Effect of bridging position on the two-photon polymerization initiating efficiencies of novel coumarin/benzylidene cyclopentanone dyes

Article abstract of DOI:10.1021/jp909745q

One- and two-photon photophysical and photochemical properties of dyes 3-DAC and 4-DAC containing coumarin and benzylidene cyclopentanone moieties were studied. Their ground state configurations were optimized using both Hartree-Fock and density functional theory (B3LYP functional) methods, and excited-state properties were calculated using time-dependent density functional theory. These two dyes share the same formula and possess similar structures, except for different bridging positions between the benzylidene cyclopentanone and coumarin moieties. The bridging position was found to have a significant effect on the electronic structure and photophysical and photochemical properties of the dyes. 3-DAC shows higher conjugation and is more planar than 4-DAC, and it exhibits a larger TPA cross section. In contrast, 4-DAC has a twisted conformation, exhibits a lower electron transfer free energy with initiator and shows higher sensitizing efficiencies in one-photon polymerization. Furthermore, the application potential of 3-DAC and 4-DAC in two-photon polymerization (TPP) was studied. Both dyes could be used directly as initiators in TPP. The TPP threshold energies of the corresponding resins were as low as the best reported results. High-resolution 2D and 3D nanopatterns containing low amounts of small molecule residue were successfully fabricated by TPP, demonstrating the extensive application prospects of these dyes in the fabrication of micromachines, microsensor arrays and biomedical devices.

Full text of DOI:10.1021/jp909745q

J. Phys. Chem. A 2010, 114, 5171–5179
5171
Effect of Bridging Position on the Two-Photon Polymerization Initiating Efficiencies of
Novel Coumarin/Benzylidene Cyclopentanone Dyes
Jianqiang Xue, Yuxia Zhao, and Feipeng Wu*
Technical Institute of Physics and Chemistry, CAS, Beijing 100190, P.R. China
De-Cai Fang
College of Chemistry, Beijing Normal UniVersity, Beijing 100875, P.R.China
ReceiVed: October 11, 2009; ReVised Manuscript ReceiVed: February 2, 2010
One- and two-photon photophysical and photochemical properties of dyes 3-DAC and 4-DAC containing
coumarin and benzylidene cyclopentanone moieties were studied. Their ground state configurations were
optimized using both Hartree-Fock and density functional theory (B3LYP functional) methods, and excited-
state properties were calculated using time-dependent density functional theory. These two dyes share the
same formula and possess similar structures, except for different bridging positions between the benzylidene
cyclopentanone and coumarin moieties. The bridging position was found to have a significant effect on the
electronic structure and photophysical and photochemical properties of the dyes. 3-DAC shows higher
conjugation and is more planar than 4-DAC, and it exhibits a larger TPA cross section. In contrast, 4-DAC
has a twisted conformation, exhibits a lower electron transfer free energy with initiator and shows higher
sensitizing efficiencies in one-photon polymerization. Furthermore, the application potential of 3-DAC and
4-DAC in two-photon polymerization (TPP) was studied. Both dyes could be used directly as initiators in
TPP. The TPP threshold energies of the corresponding resins were as low as the best reported results. High-
resolution 2D and 3D nanopatterns containing low amounts of small molecule residue were successfully
fabricated by TPP, demonstrating the extensive application prospects of these dyes in the fabrication of
micromachines, microsensor arrays and biomedical devices.
Introduction
Two-photon polymerization (TPP) has attracted much atten-
tion in recent years as a powerful tool for realizing high-density
optical data storage1,2 and three-dimensional microfabrication
of functional devices.3–5 To improve the performance of TPP,
initiators with large two-photon absorption (TPA) cross sections
and high initiating efficiencies are required.
Coumarin is a robust chromophore that is used in a broad
range of applications such as laser dyes,6–8 fluorescent probes,9,10
and photosensitizers.11–13 Our research focuses on its application
as photosensitizer in one-photon polymerization (OPP)14,15 and
TPP.16–19 Because the coumarin ring mainly absorbs in the UV
region, usually a conjugated group is introduced onto the
coumarin ring to extend its absorption into the visible region
for obtaining high performance photosensitizers.14,16,17 We have
shown that both 3-substituted18 and 4-substituted19 coumarin
derivatives exhibit high sensitizing efficiency. Such compounds
also exhibit large TPA cross sections for their strong excited
state charge transfer characteristics. However, because their
substituents at the 7-position are different, the effect of the
bridging position on the properties of these compounds cannot
be directly studied by comparing these reported compounds. In
this study, a novel coumarin derivative containing a benzylidene
cyclopentanone substituent at the 4 position of the coumarin
ring (4-DAC) was synthesized based on a reported compound
3-DAC18 (shown in Figure 1). These two dyes have the same
formula and similar structures, except that the bridging positions
Figure 1. Chemical structures of 3-DAC and 4-DAC.
between the benzylidene cyclopentanone and coumarin moieties
differ. Their photophysical, photochemical, and electrochemical
properties were studied to understand the influence of bridging
position on the properties of coumarin dyes. Furthermore, the
ground state configurations and electronic spectra of both
3-DAC and 4-DAC were calculated under optimized conditions
usingquantumchemicalcalculationmethods.Thestructure-property
relationships of 3-DAC and 4-DAC were further revealed by
considering both the experimental data and theoretical calculations.
Experimental Section
Materials. 7-Diethylamino-4-methylcoumarin (Coumarin 1)
and 3-mercapto-4-methyl-1,2,4-triazole (MMT) were from
Avocado.
Tetra-n-butylammonium
hexafluorophosphate
* To whom correspondence should be addressed. Phone: +86 10
82543569. Fax: +86 10 82543491. E-mail: fpwu@mail.ipc.ac.cn.
(TBAPF6) (Acros Organics) was recrystallized before use. 2-[4-
10.1021/jp909745q  2010 American Chemical Society
Published on Web 03/29/2010

5172 J. Phys. Chem. A, Vol. 114, No. 15, 2010
Xue et al.
