Second-order nonlinear optical activity of dipolar chromophores ased on pyrrole-hydrazono donor moieties

Article abstract of DOI:10.1002/chem.200900287

A series of highly efficient and thermally stable second-order nonlinear optical (NLO) dipolar donorauxiliary donor-acceptor chromophores have been synthesised in which a hydrazono group and a pyrrole ring act as donor and auxiliary donor components, resp

Full text of DOI:10.1002/chem.200900287

FULL PAPER
DOI: 10.1002/chem.200900287
Second-Order Nonlinear Optical Activity of Dipolar Chromophores Based
on Pyrrole-Hydrazono Donor Moieties
Alessandro Abbotto,*[a] Luca Beverina,[a] Norberto Manfredi,[a] Giorgio A. Pagani,*[a]
Graziano Archetti,[b] Hans-Georg Kuball,[b] Christian Wittenburg,[c] Jꢀrgen Heck,[c] and
Jan Holtmann[c]
Dedicated to the memory of Rꢀdiger Wortmann
Abstract: A series of highly efficient
and thermally stable second-order non-
linear optical (NLO) dipolar donor–
have been studied in detail by electro-
optical absorption (EOA) and hyper-
Rayleigh scattering (HRS) measure-
ments in 1,4-dioxane and DMSO, re-
spectively. The results originating from
the two different methods have been
compared and analysed in detail. We
found that the NLO properties mea-
sured by the EOA and HRS methods
correlate with each other and converge
to reveal the dye with the acceptor 2-
dicyanomethylene-3-cyano-4,5,5-tri-
methyl-2,5-dihydrofuran as the most ef-
ficient system. The unprecedented
combination of a strong donor hydra-
zono group and the auxiliary donor ef-
fects of p-excessive heteroaromatic
rings afforded NLO chromophores
auxiliary
donor–acceptor
chromo-
phores have been synthesised in which
a hydrazono group and a pyrrole ring
act as donor and auxiliary donor com-
ponents, respectively, in combination
with different aromatic and heteroaro-
matic acceptors. The new dyes have
been systematically investigated by
NMR spectroscopy, absorption spec-
troscopy, NLO measurements and ther-
mal stability studies. NLO properties
Keywords: chromophores · electro-
optical absorption measurements ·
heterocycles · hyper-Raleigh scat-
tering · nonlinear optics
with very high values (mgb0
to 2038ꢂ10ꢀ48 esu and b
(HRS) up to
3980ꢂ10ꢀ30 esu at 1.5 mm).
ACHTUNGTRNE(NUNG EOA) up
AHCTUNGTRENNUNG
Introduction
have been extensively investigated in the last decade for
second-order nonlinear optical (NLO) applications in pho-
tonic and electro-optic devices, including high-speed photon-
ic switching systems and electro-optic modulators.[1–4] Al-
though a large variety of donor, acceptor and spacer groups
have been used for the design of NLOphores, the use of
strong donating moieties, such as electron-rich p-excessive
heteroaromatic rings,[5] has seldom appeared in the NLO lit-
erature.[3a,6] Even more surprisingly is the absence, with very
few exceptions,[7,8] of important donor organic functionalities
Dipolar or push–pull chromophores, in which p-conjugated
systems are end-capped by donor and acceptor moieties,
[a] Prof. Dr. A. Abbotto, Dr. L. Beverina, Dr. N. Manfredi,
Prof. Dr. G. A. Pagani
Department of Materials Science and INSTM
University of Milano-Bicocca
Via Cozzi 53, 20125 Milano (Italy)
Fax : (+39)02-6448-5400
E-mail: alessandro.abbotto@mater.unimib.it
giorgio.pagani@mater.unimib.it
ꢀ
such as hydrazono moieties of the type R2N N=, which are
able to efficiently delocalise an electron pair residing on the
terminal nitrogen atom onto a p-conjugated system.
In recent years we have designed a number of di- and
multipolar dyes containing electron-rich and -poor heterocy-
cles as donor and acceptor units.[9,10] In particular, we found
that chromophores containing a pyrrole ring as a donor
moiety are endowed with remarkable NLO activity. On the
basis of INDO/1 semiempirical computations, Ratner et al.
predicted in 1997 that the NLO response of dipolar chromo-
phores would be significantly enhanced by interposing p-ex-
[b] Dr. G. Archetti, Prof. Dr. H.-G. Kuball
Physikalische Chemie, Technische Universitꢀt Kaiserlautern
Erwin-Schrçdinger-Str., 67663 Kaiserslautern (Germany)
[c] Dr. C. Wittenburg, Prof. Dr. J. Heck, J. Holtmann
Institut fꢁrAnorganische und Angewandte Chemie
Universitꢀt Hamburg
Martin-Luther-King Platz 6, 20146 Hamburg (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900287.
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6175

cessive or -deficient heterocyclic bridges between strong
donor or acceptor groups.[11] The auxiliary donor effect, gen-
erally larger than the auxiliary acceptor effect, is predicted
to be particularly strong for the electron-rich pyrrole ring.
More recently, the pyrrole ring was incorporated as an auxil-
iary donor into efficient dipolar chromophores with two-
photon absorption activity.[12]
Encouraged by the aforementioned predicted NLO en-
hancement by the auxiliary donor effect, we reasoned that
the presence of a strong electron-donating group such as a
terminal amino functionality directly attached to the elec-
tron-donating auxiliary moiety would be highly beneficial in
terms of enhancing the second-order NLO performance. It
is well known, however, that amino-functionalised p-exces-
sive heterocycles (furan, thiophene and pyrrole) have limit-
ed synthetic access and once prepared are endowed with
poor thermal stability because of easy oxidative degrada-
tion. Thus, we turned to the vinylogy concept and, in partic-
ular, aza-vinylogy. We considered that hydrazones are aza-
vinylogues[13] of amino functionalities. Phenyl- and diphenyl-
hydrazones are consequently viewed as phenyl- and diphe-
nylamino azavinylogues, the donor moiety being the phenyl-
and diphenylamino group. We decided to investigate the
second-order NLO molecular properties of a class of dipolar
chromophores combining the following design elements:
1) differently substituted hydrazono moieties as strong
donor moieties in dipolar dyes; 2) the pyrrole ring as a co-
donor (or auxiliary donor) unit and 3) strong acceptor
groups. Some of us have previously described the large elec-
tro-optic responses of hydrazono derivatives carrying a pyr-
role ring as co-donor and a pyridine ring as acceptor in hy-
drogen-bonded films grown by physical vapour deposition.[8]
We selected three different acceptor moieties on the basis of
their efficiency and stability in
the donor properties on the NLO properties to be investi-
gated.
