Synthesis of novel rigid-rod and tripodal azulene chromophores

Article abstract of DOI:10.1016/j.tetlet.2005.05.034

Two novel azulene chromophores 1 and 2 were synthesized to study dynamics of ultrafast electron injection at the interface of TiO₂ semiconductor nanoparticles. Fluorescence quenching was observed upon binding.

Full text of DOI:10.1016/j.tetlet.2005.05.034

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 4895–4899
Synthesis of novel rigid-rod and tripodal azulene chromophores
Massimiliano Lamberto, Cynthia Pagba, Piotr Piotrowiak* and Elena Galoppini*
Chemistry Department, Rutgers University, 73 Warren Street, Newark, NJ 07102, USA
Received 16 March 2005; revised 10 May 2005; accepted 11 May 2005
Abstract—Two novel azulene chromophores 1 and 2 were synthesized to study dynamics of ultrafast electron injection at the inter-
face of TiO2 semiconductor nanoparticles. Fluorescence quenching was observed upon binding.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Azulene and its derivatives possess unusual electronic
and optical properties and find application as compo-
nents in advanced materials.1,2 In particular, azulene
has a long-lived (1.4 ns) emissive second excited singlet
(S2) state and an extremely short-lived (1.6 ps) nonemis-
sive first excited singlet (S1) state, the rapid nonradiative
relaxation of which is attributed to the presence of a
conical intersection between the S1 and S0 states.3 For
this reason, azulene was selected to study electron trans-
fer into TiO2 from vibrationally nonrelaxed excited
states (Fig. 1a). The rates of injection and recombina-
tion from 1-carboxyazulene directly bound to TiO2
nanoparticles and azulene encapsulated within a host
molecule (hemicarceplex) anchored to TiO2 were re-
cently studied.4 In the case of the directly bound 1-carb-
oxyazulene injection from the S2 state was complete
within 100 fs.4 In the case of the encapsulated azulene
the injection and recombination rates are reduced by
approximately 3-orders of magnitude.4
Route A involved Suzuki-type5 coupling of dimethyl-5-
iodo-isophthalate 5b with 2-bromoazulene 7 (Scheme 2),
to afford rigid-rod 1 in 35% yield. In this reaction, in-
stead of the commercially available dimethyl-5-bromo-
isophthalate, we used the more reactive iodoisophthal-
ate 4,6 which was readily prepared from the diazonium
salt of the aminoderivative 3. The poor overall yield of
1 (ꢀ8% from 3) from this method, which was previously
employed for the synthesis of other rigid-rods,7
prompted us to use route B as an alternative.
Rigid-rod 1 was synthesized in 74% yield by Sonogash-
ira8 cross-coupling of 4 and 2-ethynylazulene9 7b
(Scheme 3). Ethynylazulene was synthesized in high
yield upon iodination of azulene with N-iodosuccin-
imide (NIS) followed by cross-coupling with trimethylsi-
lyacetylene. From this route, the overall yield of 1 from
3 was 23%. Two different cross-coupling methodologies
were used in the final step (Suzuki in route B and Sono-
gashira in route A) because Suzuki-type coupling, pro-
ceeds through an ethynylboronate intermediate formed
by reaction of the lithium acetylide of 5b, and prevents
the dimerization of 5b. The dimerization of the highly
reactive ethynylazulene 7b in Sonogashira conditions
was minimal and the cross-coupling with 4 proceeded
in high yields.
In this letter, we describe the synthesis and properties of
two novel azulene chromophores that bind to TiO2
nanoparticles through a rigid-rod (1) or a tripodal (2)
linker. Compounds 1 and 2 were designed as models
to study the effects of linkers on electron injection of
azulene into TiO2 as illustrated in Figure 1b.
Rigid-rod 1 was synthesized by the two routes shown in
Scheme 1.
The same strategy, route B involving Sonogashira cross-
coupling of the linker with the ethyne-substituted chro-
mophore, was used to synthesize azulene-tripod 2
(Scheme 4), instead of route A, which we have preva-
lently followed for the synthesis of tripodal dyes.10 Carb-
oxylation of 1,3,5,7-tetrakis-(p-iodophenyl)adamantane
811 with t-BuLi/CO2, esterification with diazomethane
and separation of the di-, tri-, and tetrasubstituted com-
pounds by silica gel column chromatography gave
Keywords: Semiconductor nanoparticles; Rigid-rod linkers; Tripodal
linkers.
*
Corresponding authors. Tel.: +1 973 353 5317; fax: +1 973 353 1264
(E.G.); e-mail: galoppin@andromeda.rutgers.edu
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.05.034