(Dimethylamino)-benzylidene]-cyclopentanone (DMA) and
3-DAC were prepared according to literature procedures.14,18
4-Dimethyldiphenyliodonium hexafluorophosphate (Omnicat
820) was from TH-UNIS Insight Co. Ltd. 2-Phenoxyethyl
acrylate (SR339), pentaerythritol triacrylate (SR444), and epoxy
acrylate (CN124A80) were from Sartomer Co. Ltd. and were
used as received. o-Cl-Hexaarylbisimidazole (HABI) was from
Tokyo Kasei Kogyo Co. Ltd. SeO2 was from Sinopharm
Chemical Reagent Co. Ltd. Fluorescein and other A.R. grade
reagents were from Beijing Chemical Reagent Company and
were used after purification by common methods.
parameters m1 and m2 can be determined by fitting ν˜a - ν˜f versus
f(ε, n) and ν˜a + ν˜f versus f(ε, n) + 2g(n), respectively.20–22
2n2 + 1 ε - 1
n2 - 1
n + 2
f(ε, n) )
-
(3)
(4)
2
2
[
]
ε + 2
n + 2
3
n4 - 1
g(n) )
2
2
[
]
2
(n + 2)
Assuming that the ground and excited states are parallel, their
dipole moments are expressed by eqs 5 and 6. Here, µg is the
ground state dipole moment, µe is the excited-state dipole
moment, and a is the Onsager radius of the solute, in which it
is assumed that the solute is in the shape of a sphere. The volume
of the solute could be calculated with the Gaussian 03 package,
using most of the calculation methods and basis sets, and thus
the radii a is very easily ready from such calculation.
7-Diethylamino-4-formylcoumarin. Coumarin 1 (4.62 g)
and SeO2 (3.3 g) were dissolved in xylene (120 mL). The
solution was protected with dry N2 and heated to reflux for 12 h.
The mixture was filtered while hot to remove selenium, and
then the solvent was evaporated under reduced pressure. The
residue was purified by column chromatography (CH2Cl2/
petroleum ether 10:1) and recrystallized (from CHCl3/hexane)
1
to give dark red needles (40%). H NMR (400 MHz, CDCl3)
δ: 1.22 (t, J 7.1 Hz, 6 H), 3.43 (q, J 7.1 Hz, 4 H), 6.45 (s, 1 H),
6.53 (s, 1 H), 6.63 (s, J 9.2 Hz 1 H), 8.30 (d, J 9.2 Hz, 1 H),
10.03 (s, 1 H).
m2 - m
1/2
1 hca3
µg )
µe )
(5)
(6)
[
]
2
2m1
4-DAC. 7-Diethylamino-4-formylcoumarin (0.37 g, 15 mmol)
and DMA (0.36 g, 16.5 mmol) were dissolved in toluene (35
mL) and protected under a dry N2 atmosphere. The mixture was
heated to reflux and then toluene-p-sulfonic acid (0.06 g) was
added. The mixture was stirred under refluxed for 30 h. After
cooling to room temperature, pyridine (4 drops) was added to
neutralize the mixture. The volatile components were removed
by distillation under reduced pressure. The crude product was
purified by column chromatography on silica gel (CH2Cl2/
m2 + m1 hca
1/2
3
[
]
2
2m1
The fluorescence quantum yields of 4-DAC in chloroform
were determined by using fluorescein (in 0.1 M aqueous NaOH)
as a standard (Φr ) 0.9); refractive index correction was
performed.23
1
petroleum ether 9:1) to give 0.33 g 4-DAC (50%). H NMR
TPA cross sections (δ) of the compounds in chloroform (2
× 10-4 M) were determined using the two-photon-excited
fluorescence (TPEF) technique with femtosecond laser pulses
following the experimental protocol described in detail by Xu
and Webb.24 The excitation light sources were a mode-locked
Tsunami Ti:sapphire laser (720-880 nm, 80 MHz, <130 fs). A
solution of fluorescein (10-4 M) in 0.1 M aqueous NaOH was
used as the reference. To avoid any contribution from other
photophysical/photochemical processes, the intensity of the input
pulses were adjusted to ensure a quadratic dependence of the
fluorescence intensity versus excitation pulse energy. δ was
calculated by the following equation:
(400 MHz, CDCl3) δ: 1.22 (t, J 7.1 Hz, 6 H), 3.07 (s, 10 H),
3.43 (q, J 7.1 Hz 4 H), 6.16 (s, 1 H), 6.55 (s, 1 H), 6.61 (d, J
8.4 Hz, 1 H), 6.78 (d, J 6.5 Hz, 2 H), 7.55 (d, J 8.9 Hz, 3 H),
7.63 (d, J 7.3 Hz, 2 H). HRMS (ESI): Anal. calcd. For
C28H31N2O3 [M+H]+: 443.23292. Found: 443.23259.
Characterization Methods. UV-vis absorption spectra were
measured on a Jasco V-530 spectrophotometer. One-photon
fluorescence and phosphorescence spectra were performed on
a Hitachi F-4500 fluorescence spectrophotometer. FT-IR spectra
were obtained on a Varian Excalibur HE 3100 spectrophotom-
1
eter. H NMR spectra were obtained on a Bruker DPX400
spectrometer. Mass spectra were obtained on a Bruker Apex
IV FT mass spectrometer. The fluorescence lifetime was
measured using time-correlated single-photon counting tech-
nique with a gated hydrogen discharge lamp as excitation source
(Edinburgh nF9000). TPP fabricated microstructures were
characterized by scanning electron microscopy (SEM, Hitachi
S-4300FEGd).
