Effect of alicyclic ring size on the photophysical and photochemical properties of bis(arylidene)cycloalkanone compounds

Article abstract of DOI:10.1039/c2cp41952a

A series of bis(arylidene)cycloalkanone compounds based on cyclobutanone, cyclopentanone, cyclohexanone and cycloheptanone, C4-C7, respectively, with a D-π-A-π-D structure containing the same donor and acceptor but different alicyclic rings was prepared. The effects of alicyclic ring size on the photophysical, photochemical and electrochemical properties of these compounds were investigated systematically. We found that an increase of the number of carbons in the central alicyclic ring leads to changes in geometry, which has significant effects on the conjugation, and photophysical and photochemical properties. These effects include decreases in the fluorescence quantum yield, transient lifetimes, peak extinction coefficients, and the singlet oxygen quantum yield with the increase of the ring size. The one-photon absorption spectra, the two-photon absorption (2PA) spectra, and the fluorescence spectra all show a hypsochromic shift with increasing ring size. The results of this study provide guidance for the design of new cycloketone-based D-π-A-π-D 2PA compounds for photopolymerization and photodynamic therapy applications.

Full text of DOI:10.1039/c2cp41952a

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PAPER
Effect of alicyclic ring size on the photophysical and photochemical
properties of bis(arylidene)cycloalkanone compounds
Qianli Zou,ab Yuxia Zhao,*a Nikolay S. Makarov,c Jochen Campo,cd Hao Yuan,ab
De-Cai Fang,e Joseph W. Perry*c and Feipeng Wu*a
Received 12th June 2012, Accepted 29th June 2012
DOI: 10.1039/c2cp41952a
A series of bis(arylidene)cycloalkanone compounds based on cyclobutanone, cyclopentanone,
cyclohexanone and cycloheptanone, C4–C7, respectively, with a D-p-A-p-D structure containing
the same donor and acceptor but different alicyclic rings was prepared. The effects of alicyclic ring
size on the photophysical, photochemical and electrochemical properties of these compounds were
investigated systematically. We found that an increase of the number of carbons in the central
alicyclic ring leads to changes in geometry, which has significant effects on the conjugation, and
photophysical and photochemical properties. These effects include decreases in the fluorescence
quantum yield, transient lifetimes, peak extinction coefficients, and the singlet oxygen quantum
yield with the increase of the ring size. The one-photon absorption spectra, the two-photon
absorption (2PA) spectra, and the fluorescence spectra all show a hypsochromic shift with
increasing ring size. The results of this study provide guidance for the design of new
cycloketone-based D-p-A-p-D 2PA compounds for photopolymerization and photodynamic
therapy applications.
It is well established that compounds with the general formula
Introduction
of D-p-A-p-D can show high 2PA activity. Bis(arylidene)-
cycloalkanone moieties have such structures, and some of them
have been shown to exhibit large 2PA cross sections (s2).20–21
Moreover, bis(arylidene)cycloalkanone moieties are versatile
Two-photon absorption (2PA) is a third order nonlinear optical
process in which an atom or a molecule absorbs two photons
simultaneously under high intensity light illumination.1 As the
pharmacophores.22–28 Compounds containing such moieties
probability of 2PA has a quadratic dependence on the excitation
intensity, any initiated photochemical or photophysical processes
have been shown to be bioactive materials with HIV-1 integrase
inhibition,22,23 cytotoxicity,24,25 cancer chemopreventive,26
will only happen in a tightly focused spot of the excitation beam,
anti-oxidant27 and two-photon excited PDT28,29 properties.
which provides high penetration depth and high resolution of the
excitation volume. Based on these characteristics, research on
The combination of 2PA activity with biological activity
2PA compounds has attracted much attention in the last
suggests that bis(arylidene)cycloalkanone moieties can have
decades for their potential applications in two-photon excited
high potential for biological applications of 2PA.
fluorescence microscopy,2,3 three-dimensional microfabrication,4,5
Though various compounds with bis(arylidene)cycloalkanone
high-density optical data storage,6 up-converted lasing7 and
moieties have been synthesized and their applications have
photodynamic therapy (PDT).8,9
been reported, studies of their structure–property relationships
Many design strategies have been explored to understand
are limited. To our best knowledge, there has been no syste-
the relationship of molecular structures and 2PA properties.10–19
matic study on the effect of alicyclic ring size on the photo-
physical and photochemical properties of such compounds.
In this paper, we report on a series of bis(benzylidene)-
a Technical Institute of Physics and Chemistry,
cycloalkanones C4–C7 (shown in scheme 1) prepared by the
aldol condensation of alicyclic ketones ((CH2)nCQO, where
n = 3 to 6) with 4-diethylamino-benzaldehyde under base
catalysis. The photophysical, photochemical, and electro-
chemical properties of these compounds have been system-
atically examined to understand the effects of alicyclic ring
size. Furthermore, their ground state geometries, electronic
transition energies, and transition dipole moments, were obtained
using quantum-chemical calculation methods, and the s2 values
Chinese Academy of Sciences, Beijing 100190, P. R. China.
E-mail: yuxia.zhao@mail.ipc.ac.cn, fpwu@mail.ipc.ac.cn
b Graduate University of Chinese Academy of Sciences,
Beijing 100049, P. R. China
c School of Chemistry and Biochemistry and Center for Organic
Photonics and Electronics, Georgia Institute of Technology, Atlanta,
GA 30332, USA. E-mail: joe.perry@gatech.edu
d Department of Physics, University of Antwerp (campus Drie Eiken),
Universiteitsplein 1, B-2610 Antwerpen, Belgium
e College of Chemistry, Beijing Normal University, Beijing 100875,
P. R. China
c
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Scheme 1 Synthetic routes to C4–C7.
were calculated using an essential-states model. The structure–
property relationships of these compounds show that there is a
significant impact of the alicyclic ring size on s2 and other
photophysical and photochemical properties.
