Two-photon photosensitized production of singlet oxygen: Sensitizers with phenylene-ethynylene-based chromophores

Article abstract of DOI:10.1021/jo0482099

(Chemical Equation Presented) Singlet molecular oxygen (a ¹Δ_g) has been produced and optically monitored in time-resolved experiments upon nonlinear two-photon excitation of photosensitizers that contain triple bonds as an integral part of the chromophore. Both experiments and ab initio computations indicate that the photophysical properties of alkyne-containing sensitizers are similar to those in the alkene-containing analogues. Most importantly, however, in comparison to the analogue that contains double bonds, the sensitizer containing alkyne moieties is more stable against singlet-oxygen-mediated photooxygenation reactions. This increased stability can be advantageous, particularly with respect to two-photon singlet oxygen imaging experiments in which data are collected over comparatively long time periods.

Full text of DOI:10.1021/jo0482099

Two-Photon Photosensitized Production of Singlet Oxygen:
Sensitizers with Phenylene-Ethynylene-Based Chromophores
Sean P. McIlroy,† Emiliano Clo´,† Lars Nikolajsen,‡ Peter K. Frederiksen,†
Christian B. Nielsen,† Kurt V. Mikkelsen,*,‡ Kurt V. Gothelf,*,† and Peter R. Ogilby*,†
Department of Chemistry, University of Aarhus, DK-8000 A° rhus, Denmark, and
Department of Chemistry, University of Copenhagen, DK-2100 Copenhagen, Denmark
kmi@theory.ki.ku.dk; kvg@chem.au.dk; progilby@chem.au.dk
Received October 11, 2004
Singlet molecular oxygen (a1∆g) has been produced and optically monitored in time-resolved
experiments upon nonlinear two-photon excitation of photosensitizers that contain triple bonds as
an integral part of the chromophore. Both experiments and ab initio computations indicate that
the photophysical properties of alkyne-containing sensitizers are similar to those in the alkene-
containing analogues. Most importantly, however, in comparison to the analogue that contains
double bonds, the sensitizer containing alkyne moieties is more stable against singlet-oxygen-
mediated photooxygenation reactions. This increased stability can be advantageous, particularly
with respect to two-photon singlet oxygen imaging experiments in which data are collected over
comparatively long time periods.
Introduction
Alternatively, a sensitizer can be specifically added to a
system, as in the case of photodynamic therapy where
light is used as a tool to selectively kill biological cells
(e.g., cancer cells).7
Until recently, sensitizer excitation has always been
achieved via a linear one-photon transition between the
sensitizer ground state, S0, and a singlet excited state of
the sensitizer, Sn.5 After excitation, singlet oxygen is most
efficiently produced upon oxygen quenching of the sen-
sitizer triplet state, T1, formed upon intersystem crossing,
S1 f T1.5
The lowest excited electronic state of molecular oxygen,
singlet molecular oxygen (a1∆g), is an intermediate in
many oxidation reactions.1,2 In many systems of practical
importance, these reactions occur in samples that contain
heterogeneous, phase-separated domains.3,4 Thus, for
work in such systems, it could be of great use to both
produce and detect singlet oxygen in time- and space-
resolved experiments.
Singlet oxygen can be efficiently produced upon ir-
radiation of a sensitizer which, in turn, transfers its
energy of excitation to ground-state oxygen (X3Σg-).5
Sensitizers can be a chromophore inherent to a given
system, as is the case with many functional polymers.6
It has been demonstrated, however, that singlet oxygen
can also be produced upon nonlinear two-photon excita-
tion of a sensitizer.8-11 In this case, an excited-state Sm
(6) Dam, N.; Scurlock, R. D.; Wang, B.; Ma, L.; Sundahl, M.; Ogilby,
P. R. Chem. Mater. 1999, 11, 1302-1305.
* To whom correspondence should be addressed.
† University of Aarhus.
(7) Dougherty, T. J.; Gomer, C. J.; Henderson, B. W.; Jori, G.; Kessel,
D.; Korbelik, M.; Moan, J.; Peng, Q. J. Natl. Cancer Inst. 1998, 90,
889-905.
‡ University of Copenhagen.
(1) Foote, C. S. Acc. Chem. Res. 1968, 1, 104-110.
(2) Frimer, A. A., Ed. Singlet Oxygen; CRC Press: Boca Raton, 1985;
Vol. I-IV.
(8) Frederiksen, P. K.; Jørgensen, M.; Ogilby, P. R. J. Am. Chem.
Soc. 2001, 123, 1215-1221.
(9) Poulsen, T. D.; Frederiksen, P. K.; Jørgensen, M.; Mikkelsen,
K. V.; Ogilby, P. R. J. Phys. Chem. A 2001, 105, 11488-11495.
(10) Frederiksen, P. K.; McIlroy, S. P.; Nielsen, C. B.; Nikolajsen,
L.; Skovsen, E.; Jørgensen, M.; Mikkelsen, K. V.; Ogilby, P. R. J. Am.
Chem. Soc. 2005, 127, 255-269.
(3) Oleinick, N. L.; Morris, R. L.; Belichenko, I. Photochem. Photo-
biol. Sci. 2002, 1, 1-21.
(4) Lissi, E. A.; Encinas, M. V.; Lemp, E.; Rubio, M. A. Chem. Rev.
1993, 93, 699-723.
(5) Schweitzer, C.; Schmidt, R. Chem. Rev. 2003, 103, 1685-1757.
10.1021/jo0482099 CCC: $30.25 © 2005 American Chemical Society
Published on Web 01/28/2005
1134
J. Org. Chem. 2005, 70, 1134-1146

Two-Photon Photosensitized Production of Singlet Oxygen
is populated as a consequence of the less-probable
simultaneous absorption of two lower energy photons.
The transition proceeds via a virtual state, S0 f Svir
to incorporate alkyne rather than alkene-containing
residues into the chromophore. In this case, one would
try to capitalize on the general fact that alkynes can be
significantly less reactive than the corresponding alkene
with respect to electrophilic reagents.19 Indeed, with
respect to singlet oxygen in particular, alkynes are
remarkably unreactive, certainly in comparison to the
corresponding alkene. To our knowledge, singlet oxygen
has been shown to react with alkynes in only two cases.
In the first, the substrate was an amine-substituted
alkyne (N,N-diethyl-1-propynylamine) and, hence, was
particularly electron rich.20 In the second, the alkyne was
strained as a consequence of being incorporated in a
ring.21
A substantial amount of work has been done in the
recent past on the development of chromophores that
have large two-photon absorption cross sections,22,23
including the development of chromophores that contain
ethynyl moieties.24-28 With respect to the latter, it has
indeed been noted that replacement of a double bond by
a triple bond leads to improved photostability of the
chromophore.24 Most of the molecules produced, however,
are highly fluorescent and are thus not well suited for
the production of singlet oxygen. For the present study,
we set out to explore issues pertinent to the development
of two-photon chromophores containing ethynyl moieties
that not only produce singlet oxygen in appreciable yield
but that are stable upon prolonged exposure to singlet
oxygen.
f
Sm, and follows selection rules that can differ from those
for a one-photon transition.12 An important aspect of this
approach is the opportunity to selectively populate an
excited state at the point of a focused laser beam where
a spatial domain of sufficiently high fluence can be
obtained. In short, two-photon excitation of a sensitizer
with a focused laser imparts spatial resolution to a singlet
oxygen experiment.
We have embarked on a program to develop techniques
by which singlet oxygen images of heterogeneous materi-
als can be produced.13-17 In one approach, we exploit the
spatial resolution available through a two-photon, pho-
tosensitized excitation scheme combined with the time-
resolved detection of singlet oxygen phosphorescence
(a1∆g f X3Σg-).16 Because the singlet oxygen phospho-
rescence signal is extremely weak, however, a limiting
aspect of this method is the need to acquire data over
long periods of time. Under these circumstances, where
the sample is exposed to prolonged irradiation, photo-
oxygenation reactions can bleach the sensitizer which,
in turn, adversely influences the production of singlet
oxygen. It would thus be useful to develop stable sensi-
tizers that produce singlet oxygen in high yield and that
have a comparatively large two-photon absorption prob-
ability (i.e., large values of the so-called two-photon
absorption cross section).
A critical component of a chromophore designed to
absorb light in the UV-vis region of the spectrum, either
by a one- or two-photon process, is a framework of
conjugated π bonds. This framework not only plays a
pivotal role in establishing transition energies (i.e.,
wavelengths of absorption) but facilitates the intramo-
lecular charge redistribution associated with the develop-
ment of transition moments, the magnitudes of which
determine the transition probability. For many common
chromophores, the network of conjugated π bonds often
contains one or more alkenyl residues. Unfortunately,
these double bonds can be quite susceptible to reactions
with singlet oxygen, particularly if they are electron
rich.18 In light of this latter statement, attempts to make
these double bonds less reactive toward singlet oxygen,
and hence make the chromophore more stable, include
the addition of electron-withdrawing substituents to the
alkenyl moiety.6
Results and Discussion
Molecules examined as singlet oxygen sensitizers are
shown in Schemes 1-3. In our system of nomenclature,
the subscript T identifies molecules that have a triple
bond as an integral part of the chromophore, whereas
the subscript D identifies molecules that contain a double
bond. In addition to considering “rodlike” molecules (i.e.,
3T(X) and 3D(X)), in which photoinduced changes in
charge distribution will principally be dipolar and/or
quadrupolar in nature, we also examined a series of “tri-
podal” molecules (i.e., 1T(X) and 1D(X)) in which changes
in charge distribution can have an octupolar component.
The two-photon behavior of a variety of octupolar mol-
(19) Streitwieser, A.; Heathcock, C. H. Introduction to Organic
Chemistry; Macmillan Publishing Co.: New York, 1985.
(20) Foote, C. S.; Lin, J. W.-P. Tetrahedron Lett. 1968, 3267-3270.
(21) Turro, N. J.; Ramamurthy, V.; Liu, K.-C.; Krebs, A.; Kemper,
R. J. Am. Chem. Soc. 1976, 98, 6758-6761.
(22) Albota, M.; Beljonne, D.; Bre´das, J.-L.; Ehrlich, J. E.; Fu, J.-
Y.; Heikal, A. A.; Hess, S. E.; Kogej, T.; Levin, M. D.; Marder, S. R.;
McCord-Maughon, D.; Perry, J. W.; Ro¨ckel, H.; Rumi, M.; Subrama-
niam, G.; Webb, W. W.; Wu, X.-L.; Xu, C. Science 1998, 281, 1653-
1656.
(23) Reinhardt, B. A.; Brott, L. L.; Clarson, S. J.; Dillard, A. G.;
Bhatt, J. C.; Kannan, R.; Yuan, L.; He, G. S.; Prasad, P. N. Chem.
Mater. 1998, 10, 1863-1874.
(24) Mongin, O.; Porres, L.; Moreaux, L.; Mertz, J.; Blanchard-Desce,
M. Org. Lett. 2002, 4, 719-722.
(25) Porres, L.; Mongin, O.; Katan, C.; Charlot, M.; Pons, T.; Mertz,
J.; Blanchard-Desce, M. Org. Lett. 2004, 6, 47-50.
(26) Iwase, Y.; Kamada, K.; Ohta, K.; Kondo, K. J. Mater. Chem.
2003, 13, 1575-1581.
(27) Pyun, O. K.; Yang, W.; Jeong, M.-Y.; Lee, S. H.; Kang, K. M.;
Jeon, S.-J.; Cho, B. R. Tet. Lett. 2003, 44, 5179-5182.
(28) Beljonne, D.; Wenseleers, W.; Zojer, E.; Shuai, Z.; Vogel, H.;
Pond, S. J. K.; Perry, J. W.; Marder, S. R.; Bredas, J.-L. Adv. Funct.
Mater. 2002, 12, 631-641.
Another option by which one might be able to ap-
preciably stabilize a singlet oxygen sensitizer would be
(11) Karotki, A.; Drobizhev, M.; Kruk, M.; Spangler, C.; Nickel, E.;
Mamardashvili, N.; Rebane, A. J. Opt. Soc. Am., B 2003, 20, 321-
332.
(12) McClain, W. M. Acc. Chem. Res. 1974, 7, 129-135.
(13) Zebger, I.; Poulsen, L.; Gao, Z.; Andersen, L. K.; Ogilby, P. R.
Langmuir 2003, 19, 8927-8933.
(14) Andersen, L. K.; Gao, Z.; Ogilby, P. R.; Poulsen, L.; Zebger, I.
J. Phys. Chem. A. 2002, 106, 8488-8490.
(15) Andersen, L. K.; Ogilby, P. R. Photochem. Photobiol. 2001, 73,
489-492.
(16) Snyder, J. W.; Zebger, I.; Gao, Z.; Poulsen, L.; Frederiksen, P.
K.; Skovsen, E.; McIlroy, S. P.; Klinger, M.; Andersen, L. K.; Ogilby,
P. R. Acc. Chem. Res. 2004, 37, 894-901.
(17) Zebger, I.; Snyder, J. W.; Andersen, L. K.; Poulsen, L.; Gao, Z.;
Lambert, J. D. C.; Kristiansen, U.; Ogilby, P. R. Photochem. Photobiol.
2004, 79, 319-322.
(18) Foote, C. S.; Clennan, E. L. In Active Oxygen in Chemistry;
Foote, C. S., Valentine, J. S., Greenberg, A., Liebman, J. F., Eds.;
Chapman and Hall: London, 1995, pp 105-140.
J. Org. Chem, Vol. 70, No. 4, 2005 1135

