Excited-State Energy-Transfer Dynamics of Self-Assembled Imine-Linked Porphyrin Dyads

Article abstract of DOI:10.1021/ic034558u

Toward the development of new strategies for the synthesis of multiporphyrin arrays, we have prepared and characterized (electrochemistry and static/time-resolved optical spectroscopy) a series of dyads composed of a zinc porphyrin and a free base porphyrin joined via imine-based linkers. One dyad contains two zinc porphyrins. Imine formation occurs under gentle conditions without alteration of the porphyrin metalation state. Five imine linkers were investigated by combination of formyl, benzaldehyde, and salicylaldehyde groups with aniline and benzoic hydrazide groups. The imine-linked dyads are quite stable to routine handling. The excited-state energy-transfer rate from zinc to free base porphyrin ranges from (70 ps)^-1 to (13 ps)^-1 in toluene at room temperature depending on the linker employed. The energy-transfer yield is generally very high (>97%), with low yields of deleterious hole/electron transfer. Collectively, this work provides the foundation for the design of multiporphyrin arrays that self-assemble via stable imine linkages, have predictable electronic properties, and have comparable or even enhanced energy-transfer characteristics relative to those of other types of covalently linked systems.

Full text of DOI:10.1021/ic034558u

Inorg. Chem. 2003, 42, 6616−6628
Excited-State Energy-Transfer Dynamics of Self-Assembled Imine-Linked
Porphyrin Dyads
Igor V. Sazanovich,† Arumugham Balakumar,‡ Kannan Muthukumaran,‡ Eve Hindin,†
,‡
,§
,†
Christine Kirmaier,† James R. Diers,§ Jonathan S. Lindsey,* David F. Bocian,* and Dewey Holten*
Departments of Chemistry, Washington UniVersity, St. Louis, Missouri 63130-4899,
North Carolina State UniVersity, Raleigh, North Carolina 27695-8204, and
UniVersity of California, RiVerside, California 92521-0403
Received May 23, 2003
Toward the development of new strategies for the synthesis of multiporphyrin arrays, we have prepared and
characterized (electrochemistry and static/time-resolved optical spectroscopy) a series of dyads composed of a
zinc porphyrin and a free base porphyrin joined via imine-based linkers. One dyad contains two zinc porphyrins.
Imine formation occurs under gentle conditions without alteration of the porphyrin metalation state. Five imine
linkers were investigated by combination of formyl, benzaldehyde, and salicylaldehyde groups with aniline and
benzoic hydrazide groups. The imine-linked dyads are quite stable to routine handling. The excited-state energy-
transfer rate from zinc to free base porphyrin ranges from (70 ps)-1 to (13 ps)-1 in toluene at room temperature
depending on the linker employed. The energy-transfer yield is generally very high (>97%), with low yields of
deleterious hole/electron transfer. Collectively, this work provides the foundation for the design of multiporphyrin
arrays that self-assemble via stable imine linkages, have predictable electronic properties, and have comparable
or even enhanced energy-transfer characteristics relative to those of other types of covalently linked systems.
Introduction
arrays: (1) stepwise syntheses of covalently linked systems
and (2) self-assembly of noncovalently linked systems in
which the components are held together via weak interactions
such as hydrogen-bonding, electrostatic interactions, hydro-
phobic forces, and/or apical metal coordination. Each ap-
proach has strengths and weaknesses. The stepwise synthesis
The organization of multiple porphyrins into arrays
provides the basis for both natural and synthetic materials
with diverse functions.1-18 Two distinct methods generally
have been employed for creating synthetic multiporphyrin
* To whom correspondence should be addressed. E-mail: holten@
wuchem.wustl.edu (D.H.); jlindsey@ncsu.edu (J.S.L.); David.Bocian@ucr.edu
(D.F.B.).
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† Washington University.
‡ North Carolina State University.
§ University of California.
(1) Harvey, P. D. In The Porphyrin Handbook; Kadish, K. M., Smith, K.
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6616 Inorganic Chemistry, Vol. 42, No. 21, 2003
10.1021/ic034558u CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/17/2003

Self-Assembled Porphyrin Dyads
Chart 1
of covalently linked arrays enables each component to be
anchored irreversibly in a specific location in a 3-dimensional
architecture. However, the construction of such arrays is often
laborious. On the other hand, the self-assembly approach only
requires synthesis of component parts and relies on the
specificity of noncovalent interactions to produce the desired
array. Disadvantages of the self-assembly approach are that
(1) the strength of the interactions between components is
often insufficient to give a high yield of the target array at
equilibrium and (2) multiple products of similar composition
may be formed. For photochemical studies, it is essential to
prepare a distinct target of known composition and archi-
tecture. The requirement of distinct molecular integrity can
limit the application of self-assembled systems because the
typical use of dilute solutions (∼µM) can result in disas-
sembly of the self-assembled array.
The inspiration for reliance on self-assembly in chemical
synthesis has often emanated from biology where self-
assembly affords elaborate molecular assemblies.19,20 The
development of chemical self-assembly processes that are
comparable to those in biological systems is a great chal-
lenge. The molecular interactions in biological self-assembly
processes typically encompass noncovalent interactions as
well as covalent bond-forming reactions (giving disulfides,
imines, etc.). Self-assembly processes built around reversible
covalent bond formation facilitate construction of elaborate
architectures while also affording more stable assemblies than
those typically obtained via noncovalent interactions. Co-
valent self-assembly19 also forms the basis for studies in
dynamic combinatorial chemistry.21 Numerous multiporphy-
rin arrays have been prepared by metal-coordination-medi-
ated self-assembly4,8-13 whereas only a few have been
prepared containing imine linkages.22 The metal-coordination
assembly processes encompass interactions that range from
relatively weak (e.g., coordination of nitrogenous bases at
the porphyrin apical metal site) to quite strong (e.g., chelation
of multivalent metals by multidentate ligands attached to the
perimeter of the porphyrin) and thus can exhibit behavior
spanning that typical of noncovalent interactions and covalent
bonds.
In covalently linked multiporphyrin arrays that exhibit
light-harvesting properties, the linker serves both a mechan-
ical function and an electronic function. The mechanical
function is to hold the porphyrins in a fixed architecture.
The electronic function is to provide a conduit for com-
munication among the porphyrins, thereby enhancing the rate
and yield of excited-state energy transfer. The rate of such
through-bond (TB) energy transfer can be much faster than
through-space (TS) energy transfer. Indeed, in dyads com-
posed of a zinc porphyrin and a free base porphyrin joined
by a 4,4′-diphenylethyne linker, the observed (24 ps)-1 rate
of transfer originates from a TB energy-transfer rate of (25
ps)-1 and a TS energy-transfer rate of (720 ps)-1.5 Thus, the
electronic communication provided by the linker is the
dominant contributor to the observed energy-transfer process.
For the covalent self-assembly of multiporphyrin light-
harvesting arrays, the ideal linker should have the following
properties: (1) afford the self-assembled product in high
yield; (2) form under neutral conditions without demetalation
of the porphyrins; (3) provide directional assembly enabling
rational formation of dyads composed of metalloporphyrins
in distinct metalation states; (4) provide efficient TB
electronic communication between the porphyrins; (5) not
participate in any deleterious excited-state quenching pro-
cesses; and (6) be stable toward oxidation and reduction. A
number of candidates for such linkers are shown in Chart 1.
Five linkers are built around imine motifs; one is a metal-
acac complex, and one is a bis(dipyrrinato)metal(II) complex.
(19) Lindsey, J. S. New J. Chem. 1991, 15, 153-180.
(20) Lawrence, D. S.; Jiang, T.; Levett, M. Chem. ReV. 1995, 95, 2229-
2260.
(21) Rowan, S. J.; Cantrill, S. J.; Cousins, G. R. L.; Sanders, J. K. M.;
Stoddart, J. F. Angew. Chem., Int. Ed. 2002, 41, 898-952.
(22) Screen, T. E. O.; Blake, I. M.; Rees, L. H.; Clegg, W.; Borwick, S.
J.; Anderson, H. L. J. Chem. Soc., Perkin Trans. 1 2002, 320-329.
In this paper, we describe the synthesis of porphyrin
building blocks bearing functional groups for formation of
Inorganic Chemistry, Vol. 42, No. 21, 2003 6617

Sazanovich et al.
Chart 2
acac- or imine-linked dyads. We investigated the assembly
properties of the building blocks and obtained the imine-
linked porphyrin dyads (Dyads-1-5; Chart 2). We also
report the energy-transfer properties of the imine-linked
6618 Inorganic Chemistry, Vol. 42, No. 21, 2003

Self-Assembled Porphyrin Dyads
Chart 3
Scheme 1
dyads containing a zinc porphyrin and a free base porphyrin,
or two zinc porphyrins. In the following paper, we describe
the formation and properties of porphyrin-dipyrrin-metal
complexes that are also prepared via a self-assembly
process.23
Results and Discussion
Synthesis. Porphyrin Building Blocks. The porphyrin
building blocks for forming self-assembled dyads are shown
in Chart 3. Each porphyrin bears one reactive functional
group (X) that gives rise to the linker, and three nonlinking
substituents (Ar) to provide enhanced solubility in organic
solvents. Two common routes were employed for the
synthesis of the porphyrins: (1) a mixed-aldehyde condensa-
tion with pyrrole followed by functional group manipulations
as needed, or (2) attachment of the reactive functional group
to the porphyrin nucleus. The mixed-aldehyde route was
employed to prepare porphyrins 1-4, while Suzuki coupling
or Vilsmeier formylation was employed to prepare porphyrin
5 or 6, respectively.
oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) typically affords a statistical mixture containing six
porphyrins. The success of the condensation with mesital-
dehyde requires use of BF3‚OEt2-ethanol cocatalysis condi-
tions, which are achieved by performing the reaction with
BF3‚OEt2 in CHCl3 containing 0.8% ethanol.25 For aldehydes
with at least moderately polar substituents, the desired
trimesitylporphyrin is typically the second of the six por-
phyrins on chromatographic elution. Note that rational
methods have not yet been developed that enable the
synthesis of porphyrins bearing three mesityl groups.26
A mixed-aldehyde condensation24 of mesitaldehyde, an
aldehyde of choice, and pyrrole in 3:1:4 ratio followed by
(24) Lindsey, J. S.; Prathapan, S.; Johnson, T. E.; Wagner, R. W.
Tetrahedron 1994, 50, 8941-8968.
(25) Lindsey, J. S.; Wagner, R. W. J. Org. Chem. 1989, 54, 828-836.
(26) Rao, P. D.; Dhanalekshmi, S.; Littler, B. J.; Lindsey, J. S. J. Org.
Chem. 2000, 65, 7323-7344.
(23) Yu, L.; Muthukumaran, K.; Sazanovich, I. V.; Kirmaier, C.; Hindin,
E.; Diers, J. R.; Boyle, P. D.; Bocian, D. F.; Holten, D.; Lindsey, J.
S. Inorg. Chem. 2003, 42, 6629-6647.
Inorganic Chemistry, Vol. 42, No. 21, 2003 6619

Sazanovich et al.
Scheme 2
The reaction of mesitaldehyde, pyrrole, and 4-(2,4-
dioxopentan-3-yl)benzaldehyde, 4-acetamidobenzaldehyde,
methyl 4-formyl benzoate, or 4-cyanobenzaldehyde afforded
the corresponding porphyrin bearing three mesityl groups
and one acac (4), acetamide (7), methyl ester (8),24 or nitrile
(9) group (Scheme 1). Hydrolysis27 of porphyrin-amide 7
with HCl/trifluoroacetic acid (TFA) at 80 °C gave porphyrin-
amine 1 in 75% yield. Treatment of the porphyrin-ester 8
with 98% hydrazine monohydrate in a mixture of THF/EtOH,
a 1.0 M solution of hydrazine in THF, or hydrazine in THF
did not readily afford the desired porphyrin-hydrazide 2.
However, the reaction of 8 with a 1:3 mixture of hydrazine/
DMF28 gave porphyrin-hydrazide 2 in 60% yield. Reduction
of porphyrin-nitrile 9 with diisobutylaluminum hydride
(DIBAL-H) gave an aluminum porphyrin. Therefore, free
base porphyrin 9 was first metalated with Zn(OAc)2‚2H2O,
the zinc chelate was reduced with DIBAL-H, and acidic
workup gave the free base porphyrin-aldehyde 3. Porphyrins
2 and 3 were metalated on treatment with Zn(OAc)2‚2H2O
to give the zinc chelates Zn-2 and Zn-3.
The synthesis of a porphyrin bearing a meso-salicylalde-
hyde group (5) was pursued by a Suzuki coupling route to
avoid the obvious potential complications of condensation
of pyrrole with the salicylaldehyde entity. The synthesis of
the porphyrin precursor for the Suzuki coupling is shown in
Scheme 2. The known diacyldipyrromethane 1026 was
reduced with NaBH4 in THF/methanol (10:1), affording the
corresponding dicarbinol 10-diol. The latter was condensed
with dipyrromethane (11)29 under new catalysis conditions
(InCl3 in CH2Cl2 at room temperature)30 followed by
oxidation with DDQ, affording the porphyrin bearing one
unsubstituted meso-position (12) in 19% yield. No other
porphyrins were observed in this reaction.
Therien et al. have reported a number of Suzuki couplings
with zinc porphyrins bearing meso-bromo groups.31 Bromi-
nation of 12 at the unsubstituted meso-position in a standard
method32 by treatment with N-bromosuccinimide (NBS) in
CHCl3 in the presence of pyridine gave bromo-porphyrin
13, which was metalated with Zn(OAc)2‚2H2O to give Zn-
13. Suzuki coupling of Zn-13 with pinacolborane under Pd-
(PPh3)2Cl2 catalysis in dichloroethane at 90 °C provided
dioxaborolane-porphyrin Zn-14 in 89% yield.
The catalytic system reported to give the highest activity
for Suzuki reactions with 5-bromosalicylaldehyde derivatives
employs a palladium reagent (Pd(dppf)Cl2 or Pd(PPh3)4) and
(27) Lindsey, J. S.; Brown, P. A.; Siesel, D. A. Tetrahedron 1989, 45,
4845-4866.
(28) Nutt, R. F.; Veber, D. F.; Saperstein, R. J. Am. Chem. Soc. 1980,
102, 6539-6545.
(29) (a) 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. (b) Wang, Q. M.; Bruce, D. W. Synlett 1995, 1267-1268.
(30) Geier, G. R., III; Callinan, J. B.; Rao, P. D.; Lindsey, J. S. J. Porphyrins
Phthalocyanines 2001, 5, 810-823.
(31) Hyslop, A. G.; Kellett, M. A.; Iovine, P. M.; Therien, M. J. J. Am.
Chem. Soc. 1998, 120, 12676-12677.
K2CO3 in dimethoxyethane (DME)/H2O (3:1).33 The reaction
of Zn-14 and 4-bromosalicylaldehyde (15; prepared by
Reimer-Tiemann formylation of 3-bromophenol;34 see Sup-
porting Information) under these conditions at 90 °C for 3 h
(32) DiMagno, S. G.; Lin, V. S.-Y.; Therien, M. J. J. Org. Chem. 1993,
58, 5983-5993.
(33) (a) Morris, G. A.; Nguyen, S. T. Tetrahedron Lett. 2001, 42, 2093-
2096. (b) Zhuravel, M. A.; Nguyen, S. T. Tetrahedron Lett. 2001, 42,
7925-7928.
6620 Inorganic Chemistry, Vol. 42, No. 21, 2003

Self-Assembled Porphyrin Dyads
Scheme 3
Scheme 4
gave the desired porphyrin-salicylaldehyde Zn-5 in 60%
yield (Scheme 3).
The porphyrin bearing a meso-formyl group was synthe-
sized as shown in Scheme 4. Conversion of porphyrin 12 to
the copper chelate followed by Vilsmeier formylation gave
the meso-formyl copper porphyrin Cu-6 in 80% yield.
Demetalation with TFA/H2SO4 afforded porphyrin 6 in 62%
yield; subsequent metalation with Zn(OAc)2‚2H2O gave the
zinc porphyrin (Zn-6) in 92% yield.
Self-Assembled Dyads. Treatment of the free base por-
phyrin-acac (4) with Zn(OAc)2‚2H2O resulted in formation
of an unknown product rather than the expected self-
assembled bis(zinc-porphyrin-acac)Zn(II) dimer (see Sup-
porting Information). We turned our attention to the imine-
linked dyads. Imines derived from small molecules are
typically formed by treating an aldehyde and an amine at
high concentration in a suitable solvent (preferably alcoholic
solvents) or in refluxing benzene or toluene under acid
catalysis with azeotropic removal of water.35 Such conditions
are not applicable for preparing Dyads-1-5 for two reasons.
(1) High concentrations are not readily achieved with
porphyrins. (2) Strong acids cause demetalation of zinc
porphyrins. Accordingly, a solution of amino-porphyrin 1
and aldehyde-porphyrin Zn-5 (5 mM each) in THF/EtOH
(3:1) was stirred at room temperature in the absence of acid
catalysis. After 40 h, the analytical size exclusion chroma-
tography (SEC) showed only ∼15% yield of the imine-dyad
product (Dyad-1). Next, reactions were performed with
attempted catalysis by Yb(OTf)3 or Dy(OTf)3 (1.0-3.2 mM)
in THF. Such catalysts were chosen because earlier studies
showed that zinc porphyrins are quite stable upon exposure
to these lanthanide triflates.36 Yields of 30-40% were
obtained upon reaction for 18-22 h.
The reaction of Zn-5 and 1 (5.0 mM each) in the presence
of Yb(OTf)3 (1.0 mM) in THF at room temperature for 5 h
afforded Dyad-1 in 38% yield (Chart 2). The analytical SEC
(35) (a) Houjou, H.; Lee, S.-K.; Hishikawa, Y.; Nagawa, Y.; Hiratani, K.
Chem. Commun. 2000, 2197-2198. (b) Mun˜oz-Herna´ndez, M.-A.;
Keizer, T. S.; Parkin, S.; Patrick, B.; Atwood, D. A. Organometallics
2000, 19, 4416-4421. (c) Roesch, K. R.; Larock, R. C. J. Org. Chem.
2001, 66, 412-420. (d) Bachmann, S.; Furler, M.; Mezzetti, A.
Organometallics 2001, 20, 2102-2108.
(34) (a) Kobayashi, S.; Azekawa, M.; Morita, H. Chem. Pharm. Bull. 1969,
17, 89-93. (b) Glennon, R. A.; McKenney, J. D.; Lyon, R. A.; Titeler,
M. J. Med. Chem. 1986, 29, 194-199.
(36) Speckbacher, M.; Yu, L.; Lindsey, J. S. Inorg. Chem. 2003, 42, 4322-
4337.
Inorganic Chemistry, Vol. 42, No. 21, 2003 6621

Sazanovich et al.
Chart 4
handling over a period of days in nonpolar or polar solvents
or in the solid state (see Supporting Information).
The absorption spectrum of Dyads-1-4 showed the
presence of the zinc and free base porphyrins. The fluores-
cence spectra are described below. The IR spectra show
negligible (for Dyad-1) or weak (for dyads Dyad-2-5) IR
absorption in the 3200-3400 cm-1 region (characteristic of
the N-H stretch) and intense bands in the region 1590-
1670 cm-1 (characteristic of CdN and/or CdO signals).
Each of the dyads was examined by 1H NMR spectroscopy.
1
The H NMR spectra of Dyad-1 and Dyad-3 (containing
the salicylaldimine unit) were unremarkable, generally show-
ing sharp lines and the superposition of the spectra derived
from the porphyrin entities of the component parts except
for the expected changes owing to the formation of the imine
linkage. The 1H NMR spectra at room temperature of Dyad-
2, Dyad-4, and the benchmark Zn-16 each showed evidence
of two conformers in 8:1, 6:1, and 6:1 ratio, respectively
(Dyad-5 gave a poor quality spectrum). The diagnostic for
the occurrence of conformers is the presence of two singlets
stemming from the NH proton, and for Dyads-4-5 and Zn-
16, from the two singlets for the imine CH proton and the
two sets of peaks for the â-pyrrole protons flanking the imine.
The identity of the NH proton signal was confirmed by D2O
exchange. Compound 18, which represents the acyl hydra-
zone linker alone, also exhibited two conformers in 10:1
ratio. The small amount of minor conformer of the same
compound was not detected in a much earlier NMR study.39
1
Variable-temperature H NMR experiments (400 MHz) of
Dyad-2, Dyad-4, and Zn-16 established the slow intercon-
version of the conformers on the NMR time scale (see
Supporting Information).
of aliquots from the reaction mixture of the dyad-forming
reactions showed two sharp peaks with a difference in
retention times of ∼0.7 min, corresponding to the dyad and
the two monomers (the latter co-chromatographed with each
other). Dyads-2-5 were each prepared in similar fashion in
yields of 30-60%.
Each compound wherein two conformers were observed
(Dyad-2, Dyad-4, Zn-16) contains an acyl hydrazone linker
derived from an aldehyde lacking the o-hydroxy group.
Conformational isomerism in acyl hydrazones is well
established.40 The acyl hydrazone motif presents two sites
for conformational isomerism: the cis/trans amide and the
E/Z imine. Multiple conformers were not observed for the
salicylaldehyde-derived dyads, regardless of whether the
linkage is an imine or an acyl hydrazone (i.e., Dyad-1, Dyad-
3). In each case, the major conformer constitutes at least
85% of the total and is assumed to be the trans-E conformer
as displayed in Charts 1 and 2. Observation of conformers
in all compounds lacking the stabilizing o-hydroxy group,
and no conformers in those containing the o-hydroxy group,
is consistent with isomerism about the imine rather than the
amide unit.
Benchmarks. For comparison purposes, we prepared the
benchmarks shown in Chart 4. Porphyrin Zn-16 contains the
same linker present in Dyad-5. A reaction of 5.0 mM each
of Zn-6 and benzoic hydrazide in the presence of Yb(OTf)3
(1.0 mM) in THF at room temperature for 46 h afforded
porphyrin Zn-16 in 70% isolated yield. Variable temperature
1H NMR experiments of Zn-16 revealed the presence of two
interconverting conformers (see below). The small molecules
17,37 18,38 and 1938 represent the linkers. These were prepared
in quantitative yield by mixing the respective aldehyde with
the amino compound under neat conditions. Compound 17
is yellow while 18 and 19 are colorless. The absorption
spectrum in CH3CN at room temperature of 17, 18, or 19
showed a broad, long-wavelength absorption band with a
peak position of 336, 293, or 325 nm, respectively.
Chemical Characterization. Each dyad was characterized
Physical Studies. In the following, the properties of each
of the dyads and reference compounds are described, with
results for Dyad-3 selected as generally exemplary for
Dyads-1-3. Additional data for each dyad (and reference
compounds) are given in the Supporting Information. For
1
by analytical SEC, LD-MS, and H NMR, IR, absorption,
and fluorescence spectroscopy. Each of the purified dyads
showed a single component by TLC and a single peak by
analytical SEC. The dyads were quite stable toward routine
(39) Palla, G.; Pelizzi, C.; Predieri, G.; Vignali, C. Gazz. Chim. Ital. 1982,
112, 339-341.
(37) Steevens, J. B.; Pandit, U. K. Tetrahedron 1983, 39, 1395-1400.
(38) Struve, G. J. Prakt. Chem. 1894, 50, 295-310.
(40) Pihlaja, K.; Simeonov, M. F.; Fu¨lo¨p, F. J. Org. Chem. 1997, 62, 5080-
5088.
6622 Inorganic Chemistry, Vol. 42, No. 21, 2003

Self-Assembled Porphyrin Dyads
Figure 1. Schematic state diagram showing the likely excited-state
processes in the dyads, which include energy transfer (EnT), electron transfer
(ET), and hole transfer (HT). The processes labeled kZn and kFb include the
intrinsic decay processes (fluorescence and internal conversion to the ground-
state plus intersystem crossing to the excited triplet state) for the respective
Zn* and Fb* excited states. The dashed arrows indicate that the (free)
energies of the charge-separated states depend on dyad and medium. In
part B, Zni and Zna represent the imine- and aryl-substituted zinc porphyrins,
respectively.
Figure 2. Electronic absorption spectra (s) and fluorescence spectra (- -
and ‚ ‚ ‚) for Dyad-3 (A) and the reference compounds 8 (B) and Zn-5 (C)
in toluene at room temperature. Absorption data between 470 and 700 nm
have been multiplied by the factors shown. Dyad emission in part A was
obtained using predominant excitation of the free base porphyrin at 517
nm (- -) or zinc porphyrin at 565 nm (‚ ‚ ‚). Spectra in the respective
regions have been normalized to the same peak intensity, and the maxima
((1 nm) indicated.
Table 1. Half-Wave Potentials (V) of the Benchmark Zn Porphyrinsa
porphyrin
E0/+1
E+1/+2
E+2/+3
E+3/+4
that (1) the meta-hydroxy group has no measurable effect
on the ground-state electronic properties of the porphyrin
and (2) the para-aldehyde exerts a modest electron-
withdrawing effect. The redox waves for Zn-6 (E0/+1 ) 0.75
V) are considerably more positive and reflect the substantial
electron-withdrawing effects of the aldehyde directly attached
to the meso-position of the porphyrin ring.
Zn-3
Zn-5
Zn-6
Zn-16
0.60
0.60
0.75
0.55
0.91
0.91
0.96
0.87
b
b
b
1.00
b
b
b
1.26
a Potentials were obtained in CH2Cl2 containing 0.1 M Bu4NPF6. E-values
reported vs Ag/Ag+, FeCp2/FeCp2+ ) 0.19 V, and scan rate ) 0.1 V s-1
Values are ( 0.03 V. b Not observed.
.
The redox characteristics of Zn-16 are unusual in two
aspects. (1) The complex exhibits four waves in the 0-1.5
V range (Table 1) instead of two. Two of these waves are
in a range typical of Zn porphyrins (<1 V), whereas two
are at higher potentials (>1 V). To ensure that the additional
waves were not due to aggregation (or other undetermined
effects), the redox characteristics of the complex were
examined after multiple dilutions of the original sample. In
all cases, the voltammetric characteristics were reproducible
under conditions of multiple redox cycling. Consequently,
the additional voltammetric waves must reflect the participa-
tion of the appended imine group. (2) The potential of the
first redox wave of Zn-16 (E0/+1 ) 0.55 V) is not appreciably
different from that of ZnTPP or ZnTMP despite the fact that
the imine moiety appended at the porphyrin meso-position
can potentially conjugate with the porphyrin π-system.
Accordingly, these results suggest that the imine group is
not appreciably conjugated in the ground electronic state of
the complex.
convenience in presenting the results, the relevant excited-
state processes in the dyads are illustrated in Figure 1.
Electrochemistry. To examine the effects of the linker
on the ground-state electronic properties of the porphyrins,
we investigated the redox properties of the benchmark zinc
porphyrins Zn-3, Zn-5, Zn-6, and Zn-16 in the 0-1.5 V
range using both square-wave and cyclic voltammetry. In
this window, all four porphyrins exhibited reversible and
reproducible voltammetry. The redox potentials are sum-
marized in Table 1. Zn-3, Zn-5, and Zn-6 each exhibit two
oxidation waves in the 0-1.5 V range, as typically found
for Zn porphyrins.41 The potentials for Zn-3 and Zn-5 are
identical (E0/+1 ) 0.60) and somewhat more positive than
those observed for either meso-tetraphenylporphinatozinc-
(II) (ZnTPP, E0/+1 ) 0.56 V) or meso-tetramesitylporphina-
tozinc(II) (ZnTMP, E0/+1 ) 0.51 V).42 These results indicate
(41) Felton, R. H. In The Porphyrins; Dolphin, D., Ed.; Academic Press:
New York, 1978; Vol. 5, pp 53-125.
(42) Loewe, R. S.; Lammi, R. K.; Diers, J. R.; Kirmaier, C.; Bocian, D.
F.; Holten, D.; Lindsey, J. S. J. Mater. Chem. 2002, 12, 1530-1552.
Absorption Spectra. The electronic ground-state absorp-
tion spectrum of ZnFb array Dyad-3 and its constituent parts
Inorganic Chemistry, Vol. 42, No. 21, 2003 6623

Sazanovich et al.
Figure 3. Electronic absorption spectra (s) and fluorescence spectra (- -
and ‚ ‚ ‚) for Dyad-4 (A) and the reference compounds 8 (B) and Zn-16
(C) in toluene. Other details are the same as those for Figure 2.
Figure 4. Electronic absorption spectra (s) and fluorescence spectra (- -
and ‚ ‚ ‚) for Dyad-5 (A) and the reference compounds Zn-16 (B) and Zn-5
(C) in toluene. Dyad emission in part A was obtained using predominant
excitation of the imine-substituted zinc porphyrin at 438 nm (- -) or the
aryl-substituted zinc porphyrin at 423 (‚ ‚ ‚). Other details are the same as
those for Figure 2.
(zinc porphyrin Zn-5 and free base porphyrin 8) are shown
in Figure 2. Superposition of the absorptions of the zinc and
free base reference compounds affords a first-order repre-
sentation of the spectrum of Dyad-3. This is especially true
in the visible Q-band (S0 f S1) region (480-650 nm),
whereas the spectrum in the Soret region (420-425 nm)
likely also contains effects (shifts/splittings) due to exciton
coupling of the strong B transition dipoles of the subunits.
A similar analysis applies to Dyad-1 and Dyad-2 in toluene.
The absorption spectra of Dyads-1-3 in benzonitrile are also
a close composite of the associated reference spectra, all
displaying the typical red shifts and intensity-ratio changes
due to solvent coordination to the metal ion of the zinc
porphyrin subunit. These findings, the redox data described
above, and the emission results described below indicate that
the inter-porphyrin electronic interactions mediated by the
linker motifs employed in Dyads-1-3 do not significantly
perturb the electronic properties of the porphyrin subunits
in the ground state or photophysically relevant S1 excited
state compared to the isolated pigments. We have drawn
similar conclusions previously for ZnFb dyads employing
diarylethyne, diphenylbutadiyne, and p-phenylene linkers.43
Figure 3 shows absorption spectra of Dyad-4 and its
reference compounds Zn-16 and 8 in toluene. The zinc
porphyrin component of Dyad-4 (and Zn-16) contains the
imine group of the linker attached directly to the macrocycle
whereas a mediating aryl ring is present in Dyads-1-3 (and
Zn-5). This direct attachment in Zn-16 and Dyad-4 results
in red shifted and broader absorption bands with an altered
Q-band intensity ratio (Figure 3 versus Figure 2). The finding
that a meso-imine group lowers the energy of the S1 excited
state of zinc porphyrin Zn-16 relative to analogues such as
Zn-5 prompted us to prepare Dyad-5, which contains two,
inequivalent zinc porphyrins (Chart 2). The spectrum of this
dyad (Figure 4) is again well approximated by the super-
imposed reference spectra. The excited-state energies implied
by the absorption spectra in Figures 3 and 4 (and the
fluorescence spectra given below) indicate that the imine-
substituted zinc porphyrin (Zni) should serve as the energy-
transfer donor to the free base porphyrin in Dyad-4 whereas
it should be the energy-transfer acceptor from the aryl-
substituted zinc porphyrin (Zna) in Dyad-5, as shown in
Figure 1. This reversed direction of excited-state energy flow
is indeed found in the studies described below.
Fluorescence Spectra and Quantum Yields. Emission
profiles for Dyad-3 and reference porphyrins 8 and Zn-5 in
toluene are shown in Figure 2 (dashed and dotted spectra).
The emission spectra of Dyad-3 (Figure 2A) clearly show
that the fluorescence occurs essentially exclusively from the
free base porphyrin moiety, with perhaps trace emission from
the zinc porphyrin component (e.g., near 600 nm). This is
true for both excitation of (predominantly) the free base
porphyrin subunit at 517 nm (- -) or the zinc porphyrin
(43) (a) 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. (b) Li, F.; Gentemann, S.; Kalsbeck, W. A.; Seth, J.; Lindsey,
J. S.; Holten, D.; Bocian, D. F. J. Mater. Chem. 1997, 7, 1245-1262.
(c) Youngblood, W. J.; Gryko, D. T.; Lammi, R. K.; Bocian, D. F.;
Holten, D.; Lindsey, J. S. J. Org. Chem. 2002, 67, 2111-2117. (d)
Yang, S. I.; Li, J.; Cho, H. S.; Kim, D.; Bocian, D. F.; Holten, D.;
Lindsey, J. S. J. Mater. Chem. 2000, 10, 283-296.
6624 Inorganic Chemistry, Vol. 42, No. 21, 2003

Self-Assembled Porphyrin Dyads
Table 2. Photophysical Properties of Dyads and Reference Compoundsa
excited-state lifetime (ps)b
fluorescence yield
-1
(kEnT
)
f
g
h
compd
solvent
toluene
benzonitrile
toluene
benzonitrile
toluene
benzonitrile
toluene
Zn*
13 ( 2
Fb*
Zn* c
Fb* d
(ps)e
ΦEnT
ΦΕΤ
ΦΗΤ
Dyads
Dyad-1
Dyad-2
Dyad-3
Dyad-4
Dyad-5
12500
10400
12300
11000
12800
10400
11100
4200
0.13 (0.13)
0.12 (0.11)
0.11 (0.12)
0.11 (0.11)
0.13 (0.16)
0.10 (0.093)
0.13 (0.13)
0.10 (0.10)
13
67
70
0.99
>0.9
0.97
>0.9
0.97
e0.01
<0.1
e0.03
<0.1
e0.03
<0.1
0.35
0.06
0.23
0.08
0.19
0.04
0.23
0.18
0.61
0.10
65 ( 10
68 ( 4
>0.9
5.4 ( 0.7
3.0 ( 0.3
2.5 ( 0.2i
1900j
8.3
6.0
3.1k
0.65
0.50
benzonitrile
toluene
0.50
e0.2
0.072
(0.058)
>0.8
benzonitrile
1600j
0.11
Monomersl
8
toluene
benzonitrile
toluene
benzonitrile
toluene
benzonitrile
13300
13500
0.11
0.13
Zn-5
Zn-16
2200
1800
2100
1800
0.040
0.043
0.049
0.046
a All data were taken at room temperature. b Lifetime of the lowest excited state. For Zn porphyrin energy-transfer donors in the dyads, lifetimes were
determined using transient absorption data; for the energy-accepting free base porphyrin in Dyads-1-4 and imine-substituted zinc porphyrin in Dyad-5, and
for the reference compounds, the lifetimes ((5%) were measured by fluorescence modulation (phase shift) spectroscopy using nitrogen-purged samples.
c Fluorescence yield ((15%) of the zinc porphyrins (in the presence of dissolved oxygen of air) relative to ZnTPP in toluene (Φf ) 0.030)46 obtained using
excitation for Dyad-5 (toluene) of the imine-substitututed zinc porphyrin at 437 nm or the aryl-substituted zinc porphyrin at 423 nm (value in parentheses),
Zn-5 at 550 nm (toluene) or 380 nm (benzonitrile), or Zn-16 at 400 nm (both solvents). d Fluorescence yield ((10%) of the free base porphyrins (in the
presence of dissolved oxygen of air) relative to (free base) meso-tetraphenylporphyrin in toluene (Φf ) 0.11)46 obtained with predominant excitation of this
moiety at 517 nm for Dyads-1-4 and 515 nm for 8. The value in parentheses for each dyad is the Fb* fluorescence yield obtained with predominant
excitation of the zinc porphyrin at 565 nm. e Inverse of the rate constant for Zn*Fb f ZnFb* energy transfer (Dyads-1-4) or Zna*Zni f ZnaZni* energy
transfer (Dyad-5). f Quantum yield of Zn*Fb f ZnFb* or Zna*Zni f ZnaZni* energy transfer. g Quantum yield of Zn*Fb f Zn+Fb- or ZnZn* f Zn+Zn-
electron transfer. h Quantum yield of ZnFb* f Zn+Fb- hole transfer for Dyads-1-4 and for ZnZn* f Zn+Zn- transfer in Dyad-5 obtained from the
excited-state lifetimes. These values are considered to be upper limits since the yields calculated from the fluorescence yields are somewhat less. i Zn*
lifetime of the aryl-substituted zinc porphyrin energy-transfer donor. j Zn* lifetime of the imine-substituted zinc porphyrin energy-transfer acceptor. k Calculated
assuming ΦEnT ) 0.8. The value is in the range (2.5-3.1 ps)-1 l The values for the reference compounds are similar to those obtained previously for other
.
zinc and free base reference porphyrins.47
component at 565 nm (‚ ‚ ‚). Furthermore, the yield of
emission from the free base porphyrin in Dyad-3 in toluene
(using excitation of either porphyrin) is comparable to that
of reference compound 8 (Table 2). These combined results
for Dyad-3 in toluene indicate (1) very efficient Zn*Fb f
ZnFb* energy transfer (>95%) with little yield of competing
Zn*Fb f Zn+Fb- electron transfer, and (2) virtually no
subsequent quenching of ZnFb* via ZnFb* f Zn+Fb- hole
transfer (Figure 1). Basically, the same emission spectra and
fluorescence yields are obtained for Dyad-1 and Dyad-2 in
toluene (Table 2).
Generally similar results are also found for Dyads-1-3
and reference compounds Zn-5 and 8 in benzonitrile (Table
2). Again, emission from the dyads occurs primarily from
the free base porphyrin moiety upon excitation of either
subunit. However, the Fb* emission yields for the dyads are
modestly reduced (up to ∼30%, using excitation of either
component) compared to the fluorescence yield of 8 in
benzonitrile. Thus, although Zn*Fb f ZnFb* energy transfer
remains efficient for Dyads-1-3 in the polar solvent, the
reduced fluorescence yield indicates that Fb* is modestly
quenched by hole transfer (Figure 1).
fluoresence mirror symmetry relationship of zinc porphyrins,
such as exhibited by Zn-5. Second, there is a larger Stokes
shift between the Q(0,0) absorption and emission maxima
(∼19 nm; ∼495 cm-1) for Zn-16 than for Zn-5 (∼10 nm;
∼280 cm-1) or Fb porphyrin 8 (∼1 nm; ∼25 cm-1). These
results likely reflect photoinduced reorientations that increase
conjugation between the porphyrin and imine units. Note that
Dyad-4, Dyad-5, and Zn-16 each contain the meso-linked
acyl hydrazone linker, which gives two conformers. That
these effects occur mainly in the excited state and not the
ground state is indicated by the similar redox potentials of
Zn-16 and Zn-5 (Table 1). Analogous results are found for
Dyad-4, Dyad-5, and Zn-16 in benzonitrile (where, ad-
ditionally, the absorption and emission bands are red-shifted
due to solvent ligation to the zinc). The fluorescence yields
for Zn-16 in toluene and benzonitrile are comparable to those
for Zn-5 (and related benchmark porphyrins), as are the Zn*
lifetimes described below (Table 2).
The emission from Dyad-4 (like Dyads-1-3) occurs
primarily from the free base porphyrin, using excitation of
either subunit and for both toluene (Figure 3A) or benzoni-
trile. Similarly, in toluene the Fb* emission yield for Dyad-4
is comparable to 8 and is reduced only slightly in benzonitrile
(again using excitation of either component).
In the ZnZn Dyad-5, the imine-substituted zinc porphyrin
(Zni) is the lower energy of the two chromophores, opposite
to the situation for ZnFb Dyad-4 as noted above. Conse-
Figures 3 and 4 show the emission profiles for ZnFb
Dyad-4 and ZnZn Dyad 5, respectively, and their reference
components in toluene. Several points regarding the emission
spectrum of Zn-16 are noteworthy. First, the emission bands
of Zn-16 do not obey the (normal) approximate absorption/
Inorganic Chemistry, Vol. 42, No. 21, 2003 6625

Sazanovich et al.
quently, emission from Dyad-5 occurs primarily from Zni*
(Figure 4A). The integrated amplitude of this emission using
Soret excitation of the aryl-substituted porphyrin (Zna) is
about 20% lower than that obtained using Soret excitation
of Zni (after correction for absorbance at λex and residual
emission from Zna). On the basis of these results, we estimate
that Zna*Zni f ZnaZni* energy transfer has a yield of g80%
for Dyad-5 in toluene (Figure 1 and Table 2).
Fluorescence Lifetimes. The excited-state lifetimes for
the free base porphyrin moiety of Dyads-1-4 in both toluene
and benzonitrile are given in Table 2 along with the lifetimes
for the reference compounds. The yields of ZnFb* f Zn+Fb-
hole transfer in Dyads-1-4 were calculated from these
lifetimes using the formula ΦHT ) 1 - (τFdyba*d/τrFebf*) and are
given in the last column of Table 2. The Fb* lifetimes for
Dyads-1-3 in toluene (12-13 ns) are comparable to the
reference values (slightly over 13 ns), consistent with the
similar emission yields, indicating minimal Fb* quenching
by hole transfer. The yield increases to ΦHT ∼ 0.2 for Dyads-
1-3 in benzonitrile on the basis of a small reduction in Fb*
lifetime compared to that in 8 (paralleling the reduction in
fluorescence yield). For Dyad-4, the Fb* lifetime is slightly
quenched in toluene (∼11 ns versus ∼13 ns; ΦHT ∼0.2) and
more so in benzonitrile (4.2 ns versus 13.5 ns; ΦHT ∼0.6).
Figure 5. Representative time-resolved absorption spectra (A) and kinetic
profile (B) for Dyad-3 in toluene at room temperature using predominant
excitation of the zinc porphyrin component with a 551 nm 130 fs flash.
The fit to the kinetic data at 515 nm in part B gives a dominant component
with a time constant of 69 ps and a minor ∼1 ps phase. The average value
for the dominant component reflecting the Zn* lifetime from measurements
at several detection wavelengths is given in Table 2.
A similar analysis for Dyad-5 gives the yield of ZnaZni*
f Zna+Zni- hole transfer (Figure 1B). The Zn* lifetime for
this dyad in toluene of 1.9 ns presumably derives mainly
from Zni* (the lower-energy unit). This lifetime is only
slightly shorter than that of 2.1 ns for imine-substituted
reference porphyrin Zn-16 and gives ΦHT ∼ 0.1; similar
results are found in benzonitrile (Table 2). The lower hole-
transfer yields for Dyad-5 compared to Dyad-4 no doubt
derive from Zna+Zni- in the former being at higher energy
than Zn+Fb- in the latter.
little change to 3 ns (‚ ‚ ‚), consistent with the Fb* (fluores-
cence) lifetime of ∼13 ns (Table 2) and the minor spectral
changes that accompany Fb* decay, which primarily involves
formation of the excited triplet state FbT (70-80% yield in
the isolated pigment45).
Figure 5B shows a kinetic profile for the decay of the Zn*
excited-state absorption and the formation of the Fb*
bleaching at 515 nm (•) for Dyad-3 in toluene. Also shown
is a fit to a dual-exponential function (convolved with an
instrument response) plus a constant (for Fb* and FbT)
affording a dominant (>90%) kinetic component having time
constant of 69 ( 4 ps and a minor 1.1 ( 0.3 ps component
that may represent some vibrational relaxation of Zn*.
Analysis of kinetic traces at a number of wavelengths gives
a time constant for the main component of 68 ( 4 ps, which
is assigned as the Zn* lifetime for Dyad-3 in toluene (Table
2). Virtually the same Zn* lifetime (65 ( 10 ps) is found
for Dyad-2, which is consistent with the similar linkers
employed in the two arrays. A 5-fold shorter Zn* lifetime
(13 ( 2 ps) is found for Dyad-1, reflecting more rapid energy
Transient Absorption Spectra and Kinetics. All the
dyads were examined by time-resolved absorption difference
spectroscopy to obtain the rate constant and yield for Zn*Fb
f ZnFb* or Zn*Zn f ZnZn* energy transfer and to assess
the contribution of competing electron transfer (Figure 1).
The data were analyzed in the same fashion as we have
previously described for other porphyrin-based arrays.43,44
Representative data for Dyad-3 and Dyad-4 in toluene are
briefly described (Figures 5 and 6) and differences for the
other dyads and solvents highlighted. The measured Zn*
lifetimes and calculated rate constants are listed in Table 2.
Dyads-1-3 in Toluene. The absorption difference spec-
trum observed at 0.5 ps after excitation of Dyad-3 in toluene
(Figure 5A, s) is ascribed principally to Zn*, with a minor
contribution of Fb* (due to direct excitation of the latter).
By 270 ps (- -), the free base bleaching at 515 nm has
increased substantially, resulting from the Zn*Fb f ZnFb*
energy-transfer process. Subsequently, the spectrum shows
347-349. (d) Kikuchi, K.; Kurabayashi, Y.; Kokubun, H.; Kaizu, Y.;
Kobayashi, H. J. Photochem. Photobiol. A: Chem. 1988, 45, 261-
263. (e) Bonnett, R. D.; McGarvey, D. J.; Harriman, A.; Land, E. J.;
Truscott, T. G.; Winfield, U. J. Photochem. Photobiol. 1988, 48, 271-
276.
(44) (a) Rodriguez, J.; Kirmaier, C.; Holten, D. J. Am. Chem. Soc. 1989,
111, 6500-6506.
(45) (a) Gradyushko, A. T.; Sevchenko, A. N.; Solovyov, K. N.; Tsvirko,
M. P. Photochem. Photobiol. 1970, 11, 387-400. (b) Hurley, J. K.;
Sinai, N.; Linschitz, H. Photochem. Photobiol. 1983, 38, 9-14. (c)
Kajii, Y.; Obi, K.; Tanaka, I.; Tobita, S. Chem. Phys. Lett. 1984, 111,
(46) Seybold, P. G.; Gouterman, M. J. Mol. Spectrosc. 1969, 31, 1-13.
(47) (a) Yang, S. I.; Seth, J.; Strachan, J.-P.; Gentemann, S.; Kim, D.;
Holten, D.; Lindsey, J. S.; Bocian, D. F. J. Porphyrins Phthalocyanines
1999, 3, 117-147. (b) Tomizaki, K.-Y.; Loewe, R. S.; Kirmaier, C.;
Schwartz, J. K.; Retsek, J. L.; Bocian, D. F.; Holten, D.; Lindsey, J.
S. J. Org. Chem. 2002, 67, 6519-6534.
6626 Inorganic Chemistry, Vol. 42, No. 21, 2003

Self-Assembled Porphyrin Dyads
The relatively minor changes at longer times are again
consistent with the long (∼11 ns) Fb* lifetime and high yield
of FbT. However, the spectra at 140 ps and longer cannot be
ascribed solely to Fb* because they clearly still display the
edge of 435-nm Soret bleaching of the zinc porphyrin
component. This bleaching must reflect a contribution from
Zn+Fb- formed by electron transfer from Zn* to the free
base porphyrin (Figure 1A). In contrast, no such bleaching
is seen in the longer-time spectra for Dyad-3 in toluene
(Figure 5A), supporting the very high energy-transfer yield
discussed above.
On the basis of the Soret bleach at ∼460 nm at 140 ps
versus 0.5 ps, the fraction of Zn* that decays by Zn*Fb f
Zn+Fb- electron transfer is estimated to be 0.35 for Dyad-4
in toluene, with the remainder (0.65) assigned to Zn*Fb f
ZnFb* energy transfer. The average Zn* lifetime of 5.4 (
0.7 ps for Dyad-4 in toluene was determined from measure-
ments at a number of probe wavelengths (e.g., Figure 6B).
From these values, the rate constants for Zn* energy and
electron transfer for Dyad-4 in toluene are approximated by
kEnT ) (5.4 ps)-1‚0.65 ) (8.3 ps)-1 and kET ) (5.4 ps)-1‚0.35
) (15.4 ps)-1 (Table 2).
Dyad-4 in Benzonitrile. The competition between energy
and electron transfer from Zn* was further probed for
Dyad-4 in benzonitrile. It can be expected that this polar
solvent will make electron transfer more facile owing mainly
to the lower energy of the charge-separated state. As for
Dyad-4 in toluene, the spectra for this array in benzonitrile
after Zn* decay show features characteristic of Fb* as well
as features involving the zinc porphyrin that we assign to
state Zn+Fb-. Again, on the basis of the magnitudes of these
spectral features, the yield of Zn*Fb f Zn+Fb- electron
transfer is estimated to be ∼50%, with a comparable yield
for Zn*Fb f ZnFb* energy transfer (Table 2). This increase
in the electron-transfer yield from ∼35% in toluene must
derive primarily from an associated increased rate constant
of this process, consistent with the observed shortening of
the Zn* lifetime from ∼5.5 ps in toluene to ∼3.0 ps in
benzonitrile. The rate constants for Zn* energy and electron
transfer for Dyad-4 in benzonitrile are thus both determined
to be (3.0 ps)-1‚0.50 ) (6.0 ps)-1.
Dyad-5 in Toluene. The time evolution of the absorption
difference spectra of Dyad-5 in toluene (see Supporting
Information) shows that the excited state of the aryl-
substituted zinc porphyrin decays with a time constant of
2.5 ( 0.2 ps, which is much shorter than the reference value
of ∼2 ns. During this time, there is concomitant formation
of bleaching in the ground-state absorption bands of the
imine-substituted zinc porphyrin. These data are consistent
with a high yield of Zna*Zni f ZnaZni* energy transfer
(Figure 1B), but due to spectral overlap, it is difficult to
assess the extent of competing charge transfer from these
data alone. The static fluorescence data give an energy-
transfer yield of g80% resulting in an electron-transfer yield
of e20% (Table 2). Combining with the Zn* lifetime gives
an energy-transfer rate constant of kEnT ∼ (2.5-3.1 ps)-1.
General Comparisons. In Dyads-1-3, the linker contains
a phenyl ring attached to each porphyrin with the imine unit
Figure 6. Representative time-resolved absorption spectra (A) and a kinetic
profile (B) for Dyad-4 in toluene at room temperature using predominant
excitation of the zinc porphyrin with a 551 nm 130 fs flash. The fit to the
kinetic data at 515 nm in (B) gives a time constant of 5.8 ps, and the average
value reflecting the Zn* lifetime from measurements at several different
detection wavelengths is given in Table 2.
transfer to the free base component than in Dyad-2 and
Dyad-3 due to the linker being two atoms shorter, along with
associated linker electronic characteristics.
Two observations indicate that Zn*Fb f ZnFb* energy
transfer is virtually quantitative for Dyads-1-3 in toluene.
First, there is no clear evidence for formation of Zn+Fb- in
the transient absorption spectra. Second, the Fb* fluorescence
yield is comparable using excitation of either subunit,
indicating ΦEnT > 90%. Third, the Zn* lifetime in each dyad
is much shorter than that of ∼2 ns for reference porphyrins
such as Zn-16. Given these findings, the rate constant and
quantum yield of Zn*Fb f ZnFb* energy transfer for Dyads-
1-3 in toluene were estimated using the expressions kEnT
)
(1/τZdyna*d) - (1/τZn*) and ΦEnT ) 1 - (τZdyna*d/τZn*). The results
ref
ref
are given in columns 7 and 8 of Table 2, with the
corresponding yield of electron transfer given in column 9.
The results for these dyads in benzonitrile given in Table 2
are based on the fluorescence data, which again indicate a
high yield of Zn*Fb f ZnFb* energy transfer and relatively
little competing Zn*Fb f Zn+Fb- electron transfer taking
place.
Dyad-4 in Toluene. Representative transient absorption
difference spectra for Dyad-4 in toluene are shown in Figure
6. Similar to Dyad-3, the 0.5 ps spectrum (s) is assigned
mainly to Zn*, with a minor contribution of Fb* due to direct
excitation. Note the bleaching of the 435-nm zinc porphyrin
Soret band on the blue edge of this spectrum. The spectrum
at 140 ps (- -) has a deep bleaching in the 515-nm ground-
state absorption band of the free base porphyrin and is
assigned largely to Fb* formed via energy transfer from Zn*.
Inorganic Chemistry, Vol. 42, No. 21, 2003 6627

Sazanovich et al.
between the phenyl rings. All three dyads afford nearly
quantitative Zn*Fb f ZnFb* energy transfer in toluene and
>90% in benzonitrile. Subsequent ZnFb* f Zn+Fb- hole-
transfer yields are <10% in toluene and <25% even in the
polar solvent benzonitrile. Together, these findings augur well
for efficient energy transfer and subsequent utilization of the
energy available at the output unit in these three self-
assembled entities when incorporated in larger architectures.
The salicylaldimine- and acyl-hydrazone-based linkers
used in Dyad-2 and Dyad-3, respectively, afford similar
Zn*Fb f ZnFb* energy-transfer rates of ∼(70 ps)-1 in
toluene. Dyad-3 differs from Dyad-2 primarily in the
presence of a hydrogen-bonding interaction that should afford
higher stability of the assembly. Dyad-1 exhibits a 5-fold
greater energy-transfer rate of (13 ps)-1 and a slightly higher
yield (99%). The more rapid energy transfer is consistent
with the imine-based linker in Dyad-1 being two-atoms
shorter than those employed in Dyad-2 and Dyad-3 (al-
though differences in electronic characteristics may also play
a role).
In Dyad-4, the imine portion of the acylhydrazone unit is
directly attached to the zinc porphyrin. Thus, Dyad-4 also
has a shorter linker than Dyad-2 and Dyad-3. This shorter
linker, perhaps in conjunction with electronic effects from
direct attachment to the macrocycle, again affords a signifi-
cantly increased energy-transfer rate for Dyad-4 in toluene
of (8.3 ps)-1 compared to ∼(70 ps)-1 for Dyad-2 and Dyad-
3. However, Dyad-4 suffers from a similar increase in the
yield of competing Zn*Fb f Zn+Fb- electron transfer even
in the nonpolar medium toluene (∼35% for Dyad-4 com-
pared to e8% for Dyads-1-3). The subsequent quenching
of Fb* via hole transfer to the zinc porphyrin is also larger
in Dyad-4 than in Dyads-1-3. Thus, the increased Zn*Fb
f ZnFb* energy-transfer rate in Dyad-4 comes at the
expense of an increased yield of two wasteful charge-transfer
processes. On the other hand, a useful aspect of the
porphyrin-imine direct attachment motif is that it allows the
generation of a module containing two electronically distinct
zinc porphyrins. The resulting Dyad-5 has the fastest energy
transfer of all the dyads [∼(3 ps)-1], a Zna*Zni f ZnaZni*
energy-transfer yield of g80% in toluene, and significantly
suppressed subsequent hole transfer (∼10% even in ben-
zonitrile) compared to ZnFb Dyad-4. Thus, Dyad-5 has
favorable characteristics that may be exploited in more
elaborate molecular architectures.
the goal is efficient energy transfer on the ∼10 ps time scale.
We note also that the energy-transfer rate in the self-
assembled Dyad-1 [(13 ps)-1] is actually larger than that
found for covalently linked dyads employing diphenylethyne
[(24 ps)-1] or diphenylbutadiyne [(37 ps)-1] linkers, and only
modestly smaller than for a p-phenylene linker [(∼3.5 ps)].43
Although the energy-transfer rate is smaller in Dyad-2 and
Dyad-3 [∼(70) ps)-1], near quantitative energy transfer is
maintained.
Conclusions
We have shown here that porphyrins can be joined using
an imine-based attachment motif that affords rapid and
efficient excited-state energy transfer while maintaining weak
linker-mediated inter-porphyrin electronic interactions. The
relatively weak interactions allow key intrinsic properties
(redox, photophysical) designed into the isolated components
to be largely retained in the arrays. Accordingly, the desired
characteristics of diverse architectures of increasing com-
plexity can be reasonably anticipated from those of simple
dyad and triad motifs. Collectively, the studies described
herein indicate that the self-assembly approach can be
implemented to afford porphyrin-dyad modules that have
comparable or even enhanced energy-transfer characteristics
relative to those of more conventional covalently linked
systems. Additional variations on the theme of self-assembly
are examined in the following paper, where arrays composed
of two porphyrins linked via an intervening bis(dipyrrinato)-
metal linker are described.
Acknowledgment. This research was supported by the
NSF (Grant CHE-9988142). Mass spectra were obtained at
the Mass Spectrometry Laboratory for Biotechnology at
North Carolina State University. Partial funding for the Mass
Spectrometry Laboratory for Biotechnology at North Caro-
lina State University was obtained from the North Carolina
Biotechnology Center and the NSF.
Supporting Information Available: Absorption/emission spec-
tra of all the dyads and reference compounds in toluene and
benzonitrile; time-resolved absorption spectra and kinetic profiles
for Dyad-1, Dyad-2, and Dyad-5 in toluene and Dyad-4 in
benzonitrile. Complete Experimental Section including the synthesis
of 4-(2,4-dioxopentan-3-yl)benzaldehyde and spectral data (absorp-
tion, fluorescence, 1H NMR, VT-NMR, LD-MS) for all new
compounds. This material is available free of charge via the Internet
at http://pubs.acs.org.
In summary, for the ZnFb dyads, the linker motif
employed in Dyad-1 is far superior to that in Dyad-4 when
IC034558U
6628 Inorganic Chemistry, Vol. 42, No. 21, 2003