2,6-Bis(4-(diethylamino)benzylidene)cyclohexanone (C6). C6
(yield 70%) was synthesized under the same conditions as for
1
C5. H NMR (400 MHz CDCl3): d (ppm) 1.19 (t, 12 H, J =
7.08 Hz), 1.80 (m, 2 H), 2.93 (t, 4 H, J = 5.48 Hz), 3.40
(q, 8 H, J = 7.08 Hz), 6.66 (d, 4 H, J = 8.88 Hz), 7.42 (d, 4 H,
J = 8.84 Hz), 7.75 (s, 2 H).30 HR-MS (ESI): m/z Calcd for
C28H37N2O [M + H]+ 417.29004; found 417.28970. Elem.
Anal.: Calcd for C28H36N2O: C, 80.73; H, 8.71; N, 6.72.
Found: C, 80.59; H, 8.67; N, 6.72.
Experimental section
Materials
Cyclobutanone, cyclopentanone, cyclohexanone, cyclohepta-
none, and 4-diethylaminobenzaldehyde were purchased from
Aladdin-Reagent Corporation. Fluorescein and Rose Bengal
were obtained from Sigma-Aldrich. Rhodamine 6G and
Coumarin 485 were purchased from Exciton. Tetra-n-butyl-
ammonium hexafluorophosphate (TBAPF6), obtained from
Acros Organics, was recrystallized before use. 4,40-Dimethyl-
diphenyliodonium hexafluorophosphate (Omnicat 820) was
from TH-UNIS Insight Co. Ltd. Other reagents were purchased
from Beijing Beihua Company and were used after purification
by common methods. The synthesis routes to C4–C7 are shown
in Scheme 1.
2,7-Bis(4-(diethylamino)benzylidene)cycloheptanone (C7). C7
(yield 38%) was synthesized under similar conditions as for C5
with longer reaction time (24 h). 1H NMR (400 MHz CDCl3):
d (ppm) 1.19 (t, 12 H, J = 7.04 Hz), 1.96 (s, 4 H), 2.75 (s, 4 H),
3.40 (q, 8 H, J = 7.04 Hz), 6.66 (d, 4 H, J = 8.80 Hz), 7.31 (s, 2 H),
7.42 (d, 4 H, J = 8.80 Hz). HR-MS (ESI): m/z Calcd for
C29H39N2O [M + H]+ 431.30569; found 431.30575. Elem.
Anal.: Calcd for C29H38N2O: C, 80.88; H, 8.89; N, 6.51.
Found: C, 80.69; H, 8.95; N, 6.41.
Characterization methods
2,4-Bis(4-(diethylamino)benzylidene)cyclobutanone (C4).
Sodium hydroxide (0.27 g, 6.8 mmol) was added to a stirred
solution of cyclobutanone (1.00 g, 14.3 mmol) and 4-diethyl-
aminobenzaldehyde (5.06 g, 28.6 mmol) in ethanol (25 ml),
and the reaction mixture was stirred at room temperature for
4 h. The precipitate was filtered out and washed with water
until the washings were of neutral pH. The crude product was
recrystallized using ethanol to give pure C4 (yield 63%).
1H NMR (400 MHz CDCl3): d (ppm) 1.20 (t, 12 H, J =
7.08 Hz), 3.42 (q, 8 H, J = 7.08 Hz), 3.72 (s, 2 H), 6.67 (d, 4 H,
J = 8.84 Hz), 7.13 (s, 2 H), 7.45 (d, 4 H, J = 8.88 Hz).
HR-MS (ESI): m/z Calcd for C26H33N2O [M + H]+ 389.25874;
found 389.25890. Elem. Anal.: Calcd for C26H32N2O: C, 80.37;
H, 8.30; N, 7.21. Found: C, 80.09; H, 8.32; N, 7.07.
UV-Vis absorption spectra were recorded using a Hitachi
U-3900 spectrophotometer. Steady-state fluorescence measure-
ments were performed on a Hitachi F-4500 fluorescence
spectrophotometer. 1H NMR spectra were obtained on a
Bruker DPX400 spectrometer. Mass spectra were obtained
on a Bruker Apex IV FT mass spectrometer. Elemental
analyses were performed on a Flash EA 1112 elemental
analyzer. Fluorescence quantum yields (Ff) of C4–C7 in
deoxygenated chloroform and dimethylformamide (DMF)
were determined by using fluorescein (Ff = 0.92 in 0.1 M
aqueous NaOH31) as a reference, and refractive index corrections
were performed using a standard procedure.32 Fluorescence
anisotropy was measured using a Jobin Yvon Fluorolog-3
spectrofluorimeter according to standard procedures in an
L-configuration.33
The singlet oxygen (1O2) quantum yields (FD) of C4–C7 were
determined by 1O2 phosphorescence emission at 1270 nm34
using Rose Bengal in ethanol (FD = 0.6835) as a reference.
A 473 nm diode laser was used as the light source and a near-
infrared fiber spectrometer (NIR-512L-1.7T1) was used to
monitor the emission. The concentrations of all the photo-
sensitizers were adjusted to yield the same absorbance at
473 nm.
2,5-Bis(4-(diethylamino)benzylidene)cyclopentanone (C5).
Sodium hydroxide (0.20 g, 5 mmol) was added to a stirred
solution of cyclopentanone (1.68 g, 20 mmol) and 4-diethyl-
aminobenzaldehyde (7.08 g, 40 mmol) in ethanol (50 ml), and
the reaction mixture was refluxed for 4 h. After cooling to
room temperature, the precipitate was filtered out and washed
with water. The crude product was recrystallized using ethanol
1
to give C5 (85%). H NMR (400 MHz CDCl3): d (ppm) 1.20
(t, 12 H, J = 7.08 Hz), 3.06 (s, 4 H), 3.42 (q, 8 H, J = 7.08 Hz),
6.69 (d, 4 H, J = 8.92 Hz), 7.50–7.52 (m, 6 H).30 HR-MS
(ESI): m/z Calcd for C27H35N2O [M + H]+ 403.27439; found
403.27454. Elem. Anal.: Calcd for C27H34N2O: C, 80.55;
H, 8.51; N, 6.96. Found: C, 80.40; H, 8.49; N, 7.00.