McIlroy et al.
SCHEME 1
SCHEME 3
Computations. It has been demonstrated, both
through our own work9,10,33 as well as through the work
of others,34,35 that ab initio computations based on
response theory can yield relatively accurate two-photon
absorption cross sections for comparatively large poly-
atomic systems. From our perspective, such computations
have become indispensable, not just as an aid in the
interpretation of experimental data, but as a tool to
predict which molecules are likely to be worth the often
extensive efforts required for their synthesis and for
making photophysical measurements.
SCHEME 2
For the present study, we set out to examine several
conjugated π systems that are either known to be, or that
could be, reasonable singlet oxygen sensitizers. Of gen-
eral concern was the extent to which two-photon absorp-
tion cross sections would be influenced by (1) the change
from an alkene to an alkyne moiety in the chromophore
and (2) a change in substituents attached to the chro-
mophore.
A. Alkene vs Alkyne Moieties in the Chromo-
phore. Our computations indicate that, for a given pair
of molecules, two-photon absorption cross sections for
the alkyne-containing analogue are not significantly
ecules has recently been examined by a number of
groups.28-32
(32) Mongin, O.; Porres, L.; Katan, C.; Pons, T.; Mertz, J.; Blanchard-
Desce, M. Tetrahedron Lett. 2003, 44, 8121-8125.
(33) Poulsen, T. D.; Ogilby, P. R.; Mikkelsen, K. V. J. Chem. Phys.
2001, 115, 7843-7851.
(34) Wang, C.-K.; Macak, P.; Luo, Y.; A° gren, H. J. Chem. Phys. 2001,
114, 9813-9820.
(35) Norman, P.; Luo, Y.; A° gren, H. J. Chem. Phys. 1999, 111, 7758-
7765.
(29) Lee, W.-H.; Lee, H.; Kim, J.; Choi, J.-H.; Cho, M.; Jeon, S.-J.;
Cho, B. R. J. Am. Chem. Soc. 2001, 123, 10658-10667.
(30) Yoo, J.; Yang, S. K.; Jeong, M.-Y.; Ahn, H. C.; Jeon, S.-J.; Cho,
B. R. Org. Lett. 2003, 5, 645-648.
(31) Kannan, R.; He, G. S.; Lin, T.-C.; Prasad, P. N.; Vaia, R. A.;
Tan, L.-S. Chem. Mater. 2004, 16, 185-194.
1136 J. Org. Chem., Vol. 70, No. 4, 2005

