Structure-property relationships for self-assembled zinc chlorin light-harvesting dye aggregates

FULL PAPER

DOI: 10.1002/chem.200800764

Structure–Property Relationships for Self-Assembled Zinc Chlorin Light-

Harvesting Dye Aggregates

Valerie Huber, Sanchita Sengupta, and Frank Würthner*^[a]

Abstract: A series of zinc 3¹-hydroxy-

methyl chlorins 10a–e and zinc 3¹-hy-

droxyethyl chlorins 17 with varied

structural features were synthesized by

modifying naturally occurring chloro-

phyll a. Solvent-, temperature-, and

concentration-dependent UV/Vis and

CD spectroscopic methods as well as

microscopic investigations were per-

formed to explore the importance of

particular functional groups and steric

effects on the self-assembly behavior of

these zinc chlorins. Semisynthetic zinc

chlorins 10a–e possess the three func-

tional units relevant for self-assembly

found in their natural bacteriochloro-

phyll (BChl) counterparts, namely, the

3¹-OH group, a central metal ion, and

the 13¹C=O moiety along the Q_yaxis,

and they contain various 17²-substitu-

ents. Depending on whether the zinc

chlorins have 17²-hydrophobic or hy-

drophilic side chains, they self-assemble

in nonpolar organic solvents or in

aqueous media, respectively. Zinc

chlorins possessing at least two long

side chains provide soluble self-aggre-

gates that are stable in solution for a

prolonged time, thus facilitating eluci-

dation of their properties by optical

spectroscopy. The morphology of the

zinc chlorin aggregates was elucidated

by atomic force microscopy (AFM)

studies, revealing well-defined nano-

scale rod structures for zinc chlorin

10b with a height of about 6 nm. It is

worth noting that this size is in good

accordance with a tubular arrangement

of the dyes similar to that observed in

their natural BChl counterparts in the

light-harvesting chlorosomes of green

bacteria. Furthermore, for the epimeric

3¹-hydroxyethyl zinc chlorins 17 with

hydrophobic side chains, the influence

of the chirality center at the 3¹-position

on the aggregation behavior was stud-

ied in detail by UV/Vis and CD spec-

troscopy. Unlike zinc chlorins 10, the

3¹-hydroxyethyl zinc chlorins 17

formed only small oligomers and not

higher rod aggregate structures, which

can be attributed to the steric effect

imposed by the additional methyl

group at the 3¹-position.

Keywords: aggregation · dyes/pig-

ments · light harvesting · self-assem-

bly · zinc chlorins

Introduction

the chlorosomes of green phototrophic bacteria (e.g., Chlo-

roflexus aurantiacus), which are constructed from the rod-

AHCTREUNG

Progressive global warming caused mainly by burning fossil

type self-assemblies of bacteriochlorophylls (BChls) and are

enveloped in a lipid monolayer.^[3]In contrast with other bac-

terial and plant antenna systems, the chlorosomes are pro-

tein-free and their structure and function rely on the self-as-

sembly of BChls c, d, and e, controlled and determined only

by pigment–pigment interactions instead of protein–pigment

interactions.^[4]The construction of protein-free pigment ag-

gregates allows the implementation of close packing of the

units and facilitates very efficient light harvesting.^[5]The effi-

cient light harvesting by chlorosomal antennae enables such

organisms to inhabit depths of up to 2300 m under the water

surface where light is extremely scarce.^[6]The intriguing ar-

chitectural principle for the supramolecular organization of

dyes in chlorosomes can be mimicked by using semisynthet-

ic molecules that are preprogrammed for self-assembly.

Thus, for the better understanding of chlorosomal light-har-

vesting systems, Tamiaki et al. developed zinc chlorin 2,

fuels is one of the major problems of the present time.^[1]

A

way out from this detrimental problem is the utilization of

renewable energy resources, particularly solar light, in a sus-

tainable way.^[2]For the exploration of this possibility, natural

light-harvesting (LH) systems can serve as archetypes be-

cause nature satisfies its whole energy demand by using sun-

light with the help of LH systems. A most fascinating exam-

ple of such natural light-harvesting systems can be found in

[a] Dr. V. Huber, S. Sengupta, Prof. Dr. F. Würthner

Universität Würzburg

Institut für Organische Chemie

Am Hubland, 97074 Würzburg (Germany)

Fax : (+49)931-888-4756

E-mail: wuerthner@chemie.uni-wuerzburg.de

Supporting information for this article is available on the WWW

under http://dx.doi.org/10.1002/chem.200800764.

Chem. Eur. J. 2008, 14, 7791 – 7807

ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

7791

which is a model compound for the naturally occurring

BChl c 1 (see Scheme 1).^[7a]Like BChls, zinc chlorin 2 forms

extended dye aggregates in nonpolar solvents, such as n-

thesized zinc chlorins 10a–c, which have one to three dode-

cyl chains at the 17²position,^[14]as well as zinc chlorins

10d,e, which contain hydrophilic polyethylene glycol chains

(see Scheme 2), and thoroughly

investigated their self-aggrega-

tion properties in nonpolar or-

ganic solvents and aqueous

media, respectively.

Bacteriochlorophylls c, d, and

e in the chlorosomes of green

bacteria occur as diastereo-

meric mixtures, to be more spe-

cific, these are epimers because

they only vary in the chirality

Scheme 1. Structures of naturally occurring bacteriochlorophyll c 1 and zinc chlorin model compound 2, and a at the 3¹carbon center and the

model showing the three noncovalent interactions involved in the formation of supramolecular rod aggregates

(right).

configurations at positions 17

and 18 are fixed.^[15a]Depending

on the type of bacteria and the

hexane or n-heptane.^[7b,c]Intensive investigations of dye ag-

gregates of BChl c 1 and zinc chlorin 2 revealed almost

identical spectral and photophysical properties compared to

those of the natural chlorosomal self-assemblies, which con-

firmed the suitability of zinc chlorin dyes for the imitation

of chlorosomal antennae systems.^[8]The major advantage of

the zinc chlorin model compounds over the native bacterio-

chlorophylls lies in their easy semisynthetic accessibility

from Chl a and their higher chemical stability.

Although the self-aggregation of BChl c, d, and e is well

known, as mentioned above, the exact structure of chloroso-

mal aggregates has been a matter of controversial discussion

for a long time.^[3i,7a,9]However, the common feature for all

of the proposed structural models is the fact that the forma-

tion of the aggregates of BChl c is achieved by self-assembly

that involves three noncovalent interactions (Scheme 1,

right):^[7a,10]1) p–p interactions between the tetrapyrrole

rings; 2) coordination between the central metal ion and the

oxygen atom of the 3¹-hydroxy group, which creates a slip-

ped arrangement of chromophores in the p stacks; and

3) hydrogen bonding between the 3¹-hydroxy and 13¹-keto

groups, which assemble the chromophores into rod-shaped

supramolecular structures. These interactions promote exci-

tonic coupling among the most closely packed and well-or-

dered molecules resulting in excitonically coupled J-aggre-

gates.^[11]

Although the in vitro aggregates of 1 and 2 possess similar

spectral properties to naturally occurring chlorosomal aggre-

gates, they are of poor solubility in nonpolar solvents and

thus prone to agglomeration,^[12]consequently, only restricted

information from spectroscopic investigations can be de-

duced. Balaban and co-workers have recently addressed this

problem by preparing related porphyrin assemblies and they

obtained microcrystalline structures that were characterized

by X-ray diffraction and electron microscopy,^[13]whereas we

have focused our work on zinc chlorin model compounds

that afford well-defined self-assembled aggregates with good

solubility and lasting stability in solution to facilitate spec-

troscopic and microscopic investigations. Thus, we have syn-

light conditions under which they prevail, the epimeric ratio

varies. Generally, the proportion of the R epimer outweighs

that of the S epimer, for example, in Chloroflexus aurantia-

cus the proportion of the R epimer is about 67%, whereas

in Chlorobium tepidium it is 85–90%.^[15]The physiological

relevance of 3¹S and 3¹R configured bacteriochlorophyll in

chlorosomes is still unclear.^[15a,16]Previous studies indicated

that the aggregate structures of the 3¹S and 3¹R stereoiso-

mers of the bacteriochlorophylls differ in size, and for the

construction of chlorosomal light-harvesting systems both

epimers are needed.^[15a]In contrast to the natural bacterio-

chlorophyll, zinc chlorin model compound 2 lacks the

methyl group at the 3¹position. Thus, to have a better un-

derstanding of the influence of this additional chirality

center on self-assembly, we have synthesized epimeric zinc

chlorins (3¹S)-17 and (3¹R)-17 with two dodecyl alkyl chains

analogous to 10b at the 17²position to achieve enhanced

solubility in nonpolar solvents such as n-hexane or n-hep-

tane. The self-assembly of the epimerically pure zinc chlor-

ins (3¹S)-17 and (3¹R)-17, and mixtures with different per-

centages of S and R epimers have been studied in nonpolar

solvents by UV/Vis and CD spectroscopies. Herein, we pro-

vide the synthetic and characterization details of zinc chlor-

ins 10a–e and 17 and report in-depth investigations of self-

assembly of these dyes into soluble biomimetic light-harvest-

ing aggregates that aim to contribute to the structural eluci-

dation of chlorosomal antennae and to a better understand-

ing of the structure–property relationships of these highly

efficient light-harvesting systems.

Results and Discussion

Synthesis of zinc chlorins 10a–c with 17²-dodecyl chainsand

10d,e with 17²-oligoethylene glycol chains: Zinc chlorins

10a–e were synthesized from Chl a, which was extracted

from cyanobacteria Spirulina platensis^[17]according to the

route outlined in Scheme 2. Chl a was converted into pheo-

phorbide a according to a literature procedure,^[17]and the

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Self-assembled Zinc Chlorins

FULL PAPER

Scheme 2. Synthesis of zinc chlorins 10 a–e: a) Collidin, 1708C, 6 h; b) concd H₂SO₄, MeOH, RT, 12h; c) concd HCl, RT, 6 h; d) appropriate benzyl alco-

hol 11a–e, DCC, DMAP, DPTS, Hünigs base, CH₂Cl₂, RT, 3–4 h; e) OsO₄, NaIO₄, AcOH, THF/H₂O, RT, 5 h; f) BH₃(tBu)NH₂, THF, RT, 2–4 h;

g) Zn(OAc)₂, MeOH, THF, RT, 3 h.

AHCTREUNG

ACHTREUNG

latter was further hydrolyzed with concentrated hydrochlo-

ric acid in a yield of 77%. This was followed by esterifica-

tion of 6 with the respective dodecyloxybenzyl alcohols

11a–c and in the case of 6a also with tetraethyleneoxyben-

zyl alcohols 11d,e in the presence of dicyclohexylcarbodi-

a yield of 68–83%. Finally, the metalation step with metha-

nolic zinc acetate in tetrahydrofuran (THF) afforded the

zinc chlorins 10a–e as turquoise green solids with yields of

56–92%. The intermediates and final products were purified

by silica gel column chromatography, and subsequently by

HPLC and characterized by ¹H NMR spectroscopy and

high-resolution mass spectrometry (HRMS).

ACHTREUNG

Hünigs base (N-ethyldiisopropylamine) in dry dichlorome-

thane,^[18]leading to the corresponding esters 7a–c in yields

of 51–76% and 8d,e in yields of 57–63%. Subsequently, the

3¹-vinyl group of 7a–c was transformed into the correspond-

ing diol with osmium tetroxide, followed by oxidative cleav-

age by sodium periodate to afford the respective formyl de-

rivatives 8a–c in yields of 65–98%. Selective reduction of

the formyl groups of 8a–e to the corresponding alcohols 9a–

e was carried out with borane-tert-butylamine complex with

Synthesis of epimeric zinc chlorins with 3¹R and 3¹S configu-

rations: The synthetic route to the epimeric zinc chlorins 17

is shown in Scheme 3. Starting from Pheo a (5), 3¹-hydroxy-

ethyl Pheo a 12 was synthesized in two steps according to a

known literature method.^[16a,19]The 3¹-vinyl group of the

Pheo a was subjected to bromination with 38% HBr in

acetic acid, followed by substitution of the bromine atom

with a hydroxyl group. In the subsequent step, the hydroxy

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F. Würthner et al.

Scheme 3. Synthesis of zinc chlorins (3¹R/S)-17: a) 38% HBr/AcOH, RT, 12h; b) MeOH, concd HCl, RT, 0.5 h; c) tetrapropylammonium perruthenate

(TPAP), N-methylmorpholine-N-oxide (NMO), CH₂Cl₂, RT, 2h; d) concd HCl, RT, 3 h; e) 11b, DCC, DMAP, DPTS, Hünigs base, CH₂Cl₂, RT, 2.5 h; f)

NaBH₄, EtOH, THF, RT, 1.5 h; g) separation of the epimers by HPLC with a chiral column (reprosil 100 chiral-NR) with n-hexane/CH₂Cl₂(1:1) as

eluent; h) Zn(OAc)₂, MeOH, THF, 1 h.

G

group was oxidized into the corresponding ketone 13 with

tetrapropylammonium perruthenate and N-methylmorpho-

line-N-oxide in a yield of 84%.^[20]Next, the 17²-propionate

ester was hydrolyzed into the corresponding carboxylic acid

with concentrated HCl, and the resulting acid was esterified

with 3,5-bis(dodecyloxy)benzyl alcohol 11b as described

before to afford the corresponding ester 14 in a yield of

31%. The 3¹R/S diastereomers of hydroxyethyl derivative 15

were obtained by the reduction of ketone compound 14

with sodium borohydride in dry ethanol in a yield of 67%.

The diastereomers were separated by semipreparative

HPLC using a chiral column in normal phase, and the inte-

gration of the HPLC chromatogram showed >99 and

>95% diastereomeric purity for 3¹S-16 and 3¹R-16, respec-

tively. The separation of the diastereomers was followed by

the metalation of 3¹R-16 and 3¹S-16 with a saturated solu-

tion of methanolic zinc acetate in THF as the solvent to

afford the diastereomeric zinc chlorins 3¹R-17 and 3¹S-17,

which were again purified by semipreparative HPLC using a

chiral column in normal phase. The intermediate products

12–15 were purified by HPLC with a reverse-phase column

and characterized by ¹H NMR spectroscopy and HRMS.

