Solvent and branching effect on the two-photon absorption properties of push-pull triphenylamine derivatives

Article abstract of DOI:10.1039/c5ra25170b

The photophysical and two-photon absorption (2PA) properties of two tri-podal molecules and of their quadrupolar and dipolar counterparts are reported for a series of solvents with varying polarity. The molecules possess a tri-phenylamine electron donating group and mono-cyano acceptors while olefinic and acetylenic π-linkers have been used. Branching led to an increase of the molar extinction coefficient and to a slight bathochromic shift of the absorption spectra while the fluorescence quantum yields decrease but they are maintained to relatively high values. Solvatochromic measurements in the tri-podal molecules revealed an emitting state with a polar nature. The 2PA cross sections in general increase upon branching but the observed behaviour strongly depends on the type of solvent. The highest 2PA cross sections are obtained in solvents of medium polarity and values as high as 1420 GM are reported.

Full text of DOI:10.1039/c5ra25170b

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PAPER
Solvent and branching effect on the two-photon
absorption properties of push–pull triphenylamine
derivatives
Cite this: RSC Adv., 2016, 6, 12819
D. Cvejn,a E. Michail,b K. Seintis,b M. Klikar,a O. Pytela,a T. Mikysek,c N. Almonasy,a
a
b
b
a
*
*
M. Ludwig, V. Giannetas, M. Fakis and F. Bures
ˇ
The photophysical and two-photon absorption (2PA) properties of two tri-podal molecules and of their
quadrupolar and dipolar counterparts are reported for a series of solvents with varying polarity. The
molecules possess a tri-phenylamine electron donating group and mono-cyano acceptors while olefinic
and acetylenic p-linkers have been used. Branching led to an increase of the molar extinction coefficient
and to a slight bathochromic shift of the absorption spectra while the fluorescence quantum yields
decrease but they are maintained to relatively high values. Solvatochromic measurements in the tri-
podal molecules revealed an emitting state with a polar nature. The 2PA cross sections in general
increase upon branching but the observed behaviour strongly depends on the type of solvent. The
highest 2PA cross sections are obtained in solvents of medium polarity and values as high as 1420 GM
are reported.
Received 26th November 2015
Accepted 17th January 2016
DOI: 10.1039/c5ra25170b
www.rsc.org/advances
The large number of applications that are based on 2PA
require the use of suitable materials with enhanced 2PA prop-
Introduction
erties at specic wavelengths accompanied, in certain cases, by
a high uorescence quantum yield. Therefore, the quest for
organic molecules satisfying the criteria needed for 2PA based
applications has received tremendous attention. It is nowadays
well established that organic molecules with increased electron
delocalization featuring intramolecular charge transfer (ICT)
are the most attractive candidates while their properties can be
systematically tuned.15–17 These molecules bear electron
donating and accepting units adopting a dipolar, quadrupolar,
octupolar or multipolar nature. Using octupolar and multipolar
compounds i.e. incorporating three or more chromophoric
units in a single multichromophoric molecule has been found
in most cases to increase their 2PA cross section through the
synergetic co-operation of the chromophores.18–27 In octupolar
or multipolar compounds, co-operation of branches is realized
through coherent and/or incoherent coupling among the
chromophoric units, the nature of the coupling being depen-
dent on the type of the core and the chemical structure of the
branches. However, some requirements such as stability, solu-
bility and cell permeability are dependent on the size of the
molecules. This leads to the need for obtaining the best 2PA
properties within a small molecular volume.
Two-photon absorption (2PA), a nonlinear optical phenomenon
related to the imaginary part of the third-order nonlinear
susceptibility, constitutes the physical origin of numerous
applications such as upconverted lasing,1,2 two-photon micros-
copy and imaging,3–5 3D microfabrication and 3D optical data
storage,6–10 optical limiters etc.11,12 2PA is the simultaneous
absorption of two photons by a molecule, atom or ion leading to
its excitation to a higher state. The absorbed photons lay typi-
cally in the near infrared spectral region (ideally from 700 to
900 nm i.e. the spectral region of Ti:sapphire fs lasers) and their
energy is half the energy between the ground and the nal
excited state. Compared to one-photon absorption (1PA) in the
UV-blue spectral region, using near infrared light in 2PA, is
advantageous because of the reduced scattering leading to an
increased penetration depth. Besides, 2PA is a process with an
intrinsic spatial connement because of its quadratic depen-
dence on the excitation power. This renders 2PA a great tool for
initiating photo-physical or photo-chemical phenomena with
a 3D resolution of the order of 100 nm.13,14
aInstitute of Organic Chemistry and Technology, Faculty of Chemical Technology,
University of Pardubice, Studentsk8 573, Pardubice, 532 10, Czech Republic; Web:
http://bures.upce.cz. E-mail: lip.bures@upce.cz; Fax: +420 46 603 7068; Tel: +420
46 603 7099
bDepartment of Physics, University of Patras, GR-26504, Patras, Greece; Web: http://
www.laserlab.physics.upatras.gr/laserlab/index.php/faculty.html. E-mail: fakis@
upatras.gr; Fax: +30 2610 997470; Tel: +30 2610 996794
In the past, several works have reported on a comparative
study of the multi-chromophoric systems with their model
chromophores. These studies, both theoretical and experi-
mental, have attempted to shed light on why multi-
chromophoric systems exhibit improved 2PA activity compared
to model compounds and why some times 2PA activity is
cDepartment of Analytical Chemistry, Faculty of Chemical Technology, University of
´
Pardubice, Studentska 573, Pardubice, 53210, Czech Republic
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reduced.20,23,24,28–33 However, several issues remain not well 1L, 1Q and 2L, 2Q were prepared in good yields as shown in
understood, because of the complexity of multi-chromophoric Scheme 1. The attained yields decrease slightly with increasing
systems, such as the exact effect of the core, the localization the multiplicity of the cross-coupling.
