Extension of the Squaraine Chromophore in Symmetrical Bis(stilbenyl)squaraines

Article abstract of DOI:10.1021/jo970284e

The bis(stilbenyl)squaraines 6d-j,d' j' represent a novel class of NIR pigments. Their synthesis was performed by the regioselective 2-fold condensation of highly nucleophilic 3,5-dihydroxystilbenes 4d-j,d'j' with squaric acid (5). Depending on the substitution with alkoxy groups, the absorption maxima range in solution from 680 to 735 nm. Reflexion measurements in the solid state reveal an exciton splitting with maxima at about 670 and 1000 nm.

Full text of DOI:10.1021/jo970284e

J . Org. Chem. 1997, 62, 4821-4826
4821
Exten sion of th e Squ a r a in e Ch r om op h or e in Sym m etr ica l
Bis(stilben yl)squ a r a in es
Herbert Meier* and Uta Dullweber
Institute of Organic Chemistry, University of Mainz, J .-J .-Becherweg 18-22, D-55099 Mainz, Germany
Received February 14, 1997X
The bis(stilbenyl)squaraines 6d -j,d ′,j′ represent a novel class of NIR pigments. Their synthesis
was performed by the regioselective 2-fold condensation of highly nucleophilic 3,5-dihydroxystilbenes
4d -j,d ′,j′ with squaric acid (5). Depending on the substitution with alkoxy groups, the absorption
maxima range in solution from 680 to 735 nm. Reflexion measurements in the solid state reveal
an exciton splitting with maxima at about 670 and 1000 nm.
In tr od u ction
Resultant water of the reaction can be removed by an
azeotropic distillation.
1,3-Disubstituted squaric acid derivatives, which are
often intensively colored, constitute the class of the
squaraines that has been known for more than 30
years.1-5 The remarkable activity for the generation of
new derivatives is due to the considerable industrial
interest in developing suitable squaraines for technical
applications such as electrophotography,3 solar energy
conversion,6 optical data storage,7 nonlinear optics,8-10
fluorescent labels,11 etc. Like the squaraines, stilbenoid
compounds qualify for the application in materials sci-
ence as optical brighteners, laser dyes, photoconductors,
photoresists, light emitting diodes, materials for nonlin-
ear optics, optical recording media, etc. owing to their
extraordinary optical and optoelectronic characteristics.12
A combination of these two building blocksssquaraine
and stilbenesshould originate very interesting new
materials.
The introduction of stilbene units with the accompany-
ing extension of conjugation in comparison to the known
diphenylsquaraines should lead to a bathochromic shift
of the vis absorption into the NIR region which would
be very useful for the applicability of GaAs diode lasers.
We reported recently on the synthesis of the first bis-
(stilbenyl)squaraines.13
In the first instance stilbenes had to be developed
which are capable of reacting with squaric acid in the
described manner. The Wittig-Horner olefination pro-
vides a convenient route. The benzyl bromides 1 were
treated with triethyl phosphite at 160 °C. The phosphon-
ates generated in the Arbuzov rearrangement could be
applied without further purification. The addition of the
substituted benzaldehydes 2 led in the presence of NaH/
DME or NaOCH3/DMF to the stilbenes 3a -j. Chroma-
tography on silica gel afforded the pure (E)-configura-
tions. The yield varied between 64 and 95% and decreased
to 39% when 3-hydroxybenzaldehyde with an unprotected
hydroxy group was used.
It turned out that the nucleophilicity of C-4 in 3a ,b
was too low for an attack on squaric acid. Therefore a
hydroxy group was placed in C-3; however, stilbene 3c
still did not react. A second activating hydroxy group in
position C-5 had to be introduced.15 In order to enhance
the yields of the Witting-Horner reaction we started
with 1-(bromomethyl)-3,5-dimethoxybenzene. The selec-
tive deprotection 3d -j f 4d -j was achieved by the
action of lithium diphenylphosphide whereas boron tri-
bromide cleaved all alkoxy groups present in 3d (Scheme
1).
Resu lts a n d Discu ssion
Compound 3j with three neighboring hexyloxy sub-
stituents represented the most difficult case. The reac-
tion time had to be carefully optimized, otherwise the
central hexyloxy group was cleaved, too (3j f 4j′). The
2-fold condensation reactions of 4d -j,d ′,j and squaric
acid (5) afforded the desired squaraines, 6d -j,d ′,j. The
yields were moderate, but the selectivity of the 1,3-attack
on 5 was perfect (Scheme 2).
The condensation of squaric acid with 2 equiv of
electron rich aromatic compounds represents the original
procedure14 for the synthesis of symmetrical squaraines.
X Abstract published in Advance ACS Abstracts, J une 15, 1997.
(1) Treibs, A; J acob, K. Angew. Chem. 1965, 77, 680.
(2) Schmidt, A. H. in West, R. Oxocarbons; Academic Press: New
York, 1980; p 185.
The squaraine pigments 6 form small blue crystals
with a metallic lustre. Their solubility in organic solvents
is very low, it just allows the determination of the vis
absorption. The narrow bands have a half-width of 60-
100 nm; the λmax values in chloroform are listed in
Scheme 2.16 Due to the low double-bond character of the
carbonyl groups in the 4-membered ring the CO stretch-
ing bands appear at 1600-1620 cm-1 in KBr. The low
solubility of the bis(stilbenyl)squaraines 6 did not permit
(3) Law, K. Y. Chem. Rev. 1993, 93, 449 and references therein.
(4) Schmidt, A. H. Synthesis 1980, 961.
(5) Frauenrath, H. Houben-Weyl; Thieme: Stuttgart, 1972; Vol.
E15,2, p 1468.
(6) Loutfy, R. O.; Hsiao, C. K.; Kazmaier, P. M. Photogr. Sci. Eng.
1983, 27, 5.
(7) Emmelius, M.; Pawlowski, G.; Vollmann, H. W. Angew. Chem.
1989, 101, 1475; Angew. Chem., Int. Ed. Engl. 1989, 28, 1445.
(8) Andrews, J . H.; Khagdarov, J . D. V.; Singer, K. D.; Hull, D. L.;
Chuang, K. C. Nonlinear Opt. 1995, 10, 227.
(9) Ashwell, G. J .; J efferies, G.; Hamilton, D. G.; Lynch, D. E.;
Roberts, M. P. S.; Bahra, G. S.; Brown, C. R. Nature 1995, 375, 385.
