Redox properties of novel tetrapyrroles: Expanded porphycenes

JOURNAL OF

ELSEVIER

Journal of ElectroanalyticalChemistry381 (1995) 159-166

Redox properties of novel tetrapyrroles: expanded porphycenes C. Bernard a, j.p. Gisselbrecht a, M. Gross a, N. Jux b, E. Vogel b a Uni~'ersitd Louis Pasteur, Laboratoire d'Electrochimie et de Chimie Physique du Corps Solide, URA au CNRS No. 405, 4 rue Blaise Pascal, 67000 Strasbourg, France b Institut fiir Organische Chemie der Unit,ersit&, Greinstrasse 4, 50939 Cologne, Germany

Received 14 July 1994

Abstract

The redox properties of four novel porphyrinoid tetrapyrrolic macrocyles (in CH2CI 2 and THF) are reported. Two of the compounds constitute expanded porphycenes exhibiting 227r main conjugation pathways but may be viewed, alternatively, as di-trans-[22]porphyrins-(2.2.2.2). Featuring linear Csp2(CspCsp)nCsp2structural units, two homologous compounds match porphycene in symmetry and are termed 22~- (n = 1) and 26~'(n = 2) acetylene-cumulene porphycenes. All of these porphyrinoids exhibit similar electrochemical behaviour in that they undergo four reversible electron transfers: two reductions and two oxidations. As expected, an increase in the number of 7r-electrons on going from porphyrin or porphycene to the four new compounds lowers the energy barrier for the electron transfers. This redox behaviour correlates with the red shifts observed in the UV-visible absorption spectra. Keywords:

Redox properties; Tetrapyrroles; Expanded porphycenes

1. Introduction

Porphyrins and metalloporphyrins, commanding interest from many points of view, have become targets of interdisciplinary research that encompasses chemistry, the biosciences, physics (especially photophysics), medicine and even materials science. In recent years, this development has spurred the design and synthesis of porphyrin structural variants, e.g. sapphyrins [1,2], texaphyrins [3], porphycenes [4] and dicationic porphyrinoids [5], expected to match or complement the established tetrapyrrolic macrocycles in their physical and chemical properties. Among the novel porphyrinoids that have already been brought to light, porphycene (1) (Fig. 1) stands out since it constitutes a hitherto overlooked true structural isomer of porphyrin [6-9]. Like the latter it is found to be planar and features an 187r main conjugation pathway. Thus its designation (in Franck's terminology [10]) as [18]porphyrin-(2.0.2.0) - - versus [18]porphyrin-(1.1.1.1) - - appears to be fully justified. Although 1 possesses a cavity smaller than that of porphyrin, it is capable of forming metal complexes with a large number of metal ions, in particular nickel(II), copper(II), cobalt(III) and iron(III). Owing 0022-0728/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0022-0728(94)03691-8

to the poor solubility of 1 and its complexes, most studies on porphycenes have been carried out utilizing 2,7,12,17-tetrapropylporphycene (2) and 2,3,6,7,13,14, 17,18-octaethylporphycene (3) (Fig. 1). Numerous investigations of porphycenes (metal complexes included) bear out the close relationship of these molecules to porphyrins but they also reveal noticeable differences between the two kinds of tetrapyrrolic macrocycles [6,11,12]. This particularly applies to the electrochemical properties of porphycenes as compared with those of porphyrins [13-17]. Although the oxidation characteristics of the porphycenes parallel those of porphyrins, they differ noticeably from the porphyrins in their reduction: only two reduction steps are observed, instead of four steps in the porphyrins, and their reductions occur more readily, i.e. at potentials less negative than in porphyrins. The finding that porphycenes are porphyrin-like pigments, and therefore promise practical applications in various domains, suggested extension of our work on porphyrin structural variants to include expanded porphycenes [18-20] and to relate them to expanded porphyrins that are currently the subject of intense research [1,10,21,22]. In order to be endowed with aromatic stability, the new porphyrinoids envisaged must