Two-Photon Photosensitized Production of Singlet Oxygen
TABLE 1. Relative Two-Photon Absorption Cross
Sections Calculated Using Response Theory
different from those for the alkene-containing analogue
(Table 1). Second, the computations indicate that a given
transition in the alkyne-containing analogue will gener-
ally be blue-shifted relative to the corresponding transi-
tion in the alkene-containing analogue. This latter point
is further discussed below in the context of our spectro-
scopic measurements.
B. Substituent Effects. For the molecules examined
in this study, our computations indicate that a change
in the substituent attached to the chromophore can have
a pronounced effect on the magnitude of the two-photon
absorption cross section. This is indeed to be expected
since the substituent’s ability to either donate or accept
electron density will influence the extent to which charge
can be redistributed in the chromophore which, in turn,
will influence the magnitude of the transition moment.
However, for any two substituents, the relative effect of
a change in substituent in the alkene-containing chro-
mophore is essentially the same as that in the alkyne-
containing chromophore. This suggests that, in the
development of the transition moment, alkene-mediated
intramolecular charge transfer does not significantly
differ from alkyne-mediated charge transfer.
two-photon cross sectionsa
(transition energy, irreducible
point
molecule
1T(H)
representation)b
groupb
1.67 × 104
(4.50, A(E))c
8.20 × 104
4.62 × 103
1.29 × 103
(5.47, A)
C1 (C3)
C1 (C3)
C1 (C3)
C1 (C3)
C1 (C3)
C1 (C3)
C1 (C3)
C1 (C3)
(4.87, A)
1T(CN)
9.20 × 104
(4.67, A)
2.54 × 103
(4.87, A)
1.26 × 103
(5.50, A)
(4.35, A(E))c
1.27 × 104
(4.50, A(E))c
6.61 × 104
(4.45, A(E))c
9.82 × 104
1T(F)
8.66 × 102
(5.47, A)
1T(F5)
6.10 × 104
(4.84, A)
1.64 × 102
(5.53, A)
1T(CHO)
1T(CO2H)
1T(HPy+)
1T(MPy+)
5.49 × 100
(4.61, A)
1.24 × 105
(4.63, A)
2.11 × 102
(5.46, A)
(4.30, A(E))c
1.03 × 105
1.27 × 105
(4.68, A)
1.44 × 106
(4.30, A)
1.48 × 106
(4.31, A)
(4.35, A(E))c
8.27 × 105
9.05 × 102
(4.84, A)
(3.90, A(E))c
8.43 × 105
7.19 × 100
(3.91, A(E))c
(4.93, A)
1D(H)
2.66 × 104
(4.15, A(E))c
1.10 × 105
1.28 × 104
1.26 × 105
(4.39, A)
1.86 × 103
(5.28, A)
C1 (C3)
C1 (C3)
(4.55, A)
1D(CN)
1.01 × 103
(5.31, A)
(4.01, A(E))c
2T(H)
3.23 × 103
8.97 × 103
(4.80, A)
9.61 × 104
(4.60, A)
1.16 × 105
(4.58, A)
1.14 × 105
(4.62, A)
5.69 × 103
(4.83, A)
C1
C1
C1
C1
C1
(4.51, A)
2T(CN)
2T(CHO)
2T(CO2H)
2T(F)
2.04 × 104
(4.35, A)
2.59 × 104
(4.33, A)
In conclusion, our computations indicate that if one has
an established chromophore with desirable nonlinear
optical properties, replacing alkene moieties with alkyne
moieties should not have a significant adverse influence
on the two-photon behavior of the system.
Synthesis. On the basis of our computational results,
selected molecules were synthesized to further examine
their potential as two-photon singlet oxygen sensitizers.
2.46 × 104
(4.37, A)
2.33 × 103
(4.54, A)
2D(H)
5.15 × 103
(4.18, A)
1.16 × 104
(4.47, A)
9.40 × 104
(4.31, A)
C1
2D(CN)
3.10 × 104
C1
(4.02, A)
Outlines of the synthetic steps used to prepare the tri-
podal molecules 1T(x) and the linear molecules 3T(x) are
given in Schemes 4-6.
The tripodal compounds were synthesized by two
different routes. In both cases, the key starting material
was tris(4-iodophenyl)amine 6, which was selectively
obtained in high yield from triphenylamine according to
a previously reported procedure.36
In the first route, 6 was coupled with arylethynes in
Sonogashira couplings37 (Scheme 4). The cyano aryl-
ethyne 5 was obtained from 4-bromobenzonitrile by a
Sonogashira coupling with trimethylsilylacetylene fol-
lowed by removal of the TMS group using tetrabutyl-
ammonium fluoride (TBAF). Subsequent use of 5 in the
coupling with 6 gave 1T(CN) in excellent yield. Reduction
of the three cyano groups in 1T(CN) using diisobutyla-
luminum hydride (DIBAL) yielded the trialdehyde
1T(CHO) in a moderate yield of 43%. The synthesis of
the arylethyne used to prepare the bromoderivative
1T(Br) is associated with a selectivity problem. Specifi-
cally, in the synthesis of 7 from 4-bromo-1-iodobenzene,
both bromo and iodo derivatives can undergo Sonogashira
coupling with acetylenes. The selectivity problem is
resolved by application of ethynylzinc bromide in a
Negishi coupling,38,39 which shows better selectivity for
iodine compared to bromine. The selectivity problem is
3T(H)
1.45 × 105
(4.90, Ag)
1.27 × 106
(4.75, Ag)
1.01 × 101
(4.42, Ag)
4.56 × 105
(4.85, Ag)
3.50 × 105
(4.87, Ag)
6.49 × 105
(4.84, Ag)
4.40 × 105
(5.02, Ag)
1.38 × 105
(4.67, Ag)
1.21 × 106
(4.52, Ag)
7.81 × 100
(4.49, Ag)
6.94 × 105
(4.65, Ag)
5.31 × 105
(4.76, Ag)
4.73 × 103
(5.50, Ag)
1.26 × 104
(5.51, Ag)
6.20 × 105
(4.81, Ag)
7.34 × 103
(5.51, Ag)
5.84 × 103
(5.50, Ag)
7.08 × 103
(5.51, Ag)
4.65 × 103
(5.78, Ag)
1.92 × 104
(5.40, Ag)
8.23 × 104
(5.40, Ag)
5.49 × 105
(4.56, Ag)
2.85 × 104
(5.41, Ag)
2.47 × 104
(5.68, Ag)
Ci
Ci
Ci
Ci
Ci
Ci
Ci
3T(CN)
3T(CHO)
3T(CO2H)
3T(F)
3T(F2)
3T(Br)d
3D(H)
Ci
Ci
Ci
Ci
Ci
3D(CN)
3D(CHO)
3D(F2)
3D(Br)d
a In atomic units. Data shown correspond to 30δ (see eq 1). For
a given molecule, the transition with the largest cross section is
shown in boldface type. b Transition energies in eV. As explained
in the section on computational methods, the calculated transition
energies are larger than those that would be experimentally
observed. For each molecule, we identify the point group used and
the corresponding irreducible representation to which the given
transition transforms. c These entries reflect transitions to a
doubly degenerate excited state. The cross sections shown are the
sum of the individual cross sections to the respective excited states.
In the point group C3, these transitions transform according to
the irreducible representation E. In DALTON, the molecules are
examined using the point group C1 and the separate transitions
transform according to the irreducible representation A. d The
3-21G basis set was used.
(36) Varanavski, O. P.; Ostrowski, J. C.; Sukhomlinova, L.; Twieg,
R. J.; Bazan, G. C.; Goodson, T. J. Am. Chem. Soc. 2002, 124, 1736-
1743.
(37) Sonogashira, K. In Comprehensive Organic Synthesis; Trost,
B. M., Flemming, I., Eds.; Pergamon Press: Oxford, 1991; Vol. 3, pp
521-549.
J. Org. Chem, Vol. 70, No. 4, 2005 1137

McIlroy et al.
SCHEME 4
not observed in the reaction between 6 and 7, and the
Sonogashira coupling gives 1T(Br) in 78% yield.
sterically hindered. For the spectroscopic measurements
in water, 1T(CO2H) was dissolved in an alkaline solution
and, thus, the pertinent compound is 1T(CO2-).
In the second route, the tripodal sensitizers were
synthesized via intermediate 9 (Scheme 5). This approach
was prompted by the fact that attempts to use the
method outlined in Scheme 4 to couple trimethylsilyl-
acetylene with electron deficient aromatic iodo com-
pounds, in particular the iodofluoro derivative, gave poor
results. On the other hand, coupling of the iodoaryl
compounds with 9 gave much better yields, even though
three iodo-substituted compounds had to be coupled to
9. Compound 9 was obtained in high yield by triple
coupling of trimethylsilylacetylene with 6 and subsequent
deprotection with TBAF. The sensitizers 1T(CO2CH3)
and 1T(F5) were obtained by standard Sonogashira
couplings with the respective iodoaryl compounds. The
coupling of 9 with 3 equiv of 4-bromopyridinium chloride
was more difficult and required a different catalyst-
ligand system to achieve an acceptable yield of 1T(Py).
It is believed that a more nucleophilic palladium catalyst
is obtained when using the t-Bu ligand, which enhances
the rate of the oxidative addition of the catalyst to the
bromopyridine.40 The sensitizer 1T(CO2H) was obtained
by hydrolysis of the three esters in 1T(CO2CH3), and
1T(MePy+) was obtained by methylation of 1T(Py). In
the latter reaction, methylation of the central nitrogen
atom is not observed because the lone pair of this nitro-
gen is delocalized and because the nitrogen atom is
The rodlike sensitizers 3T(H), 3T(F2), and 3T(Br) were
also synthesized using palladium chemistry (Scheme 6).
In the first step of the synthesis, diphenylamine was
connected to the aryl moiety in 4-bromo-1-iodobenzene
by a palladium-catalyzed C-N coupling reaction.41 With
the selected conditions, the bifunctional aryl derivative
preferably undergoes coupling by substitution of iodide
to form 11. However, the competing reaction at the
bromide cannot be completely eliminated, and the di-
amine 12 is formed as a byproduct. A Sonogashira
coupling reaction with 1,4-diethynylbenzene gives rise to
3T(H), albeit in a low yield. In the other pathway, 11 was
functionalized with TMS-acetylene and subsequent re-
moval of the TMS group gave 14 in a high yield. Coupling
2 equiv of 14 with 1,4-diiodo-2,3,5,6-tetrafluorobenzene
gave 3T(F2). To selectively obtain 3T(Br) from 1,4-
dibromo-2,5-diiodobenzene and 14, a Negishi-type cou-
pling was preferred over the Sonogashira coupling since
it has the highest preference for iodine. The zinc reagent
was made by deprotonation of the electron rich ethynyl
moiety in 14 with LDA and subsequent transmetala-
tion with ZnBr2. Cross-coupling with the diiodoaryl
derivative gave 3T(Br) with high chemoselectivity in a
yield of 80%.
Measurements. In assessing the potential of our
molecules to act as two-photon singlet oxygen sensitizers,
several issues were considered, as outlined below.
(38) Anastasia, L.; Negishi, E. Org. Lett. 2001, 3, 3111-3113.
(39) Negishi, E.; Qian, M.; Zeng, F.; Anastasia, L.; Babinski, D. Org.
Lett. 2003, 5, 1597-1600.
(40) Hundertmark, T.; Littke, A. F.; Buchwald, S. L.; Fu, G. C. Org.
Lett. 2000, 2, 1729-1731.
(41) Hartwig, J. F.; Kawatsura, M.; Hauck, S. I.; Shaughenessy, K.
H.; Alcazar-Roman, L. M. J. Org. Chem. 1999, 64, 5575-5580.
1138 J. Org. Chem., Vol. 70, No. 4, 2005

Two-Photon Photosensitized Production of Singlet Oxygen
SCHEME 5
A. Singlet Oxygen Quantum Yields. The singlet
oxygen quantum yield quantifies the efficiency of singlet
oxygen production by the excited-state sensitizer. When
using this number in the present context, we assume that
the quantum yield is independent of the method by which
the excited state was produced. Specifically, despite the
fact that the sensitizer excited state initially produced
upon two-photon irradiation may be different from that
produced upon one-photon irradiation, it is reasonable
to assume that rapid relaxation within the singlet state
manifold will yield the same state, S1, from which all
subsequent photophysics derives (i.e., Kasha’s rule42 is
followed). Thus, singlet oxygen quantum yields obtained
in a one-photon experiment should be valid in the context
of a two-photon discussion.
Of the tripodal phenylene-ethynylene-based chro-
mophores, 1T(x), five are soluble in toluene. In this
solvent, these molecules have a modest singlet oxygen
yield, ranging from 0.15 to 0.24 (Table 2). On the other
hand, the water-soluble tripodal chromophores with
charged substituents do not produce singlet oxygen in a
measurable yield (Table 2). In these latter cases, where
solvent-stabilized sensitizer-oxygen charge-transfer states
are likely to be readily accessible, charge-transfer-medi-
ated deactivation of the sensitizer excited state most
likely competes with the energy transfer process that
results in singlet oxygen formation. This is an established
phenomenon and can be quite pronounced in polar
solvents.10,43-45
In the rodlike chromophores 3T(x) and 3D(x), the
presence of the “heavy-atom” bromine on the central
aromatic ring clearly has an advantageous effect on the
singlet oxygen yield. In this case, the data reflect a heavy-
atom-mediated increase in the extent of spin-orbit
coupling that, in turn, results in a greater yield of the
sensitizer triplet state which is the immediate precursor
to singlet oxygen.
B. Stability. There are several ways one can quan-
tify the relative photostability of molecules used as
singlet oxygen sensitizers.
On one hand, one could directly irradiate the molecules
over discrete periods of time and monitor the extent to
which changes in the molecule occur (e.g., via absorption
(43) Kristiansen, M.; Scurlock, R. D.; Iu, K.-K.; Ogilby, P. R. J. Phys.
Chem. 1991, 95, 5190-5197.
(44) McGarvey, D. J.; Szekeres, P. G.; Wilkinson, F. Chem. Phys.
Lett. 1992, 199, 314-319.
(42) Gilbert, A.; Baggott, J. Essentials of Molecular Photochemistry;
CRC Press: Boca Raton, 1991.
(45) Abdel-Shafi, A. A.; Wilkinson, F. Phys. Chem. Chem. Phys. 2002,
4, 248-254.
J. Org. Chem, Vol. 70, No. 4, 2005 1139