The configurations of 3¹R-17 and 3¹S-17 were assigned by

comparing the HPLC retention times and significant

¹H NMR chemical shifts of these diastereomers with those

of similar compounds reported in the literature (for details

see the Supporting Information).^{[15b,16a,c,21]}

Optical properties of zinc chlorins 10a–c and spectroscopic

studies of their aggregation: The optical properties of mono-

mers and aggregates of zinc chlorins 10a–c have been char-

acterized by UV/Vis and CD spectroscopies. For this pur-

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Self-assembled Zinc Chlorins

FULL PAPER

pose, monomer stock solutions of the respective zinc chlorin

were prepared in THF and subsequently added to nonpolar

solvents, such as n-hexane or n-heptane, to prepare aggre-

gate solutions containing 1 vol% THF.

tion from the solution. Unlike the known literature model

compound 2, zinc chlorins 10b and 10c with two and three

alkyl chains, respectively, form soluble aggregates that are

stable for a long time in nonpolar solvents, which is evident

by the almost unchanged absorption over two days (Fig-

ure S3 in the Supporting Information). In contrast, aggre-

gates of 10a show uncontrolled agglomeration over the

course of a few hours, which leads to a decrease in absorp-

tion of the Q_yaggregate band and finally to precipitation.

Based on these observations, it can be concluded that at

least two alkyl chains at the 17²position are necessary for

properly soluble and persistent aggregates of zinc chlorins in

nonpolar solvents.

For further characterization of the self-assembly of chlor-

ins 10b and 10c, the aggregation process was investigated

by temperature-dependent UV/Vis spectroscopy in the tem-

perature range between 15 and 958C. For this purpose, the

zinc chlorins were dissolved in di-n-butyl ether (20%), fol-

lowed by the addition of the nonpolar solvent n-heptane

(80%) to initiate self-assembly. Prior to each measurement,

the solutions were allowed to equilibrate for 1.5 h at the

measuring temperature to ensure a stationary state. As

shown in Figure 2a for zinc chlorin 10c, with increasing tem-

perature the Q_yband of the aggregates at 741–742nm de-

creases and the monomer band at 648 nm increases. These

spectral changes reveal that the aggregates dissociate at

higher temperature. Upon cooling to the initial temperature

of 158C, the Q_yband was completely recovered, which is in-

dicative of the reversibility of the aggregation process. Simi-

lar observations were made for dye 10b^[14](Figure S4 and S5

in the Supporting Information). Another remarkable feature

of both dyes is that no precipitation was observed upon

heating and cooling processes, proving the thermodynamic

stability of the aggregates unlike many other self-organized

dye aggregates^[22]and model compound 2.

The formation of aggregates can be visually observed by

the instantaneous color change of turquoise-blue monomer

solutions to a pale-green color in the aggregate solutions

(Figure S2in the Supporting Information). Compared with

the absorption bands of the monomeric zinc chlorins, aggre-

gates of these dyes show redshifted absorption maxima due

to J-type excitonic coupling between the S₀-S₁transition

dipole moments of the chlorin Q_ybands.^[11]Therefore, the

aggregation properties of zinc chlorin dyes can be conven-

iently assessed by UV/Vis spectroscopy as the aggregate Q_y

band shows a large bathochromic shift (about 100 nm), com-

pared to that of the corresponding monomers. As can be

seen in Figure 1, the maxima of the Q_yabsorption bands of

Although, the aggregates of 10b and 10c in solution show

similar properties, they can be distinguished by their distinct

melting temperatures. The plot of normalized absorption at

741 nm of the Q_yband of the aggregates versus temperature

provides sigmoidal curves (Figure 2b for 10c), which can be

approximated with the Boltzmann function [Eq. (1)],^[23]

Figure 1. UV/Vis spectra of the zinc chlorin monomer 10a–c in THF

(l_Qy=647 nm) and their aggregates (l_Qy=740–743 nm) in n-heptane/THF

(100:1) at RT (c for 10a=1.210^À5m (b); c for 10b=1.610^À5

m

(g); c for 10c=2.810^À5m (d). Thin lines refer to the monomers

whereas thick lines are for the aggregates. Fluorescence emission spec-

trum (l_ex=690 nm) of the aggregates of 10b (c) in n-heptane/THF

100:1 is also shown; I_fl=fluorescence intensity.

A_aggÀA_mon

the monomers of 10a–c in THF appear at 648 nm, whereas

for the corresponding aggregates in n-heptane/THF (100:1)

the Q_y-band maxima occur at 741–743 nm for this series of

compounds. Owing to precipitation from the aggregate solu-

tion, the absorption coefficient (e) for 10a aggregates ap-

pears to be too small.

A ¼

þA_mon

ð1Þ

1þe^TÀT

m

DT

in which A is the total absorption and can be expressed as a

linear combination of absorption for the monomer and ag-

gregates [Eq. (2)].

The fluorescence emission spectrum of 10b aggregates re-

veals a small Stokes shift of about 1 nm. The bathochromic

shift of the Q_yband and the small Stokes shift are character-

istic for the formation of J-aggregates.^[11]

A ¼ A_agga_aggþA_monð1Àa_agg

Þ

ð2Þ

The long-term stability of the aggregates of 10a–c was in-

vestigated by recording time-dependent UV/Vis spectra. A

decrease in the absorbance of the aggregate solutions over a

period of two days at room temperature indicates precipita-

The term a_aggin Equation (2) is the normalized fraction

of aggregated molecules, which can be related to the melting

temperature T_m[Eq. (3)] at which a=0.5 and DT are related

to the width of the curve.

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F. Würthner et al.

737 nm and the monomer band at 650 nm increased con-

comitantly. In addition to that, the maximum of the Q_yband

shifted from 737 to 723 nm upon increasing the dichlorome-

thane content, which suggested that the p–p interactions are

disturbed by the solvent molecules that strongly interact

with p systems, leading to dissociation of the aggregates

(Figure S7 in the Supporting Information).

Further insight into the aggregation process for zinc chlor-

ins 10b and 10c was obtained by temperature-dependent

CD spectroscopy between 15 and 958C in n-heptane/di-n-

butyl ether (4:1). The CD spectra (Figure 3) show a bisig-

Figure 2. a) Temperature-dependent UV/Vis spectra of 10c in n-heptane/

di-n-butyl ether (4:1) at a concentration of 9.610^À6m. The initial tem-

perature of 158C was increased successively in 108C steps up to 958C

and at each temperature the solution was allowed to equilibrate prior to

measurement; arrows indicate changes upon increasing temperature.

b) Data points for the decrease of normalized absorption of the Q_yaggre-

gate band at 741 nm at three different concentrations of solvents with in-

creasing temperature and fitted curves with the Boltzmann function. The

Figure 3. Temperature-dependent CD spectra of a) 10b in n-heptane/di-

n-butyl ether (4:1) at a concentration of 1.110^À5m and b) 10c in n-hep-

tane/di-n-butyl ether (4:1) at a concentration of 3.110^À6m. The initial

temperature of 158C was increased successively in 108C steps up to 958C

and at each temperature the solution was allowed to equilibrate prior to

measurement; arrows indicate changes upon increasing temperature. The

anisotropy factor calculated at 158C for 10b is g₇₂₆=2.5110^À3and for

melting temperatures were obtained from fitted curves and are 748C at

À5

c=9.610^À5m ( ), 558C at c=5.810 m ( ), and 448C at c=2.9

~

*

10^À5m ( ). The UV/Vis spectra at concentrations of 5.810^À5m and 2.9

&

10^À5m are shown in Figure S6 in the Supporting Information.

10c is g₇₃₀=2.4610^À3

.

1

a_agg

¼

ð3Þ

1þe^TÀT

m

DT

Curve fitting with the Boltzmann function provided a con-

centration-dependent melting temperature in the range of

44 to 748C for the zinc chlorin 10c (Figure 2b), whereas for

10b melting temperatures of 60 and 748C at concentrations

of 3.110^À6and 1.110^À5m, respectively, were observed

(Figure S4, inset in the Supporting Information). A compari-

son of the melting temperatures of zinc chlorins 10b and

10c (60 and 448C, respectively) at very similar concentra-

tions (310^À6m) of solutions reveals a higher thermodynam-

ic stability for the aggregates of 10b.

nate signal in the region of Q_yaggregate band as a result of

an induced CD effect through excitonic coupling of the tran-

sition dipole moments^[24]of chiral zinc chlorins. With in-

creasing temperature the intensity of the CD signals de-

creases, which indicates the dissociation of aggregates. The

observed exciton couplet reversibly arises (aggregate forma-

tion) and disappears (aggregate dissociation) upon changes

to the temperature, validating the reversibility of the self-as-

sembly process, hereby the signals do not undergo any time-

dependent change at a particular temperature.

The dissociation of aggregates takes place not only at in-

creasing temperature, but also in the presence of coordinat-

ing or polar solvents. Thus, the addition of 20–30% di-

chloromethane to the aggregate solution of 10c in n-heptane

led to a pronounced decrease of the aggregate band at

Surprisingly, different CD exciton couplets of isolated

chlorosomes and in vitro BChl c aggregates were reported

in the literature.^[17a,25]Griebenow and co-workers have clas-

sified the observed CD spectra into three different types: in

type I the sign of the CD curve changes from positive at

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Self-assembled Zinc Chlorins

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smaller wavelengths to negative at longer wavelengths

A

so-called mixed type [À/+/À].^[26a]In a theoretical approach

by the groups of Holzwarth and Knoester, it has been pro-

posed that different types of CD spectra are the result of a

“size effect”.^[26b,c]According to Holzwarth et al., type II con-

verts to a mixed type at an aggregate length of over 30 mol-

ecules of BChl c.^[26b]Interestingly, zinc chlorins 10b,c show

type I CD spectra and exhibit exciton couplets, which have

close spectral resemblance to that of zinc chlorin model

compound 2.

Spectroscopic properties of the zinc chlorins 10d and 10e:

The aggregation studies for zinc chlorins 10d,e were per-

formed in aqueous media because these dyes possess 17²-hy-

drophilic side chains. For this purpose, monomer stock solu-

tions of 10d,e were prepared in THF and subsequently

added to ultrapure water Milli-Q to prepare aggregate solu-

tions in water containing 1 vol% THF. The formation of ag-

gregates of 10d was evident by an immediate color change

from turquoise-blue monomer solutions to the pale-green

color of aggregate solutions and the redshift of the Q_yaggre-

gate band from 648 (in THF) to 733 nm (in water/THF

100:1) (Figure 4a), indicating self-assembly of this dye. The

Q_yaggregate band showed a further bathochromic shift of

up to 747 nm within 24 h, which was, however, accompanied

by the formation of some precipitate. Also in the case of

10e, which has three tetraethylene glycol side chains, no

long-lasting solubility of the aggregates in water could be

achieved. In fact, a slight shift of the Q_yaggregate band

from 736 to 734 nm was observed after 24 h (Figure 4b),

which can be attributed to the additional hydrophilic side

chain in 10e, which has a higher spatial demand leading to

destabilization of the aggregates. The solubility properties of

zinc chlorins 10d,e in water/methanol were also examined,

and a similar spectral pattern to that obtained in water/THF

were observed.

Figure 4. UV/Vis spectra of zinc chlorins a) 10d at c=5.810^À6m and

b) 10e at c=1.410^À5m. Monomers (d) in THF and aggregates (c

and a lines) in water with THF (100:1). Dashed lines refer to the ag-

gregate spectra in a time course of 24 h. Decrease in the intensity of ag-

gregate bands and the concomitant increase in the baseline are indicative

of precipitation of the zinc chlorins.

sition were employed. These chlorins showed smaller batho-

chromic shifts of the Q_yband in water/THF (100:1) in the

range of 675 to 729 nm,^[28]which was attributed to the for-

mation of dimers or higher aggregates, unlike in the case of

10d for which a significantly stronger redshift of up to

747 nm indicates extended chlorosomal-type aggregates.

With zinc chlorin 10d, the formation of aggregates in

water with a Q_y-band shift of 733 to 747 nm (Figure 4a),

which lies in the same range as for chlorosomal aggregates,

is achieved. Further evidence for the formation of J-aggre-

gates of 10d in water is provided by CD spectroscopy, as a

bisignate CD couplet, similar to that of 10b, is observed.

The positive and negative peak values appear in the higher

wavelength region at l=742nm ( De=362m^À1cm^À1) and l=

753 nm (De=À544m^À1cm^À1), whereas at the lower-wave-

length region positive and negative peak values occur at l=

Spectroscopic studies on aggregation properties of epimeric

1

zinc chlorins(3 R)-17 and (3¹S)-17: To explore the impact of

the 3¹chirality center on self-assembly of zinc chlorins, the

aggregation behavior of 3¹R/S-17 was studied in detail by

UV/Vis spectroscopy. Samples were prepared as before by

dissolving (3¹R)-17 and (3¹S)-17 in THF to obtain monomer

stock solutions, followed by the addition of a nonpolar sol-

vent such as n-heptane (n-heptane/THF 100:1) to initiate

aggregate formation. In the case of aggregates of epimerical-

ly pure (3¹S)-17 (Figure 5b), a bathochromic shift of the ab-

sorption maxima of the Q_yband was observed from 648

(monomer) to 711 nm (aggregates), whereas the Q_yband for

the aggregates of (3¹R)-17 epimer shifted from 648 to

705 nm (Figure 6b). In solutions of both epimers, a signifi-

cant proportion of monomers prevailed even after two to

three days. This is in contrast with zinc chlorins 10b,c for

443 nm

(De=54m^À1cm^À1)

and

l=466 nm

(De=

À29m^À1cm^À1).

Previously, work on the water solubility of chlorosome an-

alogue aggregates was reported by Tamiakiꢁs group in which

the self-aggregates of BChl c or zinc chlorin model com-

pound 2 were embedded in silicate capsules in which alk-

A

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F. Würthner et al.

which, under comparable conditions, monomers could

barely be observed. Additionally, the bathochromic shift of

the Q_yband for (3¹R)-17 and (3¹S)-17 is less pronounced

than the shift observed in the case of 10b,c (up to 742nm)

without the 3¹-methyl group. These results indicate that

(3¹R)-17 and (3¹S)-17 may form dimers or smaller oligomers

rather than extended aggregates like rods, as in the case of

10b,c. Noteworthy, however, is the fact that the UV/Vis

spectra of the (3¹R)-17 epimer shows a shoulder at about

747 nm (Figure 6b, bold arrow), where the absorption

maxima for the extended rod aggregates of 10b,c also

appear.

