THE

PHOTOCHEMICAL

PHTHALOCYANINE V. G. M a s l o v

GENERATION

AND PORPHYRIN and

A.

N. S i d o r o v

OF

NEGATIVE

IONS UDC 541.14

The reduction of t e t r a p y r r o l e pigments usually takes place in a number of stages. A pigment anionradical and a reducing agent c a t i o n - r a d i c a l are f o r m e d in the f i r s t stage. Stabilization of the pigment anion may be achieved by p r o p e r selection of solvent, reducing agent, and t e m p e r a t u r e . Negative t e t r a p y r r o l e pigment ions have been obtained in high chemical yield by chemical and e l e c t r o c h e m i c a l means [1-7]. The possibility of the existence of pheophytin a n i o n - r a d i c a l s in the photochemical reduction of this compound in solution has also been noted [8], although adequate proof of the purely ionic nature of the products of this photoreduetion a r e lacking. We have developed a photochemical method for generating negative phthalocyanine and porphyrin ions in liquid solution in quantities sufficient for study by standard spectral instruments~ * The methods for the spectral identification of ionic and hydrated f o r m s of pigments [4, 7] allow unequivocal interpretation of the pigment photoconversion p r o d u c t s . Stabilization of the photogenerated a n i o n - r a d i c a l s in our experiments was best achieved in dimethyf o r m a m i d e (DMF) solutions using hydrazine as the reducing agent. Under these conditions, we made a spectral study of the products of the photoreduction of phthalocyanine {PC), a number of phthalocyanine metal complexes (Mg-PC, Cd-PC, P b - P C , Fe(II)-PC, and Co-PC), tetraphenylporphyrin (TPP), and its zinc complex ( Z n - T P P ) . The zinc concentration was f r o m 10-5 to 10 -4 M and the hydrazine concentration was 10 -2 M. Filtered light f r o m a DKsSh-1000 lamp was used for irradiation. The solutions w e r e f i r s t degassed and irradiation and spectral m e a s u r e m e n t s w e r e c a r r i e d out at constant t e m p e r a t u r e . Irradiation of M g - P C and PC solutions by light with wavelength k > 500 nm at -60~ leads to the appearance of the bands for the monoanions of these pigments in the absorption s p e c t r a (Fig. 1). It is possible to achieve a l m o s t complete conversion of the pigments to monoanions upon irradiation for a few minutes. Warming of the solution to +20~ leads to the partial (from 20 to 30%) r e g e n e r a t i o n of the original pigment. The formation of monoanions in PC and M g - P C solutions is also observed at r o o m t e m p e r a t u r e though at significantly reduced r a t e s and with reduced yields. In this case, the formation of side products is possible. If a solution of photogenerated PC monoanions is subjected to irradiation by light with t > 330 nm at - 6 0 ~ we find that the absorption bands for the monoanions disappear and a new product appears which gives a broad band with maximum at 528 nm (Fig. 1, c u r v e 3). This product is converted upon heating into the pigment monoanion. An analogous product was also o b s e r v e d in the case of Mg-PC though in insignificant amounts.

*The known e a s e s for the photochemical generation of negative and positive pigment ions in frozen solutions are not considered, as, f i r s t l y , these ions are unstable and disappear upon melting of the samples and, secondly, the p h o t o c h e m i s t r y of froy.en solutions has its own features and r e q u i r e s a special approach. Leningrad. Translated f r o m T e o r e t i c h e s k a y a i I~ksperimental'naya Khimiya, Vol. 7, No. 6, pp. 832835, N o v e m b e r - D e c e m b e r , 1971. Original a r t i c l e submitted July 15, 1970.

9 1974 Consultants Bureau, a division o f Plenum Publishing Corporation, !27 g'est 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.

680

AI, .,I

,,.a ..7-,

.

