Difluorenyl carbo-Benzenes: Synthesis, Electronic Structure, and Two-Photon Absorption Properties of Hydrocarbon Quadrupolar Chromophores

Article abstract of DOI:10.1002/chem.201500482

The synthesis, crystal and electronic structures, and one- and two-photon absorption properties of two quadrupolar fluorenyl-substituted tetraphenyl carbo-benzenes are described. These all-hydrocarbon chromophores, differing in the nature of the linkers b

Full text of DOI:10.1002/chem.201500482

DOI: 10.1002/chem.201500482
Full Paper
&
Chromophores
Difluorenyl carbo-Benzenes: Synthesis, Electronic Structure, and
Two-Photon Absorption Properties of Hydrocarbon Quadrupolar
Chromophores
[
a, b, c]
[d]
[d]
Iaroslav Baglai ,
Corentin Poidevin,
Manuel de Anda-Villa, Rodrigo M. Barba-Barba,
[a, b]
[d]
[a, b]
[a, b]
Gabriel Ramos-Ortíz,* ValØrie Maraval,*
Christine Lepetit,
[b]
[d]
[a, b]
Nathalie Saffon-Merceron, JosØ-Luis Maldonado, and Remi Chauvin*
Abstract: The synthesis, crystal and electronic structures,
and one- and two-photon absorption properties of two
quadrupolar fluorenyl-substituted tetraphenyl carbo-ben-
zenes are described. These all-hydrocarbon chromophores,
differing in the nature of the linkers between the fluorenyl
substituents and the carbo-benzene core (CÀC bonds for 3a,
CÀCꢀCÀC expanders for 3b), exhibit quasi–superimposable
one-photon absorption (1PA) spectra but different two-
photon absorption (2PA) cross-sections s2PA. Z-scan measure-
ments (under NIR femtosecond excitation) indeed showed
that the CꢀC expansion results in an approximately twofold
increase in the s
value, from 336 to 656 GM (1 GM=
2
PA
À1
À50
4
À1
10 cm smolecule photon ) at l=800 nm. The first ex-
cited states of A and A symmetry accounting for 1PA and
u
g
2PA, respectively, were calculated at the TDDFT level of
theory and used for sum-over-state estimations of s2PA(l), in
i
which l =2hc/E, h is Planck’s constant, c is the speed of
i
i
light, and E is the energy of the 2PA-allowed transition. The
i
calculated s2PA values of 227 GM at 687 nm for 3a and
349 GM at 708 nm for 3b are in agreement with the Z-scan
results.
Introduction
properties are associated with the corresponding abundance
of low-lying electronic excited states of versatile nature that
can be selectively targeted by suitable laser wavelengths. In
particular, the design of chromophores with the third-order
Among the development prospects envisaged for highly con-
jugated p-electron-rich compounds, nonlinear optical (NLO)
[
1]
NLO property of two-photon absorption (2PA) has recently
aroused intense activity because of the wide range of applica-
[
a] Dr. I. Baglai , C. Poidevin, Dr. V. Maraval, Dr. C. Lepetit, Prof. Dr. R. Chauvin
[
2]
tions that can be foreseen, such as microfabrication, optical
CNRS, LCC (Laboratoire de Chimie de Coordination)
[
3]
[4]
2
05 route de Narbonne, BP 44099, 31077 Toulouse CEDEX 4 (France)
limitation, three-dimensional data storage, photodynamic
[
5]
[6]
E-mail: chauvin@lcc-toulouse.fr
valerie.maraval@lcc-toulouse.fr
therapy, and two-photon fluorescence microscopy. In the
last few years, qualitative structure–property relationships have
been proposed for the design of novel organic dyes for 2PA.
For example, quadrupolar chromophores based on a D–p–D or
a D–p–A–p–D pattern, in which p denotes a p-conjugated
core bridging donor (D) or acceptor (A) subunits, have been
shown to be generally more efficient than their counterparts
[
b] Dr. I. Baglai , C. Poidevin, Dr. V. Maraval, Dr. C. Lepetit,
Dr. N. Saffon-Merceron, Prof. Dr. R. Chauvin
UniversitØ de Toulouse, UPS, ICT-FR 2599
3
1062 Toulouse CEDEX 9 (France)
[
c] Dr. I. Baglai
Kiev National Taras Shevchenko University
6
0 Volodymyrska Str., 01033 Kiev (Ukraine)
[7]
based on an A–p–A or A–p–D–p–A pattern. The nature of
[
d] M. de Anda-Villa, R. M. Barba-Barba, Dr. G. Ramos-Ortíz,
Dr. J.-L. Maldonado
both the A and D substituents and the p connectors plays sys-
tematic and complementary roles. In a specific chromophore
series, an increase in the extent of p conjugation was reported
Centro de Investigaciones en Óptica A.P.
1
-948, 37000 León, Gto. (MØxico)
[
8]
Fax: (+33)5-61-55-30-03
E-mail: garamoso@cio.mx
to strongly enhance the 2PA cross-section (s2PA). Likewise, op-
timization of the p-orbital overlap with and within the p con-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500482. It contains the synthetic princi-
ple of 5b, photoluminescence spectra of 3a and 3b, the normalized trans-
mittance curve of 3b from Z-scan measurements, the near-frontier molecu-
lar orbitals of 3a and 3b, characteristics of the first singlet excited states of
2
nectors, for example, by means of cyclic structures and p CꢀC
units of enhanced rigidity (lowering the effective degrees of
conformational freedom, in particular in thienylene–acetylene–
ethylene macrocycles), was also reported to enhance the 2PA
3
a and 3b (in vacuo and chloroform continuum), the contribution of inter-
[9]
efficiency. These structural parameters are naturally interde-
mediate 1PA-allowed states in the SOS expansion of the 2PA cross-section
[
10]
1
pendent, but can a priori guide the design of new targets. In
associated with the 4A
g
excited state of 3a and 3b, and copies of H and
13
C NMR spectra, MS analyses, and voltammograms of 3a and 3b.
this spirit, p-macrocyclic dyes based on porphyrin cores have
Chem. Eur. J. 2015, 21, 14186 – 14195
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[
11]
been widely studied as 2PA candidates. An alternative to the
ophilic fluorenyl groups were anchored to the C18 macrocycle
either directly by using 5a or through an ethyndiyl linker by
means of 5b, itself obtained from 5a (see Figure S1 in the Sup-
porting Information). Addition of the lithium salt of 5a and
the bromomagnesium salt of 5b to the [6]pericyclynedione 4
afforded the [6]pericyclynediols 6a and 6b as unresolved mix-
tures of diastereoisomers in 41 and 50% yield, respectively.
