Design and synthesis of light-harvesting rods for intrinsic rectification of the migration of excited-state energy and ground-state holes

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Design and synthesis of light-harvesting rods for intrinsic

rectiﬁcation of the migration of excited-state energy and

ground-state holes{

Robert S. Loewe,^aRobin K. Lammi,^bJames R. Diers,^cChristine Kirmaier,^b

David F. Bocian,*^cDewey Holten*^band Jonathan S. Lindsey*^a

^aDepartment of Chemistry, North Carolina State University, Raleigh, North Carolina,

27695-8204, USA

^bDepartment of Chemistry, Washington University, St. Louis, Missouri, 63130-4889, USA

^cDepartment of Chemistry, University of California, Riverside, California, 92521-0403, USA

Received 8th September 2001, Accepted 20th January 2002

First published as an Advance Article on the web 18th March 2002

We present the design of molecular materials for ultimate use in solid-state solar cells. The molecular materials

are semi-rigid oligomeric rods of deﬁned length with metalloporphyrins in the backbone and a carboxy group

at one end for attachment to a surface. The rods are designed to absorb visible light, and then undergo excited-

state energy transfer and ground-state hole transfer in opposite directions along the length of the rod. The

rational synthesis of the multiporphyrin arrays relies on joining porphyrin building blocks in an efﬁcient and

controlled manner. Several porphyrin building blocks have been synthesized that bear bromophenyl,

iodophenyl, trimethylsilylethynylphenyl and/or ethynylphenyl substituents for use in a copper-free Sonogashira

reaction using Pd₂(dba)₃and P(o-tol)₃. Competition experiments performed on equimolar quantities of an iodo-

porphyrin and a bromo-porphyrin with an ethynyl-porphyrin show iodo 1 ethyne coupling with a low amount

(35 uC) or undetectable amount (22 uC) of bromo 1 ethyne coupling. Efﬁcient coupling of bromo-porphyrins

with ethynyl-porphyrins was achieved using the same copper-free Sonogashira reaction conditions at higher

temperature (50 uC or 80 uC). These ﬁndings allow successive coupling reactions to be achieved using substrates

bearing iodo and bromo synthetic handles. Thus, a porphyrin-based tetrad (or pentad) was synthesized with a

ﬁnal convergent coupling of a bromo-substituted dyad (or triad) and an ethynyl-substituted dyad. A porphyrin

triad was prepared by sequential iodo 1 ethyne coupling reactions. The triad, tetrad, and pentad each are

comprised of a terminal magnesium porphyrin bearing one carboxy group (for surface attachment) and two

pentaﬂuorophenyl groups; the remaining porphyrins in each array are present as the zinc chelate.

Electrochemical characterization of benchmark porphyrins indicates the presence of the desired electrochemical

gradient for hole hopping in the arrays. Static absorption data indicate that the arrays are weakly coupled,

while static ﬂuorescence data indicate that the excited-state energy ﬂows in high yield to the terminal

magnesium porphyrin. Time-resolved spectroscopic analysis leads to rate constants in THF of (9 ps)²¹

(15 ps)²¹, and (30 ps)²¹for ZnMg dyad 20, Zn₂Mg triad 13, and Zn₃Mg tetrad 15, respectively, and

,

quantum efﬁciencies ¢99% for energy ﬂow to the magnesium porphyrin in each case. These design and

synthesis strategies should be useful for the construction of materials for molecular-based solar cells.

generating a transmembrane potential. In bacterial systems,

Introduction

hole transport is mediated by a series of iron porphyrins.²

Thus, four major functions performed by porphyrinic pigments

underlie photosynthetic function: light absorption, excited-

state energy migration, excited-state electron transfer, and

ground-state hole/electron migration. The efﬁciency of these

combined processes stems from the overall design, the inher-

ent optical/electronic properties of the pigments, and the

3-dimensional organization of all components.

Porphyrinic pigments lie at the heart of the photosynthetic

conversion of light into chemical energy.¹Photosynthetic

organisms employ a light-harvesting antenna complex com-

prised of numerous pigments coupled to a reaction center

for energy transduction. Absorption of light by a pigment in

the antenna is followed by rapid singlet excited-state energy

migration among the antenna chromophores until the energy

reaches the lowest energy site, the special pair of porphyrinic

pigments in the reaction center. Vectorial excited-state electron

transfer then occurs, with the electron moving along a chain

of porphyrinic and other cofactors in the reaction center

toward one side of the membrane. The hole remaining on the

special pair migrates in the opposite direction toward the other

membrane surface and out of the reaction center, thereby

Attempts to prepare molecular-based solar cells have

generally employed molecules in conjunction with semicon-

ductors. Three generic designs have been investigated: (I) a

monolayer of pigment bound to a semiconductor surface; (II) a

thin ﬁlm of pigment deposited on a semiconductor surface;

and (III) a monolayer of pigment bound to a mesoporous

semiconductor surface. The absorption of light by the pigment

results in electron injection into the semiconductor, yielding a

charge-separated state. The hole residing on the pigment is then

transferred (typically by a diffusive redox-active agent in a

liquid) to the counter electrode, thereby completing the circuit.

Achieving high efﬁciency in solar-energy conversion requires

absorption of most of the incident light across the solar

{Electronic supplementary information (ESI) available: ¹H and ¹³C

NMR spectra for all new porphyrin precursors; ¹H NMR and LD-MS

spectra for all new porphyrins and porphyrin arrays (LD-MS only for

deprotected arrays 12’ and 14’, and pentad 18); analytical SEC data for

all porphyrin arrays. See http://www.rsc.org/suppdata/jm/b1/b108168c/

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This journal is # The Royal Society of Chemistry 2002

DOI: 10.1039/b108168c

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spectrum, a high quantum yield of electron injection, a low

quantum yield of charge recombination at the electrode

surface, efﬁcient electron transport in the semiconductor to

the external circuit, and efﬁcient transport of the hole to the

counterelectrode. The three designs have

a number of

limitations. In design I, a monolayer of dye typically absorbs

only a tiny fraction of the incident light. In design II, a thin ﬁlm

can absorb much of the incident light but the resulting excited-

state energy may not reach the semiconductor surface, or the

hole generated upon electron injection at the semiconductor

may be trapped in the thin ﬁlm and not reach the counter-

electrode. In design III, although the dye is employed as a

monolayer on the semiconductor surface, the mesoporous

nature of the semiconductor material ensures high absorption

of light. However, the mesoporous structure provides many

opportunities for electron-hole recombination. The efﬁciencies

of these solar cells range from v1% (case I) to a few percent

(case II) to y10% (case III).³

We are working toward a molecular-based solar cell that

employs molecules in conjunction with semiconductors in a

manner inspired by the design of natural photosynthetic

systems. The molecular materials are light-harvesting rods

that can be sandwiched between a semiconductor (anode) and

a counterelectrode (cathode). The rod is a backbone oligomer

comprised of porphyrinic pigments joined in a covalent manner.

Absorption of light by one of the pigments yields the singlet

excited state, which migrates from pigment to pigment along

the backbone until reaching the lowest energy chromophore,

the pigment directly attached to the semiconductor. This last

pigment constitutes the electron-injection unit and undergoes

excited-state electron transfer, injecting an electron into the

semiconductor. The hole residing on the electron-injection unit

then migrates among pigments in the light-harvesting rod until

the counterelectrode is reached, thereby completing the circuit

and regenerating the rod for another photocycle.

A key feature of the molecular design is the ability to cause

excited-state energy and ground-state holes to migrate in

opposite directions along a given light-harvesting rod. Such

intrinsic rectiﬁcation is possible because the physics of the two

processes are fundamentally different, as shown in Fig. 1.

Energy transfer is an excited-state process. Energy transfer

among neighboring pigments occurs reversibly with isoener-

getic pigments and from donor to acceptor when the excited-

state energy (DE) of the acceptor is lower than that of the

donor. Thus, excited-state energy ﬂows along a rod from

pigment i 1 1 to pigment i with excited-state energies such that

Fig. 1 Molecular physics considerations for designing light-harvesting

rods. (A) The energy of the excited-state (DE) is given by the difference

between the energies of the highest occupied molecular orbital

(HOMO) and the lowest unoccupied molecular orbital (LUMO).

Energy migration occurs from a pigment with large DE to a pigment

with smaller DE. (B) The energy level of the ground-state hole is given

by the electrochemical potential (E_1/2) for the one-electron oxidation,

which depends only on the energy of the HOMO. Hole hopping occurs

from a pigment with high potential to a pigment with low potential. (C)

A light-harvesting rod composed of pigments with appropriate energy

levels supports the ﬂow of excited-state energy and ground-state holes

in opposite directions (i.e., intrinsic rectiﬁcation).

DE_(i)< DE_{(i 1 1)}

(1)

The migration of holes is a ground-state process and the direction

of transfer depends on the value of the electrochemical midpoint

potential (E_1/2) for the one-electron oxidation of each pigment

in the rod. Ground-state hole transfer among neighboring

pigments occurs reversibly with isoenergetic pigments, and

irreversibly from (oxidized) pigment i to (non-oxidized) pigment

i 1 1 when the electrochemical potentials are such that

migration. It is noteworthy that this design does not require a

diffusive charge carrier in a liquid as do many molecular-based

solar cells constructed to date. Because the rods need only be

long enough to provide efﬁcient absorption of light (antici-

pated to be less than 50 porphyrinic pigments), the active

medium in the solar cell is expected to have a thickness of less

than 100 nm. Accordingly, this design should afford an

ultrathin solid-state cell.

We have shown that excited-state energy transfer occurs

rapidly and efﬁciently in multiporphyrin arrays in which

metalloporphyrins and/or free base porphyrins are joined via

diphenylethyne^4,5or p-phenylene linkers.⁶We have also shown

that ground-state hole migration is facile among isoenergetic

porphyrins in the same types of multiporphyrin arrays.^7,8

Furthermore, both processes can be tuned by design of the

porphyrin components (which basically retain their inherent

E_1/2(i)w E_{1/2(i 1 1)}

(2)

Thus, a light-harvesting rod comprised of a chain of pigments

that satisﬁes eqns. (1) and (2) should afford intrinsic recti-

ﬁcation in the migration of excited-state energy and ground-

state holes. This is illustrated pictorially in Fig. 1. The excited

energy ﬂows along the rod toward the anode while hole

migration occurs from the pigment (electron-injection unit)

adjacent to the anode back toward the cathode.

In summary, this design provides the same four functions as

are present in natural photosynthetic systems: absorption of

light, transfer of excited-state energy to a speciﬁc site, vectorial

excited-state electron transfer, and vectorial ground-state hole

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However, the cation radicals of Fb porphyrins are generally

photophysical/electronic characteristics when joined via di-

phenylethyne or p-phenylene linkers).⁹In pioneering studies

of synthetic molecules designed for photoinduced charge-

separation processes by Gust, Moore and Moore¹⁰and by

Wasielewski,¹¹hole migration (via a charge-shift reaction)

away from the donor–acceptor pair and toward a redox-active

component at less positive potential was found to provide a

means of stabilizing the charge-separated state. The molecules

in these early studies generally were comprised of carotenoid–

porphyrin–quinone¹⁰or aniline–porphyrin–quinone¹¹species,

while more recent systems that undergo such a charge-shift

reaction have included additional features such as (1) two

porphyrins,¹²(2) arene–imides¹³or buckyballs¹⁴as electron

acceptors in conjunction with multiple porphyrins, or (3)

aniline–monoimide–diimide triads without any porphyrins.¹⁵

Taken together, these data augur well for the design of mole-

cules that support the intrinsic rectiﬁcation of the migration

of excited-state energy and ground-state holes along the length

of a linear rod as described in eqns. (1) and (2). While the design

appears robust, the synthesis of such rods requires the develop-

ment of new synthetic strategies.

In this paper, we ﬁrst describe the design strategy for

constructing linear rods comprised of 3–5 porphyrins for

intrinsic rectiﬁcation of the migration of excited-state energy

and ground-state holes. We then present the synthesis of

porphyrin building blocks for construction of the linear rods.

The porphyrins in the arrays are joined via diphenylethyne

linkers, which are constructed in Pd-mediated coupling reac-

tions of the porphyrin building blocks. Most prior syntheses of

linear multiporphyrin arrays have employed divergent iterative

coupling of iodo and ethynyl substituted porphyrin building

blocks. To achieve convergence in the synthesis of the arrays,

we performed a study of successive iodo 1 ethyne and bromo 1

ethyne coupling reactions with suitable porphyrin building

blocks. The conditions identiﬁed for such successive couplings

enabled the convergent synthesis of light-harvesting rods

consisting of 4 or 5 porphyrins. We conclude with the char-

acterization of the intrinsic properties of the components and

arrays using electrochemical, static absorption and ﬂuores-

cence, and time-resolved absorption techniques.

unstable, making these molecules poor choices to serve as the

simultaneous energy recipient (from the rod) and electron

injector (to the electrode). On the other hand, the cation

radicals of many metalloporphyrins are quite stable.¹⁶The

lowest energy absorption of a magnesium (Mg) porphyrin

typically occurs y10 nm to longer wavelength (0.035 eV

to lower energy) than that of the corresponding zinc (Zn)

porphyrin, enabling a Mg porphyrin to serve as the energy

acceptor with a Zn porphyrin (or multiple Zn porphyrins) as

the energy donor. On the other hand, a Mg porphyrin generally

exhibits an E_1/2value that is y150 mV less positive than that of

the corresponding Zn porphyrin (i.e., the former is more easily

oxidized).^17,18Thus, hole migration cannot occur from a Mg

porphyrin to a Zn porphyrin if the porphyrin macrocycles are

identical. However, the E_1/2values of porphyrins can be tuned

by incorporating electron-deﬁcient or electron-rich substituents

at the perimeter of the macrocycle with little change on the

position of the long-wavelength absorption band.¹⁸Thus, the

design we sought incorporated an electron-deﬁcient Mg

porphyrin as the electron-injection unit at the terminus of

the rod. For attachment to a metal-oxide semiconductor, a

carboxylic acid group was required on the Mg porphyrin. Zn

porphyrins of less positive E_1/2values (increasing electron-

richness) could then be incorporated in the rod with increasing

distance from the Mg porphyrin to facilitate hole migration

away from the electron-injection point and thus toward the

counter electrode.

To identify suitable peripheral substituents for Zn or Mg

porphyrins, an initial survey was performed on a small family

of metalloporphyrins. The Fb porphyrins had been prepared

previously and were metalated under standard conditions. The

results are shown in Table 1. The Mg porphyrin must have a

more positive E_1/2value and a lower energy long-wavelength

absorption band than any of the Zn porphyrins in the array.

