Linear and nonlinear optical properties of a quadrupolar carbo-benzene and its benzenic parent: The carbo-merization effect

Article abstract of DOI:10.1016/j.dyepig.2021.109133

Herein, the optical properties of thiophene-functionalized quadrupolar carbo-benzenes and a benzenic parent, of generic structure Th–C[tbnd]C–[core]–C[tbnd]C–Th, Th = R₂C₄HS, are comparatively investigated. Beyond the previously unknown dioctylthienylethynylbenzene (core = p-C₆H₄, R = nOct), two bis-dialkylthienylethynyl-carbo-benzenes (core = C₁₈Ph₄, R = nOct, nBu) are envisaged for the unique “carbo-aromatic” character of the C₁₈ macrocycle. The three targets were synthesized from the corresponding ethynylthiophenes in 47, 20 and 10% yield, respectively, then characterized by classical methods such as NMR spectroscopy, and X-ray crystallography for one of the carbo-benzenes. Regarding linear and nonlinear optical properties, our results show that the carbo-merization induces a significant shift to lower energies of the one-photon electronic excitations accompanied by an 8-fold increase of the molar extinction coefficient compared to the parent molecule. Intriguingly, these excitations lead to a broad band of photoluminescence comprising decay transitions of the type S₁ → S₀ but also of the type S₂ → S₀. This phenomenon of emission from higher excited states, which is contrary to Kasha's rule, is assigned to - or revealed by - a reduction of the internal conversion efficiency between S₂ and S₁. Two-photon induced transitions are also enhanced, the two-photon absorption cross-section (σ_2PA) being in average five times larger for the carbo-benzenes than for their benzene parent in the wavelength range 650–950 nm, with a maximum of σ_2PA = 1430 GM (1 GM = 10^?50 cm⁴ s/photon). Beyond a moderate nonlinearity, this comparative study provides quantitative insights about the way carbo-merization or insertion of a π-conjugated macrocycle between chromophoric functions (here thiophene rings) can tune optical properties of organic molecules. The optical properties of the bis-dialkylthienylethynyl-carbo-benzenes are also discussed in regard of recent reports on organic chromophores based on other types of π-conjugated macrocyclic cores.

Full text of DOI:10.1016/j.dyepig.2021.109133

Dyes and Pigments 188 (2021) 109133
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Linear and nonlinear optical properties of a quadrupolar carbo-benzene and
its benzenic parent: The carbo-merization effect
,
,
,c,*
Rodrigo M. Barba-Barbaa 1, Marwa Chammamb 1, Gabriel Ramos-Ortiza
,
Dymytrii Listunovb, Jayaramakrishnan Velusamya, Mario Rodrigueza, Ramon Carrilesa,
Carlos Silvac , Carine Duhayon , Brice Kauffmann , Valerie Maraval **, Remi Chauvinb
,
d
b
e
b,
,***
´
a
b
´
´
Centro de Investigaciones en Optica A.C., Apdo. Postal 1-948, 37000, Leon, Gto, Mexico
´
LCC-CNRS, Universite de Toulouse, CNRS, UPS, Toulouse, France
c School of Chemistry and Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive NW, Atlanta, GA, 30332, USA
d School of Physics, Georgia Institute of Technology, 837 State Street, Atlanta, GA, 30332, USA
e
´
´
Universite de Bordeaux, CNRS, INSERM, UMS3033/US001, Institut Europeen de Chimie Biologie, 33607, Pessac, France
A R T I C L E I N F O
A B S T R A C T
Herein, the optical properties of thiophene-functionalized quadrupolar carbo-benzenes and a benzenic parent, of
Keywords:
Carbo-benzenes
Macrocycles
–
–
–
C–[core]–C
–
–
generic structure Th–C
–
C–Th, Th = R2C4HS, are comparatively investigated. Beyond the previ-
ously unknown dioctylthienylethynylbenzene (core = p-C6H4, R = nOct), two bis-dialkylthienylethynyl-carbo-
benzenes (core = C18Ph4, R = nOct, nBu) are envisaged for the unique "carbo-aromatic" character of the C18
macrocycle. The three targets were synthesized from the corresponding ethynylthiophenes in 47, 20 and 10%
yield, respectively, then characterized by classical methods such as NMR spectroscopy, and X-ray crystallography
for one of the carbo-benzenes. Regarding linear and nonlinear optical properties, our results show that the carbo-
merization induces a significant shift to lower energies of the one-photon electronic excitations accompanied by
an 8-fold increase of the molar extinction coefficient compared to the parent molecule. Intriguingly, these ex-
citations lead to a broad band of photoluminescence comprising decay transitions of the type S1 → S0 but also of
the type S2 → S0. This phenomenon of emission from higher excited states, which is contrary to Kasha’s rule, is
assigned to - or revealed by - a reduction of the internal conversion efficiency between S2 and S1. Two-photon
induced transitions are also enhanced, the two-photon absorption cross-section (σ2PA) being in average five
times larger for the carbo-benzenes than for their benzene parent in the wavelength range 650–950 nm, with a
maximum of σ2PA = 1430 GM (1 GM = 10ꢀ 50 cm4 s/photon). Beyond a moderate nonlinearity, this comparative
Two-photon absorption
π
-conjugation
study provides quantitative insights about the way carbo-merization or insertion of a π-conjugated macrocycle
between chromophoric functions (here thiophene rings) can tune optical properties of organic molecules. The
optical properties of the bis-dialkylthienylethynyl-carbo-benzenes are also discussed in regard of recent reports
on organic chromophores based on other types of π-conjugated macrocyclic cores.
1. Introduction
designing molecular architectures with effective
π
-conjugated back-
bones, appropriate donor (D) and acceptor (A) electroactive sub-
During the last three decades,
π
-conjugated organic molecules and
stituents, and auxiliary functions. Classically, the π-conjugation in
polymers have emerged as functional materials for diverse applications
in the fields of optoelectronics and photonics [1a,1b, 1c]. Typically, the
optical and electronic properties of these materials are tailored by
molecules and polymers is brought by combination of unsaturated linear
chains and/or small carbo- or hetero-cycles. Recently, a special type of
π
-conjugated backbone based on shape-persistent macrocyclic
´
* Corresponding author. Centro de Investigaciones en Optica A.C., Apdo. Postal 1-948, 37000, Leon, Gto, Mexico.
** Corresponding author.
*** Corresponding author.
E-mail addresses: garamoso@cio.mx (G. Ramos-Ortiz), valerie.maraval@lcc-toulouse.fr (V. Maraval), chauvin@lcc-toulouse.fr (R. Chauvin).
1
These authors contributed equally to this work.
https://doi.org/10.1016/j.dyepig.2021.109133
Received 4 October 2020; Received in revised form 30 December 2020; Accepted 31 December 2020
Available online 11 January 2021
0143-7208/© 2021 Elsevier Ltd. All rights reserved.

R.M. Barba-Barba et al.
Dyes and Pigments 188 (2021) 109133
architectures have gained significant attention for two-photon absorp-
tion (2 PA) or other nonlinear optical effects [2a,2b, 2c,2d], optoelec-
tronics [3a,3b, 3c], near-infrared luminescence [4a,4b], sensing of
anions [5], photodynamic therapy [6], redox- or photo-responsive
switches [7a,7b] and liquid crystallinity [8]. In such compounds, the
structure-optical properties relationships in carbo-mer series is still
limited and awaits further investigation.