C. Bernard et al. /Journal of Electroanalytical Chemistry 381 (1995) 159-166

160

1

2

[18]Pn

TPr[18]Pn

OE[18]Pn

4

5

[22]P

OE[22]P

L2

4;

6 [22]ACP

L2 7 [26]ACP

Fig. 1. Porphycenes and the expanded porphycenes studied.

contain, apart from being planar or nearly so, main ~--conjugation pathways involving (4n + 2) electrons with n > 4. In pursuit of this concept, the 22~- porphyrinoids 4 and 5, to be regarded, alternatively, as laterally expanded 22~" porphycenes or as di-trans[22]porphyrins-(2.2.2.2), were synthesized in due course [18-20]. Apart from 4 and 5, the novel 22~" and 267r acetylene-cumulene porphycenes 6 and 7, respectively, which contain l i n e a r Csp2(CspCsp)nCsp2 structural units, yielded to synthesis. As expected, all of these porphyrinoids qualify as aromatic and were found to be planar. As part of a systematic investigation of 4 - 7 we report here on the redox properties of these compounds. For reasons of simplicity, compounds 1-7 are designated by the following acronyms used throughout the subsequent text (see also Fig. 1): 1 =[18]Pn; 2 = TPr[18]Pn; 3 = OE[18]Pn; 4 = [22]P; 5 = OE[22]P; 6 = [22]ACP; and 7 = [26]ACP (the numbers in brackets refer to the main ~--conjugation pathway).

2. Experimental The experiments were carried out in three solvents: tetrahydrofuran (THF) from SDS, dichloromethane (CH2C12) from SDS and N,N-dimethylformamide

(DMF) from Fluka. Before use, CH2C12 was dried under argon over molecular sieves (4 A). DMF was purified as already reported [23] and stored under argon. T H F was distilled over LiAIH 4 before each experiment. Tetrabutylammonium perchlorate (TBAP) from Fluka was purified as indicated previously [23], whereas tetrabutylammonium hexafluorophosphate (TBAPF 6) of electrochemical grade from Fluka was used as received. The electrochemical methods used here were tast polarography on Hg, stationary voltammetry on a Pt rotating disc electrode (RDE) and cyclic voltammetry on Pt. For polarographic measurements the equipment was a PRG4 device (Solea-Tacussel, Villeurbanne, France). The dropping mercury electrode was fed by a 35 cm mercury column. The mercury flow rate was 0.65 mg S

-|

Stationary voltammetric (RDE) and cyclic voltammetric experiments (10 mV s - 1 - 2 0 V s - 1 ) w e r e carried out on Pt disc electrodes. The equipment used was a potentiostatic unit interfaced with a microcomputer ( D A C F A M O V from Microtec, CNRS, Toulouse, France). Classical three electrode cells were used throughout. Pt disc electrodes had a Pt core diameter of 2 mm (EDI type; Tacussel). The auxiliary electrode was a Pt wire. The reference electrode was Ag/AgCI and all sets of experiments included ferrocene as an internal reference; the measured half-wave oxidation potential for the ferrocene was +0.48 V vs. Ag/AgC1 in CHzCI 2 + 0.1 M TBAP and + 0.58 V vs. Ag/AgC1 in T H F + 0.1 M TBAP. UV-visible absorption spectra were obtained from an HP 8452 A diode array spectrometer (HewlettPackard).

