Structural control of photoinduced energy transfer between adjacent and distant sites in multiporphyrin arrays

Article abstract of DOI:10.1021/ja001031x

A family of diphenylethyne-linked porphyrin dimers and trimers has been prepared via a building block approach for studies of energy-transfer processes. The dimers contain Mg and Zn porphyrins (MgZnU); the trimers contain an additional free base porphyrin

Full text of DOI:10.1021/ja001031x

J. Am. Chem. Soc. 2000, 122, 7579-7591
7579
Structural Control of Photoinduced Energy Transfer between Adjacent
and Distant Sites in Multiporphyrin Arrays
Robin K. Lammi,† Arounaguiry Ambroise,‡ Thiagarajan Balasubramanian,‡
Richard W. Wagner,‡ David F. Bocian,*,§ Dewey Holten,*,† and Jonathan S. Lindsey*,‡
Contribution from the Departments of Chemistry, Washington UniVersity, St. Louis, Missouri 63130-4889,
North Carolina State UniVersity, Raleigh, North Carolina 27695-8204, and UniVersity of California,
RiVerside, California 92521-0403
ReceiVed March 23, 2000. ReVised Manuscript ReceiVed May 30, 2000
Abstract: A family of diphenylethyne-linked porphyrin dimers and trimers has been prepared via a building
block approach for studies of energy-transfer processes. The dimers contain Mg and Zn porphyrins (MgZnU);
the trimers contain an additional free base porphyrin (MgZnFbU). In both the dimers and trimers, sites of
attachment to the Mg porphyrin (at the meso- or â-position) and diphenylethyne linker (at the para- or meta-
positions) were varied, producing four Mg porphyrin-Zn porphyrin arrangements with the following linker
configurations: meso-p/p-meso, meso-m/p-meso, â-p/p-meso, and â-m/p-meso. All four trimers employ a
meso-p/p-meso Zn porphyrin-Fb porphyrin connection. The ground- and excited-state properties of the
porphyrin dimers and trimers have been examined using static and time-resolved optical techniques. The rate
of energy transfer from the photoexcited Zn porphyrin to the Mg porphyrin decreases according to the following
trend: meso-p/p-meso (9 ps)-1 > â-p/p-meso (14 ps)-1 > meso-m/p-meso (19 ps)-1 > â-m/p-meso (27 ps)-1
.
In each compound, energy transfer between adjacent porphyrins occurs through a linker-mediated through-
bond process. The rate of energy transfer between Zn and Fb porphyrins is constant in each trimer ((24 ps)-1).
Energy transfer from the photoexcited Zn porphyrin branches to the adjacent Fb and Mg porphyrins, with
nearly one-half to three-fourths proceeding to the Mg porphyrin (depending on the linker). Energy transfer
from the excited Mg porphyrin to the nonadjacent Fb porphyrin occurs more slowly, with a rate that follows
the same trend in linker architecture and porphyrin connection site: meso-p/p-meso (173 ps)-1 > â-p/p-meso
(225 ps)-1 > meso-m/p-meso (320 ps)-1 > â-m/p-meso (385 ps)-1. The rate of transfer between nonadjacent
Mg and Fb porphyrins does not change significantly with temperature, indicating a superexchange mechanism
utilizing orbitals/states on the intervening Zn porphyrin. Energy transfer between nonadjacent sites may prove
useful in directing energy flow in multiporphyrin arrays and related molecular photonic devices.
I. Introduction
constituent elements.8-18 Previous studies on porphyrin dimers
have examined several architectural alterations (identity of
central metal ion, electron-donating/withdrawing character of
nonlinking substituents, linker type, steric hindrance on diaryl-
ethyne linkers, linker-porphyrin attachment sites) which have
As part of a program in molecular photonics, we have been
developing multiporphyrin arrays that serve as molecular
photonic wires,1 optoelectronic gates,2 and light-harvesting
funnels.3-8 To better understand and selectively modify rates
and efficiencies of electronic communication in these devices,
we have been investigating the photophysical properties of their
(9) Strachan, J. P.; Gentemann, S.; Seth, J.; Kalsbeck, W. A.; Lindsey,
J. S.; Holten, D.; Bocian, D. F. J. Am. Chem. Soc. 1997, 119, 11191-
11201.
(10) Strachan, J.-P.; Gentemann, S.; Seth, J.; Kalsbeck, W. A.; Lindsey,
J. S.; Holten, D.; Bocian, D. F. Inorg. Chem. 1998, 37, 1191-1201.
(11) Yang, S. I.; Lammi, R. K.; Seth, J.; Riggs, J. A.; Arai, T.; Kim, D.;
Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Phys. Chem. B 1998, 102, 9426-
9436.
† Washington University.
‡ North Carolina State University.
§ University of California.
(1) Wagner, R. W.; Lindsey, J. S. J. Am. Chem. Soc. 1994, 116, 9759-
9760.
(2) Wagner, R. W.; Lindsey, J. S.; Seth, J.; Palaniappan, V.; Bocian, D.
F. J. Am. Chem. Soc. 1996, 118, 3996-3997.
(12) Yang, S. I.; Seth, J.; Balasubramanian, T.; Kim, D.; Lindsey, J. S.;
Holten, D.; Bocian, D. F. J. Am. Chem. Soc. 1999, 121, 4008-4018.
(13) Hascoat, P.; Yang, S. I.; Lammi, R. K.; Alley, J.; Bocian, D. F.;
Lindsey, J. S.; Holten, D. Inorg. Chem. 1999, 38, 4849-4853.
(14) Yang, S. I.; Seth, J.; Strachan, J.-P.; Gentemann, S.; Kim, D. H.;
Holten, D.; Lindsey, J. S.; Bocian, D. F. J. Porphyrins Phthalocyanines
1999, 3, 117-147.
(15) Hsiao, J.-S.; Krueger, B. P.; Wagner, R. W.; Johnson, T. E.; Delaney,
J. K.; Mauzerall, D. C.; Fleming, G. R.; Lindsey, J. S.; Bocian, D. F.;
Donohoe, R. J. J. Am. Chem. Soc. 1996, 118, 11181-11193.
(16) Wagner, R. W.; Johnson, T. E.; Lindsey, J. S. J. Am. Chem. Soc.
1996, 118, 11166-11180.
(3) Li, J.; Ambroise, A.; Yang, S. I.; Diers, J. R.; Seth, J.; Wack, C. R.;
Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Am. Chem. Soc. 1999, 121,
8927-8940.
(4) Li, J.; Diers, J. R.; Seth, J.; Yang, S. I.; Bocian, D. F.; Holten, D.;
Lindsey, J. S. J. Org. Chem. 1999, 64, 9090-9100.
(5) Li, J.; Lindsey, J. S. J. Org. Chem. 1999, 64, 9101-9108.
(6) Kuciauskas, D.; Liddell, P. A.; Johnson, T. E.; Weghorn, S. J.;
Lindsey, J. S.; Moore, A. L.; Moore, T. A.; Gust, D. J. Am. Chem. Soc.
1999, 121, 8604-8614.
(7) Li, F.; Yang, S. I.; Ciringh, Y.; Seth, J.; Martin, C. H., III; Singh, D.
L.; Kim, D.; Birge, R. R.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Am.
Chem. Soc. 1998, 120, 10001-10017.
(8) Li, F.; Gentemann, S.; Kalsbeck, W. A.; Seth, J.; Lindsey, J. S.;
Holten, D.; Bocian, D. F. J. Mater. Chem. 1997, 7, 1245-1262.
(17) Seth, J.; Palaniappan, V.; Johnson, T. E.; Prathapan, S.; Lindsey, J.
S.; Bocian, D. F. J. Am. Chem. Soc. 1994, 116, 10578-10592.
(18) Seth, J.; Palaniappan, V.; Wagner, R. W.; Johnson, T. E.; Lindsey,
J. S.; Bocian, D. F. J. Am. Chem. Soc. 1996, 118, 11194-11207.
10.1021/ja001031x CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/21/2000

