Syntheses, structural characterizations, and optical and electrochemical properties of directly fused diporphyrins

10304

J. Am. Chem. Soc. 2001, 123, 10304-10321

Syntheses, Structural Characterizations, and Optical and

Electrochemical Properties of Directly Fused Diporphyrins

Akihiko Tsuda, Hiroyuki Furuta, and Atsuhiro Osuka*

Contribution from the Department of Chemistry, Graduate School of Science, Kyoto UniVersity,

Sakyo-ku, Kyoto 606-8502, Japan

ReceiVed April 30, 2001. ReVised Manuscript ReceiVed August 1, 2001

Abstract: Directly fused diporphyrins display the extensive π conjugation as evinced by highly perturbed

electronic absorption spectra as well as lowered and largely split first oxidation potentials. Such diporphyrins

prepared include meso-â doubly linked diporphyrins 7, meso-meso â-â â-â triply linked diporphyrins 8,

and meso-meso â-â doubly linked diporphyrins 9. Oxidation of 5,15-diaryl-substituted and 5,10,15-triaryl-

substituted Ni^II-, Cu^II-, and Pd^II-porphyrins with tris(4-bromophenyl)aminium hexachloroantimonate (BAHA)

in CHCl₃afforded 7, and triply linked Cu^II-diporphyrins 8a and 8g were respectively prepared by the oxidation

of meso-meso singly linked Cu^II-diporphyrins 5c and 5f with BAHA. Meso-meso â-â doubly linked Ni^II-

diporphyrin 9a was isolated along with triply linked Ni^II-diporphyrin 8e from the similar oxidation of meso-

meso singly linked Ni^II-diporphyrin 5a. Doubly linked diporphyrins 7 and 9a both exhibit significantly perturbed

electronic absorption spectra, in which the Soret-like bands are largely split at around 405-418 and 500-616

nm and the Q-bandlike absorption bands are substantially intensified and red-shifted at 748-820 nm, probably

as a consequence of symmetry lowering. Triply linked diporphyrins 8 display more strongly perturbed electronic

absorption spectra with split Soret-like bands at 408-419 and 567-582 nm and Q-bandlike absorption bands

reaching far-infrared region. Structures of three types of fused diporphyrins 7b and 7c, 8g and 8j, and 9a have

been unambiguously determined by X-ray crystallography to be nearly coplanar. Both the triply linked

diporphyrins 8g and 8j exhibit very flat structures, whereas the doubly linked diporphyrins 7b and 7c exhibit

ruffled structures. The doubly linked diporphyrin 9a shows a helically twisted conformation with larger ruffling

toward the opposite directions and has been actually separated into two enantiomers, which display strong

Cotton effects in the CD spectra. The first oxidation potentials (E_OX1) decrease in the order of 5 > 7 g 9 >

8, indicating lift-up of HOMO orbital in this order, and split potential differences ∆E ) E_OX1- E_OX2, in turn,

increase in the reverse order of 5 < 7e 9 < 8. The ¹H NMR spectra have indicated that the aromatic porphyrin

ring current becomes weakened in the order of 5 > 7 > 8. Collectively, the electronic interactions between the

diporphyrins have been concluded to increase in the other of 5 , 7 e 9 < 8.

Introduction

the constituent porphyrins and thereby to decrease HOMO-

LUMO gap in order to achieve higher electric conductivity.

Along this line, a variety of new bridging structures that can

enhance the inter-porphyrin conjugation have been intensively

exploited.^4-7A promising and straightforward synthetic strategy

is to connect two porphyrins directly with multiple covalent

In recent years, the creation of π-conjugated porphyrin arrays

in which the porphyrin π-electronic systems merge to form large

supramolecular chromophores has spurred considerable attention

because of their remarkable photophysical properties such as

high polarizability and high nonlinear optical (NLO) behavior.^1,2

Conjugated porphyrin arrays having a low-energy excited-state

extending near-infrared region and thus an extremely low

HOMO-LUMO gap are also of current interest for their po-

tential use as a molecular wire.³In these studies, one of the

central issues is to increase the electronic interaction between

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* Corresponding author. Phone: (+81)75-753-4008. Fax: (+81)75-753-

3970. E-mail: osuka@kuchem.kyoto-u.ac.jp.

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Published on Web 09/26/2001

Directly Fused Diporphyrins

J. Am. Chem. Soc., Vol. 123, No. 42, 2001 10305

Chart 1. Fused Diporphyrins

bonds in a so-called “fused diporphyrin” manner, which forces

a coplanar geometry that should be quite favorable for electronic

π conjugation.

There are, however, only a limited number of reports on fused

porphyrin arrays so far,^8-12despite the large number of

covalently and noncovalently linked porphyrin arrays made.^1,13,14

In pioneering work, Crossley et al. employed the condensation

of a porphyrin-2,3-dione with 1,2,4,5-tetraaminobenzene for the

construction of planar fused diporphyrin 1⁸(Chart 1) and

extended their strategy to the synthesis of a tetraporphyrin array

with ∼65 Å molecular length.^8cThese arrays are considered to

have potential use as a molecular wire by virtue of their rigid

shape and conjugated electronic system as evident from their

absorption spectra. The promise of this strategy was recom-

mended also from a theoretical point of view.⁹Benzene-bridged

fused diporphyrin 2 reported by Kobayashi et al. displayed a

red-shifted Q-bandlike absorption at 711 nm, also indicating

its conjugated character.¹⁰Smith et al. devised an efficient

synthetic route to directly fused diporphyrin 3 and related longer

arrays using a pyrroloporphyrin subunit.¹¹They extended this

strategy to the synthesis of a two-dimensionally extending

cruciform pentaporphyrin array with a long-wavelength absorp-

tion band at 774 nm.^11bThe strongly red-shifted absorption

bands associated with fused structures suggest the presence of

unusually large electronic interaction between the π orbitals on

the two macrocycles and further reinforce the potentials of these

molecules for optoelectronic applications. Metal coordination

was also used to assemble two porphyrins bearing a peripheral

enaminoketone function.¹²Diporphyrins thus assembled dis-

played a decreased optical HOMO-LUMO gap as well as

decreased first one-electron oxidation potential.

Recently, we reported that the one-electron oxidation of

porphyrins bearing a sterically uncongested meso position such

as 5,15-diaryl-substituted and 5,10,15-triaryl-substituted met-

alloporphyrins constitutes effective synthetic routes to directly

meso-meso and meso-â linked diporphyrins.^15,16Mg^II- and

Zn^II-porphyrins gave meso-meso linked diporphyrins 5 (Chart

2) by Ag^Isalt oxidation^15a,b,e,for electrochemical oxidation

(Scheme 1, path a),^15cwhile Cu^II-, Ni^II-, and Pd^II-porphyrins

gave meso-â singly linked diporphyrin 6 by electrochemical

oxidation (Scheme 1, path b).¹⁷Both 5 and 6 were expected to

take nearly orthogonal conformations, which may explain the

relatively weak electronic interaction between the neighboring

porphyrin.^17,18The observed different coupling regioselec-

tivity either giving 5 or 6 may be explained in terms of dif-

ferent HOMO orbital characters. Presumably, Mg^II- and

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(16) The syntheses of meso-meso linked diporphyrins were also reported

by other groups: (a) Susumu, K.; Shimidzu, T.; Tanaka, K.; Segawa, H.

Tetrahedron Lett. 1996, 37, 8399. (b) Khoury, R. G.; Jaquinod, L.; Smith,

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Gryko, D. T.; Yasseri, A. A.; Diers, J. R.; Bocian, D. F.; Kuhr, W. G.;

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10306 J. Am. Chem. Soc., Vol. 123, No. 42, 2001

Tsuda et al.

Chart 2. Porphyrins Studied in This Paper (Ar ) 3,5-Di-tert-butylphenyl, Ph ) Phenyl)

Zn^II-porphyrins favor A_2uHOMO in which there is large

electron density at the meso carbon, whereas Cu^II-, Ni^II-, and

Pd^II-porphyrins favor A_1uHOMO, which has nodal planes

through the meso positions and has significant electron density

at the â position (Figure 1).¹⁹

As an extension of these works, we have found novel effective

synthetic routes to fused porphyrins through the oxidation of

certain metalloporphyins with tris(4-bromophenyl)aminium

hexachloroantimonate (BAHA).²⁰The oxidation of Cu^II-, Ni^II-,

and Pd^II-porphyrins gave rise to formation of meso-â doubly

linked diporphyrins 7 (Scheme 1, paths b and c),^20a,21and the

oxidation of meso-meso linked Cu^II-diporphyrins 5 gave

meso-meso â-â â-â triply linked diporphyrins 8,^20bprobably

through meso-meso â-â doubly linked diporphyrin 9 (Scheme

1, paths d and e). Here we report the syntheses of these three

fused diporphyrins, 7, 8, and 9, whose structures have been all

characterized by the X-ray crystallography. Their optical and

electrochemical properties are also reported with a particular

concern about large electronic coupling between diporphyrins.

Results

(18) (a) Cho, H. S.; Song, N. W.; Kim, Y. H.; Jeoung, S. C.; Hahn, S.;

Kim, d.; Yoshida, N.; Osuka, A. J. Phys. Chem. A 2000, 104, 3287. (b)

Kim, Y. H.; Jeong, D. H.; Kim, D.; Jeoung, S. C.; Cho, H. S.; Kim, S. K.;

Aratani, N.; Osuka, A. J. Am. Chem. Soc. 2001, 123, 76.

Meso-â Doubly Linked Diporphyrins 7. BAHA is one of

one-electron oxidizing reagents frequently used to generate a

(19) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic

Press: New York, 1978; Vol. 3, p 1.

(20) (a) Tsuda, A.; Nakano, A.; Furuta, H.; Yamochi, H.; Osuka, A.

Angew. Chem., Int. Ed. 2000, 39, 558. (b) Tsuda, A.; Furuta, H.; Osuka,

A. Angew. Chem., Int. Ed. 2000, 39, 2549.

(21) Analogous meso-â doubly linked diporphyrin was reported to be

also formed in the reaction of 5,15-diaryl Ni^II-porphyrin with TeCl₄:

Sugiura, K.; Matsumoto, T.; Ohkouchi, S.; Naitoh, Y.; Kawai, T.; Takai,

Y.; Ushiroda, K.; Sakata, Y. Chem. Commun. 1999, 1957.

Directly Fused Diporphyrins

J. Am. Chem. Soc., Vol. 123, No. 42, 2001 10307

Scheme 1. Oxidative Transformations of 5,10,15-Triaryl-Substituted Metalloporphyrins (Ar ) 3,5-di-tert-butylphenyl)

To suppress a further coupling reaction, we next examined

the oxidation reaction of 5,10,15-trisubstituted porphyrins with

a single free meso position. Actually, the oxidation of 5,10,-

15-triaryl Ni^II-porphyrin 4b with BAHA afforded 7b (53%)

along with meso-halogenated monomers 4c (29%) and meso-

meso linked diporphyrin 5a (10%). Similarly, the reaction of

4d gave 7a (70%) as a major product together with 5b

(10%).

The oxidation of Cu^IIcomplex 4e with BAHA provided

meso-â doubly linked diporphyrin 7c (25%), meso-meso linked

diporphyrin 5c (33%), and meso-chlorinated monomer 4f (24%).