0

600

Fig. 1

800 X, n m

.,~,

,~00

t U

500

IiL

700

I

1000 A, nm

Fig~ 2

Fig. i. Absorption s p e c t r a of d i m e t h y l f o r m a m i d e solutions of pigments in the p r e s e n c e of hydrazine at - 6 0 ~ 1) Mg-PC after irradiation with X > 500 nm for 3 min (mostly M g - P C monoanion); 2} PC after i r r a d i a tion by light with k > 500 nm for 4 min (mostly PC monoanion); and 3) the same after additional irradiation with light with 1 > 330 nm for 30 sec (mostly PC dianion). Fig. 2. Absorption s p e c t r a of t e t r a h y d r o f u r a n solutions of Mg-PC solutions and the products of the r e a c t i o n of Mg-PC with sodium at 20~ [4]: 1) neutral pigment; 2) monoanion; and 3) dianions. The s i m i l a r i t y of the absorption s p e c t r u m shown in Fig. 1 (curve 1) with the Mg-PC monoanion absorption s p e c t r u m (Fig. 2), obtained by reaction with sodium in t e t r a h y d r o f u r a n solution [1, 4], allows us to a s s u m e that this product is the M g - P C monoaniono (Slight differences in the position of the absorption m a x i m a likely r e s u l t f r o m a solvent effect.) The photoproduct, the s p e c t r u m of which is shown in Fig. 1 (curve 2) is i n t e r p r e t e d by us as the monoanion of nonmetallated PC on the basis of the obvious spectral analogy with the M g - P C monoanion. The product with the 528 nm band is apparently the PC dianion. This hypothesis is supported, firstly, by the spectral analogy with the Mg-PC dianion [4] which has (see Fig. 2) a broad band at f r o m 520 to 535 nm and weak, not always r e s o l v e d bands on the long wavelength tail of the m a j o r band and, secondly, by the fact that the behavior of this product upon w a r m i n g the solution f r o m - 6 0 to +20~ is completely analogous to the behavior of M g - P C dianions obtained in the r e a c t i o n of sodium in t e t r a h y d r o f u r a n solution in the p r e s e n c e of a weak proton donor (methanol) [4]~ In our experiments the proton donor may be the hyd r a z i n e c a t i o n - r a d i c a l (N2H~4} f o r m e d as a r e s u l t of a photochemical electron t r a n s f e r r e a c t i o n or some product of the d a r k c o n v e r s i o n of I~2I~" 4 . It is still impossible to make any unequivocal conclusions c o n c e r n ing the m e c h a n i s m of the photoeonversion of PC monoanions to dianions. We should keep in mind that the electron donor in this r e a c t i o n may be either h y d r a z i n e which is found in excess or a product of the conversion of hydrazine. .

It should be noted that the PC photoreduction r e a c t i o n was f i r s t achieved in d i m e t h y l f o r m a m i d e solution and that this reaction was u n s u c c e s s f u l l y attempted in pyridine solution [9]. It is not impossible that PC is also r e d u c e d in pyridine but that the pigment ions a r e immediately protonated and that the hydro derivative thus f o r m e d d e c o m p o s e s with the r e g e n e r a t i o n of the original pigment. Of the other phthalocyanine metal complexes studied, Fe(II)-PC was found to be nonphotoactive under the conditions d e s c r i b e d above. F u r t h e r m o r e , the formation of complexes of this pigment with hydrazine f o r m e d in pyridine solutions [9] was also not observed. In the case of Co(TI)-PC, the addition of hydrazine to a solution of the pigment leads to the dark c o n v e r s i o n of this pigment to Co(D-PC analogously to our previous o b s e r v a t i o n in pyridine solution [9]. Solutions of Co(D-PC as Fe(TD-PC solutions were found not to be photoactive in the p r e s e n c e of h y d r a z i n e .