7580 J. Am. Chem. Soc., Vol. 122, No. 31, 2000
Lammi et al.
Chart 1. Structures of Linear and T-Shaped Optoelectronic Gates
been shown to influence interporphyrin electronic communi-
cation.8-13
porphyrins. Attachment to the Mg porphyrin is made at the
meso- or â-position, while attachment to the phenyl rings of
the linker between the Mg and Zn porphyrins is made at the
para- or meta-positions. The four resulting Mg-linker-Zn
arrangements are as follows: meso-p/p-meso, â-p/p-meso, meso-
m/p-meso, and â-m/p-meso; each trimer has a linker with meso-
p/p-meso connectivity between the Zn and Fb porphyrins (Chart
2). As benchmarks for the trimers, we have also prepared and
studied a corresponding set of Mg-Zn porphyrin dimers (Chart
3). Our goals in the investigation of these compounds were to
(1) compare the rates and efficiencies of electronic communica-
tion obtained for the four different architectures and (2)
investigate the rates and mechanism(s) of energy transfer
between the nonadjacent Mg and Fb porphyrins in the trimers.
Collectively, the present studies both improve our understanding
of the operation of the optoelectronic gates and reveal strategies
for achieving increased flexibility and control in the design of
future molecular photonic devices.
An examination of two different architectures of optoelec-
tronic gates has prompted further studies of electronic com-
munication in multiporphyrin arrays. The two types of proto-
typical molecular optoelectronic gates are shown in Chart 1.2
Each is composed of a molecular photonic wire that serves to
transmit excited-state energy and a redox-switched site where
the energy flow can be turned on and off, either chemically or
electrochemically. The wire consists of a boron-dipyrrin input
element, a Zn porphyrin transmission element, and a free base
(Fb) porphyrin output element; a Mg porphyrin acts as the
redox-switched unit. In the linear gate, the Mg porphyrin is
appended to the Fb porphyrin, while in the T gate, the Mg
porphyrin is attached to the Zn porphyrin. In both arrays,
photoexcitation of the boron-dipyrrin chromophore results in
fluorescence emission from the Fb porphyrin output element;
upon oxidation of the Mg porphyrin, this fluorescence dimin-
ishes by more than a factor of 30. The similar behavior of the
linear and T gates raised questions concerning the extent of
interaction between the Mg and Fb porphyrins via the interven-
ing Zn porphyrin. Additional evidence for such an interaction
stemmed from simple fluorescence experiments: excitation of
the Mg porphyrin in the T gate resulted in a high yield of energy
transfer to the Fb porphyrin, even though the intervening Zn
porphyrin lies at higher energy.
To our knowledge, an investigation of energy transfer between
nonadjacent sites in trimeric or larger arrays has not been
reported. While the question of whether energy transfer occurs
between nonadjacent sites in covalently linked architectures is
of great interest, characterizing such nonpairwise interactions
requires, at the minimum, an array containing three components
having different excited-state energies and distinct spectral
features. The vast majority of studies aimed at understanding
energy transfer in multiporphyrin architectures have focused on
dimers and larger arrays containing no more than two different
types of porphyrins.
II. Results
Synthesis. (a) Strategy. We have developed a modular
approach for the synthesis of multiporphyrin arrays joined by
diphenylethyne linkers.16,19 This approach relies on the use of
ethyne- or iodo-substituted porphyrin building blocks. The
porphyrins are employed in defined metalation states (Mg, Zn,
Fb) and are joined under conditions that do not alter the
metalation state. The coupling reaction proceeds under mild
conditions that are slightly basic. For the synthesis of the dimers,
the iodo or ethyne groups could be positioned equally well on
the Mg porphyrin or the Zn porphyrin; however, Mg porphyrins
are more labile than Zn porphyrins, and this difference in
chemical stability has ramifications in the synthesis plan for
the trimers. The synthesis of the MgZnFb trimers, which requires
a stepwise procedure employing three porphyrin building blocks,
in principle could proceed from the Fb porphyrin end or the
Mg porphyrin end. To minimize handling of the Mg porphyrin,
the synthesis beginning at the Fb porphyrin is strongly preferred.
Thus, a Fb mono-ethynyl porphyrin is coupled with a Zn
In this paper, we extend our investigation of energy transfer
in multiporphyrin arrays via an examination of a family of
diphenylethyne-linked porphyrin trimers comprised of one Mg,
one Zn, and one Fb porphyrin. These MgZnFbU trimers employ
different connectivities of the linker joining the Mg and Zn
(19) Lindsey, J. S. In Modular Chemistry; Michl, J., Ed.; NATO ASI
Series C: Mathematical and Physical Sciences, Vol. 499; Kluwer Academic
Publishers: Dordrecht, 1997; pp 517-528.

Photoinduced Energy Transfer Multiporphyrin Arrays
J. Am. Chem. Soc., Vol. 122, No. 31, 2000 7581
Chart 2. Structures of MgZnFbU Trimers Employing Various Attachment Architectures (Nomeclature for Linker Structure and
Site of Connection Is Listed below Each Trimer)
porphyrin bearing an iodo group and a trimethylsilyl-protected
ethyne group. Subsequent deprotection affords the mono-ethynyl
ZnFb dimer, which is then coupled with a mono-iodo Mg
porphyrin. The preparation of the four MgZnFb trimers thus
makes use of one mono-ethynyl ZnFb dimer and four different
mono-iodo Mg porphyrins (Chart 4). The same Mg porphyrins
are employed in the synthesis of the four dimers. We have
previously described the syntheses of MgZnFbU meso-p/p-meso
and MgZnU-meso-p/p-meso via this strategy.20 The synthesis
of the other trimers and dimers requires access to the requisite
mono-iodo porphyrins.
of the â-substituted porphyrins required access to the corre-
sponding â-substituted dipyrromethanes. We have previously
prepared the Fb porphyrin bearing one â-(p-iodophenyl) sub-
stituent,21 and the same strategy was employed to prepare the
Fb porphyrin bearing one â-(m-iodophenyl) substituent. Thus,
reaction of m-iodobenzaldehyde and monoethyl malonate under
Knoevenagal conditions afforded ethyl 3-iodocinnamate (1) in
85% yield, which upon treatment with tosylmethylisocyanide
under van Leusen’s methodology22 afforded 3-ethyoxycarbonyl-
4-(3-iodophenyl)pyrrole (2) in 70% yield (Scheme 1). Acid-
catalyzed coupling of 2 and N-Boc-2-hydroxymethylpyrrole in
dioxane yielded a mixture of two regioisomeric dipyrromethanes
(3a,b) in 83% yield (1:1.4 ratio). Chromatographic separation
(b) Synthesis of Monoiodo Mg Porphyrins. The Fb por-
phyrin bearing one meso-(m-iodophenyl) substituent has been
synthesized previously by mixed aldehyde condensation of
m-iodobenzaldehyde, mesitaldehyde, and pyrrole.3 The synthesis
(21) Balasubramanian, T.; Lindsey, J. S. Tetrahedron 1999, 55, 6771-
6784.
(20) Wagner, R. W.; Ciringh, Y.; Clausen, C.; Lindsey, J. S. Chem.
Mater. 1999, 11, 2974-2983.
(22) van Leusen, A. M.; Siderius, H.; Hoogenboom, B. E.; van Leusen,
D. Tetrahedron Lett. 1972, 5337-5340.

7582 J. Am. Chem. Soc., Vol. 122, No. 31, 2000
Lammi et al.
Chart 3. Structures of MgZnU Dimers Employing Various
Attachment Architectures (Nomenclature for Linker Structure
and Site of Connection Is Listed below Each Dimer)
Chart 4. Mg Porphyrin Building Blocks
The Fb porphyrins were converted to the corresponding
magnesium chelates using the heterogeneous room-temperature
magnesium insertion method.24 Thus, treatment of a Fb por-
phyrin with MgI2 and N,N-diisopropylethylamine in CH2Cl2 for
30 min, followed by chromatographic workup on alumina (grade
V) with acid-free CHCl3, gave the corresponding magnesium
porphyrin in yields of around 80%. Note that it is essential to
employ alumina rather than silica in chromatographing mag-
nesium porphyrins, as silica affords facile demetalation of
magnesium porphyrins.4,25
(c) Synthesis of the MgZn Dimers. Each MgZn dimer was
prepared by Pd-mediated coupling of the mono-ethynyl Zn
porphyrin with the desired mono-iodo Mg porphyrin (Scheme
3). We have recently developed refined conditions for the Pd-
coupling reaction that afford minimal byproducts.20 Application
of this method (2.5 mM porphyrin substrates, Pd2(dba)3, P(o-
tol)3 in toluene/triethylamine 5:1, 35 °C, no copper cocatalysts)
gave good coupling yields. The progress of the reaction was
monitored by analytical size exclusion chromatography (SEC).
The porphyrin dimers were purified by chromatographic workup
involving (1) removal of Pd species by filtration over alumina,
(2) separation by size on a preparative SEC column using
toluene, and (3) final cleanup by chromatography on alumina.
In this manner, the dimers MgZnU â-p/p-meso, MgZnU meso-
m/p-meso, and MgZnU â-m/p-meso were prepared in yields of
approximately 60%. The purity of each dimer was checked by
analytical SEC, 1H NMR spectroscopy, LD-MS, and FAB-MS.
(d) Synthesis of the MgZnFb Trimers. The synthesis of
the trimers relied on the same Pd-mediated coupling reaction
used with the dimers. Thus, the reaction of the mono-ethynyl
ZnFb dimer20 and the desired mono-iodo Mg porphyrin afforded
the MgZnFb trimer (Scheme 4). After completion of the
reaction, analytical SEC showed a major peak corresponding
to the trimer, two minor peaks corresponding to the monomer
afforded the required dipyrromethane (3b). Treatment of 3b at
200 °C in ethylene glycol containing NaOH for 1 h caused
saponification, decarboxylation, and deprotection, affording the
desired â-substituted dipyrromethane (4) in 65% yield. The
reaction of 4, 5-mesityldipyrromethane, and mesitaldehyde under
porphyrin-forming conditions (condensation via BF3‚OEt2 in
CHCl3 for 1 h followed by oxidation with DDQ)23 afforded a
mixture of three porphyrins as determined by TLC analysis and
laser desorption mass spectrometry (LD-MS). (Note that the
two putative porphyrin isomers that can form by condensation
of mesitaldehyde and 4 were not resolved by TLC and are
undetectable by LD-MS.) Separation of the porphyrins by
chromatography was difficult. Conversion of the porphyrins to
the respective zinc chelates enabled straightforward separation
by chromatography on alumina, affording the ZnI â-m product
in 12% yield. Treatment with TFA in CH2Cl2 afforded the
desired Fb porphyrin FbI â-m in 78% yield (Scheme 2).
(24) Lindsey, J. S.; Woodford, J. N. Inorg. Chem. 1995, 34, 1063-1069.
(25) O’Shea, D. F.; Miller, M. A.; Matsueda, H.; Lindsey, J. S. Inorg.
Chem. 1996, 35, 7325-7338.
(23) Lindsey, J. S.; Wagner, R. W. J. Org. Chem. 1989, 54, 828-836.

