Modulation of (non)linear optical properties in tripodal molecules by variation of the peripheral cyano acceptor moieties and the π-spacer

Article abstract of DOI:10.1039/c5tc01293g

A series of twelve tripodal push-pull molecules with a central triphenylamine donor and peripheral cyano substituted acceptors has been prepared. These molecules possess systematically altered π-linkers as well as cyano acceptors. Based on the experimental properties measured by differential scanning calorimetry, electrochemistry, one and two photon absorption/emission spectroscopy, supported by the DFT calculations, thorough structure-property relationships were elucidated.

Full text of DOI:10.1039/c5tc01293g

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Page 1 of 13
Journal Name
Journal of Materials Chemistry C
View Article Online
ARTICLE
DOI: 10.1039/C5TC01293G
Modulation of (non)linear optical properties in
tripodal molecules by variation of the peripheral
cyano acceptor moieties and the π-spacer
Cite this: DOI: 10.1039/x0xx00000x
D. Cvejn,a E. Michail,b I. Polyzos,b N. Almonasy,a O. Pytela,a M. Klikar,a T.
Mikysek,c V. Giannetas,b M. Fakisb and F. Bureš*,a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
A series of twelve tripodal pushꢀpull molecules with central triphenylamine donor and
peripheral cyano substituted acceptors has been prepared. These molecules possess
www.rsc.org/
systematically altered
πꢀlinker as well as cyano acceptors. Based on the experimental
properties measured by differential scanning calorimetry, electrochemistry, one and two
photon absorption/emission spectra, supported by the DFT calculations, thorough structureꢀ
property relationships were elucidated.
acceptor), an intramolecular charge transfer (ICT) from the
Introduction
donor via the
becomes polarized and constitutes
π
ꢀsystem to the acceptor occurs and the molecule
dipole. The DꢀA
Multiphoton absorbing (MPA)1 molecules are currently
attracting considerable attention of organic and material
chemists as well as physicists. The simultaneous absorption of
two photons by a molecule has been theoretically predicted as
early as 1931 by M. GöppertꢀMayer and experimentally
verified after the invention of the laser (1960). The fact that the
excited state of a molecule can be populated by absorbing
two/three photons of half/third the energy makes the two/threeꢀ
photon absorption (2/3PA) a very attractive phenomenon.2 The
use of a lowꢀenergy excitation source (NIR laser) along with
the intensityꢀdependent MPA processes can be considered as
two principal advantages that opened immense interest and
prospective applications of MPAꢀactive molecules across
optoelectronics and photonics.3 Thus, materials exhibiting large
2PA crossꢀsection have been utilized in the fields ranging from
material science, biology and medicine to 2PA fluorescent
microscopy while cellular imaging,4 photodynamic therapy,5
frequency upꢀconverted lasing,6 3D optical data storage,7 3D
microfabrication and lithography8 can be considered as the
currently most successful applications.
a
interaction results in formation of a new molecular orbital in
which facile electron excitation by visible light takes place.
Hence, pushꢀpull molecules are also called chromophores.9
Apart from
in linear Dꢀ
N
,
N
ꢀdialkylamino groups used as ordinary donors
π
ꢀA systems, triphenylamine (TPA) proved to be a
very useful and readily available precursor for the synthesis of
tripodal molecules. The central amino donor in TPA can easily
be connected to various peripheral acceptors to gain D(ꢀ
pushꢀpull systems. In fact, D(ꢀ ꢀA)3 systems are much
thoroughly investigated than inverse A(ꢀ
ꢀD)3 molecules10
πꢀA)3
π
π
which is most likely due to their tedious and limited synthesis
compared to possible modifications of the parent TPA (Fig. 1).
A joint feature of the most successful organic molecules is their
pushꢀpull and mostly tripodal (branched, starꢀshaped or
octupolar) arrangement. In a pushꢀpull Dꢀ
πꢀA molecule (D =
Fig. 1 Two principal arrangements of tripodal push-pull molecules.
electron donor, = conjugated system and A = electron
π
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Fig. 2 Molecular structure of all target chromophores 1-12
The vast majority of tripodal molecules have been built on TPA derivative, preparation of the acceptor moieties and their final
and, since late 90’s, an admirable number of articles reporting combination via crossꢀcoupling reaction. Scheme 1 and 2
their preparation and 2PA activity has been published.11 These shows the preparation of the particular Dꢀ and Aꢀparts 13
ꢀ
16
D(ꢀπꢀA)3 molecules possess peripheral electron withdrawing and 23ꢀ33, respectively. Threefold Sonogashira crossꢀcoupling
groups such as nitro, ester, carbonyl, trifluoromethylsulfonyl of commercially available tris(4ꢀiodophenyl)amine with an
etc. as well as (hetero)aromatic moieties such as fluorene, excess of trimethylsilylacetylene (TMSA) and subsequent TMS
perylene diimide, dimesitylboryl, pyridine/pyridinium, triazole, group removal by tetrabutylammonium fluoride (TBAF)
oxadiazole, benzothiazole, benzothiadiazole and porphyrine. afforded smoothly tris(4ꢀethynylphenyl)amine 14 in 76%
Some dendritic molecules have recently appeared as well.12 overall yield.23c A similar threefold Sonogashira reaction using
Further applications of such TPAꢀderived pushꢀpull molecules propargyl alcohol afforded triole 15 which upon oxidation with
range from 3PA,13 organic solar cells,14 molecular glasses,15 DessꢀMartin reagent provided trialdehyde 16. The synthesis of
intercalates and metalꢀorganic frameworks (MOF)16 to sensors 16 can also be accomplished via Sonogashira reaction of 13
used for detection of ions,17 amines18 and explosives.19 In with 3,3ꢀdiethoxypropyn affording triacetal 34 (90 %).