Considering zinc tetraphenylporphyrin (ZnTPP, Zn-1a) or zinc

tetramesitylporphyrin (ZnTMP, Zn-1b) as the least electron-

rich of any Zn porphyrin that might be employed, a Mg

porphyrin with two pentaﬂuorophenyl groups (Mg-1e) satisﬁes

both criteria whereas one pentaﬂuorophenyl group yields a Mg

porphyrin that is insufﬁciently electron-deﬁcient (Mg-1d). We

investigated the use of triﬂuoromethyl groups, which cause a

large positive shift in the E_1/2value (e.g., Zn-1f), but were

unable to obtain the magnesium chelate. The introduction of

two p-methoxy groups to ZnTPP (i.e., Zn-1c) shifts the E_1/2

value to less positive values by y30 mV. With these results in

hand, we considered satisfactory designs to incorporate the

following sequence of pigments: a TMP-type Zn porphyrin

Results and discussion

1 Molecular design

Designs for achieving the directed ﬂow of energy in an array of

porphyrins have frequently employed a metalloporphyrin as

the donor and a free base (Fb) porphyrin as the acceptor.

Table 1 Q(0,0) Absorption maxima and electrochemical data for representative metalloporphyrins

Porphyrin

R¹

R²

R³

M

l_absQ(0,0)^a

E_1/2/mV^b

Zn-1a

Zn-1b

Zn-1c

Mg-1d

Mg-1e

Zn-1f

Phenyl

Mesityl

p-Methoxyphenyl

Pentaﬂuorophenyl

Triﬂuoromethyl

Phenyl

Mesityl

Phenyl

p-Tolyl

Phenyl

Mesityl

Phenyl

Mesityl

p-Methoxyphenyl

Phenyl

Pentaﬂuorophenyl

Triﬂuoromethyl

Zn

Mg

Zn

590

592

604

588

0.56

0.51

0.53

0.50

0.66

0.87

b

1

^aIn toluene. E_1/2vs. Ag/Ag¹in CH₂Cl₂containing 0.1 M Bu₄NPF₆; E_1/2of FeCp₂/FeCp₂is 0.19 V; scan rate 0.1 V s²¹

.

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bearing two alkoxy groups, a TMP-type Zn porphyrin bearing

one alkoxy group, a TMP-type Zn porphyrin without other

substituents, and a Mg porphyrin bearing two pentaﬂuoro-

phenyl groups. The presence of a p-carboxy group on the Mg

porphyrin should not adversely affect the desired property of

the array, only shifting the E_1/2to a more positive value

compared with the benchmark compound Mg-1e.

Two other points that bear on design are noteworthy. First,

domains of adjacent isoenergetic pigments can be employed,

affording reversible transfer processes. Second, we have

shown that excited-state energy migration and ground-state

hole-hopping can occur rapidly between non-nearest-neighbor

porphyrins, utilizing the intervening porphyrin and linkers as

superexchange mediators.^7–9,19Thus, the presence of one

pigment (or a small domain of pigments) in a rod with slightly

inappropriate energy/E_1/2characteristics will slow, but not

cause termination of, the ﬂow of excited-state energy or

ground-state holes.

2 Synthesis

Porphyrin building blocks. Many of the porphyrin building

blocks employed in this study were synthesized following a

method for preparing porphyrins bearing up to four different

meso substituents.²⁰The starting point for such porphyrins

begins with the synthesis of 5-substituted dipyrromethanes.

Thus, the one-ﬂask reaction of an aldehyde and excess pyr-

role at room temperature afforded dipyrromethanes 2a–g

(Scheme 1).²¹Dipyrromethanes 2a–e were puriﬁed by distilla-

tion followed by recrystallization while dipyrromethanes 2f,g

were puriﬁed by column chromatography followed by recry-

stallization. These dipyrromethanes are key precursors to the

requisite porphyrin building blocks.

Scheme 2

Monoacyl dipyrromethane 3²⁰was treated with EtMgBr

followed by an acid chloride (4a–c) affording the correspond-

ing diacyldipyrromethane (5a–c) in good yield (Scheme 2).

These compounds are important precursors in the synthesis of

trans-AB₂C- and ABCD-porphyrin building blocks.

appropriate dipyrromethane (2a–g) under non-scrambling

conditions (30 mM triﬂuoroacetic acid (TFA) in CH₃CN at

room temperature) for a few minutes, followed by the addition

of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) to achieve

The dipyrromethanes 2b,d,e were treated with EtMgBr and

pentaﬂuorobenzoyl chloride, affording diacyldipyrromethanes

5d–f (Scheme 3). Compounds 5d,e serve as direct precursors to

trans-AB₂C-porphyrins while compound 5f is employed as a

building block in the synthesis of an A₃B-porphyrin.

The target porphyrin building blocks were prepared by

reducing the appropriate diacyldipyrromethane (5a–f) to the

correspondingdipyrromethane-dicarbinolusingexcessNaBH₄.²⁰

The dipyrromethane-dicarbinol was then condensed with the

Scheme 3

Scheme 1

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Scheme 4

oxidation (Scheme 4).²⁰Following this procedure, ABCD-

porphyrins 6a,b, trans-AB₂C-porphyrins 6c–f, and A₃B-porphyrin

6g were prepared in yields ranging from 18 to 26%. In each

case, analysis of the crude reaction mixture by laser desorption

mass spectrometry (LD-MS) showed no detectable formation

of new porphyrins derived from acidolysis and undesired

recombination (i.e. scrambling). The porphyrins were puriﬁed in

the following manner: (1) ﬁltration of the crude reaction mixture

through alumina to remove quinone species, (2) removal of

solvent under reduced pressure, (3) one silica gel chromato-

graphy procedure to remove non-porphyrinic pigments, and (4)

sonication of the porphyrin product suspended in methanol

followed by ﬁltration to remove trace amounts of soluble

impurities.

Porphyrins 6a–d and 6g were metalated using Zn(OAc)₂?

2H₂O in CHCl₃to afford the zinc chelates Zn-6a–d and Zn-6g

in yields ranging from 75 to 94%. Porphyrins 6e,f were

metalated using MgBr₂?O(Et)₂and triethylamine in CH₂Cl₂to

afford the magnesium chelates Mg-6e,f.²²Porphyrin 6g was

also metalated using MgI₂and N,N-diisopropylethylamine in

CH₂Cl₂to give the magnesium chelate Mg-6g in 81% yield. Remo-

val of the trimethylsilyl (TMS) protecting group of porphyrin

Zn-6a or Zn-6c was achieved using tetra-n-butylammonium

ﬂuoride (TBAF) in THF–CHCl₃(2 : 1) to afford porphyrin

Zn-6a’ or Zn-6c’ in 96% or 77% yield, respectively. Porphyrins

Mg-6h, Mg-6i’, and Zn-6i’ were each obtained from a

mixed-aldehyde condensation followed by metal insertion

and subsequent deprotection where required. These porphy-

rin building blocks were employed in the cross-coupling

experiments and array syntheses described below (see Schemes

5 and 6).

Investigation of successive ethyne 1 halo coupling as a means

of convergence. The design of the light-harvesting rods requires

control over the composition of each porphyrin unit in the

rod. Thus, a stepwise synthetic method is suggested. The

synthesis of diphenylethyne-linked multiporphyrin arrays via

the Sonogashira reaction entails the palladium-catalyzed cross-

coupling of porphyrins bearing iodophenyl and ethynylphenyl

substituents.^23–25Several routes are outlined in Fig. 2. The

distinctions among these methods include the number of

porphyrin building blocks employed, the number of coupling

steps, the severity of the conditions, and the control afforded

over the composition of the target array.

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Fig. 2 Synthetic routes for the preparation of multiporphyrin arrays. ‘‘X’’ refers to a porphyrinic component. (A) Iterative divergent coupling

yielding a porphyrin triad. (B) Iterative divergent–convergent coupling employing the use of the N,N-diethyltriazene moiety as a latent iodo group

(transformed upon treatment with methyl iodide at 100 uC), yielding a porphyrin dyad. This cycle can be repeated to construct longer arrays. (C)

Iterative divergent–convergent coupling employing successive iodo 1 ethyne and bromo 1 ethyne reactions, yielding a porphyrin tetrad.

The divergent, iterative coupling of an ethynyl-porphyrin

and a porphyrin bearing an iodo group plus a trimethylsilyl-

protected ethyne affords a porphyrin dyad (Fig. 2A). Cleavage

of the trimethylsilyl group unveils the free ethyne and sets the

stage for a second Pd-mediated coupling reaction with another

bifunctional iodo/trimethylsilylethynyl porphyrin. This strat-

egy works well for small arrays²⁶but lacks the power of

convergence needed to gain entry into larger arrays. One

convergent approach, ﬁrst demonstrated by Moore in the

synthesis of oligomers of ethynylphenyl units²⁷and extended

by Gossauer to porphyrins,²⁸employs a porphyrin building

block bearing one N,N-diethyltriazene group and one tri-

methylsilylethyne group (Fig. 2B). One batch is reacted with

methyl iodide, which results in replacement of the N,N-

diethyltriazene group with the iodo group, forming the iodo/

trimethylsilylethynyl porphyrin. The other batch is deprotected

with base or ﬂuoride, forming the N,N-diethyltriazene/ethynyl

porphyrin. Each porphyrin can be demetalated and/or metalated

as desired. The two porphyrins are then subjected to a standard

Sonogashira reaction, yielding the ethyne linkage joining the

dyad. Repetition of this cycle of divergent/convergent reactions

(not shown) has been used to create a hexameric array of

porphyrins.²⁸

Upon reaction at 35 uC, analysis of both crude and puriﬁed

samples by LD-MS showed a large mass envelope centered at

m/z ~ 1546 corresponding to dyad 7, as well as a small peak

centered at m/z ~ 1464 corresponding to dyad 8 (Fig. 3A).

These results indicate preferential but not exclusive selectivity

for Sonogashira coupling with the iodo-porphyrin versus

bromo-porphyrin. Upon reaction at room temperature (22 uC)

instead of 35 uC, both the matrix-assisted laser desorption

ionization mass spectrometry (MALDI-MS) using the matrix

1,4-bis(5-phenyloxazol-2-yl)benzene (POPOP) and LD-MS

spectra of the crude and puriﬁed samples showed a very clean

dyad mass region. Fig. 3B shows the MALDI-MS (POPOP)

spectrum of the puriﬁed dyad fraction. No mass envelope

corresponding to 8 could be detected. Thus, the lower reaction

temperature reduces the amount of bromo 1 ethyne coupling

to undetectable levels by MALDI-MS. However, the lower

reaction temperature also slowed the rate of cross-coupling as

evidenced by the analytical SEC data: after 2 h the percent

conversion to dyad was nearly twice as high at 35 uC compared

to 22 uC (63% vs. 34%).³⁰Nevertheless, the absence of a detect-

able amount of bromo coupling products at the slightly lower

reaction temperature makes this the preferred reaction condition.

A complementary approach toward achieving convergence

in the use of the Sonogashira reaction could be achieved by

performing a selective iodo 1 ethyne coupling followed by a

bromo 1 ethyne coupling (Fig. 2C). In this regard, several

examples with small organic molecules have demonstrated

selective coupling of the iodo functionality in the presence of

the bromo functional group.²⁹However, no previous attempts

have been made to examine bromo-porphyrins in this capacity.

Sonogashira coupling with bromo-porphyrins. The ability to

perform successive Sonogashira couplings with iodo and

bromo groups requires satisfactory conditions for bromo 1

ethyne coupling. In the ﬁrst experiment, porphyrins Zn-6g and

Zn-6i’ were subjected to our standard Pd-coupling conditions

with two modiﬁcations: (1) a 2 : 1 ratio of P(o-tol)₃: Pd was

employed rather than the usual 4 : 1 ratio thereby generating a

more active catalytic system, and (2) the reaction temperature

was 80 uC (Scheme 6). Aliquots were removed and analyzed at

1 and 3 h by SEC and LD-MS, then the crude reaction mix-

ture was puriﬁed according to established procedures.²⁵The

analytical SEC data showed no change in the amount of dyad

formation at 1 h and 3 h (60% dyad after 1 h; 62% dyad after

3 h).³⁰There was a signiﬁcant amount of higher molecular

weight material (HMWM) present at both timepoints (14%

after 1 h; 17% after 3 h). The LD-MS data of puriﬁed 9 showed

an intense peak centered at 1550 amu (Fig. 4). No observable

peaks resulting from tolylation (1642 amu) or homocoupling

(1512 amu) could be detected, indicating the cleanliness of this

reaction.

Chemoselectivity of iodo versus bromo coupling. To identify

conditions for performing successive Sonogashira couplings,

we ﬁrst examined the selectivity of cross-coupling of an iodo-

porphyrin and an ethynyl-porphyrin in the presence of a

bromo-porphyrin. In this competition experiment, equimolar

quantities of Mg-6g, Mg-6h and Mg-6i’ were subjected to

Sonogashira coupling conditions developed previously for the

synthesis of multiporphyrin arrays (Scheme 5).^24,25The

progress of this reaction was monitored by analytical size-

exclusion chromatography (SEC) and LD-MS. Tolyl and

mesityl groups were employed in the bromo-porphyrin and

iodo-porphyrin, respectively, to provide an adequate mass

difference between dyads 7 and 8, allowing for analysis by LD-

MS (7 has 1546 amu; 8 has 1464 amu). Dyad 7 is the product

from the desired iodo 1 ethyne coupling reaction; dyad 8 is the

product from unwanted bromo 1 ethyne coupling.

A second experiment was performed with the following

modiﬁcations: (1) the reaction was carried out at 50 uC, (2) a

4 : 1 ratio of P(o-tol)₃: Pd was employed, and (3) magne-

sium bromo-porphyrin Mg-6g was used in place of zinc

bromo-porphyrin Zn-6g to test compatibility with the more

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Fig. 4 LD-MSspectrumofpuriﬁeddyad9(reactiontemperature~80uC).

time, the analytical SEC showed a 77% yield of dyad, a

signiﬁcant improvement compared to the 1 and 2 h timepoints.

Importantly, the percentage of HMWM was much lower than

the previous experiment, which is likely due to the reduced

reaction temperature and/or higher ratio of ligand to palladium

employed. Chromatographic workup afforded dyad 10 in 75%

Fig. 3 (A) LD-MS spectrum of the dyad fraction from competition

experiments performed at 35 uC. (B) MALDI-MS spectrum of the dyad

fraction from competition experiments performed at 22 uC. Dyad 8

(undesired) is the product of bromo 1 ethyne coupling. Only dyad 7,

the product of iodo 1 ethyne coupling, is detected.