(10) Chen, C.-T.; Marder, S. R.; Cheng, L. T. J . Chem. Soc., Chem
Commun. 1994, 259; J . Am. Chem. Soc. 1994, 116, 3117.
(11) Ghazarossian, V.; Pease, J . S.; Ru, M. W. L.; Laney, M.;
Tarnowski, T. L. Europ. Pat. Appl. EP 356173 A2, 1990; Chem. Abstr.
1990, 113, 61319j.
(12) Meier, H. Angew. Chem. 1992, 104, 1425; Angew. Chem., Int.
Ed. Engl. 1992, 31, 1309.
(13) Meier, H.; Dullweber, U. Tetrahedron Lett. 1996, 37, 1191.
(14) Ziegenbein, W.; Sprenger, H. E. Angew. Chem. 1966, 78, 937;
Angew. Chem., Int. Ed. Engl. 1966, 5, 893.
(15) The substitution pattern 3-OH/5-OCH3 works, too; however, the
yield of the condensation reaction is not satisfying.
(16) The extremely low solubility did not permit an exact determi-
nation of the ꢀ values; log ꢀ is in the range of 5.
S0022-3263(97)00284-3 CCC: $14.00 © 1997 American Chemical Society

4822 J . Org. Chem., Vol. 62, No. 14, 1997
Meier and Dullweber
Sch em e 1
Sch em e 2
1H and 13C NMR measurements in solution. In order to
characterize the structures by spectroscopic methods
other than UV/vis/NIR and IR, we performed a solid state
13C NMR spectrum of compound 6d . Figure 1 shows the
normal CPMAS spectrum and additionally a section of
it that was measured with a different cross-polarization.
Thus quarternary carbon atoms could be distinguished
from aromatic or olefinic CH groups. The low-field
signals at 180 and 169 ppm belong to the 4-membered
ring. In comparison to the stilbene precursor 4d , many
signals of 6d exhibit a downfield shift. (See Experimen-
tal Section.) We attribute this effect to a partial de-
localization of the positive charge;17 however, one should
not disregard the influence of the neighbor molecules in
solid state NMR.
The most important property of the squaraines 6
concerns the absorption and the fluorescence. According
to the theory of Bigelow and Freund,18 the S0 f S1
transition is largely localized on the central 4-membered
ring. Consequently a significant bathochromic shift of
the absorption provoked by the extension of the conjuga-
tion could not be expected.19 Nevertheless, we recorded
such an effect by replacing the aryl substituents in 1,3-
position by stilbenyl groups. The λmax values listed in
Scheme 2 are between 680 and 735 nm. The comparison
of 6d and 6h /6j reveals that the electron-donating alkoxy
substituents enhance the charge transfer from the stil-
bene moieties to the central ring. The ground state S0
as well as the first excited singlet state S1 represents
intramolecular charge transfer states of the type D-A-D
(donor-acceptor-donor). Normal diarylsquaraines have
absorption maxima around 570 nm, i.e., the bathochromic
effect observed for the novelly prepared bis(stilbenyl)
systems amounts to ∆λ ) 165 nm! Therefore a strongly
localized electron transition can be ruled out. The narrow
absorption band, which is characteristic for all squaraines,
seems to be a consequence of the rigid skeleton which
causes small Franck-Condon factors for higher vibra-
tional states.
Concerning the applications of the squaraine pig-
ments, the absorption in solid state is even more impor-
tant. Figure 2 shows a comparison of the absorption of
6h dissolved in chloroform and as a dispersion, which
was spin-coated in a silane matrix. The exciton splitting
leads to a broad band with two maxima, one at λ ) 670
nm and the other at λ ) 1000 nm. Thus, the bis-
(stilbenyl)squaraines 6 represent new, interesting NIR
pigments.
Whereas a strong fluorescence is unfavorable for some
applications like the photoconductivity, it is a precondi-
tion for the use of the compounds as fluorescence mark-
ers. The dotted line in Figure 2 shows the fluorescence
of 6h in arbitrary units. The comparison of the λmax
values of the fluorescence bands of 6d , 6h , and 6j (752,
787, and 777 nm, respectively) reveals again the influence
of the alkoxy groups. A red shift (induced by an en-
hanced charge transfer) can be observed by going from
6d to 6h ; however, the third alkoxy group in 6j reduces
the effectsprobably due to steric reasons.
Finally we should mention that the squaraines 6 are
completely stable on irradiation in the range of the
intense long-wavelength absorption. In contrast to this
experiment we measured a fast degradation of the
chromophore when we used UV light (254 nm).20,21
(17) See also: Kazmaier, P. M.; Hamer, G. K.; Burt, R. A. Can. J .
Chem. 1990, 68, 530.
(18) Bigelow, R. W.; Freund, H.-J . J . Chem. Phys. 1986, 107, 159.
(19) But see also refs 10, 13, and Meyers, F.; Chen, C.-T.; Marder,
S. R.; Bre´das, J .-L. Chem. Eur. J . 1997, 530.

Extension of the Squaraine Chromophore
J . Org. Chem., Vol. 62, No. 14, 1997 4823
F igu r e 1. Solid state NMR spectrum (CPMAS) of squaraine
6d . (Measurement at νr ) 12 000 Hz, cross-polarization CP at
2000 µs for the lower spectrum and at 10 µs for the upper
section of the spectrum, which does no longer contain the
quarternary carbon atoms.)
F igu r e 2. Absorption (-) and fluorescence (‚‚‚) of squaraine
6h in solution in chloroform (upper part) and solid state
reflection spectrum of 6h in a silane matrix (lower part).
(2a ),24 3,4,5-tris(dodecyloxy)benzaldehyde,25 4-(octyloxy)ben-
zaldehyde,26 4-(decyloxy)benzaldehyde,27 4-(dodecyloxy)ben-
zaldehyde,27 3,4-bis(hexyloxy)benzaldehyde,25 and 3,4,5-tris-
(hexyloxy)benzaldehyde28 were prepared from the references
cited.
