Benzothiazoles with tunable electron-withdrawing strength and reverse polarity: A route to triphenylamine-based chromophores with enhanced two-photon absorption

Article abstract of DOI:10.1021/jo201411t

A series of dipolar and octupolar triphenylamine-derived dyes containing a benzothiazole positioned in the matched or mismatched fashion have been designed and synthesized via palladium-catalyzed Sonogashira cross-coupling reactions. Linear and nonlinear optical properties of the designed molecules were tuned by an additional electron-withdrawing group (EWG) and by changing the relative positions of the donor and acceptor substituents on the heterocyclic ring. This allowed us to examine the effect of positional isomerism and extend the structure-property relationships useful in the engineering of novel heteroaromatic-based systems with enhanced two-photon absorption (TPA). The TPA cross-sections (δ_TPA) in the target compounds dramatically increased with the branching of the triphenylamine core and with the strength of the auxiliary acceptor. In addition, a change from the commonly used polarity in push-pull benzothiazoles to a reverse one has been revealed as a particularly useful strategy (regioisomeric control) for enhancing TPA cross-sections and shifting the absorption and emission maxima to longer wavelengths. The maximum TPA cross-sections of the star-shaped three-branched triphenylamines are ～500-2300 GM in the near-infrared region (740-810 nm); thereby the molecular weight normalized δ_TPA/MW values of the best performing dyes within the series (2.0-2.4 GM?g^-1?mol) are comparable to those of the most efficient TPA chromophores reported to date. The large TPA cross-sections combined with high emission quantum yields and large Stokes shifts make these compounds excellent candidates for various TPA applications, including two-photon fluorescence microscopy.

Full text of DOI:10.1021/jo201411t

ARTICLE
pubs.acs.org/joc
Benzothiazoles with Tunable Electron-Withdrawing Strength and
Reverse Polarity: A Route to Triphenylamine-Based Chromophores
with Enhanced Two-Photon Absorption
Peter Hrobꢀarik,*,†,§ Veronika Hrobꢀarikovꢀa,†,‡ Ivica Sigmundovꢀa,‡ Pavol Zahradník,‡ Mihalis Fakis,^
Ioannis Polyzos,^,# and Peter Persephonis^
†Institut f€ur Chemie, Technische Universit€at Berlin, Strasse des 17. Juni 135, D-10623 Berlin, Germany
‡Department of Organic Chemistry, Faculty of Natural Sciences, Comenius University, Mlynskꢀa dolina, SK-84215 Bratislava, Slovakia
§Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dꢀubravskꢀa cesta 9, SK-84536 Bratislava, Slovakia
^Department of Physics, University of Patras, GR-26504 Patras, Greece
#Department of Optics and Optometry, Technological Educational Institute of Patras, GR-25100 Aigio, Greece
S Supporting Information
b
ABSTRACT: A series of dipolar and octupolar triphenylamine-
derived dyes containing a benzothiazole positioned in the
matched or mismatched fashion have been designed and
synthesized via palladium-catalyzed Sonogashira cross-coupling
reactions. Linear and nonlinear optical properties of the de-
signed molecules were tuned by an additional electron-with-
drawing group (EWG) and by changing the relative positions of
the donor and acceptor substituents on the heterocyclic ring.
This allowed us to examine the effect of positional isomerism
and extend the structureꢀproperty relationships useful in the
engineering of novel heteroaromatic-based systems with en-
hanced two-photon absorption (TPA). The TPA cross-sections
(δTPA) in the target compounds dramatically increased with the branching of the triphenylamine core and with the strength of the
auxiliary acceptor. In addition, a change from the commonly used polarity in pushꢀpull benzothiazoles to a reverse one has been
revealed as a particularly useful strategy (regioisomeric control) for enhancing TPA cross-sections and shifting the absorption and
emission maxima to longer wavelengths. The maximum TPA cross-sections of the star-shaped three-branched triphenylamines are
∼500ꢀ2300 GM in the near-infrared region (740ꢀ810 nm); thereby the molecular weight normalized δTPA/MW values of the best
performing dyes within the series (2.0ꢀ2.4 GM gꢀ1 mol) are comparable to those of the most efficient TPA chromophores
3
3
reported to date. The large TPA cross-sections combined with high emission quantum yields and large Stokes shifts make these
compounds excellent candidates for various TPA applications, including two-photon fluorescence microscopy.
’ INTRODUCTION
great potential of the TPA process, search for new compounds
exhibiting large δTPA values is still one of the hot topics in the
field of functional materials. In recent years, a variety of dipolar
(donorꢀbridgeꢀacceptor, DꢀπꢀA), quadrupolar (D-π-D,
D-π-A-π-D, A-π-D-π-A), and multibranched molecules have
been synthesized, and their structureꢀproperty relationships
were investigated.1,7 These studies revealed that the TPA cross-
section increases mainly with the conjugation length, planarity
and dimensionality of the π-center, the vibronic coupling, and
the donor/acceptor strength. Apart from the high δTPA values,
TPA chromophores have to satisfy various requirements, de-
pending on the specific application. For instance, a large TPA
cross-section combined with a high fluorescence quantum yield
Materials exhibiting large two-photon absorption (TPA)1 are
currently of great interest because of their applications in various
fields such as multiphoton fluorescence microscopy,2 three-
dimensional (3D) optical data storage,3 optical power limiting,4
photodynamic cancer therapy,5 and microfabrication.6 Particu-
larly, the two-photon excitation fluorescence (TPEF) micro-
scopy evolved into a powerful and widely used tool in bio-
imaging, given the ability to capitalize on its advantages over
conventional one-photon microscopy. Such advantages are in-
trinsic 3D resolution combined with reduced cell damage,
increased in-depth tissue penetration, and negligible background
fluorescence. However, most of the commonly used commercial
dyes and fluorescent probes have small TPA cross-sections (δTPA),
and high laser pulse energies are required to increase the prob-
ability of two-photon absorption. Therefore, to fully exploit the
Received: July 7, 2011
Published: October 03, 2011
r
2011 American Chemical Society
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in the long-wavelength range (near the ideal imaging window
650ꢀ900 nm) is required for biological imaging, whereas a
strong fluorescence is undesirable for two-photon-induced po-
lymerization (TPIP). Further design criteria include excellent
photostability, proper water solubility, and good cell permeabil-
ity, whereby the latter property is dependent upon the molecular
weight and thus limited by the size of the molecule. In this
context, a convenient strategy for achieving the desired proper-
ties is to optimize the individual fragments and their position in
the resulting chromophores to get the best TPA activity as
possible in a small molecular space.
From a literature survey, a benzothiazole unit is an attractive
building block for the construction of dyes with enhanced two-
photon absorption,8ꢀ10 or nonlinear optical (NLO) response in
general,11ꢀ14 because of its electron-withdrawing character as
well as its high chemical and photophysical stability compared to
the other heteroaromatics. This heterocyclic scaffold is often
employed as an edge substituent, but its successful use as the
π-center in quasi-quadrupolar systems with a general setup D-π-
A-π-D has been also reported very recently.8 The electron-
withdrawing strength of the benzothiazole itself is, however,
not sufficient to achieve very large TPA cross-sections, but it can
be easily adjusted by the quaternization of the azole nitrogen or
introduction of an additional acceptor. Besides, due to the
unsymmetrical nature of the benzothiazole, there are several
possibilities of how to arrange electron-donating (EDG) and
electron-withdrawing (EWG) groups on it. It has been shown
that the most efficient charge-transfer between these two func-
tionalities is achieved when they are attached to the C-2 and C-6
positions of the benzothiazole.11,13 Furthermore, as the strength
of an additional acceptor is increased, the increase in the NLO
response becomes more sensitive to its position. In general, the
stronger the acceptor introduced to the benzothiazole, the more
important the placement of this group at the C-2 position to
obtain larger NLO response. This behavior can be simply rationalized
by a concept of matched/mismatched arrangement of the 1,3-
benzothiazole unit with respect to electron-donating and elec-
tron-withdrawing substituents (Figure 1).13,15
In the matched case, a virtual dipole between the more
electron-rich benzene ring of the benzothiazole and the elec-
tron-poor C-2 position is oriented in the same direction as the
intramolecular charge-transfer (ICT) and thus reinforces the
excited-state dipole moment (cf., Figure 1). This results in a
larger change of the dipole moment between the ground and the
first excited state. Moreover, the matched alignment of the EWG
leads to a better stabilization of the lowest unoccupied molecular
orbital (LUMO) and thus to smaller excitation energy with the
increase of the EWG strength. Both these factors are essential
prerequisites for larger NLO response of the matched-type
series. The same “umpolung” strategy can be also extended to
TPA phenomena, as proved by our calculations on a series of
dipolar benzothiazole-containing molecules using the quadratic
response time-dependent density functional theory (DFT)
method16ꢀ18 (cf., Tables S1 and S2 in Supporting Information
for results obtained by using the Coulomb-attenuated CAM-
B3LYP functional in vacuo and in solution, respectively). Similar
effects of the positional isomerism on TPA properties have been
also theoretically predicted very recently in a family of triazole-
and thiazole-based chromophores, respectively.19 Although the
DFT method often provides an improper description of the
excited states with pronounced charge-transfer character, this
problem is overcome to some extent by the use of long-range
Figure 1. Schematic structure of pushꢀpull chromophores with matched
and mismatched alignment of the benzothiazole with respect to electron-
donating (EDG) and electron-withdrawing (EWG) groups.
