3, 7, 8, and i0], the negative charge is localized on the Co atom. In fact, during the formation of the Co-PC anion the absorption band in the 600-700 nm region, which is a characteristic group for all neutral M-PC, is retained but shifted to the long-wave side and reduced in intensity by approximately a half (see Fig. i, curve 3). This means that the electronic structure and energy state of the main ehromophore of the molecule, the tetrapyrrole ligand, is little altered. The double band in the 400-500 nm region which appears on formation of the~Co-PC anion may be interpreted as the band for the intermoleeular transfer of an electron from the negative charged metal atom to the ligand [ 1 0 ] . * Abbreviations adopted in the paper: PC and M - P C for phthaloeyanine and metal phthalocyanine, and THF for t e t r a h y d r ofuran. N i - P C is p r a c t i c a l l y insoluble in a mixture of THF and diethyl ether. However, the N i - P C anions which a r e f o r m e d on contacting a finely divided suspension of the pigment with sodium dissolve readily. Leningrad. T r a n s l a t e d f r o m T e o r e t i c h e s k a y a i E k s p e r i m e n t a l ' n a y a Khimiya, Vol. 8, No. 6, pp. 828831, N o v e m b e r - D e c e m b e r , 1972. Original a r t i c l e submitted July 19, 1971.

9 1974 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. A copy of this article is available from the publisher for $15.00.

685

Co-PC ~ ,

N

i l

]

\

rl

litIll I:1

t ,~ A.J \ / ~3

\\

\,,~

\ Fc-PC

_

I\

48o

.

t.g .

~ ~ to

/l'~lJ-a

600

I

I

I

800 nm

Fig. 1. Absorption s p e c t r a of solutions of m e t a l - p h t h a l o c y a n i n e s and their monoanions in a mixture of t e t r a h y d r o furan and ether: 1) monoanions of Niphthalocyanine a t - 1 9 6 ~ 2) and 3) Cophthalocyanine and its monoanions at - 1 9 6 ~ 4) and 5) monoanions of F e phthalocyanine in the p r e s e n c e of p y r i dine a t - 125 ~ a n d - 1 9 6 ~ 6) F e - p h t h a l o cyanine a t - 196 ~ The concentration of pigments and their monoanions was

Of the a t o m s Ni, Co, and Fe the latter has the least electronegativity, and a c c o r d i n g to the calculations of Taube on the reduction of F e - P C , not only the f i r s t but also the second electron must be localized on the Fe atom [3]. The absorption s p e c t r a obtained by us for the monoanions of F e - P C (see Fig. 1, curve 4) do not conform, however, with such an assumption; it is difficult to find a spectral analog both for the monoanion of C o - P C and also for those M-PC having electron localization on the ligand. We assume, therefore, that on f o r m i n g the F e - P C monoanion in a m i x lure of THF and ether, a distribution of the negative c h a r g e o c c u r s between the Fe atom and the ligand. This m a y mean, for example, that the adding electron occupies the m o l e c u l a r v orbital containing a significant contribution from the dxz and dyz atomic orbitals of the F e [9]. Of all the M - P C , F e - P C evidently p o s s e s s e s the g r e a t e s t tendency to f o r m m o l e c u l a r complexes with o r ganic bases. It is a s s u m e d that dimethyl sulfoxide is a unique solvent whose m o l e c u l e s do not form complexes with F e - P C and in which one m a y obtain an absorption spectrum not a s s o c i a t e d with the addend of the pigment. When interacting with other organic solvents F e - P C f o r m s complexes which lead to shifts of the absorption spectrum of the pigment and to the appearance of a band near 435 nm [11]. T h e r e are 2 absorption bands in this region in THF and in a mixture of THF and ether: at 412 and 468 nm (see Fig. 1, curve 6).

We have found in the f i r s t stage of the interaction of F e - P C with sodium at 20 ~ when the pigment had still not r e a c t e d completely, that together with the band of the o r i g inal pigment at 652 nm there is a shifted band near 670 nm and a band appears at 445 nm (in place of the bands at 412 and 468 nm which are lost). On freezing the solution in li~ 10 -5 M. q u i d n i t r o g e n t h e band at 670 nm disappears, the band at 652 nm i n c r e a s e s , and the band at 445 nm shifts to 410 nm. We i n t e r p r e t these spectral changes hypothetically in the following manner. The F e - P C molecules which have not r e a c t e d with sodium form complexes with the monoanions of the pigment which e m e r g e in the c o u r s e of the reaction, and this explains the appearance of the bands at 670 and 445 nm. In c o n t r a s t to the n o r m a l complexes of F e - P C with organic bases, one of the components in this complex c a r r i e s the negative charge, and this evidently explains its p r o p e r t y of decomposing on freezing: on cooling the solvation of the pigment monoanion is i n c r e a s e d and the complex is decomposed just as the a s s o c i a t e s of negative m o l e c u l a r ions with cations of alkaline m e t a l s dissociate on reducing the t e m p e r a t u r e [12]. We have also found that not only the neutral F e - P C molecules but also its anions p o s s e s s the ability to f o r m m o l e c u l a r complexes with nitrogen containing organic bases. The formation of complexes of F e PC monoanions with bases (pyridine, N-methylpiperidine, and diethylamine) at monoanion concentrations near 10 -5 M and base concentrations near 10 -z M o c c u r s at a t e m p e r a t u r e close to the freezing point of the solution and shows as a change of the original r e d color of the solution to blue; during this, g r e a t e r changes a r e found inthe s p e c t r u m , which indicate profound t r a n s f o r m a t i o n s in the electronic s t r u c t u r e Of the m o n o anion (see Fig. 1, c u r v e s 4 and 5). With the absence of bases from the solution the s p e c t r u m of the anion is unchanged on freezing. A c o m p a r i s o n shows an apparent spectral s i m i l a r i t y between F e - P C monoanion in the p r e s e n c e of base a t - 1 9 6 ~ and Ni-PC monoanion (see Fig. 1, c u r v e s 1 and 5). Such a similarity, if the strength of the specific absorption bands c o m p r i s i n g the s p e c t r a is considered, is sufficient grounds to conclude that on f r e e z i n g in the p r e s e n c e of base, that is during the formation of the m o n o a n i o n - b a s e complex, a r e d i s t r i -