3. Results The redox characteristics of the expanded porphycenes 4 - 7 were analysed by polarography and also by cyclic and stationary voltammetry. Preliminary metal cation complexation was also attempted with these ligands. 3.1. Redox properties of the expanded porphycenes Polarography in THF + 0.1 M TBAPF6 The expanded porphycenes 4 - 7 are reducible in two distinct one-electron steps (Table 1) in the potential range 0 to - 3 V vs. Ag/AgC1. This is remarkably different from the porphyrins, which are reducible in four distinct steps [24]. The polarographic reduction waves of all four expanded porphycenes exhibited an adsorption interfer-

C. Bernard et al. /Journal of Electroanalytical Chemistry 381 (1995) 159-166

/1

Table 1 Polarographic half-wave reduction potentials i n T H F + 0 . 1 M T B A P F 6 Porphycene

E{)12/V ( A g / A g C I ) ~

E,:}~/V ( A g / A g C I ) ~

OE[18]Pn [221P OE[22]P [22]ACP [26]ACP

-0.75(58) -0.54(47) -0.71(50) -0.63(50) -0.24(39)

- 1.04(70) -0.79(84) -0.97(b) -0.80(79) -0.47(76)

161

OE[18]Pn 4

EI~ 2 (ferrocene)= +0.58 V (Ag/AgCI). a Values in parentheses are arithmetic values of slope in mV, from the plot of E vs. log [l/(l d -/)]. Drop time, 1 s; mercury flow, 0.65 mg s ~; current sampling between 80 and 90% of the drop time. b Ill-defined.

[221ACP ence on the mercury electrode. This adsorption was maximum with OE[22]P.

Stationary uoltammetry on Pt electrodes in CH2CI 2 + O.1 M TBAP As shown in Fig. 2 and Table 2, the expanded porphycenes exhibited two reduction waves and two oxidation waves. All four waves were well defined, except for [22]P, in which the second oxidation was ill-defined. This second oxidation of [22]P triggered the generation of a film on the Pt electrode. The observed cathodic redissolution process of this film is shown in Fig. 2, on the return scan corresponding to this second oxidation/reduction conjugated step for [22]P. All other electron-transfer steps in [22]P and in the other four ligands were one-electron transfers. Cyclic uoltammetry in CH2CI 2 and in THF In CH 2C12 ( + 0.l M TBAP), cyclic voltammograms (Table 3) indicated that films were created on the Pt electrode surface after reduction of the expanded porphycenes. In contrast, the cyclic voltammograms (Table 4) obtained in T H F ( + 0.1 M TBAP) were characteristic of one-electron transfers in the absence of coupled reactions. In both C H 2 C 1 2 and THF, all observed redox couples were reversible one-electron reactions.

11~JA[. f

.

~

/

A

C

P

!JAI -1.0 ~

0

I

i

I

+1.0 t

I __ E / V ( v s Ag,'AgCI)

Fig. 2. Stationary voltammetry on a Pt rotating disc electrode in CHzCI 2 +0.1 M TBAP.

Table 3 Cyclic voltammetry on Pt in CH2C12 +0.1 M T B A P at u = 0.2 V s -1 Porphycene

OE[18]Pn [22]P OE[22]P [22]ACP [26]ACP

EI/2/V (Ag/AgC1)

AEI/2/V

1st red.

2nd red.

1st ox.

2nd ox.

-0.94 -0.60 - 0.87 - 0.57 0.30

- 1.26 -0.82 - 1.02 - 0.88 - 0.49

+0.87 +0.70 + 0.67 + 0.80 +0.72

1.10 b _ c - c + 1.09 +0.98

1.81 1.30 1.54 1.37 1.02

I E1/2(Ag/AgCI)I = 1/21Epoak(OX.)+ Epeak(red.)l. El~}2 (ferrocene) = +0.48 V (Ag/AgCI).

a AE,/2 = E~)t2(ox.)- El)~2(red.). b Reversible for c > 1 V s 1. c Film on the electrode surface.

Table 2 Half-wave potentials from Pt rotating disc electrode voltammetry in CH2CI 2 + 0.1 M T B A P Porphycene

OE[18]Pn [22]P OE[22]P 122]ACP [26]ACP

Ew2/V ( A g / A g C l ) a

AE1/2/V

1st red.

2nd red.

1st ox.

2nd ox.

