Swallowtail porphyrins: Synthesis, characterization and incorporation into porphyrin dyads

Article abstract of DOI:10.1021/jo049860e

The incorporation of symmetrically branched tridecyl (" swallowtail") substituents at the meso positions of porphyrins results in highly soluble building blocks. Synthetic routes have been investigated to obtain porphyrin building blocks bearing 1-4 swallowtail groups. Porphyrin dyads have been synthesized in which the zinc or free base (Fb) porphyrins are joined by a 4,4′-diphenylethyne linker and bear swallowtail (or n-pentyl) groups at the nonlinking meso positions. The swallowtail-substituted Zn ₂- and ZnFb-dyads are readily soluble in common organic solvents. Static absorption and fluorescence spectra and electrochemical data show that the presence of the swallowtail groups slightly raises the energy level of the filled a_2u(π) HOMO. EPR studies of the π-cation radicals of the swallowtail porphyrins indicate that the torsional angle between the proton on the alkyl carbon and p-orbital on the meso carbon of the porphyrin is different from that of a porphyrin bearing linear pentyl groups. Regardless, the swallowtail substituents do not significantly affect the photophysical properties of the porphyrins or the electronic interactions between the porphyrins in the dyads. In particular, time-resolved spectroscopic studies indicate that facile excited-state energy transfer occurs in the ZnFb dyad, and EPR studies of the monocation radical of the Zn₂-dyad show that interporphyrin ground-state hole transfer is rapid.

Full text of DOI:10.1021/jo049860e

Sw a llow ta il P or p h yr in s: Syn th esis, Ch a r a cter iza tion a n d
In cor p or a tion in to P or p h yr in Dya d s
Patchanita Thamyongkit,† Markus Speckbacher,† J ames R. Diers,‡ Hooi Ling Kee,§
Christine Kirmaier,§ Dewey Holten,*,§ David F. Bocian,*,‡ and J onathan S. Lindsey*,†
Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204,
Department of Chemistry, University of California, Riverside, California 92521-0403, and
Department of Chemistry, Washington University, St. Louis, Missouri 63130-4889
jlindsey@ncsu.edu; david.bocian@ucr.edu; holten@wuchem.wustl.edu
Received J anuary 24, 2004
The incorporation of symmetrically branched tridecyl (“swallowtail”) substituents at the meso
positions of porphyrins results in highly soluble building blocks. Synthetic routes have been
investigated to obtain porphyrin building blocks bearing 1-4 swallowtail groups. Porphyrin dyads
have been synthesized in which the zinc or free base (Fb) porphyrins are joined by a 4,4′-
diphenylethyne linker and bear swallowtail (or n-pentyl) groups at the nonlinking meso positions.
The swallowtail-substituted Zn2- and ZnFb-dyads are readily soluble in common organic solvents.
Static absorption and fluorescence spectra and electrochemical data show that the presence of the
swallowtail groups slightly raises the energy level of the filled a2u(π) HOMO. EPR studies of the
π-cation radicals of the swallowtail porphyrins indicate that the torsional angle between the proton
on the alkyl carbon and p-orbital on the meso carbon of the porphyrin is different from that of a
porphyrin bearing linear pentyl groups. Regardless, the swallowtail substituents do not significantly
affect the photophysical properties of the porphyrins or the electronic interactions between the
porphyrins in the dyads. In particular, time-resolved spectroscopic studies indicate that facile
excited-state energy transfer occurs in the ZnFb dyad, and EPR studies of the monocation radical
of the Zn2-dyad show that interporphyrin ground-state hole transfer is rapid.
CHART 1
In tr od u ction
One of the challenges in creating molecules with
nanoscale dimensions lies in achieving a defined 3-di-
mensional architecture while maintaining adequate solu-
bility in appropriate solvents. Solubility is essential for
carrying out synthetic transformations, purification pro-
cedures, and diverse physical studies. The solubility
problem is significant with planar aromatic compounds
such as porphyrins and is exacerbated in arrays com-
posed of multiple porphyrins in defined architectures.
Indeed, the limited solubility of multiporphyrin arrays
often constrains their synthetic manipulation and limits
their size. The most prevalent meso substituents that
have been employed to increase the solubility over that
of meso-tetraphenylporphyrins include 3,5-di-tert-butyl-
phenyl,1-4 mesityl,2,5,6 and pentyl7 groups. The develop-
ment of facile syntheses of multiporphyrin arrays would
benefit from new substituents that impart high solubility
in organic solvents.
Attractive candidates to reach the goal of relatively
high solubility are the readily available swallowtail
substituents, which are symmetrically branched alkyl
groups. Langhals introduced swallowtail substituents
into the poorly soluble 3,4:9,10-perylenediimide chro-
mophore to achieve increased solubility (Chart 1).8 The
high solubility of the resulting swallowtail perylene-
diimides prompted us to consider the use of swallowtail
substituents with porphyrins.
† North Carolina State University.
‡ University of California, Riverside.
A number of porphyrins bearing branched alkyl groups
at the meso positions have been prepared over the years.
The branched alkyl substituents include isopropyl,9
§ Washington University.
(1) Miyamoto, T. K.; Sugita, N.; Matsumoto, Y.; Sasaki, Y.; Konno,
M. Chem. Lett. 1983, 1695-1698.
(2) Volz, H.; Hassler, M.; Scha¨ffer, H. Z. Naturforsch. 1986, 41b,
1265-1272.
(3) Crossley, M. J .; Burn, P. L. J . Chem. Soc., Chem. Commun. 1987,
39-40.
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Marguerettaz, A. M. J . Org. Chem. 1987, 52, 827-836.
(8) (a) Demmig, S.; Langhals, H. Chem. Ber. 1988, 121, 225-230.