1

yield. The H NMR and LD-MS spectra of dyad 10 showed

no detectable impurities (see Electronic Supplementary Infor-

mation{). Although additional amounts of palladium and

phosphine ligand were employed to obtain the high yield, this

procedure demonstrates clean Sonogashira coupling of bromo 1

ethynyl porphyrins under gentle reaction conditions.

labile magnesium chelate (Scheme 6). SEC analysis showed

36% dyad formation within 1 h of reaction.³⁰This value

increased to only 38% after 2 h; therefore, a second batch of

Pd₂(dba)₃and P(o-tol)₃was added. After 3.5 h total reaction

Porphyrin light-harvesting rods. We have prepared three

multiporphyrin arrays that are designed to meet the criteria for

Scheme 6

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Under these conditions, the trimethylsilylethynyl-functionalized

intrinsic rectiﬁcation of the migration of excited-state energy

and ground-state holes. The terminal unit in each array is

a bis(pentaﬂuorophenyl)magnesium porphyrin bearing one

carboxy group. The procedures developed for successive

Sonogashira coupling of iodo- and bromo-substituted por-

phyrin building blocks have been employed where possible. In

each case, the porphyrin array was characterized by absorption

spectroscopy, ﬂuorescence spectroscopy, mass spectrometry,

TLC, analytical SEC, and where solubility permitted, by ¹H

NMR spectroscopy.

dyad was isolated in 57% yield. Removal of the trimethylsilyl

protecting group using TBAF in THF furnished porphyrin

dyad 11’ in 91% yield. Dyad 11’ was then allowed to react with

porphyrin Mg-6e to produce the desired porphyrin triad 12 in

75% yield. Triad 12 was puriﬁed according to a three-column

procedure: (1) one alumina column [CHCl₃–hexanes (4 : 1)] to

remove palladium and most of the ligand, (2) one SEC column

(THF) to recover the almost pure porphyrin triad, and (3) one

alumina column [CHCl₃–hexanes (4 : 1) with slow enrichment

with THF] for ﬁnal puriﬁcation. Triad 12 was found to be

poorly soluble in CHCl₃and CH₂Cl₂, but moderately soluble in

THF and toluene.

Deprotection of 12 was achieved using TBAF in DMF–THF

(1 : 1). After 24 h, LD-MS showed no starting material.

Isolation of the product was achieved by ﬁltration followed by

washing the solid with copious amounts of water, then

methanol to yield 12’ in 72% yield. The carboxy-terminated

array has the ability to bind to semiconductor substrates,

allowing for evaluation of solar cell performance.

Triad 12. The synthesis of triad 12 is shown in Scheme 7.

The synthesis of a triad does not offer the possibility for

convergence; therefore, we utilized stepwise Sonogashira

coupling of iodo-porphyrin and ethynyl-porphyrin building

blocks. Although the synthesis could be initiated from either

terminus, we introduced porphyrin Mg-6e in the last coupling

step to minimize the handling of magnesium porphyrins.

Monoporphyrin building blocks Zn-6a’ and Zn-6b were reacted

under standard Pd-coupling conditions developed in our lab.²⁵

Scheme 7

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porphyrins provide a guide to the choice of building blocks for

Tetrad 14. The convergent synthesis of tetrad 14 via

successive iodo 1 ethyne and bromo 1 ethyne coupling

reactions is shown in Scheme 8. Selective Sonogashira coupling

of bromo/ethynyl-porphyrin Zn-6c’ and iodo-porphyrin Mg-6e

at room temperature afforded bromo-porphyrin dyad 13 in

60% yield. Convergent coupling of dyad 13 and ethynyl-

porphyrin dyad 11’ at 50 uC for 5 h yielded tetrad 14 in 43%

yield. The solubility of 14 was poor in chlorinated solvents; the

best solubility was observed with mixtures of toluene and

THF. Tetrad 14’ was prepared in 50% yield by deprotection of

14 using TBAF in THF–DMF (10 : 1) at 60 uC for 17 h.

the light-harvesting rods. The trends observed for redox

characteristics of these benchmark porphyrins are consistent

with those of previous studies of metalloporphyrins.^16,18These

trends are as follows: (1) For a particular porphyrin, the Zn

complexes are more difﬁcult to oxidize than the Mg analogs. (2)

The introduction of electron withdrawing/donating substitu-

ents raises/lowers the E_1/2values. Thus, through a judicious

choice of metal and substitution pattern essentially any

potential between 0.25 and 1.0 V can be achieved.

The E_1/2values for the ﬁrst oxidation of the synthetic

building blocks of the light-harvesting rods (see Chart 2) are

included in Table 2. These values, beginning with the anode-

terminal, electron-injecting unit (Mg-6f), proceeding through

the transmission elements (Zn-6j and Zn-6b), and ending with

the cathode-terminal, hole-injecting unit (Zn-6a’), are as

follows: Mg-6f, E_1/2~ 0.60 V; Zn-6j, E_1/2~ 0.68 V; Zn-6b,

Pentad 18. Two routes to pentad 18 were investigated. In the

ﬁrst route, the successive coupling procedures were employed

in a convergent route (Scheme 9). Dyad 15 was prepared by

chemoselective Sonogashira coupling of bromo/ethynyl Zn-6c’

and iodo/trimethylsilylethynyl Zn-6d in 60% yield. Bromo/

trimethylsilylethynyl dyad 15 and ethynyl dyad 11’ were then

reacted at 50 uC for 5 h to furnish tetrad 16 in 31% yield. Tetrad

16 proved to have poor solubility.³¹In a mixture of toluene and

THF, one preparative SEC column afforded good separation

of HMWM, product and dyadic starting materials. Characteri-

zation by ¹H NMR spectroscopy proved difﬁcult, but TLC

analysis showed only one spot under a variety of solvent

systems, analytical SEC showed a single sharp peak, and LD-

MS analysis showed a strong molecule ion peak (m/z ~ 3356).

Because of the limited solubility, deprotection of 16 and

subsequent coupling with Mg-6e was not attempted and a

second route to prepare pentad 18 was undertaken.

The synthesis of pentad 18 via a convergent 3 1 2 route is

shown in Scheme 10. Bromo/ethynyl-porphyrin dyad 15’ was

prepared in 84% yield by treatment of 15 with TBAF in THF at

room temperature for 1 h. Dyad 15’ was then reacted with Mg-

6e at room temperature for 4.5 h to achieve selective iodo 1

ethyne coupling, yielding triad 17 in 51% yield. Triad 17 and

dyad 11’ were then reacted in the presence of Pd₂(dba)₃and

P(o-tol)₃at 55 uC. SEC analysis of an aliquot removed from the

crude reaction mixture after 70 min showed 25% conversion to

pentad. After another addition of catalyst and reaction for an

additional 1.5 h (3 h total reaction time), analytical SEC

revealed 38% conversion to pentad.³⁰The reaction mixture was

no longer homogeneous at this point and the reaction was

stopped after 5 h. The SEC trace showed a large number of

closely eluting peaks (see Experimental Section). The mixture

exhibited poor solubility. Large material losses occurred upon

workup and only a small amount of material (pure by SEC)

was obtained. The small amount of material obtained was

sufﬁcient for initial spectroscopic characterization.

E_1/2~ 0.57 V; Zn-6a’, E_1/2~ 0.54 V. These E_1/2values mono-

tonically decrease down the series Mg-6f w Zn-6b w Zn-6a’,

as is required for migration of the hole from the anode-

terminal (electron-injecting) unit to the cathode-terminal (hole-

injecting) unit in a short light-harvesting rod, such as the triad

12.

The fact that the E_1/2value for the transmission element Zn-

6j is larger by y80 mV than that for the anode-terminal unit

Mg-6f might at ﬁrst glance appear to restrict hole migration in

tetrameric and longer light-harvesting rods, 14 and 18,

respectively. However, this is not the case for the following

reasons: (1) The E_1/2value for the transmission element in the

actual rod is lower than that for the building block Zn-6j. In the

rod, each of the two ethyne hydrogens of Zn-6j is replaced by

the phenyl ring of an adjacent porphyrin. Previous studies of

multiporphyrin arrays have shown that the substitution of a

phenyl ring for hydrogen lowers the E_1/2value by y30 mV per

phenyl ring.⁷Thus, the E_1/2value of the transmission element

that is adjacent to the anode-terminal unit in an actual rod is

within 20 mV of the E_1/2value of this latter unit. This value is

less than thermal energy at ambient temperature; thus, facile

hole migration should occur in this last segment as in the entire

rod. (2) Previous studies of hole hopping in multiporphyrin

arrays wherein two Zn porphyrins are separated by a Fb

porphyrin have shown that the hole rapidly hops between the

Zn porphyrins (at a rate w10⁷s^{21 7,8}

)

despite the fact the redox

potential of the intervening Fb porphyrin is much higher

(w200 mV) than that of the Zn porphyrin.¹⁶The rapid hopping

occurs because the Fb porphyrin serves as superexchange

mediator (rather than acting as a cationic intermediate).^7,8,19

Accordingly, the presence of a small uphill redox gradient in a

segment of a light-harvesting rod wherein the overall end-to-

end gradient is downhill should not impede hole migration.

Finally, we note that the E_1/2value of the cathode-terminal

building block Zn-6a’ is also skewed more positive than that for

the analogous unit of an actual rod, again by the presence of an

ethynyl hydrogen in the building block (Chart 1). On the other

hand, the presence of iodo (vs. ethynylphenyl) and trimethyl-

silyl (vs. phenyl) groups in the building block Zn-6b makes the

potential slightly less positive than the E_1/2value of the

transmission element.³²Thus, the overall end-to-end electro-

chemical gradient in the light-harvesting rod is actually more

favorable for hole migration than would be suggested by the

E_1/2values of the building blocks.

Benchmark compounds. The assessment of the electroche-

mical and photodynamic properties of the triad and tetrad

requires suitable benchmark compounds for comparison

purposes. A dyad that represents the carboxy-containing end

of the triad and tetrad was prepared as shown in Scheme 11.

The Pd-mediated coupling of ethynyl porphyrin Zn-6i’ and

iodo/ester porphyrin Mg-6f afforded the requisite ZnMg dyad

19. Several porphyrin monomers employed in the synthesis of

the arrays proved to be suitable models for analogous

components of the arrays. A trans-diethynyl Zn porphyrin

(Zn-6j) was used as a model for the analogous Zn porphyrin in

the tetrad. These porphyrins are displayed in Chart 1. Other

porphyrin monomers such as ZnU and MgU were prepared

previously, as was the dyad ZnMgU which bears mesityl

groups at all non-linking meso positions (Chart 2).

Static absorption spectra. The electronic ground-state

absorption spectrum of the ZnMg dyad 19 is shown in

Fig. 5A (solid spectrum). This ﬁgure also shows spectra of

monomeric Mg and Zn reference porphyrins (Mg-6f and ZnU),

which contain the same substituents at the non-linking

positions as in the dyad (panels B and C). The spectra of the

monomers contain the typical metalloporphyrin features,

namely the intense near-UV Soret (B) band, corresponding

3 Physical properties of porphyrin light-harvesting rods

Electrochemistry. The E_1/2values for the ﬁrst oxidation of

the series of benchmark Zn and Mg porphyrins are included in

Table 1. As was noted above, the redox characteristics of these

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Scheme 11

to the S₀A S₂transition, and a series of weaker visible bands polarity and increasing ability to coordinate to the central

— Q(2,0), Q(1,0), and Q(0,0) — corresponding to the S₀A S₁

transition. These four features are at approximately 423, 510,

550 and 590 nm for the Zn porphyrin and 426, 520, 560, and

605 nm for the Mg porphyrin in toluene. The positions of the

Q(0,0) absorption bands, like the Q(0,0) ﬂuorescence features

described below, indicate that the lowest excited singlet state of

the Mg porphyrin lies y0.05 eV lower in energy than that of

the Zn porphyrin. This energy difference is an essential design

criterion to ensure the ﬂow of excited-state energy from the Zn

porphyrin(s) to the terminal Mg porphyrin when the units are

joined in the light-harvesting rods (Fig. 1).

metals of the metalloporphyrins, such as tetrahydrofuran

(THF) and acetonitrile (MeCN). In these polar solvents,

mainly due to metal-coordination effects, the energy gradient

from Zn to Mg porphyrin units is reduced compared to

nonpolar, non-ligating solvents such as toluene (more so in

MeCN than THF), but is still present. The latter fact, in

addition to other standard metal-coordination effects, is seen

from the absorption spectra, as well as the static emission and

time-resolved absorption data described below. These issues

were examined because polar (and metal-coordinating) media

may be used for some applications of the light-harvesting rods.

This energy gradient is retained in the dyad 19, as is seen

from the presence of the Q(0,0) bands of the Mg and Zn

porphyrins at the positions found in the reference monomers

(Fig. 5). In fact, the entire Q-band manifold of the dyad is

essentially the sum of the spectra of the component parts. We

have extensively documented this important characteristic of

diarylethyne-linked porphyrins, which is retained even in

branched arrays containing 21 porphyrins.³³The unchanged

position of each of the Q bands in the dyad reﬂects the

relatively weak (though signiﬁcant) electronic coupling

between the lowest excited singlet states of the porphyrin

components. On the other hand, exciton coupling between the

strong B-transition dipoles of the component porphyrins

causes the near-UV region of the dyad spectrum to deviate

from the sum of the monomer bands. This coupling results in a

splitting and shift in the center of gravity of the composite Soret

band from the monomer-band average; these properties are

seen in the spectrum of the ZnMg dyad 19 in Fig. 5 and are

even more evident in dyads containing identical subunits

studied here and previously.⁹

These results demonstrate that although the S₀A S₂

transitions of the component diarylethyne-linked porphyrins

are strongly coupled, the electronic characteristics designed

into the photophysically relevant lowest excited singlet states of

the monomeric building blocks are retained in the ZnMg dyad.

The same is found to be true for Zn₂Mg triad 12 and Zn₃Mg

tetrad 14, and can be extrapolated with conﬁdence to

progressively longer light-harvesting rods. The same properties

are retained when the arrays are placed in solvents of increasing

Emission spectra, quantum yields, and lifetimes. The emission

spectra of ZnMg dyad 19 and its component reference

monomers, Mg-6f and ZnU, in toluene are shown in Fig. 5

(dashed spectra). In analogy with the absorption spectra, the

Q(0,0) and Q(0,1) emission bands of the monomeric Mg

porphyrin (609 and 662 nm) are y15 nm to longer wavelengths

than those of the reference Zn porphyrin (594 and 645 nm).

