Novel D-π-A chromophores based on the fulvene accepting moiety

Article abstract of DOI:10.1021/ol016143n

(matrix presented) Novel D-π-A chromophores based on the fulvene accepting moiety and p-dimethylamino phenyl and 1,3-dithiole-2-ylidene donor moieties have been prepared. The X-ray structures of two representative derivatives have been determined. Examina

Full text of DOI:10.1021/ol016143n

ORGANIC
LETTERS
2001
Vol. 3, No. 15
2329-2332
Novel D-π-A Chromophores Based on
the Fulvene Accepting Moiety
,†
Emad Aqad,† Philippe Leriche,* Gilles Mabon,† Alain Gorgues,† and
,‡
Vladimir Khodorkovsky*
Inge´nierie Mole´culaire et Mate´riaux Organiques, UFR Sciences, UniVersite´ d’Angers,
49045 Angers, France, and Department of Chemistry, Ben-Gurion UniVersity,
Beer-SheVa 84105, Israel
leriche@uniV-angers.fr
Received May 21, 2001
ABSTRACT
Novel D-π-A chromophores based on the fulvene accepting moiety and p-dimethylamino phenyl and 1,3-dithiole-2-ylidene donor moieties
have been prepared. The X-ray structures of two representative derivatives have been determined. Examination of the UV−visible spectra and
cyclic voltamperometry data revealed remarkable sensitivity of the electronic structure of these derivatives to substituents at the cyclopentadiene
ring.
Conjugated donor-acceptor systems are currently the focus
of numerous studies owing to their large hyperpolarizabilities
and, therefore, the ability of second harmonic generation. A
general trend in the design of second-order nonlinear optical
materials involves tuning the donor and acceptor strength,
and a variety of strong accepting groups, such as polycar-
bonyl, nitro, and cyano derivatives have been employed.1
In particular, the high potential of polynitrofluorenes as the
electron-accepting units have recently been demonstrated.2
Fulvene derivatives3 should exhibit higher polarizability than
fluorene derivatives, which suggested the possibility of their
use as accepting moieties in the above systems, and indeed,
several fulvene compounds unsubstituted at the cyclopen-
tadiene ring, showing nonlinear optic properties, have been
described.4 Derivatives substituted at the cyclopentadiene ring
have received only scant attention. Recently, we have shown
that fulvalene derivatives are strong proaromatic electron
acceptors and their electron affinities (LUMO energies) are
highly sensitive to substitution at the cyclopentadiene ring.5
Here we report on the synthesis of a series of novel 2,3,4,5-
substituted fulvene derivatives and their spectroscopic and
electrochemical properties. Three sets of fulvene derivatives
were studied: tetrachloro-, 3,4-dimethoxy-2,5-dicyano-, and
poly(methoxycarbonyl)-fulvenes (5-8 and 10, 11). Fulvene
derivatives6 discussed here are unknown except the tetra-
chlorofulvenes 5a and 6a.7 Derivatives 5-8 were synthesized
by condensation of tetrachlorocyclopentadiene (1a),8 3,4-
* Email for second corresponding author: hodor@bgumail.bgu.ac.il.
† Universite´ d’Angers.
‡ Ben-Gurion University.
(1) Singer, K. D.; Hubbard, S. F.; Schober, A.; Hayden, L. M.; Johnson,
K. In Characterization, Techniques and Tabulations for Organic Nonlinear
Optical Materials; Kuzyk, M. G., Dirk, C. W., Eds.; Marcel Dekker: New
York, 1998; pp 310-513.
(2) Moore, A. J.; Chesney, A.; Bryce, M. R.; Batsanov, A. S.; Kelly, J.
F.; Howard, J. A. K.; Perepichka, I. F.; Perepichka, D. F.; Meshulam, G.;
Berkovich, G.; Kotler, Z.; Mazor, R.; Khodorkovsky, V. Eur. J. Org. Chem.