-

-

+ 0.89(46) + 0.75(54) + 0.69(50) + 0.78(43) + 0.71(48)

+ 1.15(61) (c) + 1.00(51) + 1.11(76) + 0.98(45)

0.91(53) 0.59(54) 0.84(58) 0.55(48) 0.30(47)

1.26(90) 0.82(60) 1.11(57) 0.90(58) 0.49(68)

E~}2 (ferrocene)= + 0.48 V (Ag/AgC1). a Values in parentheses are arithmetic values of slope in mV, from the plot of E vs. Iog[l/(l o -/)].

b ,~E1/2 = EbVox.) - Eb'2(red.) c Ill-defined.

a

1.80 1.34 1.53 1.33 1.01

C. Bernard et al. /Journal of Electroanalytical Chemistry 381 (1995) 159-166

162

Table 4 Cyclic v o l t a m m e t r y on Pt in T H F + 0 . 1

M T B A P F 6 at u = 0.2 V s i

El~ 2 / V ( A g / A g C I )

Porphycene

E

TM

-

0.76 0.61 0.90 0.61 0.30

OE[18]Pn [22]P OE[22]P [22]ACP [26]ACP

(red.)

E 2nd (red.) -

1.02 0.83 1.11 0.85 0.57

I lJA[

JE1/2(Ag/AgCI) I = 1/21Epeak(OX.) + Eoeak(red.)l E ~ 2 ( f e r r o c e n e ) = +0.58 V (Ag/AgCl).

Replacing the non-coordinating solvent CH2C12 (donor number = 0) by the more basic solvent T H F (donor number = 20) did not dramatically change the measured reduction potentials. The cyclic voltammograms obtained in CH2C12 deserve specific comments.

2.A I 1

t

I

[

E/V(vs. Ag/AgCI)

First scan in CH2Cl 2 (+0.1 M TBAP). The voltammetric curves obtained on Pt at 0.2 V s - l are shown in Fig. 3 for the expanded porphycenes [22]ACP and [26]ACP. The normal porphycene OE[18]Pn is also shown for comparison. For OE[18]Pn, the two oxidation steps became reversible when the scan rates were higher than 1 V s - 1. This behaviour has been ascribed to the nucleophilic

l

L

OE[18]Pn~ +

[22]P +

4 A[

°4 AI /

j-

4-

[26]ACP 4-

I 1 IJA

I

-1.0 ~"

0

+1.0 E/V(vs.Ag/AgCI)

Fig. 3. First scan of the s t u d i e d p o r p h y c e n e s in cyclic v o l t a m m e t r y on a Pt e l e c t r o d e in CHzC12 +0.1 M T B A P ; u = 0.2 V s - l .

Fig. 4. C~clic v o l t a m m o g r a m s of OE[22]P in C H 2 C I z +0.1 M T B A P ; ~ = 0.2 V s i. (a) M u l t i p l e scans on a p o t e n t i a l r a n g e including two r e d u c t i o n s and two oxidations; (b) m u l t i p l e scans on a p o t e n t i a l r a n g e i n c l u d i n g two r e d u c t i o n s a n d only the first oxidation; (c) m u l t i p l e scans on a p o t e n t i a l r a n g e including only the first r e d u c t i o n and the first oxidation.

attack of residual water on the electro-oxidized porphycene, as demonstrated by experiments in hyperdry media [17]. For [22]P, the return peak associated with the first oxidation was characteristic (Fig. 3) of either a redissolution process or the removal of an adsorbed film from the electrode surface. This behaviour, which is very similar to that of [18]Pn [17], explains why no second oxidation was observed in this case. For OE[22]P, the second oxidation, although uncomplicated in stationary voltammetry (Fig. 2), exhibited a very large current in cyclic voltammetry, even on the first scan (Fig. 4(a)). Multiple scans in CH2CI 2 (+0.1 M TBAP). Multiple scans were applied to OE[22]P, [22]ACP and [26]ACP. As OE[22]P exhibited enhanced currents for the second oxidation step, even in the first scan (Fig. 4(a)), multiple scans were applied to this molecule in the potential range including either the first oxidation and the two reductions (Fig. 4(b)) or the first oxidation and the first reduction (Fig. 4(c)). The same experiments were carried out with [22]ACP and [26]ACP in the potential range including either the two oxidations and the two reductions (Fig. 5(a)) or the first oxidation and the first reduction (Fig. 5(b)). Some of the voltammetric curves obtained exhibited peak currents which increased with increasing number of potential scans. Such a behaviour was obvi-

