A joint experimental and theoretical investigation on nonlinear optical (NLO) properties of a new class of push-pull spirobifluorene compounds

Article abstract of DOI:10.1002/ejoc.201000335

A new class of push-pull spirobifluorene compounds 1-5 has been synthesized. The nonlinear optical (NLO) properties (first and second hyperpolarizability β and γ, respectively) of spirobifluorene derivatives have been investigated for the first time using electric field induced second harmonic (EFISH) and third harmonic generation (THG) methods. Moreover, a comparison with the corresponding push-pull fluorene monomers 1a-5a was carried out. Besides the experimental data, ab initio theoretical calculations on structural, electronic, and optical properties were carried out for all compounds. The analyses indicate that an increase in the β and γ values occur on passing from monomers to dimers without compromising the optical transparency.

Full text of DOI:10.1002/ejoc.201000335

FULL PAPER
DOI: 10.1002/ejoc.201000335
A Joint Experimental and Theoretical Investigation on Nonlinear Optical
(NLO) Properties of a New Class of Push–Pull Spirobifluorene Compounds
Fabio Rizzo,[a,b] Marco Cavazzini,[a,b] Stefania Righetto,[c] Filippo De Angelis,[d]
Simona Fantacci,[d] and Silvio Quici*[a,b]
Keywords: Spiro compounds / Nonlinear optics / Ab initio calculations / Push–pull molecules
A new class of push–pull spirobifluorene compounds 1–5 has
been synthesized. The nonlinear optical (NLO) properties
(first and second hyperpolarizability β and γ, respectively)
of spirobifluorene derivatives have been investigated for
the first time using electric field induced second harmonic
(EFISH) and third harmonic generation (THG) methods.
Moreover, a comparison with the corresponding push–pull
fluorene monomers 1a–5a was carried out. Besides the exper-
imental data, ab initio theoretical calculations on structural,
electronic, and optical properties were carried out for all
compounds. The analyses indicate that an increase in the β
and γ values occur on passing from monomers to dimers
without compromising the optical transparency.
chromophores with D-π-A linear structure increases upon
increasing either the length of the π-conjugated system or
the strength of both donor and acceptor groups.
The organization of push–pull chromophores in structur-
ally oriented molecular architectures can be another inter-
esting factor that can be used to amplify the non linear
optical properties.
In molecular multichromophoric systems, the overall hy-
perpolarizability β arises from the contributions of the β
value of each of the chromophoric units along the molecu-
lar dipole axis, whereas in the case of bulk materials, the
NLO susceptibility is related to the β value of the chromo-
phores and on their degree of orientation in the multilayer.
In this sense, data has been reported in the literature con-
cerning multichromophoric molecular systems such as ca-
lixarenes[8–10] and cyclotetrasiloxane rings,[11] or multilay-
ered organized systems, where NLO molecules have be as-
sembled by using zirconium phosphate–phosphonate
(ZrPOx) techniques,[12–14] layer-by-layer siloxane-based self-
assembly,[15] or Langmuir–Blodgett techniques.[16]
Introduction
In recent years the design and synthesis of organic com-
pounds that exhibit nonlinear optical (NLO) properties has
received particular attention because of their possible appli-
cations in various fields, including telecommunications and
optical data storage and processing.[1] Much effort have
been aimed at identifying relationships between organic
structure and nonlinear optical activities so that these sys-
tems can be made more efficient.[2–6] In particular, since the
nonlinear optical properties of a compound stems from a
nonlinear polarization induced by a beam of high intensity
(typically a laser), it was soon clear that the ability of a
compound to show second order NLO phenomena should
be related to the degree of charge separation in the ground
state. Until now, most of the organic molecules charac-
terized by second order NLO properties contain electron-
donating (D) and electron-withdrawing (A) groups linked
through a conjugated π-spacer (“push–pull” systems). Ac-
cording to the Oudar two-states model,[7] it is well-estab-
lished that the first order hyperpolarizability β of push–pull
An interesting way to prepare geometrically oriented
push–pull bichromophoric systems is to covalently arrange
them into a highly delocalized structure with an orthogonal
spatial arrangement, as in the case of 9,9Ј-spirobifluorene,
where two fluorenes are connected through an sp3-hy-
[a] Istituto di Scienze e Tecnologie Molecolari (ISTM), Consiglio
Nazionale delle Ricerche,
via C. Golgi 19, 20133 Milano, Italy
[b] Polo Scientifico e Tecnologico (PTS), Consiglio Nazionale delle bridized carbon atom (Figure 1).
Ricerche (CNR),
Indeed, the chemical versatility of spirobifluorene allows
via Fantoli 16/15, 20138 Milano, Italy
Fax: +39-02-50314159
the introduction of different chemical functional groups in
the 2,2Ј and 7,7Ј positions, thus affording symmetric or
asymmetric compounds.[17] Therefore, when electron-donat-
ing and electron-accepting groups are introduced in these
positions, the resulting molecule formerly consists of two
push–pull halves with a mutually perpendicular arrange-
ment with a non-vanishing resulting dipole moment.[18] So
E-mail: silvio.quici@istm.cnr.it
[c] Dipartimento CIMA “Lamberto Malatesta”, Università degli
Studi di Milano and UdR dell’INSTM,
via Venezian 21, 20133, Milano, Italy
[d] Istituto di Scienze e Tecnologie Molecolari (ISTM),
c/o Dipartimento di Chimica, Università degli Studi di Perugia,
Consiglio Nazionale delle Ricerche,
via Elce di Sotto 8, 06123 Perugia, Italy
4004
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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NLO Properties of New Spirobifluorene Compounds
Results and Discussion
Synthesis of Spirobifluorenes 1–5 and Related Fluorenes
1a–5a
The push–pull spirobifluorene compounds 1–5 were de-
signed to gain information on the effect of conjugation and
on the relationship between the donor moieties and the
NLO properties. A disubstituted amino group (dialkyl or
diaryl) and a nitro group were chosen as electron-donating
and electron-withdrawing substituents, respectively.
Furthermore, we decided to introduce a structural modifi-
cation by varying the degree of unsaturation in the π-spa-
cers that connect the amino group to the spirobifluorene
Figure 1. Schematic representation of 9,9Ј-spirobifluorene and syn- unit. The synthetic strategy for the preparation of com-
thesized push–pull derivatives 1–5.
pounds 1–5 involved the synthesis of the key intermediate
2,2Ј-dibromo-7,7Ј-dinitro-9,9Ј-spirobifluorene (8), as re-
ported in Scheme 1.
this molecule represents an unconventional push–pull sys-
tem that, in principle, can show interesting second order
nonlinear optical properties.
Although several spirobifluorene compounds bearing
donor and acceptor residues have been synthesized and em-
ployed for example as dyes[19–21] or as ambipolar trans-
porting molecules,[22,23] to the best of our knowledge, no
experimental data on their hyperpolarizability β have yet
been reported. The first hyperpolarizability of spiro-linked
push–pull polyenes, where several 1,4-cyclohexadiene rings
are connected by the tetrahedral sp3 carbon atoms and elec-
tron-donating and electron-withdrawing groups are intro-
duced in the terminal 1,4-cyclohexadiene rings, has been
investigated by using ab initio and INDO/S molecular or-
bital calculations.[24] By comparing the obtained results for
the spiro-linked polyenes with those calculated for the cor-
responding linear polyenes, it was concluded that these
compounds behave as excellent second-order nonlinear op-
tical materials from the standpoint of transparency and rel-
atively large β values.[25] The second hyperpolarizability γ of
2,2Ј-diamino-7,7Ј-dinitro-9,9Ј-spirobifluorene has been ex-
perimentally measured through a Degenerate Four Wave
Mixing (DFWM) procedure.[26] The value obtained was ten
Scheme 1. Synthesis of 2,2Ј-dibromo-7,7Ј-dinitro-9,9Ј-spirobifluo-
rene (8).
times that of the corresponding 2-amino-7-nitrofluorene
monomer. Theoretical calculations reported for this same
compound showed that intermolecular vibrational modes
significantly contribute to the gas-phase second hyperpolar-
9,9Ј-Spirobifluorene (6) was prepared starting from com-
izability, although not to an extent that can explain the ex- mercially available 2-bromobiphenyl.[29] The reaction of the
perimentally observed increase in the value of the dimer Grignard derivative of 2-bromobiphenyl with 9-fluorenone
with respect to the monomer.[27,28]
was followed by a condensation under acidic conditions to
In this paper we report the synthesis of five new spirobi- give 6 in 70% overall yield. Although the direct bromina-
fluorene push–pull derivatives 1–5, bearing a disubstituted tion of 9,9Ј-spirobifluorene is described in literature,[29] we
amine and a nitro group in the 2,2Ј- and 7,7Ј-positions, had difficulty isolating pure 2,2Ј-dibromo-9,9Ј-spirobifluo-
respectively (Figure 1), in which electronic connection is rene as also reported by other groups.[30] On the other hand,
guaranteed by a range of π-bridges. In order to gain a better following the procedure described by Sutcliffe et al.,[31]
6
insight into the linear and nonlinear optical properties of was easily subjected to double electrophilic aromatic substi-
these bichromophoric systems, and to compare the related tution to afford 2,2Ј-dinitro-9,9Ј-spirobifluorene (7) in a re-
push–pull fluorene monomers 1a–5a, a joint spectroscopic, gioselective manner. This approach not only allows intro-
experimental and theoretical approach was undertaken. duction of the desired electron-withdrawing group but also
The results are presented here.
regiospecifically drives the subsequent electrophilic bromi-
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S. Quici et al.