The stability of the aggregates of zinc chlorins (3¹R)-17

and (3¹S)-17 was explored by temperature-dependent UV/

Vis measurements in the range of 15 to 808C. With increas-

ing temperature, the monomer bands of the zinc chlorins at

648 nm increased, whereas the aggregate bands at 711 and

705 nm for (3¹S)-17 and (3¹R)-17, respectively, decreased,

which is indicative of the dissociation of aggregates at

higher temperatures. Upon cooling the aggregate solutions,

the aggregate bands were completely recovered for both

epimers, which confirmed that the aggregation was reversi-

ble.

Figure 5. Spectroscopic investigation of zinc chlorin epimer (3¹S)-17 in n-

heptane/THF (100:1) at c=1.710^À5m. a) CD spectrum at 208C. b) UV/

Vis spectra in the temperature range of 20–808C measured at 58C inter-

vals starting from 208C; the sample was allowed to equilibrate for 45 min

prior to each measurement; arrows indicate changes upon increasing

temperature.

Curve fitting with the Boltzmann function^[23]provided a

melting temperature of 498C for aggregates of (3¹S)-17,

whereas a significantly lower melting temperature of 228C

was obtained for the aggregates of (3¹R)-17, revealing a

higher stability for the (3¹S)-17 aggregates despite their less-

defined J-aggregate spectra. Also, the CD spectra for the ag-

gregates of (3¹R)-17 and (3¹S)-17 show distinct differences.

Thus, the amplitude of the CD signals for (3¹R)-17 are not

only higher in magnitude than those of (3¹S)-17, but also

show a well-defined bisignate shape similar to 10b,c (type I

according to Griebenow et al.^[26a]). The negative CD signal

for the (3¹R)-17 (742nm) is also shifted more bathochromi-

cally compared with that of the (3¹S)-17 stereoisomer

(718 nm). Notably, in such partially self-assembled systems,

a variety of aggregates with different size and geometry may

prevail, which can exhibit quite distinct CD spectra. For this

reason, UV/Vis and CD spectra do not always properly

match together in such a situation. Thus, in the case of

(3¹R)-17 we attribute the intense bisignated CD signal (Fig-

ure 6a) at long wavelengths to a small fraction of extended

chlorosomal-type J-aggregates that appeared as a shoulder

in the UV/Vis spectra (Figure 6b, bold arrow).

In addition to investigations on the self-assembly of pure

(3¹R)-17 and (3¹S)-17 epimers, we have also studied the mix-

tures of these epimers by UV/Vis spectroscopy. For this pur-

pose, stock solutions of the epimers in THF were mixed in

respective ratios and added to n-hexane for the formation of

aggregates. To achieve the highest possible amount of aggre-

gates compared with monomers, solutions containing only

0.5 vol% THF were used. To make the different shifts and

forms of aggregation bands intelligible, the absorption spec-

tra (Figure 7) are normalized. The higher the percentage of

(3¹S)-17, the further the main aggregate Q_yband was batho-

chromically shifted.

Figure 6. Spectroscopic investigation of the zinc chlorin epimer (3¹R)-17

in n-heptane/THF (100:1) at c=1.710^À5m. a) CD spectrum at 208C.

b) UV/Vis spectra in the temperature range of 20–808C measured at 58C

intervals starting from 208C, the sample was allowed to equilibrate for

45 min prior to each measurement; arrows indicate changes upon increas-

ing temperature.

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Chem. Eur. J. 2008, 14, 7791 – 7807

Self-assembled Zinc Chlorins

FULL PAPER

bled rod aggregates of zinc chlorin 10b.^[14]Tapping-mode

AFM images of an aggregate solution of 10b (Figure 8) on a

highly ordered pyrolytic graphite (HOPG) surface exhibited

Figure 7. Normalized UV/Vis spectra (for clarity only the higher wave-

length region is shown) of zinc chlorin Q_yband of (3¹R)-17 and (3¹S)-17

and their mixture in n-hexane with 0.5% THF (c_S-17=c_R-17=9.610^À6m).

The spectra, from left to right, are for 100:0 R/S to 0:100 R/S with 10%

incremental increases of the S epimer. The absorption maxima for pure

(3¹R)-17 and (3¹S)-17 occur at 704 and 711 nm, respectively.

Figure 8. Tapping-mode AFM images of aggregates of zinc chlorin 10b

on HOPG. Different areas and enlargements of the sample are displayed

in A), C), and D); B) shows the height profile along the red line in A).

The sample was prepared by spin-coating of a solution of 10b in n-

hexane/THF (100:1) on HOPG and measured in air.

In several previous publications it was reported that the

aggregation modes of 3¹R and 3¹S epimers of bacteriochlo-

A

U

ent zinc chlorin 17 epimers, earlier studies on BChl c and e

revealed a larger bathochromic shift for the aggregate band

of S epimers.^[15c,16c,29]Compared with earlier studies on

BChl c and e, a more pronounced bathochromic shift for the

Q_yaggregate band for (3¹S)-17 was observed. Thus, for ag-

gregate bands of 3¹R-BChl c and 3¹S-BChl c, absorption

maxima at 703 and 750 nm, respectively, were observed in

concentrated dichloromethane.^[15c]

isolated rod aggregates with contour lengths of (300ꢀ

97) nm and heights of (5.8ꢀ0.4) nm (vertical distance be-

tween the black and green triangle in Figure 8B) were in

quite good agreement with the electron microscopy data for

the chlorosomal (C. aurantiacus)^[31]BChl c rod aggregates

and also with the tubular model by Holzwarth.^[32]

For the epimers of BChl e in cyclohexane, the 3¹R-BChl

exhibited the Q_yabsorption maximum at 706 nm and the S

epimer at 717 nm.^[16c]Two previous independent studies

showed that the addition of small amounts of the corre-

sponding S epimers to solutions of R epimers of BChl c and

e afforded aggregates with further bathochromically dis-

placed Q_yabsorption bands than those observed in the case

of pure R epimers.^[16c,d]These observations were considered

to be supportive for the bacteriochlorophyll “double-tube”

model, in which the outer layer is formed preferentially by

R epimers and the inner tube by S epimers. However, our

mixing experiments with the epimeric zinc chlorins (3¹R)-17

and (3¹S)-17 did not show such an effect.

Besides rod-aggregates, globular structures with average

heights of (3.1ꢀ0.6) and (5.6ꢀ0.7) nm (vertical distance be-

tween the black and blue and black and orange triangles, re-

spectively, in Figure 8B) can also be seen, which are either

formed by degradation of rods during the spin-coating pro-

cess or they co-exist in solution with the rod-aggregates. An

argument in favor of the first interpretation is the fact that

the number of extended rod-aggregates diminishes over

time. This indicates a low stability of these rod aggregates

on the HOPG surface in air and suggests that evaporation

of residual solvent molecules from the inner core of a tubu-

lar assembly destabilizes the rod architecture.

Furthermore, it was previously reported that a methyl

group at the 3¹position suppresses aggregation, whereas an

additional methyl group at this position (tertiary alcohol)

has no further influence.^[30]Thus, our observation that the 3¹

methyl group in zinc chlorin 17 has a strong influence on

the aggregation behavior is in accordance with previous

studies.

Conclusions

With the newly synthesized zinc chlorins it was possible to

build model systems of natural light-harvesting BChl aggre-

gates that are, in contrast to literature reported for zinc

chlorin 2 aggregates, characterized by a proficient and dura-

ble solubility. This favorable property allowed thorough

spectroscopic and microscopic investigations for the elucida-

tion of the aggregate structural and optical features. To ach-

ieve solubility in nonpolar organic solvents, zinc chlorins

with one (10a), two (10b), and three (10c) dodecyl side

chains were synthesized, thereby a durable solubility of the

Microscopic characterization of aggregates of zinc chlorins:

Followed by spectroscopic investigations, structural proper-

ties of the zinc chlorin aggregates were elucidated by atomic

force microscopy (AFM). AFM measurements under ambi-

ent conditions allowed the visualization of the self-assem-

Chem. Eur. J. 2008, 14, 7791 – 7807

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7799

F. Würthner et al.

self-assembled aggregates was

obtained with two or more side

chains (10b,c). UV/Vis and CD

spectroscopic

studies

have

shown that the formation of the

self-assemblies of 10b,c is rever-

sible. Moreover, based on the

congruent position of aggregate

Q_ybands (l_max=741–743 nm) in

UV/Vis spectra and similar bi-

signate signals in the CD spec-

tra of the zinc chlorins 10b,c

aggregates and natural BChl c

aggregates, it could be conclud-

ed that the short-range arrange-

ment of these aggregate struc-

tures is quite similar. AFM of

10b aggregates provided direct

evidence for the rod-shaped

structure of the aggregates of

this class of substances. To ach-

ieve water-soluble zinc chlorin

aggregates, hydrophilic tetra-

ethylene glycol side chains were

introduced at the 17²position

of the chlorin unit (10d,e), and

they indeed self-assemble with

similar spectral properties as

the natural BChl c counterparts.

To relate these results to the

natural BChl c aggregate struc-

tures and to investigate the in-

fluence of the chiral center at

the 3¹position on aggregation,

epimeric zinc chlorins (3¹S)-17

Scheme 4. Schematic illustration of the self-assembled aggregate structures and assignment of the zinc chlorins

to the respective aggregate structures.

and (3¹R)-17 were synthesized and isolated as pure stereo-

isomers. Spectroscopic studies revealed a strong influence of

the methyl group at the 3¹position on aggregation, leading

to a decreased aggregation propensity of both diastereomers

compared with 10b,c. These observations suggest the forma-

tion of open or closed dimers, or other small oligomers of

17. An additional Q_yband at 747 nm was only detected for

(3¹R)-17, which can be ascribed to the formation of extend-

ed rod-shaped aggregates, as observed for the chlorosomal

light-harvesting systems. Scheme 4 summarizes the struc-

ture–property relationships for the semisynthetic zinc chlor-

ins 10a–e and 17 investigated in this work.

Experimental Section

General procedures

Acidic ester hydrolysis: Pheo a or its derivatives were dissolved in small

amounts of THF and subsequently concentrated hydrochloric acid (5–

10 mL) was added. After stirring for 5–6 h at RT under an argon atmos-

phere, the reaction mixture was poured into saturated aqueous NaHCO₃

solution and solid NaHCO₃was added to adjust the pH to 6–7. The re-

sulting carboxylic acids were extracted with CH₂Cl₂and the solution was

dried with sodium sulfate. After removal of the drying agent by filtration,

the solvent was removed by rotary evaporation and the crude product

was purified by column chromatography with mixtures of CH₂Cl₂/metha-

nol as the eluent. The analytical data for the chlorin carboxylic acids ob-

tained are in accordance with those reported in literature.^[33]

To conclude, zinc chlorins with dodecyl side chains 10b,c

or containing polyethylene glycols 10d,e form higher aggre-

gates with nanorod shapes. In contrast, the aggregation pro-

pensity of zinc chlorins (3¹S)-17 and (3¹R)-17 with a methyl

group at the 3¹stereogenic center is strongly reduced, as evi-

dent from the less-pronounced aggregation and the smaller

bathochromic shift of their Q_yaggregate bands. These rela-

tionships between molecular structure and self-assembly ca-

pability toward functional J-aggregates reveal a prominent

influence of the 3¹-methyl group.

Esterification of chlorin carboxylic acids: The free carboxylic acids at the

17²position of the chlorins were esterified with the respective alcohols

by using dried coupling reagents. Chlorin carboxylic acids were dissolved

in as small amounts of dry CH₂Cl₂as possible and vacuum-dried coupling

reagents DCC, DMAP, and DPTS were added in the presence of Hünigs

base. After stirring at RT for 6 h in the dark, the resulting 17²esters were

purified by silica gel column chromatography.

Oxidation reaction: The 3¹-vinyl group of chlorins was converted into the

corresponding diol with a catalytic amount of OsO₄, followed by oxida-

tive cleavage of the diol by sodium periodate (NaIO₄).^[34]The respective

3¹-vinyl chlorin was first dissolved in THF, followed by the addition of

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Chem. Eur. J. 2008, 14, 7791 – 7807

Self-assembled Zinc Chlorins

FULL PAPER

small amounts of a 1:1 mixture of water and acetic acid and then a cata-

lytic amount of osmium tetroxide (1–2small crystals; due to the toxicity

of osmium tetroxide, weighing was avoided) was added. Subsequently,

the reaction mixture was stirred at RT under an argon atmosphere, and

the reaction was monitored by thin-layer chromatography. When signifi-

cant amounts of diol intermediate were formed after 1–1.5 h, a saturated

aqueous sodium periodate solution was added dropwise by a syringe

pump (5–10 mLh^À1) within 2–4 h. Thereby a color change from olive

green to the grey diol intermediate and then finally to brown 3¹-aldehyde

was observed. Afterwards, water (100 mL) and diethyl ether (100 mL)

were added and the acetic acid was neutralized with saturated aqueous

NaHCO₃solution. The product was extracted with diethyl ether (3

100 mL) and washed several times with water (50 mL), and finally dried

over sodium sulfate. After filtration to remove the drying agent the sol-

vent was removed with a rotary evaporator.

CH₂Cl₂(7 mL), followed by the addition of 4-(dodecyloxy)benzyl alcohol

11a (545 mg, 1.87 mmol), DCC (1.53 g, 7.42mmol), DMAP (568 mg,

4.65 mmol), and DPTS (1.37 g, 4.65 mmol). After stirring for 10 min at

RT, Hünigs base (242 mg, 1.87 mmol) was added to the reaction mixture.