(b) Langhals, H.; Demmig, S.; Potrawa, T. J . Prakt. Chem. 1991, 333,
733-748. (c) Feiler, L.; Langhals, H.; Polborn, K. Liebigs Ann. 1995,
1229-1244. (d) Langhals, H. Heterocycles 1995, 40, 477-500. (e)
Langhals, H.; Ismael, R.; Yu¨ru¨k, O. Tetrahedron 2000, 56, 5435-5441.
(4) Chardon-Noblat, S.; Sauvage, J .-P. Tetrahedron 1991, 47, 5123-
5132.
(5) Badger, G. M.; J ones, R. A.; Laslett, R. L. Aust. J . Chem. 1964,
17, 1028-1035.
(6) Lindsey, J . S.; Wagner, R. W. J . Org. Chem. 1989, 54, 828-836.
10.1021/jo049860e CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/27/2004
3700
J . Org. Chem. 2004, 69, 3700-3710

Swallowtail Porphyrins
isobutyl,10 isopentyl,11 tert-butyl,12 cyclopropyl,13 cyclo-
hexyl,14 and adamantyl15 groups. A major motivation for
many of the studies was to explore the effects of bulky
substituents on structural deformation of the porphyrin
macrocycle. Such substituents include the tert-butyl
group, which causes significant deformation, and the
isopropyl group, which causes very little deformation.
Evidence for deformation of the macrocycle stems from
X-ray characterization, electronic spectroscopy, and
electrochem-
SCHEME 1
istry.15-17 The isopropyl and isopentyl groups are ex-
amples of short swallowtail substituents. The resulting
porphyrins generally contained four identical branched-
chain substituents, without synthetic handles for elabo-
ration into arrays.
In this paper, we describe methodology for the syn-
thesis of porphyrins bearing 1-4 swallowtail substitu-
ents. The swallowtail substituents have 13 carbons, are
symmetrically branched about carbon 7, and are attached
at the porphyrin meso positions. The swallowtail groups
have been incorporated into precursors employed in the
rational synthesis of porphyrins, including dipyrro-
methanes and acyldipyrromethanes. Dyads composed of
a zinc porphyrin and a free base (Fb) porphyrin or two
zinc porphyrins have been prepared, where each porphy-
rin bears three swallowtail substituents at the nonlinking
meso positions and the porphyrins are joined via a
diphenylethyne linker. The electrochemical and photo-
physical properties of the swallowtail-containing com-
pounds have been investigated along with excited-state
energy transfer in the ZnFb dyad and ground-state hole
transfer in the monocation of the Zn2 dyad. Collectively,
these studies reveal that the swallowtail substituents
provide good solubility without significantly altering the
photophysical properties of the porphyrins or the elec-
tronic interactions in dyads.
aldehyde. The required 7-formyltridecane (1) was pre-
pared according to literature procedures18 by the Wittig
reaction of 7-tridecanone and (methoxymethyl)tri-
phenylphosphonium ylide followed by acid hydrolysis.
Condensation of 1 with pyrrole in CHCl3 (containing 0.13
M ethanol) using BF3‚O(Et)2 in the presence of NaCl19
followed by oxidation with DDQ afforded porphyrin 2 in
6% yield. The porphyrin bears four swallowtail groups
at the meso positions (Scheme 1). Attempts to improve
the yield by increasing the concentration of BF3‚O(Et)2
from 3.3 to 26 mM and that of the starting materials from
10 to 150 mM as previously reported12 for small branched
alkyl aldehydes (pivalaldehyde, 2-methylpropanal) did
not give yield improvements in this case. Indeed, the yield
of porphyrin 2 was <1% under the high-concentration
conditions. Acid screening experiments were performed
with the following acid catalysts that have been examined
in other porphyrin-forming reactions:20 TFA, BF3‚O(Et)2,
TFA/BF3‚O(Et)2, BF3‚O(Et)2/EtOH, BF3‚O(Et)2/NaCl,
SnCl4, SiCl4, TiCl4, montmorillonite K10, Sc(OTf)3, InCl3,
and MgBr2. However, these acid-catalyzed conditions
failed to give a satisfactory yield of porphyrin 2.
Resu lts a n d Discu ssion
1. Syn th esis of Sw a llow ta il P or p h yr in s. P or p h y-
r in s Bea r in g F ou r Sw a llow ta il Gr ou p s. The synthe-
sis of A4-porphyrins requires access to the precursor
(9) Tabushi, I.; Sakai, K.-I.; Yamamura, K. Tetrahedron Lett. 1978,
21, 1821-1824.
(10) Rothemund, P. J . Am. Chem. Soc. 1939, 61, 2912-2915.
(11) Runge S.; Senge, M. O. Z. Naturforsch. 1998, 53b, 1021-1030.
(12) Ema, T.; Senge, M. O.; Nelson, N. Y.; Ogoshi, H.; Smith, K. M.
Angew. Chem., Int. Ed. Engl. 1994, 33, 1879-1881.
(13) Veyrat, M.; Maury, O.; Faverjon, F.; Over, D. E.; Ramasseul,
R.; Marchon, J .-C.; Turowska-Tyrk, I.; Scheidt, W. R. Angew. Chem.,
Int. Ed. Engl. 1994, 33, 220-223.
(14) Onaka, M.; Shinoda, T.; Izumi, Y.; Nolen, E. Chem. Lett. 1993,
117-120.
(15) J entzen, W.; Simpson, M. C.; Hobbs, J . D.; Song, X.; Ema, T.;
Nelson, N. Y.; Medforth, C. J .; Smith, K. M.; Veyrat, M.; Mazzanti,
M.; Ramasseul, R.; Marchon, J .-C.; Takeuchi, T.; Goddard, W. A., III.;
Shelnutt, J . A. J . Am. Chem. Soc. 1995, 117, 11085-11097.