Photoinduced Energy Transfer Multiporphyrin Arrays
J. Am. Chem. Soc., Vol. 122, No. 31, 2000 7583
Scheme 1. Synthesis of 3-(3-Iodophenyl)dipyrromethane
Scheme 2. Synthesis of the â/m-Substituted Free Base
Porphyrin Building Block
Scheme 3. Synthesis of MgZnU Dimers with Different
Linker Structures and Sites of Connection
and dimer porphyrins, and a shoulder on the trimer peak due to
higher molecular weight material. The reaction performed in
this manner remained homogeneous. The purification scheme
employed for the trimers was identical to that for the dimers
except that THF rather than toluene was used in the preparative
SEC process. The purity of each trimer was checked by
analytical SEC, 1H NMR spectroscopy, LD-MS, and FAB-MS.
Static and Time-Resolved Absorption and Emission Spec-
troscopy. Q-region electronic absorption and emission spectra
of MgZnU meso-m/p-meso and MgZnFbU meso-m/p-meso, a
representative dimer and trimer, are shown in Figure 1. The
features correspond to transitions between the ground states and
the lowest excited singlet states of the constituent Mg, Zn, and
Fb porphyrins. The dimer absorption spectra (Figure 1a) contain
the Mg porphyrin Q(1,0) band at 562 nm, the Mg porphyrin
Q(0,0) band at 602 nm, and Zn porphyrin Q(1,0) and Q(0,0)
peaks at 551 and 589 nm, respectively. In the trimers (Figure
1b), these Mg and Zn porphyrin features are accompanied by
four Fb porphyrin bands: Qy(1,0) at 514 nm, Qy(0,0) at 551
nm, Qx(1,0) at 591 nm, and Qx(0,0) at 649 nm.
Fluorescence spectra of the dimers (Figure 1a, dashed line)
exhibit a Mg porphyrin Q(0,0) emission at 605 nm and a Mg
porphyrin Q(0,1) band at 660 nm. In addition to these features,
the trimer spectra also contain the Fb porphyrin Q(0,0) emission

7584 J. Am. Chem. Soc., Vol. 122, No. 31, 2000
Lammi et al.
Figure 1. Q-region absorption (solid line) and emission spectra (dashed
line) in toluene at room temperature: (a) MgZnU meso-m/p-meso, for
emission, λex ) 542 nm; (b) MgZnFbU meso-m/p-meso, for emission,
λex ) 569 nm.
Figure 2. (a) Representative time-resolved absorption difference
spectra for MgZnU meso-m/p-meso in toluene at room temperature
elicited by excitation with a 130 fs flash at 542 nm. (b) Representative
kinetic data (550 and 562 nm) and fits. For clarity, data before zero
time and during the instrument rise are not shown.
at 651 nm and the Fb porphyrin Q(0,1) peak at 720 nm. Note
that fluorescence from the MgZnU dimers occurs nearly
exclusively from the Mg porphyrin, even when the Zn porphyrin
is preferentially excited. This indicates that energy transfer from
the photoexcited Zn porphyrin to the ground-state Mg porphyrin
(denoted Zn* f Mg) is effectively quantitative. Similarly high
efficiencies were noted previously for the meso-p/p-meso dimers
MgZnU, ZnFbU, and MgFbU.8,13,15 In the case of the MgZnFbU
trimers (Figure 1b, dashed line), emission is detected predomi-
nantly from the free base porphyrin. No Zn* emission occurs;
however, a small amount of Mg porphyrin emission is evident
when the Mg component is excited. This suggests that photo-
induced energy transfer from Mg* to Fb is slightly less than
quantitative.
The rates and efficiencies of photoinduced energy transfer
for the series of MgZnU dimers and MgZnFbU trimers were
quantitated using time-resolved absorption measurements. Pro-
cedures and analyses were similar to those described previously
for other multiporphyrin arrays.8-15 In the dimers, the Zn
porphyrin was photoexcited in order to observe Zn* f Mg
energy transfer. Results from a representative experiment on
MgZnU meso-m/p-meso are shown in Figure 2. The absorption
difference spectrum at 1 ps after excitation with a 130 fs flash
at 542 nm shows predominantly bleaching of the Zn porphyrin
Scheme 4. Synthesis of MgZnFbU Trimers with Different Linker Structures and Sites of Connection

Photoinduced Energy Transfer Multiporphyrin Arrays
J. Am. Chem. Soc., Vol. 122, No. 31, 2000 7585
Table 1. Photophysical Data for Dimers and Monomer Reference
Table 2. Excited-State Lifetimes in MgZnFbU Trimersa
τ (ps)
Compoundsa
τ (ps)
MgZnFbU trimer
Zn* b
Zn* (calcd)c
Mg* b
dimers
MgZnU
meso-p/p-meso
â-p/p-meso
Zn* b
Mg* b
ktrans-1 (ps)c
Φtrans
d
meso-p/p-mesod
â-p/p-meso
meso-m/p-meso
â-m/p-meso
7 ( 1
11 ( 3
9 ( 1
6.5
8.8
10.3
12.7
170 ( 30
220 ( 30
310 ( 30
370 ( 50
9 ( 1f
14 ( 1
19 ( 1
27 ( 2
9f
14
19
27
>0.99f
>0.99
>0.99
0.99
13 ( 4
meso-m/p-meso
â-m/p-meso
a Data in toluene at room temperature. b Zn* and Mg* lifetimes
measured by transient absorption spectroscopy. c Zn* lifetime calculated
from eq 5 and values in Table 1. d This compound was more difficult
to work with than the other trimers, exhibiting lower solubility due to
aggregation.
ZnFbUg
meso-p/p-meso
meso-m/p-meso
â-p/p-â
24 ( 2f
39 ( 4h
55 ( 5i
24f
40h
56i
0.99f
0.98h
0.98i
MgFbUg
31 ( 3f
31f
>0.99f
two experiments were performed on each trimer. In one, the
Zn porphyrin was preferentially excited (at 542 nm, forming
MgZn*Fb in most of the molecules and Mg*ZnFb in a small
number); in the other experiment, primarily the Mg porphyrin
was excited (at 569 nm). Both experiments on a given trimer
were expected to yield similar results. More confidence was
placed in the 542 nm excitation for determination of the Zn*
lifetime and in the 569 nm excitation for the Mg* lifetime, due
to the larger amplitude of the respective kinetic component in
the dual-exponential fits. Absorption difference spectra and
kinetics traces for representative experiments on MgZnFbU
meso-m/p-meso are shown in Figure 4. Spectra in panel a (542
nm excitation) clearly show the decay of the Zn porphyrin
Q(1,0) ground-state bleaching at 541 nm and the growth of the
Fb porphyrin Qy(1,0) ground-state bleaching at 514 nm over
time. Less clear are the growth of the Mg porphyrin Q(1,0)
ground-state bleaching at 562 nm that occurs between 1 and
96 ps and its subsequent decay between 96 ps and 2.3 ns after
excitation. These processes are more evident in the spectra in
panel c (569 nm excitation), in which the Mg porphyrin was
preferentially excited. The Mg porphyrin Q(1,0) ground-state
bleaching decays, disappearing between 96 ps and 2.3 ns after
excitation, while the Fb porphyrin Qy(1,0) and Qy(0,0) ground-
state bleachings grow in with time. Kinetic analysis of data in
several spectral regions from each experiment gave similar
results, as anticipated. Both sets of kinetic data were fit to two
exponentials (plus a constant): the short component of 9 ( 1
ps corresponds to the Zn* lifetime and the longer component
of 310 ( 30 ps corresponds to the Mg* lifetime. (These are
average values that take into account the results of experiments
with 542 and 569 nm excitation.) Zn* and Mg* lifetimes for
the other trimers were determined in the same manner and are
listed in Table 2. Note that if no energy transfer occurred from
Mg*, the Mg* lifetime in each of the trimers would be expected
to be equal to that of a monomeric Mg porphyrin (9.7 ns).
meso-p/p-meso
τ (ns)
Mg* e
9.7 ( 0.4f
monomersj
Zn* e
Fb* e
MgU′
ZnU′
2.4 ( 0.2k
2.4 ( 0.2i
ZnU′-âl
FbU′
13.3 ( 0.5k
a Data in toluene at room temperature. b Zn* and Mg* (donor)
lifetimes in dimers measured by transient absorption spectroscopy.
c Inverse of the energy-transfer rate calculated from eq 3 and lifetimes
in table. d Energy-transfer efficiency calculated from eq 4. e Zn*, Mg*,
and Fb* lifetimes in monomers measured by time-resolved emission
spectroscopy. f From ref 13. g Nomenclature for ZnFbU and MgFbU
dimers is the same as for MgZnU dimers; see Chart 2. h From ref 3.
i From ref 12. j See structure of U′ monomers in Chart 5. k From ref
14. l Zn(II) 5,10,15-trimesityl-2-(4-ethynylphenyl)porphyrin.
Figure 3. Diagram of energy-transfer processes in MgZnFbU trimers.
(a) Excitation of the Zn porphyrin results in branching of the flow of
energy (Zn* f Mg and Zn* f Fb). (b) Excitation of the Mg porphyrin
results in Mg* f Fb energy transfer.
Q(1,0) ground-state band at 551 nm; there is also a small
bleaching of the Mg porphyrin Q(1,0) ground-state band at 562
nm due to Mg-porphyrin excitation in a small fraction of dimers.
The Zn porphyrin bleaching decays between 1 and 56 ps, while
the Mg porphyrin bleaching develops. Data were fit to a single-
exponential function, giving a time constant of 19 ( 1 ps for
the Zn* lifetime in the dimer. This result is listed in Table 1
along with those of the other MgZnU dimers; also included are
lifetime data for related dimers and monomer building blocks.
Low-temperature experiments were carried out on MgZnFbU
meso-m/p-meso in toluene, at 200 K (just above the freezing
point), and in 2-methyltetrahydrofuran (2-MeTHF), at 79 K.
The objective of these experiments was to examine energy
transfer from the photoexcited Mg porphyrin to the nonadjacent
Fb porphyrin, so as to elucidate the mechanism of this process:
either a two-step hopping mechanism in which Zn* is formed,
or a one-step superexchange mechanism using orbitals/states
on the Zn porphyrin. As such, we chose to excite the Mg
porphyrin preferentially (at 569 nm). Kinetic analysis of data
in several spectral regions revealed double-exponential decays;
again, the short component corresponds to the Zn* lifetime and
the long one to the Mg* lifetime. Values for the Mg* lifetimes
determined are listed in Table 3, along with room-temperature
results in the same solvents. It is apparent from these results
that the Mg* lifetime is not very temperature-dependent.
Several energy-transfer processes occur in each trimer: (1)
Zn* f Mg, (2) Zn* f Fb, both of which occur between
adjacent porphyrins, and (3) Mg* f Fb, which occurs between
nonadjacent porphyrin units (Figure 3). Since neither the Zn
nor the Mg porphyrin can be exclusively excited (due to
overlapping Q(1,0) and Q(0,0) absorption bands), dual-
exponential decays were expected. To clearly determine time
constants for the decays of the Zn* and Mg* excited states,