McIlroy et al.
SCHEME 6
TABLE 2. Singlet-Oxygen Quantum Yields, O∆, and
One-Photon Absorption Maxima, λmax(1P), for the
Sensitizers Studied
changes in the extent to which light is absorbed over the
course of the reaction.
Of course, in our case, we are particularly interested
in the relative reactivity of these molecules with singlet
oxygen itself. As such, we opted to use an independent
sensitizer to generate singlet oxygen in the presence of
the molecules whose reactivity we wished to study. We
have outlined the advantages of this approach in a study
of the reactivity of phenylene vinylenes.6 For the present
case, the independent sensitizer of choice is the fullerene
C60. It is particularly important to note that (1) upon
irradiation, C60 generates singlet oxygen in high yield,49
(2) C60 can be irradiated at wavelengths where the
molecules being degraded do not absorb, (3) the extent
of degradation can be followed using absorption spec-
molecule
λmax(1P)a (nm)
φ∆(toluene)b
φ∆(water)b
1T(CN)
393
401
390
375
382
375
448
0.18
0.22
0.15
0.24
0.19
1T(CHO)
1T(CO2CH3)
1T(Br)
1T(F5)
-
1T(CO2
)
0
0
1T(MPy+)
3T(H)
3T(Br)
3T(F2)
383
404
418
0.09
0.49
0.09
3D(H)
3D(Br)
3D(F2)
411
427
430
0.08
0.46
0.09
a Recorded in toluene, except for 1T(CO2-) and 1T(MPy+) which
were recorded in water. b Recorded in air-saturated solvents. Data
in water were recorded using perinaphthenone sulfonic acid as
the standard (φ∆ ) 1.0),46,47 whereas data in toluene were recorded
using acridine as the standard (φ∆ ) 0.83).48 Errors are (10%.
(46) Nonell, S.; Gonzalez, M.; Trull, F. R. Afinidad L 1993, 448, 445-
450.
(47) Marti, C.; Ju¨rgens, O.; Cuenca, O.; Casals, M.; Nonell, S. J.
Photochem. Photobiol., A. Chem. 1996, 97, 11-18.
(48) Wilkinson, F.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref.
Data 1993, 22, 113-262.
(49) Arbogast, J. W.; Darmanyan, A. P.; Foote, C. S.; Rubin, Y.;
Diederich, F. N.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. J. Phys.
Chem. 1991, 95, 11-12.
spectroscopy). However, this approach has the distinct
disadvantage that one must continually correct for
1140 J. Org. Chem., Vol. 70, No. 4, 2005

Two-Photon Photosensitized Production of Singlet Oxygen
FIGURE 2. One-photon absorption spectra of 3D(Br), solid
line, and 3T(Br), dashed line, in toluene.
FIGURE 1. Data illustrating the relative reactivities of 3T-
(Br) and 3D(Br) to singlet oxygen. The latter was generated
upon selective irradiation of an independent sensitizer, C60,
added to the solution. In the present experiment, the extent
of degradation was quantified by monitoring substrate dis-
appearance using absorption spectroscopy (3T(Br) λmax at 420
nm and 3D(Br) λmax at 440 nm; see Figure 2). The absorbance
at time ) t divided by the absorbance at time ) 0 was then
plotted as a function of the elapsed time of C60 irradiation.
The experiment was performed in toluene.
is a characteristic of an amplified laser that operates at
a 1 kHz pulse repetition rate to accommodate a singlet
oxygen lifetime in the microsecond domain).10 Thus, a
desirable two-photon sensitizer (1) will not have an
allowed one-photon transition in this wavelength range
but (2) will have allowed one-photon transitions in the
range ∼450-550 nm (i.e., we want λmax(1P) > (765-850
nm)/2). The latter criterion reflects the fact that, at least
for centrosymmetric molecules, states accessible via two-
photon excitation will generally be slightly higher in
energy than states accessible via one-photon excita-
tion.12,52 On the basis of these points, the data in Figure
2 indicate that, if anything, our rodlike chromophores
may have transition energies that are somewhat higher
than ideally desired (i.e., the S1 and S2 states are too high
in energy and the corresponding transitions from S0 are
shifted too far to the blue).
D. Relative Two-Photon Absorption Spectra and
Cross Sections. Of course, in the present context, the
molecules used as singlet oxygen sensitizers must also
have an appreciable probability for two-photon absorption
over the specified wavelength range of ∼765-850 nm.
To quantify this particular property of a given sensitizer,
we used an approach that has been described in detail
elsewhere.8-10 Briefly, for a given molecule under study
and for irradiation at a given wavelength, we compared
the intensity of the two-photon photosensitized singlet
oxygen phosphorescence signal to the intensity of the
singlet oxygen signal obtained upon two-photon irradia-
tion of a molecule chosen as a standard. With this
approach, it is only necessary to account for the respective
singlet oxygen quantum yields in order to obtain relative
values of the absorption cross section. For a given
molecule, accurate values of the singlet oxygen quantum
yield are readily obtained in an independent experiment.
For the present study, the molecule chosen as the two-
photon standard was 3D(CN). The two-photon absorption
profile we have independently obtained for 3D(CN) in
toluene is shown in Figure 3.10 Although values reported
for the absolute absorption cross section of 3D(CN) at its
troscopy at a wavelength where C60 does not have an
appreciable absorbance, (4) the products of the degrada-
tion reactions all absorb light at wavelengths that are
blue-shifted relative to the wavelength at which C60 is
irradiated and at which the remaining unreacted sub-
strate is monitored, and (5) C60 is stable upon prolonged
irradiation.
In Figure 1, we show the results of a study in which
the relative reactivities of 3T(Br) and 3D(Br) to singlet
oxygen were compared. The data clearly show that the
molecule containing alkyne moieties is less reactive than
the corresponding alkene-containing molecule. This
observation is also consistent with data obtained on
phenylene vinylenes in which the alkenyl residue is
identified as the functional group that initially reacts
with singlet oxygen.6,50
C. One-Photon Absorption Spectra. In the develop-
ment of singlet oxygen sensitizers, the absorption spec-
trum of a given molecule is a property that has significant
influence on where and when the molecule can be used.
As illustrated in Figure 2 and Table 2, we find that,
for a given chromophore, replacement of alkene moieties
with alkyne moieties results in a blue shift in the one-
photon absorption spectrum. This phenomenon is con-
sistent with data recorded from other systems (e.g., 1,2-
diphenylacetylene vs stilbene).51
The femtosecond laser system we currently use to
irradiate molecules in our time-resolved two-photon
singlet oxygen experiments can only be tuned over the
limited wavelength range of ∼765-850 nm (this range
(50) Scurlock, R. D.; Wang, B.; Ogilby, P. R.; Sheats, J. R.; Clough,
R. L. J. Am. Chem. Soc. 1995, 117, 10194-10202.
(51) Suzuki, H. Electronic Absorption Spectra and Geometry of
Organic Molecules; Academic Press: New York, 1967.
(52) Birge, R. R.; Bennett, J. A.; Pierce, B. M.; Thomas, T. M. J.
Am. Chem. Soc. 1978, 100, 1533-1539.
J. Org. Chem, Vol. 70, No. 4, 2005 1141