Thus, the relative Q(0,0) emission positions again reﬂect the

y0.05 eV lower energy of the lowest excited singlet state of the

Mg versus Zn porphyrin.

One of the most noteworthy ﬁndings is that emission from

ZnMg dyad 19 occurs predominantly from the Mg porphyrin

component, regardless of excitation wavelength (Fig. 5A).

Although fully selective optical pumping of either component is

not possible due to spectral overlap, absorption at 549 nm on

the blue side of the dyad’s Q(1,0) feature should favor

excitation of the Zn porphyrin while 562 nm photons on the

red side of the band should favor excitation of the Mg

porphyrin. Regardless, there is no evidence for Zn-porphyrin

emission from the dyad even when using 549 nm excitation.

This can be seen by comparison of the spectra in Fig. 5, and the

ﬁnding that the spectra using 549 and 562 nm excitation are

superimposable (not shown). Furthermore, the emission

intensity (from the Mg porphyrin) is comparable when the

excitation favors either the Zn porphyrin or Mg porphyrin

(when corrected for different absorbances at the excitation

wavelengths). Similar results for dyad 19 are found in THF and

MeCN. Similar results also were obtained for Zn₂Mg triad 12

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versus 0.14; t ~ 6.0 ns versus 8.3 ns). The values are more

substantially reduced in the dyad versus monomer in MeCN

(W_f~ 0.061 versus 0.13; t ~ 2.6 versus 7.6 ns).³⁷The reduced

emission and lifetime in 19 and the dependence on solvent

polarity indicate that Mg* in this dyad decays in part via a

channel not present in the Mg porphyrin monomer, namely

charge transfer involving the Zn porphyrin component. The

electrochemical data described above indicate that the most

viable charge-transfer process is ZnMg* A Zn¹Mg²hole

transfer. Using the standard analysis of the emission yield and

lifetime data,^38,39the hole-transfer efﬁciencies and rate

constants for dyad 19 are found to be y35% and (24 ns)²¹

in toluene and y60% and (13 ns)²¹in MeCN. This process of

hole transfer from the excited state of the Mg porphyrin is

analogous but not identical to the ground-state hole-transfer

process that is expected to occur following electron-injection by

Mg* into a semiconductor.

The decay of Mg* in dyad 19 due to hole transfer to the Zn

porphyrin (rather than electron transfer or no charge transfer

at all) is a consequence of the redox-potential tuning caused by

the inductive effect of the two pentaﬂuorophenyl rings on the

Mg porphyrin (Tables 1 and 2). The combined effects of the

porphyrin substituents clearly have moved the Zn¹Mg²

charge-separated state somewhat below the energy level of

Mg* in dyad 19 even in nonpolar media such as toluene, and no

doubt farther below in polar media such as MeCN.^40,41As

noted above, the movement of holes from the Mg to Zn

porphyrin(s) is a key characteristic for the rectiﬁcation effect in

the light-harvesting rods (Fig. 1). A limited extent of charge

transfer in the excited state of the Mg porphyrin may facilitate

electron injection into a semiconductor (by Mg²rather than

Mg*) and ensuing hole transfer down the Zn-porphyrin chain.

The fact that Mg* remains very long lived (6 ns in nonpolar

toluene and 2.6 ns in highly polar MeCN) ensures ample time

for efﬁcient excited-state electron injection in the solar cells in a

range of media.

Time-resolved absorption spectroscopy. The rate of energy

ﬂow from the Zn porphyrin(s) to the Mg porphyrin was

examined for dyad 19, triad 12, and tetrad 14. Fig. 6 shows

representative data for dyad 19 in THF, obtained using 130 fs

excitation ﬂashes at 595 nm. The absorption difference

spectrum shown at 2 ps after excitation contains the typical

characteristics of metalloporphyrin excited states.⁴²These

features include strong excited-state absorption immediately

to the red of the Soret-band bleaching that tails to longer

wavelengths and is broken by the bleaching of the ground-state

Q bands. The Q(1,0) bleaching at 2 ps is centered at y560 nm

but shows a distinct shoulder at y570 nm. Based on the static

optical properties described above, the former can be assigned

mainly to Zn* and the latter to Mg*, with each state formed by

direct excitation in independent fractions of the arrays due to

spectral overlap with the 595 nm excitation ﬂash. At 250 ps, the

bleaching on the shorter wavelength side of the feature has

decayed and that at longer wavelengths has grown. These

spectral changes are indicative of Zn*Mg A ZnMg* energy

transfer. The excited Mg porphyrins thus formed (and those

produced by direct excitation) do not decay appreciably over

the 500 ps time course of these experiments, consistent with the

multi-nanosecond Mg* lifetimes found above from the

ﬂuorescence measurements. The inset to Fig. 6 shows a typical

kinetic trace and a ﬁt to a single exponential function with a

time constant of 9 ¡ 2 ps. This value can be assigned to the Zn*

lifetime for dyad 19 in THF.

Chart 1

and Zn₃Mg tetrad 14 in THF, a solvent in which these arrays

are more soluble than in toluene. These results are all indicative

of efﬁcient energy transfer from the excited singlet state of the

Zn porphyrin (Zn*) to the ground-state Mg porphyrin to form

Mg*.³⁴Facile Zn*Mg A ZnMg* energy transfer is further

demonstrated by the transient absorption data described

below. For the triad and tetrad, the results also show that

energy input at the distant Zn porphyrin(s) migrates efﬁciently

to the terminal Mg porphyrin.

The ﬂuorescence quantum yields and lifetimes of dyad 19,

triad 12 and tetrad 14 and monomeric reference compounds are

summarized in Table 3. The emission yields and lifetimes for

monomers Mg-6f (W_f~ 0.14, t ~ 8.3 ns) and MgU (W_f~ 0.16,

t ~ 9.7 ns)³⁵in toluene show that the addition of two penta-

ﬂuorophenyl rings (to properly poise the porphyrin redox

potential for use in the light-harvesting rods) does not

signiﬁcantly affect the intrinsic excited-state properties of the

Mg porphyrin.³⁶For dyad 19 in toluene, the emission yield and

excited-state lifetime of the Mg porphyrin are reduced by 30–40%

relative to that of the reference compound Mg-6f (W_f~ 0.087

Similar results are found for triad 12 and tetrad 14 in THF,

except that the Zn* lifetime lengthens to 15 ¡ 3 ps and 30 ¡

8 ps, respectively. The progressively longer excited-state

lifetime with increasing number of Zn porphyrins in the rod

reﬂects the fact that excitation will produce a Zn* increasingly

farther from the Mg porphyrin, thereby requiring more

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Chart 2

Table 3 Photophysical data for arrays and reference compounds^a

Table 2 Q(0,0) Absorption maxima and electrochemical data for Zn

and Mg porphyrins

b

Compound

Solvent

W_F

t

_Zn*/ps^c

t

_Mg*/ns^c

Porphyrin

l_absQ(0,0)^a

E_1/2/mV^b

19 (ZnMg dyad)

Toluene 0.087

THF

—

6.0 ¡ 0.6

—

Zn-6a’

Zn-6b

Zn-6j

Mg-6f

b

593

590

604

0.54

0.57

0.68

0.60

—

9 ¡ 2^d

60 ¡ 10^e

15 ¡ 3^d

30 ¡ 8^d

9 ¡ 2^f

MeCN

THF

0.061

0.047

0.055

2.6 ¡ 0.2

—

12 (Zn₂Mg triad)

14 (Zn₃Mg tetrad) THF

ZnMgU

Mg-6f

Toluene 0.17^f

5.8 ¡ 0.3^f

—

^aIn toluene. E_1/2vs. Ag/Ag¹in CH₂Cl₂containing 0.1 M Bu₄NPF₆;

55 ¡ 10^g

1

E_1/2of FeCp₂/FeCp₂is 0.19 V.

MeCN

Toluene 0.14

MeCN 0.13

Toluene 0.012

Toluene 0.034^h2400 ¡ 200^hi

—

8.3 ¡ 0.5

7.6 ¡ 0.5

—

Mg-6e

ZnU

intervening energy-transfer steps between Zn porphyrins.⁴³

Nonetheless, these Zn* lifetimes are all considerably shorter

than the value of 2.4 ns for monomeric Zn porphyrins

THF

MeCN

—

2600 ¡ 200^h

0.063^h2400 ¡ 200^h

MgU

Toluene 0.16^h

9.7 ¡ 0.4^fh

9.7 ¡ 0.5^h

9.6 ¡ 0.5^h

THF

MeCN

0.18

0.19^h

^aAll data were acquired at room temperature (THF ~ tetrahydro-

furan; MeCN ~ acetonitrile). ^bFluorescence quantum yields (10%

error) are referenced to 0.033 for ZnTPP and 0.16 for MgTPP.^35,45

Yields for Zn monomers were determined using 549 nm excitation

and those for Mg porphyrin monomers and the Mg porphyrin in

each dyad using 563 nm excitation. ^cLifetimes w 1 ns were deter-

mined by ﬂuorescence measurements and lifetimes v 1 ns by transi-

d

e

ent absorption spectroscopy. l_exc~ 595 nm. l_exc~ 413 nm. There

is a second component with t ~ 1–4 ns that likely represents decay

of Mg*, which has t ~ 2.6 ns as determined by ﬂuorescence lifetime

f

g

i

measurements. From ref 41. l_exc~ 413 nm. ^hFrom ref. 35. From

ref. 4.

(Table 3). This ﬁnding, like the static emission data described

above, indicates that energy transfer leading to the formation

of Mg* dominates the decay of Zn* in these linear arrays.

Using the standard analysis, these lifetimes give energy-transfer

rate constants in THF of (9 ps)²¹, (15 ps)²¹, and (30 ps)²¹for

ZnMg dyad 19, Zn₂Mg triad 12, and Zn₃Mg tetrad 14,

respectively, and a quantum efﬁciency ¢99% in each case.

The energy-transfer dynamics of dyad 19 were also studied in

MeCN because the light-harvesting rods may be used in highly

polar (and metal-ligating) media for certain applications.

Transient absorption spectra similar to those obtained in

THF (Fig. 6) were obtained, with the Zn* lifetime increasing to

y60 ps. Similar results were obtained for ZnMgU, which

contains only mesityl groups at the non-linking meso positions

Fig. 5 Absorption (solid) and ﬂuorescence (dashed) spectra for dyad

19, Mg porphyrin monomer Mg-6f, and Zn porphyrin monomer ZnU

in toluene at room temperature. The near-UV Soret absorption bands,

the visible Q(1,0) absorption band, and the Q(0,1) ﬂuorescence bands

have been normalized to 1. For each compound, the Q-band region has

been multiplied relative to the Soret band by the factor indicated.

J. Mater. Chem., 2002, 12, 1530–1552

1545

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LD-MS in the presence of the matrix POPOP (MALDI-MS).

Absorption spectra were collected in toluene unless noted

otherwise. Elemental analyses were performed by Atlantic

Microlab, Inc. Melting points are uncorrected. Silica gel (Baker

40 mm average particle size) and alumina (Fisher, 80–200 mesh)

were used for column chromatography. Preparative SEC was

performed using BioRad Bio-Beads SX-1 (200–400 mesh) beads.

Analytical SEC was performed using an HP 1090 Liquid Chro-

21

˚

matograph (column size ~ 1000 A; ﬂow rate ~ 0.800 mL min

;

solvent ~ THF; quantitation at 420 nm; oven temperature

40 uC). Tris(dibenzylideneacetone)dipalladium(⁰) [Pd₂(dba)₃],

tri-o-tolylphosphine [P(o-tol)₃], and all other reagents were

purchased from Aldrich and used as received. Room tem-

perature was determined to be 21–22 uC using a calibrated

thermometer (Fisher).

Palladium coupling reactions

All Pd-mediated reactions were performed using a Schlenk line.

Standard pump-purge methods were employed to achieve

reaction under anaerobic conditions.²⁵Toluene and triethyl-

amine (TEA) were freshly distilled from CaH₂and sparged of

oxygen prior to use. The reactions were performed in dilute

solution in the absence of any copper reagents.²⁴Palladium

insertion and transmetalation have not been observed with

these conditions.

Fig. 6 Time-resolved absorption data for dyad 19 in THF at room

temperature. Samples were excited with a 130 fs excitation ﬂash at

595 nm. The line through the data in the inset is a single exponential ﬁt

with a time constant indicated; kinetic data before and during the ﬂash

and from 50 ps to 500 ps are not shown for clarity.

(rather than the other substituents including two pentaﬂuoro-

phenyl substituents on the Mg porphyrin in 19). These ﬁndings

indicate that the modest lengthening of the Zn* lifetime derives

primarily from solvent interactions (including metal ligation

effects), and that these effects are not a consequence of the

peripheral groups used to tune the redox properties of the

components in the light-harvesting rods. A Zn* lifetime of

60 ps compared to the 2.4 ns value in reference monomers

corresponds to a rate constant of (62 ps)²¹and a quantum

efﬁciency of 98% for the Zn*Mg A ZnMg* energy-transfer

process. These results indicate that the light-harvesting rods

have robust energy-transfer characteristics that are maintained

Electrochemical studies

The electrochemical studies were performed using techniques

and instrumentation previously described.⁷For all the

porphyrins, the solvent was CH₂Cl₂and 0.1 M Bu₄NPF₆

(Aldrich, recrystallized three times from methanol and dried

under vacuum at 110 uC) served as supporting electrolyte. The

potentials were measured vs. Ag/Ag¹; E_1/2(FeCp₂/FeCp₂¹) ~

0.19 V. The scan rate was 0.1 V s²¹

.

Static and time-resolved optical studies

under a variety of conditions for use in solar-energy

applications.

Static absorption, static emission, ﬂuorescence lifetime, and

time-resolved absorption measurements on samples at room

temperature were performed as described previously.^6,44

Fluorescence lifetime measurements utilized samples that

were deoxygenated by bubbling with dry N₂. Fluorescence

spectra were corrected for the instrument response. Static

emission and ﬂuorescence lifetime studies utilized 0.1–2 mM

samples in 10 cm cuvettes. For time-resolved absorption

studies, samples (y0.1–0.2 mM in 2 mm pathlength cuvettes)

were excited at 10 Hz with 130 fs pulses (4–8 mJ, 390–450 nm;

20–30 mJ, 480-590 nm) and probed with white-light ﬂashes of

comparable duration. Fluorescence quantum yields (10%

error) are referenced to 0.033 for ZnTPP and 0.16 for

MgTPP.^35,45

Conclusions

A new design has been developed for components of a

molecular solar cell. The design speciﬁes a molecular rod

comprised of pigments chosen for light absorption and the

support of the migration of excited-state energy and ground-

state holes in opposite directions. The challenge of constructing

such components motivated the development of new synthetic

methodologies based on successive iodo 1 ethyne and bromo

1 ethyne coupling reactions, which enabled a degree of

convergence to be employed in the synthesis of multiporphyrin

arrays. Porphyrin building blocks with the desired spectral,

electrochemical, and attachment groups were identiﬁed and

synthesized. The building blocks were employed in a stepwise,

rational synthesis of linear arrays comprised of 3-5 porphyrins

in the backbone of the array. The porphyrin oligomers exhibit

all the key features necessary for construction of a viable

molecular solar cell: absorption of visible light, efﬁcient

excited-state energy migration in one direction along the rod,

an electrochemical gradient for reverse migration of ground-

state holes, and a linker for surface attachment.