The solvatochromic effects of the compounds were explored in
a series of different acetonitrile-toluene solutions at a concentration
of 10-5 M. The polarity of the solvent was represented by the
solvent polarity functions f(ε, n) and g(n), which were calculated
from the refractive index (n) and dielectric constant (ε) of the
solvent using eqs 1 and 2. Based on quantum mechanical
perturbation theory, the equations
SsΦrꢀrc
δ )
r δr
(7)
SrΦsꢀscs
Here the subscripts “r” and “s” stand for the reference and
sample, respectively; S is the integrated area of the TPEF; Φ is
the quantum yield; ꢀ is the overall fluorescence collection
efficiencyoftheexperimentalapparatus;andcistheconcentration.
The oxidation potentials of 3-DAC and 4-DAC were measured
by cyclic voltammetry (CV) in acetonitrile using Ag/AgCl as a
reference electrode, a 3 mm diameter Pt working electrode, and a
Pt counter electrode. 0.1 M TBAPF6 was the supporting electrolyte,
and the compounds were dissolved in reagent grade acetonitrile
with a concentration of 0.5 mmol/L. The solutions were protected
with N2 and scanned at a rate of 100 mV/s. The free energy of
electron transfer (∆Get) from the dye to the initiator was calculated
using the Rehm-Weller equation (eq 6).
ν˜a - ν˜f ) m1f(ε, n) + const
(1)
(2)
ν˜a + ν˜f ) -m2[f(ε, n) + 2g(n)] + const
were obtained. Here, ν˜ is the wavenumber, the subscript “a” refers
to absorption and the subscript “f” refers to fluorescence, the
∆Get(kcal mol-1) ) 23.06(Eox-Ered-e2o/aꢀ-E*) (8)

Coumarin/Benzylidene Cyclopentanone Dyes
J. Phys. Chem. A, Vol. 114, No. 15, 2010 5173
2
Here, eo /aꢀ is the energy obtained when two free ions reach
distance a in the solvent with a dielectric constant of ꢀ (0.06
eV in acetonitrile).25
The sensitizing efficiencies of 3-DAC and 4-DAC were
explored by a photobleaching experiment and OPP. The
irradiation source was a 473 nm laser. The intensity of irradiation
was 22 mW/cm2. The other experiment conditions used for OPP
were same as we reported previously.14 Compounds 3-DAC and
4-DAC were used as sensitizers; Omnicat 820 was used as a
co-initiator.
The same mixed monomers were used in TPP experiments
with 3-DAC and 4-DAC as sensitizer and initiator, respectively.
The laser (Tsunami Ti:sapphire, 780 nm, 80 MHz, 80 fs) was
tightly focused into the sample using an oil-immersion objective
lens (100×, NA ) 1.4, Olympus). The sample was fixed on a
xyz-step motorized stage controlled by a computer. After laser
fabrication, the unpolymerized resin was washed out using
acetone. The obtained microstructures were characterized by
SEM.
Calculation Methods. The electronic structure of coumarin
derivatives have long been investigated theoretically by
semiempirical,26,27 ab initio and density functional theory (DFT)
methods.28,29 In 2002, Cave et al.30,31 performed a detailed
theoretical study on several coumarins using different electronic
structure methods and concluded that time-dependent density
functional theory (TD-DFT) was a powerful method for predict-
ing the spectroscopy of 7-aminocoumarins. More recently, the
electronic spectra of a series of coumarins32–35 have been studied
using TD-DFT and the calculated results are in good agreement
with various experimental measurements.
In this work, the quantum chemical calculations were carried
out using the Gaussian 03 program package.36 The ground-state
geometries of 3-DAC and 4-DAC, both in gas phase and
solution condition, were obtained from full optimizations using
the Hartree-Fock (HF) and DFT (B3LYP functional37) methods
with the 6-31+G(d) and 6-31G(d) basis sets, respectively. The
effects of solvent were considered using the polarized continuum
model (PCM)38–40 and default UA0 cavity, in which the solvents
acetonitrile, chloroform, and toluene were employed for simu-
lating real experimental conditions. In the GAUSSIAN program,
the PCM model is performed with self-consistent reaction field
(SCRF); therefore, our calculation could be denoted as B3LYP-
SCRF. On the basis of optimized configurations for the ground
state, TD-DFT calculations were performed using the B3LYP
functional (TD-B3LYP-SCRF) within the adiabatic approxima-
tion to predict the excitation energies. The oscillator strengths
(F) of the transitions and the ground to excited state transition
electric dipole moments (∆µ) were also calculated by the same
method, which are very helpful in assigning the calculated
electronic transitions to the experimental absorption bands.
Figure 2. Normalized absorption and emission spectra of 3-DAC and
4-DAC in chloroform (phosphorescence spectra were measured in
2-methyl-tetrahydrofuran at 77 K).
and triplet-state of 4-DAC is comparatively small. Correspond-
ingly, the probability of this transition may be high so the triplet
quantum yield of 4-DAC is probably larger than that of 3-DAC.
Solvatochromism. The absorption and emission maxima of
the compounds in a range of toluene/acetonitrile mixtures with
different polarities are listed in Table 2. The absorption peaks
of 3-DAC and 4-DAC show no significant shift while their
emission spectra exhibit large red shifts as the solvent polarity
increases. This implies that the polarity of their excited states
is much larger than that of their ground states. Figure 3 shows
the peak shifts (in cm-1) of V˜a - V˜f and V˜a + V˜f for all of the
compounds versus the polarity functions f(ꢀ, n) and f(ꢀ, n) +
2g(n). Excellent linear relationships are found for compound
3-DAC, but for 4-DAC the emission spectra is too sensitive to
the solvents. From Table 2, one can observe that when the
volume percentage of acetonitrile in toluene is changed from 9
to 13%, the emission peaks of 4-DAC will be red-shifted
dramatically, indicating that the data lower than 13% are much
apart from the line obtained from the data above 13% volume
percentage of acetonitrile in toluene (shown in Figure 3a).
Therefore, we have omitted these points for calculating the m1
and the dipole moment of 4-DAC. On the basis of eqs 3-6
(refer to the Experimental section), we calculated the µg and µe
of 3-DAC and 4-DAC (listed in Table 3). Both compounds
show a large change of dipole moment (∆µ ) µe - µg) between
the ground state and the first excited singlet state, which confirms
the large intramolecular charge transfer ability of their excited
states.