McIlroy et al.
FIGURE 3. Two-photon excitation spectrum of 3D(CN), the
molecule used as a standard for the wavelength-dependent
two-photon cross section measurements. The data were re-
corded using the fluorescence of 3D(CN) as the probe with a
80 MHz fs laser system that has a much wider tuning range
than that used for our time-resolved singlet oxygen experi-
ments.10
FIGURE 5. Plot of the relative absorption cross section
against the irradiation wavelength for the two-photon excita-
tion of 3T(F2) and 3T(H) in toluene. The data were obtained in
singlet oxygen phosphorescence experiments in which 3D(CN)
was used as the two-photon absorption standard.
to indicate that the difference in cross section values at
a given wavelength is simply a consequence of the fact
that the two-photon absorption spectrum of 3D(Br) is red-
shifted relative to the two-photon absorption spectrum
of 3T(Br). This interpretation is certainly consistent with
the one-photon data shown in Figure 2 and the computa-
tions shown in Table 1. Second, although 3T(Br) and
3D(Br) do not have absorption cross sections as large as
the standard 3D(CN) over this wavelength range, the
cross sections at ∼780 nm are sufficiently large to facili-
tate the use of these molecules in a range of applications.
Of the alkyne-based sensitizers that produce singlet
oxygen in measurable yield, we ascertained that 3T(F2)
has the largest two-photon absorption cross section in the
wavelength range 765-850 nm (Figure 5). In Figure 6
we show two-photon absorption spectra obtained for the
tri-podal alkyne-containing sensitizers, 1T(x). In this
case, we find that the largest absorption cross section
obtained (i.e., that for 1T(CO2CH3) at ∼770 nm) is a
factor of ∼2.5 times smaller than the largest cross section
obtained for the rod-shaped molecules (i.e., that for
3T(F2) at ∼770 nm).
FIGURE 4. Plot of the relative absorption cross section
against the irradiation wavelength for the two-photon excita-
tion of 3D(Br) and 3T(Br) in toluene. The data were obtained
in singlet oxygen phosphorescence experiments in which 3D-
(CN) was used as the two-photon absorption standard.
For all of the molecules examined, the relative differ-
ences in the maximum two-photon absorption cross
section experimentally measured are consistent with the
differences expected based on our computations (Table
1).
band maximum cover a large range,22,53,54 the data
indicate that the molecule has a comparatively large two-
photon cross section at ∼840 nm (e.g., (1890 ( 280) ×
10-50 cm4 s photon-1 molecule-1).53
In Figure 4, we show two-photon absorption spectra
obtained for 3D(Br) and 3T(Br). Several points deserve
comment. First, the data clearly indicate that, over the
wavelength range studied, the alkyne-based sensitizer,
3T(Br), does not absorb light as well as the alkene-based
sensitizer, 3D(Br). The data obtained, however, appear
It is prudent at this point to comment on the apparent
peak observed at 770 nm in the two-photon absorption
spectra shown in Figures 5 and 6. Even though the data
were recorded in a relative experiment against a stan-
dard, it is important to note that, at 765 nm, the fs
system used to irradiate the sample is operating at its
limit in which the output power, pulse temporal profile,
and spectral content have the largest uncertainty. With
respect to the latter point, it is also important to note
that, with fs lasers, the spectral bandwidth at a given
irradiation “wavelength” is quite broad (∼15 nm). As a
consequence, any subtle substituent-dependent changes
(53) Pond, S. J. K.; Rumi, M.; Levin, M. D.; Parker, T. C.; Beljonne,
D.; Day, M. W.; Bre´das, J.-L.; Marder, S. R.; Perry, J. W. J. Phys. Chem.
A 2002, 106, 11470-11480.
(54) Zhang, B.-J.; Jeon, S.-J. Chem. Phys. Lett. 2003, 377, 210-216.
1142 J. Org. Chem., Vol. 70, No. 4, 2005

Two-Photon Photosensitized Production of Singlet Oxygen
states of the molecule.56 Thus, we explicitly avoid the limita-
tions of a traditional sum-over-states calculation57-59 in which
the accuracy of the result can be compromised if one, or more,
key states of the molecule are neglected.
In our approach, the matrix elements for a two-photon
electric dipole transition, SR,â, were calculated through a
residue of the quadratic response function.56 The two-photon
cross section for linearly polarized light, δ, was then expressed
in terms of these matrix elements, summed over the molecular
axes (eq 1, where R ) x, y, z and â ) x, y, z).58,60
1
δ )
(2SR,RS* + 4SR,âS*
)
â,R
(1)
∑
â,â
30R,â
Unless stated otherwise, the response calculations were
carried out using the 6-31G* basis set.
Prior to using the DALTON package for the response
calculations, the geometries of the respective molecules were
optimized using a number of approaches available in the
Gaussian 98 program.61 We have demonstrated that, for a
given molecule, calculated two-photon transition energies and
absorption cross sections can depend significantly on the
molecular geometry used.9 This is indeed to be expected,
particularly when the geometric variable influences the extent
of conjugation in the system. For these reasons, when per-
forming the geometry optimizations, we not only tried to be
thorough in the search for “the” global minimum, but we also
tried to be consistent in an attempt to ensure that relative
changes in the respective cross sections would be accurate.
The absorption cross section obtained from the response
theory computation is in atomic units. To compare the latter
with experimental data, it is necessary to convert to a number
with the units of cm4 s photon-1molecule-1. Among other
things, this transformation involves multiplication by a band
shape function that can vary from one molecule to the
next.9,35,52,62 For our present application, in which we want only
to focus on relative changes in the magnitude of the absorption
cross section, we have opted to avoid introducing this extra
source of uncertainty.
It is likewise important to recognize that, due to the size of
the systems involved, the calculations were performed on the
uncorrelated Hartree-Fock level. Moreover, the effect of
solvent has not been included. As a consequence, the calculated
transition energies are generally larger than those that would
be experimentally observed from solution phase systems.
Although this is a general problem common to such computa-
tions,33,34 the calculated differences between transition energies
should be reasonably accurate.
Sensitizer Synthesis. Standard Schlenk techniques were
employed using argon as the inert atmosphere for all manipu-
lations of air- or moisture-sensitive compounds. Unless oth-
erwise stated, yields refer to chromatographically isolated and
FIGURE 6. Plot of the relative absorption cross section
against the irradiation wavelength for the two-photon excita-
tion of five tri-podal molecules in toluene: 1T(CO2CH3), 9;
1T(F5), b; 1T(CHO), O; 1T(CN), 0; 1T(Br), 2. The data were
obtained in singlet oxygen phosphorescence experiments in
which 3D(CN) was used as the two-photon absorption standard
(error ∼ (10-15%).
in the actual position of the absorption band maximum
may not be resolved with our available spectral resolu-
tion.
Conclusions
Not only are singlet oxygen photosensitizers that
contain a triple bond as an integral part of the chro-
mophore viable, but they also have one distinctive and
attractive feature that sets them apart from the corre-
sponding alkene-containing sensitizer; the alkynyl moiety
is significantly less susceptible to reaction with singlet
oxygen. Specifically, alkyne-containing sensitizers retain
essentially all of the desirable linear and nonlinear
optical properties of the corresponding alkene-containing
molecule but can be used over significantly longer periods
of irradiation. The latter can have significant conse-
quences in singlet oxygen imaging experiments where
prolonged periods of data acquisition may be required.
Experimental Section
(56) Olsen, J.; Jørgensen, P. J. Chem. Phys. 1985, 82, 3235-3264.
(57) Bhawalkar, J. D.; He, G. S.; Prasad, P. N. Rep. Prog. Phys. 1996,
59, 1041-1070.
Computational Methods. Features of our ab initio com-
putational approach have been described previously.9 Briefly,
calculations were performed with the DALTON program
package,55 using response theory to obtain the two-photon
absorption cross sections. For this application, the principal
attribute of response theory is the ability to obtain the two-
photon cross section in one calculation in which the virtual
state is effectively described as a linear combination of all
(58) Birge, R. R.; Pierce, B. M. J. Chem. Phys. 1979, 70, 165-178.
(59) Kogej, T.; Beljonne, D.; Meyers, F.; Perry, J. W.; Marder, S. R.;
Bre´das, J. L. Chem. Phys. Lett. 1998, 298, 1-6.
(60) McClain, W. M. J. Chem. Phys. 1971, 55, 2789-2796.
(61) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.,
Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Cioslowski, J.; Foresman, J. B.; Ortiz, J. V.; Baboul, A. G.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.;
Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.7; Gaussian Inc.:
Pittsburgh, PA, 1998.
(55) Helgaker, T.; Jensen, H. J. A.; Jørgensen, P.; Olsen, J.; Ruud,
K.; A° gren, H.; Auer, A. A.; Bak, K. L.; Bakken, V.; Christiansen, O.;
Coriani, S.; Dahle, P.; Dalskov, E. K.; Enevoldsen, T.; Fernandez, B.;
Haettig, C.; Hald, K.; Halkier, A.; Heiberg, H.; Hettema, H.; Jonsson,
D.; Kirpekar, S.; Kobayashi, R.; Koch, H.; Mikkelsen, K. V.; Norman,
P.; Packer, M. J.; Pedersen, T. B.; Ruden, T. A.; Sanchez, A.; Saue, T.;
Sauer, S. P. A.; Schimmelpfennig, B.; Sylvester-Hvid, K. O.; Taylor,
P. R.; Vahtras, O. Dalton, a molecular electronic structure program,
Release 1.2 (2001), see http://www.kjemi.uio.no/software/dalton/
dalton.html.
(62) Peticolas, W. L. Ann. Rev. Phys. Chem. 1967, 18, 233-260.
J. Org. Chem, Vol. 70, No. 4, 2005 1143