The oxidation potentials of C4–C7 and the reduction
potential of Omnicat 820 (a commercial diaryliodonium
initiator) were measured by cyclic voltammetry (CV) in aceto-
nitrile using Ag/AgCl as a reference electrode, a 3 mm diameter
c
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Pt working electrode and a Pt counter electrode. 0.1 M
TBAPF6 was the supporting electrolyte, and the compounds
Calculation methods
Quantum chemical calculations were carried out using the
Gaussian 09 program package.42 The ground-state geometries
of C4–C7, both in the gas phase and in solution, were obtained
from full optimizations using the DFT (B3LYP functional43)
method with the 6-31G(d) basis set. The effects of solvent were
included using the polarized continuum model (PCM)44,45 and
the default UFF46 cavity, in which the solvent (chloroform)
was employed for simulating the experimental conditions.
In the Gaussian program, the PCM model was performed
with a self-consistent reaction field (SCRF), therefore, our
calculation should be denoted as B3LYP-SCRF. On the
basis of optimized geometry for the ground state, TD-DFT
calculations were performed using the B3LYP functional
(TD-B3LYP-SCRF) within the adiabatic approximation to
predict the excitation energies. The oscillator strengths (f) of
the transitions and the ground to excited state electric transi-
tion dipole moments were also calculated by the same method,
which are very helpful in assigning the calculated electronic
transitions to the experimental absorption bands. In addition,
the geometric parameters of the corresponding excited states
have also been optimized with TD-B3LYP-SCRF, and the
emission spectra have been obtained in order to compare with
experimental data.
were dissolved in acetonitrile with a concentration of 1 mmol Lꢀ1
.
The solutions were purged with N2 and scanned at a rate of
100 mV sꢀ1. The free energy of the charge-shift reaction
(DGet in units of eV) from the cycloketone compound to
Omnicat 820 was calculated using the Rehm–Weller equation
(Eqn (1)),36 wherein the work term is taken to be negligible.
DGet ¼ Eox ꢀ Ered ꢀ Eꢁ
ð1Þ
Here, Eox is the oxidation potential of the bis(arylidene)-
cycloalkanone, Ered is the reduction potential of the diaryl-
iodonium and E* is the 0,0 transition energy of the excited
state (lowest singlet or triplet state, if the sensitization occurs
from the triplet) of the photosensitizer. If the sensitization
occurs from the lowest singlet excited state, E* can be obtained
from the wavelength of intersection of the normalized absorp-
tion and emission spectra.
The 2PA spectra of C4–C7 in chloroform solution were
measured over a broad spectral region using a double-arm
two-photon-excited fluorescence (2PEF) method37 relative to
Rhodamine 6G in methanol (650–1080 nm) and Coumarin 485
in methanol (600–650 nm).38 Freshly prepared samples were
excited using linearly polarized light from a tunable femto-
second (fs) optical parametric amplifier (TOPAS-C, Spectra-
Physics). The relative 2PEF signal of the sample versus the
standard was recorded as a function of excitation wavelength,
using a 901 detection geometry and an imaging spectrometer
(Acton SpectraPro-150). The spectra were scaled to the s2,
measured at a selected wavelength. The 2PA cross sections of
C4–C6 were measured at 800 nm using Rhodamine 6G in
methanol as a reference, while C7 was measured at 750 nm
using Coumarin 485 in methanol.38
If all the energy levels, transition dipole moments, permanent
dipole moments, and line shape functions of the molecule are
known, then it is possible to calculate the 2PA strength of each
transition between the initial state jii and the final state jji as:47,48
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
*
+
ꢀ
ꢁ
2
4
ð2pFÞ n2
ðnhcÞ
X
~
~
ð~mimeÞ ~mmj
e
sð2i!jÞðnÞ ¼ 2
gjð2nÞ; ð2Þ
2
ðnim ꢀ nÞ
m
O
where nij is the transition frequency, n is the solvent refractive
index, h is the Planck constant, e is the unit vector parallel to
~
-
The fs-pulsed Z-scan technique was used as another method
to measure the s2 values of C4–C7 at 790 nm and 960 nm. The
laser source used was the same as discussed above (TOPAS-C,
Spectra-Physics) that produced B75 fs (HW 1/e) pulses at a
repetition rate of 1 kHz. The optical layout for the Z-scan
technique and experimental conditions were the same as
reported previously.39,40 Solutions of C4–C7 in chloroform
(1.0 ꢂ 10ꢀ2 M) were contained within 1 mm pathlength glass
cuvettes. In order to eliminate thermal effects and reduce
contribution from excited-state absorption, the repetition
rate of the laser system was reduced to 50 Hz for the
measurements.
~
the optical electric field (E = eE), c is the speed of light, gj(n)
is the normalized line shape functions of the jth electronic
~
~
~
transition, ~mim ¼ hijerjmi and mmj ¼ hmjerjji are the dipole
3n2
2n2þ1
moments, e is the charge of the electron, and F ¼
is the
Onsager local field factor. The summation is made over all states
m of the molecule, including the initial and the final states, and
hiO represents isotropic averaging over all possible orientations of
the molecules relative to excitation optical electric field. Unfortu-
nately, only a small fraction of this information is available from
experiments, and higher-energy states are particularly difficult to
fully characterize. Nevertheless, in many practical situations
(especially if the 2PA cross section is relatively large, s2
>
Fs transient absorption spectra of C4–C7 in oxygen-free
DMF were measured using a commercially available pump–
probe spectroscopy system (Helios, Ultrafast systems). A
small portion (B5%) of a Ti:Sapphire regenerative amplifier
(Spitfire, 800 nm, 1 kHz, Spectra-physics) provided the probe
pulse to generate white-light continuum (WLC, 400–850 nm,
850–1650 nm), while the remainder was used to pump OPA
TOPAS-C, providing the pump beam (tunable over 290–2800 nm,
B75 fs HW 1/e). The pump wavelengths were 480 nm for C4–C6
and 380 nm for C7. The same experiments performed on
solutions in chloroform led to photochemical degradation of
the samples during measurements. Other experimental condi-
tions were the same as reported before.41
B100 GM) a reasonable quantitative description of the 2PA
cross sections and 2PA spectra may be achieved by considering
only a few of the lowest energy states.