contrast to linear Dꢀπ
ꢀA systems,9,20 less tripodal compounds Subsequent removal of the acetal protecting group afforded 16
bearing peripheral cyanoꢀsubstituted acceptor moieties were in 61% yield. However, the overall yield is generally lower than
prepared and investigated as 2PAꢀactive substances to date.21 that attained via the reaction path shown in Scheme 1 (see the
Beside their exceptional 2PA activity, few cyanoꢀsubstituted ESI for more details). The synthesis of diꢀ and
TPA molecules were also utilized in solar cells.22
tricyanosubstitued benzenes 23ꢀ25 (Scheme 2) has been
As only few comprehensive structureꢀproperty relationship accomplished in a modular manner starting from diꢀ and
studies on tripodal TPA molecules were reported to date23 and trimethylbromobenzenes which were oxidized to corresponding
as an extension of our recent efforts in the field of property acids 17
tuning in pushꢀpull molecules,9a we report herein on the design, with thionyl chloride and saturated with gaseous NH3 to afford
synthesis and further studies of twelve model D(ꢀ ꢀA)3 amides 20 22. This conversion for 19 was accompanied by a
molecules 12 (Fig. 2). These chromophores possess either halogen replacement and, therefore, chlorotriamide 22 was
one or two cyano groups connected to each particular TPA isolated instead of the expected bromotriamide. Final
branch and vary in the arrangement, extension and composition dehydratation to nitriles 23 25 was carried out by trifluoroacetic
of the ꢀlinker. The impact of such structural changes on the anhydride (TFAA) in dioxane/pyridine. Other precursors of the
properties of 12 were studied by means of differential electron withdrawing moieties in 12 are also shown in the
scanning calorimetry, electrochemistry, one and two photon Scheme 2. They are either commercially available (26 28 30
absorption spectroscopy and were also supported by DFT and 32) or can easily be synthesized via procedures reported in
ꢀ19 using KMnO4 in tBuOH and subsequently treated
π
ꢀ
1
ꢀ
ꢀ
π
1
ꢀ
1ꢀ
ꢀ
,
calculations.
the literature
dicarbonitrile 33 was synthesized via
(
29 and 31).24ꢀ25 2ꢀIodothiopheneꢀ3,4ꢀ
selective
a
monoiodination of thiopheneꢀ3,4ꢀdicarbonitrile which was
achieved by lithiation at ꢀ78 °C and subsequent reaction with
molecular iodine.26
Results and Discussion
Synthesis
In general, the synthesis of chromophores 1ꢀ12 involves three
reaction steps: synthesis of the properly substituted TPA
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Scheme 1 Modification of the TPA core leading to precursors 13-16
Scheme 2 Structure and selected synthesis of cyano-substituted acceptor moieties 23-33
Scheme 3 Combination of A- and D-parts leading to tripodal push-pull molecules 1-12
The synthetic details and spectral characterization of precursors subsequent Beckmann rearrangement.28 Chromophore
2
was
and intermediates 13
ꢀ
25 are given in the ESI. Target tripodal prepared by HeckꢀMizoroki crossꢀcoupling reaction of 13 and
pushꢀpull molecules
1
ꢀ
12 were obtained by a combination of acrylonitrile 26 with a similar outcome as described by Tian et
ꢀlinker in chromophore has been generated by
the aforementioned Aꢀ and Dꢀprecursors, mainly in terms of al.29 Biphenyl
π
3
crossꢀcoupling reactions as shown in Scheme 3. Threefold SuzukiꢀMiyaura reaction between 13 and boronic acid 27 (76
cyanation27 of triiodotriphenylamine proved to be tedious and, %).
therefore, chromophore
1 was synthesized from trialdehyde 35
(Ref.4g) by oneꢀpot reaction with hydroxylamine and
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Table 1 Summarized properties of chromophores 1ꢀ12
A
F
ΦF
(%)
0.62
Comp. Tm
Td
E1/2(ox1) E1/2(red1)
EHOMO
ELUMO
Stokes shift
(cmꢀ1)
ꢁ2PA (GM)g
ꢀE
λmax
λmax
(°C)a
(°C)b
(V)c
1.49
1.12
1.02
1.04
0.85
1.41
1.29
0.97
1.16
1.15
1.09
1.17
(V)c
(eV)d
(eV)d
(V)c
(nm/eV)f (nm/eV)f
1
2
339/3.66
394/3.15
372/3.33
387/3.20
403/3.08
453/2.74
470/2.64
413/3.00
420/2.95
414/3.00
397/3.12
412/3.01
377/3.29
460/2.70
448/2.77
458/2.71
510/2.43
549/2.26
611/2.03
497/2.49
533/2.33
499/2.48
475/2.61
516/2.40
2970
ꢀ
343
275
329
217
291
145
84
ꢀ
–
2.01
1.74
2.02
1.85
1.80
ꢀd
3.50
2.86
3.04
2.89
2.65
ꢀ
–
–
–
–
–
–
–
–
–
–
–
–
5.92
5.55
5.45
5.47
5.28
5.84
5.72
5.40
5.59
5.58
5.52
5.60
–
–
–
–
–
2.42
2.69
2.41
2.58
2.63
ꢀ
0.48
0.64
0.59
0.51
0.29
0.29
0.59
0.69
0.55
0.46
0.49
3640
4560
4010
5210
3860
4910
4090
5050
4126
4140
4890
260 (770)
566 (760)
785 (780)
1100 (810)
757 (830)
667 (810)
620 (810)
852 (810)
631 (810)
148 (750)
375 (830)
317
340
ꢀ
–
–
–
–
3
4
5
477
322
176
366
333
282
240
252
6
ꢀd
ꢀ
7
ꢀ
8
188
327
ꢀ
–
–
–
–
–
1.66
1.55
1.44
1.45
1.41
2.63
2.71
2.59
2.54
2.58
–
–
–
–
–
2.77
2.88
2.99
2.98
3.02
9
10
11
12
191
198
a Tm = melting point (the point of intersection of a baseline before the thermal effect with a tangent). b Td = thermal decomposition (pyrolysis in N2 atmosphere). c E1/2(ox1) and E1/2(red1)
d
are halfꢀwave potentials of the first oxidation and reduction, respectively; all potentials are given vs. SCE; ꢀE = E1/2(ox1) ꢀ E1/2(red1)
.
Electrode surface inhibition. e –EHOMO/LUMO
=
E1/2(ox1/red1) + 4.429 (Ref.32). f Measured in THF; g The wavelength of the maximum 2PA cross section is shown in parenthesis (measured in THF).
A Sonogashira reaction of 13 and terminal acetylene 28
provided chromophore
by Ogilby et al. (69 vs. 94 %).5d The synthesis of chromophore
has been described by Cho et al. via HornerꢀWadsworthꢀ
Emmons reaction.21b However, the SuzukiꢀMiyaura crossꢀ
coupling on 13 using boronic acid pinacol ester 29 delivered
in a slightly improved yield of 76 %. Al2O3ꢀcatalyzed
Knoevenagel condensation30 of aldehydes 35
16 and
malononitrile afforded smoothly target chromophores and
with dicyanovinyl (DCV) moieties in the yields of 42 and 94 %.