Su m m a r y a n d Con clu sion
A series of symmetrical bis(stilbenyl)squaraines has
been synthesized by a 2-fold condensation reaction in the
1,3-position of squaric acid with highly nucleophilic 3,5-
dihydroxystilbenes bearing alkoxy side chains. The
absorption maxima in solution and in solid state (reflex-
ion measurement) as well as the fluorescence maxima
show a very strong bathochromic shift in comparison to
normal diarylsquaraines. Thus, these new NIR pigments
are highly interesting compounds for applications in
materials science. The introduction of dendrimeric side
chains should lead to an enhanced solubility and should
therefore enable also an application as fluorescence
marker.
3,5-Bis(h exyloxy)ben za ld eh yd e. The preparation was
performed as described for 3,5-bis(dodecyloxy)benzaldehyde.29
Methyl 3,5-dihydroxybenzoate was transformed in the first
step into methyl 3,5-bis(hexyloxy)benzoate: yield 71%, oil; 1H
NMR δ 7.12 (d, 2 H), 6.60 (t, 1 H), 3.92 (t, 4 H), 3.85 (s, 3 H),
1.73 (m, 4 H), 1.45-1.23 (m, 12 H), 0.87 (t, 6 H); 13C NMR δ
166.8, 160.1 (2 C), 131.7, 107.5 (2 C), 106.4, 68.2 (2 C), 52.0,
31.5 (2 C), 29.1 (2 C), 25.7 (2 C), 22.6 (2 C), 14.0 (2 C); MS (EI)
m/ e (relative intensity) [M+•] 336 (39), 168 (100). Anal. Calcd
for C20H32O4 (336.5): C, 71.39; H 9.59. Found: C, 71.42; H
9.66. The reduction with LiAlH4 led to 3,5-bis(hexyloxy)benzyl
1
alcohol: yield 80%, oil; H NMR δ 6.45 (d, 2 H), 6.34 (t, 1 H),
4.52 (s, 2 H), 3.88 (t, 4 H), 2.89 (s, 1 H), 1.74 (m, 4 H), 1.50-
1.22 (m, 12 H), 0.91 (t, 6 H); 13C NMR δ 160.4 (2 C), 143.3,
104.9 (2 C), 100.4, 68.0 (2 C), 65.0, 31.6 (2 C), 29.2 (2 C), 25.7
(2 C), 22.6 (2 C), 14.0 (2 C); MS (EI) m/ e (relative intensity)
[M+•] 308 (1), 56 (100). Anal. Calcd for C19H32O3 (308.5): C,
73.98; H 10.46. Found: C, 73.95; H, 10.27. Finally the
primary alcohol was oxidized with DDQ to yield 82% of 3,5-
bis(hexyloxy)benzaldehyde as a colorless oil: 1H NMR δ 9.84
Exp er im en ta l Section
Gen er a l. Melting points are uncorrected. 1H and 13C NMR
spectra in solution (in CDCL3 or solvent indicated) were
obtained at 400 and 100 MHz, respectively, and 13C NMR
spectra in solid state at 75 MHz. Fluorescence excitation and
emission spectra are not corrected. The solvents were dried
using standard methods.
Sta r tin g Com p ou n d s. 1, 2: benzyl bromide (1a ) is
commercially available. 3-(Bromomethyl)phenol,22 1-(bromo-
methyl)-3,5-dimethoxybenzene,11,23 4-(hexyloxy)benzaldehyde
(24) Weygand, C.; Gabler, R. J . Prakt. Chem. 1940, 155, 332.
(25) Kosteyn, F.; Zerban, G.; Meier, H. Chem.Ber. 1992, 125, 893.
(26) Shimizu, K.; Kizawa, K.; Yoshimoto, T.; Imamura, J . Sekiyu
Gakkaishi 1982, 25, 7; Chem. Abstr. 1982, 96, 122333p.
(27) Gray, G. W.; J ones, B. J . Chem. Soc. 1954, 1467.
(28) Kretzschmann, H.; Mu¨ller, K.; Kolshorn, H.; Schollmeyer, D.;
Meier, H. Chem. Ber. 1994, 127, 1735.
(29) Zerban, G.; Meier, H. Z. Naturforsch. 1993, 48b, 171. Lifka, T.
Dissertation, Mainz, 1994.
(30) The bis(stilbenyl)squaraine crystals usually contain up to 1%
H2O. Extensive drying at temperatures close to the melting point does
not increase the purity.
(20) The photochemistry of the squaraines will be published else-
where.
(21) See also: Das, S.; Thomas, K. G.; Kamat, P. V.; George, M. V.
J . Phys. Chem. 1994, 98, 9291.
(22) Przybilla, K. J .; Vo¨gtle, F. Chem. Ber. 1989, 122, 347.
(23) Bhati, A. Tetrahedron 1962, 18, 1519.

4824 J . Org. Chem., Vol. 62, No. 14, 1997
Meier and Dullweber
(s, 1 H), 6.94 (d, 2 H), 6.64 (t, 1 H), 3.93 (t, 4 H), 1.72 (m, 4 H),
1.51-1.19 (m, 12 H), 0.87 (t, 6 H); 13C NMR δ 192.1, 162.5 (2
C), 138.0, 108.0, 107.6 (2 C), 68.4 (2 C), 31.5 (2 C), 29.1 (2 C),
25.6 (2 C), 22.6 (2 C), 14.0 (2 C); MS (EI) m/ e (relative
intensity) [M+•] 306 (7), 138 (30), 43 (100). Anal. Calcd for
m/ e (relative intensity) [M+•] 340 (100), 256 (82). Anal. Calcd
for C22H28O3 (340.46): C, 77.61; H, 8.29. Found: C, 77.41; H,
8.22.
(E)-1-(3,5-Dim et h oxyp h en yl)-2-[4-(oct yloxy)p h en yl]-
eth en e (3e): yield 81% (method B); mp 67 °C; IR (KBr, cm-1
)
1
1590, 1510, 955; H NMR δ 7.44 (d, 2 H), 7.06 (d, 3J ) 16.3
C
19H30O3 (306.5): C, 74.47; H, 9.87. Found: C, 74.35; H, 9.83.
3
Hz, 1 H), 6.90 (d, J ) 16.3 Hz, 1 H), 6.89 (d, 2 H), 6.66 (d, 2
Gen er a l P r oced u r e for th e P r ep a r a tion of th e Stil-
H), 6.39 (t, 1 H), 3.97 (t, 2 H), 3.83 (s, 6 H), 1.80 (m, 2 H),
1.50-1.31 (m, 10 H), 0.91 (t, 3 H); 13C NMR δ 161.0 (2 C),
159.1, 139.8, 129.7, 128.9, 127.8 (2 C), 126.4, 114.7 (2 C), 104.3
(2 C), 99.6, 68.1, 55.4 (2 C), 31.9, 29.4, 29.3 (2 C), 26.1, 22.7,
14.2; MS (EI) m/ e (relative intensity) [M+•] 368 (100), 256 (45).