It was further stirred for 3.5 h and was then directly subjected to column

chromatography on silica gel with n-pentane/diethyl ether (1:1) as the

eluent mixture. The olive-green solid obtained following purification by

column chromatography was subjected to further purification by semipre-

parative HPLC (451 mg, 0.56 mmol, 61%). M.p. 38–438C; ¹H NMR

(400 MHz, CDCl₃, 2 58C): d=9.42(s, 1H; 10-H), 9.32(s, 1H; 5-H), 8.53

(s, 1H; 20-H), 7.96 (dd, ³J_trans=17.6 Hz, ³J_cis=11.6 Hz, 1H; 3¹-H), 7.14

(m, 2H; 6’-H, 2’-H), 6.76 (m, 2H; 3’-H, 5’-H), 6.25 (dd, ³J_trans=17.8 Hz,

2H, J=1.5 Hz, 1H; 3²-H), 6.15 (dd, ³J_cis=11.5 Hz, ²J=1.5 Hz, 1H; 3-H),

5.25 (d, ²J=19.8 Hz, 1H; 13²-H), 5.06 (d, ²J=19.8 Hz, 1H; 13²-H), 5.01

(d, ²J=12.0 Hz, 1H; 1’’-H), 4.95 (d, ²J=12.0 Hz, 1H; 1’’-H), 4.47 (dq,

³J=7.3, 2.0 Hz, 1H; 18-H), 4.27 (td, ³J=8.7, 2.1 Hz, 1H; 17-H), 3.87 (t,

³J=6.6, 2H; OCH₂), 3.64 (s, 3H; 12¹-H), 3.66–3.60 (m, 2H; 8¹-H), 3.39 (s,

3H; 2¹-H), 3.19 (s, 3H; 7¹-H), 2.75–2.54 and 2.35–2.26 (m, 4H; 17¹-H,

17²-H), 1.79 (d, ³J=7.3 Hz, 3H; 18¹-H), 1.74–1.66 (m, 5H; CH₂, 8²-H),

Reduction reaction: The 3¹-formyl group of chlorins was selectively re-

duced to the corresponding primary alcohol with 10 equiv of borane-tert-

butylamine complex (BH₃

(tBu)NH₂) reagent.^[17a]The respective 3¹-formyl

G

chlorin was dissolved in THF, followed by the addition of the boron re-

agent and the mixture was stirred at RT under an argon atmosphere. The

color of the reaction mixture changed from dark brown to olive green,

which indicated the formation of the chlorin with a 3¹-alcohol functionali-

ty. Diethyl ether (100 mL) and water (50 mL) were added to the reaction

mixture and the amine complex was neutralized with saturated ammoni-

um chloride aqueous solution (20 mL). The organic phases were washed

several times with water and dried over sodium sulfate. The desiccant

was filtered off and the solvent was removed under vacuum to give the

3¹-hydroxymethyl chlorin derivatives.

3

1.39–1.26 (m, 18H; 9CH₂), 0.89 (t, J=6.7 Hz, 3H; OCH₃), 0.38 (s, 1H;

NH), À1.75 ppm (s, 1H; NH); HRMS (ESI): m/z calcd for

C₅₂H₆₄N₄O₄Na [M+Na]⁺: 831.4825; found: 831.4819.

3-Devinyl-3-formyl-13²-demethoxycarbonylpheophorbide

oxy) benzyl ester 8a: According to the general procedure for the oxida-

a (4’-dodecyl-

AHCTREUNG

tion reaction, the oxidative cleavage of the 3¹-vinyl group of chlorin 7a

was carried out by dissolving 7a (253 mg, 0.31 mmol) in THF (40 mL),

followed by the addition of water (0.5 mL) and concentrated acetic acid,

and subsequently, a small crystal of osmium tetroxide was added. After

stirring the reaction mixture for 2.5 h at RT, saturated aqueous NaIO₄so-

lution (6 mLh^À1) was added dropwise. The reaction mixture was extract-

ed as described in the general procedure and subjected to silica gel

column chromatography with a mixture of n-pentane/diethyl ether (3:2),

and then further purified by semipreparative HPLC to obtain a brown

solid (254 mg, 0.31 mmol, 98%). Analytical HPLC: 8a eluted after

8.1 min at a flow of 1 mLmin^À1with a solvent mixture of methanol/

Metalation reaction: The metalation of chlorins was carried out in THF

and with a saturated solution of zinc acetate in methanol and by stirring

for 2–3 h at RT under an argon atmosphere. The incorporation of zinc

ion into the chlorin center leads to a color change from olive green to

turquoise blue. To terminate the reaction, water (100 mL) and diethyl

ether (100 mL) were added. After adding a saturated aqueous NaHCO₃

solution (100 mL) for the neutralization of the acetate, the mixture was

washed with water at least four times to remove the excess amount of

zinc acetate. The organic phase was dried over sodium sulfate, the desic-

cant was filtered off and the solvent was then removed with a rotary

evaporator to afford the zinc chlorins.

13²-Demethoxycarbonylpheophorbide a methyl ester (Pheo a): The start-

ing material for all the newly synthesized zinc chlorins was Pheo a, which

was synthesized from natural chlorophyll a according to a known litera-

ture procedure.^[17b,35]The provision of chlorophyll a was made by Soxh-

let-extraction of dried algae Spirulina platensis. The extraction was per-

formed with 800 g of algae and extracted for 2–3 d with acetone (1.5–

2L). After removal of acetone with a rotary evaporator, a dark green

oily residue was obtained, which was heated at reflux with collidin

(200 mL) for 6 h, afterwards collidin was removed under vacuum

(30 mbar) and the residue was immediately cooled to room temperature.

Subsequently, the residue was dissolved in methanol (200 mL) and

cooled to 58C and 20% sulfuric acid (150 mL) was added. After stirring

for 12h at RT, diethyl ether (500 mL) was added and the pH of the aque-

ous phase was adjusted to 6–7 by careful addition of saturated sodium

hydrogen carbonate solution. The product was repeatedly extracted with

diethyl ether and the combined organic phases were washed with water.

The organic layer was dried over sodium sulfate, filtered, and the solvent

was removed with a rotary evaporator. The crude product obtained was

purified on a silica gel column using n-pentane as the eluent to remove

the carotenoids and plant oils. Diethyl ether was used as the eluent to

obtain the desired Pheo a. The latter has the tendency to aggregate on

the column, thus the aggregated portions were collected from the column

with the help of a spatula and washed with diethyl ether and after filtra-

tion the solvent was removed under vacuum. Starting from 800 g of Spir-

ulina platensis, 3–4 g of Pheo a was obtained, which was then character-

ized by NMR and UV/Vis spectroscopies and mass spectrometry.^[17b]

1

CH₂Cl₂(7:3). M.p. 43–528C; H NMR (400 MHz, CDCl₃, 2 58C): d=11.53

(s, 1H; 3¹-H), 10.28 (s, 1H; 10-H), 9.59 (s, 1H; 5-H), 8.81 (s, 1H; 20-H),

7.12(m, 2H; 6 ’-H, 2’-H), 6.75 (m, 2H; 3’-H, 5’-H), 5.31 (d, ²J=20.0 Hz,

1H; 13¹-H), 5.13 (d, ²J=20.0 Hz, 1H; 13²-H), 4.99 (d, ²J=12.0 Hz, 1H;

1’’-H), 4.92(d, ²J=11.9 Hz, 1H; 1’’-H), 4.55 (dq, ³J=7.3, 1.9 Hz, 1H; 18-

H), 4.36 (td, ³J=8.6, 2.4 Hz, 1H; 17-H), 3.87 (t, ³J=6.6 Hz, 2H; OCH₂),

3.76 (s, 3H; 12¹-H), 3.74–3.68 (m, 5H; 8¹-H, 2¹-H), 3.30 (s, 3H; 7¹-H),

2.77–2.56 and 2.36–2.27 (m, 4H; 17¹-H, 17²-H), 1.83 (d, ³J=7.3 Hz, 3H;

18¹-H), 1.73–1.68 (m, 5H; CH₂, 8²-H), 1.41–1.24 (m, 18H; 9CH₂), 0.87

(t, ³J=6.9 Hz, 6H; CH₃), À0.19 (s, 1H; NH), À2.1 ppm (s, 1H; NH);

HRMS (ESI): m/z calcd for C₅₁H₆₃N₄O₅[M+H]⁺: 811.4798; found:

811.4793.

3-Devinyl-3-hydroxymethyl-13²-demethoxycarbonylpheophorbide

a (4’-

dodecyloxy)benzyl ester 9a: Borane-tert-butylamine complex (261 mg,

3.00 mmol) was added to a solution of 8a (250 mg, 0.30 mmol) in THF

(40 mL), and stirred for 3 h at RT in the dark, and the reaction mixture

was extracted as described in the general procedure for the reduction re-

action. The resulting olive-grey solid was purified by semipreparative

HPLC (203 mg, 0.25 mmol, 83%). Analytical HPLC: Compound 9a was

eluted after 5.2min at a flow of 1 mLmin ^À1with a solvent combination

of methanol/CH₂Cl₂(7:3). M.p. 60–648C; ¹H NMR (400 MHz, CDCl₃,

258C): d=9.53 (s, 1H; 10-H), 9.47 (s, 1H; 5-H), 8.57 (s, 1H; 20-H), 7.10

(m, 2H; 2’-H, 6’-H), 6.74 (m, 2H; 3’-H, 5’-H), 5.90 (s, 2H; 3¹-H), 5.23 (d,

²J=19.8 Hz, 1H; 13²-H), 5.05 (d, ²J=19.8 Hz, 1H; 13²-H), 4.97 (d, ²J=

12.0 Hz, 1H; 1’’-H), 4.90 (d, ²J=12.4 Hz, 1H; 1’’-H), 4.47 (dq, ³J=7.3,

2.0 Hz, 1H; 18-H), 4.27 (td, ³J=6.3, 2.5 Hz, 1H; 17-H), 3.78 (t, ³J=

6.6 Hz, 2H; OCH₂), 3.70 (q, ³J=7.6 Hz, 2H; 8¹-H), 3.66 (s, 3H; 12¹-H),

3.41 (s, 3H; 2¹-H), 3.26 (s, 3H; 7¹-H), 2.73–2.52 and 2.33–2.25 (m, 4H;

17¹-H, 17²-H), 1.77 (d, ³J=7.3 Hz, 3H; 18¹-H), 1.71–1.68 (m, 5H; CH₂,

3

8²-H), 1.41–1.24 (m, 18H; 9CH₂), 0.87 (t, J=6.9 Hz, 3H; CH₃), 0.21 (s,

13²-Demethoxycarbonylpheophorbide a (4’-dodecyloxy)benzyl ester 7a:

According to the general procedure for esterification, the 17²-carboxylic

acid derivative of chlorin 6 (498 mg, 0.93 mol) was dissolved in dry

1H; NH), À1.83 ppm (s, 1H; NH); HRMS (ESI): m/z calcd for

C₅₁H₆₄N₄O₅Na [M+Na]⁺: 835.4774; found: 835.4770.

Chem. Eur. J. 2008, 14, 7791 – 7807

ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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7801

F. Würthner et al.

3-Devinyl-3-hydroxymethyl-13²-demethoxycarbonylpheophorbide

a

(4’-

4H; 17¹-H, 17²-H), 1.82(d, ³J=7.3 Hz, 3H; 18¹-H), 1.72–1.65 (m, 7H;

CH₂, 8²-H), 1.34–1.17 (m, 36H; 18CH₂), 0.87 (t, ³J=6.7 Hz, 6H; CH₃),

À0.13 (s, 1H; NH), À2.10 ppm (s, 1H; NH); HRMS (ESI): m/z calcd for

C₆₃H₈₇N₄O₆[M+H]⁺: 995.6625; found: 995.6619.

dodecyloxy)benzyl ester zinc complex 10a: According to the general pro-

cedure for metalation, 3¹-hydroxy chlorin 9a (50 mg, 0.06 mmol) was dis-

solved in THF (5 mL), followed by the addition of a saturated solution of

zinc acetate in methanol (10 mL) and the reaction mixture was stirred

for 2h at RT. The reaction mixture was extracted as described in the gen-

eral procedure and the turquoise-colored product was purified by semi-

preparative HPLC (41 mg, 0.05 mmol, 78%). Analytical HPLC: 10a

eluted after 7.4 min at a flow of 1 mLmin^À1with a solvent mixture of

methanol/CH₂Cl₂(9:1). M.p. 2338C; ¹H NMR (400 MHz, CDCl₃,

[D₅]pyridine, 258C): d=9.56 (s, 1H; 10-H), 9.37 (s, 1H; 5-H), 8.31 (s,

1H; 20-H), 7.12 (m, 2H; 2’-H, 6’-H), 6.76 (m, 2H; 3’-H; 5’-H), 5.85 (s,

2H; 3¹-H), 5.17 (d, ²J=19.7 Hz, 1H; 13²-H), 5.02(d, ²J=19.6 Hz, 1H;

13²-H), 4.94 (d, ²J=11.9 Hz, 1H; 1’’-H), 4.89 (d, ²J=11.9 Hz, 1H; 1’’-H),

3-Devinyl-3-hydroxymethyl-13²-demethoxycarbonylpheophorbide a (3’,5’-

bis-dodecyloxy)benzyl ester 9b: According to the general procedure for

the reduction reaction, chlorin 8b (80.2mg, 0.08 mmol) was dissolved in

CH₂Cl₂(10 mL), followed by the addition of borane-tert-butylamine com-

plex (70.1 mg, 0.08 mmol) and the reaction mixture was stirred for 1 h at

RT. The reaction mixture was extracted according to the general proce-

dure and the olive-grey product 9b was purified by column chromatogra-

phy on silica gel column with a solvent mixture of CH₂Cl₂/methanol

(9:1), and subsequently by semipreparative HPLC (yield: 59.1 mg,

0.06 mmol, 74%). Analytical HPLC: 9b eluted after 8.7 min at a flow of

1 mLmin^À1with solvent mixture methanol/CH₂Cl₂(7:3). M.p. 51–548C;

¹H NMR (400 MHz, CDCl₃, 2 58C): d=9.50 (s, 1H; 10-H), 9.45 (s, 1H; 5-

H), 8.56 (s, 1H; 20-H), 6.34 (s, 3H; 2’-H, 4’-H, 6’-H), 5.89 (s, 2H; 3¹-H),

5.23 (d, ²J=19.8 Hz, 1H; 13²-H), 5.05 (d, ²J=19.7 Hz, 1H; 13²-H), 4.99

(d, ²J=12.4 Hz, 1H; 1’’-H), 4.88 (d, ²J=12.4 Hz, 1H; 1’’-H), 4.29 (dq,

3

4.36 (dq, J=7.2, 1.9 Hz, 1H; 18-H), 4.19 (td, J=7.7, 2.1 Hz, 1H; 17-H),

3.87 (t, ³J=6.6 Hz, 2H; OCH₂), 3.74 (q, ³J=7.5 Hz, 2H; 8¹-H), 3.69 (s,

3H; 12¹-H), 3.30 (s, 3H; 2¹-H), 3.19 (s, 3H; 7¹-H), 2.62–2.24 and 1.94–

1.88 (m, 4H; 17¹-H, 17²-H), 1.74–1.68 (m, 8H; 18¹-H, 8²-H, CH₂), 1.41–

1.24 (m, 18H; 9CH₂), 0.88 ppm (t, ³J=7.0 Hz, 3H; CH₃); HRMS

(ESI): m/z calcd for C₅₁H₆₂N₄O₅Zn [M]⁺: 874.4011; found: 874.4004; UV/

Vis (THF): l_max(e_max)=648 nm (91000m^À1cm^À1).