681

The f i r s t s p e e t r a l l y o b s e r v e d photoreduction product of Cd-PC at -60~ is identical with the PC monoaniono Apparently, in this case, the initial photochemical act of the molecular interaction of pigment and hydrazine takes place s i m i l a r l y to that in the case of Mg-PC though the Cd-PC monoanion thus f o r m e d immediately loses metal and is converted to the monoanion of nometallated PC. The behavior of P b - p C is r a t h e r similar to the behavior of C d - P C except that in this case for short exposures, along with (PC)bands, bands a r e also o b s e r v e d at 905 and 615 nm which do not belong to (PC)-. These bands, which p o s s i bly c o r r e s p o n d to the P b - P C monoanion, disappear upon warming the solution to + 2 0 ~ is a partial r e g e n e r a t i o n of the original P b - P C . The loss of the central metal atom in the c o u r s e of the photoreduction reactions of Cd-PC and P b - P C is not unusual as, according to Berezin [10], these metal complexes belong to the group of less stable complexes and, in this case, this loss is analogous to the photopheophytinization r e a c t i o n of chlorophyll [11]. It is interesting to c o m p a r e our r e s u l t s with the r e s u l t s of a study of the photoreduction of metallophthalocyanines by hydrazine in pyridine solution [9]. We have already shown that the photosensitivity of PC in DMF and in pyridine differs. It should also be noted that in the photoreduction of C d - P C in pyridine solution [9], a s w e l l as in DMF, the formation of PC monoanion is observed in the f i r s t stage. Not having the spectral c h a r a c t e r i s t i c s of the ionic f o r m s of PC and its metal derivatives, we could not give any explanations for these r e s u l t s . It is interesting that in DMF we could not obtain the dihydro derivatives of the metallophthalocynanines with bands in the region f r o m 550 to 580 nm which are o b s e r v e d in pyridine. The r e a s o n for the differences in the c o u r s e of the photoreduction of the pigments in pyridine and DMF apparently should be sought in the different solvating p r o p e r t i e s and p r o t o n - a c c e p t o r capacity of these solvents. This method for the photogeneration of ions was used e a r l i e r far a number of porphyrins, namely, T P P and Z n - T P P [12]. The photoconversion of these p o r p h y r i n s has a number of special features relative to the conversions of PC and its metal derivatives. The optimum conditions for obtaining Z n - T P P and T P P monoanions a r e the same as in the case of PC. However, we w e r e not able to achieve complete conversion of the original pigment to the monoanion (the yield of monoanion in the best case reached 50%) even upon prolonged (from 10 to 15 rain) exposures. It is possible that this result is related to a photoinduced back reaction. In contradistinction to PC and Mg-PC monoanions, photogenerated TPP and Z n - T P P monoanions in DMF disappear upon warming the solution to +20~ and a r e partially converted to the stable hydro d e r i v ative of the pigment and partially to to the original pigment~ This conversion may be r e p r e s e n t e d as a protonation reaction of the monoanions: H - ~- H+-~ HH (H is the neutral p o r p h y r i n molecule) with rapid subsequent disproportionation of the neutral radicals formed: 2//H ~+ HH 2 + / / We have not o b s e r v e d similar phenomena in the c a s e of PC and its metal complexes in DMF. This failure is apparently explained by ' the g r e a t e r stability of the monoanions of these pigments relative to porphyrin monoanions in r e g a r d to protonation. LITERATURE 1. 2. 3. 4. 5. 6. 7. 8. 9.

682

CITED

A . V . Shablya and A. N. Terenin, Optikai Spektroskopiya, 9, 533 (1960). G . L . Closs and L. E. Closs, J. A m e r . Chem. Soc., 8__55,819 (1963). A . N . Sidorov, in: Molecular Photobehavior [in Russian], Izd. Nauka, Leningrad (1970). A . N . Sidorov and V. E. Kholmogorov, Teor. i t~ksperim. Khim., 7, 332 (1971). Ro H. Felton and J. Linschitz, J. A m e r . Chem. Soc., 8._~8, 1113 (1966). L . D . Rollman and R. T. Iwamoto, J. A m e r . Chem. Soc., 90, 1455 (1968). V . G . Maslov, A . N . Sidorov, and V. E. Kholmogorov, in: Molecular Photobehavior [in Russian], Izd. Nauka, Leningrad (1970). V . B . Evstigneev and V. A. Gavrilova, Dokl. Akad. Nauk SSSR, 9._.66,1201 (1954). D . A . Savel'ev, I. P. Kotlyar, and A. N. Sidorov, P a p e r Deposited at No. 542-69; Zh. Fiz. Khim., 4__33, 1914 (1969).

10. 11. 12,

B . D . Berezin, Tr. Ivansk. Khimiko-Tekhnol. Inst., 10th ed. (1968), p. 62. E . V . Pakshina and A. Ao Krasnovskii, B[okhimiya, 29, 1132 (1964). V.G. Maslov and Ao No Sidorov, Biofizika, 16, 718 (1971).

683