Synthesis and Characterization of Tetrachlorodiarylethyne-Linked Porphyrin Dimers. Effects of Linker Architecture on Intradimer Electronic Communication

Article abstract of DOI:10.1021/ic970967c

The effects of incorporating chloro groups at all ortho positions of a diphenylethyne linker that bridges the zinc and free base (Fb) components of a porphyrin dimer (ZnFbB(Cl₄)) have been investigated in detail via various static and time-resolved spectroscopic methods. The excited-state energy-transfer rate in ZnFbB(Cl₄) ((134 ps)^-1) is 5-fold slower than that in the corresponding dimer having an unsubstituted linker (ZnFbU, (24 ps)^-1) but is only modestly slower than that in the dimer having o-methyl groups on the linker (ZnFbB(CH₃)₄, (115 ps)^-1). The ground-state hole/electron-hopping rates in the oxidized bis-Zn analogues of all three dimers are much slower than the excited-state energy-transfer rates. There is no discernible difference between the hole/electron-hopping rates in the o-chloro- and o-methyl-substituted arrays. The similar ground- and excited-state dynamics observed for the o-chloro- and o-methyl-substituted arrays is attributed to the dominance of torsional constraints in mediating the extent of through-bond electronic communication. These constraints attenuate intradimer communication by restricting the rotation toward coplanarity of the phenyl rings of the linker and the porphyrin rings. Thus, the o-chloro groups on the linker decrease electronic communication via a steric, rather than purely electronic, mechanism.

Full text of DOI:10.1021/ic970967c

Inorg. Chem. 1998, 37, 1191-1201
1191
Synthesis and Characterization of Tetrachlorodiarylethyne-Linked Porphyrin Dimers.
Effects of Linker Architecture on Intradimer Electronic Communication
Jon-Paul Strachan,† Steve Gentemann,‡ Jyoti Seth,§ William A. Kalsbeck,§
Jonathan S. Lindsey,*,† Dewey Holten,*,‡ and David F. Bocian*,§
Departments of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204,
Washington University, St. Louis, Missouri 63130-4899, and University of California,
Riverside, California 92521-0403
ReceiVed July 31, 1997
The effects of incorporating chloro groups at all ortho positions of a diphenylethyne linker that bridges the zinc
and free base (Fb) components of a porphyrin dimer (ZnFbB(Cl4)) have been investigated in detail via various
static and time-resolved spectroscopic methods. The excited-state energy-transfer rate in ZnFbB(Cl4) ((134 ps)-1
)
is 5-fold slower than that in the corresponding dimer having an unsubstituted linker (ZnFbU, (24 ps)-1) but is
only modestly slower than that in the dimer having o-methyl groups on the linker (ZnFbB(CH3)4, (115 ps)-1).
The ground-state hole/electron-hopping rates in the oxidized bis-Zn analogues of all three dimers are much slower
than the excited-state energy-transfer rates. There is no discernible difference between the hole/electron-hopping
rates in the o-chloro- and o-methyl-substituted arrays. The similar ground- and excited-state dynamics observed
for the o-chloro- and o-methyl-substituted arrays is attributed to the dominance of torsional constraints in mediating
the extent of through-bond electronic communication. These constraints attenuate intradimer communication by
restricting the rotation toward coplanarity of the phenyl rings of the linker and the porphyrin rings. Thus, the
o-chloro groups on the linker decrease electronic communication via a steric, rather than purely electronic,
mechanism.
Introduction
Chart 1. Dimeric Arrays for Probing Energy Transfer
Our groups have initiated a program in the design, synthesis,
and characterization of multipigment arrays for studies of
artificial photosynthesis and molecular photonics. Toward that
end, we have investigated a variety of multiporphyrin arrays
comprised of free base (Fb) porphyrins, metalloporphyrins, and
other pigments that are joined by semirigid1 diarylethyne linkers.
These systems include a star-shaped pentameric array that serves
as a light-harvesting device;2 a linear array comprised of a boron
dipyrromethene dye, three Zn porphyrins, and one Fb porphyrin
that functions as a molecular photonic wire;3 and a related array
including a Mg porphyrin as a redox switch that serves as an
optoelectronic gate.4 These and related architectures are quite
robust in terms of their convenient building-block syntheses,
their stability and solubility, and their ultrafast and efficient
energy-transfer properties. These favorable qualities indicate
that the diphenylethyne-linker motif is ideal for incorporation
into even larger and more complex arrays.
Toward optimizing our building-block strategy and exploring
its full potential, we have investigated the factors that control
electronic communication in three sets of diarylethyne-linked
porphyrin dimers (Chart 1). An overarching finding is that both
the singlet excited-state energy-transfer process in the neutral
arrays and the ground-state hole/electron-hopping process in the
oxidized complexes predominantly involve through-bond elec-
tronic communication mediated by the diarylethyne linker.5-9
More specific findings of these studies are as follows.
† North Carolina State University.
‡ Washington University.
§University of California.
(1) Bothner-By, A. A.; Dadok, J.; Johnson, T. E.; Lindsey, J. S. J. Phys.
Chem. 1996, 100, 17551-17557.
(2) Prathapan, S.; Johnson, T. E.; Lindsey, J. S. J. Am. Chem. Soc. 1993,
115, 7519-7520.
(1) The first series of dimers explored the effects of torsional
constraints on the electronic communication. In particular, steric
(3) Wagner, R. W.; Lindsey, J. S. J. Am. Chem. Soc. 1994, 116, 9759-
9760.
(4) Wagner, R. W.; Lindsey, J. S.; Seth, J.; Palaniappan, V.; Bocian, D.
F. J. Am. Chem. Soc. 1996, 118, 3996-3997.
(5) Seth, J.; Palaniappan, V.; Johnson, T. E.; Prathapan, S.; Lindsey, J.
S.; Bocian, D. F. J. Am. Chem. Soc. 1994, 116, 10578-10592.
S0020-1669(97)00967-1 CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/03/1998

1192 Inorganic Chemistry, Vol. 37, No. 6, 1998
Strachan et al.
Chart 2. Structures of Dimeric Arrays and Porphyrin
constraints that restrict the phenyl rings of the linker from
rotating toward coplanarity with the porphyrin constituents result
in a marked slowing of the energy-transfer rate. For example,
the energy-transfer rate from the photoexcited Zn porphyrin
(Zn*) to the Fb porphyrin is reduced from ∼(24 ps)-1 in the
unhindered dimer ZnFbU to ∼(115 ps)-1 in the dimer having
methyl groups at each of the ortho positions of the linker
ZnFbB(CH3)4.6,10 This difference derives predominantly from
the sterically induced reduction in porphyrin-linker orbital
overlap which attenuates through-bond electronic communica-
tion. This effect also modulates the ground-state hole/electron-
hopping process in the oxidized bis-Zn analogues of the arrays
[Zn2U]+ and [Zn2B(CH3)4]+. However, this latter process is
much slower in both arrays ((10-100 ns)-1) than the excited-
state energy transfer.
Building Blocks
(2) The second series of dimers explored the effects of metal
substitution (Mg vs Zn) on the photodynamics of the torsionally
unconstrained array.8 The ZnFbU and MgFbU dimers exhibit
comparable energy-transfer rates independent of solvent, al-
though the longer intrinsic excited-state lifetime of the Mg
porphyrin relative to that of the Zn porphyrin (∼10 vs ∼2 ns)
affords a slightly greater energy-transfer yield for the Mg-
containing dimer. This difference, although slight for the
isolated dimer, would be amplified in arrays having large
numbers of energy-transfer steps. On the other hand, architec-
tures having a Mg versus Zn porphyrin component suffer the
disadvantages of slightly lower stability and enhanced electron
transfer competing with energy migration.
(3) The third series of dimers revealed the novel effects of
the nature and electron-density distribution of the highest
occupied molecular orbital (HOMO) of the porphyrin constitu-
ents.9 The energy-transfer rate in F30ZnFbU ((240 ps)-1), which
contains pentafluorophenyl groups at all nonlinking meso
carbons, is an order of magnitude slower than in ZnFbU ((24
ps)-1), which contains mesityl groups at these positions. The
a1u HOMOs of the porphyrins in the former dimer have nodes
at the meso positions, including those to which the linker is
appended. In contrast, the a2u HOMOs of the constituents of
the latter array have substantial electron density at these
positions. This reversal of orbital ordering, together with a
redistribution in electron density accompanying incorporation
of the electron-withdrawing fluorines, substantially diminishes
electronic coupling via the linker and reduces the rates of both
excited-state energy transfer and ground-state hole/electron
hopping.
orbitals of the meso-carbon atoms of the porphyrin.11 In order
to examine whether this interaction could open a new conduit
for enhanced electronic communication in diarylethyne-linked
multiporphyrin arrays, we prepared ZnFb and bis-Zn dimers
bearing chloro substituents at all ortho positions on the
diphenylethyne linkers ZnFbB(Cl4) and Zn2B(Cl4), as shown
in Chart 2. We have examined these and related compounds
by static spectroscopy (absorption, fluorescence, resonance
Raman, electron paramagnetic resonance), time-resolved absorp-
tion spectroscopy, and electrochemical (cyclic and square-wave
voltammetry, coulometry) techniques.
Our previous studies of diarylethyne-linked porphyrin arrays
have afforded insights into the predominant mechanism of
electronic communication and have revealed new strategies for
optimizing these tetrapyrrole-based architectures for materials
applications. Recently, one of our groups observed that o-chloro
substituents on meso-phenyl rings directly interact with the p
Experimental Section
A. Synthetic Procedures. 1. General. 1H NMR spectra (300
MHz, IBM FT-300), absorption spectra (HP 8453A, Cary 3), and
fluorescence spectra (Spex FluoroMax) were collected routinely.
Porphyrins were analyzed in neat form by laser desorption mass
spectrometry (LD-MS)12 or in a matrix (4-hydroxy-R-cyanocinnamic
acid) (MALDI-TOF-MS)12 using a Bruker Proflex II. Elemental
analyses were obtained from Atlantic Micro Labs Inc. Pyrrole was
distilled at atmospheric pressure from CaH2. Commercial sources
(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) Li, F.; Gentemann, S.; Kalsbeck, W. A.; Seth, J.; Lindsey, J. S.; Holten,
D.; Bocian, D. F. J. Mater. Chem. 1997, 7, 1245-1262.
(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) The previous value for the energy-transfer rate for ZnFbB(CH3)4, ktrans
(11) Kalsbeck, W. A.; Seth, J.; Bocian, D. F. Inorg. Chem. 1996, 35, 7935-
7937.
(12) Fenyo, D.; Chait, B. T.; Johnson, T. E.; Lindsey, J. S. J. Porphyrins
Phthalocyanines 1997, 1, 93-99.
) (88 ps)-1, has been remeasured by transient absorption spectroscopy
over a longer time frame. The revised value is ktrans ) (115 ps)-1
,
which is used throughout this paper.

Tetrachlorodiarylethyne-Linked Porphyrin Dimers
Inorganic Chemistry, Vol. 37, No. 6, 1998 1193
provided (trimethylsilyl)acetylene (Janssen Chimica), 4-iodobenzalde-
hyde (Karl Industries Inc.), 2,6-dichlorobenzaldehyde (Aldrich), and
all other reagents and starting materials (Aldrich). Melting points are
uncorrected.
90 °C/0.1 mmHg to give a white solid (1.77 g, 24%). Mp 117-119
°C. Anal. Found: C, 28.3; H, 0.8; N, 4.7; Cl, 23.8; I, 42.6; (M+),
296.8604. C7H2Cl2IN requires the following: C, 28.2; H, 0.7; N, 4.7;
Cl, 23.8; I, 42.6; (M+), 296.8609. 1H NMR (300 MHz, CDCl3): 7.82
(2 H, s, Ar-H). 13C NMR (75 MHz, CDCl3): 99.2 (quat, C-I), 112.9
(quat, *C-CN), 114.0 (quat, CN), 137.0 (2 × CH), 138.4 (2 × quat,
C-Cl). m/z (EI): 299 (M+, 60%), 297 (100), 172 (55), 170 (100),
134 (50), 127 (15), 99 (40). IR νmax(Nujol)/cm-1: 2234 (CN).
2,6-Dichloro-4-iodobenzaldehyde (4). A sample of 2,6-dichloro-
4-iodobenzonitrile (3) (1.5 g, 5.04 mmol) was dissolved in 5 mL of
dry CH2Cl2 (distilled from CaH2), and the solution was cooled to 0
°C. A 6.04 mL solution of DIBAL-H (6.04 mmol, 1 M in CH2Cl2)
was added dropwise. After the addition was complete, the reaction
mixture was allowed to warm to room temperature and was stirred for
30 min. The reaction mixture was then poured into a conical flask
containing 20 g of ice and 20 mL of 6 N HCl. After 1 h of stirring,
the aqueous layer was separated and extracted with CH2Cl2 (2 × 25
mL). The organic layers were combined, washed with 5% w/v NaHCO3
(20 mL) and brine (20 mL), and dried (MgSO4), and the solvent was
removed in vacuo. Gravity column chromatography on silica (hex-
anes-ether (9:1)) gave a pale orange solid which was sublimed at 80
°C/0.1 mmHg, affording a white solid (1.36 g, 90%). Mp: 86-87
°C. Anal. Found: C, 28.0; H, 1.0; Cl, 23.5; I, 42.1; (M+), 299.8602.
C7H3Cl2IO requires the following: C, 27.9; H, 1.0; Cl, 23.6; I, 42.2;
(M+), 299.8606. 1H NMR (300 MHz, CDCl3): 7.77 (2 H, s, Ar-H),
10.41 (1 H, s, CHO). 13C NMR (75 MHz, CDCl3): 99.0 (quat, C-I),
129.8 (quat, *C-CHO), 137.1 (2 × quat, C-Cl), 138.3 (2 × CH),
187.8 (CHO) m/z (EI): 301 (M+, 40%), 300 (50), 299 (100), 272 (10),
172 (25), 109 (60), 74 (75). IR νmax(Nujol)/cm-1: 1684 (CHO).
2,6-Dichloro-4-[2-(trimethylsilyl)ethynyl]benzaldehyde (5). Samples
of 2,6-dichloro-4-iodobenzaldehyde (4) (0.8 g, 2.66 mmol) and AsPh3
(195 mg, 0.64 mmol) were dissolved in 20 mL of Et3N at room
temperature. The solution was deaerated with argon for 15 min.
(Trimethylsilyl)acetylene (0.45 mL, 3.18 mmol, 1.2 molar equiv) was
added to the solution via syringe. Pd2(dba)3 (72 mg, 79 µmol) was
added, and the reaction vessel was capped and placed in an oil bath at
35 °C. The reaction mixture was stirred at 35 °C under an argon
atmosphere until GC-MS analyses showed the absence of starting
material (48 h); an additional sample of (trimethylsilyl)acetylene (0.2
mL) was added after 24 h. The solvents were removed in vacuo and
wet-flash column chromatography on silica (hexanes) gave an off-white
solid, which was sublimed at 70 °C/0.1 mmHg (0.56 g, 78%). Mp 58
°C. Anal. Found: C, 53.0; H, 4.4; Cl, 26.0; (M+), 270.0031.
C12H12Cl2OSi requires the following: C, 53.4; H, 4.5; Cl, 25.8; (M+),
270.0034). 1H NMR (300 MHz, CDCl3): 0.25 (9 H, s, Si(CH3)3), 7.44
(2 H, s, Ar-H), 10.44 (1 H, s, CHO). 13C NMR (75 MHz, CDCl3):
-0.4 (3 × CH3, Si(CH3)3), 100.9 (quat, *C-ethyne), 101.5 (quat), 129.2
(quat, *C-CHO), 129.7 (quat), 132.6 (2 × CH), 136.7 (2 × quat,
C-Cl), 188.0 (CHO). m/z (EI): 272 (M+, 40%), 270 (75), 255 (100),
127 (20), 113 (10). IR νmax(Nujol)/cm-1: 1710 (CHO).
2. Chromatography. Preparative chromatography was performed
using the “wet-flash” column technique13 using flash silica (Baker) or
alumina (Fisher A540, 80-200 mesh) and eluting solvents based on
hexanes admixed with ether or CH2Cl2. Porphyrins were dissolved in
CH2Cl2 and preadsorbed onto silica or alumina in a round-bottom flask.
The solvent was removed on a rotary evaporator with gentle heating
to avoid bumping. With the eluant level in the column a few
centimeters above the adsorbent bed, the preadsorbed sample was added.
Preparative scale size exclusion chromatography (SEC) was performed
using BioRad Bio-Beads SX-1.8,14 Analytical scale SEC was performed
to assess the purity of the array-forming reactions and to monitor the
8,14
preparative purification of the arrays.
3. Solvents. CH2Cl2 (Fisher, reagent grade) was subjected to simple
distillation from K2CO3. Dry CH2Cl2 (Fisher certified ACS), toluene
(Fisher certified ACS), and triethylamine (Fluka puriss) were distilled
from CaH2. Other solvents were used as received.
4. Synthetic Details. 2,6-Dichloro-4-iodoaniline (1). To a solution
of 2,6-dichloroaniline (9.05 g, 55.8 mmol) in 250 mL of dichlo-
romethane and 100 mL of methanol were added benzyltrimethylam-
monium dichloroiodate (BTMAICl2)15 (22.50 g, 64.6 mmol) and
calcium carbonate (7.23 g). The mixture was heated at reflux for 2 h
under an argon atmosphere, during which time the solution gradually
changed from yellow to orange. The solution was then stirred at room
temperature overnight. Excess calcium carbonate was filtered, and the
solvent was removed in vacuo. To the residue was added 5% w/v
NaHSO4 (25 mL), and the mixture was extracted with CH2Cl2 (4 ×
100 mL). The organic layer was dried and filtered and the solvent
removed in vacuo. Wet-flash column chromatography on silica
(hexanes-ether (9:1)) gave a white solid (6.4 g, 40%). Mp: 96 °C
from methanol (lit.16 mp 96-97 °C from 30% alcohol). Anal.
Found: (M+), 286.8758. C6H4Cl2IN requires the following: (M+),
286.8766. 1H NMR (300 MHz, CDCl3): 4.48 (2 H, br, NH2), 7.45 (2
H, s, Ar-H). 13C NMR (75 MHz, CDCl3): 75.8 (quat, C-I), 120.2
(2 × quat, C-Cl), 135.7 (2 × CH), 140.0 (quat, C-NH2). m/z (EI):
289 (M+, 64%), 287 (100), 162 (24), 160 (39), 127 (30), 124 (41), 97
(10).
2,6-Dichloro-4-iodobenzonitrile (3). A hot mixture of equal
amounts of concentrated H2SO4 and water (70 mL) was added to 2,6-
dichloro-4-iodoaniline (1) (7.11 g, 24.7 mmol), and the resulting mixture
was stirred for 10 min before cooling to 0 °C. On cooling a mass of
the anilinium sulfate was obtained. Diazotization was effected at 0-5
°C with a solution of NaNO2 (1.77 g, 25.6 mmol) in 25 mL of water.17
The solution was filtered and diluted with 150 g of ice. A solution of
NaBF4 (32.4 g, 295 mmol) dissolved in the minimum amount of H2O
was then added to the diazonium solution. The precipitate was filtered
and washed with 25 mL of a saturated NaBF4 solution, 10 mL of
methanol and 25 mL of ether. The resultant yellow solid was dried in
vacuo. The tetrafluoroborate diazonium salt (2) dissolved in ∼400 mL
of H2O was added dropwise at 0 °C with vigorous stirring to a cuprous
cyanide mixture. [Cuprous cyanide (2.32 g, 25.9 mmol) was suspended
in 120 mL of water. Aqueous sodium cyanide (15 mL, 3.2 g, 65.3
mmol) was added and the mixture stirred at room temperature until
the cuprous cyanide dissolved.] When the addition was complete, the
mixture was stirred at 0 °C for 1 h and at room temperature for 1 h.
CH2Cl2 (3 × 100 mL) was added, and the organic layer was separated,
washed with 50 mL of water, and dried (MgSO4), and the solvent was
removed in vacuo. Gravity column chromatography on silica (hex-
anes-ethyl acetate (9:1)) gave an orange solid which was sublimed at
5,10,15-Triphenyl-20-(2,6-dichloro-4-iodophenyl)porphyrin (FbI-
H(Cl2)). Pyrrole (92 µL, 1.33 mmol), benzaldehyde (101.5 µL, 0.997
mmol), and 2,6-dichloro-4-iodobenzaldehyde (5) (0.1 g, 0.332 mmol)
were dissolved in 135 mL of CH2Cl2 under an argon atmosphere. After
10 min, BF3‚O(Et)2 (0.2 mL of a 2.65 M stock solution, 0.4 mmol)
was added via syringe with vigorous stirring. After addition was
complete, the reaction mixture was stirred for 1 h at room temperature.
DDQ (0.30 g, 1.33 mmol) was added, and after 1 h of stirring at room
temperature, the solvent was removed in vacuo. Wet-flash column
chromatography on silica (hexanes-CH2Cl2 (4:1)) gave a mixture of
porphyrins. Wet-flash column chromatography on alumina (hexanes-
CH2Cl2 (4:1)), gave a mixture of two porphyrins. These were separated
on alumina (hexanes-CH2Cl2 (4:1)) affording a purple solid (50 mg,
18.4%). 1H NMR (300 MHz, CDCl3): -2.71 (2 H, s, NH), 7.76 (9 H,
m, Ar-H), 8.15 (2 H, s, Ar-H), 8.20 (6 H, m, Ar-H), 8.65 (2 H, d,
J ) 4.8 Hz, â-pyrrole), 8.82 (4 H, s, â-pyrrole), 8.88 (2 H, d, J ) 4.8
Hz, â-pyrrole). MALDI-TOF-MS C44H27Cl2IN4: calcd av mass, 808.1;
obsd m/z, 809.0. λabs(toluene)/nm: 421, 514, 547, 590, 646.
(13) Still, W. C.; Kahn, M.; Mitra, A. J. Org. Chem. 1978, 43, 2923.
(14) (a) Wagner, R. W.; Johnson, T. E.; Lindsey, J. S. J. Am. Chem. Soc.
1996, 118, 11166-11180. (b) Nishino, N.; Wagner, R. W.; Lindsey,
J. S. J. Org. Chem. 1996, 61, 7534-7544.
(15) Kajigaeshi, S.; Kakinami, T.; Yamasaki, H.; Fujisaki, S.; Okamoto,
T. Bull. Chem. Soc. Jpn. 1988, 61, 600-602.
(16) Kutepov, D. F.; Kholkhlov, D. N.; Tuzhilkina, V. L. Zh. Obshch. Khim.
1960, 30, 2470.
Zinc(II) 5,10,15-Triphenyl-20-[2,6-dichloro-4-{2-(trimethylsilyl)-
ethynyl}phenyl]porphyrin (ZnH(Cl2)). Pyrrole (349 µL, 5.05 mmol),
benzaldehyde (384 µL, 3.78 mmol), and 2,6-dichloro-4-[2-(trimethyl-
(17) Lamm, B.; Andersson, B. Ark. Kemi 1966, 25, 367.

1194 Inorganic Chemistry, Vol. 37, No. 6, 1998
Strachan et al.
silyl)ethynyl]benzaldehyde (5) (0.34 g, 1.25 mmol) were dissolved in
515 mL of CH2Cl2 under an argon atmosphere. After 10 min,
BF3‚O(Et)2 (0.76 mL of a 2.65 M stock solution, 1.51 mmol) was added
via syringe with vigorous stirring. After addition was complete, the
reaction mixture was stirred for 1 h at room temperature. DDQ (1.15
g, 5.05 mmol) was added and, after 1 h of stirring at room temperature,
the solvent was removed in vacuo. The crude reaction mixture was
passed through a short silica column (hexanes-CH2Cl2 (3:2)). The
mixture of porphyrins was dissolved in 25 mL of CH2Cl2, and a solution
of Zn(OAc)2‚2H2O (445 mg, 2.03 mmol) in 5 mL of methanol was
added. The reaction mixture was stirred under an argon atmosphere
at room temperature overnight. The solvents were removed in vacuo,
and wet-flash column chromatography on silica (hexanes-CH2Cl2 (4:
1)) afforded a pink/purple solid (190 mg, 18.0%). 1H NMR (300 MHz,
CDCl3): 0.36 (9 H, s, Si(CH3)3), 7.75 (9 H, m, Ar-H), 7.90 (2 H, s,
Ar-H), 8.21 (6 H, m, Ar-H), 8.74 (2 H, d, J ) 4.7 Hz, â-pyrrole),
8.92 (4 H, s, â-pyrrole), 8.95 (2 H, d, J ) 4.7 Hz, â-pyrrole). MALDI-
TOF-MS C49H34Cl2N4SiZn: calcd av mass, 840.1, obsd m/z, 841.1.
dimer eluted as the second band, contaminated with some HMWM
and monomer. The dimer was reloaded onto the SEC column and
eluted with THF. The desired fraction was concentrated to dryness
and passed through a silica column (hexanes-CH2Cl2 (3:2)) to give
the title compound (free of HMWM and monomer by analytical SEC)
as a purple solid (67 mg, 75%) 1H NMR (300 MHz, CDCl3): -2.65 (2
H, s, NH), 7.78 (18 H, m, Ar-H), 8.11 (4 H, m, Ar-H), 8.24 (12 H,
m, Ar-H), 8.75 (4 H, d, J ) 4.7 Hz, â-pyrrole), 8.84 (4 H, s, â-pyrrole),
8.95 (4 H, s, â-pyrrole), 9.02 (4 H, d, J ) 4.7 Hz, â-pyrrole); MALDI-
TOF-MS C90H52Cl4N8Zn: calcd av mass, 1448.2, obsd m/z, 1447.1.
λ
abs(toluene)/nm: 426, 515, 550, 591, 646.
4,4′-Bis[10,15,20-triphenyl-5-porphinyl]-3,5,3′,5′-tetramethyl-
diphenylethyne (Fb2B(CH3)4). Samples of FbIH(CH3)2 (33 mg, 43
µmol) and FbH(CH3)2′ (30 mg, 45 µmol) were coupled following the
standard procedure (2 h), affording the title compound (free of HMWM
1
and monomer by analytical SEC) as a purple solid (39 mg, 70%). H
NMR (300 MHz, CDCl3): -2.66 (4 H, s, NH), 1.95 (12 H, s, CH3),
7.77 (18 H, m, Ar-H), 7.82 (4 H, s, Ar-H), 8.26 (12 H, m, Ar-H),
8.75 (4 H, d, J ) 4.7 Hz, â-pyrrole), 8.85 (8 H, s, â-pyrrole), 8.87 (4
H, d, J ) 4.7 Hz, â-pyrrole), LD-MS C94H66N8 calcd av mass, 1306.5;
obsd m/z, 1305.6. λabs(toluene)/nm: 422, 513, 547, 592, 646.
4,4′-Bis[zinc(II) 10,15,20-triphenyl-5-porphinyl]-3,5,3′,5′-tetra-
chlorodiphenylethyne Zn2B(Cl4). A sample of ZnFbB(Cl4) (14 mg,
9.64 µmol) was dissolved in 5 mL of CH2Cl2, and then a methanolic
solution of Zn(OAc)2‚2H2O (5 mg, 22.8 µmol, 500 µL of methanol)
was added. The reaction mixture was stirred overnight at room
temperature and was checked prior to workup by TLC (hexanes-
CH2Cl2 (1:1)). The solvents were removed in vacuo, and wet-flash
column chromatography on silica (hexanes-CH2Cl2 (3:2)) afforded a
pink/purple solid (14.6 mg, 100%). 1H NMR (300 MHz, CDCl3): 7.78
(18 H, m, Ar-H), 8.11 (4 H, s, Ar-H), 8.24 (12 H, m, Ar-H), 8.84
(4 H, d, J ) 4.7 Hz, â-pyrrole), 8.94 (8 H, s, â-pyrrole), 9.02 (4 H, d,
J ) 4.7 Hz, â-pyrrole). LD-MS C90H50Cl4N8Zn2: calc av mass, 1510.2;
obsd m/z, 1510.2. λabs(toluene)/nm: 427, 550.
4,4′-Bis[cupric(II) 10,15,20-triphenyl-5-porphinyl]-3,5,3′,5′-tet-
rachlorodiphenylethyne (Cu2B(Cl4)). A sample of ZnFbB(Cl4) (10
mg, 6.9 µmol) was dissolved in 5 mL of CH2Cl2 and treated with TFA
(5 µL, 65 µmol). The demetalation was complete after 10 min, as
evidenced by TLC. Et3N (14 µL, 98 µmol) was added and the reaction
mixture stirred for another 10 min. The solution was washed with
10% w/v NaHCO3 (5 mL) and dried (Na2SO4) and the solvent removed
in vacuo. Wet-flash column chromatography on silica (hexanes-
CH2Cl2 (4:1)) afforded Fb2B(Cl4) as a purple solid. A sample of
Fb2B(Cl4) (9.5 mg, 6.9 µmol) was dissolved in 5 mL of CH2Cl2, and
then a methanolic solution of Cu(OAc)2‚2H2O (28 mg, 138 µmol, 500
µL of methanol) was added. The reaction mixture was stirred overnight
at room temperature and was checked prior to work-up by TLC
(hexanes-CH2Cl2 (2:1)). The solvents were removed in vacuo, and
wet-flash column chromatography on silica (hexanes-CH2Cl2 (4:1))
afforded a pink/purple solid (10 mg, 99%). LD-MS C90H50Cl4Cu2N8:
calcd av mass, 1508.2; obsd m/z, 1509.5. λabs(toluene)/nm: 421, 542.
4,4′-Bis[cupric(II) 10,15,20-triphenyl-5-porphinyl]-3,5,3′,5′-tet-
ramethyldiphenylethyne (Cu2B(CH3)4). A sample of Fb2B(CH3)4 (15
mg, 11.4 µmol) was dissolved in 5 mL of CH2Cl2, and then a methanolic
solution of Cu(OAc)2‚2H2O (46 mg, 230 µmol, 500 µL of methanol)
was added. The reaction mixture was stirred overnight at room
temperature and was checked prior to work-up by TLC (hexanes-
CH2Cl2 (2:1)). The solvents were removed in vacuo, and wet-flash
column chromatography on silica (hexanes-CH2Cl2 (4:1)) afforded a
pink/purple solid (16 mg, 99%). LD-MS C94H62Cu2N8: calcd av mass,
1428.4; obsd m/z, 1426.9. λabs(toluene)/nm: 420, 540.
λ
abs(toluene)/nm: 423, 558.
Zinc(II) 5,10,15-Triphenyl-20-[2,6-dichloro-4-ethynylphenyl]por-
phyrin (ZnH(Cl2)′). A sample of ZnH(Cl2) (145 mg, 0.17 mmol) was
dissolved in 10 mL of freshly distilled THF. Tetrabutylammonium
fluoride on silica (345 mg, 1.0-1.5 mmol F/g resin) was added and
the reaction mixture stirred under argon for 30 min. The solvent was
removed in vacuo, and wet-flash column chromatography on silica
(hexanes-CH2Cl2 (7:3)) afforded a pink/purple solid (118 mg, 90%).
1H NMR (300 MHz, CDCl3): 3.37 (1 H, s, CH), 7.75 (9 H, m, Ar-
H), 7.92 (2 H, s, Ar-H), 8.21 (6 H, m, Ar-H), 8.74 (2 H, d, J ) 4.7
Hz, â-pyrrole), 8.92 (4 H, s, â-pyrrole), 8.96 (2 H, d, J 4.7 ) Hz,
â-pyrrole). LD-MS C46H26Cl2N4Zn: calcd av mass, 768.0; obsd m/z,
767.4. λabs(toluene)/nm: 424, 550.
5,10,15-Triphenyl-20-[2,6-dichloro-4-ethynylphenyl]porphyrin (Fb-
H(Cl2)′). A sample of zinc porphyrin ZnH(Cl2)′ (15 mg, 19.5 µmol)
was dissolved in 5 mL of CH2Cl2 and treated with TFA (12 µL, 156
µmol). The demetalation was complete after 10 min, as evidenced by
TLC. Triethylamine (33 µL, 234 µmol) was added and the reaction
mixture stirred for another 10 min. The solution was washed with
10% w/v NaHCO3 (5 mL) and dried (Na2SO4) and the solvent removed
in vacuo. Wet-flash column chromatography on silica (hexanes-
CH2Cl2 (4:1)) afforded a purple solid (13.7 mg, 100%). 1H NMR (300
MHz, CDCl3): -2.70 (2 H, s, NH), 3.38 (1 H, s, CH), 7.75 (9 H, m,
Ar-H), 7.92 (2 H, s, Ar-H), 8.25 (6 H, m, Ar-H), 8.65 (2 H, d, J 4.7
Hz, â-pyrrole), 8.82 (4 H, s, â-pyrrole), 8.88 (2 H, d, J ) 4.7 Hz,
â-pyrrole). LD-MS C46H28Cl2N4: calcd av mass, 706.2; obsd m/z,
706.7. λabs(toluene)/nm: 420, 514, 546, 590, 646.
5,10,15-Triphenyl-20-[4-iodo-2,6-dimethylphenyl[porphyrin (FbI-
H(CH3)2). See ref 14a.
5,10,15-Triphenyl-20-[4-ethynyl-2,6-dimethylphenyl]porphyrin (Fb-
H(CH3)2′). See ref 14a.
Procedure for Performing Pd-Coupling Reactions: 4-[Zinc(II)
10,15,20-triphenyl-5-porphinyl]-4′-[10,15,20-triphenyl-5-porphinyl]-
3,5,3′,5′-tetrachlorodiphenylethyne (ZnFbB(Cl4)). 5,10,15-Triphenyl-
20-(2,6-dichloro-4-iodophenyl)porphyrin (FbIH(Cl2)) (50 mg, 61.7
µmol) and zinc(II) 5,10,15-triphenyl-20-[2,6-dichloro-4-ethynylphenyl]-
porphyrin (ZnH(Cl2)′) (55 mg, 64.9 µmol) were added to a 50 mL
round-bottom flask containing 20.8 mL of toluene and 4.2 mL of Et3N.
The flask was fitted with a reflux condenser, and a glass pipet was
inserted through the top. A stream of argon was passed through the
pipet into the solution for 15 min. The pipet was raised and the reaction
vessel purged for a further 10 min. AsPh3 (19.0 mg, 62.1 µmol) and
Pd2(dba)3 (8.3 mg, 9.1 µmol) were then added and the reaction vessel
was placed in an oil bath at 35 °C. The reaction mixture was stirred
at 35 °C until SEC analyses showed the absence of starting material (2
h). The solvents were removed in vacuo, and wet-flash column
chromatography on silica (hexanes-CH2Cl2 (4:1)) removed all traces
of the AsPh3. The eluting solvent was then changed (hexanes-
CH2Cl2 (1:1)) to elute monomeric porphyrins, desired dimer, and higher
molecular weight material (HMWM). The mixture of porphyrins was
concentrated to dryness, dissolved in the minimum amount of THF,
loaded onto the top of a preparative SEC column (2.5 cm × 45 cm)
(Bio-Beads SX-1 poured in THF), and eluted with THF. The desired
B. Physical Methods. The static and time-resolved absorption and
fluorescence studies were performed on samples prepared in toluene
or DMSO at room temperature. Toluene (EMI, Omnisolve) was
distilled from sodium; DMSO (Mallinckrodt) was used without further
purification. The samples for time-resolved fluorescence measurements
were degassed by several freeze-pump-thaw cycles on a high-vacuum
line. The samples for static fluorescence and time-resolved absorption
measurements were not degassed. The RR, electrochemical, and EPR
studies were performed on samples prepared in CH2Cl2. CH2Cl2
(Aldrich, HPLC Grade) was purified by vacuum distillation from P2O5

Tetrachlorodiarylethyne-Linked Porphyrin Dimers
Inorganic Chemistry, Vol. 37, No. 6, 1998 1195
followed by another distillation from CaH2. For the electrochemical
studies, tetrabutylammonium hexafluorophosphate (TBAH; Aldrich,
recrystallized three times from methanol and dried under vacuum at
110 °C) was used as the supporting electrolyte. The solvents were
degassed thoroughly by several freeze-pump-thaw cycles prior to use.
Scheme 1. Aldehyde Precursors for Preparing the
Porphyrinsa
1. Static Absorption and Fluorescence Spectroscopy. Static
absorption and emission measurements were performed as described
previously.6,8,9 The extinction coefficients (M-1cm-1) for the porphyrin
monomers in toluene at room temperature at the Soret band and
Q-region maximum (510 nm for Fb complexes or 550 nm for the Zn
complexes) were determined to be as follows: ZnH(Cl2), 325 000 and
18 000; ZnH(Cl2)′, 349 000 and 21 000; FbH(Cl2)′, 387 000 and 19 500;
FbIH(Cl2), 211 000 and 11 000.
2. Time-Resolved Absorption and Fluorescence Spectroscopy.
Transient absorption data were acquired as described elsewhere.8,9,18
Samples were excited with 0.2-ps, 582-nm, 100-µJ flashes at 10 Hz
and had a concentration of ∼0.2 mM for measurements between 450
and 560 nm and of ∼0.6 mM for measurements to the red of 595 nm.
Fluorescence lifetimes were acquired as described previously8,9 on an
apparatus having a time response of ∼0.5 ns, using samples having a
concentration of ∼0.2 mM.
a Reagents and conditions: (i) Benzyltrimethylammonium dichlor-
oiodate in MeOH/CH2Cl2, ∆, 2 h; (ii) H2SO4/H2O, NaNO2, NaBF4, 0
°C; (iii) CuCN/NaCN, H2O; (iv) DIBAL-H, CH2Cl2, 0 °C, HCl/H2O;
(v) (trimethylsilyl)acetylene, Pd2(dba)3, AsPh3, toluene/Et3N, 35 °C.
3. RR Spectroscopy. The RR spectra were acquired as described
previously,9 on samples that typically had a concentration of 0.05 mM
(B-state excitation) or 0.5 mM (Q-state excitation). The laser power
at the sample was typically 3 mW, and the spectral resolution was ∼3
attribute to the formation of byproducts resulting from halogen-
halogen exchange.23 To avoid this problem the stable tetrafluo-
roborate diazonium salt (2) was synthesized.17 Compound 1
was added to hot sulfuric acid to form the anilinium sulfate
and then reacted with sodium nitrite to give the diazonium ion,
which was isolated as the salt upon addition of sodium
tetrafluoroborate. Treatment of the diazonium salt with a
cuprous cyanide mixture gave the 4-iodobenzonitrile (3) in 24%
yield. The major byproducts as seen by GC-MS were 2,4,6-
trichloroaniline, dichlorodiiodobenzene, and 3,5-dichloroiodo-
benzene. The benzonitrile 3 was reduced using 1.1 molar equiv
of DIBAL-H, affording aldehyde 4 in 80% yield. Treatment
of 2,6-dichloro-4-iodobenzaldehyde (4) with (trimethylsilyl)-
acetylene under Pd-mediated coupling conditions (35 °C for 48
h under argon)24 gave 5 in 75% yield after Kugelrohr distillation
(Scheme 1). The unwanted enyne product, present at ∼3% as
evidenced by GC-MS, was readily removed by column chro-
matography on silica.
cm-1 at a Raman shift of 1600 cm-1
.
4. Electrochemistry. The oxidized complexes were prepared and
manipulated in a glovebox as described previously.9 The integrity of
the samples was checked by cyclic voltammetry after each successive
oxidation. In all cases, the cyclic voltammograms were reproducible
upon repeated scans and exhibited no scan rate dependence in the 20-
100 mV/s range. Studies were performed immediately after oxidation
and transfer of the samples to an optical cuvette (absorption) or quartz
capillary (EPR).
5. EPR Spectroscopy. The EPR spectra were recorded as described
previously.9 The sample concentration for all the experiments was
typically 0.05 mM. The microwave power and magnetic field
modulation amplitude were typically 5.7 mW and 0.32 G, respectively.
Results and Discussion
A. Synthesis of the Porphyrin Building Blocks and
Arrays. The synthesis of the arrays relies on a modular
building-block approach that involves repetitive use of a small
set of chemical reactions.14,19,20 The arrays are constructed by
joining free base or metalloporphyrins that bear iodo or ethyne
groups. The porphyrins in turn are constructed from pyrrole
and the appropriate benzaldehyde using a room-temperature
synthetic method.21,22 Incorporating the desired porphyrin
substituents in the aldehyde precursor minimizes synthetic
manipulation of the porphyrins.
2. A3B Porphyrin Building Blocks. Porphyrin building
blocks bearing a functional group (iodo, ethyne) at one of the
four meso positions provide the basis for constructing the
dimeric arrays. These A3B porphyrin building blocks can be
obtained through mixed aldehyde-pyrrole condensations using
the two-step one-flask room-temperature synthesis21,22 (Scheme
2). The method of mixed aldehyde-pyrrole condensations
affords a mixture of six porphyrins. The separability of mixtures
of porphyrins by adsorption chromatography depends on the
facial encumbrance due to ortho substituents and on the different
polarity of all of the aryl substituents.20 For mixtures that are
difficult to separate, metalation of the mixture of Fb porphyrins
affords the Zn porphyrins, which often can be easily separated
via column chromatography.8,14 The introduction of zinc
imparts a polar site which enhances the affinity of the porphyrins
for silica gel and accentuates effects of facial encumbrance.
Thus, for the preparation of ethyne-substituted porphyrins, crude
aldehyde-pyrrole condensation mixtures were concentrated and
passed over a flash silica column to remove reagents and
byproducts. The porphyrin mixtures were treated with metha-
nolic zinc acetate, and the resulting Zn porphyrins were
1. Aldehydes. The 2,6-dichloro-substituted benzaldehydes
required for the synthesis of arrays bearing chloro substituents
in the linker were prepared according to Scheme 1. The reaction
15
of 2,6-dichloroaniline with BTMAICl2 in CH2Cl2-MeOH at
reflux for 2 h gave the 2,6-dichloro-4-iodoaniline (1) in 40%
yield. The diazotization reaction failed when concentrated
hydrochloric acid and sodium nitrite were used, which we
(18) Kirmaier, C.; Holten, D. Biochemistry 1991, 30, 609-613.
(19) Lindsey, J. S. In Modular Chemistry; Michl, J., Ed., NATO ASI Series
C: Mathematical and Physical Sciences; Vol. 499, Dordrecht, 1997;
Kluwer Academic Publishers: pp 517-528.
(20) Lindsey, J. S.; Prathapan, S.; Johnson, T. E.; Wagner, R. W.
Tetrahedron 1994, 50, 8941-8968.
(21) Lindsey, J. S.; Wagner, R. W. J. Org. Chem. 1989, 54, 828-836.
(22) Lindsey, J. S.; Schreiman, I. C.; Hsu, H. C.; Kearney, P. C.;
Marguerettaz, A. M. J. Org. Chem. 1987, 52, 827-836.
(23) Lamm, B.; Andersson, B. Acta Chem. Scand. 1969, 23, 3355-3360.
(24) Wagner, R. W.; Johnson, T. E.; Li, F.; Lindsey, J. S. J. Org. Chem.
1995, 60, 5266-5273.

1196 Inorganic Chemistry, Vol. 37, No. 6, 1998
Scheme 2. Separation of Porphyrin Building Blocks
Strachan et al.
Scheme 3. Synthesis of Dimeric Arrays
Figure 1. Absorption spectra in toluene at room temperature of the
monomeric building blocks and the dimeric arrays.
building blocks afforded the dimer Fb2B(CH3)4 in comparable
yield (Chart 2). In each case there were no detectable impurities
(>99%) based on integrated peak area in the analytical SEC.
The bis-Zn dimer Zn2B(Cl4) was prepared in quantitative yield
by treatment of the corresponding ZnFb dimer with methanolic
zinc acetate. The zinc insertion was quantitative. The bis-Cu
dimers (Cu2B(Cl4), Cu2B(CH3)4) were prepared by treating the
corresponding Fb2 dimers (obtained by direct synthesis or by
demetalating the ZnFb dimers with trifluoroacetic acid) with
methanolic cupric acetate (Scheme 3). These metal insertions
also were quantitative. This building block approach provided
ready access to small quantities of the various dimers.
separated on a silica column to give the desired A3B porphyrin
(Chart 2). The corresponding Fb porphyrins were prepared by
demetalation. On the other hand, the desired Fb iodo porphyrin
FbIH(Cl2) (Chart 2) could be separated from its mixture as the
Fb porphyrin by column chromatography on silica without the
need for metalation and demetalation. The Zn ethynyl porphyrin
ZnH(Cl2)′ was prepared by deprotecting the trimethylsilyl-
protected porphyrin with tetrabutylammonium fluoride on silica.
3. Dimeric Arrays. The preparation of the dimeric arrays
relies on Pd-mediated coupling reactions24 of ethynylporphyrins
and iodoporphyrins (Scheme 3). The reaction of the Fb
iodoporphyrin FbIH(Cl2) and the Zn ethynylporphyrin ZnH-
(Cl2)′ with Pd2(dba)3 and AsPh3 in toluene/triethylamine (5:1)
at 35 °C under argon for 2 h afforded the diphenylethyne-linked
dimer ZnFbB(Cl4). The crude reaction mixture consisted of
small amounts of higher molecular weight material (uncharac-
terized), desired ZnFbB(Cl4), monomeric porphyrins, and
reagents. The progress of the reaction could be assessed by
silica TLC or by analytical SEC. For preparative purification,
a flash silica column with hexanes-CH2Cl2 (1:1) removed
AsPh3 and left the Pd residue on the top of the column. Passing
the resultant mixture over a preparative SEC column in THF
(twice) removed the unwanted higher molecular weight material
and porphyrin monomers. The fraction containing the desired
ZnFbB(Cl4) dimer was then passed over a silica column eluting
with hexanes-CH2Cl2 (2:1). Fractions were assessed for purity
by analytical SEC. This method afforded 67 mg of ZnFbB-
(Cl4) in 75% yield. This same approach with other porphyrin
B. Physical Properties of the Neutral Complexes. 1.
Absorption Spectra. The absorption spectra of selected
monomeric and dimeric chlorine-containing porphyrins are
shown in Figure 1. The general spectral features of all the
compounds (strong B- and weak Q-bands) are similar to those
we have previously reported for other arrays and their building
blocks.2-9 The spectra of the dimers are approximately a
superposition of the absorption spectra of the Zn and Fb
porphyrin constituents, indicative of the relatively weak interac-
tions between the porphyrins. The absorption spectra of the
chlorine-containing arrays are very similar to those of their TPP
analogues.
2. Fluorescence Quantum Yields and Lifetimes. The
fluorescence yields and lifetimes of ZnFbB(Cl4), the non-
chlorinated, hindered analogue ZnFbB(CH3)4, and a number of
monomeric building blocks in toluene and DMSO are collected
in Table 1. The fluorescence spectra of ZnFbB(Cl4) and the
chlorine-containing monomers are very similar to those reported
previously for the non-chlorinated complexes.6,8,9 The emission
from ZnFbB(Cl4) comes predominantly from the photoexcited
Fb porphyrin (Fb*) independent of whether the Zn or Fb
porphyrin is excited, as has been reported for ZnFbB(CH3)4 and

Tetrachlorodiarylethyne-Linked Porphyrin Dimers
Inorganic Chemistry, Vol. 37, No. 6, 1998 1197
Table 1. Singlet Excited-State Lifetimes and Fluorescence Yieldsa
Zn porphyrin
τ (ns) Φf
Fb porphyrin
τ (ns) Φf
porphyrin
dimers
ZnFbB(Cl4)
solvent
toluene 0.13
DMSO 0.19b
<0.002 4.6
0.038
0.033
4.0
ZnFbB(CH3)4 toluene 0.11, 0.090c <0.005c 12.9, 12.2c 0.10c
DMSO 0.12, 0.085c <0.006c 10.1, 10.8c 0.092c
monomers
ZnH(Cl2)
toluene 4.4
0.059
0.050
0.053
0.039c
0.033d
ZnH(Cl2)′ toluene 4.3
DMSO 3.4
ZnH(CH3)2 toluene 2.3c
ZnTPP
toluene 2.2, 2.0c
FbH(Cl2)′ toluene
DMSO
FbH(CH3)2 toluene
4.9
0.034
0.037
0.11c
Figure 2. Transient difference spectra acquired following excitation
of ZnFbB(Cl4) in toluene at room temperature with a 0.2-ps flash at
582 nm. The spectra for the array was acquired with time delays of 1
ps (solid traces) or 800 ps (dashed traces). Note that the data in the red
region were acquired with a sample concentration ∼10-fold greater
than that used for the blue region. Consequently, a quantitative
comparison of the absorption changes in the two regions should not
be made. The inset shows a representative kinetic trace at 515 nm with
a fit to a dual exponential which takes into account both the fast decay
of Zn* and the slow decay of Fb*. The Fb* lifetime was fixed at the
value of 4.6 ns obtained from the fluorescence lifetime measurements
(Table 1).
4.1
12.7c
TPP
toluene
13.2, 11.5c 0.11d
a Data were collected at ∼295 K. The excited-state lifetimes were
determined by time-resolved fluorescence ((5%) in degassed solutions,
except for lifetimes of the excited Zn porphyrins in the dimers, which
were measured using transient absorption spectroscopy ((10%). The
Zn* lifetimes reported from the transient absorption studies are derived
from dual exponential fits to the data (see Figure 2), with the longer
component fixed at the Fb* lifetime determined via fluorescence decay.
b For ZnFbB(Cl4) in DMSO, the time constant obtained in the Soret-
region transient absorption (450-490 nm) is somewhat longer (270
ps) than that obtained in the 500-550-nm region of the Q-band
bleachings (115 ps). An average of the time constants determined in
the two regions is reported for each sample. Further studies in other
polar solvents are required to ascertain the origin of this wavelength
dependence, which could derive from inhomogeneities associated with
differing states of metal ligation in DMSO. c Reference 6. Note that
the fluorescence yields of the Zn porphyrins were scaled from those in
that paper by a factor of 0.033/0.030 to account for a revised value
(see footnote d) of the fluorescence yield of the ZnTPP standard. d The
yield of 0.033 for ZnTPP was used as the reference for the Zn
porphyrins, and the yield of 0.11 for TPP was used as the reference
for the Fb porphyrins.38,39
properties of photonic devices, has been explored in a set of 40
Fb porphyrins and metalloporphyrins containing differing
numbers of various halogenated aryl units.25
3. Time-Resolved Absorption Spectra. The energy-transfer
rate from Zn* to Fb in ZnFbB(Cl4) was probed using ultrafast
absorption difference spectroscopy. Representative data are
shown in Figure 2. The 582-nm excitation flashes excite both
Zn and Fb porphyrins. Therefore, the absorption difference
spectra acquired shortly after excitation contain features char-
acteristic of Zn* in those arrays in which the Zn porphyrin was
excited and features characteristic of Fb* in those arrays in
which the Fb porphyrin was excited.
Assignments of the spectra shown in Figure 2 were made by
direct analogy with the those previously described for other Zn
porphyrin arrays such as ZnFbB(CH3)4 and the unhindered
parent dimer ZnFbU.6,8,9 In particular, between 1 and 800 ps
there is simultaneous decay of the Zn porphyrin bleaching at
550 nm, increased Fb porphyrin bleaching at 515 nm, and
increased Fb* stimulated emission at 715 nm. These changes
reflect simultaneous decay of Zn* and formation of Fb* in the
fraction of dimers in which the Zn porphyrin was originally
excited. The results demonstrate that energy transfer, Zn*Fb
f ZnFb*, is the primary mode of Zn* decay in these arrays
(vide infra). The absorption difference spectra observed fol-
lowing complete decay of Zn* (e.g., 800-ps spectrum) can be
assigned to Fb*.
The insets in Figure 2 show representative kinetic data and
a fit giving a time constant of 130 ( 15 ps for the Zn* lifetime
of ZnFbB(Cl4) in toluene. The same time constant is obtained
in other regions of the spectrum where there are sufficiently
large changes in ∆A as a function of time. Subsequent decay
of Fb* occurs with a time constant of 4.6 ns for ZnFbB(Cl4) in
toluene, as determined via the fluorescence decay profiles (vide
supra). Similar results are obtained in DMSO (Table 1).
4. The Rates, Yields, and Mechanism of Energy Transfer.
The intrinsic rate constant (ktrans) and quantum yield (Φtrans) for
the Zn*Fb f ZnFb* energy-transfer process were obtained from
a number of other ZnFb arrays.6,8,9 This observation is
indicative of fast and highly efficient energy transfer from the
photoexcited Zn porphyrin (Zn*) to the Fb porphyrin, a process
that is quantitated from the time-resolved absorption data (vide
infra).
Although this study focuses on the energy-transfer process
from Zn* to the Fb porphyrin in ZnFbB(Cl4) relative to that in
ZnFbB(CH3)4, we also examined the photophysical properties
of Fb* in the dimers and their building blocks, as well as the
Zn* lifetime in the building blocks (Table 1). The primary
findings are as follows: (1) The Fb* lifetime and fluorescence
yield of ZnFbB(Cl4) are reduced by about 60% from the values
for ZnFbB(CH3)4. The Fb* lifetimes and emission yields of
both dimers are comparable to those of the corresponding
monomeric building blocks. These results, together with the
static absorption and emission spectra, indicate that the reduced
Fb* lifetimes and emission yields in ZnFbB(Cl4) versus those
for ZnFbB(CH3)4 result from enhanced nonradiative decay of
Fb* rather than from a change in the natural radiative rate. (2)
The lifetimes of Fb* in the monomers and dimers are similar
to one another both in toluene and in the polar solvent DMSO,
indicating that charge-transfer quenching of Fb* by the Zn
porphyrin in ZnFbB(Cl4) is not appreciable (<10%), as has also
been observed for ZnFbB(CH3)4.6 (3) The Zn* lifetimes and
fluorescence yields of ZnH(Cl2)′ and ZnH(Cl2) are increased
almost 2-fold relative to ZnH(CH3)2 and ZnTPP. Thus, the
presence of one 2,6-dichloroaryl group has multiple opposing
effects on excited-state lifetimes and yields. The interplay of
these factors, which offer the prospect of fine-tuning the
(25) Yang, S. I.; Seth, J.; Strachan, J. P.; Gentemann, S.; Kim, D.; Holten,
D.; Lindsey, J. S.; Bocian, D. F. J. Phys. Chem. submitted for
publications.

1198 Inorganic Chemistry, Vol. 37, No. 6, 1998
Strachan et al.
Table 2. Half-Wave Potentialsa for Oxidation of the Porphyrins of
the Various Arrays
the kinetic data using the following relationships:
-1
ktrans ) (τZn*
)
-1 - (τ°Zn*
)
(1)
(2)
Zn porphyrin
Fb porphyrin
E1/2 (1)
E1/2 (2)
E1/2 (1)
E1/2 (2)
Φtrans ) ktrans τZn* ) 1 - τZn*/τ°Zn*
FbH(Cl2)′
ZnH(Cl2)′
ZnFbB(Cl4)
Zn2B(Cl4)b
0.83
1.19
0.68
0.68
0.68
1.00
1.00
1.00
Here, τZn* is the measured lifetime of the excited Zn porphyrin
and τ°Zn* is the Zn* lifetime of the relevant monomer in toluene
(Table 1). This analysis assumes that the dramatic reduction
in the Zn* lifetime in the dimer exclusively reflects energy
transfer to the Fb porphyrin. In particular, it assumes that the
inherent rate constants for the fluorescence, internal conversion,
and intersystem crossing decay pathways of Zn* have the same
values in the dimer as in the corresponding monomer, and that
charge-transfer quenching of Zn* by the Fb porphyrin makes a
minimal (<10%) contribution to the photodynamics in toluene.
The latter is evidenced by the data given above for ZnFbB-
(Cl4) and previously for a number of other ZnFb dimers.6,8,9
Using eqs 1 and 2 and the measured lifetimes in Table 1, the
rate and yield of energy transfer from Zn* to the Fb porphyrin
in ZnFbB(Cl4) are calculated to be ktrans ) (134 ps)-1 and Φtrans
) 97%. Energy transfer to the Fb porphyrin dominates the
decay of Zn*. This fact is evident from the observation that
the calculated energy-transfer rate constant is nearly equal to
the observed excited-state decay rate of (130 ps)-1. The energy-
transfer rate and yield for ZnFbB(Cl4) are comparable to that
of the torsionally constrained dimer ZnFbB(CH3)4 (ktrans ) (115
ps)-1, Φtrans ) 95%) but differ significantly from the values
0.83
1.19
a Obtained in CH2Cl2 containing 0.1 M TBAH. E1/2 vs Ag/Ag+; E1/2
of FeCp2/FeCp2+ ) 0.22 V; scan rate ) 0.1 V/s. Values are (0.01 V.
bThe redox waves of the two Zn porphyrins are not resolved by cyclic
voltammetry.
tion factor that might occur due to the presence of linker
substituents are insufficient to account for the differences in
energy-transfer rates of the various dimers.
The dominance of the TB mechanism for ZnFbB(Cl4) (and
the other arrays) can be seen by assuming that the observed
energy-transfer rate (ktrans) is determined by the additive effects
of TB and TS processes (eq 3).6,8,9 The fractional amounts of
the TB (øTB) and TS (øTS) contributions can then be estimated
(eqs 4 and 5). The resulting TB energy-transfer rate for ZnFbB-
ktrans ) kTB + kTS
øTB ) kTB/ktrans
øTB + øTS ) 1
(3)
(4)
(5)
for the unconstrained dimer ZnFbU (ktrans ) (24 ps)-1, Φtrans
99%).
)
(Cl4) is (153 ps)-1, and this pathway accounts for ∼88% of the
total energy transfer. A similarly high percentage is found for
the other dimers (Table 3). Consequently, the TB rate in
ZnFbB(Cl4) is nearly identical to that in its sterically similar
counterpart ZnFbB(CH3)4 ((140 ps)-1) but is about 5-fold slower
than in the unhindered parent complex ZnFbU ((25 ps)-1).
Again, these comparisons reflect (1) the similar steric constraints
imposed by o-chloro and -methyl groups incorporated into the
diphenylethyne linker on electronic communication and (2) the
dominance of the sterically mediated electronic effect in ZnFbB-
(Cl4) over other potential effects of the o-chloro substituents.
5. RR Spectra. The high-frequency regions of the B-state
excitation (λexc ) 457.9 nm) RR spectra of ZnFbB(Cl4) and
Zn2B(Cl4) are shown in Figure 3. The scattering characteristics
of the porphyrin skeletal modes of the chlorine-containing arrays
are similar to those observed for other arylethyne-linked arrays
and are generally unremarkable.5,7 The key spectral feature
The finding of comparable energy-transfer rates in ZnFbB-
(Cl4) and ZnFbB(CH3)4 argues that the attenuation in electronic
communication in these dimers relative to ZnFbU stems from
steric constraints imposed by the o-chloro or o-methyl groups
of the linker. The ortho substituents impede rotation of the
linker aryl groups with respect to the porphyrin macrocycles,
as we have previously discussed for ZnFbB(CH3)4.6 Hindered
rotation increases the average torsional angle between the linker
and the porphyrin, thereby diminishing orbital overlap. This
effect dominates other effects of the o-chloro substituents, such
as electron withdrawal and interaction of the p orbitals of the
o-chloro substituents with the p orbitals of the meso-carbon11
(vide infra).
The realization that steric constraints on electronic com-
munication in ZnFbB(Cl4) (as in ZnFbB(CH3)4) underlie the
reduced rate and efficiency of energy transfer relative to those
observed for the unhindered analogue ZnFbU has important
consequences. In particular, it provides additional experimental
support for the contention that the predominant mechanism for
energy transfer in the diphenylethyne-linked porphyrin arrays
is through-bond (TB) rather than through-space (TS) in nature.6
Indeed, calculations based on the Fo¨rster TS mechanism26
indicate that the various dimers (ZnFbB(Cl4), ZnFbB(CH3)4, and
ZnFbU) have TS rates (kTS) that differ by less than a factor of
2 (Table 3), far smaller than the experimentally observed 5-fold
difference in energy-transfer rates in the torsionally constrained
versus unconstrained arrays. The Fo¨rster TS rate for ZnFbB-
(Cl4) is calculated to be (1.071 ns)-1, in contrast to the much
shown in the figure is the ethyne stretching mode, νCtC,
5,7 which
is observed at ∼2215 cm-1. The RR intensities of the νCtC
modes of ZnFbB(Cl4) and Zn2B(Cl4) are modest compared with
those of the porphyrin skeletal modes. However, the RR
intensities of the νCtC modes (compared with the porphyrin
modes) of the chlorine-containing dimers are somewhat larger
(∼3-fold) than those reported previously6 for the analogous
modes of ZnFbB(CH3)4 and Zn2B(CH3)4. The increased RR
intensities observed for the νCtC modes of the two different
classes of torsionally constrained dimers indicate that the
B-excited-state electronic coupling between the ethyne group
and the π system of the porphyrin ring (which dictates the
relative RR intensities of the νCtC versus porphyrin skeletal
modes5,7) is somewhat larger for the chlorine-containing arrays.
Although the linker-mediated electronic coupling in the
B-excited state is interesting in its own right, the energy transfer
involves the Q-excited states. However, as is the case for other
ZnFb and bis-Zn porphyrins, Q-state excitation RR spectra
cannot be obtained for the chlorine-containing dimers owing
faster observed rate of (0.134 ns)-1
. The Fo¨rster analysis
assumes that each of the dimers has the same value for the
orientation term of the transition dipoles of the Zn and Fb
porphyrins (κ2 ) 1.125).6 However, any differences in orienta-
(26) Lamola, A. A. In Energy Transfer and Organic Photochemistry;
Lamola, A. A., and Turro, N. J., Eds.; Interscience: New York 1969.

Tetrachlorodiarylethyne-Linked Porphyrin Dimers
Inorganic Chemistry, Vol. 37, No. 6, 1998 1199
Table 3. Through-Bond (TB) vs Through-Space (TS) Energy-Transfer Rates and Efficiencies for Various ZnFb Dimersa
b
b
d
e
porphyrin
ZnFbU
ZnFbB(CH3)4
ZnFbB(Cl4)
ꢀ510
ꢀ550
Φfc
τ (ns)c
J (cm6 mmol-1
)
ktrans-1 (ps)
Φtrans
kTS-1 (ps)
kTB-1 (ps)
øTB
øTS
19000
22000
19500
7800
8900
7750
0.035
0.032
0.050
2.38
2.26
4.4
2.94 × 10-14
3.53 × 10-14
2.65 × 10-14
24
115
134
0.99
0.95
0.97
745
644
1071
25
140
153
0.96
0.82
0.88
0.04
0.18
0.12
a All data were obtained in toluene at room temperature. b Peak extinction coefficients in M-1 cm-1 c For the Zn porphyrin in the absence of a
.
Fb porphyrin. d The Fo¨rster spectral overlap term is computed using selected monomers which best approximate those units in the dimer. The
spectral overlaps are generally in accord with those obtained for other porphyrin systems.2,6,40 e Calculated from eq 4.
Figure 3. High-frequency regions of the B-state excitation (λexc
457.9 nm) RR spectra of ZnFbB(Cl4) and Zn2B(Cl4) at room temper-
ature. The bands marked by asterisks are due to solvent.
)
Figure 4. RR spectra of Cu2B(Cl4) (left panel) and Cu2B(CH3)4 (right
panel) obtained with B-state (λexc ) 457.9 nm) and Q-state (λexc
530.9 nm) excitation at room temperature.
)
to interference from fluorescence.5,9 Therefore, complete sets
of RR spectra (B- and Q-state excitation) were instead acquired
for the bis-Cu complexes of the chlorine-containing dimers,
which exhibit little or no emission in the Q-state region. In
order to make a more direct comparison with the RR spectra of
Cu2B(Cl4), B- and Q-state excitation RR spectra were also
obtained for the Cu(II) analogue of the bis-hindered array we
have previously studied,7 Cu2B(CH3)4.
ethyne Q-excited-state electronic coupling is observed for the
unhindered dimer Cu2U, as is indicated by the significant
enhancement of its νCtC mode.9 These observations are in
accord with the relatively slow energy-transfer rates observed
for both ZnFbB(Cl4) and ZnFbB(CH3)4 relative to that of ZnFbU
(vide infra).
C. Physical Properties of the Oxidized Complexes. The
oxidized arrays were investigated to gain insight into how the
incorporation of o-chloro substituents on the linker modulates
the rates of hole/electron hopping.5,7 These rates are of interest
because no independent assessment of the rates of charge
transfer is available for the neutral arrays. Although the hole/
electron-hopping rates in the ground electronic states of the
cations are not expected to be equal to the charge-transfer rates
in the excited states of the neutrals, they at least provide some
measure of the factors which control this type of process.
1. Electrochemistry. The E1/2 values for oxidation of the
chlorine-containing dimeric arrays and the monomeric building
blocks are summarized in Table 2. The two E1/2 values listed
in the table correspond to the first and second oxidations of the
porphyrin ring.27 The redox characteristics of the chlorine-
Figure 4 shows the high-frequency regions of the RR spectra
of Cu2B(Cl4) (left panel) and Cu2B(CH3)4 (right panel) obtained
with B- and Q-state excitation. The absolute RR intensities
observed at the different excitation wavelengths cannot be
compared directly because the sample concentrations required
to obtain reasonable quality Q-state spectra are higher than those
necessary for B-state studies. The RR data obtained for both
Cu2B(Cl4) and Cu2B(CH3)4 show that the νCtC mode exhibits
appreciable RR intensity with excitation in the B-state region
(457.9 nm). This behavior parallels that observed for the
chlorine-containing bis-Zn and ZnFb arrays (Figure 3). In
contrast, both Cu2B(Cl4) and Cu2B(CH3)4 exhibit little enhance-
ment of the νCtC mode with excitation resonant with the Q-state
absorption. The minimal (or negligible) RR enhancement
observed for the νCtC modes of Cu2B(Cl4) and Cu2B(CH3)4
indicates that the Q-excited-state electronic coupling between
the ethyne group and the π system of the porphyrin is weak for
both of these bis-hindered arrays. Much stronger porphyrin-
(27) (a) Felton, R. H. In The Porphyrins; Dolphin, D., Ed.; Academic Press:
New York, 1978; Vol. V, pp 53-126. (b) Davis, D. G. Ibid.; pp 127-
152.

1200 Inorganic Chemistry, Vol. 37, No. 6, 1998
Strachan et al.
containing arrays closely resemble those of other arylethyne-
linked arrays we have previously investigated.5,7 The potentials
are positively shifted relative to the non-chlorinated analogues;
however, this shift is relatively small (∼0.06 V) and com-
mensurate with the small number of modestly electron-
withdrawing chloro substituents.28 In particular, for ZnFbB(Cl4),
four overlapping redox waves are observed, two each for the
Zn and Fb porphyrins. For Zn2B(Cl4), two redox waves are
observed whose peak-to-peak separations are ∼70 mV. Quan-
titative coulometry confirms that each wave corresponds to the
removal of two electrons. Differential pulse and square-wave
voltammetry on the Zn2B(Cl4) fails to resolve peaks due to the
individual one-electron oxidations; however, the peaks exhibit
asymmetries which indicate small inequivalences between the
redox potentials of the individual Zn porphyrins (50 mV or less).
As observed for non-chlorinated arylethyne-substituted por-
phyrins,5,7 the chlorine-containing arylethyne-substituted por-
phyrins do not exhibit any redox characteristics which would
suggest that there is any significant ground-state interaction
between the porphyrin and arylethyne group π systems. Indeed,
such porphyrin-arylethyne group interaction is not anticipated
because the E1/2 value for oxidation of diphenylethyne is 1.64
V versus SCE29 and its absorption λmax occurs at ∼300 nm,30
indicating that the highest occupied and lowest unoccupied
molecular orbitals of diphenylethyne span those of the porphy-
rins. The presence of four o-chloro substituents on the
diarylethyne group would tend to lower its potential and red-
shift its λmax; however, these effects are necessarily quite small
considering that the redox and optical characteristics of the
chloro-substituted porphyrin monomers and dimers are very
similar to those of the non-chlorine-containing analogues.5,7
Collectively, the redox characteristics of ZnFbB(Cl4) and Zn2B-
(Cl4) are indicative of relatively small ground-state electronic
interaction between the porphyrin constituents.