Treatment of 6a and 6b with the acidic reducing system
Hückel-aromatic 18-p-electron C N circuits of the square-
1
6
2
shaped heteropentacyclic porphyrins is the 18-p -electron C18
z
[
21]
circuit of the hexagon-shaped all-carbon macrocyclic carbo-
[
12]
benzenes. Beyond the topological analogy, porphyrins and
carbo-benzenes were shown to exhibit a strong analogy after
Gouterman’s four-orbital model for their one-photon absorp-
[
12d,13]
tion (1PA) spectra.
In the recent years, efforts in carbo-mer chemistry have fo-
cused on the synthesis of quadrupolar para-disubstituted
SnCl /HCl in dichloromethane and then with aqueous NaOH
2
produced the targeted p-difluorenyl carbo-benzenes 3a and
3b in 59 and 51% yield, respectively (Scheme 1). These carbo-
chromophores were isolated as dark green-gold solids, soluble
in most of the classical organic solvents, giving intense violet
solutions. They were fully characterized by NMR and UV/Vis
spectroscopy (see below), mass
[12d,14]
carbo-benzenic chromophores.
theoretical studies on the p-dianisyl carbo-benzene 1,
is, the central-ring carbo-mer of the p-terphenyl fluorophore 2
Previous experimental and
[
12d]
that
[15]
(
Figure 1), suggested 1 as a promising 2PA candidate. Unlike
spectrometry, and voltammetry
(
see Experimental Section and
Supporting Information). Their
structures were confirmed by X-
ray diffraction analysis of single
crystals deposited from dichloro-
methane (Figure 2, Table 2).
Figure 1. p-Dianisyl carbo-benzene 1 and its parent terphenyl fluorophore 2.
2
, however, 1 is not fluorescent, and this prevents 2PA cross-
section measurements by the two-photon excitation fluores-
[
16]
cence method. Moreover, the poor solubility of 1 prevented
the determination of s2PA by the alternative Z-scan method,
À2
which requires the use of highly concentrated (10 m) solu-
[17]
tions of the chromophore. Quadrupolar carbo-benzenes sub-
stituted by solubilizing hydrocarbon fluorophoric motifs were
thus suggested as alternative targets.
In this regard, alkyl-substituted fluorenyl groups have been
widely used as weak donor substituents in D–(p–A)–p–D sys-
tems, because of their intrinsic physicochemical properties
such as high fluorescence quantum yield and pronounced op-
[
18]
tical nonlinearities. p-Difluorenyl carbo-benzenes were thus
envisaged. The proposed targets 3a and 3b (Scheme 1) differ
in their extent of conjugation: the fluorenyl groups are con-
nected to the carbo-benzene core by CÀC single bonds in 3a
and CÀCꢀCÀC expanders in 3b. In this work, the synthesis,
spectroscopic and structural characteristics, and 2PA optical
properties of these chromophores are addressed by experi-
mental and theoretical investigations.
Results and Discussion
Synthesis of carbo-chromophores 3a and 3b
The synthesis of 3a and 3b started from common [6]peri-
cyclynedione precursor 4, the synthesis of which had been op-
[
19]
timized. With a view to enhancing the solubility of the tar-
geted carbo-chromophores, C9-dialkylated fluorenyl substitu-
Scheme 1. Synthesis of the p-difluorenyl-conjugated carbo-benzenes 3a and
3b.
[20]
ents were envisaged, starting from 2-bromofluorene. The lip-
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Figure 3. UV/Vis absorption spectra of p-difluorenyl carbo-benzenes 3a and
3
3
b (in CHCl ).
Information), with quasi-degenerate HOMO (H) and HOMOÀ1
HÀ1) on the one hand, and LUMO (L) and LUMOÀ1 (LÀ1) on
the other. The energy and nature of the first excited states of
a and 3b are also very similar (Tables S2 and S3 in the Sup-
(
3
Figure 2. XRD molecular views of the p-difluorenyl carbo-benzenes 3a (top)
and 3b (bottom) with thermal ellipsoids drawn at the 50% probability level.
H atoms, disordered atoms, and solvent (3b) are omitted for clarity.
porting Information). For both the chromophores solvato-
chromism appears to be stronger for the 2A and 3A excited
u
u
states responsible for the UV/Vis absorption maxima (Support-
ing Information, Table S3).
1PA electronic spectra and first excited states of 3a and 3b
According to the Laporte rules, only transitions from the
Electronic spectra of 3a and 3b in CHCl solutions show that
ground state of gerade symmetry A to excited states of un-
3
g
the increase of the extent of conjugation from 3a to 3b does
gerade symmetry A are 1PA-allowed. In both 3a and 3b, the
u
not affect significantly the UV/Vis absorption properties
first four excited states are thus allowed for 1PA and can be
described by linear combinations of one-electron excitations
from the two highest occupied MOs (HÀ1, H) to the two
lowest unoccupied ones (L, L+1). The 1A and 4A excited
(
Figure 3). The two chromophores exhibit almost identical
maximum absorption wavelengths of lmax =493Æ1 nm and
À1
À1
similar absorption coefficients (eꢁ330000 Lcm mol for 3a,
u
u
À1
À1
eꢁ380 000 Lcm mol
for 3b). This unexpected finding
states involve the same jHÀ1!L> and jH!L+1> excita-
tions in complementary additive and subtractive manners, cor-
responding respectively to destructive and constructive transi-
tion densities and finally to the related vanishing or enhanced
oscillator strengths. Likewise, the excited states 2A and 3A in-
prompted theoretical analysis of the first excited states (see
below). Weak absorption bands also occur in the NIR region at
8
82 and 891 nm for 3a and 3b, respectively (see inset in
Figure 3).