2001, in press.
(4) Peterson, M. L.; Strnad, J. Y.; Markotan, T. P.; Morales, C. A.;
Scaltrito, D. V.; Staley, S. W. J. Org. Chem. 1999, 64, 9067. Kawabe, Y.;
Ikeda, H.; Sakai, T.; Kawasaki, K. J. Mater. Chem. 1992, 2, 1025. Kondo,
K.; Goda, H.; Takemoto, K.; Aso, H.; Sakaki, T.; Kawakami, K.; Yoshida,
H.; Yosida, K. J. Mater. Chem. 1992, 2, 1097.
(5) Aqad, E.; Leriche, P.; Mabon, G.; Gorgues, A.; Khodorkovsky, V.
Tetrahedron Lett. 2001, 42, 2813.
(3) Zeller, K. P. In Methoden der Organischen Chemie; Houben-Weyl:
1985; 512C, pp 504-684. Gompper, R.; Guggenberger, R. Tetrahedron
1986, 42, 839. Gompper, R.; Kutter, E. Ber. Dtsch. Chem. Ges. 1965, 98,
2825. Lu¨ttringhaus, A.; Prinzbach, H. Tetrahedron Lett. 1965, 25, 2121
and references therein. Tacke, M.; Fox, S.; Cuffe, L.; Dunne, J. P.; Hartl,
F.; Mahabiersing, T. J. Mol. Struct. 2001, 559, 331.
(6) All new fulvenes were characterized by spectroscopic means and
HRMS. The molecular structure of cyclopentadiene 1b and fulvenes 10
and 11 were determined by X-ray analysis and will be described elsewhere.
(7) Agranat, I.; Hayek, M.; Gill, D. Tetrahedron 1977, 33, 239.
(8) Smith, R.; West, R. J. Org. Chem. 1970, 35, 2681 and references
therein.
10.1021/ol016143n CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/03/2001

Scheme 1
Figure 1. Molecular structure of 6b. Selected bond lengths (in
Å): 1.338(4) (C1-C2), 1.433(4) (C2-C3), 1.357(4) (C3-C4),
1.442(4) (C1-C13), 1.388(4) (C13-C14), 1.396(5) (C13-C18),
1.373(4) (C14-C15), 1.393(5) (C15-C16), 1.406(4) (C16-C17),
1.374(4) (C17-C18).
corresponding 1,3-indanedione-2-ylidene analog: 1.363(4),
1.423(5), 1.352(4), and 1.439(4) Å for the bonds correspond-
ing to C3-C4, C2-C3, C1-C2, and C1-C13.12
The length of the central double bond (1.380 Å) of
derivative 7b (Figure 2) is also close to that found in the
dimethoxy-2,5-dicyanocyclopentadiene (1b), or tetrakis-
(methoxycarbonyl) cyclopentadiene (1c) (Scheme 1) with the
corresponding aldehyde or 1,3-dithiole precursors.9
The synthesis of 1c was achieved by reproducing the
recently reported10 partial hydrolysis/decarboxylation of
pentakis (methoxycarbonyl) cyclopentadiene (9). The overall
yield of 1c did not exceed 5%, which makes its use as a
precursor not practical. Alternatively, we found that deriva-
tive (9) reacts smoothly with aromatic aldehydes and
2-methylthio-1,3-dithiolium salt 3 in methanol at room
temperature, providing an efficient one-pot synthesis of
tetrakis(methoxycarbonyl) fulvenes (Scheme 2). The reaction
Scheme 2
Figure 2. Molecular structure of 7b. Selected bond lengths (in
Å): 1.747(3) (S1-C2), 1.742(2) (S1-C3), 1.380(5) (C3-C4),
1.454(3) (C4-C6), 1.382(3) (C6-C7).
corresponding 1,3-indandione-2-ylidene analog13 (1.370 Å),
and therefore the accepting ability of the 1,4-dicyano-2,3-
dimethoxycyclopentadiene-2-ylidene moiety is close to that
of 1,3-indandione-2-ylidene moiety.
Results of the electrochemical and spectroscopic studies
are collated in Table 1. All new derivatives undergo one-
of 9 with 2a in refluxing acetic acid was accompanied by
hydrolysis and decarboxylation, and di- and triesters 10 and
11 were isolated from the reaction mixture.
The crystal structures of derivatives 6b and 7b were
determined by the X-ray experiments.11 Both derivatives are
planar in the solid state. The geometry of the conjugating
bridge linking the donor and acceptor moieties within
derivative 6b (Figure 1) is close to that found for the
(9) Condensation of tetrachlorocyclopentadiene (1a) with aldehydes 2a
and 2b was carried out in MeOH in the presence of a catalytic amount of
DMF. Reaction of 1a with 2-methylthio-1,3-dithiolium salt (3) was carried
out in refluxing AcOH. Condensation of 3,4-dimethoxy-2,5-dicyanocyclo-
pentadiene (1b) with 2a, 2b and 3 carried out in refluxing AcOH.
Condensation of tetrakis(methoxycarbonyl) cyclopentadiene (1c) with 2a,
2b, and 2c was carried out in methanol at room temperature. Cyclopenta-
dienes 1a and 1b were condensed with aldehyde 4 in refluxing AcOH in
the presence of catalytic amount of piperidine.
(10) Lei, Y. Z.; Cerioni, G.; Rappoport, Z. J. Org. Chem. 2000, 65, 4028.
2330
Org. Lett., Vol. 3, No. 15, 2001

maximum in the case of the c series. However, the derivatives
of the b series exhibit fully unexpected negative solvato-
chromism.
Table 1. Redox Potentials (V vs Ag/AgCl) in CH2Cl2 Solution
and Absorption Spectra (nm) for 10-5 mol L-1 Solutions
Indeed, whereas the positive solvatochromism is expected
for the longest wave absorption band that should have a
charge-transfer nature, the above behavior of derivatives of
the b series is puzzling. Quantum mechanical calculations14
were performed for derivatives 6a, 6b, and 6d (X ) Y )
CN) to gain deeper insight into the remarkable sensitivity
of the cyclopentadiene ring to the substituent effects.
Geometry optimization on RHF/6-31G(p,d) level yielded
planar structures, and a good agreement with the experi-
mental geometry found for 6b was obtained. All bond lengths
were predicted within 0.01 Å, except the C1-C13 bond for
which the length of 1.461 Å was calculated. The calculated
dipole moments for compounds 6a, 6b, and 6d were µg )
9.15 , 2.56, and 19.44 D, respectively. HOMOs of all three
derivatives feature zero coefficients at 1,4-positions of the
cyclopentadiene ring (see Figure 3 for an example), which
b
c
d
compound
Ered
Eox
λmax
λmax
λmax
7a
7b
7c
8a
8b
5a
5b
5c
6a
6b
6c
10
11
502
514
520
541
539
474
485
523
511
512
600
506
521
511
511
519
550
525
485
479
562
527
503
633
522
540
-1.07
-0.77a
-0.99
-0.93
-1.26
-1.20
-0.87a
-1.05
-1.07
-0.72
-1.16a
-1.04
1.11a,e
1.41
1.04
0.95a
518
521
545
537
479
489
556
518
517
625
518
536
1.06
0.97a
1.19a
0.85
0.79a
0.96a
1.04
1.11
a Reversible. b In CH3Ph. c In CH3CN. d In CH2Cl2. Eox(2) ) 1.49 V.
e
electron reduction at potentials between -1.26 (5a) and
-0.72 V (6c) and one-electron oxidation between 1.41 (7c)
and 0.79 V (6b). Derivatives 5c, 7c, and 10 undergo
reversible reduction.
The reduction potentials within the series a (X ) Y )
Cl) and b (X ) CN, Y ) OMe) are very close and
approximately 300 mV more negative than observed for the
series c (X ) Y ) CO2Me). Thus, the electron-withdrawing
effect of four chlorine atoms is close to that of the two cyano
and two methoxy groups and, expectedly, weaker than the
effect of four methoxycarbonyl groups. The trend in oxida-
tion potentials is more difficult to rationalize. All derivatives
of the b series undergo reversible oxidation at considerably
lower potentials than derivatives from a and c series (Table
1); two oxidation peaks were observed for derivative 7b.
The position of the longest wave absorption bands of
derivatives 5-8, 10, and 11 within the series undergoes shifts
in agreement with the differences in redox potentials. Thus,
the largest difference in redox potentials observed for 5a
(2.32 V) corresponds to the absorption at the shortest
wavelength (479 nm), and the smallest difference observed
for 6c (1.68 V) corresponds to the absorption at 625 nm (all
values for methylene chloride solutions). This observation
supports indirectly the assignment of these bands as charge-
transfer bands (HOMO f LUMO transitions). Derivatives
of the a series exhibit moderate positive solvatochromism,
which increases for derivative 10 and 11 and reaches its
Figure 3. LUMO (a) and HOMO (b) diagrams for derivative 6a.
means that the HOMO energies are not sensitive to the
substituents mesomeric effect at these positions. This result
accounts for the observed trend in oxidation potentials.
Absorption spectra were calculated using TD B3LYP/6-31G-
(d,p) (see, e.g., ref 15 and references therein), which gave
excellent predictions for molecules of a similar type.16 Strong
charge-transfer bands at 431 nm (f ) 1.27) and 490 nm (f )
(11) 6a: dark violet prism (0.4 × 0.4 × 0.1), monoclinic, C20H13N3O2,
space group C2/c, Z ) 8, at 294 K, a ) 21.474(6), b ) 13.828(1), c )
15.723(7) Å, V ) 3565(3) Å3, Fcalcd ) 1.24 Mg/m3, F(000) ) 1408; 3410
reflections were collected of which 1342 reflections were independent.
Structure was refined to R1 ) 0.044, wR2 ) 0.057, GOF ) 1.117. 7b:
dark ruby prism (0.5 × 0.2 × 0.2), orthorhombic, C14H10N2O2S4, space
group Cmcm, Z ) 4, at 293(2) K, a ) 15.917(1), b ) 13.513(2), c ) 7.1891-
(7) Å, V ) 1546.4(3) Å3, Fcalcd ) 1.574Mg/m3, F(000) ) 752; 6894
reflections were collected of which 1065 reflections were independent.
Structure was refined to R1 ) 0.0388, wR2 ) 0.1013, GOF ) 0.933.
Atomic coordinates, bond lengths and angles, and anisotropic parameters
have been deposited at the Cambridge Crystallographic Data Center.
(12) Mazor, R.; Ellern, A.; Khodorkovsky, V. Unpublished results.
(13) Bulgarovska, I. V.; Sobolev, A. N.; Zavodnik, V. E.; Khodorkovsky,
V.; Neilands, O. IzV. Akad. Nauk LatV. SSR, Ser. Khim. 1988, 349.
(14) Gaussian 98, revision A.9; Frish, M. et al.; Gaussian, Inc.; Pittsburgh,
PA, 1998.
(15) Bauernschmidt, R.; Ahlrichs, R. Chem. Phys. Lett. 1996, 256, 454.
(16) Schwartz, H.; Mazor, R.; Khodorkovsky, V.; Shapiro, L.; Klug, J.
T.; Kovalev, E.; Meshulam, G.; Berkovic, G.; Kotler, Z.; Efrima, S. J. Phys.
Chem. B 2001, 105, 5914.
Org. Lett., Vol. 3, No. 15, 2001
2331

1.40) (HOMO f LUMO, π f π* transition), which should
undergo a red shift at increasing polarity of solvents, were
predicted for derivative 6a and 6d, correspondingly. In
contrast, two intense transitions in the visible part of the
spectrum, 467 nm, f ) 1.0 (HOMO f LUMO, π-π*) and
323 nm, f ) 0.45 (HOMO2- f LUMO, π f π*), were
predicted for derivative 6b. This result is consistent with the
observed asymmetric longest wave absorption bands of 6b
and other derivatives of the b series. The transition dipole
moments for both transitions involve predominantly the
X-component and have the opposite signs (charge transfer
from NMe2 donor moiety for the first transition and from 2
MeO donors for the second transition to the accepting
moiety). The calculated energy and intensity of the first
transition strongly depends on the conformation of the
molecule. For instance, the intensity of the first transition
for a nonplanar conformer of 6b (with the C14-C13-C1-
C2 dihedral angle of 45°) drops down considerably (f )
0.60), whereas the intensity of the second transition increases
(f ) 0.67). The nonplanar conformers of 6b should always
be present in solution at room temperature, since the rotation
barriers around C1-C13 and C2-C3 bonds do not exceed
4 kcal/mol according to the results of RHF/6-31G(p,d)
calculations. In contrast, HOMO2- f LUMO transition does
not depend on the conformation.17 The above difference in
the transition nature gives rise to the observed negative
solvatochromism. Similar behavior was recently observed
for the D-π-A systems involving a carbazole moiety for
which negative solvatochromism and negative µâ0 values
were found.18
acceptor and studied their electrochemical and electronic
spectra. These compounds exhibit a remarkable sensitivity
to the substituents’ nature and position. Fine-tuning of the
substituents at the cyclopentadiene ring and the conjugating
bridge can afford new dyes possessing a variety of solva-
tochromic effects and high hyperpolarizability. Indeed, the
initial EFISH measurements performed for derivative 6a19
afforded a value of µâ0 ) 548 × 10-48 esu, about two times
larger than for the analogue of 6a involving the 1,3-
indandione accepting moiety.20 Detailed studies of the
donor-acceptor substituted fulvene derivatives are currently
under way.
Acknowledgment. We thank Dr. A. Ellern and M. Allain
for the X-ray experiments. V.K. thanks Pole de recherche et
d’Innovation d’Angers (PRIA) for financial support of his
stay. E.A. thanks the French foreign office (Bourse Cha-
teaubriand) and re´gion pays de la Loire for financial support
during his postdoctoral stay.
OL016143N
(17) Calculations on the semiempirical (PM3) level provided essentially
the same results. However, the energies of both transitions calculated at
this level using CI are very close: 412 and 408 nm for the planar
conformation. The second transition becomes the first (409 nm), whereas
the first transition undergoes considerable hypsochromic shift (389 nm) for
the above nonplanar conformation, which is less stable by 1.4 kcal/mol.
The large energy gap between the first and the second transitions predicted
by the TD DFT method stems probably from the propensity of this method
to overestimate energies of the higher energy transitions, as it is the case
with the HF-based methods.
(18) Meshulam, G.; Shaier, P.; Berkovic, G.; Ben-Asuli, A.; Mazor, R.;
Shapiro, L.; Khodorkovsky, V.; Kotler, Z. Nonlinear Optics 2000, 25, 321.
(19) Kotler, Z.; Berkovic, G.; Khodorkovsky, V. Unpublished results.
The setup and conditions described in ref 2 were used.
In summary, we synthesized a series of D-π-A chro-
mophores involving a substituted fulvene moiety as electron
(20) Moylan, C. R.; Twieg, R. J.; Lee, V. Y.; Swanson, S. A.; Betterton,
K. M.; Miller, R. D. J. Am. Chem. Soc. 1993, 115, 12599.
2332
Org. Lett., Vol. 3, No. 15, 2001