Anal. Calcd for C24H32O3 (368.52): C, 78.22; H, 8.75. Found:
C, 78.26; H, 8.68.
ben es 3a -j. The corresponding benzyl bromide (1) (0.1 mol)
was dissolved in excess triethyl phosphite (34.2 g, 0.2 mol),
and the solution was heated to 160 °C for 3 h. The ethyl
bromide resulting from the Arbuzov reaction was distilled
continuously through a to 60 °C tempered reflux condenser.
The surplus triethyl phosphite was removed under high
vacuum (0.1 Torr), and the remaining colorless phosphonate
was used without further purification. The following Horner
olefination was carried out either with NaH/dimethoxyethane
(method A) or with NaOMe/DMF (method B). (A) A solution
of the phosphonate in dry dimethoxyethane (DME) was added
dropwise to a suspension of 3.0 equiv of NaH (80% in paraffin)
in the same solvent. After 15 min of stirring at room
temperature, a solution of the corresponding benzaldehyde 2
(0.1 mol) in DME was added. The reaction mixture was heated
under reflux for 2 h, carefully quenched with methanol/water,
and extracted with ethyl acetate. The combined organic
phases were washed with brine, dried over MgSO4, and
concentrated to leave a residue which was purified chromato-
graphically (silica gel, toluene). (B) A solution of the phos-
phonate in DMF was treated with 3.5 equiv of NaOMe and
cooled to 0 °C prior to the addition of the corresponding
benzaldehyde 2. After the reaction mixture was heated to 100
°C for 5 h and the reaction quenched with methanol/water,
the precipitated crystals were filtered off and washed with
water. Chromatography (silica gel, toluene) afforded the pure
(E)-stilbene 3.
(E)-1-[4-(Decyloxy)p h en yl]-2-(3,5-d im et h oxyp h en yl)-
eth en e (3f): yield 95% (method B); mp 69 °C; IR (KBr, cm-1
)
1
1590, 1510, 955; H NMR δ 7.43 (d, 2 H), 7.05 (d, 3J ) 16.3
3
Hz, 1 H), 6.89 (d, J ) 16.3 Hz, 1 H), 6.88 (d, 2 H), 6.66 (d, 2
H), 6.38 (t, 1 H), 3.96 (t, 2 H), 3.82 (s, 6 H), 1.79 (m, 2 H),
1.50-1.25 (m, 14 H), 0.90 (t, 3 H); 13C NMR δ 160.9 (2 C),
158.9, 139.7, 129.6, 128.8, 127.7 (2 C), 126.3, 114.6 (2 C), 104.2
(2 C), 99.5, 68.0, 55.3 (2 C), 31.9, 29.5 (2 C), 29.4, 29.3, 29.2,
26.0, 22.6, 14.1; MS (EI) m/ e (relative intensity) [M+•] 396
(100), 256 (67). Anal. Calcd for C26H36O3 (396.57): C, 78.75;
H, 9.15. Found: C, 78.77; H, 9.25.
(E)-1-[4-(Dodecyloxy)p h en yl]-2-(3,5-dim eth oxyp h en yl)-
eth en e (3g): yield 86% (method B); mp 67 °C; IR (KBr, cm-1
)
1
1585, 1500, 950; H NMR δ 7.43 (d, 2 H), 7.05 (d, 3J ) 16.2
3
Hz, 1 H), 6.89 (d, J ) 16.2 Hz, 1 H), 6.88 (d, 2 H), 6.65 (d, 2
H), 6.38 (t, 1 H), 3.96 (t, 2 H), 3.82 (s, 6 H), 1.79 (m, 2 H),
1.45-1.21 (m, 18 H), 0.90 (t, 3 H); 13C NMR δ 161.1 (2 C),
159.1, 139.8, 129.8, 128.9, 127.8 (2 C), 126.5, 114.8 (2 C), 104.5
(2 C), 99.7, 68.2, 55.3 (2 C), 32.0-22.7, (10 C) 14.1; MS (EI)
m/ e (relative intensity) [M+•] 424 (100), 256 (21). Anal. Calcd
for C28H40O3 (424.62): C, 79.20; H, 9.49. Found: C, 79.01; H,
9.48.
(E)-1-[4-(Hexyloxy)p h en yl]-2-p h en yleth en e (3a ): yield
64% (method A); mp 110 °C; IR (KBr, cm-1) 1600, 1500, 960;
1H NMR δ 7.51 (d, 2 H), 7.46 (d, 2 H), 7.36 (t, 2 H), 7.25 (t, 1
H), 7.09 (d, 3J ) 16.3 Hz, 1 H), 6.99 (d, 3J ) 16.3 Hz, 1 H),
6.91 (d, 2 H), 3.97 (t, 2 H), 1.82 (m, 2 H), 1.50-1.30 (m, 6 H),
0.91 (t, 3 H); 13C NMR (CDCl3) δ 158.9, 137.7, 130.0, 128.6 (2
C), 128.3, 127.7 (2 C), 127.1, 126.5, 126.2 (2 C), 114.7 (2 C),
68.1, 31.6, 29.2, 25.7, 22.6, 14.0; MS (EI) m/ e (relative
(E )-1-[3,4-B is (h e x y lo x y )p h e n y l]-2-(3,5-d im e t h o x y -
p h en yl)eth en e (3h ): yield 76% (method B); mp 54 °C; IR
(KBr, cm-1) 1590, 1510, 955; 1H NMR δ 7.07 (d, 1 H), 7.01 (d,
3
3J ) 16.2 Hz, 1 H), 7.01 (dd, 1 H), 6.88 (d, J ) 16.2 Hz, 1 H),
6.84 (d, 1 H), 6.65 (d, 2 H), 6.37 (t, 1 H), 4.04 (t, 2 H), 4.00 (t,
2 H), 3.81 (s, 6 H), 1.83 (m, 4 H), 1.54-1.32 (m, 12 H), 0.92 (t,
3 H), 0.91 (t, 3 H); 13C NMR δ 160.9 (2 C), 149.2 (2 C), 139.6,
130.2, 129.1, 126.5, 120.1, 113.6, 111.5, 104.2 (2 C), 99.6, 69.3,
69.2, 55.2 (2 C), 31.6 (2 C), 29.3, 29.2, 25.7 (2 C), 22.6 (2 C),
14.0 (2 C); MS (EI) m/ e (relative intensity) [M+•] 440 (100),
356 (23), 272 (33). Anal. Calcd for C28H40O4 (440.62): C,
76.33; H, 9.15. Found: C, 76.54; H, 9.05.
intensity) [M+•] 280 (53), 196 (100). Anal. Calcd for C20H24
(280.41): C, 85.67; H, 8.63. Found: C, 85.39; H, 8.80.
