Investigation of stepwise covalent synthesis on a surface yielding porphyrin-based multicomponent architectures

Article abstract of DOI:10.1021/jo052650x

Porphyrins have been shown to be a viable medium for use in molecular-based information storage applications. The success of this application requires the construction of a stack of components ("electroactive surface/tether/ charge-storage molecule/linker

Full text of DOI:10.1021/jo052650x

Investigation of Stepwise Covalent Synthesis on a Surface Yielding
Porphyrin-Based Multicomponent Architectures
Izabela Schmidt,‡ Jieying Jiao,§ Patchanita Thamyongkit,‡ Duddu S. Sharada,‡
David F. Bocian,*,§ and Jonathan S. Lindsey*,‡
Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204, and
Department of Chemistry, UniVersity of California, RiVerside, California 92521-0403
jlindsey@ncsu.edu; daVid.Bocian@ucr.edu
ReceiVed December 24, 2005
Porphyrins have been shown to be a viable medium for use in molecular-based information storage applica-
tions. The success of this application requires the construction of a stack of components (“electroactive
surface/tether/charge-storage molecule/linker/electrolyte/top contact”) that can withstand high-temperature
conditions during fabrication (up to 400 °C) and operation (up to 140 °C). To identify suitable chemistry
that enables in situ stepwise synthesis of covalently linked architectures on an electroactive surface,
three sets of zinc porphyrins (22 altogether) have been prepared. In the set designed to form the base
layer on a surface, each porphyrin incorporates a surface attachment group (triallyl tripod or vinyl monopod)
and a distal functional group (e.g., pentafluorophenyl, amine, bromo, carboxy) for elaboration after surface
attachment. A second set designed for in situ dyad construction incorporates a single functional group
(alcohol, isothiocyanato) that is complementary to the functional group in the base porphyrins. A third
set designed for in situ multad construction incorporates two identical functional groups (bromo, alcohol,
active methylene, amine, isothiocyanato) in a trans configuration (5,15-positions in the porphyrin). Each
porphyrin that bears a surface attachment group was found to form a good quality monolayer on Si(100)
as evidenced by the voltammetric and vibrational signatures. One particularly successful chemistry iden-
tified for stepwise growth entailed reaction of a surface-tethered porphyrin-amine with a dianhydride
(e.g., 3,3′,4,4′-biphenyltetracarboxylic dianhydride), forming the monoimide/monoanhydride. Subsequent
reaction with a diamine (e.g., 4,4′-methylene-bis(2,6-dimethylaniline)) gave the bis(imide) bearing a
terminal amine. Repetition of this stepwise growth process afforded surface-bound oligo-imide architectures
composed of alternating components without any reliance on protecting groups. Taken together, the ability
to prepare covalently linked constructs on a surface without protecting groups in a stepwise manner
augurs well for the systematic preparation of a wide variety of functional molecular devices.
Introduction
storage molecules replace the material that presently serves as
the charge-storage medium in existing memory chips. As part
of this program, we have prepared a wide variety of redox-
active molecular architectures, particularly porphyrinic mol-
ecules, as candidates for information-storage applications.1-8
The reader is referred to refs 1-8 for recent studies; references
to earlier work can be found in these papers.
Over the past few years, we have been working to develop
approaches for molecular-based information storage. In this
approach, redox-active molecules are employed to store charge;
the presence of stored charge at a given potential represents
the storage of information. This approach is amenable to
implementation in a hybrid technology wherein the charge-
The design of the information-storage molecules includes a
redox-active unit and a tether for attachment to a surface. In
laboratory studies, the information-storage molecule is attached
‡ North Carolina State University.
§ University of California.
10.1021/jo052650x CCC: $33.50 © 2006 American Chemical Society
Published on Web 03/14/2006
J. Org. Chem. 2006, 71, 3033-3050
3033

Schmidt et al.
SCHEME 1
to an electroactive surface. A liquid or gel electrolyte is added,
and a counterelectrode is contacted to the electrolyte to complete
the electrochemical cell that constitutes the memory device.9,10
For real-world implementation in a hybrid molecular-semicon-
ductor device, the fabrication of the memory device presents a
number of challenges: (1) The charge-storage molecules must
be incorporated as components in a stack that consists of
electroactive surface/tether/charge-storage molecule/linker/
electrolyte/top contact. (2) The electrolyte should be sufficiently
thin to afford rapid electron-transfer reactions yet sufficiently
thick to minimize electric shorts (via pinholes) between the
electroactive surface and the top contact. The ideal dimensions
of the electrolyte are considered to be ∼10 nm. (3) Sufficient
charge density must be present to reliably read the stored
information.11 For the feature sizes in current memory cells,
the requisite charge density can be achieved with a monolayer
of monomeric porphyrins on a planar substrate.12,13 Nevertheless,
as feature sizes for memory cells become smaller, assemblies
of monomeric porphyrins will not have the requisite charge
density for reliable read out.8 Accordingly, multilayer architec-
tures of redox-active molecules will be needed to maintain the
required amount of charge stored in each cell.
Two distinct methods can be considered for construction of
the stack of components in a memory cell:
(1) Attach a presynthesized molecular architecture to the
electroactive surface. We have explored this approach at length;
although porphyrin dyads and triads have been prepared, such
constructs attach to the surface with an increased molecular
footprint that partially attenuates the greater charge density
expected from the multad design.8,14
(2) Build the stack in a stepwise process on the electroactive
surface. A number of multilayer assembly procedures have been
developed over the years, epitomized by the Langmuir-Blodgett
method.15 Most such approaches afford noncovalent assemblies.
Given the excursions in temperature to which the chip is
exposed, during both fabrication (up to 400 °C) and operation
(up to 140 °C),12 we focused on the fabrication of covalently
linked architectures.
A conceptual outline for in situ formation of covalently linked
architectures is shown in Scheme 1. A charge-storage molecule
that bears a surface attachment group (SAG) and distal
functional group (Y) is attached to an electroactive surface (step
1). A monomer bearing a functional group (X) complementary
to Y is then attached (step 2). In considering approaches for
further elaboration of the stack, we realized that the stepwise
growth process might be carried out with a pair of difunctional
monomers wherein each monomer bears two identical functional
groups. For example, with monomers X-M1-X and Y-M2-
Y, the coupling could be carried out step-by-step without use
of protecting groups to generate oligomers composed of -M1-
M2-M1-M2...Mi-. The success of this method would stem
from (1) growth on a surface, which effectively blocks one site
of reactivity on the initial monomer, (2) monomers that are
inherently symmetrical with respect to their two functional
groups, and (3) the use of relatively rigid monomers wherein
reactivity between components at different sites on the surface
is effectively suppressed. This approach differs from the
synthesis of biomolecules on a polymeric resin (e.g., solid-phase
peptide synthesis) or on a surface (e.g., DNA chip fabrication).
In the synthesis of peptides, for example, each amino acid
contains an amino group and a carboxylic acid moiety (x-m-
y), one of which must be protected to prevent self-condensation
while the other is activated. The advantage of avoiding
protecting groups lies in synthetic convenience and efficiency,
where every reaction carried out contributes to the growth of
the stack of components necessary to construct the memory cell.
Note that in principle the monomer M can comprise a charge-
storage entity, a spacer, an electrolyte, a tether for surface
attachment, and so forth.
(1) Balakumar, A.; Lysenko, A. B.; Carcel, C.; Malinovskii, V. L.; Gryko,
D. T.; Schweikart, K.-H.; Loewe, R. S.; Yasseri, A. A.; Liu, Z.; Bocian, D.
F.; Lindsey, J. S. J. Org. Chem. 2004, 69, 1435-1443.
(2) Muthukumaran, K.; Loewe, R. S.; Ambroise, A.; Tamaru, S.-I.; Li,
Q.; Mathur, G.; Bocian, D. F.; Misra, V.; Lindsey, J. S. J. Org. Chem.
2004, 69, 1444-1452.
(3) Loewe, R. S.; Ambroise, A.; Muthukumaran, K.; Padmaja, K.;
Lysenko, A. B.; Mathur, G.; Li, Q.; Bocian, D. F.; Misra, V.; Lindsey, J.
S. J. Org. Chem. 2004, 69, 1453-1460.
(4) Wei, L.; Padmaja, K.; Youngblood, W. J.; Lysenko, A. B.; Lindsey,
J. S.; Bocian, D. F. J. Org. Chem. 2004, 69, 1461-1469.
(5) Liu, Z.; Yasseri, A. A.; Loewe, R. S.; Lysenko, A. B.; Malinovskii,
V. L.; Zhao, Q.; Surthi, S.; Li, Q.; Misra, V.; Lindsey, J. S.; Bocian, D. F.
J. Org. Chem. 2004, 69, 5568-5577.
(6) Lysenko, A. B.; Malinovskii, V. L.; Kisari, P.; Wei, L.; Diers, J. R.;
Bocian, D. F.; Lindsey, J. S. J. Porphyrins Phthalocyanines 2005, 9, 491-
508.
(7) Wei, L.; Syomin, D.; Loewe, R. S.; Lindsey, J. S.; Zaera, F.; Bocian,
D. F. J. Phys. Chem. B 2005, 109, 6323-6330.
(8) Thamyongkit, P.; Yu, L.; Padmaja, K.; Jiao, J.; Bocian, D. F.; Lindsey,
J. S. J. Org. Chem. 2006, 71, 1156-1171.
(9) Roth, K. M.; Dontha, N.; Dabke, R. B.; Gryko, D. T.; Clausen, C.;
Lindsey, J. S.; Bocian, D. F.; Kuhr, W. G. J. Vac. Sci. Technol., B 2000,
18, 2359-2364.
(10) Roth, K. M.; Yasseri, A. A.; Liu, Z.; Dabke, R. B.; Malinovskii,
V.; Schweikart, K.-H.; Yu, L.; Tiznado, H.; Zaera, F.; Lindsey, J. S.; Kuhr,
W. G.; Bocian, D. F. J. Am. Chem. Soc. 2003, 125, 505-517.
(11) Mandelman, J. A.; Dennard, R. H.; Bronner, G. B.; DeBrosse, J.
K.; Divakaruni, R.; Li, Y.; Radens, C. J. IBM J. Res. DeV. 2002, 46, 187-
212.
(12) Kuhr, W. G.; Gallo, A. R.; Manning, R. W.; Rhodine, C. W. Mater.
Res. Soc. Bull. 2004, 838-842.
(13) Padmaja, K.; Wei, L.; Lindsey, J. S.; Bocian, D. F. J. Org. Chem.
2005, 70, 7972-7978.
(14) Clausen, C.; Gryko, D. T.; Dabke, R. B.; Dontha, N.; Bocian, D.
F.; Kuhr, W. G.; Lindsey, J. S. J. Org. Chem. 2000, 65, 7363-7370.
(15) Ulman, A. An Introduction to Ultrathin Organic Films; Academic
Press: Boston, 1991.
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Stepwise CoValent Synthesis on a Surface
TABLE 1. Prototypical Reactions for Stepwise Synthesis on a Surface
a Reaction requires an oxidant.
To successfully implement the in situ covalent synthesis
method, the following criteria must be met: (1) The Y group
in the compound forming the base layer must not attach to the
surface and must survive the surface attachment process. In this
regard, the method of attachment of the porphyrinic-based
charge-storage molecules to Si(100) entails a high-temperature
(300-400 °C) baking or sublimation process.5,10 (2) The Y
group must enable derivatization with the next layer. (3) The
derivatization chemistry must be compatible with the surface,
the components in the preceding layer (e.g., the porphyrinic
charge-storage material), and the modular components that will
be attached or deposited in subsequent layers. (4) The entire
construct must be compatible with device operation. There is
little precedent in organic chemistry to guide the selection of
Y groups and derivatization chemistries that satisfy the afore-
mentioned criteria, particularly the high temperatures of surface
attachment and device operation. Indeed, an early effort toward
this goal encountered inadequate selectivity of the SAG vs the
Y group in the base porphyrin upon attempted surface attach-
ment as well as competing surface reactions upon addition of
the components designated to form the second layer.16 Accord-
ingly, we embarked on a program to examine a set of porphyrins
for suitability in these applications. The porphyrins of interest
include the base porphyrin, which is attached to the electroactive
surface, and porphyrins that can be attached to the base
porphyrin in building a stack of redox-active compounds for
increased charge density.
CHART 1
Herein, we describe the preparation of a series of 22
porphyrins for studies of in situ covalent synthesis accompanied
by a survey of their use. The paper is divided into three parts.
Part I describes the synthesis of a family of molecules built
around a zinc porphyrin bearing a tripodal allyl tether (for
surface attachment) and a distal functional group (for subsequent
elaboration). We previously prepared a porphyrin bearing a
“triallyl” tether (Chart 1), which was found to afford a compact
molecular footprint upon attachment to Si(100).13 Part II
describes the synthesis of porphyrins that bear one or two
functional groups for use in the in situ construction of porphyrin
dyads or multads, respectively, to give high-charge density
(16) Carcel, C. M.; Laha, J. K.; Loewe, R. S.; Thamyongkit, P.;
Schweikart, K.-H.; Misra, V.; Bocian, D. F.; Lindsey, J. S. J. Org. Chem.
2004, 69, 6739-6750.
J. Org. Chem, Vol. 71, No. 8, 2006 3035