Results and Discussion
Synthesis: Mono- and diphenylhydrazonopyrrole derivatives
carrying a terminal acceptor moiety have been scantly re-
ported in the literature.[8] The previously unknown dyes 1–3
were synthesised following the two different general routes
depicted in Scheme 1. Route A is based on the Vilsmeier–
Haack formylation of the push–pull ethene derivative 5 and
the final condensation of 6 with phenylhydrazine or diphe-
nylhydrazine. This route cannot be applied to the synthesis
of chromophore 3b due to the strong accepting properties
of the tricyanofuran moiety, which deactivates the pyrrole
NLO applications: the pyridini-
um group,
a very common
group in stilbazolium and other
efficient NLO derivatives,[2,9]
2,4-dinitrophenyl,[14] and 2-di-
cyanomethylene-3-cyano-4,5,5-
trimethyl-2,5-dihydrofuran,[15]
successfully embedded in
a
number of NLO chromophores
with exceptionally large
values.[2,16]
b
We herein describe the syn-
thesis, optical and nonlinear op-
tical characterisation and ther-
mal stability of the new hydra-
zono chromophores 1a, 2a and
1b–3b. The two sets of dyes a
and b carry a different substitu-
tion pattern on the terminal hy-
drazono moiety, that is, mono-
phenyl- (a) versus diphenylhy-
drazono (b) derivatives, to
enable the effect of tuning of Scheme 1.
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Chem. Eur. J. 2009, 15, 6175 – 6185

Second-Order Nonlinear Optical Activity of Dipolar Chromophores
FULL PAPER
Chromophore 3b was ob-
tained by Route B (Scheme 1)
according to Scheme 4. Diphe-
nylhydrazone 14[17] was submit-
ted to Vilsmeier–Haack formy-
lation to give the corresponding
aldehyde 15. Unfortunately, for-
mylation of the monophenylhy-
drazone derivative 7 (R=H)
gave the N-formyl product,
which prevented the formation
of the monophenylhydrazone
chromophore 3a. Acid-cata-
lysed condensation of 15 with
3-cyano-2-dicyanomethylene-
Scheme 2.
4,5,5-trimethyl-2,5-dihydrofur-
an[16f] afforded the chromo-
phore 3b (isomer A) in good
yield. However, when base-free
microwave-assisted condensa-
tion was performed a different
isomer of 3b (isomer B) was
isolated. NMR spectroscopy,
mass spectrometry and UV/Vis
spectroscopy analysis suggested
that isomers A and B are likely
to be E and Z stereoisomers
around the C=N hydrazone
bond, respectively. All of the
new dyes were characterised by
NMR spectroscopy and high-resolution mass spectrometry.
Scheme 3.
ring towards the Vilsmeier–Haack formylation and thus in-
hibits the formation of 6. We have therefore envisaged the
alternative Route B, which involves formylation of the 5-po-
sition of the pyrrolyl ring of the hydrazone 7 of N-methyl-2-
pyrrolecarbaldehyde and condensation to introduce the ac-
ceptor group.
Route A (Scheme 1) was applied to the synthesis of the
pyridinium derivatives 1a and 1b and the dinitrophenyl de-
rivatives 2a and 2b, according to Schemes 2 and 3, respec-
tively. Acid-catalysed or microwave-promoted condensation
of aldehyde 9[10c] with diphenyl-
1
Selected H NMR chemical shifts of dyes 1a, 2a and 1b–3b
are collected in Table 1.
The 1H NMR spectra of mono- and diphenylhydrazono
derivatives carrying the same acceptor group are rather sim-
ilar to each other. The most apparent difference is the
chemical shift of the olefin proton bound to the carbon–ni-
trogen hydrazone bond, which is centred around d=7.9 ppm
in the monophenylhydrazono dyes and around d=7.0–
7.3 ppm in their diphenylhydrazono counterparts. This effect
or phenylhydrazine gave the
corresponding hydrazones 10a
and 10b. Final alkylation with
methyl triflate afforded the
products 1a and 1b. Alterna-
tively, the hydrazones were
formed from the quaternised
pyridinium precursor 11[10b] di-
rectly generating chromophores
1a and 1b. Similarly, formyla-
tion of the ethene derivative 12
and condensation of the result-
ing aldehyde 13 with the appro-
priate arylhydrazine yielded
dyes 2a and 2b.
Scheme 4.
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6177

A. Abbotto, G. A. Pagani et al.
Table 1. Selected 1H NMR chemical shifts [ppm] of chromophores 1a,
2a and 1b–3b in DMSO.[a]
ꢀ
ꢀ
Compound
a
b
3
4
H C=N
1a
7.94
7.13
6.61
6.99
7.90
A
E
N
ACHTUNGTRENNUNG
2a
7.88
7.15
7.04
7.16
7.32
G
(15.7)
G
ACHTUNGTRENNUNG
1b
7.15[b]
E
ACHTUNGTRENNUNG
2b
N
ACHTUNGTRENNUNG
(E)-3b (isomer A)
(Z)-3b (isomer B)
U
ACHTUNGTRENNUNG
A
E
N
ACHTUNGTRENNUNG
[a] Coupling constants in parentheses [Hz]. [b] Mean value of a multiplet
corresponding to six protons.
Figure 1. PM3-optimised structures of (E)-3b (isomer A) (energies of op-
timised structures of conformers a,
b and g differ by less than
0.5 kcalmolꢀ1).
is likely to be due to the shielding effect of two versus one
phenyl ring bound to the terminal nitrogen atom of the hy-
drazono moiety. In addition, the NMR spectra of the mono-
phenylhydrazono derivatives are characterised by the pres-
ence of a singlet at around d=10 ppm, assigned to the NH
proton of the hydrazono moiety. Rather unexpectedly,
ethene and pyrrole protons are shifted upfield, though very
slightly, on going from the mono- to the corresponding di-
phenylhydrazono derivatives, which suggests a somewhat
stronger donor capacity of the diphenylhydrazono compared
with the monophenylhydrazono moiety. This result is in con-
trast to the hypothesis that the presence of two phenyl rings
in place of one should make the electron pair on the termi-
nal nitrogen atom less available for delocalisation onto the
p framework. In agreement with the NMR spectroscopy re-
sults, the PM3-optimised structures of 3b (Figure 1) show
that the two phenyl rings of the diphenylhydrazono moiety
are not coplanar with the p framework of the rest of the
molecule, thus the adjacent nitrogen electron pair is delocal-
ised less efficiently onto the phenyl rings and allows prefer-
ential delocalisation onto the pyrrole ring. Recent electron-
donor strength studies of aromatic amines have established
that, because of steric effects, the diphenylamino group is a
poorer p donor than dialkylamino when directly bonded to
an aromatic p framework.[18] Our finding, in apparent con-
trast, that diphenylamino behaves as a stronger donor than
monophenylamino can be explained by the presence of a
C=N spacer between the donor and the remaining p frame-
work, thus bypassing group hindrance.