Non-commercial compounds

Compounds 2a,b,d,e;²¹2f;⁴⁶3, 5c, and 5e;²⁰5f;⁴⁷Mg-6h;²⁵Mg-

6i’ and MgU;³⁵Zn-6j;⁴⁸ZnU²⁶and ZnMgU⁴¹were prepared as

described in the literature. Compound 2c⁴⁹was prepared using

a reﬁned method.²¹Compounds Zn-1a (ZnTPP) and Zn-1b

(ZnTMP) are standard compounds. Porphyrins 1c;⁴⁶1d and

1e;²⁰and 1f⁵⁰were prepared as described in the literature and

were metalated as needed.

Experimental

5-[4-(Hexyloxy)phenyl]dipyrromethane (2g). Following

a

general procedure,²¹a mixture of pyrrole (65 mL, 1.0 mol)

and 4-(hexyloxy)benzaldehyde (7.94 g, 38.5 mmol) was treated

with TFA (0.296 mL, 3.84 mmol). The mixture was stirred for

5 min. Ethyl acetate (70 mL) and a solution of 0.1 M aq. NaOH

(70 mL) were added. The layers were separated. The aqueous

layer was washed with additional ethyl acetate (50 mL). The

General

¹H (300 MHz) and ¹³C (75 MHz) NMR spectra were recorded

in CDCl₃unless noted otherwise. Mass spectra of porphyrins

were obtained by high-resolution fast atom bombardment

(FAB), by laser desorption mass spectrometry (LD-MS), or by

1546

J. Mater. Chem., 2002, 12, 1530–1552

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(2.00 g, 3.34 mmol) in dry THF–methanol (10 : 1, 132 mL)

organic layers were collected, dried (Na₂SO₄), and concen-

trated. Column chromatography (silica, CH₂Cl₂) afforded a

yellow, viscous oil which was recrystallized from ethanol–H₂O

(10 : 1), affording an amorphous, pale yellow solid (4.88 g,

40%): mp 58–60 uC; ¹H NMR d 0.91 (t, 3H), 1.33 (m, 4H), 1.46

(m, 2H), 1.77 (m, 2H), 3.94 (t, 2H), 5.42 (s, 1H), 5.92 (m, 2H),

6.15 (m, 2H), 6.69 (m, 2H), 6.85 (d, J ~ 8.7 Hz, 2H), 7.11 (d,

J ~ 8.7 Hz, 2H), 7.91 (br s, 2H); ¹³C NMR d 14.0, 22.6, 25.7,

29.2, 31.5, 43.0, 68.0, 107.0, 108.3, 114.5, 117.0, 129.3, 132.9,

133.9, 158.0; HRMS (FAB) obsd 322.2053, calcd 322.2045.

Anal. Calcd (C₂₁H₂₆N₂O): C, 78.22; H, 8.13; N, 8.69. Found:

C, 77.32; H, 8.08; N, 8.62.

was reduced with NaBH₄(2.53 g, 66.8 mmol) affording the

corresponding dipyrromethane-dicarbinol. The dipyrro-

methane-dicarbinol was condensed with dipyrromethane 2g

(1.08 g, 3.34 mmol) in acetonitrile (1.34 L) containing TFA

(3.09 mL, 40.1 mmol). After 4 min, DDQ (2.27 g, 10.0 mmol)

was added and the mixture was stirred at room temperature for

1 h. The standard workup afforded a purple solid (662 mg,

1

22%): H NMR d 22.70 (br s, 2H), 0.38 (s, 9H), 0.99 (t, 3H),

1.53 (m, 4H), 1.63 (m, 2H), 1.84 (s, 6H), 1.99 (m, 2H), 2.63 (s,

3H), 4.10 (s, 3H), 4.25 (t, 2H), 7.26–7.29 (m, 6H), 7.87 (d, J ~

8.1 Hz, 2H), 8.09 (d, J ~ 9.0 Hz, 2H), 8.13 (d, J ~ 9.0 Hz, 2H),

8.17 (d, J ~ 8.1 Hz, 2H), 8.7–8.9 (m, 8 H); LD-MS obsd 881.6;

HRMS (FAB) obsd 882.4367, calcd 882.4329 (C₅₉H₅₈N₄O₂Si);

l_abs423, 517, 553, 594, 651 nm.

5-Mesityl-1-(4-methoxybenzoyl)-9-{4-[2-(trimethylsilyl)ethynyl]-

benzoyl}dipyrromethane (5a). Following a general procedure,²⁰

a solution of 4a (2.85 g, 6.13 mmol) in dry toluene (25 mL) was

treated with EtMgBr (12.2 mL, 12.2 mmol, 1 M in THF)

followed by 4-anisoyl chloride (1.04 g, 6.10 mmol). Second and

third additions (identical amounts; half the amount, respec-

tively) were performed after 10 and 20 min, followed by the

standard workup. Chromatography [column 1: silica, CH₂Cl₂,

followed by CH₂Cl₂–ethyl acetate (9 : 1); column 2: silica, ethyl

acetate] afforded a red solid (2.59 g, 71%): mp 136–138 uC; ¹H

NMR d 0.26 (s, 9H), 2.19 (s, 6H), 2.31 (s, 3H), 3.85 (s, 3H), 6.03

(m, 2H), 6.11 (s, 1H), 6.67 (m, 2H), 6.90 (d, J ~ 8.1 Hz, 2H),

6.92 (s, 2H), 7.49 (d, J ~ 8.7 Hz, 2H), 7.70 (d, J ~ 8.1 Hz, 2H),

7.78 (d, J ~ 8.7 Hz, 2H), 10.46 (br s, 2H); ¹³C NMR d 20.2,

20.9, 39.2, 55.3, 96.9, 104.3, 110.3, 110.5, 113.2, 120.2, 121.1,

126.1, 129.2, 130.1, 130.3, 130.4, 130.9, 131.4, 133.2, 136.9,

137.5, 137.9, 139.1, 140.7, 162.3, 182.6. Anal. Calcd

(C₃₈H₃₈N₂O₃Si): C, 76.22; H, 6.40; N, 4.68. Found: C, 76.06;

H, 6.53; N, 4.62.

5-[4-(Hexyloxy)phenyl]-10-(4-iodophenyl)-15-mesityl-20-{4-

[2-(trimethylsilyl)ethynyl]phenyl}porphyrin (6b). Reduction of 5c

(2.57 g, 3.70 mmol) followed by condensation with 2g (1.19 g,

3.70 mmol) for 3 min, oxidation with DDQ (2.52 g, 11.1 mmol),

and standard workup furnished a purple solid (790 mg, 22%):

¹H NMR d 22.73 (br s, 2H), 0.38 (s, 9H), 1.00 (t, 3H), 1.47

(m, 4H), 1.65 (m, 2H), 1.84 (s, 6H), 1.99 (m, 2H), 2.64 (s, 3H),

4.26 (t, 2H), 7.29 (s, 4H), 7.87 (d, J ~ 8.1 Hz, 2H), 7.95 (d,

J~8.1Hz,2H),8.09(d, J~7.8Hz,4H),8.17(d, J~8.1Hz,2H),

8.7–8.9 (m, 8H); LD-MS obsd 979.8; HRMS (FAB) obsd

979.3278, calcd 979.3268 (C₅₈H₅₅IN₄OSi); l_abs422, 516, 552,

594, 651 nm.

5-(4-Bromophenyl)-10,20-dimesityl-15-{4-[2-(trimethylsilyl)-

ethynyl]phenyl}porphyrin (6c). Reduction of 5b (1.40 g, 2.16

mmol) followed by condensation with 2a (0.57 g, 2.2 mmol) for

3 min, oxidation with DDQ (1.47 g, 6.47 mmol), and standard

1

1-(4-Bromobenzoyl)-5-mesityl-9-{4-[2-(trimethylsilyl)ethynyl]-

benzoyl}dipyrromethane (5b). Following a general procedure,²⁰

reaction of 4a (2.79 g, 6.00 mmol) and 4-bromobenzoyl

chloride (1.32 g, 6.00 mmol) followed by column chromato-

graphy (silica, CH₂Cl₂) and recrystallization (CH₂Cl₂–methanol)

afforded a yellow solid (2.66 g, 68%): mp 144–146 uC; ¹H NMR

d 0.26 (s, 9H), 2.19 (s, 6H), 2.32 (s, 3H), 6.06 (m, 2H), 6.11

(s, 1H), 6.68 (m, 2H), 6.93 (s, 2H), 7.49 (d, J ~ 7.8 Hz, 2H),

7.55 (d, J ~ 8.1 Hz, 2H), 7.63 (d, J ~ 8.1 Hz, 2H), 7.69 (d, J ~

7.8 Hz, 2H), 10.43 (br s, 2H); ¹³C NMR d 20.2, 20.8, 39.2, 97.1,

104.3, 110.7, 121.2, 126.3, 129.2, 130.0, 130.3, 130.5, 130.8,

131.2, 131.4, 133.0, 136.9, 137.0, 137.4, 137.7, 140.3, 140.6,

182.4, 182.7. Anal. Calcd (C₃₇H₃₅BrN₂O₂Si): C, 68.61; H, 5.45;

N, 4.33. Found: C, 68.88; H, 5.26; N, 4.08.

workup furnished a purple solid (484 mg, 26%): H NMR d

22.68 (br s, 2H), 0.37 (s, 9H), 1.83 (s, 12 H), 2.63 (s, 6H), 7.28

(s, 4H), 7.84–7.89 (m, 4H), 8.08 (d, J ~ 8.1 Hz, 2H), 8.16 (d,

J ~ 8.1 Hz, 2H), 8.69–8.77 (m, 8H); LD-MS obsd 873.0;

HRMS (FAB) obsd 872.2898, calcd, 872.2910 (C₅₅H₄₉BrN₄Si);

l_abs420, 515, 549, 592, 649 nm.

5-(4-Iodophenyl)-10,20-dimesityl-15-{4-[2-(trimethylsilyl)-

ethynyl]phenyl}porphyrin (6d). Reduction of 5c (2.30 g, 3.31

mmol) followed by condensation with 2a (0.87 g, 3.3 mmol) for

3 min, oxidation with DDQ (2.31 g, 9.93 mmol), and standard

1

workup furnished a purple solid (0.66 g, 22%). The H NMR

and LD-MS data were identical to samples prepared by a

mixed-aldehyde condensation.⁵¹

1,9-Bis(pentaﬂuorobenzoyl)-5-(4-iodophenyl)dipyrromethane

(5d). Following a general procedure,²⁰a solution of 2b (2.00 g,

5.74 mmol) in dry toluene (95 mL) was treated with EtMgBr

(11.5 mL, 11.5 mmol, 1 M in THF) followed by a solution of

pentaﬂuorobenzoyl chloride (1.34 g, 5.81 mmol) in toluene

(6 mL). Second and third additions (identical amounts; half

the amounts, respectively) were performed after 10 and 20 min,

followed by the standard workup. Column chromatography

[silica, CH₂Cl₂–ethyl acetate (95 : 5)] afforded the crude diacyl-

dipyrromethane, which was dissolved in CH₂Cl₂and then

precipitated with hexanes. Filtration and washing with hexanes

afforded a ﬂuffy white powder (1.70 g, 40%): mp 138–140 uC;

¹H NMR d 5.60 (s, 1H), 6.12 (m, 2H), 6.67 (m, 2H), 6.97

(d, J ~ 8.1 Hz, 2H), 7.71 (d, J ~ 8.1 Hz, 2H), 9.80 (br s, 2H);

¹³C NMR d 44.5, 94.3, 112.9, 114.3, 123.6, 130.9, 132.2,

138.8, 139.9, 140.0, 142.9, 143.2, 146.3, 173.1. Anal. Calcd

(C₂₉H₁₁F₁₀IN₂O₂): C, 47.31; H, 1.51; N, 3.80. Found: C, 47.49;

H, 1.73; N, 3.64.

5-(4-Iodophenyl)-10,20-bis(pentaﬂuorophenyl)-15-{4-[2-(tri-

methylsilyl)ethoxycarbonyl]phenyl}porphyrin (6e). Reduction

of 5d (2.21 g, 3.00 mmol) followed by condensation with 2f

(1.10 g, 3.00 mmol) for 5 min, oxidation with DDQ (2.04 g,

9.00 mmol), and standard workup furnished a purple solid. A

second column [silica, CH₂Cl₂–hexanes (1:1)] yielded a purple

1

solid (562 mg, 18%): H NMR d 22.89 (s, 2H), 0.18 (s, 9H),

1.30 (t, 2H), 4.62 (t, 2H), 7.95 (d, J ~ 8.7 Hz, 2H), 8.13 (d, J ~

8.7 Hz, 2H), 8.30 (d, J ~ 8.7 Hz, 2H), 8.46 (d, J ~ 8.7 Hz, 2H),

8.8–9.0 (m, 8H); LD-MS obsd 1066.7; HRMS (FAB) obsd

1064.1105, calcd 1064.1101 (C₅₀H₃₁F₁₀IN₄O₂Si); l_abs419, 512,

545, 589, 644 nm.