3

³J=7.3, 2.0 Hz, 1H; 18-H), 4.29 (td, J=8.3, 2.3 Hz, 1H; 17-H), 3.84–3.78

3

(m, 4H; OCH₂), 3.72–3.66 (q, J=7.6 Hz, 2H; 8¹-H), 3.66 (s, 3H; 12¹-H),

13²-Demethoxycarbonylpheophorbide

a

(3’,5’-bis-dodecyloxy)benzyl

3.40 (s, 3H; 2¹-H), 3.26 (s, 3H; 7¹-H), 2.73–2.53 and 2.37–2.26 (m, 4H;

17¹-H, 17²-H), 1.77 (d, ³J=7.3 Hz, 3H; 18¹-H), 1.69–1.63 (m, 7H; CH₂,

8²-H), 1.35–1.21 (m, 36H; 18CH₂), 0.88 (t, ³J=7.1 Hz, 6H; CH₃), 0.24

(s, 1H; NH), À1.83 ppm (s, 1H; NH); HRMS (ESI): m/z calcd for

C₆₃H₈₉N₄O₆[M+H]⁺: 997.6782; found: 997.6777.

ester 7b: In dry CH₂Cl₂(6 mL), 13²-demethoxycarbonylpheophorbide a 5

(150 mg, 0.28 mmol) was dissolved, followed by the addition of 3,5-bis-

(dodecyloxy)benzyl alcohol 11b (200 mg, 0.42 mmol), DCC (289 mg,

1.40 mmol), DMAP (69.1 mg, 0.56 mmol), and DPTS (165 mg,

0.56 mmol). After 30 min of stirring at RT, Hünigs base (57.2mg,

0.43 mmol) was added. The reaction was stirred for another 3 h, followed

by the addition of water (100 mL) and a saturated solution of NH₄Cl

(20 mL) to terminate the reaction. The product was extracted with

CH₂Cl₂(3100 mL) and then washed with water. After removal of the

solvent, the residue was loaded on a silica gel column and purified by

using 1:1 n-pentane/diethyl ether as the eluent. It was further purified by

semipreparative HPLC (212 mg, 0.21 mmol, 76%). Analytical HPLC:

3-Devinyl-3-hydroxymethyl-13²-demethoxycarbonylpheophorbide a (3’,5’-

bis-dodecyloxy)benzyl ester zinc complex 10b: According to the general

procedure for metalation, chlorin 9b (50 mg, 0.05 mmol) was dissolved in

a small amount of THF followed by the addition of saturated solution of

zinc acetate in methanol (12mL) and was stirred at RT. The reaction

mixture was extracted according to the general procedure with CH₂Cl₂

instead of diethyl ether and purified by column chromatography with an

eluting solvent mixture of CH₂Cl₂/methanol (9:1), and further purified

using semipreparative HPLC ( 40.3 mg, 0.04 mmol, 75%). Analytical

HPLC: 10b was eluted after 6.1 min at a flow of 1 mLmin^À1with solvent

mixture of methanol/CH₂Cl₂(7:3). M.p. 2418C; ¹H NMR (400 MHz,

[D₈]THF, 258C): d=9.62(s, 1H; 10-H), 9.49 (s, 1H; 5-H), 8.49 (s, 1H;

The olive-grey chlorin 7b was eluted after 21.9 min at

a flow of

1 mLmin^À1with a solvent mixture of methanol/CH₂Cl₂(7:3). M.p. 51–

558C; ¹H NMR (400 MHz, CDCl₃, 2 58C): d=9.46 (s, 1H; 10-H), 9.35 (s,

3

1H; 5-H), 8.54 (s, 1H; 20-H), 7.98 (dd, J_trans=17.6 Hz, J_cis=11.6 Hz, 1H;

4

3¹-H), 6.39 (d, J=2.3 Hz, 2H; 6’-H, 2’-H), 6.34 (t, J=2.3 Hz, 1H; 4’-H),

6.26 (dd, ³J_trans=17.8 Hz, ²J=1.5 Hz, 1H; 3²-H), 6.15 (dd, ³J_cis=11.6 Hz,

²J=1.5 Hz, 1H; 3²-H), 5.24 (d, ²J=19.8 Hz, 1H; 13²-H), 5.09 (d, ²J=

19.8 Hz, 1H; 13²-H), 5.01 (d, ²J=12.2 Hz, 1H; 1’’-H), 4.95 (d, ²J=

12.2 Hz, 1H; 1’’-H), 4.48 (dq, ³J=7.3, 2.0 Hz, 1H; 18-H), 4.29 (td, ³J=

8.7, 2.4 Hz, 1H; 17-H), 3.83 (t, ³J=5.3, 4H; OCH₂), 3.69–3.63 (q, ³J=

6.7 Hz, 2H; 8¹-H), 3.65 (s, 3H; 12¹-H), 3.40 (s, 3H; 2¹-H), 3.21 (s, 3H; 7¹-

H), 2.78–2.58 and 2.38–2.29 (m, 4H; 17¹-H, 17²-H), 1.79 (d, ³J=7.3 Hz,

3H; 18¹-H), 1.71–1.64 (m, 7H; CH₂, 8²-H), 1.37–1.22 (m, 36H; 18CH₂),

4

20-H), 6.40 (d, J=2.3 Hz, 2H; 2’-H, 6’-H), 6.32(t, ⁴J=2.2 Hz, 1H; 4’-H),

5.71 (d, ³J=5.8 Hz, 2H; 3¹-H), 5.11 (d, ²J=19.5 Hz, 1H; 13²-H), 5.00–

4.92(m, 3H; 13 ²-H, 1’’-H), 4.53 (dq, ³J=7.2, 2.1 Hz, 1H; 18-H), 4.40 (t,

³J=5.8 Hz, 1H; 3¹-OH), 4.31 (td, ³J=8.3, 2.3 Hz, 1H; 17-H), 3.84 (m,

6H; OCH₂, 8¹-H), 3.61 (s, 3H; 12¹-H), 3.31 (s, 3H; 2¹-H), 3.29 (s, 3H; 7¹-

H), 2.74–2.59 and 2.36–2.26 (m, 4H; 17¹-H, 17²-H), 1.78 (d, ³J=7.3 Hz,

3H; 18¹-H), 1.71–1.63 (m, 7H; 8²-H, CH₂), 1.40–1.22 (m, 36H; 18CH₂),

0.88 ppm (t, ³J=6.9 Hz, 6H; CH₃); HRMS (ESI): m/z calcd for

C₆₃H₈₆N₄O₆Zn [M]⁺: 1058.5839; found: 1058.5830; UV/Vis (THF): l_max

(e_max)=648 nm (90,000m ^À1cm^À1).

3

0.88 (t, J=6.8 Hz, 6H; CH₃), 0.42(s, 1H; NH), À1.72ppm (s, 1H; NH);

HRMS (ESI): m/z calcd for C₆₄H₈₈N₄O₅Na [M+Na]⁺: 1015.6650; found:

1015.6647.

13²-Demethoxycarbonylpheophorbide

a (3’,4’,5’-tris-dodecyloxy)benzyl

3-Devinyl-3-formyl-13²-demethoxycarbonylpheophorbide

a (3’,5’-bis-do-

ester 7c: 3,4,5-Tris(dodecyloxy)benzyl alcohol 11c (743 mg, 1.12mmol),

DCC (1.24 g, 5.99 mmol), DMAP (458 mg, 3.75 mmol), and DPTS

(1.10 g, 3.75 mmol) were added to a solution of 6 (400 mg, 0.75 mmol) in

dry CH₂Cl₂(20 mL). After stirring for 5 min at RT, Hünigs base (194 mg,

1.5 mmol) was added and the reaction mixture was stirred for a further

4 h at RT. The reaction was terminated by the addition of water

(100 mL) and a solution of NH₄Cl (20 mL), and the reaction mixture was

extracted with diethyl ether (3100 mL). The united organic phases were

washed with water (100 mL) and dried over sodium sulfate. After remov-

al of the desiccating agent, the olive-grey solid was purified by silica gel

column chromatography with n-pentane/diethyl ether (3:2) as the eluent,

and further purified by semipreparative HPLC (447 mg, 0.38 mmol,

51%). Analytical HPLC: 7c eluted after 6.7 min at a flow of 1 mLmin^À1

decyloxy)benzyl ester 8b: Chlorin 7b (100 mg, 0.10 mmol) was dissolved

in THF (30 mL), and water (0.5 mL) and concentrated acetic acid

(0.5 mL) were added. Subsequently, two small crystals of OsO₄were

added to the reaction mixture and stirred at RT for 1.5 h. Within another

2h, a saturated solution of NaIO ₄was added dropwise (6 mLh^À1). The

reaction mixture was extracted according to the general procedure for

oxidation reaction. The brown solid was purified on a silica gel column

by eluting with a mixture of n-pentane/diethyl ether (3:2), and also by

semipreparative HPLC (98 mg, 0.1 mmol, 98%). Analytical HPLC: 8b

was eluted after 15.4 min at a flow of 1 mLmin^À1with methanol/CH₂Cl₂

1

(7:3). M.p. 54–588C; H NMR (400 MHz, CDCl₃, 2 58C): d=11.53 (s, 1H;

3¹-H), 10.27 (s, 1H; 10-H), 9.58 (s, 1H; 5-H), 8.82 (s, 1H; 20-H), 6.37 (d,

4

2

⁴J=2.15 Hz, 2H; 6’-H, 2’-H), 6.34 (t, J=2.14 Hz, 1H; 4’-H), 5.33 (d, J=

20.0 Hz, 1H; 13²-H), 5.17 (d, ²J=20.0 Hz, 1H; 13²-H), 5.00 (d, ²J=

12.3 Hz, 1H; 1’’-H), 4.91 (d, ²J=12.3 Hz, 1H; 1’’-H), 4.56 (dq, ³J=7.3,

1.8 Hz, 1H; 18-H), 4.38 (td, ³J=8.5, 2.4 Hz, 1H; 17-H), 3.83 (t, ³J=

with

a solvent mixture of methanol/CH₂Cl₂(1:1). M.p. 59–648C;

¹H NMR (400 MHz, CDCl₃, 2 58C): d=9.49 (s, 1H; 10-H), 9.38 (s, 1H; 5-

H), 8.54 (s, 1H; 20-H), 8.00 (dd, ³J_trans=17.8 Hz, ³J_cis=11.6 Hz, 1H; 3¹-

H), 6.46 (s, 2H; 6’-H, 2’-H), 6.28 (dd, ³J_trans=17.9 Hz, J=1.5 Hz, 1H; 3²-

2

6.7 Hz, 4H; OCH₂), 3.76 (s, 3H; 12¹-H), 3.73–3.68 (q, J=6.7 Hz, 2H; 8¹-

H), 6.17 (dd, ³J_cis=11.6 Hz, ²J=1.5 Hz, 1H; 3²-H), 5.25 (d, ²J=19.8 Hz,

1H; 13²-H), 5.09 (d, ²J=19.8 Hz, 1H; 13²-H), 4.99 (d, ²J=12.0 Hz, 1H;

3

H), 3.70 (s, 3H; 2¹-H), 3.30 (s, 3H; 7¹-H), 2.80–2.62 and 2.39–2.28 (m,

7802

www.chemeurj.org

ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 2008, 14, 7791 – 7807

Self-assembled Zinc Chlorins

FULL PAPER

1’’-H), 4.92(d, ²J=12.0 Hz, 1H; 1’’-H), 4.47 (dq, ³J=7.3, 2.0 Hz, 1H; 18-

H), 4.29 (td, ³J=8.5, 2.4 Hz, 1H; 17-H), 3.87 (t, ³J=6.6 Hz, 6H; OCH₂),

3.71–3.65 (q, ³J=7.7 Hz, 2H; 8¹-H), 3.67 (s, 3H; 12¹-H), 3.40 (s, 3H; 2¹-

H), 3.23 (s, 3H; 7¹-H), 2.76–2.57 and 2.37–2.28 (m, 4H; 17¹-H, 17²-H),

1.78 (d, ³J=7.3 Hz, 3H; 18¹-H), 1.73–1.64 (m, 9H; CH₂, 8²-H), 1.43–1.21

(m, 54H; 27CH₂), 0.88 (m, 9H; CH₃), 0.45 (s, 1H; NH), À1.70 ppm (s,

1H; NH); HRMS (ESI): m/z calcd for C₇₆H₁₁₂N₄O₆Na [M+Na]⁺:

1199.8480; found: 1199.8471.

2H; 8¹-H), 3.73 (s, 3H; 12¹-H), 3.32(s, 3H; 2 ¹-H), 3.20 (s, 3H; 7¹-H),

2.69–2.60, 2.53–2.45, 2.39–2.29, and 2.12–2.04 (m, 4H; 17¹-H, 17²-H),

1.79–1.71 (m, 12H; 18¹-H, CH₂, 8²-H), 1.50–1.27 (m, 54H; 27CH₂),

0.91 ppm (m, 9H; CH₃); HRMS (ESI): m/z calcd. for C₇₅H₁₁₀N₄O₇Zn

[M+Na]⁺: 1265.7563; found: 1265.7558; UV/Vis (THF): l_max(e_max)=

648 nm (92000 m^À1cm^À1).