As expected, larger differences are recorded by varying
the acceptor moiety. The effect of the acceptor end group
1
on the H NMR chemical shifts is evident both for the eth-
enyl and the pyrrolyl positions. Analysis of the chemical
shifts in the series 1b–3b shows that positions in compounds
1b and 3b are shifted downfield with respect to 2b, which
confirms that the pyridinium and the dihydrofuran moieties
are stronger acceptors than the dinitrophenyl group. This
conclusion is particularly evident for the ethenyl a position
and the pyrrolyl 4-position, which are the two sites in direct
conjugation (positively charged sites in resonance struc-
tures) with the acceptor group.
1
Comparison of the H NMR spectra of the two isomers of
3b is useful to obtain some insights into their structures.
1
The two H (Figure 2) and 13C NMR (see the Supporting In-
formation) spectra are very similar to each other. The ethen-
1
yl and pyrrolyl H chemical shifts of isomer B are shifted
slightly upfield with respect to isomer A. This result is con-
sistent with a higher donor capacity of the diphenylhydrazo-
no end group in isomer B compared with isomer A. There-
fore, we hypothesise that the two isomers are likely to be
the E (isomer A) and Z (isomer B) geometrical isomers of
3b around the hydrazone p bond. The identical values of
the vicinal coupling constants of the ethene bridge adjacent
to the acceptor group rule out the hypothesis that geometri-
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Second-Order Nonlinear Optical Activity of Dipolar Chromophores
FULL PAPER
Figure 2. Comparison of 1H NMR spectra of the ethenylic and aromatic
region of 3b (isomer A) and 3b (isomer B) (main differences are out-
lined).
cal isomerism involves this double bond. The existence of
the two isomers is supported by the analysis of the chemical
shifts of the methine proton bound to the carbon–nitrogen
hydrazone bond (d=7.16 and 7.32 ppm in isomers A and B,
respectively). Indeed, the shielding effect of the two phenyl
rings is expected to be less efficient in the Z isomer than in
the E isomer.
Linear optical properties: The compounds 1a, 2a and 1b–
3b present an intense transition in the visible region of their
UV/Vis spectra. The absorption data for this band are col-
lected in Table 2 for selected solvents. Figure 3a shows the
Table 2. Solvatochromism of the CT band (lmax [nm]) for the chromo-
phores 1a, 2a and 1b–3b in selected solvents (polarity index is reported
in brackets).
Figure 3. a) UV/Vis spectra of (Z)-3b in different solvents. b) UV/Vis ab-
sorption spectra of 2b (g), (Z)-3b (a) and (E)-3b (c) in 1,4-diox-
ane (T=298 K, ca. 10ꢀ5 m). Compound 1b was not soluble enough in 1,4-
dioxane to obtain the corresponding molar extinction coefficient.
Compound
Dioxane CHCl3 EtOH DMF DMSO Dn
(4.8) (4.1) (5.2) (6.4) (7.2) ]
[cmꢀ1
[a]
A
E
R
G
N
U
1a
1b
2a
2b
522
538
529
528
564
578
538
558
660
597
559
546
519
518
652
605
538
528
545
540
652
590
539
528
554
548
656
596
1427
1286
316
1018
772
Some important insights can be derived by comparing the
spectral position of the CT band as the acceptor or the
donor end group is varied. This absorption band is batho-
chromically shifted on going from chromophores 1 and 2 to
3b. The larger bathochromic effect of the tricyanodihydro-
furan group compared with the other two acceptors is likely
to be the consequence of the stronger electron-withdrawing
strength of the former, which enhances the intramolecular
charge transfer from the donor end group. Furthermore, by
using the tricyanodihydrofuran moiety as an acceptor group
an additional shoulder appears in the CT band, which sug-
gests the existence of overlapping transitions.
The effect of the donor monophenyl- versus diphenylhy-
drazono group (series a vs. series b) depends on the solvent
polarity. For solvents of low polarity (dioxane, CHCl3) the
spectral band position of the diphenylhydrazono derivative
1b is shifted to lower energies with respect to the monophe-
nylhydrazono counterpart 1a. However, with the exception
(E)-3b (isomer A) 629
(Z)-3b (isomer B) 568
855
[a] Dn=n
(dioxane)ꢀn
ACHTUNGTRENNUNG(CHCl3).
absorption spectra of compound (Z)-3b in five solvents of
different polarity (er =2.21–46.7) and Figure 3b compares
the spectra of compounds 2b, (E)-3b and (Z)-3b in 1,4-di-
oxane (the spectra in DMSO are available in the Supporting
Information). All the chromophores show a positive solvato-
chromism in their low-energy absorption band, which indi-
cates that the dipole moment in the excited state is larger
than in the ground state (Table 2).[19,20] The size of the solva-
tochromism suggests that the long-wavelength band possess-
es charge-transfer (CT) character.
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6179

A. Abbotto, G. A. Pagani et al.
of 2 in CHCl3, the band position of 2a is always lower in
energy than that of 2b. The reason for such behaviour is not
obvious, but it clearly suggests a strong perturbation of the
electronic structures of the chromophores and an influence
of the conformational distribution, as shown in Figure 1.
Some other insights into the effect of the configurational
change for 3b become evident by comparing the shapes and
spectral positions of the CT bands of the E and Z isomers
(Table 2, Figure 3). Firstly, the shoulder appears in different
positions: At a lower energy for (Z)-3b and at a higher
energy for (E)-3b with respect to the CT absorption maxi-
mum. Secondly, the maximum of the CT band of (Z)-3b is
always hypsochromically shifted by around 60 nm compared
with (E)-3b, regardless of solvent polarity.
The EOA spectrum LACTHNGUTERNNUG
(f,n˜)e(n˜)n˜ꢀ1 is determined by three
effects: the electrodichroic effect, the Stark effect and the
field dependence of the transition dipole moment. The elec-
trodichroic effect is the dependence of the intensity of ab-
sorption for differently polarised light originated by the in-
duced orientational order of the molecules in the electric
field, modified by the field dependence of the absorption co-
efficient. The electrodichroism is positive (negative) when
the absorption in the electric field increases (decreases) for
parallel (f=08) and decreases (increases) for perpendicular
(f=908) detection in comparison with the isotropic solu-
tion. The Stark effect is the shift of the absorption band in
the presence of the electric field, which only occurs when
the optical excitation is accompanied by a change in the
dipole moment (Dm=meꢀmg). A bathochromic (hypsochro-
mic) shift relative to the isotropic absorption spectrum is
due to an increase (a decrease) in the molecular dipole
moment on going from the ground to the excited state
(Dm>0 and Dm<0, respectively).
Nonlinear optical properties: Second-order molecular NLO
properties have been investigated by electro-optical absorp-
tion (EOA)[21,22] and hyper-Rayleigh scattering (HRS) meas-
urements in 1,4-dioxane and DMSO, respectively.