(16) (a) Barkigia, K. M.; Berber, M. D.; Fajer, J .; Medforth, C. J .;
Renner, M. W.; Smith, K. M. J . Am. Chem. Soc. 1990, 112, 8851-
8857. (b) Ravikanth, M.; Chandrashekar, T. K. Struct. Bonding (Berlin)
1995, 82, 105-188. (c) Shelnutt, J . A.; Song, X.-Z.; Ma, J .-G.; J ia, S.-
L.; J entzen, W.; Medforth, C. J . Chem. Soc. Rev. 1998, 27, 31-41. (d)
Senge, M. O. In The Porphyrin Handbook; Kadish, K. M., Smith, K.
M., Guilard, R., Eds.; Academic Press: San Diego, 2000; Vol. 1, pp
239-347.
(17) (a) Gentemann, S.; Medforth, C. J .; Ema, T.; Nelson, N. Y.;
Smith, K. M.; Fajer, J .; Holten, D. Chem. Phys. Lett. 1995, 245, 441-
447. (b) Gentemann, S. G.; Nelson, N. Y.; J aquinod, L. A.; Nurco, D.
J .; Leung, S. H.; Medforth, C. J .; Smith, K. M.; Fajer, J .; Holten, D. J .
Phys. Chem. 1997, 101, 1247-1254.
Treatment of 2 under standard conditions with Zn-
(OAc)2‚2H2O at room temperature for 2 h afforded
porphyrin Zn -2 in 96% yield. The analogous magnesium
(18) Kato, M.; Komoda, K.; Namera, A.; Sakai, Y.; Okada, S.;
Yamada, A.; Yokoyama, K.; Migita, E.; Minobe, Y.; Tani, T. Chem.
Pharm. Bull. 1997, 45, 1767-1776.
(19) Li, F.; Yang, K.; Tyhonas, J . S.; MacCrum, K. A.; Lindsey, J . S.
Tetrahedron 1997, 53, 12339-12360.
(20) Geier, G. R., III.; Lindsey, J . S. J . Porphyrins Phthalocyanines
2002, 6, 159-185.
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Thamyongkit et al.
SCHEME 2
SCHEME 3
Treatment of 1,9-diacyldipyrromethane 422 with NaBH4
in dry THF/methanol (10:1) afforded the corresponding
dipyrromethane-dicarbinol 4-d iol as a colorless foam.
Condensation of the latter with dipyrromethane 3 using
new acid catalysis conditions (InCl3 in CH2Cl2 at room
temperature)25 followed by oxidation with DDQ gave
porphyrin 5 (33% yield), an A3B-porphyrin that bears one
swallowtail substituent. Treatment of 5 with Zn(OAc)2‚
2H2O gave Zn -5 in 95% yield.
P or p h yr in s Bea r in g Tw o Sw a llow ta il Gr ou p s.
trans-A2B2-Porphyrins are readily prepared by the reac-
tion of a dipyrromethane and an aldehyde, given that the
dipyrromethane does not undergo acidolysis followed by
undesired recombination of reactive fragments (i.e.,
scrambling) during the condensation.26 Condensation of
dipyrromethane 3 and benzaldehyde in CH2Cl2 under
TFA catalysis followed by oxidation with DDQ gave the
desired trans-A2B2-porphyrin 6 in 15% yield. No other
porphyrins were observed by analysis of the crude
reaction mixture with laser-desorption mass spectrom-
etry (LD-MS).27 These conditions are known to afford
reaction without noticeable acidolytic scrambling for
sterically hindered dipyrromethanes,27 and apparently
work well for dipyrromethane 3. Treatment with Zn-
(OAc)2‚2H2O gave porphyrin Zn -6 in 94% yield (Scheme
3).
insertion was achieved by treatment of a solution of 2 in
CH2Cl2 with a freshly prepared solution of MgI2 in diethyl
ether containing DIEA,21 affording porphyrin Mg-2 in
92% yield.
P or p h yr in s Bea r in g On e Sw a llow ta il Gr ou p . A3B-
porphyrins are readily available by reaction of an A3-
dipyrromethane-dicarbinol and a B-dipyrromethane.22
Dipyrromethanes are key intermediates in the synthesis
of porphyrins bearing distinct patterns of substituents.
The condensation of aldehyde 1 with excess pyrrole23,24
was carried out at room temperature in CH2Cl2 contain-
ing TFA. The resulting 5-(tridec-7-yl)dipyrromethane (3)
was obtained by Kugelrohr distillation as a viscous yellow
liquid in 42% yield (Scheme 2).
An alternative route to trans-A2B2-porphyrins employs
the self-condensation of an AB-dipyrromethane-mono-
(21) O’Shea, D. F.; Miller, M. A.; Matsueda, H.; Lindsey, J . S. Inorg.
Chem. 1996, 35, 7325-7338.
(22) Rao, P. D.; Dhanalekshmi, S.; Littler, B. J .; Lindsey, J . S. J .
Org. Chem. 2000, 65, 7323-7344.
(23) 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.
(24) (a) Loewe, R. S.; Tomizaki, K.-Y.; Youngblood, W. J .; Bo, Z.;
Lindsey, J . S. J . Mater. Chem. 2002, 12, 3438-3451. (b) Tomizaki, K.-
Y.; Thamyongkit, P.; Loewe, R. S.; Lindsey, J . S. Tetrahedron 2003,
59, 1191-1207.
(25) Geier, G. R., III.; Callinan, J . B.; Rao, P. D.; Lindsey, J . S. J .
Porphyrins Phthalocyanines 2001, 5, 810-823.
(26) Littler, B. J .; Ciringh, Y.; Lindsey, J . S. J . Org. Chem. 1999,
64, 2864-2872.
(27) (a) Srinivasan, N.; Haney, C. A.; Lindsey, J . S.; Zhang, W.;
Chait, B. T. J . Porphyrins Phthalocyanines 1999, 3, 283-291. (b)
Fenyo, D.; Chait, B. T.; J ohnson, T. E.; Lindsey, J . S. J . Porphyrins
Phthalocyanines 1997, 1, 93-99.