This result implied that the initial coupling regioselectivity

(meso-meso versus meso-â) of Cu^II-porphyrin was lower in

comparison to that of Ni^II-porphyrin. In contrast, the coupling

regioselectivity was shown to be quite high for Pd^II-porphyrin,

in that the oxidation of 4g gave only meso-â doubly linked

diporphyrins 7d (chlorine free, 20%), 7e (monochlorinated,

20%), and 7f (dichlorinated, 19%). The product distribution of

7d, 7e, and 7f was found to depend on the reaction time and

the amounts of BAHA used. Thus, the use of 3 equiv of BAHA

for 48 h led to the formation of 7f (49%) as a sole diporphyrin

product along with meso-brominated monomer 4h.²³Interest-

ingly, the chlorination regioselectivity that was manifest on the

Figure 1. Schematic representation of the two HOMO orbitals of the

D_4hporphyrin ring.

metalloporphyrin cation radical.²²We found, however, that one-

electron oxidation of 5,15-diaryl-substituted and 5,10,15-triaryl-

substituted Ni^II-, Cu^II-, and Pd^II-porphyrins bearing a steri-

cally uncongested meso position with BAHA led to formation

of meso-â doubly linked diporphyrins 7. Initially, 5,15-diaryl-

substituted Ni^II-porphyrin 4a was oxidized with an equivalent

amount of BAHA in CHCl₃for 12 h. Separation by preparative

size exclusion chromatography (SEC) and the analysis by

MALDI-TOF mass spectroscopy revealed formation of dimeric,

trimeric, tetrameric, and pentameric porphyrins. Subsequent

flash chromatography over a silica gel column provided

diporphyrin 7a (11%), which exhibited a parent molecular ion

peak at m/z ) 1550 (calcd for C₉₆H₉₈N₈Cl₂Ni₂, 1549) by FAB

1

basis of the H NMR data and the preliminary X-ray analysis

(Supporting Information 1) was quite high, only occurring at

(23) In the reaction of Pd^II-porphyrin, meso-chlorinated product was

not obtained. A possible mechanism for the meso bromination may be

bromine atom abstraction by a metalloporphyrin meso radical, since the

only available bromine source is BAHA. As described in the text, the

oxidation of Ni^II-porphyrin 4b with BAHA gave a .∼1:1 mixture of meso-

chlorinated and -brominated monoporphyrins 4c as side products. Since

meso-brominated Ni^II-porphyrin was found to undergo a halogen exchange

reaction to give meso-chlorinated products under the reaction conditions,

it is likely that meso-brominated monoporphyrin was initially formed and

was converted to meso-chlorinated porphyrin. In contrast, such a halogen

exchange reaction was not observed for Pd^II-porphyrins as seen for the

reactions of 4g and 5d. The observed â chlorination for meso-â doubly

linked diporphyrin might be ascribed to steric hindrance of the chlorinated

â position or different halogenation mechanism. As suggested by one of

the reviewers, we have examined the reaction of 4g with tris(4-bromophen-

yl)aminium perchlorate (Bell, F. A.; Ledwith, A.; Sherrington, D. C. J.

Chem. Soc. C 1969, 2729) with the aim of suppressing the peripheral â

chlorination. But 4g was recovered after the reaction with tris(4-bromophe-

nyl)aminium perchlorate for 2 days. We have also examined the reaction

of 5c with the percholorate salt, which resulted in the formation of 8g in a

trace amount with the recovery of 5c (56%) and some insoluble products.

mass spectrometry and a broad absorption spectrum (λ_max

-

(CHCl₃) 418, 534, and 756 nm). The 500-MHz ¹H NMR

spectrum provided three sets of mutually coupled doublets for

the peripheral â-protons at 9.42 and 9.11, 9.08 and 8.74, and

8.46 and 8.41 ppm, and a singlet for the â-proton (R²and R³)

at 8.98 ppm. A separate BAHA oxidation of meso-â singly

linked diporphyrin 6, which was obtained by the electrochemical

oxidation of 4a,¹⁷provided 7a quantitatively, suggesting 6 to

be a precursor of 7a. The obtained higher oligomeric porphyrin

arrays formed in the reaction of 4a with BAHA have been not

fully characterized mostly due to their extremely poor solubilities

and possible existence of inseparable syn-anti isomers.

(22) (a) Bell, F. A.; Ledwith, A.; Sherrington, D. C. J. Chem. Soc. C

1969, 2719. (b) Smith, K. M.; Barnett, G. H.; Evans, B.; Martynenko, Z. J.

Am. Chem. Soc. 1979, 101, 5939.

10308 J. Am. Chem. Soc., Vol. 123, No. 42, 2001

Tsuda et al.

Figure 2. Molecular structures of 7b. (A) Top view. (B) Side view. Hydrogen atoms have been omitted for clarity.

the â position adjacent to the fused ring. Here it is noteworthy

that the fused Cu^II-diporphyrin 7c is a useful precursor for other

metalated meso-â doubly linked diporphyrins by virtue of its

easy and quantitative transformation into the corresponding free-

base diporphyrin 7g, from which Zn^II-diporphyrin 7h was

indeed prepared.

and III), probably as a reflection of extensive conjugation over

the diporphyrin π-electronic system. The positions of bands I

(420-430 nm) are quite similar to those of the Soret bands of

porphyrin monomers, but the intensities are attenuated compared

with those of the porphyrin monomers. Bands II are red-shifted

(500-600 nm) relative to bands I and are more broad and

complicated, exhibiting several components. Bands III are more

red-shifted (750-830 nm) and significantly intensified in

comparison to the Q-bands of the porphyrin monomers. The

central metals in the porphyrin core have influence on the

absorption spectra, particularly on the position of bands III.

Those of Ni^II- and Pd^II-diporphyrins 7b and 7d are observed

at high-energy positions (756 and 748 nm) and those of Zn^II-

and free-base diporphyrins 7h and 7g are observed at low-energy

positions (798 and 820 nm).

The molecular structures of 7b and 7c have been confirmed

by X-ray crystallography. In 7b, the two porphyrin rings are

forced to take a nearly coplanar conformation with large ruffling

and mean plane deviations of the 25 core atoms of 0.30 Å

(Figure 2). The mean NisN distance is 1.92 Å, similar to the

reported 1.929 Å for Ni^II-TPP;²⁴the two newly formed meso-â

bonds are l.45 Å long, similar to the C₂-C₃bond length (1.48

Å) of 1,3-butadiene; the Ni-Ni distance is 8.61 Å. The X-ray

crystal structure of 7c also shows almost planar but slightly

ruffled conformation with mean plane deviations of 0.26 Å

(Figure 3). The meso-â C-C bonds are 1.46 Å long, the Cu-

Cu distance is 8.66 Å, and the mean Cu-N distance is 1.98 Å,

again similar to the reported Cu-N distance for Cu^II-TPP.^24a,25

The electronic absorption spectra of the meso-â doubly linked

diporphyrins 7b, 7c, 7d, 7g, and 7h are shown in Figure 4. The

absorption spectra display a similar pattern among the series,

showing broad and red-shifted three major bands (bands I, II,

(24) (a) Scheidt, W. R. In The Porphyrin Handbook; Kadish, K. M.,

Smith, K. M., Guilard, R., Eds.; Academic Press: Boston, MA, 2000; Vol.

3, p 49. (b) Maclean, A. L.; Foran, G. J.; Kennedy, B. J.; Turner, P.;

Hambley, T. W. Aust. J. Chem. 1996, 49, 1273. (c) Jentzen, W.; Unger, E.;

Song, X.-Z.; Jia, S.-L.; Turowska-Tyrk, I.; Schweitzer-Stenner, R.; Drey-

brodt, W.; Scheidt, W. R.; Shelnutt, J. A. J. Phys. Chem. A 1997, 101,

5789.

(25) Fleischer, E. B.; Miller, C. K.; Webb, L. E. J. Am. Chem. Soc. 1964,

86, 2342.

Directly Fused Diporphyrins

J. Am. Chem. Soc., Vol. 123, No. 42, 2001 10309

Figure 3. Molecular structures of 7c. (A) Top view. (B) Side view. Hydrogen atoms have been omitted for clarity.

Figure 4. Absorption spectra of meso-â doubly linked diporphyrins in CHCl₃.

As an application, meso-meso linked Pd^II-diporphyrin 5d

was reacted with BAHA under similar conditions to give

tetraporphyrin 7i (29%) along with meso-brominated porphyrin

5e (60%) and recovery of 5d (20%) (Scheme 2). Tetraporphyrin

10310 J. Am. Chem. Soc., Vol. 123, No. 42, 2001

Scheme 2. Synthesis of Tetraporphyrin 7i from 5d

Tsuda et al.

Table 1. Synthesis of 8a from the Oxidation of 5c with BAHA.

suppression of the peripheral chlorination and slight improve-

ment in the yield of 8a (entries 6 and 7). These results indicated

that the chlorine was coming not only from the solvents but

also from the oxidizing reagent, BAHA itself. Meanwhile, we

found that the peripheral chlorination was fairly suppressed in

C₆F₆, allowing the formation of 8a in 55% yield (entry 8).

Longer reaction time and use of 2-3 equiv of BAHA resulted

in the formation of 8a in 62-74% yield (entries 9 and 10).

Cu^II-diporphyrin 8a was synthetically useful, as it was quan-

titatively demetalated with concentrated H₂SO₄to give free-

base form 8c from which Zn^II-diporphyrin 8d, Ni^II-diporphyrin

8e, and Pd^II-diporphyrin 8f were prepared.

yields (%)^a

temp time BAHA

entry solvent (°C) (h) (equiv) 8a 8b

dichlorinated

product^b

5c^c

1

2

3

4

5

6

7

8

9

CHCl₃

CH₂Cl₂

THF

C₆H₆

CF₃C₆H₅20

20

61

0

20

12

10

30

5

2

3

8

0

5

0

29

0

15

0

34

58

19

0

14

0

trace

10

40

60

98

96

45

16

30

21

10

5

0

12

30

48

10 20

36 34

55

62

C₆F₆

20

5

6

To confirm the X-ray structure of a parent triply linked fused

diporphyrin, we converted meso-meso Cu^II-diporphyrin 5f to

triply linked diporphyrins 8g (chlorine free, 76%; m/z ) 1645,

calcd for C₁₀₈H₁₀₆N₈Cu₂, 1641) and 8h (monochlorinated, 6%;

m/z ) 1677, calcd for C₁₀₈H₁₀₅N₈Cu₂Cl, 1675). The X-ray

structure of 8g is shown in Figure 5. The two porphyrin rings

are fused to form a nearly complete coplanar conformation with

a very small mean plane deviation (0.03 Å). The C(â)-C(â)

bonds are 1.41 Å long and the C(meso)-C(meso) bond is 1.44

Å long; the Cu-Cu distance is 8.38 Å; the mean Cu-N distance

is 1.99 Å. It is interesting to note that fused diporphyrin units

show nearly orthogonal crystal packing in the solid state with

the 3,5-di-tert-butylphenyl groups pointing to the Cu^II-dipor-

phyrin plane. The Cu^II-diporphyrin 8g was demetalated to free-

base diporphyrin 8i, from which Zn^II-diporphyrin 8j was

prepared. The monochlorinated triply linked diporphyrin 8b was

similarly demetalated to 8k. The orthogonal crystal packing was

also observed for the free-base diporphyrin 8k (Supporting

Information 3), whereas Zn^II-diporphyrin 8j displayed a flat

structure with a mean plane deviation of 0.23 Å, a Zn-Zn

distance of 8.50 Å, a mean Zn-N distance of 2.07 Å, two

C(â)-C(â) bonds of 1.44 Å, and the C(meso)-C(meso) bond

of 1.51 Å, packed in a parallel manner with the interporphyrin

separation of ∼5.44 Å (Figure 6). In 8j, the central Zn^IIion has

been revealed to coordinate with ethanol (not shown for clarity).

The average Zn-N distance observed is slightly longer than

the reported 2.037 Å distance of Zn^II-TPP.²⁶

10 C₆F₆

74 16

^aIsolated yields. ^bThe secondary chlorination site has not been

determined. ^cRecovered.

7i exhibits a parent molecular ion peak at m/z ) 3319 (calcd

for C₁₉₂H₁₉₈Br₂N₁₆Pd₄, 3315) in its MALDI-TOF mass spectrum

and absorption spectrum with bands at 419, 554, 752, and 1036

nm (Supporting Information 2). The tetraporphyrin 7i has an

interesting structure possessing both meso-meso single linkage

and meso-â fused linkage.