In the context of the Laporte selection rules, the geometries
of chromophores 3a and 3b were calculated at the B3PW91/6-
1G** level of theory under centrosymmetry constraint (the Ci
u
u
volve the same jH!L> and jHÀ1!L+1> excitations corre-
sponding to destructive and constructive transition densities,
respectively. This picture matches with the Gouterman four-or-
bital model for the interpretation of the absorption spectra of
3
symmetry occurring in the experimental crystal structure:
Figure 2). 1PA spectra of 3a and 3b, calculated at the TDDFT
level of theory in vacuo, exhibit a weak hypsochromic (blue)
shift with respect to the experimental spectra (Figure 4). The
matching between calculated and experimental spectra is im-
proved by taking into account the chloroform solvent through
[
13]
porphyrins, in which the intense Soret band (B band) corre-
sponds to quasi-degenerate 4A and 3A excited states, and
u
u
the weaker Q bands correspond to the lower 1A and 2A ex-
u
u
cited states. In the case of the carbo-benzenic chromophores
3a and 3b, whereas the 1A and 2A excited states exhibit
u
u
a
continuum of dielectric constant eCHCl3 =4.7113 (PCM
vanishing oscillator strengths (and thus underestimate the
weak pseudo-Q bands experimentally recorded at 550 and
600 nm; see Figure 3), the 3A and 4A excited states account
method). Notably, the calculations reproduce the experimental-
ly observed negligible influence of the insertion of two CꢀC
conjugation expanders on the absorption spectra (Figure 3),
and more precisely on the lmax values (ca. 10 nm residual red-
shift for 3b vs. 3a).
u
u
almost perfectly for the maximum absorption bands observed
at 493 nm (Supporting Information, Table S3 and Figure 3).
The scheme of the Gouterman model actually holds for the
interpretation of the maximum UV/Vis absorption band of all
Accordingly, the near-frontier p MOs of 3a and 3b are very
similar in both shape and energy (Table S1 in the Supporting
[
12c]
carbo-benzenic chromophores studied to date.
In the pres-
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carbo-benzenic core (the C units of 3b make no contribution
2
to HÀ1 and L+1, and only secondary contributions to H and
L; Supporting Information, Table S1). Residual discrepancies be-
tween calculated and experimental spectra (regarding the in-
tensity of the pseudo-Q bands in particular), might be tenta-
tively attributed to vibronic couplings, which were not taken
into account in the present calculations.
Finally, the fluorenyl-substituted carbo-benzenes 3a and 3b
were found to exhibit very weak fluorescence at about 650 nm
[
22]
on excitation at 532 nm (Supporting Information, Figure S2).
Z-scan measurements of the 2PA of 3a and 3b
[
22]
Because of their very poor fluorescence, the 2PA cross-sec-
tions of carbo-chromophores 3a and 3b were measured by
the Z-scan technique. The Z-scan method indeed allows con-
[
17b]
venient investigations of third-order NLO properties,
not
only nonlinear refraction (self-focusing or self-defocusing phe-
nomena), but also nonlinear absorption of materials. The
method consists of monitoring the transmittance of the NLO-
active sample scanned in the vicinity (Z position) of the focus
point of a focused laser beam. In the study of nonlinear ab-
sorption properties of 3a and 3b in solution, Z-scan measure-
ments were performed with a train of femtosecond laser
pulses at a wavelength of 800 nm and extended to different
wavelengths in the case of 3b, which reached the largest
values of the 2PA cross-section s2PA (see below).
Figure 5a shows the open-aperture Z-scan curves for 3a and
À2
3
b in chloroform at a concentration of 10 m and a laser peak
À2
intensity at the focal region of 25 GWcm . Clearly, the nonlin-
ear absorption is more intense in 3b than in 3a. Owing to the
type of excitation (femtosecond pulses far from the linear ab-
sorption band, see Figure 3), the recorded nonlinear absorp-
tions may originate from a 2PA process. By fitting data to
Equation (5) given in the Experimental Section, it was found
Figure 4. Experimental absorption spectra of 3a (top) and 3b (bottom) in
CHCl solution (solid line) and calculated counterparts. Level of calculation:
TD-CAM-B3LYP/6-31G** in vacuo (dashed line) or PCM-TD-CAM-B3LYP/6-
1G** in the CHCl continuum (dash-dotted line).
that the nonlinear absorption coefficients b at 800 nm are
3
À1
about 0.082 and 0.161 cmGW , resulting in s2 values of 336
PA
3
3
and 656 GM for 3a and 3b, respectively. This twofold increase
in the s2PA value from 3a to 3b is explained by the more ex-
tended p conjugation in 3b due to the CꢀC expansion. Al-
though these values are smaller than those measured for por-
phyrin arrays, they are similar to those found for other quadru-
polar structures such as stilbene, fluorene, and dihydrophenan-
ent case, since the p-conjugation extent of 3b is larger than
that of 3a, the maximum absorption wavelength and the cor-
responding calculated oscillator strengths are slightly higher
for the former, as observed experimentally for the correspond-
ing lmax and e values (Figures 3 and 4, Table S2 of the Support-
ing Information). The increase in oscillator strength from 3a to
threne derivatives, for which the s
value remains in the
PA
2
2
3
[1b]
range 10 –10 GM.
The Z-scan experiments on 3b were extended to other
wavelengths. By pumping the sample with the beam of an op-
tical parametric amplifier it was possible to evaluate nonlineari-
ties and to determine s2PA values of this compound in the
wavelength range 650–900 nm. Figure 5b shows that the non-
linearities tend to increase at shorter wavelengths. For in-
stance, the value of s2PA is artificially enhanced at 650 nm by
a resonant effect of the nonlinear absorption, since at this
wavelength 3b exhibits an absorption tail (see Figure 3).
3
b can be correlated with an increase of the overall p-conju-
gation efficiency due to steric relaxation between the substitu-
ents on insertion of the CꢀC units: the dihedral angles be-
tween the phenyl substituents and the carbo-benzene core are
indeed smaller in 3b (ca. 128) than in 3a (ca. 228).
The very small experimental shift in lmax between 3a and
3
b (ca. 2 nm; Figure 3) is in agreement with the weak corre-
sponding calculated value (12 nm in vacuo, 16 nm in CHCl , on
3
average for the 3A and 4A states; Supporting Information, Ta-
To further corroborate the two-photon excitation of 3b,
femtosecond transient absorption (TA) experiments were per-
u
u
bles S2 and S3), and is accounted for by excitations involved in
the corresponding excited states, confined in the common C18
[
23]
formed in a pump–probe configuration.
The TA was ob-
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Figure 6. Transient absorption of 3b for one-photon and two-photon excita-
tion. The excited states were monitored with an energy corresponding to
7
35 nm of the probe beam. The biexponential fitting to experimental data is
À2
shown. The solution used was 10 m in CHCl
3
. See details in Experimental
Section.