C. Bernard et al. /Journal of Electroanalytical Chemistry 381 (1995) 159-166

with those observed in normal free-base porphycenes TPr[18]Pn and OE[18]Pn [17]. Each reduction step revealed well defined isosbestic points, as expected for two species in equilibrium. The generated two oneelectron reduced species can be oxidized stepwise to the one-electron reduced species and to the initial species. Such a behaviour indicates that the generated species are stable on the time-scale of O T T L E electrolysis. Fig. 6 represents, as an example, the UV-visible absorption spectra of the studied expanded porphycenes and their reduced forms generated on the OTTLE. Qualitatively, the spectral changes associated with the successive two reductive one-electron transfers to each expanded porphycene were very similar to those previously reported for the normal free-base porphycenes [17]. Typically, after the first reduction, the spectrum was that expected for a radical anion, namely with the Soret band replaced by less intense bands in the same region, and the disappearance of the Q bands while much weaker bands emerged at shorter wavelengths. After the second reduction (i.e. a total of two electrons received per molecule), no absorption occurred

S

I

163

I

E/V(vs. Ag/AgC])

Fig. 5. Cyclic voltammograms of [26]ACP in CH2CI 2 +0.1 M TBAP. (a) Multiple scans covering two reductions and twc oxidations; (b) multiple scans covering only the first reduction and the first oxidation.

2 1

ously reminiscent of film generation on the electrode surface [25]. Typically, such voltammetric patterns were observed for the second reduction and the first oxidation of OE[22]P (Fig. 4(b) and 4(c)), for the first reduction and the first oxidation of [22]ACP and for the second reduction and the second oxidation of [26]ACP (Fig. 5(a)). In contrast, multiple scans had no effect on the peak currents of the first reduction and the first oxidation of [26]ACP (Fig. 5(b)) and also no effect on the first reduction of OE[22]P. The film generated showed poor adherence to the electrode surface: simple movement, such as rotation, of the electrode in the solution eliminated the film and restored the initial voltammogram, thus preventing, in these conditions, further characterization of the film. Similar results were observed on glassy carbon electrodes.

3 3 2

o0

I

oc

I

Thin-layer spectroelectrochemical measurements were carried out on an O T T L E (optically transparent thin-layer electrode) in order to compare the spectral changes resulting from the electron-transfer reactions

I

2

/

..........

1 i

I

Spectroelectrochemistry

I

c~

300

400

•

.500

COC

i-~

700

800

Wavelength/nm

Fig. 6. Spectroelectrochemistry of the expanded porphycenes (thick solid lines) and of their two reduced forms (monoradical anion (thin solid lines) and dianion (dashed lines)). (a) [22]P, in THF; (b) OE[22]P, in THF; (c) [22]ACP, in CH2C12; (d) [26]ACP, in CH2CI 2.

C. Bernardet al. /Journal of ElectroanalyticalChemistry381 (1995) 159-166

164

[22]ACP 2O

1.5 t,

°m l o c~

<

i 05

0@ ?300

400

500

600

700

800

Wavelengt.h / n m Fig. 7. UV-visible absorption changes associated with the complexation of Cun by OEPn (3× 10 4 M ) . StoichiometryCuII/OEPn = 20.