In this study, though C4–C7 are non-centrosymmetric, they
all have approximate C2v symmetry. The calculated permanent
dipole moment differences, Dmgf, are significantly smaller than
their transition dipole moments, mif (listed in Table 1), so in a
first approximation it is possible to calculate the s2 using an
essential-states model assuming only 3 lowest-energy states:48
ꢂ
ꢂ
2
2
4
ꢂ
ꢂ
ꢀ
ꢁ
~mgi j~mif j
2ð2pFÞ
s32-st:ðnÞ ¼
1 þ 2 cos2 a
2 gð2nÞ; ð3Þ
ꢀ
ꢃ ꢁ
2
15ðnhcÞ
1 ꢀ ngi
n
c
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Table 1 Optical properties of diarylidenecycloalkanones obtained
using TD-B3LYP-SCRFa
most common organic solvents, such as chloroform, acetonitrile
and DMF.
No. of excited state
UV-Vis absorption and steady-state fluorescence
1
2
3
4
5
6
7
Compound
C4
C5
C6
C7
l
f
max/nm 477
407
1.315 0.232
298
0.180
3.4
288 286
0.209 0.401
The experimental absorption and emission spectra of the
compounds in chloroform are shown in Fig. 1 and the relevant
data are listed in Table 2. The main electronic absorption band
shifts to the blue with increasing ring size. This absorption
band is attributed to symmetrical charge transfer from the
electron-donating amino groups on both ends to the central
electron-withdrawing carbonyl group. The increase of the
ring size results in twisting the cross-conjugated system and
disturbing the delocalization of the p electrons. Such ring size-
dependent behavior has been observed for a variety of diaryl-
idenecycloalkanones and has been discussed by Farrell et al.51
The maximum molar extinction coefficient (emax) increases
somewhat from C4 to C5, and then decreases significantly
from C5 to C6 and C7. This decrease in emax is consistent with
the calculated trend of a decrease in the magnitude of the
transition dipole moments (mgf, see Table 1). The wavelengths
calculated for the one-photon absorption (1PA) transitions of
C4, shown in Table 1, are 477, 407 and 286 nm, which agrees
with the experimental peak positions (492, 426 and 284 nm
respectively, see Fig. 1). There is also a shoulder in the
experimental 1PA spectrum at B330 nm. These peaks are
assigned to the excitations of HOMO to LUMO, HOMOꢀ1
to LUMO and HOMOꢀ1 to LUMO+1 orbitals, respectively.
The assignment of the peak at 426 nm to a second electronic
transition and not to a vibronic satellite of the first transition is
supported by the measurements of fluorescence anisotropy
(see Fig. 2). The transition at 426 nm is polarized perpendi-
cularly to the S0 - S1, while a vibronic transition would have
parallel polarization. The two main peaks observed for C5–C7
come from the 1st and 7th (for C5 and C6) and 1st and 4th
(for C7) excited states, which correspond to the excitations of
HOMO to LUMO orbital and HOMOꢀ1 to LUMO+1
orbital, respectively. The experimental wavelengths for the
main peaks of C4–C7 in chloroform are 492, 480, 451 and
395 nm, respectively, (see Table 2), which are quite close to our
calculated values of 477, 472, 459 and 411 nm, respectively.
The fluorescence anisotropies of C5 and C6 also reveal a weak
S0 - S2 transition at around 400 nm, polarized orthogonally
to the S0 - S1 transition. For C7, the fluorescence anisotropy
reveals two small bands polarized perpendicular to the
S0 - S1 transition, which might correspond to S0 - S2 and
S0 - S3 transitions. The frontier orbitals depicted in Fig. 3
illustrate that the transitions between HOMO and LUMO for
C4–C7 all belong to p - p* excitations, where the p* orbitals
consist of CQC anti-bonding and CQO anti-bonding
orbitals.
mgf/D
Dmgf/D
s2
11.6
0.90
4.5
3.6
4.9
3-st.
233
401
lmax/nm 472
297
0.068
2.3
291 286
0.015 0.464
f
1.784 0.053
mgf/D
Dmgf/D
s2
13.4
0.52
2.1
1.0
5.3
3-st.
584
401
lmax/nm 459
296 294
0.030 0.059
1.2
289
0.613
6.1
f
1.638 0.013
mgf/D
Dmgf/D
s2
12.6
0.36
1.0
1.9
3-st.
453
390
lmax/nm 411
347 315
306
291
289
f
1.137 0.024 0.01 0.780 0.040 0.048 0.037
mgf/D
Dmgf/D
s2
10.0
—
1.4
0.8
7.2
1.6
1.7
1.5
3-st.
234
268
a
lmax is the calculated absorption peak of the corresponding electric
transition band; f is the oscillator strength; mgf is the transition dipole
moment from ground state to corresponding excited state; Dmgf is the
change in the static dipole moment in the corresponding excited state
3-st.
relative to the ground state; s2
section (GM) calculated by using eqn (3).
is the two-photon absorption cross
where ~mgi is the transition dipole moment vector between the
ground state g and the intermediate excited state i, ~mif is the
transition dipole moment vector between the intermediate
excited state i and the final state f, a is the angle between
the two transition dipole moment vectors, and g(2n) is the
normalized line shape function, determined from the experi-
mental spectra.
Results and discussion
Synthetic strategy
C4–C7 were synthesized using the approach shown in Scheme 1.