3
a and 3b exhibit strong linear (one-photon) absorption. For
instance, at 532 nm the samples showed saturable absorption
even at very low peak intensities in dilute solutions. In this
case, the effect can be due to depletion of the ground state,
since the use of femtosecond pulses implies high photon den-
sity. In fact, depending on the laser-pulse duration, contribu-
tions corresponding to different physical mechanisms can be
part of the nonlinear absorption. The chromophore 3b was
therefore tested in a Z-scan experiment at 532 nm, but now
with subnanosecond pulses (5 Hz repetition rate) with tempo-
ral duration comparable with the lifetime of excited states so
that other nonlinear process, such as reverse saturable absorp-
tion due to absorption of excited states, can occur. Neverthe-
less, using laser pulses of 0.3 ns resulted again in saturable ab-
sorption (see Figure S3 in the SI showing the open-aperture Z-
scan curves for 3b in chloroform for solutions of linear trans-
mittance within the range 30-60%). In this case nonlinear ab-
Figure 5. a) Normalized transmittance T(Z) of 3a and 3b from open-aperture
Z-scan experiments (femtosecond excitation, l=800 nm) at a peak intensity
À2
of I
p
=25 GWcm . Continuous lines correspond to theoretical fits of experi-
mental data. b) 2PA cross-section of 3b as a function of wavelength. The sol-
À2
utions used were 10 m in CHCl
3
.
tained for two different pump energies, the first one at 800 nm
inducing nonlinear absorption (Figure 5), and the second one
at 400 nm corresponding to one-photon excitation energies
(
Figure 3). In both cases, the dynamics of the produced excited
states were monitored through the TA of the sample by set-
ting the probe beam at 735 nm. The decay of the TA signals
for the two pump energies fitted satisfactorily with biexponen-
tial expressions (Figure 6). From these results, the excited
states produced by one-photon excitation (400 nm) exhibit
a fast decay of 769 fs followed by a slow decay of 17 ps, and
those produced by two-photon excitation (800 nm) exhibit
a much faster decay of 170 fs followed by a slow decay of
À1
sorption coefficients b in the range À(0.365–1.14) cmGW
were found.
Theoretical studies on the 2PA efficiency of the carbo-chro-
mophores 3a and 3b
1
4.8 ps. On excitation at 800 nm, a coherent interaction be-
tween the pump and probe beams is clearly evidenced by the
spike at the beginning of the TA signal. This is due to a nonde-
generate 2PA process in which the molecule is excited by si-
multaneous absorption of one photon at 800 nm and one
photon at 735 nm. As expected, such coherent interaction
does not occur on excitation at 400 nm. This validates the as-
sumption that the nonlinear absorption by fluorophore-substi-
tuted carbo-benzene 3b (see Figure 5b) is due to 2PA when
femtosecond excitation in the wavelength range 650–900 nm
is utilized. In contrast, an increase in normalized transmittance
was observed in the open-aperture Z-scan traces as the femto-
second excitation was tuned to shorter wavelengths at which
For centrosymmetric chromophores 3a and 3b, which have
a ground state of A symmetry, only the excited states of the
g
same parity, namely, Ag excited states, are 2PA-allowed (re-
versed Laporte rules for 1PA). The corresponding 2PA cross-
sections were therefore calculated by using the sum-over-state
(SOS) approach implemented in the DALTON2011 package
[
24]
[Eqs. (1)–(3) in the Experimental Section]. Excited states were
calculated at the same TD-CAM-B3LYP/6-31G** level of theory
as used for 1PA studies (see above). To match the experimental
window of the Z-scan 2PA measurements, the calculations
were restricted to the first four excited states of A symmetry.
g
Results are summarized in Table 1. In both cases, the maximum
Chem. Eur. J. 2015, 21, 14186 – 14195
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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nate the 2PA response because of its proximity to half the
Table 1. Calculated 2PA cross-sections of 3a and 3b at the TD-CAM-
B3LYP/6-31G** level of theory.
energy of the 4A excited state, that is, DE close to zero in
g
Equation (4). Like many organic counterparts, the carbo-chro-
SOS
Chromophore Excited
state
Energy [eV] Photon wavelength [n- s [GM]
2PA
mophores 3a and 3b thus fit into the three-level model (Fig-
[
a]
m]
[1c,10,25]
ure 7).
Indeed, for both of them, the contribution of the
2
3
4
5
2
3
4
5
A
A
A
A
A
A
A
A
g
g
g
g
g
g
g
g
3.18
3.40
3.61
3.76
3.01
3.33
3.50
3.66
780
729
687
659
824
745
708
678
28
1
228
35
51
12
2
A excited state to the 2PA cross-section amounts to about
u
3
3
a
b
349
49
[
a] Wavelength corresponding to half the energy of the excited state.
2
PA cross-section corresponds to the 4A excited state and is
g
found to occur for a photon wavelength of 687 nm for 3a and
7
7
08 nm for 3b, in agreement with experiment for 3b (ca
90 GM at 750 nm, see Figure 5a). The chromophore 3b is ac-
cordingly predicted to be more efficient than 3a, but the cal-
SOS
SOS
culated relative cross-section (s2PA(3b)/s2PA(3a)=1.53) is lower
EXP
than the experimental value measured at 800 nm s2PA(3b)/
EXP
s2PA(3a)=1.93).
Table 2. Crystallographic data for 3a and 3b.
Figure 7. Three essential states accounting for 80% of the 2PA efficiency of
a (left) and 3b (right). Transition dipole moments, energies, and 2PA cross-
sections s2PA [GM] calculated at the TD-CAM-B3LYP/6-31G** level of theory.
3
3
a
3b
SOS
formula
C
92
H
86
96 2 2
C H86·0.25CH Cl
M
r
1191.61
1260.88
crystal system
space group
a [Š]
b [Š]
c [Š]
monoclinic
tetragonal
P2
1
/n
P4
2
/n
81% (184 versus 227 GM for 3a and 285 versus 349 GM for
3b). The relevance of Equation (4) also allows further analysis
of the origin of the exaltation of the 2PA efficiency by insertion
8.6508(3)
23.1726(7)
17.3511(6)
90
26.535(2)
26.535(2)
11.1134(9)
90
of C units between the carbo-benzenic core and the fluorenyl
2
a [8]
b [8]
92.018(2)
90
3476.1(2)
2
53164
8580
90
90
extremities of 3a in 3b (Figure 7). The DE value of 3a (0.12 eV)
is very close to but slightly smaller than that of 3b (0.13 eV),
and is thus assumed to modify the 2PA cross-section of 3a as
compared to 3b by a factor of 0.85. The transition dipole mo-
ments of the two intermediate 1PA transitions of 3b (3.49 and
2.36 a.u.) are higher than those of 3a (3.40 and 1.70 a.u.). The
corresponding enhancement factor expected from Equation (4)
g [8]
3
V [Š ]
7824.7(10)
4
95276
7932
[R(int)=0.2477]
1.070
0.077
0.30”0.04”0.02
0.977
0.0745
0.2597
0.205/À0.246
Z
reflns collected
independent reflns
[
R(int)=0.0509]
À3
1calcd [gcm
]
À1
1.138
0.064
0.50”0.50”0.50
1.021
0.0534
0.1502
0.315/À0.297
m(MoKa) [mm
crystal size [mm]
GOF on F
]
(
see Experimental Section) is therefore estimated to be 2.11.