McIlroy et al.
spectroscopically pure materials. Commercially available start-
ing materials were used as received without further purifica-
tion. Flash chromatography was performed using Merck silica
gel 60 (230-400 mesh). THF was freshly distilled over sodium
wire and DCM was freshly distilled over CaH2. Toluene,
triethylamine and diisopropylamine were distilled over CaH2
and stored under Argon over molecular sieves. ZnBr2 was
flame-dried at <1 mmHg shortly before use. Sodium tert-
butoxide was stored in a glovebox and small portions were
removed in glass vials, stored in the air in desiccators, weighed
in the air, and consumed within a week. 1,3-Diethynylben-
zene,63,64 tris-(4-iodophenyl)amine (6),36 and 4-iodobenzoic acid
methyl ester (10)65 were synthesized according to literature
procedures.
Tris-4,4′,4′′-(4-formylphenylethynyl)triphenylamine (1T-
(CHO)). In a flame-dried Schlenk flask, 1T(CN) (200 mg, 0.32
mmol) was dissolved in DCM (6 mL) and cooled to -78 °C. A
1.5 M solution of DIBAL in toluene (1.16 mL, 1.74 mmol) was
added dropwise via syringe. The mixture was warmed to rt
over 2 h under constant stirring. An excess of 5 M HCl (6 mL)
was slowly added, and stirring was continued for 1 h. Aqueous
NaOH was used to render the solution alkaline, and the
organic phase was separated, washed once with water, dried
over MgSO4, filtered, and concentrated. Chromatography on
silica gel (neat DCM) gave 84 mg (43%) of 1T(CHO): mp 210-
211 °C; 1H NMR (CDCl3, 400 MHz) δ 7.11 (d, 6H, J ) 8.5 Hz),
7.48, (d, 6H, J ) 8.5 Hz), 7.66 (d, 6H, J ) 8.1 Hz), 7.87 (d, 6H,
J ) 8.1 Hz), 10.02 (s, 3H); 13C NMR (CDCl3, 100 MHz) δ 88.7,
93.3, 117.3, 124.1, 129.6, 131.9, 133.1, 135.2, 147.0, 191.4;
HRMS m/z calcd for C45H27NO3 629.7008, found 629.2016.
4-Bromoethynylbenzene (7). A flame-dried Schlenk flask
was charged with a solution of ethynylmagnesium bromide (2.4
mL of a 0.5 M solution in THF, 1.22 mmol). A solution of
anhydrous ZnBr2 (293 mg, 1.22 mmol) in THF (3 mL) was
added via cannula at 0 °C. The mixture was stirred for 30 min
and added, via cannula, to a solution of 4-bromoiodobenzene
(345 mg, 1.22 mmol) and Pd(PPh3)4 (75 mg, 0.065 mmol) in
THF (3 mL). The reaction mixture was stirred at rt for 15 h,
diluted with Et2O (7 mL), washed once with aqueous NH4Cl,
once with aqueous NaHCO3, and once with brine, dried over
MgSO4, filtered, and concentrated. Chromatography on silica
gel (pentane/DCM ) 90/10, v/v) gave 150 mg (68%) of 7.
Spectroscopic data were consistent with those reported previ-
ously for this compound.68,69
The 1H, 13C, and 19F NMR spectra were recorded at 400,
100, and 376 MHz, respectively. For the fluorinated compounds
1T(F5), 3T(F2), and 3D(F2), the 13C NMR signals from the atoms
on and near the fluorinated phenyl ring were not discernible
from the background noise. Specifically, when using traditional
13C NMR techniques that lack special broadband C-F decou-
pling schemes, the pertinent carbon signals are split into a
number of low intensity peaks due to the comparatively large
carbon-fluorine coupling constants.66
4-Trimethylsilanylethynylbenzonitrile (4). A flame-
dried Schlenk flask was charged with 4-bromobenzonitrile (200
mg, 1.105 mmol), Pd(PPh3)2Cl2 (40 mg, 0.055 mmol), and CuI
(26 mg, 0.11 mmol). After three successive vacuum/argon
cycles, THF (4 mL), trimethylsilylacetylene (0.321 mL, 2.21
mmol), and triethylamine (1 mL) were introduced via syringe.
Once triethylamine was added, the reaction mixture turned
black. The mixture was stirred at rt for 15 h, diluted with Et2O,
washed with aqueous NH4Cl and then with aqueous NaHCO3,
dried over MgSO4, filtered, and concentrated. Chromatography
on silica gel (pentane/Et2O ) 95/5, v/v) gave 168 mg (83%) of
4. Spectroscopic data were consistent with those reported
previously for this compound.67
4-Ethynylbenzonitrile (5). In a one-neck round-bottom
flask, 4 (223 mg, 1.12 mmol) was dissolved in DCM (10 mL).
The flask was cooled to 0 °C and a 1 M solution of tetrabutyl-
ammoniumfluoride in THF (1.34 mL, 1.34 mmol) was added
via syringe. The reaction mixture was stirred for 2 h at 0 °C,
washed twice with water, dried over MgSO4, filtered and
concentrated. Chromatography on silica gel (pentane/Et2O )
95/5, v/v) gave 115 mg (81%) of 5. Spectroscopic data were
consistent with those reported previously for this compound.67
Tris-4,4′,4′′-(4-cyanophenylethynyl)triphenylamine (1T-
(CN)). A flame-dried Schlenk flask was charged with 4-eth-
ynylbenzonitrile 5 (115 mg, 0.907 mmol), tris(4-iodophenyl)-
amine (146 mg, 0.234 mmol), Pd(PPh3)2Cl2 (25 mg, 0.035
mmol), and CuI (13 mg, 0.070 mmol). After three successive
vacuum/argon cycles, THF (6 mL) and triethylamine (1.5 mL)
were introduced via syringe. Once triethylamine was added,
the reaction mixture turned black. The mixture was stirred
at room temperature for 15 h and concentrated in vacuo, and
the crude product was taken up in DCM, washed with aqueous
NH4Cl and then with aqueous NaHCO3, dried over MgSO4,
filtered, and concentrated. Chromatography on silica gel (neat
DCM) gave 136 mg (94%) of 1T(CN): mp 234-236 °C; 1H NMR
(CDCl3, 400 MHz) δ 7.10 (d, 6H, J ) 8.6 Hz), 7.46 (d, 6H, J )
8.6 Hz), 7.59 (d, 6H, J ) 8.4 Hz), (d, 6H, J ) 8.5 Hz); 13C NMR
(CDCl3, 100 MHz) δ 87.9, 93.6, 111.3, 117.1, 118.5, 124.1,
128.2, 131.9, 132.0, 133.1, 147.0; HRMS m/z calcd for C45H24N4
620.2001, found 620.2010.
Tris-4,4′,4′′-(4-bromophenylethynyl)triphenylamine (1T-
(Br)). A flame-dried Schlenk flask was charged with tris(4-
iodophenyl)amine (156 mg, 0.25 mmol), Pd(PPh3)2Cl2 (17.5 mg,
0.025 mmol), and CuI (4.8 mg, 0.025 mmol). After three
successive vacuum/argon cycles, THF (6 mL), 4-bromoethy-
nylbenzene 7 (150 mg, 0.8 mmol), and triethylamine (2 mL)
were introduced via syringe. Once triethylamine was added,
the reaction mixture turned black. The mixture was stirred
at rt for 15 h, diluted with DCM, and forced through a short
pad of silica. The solvent was removed in vacuo and the crude
product was absorbed onto silica. Chromatography on silica
gel with gradient elution (pentane/DCM from 95/5 to 80/20,
1
v/v) gave 152 mg (78%) of 1T(Br): mp 238-240 °C; H NMR
(CDCl3, 400 MHz) δ 7.07 (d, 6H, J ) 8.6 Hz), 7.37 (d, 6H, J )
8.5 Hz), 7.42 (d, 6H, J ) 8.6 Hz), 7.48 (d, 6H, J ) 8.5 Hz); 13
C
NMR (CDCl3, 100 MHz) δ 88.3, 90.3, 117.6, 122.2, 122.3, 124.0,
131.6, 132.8, 132.9, 146.7; HRMS m/z calcd for C42H24Br3N
778.9459, found 778.9479.
Tris(4-ethynylphenyl)amine (9). A flame-dried Schlenk
flask was charged with tris(4-iodophenyl)amine (623 mg, 1.0
mmol), Pd(PPh3)2Cl2 (70 mg, 0.1 mmol), and CuI (19 mg, 0.1
mmol). After three successive vacuum/argon cycles, THF (8
mL), trimethylsilylacetylene (0.57 mL, 4.0 mmol), and trieth-
ylamine (2 mL) were introduced via syringe. Once trieth-
ylamine was added the reaction mixture turned black. The
mixture was stirred at rt for 15 h, diluted with Et2O, and forced
through a short pad of silica. The solvent was removed in
vacuo, and the crude 8 was placed in a round-bottom flask
and dissolved in DCM (10 mL). The flask was cooled to 0 °C,
and tetrabutylammonium fluoride (3.0 mL of a 1 M solution
in THF, 3.0 mmol) was added via syringe. The reaction mixture
was stirred for 2 h at 0 °C and successively absorbed onto
silica. Chromatography on silica gel (pentane/DCM ) 90/10,
v/v) gave 252 mg (80%) of 9. Spectroscopic data were consistent
with those reported previously for this compound.70
(63) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N.
Synthesis 1980, 627-630.
Tris-4,4′,4′′-(pentafluorophenylethynyl)triphenyl-
amine (1T(F5)). A flame-dried Schlenk flask was charged with
tris(4-ethynylphenyl)amine 9 (63.5 mg, 0.2 mmol), iodopen-
(64) Yu, C. J.; Chong, Y.; Kayyem, J. F.; Gozin, M. J. Org. Chem.
1999, 64, 2070-2079.
(65) Kato, Y.; Conn, M. M.; Rebeck, J. J. Am. Chem. Soc. 1994, 116,
3279-3284.
(66) Krebs, F. C.; Jørgensen, M. Macromolecules 2002, 35, 7200-
7206.
(68) Okuhara, K. J. Org. Chem. 1976, 41, 1487-1494.
(69) Kamikawa, T.; Hayashi, T. J. Org. Chem. 1998, 63, 8922-8925.
(70) Wu¨rthner, F.; Schmidt, H.-W.; Haubner, F. German Patent,
1998, DE 19643097 A1.
(67) Hirsch, K. A.; Wilson, S. R.; Moore, J. S. J. Am. Chem. Soc.
1997, 119, 10401-10412.
1144 J. Org. Chem., Vol. 70, No. 4, 2005