Two-Photon Absorption. The two-photon fluorescence
excitation spectra of 3-DAC and 4-DAC were measured over
a broad range of 720-880 nm, and the results are shown in
Figure 4. It is obvious that the TPA cross-section of 3-DAC is
much larger than that of 4-DAC, which means that increase
the degree of conjugation could enhance the stability of the
excited state and in turn increase the ratio of the transition that
enlarges the TPA cross-section. Also, the two-photon excitation
peak wavelengths (δmax) of 3-DAC and 4-DAC show a
significant blue-shift compared to the double wavelength of their
linear absorption peaks. This suggests that the TPA in these
compounds might correspond to the S0-S2 transition.
Results and Discussion
Photophysical Properties. Absorption and emission spectra
of 3-DAC and 4-DAC are shown in Figure 2. Their corre-
sponding photophysical data are listed in Table 1. The absorption
maxima of 3-DAC is significantly red-shifted and its molar
absorption coefficient is larger than that of 4-DAC, which
suggests that 3-DAC exhibits increased conjugation over
4-DAC. The larger Stoke’s shift of 4-DAC compared with that
of 3-DAC indicates a larger change in geometry between its
ground state and first excited singlet state. On the other hand,
the shift between the phosphorescent and fluorescent peaks of
4-DAC is much smaller than that of 3-DAC, indicating that
the change in geometry between the first excited singlet-state
Electrochemical Properties. Typical cyclic voltammograms
(CVs) of 3-DAC and 4-DAC are shown in Figure 5. The scans
show irreversible oxidation peaks in the 0.0-1.8 V (vs Ag/
AgCl) range. The peak shapes of 3-DAC and 4-DAC are quite
similar; however, the oxidative potential of 3-DAC is smaller
than that of 4-DAC. A reduction potential for Omincat 820 of

5174 J. Phys. Chem. A, Vol. 114, No. 15, 2010
Xue et al.
TABLE 1: One- and Two-Photon Optical Properties of 3-DAC and 4-DAC in Chloroforma
compound λma ax (nm) εmax (104 M-1cm-1) λflmax (nm) λ (nm) ∆Vss (103cm-1
)
Φ
kf (108 s-1) kr (108 s-1) δmax (GM) δavg (GM) τt (ns)
ph
max
3-DAC
4-DAC
500.0
471.0
6.6
4.2
564.0
617.4
700
659
2.27
5.03
0.09
0.05
0.67
1.17
12.7
11.8
769
199
300
107
0.77
0.75
a λamax, λflmax, and λ are the absorption maxima, fluorescence maxima and phosphorescent maxima, respectively; εmax is the molar absorption
ph
max
coefficient; ∆Vss is the Stokes shift; Φ is the fluorescence quantum yield; kf and kr are the radiative and nonradiative rate constants; δ is the
two-photon absorption cross section; τt is the fluorescence lifetime.
TABLE 2: Absorption and Emission Maxima of 3-DAC and 4-DAC in Toluene/Acetonitrile Mixtures ([dye] ) 1 × 10-5 M)
solventa (%)
0
2
5
9
13
16
20
40
60
80
100
λamax (nm)
485.5
488.5
489.5
492.5
494.0
494.5
495.5
497.5
497.5
497.0
495.0
3-DAC
551.2
457.5
λflmax (nm)
λamax (nm)
520.0
452.5
526.4
454.0
534.6
456
546.4
457.5
558.8
459.5
562.0
460.0
567.0
462.0
574.6
460.0
581.6
461.0
579.0
457.5
4-DAC
525.3
λflmax (nm)
481.0
482.0
492.7
505.6
532.4
538.0
544.2
549.0
551.0
557.0
a The numbers denote the volume percentage of acetonitrile in toluene.
smaller than 3-DAC. This indicates that electron transfer from
4-DAC to Omnicat 820 is more facile than from 3-DAC to
Omnicat 820. The photosensitizing efficiency of 4-DAC may
be higher than that of 3-DAC.
Photobleaching and One-Photon Polymerization. The
primary photoreaction between the dyes and the initiator
Omnicat 820 was studied using a common photobleaching
experiment. When the dye/Omnicat 820 system was exposed
to a 473 nm laser, the dye was oxidized and its absorption peak
would decrease with irradiation time because of photoinduced
electron transfer from the dye to Omnicat 820. A kinetic study
of the photoreaction was carried out by monitoring the relative
change in the optical density (OD) at the absorption maxima
of the dye over the irradiation time. It was found that the
photobleaching rate Rb (Rb ) (OD0 - OD)/(OD0t)) of 4-DAC
is faster than that of 3-DAC with Omnicat 820 (shown in Figure
6a).
The initiating efficiency of the dye/Omnicat 820 system was
further investigated by OPP. Through monitoring the relative
change in the absorption of the double bond of acrylate
monomers at 6164 cm-1 in near-IR region, the sensitizing
efficiencies of both dyes were determined. Similar results were
obtained (Figure 6b) to the photobleaching experiment, which
further confirmed that 4-DAC exhibited higher sensitizing
efficiency than 3-DAC.
Two-Photon Polymerization. For comparison, three resins
(R1-R3) with different components were investigated. The
polymerized components of these resins were the same as the
OPP resins except for the initiating components. R1 contained
0.4 wt % 3-DAC, 1 wt % HABI, and 1 wt % MMT. R2 and
R3 contained 0.4 wt % 3-DAC and 4-DAC, respectively. Their
efficiency at initiating the polymerization of the mixed acrylate
Figure 3. Plot of (a) V˜a - V˜f (cm-1) vs f(ε, n) and (b) V˜a + V˜f (cm-1
vs f(ε, n) + 2g(n) for 3-DAC and 4-DAC in a mixture of solvents.
