Photodimerizable ditopic ligand

Article abstract of DOI:10.1021/ol060253i

The synthesis, photophysical properties, and structural characterization of a photodimerizable ditopic ligand are described. Upon irradiation at 366 nm, ligand 1 dimerizes to the head-to-tail tetra-bpy ligand 2. This thermally stable photodimer can be dis

Full text of DOI:10.1021/ol060253i

ORGANIC
LETTERS
2006
Vol. 8, No. 10
1987-1990
Photodimerizable Ditopic Ligand
Damien Jouvenot, Edith C. Glazer, and Yitzhak Tor*
Department of Chemistry and Biochemistry, UniVersity of California, San Diego,
La Jolla, California 92093
ytor@ucsd.edu
Received January 29, 2006
ABSTRACT
The synthesis, photophysical properties, and structural characterization of a photodimerizable ditopic ligand are described. Upon irradiation
at 366 nm, ligand 1 dimerizes to the head-to-tail tetra-bpy ligand 2. This thermally stable photodimer can be dissociated back to 1 using higher
energy irradiation (254 nm).
Bridging ligands, capable of coordinating multiple metal ions,
have been attracting considerable interest because of their
potential use as building blocks for the fabrication of
functional multimetallic assemblies.1 Of particular interest
are systems that can respond to external stimuli, such as
photons,2,3 protons,4,5 electrons,6 or other reactive species.
Such responsive ligands and their complexes can, in prin-
ciple, be employed for the on/off switching or fine tuning
of the interactions between the coordinating centers. In
contrast to the abundance of unreactive polypyridine mul-
titopic ligands, the repertoire of responsive ditopic bridging
ligands that can be modulated by external stimuli is limited.
Here, we report the synthesis and reactivity of a new pho-
todimerizable ligand 1, where a diimine core is incorporated
into an anthracene unit. Near-ultraviolet irradiation (λ ∼ 360
nm) results in exclusive formation of a head-to-tail tetra-
bpy ligand 2.
Anthracene photodimerization is one of the oldest pho-
tochemical reactions known.7 Since it was first observed in
1867, it has been employed in numerous responsive systems,
including metal coordinating ones.8 Most anthracene-contain-
ing ligands, however, consist of isolated coordinating sites
that are covalently linked to an anthracene unit, where the
latter serves as the dimerizing module only. Examples include
anthracene-containing crown ethers or cryptands,9,10 as well
as systems with pendant polypyridyl ligands.11,12 In contrast
to the majority of these systems, ligand 1 encompasses a
conjugating diazaanthracene unit that serves as the common
core of the ditopic bis-bpy ligand. As such, the electronic
(1) (a) Ruben, M.; Rojo, J.; Romero-Salguero, F. J.; Uppadine, L. H.;
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(7) Becker, H. D. Chem. ReV. 1993, 93, 145-172.
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(10) Another system based on a [2.2](9,10)anthracenophane is used to
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10.1021/ol060253i CCC: $33.50
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properties of the ligand and its metal complexes are expected
to change upon photodimerization.
To facilitate the synthesis of the bridging ligand 1, a
concise synthesis of the hitherto unknown dibromodiazaan-
thracene building block 7 was needed (Scheme 1). It was
Scheme 1. Synthesis of Ligand 1
Figure 1. Crystal structure of 1.
solution of the ligand in chloroform in the dark. As seen in
the solved structure (Figure 1), the bipyridine units adopt
the expected trans geometry, thus minimizing the global di-
pole moment of the molecule and the electrostatic interaction
between the nitrogen atoms’ lone pairs. In addition, the diaza-
anthracene core and the individual pyridine rings are also
out of plane (torsion angles: 134.1° and 151.5°). Inspection
of the crystal lattice reveals intermolecular contacts between
the pyridinic moieties of the anthracene unit. The average
distance between planes is 3.44 Å, allowing for an efficient
π-π stacking along the b axis. Note, however, the absence
of stacked diazaanthracene units, an arrangement that is likely
to be necessary for photodimerization in the solid state.20
The absorption spectrum of ligand 1 is shown in Figure
2a. It is characterized by an intense absorption at 250 nm
with a shoulder centered around 290 nm. There is also a
broad, featureless band tailing into the visible region with a
maximum at 392 nm. The molecule’s emission spectrum
(Figure 2a, inset) lacks the fine features associated with
typical anthracene derivatives, exhibiting a broad band with
a λmax at 470 nm.21 The excitation spectrum agrees well with
the absorption spectrum, with a peak maximum at 397 nm
(Figure 2a, inset).
When crystallization attempts were carried out in the
presence of light, the photodimer 2, less soluble than 1,
crystallized out exclusively. The head-to-tail dimeric structure
of the photoproduct could be unambiguously determined by
X-ray crystallography (Figure 3). As with the parent ligand,
the bipyridine units adopt an expected trans geometry with
significant deviation from coplanarity (torsion angles: 143.0°
and 165.4°). In the crystal structure, the dimer displays an
inversion center located in the center of the molecule. The
crystal packing is ensured mainly by two π-π interactions
(Figure 4). The principal π-π stacking is observed between
prepared from the diester 3 which, in turn, can be made from
resorcinol using established procedures.13,14 Following acti-
vation of the phenolic groups as triflates to give 4, a
Sonogashira cross-coupling reaction with trimethylsilylacety-
lene yielded 5. A sealed tube reaction with saturated ethanolic
ammonia afforded the bislactam 6.15 As 6 is rather insoluble,
a simple filtration provided the pure product. Bromination
of the lactam using phosphorus oxybromide and potassium
carbonate in acetonitrile16 produced the desired 1,8-dibromo-
2,7-diazaanthracene 7. These milder conditions were neces-
sary because refluxing the bislactam in pure phosphorus
oxybromide led primarily to the tribrominated derivative.
Final assembly of the bridging ligand 1 was achieved by a
Negishi cross-coupling reaction17 between 2-bromopyridine
and the dibromo derivative 7 to give the desired product in
88% yield (Scheme 1).14,18,19
Single crystals of 1, suitable for X-ray crystallography,
were obtained by slowly diffusing diethyl ether into a
(11) Beyerler, A.; Belser, P.; De Cola, L. Angew. Chem., Int. Ed. Engl.
1997, 36, 2779-2781.
(12) Trouts, T. D.; Tyson, D. S.; Pohl, R.; Kozlov, D. V.; Waldron, A.
G.; Castellano, F. N. AdV. Funct. Mater. 2003, 13, 398-402.
(13) Zeng, H.; Miller, R. S.; Flowers, R. A.; Gong, B. J. Am. Chem.
Soc. 2000, 122, 2635-2644.
(14) See Supporting Information for additional experimental details and
data.
(15) Sakamoto, T.; An-naka, M.; Kondo, Y.; Yamanaka, H. Chem.
Pharm. Bull. 1986, 34, 2754-2759.
(16) Janin, Y. L.; Roulland, E.; Beurdeley-Thomas, A.; Decaudin, D.;
Monneret, C.; Poupon, M.-F. J. Chem. Soc., Perkin Trans. 1 2002, 529-
532.
(18) A successful double cross-coupling reaction requires thoroughly
dried zinc chloride (24 h at 55 °C under reduced pressure).
(19) Interestingly, the usual stain employed to visualize bipyridines
(Fe(NH4)2(SO4)2 in water) does not yield the typical red color but rather a
bright green color. See Supporting Information Figure S.15.
(20) Single-crystal-single-crystal reactions are very rare for anthracene
derivatives; however, dimerization can be performed in the crystalline state
followed by recrystallization. See: Turowska-Tyrk, I.; Grzesniak, K. Acta
Crystallogr. 2004, C60, o146-o148.
(17) Loren, J. C.; Siegel, J. S. Angew. Chem, Int. Ed. 2001, 40, 754-
757.
(21) The emission quantum yield was determined to be 0.02.
1988
Org. Lett., Vol. 8, No. 10, 2006