Schmidt et al.
CHART 2
CHART 3
of an amine with an anhydride can afford the acid-amide (not
shown), which proceeds to the imide (entry 2).18 Porphyrins
upon electrochemical oxidation are known to undergo nucleo-
philic substitution (entry 3).19 In the same vein, a pyrrole
attached to a porphyrin can undergo oxidative oligomerization
with pyrrole in solution (entry 4).20 A porphyrin bearing a
directly attached halo substituent may undergo nucleophilic
displacement (entry 5).19 A carboxylic acid substituent may
enable electrostatic assembly of a cationic oligomer, as required
for placement of the electrolyte material (entry 6). The lone
pair of electrons on nitrogen in a pyridyl or benzonitrile group
can be exploited for binding a metal coordination complex
(entries 7 and 8).21
A set of porphyrins (Zn-2-Zn-12) for investigation of these
derivatization processes is shown in Chart 2. Each molecule
incorporates a zinc porphyrin as an electroactive unit, a triallyl
tether, and a distal functional group. The members of a second
set of porphyrins (Zn-13, Zn-14) each bear a single vinyl tether
and a distal functional group (Chart 3).
B. Synthesis. The syntheses described herein generally make
use of mild rational methods developed over the past few years
for preparing porphyrins bearing up to four different meso
substituents.22-25 Each of Zn-2-Zn-14 is a trans-AB2C-
porphyrin. The core porphyrin-forming reaction requires access
to dipyrromethanes and 1,9-diacyldipyrromethanes. The syn-
thesis of dipyrromethanes proceeds by condensation of an
aldehyde with excess pyrrole in the presence of an acid catalyst
(e.g., TFA or InCl3) at room temperature.26,27 In this manner,
aldehydes 15a-j afforded the corresponding dipyrromethanes
(17) Battioni, P.; Brigaud, O.; Desvaux, H.; Mansuy, D.; Traylor, T. G.
Tetrahedron Lett. 1991, 32, 2893-2896.
architectures. Part III describes the results of studies that assess
the suitability of the functional groups for derivatization
purposes. This work establishes a new approach for building
molecular architectures on a surface that is particularly ap-
propriate for creating the vertical stack of components required
for hybrid molecular-semiconductor information-storage devices.
(18) Polyimides: Fundamentals and Applications; Ghosh, M. K., Mittal,
K. L., Eds.; Marcel Dekker: New York, 1996.
(19) Jaquinod, L. In The Porphyrin Handbook; Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 1,
pp 201-237.
(20) Carvalho de Medeiros, M. A.; Cosnier, S.; Deronzier, A.; Moutet,
J.-C. Inorg. Chem. 1996, 35, 2659-2664.
(21) Chambron, J.-C.; Heitz, V.; Sauvage, J.-P. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: San Diego, CA, 2000; Vol. 6, pp 1-42.
Results and Discussion
(22) Littler, B. J.; Ciringh, Y.; Lindsey, J. S. J. Org. Chem. 1999, 64,
2864-2872.
Part I. Porphyrins for Surface Attachment and Subse-
quent Elaboration. A. Molecular Design. The key reactions
we sought to examine are shown in Table 1. The reactions
selected for examination require few or no added reagents. The
reactions are primarily aimed at derivatization of porphyrins,
although a number may have a broader scope. Most of the
reactions are well precedented. For example, pentafluorophenyl-
substituted porphyrins are known to undergo nucleophilic
displacement of the p-fluoro substituent (entry 1).17 The reaction
(23) Rao, P. D.; Dhanalekshmi, S.; Littler, B. J.; Lindsey, J. S. J. Org.
Chem. 2000, 65, 7323-7344.
(24) Geier, G. R., III; Callinan, J. B.; Rao, P. D.; Lindsey, J. S. J.
Porphyrins Phthalocyanines 2001, 5, 810-823.
(25) Fan, D.; Taniguchi, M.; Yao, Z.; Dhanalekshmi, S.; Lindsey, J. S.
Tetrahedron 2005, 61, 10291-10302.
(26) Littler, B. J.; Miller, M. A.; Hung, C.-H.; Wagner, R. W.; O’Shea,
D. F.; Boyle, P. D.; Lindsey, J. S. J. Org. Chem. 1999, 64, 1391-1396.
(27) Laha, J. K.; Dhanalekshmi, S.; Taniguchi, M.; Ambroise, A.;
Lindsey, J. S. Org. Process Res. DeV. 2003, 7, 799-812.
3036 J. Org. Chem., Vol. 71, No. 8, 2006

Stepwise CoValent Synthesis on a Surface
SCHEME 2
SCHEME 3
16a, 16b,13 16c,28 16d,29 16e,23 16f, 16g, 16h,27 16i,5 and 16j,30
of which 16f and 16g are new compounds (Scheme 2).
Commercially available aldehydes were employed with the
exception of 15b,13 15f,31 and 15g,32 which were prepared as
described in the literature.
SCHEME 4
The synthesis of porphyrin Zn-2 is shown in Scheme 3.
Diacylation33 of 5-(pentafluorophenyl)dipyrromethane (16a)
followed by complexation with dibutyltin dichloride (to facilitate
workup) afforded the dipyrromethane-tin complex 17 in 18%
yield. Reduction of 17 with NaBH4 afforded the corresponding
dipyrromethane-dicarbinol. Reaction of the latter with the triallyl
dipyrromethane 16b in CH2Cl2 containing Yb(OTf)3 followed
by oxidation with DDQ gave porphyrin 2 in 30% yield. Zinc
metalation of 2 afforded Zn-2 in 76% yield.
The synthesis of porphyrin Zn-3 is shown in Scheme 4.
Reduction of the meso-unsubstituted 1,9-diacyldipyrromethane
1833 with NaBH4 afforded the corresponding dipyrromethane-
dicarbinol. The latter was condensed with 16b in the presence
of Yb(OTf)3 followed by oxidation with DDQ, affording the
free-base porphyrin 3. Treatment of 3 with NBS34,35 at 0 °C
(28) Tomizaki, K.-y.; Yu, L.; Wei, L.; Bocian, D. F.; Lindsey, J. S. J.
Org. Chem. 2003, 68, 8199-8207.
(29) Gryko, D.; Lindsey, S. J. J. Org. Chem. 2000, 65, 2249-2252.
(30) Balakumar, A.; Muthukumaran, K.; Lindsey, J. S. J. Org. Chem.
2004, 69, 5112-5115.
afforded the meso-bromo porphyrin 4 in 86% yield, showing
the chemoselectivity of the porphyrin meso-position vs the three
allyl groups. Zinc metalation of 3 or 4 afforded Zn-3 or Zn-4,
respectively.
(31) Rai, R.; Katzenellenbogen, J. A. J. Med. Chem. 1992, 35, 4150-
4159.
(32) Lal, B.; Gidwani, R. M.; Reden, J.; de Souza, N. J. Tetrahedron
Lett. 1984, 25, 2901-2904.
(33) Tamaru, S. I.; Yu, L.; Youngblood, W. J.; Muthukumaran, K.;
Taniguchi, M.; Lindsey, J. S. J. Org. Chem. 2004, 69, 765-777.
(34) (a) Nudy, L. R.; Hutchinson, H. G.; Schieber, C.; Longo, F. R.
Tetrahedron 1984, 40, 2359-2363. (b) DiMagno, S. G.; Lin, V. S.-Y.;
Therien, M. J. J. Org. Chem. 1993, 58, 5983-5993.
(35) Yu, L.; Muthukumaran, K.; Sazanovich, I. V.; Kirmaier, C.; Hindin,
E.; Diers, J. R.; Boyle, P. D.; Bocian, D. F.; Holten, D.; Lindsey, J. S.
Inorg. Chem. 2003, 42, 6629-6647.
J. Org. Chem, Vol. 71, No. 8, 2006 3037