FULL PAPER
nation towards the 7,7Ј-positions. As expected, the presence
The application of Suzuki cross-coupling conditions, i.e.,
of the nitro groups prevents the formation of undesired by- [Pd(PPh3)4] as catalyst and aqueous K2CO3 as base, pro-
products on phenyl rings that already bear nitro functions. vided an easy way to interpose a single phenyl ring between
2,2Ј-Dibromo-7,7Ј-dinitro-9,9Ј-spirobifluorene (8) was thus the donor and acceptor groups. The reaction of 8 with 4-
obtained by reaction of 7 with bromine, in the presence of (N,N-diphenylamino)phenyl boronic acid (9) afforded 2,2Ј-
a catalytic amount of ferric chloride, in almost quantitative dinitro-7,7Ј-bis(4-N,N-diphenylaminophenyl)-9,9Ј spirobi-
yield.
fluorene (2) in 77% yield. Using the same synthetic pro-
The synthesis of chromophores 1–5 is reported in cedure, the reaction of 8 and 4-(dimethylamino)phenyl
Scheme 2. We were able to prepare compounds with a range boronic acid (10)[32] gave 2,2Ј-bis[4-(dimethylamino)phen-
of conjugation lengths through several palladium-catalyzed yl]-7,7Ј-dinitro-9,9Ј-spirobifluorene (3) in 67% yield.
cross-coupling reactions between 8 and a suitable amino
In addition to changing the electron-donating properties
intermediate. In order to estimate the influence of the elec- of the amino residue, we decided to introduce a double and
tronic connection between the acceptor and donor moieties a triple bond between the dimethylaminophenyl portion
in the spirobifluorene spacer itself, we synthesized 2,2Ј-dini- and the nitrospirobifluorene to verify the effect of increased
tro-7,7Ј-dipiperidin-1-yl-9,9Ј-spirobifluorene (1) in 53% conjugation length on the NLO properties. In particular,
yield by Hartwig–Buchwald amination of 8 with piperidine, (E,E)-2,2Ј-bis{2-[4-(dimethylamino)phenyl]ethen-1-yl}-7,7Ј-
in the presence of Cs2CO3 as base and a combination of dinitro-9,9Ј-spirobifluorene (4) was synthesized in 44 %
tris(dibenzylidenacetone)palladium [Pd2(dba)3] and tris- yield by Heck coupling between 8 and N,N-dimethyl-4-
tert-butylphosphane (PtBu3) as catalyst.
vinylaniline (11)[33] in anhydrous dimethylformamide
Scheme 2. Synthesis of push–pull spirobifluorenes 1–5.
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NLO Properties of New Spirobifluorene Compounds
(DMF) with Pd(OAc)2 as catalyst and K2CO3 as base, in Linear Optical Properties
presence of tetrabutylammonium bromide (TBAB). We ap-
To gain information on the electronic connection be-
plied the Sonogashira coupling as described in the literature
for different spirobifluorene derivatives[34] to generate 2,2Ј-
bis{2-[4-(dimethylamino)phenyl]ethyn-1-yl}-7,7Ј-dinitro-
9,9Ј-spirobifluorene (5) in 46% yield. The reaction was per-
formed in anhydrous DMF between 8 and 4-ethynyl-N,N-
dimethylaniline (12) by using bis(triphenylphosphane)palla-
dium(II) dichloride [Pd(PPh3)2Cl2] as catalyst and triethyl-
amine as base. In this case, the use of a copper salt as co-
catalyst was not necessary, probably because of the very
high reactivity of 12.
tween donor and acceptor groups, experimental UV/Vis ab-
sorption spectra and time-dependent DFT (TDDFT) calcu-
lations of all compounds were performed. As reported in
Figure 2, the absorption spectra of spirobifluorenes 1–5 in
CHCl3 solutions showed the presence of two broad bands,
one lying in the UV region (270–325 nm) that can be attrib-
uted to π–π* transitions and another at lower energy (410–
438 nm) assigned to charge-transfer transitions (CT).
To study the relationship between β and γ values and the
number of chromophores present in the spiro system, we
synthesized the corresponding monomers 1a–5a through
analogous synthetic pathways (Scheme 3). These com-
pounds were prepared starting from 2-iodo-9,9-dimethyl-7-
nitro-9H-fluorene (13),[35] which in turn was obtained by
reaction of 2-iodo-7-nitro-9H-fluorene[36] with methyl io-
dide in dimethyl sulfoxide (DMSO) in the presence of KOH
as base.
Figure 2. Absorption profiles of spirobifluorenes 1–5 (top) and
fluorenes 1a–5a (bottom).
The absorption maxima wavelengths of 1 coincides with
that of the reference monomer 1a (Table 1). Replacement
of the piperidinyl donor groups with triphenylamines in spi-
robifluorene 2 and fluorene 2a leads to a hypsochromic
shift of the CT absorption band together with a batho-
chromic shift of the π–π* maximum.
Moving from 2 to 3, substitution of the phenyl moiety
with alkyl residues should increase the electron density on
the nitrogen atom through an inductive effect. At the same
time, the electron density at the nitrogen should be reduced
by loss of resonance delocalization on the missing phenyl
groups.[39] The net effect we observed by changing the do-
nor properties of the amine substituents in 3 and 3a, is an
unperturbed charge-transfer band compared to 2 and 2a.
Conversely, a significant decrease in the intensity of the π–
Scheme 3. Synthesis of push-pull fluorenes 1a–5a.
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S. Quici et al.
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Table 1. Experimental and computed absorption maxima of push– chromophores, the scaling of absorptive properties passing
pull fluorene (1a–5a) and spirobifluorene (1–5) derivatives.
from compounds 1–5 to 1a–5a does not completely agree
λexp [nm][a]
ε [Lcm–1 mol–1][a]
λtheor [nm][b]
with this hypothesis. In fact, the ratio of molar extinction
coefficients between the spiro-compounds and the reference
fluorenes is less than 2 in most cases. This result can be
interpreted as an indication of some weak interaction tak-
ing place between the two molecular halves of 1–5.
Quantum mechanical calculations were performed to
characterize the excited states and the electronic transitions
giving rise to the absorption bands of both push–pull fluo-
renes and spirobifluorenes spectra.
The DFT-optimized molecular structures of all spirobi-
fluorenes show that the two halves are essentially mutually
orthogonal, being characterized by a dihedral angle be-
tween the planes containing the two fluorene units that var-
ies from 89.0° for 1 to 88.4° for 4. The optimized geometri-
cal parameters do not significantly vary on passing from
the fluorene to the spirobifluorene systems, where a slight
lengthening by 0.02 Å is found for the C–C bonds involving
the spiro center in the spirobifluorene derivatives and a de-
crease of the related angle by ca. 2° is computed.
Compound
1a
1aЈ
1
1Ј
2a
2
3a
3
4a
4
272, 418
16100, 25800
249, 373
251, 393
248, 378
252, 396
303, 388
303, 397
286, 387
279, 394
320, 426
322, 434
303, 404
306, 413
270, 420
27800, 38700
311, 401
311, 410
297, 403
297, 410
325, 428
325, 438
311, 403
315, 415
31700, 25800
64400, 46400
23300, 26100
40900, 42600
18300, 27900
29200, 43800
26400, 27700
52400, 49600
5a
5
[a] CHCl3 solutions. [b] Simulated TDDFT absorption bands cal-
culated by using MPW1K[37] functional, including solvation effects
(CHCl3) by the conductor polarizable continuum model (C-PCM)
as implemented in G03.[38]
π* band and its displacement to higher energies were mea-
sured. Absorption profiles of spiro-compound 4 and fluo-
rene 4a, which both have an additional double bond linker,
showed a bathochromic shift of 25 nm for the π–π* band
and of 28 nm for the CT band, reflecting the increased con-
jugation. Finally, the replacement of a double bond in 4
and 4a with a triple bond spacer in 5 and 5a leads to a
blueshift of both the CT and π–π* bands.
Interestingly, for fluorene 1a and spiro-compound 1, two
minima were found according to the conformation adopted
by the piperidine, the chair form being more stable by ap-
proximately 4 and 3 kcal/mol, respectively, than the half-
chair piperidine conformation (hereafter labelled 1aЈ and
1Ј).
In addition to these spectroscopic details, two more im-
portant observations can be noted. With the exception of
The frontier molecular orbital energies and HOMO and
the pair 1 and 1a, a slight bathochromic displacement (7– LUMO isodensity plots of all investigated fluorene 1a–5a
12 nm) of the CT maxima bands was measured for spirobi- derivatives are reported in Figure 3. It is worth noting that
fluorenes 2–5. Although these spectral shifts are small the energy of the LUMOs is quite similar, being only
enough to suggest the presence of quasi-independent active slightly lower in the case of compounds 2a, 4a, and 5a. The
Figure 3. Frontier orbital energies of the fluorene compounds 1a–5a. The isodensity plot of HOMOs and LUMOs are reported.