2
The overall 2PA probability enhancement factor of 0.85”
2.03=1.73 thus originates mainly from the difference in the
transition dipole moments resulting from insertion of the CꢀC
units (Figure 7), which increases the p-conjugation extent in
the chromophore 3b.
R [I>2s(I)]
2
wR (all data)
largest difference peak/
hole [eŠ
À3
]
Conclusion
3
PA
L
The s2 contributions of each intermediate excited state of
SOS
A symmetry to the 2PA cross-section s2PA related to the 4Ag
The above-described chromophores 3a and 3b, differing by
u
excited state were estimated by using the approximation of
Equation (4) and the transition dipole moments and excited
states energies, calculated at the TD-CAM-B3LYP/6-31G** of
theory level with DALTON2011 (Supporting Information, Ta-
bles S4 and S5). For both 3a and b, the major contribution
the presence or not of C expanders between the carbo-ben-
2
zene core and the fluorenyl groups, extend the scope of quad-
rupolar carbo-benzenes for linear and nonlinear optical proper-
ties in both the all-hydrocarbon series and the fluorophore-
substituted series. Quenching of the intrinsic fluorophoric char-
acter of substituents of the carbo-benzene core has now fur-
arises from the 2A excited state, which was expected to domi-
u
Chem. Eur. J. 2015, 21, 14186 – 14195
www.chemeurj.org
14191
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
ther been illustrated in the fluorenyl series. The low absolute
ꢂ
value of the first reduction potential of 3a (E =À0.75 V vs.
1
=2
ꢂ
SCE) and 3b (E1=2 =À0.61 V vs. SCE, see Experimental Section
and Supporting Information) suggests that the carbo-benzene
core, although highly electron rich, could play the role of an
ꢂ
acceptor, just like fullerenes do (e.g., C : E =À0.41 V vs.
6
0
1=2
[
26]
SCE). As previously evidenced for structurally related fluoren-
[27]
[28]
yl-substituted benzothiadiazole
and porphyrin
chromo-
phores, expansion of the links between the central accepting
Synthesis
core and the fluorenyl donating units of 3a by C units in 3b
2
results in a significant (ca. twofold) enhancement of the 2PA ef-
ficiency. This was shown by Z-scan measurements of the s2PA
cross-section (femtosecond excitation at l=800 nm). The oc-
currence of 2PA excitation was experimentally corroborated by
femtosecond TA experiments, and further confirmed by TDDFT
calculations of the corresponding excited states and of the
third-order NLO response, which was analyzed in the frame-
work of the three-level model by using the SOS formalism. At
shorter wavelengths, the observed increase in nonlinearity of
1,10-Bis(9,9-dihexyl-9H-fluoren-2-yl)-4,7,13,16-tetramethoxy-
4,7,13,16-tetraphenylcyclooctadeca-2,5,8,11,14,17-hexayne-1,10-
diol (6a): nBuLi (96 mL, 0.24 mmol) was added to a solution of 2-
[20]
bromo-9,9-dihexyl-9H-fluorene
(116 mg, 0.28 mmol) in THF
(
15 mL) with stirring at À788C. The mixture was stirred for 1 h at
À788C, when 5a was assumed to be quantitatively generated, and
a solution of [6]pericyclynedione 4 (68 mg, 0.1 mmol) in THF (3 mL)
was added at the same temperature. The mixture was allowed to
warm slowly to 08C over 3 h. Then, saturated aqueous NH Cl was
4
added. The aqueous layer was extracted with diethyl ether and the
combined organic layers were washed with brine, dried over
3
b was shown to mainly result from the resonant effect.
MgSO , and concentrated to dryness under reduced pressure. The
residue was purified by silica-gel chromatography (EtOAc:pentane
These results encourage further investigations of the scope
4
of carbo-meric structures in the design of organic NLO materi-
als. In particular, the specific quadrupolar all-hydrocarbon
nature of 3a and 3b deserves to be delineated with respect to
1
6
:9) to give 6a as a light yellow solid (55 mg, 41% yield). M.p.
1
98C; R (EtOAc:heptane 4:6)=0.53; H NMR (CDCl ): d=0.52–1.11
f
3
(
m, 44H, (CH ) CH ), 1.96 (m, 8H, C1-Fluo(CH ) ), 3.00–3.30 (m, 2H,
2 4 3 2 2
[29]
related octupolar all-hydrocarbon systems. Finally, the influ-
ence of the aromaticity of the central C18 core on the enhance-
ment of the TPA efficiency deserves to be investigated in com-
OH), 3.30–3.80 (m, 12H, OCH ), 7.29–7.53 (m, 18H, H Ar), 7.57–8.03
3
13
1
(m, 16H, H Ar); C{ H} NMR (CDCl ): d=14.0 (CH CH ), 22.6, 23.8,
3
2
3
29.7, 31.5 ((CH ) CH ), 40.3 (CH -C1-Fluo), 53.4 (OCH ), 55.2 (C1-
2
4
3
2
3
[30]
parison to nonaromatic analogues of 3a and 3b.
Fluo), 65.5 (C-(Fluo)OH), 72.0 (C-(Ph)OMe), 82.8, 84.6, 87.1 (ÀCꢀCÀ),
119.7, 120.0, 120.2, 123.0, 124.8 (C6-, C9-, C10-, C12-, C13-Fluo),
1
1
26.5, 126.9, 127.5, 128.6, 129.1 (o-, m-, p-Ph, C7-, C8-Fluo), 139.4,
39.7, 140.3, 142.0 (i-Ph, C3-, C4-, C11-Fluo), 151.1, 151.3 (C2-, C5-
Experimental Section
+
Fluo); MS (MALDI-TOF/DCTB): m/z: 1371.7 [M+Na] ; HRMS (MALDI-
TOF/DCTB): m/z calcd for C H O Na [M+Na] : 1371.7418, found:
+
General
96
100
6
1
371.7491.