In this work, the effects of carbo-merization are evaluated through
the comparison of the optical properties of the quadrupolar carbo-ben-
zenes 2a,b with those of the benzenic parent molecule 2P, in which the
central C18 macrocycle is replaced by a C6 benzene ring (Fig. 1). In these
molecules, the thienyl substituents, largely used in the design of 2 PA
chromophores, behave as electronic donors that are anchored to the C18
macrocycle through an acetylenic linker. Dialkylthienylethynyl sub-
stituents were selected for two reasons: i) two alkyl chains are attached
to the thiophene ring with the view of enhancing the solubility of the
targeted carbo-benzenes 2a,b to allow sufficiently high concentrations
for the measurement of σ2PA by the Z-scan technique (ca 10ꢀ 3 - 10ꢀ 2 mol
Lꢀ 1), and ii) the ethynyl linkers serve to both facilitate the synthesis key
number of π-electrons varies with the size of the macrocycle and in some
cases intriguing electronic and optical properties are achieved even
without the use of electroactive substituents [9]. Over the years, various
π
-conjugated macrocyclic architectures have been investigated,
including macrocyclic annulenes [10], cyclic oligophenylenes and
belt-shaped
paraphenylenes
[11a,11b,
11c],
macrocyclic
oligophenylene-ethynylenes [12a,12b, 12c], porphyrins and expanded
porphyrins [13a,13b, 13c], and cyclic oligothiophenes [14]. More
recently,
novel
macrocyclic
architectures
exhibiting
aggregation-induced emission (AIE) properties have been designed [15].
Oligofuran-based macrocycles [16], polyradicaloids [17], hexagonal
ring carbo-mers of benzene [18] and many others have been also
considered.
step, and increase the length of the π-conjugated system, thus a priori
favoring high 2 PA efficiency, as previously evidenced by the compari-
son between 1a and 1b (Fig. 1). The linear photophysical properties
(UV–vis absorption, emission, excitation) and nonlinear optical prop-
erties (2 PA) of the core-carbo-mers 2a and 2P are here reported in
detail.
The term carbo-mer designates "carbon enriched" molecules formally
generated by systematic insertion of C2 units into each (or a selected
part) of the covalent bonds of a representative Lewis structure of any
parent molecule [19]. At the Lewis-Gillespie level of representation, the
carbo-merization process preserves the topology, symmetry, shape and
2. Results and discussion
π
-electronic resonance of the parent molecule, while its bonding size is
2.1. Synthesis
approximately expanded by a factor of three. Ring carbo-mers of ben-
zene, simply called "carbo-benzenes", constitute the most exemplified
series of carbo-mers [20a,20b, 20c,20d], and have been studied both
theoretically, in particular for their aromaticity [21a,21b, 21c,21d,21e],
and experimentally, e. g. for redox [18,22], conductive [23], and
mesogenic properties [24]. It was also demonstrated that carbo--
benzenes exhibit unique chromophoric [18,25a,25b,25c] and nonlinear
optical properties. In particular, the two-photon absorption (2 PA) by
the quadrupolar representatives (p-difluorenyl)tetraphenyl-carbo--
benzene 1a and (p-difluorenylethynyl)tetraphenyl-carbo-benzene 1b
(Fig. 1) was recently evaluated, with a 2 PA cross-section σ2PA = 656 GM
at 800 nm for 1b [26]. This first attempt to assess the optical response of
carbo-benzenes at infrared wavelengths showed their relevance as
nonlinear materials. In this context, the understanding of the
The synthesis of the bis-dialkylthiophenylethynyl-carbo-benzenes
2a,b required the preparation of known key precursors, namely the [6]
pericyclynedione 3 [27a,27b, 27c], a classical intermediate in the syn-
thesis of most quadrupolar carbo-benzenes [20], and the dialkylth-
ienylacetylenes 4a,b.
Though a synthesis of 4b was previously published [28], 4a and 4b
were not prepared following exactly the reported methodology. First,
the described nickel-catalyzed Kumada coupling of commercial 3,
4-dibromothiophene with octyl- or butyl-magnesium bromide led to
mixtures of dialkylthiophenes 5a,b and monoalkylthiophenes 5a′,b′ in a
95:5 ratio (Scheme 1). In both cases, the separation of the two products
by silica gel column chromatography appeared problematic because of
their similar low polarity. The direct formylation of the mixtures 5a,b
Fig. 1. Quadrupolar carbo-benzenes 1a and 1b previously studied [26] for their 2 PA properties (top); bis-dithientlethynyl-carbo-benzenes 2a,b and parent molecule
2P considered in this work (bottom).
2

R.M. Barba-Barba et al.
Dyes and Pigments 188 (2021) 109133
Scheme 1. Synthesis of the dialkylthinylethynylacetylenes 4a,b.
and 5a′,b′ leading to inseparable mixtures of aldehydes 6a,b and 6a′,b′,
the preparation of more polar intermediates was envisioned with the
view to facilitating removal of the monoalkyl-thiophene contaminants.
The mixtures of 5a,b and 5a′,b′ were thus treated with one equivalent of
n-butyllithium, followed by addition of paraformaldehyde, to give
mixtures of the alcohols 7a,b and 7a′,b′ which could then be separated
by silica gel column chromatography. The pure alcohols 7a and 7b,
isolated with 83% and 95% yield, respectively, were then oxidized by
treatment with MnO2 to give the corresponding aldehydes 6a and 6b
almost quantitatively. Finally, the aldehydes were converted to the
terminal alkynes 4a and 4b by a Corey-Fuchs reaction, in two steps via
the dibrominated intermediates 8a,b [29].
The carbo-benzenes 2a,b were then prepared from 3 and 4a,b by
using the classical two-step process developed for the synthesis of other
quadrupolar carbo-benzenes [20]. First, two equivalents of the lithium
dialkylthienylacetylide of 4a or 4b, deprotonated with n-butyllithium,
were added to the diketone 3, leading to the pericyclynediols 9a,b
bearing the two thienylethynyl substituents. The last reductive
Scheme 2. Two-step synthesis of the carbo-benzenes 2a,b from the known [6]pericyclynedione 3.
3

R.M. Barba-Barba et al.
Dyes and Pigments 188 (2021) 109133
aromatization step was performed with SnCl2 alone to avoid the
reduction and hydrochlorination of the exocyclic triple bonds, affording
the carbo-benzenes 2a and 2b in 35% and 20% yield respectively
(Scheme 2). In the presence of SnCl2 and HCl, an inseparable mixture of
chromophoric products was formed from 9a, among which 2a′ could be
identified by X-ray diffraction analysis (see SI, Figure S1). Such a
hydrochlorination side-reaction was previously observed for [6]peri-
cyclynediols bearing indolylethynyl donor groups, and its occurrence
here with donor thienylethynyl groups is thus consistent [25c]. The
unexpected reduction of the other triple bond to a trans-substituted
double bond can be compared to a previously reported SnCl2-mediated
2.3. Electrochemistry
The electrochemical properties of the carbo-benzene 2b were inves-
tigated by square-wave (SWV) and cyclic voltammetry (CV; Table 1).