Schmidt et al.
SCHEME 5
SCHEME 6
Diacylation of triallyl dipyrromethane 16b followed by tin
complexation afforded the dipyrromethane-tin complex 19
(Scheme 5). This valuable compound was reduced with NaBH4
to give the corresponding dipyrromethane-dicarbinol. Condensa-
tion of the latter with a dipyrromethane (16c, 16d, 16e, 16f, or
16g) followed by oxidation and metalation gave the correspond-
ing porphyrin (Zn-5, Zn-7, Zn-8, Zn-9, or Zn-11). Treatment
of Zn-5 with TBAF cleaved the trimethylsilylethyl group,
thereby providing Zn-6. Treatment of BOC-protected amino-
porphyrin 9 with TFA afforded the free-base aminoporphyrin
10 in quantitative fashion. Metalation of 10 afforded Zn-10 or
Cu-10 in 87% or 90% yield.
The condensation of 5-mesityldipyrromethane (16h) with the
aldehyde tripod 15b using TFA catalysis22 followed by oxidation
with DDQ afforded the free-base porphyrin 12 in 44% yield.
Subsequent metalation with Zn(OAc)2‚2H2O afforded Zn-12 in
97% yield (Scheme 6).
The synthesis of a porphyrin bearing a 2-pyrrolyl group and
a 4-vinylphenyl tether is outlined in Scheme 7. Tri(pyrrol-2-
yl)methane (16k) was prepared by the known reaction of triethyl
orthoformate and pyrrole with chloroacetic acid.36 Diacylation
of 5-(4-vinylphenyl)dipyrromethane (16i) followed by tin com-
plexation provided the dipyrromethane-tin complex 20 in 34%
yield. Reduction of 20 with NaBH4 afforded the corresponding
dipyrromethane-dicarbinol, which upon Yb(OTf)3-mediated
condensation with 16k followed by oxidation with DDQ gave
porphyrin 13 in 10% yield. Treatment of 13 with Zn(OAc)2‚
2H2O provided Zn-13 in 52% yield.
The synthesis of two pyridyl-substituted porphyrins (Zn-14
and Zn-22) is outlined in Scheme 8. Porphyrin Zn-14 bears a
vinyl group for surface attachment, a pyridyl group for
subsequent elaboration, and two p-tolyl groups. Porphyrin Zn-
22 is a control compound that bears a pyridyl group and three
phenyl groups. Attempts to diacylate 5-(4-pyridyl)dipyr-
romethane (16d) were unsuccessful. Accordingly, the dipyr-
romethane representing the distal side of the porphyrin was
The strategy for preparing porphyrins Zn-2, Zn-3, and Zn-4
entailed use of the triallyl dipyrromethane and the diacyldipyr-
romethane bearing the distal functional group. The preparation
of the porphyrins bearing sensitive functional groups destined
for the distal site required reversal of this strategy, wherein the
triallyl dipyrromethane carried the 1,9-diacyl moieties. The sole
consideration in the two approaches centers around the compat-
ibility of the functional group with the conditions for 1,9-
diacylation (EtMgBr/ArCOCl) and reduction to the dicarbinol
(NaBH4 in THF/MeOH).
(36) Reese, C. B.; Yan, H. Tetrahedron Lett. 2001, 42, 5545-5547.
3038 J. Org. Chem., Vol. 71, No. 8, 2006

Stepwise CoValent Synthesis on a Surface
SCHEME 7
SCHEME 8
tively) to form ether-linked dyads on the surface. A porphyrin
bearing an isothiocyanatophenyl group can be used for in situ
reaction with an aminoporphyrin (Zn-10 or Zn-11) to form
thiourea-linked dyads on the surface.
subjected to diacylation. The diacyldipyrromethane-tin complex
20 was described above, and diacyldipyrromethane-tin complex
2137 has been reported previously. Reduction of 20 or 21 with
NaBH4 afforded the corresponding dipyrromethane-dicarbinol,
which upon reaction with the pyridyl-dipyrromethane 16d in
CH2Cl2 containing Yb(OTf)3, oxidation with DDQ, and zinc
metalation afforded Zn-14 or Zn-22, respectively. It is note-
worthy that the free-base analogue (22)29,38 and Zn-2239 were
previously prepared by different routes.
Part II. Porphyrins for in Situ Synthesis. A. Porphyrins
for in Situ Dyad Formation. Two porphyrins were designed
for studies of in situ dyad formation, where the porphyrin would
be attached to the distal functional group of the base porphyrin.
Each porphyrin bears one reactive group and three nonlinking
substituents. A porphyrin containing an alcohol group can be
used for in situ reaction with a porphyrin containing a
pentafluorophenyl or bromo substituent (Zn-2 or Zn-4, respec-
We have prepared several porphyrins, each bearing a single
alcohol substituent (Zn-23,40 Zn-24,1 and Zn-25,1 Chart 4). A
porphyrin bearing a biphenylmethanol group was attractive given
the ample distance between the porphyrin and the reactive
functional group. The Suzuki coupling reaction of porphyrin
Zn-2641 and 4-(hydroxymethyl)phenylboronic acid (27) was
carried out using Pd(PPh3)4 and K2CO3 in toluene/DMF,
affording porphyrin-biphenylmethanol Zn-28 (eq 1). The condi-
tions employed were typical of those for Suzuki reactions with
porphyrins, where limited solubility requires reaction in dilute
solution.42
Porphyrins bearing isothiocyanato groups have been described
by Boyle for use in bioconjugation procedures.43 Porphyrin 2944
was metalated with zinc, and the zinc chelate was treated with
(40) (a) Yasseri, A. A.; Syomin, D.; Loewe, R. S.; Lindsey, J. S.; Zaera,
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Erratum: J. Am. Chem. Soc. 2005, 127, 9308.
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(42) (a) Yu, L.; Lindsey, J. S. Tetrahedron 2001, 57, 9285-9298. (b)
Zhou, X.; Chan, K. S. J. Chem. Soc., Chem. Commun. 1994, 2493-2494.
(43) Sutton, J. M.; Clarke, O. J.; Fernandez, N.; Boyle, R. W. Biocon-
jugate Chem. 2002, 13, 249-263.
(44) Sazanovich, I. V.; Balakumar, A.; Muthukumaran, K.; Hindin, E.;
Kirmaier, C.; Diers, J. R.; Lindsey, J. S.; Bocian, D. F.; Holten, D. Inorg.
Chem. 2003, 42, 6616-6628.
(37) Liu, Z.; Schmidt, I.; Thamyongkit, P.; Loewe, R. S.; Syomin, D.;
Diers, J. R.; Lindsey, J. S.; Bocian, D. F. Chem. Mater. 2005, 17, 3728-
3742.
(38) Barton, M. T.; Rowley, N. M.; Ashton, P. R.; Jones, C. J.; Spencer,
N.; Tolley, M. S.; Yellowlees, L. J. New J. Chem. 2000, 24, 555-560.
(39) Barton, M. T.; Rowley, N. M.; Ashton, P. R.; Jones, C. J.; Spencer,
N.; Tolley, M. S.; Yellowlees, L. J. J. Chem. Soc., Dalton Trans. 2000,
3170-3175.
J. Org. Chem, Vol. 71, No. 8, 2006 3039

Schmidt et al.
CHART 4
1,1′-thiocarbonyldi-2(1H)-pyridone (TDP) to give the isothio-
cyanatoporphyrin Zn-30 (eq 2).
B. Porphyrins for in Situ Oligomer Formation without
Protecting Groups. The porphyrins for in situ synthesis of
oligomers bear two identical functional groups on opposing sides
of the porphyrin. A set of seven such trans-A2-porphyrins (Zn-
31-Zn-37) is shown in Chart 5. The functional groups were
chosen for complementarity to the distal functional group in
the set of base porphyrins (Chart 2). The syntheses of trans-
A2-porphyrins Zn-31-Zn-36 were initiated by condensation of
an aldehyde and a dipyrromethane that is resistant to acidolysis
(e.g., 5-mesityldipyrromethane (16h) or dipyrromethane itself).22
The trans-A2-porphyrin Zn-37 relied on self-condensation of
an acetal-substituted dipyrromethane-carbinol.30 The syntheses
are described in more detail below.
The synthesis of bis(hydroxymethyl)porphyrin Zn-32 and bis-
(cyanomethyl)porphyrin Zn-33 is shown in Scheme 9. The
CHART 5
The synthesis of dibromoporphyrin Zn-31 is shown in eq 3.
Treatment of porphyrin 38 (available by condensation of
dipyrromethane and mesitaldehyde)35 with NBS afforded crude
dibromoporphyrin 31,35 which was directly metalated with Zn-
(OAc)2‚2H2O to give Zn-31 (71% yield from 38).
3040 J. Org. Chem., Vol. 71, No. 8, 2006

Stepwise CoValent Synthesis on a Surface
SCHEME 10
condensation of 5-mesityldipyrromethane (16h) and aldehyde
15l or 15m45 in CH2Cl2 containing TFA followed by oxidation
with DDQ and zinc insertion led to Zn-39 or Zn-40 in 21% or
10% yield, respectively. Free-base porphyrin 39 is known.16
Porphyrin Zn-40 has been prepared previously in higher yield
using different reaction conditions.46 Reaction of Zn-39 with
LiAlH4 gave Zn-32 in 93% yield. Treatment of Zn-40 with
KCN in the presence of 18-crown-647 afforded Zn-33 in 98%
yield.
The synthesis of diaminoporphyrin Zn-34 and diisothiocy-
anatoporphyrin Zn-35 is outlined in Scheme 10. The condensa-
tion of 5-mesityldipyrromethane with 15f using TFA catalysis
followed by oxidation with DDQ afforded 41 in 40% yield.
Treatment of this BOC-protected free-base porphyrin with TFA
afforded free-base diaminoporphyrin 34 in quantitative fashion.
Zinc metalation of the latter afforded Zn-34 in 92% yield. The
isothiocyanato group was introduced to porphyrin 34 using a
different reagent than that employed for porphyrin 29. Following
an older literature procedure,48 treatment of 34 with di-2-pyridyl
thiocarbonate (DPTC) gave diisothiocyanatoporphyrin 35 in
73% yield. Zinc metalation of 35 afforded Zn-35 in 93% yield.
The synthesis of a sterically hindered diaminoporphyrin (Zn-
36) is shown in Scheme 11. The condensation of dipyrromethane
16h and aldehyde 15g32 in CH2Cl2 containing TFA followed
SCHEME 11
SCHEME 9
by oxidation with p-chloranil led to 36 in 5% yield. Zinc
metalation of 36 afforded Zn-36 in 93% yield.
The synthesis of diformylporphyrin Zn-37 is shown in eq 4.
Porphyrin 42 was obtained by self-condensation of the carbinol
(45) Wen, L.; Li, M.; Schlenoff, J. B. J. Am. Chem. Soc. 1997, 119,
7726-7733.
(46) Jiang, B.; Jones, W. E., Jr. Macromolecules 1997, 30, 5575-5581.
J. Org. Chem, Vol. 71, No. 8, 2006 3041