7586 J. Am. Chem. Soc., Vol. 122, No. 31, 2000
Lammi et al.
Figure 4. Representative time-resolved absorption spectra, kinetic data at 513 nm, and fits for MgZnFbU meso-m/p-meso in toluene. Panels a and
b show the results of excitation at 542 nm; spectra and kinetics shown in panels c and d were obtained by exciting at 569 nm. Average Zn* and
Mg* lifetimes were determined from analysis of several spectral regions in each experiment and are listed in Table 2.
Table 3. Temperature Dependence of Mg* Lifetimes in MgZnFbU
meso-m/p-meso
Chart 5. Structures of MU′ Monomers
solvent
temperature (K)
τ(Mg*)a (ps)
toluene
2-MeTHF
toluene
294
294
200
79
310 ( 30
250 ( 50
340 ( 120
180 ( 30
2-MeTHF
a Mg* lifetimes obtained from transient absorption experiments.
Excited-State Energy-Transfer Rates and Yields. The
parameters for photoinduced energy transfer in the MgZnU
dimers and MgZnFbU trimers were obtained using eqs 1-5,
which do not also exist in the porphyrin monomers (radiative
decay (rad), intersystem crossing (isc), and internal conversion
(ic)). The transient absorption and static emission data support
this assumption; however, we cannot exclude the possibility of
a small amount of charge transfer from donor to acceptor in
each array.13
Using eqs 1-4 and lifetime data in Table 1, photoinduced
energy transfer from Zn* to Mg in MgZnU meso-m/p-meso
was calculated to occur with a rate constant of (19 ps)-1; the
meso-p/p-meso and â-p/p-meso dimers have faster rates of
energy transfer, while that of the â-m/p-meso compound is
slower. Energy transfer is very efficient in all four dimers
(g99%). Rates and efficiencies of Zn* f Mg, Zn* f Fb, and
Mg* f Fb energy transfer for these dimers and a variety of
others (studied previously) are listed in Table 1.
1/τD ) krad + kisc + kic
1/τDA ) krad + kisc + kic + ktrans
ktrans ) 1/τDA - 1/τD
(1)
(2)
(3)
(4)
Φtrans ) ktransτDA ) 1 - τDA/τD
tri
Zn
1/τ ) 1/τD + kMgZn + kZnFb
(5)
where τDA is the excited-state lifetime of the donor in the
presence of the acceptor (e.g., Zn* in MgZnU meso-m/p-meso),
τD is the excited-state lifetime of the (donor) porphyrin monomer
(e.g., Zn* in ZnU′; see Chart 5), ktrans is the energy-transfer
rate, Φtrans is the energy-transfer efficiency, and τtZrni is the Zn*
lifetime in the trimer. These equations assume that, other than
energy transfer, there are no pathways for depopulating the
excited state of the donor porphyrin in the multiporphyrin arrays
Calculated Zn* lifetimes for the trimers that incorporate both
the Zn* f Mg and Zn* f Fb pathways are listed in Table 2.
These values were determined using eq 5 and the calculated
energy-transfer rates for the appropriate MgZnU and ZnFbU
dimers, in addition to the measured lifetime of the ZnU′
monomer (Table 1). The calculated Zn* lifetime of 10.3 ps for

Photoinduced Energy Transfer Multiporphyrin Arrays
J. Am. Chem. Soc., Vol. 122, No. 31, 2000 7587
Table 4. Rates and Yields of Energy Transfer in MgZnFbU
comprised of multiple components in covalently linked archi-
tectures. These topics are discussed in more detail in the
following sections.
Trimersa
Zn* f Mg
Zn* f Fb
Mg* f Fb
MgZnFbU trimer k1-1 (ps)b φ1 k2-1 (ps)d φ2 k3-1 (ps)f Φ3
c
e
g
Effects of Varying Porphyrin and Linker Attachment
Sites. The rates and efficiencies determined for the dimers and
trimers reveal an underlying trend that depends on porphyrin-
linker connectivity. In the MgZnU dimers (Table 1), the rate
of Zn* f Mg energy transfer decreases according to the
following pattern: meso-p/p-meso (9 ps)-1 > â-p/p-meso (14
meso-p/p-meso
â-p/p-meso
9
14
19
27
0.73
0.63
0.56
0.47
24
24
24
24
0.27
0.37
0.44
0.53
173
225
320
385
0.98
0.98
0.97
0.96
meso-m/p-meso
â-m/p-meso
a All values calculated from results in toluene at room temperature.
Processes 1, 2, and 3 are identified in Figure 3. b Inverse of energy-
transfer rate in MgZnU dimer employing the same linker (Table 1).
c Branching ratio for flow of energy from Zn* to Mg; for MgZnFbU
meso-p/p-meso, φ1 ) (1/9)/(1/9 + 1/24) ) 0.73. d Inverse of energy-
transfer rate in ZnFbU meso-p/p-meso. e Branching ratio for flow of
energy from Zn* to Fb, calculated in the same manner as in footnote
c. f Inverse of energy-transfer rate calculated from eq 3, using Mg*
lifetimes in trimer and MgU′ monomer (Tables 1 and 2). g Energy-
transfer efficiency, calculated from eq 4.
ps)-1 > meso-m/p-meso (19 ps)-1 > â-m/p-meso (27 ps)-1
.
The efficiency of energy transfer follows the same trend,
decreasing only slightly along the series, from 0.996 for MgZnU
meso-p/p-meso to 0.989 for the â-m/p-meso dimer. The family
of MgZnFbU trimers exhibits the same trend in rate and
efficiency of Mg* f Fb energy transfer, decreasing from (173
ps)-1 and 0.98 for the meso-p/p-meso trimer to (385 ps)-1 and
0.96 for the â-m/p-meso compound (Table 4). It is more difficult
to clearly discern a trend in Zn* lifetime in the trimers, because
the error bars in the experimental data are large enough to render
the values nearly indistinguishable; however, the calculated Zn*
lifetimes fit the pattern mentioned above (Table 2). Thus, the
same trend in rate and efficiency of energy transfer is evident
in both the dimers and trimers, and for both the Zn* f Mg and
Mg* f Fb energy-transfer processes.
MgZnFbU meso-m/p-meso is in good agreement with the
measured value of 9 ( 1 ps. Values calculated for the other
three trimers also agree well with experimental results (Table
2).
In the MgZnFbU trimers, branching ratios are readily
calculated (eqs 6-8) for energy transfer from Zn* to the Mg
(φ1) and Fb (φ2) porphyrins (see Figure 3).
The findings concerning the effects of structure and con-
nectivity on electronic communication can largely be understood
by consideration of orbital interactions. In particular, attachment
at the meso-position of the Mg porphyrin results in more
efficient electronic communication than does connection at the
â-position, and connection at the p-position of the linker is
preferable to attachment at the m-position. The latter result is
in agreement with previous work on ZnFbU dimers (Table 1),
in which the meso-p/p-meso compound was found to have a
faster rate of energy transfer than the meso-m/p-meso array ((24
ps)-1 versus (40 ps)-1).3 Consideration of HOMO electron
density distributions for the Mg porphyrin and the diphenyl-
ethyne linker provides a likely explanation for these findings.
The majority of the electron density in the porphyrin a2u(π)
HOMO is located at the meso-positions with minimal density
at the â-positions.26 Similarly, the HOMO in diphenylacetylene
has electron density concentrated at the p-positions, with
significantly less density at the m-positions.27 Thus, it is
reasonable that communication will be fastest and most efficient
in the compounds that take advantage of both high-electron-
density attachment sites (the meso-p/p-meso dimer and trimer),
and slowest and least efficient in those that utilize the attachment
sites with less HOMO electron density (the â-m/p-meso
compounds). It is more difficult to predict the relative merit of
the other two porphyrin-linker connectivities (â-p/p-meso vs
meso-m/p-meso) because each of these architectures contains
both favorable and unfavorable structural elements for maximiz-
ing energy-transfer rate and efficiency. Our experimental results
consistently show that communication is better in â-p/p-meso
compounds than in their meso-m/p-meso counterparts.
k1/(k1 + k2) ) φ1
k2/(k1 + k2) ) φ2
φ1 + φ2 ) 1
(6)
(7)
(8)
In each trimer, the Zn* f Fb energy-transfer rate is constant
((24 ps)-1), due to use of the same meso-p/p-meso linker
architecture, while the Zn* f Mg energy-transfer rate is altered
with the different arrangements. This latter rate ranges from (9
ps)-1 to (27 ps)-1, giving a branching ratio that ranges from
0.73:0.27 (MgZnFbU meso-p/p-meso) to 0.47:0.53 (MgZnFbU
â-m/p-meso) for flow of energy to the Mg vs Fb porphyrin
(Table 4).
The rates and efficiencies of Zn porphyrin-mediated Mg* f
Fb energy transfer were calculated using the measured Mg*
lifetimes in the trimers and that of the MgU′ reference monomer
(Tables 1 and 2). For example, MgZnFbU meso-m/p-meso has
a Zn porphyrin-mediated Mg* f Fb energy-transfer rate of (320
ps)-1, with an efficiency of 97%. Note that the slower rate of
Mg* f Fb transfer compared to that of Zn* f Fb transfer ((24
ps)-1) is mitigated by the longer lifetime of the Mg porphyrin
(9.7 ns) compared to the Zn porphyrin (2.4 ns). The rates and
efficiencies for Mg* f Fb transfer for the four trimers are listed
in Table 4.
III. Discussion
The availability of a systematic family of arrays comprised
of three porphyrins with distinct energies and spectral features
has enabled study of the flow of energy between adjacent and
nonadjacent sites. Static emission spectra show that excitation
of the Mg or Zn porphyrins in the MgZnFbU trimers results in
efficient energy transfer to the Fb porphyrin. Time-resolved data
have revealed that the photoexcited Zn porphyrin transfers nearly
one-half to three-fourths of its energy to the adjacent Mg
porphyrin; this is followed by slower energy transfer from the
excited Mg porphyrin to the nonadjacent Fb porphyrin. The
observation of energy transfer between nonadjacent sites has
implications for the design of molecular photonic devices
Mechanisms of Electronic Communication. Understanding
the mechanism of electronic communication is essential for
the rational design of molecular photonic devices. Our prior
work on various families of dimers has necessarily enabled
characterization of pairwise (adjacent) interactions only. The
MgZnFbU trimers, which contain three porphyrins of unequal
(26) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic:
New York, 1978; Vol. III, pp 1-153.
(27) Ferrante, C.; Kensy, U.; Dick, B. J. Phys. Chem. 1993, 97, 13457-
13463.