(or not) of the excited state on a single chromophore (branch),
the effect of solvent etc. Localization of the excited state on Thermal properties
a single branch may hinder signicant branch co-operation and
enhancement of the 2PA properties. On the other hand,
the proximity of chromophores allow branch incoherent
co-operation by means of through space interactions such as
energy hopping. The solvent is of great importance because due
to the polar nature of the chromophores used, large charge
displacements take place upon one- or two-photon excitation,
while the magnitude and nature of chromophore coupling can
Thermal behaviour of target chromophores 1L, 1Q, 1T and 2L,
2Q, 2T was studied by differential scanning calorimetry (DSC).
The measured melting temperatures (Tm) and temperatures of
thermal decomposition (Td) are listed in Table 1. Fig. 1 shows
two representative spectra of 1L and 1Q. Both these compounds
showed sharp melting points at 101 and 232 ꢀC, which is for 1Q
immediately followed by decomposition. In contrast to this,
compounds 1T, 2L and 2Q showed wide peaks corresponding to
be tuned by changing the solvent polarity. Therefore, 2PA
their melting points at 217, 131 and 203 ꢀC and surprisingly no
molecules are very sensitive on the polarity of the environment
decomposition peaks. However, a slow decrease of the heat ow
was observed and the samples were clearly carbonized aer
heating up to 400 ꢀC. Compounds 1T and 2Q showed a wide
which inuences both their linear and nonlinear optical prop-
erties. In the past, it has been found that large 2PA cross
sections, d2PA, are achieved in medium polar solvents where
enantiotropic solid–solid transition at 37–82 and 150–190 ꢀC. As
usually a relatively high uorescence quantum yield is also
a main feature, the branching of chromophores increases their
melting temperatures (except 1T), but the decomposition
remained unresolved. However, chromophore 2T resisted
thermal decomposition up to 477 ꢀC. Chromophores in series 1
bearing acetylenic p-linkers proved to be generally less ther-
mally stable than the corresponding olens in series 2.
exhibited.34–36
In our very recent work, we have studied the 2PA properties
of a large series of tri-podal molecules based on a central tri-
phenylamine (TPA) electron donor and various peripheral
cyano-based acceptors.37 It has been found that two tripodal
molecules with mono-cyano acceptors and acetylenic or olenic
p-conjugated bridges exhibited the largest 2PA cross section. In
Electrochemistry
this respect and as an extension of our recent work, we report
Electrochemical studies of 1L, 1Q, 1T and 2L, 2Q, 2T were
herein a comparative study of two tripodal molecules (T) with
carried out by cyclic voltammetry (CV), rotating disc voltam-
metry (RDV) and polarography. All measurements were carried
out in acetonitrile containing 0.1 M Bu4NPF6 as electrolyte. The
acquired rst oxidation and reduction potentials are summa-
rized in Table 1. As a main feature, they showed only negligible
changes upon branching. When going from 1L to 1Q and 1T,
the rst oxidation potential is slightly shied to more positive
their quadrupolar (Q) and dipolar/linear (L) analogues. The
synthesis of the quadrupolar and dipolar compounds is
described and the effect of branching on thermal, electro-
chemical, photophysical and 2PA properties is studied. Finally,
solvent effects on linear and non-linear optical properties are
described. The experimental outcomes are further supported by
DFT calculations.