THF and diethyl ether were dried and distilled over sodium/benzo-
phenone, and pentane and dichloromethane over P O . All other
2
5
1,10-Bis[2-(9,9-dihexyl-9H-fluoren-2-yl)ethynyl]-4,7,13,16-tetra-
methoxy-4,7,13,16-tetraphenylcyclooctadeca-2,5,8,11,14,17-hex-
ayne-1,10-diol (6b): EtMgBr (102 mL, 0.3 mmol) was added to a so-
lution of 2-ethynyl-9,9-dihexyl-9H-fluorene (120 mg, 0.33 mmol)
in THF (10 mL) at 08C. The reaction mixture was stirred for 1 h at
reagents were used as commercially available. In particular, com-
mercial solutions of nBuLi were 2.5m in hexane, solutions of ethyl-
magnesium bromide were 3m in THF, and solutions of HCl were
[21]
2
m in diethyl ether. Previously described procedures were used for
[19a]
the preparation of pericyclynedione 4.
All reactions were carried
0
8C, when 5b was assumed to be quantitatively generated, and
a solution of [6]pericyclynedione 4 (95 mg, 0.14 mmol) in THF
3 mL) was added at the same temperature. The reaction mixture
out under nitrogen or argon by using Schlenk-tube and vacuum-
line techniques. Column chromatography was carried out on silica
gel (60 P, 70–200 mm). Analytical silica-gel TLC plates (60F254,
(
was allowed to warm slowly to room temperature and was stirred
0
.25 mm) were visualized by treatment with an ethanolic solution
overnight before hydrolysis with saturated aqueous NH Cl. The
4
of phosphomolybdic acid (20%). The following analytical instru-
aqueous layer was extracted with diethyl ether and the combined
1
13
ments were used: H and C NMR: Bruker Avance 300 or Avance
organic layers were washed with brine, dried over MgSO , and con-
4
4
1
00 spectrometer; mass spectrometry: Quadrupolar Nermag R10-
0H spectrometer; UV/Vis absorption: PerkinElmer UV/Vis Win-Lab
centrated to dryness under reduced pressure. The residue was pu-
rified by silica gel chromatography (EtOAc:pentane 1:9) to give 6b
Lambda 35 spectrometer; Yvon fluorescence: HORIBA Jobin Fluoro-
max-4 spectrofluorometer . The atom numbering Scheme used for
NMR assignments of the fluorenyl H and C nuclei is given below.
as a light yellow solid (98 mg, 50% yield). M.p. 648C; R (EtOAc:
heptane 2:8)=0.21; H NMR (CDCl3): d=0.58–1.13 (m, 44H,
f
1
1
13
(
CH ) CH ), 1.98 (m, 8H, C1-Fluo-CH ), 3.01–3.29 (m, 2H, OH), 3.38–
2 4 3 2
Cylclic and square-wave voltammetric measurements were per-
3.78 (m, 12H, OCH ), 7.31–7.58 (m, 22H, H Ar), 7.60–7.93 (m, 12H,
H Ar). C{ H} NMR (CDCl ) d 14.0 (CH ), 22.6, 23.7, 29.7, 31.5, 40.4,
3 3
(CH ), 53.6 (OCH ), 54.9 (C-(Fluo)OH), 55.2 (C1-Fluo), 71.8 (C-
2 3
3
13
1
formed with a Autolab PSSTAT100 potentiostat controlled by GPES
À3
4
.09 software at room temperature in dichloromethane (10 m in
3
a); supporting electrolyte: [nBu N][PF ] (0.1m); working electrode:
(Ph)OMe), 80.5, 80.6, 80.7, 84.5, 84.9, 85.1, 85.2 (CꢀC), 119.1 (C11-
Fluo), 119.6, 120.2, 122.9 (C6-, C9-, C13-Fluo), 126.5, 126.6, 126.9,
127.8, 128.6, 129.1, 131.1 (C7-, C8-, C10-, C12-Fluo, o-, m-, p-Ph),
4
6
Pt; reference electrode: SCE (0.242 V vs. the hydrogen electrode);
scan rate: 0.2 Vs . For reversible processes, half-wave potentials
measured by CV E1/2 =(Ep +E )/2 are given in volts versus SCE.
For irreversible processes, E values measured by CV are also given
À1
red
ox
139.3, 140.1, 142.5 (C -, C4-Fluo, i-Ph), 150.8, 151.1 (C2-, C5-Fluo);
p
3
+
HRMS (MALDI-TOF/DCTB): m/z calcd for C100H100O Na [M+Na] :
p
6
in volts versus SCE.
1419.7418, found: 1419.7356.
Chem. Eur. J. 2015, 21, 14186 – 14195
www.chemeurj.org
14192
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
2
-[10-(9,9-Dihexyl-9H-fluoren-2-yl)-4,7,13,16-tetraphenylcyclo-
octadeca-1,2,3,7,8,14,15-nonaen-5,11,17-triyn-1-yl]-9,9-dihexyl-
H-fluorene (3a): SnCl2 (85 mg, 0.4 mmol) and HCl/Et O (0.4 mL,
cooled microfocus source, by using MoKa radiation (l=0.71073 Š).
Phi and omega scans were used. The data were integrated with
[31]
9
SAINT, and an empirical absorption correction with SADABS was
2
[32]
0
0
.8 mmol) were successively added to a solution of diol 6a (53 mg,
.04 mmol) in dry dichloromethane (12 mL) at À788C. The temper-
applied. The structures were solved by direct methods (SHELXS-
[
33]
2
97)
and refined by the least-squares method on F (SHELXL-
[33]
ature of the mixture was allowed to slowly increase to À208C over
97). All non-H atoms were refined with anisotropic displacement
parameters. The H atoms were refined isotropically at calculated
positions using a riding model.