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NLO Properties of New Spirobifluorene Compounds
character of the LUMOs is also very similar along the bilization. In all cases, the electronic structure of the spiro-
series, with the electronic charge delocalized on the ac- dimers can be described as pairs of orbitals, which are the
ceptor part of the molecule with a different involvement of bonding/antibonding combination of the corresponding
the fluorene core. On the other hand, the HOMO reflects monomer orbitals. They are delocalized in the two substi-
the different donor substituents on the investigated com- tuted fluorene units and possess similar energy with respect
pounds and shows a different charge delocalization. In par- to the corresponding monomer orbitals. In general, on
ticular, the HOMOs are delocalized on the donor moiety of passing from the fluorene to the spirobifluorene systems,
the fluorenes for 2a–5a, whereas they show a contribution we did not observe the spiro-conjugation coupling for the
within the fluorene unit for 1a and 1aЈ. The HOMO ener- HOMO; the computed spirobifluorene HOMO and
gies for all the compounds show a higher variability, lying
in the range of 5.7–6.3 eV. The HOMO of 1a (4a) is the
most stabilized (destabilized), and the HOMO–LUMO gap
energies (∆EH–L) are directly affected by the position of the
HOMO. Indeed, the largest and the smallest ∆EH–L were
computed to be 4.37 and 3.81 eV for 1a and 4a, respectively,
which reflects the stronger donor ability of the NMe2 frag-
ment connected to the fluorene moiety by a C=C double
bond.
Notably, a general trend can be seen upon going from the
monomers to the spiro-dimers, which can be exemplified by
comparing the electronic structures of 3a and 3. As already
mentioned, the HOMO of 3a is localized on the dimeth-
ylaniline Ϫ the donor part of the molecule Ϫ whereas the
LUMO is localized on the acceptor part. In the spirobi-
fluorene compound 3, the π orbitals of the monomer
moieties combine in a symmetric and anti-symmetric way
to provide two almost degenerate orbitals (Figure 4). The
HOMO–LUMO gap only slightly reduces on passing from
3a to 3 (4.11 eV/4.04 eV), mainly by virtue of a LUMO sta- plots of selected orbitals are reported.
Figure 4. Molecular orbital energies of 3a and 3. The isodensity
Figure 5. Frontier orbital energies of the fluorene compounds 1–5. The isodensity plot of HOMOs and LUMOs are reported.
Eur. J. Org. Chem. 2010, 4004–4016
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HOMO-1 orbitals were almost degenerate and had essen- is represented by a transition within the acceptor part of
tially the same energy with respect to the fluorene HOMO. the molecule, i.e., from the phenyl ring to the nitro group.
This is due to the character of the fluorene HOMO, which
For spirobifluorene 3, the lowest absorption band is
shows negligible electron density on the atoms adjacent to slightly redshifted with respect to that of the monomer 3a,
the spiro carbon. On the other hand, the monomer being centered at 394 nm (3.15 eV) for the former versus
HOMO-1 is mainly localized on the fluorene moiety. In the 387 nm (3.21 eV) for the latter, the same trend was observed
dimer the correspondig orbitals, HOMO-2 and HOMO-3, experimentally, with maxima at 410 nm (3.03 eV) and
are split by virtue of the spiro-conjugation.[40,41] In Fig- 403 nm (3.08 eV), respectively.
ure 5, the frontier molecular orbital energies with HOMO
As can be seen, apart from a quasi-rigid blueshift of the
and LUMO isodensity plots of all the spirobifluorene deriv- calculated data of approximately 0.15 eV, the small shift of
atives 1–5 are reported. In line with the discussion above, the absorption band going from 3a to 3 is nicely repro-
the ∆EH–L decreases from the fluorene to the spirobifluor- duced. It is worth noting that in the case of 3, the lowest
ene for 1Ј and 2–5 compounds, which is essentially due to absorption band originates from two degenerate CT transi-
a stabilization of the LUMO; a fact that partly justifies the tions of comparable intensity involving the HOMO/
bathochromic shift of the CT band of the spirobifluorenes LUMO+1 and the HOMO–1/LUMO excitations. These
spectra.
transitions dominate the (second order) NLO response of
The overall picture of the molecular frontier orbitals of the investigated systems, which is expected to slightly in-
the spirocompounds describes a partial coupling that oc- crease compared to the monomer by virtue of the lower
curs between the two orthogonal units, a fact that has impli- energy excitation, at only little expense to the optical trans-
cations on the nonlinear optical properties. We then simu- parency.
lated the UV/Vis spectra of all the investigated systems and
The spectra of all the spirobifluorene compounds show
analyzed the computed TDDFT eigenvalues and eigenvec- the same trends, which can be seen in Table 1 as a compari-
tors. The absorption band maxima of the simulated son between theory and experiment. The calculated spectra
TDDFT spectra are compared with the experimental values of compounds 5a, 5, 4a, and 4 are in excellent agreement
in Table 1. As an example, the calculated absorption spectra with the experimental values. Deviations within 0.2 eV are
of 3a and 3 are reported in Figure 6. We computed that the calculated for the other chromophores, with compounds 1
lowest absorption band of 3a at 387 nm originated from a and 1a showing the largest blue-shifts compared to the ex-
single transition of charge-transfer (CT) character from the perimental data. This is possibly due to the influence of
HOMO to the LUMO. This computed band is blueshifted dynamic effects on the calculated spectra, as suggested by
compared to the experimental value (403 nm). With refer- the sensitivity of the calculated data to the selected piperi-
ence to the results obtained with 3a, the experimental ab- dine conformation.
sorption band at 297 nm has been computed to occur at
286 nm and originates essentially from two transitions at
Nonlinear Optical Properties
294 nm (f = 0.26) and 275 nm (f = 0.45), involving the lower
lying HOMOs and the LUMO/LUMO+1. The former tran-
sition, therefore, has the π orbital delocalized on the
fluorene unit of the 3a compound in the starting state and
the π* orbital on the acceptor part of the molecule in the
arriving state. The main contribution of the latter transition
The molecular NLO responses of the push–pull spirobi-
fluorene derivatives 1–5 and those of the corresponding
monomers are reported in Table 2. All compounds were
measured in liquid solution by the direct current electric-
field-induced second-harmonic (EFISH) generation
method,[42–44] which can provide γEFISH values, that is, di-
rect information on the molecular NLO properties, by using
Equation 1.
µβλ
γEFISH
=
+ γ0(–2ω; ω, ω, 0)
(1)
5kT
µβλ/5kT is the dipolar orientational contribution, with βλ
being the projection along the dipole moment axis of the
vectorial component βVEC of the quadratic hyperpolariz-
ability tensor, µ is the static dipole moment, ω is the funda-
mental wavelength of the incident photon, and γ0(–2ω; ω,
ω, 0), which is a third-order term at frequency ω of the
incident light, is the cubic electronic contribution to γEFISH
.
The values of µβλ were directly obtained from the EFISH
measurement by considering the γ0 contribution to γEFISH
to be negligible. The purely electronic cubic hyperpolariz-
abilities, γTHG(–3ω; ω, ω, ω), were measured by third har-
monic generation (THG) experiments.
Figure 6. Simulated absorption spectra of 3a (red) and 3 (blue);
MPW1K data in solution.
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NLO Properties of New Spirobifluorene Compounds
Table 2. Experimental and calculated molecular NLO properties of push–pull fluorenes and spirobifluorene derivatives.
γEFISH
γTHG
µD[b]
µβλ
µtheor
[D]
βtheor
βyyy theor
βzzz theor
µβtheor
[ϫ10–34 esu][a]
[ϫ10–34 esu][a]
[ϫ10–48 esu][a]
[ϫ10–30 esu][c] [ϫ10–30 esu][c] [ϫ10–30 esu][c] [ϫ10–48 esu]
1a
1.75
2.59
4.2
342
9.8
11.6
8.3
43
57
65
421
661
540
2a
3a
4a
5a
1
1.65
1.90
1.40
1.35
2.48
2.70
3.30
3.10
3.05
8.89
3.3
5.9
2.1
4.2
4.4
325
355
270
250
567
10.6
11.9
11.4
13.9
16.2
11.8
14.8
16.6
16.2
65
689
138
109
59
76
103
98
1642
1243
820
1231
1215
1450
3453
2803
41
54
73
69
148
121
43
53
72
69
146
123
1Ј
2
2.91
1.89
4.20
3.90
9.63
12.88
11.00
10.71
6.6
6.9
5.3
4.7
600
390
850
800
3
4
208
173
5
[a] In CHCl3 solutions at the same concentration (10–3 ) working at a nonresonant incident wavelength of 1.907 µm. [b] Determined in
anhydrous CHCl3 by the Guggenheim method.[47,48] [c] Calculated by a combined Finite Fields and Coupled Perturbed Kohn Sham
formalism as implemented in G03, in which numerical differentiation of analytic polarizabilities in the presence of an external electric
field is performed.