Four reduction and three oxidation waves were observed by SWV, the
two first reductions at – 0.62 and – 1.02 V/SCE being reversible.
The first reduction is observed at a quite low potential (in absolute
value), which is consistent with the extended π-conjugated system of 2b
a priori associated with a low LUMO level, and similar to the corre-
sponding potential previously reported for 1b (see Fig. 1) [26].
The first oxidation, observed at 1.13 V/SCE, is a non-reversible
process, as observed for 1b, which is first oxidized irreversibly at
1.17 V/SCE.
–
reduction of a spC-spC bond of a carbo-meric C18 ring to a trans-CH CH
–
bond [18].
The carbo-chromophores 2a,b were isolated as violet solids, soluble
in most of the conventional organic solvents while giving intense violet
solutions. 2a was characterized by NMR and UV–vis spectroscopy, and
by mass spectrometry. Its structure was also determined by X-ray
diffraction analysis of a single crystal deposited upon slow evaporation
of a dichloromethane solution at room temperature (Fig. 2 and Section
2.2) [30].
2.4. Optical properties
2.4.1. Linear optical properties
Effects of carbo-merization on optical properties can be evaluated by
comparing the photoinduced electronic transitions of 2P and 2a (Fig. 3).
While 2P exhibits an ordinary absorption band with a peak at 350 nm
The parent molecule 2P was prepared by a Sonogashira coupling of
two equivalents of 4a with diiodobenzene, and isolated with 47% yield
(Scheme 3).
assigned to a π→ π* transition, 2a presents the characteristic features of
carbo-benzenes [18], with a sharp and intense band, in this case at 495
nm (for the simplicity of discussion this main band is denoted as M in
Fig. 3), a shoulder at 542 nm, and a secondary band with a peak at 608
nm (denoted as S band). The effect of dioctylthienylethynyl substitution
at the para positions of the C18 macrocycle (C1 and C10 positions in 2a)
is clearly distinguishable by comparison with the absorption spectrum of
the previously reported para-dialkyl-substituted analogue 1c (Supple-
2.2. X-ray crystallography
In the X-ray crystal structure of 2a [30], the macrocycle is quasi
planar, the maximum deviation from planarity being of 0.02 Å, with
almost coplanar phenyl substituents (maximal torsion angle of 12.9◦).
The shape of the C18 ring is however quite distorted from a regular
hexagon, the three sp2C … sp2C diagonal distances varying from 7.69 to
7.87 and 8.33 Å, while the internal bond angles vary from 116.9 to
121.8◦, with two concave PhC… C⋯C⋯C(C2Th) edges.
mentary Information, Figure S2) with a much shorter π-conjugation
extent [31]. The overall effect of substitution of the carbo-benzene ring
with thiophenylethynyl units produces a bathochromic shift of ca 50 nm
of both M and S bands as compared to 1c accompanied by a moderate
spectral broadening of the M band and vanishing of minor peaks be-
tween the M and S bands. The peak of maximum absorption for 2a is
practically at the same wavelength (495 nm) as the one observed for the
fluorenylethynyl-substituted carbo-benzene 1b [26] but with a ca 30%
In the crystal packing, the molecules of 2a are shifted one from the
other, but the mean planes of two successive macrocycles are parallel,
albeit without normal overlapping between them. The distance between
the mean planes of two successive molecules is of 3.26 Å. As one of the
octyl groups is bent, some H atoms of the chain point towards the center
of the closest carbo-benzene ring, the shortest distance measured be-
tween the centroid of the C18 ring and a H atom of the octyl chain being
of 2.21 Å.
reduction of the molar extinction coefficient ε. Nevertheless, ε increases
from 2.8 × 104 Mꢀ 1 cmꢀ 1 in 2P to 2.25 × 105 Mꢀ 1 cmꢀ 1 in 2a due to the
expansion of the
magnitude of
quencies, which correlates with the degree of
backbone and with substituents. For reference, Table 2 presents the
π-conjugation. For organic molecules in general, the
ε
is a signature of their polarizability at optical fre-
-conjugation within the
π
Fig. 2. Molecular view of the X-ray crystal structure of 2a (H atoms were omitted for clarity). See note [30], CCDC 2027203.
4

R.M. Barba-Barba et al.
Dyes and Pigments 188 (2021) 109133
Scheme 3. Synthesis of the parent molecule 2P of 2a.
Table 1
SWV and CV data for the carbo-benzene 2b.
Reduction
Oxidation
E1/2a,b (ΔEp)
- 0.62 (69)
- 1.02 (59)
RIp
Erpedd
Eopxd
1.13
1.75
1.89
c
0.99
0.76
- 1.52
- 1.74
a
Measurements performed at room temperature in DCM; supporting elec-
trolyte: [nBu4N][PF6] (0.1 M); working electrode: Pt; reference electrode:
saturated calomel electrode (SCE; 0.242 V versus the hydrogen electrode); scan
rate: 0.2 V sꢀ 1, unless otherwise noted.
Half-wave potential, E1/2 = (Erped + Epox)/2, in V/SCE.
b
c
Peak current ratio RIp = | Iopx/Ipred |.
d
Ep values measured from CV in V/SCE.
optical properties and the molecular structure of some representative
π
-conjugated macrocycles. In these examples the number of π-electrons
(m) in a conjugated circuit inscribed in the corresponding macrocycle
varies from 18 to 180, producing molar extinction coefficients in the
range 0.2–6.39 × 105 Mꢀ 1 cmꢀ 1. In spite of a relatively low out-of-plane
πz-electron count m = 18 (unconjugated with the 12 in-plane πxy elec-
trons of the 6 spC-spC bonds), the
ε values of carbo-benzenes are thus
among the largest. According to the parity of m/2, the macrocycles of
Table 2 can be qualitatively identified as aromatic vs antiaromatic using
the well-known m = 4n + 2 vs 4n Hückel rule.
It should be noted that characteristic features of the absorption
spectra of carbo-benzenes, i.e., a sharp and intense M band followed by a
secondary S band of weaker intensity, are not observed in other families
of π-conjugated macrocycles (as those exemplified in Table 2) since they
generally exhibit broad and structureless absorptions, except in the
porphyrinoids whose spectra present the well-known Soret (B Band) and
Q bands [(13a–c),45]. The main band of the absorption spectra of
substituted C2-symmetrical carbo-benzenes was previously interpreted
[26] as additive and subtractive combinations of excitations from the
two highest occupied molecular orbitals (HOMO-1, HOMO) to the two
lowest unoccupied molecular orbitals (LUMO, LUMO+1), as schemati-
cally shown in Fig. 4. This approach invokes the Gouterman four-orbital
model developed to explain the origin of the Soret and Q bands in the
absorption spectra of porphyrins [45].
Fig. 3. Absorption, PL (with excitation wavelength λexc) and excitation spectra
(with detection of PL at the wavelength λdet) obtained for a) 2P and b) 2a in
THF solutions prepared at 8.43 × 10ꢀ 5 M.
Fig. 3 presents the photoluminescence (PL) spectra of 2P and 2a
under excitation at their main absorptions, 350 and 497 nm, respec-
tively. The PL spectrum of the parent molecule 2P exhibits a band with a
relatively small Stokes shift of 36 nm and noticeable vibronic structure.