7588 J. Am. Chem. Soc., Vol. 122, No. 31, 2000
Lammi et al.
excited-state energies, enable characterization of interactions
between nonadjacent porphyrin units.
of non-nearest-neighbor energy transfer in multiporphyrin
systems. The speed and efficiency noted here indicate that such
processes may make a significant contribution to the overall
energy flow in many multiporphyrin arrays.
Our previous studies have shown that photoinduced energy
transfer between adjacent donor and acceptor porphyrins in
diarylethyne-linked dimers (e.g., ZnFbU, MgFbU) occurs
predominantly via an exchange-mediated through-bond (Dexter)
mechanism, with a much smaller through-space (Fo¨rster)
contribution.8,9,15,28 This assessment is based on the following
factors: (1) the donor excited-state lifetimes predicted by Fo¨rster
calculations are on the order of 1 ns, nearly 2 orders of
magnitude larger than the measured values (tens of picoseconds),
and (2) energy-transfer rates depend on porphyrin-linker steric
hindrance, and orbital spacing and ordering in a manner
consistent with through-bond rather than through-space pro-
cesses. The fact that the Zn* lifetimes in the various MgZnU
dimers studied herein are also on the picosecond time scale
indicates that the through-bond mechanism is likely the primary
contributor in these dimers, as well. In addition, the measured
Zn* lifetimes in the MgZnFbU trimers agree well with values
calculated from the energy-transfer rates of constituent MgZnU
and ZnFbU dimers (Table 1); therefore, it may be inferred that
energy transfer between adjacent, diarylethyne-linked porphyrins
in trimers and larger arrays also occurs primarily via a through-
bond mechanism.
Results of static emission and transient absorption experiments
(namely, emission largely due to the Fb porphyrin upon
excitation of the Mg porphyrin and energy-transfer rates of
(100-400 ps)-1) have shown that there is good communication
between the nonadjacent Mg and Fb porphyrins in the trimers.
The finding of Mg* f Fb rates that are 2 orders of magnitude
faster than the (50 ns)-1 rate predicted for a through-space
(Fo¨rster) process indicates that a through-bond mechanism
involving the intervening Zn porphyrin is operable. This energy
transfer between nonadjacent sites could occur by one or both
of two possible mechanisms: (1) a superexchange mechanism
utilizing orbitals/states on the intervening Zn porphyrin, and
(2) a hopping mechanism whereby the Zn* state is actually
formed from Mg*. The latter route is possible in this case
because the lowest-lying Zn* excited state is only slightly higher
in energy than the lowest-lying Mg* state, within 340 cm-1 for
MgZnFbU meso-m/p-meso, a difference of 1.6 kBT at room
temperature. The rationale for the low-temperature transient
absorption experiments was to examine the Mg* f Fb energy-
transfer process under conditions in which thermal repopulation
of Zn* was nearly impossible. (At 200 K, 340 cm-1 corresponds
to 2.4 kBT; at 79 K, 6.1 kBT.) If hopping were the predominant
mechanism, experiments performed at these low temperatures
would show a much slower rate of Mg* f Fb energy transfer
than those measured at room temperature in the same solvents.
This prediction is contrary to our observations, which show that
the Mg* lifetimes measured for MgZnFbU meso-m/p-meso at
low temperatures were very similar to those measured at room
temperature, in both toluene and 2-MeTHF (Table 3). If a very
small amount of hopping does occur, it could explain why the
Mg* lifetime at 79 K in 2-MeTHF may be slightly longer than
that measured at 294 K (250 ( 50 versus 180 ( 30 ps). Still,
it is clear that the hopping mechanism makes only a minor
contribution to the observed Mg* f Fb energy-transfer process.
The principal method of electronic communication between the
nonadjacent, diarylethyne-linked Mg and Fb porphyrins appears
to be superexchange through the orbitals/states of the intervening
Zn porphyrin. To our knowledge, this is the first demonstration
The observation of through-bond energy transfer between
nonadjacent sites in multiporphyrin arrays has significant
implications for the design and characterization of molecular
photonic devices. (1) Simulations aimed at accurately modeling
the flow of energy in multicomponent architectures must
consider the rates of pairwise and nonadjacent transfer processes.
(2) The ability to transfer energy between nonadjacent sites may
have advantages in some applications. For example, energy
could be shunted to a pigment having a long intrinsic excited-
state lifetime and subsequently released at a slower rate to a
nonadjacent site. The rate of transfer can be controlled by
structure, energetics, and porphyrin-linker connectivity. The
MgZnFbU trimers provide a limited illustration of this case,
where the monomeric Mg and Zn porphyrins have excited-state
lifetimes of ∼10 and 2.4 ns, respectively. The use of other
pigments with longer excited-state lifetimes can be envisioned.
IV. Conclusions
Our studies of the MgZnFbU trimers and their constituent
dimers have revealed the following: (1) Attachment at the
porphyrin meso-position and the p-phenyl position of the linker
leads to the fastest, most efficient communication. (2) Rapid,
efficient energy transfer occurs between nonadjacent Mg and
Fb porphyrins. (3) This energy-transfer process between non-
adjacent sites occurs via superexchange using orbitals/states on
the Zn porphyrin. In conjunction with our previous findings
regarding the through-bond energy transfer that occurs between
adjacent porphyrins in diarylethyne-linked dimers (and trimers),
we now have a complete picture of the processes occurring in
the neutral forms of the optoelectronic gates, particularly the T
gate, with its Mg-Zn-Fb porphyrin arrangement. Further
understanding of gate operation will be obtained through
experiments underway on dimers and trimers in which the Mg
porphyrin is oxidized. Collectively, these results regarding the
efficacy of different linker motifs and energy transfer between
adjacent and distant porphyrins will aid in the design of larger
and more complex multiporphyrin arrays for various applications
in molecular photonics.
V. Experimental Methods
1
Synthesis. (a) General. H NMR spectra (300 MHz), absorption
spectra (HP 8451A, Cary 3), and fluorescence spectra (Spex FluoroMax)
were collected routinely. Porphyrins were analyzed by laser desorption
mass spectrometry (LD-MS) in the absence of a matrix.29 Pyrrole was
distilled at atmospheric pressure from CaH2. 3-Iodobenzaldehyde was
obtained from Karl Industries, Ltd.
(b) Chromatography. Adsorption column chromatography was
performed using flash silica (Baker) or alumina (grade V).8 Porphyrins
were dissolved in CH2Cl2 for column chromatography at high concen-
tration and then hexanes was added to achieve a nonpolar loading
solvent. Preparative-scale size exclusion chromatography (SEC) was
performed using Bio-Rad Bio-Beads SX-1 in a glass column (4.8 ×
60 cm) packed with THF. The chromatography was performed with
gravity flow (∼4 mL/min). A typical separation required ∼3 h.
Following purification, the SEC column was washed with two volume
equivalents of THF. Unlike adsorption columns, the SEC columns can
be used repeatedly, although with continued use for porphyrin separa-
tions the columns become slightly pink-brown.
(29) (a) Fenyo, D.; Chait, B. T.; Johnson, T. E.; Lindsey, J. S. J.
Porphyrins Phthalocyanines 1997, 1, 93-99. (b) Srinivasan, N.; Haney,
C. A.; Lindsey, J. S.; Zhang, W.; Chait, B. T. J. Porphyrins Phthalocyanines
1999, 3, 283-291.
(28) Wagner, R. W.; Seth, J.; Yang, S. I.; Kim, D.; Bocian, D. F.; Holten,
D.; Lindsey, J. S. J. Org. Chem. 1998, 63, 5042-5049.