Under basic conditions, aldol condensation occurred between
1 equivalent of cycloalkanone and 2 equivalents of 4-diethyl-
aminobenzaldehyde. Different reaction conditions were used
according to the fact that the activity of cycloalkanone
changed a lot with different alicyclic ring sizes. Nielsen
et al.49 reported that the condensation of cyclobutanone with
4-(dimethylamino)benzaldehyde only afforded bis(benzylidene)-
cyclobutanone in a low yield (14%) and the product was
insoluble in most organic solvents. In this work, we found
that when 4-(diethylamino)benzaldehyde instead of 4-(dimethyl-
amino)benzaldehyde was used, the aldol condensation
occurred smoothly at room temperature and produced C4 at
high yield (63%). C5–C7 were synthesized in similar processes
and the yields decreased with increasing ring size. This ring
size-dependent phenomenon was also observed in other kinds
of bis(benzylidene)cycloalkanone and explained by the com-
bined unfavourable equilibrium and solubility factors.50 The
four compounds synthesized in this study are all soluble in
Similarly to the UV-vis spectra, the fluorescence emission
band of C4–C7 shifts to the blue with increasing ring size, with
the experimental peak wavelengths being 568, 550, 534 and
528 nm, respectively. The results of TD-B3LYP-SCRF calcu-
lations predict 514, 509 and 497 nm wavelengths for C4–C6,
which are in fairly good agreement with the experimental
values. Our attempts to optimize the geometric parameters
for the first excited state of C7 did not converge because the
c
11746 Phys. Chem. Chem. Phys., 2012, 14, 11743–11752
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Fig. 1 2PA, linear absorption and normalized fluorescence spectra for C4–C7 in chloroform. 2PA cross sections are given in GM (1 GM =
1 ꢂ 10ꢀ50 cm4 s photonꢀ1).
Table 2 Photophysical and photochemical properties of C4–C7 in chloroforma
emax (104 Mꢀ1 cmꢀ1
)
lmax (nm)
Dn (cmꢀ1
)
Ffb
Ffc
FD
lmax
s2
(GM)
1PA
fl
2PA
max
Comp.
lmax
(nm)
C4
C5
C6
C7
492
480
451
395
6.96
7.01
4.57
3.63
568
550
534
528
2720
2652
3446
6377
0.40
0.26
0.023
0.0088
0.23
0.18
0.040
0.021
0.44
0.26
0.028
0.021
849
837
798
744
466
614
393
574
a
1PA
lmax1PA, lmaxfl and lmax2PA are the 1PA, fluorescence emission and 2PA maxima, respectively; emax is the molar extinction coefficient at lmax
;
Dv is the Stokes shift; Ffb is the fluorescence quantum yield in chloroform; Ffc is the fluorescence quantum yield in DMF; FD is singlet oxygen
quantum yield; s2
max
is the experimental 2PA cross section maxima.
first two excited states of C7 are too close to each other (see the
lmax in Table 1). The increase in the Stokes shifts of com-
pounds with increasing ring size suggests that the difference in
molecular geometry between their ground state and first
excited state becomes more significant from C5 to C7.
to the data in chloroform. The different changes in quantum
yields are likely due to the opposite influence on Ff of solvent
polarity and viscosity. The increased solvent polarity from
chloroform to DMF reduces the energy gap between the
excited states and the ground states, leading to the enhance-
ment of the internal conversion rate constant (kIC) of the
excited state. In contrast, the larger solvent viscosity on
going from chloroform to DMF decreases kIC of the excited
states.
The fluorescence quantum yields (Ff) of C4 and C5 in
chloroform are 0.40 and 0.26, respectively. The Ff of C6 and
C7 in chloroform are much smaller, only 0.023 and 0.009,
respectively. The decrease of Ff from C4 to C7 is attributed to
the increase of skeletal vibration and torsional motions around
the central ring, with increasing the ring size, which con-
sequently enhances nonradiative relaxation of excited states.
The Ff of C4 and C5 in DMF (0.23 and 0.18, respectively) are
lower than the data in chloroform, while the Ff of C6 and C7
in DMF (0.040 and 0.021, respectively) are higher compared
Two-photon absorption
2PA spectra and corresponding 1PA spectra of C4–C7 are
shown in Fig. 1. The s2 values were obtained by the 2PEF and
Z-scan techniques. The corresponding data are listed in Table 2.
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Fig. 2 Polarized fluorescence excitation spectra and fluorescence anisotropies of C4–C7 in chloroform solution. The curves in red and blue are the
parallel and perpendicular polarized excitation spectra, respectively. The upper plots for each compound are the steady-state fluorescence
excitation anisotropies.
Fig. 3 The frontier orbitals for C4–C7, along with the orbital energies (eV) in parenthesis.
The s2 of C4–C7 at 790 nm and C7 at 960 nm obtained by the
fs-pulsed Z-scan technique correlate well with the values
obtained by the 2PEF method, while the Z-scan values of s2
of C4–C6 at 960 nm deviate from the corresponding 2PEF
data. The wavelength region between 800 and 1000 nm
corresponds to a triplet-state transition in the excited-state
absorption (ESA) spectra of C4–C6 (see Fig. 4, next section),
and therefore the Z-scan results at 960 nm are affected by the
triplet–triplet ESA and ground-state depletion. The 2PA peak
of C4 at 849 nm overlaps with the second linear absorption
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Fig. 4 Excited state absorption spectra (left panel) and kinetics (right panel) of C4–C7 dissolved in deoxygenated DMF.
peak at 426 nm (S0 - S2). In addition to the 2PA peak at
849 nm, there is another weak 2PA absorption band around
970 nm, which appears as a weak shoulder on the red side of
the 849 nm peak, and corresponds to the S0 - S1 absorption
band of C4. This indicates that the 2PA absorption spectrum
of C4 in the range of 650–1080 nm includes at least two
electronic transitions, a strong S0 - S2 and a weak S0 - S1
transition. Similarly, the 2PA peaks of C5–C6 are mainly due
to the S0 - S2 transition, but also include a weak component
corresponding to the S0 - S1 transition. For C7, in addition
to the S0 - S1 and S0 - S2 transitions, the S0 - S3 transi-
tion is also observed in the experimental wavelength range,
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as was mentioned above in the discussion of the fluorescence
anisotropy.
and 0.14 ns for C6. Also, the strength of the triplet state
absorption at B850 nm decreased significantly for C6 com-
pared to C4 and C5. Additionally for C6, the ESA band near
850 nm broadened and showed a fairly rapid decay as well as a
long-lived component, suggesting that there is a singlet ESA
contribution in addition to that of the triplet.
