π-expanded α,β-unsaturated ketones: Synthesis, optical properties, and two-photon-induced polymerization

Article abstract of DOI:10.1002/cphc.201402646

A library of π-expanded α,β-unsaturated ketones was designed and synthesized. They were prepared by a combination of Wittig reaction, Sonogashira reaction, and aldol condensation. It was further demonstrated that the double aldol condensation can be performed effectively for highly polarized styrene- and diphenylacetylene-derived aldehydes. The strategic placement of two dialkylamino groups at the periphery of D-π-A-π-D molecules resulted in dyes with excellent solubility. These ketones absorb light in the region 400-550 nm. Many of them display strong solvatochromism so that the emission ranges from 530-580 nm in toluene to the near-IR region in benzonitrile. Ketones based on cyclobutanone as central moieties display very high fluorescence quantum yields in nonpolar solvents, which decrease drastically in polar media. Photophysical studies of these new functional dyes revealed that they possess an enhanced two-photon absorption cross section when compared with simpler ketone derivatives. Due to strong polarization of the resulting dyes, values of two-photon absorption cross sections on the level of 200-300 GM at 800 nm were achieved, and thanks to that as well as the presence of the keto group, these new two-photon initiators display excellent performance so that the operating region is 5-75 mW in some cases.

Full text of DOI:10.1002/cphc.201402646

CHEMPHYSCHEM
ARTICLES
DOI: 10.1002/cphc.201402646
p-Expanded a,b-Unsaturated Ketones: Synthesis, Optical
Properties, and Two-Photon-Induced Polymerization
[a]
[b]
[b]
[d]
[b]
ˇ
´
Rashid Nazir, Florent Bourquard, Evaldas Balciunas, Sabina Smolen, David Gray,
¯
Nikolai V. Tkachenko,*[c] Maria Farsari,*[b] and Daniel T. Gryko*[a, d]
A library of p-expanded a,b-unsaturated ketones was designed
and synthesized. They were prepared by a combination of
Wittig reaction, Sonogashira reaction, and aldol condensation.
It was further demonstrated that the double aldol condensa-
tion can be performed effectively for highly polarized styrene-
and diphenylacetylene-derived aldehydes. The strategic place-
ment of two dialkylamino groups at the periphery of D-p-A-p-
D molecules resulted in dyes with excellent solubility. These
ketones absorb light in the region 400–550 nm. Many of them
display strong solvatochromism so that the emission ranges
from 530–580 nm in toluene to the near-IR region in benzoni-
trile. Ketones based on cyclobutanone as central moieties dis-
play very high fluorescence quantum yields in nonpolar sol-
vents, which decrease drastically in polar media. Photophysical
studies of these new functional dyes revealed that they pos-
sess an enhanced two-photon absorption cross section when
compared with simpler ketone derivatives. Due to strong po-
larization of the resulting dyes, values of two-photon absorp-
tion cross sections on the level of 200–300 GM at 800 nm were
achieved, and thanks to that as well as the presence of the
keto group, these new two-photon initiators display excellent
performance so that the operating region is 5–75 mW in some
cases.
1. Introduction
The increase in human life expectancy has increased demand
for materials for tissue engineering and regenerative medicine.
New biomaterials have been developed through object-orient-
ed synthesis, blending, and modification that possess charac-
teristics tailor-made for the applications they are aimed to-
wards. One of the techniques used for the construction of bio-
medical implants and scaffolds for tissue engineering is direct
laser writing (DLW) by two-photon-induced polymerization
(TPIP). Since its first demonstration by Maruo in 1997, TPIP ma-
terial engineering has developed to a level at which it is now
possible to produce high-quality products of suitable design
by computer-aided design.[1–6]
In TPIP, the tightly focused laser beam is used for the gener-
ation of radicals through the two-photon absorption (2PA) of
a photoinitiator; these radicals induce the polymerization. The
photopolymerization voxel is minimized by tightly focusing
the laser beam using a high numerical aperture (NA) objective
lens, and reducing the laser power and exposure time.[7,8]
Though a significant number of compounds possessing high
2PA cross sections (s2) have been developed, the majority of
them have low solubility and/or a low yield of radical genera-
tion.[9–14] Consequently, the majority of TPIP research is still car-
ried out using commercially available radical photoinitiators,
usually with 2PA cross sections lower than 40 GM (GM=Goep-
pert–Mayer units), and hence they need large excitation
powers and high exposure times.[15] In addition, many of these
are compounds that are either toxic, or have components that