Photoinduced Energy Transfer Multiporphyrin Arrays
J. Am. Chem. Soc., Vol. 122, No. 31, 2000 7589
Analytical-scale SEC was performed to assess the purity of the
trimer-forming reactions and to monitor the preparative purification of
the trimers. Analytical SEC columns (styrene-divinylbenzene copoly-
mer) were purchased from Hewlett-Packard and Phenomonex. Analyti-
cal SEC was performed with a Hewlett-Packard 1090 HPLC using a
1000 Å (260 × 12 mm) column (5 µm) eluting with THF (flow rate )
0.8 mL/min). Reactions were monitored by removing aliquots from
the reaction mixture and diluting with THF (Fisher, HPLC grade).
Sample detection was achieved by absorption spectroscopy using a
diode array detector with quantitation at 420 nm ((10 nm bandwidth),
which best captures the peaks of monomeric porphyrins and the
multiporphyrin arrays.
(c) Solvents. CH2Cl2 (Fisher, reagent grade) and CHCl3 (Fisher
certified ACS) were subjected to simple distillation from K2CO3. The
commercially available CHCl3 contained ethanol (0.75%) as a stabilizer.
All references to CHCl3 in this paper pertain to CHCl3 containing 0.75%
ethanol. Simple distillation does not significantly alter the ethanol
content. THF (Fisher certified ACS) was distilled from sodium
benzophenone ketyl as required. Toluene (Fisher certified ACS) and
triethylamine (Fluka puriss) were distilled from CaH2. THF (Fisher
HPLC grade) was used for preparative SEC. Other solvents were used
as received.
phase was extracted with ether (4 × 50 mL). The combined ethereal
fraction was successively washed with water (3 × 100 mL) and brine
(100 mL), dried (Na2SO4), filtered, and evaporated. Purification by flash
chromatography (silica, hexanes/ethyl acetate, 4:1) separated the two
compounds, with regioisomer 3a eluting first as a colorless solid (0.80
g, 35%) and 3b eluting second as a colorless solid (1.1 g, 48%). Data
1
for 3a: mp 55-56 °C; (the H NMR and 13C NMR spectra exhibited
the effects of rotamers) 1H NMR (CDCl3) δ 1.17 (t, J ) 7.2 Hz, 3H),
1.57 (s, 9H), 4.18 (q, J ) 7.2 Hz, 2H), 4.58 (s, 2H), 6.10 (m, 1H),
6.24 (t, J ) 1.5 Hz, 1H), 6.55 (s, 1H), 7.03 (dt, J ) 8.1 Hz, 1.5 Hz,
1H), 7.18 (t, J ) 1.5 Hz, 1H), 7.33 (d, J ) 8.1 Hz, 1H), 7.56 (d, J )
8.1 Hz, 1H) 7.73 (d, J ) 1.5 Hz, 1H), 9.14 (brs, 1H); 13C NMR (CDCl3)
δ 14.2, 25.8, 27.9, 59.4, 77.2, 84.3, 93.3, 109.4, 110.6, 114.1, 115.8,
121.4, 125.2, 128.6, 129.1, 131.5, 134.9, 138.2, 138.4, 150.3, 165.3;
FAB-MS obsd 520.0880, calcd 520.0859 (C23H25N2O4I). Anal. Calcd:
C, 53.07; H, 4.84; N, 5.38. Found: C, 53.71; H, 4.96; N, 5.43. Data
1
for 3b: mp 65-66 °C; (the H NMR and 13C NMR spectra exhibited
the effects of rotamers) 1H NMR (CDCl3) δ 1.14 (t, J ) 7.2 Hz, 3H),
1.60 (s, 9H), 4.03 (s, 2H), 4.12 (q, J ) 7.2 Hz, 2H), 5.81 (s, 1H), 6.06
(m, 1H), 7.18 (m, 2H), 7.36 (m, 2H), 7.62 (m, 1H), 7.69 (m, 1H), 9.28
(brs, 1H); 13C NMR (CDCl3) δ 14.2, 24.6, 27.9, 59.3, 77.2, 84.3, 93.2,
110.5, 113.3, 114.5, 120.8, 121.2, 122.8, 129.1, 130.1, 132.7, 135.2,
137.4, 139.5, 150.3, 164.7; FAB-MS obsd 520.0878, calcd 520.0859
(C23H25N2O4I). Anal. Calcd: C, 53.07; H, 4.84; N, 5.38. Found: C,
53.25; H, 4.87; N, 5.20.
Ethyl 3-Iodocinnamate (1). A solution of 3-iodobenzaldehyde (5.80
g, 25.0 mmol), monoethyl malonate (5.00 g, 37.9 mmol), piperidine
(0.3 mL), and pyridine (14 mL) was heated at reflux for 7 h in a 100-
mL round-bottom flask. The reaction mixture was cooled to room
temperature, poured into 2 N aqueous HCl (100 mL), and extracted
with ether (3 × 40 mL). The ethereal portions were combined and
washed successively with water (2 × 40 mL), saturated aqueous
NaHCO3 (2 × 40 mL), water (2 × 40 mL), and brine (50 mL). The
ether portion was dried (Na2SO4) and evaporated to give a brownish
solid. Recrystallization from absolute ethanol gave 6.50 g (85%) of a
3-(3-Iodophenyl)dipyrromethane (4). A 25-mL round-bottom flask
was charged with 3b (1.00 g, 1.92 mmol) and 8 mL of ethylene glycol.
The system was flushed with argon for 10 min, and then powdered
NaOH (0.20 g, 5.0 mmol) was added and the flask was immersed in
an oil bath at 180 °C. The oil bath temperature was increased to 195-
200 °C and maintained (total time, 1 h). The reaction flask was then
cooled to room temperature and 30 mL of 10% aqueous NaCl was
added. The aqueous portion was extracted with CH2Cl2 (3 × 25 mL),
and the combined organic layers were washed with 50 mL of brine,
dried (Na2SO4), filtered, and evaporated. Purification by flash chro-
matography (silica, hexanes/ethyl acetate (3:1) containing 1-2%
triethylamine) afforded a gummy solid (0.45 g, 65%): 1H NMR (CDCl3)
δ 4.04 (s, 2H), 6.06 (s, 1H), 6.15 (dd, J ) 6.0, 3.0 Hz, 1H), 6.30 (t, J
) 3.0 Hz, 1H), 6.58 (m, 2H), 7.08 (t, J ) 8.1 Hz, 1H), 7.34 (d, J )
8.1 Hz, 1H), 7.54 (dd, J ) 8.1 Hz, 1.5 Hz, 1H), 7.67 (brs, 1H), 7.76
(brs, 2H); 13C NMR (CDCl3) δ 25.2, 94.7, 107.1, 108.6, 108.8, 117.0,
117.6, 120.1, 125.6, 126.8, 128.3, 130.2, 134.4, 136.6, 138.8; FAB-
MS obsd 348.0144, calcd 348.0124 (C15H13N2I).
Zn(II) 5,10,15-Trimesityl-2-(3-iodophenyl)porphyrin (ZnI â-m).
To a solution of 5-mesityldipyrromethane31 (0.16 g, 0.60 mmol), 4 (0.21
g, 0.60 mmol), and mesitaldehyde (0.18 g, 1.2 mmol) in 100 mL of
CHCl3 under argon was added BF3-etherate (132 µL of a 2.5 M stock
solution in CHCl3, 3.3 mM). The reaction mixture was stirred at room
temperature for 1 h. DDQ (0.41 g, 1.8 mmol, 3 equiv) was added and
stirring was continued for an additional 2 h. The solvent was evaporated
under reduced pressure, and the residue was filtered through a silica
gel column (hexanes/CH2Cl2, 1:1) to remove quinone species and dark
pigments. The porphyrin band that eluted first from the column was
collected, concentrated to dryness (0.15-0.20 g), dissolved in 50 mL
of CH2Cl2, and treated with methanolic Zn(OAc)2‚2H2O (0.12 g in 2
mL of methanol). The mixture was heated at reflux for 1 h, then diluted
with CH2Cl2 (100 mL) and washed successively with saturated aqueous
NaHCO3 (2 × 60 mL), H2O (2 × 60 mL), and brine (60 mL), and
then dried (Na2SO4), filtered, and concentrated. Chromatography of
the crude porphyrin mixture (alumina, hexanes/ethyl acetate, 2:1)
afforded the desired porphyrin as the second band (55 mg, 12%): 1H
NMR (CDCl3) δ 1.84 (m, 18H), 2.64 (s, 9H), 7.30 (m, 6H), 7.52 (t, J
) 8.1 Hz, 1H), 7.97 (d, J ) 7.2 Hz, 1H), 8.25 (d, J ) 7.5 Hz, 1H),
8.60 (t, J ) 2.1 Hz, 1H), 8.73 (s, 4H), 8.87 (m, 2H), 9.30 (m, 1H),
10.15 (s, 1H); LD-MS obsd 931.1; FAB-MS obsd 928.2001, calcd
1
colorless solid: mp 38-39 °C; H NMR (CDCl3) δ 1.33 (t, J ) 7.2
Hz, 3H), 4.26 (q, J ) 7.2 Hz, 2H), 6.40 (d, J ) 16.2 Hz, 1H), 7.09 (t,
J ) 7.2 Hz, 1H), 7.45 (d, J ) 7.2 Hz, 1H), 7.48 (d, J ) 16.2 Hz, 1H)
7.67 (d, J ) 7.2 Hz, 1H), 7.84 (s, 1H); 13C NMR (CDCl3) δ 14.2,
60.5, 94.6, 119.5, 127.0, 130.3, 136.4, 136.6, 138.7, 142.6, 166.3; FAB-
MS obsd 302.9885, calcd 302.9882 (C11H11O2I). Anal. Calcd: C, 43.71;
H, 3.67, Found: C, 43.92; H, 3.67.
3-Ethoxycarbonyl-4-(3-iodophenyl)pyrrole (2). To a suspension
of NaH (0.60 g, 60% dispersion in mineral oil, 1.2 equiv) in 25 mL of
dry ether under argon was added dropwise over 30 min a solution of
1 (3.8 g, 12.5 mmol) and tosylmethylisocyanide (TosMIC) (2.5 g, 12.8
mmol) in 60 mL of dry ether/DMSO (2:1). Care must be exercised
during addition as an exothermic reaction ensues. The reaction mixture
was stirred for an additional 30 min, carefully quenched with ice-cold
water (60 mL), and extracted with ether (3 × 60 mL). The ethereal
portion was dried (Na2SO4), filtered, and evaporated under vacuum to
give a brownish solid. The crude product was purified by flash
chromatography (silica, hexanes:ethyl acetate, 3:1) to give 3.0 g (70%)
of a light brownish solid: mp 152-153 °C; 1H NMR (CDCl3) δ 1.26
(t, J ) 7.2 Hz, 3H), 4.22 (q, J ) 7.2 Hz, 2H), 6.76 (t, J ) 2.7 Hz, 1H),
7.08 (t, J ) 7.2 Hz, 1H), 7.47 (d, J ) 7.2 Hz, 1H), 7.48 (t, J ) 2.7 Hz,
1H) 7.60 (d, J ) 8.1 Hz, 1H), 7.83 (s, 1H), 8.65 (brs, 1H); 13C NMR
(CDCl3) δ 14.3, 59.7, 93.5, 113.8, 118.5, 124.9, 125.5, 128.7, 129.3,
135.3, 137.0, 138.0, 164.7; FAB-MS obsd 340.9934, calcd 340.9913
(C13H12NO2I). Anal. Calcd: C, 45.75; H, 3.55; N, 4.11. Found: C,
46.30; H, 3.81; N, 3.95.
2-Ethoxycarbonyl-3-(3-iodophenyl)-11-N-(tert-butoxycarbonyl)-
dipyrromethane (3b). To a solution of 2-hydroxymethyl-N-(tert-
butoxycarbonyl)pyrrole30 (0.90 g, 4.57 mmol) and 2 (1.50 g, 4.40 mmol)
in 45 mL of 1,4-dioxane was added aqueous HCl (10%, 9 mL). The
reaction mixture was stirred at room temperature for 6 h. Another
portion of 2-hydroxymethyl-N-(tert-butoxycarbonyl)pyrrole (0.30 g,
1.52 mmol) was added, and the reaction mixture was stirred for an
additional 12 h. Upon completion of the reaction as judged by TLC
(hexanes/ethyl acetate, 4:1), saturated aqueous NaHCO3 (25 mL) and
H2O (50 mL) were carefully added to quench the reaction. The aqueous
928.1980 (C53H45N4ZnI); λabs (toluene) 422, 548, 582 nm; λem (λexc
550 nm, toluene) 587, 642 nm.
)
5,10,15-Trimesityl-2-(4-iodophenyl)porphyrin (FbI â-m). A sample
of ZnI â-m (50 mg, 54 µmol) was dissolved in CH2Cl2 (15 mL) and
(30) Tietze, L. F.; Kettschau, G.; Heitman, K. Synthesis 1996, 851-
857.
(31) Littler, B. J.; Miller, M. A.; Hung, C.-H.; Wagner, R. W.; O’Shea,
D. F.; Boyle, P. D.; Lindsey, J. S. J. Org. Chem. 1999, 64, 1391-1396.