3-Devinyl-3-hydroxymethyl-13²-demethoxycarbonylpheophorbide

a

methyl ester (5a): The synthesis was performed according to the litera-

ture and the analytical data are in accordance with those reported.^[17a]

3-Devinyl-3-formyl-13²-demethoxycarbonylpheophorbide

a (3’,4’,5’-tris-

3-Devinyl-3-methoxymethyl-13²-demethoxycarbonylpheophorbide

a

dodecyloxy)benzyl ester 8c: According to the general procedure for the

oxidation reaction, chlorin derivative 7c (300 mg, 0.26 mmol) was dis-

solved in THF (10 mL), followed by the addition of water (0.5 mL), con-

centrated acetic acid, and a small crystal of osmium tetroxide. The reac-

tion mixture was stirred at RT for further 2h, afterwards a saturated so-

lution of NaIO₄(7 mLh^À1) was added dropwise. Extraction of the reac-

tion mixture was performed according to the general procedure and the

resulting brown-colored formyl chlorin 8c was purified by column chro-

matography with a mixture of n-pentane/diethyl ether (3:2), and was ad-

ditionally purified by semipreparative HPLC (194 mg, 0.16 mmol, 65%).

Analytical HPLC: 8c eluted after 23.4 min at a flow of 1 mLmin^À1with

solvent mixture of methanol/CH₂Cl₂(7:3). M.p. 132–1388C; ¹H NMR

(400 MHz, CDCl₃, 2 58C): d=11.57 (s, 1H; 3¹-H), 10.35 (s, 1H; 10-H),

9.65 (s, 1H; 5-H), 8.82(s, 1H; 20-H), 6.46 (s, 2H; 6 ’-H, 2’-H), 5.34 (d,

²J=20.0 Hz, 1H; 13²-H), 5.17 (d, ²J=20.0 Hz, 1H; 13²-H), 4.99 (d, ²J=

12.0 Hz, 1H; 1’’-H), 4.90 (d, ²J=12.0 Hz, 1H; 1’’-H), 4.56 (dq, ³J=7.3,

1.9 Hz, 1H; 18-H), 4.38 (td, ³J=9.0, 2.5 Hz, 1H; 17-H), 3.88–3.85 (t, ³J=

6.6 Hz, 6H; OCH₂), 3.78–3.72(m, 8H; 8 ¹-H, 12¹-H, 2¹-H), 3.35 (s, 3H;

methyl ester (6a): The synthesis was performed according to the litera-

ture and the analytical data are in accordance with those reported.^[36]

3-Devinyl-3-formyl-13²-demethoxycarbonylpheophorbide

a [3’,5’-bis-(2-

{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}ethoxy)]benzyl ester 8d: Accord-

ing to the general procedure for esterification, the acid of 6a (90.3 mg,

0.17 mmol) was dissolved in dry CH₂Cl₂(10 mL) and 11d (233 mg,

0.45 mmol) was added, followed by the addition of DCC (277 mg,

1.34 mmol), DMAP (103 mg, 0.84 mmol), and DPTS (247 mg,

0.84 mmol). After stirring at RT for 5 min, Hünigs base (65.2mg,

0.50 mmol) was added. The reaction mixture was stirred for a further 3 h

and then treated with CH₂Cl₂(100 mL) and a saturated solution of

NH₄Cl (20 mL), then it was washed with water (50 mL) several times.

Because of the highly polar ethyleneglycol side chains, the aqueous phase

was re-extracted with CH₂Cl₂(2100). The combined organic phases

were dried with Na₂SO₄, solvent was removed and the residue was puri-

fied by column chromatography with a mixture of CH₂Cl₂/ethanol (95:5)

as the eluent, and was further purified by semipreparative HPLC

(100 mg, 0.10 mmol, 57%). Analytical HPLC: 8d eluted after 10.2min at

3

7¹-H), 2.80–2.91 and 2.40–2.28 (m, 4H; 17¹-H, 17²-H), 1.81 (d, J=7.3 Hz,

3H; 18¹-H), 1.75–1.61 (m, 9H; CH₂, 8²-H), 1.40–1.21 (m, 54H; 27CH₂),

0.88 (m, 9H; CH₃), À0.09 (s, 1H; NH), À2.03 ppm (s, 1H; NH); HRMS

(ESI): m/z calcd for C₇₅H₁₁₁N₄O₇[M+H]⁺: 1179.8452; found: 1179.8446.

1

a flow of 1 mLmin^À1with methanol as the solvent. M.p. 1738C; H NMR

(400 MHz, CDCl₃, TMS, 258C): d=11.55 (s, 1H; CHO), 10.31 (s, 1H; 10-

H), 9.62(s, 1H; 5-H), 8.84 (s, 1H; 20-H), 6.38 (s, 3H; 2 ’-H, 4’-H, 6’-H),

5.32(d, ²J=19.9 Hz, 1H; 13²-H), 5.16 (d, ²J=19.8 Hz, 1H; 13²-H), 4.97

(d, ²J=12.5 Hz, 1H; 1’’-H), 4.88 (d, ²J=12.4 Hz, 1H; 1’’-H), 4.57 (dq,

3-Devinyl-3-hydroxymethyl-13²-demethoxycarbonylpheophorbide

(3’,4’,5’-tris-dodecyloxy)benzyl ester 9c: Borane-tert-butylamine complex

(110 mg, 1.27 mmol) was added to solution of formyl chlorin 8c

a

3

a

³J=7.3, 2.0 Hz, 1H; 18-H), 4.39 (td, J=8.4, 2.2 Hz, 1H; 17-H), 4.01–3.98

(150 mg, 0.13 mmol) in CH₂Cl₂(20 mL), and stirred for 1 h at RT. The re-

action mixture was extracted as described in the general procedure for

reduction. The olive-grey reduced product 9c was purified by column

chromatography by eluting with a mixture of diethyl ether/n-pentane

(3:2), and was further purified by semipreparative HPLC (102 mg,

0.09 mmol, 68%). Analytical HPLC: 9c was eluted after 6.8 min at a

flow of 1 mLmin^À1with a solvent mixture of methanol/CH₂Cl₂(3:2).

M.p. 104–1068C; ¹H NMR (400 MHz, CDCl₃, TMS, 258C): d=9.51 (s,

1H; 10-H), 9.46 (s, 1H; 5-H), 8.50 (s, 1H; 20-H), 6.25 (s, 2H; 6’-H, 2’-H),

5.90 (m, 2H; 3¹-H), 5.25 (d, ²J=19.8 Hz, 1H; 13²-H), 5.08 (d, ²J=

19.8 Hz, 1H; 13²-H), 4.91 (d, ²J=12.1 Hz, 1H; 1’’-H), 4.67 (d, ²J=

12.3 Hz, 1H; 1’’-H), 4.48 (dq, ³J=7.3, 2.1 Hz, 1H; 18-H), 4.30 (td, ³J=

8.1, 2.4 Hz, 1H; 17-H), 3.84 (t, ³J=6.6 Hz, 2H; OCH₂), 3.79–3.64 (m,

9H; 8¹-H, OCH₂, 12¹-H), 3.37 (s, 3H; 2¹-H), 3.26 (s, 3H; 7¹-H), 2.77–2.68

and 2.54–2.28 (m, 4H; 17¹-H, 17²-H), 1.76 (d, ³J=7.3 Hz, 3H; 18¹-H),

1.72–1.63 (m, 9H; CH₂, 8²-H), 1.34–1.21 (m, 54H; 27CH₂), 0.87 (m,

9H; CH₃), 0.34 (s, 1H; NH), À1.81 ppm (s, 1H; NH); HRMS (ESI): m/z

calcd for C₇₅H₁₁₃N₄O₇[M+H]⁺: 1181.8608; found: 1181.8606.

(m, 4H; OCH₂), 3.77–3.70 (m, 13H; CH₂O, CH₂, 8¹-H, 12¹-H), 3.61 (s,

1

3H; 2¹-H), 3.65–3.48 (m, 20H; CH₂), 3.34 (s, 6H; CH₃), 3.32(s, 3H; 7

-

H), 2.79–2.58 and 2.38–2.30 (m, 4H; 17¹-H, 17²-H), 1.83 (d, ³J=7.3 Hz,

3H; 18¹-H), 1.72(t, ³J=7.6 Hz, 3H; 8²-H), overlap with TMS (1H; NH),

À2.06 ppm (s, 1H; NH); HRMS (ESI): m/z calcd for C₅₇H₇₄N₄O₁₄Na:

1061.5098 [M+Na]⁺; found: 1061.5094.

3-Devinyl-3-hydroxymethyl-13²-demethoxycarbonylpheophorbide a [3’,5’-

bis-(2-{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}ethoxy)]benzyl ester 9d:

Borane-tert-butylamine complex (84.1 mg, 0.96 mmol) was added to a so-

lution of 8d (100 mg, 0.10 mmol) in dry CH₂Cl₂(50 mL), and stirred for

1.5 h at RT. The reaction mixture was then diluted with CH₂Cl₂(100 mL)

and water (50 mL), and the neutralization of the amine complex was per-

formed with a saturated aqueous solution of NH₄Cl (20 mL). The organic

phase was washed several times with water (50 mL) and then dried over

sodium sulfate. The desiccant was removed by filtration and the solvent

was removed under vacuum. The olive-grey colored chlorin 9d was fur-

ther purified by semipreparative HPLC (80.0 mg, 0.07 mmol, 80%). Ana-

lytical HPLC: 9d eluted after 5.6 min at a flow of 1 mLmin^À1using meth-

1

3-Devinyl-3-hydroxymethyl-13²-demethoxycarbonylpheophorbide

a

anol as solvent. M.p. 1648C; H NMR (400 MHz, THF, 258C): d=9.72(s,

1H; 10-H), 9.70 (s, 1H; 5-H), 8.82(s, 1H; 20-H), 6.55 (d, ⁴J=2.3 Hz, 2H;

(3’,4’,5’-tris-dodecyloxy)benzyl ester zinc complex 10c: According to the

general procedure for metalation, 3¹-hydroxy chlorin 9c (80.1 mg,

0.07 mmol) was dissolved in THF (3 mL) followed by the addition of a

saturated solution of zinc acetate in methanol (12mL). After 2h reaction

time, the mixture was worked up according to the general procedure, and

the resulting turquoise product was purified by column chromatography

by using n-pentane/diethyl ether (3:2), and further purified by semipre-

parative HPLC (62.2 mg, 0.05 mmol, 73%). Analytical HPLC: 10c

eluted after 9.8 min at a flow of 1 mLmin^À1with solvent mixture of meth-

anol/CH₂Cl₂(7:3). M.p. 187–1898C; ¹H NMR (400 MHz, [D₅]pyridine,

CDCl₃, 2 58C): d=9.62(s, 1H; 10-H), 9.48 (s, 1H; 5-H), 8.36 (s, 1H; 20-

4

3

2’-H, 6’-H), 6.51 (t, J=2.3 Hz, 1H; 4’-H), 5.93 (d, J=5.7 Hz, 2H; 3¹-H),

5.31 (d, ²J=19.7 Hz, 1H; 13²-H), 5.13 (d, ²J=19.8 Hz, 1H; 13²-H), 5.11

(d, ²J=12.4 Hz, 1H; 1’’-H), 5.03 (d, ²J=12.4 Hz, 1H; 1’’-H), 4.83 (t, ³J=

5.7 Hz, 1H; 3¹-OH), 4.68 (dq, ³J=7.2, 1.9 Hz, 1H; 18-H), 4.46 (td, ³J=

9.1, 2.4 Hz, 1H; 17-H), 4.10–4.07 (m, 4H; OCH₂), 3.83 (q, ³J=7.7 Hz,

2H; 8¹-H), 3.81–3.77 (m, 4H; CH₂O), 3.73 (s, 3H; 12¹-H), 3.71–3.69 (m,

8H; CH₂), 3.65–3.58 (m, 12H; CH₂), 3.53 (s, 3H; 2¹-H), 3.51–3.48 (m,

4H; CH₂), 3.37 (s, 3H; 7¹-H), 3.34 (m, 6H; CH₃), 2.90–2.73 and 2.54–2.35

(m, 4H; 17¹-H, 17²-H), 1.90 (d, ³J=7.2Hz, 3H; 18 ¹-H), 1.81 (t, ³J=

7.6 Hz, 3H; 8²-H), 0.42(s, 1H; NH), À1.74 ppm (s, 1H; NH); HRMS

(ESI): m/z calcd for C₅₇H₇₆N₄O₁₄Na: 1063.5255 [M+Na]⁺; found:

1063.5250.

2

H), 6.53 (s, 2H; 6’-H, 2’-H), 5.91 (s, 2H; 3¹-H), 5.23 (d, J=19.6 Hz, 1H;

2

13²-H), 5.08 (d, J=19.7 Hz, 1H; 13²-H), 5.02(d, ²J=12.0 Hz, 1H; 1’’-H),

2

4.96 (d, J=12.0 Hz, 1H; 1’’-H), 4.42(dq, ³J=7.3, 2.2 Hz, 1H; 18-H), 4.23

3-Devinyl-3-hydroxymethyl-13²-demethoxycarbonylpheophorbide a [3’,5’-

bis-(2-{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}ethoxy)]benzyl ester zinc

3

(td, J=7.9, 2.4 Hz, 1H; 17-H), 3.93 (m, 6H; OCH₂), 3.77 (q, J=7.6 Hz,

Chem. Eur. J. 2008, 14, 7791 – 7807

ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

7803

F. Würthner et al.

complex 10d: According to the general procedure for metalation, chlorin

derivative 9d (40 mg, 0.04 mmol) was dissolved in THF (10 mL), fol-

lowed by the addition of a saturated solution of zinc acetate in methanol

(10 mL), the reaction mixture was stirred for 3 h at RT and for additional

30 min at 408C . The reaction was then terminated by the addition of

CH₂Cl₂(100 mL), water (100 mL), and a saturated aqueous solution of

NaHCO₃(20 mL). The resulting turquoise zinc chlorin 10d was then ex-

tracted several times with CH₂Cl₂(150 mL), and the combined organic

phases were dried over sodium sulfate, the desiccant was removed by fil-

tration and the solvent was removed under vacuum. Zinc chlorin 10d

was further purified by semipreparative HPLC (39.0 mg, 0.04 mmol,

92%). Analytical HPLC: 10d was eluted after 4.0 min at a flow of

1 mLmin^À1with methanol as the solvent. M.p. 231–2348C; ¹H NMR

(400 MHz, THF, 258C): d=9.71 (s, 1H; 10-H), 9.58 (s, 1H; 5-H), 8.58 (s,

1H; 20-H), 6.52 (d, ⁴J=2.3 Hz, 2H; 2’-H, 6’-H), 6.48 (t, ⁴J=2.3 Hz, 1H;

4’-H), 5.81 (d, ³J=6.1 Hz, 2H; 3¹H), 5.20 (d, ²J=19.6 Hz, 1H; 13²-H),

4.51 (q, ³J=7.3 Hz, 1H; 18-H), 4.33 (d, ³J=7.8 Hz, 1H; 17-H), 4.04–3.87

(m, 6H; CH₂O), 3.74–3.47 (m, 47H; CH₂, 8¹-H, 12¹-H), 3.40 (s, 3H; 2¹-

H), 3.35–3.33 (m, 9H; CH₃), 3.28 (s, 3H; 7¹-H), 2.54–2.26 (m, 4H; 17¹-H,

17²-H), 1.80 (d, ³J=7.3 Hz, 3H; 18¹-H), 1.71 ppm (t, ³J=7.6 Hz, 3H; 8²-

H), overlap with TMS (1H; NH), À1.82(s, 1H; NH); HRMS (ESI): m/z

calcd for C₆₆H₉₄N₄O₁₉: 1269.6409 [M+Na]⁺; found: 1269.6405.