could cause long-term damage to humans. Novel generations
of photoinitiators tailored towards TPIP have been investigated
over the last decade.[16–20] Herein, we present our research into
photoinitiator molecules that at the same time possess large
s2, strong TPIP photoinitiation potential, and excellent solubili-
ty in monomers, another key factor that is often
overlooked.[16e]
[a] R. Nazir, Prof. Dr. D. T. Gryko
Warsaw University of Technology Faculty of Chemistry
Noakowskiego 3, 00-664 Warsaw (Poland)
E-mail: dtgryko@icho.edu.pl
ˇ
[b] Dr. F. Bourquard, E. Balciunas, Dr. D. Gray, Dr. M. Farsari
¯
Institute of Electronic Structure and Laser (IESL)
Foundation 50 for Research and Technology Hellas (FORTH)
711 10 Heraklion, Crete (Greece)
E-mail: mfarsari@iesl.forth.gr
[c] Prof. Dr. N. V. Tkachenko
Department of Chemistry and Bioengineering
Tampere University of Technology
33720 Tampere (Finland)
We focused on ketones, since carbonyl groups are simulta-
neously one of the classic elements in photoinitiated polymeri-
zation[21] and also very strong electron acceptors. In addition,
these molecules do not possess any moieties harmful to
humans; they rather resemble the widely used natural food
colorant turmeric. The aim of this study is to investigate sys-
tematically the effect of rigidity, size of p-conjugated systems,
E-mail: nikolai.tkachenko@tut.fi
´
[d] Dr. S. Smolen, Prof. Dr. D. T. Gryko
Institute of Organic Chemistry
Polish Academy of Sciences
Kasprzaka 44/52, Warsaw (Poland)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cphc.201402646.
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and type of linker on the photoinitiating activity of quasi-quad-
rupolar a,b-unsaturated ketones.
2. Results and Discussion
Knowing that quadrupolar-like molecules possessing the same
structural elements as dipolar molecules always have higher
s2,[14] we focused our research on quadrupolar a,b-unsaturated
ketones. To minimize conformational flexibility we selected
only cycloalkanones as central units of these dyes.
Examples of bis(arylidene)cycloalkanones having the general
formula D-p-A-p-D have already been reported and a number
of them possess large s2.[22,23] Needless to say, the size of the
middle ring has a major impact on the photochemical and
photophysical properties of bis(arylidene)cycloalkanones.[17,24]
The structural elements have been chosen in such a way to
achieve high diversity and at the same time to allow us to
cross-compare both optical properties and performance in
TPIP of these compounds. Three different cyclic ketones,
namely cyclobutanone (8), cyclopentanone (9), and 4-methyl-
cyclohexanone (10), were selected as key building blocks,
which allowed us to obtain dyes of different rigidity. As second
building blocks we have chosen D-p-A-type aldehydes differ-
ing in the type of linker. Altogether six a,b-unsaturated ke-
tones based on cycloalkanones were designed and
synthesized.
4-(Didodecylamino)benzaldehyde was synthesized from ani-
line following a procedure described in the literature.[25] The di-
alkylamino styrene 4 was prepared through a Wittig-type ap-
proach starting from N,N-didodecylaniline (1).[25] The prepared
phosphonium salt 2[26] was subjected to Wittig condensation
with terephthaldehyde diethyl acetal (3), followed by the re-
moval of the acetal group under mild acidic conditions[27]
(Scheme 1). Intermediate 5 (prepared in two steps from 4-io-
doaniline[28]) was reacted with aldehyde 6 under Sila–Sonoga-
shira reaction conditions[29] at room temperature to afford the
desired push–pull aldehyde 7 (Scheme 2). All a,b-unsaturated
ketones 11–16 were synthesized from 4-(didodecylamino)ben-
zaldehyde or aldehydes 4 and 7 through classical aldol con-
densation using commercially available 4-N,N-dimethylamino-
benzaldehyde or prepared aldehydes and the corresponding
cycloketones (Table 1). Except for dye 16, all double aldol con-
densations occurred smoothly and furnished final products at
a yield of 69–99% as orange-red low-melting solids. All six
dyes displayed excellent solubility in a majority of solvents.
Despite the strong structural similarity between ketones 11
and 12 (identical p system), the absorption spectra of these
compounds are different (Figure 1, Table 2). The absorption
maxima of 12, the compound bearing long n-alkyl chains
(ꢀC12H25), are redshifted relative to 11 (ꢀCH3) both in toluene
(587 cmꢀ1) and benzonitrile (611 cmꢀ1). On comparing com-
pound 12 with 13, it is well visible that p expansion of a,b-un-
saturated ketones sometimes does not lead to an increase in
their molar absorption coefficients (Figure 1). Higher absorptiv-
ity has been observed for dyes 14–16, which possess different
conformation being derivatives of more flexible cyclopenta-
none and 4-methylcyclohexanone (Figure 1). For compounds
Scheme 1. Synthesis of (E)-4-[4-(didodecylamino)styryl]benzaldehyde.