7590 J. Am. Chem. Soc., Vol. 122, No. 31, 2000
Lammi et al.
treated with TFA (30 µL, >5 equiv). The reaction was found to be
complete after 1 h as judged by silica TLC, absorption spectroscopy,
and fluorescence excitation spectroscopy. Triethylamine (100 µL) was
added, and the reaction mixture was stirred for another 10 min. It was
then diluted with CH2Cl2 (50 mL), washed with 10% aqueous NaHCO3
(2 × 50 mL), H2O (50 mL), and brine (50 mL), dried over Na2SO4,
and filtered, and the solvent was removed under vacuum to give a purple
solid (35 mg, 78%): 1H NMR (CDCl3) δ -2.72 (brs, 2H), 1.86 (m,
18H), 2.63 (m, 9H), 7.30 (m, 6H), 7.51 (t, J ) 7.8 Hz, 1H), 7.98 (d,
J ) 8.1 Hz, 1H), 8.22 (d, J ) 8.1 Hz, 1H), 8.58 (d, J ) 1.5 Hz, 1H),
8.64 (m, 4H), 8.77 (s, 1H), 8.81 (d, J ) 4.2 Hz, 1H), 9.26 (d, J ) 5.1
Hz 1H), 10.10 (s, 1H); LD-MS obsd 868.9; FAB-MS obsd 866.2834,
calcd 866.2845 (C53H47N4I); λabs (toluene) 419, 512, 544, 589, 644 nm;
λem (λexc ) 550 nm, toluene) 646, 715 nm.
Mg(II) 5,10,15-Trimesityl-2-(3-iodophenyl)porphyrin (MgI â-m).
A solution of FbI â-m (25 mg, 29 µmol) in 5 mL of CH2Cl2 was treated
with N,N-diisopropylethylamine (100 µL, 58 µmol) and MgI2 (80 mg)
with stirring at room temperature. After 30 min, the reaction mixture
was diluted with CH2Cl2 (30 mL), and then washed with 10% aqueous
NaHCO3 (2 × 25 mL), dried (Na2SO4), filtered, and concentrated under
vacuum to 5 mL. Chromatography (alumina grade V, CHCl3) afforded
22 mg (85%): 1H NMR (CDCl3) δ 1.85 (m, 18H), 2.63 (m, 9H), 7.29
(m, 6H) 7.50 (t, J ) 8.1 Hz, 1H), 7.95 (d, J ) 8.1 Hz, 1H), 8.27 (d,
J ) 8.1 Hz, 1H), 8.64 (m, 5H), 8.79 (m, 2H), 9.22 (d, J ) 4.2 Hz,
1H), 10.06 (s, 1H); LD-MS obsd 890.8; FAB-MS obsd 888.2556, calcd
material as the first band, which was collected and concentrated to
dryness. The Pd species remained bound on the top of the column.
The porphyrin mixture was dissolved in a minimum amount of toluene,
loaded on a preparative SEC column (5 × 65 cm), and eluted by gravity
flow with toluene. The desired dimer was obtained as the second band.
The dimer solution was concentrated to dryness, dissolved in a
minimum of toluene, and chromatographed (alumina grade V, toluene,
then toluene/CHCl3, 1:1), affording 20 mg (57%): 1H NMR (CDCl3)
δ 1.86 (m, 36H), 2.64 (m, 18H), 7.27 (m, 12H), 8.05 (m, 4H), 8.29 (d,
J ) 8.1 Hz, 2H), 8.40 (d, J ) 8.1 Hz, 2H), 8.65-8.94 (m, 14H), 9.26
(d, J ) 4.5 Hz, 1H), 10.20 (s, 1H); LD-MS obsd 1592.79; FAB-MS
obsd 1586.61, calcd 1586.64 (C108H90MgN8Zn); λabs (toluene) 425, 554,
596 nm; λem (λex 550 nm, toluene) 604, 661 nm.
MgZnU â-m/p-meso. Following the same approach as that used
for the preparation and purification of MgZnU â-p/p-meso, the reaction
of MgI â-m (20 mg, 23 µmol), ZnU′ (19 mg, 23 µmol), Pd2(dba)3 (3.2
mg, 3.5 µmol), and P(o-tol)3 (8.0 mg, 28 µmol) afforded 20 mg
(57%): 1H NMR (CDCl3) δ 1.84 (m, 36H), 2.62 (m, 18H), 7.27 (m,
12H), 7.84 (t, J ) 7.8 Hz, 1H), 7.92 (d, J ) 7.2 Hz, 1H), 8.00 (d, J )
8.1 Hz, 2H), 8.26 (d, J ) 7.8 Hz, 2H), 8.37 (d, J ) 7.5 Hz, 1H), 8.60
(s, 1H), 8.65-8.90 (m, 14H), 9.28 (d, J ) 3.9 Hz, 1H), 10.21 (s, 1H);
LD-MS obsd 1592.38; FAB-MS obsd 1586.55, calcd 1586.64 (C108H90-
MgN8Zn); λabs (toluene) 425, 553, 592 nm; λem (λex 550 nm, toluene)
602, 659 nm.
MgZnU meso-p/p-meso. Prepared as described in the literature.13
MgZnFbU meso-p/p-meso. Prepared as described in the literature.20
MgZnFbU meso-m/p-meso. MgI meso-m (10.0 mg, 11.3 µmol),
mono-ethynyl ZnFb dimer20 (17.5 mg, 11.3 µmol), Pd2(dba)3 (1.6 mg,
1.7 µmol), and P(o-tol)3 (4.7 mg, 14 µmol) were added to a 25-mL
Schlenk flask. The flask was evacuated and purged with argon three
times. After that, 4.5 mL of deaerated toluene/triethylamine (5:1) was
added by syringe. The flask was immersed in an oil bath at 37 °C, and
the reaction was followed by analytical SEC and found to be complete
at 24 h. The solvent was removed, and the crude reaction mixture was
dissolved in toluene and loaded on an alumina column (grade V).
Elution with toluene/CHCl3 (1:1) afforded the monomeric porphyrin,
the dimer, and the desired trimer along with higher molecular weight
material as the first band, which was collected and concentrated to
dryness. The Pd species remained bound on the top of the column.
The porphyrin mixture was dissolved in a minimum amount of THF,
loaded on a preparative SEC column (5 × 65 cm), and eluted by gravity
flow with THF. The desired trimer was obtained as the second band.
The trimer solution was concentrated to dryness, dissolved in a
minimum of toluene, and chromatographed (alumina grade V, toluene,
then toluene/CHCl3, 1:1), affording 12.0 mg (46%): 1H NMR (CDCl3)
δ -2.53 (brs, 2H), 1.85 (m, 48H), 2.64 (s, 24H), 7.29 (s, 16H), 7.79
(t, J ) 7.5 Hz, 1H), 7.90 (d, J ) 7.2 Hz, 1H), 8.05 (m, 6H), 8.26 (m,
7H), 8.57 (s, 1H), 8.63-8.97 (m, 24H); LD-MS obsd 2306.6; FAB-
MS obsd 2306.90, calcd 2306.97 (C160H130MgN12Zn); λabs (toluene) 427,
516, 553, 595, 650 nm; λem (λex 550 nm, toluene) 607, 651, 721 nm.
MgZnFbU â-p/p-meso. Following the same approach as that used
for the preparation and purification of MgZnFbU meso-m/p-meso, the
reaction of MgI â-p (10.0 mg, 11.3 µmol), mono-ethynyl ZnFb dimer20
(17.5 mg, 11.3 µmol), Pd2(dba)3 (1.6 mg, 1.7 µmol), and P(o-tol)3 (4.7
mg, 14 µmol) afforded 12 mg (46%): 1H NMR (CDCl3) δ -2.53 (brs,
2H), 1.89 (m, 48H), 2.65 (m, 24H), 7.31 (m, 16H), 8.07 (m, 8H), 8.27-
8.42 (m, 8H), 8.65-8.99 (m, 22H), 9.27 (m, 1H), 10.21 (m, 1H); LD-
888.2539 (C53H45N4MgI); λabs (toluene) 427, 563, 598 nm; λem (λexc
550 nm, toluene) 605, 660 nm.
)
Mg(II) 5,10,15-Trimesityl-2-(4-iodophenyl)porphyrin (MgI â-p).
Under conditions similar to those described above, a solution of FbI
â-p21 (25 mg, 29 µmol) in 5 mL of CH2Cl2 was treated with N,N-
diisopropylethylamine (100 µL, 58 µmol) and MgI2 (80 mg) to afford
20 mg (78%): 1H NMR (CDCl3) δ 1.83 (m, 18H), 2.62 (m, 9H), 7.29
(m, 6H), 8.09 (AB quartet, J ) 6.6 Hz, 4H), 8.62 (s, 4H), 8.75 (d, J )
4.2 Hz, 1H), 8.79 (s, 1H), 9.22 (d, J ) 4.2 Hz, 1H), 10.09 (s, 1H);
LD-MS obsd 890.0; FAB-MS obsd 888.2557, calcd 888.2539 (C53H45N4-
MgI); λabs (toluene) 428, 563, 598 nm; λem (λexc ) 550 nm, toluene)
604, 659 nm.
Mg(II) 5,10,15-Trimesityl-20-(3-iodophenyl)porphyrin (MgI meso-
m). Under conditions similar to those described above, a solution of
5,10,15-trimesityl-20-(3-iodophenyl)porphyrin (FbI meso-m)3 (31 mg,
36 µmol) in 8 mL of CH2Cl2 was treated with N,N-diisopropylethyl-
amine (120 µL, 0.70 mmol) and MgI2 (100 mg) to give 25 mg (78%):
1H NMR (CDCl3) δ 1.81, 1.86 (s, 18H), 2.62 (m, 9H), 7.27 (s, 6H),
7.46 (t, J ) 7.5 Hz, 1H), 8.07 (d, J ) 8.1 Hz, 1H), 8.18 (d, J ) 7.5
Hz, 1H), 8.60 (s, 5H), 8.70 (m, 4H); LD-MS obsd 887.9; FAB-MS
obsd 888.25, calcd 888.25 (C53H45IN4Mg); λabs (toluene) 428, 525, 565,
605 nm; λem (λexc ) 550 nm, toluene) 607, 663 nm.
MgZnU meso-m/p-meso. A solution of FbZnU meso-m/p-meso3
(12.0 mg, 7.6 µmol) in 5 mL of CH2Cl2 was treated with N,N-
diisopropylethylamine (20.0 mg, 0.15 mmol) and MgI2 (21 mg, 77
µmol) to give 10.0 mg (83%): 1H NMR (CDCl3) δ 1.82 (m, 36H),
2.62 (m, 18H), 7.28 (m, 12H), 7.78 (t, J ) 8.1 Hz, 1H), 7.93 (d, J )
8.1 Hz, 2H), 8.06 (d, J ) 8.1 Hz, 1H), 8.20 (d, J ) 8.1 Hz, 2H), 8.27
(d, J ) 7.5 Hz, 1H), 8.57 (s, 1H), 8.63-8.87 (m, 16H); LD-MS obsd
1591.54; FAB-MS obsd 1586.61, calcd 1586.64 (C108H90MgN8Zn); λabs
(toluene) 430, 555, 563, 604 nm; λem (λexc ) 550 nm, toluene) 608,
664 nm.
MgZnU â-p/p-meso. MgI â-p (20 mg, 23 µmol), Zn(II) 5,10,15-
trimesityl-20-(4-ethynylphenyl)porphyrin (ZnU′)32 (19 mg, 23 µmol),
Pd2(dba)3 (3.2 mg, 3.5 µmol), and P(o-tol)3 (8.0 mg, 28 µmol) were
added to a 25-mL Schlenk flask. The flask was evacuated and purged
with argon three times. Then 9 mL of deaerated toluene/triethylamine
(5:1) was added by syringe. The flask was immersed in an oil bath at
37 °C, and the reaction was followed by analytical SEC and found to
be complete at 15 h. The solvent was removed, and the crude reaction
mixture was dissolved in toluene and loaded on an alumina column
(grade V). Elution with toluene/CHCl3 (1:1) afforded the monomeric
porphyrin and the desired dimer along with higher molecular weight
MS obsd 2308.6; FAB-MS obsd 2307.05, calcd 2306.97; (C160H130
-
MgN12Zn); λabs (toluene) 427, 515, 553, 595, 649 nm; λem (λex 550
nm, toluene) 604, 651, 720 nm.
MgZnFbU â-m/p-meso. Following the same approach as that used
for the preparation and purification of MgZnFbU meso-m/p-meso, the
reaction of MgI â-m (10.0 mg, 11.3 µmol), mono-ethynyl ZnFb dimer20
(17.5 mg, 11.3 µmol), Pd2(dba)3 (1.6 mg, 1.7 µmol), and P(o-tol)3 (4.7
mg, 14 µmol) afforded 13.0 mg (50%): 1H NMR (CDCl3) δ -2.53
(brs, 2H), 1.87 (m, 48H), 2.65 (m, 24H), 7.30 (m, 16H), 7.85 (t, J )
8.1 Hz, 1H), 7.94 (m, 1H), 8.05 (m, 6H), 8.31 (m, 7H), 8.61 (s, 1H),
8.65-8.98 (m, 22H), 9.30 (d, J ) 4.2 Hz, 1H), 10.22 (s, 1H); LD-MS
obsd 2310.7; FAB-MS obsd 2306.89, calcd 2306.97 (C160H130MgN12-
Zn); λabs (toluene) 426, 516, 552, 594, 650 nm; λem (λex 550 nm, toluene)
603, 651, 720 nm.
(32) Lindsey, J. S.; Prathapan, S.; Johnson, T. E.; Wagner, R. W.
Tetrahedron 1994, 50, 8941-8968.