Meso-Meso â-â â-â Triply Linked Diporphyrins 8. In

the course of these studies, it occurred to us that the oxidation

of meso-meso linked metalloporphyrins might lead to formation

of meso-meso â-â â-â triply linked diporphyrins. This is more

likely for the diporphyrins with an A_1uHOMO orbital, since

they have a large spin density at the â positions in the cation

radical part. In the first attempt, meso-meso singly linked Cu^II-

diporphyrin 5c was oxidized with 2 equiv of BAHA in CHCl₃

at room temperature for 12 h. Repeated separations over silica

gel column chromatography provided triply linked diporphyrins

8a (chlorine free, 8%) and 8b (monochlorinated, 29%), together

with dichlorinated triply linked diporphyrins (34%) (Table 1,

entry 1). The structures of 8a and 8b have been suggested by

their absorption spectra (8a, λ_max) 411, 576, and 996 nm

(Figure 7); 8b, λ_max) 409, 573, and 990 nm) and the parent

molecular ion peaks detected by FAB-MS (or MALDI-TOF)

(8a, m/z ) 1866, calcd for C₁₂₄H₁₃₈N₈Cu₂,1865) and (8b, m/z

) 1900, calcd for C₁₂₄H₁₃₇N₈ClCu₂, 1899). Analogously, the

structure of the dichlorinated triply linked diporphyrins has been

suggested on the grounds of the molecular weight (m/z ) 1936,

calcd for C₁₂₄H₁₃₆N₈Cl₂Cu₂, 1933) and the absorption spectrum

(λ_max) 408, 571, and 987 nm) that were nearly the same as

those of 8a and 8b, but its secondary chlorination site has not

been determined. In the next step, we attempted to suppress

the concurrent peripheral chlorination (Table 1). The chlorination

was not suppressed in CHCl₃by temperature variation (entries

1-3), and the ring closure was not effected in CH₂Cl₂and THF

(entries 4 and 5). In C₆H₆and CF₃C₆H₅, we noted some

The electronic absorption spectra of the triply linked dipor-

phyrins are shown in Figure 7. As in the case of the meso-â

doubly linked diporphyrins, all the spectra feature three major

bands (I, II, III). Absorption bands I remain at positions (408-

419 nm) similar to those of the Soret band of porphyrin

monomers, bands II are observed at 550-560 nm, and bands

III are exceedingly red-shifted at 850-1050 nm. Bands II

become sharper as compared with those for the meso-â doubly

linked diporphyrins 7. The molecular extinction coefficients of

(26) Scheidt, W. R.; Mondal, J. U.; Eigenbrot, C. W.; Adler, A.;

Radonovich, L. J.; Hoard, J. L. Inorg. Chem. 1986, 25, 795.

Directly Fused Diporphyrins

J. Am. Chem. Soc., Vol. 123, No. 42, 2001 10311

Figure 5. Molecular structures of 8g. (A) Top view. (B) Side view. Hydrogen atoms have been omitted for clarity.

Figure 6. Molecular structures of 8j. (A) Top view. (B) Side view. Hydrogen atoms have been omitted for clarity.

bands I in 8 are similar to those of 7 but the molecular extinction

coefficients of bands II are larger in comparison to those in 7.

In the particular case of the Pd^II-diporphyrin 8f, band II is

higher than band I. Effects of the central metals upon the

10312 J. Am. Chem. Soc., Vol. 123, No. 42, 2001

Tsuda et al.

Figure 7. Absorption spectra of meso-meso â-â â-â triply linked diporphyrins in CHCl₃.

positions of bands III are similar to those found for 7. Namely,

the bathochromic shifts are modest for the Ni^II- and Pd^II-

diporphyrins 8e and 8f and are large for the Zn^II- and free-

base diporphyrin 8d and 8c, which exhibit the bands III at 1068

and 1086 nm, respectively.

Meso-Meso â-â Doubly Linked Diporphyrins 9. We have

also examined the oxidative transformation of meso-meso

singly linked Ni^II-diporphyrin 5a to 8e, since the cation radicals

formed from Ni^II-porphyrins are known to favor A_1uHOMO.

Curiously, the similar reaction of 5a with BAHA afforded

meso-meso â-â doubly linked diporphyrin 9a (chlorine free,

10%) and 9b (monochlorinated, 20%) as the third type of fused

diporphyrin in addition to triply linked diporphyrin 8e (10%)

and 8l (monochlorinated, 20%). The doubly linked diporphyrin

9a displayed a parent molecular ion peak at m/z ) 1862 (calcd

for C₁₂₄H₁₄₀N₈Ni₂, 1860) in its FAB-MS (or MALDI-TOF) mass

spectrum and exhibited six doublets for the â-protons at 7.51,

8.48, 8.50 (for 4H), 8.51, and 8.53 ppm and a singlet for the

â-protons adjacent to the â-â connection at 8.84 ppm in its

¹H NMR spectrum. The electronic absorption spectrum of 9a

is similar to that of the meso-â doubly linked diporphyrin 7b,

exhibiting also three main bands λ_max) 410, 490, and 735 nm

(Figure 8). The structure of the monochlorinated 9b has been

inferred by the analogy of its properties to those of 9a; m/z )

1891 (calcd for C₁₂₄H₁₃₉N₈ClNi₂, 1891; λ_max) 407, 492, and

728 nm).

The final structural determination of 9a has come from its

X-ray crystal structure (Figure 9). The two porphyrin rings take

a nearly coplanar conformation but with large ruffling toward

the opposite directions. The mean plane deviation is 0.34 Å,

the Ni-Ni distance is 8.20 Å, and the mean Ni-N distance is

1.90 Å. As a result of the opposite ruffling, 9a has a helical

structure with nearly C₂symmetry and the remaining two

â-hydrogens adjacent to the meso-meso connection are fixed

to be about 3.20 Å apart. In the crystal packing structure, 9a is

arranged like a slipped parallel sheet with pairing of chiral

enantiomers.

The helical structure of 9a has been confirmed by its

separation into the two enatiomers by HPLC with a chiral

preparative column. The isolated two enantiomers displayed

opposite Cotton effects in the CD spectra (Figure 8). In the CD

spectrum of the first eluting isomer, 9a(A), the first Cotton effect

Figure 8. Bottom, absorption spectrum of 9a. Top, CD spectra of 9a

in CHCl₃; the first eluting isomer 9a(A) (s); the secondary eluting

isomer 9a(B) (- - -).

with the negative sense was observed around 700 nm, which

corresponds to absorption band III. But the peak position in

the CD spectrum was at 725 nm, being blue-shifted by 11 nm

from absorption band III. The second and third ones were

observed as bisignate split Cotton effects, both of which coincide

very closely with absorption bands I and II seen in the absorption

spectra. These bisignate two Cotton effects had the opposite

sign, and the intensity of the CD effect at band I was distinctly

stronger than that at band II. These CD spectra provide rich

information on the absorption properties of the meso-meso â-â

doubly linked diporphyrins, which will be discussed below. The

activation barrier for the interconversion between the two

enantiomers has been determined by monitoring the CD intensity

decrease at 416 nm with a toluene solution containing 9a (1.04

× 10^-2mM) at 333-363 K. The racemization rate constants

k_Rthat obey first-order kinetics have been determined at different

temperatures (Supporting Information 4). The activation pa-

rameters obtained (∆G^‡) 111 kJ mol^-1, ∆H^‡) 101 kJ mol^-1

∆S^‡) -30.4 J mol^-1K^-1, ∆E ) 104 kJ mol^-1) are very similar

,

to those determined for the pentahelicene (∆G^‡) 101 kJ mol^-1

,

∆H^‡) 95.8 kJ mol^-1, ∆S^‡) -17.2 J mol^-1K^-1, ∆E ) 98.3

kJ mol^-1) but distinctly smaller than those of the hexahelicene

(∆G^‡) 146 kJ mol^-1, ∆H^‡) 152 kJ mol^-1, ∆S^‡) -17.6 J

Directly Fused Diporphyrins

J. Am. Chem. Soc., Vol. 123, No. 42, 2001 10313

Figure 9. Molecular structures of 9a. (A) Top view. (B) Side view. Hydrogen atoms have been omitted for clarity.

1

mol^-1K^-1, ∆E ) 149 kJ mol^-1) and other higher ordered

helicenes.²⁷

counterpart porphyrin. In contrast, the H NMR spectrum of

the meso-â doubly linked diporphyrin 7g shows the upfield

shifts of the outer â-protons (H^b, H^c, H^d, H^e) at 8.42-8.53 ppm

and the downfield shifts of the inner â-protons (H^a, H^g) at 9.18

and 9.55 ppm relative to those in 4i and 4j. The downfield shifts

of H^aand H^gmay be ascribed due to the deshielding ring cur-

rent effect of the other porphyrin, since they are positioned in

the same plane with the other porphyrin ring, while the upfield

shifts of the outer â-protons (H^b, H^c) suggest the decreased

aromatic ring current probably associated with enhanced

conjugation over the array. Such a tendency seems to be stronger

in the spectrum of 8c with distinct upfield shifts of the outer

â-protons at 7.60-7.63 ppm. The inner NH protons are clearly

downfield shifted in order of 4i and 4j < 5g < 7g < 8c. The

similar trends of the decreased ring current effect upon the

increase of the π-conjugation were also suggested from the ¹H

NMR spectra of the Ni^II- and Zn^II-diporphyrins (See Experi-

mental Section).

Oxidation Reactions of Other Meso-Meso Singly Linked

Diporphyrins. In contrast to the successful formation of the

triply linked diporphyrin from the meso-meso linked Cu^II-

and Ni^II-diporphyrins, neither free-base diporphyrin 5g, Zn^II-

diporphyrin 5h, nor Pd^II-diporphyrin 5i gave the triply linked

diporphyrins under similar oxidation conditions. Diporphyrins

5g and 5i were almost recovered and 5h was merely demetalated

to 5g.

¹H NMR Spectra. The chemical shifts of the peripheral

â-protons and the inner NH protons provide important informa-

tion on the aromaticity of the electronic π system of porphyrins.

Figure 10 shows some representative chemical shifts of the free-

base porphyrins and diporphyrins in CDCl₃. In 4i and 4j, the

â-protons appear at 8.90-9.33 ppm and the inner NH protons

appear at -2.90 and -2.66 ppm. In 5g, the inner â-protons

(H^aand H^b) and inner NH protons are upfield shifted to 8.10

and 8.61 ppm and -2.11 ppm, respectively, while the outer

â-protons (H^cand H^d) remain at chemical shifts similar in

comparison to those of the monomer. The observed upfield shifts

of the inner â-protons indicate their location in the shielding

region of the ring current of the perpendicularly arranged

Electrochemistry. The electrochemical properties, particu-

larly the one-electron oxidation potentials, were studied by cyclic

(27) (a) Goediche, C.; Stegemeyer, H. Tetrahedron Lett. 1970, 12, 937.

(b) Martin, R. H.; Marchant, M.-J. Tetrahedron Lett. 1972, 35, 937. (c)

Martin. R. H.; Marchant. M-.J. Tetrahedron 1974, 30, 347.

10314 J. Am. Chem. Soc., Vol. 123, No. 42, 2001

Tsuda et al.

Figure 11. Cyclic voltammograms (E_ox(V) vs AgClO₄/Ag) for the

oxidation of Zn^II-porphyrin 4k and Zn^II-diporphyrins, 5h, 7h, and

8d in CHCl₃. Scan rate, 30 mV/s; working electrode, Pt; supporting

electrolyte, 0.1 M Bu₄N‚BF₄.

Figure 10. Representative ¹H NMR chemical shifts of free-base

porphyrins 4i, 4j, 5g, 7g, and 8c.

order of 4 > 5 > 7 > 8 as similar to the trend found for the

Zn^II-diporphyrin series (Figure 12) and the increase of the ∆E

values also followed the same order of 5 < 7 < 8. The ∆E

values were also a function of the central metal in the porphyrin

core. A similar trend was found for the Pd^II-, Ni^II-, (free-

base), and Cu^II-diporphyrin series, while a somewhat dif-

ferent trend was found for the Zn^II-diporphyrins. In the former

group, the ∆E values of 5 were 0.18-0.21 V, those of 7 were

0.31-0.37 V, and those of 8 were 0.39-0.44 V, whereas in

the Zn^II-diporphyrin series, the ∆E values were 0.13, 0.16, and

0.25 for 5h, 7h, and 8d, respectively. The ∆E values of 7h and

8d were considerably smaller compared with those of the other

diporphyrins.

voltammetry in CHCl₃containing 0.1 M tetrabutylammonium

tetrafluoroborate (Bu₄N‚BF₄) as supporting electrolyte. Unfor-

tunately, at the present stage, waves associated with electro-

chemical reduction could not be measured precisely in our

hands. The cyclic voltammograms of the Zn^II-porphyrins, 4k,

5h, 7h, and 8d, are shown in Figure 11. The monomer 4k

underwent reversible first and second oxidations at 0.52 and

0.83 V versus Ag/AgClO₄, while the two reversible oxidation

waves at 0.47 and 0.59 V in the meso-meso singly linked

diporphyrin 5h have been assigned as split first oxidation waves

(one electron per porphyrin) as judged from the results of

other electronically coupled diporphyrins.^2,4Here we define the

value of potential difference, ∆E ) E_OX1- E_OX2, where E_OX1

and E_OX2represent split potentials for the first oxidation. E_OX1

of 5h was lowered with respect to that of 4k, indicating the lift

up of HOMO orbital in 5h, while E_OX2was higher than that of

4k, probably as an influence of the positive charge first

generated. The one-electron oxidation potentials were detected

at 0.31 and 0.47 V for 7h and at 0.17 and 0.42 V for 8d,

respectively.