5,15-Bis(pentaﬂuorophenyl)-10-phenyl-20-{4-[2-(trimethylsilyl)-

ethoxycarbonyl]phenyl}porphyrin (6f). Reduction of 5e (134

mg, 0.22 mmol) followed by condensation with 2f (80.6 mg,

0.22 mmol) for 7 min, oxidation with DDQ (150 mg, 0.66

mmol), and standard workup furnished a purple solid. A

second column [silica, CH₂Cl₂–hexanes (1 : 1)] yielded a purple

1

5-[4-(Hexyloxy)phenyl]-15-mesityl-10-(4-methoxyphenyl)-20-

{4-[2-(trimethylsilyl)ethynyl]phenyl}porphyrin (6a). Following a

general procedure,²⁰a solution of diacyldipyrromethane 5a

solid (39.2 mg, 19%): H NMR (400 MHz) d 22.86 (s, 2H),

0.18 (s, 9H), 1.30 (t, 2H), 4.62 (t, 2H), 7.78–7.82 (m, 3H) 8.21 (d,

J ~ 8.0 Hz, 2H), 8.30 (d, J ~ 8.4 Hz, 2H), 8.46 (d, J ~ 7.2 Hz,

J. Mater. Chem., 2002, 12, 1530–1552

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(690 mL, 4.92 mmol) followed by MgBr₂?O(Et)₂(639 mg,

2H), 8.80–8.82 (m, 4H), 8.90 (d, J ~ 4.4 Hz, 2H), 8.95 (d, J ~

4.4 Hz, 2H); LD-MS obsd 938.0; HRMS (FAB) obsd

1064.1105, calcd 1064.1101 (C₅₀H₃₂F₁₀N₄O₂Si); l_abs418, 512,

544, 589, 643 nm.

2.47mmol).After30min,themixturewasdilutedwithCH₂Cl₂(20

mL), washed with 5% NaHCO₃, dried (Na₂SO₄), and

concentrated. Column chromatography (alumina, CH₂Cl₂)

1

afforded a purple solid (83 mg, 77%): H NMR (THF-d₈) d

5-(4-Bromophenyl)-10,15,20-tri-p-tolylporphyrin (6g). Reduc-

tion of 5f (0.54 g, 1.2 mmol) followed by condensation with 2c

(0.35 g, 1.2 mmol) for 3 min, oxidation with DDQ (0.79 g, 3.4

mmol), and standard workup furnished a purple solid (210 mg,

0.20 (s, 9H), 1.31 (t, 2H), 4.61 (t, 2H), 7.98 (d, J ~ 8.1 Hz, 2H),

8.14 (d, J ~ 8.1 Hz, 2H), 8.31 (d, J ~ 8.1 Hz, 2H), 8.42 (d, J ~

8.1 Hz, 2H), 8.80–8.90 (m, 8H); LD-MS obsd 1085.0; HRMS

(FAB) obsd 1086.077, calcd 1086.079 (C₅₀H₂₉F₁₀IMgN₄O₂Si);

l_abs427, 522, 561, 604 nm.

1

25%): H NMR d 22.79 (s, 2H), 2.71 (br s, 9H), 7.56 (d, J ~

8.4 Hz, 6H), 7.89 (d, J ~ 9.0 Hz, 2H), 8.08–8.11 (m, 8H), 8.80

(d, J ~ 4.2 Hz, 2H), 8.87–8.90 (m, 6H); LD-MS obsd 734.3;

HRMS (FAB) obsd 734.2083, calcd 734.2045 (C₄₇H₃₅BrN₄);

Magnesium(^II)-5,15-Bis(pentaﬂuorophenyl)-10-phenyl-20-{4-[2-

(trimethylsilyl)ethoxycarbonyl]phenyl}porphyrin (Mg-6f). Fol-

lowing a general procedure,²²a solution of 6f (16.0 mg,

17.0 mmol) in CH₂Cl₂(5 mL) was treated with TEA (94 mL,

0.68 mmol) followed by MgBr₂?O(Et)₂(88.0 mg, 0.34 mmol).

The mixture was stirred at room temperature for 15 h. Column

chromatography (silica, CHCl₃) afforded a purple solid

(14.4 mg, 88%): ¹H NMR (400 MHz) d 0.00 (s, 9H), 0.79

(t, 2H), 3.40 (br s, 2H), 7.75–7.79 (m, 5H), 8.03 (d, J ~ 8.4 Hz,

2H), 8.23 (d, J ~ 6.8 Hz, 2H), 8.76 (d, J ~ 4.8 Hz, 2H),

8.81–8.84 (m, 4H), 8.98 (d, J ~ 4.4 Hz, 2H); LD-MS

obsd 958.4; HRMS (FAB) obsd 960.1891, calcd 960.1829

(C₅₀H₃₀F₁₀MgN₄O₂Si); l_abs426, 522, 561, 604 nm.

l_abs421, 516, 551, 593, 648 nm.

Zinc(^II)-5-[4-(Hexyloxy)phenyl]-15-mesityl-10-(4-methoxyphenyl)-

20-{4-[2-(trimethylsilyl)ethynyl]phenyl}porphyrin (Zn-6a). To a

solution of 6a (147 mg, 0.166 mmol) in CHCl₃(50 mL) was

added a solution of Zn(OAc)₂?2H₂O (182 mg, 0.831 mmol) in

methanol (6 mL). The reaction mixture was stirred at room

temperature for 16 h. The reaction mixture was washed with

water, dried (Na₂SO₄), and concentrated. Column chromato-

graphy (silica, CH₂Cl₂) afforded a purple solid (142 mg, 91%):

¹H NMR d 0.38 (s, 9H), 1.00 (t, 3H), 1.49 (m, 4H), 1.64 (m,

2H), 1.83 (s, 6H), 2.00 (m, 2H), 2.64 (s, 3H), 4.09 (s, 3H), 4.26

(t, 2H), 7.29 (s, 6H), 7.87 (d, J ~ 8.1 Hz, 2H), 8.10 (d, J ~ 8.7 Hz,

2H), 8.14 (d, J ~ 8.7 Hz, 2H), 8.18 (d, J ~ 8.1 Hz, 2H), 8.8–9.0

(m, 8H); LD-MS obsd 942.7; HRMS (FAB) obsd 944.3470,

calcd 944.3464 (C₅₉H₅₆N₄O₂SiZn); l_abs427, 552, 593 nm.

Zinc(^II)-5-(4-Bromophenyl)-10,15,20-tri-p-tolylporphyrin (Zn-

6g). A sample of 6g (54 mg, 74 mmol) was treated with

Zn(OAc)₂?2H₂O (80 mg, 0.36 mmol) following the general

procedure. Column chromatography (silica, CHCl₃) afforded

1

a purple solid (44 mg, 75%): H NMR d 2.72 (s, 9H), 7.56 (d,

Zinc(^II)-5-[4-(Hexyloxy)phenyl]-10-(4-iodophenyl)-15-mesityl-

20-{4-[2-(trimethylsilyl)ethynyl]phenyl}porphyrin

J ~ 7.2 Hz, 6H), 7.89 (d, J ~ 8.1 Hz, 2H), 8.11 (d, J ~ 7.5 Hz,

8H), 8.89–8.96 (m, 8H); LD-MS obsd 797.7; HRMS (FAB)

obsd 796.1185, calcd 796.1180 (C₄₇H₃₃BrN₄Zn); l_abs424, 514,

551, 590 nm.

(Zn-6b). A

sample of 6b (160 mg, 0.163 mmol) was treated with

Zn(OAc)₂?2H₂O (179 mg, 0.816 mmol) following the general

procedure. Column chromatography (silica, CH₂Cl₂) afforded

1

a purple solid (149 mg, 88%): H NMR d 0.38 (s, 9H), 1.00

Magnesium(^II)-5-(4-Bromophenyl)-10,15,20-tri-p-tolylporphyrin

(Mg-6g). Following a general procedure,²²a solution of 6g

(100 mg, 136 mmol) in CH₂Cl₂(30 mL) was treated with

N,N-diisopropylethylamine (1.18 mL, 6.80 mmol) and MgI₂

(1.89 g, 6.80 mmol). After 2 h, the mixture was washed with 5%

NaHCO₃, dried (Na₂SO₄), and concentrated. Column chro-

matography (alumina, CHCl₃) afforded a purple solid (84 mg,

(t, 3H), 1.48 (m, 4H), 1.65 (m, 2H), 1.83 (s, 6H), 1.99 (m, 2H),

2.64 (s, 3H), 4.26 (t, 2H), 7.29 (s, 4H), 7.87 (d, J ~ 8.1 Hz, 2H),

7.97 (d, J ~ 8.7 Hz, 2H), 8.10 (m, 4H), 8.18 (d, J ~ 8.1 Hz,

2H), 8.8–9.0 (m, 8H); LD-MS obsd 1040.7; HRMS (FAB) obsd

1040.2318, calcd 1040.2325 (C₅₈H₅₃IN₄OSiZn); l_abs427, 552,

594 nm.

1

81%): H NMR d 2.72 (s, 9H), 7.54 (d, J ~ 7.2 Hz, 6H), 7.87

Zinc(^II)-5-(4-Bromophenyl)-10,20-dimesityl-15-{4-[2-(trimethylsilyl)-

ethynyl]phenyl}porphyrin (Zn-6c). A sample of 6c (262 mg,

300 mmol) was treated with Zn(OAc)₂?2H₂O (329 mg,

1.50 mmol) following the general procedure. Column chroma-

tography [silica, CHCl₃–hexanes (7 : 3)] afforded a purple solid

(d, J ~ 8.1 Hz, 2H), 8.09–8.12 (m, 8H), 8.82 (d, J ~ 5.1 Hz,

2H), 8.89–8.92 (m, 6H); LD-MS obsd 758.5; HRMS (FAB)

obsd 756.1766, calcd 756.1739 (C₄₇H₃₃BrMgN₄); l_abs428, 526,

565, 605 nm.

1

(265 mg, 94%). H NMR d 0.37 (s, 9H), 1.82 (s, 12H), 2.63

Zinc(^II)-5-(4-Ethynylphenyl)-10-[4-(hexyloxy)phenyl]-20-mesityl-

15-(4-methoxyphenyl)porphyrin (Zn-6a’). A mixture of Zn-6a

(138 mg, 0.146 mmol) and TBAF (175 mL, 175 mmol, 1.0 M in

THF) in THF–CHCl₃(12 mL, 2 : 1) was stirred at room

temperature for 3 h. The solvent was removed and the residue

was chromatographed (silica, CH₂Cl₂) yielding a purple solid

(122 mg, 96%): ¹H NMR d 0.99 (t, 3H), 1.47 (m, 4H), 1.64

(m, 2H), 1.84 (s, 6H), 1.99 (m, 2H), 2.64 (s, 3H), 3.31 (s, 1H),

4.09 (s, 3H), 4.26 (t, 2H), 7.26–7.29 (m, 4H), 7.88 (d, J ~ 8.1 Hz,

2H), 8.10 (d, J ~ 8.7 Hz, 2H), 8.14 (d, J ~ 8.7 Hz, 2H), 8.20

(d, J ~ 8.1 Hz, 2H), 8.70–8.90 (m, 10 H); LD-MS obsd

870.7; HRMS (FAB) obsd 872.3070, calcd 872.3069

(C₅₆H₄₈N₄O₂Zn); l_abs426, 552, 593 nm.

(s, 6H), 7.28 (s, 4H), 7.84–7.89 (m, 4H), 8.10 (d, J ~ 8.1 Hz,

2H), 8.18 (d, J ~ 8.7 Hz, 2H), 8.77–8.79 (m, 4H), 8.84–8.86

(m, 4H); LD-MS obsd 936.5; HRMS (FAB) obsd 934.2055,

calcd 934.2045 (C₅₅H₄₇BrN₄SiZn); l_abs419, 550, 590 nm.

Zinc(^II)-5-(4-Iodophenyl)-10,20-dimesityl-15-{4-[2-(trimethyl-

silyl)ethynyl]phenyl}porphyrin (Zn-6d). A sample of 6d (0.38 g,

0.41 mmol) was treated with Zn(OAc)₂?2H₂O (0.45 g,

2.1 mmol) following the general procedure. Puriﬁcation was

achieved by suspension in methanol, followed by ﬁltration to

1

afford a purple solid (0.36 g, 90%): H NMR d 0.37 (s, 9H),

1.82 (s, 12H), 2.63 (s, 6H), 7.28 (s, 4H), 7.86 (d, J ~ 8.1 Hz,

2H), 7.97 (d, J ~ 8.1 Hz, 2H), 8.08 (d, J ~ 8.1 Hz, 2H), 8.18

(d, J ~ 7.8 Hz, 2H), 8.77–8.79 (m, 4H), 8.84–8.87 (m, 4H);

LD-MS obsd 983.1; HRMS (FAB) obsd 982.1943, calcd

982.1906 (C₅₅H₄₇IN₄SiZn); l_abs425, 550, 592 nm.

Zinc(^II)-5-(4-Bromophenyl)-15-(4-ethynylphenyl)-10,20-di-

mesitylporphyrin (Zn-6c’). A mixture of Zn-6c (123 mg,

131 mmol) and TBAF (157 mL, 157 mmol, 1.0 M in THF) in

THF–CHCl₃(12 mL, 2 : 1) was stirred at room temperature

for 30 min. The solvent was removed and the residue was

chromatographed [silica, CH₂Cl₂–hexanes (7 : 3)] yielding a

purple solid (87 mg, 77%): ¹H NMR d 1.81 (s, 12H), 2.63

(s, 6H), 3.31 (s, 1H), 7.28 (s, 4H), 7.87 (d, J ~ 8.1 Hz, 4H), 8.10

Magnesium(^II)-5-(4-Iodophenyl)-10,20-bis(pentaﬂuorophenyl)-

15-{4-[2-(trimethylsilyl)ethoxycarbonyl]phenyl}porphyrin (Mg-

6e). Following

(106 mg, 99.4 mmol) in CH₂Cl₂(15 mL) was treated with TEA

a a solution of 6e

general procedure,²²

1548

J. Mater. Chem., 2002, 12, 1530–1552

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Triad 12. Samples of 11’ (35.5 mg, 20.7 mmol) and Mg-6e

(d, J ~ 8.1 Hz, 2H), 8.19 (d, J ~ 8.7 Hz, 2H), 8.78 (d, J ~

6.0 Hz, 4H), 8.84 (d, J ~ 3.6 Hz, 4H); LD-MS obsd 859.1;

HRMS (FAB) obsd 862.1624, calcd 862.1650 (C₅₂H₃₉BrN₄Zn);

l_abs419, 550, 590 nm.

(26.1 mg, 24.0 mmol) were coupled with Pd₂(dba)₃(5.2 mg,

5.7 mmol) and P(o-tol)₃(8.9 mg, 29 mmol) in toluene–TEA (5 : 1,

7.0 mL) at 35 uC for 2 h. The volatile components were

removed and the solid was ﬁltered through an alumina column

(CHCl₃–hexanes, 4 : 1). Porphyrin-containing fractions were

concentrated and the mixture was chromatographed (SEC,

THF). Column chromatography [alumina, CHCl₃–hexanes

(4 : 1) followed by slow enrichment with THF] afforded a

Dyad 9. To samples of Zn-6g (10 mg, 13 mmol), Zn-6i’

(10 mg, 12 mmol), Pd₂(dba)₃(2.1 mg, 2.3 mmol), and P(o-tol)₃

(4.8 mg, 16 mmol) was added a degassed solution of toluene–

TEA (5 : 1, 6 mL). The mixture was placed in an oil bath at

80 uC. Aliquots were removed at 1 h and 3 h and analyzed by

analytical SEC and LD-MS. The reaction mixture was passed

over a silica column (THF), then loaded onto a preparative

SEC column (THF). The dyad fraction was concentrated and

analyzed by analytical SEC and LD-MS (with or without

added POPOP, see text).

