Synthesis, crystal structure, and nonlinear optical behavior of β-unsubstituted meso-meso E-vinylene-linked porphyrin dimers

Article abstract of DOI:10.1021/ol0520525

(Chemical Equation Presented) A vinylene-linked porphyrin dimer, with no substituents at the β-positions, has been synthesized by CuI/CsF promoted Stille coupling. In the crystal structure of this dimer, the C₂H ₂ bridge is twisted by 45° relative to the plane of the porphyrins. The absorption, emission spectra, and electrochemistry reveal substantial porphyrin-porphyrin π-conjugation. The triplet excited-state absorption spectrum of this dimer makes it suitable for reverse saturable absorption at 710-900 nm.

Full text of DOI:10.1021/ol0520525

ORGANIC
LETTERS
2005
Vol. 7, No. 24
5365-5368
Synthesis, Crystal Structure, and
Nonlinear Optical Behavior of
â
-Unsubstituted meso−meso
E-Vinylene-Linked Porphyrin Dimers
Michael J. Frampton,† Huriye Akdas,† Andrew R. Cowley,† Joy E. Rogers,‡
Jonathan E. Slagle,‡ Paul A. Fleitz,‡ Mikhail Drobizhev,§ Aleksander Rebane,§ and
Harry L. Anderson*,†
Department of Chemistry, Oxford UniVersity, Chemistry Research Laboratory,
Mansfield Road, Oxford OX1 3TA, U.K., Air Force Research Laboratory, AFRL/MLPJ,
3005 Hobson Way, Wright-Patterson Air Force Base, Dayton, Ohio 45433-7702, and
Department of Physics, Montana State UniVersity, Bozeman, Montana 59717
harry.anderson@chem.ox.ac.uk
Received August 25, 2005
ABSTRACT
A vinylene-linked porphyrin dimer, with no substituents at the
â
-positions, has been synthesized by CuI/CsF promoted Stille coupling. In the
relative to the plane of the porphyrins. The absorption, emission spectra, and
π-conjugation. The triplet excited-state absorption spectrum of this dimer makes it
crystal structure of this dimer, the C2H2 bridge is twisted by 45
°
electrochemistry reveal substantial porphyrin
−porphyrin
suitable for reverse saturable absorption at 710
−900 nm.
Conjugated porphyrin oligomers1 display outstanding char-
acteristics for a variety of applications, including two-photon
absorption,2 reverse-saturable absorption in the near-infrared,3
and single-molecule conductivity.4 All these applications
require efficient π-orbital overlap between the porphyrins,
and this electronic coupling is generally maximized by
locating the conjugated connections at the meso-positions
of the porphyrin units. Meso-meso alkyne-linked porphyrin
oligomers have been extensively investigated,5-7 whereas
meso-meso vinylene-linked structures have received little
attention, except for â-alkyl systems such as Ni21,8 in which
conjugation is severely limited by steric interactions. For
example, in Ni21, the ethyl substituents force the vinylene
† Oxford University.
‡ Air Force Research Laboratory.
§ Montana State University.
(1) (a) Tsuda, A.; Osaka, A. Science 2001, 293, 79. (b) Arnold, D. P.
Synlett 2000, 296. (c) Anderson, H. L. Chem. Commun. 1999, 1771.
(2) (a) Drobizhev, M.; Stepanenko, Y.; Dzenis, Y.; Karotki, A.; Rebane,
A.; Taylor, P. N.; Anderson, H. L. J. Am. Chem. Soc. 2004, 126, 15352.
(b) Drobizhev, M.; Stepanenko, Y.; Dzenis, Y.; Karotki, A.; Rebane, A.;
Taylor, P. N.; Anderson, H. L. J. Phys. Chem. B 2005, 109, 7223.
(3) McEwan, K. J.; Fleitz, P. A.; Rogers, J. E.; Slagle, J. E.; McLean,
D. G.; Akdas, H.; Katterle, M.; Blake, I. M.; Anderson, H. L. AdV. Mater.
2004, 16, 1933.
(5) (a) Anderson, H. L. Inorg. Chem. 1994, 33, 972. (b) Screen, T. E.
O.; Blake, I. M.; Rees, L. H.; Clegg, W.; Borwick, S. J.; Anderson, H. L.
J. Chem. Soc., Perkin Trans. 1 2002, 320.
(6) Arnold D. P.; James, D. A. J. Org. Chem. 1997, 62, 3460.
(7) (a) Lin, V. S.-Y.; DiMagno, S. G.; Therien, M. J. Science 1994, 264,
1105. (b) Susumu, K.; Duncan, T. V.; Therien, M. J. J. Am. Chem. Soc.
2005, 127, 5186.
(4) Yoon, D. H.; Lee, S. B.; Yoo, K.-H.; Kim, J.; Lim, J. K. Aratani,
N.; Tsuda, A.; Osuka, A.; Kim, D. J. Am. Chem. Soc. 2003, 125, 11062.
10.1021/ol0520525 CCC: $30.25
© 2005 American Chemical Society
Published on Web 10/29/2005

bridge to twist out of conjugation with the porphyrin
π-system.9-11 Here we report the synthesis of meso-meso
vinylene-linked porphyrin dimers Zn22a,b without â-sub-
stituents, together with an analysis of the crystal structure
of one of these dimers, Zn22b.12 We compare the linear and
nonlinear absorption behavior of Zn22a with those of its
alkyne-linked analogue Zn23. Although the vinylene bridge
is twisted by 45° relative to the plane of the porphyrin
π-systems in the solid state, the degree of conjugation (as
estimated from absorption spectra, emission spectra, and
redox potentials) in these E-vinylene-linked dimers is similar
to that of their alkyne-linked analogues. The vinylene-linked
dimers exhibit superior reverse-saturable absorption in the
near-infrared (710-900 nm).
converted to Zn22a and Zn22b in yields of 44 and 58%,
respectively (Scheme 1).16 The yield drops to around 10%
Scheme 1. Synthesis of Dimers Zn22a and Zn22ba
a Ar ) 3,5-Di-tert-butylphenyl.
in the absence of cesium fluoride, and no product is formed
in the absence of copper iodide. We also used this method
to synthesize the C2-linked dimer Zn23 from Zn4a using bis-
(tributylstannyl)ethyne, although the yield of Zn23 is only
12%, so this approach may be inferior to the Heck-
Sonogashira route pioneered by Therien and co-workers.7
The crystal structure of Zn22b‚2(C6H5N) was determined
using single crystals grown from a solution in chlorobenzene/
pyridine.17 One pyridine molecule is coordinated to each zinc
atom (not shown in Figure 1). There is a crystallographic
inversion center at the center of the dimer molecule, so the
mean planes of the two porphyrins are exactly parallel, but
they are off-set by a distance of 1.47 Å, rather than being
coplanar. The mean plane of the C-CHdCH-C bridge
makes an angle of 45° to the mean plane of each porphyrin
macrocycle. The main cause for this twist is a 1,5-C‚‚‚C
interaction between carbon atoms of the bridge and the
Initial attempts at synthesis of the C2H2-linked porphyrin
dimer Zn22a by McMurry and Wittig couplings failed.13
Palladium-catalyzed Stille coupling of bromoporphyrins with
bis(tributylstannyl)ethene also failed to give the desired
dimers until we tested Baldwin’s protocol (Pd(PPh3)4, CuI,
CsF).14,15 When copper(I) iodide and cesium fluoride were
added to the reaction mixture, Zn4a and Zn4b were
(8) (a) Ponomarev, G. V.; Shul’ga, A. M. Chem. Heterocyl. Compd.
(Engl. Transl.) 1986, 22, 228. (b) Vicente, M. G. H.; Smith, K. M. J. Org.
Chem. 1991, 56, 4407. (c) Yashunsky, D. V.; Ponomarev, G. V.; Arnold,
D. P. Tetrahedron Lett. 1995, 36, 8485.
(9) (a) Kitagawa, R.; Kai, Y.; Ponomarev, G. V.; Sugiura, K.; Borovkov,
V. V.; Kaneda, T.; Sakata, Y. Chem. Lett. 1993, 1071. (b) Senge, M. O.;
Vicente, M. G. H.; Gerzevske, K. R.; Forsyth, T. P.; Smith, K. M. Inorg.
Chem. 1994, 33, 5625.
(10) Several related structures of â-alkyl meso-vinylene-linked porphyrin-
chlorin dimers and bischlorins have been reported, as well as a bispy-
ropheophorabide-a: (a) Jaquinod, L.; Nurco, D. J.; Medforth, C. J.; Pandey,
R. K.; Forsyth, T. P.; Olmstead, M. M.; Smith, K. M. Angew. Chem., Int.
Ed. 1996, 35, 1013. (b) Kalisch, W. W.; Senge, M. O.; Ruhlandt-Senge, K.
Photochem. Photobiol. 1998, 67, 312. The angles between the mean planes
of the C-CHdCH-C bridge and the macrocycle are 44° and 42° in the
Ni2 porphyrin-chlorin, 53° and 80° in the Ni2 bischlorin, 27° in the Cu2
bischlorin, and 79° in the free-base bispyropheophorabide-a. The presence
of a saturated tetrahedral center at the â-position adjacent to the E-vinylene
bridge reduces the steric interaction of a â-ethyl substituent at this position
with the E-vinylene bridge and favors a more conjugated conformation.
(11) (a) Chachisvilis, M.; Chirvony, V. S.; Shulga, A. M.; Ka¨llebring,
B.; Larsson, S.; Sundstro¨m, V. J. Phys. Chem. 1996, 100, 13857. (b)
Chachisvilis, M.; Chirvony, V. S.; Shulga, A. M.; Ka¨llebring, B.; Larsson,
S.; Sundstro¨m, V. J. Phys. Chem. 1996, 100, 13867.
(12) To the best of our knowledge, meso-meso E-vinylene-linked
porphyrin dimers without â-substituents, such as Zn22a/b, have not been
previously reported in the literature, although D. P. Arnold’s group has
prepared several analogues of 2a by Suzuki coupling (unpublished, personal
communication). The synthesis of a Z-vinylene-linked analogue of Zn22b
(with Ph rather than Ar substituents), by a route similar to Scheme 1, has
been reported in a patent: (a) Therien, M. J.; DiMagno, S. G. U.S. Patent
5,371,199, 1994. â-â E-vinylene-linked porphyrin dimers have also been
investigated: (b) Johnson, S. G.; Small, G. J.; Johnson, D. G.; Svec, W.
A.; Wasielewski, M. R. J. Phys. Chem. 1989, 93, 5437. (c) Zhilina, Z. I.;
Ishkov, Y. V.; Voloshanovskii, I. S.; Andronati, S. A.; Fel’dman, S. V. J.
Org. Chem. USSR (Engl. Transl.) 1989, 25, 2444.
(13) Screen, T. E. O., D.Phil. Thesis, Oxford University, Oxford, 2002.
(14) (a) Mee, S. P. H.; Lee, V.; Baldwin, J. E. Angew. Chem., Int. Ed.
2004, 43, 1132. (b) Mee, S. P. H.; Lee, V.; Baldwin, J. E. Chem.-Eur. J.
2005, 11, 3294.
(15) For previous examples of Stille coupling with meso-bromo por-
phyrins, see: (a) DiMagno, S. G.; Lin, V. S.-Y.; Therien, M. J. J. Am.
Chem. Soc. 1993, 115, 2513. (b) DiMagno, S. G.; Lin, V. S.-Y.; Therien,
M. J. J. Org. Chem. 1993, 58, 5983. (c) Shi, X.; Amin, S. R.; Liebeskind,
L. S. J. Org. Chem. 2000, 65, 1650.
5366
Org. Lett., Vol. 7, No. 24, 2005

Figure 1. Molecular structure of Zn22b‚2(C6H5N) viewed per-
pendicular to the mean plane of the porphyrin macrocycles (omitting
coordinated pyridine ligands, 50% probability ellipsoids).
neighboring â-pyrrole carbons; this C‚‚‚C distance is 3.09
Å (compared to 3.54 Å for twice the van der Waals radius
of carbon). The 45° twist in Zn22b does not completely block
conjugation between the two porphyrin π-systems, because
the orbital overlap depends on the cosine of the angle. The
two reported crystal structures of Ni21 have porphyrin to
C-CHdCH-C bridge mean plane twist angles of 89° and
74° (for the toluene and chloroform solvates, respectively),9,10
and therefore the absence of â-ethyl substituents clearly leads
to a more planar conformation in Zn22b.
Figure 2. Ground-state and excited-state absorption spectra of
Zn22a and Zn23 in benzene containing 1% pyridine.
at 765 nm (ΦF 0.041, τS 0.76 ns) and 731 nm (ΦF 0.10, τS
The degree of porphyrin-porphyrin π-conjugation in
Zn22a and Zn23 can be evaluated from their absorption and
emission spectra and redox potentials. The ground-state S0-
Sn absorption spectra of Zn22a and Zn23 are compared in
Figure 2 (solid lines). The maxima of Zn22a (422, 480, and
665 nm) are similar to those of Zn23 (408, 483, and 710
nm), although the spectrum of Zn22a is broader. Under the
same conditions, Zn22a and Zn23 give fluorescence maxima
1.1 ns).18,19 The first oxidation and reduction potentials for
ox
red
these dimers are: Zn22a: E1 +0.39 V; E1 -1.53 V;
ox
red
Zn23: E1 +0.29 V; E1 -1.69 V (all potentials versus
Fc/Fc+ in THF containing 0.1 M Bu4NBF4), corresponding
to electrochemical gaps (E1 - E1red) of 1.92 and 1.98 V,
ox
respectively.19 The greater Stokes shift in Zn22a together with
its lower fluorescence quantum yield and broader absorption
spectrum all indicate greater flexibility in the E-vinylene
bridge. Conformations are populated with a range of torsional
twist angles at the vinylene bridge, leading to a distribution
in the efficiency of porphyrin-porphyrin π-conjugation.11
The more widely split B band and more red-shifted Q-band
of Zn23 demonstrate that its average ground state conforma-
tion is more conjugated than that of Zn22a, whereas the more
red-shifted emission and smaller electrochemical gap of
Zn22a indicate that its S1 excited state, radical cation, and
radical anion are more conjugated than those of Zn23.
The nonlinear optical behavior of conjugated porphyrin
oligomers leads to important applications, such as reverse
(16) Experimental procedure for synthesis of Zn22b: (E)-1,2-Bis(tri-n-
butylstannyl)ethene (25 µL, 47 µmol) was added to Zn4b (80 mg, 79 µmol),
cesium fluoride (29 mg, 0.19 mmol), copper(I) iodide (5.5 mg, 29 µmol),
tetrakis(triphenylphosphine)palladium(0) (17 mg, 15 µmol), and DMF (1.5
cm3). The mixture was stirred at 115 °C under nitrogen for 42 h. Purification
by chromatography over silica with light petroleum (bp 40-60 °C)/ethyl
acetate/pyridine (10:1:1) as eluent and recrystallization from dichlo-
romethane by layered addition of methanol gave dimer Zn22b (35 mg, 58%)
as a purple powder. λmax (C6H6/pyridine 99:1)/nm 432 (log(ꢀ/M-1 cm-1
)
5.31), 487 (5.34), 570 (4.35), and 674 (4.55); δH (500 MHz, CDCl3/C5D5N)
1.52 (72H, s), 1.53 (36H, s), 7.76 (4H, t, J ) 1.6), 7.77 (2H, t, J ) 1.6),
8.08 (4H, d, J ) 1.6), 8.11 (8H, d, J ) 1.6), 8.91 (8H, ABq), 9.06 (4H, d,
J ) 4.7), 10.02 (4H, d, J ) 4.7), 10.03 (2H, s); δC (125.7 MHz, CDCl3/
C5D5N) 31.8, 35.0, 118.1, 120.5, 122.2, 122.4, 129.6, 129.8, 130.1, 131.8,
132.4, 142.57, 142.64, 144.1, 148.2, 149.8, 150.0, 150.2, 150.3; m/z
(MALDI-TOF, dithranol) 1900.5; (M+, C126H144N8Zn2 requires 1901.0).
(17) Crystal data for Zn22b‚2(C5H5N): Crystals were grown from
chlorobenzene/pyridine by vapor diffusion of ethanol. The structure was
solved on an Enraf-Nonius Kappa CCD diffractometer using Mo KR
radiation. C136H154N10Zn2‚3(C6H5Cl)‚3(H2O), (Mr ) 2451.27): triclinic,
space group P1h, Dc ) 1.131 g cm-3, a ) 10.3948(2), b ) 13.8919(2), c )
25.1936(3) Å, R ) 90.6016(4), â ) 91.8742(4), γ ) 97.9594(5)°, V )
3600.65(10) Å3, Z ) 1, λ ) 0.71073 Å, µ ) 0.442 mm-1, T ) 150 K, R
) 0.0805 for 10 198 observed reflections [I > 3σI] and Rw ) 0.0878 for
all 16 237 unique reflections. The data have been deposited with the
Cambridge Crystallographic Data Center, CCDC No. 276947.
(18) Fluorescence quantum yields were standardized to tetraphenyl
porphyrin in benzene, ΦF ) 0.11; Seybold, P. G.; Gouterman, M. J. Mol.
Spectrosc. 1969, 31, 1.
(19) 5,15-Bis(3,5-di-tert-butylphenyl)porphyrinato zinc(II) has absorption
spectrum: λmax 420, 588 nm; fluorescence spectrum: λmax 596, 649 nm
red
(both in 1% pyridine/benzene); its redox potentials are: E1ox +0.39 V; E1
-1.87 V (versus Fc/Fc+ in CH2Cl2 containing 0.1 M Bu4NBF4). Thus, the
longest wavelength absorption band of Zn22a is red-shifted by 77 nm,
relative to the corresponding porphyrin monomer, and its electrochemical
gap is reduced by 0.34 V.
Org. Lett., Vol. 7, No. 24, 2005
5367

saturable absorption (RSA)3 and two-photon absorption
(TPA),2 and therefore we have compared the RSA and TPA
characteristics of the C2H2- and C2-linked dimers Zn22a and
Zn23. A compound exhibits RSA if the excited-state T1-Tn
absorption is stronger than the ground-state S0-Sn absorption,
provided that it has a significant triplet yield (ΦT) and
provided that the S0-Sn absorption is high enough to achieve
significant population of excited states.20 The T1-Tn absorp-
tion spectra of Zn22a and Zn23 were measured by laser flash
photolysis, in deoxygenated benzene with 1% pyridine
(Figure 2, dashed curves, excitation at 355 nm with a 5-ns
pulse). Zn22a has a triplet yield of ΦT ) 0.21 and a triplet
lifetime of τT ) 10 µs, whereas Zn23 has ΦT ) 0.44 and
biexponential decay, τT ) 143 µs and 1.65 ms. The T1-Tn
absorption of the C2-linked dimer Zn23 peaks beyond 1000
nm, where there is essentially no S0-Sn absorption, making
it unsuitable for RSA. In contrast, the C2H2-linked dimer
Zn22a exhibits a T1-Tn band at 880 nm. There is good
overlap between this excited-state absorption and the broad
tail of the ground-state Q-band, making it suitable for RSA
in the near-infrared at 710-900 nm.
of Zn23, is consistent with the weaker conjugation in the
C2H2-linked dimer. Unfortunately, it was impossible to
measure the TPA spectrum of Zn22a at excitation wave-
lengths shorter than 940 nm because the long tail on the S0-
S1 absorption makes the power dependence of the fluores-
cence nonquadratic in this region. This weak one-photon
absorption at 710-900 nm, which prevents us from measur-
ing the TPA spectrum of Zn22a, is the same feature that
makes this chromophore promising for RSA.
In conclusion, we have developed an efficient synthetic
route to meso-meso E-vinylene-linked porphyrin dimers.
While the most populated conformation of the C2H2-linked
dimer Zn22a has the vinylene bridge partly twisted out of
conjugation with the porphyrins (45° twist in the solid state),
making it less conjugated than the C2-linked dimer Zn23,
more planar conformations are also populated, which are
more conjugated than the C2-linked dimer, resulting in a long
red tail in the absorption at 710-900 nm. In combination
with a strong excited-state T1-Tn absorption band at 700-
1000 nm, these photophysical characteristics make C2H2-
linked dimers promising materials for reverse saturable
absorption.
We also tested the two-photon absorption behavior of
Zn22a using two-photon excited fluorescence. Previously we
used this technique to show that Zn23 has a remarkably strong
TPA band at 821 nm (σ2max ) 8200 GM), as well as a weaker
red-shifted band at 940 nm (255 GM).2 Zn22a shows a similar
broad red-shifted TPA transition peaking at ca. 975 nm (60
GM) in the excitation wavelength range 940-1100 nm. The
4-fold lower cross section of this peak, as compared to that
Acknowledgment. We thank EPSRC and EOARD for
support, and the EPSRC Mass Spectrometry Service (Swansea)
for mass spectra.
Supporting Information Available: Experimental pro-
cedures, full spectroscopic data for all new compounds, and
crystal data (CIF) for porphyrin dimer 2b. This material is
available free of charge via the Internet at http://pubs.acs.org.
(20) (a) Miles, P. A. Appl. Opt. 1994, 33, 6965. (b) Miles, P. A. Appl.
Opt. 1999, 38, 566. (c) Kobyakov, A.; Hagan, D. J.; Van Stryland, E. W.
J. Opt. Soc. Am. B 2000, 17, 1884.
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