Discussion

As described above, the unsubstituted peripheral side of 5,-

15-diaryl- and 5,10,15-triarylmetalloporphyrins can be made

reactive for the direct coupling upon treatment with BAHA.

Taking advantage of this, we have developed the synthetic routes

to the three novel fused diporphyrins 7, 8, and 9. These three

fused diporphyrins are forced to take nearly coplanar geometries,

which cause extensive π conjugation over the diporphyrins. In

the synthesis of 7, the use of Ni^II- or Pd^II-porphyrins is

recommended because of their highly regioselective initial

meso-â coupling arising from their A_1uHOMO orbitals,

although meso-â doubly linked Pd^II-diporphyrin has been

The oxidation potentials were also determined for Cu^II-, free-

base, Ni^II-, and Pd^II-porphyrins, and the results are listed in

Table 2.

These studies revealed that within the same metalated series

the decrease of the first oxidation potential followed the same

Directly Fused Diporphyrins

J. Am. Chem. Soc., Vol. 123, No. 42, 2001 10315

a

Table 2. Oxidation Potentials of 4, 5, 7, 8 and 9 in CHCl₃

Zn

Cu

H₂

E_OX1E_OX2

0.75

Ni

Pd

E_OX1E_OX2

∆E

E_OX1E_OX2

∆E

E_OX1E_OX2

∆E

E_OX1E_OX2

∆E

4k 0.52

4e 0.72

4i

4b 0.76

4g 0.82

0.79 1.01 0.21

5h 0.47 0.59 0.13 5c 0.68 0.86 0.18 5g 0.71 0.88 0.17 5a 0.73 0.91 0.18 5i

7h 0.31 0.47 0.16 7c 0.47 0.78 0.31 7g 0.46 0.79 0.33 7b 0.52 0.84 0.32 7d 0.57 0.94 0.37

9a 0.48 0.87 0.39

8d 0.17 0.42 0.25 8a 0.39 0.78 0.39 8c 0.35 0.75 0.40 8e

0.46 0.87 0.41 8f

0.51 0.95 0.44

^aE_OX1, first oxidation potential (V); E_OX2, second oxidation potential (V); ∆E ) E_OX2- E_OX1

.

Figure 12. Comparison of E_OX1(V) vs AgClO₄/Ag in CHCl₃.

shown more prone to peripheral â chlorination. This is in

contrast to the successful synthesis of meso-meso singly linked

diporphyrin and higher porphyrin arrays from the corresponding

Zn^II-porphyrins which seem to have A_2uHOMO orbital.¹⁵In

the oxidative transformation of the meso-meso singly linked

diporphyrin to the meso-meso â-â â-â triply linked dipor-

phyrins, the Cu^II-diporphyrin substrates such as 5c and 5f are

of preferable choice, in that they give triply linked diporphyrins

in good yields without noticeable demetalation during the

oxidation with BAHA and are quantitatively demetalated to the

corresponding free-base diporphyrins, from which a variety of

metalated triply linked diporphyrin can be prepared.

linked diporphyrin product was never detected in the reaction

of the Cu^II-porphyrins 5c and 5f. Accumulation of 9 may be

rationalized in terms of its constrained helical structure in which

the two â-hydrogens adjacent to the meso-meso linkage are

kept apart from the coupling distance. This may be brought

about by the large ruffling of the porphyrin core which is

characteristic of Ni^II-porphyrins.²⁴By contrast, Cu^II-porphyrins

usually tend to take more planar structures.^25,28If the similar

planar structure is favored also for the meso-meso â-â doubly

linked Cu^II-diporphyrin, the two â-hydrogens adjacent to the

meso-meso connection would be located closer, hence facilitat-

ing subsequent â-â coupling to complete the triple linkage.

In the course of this study, we noted several interesting

reactivities of the metal porphyrins under the present oxidation

conditions. First, in the transformation of 5c to 8a, we found

that the peripheral â chlorination was fairly suppressed in C₆F₆.

The results in C₆H₆and CF₃C₆H₅(Table 1, entries 6 and 7)

suggested that the chlorine atom abstraction was taking place

from BAHA. Thus, the observed suppression of the peripheral

â chlorination in C₆F₆can be attributed to suppression of the

reaction of 8a with BAHA, while 8a is very susceptible to the

peripheral chlorination due to its high HOMO energy in

comparison to 5c. Here it is interesting to note that the solubility

of 8a is quite poor, almost insoluble in C₆F₆, but certainly

soluble in CHCl₃, C₆H₆, and CF₃C₆H₅. The starting substrate

5c is much soluble in C₆F₆. It is therefore conceivable that,

during the reaction in C₆F₆, the oxidation product 8a once

formed may be precipitated out from the reaction mixture,

thereby escaping from the undesirable reaction with BAHA. In

line with this consideration, the formation of turbid material

was indeed observed in the reaction of 5c with BAHA in C₆F₆.

The second interesting reactivity is that the meso-meso â-â

doubly linked diporphyrin 9 was isolated only in the reaction

of Ni^II-diporphyrin 5a. The similar meso-meso â-â doubly

Fortunately, we have obtained X-ray quality single crystals

for the three types of fused diporphyrins, meso-â doubly linked

Ni^II-diporphyrin 7b and Cu^II-diporphyrin 7c, meso-meso

â-â â-â triply linked Cu^II-diporphyrin 8g and Zn^II-dipor-

phyrin 8j, and meso-meso â-â doubly linked Ni^II-diporphyrin

9a. In most cases, the structural parameters including bond

lengths, bond angles, mean plane deviations, and mean metal-

nitrogen distances that relate with porphyrin ring distortion are

quite similar to those of the reference Ni^II-, Cu^II-, and Zn^II-

TPP, respectively (Supporting Information 5). Therefore, it may

be concluded that the fused connection scarcely alters the basic

skeletons of the each porphyrin ring but serves to force coplanar

arrangements of the diporphyrins.

The doubly linked diporphyrins 7b, 7c, and 9a show ruffling

depending on the linkage pattern and the coordinated metal,

whereas the triply linked diporphyrins 8g and 8j display more

planar geometries. The ruffling for the Ni^II-diporphyrins 7b

(28) Cu^II-TPP was reported to take a ruffled structure with a mean plane

deviation of 0.205 Å (ref 25), but we found that 5,15-bis(3,5-dioctyloxy)-

phenyl Cu^II-porphyrin was more planar with a mean plane deviation of

0.014 Å and a mean Cu-N distance of 1.99 Å: Aratani, N.; Yoshida, N.;

Osuka, A., unpublished results.

10316 J. Am. Chem. Soc., Vol. 123, No. 42, 2001

Tsuda et al.

Figure 13. Absorption spectra of Ni^II-porphyrin series including 4b, 5a, 6, 7b, 8e, and 9a in CHCl₃. Concentrations are 2 × 10^-6M for 4b and

1 × 10^-6M for the other diporphyrins.

and 9a may be ascribed largely to a smaller size of the Ni^II

cation toward the central porphyrin cavity.²⁴In line with this

interpretation, the average Ni-N distances (1.92 and 1.90 Å

for 7b and 9a) are similar to the reported 1.929 Å for Ni^II-

TPP.²⁴In 9a, the steric congestion between the remaining

peripheral â-hydrogens adjacent to the meso-meso bond causes

additional distortion. Compared with Ni^II-porphyrins, Cu^II-

porphyrins have been known to take more planar structures.^25,28

Thus, the ruffled structure of 7c suggests some intrinsic

distortion arising from the meso-â double linkage, while such

distortion would be smaller in triply linked diporphyrins, judging

from more planar structure of 8g. Another interesting observa-

tion is the orthogonal crystal packing of 8g and 8k, which

suggests attractive CH-π interaction between the 3,5-di-tert-

butylphenyl groups and diporphyrin π system.²⁹The distances

between the carbon atoms of the tert-butyl group and the

porphyrin plane are 3.41 and 3.55 Å, in the reported range of

CH-π interaction.²⁹This interaction may be enhanced by the

large polarizability of the triply linked diporphyrins associated

with their wide π-electronic system. On the other hand, 8j shows

not orthogonal but parallel packing with large offset. This may

be ascribed to ethanol coordination to the zinc ion.

exciton coupling energies in the Soret band region were

evaluated to be 4250 and 2530 cm^-1, respectively, for the

meso-meso linked and meso-â linked diporphyrins on the basis

of the deconvolution of the split Soret bands.¹⁷Analogously,

the exciton coupling energies have been evaluated to be 3300

and 2050 cm^-1for 5a and 6, respectively. Smaller exciton

coupling energies in the Ni^II-porphyrins arise from the smaller

oscillator strength of the Ni^II-porphyrin monomer compared

with that in the Zn^II-porphyrin monomer, and the different

values for 5a and 6 can be ascribed to the different geometries.

From this analysis it may be concluded that there is large

electronic interaction between the two porphyrin in 5a and 6

but they are not conjugated.

In contrast, the electronic π systems in the fused diporphyrins

7b, 8e, and 9a can be regarded conjugated, judging from their

entirely perturbed absorption spectra. Evidently, the forced

coplanar geometries of the fused diporphyrins allow the π

conjugation. The absorption spectra of the doubly linked

diporphyrins 7b and 9a are similar to each other, and the triply

linked diporphyrin 8e displays the most altered absorption

spectrum with the largest splitting of the Soret-like band and

extension of its Q-band into far-infrared region.

All of the diporphyrin set is available only for Ni^II-por-

phyrins, and thus, their absorption spectra are compared in

Figure 13. In contrast to the sharp Soret band of the monomer

4b, the singly linked diporphyrins 5a and 6 exhibit perturbed

Soret bands but the spectral changes in the Q-bands are rather

modest despite the direct connections. As seen for the corre-

sponding Zn^II-diporphyrins,¹⁷the Soret band of 5a is split at

419 and 450 nm, and that of 6 is observed as a broad band

with a peak at 412 nm with a shoulder at 425 nm, and the

Q-bands are observed at 523, 538, and 526 nm for 4b, 5a, and

6, respectively. The relatively small spectral changes may be

ascribed to their almost perpendicular conformations. The

spectral changes in 5a and 6 can be qualitatively understood in

terms of the exciton coupling between the transition dipole

moments of the porphyrin, which depends on the oscillator

strength, and the relative geometry of the interacting transition

dipole moments. In the case of the Zn^II-diporphyrins, the

The absorption spectral pattern consisting of the three main

bands (I, II, III) is common for all the fused diporphyrins.

Although the assignment of these bands has not been fixed yet,

it may be possible to qualitatively understand these spectral

characteristics on the basis of the well-established Gouterman

four-orbital theoretical model for the absorption spectra of

porphyrins.³⁰In the four-orbital theory, the B- and Q-bands both

arise from π-π* transitions and can be explained in terms of

a linear combination of transitions from a_1uand a_2uHOMO

orbitals to a degenerate pair of e_gxand e_gyLUMO orbitals. The

two HOMO orbitals happen to have the same energy and the

configurational interaction results in two bands with very

different intensities and wavelengths; the intense short-

wavelength B-band and the weak long-wavelength Q-band. On

the other hand, the fused connection causes a significant

perturbation that breaks down the degeneracy of the e_gxand e_gy

(30) (a) Gouterman, M. J. Mol. Spectrosc. 1961, 6, 138. (b) Bilsel, O.;

Rodriguez, J.; Milam, S. N.; Gorlin, P. A.; Girolami, G. S.; Suslick, K. S.;

Holten, D. J. Am. Chem. Soc. 1992, 114, 6528.

(29) (a) Aurivillius, B.; Carter, R. E. J. Chem. Soc., Perkin 2 1978, 1033.

(b) Nishino, M.; Hirota, M. Tetrahedron 1989, 45, 7201.