The γEFISH and µβλ values of fluorene derivatives 1a–5a vide a rationale for the nonlinear optical properties of
(Table 2) are very similar, and a clear trend cannot be in-
ferred, especially considering that the difference between
consecutive values is less than the experimental error of the
EFISH measurement, which allows the values to be esti-
mated with a standard deviation in the order of 10–15%.
However, taking into account the data obtained for spirobi-
fluorenes 1–5, a trend can be observed that could be ex-
plained on the basis of theoretical considerations of the
conjugation allowed by the spacers involved. As other au-
thors suggested,[45] the enhancement in hyperpolarizability
upon going from 1 and 3 to 2 can be interpreted on the
basis of the larger perturbation in the energy of the donor
orbital by the phenyl substituent and on the additional con-
tribution of π-electron density from the phenyl, as also con-
firmed by the observed decrease of ∆EH–L. Since a double
bond is superior to a triple bond in transmitting charge
from the donor to the acceptor moieties,[46] the improved
electronic communication along the series 4 Ǟ 5 Ǟ 3 finds
its counterpart in the increased values of µβλ.
fluorenes and related spirobifluorene, we have computation-
ally investigated the ground-state dipole moment, the static
hyperpolarizability, and µβλ, which is experimentally ob-
servable, of all the investigated systems.
In Table 2, the computed ground-state dipole moments
of fluorenes and spirobifluorenes under study are reported.
The calculated dipole moment of all the investigated
fluorenes originated from a charge asymmetry from the
partly negatively charged NO2 ligand, which prevails over
donation from the disubstituted amine ligand, similar to the
para-nitroaniline case. The dipole moments of the mono-
mers are in the range of 8.3–11.9 D, and they essentially
coincide with the value of their z component, i.e., they were
oriented along the main molecular axis. The dipole mo-
ments of the spirobifluorene derivatives are increased com-
pared to the monomers, with values in the range of 11.8–
16.6 D. As expected, the dipole is the resulting vector of
the two almost equivalent µy and µz components, which are
characterized by a similar value and the same direction as
those of the monomers. Overall, we observe that the dipole
moment of the investigated spirobifluorene is essentially
equal to µspiro = [cos(π/4)ϫ(µy + µz)], it therefore crosses
the spiro region and is directed between the two –NR2 ends.
The comparison between the calculated and experimental
values shows a general overestimation of calculated dipole
moments, although the experimental trends are roughly re-
produced considering that the dipole moments of all com-
pounds are quite similar and that an increase is observed
passing from the fluorenes to the related spirobifluorenes.
As excited states, the absolute values of these results are
clearly sensitive to the level of theory and therefore to the
xc functional employed. Nevertheless, the same conclusion
holds for the trends going from the monomer to the spiro-
dimer, independent of the xc functional used and whether
the calculations involved in vacuo or solution methods. Ac-
tually, the measured dipole moments for 2a and 4a were
surprisingly low.
Particularly interesting are the experimental results on
the purely electronic cubic hyperpolarizabilities γTHG. In the
hypothesis of no electronic coupling between two monomer
units, the γ value of a dimer can be related to that of a
monomer by Equation 2.[26]
͗γ͘
spiro
= ͗γ͘monomer ϫ4(1 + cosϑ)2
(2)
ϑ is the dihedral angle between two dipoles. Thus, when ϑ
is approximately 90° for spirobifluorene derivatives, a
γTHGspiro/γTHGmonomer ratio equal to 4 is expected. Our ex-
perimental results give a very reproducible ratio that always
falls in the interval 3.43–3.9.
Although a substantial increase of µβλ was measured for
our spiro-compounds with respect to the corresponding
fluorene (with the exception of the pair 3a/3), the analysis
of experimental dipole moments and the quadratic hyper-
polarizability values is less straightforward. In order to pro-
Eur. J. Org. Chem. 2010, 4004–4016
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4011

S. Quici et al.
FULL PAPER
The calculated static hyperpolarizabilities β of the investi- increasing the NLO response without compromising the
gated fluorene series are reported in Table 2 together with optical transparency.
µβ values. The obtained values are in satisfactory agreement
with the experimental data, taking into account the general
overestimation of the theoretical values due to the com-
puted dipole moments. We notice, however, that a direct
Conclusions
comparison between the theoretical and experimental data
A new class of push–pull chromophores with disubsti-
is not straightforward due to the neglect in the former of tuted amine as donor and nitro group as acceptor con-
the dispersion and vibrational contribution to the second- nected to a spirobifluorene core has been synthesized. A
order NLO response.
simple synthetic approach was employed to obtain geomet-
For the spirobifluorenes, analogously to the calculations rically oriented push–pull bichromophoric systems and the
on the dipole moment, the only relevant components of the corresponding push–pull fluorene monomers with good
β tensor are βyyy and βzzz, which are almost equivalent and yields. Using electric field second harmonic and third har-
correspond to the β components along the z and y axes of monic generation methods, the nonlinear optical properties
the dipole moments. We therefore evaluated the values of β of spirobifluorene derivatives have been experimentally in-
using the following equation: β = [cos(π/4) (βyyy
+
vestigated for the first time. Furthermore, we compared the
βzzz)ϫ10–48 esu]. The values of the two spirobifluorenes hy- first and second hyperpolarizability (β and γ) of the spiro-
perpolarizability components are related to the monomer compounds with the corresponding fluorene monomers. We
values, and show an increase with respect to the value of found that the first hyperpolarizability β is enhanced on
the corresponding fluorene directly depending on the rigid passing from the monomer to the corresponding spiro-di-
conformation of the spiro-compounds. Looking at the theo- mer without substantial variation of the maximum of the
retical µβ values, analogously to the fluorene case, we ob- absorption bands. Particularly interesting are the experi-
serve a general overestimation with respect to the EFISH mental results on the γTHG. In fact, the ratio γTHGspiro
/
measurements, which is again due to the overestimated di- γTHGmonomer is expected to be equal to 4[26] and our experi-
pole moments. However, with the exception of 3, a reason- ments have given very reproducible data that was always in
able correlation between theory and experiment is observed. the range 3.43–3.9.
Overall, some general trends can be drawn within the spiro-
By calculating the optimized molecular structures and
bifluorene series. In particular, it is interesting that a rela- conducting subsequent analyses of the electronic structure
tionship can be found between the NLO behavior and the of all compounds, the HOMO levels were found to be local-
optical properties among the series 3Ǟ4Ǟ5. The (dimeth- ized on the donor part of the molecule and reflected the
ylamino)phenyl donor group in 3 is connected directly to different donor substituents, whereas the LUMOs were
the spiro moiety, whereas in 4 and 5 a double and triple found to be quite similar, with the electronic charge being
bond spacer, respectively, is inserted between the donor delocalized on the nitro group and the adjacent phenyl ring.
group and the spirobifluorene moiety. Along this series, Remarkably, the electronic structure of the spirobifluorene
both the monomer and dimer NLO response follow the compounds can be described as pairs of orbitals that are
trends in HOMO–LUMO and lowest transition energies, delocalized on the two molecular halves and which are at
which is in line with the two-level model, which predicts similar energy with respect to the corresponding monomer
that the hyperpolarizability will increase as the transition orbitals; this fact has a large impact on the related linear
energies decrease. The effect of the conjugate spacer induces and nonlinear optical properties of the spiro-derivatives
a substantial increase in the NLO response of both the compared to the fluorenes. In fact, the computed lowest
monomers and dimers. Besides the transition red-shift al- energy band of the spirobifluorene UV/Vis spectra origi-
ready mentioned, this effect is also due to the higher polar- nates from two equivalent CT transitions, each one taking
izability introduced in the molecules by the conjugated spa- place in a fluorene unit.
cers. Changing a double bond to a triple bond as spacer
implies a decrease of the NLO response, due to the smaller been computed to arise as a result of the two almost equiva-
conjugating ability of the triple bond.[46]
lent components (βyyy and βzzz) along the dipole moments
The first hyperpolarizability β of the spirobifluorene has
It is also interesting to analyze the effect of replacing z and y axes, thus confirming the experimentally observed
the methyl substituents in 3 with phenyl substituents in 2. increase in the dipole moment of the spiro-dimer compared
This substitution, which does not have any impact on the to the fluorene. Moreover, analysis of the spirobifluorene
NLO response of the fluorene monomers (65ϫ10–30 esu series 3/4/5, in which a double or triple bond is inserted
for both 2a and 3a), slightly affects the computed hyperpo- between the donor group and the spirobifluorene core, re-
larizability of the spirobifluorenes, with the increase re- vealed a significant increase in the β values. Finally, substi-
lated to the presence of the phenyl substituents. This in- tution of the dimethylamine 3 with the diphenylamine 2 re-
crease comes at basically no cost to transparency, because sults in a sizeable enhancement of the first hyperpolarizabil-
both species show essentially the same absorption max- ity, which can be ascribed to a “non-traditional” π-conjuga-
ima. This interesting observation, which is possibly due to tion effect as described in the literature.[37,45]
the polarizable nature of the phenyl substituents, can offer
This study covers a number of spirobifluorene com-
additional guidance for future design strategies aimed at pounds that have not previously been fully investigated. The
4012
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 4004–4016

NLO Properties of New Spirobifluorene Compounds
Coupled Perturbed Kohn Sham formalism as implemented in G03,
in which numerical differentiation of analytic polarizabili-
ties in the presence of an external electric field was performed.
geometrically rigid structure of these bichromophoric sys-
tems could be used as a base from which to design molecu-
lar materials with high NLO properties without significant
modifications to the optical behavior of the single chromo-
phore.
