Syntheses and 1H NMR spectroscopy of rigid, cofacially aligned, porphyrin-bridge-quinone systems in which the interplanar separations between the porphyrin, aromatic bridge, and quinone are less than the sum of their respective van der waals ra

Article abstract of DOI:10.1021/ja000759a

Unusually rigid π-stacked porphyrin-spacer-quinone systems have been synthesized using an approach that enables extensive control over the nature of electronic interactions between donor, aromatic spacer, and acceptor. This new class of porphyrin-based st

Full text of DOI:10.1021/ja000759a

J. Am. Chem. Soc. 2000, 122, 8717-8727
8717
1
Syntheses and H NMR Spectroscopy of Rigid, Cofacially Aligned,
Porphyrin-Bridge-Quinone Systems in Which the Interplanar
Separations between the Porphyrin, Aromatic Bridge, and Quinone
Are Less than the Sum of Their Respective van der Waals Radii
Peter M. Iovine, Matthew A. Kellett, Naomi P. Redmore, and Michael J. Therien*
Contribution from the Department of Chemistry, UniVersity of PennsylVania,
Philadelphia, PennsylVania 19104-6323
ReceiVed March 1, 2000. ReVised Manuscript ReceiVed July 7, 2000
Abstract: Unusually rigid π-stacked porphyrin-spacer-quinone systems have been synthesized using an
approach that enables extensive control over the nature of electronic interactions between donor, aromatic
spacer, and acceptor. This new class of porphyrin-based structures is distinct from related assemblies designed
to probe electronic interactions between cofacial π-stacked, aromatics: the donor (D), spacer (Sp), and acceptor
(A) components of the assembly are held fixed at sub van der Waals contact distances, restricting severely the
range of dynamical processes that modulate typically the magnitude of inter-ring separation and the extent of
the lateral shift between juxtaposed aromatic units in the condensed phase. NMR spectroscopic studies
demonstrate that these structures manifest disparate shielding environments which distribute uniformly the
1
aromatic H resonances for these diamagnetic D-Sp-A compounds over spectral windows that exceed 9.0
ppm.
Introduction
the construction of elaborate porphyrin-spacer-quinone sys-
tems.15,16 While there has been extensive interest in utilizing
oligonucleotide-based scaffolds to build D-Sp-A complexes
that feature stacked aromatic entities separating D from A,17-24
constructs that manifest electronic interactions between cofa-
cially aligned D, Sp, and A moieties that differ radically from
deoxyribonucleic acid-based systems have yet to be developed.
This is perhaps surprising, given the potential impact that both
the magnitude of respective D, Sp, and A interplanar separations,
and the nature of the quadrupolar interactions between these
aromatics, may have on the regulation of the distance depen-
dence of D-A electronic coupling in such cofacially organized
π manifolds.25-27
Fundamental issues regarding the role played by the medium
(spacer) electronic structure in modulating donor-acceptor
coupling in the charge tunneling regime, as well as the desire
to control the magnitude of photoinduced charge separation and
thermal charge recombination rate constants in synthetic donor-
spacer-acceptor (D-Sp-A) systems, motivate the design of
increasingly more complex electron transfer (ET) assemblies.
Due to the long lifetimes and well characterized nature of both
their electronically excited singlet and triplet states, porphyrinic
components have figured prominently in such D-Sp-A as-
semblies; as a result, a plethora of synthetic approaches,
including Diels-Alder cycloaddition,1-5 condensation of ap-
propriately substituted aldehydes and pyrroles,6-10 self-assembly
of complimentary H-bonding components,11-13 and metal-
catalyzed cross-coupling schemes,9,12,14 have been applied to
(11) Sessler, J. L.; Wang, B.; Harriman, A. J. Am. Chem. Soc. 1993,
115, 10418-10419.
(12) De Rege, P. J. F.; Williams, S. A.; Therien, M. J. Science 1995,
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K. Angew. Chem., Int. Ed. Engl. 1996, 35, 1964-1966.
(14) Hyslop, A. G.; Kellett, M. A.; Iovine, P. M.; Therien, M. J. J. Am.
Chem. Soc. 1998, 120, 12676-12677.
(15) Wasielewski, M. R. Chem. ReV. 1992, 92, 435-461.
(16) Piotrowiak, P. Chem. Soc. ReV. 1999, 28, 143-150.
(17) Purugganan, M. D.; Kumar, C. V.; Turro, N. J.; Barton, J. K. Science
1988, 241, 1645-1649.
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10.1021/ja000759a CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/26/2000

8718 J. Am. Chem. Soc., Vol. 122, No. 36, 2000
IoVine et al.
either at the Microanalytical Laboratory in the Department of Chemistry
at the University of Pennsylvania or at Robertson Microlit Laboratories,
Inc. (Madison, NJ).
Instrumentation. Electronic spectra were recorded on an OLIS UV/
vis/NIR spectrophotometry system that is based on the optics of a Carey
14 spectrophotometer.
We describe in this work the syntheses of unusually rigid
π-stacked porphyrin-quinone (P-Q) systems via an approach
that enables extensive control over the nature of electronic
interactions between D, Sp, and A. This work exploits metal-
mediated cross-coupling methodologies,28-35 a valuable, new
porphyrinic synthon,14 and a 1,8-naphthyl pillaring motif, to
provide entry into a new class of porphyrin-based supramo-
lecular structures featuring cofacially aligned aromatic moieties
fixed at sub van der Waals contact distances. The syntheses
NMR Data. NMR Spectra were recorded on either a 500-MHz or
AC 250-MHz Bruker spectrometer. Chemical shifts for 1H NMR spectra
are relative to TMS (δ ) 0.00 ppm); 13C NMR spectra are referenced
to deuteriochloroform solvent (CDCl3, δ ) 77.00 ppm), while those
for 19F NMR spectra are relative to fluorotrichloromethane (CCl3F, δ
) 0.00 ppm). The 1H NMR spectra of compounds 8b, 8c, 9b, 9c, 10b,
10c, 2b, and 2c were recorded as diastereomeric mixtures; the syn and
anti components of the mixture were not separable using the chro-
matographic media described above. 1H NMR data for these compounds
are therefore presented in the following manner: (i) resonances
attributed to the anti isomer have their multiplicity labeled with a prime
notation (′) [e.g., (s′, 3 H)], (ii) resonances attributed to the syn isomer
have their multiplicity doubly primed (′′) [e.g. (s′′, 3 H)], and (iii)
resonances that correspond to overlapping signals of syn and anti
isomers are denoted with unprimed notation [e.g., (s, 3 H)]. The
diastereomeric ratio was determined by integration of well-resolved
1
and spectacular 1-D H NMR spectroscopic properties of four
such species are reported, and include P-Q species, 1, the
prototype complex of this new family of D-Sp-A assemblies
designed to probe the electronic coupling modulated through
compressed, stacked π manifolds, along with tri-level systems
2a-c, in which intervening phenyl, xylyl, and difluorophenyl
units respectively separate the porphyryl and quinonyl moieties.
These structures highlight the utility of this modular synthetic
approach to generate supramolecular systems that manifest
unusual electronic structures and demonstrate the ease at which
bridge electronic structure can be modulated in this class of
π-stacked D-Sp-A systems.
1
anti and syn resonances in the H NMR spectrum. The specific anti/
syn resonance pair utilized to determine a diastereomeric ratio varied
from compound to compound.
1,4-Di(4′,4′,5′,5′-tetramethyl[1′,3′,2′]dioxaborolan-2′-yl)benzene
(7a).37 A 50 mL Schlenk reaction vessel was charged with pinacoldi-
boron ester (500 mg, 1.97 mmol), KOAc (263 mg, 2.68 mmol), and
PdCl2(dppf) (20 mg, 0.027 mmol). A deoxygenated solution of 1,4-
dibromobenzene (211 mg, 0.89 mmol) in DMF (10 mL) was cannula
transferred into this flask; the mixture was heated at 80 °C for 2 h, and
subsequently cooled, filtered, and partitioned with benzene and saturated
aqueous NaCl. The organic layer was washed with saturated NaCl (3
× 75 mL) and H2O (1 × 50 mL), following which it was dried over
CaCl2, and evaporated. The recovered dark solid was washed repeti-
tively with 10 mL aliquots of cold n-pentane to remove unreacted
pinacoldiboron. Sublimation of the crude solid in vacuo at 120 °C gives
the product as a white crystalline material. Recrystallization of this
material from hexanes affords pure white crystals; isolated yield ) 93
mg (31% based on 211 mg 1,4-dibromobenzene): mp 235-236 °C.
1H NMR (250 MHz, CDCl3): δ 7.80 (s, 4 H), 1.35 (s, 24 H). 13C NMR
(500 MHz, CDCl3): δ 24.84, 83.83, 133.88. HRMS (CI+) m/z: 331.2252
(calcd for C18H29B2O4 (MH+) 331.2252). Anal. Calcd for C18H28B2O4:
C, 65.41; H, 8.55. Found: C, 65.31; H, 8.73.
Experimental Section
Materials. All manipulations were carried out under nitrogen
previously passed through an O2 scrubbing tower (Schweizerhall R3-
11 catalyst) and a drying tower (Linde 3-Å molecular sieves) unless
otherwise stated. Air-sensitive solids were handled in a Braun 150-M
glovebox. Standard Schlenk techniques were employed to manipulate
air-sensitive solutions. All solvents utilized in this work were obtained
from Fisher Scientific (HPLC Grade). Tetrahydrofuran (THF), and 1,2-
dimethoxyethane (DME) were predried over 4-Å molecular sieves and
then distilled from Na/benzoylbiphenyl under N2, while CH2Cl2 was
distilled from CaH2 under N2. Anhydrous dimethylformamide (DMF)
(Aldrich Chemical Co.) was stirred over MgSO4 prior to distillation
under vacuum. Bis(pinacolato)diboron (Frontier Scientific) and halo-
genated reagents (Aldrich Chemical Co.) 1,4-dibromobenzene, 1,4-
dibromo-2,5-dimethylbenzene, 1,4-dibromo-2,5-difluorobenzene were
used as received. Ba(OH)2‚8H2O (Aldrich Chemical Co.) was recrystal-
lized from distilled H2O. The catalysts, Pd[(PPh)3]4 and Pd(dppf)Cl2,
were obtained from Strem. 1,8-Diiodonaphthalene was prepared using
the procedure developed by House.36 Chromatographic purification
(Silica Gel 60, ICN 32-630) of all newly synthesized compounds was
accomplished on the benchtop. Elemental analyses were performed
1,4-Dimethyl-2,5-di(4′,4′,5′,5′-tetramethyl[1′,3′,2′]dioxaborolan-
2′-yl)benzene (7b). A 50 mL Schlenk flask was charged with 1,4-
dibromo-2,5-dimethylbenzene (2.0 g, 7.6 mmol), dioxoboralane (4.4
mL, 30 mmol), Et3N (6.3 mL, 45 mmol), and toluene (15 mL). This
solution was degassed via three freeze-pump-thaw cycles, following
which PdCl2(PPh3)2 (319 mg, 0.45 mmol) was added. The reaction was
heated at 80 °C for 12 h, cooled, and filtered. The solution was then
diluted with additional benzene, washed with saturated NaCl (3 × 75
mL), H2O (1 × 50 mL), and subsequently dried over CaCl2, filtered,
and evaporated, to give a tan solid. The crude material was refluxed in
hexanes, filtered hot, and cooled to room temperature. The crystalline
product was collected by vacuum filtration; isolated yield ) 1.1 g (39%
1
based on 2.0 g 1,4-dibromo-2,5-dimethylbenzene): mp > 250 °C. H
NMR (250 MHz, CDCl3): δ 7.52 (s, 2 H), 2.47 (s, 6 H), 1.32 (s, 24
H). 13C NMR (500 MHz, CDCl3): δ 21.46, 24.87, 83.39, 136.90, 140.52.