Directly Fused Diporphyrins

J. Am. Chem. Soc., Vol. 123, No. 42, 2001 10317

voltammetric behaviors of the present Zn, Cu, H₂, Ni, and Pd

diporphyrins as a group are remarkably similar, thus suggesting

electron removal is commonly occurring from the diporphyrin

ligand but not from the metal center. The dependence of

oxidation potentials of the meso-meso linked and fused

diporphyrins on the identity of the central metal follows that

seen in 5,10,15-triaryl-substituted porphyrin monomer, becoming

harder in the order of Zn, Cu, H₂, Ni, and Pd, probably reflecting

the electronegativity of the metals. Judging from the Ni^II-

porphyrin series, the electronic interaction of the meso-meso

â-â doubly linked diporphyrins 9 may be placed between 7

and 8. Within the same metalated diporphyrin series, the first

oxidation potentials decrease in the order of 4 > 5 > 7 g 9 >

8, suggesting the lift-up of the HOMO orbital in this order. The

∆E values were also found to increase in the reversed order of

5 < 7 e 9 < 8, probably reflecting the increasing delocalized

nature of one-electron oxidized diporphyrins. It is interesting

to note that the ∆E values are also a function of the central

metals. There is a distinct difference between the Zn^IIseries

and the other diporphyrin series, which, we believe, may be

derived from a difference in the favored HOMO orbital

characteristics of the porphyrin monomer unit, since only the

Zn^II-porphyrin favors A_2uHOMO.

Figure 14. Transition dipole moment arrangements in 9a.

orbitals. It may be pertinent to assign bands I and II to the

allowed Soret-like transitions along the shorter and longer

molecular axes of the diporphyrins, respectively, while band

III may be assigned to a formally forbidden Q-bandlike transition

in the D₄symmetry Zn^II-porphyrin monomer. Therefore, the

enhancement of bands III in the fused diporphyrins can be

understood in terms of symmetry lowering. The CD spectra of

the enationmers 9a(A) and 9a(B) are quite informative on the

absorption characteristics. Consideration of the two transition

dipole moments, B_xand B_y, respectively, nearly parallel and

perpendicular to the long molecular axis of the diporphyrin as

shown in Figure 14 can explain the perturbed absorption spectra

and CD Cotton effects through the excitonic interactions of the

resultant four transition dipole moments. The coupling of the

two B_ytransition dipole moments does not lead to the substantial

spectral change nor spectral shift because of their almost

perpendicular arrangement but gives rise to the strong bisignate

Cotton effect at the absorption band I position, since the two

B_ytransition dipole moments are held at a chiral perpendicular

arrangement. On the other hand, the coupling between the nearly

parallel two B_xtransition dipole moments leads to a dipole-

allowed red-shifted transition that corresponds to the band II

and almost dipole-prohibited blue-shifted transition which cannot

be detected clearly. The coupling of the B_xdipole moments also

brings about the bisignate CD signal at the band II region, but

its sign is opposite to the CD effect of band I and its intensity

is weaker because of the unfavorable arrangement of the two

B_xfor the Cotton effects. According to the empirical chirality

exciton methods and the related studies on the helical molecules,

the first-eluting isomer 9a(A) that exhibits the split CD Cotton

effects of positive chirality at band I and those of negative

chirality at band II can be assigned to the counterclockwise-

skew structure A shown in Figure 14, and the secondary-eluting

isomer 9a(B) that exhibits the split CD Cotton effects of the

opposite sense can be assigned to the clockwise-skew structure

B.³¹These interpretations seem to help us to understand the

absorption spectra of the triply linked diporphyrins 8, in which

two B_xtransition dipole moments are completely in a line along

the meso-meso bond, thereby giving rise to sharper and

intensified bands II, as seen in the absorption spectrum of 8f.

As described above, the oxidation potentials of the singly

and doubly linked diporphyrins exhibit systematic changes upon

the structural changes and the central metals. The first question

may be the site of oxidation, i.e., the metal or the ligand.

Although we do not have definitive evidence for it, the

1

Inspection of the H NMR chemical shifts shown in Figure

10 has revealed that the shielding and deshielding effects of

porphyrin ring currents are weakened upon increasing connec-

tion between the two units, since the outer â-protons are shifted

to high field and the inner NHs are shifted to downfield in the

order of 4i and 4j < 5g < 7g < 8c. Similar results have recently

been reported by Smith et al. for the directly fused porphyrin

trimers that bear the same connection as that of 3.¹¹Taken to-

gether, the reduced ring current may be ascribed to the increased

conjugation over the π network of the diporphyrins. But, as

discussed above, the structural parameters of the porphyrin cores

are scarcely affected by direct fused connection. Therefore, it

may be concluded that the fused connection only serves to

provide the coplanar π network without affecting the porphyrin

cores to a significant extent, particularly in the case of 8.

Collectively, the electronic absorption spectra, the first one-

electron potential, the splitting of the first one-electron oxidation

1

potential, and the H NMR spectra indicated the increase of

the electronic interaction between the two porphyrins in the order

of 5 < 7 e 9 < 8. Judging from these properties, the triply

linked diporphyrins 8 have been concluded to exert the strongest

electronic interactions and the smallest optical HOMO-LUMO

gaps (1.14-1.33 eV) among the fused diporphyrins studied so

far.^8-12In addition, the triply linked diporphyrins 8 have the

highly symmetric structure and are likely to be free from severe

structural distortion. Therefore, extension of this synthetic

strategy to higher oligomeric and polymeric porphyrin arrays

will be a fascinating next project.³²

Experimental Section

General Information. All reagents and solvents were of the

commercial reagent grade and were used without further purification

(31) (a) Nakanishi, K.; Berova, N. In Circular Dichroism: Pronciples

and Applications; Nakanish, K., Berova, N., Woody, R. W., Eds.: VCH

Publishers Inc.: NewYork, 1994; p 361. (b) Mason, S. F. Molecular Optical

ActiVity and the Chiral Discriminations; Cambridge University Press:

Cambridge, U.K., 1982. (c) Harada, N.; Nakanishi, K. Acc. Chem. Res.

1972, 5, 257.

(32) Recently we found meso-meso linked diporphyrin 5h can be

converted to 8d by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzo-

quinone (DDQ) in the presence of Sc^III(OTf)₃, and this method was

applicable to longer arrays: Tsuda, A.; Osuka, A. Science 2001, 293, 79.

10318 J. Am. Chem. Soc., Vol. 123, No. 42, 2001

Tsuda et al.

except where noted. Dry CH₂Cl₂, CHCl₃, and acetonitrile were obtained

by heating under reflux and distillation over CaH₂. Solvents used for

spectroscopic measurements were all spectroscopic grade. Preparative

separations were performed by silica gel gravity flow column chro-

matography (Wakogel C-400), silica gel flash column chromatography

(Merck Kieselgel 60H Art. 7736), and size exclusion gel permeation

chromatography (Bio-Rad Bio-Beads S-XI, packed with toluene or

CHCl₃in a 4.5 × 95 cm gravity flow column; flow rate, 3.8 mL min^-1).

Separation of the chiral isomers was performed by preparative HPLC

with Sumichiral OA3100 preparative column (eluent, hexane:CHCl₃

) 97:3; flow rate, 1.2 mL min^-1; detected at 400-700 nm) and with

a Jasco HPLC apparatus with a multiwavelength detector SHIMADZU

SPD-M10A. ¹H NMR spectra were recorded in a CDCl₃solution on a

JEOL Alpha-500 spectrometer (operating at 500 MHz), and chemical

shifts were represented as δ values in ppm relative to the internal

standard of CHCl₃(δ ) 7.260 ppm). FAB mass spectra were recorded

on a JEOL HX-110 spectrometer with the positive-FAB ionization

method (accelerating voltage, 10 kV; primary ion sources, Xe) and

3-nitrobenzyl alcohol matrix, and high-resolution mass spectra (HRMS)

were taken with CsI clusters as reference. MALDI-TOF mass spectra

were recorded on a Kratos PC-Kompact Shimadzu MALDI 4 spec-

trometer using a positive-MALDI-TOF method with/without a sinapinic

acid matrix. UV/visible/NIR absorption spectra were recorded on a

Simadzu UV-3100PC UV/visible/NIR scanning spectrophotometer in

a range of 200 and 2000 nm. CD spectra were recorded on a Jasco

J-720WI spectrometer. Redox potentials were measured by the cyclic

voltammetry method and differential pulse voltammetry method on a

ALS electrochemical analyzer model 660. X-ray crystallography was

performed on a Rigaku-Raxis imaging plate system.

General Procedure for Porphyrin Metalation. For Zn^IImetalation,

a saturated solution of Zn(OAc)₂in MeOH was added to a solution of

free-base porphyrin in CHCl₃, and the resulting mixture was refluxed

for 1-2 h. For Ni^IImetalation, a solution of free-base porphyrin in

toluene was refluxed with Ni(acac)₂for 5-6 h in the dark. For Cu^II

metalation, a saturated solution of Cu(OAc)₂in MeOH was added to a

solution of free-base porphyrin in CHCl₃, and the resulting mixture

was refluxed for 1-2 h in the dark. For Pd^IImetalation, a solution of

free-base porphyrin in CHCl₃was stirred in the presence of Pd(OAc)₂

powder at room temperature for 2 days in the dark. After the complete

metalation was confirmed by TLC and UV analyses, the mixture was

poured into water, and the porphyrin products were extracted with

CHCl₃. The organic layer was separated, and the combined extracts

were washed with water and brine and dried over anhydrous Na₂SO₄.

Finally, column chromatography over a silica gel column gave the

respective pure metalated porphyrins.

of inseparable chlorinated products, and possible existence of insepa-

rable syn-anti isomers. 7a: mp >300 °C; H NMR (CDCl₃) δ 1.52

1

(s, 36H, t-Bu), 1.54 (s, 36H, t-Bu), 7.74 (t, J ) 1.8 Hz, 2H, Ar-H),

7.77 (t, J ) 1.8 Hz, 2H, Ar-H), 7.80 (d, J ) 1.8 Hz, 4H, Ar-H), 7.92

(d, J ) 1.8 Hz, 4H, Ar-H), 8.41 (d, J ) 4.9 Hz, 2H, Por-â), 8.46 (d,

J ) 4.9 Hz, 2H, Por-â), 8.73 (d, J ) 4.9 Hz, 2H, Por-â), 8.98 (s, 2H,

Por-â), 9.08 (d, J ) 4.9 Hz, 2H, Por-â), 9.11 (d, J ) 4.9 Hz, 2H,

Por-â), and 9.42 (d, J ) 4.9 Hz, 2H, Por-â); HRMS (FAB) found m/z

1548.59(2), calcd for C₉₆H₉₈Cl₂N₈Ni₂, m/z 1548.5998; UV/visible

(CHCl₃) λ_max(ꢀ) ) 418 (55 400), 500 (46 200), and 756 (36 200) nm.

Meso-â Doubly Linked Ni^II-Diporphyrin 7a. A 50-mL round-

bottled flask was charged with a solution of 4d (10 mg, 13 µmol) in

CHCl₃(20 mL). The reaction vessel was covered with foil. BAHA (12

mg, 15 µmol) was added in one portion, and the resulting mixture was

stirred for 10 h at room temperature. The mixture was diluted with

water, and the organic layer was separated off, washed with water,

and dried over anhydrous MgSO₄. The products were initially separated

by preparative SEC with CHCl₃as an eluant. The first eluting fraction

was diporphyrin, and the second slowly eluting fraction was monomeric

porphyrins. The first fraction was further separated by flash chroma-

tography over a silica gel (Wakogel FC40) column (hexane/CH₂Cl₂

(95:5)) to give 5b (1 mg, 10%) as the first fraction and 7a (7 mg,

70%) as the second fraction.

Meso-â Doubly Linked Ni^II-Diporphyrin 7b. Porphyrin 4b (30

mg, 32 µmol) was oxidized with BAHA (32 mg, 39 µmol) in 20 mL

of CHCl₃for 12 h at room temperature. The products were separated

into diporphyrin and monoporphyrin fractions by SEC. The latter was

identified as a 1:1 mixture of meso-chlorinated and meso-brominated

porphyrins (4c, 9 mg, 28%). The former was further separated by flash

chromatography over a silica gel (Wakogel FC40) column (hexane/

CH₂Cl₂(95:5)) to give 5a (3 mg, 10%) as the first fraction and 7b (16

mg, 53%) as the second fraction. 7b: mp >300 °C; ¹H NMR (CDCl₃)

δ 1.45 (s, 36H, t-Bu), 1.51 (s, 36H, t-Bu), 1.54 (s, 36H, t-Bu), 7.67 (t,

J ) 1.8 Hz, 2H, Ar-H), 7.72 (t, J ) 1.8 Hz, 2H, Ar-H), 7.75 (t, J )

1.8 Hz, 2H, Ar-H), 7.79 (d, J ) 1.8 Hz, 4H, Ar-H), 7.84 (d, J ) 1.8

Hz, 4H, Ar-H), 7.96 (d, J ) 1.8 Hz, 4H, Ar-H), 8.37 (d, J ) 4.9 Hz,

2H, Por-â), 8.40 (s, 4H, 8.46, Por-â), 8.45 (d, J ) 4.9 Hz, 2H, Por-â),

8.77 (d, J ) 4.9 Hz, 2H, Por-â), 9.05 (s, 2H, Por-â), and 9.49 (d, J )

4.9 Hz, 2H, Por-â); HRMS (FAB) found m/z 1856.96(3), calcd for

C

₁₂₄H₁₄₀N₈Ni₂, m/z 1856.9908; UV/visible (CHCl₃) λ_max(ꢀ) ) 417

(55 100), 501 (47 600), 538 (43 900), and 756 (35 000) nm.