Two-Photon Photosensitized Production of Singlet Oxygen
tafluorobenzene (0.093 mL, 0.7 mmol), Pd(PPh3)4 (23.1 mg,
0.02 mmol), and CuI (7.6 mg, 0.04 mmol). After three succes-
sive vacuum/argon cycles, toluene (4 mL) and triethylamine
(1 mL) were added via syringe. The reaction mixture was
stirred at 45 °C for 72 h, diluted with DCM, and forced through
a short pad of silica. The solvent was removed in vacuo, and
the crude product was absorbed onto silica. Chromatography
on silica gel with gradient elution (pentane/DCM from 90/10
to 80/20, v/v) gave 85.2 mg (52%) of 1T(F5): mp 225-227 °C;
1H NMR (CDCl3, 400 MHz) δ 7.11 (d, 6H, J ) 8.7 Hz), 7.50 (d,
6H, J ) 8.7 Hz); 13C NMR (CDCl3, 100 MHz) δ 73.9, 102.0,
116.5, 124.1, 133.3, 137.6 (d, J ) 252.3 Hz), 141.3 (d, J ) 257.0
Hz), 147.0 (d, J ) 252.8 Hz), 148.1; 19F NMR (CDCl3, 376.256
MHz) δ - 136.2 (dd, Jo,p ) 7.0 Hz, Jo,m ) 21.6 Hz), -153.3 (t,
J ) 20.8 Hz), -162.2 (ddd, Jo,o′ ) 6.5 Hz, Jm,o ) Jm,p ) 20.5
Hz); HRMS m/z calcd for C42H12F15N 815.0730, found 815.0670.
Tris-4,4′,4′′-(pyridine-4-ylethynyl)triphenylamine (1T-
(Py)). In a flame-dried Schlenk flask, 4-bromopyridine hydro-
chloride (156 mg, 0.8 mmol) and triethylamine (1.5 mL) were
vigorously stirred for 15 min. During this time a white
precipitate formed. Thereafter, Pd(PhCN)2Cl2 (10.7 mg, 0.028
mmol), CuI (3.80 mg, 0.02 mmol), and tris(4-ethynylphenyl)-
amine 9 (63.5 mg, 0.2 mmol) were successively added. Toluene
(3 mL) and P(t-Bu)3 (0.14 mL of a 0.4 M solution in hexane,
0.07 mmol) were then introduced via syringe. The Schlenk
flask was wrapped in aluminum foil to keep the reaction
mixture in the dark. The reaction mixture was stirred at rt
for 15 h and then partitioned between DCM (10 mL) and
aqueous NaHCO3 (10 mL). The organic phase was dried over
MgSO4, filtered, and concentrated. Chromatography on silica
gel (EtOAc/triethylamine ) 90/10, v/v) gave 62 mg (56%) of
1T(Py). Spectroscopic data were consistent with those reported
previously for this compound.71
was stirred at rt for 15 h. A 1 M HCl solution was then added
dropwise to render the solution acidic. The volume was reduced
under vacuo to 2 mL, and water (10 mL) was added causing
the formation of green-yellow crystals. Filtration and drying
under vacuum yielded 79 mg (96%) of 1T(CO2H): mp 259-
1
262 °C; H NMR (DMSO-d6, 400 MHz) δ 3.35 (br, 3H), 7.10
(d, 6H, J ) 8.3 Hz), 7.56 (d, 6H, J ) 8.3 Hz), 7.65 (d, 6H, J )
8.0 Hz), 7.63 (d, 6H, J ) 8.0 Hz); 13C NMR (DMSO-d6, 100
MHz) δ 88.5, 91.9, 116.6, 124.1, 126.7, 129.5, 130.3, 131.3,
133.1, 146.5, 166.7.
(4-Bromophenyl)diphenylamine (11) and N,N,N′,N′-
Tetraphenyl-1,4-phenylenediamine (12). A flame-dried
Schlenk flask was charged with diphenylamine (677 mg, 4.0
mmol), 4-bromoiodobenzene (1132 mg, 4.0 mmol), Pd2(dba)3‚
CHCl3 (41.4 mg, 0.04 mmol), and NaO-t-Bu (577 mg, 6.0
mmol). After three successive vacuum/argon cycles, toluene
(6 mL) and P(t-Bu)3 (0.2 mL of a 0.4 M solution in hexane,
0.08 mmol) were added via syringe. The reaction mixture was
stirred at rt for 4 h and the solvent removed in vacuo. The
crude product was taken up with some DCM and absorbed
onto silica gel. Chromatography on silica gel with gradient
elution (pentane/DCM from 95/5 to 80/20, v/v) gave 582 mg
(45%) of 11 and 399 mg (25%) of 12. Spectroscopic data for 11
and 12 were consistent with those reported previously for these
compounds.41,72,73
Bis[4-(N,N-diphenylamino)phenylethynyl]benzene(3T-
(H)). A flame-dried Schlenk tube was charged with 11 (324
mg, 1.0 mmol), 1,4-diethynylbenzene (57 mg, 0.45 mmol), Pd-
(PhCN)2Cl2 (10.4 mg, 0.027 mmol), and CuI (3.4 mg, 0.018
mmol). After three successive vacuum/argon cycles, toluene
(6 mL), P(t-Bu)3 (0.2 mL of a 0.4 M solution in hexane, 0.08
mmol) and triethylamine (1 mL) were added via syringe. The
reaction mixture was stirred at rt for 48 h and the solvent
removed in vacuo. The crude product was taken up in DCM,
washed with aqueous NH4Cl and then with aqueous NaHCO3,
dried over MgSO4, filtered, and concentrated. Chromatography
on silica gel (pentane/DCM ) 70/30, v/v) gave 60 mg (22%) of
3T(H): mp 196-198 °C; 1H NMR (CDCl3, 400 MHz) δ 7.00 (d,
4H, J ) 8.6 Hz), 7.07 (t, 4H, J ) 7.4 Hz), 7.12 (d, 8H, J ) 8.0
Hz), 7.28 (t, 8H, J ) 7.9 Hz), 7.37 (d, 4H, J ) 8.6 Hz), 7.46 (s,
4H); 13C NMR (CDCl3, 100 MHz) δ 88.5, 91.5, 115.7, 122.1,
123.0, 123.6, 125.5, 129.4, 131.3, 132.5, 147.1, 148.0; HRMS
m/z calcd for C46H32N2 612.2565, found 612.2554.
(4-Ethynylphenyl)diphenylamine (14). A flame-dried
Schlenk flask was charged with 11 (324 mg, 1.0 mmol),
trimethylsilylacetylene (170 mL, 1.2 mmol), Pd(PhCN)2Cl2
(11.5 mg, 0.03 mmol), and CuI (3.82 mg, 0.02 mmol). After
three successive vacuum/argon cycles, toluene (6 mL), P(t-Bu)3
(0.2 mL of a 0.4 M solution in hexane, 0.08 mmol), and
triethylamine (1 mL) were added via syringe. The reaction
mixture was stirred at rt for 15 h, diluted with Et2O, and forced
through a short pad of silica. The solvent was removed in
vacuo, and the crude product was placed in a round-bottom
flask and dissolved in DCM (6 mL). The flask was cooled to 0
°C, and a 1 M tetrabutylammonium fluoride solution in THF
(1.5 mL, 1.5 mmol) was added via syringe. The reaction
mixture was stirred for 2 h at 0 °C and successively absorbed
onto silica. Chromatography on silica gel (pentane/DCM ) 93/
7, v/v) gave 259 mg (96%) of 14: mp 80-82 °C; 1H and 13C
NMRs were consistent with those reported previously for these
compounds;72 HRMS m/z calcd for C20H15N 269.1204, found
269.1212.
Tris-4,4′,4′′-[N-(4-methyl)-4-pyridinioethynyl]triphen-
ylamine Trisiodidide (1T(MePy+)). A flame-dried Schlenk
tube was charged with 1T(Py) (62 mg, 0.113 mmol), methyl
iodide (0.5 mL, 8.0 mmol), and DCM (5 mL). The reaction
mixture was stirred at 60 °C for 1 h and then at 40 °C for 12
h. The solvent was removed in vacuo and the dark red residue
taken up with a mixture of MeOH (7 mL) and EtOAc (7 mL).
The solvent was removed under vacuum furnishing 70 mg
1
(64%) of 1T(MePy+): mp 210-215 °C dec; H NMR (DMSO-
d6, 400 MHz) δ 4.31 (s, 9H), 7.23 (d, 6H, J ) 8.7 Hz), 7.72 (d,
6H, J ) 8.7 Hz), 8.22 (d, 6H, J ) 6.8 Hz), 8.98 (d, 6H, J ) 6.8
Hz); 13C NMR (DMSO-d6, 100 MHz) δ 47.7, 85.6, 102.2, 115.0,
124.5, 128.6, 134.3, 138.2, 145.4, 147.7.
Tris-4,4′,4′′-(4-methoxycarbonylphenylethynyl)triphen-
ylamine (1T(CO2CH3)). A flame-dried Schlenk flask was
charged with 9 (63.5 mg, 0.2 mmol), 10 (202.6 mg, 0.8 mmol),
Pd(PPh3)4 (23.1 mg, 0.02 mmol), and CuI (3.8 mg, 0.02 mmol).
After three successive vacuum/argon cycles, toluene (4 mL)
and triethylamine (1 mL) were added via syringe. The Schlenk
flask was sealed, and the reaction mixture was stirred at 40
°C for 15 h. After being cooled to rt, the mixture was diluted
with DCM and forced through a short pad of silica. The solvent
was removed in vacuo, and the crude product was absorbed
onto silica. Chromatography on silica gel (neat DCM) gave 133
1
mg (92%) of 1T(CO2CH3): mp 210-215 °C; H NMR (CDCl3,
400 MHz) δ 3.93 (s, 9H), 7.10 (d, 6H, J ) 8.5 Hz), 7.46 (d, 6H,
J ) 8.5 Hz), 7.57 (d, 6H, J ) 8.3 Hz), 8.02 (d, 6H, J ) 8.3 Hz);
13C NMR (CDCl3, 100 MHz) δ 52.2, 88.7, 92.2, 117.5, 124.1,
128.0, 129.3, 129.5, 131.3, 132.9, 146.9, 166.5; HRMS m/z calcd
for C48H33NO6 719.2308, found 719.2308.
Bis[4-(N,N-diphenylamino)phenylethynyl]tetra-
fluorobenzene (3T(F2)). A flame-dried Schlenk flask was
charged with 14 (100 mg, 0.37 mmol), 1,4-diodotetrafluoroben-
zene (70 mg, 0.176 mmol), Pd(PPh3)4 (16.3 mg, 0.08 mmol),
and CuI (2.70 mg, 0.08 mmol). After three successive vacuum/
argon cycles, toluene (5 mL) and triethylamine (1 mL) were
Tris-4,4′,4′′-(4-carboxyphenylethynyl)triphenyl-
amine (1T(CO2H)). A round-bottom flask was charged with
1T(CO2CH3) (88 mg, 0.12 mmol) and THF (10 mL). Once a
clear solution was obtained, water (1.7 mL), MeOH (1.7 mL),
and LiOH (24 mg, 0.72 mmol) were added, and the solution
(71) Lambert, C.; Gaschler, W.; Noll, G.; Weber, M.; Schma¨lzlin, E.;
Bra¨uchle, C.; Meerholz, K. J. Chem. Soc., Perkin Trans. 2 2001, 964-
974.
(72) Suh, S. C.; Suh, M. C.; Shim, S. C. Macromol. Chem. Phys. 1999,
200, 1991-1997.
(73) Paine, A. J. J. Am. Chem. Soc. 1987, 109, 1496-1502.
J. Org. Chem, Vol. 70, No. 4, 2005 1145