1

purple solid (41.2 mg, 75%): H NMR (500 MHz) (THF-d₈) d

0.17 (s, 9H), 0.99 (m, 6H), 1.28 (t, 2H), 1.60–1.75 (m, 8H), 1.80-

1.90 (m, 4H), 1.85 (s, 6H), 1.88 (s, 6H), 1.97 (m, 4H), 2.59

(s, 3H), 2.61 (s, 3H), 4.04 (s, 3H), 4.26 (m, 4H), 4.59 (t, 2H),

7.26–7.31 (m, 12H), 8.04–8.11 (m, 12H), 8.28–8.32 (m, 8H),

8.40 (d, J ~ 7.8 Hz, 4H), 8.65 (d, J ~ 4.5 Hz, 2H), 8.71 (d,

J ~ 4.5 Hz, 2H), 8.74 (d, J ~ 4.5 Hz, 4H), 8.81–8.95 (m,

16H); MALDI-MS obsd 2676.2, calcd avg mass 2675.9

(C₁₆₁H₁₂₀F₁₀MgN₁₂O₅SiZn₂); l_abs427, 435, 521, 558, 599 nm.

Dyad 10. To samples of Mg-6g (9.4 mg, 11.3 mmol), Zn-6i’

(8.4 mg, 11.1 mmol), Pd₂(dba)₃(1.6 mg, 1.7 mmol), and P(o-tol)₃

(3.7 mg, 12 mmol) was added a degassed solution of toluene–

TEA (5 : 1, 6 mL). The reaction mixture was placed in an oil

bath at 50 uC. Aliquots were removed at 1 h and 2 h timepoints

for analysis (LD-MS and SEC, 1 h timepoint, SEC, 2 h

timepoint). After 2 h, the reaction appeared to stop (based on

SEC data); therefore, another batch of catalyst was added

[1.8 mg Pd₂(dba)₃; 3.9 mg P(o-tol)₃]. After an additional 1 h,

SEC analysis of a reaction aliquot indicated the reaction was

complete. The mixture was ﬁltered through a short alumina

column (CHCl₃), chromatographed on SEC (THF) and ﬁltered

through a silica column (CHCl₃) affording a purple powder

Triad 12’. To a sample of 12 (15 mg, 5.6 mmol) in THF–

DMF (4 mL, 1 : 1) was added TBAF (6.7 mL, 6.7 mmol, 1.0 M

in THF). The mixture was stirred at room temperature until

LD-MS showed no starting material (24 h). Additional THF

(5 mL) was added and the solution was poured into 50 mL of

water. The resulting solid was ﬁltered, washed copiously

with water and then with methanol, yielding a purple solid

(10.4 mg, 72%): LD-MS obsd 2571.46, calcd avg mass 2575.9

(C₁₅₆H₁₀₈F₁₀MgN₁₂O₅Zn₂); l_abs427, 435, 520, 561, 602 nm.

Dyad 13. Samples of Zn-6c’ (90.0 mg, 104 mmol) and Mg-6e

(113.1 mg, 104 mmol) were coupled with Pd₂(dba)₃(14.3 mg,

15.6 mmol) and P(o-tol)₃(38.0 mg, 125 mmol) in toluene–TEA

(5 : 1, 40 mL). The mixture was stirred at room temperature

for 105 min, at which time additional Pd₂(dba)₃(14.3 mg,

15.6 mmol) and P(o-tol)₃(38.0 mg, 125 mmol) were added. After

a total of 4 h 20 min, the reaction mixture was concentrated,

then loaded onto an alumina column (CHCl₃). The resulting

porphyrin-containing fractions were then loaded onto a

preparative SEC column (THF). The nearly pure dyad fraction

was then loaded onto a silica column [CHCl₃–hexanes–TEA

(95 : 5 : 1)]. Trituration with hexanes followed by ﬁltration

afforded a purple solid (115 mg, 60%): ¹H NMR (THF-d₈)

d 0.20 (s, 9H), 1.31 (t, 2H), 1.86 (s, 12H), 2.62 (s, 6H), 4.61

(t, 2H), 7.32 (s, 4H), 7.91 (d, J ~ 9.0 Hz, 2H), 8.05–8.14 (m,

6H), 8.31 (dd, J¹~ 2.1 Hz, J²~ 1.5 Hz, 6H), 8.42 (d, J ~

7.8 Hz, 2H), 8.69 (d, J ~ 4.2 Hz, 2H), 8.72 (d, J ~ 4.2 Hz, 2H),

8.78 (d, J ~ 4.5 Hz, 2H), 8.83 (d, J ~ 5.1 Hz, 2H), 8.88–8.97

(m, 8H); LD-MS obsd 1824.7, calcd avg mass 1824.3

(C₁₀₂H₆₇BrF₁₀MgN₈O₂SiZn); l_abs430, 554, 591 nm.

1

(12.6 mg, 75%): H NMR d 1.88 (s, 18H), 2.65 (s, 9H), 2.73

(s, 9H), 7.28–7.30 (m, 6H), 7.55–7.58 (m, 6H), 8.06 (d, J ~

8.1 Hz, 4H), 8.12–8.15 (m, 6H), 8.30–8.33 (m, 4H), 8.72 (s, 4H),

8.80 (d, J ~ 4.2 Hz, 2H), 8.91–8.96 (m, 10H); LD-MS

obsd 1503.2; FAB-MS obsd 1505.555, calcd 1505.549

(C₁₀₂H₇₈MgN₈Zn); l_abs431, 553, 564, 606 nm.

Dyad 11’. Following a general procedure,²⁵Zn-6a’ (54.1 mg,

61.9 mmol) and Zn-6b (62.5 mg, 60.0 mmol) were coupled using

Pd₂(dba)₃(11.7 mg, 12.8 mmol) and P(o-tol)₃(22.7 mg,

74.5 mmol) in toluene–TEA (20 mL, 5 : 1) at 35 uC for 3 h.

Concentration of the mixture afforded a purple–brown solid.

The solid was ﬁltered [silica, 4 6 8 cm, hexanes–CH₂Cl₂(1 : 1)]

to remove P(o-tol)₃, and the porphyrinic products were eluted

(CH₂Cl₂). After a preparative SEC column (THF), the dyad

fraction was concentrated, dissolved in CHCl₃, and chromato-

graphed [silica, 4 6 8 cm, CHCl₃–hexanes (4 : 1)], yielding a

purple solid (61 mg, 57%): ¹H NMR d 0.38 (s, 9H), 1.00 (t, 6H),

1.48 (m, 8H), 1.65 (m, 4H), 1.87 (s, 12H), 2.00 (m, 4H), 2.65

(s, 6H), 4.10 (s, 3H), 4.27 (t, 4H), 7.26–7.31 (m, 8H), 7.90

(d, J ~ 8.1 Hz, 2H), 8.06–8.23 (m, 14H), 8.32 (d, J ~ 7.2 Hz,

4H), 8.80–9.06 (m, 16H); LD-MS obsd 1783.8; HRMS (FAB)

obsd 1788.63, calcd 1788.63 (C₁₁₄H₁₀₀N₈O₃SiZn₂); l_abs430,

516, 552, 593 nm.

Tetrad 14. Samples of 11’ (50.0 mg, 29.1 mmol) and 13

(53.1 mg, 29.1 mmol) were coupled using Pd₂(dba)₃(4.0 mg,

4.4 mmol) and P(o-tol)₃(10.6 mg, 34.9 mmol) in toluene–TEA

(5 : 1, 18 mL) at 50 uC for 2.5 h. Then additional Pd₂(dba)₃

(4.0 mg, 4.4 mmol) and P(o-tol)₃(10.6 mg, 34.9 mmol) were

added. After a total of 5 h, the reaction mixture was

concentrated, then chromatographed on alumina [toluene–

THF (9 : 1)]. The porphyrin-containing fractions were then

loaded onto a preparative SEC column (THF). Some streaking

occurred during puriﬁcation. A second SEC column was run

(THF). The tetrad-containing fractions were passed over an

alumina column [toluene–THF (9 : 1)] to afford a purple solid

(43 mg, 43%): ¹H NMR (THF-d₈) d 0.22 (s, 9H), 1.01 (m, 6H),

1.83 (s, 8H), 1.88 (s, 16H), 2.63 (s, 4H), 2.65 (s, 8H), remaining

aliphatic CH₂resonances are broad and cannot be integrated

with accuracy, 4.07 (s, 3H), 4.24 (m, 4H), 6.91 (s, 8H), 7.34

(m, 12H), 8.10 (m, 16H), 8.34–8.42 (m, 12H), 8.75–8.97

(m, 32H); MALDI-MS obsd 3464.1, calcd avg mass 3460.2

(C₂₁₃H₁₅₈F₁₀MgN₁₆O₅SiZn₃); l_abs436, 551, 592 nm.

A solution of the porphyrin (61 mg, 34 mmol) in THF–

CHCl₃(8 mL, 2 : 1) was treated with TBAF (40 mL, 40 mmol,

1.0 M in THF). The mixture was stirred at room temperature

for 2 h. The solvent was removed and the sample was

redissolved in CHCl₃(20 mL). The solution was washed with

10% aqueous NaHCO₃(20 mL) and water. The organic layer

was dried (Na₂SO₄), ﬁltered, and concentrated. Puriﬁcation by

column chromatography [silica, 4 6 10 cm, CHCl₃–hexanes

1

(4 : 1)] afforded a purple solid (53 mg, 91%): H NMR d 1.00

(t, 6H), 1.48 (m, 8H), 1.65 (m, 4H), 1.87 (s, 12H), 2.00 (m, 4H),

2.65 (s, 6H), 3.32 (s, 1H), 4.10 (s, 3H), 4.27 (t, 4H), 7.26–7.31

(m, 8H), 7.90 (d, J ~ 8.1 Hz, 2H), 8.06–8.23 (m, 14H), 8.32

(d, J ~ 7.2 Hz, 4H), 8.80–9.06 (m, 16H); LD-MS obsd 1710.7;

HRMS (FAB) obsd 1712.58, calcd 1712.59 (C₁₁₁H₉₂N₈O₃Zn₂);

l_abs430, 516, 553, 594 nm.

J. Mater. Chem., 2002, 12, 1530–1552

1549

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mesityl resonances, 4.07 (s, 3H), 4.29 (m, 4H), 7.32 (m, 20H),

8.09 (m, 24H), 8.32 (m, 8H), 8.68–8.78 (m, 12H), 8.83–8.97

(m, 20H); LD-MS obsd 3355.9, calcd avg mass, 3357.48

(C₂₁₈H₁₇₆N₁₆O₃SiZn₄); l_abs427, 435, 513, 552, 593 nm.

Tetrad 14’. To a solution of 14 (33.6 mg, 9.7 mmol) in THF–

DMF (40 mL, 10 : 1) was added TBAF (14.6 mL, 14.6 mmol,

1.0 M in THF). The solution was stirred at 60 uC for 12 h. LD-

MS still showed a signiﬁcant amount of starting material;

therefore another 15 mL of TBAF was added. The mixture was

stirred for another 5 h (LD-MS did not detect any starting

material). The mixture was concentrated to a purple solid and

triturated with methanol. The suspension was ﬁltered, then

washed with methanol, H₂O, then methanol again to recover

16.3 mg (50%) of a purple solid: MALDI-MS obsd 3362.1,

calcd avg mass 3360.0 (C₂₀₈H₁₄₆F₁₀MgN₁₆O₅Zn₃); l_abs435,

553, 594 nm.

Triad 17. Samples of 15’ (46.6 mg, 28.2 mmol) and Mg-6e

(30.7 mg, 28.2 mmol) were coupled using Pd₂(dba)₃(3.9 mg,

4.2 mmol) and P(o-tol)₃(7.7 mg, 25 mmol) in toluene–TEA

(5 : 1, 18 mL) at room temperature for 2 h. Then additional

Pd₂(dba)₃(4.0 mg, 4.4 mmol) and P(o-tol)₃(7.7 mg, 25 mmol)

were added. After a total of 4.5 h, the reaction mixture was

concentrated and chromatographed on alumina [toluene–THF

(9 : 1)]. The porphyrin-containing fractions were then loaded

onto a preparative SEC column (THF). The triad-containing

fractions were passed over an alumina column [toluene–THF

Dyad 15. Samples of Zn-6c’ (52.5 mg, 60.6 mmol) and Zn-6d

(59.7 mg, 60.6 mmol) were coupled using Pd₂(dba)₃(8.3 mg,

9.1 mmol) and P(o-tol)₃(22.1 mg, 72.7 mmol) in toluene–TEA

(5 : 1, 20 mL) at room temperature. An aliquot was removed

after 1 h and analyzed by SEC. Because of the modest

conversion to dyad, another identical batch of Pd₂(dba)₃and

P(o-tol)₃was added after 1.5 h. After an additional 2 h (3.5 h

total reaction time), an aliquot was removed and analyzed by

SEC. After 4 h, the mixture was concentrated and passed through

an alumina column (toluene). The porphyrin-containing frac-

tions were concentrated, then loaded onto a preparative SEC

column (THF). The dyad fraction was then passed through a

silica column [toluene–THF (95 : 5)]. The purple solid was

triturated with hexanes, ﬁltered, then washed with hexanes,

methanol, and then hexanes again. The solid was then dis-

solved in THF and concentrated, affording a purple solid

(62.1 mg, 60%): ¹H NMR (THF-d₈) d 0.36 (s, 9H), 1.87 (s, 24H),

2.62 (s, 12H), 7.32 (s, 8H), 7.83 (d, J ~ 8.1 Hz, 2H), 7.91 (d, J ~

8.1 Hz, 2H), 8.06 (d, J ~ 8.1 Hz, 4H), 8.12 (d, J ~ 8.1 Hz, 2H),

8.19 (d, J ~ 8.1 Hz, 2H), 8.31 (d, J ~ 8.1 Hz, 4H), 8.68–8.73

(m, 8H), 8.78–8.80 (m, 4H), 8.89 (d, J ~ 4.5 Hz, 4H); LD-MS

calcd avg mass 1722.1, obsd 1722.1 (C₁₀₇H₈₅BrN₈SiZn₂); l_abs

429, 513, 550, 591 nm.