2,2Ј-Dibromo-7,7Ј-dinitro-9,9Ј-spirobifluorene (8): Bromine (1 mL,
19.52 mmol) was added dropwise to a stirred solution of 7 (1.87 g,
4.61 mmol) and anhydrous FeCl3 (40 mg, 0.247 mmol) in CH2Cl2
(45 mL). The mixture was refluxed for 5 h and then cooled to r.t.
The solution was washed with saturated NaHCO3 (2ϫ30 mL), sat-
urated Na2S2O3 (2ϫ30 mL) and water (2ϫ30 mL). The organic
layer was dried with Na2SO4 and the solvent was removed under
Experimental Section
General: All commercially available solvents and reagents were used
as received without further purification. 2,2Ј-Dinitro-9,9Ј-spirobi-
fluorene (7),[31] 4-(dimethylamino)phenylboronic acid (10),[32] N,N-
dimethyl-4-vinylaniline (11),[33] 2-iodo-7-nitro-9H-fluorene,[36] and
2-iodo-9,9-dimethyl-7-nitro-9H-fluorene (13)[35] were synthesized
according to literature procedures. Coupling reactions were per-
formed under nitrogen using standard Schlenk techniques. Thin
layer chromatography (TLC) was performed on plates precoated
with silica gel Si60-F254 (Merck, Darmstadt, Germany). Column
chromatography was carried out with silica gel Si60, mesh size
0.040–0.063 mm (Merck, Darmstadt, Germany). Flash chromatog-
raphy was carried out on silica gel, mesh size 230–400 (J. T. Baker).
1H and 13C NMR spectra were recorded with a Bruker Avance 400
(400 and 100.6 MHz, respectively) spectrometer. Chemical shifts
are indicated in parts per million downfield from SiMe4, using the
residual proton (CHCl3 δ = 7.26 ppm) and carbon (CDCl3 δ =
77.0 ppm) solvent resonances as internal reference. The coupling
constants J are given in Hz. Proton and carbon atom assignments
were determined by performing COSY and HSQC correlation ex-
periments. ESI mass spectra were obtained with an electrospray
ion-trap mass spectrometer ICR-FTMS APEX II (Bruker Dalton-
ics) by the Centro Interdipartimentale Grandi Apparecchiature
(C.I.G.A.) of the University of Milano. Melting points were deter-
mined with a Büchi Melting Point B-540 capillary melting point
apparatus. UV/Vis spectra were recorded with a Nicolet Evolution
500 (Thermo Electron Corporation) spectrometer. All EFISH and
THG measurements were carried out in CHCl3 solutions at the
same concentration (10–3 ) working at a nonresonant incident
vacuum to give 8 (2.54 g, Ͼ 95%) as a pale-yellow solid; m.p. 360–
1
365 °C. H NMR (400 MHz, CDCl3, 25 °C): δ = 8.37 (dd, JH,H
=
8.5, JH,H = 2.1 Hz, 2 H, 6-H, 6Ј-H), 8.00 (d, JH,H = 8.5 Hz, 2 H,
5-H, 5Ј-H), 7.85 (d, JH,H = 8.2 Hz, 2 H, 4-H, 4Ј-H), 7.66 (dd, JH,H
= 8.2, JH,H = 1.8 Hz, 2 H, 3-H, 3Ј-H), 7.56 (d, JH,H = 2.1 Hz, 2 H,
8-H, 8Ј-H), 6.91 (d, JH,H = 1.7 Hz, 2 H, 1-H, 1Ј-H) ppm. 13C NMR
(101 MHz, CDCl3, 25 °C): δ = 149.41 (q), 147.99 (q), 147.23 (q),
146.88 (q), 138.32 (q), 132.72 (6-C, 6Ј-C), 127.63 (8-C, 8Ј-C), 125.12
(3-C, 3Ј-C), 124.58 (q), 123.25 (5-C, 5Ј-C), 120.99 (4-C, 4Ј-C),
119.65 (1-C, 1Ј-C), 65.18 (9-C) ppm. HRMS (ESI+): calcd. for
C25H12N2O4Br2 [M
+
H]+ 563.9140; found 563.9144.
C25H11Br2N2O4 (563.17): calcd. C 53.22, H 2.14, N 4.97; found C
53.35, H 2.14, N 4.98.
2,2Ј-Dinitro-7,7Ј-di(piperidin-1-yl)-9,9Ј-spirobifluorene (1): In
a
Schlenk tube, a solution of Pd2(dba)3 (22 mg, 0.024 mmol) and
PtBu3 (1  in toluene, 40 µL, 0.04 mmol) in anhydrous toluene
(2 mL) was stirred at r.t. for 20 min. The mixture was transferred
by using a cannula to a second Schlenk tube containing a suspen-
sion of 8 (250 mg, 0.445 mmol), Cs2CO3 (510 mg, 1.566 mmol),
and piperidine (108 µL, 1.092 mmol) in anhydrous toluene (4 mL).
The mixture was heated to reflux under vigorous stirring for 24 h.
After cooling to r.t., saturated NH4Cl (10 mL) was added and the
mixture was extracted with ethyl acetate (4ϫ15 mL). The collected
organic phases were dried with Na2SO4 and the solvent was re-
moved under vacuum. The crude material was purified by flash
chromatography (hexane/ethyl acetate, 9:1) to give 1 (135 mg, 53%)
as a red solid; m.p. 175–195 °C. 1H NMR (400 MHz, CDCl3,
25 °C): δ = 8.26 (dd, JH,H = 8.5, JH,H = 2.0 Hz, 2 H, 3-H, 3Ј-H),
7.78 (d, JH,H = 8.6 Hz, 2 H, 5-H, 5Ј-H), 7.77 (d, JH,H = 8.5 Hz, 2
H, 4-H, 4Ј-H), 7.47 (d, JH,H = 2.1 Hz, 2 H, 1-H, 1Ј-H), 7.0 (dd,
JH,H = 8.6, JH,H = 1.8 Hz, 2 H, 6-H, 6Ј-H), 6.97 (d, JH,H = 1.9 Hz,
2 H, 8-H, 8Ј-H), 3.11–3.09 (m, 8 H, piperidine), 1.60–1.50 (m, 12
H, piperidine) ppm. 13C NMR (101 MHz, CDCl3, 25 °C): δ =
153.85 (q), 150.99 (q), 148.88 (q), 148.34 (q), 146.09 (q), 129.66 (q),
124.69 (3-C, 3Ј-C), 122.53 (4-C, 4Ј-C), 119.35 (1-C, 1Ј-C), 118.75
(5-C, 5Ј-C), 115.96 (6-C, 6Ј-C), 110.48 (8-C, 8Ј-C), 65.51 (9-C),
49.60 (2-C piperidine, 6-C piperidine), 25.57 (3-C piperidine, 5-C
piperidine), 24.05 (4-C piperidine) ppm. UV/Vis (CHCl3): λmax (ε,
mol–1 dm3 cm–1) = 270 (27800), 420 (38700) nm. HRMS (ESI+):
calcd. for C35H33N4O4 [M + H]+ 573.2496; found 573.2484.
C35H32N4O4 (572.65): calcd. C 73.41, H 5.63, N 9.78; found C
73.55, H 5.64, N 9.77.
wavelength of 1.907 µm, using
a Q-switched, mode-locked
Nd3+:YAG laser manufactured by Atalaser equipped with a Raman
shifter; the apparatus used for the EFISH measurements was made
by SOPRA (France). All experimental EFISH βλ values are defined
according to the “phenomenological” convention.[49] Dipole mo-
ments µ were determined in anhydrous CHCl3 by the Guggenheim
method.[47,48]
Theoretical Calculations: The molecular structure of all fluorenes
1a–5a and related spirobifluorenes 1–5 were optimized using a 6-
31g** basis set[50] and B3LYP exchange-correlation functional[51] in
vacuo, without any symmetry constraints, by using the Gaussian03
(G03) program package.[52] On all the optimized molecular geome-
tries, the analysis of the electronic structure and Time-Dependent
DFT (TDDFT) calculations were performed using the 6-31g** ba-
sis set, and the hybrid B3LYP and MPW1K[38] functionals and in-
cluded the solvation effects by the conductor polarizable contin-
uum model (C-PCM) as implemented in G03.[39] The MPW1K
functional was chosen because, due to increased amounts of
Hartree–Fock exchange, the results were particularly effective in
accurately describing the charge transfer systems.[38,53] The lowest
20 (60) TDDFT singlet–singlet excitations for all the monomers
(dimers) were computed. To simulate the absorption spectra, thus
allowing a direct comparison with the experimental data, the com-
puted transition energies and oscillator strengths were convoluted
by Gaussian functions with a σ of 0.2 eV. The static quadratic hy-
perpolarizability β were calculated by a combined Finite Fields and
7,7Ј-Bis[4-(diphenylamino)phenyl]-2,2Ј-dinitro-9,9Ј-spirobifluorene
(2): In a Schlenk tube, a stirred mixture of 8 (112 mg, 0.198 mmol),
boronic acid 9 (233 mg, 0.806 mmol), aqueous K2CO3 (1 ,
2.4 mL, 2.4 mmol), and Pd(PPh3)4 (12 mg, 0.011 mmol) in anhy-
drous THF (5 mL) was refluxed overnight under an inert atmo-
sphere. After cooling to r.t., THF was removed under vacuum and
the aqueous phase was extracted with CH2Cl2 (3ϫ20 mL). The
collected organic phases were dried with Na2SO4 and the solvent
was removed under vacuum. The crude material was purified by
column chromatography over silica (hexane/ethyl acetate, 5:1) to
Eur. J. Org. Chem. 2010, 4004–4016
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4013

S. Quici et al.
FULL PAPER
give the product 2 (137 mg, 77%) as an orange solid; m.p. 195–
NMR (101 MHz, CDCl3, 25 °C): δ = 150.29 (q), 149.25 (q), 148.55
(q), 148.16 (q), 147.28 (q), 140.88 (q), 137.75 (q), 130.66 (1-C eth-
1
197 °C. H NMR (400 MHz, CDCl3, 25 °C): δ = 8.35 (dd, JH,H
=
8.4, JH,H = 2.0 Hz, 2 H, 3-H, 3Ј-H), 8.01 (d, JH-H = 8.1 Hz, 2 H, enyl), 127.84 (2-C phenyl, 6-C phenyl), 127.10 (3-C, 3Ј-C), 125.09
5-H, 5Ј-H), 8.00 (d, JH-H = 8.4 Hz, 2 H, 4-H, 4Ј-H), 7.72 (dd, JH- (q), 124.76 (6-C, 6Ј-C), 123.19 (2-C ethenyl), 122.07 (4-C, 4Ј-C),
= 8.0, JH-H = 1.2 Hz, 2 H, 6-H, 6Ј-H), 7.60 (d, JH-H = 1.9 Hz, 2 121.38 (1-C, 1Ј-C), 120.23 (5-C, 5Ј-C), 119.56 (8-C, 8Ј-C), 112.29
H
H, 1-H, 1Ј-H), 7.31 (d, JH-H = 8.6 Hz, 4 H, Ph-H), 7.24–7.21 (m,
(3-C phenyl, 5-C phenyl), 65.43 (9-C), 40.35 (CH3) ppm. UV/Vis
8 H), 7.07–6.98 (m, 16 H) ppm. 13C NMR (101 MHz, CDCl3, (CHCl3): λmax (ε, mol–1 dm3 cm–1) = 325 (29200), 438 (43800) nm.
25 °C): δ = 149.28 (q), 148.53 (q), 148.02 (q), 147.87 (q), 147.50
HRMS (ESI+): calcd. for C45H37N4O4 [M + H]+ 697.2809; found
(q), 147.36 5 (q), 143.13 (q), 137.97 (q), 133.48 (q), 129.31 (Ph), 697.2808; calcd. for [M + H]2+ C45H38N4O4 349.1441; found
127.81 (2-C phenyl-spiro, 6-C phenyl-spiro), 127.59 (6-C spiro), 349.1445. C45H36N4O4 (696.79): calcd. C 77.57, H 5.21, N 8.04;
124.80 (3-C spiro), 124.58 (Ph), 123.38 (Ph), 123.24 (Ph), 122.23 found C 77.55, H 5.22, N 8.06.
(5-C, 5Ј-C, 8-C, 8Ј-C), 120.52 (4-C, 4Ј-C), 119.62 (1-C, 1Ј-C), 65.71
2,2Ј-Bis{2-[4-(dimethylamino)phenyl]ethyn-1-yl}-7,7Ј-dinitro-9,9Ј-
(9-C) ppm. UV/Vis (CHCl3): λmax(ε, mol–1 dm3 cm–1) = 311 (64400),
spirobifluorene (5): In a Schlenk tube, a mixture of 8 (230 mg,
410 (46400) nm. HRMS (ESI+): calcd. for C61H41N4O4 [M + H]+
0.4085 mmol), Pd(PPh3)2Cl2 (31 mg, 0.044 mmol), anhydrous
893.3122; found 893.3149. C61H40N4O4 (892.99): calcd. C 82.04, H
DMF (20 mL), and NEt3 (1 mL, 7.184 mmol) was stirred at r.t. for
4.51, N 6.27; found C 82.25, H 4.50, N 6.28.
5 min under an inert atmosphere. Compound 12 (241 mg,
2,2Ј-Bis[4-(dimethylamino)phenyl]-7,7Ј-dinitro-9,9Ј-spirobifluorene 1.657 mmol) was added and the solution was stirred at 90 °C for
(3): In a Schlenk tube, a stirred mixture of 8 (202 mg, 0.359 mmol),
10 (358 mg, 2.170 mmol), aqueous K2CO3 (1 , 5 mL, 5 mmol),
and Pd(PPh3)4 (23 mg, 0.020 mmol) in anhydrous THF (10 mL)
was refluxed overnight under an inert atmosphere. After cooling to
r.t., THF was removed under vacuum and the aqueous phase was
extracted with CH2Cl2 (3ϫ25 mL). The collected organic phases
were dried with Na2SO4 and the solvent was removed under vac-
uum. The crude material was purified by chromatography (hexane/
5 h. After cooling to r.t., the solvent was removed under vacuum
and the crude material was dissolved in CH2Cl2 and washed with
water (3ϫ25 mL). The organic phase was dried with Na2SO4 and
the solvent was removed under vacuum. The crude material was
purified by flash chromatography (CH2Cl2/hexane, 2:1) to afford 5
(130 mg, 46%) as an orange solid; m.p. Ͼ400 °C. 1H NMR
(400 MHz, CDCl3, 25 °C): δ = 8.25 (dd, JH,H = 8.4, JH,H = 2.1 Hz,
2 H, 6-H, 6Ј-H), 7.92 (d, JH,H = 8.4 Hz, 2 H, 5-H, 5Ј-H), 7.91 (d,
ethyl acetate, 5:1) to give 3 (167 mg, 67%) as an orange solid; m.p. JH,H = 8.0 Hz, 2 H, 4-H, 4Ј-H), 7.57 (dd, JH,H = 8.0, JH,H = 1.4 Hz,
281–283 °C. 1H NMR (400 MHz, CDCl3, 25 °C): δ = 8.34 (dd, JH,H 2 H, 3-H, 3Ј-H), 7.52 (d, JH,H = 2.1 Hz, 2 H, 8-H, 8Ј-H), 7.22 (d,
= 8.4, JH,H = 2.1 Hz, 2 H, 6-H, 6Ј-H), 7.98 (d, JH,H = 8.1 Hz, 2 H,
4-H, 4Ј-H), 7.97 (d, JH,H = 8.4 Hz, 2 H, 5-H, 5Ј-H), 7.70 (dd, JH,H
= 8.1, JH,H = 1.6 Hz, 2 H, 3-H, 3Ј-H), 7.58 (d, JH,H = 2.1 Hz, 2 H,
JH,H = 9.0 Hz, 4 H, 2-H phenyl, 6-H phenyl), 6.95 (d, JH,H =
1.2 Hz, 2 H, 1-H, 1Ј-H), 6.54 (br. d, JH,H = 9.0 Hz, 4 H, 3-H
phenyl, 5-H phenyl), 2.95 (s, 6 H, CH3) ppm. 13C NMR (101 MHz,
8-H, 8Ј-H), 7.50 (d, JH,H = 8.9 Hz, 4 H, 2-H phenyl, 6-H phenyl), CDCl3, 25 °C): δ = 150.24 (q), 148.25 (q), 148.00 (q), 147.66 (q),
6.96 (d, JH,H = 1.6 Hz, 2 H, 1-H, 1Ј-H), 6.58 (br. d, JH,H = 8.8 Hz,
147.46 (q), 138.28 (q), 132.73 (2-C phenyl, 6-C phenyl), 132.10 (3-
C, 3Ј-C), 126.84 (1-C, 1Ј-C), 126.20 (q), 124.81 (6-C, 6Ј-C), 121.65
4 H, 3-H phenyl, 5-H phenyl), 2.83 (s, 6 H, CH3) ppm. 13C NMR
(101 MHz, CDCl3, 25 °C): δ = 150.36 (q), 149.42 (q), 148.61 (q), (5-C, 5Ј-C), 120.63 (4-C, 4Ј-C), 119.52 (8-C, 8Ј-C), 111.61 (3-C
148.34 (q), 147.28 (q), 143.75 (q), 137.15 (q), 127.79 (2-C phenyl,
phenyl, 5-C phenyl), 109.07 (q), 93.61 (2-C ethynyl), 87.24 (1-C
6-C phenyl), 127.72 (q), 127.01 (3-C, 3Ј-C), 124.71 (6-C, 6Ј-C),
ethynyl), 65.14 (9-C), 40.09 (CH3) ppm. UV/Vis (CHCl3): λmax (ε,
122.09 (4-C, 4Ј-C), 121.81 (1-C, 1Ј-C), 120.22 (5-C, 5Ј-C), 119.61 mol–1 dm3 cm–1) = 315 (52400), 415 (49600) nm. HRMS (ESI+):
(1-C, 1Ј-C), 112.55 (3-C phenyl, 5-C phenyl), 65.70 (9-C), 40.39
calcd. for C45H33N4O4 [M + H]+ 693.2496; found 693.2485.
C45H32N4O4 (692.76): calcd. C 78.02, H 4.66, N 8.09; found C
(CH3) ppm. UV/Vis (CHCl3): λmax (ε, mol–1 dm3 cm–1) = 300
(40900), 410 (42600) nm. HRMS (ESI+): calcd. for C41H32N4O4Na 78.25, H 4.68, N 8.11.
[M + Na]+ 667.2316; found 667.2305. C41H32N4O4 (644.72): calcd.
C 76.38, H 5.00, N 8.69; found C 76.55, H 5.09, N 8.71.
9,9-Dimethyl-2-nitro-7-(piperidin-1-yl)fluorene (1a): In a Schlenk
tube, a solution of Pd2(dba)3 (11 mg, 0.012 mmol) and PtBu3 (1 
in toluene, 19 µL, 0.019 mmol) in anhydrous toluene (2 mL) was
stirred at r.t. for 20 min. The solution was transferred by using a
(E,E)-2,2Ј-Bis{2-[4-(dimethylamino)phenyl]ethen-1-yl}-7,7Ј-dinitro-
9,9Ј-spirobifluorene (4): In a Schlenk tube, a mixture of 8 (249 mg,
0.442 mmol), K2CO3 (124 mg, 0.900 mmol), TBAB (285 mg, cannula to a second Schlenk tube containing a mixture of 13
0.885 mmol), anhydrous DMF (8 mL), and 11 (148 mg,
1.010 mmol) dissolved in anhydrous DMF (2 mL) was stirred at r.t.
for 30 min. Then Pd(OAc)2 (10 mg, 0.046 mmol) was added and
the solution was vigorously stirred at 105 °C for 6 h. After cooling
to r.t., DMF was removed under vacuum and the crude material
(196 mg, 0.535 mmol) and Cs2CO3 (328 mg, 1.006 mmol) and pi-
peridine (65 µL, 0.657 mmol) in anhydrous toluene (5 mL). The
solution was heated to reflux under vigorous stirring for 6 h. After
cooling to r.t., saturated NH4Cl (15 mL) was added and the mix-
ture was extracted with ethyl acetate (3ϫ15 mL). The collected
was dissolved in CH2Cl2 (50 mL). After washed with water organic phases were dried with Na2SO4 and the solvent was re-
(2ϫ20 mL), the organic phase was dried with Na2SO4 and the sol-
vent was removed under vacuum. The crude material was purified
by flash chromatography (hexane/ethyl acetate, 4:1) to give 4
(136 mg, 44 %) as a dark solid; m.p. 234–239 °C. 1H NMR
(400 MHz, CDCl3, 25 °C): δ = 8.33 (dd, JH,H = 8.4, JH,H = 2.1 Hz,
moved under vacuum. The crude material was purified by
chromatography on silica (hexane/ethyl acetate, 96:4) to give 1a
(126 mg, 73%) as
a
red solid; m.p. 157–162 °C. 1H NMR
(400 MHz, CDCl3, 25 °C): δ = 8.22–8.20 (m, 2 H, 1-H, 3-H), 7.65–
7.61 (m, 2 H, 4-H, 5-H), 6.98 (d, JH,H = 2.3 Hz, 1 H, 8-H), 6.95
2 H, 6-H, 6Ј-H), 7.96 (d, JH,H = 8.4 Hz, 2 H, 5-H, 5Ј-H), 7.93 (d, (dd, JH,H = 8.5, JH,H = 2.3 Hz, 1 H, 6-H), 3.31 (m, 4 H, piperidine),
JH,H = 8.1 Hz, 2 H, 4-H, 4Ј-H), 7.61 (dd, JH,H = 8.1, JH,H = 1.5 Hz, 1.76–1.72 (m, 4 H, piperidine), 1.68–1.64 (m, 2 H, piperidine), 1.50
2 H, 3-H, 3Ј-H), 7.57 (d, JH,H = 2.1 Hz, 2 H, 8-H, 8Ј-H), 7.29 (d, (s, 6 H, CH3) ppm. 13C NMR (101 MHz, CDCl3, 25 °C): δ = 157.00
JH,H = 8.9 Hz, 4 H, 2-H phenyl, 6-H phenyl), 6.94 (d, JH,H
16.2 Hz, 1 H, 1-H ethenyl), 6.90 (d, JH,H = 1.1 Hz, 2 H, 1-H, 1Ј-
H), 6.77 (d, JH,H = 16.2 Hz, 1 H, 2-H ethenyl), 6.64 (br. d, JH,H
=
(q), 153.86 (q), 153.48 (q), 146.53 (q), 145.72 (q), 127.47 (q), 123.69
(3-C), 122.32 (5-C), 118.45 (4-C), 118.06 (1-C), 115.25 (6-C), 109.65
(8-C), 50.19 (2-C piperidine, 6-C piperidine), 47.11 (9-C), 27.04
=
8.9 Hz, 4 H, 3-H phenyl, 5-H phenyl), 2.95 (s, 6 H, CH3) ppm. 13C (CH3), 25.75 (3-C piperidine, 5-C piperidine), 24.31 (4-C piperi-
4014
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 4004–4016

NLO Properties of New Spirobifluorene Compounds
dine) ppm. UV/Vis (CHCl3): λmax (ε, mol–1 dm3 cm–1)
=
272
was removed under vacuum and the crude material was dissolved
in CH2Cl2 (50 mL), washed with water (2ϫ20 mL), and the or-
(16100), 418 (25800) nm. HRMS (ESI+): calcd. for [M + H]+
C20H23N2O2 323.1754; found 323.1755. C20H22N2O2 (322.40): ganic phase was dried with Na2SO4 and the solvent removed under
calcd. C 74.51, H 6.88, N 8.69; found C 74.55, H 6.89, N 8.71.
vacuum. The crude material was purified by chromatography on
silica (hexane/ethyl acetate, 9:1) to give 4a (80 mg, 47%) as an
orange solid; m.p. 212–219 °C. 1H NMR (400 MHz, CDCl3,
25 °C): δ = 8.29 (d, JH,H = 2.0 Hz, 1 H, 8-H), 8.26 (dd, JH,H = 8.3,
JH,H = 2.1 Hz, 1 H, 6-H), 7.75 (br. t, JH,H = 8.1 Hz, 2 H, 4-H, 5-
H), 7.59 (d, JH,H = 1.1 Hz, 1 H, 1-H), 7.52 (dd, JH,H = 8.0, JH,H
= 1.5 Hz, 1 H, 3-H), 7.46 (d, JH,H = 8.8 Hz, 2 H, 2-H phenyl, 6-H
phenyl), 7.17 (d, JH,H = 16.2 Hz, 1 H, 1-H ethenyl), 7.01 (d, JH,H
= 16.2 Hz, 1 H, 2-H ethenyl), 6.74 (d, JH,H = 8.8 Hz, 2 H, 3-H
phenyl, 5-H phenyl), 3.02 (s, 6 H, N-CH3), 1.58 (s, 6 H, fluorene-
CH3) ppm. 13C NMR (101 MHz, CDCl3, 25 °C): δ = 155.72 (q),
154.72 (q), 150.36 (q), 146.84 (q), 145.78 (q), 139.83 (q), 135.41 (q),
129.98 (1-C ethenyl), 127.82 (2-C phenyl, 6-C phenyl), 125.87 (3-
C), 125.40 (q), 123.99 (2-C ethenyl), 123.53 (6-C), 121.68 (4-C),
120.09 (1-C), 119.76 (5-C), 118.27 (8-C), 112.41 (3-C phenyl, 5-C
phenyl), 47.22 (9-C), 40.42 (CH3-N), 26.89 (CH3-fluorene) ppm.
UV/Vis (CHCl3): λmax (ε, mol–1 dm3 cm–1) = 325 (18300), 429
(27900) nm. HRMS (ESI+): calcd. for [M + H]+ C25H25N2O2
385.1910; found 385.1902. C25H24N2O2 (384.47): calcd. C 78.10, H
6.29, N 7.29; found C 78.30, H 6.30, N 7.31.
7-[4-(Diphenylamino)phenyl]-9,9-dimethyl-2-nitrofluorene (2a): In a
Schlenk tube, a stirred mixture of 13 (158 mg, 0.434 mmol), 9
(254 mg, 0.878 mmol), aqueous K2CO3 (1 , 2.6 mL, 2.6 mmol),
and Pd(PPh3)4 (22 mg, 0.019 mmol) in anhydrous THF (5 mL) was
refluxed for 5 h under an inert atmosphere. After cooling to r.t.,
THF was removed under vacuum and the aqueous phase was ex-
tracted with CH2Cl2 (2ϫ10 mL). The collected organic phases
were dried with Na2SO4 and the solvent was removed under vac-
uum. The crude material was purified by chromatography (hexane/
ethyl acetate, 95:5) to afford 2a (145 mg, 69%) as an orange solid;
m.p. 224–228 °C. 1H NMR (400 MHz, CDCl3, 25 °C): δ = 8.31 (d,
JH,H = 2.0 Hz, 1 H, 1-H), 8.28 (dd, JH,H = 8.3, JH,H = 2.1 Hz, 1
H, 3-H), 7.84 (d, JH,H = 7.9 Hz, 1 H, 5-H), 7.82 (d, JH,H = 8.3 Hz,
1 H, 4-H), 7.69 (d, JH,H = 1.5 Hz, 1 H, 8-H), 7.63 (dd, JH,H = 7.9,
JH,H = 1.6 Hz, 1 H, 6-H), 7.55 (d, JH,H = 8.7 Hz, 2 H, PhH), 7.32–
7.28 (m, 4 H), 7.20–7.16 (m, 6 H), 7.07 (br. t, JH,H = 7.3 Hz, 2
H), 1.59 (s, 6 H, fluorene-CH3) ppm. 13C NMR (101 MHz, CDCl3,
25 °C): δ = 155.84 (q), 154.78 (q), 147.71 (q), 147.57 (q), 147.06
(q), 145.62 (q), 142.18 (q), 135.53 (q), 134.57 (q), 129.38 (Ph),
127.96 (Ph), 126.32 (6-C), 124.62 (Ph), 123.70 (Ph), 123.55 (3-C),
123.21 (Ph), 121.80 (5-C), 121.13 (8-C), 120.01 (4-C), 118.33 (1-C),
47.42 (9-C), 26.91 (CH3) ppm. UV/Vis (CHCl3): λmax (ε,
mol–1 dm3 cm–1) = 311 (31700), 403 (25800) nm. HRMS (ESI+):
calcd. for [M + H]+ C33H27N2O2 483.2067; found 483.2053.
C33H27N2O2 (483.58): calcd. C 82.13, H 5.43, N 5.81; found C
82.00, H 5.44, N 5.80.
2-{2-[4-(Dimethylamino)phenyl]ethyn-1-yl}-9,9-dimethyl-7-nitrofluor-
ene (5a): In a Schlenk tube, a mixture of 13 (151 mg, 0.415 mmol),
Pd(PPh3)2Cl2 (15 mg, 0.022 mmol), anhydrous DMF (15 mL), and
NEt3 (0.59 mL, 4.24 mmol) was stirred at r.t. for 5 min under an
inert atmosphere. Compound 12 (123 mg, 0.846 mmol) was added
and the solution was stirred at 90 °C for 6 h. After cooling to r.t.,
the solvent was removed under vacuum and the crude material was
dissolved in ethyl acetate (50 mL) and washed with water
(3ϫ20 mL). The organic phase was dried with Na2SO4 and the
solvent was removed under vacuum. The crude material was puri-
fied by flash chromatography (hexane/CH2Cl2, 9:1) to give 5a
(86 mg, 54%) as an orange solid; m.p. 236–239 °C. 1H NMR
(400 MHz, CDCl3, 25 °C): δ = 8.29 (d, JH,H = 2.0 Hz, 1 H, 8-H),
2-[4-(Dimethylamino)phenyl]-9,9-dimethyl-7-nitrofluorene (3a): In a
Schlenk tube, a stirred mixture of 13 (201 mg, 0.552 mmol), 10
(272 mg, 1.650 mmol), aqueous K2CO3 (1 , 3.3 mL, 3.3 mmol),
and Pd(PPh3)4 (25 mg, 0.022 mmol) in anhydrous THF (6 mL) was
refluxed overnight under an inert atmosphere. After cooling to r.t.,
THF was removed under vacuum and the aqueous phase was ex-
tracted with CH2Cl2 (2ϫ10 mL). The collected organic phases
were dried with Na2SO4 and the solvent was removed under vac-
uum. The crude material was purified by chromatography (hexane/
ethyl acetate, 95:5) to give 3a (126 mg, 64%) as an orange solid;
m.p. 215–221 °C. 1H NMR (400 MHz, CDCl3, 25 °C): δ = 8.30 (d,
JH,H = 2.0 Hz, 1 H, 8-H), 8.27 (dd, JH,H = 8.3, JH,H = 2.1 Hz, 1
H, 6-H), 7.80 (m, 2 H, 4-H fluorene, 5-H fluorene), 7.66 (d, JH,H
= 1.5 Hz, 1 H, 1-H), 7.63–7.58 (m, 3 H, 3-H, 2-H phenyl, 6-H
phenyl), 6.85 (br. d, JH,H = 8.1 Hz, 2 H, 2-H phenyl, 6-H phenyl),
3.04 (s, 6 H, N-CH3), 1.59 (s, 6 H, fluorene-CH3) ppm. 13C NMR
(101 MHz, CDCl3, 25 °C): δ = 155.83 (q), 154.70 (q), 146.87 (q),
145.87 (q), 142.78 (q), 134.74 (q), 127.92 (2-C phenyl, 6-C phenyl),
125.87 (3-C), 123.52 (6-C), 121.72 (5-C), 120.62 (1-C), 119.78 (4-
C), 118.28 (8-C), 112.79 (3-C phenyl, 5-C phenyl), 47.34 (9-C),
40.57 (CH3-N), 26.92 (CH3-fluorene) ppm. UV/Vis (CHCl3): λmax
(ε, mol–1 dm3 cm–1) = 297 (23300), 402 (26100) nm. HRMS (ESI+):
calcd. for [M + H]+ C23H23N2O2 359.1754; found 359.1754.
C23H22N2O2 (358.43): calcd. C 77.07, H 6.19, N 7.82; found C
77.00, H 6.20, N 7.84.
8.26 (dd, JH,H = 8.3, JH,H = 2.1 Hz, 1 H, 6-H), 7.78 (d, JH,H
=
8.3 Hz, 1 H, 5-H), 7.75 (d, JH,H = 7.9 Hz, 1 H, 4-H), 7.62 (d, JH,H
= 0.9 Hz, 1 H, 1-H), 7.54 (dd, JH,H = 7.9, JH,H = 1.4 Hz, 1 H, 3-
H), 7.44 (d, JH,H = 8.9 Hz, 2 H, 2-H phenyl, 6-H phenyl), 6.68 (d,
JH,H = 8.9 Hz, 2 H, 3-H phenyl, 5-H phenyl), 3.02 (s, 6 H, N-CH3),
1.56 (s, 6 H, fluorene-CH3) ppm. 13C NMR (101 MHz, CDCl3,
25 °C): δ = 155.07 (q), 154.82 (q), 150.29 (q), 147.17 (q), 145.28
(q), 135.98 (q), 132.85 (2-C phenyl, 6-C phenyl), 130.84 (3-C),
125.78 (1-C), 125.24 (q), 123.51 (6-C), 121.33 (4-C), 120.19 (5-C),
118.32 (8-C), 111.85 (3-C phenyl, 5-C phenyl), 109.65 (q), 92.55 (2-
C ethynyl), 87.82 (1-C ethynyl), 47.33 (9-C), 40.19 (CH3-N), 26.73
(CH3-fluorene) ppm. UV/Vis (CHCl3): λmax (ε, mol–1 dm3 cm–1) =
311 (26400), 405 (27700) nm. HRMS (ESI+): calcd. for [M + H]+
C25H23N2O2 383.1754; found 323.1746. C25H22N2O2 (382.45):
calcd. C 78.51, H 5.80, N 7.32; found C 78.70, H 5.80, N 7.34.
Acknowledgments
Financial support by Fondazione CARIPLO (project title: “Prepa-
razione strutture altamente organizzate – PRESTO”) and Fondo
per gli Investimenti della Ricerca di Base (FIRB), project number
RBIP0642YL (title: “Sorgenti di luce innovative ad alta efficienza
(E)-2-{2-[4-(Dimethylamino)phenyl]ethen-1-yl}-9,9-dimethyl-7-nitro-
fluorene (4a): In a Schlenk tube, a mixture of 13 (162 mg,
0.443 mmol), K2CO3 (65 mg, 0.472 mmol), TBAB (144 mg, per dispositivi illuminanti a stato solido con impiego civile ed auto-
0.447 mmol), anhydrous DMF (5 mL), and 11 (73 mg, 0.496 mmol)
dissolved in anhydrous DMF (2 mL) was stirred at r.t. for 30 min.
Then Pd(OAc)2 (5 mg, 0.024 mmol) was added and the solution
was vigorously stirred at 105 °C for 6 h. After cooling to r.t., DMF
motive – LUCI” and “Sviluppo di componenti e soluzioni tecnolo-
giche integrate per display di nuova generazione – NODIS”) of
Ministero dell’Università e della Ricerca (MIUR) is gratefully ac-
knowledged.
Eur. J. Org. Chem. 2010, 4004–4016
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4015

S. Quici et al.
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Received: March 10, 2010
Published Online: June 11, 2010
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