In the case of the carbo-benzene 2a, the broad PL band is distorted due to
absorption inner effects from the S band. Inner filter effects occurred
Fig. 3b clearly indicates that the electronic transitions corresponding to
the M and S bands contribute to the PL emission. Furthermore, signifi-
cant PL still occurs at wavelengths with small shifts with respect to the
excitation, suggesting small geometric changes between the excited and
ground states. This is not surprising as we consider the planar and quite
rigid monocyclic carbo-benzene ring, identified previously as both aro-
matic and flexible (“carbo-aromatic") [20c]. Small Stokes shifts are also
observed in other planar conjugated systems such as the more rigid
pentacyclic porphyrins and expanded porphyrins with residual
inevitably in all the PL measurements of 2a given its high
ε and poor
emission properties. Nevertheless, PL characterization of 2a is worth-
while since it can provide information on the excited states that lead to
radiative decays in carbo-benzenes. For example, radiative decays occur
not only at longer wavelengths (lower energies) than the low-lying band
S, but also at shorter wavelengths (higher energies). The matching be-
tween the PL excitation spectra and the absorption spectrum shown in
5

R.M. Barba-Barba et al.
Dyes and Pigments 188 (2021) 109133
Table 2
Comparative properties for various π-conjugated macrocycles. The molecular structure of each of these macrocycles is presented in Table S1.
π-conjugated macrocycle
λabs (nm)
λem (nm)
ε
× 105 (Mꢀ 1 cmꢀ 1
)
Conjugated
π
-electron count m and
σ2PA (GM)
λmax(2PA)
Ref.
Hückel aromatic charactera
σ2PA/m
π
-e
(nm)
σ2PA/MW
Thienylene-phenanthrylene
Tetraphenylethylene based
Oligofuran based
385
330
401
364
462
403
380
495
520
530
527
437
.
1.51
–
56 antiaromatic
non-aromatic
32 antiaromatic
32 antiaromatic
non-aromatic
non-aromatic
non-aromatic
–
–
[32]
[15]
[16]
[33]
[34]
[35]
[36]
–
–
1.5
0.26
–
–
–
BODIPY-phenylacetylene
–
–
Phenanthrylene-ethynylidene
Expanded 1,3-Dithiolane [5]radialene
Triphenylamine phenylacetylene
–
–
1.2
–
–
–
430
1300
19.7
0.81
11.5
0.64
0.031
2800
42.42
0.85
125
650
Triangle-shaped annulene
332
375
413
480
471
440
712
427
567
496
389
469
486
559
–
–
18 aromatic
66 aromatic
non-aromatic
180 antiaromatic
18 aromatic
18 aromatic
22 aromatic
30 aromatic
26 aromatic
18 aromatic
372
[37]
Triangular –[D–π–A]3–
2.0
1.1
6.18
–
750
[38]
Phenylacetylene-based macrocycle
820
[39]
4.2
0.069
107 800
598
Giant expanded oligothiophenes Thiophene [30]mer
Zinc(II) porphyrin
710
[14,40]
[41]
16.30
3520
195
1360
800
2.66
42 600
2366
38.33
2900
131.8
4.81
3030
101
Hydrocarbon-substituted metal-free porphyrins
Pentapyrrolic expanded porphyrin-isosmaragdyrin
Cyclo [8]pyrrole expanded porphyrin
[26]Hexaphyrin expanded porphyrin
Quadrupolar carbo-benzene 2a
660
716
1496
1036
–
[42]
–
1280
1600
1200
650
[43]
1.2
4.2
2.25
[44a, 44b]
[44a, 44b]
This work
3.12
9890
380
6.77
1430
83.3
2.12
a
Number m of conjugated π-electrons inscribed exclusively in the circuit of the corresponding macrocycle.
flexibility [44]. Unexpectedly, the observation that PL can occur directly
from the excitation of the M band is in contraposition with Kasha’s rule
[46] since it is assumed that such excitation leads to the
quasi-degenerated exited state S2 (see Fig. 4 for the Jablonsky diagram
proposed for 2a in analogy with the Gouterman four-orbital model). PL
from higher excited states (S2 → S0) is a quite rare phenomenon that
mainly occurs in small organic compounds [47] although it has also
been reported in macrocycles such as [18]annulene C18H18 [48], akin to
carbo-benzene also identifiable as the most symmetric dodecadehydro-
S1) ~ 6272 cmꢀ 1. This suggests that the fluorescence decay S2 → S0 in 2a
could be assigned to the relatively wide separation between the M and S
bands, which decreases, to some extent, the IC efficiency between S2 and
S1, thus increasing the probability of bypassing S1 during relaxations. It
must be noted that the conformation of the macrocycle determines the
density and shape of vibronic levels, and, in turn, the Franckꢀ Condon
factor. In consequence, the fluorescence in carbo-benzenes originates
simultaneously from both the first and second excited states (S1 → S0 and
S2 → S0) as depicted in Fig. 4. All this characterization of the linear
optical properties for 2P and 2a was obtained in diluted solutions such
that self-assembly effects were discarded.
[18]annulene
C18H6, BODYPI-based macrocycles [33], metal-
loporphyrins [49a,49b], and theoretically predicted in cyclo[n]thio-
phenes [50]. Anomalous emission had been also already observed for a
dichromic carbo-cyclohexadiene [18] having a green PL band appearing
between its blue and red absorption bands. However, this PL band was
not explicitly identified as a radiative decay of the type S2 → S0. The
accepted interpretation for anomalous PL in organic molecules is that it
can occur when the energy gap ΔE(S2– S1) is sufficiently large to reduce
the Franck-Condon factor [51], the latter expressing the overlap integral
of vibrational wavefunctions involved in the internal conversion (IC)
process between S2 and S1, to a level where the rate of radiative S2 → S0
transitions competes with the IC rate. Azulene, a polar hydrocarbon
molecule, is a well-known prototype that exhibits efficient PL from S2
due to a very large ΔE(S2– S1) ~ 10 000 cmꢀ 1 [47]. The carbo-chro-
mophore 2a also exhibits significant ΔE(S2– S1) ~ 3673 cmꢀ 1. This value
is even larger for the carbo-cyclohexadiene reported in Ref. [18], ΔE(S2–
As a consequence of carbo-merization, 2a exhibited significantly
longer fluorescence lifetime than its parent molecule 2P. Fig. 5 presents
the PL decays of the two molecules using the technique of time-
correlated single photon counting (TCSPC); in this figure, the dotted
lines denote the instrument response, the solid lines the lifetimes of 2P
(black) and 2a (blue). The average lifetime measured in 2a was 5.04 ns,
while the measurement for 2P gave a significantly shorter value (1.02
ns), just barely different from the instrument response function.
Although radiative decays from S2 and S1 should imply differentiated
lifetimes, no wavelength dependence of the PL was observed, the reso-
lution of the TCSPC technique probably being insufficient to allow their
discrimination. Further investigation using techniques of higher tem-
poral resolution will thus be necessary.
6

R.M. Barba-Barba et al.