3-Devinyl-3-hydroxymethyl-13²-demethoxycarbonylpheophorbide

a

[3’,4’,5’-tris-(2-{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}ethoxy)]benzyl

ester zinc complex 10e: According to the general procedure for metala-

tion, chlorin 9e (76.1 mg, 0.06 mmol) was dissolved in THF (15 mL), fol-

lowed by the addition of a saturated solution of zinc acetate in methanol

(10 mL). The reaction mixture was stirred for 3 h at RT and for an addi-

tional 15 min at 408C. The reaction was terminated by the addition of

CH₂Cl₂(100 mL), water (100 mL), and an aqueous solution of NaHCO₃

(20 mL) and the turquoise zinc chlorin was extracted with CH₂Cl₂

(150 mL) several times. The combined organic phases were dried over

sodium sulfate, the desiccant was filtered off and the solvent was re-

moved under vacuum. The product was purified by semi-preparative

HPLC (45.2mg, 0.03 mmol, 56%). Analytical HPLC: 10e was eluted

after 3.7 min at a flow of 1 mLmin^À1with methanol as the solvent. M.p.

234–2368C; ¹H NMR (400 MHz, CDCl₃, [D₅]pyridine, TMS, 258C): d=

9.56 (s, 1H; 10-H), 9.35 (s, 1H; 5-H), 8.32(s, 1H; 20-H), 6.44 (s, 2H; 2 ’-

H, 6’-H), 5.83 (s, 2H; 3¹-H), 5.18 (d, ²J=19.7 Hz, 1H; 13²-H), 5.04 (d,

²J=19.7 Hz, 1H; 13²-H), 4.91 (d, ²J=12.1 Hz, 1H; 1’’-H), 4.84 (d, ²J=

12.3 Hz, 1H; 1’’-H), 4.39 (dq, ³J=7.3, 2.1 Hz, 1H; 18-H), 4.20 (dt, ³J=

7.8, 2.7 Hz, 1H; 17-H), 4.07–4.01 (m, 6H; OCH₂), 3.74–3.57 (m, 38H;

CH₂, 8¹-H), 3.68 (s, 3H; 12¹-H), 3.52–3.48 (m, 6H; CH₂), 3.33 (m, 9H;

CH₃), 3.30 (s, 3H; 2¹-H), 3.21 (s, 3H; 7¹-H), 3.02(s, 1H; 3 ¹-OH), 2.61–

2.56 and 2.44–2.25 and 2.03–1.96 (m, 4H; 17¹-H, 17²-H), 1.72–1.68 ppm

(m, 6H; 18¹-H, 8²-H); HRMS (ESI): m/z calcd for C₆₆H₉₂N₄O₁₉Zn:

1308.5647 [M]⁺; found: 1308.5643; UV/Vis (THF): l_max(e _max)=647 nm

(94000m^À1cm^À1).

2

5.04 (d, J=19.5 Hz, 1H; 13²-H), 5.08 (d, J=12.4 Hz, 1H; 1’’-H), 5.02(d,

3

²J=12.3 Hz, 1H; 1’’-H), 4.63 (dq, J=7.2, 2.1 Hz, 1H; 18-H), 4.50 (t, J=

5.7 Hz, 1H; 3¹-OH), 4.41 (td, ³J=8.1, 1.9 Hz, 1H; 17-H), 4.07–4.05 (m,

4H; OCH₂), 3.89 (q, ³J=7.7 Hz, 2H; 8¹-H), 3.79–3.76 (m, 4H; CH₂O),

3.71 (s, 3H; 12¹-H), 3.68–3.66 (m, 12H; 6CH₂), 3.65–3.58 (m, 8H; 4

CH₂), 3.50–3.47 (m, 4H; CH₂), 3.41 (s, 3H; 2¹-H), 3.38 (s, 3H; 7¹-H), 3.33

(m, 6H; CH₃), 2.83–2.68 and 2.50–2.36 (m, 4H; 17¹-H, 17²-H), 1.88 (d,

³J=7.2Hz, 3H; 18 ¹-H), 1.81 ppm (t, ³J=7.7 Hz, 3H; 8²-H); HRMS

(ESI): m/z calcd for C₅₇H₇₄N₄O₁₄ZnNa: 1125.4390 [M+Na]⁺; found:

1125.4385; UV/Vis (THF): l_max(e_max)=648 nm (91000m^À1cm^À1).

3-Devinyl-3-formyl-13²-demethoxycarbonylpheophorbide

a [3’,4’,5’-tris-

(2-{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}ethoxy)]benzyl ester 8e: Ac-

cording to the general procedure for esterification, chlorin derivative 6a

(80.1 mg, 0.15 mmol) was dissolved in dry CH₂Cl₂(10 mL), followed by

the addition of 11e (218 mg, 0.30 mmol), DCC (247 mg, 1.20 mmol),

DMAP (92.0 mg, 0.75 mmol), and DPTS (221 mg, 0.75 mmol). The reac-

tion mixture was stirred for 10 min at RT, and subsequently, Hünigs base

(62.0 mg, 0.48 mmol) was added. After stirring for 3 h, the reaction mix-

ture was treated with CH₂Cl₂(100 mL) and a saturated solution of

NH₄Cl (20 mL), then washed with water (50 mL) several times. The

aqueous phase was re-extracted with CH₂Cl₂(2100 mL). The combined

organic phases were dried over Na₂SO₄and the solvent was removed

under vacuum. The resulting product was purified on a silica gel column

with a mixture of CH₂Cl₂/methanol (98:2) as the eluent and further puri-

fied by semipreparative HPLC (118 mg, 0.09 mmol, 63%). Analytical

HPLC: 8e was eluted after 8.5 min at a flow of 1 mLmin^À1with metha-

nol as the solvent. M.p. 88–928C; ¹H NMR (400 MHz, CDCl₃, TMS,

258C): d=11.56 (s, 1H; CHO), 10.36 (s, 1H; 10-H), 9.66 (s, 1H; 5-H),

3-Devinyl-3¹-hydroxyethyl-13²-demethoxycarbonylpheophorbide

a

methyl ester 12: The synthesis was performed according to the known lit-

erature methods developed by Tamiaki and Hynninen.^[16a,19]Pheo a 5

(460 mg, 0.840 mmol) was dissolved in 38% HBr in acetic acid (10 mL)

and stirred at RT overnight. The reaction mixture was poured into ice

water (100 mL) and extracted with CHCl₃(4100 mL). The combined

organic phases were washed several times with a saturated solution of

NaHCO₃(100 mL) and water (2100 mL) and dried over Na₂SO₄. After

removal of the drying agent by filtration, solvent was removed under

vacuum, the residue dissolved in methanol (60 mL), followed by addition

of concentrated HCl (6 mL), and stirred for 30 min. Finally, the olive-

green colored product was purified by column chromatography on a

silica gel column with a mixture of CH₂Cl₂/methanol (9:1) as the eluent

(377 mg, 0.67 mmol, 79%). The analytical data were in accordance with

those reported in the literature.^[19]

3-Devinyl-3-acetyl-13²-demethoxycarbonylpheophorbide a methyl ester

13: Compound 13 was synthesized from 12 according to a known litera-

ture method, and the analytical data are in accordance with those report-

ed.^[20]

3-Devinyl-3-acetyl-13²-demethoxycarbonylpheophorbide a (3’,5’-bis-dode-

cyloxy) benzyl ester 14: Starting with 3-acetylchlorin 13 (115 mg,

0.20 mmol), the free carboxylic acid derivative was generated by adding

concentrated hydrochloric acid (5–10 mL) and stirring for 5–6 h at RT.

The reaction mixture was adjusted to pH 6–7 by careful addition of a sa-

turated aqueous solution of NaHCO₃. The intermediate product was ex-

tracted with CH₂Cl₂and dried over sodium sulfate. After removal of the

drying agent, the solvent was removed, and the carboxylic acid was puri-

fied by column chromatography with a mixture of CH₂Cl₂/methanol (9:1)

as the eluent (82.3 mg, 0.15 mmol, 73%).

2

8.87 (s, 1H; 20-H), 6.50 (s, 2H; 2’-H, 6’-H), 5.35 (d, J=20.0 Hz, 1H; 13²-

H), 5.18 (d, ²J=20.0 Hz, 1H; 13²-H), 4.91 (d, ²J=12.3 Hz, 1H; 1’’-H),

4.87 (d, ²J=12.1 Hz, 1H; 1’’-H), 4.58 (q, ³J=7.4 Hz, 1H; 18-H), 4.39 (d,

³J=8.3 Hz, 1H; 17-H), 4.08–4.05 (m, 6H; OCH₂), 3.78–3.58 (m, 47H;

CH₂, 8¹-H, 12¹-H, 2¹-H), 3.53–3.48 (m, 9H; CH₂), 3.35 (s, 3H; 7¹-H), 3.34

3

(s, 9H; CH₃), 2.80–2.60 and 2.38–2.30 (m, 4H; 17¹-H, 17²-H), 1.83 (d, J=

7.3 Hz, 3H; 18¹-H), 1.73 (t, ³J=7.6 Hz, 3H; 8²-H), overlap with TMS

signal (1H; NH), À2.05 ppm (s, 1H; NH); HRMS (ESI): m/z calcd for

C₆₆H₉₂N₄O₁₉Na: 1267.6252 [M+Na]⁺, found: 1267.6248.

3-Devinyl-3-hydroxymethyl-13²-demethoxycarbonylpheophorbide

a

[3’,4’,5’-tris-(2-{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}ethoxy)]benzyl

ester 9e: Borane-tert-butylamine complex (82.3 mg, 0.95 mmol) was

added to a solution of 8e (118 mg, 0.09 mmol) in dry CH₂Cl₂(20 mL),

and stirred for 2h at RT. The reaction mixture was then extracted with

CHCl₃(100 mL) and the amine complex was neutralized with a saturated

aqueous solution of NH₄Cl. The organic phase was washed several times

with water and then dried over sodium sulfate. The desiccant was filtered

and the solvent was removed under vacuum. The olive-grey product 9e

was purified on a silica gel column, and then by semipreparative HPLC

(86.0 mg, 0.07 mmol, 73%). Analytical HPLC: 9e was eluted after

5.2min at a flow of 1 mLmin ^À1with methanol as the solvent. M.p.

1198C; ¹H NMR (400 MHz, CDCl₃, TMS, 258C): d=9.58 (s, 1H; 10-H),

9.53 (s, 1H; 5-H), 8.57 (s, 1H; 20-H), 6.27 (s, 2H; 2’-H, 6’-H), 5.92(m,

2H; 3¹-H), 5.27 (d, ²J=20.1 Hz, 1H; 13²-H), 5.11 (d, ²J=19.8 Hz, 1H;

13²-H), 4.85 (d, ²J=12.5 Hz, 1H; 1’’-H), 4.61 (d, ²J=12.4 Hz, 1H; 1’’-H),

Because the acid was difficult to purify by HPLC, it was used for esterifi-

cation without further characterization. For this purpose, the carboxylic

acid (75.1 mg, 0.14 mmol) was dissolved in dry CH₂Cl₂(15 mL), and 11b

(212 mg, 0.44 mmol) was added, followed by the addition of the vacuum-

dried coupling reagents DCC (184 mg, 0.89 mmol), DMAP (67.0 mg,

0.55 mmol), and DPTS (161 mg, 0.55 mmol) and the reaction mixture

was stirred for 2.5 h at RT. The reaction mixture was directly subjected

7804

www.chemeurj.org

ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 2008, 14, 7791 – 7807

Self-assembled Zinc Chlorins

FULL PAPER

to purification by column chromatography with a mixture of diethyl

ether/n-pentane (1:1) as the eluent. It was additionally purified by semi-

preparative HPLC (57.3 mg (0.06 mmol, 42%). Analytical HPLC: 14 was

eluted after 15.59 min at a flow of 1 mLmin^À1with a solvent mixture of

methanol/CH₂Cl₂(7:3). M.p. 648C; ¹H NMR (400 MHz, CDCl₃, 2 58C):

d=10.01 (s, 1H; 5-H), 9.62(s, 1H; 10-H), 8.76 (s, 1H; 20-H), 6.38 (d,

⁴J=2.3 Hz, 2H; 2’-H, 6’-H), 6.34 (t, ⁴J=2.2 Hz, 1H; 4’-H), 5.31 (d, ²J=

19.8 Hz, 1H; 13²-H), 5.14 (d, ²J=19.7 Hz, 1H; 13²-H), 5.01 (d, ²J=

12.3 Hz, 1H; 1’’-H), 4.93 (d, ²J=12.3 Hz, 1H; 1’’-H), 4.53 (dq, ³J=7.2,

1.4 Hz, 1H; 18-H), 4.36 (d, ³J=7.6 Hz, 1H; 17-H), 3.83 (t, ³J=6.6 Hz,

4H; OCH₂), 3.74 (q, ³J=7.7 Hz, 2H; 8¹-H), 3.72(s, 3H; 12 ¹-H), 3.65 (s,

3H; 2¹-H), 3.30 (s, 3H; 7¹-H), 3.29 (s, 3H; 3²-H), 2.79–2.59 and 2.37–2.31

(m, 4H; 17¹-H, 17²-H), 1.81 (d, ³J=7.3 Hz, 3H; 18¹-H), 1.74–1.66 (m,

7H; CH₂, 8²-H), 1.35–1.22 (m, 36H; 18CH₂), 0.87 (t, ³J=6.9 Hz, 6H;

CH₃), À0.06 (s, 1H; NH), À2.01 ppm (s, 1H; NH); HRMS (ESI): m/z

calcd for C₆₄H₈₈N₄O₆[M]⁺: 1008.6703; found: 1008.6647 [M]⁺and

1009.6794 [M+H]⁺.

togram showed 95% diastereomeric purity (7.98 mg, 0.008 mmol, 56%).