3
h. Then 1m aqueous NaOH (0.9 mL, 0.9 mmol) was added. The
aqueous layer was extracted with dichloromethane and the com-
bined organic layers were washed with brine, dried over MgSO4
CCDC 1047701 (3a) and 1047702 (3b) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
and concentrated to dryness under reduced pressure. The residue
was purified by silica-gel chromatography (EtOAc:pentane 5:95) to
give 3a as a dark solid (28 mg, 59% yield). M.p. >3008C; R (EtOA-
f
1
c:heptane 5:96)=0.26; H NMR (CDCl ): d=0.71 (brs, 12H, CH ),
3
3
0
2
.91–1.28 (m, 32H, ((CH ) CH ) 2.34–2.47 (m, 4H, (CH -C1-Fluo)
2 4 3 2
Computational details
.47–2.60 (m, 4H, CH -C1-Fluo), 7.45–7.62 (m, 6H, H6-, H7-, H8-
2
Fluo), 7.79 (t, J=6.75 Hz, 4H, p-Ph), 7.96–8.12 (m, 10H, m-Ph, H9-
Fluo), 8.33 (d, J=8.20 Hz, 2H, H13-Fluo), 9.47–9.63 (m, 12H, o-Ph,
Geometries of the ground states were fully optimized at the
B3PW91/6-31G** level of theory by using Gaussian 09. Vibration-
[34]
1
3
1
H10-, H12-Fluo). C{ H} NMR (CDCl ): d=13.95 (CH ), 22.58, 24.07,
3
3
al analysis was performed at the same level of theory as the geom-
etry optimization. Vertical excitation energies were subsequently
calculated at the TD-CAM-B3LYP/6-31G** level of theory by using
2
9.94, 31.58 ((CH ) ), 40.97 ((CH )-C1-Fluo), 55.72 (C1-Fluo), 104.03
2 4 2
(
C-Ph), 105.89 (C-Fluo) 117.88, 117.92, 118.87 (C=C=C=C, CÀCꢀCÀ
C), 120.56, 121.11, 123.25, 124.54, 125.93, 127.23, 127.99 (C6-, C7-,
[34]
Gaussian 09. Solvent effects were included by using the polariza-
C8-, C9-, C10-, C12-, C13-Fluo), 129.40 (p-Ph), 129.92 (o-Ph), 130.53
ble continuum model (PCM) implemented in Gaussian 09 for
(m-Ph), 139.34, 140.49, 140.61, 142.98 (i-Ph, C3-, C4-, C11-Fluo),
[34]
chloroform (e=4.7113). Molecular orbitals were plotted with GA-
1
51.66, 152.58 (C2-, C5-Fluo); MS (MALDI-TOF/DCTB): m/z 1191.8
[35]
BEDIT.
+
+
[M+H] ; HRMS (MALDI-TOF/DCTB): m/z calcd for C H [M] :
SOS
9
2
86
Two-photon absorption cross-sections s2PA were calculated using
1
190.6730; found: 1190.6730; UV/Vis (CHCl3): lmax =494 nm
DALTON2011 (“.TWO-PHOTON” keyword within quadratic re-
À1
À1
(
eꢁ332000 Lmol cm ); voltammetry: reduction: À0.75 (rev.),
À1.14 (rev.), À1.62 (irrev.); oxidation: 1.08 (rev), 1.39 (irrev.), 1.84
irrev.), 1.98 (irrev.).
-[10-(9,9-Dihexyl-9H-fluoren-2-yl)ethynyl-4,7,13,16-tetraphenyl-
[36]
sponse), from the SOS scheme by computing the 2PA transition
matrix elements Smn between the ground state j0i and the final
(
[24]
state jfi according to Equation (1):
2
cyclooctadeca-1,2,3,7,8,14,15-nonaen-5,11,17-triyn-1-yl]-9,9-di-
hexyl-9H-fluorene (3b): SnCl2 (70 mg, 0.36 mmol) and HCl/Et O
ꢀ
ꢁ
2
X
h0jm jiihijm jfi h0jm jiihijm jfi
m
n
n
m
Smn
¼
þ
ð1Þ
(
(
0.36 mL, 0.72 mmol) were successively added to a solution of 6b
50 mg, 0.036 mmol) in dry dichloromethane (12 mL) with stirring
ðw À wÞ
ðw À wÞ
i
i
i
at À788C. The temperature was slowly increased up to 08C over
in which w is the fundamental frequency of the laser beam which
3
h. Then 1m aqueous NaOH (0.8 mL, 0.8 mmol) was added. The
is equal to the half of the excitation energy to the final state w /2.
f
aqueous layer was extracted with dichloromethane and the com-
bined organic layers were washed with brine, dried over MgSO4,
and concentrated under reduced pressure. The residue was puri-
fied by silica-gel chromatography (dichloromethane:pentane 1:9)
For a given final excited state jfi, the summation runs over all in-
termediate states jii of energy w. The 2PA probability d
for
2PA
i
a given molecular structure in the gas or liquid phase can be ob-
tained by orientational averaging [Eq. (2)]:
to give 3b as a dark solid (23 mg, 51% yield). Mp >3008C; R (di-
f
1
chloromethane:pentane 2:8)=0.24; H NMR (400 MHz, CD Cl ): d=
X
X
2
2
SOS
2PA
d
¼ A
SmmSnn þ B
SmnSmn
0
7
.75–1.28 (m, 44H, (CH ) CH ), 2.15–2.38 (m, 8H, CH -Fluo), 7.44–
.58 (m, 6H, Fluo), 7.80 (t, J=7.3 Hz, 4H, p-Ph), 7.91–7.95 (m, 2H,
ð2Þ
2
4
3
2
mn
mn
Fluo), 8.04 (m, 10H, m-Ph, Fluo), 8.17 (m, 4H, H9, H13-Fluo), 9.53
1
3
1
The coefficients A and B depend on the polarization of the light:
A=2, B=4 for linearly polarized light and A=À2, B=6 for circu-
larly polarized light. For direct comparison with experimental
values, the 2PA probability can then be converted into a cross-sec-
(
(
(
1
1
d, J=7.3 Hz, 8H, o-Ph). C{ H} NMR (101 MHz, CD Cl ): d=13.8
2 2
CH ), 40.5, 31.6, 29.7, 24.0, 22.6 ((CH ) ), 55.5 (>C-Hex ), 91.1, 84.7
3
2 5
2
Fluo-CꢀC-), 100.5 (CÀCꢀC-Fluo), 104.9 (C-Ph), 123.1, 121.0, 120.4,
20.1, 118.7, 113.8 (C6-, C9-, C13-, C11-Fluo, C=C=C=C, CÀCꢀCÀC),
4
À2
tion in GM (cm sphoton ) by using Equation (3):
31.4, 130.1, 130.0, 129.9, 128.1, 127.1, 126.7 (C7-, C8-, C10-, C12-
Fluo, o-, m-, p-Ph), 143.0, 140.3, 139.0 (C3-, C4-Fluo, i-Ph), 151.4,
+
1
51.3 (C2-, C5-Fluo); MS (MALDI-TOF/DCTB): m/z 1238.7 [M] ;
SOS
PA
3
4
0
2
2
SOS
2PA
+
s2 ¼ 8p a t
0
a w d
ð3Þ
HRMS (MALDI-TOF/DCTB): m/z calcd for C H [M] : 1238.6730;
9
6
86
found:
eꢁ381900 Lmol cm ); voltammetry: reduction: À0.61 (rev.),
À0.99 (rev), À1.57 (irrev.), À1.94 (irrev.); oxidation: 1.17 (irrev.), 1.46
irrev.), 1.75 (irrev.), 1.89 (irrev.).