)
monomers SR339, SR444, and CN124A80 (mSR339:mSR444
:
mCN124A80 ) 1:3:5) were characterized by TPP. The threshold
energies (Eth) of the resins were determined using a line scan
method.42 Here, Eth was defined by the lowest laser power at
the focus point that ensures the production of a solid line at a
line scan speed of 10 µm/s. 2D nanopatterns were fabricated in
the resins by changing the incident energy. The lowest Eth of
22 µW was obtained by R1 (Figure 7d). As far as we know,
this is the lowest threshold energy ever reported for TPP.
It is found that 3-DAC and 4-DAC can also be directly used
as single component initiators for TPP. The Eth of R2 and R3
were 99 and 198 µW, respectively, larger than the Eth of R1,
but also comparatively small among reported results. Compared
-0.5 V was obtained by scanning in the range -1.8 to 0 V (vs
Ag/AgCl). Table 4 shows the resulting redox potentials and free
energy of electron transfer (∆Get) from the dyes to the initiator
calculated from the Rehm-Weller equation (see eq 8 in the
Experimental section). The ∆Get of both sensitize-initiating
systems are much smaller than -10 kcal/mol, indicating that
electron transfer from the dyes to the initiator occurs readily
after excitation of the dye.41 Though the oxidative potential of
3-DAC is smaller than that of 4-DAC, the excitation energy of
4-DAC is higher than that of 3-DAC, so the ∆Get of 4-DAC is

Coumarin/Benzylidene Cyclopentanone Dyes
J. Phys. Chem. A, Vol. 114, No. 15, 2010 5175
TABLE 3: Dipole Moments (µ), Slope (m) and Correlation Factor (r) of Compounds in Mixed Solventsa
molecule
3-DAC
a (Å)
m1 (cm-1
)
m2 (cm-1
)
µg (D)
µe (D)
∆µ (D)
r
6.4b
6.3c
5.9d
6.0b
6.4c
5.7d
1852.9
1852.9
1852.9
2033.1
2033.1
2033.1
3601.5
3601.5
3601.5
5078.8
5078.8
5078.8
3.3
3.2
2.9
5.0
5.5
4.6
10.3
10.0
8.9
11.6
12.8
10.7
7.0
6.8
6.0
6.6
7.3
6.1
0.989, 0.996
4-DAC
0.958, 0.996
a a is the Onsager radius of solute; ∆µ is the change of dipole moment between ground state (µg) and the first excited state (µe).
b B3LYP/6-31G(d). c B3LYP/6-31+G(d). d HF/6-31G(d).
TABLE 4: Electrochemical Data, Absorption Maxima, and
∆Get of the Dyes to the Initiator
compound
3-DAC
4-DAC
Omnicat 820(Ered
-0.50
)
λmax (nm)a
Eox (V)
∆Get (kcal/mol)
495
0.88
-26.7
457
0.96
-29.7
a Data in acetonitrile.
Figure 4. Two-photon excitation spectra of 3-DAC and 4-DAC in
chloroform.
Figure 5. Cyclic voltammograms of 3-DAC and 4-DAC using a Pt
electrode at a scan speed 100 mV/s. The solutions contained 0.5 mmol/L
of the compounds in acetonitrile with 0.1 M TBAPF6 as the supporting
electrolyte.
to R1, R2 has obvious the advantages of a limited amount of
residue of small molecules left after polymerization. Normally,
1-2 wt % of the initiators are used in reported TPP materials.
Here, only 0.4 wt % of initiator is used in R2, which evidently
decreases the residue of small molecules. In addition, the line
fabricated by TPP line in R2 is thinner than that of R1 at the
same laser energy (shown in Figure 7, panels c and a), so a
much higher resolution is obtained.
It was also found that the incident energy has a significant
effect on the width of the fabricated lines (Figure 7, panels a
and b). The width of the fabricated lines reduced dramatically
as the average laser power decreased. A 3D giraffe with fine
Figure 6. (a) Photobleaching of dyes with Omnicat 820. [Dye] ) 2
× 10-5 mol/L, [Omnicat 820] ) 8 × 10-5 mol/L in chloroform
protected with N2; (b) Double-bond conversion rate vs irradiation time
of dyes with Omnicat 820.
structure was successfully fabricated in R2 at an energy of 163
µW and a scan speed of 110 µm/s (Figure 7e). These results
indicate great potential for the application of 3-DAC, 4-DAC,
and their corresponding resins in TPP.
Calculated Molecular Geometries and Electronic Spectra.
To learn more about the structure-property relationship of these
dyes, the ground state geometry and excited-state properties of

5176 J. Phys. Chem. A, Vol. 114, No. 15, 2010
Xue et al.
Figure 7. 2D nanopattern fabricated in (a) R2 and (b) R3 by changing the incident energy with a fixed line scan speed of 10 µm/s; in R1 by
changing (c) the incident energy and (d) the incident energy and adjusting the focal point. (e) 3D giraffe structure fabricated in R2 at a power of
163 µW and a scan speed of 110 µm/s. (a-d) Labels show the incident energy (mW) and line width (nm) respectively.
3-DAC and 4-DAC were determined by theoretical calculations,
and the numbering systems for these two compounds are
indicated in Figure 8. The optimized dihedral angles of various
moieties are listed in Table 5, from which one can reveal that
the optimized geometries by Hatree-Fock (HF) and DFT
(B3LYP functional) are slightly different. HF method predicts
3-DAC and 4-DAC as a less-conjugated structure, in which the
largest torsional angle (out of plane) is about 50°. The reason
for this is probably due to that HF neglects the electron
correlations. Although the geometric parameters of 3-DAC and
4-DAC are not very dependent on the solvents and basis sets
employed, the calculated dipole moments are quite sensitive to
the basis sets, solvents, and calculation methods (see Table 6).