The theoretical 2PA cross sections (s23-st.) calculated using
max
eqn (3) and the experimental peak 2PA cross sections (s2
are listed in Table 1 and Table 2. C5 shows the highest s2
value among the studied compounds, which is consistent with
)
max
the calculated results for the S0 - S2 transition for C4–C6.
The ESA of C7 shows significant blue shifts of the SE and
ESA transitions. The broad transition at B1350–1400 nm is
narrower than that of the other molecules and peaks at
1280 nm. The triplet state ESA band is not observed, and
the SE band is shifted to B460 nm. There appear to be weak
singlet ESA bands at B700–900 nm, and a shoulder at B600 nm.
The peak at 1280 nm corresponds roughly to the S1 - S4
transition, where S4 is the upper state for the 1PA band at
308 nm. The broad band at B700–900 nm corresponds to
a transition into the upper state for the 1PA peak of C7 at
B250 nm. The lifetime of the 1280 nm peak is only 70 ps.
It is apparent that the decay rates for internal conversion
increase substantially as the cycloketone ring size is increased
and the system becomes less planar in the ground state,
which is consistent with the decrease in fluorescence quantum
yields.
max
For C7, however, the experimental s2
value is observed to
be the second highest, while s23-st. for the S0 - S2 transition is
only 234 GM, which is lower than the calculated value for C6
(453 GM). A possible reason for this is the close proximity
of the 2PA band associated with the S0 - S3 transition,
3-st.
discussed above, that has a calculated s2
of 268 GM and
likely overlaps with the S0 - S2 transition. The additive sum
of the contributions of the S0 - S2 and S0 - S3 transitions
gives a value of 502 GM, which is reasonably close to the
observed s2max. On the other hand, the bandwidth (FWHM)
of the 2PA band for C7 is B1.5 times smaller than that of C5.
Thus, the narrower bandwidth itself would yield an increase in
peak cross section. These results suggest that a large peak s2
may be obtained in some compounds with reduced conjuga-
tion that blue shifts the lowest 2PA absorption band such that
it overlaps with a higher lying 2PA band.
It is interesting to note that the position of the strong low
energy 2PA band of C4–C7 is somewhat higher in energy than
the lowest 1PA band, which is similar to the energy level
structure of the 1PA and 2PA bands in centrosymmetric DAD
compounds.10 While the 2PA transition to the lowest excited
electronic state of the DAD cycloalkanones is not forbidden
by symmetry, it is rather weak. This is likely due to the two-
state contribution to the 2PA cross section of the lowest
energy transition (s22-state = Amg2f (Dmgf)2/3, where A is a
constant collecting several factors) being rather small as
compared to the three-state contribution for the transition to
the next higher energy excited state, as a result of Dmgf (change
in dipole moment along short molecular axis, z) being sub-
stantially smaller than the excited state transition moment
(along the long molecular axis, x) for the cycloketone DAD
systems.
Singlet oxygen generation
The singlet oxygen quantum yields (FD) of C4–C7 were
determined by 1O2 phosphorescence emission at 1270 nm.
The results are listed in Table 2 and shown in Fig. 5. The FD
is an important factor for photodynamic therapy. In previous
research, we have shown that cyclopentanone dyes have a
large potential in 2PE-PDT applications with moderate FD.28
In this study, C4 exhibits higher FD than that of C5, which
means that cyclobutanone dyes could be better candidates for
2PE-PDT. The FD values of C6 and C7 are much smaller than
those of C4 and C5. This tendency is similar to that of Ff.
Again, the enhanced nonradiative relaxation of excited states
with increasing ring size (C6 and C7) gives rise to a very fast
internal conversion decay channel that largely exceeds the
intersystem crossing rate.
Excited-state absorption
The ESA spectrum of C4 shown in Fig. 4, displays a ground-
state bleach (which at longer times evolves into ESA) at
wavelengths below 550 nm, followed by a longer-lived excited
state at B550–700 nm, which overlaps with a shorter-lived
stimulated emission (SE) peak at B590 nm. There is also a
strong ESA peak corresponding to the triplet state at B850 nm
and a broad band associated with a singlet excited state at
B1360 nm. The peak at 1360 nm roughly corresponds to a
transition from the S1 to the shoulder in the 1PA spectrum at
330 nm, which we attribute to S3. The S1 - S4 transition is
expected to be at B670 nm, where it partially overlaps with
the stimulated emission. The S1 - S2 transition is expected to
be at B3175 nm, and is not accessible with our experimental
setup. The kinetics of the S1 - S3 transition can be fitted with
a single-exponential decay function with the lifetime of 1.1 ns.
Similar ESA spectral behavior is observed for C5 and C6. The
lifetimes of the S1 - S3 transition decrease to 0.62 ns for C5
Fig. 5 The emission of singlet oxygen at 1270 nm generated by
irradiating C4–C7 in chloroform with Rose Bengal (RB) in ethanol
as reference.
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size dramatically facilitates nonradiative deactivation of the
excited singlet state, which is clearly reflected in the progres-
sive decrease of singlet and triplet excited state lifetimes, Ff
and FD on going from C4 to C7. The first oxidation potential
of C4–C7 became larger with increasing ring size. The DGet of
the four compounds are lower than ꢀ0.5 eV, which indicates
that these compounds may be highly efficient sensitizers. The
max
s2 of C5 was found to be the highest, consistent with the
theoretical predictions of the three-level model and its high
max
transition dipole moments. C7 shows the second highest s2
value among these compounds with the lowest conjugation
and transition dipole moment values, but the narrowest
bandwidth. The results presented should facilitate the design
and development of cycloalkanone compounds for a wide
range of two-photon applications.
Fig. 6 Cyclic voltammograms of C4–C7 using a Pt electrode at a scan
speed of 100 mV sꢀ1. The solutions contained 1 mmol Lꢀ1 of the
compounds in acetonitrile with 0.1 M TBAPF6 as the supporting
electrolyte.