Scheme 2. Synthesis of 4-{[4-(dihexylamino)phenyl]ethynyl}benzaldehyde.
TMS=trimethylsilyl, TBAF=tetrabutylammonium fluoride.
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Table 1. Preparation of a,b-unsaturated ketones 11–16.
Aldehyde
Ar
Cyclic ketone
Unsaturated ketone
Yield [%]
99
8
8
11
12
13
93
87
4
4
8
9
14
91
4
7
10
9
15
16
69
29
Table 2. Photophysical properties of bis(arylidene)cycloalkanones 11–16
in toluene and benzonitrile.
Compound
Solvent
labs
lem
Dn
[cmꢀ1
]
Ffl
[nm]
[nm]
toluene
benzonitrile
toluene
benzonitrile
toluene
benzonitrile
toluene
benzonitrile
toluene
benzonitrile
toluene
476
488
477
503
467
488
466
487
444
460
440
458
530
591
533
591
583
705
556
764
540
742
536
793
2100
3600
2200
3100
4300
6300
3500
7400
4000
8300
4100
9200
0.16
0.36
0.19
0.29
0.74
<0.01
0.72
<0.01
0.40
<0.01
0.14
<0.01
11
12
13
14
15
16
Figure 1. Absorption spectra of 11–16 in toluene.
13–15, which possess extended p systems (styryl–benzylidene),
one can observe a correlation between the size of the alicyclic
middle ring and the position of the absorption band (Figure 1,
Table 2). Absorption maxima of 15 (cyclohexanone ring) are
significantly blueshifted (ꢁ1100 cmꢀ1) relative to 13 (cyclobu-
tanone ring) and 14 (cyclopentanone ring) both in toluene and
benzonitrile. These results are in agreement with the litera-
ture.[24] On comparing compounds 14 and 16, which possess
the same alicyclic ring (cyclopentanone), one can observe
a blueshift of absorption maximum for 16, the dye bearing
two CꢂC triple bonds. This is in full agreement with the work
presented by Meier,[30] who pointed out that C=C double
bonds are more polarizable than CꢂC triple bonds (which
benzonitrile
result in a decrease in the HOMO–LUMO gap). Furthermore, all
newly synthesized dyes 11–16 exhibit positive solvatochrom-
ism, thus suggesting a larger polarization in the excited state
than in the ground state.
There is no significant difference in the emission spectra of
11 and 12 (Figure 2, Table 2). For all compounds the Stokes
shift is larger in benzonitrile than in toluene, in agreement
with the assumption that these newly synthesized dyes exhibit
a greater dipole moment in the excited state than in the
ground state. A change in dipole moment requires solvent re-
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cases there is one dominating component. The lifetimes are
given in Table 3, together with the relative amplitudes of each
decay component with the first being the dominating one. In
the case of 11 and 12, the fluorescence lifetimes in benzoni-
trile are appreciably longer than in toluene. One reason for this
might be the significant difference in viscosity between benzo-
nitrile and toluene, which can result in limiting the nonradia-
tive decay mechanism.
The fluorescence lifetimes of 13–16 vary in a similar manner
to the fluorescence quantum yields: the lifetime shortens with
an increase in size of the alicyclic ring. For 13–16 in benzoni-
trile, a short-lived component, t=190–250 ps, was observed,
which indicates faster nonradiative decay of the excited state
compared to that of 11 and 12 in qualitative agreement with
the fluorescence quantum yields presented in Table 2.
For compounds 11–16, the transient absorption spectra
were measured with the pump–probe technique in toluene
and benzonitrile. In the case of 11 and 12 in both solvents, de-
composition of samples by laser beam was observed. For
other compounds reasonably good agreement between emis-
sion decays and transient absorption measurements was ob-
served. Fluorescence decay measurements for 13 in toluene
Figure 2. Normalized emission spectra of 11–16 in: a) toluene, b) benzoni-
trile.
organization, which makes the
Table 3. Fluorescence emission lifetimes obtained by TCSPC for 11–16 (lex =483 nm).
Stokes shift sensitive to the sol-
vent polarity. The Stokes shift
depends on the reorganization
energy and the reorganization
energy is greater in solvents
with greater dielectric constant.
The emission maxima of com-
pounds 13–15 are blueshifted,
as are their absorption spectra.