Dyes and Pigments 188 (2021) 109133
Fig. 4. Jablonsky diagram (left) and Gouterman four-orbital model (right) for 2a.
comparison of the nonlinearities obtained for the carbo-meric and parent
molecules through different techniques, the σ2PA values of 2P in solution
(8.43 × 10ꢀ 2 M in THF) were also determined by Z-scan at three
different wavelengths (black dots in Fig. 6a), showing the agreement
between the two techniques. In spite the high concentration of the so-
lutions used in Z-scan experiments, self-assembly effects in 2P and 2a
were not detected. For instance, light scattering, an affect that is typi-
cally produced in presence of self-assembled molecules, was not
observed in the tested solutions. Of course, self-assembly could be pre-
sent if the molecules are deposited in solid films, but such studies were
beyond our present work. Larger nonlinearities are observed in 2a with
respect to 2P, with a ca 7-fold increase in σ2PA at the infrared (IR) range
of wavelengths (which is, in fact, the region of interest for practical
applications of the 2 PA process). We can see that the onset of detectable
nonlinear absorption in 2a takes place at a significantly longer wave-
length (950 nm, as shown in Figure S4) compared with 2P (780 nm). Out
of the IR range, the maximum values of σ2PA were 506 GM for 2P at 680
nm and 1430 GM for 2a at 650 nm, with a tendency to increase at
shorter wavelengths for both molecules. It is also illustrative to evaluate
the enhancement of nonlinearities by comparison of the ratio between
Fig. 5. Lifetime measurements of 2a (blue line) and 2P (black line). As reference,
the response function of the instrument (dotted lines) is presented. The emission
lifetime of 2a increases 5 times in comparison to the lifetime of 2P. The lifetime
of 2a was monitored at 550 nm and 2P at 414 nm. Inset: Plots in semilog scale.
(For interpretation of the references to color in this figure legend, the reader is
referred to the Web version of this article.)
σ2PA and the number m of π-electrons inscribed inclusively in the
π
-conjugated circuit. For instance, at IR wavelengths we have the rela-
tion σ2PA/m (2a) ~ 2 × σ2PA/m (2P). Thus, the overall linear and
nonlinear characterization of 2a shows that carbo-merization, though
preserving the shape, symmetry and topology of a parent molecule, can
induce enhancements of the polarizability (
σ2PA) due to a wider
optical effects per -conjugated electron. Table 2 presents σ2PA/m values
ε) and hyperpolarizability
(
π-conjugation extent and an improvement of the
2.4.2. Nonlinear optical properties
π
Comparison of one- and two-photon excitations of 2P and 2a pro-
vides insight into the effects of carbo-merization on the energy level
configuration and nonlinear polarizability. To measure σ2PA in 2P, the
TPEF technique was used since that molecule fluoresces appreciably
(quantum efficiency of 22%) and this technique is of higher sensitivity
than Z-scan for molecules with small σ2PA values. TPEF spectra of 2P
were recorded from 680 nm to 780 nm in a THF solution (8.43 × 10ꢀ 5
M), as shown in Figure S3, and used to calculate the σ2PA spectrum
presented in Fig. 6. In the case of 2a, it was not possible to measure σ2PA
through the TPEF technique because of its very weak fluorescence. The
σ2PA spectrum of 2a (THF solution at 8.43 × 10ꢀ 2 M) was thus acquired
from 650 nm to 950 nm through the Z-scan technique. To ensure a fair
for a variety of macrocyclic chromophores. The carbo-benzene 2a shows
intermediate nonlinearities as compared to its counterparts. A similar
trend is observed when σ2PA is normalized by the molecular weight. Note
that in some series of macrocycles of increasing number m, an
enhancement of nonlinearities is observed: giant oligothiophenes with
m = 72 and 180 showed σ2PA/m ~209 and ~598 GM, respectively [14].
In line with these observations, it would thus be of interest, at least at the
theoretical level, to study the effect of enhancement or saturation of σ2PA
in carbo-mers of larger size than 2a, for instance in carbo-benzenes of
second generation exhibiting 30 cyclically
π-conjugated electrons [20b].
The excited states accessible by two-photon transitions in substituted
7

R.M. Barba-Barba et al.
Dyes and Pigments 188 (2021) 109133
250 GW/cm2. Even at energies as low as 20 GW/cm2, 2a begins to
deviate from the linear regime getting to a maximum deviation at 60
GW/cm2 (Fig. 6b). For intensities as high as 250 GW/cm2 no damage or
degradation was detected in the sample, making 2a appropriate for
optical damage control [1c].
3. Conclusions
To get insights into the underlying optical features that emerge in a
given
π-conjugated molecular topology when a C6 benzene ring is
replaced by a C18 aromatic macrocycle, the one- and two-photon-
promoted electronic transitions were characterized in a quadrupolar
carbo-benzene, namely 2a, and compared with those of its parent
molecule 2P. In passing, but not less importantly, the synthesis and
extensive characterization of the previously unknown carbo-benzene 2a,
homologue 2b, and parent molecule 2P are disclosed. In the linear
regime, a notorious increment in the polarizability was observed,
whereas, in the nonlinear regime, a higher 2 PA activity per π-electron
was determined. The presence of the hexagonal carbo-benzene ring in
the conjugated backbone introduces anomalous PL due to simultaneous
radiative decays S1 → S0 and S2 → S0. In view of these results, the C18
π
-conjugated macrocycle can be considered as a versatile platform on
which substituents can be attached at various positions in dipolar,
quadrupolar or octupolar geometries, the two latter being relevant for
potential applications involving 2 PA as an initial process, or possibly for
energy transfer from either the S1 or S2 excited states. Finally, the results
also suggest that lowering the symmetry of substituted carbo-benzenes,
through replacement of some peripheral phenyl groups by other sub-
stituents, might allow tuning the peak of 2 PA to longer wavelengths.
4. Experimental section
4.1. General
THF, Et2O, pentane and DCM were dried with a PureSolv-MD-5
Innovative Technology system for the purification of solvents. All
other reagents were used as commercially available. In particular, so-
lutions of n-BuLi were 1.6 or 2.5 M in hexane, solutions of EtMgBr were
3.0 M in THF, solution of n-OctMgBr were 2 M in Et2O, solution of n-
BuMgCl were 2.0 M in THF. All the reactions were carried out under an
argon atmosphere by using Schlenk tube and vacuum line techniques.
Column chromatography was carried out on silica gel (60 Å, C.C.
Fig. 6. a) Spectra of the 2 PA cross section for 2P and 2a measured in THF
solution by TPEF and Z-scan techniques at the concentration of 8.43 × 10ꢀ 5 and
8.43 × 10ꢀ 2 M, respectively. 2 PA cross sections were also measured for 2P by
Z-scan technique is a THF solution at 8.43 × 10ꢀ 2 M; b) Optical limiting at 800
nm for 2a emphasizing that, at the relatively low intensity of 20 GW/cm2, its
transmission diverges from a linear behavior.