Schmidt et al.
SCHEME 12
derived from a 1-acyldipyrromethane bearing a 5-acetal sub-
stituent.30 An alternative synthesis of 42 is described in the
Supporting Information. Hydrolysis of the two acetal groups
of 42 with CH2Cl2/TFA/H2O gave crude 5,15-diformylporphyrin
3730 which upon metalation gave Zn-37 (88% yield from 42).
A porphyrin containing one amino group and one formyl
substituent was prepared for examination of in situ polymeri-
zation as a complement to in situ stepwise growth. A new
synthesis of trans-AB-porphyrins was employed.25 Reaction of
16f with Eschenmoser’s reagent at room temperature followed
by hydrolysis with aqueous NaHCO3 gave the 1,9-bis(N,N-
dimethylaminomethyl)dipyrromethane (43) in 52% yield (Scheme
12). Condensation of 43 and 16j in ethanol containing Zn(OAc)2
under reflux for 2 h followed by oxidation with DDQ afforded
the porphyrin Zn-44 in 10% yield. Treatment of Zn-44 with
TFA and subsequent zinc metalation afforded Zn-45 in 20%
yield. Porphyrin Zn-45 was unstable on chromatography and
was isolated in ∼95% purity in low yield.
Part III. Physical Studies. A. Surface Coverage, Adsorp-
tion Geometry, and Binding Motif of the Zn Porphyrin
Monolayers. Prior to investigating the suitability of the surface-
tethered Zn porphyrins as base layers for in situ formation of
covalent architectures, the redox characteristics, surface cover-
age, adsorption geometry, and binding motif of each porphyrin
(Zn-2-Zn-14, Charts 2 and 3) were examined on Si(100). The
surface coverage was evaluated using electrochemical methods;
the adsorption geometry and binding motif were examined using
FTIR spectroscopy. Voltammetric and FTIR data for two
representative molecules, Zn-2 and Zn-10, are shown in Figures
1 and 2, respectively. A summary of the redox potentials, surface
coverage, adsorption geometry (expressed as the average tilt
angle with respect to the surface normal), and binding motif of
each molecule is provided in Table 2.
The general electrochemical and vibrational characteristics
of the Zn porphyrin monolayers are similar to those we have
previously reported for other carbon-tethered porphyrin mono-
layers on Si(100) and will not be elaborated on herein.7,8,13
Instead, we summarize the general characteristics and focus on
the key features of the molecules that are germane to their
suitability for in situ covalent synthesis. These characteristics
and features are as follows.
of Zn-7 and Zn-14) is in the range of (1-2) × 10-10 mol cm-2
,
which is comparable to that of other carbon-tethered Zn
porphyrins on Si(100).7,8,13 The surface coverages for Zn-7 and
Zn-14, both of which are functionalized with pyridine, are 3-4-
fold lower; we have no explanation for this observation.
(2) The adsorption geometries of all the Zn porphyrins are
similar to one another and similar to those of other carbon-
tethered Zn porphyrins on Si(100) as determined by their
vibrational signatures.7,8,13 In particular, the average tilt angle
of the porphyrin plane with respect to the surface normal is
∼38° for all the molecules.
(3) For a number of the Zn porphyrins, it appears that binding
can occur via either the targeted triallyl (T) (or vinyl (V)) group
or the functional group (Y); only Zn-2, Zn-3, Zn-5, and Zn-6
(and Zn-12, for which Y is also a T group) bind exclusively
via the targeted alkenyl group. This assessment is based on
features observed in the FTIR spectra as is illustrated by the
spectra shown for Zn-2 vs Zn-10 in Figure 2. In particular, the
spectra of the Zn-2 and Zn-10 solids exhibit bands characteristic
of the alkenyl group near 1638 (CdC stretch) and 917 cm-1
(CH deformation). Attachment to the surface via a hydrosily-
lation reaction eliminates these bands,7,8,13 as is observed for
the Zn-2 (and Zn-3, Zn-5, and Zn-6) monolayer(s). In contrast,
bands due to the alkenyl CdC stretch and CH deformation
clearly remain in the spectrum of the Zn-10 (and other)
(1) All of the Zn porphyrins that bear a surface attachment
group form good quality monolayers on Si(100) as evidenced
by both their voltammetric and their vibrational signatures. The
surface coverage of all the Zn porphyrins (with the exception
(47) Cook, F. L.; Bowers, C. W.; Liotta, C. L. J. Org. Chem. 1974, 39,
3416-3418.
(48) (a) Kim, S.; Yi, K. Y. Tetrahedron Lett. 1985, 26, 1661-1664. (b)
Han, W.; Li, S.; Lindsay, S. M.; Gust, D.; Moore, T. A.; Moore, A. L.
Langmuir 1996, 12, 5742-5744.
3042 J. Org. Chem., Vol. 71, No. 8, 2006

Stepwise CoValent Synthesis on a Surface
FIGURE 2. FTIR spectra of solid Zn-2 and Zn-10 in KBr pellets
and the corresponding Zn-2 and Zn-10 monolayers on Si(100).
used for surface attachment, as evidenced by their vibrational
signatures in the spectrum of the monolayer (e.g., C-F stretch
at 944 cm-1 for Zn-2; CtN stretch for Zn-8 at 2213 cm-1).
One clear exception is the carboxyl functional group. The spectra
of the monolayers of Zn-5, Zn-6, and Zn-9 show no evidence
of the characteristic CdO stretches (∼1700 cm-1) that are
observed for these molecules in the solids.
The observation that the functional groups of many of the
Zn porphyrins that appear to bind and/or react with the surface
thermally decompose limits the choices for the base porphyrins
in the studies of in situ formation of vertical architectures.
Indeed, the only porphyrins that bind exclusively via the alkenyl
group and exhibit thermally stable functional groups are Zn-2
and Zn-3. From this pair, Zn-2 was selected for studies of in
situ porphyrin dyad formation because the p-fluorine atom (of
the pentafluorophenyl group) should be more reactive than the
meso-H atom (of the porphyrin macrocycle) toward ipso
substitution. We also chose to investigate dyad formation using
FIGURE 1. Fast-scan (100 V s-1) voltammograms of the Zn-2 (top
panel) and Zn-10 (bottom panel) monolayers on Si(100).
monolayer(s), indicating that some of the porphyrins bind via
an alternative motif, namely, the functional group. The relative
number of molecules that bind via the alkenyl vs the functional
group cannot be readily determined from the FTIR data because
the intensities of the vibrational bands for the monolayer are
sensitive to the orientation of the transition dipoles with respect
to the surface as well as the relative number of dipoles. For
example, the strong residual alkenyl features that appear in the
spectrum of the Zn-10 monolayer would nominally suggest that
the majority of the molecules bind via the NH2 group. However,
the in situ synthesis studies described below show that this
cannot be the case (vide infra).
(4) Most of the functional groups (that do not bind to the
surface) remain intact under the high-temperature conditions
TABLE 2. Redox Potentials,a Surface-Coverage Values,b Average Tilt Angles,c and Binding Motifd for the Zn-Porphyrin Monolayers on
Si(100)
E 0/+1 (V)
monolayer
E +1/+2 (V)
monolayer
surface coverage
tilt
(deg)
porphyrin
soln
soln
(10-10 mol cm-2
)
binding motif
Zn-2
0.61
0.56
0.54
0.59
0.53
0.54
0.54
0.55
0.52
0.55
0.52
0.48
0.52
0.64
0.59
0.56
0.66
0.55
0.62
0.55
0.65
0.58
0.58
0.55
0.52
0.58
0.90
0.88
0.87
0.86
0.84
0.86
0.85
0.87
0.80
0.87
0.85
0.73
0.89
0.95
0.99
0.90
0.99
0.88
0.90
0.87
0.94
0.88
0.92
0.93
0.80
0.94
2.1
1.8
1.7
1.8
1.7
0.4
2.2
2.1
2.1
2.0
1.4
1.3
0.6
36
38
39
37
39
37
38
41
38
37
36
41
40
T
Zn-3
T
Zn-4
T or Y
T
T
Zn-5
Zn-6e
Zn-7
T or Y
T or Y
T or Y
T or Y
T or Y
T or Y
V or Y
V or Y
Zn-8
Zn-9
Zn-10f
Zn-11g
Zn-12
Zn-13h
Zn-14
a Solution potentials were obtained in CH2Cl2 containing 0.1 M n-Bu4NPF6; scan rate ) 0.1 V s-1. Values are referenced vs Ag/Ag+; FeCp2/FeCp2+ 0.20
b Porphyrin surface concentration was
V. Monolayer potentials were obtained in propylene carbonate containing 1.0 M n-Bu4NPF6; scan rate ) 100 V s-1
.
calculated from the integrated area of the E0/+1 and E+1/+2 anodic waves and using the geometrical area of the microelectrode (10-4 cm2). c Average tilt
angle determined on the basis of the intensity ratio of the in-plane pyrrole breathing (998 cm-1) and the out-of-plane â-pyrrole hydrogen deformation (797
cm-1) bands in the IR spectra. d Binding motif: T ) tripod; V ) vinyl; Y ) functional group (Charts 2 and 3). e A third redox wave is observed at ∼1.18
V in solution that is absent in the monolayer. f A third redox wave is observed at ∼1.17 V for both solution and monolayer. g A third redox wave is observed
at ∼1.35 V for both solution and monolayer. h Two additional redox waves are observed at ∼1.18 and ∼1.30 V for both solution and monolayer.
J. Org. Chem, Vol. 71, No. 8, 2006 3043

Schmidt et al.
Zn-4, which is functionalized with a meso-Br atom, as the base
porphyrin. In another series of studies, we investigated in situ
formation of polyimides on a porphyrin base layer. These studies
used Zn-10, which is functionalized with a p-aminophenyl
group. These studies are described in more detail below.
B. In Situ Formation of Zn Porphyrin Dyad Monolayers.
We attempted to form porphyrin dyads by reacting the surface-
attached monolayers of Zn-2 and Zn-4 with the three alcohol-
functionalized Zn porphyrins shown in Chart 4. The porphyrin
alcohol was deposited on the monolayer and heated at 400 °C
(see Experimental Section for details). The samples were then
interrogated using both voltammetry and FTIR spectroscopy.
The voltammetric signatures showed modest increases in charge
density (20-40%) depending on the choice of porphyrin base
layer and alcohol. In no case was dyad formation quantitative.
The changes in the FTIR spectra were less apparent, and no
signature bands could be identified that are characteristic of
formation of an ether linkage between the two porphyrins.
However, these bands would be difficult to detect because they
would fall in a spectrally congested region. We have not yet
attempted to optimize the conditions for in situ dyad formation
because this will require exploration of a relatively large
parameter space.
C. In Situ Formation of Polyimide Functionalized Zn
Porphyrin Monolayers. We examined in situ formation of
polyimides by reacting the surface-attached Zn-10 monolayers
with successive applications of 3,3′,4,4′-biphenyltetracarboxylic
dianhydride (BPTC) and either 4,4′-methylenedianiline (MDA)
or 4,4′-methylene-bis(2,6-dimethylaniline) (MMDA). The por-
phyrin dianhydride and dianiline (“imide reagents”) were
successively deposited on the monolayer and heated at 280 °C
(see Experimental Section for details). The samples were then
interrogated using both voltammetry and FTIR spectroscopy.
The voltammetric and FTIR data for the stepwise addition of
one-four aliquots of imide reagents are shown in Figures 3
and 4, respectively. The structures of the molecules that would
result from the imide formation reaction are shown in Scheme
13.
FIGURE 3. Fast-scan (100 V s-1) voltammograms of the Zn-10
monolayers on Si(100) before and after successive stepwise additions
of BPTC and MDA (top panel) or MMDA (bottom panel).
group free to couple to one end of the BPTC molecule. [Neat
BPTC only exhibits bands at 1852 and 1778 cm-1 due to the
anhydride.] Upon addition of MDA/MMDA, the 1853-cm-1
band of the anhydride disappears; the intensity of the 1778-
cm-1 band is greatly attenuated, and the intensity of the 1723-
cm-1 band of the imide increases, consistent with loss of
anhydride and formation of an additional imide linkage. Upon
addition of the next aliquots of BPTC and MDA/MMDA, the
band-intensity pattern exhibits a similar alteration. As the
number of imide linkages increases, the imide band at 1723
cm-1 gains intensity relative to the porphyrin bands. These data
do not, however, address the issue of whether the imide-forming
reaction is quantitative. Finally, we note that the successive
addition of imide reagents appears to result in an overall loss
of the intensity of the porphyrin vibrational bands, qualitatively
consistent with the loss of voltammetric data and reinforcing
the notion that the conditions used for polyimide formation are
not totally benign toward the porphyrin base monolayer. We
are continuing to investigate strategies for mitigating the
deleterious processes.
Inspection of the voltammetric data in Figure 3 shows that
subjecting the Zn-10 monolayer to BPTC and MDA/MMDA
results in an apparent successive loss of signal. The signal loss
is particularly large upon addition of MDA to the BPTC-
modified monolayer but far less severe when MMDA is added.
The attenuation of the voltammetric signal suggests that the
imide reagents may partially compromise the integrity of the
monolayer. However, these data do not provide structural
information.
The FTIR spectra shown in Figure 4 provide a much clearer
picture of the effects of the imide reagents on the porphyrin
monolayer and indeed confirm that the imide formation reaction
occurs. Upon addition of the BPTC, the vibrational signatures
of the porphyrin remain and a number of new features appear
due to the appended BPTC. The key bands are observed at 1723,
1778, and 1852 cm-1, all of which are due to CdO stretches
of the anhydride and/or imide. The 1853-cm-1 band is due to
the symmetric CdO stretch of the anhydride; the 1778-cm-1
band is due primarily to the asymmetric CdO stretch of the
anhydride, which overlaps the weaker symmetric CdO stretch
of the imide; the 1723-cm-1 band is due to the asymmetric Cd
O stretch of the imide. This latter feature provides direct
evidence that a significant number of Zn-10 molecules bind to
the surface via the triallyl group, leaving the porphyrin amino
Outlook
A major impediment to the development of hybrid molecular-
semiconductor devices resides in the identification of molecular
chemistry that is compatible with the daunting temperatures
encountered in semiconductor fabrication (up to 400 °C) and
operation (up to 140 °C). Our prior work has established that
porphyrins (1) bearing appropriate tethers undergo attachment
to surfaces at elevated temperatures (200-400 °C), (2) operate
at elevated temperatures (100 °C), and (3) can be cycled
repeatedly (>1010 cycles).49 Little precedent is available,
3044 J. Org. Chem., Vol. 71, No. 8, 2006