EOA Measurements: EOA
spectroscopy is
a
suitable
method for obtaining dipole
moments of the ground (mg)
and excited (me) electronic
states and allows for the evalua-
tion of (hyper)polarisabilities of
uniformly polarised absorption
band of a single compound in a
dilute solution in an electric
field E. Experimentally the
very small absorption differ-
ence e ACHTUNGTRENNUNG
with light linearly polarised par-
allel (f=08) and perpendicular
(f=908) to E as a function of
the wavenumber n˜ [Eq. (1)]. eE-
ACHTUNGTRENNUNG(f,n˜) and eACHTUNGTRENNUNG(f,n˜) are the molar
decadic absorption coefficients
of the solution in the presence
and in the absence of the exter-
nally applied electric field, re-
spectively.
E
~
~
e ðf; nÞ ꢀ eðnÞ 1
~
Lðf; nÞ ¼
~
E2
eðnÞ
ð1Þ
The EOA spectra of com-
pounds 2a, 2b, (E)-3b and (Z)-
3b are shown in Figure 4 and
the Supporting Information.
Owing to the applied strong
electric field needed for the
measurements, EOA spectra
cannot be recorded for salt-like
derivatives such as 1.
Figure 4. Fitted EOA spectra (solid lines, left scale) at 298 K in 1,4-dioxane for (E)-3b (a) and (Z)-3b (b). The
~
*
open symbols (
, ,&) show the measured change of absorption for light polarised parallel to the electric field.
~
&
*
,
The filled symbols (
, ) depict the change of absorption for light polarised perpendicular to the electrical
field. Open and full circles and squares have been used to indicate the first and second bands, respectively, in
the different spectral regions used for two independent fits. For comparison, the UV/Vis absorption spectrum
is also shown (dashed lines, right scale).
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Second-Order Nonlinear Optical Activity of Dipolar Chromophores
FULL PAPER
From the multilinear regression coefficients D to H (see
the Experimental Section and the Supporting Information)
the dipole moment of the ground state mg and the dipole
moment difference Dm for 2a and 2b were calculated under
the assumption that the electronic transition/transitions (meg)
are polarised parallel to the dipole moments. This is not
true for (E)-3b and (Z)-3b. In particular, for (Z)-3b the
EOA measurements show that there are two differently po-
larised transitions superposed for which the electric dipole
moment and the transition moment are oriented differently
(Q¼1, see the Supporting Information). Whereas the dipole
differences Dm were calculated from the average values of
the regression coefficients F and G, the ground-state dipole
moment mg was obtained from the difference Eꢀ6D.[23] Sol-
vent effects were corrected by local fields within the approx-
imations of Onsagerꢄs continuum model.[24] The dipole mo-
ments of 2 and 3 increase with the polarity of the solvent,
that is, about 20–30% in 1,4-dioxane solution (see the Sup-
porting Information). Polarisability effects were neglected in
1,4-dioxane. The results of the EOA analysis are collected in
Table 3.
14300–15600 cmꢀ1 and 16400–18900 cmꢀ1), in which the
contribution from two overlapping bands was found not to
be significant.
The dipole moments obtained from the two overlapping
bands are identical for (E)-3b and (Z)-3b within experimen-
tal error. This suggests that the bands must belong to the
same molecule and not to two different species. There are
two possible explanations for these bands: 1) different excit-
ed states are reached upon electronic excitation and 2) the
two transitions have a vibronic origin. The differences in the
dipole changes of the Franck–Condon excited states belong-
ing to the two overlapping bands of the two isomers (DDm=
3.4 and 1.3 debye for (E)-3b and (Z)-3b, respectively; see
Table 3) is probably too large to be of vibronic origin. Unex-
pectedly, (E)-3b and (Z)-3b also possess the same ground-
state dipole moment, which is a restriction to the conclusion
discussed before. On the other hand, the nonparallel orient-
ed transition dipole moment and dipole moments are consis-
tent with the assumption of a Z conformer.
Within the assumption of a one-dimensional chromo-
phore,[25] the static hyperpolarisability tensor coordinate
along the direction mg can be calculated according to the
two-level model [Eq. (2)] by using the Taylor (T) or the phe-
nomenological convention (X).[20,22,26]
Table 3. UV/Vis and EOAM data[a] of 2a, 2b and 3b in 1,4-dioxane.
Compound leg [nm] meg [D] Fitting range [nm] mg[a] [D] Dm [D]
2a
2b
528.5
527.7
8.2
8.4
640–480
620–500
700–635
605–570
700–645
615–490
5.6ꢁ0.1 27.3ꢁ1.4
5.9ꢁ0.1 31.2ꢁ1.4
13.0ꢁ0.1 10.8ꢁ0.4
13.1ꢁ0.1 14.2ꢁ0.5
12.9ꢁ0.1 11.6ꢁ0.2
13.0ꢁ0.1 10.3ꢁ1.1
2
6ðmeglegÞ Dm
ð2Þ
bð00; 0Þ ¼ b0 ¼
2
(E)-3b
(Z)-3b
628.6
568.0
11.2
6.7
ðhcÞ
[a] Measurements have been performed in 1,4-dioxane at a concentration
of about 10ꢀ5 m at 298 K. The solvent effect has been corrected within the
Onsagerꢄs continuum model.
The b0 values for 2a, 2b, (E)-3b and (Z)-3b are collected
in Table 4. Since the transition dipole moment for each
single overlapping transition of 3b is not known, for the cal-
culation of b0 the average value of Dm (Table 3) was used
and assumed that for (E)-3b and (Z)-3b a Two-Level Model
can be applied. According to the figure-of-merit mgb0, the
most promising chromophore for non-linear optical applica-
tions is 3b in its E form, while the less efficient one is 3b in
its Z form.
All the chromophores show positive electrodichroism and
high values of Dm. The latter confirms the CT character of
the lowest-energy transitions. The ground-state dipole
moment in 1,4-dioxane more than doubles by moving from
the dinitrophenyl acceptor to the tricyanodihydrofuran
moiety, whereas the dipole difference Dm becomes much
smaller. This finding is consistent with the hypothesis that
the extent of the charge transfer in their ground states is
higher for derivative 3b than for 2a and 2b. The effect of
the monophenylamino donor compared with the diphenyl-
amino donor group was found to slightly affect the ground-
state dipole moment, in agreement with the results of the
NMR spectroscopy analysis. Compound 2b has an excited-
state dipole moment 4 debye higher than 2a, which could
also be an effect of the stronger donor character of the di-
phenylamino group. The solvatochromism for compounds 2
and 3 (Table 2) is consistent with the approximately equal
product mgDm (Table 3) derived from the EOA spectra.