values (DE1/2(ox1) ¼ 0.06 V). Considering the central amino
donor as a main oxidation centre, this observation reects its
Results and discussion
lowered electron density caused by attaching more electron
Synthesis
acceptors (enhanced/spread ICT). The rst oxidation potentials
of 2L, 2Q and 2T are almost identical. The rst reductions
within both series 1 and 2 became slightly facilitated (E1/2(red1)
are shied to more positive values) upon branching, which
implies that attaching more electron acceptors to one electron
donor results in their competition in the ICT. The nature of the
p-linker showed much more signicant effect. When
comparing series 1 and 2, the latter showed signicantly lower
rst oxidation potentials and thus also narrowed HOMO–LUMO
gap (DE ¼ 2.86–2.89 vs. 2.65–2.70 V). This is in accordance with
our observations on enhanced D–A interaction across olenic
than acetylenic p-linkers.41
In general, the synthesis of target chromophores 1–2 consists of
two steps (Scheme 1). The rst step involves preparation of
properly halogenated triphenylamine, while the subsequent
one comprises of one-, two- or threefold Sonogashira and
Suzuki–Miyaura cross-coupling reactions. Commercially avail-
able triphenylamine was iodinated in two different ways. The
well-established threefold iodination of TPA (3) using the HgO/
I2 system afforded tris(4-iodophenyl)amine 6 in 88% yield.38
However, the iodination using HgO as a Lewis acid lacks
selectivity towards mono- and bisiodinated products 4 and 5.
Hence, the iodination was carried out using AgNO3/I2/EtOH
system, which was formerly used for iodination of carbazole.39
In this way, the desired mono- and bisiodinated triphenyl-
Steady state absorption and uorescence properties
amines 4 and 5 were prepared in reasonable yields of 51 and The optical properties of all target compounds were investi-
77%. The nal one-, two- and threefold Sonogashira and gated by electronic absorption and emission spectra in various
Suzuki–Miyaura cross-coupling reactions were carried out solvents. Fig. 2 shows the absorption and uorescence spectra
under standard condition used for the construction of tripodal in THF; the principal photophysical parameters are given in
molecules 1T and 2T.37 The linear and quadrupolar molecules Table 1.
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Scheme 1 Synthetic pathway leading to target linear, quadrupolar and tripodal chromophores.
Compared to all chromophores in series 1, molecules 2L, 2Q nm (Table 2) i.e. close to that of 2Q. In the absorption spectra of
and 2T showed bathochromically shied longest-wavelength the quadrupolar compounds 1Q and 2Q, apart from the main
absorption maxima (DlAmax ¼ 14–24 nm), which reects more peak, a shoulder located at the high-energy side of the peak, is
efficient ICT over olenic than acetylenic p-linkers as seen by also observed pointing to a splitting of the absorption bands.
electrochemistry (see above). Besides, when going from linear This splitting results from the coupling between branches in the
(L) to quadrupolar (Q) chromophores, the absorption spectra case of 1Q and 2Q and it is explained by the Frenkel exciton
are red-shied as a result of attaching more electron acceptors/ model, which is also applied to H- and J-aggregates and
branches and indicate an electronic coupling between the assumes electrostatic interaction of the chromophores.20,32,42–44
branches.21,23,31
The model predicts that in the quadrupolar V-shaped mole-
In addition, when going from quadrupolar (Q) to tri-podal cules, the singlet excited state splits into two bands which are
(T), the absorption spectra are slightly blue-shied implying positioned themselves symmetrically with respect to the excited
that attaching a third cyano-substituted branch on the central state of the linear molecule. Both states are one photon allowed
amino donor has a diminished effect on the absorption prop- while the lower energy one has stronger oscillator strength. On
erties. This blue shi is more signicant for 2T in THF, where the other hand, the excited state of tripodal compounds (1T and
specic solvent–solute interactions should play a role because 2T) is split into three states. The two states are degenerate and
in other solvents the absorption peak of 2T is found at 411–413 lay at the low-energy side of the linear chromophore's excited
Table 1 Summarized properties of target chromophores
DEc
(V)
EHOMO
(eV)
ELUMO
(eV)
lAmax
3e
lFmax
FFe
Stokes shi d2PA
(cmꢁ1
(GM)
a
b
c
c
d
d
e
e
f
Tm
Comp. (ꢀC) (ꢀC) (V)
Td
E1/2(ox1)
E1/2(red1)
(V)
(nm eVꢁ1
)
(M cmꢁ1
)
(nm eVꢁ1
)
(%)
)
1L
1Q
1T
2L
2Q
2T
101
232
217
131
203
291
303
235
—
—
—
0.98
1.01
1.04
0.84
0.84
0.85
ꢁ1.88
ꢁ1.88
ꢁ1.85
ꢁ1.85
ꢁ1.86
ꢁ1.80
2.86 ꢁ5.41
2.89 ꢁ5.44
2.89 ꢁ5.47
2.69 ꢁ5.27
2.70 ꢁ5.27
2.65 ꢁ5.28
ꢁ2.55
ꢁ2.55
ꢁ2.58
ꢁ2.58
ꢁ2.57
ꢁ2.63
378/3.28
390/3.18
387/3.20
392/3.16
414/3.00
403/3.08
13 120
57 621
80 141
20 299
31 033
53 613
472/2.63
461/2.69
458/2.71
495/2.51
504/2.46
510/2.43
0.80 5270
0.73 3950
0.59 4006
0.63 5310
0.61 4310
0.51 5206
641/750
1025/750
1195/760
284/750
1050/750
1100/810
477
a
Tm ¼ melting point (the point of intersection of a baseline before the thermal effect with a tangent). b Td ¼ thermal decomposition (pyrolysis in
a N2 atmosphere). c E1/d2(ox1) and E1/2(red1) are half-wave potentials of the rst oxidation and reduction, respectively; all potentials are given vs. SCE; DE
e
f
¼ E1/2(ox1) ꢁ E1/2(red1)
.
