10002 J. Am. Chem. Soc., Vol. 120, No. 39, 1998
Li et al.
and purification. Issues relating to the overall architecture
include chromophore substituents, linker types, pigment con-
nection sites,2 and interpigment distances and orientations.
Collectively, these factors dictate the spectral coverage and light-
absorption efficiency; the rates, efficiencies, and mechanisms
of energy transfer (through-bond and/or through-space);2,3,5-8
the direction of energy flow;9 and the solubility (for chemical
processing10). The synthetic challenge is to realize the design
objective. Synthesis issues include preparing the individual
pigments, joining the pigments using selective coupling methods,
and purifying the resulting molecular photonic device.
Synthetic porphyrins have been frequently employed as light-
absorbing and energy-transfer components of molecular devices.
Porphyrins absorb strongly in the blue and weakly in the green
region, mimicking the absorption properties of (bacterio)-
chlorophyll pigments, though the former differ from the latter
in absorbing only weakly in the red. However, porphyrins are
more accessible synthetically than are (bacterio)chlorins. Nu-
merous compounds have been prepared to investigate energy
transfer between covalently linked porphyrins,11 from one
accessory pigment (anthracenyl-polyenes,12 anthracenyl-poly-
ynes,13 boron-dipyrrin dyes,14 carbocyanines,15 carotenoids,16
and polyynes17) to a porphyrin, or from four ruthenium
coordination complexes to a porphyrin.18
A desirable architecture for efficient light-harvesting employs
large numbers of accessory pigments that absorb light and
transfer the energy directly to the acceptor.9 We have initiated
studies of boron-dipyrrin dyes as accessory pigments in por-
phyrin-containing arrays because these chromophores have a
number of favorable properties. These characteristics include
sharp (fwhm ∼ 25 nm) and moderately strong (ꢀ ) 40 000-
100 000 M-1cm-1) absorption near 500 nm, long excited-state
lifetimes (∼5 ns for boron-dipyrrin dyes studied previously19),
good solubility in organic solvents, and amenability toward
chromatography on silica and alumina. The blue-green absorp-
Chart 1. Boron-Dipyrrin Dyes
tion of the boron-dipyrrin pigments (1) enables the pigment to
enhance the absorption properties of associated arrays for light-
harvesting applications and (2) facilitates relatively selective
excitation of the pigment in the presence of free base or
metalloporphyrins for applications in molecular photonic de-
vices. Boron-dipyrrin dyes bearing pyrrole substituents, but
lacking substituents at the methine (5-) position, were first
prepared by Treibs and Kreuzer in 1968,20 and have been widely
used as fluorescent probes (Chart 1).21 We recently developed
a facile route to boron-dipyrrin dyes bearing substituents at the
5-position22 that facilitates incorporation of these accessory
pigments into porphyrin-containing molecular photonic wires14a
and optoelectronic gates.14b
In this paper, we describe the synthesis and detailed photo-
physical characterization of arrays comprised of a porphyrin
and one, two, or eight boron-dipyrrin pigments. The synthetic
approach capitalizes on a building block strategy that has
afforded light-harvesting arrays and molecular photonic devices
comprised of metalloporphyrins and free base (Fb) porphyrins
joined via p,p′-diarylethyne linkers.11,23 In these latter multi-
porphyrin arrays, energy transfer was found to occur predomi-
nantly via a linker-mediated through-bond (TB) process rather
than a through-space (TS) mechanism.2,3,5-8 The photophysical
studies of the boron-dipyrrin-porphyrin arrays studied herein
elucidate a variety of fundamental properties of these light-
harvesting architectures, including (1) the excited-state properties
of boron-dipyrrin pigments, (2) the rate, efficiency, and mech-
anism of energy transfer from such an accessory pigment to a
porphyrin, (3) the differences in the rates and efficiencies of
energy transfer for p,p′-substituted versus p,m′-substituted diaryl-
ethyne linkers, and (4) the changes in the photophysical proper-
ties of the arrays as the number of pigments appended to one
porphyrin is increased from one to eight. The synthetic and
photophysical studies are augmented by theoretical calculations
concerning the excited-state properties of the boron-dipyrrin
pigments.
(4) Yang, S. I.; Seth, J.; Strachan, J. P.; Gentemann, S.; Kim, D.; Holten,
D.; Lindsey, J. S.; Bocian, D. F. J. Porphyrins Phthalocyanines, in press.
(5) Seth, J.; Palaniappan, V.; Johnson, T. E.; Prathapan, S.; Lindsey, J.
S.; Bocian, D. F. J. Am. Chem. Soc. 1994, 116, 10578-10592.