¹H NMR (400 MHz, CDCl₃, 2 58C): d=9.87 (s, 1H; 5-H), 9.63 (s, 1H; 10-

H), 8.60 (s, 1H; 20-H), 6.45 (m, 1H; 3¹-H), 6.32(m, 3H; 2 ’-H, 4’-H, 6’-

H), 5.27 (d, ²J=20.0 Hz, 1H; 13²-H), 5.12(d, ²J=20.0 Hz, 1H; 13²-H),

2

4.98 (d, J=12.4 Hz, 1H; 1’’-H), 4.85 (d, J=12.3 Hz, 1H; 1’’-H), 4.53 (m,

1H; 18-H), 4.34 (m, 1H; 17-H), 3.82–3.74 (m, 6H; OCH₂, 8¹-H), 3.70 (s,

3H; 12¹-H), 3.41 (s, 3H; 2¹-H), 3.29 (s, 3H; 7¹-H), 2.78–2.53 and 2.39–

2.30 (m, 4H; 17¹-H, 17²-H), 2.17 (d, ³J=6.5 Hz, 3H; 3²-H), 1.79 (d, ³J=

7.2Hz, 3H; 18 ¹-H), 1.73–1.65 (m, 7H; 8²-H, CH₂), 1.34–1.23 (m, 36H;

18CH₂), 0.87 (t, ³J=6.8 Hz, 6H; CH₃), broad signal (1H; NH),

À1.83 ppm (s, 1H; NH).

3-Devinyl-(3¹S)-hydroxymethyl-13²-demethoxycarbonylpheophorbide

a

(3’,5’-bis-dodecyloxy)benzyl ester zinc complex (3¹S)-17: According to

the general procedure for metalation, chlorin (3¹S)-16 (3.98 mg,

0.004 mmol) was dissolved in THF (3.5 mL), followed by the addition of

a saturated solution of zinc acetate in methanol (6 mL) and the mixture

was stirred for 1 h at RT. After extraction according to the general proce-

dure, the turquoise zinc chlorin (3¹S)-17 was purified by semipreparative

chiral HPLC column (2.98 mg, 0.002 mmol, 55%). Analytical HPLC:

(3¹S)-17 was eluted after 13.0 min at a flow of 1 mLmin^À1with a solvent

mixture of n-hexane/CH₂Cl₂/methanol (70:25:5). The integration of

HPLC chromatogram showed 100% diastereomeric purity. M.p. 2148C;

¹H NMR: (400 MHz, [D₈]THF, 258C): d=9.70 (s, 1H; 5-H), 9.60 (s, 1H;

10-H), 8.45 (s, 1H; 20-H), 6.40 (d, ⁴J=2.3 Hz, 2H; 2’-H, 6’-H), 6.32–6.30

3-Devinyl-(3¹R/S)-hydroxymethyl-13²-demethoxycarbonylpheophorbide a

(3’,5’- bis-dodecyloxy)benzyl ester 15: 3-Acetyl chlorin 14 (28.0 mg,

0.03 mmol) was dissolved in dry THF (2mL) and dry ethanol (10 mL)

was added. Subsequently, sodium borohydride (4.40 mg, 0.12mmol) was

added and was stirred for 70 min at RT. The reaction was terminated by

the addition of water (15 mL) and the pH of the reaction mixture was ad-

justed to 6–7 with a saturated solution of NH₄Cl. The product was ex-

tracted with diethyl ether (3100 mL) and the combined organic phases

were dried over sodium sulfate. After filtration of the drying agent, the

solvent was removed under vacuum and the olive-grey product was puri-

fied by column chromatography with a solvent mixture of diethyl ether/

n-pentane (3:2), and additionally by semipreparative HPLC (19.1 mg,

0.02mmol, 67%). Analytical HPLC: 15 was eluted after 7.0 min

(21.1 min) with a flow of 1 mLmin^À1with a solvent mixture of methanol/

CH₂Cl₂(8:2). ¹H NMR of (R/S)-diastereomer mixture (400 MHz, CDCl₃,

258C): d=9.70/9.65 (s, 1H; 5-H), 9.48/9.47 (s, 1H; 10-H), 8.51/8.49 (s,

1H; 20-H), 6.40 (q, ³J=6.8, 1H; 3¹-H), 6.34–6.32(m, 3H; 2 ’-H, 4’-H, 6’-

H), 5.25/5.23 and 5.20/5.18 (d, ²J=19.7 Hz, 1H; 13²-H), 5.09/5.08 and

5.04/5.03 (d, ²J=19.7 Hz, 1H; 13²-H), 5.00 and 4.97 (d, ²J=12.8 Hz, 1H;

1’’-H), 4.90/4.88 and 4.87/4.85 (d, ²J=12.4 Hz, 1H; 1’’-H), 4.46 (q, ³J=

7.3 Hz, 1H; 18-H), 4.26 (m, 1H; 17-H), 3.84–3.79 (m, 4H; OCH₂), 3.69

2

(m, 1H; 4’-H), 6.29–6.28 (m, 1H; 3¹-H), 5.10 (d, J=19.6 Hz, 1H; 13²-H),

4.99–4.90 (m, 3H; 13²-H, 1’’-H), 4.52(dq, ³J=7.2, 2.2 Hz, 1H; 18-H), 4.30

(dt, ³J=8.1, 2.6 Hz, 1H; 17-H), 3.86–3.76 (m, 6H; OCH₂, 8¹-H), 3.61 (s,

3H; 12¹-H), 3.34 (s, 3H; 2¹-H), 3.28 (s, 3H; 7¹-H), 2.68–2.61 and 2.31–

2.26 (m, 4H; 17¹-H, 17²-H), 2.00 (d, ³J=6.6 Hz, 3H; 3²-H), 1.78 (d, ³J=

7.3 Hz, 3H; 18¹-H), 1.73–1.63 (m, 7H; 8²-H, CH₂), 1.41–1.27 (m, 36H;

18CH₂), 0.88 ppm (t, ³J=6.9 Hz, 6H; CH₃); HRMS (ESI): m/z calcd

for C₆₄H₈₈N₄O₆Zn [M]⁺: 1072.5995; found: 1072.5990; UV/Vis (THF):

l_max(e_max)=648 nm (83700m^À1cm^À1).

3-Devinyl-(3¹R)-hydroxymethyl-13²-demethoxycarbonylpheophorbide

a

(3’,5’-bis-dodecyloxy)benzyl ester zinc complex (3¹R)-17: According to

the general procedure of metalation, (3¹R)-16 (8.001 mg, 0.008 mmol)

was dissolved in THF (4 mL), followed by the addition of a saturated so-

lution of zinc acetate in methanol (6 mL) and the reaction mixture was

stirred for 1 h at RT. After extraction of the reaction mixture, the result-

ing zinc chlorin (3¹R)-17 was purified by semipreparative chiral HPLC

column (3.50 mg, 0.003 mmol, 41%). Analytical HPLC: (3¹R)-17 was

eluted after 13.8 min at a flow of 1 mLmin^À1with a solvent mixture of n-

hexane/CH₂Cl₂/methanol (70:25:5). The integration of HPLC chromato-

gram showed 100% diastereomeric purity. M.p. 2098C; ¹H NMR

(400 MHz, [D₈]THF, 258C): d=9.75 (s, 1H; 5-H), 9.60 (s, 1H; 10-H),

8.44 (s, 1H; 20-H), 6.40 (d, ⁴J=2.3 Hz, 2H; 2’-H, 6’-H), 6.32(t, ⁴J=

2.3 Hz, 1H; 4’-H), 6.26 (dq, ³J=6.7, 2.6 Hz, 1H; 3¹-H), 5.09 (d, ²J=

19.6 Hz, 1H; 13²-H), 4.97–4.90 (m, 3H; 13²-H, 1’’-H), 4.53 (dq, ³J=7.3,

2.0 Hz, 1H; 18-H), 4.29 (dt, ³J=8.3, 2.7 Hz, 1H; 17-H), 3.86–3.76 (m,

6H; OCH₂, 8¹-H), 3.60 (s, 3H; 12¹-H), 3.31 (s, 3H; 2¹-H), 3.28 (s, 3H; 7¹-

H), 2.73–2.59 and 2.35–2.27 (m, 4H; 17¹-H, 17²-H), 2.01 (d, ³J=6.6 Hz,

3H; 3²-H), 1.77 (d, ³J=7.3 Hz, 3H; 18¹-H), 1.69–1.63 (m, 7H; 8²-H,

CH₂), 1.38–1.27 (m, 36H; 18CH₂), 0.88 ppm (t, ³J=6.8 Hz, 6H; CH₃);

HRMS (ESI): m/z calcd for C₆₄H₈₈N₄O₆Zn [M]⁺: 1072.5995; found:

1072.5990; UV/Vis (THF): l_max(e_max)=648 nm (87600m^À1cm^À1).

3

(q, J=6 Hz, 2H; 8¹-H), 3.64 (s, 3H; 12¹-H), 3.40/3.37 (s, 3H; 2¹-H), 3.25

(s, 3H; 7¹-H), 2.74–2.51 and 2.36–2.27 (m, 4H; 17¹-H, 17²-H), 2.15/2.14

and 2.13/2.12 (d, ³J=6.7 Hz, 3H; 3²-H), 1.76 (t, ³J=7.7 Hz, 3H; 8²-H),

1.78–1.63 (m, 7H; 18¹-H, CH₂), 1.34–1.22 (m, 36H; 18CH₂), 0.87 (t,

³J=6.9 Hz, 6H; CH₃), (undefined broad signal, 1H; NH), À1.85 ppm (s,

1H; NH); HRMS (ESI): m/z calcd for C₆₄H₉₀N₄O₆[M]⁺: 1010.6860;

found: 1010.6855 [M]⁺and 1011.6961 [M+H]⁺.

The diastereoisomers were in a 1:1 ratio and they were successfully sepa-

rated by HPLC using a semipreparative chiral column (reprosil 100

chiral-NR) with a solvent mixture of n-hexane/CH₂Cl₂(1:1).

3-Devinyl-(3¹S)-hydroxymethyl-13²-demethoxycarbonylpheophorbide

a

(3’,5’-bis-dodecyloxy)benzyl ester (3¹S)-16: Analytical HPLC on chiral

column: (3¹S)-16 was eluted after 17.7 min at a flow of 1 mLmin^À1in

eluting mixture of n-hexane/CH₂Cl₂(1:1). The integration of the HPLC

chromatogram showed almost 100% diastereomeric purity (4.01 mg,

1

0.004 mmol, 28%). H NMR (400 MHz, CDCl₃, 2 58C): d=9.88 (s, 1H; 5-

3

H), 9.68 (s, 1H; 10-H), 8.66 (s, 1H; 20-H), 6.48 (q, J=6.6 Hz, 1H; 3¹-H),

6.35 (m, 3H; 2’-H, 4’-H, 6’-H), 5.29 (d, ²J=19.8 Hz, 1H; 13²-H), 5.12(d,

²J=19.8 Hz, 1H; 13²-H), 4.98 (d, ²J=12.4 Hz, 1H; 1’’-H), 4.89 (d, ²J=

12.3 Hz, 1H; 1’’-H), 4.53 (q, ³J=7.2Hz, 1H; 18-H), 4.34 (d, ³J=8.08 Hz,

1H; 17-H), 3.82(dt, ³J=6.6 Hz, ²J=2.2 Hz, 4H; OCH₂), 3.76 (q, ³J=

7.8 Hz, 2H; 8¹-H), 3.71 (s, 3H; 12¹-H), 3.44 (s, 3H; 2¹-H), 3.30 (s, 3H; 7¹-

H), 2.80–2.57 and 2.39–2.31 (m, 4H; 17¹-H, 17²-H), 2.16 (d, ³J=6.7 Hz,

3H; 3²-H), 1.81 (d, ³J=7.3 Hz, 3H; 18¹-H), 1.73–1.66 (m, 7H; 8²-H,

Acknowledgements

We thank Dr. Marina Knoll for AFM measurements. We are grateful to

the Volkswagen Foundation for the financial support of this work within

the research priority program “Complex Materials: Cooperative Projects

of the Natural, Engineering and Biosciences”.

3

CH₂), 1.39–1.23 (m, 36H; 18CH₂), 0.87 (t, J=6.7 Hz, 6H; CH₃), broad

signal (1H; NH), À1.84 ppm (s, 1H; NH).

3-Devinyl-(3¹R)-hydroxymethyl-13²-demethoxycarbonylpheophorbide

a

(3’,5’-bis-dodecyloxy)benzyl ester (3¹R)-16: Analytical HPLC on chiral

column: (3¹R)-16 was eluted after 19.7 min at a flow of 1 mLmin^À1with

solvent mixture n-hexane/CH₂Cl₂(1:1). Integration of the HPLC chroma-

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Self-assembled Zinc Chlorins

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Received: April 22, 2008

Published online: July 18, 2008

Chem. Eur. J. 2008, 14, 7791 – 7807

ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

7807

Article Doi

Structure-property relationships for self-assembled zinc chlorin light-harvesting dye aggregates

DOI: 10.1002/chem.200800764

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