Chromophore has also been previously synthesized by the
pyridine/acetic acidꢀcatalyzed condensation in similar
yield.11q,31 Target chromophore
bearing peripheral
4 in generally lower yield than reported
5
5
/
6
7
6
a
8
Fig. 3 Representative DSC curves of compounds
scanning rate of 3 °C/min under N2
2
and
9
determined with a
dicyanoimidazole (DCI) acceptor moieties was prepared by
HeckꢀMizoroki olefination of 13 and vinazene 31. Although
carried out under optimized conditions (dioxane, elevated
temperature and Pd(0) precatalyst), Sonogashira reaction of
triacetylene 14 and halogenated dinitriles 23 and 24 afforded
The measured melting points of
The temperature of decomposition was estimated within the
range of 176ꢀ477 °C. Compound provided very sharp peak of
1ꢀ12 range from 84 to 343 °C.
1
chromophores
9 and 10 only in modest yields. This reaction
melting/crystallization at 343/263 °C and do not undergo
decomposition until its boiling point (436 °C). Thermal
was complicated by predominant formation of oneꢀ and twoꢀ
fold crossꢀcoupling products which were tedious to separate
behaviour of compounds
with the melting point peak followed by a subsequent
decomposition at a higher temperature. Compound showed
2, 3, 5ꢀ9 and 11ꢀ12 is very similar
and to suppress even in case of an excess of 23 and 24
.
Moreover, similar Sonogashira reaction with trinitrile 25
proved to be unsuccessful at all, which is most likely caused by
the presence of chloro instead of bromo substituents (see
above). Crossꢀcoupling reactions with thiophene analogues 32
and 33 afforded chromophores 11 and 12 in similar modest
yields of 25 and 16 %, respectively.
4
wide enantiotropic solidꢀsolid transition (37ꢀ82 °C), melting
point at 217 °C and surprisingly no decomposition peak.
However, the sample was clearly carbonized after heated up to
500 °C. For compound 10, thermal decomposition preceded the
melting point. As can be seen from the whole series (Table 1),
DCVꢀterminated tripodal compounds
6 and 7 showed the
Thermal properties
lowest melting points and temperatures of thermal
Thermal behaviour of compounds
differential scanning calorimetry (DSC). Figure 3 shows
thermograms of representative compounds and while Table
lists the measured melting temperatures Tm and
1ꢀ12 was studied by
decompositions.
cyanoaromates
TPA
and
derivatives
9ꢀ10 resisted thermal decomposition
substituted
with
3
ꢀ
5
2
9
above 300 °C. Heterocyclic acceptor moieties such as DCI and
(di)cyanothiophene as in and 11 12 brings also noticeable
1
(
)
8
ꢀ
temperatures of thermal decompositions (Td).
thermal stability up to 366 °C, especially for the imidazoleꢀ
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terminated compound
compounds 12 reflects their different molecular structure and planarization of the
(varied peripheral acceptors and ꢀlinker). The following subunits ( vs. and ) lowered the electrochemical gap up to
relations can be deduced from the DSC measurements: 2.65 V. Dicyanoꢀsubstituted (hetero)aromatic moieties attached
The melting temperatures within the most evaluated to TPA core affected both the first oxidation and the reduction
subseries of compounds are mostly affected by an potentials ( 12, Table 1). The evaluation of the ICT based on
insertion of multiple bonds. In general, chromophores the overall electrochemical gap implies that these
bearing triple bond proved less stable than the (hetero)aromates impart stronger resonance effect than
corresponding olefinic derivatives (e.g. vs. and aromates ( of 2.71ꢀ2.54 vs. 3.50ꢀ2.65 V for vs. 12).
vs. ). Positioning of the CN groups along the (hetero)aromatic
This can be further demonstrated by the DCV acceptor scaffold plays also significant role as can be seen for instance
linked to the TPA core via acetylenic spacer in on chromophores and 10. The latter, 2,4ꢀdicyanoꢀsubstituted
which lowered the thermal stability very significantly chromophore showed a reduced gap by 0.12 V. The lowest
when compared to Tm = 61 °C, Td = 146 °C). electrochemical gap ( = 2.58 V) was measured for
Chromophores bearing benzeneꢀderived acceptors chromophore 11 bearing cyanoꢀacceptor based on polarizable
(e.g. or 10) are generally more thermally stable thiophene with the cyano group attached at the resonant and
than the corresponding thiophene derivatives (e.g. 11 most extended C5 position (as compared to dicyano acceptor in
8
.
The measured DSC data of comparison to aromatic 1,4ꢀphenyle moiety.33 Further extension
ꢀlinker by acetylenic and olefinic
1
ꢀ
π
π
3
4
5
•
1
ꢀ
5
8ꢀ
ꢀ
E
4
3
5
ꢀ
E
1
ꢀ
5
8ꢀ
3
•
•
•
7
,
9
6
(
ꢀ
ꢀ
ꢀE
4
9/
and 12).
Chromophore
12). A comparison of chromophores
4
and 11 also allows
ꢀsystems.34
8
with the imidazoleꢀbased acceptor comparison of 1,4ꢀphenylene and 2,5ꢀthienylene π
(DCI) showed the highest thermal resistance among With very similar oxidation potentials (1.04 and 1.09 V), the
all heterocyclic compounds (
°C.
8, 11ꢀ12) with Td = 366 later showed significantly lowered first reduction potential
(–1.45 vs. –1.85 V) as well as the electrochemical gap (
2.89 vs. 2.54 V).
ꢀE =
Electrochemistry
Electrochemical measurements of chromophores
One photon absorption and emission
1ꢀ12 were
carried out in acetonitrile containing 0.1 M Bu4NPF6 in a three Target chromophores are coloured molecules, their colours
electrode cell by cyclic voltammetry (CV), rotating disk range from yellowish to red (see the ESI). Optical properties of
voltammetry (RDV) and polarography. The working electrode
was platinum disk (2 mm in diameter) for CV and RDV spectra; the longestꢀwavelength absorption/emission maxima
experiments. A saturated calomel electrode (SCE) separated by λmax
A λmaxF, the fluorescence quantum yields F and the Stokes
a bridge filled with a supporting electrolyte and a Pt wire were shifts are summarized in Table 1. Figure 4 shows absorption
used as reference and auxiliary electrodes. The acquired data and emission spectra of representative chromophores 7-9 and
are summarized in Table 1, while representative CV diagrams 11
of compounds and 11 are given in the ESI.