O
(E)-1-[3,4,5-Tr is(d od ecyloxy)p h en yl]-2-p h en ylet h en e
(3b): yield 81% (method A); mp 45 °C; IR (KBr, cm-1) 1570,
1500, 955; 1H NMR δ 7.49 (d, 2 H), 7.34 (t, 2 H), 7.25 (m, 1 H),
7.01 (d, 3J ) 16.3 Hz, 1 H), 6.96 (d, 3J ) 16.3 Hz, 1 H), 6.71 (s,
2 H), 4.02 (t, 4 H), 3.97 (t, 2 H), 1.83 (m, 4 H),1.76 (m, 2 H),
1.49 (m, 6 H), 1.27 (m, 48 H), 0.89 (t, 9 H); 13C NMR δ 153.4
(2 C), 138.7, 137.5, 132.6, 128.6 (2 C), 129.0, 127.7, 127.4, 126.4
(2 C), 105.6 (2 C), 73.5, 69.3 (2 C), 31.9-22.7 (30 C), 14.1 (3
C); MS (EI) m/ e (relative intensity) [M+•] 733 (100), 565 (18).
(E )-1-[3,5-B is (h e x y lo x y )p h e n y l]-2-(3,5-d im e t h o x y -
p h en yl)eth en e (3i): yield 88% (method B); mp 41 °C; IR
1
(KBr, cm-1) 1590, 1450, 960; H NMR δ 6.98 (s, 2 H), 6.63 (2
d, 4 H), 6.37 (2 t, 2 H), 3.97 (t, 4 H), 3.81 (s, 6 H), 1.80 (m, 4
H), 1.48-1.34 (m, 12 H), 0.93 (t, 6 H); 13C NMR δ 161.0 (2 C),
160.6 (2 C), 139.3, 139.0, 129.4, 129.0, 105.3 (2 C), 104.7 (2
C), 101.2, 100.1, 68.1 (2 C), 55.2 (2 C), 31.6 (2 C), 29.3 (2 C),
25.7 (2 C), 22.6 (2 C), 14.0 (2 C); MS (EI) m/ e (relative
intensity) [M+•] 440 (79), 44 (100). Anal. Calcd for C28H40O4
(440.62): C, 76.33; H, 9.15. Found: C, 76.40; H, 9.17.
(E)-1-[3,4,5-Tr is(h exyloxy)p h en yl]-2-(3,5-d im et h oxy-
p h en yl)eth en e (3j): yield 84% (method A); mp 38 °C; IR
Anal. Calcd for
C50H84O3 (733.22): C, 81.91; H, 11.55.
Found: C, 81.76; H, 11.54.
(E )-1-[4-(H e x y lo x y )p h e n y l]-2-(3-h y d r o x y p h e n y l)-
eth en e (3c): yield 39% (method A); mp 131 °C; IR (KBr, cm-1
)
1
3460, 3400, 1600, 1505, 970; H NMR δ 7.40 (d, 2 H), 7.20 (t,
3
1 H), 7.06 (d, 1 H), 6.98 (d, J ) 16.4 Hz, 1 H), 6.97 (d, 1 H),
3
(KBr, cm-1) 1580, 1490, 950; 1H NMR δ 6.99 (d, J ) 16.2 Hz,
3
6.87 (d, 2 H), 6.87 (d, J ) 16.4 Hz, 1 H), 6.70 (dd, 1 H), 5.12
3
1 H), 6.88 (d, J ) 16.2 Hz, 1 H), 6.70 (s, 2 H), 6.65 (d, 2 H),
(s, 1 H), 3.96 (t, 2 H), 1.87 (m, 2 H), 1.49-1.29 (m, 6 H), 0.90
(t, 3 H); 13C NMR δ 159.1, 155.9, 139.6, 130.0, 129.8, 128.9,
126.2, 119.3, 127.8 (2 C), 114.9 (2 C), 114.3, 112.8, 68.3, 31.6,
6.38 (t, 1 H), 4.01 (t, 4 H), 3.97 (t, 2 H), 3.81 (s, 6 H), 1.80 (m,
6 H), 1.52-1.26 (m, 18 H), 0.91 (t, 9 H); 13C NMR δ 160.9 (2
C), 153.2 (2 C), 139.4, 138.3, 132.3, 129.3, 127.6, 105.2 (2 C),
104.3 (2 C), 99.8, 73.4, 69.1 (2 C), 55.2 (2 C), 31.7 (2 C), 31.5
(2 C), 30.2 (2 C), 29.3 (2 C), 25.7 (2 C), 22.6 (2 C), 14.0 (3 C);
MS (EI) m/ e (relative intensity) [M+•] 540 (100), 456 (29). Anal.
Calcd for C34H52O5 (540.78): C, 75.52; H, 9.69. Found: C,
75.52; H, 9.81.
29.3, 25.7, 22.6, 13.9; MS (EI) m/ e (relative intensity) [M+•
]
296 (89), 212 (100). Anal. Calcd for C20H24O2 (296.41): C,
81.04; H, 8.16. Found: C, 80.99; H, 8.11.