70–200 μm). Silica gel TLC plates (60F254, 0.25 mm) were observed
carbo-benzenes were examined. In Fig. 6a, the total energy for a pair of
under UV light and/or by treatment with a solution of phosphomolybdic
acid (20%) in ethanol. The following analytical instruments were used:
1H and 13C NMR spectroscopy: Avance 300, Avance 400 and Avance 400
HD spectrometers; MS: TSQ 7000 Thermo Electron and Voyager DE-ST
Perseptive Biosystems spectrometers; UV/vis: PerkinElmer UV–vis
Win-Lab Lambda 35; FT-IR: PerkinElmer 1725.
infrared photons (2ℏω) exciting 2a is higher than the energy corre-
sponding to the one-photon peak at the M band. This blue-shifting of the
2 PA resonance with respect to one-photon resonance is expected in
symmetric molecules like 2a because one-photon and two-photon
transitions are mutually exclusive according to Laporte’s rule [51]. In
the four-orbital model used to construct the excited states diagram of 2a,
the quasi-degenerate states 3Au and 4Au of the M band have an ungerade
parity and are thus two-photon forbidden from the ground state of
gerade parity (Ag). A strategy to enhance the nonlinearities at IR
wavelengths of equivalent energies to the M band would thus be to
lower the symmetry of carbo-benzenes by modifying the positions of the
substituents of the C18 hexagonal core, strategy that was successfully
applied with other macrocyclic molecules to bring 2 PA states closer in
energy to one-photon allowed states [52]. Of course, not only the
symmetry of the molecule but also the nature and position of the sub-
NMR chemical shifts are given in ppm, with positive values to high
frequency relative to the tetramethylsilane (TMS) reference. Coupling
constants, J, are in Hz, UV–vis extinction molar coefficients,
ε
, are in M-1
cmꢀ 1, wavelengths, λ, in nm.
4.2. Voltammetry
Voltammetric measurements were carried out with an Autolab
PGSTAT100 potentiostat controlled by GPES 4.09 software. Experi-
ments were performed at room temperature in a homemade, air-tight,
three-electrode cell connected to a vacuum/argon line. The reference
electrode consisted of a SCE separated from the solution by a bridge
compartment. The counter electrode was a platinum wire of about 1 cm2
apparent surface. The working electrode was a Pt microdisk (0.5 mm
diameter). The supporting electrolyte, [nBu4N][PF6], was used as
received (Fluka, 99% electrochemical grade) and simply degassed under
argon. The solution used in the electrochemical study was 10ꢀ 3 M in
stituents and
π connectors play complementary roles that have to be
globally taken into account in the design of new carbo-chromophore
targets.
Finally, the 2 PA properties of the carbo-benzene 2a allowed imple-
mentation of optical limiting, an application of utility for the protection
of sensors, optical elements and human eyes. Measurements were per-
formed at 800 nm with pulses of 80 fs and intensities ranging from 0.1 to
8

R.M. Barba-Barba et al.
Dyes and Pigments 188 (2021) 109133
carbo-benzene and 0.1 M in supporting electrolyte. Before each mea-
surement, the solution is degassed by bubbling with argon, and the
working electrode was polished by using a polishing machine (Presi
P230). Typical instrumental parameters for recording SW voltammo-
grams were: SW frequency, f = 20 Hz; amplitude, Esw = 20 mV; scan
increment dE = 0.5 mV.
121.7, 123.0, 131.0, 142.2, 147.3.
HRMS (MALDI-TOF/DCTB): m/z calcd for C50H74S2 [M*]+
738.5232, found: 738.5288.
• 2-ethynyl-3,4-dialkylthiophene (4a,b).
n-BuLi (1.2 eq.) was added at ꢀ 78 ◦C to a solution of dibromoolefin
8a or 8b (13 mmol,1 eq.) in pentane (300 mL). The temperature was
increased up to ca ꢀ 40 ◦C and stirring was continued for 30 min. Then,
the mixture was cooled again to ꢀ 78 ◦C and treated with a saturated
aqueous solution of NH4Cl. After extractions of the aqueous layer with
diethyl ether, the combined organic layers were washed with brine,
dried over MgSO4 and evaporated under reduced pressure to give the
expected product 4a or 4b as a yellow liquid (4a: 99%, 4b: 91%).
4a: 1H NMR (400 MHz, CDCl3) δ 0.90–0.93 (m, 6H), 1.29–1.41 (m,
24H), 1.55–1.63 (m, 4H), 2.51 (t, J = 8.0 Hz, 2H), 2.66 (t, J = 8.0 Hz,
2H), 3.43 (s, 1H), 6.84 (s, 1H).
4.3. Crystallography
Data collections were performed on a high flux microfocus Rigaku
FRX rotating anode at the copper kα wavelength equipped with a Dectris
Pilatus 200 K hybrid detector at 130 K. The structure was solved using
Superflip [53a], and refined by full-matrix least-squares procedures on F
using the programs of CRYSTALS [53b]. Atomic scattering factors were
taken from the International tables for X-ray Crystallography [53c]. All
non-hydrogen atoms were refined anisotropically. Hydrogen atoms were
refined using a riding model. Absorption corrections were introduced
using the program MULTISCAN.
13C {1H} NMR (CDCl3, 100 MHz) δ 14.1, 22.7, 28.1, 29.0, 29.3, 29.3,
29.4, 29.5, 29.5, 29.6, 29.8, 30.0, 31.9, 31.9, 82.6, 117.3, 121.4, 141.9,
148.1.
HRMS (DCI/CH4): m/z calcd for C22H37S [M ꢀ H]+ 333.2616, found:
333.2603.
4.4. Synthesis and characterization
IR (cmꢀ 1):
ν = 3313, 2954, 2923, 2854, 2101, 1465, 1155, 875, 744,
655, 575.
•
2-(2-{10-[2-(3,4-dialkylthien-2-yl)ethynyl]-4,7,13,16-tetraphe-
4b: the obtained data were in accordance with those previously
nylcyclooctadeca-1,2,3,7,8,9,13,14,15-nonaen-5,11,17-triyn-1-yl}ethy-
nyl)-3,4-dialkylthiophene (2a, 2b).
described [28].
•
3,4-dialkylthiophenes (5a, 5b) were prepared following
To the solution of 9a or 9b (0.025 mmol) in 20 mL of dry DCM at
ꢀ 78 ◦C was added anhydrous SnCl2 (20 eq.), and the resulting mixture
was allowed warming to room temperature and stirred at this temper-
ature for 3 h. Then the mixture was filtered through a pad of silica gel
using DCM until complete elution of colored fractions. The resulting
colored solutions were joined and concentrated under vacuum and the
residue were washed with pentane to give 2a or 2b as a dark violet solid
(2a: 35%, 2b: 20%).
described procedures. Physical properties of the obtained products are
in accordance with those previously reported [28,54].
• 3,4-dialkyl-2-thiophenocarboxaldehyde (6a, 6b).
A solution of the alcohol 7a or 7b (1 eq., 15 mmol) in DCM (300 mL)
was treated with MnO2 (10 eq.) at 0 ◦C. The mixture was stirred at 0 ◦C
for 10 min, then 1.5 h at room temperature, before being filtered
through Celite®. The solution was concentrated under vacuum to give
the expected spectroscopically pure aldehydes 6a or 6b as a yellow oil
(6a: 95%, 6b: 94%)
2a: 1Н NMR (CD2Cl2, 400 MHz): δ 0.61 (t, J = 7.1 Hz, 6H), 0.94 (t, J
= 6.6 Hz, 6H), 1.01–1.13 (m, 8H), 1.17–1.29 (m, 4H), 1.32–1.58 (m,
24H), 1.65 (quint, J = 7.2 Hz, 4H), 1.81 (quint, J = 7.6 Hz, 4H), 1.99
(quint, J = 8.7, Hz, 4H), 2.75 (t, J = 7.8 Hz, 4H), 3.28 (t, J = 7.7 Hz, 4H),
7.26 (s, 2H), 7.73 (t, J = 7.5 Hz, 4H), 7.96 (t, J = 7.8 Hz, 8H), 9.43 (d, J
= 7.5 Hz, 8H).
6a: the obtained data were in accordance with those previously
described [54].
6b: the obtained data were in accordance with those previously
described [55].
13C {1H} NMR (CD2Cl2, 100 MHz): δ 14.1, 14.3, 22.9, 23.2, 29.3,
29.6, 29.7, 29.8, 30.0, 30.0, 30.1, 30.1, 30.4, 30.9, 32.1, 32.4, 85.1,
94.0, 97.6, 105.1, 114.0, 118.7, 119.4, 120.0, 124.8, 130.3, 130.3,
130.4, 139.4, 143.7, 150.2.
• 2-Hydroxymethyl-3,4-dialkylthiophene (7a,7b).
n-BuLi (1.2 eq.) was added at 0 ◦C to a solution of the 6a, 6a′ or 6b,
6b’ mixture (1 eq., 16 mmol) in THF (50 mL). After 45 min of stirring,
the mixture was brought to room temperature and a suspension of
paraformaldehyde (1.8 eq.) in THF (10 mL) was added. The mixture was
stirred for 3 h at room temperature, and poured into ice and water. Then,
the pH was adjusted to 6 by addition of concentrated HCl. After ex-
tractions of the aqueous layer with diethyl ether, the combined organic
layers were washed with water, dried over MgSO4 and evaporated under
reduced pressure. After purification by silica gel chromatography
(pentane: diethyl ether, 98 : 2), the alcohols 7a or 7b were obtained as a
yellow liquid (7a: 83%, 7b: 95%)
HRMS (MALDI-TOF/DCTB): m/z calcd for
C86H90S2 [M*]+
1186.6484, found: 1186.6416.
UV–Visible (CHCl3): λmax = 497 nm (
ε
= 225 000 L molꢀ 1 cmꢀ 1).
2b: 1Н NMR (CDCl3, 300 MHz): δ 1.06–1.13 (m, 12H), 1.57 (m, 4H),
1.72–2.01 (m, 12H), 2.75 (t, J = 7.8 Hz, 4H), 3.28 (t, J = 7.6 Hz, 4H),
7.22 (s, 2H), 7.75 (t, J = 7.3 Hz, 4H), 7.97 (t, J = 7.7 Hz, 8H), 8.46 (d, J
= 7.3 Hz, 8H).
HRMS (MALDI-TOF/DCTB): m/z calcd for
C70H58S2 [M*]+
7a: t he obtained data were in accordance with those previously
described [54].
962.3943, found: 962.3980.
•
2-(2-{4-[2-(3,4-dioctylthien-2-yl)ethynyl]phenyl}ethynyl)-3,4-
7b: the obtained data were in accordance with those previously
described [56].
dioctylthiophene (2P).
To a degassed solution of 1,4-diiodobenzene (48 mg, 0.143 mmol)
and 4a (100 mg, 0.301 mmol) in a mixture of dry THF/Et3N (5 mL, 4/1)
under an argon atmosphere were added copper(I) iodide (3 mg, 0.0143
mmol) and bis(triphenylphosphine)palladium(II) dichloride (5 mg,
0.00715 mmol). The mixture was heated in a sealed tube at 75 ◦C for 12
h before cooling to room temperature and concentration under vacuum.
The residue was purified by SiO2 column chromatography (pentane:
EtOAc = 10 : 1) to give 2P as a waxy solid (49 mg, 47%).
• ((1,1-Dibromo) -2-ethene) ꢀ 3,4-dialkylthiophene (8a,8b).
A solution of aldehyde 6a or 6b (1 eq., 15 mmol) in DCM (200 mL)
was added at 0 ◦C to a mixture of PPh3 (4 eq.) and CBr4 (2 eq.) in DCM
(50 mL). The resulting mixture was stirred for 5 min before being
concentrated to dryness. The residue was purified by silica gel chro-
matography (pentane: EtOAc 100 : 1) to give the expected product 8a or
8b as a yellowish liquid (8a: 91%, 8b: 66%).
8a: 1H NMR (300 MHz, CDCl3) δ 0.96 (t, J = 8.0 Hz, 6H), 1.34–1.40
(m, 22H), 1.64–1.70 (m, 2H), 2.53–2.60 (m, 4H), 7.04 (s, 1H), 7.68 (s,
1H).
1Н NMR (CDCl3, 400 MHz): δ 0.84–0.95 (m, 12H), 1.22–1.47 (m,
40), 1.56–1.69 (m, 8H), 2.51 (t, J = 7.6 Hz, 4H), 2.71 (t, J = 7.6 Hz, 4H),
6.88 (s, 2H), 7.42–7.48 (s, 4H).
13C {1H} NMR (CDCl3) δ 14.2, 22.8, 27.4, 28.8, 29.3, 29.4, 29.4,
29.6, 29.7, 29.7, 29.8, 31.0, 32.0, 85.9, 121.7, 129.9, 131.9, 141.8,
13C {1H} NMR (CDCl3, 100 MHz): δ 14.1, 22.7, 28.3, 29.1, 29.3,
29.3, 29.4, 29.5, 29.5, 29.6, 29.8, 30.0, 31.9, 31.9, 85.2, 94.6, 118.3,
9

R.M. Barba-Barba et al.
Dyes and Pigments 188 (2021) 109133
143.2.
The Z-scan apparatus was calibrated with traces in the close aperture
HRMS (DCI/CH4): m/z calcd for C22H36OSBr2 [M] 490.0904, found:
490.0904.
scheme using CS2 as standard nonlinear refractive material with n2 = 2
× 10ꢀ 15 cm2/W [58]. Finally, σ2PA
=
ω β, where ℏ
ω
is the photon en-
ℏ
ergy and N the density of molecules in Nthe solution.
8b: 1H NMR (300 MHz, CDCl3) δ 0.97 (m, 6H), 1.39–1.62 (m, 12H),
2.50–2.60 (m, 4H), 7.03 (s, 1H), 7.64 (s, 1H).
13C {1H} NMR (75 MHz, CDCl3): δ 13.9, 14.0, 22.6, 22.7, 27.2, 28.4,
31.9, 33.1, 85.9, 121.7, 129.8, 131.8, 141.8, 143.2.
HRMS (DCI/CH4): m/z calcd for C14H20OSBr2 [M] 377.9639, found:
377.9652.
Two-photon excited fluorescence (TPEF) was implemented with 140
fs pulses from a Ti:Sa oscillator (Chameleon ultra, from Coherent) in the
range 680–1080 nm. THF solutions were tested at a concentration of
8.43 × 10ꢀ 5 M. Laser beam was focused in the frontal face of a 1 cm
quartz cell with a 35 mm focal length lens. TPEF signal was detected in
the perpendicular direction to the excitation with a hand-held spec-
trometer (Ocean optics, 2000+) and using a low pass filter to block the
excitation. The σ2PA value was obtained using the equation [59]:
•
1,10-bis[2-(3,4-dialkylthien-2-yl)ethynyl]-4,7,13,16-tetrame-
thoxy-4,7,13,16-tetraphenylcyclooctadeca-2,5,8,11,14,17-hexayne-
1,10-diol (9a, 9b).