B3LYP-SCRF/6-31G(d) calculation obtained the dipole moment
for 3-DAC to be 3.4 D in toluene solution, which is in good
agreement with the value (3.3 D) calculated from eq 5. The
configuration of 3-DAC is more planar than 4-DAC, exhibiting
the potential for good polarizability. The dipole moments for
4-DAC calculated by different methods, solvents, and basis sets
range from 6.9 to 10.7 D, with a little bigger deviation from
the value calculated with eq 5. This gap might be caused by
the approximation of eq 5, the accuracy of the data for fitting
the line, and accuracy of DFT calculation for the dipole moment.
However, the most important is the relative dipole moment, that
is, the transition dipole moment from ground to excited states,
which can be calculated with TD-DFT method.
that of 4-DAC, the same trend as calculated from eqs 5 and 6
(see Table 3). For 3-DAC, the excitation energies, obtained from
TD-B3LYP-SCRF/6-31G(d) calculations at B3LYP-SCRF/6-
31G(d) geometries, are in good agreement with experimental
observations. For 4-DAC, the excitation energies with the same
calculation methods are slightly different from the experimental
values, which is about 40-60 nm. However, when the geom-
etries obtained with HF-SCRF/6-31G(d) are employed, the
calculated excitation energies are quite close to the experimental
values. As we know, 3-DAC exhibits a high degree of
conjugation, the B3LYP method that fully considers electronic
correlation reasonably predicts the real molecular structure of
3-DAC. However, for the structure of 4-DAC, the calculated
excitation energies are sensitive to the dihedral angle of the
coumarin moiety and the central cyclopentanone fragment. The
smaller the dihedral angle, the bigger the energy gap between
HOMO and LUMO, that is, the bigger the excitation energy.
For example, when we rotated the coumarin fragment along
the C8-C18 bond for making the dihedral angle C1C8C18C19
change from 156 to 120° and kept other geometric parameters
unchanged, the first excitation energy of 4-DAC calculated in
chloroform changes from 507 to 470 nm, which is very close
to the experimental value (471 nm) in chloroform solution. The
rotation barrier along the C8-C18 bond is very low (about 1
kcal/mol), but the rotation along the corresponding bond of
3-DAC is not so easy because it will cost more than 6 kcal/
mol energy barrier. Therefore, it is reasonable to presume that
the structure of 4-DAC should be even less conjugated than
what the DFT calculation obtained, and the structure of 3-DAC
could be more coplanar; this is waiting for the further confirma-
tion from the experiments. This can also be observed that the
B3LYP-SCRF/6-31G(d) calculation in chloroform solvent pre-
The calculated excitation energies (λamax), their corresponding
oscillator strengths (F), and ground to excited state transition
electric dipole moments (∆µ) for 3-DAC and 4-DAC are listed
in Table 6, along with the measured first absorption spectra data
for comparison. One can observe from this Table that the
transition electric dipole moments for 3-DAC is bigger than

Coumarin/Benzylidene Cyclopentanone Dyes
J. Phys. Chem. A, Vol. 114, No. 15, 2010 5177
Figure 8. The numbering systems of 3-DAC and 4-DAC for all the calculations.
TABLE 5: Calculated Dihedral Angles (°) of the Ground States of 3-DAC and 4-DAC at Various Calculation Methodsa
HF-SCRF/6-31G(d)
B3LYP-SCRF/6-31G(d)
B3LYP-SCRF/6-31+G(d)
C
T
A
C
T
A
C
3-DAC
2.2
-178.9
-176.1
7.6
O7C5C4C6
C5C4C6C10
C4C6C10C28
O7C5C1C8
6.1
-179.3
-167.3
11.6
2.8
2.3
2.8
2.7
2.8
-178.7
-175.5
7.7
-178.8
-175.8
7.2
-178.7
-175.5
7.7
-178.3
-175.7
9.4
-178.5
-174.8
9.1
C5C1C8C9
C1C8C9C11
176.7
177.2
-154.4
5.5
-174.7
-180.0
-5.5
174.6
-179.6
177.3
-155.3
4.7
-175.5
-179.9
-4.5
175.5
-179.8
177.3
-157.2
5.3
-174.9
-180.1
-5.8
174.8
-179.6
177.2
-154.4
5.5
-174.7
-180.0
-5.5
174.6
-179.6
177.4
-150.6
4.0
-176.2
-180.0
-4.2
175.7
-179.9
176.9
-151.4
5.1
-175.2
-180.2
-4.4
175.6
-179.7
-147.3
C27C26N29C31
C25C26N29C30
C26N28C30C31
C17C18N21C22
C19C18N21C23
C18N21C22C23
7.0
-173.3
-179.7
-5.5
177.4
-179.4
4-DAC
-2.1
O7C5C4C6
C5C4C6C9
C4C6C9C10
O7C5C1C8
-6.7
179.8
166.0
-10.9
-178.2
132.7
-3.0
178.7
174.1
-9.6
-177.8
151.0
5.5
-174.5
-179.8
4.7
-2.7
178.2
175.0
-10.4
-177.7
148.2
-2.7
178.2
175.0
-10.2
-177.7
149.6
-2.3
178.0
175.1
-11.4
-177.8
145.9
-2.5
178.0
174.9
-11.0
-177.8
147.5
4.8
-174.9
-180.1
4.0
178.1
175.1
-11.3
-177.5
146.9
4.4
-175.3
-180.0
4.0
C5C1C8C18
C1C8C18C19
C11C12N15C17
C13C12N15C16
C12N15C16C17
C27C26N28C30
C25C26N28C31
C26N28C30C31
6.0
5.0
5.8
3.8
-173.5
-179.6
5.0
-174.6
-179.4
-174.7
-180.0
4.1
-175.5
-179.5
-173.8
-180.0
4.4
-175.3
-179.2
-175.9
-180.1
3.3
-176.4
-179.7
-174.9
-179.3
-175.6
-179.7
-175.6
a T: toluene; A: acetonitrile; C: chloroform.
dicts that the LUMO orbital of 4-DAC is lower than that of
3-DAC, as depicted in Figure 9, indicating that TD-DFT
calculation on B3LYP-SCRF/6-31G(d) geometries would un-
derestimate the first excitation energy of 4-DAC. Furthermore,
the calculated oscillator strength of first transition (HOMO f
LUMO) of 4-DAC is significantly smaller than that of 3-DAC,
which is in good agreement with experimental data that the
molar absorption coefficient of 4-DAC is smaller than that of
3-DAC (see Figure 2). It should be noted that 4-DAC has
another absorption peak around 330 nm in Figure 2, which

5178 J. Phys. Chem. A, Vol. 114, No. 15, 2010
Xue et al.