3702 J . Org. Chem., Vol. 69, No. 11, 2004

Swallowtail Porphyrins
SCHEME 4
SCHEME 5
one ethyne group is shown in Scheme 5. Acylation of the
ethynyl-substituted 1-acyldipyrromethane 8 with 4-
iodobenzoyl chloride gave 1,9-diacyldipyrromethane 11.
Reduction of the latter and condensation of the resulting
dipyrromethane-dicarbinol with dipyrromethane 3 af-
forded the trans-AB2C-porphyrin 12 in 31% yield. Zinc
insertion and cleavage of the TMS group gave porphyrin
Zn -13, which can be used for Sonogashira coupling.
It is noteworthy that an alternative approach toward
trans-A2B2-porphyrins or trans-AB2C-porphyrins bearing
two swallowtail substituents employs the swallowtail
substituents attached to the carbons at the 1- and
9-positions of the dipyrromethane-dicarbinol. However,
this route proved infeasible on the basis of the results
described in the next section.
carbinol.28 Dipyrromethane 3 was subjected to mono-
acylation with the pyridyl benzothioate 7,28 affording the
ethynyl-substituted 1-acyldipyrromethane 8. Reduction
of 8 gave the dipyrromethane-monocarbinol, which un-
derwent self-condensation in the presence of InCl3 in CH2-
Cl2. Subsequent oxidation with DDQ gave the desired
trans-A2B2-porphyrin 9 in 26% yield (Scheme 4). The
yield of the dipyrromethane-monocarbinol self-condensa-
tion approach giving 9 was higher (26% vs 15%) than that
of the aldehyde + dipyrromethane route giving 6. Meta-
lation and deprotection gave the diethynyl porphyrin Zn -
10, which can be used for Glaser coupling.
trans-AB2C-Porphyrins are readily available by reac-
tion of an AB2-dipyrromethane-dicarbinol and a C-
dipyrromethane.22 The synthesis of such a porphyrin
bearing two swallowtail substituents, one iodo group, and
P or p h yr in s Bea r in g Th r ee Sw a llow ta il Gr ou p s.
A3B-Porphyrins are readily available by reaction of an
A3-dipyrromethane-dicarbinol and a B-dipyrromethane.22
We explored the rational synthesis of porphyrins bearing
three swallowtail groups, which requires the preparation
(28) Rao, P. D.; Littler, B. J .; Geier, G. R., III; Lindsey, J . S. J . Org.
Chem. 2000, 65, 1084-1092.
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Thamyongkit et al.
SCHEME 6
SCHEME 8
SCHEME 7
6). Application of a tin-complexation method,31 which
facilitates purification of a wide variety of 1,9-diacyl-
dipyrromethanes, was fruitless as 17 failed to give a tin
complex.
Treatment of 17 (95% purity) with NaBH4 in THF/
methanol (10:1) at room temperature or at 40 °C failed
to give clean reduction to the corresponding dicarbinol.
Although no evidence was obtained to confirm the pres-
ence of the dicarbinol, the crude mixture was worked up
and reacted with dipyrromethane 14 in the presence of
InCl3 in CH2Cl2 at room temperature, followed by the
addition of DDQ. No trace of porphyrin was observed.
We examined a simpler substrate, a pyrrole-carbinol
bearing a swallowtail substituent, because pyrrole-
carbinols are known to readily give porphyrin.32 Reaction
of the swallowtail acid chloride 16 and the pyrrole
of a dipyrromethane-dicarbinol bearing swallowtail groups
at the R-carbon positions. Oxidation of aldehyde 1 with
KMnO4 led to carboxylic acid 1529 (81% yield), which was
converted to acid chloride 16 upon reaction with SOCl2
as previously reported.30 Acylation of 5-mesityldipyr-
romethane (14)23 via the dipyrromethane Grignard re-
agent22 proceeded slowly, affording a mixture of the
1-acyldipyrromethane and the target 1,9-diacyldipyr-
romethane (17). The mixture proved difficult to separate
and the corresponding 1,9-diacyldipyrromethane (17) was
obtained in only ∼10% yield and ∼95% purity (Scheme
(29) (a) Asinger, F.; Berger, W.; Fangha¨nel, E.; Mu¨ller, K. R. J .
Prakt. Chem. 1963, 22, 153-172. (b) Hwang, Y.-S.; Mulla, M. S.; Arias,
J . R. J . Agric. Food Chem. 1974, 22, 1004-1006.
(30) Asano, M.; Yamakawa, T. Yakugaku Zasshi 1950, 70, 474-
477.
(31) Tamaru, S.-I.; Yu, L.; Youngblood, W. J .; Muthukumaran, K.;
Taniguchi, M.; Lindsey, J . S. J . Org. Chem. 2004, 69, 765-777.
(32) Lindsey, J . S. In The Porphyrin Handbook; Kadish, K. M.,
Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000;
Vol. 1, pp 45-118.
3704 J . Org. Chem., Vol. 69, No. 11, 2004

Swallowtail Porphyrins
SCHEME 9
Grignard reagent afforded acyl-pyrrole 18 (Scheme 7).
Treatment of 18 with LiAlH4 for 90 min in refluxing
diethyl ether gave incomplete reduction. The resulting
crude product was subjected to acid catalysis (TFA,
BF3‚O(Et)2, or InCl3) and oxidation; however, no porphy-
rinic product was observed. Further studies are required
to develop suitable reaction conditions for use with
dipyrromethanes bearing swallowtail groups at the 1-
and 9-positions. The absence of such conditions precludes
rational syntheses of porphyrins bearing three meso-
swallowtail substituents.