HRMS (CI+) m/z: 359.2566 (calcd for C20H33B2O4 (MH+) 359.2565).
Anal. Calcd for C20H32B2O4: C, 66.99; H, 9.00. Found: C, 66.81; H,
9.09.
1,4-Difluoro-2,5-di(4′,4′,5′,5′-tetramethyl[1′,3′,2′]dioxaborolan-2′-
yl)benzene (7c). A 50 mL Schlenk reaction vessel was charged with
pinacoldiboron ester (616 mg, 2.4 mmol), KOAc (325 mg, 3.3 mmol),
and PdCl2(dppf) (40 mg, 0.055 mmol). A deoxygenated solution of
1,4-dibromo-2,5-difluorobenzene (300 mg, 1.1 mmol) in DMF (10 mL)
was cannula transferred into this flask; the mixture was heated at 80
°C for 15 h, and subsequently cooled, filtered, and partitioned with
benzene and saturated aqueous NaCl. The organic layer was washed
with saturated NaCl (3 × 75 mL) and H2O (1 × 50 mL) following
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37, 1003-1011.

Syntheses of Porphyrin-Bridge-Quinone Systems
J. Am. Chem. Soc., Vol. 122, No. 36, 2000 8719
which it was dried over CaCl2, and evaporated. The recovered dark
solid was washed repetitively with 10 mL aliquots of cold n-pentane
to remove unreacted pinacoldiboron. Sublimation of the crude solid in
vacuo at 135 °C gives a white crystalline material. Recrystallization
of this solid from hexanes affords pure white crystals of the product;
isolated yield ) 158 mg (39% based on 300 mg of 1,4-dibromo-2,5-
difluorobenzene). 1H NMR (250 MHz, CDCl3): δ 7.33 (t, 2 H, J ) 6.6
Hz), 1.35 (s, 24 H). 13C NMR (500 MHz, CDCl3): δ 24.79, 84.27,
122.23, 122.34, 122.40, 122.50, 161.46, 161.49, 163.44, 163.48. 19F
NMR (200 MHz, CDCl3): δ -111.52 (s, 2 F). HRMS (CI+) m/z:
367.2068 (calcd for C18H27B2F2O4 (MH+) 367.2064). Anal. Calcd for
C18H26B2F2O4: C, 58.98; H, 7.16. Found: C, 58.80; H, 7.03.
yellow residue was chromatographed on silica gel (9:1 hexanes:THF),
giving a white solid; isolated yield ) 17 mg (35% based on 38 mg of
1
compound 8a). H NMR (250 MHz, CDCl3): δ 8.18 (d, 1 H, J ) 6.8
Hz), 7.98-7.85 (m, 4 H), 7.60-7.51 (m, 3 H), 7.44 (dd, 1 H, J ) 1.3
Hz, J ) 7.0 Hz), 7.37 (dd, 2 H, J ) 1.1 Hz, J ) 7.05 Hz), 7.18 (d, 1
H, J ) 7.6 Hz), 7.08 (t, 1 H, J ) 7.7 Hz), 6.96-6.84 (m, 3 H), 6.62
(dd, 1 H, J ) 3.0 Hz, J ) 8.7 Hz), 6.58 (d, 1 H, J ) 3.0 Hz), 6.30 (d,
1 H, J ) 8.7 Hz), 3.67 (s, 3 H), 3.48 (s, 3 H). 13C NMR (500 MHz,
CDCl3): δ 55.03, 55.78, 92.19, 110.34, 112.83, 117.59, 124.80, 125.03,
125.07, 126.45, 127.78, 128.60, 128.71, 128.79, 128.84, 129.67, 129.79,
130.03, 130.19, 130.40, 130.67, 130.75, 131.35, 133.25, 134.93, 135.43,
136.59, 138.01, 140.79, 141.99, 142.26, 142.34, 150.24, 152.65. HRMS
(CI+) m/z: 592.0898 (calcd for C34H25IO2 (M+) 592.0899).
1-Iodo-8-(2,5-dimethoxyphenyl)naphthalene (3). A 1.0 M THF
solution of (2,5-dimethoxyphenyl)magnesium bromide was prepared
from 1-bromo-2,5-dimethoxybenzene and Mg turnings. The Grignard
reagent (2.50 mL, 2.50 mmol) was diluted with THF (5 mL) in a 25
mL Schlenk flask and added dropwise to a solution of ZnCl2 (1.02 g,
7.5 mmol) in THF (15 mL). After the solution stirred for 15 min, a
heavy white precipitate formed; the organozinc reagent was then
transferred dropwise via cannula to a 100 mL Schlenk flask containing
1,8-diiodonaphthalene (1.00 g, 2.63 mmol) and Pd(PPh3)4 (152 mg,
0.13 mmol) in THF (25 mL). The reaction was followed by TLC; after
12 h, no further changes were observed in the product distribution.
Silica gel chromatography (19:1 hexanes:THF) yielded three frac-
tions: residual 1,8-diiodonaphthalene, the desired mono-coupled
product, and 1,8-bis(2,5-dimethoxyphenyl)naphthalene. The product was
dissolved in ethanol (10 mL) and cooled to -38 °C, yielding a white
precipitate, which was collected via filtration; isolated yield ) 595 mg
(61% based on 543 mg of 1-bromo-2,5-dimethoxybenzene): mp 97-
[5-(8′-[4′′-(8′′′-[2′′′′, 5′′′′-Dimethoxyphenyl]-1′′′-naphthylaphthyl)-
10,20-diphenylporphinato]zinc(II) (10a). A 25 mL Schlenk tube
was charged with 9a (17 mg, 0.029 mmol), [5-(4′,4′,5′,5′-tetramethyl-
[1′,3′,2′]dioxaborolan-2′-yl)-10,20-diphenylporphinato]zinc(II) 4 (31
mg, 0.048 mmol), Ba(OH)2‚8H2O (16 mg, 0.050 mmol), distilled H2O
(0.5 mL), and dimethoxyethane (DME) (5 mL). This solution was
degassed via three freeze-pump-thaw cycles; following the addition
of Pd(PPh3)4 (3 mg, 0.0026 mmol), the mixture was heated at 80 °C
for 1 h. After partitioning the reaction mixture with benzene and
saturated NaCl, the organic layer was washed with saturated NaCl (3
× 50 mL) and H2O (1 × 50 mL), dried over CaCl2, filtered, and
evaporated. The crude material was chromatographed on silica gel (17:3
hexanes:THF). A small fraction of (5,15-diphenylporphinato)zinc(II)
eluted prior to the main product. The desired product eluted with a
trace amount of an orange porphyrinic impurity; rechromatographing
this material on silica gel gave pure compound 10a; isolated yield )
1
1
99 °C. H NMR (250 MHz, CDCl3): δ 8.19 (dd, 1 H, J ) 1.1 Hz, J
23 mg (81% based on 17 mg of compound 9a). H NMR (500 MHz,
) 7.3 Hz), 7.87 (m, 2 H), 7.47 (m, 2 H), 7.06 (t, 1 H, J ) 7.7 Hz),
6.96 (dd, 1 H, J ) 3.0 Hz, J ) 8.8 Hz), 6.85 (d, 1 H, J ) 8.8 Hz), 6.80
(d, 1 H, J ) 3.0 Hz), 3.79 (s, 3 H), 3.65 (s, 3 H). 13C NMR (500 MHz,
CDCl3): δ 55.92, 56.02, 91.82, 111.64, 114.18, 118.47, 125.23, 126.28,
129.53, 129.96, 130.98, 131.09, 131.50, 135.07, 137.63, 142.03, 152.94,
153.39. HRMS (ESI+) m/z: 390.011843 (calcd for C18H15IO2 (M+)
390.011682). Anal. Calcd for C18H15IO2: C, 55.38; H, 3.88. Found:
C, 55.53; H, 4.05.
CDCl3, see Figure 2 for proton labeling schematic): δ 10.15 (s, 1 H,
Hmeso), 9.36 (d, 1 H, J ) 4.5 Hz, Hâ), 9.32 (d, 1 H, J ) 4.4 Hz, Hâ),
9.05 (d, 1 H, J ) 4.4 Hz, Hâ), 8.98 (d, 1 H, J ) 4.4 Hz, Hâ), 8.77 (d,
1 H, J ) 4.6 Hz, Hâ), 8.70 (d, 1 H, J ) 4.6 Hz, Hâ), 8.67 (d, 1 H, J
) 4.6 Hz, Hâ), 8.50 (d, 1 H, J ) 4.6 Hz, Hâ), 8.35 (d, 1 H, J ) 7.1 Hz,
H11), 8.27 (d, 1 H, J ) 6.8 Hz, H14), 8.21-8.19 (m, 2 H, H13 + Hortho,
10,20 phenyls), 8.17-8.14 (m, 2 H Hortho, 10,20 phenyls × 2), 8.06 (d, 1 H, J )
7.3 Hz, Hortho, 10,20 phenyls), 7.82 (t, 1 H, J ) 7.6 Hz, H12), 7.74-7.67 (m,
6 H, Hmeta/para, 10,20 phenyls), 7.60 (t, 1 H, J ) 7.6 Hz, H15), 7.25 (d, 1 H,
J ) 8.1 Hz, H8), 6.98 (t, 1 H, J ) 7.5 Hz, H9), 6.88-6.87 (m, 2 H, H16
+ H7), 6.64 (d, 1 H, J ) 6.9 Hz, H10), 6.20 (dd, 1 H, J ) 3.1 Hz, J )
8.8 Hz, H17), 5.75 (d, 1 H, J ) 8.8 Hz, H18), 5.55 (d, 1 H, J ) 3.0 Hz,
H19), 5.29 (dd, 1 H, J ) 1.8 Hz, J ) 7.7 Hz, H2), 5.18 (dd, 1 H, J )
1.8 Hz, J ) 7.7 Hz, H1), 4.95 (t, 1 H, J ) 7.6 Hz, H6), 4.08 (dd, 1 H,
J ) 1.8 Hz, J ) 7.7 Hz, H4), 3.86 (dd, 1 H, J ) 1.8 Hz, J ) 7.7 Hz,
H3), 3.27 (s, 3 H, OMe), 2.85 (s, 3 H, OMe), 0.93 (d, 1 H, J ) 7.0 Hz,
H5). Vis (CH2Cl2): 423 (5.54), 467 (3.74), 547 (4.26), 584 (3.33).