TABLE 6: The dipole moments (D) calculated with
different basis sets
3-DAC
6-31G(d) 6-31+G(d)
4-DAC
6-31G(d)
6-31+G(d)
gas
3.8
3.4
6.1
7.2
4.7
6.4
7.8
9.0
6.9
8.4
9.6
7.6
9.4
10.7
12.1
tolunene
chloroform
acetonitrille
10.7
transition from its ground state to excited state might result in
a large conformational change, which can explain the larger
Stoke’s shift of 4-DAC compared with that of 3-DAC in Figure
2. At present, it is still difficult to calculate the geometric
parameters of the excited state for such big system, and thus
we cannot calculate the fluorescence spectra for comparison in
this paper.
TPA in 3-DAC and 4-DAC might correspond to the second
excited state (S2), which is mainly the transition from HOMO-1
to LUMO in our calculation. It is clear that TPA cross-section
should be related to the transition dipole moments (∆µ) and
oscillator strength of second excited state. The data in Table 7
indicate that the value for 3-DAC are obviously larger than that
of 4-DAC, as we expected that the larger change in conforma-
tion for 4-DAC could be happened compared with that of
3-DAC.
Conclusion
Figure 9. Electron density plots of HOMO-1, HOMO, LUMO, and
LUMO+1 of 3-DAC and 4-DAC.
Photophysical and photochemical studies show that both
3-DAC and 4-DAC possess strong polarizability. 3-DAC shows
increased conjugation and larger TPA cross sections, whereas
4-DAC exhibits higher sensitizing efficiencies in OPP. Elec-
trochemical experiments show that 4-DAC exhibits a lower
electron transfer free energy with Omnicat 820, explaining its
higher sensitizing efficiency than 3-DAC. Theoretical calcula-
tions show great differences in the configuration, electron
density, and electron distribution between 3-DAC and 4-DAC.
3-DAC has a near-planar conformation while the conformation
of 4-DAC is twisted. The difference of conformations between
3-DAC and 4-DAC will result in different transition electric
dipole moments and oscillator strength, which accounts for the
smaller one-photon and two-photon absorption coefficients of
4-DAC compared with those of 3-DAC.
corresponds to the excitation between HOMO and LUMO+1
(the fourth excited state). TD-DFT-SCRF calculation in chlo-
roform has also confirmed this peak, and the ratio of the
oscillator strength for this excitation over the first excitation is
only 0.86, roughly similar to the ratio of the peak heights in
Figure 2. The oscillator strength is proportional to the transition
electric dipole moment, as indicated in Table 7, which is in
good agreement with the traditional point-view.
The dihedral angle between cyclopentanone and coumarin
moieties is larger in 4-DAC than in 3-DAC (Table 5), and thus
the electron densities of the frontier orbitals for 4-DAC are much
more localized than those of 3-DAC (see Figure 9). The
TABLE 7: Calculated Absorption Maxima, Transition Electric Dipole Moments, and Oscillator Strengths (F) of 3-DAC and
4-DAC at the TD-B3LYP-SCRF/6-31G(d) Level of Theory in Various Solvents, along with the Experimental Absorption
Maxima Valuesa
solvent
toluene
acetonitrile
chloroform
3-DACb
13.6(1)
509
1.715
495
4-DACb
10.1(1)
∆µ (D)
13.3(1)
499
1.675
484
5.9(2)
426
0.390
4.8(2)
436
0.242
13.3(1)
502
1.653
500
5.4(2)
431
0.316
λamax(calc) (nm)
F
λamax(exp) (nm)
∆µ (D)
9.8(1)
497
0.902
449
2.7(2)
6.5(4)
2.6(2)
7.7(4)
10.0(1)
507
0.919
471
2.6(2)
7.7(4)
λamax(calc) (nm)
487
0.071
350
0.561
518
0.919
458
500
0.066
359
0.768
492
0.065
354
0.787
F
λamax(exp) (nm)
4-DACc
∆µ (D)
8.9(1)
451
0.822
2.2(2)
445
0.053
6.3(4)
333
0.556
9.0(1)
2.3(2)
460
0.053
5.9(4)
345
0.474
9.0(1)
463
0.827
2.2(2)
453
0.020
5.7(4)
339
0.510
λamax(calc) (nm)
471
0.816
F
a (1), (2) and (4) refer to the first, second and fourth excited states, respectively. b TD-B3LYP-SCRF/6-31G(d) calculation based on
B3LYP-SCRF/6-31G(d). c TD-B3LYP-SCRF/6-31G(d) calculation based on HF-SCRF/6-31G(d).

Coumarin/Benzylidene Cyclopentanone Dyes
J. Phys. Chem. A, Vol. 114, No. 15, 2010 5179
(20) Bilot, L.; Kawski, A. Z. Naturforsch. 1962, 17A, 621–627.
(21) Kawski, A. In Progress in Photochemistry and Photophysics;
Rabek, J. F., Ed.; CRC Press: Boca Raton, FL, 1992; Vol 5, pp 1-47.
(22) Kawski, A. Z. Naturforsch. 2002, 57A, 255–265.
(23) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991–1024.
(24) Xu, C.; Webb, W. W. J. Opt. Soc. Am. B. 1996, 13, 481–491.
(25) Faulkner, L. R.; Tachikawa, H.; Bard, A. J. J. Am. Chem. Soc.
1972, 94, 691–699.
(26) Moylan, C. R. J. Phys. Chem. 1994, 98, 13513–13516.
(27) McCarthy, P. K.; Blancbard, G. J. J. Phys. Chem. 1993, 97, 12205–
12209.
(28) Petersilka, M.; Gossman, U. J.; Gross, E. K. U. Phys. ReV. Lett.
1996, 76, 1212–1215.
Furthermore, the application of these two dyes in TPP was
studied. 3-DAC and 4-DAC can be used directly as photoini-
tiators in TPP, and the threshold energies of the corresponding
resins are as low as the best results reported. High-resolution
2D and 3D nanopatterns containing low amounts of small
molecule residue were successfully fabricated by TPP. These
dyes should be useful for devices or materials fabricated by
TPP that are currently limited by the presence of small molecule
impurities.
Acknowledgment. This work was supported by NSFC (No.
50403030), NSAF of China (No. 10776033), and the Science
and Technology Innovation Fund of CAS.
(29) Georgieva, I.; Trendafilova, N.; Aquino, A.; Lischka, H. J. Phys.
Chem. A. 2005, 109, 11860–11869.
(30) Cave, R. J.; Burke, K.; Castner Jr, E. W. J. Phys. Chem. A. 2002,
106, 9294–9305.
(31) Cave, R. J.; Castner Jr, E. W. J. Phys. Chem. A. 2002, 106, 12117–
12123.
References and Notes
(32) Sulpizi, M.; Carloni, P.; Hutterd, J.; Rothlisberger, U. Phys. Chem.
Chem. Phys. 2003, 5, 4798–4805.
(33) Jacquemin, D.; Perpe`te, E. A.; Scalmani, G.; Frisch, M. J.; Assfeld,
X.; Ciofini, I.; Adamo, C. J. Chem. Phys. 2006, 125, 164324.
(34) Preat, J.; Jacquemin, D.; Wathelet, V.; Andre´, J.; Perpe`te, E. A. J.
Phys. Chem. A. 2006, 110, 8144–8150.
(1) Parthenopoulos, D. A.; Rentzepis, P. M. Science. 1989, 245, 843–
845.
(2) Kawata, S.; Kawata, Y. Chem. ReV. 2000, 100, 1777–1788.
(3) Cumpston, B. H.; Ananthavel, S. P.; Barlow, S.; Dyer, D. L.; Ehrich,
J. E.; Erskine, L. L.; Heikal, A. A.; Kuebler, S. M.; Lee, I.-Y. S.; Maughon,
D. M.; Qin, J.; R′′ckel, H.; Rumi, M.; Wu, X. L.; Marder, S. R.; Perry,
J. W. Nature 1999, 398, 51–54.
(4) Wang, I.; Bouriau, M.; Baldeck, P. L.; Martineau, C.; Andraud, C.
Opt. Lett. 2002, 27, 1348–1350.
(5) Pruzinsky, S. A.; Braun, P. V. AdV. Funct. Mater. 2005, 15, 1995–
2004.
(6) Fletcher, A. N.; Bliss, D. E. Appl. Phys. 1978, 16, 289–295.
(7) Halstead, J. A.; Reeves, R. R. Opt. Commun. 1978, 27, 273–276.
(8) Drexhage, K. H. Dye Lasers; Schafer, F. P. Ed.; Springer-Verlag:
Berlin, 1990; Ch 5.
(9) Satpati, A. K.; Kumbhakar, M.; Maity, D. K.; Pal, H. Chem. Phys.
Lett. 2005, 407, 114–118.
(10) El-Kemary, M.; Rettig, W. Phys. Chem. Chem. Phys. 2003, 5, 5221–
5228.
(11) Specht, D. P.; Martic, P. A.; Farid, S. Tetrahedron. 1982, 38, 1203–
1211.
(12) Jones, G.; Xuan, P. T.; Schwarz, W. Tetrahedron Lett. 1982, 23,
5505–5508.
(13) Fouassier, J. P.; Wu, S. K. J. Appl. Polym. Sci. 1992, 44, 1779–
1786.
(14) Wang, T.; Wu, F.-P.; Shi, M.-Q.; Gao, F.; Yang, Y.-Y. Acta. Chim.
Sin. 2004, 62, 527–531.
(15) Li, X.; Shi, M.-Q.; Wu, F.-P. Photogra Sci Photochem. 2006, 24,
245–253.
(35) Zhao, W.-W.; Bian, W.-S. J. Mol. Struct: THEOCHEM. 2007, 818,
43–49.
(36) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
ReVision B.02: Gaussian, Inc., Pittsburgh PA, 2003.
(37) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(38) Mierts, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55, 117–
129.
(39) Mierts, S.; Tomasi, J. Chem. Phys. 1982, 65, 239–245.
(40) Cossi, M.; Barone, V.; Cammi, R.; Tomasi, J. Chem. Phys. Lett.
1996, 255, 327–335.
(41) Liu, A.; Trifunac, A. D.; Krongauz, V. V. J. Phys. Chem. 1992,
96, 207–211.
(42) Xing, J.-F.; Dong, X.-Z.; Chen, W.-Q.; Duan, X.-M.; Takeyasu,
N.; Tanaka, T.; Kawata, S. Appl. Phys. Lett. 2007, 90, 131106.
(16) Li, X.; Zhao, Y.-X.; Wu, J.; Shi, M.-Q.; Wu, F.-P. J. Photochem.
Photobio. A: Chem. 2007, 190, 22–28.
(17) Zhao, Y.-X.; Li, X.; Wu, F.-P.; Fang, X.-Y. J. Photochem. Photobio.
A: Chem. 2005, 177, 12–16.
(18) Xue, J.-Q.; Zhao, Y.-X.; Wu, J.; Wu, F.-P. J. Photochem. Photobio.
A: Chem. 2008, 195, 261–266.
(19) Wang, T.; Zhao, Y.-X.; Wu, F.-P.; Shi, M.-Q. Dyes and Pigments.
2007, 75, 104–110.
JP909745Q