(E)-1-[4-(H exyloxy)p h en yl]-2-(3,5-d im et h oxyp h en yl)-
eth en e (3d ): yield 87% (method B); mp 62 °C; IR (KBr, cm-1
)
1
1590, 1505, 955; H NMR δ 7.44 (d, 2 H), 7.04 (d, 3J ) 16.3
Gen er a l P r oced u r e for th e P r ep a r a tion of th e 3,5-
Dih yd r oxystilben es 4d-j,j′. To a flame-dried flask equipped
with a magnetic stirbar was added under nitrogen a solution
of 5 mL (5.4 g, 29.0 mmol) diphenylphosphine in dry THF (20
mL). After the flask was cooled to 0 °C and 19.9 mL (31.9
mmol) of n-butyllithium (1.6 M in hexane) was introduced via
3
Hz, 1 H), 6.90 (d, J ) 16.3 Hz, 1 H), 6.89 (d, 2 H), 6.66 (d, 2
H), 6.38 (t, 1 H), 3.97 (t, 2 H), 3.83 (s, 6 H), 1.79 (m, 2 H),
1.47-1.32 (m, 6 H), 0.92 (t, 3 H); 13C NMR δ 160.9 (2 C), 158.9,
139.7, 129.6, 128.8, 127.7 (2 C), 126.3, 114.6 (2 C), 104.2 (2
C), 99.5, 68.0, 55.3 (2 C), 31.6, 29.2, 25.7, 22.6, 14.0; MS (EI)

Extension of the Squaraine Chromophore
J . Org. Chem., Vol. 62, No. 14, 1997 4825
1
3
a syringe, the red reaction mixture was allowed to warm to
room temperature, treated with a solution of the corresponding
dimethoxystilbene (3d -j) in 10 mL of THF, and refluxed for
5 h (unless otherwise stated). After dilution with water, the
product was extracted into ethyl acetate and the combined
organic phases were washed with brine, dried, and evaporated.
The separation of small portions of monodeprotected stilbenes
from the 3,5-dihydroxystilbenes was realized chromatographi-
cally on silica gel with petroleum ether/ethyl acetate (70:30).
(E)-1-[4-(H exyloxy)p h en yl]-2-(3,5-d ih yd r oxyp h en yl)-
eth en e (4d ): yield 50%; mp 155 °C; IR (KBr, cm-1) 3380, 3250,
1600, 1575, 1505, 960; 1H NMR (CD3OD) δ 7.38 (d, 2 H), 6.98
(d, 3J ) 16.3 Hz 1 H), 6.81 (d, 3J ) 16.3 Hz 1 H), 6.82 (d, 2 H),
6.49 (d, 2 H), 6.22 (t, 1 H), 3.87 (t, 2 H), 1.70 (m, 2 H), 1.45-
1.27 (m, 6 H), 0.90 (t, 3 H); 13C NMR (CD3OD) δ 160.2, 159.6
(2 C), 141.2, 131.2, 129.2, 128.7 (2 C), 127.5, 115.6 (2 C), 105.9
(2 C), 102.7, 69.0, 32.7, 30.3, 26.8, 23.6, 14.4; MS (EI) m/ e
(relative intensity) [M+•] 312 (86), 228 (100). Anal. Calcd for
C20H24O3 (312.41): C, 76.89; H, 7.74. Found: C, 76.85; H, 7.69.
(E)-1-(3,5-Dih yd r oxyp h en yl)-2-[4-(oct yloxy)p h en yl]-
eth en e (4e): yield 47%; mp 157 °C; IR (KBr, cm-1) 3380, 3225,
1600, 1575, 1500, 960; 1H NMR (CD3OD) δ 7.36 (d, 2 H), 6.98
(d, 3J ) 16.2 Hz 1 H), 6.81 (d, 3J ) 16.2 Hz 1 H), 6.81 (d, 2 H),
6.50 (d, 2 H), 6.23 (t, 1 H), 3.84 (t, 2 H), 1.67 (m, 2 H), 1.41-
1.26 (m, 10 H), 0.88 (t, 3 H); 13C NMR (CD3OD) δ 160.3, 159.7
(2 C), 141.3, 131.3, 129.3, 128.8 (2 C), 127.6, 115.8 (2 C), 106.1
(2 C), 102.9, 69.1, 33.1, 30.6, 30.5 (2 C), 27.3, 23.8, 14.6; MS
(EI) m/ e (relative intensity) [M+•] 340 (73), 228 (100). Anal.
Calcd for C22H28O3 (340.46): C, 77.61; H, 8.29. Found: C,
77.59; H, 8.22.
(neat, cm-1) 3300, 1580, 960; H NMR (CD3OD) δ 6.96 (d, J
) 16.3 Hz, 1 H), 6.86 (d, 3J ) 16.3 Hz, 1 H), 6.73 (s, 2 H), 6.50
(d, 2 H), 6.21 (t, 1 H), 3.98 (t, 4 H), 3.95 (t, 2 H), 1.84-1.62 (m,
6 H), 1.50-1.34 (m, 18 H), 0.93 (t, 9 H); 13C NMR (CD3OD) δ
159.7 (2 C), 154.4 (2 C), 140.8, 139.6, 134.6, 129.7, 129.3, 106.1
(2 C), 105.7 (2 C), 103.1, 74.6, 70.1 (2 C), 33.0-23.8 (12 C),
15.0-14.5 (3 C); MS (EI) m/ e (relative intensity) [M+•] 512
(1), 428 (2), 43 (100). Anal. Calcd for C32H48O5 (512.73): C,
74.96; H, 9.44. Found: C, 75.21; H, 9.33.
(E)-1-[3,5-Bis(h exyloxy)-4-h yd r oxyp h en yl]-2-(3,5-d ih y-
d r oxyp h en yl)eth en e (4j′): reaction time 18 h; yield 8%; mp
60 °C; IR (KBr, cm-1) 3300, 1580, 955; 1H NMR (CD3OD) δ
7.00 (d, 3J ) 16.4 Hz, 1 H), 6.86 (d, 3J ) 16.4 Hz, 1 H), 6.82 (s,
2 H), 6.53 (d, 2 H), 6.23 (t, 1 H), 4.10 (t, 4 H), 1.86 (m, 4 H),
1.58-1.33 (m, 12 H), 0.98 (t, 6 H); 13C NMR (CD3OD) δ 159.6
(2 C), 148.8 (2 C), 141.2, 137.4, 129.9 (2 C), 127.6, 106.3 (2 C),
105.9 (2 C), 102.8, 70.6 (2 C), 32.9 (2 C), 30.4 (2 C), 26.8 (2 C),
23.7 (2 C), 14.5 (2 C); MS (EI) m/ e (relative intensity) [M+•
428 (100), 344 (16), 260 (37). Anal. Calcd for C26H36O5
(428.57): C, 72.87; H, 8.47. Found: C, 73.02; H, 8.54.
]
(E )-1-(3,5-Dih yd r oxyp h e n yl)-2-(4-h yd r oxyp h e n yl)-
eth en e (4d ′). Compound 3d (136 mg, 0.40 mmol) was
dissolved in dichloromethane (10 mL), cooled to -78 °C, and
treated dropwise with boron tribromide (0.11 mL, 0.30 g, 1.20
mmol). The red reaction mixture was allowed to warm to room
temperature, stirred for 4 h, quenched with KOH solution
(10%), acidified with HCl, and extracted three times with
dichloromethane. The combined organic phases were dried
and concentrated prior to purification by chromatography on
silica gel (50% ethyl acetate in petrolether). Product 4d ′ could
be isolated (63 mg, 70%) as colorless crystals: mp 247 °C; IR
(KBr, cm-1) 3250, 1600, 1580, 1505, 965; 1H NMR (CD3SOCD3)
(E)-1-[4-(Decyloxy)p h en yl]-2-(3,5-d ih yd r oxyp h en yl)-
eth en e (4f): reaction time 3 h; yield 11%; mp 150 °C; IR (KBr,
3
1
cm-1) 3400, 3250, 1600, 1510, 965; H NMR (CD3OD) δ 7.41
δ 9.57 (s, 1 H, OH), 9.22 (s, 2 H, OH), 7.36 (d, 2 H), 6.92 (d, J
) 16.3 Hz, 1 H), 6.80 (d, 3J ) 16.3 Hz, 1 H), 6.75 (d, 2 H), 6.39
(d, 2 H), 6.12 (t, 1 H); 13C NMR δ 158.5 (2 C), 157.2, 139.2,
128.1, 127.9, 127.8 (2 C), 125.7, 115.5 (2 C), 104.3 (2 C), 101.8;
MS (EI) m/ e (relative intensity) [M+•] 228 (6), 69 (100). Anal.
Calcd for C14H12O3 (228.25): C, 73.67; H, 5.30. Found: C,
73.39; H, 5.48.
3
(d, 2 H), 6.98 (d, 3J ) 16.3 Hz 1 H), 6.86 (d, 2 H), 6.82 (d, J )
16.3 Hz 1 H), 6.47 (d, 2 H), 6.18 (t, 1 H), 3.94 (t, 2 H), 1.75 (m,
2 H), 1.50-1.25 (m, 14 H), 0.90 (t, 3 H); 13C NMR (CD3OD) δ
160.3, 159.7 (2 C), 141.2, 131.4, 129.2, 128.7 (2 C), 127.7, 115.7
(2 C), 105.9 (2 C), 102.8, 69.1, 33.1, 30.7 (2 C), 30.6, 30.5 (2 C),
27.2, 23.8, 14.5; MS (EI) m/ e (relative intensity) [M+•] 368 (79),
228 (100). Anal. Calcd for C24H32O3 (368.52): C, 78.22; H,
8.75. Found: C, 78.33; H, 8.76.
Gen er a l P r oced u r e for th e P r ep a r a tion of Squ a r a in es
6d -j,d ′,j′. Squaric acid was added to a solution of 2 equiv of
the corresponding dihydroxystilbene 4d -j,d ′,j′ in toluene/1-
butanol (2:1), and the reaction mixture was refluxed for 4 h
(unless otherwise stated). Squaric acid should not get in
contact with the skin because it is a strong skin irritant. The
resultant water of condensation was removed by a filter
between flask and reflux condenser containing Na2SO4. After
the blue reaction mixture was cooled to room temperature, the
precipitated blue-green crystals were filtered off and washed
with ether.
(E ,E )-B is {4-{2-[4-(h e x y lo x y )p h e n y l]e t h e n y l}-2,6-
d ih yd r oxyp h en yl}squ a r a in e (6d ): yield 33%; mp 303 °C;
IR (KBr, cm-1) 3500-3200, 1630, 1610, 1590, 1560, 1510, 1410,
965; MS (FAB) m/ e (702) [M+•]. Anal. Calcd for C44H46O8
(702.84): C, 75.19; H, 6.60. Found: C, 74.84; H, 6.69.
(E,E)-Bis{2,6-dih ydr oxy-4-[2-(4-h ydr oxyph en yl)eth en yl]-
p h en yl}squ a r a in e (6d ′): reaction time 2 h; yield 49%; mp
329 °C; IR (KBr, cm-1) 3500-3100, 1630, 1600, 1580, 1500,
1420, 940; MS (FD) m/ e (relative intensity) [M + H+] 535 (56),
[M + 2 H+] 268.5 (100). Anal. Calcd for C32H22O8 (534.52):
C, 71.91; H, 4.15. Found: C, 71.07; H, 4.20.30
(E ,E )-Bis{2,6-d ih yd r oxy-4-{2-[4-(oct yloxy)p h e n yl]-
eth en yl}p h en yl}squ a r a in e (6e): yield 23%; mp 289 °C; IR
(KBr, cm-1) 3400, 1630, 1610, 1590, 1550, 1500, 1410, 960;
MS (FAB) m/ e (758) [M+•]. Anal. Calcd for C48H54O8
(758.95): C, 75.96; H, 7.17. Found: C, 75.94; H, 7.28.
(E ,E )-B is {4-{2-[4-(d e c y lo x y )p h e n y l]e t h e n y l}-2,6-
d ih yd r oxyp h en yl}squ a r a in e (6f): yield 9%; mp 284 °C; IR
(KBr, cm-1) 3400, 1630, 1610, 1590, 1560, 1500, 1410, 965.;
MS (FAB) m/ e (814) [M+•] 815. Anal. Calcd for C52H62O8
(815.06): C, 76.63; H, 7.67. Found: C, 75.94 H, 7.66.
(E ,E )-Bis{4-{2-[4-(d od e cyloxy)p h e n yl]e t h e n yl}-2,6-
d ih yd r oxyp h en yl}squ a r a in e (6g): yield 8%; mp 283 °C; IR
(KBr, cm-1) 3400, 1630, 1610, 1590, 1555, 1500, 1410, 960;
MS (FAB) m/ e (871) [M+•]. Anal. Calcd for C56H70O8
(871.17): C, 77.21; H, 8.10. Found: C, 76.65; H, 8.03.