corrected functionals such as CAM-B3LYP.20 The reliability of
this method for predicting the correct structureꢀproperty rela-
tions for TPA cross-sections of organic chromophores has been
shown in previous systematic studies.8,17
Aware of these findings, we were astonished that most of the
benzothiazole-derived NLO-phores or TPA dyes known to date
are designed in the mismatched alignment (“commonly-used
polarity”). Only a few compounds showing the matched setup
(“reverse polarity”) with the confined conjugation length or without
an additional EWG were prepared very recently.21 However,
because of the absence of the corresponding mismatched-type
congeners or an auxiliary acceptor, these systems did not allow to
examine the effect of positional isomerism on NLO properties.
To verify the trends predicted by quantum-chemical calcula-
tions, we present herein the synthesis and photophysical proper-
ties of a series of dipolar D1ꢀD6 and three-branched (octupolar,
C3-symmetric) T1ꢀT6 chromophores containing a benzothia-
zole moiety positioned in the matched or mismatched fashion
(Scheme 1). An additional substituent R introduced to the
benzothiazole allows for the fine-tuning of the linear and non-
linear optical properties at the same time. In view of the good
performance of the triphenylamine framework in TPA dyes,22
this moiety, acting as a strong electron donor and efficient π-
electron bridge, was employed in each structure, either as the
core or the peripheral unit. Despite the fact that a double bond is
a somewhat more efficient π-conjugation bridge than a triple
bond, we chose to incorporate an ethynylene π-spacer into our
chromophores because of the improved photostability, absence
of E/Z isomerization, and generally higher fluorescence quantum
yields.8,23
’ RESULTS AND DISCUSSION
1. Synthesis. The synthetic route to the target molecules takes
advantage of a palladium-catalyzed Sonogashira cross-coupling
reaction between (4-N,N-diphenylamino)phenylacetylene (1)
or tris(4-ethynylphenyl)amine (2) and a benzothiazole with a
halogen atom (Br or I, respectively) in the C-2 or C-6 position
(3a,b and 4a,b, respectively) (Scheme 1). The dicyanovinyl
(DCV)-substituted derivatives were prepared from the corre-
sponding carbaldehydes by a base-catalyzed Knoevenagel con-
densation with malononitrile.
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Scheme 1. Structure and Synthesis of the Target Chromophores D1ꢀD6 and T1ꢀT6
Scheme 2. Synthesis of the Halogen-Substituted Benzothiazoles 3a,b and 4a,b
The starting phenylacetylene 1 was obtained from the 4-(N,N-
diphenylamino)benzaldehyde by using a Wittig-type condensa-
tion with (bromomethyl)triphenylphosphonium bromide in the
presence of excess potassium tert-butoxide.8 Tris(4-ethynylphe-
nyl)amine (2) was prepared in high yield via triple Sonogashira-
type reaction of tris(4-iodophenyl)amine with trimethylsilylace-
tylene (TMSA) and subsequent deprotection of the TMS moiety
with tetrabutylammonium fluoride (TBAF)24 (Scheme 1).
6-Iodobenzothiazole (3a) was prepared by the nitration of
benzothiazole, subsequent sonochemical reduction of 6-nitro-
benzothiazole (5) to amine 6, and replacement of the NH2 group
with iodine via diazonium salt (Scheme 2). 2-Iodobenzothiazole
(4a) was obtained in good yield (58%) from the commercially
available 2-chlorobenzothiazole by nucleophilic substitution of
chlorine by iodine in the presence of acetyl chloride.
The hitherto unknown 6-iodobenzothiazole-2-carbaldehyde
(3b), which can serve as a versatile building block for the
synthesis of benzothiazole-derived NLO-phores with extended
π-conjugation and reverse polarity, was obtained by a sequence
of reactions shown in Scheme 2: 4-Iodoaniline was almost
quantitatively converted to N-(4-iodophenyl)acetamide (7) with
acetic acid anhydride. The subsequent substitution of oxygen by
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Table 1. Photophysical Properties Measured in Toluene
a
compd λabs (nm) Emaxb (eV) εmaxc (Mꢀ1 cmꢀ1
)
λfd (nm)
Φfe
Δν~f (cmꢀ1
)
λTPAg (nm) δTPAh (GM) δTPA/MW i (GM gꢀ1.mol)
3
3
D1
D2
D3
365
412
494
3.40
3.01
2.51
40000
26400
6200
428
519
617
0.23
0.57
0.07
4033
5004
4035
740
810
ꢀ
<50
938
ꢀ
<0.12
2.18
ꢀ
740
740
(740)j
810
840
740
810
ꢀ
740
740
(740)j
810
800
256
<50
(16)j
483
821
570
1607
ꢀ
922
459
(345)j
538
2275
0.53
<0.12
(0.04)j
1.12
1.72
0.79
2.00
ꢀ
0.98
0.64
(0.48)j
0.67
2.41
D4
388
(403)j
411
457
380
414
480
3.20
(3.08)j
3.02
37500
(19700)j
37400
437
(531)j
473
535
414
500
594
0.62
(0.24)j
1.00
2890
(5981)j
3189
D5
D6
T1
T2
T3
2.71
35700
0.47
3190
3.26
91000
0.53
2161
3.00
46600
0.65
4155
2.58
45700
0.29
3998
T4
404
(433)j
3.07
(2.86)j
2.95
115400
(89200)j
87500
434
(538)j
0.80
(0.54)j
1.00
1711
(4507)j
1919
T5
T6
421
458
440
2.82
54100
512
0.61
3196
a Wavelength of maximum absorbance (measured at the concentration of 1 ꢁ 10ꢀ6 M at 298 K). b Excitation energy. c Molar extinction coefficient.
d Wavelength of maximum emission. e Fluorescence quantum yield. f Stokes shift, Δν~ = (1/λabs ꢀ 1/λf) ꢁ 107. g Wavelength of maximum TPA cross-
section. h TPA cross-section (1 GM = 10ꢀ50 cm4 sꢀ1 photonꢀ1). i Normalized TPA cross-section (MW is the molecular weight). j Experimental data
3
3
for analogous systems with ethenylene π-linkages instead of ethynylene ones measured in dichloromethane. Values taken from ref 9.
Figure 2. Normalized one-photon absorption (a) and emission (b) spectra of the three-branched chromophores T1ꢀT6 with matched (solid line) and
mismatched (dashed line) setup of the benzothiazole.
sulfur, to provide the thioacetamide 8, was examined using two
different methods, either by treatment with Lawesson's reagent
under microwave irradiation25 (70% yield) or by heating under
reflux with P4S10 and Al2O3 as a solid support26 (80% yield). The
latter method has been found to be more efficient, not only due
to higher yields but also because of the inexpensive reagent,
simple reaction conditions, and a cleaner product. The reaction
with Lawesson’s reagent resulted in the formation of byproducts
derived from the reagent itself, which cannot be easily removed.
6-Iodo-2-methylbenzothiazole (9) was prepared from 8via Jacobson's
cyclization using potassium ferricyanide in an aqueous solution
of NaOH (60% yield). The synthetic route to 9 presented herein
offers a somewhat improved overall yield and allows the synthesis
on a larger scale compared to the procedure described in ref 8.
Finally, the last step involved a microwave-assisted oxidation of
the 2-methyl group of 9 with selenium dioxide. We have found
that 9 was largely immune to the action of SeO2 in common
solvents such as ethanol, dioxane, or a mixture of dioxane/water
under conventional heating or microwave irradiation. Howewer,
a combination of glacial acetic acid as a solvent and microwave
irradiation made the oxidation by SeO2 successful and afforded
the desired product 3b in 55% yield and a short time (20 min).
This approach has been found very useful and applicable also to
other 2-methyl-substituted benzothiazoles. Nevertheless, the
methyl oxidation of the analogous 6-(N,N-dimethylamino)-2-
methylbenzothiazole12 by SeO2 proceeded only in nonpolar
dioxane with a somewhat lower yield (25%).
2-Bromobenzothiazole-6-carbaldehyde (4b), which can be viewed
as a counterpart of 3b, was synthesized as follows (cf., Scheme 2):
4-Toluidine was reacted with thiocyanogen, generated in situ from
NH4SCN and Br2 in acetic acid, to give 2-amino-substituted
benzothiazole 10 in 80% yield. This was then smoothly transformed
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Figure 4. Two-photon absorption cross-sections δTPA (in GM units) of
selected chromophores in toluene.
Figure 3. Frontiers orbitals of D2 and D5.
to 2-bromo-6-methylbenzothiazole (11) by means of a Sandmeyer
reaction with tert-butyl nitrite and copper(II) bromide. In this case,
we chose bromine instead of iodine because of generally higher yields
of a Sandmeyer-type reaction with the former halogen and also were
aware of the increased reactivity of the C-2 position in cross-coupling
reactions. Microwave-assisted oxidation of 11 by SeO2 (vide supra)
afforded the expected carbaldehyde in very low yield (<10%), and
therefore alternative approaches were explored. Among them, light-
catalyzed bromination27 of 11 using N-bromosuccinimide (NBS)
and a subsequent hydrolysis of the geminal dibromo intermediate 12
was revealed as the most efficient method, providing 4b in an overall
good yield (50%).
2. One-Photon Absorption and Emission Properties. The
one-photon absorption and emission characteristics of all target
chromophores measured in toluene are summarized in Table 1.
Corresponding UVꢀvis spectra of the three-branched tripheny-
lamines, providing the first insight into the electronic structure,
are shown in Figure 2a.
to the corresponding linear counterparts (D1, D2, D4 and D5),
indicating some degree of conjugation between the dipolar arms.
Interestingly, octupolar systems containing the DCV group (T3,
T6) behave in an opposite way and exhibit a blue-shift with
respect to their dipolar congeners (D3, D6).
Comparing extinction coefficients εmax, stronger absorption is
generally observed for compounds with the commonly used
polarity (with εmax up to 115 400 Mꢀ1 cmꢀ1 for T4). This
3
correlates well with their larger calculated oscillator strengths,
fosc, and can be qualitatively understood by inspecting the
HOMO and LUMO orbitals in Figure 3. While no obvious
difference between the HOMOs of the D2 and D5 can be
noticed, in the case of LUMOs, there is a larger contribution from
the atomic pz orbitals of the triphenylamine moiety in the
mismatched-type compound D5. This results in a better overlap
of the frontier orbitals and thus larger oscillator strengths of this
series.
All chromophores under study display moderate to excellent
emission in toluene solution with fluorescence quantum yields
ranging from 0.07 (D3) to 1.00 (D5, T5). The quantum yield
generally increases with the dimensionality of the molecule, and
it reaches higher values for the compounds with commonly used
polarity. The fluorescence spectra exhibit a single peak (cf.,
Figure 2b), suggesting that the emission occurs from the lowest
excited state. The emission maxima are spread over a wide range of
wavelengths (428ꢀ617 nm), from the blue (D1, D4, T1, T4) to
the orange-red spectral region (D3, T3). Similarly to absorption,
the emission spectra show systematic bathochromic shifts with the
EWG strength increase, that is H < CHO < CHdC(CN)2.
In contrast, the fluorescence Stokes shift, Δν~, displays a
nonmonotonous behavior, particularly within the series of dyes
with reverse polarity (D1ꢀD3, T1ꢀT3). Furthermore, the
compounds from this series exhibit Stokes shifts larger than
those observed for their counterparts with the commonly used
polarity (D4ꢀD6, T4ꢀT6). That is presumably because the
ground-state and the first excited-state structures are more
similar in the compounds with mismatched setup (commonly
used polarity). This would corroborate the larger change of the
dipole moment Δμ01 upon photoexcitation for the reverse
polarity and will be the subject of our further investigations.
3. Two-Photon Absorption Properties. Two-photon ab-
sorption cross-sections (δTPA) were measured by the two-
photon-induced fluorescence technique using femtosecond (fs)
laser pulses. Due to the laser availability (Ti:sapphire laser), the
In general, all the compounds show an intense πꢀπ* absorp-
tion band in the visible region, which is associated with the
intramolecular-charge transfer (ICT) from the electron-donating
triphenylamine moiety to the electron-withdrawing benzothia-
zole and adjacent EWG, if present (cf., Figure 3 for HOMO and
LUMO orbitals of selected chromophores).
The maximum of this ICT band is positioned at 365 to
494 nm, depending on the substituent from the violet (R = H)
to the blue-green (R = DCV) spectral region. A systematic red-
shift in the λabs along the series H < CHO < CHdC(CN)2 is
consistent with the trend of the increasing electron-withdrawing
ability and can be primarily explained by the significant stabiliza-
tion of the LUMO leading to a smaller HOMOꢀLUMO
energy gap (cf., Supporting Information, Table S3). Comparing
the two regioisomeric series with matched and mismatched setup
(D1ꢀD3 vs D4ꢀD6 and T1ꢀT3 vs T4ꢀT6, respectively), it is
evident that the λabs increases along the series R = H, CHO, DCV
more dramatically for the compounds with reverse polarity
(D1ꢀD3, T1ꢀT3). In the case of D6 and T6, the excitation
energy of the lowest energy band markedly exceeds that of the
analogous DCV-substituted compounds with reverse polarity
(D3, T3) by ca. 0.2 eV, that is in good agreement with our
time-dependent DFT calculations (cf., Supporting Information,
Table S2).
Furthermore, the three-branched compounds with R = H or
CHO (T1, T2, T4, and T5) are slightly red-shifted as compared
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solutions were excited in the range of 740 to 850 nm. In all cases,
the output intensity of two-photon excited fluorescence was
linearly dependent on the square of the input laser intensity,
thereby confirming the TPA process.
absorption by introducing an auxiliary acceptor to the pendant
benzothiazole side arm and by changing the relative positions of
the donor and acceptor substituents on the heteroaromatic ring.
A change from the commonly used polarity in pushꢀpull benzo-
thiazoles to a reverse one has been revealed as a particularly useful
strategy (regioisomeric control) for enhancing the TPA cross-
sections and shifting the absorption and emission maxima to
longerwavelengths. The values δTPA/MW determined for D2, T2,
All compounds, except the dipolar ones without an additional
EWG substituent (D1 and D4), show moderate to excellent TPA
cross-sections in the range of 256ꢀ2275 GM (cf., Table 1,
Figure 4). Table 1 shows that the λTPA/2 values of compounds
where R = H or CHO are located at nearly the same wavelengths
as the one-photon absorption maxima. This is indeed in accord
with the selection rules that the two-photon allowed states of the
dipolar and octupolar molecules are similar to those of the one-
photon allowed states. Closer inspection of the TPA spectra
reveals, however, that λTPA appears slightly blue-shifted with
respect to twice the λabs wavelength, and the difference is even
more pronounced for DCV derivatives. This might indicate that
the transitions to higher-energy excited states, allowed by vibronic
coupling, are also contributing to the signal.28
In general, the three-branched triphenylamines display larger
TPA cross-sections than their dipolar counterparts, with the
highest δTPA values for T2 (1607 GM) and T6 (2275 GM),
located at 810 and 800 nm, respectively. These values are higher
than those of the most triphenylamine-derived molecules of a
similar molecular weight. Furthermore, both compounds exhibit a
relatively high fluorescence quantum yield (>0.60) and could be
thus promising candidates for two-photon imaging.
When the structures without an additional EWG are com-
pared, it is obvious that the δTPA value increased by approximately 1
order of magnitude as the dimensionality was increased from
mono- to three-branched structures (cf., D1 vs T1 and D4 vs T4,
respectively). The TPA cross-section of the fluorophore T1,
possessing a matched setup, is somewhat larger than that of T4.
This already demonstrates the superiority of the matched setup
(reverse polarity) in terms of TPA cross-section, although the
product ΦfδTPA is higher for T4. The difference between these
regioisomeric series is further increased by CHO substitution, which
dramatically boosts the δTPA value of the dipolar systems by ca. 20-
fold for compounds with reverse polarity (D1 vs D2) and by ca. 10-
fold for compounds with commonly used polarity (D4 vs D5). In
the case of three-branched systems, this increase is also more
pronounced for compounds with the reverse polarity (ca. 3-fold,
T1 vs T2), while only a small TPA enhancement (ca. 1.3-fold) is
observed when passing from T4 to T5. Further increase of the EWG
strength from CHO to DCV leads to a dramatic TPA enhancement
for the compounds with commonly used polarity (cf., D6 vs D5 and
T6 vs T5). In contrast, a remarkable drop of the TPA activity can be
noticed when comparing the couples with reverse polarity D3/D2
and T3/T2, respectively. The striking behavior can be, however,
rationalized as follows: While most of the target chromophores have
their TPA maximum associated with the first excited state in or close
to the measurement range (740ꢀ850 nm), the maxima of the DCV
derivatives D3 and T3 are anticipated, on the basis of their λabs, at
longer wavelengths (>900 nm), which are not covered in the
scanned optical window. This unfortunately prevents us from direct
comparison of their TPA cross-sections with those of their analo-
gues D6 and T6, and it would be of interest to test these dyes also at
higher excitation wavelengths in future studies.
and T6 (ca. 2.0ꢀ2.4 GM gꢀ1.mol) are comparable to those of the
3
most efficient TPA materials reported to date and can be easily
improved further by planarization of the triphenylamine core via
dialkylmethylene connectors or increasing the strength of the
EWG. High TPA cross-sections combined with the easy synthetic
access of these structures and with their high quantum yields make
these compounds worth considering for various TPA applications.
Furthermore, large hyperpolarizabilities β0 predicted for the dipo-
lar compounds with an additional EWG substituent (cf., Sup-
porting Information, Tables S1 and S2) make these systems
attractive simultaneously for application in TPEF as well as in
second harmonic generation (SHG) imaging microscopy.
’ EXPERIMENTAL SECTION
1. Synthetic Procedures. Tris(4-iodophenyl)amine. To a solu-
tion of triphenylamine (3.0 g, 12 mmol) in ethanol (150 mL) were added
HgO (12.2 g, 56 mmol) and finely powdered iodine (15.3 g, 60 mmol). The
reaction mixture was stirred at room temperature for 24 h, and then the
solvent was removed on a rotary evaporator. The orange solid residue was
mixed with hot toluene, refluxed for 10 min, and consequently hot-filtered
through a short column of Al2O3. Then, methanol was added to the hot
toluene solution until crystals were formed. The solution was left for 12 h,
and the crystals were collected by filtration to give the title product in 77%
yield (5.8 g). Mp 180ꢀ182 ꢀC(lit.29 mp 182ꢀ184 ꢀC); 1H NMR (300 MHz,
CDCl3) δ 7.54 (d, J = 9.0 Hz, 6H, Har), 6.81 (d, J = 8.8 Hz, 6H, Har).
Tris(4-ethynylphenyl)amine (2). To a mixture of (4-iodophenyl)amine
(0.58 g, 0.93 mmol), Pd(PPh3)2Cl2 (98 mg, 0.14 mmol), and copper iodide
(13 mg, 0.07 mmol) was added THF (8 mL) under nitrogen, and the
suspension was stirred at room temperature for 20 min. Consequently,
triethylamine (5 mL, freshly distilled from CaH2) was added, and the
reaction mixture was stirred for an additional 30 min at room temperature.
After this time, trimethylsilylacetylene (0.53 mL, 3.72 mmol) was added
dropwise via syringe, and the reaction mixture turned black within 5 min.
The mixture was stirred at room temperature for 36 h, diluted with ether
(10 mL), and forced through a short pad of silica. The solid part was washed
with ether, the solvent from the combined organic solutions was evaporated,
and the obtained crude intermediate was dissolved in CH2Cl2 (10mL). The
solution was cooled to 0 ꢀC, and TBAF (2.8 mL of 1 M solution in THF, 2.8
mmol) was added via syringe. The reaction mixture was stirred at 0ꢀ5 ꢀC
for 2.5 h and consequently absorbed onto silica. The title compound was
obtained by column chromatography on silica gel (eluent hexanes/CHCl3
= 3:1) in 62% yield (0.18 g). Rf 0.25 (SiO2, hexanes/CHCl3 = 3:1); mp
105.5ꢀ107 ꢀC; 1H NMR (300 MHz, CDCl3) δ 7.38 (d, J = 8.6 Hz, 6H,
Har), 7.01 (d, J = 8.6 Hz, 6H, Har), 3.06 (s, 3H, CtCH).
6-Nitrobenzothiazole (5). Benzothiazole (18.2 mL, 0.17 mol) was
dissolved in concentrated H2SO4 (46 mL). The solution was cooled to
0 ꢀC, and 100% HNO3 (18.2 mL) was added dropwise at such a rate to
keep the temperature below 5 ꢀC throughout the addition. After the
addition was completed, the reaction mixture was stirred under cooling
for 15 min and then at room temperature for an additional 2 h.
Subsequently, the mixture was poured into ice-cold water (400 mL).
The precipitate was collected by filtration, washed with cold water until
the pH was neutral, and crystallized from ethanol to give the title
compound 5 as a white solid in 68% yield (20.8 g). Mp 176ꢀ177 ꢀC
’ CONCLUSIONS
In summary, we demonstrated here a facile route to triphe-
nylamine-based chromophores with enhanced two-photon
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ARTICLE
(lit.30 mp 177 ꢀC); 1H NMR (300 MHz, CDCl3) δ 9.28 (s, 1H, H-2),
8.93 (d, J = 2.2 Hz, 1H, H-7), 8.42 (dd, J = 9.3, 2.2 Hz, 1H, H-5), 8.26
(d, J = 9.3 Hz, 1H, H-4).
Method B. A suspension of N-(4-iodophenyl)acetamide (7) (1.0 g,
3.8 mmol), P4S10 (1.69 g, 3.8 mmol), Al2O3 (200 mg), and elementary
sulfur (10 mg) in dichloromethane (35 mL) was refluxed under
intensive stirring for 10 h. Consequently, the hot solution was poured
into water (200 mL), the organic layer was separated and the aqueous
layer was extracted with CH2Cl2 (3 ꢁ 50 mL). The combined organic
layers were washed with hot water (3 ꢁ 50 mL) and brine (50 mL) and
then dried over Na2SO4, and solvents were removed under vacuum. The
crude product was purified by crystallization from 60% methanol
(40 mL) to give the title compound in 80% yield (0.84 g). Mp
6-Aminobenzothiazole (6). To a mixture of 6-nitrobenzothiazole (5)
(4.8 g, 26.6 mmol) and concentrated HCl (3.8 mL) in 80% ethanol
(150 mL) was added powdered iron (6.0 g, 107 mmol). The reaction
mixture was sonicated for 40 min and then cooled down to room
temperature. Precipitated iron oxides were removed by filtration
through a short pad of silica gel and washed with ethanol (2 ꢁ 30 mL).
The solvent was removed, and the solid residue was extracted into a
heterogeneous mixture of EtOAc and 10% aq solution of Na2CO3. The
organic layer was dried (Na2SO4), and the solvent was removed under
vacuum. The crude product was purified by short-column chromatog-
raphy on silica gel (eluent hexanes/EtOAc = 2:1) to yield 6 (3.5 g, 88%)
as a white-gray solid. Rf 0.11 (SiO2, hexanes/EtOAc = 1:1); mp
1
148ꢀ149 ꢀC (lit.34 mp 147ꢀ149 ꢀC); H NMR (300 MHz, DMSO)
δ 11.62 (s, 1H, NH), 7.75 (d, J = 8.8 Hz, 2H, Har), 7.65 (d, J = 8.8 Hz,
2H, Har), 2.59 (s, 3H, CH3).
6-Iodo-2-methylbenzothiazole (9). N-(4-Iodophenyl)thioacetamide
(8) (4.0 g, 14.4 mmol) was dissolved in a 30% aqueous solution of
NaOH (20 mL) under vigorous stirring and cooling in an ice bath. The
resulting solution was diluted with iceꢀwater (40 mL) and then added
dropwise to an aqueous solution of K3[Fe(CN)6] (19.0 g, 58 mmol)
under intensive stirring and cooling during 4 h (note: temperature of the
added solution has to be maintained below 15 ꢀC throughout the addition).
After the addition was completed, the reaction mixture was stirred at
room temperature for 30 min. The precipitate was collected by filtration,
washed with water, and dried. The crude product was purified by column
chromatography on silica gel, eluting with dichloromethane to yield 6-iodo-
2-methylbenzothiazole (9) as a white solid (2.18 g, 55%). Rf 0.53 (SiO2,
CH2Cl2); mp 140ꢀ141 ꢀC (lit.35 mp 140ꢀ141 ꢀC); 1H NMR (300 MHz,
CDCl3) δ 8.16 (d, J = 1.6 Hz, 1H, H-7), 7.73 (dd, J = 8.4, 1.6 Hz, 1H, H-5),
7.67 (d, J = 8.4 Hz, 1H, H-4), 2.82 (s, 3H, CH3); 13C NMR (75 MHz,
CDCl3) δ 162.3, 147.6, 132.6, 129.8, 124.6, 118.7, 83.7, 14.8.
1
86ꢀ87 ꢀC (lit.12 mp 86ꢀ87 ꢀC); H NMR (300 MHz, CDCl3) δ
8.70 (s, 1H, H-2), 7.89 (d, 1H, J = 8.9 Hz, H-4), 7.16 (d, 1H, J = 2.3 Hz,
H-7), 6.87 (dd, 1H, J = 8.9, 2.3 Hz, H-5), 3.86 (bs, 2H, NH2).
6-Iodobenzothiazole (3a). A mixture of concentrated H2SO4
(20 mL) and NaNO2 (1.3 g, 18.5 mmol) was prepared at 70 ꢀC. After
cooling to 40 ꢀC, a solution of 6-aminobenzothiazole (6) (2.5 g, 16.6
mmol) in acetic acid (35 mL) was added dropwise. The reaction mixture
was stirred at room temperature for 30 min, and after this time the
formed diazonium salt was added to the stirred solution of KI (3.32 g, 20
mmol) in water (35 mL) and heated to 70 ꢀC. The reaction mixture was
stirred at 70 ꢀC for 30 min and then poured onto ice. The precipitate was
collected by filtration, washed with water, and dissolved in CHCl3
(50 mL). The organic solution was washed with 10% aq solution of
Na2S2O3 (2 ꢁ 30 mL), dried over Na2SO4, and the solvent was
evaporated. The crude product was crystallized from cyclohexane (20 mL)
or purified by column chromatography on silica gel (eluent hexanes/
EtOAc = 7:1) to yield 3a as a white solid (2.33 g, 55%). Rf 0.55 (SiO2,
6-Iodobenzothiazole-2-carbaldehyde (3b). A suspension of 6-iodo-
2-methylbenzothiazole (9) (1.5 g, 5.45 mmol) and SeO2 (1.8 g, 16
mmol) in glacial acetic acid (15 mL) was heated under microwave
irradiation (150 ꢀC) for 20 min, and then the hot suspension was filtered
through short column of silica. The solid part absorbed on silica was
washed with boiling hot water, and the filtrate was diluted with cold
water until a precipitate was formed. The precipitate was collected by
filtration, washed with cold water, and dried. The solid part absorbed on
silica was washed with a 30% aq solution of Na2CO3 (100 mL) and then
with hot chloroform (100 mL). The chloroform layer was separated and
dried over Na2SO4, and the solvent was evaporated. The solid residue
was combined with the precipitate and purified by column chromatog-
raphy on silica gel, eluting with dichloromethane, to yield the title
compound 3b (0.87 g, 55%) as a pale yellow solid. Rf 0.52 (SiO2,
1
hexanes/EtOAc = 4:1); mp 78ꢀ80 ꢀC (lit.31 mp 79ꢀ80ꢀ); H NMR
(300 MHz, CDCl3) δ 8.93 (s, 1H, H-2), 8.32 (d, J = 1.1 Hz, 1H, H-7),
7.89 (d, J = 8.8 Hz, 1H, H-4), 7.81 (dd, J = 8.8, 1.1 Hz, 1H, H-5).
4-Iodoaniline. To a solution of sodium bicarbonate (4.0 g, 48 mmol)
in water (350 mL) was added aniline (2.94 mL, 32 mmol). The reaction
mixture was cooled in an ice water bath, and finely powdered iodine
(6.1 g, 24 mmol) was added in small portions at 3-min intervals under
intensive stirring (temperature was maintained at 12ꢀ15 ꢀC). After the
addition was finished, the reaction mixture was stirred at room tem-
perature for 40 min, and the crude product was collected by filtration and
crystallized from cyclohexane (20 mL) to yield 4-iodoaniline (6.4 g,
91%) as white-gray crystals. Mp 62ꢀ63 ꢀC (lit.32 mp 62ꢀ63 ꢀC); 1H
NMR (300 MHz, CDCl3) δ 7.41 (d, J = 9.0 Hz, 2H, Har), 6.48 (d, J = 9.0
Hz, 2H, Har), 3.67 (bs, 2H, NH2).
1
CH2Cl2); mp 133ꢀ134 ꢀC; H NMR (300 MHz, CDCl3) δ 10.16
(s, 1H, CHO), 8.39 (d, J = 1.6 Hz, 1H, H-7), 7.97 (d, J = 8.8 Hz, 1H, H-4),
7.90 (dd, J = 8.8, 1.6 Hz, 1H, H-5); 13C NMR (75 MHz, CDCl3) δ 185.1,
165.5, 152.8, 138.2, 136.7, 131.3, 127.0, 94.1. Anal. Calcd for C8H4I-
NOS: C 33.24, H 1.39, N 4.85. Found: C 33.41, H 1.22, N 4.99.
2-Iodobenzothiazole (4a). Acetyl chloride (7.2 mL, 0.1 mol) was
added dropwise to the cooled and stirred 2-chlorobenzothiazole (17 g,
0.1 mol). After the mixture was stirred for 10 min, a solution of dry NaI
(30.0 g, 0.2 mol) in acetonitrile (210 mL, freshly distilled from P2O5)
was added. After a yellow precipitate was formed, the suspension was
stirred for an additional 5 min, and consequently the precipitate was
collected by filtration. The precipitate was dissolved in chloroform
(150 mL), and the resulting solution was washed with 15% aq solution of
K2CO3 (100 mL) and then with a 20% aq solution of Na2S2O5 until the
organic layer became colorless. The organic layer was separated and
dried over Na2SO4, and chloroform was evaporated. The crude product
was purified by crystallization from 80% ethanol to yield the title
compound 4a (15.1 g, 58%) as a white solid. Mp 77ꢀ78 ꢀC (lit.36 mp
78ꢀ82 ꢀC); 1H NMR (300 MHz, CDCl3) δ 8.03 (dd, J = 7.3, 1.5 Hz,
1H), 7.85 (dd, J = 7.3, 1.5 Hz, 1H), 7.44 (ddd, J = 7.3, 7.3, 1.5 Hz, 1H),
7.39 (ddd, J = 7.3, 7.3, 1.5 Hz, 1H).
N-(4-Iodophenyl)acetamide (7). To a solution of 4-iodoaniline
(7.8 g, 36 mmol) in THF (20 mL) was added acetic anhydride
(72 mmol, 6.8 mL) and 3 drops of pyridine. The reaction mixture was
stirred and heated spontaneously. After cooling to room temperature,
the precipitate was collected by filtration, washed with ice-cold THF and
dried. The title compound was obtained in 95% yield (8.9 g). Mp
182ꢀ184 ꢀC (lit.33 mp 180ꢀ182 ꢀC); 1H NMR (300 MHz, DMSO) δ
10.02 (s, 1H, NH), 7.62 (d, J = 8.8 Hz, 2H, Har), 7.42 (d, J = 8.8 Hz, 2H,
Har), 2.03 (s, 3H, CH3).
N-(4-Iodophenyl)thioacetamide (8). Method A. AsuspensionofN-(4-
iodophenyl)acetamide (7) (1.0g, 3.8mmol) andLawesson’s reagent (1.1 g,
2.7 mmol) in THF (15 mL) was heated under microwave irradiation
(105ꢀC) for 15 min. The reaction mixture was poured into water (200 mL),
and the aqueous layer was extracted with ether (3 ꢁ 50mL). Thecombined
organic layers were washed with hot water (3 ꢁ 50 mL) and brine (50 mL)
and dried over Na2SO4, and solvents were removed under vacuum. The
crude product was purified by crystallization from 60% methanol (40 mL)
to give the title compound in 70% yield (0.74 g).
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ARTICLE
2-Amino-6-methylbenzothiazole (10). A mixture of 4-methylaniline
(5 g, 47 mmol) and NH4SCN (17.8 g, 234 mmol) in glacial acetic acid
(50 mL) was cooled and stirred. To this solution was dropwise added
bromine (2.6 mL, 51 mmol) dissolved in acetic acid (5 mL) at such a rate
to keep the temperature below 10 ꢀC throughout the addition (ca. 2ꢀ3 h).
Stirring at 10ꢀ15 ꢀC was continued for an additional 3 h, and then the
reaction mixture was poured into cold water and neutralized with a 25%
aqueous solution of ammonia. The solid precipitate was collected by
filtration and washed with water. The crude product was purified by
crystallization from benzene, respectively, by column chromatography
on silica gel (eluent hexanes/EtOAc = 3:1), affording 10 in 80% yield
(6.13 g). Rf 0.25 (SiO2, hexanes/EtOAc = 1:1); mp 135ꢀ137 ꢀC (lit.37
135ꢀ137 ꢀC); 1H NMR (300 MHz, CDCl3) δ 7.43 (d, J = 8.4 Hz, 1H,
H-4), 7.40 (d, J = 1.2 Hz, 1H, H-7), 7.12 (dd, J = 8.4, 1.2 Hz, 1H, H-5),
5.26 (bs, 2H, NH2), 2.40 (s, 3H, CH3).
2-Bromo-6-methylbenzothiazole (11). To a suspension of copper-
(II) bromide (6.52 g, 29 mmol) in acetonitrile (60 mL) was added tert-
butyl nitrite (4.30 mL, 36 mmol). The resulting mixture was stirred at
55 ꢀC for 10 min, and then the solid 2-amino-6-methylbenzothiazole (10)
(4 g, 24 mmol) was added in small portions. The reaction mixture was
stirred at 60 ꢀC for 1 h. After cooling to room temperature, the reaction
mixture was poured into ethyl acetate (100 mL) and washed with 1 M
solution of HCl and saturated brine. The organic solution was dried over
Na2SO4, solvents were evaporated, and the crude product was purified by
column chromatography on silica gel (eluent hexanes/EtOAc = 10:1) to
yield 11 as a white solid (2.58 g, 47%). Rf 0.33 (SiO2, hexanes/EtOAc =
10:1); mp 54ꢀ56 ꢀC; 1H NMR (300 MHz, CDCl3) δ 7.86 (d, J = 8.4
Hz, 1H, H-4), 7.27 (d, J = 1.5 Hz, 1H, H-7), 7.12 (dd, J = 8.4, 1.5 Hz, 1H,
H-5), 2.47 (s, 3H, CH3).
2-Bromo-6-(dibromomethyl)benzothiazole (12). A suspension of 2-bro-
mo-6-methylbenzothiazole (11) (1.38 g, 5.92 mmol) and N-bromosuccini-
mide (2.10 g, 11.84 mmol) in carbon tetrachloride (130 mL) was irradiated
with a tungsten lamp (125 W) for 4 h. After cooling to room temperature, the
solid part was filtered off and washed with CCl4. The organic solutions were
combined, and the solvent was removed under reduced pressure. The crude
product was purified by column chromatography on silica gel (eluent hexanes/
EtOAc = 10:1) to yield 12 as a white solid (1.58 g, 69%). Rf 0.27 (SiO2,
hexanes/EtOAc = 10:1); mp 125ꢀ126 ꢀC; 1H NMR (300 MHz, CDCl3) δ
8.03 (d, J = 1.8 Hz, 1H, H-7), 7.98 (d, J = 8.4 Hz, 1H, H-4), 7.70 (dd, J = 8.4,
1.8 Hz, 1H, H-5), 6.75 (s, 1H, CHBr2).
2-Bromobenzothiazole-6-carbaldehyde (4b). To a refluxed suspen-
sion of 2-bromo-6-(dibromomethyl)benzothiazole (12) (1.0 g, 2.58
mmol) in ethanol (15 mL) was added a solution of AgNO3 (4.39 g 25.8
mmol) in water (15 mL) dropwise over 20 min. After the addition was
completed, the reaction mixture was stirred at reflux for an additional 1 h,
cooled to room temperature, and then poured into water (150 mL). The
formed precipitate was filtered off and washed with chloroform (80 mL),
and the filtrate was extracted with chloroform (3 ꢁ 20 mL). Combined
organic layers were dried over Na2SO4, and the solvent was evaporated.
The crude product was purified by column chromatography on silica gel
(eluent hexanes/EtOAc = 5:1) to yield 4b as a white solid (0.43 g, 69%).
Rf 0.48 (SiO2, hexanes/EtOAc = 5:1); mp 148ꢀ150 ꢀC; 1H NMR (300
MHz, CDCl3) δ 10.13 (s, 1H, CHO) 8.37 (d, J = 1.5 Hz, 1H, H-7), 8.13
(d, J = 8.4 Hz, 1H, H-4), 8.00 (dd, J = 8.4, 1.5 Hz, 1H, H-5); 13C NMR
(75 MHz, CDCl3) δ 190.8, 156.0, 143.6, 138.0, 133.6, 127.6, 123.4,
123.3. Anal. Calcd for C8H4BrNOS: C 39.69, H 1.67, N 5.79, S 13.24.
Found: C 39.74, H 1.65, N 5.70.
mixture, and stirring was continued for an additional 30ꢀ90 min.
Afterward, corresponding acetylene 1 (0.65 g, 2.4 mmol) or 2 (0.21 g,
0.66 mmol) dissolved in THF (3ꢀ8 mL) was added dropwise (over 30
min) with intensive stirring under an atmosphere of nitrogen (note: a
large amount of white precipitate appeared after the addition of
acetylene 2). After being stirred for 24ꢀ48 h at room temperature under
nitrogen, the reaction mixture was diluted with THF (10ꢀ15 mL). The
precipitate was filtered and washed with CH2Cl2 and consequently with
the mixture CH2Cl2/MeOH (20:1). The solvent from the combined
organic solutions was evaporated under vacuum, and the crude product
was purified by column chromatography on silica gel.
6-[(4-Diphenylaminophenyl)ethyn-1-yl]benzothiazole (D1). Eluent
CHCl3/hexanes = 2:1; yield 77% (0.62 g); Rf 0.43 (SiO2, CHCl3/
hexanes = 2:1); mp 150ꢀ152 ꢀC; 1H NMR (300 MHz, CDCl3) δ 9.02
(s, 1H, H-2), 8.11 (dd, J = 1.5 Hz, 0.5 Hz, 1H, H-7), 8.08 (dd, J = 8.5 Hz,
0.5 Hz, 1H, H-4), 7.64 (dd, J = 8.5, 1.5 Hz, 1H, H-5), 7.39 (d, J = 8.8 Hz,
2H, Har), 7.31ꢀ7.27 (m, 4H, Har), 7.14ꢀ7.04 (m, 6H, Har), 7.02 (d, J =
8.8 Hz, 2H, Har); 13C NMR (75 MHz, CDCl3) δ 152.6, 148.1, 147.1,
133.9, 132.6, 129.6, 129.4, 125.0, 124.8, 123.6, 123.4, 122.2, 121.2, 115.6,
90.7, 88.1 (the number of 13C NMR peaks is reduced due to signal
overlapping). Anal. Calcd for C27H18N2S: C 80.57, H 4.51, N 6.96.
Found: C 80.50, H 4.43, N 6.90.
6-[(4-Diphenylaminophenyl)ethyn-1-yl]benzothiazole-2-carbalde-
hyde (D2). Eluent CHCl3/MeOH = 30:1; yield 64% (0.55 g); Rf 0.73
(SiO2, CHCl3/MeOH = 30: 1); mp 171ꢀ172 ꢀC; 1H NMR (300 MHz,
CDCl3) δ 10.16 (s, 1H, CHO), 8.18 (d, J = 8.5 Hz, 1H, H-4), 8.13 (d, J =
1.6 Hz, 1H, H-7), 7.71 (dd, J = 8.5, 1.6 Hz, 1H, H-5), 7.39 (d, J = 8.8 Hz,
2H, Har), 7.32ꢀ7.27 (m, 4H, Har), 7.14ꢀ7.06 (m, 6H, Har), 7.02 (d, J =
8.8 Hz, 2H, Har); 13C NMR (75 MHz, CDCl3) δ 185.2, 165.7, 152.7,
148.5, 147.0, 136.6, 132.7, 130.7, 129.5, 125.5, 125.23, 125.20, 124.3,
123.8, 121.9, 114.9, 93.0, 87.9. Anal. Calcd for C28H18N2OS: C 78.11, H
4.21, N 6.51. Found: C 78.28, H 4.02, N 6.68.
2-[(4-Diphenylaminophenyl)ethyn-1-yl]benzothiazole (D4). Eluent
CHCl3/hexanes = 3:1; yield 77% (0.62 g); Rf 0.48 (SiO2, CHCl3/
hexanes = 3:1); mp 199ꢀ201 ꢀC; 1H NMR (300 MHz, CDCl3) δ 8.05
(d, J = 7.9 Hz, 1H, HBTZ), 7.86 (d, J = 7.9 Hz, 1H, HBTZ), 7.54ꢀ7.40 (m,
4H, 2HBTZ, 2Har), 7.34ꢀ7.28 (m, 4H, Har), 7.15ꢀ7.09 (m, 6H, Har),
7.00 (d, J = 8.7 Hz, 2H, Har); 13C NMR (75 MHz, CDCl3) δ 153.1,
149.4, 149.1, 146.7, 135.3, 133.3, 129.5, 126.6, 125.9, 125.6, 124.2, 123.4,
121.2, 121.1, 112.7, 97.1, 82.3. Anal. Calcd for C27H18N2S: C 80.57, H
4.51, N 6.96. Found: C 80.66, H 4.39, N 7.09.
2-[(4-Diphenylaminophenyl)ethyn-1-yl]benzothiazole-6-carbalde-
hyde (D5). Eluent hexanes/EtOAc = 5:1; yield 71% (0.61 g); Rf 0.44
(SiO2, hexanes/EtOAc = 3: 1); mp 165ꢀ167 ꢀC; 1H NMR (300 MHz,
CDCl3) δ 10.12 (s, 1H, CHO), 8.4 (d, J = 1.5 Hz, 1H, H-7), 8.16 (d, J =
8.5 Hz, 1H, H-4), 8.02 (dd, J = 8.5, 1.5 Hz, 1H, H-5), 7.47 (d, J = 8.9 Hz,
2H, Har), 7.35ꢀ7.30 (m, 4H, Har), 7.17ꢀ7.11 (m, 6H, Har), 7.00 (d, J =
8.9 Hz, 2H, Har); 13C NMR (75 MHz, CDCl3) δ 191.0, 157.0, 153.4,
149.9, 146.5, 135.8, 133.7, 133.5, 129.6, 127.4, 125.8, 124.5, 124.1, 123.8,
120.7, 111.9, 99.9, 82.4. Anal. Calcd for C28H18N2OS: C 78.11, H 4.21,
N 6.51. Found: C 78.30, H 4.10, N 6.45.
Tris[4-(benzothiazol-6-ylethynyl)phenyl]amine (T1). Eluent hexanes/
EtOAc = 3:1; yield 47% (0.22 g); Rf 0.32 (SiO2, hexanes/EtOAc = 2:1);
mp 168ꢀ169 ꢀC; 1H NMR (300 MHz, CDCl3) δ 9.03 (s, 3H, H-2), 8.14
(d, J = 1.2 Hz, 3H, H-7), 8.11 (d, J = 8.5 Hz, 3H, H-4), 7.67 (dd, J = 8.5, 1.2
Hz, 3H, H-5), 7.48 (d, J = 8.6 Hz, 6H, Har), 7.11 (d, J = 8.6 Hz, 6H, Har);
13C NMR (75 MHz, CDCl3) δ 152.7, 146.8, 134.0, 132.9, 129.7, 125.0,
124.1, 123.5, 120.9, 117.8, 90.2, 88.8 (the number of 13C NMR peaks is
reduced due to signal overlapping). Anal. Calcd for C45H24N4S3: C 75.39,
H 3.37, N 7.82. Found: C 75.50, H 3.40, N 7.85.
General Procedure for Synthesis of D1ꢀD2, D4ꢀD5, T1ꢀT2, and
T4ꢀT5 (Sonogashira Cross-Coupling Reaction). To a mixture of
halogen-substituted benzothiazole (3a, 3b, 4a, or 4b, 2.0 mmol),
Pd(PPh3)2Cl2 (84 mg, 0.12 mmol), and CuI (8 mg, 0.04 mmol) was
added THF (22 mL) under nitrogen, and the resulting suspension was
stirred at room temperature for 20 min. Consequently, triethylamine
(10 mL, freshly distilled from CaH2) was injected into the reaction
Tris[4-(2-formylbenzothiazol-6-ylethynyl)phenyl]amine (T2). Column
chromatography on a short pad of silica gel, eluent CH2Cl2/MeOH = 20:1;
yield 46% (0.24 g); Rf 0.20 (SiO2, CH2Cl2/MeOH = 20:1); mp >300 ꢀC;
1H NMR (300 MHz, CDCl3) δ 10.17 (s, 3H, CHO), 8.21 (d, J = 8.6 Hz,
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3H, H-4), 8.16 (d, J = 1.6 Hz, 3H, H-7), 7.73 (dd, J = 8.6, 1.6 Hz, 3H, H-5),
7.50 (d, J = 8.8 Hz, 6H, Har), 7.14 (d, J = 8.8 Hz, 6H, Har); 13C NMR (75
MHz, CDCl3) δ 185.4, 166.1, 153.1, 147.2, 136.9, 133.3, 131.0, 125.8,
125.7, 124.4, 124.1, 117.7, 92.4, 88.9. Anal. Calcd for C48H24N4O3S3: C
71.98, H 3.02, N 7.00. Found: C 72.10, H 2.99, N 7.11.
Tris[4-(benzothiazol-2-ylethynyl)phenyl]amine (T4). Eluent hexanes/
EtOAc = 9:1ꢀ4:1; yield 59% (0.28 g); Rf 0.32 (SiO2, MeOH/CH2Cl2 =
1:60); mp 259ꢀ260 ꢀC; 1H NMR (300 MHz, CDCl3) δ 8.08 (d, J = 8.2
Hz, 3H, HBTZ), 7.88(d, J=8.2 Hz, 3H, HBTZ), 7.59(d, J=8.6 Hz, 6H, Har),
7.56ꢀ7.43 (m, 6H, HBTZ), 7.16 (d, J = 8.5 Hz, 6H, Har); 13C NMR (75
MHz, CDCl3) δ 153.0, 148.7, 147.6, 135.4, 133.7, 126.7, 126.2, 124.3,
123.6, 121.3, 116.1, 95.8, 83.1. Anal. Calcd for C45H24N4S3: C 75.39, H
3.37, N 7.82. Found: C 75.45, H 3.31, N 7.79.
Tris[4-(6-formylbenzothiazol-2-ylethynyl)phenyl]amine (T5). Eluent
hexanes/THF = 6:5; yield 42% (0.22 g); Rf 0.57 (SiO2, hexanes/EtOAc
= 2:1); mp >300 ꢀC; 1H NMR (300 MHz, CDCl3) δ 10.14 (s, 3H, CHO),
8.43 (s, 3H, HBTZ), 8.19 (d, J = 8.5 Hz, 3H, HBTZ), 8.05 (d, J = 8.5 Hz, 3H,
HBTZ), 7.62(d,J= 8.6 Hz, 6H, Har), 7.18(d, J= 8.6 Hz, 6H, Har);13CNMR
(75MHz, CDCl3) δ190.9, 156.9, 152.9, 147.9, 135.9, 133.99, 133.96, 127.5,
124.4, 124.12, 124.06, 115.8, 98.1, 83.1. Anal. Calcd for C48H24N4O3S3:
C 71.98, H 3.02, N 7.00. Found: C 72.17, H 3.06, N 7.11.
General Procedure for Synthesis of D3, D6, T3, and T6 (Knoevenagel-
type condensation). To a solution of the dipolar (D2 or D5, 0.23 mmol) or
the three-branched carbaldehyde (T2 or T5, 0.125 mmol) dissolved in
methanol (5 mL) and 2-propanol (12 mL, freshly distilled from CaH2),
respectively, were added malononitrile (1.25 equiv and 3.75 equiv, respectively)
and 3ꢀ4 drops of pyridine. The reaction mixture was heated to reflux for 2
h, poured into water (30 mL), and extracted with CH2Cl2 (4 ꢁ 20 mL).
The combined organic layers were dried over Na2SO4, the solvent was
evaporated, and the crude product was purified by chromatography on a
short column of silica gel or crystallized from ethanol.
2-{{6-[(4-Diphenylaminophenyl)ethyn-1-yl]benzothiazol-2-yl}
methylene}malononitrile (D3). The crude product was crystallized
from ethanol to obtain the target compound as a dark solid in 84% (92
mg) yield. Mp 190ꢀ193 ꢀC; 1H NMR (300 MHz, CDCl3) δ 8.17 (d, J =
8.7 Hz, 1H, H-4), 8.11 (d, J = 1.5 Hz, 1H, H-7), 8.08 (s, 1H, CHdC-
(CN)2), 7.74 (dd, J = 8.7, 1.5 Hz, 1H, H-5), 7.40 (d, J = 8.9 Hz, 2H, Har),
7.33ꢀ7.27 (m, 4H, Har), 7.15ꢀ7.07 (m, 6H, Har), 7.02 (d, J = 8.9 Hz,
2H, Har); 13C NMR (75 MHz, CDCl3) δ 157.4, 152.5, 150.8, 148.9,
147.1, 137.5, 133.0, 131.7, 129.7, 125.5 (2 ꢁ C), 125.3, 124.7, 124.2,
121.9, 114.8, 112.9, 111.9, 94.5, 88.2, 87.4. Anal. Calcd for C31H18N4S:
C 77.80, H 3.79, N 11.71. Found: C 77.99, H 3.62, N 11.70.
2-{{2-[(4-Diphenylaminophenyl)ethyn-1-yl]benzothiazol-6-yl}-
methylene}malononitrile (D6). Eluent hexanes/EtOAc = 5:1; yield
68% (75 mg); Rf 0.42 (SiO2, hexanes/EtOAc = 4:1); mp 201ꢀ203 ꢀC;
1H NMR (300 MHz, CDCl3) δ 8.49 (d, J = 1.8 Hz, 1H, H-7), 8.13 (d, J =
8.6 Hz, 1H, H-4), 7.99 (dd, J = 8.6, 1.8 Hz, 1H, H-5), 7.85 (s, 1H,
CHdC(CN)2), 7.48 (d, J = 8.9 Hz, 2H, Har), 7.35ꢀ7.30 (m, 4H, Har),
7.17ꢀ7.11 (m, 6H, Har), 7.01 (d, J = 8.8 Hz, 2H, Har); 13C NMR (75
MHz, CDCl3) δ 158.7, 156.8, 154.3, 150.1, 146.4, 136.5, 133.6, 129.6,
129.1, 128.2, 125.8, 124.6, 124.2, 124.1, 120.5, 113.8, 112.8, 111.5, 101.3,
82.5, 82.3. Anal. Calcd for C31H18N4S: C 77.80, H 3.79, N 11.71. Found:
C 78.00, H 3.85, N 11.78.
0.38 (SiO2, CH2Cl2/MeOH = 60:1); mp >300 ꢀC; 1H NMR (300 MHz,
CDCl3) δ 8.53 (d, J = 1.5 Hz, 3H, H-7), 8.17 (d, J = 8.7 Hz, 3H, H-4),
8.01 (dd, J = 8.7, 1.5 Hz, 3H, H-5), 7.88 (s, 3H, CHdC(CN)2), 7.63 (d,
J = 8.7 Hz, 6H, Har), 7.19 (d, J = 8.7 Hz, 6H, Har); 13C NMR (75 MHz,
CDCl3) δ 158.6, 156.6, 153.7, 147.9, 136.6, 134.1, 129.2, 128.5, 124.4,
124.1, 115.7, 113.6, 112.7, 99.2,83.1, 82.8 (the number of13C NMRpeaks
is reduced due to signal overlapping). Anal. Calcd for C57H24N10S3: C
72.44, H 2.56, N 14.82. Found: C 72.64, H 2.41, N 14.71.
2. Two-Photon Absorption Measurements. The two-photon
absorption cross-sections, δTPA, of the dyes were determined via a two-
photon excited fluorescence (TPEF) technique using femtosecond
pulsed excitation.8,38 Femtosecond pulses are ideal for this study due
to high peak power, capable of inducing nonlinear optical phenomena,
combined with low pulse energy. Additionally, the TPEF method is
advantageous over other two-photon absorption techniques because the
order of the nonlinear absorption can be monitored, avoiding the
contribution from excited state absorption or other absorption mechan-
isms and saturation. A mode-locked Ti:sapphire laser emitting 80 fs
pulses tunable from 740 to 850 nm was used as the excitation source.
The laser beam was shaped to have a nearly top-hat spatial distribution
using a 5-fold beam expander in order to uniformly illuminate a 0.32 NA
objective lens used for the excitation of the samples. The TPA-induced
fluorescence was collected backward by the same lens and was separated
from the excitation beam by using a dichroic mirror and a series of short-
pass filters. Finally, it was detected by a photomultiplier connected
to photon-counting electronics. The TPA fluorescence intensity was
measured as a function of the excitation power every 10 nm within the
wavelength range of the excitation laser. In all cases, the calculations of
δTPA were made within the power regime, where the fluorescence was
proportional to the square of the excitation power, in order to ensure
that the observed fluorescence was only due to two-photon absorption.
The samples were measured as 1 ꢁ 10ꢀ5 M solutions of the dyes in
toluene. In a reference experiment, the scattered light from a cuvette with
toluene was measured, through the same setup, as a function of excitation
power. This signal was consequently subtracted from the TPA fluorescence
signal of the dyes. The values of δTPA were determined using rhodamine B
(1 ꢁ 10ꢀ4 M in methanol) as a reference, which has a well-known TPA
spectrum. For the calculation of δTPA, it was assumed that the fluorescence
quantum yield is the same under two-photon or one-photon excitation
following the analysis of Xu et al.39
3. Computational Details. The ground-state structures of
pushꢀpull triphenylamines containing a benzothiazole subunit were
fully optimized at the DFT level employing the B3LYP40 functional and
TZVP41 basis set with the Turbomole program.42 The time-dependent
DFT method with the 6-31G(d) basis set and the Coulomb-attenuated
CAM-B3LYP43 exchange-correlation functional, as implemented in the
Gaussian 09 program package,44 was applied to get excitation energies
(Emax), transition dipole moments (μ01), oscillator strengths (fosc), and
adiabatic dipole moment changes (Δμ01) between the ground state and
the first excited state. Solvent effects were simulated by employing a
polarizable continuum model (PCM).18,45
The TPA cross-sections δTPA were evaluated by calculating the
two-photon transition moment matrix elements Sαβ in the Dalton
program.46,47 With the Sαβ elements in hand, the TPA cross-section
of the molecule for linearly polarized monochromatic light was calcu-
lated (in atomic units) as:
Tris{4-[2-(2,2-dicyanoethen-1-yl)benzothiazol-6-ylethynyl]phenyl}-
amine (T3). Eluent CH2Cl2/MeOH = 60:1; yield 45% (53 mg); Rf 0.80
1
(SiO2, CH2Cl2/MeOH = 60:1); mp >300 ꢀC; H NMR (300 MHz,
CDCl3) δ 8.19 (d, J = 8.6 Hz, 3H, H-4), 8.14 (d, J = 1.5 Hz, 3H, H-7), 8.09
(s, 3H, CHdC(CN)2), 7.75 (dd, J = 8.6, 1.5 Hz, 3H, H-5), 7.51 (d,
J = 8.6 Hz, 6H, Har), 7.14 (d, J = 8.6 Hz, 6H, Har); 13C NMR (75 MHz,
CDCl3) δ 157.7, 152.7, 150.8, 147.3, 137.4, 133.4, 131.7, 125.6, 124.9,
124.8, 124.4, 117.6, 112.8, 111.9, 93.5, 89.0, 87.7. Anal. Calcd for
C57H24N10S3: C 72.44, H 2.56, N 14.82. Found: C 72.59, H 2.44, N 14.66.
Tris{4-[6-(2,2-dicyanoethen-1-yl)benzothiazol-2-ylethynyl]phenyl}-
amine (T6). Eluent CH2Cl2/MeOH = 60:1; yield 42% yield (50 mg); Rf
1
ꢂ
ꢂ
βαÞ
δa:u:
¼
ð2Sαα
S
þ 4Sαβ
S
ββ
∑
30 α, β
The final expression for the TPA cross-section, which is directly
comparable to experiment, was used
3
ð2πÞ αa50 ω2
δTPA
¼
δa:u:
c
πΓ
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The Journal of Organic Chemistry
ARTICLE
Here, α is the fine structure constant, a0 the Bohr radius, c the speed of
light, ω the energy of the exciting photon (in case of the TPA process,
one-half of the excitation energy), and πΓ a normalization factor due to
the Lorentzian line-shape broadening of the excited state (Γ =0.2 eV was
assumed throughout this work). The units of δTPA will become GM
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’ ASSOCIATED CONTENT
S
Supporting Information. General experimental meth-
b
ods, H and 13C NMR spectra of new compounds, linear and
1
nonlinear optical properties of dipolar compounds calculated at
the DFT level, and Cartesian coordinates of the optimized
ground-state structures. This material is available free of charge
via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: peter.hrobarik@savba.sk.
’ ACKNOWLEDGMENT
This work has been supported by the Slovak Grant Agencies
APVV (No. 0424-10) and VEGA (No. 1/4470/07) and by the
Research & Development Operational Programme (“Center of
excellence for design, preparation, and diagnostics of nanostruc-
tures for electronics and photonics”, NanoNet) funded by the
European Regional Development Fund (ERDF). Dr. M. Cigꢀaꢁn is
acknowledged for measurement of fluorescence quantum yields
and Dr. M. Zajac for fruitful discussions. P.H. thanks the Alexander
von Humboldt Foundation for a research fellowship.
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