4896
M. Lamberto et al. / Tetrahedron Letters 46 (2005) 4895–4899
E (eV)
3.5
(a)
Az*( S2)
Az*( S1)
1.8
conduction
band
hv
TiO2
bandgap
3.2 eV
Az S0
valence
band
O
TiO2
O
Az bound to
TiO2 nanoparticle
(b)
A
A
A
A
A
A
A
A
bound through
a cage
directly
bound
bound through a rigid-rod or
tripodal linker
Figure 1. (a) Electron injection into TiO2 nanoparticles from the bound photoexcited azulene (Az*). (b) Models used to study this process: azulene
bound through different types of linkers. A = COOH anchoring group.
NH2
I
H
i
Br
A
34%
MeOOC
COOMe
R
MeOOC
COOMe
Suzuki
3
4
Coupling
overall ~8%
MeOOC
COOMe
COOMe
ii
Br
iv
35%
1
MeOOC
COOMe
80%
I
B
H
1
MeOOC
5a: R = Me3Si
5b: R = H
COOMe
Sonogashira
Coupling
iii
MeOOC
overall ~23%
90%
Scheme 1. Synthetic routes to azulene rigid-rod 1.
Scheme 2. Synthesis of 1 by route A. Reagents and conditions: (i) (a)
NaNO2, HCl (aq), ꢁ6 to ꢁ3 ꢁC; (b) KI, toluene, ꢁ6 to 0 ꢁC, (c) 12 h at
rt, (d) 1 h at reflux; (ii) PdCl2(PPh3)2, CuBr, i-Pr2NH, TMSA, reflux
24 h; (iii) TBAF, THF, 2 h at rt; (iv) Pd(PPh3)4, (TMS)2NLi, MeO-9-
BBN, THF.
monoiodide 9 in 6–10% yields.12 Sonogashira cross-cou-
pling of 9 with 2-ethynylazulene 7b gave 2 in 37% yield.
While azulene derivatives 6 and 7b easily decomposed
and were used immediately, both 1 and 2, isolated as
green powders, were remarkably stable.13 Their spectral
properties are reported in Table 1. The absorption spec-
tra of 1 and 2, where azulene is substituted with a
p-ethynylenephenylene group, showed a significant
(60 nm) red shift of the S0 ! S2 band compared to
unsubstituted azulene. The weaker S0 ! S1 lower energy
band, responsible for the blue color of unsubstituted
azulene, was also red shifted by ꢀ30 nm in 1 and 2.