686

bution of the negative c h a r g e in the s t r u c t u r e of the F e - P C monoanion o c c u r s , namely, its complete c e n t e r ing on the t e t r a p y r r o l e ligand. Schematically this can be r e p r e s e n t e d thus: (Fe-

PC) -

B.--196oc >- F r

)--,

where B is base. The F e - ( P C ) - monoanion with a localized c h a r g e on the ligand is unstable. On melting the sample, the complex of monoanion and molecule B is d e c o m p o s e d and a r e v e r s i b l e t r a n s f o r m a t i o n o c c u r s : the charge is again r e d i s t r i b u t e d between the Fe atom and the ligand; the t r a n s f o r m a t i o n cycle can be r e p e a t e d s e v e r a l t i m e s by a l t e r n a t e l y f r e e z i n g and thawing the solution. In one of the e x p e r i m e n t s when the p y r i dine concentration was near 0.1 M and with the p r e s e n c e of negative ions in the solution the pyridine was successful in stabilizing the F e - ( P C ) - f o r m in the liquid solution at - 1 0 0 ~ at a level of about 50% of the t o tal quantity of pigment monoanions. The F e - P C dianions a r e unchanged in the p r e s e n c e of bases. Evidently the interaction of the dianion with the base molecule o b s t r u c t s the c h a r g e on the Fe atom which apparently in the dianion has a g r e a t e r value in c o m p a r i s o n with the charge on the Fe atom in the rnonoanion s t r u c t u r e . We did not observe changes in the s p e c t r a of the C o - P C rnonoanions either at 20 ~ o r at - 1 9 6 ~ in the p r e s e n c e of pyridine or N-rnethylpiperidine. I r r a d i a t i o n of the frozen solutions of the m o l e c u l a r anions, among them the t e t r a p y r r o l e pigment anions, as a r u l e , l e d to their discharge [1]. We c o m p a r e d qualitatively the photochemical p r o p e r t i e s of the rnonoanions of the different M - P C studied by us at - 1 9 6 ~ The C o - P C monoanion p r o v e d to be the m o s t s t a ble in r e s p e c t to e m i s s i o n in the 240-400 nm region; in general we did not observe its photodischarge. The N i - P C anion, which c a r r i e d the c h a r g e on the ligand, split off the electron c o m p a r a t i v e l y e a s i l y under analogous conditions. As was to be expected, the various f o r m s of the F e - P C monoanions had different photosensitivities. The F e - ( P C ) - f o r m on i r r a d i a t i o n in the p r e s e n c e of bases split off the electron app r o x i m a t e l y the s a m e as the N i - P C rnonoanion. The ( F e - P C ) - f o r m split off the electron, b y f a r , m o r e difficultly: duringidentieal exposures the fraction of ( F e - P C ) - ions which d i s c h a r g e d was approximately an o r d e r less than the fraction of F e - ( P C ) - ions which discharged. The c o m p a r i s o n c a r r i e d out indicated that the central metal atom r e t a i n s the electron localized on it m o r e f i r m l y than the t e t r a p y r r o l e ligand. I r r a d i a t i o n of the frozen solutions of F e - P C dianions both in the p r e s e n c e of pyridine and in its absence led to d i s c h a r g e of the dianions, and as a r e s u l t monoanions in the form ( F e - P C ) - were obtained. The yield of the r e a c t i o n is small: in the best c a s e about 30% of the original quantity of dianions was d i s charged. LITERATURE

i. 2. 3. 4.

5 84 6. 7. 8. 9.

i0. ii. 12.

CITED

A.N. Sidorov and V. G. Maslov, in: Transactions of the ist Symposium on BiophotochemicalProblems [in Russianh Moscow (1970). A.N. Sidorov and V. E. Kholrnogorov,Teoret. i Eksperim. Khim., 7, 332 (1971). R. Taube, Z. Chem., 6, 8 (1966). V.G. Maslov, A. N. Sidorov, and V. E. Kholmogorov, in: MolecularPhotonies [in Russian], Nauka, Leningrad (1970), p. 208. V. G. Maslov and A. N. Sidorov, Teoret. i ]~ksperim. Khim., 7, 832 (1971). J. W. Dodd and N. S. Hush, J. Chem. Soc., 4607 (1964). L. D. Rollman andR. T. lwamoto, J. Arner. Chem, Soe., 90, 1455 (1968). D. C. P. B. M.

W. Clack, N. S. Hush, and J. R. Jandle, Chem. Phys. Lett., 1, 157 (1967). M. Guzy, J. B. Ray-nor, L. P. Stodulski, and M. C. K. Symons, J. Chem. Soc., 6A, 997 (1969). Day, It. A. O. Hill, and M. G. P r i c e , J. Chem. Soc., 1A, 90 (1968). W. Dale, Trans. F a r a d . Soc., 65, 331 (1969). Shvarts, Usp. Khirn., 39, 1260 (1970).

687