The complexation of Cu n was also observed in the presence of [22]ACP ( M / L = 100). Spectral changes were also clear: the intensity of the Soret band around 400 nm decreased while its shoulder increased, and a new band emerged at 470 nm. Also, the Q bands merged into a single band. The kinetics of the global reaction were very slow: quantitative complexation was not achieved after 5 days. After 15 days, the complexation of Cu ~l by [22]ACP seemed complete. However the specific absorption bands of uncomplexed ligand around 404, 438 and 700 nm, together with the large excess of Cu II with respect to the expected complex (100:1), prevented precise spectral measurements on the complex. It is clear, however, that the complexation reactions of Cu I1 with the porphycene OE[18]Pn, and also with the expanded porphycene [22]ACP, are much slower than with the porphyrin H2TPP.

4. Discussion beyond 500 nm but a set of bands was observed between 350 and 450 rim, as shown in Fig. 6, which is characteristic of the generated dianion. These results were very similar to those obtained previously [17] with normal porphycenes except that, in the expanded porphycenes, all bands were red shifted.

3.2. Complexing properties of porphycenes and of expanded porphycenes towards Cu¢z, Ag i and Ca ll OE[18]Pn The complexation studies were monitored in situ by UV-visible spectrophotometry in D M F on all the porphycenes shown in Fig. 1. In these measurements the stoichiometry in the solution was 20 or 100 metal cations for each molecule of ligand. The salts added in excess were Ca n perchlorate, Ag I acetate and Cu n bistrifluoromethanesulphonate. Spectral changes were only observed after addition of excess Cu I1 ( M / L = 20) to solutions either of OE[18]Pn or of [22]ACP. In the case of OEPn, the Soret band around 400 nm was slightly red shifted (2 nm) and the three Q bands were replaced by a single band at 625 nm (Fig. 7). The absorbances of the new bands reached invariant values after about 24 h, indicating that the complexation reaction leading to Cu II OE[18]Pn was slow. A comparison was made with the complexation of Cu I1 by the porphyrin H2TPP (meso-tetraphenylporphyrin) under the same conditions ( M / L = 20). In the latter case, the spectral changes were much faster, and the complexation of the copper(II) by the porphyrin was almost quantitative after 30 min. Hence the copper(II) complexation reaction is much slower for the porphycene than the porphyrin.

The measured potentials on the expanded porphycenes indicate that all electron transfers - - reductions and oxidations - - require less energy as the size of the tetrapyrrolic macrocycle and the number of 7r electrons in the aromatic system increase. In the studied series, the important variation of the potential difference (AE1/2) between the first oxidation and the first reduction is remarkable. As can be seen in the last column of Table 5, AE1/2 decreases from 1.8 V in OE[18]Pn to 1.01 V in [26]ACP, obviously paralleling the increase in 7r electrons and in the size of the macrocycles. As the first reduction and the first oxidation reactions of porphyrins [26,27], and more generally the alternant aromatic hydrocarbons [28], are known to involve the frontier orbitals H O M O and LUMO, it is reasonable to infer here that, on going from 18rr to 26~" porphycenes, the energy gap decreases between H O M O and LUMO. Another conclusion is that the

Table 5 UV-visible absorption spectra of studied porphycenes in CHeCIz Porphycene A/nm AE~/2/V Soret bands, Q bands, TPr[18]Pn OE[18]Pn [18]Pn OE[22]P [221P [22]ACP [26]ACP

370 365 a 430 442 430 404 448

384 a 384 452 466 452 438 496

562 576 680 674 680 680 804

602 626 722 722 724 726 839

lIE1~~= E~)t2(ox.)- E~)t~(red.) from Table 2. a Shoulder.