1238.6772;
UV/Vis
(CHCl3):
lmax =492 nm
À1
À1
in which a is the fine-structure constant, a
0
the Bohr radius (in cm/
(
a.u.), t the atomic unit of time (in s/a.u.), and w the photon energy
0
(
in a.u.), that is, w=w /2.
f
(
In many cases, centrosymmetric organic molecules fit into the
[1c,10,25]
three-level model.
The 2PA probability can therefore be ap-
proximated to the truncation of the summation of Equation (1) to
a single intermediate excited state, and involving the transition
dipole moments of the two related successive 1PA processes and
the energy difference between the intermediate state and a virtual
state lying halfway to the 2PA state, referred to as DE [Eq. (4)]:
Crystallographic data collection and structure determination
for 3a and 3b
The data were collected at low temperature (193 K) on a Bruker-
AXS APEX II QUAZAR diffractometer equipped with a 30W air-
Chem. Eur. J. 2015, 21, 14186 – 14195
www.chemeurj.org
14193
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
2
2
2
2
h0jmjii hijmjfi
m m
grant no 215708. A determining financial support was also pro-
vided by the ANR program (ANR-11-BS07-016-01). The theoreti-
cal studies were performed using HPC resources from CALMIP
(Grant 2013-2014 [0851]) and from GENCI-[CINES/IDRIS] (Grant
3
PA
L
0i if
DE2
d2
¼
À
Á
¼
ð4Þ
wf
2
w À
i
2
in which DE=wÀw /2. The transition dipole moments m and mif
i
f
0i
involved in Equation (4) were calculated with DALTON2011 by
2
013-2014 [085008]). The French Embassy in Kiev and the
using the “.DOUBLE RESIDUE” and “.DIPLEN’’ keywords. The 2PA
probability is then converted to the cross-section s2 (in GM units)
through Equation (3).
French Minist›re de l’Enseignement SupØrieur et de la Recher-
che are acknowledged for the doctoral fellowships of I.B. and
C.P., respectively. The authors are finally indebted to Dr.
Arnaud Rives and Mr. Kevin Cocq for discussions, and to Mrs
Alix Saquet for electrochemical analyses.
3
L
PA
Optical experiments
A standard Z-scan setup was utilized. Briefly, a femtosecond Ti:sap-
phire regenerative amplifier (Libra, Coherent Inc.) delivered pulses
of 80 fs (1 kHz repetition rate) at 800 nm. These laser pulses were
focused into a beam-waist radius of 20 mm. For Z-scan curves, the
samples were poured into a 1 mm-thick quartz cell. To avoid
Keywords: carbo-mers · charge transfer · chromophores ·
density functional calculations · nonlinear optics
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330; b) H. M. Kim, B. R. Cho, Chem. Commun. 2009, 153–164; c) F. Ter-
À2
1
00 GWcm . Z-scan measurements were also performed at other
enziani, C. Katan, E. Badaeva, S. Tretiak, M. Blanchard-Desce, Adv. Mater.
2008, 20, 1–38.
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(
9
TOPAS, Light Conversion), which was tuned in the range 650–
00 nm.
The nonlinear absorption coefficient b for open-aperture Z-scan
[
[
[
[
17b]
data was obtained by using Equation (5)
2012, 134, 17486–17489.
[6] W. Denk, J. H. Strickler, W. W. Webb, Science 1990, 248, 73–77.
1
I
p
L
eff
[7] O. Mongin, L. Porr›s, M. Charlot, C. Katan, M. Blanchard-Desce, Chem.
Eur. J. 2007, 13, 1481–1498.
TðZÞ ¼ 1 À pffi ffiffi b
ð5Þ
2
2
2
1 þ ðZ=Z Þ
0
[8] a) L. Ventelon, S. Charier, L. Moreaux, J. Mertz, M. Blanchard-Desce,
Angew. Chem. Int. Ed. 2001, 40, 2098–2101; Angew. Chem. 2001, 113,
in which I is the peak intensity, L =[1Àexp(Àa L)]/a the effec-
p
eff
0
0
2156–2159; b) M. Drobizhev, Y. Stepanenko, Y. Dzenis, A. Karotki, A.
tive thickness with L the sample depth, Z the sample position, and
Rebane, P. N. Taylor, H. L. Anderson, J. Phys. Chem. B 2005, 109, 7223–
7236.
p
2
Z (¼ w ) the Rayleigh range. Both parameters I and Z were de-
0
l
0
p
0
termined by measuring the beam waist w0 by the knife-edge
method. To verify the validity of the obtained results, the Z-scan
apparatus was used in closed-aperture mode to measure the non-
linear refractive index of the standard CS , resulting in n =2.5”
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2
2
À15
2
À1
1
0
cm W , which is in the range of the accepted values report-
[
37]
ed in the literature. This confirmed good calibration of our Z-
ꢀh w
scan. Thus, s2PA can be evaluated from s2PA
the photon energy and N the density of molecules in the solution.
¼
b, in which ꢀh w is
N
[
9
, 13–28; b) H. Uoyama, K. S. Kim, K. Kuroki, J.-Y. Shin, T. Nagata, T. Oku-
For the ultrafast time measurements, a femtosecond TA experi-
ment was implemented in a pump–probe configuration. In short,
the transmitted intensity through the sample of a probe pulse was
measured as a function of its time of arrival with respect to a stron-
ger pump pulse. The wavelength of the probe pulse was set at
jima, H. Yamada, N. Ono, G. Kim, H. Uno, Chem. Eur. J. 2010, 16, 4063–
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3
5
6, 397–400; b) V. Maraval, R. Chauvin, Chem. Rev. 2006, 106, 5317–
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7
35 nm, while the pump pulse was fixed at 400 or 800 nm. The TA
blance between carbo-benzenes and porphyrins, see: d) L. Leroyer, C.
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signal was defined as TA =ÀDT/T =À(TÀT )/T , in which T is the
0
0
0
0
transmission through the sample in the absence of the pump
pulse and T the transmission in the presence of the pump pulse.
According to this definition, a positive transient absorption signal
is due to increased absorption of the probe beam in the sample
after excitation by the pump pulse.
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GDRI) in Molecular Chemistry” funded by the CNRS. Thanks
[
[
are also due to the Laboratory of Ultrafast Optics at the Optical
Research Center for working facilities and to CONACYT for
Chem. Eur. J. 2015, 21, 14186 – 14195
www.chemeurj.org
14194
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Published online on August 12, 2015
Chem. Eur. J. 2015, 21, 14186 – 14195
www.chemeurj.org
14195
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim