Avenues into the Synthesis of Illusive Poly(m-phenylene-alt-squaraine)s: Polycondensation of m-Phenylenediamines with Squaric Acid Intercepted by Intermediate Semisquaraines of Exceptionally Low Reactivity

Article abstract of DOI:10.1021/jo035399z

The synthesis and properties of a novel class of ortho-dialkylamino-substituted semisquaraines are described. The exceptionally low reactivity of the investigated compounds is caused by an intramolecular hydrogen bond as evidenced by experimental and computational studies. Although this constitutes the reason for our failed attempts to prepare poly(m-phenylene-alt-squaraine)s, the discovered influence of hydrogen bonding on the photophysical properties of these semisquaraines provides a promising new motif for sensor design.

Full text of DOI:10.1021/jo035399z

CHART 1
Aven u es in to th e Syn th esis of Illu sive
P oly(m -p h en ylen e-a lt-squ a r a in e)s:
P olycon d en sa tion of m -P h en ylen ed ia m in es
w ith Squ a r ic Acid In ter cep ted by
In ter m ed ia te Sem isqu a r a in es of
Excep tion a lly Low Rea ctivity
Marco A. Balbo Block, Anzar Khan, and Stefan Hecht*
Institut fu¨r Chemie/ Organische Chemie, Freie Universita¨t
Berlin, Takustr. 3, 14195 Berlin, Germany
shecht@chemie.fu-berlin.de
Received September 24, 2003
Abstr a ct: The synthesis and properties of a novel class of
ortho-dialkylamino-substituted semisquaraines are de-
scribed. The exceptionally low reactivity of the investigated
compounds is caused by an intramolecular hydrogen bond
as evidenced by experimental and computational studies.
Although this constitutes the reason for our failed attempts
to prepare poly(m-phenylene-alt-squaraine)s, the discovered
influence of hydrogen bonding on the photophysical proper-
ties of these semisquaraines provides a promising new motif
for sensor design.
bisnucleophile.7,8 However, the prepared polymers, mostly
of the poly(pyrrole-alt-squaraine) type,7 exhibit relatively
low degrees of polymerization and poor solubility, contain
regiodefects arising from â-substitution, and allow for
only limited architectural control.
Our interest in this field originates from the idea of
utilizing phenylenediamine derivatives to obtain poly-
(phenylene-alt-squaraine)s of various topologies such as
rodlike, cyclic, and helical structures 1-3 (Chart 1),
respectively, depending on the employed regioisomer, i.e.,
o-, m-, or p-phenylenediamine. Due to their expected
advantageous photophysical properties3 and the readily
tunable substitution to introduce, for instance, solubiliz-
ing groups, these compounds would be promising candi-
dates for optoelectronic, sensory, and transport applica-
tions. Here, we report on our attempts toward the
synthesis of poly(m-phenylene-alt-squaraine)s and sub-
sequently performed experiments devoted to understand
the unusual reactivity of intermediate phenylenedi-
amine-squaric acid monoadducts. We focus on the case
of m-phenylenediamines as activated aromatic moieties
since both amino substituents donate electron density to
the same carbon centers leading to a strong directing
effect, thereby affording meta-linked chain segments that
In tr od u ction
Squaraine dyes1 are well-known for their exceptional
properties,2 including their relatively small HOMO-
LUMO gaps, high extinction coefficients, photoconductiv-
ity, and net charge neutrality, that have been exploited
in a number of applications. The incorporation of the
squaraine chromophore into a conjugated framework
promises improved properties due to the predicted low
band gaps3 and polymer processibility. Therefore, to
access intrinsically conducting as well as IR-emitting
polymers based on donor-acceptor substituted conju-
gated polymers,4 poly(squaraine)s5 have received consid-
erable attention.6 In all cases, the polymers have been
accessed via a standard A2 + B2 polycondensation route
involving squaric acid as the biselectrophile and an
electron-rich aromatic or bisenamine moiety as the
(1) (a) Treibs, A.; J acob, K. Angew. Chem., Int. Ed. Engl. 1965, 4,
694-695. (b) A review is given in: Schmidt, A. H. Synthesis 1980, 961-
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(2) For reviews, consult: (a) Law, K.-Y. Chem. Rev. 1993, 93, 449-
486. (b) Das, S.; Thomas, K. G.; George, M. V. Mol. Supramol.
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Brocks, G.; Tol, A. Synth. Met. 1996, 76, 213-216.
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2003, 32, 181-191.
(6) Representative reviews on conjugated polymers and their use
in organic electronics are given in: (a) Mu¨llen, K.; Wegner, G.
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(b) Lynch, D. E.; Geissler, U.; Peterson, I. R.; Floersheimer, M.;
Terbrack, R.; Chi, L. F.; Fuchs, H.; Calos, N. J .; Wood, B.; Kennard,
C. H. L.; Langley, G. J . J . Chem. Soc., Perkin Trans. 2 1997, 827-
832. (c) Lynch, D. E.; Peterson, I. R.; Floersheimer, M.; Essing, D.;
Chi, L.-F.; Fuchs, H.; Calos, N. J .; Wood, B.; Kennard, C. H. L.; Langley,
G. J . J . Chem. Soc., Perkin Trans. 2 1998, 779-784. (d) Lynch, D. E.;
Geissler, U.; Calos, N. J .; Wood, B.; Kinaev, N. N. Polym. Bull. 1998,
40, 373-380. (e) Ajayaghosh, A.; Chenthamarakshan, C. R.; Das, S.;
George, M. V. Chem. Mater. 1997, 9, 644-646. (f) Chenthamarakshan,
C. R.; Ajayaghosh, A. Chem. Mater. 1998, 10, 1657-1663. (g)
Chenthamarakshan, C. R.; Eldo, J .; Ajayaghosh, A. Macromolecules
1999, 32, 251-257.
(8) (a) Chen, Y. Y.; Hall, H. K., J r. Polym. Bull. 1986, 16, 419-425.
(b) Havinga, E. E.; Ten Hoeve, W.; Wynberg, H. Polym. Bull. 1992,
29, 119-126. (c) Havinga, E. E.; ten Hoeve, W.; Wynberg, H. Synth.
Met. 1993, 55, 299-306. (d) Havinga, E. E.; Pomp, A.; Ten Hoeve, W.;
Wynberg, H. Synth. Met. 1995, 69, 581-582. (e) Ajayaghosh, A.; Eldo,
J . Org. Lett. 2001, 3, 2595-2598. (f) Eldo, J .; Ajayaghosh, A. Chem.
Mater. 2002, 14, 410-418.
10.1021/jo035399z CCC: $27.50 © 2004 American Chemical Society
Published on Web 12/12/2003
184
J . Org. Chem. 2004, 69, 184-187