Meso-â Doubly Linked Cu^II-Diporphyrin 7c. Porphyrin 4e (12

mg, 13 µmol) was oxidized with BAHA (13 mg, 15 µmol) in 10 mL

of CHCl₃for 12 h at room temperature. The products were separated

into diporphyrin and monoporphyrin fractions by SEC. The latter was

identified as meso-chlorinated porphyrin 4f (3 mg, 24%). The former

was further separated by flash chromatography over a silica gel

(Wakogel FC40) column (hexane/CH₂Cl₂(95:5)) to give 5c (4 mg,

33%) as the first fraction and 7c (3 mg, 25%) as the second fraction.

7c: mp >300 °C; HRMS (FAB) found m/z 1866.99(2), calcd for

Porphyrin Syntheses. All the porphyrin monomers were prepared

according to the published methods.^15,33Meso-meso singly linked

diporphyrins were prepared by the metalation of the corresponding free-

base diporphyrins, which were prepared by demetalation of meso-

meso linked Zn^II-diporphyrins.¹⁵

Meso-â Doubly Linked Ni^II-Diporphyrin 7a. A 50-mL round-

bottom flask was charged with a solution of 4a (55 mg, 74 µmol) in

CHCl₃(20 mL). The reaction vessel was covered with foil. BAHA (73

mg, 89 µmol) was added in one portion, and the resulting mixture was

stirred for 10 h at room temperature. The mixture was diluted with

water, and the organic layer was separated off, washed with water,

and dried over anhydrous MgSO₄. The products were initially separated

by preparative SEC with CHCl₃as an eluant, giving porphyrin pentamer

C

₁₂₄H₁₄₀N₈Cu₂, m/z 1866.9993; UV/visible (CHCl₃) λ_max(ꢀ) ) 419

(130 600), 551 (99 300), and 781 (76 300) nm.

Meso-â Doubly Linked Pd^II-Diporphyrins 7d, 7e, and 7f.

Porphyrin 4g (10 mg, 10 µmol) was oxidized with BAHA (10 mg, 12

µmol) in 10 mL of CHCl₃for 12 h at room temperature. The products

were separated into diporphyrin and monoporphyrin fractions by SEC.

The former was further separated by flash chromatography over a silica

gel (Wakogel FC40) column (hexane/CH₂Cl₂(95:5)) to give 7f (2 mg,

19%) as the first fraction, 7e (2 mg, 20%) as the second fraction, and

7d (2 mg, 20%) as the third fraction. 7d: mp >205-208 °C; ¹H NMR

(CDCl₃) δ 1.50 (s, 36H, t-Bu), 1.56 (s, 36H, t-Bu), 1.60 (s, 36H, t-Bu),

7.73 (t, J ) 1.8 Hz, 2H, Ar-H), 7.79 (t, J ) 1.8 Hz, 2H, Ar-H), 7.83

(t, J ) 1.8 Hz, 2H, Ar-H), 7.92 (d, J ) 1.8 Hz, 4H, Ar-H), 8.00 (d, J

) 1.8 Hz, 4H, Ar-H), 8.14 (d, J ) 1.8 Hz, 4H, Ar-H), 8.47 (d, J ) 4.9

Hz, 4H, Por-â), 8.51 (d, J ) 4.9 Hz, 2H, Por-â), 8.58 (d, J ) 4.9 Hz,

2H, Por-â), 8.76 (d, J ) 4.9 Hz, 2H, Por-â), 9.31 (s, 2H, Por-â), and

9.73 (d, J ) 4.9 Hz, 2H, Por-â); HRMS (FAB) found m/z 1952,92(6),

calcd for C₁₂₄H₁₄₀N₈Pd₂, m/z 1952.9270; UV/visible (CHCl₃) λ_max(ꢀ)

) 420 (95 500), 544 (93 000), and 748 (61 500) nm. 7e: mp >215-

217 °C; ¹H NMR (CDCl₃) δ 1.46-1.56 (br, 36H, t-Bu), 1.49 (s, 36H,

t-Bu), 1.55 (s, 18H, t-Bu), 1.56 (s, 18H, t-Bu), 7.73 (m, J ) 1.8 H z,

(MALDI-TOF MS m/z ) 3709 and 3741, UV/visible (CHCl₃) λ_max

)

408, 559, 777, 899, and 1151 nm) as the first fraction, porphyrin

tetramer (MALDI-TOF MS m/z ) 2968, 3000 and 3032, UV/visible

(CHCl₃), λ_max) 407, 559, 776, 910, and 1151 nm) as the second

fraction, porphyrin trimer (MALDI-TOF MS m/z ) 2229, 2260 and

2295, UV/visible (CHCl₃) λ_max) 405, 555, 765, 899, and 1151 nm)

as the third fraction, and porphyrin dimer as the fourth fraction. The

final fraction was recrystallized with CHCl₃and MeOH to give 7a (7

mg, 11%). Further characterization of the higher oligomeric porphyrins

was abandoned due to their extremely poor solubilities, contamination

(33) (a) Wang, Q. M.; Bruce, D. W. Synlett 1995, 1267. (b) Littler, B.

J.; Ciringh, Y.; Lindsey, J. S. J. Org. Chem. 1999, 64, 2864.

Directly Fused Diporphyrins

J. Am. Chem. Soc., Vol. 123, No. 42, 2001 10319

4H, Ar-H), 7.75 (t, J ) 1.8 Hz, 1H, Ar-H), 7.79 (d, J ) 1.8 Hz, 2H,

Ar-H), 7.85-8.12 (br; 5H, Ar-H), 7.91 (d, J ) 1.8 Hz, 2H, Ar-H),

7.93 (d, J ) 1.8 Hz, 2H, Ar-H), 7.99 (s, 2H, Ar-H)), 8.44 (d, J ) 1.8

Hz, 1H, Por-â), 8.46 (s, 3H, Por-â), 8.47 (d, J ) 4.9 Hz, 1H, Por-â),

8.49 (d, J ) 4.9 Hz, 1H, Por-â), 8.52 (d, J ) 4.9 Hz, 1H, Por-â), 8.55

(d, J ) 4.9 Hz, 1H, Por-â), 8.56 (d, J ) 4.9 Hz, 1H, Por-â), 8.75 (d,

J ) 4.9 Hz, 1H, Por-â), 9.13 (s, 1H, Por-â), 9.64 (d, J ) 4.9 Hz, 1H,

Por-â), 9.65 (d, J ) 4.9 Hz, 1H, Por-â); HRMS (FAB) found m/z

1986.82(9) calcd for C₁₂₄H₁₃₉ClN₈Pd₂, m/z 1986.8881; UV/visible

mg, 10 µmol) in C₆F₆(20 mL). The reaction vessel was covered with

foil and then BAHA (25 mg, 30 µmol) was added in one portion. After

being stirred for 2 days at room temperature, the reaction mixture was

diluted with a mixture of methanol and THF. The solvent was removed

on a rotary evaporator, and the residue was precipitated by treatment

with a mixture of benzene and methanol. The product was finally

separated by flash chromatography on silica gel (Wakogel C-400).

Elution with hexane/CH₂Cl₂(95:5) gave recovered 5c (2 mg, 10%) as

the first fraction, monochlorinated 8b (3 mg, 16%) as the second

fraction, and 8a (14 mg, 74%) as the third fraction. 8a: mp >300 °C;

HRMS (FAB) found m/z 1865.01(5), calcd for C₁₂₄H₁₃₈N₈Cu₂, m/z

1864.9836; UV/visible (CHCl₃) λ_max(ꢀ) ) 411 (150 800), 576

(131 400), and 996 (35 200) nm. 8b: mp >300 °C; HRMS (FAB) found

m/z 1898.84(9), calcd for C₁₂₄H₁₃₇ClN₈Cu₂, m/z 1898.9447; UV/visible

(CHCl₃) λ_max) 409, 573, and 990 nm.

Meso-Meso â-â â-â Triply Linked Free-Base Diporphyrin 8c.

Triply linked Cu^II-diporphyrin 8a (13 mg, 7.0 µmol) was dissolved

in a mixture of CHCl₃and concentrated H₂SO₄. After being stirred for

30 min, the mixture was poured into water and the porphyrin products

were extracted with CHCl₃. The combined organic extracts were washed

with water, aqueous NaHCO₃, and brine and dried over Na₂SO₄.

Separation by silica gel column chromatography gave the corresponding

triply linked free-base diporphyrin 8c (9 mg, 74%). 8c: mp >300 °C;

¹H NMR (CDCl₃) δ 1.41 (s, 36H, t-Bu), 1.42 (s, 4H, NH), 1.45 (s,

72H, t-Bu), 7.36 (s, 4H, Por-â), 7.59 (t, J ) 1.8 Hz, 2H, Ar-H), 7.60

(d, J ) 4.9 Hz, 4H, Por-â), 7.62 (t, J ) 1.8 Hz, 4H, Ar-H), 7.63 (d, J

) 4.9 Hz, 4H, Por-â), 7.65 (d, J ) 4.9 Hz, 4H, Por-â), and 7.67 (d, J

) 1.8 Hz, 8H, Ar-H); HRMS (FAB) found m/z 1743.10(4), calcd for

1

(CHCl₃) λ_max) 423, 543, and 747 nm. 7f: mp >217-219 °C; H

NMR (CDCl₃) δ 1.43-1.55 (br, 72H, t-Bu), 1.48 (s, 36H, t-Bu), 7.66

(t, J ) 1.8 Hz, 2H, Ar-H), 7.72-7.74 (m, 4H, Ar-H), 7.70-8.00 (br,

12H, Ar-H), 8.35 (d, J ) 1.8 Hz, 2H, Ar-H), 8.40 (d, J ) 4.9 Hz, 2H,

Por-â), 8.47 (s, 4H, Por-â), 8.53 (d, J ) 4.9 Hz, 2H, Por-â), and 9.50

(d, J ) 4.9 Hz, 2H, Por-â); HRMS (FAB) found m/z 2020.92(7), calcd

for C₁₂₄H₁₃₈Cl₂N₈Pd₂, m/z 2020.8491; UV/visible (CHCl₃) λ_max) 426,

542, and 734 nm.

Meso-â Doubly Linked Free-Base Diporphyrin 7g. The meso-â

doubly linked Cu^II-diporphyrin 7c (30 mg, 16 µmol) was dissolved

in a mixture of CHCl₃, TFA, and 10% H₂SO₄. After being stirred for

30 min, the mixture was poured into water and the porphyrin products

were extracted with CHCl₃. The combined organic extract was washed

with water, aqueous NaHCO₃, and brine and dried over Na₂SO₄.

Separation by chromatography through an alumina column gave the

corresponding meso-â doubly linked free-base diporphyrin 7g (25 mg,

1

90%). 7g: mp >300 °C; H NMR (CDCl₃) δ 0.62 (s, 4H, NH), 1.50

(s, 36H, t-Bu), 1.56 (s, 36H, t-Bu), 1.61 (s, 36H, t-Bu), 7.74 (t, J )

1.8 Hz, 2H, Ar-H), 7.79 (t, J ) 1.8 Hz, 2H, Ar-H), 7.84 (t, J ) 1.8

Hz, 2H, Ar-H), 7.95 (d, J ) 1.8 Hz, 4H, Ar-H), 8.02 (d, J ) 1.8 Hz,

4H, Ar-H), 8.15 (d, J ) 1.8 Hz, 4H, Ar-H), 8.42 (d, J ) 4.9 Hz, 2H,

Por-â), 8.44 (d, J ) 4.9 Hz, 2H, Por-â), 8.47 (d, J ) 4.9 Hz, 2H,

Por-â), 8.53 (d, J ) 4.9 Hz, 2H, Por-â), 8.63 (d, J ) 4.9 Hz, 2H,

Por-â), 9.18 (s, 2H, Por-â), and 9.55 (d, J ) 4.9 Hz, 2H, Por-â); HRMS

(FAB) found m/z 1745.12(3), calcd for for C₁₂₄H₁₄₄N₈, m/z 1745.1514;

UV/visible (CHCl₃) λ_max(ꢀ) ) 425 (144 500), 501 (75 400), 546

(61 600), 616 (49 900), and 820 (67 800) nm.

C

₁₂₄H₁₄₂N₈, m/z 1743.1357; UV/visible (CHCl₃) λ_max(ꢀ) ) 410

(117 900), 567 (112 200), and 1086 (26 800) nm.

Meso-Meso â-â â-â Triply Linked Zn^II-Diporphyrin 8d.

Triply linked free-base diporphyrin 8c (10 mg, 5.7 µmol) was converted

to Zn^II-diporphyrin 8d (10 mg, 93%) by the standard method. 8d:

1

mp >300 °C; H NMR (CDCl₃) δ 1.41 (s, 36H, t-Bu), 1.45 (s, 72H,

t-Bu), 7.35 (s, 4H, Por-â), 7.59 (t, J ) 1.8 Hz, 2H, Ar-H), 7.61(t, J )

1.8 Hz, 4H, Ar-H), 7.63 (d, J ) 1.8 Hz, 4H, Ar-H), 7.67 (d, J ) 1.8

Hz, 8H, Ar-H), 7.70 (d, J ) 4.9 Hz, 4H, Por-â), and 7.75 (d, J ) 4.9

Hz, 4H, Por-â); HRMS (FAB) found m/z 1866.94(4), calcd for

C₁₂₄H₁₃₈N₈Zn₂, m/z 1866.9627; UV/visible (CHCl₃) λ_max(ꢀ) ) 419

(113 000), 582 (101 600), and 1068 (22 900) nm.

Meso-â Doubly Linked Zn^II-Diporphyrin 7h. The meso-â doubly

linked free-base diporphyrin 7g (42 mg, 24 µmol) was converted to

Zn^II-diporphyrin 7h (42 mg, 93%) by the standard method. 7h: mp

>300 °C; ¹H NMR (CDCl₃) δ 1.51 (s, 36H, t-Bu), 1.57 (s, 36H, t-Bu),

1.62 (s, 36H, t-Bu), 7.73 (t, J ) 1.8 Hz, 2H, Ar-H), 7.79 (t, J ) 1.8

Hz, 2H, Ar-H), 7.83 (t, J ) 1.8 Hz, 2H, Ar-H), 7.96 (d, J ) 1.8 Hz,

4H, Ar-H), 8.04 (d, J ) 1.8 Hz, 4H, Ar-H), 8.19 (d, J ) 1.8 Hz, 4H,

Ar-H), 8.56 (d, J ) 4.9 Hz, 4H, Por-â), 8.60 (d, J ) 4.9 Hz, 2H, Por-

â), 8.65 (d, J ) 4.9 Hz, 2H, Por-â), 8.77 (d, J ) 4.9 Hz, 2H, Por-â),

9.37 (s, 2H, Por-â), and 9.77 (d, J ) 4.9 Hz, 2H, Por-â); HRMS (FAB)

found m/z 1868.97(8), calcd for C₁₂₄H₁₄₀N₈Zn₂, m/z 1868.9784; UV/

visible (CHCl₃) λ_max(ꢀ) ) 424 (157 400), 496 (74 000), 562 (88 900),

and 798 (75 000) nm.

Meso-Meso â-â â-â Triply Linked Ni^II-Diporphyrin 8e and

Meso-Meso â-â Doubly Linked Ni^II-Diporphyrin 9a. The por-

phyrin 5a (10 mg, 5.4 µmol) was oxidized with BAHA (9 mg, 11 µmol)

in 20 mL of C₆F₆for 2 days at room temperature. The product was

separated by flash chromatography on silica gel (Wakogel C-400).

Elution with hexane/CH₂Cl₂(95:5) gave recovered 5a (3 mg, 30%) as

the first fraction, monochlorinated 9b (2 mg, 20%) as the second

fraction, monochlorinated 8l (2 mg, 20%) as the third fraction, 9a (1

mg, 10%) as the fourth fraction, and 8e (1 mg, 10%) as the fifth fraction.

Pd^II-Porphyrin Tetramer 7i. Meso-monobrominated meso-meso

linked diporphyrin 5d (5 mg, 3 µmol) was oxidized with BAHA (3

mg, 3.6 µmol) in 20 mL of CHCl₃for 24 h at room temperature. The

products were separated into tetraporphyrin and diporphyrin fractions

by SEC. The latter fraction was further separated by flash chromatog-

raphy over a silica gel (Wakogel FC40) column and was identified as

meso-brominated diporphyrin 5e (3 mg, 60%) and recovered 5d (1 mg,

20%). The former was further purified by flash chromatography to give

7i (1 mg, 20%) over a silica gel column (Wakogel FC40). 7i: mp >300

°C; ¹H NMR (CDCl₃) δ 1.43 (s, 36H, t-Bu), 1.47 (s, 72H, t-Bu), 1.48

(s, 36H, t-Bu), 7.66(d, J ) 4.9 Hz, 4H, Por-â), 8.51 (d, J ) 4.9 Hz,

2H, Por-â), 8.58 (d, J ) 4.9 Hz, 2H, Por-â), 7.70 (t, J ) 1.8 Hz, 2H,

Ar-H), 7.74 (t, J ) 1.8 Hz, 4H, Ar-H), 7.77 (t, J ) 1.8 Hz, 2H, Ar-H),

8.00 (d, J ) 1.8 Hz, 4H, Ar-H), 8.03 (d, J ) 1.8 Hz, 8H, Ar-H), 8.17

(d, J ) 1.8 Hz, 4H, Ar-H), 8.26 (d, J ) 4.9 Hz, 2H, Por-â), 8.34 (d,

J ) 4.9 Hz, 2H, Por-â), 8.64 (d, J ) 4.9 Hz, 4H, Por-â), 8.81 (d, J )

4.9 Hz, 2H, Por-â), 8.96 (d, J ) 4.9 Hz, 4H, Por-â), 9.42 (s, 2H, Por-

â), 9.74 (d, J ) 4.9 Hz, 4H, Por-â),and 9.83 (d, J ) 4.9 Hz, 2H, Por-

â); MALDI-TOF MS m/z 3319; calcd for C₁₉₂H₁₉₈Br₂N₁₆Pd₄, m/z 3315;

UV/visible (CHCl₃) λ_max) 419, 554, 752 and 1036 nm.

1

8e: mp >300 °C; H NMR (CDCl₃) δ 1.40 (s, 36H, t-Bu), 1.43 (s,

72H, t-Bu), 7.54 (s, 4H, Por-â), 7.57-7.60 (m, 18H, Ar-H), 7.73 (d, J

) 4.9 Hz, 4H, Por-â), and 7.77 (d, J ) 4.9 Hz, 4H, Por-â); HRMS

(FAB) found m/z 1854.91(3), calcd for C₁₂₄H₁₃₈N₈Ni₂, m/z 1854.9751;

UV/visible (CHCl₃) λ_max(ꢀ) ) 408 (76 900), 575 (82 500), and 933

(12 000) nm. 8l: mp >300 °C; ¹H NMR (CDCl₃) δ 1.39 (s, 18H, t-Bu),

1.40 (s, 18H, t-Bu), 1.41 (s, 18H, t-Bu), 1.42 (s, 18H, t-Bu), 1.43 (s,

36H, t-Bu), and complicated overlapping Por-â and Ar protons in the

range of 7.40-7.85 ppm; HRMS (FAB) found m/z 1888.87(4), calcd

for C₁₂₄H₁₃₇ClN₈Ni₂, m/z 1888.9362; UV/visible (CHCl₃) λ_max) 408,

1

575, and 932 nm. 9a: mp >300 °C; H NMR (CDCl₃) δ 1.33-1.43

(br, 108H, t-Bu), 7.43 (d, J ) 4.9 Hz, 2H, Por-â), 7.56 (t, J ) 1.8 Hz,

2H, Ar-H), 7.61 (t, J ) 1.8 Hz, 2H, Ar-H), 7.67 (t, J ) 1.8 Hz, 2H,

Ar-H), 8.42-8.44 (m, 8H, Por-â), 8.47 (d, J ) 4.9 Hz, 2H, Por-â),

and 8.77 (s, 2H, Por-â); HRMS (FAB) found m/z 1856.90(8), calcd

for C₁₂₄H₁₄₀N₈Ni₂, m/z 1856.9908; UV/visible (CHCl₃) λ_max(ꢀ) ) 410

1

(57 200), 490 (77 200), and 735 (40 300) nm. 9b: mp >300 °C; H

NMR (CDCl₃) δ 1.40-1.50 (br, 108H, t-Bu), 7.02 (t, J ) 1.8 Hz, 1H,

Ar-H), 7.47 (d, J ) 4.9 Hz, 2H, Por-â), 7.63 (m, 2H, Ar-H), 7.69 (d,

J ) 1.8 Hz, 2H, Ar-H), 7.70 (t, J ) 1.8 Hz, 1H, Ar-H), 7.71 (t, J )

1.8 Hz, 1H, Ar-H), 8.41 (t, J ) 1.8 Hz, 1H, Ar-H), 8.48-8.52 (m, 7H,

Por-â), 8.54 (d, J ) 4.9 Hz, 1H, Por-â), 8.56 (d, J ) 4.9 Hz, 1H,

Meso-Meso â-â â-â Triply Linked Cu^II-Diporphyrin 8a. A

50-mL round-bottomed flask was charged with a suspension of 5c (19

10320 J. Am. Chem. Soc., Vol. 123, No. 42, 2001

Tsuda et al.

Table 3. Crystal Data and Structure Refinements of 7b, 7c, 8g, 8j and 9a^a

ID code

chem formula

7b

₁₂₄H₁₄₀N₈Ni₂‚

2C₆H₁₄

2029.21

105

7c

8g

8j

9a

C

C₁₂₄H₁₄₀N₈Cu₂‚

C₁₀₈H₁₀₆Cu₂N₈‚

C₁₀₈H₁₀₆Zn₂N₈‚

C₆H₆‚2C₂H₆O

1720.76

296

C₁₂₄H₁₄₀N₈Ni₂

4C₆H₆

2C₆H₆

formula wt

temp, K

2179.19

296

1796.83

296

1856.99

296

wavelength, Å

cryst syst

space group

unit cell dimens

a, Å

b, Å

c, Å

0.710 69

triclinic

P-1

0.710 69

triclinic

P-1

0.7107

monoclinic

C2/c

0.7107

triclinic

P-1

0.710 69

triclinic

P-1

14.807(1)

15.293(1)

16.595(1)

68.256(4)

77.287(5)

75.368(5)

3344

1

1.09

3.33

1196

16.2904(5)

17.5885(3)

13.378(1)

95.848(5)

101.181(6)

94.798(5)

3719

1

0.97

3.31

1166

44.268(2)

17.791(6)

17.326(5)

90

98.365(2)

90

13500

6

1.21

17.428(7)

20.324(2)

8.566(2)

91.696(2)

96.988(4)

99.278(4)

2968

1

1.02

4.51

964

19.9822(9)

29.361(1)

12.8584(4)

99.425(1)

102.2310(4)

105.9260(6)

6889.3

2

0.90

3.14

1992

R, deg

â, deg

γ, deg

volume, Å³

Z

density (calcd), g/cm³

abs coeff, cm^-1

F(000)

5.26

5964

cryst size, mm³

2θ_max, deg

obsd reflctns

total reflctns

completeness to

θ ) 55.0°, %

absorpn corrn

refinement

method

0.20 × 0.30 × 0.25

0.50 × 0.30 × 0.1

0.30 × 0.30 × 0.05

0.30 × 0.20 × 0.1

0.40 × 0.20 × 0.10

52.7

55.0

8446

11968

100.0

7564

16058

100.0

3555

15223

100.0

4134

11695

100.0

10877

28184

100.0

empirical

full-matrix least-

squares on F²

8446/0/616

full-matrix least-

squares on F²

7564/0/604

full-matrix least-

squares on F²

3555/0/557

Full-matrix least-

squares on F²

4134/0/568

Full-matrix least-

squares on F²

10877/0/883

Data/restraints/

params

goodness of fit

on F²

5.17

0.86

2.42

3.19

0.97

final R indices

R₁) 0.093,

wR₂) 0.114

2.50 and -0.63

R₁) 0.098,

wR₂) 0.136

0.70 and -0.63

R₁) 0.096,

wR₂) 0.137

0.68 and -0.40

R₁) 0.098,

wR₂) 0.119

1.21 and -0.55

R₁) 0.123,

wR₂) 0.212

1.50 and -1.91

[I>3σ(I)]

largest diff peak

and hole, e/Å³

^aHydrogen atom treatment: Hydrogen atoms were generated by their idealized geometry and refined with a riding model with the exception of

the following:

Por-â), 8.61 (d, J ) 4.9 Hz, 1H, Por-â), and 9.35 (s, 1H, Por-â); HRMS

(FAB) found m/z 1890.95(9), calcd for C₁₂₄H₁₃₉ClN₈Ni₂, m/z 1890.9518;

UV/visible (CHCl₃) λ_max(ꢀ) ) 407 (60 800), 492 (74 000), and 728

(32 000) nm.