Photoinduced Energy Transfer Multiporphyrin Arrays
J. Am. Chem. Soc., Vol. 122, No. 31, 2000 7591
Physical Methods. (a) General. Static and time-resolved absorption
and fluorescence studies were performed on samples prepared in toluene
at room temperature. Samples for time-resolved fluorescence measure-
ments were degassed by several freeze-pump-thaw cycles on a high-
vacuum line; samples for static fluorescence and static and time-resolved
absorption measurements were not degassed.
were generated by averaging the ∆A values in specific wavelength
ranges for the entire set of pump-probe delay times and plotting these
values as a function of time. A nonlinear least-squares algorithm was
then used to fit the kinetic traces to functions consisting of either a
single or a double exponential plus a constant. An Oxford Instruments
liquid-nitrogen cryostat was used for low-temperature measurements.
(b) Static Absorption Spectroscopy. Static absorption (Cary 100)
measurements were performed as described previously.8-10,15,28
(c) Static and Time-Resolved Fluorescence Spectroscopy. Static
and time-resolved fluorescence (Spex Fluoromax or Fluorolog II)
measurements were performed as described previously.8-10,15,28
(d) Time-Resolved Absorption Spectroscopy. Transient absorption
data were acquired as discussed elsewhere.8,9,33 Samples (∼0.1-0.2
mM in toluene or 2-methyltetrahydrofuran) were placed in 2- or 5-mm
path length cuvettes at room temperature. They were excited at 10 Hz
with 130 fs, 4-6 µJ pump pulses and probed with white light pulses
of comparable duration. The kinetic data shown in Figures 2 and 4
Acknowledgment. This research was supported by grants
from the NSF (CHE-9707995 and CHE-9988142). Mass spectra
were obtained at the Mass Spectrometry Laboratory for Bio-
technology at North Carolina State University. Partial funding
for the Facility was obtained from the North Carolina Biotech-
nology Center and the NSF.
Supporting Information Available: 1H NMR spectra for
all compounds, and LD-MS spectra for all porphyrins and
porphyrin arrays (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
(33) (a) Kirmaier, C.; Holten, D. Biochemistry 1991, 30, 609-613. (b)
Drain, C. M.; Kirmaier, C.; Medforth, C. J.; Nurco, D. J.; Smith, K. M.;
Holten, D. J. Phys. Chem. 1996, 100, 11984-11993.
JA001031X