(6) Hsiao, J.-S.; Krueger, B. P.; Wagner, R. W.; 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.
(7) Seth, J.; Palaniappan, V.; Wagner, R. W.; Johnson, T. E.; Lindsey,
J. S.; Bocian, D. F. J. Am. Chem. Soc. 1996, 118, 11194-11207.
(8) Strachan, J. P.; Gentemann, S.; Seth, J.; Kalsbeck, W. A.; Lindsey,
J. S.; Holten, D.; Bocian, D. F. Inorg. Chem. 1998, 37, 1191-1201.
(9) Van Patten, P. G.; Shreve, A. P.; Lindsey, J. S.; Donohoe, R. J. J.
Phys. Chem. B 1998, 102, 4209-4216.
Results
Synthesis of Light-Harvesting Arrays. (1) Synthetic
Strategy. In our modular building block approach for synthe-
sizing multiporphyrin light-harvesting arrays and related mo-
lecular devices,3,8,10,11,14,23 an aryl aldehyde bearing ethyne or
iodo groups is reacted with pyrrole or a dipyrromethane for
conversion to the porphyrin building block. Arrays are then
formed by joining the porphyrin building blocks (each in a
defined metalation state) via a Pd-mediated coupling method.24
To cluster up to eight boron-dipyrrin dyes around the porphyrin,
(10) Lindsey, J. S.; Prathapan, S.; Johnson, T. E.; Wagner, R. W.
Tetrahedron 1994, 50, 8941-8968.
(11) Wagner, R. W.; Johnson, T. E.; Lindsey, J. S. J. Am. Chem. Soc.
1996, 118, 11166-11180.
(12) (a) Effenberger, F.; Schlosser, H.; Bauerle, P.; Maier, S.; Port, H.;
Wolf, H. C. Angew. Chem., Int. Ed. Engl. 1988, 27, 281-284. (b) Bon-
fantini, E. E.; Officer, D. L. J. Chem. Soc., Chem. Commun. 1994, 1445-
1446. (c) Wurthner, F.; Vollmer, M. S.; Effenberger, F.; Emele, P.; Meyer,
D. U.; Port, H.; Wolf, H. C. J. Am. Chem. Soc. 1995, 117, 8090-8099.
(13) Kawabata, S.; Yamazaki, I.; Nishimura, Y. Bull. Chem. Soc. Jpn.
1997, 70, 1125-1133.
(14) (a) Wagner, R. W.; Lindsey, J. S. J. Am. Chem. Soc. 1994, 116,
9759-9760. (b) Wagner, R. W.; Lindsey, J. S.; Seth, J.; Palaniappan, V.;
Bocian, D. F. J. Am. Chem. Soc. 1996, 118, 3996-3997.
(15) Lindsey, J. S.; Brown, P. A.; Siesel, D. A. Tetrahedron 1989, 45,
4845-4866.
(16) (a) Gust, D.; Moore, T. A.; Moore, A. L.; Devadoss, C.; Liddell, P.
A.; Hermant, R.; Nieman, R. A.; Demanche, L. J.; DeGraziano, J. M.; Gouni,
I. J. Am. Chem. Soc. 1992, 114, 3590-3603. (b) Moore, T. A.; Gust, D.;
Moore, A. L. Pure Appl. Chem. 1994, 66, 1033-1040.
(17) Maruyama, K.; Kawabata, S. Bull. Chem. Soc. Jpn. 1989, 62, 3498-
3507.
(19) Karolin, J.; Johansson, L. B.-A.; Strandberg, L.; Ny, T. J. Am. Chem.
Soc. 1994, 116, 7801-7806.
(20) Treibs, A.; Kreuzer, F. H. Liebigs Ann. Chem. 1968, 718, 208-
223.
(21) (a) Wories, H. J.; Koek, J. H.; Lodder, G.; Lugtenburg, J.; Fokkens,
R.; Driessen, O.; Mohn, G. R. Recl. TraV. Chim. Pays-Bas 1985, 104, 288-
291. (b) Haugland, R. P.; Kang, H. C. U.S. Patent 4, 774, 339, 1988. (c)
Molecular Probes, Inc., Eugene, OR.
(22) Wagner, R. W.; Lindsey, J. S. Pure Appl. Chem. 1996, 68, 1373-
1380.
(23) Lindsey, J. S. In Modular Chemistry; Michl, J., Ed.; NATO ASI
Series C: Mathematical and Physical Sciences 499; Kluwer Academic
Publishers: Dordrecht, 1997; pp 517-528.
(18) Collin, J.-P.; Harriman, A.; Heitz, V.; Odobel, F.; Sauvage, J.-P. J.
Am. Chem. Soc. 1994, 116, 5679-5690.