Table 4. First hyperpolarisabilities and product mgb0 for the chromo-
phores 1a, 2a and 1b–3b from EOA (in dioxane) and HRS (in DMSO)
measurements. All data are solvent corrected by equations derived from
continuum theory.[a] EOA and HRS data have been converted to the
phenomenological convention (X convention).[25]
EOA measurements
b0/10ꢀ30 mgb0/10ꢀ48
[esu] [esu]
HRS measurements
b@1.5 mm/10ꢀ30 b0/10ꢀ30
[esu] [esu]
Compound
G
E
G
ACHTUNGTRENNUNG
[b]
[b]
[c]
[c]
1a
2a
1b
161
723
low[d]
low[d]
[b]
[b]
[c]
[c]
2b
197
194
49
932
2038
510
203
3980
720
89
757
224
(E)-3b (isomer A)
(Z)-3b (isomer B)
As is already clear from the UV/Vis spectra (Figure 3),
the EOA spectra of (E)-3b and (Z)-3b (Figure 4) are a
result of the contribution of at least two overlapping transi-
tions. To apply Equation (1), a multilinear fitting analysis
was performed in two different frequency ranges (ca.
[a] See Table S2 in the Supporting Information and references cited
therein. [b] Salts cannot be investigated by the EOA technique due to
the presence of the strong external electric field. [c] HRS measurements
are not feasible due to strong dye fluorescence. [d] Signal is slightly
above the detection limit of approximately 40.
Chem. Eur. J. 2009, 15, 6175 – 6185
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6181

A. Abbotto, G. A. Pagani et al.
HRS measurements of the first hyperpolarisabilities: The
measurements at a wavelength of 1500 nm were carried out
as previously described.[27] Instead of the third harmonic
(355 nm) generated from an Nd:YAG laser with a wave-
length of 1064 nm, the optical parametric oscillator
(OPO)[28] in use was pumped with the second harmonic
(532 nm). The signal intensity at 824 nm and the fundamen-
tal at 532 nm were removed from the Idler using dichroic
mirrors, a green-light filter (HR 532) and a silicon filter
(HR 650–850; transparent>1000 nm). An additional Glan-
Taylor polariser ensured the vertical polarisation of the
beam into the measurement cell. Measurements were per-
formed with 10ꢀ4–10ꢀ6 m solutions. UV/Vis measurements
displayed a linear correlation between extinction and con-
centration according to Lambert–Beerꢄs Law. Disperse
Red 1 (DR1) was used as an external standard. The hyper-
polarisability was obtained by comparing the slopes of the
Figure 5. Temperature-dependent degradation in air of chromophores 1b
~
&
*
(
), 2b ( ) and 3b isomers A ( ) and B (N) in host–guest polysulfone
films in terms of decreasing of absorption after thermal treatment (A)
with respect to starting value (A0).
reference in CHCl3 and DMSO to obtain the ratio of
[29]
bsolute
.
(CHCl3)=80ꢂ10ꢀ30 esu[30]
By using the value of bACHTUNGRTNEGNU ,
the hyperpolarisability of DR1 in DMSO is estimated to be
253ꢂ10ꢀ30 esu. Interference filters with a bandwidth of 2 nm
were used to ensure that only the frequency doubled radia-
tion at 750 nm was detected. The absence of fluorescence ra-
diation was checked by the use of interference filters at
700 nm. The effect of the refractive indices of the solvents
was corrected by using the simple Lorentz local field.[31] Ex-
perimental static optical first hyperpolarisability b0 values of
HRS are collected in Table 4 and compared with the results
of the EOA spectroscopy.
both isomers of 3b is less than 20% at 2008C, to be com-
pared with a 70% degradation or more of the other com-
pounds. Chromophore (Z)-3b is the most stable compound,
with no significant degradation up to 1608C. The stability is
expected to be significantly improved when the NLOphores
are incorporated in a device-type structure by operating in
an inert gas atmosphere.[16e] This result clearly suggests that
the two dyes 3b are the best candidates to successfully with-
stand poling conditions without significant degradation.[32]
The hyperpolarisability calculated from the EOA is the
contribution of the long-wavelength CT band to the static
b0, which corresponds to the HRS extrapolated to w=0. In
spite of the fact that the contribution of the long-wavelength
CT band is by far the largest contribution from all transi-
tions in the short wavelength region, one has to expect that
these values are normally distinctly smaller than the HRS
results. Because the ratios of the b0 values of the (E)-3b and
(Z)-3b isomers are consistent, the HRS as well as the EOA
results reflect the NLO efficiency of the synthesised com-
pounds. The efficiencies of 2a and 2b obtained from EOA
are of the same size as for (E)-3b (Table 4). Their figures of
merit are smaller than those of (E)-3b because the smaller
dipole moments and their larger Dm do not compensate the
smaller intensity of absorption. The low HRS signal suggests
that the contributions of the higher absorption bands are
smaller in 2a and 2b than in (E)-3b and, thus, the contribu-
tion of the CT band completely determines the static b0.
Conclusions
A combination of organic design and synthesis, NMR inves-
tigation, computations, optical absorption spectroscopy,
EOA and HRS measurements of NLO properties and ther-
mal stability has been applied to the investigation of a new
family of dipolar donor–acceptor compounds based on pyr-
role-hydrazono donor end groups. Different acceptor groups
have been embedded in the dipolar structure to study the
effect on linear and nonlinear optical properties. Despite
the huge number of dipolar compounds showing NLO activ-
ity reported in the literature, the concept of combining the
donor properties of the hydrazono group and the auxiliary
donor effects of electron-rich heteroaromatic rings is unpre-
cedented in the literature, giving rise to the novel family of
NLOphores herein described.
Indeed, this multidisciplinary study has demonstrated that
this class of chromophores is endowed with good to large
second-order molecular NLO activity. The hyperpolarisabili-
ties determined by EOA originate from the contribution of
long-wavelength CT absorption, whereas the total hyperpo-
larisability is obtained as an elastic quantity by the HRS ap-
proach. In many cases such long-wavelength CT band gives
the dominating contribution to the static hyperpolarisability
in comparison to the effect of all other transitions from the
shorter wavelength region. Then, the NLO properties mea-
Thermal stability: Thermal stability of chromophores 1b–3b
was checked by isothermal heating in the air of host–guest
polysulfone films doped with 50 wt% of chromophore. Rel-
ative degradation of host–guest films as a function of tem-
perature is depicted in Figure 5. It is evident that thermal
stability is strongly dependent on the nature of the acceptor
moiety. Both isomers 3b, carrying the tricyano-dihydrofuran
group, are found to be much more stable than the pyridini-
um and dinitrophenyl derivatives. Thermal degradation of
6182
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Chem. Eur. J. 2009, 15, 6175 – 6185

Second-Order Nonlinear Optical Activity of Dipolar Chromophores
FULL PAPER
(39000mꢀ1 cmꢀ1); HR-ESIMS: m/z calcd for C20H21N4: 317.17607
[MꢀCF3SO3]+; found: 317.17581.