The measured halfꢀwave potentials of the first oxidation and
reduction (E1/2(ox1) and E1/2(red1)) of chromophores 12 were
1ꢀ12 were investigated by electronic absorption and emission
/
Φ
5
,
.
5, 7ꢀ9
1
ꢀ
found within the range of 0.85 to 1.49 and –1.41 to –2.02 V,
respectively. The first reduction and oxidation are oneꢀelectron
processes, followed by subsequent oxidations and reductions,
and are obviously a function of the peripheral acceptor and the
π
ꢀlinker (Table 1). Whereas the first oxidation can be attributed
to the central TPA donor, the first reduction is mostly localized
on the peripheral CN groups and the adjacent
first oxidation potentials of chromophores
π
ꢀsystem. The
bearing
1ꢀ5
monocyanoꢀsubstituted acceptor moieties steadily decreased
from 1.49 to 0.85 V as a function of the ꢀsystem elongation.
The first reduction potentials showed similar trends with
exception for in which nonplanar biphenyl ꢀlinker allows
only reduced ICT. The difference between the first oxidation
and reduction potentials (electrochemical gap, ) represents a
straightforward way for the evaluation of the ICT. When going
from = 3.50 V) to = 2.86 V), the electrochemical
gap decreased by 0.64 V, as a consequence of the insertion of
an additional olefinic subunit. However, further ꢀsystem
extension as in resulted in an increased gap of 3.04 V. This
π
3
π
ꢀ
E
Fig.
chromophores
4
Representative absorption (left) and fluorescence (right) spectra of
and 11 measured in THF
5, 7-9
1
(
ꢀ
E
2 (ꢀE
Whereas the absorption maxima range from 339 to 453 nm, the
emission maxima are redꢀshifted to 377ꢀ611 nm. The
absorption spectra are dominated by one intense CTꢀband
π
3
demonstrates “ICT transparency” of the olefinic unit in
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whose position is clearly dependent on the nature of the Figure 5. For a better comparison of the 2PA properties, target
appended cyanoꢀacceptors and the ꢀsystem. When considering chromophores 12 were split in four groups according to the
the monocyanoꢀsubstituted series of molecules peripheral acceptor moieties. Whereas the first group includes
values showed very similar trend as seen for the chromophores with monocyano acceptors, chromophores
electrochemical gaps . Namely, extension of the ꢀsystem by and in the second group possess DCV acceptors. Third and
olefinic ( vs. ) or aromatic units ( vs. 2), planarization ( fourth group of molecules consist of dicyano aromatic (
vs. ) and further elongation by acetylenic subunit ( ) affected 10) and (di)cyano heteroaromatic acceptor moieties ( 11
π
1ꢀ
A
1ꢀ5, the λmax
a
2
ꢀ
5
6
ꢀ
E
π
7
1
2
3
4
/5
9
and
3
4
8
,
ꢀ
12),
the absorption maxima within the range of 339 to 403 nm. It respectively. In all cases, good 2PA properties were observed
should be noted that the longestꢀwavelength absorption maxima with the 2PA crossꢀsection exceeding 1000 GM for compound
of
1
ꢀ
12 correlate very tightly with the measured
(see the ESI). Chromophores and 7
5
. In general, chromophores
4 and 5 with monocyano acceptors
electrochemical gap
ꢀ
E
6
displayed larger 2PA cross section than the other tripodal
bearing strong electron DCV acceptors showed the most molecules in the whole series. Moreover, these two
A
bathochromically shifted CTꢀbands with λmax at 453 and 470 chromophores also exhibited good fluorescence quantum yields
nm, respectively. In contrast to 1ꢀ5, chromophores 8ꢀ10 and 12 (>0.50). The 2PA results are summarized in Table 1.
exhibited redꢀshifted absorption maxima up to 412ꢀ420 as a
result of the appended dicyanoꢀsubstituted (hetero)aromatic
acceptor moieties. However, the differences in λmax are just
within the range of 8 nm, which implies that, in contrast to
electrochemical gap, the central (hetero)aromatic πꢀsystem and
1200
1000
800
600
400
200
0
800
700
600
500
400
300
200
100
0
2
5
3
11
4
6
7
(b)
(a)
A
positioning of the CN groups do not affect the absorption
properties very significantly.
740
760
780
800
820
840
(c)
860
740
760
780
800
820
840
860
The fluorescence spectra exhibited one single band with the
Wavelength (nm)
Wavelength (nm)
F
λmax appearing at 377ꢀ611 nm. In general, its position is
700
600
500
400
300
200
100
0
8
800
600
400
200
0
11
12
affected by the same structural features as the aforementioned
absorption bands. Both positions of the longestꢀwavelength
absorption and fluorescence maxima showed tight correlations
(see the ESI). The quantum yields lie within the range of 0.29
to 0.69. The lowest quantum yields were measured for
(d)
9
10
chromophores
moieties. The weaker (di)cyano acceptors in
6
and
7
with strongly electron withdrawing DCV
and 12 lead
740
760
780
800
820
840
860
740
760
780
800
820
840
860
Wavelength (nm)
Wavelength (nm)
1
ꢀ
5
8ꢀ
generally to blueꢀshifted emission with high quantum yield.
This is in accordance with our previous observations on linear
and Xꢀshaped pushꢀpull molecules feat. strong/weak
ICT.33a,20g,h
Fig. 5. Two-photon absorption cross-sections, δ2PA, of (a)
and (c) and 10 and (d) 11 12. All compounds have been measured in THF
solutions at 10-4 M concentration.
2, 3, 4, 5 and 11 (b) 6
7
9
8
,
,
Figure 5a compares the 2PA spectra of monoꢀcyano terminated
chromophores and 11. Compound showed negligible 2PA
crossꢀsections within the investigated spectral area. It was
observed that by increasing the length of the ꢀlinker, i.e
passing from to and , the 2PA cross sections increased. In
addition, it is also obvious that upon changing the acetylenic
πꢀconjugated bridge by an olefinic one ( vs. ), the 2PA crossꢀ
sections significantly increased up to 1100 GM (at 810 nm) for
while this is 785 GM (at 780 nm) for . Since the
fluorescence quantum yields of and are similar (Table 1),
the 2PA action crossꢀsection product Φ×δTPA is again higher for
than for . This renders molecule as a promising candidate
for upꢀconverted lasing or twoꢀphoton imaging. A replacement
of the 1,4ꢀphenylene moiety in with 2,5ꢀthienylene one (11),
Two photon absorption
2
ꢀ
5
1
The 2PA properties of tripodal chromophores 1ꢀ12 were studied
π
by means of twoꢀphoton excited fluorescence spectroscopy
within the spectral range from 750 to 850 nm. The advantage of
this method is that low concentrations are used and that the
order of the nonꢀlinear effect is monitored by detecting the
intensity of the emitted fluorescence as a function of excitation
power. In the case of twoꢀphoton excitation, the emitted
fluorescence is dependent on the square of the excitation laser
power. This was checked in all experiments to confirm the 2PA
process. The excitation power was in all cases kept below 20
mW in order to minimize thermal effects and in most cases the
2PA crossꢀsections were measured within the range of 10ꢀ20
mW. The samples were 10ꢀ4 M solutions of the dyes in THF.
The contribution of the solvent was taken into account by
subtracting the intensity of scattered light, obtained by a cuvette
containing the solvent only, from the measurements. The twoꢀ
photon absorption crossꢀsections were determined by using
Rhodamine B in methanol (10ꢀ4 M concentration), as reference.
The 2PA spectra of all compounds, i.e the 2PA crossꢀsections
(δ2PA) as a function of excitation wavelength, are shown in
2
4
5
4
5
5
4
4
5
5
4
5
4
led to more than sevenꢀtimes decrease of the 2PA cross sections
(Table 1, 1100 vs. 148 GM). Hence, heteroaromatic polarizable
moiety based on thiophene in 11 did not bring expected higher
2PA activity. Another conclusion drawn from Figure 5a is that
the peaks of the 2PA spectra appear at approximately twice the
wavelengths of the one photon absorption spectra. This is in
accordance with the twoꢀphoton allowed states for octupolar
molecules. Specifically, the 2PA spectrum of
4 is blueꢀshifted
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J. Name., 2012, 00, 1-3 | 6

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Journal of Materials Chemistry C
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DOI: 10.1039/C5TC01293G
compared to
in the case of their oneꢀphoton absorption spectra.
The 2PA spectra of chromophores and bearing the strongest
DCV acceptor are shown in Figure 5b. Chromophore
exhibited a large 2PA crossꢀsection of 760 GM at 830 nm. The spatial and electronic properties of the parent TPA and
Interestingly, an extension of the πꢀconjugated path by an chromophores 12 were investigated using Gaussian W09
additional acetylene subunit as in
did not lead to enhanced package35 at the DFT level. The initial geometries of molecules
2PA crossꢀsections. The fluorescence quantum yields of and 12 were estimated by PM3 method implemented in
are relatively low making the Φ×δ2PA values significantly ArgusLab36 and these were subsequently optimized by DFT
smaller than those of compounds and . However, it should B3LYP/6ꢀ311++G(2d,p) method. The energies of the HOMO
be noted that, according to their oneꢀphoton absorption spectra, and LUMO, their differences and ground state dipole moments
the maximum of the 2PA spectra of and is expected at were calculated with the DFT B3LYP/6ꢀ311++G(2d,p) and are
wavelengths close to 900 nm, which is not covered by our summarized in Table 2.
experiment. On the other hand, chromophore with DCV
acceptor exhibited much larger 2PA crossꢀsection than
structurally related , which possesses only one cyano group.
Figure 5c presents the 2PA crossꢀsections of and 10 bearing
5
while that of
3
is blue shifted compared to
2
as triꢀbranched compounds with D(ꢀπꢀA)3 arrangement and similar
molecular weight.11b,11h
6
7
Quantum chemical calculations
6
1
ꢀ
7
6
7
1ꢀ
4
5
6
7
6
Table 2 DFT calculated propertiesa of TPA and chromophores 1ꢀ12
Comp. Symmetry
group
EHOMO
ELUMO
2
ꢀE
ꢁ
(eV)a
(eV)a
(eV)
(D)
9
TPA
1
D3
D3
C3
D3
D3
C3
C3
C3
C3
D3
C3
C3
C3
–
–
–
–
–
–
–
–
–
–
–
–
–
5.29
6.53
6.14
5.86
5.80
5.62
6.76
6.49
6.30
5.78
6.05
5.81
6.07
–
–
–
–
–
–
–
–
–
–
–
–
–
0.90
2.53
2.90
2.39
2.67
2.75
3.80
3.81
3.01
2.84
3.17
2.82
3.13
4.39
4.00
3.24
3.47
3.13
2.87
2.94
2.68
3.29
2.94
2.88
2.99
2.94
0.024
0.000
0.227
0.002
0.001
0.221
5.675
7.199
7.603
0.035
6.034
5.199
11.914
dicyano aromatic acceptors with different orientation of the
appended CN groups. The position of the cyano groups played
2
significant role as
larger 2PA crossꢀsections compared to 10. Moreover, the
fluorescence quantum yield is higher for (0.69) compared to
10 (0.55), which provides a larger Φ×δ2PA value. A
comparison of the structurally related chromophores 10 and
with diꢀ and monocyano aromatic acceptor moieties, reveals
that the second cyano group has broadened and redꢀshifted the
2PA spectra. This is in accordance with one photon absorption
spectra, but, on the other hand, it also causes a decrease of the
2PA response.