M. Lamberto et al. / Tetrahedron Letters 46 (2005) 4895–4899
4897
Table 1. Absorption and fluorescence solution data for 1, 2, and
azulene
i
98%
Chromophore
kmax, nm
kF, nm
S0 ! S2 (e401, Mꢁ1 cmꢁ1
340
)
S0 ! S1
572
598
600
I
6
Azulenea
1b
2b
373
429
425
401 (9964)
402 (8216)
iv
ii
+
4
1
100%
a MeOH, 1 · 10ꢁ4 M.
b EtOH, 2.5 · 10ꢁ5 M.
74%
iii
7a: R = Me3Si
7b: R = H
R
93%
Scheme 3. Synthesis of 1 by route B. Reagents and conditions: (i) NIS,
CH2Cl2, 2 h at rt; (ii) TMSA, Pd(dba)2, CuI, i-Pr2NH, PPh3, toluene,
24 h at rt; (iii) KOH in MeOH/THF (1:1), 4 h at rt; (iv) Pd(dba)2, CuI,
i-Pr2NH, PPh3, toluene, 24 h at rt.
1-COOH (2.5e-5M)
0.5
0.4
0.3
0.2
0.1
1-COOH (5e-5M)+TiO2(250mg/L)
TiO2(125mg/L)
The methyl esters 1 and 2 were converted into the corre-
sponding carboxylic acids (1-COOH and 2-COOH)14
and added to diluted (250 or 500 mg/L) colloidal EtOH
solutions4 of TiO2 nanoparticles. The colloidal solutions
(pH ꢀ 3) were optically transparent and light scattering
was minimal.15 The ground state absorption spectra of
1-COOH and 2-COOH did not change considerably
upon binding (ꢀ2 nm red shift), as shown in Figure 2.
The absence of charge transfer bands or large shifts
upon binding suggests weak electronic coupling of the
azulene unit with the semiconductor.
x 10
500
600
700
0.0
300
400
500
600
700
800
Wavelength [nm]
The fluorescence emission of the azulene chromophores
was clearly quenched by addition of TiO2 (Fig. 3), indi-
cating that the molecules bind to the nanoparticles and
that binding results in electron injection in the semicon-
ductor. The residual fluorescence is likely to originate
entirely from nonbound molecules.
Figure 2. Absorption spectra of MeOH solutions of 1-COOH (black
solid line)and 1-COOH/colloidal TiO2 in EtOH (red dotted line). The
spectra are overlayed with the absorption of the TiO2 colloidal
solution in EtOH (green dashed line). An identical behavior was
observed for 2-COOH.
I
I
i
6-10%
MeOOC
I
COOMe
I
9
8
MeOOC
I
ii
37%
MeOOC
COOMe
2
MeOOC
Scheme 4. Synthesis of azulene tripod 2. Reagents and conditions: (i) (a) t-BuLi, (b) CO2 (g), H+, (c) CH2N2, (d) separation (silica gel, hexane/ethyl
acetate 4:1, gradient elution); (ii) Pd(dba)2, CuI, i-Pr2NH, PPh3, 7b, toluene, 24 h at rt.

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M. Lamberto et al. / Tetrahedron Letters 46 (2005) 4895–4899
Chapter 13; (d) Mochalin, V. B.; Porshnev, Yu. N. Russ.
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250
200
150
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HOOC
COOH
1-COOH (2.5 x 10-5M)
1-COOH (5 x 10-5M) + TiO2 (500 mg/L)
3. (a) Beer, M.; Longuet-Higgins, H. C. J. Chem. Phys. 1955,
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Hore, S.; Deshayes, K.; Piotrowiak, P. J. Am. Chem. Soc.
2004, 126, 9888.
0
400
450
500
550
600
650
700
a
Wavelength [nm]
350
5. (a) Brown, H. C.; Bhat, N. G.; Srebnik, M. Tetrahedron
Lett. 1988, 29, 2631; (b) Soderquist, T. A.; Matos, K.
Tetrahedron Lett. 1995, 36, 2401; (c) Myiaura, N.; Suzuki,
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Chem. Eur. J. 1999, 5, 345.
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2002, 58, 6027–6032; (b) Hoertz, P. G.; Carlisle, R.;
Meyer, G. J.; Wang, D.; Piotrowiak, P.; Galoppini, E.
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ed.; de Meijere, A., Diederich, F., Eds., Wiley-VCH:
Weinheim, Germany, 2004.
300
250
200
150
100
50
2-COOH (2.5 x 10-5M)
HOOC
COOH
HOOC
2-COOH (5 x 10-5M) + TiO2 (500 mg/L)
0
9. (a) Fabian, K. H. H.; Elwahy, A. H. M.; Hafner, K.
Tetrahedron Lett. 2000, 41, 2855; (b) Anderson, A. G.;
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Ann. Chem 1962, 656, 24.
400
450
500
550
600
650
700
b
Wavelength [nm]
Figure 3. Fluorescence emission spectra of (a) 1-COOH (black solid
line) and 1-COOH/colloidal TiO2 (red dotted line). (b) 2-COOH
(black solid line) and 2-COOH/colloidal TiO2 (red dotted line). In all
cases, the solvent was EtOH and kexc = 382 nm.
10. Guo, W.; Galoppini, E.; Rydja, G.; Pardi, G. Tetrahedron
Lett. 2000, 41, 7419.
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Mater. 1993, 5, 4.
12. The synthesis of 9 has been reported: Wei, Q.; Galoppini,
E. Tetrahedron 2004, 60, 8497. Compound 9 was contam-
inated by 1-phenyl-3,5,7-tris(4-carbomethoxyphenyl)ada-
mantane, which was easily separated from 2 after the
cross-coupling step. The yield of 9 is dependent on the
reactions conditions as the solvent and the apparatus must
be rigorously anhydrous.
In conclusion two novel azulene chromophores, rigid-
rod 1 and tripod 2, have been synthesized and bound
to colloidal solutions of TiO2 nanoparticles. Steady-
state and time-resolved fluorescence quenching studies
are in progress.
13. Compound 1: Mp 178–179 ꢁC; IR 3021, 2195, 1726, 1439,
1396, 1344, 1249, 1215 cmꢁ1; 1H NMR (CDCl3) d 8.69 (1H,
d, J = 10 Hz), 8.61 (1H, s), 8.44 (2H, s), 8.34 (1H, d,
J = 9 Hz ), 8.05 (1H, d, J = 4 Hz), 7.71 (2H, t, J = 10 Hz),
7.37 (1H, dd, J = 9, 6 Hz), 7.29 (1H, t, J = 10 Hz), 3.99 (6H,
s); 13C NMR (CDCl3) d 166.0, 141.9, 141.8, 139.8, 139.1,
137.7, 136.6, 136.2, 131.1, 129.4, 125.6, 125.3, 124.7, 118.2,
109.8, 92.4, 88.4, 52.7; FAB-MS m/z 344 (M+, 40); FAB-
HRMS calcd for C22H16O4 (M+) 344.1049, found 344.1048.
Compound 2: Mp 142–144 ꢁC; IR 3414, 2973, 2866, 1725,
1459, 1364, 1181, 1066 cmꢁ1; 1H NMR (CDCl3) d 8.64 (1H,
d, J = 10 Hz), 8.29 (1H, d, J = 10 Hz), 8.04 (6H, d,
J = 8 Hz), 8.02 (1H, d, J = 4 Hz), 7.65 (1H, t, J = 10 Hz),
7.61 (2H, d, J = 8 Hz), 7.57 (6H, d, J = 8 Hz), 7.48 (2H, d,
J = 8 Hz), 7.33 (1H, d, J = 4 Hz), 7.27 (1H, t, J = 10 Hz),
7.22 (1H, t, J = 10 Hz), 3.92 (9H, s), 2.21 (12H, 2s); 13C
NMR (CDCl3) d 166.9, 153.9, 148.1, 141.4, 141.2, 139.4,
138.7, 137.3, 136.4, 131.4, 129.8, 128.3, 125.1, 125.0, 124.6,
123.9, 122.3, 117.8, 110.6, 93.7, 85.9, 52.0, 46.7, 39.6, 39.4;
Acknowledgments
Support of this research by the National Science Foun-
dation NIRT Grant 0303829 is gratefully acknowl-
edged. We thank Mr. Dong Wang and Dr. Giovanni
Zordan for their early contribution to the project.
References and notes
1. For reviews, see: (a) Zeller, K.-P. In Methoden der
Organischen Chemie (Houben-Weyl); Kropf, H., Ed.;
Georg Thieme: Stuttgart, Germany, 1985; Vol. V/2c, p
127; (b) Lloyd, D. Nonbenzenoid Conjugated Carbocyclic
Compounds; Elsevier: Amsterdam, The Netherlands, 1984;
p 352; (c) Lloyd, D. The Chemistry of Conjugated Cyclic
Compounds; John Wiley and Sons: Chichester, UK, 1989;

M. Lamberto et al. / Tetrahedron Letters 46 (2005) 4895–4899
4899
FAB-MS m/z 765 (M+H+, 11); FAB-HRMS calcd for
C52H45O6 (M+H)+ 765.3216, found 765.3213.
14. Methyl esters 1 and 2 were converted to the corresponding
carboxylic acids 1-COOH and 2-COOH by standard basic
hydrolysis (2 M KOH in MeOH, 12 h at rt).
diameter, is described in Ref. 4. For a review about
the properties of the TiO2 colloids, see: Kalyanasundar-
aman, K.; Gratzel, M. Coord. Chem. Rev. 1998, 177, 347.
Given the large excess of the colloid and the very low
(10ꢁ5 M) dye concentration it is estimated that the
coverage is low, with a few to less than one chromophore
per particle.
15. The preparation of the colloidal solution employed in
this work, that yielded TiO2 particles about 15 nm