SCHEME 1
F IGURE 1. 1H NMR comparison of the aromatic region before
(4a , top) and after (7a , bottom) introduction of the squaraine
moiety to the m-phenylenediamine system (solvent: CDCl3).
followed by hydrolysis10 to afford 7a ,c in moderate yields
(Scheme 1).9 Detailed characterization was performed,
1
including H and 13C NMR, EI-HRMS, IR, UV/vis, and
emission spectroscopy, while purity was confirmed by
HPLC. To our knowledge, these compounds are the first
reported semisquaraines carrying ortho-dialkylamino
substituents. Semisquaraines 7a ,c serve as ideal model
compounds since they potentially contain both electro-
philic (semisquaraine carbonyl) and nucleophilic (electron-
rich aromatic) sites and therefore resemble AB mono-
mers. Surprisingly, even under vigorous conditions (vide
supra), self-condensation (AB polycondensation) could not
be affected, indicating a lack of reactivity on one or both
sites. The phenylenediamine system is significantly
deactivated by the introduction of the squaric acid moiety.
encode oligomeric macrocycles and polymeric helices as
targets of our interest.
Resu lts a n d Discu ssion
1
This is indicated by the downfield H NMR shifts of the
aromatic ring protons in 7a as compared to 4a (Figure
1).
P olycon d en sa tion . Tertiary m-phenylenediamines
4a -c were readily accessible by standard alkylation and
reductive amination protocols, respectively.9 The asym-
metrically substituted derivative 4d was prepared via a
three-step amidation, N-alkylation, reduction sequence.
It should be noted that tertiary amines have to be
employed in squaraine condensations to prevent compet-
ing squaramide formation. With these bifunctional mono-
mers in hand, A2 + B2 polycondensation reactions with
squaric acid 5 were carried out (Scheme 1). However,
employing various conditions, i.e., dehydrating agents
(e.g., tributyl orthoformate), solvents (e.g., n-butanol/
toluene mixtures, DMSO), and temperatures (e.g., 110-
180 °C), only short (2 e n e 5), highly colored oligomers
6 were formed according to GPC9 as well as MALDI-TOF
MS analysis.
The observed low degrees of polymerization indicate
inefficient elementary condensation steps. To investigate
the polycondensation limiting step, semisquaraines 7a ,c
resembling key intermediates (AB monomers) in the
polycondensation process were prepared and their reac-
tivities studied.
Sem isqu a r a in e Mod el Stu d ies. Two semisquaraines
7a ,c were synthesized using a newly developed three-
step sequence involving lithiation of tertiary phenylene-
diamines 4a ,c and coupling with diethyl squarate 8
Interestingly, also the electrophilic character of the
investigated semisquaraines 7a ,c is greatly diminished
since virtually no reaction took place with highly active
aromatic systems such as N,N-dimethylaniline, pyrrole,
and phenylenediamine 4c to give the expected squaraine
dimers. Even under extremely forcing conditions involv-
ing reflux in n-butanol/toluene employing a Dean-Stark
trap for several days, only faint product spots were
observed on TLC, with the overwhelming majority being
unreacted starting material. This was surprising since
other known semisquaraines lacking the ortho-amino
group readily undergo such coupling reactions.2
The observed extraordinarily low reactivity of ortho-
dialkylamino-substituted semisquaraines 7a ,c at both
nucleophilic and electrophilic termini is probably due to
an intramolecular hydrogen bonding interaction between
the fairly acidic semisquaraine hydroxyl group and the
aromatic ortho-nitrogen atom. Such interaction would
lock the structure in a planar conformation, in which the
squaryl ring in conjugation to the aromatic system acts
as a strong acceptor and the ortho-amino group can no
longer act as a donor since its lone pair is engaged in
intramolecular hydrogen bonding. The net effect would
(10) For a related procedure, see: Reed, M. W.; Pollart, D. J .; Perri,
S. T.; Foland, L. D.; Moore, H. W. J . Org. Chem. 1988, 53, 2477-2482.
(9) See Supporting Information for details.
J . Org. Chem, Vol. 69, No. 1, 2004 185