9 exhibited an approximately 1.35ꢀtimes
3
4
9
5
9
6
4
,
7
8
9
10
11
12
Finally, the 2PA crossꢀsections of chromophores 8, 11 and 12
a Calculated at the DFT B3LYP/6ꢀ311++G(2d,p) level.
with (di)cyanoꢀheteroaromatic acceptors are compared in
Figure 5d. Chromophores 11 and 12 with thiophene acceptors
exhibited relatively small 2PA crossꢀsection values compared
The calculated energies of the HOMO/LUMO of
1ꢀ12 range
from 6.76/ 3.81 to 5.62/ 2.39 eV and are obviously a function
–
–
–
–
to their analogues
4 and 10 having a phenyl ring. Thus, a
of the structure of the particular chromophore. Both energies
correlate tightly with the experimental data obtained by the
electrochemical measurements (Table 1). Hence, the used DFT
method can be considered as a reasonable tool for the
replacement of 1,4ꢀphenylene by 2,5ꢀthienylene moieties
coupled either to cyano or dicyano groups, is a not useful
strategy towards increasing the 2PA activity in TPAꢀderived
tripodal chromophores. This is in contrast to general trends
observed for second order NLO chromophores.34 However, an
addition of the second cyano group into 11 leads to 12 with
more than threeꢀtimes higher 2PA crossꢀsection without
affecting the florescence quantum yield. This makes the
thiophene substituted chromophores 11 and 12 extremely
sensitive to the number of cyano groups. On the other hand,
description of electronic properties of
going from parent TPA to molecule
1
ꢀ
12 (see the ESI). When
1
, the HOMOꢀLUMO gap
decreased from 4.39 to 4.00 eV as a result of attaching three
cyano groups and generating a pushꢀpull system. Further
extension of the
πꢀsystem within the series 1ꢀ5 affected the
HOMO and LUMO levels as well as the gap in very similar
way as observed by the electrochemistry. The lowest HOMOꢀ
2PA crossꢀsections measured for the analogous chromophores
and 10 with 1,4ꢀphenylene parent moieties showed less
sensitivity. Chromophore bearing three peripheral DCI
acceptor units exhibited a relative large 2PA crossꢀsection of
620 GM at 810 nm accompanied by a good quantum yield of
0.59. Although, a straightforward comparison of the TPA
properties of the herein studied compounds with literature data
is not easy because of various experimental approaches used
(sample concentration, solvent used, pulse duration, laser
4
LUMO gap within this subseries has been calculated for
bearing cyanoꢀsubstituted styryl linker. Further attachment of
an additional CN group ( vs. and 10) resulted in drop of the
calculated gap from 3.13 to 2.94 and 2.88 eV. DCV acceptors
in chromophores impart also strong ICT and reduce the
gaps up to 2.94 and 2.68 eV, respectively. A replacement of the
terminal 1,4ꢀphenylene moiety in by 2,5ꢀthienylene one in 11
slightly decreases the HOMOꢀLUMO gap from 3.13 to 2.99 eV
in a similar way as observed by CV measurements. In general,
both experimental and calculated HOMOꢀLUMO gaps correlate
tightly as shown in the ESI.
5
8
4
9
6ꢀ7
4
power, spectral range etc.), chromophores
4 and 5 are found to
exhibit similar or better TPA activity compared with other
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J. Name., 2012, 00, 1-3 | 7

Journal of Materials Chemistry C
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Journal Name
ARTICLE
DOI: 10.1039/C5TC01293G
Fine tuning of the properties has been achieved either by
modulation of the acceptors as well as by the systematic
elongation of the πꢀsystem. The synthesis has been carried out
in a modular manner by connecting donor and acceptor parts
via crossꢀcoupling reactions. Target chromophores were further
studied by differential scanning calorimetry, electrochemistry,
one and two photon absorption and emission spectra and also
quantum chemical calculations. Thorough structureꢀproperty
relationships were subsequently evaluated.
Compounds 1ꢀ12 showed melting point and subsequent thermal
decomposition within the range of 84ꢀ343 and 176ꢀ477 °C.
These values indicate relatively high thermal resistance of TPA
derived tripodal molecules.
The electrochemical properties (HOMO and LUMO levels and
gap) were mainly affected by the type of the peripheral
acceptor, length and composition of the πꢀlinker as well as its
planarity. Using these structural changes as a tool, the
electrochemical HOMOꢀLUMO gap could be finely tuned by
almost 1 V (3.50 to 2.54 V).
One photon absorption and emission properties studied by
electronic absorption/emission spectra revealed significant
CTꢀbands appearing between 339/377 and 470/611 nm. Hence,
optical tuning could be carried out within the range of 130/230
nm. Quantum yields ranging from 0.29ꢀ0.69 were measured for
1
ꢀ
12
.
The two photon absorption properties of
1ꢀ12 were measured
by twoꢀphoton excited fluorescence spectroscopy within the
spectral range 750 to 850 nm. A critical assessment has been
carried out within/across the particular series of chromophores
in order to evaluate structureꢀproperty relationships. Extension
Fig. 6. HOMO/HOMO-1 (red) and LUMO/LUMO+1 (blue) localizations in
HOMO/LUMO mix in 7-9 and 11.
5 and
of the
polarizable olefinic units led to a significant increase of 2PA
activity. Hence, chromophore bearing cyano substituted styryl
linker showed the most enhanced NLO activity within the
whole series. An attachment of aromatic acceptors bearing two
πꢀsystem and replacement of acetylenic by more
The calculated ground state dipole moments are almost zero for
molecules having D3 group of symmetry, whereas noticeable
5
values were obtained for molecules
6ꢀ8 and 10ꢀ11 having
branched and unsymmetrical acceptor moieties (DCV, DCI,
2,4ꢀdicyanobenzene and 5(2,4)ꢀ(di)cyanothiophene).
The HOMO and LUMO localizations in representative
CN groups (
units in and
compared to
9
and 10) as well as strongly withdrawing DCV
did not lead to increased δTPA coefficients
. A similar conclusion can be made for
6
7
5
chromophores
5, 7ꢀ9 and 11 are shown in Figure 6. For
complete listing see the ESI. As expected, the HOMO is
localized on the central amino donor and adjacent alternating
positions of the TPA, whereas the LUMO is spread over the
heterocyclic cyano acceptors such as DCI and 5(2,4)ꢀ
(di)cyanothiophene.
The DFT calculate HOMO and LUMO levels and their
differences correlates tightly with the linear properties
measured by electrochemistry and absorption/emission spectra.
peripheral cyano acceptor and the adjacent
shows a clear charge separation and thus confirm the ICT
character of chromophores 12. As can be seen, the LUMO is
πꢀlinker. Figure 6
1
ꢀ
Considering all the aforementioned structural features of 1ꢀ12,
predominantly localized over one or two particular branche(s),
while the third branch is occupied by the LUMO+1. The
HOMO as well the HOMOꢀ1 remained on the central amino
donor. This is a common feature of tripodal molecules based on
TPA.23bꢀc,29
we believe that this work could be treated as a useful guide in
designing/tuning tripodal pushꢀpull molecules with central
donor and peripheral acceptors.
Experimental
Conclusions
Materials and methods
A series of twelve tripodal D(ꢀπꢀA)3 molecules based on
triphenylamine has been designed and synthesized. The
investigated pushꢀpull systems possess one central amino donor
connected via a πꢀlinker to three peripheral cyano acceptors.
For detailed description of used materials and methods used in
this work see the ESI. Synthesis and full spectral
characterization of target chromophores
Two general methods were used for the synthesis of
chromophores and 12
1ꢀ5 is given in the ESI.
6
ꢀ
7
9
ꢀ
.
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J. Name., 2012, 00, 1-3 | 8

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Journal Name
Journal of Materials Chemistry C
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DOI: 10.1039/C5TC01293G
General method for Knoevenagel condensation (chromophores 6 CH=CH). 13CꢀNMR: δC (100 MHz; (CD2Cl2) 33.2, 109.1,
and 7)
109.5, 112.8, 113.3, 122.8, 125.0, 129.5, 130.9, 139.7, 148.6,
151.3. HRꢀMALDIꢀMS (DCTB): calcd for C42H27N13 (M+)
713.2507 found 713.2526.
Triphenylamineꢀ4,4’,4’’ꢀtriscarboxaldyde 35 (165 mg, 0.5
mmol) or aldehyde 16 (201 mg, 0.5 mmol) and malononitrile
30 (150 mg, 2.27 mmol) were dissolved in DCM (20 ml) and
alumina (930 mg, 8.9 mmol, Brockmann activity IIꢀIII) was
added. The reaction was stirred for 4h and monitored by TLC
(SiO2, DCM/EtOAc 10:1). Anhydrous Na2SO4 (350 mg, 2.46
mmol) can eventually be added to accelerate the reaction. The
resulting coloured suspension was filtered and the filtrate was
concentrated in vacuo. The crude products were purified by
column chromatography (SiO2, DCM/EtOAc 10:1 or DCM) to
General method of Sonogashira cross-coupling (chromophores
9–12)
Tris(4ꢀethynylphenyl)amine 14 (317 mg, 1.00 mmol) and the
corresponding halogen derivative 23, 24, 32 or 33 (3.6 mmol)
were dissolved in dry 1,4ꢀdioxane (40 ml) and triethylamine (10
ml, 71.8 mmol). Argon was bubbled through the mixture for 10
min, whereupon Pd(PPh3)4 (116 mg, 0.10 mmol) and CuI (38
mg, 0.2 mmol) were added and the reaction was heated to 92 °C
for 8 h. The reaction mixture was quenched with saturated
solution of NH4Cl (150 ml) and extracted with DCM (2 × 150
ml). The combined organic extracts were dried over anhydrous
Na2SO4 and concentrated in vacuo. The crude product was
purified by column chromatography (SiO2, EtOAc/Hexane 2:3
afford
6 or 7.
Chromophore 6
The title compound was synthesized from 35 and 30 following
the general method for Knoevenagel condensation. Reddishꢀ
orange solid. Yield (122 mg, 42 %). Rf 0.3 (SiO2; DCM/EtOAc
10:1) Mp 145 °C. IR (HATR)
1559, 1496, 1269, 1186, 827. HꢀNMR:
7.25 (6 H, d, 3J 8.8, CHAr), 7.73 (3H, s, CH=C), 7.90 (6 H, d, 3J
8.8, CHAr). 13CꢀNMR: δC (100 MHz; CD2Cl2) 81.6, 113.6,
114.7, 125.4, 128.2, 133.3, 150.7, 158.5. HRꢀMALDIꢀMS
(DHB): calcd for C30H15N7 (M+) 473.1384 found 473.1385.
or EtOAc/Hexane 3:4) to afford target chromophores 9–12.
ν
max/cmꢀ1 3087, 2220 (CN),
H (400 MHz; CD2Cl2)
1
δ
Chromophore 9
The title compound was synthesized from 14 and 23 following
the general method for Sonogashira crossꢀcoupling. Dark
yellow solid. Yield (204 mg, 29 %). Rf 0.35 (SiO2; 2:3). Mp
327 °C (dec.). IR (HATR), ν
max/cmꢀ1 2921, 2206 (CN), 1593,
Chromophore 7
1504, 1320, 1289, 1145, 836, 801, 736. 1HꢀNMR: δH (400
MHz; CD2Cl2) 7.19 (6 H, d, 3J 8.8, CHAr), 7.52 (3 H, t, 3J 8.0,
CHAr), 7.64 (6 H, d, 3J 8.8, CHAr), 7.90 (6 H, d, 3J 8.0, CHAr).
13CꢀNMR: δC (100 MHz; CD2Cl2) 83.4, 103.5, 116.2, 116.5,
124.5, 128.3, 131.2, 134.3, 136.4 148.1, 149.5. HRꢀMALDIꢀ
MS (DHB): calcd for C48H21N7 (M+) 695.1853 found 695.1852.
The title compound was synthesized from 16 and 30 following
the general method for Knoevenagel condensation. Dark red
solid. Yield 96 mg (94%). Rf = 0.35 (SiO2; CH2Cl2), M.p. 84
°
C. IR (HATR),
ν
max/cmꢀ1 3010, 2216 (CN), 1539, 1495, 1316,
1
1266, 1176, 1026, 832. HꢀNMR:
δ
H (400 MHz; CD2Cl2) 7.14
(6H, d, 3J 9.0, CHAr), 7.36 (3H, s, CH=C), 7.56 (6H, d, 3J 9.0,
CHAr). 13CꢀNMR: δC NMR (100 MHz; CD2Cl2): 86.2, 93.0,
111.4, 112.47, 114.9, 115.6, 124.5, 135.0, 140.8, 148.5. HRꢀ
MALDIꢀMS (DHB): calcd for C36H15N7 (M+) 545.1384 found
545.1386.
Chromophore 10
The title compound was synthesized from 14 and 24 following
the general method for Sonogashira crossꢀcoupling. Orange
solid. Yield (144 mg, 21 %). Rf 0.4 (SiO2; EtOAc/Hexane 2:3).