Acknowledgements
This work was supported in part by grants (to YZ) by the
National Natural Science Foundation of China (No. 60978057),
the Visiting Scholarship Fund of Chinese Academy of Sciences,
and in the U.S. (to JWP) by the DARPA ZOE program
(Grant No. W31P4Q-09-1-0012) and the National Science
Foundation PREM (Grant No. DMR-0934212). We thank
Dr Joel M. Hales for his help on Z-scan measurement, and
thank Ariel Marshall and Dr Matthew Sartin for their help on
excited-state absorption measurement.
Electrochemical properties
The cyclic voltammograms (CVs) of the compounds are
shown in Fig. 6. The first oxidation potentials of C4–C7 are
enhanced with increasing ring size, consistent with a reduction
of conjugation and electronic coupling of the ends on going
from C4 to C7. Table 3 lists the redox potentials and free
energy of electron transfer (DGet) from the dyes to the Omnicat
820 initiator calculated from eqn (1). The DGet values of all
sensitizer–initiator systems are much lower than ꢀ0.5 eV,
which indicates that these dyes can be highly efficient sensiti-
zers for common iodonium and sulfonium initiators. On the
basis of the singlet oxygen quantum yield data, combined with
the observations from the ESA measurements, it is reasonable
to conclude that C4 has the highest triplet-state quantum
yield, indicating that it may have the best photosensitizing
efficiency among these compounds.
References
1 M. Goppert-Mayer, Ann. Phys., 1931, 401, 273–294.
¨
2 W. Denk, J. H. Strickler and W. W. Webb, Science, 1990, 248,
73–76.
3 W. R. Zipfel, R. M. Williams and W. W. Webb, Nat. Biotechnol.,
2003, 21, 1369–1377.
4 W. Zhou, S. M. Kuebler, K. L. Braun, T. Yu, J. K. Cammack,
C. K. Ober, J. W. Perry and S. R. Marder, Science, 2002, 296,
1106–1109.
5 S. Kawata, H.-B. Sun, T. Tanaka and K. Takada, Nature, 2001,
412, 697–698.
6 D. A. Parthenopoulos and P. M. Rentzepis, Science, 1989, 245,
843–845.
Conclusion
7 J. D. Bhawalkar, G. S. He, C.-K. Park, C. F. Zhao, G. Ruland and
P. N. Prasad, Opt. Commun., 1996, 124, 33–37.
8 J. D. Bhawalkar, N. D. Kumar, C. F. Zhao and P. N. Prasad,
J. Clin. Laser Med. Surg., 1997, 15, 201–204.
9 Hazel A. Collins, M. Khurana, E. H. Moriyama, A. Mariampillai,
E. Dahlstedt, M. Balaz, M. K. Kuimova, M. Drobizhev,
X. D. YangVictor, D. Phillips, A. Rebane, B. C. Wilson and
H. L. Anderson, Nat. Photonics, 2008, 2, 420–424.
A series of bis(arylidene)cycloalkanone compounds C4–C7
with a D-p-A-p-D structure containing the same donor and
acceptor but different sized central alicyclic rings was pre-
pared. In addition to the widely used cyclopentanone and
cyclohexanone, cyclobutanone and cycloheptanone were also
introduced to investigate the effect of cycloketone ring size on
the properties of such compounds. The change of alicyclic ring
size affects both the planarity of the compounds and the
delocalization of the p electrons. The absorption and fluores-
cence emission bands shift to shorter wavelength with the
increase of the ring size. In addition, the increase of the ring
´
10 M. Albota, D. Beljonne, J.-L. Bredas, J. E. Ehrlich, J.-Y. Fu,
A. A. Heikal, S. E. Hess, T. Kogej, M. D. Levin, S. R. Marder,
D. McCord-Maughon, J. W. Perry, H. Rockel, M. Rumi,
¨
G. Subramaniam, W. W. Webb, X.-L. Wu and C. Xu, Science,
1998, 281, 1653–1656.
11 L. Ventelon, S. Charier, L. Moreaux, J. Mertz and M. Blanchard-
Desce, Angew. Chem., Int. Ed., 2001, 40, 2098–2101.
12 M. Charlot, L. Porres, L. C. D. Entwistle, A. Beeby, T. B. Marder
and M. Blanchard-Desce, Phys. Chem. Chem. Phys., 2005, 7,
600–606.
Table 3 Electrochemical data, E*, and DGet of the compounds to the
initiator Omnicat 820
13 K. J. Schafer, J. M. Hales, M. Balu, K. D. Belfield, E. W.
Van Stryland and D. J. Hagan, J. Photochem. Photobiol., A,
2004, 162, 497–502.
14 B. R. Cho, K. H. Son, S. H. Lee, Y.-S. Song, Y.-K. Lee, S.-J. Jeon,
J. H. Choi, H. Lee and M. Cho, J. Am. Chem. Soc., 2001, 123,
10039–10045.
C4
2.34
C5
2.40
C6
2.50
C7
2.67
0.84 ꢀ0.50
Omnicat 820 (Ered)
E* (eV)
EOX (V)
—
0.75
0.77
0.80
DGet (eV) ꢀ1.09 ꢀ1.13 ꢀ1.20 ꢀ1.33
—
c
This journal is the Owner Societies 2012
Phys. Chem. Chem. Phys., 2012, 14, 11743–11752 11751

View Article Online
15 S.-J. Chung, T.-C. Lin, K.-S. Kim, G. S. He, J. Swiatkiewicz,
P. N. Prasad, G. A. Baker and F. V. Bright, Chem. Mater., 2001,
13, 4071–4076.
16 J. Yoo, S. K. Yang, M. Y. Jeong, H. C. Ahn, S. J. Jeon and
B. R. Cho, Org. Lett., 2003, 5, 645–648.
35 P. Gottschalk, J. Paczkowski and D. C. Neckers, J. Photochem.,
1986, 35, 277–281.
36 D. Rehm and A. Weller, Isr. J. Chem., 1970, 8, 259.
37 N. S. Makarov, J. Campo, J. M. Hales and J. W. Perry, Opt.
Mater. Express, 2011, 1, 551–563.
17 M. Drobizhev, A. Karotki, Y. Dzenis, A. Rebane, Z. Suo and
C. W. Spangler, J. Phys. Chem. B, 2003, 107, 7540–7543.
18 E. Collini, S. Carlotto, C. Ferrante, R. Bozio, A. Polimeno,
J. Bloino, V. Barone, E. Ronchi, L. Beverina and G. A. Pagani,
Phys. Chem. Chem. Phys., 2011, 13, 12087–12094.
38 N. S. Makarov, M. Drobizhev and A. Rebane, Opt. Express, 2008,
16, 4029–4047.
39 M. Sheik-Bahae, A. A. Said, T.-H. Wei, D. J. Hagan and
E. W. Van Stryland, IEEE J. Quantum Electron., 1990, 26,
760–769.
19 C. Barsu, R. Fortrie, K. Nowika, P. L. Baldeck, J.-C. Vial,
A. Barsella, A. Fort, M. Hissler, Y. Bretonniere, O. Maury and
C. Andraud, Chem. Commun., 2006, 4744–4746.
20 E. A. Badaeva, T. V. Timofeeva, A. Masunov and S. Tretiak,
J. Phys. Chem. A, 2005, 109, 7276–7284.
40 J. M. Hales, J. Matichak, S. Barlow, S. Ohira, K. Yesudas,
J.-L. Bredas, J. W. Perry and S. R. Marder, Science, 2010, 327,
´
1485–1488.
41 J. M. Hales, M. Cozzuol, T. E. O. Screen, H. L. Anderson and
J. W. Perry, Opt. Express, 2009, 17, 18478–18488.
21 J. Xue, Y. Zhao, F. Wu and D.-C. Fang, J. Phys. Chem. A, 2010,
114, 5171–5179.
22 M. Artico, R. Di Santo, R. Costi, E. Novellino, G. Greco,
S. Massa, E. Tramontano, M. E. Marongiu, A. De Montis and
P. La Colla, J. Med. Chem., 1998, 41, 3948–3960.
23 R. Costi, R. D. Santo, M. Artico, S. Massa, R. Ragno, R. Loddo,
M. La Colla, E. Tramontano, P. La Colla and A. Pani, Bioorg.
Med. Chem., 2004, 12, 199–215.
24 J. R. Dimmock, P. Kumar, A. J. Nazarali, N. L. Motaganahalli,
T. P. Kowalchuk, M. A. Beazely, J. Wilson Quail, E. O. Oloo,
T. M. Allen, J. Szydlowski, E. De Clercq and J. Balzarini, Eur. J.
Med. Chem., 2000, 35, 967–977.
42 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li,
H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta,
F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin,
V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari,
A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi,
N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross,
V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma,
V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg,
25 J. R. Dimmock, M. P. Padmanilayam, G. A. Zello,
K. H. Nienaber, T. M. Allen, C. L. Santos, E. De Clercq,
J. Balzarini, E. K. Manavathu and J. P. Stables, Eur. J. Med.
Chem., 2003, 38, 169–177.
¨
S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz,
J. Cioslowski and D. J. Fox, Gaussian 09, Revision A.02, Gaussian,
Inc., Wallingford CT, 2009.
26 S. Gafner, S. K. Lee, M. Cuendet, S. Barthe
´
le
´
my, L. Vergnes,
S. Labidalle, R. G. Mehta, C. W. Boone and J. M. Pezzuto,
Phytochemistry, 2004, 65, 2849–2859.
43 A. D. Becke, J. Chem. Phys., 1993, 98, 5648–5652.
44 S. Miertus, E. Scrocco and J. Tomasi, Chem. Phys., 1981, 55,
117–129.
45 G. Scalmani and M. J. Frisch, J. Chem. Phys., 2010, 132, 114110.
46 A. K. Rappe, C. J. Casewit, K. S. Colwell, W. A. Goddard and
W. M. Skiff, J. Am. Chem. Soc., 1992, 114, 10024–10039.
47 M. Drobizhev, F. Meng, A. Rebane, Y. Stepanenko, E. Nickel and
C. W. Spangler, J. Phys. Chem. B, 2006, 110, 9802–9814.
48 A. Karotki, PhD thesis, Montana State University, Bozeman,
Montana, 2003.
49 A. T. Nielsen, R. C. Weiss and D. W. Moore, J. Org. Chem., 1972,
37, 1086–1092.
50 R. J. Cremlyn, M. J. Frearson and S. Graham, Phosphorus, Sulfur
Silicon Relat. Elem., 1995, 107, 205–217.
51 P. G. Farrell and B. A. Read, Can. J. Chem., 1968, 46, 3685–3690.
27 W. M. Weber, L. A. Hunsaker, S. F. Abcouwer, L. M. Deck and
D. L. Vander Jagt, Bioorg. Med. Chem., 2005, 13, 3811–3820.
28 Y. Zhao, W. Wang, F. Wu, Y. Zhou, N. Huang, Y. Gu, Q. Zou
and W. Yang, Org. Biomol. Chem., 2011, 9, 4168–4175.
29 W. Yang, Q. Zou, Y. Zhou, Y. Zhao, N. Huang, Y. Gu and F. Wu,
J. Photochem. Photobiol., A, 2011, 222, 228–235.
30 K. Yamashita, S. Imahashi and S. Ito, Dyes Pigm., 2008, 76,
748–753.
31 D. Magde, R. Wong and P. G. Seybold, Photochem. Photobiol.,
2002, 75, 327–334.
32 G. A. Crosby and J. N. Demas, J. Phys. Chem., 1971, 75, 991–1024.
33 J. R. Lakowicz, Principles of fluorescence spectroscopy, Plenum
Press, New York, 1983.
34 A. A. Krasnovsky, Jr., Photochem. Photobiol., 1979, 29, 29–36.
c
11752 Phys. Chem. Chem. Phys., 2012, 14, 11743–11752
This journal is the Owner Societies 2012