F IGURE 2. UV/vis absorption and fluorescence emission spectra of 7a in CH2Cl2 (abs, solid blue line; emi, broken blue line),
CH3CN (abs, solid green line; emi, broken green line), and CH3OH (abs, solide red line; emi, broken red line) (λexc ) 355 nm for
CH2Cl2 and CH3CN and 269 nm for CH3OH). The inset illustrates a simplified attempt to rationalize the dependence of
photophysical properties on the protic/nonprotic medium.
be a shift of electron density from the phenylenediamine
to the squaryl moiety via both conjugation and hydrogen-
bonding pathways and hence a loss in nucleophilicity and
electrophilicity, respectively. The UV/vis absorption and
emission spectra of semisquaraine 7a in different sol-
vents provide experimental support for the existence of
an intramolecular hydrogen bond. While the absorption
spectra of 7a displaying a maximum at λmax ) 355 nm
are comparable for both extremes of nonpolar (methylene
chloride) and polar nonprotic solvents (acetonitrile), polar
protic solvents (methanol) cause a dramatic hypsochromic
shift to λmax ) 269 nm (Figure 2). Furthermore, strong
The almost planar (phenyl-squaraine dihedral angle
∼7°), hydrogen bonded structure 7a -0 is clearly the most
stable conformation according to both initial computation
and variation of the CdC-O-H dihedral angle. The
stability of the planar structure arises from a strong
intramolecular hydrogen bond as evidenced by compari-
son with the 18 kcal/mol less stable orthogonal conforma-
tion (nonoptimized). Geometry optimization for the given
angle reduces the depth of the well (∆∆G ≈ 12 kcal/mol),
i.e., stabilizes the twisted conformations, mainly due to
rehybridization of the ortho-nitrogen atom (sp3fsp2) and
hence enhanced resonance energy. Further support for
the key stabilizing hydrogen-bonding interaction arises
from the existence of a second minimum on the potential
energy surface that is approximately 6 kcal/mol less
stable than 7a -0 and associated with the non-hydrogen-
bonded structure 7a -180, which shows significant devia-
tion from planarity (phenyl-squaraine dihedral angle ≈
36°). Natural bond order (NBO) analyses were performed
to deduce charge distributions of the optimized rotamers.
In particular, minimum hydrogen-bonded structure 7a -0
displays an increased natural charge on the ortho-
nitrogen and a decrease in natural charge on the meta-
carbon atom when compared to maximum orthogonal
emission occurs only in methanol (λexc ) 269 nm, λem
)
354 nm). These results suggest the presence of an
intramolecular N-H-O hydrogen bond in nonprotic
media, while protic solvents destabilize the planar con-
formation and lead to the formation of a decoupled
system with strongly altered electronic properties that
are reflected in its drastically different photophysical
1
behavior. Alternative techniques such as IR or H NMR
unfortunately failed to provide more direct evidence for
an existing hydrogen bond; therefore, theoretical ap-
proaches were sought to gain more insight into the
structure and electronic properties of semisquaraines 7.
Com p u ta tion a l Stu d ies. To investigate the influence
of a hydrogen-bonding interaction on the structural
preferences of semisquaraines 7, ab initio calculations
using Gaussian 9811 with the Hartree-Fock method and
the 6-31G* basis set were carried out.9 Starting from a
fully optimized geometry, a rotamer series of 7a was
generated by fixing the CdC-O-H dihedral angle be-
tween 0 and 180°. Thereby, the hydrogen atom is moved
from a planar hydrogen-bonded structure 7a -0 (0°) to
orthogonal 7a -90 (90°) and finally planar 7a -180 (180°)
non-hydrogen-bonded structures. Single-point energies as
well as energies of the optimized geometries were calcu-
lated yielding the nonoptimized and optimized ground-
state potential energy diagrams (Figure 3).12
(11) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J . A.,
J r.; 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.;
Foresman, J . B.; Cioslowski, J .; Ortiz, J . V.; 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.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; J ohnson, B. G.; Chen,
W.; Wong, M. W.; Andres, J . L.; Head-Gordon, M.; Replogle, E. S.;
Pople, J . A. Gaussian 98, revision A7; Gaussian, Inc.: Pittsburgh, PA,
1998.
(12) Diagram reflects only enthalpic contributions, i.e., differences
in calculated heats of formation. However, entropic terms should be
negligible when comparing (rotational) isomers.
186 J . Org. Chem., Vol. 69, No. 1, 2004

