Octupolar merocyanine dyes: A new class of nonlinear optical chromophores

Article abstract of DOI:10.1002/chem.201200718

A set of new octupolar merocyanine chromophores was designed and synthesized. These compounds were prepared from the reaction of 1,3,5-triformyl-2,4,6-trihydroxybenzene with heterocyclic nucleophiles. Octupolar dyes were formed exclusively in their open-dye form. The one- and two-photon-absorption spectra of the dyes consist of two bands: The long-wavelength band in the two-photon absorption spectrum (a few hundreds GM above 1000 nm) matches well with the intense, long-wavelength-absorption band that is located in the visible region in the linear spectrum. Interestingly, an additional, much-more-intense TPA band in the NIR region is observed at higher energy, which corresponds to a weakly allowed one-photon electronic transition. Changing the peripheral heterocyclic moieties allows tuning of the optical properties to approach the cyanine limit (i.e., polymethine state), thus resulting in a red-shift of the low-energy one-photon-absorption band as well as to the rise of an intense two-photon-absorption band in the NIR region. To the best of our knowledge, this is the first synthesis and TPA characterization of octupolar merocyanine chromophores with typical low-bond-length alternation. Copyright

Full text of DOI:10.1002/chem.201200718

FULL PAPER
DOI: 10.1002/chem.201200718
Octupolar Merocyanine Dyes: A New Class of Nonlinear Optical
Chromophores
Yevgen M. Poronik,[a] Vincent Hugues,[b] Mireille Blanchard-Desce,*[b] and
Daniel T. Gryko*[a]
Dedicated to Professor Jonathan Lindsey on the occasion of his 55th birthday
Abstract: A set of new octupolar mero-
cyanine chromophores was designed
and synthesized. These compounds
were prepared from the reaction of
1,3,5-triformyl-2,4,6-trihydroxybenzene
with heterocyclic nucleophiles. Octupo-
lar dyes were formed exclusively in
their open-dye form. The one- and
two-photon-absorption spectra of the
dyes consist of two bands: The long-
wavelength band in the two-photon ab-
sorption spectrum (a few hundreds GM
above 1000 nm) matches well with the
intense,
long-wavelength-absorption
ripheral heterocyclic moieties allows
tuning of the optical properties to ap-
proach the cyanine limit (i.e., polyme-
thine state), thus resulting in a red-shift
of the low-energy one-photon-absorp-
tion band as well as to the rise of an in-
tense two-photon-absorption band in
the NIR region. To the best of our
knowledge, this is the first synthesis
and TPA characterization of octupolar
merocyanine chromophores with typi-
cal low-bond-length alternation.
band that is located in the visible
region in the linear spectrum. Interest-
ingly, an additional, much-more-intense
TPA band in the NIR region is ob-
served at higher energy, which corre-
sponds to a weakly allowed one-photon
electronic transition. Changing the pe-
Keywords: dyes
imaging agents · solvatochromism ·
two-photon absorption
· fluorescence ·
Introduction
low concentrations, it is necessary to design fluorescent
labels and probes that have much-larger two-photon bright-
ness (s2F, where F is the fluorescence quantum yield) than
classical fluorescent probes (typically thousands of GM).
Whereas fluorophores that have very large two-photon
brightness in the red-NIR range also have very large one-
photon brightness in the near UV/Vis region,[7,8] the oppo-
site is not true. Indeed, 2PA chromophores belong to very
unique class of functional dyes that have some structural pe-
culiarities that are different from usual chromophore sys-
tems.[1,9] It has become clear that, for the two-photon tech-
nology to realize its full potential, the development of more
two-photon-active chromophores that also possess other
useful optical or chemical properties, such as high fluores-
cence quantum yields, easy processability, good photostabili-
ty, and durability will play a vital role.
There are some principles in the construction of NLO
chromophores, which are imposed by the selection rules for
2PA, unlike for 1PA, that result in the particular structure of
2PA systems.[1,9] Large two-photon-absorption cross-sections
have often been associated with the extent of conjugation
(effective p-delocalization), which leads to an extended
charge-separation (a large p-conjugated system). Adding an
electron donor (D) and acceptor (A) onto the ends of a con-
jugated chromophore to give a D-p-A system enhances the
transition dipole moment as a consequence of the increase
in the displacement of charge during the transition from
HOMO to LUMO.