1

(9 : 1)] to afford a purple solid (38 mg, 51%): H NMR (THF-

d₈) d 0.21 (s, 9H), 1.32 (t, 2H), 1.88 (s, 12H), 1.91 (s, 12H), 2.63

(s, 6H), 2.65 (s, 6H), 4.63 (t, 2H), 7.33 (s, 4H), 7.35 (s, 4H),

7.91 (d, J ~ 8.1 Hz, 2H), 8.07–8.15 (m, 10H), 8.31–8.35

(m, 10H), 8.44 (d, J ~ 8.1 Hz, 2H), 8.70–8.77 (m, 8H), 8.80

(d, J ~ 4.2 Hz, 2H), 8.85 (d, J ~ 4.5 Hz, 2H), 8.89–8.99

(m, 12H), multiplet centered at 7.15 and singlet at 2.33 are

due to toluene; LD-MS obsd 2621.5, calcd avg mass 2608.6

(C₁₅₄H₁₀₅BrF₁₀MgN₁₂O₂SiZn₂); l_abs435, 553, 594 nm.

Pentad 18. Samples of 17 (19.5 mg, 11.4 mmol) and 11’

(27.0 mg, 10.4 mmol) were coupled using Pd₂(dba)₃(1.4 mg,

1.6 mmol) and P(o-tol)₃(2.9 mg, 9.4 mmol) in toluene–TEA

(5 : 1, 8.5 mL) at 50 uC. THF (2 mL) was added and the mixture

became homogeneous. An aliquot was removed after 70 min

and analyzed by SEC, which revealed 25% conversion to

pentad. After 1.5 h, another identical batch of catalyst was

added. After 3 h, SEC revealed 38% conversion to pentad. The

reaction was stopped after 5 h and the mixture was poured

without concentration over an alumina column [toluene–THF

(8 : 2)]. Some purple material remained bound to the top of the

column. The porphyrin-containing fractions were loaded onto

a preparative SEC column (THF). Puriﬁcation was hampered

by severe streaking. Only a small fraction of pentad was

recovered (y2 mg) and due to its very poor solubility, could

not be completely analyzed for purity. LD-MS obsd 4249.8,

calcd avg mass 4244.5 (C₂₆₅H₁₉₆F₁₀MgN₂₀O₅SiZn₄); l_abs436,

563, 593 nm.

Dyad 15’. A solution of dyad 15 (65 mg, 38 mmol) in THF

(7 mL) was treated with TBAF (45 mL, 45 mmol, 1.0 M in THF)

and the mixture was stirred at room temperature for 1 h. The

solvent was removed by rotary evaporation. Column chroma-

tography (silica, toluene) afforded a purple solid (52 mg, 84%):

¹H NMR d (toluene-d₈, 400 MHz) 2.11 (s, 24H) 2.55 (s, 12H),

2.93 (s, 1H), 7.27 (s, 8H), 7.58 (d, J ~ 8.0 Hz, 2H), 7.72 (d, J ~

7.6 Hz, 2H), 7.78 (d, J ~ 8.0 Hz, 2H), 7.94 (d, J ~ 8.4 Hz, 2H),

8.00 (d, J ~ 8.0 Hz, 4H), 8.12 (d, J ~ 7.8 Hz, 4H), 8.84 (d, J ~

4.4 Hz, 2H), 8.86 (d, J ~ 4.4 Hz, 2H), 8.90 (d, J ~ 4.8 Hz, 2H),

8.92 (d, J ~ 4.8 Hz, 2H), 8.95 (d, J ~ 4.8 Hz, 4H), 9.02 (d, J ~

4.4 Hz, 4H); LD-MS obsd 1651.2; calcd avg mass 1649.5

(C₁₀₄H₇₇BrN₈Zn₂); l_abs429, 513, 550, 591 nm.

Dyad 19. Samples of Zn-6i’ (30.0 mg, 36.2 mmol) and Mg-6f

(39.4 mg, 36.2 mmol) were coupled using Pd₂(dba)₃(6.6 mg,

7.2 mmol) and P(o-tol)₃(13.2 mg, 43.4 mmol) in toluene–TEA

(5 : 1, 15 mL) at 35 uC for 2.5 h. The mixture was passed

through an alumina column (CHCl₃). Preparative SEC (THF)

followed by column chromatography [silica, toluene–THF

(98 : 2)] afforded a purple solid. Trituration with hexanes

afforded a purple solid (38.7 mg, 60%): ¹H NMR (400 MHz) d

0.06 (s, 9H), 1.87 (s, 18H), 2.63 (s, 3H), 2.65 (s, 6H), 3.71

(br s, 2H), 7.27 (s, 2H), 7.30 (s, 4H), 7.54 (br s, 2H), 8.03–8.06

(m, 6H), 8.12 (d, J ~ 7.6 Hz, 2H), 8.30 (d, J ~ 7.6 Hz, 4H),

8.73 (s, 4H), 8.81–8.87 (m, 6H), 8.91 (d, J ~ 4.4 Hz, 2H), 8.94

(d, J ~ 4.4 Hz, 2H), 9.08 (d, J ~ 4.4 Hz, 2H); LD-MS obsd

1790.16, calcd avg mass 1787.53 (C₁₀₅H₇₄F₁₀MgN₈O₂SiZn₄);

l_abs430, 554, 590, 604 nm.

Tetrad 16. Samples of 15 (16.6 mg, 9.69 mmol) and 11’

(16.7 mg, 9.70 mmol) were coupled using Pd₂(dba)₃(1.3 mg,

1.4 mmol) and P(o-tol)₃(3.5 mg, 12 mmol) in toluene–TEA (5 : 1,

6 mL) at 50 uC. The reaction mixture was not homogeneous.

Analytical SEC after 1 h showed 23% conversion to tetrad.

After 90 min, additional Pd₂(dba)₃(1.3 mg, 1.4 mmol) and

P(o-tol)₃(3.5 mg, 12 mmol) were added. Analytical SEC after

4.5 h showed 51% conversion to tetrad. After 5 h, the reaction

mixture was loaded onto an alumina column [toluene–THF

(95 : 5)]. The porphyrin fractions were dissolved in THF–

toluene (95 : 5), then loaded onto a preparative SEC column

(THF). The tetrad fraction was triturated with hexanes. The

mixture was ﬁltered. The ﬁltered material was washed with

hexanes, methanol, and then hexanes again. The solid was then

dissolved in THF and concentrated affording a purple solid

(10.1 mg, 31%): ¹H NMR (THF-d₈) d 0.36 (s, 9H), 0.91 (t, 6H),

1.50 (m 4H), 1.88 (d, 36H), 2.63 (d, 18H), remaining aliphatic

CH₂resonances are buried under the large THF signal and the

Acknowledgement

This work was supported by the Division of Chemical Sciences,

Ofﬁce of Basic Energy Sciences, Ofﬁce of Energy Research,

U.S. Department of Energy. Mass spectra were obtained at the

Mass Spectrometry Laboratory for Biotechnology at North

Carolina State University. Partial funding for the Facility was

obtained from the North Carolina Biotechnology Center and

1550

J. Mater. Chem., 2002, 12, 1530–1552

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R. W. Wagner, J. R. Diers, D. F. Bocian, D. Holten and

J. S. Lindsey, Chem. Phys. Lett., 2001, 341, 35.

the National Science Foundation. We thank Dr. Richard

Wagner for providing samples of Mg-6g, Mg-6h’, and Zn-6h’.

20 P. D. Rao, S. Dhanalekshmi, B. J. Littler and J. S. Lindsey, J. Org.

Chem., 2000, 65, 7323.

21 B. J. Littler, M. A. Miller, C.-H. Hung, R. W. Wagner,

D. F. O’Shea, P. D. Boyle and J. S. Lindsey, J. Org. Chem.,

1999, 64, 1391.

22 J. S. Lindsey and J. N. Woodford, Inorg. Chem., 1995, 34, 1063.

23 W. M. Sharman and J. E. Van Lier, J. Porphyrins Phthalocyanines,

2000, 4, 441.

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24 The coupling of porphyrin building blocks requires reaction

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30 The analytical SEC data are uncorrected.

31 Tetrad 16 was almost totally insoluble in CH₂Cl₂and CHCl₃, only

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32 The E_1/2value for the transmission element in the rod can be

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with a p-methoxy group lowers the potential by y20 mV per

p-methoxy group; whereas replacing three methyl groups on a

phenyl ring with hydrogens raises the potential by y10 mV.

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34 The rate constant (k_ENT) and quantum efﬁciency (W_ENT) of energy

transfer (Zn*Mg A ZnMg*) in dyad 19 are calculated using the

observed Zn* lifetime in the dyad and the value of 2.4 ns in

monomeric Zn porphyrin reference compounds (Table 3) using

standard equations.⁵In THF, the values are k_ENT~ (9 ps)²¹

–

(2.4 ns)²¹~ (9 ps)²¹and W_ENT~ 1 – (9 ps)/(2.4 ns) w 0.99.

Similar calculations for energy ﬂow to the Mg porphyrin in the

triad and tetrad in THF give k_ENT~ (15 ps)²¹and W_ENTw 0.99

for triad 12 and k_ENT~ (30 ps)²¹and W_ENT~ 0.99 for tetrad 14.

35 F. Li, S. Gentemann, W. A. Kalsbeck, J. Seth, J. S. Lindsey,

D. Holten and D. F. Bocian, J. Mater. Chem., 1997, 7, 1245.

36 Small changes in the emission properties of Mg-6f compared to

MgU can be understood by a small effect on the natural radiative

(ﬂuorescence) rate constant k_f. This effect is evident from the small

change in the oscillator strength of the Q(0,0) absorption and

emission transitions compared to MgU, and can be traced to

ﬂuorination effects on the porphyrin orbital energies, as has been

described for other halogenated porphyrin monomers.¹⁸

18 S. I. Yang, J. Seth, J.-P. Strachan, S. Gentemann, D. Kim,

D. Holten, J. S. Lindsey and D. F. Bocian, J. Porphyrins

Phthalocyanines, 1999, 3, 117.

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J. R. Diers, D. F. Bocian, D. Holten and J. S. Lindsey, J. Phys.

Chem. B, 2001, 105, 5341; (c) R. K. Lammi, A. Ambroise,

37 The more polar solvent MeCN can also coordinate to the central

J. Mater. Chem., 2002, 12, 1530–1552

1551

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Zn porphyrin to form Zn²Mg¹. This is the opposite direction of

charge ﬂow to that required for the desired rectiﬁcation effect (in

either the ground or excited state) in the light-harvesting rods.

41 P. Hascoat, S. I. Yang, R. K. Lammi, J. Alley, D. F. Bocian,

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metal. Such coordination causes a change in the porphyrin

radiative rate constant k_f, as is seen in the changes in the oscillator

strength of the Q(0,0) absorption and emission bands (relative to

the nearly constant Q(1,0) absorption and Q(0,1) emission

strengths). The change in k_fmust be taken into account in

assessment of the quantum yields and lifetimes in different

solvents.

38 The efﬁciency of ZnMg* A Zn¹Mg²hole transfer (W_HT) in dyad

19 can be calculated using the emission yield and lifetime data in

Table 3 and the standard equations.³⁹In toluene, the hole-transfer

yield is found to be W_HT~ 1 – (6.0 ns)/(8.3 ns) ~ 0.28

from the lifetimes and W_HT~ 1 – 0.087/0.14 ~ 0.38 from the

emission yields, giving an average value of y0.35. The same

analysis for 19 in MeCN gives W_HT~ 1 – (2.6 ns)/(7.6 ns) ~ 0.66

and W_HT~ 1 – 0.061/0.13 ~ 0.53 for an average value of 0.60.

These hole-transfer yields along with the Mg* lifetimes give hole-

transfer rate constants of k_HT~ 0.35/(8.3 ns) ~ (24 ns)²¹in

toluene and k_HT~ 0.60/(7.6 ns) ~ (13 ns)²¹in MeCN.

43 In the triad 12 and tetrad 14, a faster component with a time

constant of 2–3 ps is often seen in the kinetic data. This component

includes energy transfer between Zn porphyrins and Zn*Zn*

annihilation between adjacent units if more than one photon is

absorbed in a given array. This latter effect is eliminated at very

low excitation intensities, as we have found recently in studies of

porphyrins containing 8 or 20 Zn porphyrins around a single Fb

porphyrin in a dendrimeric architecture.³³

44 S. I. Yang, J. Li, H. S. Cho, D. Kim, D. F. Bocian, D. Holten and

J. S. Lindsey, J. Mater. Chem., 2000, 10, 283.

45 P. G. Seybold and M. Gouterman, J. Mol. Spectrosc., 1969, 31, 1.

46 P. D. Rao, B. J. Littler, G. R. Geier III and J. S. Lindsey, J. Org.

Chem., 2000, 65, 1084.

39 S. Prathapan, S. I. Yang, M. A. Miller, D. F. Bocian, D. Holten

and J. S. Lindsey, J. Phys. Chem. B, 2001, 105, 8237.

40 As a point of reference, Mg* in ZnMgU (which contains only

mesityl groups at the non-linking meso positions) does not decay

to any measurable degree by charge transfer, indicating that the

charge-separated states lie above the excited state;⁴¹furthermore,

based on the redox potentials of these porphyrins (and related

molecules depicted in Tables 1 and 2) it is clear that if excited-state

charge transfer were to occur in ZnMgU and similar dyads (as in

polar media due to lowering of the charge-separated state below

that of Mg*) then Mg* would transfer instead an electron to the

47 D. Gryko and J. S. Lindsey, J. Org. Chem., 2000, 65, 2249.

48 J. Li, A. Ambroise, S. I. Yang, J. R. Diers, J. Seth, C. R. Wack,

D. F. Bocian, D. Holten and J. S. Lindsey, J. Am. Chem. Soc.,

1999, 121, 8927.

49 C.-H. Lee and J. S. Lindsey, Tetrahedron, 1994, 50, 11427.

50 N. Nishino, R. W. Wagner and J. S. Lindsey, J. Org. Chem., 1996,

61, 7534.

51 M. Ravikanth, J.-P. Strachan, F. Li and J. S. Lindsey

Tetrahedron, 1998, 54, 7721.

1552

J. Mater. Chem., 2002, 12, 1530–1552

Article Doi

Design and synthesis of light-harvesting rods for intrinsic rectification of the migration of excited-state energy and ground-state holes

DOI: 10.1039/b108168c

Source and publish data:

Authors:

Article abstract of DOI:10.1039/b108168c

Full text of DOI:10.1039/b108168c

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