With an increase in the size of
the middle ring, for 13–15, com-
pounds possessing the same
p system, one can observe a de-
crease in fluorescence quantum
yield (Ffl). A larger ring provides
greater movement flexibility,
thus promoting nonradiative re-
Compound Solvent
t1
Relative ampli-
t2
Relative ampli-
t3
Relative ampli-
c2[a]
[ns] tude
[ns] tude
[ns] tude
toluene
0.29
0.97
0.72
0.83
0.82
1
0.82
0.22
0.68
0.35
–
0.03
0.28
0.17
0.18
–
–
–
–
–
1.07
1.10
1.17
1.18
1.09
1.04
1.05
1.09
1.09
1.23
0.83
1.00
11
benzonitrile 1.34
toluene 0.34
benzonitrile 1.27
toluene 1.93
12
13
14
15
16
–
–
benzonitrile 0.06 51
0.58 35
–
1.38 14
toluene
benzonitrile 0.19
toluene 0.68
benzonitrile 0.16
toluene 0.33
benzonitrile 0.25
1.35
1
–
0.91
0.73
0.99
0.72
0.99
1.40
0.96
1.92
1.03
1.10
0.09
0.27
0.01
0.28
0.01
–
–
–
–
–
–
[a] Data fit weighted mean square deviation.
laxation of electronically excited states. A significant redshift of
the emission maxima in benzonitrile in 13–16 may be due to
the formation of excited charge-transfer states. Furthermore,
their fluorescence quantum yields decrease significantly when
the solvent polarity increases, and do not exceed 1% (Table 1),
thus indicating that for 13–16 in benzonitrile the charge sepa-
ration is almost complete. The latter also explains an interest-
ing phenomenon observed on comparing the optical proper-
ties of ketones 11 and 12 with p-expanded ketones 13–16. On
moving from 11 and 12 to 13–16 one can observe a small
bathochromic shift of absorption;[30] at the same time there is
a strong bathochromic shift of emission depending strongly
on the solvent.
give a 1.93 ns lifetime for the singlet state; therefore, the tran-
sient absorption spectrum component with time constant of
2.2 ns can definitely be attributed to the decay of the singlet
state (Figure S1a in the Supporting Information). The triplet-
state spectrum (after 2.2 ns decay) is very weak. For 14 in tolu-
ene the singlet state decays with a time constant of 100 ps
and the long-lived state (after 100 ps decay) is the triplet state
(Figure S1b). Similarly to 14, the longest-lived state for 15 is
the triplet excited state, and the spectrum at 550 ps is the
spectrum of the thermally equilibrated excited singlet state
(Figure S1c). In the case of 16, the spectrum of the triplet state
has much lower relative intensity than that for 14 and 15. Oth-
erwise its behavior is similar to that of the previously men-
tioned compounds. Like before, the lifetimes of the singlet
state obtained from transient absorption and fluorescence
The fluorescence lifetimes for 11–16 were obtained after
biexponential fitting of the emission decays, though in most
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one has to remember that the maximum of the 2PA can be lo-
cated at different values from 800 nm for two reasons. First of
all, dyes 11–16 differ in the size of their p-conjugated system.
Secondly, although all studied molecules are symmetrical (C2)
they are not centrosymmetrical. Consequently g!c and g!
e transitions should both be observable in the 2PA spec-
trum.[31] As a result, two-photon spectra of these dyes have to
possess two bands, and the band that could be ascribed to
the S0–S1 transition is most probably more intense than the
higher-energy S0–S2 transition.[31]
Table 4. Comparison of excited singlet-state lifetimes obtained by
pump–probe system (tp-p) and fluorescence emission (tem) measurements.
Compound
Solvent
Singlet tp-p
Singlet tem Triplet/singlet[a]
[ps]
[ps]
13
toluene
2200ꢃ340
(320ꢃ75)
730ꢃ220
1930
ꢁ0.1
benzonitrile
1110
0
(100ꢃ25) 190
880ꢃ60 1350
150ꢃ15 190
550ꢃ30 680
310ꢃ30 330
195ꢃ12 250
14
toluene
benzonitrile
toluene
toluene
benzonitrile
0.3
0
0.5
0.15
0
15
16
The highest s2 value (at 800 nm) has been measured for dye
16, which possesses cyclopentanone as a central unit and a
CꢂC bond as linker.
[a] Triplet/singlet is calculated as the ratio of the maximum of triplet-state
differential absorption in the red–near-IR part of the spectrum to that of
the singlet after fast (<50 ps) relaxation.
2.2. Fabrication Properties
The one-photon potential of the synthesized photoinitiators
was investigated by polymerizing drop-cast films of the materi-
al under a wide-spectrum UV lamp. The fabrication window
(FW) is the power difference between the polymerization
threshold and the burning threshold. Having a large FW is ben-
eficial, as it allows the use of higher powers and indicates the
possibility of using the material at industrial scales.
emission measurements agree well with each other, in this
case, 310 and 330 ps, respectively (Figure S1d, Table 4).