Figure 5. EPR spectra of [ZnH(Cl2)′]+ and [Zn2B(Cl4)]+n (n ) 1, 2)
obtained at 295 and 100 K (solid lines). The additional traces shown
for [ZnH(Cl2)′]+ and [Zn2B(Cl4)]+ at 295 K are simulated spectra. For
[ZnH(Cl2)′]+, the simulated spectrum was obtained with the following
parameters: a(14N) ) 1.39 G, a(1Ho-aryl) ) 0.36 G, Lorentzian fwhm
) 1.25 G. For [Zn2B(Cl4)]+, the simulated spectrum was obtained by
reducing the hyperfine splittings by a factor of 2, doubling the number
of interacting nuclei, and holding the line width constant.
line shape were observed for the concentrations used for the
EPR studies (e0.05 mM). Consequently, the differences
observed in the spectral features of the various complexes are
intrinsic and cannot be ascribed to intermolecular interactions.
The liquid-solution EPR spectrum of [ZnH(Cl2)′]+ exhibits
a poorly resolved nine-line hyperfine pattern due to interaction
of the unpaired electron with the four pyrrole 14N nuclei (Figure
2
5). This spectral pattern is characteristic of a A2u porphyrin
π-cation radical.35 This ground electronic state is expected for
the cation radical of a porphyrin containing a single 2,6-
dichloroaryl group. This is so because porphyrins containing
four 2,6-dichlorophenyl groups still exhibit a ground electronic
2. Absorption Spectra. The UV-vis absorption charac-
teristics of the mono- and dications of the arrays (not shown)
are typical of those of other porphyrin π-cation radicals, namely
weaker, blue-shifted B bands (relative to the neutral complexes)
and very weak, broad bands in the visible and near-infrared
regions.27a The absorption spectra of the oxidized complexes
appear to be a superposition of the spectra of the neutral and
cationic species of the different porphyrin units. This observa-
tion, like the results of the electrochemical studies,31-34 is
consistent with weak interactions between the constituent
porphyrins.
2
state which is predominantly A2u in character.11,36,37 Simula-
tions of the EPR spectrum of [ZnH(Cl2)′]+ (Figure 5, top left
panel) indicate that the 14N hyperfine splittings (a(14N) ∼ 1.39
G) are generally comparable to, but slightly smaller than, those
measured for [ZnTPP]+ or any other Zn porphyrin monomer
radical cations of this family (a(14N) ∼ 1.45-1.65 G).5,7,35 The
relatively small reduction in the 14N splitting relative to that of
non-chlorine-containing porphyrins is commensurate with the
modest electron-withdrawing capabilities afforded by the single
2,6-dichloroaryl group on the porphyrin (vide supra). In frozen
solution, no hyperfine structure is resolved for [ZnH(Cl2)′]+;
3. EPR Spectra. The EPR spectra of the oxidation products
of ZnH(Cl2)′ and Zn2B(Cl4) are shown in Figure 5. The EPR
spectra of all of the cations were examined as a function of
sample concentration. No changes in hyperfine splittings or
(35) Fajer, J.; Davis, M. S. In The Porphyrins; Dolphin, D., Ed.; Academic
Press: New York, 1979; Vol. IV, pp 197-256.
(28) Ghosh, A. J. Am. Chem. Soc. 1995, 117, 4691-4699.
(29) Cariou, M.; Simonet, J. J. Chem. Soc., Chem. Commun. 1990, 445-
446.
(36) (a) Fujii, H. J. Am. Chem. Soc. 1993, 115, 4641-4648. (b) Fujii, H.;
Yoshimura, T.; Kamada, H. Inorg. Chem. 1996, 35, 2373-2377.
(37) Jayaraj, K.; Terner, J.; Gold, A.; Roberts, D. A.; Austin, R. N.; Mandon,
D.; Weiss, R.; Bill, E.; Mu¨ther, M.; Trautwein, A. X. Inorg. Chem.
1996, 35, 1632-1640.
(30) Armitage, J. B.; Entwistle, N.; Jones, E. R. H.; Whiting, M. C. J.
Chem. Soc. 1954, 147-154.
(31) Heath, G. A.; Yellowlees, L. J.; Braterman, P. S. J. Chem. Soc., Chem.
Commun. 1981, 287-289.
(38) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic Press:
New York, 1978; Vol. III, pp 1-165.
(32) Elliot, C. M.; Hershenhart, E. J. Am. Chem. Soc. 1982, 104, 7519-
7526.
(39) Seybold, P. G.; Gouterman, M. J. Mol. Spectrosc. 1969, 31, 1-13.
(40) (a) Anton, J. A.; Loach, P. A.; Govindjee. Photochem. Photobiol. 1978,
28, 235-242. (b) Brookfield, R. L.; Ellul, H.; Harriman, A.; Porter,
G. J. Chem. Soc., Faraday Trans. 2 1986, 82, 219-233. (c) Osuka,
A.; Maruyama, K.; Yamazaki, I.; Tamai, N. Chem. Phys. Lett. 1990,
165, 392-396. (d) Gust, D.; Moore, T. A.; Moore, A. L.; Gao, F.;
Luttrull, D.; DeGraziano, J. M.; Ma, X. C.; Makings, L. R.; Lee, S.-
J.; Trier, T. T.; Bittersmann, E.; Seely, G. R.; Woodward, S.;
Bensasson, R. V.; Rouge´e, M.; De Schryver, F. C.; Van der Auweraer,
M. J. Am. Chem. Soc. 1991, 113, 3638-3649. (e) Sessler, J. L.; Wang,
B.; Harriman, A. J. Am. Chem. Soc. 1995, 117, 704-714.
(33) Edwards, W. D.; Zerner, M. C. Can. J. Chem. 1985, 63, 1763-1772.
(34) (a) Angel, S. M.; DeArmond, M. K.; Donohoe, R. J.; Wertz, D. W. J.
Phys. Chem. 1985, 89, 282-285. (b) Donohoe, R. J.; Tait, C. D.;
DeArmond, M. K.; Wertz, D. W. Spectrochim. Acta 1986, 42A, 233-
240. (c) Tait, C. D.; MacQueen, D. B.; Donohoe, R. J.; DeArmond,
M. K.; Hanck, K. W.; Wertz, D. W. J. Phys. Chem. 1986, 90, 1766-
1771. (d) Donohoe, R. R.; Tait, C. D.; DeArmond, M. K.; Wertz, D.
W. J. Phys. Chem. 1986, 90, 3923-3926. (e) Donohoe, R. J.; Tait, C.
D.; DeArmond, M. K.; Wertz, D. W. J. Phys. Chem. 1986, 90, 3927-
3930.

Tetrachlorodiarylethyne-Linked Porphyrin Dimers
Inorganic Chemistry, Vol. 37, No. 6, 1998 1201
however, the peak-to-peak line width of the unresolved signal
(∼5.3 G) is similar to the full width of the structured signal
observed at room temperature.
incorporation of o-chloro groups on the diphenylethyne linker
of ZnFb and bis-Zn dimers has significant steric, but not
electronic, effects on both ground- and excited-state electronic
communication. The magnitude of the effect of o-chloro group
incorporation is comparable to that observed for o-methyl group
incorporation. Other effects of the chloro groups, such as
electron withdrawal and overlap with the π orbitals of the
porphyrin at the meso-carbons, are of little consequence. The
results on the chlorinated dimers provide additional evidence
that energy transfer in the diarylethyne-linked dimers predomi-
nantly occurs via a through-bond electron-exchange-mediated
mechanism. The electron-withdrawing effect of the o-chloro
groups enhances the robustness of the arrays by diminishing
oxidative lability. These factors and the protocols developed
to prepare and purify the complexes studied herein should find
utility in the synthesis of the next generation of multiporphyrin
arrays.
The liquid and frozen solution EPR signatures observed for
[Zn2B(Cl4)]+ and [Zn2B(Cl4)]+2 are generally similar to those
previously reported for [Zn2B(CH3)4]+ and [Zn2B(CH3)4]+2 5,7
.
In particular, the liquid solution EPR signals of both [Zn2B-
(Cl4)]+ and [Zn2B(Cl4)]+2 are much narrower (∼3.8 and ∼4.1
G, respectively) than that of [ZnH(Cl2)′]+ (∼5.4 G) and do not
exhibit any hyperfine structure. Upon freezing the solution, the
EPR signal of [Zn2B(Cl4)]+ broadens (∼5.3 vs ∼3.8 G) and
becomes comparable in width to that of [ZnH(Cl2)′]+ (∼5.3 G),
whereas the signal of [Zn2B(Cl4)]+2 exhibits additional narrow-
ing (∼3.0 vs ∼4.1 G). The spectral signatures observed for
[Zn2B(Cl4)]+ and [Zn2B(Cl4)]+2 indicate that the nature and rates
of the hole/electron-hopping and exchange processes which are
operative in the cations of these chlorine-containing arrays are
qualitatively similar to those operative in [Zn2B(CH3)4]+ and
[Zn2B(CH3)4]+2. In this connection, the liquid solution EPR
spectrum of [Zn2B(Cl4)]+ is well accounted for by halving the
hyperfine coupling present in the monomer and doubling the
number of interacting nuclei (while holding the line width
constant) (Figure 5, bottom left panel). This result indicates
that the hole/electron in [Zn2B(Cl4)]+ is delocalized on the EPR
time scale. The same conclusion has been drawn for [Zn2B-
(CH3)4]+ and [Zn2U]+.6
Acknowledgment. This work was supported by the Division
of Chemical Sciences, Office of Basic Energy Sciences, Office
of Energy Research, U.S. Department of Energy (J.S.L.), the
LACOR Program (D.F.B.) from Los Alamos National Labora-
tory, and Grants GM36243 (D.F.B.) and GM34685 (D.H.) from
the National Institute of General Medical Sciences. 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.
Conclusions
The combined static and time-resolved spectroscopic and
electrochemical investigations reported herein indicate that the
IC970967C