(E)-1-[4-(Dod ecyloxy)p h en yl]-2-(3,5-d ih yd r oxyp h en yl)-
eth en e (4g): yield 58%; mp 148 °C; IR (KBr, cm-1) 3380, 3250,
1600, 1575, 1505, 960; 1H NMR (CD3OD) δ 7.40 (d, 2 H), 6.99
3
3
(d, J ) 16.3 Hz, 1 H), 6.82 (d, 1 H), 6.86 (d, J ) 16.3 Hz, 2
H), 6.47 (d, 2 H), 6.19 (t, 1 H), 3.93 (t, 2 H), 1.74 (m, 2 H),1.28
(m, 18 H), 0.89 (t, 3 H); 13C NMR (CD3OD) δ 160.3, 159.7 (2
C), 141.2, 131.3, 129.2, 128.7 (2 C), 127.7, 115.7 (2 C), 105.9
(2 C), 102.8, 69.1, 33.1-23.8 (10 C), 14.5; MS (EI) m/ e (relative
intensity) [M+•] 396 (2), 279 (22), 228 (41), 149 (100). Anal.
Calcd for C26H36O3 (396.57): C, 78.75; H, 9.15. Found: C,
78.61; H, 9.12.
(E )-1-[3,4-B is (h e x y lo x y )p h e n y l]-2-(3,5-d ih y d r o x y -
p h en yl)eth en e (4h ): reaction time 18 h; yield 48%; mp 114
°C; IR (KBr, cm-1) 3360, 1585, 1500, 955; 1H NMR (CD3OD) δ
3
7.08 (d, 1 H), 6.98 (d, J ) 16.3 Hz, 1 H), 6.99 (dd, 1 H), 6.86
(d, 3J ) 16.3 Hz, 1 H), 6.82 (d, 1 H), 6.54 (d, 2 H), 6.26 (t, 1 H),
3.98 (t, 2 H), 3.91 (t, 2 H), 1.75 (m, 4 H), 1.47-1.24 (m, 12 H),
0.92 (t, 6 H); 13C NMR (CD3OD) δ 159.7 (2 C), 150.5, 150.4,
141.2, 132.1, 129.5, 128.0, 121.4, 115.2, 113.2, 106.1 (2 C),
103.0, 70.6, 70.4, 32.9 (2 C), 30.6, 30.5, 27.0 (2 C), 23.8 (2 C),
14.6 (2 C); MS (EI) m/ e (relative intensity) [M+•] 412 (100),
328 (30), 244 (50). Anal. Calcd for C26H36O4 (412.57): C,
75.69; H, 8.80. Found: C, 75.60; H, 8.89.
(E )-1-[3,5-B is (h e x y lo x y )p h e n y l]-2-(3,5-d ih y d r o x y -
p h en yl)eth en e (4i): reaction time 18 h; yield 42%; oil; IR
(neat, cm-1) 3300, 1580, 955; 1H NMR (CD3OD) δ 6.94 (s, 2
H), 6.62 (d, 2 H), 6.50 (d, 2 H), 6.34 (t, 1 H), 6.21 (t, 1 H), 3.92
(t, 4 H), 1.74 (m, 4 H), 1.49-1.26 (m, 12 H), 0.92 (t, 6 H); 13C
NMR (CD3OD) δ 161.8 (2 C), 159.7 (2 C), 140.7 (2 C), 130.2,
129.7, 106.2 (2 C), 106.1 (2 C), 103.3, 101.9, 69.1 (2 C), 32.9 (2
C), 30.5 (2 C), 26.9 (2 C), 23.7 (2 C), 14.5 (2 C); MS (FD) m/ e
(relative intensity) [M+•] 412 (100). Anal. Calcd for C26H36O4
(412.57): C, 75.69; H, 8.80. Found: C, 75.51; H, 8.69.
(E)-1-[3,4,5-Tr is(h exyloxy)p h en yl]-2-(3,5-d ih yd r oxy-
p h en yl)eth en e (4j): reaction time 90 min; yield 20%; oil; IR

4826 J . Org. Chem., Vol. 62, No. 14, 1997
Meier and Dullweber
(E,E)-Bis{4-{2-[3,4-b is(h exyloxy)p h en yl]et h en yl}-2,6-
d ih yd r oxyp h en yl}squ a r a in e (6h ): reaction time 6 h; yield
27%; mp 295 °C; IR (KBr, cm-1) 3400, 1620, 1600, 1580, 1500,
1410, 950; MS (FAB) m/ e (903) [M+•]. Anal. Calcd for
C56H70O10 (903.17): C, 74.47; H, 7.81. Found: C, 73.81; H,
7.52.
(E,E)-Bis{4-{2-[3,5-b is(h exyloxy)p h en yl]et h en yl}-2,6-
d ih yd r oxyp h en yl}squ a r a in e (6i): reaction time 2 h, yield
16%; mp 245 °C; IR (KBr, cm-1) 3400, 1600, 1435, 960; MS
(FAB) m/ e (903) [M+•]. Anal. Calcd for C56H70O10 (903.17):
C, 74.47; H, 7.81. Found: C, 73.89; H, 7.83.
(E,E)-Bis{4-{2-[3,4,5-tr is(h exyloxy)p h en yl]eth en yl}-2,6-
d ih yd r oxyp h en yl}squ a r a in e (6j): reaction time 8 h; yield
20%; mp 222 °C; IR (KBr, cm-1) 3400, 1595, 1490, 1405, 950;
MS (FD) m/ e (relative intensity) [M2+] 552 (100). Anal. Calcd
for C68H94O12 (1103.49): C, 74.02; H, 8.59. Found: C, 73.87;
H, 8.51.
(E,E)-Bis{4-{2-[3,5-b is(h exyloxy)-4-h yd r oxyp h en yl]-
eth en yl}-2,6-d ih yd r oxyp h en yl}squ a r a in e (6j′): reaction
time 6 h; yield 20%; mp 138 °C; IR (KBr, cm-1) 3400, 1600,
1510, 1410, 960; MS (FAB) m/ e (935) [M+•]. Anal. Calcd for
C56H70O12 (935.16): C, 71.92; H, 7.54. Found: C, 72.10; H,
7.65.
Ack n ow led gm en t. This work was supported by the
Deutsche Forschungsgemeinschaft and the Fonds der
Chemischen Industrie. We thank the MPI fu¨r Poly-
merforschung Mainz for recording solid state CPMAS
13C NMR spectra and solid state UV/vis/NIR spectra.
J O970284E