Method B: A solution of p-toluenesulfonic acid (few mg) in EtOH
(3 mL) was added dropwise to a solution of 11[10b] (0.256 g, 0.68 mmol)
and N-phenylhydrazine (0.076 g, 0.70 mmol) in EtOH (20 mL). After stir-
ring overnight at room temperature the solvent was partially removed
and the precipitate collected to give the product (0.156 g, 0.33 mmol,
48%).
sured by the EOA and HRS investigation correlate to each
other as previously experimentally shown by Wortmann and
Goebel.[33] The different solvents may also account for the
deviations in the two types of measurements. Both methods
show that chromophore (E)-3b has the largest NLO activity
in the series.
The new efficient NLOphore combines three strategically
important design contributions: 1) the use of diphenylhydra-
zono as an efficient donor group; 2) the use of 2-dicyano-
methylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran as effi-
cient acceptor groups; 3) the use of the electron-rich pyrrole
ring as an efficient auxiliary donor spacer. The three sub-
units are also combined in the isomer (Z)-3b, but in this
case the NLO activity is approximately four-fold smaller, as
measured by both EOA and HRS studies, which is likely to
be due to the effect of the cis double-bond arrangement on
the p-conjugated framework. The first hyperpolarisability of
compound (E)-3b is very promising for its use in electro-op-
tical devices. Investigation of bulk NLO properties of poly-
meric and sol–gel host–guest and side-chain materials based
on the most performing chromophores of this series is in
progress.
2-(N,N-Diphenylhydrazonomethyl)-N-methyl-5-[2-(pyrid-4-yl)vinyl]pyr-
role (10b): A mixture of N,N-diphenylhydrazine hydrochloride (0.520 g,
2.36 mmol) and aldehyde 9[10c] (0.250 g, 2.36 mmol) in EtOH (5 mL) was
heated in a microwave oven under stirring (150 W, 15 min). The deep
violet solid was collected by filtration and washed with H2O (15 mL) to
give 10b·HCl (0.712 g, 1.72 mmol); 1H NMR (500 Hz, [D6]DMSO): d=
8.67 (d, J=6.2 Hz, 2H), 8.10 (d, J=6.1 Hz, 2H), 7.95 (d, J=15.7 Hz,
1H), 7.44 (t, J=7.7 Hz, 4H), 7.23 (t, J=7.1 Hz, 2H), 7.18–7.05 (m, 6H),
6.97 (d, J=4.1 Hz, 1H), 6.51 (d, J=4.0 Hz, 1H), 4.00 ppm (s, 3H). The
product 10b was obtained by eluting a solution of its hydrochloride in
MeOH through basic Amberlite and upon removal of the solvent under
reduced pressure (0.336 g, 0.89 mmol, 38%). The compound was used in
the subsequent step without any further purification.
2-(N,N-Diphenylhydrazonomethyl)-N-methyl-5-[2-(N-methylpyrid-4-yl)-
vinyl]pyrrole triflate (1b)
Method A: A solution of methyl triflate (0.150 g, 0.91 mmol) in anhy-
drous CH3CN (2 mL) was added dropwise to a solution of 10b (0.266 g,
0.70 mmol) in the same solvent (45 mL). After stirring overnight the sol-
vent was evaporated under reduced pressure to give the product as a
deep violet solid (0.140 g, 0.26 mmol, 37%): M.p. 2758C (EtOH);
1H NMR (500 MHz, [D6]DMSO): d=8.67 (d, J=6.6 Hz, 2H), 8.10 (d, J=
6.6 Hz, 2H), 7.96 (d, J=15.7 Hz, 1H), 7.48 (t, J=7.8 Hz, 4H), 7.24 (t, J=
7.4 Hz, 2H), 7.20–7.10 (m, 6H), 6.97 (d, J=4.2 Hz, 1H), 6.55 (d, J=
4.2 Hz, 1H), 4.17 (s, 3H), 3.99 ppm (s, 3H); UV/Vis (DMSO): lmax (e)=
528 nm, (40100mꢀ1 cmꢀ1); HR-ESIMS: m/z calcd for C26H25N4: 393.20737
[MꢀCF3SO3]+; found: 393.20704.
Experimental Section
General: 1H NMR coupling constants are presented in Hertz. Extracts
were dried over Na2SO4. Anhydrous DMF was supplied by FLUKA and
stored over molecular sieves. Anhydrous acetonitrile was supplied by Al-
drich and stored under nitrogen. All other commercial chemicals were
supplied by Aldrich and used without purification. Melting points are un-
corrected. NMR spectra were recorded with a Bruker 500 MHz spec-
trometer. UV/Vis spectra were performed with a Jasco V570 spectropho-
tometer at room temperature. HR-ESIMS were carried out using a
Bruker ICR-FTMS Apex II spectrometer.
Method B:
A suspension of N,N-diphenylhydrazine hydrochloride
(0.154 g, 0.70 mmol) in EtOH (5 mL) was added dropwise to a solution
of 11[10b] (0.256 g, 0.68 mmol) in EtOH (20 mL). After stirring for 4 h at
room temperature the solvent was partially removed to give the product
as a precipitate, which was collected by filtration (0.180 mg, 0.33 mmol,
48%).
N-Methyl-2-(N-phenylhydrazonomethyl)-5-[2-(pyrid-4-yl)vinyl]pyrrole
(10a): A solution of N-phenylhydrazine (0.330 g, 3.07 mmol) in EtOH
(5 mL) was added dropwise to a mixture of aldehyde 9[10c] (0.650 g,
3.07 mmol) and p-toluenesulfonic acid (0.530 g, 3.07 mmol) in EtOH
(50 mL). After stirring for 3 days at room temperature the deep-red pre-
cipitate was collected by filtration. A solution of the precipitate in
AcOEt (200 mL) was filtered through basic Amberlite. The solvent was
evaporated under reduced pressure to give the product as a red-orange
solid (0.461 g, 1.52 mmol, 50%). M.p. 1688C; 1H NMR (500 MHz,
[D6]DMSO): d=10.10 (s, 1H), 8.50 (d, J=6.0 Hz,2H), 7.87 (s, 1H), 7.55
(d, J=6.0 Hz, 2H), 7.53 (d, J=16.5 Hz, 1H), 7.22 (t, J=7.8 Hz, 2H), 6.99
(d, J=8.0 Hz, 2H), 6.98 (d, J=16.5 Hz, 1H), 6.74 (d, J=4.5 Hz, 1H),
6.73 (t, J=7.3 Hz, 1H), 6.45 (d, J=4.1 Hz, 1H), 4.00 ppm (s, 3H); HR-
ESIMS: m/z calcd for C19H19N4: 303.16097 [M+H]+, 325.14292 [M+
Na]+; found: 303.16048, 325.14240.
2-(2,4-Dinitrostyryl)-N-methylpyrrole (12): Piperidine (6 mL) was added
to a solution of 2,4-dinitrotoluene (1.00 g, 5.50 mmol) and N-methylpyr-
role-2-carbaldehyde (1.20 g, 11.0 mmol) in toluene (20 mL). After stirring
for 6 h at 808C the red precipitate was collected by filtration and washed
with hexane to yield the product as a red solid (0.870 g, 3.19 mmol,
58%). M.p. 1768C; 1H NMR (500 MHz, [D6]DMSO): d=8.70 (d, J=
2.4 Hz, 1H), 8.41 (dd, J=9.0, 2.4 Hz, 1H), 8.33 (d, J=9.0 Hz, 1H), 7.61
(d, J=15.8 Hz, 1H), 7.15 (d, J=15.8 Hz, 1H), 6.99 (dd, J=2,1, 1.8 Hz,
1H), 6.70 (dd, J=3.9, 1.5 Hz, 1H), 6.15 (dd, J=3.6, 2.7 Hz, 1H),
3.78 ppm (s, 3H).
5-(2,4-Dinitrostyryl)-N-methylpyrrole-2-carbaldehyde (13): Freshly dis-
tilled POCl3 (1.41 g, 9.23 mmol) was added dropwise to anhydrous DMF
(0.67 g, 9.2 mmol) at ꢀ208C under a dry atmosphere. The resulting solu-
tion was stirred at 08C until a glassy white solid was obtained, which was
dissolved in anhydrous CH3CN (3 mL). A suspension of 12 (1.80 g,
6.59 mmol) in anhydrous CH3CN (200 mL) was added dropwise to the re-
sulting mixture at 08C over a period of 15 min. After stirring overnight
at room temperature, the reaction mixture was poured into a 10% aque-
ous solution of K2CO3 (100 mL) and, after 1 h, the red precipitate was
collected by filtration and washed with H2O (100 mL) to give the product
N-Methyl-2-(N-phenylhydrazonomethyl)-5-[2-(N-methylpyrid-4-yl)vinyl]-
pyrrole triflate (1a)
Method A: A solution of methyl triflate (0.172 g, 1.06 mmol) in anhy-
drous CH3CN (5 mL) was added dropwise to a solution of 10a (0.320 g,
1.06 mmol) in the same solvent (45 mL). After stirring overnight the sol-
vent was evaporated under reduced pressure to leave a solid that was dis-
solved in EtOH (10 mL) to give the product as a dark violet solid
(0.359 g, 0.77 mmol, 73%). M.p. 1788C; 1H NMR (500 MHz,
[D6]DMSO): d=10.46 (s, 1H), 8.65 (d, J=6.7 Hz, 2H), 8.08 (d, J=
6.7 Hz, 2H), 7.94 (d, J=15.7 Hz, 1H), 7.90 (s, 1H), 7.21 (t, J=7.8 Hz,
2H), 7.13 (d, J=15.7 Hz, 1H), 7.01 (d, J=8.0 Hz, 2H), 6.99 (d, J=
4.2 Hz, 1H), 6.77 (t, J=7.3 Hz, 1H), 6.61 (d, J=4.2 Hz, 1H), 4.15 (s,
3H), 3.99 ppm (s, 3H); UV/Vis (EtOH): lmax (e)=559 nm
1
as a red solid (1.51 g, 5.01 mmol, 76%). M.p. 1998C; H NMR (500 MHz,
[D6]DMSO): d=9.59 (s, 1H), 8.76 (d, J=2.4 Hz, 1H), 8.52 (dd, J=6.2,
2.6 Hz, 1H), 8.41 (d, J=8.8 Hz, 1H), 7.72 (d, J=15.9 Hz, 1H), 7.53 (d,
J=15.9 Hz, 1H), 7.11 (d, J=4.4 Hz, 1H), 6.88 (d, J=4.2 Hz, 1H),
4.07 ppm (s, 3H).
5-(2,4-Dinitrostyryl)-N-methyl-2-[(N-phenylhydrazono)methyl]pyrrole
(2a): p-Toluenesulfonic acid (few mg) has been added to a mixture of N-
Chem. Eur. J. 2009, 15, 6175 – 6185
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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6183

A. Abbotto, G. A. Pagani et al.
phenylhydrazine (0.029 g, 0.26 mmol) and aldehyde 13 (0.80 g,
0.26 mmol) in EtOH (10 mL). After heating at reflux for 4 h the mixture
was poured into a saturated aqueous solution of K2CO3 (20 mL) and ex-
tracted with CH2Cl2 (3ꢂ15 mL). The organic layers were combined,
dried and evaporated to dryness to leave the product as a violet solid
(0.070 g, 0.18 mmol, 69%). M.p. 2308C; 1H NMR (500 MHz,
[D6]DMSO): d=10.27 (s, 1H), 8.70 (d, J=2.0 Hz, 1H), 8.43 (dd, J=9.0,
2.0 Hz, 1H), 8.38 (d, J=9.0 Hz, 1H), 7.88 (s, 1H), 7.70 (d, J=15.7 Hz,
1H), 7.27 (d, J=15.7 Hz, 1H), 7.23 (t, J=7.8 Hz, 2H), 7.01 (d, J=8.0 Hz,
2H), 6.83 (d, J=4.1 Hz, 1H), 6.75 (t, J=7.5 Hz, 1H), 6.53 (d, J=4.2 Hz,
1H), 4.02 ppm (s, 3H); UV/Vis (DMSO): lmax (e)=554 nm,
(24000mꢀ1 cmꢀ1); HR-ESIMS: m/z calcd for C20H17N5O4Na: 414.11728
[M+Na]+; found: 414.11746.
ESIMS: m/z calcd for C30H24N6ONa: 507.19038 [M+Na]+; found:
507.19117.
Semiempirical calculations: Geometries have been fully optimised using
the PM3 Hamiltonian as available in the Gaussian 03 program pack-
age.[35] Different structures have been used as starting geometries. Opti-
misation was carried out with no symmetry constraints.
EOA measurements: All EOA and UV/Vis spectra were recorded in an-
hydrous 1,4-dioxane prepared by distillation from Na under Argon prior
to use. The UV/Vis spectra required for the evaluation of the integral ab-
sorption, band maxima and the first [t(n˜)] and second [u(n˜)]derivatives of
the optical absorption spectrum were recorded with a Perkin–Elmer
Lambda 900 spectrophotometer at 298 K (scan speed of 50 nmminꢀ1
;
3 cm quartz glass cuvette). The EOA signal has been recorded with an in-
tegration time of 0.5–1 min (depending on the quality of the signal) and
p-amino-p’-nitrobiphenyl was used as a calibrating reference. From multi-
linear fittings of Len˜ꢀ1 as a function of the first [t(n˜)] and second [u(n˜)]
derivatives of the optical absorption spectrum five parameters, E, D, F,
G, H and I, can be obtained and are the basis for the evaluations of the
dipole moments mg and Dm=meꢀmg and by this me along the transition
dipole moment direction.