F IGURE 3. Potential energy diagram as a function of the CdC-O-H dihedral angle derived from ab initio calculations (Gaussian
98, HF, 6-31G*) on a preoptimized geometry of 7a . Nonoptimized, single-point calculations (red circles) and fully optimized
calculations for a given dihedral angle (blue squares) as well as selected respective fully optimized structures for 0° (7a -0), 45°
(7a -45), 90° (7a -90), 135° (7a -135), and 180° (7a -180) are shown.
structure 7a -90. Therefore, the computational results
agree with the postulated effect of torsion on electron
density distribution and therefore support the experi-
mentally found unusual reactivity.
Gratifyingly, the novel intramolecular hydrogen-bonding
motif discovered during the course of our investigations
displays a high sensitivity of photophysical properties on
structural parameters and therefore offers potential
applications in sensor design.
Con clu sion
Ack n ow led gm en t. We are indebted to Christian
Hu¨bschle as well as Hans-Eric Daroczi and Torsten
Bartz for initial polycondensation experiments and
semisquaraine syntheses, respectively, and to Prof.
Amnon Stanger (Technion, Haifa) for helpful discussions
regarding the computational studies. Generous support
by the Sofja Kovalevskaja Award of the Alexander von
Humboldt Foundation, endowed by the Federal Ministry
of Education and Research (BMBF) within the Program
for Investment in the Future (ZIP) of the German
Government, is gratefully acknowledged.
A novel class of ortho-dialkylamino-substituted semi-
squaraines has been synthesized and exhibits an in-
tramolecular hydrogen bond that controls the conforma-
tional preference and hence the reactivity of the molecule.
This finding, unfortunately, is the reason for our unsuc-
cessful attempts to prepare poly(m-phenylene-alt-
squaraine)s and is manifested in the fact that the
semisquaraines themselves do not undergo an AB poly-
condensation reaction. The fact that alternative methods
such as ketene dimerization followed by oxidation13 also
failed in our hands illustrates the need for conceptually
new routes to assemble squaraine moieties and access
the attractive family of poly(phenylene-alt-squaraine)s.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures, chemical characterization data, and computational
details. This material is available free of charge via the
Internet at http://pubs.acs.org.
(13) Farnum, D. G.; Webster, B.; Wolf, A. D. Tetrahedron Lett. 1968,
8, 5003-5006.
J O035399Z
J . Org. Chem, Vol. 69, No. 1, 2004 187