ꢁEHOMO/LUMO ¼ E1/2(ox1/red1) + 4.429 (ref. 40). Measured in THF. The wavelength of the maximum 2PA cross section is
shown aer slash (measured in THF).
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Table 2 Photophysical parameters of 1T and 2T in various solvents
lAmax
(nm eVꢁ1
lFmax
(nm eVꢁ1
Stokes shi
Comp. Solvent
)
)
(cmꢁ1
)
F
F (%)
1T
Toluene
THF
Acetone
Acetonitrile 387
Toluene
THF
392
387
387
428
458
482
505
464
510
550
564
2146
4006
5093
6038
2661
5206
6083
6569
0.52
0.59
0.37
0.35
0.54
0.51
0.37
0.28
2T
413
403
411
Acetone
Acetonitrile 411
Fig.
1 Representative DSC curves of chromophores 1L and 1Q
determined with a scanning rate of 3 ꢀC minꢁ1 under N2.
noting that FF for the tri-podal molecules remains relatively
high even in solvents of high polarity such as acetone and
acetonitrile (Table 2).
Fig. 3 shows the absorption and uorescence spectra of
tripodal chromophores 1T and 2T in toluene (TOL), THF,
state while the third state is located at the high-energy side and
is predicted to have zero oscillator strength for 1PA. This
explains why the spectra of tripodal compounds 1T and 2T
exhibit a single peak without any shoulder and/or broadening. acetone (ACT) and acetonitrile (ACN); the measured photo-
As expected, the molar extinction coefficient of the chromo- physical data are given in Table 2. The increased solvent polarity
phores increases upon branching. In series 1, though, the has only diminished effect on the absorption spectra. On the
increase is not linear to the number of branches indicating
a strong coupling. In series 2, the value of extinction coefficient
is approximately proportional to the number of branches indi-
cating a nearly independent behaviour of chromophores.23,31,45
Regarding the uorescence spectra, 1Q and 1T exhibit
other hand, the uorescence spectra exhibit a positive sol-
vatochromism (i.e. a bathochromic shi with DlFmax ¼ 77/99 nm
upon increasing the solvent polarity). The spectra in solvents of
low polarity (toluene) are narrow with a weak shoulder (more
obvious in the spectra of 2T), while they become featureless and
a hypsochromically shied spectrum compared to that of 1L broad in high polarity solvents.
attributed to a less polar excited state. In series 2, on the other
The red shi and broadening of the uorescence spectra
hand, a red-shied uorescence is observed for 2Q and 2T vs. 2L with increasing solvent polarity resembles the behaviour
meaning that the excited state in the branched compounds is
more polar and more stabilized. In both cases, the observed
differences in the uorescence spectra indicate a coupling
between branches. The uorescence quantum yields, FF, in
series 2 are lower than in series 1 perhaps due to the more
observed in linear D–p–A compounds and indicates that the
emission originates from an excited state with polar nature i.e.
that the main emitting chromophoric unit is the single
branch.23,32 The Lippert–Mataga plots46,47 for 1T and 2T in Fig. 4
show that the Stokes shi increases linearly with the solvent
efficient ICT formation (Table 1). In both series, FF decreases polarizability (Df) indicating a stabilization of the excited state
upon branching, implying increased non-radiative pathways, in polar solvents; the correlation between the Stokes shi and
but is maintained to relatively high values similar to those the solvent polarizability is especially tight for 2T. The slope of
found for other TPA-based tri-podal molecules.21,30,31 It is worth
Fig. 3 Steady state absorption (full points) and fluorescence spectra
Fig. 2 Steady state absorption (full points) and fluorescence spectra (empty points) of chromophores 1T (a) and 2T (b) in toluene, THF,
(empty points) of chromophores in series 1 (a) and 2 (b) in THF.
acetone and acetonitrile.