5-(2,4-Dinitrostyryl)-N-methyl-2-[(N,N-diphenylhydrazono)methyl]pyr-
role (2b): A mixture of N,N-diphenylhydrazine hydrochloride (0.220 g,
1.00 mmol) and aldehyde 13 (0.300 g, 1.00 mmol) was suspended in
EtOH (30 mL) and heated at reflux for 4 h. The solid was collected by
filtration and washed with hot H2O (20 mL) to give the product as a
deep violet solid (0.230 g, 0.49 mmol, 49%). M.p. 1938C; 1H NMR
(500 MHz, [D6]DMSO): d=8.71 (d, J=2.4 Hz, 1H), 8.43 (dd, J=9.0,
2.5 Hz, 1H), 8.39 (d, J=9.0 Hz, 1H), 7.70 (d, J=15.5 Hz, 1H), 7.49 (t,
J=7.9 Hz, 4H), 7.27 (d, J=15.5 Hz, 1H), 7.23 (t, J=7.4 Hz, 2H), 7.15 (d,
J=6.5 Hz, 4H), 7.14 (s, 1H), 6.80 (d, J=4.2 Hz, 1H), 6.46 (d, J=4.2 Hz,
1H), 4.01 ppm (s, 3H); UV/Vis (DMSO) lmax (e)=548 nm,
(26000mꢀ1 cmꢀ1); HR-ESIMS: m/z calcd for C26H21N5O4Na: 490.14858
[M+Na]+; found: 490.14893.
Thermal stability studies of compounds 1b–3b: Cast or spin-coated films
on glass substrates of host-guest polymer composites were prepared by
doping commercial polysulfone (Aldrich, average Mn ca. 22000) with
50 wt% chromophore. Films were heated at 100, 120, 140, 160, 180 and
2008C for 20 min at each temperature value in the air. Thermal degrada-
tion of the chromophore in the host polymeric matrix was monitored by
measuring the decrease of the UV/Vis absorption at lmax
.
5-[(N,N-Diphenylhydrazono)methyl]-N-methylpyrrole-2-carbaldehyde
(15): Freshly distilled POCl3 (0.62 g, 4.0 mmol) was added dropwise to
anhydrous DMF (0.31 mL) under a dry atmosphere at ꢀ108C. After
30 min the glassy solid that formed was dissolved in dry 1,2-dichloro-
ethane (20 mL). A solution of 14[17] (1.00 g, 3.66 mmol) in dry 1,2-di-
chloroethane (20 mL) at room temperature was then added dropwise.
After stirring overnight at room temperature, the reaction mixture was
treated with CH2Cl2 (20 mL) and poured into a saturated aqueous solu-
tion of K2CO3 (100 mL). The organic layer was separated, dried and
evaporated to dryness to leave a dark-yellow oily residue which was fil-
trated through silica gel (CH2Cl2). The solvent was removed under re-
duced pressure to give the product as a yellow solid (1.05 g, 3.46 mmol,
95%): mp 1048C;[34] 1H NMR ([D6]DMSO) d 9.52 (s, 1H), 7.49 (t, 4H,
J=7.8), 7.26 (t, 2H, J=7.4), 7.17 (d, 4H, J=8.0), 7.15 (s, 1H), 7.02 (d,
1H, J=4.2), 6.55 (d, 1H, J=4.2), 4.08 ppm (s, 3H).
Acknowledgements
Research was supported by the European Commission through NMP3-
CT-2003-505478 contract (STRP-ODEON) and MIUR-FIRB
RBNE033KMA (Molecular compounds and hybrid nanostructured mate-
rials with resonant and non-resonant optical properties for photonic devi-
ces).
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5-[2-(3-Cyano-2-dicyanomethylene-5,5-dimethyl-2,5-dihydrofuran-4-yl)-
vinyl]-2-(N,N-diphenylhydrazonomethyl)-N-methylpyrrole—Isomer
A
(3b-isomer A): A solution of AcONH4 (7 mg) and AcOH (20 mg) in
EtOH (1 mL) was added to a solution of aldehyde 15 (1.10 g, 3.62 mmol)
[2] For reviews, see: a) L. R. Dalton, J. Phys. Condens. Matter 2003, 15,
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and
3-cyano-2-dicyanomethylene-4,5,5-trimethyl-2,5-dihydrofuran[16f]
(0.722 g, 3.62 mmol) in the same solvent (130 mL). After stirring for 18 h
at 608C, the green precipitate was isolated by filtration and washed with
hot H2O (20 mL) to afford the product as
a green solid (0.770 g,
1.59 mmol, 44%). M.p. 2708C (d); 1H NMR (500 MHz, [D6]DMSO): d=
7.99 (d, J=15.5 Hz, 1H), 7.53 (d, J=4.7 Hz, 1H), 7.52 (t, J=7.5 Hz, 4H),
7.30 (t, J=7.4 Hz, 2H), 7.21 (d, J=7.8 Hz, 4H), 7.16 (s, 1H), 6.86 (d, J=
15.5 Hz, 1H), 6.84 (d, J=4.7 Hz, 1H), 3.89 (s, 3H), 1.72 ppm (s, 6H);
UV/Vis (DMSO) lmax (e)=656 nm, (63900mꢀ1 cmꢀ1); HR-ESIMS m/z
calcd for C30H24N6ONa: 507.19038 [M+Na]+; found: 507.19091.
5-[2-(3-Cyano-2-dicyanomethylene-5,5-dimethyl-2,5-dihydrofuran-4-yl)-
vinyl]-2-(N,N-diphenylhydrazonomethyl)-N-methylpyrrole—Isomer
B
(3b-isomer B): A mixture of aldehyde 15 (1.05 g, 3.46 mmol) and 3-
cyano-2-dicyanomethylene-4,5,5-trimethyl-2,5-dihydrofuran[16f] (0.689 g,
3.46 mmol) in EtOH (20 mL) was heated at reflux under microwave con-
ditions in an open vessel at a constant power of 40 W for 15 min with si-
multaneous cooling and stirring. After cooling to 08C the precipitate was
collected by filtration to give the product as a deep-blue solid (1.10 g,
2.27 mmol, 66%). M.p. 251–2528C; 1H NMR (500 MHz, [D6]DMSO): d=
7.86 (d, J=15.5 Hz, 1H), 7.51 (t, J=7.8 Hz, 4H), 7.32 (s, 1H), 7.28 (t, J=
7.4 Hz, 2H), 7.18 (d, J=6.7 Hz, 4H), 7.12 (d, J=2.8 Hz, 1H), 6.84 (d, J=
2.8 Hz, 1H), 6.73 (d, J=15.5 Hz, 1H), 3.83 (s, 3H), 1.61 ppm (s, 6H);
UV/Vis (DMSO) lmax (e)=596 nm, ((26600ꢁ900)mꢀ1 cmꢀ1); HR-
6184
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ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 6175 – 6185

Second-Order Nonlinear Optical Activity of Dipolar Chromophores
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Received: February 3, 2009
Published online: May 6, 2009
Chem. Eur. J. 2009, 15, 6175 – 6185
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6185