Figure 4. Packing of ligand 2 in the crystal.
Upon irradiation at 366 nm, ligand 1 undergoes dimer-
ization at the 9 and 10 carbons. This photochemical trans-
formation can be easily monitored by fluorescence spectros-
copy as the anthracene core responsible for the molecule’s
emission at 470 nm is disrupted upon dimerization, thereby
providing a probe for following the progress of the reaction
(Figure 2b).22 A considerable amount of dimer 2 was
prepared by irradiation (366 nm) of ligand 1 in a degassed
acetonitrile solution followed by purification using prepara-
Figure 2. (a) Absorption spectrum of ligand 1 in CH3CN. Inset:
emission (blue line) (excitation at 395 nm) and excitation (green
line) (monitoring at 470 nm) spectra of 1 in CH3CN. (b) Fluor-
escence intensity continuously decreases upon irradiation at 366
nm: t ) 0 (red line); t ) 45 min (blue line); t ) 90 min (green
line); t ) 180 min (black line); t ) 360 min (yellow line). Sample
temperature is initially stabilized at 25 °C. A photostationary state
can be estimated around 50%. Irradiation was performed in degassed
acetonitrile solution in a quartz cell with a portable UV lamp of 4
W at 366 nm.
1
tive TLC.14 Most signals in the H NMR spectrum of the
dimer 2 feature an upfield shift when compared to the corre-
sponding protons in ligand 1 (Figure 5). As expected, protons
two adjacent bipyridine fragments with a distance of 3.53 Å
between the planes. An additional π-π interaction involves
the other pyridyl substituent stacking with its analogue of
the neighboring molecule. This effect is weaker as the
interplanar separation is 3.62 Å.
Figure 5. 1H NMR spectra in CD2Cl2 and assignment of ligand 1
(top) and the photodimer 2 (bottom).
Figure 3. Crystal structure of ligand 2.
Org. Lett., Vol. 8, No. 10, 2006
1989