634 664 790 790 790 768 882

1.85 1.80 1.73 1.53 1.34 1.33 1.01

C. Bernard et al. /Journal of Electroanalytical Chemistry 381 (1995) 159-166

:

4 -m

ca k,

© 30 ;,,< o

<5

,/

/

//• 5

/

Cal

:5

//

°s

/ /"

2

O

Sorer

•

c~ b a n d

band

/

0L/

I 4

2

S

19

10 h ~ / J Fig. 8. Plot of the energy gap corresponding to AEI/z versus the energies of the UV-visible absorption bands. (1) OEll8]Pn; (2) [22]P; (3) OE[22]P; (4) [22]ACP; (5) [26]ACP; (6) H2TPrPc; (7) [18]Pn.

ligand field strength decreases in the ligands as the number of ~" electrons increases. The observed trends in the wavelengths of UV-visible absorption spectra (Table 5) support the above conclusions, as all characteristic bands were red shifted in the 18-, 22- and 26~- porphycene series; this is a clear indication that the transition energies decrease along this sequence. A correlation was expected between the above reported decrease in AE~/2 and the observed shifts in the UV-visible absorption spectra, if both involved the frontier orbitals H O M O and L U M O . Therefore, the energy associated with the gap A E t / 2 was plotted versus the energies of the Soret band and of the a band (that with the smallest energy among the three Q bands). The slopes of the linear correlations (Fig. 8) were close to unity (0.94 for the Soret band and 1.29 for the a band). Hence the correlation holds, indicating that the same orbitals were involved in the electron transfers (first reduction and first oxidation) and in the spectral transitions. Additionally, it may also be noted (Tables 2 and 3) that the extension of the ~--conjugation pathway of the tetrapyrrolic macrocycle from 18- to 22- and 26rr electrons had varying effects on the first reduction and the first oxidation potentials. The first oxidation potentials were much less modified than the first reduction potentials. This may be correlated with smaller energy changes in the H O M O than in the L U M O for the set of molecules studied.

5. Conclusions T h e expanded porphycenes undergo four distinct one-electron reversible transfers, namely two oxida-

165

tions and two reductions. This is similar to the behaviour of the free-base porphycenes [18]Pn, TPr[18]Pn and OE[18]Pn [14,17]. Remarkable differences are noticed between the redox characteristics of the expanded porphycenes and those of the porphyrins: (1) Only two distinct reduction steps occur in the expanded porphycenes, whereas four steps are observed in the porphyrins. These results may be related to the lower symmetry of the porphycenes as compared with porphyrins. (2) The energy difference between the first reduction and first oxidation is smaller (1-1.5 eV) in the expanded porphycenes than in the 18~r electron porphycenes [13,14,17] (1.85 eV) and in the porphyrins (2.25 eV) [26,27]. This energy difference obviously decreases as both the size of the macrocycle and the number of the :r electrons increases. (3) All electron transfers are facilitated in the expanded porphycenes when compared with those of porphyrins: the reductions become significantly easier as the tetrapyrrolic macrocycle becomes larger and as the number of ~- electrons increases in the expanded porphycenes. This behaviour is reminiscent of redox properties observed in other molecules encompassing large :r electron systems, even in non-cyclic compounds such as polyacetylenes[29], polyphenylacetylenes[30] and polytriacetylenes[31]. Many expanded porphyrins have been synthesized [1,2,10,21,22] but the lack of electrochemical data hindered any comparison with our species. The reported redox behaviour of the expanded porphycenes is consistently supported by the observed UV-visible spectrophotometric results.

Acknowledgements This work was made possible thanks to the financial support of CNRS (Centre National de la Recherche Scientifique). N.J. is indebted to the Fonds der Chemischen Industrie for a fellowship.

References [1] J.L. Sessler, M.J. Cyr and A.K. Burrell, Synlen, (1991) 127. [2] V.J. Bauer, D.L.J. Clive, D. Dolphin, J.B. Paine, III, F.L. Harris, M.M. King, H. Loder, S.W.C. Wang and R.B. Woodward, J. Am. Chem. Soc., 105 (1983) 6429. [3] J.L. Sessler, T. Murai, V. Lynch and M, Cyr, J. Am. Chem. Soc., 110 (1988) 5586. [4] E. Vogel, M. K6cher, H. Schmickler and J. Lex, Angew. Chem., Int. Ed. Engl., 25 (1986) 257. [5] E. Vogel, Pure Appl. Chem., 62 (1990) 557.

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