To the solution of 4a or 4b (3 eq.) in dry THF (10 mL) at ꢀ 78 ◦C was
added dropwise a n-BuLi solution (1.6 M in hexane, 2.5 eq.). Stirring was
continued at ꢀ 78 ◦C for 30 min, then at room temperature for 1 h. After
cooling to ꢀ 78 ◦C, a solution of 3 (0.15 mmol) in dry THF (2 mL) was
added dropwise and the mixture was allowed warming slowly to room
temperature overnight. The reaction was treated with a saturated
aqueous solution of NH4Cl and the aqueous layer was extracted with
Et2O. The combined organic layers were dried with MgSO4, concen-
trated under vacuum and purified by SiO2 column chromatography
(pentane: EtOAc 10 : 1) leading to the diol 9a or 9b as a waxy solid (9a:
58%, 9b: 48%).
FsCRΦR
η
RFRCRΦSηR
σTPA
=
σ
(2)
S
where, F is the integrated area of the emission, C is the molar concen-
tration (mol. Lꢀ 1), Φ is the fluorescence QY, and
η is the refraction index
of the solvent, the subscripts S and R denoting sample and reference,
respectively. The Rodhamine 6G at a concentration of 1 × 10ꢀ 5 M was
used as reference for calibration.
4.6. Optical limiting
9a: 1Н NMR (CDCl3, 300 MHz): δ 0.84–0.94 (m, 12H), 1.18–1.65 (m,
24H), 2.49–2.67 (8H), 2.93–3.21 (m, 2H), 3.41–3.68 (m, 12H), 6.92 (s,
2H), 7.28–7.42 (m, 12H), 7.74–7.82 (m, 8H).
For optical limiting measurements, the same laser source as the Z-
scan experiments was used at 800 nm. The setup consisted in a laser
beam hitting a 1 mm quartz cell containing a solution of 2a at a con-
centration of 8.43 × 10ꢀ 2 M in THF.
13C {1H} NMR (CDCl3, 100 MHz): δ 14.1, 22.7, 28.3, 28.9, 29.2,
29.4, 29.5, 29.7, 29.8, 30.0, 30.1, 31.9, 53.6, 54.9, 71.9, 78.3, 80.4,
84.4, 90.5, 116.1, 123.0, 126.5, 128.5, 129.1, 139.2, 142.2, 149.0.
HRMS (MALDI-TOF/DCTB): m/z calcd for
C
86H90S2 [M]+
Author contribution
1186.6484, found: 1186.6460.
9b: 1Н NMR (CDCl3, 300 MHz): δ 0.88–0.98 (m, 12H), 1.27–1.59 (m,
16H), 2.48–2.62 (m, 8H), 3.52–3.54 (m, 12H), 6.88 (s, 2H), 7.29–7.33
(m, 12H), 7.74 (m, 8H).
Rodrigo M. Barba-Barba: Investigation (general optical studies,
linear and nonlinear); Formal analysis; Visualization; Writing – original
draft. Marwa Chammam: Investigation (general chemical studies and
synthesis); Formal analysis; Visualization; Writing – original draft.
Gabriel Ramos-Ortiz: Conceptualization; Supervision; Investigation;
Formal analysis; Funding acquisition; Resources; Project administration;
Writing – review & editing. Dymytrii Listunov: Investigation (chemical
synthesis). Jayaramakrishnan Velusamy: Investigation (nonlinear opti-
cal characterization). Mario Rodriguez: Conceptualization (molecular
design). Ramon Carriles: Investigation (linear optical characterization).
Carlos Silva: Validation; Resources, Funding acquisition; Writing – re-
view & editing. Carine Duhayon: Investigation (X-ray crystallography).
MS (MALDI-TOF/DCTB): m/z [M ꢀ Na]+ 1143.7 [M ꢀ K]+ 1159.7.
4.5. Linear and nonlinear spectroscopic characterization
Spectra of linear absorption, emission, excitation, and lifetime of
fluorescence were obtained with the use of a fluorometer (FS5, Edin-
burgh Instruments). Emission was also obtained by pumping the sample
with short optical pulses (100 kHz repetition rate) from an optical
parametric amplifier (Orpheus, Light Conversion, Lithuania) pumped at
1030 nm by a mode-locked oscillator (Pharos—Model PH1-20-0200-02-
10, Light Conversion, Lithuania). To minimize absorption inner effects,
the excitation and recollection of emission was produced in the front
face of a 1 mm-thick cuvette containing a solution of the sample. The
emission analyzed with an imaging spectrograph (Shamrock 193i,
Andor Technology) and detected with a multichannel detector. The σ2PA
was obtained by Z-scan method with 80 fs (800 nm) laser pulses deliv-
ered by a Ti:Sa amplifier (Libra HE from Coherent) at 1 KHz. At the
´
Brice Kauffmann: Investigation (X-ray crystallography). Valerie Mar-
aval: Conceptualization; Supervision; Formal analysis; Funding acqui-
sition; Resources; Project administration; Writing – review & editing.
Remi Chauvin: Conceptualization; Supervision; Formal analysis; Fund-
ing acquisition; Resources; Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
position Z = 0 the beam waist was 20 μm. THF solutions were tested at
the concentration of 8.43 × 10ꢀ 2 M in a 1 mm quartz cell. For wave-
lengths other than 800 nm in the range 650–950 nm, the Z-scan method
was implemented with an optical parametric amplifier (TOPAs, from
Light Conversion). The nonlinear coefficient β was calculated from the
equation [57a,57b]:
Acknowledgements
The work has been performed within the framework of ECOS-NORD
project (M16P02 and 291434) and the French Mexican International
Laboratory LIA-LCMMC with financial support from CNRS (France) and
CONACYT (Mexico). M. B.-B. thanks CONACYT for his PhD fellowship
1
I0Leff
√̅̅̅
T(z) = 1 ꢀ
β
(1)
1 + (Z/Z0)2
2
2
where I0 is the peak intensity, Leff is the effective length of the sample
`
(565612). M. C. thanks the French Ministere de l’Enseignement
with Leff = [1 ꢀ exp( ꢀ
α0L)]/
α
0, where L is the sample thickness and
´
Superieur de la Recherche for her PhD scholarship. The authors thank
(
)
the IDEX Emergence program 2014 (Carbo-device) for the post-doctoral
grant of D. L. and for funding. The Centre National de la Recherche
Scientifique (CNRS) is also acknowledged for funding. The work at the
/
α0 the absorption coefficient, Z the sample position and Z0
=
πω20
λ
the Rayleigh range. To determine ω0 the knife edge method was used.
10

R.M. Barba-Barba et al.
Dyes and Pigments 188 (2021) 109133
[15] Liu Y, Lin FX, Feng Y, Liu X, Wang L, Yu ZQ, Tang BZ. Shape-persistent
Georgia Institute of Technology was supported by the National Science
Foundation (DMR-1729737). This work has benefited from the X-ray
facility of the Biophysical and Structural Chemistry platform at IECB,
CNRS UMS3033, INSERM US001, Bordeaux University.
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