Stepwise CoValent Synthesis on a Surface
FIGURE 4. FTIR spectra of the Zn-10 monolayers on Si(100) before and after successive stepwise additions of BPTC and MDA (left panel) or
MMDA (right panel).
however, concerning chemistry suitable for in situ synthesis of
covalently linked molecular architectures on an electroactive
surface, particularly where the first step requires a high-
temperature attachment procedure.
The current studies further demonstrate that, among the
various chemistries explored for covalent multad synthesis, the
imide-forming reaction is particularly attractive. Coupling to
the surface-tethered molecule is readily achieved using this
reaction, and the stepwise growth of the overlayers is conve-
niently monitored via the IR signatures of the building blocks.
Polyimides are well established as polymers with high thermal
stability and a wide range of applications.18 We are currently
in the process of exploring this reaction as a means of preparing
covalently linked multiporphyrin architectures. In this approach,
a diamino-functionalized porphyrin, such as Zn-34 or Zn-36,
is substituted for MDA/MMDA and BPTC is retained as the
dianhydride. The results of these studies are beyond the scope
of the current work and will be the subject of a separate
publication.
The library of porphyrins prepared herein has enabled an
initial survey of a variety of chemistries for in situ synthesis of
molecular architectures on an electroactive surface. More
extensive studies and examination of reaction conditions with
this library are now possible. A chief finding is that multads of
porphyrinic macrocycles (and/or spacers of various composition)
can be constructed in a stepwise manner without the use of
protecting groups. Such a synthesis process has heretofore
required the use of protecting groups, wherein one cycle of
coupling has entailed three reactions: protecting group introduc-
tion, coupling, and protecting group removal. The avoidance
of protecting groups provides a more efficient process and
lessens the burden of identifying suitable conditions for protect-
ing group removal that are compatible with the components in
the nascent molecular architecture as well as the underlying
substrate.
Experimental Section
A. General Procedures for Porphyrin Formation. 5-[4-(4-
Allylhepta-1,6-dien-4-yl)phenyl]-10,20-di-p-tolyl-15-(pentafluo-
rophenyl)porphyrin (2). Following a standard procedure23 with
improved acid catalysis conditions,24 a solution of tin complex 17
(49) Liu, Z.; Yasseri, A. A.; Lindsey, J. S.; Bocian, D. F. Science 2003,
302, 1543-1545.
J. Org. Chem, Vol. 71, No. 8, 2006 3045

Schmidt et al.
SCHEME 13
(108 mg, 0.139 mmol) in dry THF/MeOH (10 mL, 10:1) was treated
with NaBH4 (303 mg, 8.01 mmol) in small portions with rapid
stirring at room temperature. After 3 h, the reaction was quenched
by slow addition of saturated aqueous NH4Cl. The reaction mixture
was extracted with CH2Cl2. The organic solution was dried (K2-
CO3) and concentrated, affording 17-diol as a slightly yellow
foamlike solid. The freshly prepared 17-diol was condensed with
16b (50 mg, 0.14 mmol) in CH2Cl2 (55 mL) containing Yb(OTf)3
(109 mg, 0.176 mmol) at room temperature for 30 min. DDQ was
added, and the reaction mixture was stirred for 1 h. TEA was added.
The mixture was filtered through a pad of alumina (CH2Cl2). The
eluted crude product was chromatographed (silica, CH2Cl2), af-
fording a purple solid (36 mg, 30%): 1H NMR δ -2.55 (s, 2H),
2.74-2.71 (overlapping peaks, 12H), 5.22-5.18 (m, 6H), 5.92-
5.81 (m, 3H), 7.56 (d, J ) 7.6 Hz, 4H), 7.69 (d, J ) 7.9 Hz, 2H),
8.10 (d, J ) 7.6 Hz, 4H), 8.15 (d, J ) 7.9 Hz, 2H), 8.76 (d, J )
4.6 Hz, 2H), 8.83 (d, J ) 4.6 Hz, 2H), 8.88 (d, J ) 4.6 Hz, 2H),
8.96 (d, J ) 4.6 Hz, 2H); LD-MS obsd 866.2; FAB-MS obsd
866.3432, calcd 866.3422 (C56H43F5Ν4); λabs 418, 485, 515, 548,
588, 644 nm.
3046 J. Org. Chem., Vol. 71, No. 8, 2006

Stepwise CoValent Synthesis on a Surface
5-[4-(4-Allylhepta-1,6-dien-4-yl)phenyl]-10,20-di-p-tolyl-15-[4-
[2-(trimethysilyl)ethoxycarbonyl]phenyl]porphinatozinc(II) (Zn-
5). Following a standard procedure,23,24 a solution of tin complex
19 (170 mg, 0.206 mmol) in dry THF/MeOH (20 mL, 10:1) was
treated with NaBH4 (390 mg, 10.3 mmol) in small portions with
rapid stirring at room temperature. After 4 h, the reaction was
quenched by slow addition of saturated aqueous NH4Cl. The
reaction mixture was extracted with CH2Cl2. The organic solution
was dried (Na2SO4) and concentrated, affording 19-diol as a slightly
yellow foamlike solid. The freshly prepared 19-diol was condensed
with 16c (75.9 mg, 0.206 mmol) in CH2Cl2 (82 mL) containing
Yb(OTf)3 (162 mg, 3.2 mM, 0.261 mmol) at room temperature for
20 min. DDQ (139 mg, 0.61 mmol) was added, and the reaction
mixture was stirred for 1 h. TEA was added. The reaction mixture
was filtered through a pad of alumina (CH2Cl2). The first fraction
was collected and concentrated. The purple solid was dissolved in
CHCl3 (20 mL), and a solution of Zn(OAc)2‚2H2O (300 mg, 1.37
mmol) in methanol (10 mL) was added. The reaction mixture was
stirred overnight at room temperature. Chromatography (silica, CH2-
Cl2) afforded a purple solid. Methanol was added, and the resulting
suspension was sonicated. Filtration afforded a purple solid (24.4
mg, 12%): 1H NMR δ 0.18 (s, 9H), 1.25-1.32 (m, 2H), 2.72 (s,
6H), 2.74 (d, J ) 7.0 Hz, 6H), 4.59 (t, J ) 8.42 Hz, 2H), 5.18-
5.23 (m, 6H), 5.83-5.92 (m, 3H), 7.56 (d, J ) 7.7 Hz, 4H), 7.68
(d, J ) 8.1 Hz, 2H), 8.11 (d, J ) 7.7 Hz, 4H), 8.16 (d, J ) 8.1 Hz,
2H), 8.30 (d, J ) 8.4 Hz, 2H), 8.41 (d, J ) 8.1 Hz, 2H), 8.87 (d,
J ) 4.8 Hz, 2H), 8.93 (d, J ) 4.8 Hz, 2H), 9.00 (d, J ) 4.8 Hz,
4H); LD-MS obsd 982.0; FAB-MS obsd 982.3628, calcd 982.3621
(C62H58N4O2SiZn); λabs 422, 549, 589 nm.
5,15-Bis[4-(4-allylhepta-1,6-dien-4-yl)phenyl]-10,20-dimesi-
tylporphyrin (12). Following a standard procedure,22 samples of
15b (17.9 mg, 0.074 mmol) and 16h (19.7 mg, 0.074 mmol) were
reacted at room temperature in CH2Cl2 (7.5 mL) containing TFA
(10 µL, 18 mM, 0.130 mmol). After 30 min, DDQ (30 mg, 15
mM, 0.13 mmol) was added, and the reaction mixture was stirred
at room temperature for 1 h. The reaction mixture was neutralized
by addition of TEA. The mixture was filtered through a pad of
alumina (CH2Cl2). The filtrate was concentrated under reduced
pressure. The residue was chromatographed (silica, CH2Cl2) to give
a purple solid. The solid was suspended in methanol. The suspension
was sonicated with methanol and filtered, affording a purple solid
(16 mg, 44%): 1H NMR δ -2.61 (s, 2H), 1.84 (s, 12H), 2.62 (s,
6H), 2.72 (d, J ) 7.3 Hz, 12H), 5.16-5.23 (m, 12H), 5.80-5.90
(m, 6H), 7.27 (s, 4H), 7.67 (d, J ) 8.2 Hz, 4H), 8.17 (d, J ) 8.4
Hz, 4H), 8.70 (d, J ) 4.8 Hz, 4H), 8.78 (d, J ) 4.8 Hz, 4H); LD-
MS obsd 966.5; FAB-MS obsd 966.5612, calcd 966.5600 (C70H70Ν4);
λabs 419, 516, 550, 593, 647 nm.
5-[4-(N-(tert-Butyloxycarbonyl)amino)phenyl]-15-(5,5-dimeth-
yl-1,3-dioxan-2-yl)porphinatozinc(II) (Zn-44). Following a stan-
dard procedure,25 a solution of 16f (168 mg, 0.500 mmol) in CH2Cl2
(5.0 mL) was treated with N,N-dimethylmethyleneammonium iodide
(Eschenmoser’s reagent, in fine powder form; 194 mg, 1.05 mmol)
at room temperature for 1 h. After standard workup, addition of
hexanes/CH2Cl2 to the crude product afforded a precipitate, which
upon filtration gave 5-[4-(N-(tert-butyloxycarbonyl)amino)phenyl]-
1,9-bis(N,N-dimethylaminomethyl)dipyrromethane (43) as a pale
yellow solid (112 mg, 52%): 1H NMR δ 1.49 (s, 9H), 2.24 (s,
12H), 3.40-3.51 (m, 4H), 5.33 (s, 1H), 5.74-5.76 (m, 2H), 5.92-
5.94 (m, 2H), 6.58 (s, 1H), 7.12 (d, J ) 8.6 Hz, 2H), 7.25 (d, J )
8.9 Hz, 2H), 8.68 (br, 1H); 13C NMR δ 28.4, 43.8, 44.8, 56.6, 107.0,
108.4, 118.9, 127.5, 129.1, 133.4, 137.1, 137.3; FAB-MS (LR) obsd
450.29, calcd 451.2947 (C26H37N5O2). A solution of 43 (230 mg,
0.500 mmol) and 16j (110 mg, 0.500 mmol) in ethanol (50 mL) at
room temperature was treated with Zn(OAc)2 (915 mg, 5.00 mmol)
and heated to reflux. After 2 h, the reaction mixture was allowed
to cool to room temperature. A sample of DDQ (340 mg, 1.50
mmol) was added, and the mixture was stirred for 15 min. TEA
(0.348 mL, 2.50 mmol) was added. The reaction mixture was
concentrated and chromatographed [column 1; silica, CH2Cl2/ethyl
acetate (3:2); column 2: silica, CH2Cl2/MeOH/TEA (50:20:1)] to
give a purple solid (32 mg, 10%): 1H NMR (THF-d8) δ 1.15 (s,
3H), 1.64 (s, 9H), 2.00 (s, 3H), 4.27 (d, J ) 11.3 Hz, 2H), 4.44 (d,
J ) 11.0 Hz, 2H), 7.93 (d, J ) 8.4 Hz, 2H), 8.11 (d, J ) 8.4 Hz,
2H), 8.17 (s, 1H), 8.92 (s, 1H), 9.08 (d, J ) 4.4 Hz, 2H), 9.36 (d,
J ) 4.4 Hz, 2H), 9.45 (d, J ) 4.8 Hz, 2H), 10.22 (d, J ) 4.8 Hz,
2H), 10.24 (s, 2H); LD-MS obsd 677.7; FAB-MS obsd 677.2037,
calcd 677.1981 (C37H35N5O4Zn); λabs 406, 536, 571 nm.
B. General Procedure for Porphyrin Metalation. 5-[4-(4-
Allylhepta-1,6-dien-4-yl)phenyl]-10,20-di-p-tolyl-15-(pentafluo-
rophenyl)porphinatozinc(II) (Zn-2). A solution of 2 (27 mg, 0.031
mmol) in CHCl3 (30 mL) was treated with a solution of Zn(OAc)2‚
2H2O (200 mg, 0.911 mmol) in methanol (6 mL). After stirring
overnight at room temperature, the mixture was concentrated. The
residue was dissolved in CH2Cl2. Chromatography (silica, CH2-
Cl2) afforded a purple powder (22 mg, 76%): 1H NMR δ 2.75-
2.72 (overlapping peaks, 12H), 5.23-5.17 (m, 6H), 5.89-5.84 (m,
3H), 7.57 (d, J ) 7.6 Hz, 4H), 7.69 (d, J ) 7.9 Hz, 2H), 8.10 (d,
J ) 7.6 Hz, 4H), 8.16 (d, J ) 7.9 Hz, 2H), 8.85 (d, J ) 4.6 Hz,
2H), 8.94 (d, J ) 4.6 Hz, 2H), 8.99 (d, J ) 4.6 Hz, 2H), 9.07 (d,
J ) 4.6 Hz, 2H); LD-MS obsd 928.2; FAB-MS obsd 928.2527,
calcd 928.2543 (C56H41F5Ν4·n); λabs 419, 547 nm.
5-[4-(4-Allylhepta-1,6-dien-4-yl)phenyl]-15-bromo-10,20-di-
p-tolylporphinatozinc(II) (Zn-4). A solution of 4 (24 mg, 0.03
mmol) in CHCl3 (25 mL) was treated with a solution of Zn(OAc)2‚
2H2O (300 mg, 1.37 mmol) in methanol (10 mL). The mixture
was stirred overnight at room temperature. The mixture was poured
into water, and the porphyrin product was extracted with CH2Cl2.
The organic extracts were washed (aqueous NaHCO3 and water)
and dried (Na2SO4). Chromatography (silica, hexanes/CH2Cl21:1)
afforded a purple solid (23 mg, 89%): 1H NMR δ 2.72-2.75
(overlapping peaks, 12H), 5.17-5.22 (m, 6H), 5.83-5.91 (m, 3H),
7.56 (d, J ) 7.7 Hz, 4H), 7.67 (d, J ) 8.4 Hz, 2H), 8.08 (d, J )
7.7 Hz, 4H), 8.12 (d, J ) 8.1 Hz, 2H), 8.87 (d, J ) 4.8 Hz, 2H),
8.93 (d, J ) 4.8 Hz, 2H), 9.03 (d, J ) 4.8 Hz, 2H), 9.77 (d, J )
4.8 Hz, 2H); LD-MS obsd 840.9; FAB-MS obsd 840.1868, calcd
840.1806 (C50H41Ν4BrZn); λabs 421, 552, 591 nm.
5-[4-(4-Allylhepta-1,6-dien-4-yl)phenyl]-15-(4-aminophenyl)-
10,20-di-p-tolylporphinatocopper(II) (Cu-10). A solution of 10
(7 mg, 0.009 mmol) in CHCl3 (20 mL) was treated with a solution
of Cu(OAc)2‚H2O (50 mg, 0.25 mmol) in methanol (6 mL). The
mixture was stirred overnight at room temperature. The mixture
was poured into water, and the porphyrin product was extracted
with CH2Cl2. The organic extract was washed (aqueous NaHCO3
and water), dried (Na2SO4), concentrated, and chromatographed
(silica, CH2Cl2), affording a purple solid (7 mg, 90%): 1H NMR δ
2.54 (s, 6H), 2.63 (br, 6H), 3.89 (s, 2H), 5.11 (br, 6H), 5.76 (br,
3H), 6.83 (br, 2H), 7.26 (br, 6H), 7.45 (br, 8H); MALDI-MS
(dithranol) obsd 852.8; FAB-MS obsd 852.3193, calcd 852.3127
(C56H47Ν5Cu); λabs 419, 541, 578 nm.
C. Other Synthetic Procedures. 5-[4-(4-Allylhepta-1,6-dien-
4-yl)phenyl]-15-bromo-10,20-di-p-tolylporphyrin (4). Following
a standard procedure,34,35 a solution of 3 (25.0 mg, 0.035 mmol) in
CHCl3 (12 mL) and pyridine (60 µL) was treated with NBS (10.0
mg, 0.057 mmol) at 0 °C. After 30 min, the reaction was quenched
with acetone (10 mL). The reaction mixture was washed with H2O
and dried (Na2SO4). Chromatography (silica, CH2Cl2) afforded a
purple solid (24.0 mg, 87%): 1H NMR δ -2.73 (s, 2H), 2.71-
2.73 (m, 12H), 5.17-5.22 (m, 6H), 5.80-9.91 (m, 3H), 7.57 (d, J
) 7.7 Hz, 4H), 7.66 (d, J ) 8.4 Hz, 2H), 8.08 (d, J ) 7.7 Hz, 4H),
8.12 (d, J ) 8.1 Hz, 2H), 8.77 (d, J ) 4.8 Hz, 2H), 8.85 (d, J )
4.8 Hz, 2H), 8.93 (d, J ) 4.8 Hz, 2H), 9.66 (d, J ) 4.8 Hz, 2H);
LD-MS obsd 779.3; FAB-MS obsd 778.2721, calcd 778.2671
(C50H43Ν4Br); λabs 421, 519, 555, 597, 653 nm.
5-[4-(4-Allylhepta-1,6-dien-4-yl)phenyl]-15-(4-carboxyphenyl)-
10,20-di-p-tolylporphinatozinc(II) (Zn-6). A solution of Zn-5 (12
mg, 12 mmol) in DMF (10 mL) was treated with TBAF (60 µL,
1.0 M solution in THF) at room temperature for 3 h. The reaction
mixture was washed with 10% NaHCO3 and water. The organic
J. Org. Chem, Vol. 71, No. 8, 2006 3047

Schmidt et al.
layer was dried (Na2SO4), concentrated, and chromatographed
[silica, CHCl2, then CH2Cl2/MeOH (3:1)]. The eluent was concen-
trated. The residue was treated with a mixture of hexanes/EtOH
(1:1) yielding a suspension that was sonicated. Filtration afforded
a purple solid (8.0 mg, 75%): 1H NMR (THF-d8) δ 2.68 (s, 6H),
2.78 (d, J ) 7.0 Hz, 6H), 5.15-5.23 (m, 6H), 5.87-5.97 (m, 3H),
7.56 (d, J ) 7.7 Hz, 2H), 7.76 (d, J ) 8.4 Hz, 4H), 8.07 (d, J )
7.7 Hz, 4H), 8.16 (d, J ) 8.1 Hz, 2H), 8.27 (d, J ) 7.7 Hz, 2H),
8.41 (br, 2H), 8.82-8.87 (overlapping peaks, 8H); LD-MS obsd
881.8; FAB-MS obsd 882.2912, calcd 882.2912 (C57H46Ν4O2Zn);
λabs (THF) 424, 557, 587 nm.
5-[4-(4-Allylhepta-1,6-dien-4-yl)phenyl]-15-(4-aminophenyl)-
10,20-di-p-tolylporphyrin (10). A solution of 9 (20.0 mg, 0.022
mmol) in CHCl3 (20 mL) was treated with TFA (5 mL) at 0 °C.
The mixture was stirred for 45 min. Then, the mixture was poured
into water, and the porphyrin product was extracted with CH2Cl2.
The organic extract was washed (aqueous NaHCO3 and water),
dried (Na2SO4), concentrated, and chromatographed (silica, CH2-
Cl2), affording a purple solid (17 mg, 96%): 1H NMR δ -2.75 (s,
2H), 2.71 (s, 6H), 2.73 (d, J ) 7.0 Hz, 6H), 4.01 (s, 2H), 5.17-
5.22 (m, 6H), 5.83-5.92 (m, 3H), 7.06 (d, J ) 8.1 Hz, 2H), 7.55
(d, J ) 8.1 Hz, 4H), 7.68 (d, J ) 8.4 Hz, 2H), 8.00 (d, J ) 8.4 Hz,
2H), 8.10 (d, J ) 7.7 Hz, 4H), 8.16 (d, J ) 8.1 Hz, 2H), 8.81 (d,
J ) 4.8, 2H), 8.86-8.88 (m, 4H), 8.91 (d, J ) 4.8, 2H); LD-MS
obsd 791.4; FAB-MS obsd 792.4125, calcd 792.4066 [(M + H)+;
C56H49Ν5]; λabs 422, 519, 555, 593, 649 nm.
6.12 (s, 1H), 7.01 (d, J ) 3.8 Hz, 2H), 7.30 (d, J ) 8.0 Hz, 4H),
7.83 (d, J ) 8.0 Hz, 4H); 13C NMR δ 13.7, 13.8, 21.8, 24.4, 24.7,
26.3, 26.4, 27.2, 25.5, 34.1, 114.0, 123.7, 129.3, 129.4, 134.9, 135.7,
142.6, 147.3, 185.2; FAB-MS obsd 781.1915, calcd 781.1875 [(M
+ H)+; M ) C39H37F5N2O2Sn]. Anal. Calcd for C39H37F5N2O2Sn:
C, 60.10; H, 4.78; N, 3.59. Found: C, 60.12; H, 4.76; N, 3.67.
5-(4-Hydroxymethylbiphen-4′-yl)-10,15,20-tri-p-tolylporphy-
rinatozinc(II) (Zn-28). Following a standard procedure,42 a mixture
of Zn-26 (100 mg, 0.125 mmol), 4-(hydroxymethyl)phenylboronic
acid (27) (38.0 mg, 0.250 mmol), anhydrous K2CO3 (138 mg, 0.998
mmol), and Pd(PPh3)4 (21.7 mg, 0.0188 mmol) in DMF/toluene
(12.5 mL) was reacted at 85 °C for 16 h using Schlenk techniques.
The reaction mixture was concentrated to dryness. The resulting
crude product was chromatographed (silica, CH2Cl2), affording a
purple solid (93.0 mg, 90%): 1H NMR (THF-d8) δ 2.54 (s, 6H),
2.69 (s, 3H), 4.32 (t, J ) 6.0 Hz, 1H), 4.71 (d, J ) 6.0 Hz, 2H),
7.56 (d, J ) 7.6 Hz, 8H), 7.93 (d, J ) 8.0 Hz, 2H), 8.04 (d, J )
7.6 Hz, 2H), 8.08 (d, J ) 8.0 Hz, 6H), 8.26 (d, J ) 7.6 Hz, 2H),
8.84 (s, 4H), 8.87 (d, J ) 4.8 Hz, 2H), 8.92 (d, J ) 4.8 Hz, 2H);
LD-MS obsd 824.5; FAB-MS obsd 824.2430, calcd 824.2494
(C54H40N4OZn); λabs (toluene) 425, 552, 593 nm; λem (λex 550 nm)
603, 650 nm.
5-(4-Isothiocyanatophenyl)-10,15,20-trimesitylporphinatozinc-
(II) (Zn-30). A solution of 29 (40 mg, 0.053 mmol) in CHCl3 (5
mL) was treated with a solution of Zn(OAc)2‚2H2O (58 mg, 0.26
mmol) in methanol (1 mL) with stirring at room temperature for
15 h. Chromatography [silica, CHCl3/THF (98:2)] afforded 5-(4-
aminophenyl)-10,15,20-trimesitylporphinatozinc(II) (Zn-29) as a
5-[4-(N-(tert-Butoxycarbonyl)amino)phenyl]dipyrrometh-
ane (16f). Following a general procedure,26 a mixture of 15f (2.00
g, 9.04 mmol) and pyrrole (16 mL, 0.23 mol) was treated with
TFA (70 µL, 0.90 mmol) and stirred at room temperature for 30
min. NaOH (0.1 M, 20 mL) and ethyl acetate (50 mL) were added,
and the organic layer was separated. After washing with brine and
water, the organic extract was dried (Na2SO4) and concentrated.
The resulting brown residue was chromatographed (silica, CH2-
Cl2) to obtain a pale yellow solid (2.48 g, 81%): mp 141-144 °C
purple solid (41 mg, 95%). Following a literature procedure,43
a
solution of this sample of Zn-29 (40 mg, 0.049 mmol) in dry CH2-
Cl2 (10 mL) was treated with 1,1′-thiocarbonyldi-2(1H)-pyridone
(TDP) (23 mg, 0.098 mmol) with stirring at room temperature under
argon for 2 h. Chromatography (silica, CH2Cl2) afforded a purple
solid (42 mg, 100%): 1H NMR (300 MHz, CD2Cl2) δ 1.83-1.86
(overlapping peaks, 18H), 2.63 (s, 9H), 7.31 (s, 6H), 7.65 (d, J )
8.4 Hz, 2H), 8.24 (d, J ) 8.1 Hz, 2H), 8.70-8.75 (m, 4H), 8.77
(d, J ) 4.8 Hz, 2H), 8.84 (d, J ) 4.5 Hz, 2H); LD-MS obsd 860.1;
FAB-MS obsd 859.2712, calcd 859.2687 (C54H45N5SZn); λabs
(toluene) 423, 550 nm.
5,15-Dibromo-10,20-dimesitylporphinatozinc(II) (Zn-31). Fol-
lowing a standard procedure,34,35 a solution of 38 (50 mg, 0.091
mmol) in CHCl3 (30 mL) and pyridine (40 µL) was treated with
NBS (40 mg, 0.23 mmol) at 0 °C. After 1 h, the reaction was
quenched by addition of acetone (5 mL). The reaction mixture was
washed with H2O, dried (Na2SO4), and concentrated. The crude
solid was dissolved in CHCl3 (20 mL) and treated overnight at room
temperature with a solution of Zn(OAc)2‚2H2O (200 mg, 0.911
mmol) in methanol (6 mL). Chromatography [silica, hexanes/CHCl3
(2:1)] afforded a purple solid (50 mg, 71%): 1H NMR (THF-d8) δ
1.84 (s, 12H), 2.64 (s, 6H), 7.33 (s, 4H), 8.67 (d, J ) 4.8 Hz, 4H),
9.59 (d, J ) 4.4 Hz, 4H); LD-MS obsd 767.4; FAB-MS obsd
764.0140, calcd 764.0129 (C38H30Br2N4Zn); λabs (THF) 428, 565,
607 nm.
5,15-Bis[(4-hydroxymethyl)phenyl]-10,20-dimesitylporphin-
atozinc(II) (Zn-32). A solution of Zn-39 (28 mg, 0.032 mmol) in
dry THF (15 mL) was treated with LiAlH4 (20 mg, 0.53 mmol)
under argon at room temperature for 1 h. Methanol was slowly
added to destroy the excess LiAlH4. The solvent was evaporated
under reduced pressure. Chromatography (silica, CH2Cl2) afforded
a purple powder (21 mg, 80%): 1H NMR δ 1.83 (s, 12H), 2.63 (s,
6H), 4.95 (d, J ) 5.12 Hz, 4H), 7.28 (s, 4H), 7.68 (d, J ) 7.7 Hz,
4H), 8.22 (d, J ) 8.1 Hz, 4H), 8.77 (d, J ) 4.4 Hz, 4H), 8.87 (d,
J ) 4.8 Hz, 4H); LD-MS obsd 820.3; FAB-MS obsd 820.2736,
calcd 820.2756 (C52H44N4O2Zn); λabs 420, 548 nm.
5,15-Bis[4-(cyanomethyl)phenyl]-10,20-dimesitylporphinatozinc-
(II) (Zn-33). Following a standard procedure,47 a solution of Zn-
40 (20 mg, 0.022 mmol) in acetonitrile/THF [20 mL, 1:1] was
treated with KCN (24 mg, 0.37 mmol) and 18-crown-6 (3 mg, 0.01
mmol) with stirring at room temperature for 2 days. The reaction
1
(dec); H NMR δ 1.51 (s, 9H), 7.43 (s, 1H), 5.92 (m, 2H), 6.14-
6.16 (m, 2H), 6.46 (m, 1H), 6.69-6.70 (m, 2H), 7.14 (d, J ) 8.4
Hz, 2H), 7.30 (d, J ) 8.4 Hz, 2H), 7.93 (br, 2H); 13C NMR δ 28.3,
43.3, 80.6, 107.1, 108.4, 117.1, 118.9, 128.9, 132.5, 136.7, 137.1,
152.8; FAB-MS obsd 337.1791; calcd 337.1790 (C20H23N3O2).
5-(4-Amino-3,5-dimethylphenyl)dipyrromethane (16g). Fol-
lowing a general procedure,26 a mixture of aldehyde 15g (500 mg,
3.35 mmol) and pyrrole (16 mL, 0.23 mol) was treated with TFA
(70 µL, 0.90 mmol) and stirred at room temperature for 16 h. The
mixture was concentrated. Chromatography (silica, CH2Cl2) gave
unreacted aldehyde followed by the title compound as a pale yellow
1
solid (175 mg, 20%): mp 137-143 °C (dec); H NMR δ 2.14 (s,
6H), 3.47 (s, 2H), 5.34 (s, 1H), 5.94-5.95 (m, 2H), 6.15-6.16
(m, 2H), 6.67-6.68 (m, 2H), 6.81 (s, 2H), 7.90 (br, 2H); 13C NMR
δ 17.9, 43.4, 106.9, 108.5, 117.0, 122.2, 128.4, 133.5, 141.8; FAB-
MS obsd 265.1572; calcd 265.1579 (C17H19N3).
Dibutyl[5,10-dihydro-5-(pentafluorophenyl)-1,9-di-p-toluoyl-
dipyrrinato]tin(IV) (17). Following a standard procedure,33 Et-
MgBr (6.4 mL, 6.4 mmol, 1.0 M in THF) was added slowly to a
tap-water cooled flask containing a solution of 16a (400 mg, 1.28
mmol) in toluene (25 mL) under argon. The reaction mixture was
stirred at room temperature for 30 min. A sample of p-toluoyl
chloride (0.42 mL, 3.2 mmol) was added over 10 min. The mixture
was stirred for an additional 1 h and then was poured into a mixture
of saturated aqueous NH4Cl and ethyl acetate. The organic layer
was washed (water and brine), dried (Na2SO4), and concentrated
to dryness. The residue was treated with TEA (0.4 mL) and Bu2-
SnCl2 (389 mg, 1.28 mmol) in CH2Cl2 (15 mL). The mixture was
stirred at room temperature for 30 min and then concentrated.
Chromatography [silica, hexanes/CH2Cl2 (1:3)] and then crystal-
lization (diethyl ether/methanol) afforded pale pink crystals (180
mg, 18%): mp 139-142 °C (dec); 1H NMR δ 0.72 (t, J ) 7.3 Hz,
3H), 0.75 (t, J ) 7.3 Hz, 3H), 1.25-1.15 (m, 4H), 1.48-1.34 (m,
4H), 1.63-1.55 (m, 4H), 2.45 (s, 6H), 7.01 (d, J ) 3.8 Hz, 2H),
3048 J. Org. Chem., Vol. 71, No. 8, 2006