9 and 10 display the most dramatic shift as their hybridization
changes from sp2 to sp3 (Figure 5, blue line). Their multi-
plicity also changes from singlets to doublets, as expected
for a head-to-tail dimer. A group of three protons (4′, 5′,
and 6′) are only slightly shifted (red line), consistent with a
minimal change in their environment upon dimerization.
Protons 3 and 4, along with proton 3′, experience a significant
shift (0.4-0.6 ppm) (green line). For protons 3 and 4, the
significant shielding can be attributed to ring current effects
of the aromatic rings located directly below. On the other
hand, the shift of proton 3′ could be attributed to the absence
of the deshielding cone produced by the central ring as it
loses aromaticity upon dimerization.
S.12).14 This finding is in agreement with previous literature
reports of photoreversible dimerization.11,23 Note, however,
that further optimization is required because under the current
unoptimized conditions the reaction is only partially revers-
ible and repetitive cycling appears to result in sample
degradation.14
To the best of our knowledge, ligand 1 is the first example
of a photodimerizable ligand, in which the diimine core is
embedded within the bridging anthracene unit. Akin to such
systems that undergo photoinduced structural modifications
(e.g., cis-trans isomerization,24 photocyclization25), this
paradigm system could be considered as a photoswitch to
which added properties may eventually be incorporated upon
binding to metal ions.
The photodimer 2 was found to be thermally stable, with
no indication for dissociation to monomer 1 over 12 h in
solution at room temperature. Preliminary results suggest,
however, that the dimerization may be reversed by irradiation
at 254 nm (Scheme 2 and Supporting Information Figure
Acknowledgment. The authors thank the National Insti-
tutes of Health (GM 069773) and NSF (CHE 0213323) for
support.
Supporting Information Available: Full synthetic details
and analytical data for all new compounds; photoinduced
dimerization and dissociation; crystal data and refinement
information for 1 and 2; spectra of mono- and dinuclear Ru2+
complexes of 1; and reactions of 1 and 2 with Fe2+. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Scheme 2. Reversible Photoinduced Dimerization and
Dissociation Reactions
OL060253I
(22) Note the photodimer 2 is nonemissive under these conditions.
(23) European Patent Application 0 394 846 A2.
(24) Shinkai, S.; Nakaji, T.; Nishida, Y.; Ogawa, T.; Manabe, O. J. Am.
Chem. Soc. 1980, 102, 5860-5865.
(25) (a) Irie, M.; Fukaminato, T.; Sasaki, T.; Tamai, N.; Kawai, T. Nature
2002, 420, 759-760. (b) Fraysse, S.; Coudret, C.; Launay, J.-P. Eur. J.
Inorg. Chem. 2000, 1581-1590.
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Org. Lett., Vol. 8, No. 10, 2006