HRMS (ESI+) m/z: 1011.2655 (calcd for C66H44N4O2NaZn (M + Na)
1011.2653).
1-(2,5-Dimethoxyphenyl)-8-[4-(4′,4′,5′,5′-tetramethyl[1′,3′,2′]-
dioxaborolan-2′-yl)-1-phenyl]naphthalene (8a). A 25 mL Schlenk
tube was charged with compound 3 (59 mg, 0.15 mmol), K3PO4 (48
mg, 0.23 mmol), compound 7a (100 mg, 0.30 mmol), and Pd(PPh3)4
(9 mg, 7.8 × 10-3 mmol). DMF (5 mL), that had previously been
deoxygenated via three freeze-pump-thaw cycles, was cannula
transferred to the tube containing the solids. The reaction was heated
at 100 °C for 2 h, cooled, filtered, and partitioned with benzene and
saturated NaCl. The organic layer was washed with saturated NaCl (3
× 75 mL) and H2O (1 × 50 mL), following which it was dried over
CaCl2, filtered, and evaporated, giving a light yellow solid. This material
was chromatographed on silica gel (CHCl3). The product eluted after
residual compound 3; isolated yield ) 40 mg (57% based on 59 mg of
[5-(8′-[2′′,5′′-Dimethoxyphenyl]-1′-naphthyl)-10,20-diphenylpor-
phinato]zinc(II) (5). A 50 mL Schlenk reaction vessel was charged
with [5-(4′,4′,5′,5′-tetramethyl[1′,3′,2′]dioxaborolan-2′-yl)-10,20-diphen-
ylporphinato]zinc(II) (250 mg, 0.38 mmol), 3 (100 mg, 0.26 mmol),
Ba(OH)2‚8H2O (122 mg, 0.38 mmol), distilled water (1.0 mL), and
dimethoxyethane (DME) (10 mL). The solution was degassed via three
freeze-pump-thaw cycles, following which Pd(PPh3)4 (15 mg, 0.013
mmol) was added. The reaction mixture was heated at 80 °C for 6 h,
quenched, and loaded onto a silica gel column (19:1 hexanes:THF).
During the chromatographic separation, the eluant polarity was
increased gradually to (9:1 hexanes:THF); a small fraction of (5,15-
diphenylporphinato)zinc(II) eluted prior to the main product. After
removal of the volatiles, the product was washed with hexanes and
isolated via vacuum filtration on a fine glass frit; isolated yield ) 185
mg (92% based on 100 mg of compound 3). 1H NMR (250 MHz,
CDCl3, see Figure 3 for proton labeling schematic): δ 10.19 (s, 1 H,
1
compound 3). H NMR (250 MHz, CDCl3): δ 7.92 (d, 1 H, J ) 8.1
Hz), 7.91 (d, 1 H, J ) 8.2 Hz), 7.56-7.45 (m, 3 H), 7.36-7.33 (m, 2
H), 7.28 (d, 1 H, J ) 7.1 Hz), 7.16 (d, 1 H, J ) 7.5 Hz), 6.85 (d, 1 H,
J ) 7.7 Hz), 6.55 (d, 1 H, J ) 3.1 Hz), 6.38 (dd, 1 H, J ) 3.1 Hz, J
) 9.0 Hz), 6.18 (d, 1 H, J ) 9.0 Hz), 3.68 (s, 3 H), 3.45 (s, 3 H), 1.35
(s, 6 H), 1.34 (s, 6 H). 13C NMR (500 MHz, CDCl3): δ 24.66, 24.96,
54.93, 55.86, 83.45, 110.32, 112.78, 117.96, 124.72, 125.07, 127.62,
128.66, 128.76, 129.87, 130.08, 130.12, 130.65, 132.69, 132.82, 132.95,
133.87, 134.82, 136.57, 140.82, 145.53, 150.00, 152.77. HRMS (CI+)
m/z: 467.2394 (calcd for C30H32BO4 (MH+) 467.2356).
1-Iodo-8-[4′- (8′′-[2′′′, 5′′′-dimethoxyphenyl]-1′′-naphthyl)-1′-phen-
yl]naphthaene (9a). A 25 mL Schlenk flask was charged with 8a (38
mg, 0.082 mmol), 1,8-diiodonaphthalene (54 mg, 0.14 mmol), Ba(OH)2‚
8H2O (39 mg, 0.12 mmol), and Pd(PPh3)4 (8 mg, 6.9 × 10-3 mmol).
A degassed solution of dimethoxyethane (DME) (6 mL) and distilled
water (1 mL) was cannula transferred to the reaction vessel. After
heating at 80 °C for 1 h, the reaction mixture was partitioned with
benzene and saturated NaCl. The organic layer was washed with
saturated NaCl (3 × 75 mL) and H2O (1 × 50 mL), following which
it was dried over CaCl2, filtered, and evaporated. The recovered oily
Hmeso), 9.39 (d, 1 H, J ) 4.5 Hz, Hâ), 9.36 (d, 1 H, J ) 4.4 Hz, Hâ),
9.07 (d, 1 H, J ) 4.4 Hz, Hâ), 9.05 (d, 1 H, J ) 4.4 Hz, Hâ), 8.92 (d,
1 H, J ) 4.7 Hz, Hâ), 8.80 (m, 2 H, Hâ × 2), 8.56 (d, 1 H, J ) 6.8 Hz,
H1), 8.47 (d, 1 H, J ) 6.6 Hz, Hortho, 10,20 phenyls), 8.45 (d, 1 H, J ) 4.5
Hz, Hâ), 8.34 (m, 2 H, Hortho,
+ H4), 8.13 (d, 1 H, J ) 8.2,
10,20 phenyls
H3), 8.07 (d, 2 H, J ) 6.9 Hz, Hortho, 10,20 phenyls × 2), 7.89 (t, 1 H, J )

8720 J. Am. Chem. Soc., Vol. 122, No. 36, 2000
7.6 Hz, H2), 7.74 (m, 6 H, Hmeta/para, phenyls), 7.45 (t, 1 H, J ) 7.6
IoVine et al.
2 H, N-H). Vis (CH2Cl2): 422 (5.37), 516 (4.18), 550 (3.85), 590
(3.85), 648 (3.78). HRMS (ESI+) m/z: 919.3088 (calcd for C64H40N4O2-
Na (M + Na) 919.3049).
10,20
Hz, H5), 6.71 (d, 1 H, J ) 7.0 Hz, H6), 4.88 (d, 1 H, J ) 3.1 Hz, HC),
3.13 (d, 1 H, J ) 9.0 Hz, HB), 2.43 (dd, 1 H, J ) 3.1 Hz, J ) 8.9 Hz,
HA), 2.20 (s, 3 H, OMe), 1.26 (s, 3 H, OMe). 13C NMR (500 MHz,
CDCl3): δ 151.15, 150.09, 149.95, 149.70, 149.64, 149.55, 149.50,
149.26, 147.96, 142.97, 142.91, 139.34, 137.48, 135.59, 134.59, 134.51,
134.40, 133.97, 133.32, 133.25, 132.31, 132.07, 132.05, 131.77, 131.72,
131.43, 131.19, 130.86, 130.36, 130.28, 129.67, 128.79, 127.57, 127.40,
127.34, 126.72, 126.57, 126.48, 125.29, 123.26, 121.22, 120.82, 119.63,
114.90, 105.93, 105.53, 104.75, 53.59, 52.45. Vis (CH2Cl2): 423 (5.42),
549 (4.23). HRMS (FAB) m/z: 786.1973 (calcd for C50H34N4O2Zn (M+)
786.1996).
5-[8′-(2′′,5′′-Dimethyl-4′′-[8′′′-(2′′′′, 5′′′′-benzoquinonyl)-1′-naph-
thyl]-10,20-diphenylporphyrin (2b). A dry CH2Cl2 solution of 10b
(33 mg, 0.032 mmol) was cooled to -78 °C, following which 10
equivalents of BBr3 (324 µL of a 1.0 M CH2Cl2 solution, 0.32 mmol)
were added dropwise. The reaction was stirred for 1 h at -78 °C and
then slowly warmed to room temperature. After stirring for two
additional h, a small quantity of methanol was added, and the reaction
stirred for an additional 15 min. The reaction was partitioned between
CH2Cl2 and saturated Na2CO3. The organic layer was washed with
saturated Na2CO3, (3 × 20 mL) dried, and filtered; PbO2 (100 mg)
was added, and the heterogeneous CH2Cl2 solution was stirred for 30
min. After filtering and removal of the volatiles, the red material was
chromatographed on silica gel (CH2Cl2). After consolidation of the
product fractions, 2b was isolated as a glassy red solid; yield ) 12 mg
(40% based on 33 mg of compound 10b). anti:syn 1.3:1 1H NMR (250
MHz, CDCl3, see Figure 2 for proton labeling schematic): δ 10.24 (s′′,
1 H, Hmeso), 10.20 (s′, 1 H, Hmeso), 9.41 (d′, 1 H, J ) 4.7 Hz, Hâ), 9.37
(d′′, 1 H, J ) 5.0 Hz, Hâ), 9.33 (d′′, 1 H, J ) 4.9 Hz, Hâ), 9.25 (d′, 1
H, J ) 4.5 Hz, Hâ), 9.04 (d′, 1 H, J ) 4.7 Hz, Hâ), 8.98 (d′′, 1 H, J )
4.2 Hz, Hâ), 8.90 (d′′, 1 H, J ) 4.4 Hz, Hâ), 8.85 (d′, 1 H, J ) 5.5 Hz,
Hâ), 8.77 (d′′, 1 H, J ) 4.0 Hz, Hâ), 8.75 (d′, 1 H, J ) 4.0 Hz, Hâ),
8.66 (d′, 2 H, J ) 4.7 Hz, Hâ), 8.61 (d′′, 1 H, J ) 4.9 Hz, Hâ), 8.54
(d′′, 1 H, J ) 4.7 Hz, Hâ), 8.49 (d′′, 1 H, J ) 4.8 Hz, Hâ), 8.38 (d, 1
H, J ) 7.8 Hz, H11), 8.33 (d′, 1 H, J ) 5.0 Hz, Hâ), 8.27-8.06 (m, 4
H), 7.99-7.90 (m, 2 H), 7.84-7.63 (m, 8 H, H12 + Hmeta/para, 10,20 phenyls
+ H15), 7.37 (d′′, 1 H, J ) 8.5 Hz, H8), 7.34 (d′, 1 H, J ) 8.4 Hz, H8),
7.02-6.79 (m, 3 H, H9 + H7 + H16), 6.54 (d′′, 1 H, J ) 7.0 Hz, H10),
6.51 (d′, 1 H, J ) 7.3 Hz, H10), 6.20 (dd′, 1 H, J ) 2.8 Hz, J ) 10.0
Hz, H17-Quinone), 6.15 (dd′′, 1 H, J ) 2.7 Hz, J ) 10.6 Hz, H17-Quinone),
5.98 (d′, 1 H, J ) 9.8 Hz, H18), 5.77 (d′′, 1 H, J ) 10.0 Hz, H18), 5.56
(d′, 1 H, J ) 2.7 Hz, H19), 5.53 (d′′, 1 H, J ) 2.9 Hz, H19), 5.42 (s′′,
1 H, H6′′-xylene), 5.19 (s′, 1 H, H6′′-xylene), 4.68 (t, 1 H, J ) 7.6 Hz, H6),
4.32 (s′′, 1 H, H3′′-xylene), 4.30 (s′, 1 H, H3′′-xylene), 1.47 (s′, 3 H, Me),
1.37 (s′′, 3 H, Me), 0.53 (d′′, 1 H, J ) 7.6 Hz, H5), 0.51 (d′, 1 H, J )
5.4 Hz, H5), -0.89 (s′′, 3 H, Me), -1.25 (s′, 3 H, Me), -1.98 (s (br),1
H, N-H), -2.04 (s (br), 1 H, N-H). Vis (CH2Cl2): 422 (5.38), 516
(4.09), 552 (3.71), 591 (3.65), 646 (3.47). HRMS (ESI+) m/z: 925.3498
(calcd for C66H45N4O2 (M + H) 925.3543).