Stepwise CoValent Synthesis on a Surface
mixture was concentrated and chromatographed (silica, CH2Cl2)
to give a purple solid (17 mg, 96%): 1H NMR δ 1.83 (s, 12H),
2.61 (s, 6H), 4.24 (s, 4H), 7.30 (s, 4H), 7.75 (d, J ) 7.7 Hz, 4H),
8.21 (d, J ) 8.1 Hz, 4H), 8.67 (d, J ) 4.4 Hz, 4H), 8.78 (d, J )
4.4 Hz, 4H); LD-MS obsd 838.5; FAB-MS obsd 838.2768, calcd
838.2762 (C54H42N6Zn); λabs (THF) 423, 557, 596 nm; λabs 421,
549 nm.
5,15-Bis(4-aminophenyl)-10,20-dimesitylporphyrin (34). A
solution of 41 (60 mg, 0.065 mmol) in CHCl3 (20 mL) was treated
with TFA (5 mL) at 0 °C with stirring for 45 min at room
temperature. The mixture was poured into water, and the porphyrin
product was extracted with CH2Cl2. The organic extract was washed
(aqueous NaHCO3 and water), dried (Na2SO4), concentrated, and
chromatographed (silica, CH2Cl2) to give a purple solid (47 mg,
99%): 1H NMR δ -2.59 (s, 2H), 1.84 (s, 12H), 2.63 (s, 6H), 4.02
(s, 4H), 7.06 (d, J ) 8.1 Hz, 4H), 7.28 (s, 4H), 7.99 (d, J ) 8.1
Hz, 4H), 8.66 (d, J ) 4.8 Hz, 4H), 8.87 (d, J ) 4.8 Hz, 4H); LD-
MS obsd 728.9; FAB-MS obsd 729.3712, calcd 729.3706 [(M +
H)+; M ) C50H44N6); λabs 423, 519, 556, 595, 652 nm.
5,15-Bis(4-isothiocyanatophenyl)-10,20-dimesitylporphyrin (35).
Following a literature procedure,48 a solution of 34 (32 mg, 0.044
mmol) in CHCl3 (20 mL) was treated with di-2-pyridyl thiocar-
bonate (DPTC) (21 mg, 0.090 mmol) with stirring at room
temperature. TLC analysis (silica, CH2Cl2) after 2 h indicated
incomplete reaction, whereupon additional DPTC (10 mg, 0.043
mmol) was added. The reaction mixture was stirred for 1 h. The
reaction mixture was concentrated and chromatographed (silica,
CH2Cl2) to give a purple solid (26 mg, 73%): 1H NMR δ -2.65
(s, 2H), 1.84 (s, 12H), 2.64 (s, 6H), 7.30 (s, 4H), 7.62 (d, J ) 8.1
Hz, 4H), 8.21 (d, J ) 8.4 Hz, 4H), 8.73-8.75 (m, 8H); LD-MS
obsd 812.7; FAB-MS obsd 813.2823, calcd 813.2834 [(M + H)+;
M ) C52H40N6S2]; λabs (THF) 420, 516, 551, 559, 647, 601 nm.
5,15-Diformyl-10,20-di-p-tolylporphinatozinc(II) (Zn-37). Fol-
lowing a standard procedure,30 a solution of 42 (20.0 mg, 0.028
mmol) in CH2Cl2 (24 mL) was treated with TFA/H2O (2.8 mL,
2:1) at room temperature. TLC analysis after 16 h indicated
incomplete reaction. An additional amount of TFA/H2O [5 mL (2:
1)] was added, and the reaction mixture was stirred at room
temperature for 16 h. After standard workup, the crude 37 was
dissolved in CHCl3 (20 mL) and treated overnight with a solution
of Zn(OAc)2‚2H2O (100 mg, 0.456 mmol) in methanol (6 mL) at
room temperature. The product obtained upon chromatography
(silica, CH2Cl2) was washed with hexanes and with ethanol,
affording a purple powder (10.0 mg, 60%): 1H NMR (THF-d8) δ
2.73 (s, 6H), 7.62 (d, J ) 7.3 Hz, 4H), 8.07 (d, J ) 8.1 Hz, 4H),
8.97 (d, J ) 4.8 Hz, 4H) 10.16 (d, J ) 4.8 Hz, 4H), 12.65 (s, 2H)
LD-MS obsd 608.7; FAB-MS obsd 608.1211, calcd 608.1191
(C36H24N4O2Zn); λabs (THF) 432, 633 nm.
5-(4-Aminophenyl)-15-formylporphinatozinc(II) (Zn-45). A
solution of Zn-44 (20 mg, 0.029 mmol) in CH2Cl2 (20 mL) was
treated with TFA/H2O (1.4 mL, 1:1) at room temperature for 16 h.
The organic layer was washed (5% aqueous NaHCO3 and water),
dried (Na2SO4), and concentrated. The 1H NMR spectrum showed
incomplete deprotection of the amino group. The solid was
dissolved in CH2Cl2 (20 mL), and TFA (4 mL) was slowly added.
After 1 h, the organic layer was washed (5% aqueous NaHCO3
and water), dried (Na2SO4), and concentrated. The resulting residue
was dissolved in CHCl3 (20 mL) and treated with Zn(OAc)2‚2H2O
(200 mg, 0.911 mmol) at room temperature for 16 h. Chromatog-
raphy (silica, CH2Cl2) afforded a purple-green solid, which proved
somewhat unstable on chromatography (3 mg, 20%): 1H NMR
(THF-d8) δ 5.00 (br, 2H), 7.01 (d, J ) 8.4 Hz, 2H), 7.88 (d, J )
8.1 Hz, 2H), 9.07 (d, J ) 4.4, 2H), 9.25 (d, J ) 4.4 Hz, 2H), 9.48
(d, J ) 4.4 Hz, 2H), 10.20 (s, 2H), 10.30 (d, J ) 4.8 Hz, 2H),
12.61 (s, 1H); MALDI-MS (dithranol) obsd 491.3, calcd 491.1
(C27H17N5O); λabs (THF) 421, 557, 597 nm.
used in the preparation of the porphyrin monolayers, the in situ
synthesis studies, and the electrochemical and FTIR characterization
include benzonitrile, CH2Cl2, N,N-dimethylacetamide (DMAc),
3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPTC), 4,4′-meth-
ylenedianiline (MDA), and 4,4′-methylene-bis(2,6-dimethylaniline)
(MMDA); all were used as received. The propylene carbonate used
for the electrochemical studies was dried on molecular sieves before
use. The Bu4NPF6 supporting electrolyte was recrystallized three
times from methanol and dried at 110 °C under vacuum.
Porphyrin Monolayer Preparation. The porphyrin monolayers
on Si(100) were prepared using a high-temperature (400 °C), short
time (2 min) “baking” attachment procedure described previously.5
The monolayers for the electrochemical experiments were prepared
by dispensing a 2 µL drop of the porphyrin solution onto the surface
of a microelectrode contained in a sparged VOC vial sealed under
Ar. The monolayers prepared for the FTIR experiments utilized
much larger platforms (∼1 cm2) and consequently required a larger
drop size, ∼50 µL. After deposition, the vial containing the Si
substrate was heated on a hotplate at 400 °C for 2 min and then
removed and purged with Ar until cooling to room temperature.
Finally, the Si substrate was rinsed, sonicated five times with
anhydrous CH2Cl2, and purged dry with Ar.
In Situ Synthesis Studies. The studies of in situ formation of
porphyrin dyads were performed by first preparing a particular
porphyrin monolayer as described above. The cleaned and washed
substrate was then placed in a sealed vial and purged with Ar, and
a drop of the solution containing the second porphyrin was
introduced (5 and 50 µL for the electrochemical and FTIR
substrates, respectively). After deposition, the vial containing the
Si substrate was heated on a hotplate at 400 °C for 2 min and then
removed and purged with Ar until cooling to room temperature.
Finally, the Si substrate was rinsed, sonicated five times with
anhydrous CH2Cl2, and purged dry with Ar.
The studies of in situ formation of polyimides on the Zn-10
monolayers followed the same general procedure as that described
above for the porphyrin dyads with the following modifications.
(1) After deposition of the BPTC solution (5 mM in DMAc) onto
the porphyrin-modified substrate, the substrate was heated on a
hotplate at 280 °C for 2 min, removed and purged under Ar until
cooling to room temperature, washed and sonicated with CH2Cl2,
and dried with Ar. (2) After electrochemical or spectroscopic
interrogation, the sample was again washed and dried, and a solution
of MDA/MMDA (5 mM in DMAc) was introduced onto the
substrate. The sample was then heated at 280 °C for 2 min, removed
and purged under Ar until cooled to room temperature, washed
and sonicated with CH2Cl2, and dried with Ar. (3) After the second
electrochemical and spectroscopic interrogation, the sample was
rewashed and dried, and the above procedure was repeated with
alternating doses of BPTC and MDA/MMDA.
Electrochemical Measurements. The electrochemical measure-
ments of the porphyrins in solution were made in a standard three-
electrode cell using Pt working and counterelectrodes and a Ag/
Ag+ reference electrode. The solvent/electrolyte was CH2Cl2
containing 0.1 M n-Bu4NPF6.
The electrochemical measurements on the porphyrin monolayers
were performed in a two-electrode configuration using highly doped
p-type Si(100) working electrodes (100 × 100 µm) and a Ag
counter-/reference electrode, fabricated as described earlier.10
Propylene carbonate containing 1.0 M n-Bu4NPF6 was used as
solvent/electrolyte. The cyclic voltammograms were recorded using
a Gamry Instruments PC4-FAS1 femtostat running PHE 200
framework and Echem Analyst software. The charge density in the
monolayer was determined by integration of the total charge of
both anodic waves and by using the geometrical dimensions of the
microelectrode. The surface coverage of the porphyrin monomers
and dyads was determined by scaling the charge density by a factor
of 2 or 4, respectively.
D. Physical Studies. Materials. The substrates for surface
attachment were prepared from commercially available highly
doped p-type Si(100) wafers. The anhydrous solvents and chemicals
FTIR Spectroscopy. The FTIR spectra of the porphyrins in both
solid and monolayer forms were collected at room temperature with
J. Org. Chem, Vol. 71, No. 8, 2006 3049

Schmidt et al.
a spectral resolution of 4 cm-1. The spectra of the solid porphyrin
samples were obtained in KBr pellets (∼1-2 wt % porphyrin).
These spectra were collected in transmission mode using a room-
temperature DTGS detector by averaging over 32 scans.
Acknowledgment. This work was supported by the DARPA/
DMEA (Award Nos. H94003-04-2-0404 and H94003-05-2-
0504) and by ZettaCore, Inc. We thank Dr. Robert S. Loewe
for the preparation of porphyrin Zn-30. Mass spectra were
obtained at the Mass Spectrometry Laboratory for Biotechnology
at North Carolina State University. Partial funding for the facility
was obtained from the North Carolina Biotechnology Center
and the NSF.
The IR spectra of the monolayers were obtained using a Harrick
Scientific horizontal reflection Ge-attenuated total reflection ac-
cessory (GATR, 65° incidence angle). The Si substrates were placed
in contact with the flat surface of a semispherical Ge crystal that
serves as the optical element, and IR spectra were collected with p
polarized light using a liquid-nitrogen cooled medium-bandwidth
MCT detector (600-4000 cm-1) and averaging 256 scans. The Ge
crystal was cleaned with neat 2-butanone before every experiment,
and the GATR accessory was purged with dry N2 during data
acquisition. The spectra of porphyrin monolayers were referenced
against that of a hydrogen-terminated Si(100) surface previously
subjected to the same deposition conditions as those used to obtain
the monolayer but using only the neat deposition solvent.
Supporting Information Available: Complete Experimental
Section including an alternative synthesis of 42; 1H NMR and 13
C
NMR spectra for selected compounds; and LD-MS and/or MALDI-
MS spectra for all new porphyrins. This material is available free
of charge via the Internet at http://pubs.acs.org.
JO052650X
3050 J. Org. Chem., Vol. 71, No. 8, 2006