We turned to the use of statistical condensations for
the synthesis of A3B-porphryins bearing three swallowtail
groups. Condensation of pyrrole, aldehyde 1 and 4-iodo-
benzaldehyde in 4:8:1 ratio with BF3‚O(Et)2 in CHCl3
followed by oxidation with DDQ gave 19 in 20% yield
(Scheme 8). This ratio was found to be effective in terms
of yield and convenience of isolating porphyrin 19 from
the byproduct tetra-swallowtail porphyrin 2, which have
similar chromatographic Rf values. Subsequent reaction
with Zn(OAc)2‚2H2O gave Zn -19 in 96% yield. The similar
mixed-condensation strategy with pyrrole, aldehyde 1
and 4-[2-(trimethylsilyl)ethynyl]benzaldehyde predomi-
nantly afforded porphyrins containing three or four
ethyne groups without significant amounts of the desired
porphyrin 20, despite employing up to a 4:15:1 ratio of
pyrrole, aldehyde 1, and 4-[2-(trimethylsilyl)ethynyl]-
benzaldehyde. However, the Pd-coupling of porphyrin 19
and trimethylsilylacetylene gave porphyrin 20 in 51%
yield. Subsequent TMS cleavage with TBAF led to
ethynyl-porphyrin 21 in 72% yield. Similarly, the Pd-
coupling of Zn -19 and (trimethylsilyl)acetylene gave Zn -
20, which upon treatment with TBAF in THF gave
ethynyl-porphyrin Zn -21 in good yield.
2. Syn th esis of P or p h yr in s Dya d s. Sw a llow ta il-
P or p h yr in Dya d s. Diphenylethyne-linked dyads in
which each porphyrin bears three swallowtail groups
were prepared to investigate the electronic communica-
tion in these types of arrays. Both Zn2 and ZnFb dyads
were prepared, the former to investigate ground-state
hole-hopping in monocation radical species and the latter
to investigate excited-state energy transfer in neutral
complexes. The syntheses of Dya d -1 and Dya d -2 are
summarized in Scheme 9.
The Pd-coupling reaction was performed in toluene/
TEA (5:1) at 35 °C in the presence of Pd2(dba)3 and P(o-
tol)3 under standard conditions for Sonogashira coupling
of porphyrins in dilute solution in the absence of copper.33
J . Org. Chem, Vol. 69, No. 11, 2004 3705

Thamyongkit et al.
SCHEME 10
Porphyrin Zn -19 was reacted with 21 or Zn -21, affording
the ZnFb array Dya d -1 or the Zn2 array Dya d -2 in 39%
or 42% yield, respectively. The reactions were monitored
by analytical size exclusion chromatography (SEC).33 The
dyads were purified by a three-column chromatography
sequence of silica, preparative SEC, and silica columns.
As expected, both porphyrinic arrays exhibit good solubil-
ity in common organic solvents such as CH2Cl2, CHCl3,
and THF.
P en tyl-P or p h yr in Dya d s. To compare the effect of
the branched alkyl groups on the electronic communica-
tion within the porphyrin dyads versus that of linear
alkyl chains, we synthesized Dya d -3 and Dya d -4, which
contain n-pentyl substituents at the meso positions of the
macrocycle framework instead of swallowtail groups as
in Dya d -1 and Dya d -2. Reduction of diacyldipyr-
romethane 2222 with NaBH4 in THF/methanol followed
(33) Wagner, R. W.; Ciringh, Y.; Clausen, C.; Lindsey, J . S. Chem.
Mater. 1999, 11, 2974-2983.
3706 J . Org. Chem., Vol. 69, No. 11, 2004

Swallowtail Porphyrins
by condensation of the resulting dipyrromethane-di-
carbinol 22-d iol and dipyrromethane 2334 in CH3CN
containing TFA gave porphyrin 24 in 21% yield (Scheme
10). Porphyrin Zn -24, a benchmark compound for Dya d -
3, was obtained in 91% yield upon treatment of 24 with
Zn(OAc)2‚2H2O. Cleavage of the TMS group by TBAF
afforded porphyrin 25 (94% yield), which has been
prepared previously by a slightly different route.35 The
Pd-coupling of porphyrins 25 and Zn -26 (prepared in 95%
yield by metalation of its free base counterpart 26)36 was
performed under the same conditions as described for
Dya d -1 and Dya d -2. The desired ZnFb Dya d -3 was
obtained in 47% yield. Treatment of Dya d -3 with Zn-
(OAc)2‚2H2O afforded the Zn2 product Dya d -4 in 91%
yield. Reaction monitoring and purification was per-
formed as described for Dya d -1 and Dya d -2. Dya d -3 is
soluble in common organic solvents such as CH2Cl2,
CHCl3, and THF, whereas Dya d -4 could be dissolved in
THF but only partially in CH2Cl2 and CHCl3.
3. P h ysicoch em ica l P r op er ties of th e Neu tr a l
Sw a llow ta il P or p h yr in s. Sta tic Absor p tion a n d
Flu or escen ce Spectr a. Figure 1 shows electronic ground-
state absorption spectra (solid) and excited-state fluo-
rescence spectra (dashed) of Zn porphyrins containing 0
(Zn TP P ), 1 (Zn -5), 2 (Zn -6 and Zn -9), and 4 (Zn -2)
swallowtails, with aryl groups at the other meso posi-
tions. The optical spectra for ZnFb Dya d -1 along with
monomeric reference porphyrins (Zn -20 and 20) contain-
ing three swallowtails per porphyrin are shown in Figure
2. The corresponding spectra for the analogues containing
three pentyl groups per porphyrin instead of swallowtails
(Dya d -3, Zn -24, and 24) are given in Figure 3. The
wavelengths of the Q(0,0) absorption and emission maxima
along with fluorescence quantum yields and lifetimes of
the lowest excited singlet state for all of these compounds
are summarized in Table 1. The key aspects of these
static optical data are described in the following:
(1) Swallowtails do not significantly perturb the pho-
tophysical properties of the porphyrins relative to meso-
tetraarylporphyrins (e.g., Zn TP P ) and analogues bearing
meso alkyl substituents such as n-pentyl groups. For
example, the replacement of four aryl rings by four
swallowtails in Zn porphyrins causes only a ∼5 nm red
shift in the origin optical transitions (Figure 1A and E).
This effect can be accounted for by minor electronic
perturbations of alkyl versus aryl substituents, given that
compounds with three swallowtails have virtually the
same spectra as the pentyl analogues (Figures 2A versus
3A).
F IGURE 1. Electronic absorption spectra (solid) and fluores-
cence spectra (dashed) for the indicated porphyrins in toluene
at room temperature. The absorption spectra in the 450-750
nm region have been multiplied by the factors shown. The
emission spectra were acquired using Soret excitation. Spectra
in the respective regions have been normalized to the same
peak intensity, and the maxima (( 1 nm) are indicated.
HOMO slightly further above the filled a1u(π) orbital
(relative to meso aryl-substituted porphyrins).
(2) The minor Q-band red shifts observed upon replace-
ment of multiple aryl rings by either swallowtails or
pentyl groups is accompanied by a slight increase in the
intensity of the Q(0,0) absorption band (∼595 nm) with
respect to the Q(1,0) band (cf. features at ∼595 and ∼555
nm in Figure 1A versus E). According to the four-orbital
model,37 this change can also be accounted for by the
effect of meso alkyl groups, which raise the filled a2u(π)
(3) The above-mentioned comparisons of optical spectra
as well as the fluorescence quantum yields (∼0.03) and
excited-state lifetimes (∼2 ns) in Table 1 demonstrate
that swallowtail groups cause little if any of the nonpla-
nar distortions that occur in molecules with multiple
bulky substituents such as zinc(II)-meso-tetra-tert-bu-
tylporphyrin. These nonplanar distortions result in broad-
ened and red-shifted (by 50 nm or more) optical spectra
as well as fluorescence yields and excited-state lifetimes
that are reduced by over an order of magnitude from
planar analogues (Φf < 10-4 and τ ∼ 10 ps).15,17
(34) Cho, W.-S.; Kim, H.-J .; Littler, B. J .; Miller, M. A.; Lee, C.-H.;
Lindsey, J . S. J . Org. Chem. 1999, 64, 7890-7901.
(35) Gryko, D.; Li, J .; Diers, J . R.; Roth, K. M.; Bocian, D. F.; Kuhr,
W. G.; Lindsey, J . S. J . Mater. Chem. 2001, 11, 1162-1180.
(36) Li, J .; Gryko, D.; Dabke, R. B.; Diers, J . R.; Bocian, D. F.; Kuhr,
W. G.; Lindsey, J . S. J . Org. Chem. 2000, 65, 7379-7390.
(37) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Aca-
demic: New York, 1978; Vol. III, pp 1-165.
(4) The spectra of the diphenylethyne-linked Dya d -1
that contains three swallowtails at the nonlinking meso
positions of the macrocycles is well approximated by the
sum of the spectra of the Zn porphyrin and Fb porphyrin
J . Org. Chem, Vol. 69, No. 11, 2004 3707

Thamyongkit et al.
TABLE 1. Absor p tion , F lu or escen ce, a n d
Electr och em ica l P r op er ties of Sw a llow ta il P or p h yr in sa
no. of
alkyl
no. of
aryl
τe
(ns)
E1/2
(V)
f
λabs
λem
porphyrin groupsb groupsc (nm) (nm)
Φfd
Zn -2
Zn -6
Zn -9
Zn -5
4 ST
2 ST
2 ST
1 ST
3 ST
3 ST
3 Pn
3 Pn
0
0
2
2
3
1
1
1
1
4
4
1
1
1
1
595
591
596
590
595
658
595
659
588
602 0.023
599 0.031
605 0.045
597 0.032
603 0.026
660 0.13
603 0.053
660 0.071
596 0.035h
0.11i
1.9
2.3
2.2
2.1
2.3
10.3
2.4
11.2
+0.25
+0.38
g
+0.42
+0.33
g
g
g
Zn -20
20
Zn -24
24
Zn TP P
F bTP P
Dya d -1
Dya d -2
Dya d -3
Dya d -4
2.3h +0.50
0
13i
g
3 ST
3 ST
3 Pn
3 Pn
657
658
660 0.14j
10.4
+0.34
+0.34
+0.37
+0.36
659 0.13k
9.6
a
In toluene at room temperature except for the E1/2 values. b ST
is a swallowtail group and Pn is an n-pentyl chain. c The aryl
groups are combinations of phenyl, p-tolyl, mesityl, and H- or TMS-
terminated ethynylphenyl groups; the differences can give rise to
d
small variations in electronic properties. Fluorescence quantum
yield determined using Soret excitation relative to Φf ) 0.030 for
zinc tetraphenylporphyrin (ZnTPP) and/or Φf ) 0.11 for free base
tetraphenylporphyrin (FbTPP) in toluene.43 eLifetime of the lowest
excited singlet state determined using fluorescence modulation
(phase shift) techniques.44 f First oxidation potential for the
compound in CH2Cl2/THF (9:1) containing 0.1 M Bu4NPF4 vs Ag/
Ag+; FeCp2/FeCp2+ ) 0.19; scan rate ) 0.1 V s-1; values are (0.03
g
h
V. Not determined. Values in the ranges Φf ) 0.02-0.05 and
τ ) 1.9-2.6 ns have been found for reference zinc porphyrins
including ZnTPP, zinc tetramesitylporphyrin, and analogues bear-
F IGUR E 2. Optical spectra for swallowtail-containing
Dya d -1 and reference porphyrins. The other conditions are
as in Figure 1.
i
ing ethynylphenyl or alkyl groups.39 Values in the ranges Φf )
0.09-0.12 and τ ) 12.5-13.5 ns have been found for reference Fb
porphyrins including FbTPP, tetramesitylporphyrin, and ana-
j
logues bearing ethynylphenyl or alkyl groups.45 Obtained using
primary excitation of the Fb unit at 515 nm; the value obtained
using excitation of the Zn unit (along with some Fb excitation) at
k
551 nm is 0.11. Obtained using primary excitation of the Fb unit
at 519 nm; the value obtained using excitation of the Zn unit (along
with some Fb excitation) at 554 nm is 0.11.
the porphyrin constituents in the arrays, as is further
demonstrated by the excited-state energy-transfer and
ground-state hole-hopping dynamics described below.
Tim e-Resolved Absor p tion Sp ectr a a n d En er gy-
Tr a n sfer Ch a r a cter istics of Dya d s. Facile energy
transfer from the Zn porphyrin to the Fb porphyrin in
the swallowtail-containing array Dya d -1 is demonstrated
by several observations. First, fluorescence from this
dyad is observed basically exclusively from the Fb
constituent even upon excitation of the Zn porphyrin, just
as is observed for the pentyl analogues and previously39
for the aryl counterparts (see fluorescence spectra in
Figures 2C and 3C and Fb porphyrin fluorescence yields
with either excitation wavelength in Table 1). Second,
the transient absorption difference spectra in Figure 4
show that bleaching of the Qy(1,0) band of the Fb
porphyrin in Dya d -1 develops with a ∼20 ps time
constant as energy flows to this component following
F IGURE 3. Optical spectra for pentyl-containing Dya d -3 and
reference porphyrins. The other conditions are as in Figure 1.
constituents in the photophysically relevant Q-band
region (500-650 nm; Figure 2). This finding reflects the
relatively weak interporphyrin electronic interactions in
these dyads. The same result is found here for the pentyl-
containing Dya d -3 (Figure 3) and previously38 for aryl-
containing analogues. Thus, the swallowtails do not
significantly modulate the electronic interactions between
(38) Holten, D.; Bocian, D. F.; Lindsey, J . S. Acc. Chem. Res. 2002,
35, 57-69.
(39) (a) Hsiao, J .-S.; Krueger, B. P.; Wagner, R. W.; J ohnson, T. E.;
Delaney, J . K.; Mauzerall, D. C.; Fleming, G. R.; Lindsey, J . S.; Bocian,
D. F.; Donohoe, R. J . J . Am. Chem. Soc. 1996, 118, 11181-11193. (b)
Li, F.; Gentemann, S.; Kalsbeck, W. A.; Seth, J .; Lindsey, J . S.; Holten,
D.; Bocian, D. F. J . Mater. Chem. 1997, 7, 1245-1262. (c) Tomizaki,
K.-Y.; Loewe, R. S.; Kirmaier, C.; Schwartz, J . K.; Retsek, J . L.; Bocian,
D. F.; Holten, D.; Lindsey, J . S. J . Org. Chem. 2002, 67, 6519-6534.
3708 J . Org. Chem., Vol. 69, No. 11, 2004

Swallowtail Porphyrins
F IGURE 5. Room-temperature X-band EPR spectra of [Zn -
6]+ and [Zn -2]+ (left panel) and [Dya d -1]+ and [Dya d -2]+
(right panel).
potential as the number of alkyl groups increases,41
reflecting the electron-donating characteristics of the
swallowtail group.
F IGURE 4. Representative time-resolved absorption spectra
and kinetic profiles for Dya d -1 in toluene at room temperature
obtained using excitation with a 565-nm 130-fs flash (top
panel). The fit to the data at 525 nm for Dya d -1 in the lower
panel is the convolution of the instrument response with an
exponential plus a constant, giving a time constant of 22 ps.
The average of this value with the time constant of 18 ps from
the fit to the kinetic profile at 465 nm gives a lifetime of the
excited Zn porphyrin of 20 ( 3 ps. The same lifetime is
obtained for Dya d -3, as can be seen from the data at 465 nm
in the inset to the lower panel.
EP R Sp ectr a a n d Hole-Hop p in g Ch a r a cter istics
of th e Dya d s. The EPR spectra of the π-cation radicals
of the swallowtail porphyrins bearing two and four
swallowtail groups, [Zn -6]+ and [Zn -2]+, are shown in
Figure 5 (left panel). The spectra are quite similar and
exhibit a nine-line 14N hyperfine pattern that is typical
for porphyrins in which the electronic ground state is 2A2u
(as is expected for meso-substituted complexes).41,4242-45
The spectra of the π-cation radicals of the swallowtail
porphyrins containing one and three groups (not shown)
are very similar to those shown in Figure 5 (left panel).
The most notable characteristic of the EPR spectra of the
π-cation radicals of the monomeric swallowtail porphy-
rins is that no hyperfine features are observed for the
proton(s) on the branching meso-connected carbon(s) of
the swallowtail(s). In this respect, the EPR spectra of the
swallowtail porphyrins are from those of porphyrins
bearing linear meso-alkyl groups. The EPR spectra of
these latter complexes typically exhibit substantial hy-
perfine splittings due to the protons on the first carbon
of the alkyl chain.41,42
principal excitation of the Zn porphyrin with a 130-fs 550
nm flash. Third, the lifetime of the excited singlet state
of the Zn porphyrin in swallowtail Dya d -1 is 20 ( 3 ps
(Figure 4), the same as in pentyl Dya d -3 (Figure 4 inset)
and basically the same as the value of 24 ( 4 ps found
previously39 for the dyad containing mesityl groups at
the nonlinking meso positions. These lifetimes are all
significantly shorter than the value of ∼2 ns in the
reference Zn porphyrins, affording energy-transfer rates
on the order of ∼(20 ps)-1 and efficiencies of >99%. Thus,
the use of swallowtails at the nonlinking positions in
diphenylethyne-linked dyads does not diminish energy
transfer in these systems.
4. P h ysicoch em ica l P r op er ties of th e Oxid ized
Sw a llow ta il P or p h yr in s. Electr och em istr y. The E1/2
values for the first oxidation of various monomers and
dyads are given in Table 1. In the case of the ZnFb dyads,
E1/2 values are for the Zn porphyrins. The E1/2 values for
the swallowtail porphyrins are generally in the range of
those found previously for porphyrins substituted with
alkyl and/or aryl groups at the meso carbons.40,41 Con-
sistent with previous studies, the E1/2 value shifts to lower
The absence of proton hyperfine structure for the
π-cation radicals of the swallowtail porphyrins provides
insight into the torsional angle of the CR2H group with
respect to the porphyrin plane. In particular, the proton
(41) Atamian, M.; Wagner, R. W.; Lindsey, J . S.; Bocian, D. F. Inorg.
Chem. 1988, 27, 1510-1512.
(42) Fajer, J .; Davis, M. S. In The Porphyrins; Dolphin, D., Ed.,
Academic Press: New York, 1979; Vol. 4, pp 197-256.
(43) Seybold, P. G.; Gouterman, M. J . Mol. Spectrosc. 1969, 31,
1-13.
(44) (a) Prathapan, S.; Yang, S. I.; Seth, J .; Miller, M. A.; Bocian,
D. F.; Holten, D.; Lindsey, J . S. J . Phys. Chem. B 2001, 105, 8237-
8248. (b) Yang, S. I.; Li, J .; Cho, H. S.; Kim, D.; Bocian, D. F.; Holten,
D.; Lindsey, J . S. J . Mater. Chem. 2000, 10, 283-296.
(45) Yang, S. I.; Seth, J .; Strachan, J .-P.; Gentemann, S.; Kim, D.;
Holten, D.; Lindsey, J . S.; Bocian, D. F. J . Porphyrins Phthalocyanines
1999, 3, 117-147.
(40) (a) Kadish, K. M.; Van Caemelbecke, E.; Royal, G. In The
Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.;
Academic Press: San Diego, CA, 2000; Vol. 8, pp 1-114. (b) Felton,
R. H. In The Porphyrins; Dolphin, D., Ed., Academic Press: New York,
1978; Vol. 5, pp 53-125.
J . Org. Chem, Vol. 69, No. 11, 2004 3709

Thamyongkit et al.
porphyrin.40 As can be seen, the EPR signal of [Dya d -
2]+ exhibits no resolved hyperfine structure and is
narrower than that of the [Dya d -1]+. The spectrum of
[Dya d -1]+ is similar to that of the monomeric swallowtail
porphyrin π-cations (Figure 5, left panel). Accordingly,
the swallowtail substituents do not have any effect on
the qualitative aspects of the hole-transfer process.
Ou tlook
The results obtained define approaches for the syn-
thesis of porphyrins bearing 1-4 swallowtail substitu-
ents. A key limitation is the inability to reduce the
7-tridecyl group attached to the keto group of a 1-acyl-
dipyrromethane with NaBH4, though the swallowtail
substituent can be located at the 5-position of the
dipyrromethane. In the synthesis of porphyrin building
blocks, this present limitation leads to the use of statisti-
cal methods for the synthesis of porphyrins bearing three
swallowtail substituents (A3B-porphyrins) or two swal-
lowtail substituents at adjacent meso positions (cis-A2B2-
porphyrins), while rational methods can be employed for
porphyrins that bear one swallowtail substituent (AB3-
porphyrin) or two swallowtail substituents at opposite
meso positions (trans-A2B2- or trans-AB2C-porphyrins).
The fact that the swallowtail substituents do not signifi-
cantly alter the electronic or photophysical properties of
porphyrins makes these molecules excellent building
blocks for the construction of large arrays, where the
solubility imparted by the swallowtails is beneficial for
performing detailed physicochemical studies. We are
currently implementing the swallowtail design in large
multiporphyrin arrays and probing the ground and
excited-state electronic communication in these archi-
tectures.
F IGURE 6. Newman projections of the torsional angles for
meso substituents in swallowtail (left panel) versus linear alkyl
(right panel) substituted porphyrins.
hyperfine coupling exhibits a cos2 θ dependence, where
θ is the torsional angle between the proton on the alkyl
carbon and the p-orbital on the meso carbon atom of the
porphyrin. The absence of proton hyperfine splitting in
the swallowtail porphyrins indicates that θ ≈ 90°, i.e.,
the proton is in the plane of the porphyrin ring, as shown
in Figure 6 (left structure). Thus, the two hexyl chains
of the swallowtails project above and below the porphyrin
ring, respectively. In the case of porphyrins bearing linear
meso alkyl-substituted porphyrins, θ ≈ 60°, i.e., the two
protons on the first carbon of the alkyl chain are on the
same side of the porphyrin plane and the second carbon
of the alkyl chain is in the same plane as the p-orbital of
the porphyrin meso carbon, as shown in Figure 6 (right
structure).42 The different torsional angles for the swal-
lowtail versus linear alkyl groups reflect the different
steric constraints experienced by these two types of
groups.
The EPR characteristics of the monocation radical of
the Zn2-dyad, [Dya d -2]+, permit the investigation of hole
transfer between the two swallowtail porphyrins in the
array. In this regard, our previous EPR studies of
monocation radicals of Zn2 porphyrin dyads in which the
Ack n ow led gm en t. This research was supported by
the NIH (GM-36238 (J .S.L.), GM-34685 (D.H.), and GM-
36243 (D.F.B.)). 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 Biotech-
nology Center and the National Science Foundation.
2
ground state of the porphyrin is A2u have shown that
the hole rapidly transfers between the two porphyrins
on the EPR time scale.38 The rapid hole transfer is
evidenced by a narrowed EPR line shape with no resolved
hyperfine structure. This same behavior is exhibited by
the monocation radical of the swallowtail Zn2-dyad,
[Dya d -2]+, as can be seen via comparison of the EPR
spectrum of this array with that of the ZnFb analogue,
[Dya d -1]+ (Figure 5, right panel). In the latter dyad, the
hole is localized on the Zn porphyrin and cannot transfer
owing to the much higher oxidation potential of the Fb
Su p p or tin g In for m a tion Ava ila ble: Complete Experi-
mental Section including procedures for the synthesis of all
new compounds; 1H NMR spectra for all new compounds; 13
C
NMR spectra for all new non-porphyrin 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.
J O049860E
3710 J . Org. Chem., Vol. 69, No. 11, 2004