Meso-Meso â-â â-â Triply Linked Pd^II-Porphyrin 8f. The

triply linked free-base diporphyrin 8c (10 mg, 5.7 µmol) was converted

to 8f (9 mg, 80%) by the standard method. 8f: mp >300 °C; ¹H NMR

(CDCl₃) δ 1.43 (s, 36H, t-Bu), 1.47 (s, 72H, t-Bu), 7.36 (s, 4H, Por-

â), 7.59-7.67 (m, 10H, Ar-H), 7.69 (s, 4H, Por-â), 7.74 (d, J ) 1.8

Hz, 8H, Ar-H), and 7.83 (s, 4H, Por-â); HRMS (FAB) found m/z

1950.91(7), calcd for C₁₂₄H₁₃₈N₈Pd₂, m/z 1950.9114; UV/visible

(CHCl₃) λ_max(ꢀ) ) 408 (147 400), 569 (205 500), and 935 (31 900)

nm.

Monochlorinated Meso-Meso â-â â-â Triply Linked Dipor-

phyrin 8k. Triply linked Cu^II-diporphyrin 8b (10 mg, 5.3 µmol) was

dissolved in a mixture of CHCl₃and concentrated H₂SO₄. After being

stirred for 30 min, the mixture was poured into water and the porphyrin

products were extracted with CHCl₃. The combined organic extracts

were washed with water, aqueous NaHCO₃, and brine and dried over

Na₂SO₄. Separation by silica gel column chromatography gave the

corresponding triply linked free-base diporphyrin 8k (8 mg, 85%). The

structure of 8k was determined in X-ray crystallography (Supporting

1

Information 3). 8k: mp >300 °C; H NMR (CDCl₃) δ 1.41 (s, 18H,

t-Bu), 1.42 (s, 18H, t-Bu), 1.43 (s, 18H, t-Bu), 1.44 (s, 18H, t-Bu),

1.45 (s, 36H, t-Bu), and complicated overlapping Por-â and Ar protons

in the range of 7.45-7.75 ppm; HRMS (FAB) found m/z 1777.08(3),

calcd for C₁₂₄H₁₄₁ClN₈, m/z 1777.0968; UV/visible (CHCl₃) λ_max(ꢀ) )

408 (121 000), 567 (116 000), and 1081 (27 500) nm.

Meso-Meso â-â â-â Triply Linked Cu^II-Diporphyrin 8g. The

porphyrin 5f (16 mg, 10 µmol) was oxidized with BAHA (18 mg, 22

µmol) in 20 mL of C₆F₆for 2 days at room temperature. The product

was finally separated by flash chromatography on silica gel (Wakogel

C-400). Elution with hexane/CH₂Cl₂(95:5) gave recovered 5f (2 mg,

13%) as the first fraction, monochlorinated 8h (1.0 mg, 6%) as the

second fraction, and 8g (12.2 mg, 76%) as the third fraction. 8g: mp

>300 °C; HRMS (FAB) found m/z 1640.71(5), calcd for C₁₀₈H₁₀₆N₈-

Cu₂, m/z 1640.7332; UV/visible (CHCl₃) λ_max(ꢀ) ) 411 (161 800),

575 (136 100), and 994 (38 700) nm. The diporphyrin 8g was

Meso-Meso â-â â-â Triply Linked Zn^II-Diporphyrin 8j. The

triply linked Cu^II-diporphyrin 8g (5 mg, 3.1 µmol) was demetalated

by stirring in a mixture of CHCl₃, concentrated H₂SO₄for 30 min to

provide the corresponding free-base diporphyrin, from which Zn^II-

diporphyrin 8j was prepared by the standard method (4 mg, 80%). 8j:

1

mp >300 °C; H NMR (CDCl₃) δ 1.45 (s, 72H, t-Bu), 7.35 (s, 4H,

Por-â), 7.56 (d, J ) 4.9 Hz, 4H, Por-â), 7.62 (t, J ) 1.8 Hz, 4H, Ar-

H), 7.66 (d, J ) 1.8 Hz, 8H, Ar-H), 7.69 (s, 10H, Ar-H), and 7.80 (d,

J ) 4.9 Hz, 4H, Por-â); HRMS (FAB) found m/z 1642.67(3), calcd

for C₁₀₈H₁₀₆N₈Zn₂, m/z 1642.7123; UV/visible (CHCl₃) λ_max(ꢀ) ) 418

(10 800), 581 (91 000), and 1068 (19 700) nm.

1

demetalated to free-base diporphyrin 8i. 8i: mp >300 °C; H NMR

(CDCl₃) δ 1.45 (s, 72H, t-Bu), 1.51(s, 4H, NH), 7.55 (d, J ) 4.9 Hz,

4H, Por-â), 7.56 (s, 4H, Por-â), 7.58 (s, 10H, Ar-H), 7.62 (t, J ) 1.8

Hz, 4H, Ar-H), 7.64 (d, J ) 1.8 Hz, 8H, Ar-H), and 7.79 (d, J ) 4.9

Hz, 4H, Por-â). HRMS (FAB) found m/z 1518.91(7), calcd for

Crystal Structure Data for 7b, 7c, 8 g, 8j, and 9a. Data of the

crystal structure of 7b‚2C₆H₁₄: C₁₃₆H₁₆₈Ni₂N₈, M_W) 2029, crystal from

CHCl₃/C₆H₁₄, crystal dimensions 0.2 × 0.3 × 0.25 mm, space group

P-1, a ) 14.807 (1) Å, b ) 15.293 (1) Å, c ) 16.595 (1) Å, R )

68.256 (4)°, â ) 77.287 (5)°, γ ) 75.368 (5)°, V ) 3344 Å³. Z ) 1,

F_calc) 1.09 g cm^-3; µ_Mo) 3.23 cm^-1; θ_max) 27.5°. For 11 968

C₁₀₈H₁₁₀N₈, m/z 1518.8853; UV/visible (CHCl₃) λ_max(ꢀ) ) 410 (119 300),

567 (114 400), and 1086 (26 100) nm. 8h: mp >300 °C; HRMS (FAB)

found m/z 1674.68(2), calcd for C₁₀₈H₁₀₅ClN₈Cu₂, m/z 1674.6943; UV/

visible (CHCl₃) λ_max(ꢀ) ) 409, 573, and 993 nm.

Directly Fused Diporphyrins

J. Am. Chem. Soc., Vol. 123, No. 42, 2001 10321

reflections measured; R_l) 0.093 for 8446 data [I > 3σ(I)], wR₂)

0.114 for all measured data. (See Table 3.)

Diffraction data of 7b were collected on a Mac Science DIP-2020K

imaging plate system diffractometer (105 K, Mo KR radiation λ )

0.7107 Å). Whereas, diffraction data for 7c, 8g, 8j, and 9a were

collected on a Rigaku-Raxis imaging plate system diffractometer (296

K, Mo KR radiation λ ) 0.7107 Å).

The structures of all the compounds were solved by direct methods

and refined by F²with all observed reflections. All non-hydrogen atoms

were refined anisotropically, and hydrogen atoms were added to

calculated positions. The programs used were structure determination

by SIR92 and refined by teXane for Windows.

Data of the crystal structure of 7c‚4C₆H₆: C₁₄₈H₁₆₄Ni₂N₈Cu₂, M_W

) 2182, crystal from C₂H₅OH/C₆H₆, crystal dimensions 0.5 × 0.3 ×

0.1 mm, space group P-1, a ) 16.2904 (5) Å, b ) 17.5885(3) Å, c )

13.378(1) Å, R ) 95.848(5)°, â ) 101.181(6)°, γ ) 94.798(2)°, V )

3719.7(3) Å³. Z ) 1, F_calc) 0.974 g cm^-3; µ_Mo) 3.31 cm^-1; θ_max

)

27.5°. For 16 058 reflections measured; R_l) 0.098 for 7564 data [I >

3σ(I)], wR₂) 0.136 for all measured data.

Data of the crystal structure of 8g‚2C₆H₆: C₁₂₀H₁₁₈Cu₂N₈, M_W

)

1796, crystal from C₆H₆/C₂H₅OH, crystal dimensions 0.3 × 0.3 × 0.1

mm, space group C2/c, a ) 44.268 (2) Å, b ) 17.791 (6) Å, c )

17.326 (5) Å, R ) 90°, â ) 98.365 (2)°, γ ) 90°, V ) 13500 Å³. Z )

6, F_calc) 1.21 g cm^-3; µ_Mo) 5.26 cm^-1; θ_max) 27.5°. For 15 223

reflections measured; R_l) 0.096 for 3555 data [I > 3σ(I)], wR₂)

0.137 for all measured data.

Data of the crystal structure of 8j‚C₆H₆‚2C₂H₆O: C₁₁₈H₁₂₄N₈O₂Zn₂,

M_W) 1817, crystal from C₆H₆/C₂H₅OH, crystal dimensions 0.3 × 0.3

× 0.1 mm, space group P-1, a ) 17.428 (7) Å, b ) 20.324 (2) Å, c )

8.566 (2) Å, R ) 91.696 (2)°, â ) 96.988 (4)°, γ ) 99.278 (4)°, V )

2968 Å³. Z ) 1, F_calc) 1.02 gcm^-3; µ_Mo) 4.51 cm^-1; θ_max) 27.5°.

For 11 695 reflections measured; R_l) 0.098 for 4134 data [I > 3σ

(I)], wR₂) 0.119 for all measured data.

Acknowledgment. This work was supported by Grant-in-

Aids for Scientific Research from the Ministry of Education,

Science, Sports and Culture of Japan (11223205, 12440196, and

12874076) and by CREST (Core Research for Evolutional

Science and Technology) of Japan Science and Technology

Corporation (JST). A.T. thanks JSPS for a Research Fellowship

for Young Scientists. We thank Prof. M. Inouye (Toyama

Medical and Pharmaceutical University) for his help in mea-

surements of the CD spectra.

Supporting Information Available: Preliminary X-ray

structures of 7f and 8k, absorption spectrum of 7i, racemization

experimental data on 9a, and X-ray structural details on 7b,

7c, 8g, 8j, and 9a. This material is available free of charge via

the Internet at http://pubs.acs.org. See any current masthead page

for ordering information and Web access instructions.

Data of the crystal structure of 9a: C₁₂₄H₁₄₀Ni₂N₈, M_W) 1857,

crystal from C₇H₈/C₂H₆O, crystal dimensions 0.4 × 0.2 × 0.1 mm,

space group P-1, a ) 19.9822 (9) Å, b ) 29.361 (1) Å, c ) 12.8584

(4) Å, R ) 99.425 (1)°, â ) 102.2310 (4)°, γ ) 105.9260 (6)°, V )

6889.3 Å³. Z ) 2, F_calc) 0.940 g cm^-3; µ_Mo) 3.16 cm^-1; θ_max

)

27.5°. For 28 184 reflections measured; R_l) 0.123 for 10877 data [I

> 3σ(I)], wR₂) 0.212 for all measured data.

JA0110933

Article Doi

Syntheses, structural characterizations, and optical and electrochemical properties of directly fused diporphyrins

DOI: 10.1021/ja0110933

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Article abstract of DOI:10.1021/ja0110933

Full text of DOI:10.1021/ja0110933

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