The design of new chromophores that possess nonlinear op-
tical (NLO) properties, such as two-photon absorption
(2PA), has been attracting considerable attention in recent
years owing to the prospect of using them in a wide range of
applications, including the two-photon excited fluorescence
(TPEF) microscopy of biological structures,[1,2] 3D data stor-
age,[3] photodynamic therapy,[4] optical limiting,[5] 3D micro-
fabrication,[1a,5f,6] etc. On the other hand, classical one-
photon-absorption (1PA) chromophores, such as cyanine
and rhodamine dyes, which are very popular in fluorescence
microscopy techniques, usually have relatively low two-
photon-absorption cross-sections (s2) in the spectroscopic
range of interest for biological imaging (700–1000 nm);
therefore, they require high excitation intensity and/or high
concentrations when used as fluorochromes in the multipho-
ton imaging of biological systems. To lower the excitation in-
tensity (for reduced damage) or scan more rapidly (for dy-
namic imaging) for the detection of species that have very
[a] Dr. Y. M. Poronik, Prof. Dr. D. T. Gryko
Institute of Organic Chemistry of the
Polish Academy of Sciences
44/52 Kasprzaka Str., 01-224 Warsaw (Poland)
E-mail: dtgryko@icho.edu.pl
[b] V. Hugues, Dr. M. Blanchard-Desce
Universitꢀ de Bordeaux, ISM (UMR5255 CNRS)
33400, Bordeaux (France)
Compared to dipolar systems, centrosymmetric dyes (D-
p-A-p-D or A-p-D-p-A) tend to have larger TPA respons-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200718.
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es,[1,7] partly because the transition moment in a D-p-A-p-D
system is larger than those for the corresponding D-p-A
unit, because D-p-A-p-D will be effectively polarized by
both parts of the optical field cycle, whereas D-p-A is easily
polarized in just one direction. Similarly, centrosymmetric
octupolar dyes that are typically built from the branching of
several dipolar chromophores through a common donor or
polarity of the solvent. When the two resonance forms con-
tribute equally to the ground-state structure, the molecule
exhibits essentially no BLA (the so-called “cyanine limit”,
that is, state II). In their electronic spectra, merocyanine
dyes are characterized by intense and sharp cyanine-like ab-
sorption bands. Compared to polyene molecules, merocya-
nine dyes may have more-efficient intramolecular charge-
transfer (ICT) between the donor and the acceptor and,
hence, a higher dipole moment (or polarization) in the
ground state, as well as a larger 2PA cross-section for
a shorter conjugated chain.[12] Overall, only a few papers on
the study of 2PA of merocyanine dyes have been pub-
lished.[13–15]
acceptor core (i.e., DACHTUNGRTENNUG(-p-A)3 or AACHUTGNTREN(NGUN -p-D)3) can lead to
much-larger TPA[10] responses than their dipolar counter-
part.
From the point of view of electronic structure, a number
of 2PA chromophores belong to a family of push–pull poly-
enes (structure A, Figure 1) that contain an electron donor
This paper addresses the synthesis and investigation of
new octupolar conjugated merocyanine dyes as a combina-
tion of three dipolar merocyanines in one system. To the
best of our knowledge, no such octupolar merocyanines
have been reported previously.
Results and Discussion
Figure 1. Resonance forms for the polyene (A) and merocyanine chromo-
phores (B).
To obtain the desired merocyanines, phloroglucinol trialde-
hyde (1)[16] was chosen as the building block. The reactivity
of the aldehyde groups was high enough to undergo conden-
sation with three equivalents of quaternary heterocyclic
salts. The reaction between trialdehyde 1 and methylindoli-
um salt 2a in boiling pyridine gave octupolar merocyanine
3a (Scheme 1).
and an electron acceptor grafted onto opposite ends of the
conjugated system. The contribution of the mesomeric form
with charge separation (zwitterionic form) is usually very
low and such polyenes have high bond-length alternation
(BLA) and moderate charge separation (and polarization)
in the ground state.[1,10] The electronic spectra of polyenes
are characterized by broad, low- or moderately intense ab-
sorption bands. Merocyanine dyes[11] (structure B, Figure 1),
although formally similar to polyenes, are completely differ-
ent from an electronic point of view and belong to a family
of polymethine dyes. Indeed, merocyanine dyes contain an
odd number of p-centers within the conjugated system,
whereas polyenes contain an even number of p-centers.
The structure of merocyanine dyes is the superposition of
two mesomeric forms: neutral (I) and zwitterionic (III,
Figure 1). The relative contribution of these forms depends
on the strength of the donor and the acceptor and on the
Scheme 1. Reagents and conditions: pyridine (Py), reflux, 10 min; yield:
59%.
Abstract in Polish: Otrzymano barwniki funkcjonalne typu
merocyjanin o budowie oktupolowej. Synteza oparta jest o
reakcje˛ 1,3,5-triformylo-2,4,6-trihydroksybenzenu z nukleofi-
lami heterocyklicznymi. Udowodniono, z˙e barwniki istnieja˛
wyła˛cznie w formie otwartej. Otrzymane zwia˛zki absorbuja˛
The generic dipolar merocyanine structure commonly
exists in the form of isomeric spiropyran I (Figure 2) in
which the conjugation chain is broken. Spiropyran I can un-
dergo reversible isomerization to form merocyanine II
under UV irradiation.[17a,b]
´
´
silnie swiatło czerwone i pomaranczowe, wykazuja˛ fluores-
cencje˛ (lem =590–690 nm) oraz dwa silne pasma w absorpcji
dwufotonowej (s2 =500–2300 GM). Zmiana jednostki hetero-
1
The H NMR spectrum of compound 3a reveals six sig-
´
´
nals in the aromatic region (see the Supporting Informa-
tion); moreover, the absorption spectrum in solution shows
an intense and sharp absorption band at around 570 nm,
which suggests a symmetrical cyanine-type electronic struc-
cyklicznej pozwala na modulowanie własciwosci optycznych
´
i zbliz˙enie sie˛ do tzw. granicy cyjaninowej, co bezposrednio
skutkuje batochromowym przesunie˛ciem maksimum absorp-
cji jednofotonowej i zwie˛kszeniem intensywnosci absorpcji
´
1
ture. According to the H NMR spectrum, no isomerization
dwufotonowej.
occurred in the solution of compound 3a after several days.
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Unique Octupoles for Two-Photon Absorption
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instead of desired dye 3b, gave rise to pyrylocyanine triflate
4b as a major product, accompanied by traces of merocya-
nine 3b (Scheme 2). Subsequently, we found that this reac-
tion should be performed in two steps: 1) heating in boiling
EtOH in the absence of base and 2) the subsequent addition
of triethylamine afforded the desired merocyanine (3b),
which was isolated as the major product (Scheme 2).
Figure 2. Tautomerism between the spiropyran and merocyanine forms.
Dye 3a was also synthesized in high yield under standard
conditions (boiling EtOH) by using Fischer’s base (2a)[17c]
and no spiropyran derivative was detected.
Compound 3a belongs to a class of merocyanine dyes
with an octupolar structure that, until now, has no reported
analogues in the literature. This fact requires the additional
consideration of the electronic structure of this dye. The
structure of such octupolar chromophores can be depicted
in four limiting forms (Figure 3): one neutral (N) and three
Scheme 2. Reagents and conditions: a) Py, RT, 12 h; yield: 3b: 5%, 4b:
77%. b) (i) EtOH, reflux, 15 min; (ii) addition of NEt3, reflux, 10 min;
yield: 3b: 27%, 4b: traces.
Interestingly, in this case, the contribution of the various
zwitterionic forms in the description of the electronic struc-
ture is expected to be higher because aromaticity is also
gained in both the electron-donating peripheral moieties
and in the central ring upon charge separation. Based on
a simplified four-essential-state model, getting closer to the
cyanine limit should lead to both a red-shift in the 1PA re-
sponse and a marked increase in the 2PA responses for such
octupolar systems.[19] Because charge separation (and charge
multiplication) costs energy, in most situations, the neutral
form dominates in the ground state. This situation is, in par-
ticular, the case for the recently reported 2PA-active octupo-
lar derivatives, which were all built from either a typical do-
nor[7d,m] or acceptor aromatic core.[7i,10a] The octupolar mero-
cyanines investigated herein do not contain such an aromat-
ic core. This property gives more chance to shift the elec-
tronic structure towards the cyanine limit and, possibly,
overpass it by increasing the donor strength of the peripher-
al moieties.
Figure 3. Limiting forms for octupolar merocyanine 3a that was derived
from a phloroglucinol core.
Following this line, other peripheral moieties that confer
aromaticity upon charge separation were investigated to
tune the contribution of the zwitterionic forms in the
ground state and to probe the resulting optical properties.
Trialdehyde 1 readily reacted with 2,3-dimethylbenzothiazo-
lium and 1,2-dimethylquinolinium salts to form the desired
merocyanines 3c and 3d, respectively (Figure 4). However,
these latter dyes seemed to be insoluble in most organic sol-
vents, thus making it impossible to obtain analytical pure
samples.
To overcome this obstacle, trialdehyde 1 was condensed
with benzothiazolium and quinolinium salts, which con-
tained longer N-alkyl groups (2e and 2 f, respectively). This
approach allowed us to obtain isoelectronic analogues of
dyes 3c and 3d (3e and 3 f, respectively, Scheme 3).
mesoionic forms in which one dipole arm possesses zwitter-
ionic character (Z1–Z3)[18] and where the negative charge is
delocalized between the oxygen atoms of the central elec-
tron-acceptor framework. However, the energy of the limit-
ing forms that contain multiple charge-transferred dipole
arms is too high; therefore, these latter forms do not make
any significant contribution. As the result of the superposi-
tion of these forms, the electron density is partly shifted
from the end moieties towards the central part of molecule.
For a better understanding of the nature of this chromo-
phore, we attempted to obtain analogous dyes that contain
peripheral end-groups with lower and higher electron-donor
ability. It appeared that the reaction between methylpyryli-
um salt 2b and aldehyde 1 in pyridine at room temperature,
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M. Blanchard-Desce, D. T. Gryko et al.
Figure 4. Structures of compounds 3c and 3d.
Figure 5. Absorption spectra of octupolar merocyanine dyes 3a (solid
line), 3b (dashed line), 3e (dashed dotted line), and 3 f (dotted line) in
toluene.
Table 1. Linear optical properties of the synthesized dyes.
max
max
Compound
labs
[nm]
emax ꢂ10ꢁ3 FWHM lem
Stokes shift
[cmꢁ1
F
A
]
E
]
[nm]
G
]
[%]
Scheme 3. Reagents and conditions: Py, reflux. e: Y=S, yield: 14%; f:
Y=CH=CH, yield: 50%.
3a (toluene) 567
195
45.8
141
36.5
160
44.2
152
571.5 105
456.5 46.0
220
46.0
1356
2026
2272
2524
596
598
648
655
858
3.4
419
3a (THF)
562
1070
723
6.7
418.5
3b (toluene) 619
0.91
Only a few dye molecules with some similarity to mero-
cyanines 3a–3 f are known in the literature. The tetraone de-
rivative[15] is a quadrupolar analogue of octupolar dyes 3a–
3 f. Analogously, it contains a central electron acceptor
moiety that is conjugated with two electron-donating periph-
eral heterocycles. The major difference between octupolar
merocyanines 3a–3 f and tetraone is that tetraone contains
two chromophores that are formally non-conjugated with
each other, that is, the zwitterionic forms do not result in ex-
tended p-conjugated systems, as is the case here. Tetraone
shows a 1PA absorption maximum at 624 nm and its 2PA
spectrum consists of a broad band that overlaps the whole
457
3b (THF)
615
990
1.9
3e (toluene) 599
1184
1432
2172
2194
627
627
690
685
745
750
636
550
17.2
12.3
440
3e (CHCl3)
602.5 215
451.5 45.3
187
24.0
3 f (toluene) 661
0.62
0.28
506
3 f (CHCl3)
659.5 213
609.5 128
511.5
31.2
max
700–1150 nm spectroscopic range with s2 ꢀ2700 GM at
820 nm.
Ketocyanines[20] can also be considered as analogues of
octupolar merocyanines 3a–3 f. Their structure can also be
considered to be quadrupolar with the carbonyl group as an
electron acceptor. In the case of these dyes, two chromo-
phore arms are conjugated in one p-system (via the inter-
mediate zwitterionic form). Ketocyanines are sensitive to
the pH value and, in acidic media, they are reversibly pro-
tonated, thus switching to a carbocyanine form. Ketocya-
nines exhibit moderate linear absorption in the spectroscop-
ic range 550–650 nm and show large quadratic hyperpolari-
zabilities.[20d]
When comparing the linear absorptions of dyes 3a–3 f,
some similarities can be found. Because the synthesized
dyes consist of end moieties that possess different donor–ac-
ceptor properties, the electron densities in the structures of
the merocyanines is distributed in different ways. Let us an-
alyze the influence of the solvent on the shape of the ab-
sorption bands. According to the Franck–Condon principle,
the smaller BLA in the polymethine chain in the ground
state, the smaller the change in the bond lengths in the ex-
cited state. Consequently, the absorption spectra of such
electron-symmetrical dyes should exhibit strong 0–0 transi-
tions at the expense of the vibronic transitions.[21]
Linear optical properties: The synthesized octupolar mero-
cyanine dyes display two absorption bands in the visible
region: an intense band at long-wavelength with a well-re-
solved vibrational structure and a much-weaker band at
shorter wavelength (Figure 5, Table 1).
The absorption spectrum of the octupolar merocyanine
that contains the weakest electron-donating peripheral moi-
eties (3b) displays a relatively broad, long-wavelength ab-
sorption band with well-resolved vibronic structure in low-
polarity solvents. The value of the band width at half maxi-
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Unique Octupoles for Two-Photon Absorption
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mum (FWHM) is relatively high (Table 1). In addition, in-
creasing the solvent polarity gives rise to a definite red-shift
and broadening of the low-energy absorption band
(Figure 6), as well as an increase in Stokes shift, whereas the
intensity ratio of the 0–1 sub-band relative to that of the 0–0
sub-band (I0–1/I0–0) is not much affected (0.66 in 1,4-diox-
ane). From this variation, we can speculate that the elec-
tronic structure of merocyanine 3b is characterized by
a dominant contribution of the neutral form in the ground
state (i.e., 1<0.5 in a four-essential-state model).[19]
Figure 7. Normalized absorption spectra of merocyanine 3a in DMSO
(solid line), toluene (dotted line), 1,4-dioxane (dashed line), and cyclo-
hexane (dashed dotted line).
Figure 6. Normalized absorption spectra of merocyanine 3b in DMSO
(solid line), toluene (dotted line), 1,4-dioxane (dashed line), and cyclo-
hexane (dashed dotted line).
Merocyanine 3a contains peripheral heterocyclic moieties
with increased electron-donating ability compared to dye
3b. The absorption spectra of merocyanine 3a exhibits
somewhat narrower absorption bands at long-wavelength
and its FWHM is smaller than that of merocyanine 3b. In
addition, increasing the solvent polarity gives rise to a defi-
nite red-shift of the low-energy absorption band, whilst the
I0–1/I0–0 ratio decreases (Figure 7).
Figure 8. Normalized absorption spectra of merocyanine 3e in DMSO
(solid line), toluene (dotted line), 1,4-dioxane (dashed line), and cyclo-
hexane (dashed dotted line).
The fine vibronic structure is only resolved in cyclohexane
This electronic structure also results in the smallest Stokes
shift and the largest fluorescent quantum yield among the
other dyes in the series. This result is corroborated by the
value of the coupling constant (J) between the vicinal vinylic
solution (with a corresponding intensity ratio of I0–1/I0–0
=
0.57). This result suggests that the electronic structure of
merocyanine 3a in nonpolar solvents is characterized by
a dominant neutral form but get closer to the cyanine limit
(or polymethine state) in polar solvents (i.e., 1ꢂ0.5 in the
four-essential-state model).[19]
ꢁ
protons in the C C bond that connects the phloroglucinol
core and the peripheral moieties in the case of merocyanine
3e. Indeed, the J values (CDCl3) are intermediate between
the coupling constants of the vicinal vinylic protons through
Further increasing the electron-donating ability of the pe-
ripheral heterocyclic moieties (dye 3e) gives rise to symmet-
rization of the p-system of the merocyanines. In their elec-
tronic spectra, dye 3e exhibits a narrow absorption band at
long-wavelength with a minimal FWHM among other octu-
polar merocyanines (Table 1). Dye 3e demonstrates the
strongest linear absorption among the other octupoles and
its absorption hardly depends on the polarity of the solvent.
The intensity ratio (I0–1/I0–0 =0.43 in cyclohexane) is the
smallest within the series (Figure 8). Evidently, the electron-
ic structure of dye 3e approaches the cyanine limit (i.e., 1
ꢀ0.5 in the four-essential-state model).[19]
ꢁ
a C C single bond (10 Hz, (anticoplanar geometry, as in the
neutral form) or through a C=C double bond (16 Hz, trans
geometry, as in the zwitterionic forms).
Merocyanine 3 f contains peripheral heterocyclic moieties
that have the strongest electron-donating character. The ab-
sorption spectrum of dye 3 f display a well-resolved vibronic
structure and the intensity ratio (I0–1/I0–0 =0.73, in cyclohex-
ane) is the highest within the series (Figure 9). This latter
result indicates some degree of BLA in the ground state of
merocyanine 3 f, which is corroborated by the larger FWHM
value compared to octupolar merocyanine 3e (similar to
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M. Blanchard-Desce, D. T. Gryko et al.
Figure 9. Normalized absorption spectra of merocyanine 3 f in DMSO
(solid line), toluene (dotted line), 1,4-dioxane (dashed line), and cyclo-
hexane (dashed dotted line).
Figure 11. Linear absorption (blue), fluorescence (green) and two-photon
absorption spectra (red) of merocyanine 3b.
that of dye 3b). Increasing the solvent polarity does not sig-
nificantly affect the position of the absorption band but
rather induces a decrease in the I0–1/I0–0 ratio and in the
Stokes shift.
Consequently, in the case of dye 3 f, we speculate that the
contribution of the limiting forms that have zwitterionic
character is higher than that of the neutral form (i.e., 1ꢃ0.5
in the four-essential-state model).[19]
S0!S1 transition (two-photon allowed for octupolar chro-
mophores),[7c] which is typical of polymethine dyes. Con-
versely, the 2PA spectra of octupolar merocyanines also dis-
play high-intensity, short-wavelength 2PA bands that match
well with the short-wavelength absorption band in the linear
electronic spectra and are most-probably related to the one-
photon forbidden transition in C3 molecules (and the two-
photon allowed electronic transition S0!S2). Here, this tran-
sition becomes partially 1PA allowed owing to relaxation of
the molecular geometry to a conformation with a lower
symmetry.[7c]
Nonlinear optical properties: There are some parallels be-
tween the 1PA and 2PA of merocyanines within the series.
For better comparison, the 2PA spectra are shown on the
same graph as the 1PA spectra, with separate axes, by multi-
plying the wavelength scale by 2 (Figure 10 and Figure 11).
The linear absorption spectra of dyes 3a–3 f consist of two
bands: an intense, long-wavelength band and a weaker,
short-wavelength band. At the same time, the 2PA spectra
of the octupolar merocyanines display low-intensity long-
wavelength maxima (150–200 GM) that match well with the
For merocyanines 3a and 3b, which have the weakest
electron-donating heterocyclic moieties, the maximum 2PA
max
cross-sections in the NIR region are comparable: s2
ꢀ1300 GM (Table 2). In the case of merocyanine 3e, the
Table 2. Two-photon absorption of octupolar merocyanines.
max
max1
max1
max2
max2
Compound
2lOPA
[nm]
lTPA
[nm]
s2
lTPA
[nm]
s2
[GM]
[GM]
3a (THF)
1124
1150
1110
1070
1200
1150
1050
>1200
>1200
98
129
167
162
214
166
–
850
1315
3b (THF)
1230
930
1293
3e (CHCl3)
3 f (CHCl3)
1205
1319
<860
890
>485[a]
2274
–
[a] The 1PA transition is close to the wavelength of the 2PA laser,
making it difficult to resolve the 2PA band.
linear absorption band is close to the higher-energy 2PA
band, thereby making it difficult to reach the maxima value
by using TPEF experimental methods. This result is consis-
tent with merocyanine 3e, which is very close to the cyanine
limit (or the ideal polymethine state), where 2PA become
1PA resonant, thereby giving rise to giant 2PA responses.[19]
On the other hand, merocyanine 3 f contains the strongest
electron-donating peripheral heterocyclic moieties and, con-
Figure 10. Linear absorption (blue), emission (green), and two-photon
absorption spectra (red) of merocyanine 3a.
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dolinium salt 2a (450 mg, 1.65 mmol) were heated at reflux in pyridine
(4 mL) for 10 min. After cooling, pyridine was evaporated and the crude
product was eluted through silica gel (CH2Cl2/CH3OH, 95:5). The solvent
was evaporated and the product was purified for a second time by
column chromatography on silica gel (CH2Cl2/Et2O, 3:1). After evaporat-
ing the eluent, the product was dried to give 200 mg (59%) of merocya-
nine 3a. M.p.: 2388C; 1H NMR (500 MHz, CD2Cl2, 258C, TMS): d=8.70
sequently, has the strongest charge separation in the ground
state with the largest 2PA response (s2max =2300 GM).
Conclusions
(d, 3J
7.31 (m, 6H; ArH), 7.12 (t, 3J
(H,H)=7.5 Hz, 3H; ArH), 3.54 (s, 9H; NCH3), 1.78 ppm (s, 18H; C-
AHCTUNGTRENNUNG
(CH3)2); 13C NMR (125 MHz, CD2Cl2, 258C, TMS): d=173.6, 146.8,
A
ACHTUNGERTN(NUNG H,H)=14 Hz, 3H CH), 7.36–
A number of new octupolar merocyanine dyes were de-
signed and synthesized. These molecules consist of three
merocyanine chromophores that are conjugated in one p-
system to form octupolar frameworks with a central elec-
tron-withdrawing fragment and peripheral electron-donor
moieties. Their main unique structural feature comes from
the nature of the (acceptor) core, which gains some aromat-
ic character upon charge-separation. As a result, it becomes
possible to modulate the relative contributions of the neu-
tral (polyene like) and zwitterionic forms (charge-separated)
by using peripheral heterocyclic donors of various strengths
and investigate derivatives in the same series on both side
of the cyanine limit. In addition, this procedure allowed us
to obtain octupolar structures that have electronic structures
that are close to the cyanine limit, in which the higher-
energy 2PA band (close to 1PA resonance) is expected to be
extremely intense.
The linear absorption spectra of the dyes consist of two
bands: an intense, long-wavelength band and a weak, short-
wavelength band. The two-photon-absorption spectra also
display two maxima. The low-intensity, long-wavelength
band corresponds to a two-photon allowed transition for oc-
tupolar chromophores (S0!S1) and matches well with the
long-wavelength absorption band in the linear spectrum.
The intense higher-energy 2PA band matches well with the
short-wavelength absorption band in the linear electronic
spectra, which corresponds to a one-photon forbidden elec-
tronic transition (S0!S2).
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
143.7, 140.8, 127.9, 122.8, 121.8, 108.6, 98.9, 48.0, 30.4, 28.8 ppm; UV/Vis
(toluene): lmax(e)=567 (195000), 419 nm (46000 molꢁ1 m3 cmꢁ1); HRMS
(FD): m/z calcd for C45H45N3O3: 675.3461 [M]+; found: 675.3486.
AHCTUNGTRENNUNG
1
pyrylium salt 2b (550 mg, 1.55 mmol) were heated at reflux in EtOH
(5 mL) for 15 min. After cooling, triethylamine (200 mg, 2 mmol) was
added and the reaction was heated at reflux for an additional 10 min.
The solvent was evaporated and cyclohexane (20 mL) was added to the
residue. The product was dissolved, the solid material was filtered off,
and cyclohexane was evaporated. The residue was purified by column
chromatography on silica gel (CH2Cl2/Et2O, 9:1). After evaporating the
eluent, the product was dried to give 110 mg (27%) of merocyanine 3b.
M.p.: 3228C (dec.); 1H NMR (500 MHz, CD2Cl2, 258C, TMS): d=8.45
(br s, 3H; CH), 7.70 (d, 3J
6.33 (s, 3H; ArH), 1.34 (s, 27H; C
ACHTUNGTRENNUNG
N
ACHTUNGTRENNUNG
13C NMR (125 MHz, CD2Cl2, 258C, TMS): d=185.4, 169.5, 150.8, 144.0,
122.7, 113.1, 108.7, 101.3, 36.4, 36.1, 27.7, 27.6 ppm; UV/Vis (toluene):
lmax (e)=619 (160000), 572 (102000), 457 (44000), 435 nm
(39000 molꢁ1 m3 cmꢁ1); HRMS (FD) m/z calcd for C51H66O6: 774.4859
[M]+; found: 774.4843; elemental analysis calcd (%) for C51H66O6:
C 79.03, H 8.58; found: C 78.81, H 8.55.
General procedure for preparation of dyes 3e and 3 f: Trialdehyde
1 (105 mg, 0.5 mmol), the heterocyclic salt (1.65 mmol), and piperidine
(250 mg, 3 mmol) were heated at reflux in pyridine (4 mL) for 5 min.
After cooling, the reaction was diluted with Et2O (30–40 mL).
ACHUTNGRENUN(G 2E,4E,6E)-2,4,6-tris((Z)-2-(3-butylbenzo[d]thiazol-2ACHTUNGTNER(NUGN 3H)-ylidene)ethyli-
dene)cyclohexane-1,3,5-trione (3e): The product was filtered and the
crude product was eluted through silica gel (CH2Cl2/CH3OH 95:5). The
solvent was evaporated and the residue was recrystallized from toluene
to give 90 mg (14%) of merocyanine 3e. M.p.: 1788C; 1H NMR
Analysis of the linear absorption spectra suggests that,
whilst the merocyanine that contains peripheral pyrylium
moieties possesses partial polyene character, the structure of
the dye that contains peripheral benzothiazole moieties is
close to the “polymethine state”. Moreover, the merocya-
nine that contains peripheral quinoline moieties exists
mainly in a form in which the charges are partially separat-
ed; this form results in the largest 2PA cross-section in the
NIR range among the other dyes investigated.
(500 MHz, CDCl3, 258C, TMS): d=8.43 (d, 3J
-CH=), 8.13 (d, 3J(H,H)=13 Hz, 3H, -CH=), 7.52 (d, 3J
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
A
(H,H)=8.0 Hz, 3H, ArH), 7.18 (t, 3J
3H, ArH), 7.33 (t, JACTHNGUTRENNUNG CATHUNGTRENNNUG
3
3
3
3H, ArH), 7.12 (d, J
6H, NCH2), 1.89 (quintet, 3J
(H,H)=7.5 Hz, 6H, CH2), 1.05 ppm (t, 3J
ACHTUNGTRENNUNG
N
ACHTUNGTREN(NUNG H,H)=7.5 Hz,
AHCTUNGTRENNUNG
N
13C NMR (125 MHz, CDCl3, 258C, TMS): d=165.8, 146.1, 141.9, 126.9,
126.0, 123.5, 122.1, 111.0, 97.8, 46.0, 29.2, 20.3, 13.8 ppm; UV/Vis (tolu-
ene): lmax (e)=599 (220000), 440 nm (46000 molꢁ1 m3 cmꢁ1); HRMS (FD)
m/z calcd for C45H45N3O3S3: 771.2623 [M+]; found: 771.2628.
ACHUTNGRENUN(G 2E,4E,6E)-2,4,6-tris((E)-2-(1-butylquinolin-2ACHUTNGTREN(NUGN 1H)-ylidene)ethylidene)cy-
clohexane-1,3,5-trione (3 f): The solvents were decanted and the oily ma-
terial was washed several times with Et2O. The product was purified by
column chromatography on aluminum oxide (CH2Cl2/CH3OH, 98:2) to
Experimental Section
1
give 190 mg (50%) of merocyanine 3 f. M.p.: 1838C; H NMR (500 MHz,
CDCl3, 258C, TMS): d=8.78 (br s, 3H; CH), 8.29 (d, 3J
ACHTUNGTRENNUNG
All chemicals were used as received unless otherwise noted. Reagent-
grade solvents (CH2Cl2, hexanes) were distilled prior to use. All of the
reported 1H NMR spectra were collected on 400 MHz or 500 MHz spec-
trometers. Chemical shifts (d, ppm) were determined by using TMS as an
internal reference; J values are given in Hz. Column chromatography
was either performed on silica gel (200–400 mesh) or on neutral alumi-
num oxide (70–230 mesh). Mass spectra were obtained by field-desorp-
tion MS (FDMS). Quaternary heterocyclic salts were prepared according
to the following literature procedures: 2b,[22] 2e,[23] and 2 f.[24]
3H; CH), 7.93 (d, 3J
N
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
3
3
AHCTUNGTERG(NNUN H,H)=7.5 Hz, 6H; CH2), 1.13 ppm (t, J-
AHCTUNGTRENNUNG
185.6, 153.7, 145.1, 140.0, 132.3, 131.0, 128.6, 124.5, 123.1, 120.4, 114.4,
102.6, 47.3, 28.4, 20.3, 14.0 ppm; UV/Vis (toluene): lmax (e)=661
(187000), 608 (122000), 507 (24000), 467 (26000), 416 nm
(30000 molꢁ1 m3 cmꢁ1); HRMS (FD): m/z (%) calcd for C51H51N3O3:
753.3930 [M]+; found: 753.3928.
ACHTUNGTRENNUNG
Chem. Eur. J. 2012, 00, 0 – 0
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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Acknowledgements
This work was funded by the Foundation for Polish Science (TEAM-
2009-4/3). M.B.D. thanks the Conseil Regional d’Aquitaine for financial
support (Chaire d’excellence grant).
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Unique Octupoles for Two-Photon Absorption
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Received: April 9, 2012
Published online: && &&, 0000
Chem. Eur. J. 2012, 00, 0 – 0
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M. Blanchard-Desce, D. T. Gryko et al.
Octupolar merocyanine chromophores
displayed very high molar absorption
coefficients and high two-photon
absorption cross-sections in the spec-
tral range of interest for biological
imaging. Changing the end heterocy-
clic moieties allowed the tuning of
optical properties to approach the cya-
nine limit (i.e., polymethine state); this
resulted in a red-shift of the linear
absorption band, as well as in the rise
of a two-photon band in the NIR
region.
Dyes
Y. M. Poronik, V. Hugues,
M. Blanchard-Desce,*
D. T. Gryko* .................. &&&& — &&&&
Octupolar Merocyanine Dyes: A New
Class of Nonlinear Optical Chromo-
phores
&
10
&
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
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