McIlroy et al.
added via syringe. The reaction mixture was stirred at 50 °C
for 48 h, diluted with DCM, and forced through a short pad of
silica. The solvent was removed in vacuo and the crude product
was absorbed onto silica. Chromatography on silica gel with
gradient elution (pentane/DCM from 90/10 to 70/30, v/v) gave
under Ar atmosphere. After being cooled to rt, the reaction
was quenched with water. The precipitated product was
filtered and dried yielding 3.46 g (87%) of 3D(H) as a yellow
1
powder: mp 206-208 °C; H NMR(CDCl3) 7.49 (s, 4H), 7.41
(d, 4H, J ) 8.7 Hz), 7.33-7.20 (m, 10H), 7.19-6.96 (m, 18H);
13C NMR(CDCl3) 147.6, 147.4, 136.7, 131.6, 129.3, 127.9, 127.3,
126.7, 126.6, 124.5, 123.6, 123.0. Anal. Calcd for C46H36N2: C,
89.58; H, 5.88; N, 4.54. Found: C, 89.26; H, 5.97; N, 4.40.
(E,E)-2,3,5,6-Tetrafluoro-1,4-bis(2-(4-diphenylamino-
phenyl)vinyl)benzene (3D(F2)). [4-(Diethoxyphosphoryl-
methyl)-2,3,5,6-tetrafluorobenzyl]phosphonic acid diethyl ester
(0.44 g, 0.89 mmol), prepared using a published procedure,77
and 4-diphenylaminobenzaldehyde (0.55 g, 2.01 mmol) were
dissolved in 80 mL of DMF and purged with Ar for 15 min.
KO-t-Bu (0.50 g, 4.46 mmol) was then added, and the reaction
mixture was refluxed for 30 min under Ar atmosphere. After
being cooled to rt, the reaction was quenched with water. The
precipitated product was filtered and recrystallized from
toluene. The isolated product was further purified using dry-
column vacuum chromatography78 performed on silica gel 60
using gradient elution (n-heptane/CHCl3) and recrystallized
once more from benzene yielding 0.26 g (42%) of 3D(F2) as a
light green microcrystalline powder: mp >250 °C; 1H NMR
(CDCl3) 7.49-7.35 (m, 6H), 7.32-7.22 (m, 10H), 7.17-7.00 (m,
14H), 6.95 (d, 2H, J ) 16.6 Hz); 13C NMR(CDCl3) 148.7, 147.5,
130.9, 129.4, 127.9, 125.0, 123.6, 123.0; 19F NMR(CDCl3, 330
K) -141.7. Anal. Calcd for C46H32F4N2‚(H2O)1/2: C, 79.18; H,
4.77; N, 4.01. Found: C, 79.18; H, 4.55; N, 3.88.
Apparatus and Techniques. The details of the instru-
mentation and approach we use to produce and optically detect
singlet oxygen in a two-photon experiment are presented
elsewhere.10 Likewise, the details of the approach we use to
quantify singlet oxygen yields are presented elsewhere.8
In the experiment to quantify the relative reactivities of a
given molecule to singlet oxygen, the independent sensitizer,
C60, was irradiated at 600 nm using the output of an optical
parametric oscillator pumped by a Nd:YAG laser (Quanta-Ray
GCR 230, MOPO 710). The experiment was performed at a
pulse repetition rate of 10 Hz using an incident laser energy
of 2.7 mJ/pulse and a beam that was 5 mm in diameter. The
fullerene C60 was obtained from a commercial supplier and
used as received.
1
85.3 mg (70%) of 3T(F2): mp 258-260 °C; H NMR (CDCl3,
400 MHz) δ 7.01 (d, 4H, J ) 8.3 Hz), 7.10 (t, 4H, J ) 7.5 Hz),
7.13 (d, 8H, J ) 8.1 Hz) 7.30 (t, 8H, J ) 7.6 Hz) 7.41 (d, 4H,
J ) 8.3); 13C NMR (CDCl3, 100 MHz) δ 103.6, 113.7, 121.4,
124.02, 125.4, 129.5, 132.9, 146.8, 149.1; 19F NMR (CDCl3,
376.256 MHz) δ -138.4 (s, 4F); HRMS m/z calcd for C46H28F4N2
684.2189, found 684.2136.
1,4-Bis[4-(N,N-diphenylamino)phenylethynyl]2,5-di-
bromobenzene (3T(Br)). A flame-dried Schlenk flask was
charged with N,N-diisopropylamine (0.141 mL, 1.0 mmol) and
THF (5 mL) and cooled to 0 °C. Thereafter, n-BuLi (0.625 mL
of 1.6 M solution in hexane, 1.0 mmol) was added via syringe
dropwise. After 30 min, a THF solution of 14 (269 mg, 1.0
mmol in 2 mL) was added to the solution via cannula at -78
°C. The reaction mixture was stirred at -78 °C for 30 min,
treated with a solution of anhydrous ZnBr2 (225 mg, 1.0 mmol)
in THF (2 mL), and warmed gradually to 0 °C over 30 min.
1,4-Dibromo-2,5-diiodobenzene (244 mg, 0.5 mmol)74 and Pd-
(PPh3)4 (58 mg, 0.05 mmol) were added to the reaction mixture
at 0 °C, which was then stirred at rt for 15 h. The reaction
mixture was then diluted with DCM (20 mL) and washed once
with aqueous NH4Cl and once with aqueous NaHCO3. The
combined aqueous phases were backwashed with 10 mL of
DCM, and then the combined organics were dried over MgSO4,
filtered, and concentrated. Chromatography on silica gel with
gradient elution (pentane/DCM from 90/10 to 70/30, v/v) gave
311 mg (80%) of 3T(Br): mp 267-269 °C; 1H NMR (CDCl3,
400 MHz) δ 7.01 (d, 4H, J ) 8.8 Hz), 7.08 (t, 4H, J ) 7.3 Hz),
7.12 (d, 8H, J ) 8.4 Hz), 7.29 (t, 8H, J ) 8.0 Hz), 7.40 (d, 4H
J ) 8.8 Hz); 13C NMR (CDCl3, 100 MHz) δ 86.4, 97.2, 114.7,
121.7, 123.4, 123.9, 125.2, 126.2, 129.4, 132.7, 135.6, 146.9,
148.6; HRMS m/z calcd; for C46H30Br2N2 768.0776, found
768.0806.
(E,E)-2,5-Dicyano-1,4-bis(2-(4-diphenylaminophenyl)-
vinyl)benzene (3D(CN)) and (E,E)-2,5-dibromo-1,4-bis(2-
(4-diphenylaminophenyl)vinyl)benzene (3D(Br)) were syn-
thesized via Horner-Wadsworth-Emmons reactions in an
approach that has been outlined elsewhere.8,53
(E,E)-1,4-Bis(2-(4-diphenylaminophenyl)vinyl)ben-
zene (3D(H)). [4-(Diethoxyphosphorylmethyl)benzyl]phosphonic
acid diethyl ester (2.44 g, 6.45 mmol) and 4-diphenylami-
nobenzaldehyde (3.20 g, 11.7 mmol), both prepared using
published procedures,75,76 were dissolved in 80 mL of DMF and
purged with Ar for 15 min. KO-t-Bu (2.61 g, 23.3 mmol) was
then added, and the reaction mixture was refluxed for 30 min
Acknowledgment. This work was funded by the
Danish Natural Science Research Council through a
block grant for The Center for Oxygen Microscopy, the
Danish Center for Scientific Computing, and the Villum
Kann Rasmussen Foundation.
Supporting Information Available: NMR spectra of
compounds synthesized. This material is available free of
charge via the Internet at http://pubs.acs.org.
(74) Hart, H.; Harada, K.; Du, C. J. F. J. Org. Chem. 1985, 50, 3104-
3110.
(75) Plater, M. J.; Jackson, T. Tetrahedron 2003, 59, 4673-4686.
(76) Wang, X.-M.; Zhou, Y.-F.; Yu, W.-T.; Wang, C.; Fang, Q.; Jiang,
M.-H.; Lei, H.; Wang, H.-Z. J. Materials Chem. 2000, 10, 2698-2703.
JO0482099
(77) Krebs, F. C.; Jensen, T. J. Fluorine Chem. 2003, 120, 77-84.
(78) Pedersen, D. S.; Rosenbohm, C. Synthesis 2001, 16, 2431-2434.
1146 J. Org. Chem., Vol. 70, No. 4, 2005