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Fig. 4 Lippert–Mataga plots for chromophores 1T and 2T.
this diagram depends on the difference between the excited and
ground state dipole moments and the radius of the molecular
cavity.46,47 Considering small structural changes between 1T and
2T (olenic vs. acetylenic p-linkers), we can assume that the
radius of the cavity is similar for both molecules. The DFT-
calculated box or molar volumes of 1T/2T equal to 2.35 ꢂ
104/2.57 ꢂ 104 A3 or 542.5/552.1 cm3 molꢁ1, respectively, which
further conrms their close spatial arrangement. Hence, the
slope is a quantitative measure of the polar nature of the excited
state. Higher slope is obtained for 2T, which indicates its larger
dipole moment change relative to 1T and suggests a more polar
excited state of 2T.
This is corroborated by the frontier molecular orbitals of 1T
and 2T, reported in a previous work of ours,37 where it was
shown that the LUMO level of 2T is located on a single dipolar
branch while that of 1T is located on a single branch but also
has a small contribution on a second branch, rendering the
excited state less polar.
Fig. 5 2PA spectra of (a) 1L, (b) 1Q and (c) 1T in various solvents.
efficiency is expected in solvents of medium polarity where
intramolecular charge transfer process takes place and excited
states posses a highly polar nature.34,35,49,50 However, it is unclear
why 1L exhibits enhanced 2PA properties in toluene and its 2PA
efficiency decreases monotonically as the solvent polarity
increases. Maybe this is due to specic solvent–solute interac-
tions in polar solvents redistributing the energetics of the
excited state and changing the excitation transition dipole
moments.
Next, in order to better evaluate the effect of branching and
to assess whether the observed behaviour of branched
compounds is due to a cooperative or additive effect, we
2PA properties
Fig. 5 and 6 show the 2PA spectra of 1L, 1Q and 1T and of 2L, 2Q
and 2T respectively in various solvents. In compounds 1L, 1Q
and 1T the maximum 2PA cross section is observed at the short
wavelength region of our excitation range while in 2L, 2Q and
2T, the 2PA peaks are observed at short to medium wavelengths
across our scanning spectral window which is in agreement
with the shi in the 1PA spectra. Interestingly, 1L exhibits
a maximum 2PA cross section (800 GM) in the low polarity
solvent toluene. However, toluene is not the optimum choice
upon increasing the number of branches since both 1Q and 1T
exhibit large values of 2PA cross sections in the more polar
solvents THF and acetone. Specically, 1T has a 2PA cross
section of 1400 GM in acetone which is almost twice larger than
the value found for 1T in toluene. In addition, 2L, 2Q and 2T
exhibit the maximum 2PA cross sections in acetone and THF
while the largest 2PA cross section is observed for 2Q in acetone
(1420 GM). The above ndings are in agreement with our
previous works where we have studied the 2PA properties of
linear quadrupolar molecules and have found their maximum
2PA cross section values in solvents of medium polarity.36,48
These results are also in agreement with other experimental and
theoretical works by Woo et al. Orłowski et al. Wang et al. and
Luo et al. in which a general conclusion was that enhanced 2PA Fig. 6 2PA spectra of (a) 2L, (b) 2Q and (c) 2T in various solvents.
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compare in series 1 and 2, the 2PA cross sections normalized the high energy one.19 Therefore, in our case, although a 2PA
per branch number.23,31 This comparison led to varied results enhancement that is above the additive limit is observed for 2Q,
depending on the series and solvent.
In series 1, the 2PA cross sections/branch values decrease or to the limited spectral window of our experimental system.
remain approximately constant in quadrupolar and octupolar Finally, another feature that warrants discussion is the
the fact that 2T does not exhibit a similar behaviour may be due
compounds compared to dipolar one. The most signicant comparison of the 1PA and 2PA spectra at half wavelengths. In
decrease in series 1 was observed in toluene where the cross dipolar molecules, the same excited state is populated both
section/branch value of 800 GM in 1L decreases to approxi- under 1PA and 2PA, so a similarity of 1PA and 2PA spectra is
mately 200 GM for 1Q and 1T. Similarly, in acetonitrile observed. In V-shaped molecules the 2PA are blue shied
a decrease of the normalized 2PA cross sections is also observed compared to the 1PA ones.19 In the case of octupolar molecules,
while, in THF and acetone, a nearly additive behaviour is the excitonic model predicts that 1PA is expected mainly to the
exhibited meaning that the 2PA cross sections/branch are two lowest degenerate excited states while 2PA to the higher
almost the same in 1L, 1Q and 1T. Interestingly, in series 2, 2Q energy excited state. However, previous works have shown
compared to 2L, exhibits 2PA cross section/branch values that a similarity of 1PA and 2PA spectra21,23 while a shi between
are above the additive behaviour in all solvents except from these spectra has also been reported.31 In the compounds
acetonitrile. This 2PA enhancement is due to a co-operative studied herein, 1T exhibits the peak of the 2PA spectra at 760–
behaviour of branches and leads to an increase of the 2PA 770 nm and 2T at 810–830 nm in all solvents. Both these regions
cross section/branch values of approximately 4, 1.8 and 1.7 are at twice the peak wavelengths of 1PA spectra of 1T and 2T
times in toluene, THF and acetone respectively. Unfortunately, (Table 2). This good similarity of the 1PA and 2PA spectra at half
this co-operative enhancement is not observed in 2T where wavelengths is an indication of weak coupling among branches.
a decrease of the 2PA cross sections/branch compared to 2Q is
observed in all solvents. Besides, in 2T, an enhancement of the
2PA spectrum at long wavelengths is observed compared to 2L
Quantum chemical calculations
The spatial and electronic properties of chromophores in both
series 1 and 2 were investigated using the Gaussian W09
package55 at the DFT level. The initial geometries of molecules
were estimated using the PM3 method implemented in Argus-
Lab50,56 and these were subsequently optimized using the DFT
B3LYP/6-311++G(2d,p) method. The energies of the HOMO and
LUMO, their differences and ground state dipole moments were
calculated with the DFT B3LYP/6-311++G(2d,p) and are
summarized in Table 3. The calculated energies of the HOMO/
LUMO and their differences correlate well with the experi-
mental data obtained by the electrochemical measurements
(Table 1). Hence, the used DFT method can be considered as
and 2Q, which probably may be due to breaking of symmetry.
In the past, several works have reported on the enhancement
of 2PA in branched compounds, that is more than the additive
limit, due to the increase in dimensionality, delocalization and
co-operation of chromophores.31,51–53 Goodson and co-workers
have associated such a co-operative enhancement in branched
compounds with an ultrafast depolarization of emission.38
However, in other works, an additive behaviour or even decrease
of 2PA efficiency i.e. of the 2PA cross section/branch value was
also reported.19,27,32,33,54 The understanding of why branching is
not always the optimum design strategy remains a puzzling
issue. It is mentioned, though, that the above comparison of the
branched compounds vs. the linear ones should be made over
a wide wavelength range. Katan et al. have shown that a tripodal
compound with dipolar branches showed only
a slight
enhancement of its 2PA cross section close to 800 nm compared
to its linear counterpart, while a signicant enhancement was
observed at the higher energy region, close to 700 nm.31
Makarov et al. have reported that in their tripodal compounds,
the 2PA spectrum is strongly shied to high energies compared
to the linear molecules i.e. the 2PA cross sections decrease in
the low-energy side of the spectrum while they are enhanced in
Table 3 DFT calculated propertiesa of target chromophores
a
a
Comp.
EHOMO (eV)
ELUMO (eV)
DE (eV)
m (D)
1L
1Q
1T
2L
2Q
2T
ꢁ5.51
ꢁ5.67
ꢁ5.80
ꢁ5.42
ꢁ5.52
ꢁ5.62
ꢁ2.27
ꢁ2.54
ꢁ2.67
ꢁ2.34
ꢁ2.59
ꢁ2.75
3.24
3.13
3.13
3.08
2.93
2.87
7.94
7.01
0.001
7.80
7.12
0.221
Fig.
7 HOMO (red) and LUMO (blue) visualizations in target
a
Calculated at the DFT B3LYP/6-311++G(2d,p) level.
chromophores.
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a reasonable tool for the description of trends in electronic conducted on aluminum sheets coated with silica gel 60 F254
1
properties of chromophores 1–2.
with visualization by a UV lamp (254 or 365 nm). H and 13C
As a main feature, one can observe that the energies of the NMR spectra were recorded at 400 and 100 MHz, respectively,
HOMO and LUMO steadily increase when going from linear to with a Bruker AVANCE 400 instrument, or at 500 and 125 MHz,
1
quadrupolar and tripodal molecules. This results in narrowed respectively, with Bruker Ascend™ 500. Chemical shis in H,
HOMO–LUMO gap (DE), however the effect of branching seems 13C spectra are reported in ppm relative to the signal of Me4Si.
to be negligible similar to that observed by electrochemical The residual solvent signal in the 1H and 13C-NMR spectra was
measurements (Table 1). The olenic units in molecules in used as an internal reference (CDCl3 7.25 and 77.23 ppm,
series 2 allow higher D–A coupling and, therefore, chromo- CD2Cl2 5.32 and 54.00 ppm). Apparent resonance multiplicities
phores 2L, 2Q and 2T showed reduced HOMO–LUMO gap.
are described as s (singlet), d (doublet), and m (multiplet),
The HOMO and LUMO distributions in all target molecules apparent coupling constants of multiplets (3J or 4J) are given in
are shown in Fig. 7. Whereas the HOMO is localized on the Hz. IR spectra were recorded as neat using HATR adapter on
central amino donor, the LUMO occupies the peripheral CN a Perkin-Elmer FTIR Spectrum BX spectrometer. High resolu-
acceptor(s) and the adjacent p-system. The LUMO is predomi- tion MALDI MS spectra were measured on a MALDI mass
nantly localized over one particular branch and only negligibly spectrometer LTQ Orbitrap XL (Thermo Fisher Scientic, Bre-
over the second branch in Q and T series of molecules with the men, Germany) equipped with nitrogen UV laser (337 nm,
contribution over the second branch being larger in series 1. 60 Hz). The LTQ Orbitrap instrument was operated in positive-
The second/third branch in Q and T compounds is occupied by or negative-ion mode over a normal mass range (m/z 50–2000)
LUMO+1. Both frontiers orbitals are clearly separated which with resolution 100 000 at m/z ¼ 400. The survey crystal posi-
further conrms the ICT character of all chromophores. When tioning system (survey CPS) was set for the random choice of
going from unsymmetrical L and Q chromophores to tripodal shot position by automatic crystal recognition. 2,5-Dihydroxy-
molecules 1T and 2T, having D3 and C3 group of symmetry, the benzoic acid (DHB) was used as a matrix. Mass spectra were
calculated ground state dipole moments diminish signicantly averaged over the whole MS record for all measured samples.
from about 7–8 to almost 0 D for 1T.
Thermal properties of target molecules were measured by DSC
with a Mettler-Toledo STARe System DSC 2/700 equipped with
FRS 6 ceramic sensor and cooling system HUBERT TC100-MT
RC 23.
Conclusions
A study of the effect of solvent polarity and branching strategy
The absorption spectra were measured on a UV/vis Perkin-
on the photophysical and 2PA properties of triphenylamine Elmer Lambda 35 spectrophotometer at room temperature.
push–pull molecules was presented. For this reason, the prop- The steady-state uorescence spectra were measured using
erties of two tripodal molecules were compared with their a Perkin-Elmer LS 55 spectrophotometer. The spectra were
quadrupolar and dipolar analogues in various solvents. The recorded by setting the excitation wavelength at the absorption
photophysical properties have shown a dipolar nature of the maxima of the measured compounds. The instrument provides
emitting state in the tripodal molecules, a behaviour resem- corrected excitation spectra directly; the uorescence spectra
bling that of the linear dipolar molecules. A small coupling were corrected for the characteristics of the emission mono-
among branches was found as a result of a slight red-shi of the chromator and for the photomultiplier response. For uores-
absorption spectra of the branched compounds. The investi- cence measurements, the solutions of very low concentration
gation of the 2PA properties has shown that the 2PA cross about 10ꢁ6 mol Lꢁ1 (optical density ꢃ 0.05 at the excitation
section of the branched compounds increases compared to that wavelength in 1 cm cell) were used; the following uorescence
of the dipolar ones. In some cases, depending on the solvent standards were used for the determination of the uorescence
used and molecule, this increase is higher than what was ex- quantum yields: quinine sulphate in 0.5 M aqueous H2SO4
pected based on an additive behaviour of the branches, (lFmax ¼ 445 nm, FF ¼ 0.546)57 and coumarin 153 in ethanol
implying a co-operative effect. The highest 2PA cross section (lFmax ¼ 536 nm, FF ¼ 0.38).58
was observed in solvents of medium polarity and a value as high
The two-photon absorption properties of the dyes were
as 1420 GM was reported for the quadrupolar molecule 2Q in studied by means of a two-photon excited uorescence (TPEF)
acetone.
technique.21,59–61 A mode-locked Ti:sapphire laser emitting 80 fs
pulses tunable from 750 to 850 nm was used as the excitation
source. The laser beam was focused on the samples by a 0.32 NA
objective lens while the 2PA induced uorescence was collected
backward by the same objective lens. It was then separated from
Experimental
General methods
For the synthesis and characterization of tripodal chromo- the excitation beam by using a dichroic mirror and a series of
phores 1T and 2T see the literature.37 THF was dried in Pure- short-pass lters. A photomultiplier connected to photon-
solv™ micro solvent purication system. All commercial counting electronics was used to detect the emitted uores-
chemicals and solvents were purchased from suppliers such as cence. The detected uorescence intensity was in all cases
Sigma Aldrich, Acros and TCI at reagent grade and were used as proportional to the square of the excitation power verifying that
obtained. All cross-coupling reactions were carried out in ame- the observed uorescence was due to two-photon absorption.
dried asks under argon. Thin-layer chromatography (TLC) was The samples were 10ꢁ4 M solutions of the chromophores in
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3
various solvents. Finally, the 2PA cross sections, dTPA, were CH–PhCN), 7.08 (4H, d, J ¼ 8.8, N–PhH–CH]CH), 7.12–7.15
determined by using Rhodamine B (10ꢁ4 M in MeOH) as (3H, m, N–PhH), 7.17 (2H, d, 3J ¼ 16.4, Ph–CH]CH–PhCN),
3
3
reference.
7.31 (3H, t, J ¼ 7.0, N–PhH), 7.43 (4H, d, J ¼ 9.0, N–PhH–Ar),
3
3
7.59 (4H, d, J ¼ 8.6, Ph–PhH–CN), 7.62 (4H, d, J ¼ 8.6, Ph–
PhH–CN). 13C-NMR: dC (125 MHz, CD2Cl2) 88.0, 94.3, 111.7,
116.14, 119.1, 123.6, 125.4, 126.6, 128.8, 130.2, 132.3, 132.6,
133.4, 146.8, 148.4. HR-MALDI-MS (DHB): calcd for C27H20N2
(M+) 372.16210 found 372.16248.
General method for the synthesis of 1L and 1Q
Iodinated triphenylamine derivatives 4 or 5 (0.23 mmol) and
4-ethynylbenzonitrile (0.3 or 0.6 mmol) were dissolved in dry
THF (15 ml) and TEA (5 ml, 36 mmol). Argon was bubbled
through the mixture for 10 min, whereupon PdCl2(PPh3)2
(16 mg, 0.023 mmol) and CuI (13 mg, 0.069 mmol) were added
and the reaction was heated to 70 ꢀC for 8 h. The reaction
mixture was poured into the sat. solution of NH4Cl (100 ml) and
was extracted with DCM (3 ꢂ 100 ml). The combined organic
extracts were dried over anhydrous Na2SO4 and concentrated in
vacuo. The crude product was puried by column chromatog-
raphy (SiO2, hexane/EtOAc 4 : 1).
Chromophore 2Q. Title compound was synthesized from 5
(169 mg) following the general method. Yellow solid. Yield
(139 mg, 82%). Rf 0.55 (SiO2; EtOAc/hexane 1 : 4). Mp 203 ꢀC. IR
(HATR) nmax/cmꢁ1 3180, 2221 (CN), 1585, 1484, 1276, 1171, 958,
831. 1H-NMR: dH (400 MHz, CD2Cl2) 7.02 (2H, d, 3J ¼ 16.4, Ph–
CH]CH–PhCN), 7.08 (4H, d, 3J ¼ 8.8, N–PhH–CH]CH), 7.12–
7.15 (3H, m, N–PhH), 7.17 (2H, d, 3J ¼ 16.4, Ph–CH]CH–
3
3
PhCN), 7.31 (3H, t, J ¼ 7.0, N–PhH), 7.43 (4H, d, J ¼ 9.0, N–
3
3
PhH–Ar), 7.59 (4H, d, J ¼ 8.6, Ph–PhH–CN), 7.62 (4H, d, J ¼
8.6, Ph–PhH–CN). 13C-NMR: dC (125 MHz, CD2Cl2) 88.0, 94.3,
111.7, 116.14, 119.1, 123.6, 125.4, 126.6, 128.8, 130.2, 132.3,
132.6, 133.4, 146.8, 148.4. HR-MALDI-MS (DHB): calcd for
Chromophore 1L. Title compound was synthesized from 4
(85 mg) following the general method. Pale yellow solid. Yield
(80 mg, 94%). Rf 0.7 (SiO2; EtOAc/hexane 1 : 4). Mp 101 ꢀC (lit.62
159 ꢀC). IR (HATR) nmax/cmꢁ1 3030, 2212 (CN), 1578, 1486, 1266,
C
36H25N3 (M+) 499.20430 found 499.20466.
1
1176, 1073, 755, 694; H-NMR: dH (400 MHz, CD2Cl2) 7.01 (2H,
3
d, J ¼ 8.8, N–PhH–Ar), 7.12–7.16 (6H, m, N–PhH), 7.32 (4H, t,
3J ¼ 7.6, N–PhH), 7.40 (4H, d, 3J ¼ 8.8, N–PhH–Ar), 7.60 (2H, d,
3J ¼ 8.4, Ph–PhH–CN), 7.65 (4H, d, 3J ¼ 8.4, Ph–PhH–CN).
Acknowledgements
13C-NMR: dC 87.6, 94.7, 111.5, 114.8, 119.1, 121.9, 124.5, 125.9, This research was supported by the Czech Science Foundation
129.0, 130.0, 132.3, 132.6, 133.2, 147.4, 149.3. HR-FT-MALDI-MS (13-01061S). M. L. and F. B. are indebted to the Ministry of
(DHB): calcd for C27H18N2 (M+) 370.14645 found 370.14592.
Chromophore 1Q. Title compound was synthesized from 5
(114 mg) following the general method. Yellow solid. Yield
(91 mg, 80%). Rf 0.65 (SiO2; EtOAc/hexane 1 : 4). Mp 232 ꢀC. IR
(HATR) nmax/cmꢁ1 2206 (CN), 1585, 1485, 1269, 1110, 1021, 830.
Education, Youth and Sports of the Czech Republic (LG13053).
Notes and references
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ˇ
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