The spectra of 13–16 in benzonitrile vary considerably from
those measured in toluene. For all samples there is no detecta-
ble trace of triplet state formed after singlet-state relaxation. In
the case of 14, the singlet excited state decays with a time
constant of 150 ps, which is in good agreement with fluores-
cence lifetime measurements of 190 ps (Figure S2b). For 15 in
benzonitrile, the negative absorbance in the 750–1000 nm
range is definitely due to the stimulated emission, since there
is no ground-state absorption in this range (Figure S2c). There-
fore the process with 130 ps time constant is a decay of the
singlet excited state. The emission decay time constant ob-
tained from time-correlated single photon counting (TCSPC)
measurements is 160 ps, which also agrees well with the
130 ps time constant and its interpretation as the singlet excit-
ed-state lifetime (Table 4). No indication of triplet state is ob-
served for 16 in benzonitrile (Figure S2d). The singlet-state life-
times obtained from fluorescence and absorption decay meas-
urements are in reasonably good agreement at 250 and
200 ps, respectively (Table 4).
The threshold value, damage threshold, and FW of the syn-
thesized compounds are presented in Figure 3. As reference,
we used 4,4’-bis(diethylamino)benzophenone, a commercially
available photoinitiator used widely in TPIP.
2.1. Two-Photon Absorption
Figure 3. 2PA FW values of the compounds 11–16 and R (4,4’-bis(diethyl-
amino)benzophenone). The low-power end shows polymerization thresholds
(red and green interface), whereas the burning threshold is represented in
the high-power end of the scale.
Two-photon absorption cross sections were measured through
the Z-scan technique at 800 nm (Table 5). The values are gener-
ally in the range 50–370 GM, that is, significantly higher than
in the case of 4,4’-bis(diethylamino)benzophenone (R), though
Compounds 11–16 (1%, w/w) were added to the hybrid ma-
terial and the photopolymerization thresholds were deter-
mined by polymerizing structures at different laser powers.
The upper limit of the polymerization window corresponds to
material being burned. The overall performance in TPIP can be
evaluated by two factors: 1) the increase of 2PA cross section;
and 2) photofragmentation of ketones, which is the key step
since it affects the production of radicals. It is clearly visible
that compounds 14 and 16 possess the broadest FW
(Figure 3). p Expansion improves the polymerization threshold
from about 15 mW to approximately 5 mW (14–16 versus 11).
Table 5. 2PA cross section values of the compounds 11–16 measured by
Z-scan at 800 nm.
Compound
Solvent
Concentration [molLꢀ1
]
s2 [GM]
11
12
13
14
15
16
R
CH3Cl3
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
1-propanol
0.05
0.01
0.01
0.01
0.01
0.01
0.25
70
140
50
130
200
370
7
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For the derivative of 4-methylcyclohexanone 15, the burning
threshold is the lowest, which may originate from the lower
yield of photofragmentation into radicals.
3. Conclusions
A synthetic route to novel two-photon photoinitiators has
been presented. We have demonstrated that p-expanded a,b-
unsaturated ketones with D-p-A-p-D structure possess large in-
trinsic 2PA in the near-IR range of wavelengths, with maximum
values of s2 ꢁ370 GM. The introduction of peripheral dialkyl-
amino groups proved to be an excellent strategy to achieve at
the same time strong charge-transfer character and good solu-
bility of p-expanded ketones. As a result, a very broad FW has
been achieved in DLW (5–75 mW), which compares favorably
with that of previous TPIP initiators. The other notable findings
are as follows: 1) the p expansion of quadrupolar a,b-unsatu-
rated ketones typically leads to only small changes in both
molar absorption coefficients and position of the absorption
maxima (Figure 1); 2) the fluorescence quantum yield of these
solvatochromic dyes depends strongly on the polarity of the
solvent; and 3) the ketone possessing the highest 2PA cross
section is the best overall performer, which emphasizes the
critical influence of the 2PA cross section.
To explore the effectiveness of the synthesized TPIP photo-
initiators, 2D and 3D grid structures (Figure 4) were fabricated
by DLW using different laser powers. Figure 4A shows a 3D
Experimental Section
Materials and Synthesis
All commercially available compounds were used as purchased.
The reaction progress was monitored by thin-layer chromatogra-
phy (TLC) with silica gel 60 F254 (Merck) with detection by UV
lamp. N,N-Didodecylaniline[25] (1), 4-(didodecylamino)benzalde-
hyde,[25] N,N-dihexyl-4-iodoaniline,[32] and N,N-dihexyl-4-[(trimethyl-
silyl)ethynyl]aniline (5)[33] were obtained by the reported methods.
N,N-Didodecyl-4-[(iodotriphenylphosphoranyl)methyl]aniline (2): A
mixture of N,N-didodecylaniline (4.71 g, 11 mmol), triphenylphos-
phine (2.88 g, 11 mmol), paraformaldehyde (0.33 g, 3.7 mmol), po-
tassium iodide (1.65 g, 11 mmol), water (0.75 mL), and acetic acid
(2.22 mL) in toluene (30 mL) was heated at reflux for 7 h. After ad-
dition of water (40 mL), the resulting mixture was extracted with
CH2Cl2, and the combined organic layers were washed with aq
NaHCO3 and water and dried over Na2SO4. The solvent was evapo-
rated, and the residue was recrystallized with methanol and used
as such for the next step. HRMS (ESI): m/z: calcd for C49H72NP:
704.5324 [M+H]+; found: 704.5326.
Figure 4. Typical microstructures fabricated under three different conditions
using compound 16 as initiator: A) good structuring 3D, B) polymerization
threshold 3D, C) 3D structures under different laser power, D) good structur-
ing 2D, E) polymerization threshold 2D, F) 2D structures under different laser
power. UoC=University of Crete
(E)-4-[4-(Didodecylamino)styryl]benzaldehyde (4): Compound
2
(1.40 g, 2 mmol), terephthalaldehyde mono(diethylacetal) (3,
0.499 g, 2.40 mmol), and tBuOK (0.268 g, 2.40 mmol) were mixed in
dry CH2Cl2 (20 mL) at 08C under argon. The resulting mixture was
stirred at room temperature for 24 h, then the solvent was re-
moved under reduced pressure and the crude acetal was hydro-
lyzed using 10% HCl (25 mL) in CHCl3 (50 mL) at 258C for 1 h. The
two layers were separated and the organic layer was washed with
aq NaHCO3, dried over Na2SO4, and evaporated to dryness. The res-
idue was dissolved in CH2Cl2, a catalytic amount of iodine was
added, and this solution was stirred at 258C for 3 h under illumina-
tion (75 W lamp). The resulting mixture was washed with aq
Na2S2O3 and dried over Na2SO4. After evaporation of the solvent,
the crude product was purified by dry column vacuum chromatog-
raphy (DCVC;[34] silica, hexanes/CH2Cl2 1:1) to give 4 in the form of
a yellow solid with 79% yield. Physical properties corresponded
with the published data.[35]
structure obtained by using a laser power in the middle of the
FW; it can be seen that it is built accurately and has a well-de-
fined topography. Figure 4B presents a structure built by using
a laser power near the polymerization threshold. As the materi-
al had not polymerized fully, the structure could not survive
the development process and collapsed. Figure 4C shows 3D
structures under different laser powers. The laser power in-
creases from the left to the right. Figure 4D and 4E show 2D
structures built using powers in the middle of the FW and at
the polymerization threshold, respectively. Again, the structure
fabricated at the polymerization threshold is distorted due to
incomplete polymerization. Figure 4F presents 2D structures
under different laser powers increasing from the left to the
right.
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4-{[4-(Dihexylamino)phenyl]ethynyl}benzaldehyde (7): A stirred so-
lution of 6 (370 mg, 2.0 mmol), 5 (714 mg, 2.0 mmol), PdCl2(PPh3)2
(100 mg, 0.1 mmol), and CuI (20 mg, 0.1 mmol) in triethylamine
(10 mL) and tetrahydrofuran (10 mL) was flushed with argon for
15 min at room temperature. Then tetrabutylammonium fluoride
(2.5 mL, 1m in THF, 2.5 mmol) was added dropwise and the result-
ing solution was stirred overnight at room temperature. Subse-
quently, the reaction mixture was extracted with ethyl acetate and
water (50 mL each), and the combined organic layers were washed
with water and brine and dried over Na2SO4. Purification by DCVC
(silica, dichloromethane/hexanes 1:2 to 1:1) gave the desired com-
pound 7 as an orange oil (719 mg, 92%). Physical properties corre-
sponded with the published data.[28]
(2E,6E)-2,6-Bis{4-[(E)-4-(didodecylamino)styryl]benzylidene}-4-
methylcyclohexanone (15): Yield 69%. Purification by DCVC (silica,
CH2Cl2/hexanes 2:1) afforded the product as a red gel. 1H NMR
(500 MHz, CDCl3): d=7.78 (s, 2H), 7.50–7.37 (m, 12H), 7.11 (d, J=
16 Hz, 2H) 6.89 (d, J=16 Hz, 2H), 6.62 (d, J=9 Hz, 4H), 3.29 (t, J=
15 Hz, 8H), 3.12–3.09 (m, 2H) 2.57–2.52 (m, 2H), 1.91–1.89 (m, 1H),
1.60 (m, 8H), 1.32–1.22 (m, 72H), 1.11 (d, J=6.5 Hz, 3H), 0.89 ppm
(t, J=14 Hz, 12H); 13C NMR (125 MHz, CDCl3) d=190.0, 148.1,
139.0, 137.0, 134.8, 134.2, 131.1, 130.2, 128.1, 125.9, 124.2, 122.9,
111.7, 51.1, 36.8, 32.0, 29.8, 29.76, 29.75, 29.67, 29.49, 29.3, 27.45,
27.30, 22.8, 21.8, 14.2 ppm; HRMS (ESI): m/z: calcd for C85H131N2O:
1196.0261 [M+H]+; found: 1196.0255.
(2E,5E)-2,5-Bis(4-{[4-(dihexylamino)phenyl]ethynyl}benzylidene)-
cyclopentanone (16): Purification by DCVC (silica, CH2Cl2/hexanes
2:1) afforded the product as a red gel with 29% yield. 1H NMR
(500 MHz, CDCl3): d=7.57–7.52 (m, 10H), 7.38 (d, J=9 Hz, 4H),
6.58 (d, J=9.5 Hz, 4H), 3.29 (t, J=15 Hz, 8H), 3.14 (s, 4H), 1.60–
1.57 (m, 8H), 1.33–1.32 (m, 24H), 0.92 ppm (t, J=13.5 Hz, 12H);
13C NMR (125 MHz, CDCl3) d=196.1, 148.3, 137.5, 134.7, 133.4,
133.1, 131.5, 130.7, 125.6, 111.3, 108.4, 93.9, 87.5, 51.1, 31.9, 27.3,
26.9, 26.7, 22.8, 14.1 ppm; HRMS (ESI): m/z: calcd for C59H75N2O:
827.5879 [M+H]+; found: 827.5886.
General Procedure for Synthesis of a,b-Unsaturated Ketones
11–16
A mixture of aromatic aldehyde (1 mmol), cyclic ketone (0.5 mmol),
and KOH (56 mg, 1 mmol) in ethanol (5 mL) was stirred at 408C for
8 h (the reaction progress was monitored by TLC analysis). Then,
solvent was removed and the crude product was purified by re-
crystallization or by column chromatography.
(2E,4E)-2,4-Bis[4-(dimethylamino)benzylidene]cyclobutanone (11):
Resin Preparation
Yield 99%. The product was purified by crystallization from abso-
lute ethanol to afford 11 as red crystals. H NMR (500 MHz, CDCl3):
d=7.48 (d, J=9 Hz, 4H), 7.14 (s, 2H), 6.72 (d, J=8.5 Hz, 4H), 3.73
(s, 2H), 3.04 ppm (s, 12H); 13C NMR (125 MHz, CDCl3) d=190.7,
151.3, 141.3, 131.6, 127.1, 123.2, 112.1, 40.3, 35.2 ppm; HRMS (ESI):
m/z: calcd for C22H25N2O: 332.1889 [M+H]+; found: 332.1894.
The resin used in this work was an organic–inorganic zirconium sili-
cate described in detail earlier.[12] For these experiments, it was pre-
pared as described previously.[19]
1
Photophysics
(2E,4E)-2,4-Bis[4-(didodecylamino)benzylidene]cyclobutanone (12):
All photophysical studies were conducted at room temperature in
spectrometric-grade solvents: toluene with dielectric constant e=
2.38 (238C) and benzonitrile with e=25.9 (208C).[36] Steady-state
absorption spectra of 11–16 were recorded in a 1 cm quartz cuv-
ette by using a Shimadzu UV-3600 spectrophotometer in the 300–
800 nm wavelength region. Absorption and fluorescence measure-
ments for compounds 11–16 were performed at a dye concentra-
tion of 0.01–0.05 molLꢀ1 depending on the dye. Steady-state emis-
sion spectra were recorded using a Horiba Jobin Yvon Fluorolog-3-
111 fluorescence spectrophotometer. The samples were excited at
the first absorption maximum. The fluorescence quantum yields of
11–16 were calculated using fluorescein (Ff =0.79 in ethanol) as
a standard.[37] For each sample, refractive index corrections were
taken into account. Fluorescence lifetimes in the nanosecond time-
scale were determined by using a time-correlated single photon
counting (TCSPC) method (PicoQuant GmbH, including the Pico-
Harp 300 controller, LDH-P-C-485 diode laser, and PDL 800-B
driver). Samples were excited near the absorption maximum
(483 nm) and monitored at the emission maximum. The excitation
repetition rate was 10 MHz and a typical time resolution was
150 ps. The transient absorption spectra for 11–16 were measured
with a pump–probe system, with a time resolution of approximate-
ly 100 fs. The pump wavelength was 420 nm. Absorption was
monitored in both 470–760 and 840–1060 nm wavelength ranges.
Yield 93%. Purification by DCVC (silica, CH2Cl2/hexanes 1:1) provid-
1
ed the product as a red gel. H NMR (500 MHz, CDCl3): d=7.45 (d,
J=8.5 Hz, 4H), 7.12 (s, 2H), 6.63 (d, J=8.5 Hz, 4H), 3.72 (s, 2H),
3.32 (t, J=14.5 Hz, 8H), 1.61–160 (m, 8H), 1.32–1.26 (m, 72H),
0.89 ppm (t, J=14 Hz, 12H); 13C NMR (125 MHz, CDCl3) d=190.7,
149.3, 141.1 131.8, 129.7, 124.3, 111.6, 51.2, 45.1, 35.2, 32.0, 29.79,
29.75, 29.6, 29.4, 27.4, 27.2, 26.9, 22.8, 14.2 ppm; HRMS (ESI): m/z:
calcd for C66H112N2ONa: 971.8679 [M+Na]+; found: 971.8672.
(2E,4E)-2,4-Bis{4-[(E)-4-(didodecylamino)styryl]benzylidene}cyclobu-
tanone (13): Yield 87%. Purification by DCVC (silica, CH2Cl2/hexanes
1
2:1) afforded the product as a red gel. H NMR (500 MHz, CDCl3):
d=7.55–7.50 (m, 8H), 7.41 (d, J=9 Hz, 4H), 7.23 (d, J=4.5 Hz, 2H),
7.14 (d, J=16.5 Hz, 2H), 6.90 (d, J=17 Hz, 2H), 6.63 (d, J=9 Hz,
4H), 3.88 (s, 2H), 3.30 (t, J=15 Hz, 8H), 1.59–1.56 (m, 8H), 1.32–
1.27 (m, 72H), 0.90 ppm (t, J=13.5 Hz, 12H); 13C NMR (125 MHz,
CDCl3) d=189.8, 148.0, 138.8, 136.9, 134.6, 134.1, 131.0, 130.1,
128.0, 125.7, 124.1, 122.7, 111.5, 51.0, 36.7, 31.9, 29.67, 29.64, 29.62,
29.54, 29.36, 27.3, 27.1, 22.7, 21.7, 14.1 ppm; HRMS (ESI): m/z: calcd
for C82H125N2O: 1153.9792 [M+H]+; found: 1153.9788.
(2E,5E)-2,5-Bis{4-[(E)-4-(didodecylamino)styryl]benzylidene}cyclopen-
tanone (14): Purification by DCVC (silica, CH2Cl2/hexanes 2:1) af-
1
forded the product as a red gel with 91% yield. H NMR (500 MHz,
CDCl3): d=7.59–7.51 (m, 10H), 7.40 (d, J=9, 4H), 7.10 (d, J=
16.5 Hz, 2H), 6.90 (d, J=16 Hz, 2H), 6.63 (d, J=9.5 Hz, 4H), 3.30 (t,
J=15 Hz, 8H), 3.15 (s, 4H), 1.59–1.53 (m, 8H), 1.32–1.27 (m, 72H),
0.901 ppm (t, J=14 Hz, 12H); 13C NMR (125 MHz, CDCl3) d=196.3,
148.2, 139.7, 136.8, 134.2, 133.6, 131.4, 130.6, 128.2, 126.3,
124.2,122.7, 111.7, 51.2, 32.0, 29.8, 29.78, 29.76, 29.6, 29.57, 29.50,
27.4, 27.3, 26.8, 22.8, 14.2 ppm; HRMS (ESI): m/z: calcd for
C83H127N2O: 1167.9948 [M+H]+; found: 1167.9935.
Z-Scan Technique
The experimental setup to measure 2PA cross sections using the Z-
scan technique was described earlier;[19b] however, the laser pulse
duration used was shorter (60 fs instead of 250 fs; see the Sup-
porting Information for details). Z-scan measurements for com-
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Acknowledgements
The research leading to these results has received funding from
the European Community’s Seventh Framework Marie Curie ITN
programs TopBio (PITN-GA-2010-264362) and AngioMatTrain
(PITN-GA-2012- 317304).
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Keywords: aldol reaction
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Received: September 18, 2014
Revised: November 17, 2014
Published online on && &&, 2014
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ARTICLES
ˇ
R. Nazir, F. Bourquard, E. Balciunas,
Expansion benefits: Photoinitiators for
two-photon-induced polymerization,
which comprise p-expanded a,b-unsa-
turated ketones with D-p-A-p-D struc-
ture, possess large intrinsic two-photon
absorption cross sections (s2) in the
near-IR range, strong solvatochromism,
good solubility, and a broad fabrication
window (FW; see figure).
¯
´
S. Smolen, D. Gray, N. V. Tkachenko,*
M. Farsari,* D. T. Gryko*
&& – &&
p-Expanded a,b-Unsaturated Ketones:
Synthesis, Optical Properties, and
Two-Photon-Induced Polymerization
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers!