542 J. Phys. Chem. B, Vol. 101, No. 4, 1997
Liang et al.
addition was completed, the mixture was refluxed for 1 h. The
solvent was removed and the product was purified by column
chromatography on silica gel, affording a yellow solid identified
as 1-chloro-2-(p-didodecylaminophenyl)cyclobutene-3,4-dione.
This yellow solid was then dissolved in a mixture of ethanol
and water (1:1). After a small quantity of concentrated
hydrochloric acid was introduced, the mixture was refluxed for
1 h, resulting in a yellow solid suspension. The yellow solid
was isolated by filtration and was washed throughly with water
until the filtrate was neutral. It was then dissolved in chloroform
and the residual water was removed by adding CaCl2 into the
chloroform solution, yielding pure 1-(p-didodecylaminophenyl)-
2-hydroxycyclobutene-3,4-dione 0.9 g (43%) after removing the
chloroform solvent.
Synthesis of 4-N-Methyl-N-(3-bromopropyl)aminophenyl-
4′-(N′,N′-dibutylamino)phenylsquaraine. The N-methyl-N-
(3-bromopropyl)aniline used in the synthesis was synthesized
by condensing N-methylaniline with 1,3-dibromopropane in
1-butanol in the presence of potassium bicarbonate and iodine.5
The yield was 70%. NMR (ppm in CDCl3): 2.21 (m, 2H),
3.03 (s, 3H), 3.55 (m, 4H), 6.80 (m, 3H), and 7.38 (m, 2H).
Figure 1. Absorption spectrum of C4Sq-By in acetonitrile.
(CD3CN): 0.87 (t, 6H), 1.27 (m, 32H), 1.93 (m, 4H), 2.12 (m,
4H), 2.40 (m, 2H), 3.10 (s, 3H), 3.49 (t, 4H), 3.70 (t, 2H), 4.39
(s, 3H0, 4.69 (t, 2H), 6.85 (m, 4H), 8.15 (m, 4H), 8.30 (m,
4H), 8.85 (m, 4H). FAB-MS: 826.6 (M+). Maximum absorp-
tion: 636 nm (3.1 × 105 M-1 cm-1).
4-N-Methyl-N-(3-bromopropyl)aminophenyl-4′-(N′,N′-di-
butylamino)phenylsquaraine was prepared by condensing 1-(di-
butylamino)phenyl-2-hydroxycyclobutene-3,4-dione (0.48 g, 1.6
mmol, synthesized according to the procedure of Chen et al.8)
with N-methyl-N-(3-bromopropyl)aniline (0.34 g, 1.48 mmol)
in the presence of tributylorthoformate (1 mL) in about 50 mL
of 1-butanol at reflux for 2 h.9 The progress of the synthesis
was monitored by absorption spectroscopy. The product
mixture was cooled in a refrigerator overnight. The solid
product was isolated by filtration and was purified by washing
first with 1-butanol and then ether. The yield was 0.28 g (37%).
NMR (CDCl3): 1.01 (t, 6H), 1.42 (m, 4H), 2.22 (m, 2H), 3.20
(s, 3H), 3.47 (t, 4H), 3.70 (t, 2H), 6.77 (m, 4H), and 8.41 (d,
4H).
The PF6 salt of C12Sq-By, C12Sq-By/PF6, could be prepared
by an ion exchange process using KPF6 and C12Sq-By in
acetonitrile. The spectroscopic properties of C12Sq-By/PF6
were found to be identical with those of the bromide iodide
compound.
Results and Discussion
Synthesis and Characterization of C4Sq-By and C12Sq-
By. The synthetic path for C4Sq-By and C12Sq-By is given
in Scheme 1. The entire synthesis is dictated by our ability to
purify the intermediates and the final products. For example,
we attempted to synthesize C4Sq-By by condensing the
covalently linked aniline-bipyridinium derivative with the
corresponding “half squaric acid”. Although C4Sq-By was
formed and detected spectroscopically and chromatographically,
we failed to isolate any pure material in this study. The
synthetic route in Scheme 1, although not the shortest, offers
the advantage that each intermediate is amenable to purification.
Both C4Sq-By and C12Sq-By are characterized by proton
NMR and FAB mass spectrometric analysis. In the absorption
spectra, both compounds exhibit sharp and intense absorption
very typical of that of the monomer of squaraine (Figure 1).10
The absorption maxmium is at 636 nm in dilute acetonitrile
solution, and the molar extinction coefficient is 3.1 × 105 cm-1
M-1. The absorption data suggest that there is no ground state
interaction between the squaraine chromphore and the bipyri-
dinium group in C4Sq-By and C12Sq-By.
The lack of any ground-state charge transfer interaction
between the squaraine chromophore and the bipyridinium group
in C4Sq-By and C12Sq-By is also supported by the cyclic
voltammetry (CV) data. Figure 2 depicts a cyclic voltammo-
gram of C4Sq-By in acetonitrile. The redox potentials are
basically the combination of the two chromphores in the diads,
with the oxdiation potenial at 0.71 and 1.04 V vs Ag/AgCl,
attributable to the squaraine chromophore,11 and the reduction
potential at -0.4 V vs Ag/AgCl is attributable to the bi-
pyridinium group.12
Synthesis of 4-N-Methyl-N-(N′-methyl-N′′-(4,4′-bipyridin-
iumyl)-3-propyl)aminophenyl-4′′-dibutylaminophen-
ylsquaraine Bromide Iodide (C4Sq-By). There was first
prepared 4-N-methyl-N-(4-(4-pyridyl)pyridiniumyl)-3-propyl-
aminophenyl-4′-(dibutylamino)phenylsquaraine by reacting 4-(N-
methyl-N-(3-bromopropyl)amino)phenyl-4′-(dibutylamino)-
phenylsquaraine (0.2 g, 0.39 mmol) with 1.2 g (7.82 mmol) of
4,4′-bipyridyl in 40 mL of acetonitrile at reflux for 2 days. After
the solvent was removed, excess bipyridyl was removed by
extracting with benzene repeatedly. The pure desired squaraine
product was obtained in 69% yield (0.18 g). NMR (DMSO-
d6): 0.88 (t, 6H), 1.26 (m, 4H), 1.53 (m, 4H), 2.33 (m, 2H),
3.12 (s, 3H), 3.53 (t, 4H), 3.75 (t, 2H), 4.71 (t, 2H), 6.68 (m,
4H), 8.06 (m, 2H), 8.11 (m, 4H), 8.63 (m, 2H), 8.81 (d, 2H),
and 9.25 (d, 2H).
C4Sq-By was then prepared by reacting the above squaraine
product (0.03 g, 0.045 mmol) with 2 mL of methyl iodine in
10 mL of acetonitrile at room temperature for 2 days. After
the solvent was removed, the product was purified by column
chromatography on celullose using acetonitrile as eluent,
affording 10 mg of C4Sq-By (27%). NMR (CD3CN): 0.96
(t, 6H), 1.39 (m, 4H0, 1.93 (m, 4H0, 2.42 (m, 2H), 3.11 (s,
3H), 3.50 (t, 4H), 3.71 (t, 2H), 4.40 (s, 3H), 4.74 (t, 2H), 6.88
(m, 4H), 8.15 (m, 4H), 8.35 (d, 4H), and 8.90 (d, 4H). FAB-
MS: 602.3 (M+). Maximum absorption (in CHCl3): 636 nm
(3.1 × 105 M-1 cm-1).
Synthesis of 4-N-Methyl-N-(N′-methyl-N′′-(4,4′-bipyridin-
iumyl)-3-propylaminophenyl-4′′-(didodecylamino)phen-
ylsquaraine Bromide and Iodide (C12Sq-By). The titled
compound was synthesized analogously to C4Sq-By. NMR
Aggregation Studies. Unlike alkyl substitued squaraines,
which are readily soluble in chloroform, C4Sq-By is insoluble
in chloroform and C12Sq-By is only slightly soluble in
choroform, forming a mixture of monomer and blue-shifted