5-[8′-(2′′,5′′- Benzoquinonyl)-1′-naphthyl]-10,20-diphenylporphy-
rin (1). A 25 mL Schlenk reaction vessel was charged with 5 (31 mg,
0.039 mmol) and dry benzene (5 mL). Ten equivalents of BBr3 (390
µL of a 1.0 M CH2Cl2 solution, 0.39 mmol) were added dropwise to
this solution. After stirring for 24 h at room temperature, the reaction
mixture was diluted with additional benzene (5 mL), washed with
saturated Na2CO3 (3 × 50 mL) and H2O (1 × 50 mL), following which
it was dried over CaCl2, filtered, and evaporated. The recovered red
residue was chromatographed on silica gel (99:1 CH2Cl2:MeOH),
affording a separation of two major bands. The first non-fluorescent
band was the desired product 1; the second band contained monode-
methylated 6. The combined fractions of 6 were concentrated to dryness
and resubjected to the above reaction conditions; total isolated yield )
2 mg (7% based on 31 mg of compound 5). 1H NMR (500 MHz, CDCl3,
see Figure 3 for proton labeling schematic): δ 10.19 (s, 1 H, Hmeso),
9.33 (d, 1 H, J ) 4.9 Hz, Hâ), 9.30 (d, 1 H, J ) 4.7 Hz, Hâ), 9.02 (d,
1 H, J ) 4.9 Hz, Hâ), 8.97 (d, 1 H, J ) 4.7 Hz, Hâ), 8.83 (d, 1 H, J
) 5.1 Hz, Hâ), 8.61-8.60 (m, 3 H, Hâ + Hortho, 10,20 phenyls + H1), 8.58
(d, 1 H, J ) 4.9 Hz, Hâ), 8.49 (d (br), 1 H, J ) 8.1 Hz, Hortho,
10,20
phenyls), 8.36 (d, 1 H, J ) 8.5 Hz, H4), 8.25 (d, 1 H, J ) 4.9 Hz, Hâ),
8.20 (d, 1 H, J ) 8.5 Hz, H3), 8.17 (d (br), 2 H, J ) 7.7 Hz, Hortho, 10,20
phenyls), 7.96 (t, 1 H, J ) 7.8 Hz, H2), 7.85-7.66 (m, 6 H, Hmeta/para, 10,20
phenyls), 7.48 (t, 1 H, J ) 7.7 Hz, H5), 6.78 (d, 1 H, 7.0 Hz, H6), 4.97 (d,
1 H, J ) 2.8 Hz, HC-Quinone), 2.02 (d, 1 H, J ) 10.0 Hz, HB-Quinone),
1.65 (dd, 1 H, J ) 2.8 Hz, J ) 10.1 Hz, HA-Quinone), -3.35 (s (br), 2
H, N-H). UV-Vis (CH2Cl2): 414 (5.26), 500 (br) (3.92), 596 (3.66),
661 (3.39). HRMS (ESI+) m/z: 695.2454 (calcd for C48H31N4O2 (M +
H) 695.2454).
5-[8′-(4′′-[8′′′-(2′′′′, 5′′′′-Benzoquinonyl)-1′′′-naphthyhthyl]-10,20-
diphenylporphyrin (2a). A dry CH2Cl2 solution of 10a (11 mg, 0.011
mmol) was cooled to -78 °C, following which 10 equivalents of BBr3
(111 µL of a 1.0 M CH2Cl2 solution, 0.11 mmol) were added dropwise.
The reaction was stirred for 1 h at -78 °C and then slowly warmed to
room temperature. After stirring for an additional h, a small quantity
of methanol was added and the reaction stirred for 15 min. The reaction
was then partitioned between CH2Cl2 and saturated Na2CO3. The
organic layer was washed with saturated Na2CO3 (3 × 20 mL), dried,
and filtered; PbO2 (100 mg) was added, and the heterogeneous CH2-
Cl2 solution was stirred for 1 h. After filtering and removal of the
volatiles, the red material was chromatographed on silica gel (CH2-
Cl2). The quinonyl compound 2a elutes first, followed by mono-
demethylated 10a. After consolidation of the product fractions, 2a was
isolated as a glassy red solid; yield ) 6.7 mg (67% based on 11 mg of
5-[8′-(2′′,5′′-Difluoro-4′′-[8′′′-(2′′′′, 5′′′′-benzoquinonyl)-1′-naph-
thyl]-10,20-diphenylporphyrin (2c). A dry CH2Cl2 (5 mL) solution
of 10c (11 mg, 0.011 mmol) was cooled to -78 °C, following which
10 equiv of BBr3 (100 µL of a 1.0 M CH2Cl2 solution, 0.1 mmol)
were added dropwise. The reaction was stirred for 1 h at -78 °C, and
then slowly warmed to room temperature. After stirring for an additional
h, a small quantity of methanol was added, and the reaction stirred for
an additional 15 min. The reaction was partitioned between CH2Cl2
and saturated Na2CO3. The organic layer was washed with saturated
Na2CO3, (3 × 20 mL) dried, and filtered; PbO2 (100 mg) was added,
and the heterogeneous CH2Cl2 solution was stirred for 30 min. After
filtering and removal of the volatiles, the red material was chromato-
graphed on silica gel (CH2Cl2). The product was collected as the first
red band. After consolidation of the product fractions, 2c was isolated
as a glassy red solid; yield ) 3.9 mg (39% based on 11 mg of compound
10c). anti:syn 5.1:1 1H NMR (500 MHz, CDCl3, see Figure 2 for proton
labeling schematic): δ 10.24 (s′′, 1 H, Hmeso), 10.22 (s′, 1 H, Hmeso),
9.40 (d′, 1 H, J ) 4.5 Hz, Hâ), 9.37 (d′′, 1 H, J ) 4.6 Hz, Hâ), 9.31
(d′′, 1 H, J ) 4.5 Hz, Hâ), 9.25 (d′, 1 H, J ) 4.7 Hz, Hâ), 9.04 (d′, 1
H, J ) 4.6 Hz, Hâ), 9.00 (d′′, 1 H, J ) 4.5 Hz, Hâ), 8.90 (m, 1 H, Hâ),
8.83 (d′, 1 H, J ) 4.5 Hz, Hâ), 8.80 (d′, 1 H, J ) 4.8 Hz, Hâ), 8.73
(d′′, 1 H, J ) 4.8 Hz, Hâ), 8.63 (d′′, 1 H, J ) 4.8 Hz, Hâ), 8.58 (d′, 1
H, J ) 4.6 Hz, Hâ), 8.51 (d′′, 1 H, J ) 4.8 Hz, Hâ), 8.39 (dd, 1 H, J
) 1.2 Hz, J ) 8.1 Hz, H11), 8.31 (dd, 1 H, J ) 1.3 Hz, J ) 8.4 Hz,
1
compound 10a). H NMR (500 MHz, CDCl3, see Figure 2 for proton
labeling schematic): δ 10.20 (s, 1 H, Hmeso), 9.36 (d, 1 H, J ) 4.5 Hz,
Hâ), 9.28 (d, 1 H, J ) 4.5 Hz, Hâ), 8.99 (d, 1 H, J ) 4.6 Hz, Hâ), 8.88
(d, 1 H, J ) 4.3 Hz, Hâ), 8.71 (d, 1 H, J ) 4.8 Hz, Hâ), 8.67 (d, 1 H,
J ) 4.7 Hz, Hâ), 8.58 (d, 1 H, J ) 4.8 Hz, Hâ), 8.38 (d, 1 H, J ) 7.7
Hz, H11), 8.35 (d, 1 H, J ) 4.7 Hz, Hâ), 8.26-8.17 (m, 3 H, H14 + H13
+ Hortho,
phenyls), 8.15 (d (br), 1 H, J ) 6.8 Hz, Hortho,
phenyls),
10,20
10,20
8.09 (d (br), 1 H, J ) 7.4 Hz, Hortho, 10,20 phenyls), 7.95 (d (br), 1 H, J )
7.4 Hz, Hortho, 10,20 phenyls), 7.83 (t, 1 H, J ) 7.6 Hz, H12), 7.77-7.65 (m,
7 H, H15 + Hmeta/para, 10,20 phenyls), 7.34 (d, 1 H, J ) 8.2 Hz, H8), 6.99 (t,
1 H, J ) 7.6 Hz, H9), 6.93 (d, 1 H, J ) 6.8 Hz, H16), 6.85 (d, 1 H, J
) 8.2 Hz, H7), 6.55 (d, 1 H, J ) 6.6 Hz, H10), 6.11 (dd, 1 H, J ) 2.6
Hz, J ) 10.0 Hz, H17), 5.87 (d, 1 H, J ) 7.5 Hz, H18), 5.76 (d, 1 H, J
) 8.8 Hz, H1), 5.73 (d, 1 H, J ) 7.6 Hz, H2), 5.49 (d, 1 H, J ) 2.5 Hz,
H19), 4.65 (t, 1 H, J ) 7.7 Hz, H6), 4.23 (d, 1 H, J ) 8.8 Hz, H4), 4.07
(d, 1 H, J ) 7.2 Hz, H3), 0.62 (d, 1 H, J ) 7.2 Hz, H5), -3.30 (s (br),
H14), 8.28-8.26 (m, 1 H, Hortho, 10,20 phenyls), 8.21-8.20 (m, 2 H, H13
Hâ), 8.15 (d′ (br), 2 H, Hortho, 10,20 phenyls), 8.08 (d′′ (br), 2 H, Hortho, 10,20
phenyls), 7.91 (d′′ (br), 1 H, Hortho, phenyls), 7.88 (d′ (br), 1 H, Hortho,
+
10,20
10,20 phenyls), 7.84 (t′, 1 H, J ) 7.5 Hz, H12), 7.83 (t′′, 1 H, J ) 7.5 Hz,
H12), 7.78-7.60 (m, 7 H, H15 + Hmeta/para, 10,20 phenyls), 7.32 (dd, 1 H, J
) 1.1 Hz, J ) 8.2 Hz, H8), 6.98 (t, 1 H, J ) 8.2 Hz, H9), 6.95 (dd′′,

Syntheses of Porphyrin-Bridge-Quinone Systems
J. Am. Chem. Soc., Vol. 122, No. 36, 2000 8721
1 H, J ) 1.1 Hz, J ) 6.9 Hz, H16/7), 6.93 (dd′, 1 H, J ) 1.1 Hz, J )
6.9 Hz, H16/7), 6.89 (dd′′, 1 H, J ) 1.1 Hz, J ) 8.6 Hz, H16/7), 6.85
(dd′, 1 H, J ) 1.1 Hz, J ) 7.2 Hz, H16/7), 6.60 (d′′, 1 H, J ) 1.1 Hz,
J ) 6.9 Hz, H10), 6.57 (dd′, 1 H, J ) 1.1 Hz, 6.8 Hz, H10), 6.25 (dd′′,
1 H, J ) 2.5 Hz, J ) 10.3 Hz, H17-Quinone), 6.24 (dd′, 1 H, J ) 2.5 Hz,
J ) 10.0 Hz, H17-Quinone), 6.00 (d′′, 1 H, J ) 10.0 Hz, H18-Quinone), 5.94
(d′, 1 H, J ) 10.0 Hz, H18-Quinone), 5.89-5.88 (m, 1 H, H19-Quinone),
5.64 (d′′, 1 H, J ) 2.5 Hz, H19-Quinone), 5.53 (dd′′, 1 H, J ) 6.2 Hz, J
) 9.2 Hz, H6′′-difluorophenyl), 5.43 (dd′, 1 H, J ) 6.0 Hz, J ) 9.2 Hz,
In these assemblies, the arene-based electrophores are held at
an interplanar separation of 3.1 Å, 0.3 Å below the van der
Waals contact distance51 for two cofacial aromatics. While these
systems do in fact enforce a sub-van der Waals interplanar
separation between juxtaposed arenes, systematic modulation
of the electronic structure of the components of these assemblies,
as well as the expansion of this motif to multiple levels of
π-stacking interactions, clearly define arduous synthetic tasks.
Due to the modular nature of the route exploited in the
fabrication of π-stacked compounds 1 and 2a-c, (Schemes
1-3) structure-function relationships that determine the mag-
nitude of D-A electronic coupling can be more readily probed.
Compounds 1 and 2a-c highlight the utility of the simple
and versatile 1,8-diarylnaphthalene molecular scaffold in the
fabrication of D-Sp-A systems designed to interrogate how
electronic interactions that derive from π-stacking effects impact
D-A electronic coupling. Seminal work by House36 and
Roberts52-54 detailed the molecular structure of 1,8-diphenyl-
naphthalene; X-ray crystallographic studies show that these
compounds typically manifest a structure in which the two aryl
rings are held in a cofacial arrangement with a torsional angle
of approximately 70° with respect to the naphthalene plane.
Because these compact structures manifest considerable elec-
trostatic repulsion between the compressed π-aromatic systems,
the two aryl units splay outward, resulting in an enlargement
of the naphthalene C(1)-C(9)-C(8) angle by approximately
5°. The effect of this splaying of the aryl moieties is also
apparent in the differing distances that separate the C(1) and
C(8) carbon atoms of the naphthalene ring (2.563 Å), and the
C(1′) aryl carbon atoms of the 1,8-diphenyl substituents (2.993
Å). Congruently, the naphthalene carbon framework exhibits
distorted bond lengths and angles, the most notable of which
are the displacements of the C(1) (-0.012 Å) and C(8) (+0.017
Å) atoms from the naphthalene least-squares plane. Despite these
sterically driven deformations of the naphthalene bridge, it is
noteworthy that the inter-planar separation between the C(l′)
atoms of the cofacial 1,8-aryl substituents is ∼0.4 Å less than
the arene-arene van der Waals contact distance.
H6′′-difluorophenyl), 4.60 (t′′, 1 H, J ) 7.1 Hz, H6), 4.50 (t′, 1 H, J ) 7.1
Hz, H6), 3.95 (dd′′, 1 H, J ) 6.0 Hz, J ) 8.9 Hz, H3′′-difluorophenyl), 3.85
(dd′, 1 H, J ) 6.0 Hz, J ) 8.8 Hz, H3′′-difluorophenyl), 0.56 (d (br), 1 H,
J ) 6.8 Hz, H5), 0.40 (dd′, 1 H, J ) 1.1 Hz, J ) 6.9 Hz, H5), -3.19
(s (br), 2 H, N-H). 19F NMR (200 MHz, CDCl3): δ -116.62 (d′, 1 F,
J ) 13.8 Hz), -117.90 (m′′, 1 F), -121.77 (d′, 1 F, J ) 15.8 Hz),
-123.19 (d′′, 1 F, J ) 16.2 Hz). Vis (CH2Cl2): 420 (5.40), 515 (4.08),
548 (3.64), 589 (3.64), 645 (3.49). HRMS (ESI+) m/z: 933.3070 (calcd
for C64H39F2N4O2 (M + H) 933.3041).
Results and Discussion
Design. Several criteria were considered in the design of these
D-Sp-A systems. For example, to potentially assess how the
nature of quadrupolar interactions and the magnitude of inter-
planar separations between stacked aromatic entities impact the
magnitude of D-A electronic coupling in these π-cofacially
aligned ET systems, a rigorously rigid D-Sp-A assembly is
required. These D-Sp-A compounds stand in sharp contrast
to previously synthesized porphyrin- and quinone-based ET
systems in which flexible tethers hold porphyryl and quinonyl
moieties at distances greater than or equal to the sum of their
respective van der Waals radii.6,11,15,38-41 Likewise, tethered
P-Q assemblies that are structurally reminiscent of archetypal
cyclophane compounds42-46 do not preclude dynamical pro-
cesses that modulate the magnitude of inter-ring separation or
the extent of the lateral shift47 between aromatic units on the
photoinduced charge separation and thermal charge recombina-
tion time scales and hence complicate detailed analyses of ET
rate data obtained in such systems.
The synthetic challenge of covalently linking multiple (>2)
arene units in a closely held cofacial π-stacked arrangement is
formidable; few molecular designs meet this requirement, which
is underscored by the fact that traditional routes to cyclophanic
architectures make difficult the design and synthesis of such
systems. Recent work by Wartini and Neugebauer exploited the
[2.2] paracyclophane motif to investigate intramolecular charge
delocalization between cofacial aromatic units using electro-
chemical and electron paramagnetic resonance methods.48-50
The substantial geometric constraints imposed by the 1,8-
naphthalene skeleton make this unit an ideal scaffold upon which
to elaborate an extensive series of model compounds to probe
the nature of electronic interactions in π-stacked arene systems.
As a case in point, Cozzi and Siegel have carried out detailed
nuclear magnetic resonance studies that correlate the magnitude
of arene ring rotational barriers with phenyl-ring-substituent
Hammett parameters; this work evinced the dominance of
polar-π effects over charge-transfer (CT) interactions in deter-
mining the extent of π-π delocalization in 1,8-diarylnaphtha-
lene systems,55-58 and highlighted the degree to which nuclear
dynamics can be attenuated in a π-stacked D-A system that
takes advantage of a 1,8-naphthyl pillaring motif. Given the
(37) Ishiyama, T.; Murata, M.; Miyaura, N. J. Org. Chem. 1995, 60,
7508-7510.
(38) Ganesh, K. N.; Sanders, J. K. M. J. Chem. Soc., Perkin Trans. I
1982, 1611.
(39) Leighton, P.; Sanders, J. K. M. J. Chem. Soc., Chem. Commun.
1984, 856.
(40) Abraham, R. J.; Leighton, P.; Sanders, J. K. M. J. Am. Chem. Soc.
1985, 107, 3472.
(41) Sessler, J. L.; Johnson, M. R.; Creager, S. E.; Fettinger, J. C.; Ibers,
J. A. J. Am. Chem. Soc. 1990, 112, 9310-9329.
(42) Weiser, J.; Staab, H. A. Angew. Chem., Int. Ed. Engl. 1984, 23,
623.
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J. Org. Chem. 1998, 139, 9-148.
(51) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell
University Press: Ithaca, NY, 1960.
(43) Weiser, J.; Staab, H. A. Tetrahedron Lett. 1985, 26, 6059-6062.
(44) Staab, H. A.; Weikard, J.; Ruckemann, A.; Schwogler, A. Eur. J.
Org. Chem. 1998, 2703, 3-2712.
(52) Clough, R. L.; Kung, W. J.; Marsh, R. E.; Roberts, J. D. J. Org.
Chem. 1976, 41, 3603-3609.
(53) Clough, R. L.; Roberts, J. D. J. Am. Chem. Soc. 1976, 98, 1018-
(45) Staab, H. A.; Kratzer, B.; Quazzotti, S. Eur. J. Org. Chem. 1998,
2149-2160.
1020.
(54) Clough, R. L.; Roberts, J. D. J. Org. Chem. 1978, 43, 1328-1331.
(55) Cozzi, F.; Cinquini, M.; Annunziata, R.; Dwyer, T.; Siegel, J. S. J.
Am. Chem. Soc. 1992, 114, 5729-5733.
(46) Staab, H. A.; Hauck, R.; Popp, B. Eur. J. Org. Chem. 1998, 631,
1-642.
(47) Scheidt, R. W.; Lee, Y. J. Struct. Bonding 1987, 64, 1-70.
(48) Wartini, A. R.; Staab, H. A.; Neugebauer, F. A. Eur. J. Org. Chem.
1998, 1161-1170.
(56) Cozzi, F.; Cinquini, M.; Annuziata, R.; Siegel, J. S. J. Am. Chem.
Soc. 1993, 115, 5330-5331.
(57) Cozzi, F.; Ponzini, F.; Annunziata, R.; Cinquini, M.; Siegel, J. S.
Angew. Chem., Int. Ed. Engl. 1995, 34, 1019-1020.
(58) Cozzi, F.; Siegel, J. S. Pure Appl. Chem. 1995, 67, 683-689.
(49) Wartini, A. R.; Valenzuela, J.; Staab, H. A.; Neugebauer, F. A. Eur.
J. Org. Chem. 1998, 221, 1-227.

8722 J. Am. Chem. Soc., Vol. 122, No. 36, 2000
IoVine et al.
size of the porphyrin macrocycle and the fact that multiple,
substantial dynamical restraints are enforced by a 1,8-diaryl-
naphthyl backbone, it is expected that combination of a
porphyrin basal unit with 1,8-naphthyl linkers should give rise
to D-Sp-A systems (1, 2a-c) featuring structural rigidity that
exceeds that evinced in analogous 1,8-naphthyl-linked phen-
ylenes for identical levels of π stacking.
Scheme 1a
From a synthetic perspective, the availability of 1,8-
dihalonaphthalenes36,59-62 allows exploitation of metal-catalyzed
cross-coupling strategies in the fabrication π-stacked D-Sp-A
systems that feature naphthyl scaffolds linking successive
aromatic moieties; notably in this regard, 1,8-diarylnaphthalene
syntheses have been accomplished previously via Ni(II)-
catalyzed coupling reactions utilizing Grignard reagents,52,53,59,63
as well as by Ullmann coupling.36,55,56 Corresponding Suzuki-
type synthetic routes to such compounds have also appeared in
the literature;64-66 given both the versatility and facile reaction
conditions of the Suzuki reaction we chose to build structures
1 and 2a-c using a modular synthetic approach based on a
family of aryl halide, porphyryl bromide,34,35 arylboronate,37,67,68
and porphyryl boronic acid ester reagents.14
Synthesis. Schemes 1-3 highlight the reagents, coupling
protocols, and reaction conditions that were employed in the
syntheses of compounds 1 and 2a-c. Modified Suzuki condi-
tions,28,29,69,70 developed for systems in which hydrolytic de-
boronation is a significant side reaction, were required for many
of the coupling reactions outlined herein. This cross-coupling
protocol is carried out under anhydrous conditions, employing
a dry polar solvent (DMF) and weakly soluble base (K3PO4);
all carbon-carbon bond-forming reactions involving the 1-iodo-
8-arylnaphthalenic substrate were carried out under these
conditions, which effectively suppressed deleterious dehaloge-
nation and deboronation processes.
Interestingly, the modified Suzuki conditions described above
were not effective in coupling reactions involving [5-(4′,4′,5′,5′-
tetramethyl[1′,3′,2′]dioxaborolan-2′-yl)-10,20-diphenylporphy-
rinato]zinc(II) 4 and dimethoxy-protected naphthyl halide
substrates such as 3 (Scheme 1) or 9a-c (Scheme 2). If,
however, conventional Suzuki conditions that utilize strongly
basic conditions (aqueous Ba(OH)2) are employed, coupling
proceeds in good yield. The presence of hydroxide ions,
however, does result in observable deboronation of the por-
phyrinic transmetalating reagent in this sterically encumbered
cross-coupling reaction.68,69 In a typical experiment, it was found
that an excess of porphyryl boronic ester 4 (1.5-2.0 equiv) was
sufficient to compensate for any loss of this reagent due to
hydrolytic deboronation over the time scale of the metal-
catalyzed cross-coupling reaction.
a Key: (a) Pd(Ph3)4, THF, rt (61%); (b) Ba(OH)2‚8H2O, Pd(Ph3)4,
DME/H2O, 80 °C (92%); (c) (i) BBr3, C6H6, rt; (ii) Na2CO3 (aq).
Aryl diboronic esters 7a-c proved to be key building blocks
in the fabrication of compounds 2a-c. Such diboronic acids
and their respective ester derivatives have been exploited in the
synthesis of terphenyl compounds, fused polycyclic aromatic
compounds,71 and sugar conjugates,68 and have typically
been prepared by transmetalation of a halogenated aromatic
starting material and subsequent reaction with trialkoxyboron
reagents;72-75 after hydrolysis, the conversion of the resulting
diboronic acid derivative to the corresponding diboronic ester
is accomplished via simple esterification.76
Recently, Miyaura and Masuda have reported alternative
routes to synthesize arene diboronic ester compounds,37,67 which
exploit Pd-catalyzed coupling reaction of bis(pinacolato)-
diboron37 or dialkoxyhyroborane67 species with halogenated
aromatics. This methodology not only provides a direct, one-
step conversion of dihaloaryl compounds to their corresponding
diboronic ester derivatives; it enables the high yield preparation
of arylboronates in the presence of functional groups sensitive
to strong base, and circumvents the need to isolate and
characterize the corresponding diboronic acid derivatives.68 The
wide range of commercially available 1,4-dihalogenated arene
precursors coupled with Miyaura and Masuda methodology
allows for the facile introduction of a variety of intervening
aromatic moieties that differ in their respective electronic and
(59) Kuroda, M.; Nakayama, J.; Hoshino, M.; Furusho, N.; Kawata, T.;
Ohba, S. Tetrahedron 1993, 49, 3735-3748.
(60) Letsinger, R. L.; Gilpin, J. A.; Vullo, W. J. J. Org. Chem. 1962,
27, 672-674.
(61) Kiely, J. S.; Nelson, L.; Boudjouk, P. J. Org. Chem. 1977, 42, 1480.
(62) Fieser, L. F.; Seligman, M. J. Am. Chem. Soc. 1939, 61, 136-142.
(63) Cosmo, R.; Sternhell, S. Aust. J. Chem. 1987, 40, 1107-1126.
(64) Bahl, A.; Grahn, W.; Stadler, S.; Feiner, F.; Bourhill, G.; Brauchle,
C.; Reisner, A.; Jones, P. G. Angew. Chem., Int. Ed. Engl. 1995, 34, 1485-
1488.
(71) Goldfinger, M. B.; Crawford, K. B.; Swager, T. M. J. Am. Chem.
Soc. 1997, 119, 4578-4593.
(65) Zoltewicz, J. A.; Maier, N. M.; Fabian, W. M. F. J. Org. Chem.
1997, 62, 3215.
(72) Clement, R.; Champetier, G. C. R. Acad. Sci. Paris (C) 1966, 1398-
(66) Zoltewicz, J. A.; Maier, N. M.; Fabian, W. M. F. Tetrahedron 1996,
52, 8703-8706.
1400.
(73) Musgrave, O. C. Chem. Ind. 1957, 1152.
(74) Nielsen, D. R.; McEwan, W. E. J. Am. Chem. Soc. 1957, 79, 3081-
3084.
(75) Coutts, I. G. C.; Goldschmid, H. R.; Musgrave, O. C. J. Chem.
Soc., C 1970, 488-493.
(76) Waas, J. R.; Sidduri, A. R.; Knochel, P. Tetrahedron Lett. 1992,
33, 3717-3720.
(67) Murata, M.; Watanabe, S.; Masuda, Y. J. Org. Chem. 1997, 62,
6458-6459.
(68) Todd, M. H.; Balasubramanian, S.; Abell, C. Tetrahedron Lett. 1997,
38, 6781-6784.
(69) Suzuki, A. Pure Appl. Chem. 1994, 66, 213-222.
(70) Watanabe, T.; Miyaura, N.; Suzuki, A. Synlett 1992, 207-210.

Syntheses of Porphyrin-Bridge-Quinone Systems
J. Am. Chem. Soc., Vol. 122, No. 36, 2000 8723
Scheme 2a
a Key: (a) K3PO4, Pd(Ph3)4, DMF, 100 °C; (b) Ba(OH)2‚8H2O, Pd(Ph3)4, DME/H2O, 80 °C.
steric properties between porphyrin and quinone in compound
2-type structures. The one-to-one coupling of 1,4-diboronic ester
derivatives 7a-c (Scheme 2) with 1-iodo-8-(2,5-dimethoxy-
phenyl)naphthalene 3 gives compounds 8a-c, which notably
retain boronic acid ester functionality at the 4′ position of the
naphthalene-1-aryl substituent, enabling further elaboration of
these building blocks to give compounds 9a-c.
The synthesis of the 1-iodo-8-(2,5-dimethoxyphenyl)naph-
thalene (3, Scheme 1) relies on the stoichiometric reaction of
Figure 1. Top-down representations of the syn (A) and anti (B)
1,8-diiodonaphthalene with 2,5-dimethoxyphenylzinc chloride,
employing standard Negishi cross-coupling protocols.31 The
reaction proceeds smoothly at ambient temperature and yields
mono-aryl-substituted 3 in 57% yield. Under these reaction
conditions, significant dehalogenation of the starting material
does not occur, enabling any unreacted 1,8-diiodonaphthalene
to be recovered. After successful monoarylation of the naphthyl
peri position, the iodide remaining at the 1 position becomes
extremely labile, due likely to the substantial steric crowding
enforced by the naphthyl 8-dimethoxyphenyl substituent. Suc-
cessful Suzuki coupling of 1,4-diboronic esters 7a-c at this
sensitive position was only realized when the reaction was
carried out in anhydrous DMF in the presence of phosphate
base.29,69,70 Other coupling protocols29,77 developed for arylbo-
ronic acid and arylboronate transmetalating reagents gave
dehalogenated 1-(2,5-dimethoxyphenyl)naphthalene as the sole
product for reactions involving 3 and 7a-c.
As shown in Scheme 2, the coupling of substrate 3 with 1,4-
diboronic esters 7a-c under weakly basic conditions generates
π-stacked 1-(4-[4′,4′,5′,5′-tetramethyl[1′,3′,2′]dioxaborolan-2′-
yl]aryl)-8-(2,5-dimethoxyphenyl)naphthalene structures 8a-c.
As expected, the isolated 8b and 8c products exist as a mixture
of syn and anti isomers. The syn isomer (Figure 1a) is
characterized by an eclipsed orientation of the methoxy sub-
diastereomers of 1,8-di[(2,5-disubstituted)aryl]naphthalenes.
stituents on the 8-aryl ring with the methyl (8b) or fluoro
substituents (8c) of the phenyl ring attached to the naphthyl
1-position. Congruent with earlier reports that detail the
syntheses of 1,8-diarylnaphthalenes featuring 2′,5′-substituted
phenyl groups,53,55 the anti isomer (Figure 1b) is the major
component of the diastereomeric mixture due to the reduced
steric repulsions associated with a staggered orientation of the
naphthyl 1- and 8-aryl substituents.
Similar to that observed for 8b-c, compounds 10b-c, and
quinone products 2b-c are isolated as a mixture of syn and
anti diastereomers. The observed anti:syn diasteriomeric ratios
for xylyl-spacer containing compounds 10b and 2b are similar
(1.8:1 and 1.3:1, respectively). The corresponding anti:syn
diasteriomeric ratios for the analogous difluorophenyl-bridged
species 10c and 2c are 3.3:1 and 5.1:1, respectively.78
The conversion of compounds 10a-c (Scheme 3) to their
corresponding quinoidal derivatives was accomplished using the
standard deprotection-oxidation protocol.79-83 Treatment of
(78) The degree to which syn/anti diastereomerism affects conformational
dynamics as well as donor-acceptor electronic coupling will be discussed
elsewhere.
(79) Vickery, E. H.; Pahler, L. F.; Eisenbraun, E. J. J. Org. Chem. 1979,
44, 4444-4446.
(77) Kowitz, C.; Wegner, G. Tetrahedron 1997, 53, 15553-15574.

8724 J. Am. Chem. Soc., Vol. 122, No. 36, 2000
IoVine et al.
Scheme 3a
a Key: (a) (i) BBr3, CH2Cl2, rt; (ii) Na2CO3 (aq); (iii) PbO2, CH2Cl2,
rt.
Figure 2. Proton labeling schematic for compound 10a.
porphyrin-spacer-dimethoxybenzene species 10a-c with the
demethylating agent BBr3 followed by subsequent aqueous work
up affords the intermediate hydroquinone derivatives, which can
then be isolated and oxidized with PbO2. In contrast, this simple
deprotection-oxidation sequence proved ineffective for com-
pound 5 (Scheme 1), possibly due to a combination of factors
that include: (i) the limited allowed dynamical motions of the
two stacked aromatic moieties relative to each other that derives
from the sub van der Waals interplanar separation between the
2,5-dimethoxyphenyl group and the larger porphyrin ring
system, and (ii) the cavity like structure created by the three
flanking porphyrin meso substituents (2 phenyl rings and 1
naphthyl unit), which limits access to the methoxy group
oxygens by the demethylating reagent. Congruent with this latter
plane with the other aryl ring fixed orthogonal to it.63 In this
model, the outward splaying of the 1,8-diaryl groups serves to
reduce the steric interactions between meta substituents of the
juxtaposed phenyl groups, while unavoidable, severe steric
clashes occur if ortho substituents are present on these rings.
Both calculated and experimentally derived rotational barriers53-56
demonstrate that the magnitude of ∆Gq for such a rotation in a
1,8-diarylnaphthalene structure is considerably higher when the
phenyl-ring substituents occupy ortho positions.
Because such rotational barriers are increased substantially
in simple ortho-substituted 1,8-diarylnaphthalenes (e.g., ∆Gq
(1,8-di-o-tolylnaphthalene) ) 24.1 kcal/mol)53,55 relative to that
determined for 1,8-diphenylnaphthalene and meta-substituted
1,8-diarylnaphthalenes (e.g., ∆Gq (1-(3′-isopropylphenyl)-8-
phenylnaphthalene ) 16.4 kcal/mol),54,63 steric arguments dictate
that compounds 5 and 10a-c should possess considerably
augmented barriers to aryl ring rotation relative to that elucidated
for 1,8-diphenylnaphthalenes, due to the presence of a basal
porphyrin unit in these π-stacked ET systems. Factors respon-
sible for the rigidifying effect that the porphyryl unit has in
these systems can be seen in the atom-labeled 10a structure
shown in Figure 2. The positioning and relative orientation of
naphthalenes I and II (Figure 2) function synergistically to
augment rotational barriers of their respective aryl and porphyryl
substituents. For example, from the perspective of the (porphi-
nato)zinc(II) moiety, the Câ carbon atoms that flank the
porphyrin meso carbon fused to naphthalene I’s 1-position
constitute effective ortho substituents; porphyryl ring rotations
are thus coupled to steric clashes involving the corresponding
porphyrin Câ protons with the H11 atom of naphthalene I. While
such interactions would be expected to enforce rotational barriers
similar to that observed for ortho-substituted 1,8-diarylnaph-
thalenes, it is crucial to note that naphthalene II also serves to
1
point, (Figure 3), H NMR chemical shift data (vide infra) for
the three highly shielded dimethoxyphenyl protons (δ ) 4.88,
3.13, 2.42 ppm) and the two methoxy groups (δ ) 2.20, 1.26
ppm) indicate a time-averaged structure in which the dimethoxy-
phenyl ring is canted toward the porphyrin core, thereby
orienting one of the methoxy substituents in the porphyrin cavity
and causing the other such group to be more heavily solvated.
As shown in Scheme 1, treatment of 5 with excess BBr3
generates monodemethylated compound 6 and a trace amount
of quinone 1. Resubjecting the monomethoxy species 6 to the
BBr3 deprotection (Scheme 1), enables small amounts of the
benzoquinonyl derivative (1) of compound 5 to be generated
(7%). The hydroquinone intermediate air oxidizes to the product
porphyrin-quinone complex 1 as it is formed; no attempt was
made to elucidate reaction conditions that would enable the
isolation of 5-[8′-(2′′,5′′- dihydroxyphenyl)-1′-naphthyl]-10,20-
diphenylporphyrin.
Structural Comparisons with 1,8-Diarylnaphthalene Bench-
marks. Dynamical studies that probe the extent to which rotation
is hindered in 1,8-diarylnaphthalenes36,63 and analogous asym-
metrically substituted species59,65,66,84 have been used as a tool
to probe the factors important in determining the strength of
π-π interactions.55-58 The interconversion of anti and syn
isomers requires a 180° rotation by one of the aryl ring
substituents; the transition state for this process is generally
assumed to feature one phenyl substituent in the naphthalene
restrict rotation of the (porphinato)zinc(II) unit about the Cmeso
-
to-C1-Naphthyl bond; any porphyryl ring librating motion causes
enhanced steric clashes with naphthalene II’s H6 and H5 atoms
with the macrocycle plane; consistent with such a conforma-
tional restraint playing a key role in restricting nuclear dynamics,
it is worth emphasizing at this point that the time-averaged
positions of H5 and H6 account for two of the most dramatic
1H NMR signatures observed for compounds 2a-c (vide infra).
NMR Spectroscopy. The 1-D 1H NMR spectral data obtained
for the porphyrin-(protected quinone) complexes (5 and 10a-
c) as well as the porphyrin-quinone charge-transfer systems
(1 and 2a-c) are noteworthy (Figures 3 and 4). The rigid
geometry coupled with the sub-van der Waals interplanar
separations manifest in these porphyrin-based D-Sp-A systems
(80) Chan, A. C.; Dalton, J.; Milgrom, L. R. J. Chem. Soc., Perkin Trans.
2 1982, 2.
(81) McIntosh, A. R.; Siemiarczuk, A.; Bolton, J. R.; Stillman, M. J.;
Ho, T. F.; Weedon, A. C. J. Am. Chem. Soc. 1983, 105, 7215-7223.
(82) Schmidt, J. A.; Siemiarczuk, A.; Weedon, A. C.; Bolton, J. R. J.
Am. Chem. Soc. 1985, 107, 6112-6114.
(83) Zhang, Y.; Hoernfeldt, A.-B.; Gronowitz, S. J. Heterocycl. Chem.
1995, 32, 435-444.
(84) Zoltewicz, J. A.; Maier, N. M.; Fabian, W. M. F. J. Org. Chem.
1996, 61, 7018-7021.

Syntheses of Porphyrin-Bridge-Quinone Systems
J. Am. Chem. Soc., Vol. 122, No. 36, 2000 8725
Figure 3. Upfield region of the 500 MHz 1H NMR spectrum of [5-(8′-(2′′,5′′-dimethoxyphenyl)-1′-naphthyl)-10,20-diphenylporphinato]zinc(II) 5
1
in CDCl3 at room temperature. The inset shows the H NMR spectrum of the dimethoxyphenyl region of precursor molecule 3. The 20-phenyl
substituent on the porphyrin ring has been omitted for clarity.
1
Figure 4. 500 MHz H NMR spectrum of [5-(8′-[4′′-(8′′′-[2′′′′, 5′′′′-dimethoxyphenyl]-1′l)-1′′-phenyl]-1′-naphthyl)-10,20-diphenylporphinato]-
zinc(II) (10a) in CDCl3. The proton labeling scheme is described in Figure 2.
gives rise to disparate shielding effects which distribute the
aromatic H resonances for these species over wide spectral
windows.
dimethoxyphenyl ring protons of 5 relative to the HA, HB, and
HC proton resonances of 1-iodo-8-(2,5-dimethoxyplenyl)naph-
thalene (3) which exhibit chemical shift values of 6.96, 6.85,
and 6.80 ppm respectively (Figure 3, inset). After cross-coupling
compound 3 with the porphyryl boronate complex 4, the HA,
HB, and HC dimethoxy aryl proton signals of 5 shift upfield to
2.43, 3.13, and 4.88 ppm respectively; note that the shielding
effect experienced by HA in 5 is particularly pronounced (∆δ
) 4.53 ppm with respect to the analogous resonance of 3). Note
1
Such unusually large shielding effects are apparent in
compound 5’s 1H NMR spectrum, the upfield region of which
is shown in Figure 3. This spectrum shows a number of
1
signatures that differ from the analogous H NMR spectra of
compounds 8a-c and related 1,8-diarylnaphthalenes. Most
striking is the large upfield shift observed for the three

8726 J. Am. Chem. Soc., Vol. 122, No. 36, 2000
IoVine et al.
also that the methoxy protons are shielded by the porphyrin
aromatic π system; in the precursor molecule 3, the methoxy
resonances are located at δ ) 3.79 and 3.65 ppm, while in por-
phyrin compound 5 they are observed to resonate at δ ) 2.20
and include the signals corresponding to the ortho and meta,
para protons of the porphyrin meso-phenyl substituents, as well
as the resonances assigned to the naphthalene I pillar (Figure
2). Note that each of the porphyrin â-proton resonances exhibits
a unique chemical shift, and appears as a well-resolved doublet
within the 9.36-8.50 ppm spectral domain. The naphthalenic
signals displayed in region A of Figure 4 (H11-H16, Figure 2)
resonate at chemical shift values close to that observed for
simple 1,8-disubstituted naphthalenes, as expected, given that
the protons on naphthalene I reside outside the porphyrin-
shielding region.
1
and 1.26 ppm. Congruently, the H NMR spectrum of the co-
facial porphyrin-quinone complex 1 shows that the three quin-
oidal protons resonate at chemical shifts of δ ) 1.65, 2.02, and
4.97 ppm, while the benchmark 1H chemical shift value for 1,4-
benzoquinone lies substantially downfield at δ ) 6.83 ppm.
Given the relative structural simplicity of 5 in this series of
porphyrin-based stacked aromatic structures, variable temper-
1
ature H NMR spectroscopic studies (data not shown) were
In contrast, the naphthalenic resonances highlighted in region
B (naphthalene II, Figure 2) of compound 10a’s 1H NMR
spectrum lie substantially upfield from the naphthalene I protons
and are observed over a large spectral window (7.25-0.93 ppm).
The signal corresponding to proton H5 (Figure 4) at 0.93 ppm
is shifted upfield by approximately 6.5 ppm from the analogous
resonance of compound 9a and approximately 6.4 ppm from
the corresponding resonance of compound 8a; few, if any,
diamagnetic aromatic compounds manifest shielding effects of
this magnitude. The assignment of the 0.93 ppm doublet to the
H5 nucleus was verified by a 500 MHz homonuclear COSY
experiment;88 this resonance displays a clear cross-peak to the
triplet centered at 4.95 ppm (H6). The unusually large upfield
shift experienced by H5 (and the corresponding resonances for
compounds 10b-c and 2a-c) highlight the impact that the
close, sub van der Waals separation between the porphyrin and
the edge of the upright naphthalene II pillar has upon the local
magnetic environment sampled by these nuclei (Figure 4);
importantly, the assignment of the 0.93 ppm doublet to H5
coupled with the clear trend with respect to the degree of
shielding experienced by the naphthalene II protons (H5 . H6
. H7 > H8, H9, H10) provide important structural information
regarding the orientation of naphthalene II relative to the
porphyrin plane.
performed in order to estimate the minimum barrier height for
rotation of the 1,8-naphthyl moiety’s porphyryl and 2,5-
dimethoxyphenyl substituents. Compound 5 has eight assignable
â-proton resonances (Experimental Section). Free rotation of
the porphyryl and aryl naphthalene substituents on the NMR
time scale would result in the coalescence of symmetry related
pairs of these â-proton signals to give four resonances. Interest-
ingly, the 1H NMR spectrum of 5 recorded at 120 °C is virtually
identical to the ambient temperature spectrum; the only NMR
time scale dynamics evident correlate with librations and
rotations of the porphyryl 10-and 20-phenyl substituents.85-87
All of the â-proton resonances remain sharp, with their chemical
shift values unperturbed relative to the spectrum recorded at
30 °C. Selecting a pair of well-resolved â proton resonances
that would be chemically equivalent in the advent of free rotation
of the 2,5-dimethoxyphenyl moiety that exhibited the smallest
chemical shift difference at 30 °C provides an estimate for the
activation barrier for rotation of at least 22 kcal/mol88 in these
porphyrin-based π-stacked systems. Three conclusions can be
drawn from this analysis: (i) the barrier to rotation in 5 is at a
minimum comparable to that elucidated by Clough and Roberts53
as well as Siegel and Cozzi55 for 1,8-di-(o-tolyl)-naphthalene
derivatives; (ii) replacing a simple phenyl ring with a porphyryl
group in 1,8-diarylnaphthalene systems augments the rotational
barrier height by at least 6 kcals/mol; and (iii) use of NMR
techniques (such as 2-D EXSY89) will likely not be useful probes
of rotational barriers in compounds related structurally to 5.
More dramatic shielding effects are manifest in the spectra
observed for tri-level π-stacked structures 10a-c and 2a-c.
As a case in point, note that compound 10a possesses a total of
44 protons, of which 38 are aromatic; remarkably, 38 of the 44
proton resonances are directly assignable in the 1-D spectrum.
Note that only the signals for meta and para protons of the
porphyrin 10- and 20-phenyl groups overlap (Figure 4). The
aromatic resonances for 10a have chemical shift values that
range from 0.93 ppm (H5 in Figure 4) to 10.15 ppm (the por-
phyrin meso proton). Note the extraordinary spectral resolution
evident in Figure 4, and the fact that analogous spectroscopic
features are manifest in the 1-D 1H NMR spectra of the related
10b-c and 2a-c structures as well (Experimental Section;
Supplementary Material).
Compounds 10a-c (2a-c) were engineered to provide a
cofacial arrangement of porphyrin, spacer, and dimethoxy arene
(quinone) at sub van der Waals interplanar separations while
maintaining an approximately orthogonal arrangement of the
planes of naphthalene pillars with respect to the porphyrin
macrocycle. In such a structure, electronic interactions between
the porphyrin, spacer, and dimethoxyarene (quinone) ring
systems are presumably extensive, while electronic coupling
between the naphthalene pillars and the cofacial aromatic
moieties is minimized due to unfavorable π overlap. The 1-D
1H NMR spectroscopic data obtained for compounds 10a-c
and 2a-c is consistent with expectations based on molecular
modeling that naphthalene II will be forced to adopt an upright
1
orientation. The H NMR data for these species manifest a
distribution of resonances over a wide spectral window that
derives from radically different magnetic environments sampled
by their respective aromatic protons. Certainly, if the structure
probed on the NMR time scale for molecules 2a-c and 10a-c
were consistent with a relatively small dihedral angle between
the porphyrin and naphthalene II least squares planes, one would
expect to see a more clustered pattern of chemical shifts due to
similar shielding effects experienced by the H5-H10 nuclei; this
is inconsistent with the data presented in Figure 4 (compound
10a) and analogous tabulated spectroscopic data presented in
the Experimental Section and Supplementary Material for
compounds 2a-c and 10b-c. Because both naphthalene pillars
manifest an essentially perpendicular orientation with respect
to the porphyrin least-squares plane, highly anisotropic shielding
environments for these ring systems are assured, thereby
1
Figure 4 divides the H NMR spectrum of compound 10a
into two regions labeled A and B; resonance assignments are
denoted using the labeling scheme of Figure 2. Region A of
the spectrum highlights primarily the porphyrinic resonances,
(85) Eaton, S. S.; Eaton, G. R. J. Am. Chem. Soc. 1975, 97, 3660-
3666.
(86) Eaton, S. S.; Eaton, G. R. J. Am. Chem. Soc. 1977, 99, 6594-
6599.
(87) Noss, L.; Lidell, P. A.; Moore, A. L.; Moore, T. A.; Gust, D. J.
Phys. Chem. B 1997, 101, 458-465.
(88) Iovine, P. M.; Veglia, G.; Furst, G. T.; Therien, M. J. Manuscript
in preparation.
(89) Perrin, C. L.; Dwyer, T. J. Chem. ReV. 1990, 90, 935-967.

Syntheses of Porphyrin-Bridge-Quinone Systems
J. Am. Chem. Soc., Vol. 122, No. 36, 2000 8727
1
enabling facile and unambiguous assignment of the H NMR
spectra of compounds 2a-c and 10a-c.
D-Sp-A system components in photoinduced electron transfer
and thermal charge recombination reactions.
1
Additional aspects of the 1-D 1H NMR spectrum of Figure 4
deserve further comment; note that the region B NMR signals
feature resonances that are ascribed to the spacer moiety (H1-
H4) and the dimethoxyphenyl ring (H17-H19) protons. The H1,
H2, H3, and H4 phenyl protons are shifted significantly upfield
by 2.09, 1.98, 3.41, and 3.19 ppm with respect to the proton
resonance frequency of benzene (7.27 ppm). Importantly,
resonances H1-H4 are extraordinarily sharp, indicating that the
central π -stacked phenyl ring of compound 10a remains static
on the NMR time scale. The dimethoxyphenyl ring (H17-H19)
protons, likewise, by virtue of the sub van der Waals, cofacial
arrangement of the phenyl and dimethoxyaryl ring systems, are
shielded relative to analogous aromatic resonances of compound
3 by 0.76, 1.10, and 1.25 ppm, respectively. These shielding
effects derive from the respective magnetic contributions of the
neighboring cofacial phenyl spacer and the aromatic porphyrin
moiety, as well as conformational constraints that limit nuclear
dynamics of the 2,5-dimethoxyphenyl moiety in 10a relative
to simple 1,8-aryl-substituted naphthyl derivatives. Comparison
of the relative resonance frequencies for the dimethoxyphenyl
arene protons of compounds 8a, 3, and 10a demonstrates that
the approximate shielding contribution from the phenyl moiety
only accounts for 0.58, 0.67, and 0.25 ppm of the upfield shifts
manifest by 10a’s respective H17, H18, and H19 hydrogen atoms.
Perturbations to the local magnetic environment that emanate
from the porphyrin ring current and a more rigid structure
account for the additional respective 0.18, 0.43, and 1.0 ppm
shielding experienced by H17-H19 nuclei of 10a relative to that
manifest by the analogous nuclei in compound 8a.
The 1-D H NMR experiments described herein evince that
the 1,8-naphthyl moieties cofacially align aromatic D, Sp, A
units and constrain the nuclei to reside in unusual and diverse
local magnetic environments. The quality of the NMR data
obtained for these assemblies of stacked aromatic residues is
unusually high, due in part to the fact that diamagnetic aromatic
proton NMR signals are spread over spectral windows that
1
exceed 9.0 ppm; these exceptional H NMR spectra enable
straightforward assignment of a high percentage of the aromatic
resonances in these D-Sp-A assemblies.90
The modular synthetic approach outlined herein, that exploits
metal-catalyzed cross-coupling methodologies29-35 and the
porphyryl boronate synthon14 is well suited for systematic
modification of electronic and steric properties of the donor,
bridge, and acceptor components of these cofacial ET as-
semblies. Of equal importance, this synthetic strategy allows
for control over the absolute energetics of the spacer bonding
and antibonding states, as well as the nature of the quadrupolar
interactions that exist between adjacent aromatic entities in these
π-stacked assemblies; this flexibility to engineer a wide range
of electronic factors in these systems while simultaneously
enforcing sub van der Waals contacts and eliminating lateral
shift between juxtaposed, cofacial aromatic moieties, distin-
guishes these D-Sp-A compounds from other classes of
π-stacked structures synthesized and probed to date.
Acknowledgment. M.J.T. is indebted to the United States
Department of Energy (DE-FG02-94ER14494) for their gener-
ous financial support of this work, and thanks the Alfred P.
Sloan and Camille and Henry Dreyfus Foundations for research
fellowships.
Summary and Conclusions
A prototype series of rigid π-stacked porphyrin-spacer-
quinone (P-Sp-Q) systems have been synthesized; these
systems differ in key respects from assemblies designed
previously to probe the extent of electronic coupling afford by
cofacial alignment of aromatic donor, bridge, and acceptor
moieties. The 1,8-diarylnaphthalene pillaring motif, coupled with
the presence of a basal porphyryl moiety, ensures significant
molecular rigidity in these D-Sp-A systems and limits
conformational heterogeneity in solution. These constructions
fix sub van der Waals separations between juxtaposed donor,
spacer, and acceptor units; as such, these systems will likely
manifest increased π orbital electronic mixing between the
Supporting Information Available: Syntheses and char-
acterization data for compounds 8b-c, 9b-c, and 10b-c, as
well as 1H NMR spectra for compounds 1, 2a-c, 5, and 10a-c
are available (PDF). This material can be obtained free of charge
via the Internet at http://pubs.acs.org.
JA000759A
(90) These facts, coupled with the observed sharp 1H resonances and
the plethora of short through space proton-proton distances manifest in
these structures, allow rigorous solution-phase structural determinations for
these species as a function of temperature via modern NMR methods. See
ref 88.