Mp 282 °C (dec.). IR (HATR),
1593, 1503, 1320, 1277, 1145, 836, 800, 735. 1HꢀNMR:
ν
max/cmꢀ1 2922, 2204 (CN),
H (400
Chromophore 8
δ
MHz; CD2Cl2) 7.24 (6 H, d, 3J 8.8, CHAr), 7.66 (6 H, d, 3J 8.8,
CHAr), 7.81 (3 H, d, 3J 8.4, CHAr), 7.92 (3 H, dd, 3J 8.4, 4J 1.2,
CHAr), 8.04 (3 H, d, 4J 1.2, CHAr). 13CꢀNMR: δC (100 MHz;
(CD2Cl2) 85.7, 101.1, 112.5, 116.4, 116.7, 116.8, 117.2, 124.9,
132.0, 133.2, 134.3, 136.1, 136.5, 148.4. HRꢀMALDIꢀMS
(DHB): calcd for C48H21N7 (M+) 695.1853 found 695.1872.
Triiodotriphenylamine 13 (300 mg, 0.48 mmol) and 1ꢀmethylꢀ
2ꢀvinylꢀ1Hꢀimidazoleꢀ4,5ꢀdicarbonitrile 31 (236 mg, 1.49
mmol) were dissolved in dry DMF (5 ml) and DIPEA (1 ml ,
5.75 mmol). Argon was bubbled through the mixture for 10
min, whereupon Pd(PtBu)2 (20 mg, 0.04 mmol) was added and
the reaction mixture was heated to 85 °C for 8 h. The reaction
mixture was poured into saturated NH4Cl solution (100 ml) and
was extracted with DCM (2 × 100 ml). The combined organic
extracts were washed with brine (150 ml) and water (100 ml),
dried over anhydrous Na2SO4 and concentrated in vacuo. The
crude product was purified by column chromatography (SiO2,
Chromophore 11
The title compound was synthesized from 14 and 32 following
the general method for Sonogashira crossꢀcoupling. Orange
solid. Yield (160 mg, 25 %). Rf 0.35 (SiO2; EtOAc/Hexane
3:4). Mp 191 °C. IR (HATR), ν
max/cmꢀ1 2925, 2208 (CN), 1593,
EtOAc/Hexane 1:3) to afford
8 as orange solid. Yield (144 mg,
1504, 1320, 1291, 834, 808, 737. 1HꢀNMR: δH (400 MHz;
CD2Cl2) 7.18 (6 H, d, 3J 8.8, CHAr), 7.31 (3 H, d, 3J 4.0, Th),
7.55 (6 H, d, 3J 8.8, CHAr), 7.62 (3 H, d, 3J 4.0, Th). 13CꢀNMR:
42 %). Rf 0.4 (SiO2; EtOAc/Hexane 1:3). Mp 188 °C. IR
(HATR),
1179, 972, 818. HꢀNMR:
CH3), 6.77 (6 H, d, 3J 16.0, CH=CH), 7.16 (6 H, d, 3J 8.5,
CHAr), 7.54 (6 H, d, 3J 8.5, CHAr), 7.81 (6 H, d, 3J 16.0,
ν
max/cmꢀ1 2294, 2231 (CN), 1592, 1506, 1324, 1285,
1
δ
H (400 MHz; CD2Cl2) 3.83 (9 H, s,
δ
C (100 MHz; CD2Cl2) 81.2, 97.0, 110.4, 114.1, 117.1, 124.8,
131.2, 132.0, 133.6, 138.0, 147.9. HRꢀMALDIꢀMS (DHB):
calcd for C39H18N4S3 (M+) 638.0688 found 638.0705.
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 9

Journal of Materials Chemistry C
Page 10 of 13
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Journal Name
DOI: 10.1039/C5TC01293G
Chromophore 12
Wong, Chem. Commun., 2012, 48, 3442; (
Xu, K. P. Loh and Z.ꢀK. Chen, Org. Lett., 2009, 11, 1257; (
Wang, X. Zhang, J. Geng, K.. Li, D. Ding, K.ꢀY. Pu, L. Cai, Y.ꢀH.
Lai and B. Liu, Chem. Eur. J., 2012, 18, 9705; ( ) G. Wang, K.ꢀY.
Pu, X. Zhang, K. Li, L. Wang, L. Cai, D. Ding, Y.ꢀH. Lai and B. Liu,
Chem. Mater., 2011, 23, 4428; ( ) O. P. Varnavski, J. C. Ostrowski,
L. Sukhomlinova, R. J. Twieg, G. C. Bazan and T. Goodson III, J.
Am. Chem. Soc., 2002, 124, 1736; ( ) W. Denk, J. H. Strickler and
W. W. Webb, Science, 1990, 248, 73.
) F. Hammerer, G. Garcia, S. Chen, F. Poyer, S. Achelle, C.
FioriniꢀDebuisschert, M.ꢀP. TeuladeꢀFichou and P. Maillard, J. Org.
Chem., 2014, 79, 1406; ( ) S. Achelle, P. Couleaud, P. Baldeck, M.
P. TeuladeꢀFichou and P. Maillard, Eur. J. Org. Chem., 2011, 1271;
) M. GaryꢀBobo, Y. Mir, C. Rouxel, D. Brevet, I. Basile, M.
i
) X. Zhang, X. Ren, Q.ꢀH.
j) G.
The title compound was synthesized from 14 and 33 following
the general method for Sonogashira crossꢀcoupling. Dark
orange solid. Yield (111 mg, 16 %). Rf 0.25 (SiO2;
k
EtOAc/Hexane 3:4). Mp 198 °C (dec.). IR (HATR),
ν
max/cmꢀ1
1
l
2848, 2236 (CN), 1592, 1472, 1377, 1169, 831, 728, 718. Hꢀ
NMR: δ
H (500 MHz; CD2Cl2) 7.19 (6 H, d, 3J 9.0, CHAr), 7.59
m
(6 H, d, 3J 9.0, CHAr), 7.99 (3H, s, Th). 13CꢀNMR: δC (100
MHz; CD2Cl2) 78.2, 103.1, 111.8, 111.9, 112.3, 114.9, 115.8,
124.6, 133.7, 135.5, 136.6, 148.0. HRꢀMALDIꢀMS (DHB):
calcd for C42H27N13 (M+) 713.2507 found 713.2532.
5
(a
b
Acknowledgements
This research was supported by the Czech Science Foundation
(13ꢀ01061S).
(c
Maynadier, O. Vaillant, O. Mognin, M. BlanchardꢀDesce, A. Morère,
M. Garcia, J.ꢀO. Durand and L. Raehm, Angew. Chem. Int. Ed., 2011,
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