Bromine-substituted p-nitrostilbene derivatives: Synthesis, crystal structural studies, photoluminescence and the heavy atom effect on the singlet oxygen generation by two-photon absorption

Article abstract of DOI:10.1016/j.tet.2013.02.010

A variety of linear and side-chained p-nitrostilbene derivatives with various numbers of bromine atoms were prepared to survey the crystal structural properties, the photoluminescence, and heavy atom effect on the singlet oxygen generation by two-photo ab

Full text of DOI:10.1016/j.tet.2013.02.010

Tetrahedron 69 (2013) 2720e2732
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Bromine-substituted p-nitrostilbene derivatives: synthesis, crystal
structural studies, photoluminescence and the heavy atom effect on
the singlet oxygen generation by two-photon absorption
,
a
b
a
a
a
Fang Gao a , Xinchao Wang , Suna Wang , Meng Liu , Xiaojiao Liu , Xiaojuan Ye ,
*
Hongru Li a
,
*
a School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China
b School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A variety of linear and side-chained p-nitrostilbene derivatives with various numbers of bromine atoms
were prepared to survey the crystal structural properties, the photoluminescence, and heavy atom effect
on the singlet oxygen generation by two-photo absorption (TPA). Single crystals of 4-nitro-40-(400-bromo-
phenyl-methyl-oxy)-diphenylethylene (C2), 4-nitro-40-(300,500-dibromo-phenyl-methyl-oxy)-diphenyl-
ethylene (C3), 4-nitro-40-(20,300,40,500,600-pentabromo-phenyl-methyl-oxy)-diphenylethylene (C4) were
obtained, and the structural characteristics were analyzed by X-ray diffraction. One- and two-photon
optical properties of the photosensitizers are shown dependence on the numbers of substituted
bromine atoms. While TPA cross-sections of the photosensitizers are diminished more considerably,
singlet oxygen quantum yields of the photosensitizers are enhanced at some extents by the substituted
bromine atoms. Side-chained photosensitizers display correspondingly higher singlet oxygen quantum
yields and larger TPA cross-sections than the molecules with single bromine-substituted aromatic
segment. Molecular modeling was also performed to reveal the fundamental reasons of the experimental
observation. Photooxidation reaction of singlet oxygen with some substrates was employed to confirm
singlet oxygen generation under one- and two-photon irradiation.
Received 7 November 2012
Received in revised form 22 January 2013
Accepted 1 February 2013
Available online 8 February 2013
Keywords:
Two-photon absorption
Crystal structure
Photoluminescence
Singlet oxygen
Near-IR laser
Ó 2013 Published by Elsevier Ltd.
1. Introduction
the absence of TPA photosensitizers with ideal properties such
as high singlet oxygen quantum yields, large TPA cross-sections,
Chemistry and biochemistry of singlet oxygen (1O2) have been
drawing considerable attention since its discovery in the 1960s. An
electronically excited molecule of oxygen (1O2) is produced nor-
mally through photoinduced energy transfer between the triplet-
sensitizing molecule absorbing incident near ultraviolet or visible
light irradiation and the ground state oxygen.1 Singlet oxygen ex-
hibits distinguished reactivity with a wide variety of electron-rich
molecules, which has been found in many synthetic fields.2e6 Of
particular interest for scientists is that singlet oxygen plays signif-
icant photodynamic therapy (PDT) role due to its efficiently treating
tumor cells.7e12 In recent years, the utilization of available near-IR
laser to generate singlet oxygen becomes one of the hot topics as
near-IR light has several unique advantages such as low energy,
deep penetration, and negligible damage to the normal biological
tissues. TPA dyes open up the future of near-IR PDT application.13e15
However, near-IR TPA photodynamic therapy is limited because of
and excellent biological compatibility. A number of chemical ap-
proaches have been tried to obtain TPA singlet oxygen photo-
sensitizers.15e23 For example, dimer or trimer porphyrins were
demonstrated with large TPA cross-sections.16e18 Heavy atom effect
is regarded as an efficient strategy to improve the quantum yield of
single oxygen of photosensitizers under two-photon irradiation.19
It has been reported that non-linear optical properties of the or-
ganic molecules exhibited certain dependence on heavy atom ef-
fect.24 However, it is some surprised that the effects of the numbers
of substituted bromine atoms on TPA generation of singlet oxygen
by the photosensitizers have been reported seldomly so far. It is
necessary to optimize the numbers of substituted bromine atoms
for a new TPA singlet oxygen photosensitizer.
The introduction of side chain could improve the density of some
functional segments in a molecule. p-Nitrostilbene can be utilized as
parent segment to construct TPA singlet oxygen photosensitizers
due to its remarkable TPA nature. In this study, a variety of p-nitro-
stilbene derivatives bearing with various numbers of substit-
uted bromine atoms were synthesized. As shown in Scheme 1, the
* Corresponding authors. E-mail address: fanggao1971@gmail.com (F. Gao).
0040-4020/$ e see front matter Ó 2013 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.tet.2013.02.010

F. Gao et al. / Tetrahedron 69 (2013) 2720e2732
2721
Br
Br
Br
Br
Br
Br
Br
Br
O
O2N
O2N
O2N
O2 N
O
O
O
C3
C4
C1
C2
Group 1 (G 1)
Br
Br
Br
Br
Br
Br
Br
O2N
O2 N
O2N
O2N
O
O
O
O
Br
O
O
O
O
Br
C7
C6
Br
C8
C5
Br
Br
Br
Br
Br
Br
Group 2 (G 2)
Scheme 1. Chemical structures of the photosensitizers studied in this article.
numbers of bromine atoms in the side-chained photosensitizers
(Group 2, C5eC8) are correspondingly twice as many as those in the
linear ones (Group 1, C1eC4). It is assumed that heavyatom effect on
one- and two-photon optical properties could be more remarkable if
TPA photosensitizers contain more bromine atoms, which are con-
sidered to be favorable for intersystem crossing of the excited states.
Consequently, it can be assumed that one- and two-photon fluo-
rescence emission could be reduced by the substituted bromine
atoms, and the quantum yields of singlet oxygen could be improved.
Furthermore, the effects of the substituted bromine atoms on TPA
cross-sections of the photosensitizers could be revealed. This study
may provide chemical insight to design and synthesis TPA singlet
oxygen photosensitizers. In this work, we performed cooperative
experimental and molecular modeling to reveal the interrelation-
ship between the numbers of substituted bromine atoms and the
structural characteristics and one- and two-photon optical proper-
ties of the photosensitizers.
Singlet oxygen generation by near-IR TPA photosensitizers is
commonly detected by the determination of characteristics phos-
phorescence photoluminescence (1270 nm). This study utilized the
chemical reaction of singlet oxygen with some substrates to pro-
duce photooxidation products, which in turn confirm the genera-
tion of singlet oxygen by these photosensitizers under near-IR
femtosecond laser irradiation.
2. Results and discussion
2.1. X-ray crystallography
Fig. 1. ORTEP drawing of C2, C3, and C4 with thermal ellipsoids at 50% probability.
1ꢁz; C4, symmetry codes: A: x, 0.5ꢁy, 0.5þz; B: x, 0.5ꢁy, ꢁ0.5þz),
which could be the major factor dominating the growth and sta-
bilizations of the crystals. The analysis shows that the distance of
Although some crystallographic disorder was formed in single
crystals of C2 and C4 (Fig. 1a and c), for example, two oxygen atoms
occupied two positions (O(3), and O(3)0) with half rate in the single
crystal of C4, the correct molecular structures of C2, C3, and C4
could be analyzed (Fig. 1, also see Supplementary data). Fig. 1 also
suggests that the two adjacent benzene rings in diphenylethylene
parts of C2, C3, and C4 tend to be coplanar, and the dihedral angles
between them in C2, C3, and C4 are 22.93ꢀ, 9.44ꢀ, and 10.45ꢀ. The
bromine-containing-phenyl rings are deviated considerably from
the adjacent benzene rings (connecting via ether bond) in the
diphenylethylene parts of C2, C3, and C4 (Fig. 1), and the dihedral
angles between them are increased by the substituted bromine
atoms (67.33ꢀ, 73.53ꢀ, and 83.82ꢀ for C2, C3, and C4). The results
reflect the effect of heavy weight bromine atoms on the crystal
ꢀ
ꢀ
CeH/
C3, and 2.946(2) A in the crystal of C4. We further observed in-
termolecular stacking interaction between nitro-phenyl rings
in the single crystal of C3, and the distance between two rings
p is 2.810(2) A in the crystal of C2, 3.220(3) A in the crystal of
ꢀ
p
/p
ꢀ
centers (d1) was 3.982(1) A. Similar CH/
p or p/p stacking in-
teraction has been ever observed in other AreOeCH2estilbene
derivatives.25
2.2. Effects on one-photon absorption and emission spectra
One-photon absorption and emission spectra of the photosen-
sitizers were investigated in various solvents. Representative linear
absorption and emission spectra of the photosensitizers in tetra-
hydrofuran (THF) are available in Fig. 3. Typical one-photon
geometry of C2, C3, and C4. Fig. 2 shows that there is CH/
(namely ) stacking intermolecular interaction in the crystals of
C2, C3, and C4 (C3, symmetry codes: A: ꢁx, ꢁy, 1ꢁz; B: 1ꢁx, 1ꢁy,
p
sep

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F. Gao et al. / Tetrahedron 69 (2013) 2720e2732
CH2Cl2 and CH3CN), the maximal molar extinction coefficients of
C5eC8 are accordingly similar to those of C1eC4. Although the
solvent refractive index and its dispersion could be the part of
reason for the above results,27 the main reason could be due to
internal charge transfer.28 The absorption spectra determined in
various solvents suggest that the maximal peaks of the photosen-
sitizers could be from internal charge transfer.28 In benzene, the
difference on the internal charge transfer between C5eC8 and
C1eC4 could not be obvious due to the low polarity of the solvent.
Hence, the maximal molar extinction coefficients are correspond-
ingly similar. While in very strong polar solvents such as CH2Cl2 or
CH3CN, internal charge transfer tends to be diminished by adiabatic
molecular torsion, hence, the maximal molar extinction coefficients
of C5eC8 are also accordingly close to those of C1eC4 as well. But in
the modest polar solvents such as THF and EtOAc, the difference on
the internal charge transfer between C5eC8 and C1eC4 could ex-
hibit the most remarkable due to the absence of large adiabatic
molecular deviation. As a consequence, the maximal molar ex-
tinction coefficients of C5eC8 are correspondingly larger than
those of C1eC4.
2.3. Molecular modeling
Molecular geometry optimization also suggests that the co-
planarity of C1eC8 are decreased by the substituted bromine atoms
(typically shown in Fig. 5, also see Supplementary data). It is ob-
vious that C5eC8 exhibit more extension in the space than C1eC4.
Table 2 presents representative optimized geometry parameters of
the photosensitizers. The data demonstrate that the dihedral angles
between the phenyl ring carrying bromine atoms and its adjacent
phenyl ring connected via ether linkage bond (i.e., AreCH2eOePh)
are enhanced by the substituted bromine atoms, which is consis-
tent with the single crystal analysis.
The oscillator strengths (f), and the composition (weightage) of
the most important microstates in the anterior excited states of the
photosensitizers were calculated out to analyze the relationship
between the frontier orbital transition and the properties of the
ground and excited states (see Supplementary data). The most
important states mean that there is some transition probability
(namely weightages) between the frontier orbitals.29 It is noticed
that HOMO/LUMO transition of the photosensitizers possess the
largest f values for both the absorption (such as C2, 0.7528; C6,
0.7018) and the emission spectra (such as C2, 1.284; C6, 1.170) with
remarkable weightages (such as for absorption: C2, C6, 99%; for
emission: C2, C6, 100%). The results suggest that the absorption and
emission of C1eC8 are from HOMO/LUMO electron transition.
Symmetric HOMO and LUMO orbitals with the electron cloud
density distribution of the photosensitizers mean that the ground
and excited states of the photosensitizers are characterized with
Fig. 2. View of the CeH/
p and p/p stacking interactions in C2, C3, and C4.
absorption and emission spectral parameters of C1eC8 in various
solvents are given in Table 1. The absorption and emission maxima
of C1eC8 shift to longer wavelengths in polar solvents (such as from
benzene to CH3CN). This suggests that internal charge transfer
could exist in the ground and excited states of C1eC8.26
The maximal absorption and emission wavelengths of C1eC8
are shifted hypsochromically by the substituted bromine atoms
(Fig. 3, also see Supplementary data). These could be caused by the
reduced intramolecular charge transfer and lowered coplanarity of
C1eC8 by bromine atoms. We need mention that the number of
bromine atoms in the phenyl unit could not seriously affect the
donating capability of the PheCH2eOe segment, because it is not
a conjugated structure and there is an sp3 eCH2e between the
phenyl unit and oxygen atom. The main reason could be due to the
lowered coplanarity of C1eC8 by bromine atoms, and it increases
the difficulty of internal charge transfer accordingly. It is also no-
ticed that the maximal absorption and emission wavelengths of
C5eC8 are red-shifted accordingly as comparison with C1eC4 in
various solvents (ca. 10e30 nm, Table 1, or typically shown in
Fig. 4a). It is assumed that internal charge transfer of C5eC8 could
be improved due to more numbers of oxygen donors. This could
explain the corresponding red-shift of the absorption and emission
maxima of C5eC8 with respect to those of C1eC4.
(p,p) transition with internal charge transfer nature (typically
shown in Fig. 6, also see Supplementary data).
The sum of charge (e) in phenyl ring containing bromine atoms
tends to be more negative due to electron-withdrawing effect of
bromine atoms (Table 3). More important, the coplanarity of C1eC8
is seriously diminished by bromine atoms. As a consequence, the
electron-donating properties of AreCH2eOe part in C1eC8 could
be lowered by bromine substitution. It could be seen from the di-
pole moment differences between the ground and excited states of
the photosensitizers. Table 3 also shows that the dipole moment
differences of the photosensitizers are lowered by the substituted
bromine atoms. The data indicate that the extent of internal charge
transfer of C1eC4 and C5eC8 is decreased gradually by bromine
atoms. C5eC8 have more numbers of oxygen donors (namely
AreCH2eOe), hence, the electronic delocalization and corre-
sponding properties are truly affected. The extent of internal charge
transfer during HOMO/LUMO of C5eC8 are correspondingly
Seen from Table 1, the maximal molar extinction coefficients
of C5eC8 are correspondingly larger than those of C1eC4 in
modest polar solvents such as THF and EtOAc (such as C1:
0.146ꢂ105 molꢁ1 cmꢁ2; C5: 0.216ꢂ105 molꢁ1 cmꢁ2). While in non-
polar solvents (such as benzene) or stronger polar solvents (such as

F. Gao et al. / Tetrahedron 69 (2013) 2720e2732
2723
0.25
0.20
0.15
0.10
0.05
0.00
0.35
0.30
C5
C6
C7
C8
C1
C2
0.25
C3
C4
0.20
0.15
0.10
0.05
0.00
290
340
390
440
490
290
340
390
440
490
Wavelength (nm)
Wavelength (nm)
(a)
(b)
150
120
90
60
30
0
1200
C1
C2
C3
C4
1000
800
600
400
200
0
C5
C6
C7
C8
425
475
525
575
625
675
400
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
(d)
(c)
Fig. 3. Representative UV/visible absorption and emission spectra of the photosensitizers in THF (c: 1ꢂ10ꢁ5 mol/L) determined under the same experimental condition for ab-
sorption and emission spectra, respectively. (a) and (b): The absorption spectra; (c) and (d): the emission spectra.
Table 1
Representative one-photon spectral parameters of the photosensitizers in various solvents
Solvents
Photosensitizers
G1
G2
C1
C2
C3
C4
C5
C6
C7
C8
Benzene
EtOAc
THF
labs,max
360
0.127
d
357
0.112
d
356
0.121
d
354
0.099
d
373
0.124
494
0.062
375
0.174
531
0.290
379
0.216
531
372
0.104
492
0.017
374
0.236
525
0.202
378
0.208
526
369
0.131
483
0.002
369
0.194
523
0.123
371
0.172
523
369
0.109
472
0.001
368
0.146
514
0.057
370
0.139
517
3
(ꢂ105)
max
lem,max
F
Weak
362
0.111
514
0.049
366
0.146
513
Weak
360
0.114
510
0.030
364
0.136
508
Weak
358
0.102
503
0.024
356
0.159
506
Weak
357
0.128
502
0.016
355
0.125
501
labs,max
3
(ꢂ105)
max
lem,max
F
labs,max
3
(ꢂ105)
max
lem,max
F
0.097
376
0.200
545
0.062
367
0.355
541
0.039
364
0.272
538
0.034
362
0.289
537
0.390
381
0.221
560
0.309
380
0210
556
0.177
376
0.193
549
0.102
374
0.138
548
CH2Cl2
CH3CN
labs, max
3
(ꢂ105)
max
lem,max
F
0.193
371
0.273
562
0.190
367
0.242
561
0.176
366
0.249
554
0.174
364
0.161
553
0.151
379
0.276
d
0.073
376
0.247
d
0.065
375
0.163
d
0.045
372
0.106
d
labs,max
3
(ꢂ105)
max
lem,max
F
0.083
0.072
0.056
0.043
Weak
Weak
Weak
Weak
labs,max: nm, the maximal absorption wavelength;
3
, molꢁ1 cmꢁ2, the maximal molar extinction coefficient; lem,max: nm, the maximal emission wavelength.
F
: the fluo-
rescence quantum yield.
larger than those of C1eC4 due to containing more oxygen donors.
Hence, C5eC8 show accordingly more dipole moment differences
between the ground and excited states than C1eC4 (Table 3).
It is assumed that the values of absorption maxima could be from
the vertical transition from S0 states to the FranckeCondon S1 states,
and the numbers of the normal emission maxima may be identical
to the decay of S1 states to the corresponding FranckeCondon S0.30
Hence, the calculated absorption and emission maxima of the
photosensitizers are considered to be equal to the corresponding
frontier orbital transition energy differences, which represent as
singletesinglet transition energies. HOMOeLUMO energy gaps
(E: eV) of C1eC4 and C5eC8 in the ground and excited states are
improved by the bromine substitution (Table 4), which could in-
terpret that the maximal absorption and emission wavelengths of
C1eC8 are blue-shifted by bromine substitution. Table 4 further
shows that C5eC8 have correspondingly less HOMOeLUMO gaps in
the ground and excited states than C1eC4. These are further evi-
dences that the maximal absorption and emission wavelengths of

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F. Gao et al. / Tetrahedron 69 (2013) 2720e2732
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
600
500
400
300
200
100
0
OPA
C2
C2
C6
C6OPA
OPF
C2
OPF
C6
400 450 500 550 600 650 700
Wavelength (nm)
280
363
446
Wavelength (nm)
529
612
695
(a)
(b)
Fig. 4. Typical comparisons one-photon absorption and emission spectra of C2 and C6 in THF. (a): Normalized linear absorption and emission spectra of C2 and C6 in THF; (b): actual
one-photon emission spectra of C2 and C6 in THF, performed at the same experimental condition.
Fig. 5. Typical comparisons of the theoretically optimized geometry of C2, C4, C6, and C8.
Table 2
Theoretically optimized geometry parameters of C1eC8
Dihedral angles between
phenyl rings (ꢀ)
G1
G2
C1
C2
0.0926
43.171
C3
0.0513
49.931
C4
0.0553
71.892
C5
C6
C7
C8
1e2
2e3
2e4
0.175
39.102
0.103
0.942
1.077
1.600
40.301
ꢁ42.685
44.102
ꢁ45.156
52.781
ꢁ56.632
75.315
ꢁ73.645

F. Gao et al. / Tetrahedron 69 (2013) 2720e2732
2725
Fig. 6. The electron cloud density distributions in the frontier orbital of the photosensitizers C2, C4, C6, and C8.
C5eC8 are accordingly red-shifted with respect to those of C1eC4. It
is worth pointing out that the molecular modeling was assumed in
vacuum condition, so the calculated absorption and emission
maxima of the photosensitizers are smaller than those measured
experimentally in the solvents.
(typically shown in Fig. 4; in EtOAc, F: C2, 0.030, C6, 0.202; inTHF, F:
C2, 0.062, C6, 0.309). It looks unusual because more numbers of
bromine atoms mean the larger possibility to diminish the emission
of the photosensitizers due to heavy atom effect. The results indicate
that heavy atom effect could be not ONLY one factor inherently
influencing the deactivation of the excited singlet states of C1eC8.31
As observed in many similar diphenylethylene derivatives,
twisted intramolecular charge transfer could be the other mecha-
nism to quench the singlet states of C1eC8.32,33 On the other hand,
twisted intramolecular charge transfer could be varied by the mo-
lecular structural characteristics and the polarity of the solvents.32 It
is obvious that the molecular twist in the excited states of C5eC8
could be sterically hindered by the heavy chains, although they have
larger internal charge transfer ability. Hence, in low or modest polar
2.4. Effects on photoluminescence yields
As our assumption, the emission intensities and the fluorescence
quantum yields (F) of C1eC4 and C5eC8 are lowered by bromine
atoms due to heavy atom effect (Fig. 3c and d, Table 1). It is un-
expected that C5eC8 display stronger emission and larger fluores-
cence quantum yields (
F) than C1eC4 in modest polar solvents such
as THF and EtOAc, although they have more bromine atoms

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F. Gao et al. / Tetrahedron 69 (2013) 2720e2732
Table 3
Theoretically calculated dipole moment differences between the excited states and ground states of the photosensitizers
R3
R2
R4
R1
c
R5
R4
R2
R5
R1
O
R2
R4
R1
NO2
R3
a
O
O
R3
NO2
b
A
R5
B
Photosensitizers
G1
Sum of charge in phenyl ring
carrying bromine atoms
Dipole moment
differences (Dm, D)
C1
C2
C3
C4
C5
C6
C7
C8
Ring a: ꢁ0.014
Ring a: ꢁ0.029
Ring a: ꢁ0.054
Ring a: ꢁ0.109
1.680
1.424
1.121
0.533
2.231
1.824
1.575
1.082
G2
Ring b: ꢁ0.023; ring c: ꢁ0.027
Ring b: ꢁ0.036; ring c: ꢁ0.042
Ring b: ꢁ0.059; ring c: ꢁ0.060
Ring b: ꢁ0.124; ring c: ꢁ0.085
Table 4
Theoretically calculated singlet-singlet transition energies (HeL gaps,
In contrast, C1eC4 display accordingly much stronger emission
and higher fluorescence quantum yields than C5eC8 in strong polar
solvents, such as acetonitrile (Table 4). It is well-accepted that the
strong polar solvents are much more favorable for internal charge
transfer for a molecule.32 This means that even if the molecular
twist in the excited states of C5eC8 could be prohibited by the
heavy chains, they still could have larger possibility to undergo
twisted intramolecular charge transfer in very strong polar solvents
such as acetonitrile due to containing more oxygen donors.32,33 As
a consequence, the fluorescence emissions of C1eC4 are corre-
spondingly stronger than C5eC8 in acetonitrile (typically shown in
Fig. 7).
D
E, eV), ab-
sorption and fluorescence maxima (
l, nm) of the photosensitizers at the TDDFT//HF
and TDDFT//CIS//HF level
Photosensitizers
Absorption and emission
HeL gaps (E, eV)
l(nm)
C1
A
E
A
E
A
E
A
E
A
E
A
E
A
E
A
E
3.42
3.07
3.52
3.11
3.55
3.14
3.62
3.20
3.26
2.92
3.34
2.96
3.41
2.97
3.46
3.00
362.8
404.2
352.4
398.9
349.4
395.2
343.1
387.8
380.1
424.2
370.7
419.5
364.0
418.0
358.7
413.3
C2
C3
C4
C5
C6
C7
C8
2.5. Effects on two-photon optical properties
Femtosecond near-IR Ti:squassier laser tuning from 700 nm to
880 nm at 20 nm step was used to determine two-photon optical
properties of C1eC8. Fig. 8 presents representative TPA fluores-
cence emission spectra of the photosensitizers in THF excited by
760 nm femtosecond laser. As given in Fig. 8, two-photon emission
intensities of C1eC8 are reduced by the bromine atoms. TPA cross-
sections of C1eC8 were determined according to TPA fluorescence
emission method. Fig. 9 shows that TPA cross-sections of C1eC8 are
diminished by bromine atoms under various near-IR femtosecond
laser frequencies (such as C1, 56 GM C2, 37 GM, C3, 26 GM, C4,
20 GM under 760 nm laser excitation). As discussed above, the
extent of intramolecular charge transfer of C1eC4 and C5eC8 could
be lowered by the substituted bromine atoms. This means that the
transition dipole moment changes between the excited and ground
states of C1eC4 and C5eC8 could be diminished by bromine sub-
stitution accordingly. On the other hand, the coplanarity of C1eC8
could be decreased by bromine atoms. These could cause the di-
minishments of TPA cross-sections of C1eC4 and C5eC8 by bro-
mine atoms.35
solvents such as THF, it could not be easy to form twisted intra-
molecular charge transfer state for C5eC8. Normally, the sol-
uteesolute intermolecularcollisionpossibilitycould be limited by its
large size, which could lead to diminish photoinduced soluteesolute
intermolecular interaction (energy and electron transfer).34 The
above two factors could explain that the emission of C5eC8 is
stronger than that of C1eC4 in THF (typically shown in Fig. 7).
We would point out that C5eC8 show much stronger TPA un-
converted fluorescence emission with longer wavelengths and
larger TPA cross-sections than C1eC4 (typically shown in Fig. 10,
TPA, C2, 37 GM, C6, 153 GM in THF under 760 nm laser excitation),
although C5eC8 contain more bromine atoms. These phenomena
demonstrate that side-chained structure could increase TPA cross-
sections of the molecules. The results could be ascribed to more
extent of internal charge transfer due to containing more oxygen
Fig. 7. Photoluminescence of C2 and C6 in the solvents excited by 365 nm. From left to
right: C2, C6 in THF; C2, C6 in acetonitrile.

F. Gao et al. / Tetrahedron 69 (2013) 2720e2732
2727
300
100
80
60
40
20
0
250
C5
C6
C7
C8
C1
C2
200
C3
C4
150
100
50
0
420
470
520
570
620
Wavelength (nm)
670
720
400
450
500
550
Wavelength (nm)
600
650
700
(a)
(b)
Fig. 8. TPA fluorescence emission of C1eC8 excited by 760 nm femtosecond laser in THF, c: 1ꢂ10ꢁ4 mol/L.
500
115
C5
C6
C7
C8
C1
400
300
200
100
0
95
75
55
35
15
C2
C3
C4
720 740 760 780 800 820 840 860 880
740 760 780 800 820 840 860
Wavelength (nm)
Wavelength(nm)
(a)
(b)
Fig. 9. TPA cross-sections of C1eC8 under femtosecond laser wavelengths from 700 to 880 nm in THF.
1.5
1.0
0.5
0.0
300
250
C2
C6
200
150
100
50
C2
C6
0
400
450
500
550
600
650
700
400 450 500 550 600 650 700
Wavelength (nm)
Wavelength (nm)
(a)
(b)
Fig. 10. Typical comparisons of two-photon emission spectra of C2 and C6 in THF, performed at the same experimental condition, under 760 nm, c: 1ꢂ10ꢁ4 mol/L. (a): Normalized
TPA emission spectra of C2, C6; (b): real TPA emission spectra of C2, C6.
donors and more extended structure in the three-dimensional
space for C5eC8.36 It could also interpret that one- and two-
photon emission maxima of C5eC8 are correspondingly shifted
bathochromically with respect to those of C1eC4. TPA emission
maxima of C1eC8 under various near-IR femtosecond laser wave-
lengths are irrespective of excited near-IR laser wavelengths, and
they are similar to the maximal one-photon emission wavelengths
(e.g., the maximal emission wavelength of C6 in THF, one-photon is
526 nm, two-photon under 760 nm laser excitation is 536 nm).
In a word, the experimental and molecular modeling give
powerful evidences that the absorption and emission maxima
(one- and two-photon) of C1eC8 are blue-shifted by bromine
atoms. And, it is reasonable that TPA cross-sections of C1eC8 are
diminished by bromine atoms as well. The calculation could explain
that C5eC8 possess correspondingly longer absorption and emis-
sion maxima (one- and two-photon) and larger TPA cross-sections
as comparison with C1eC4.
2.6. Effects on photosensitized generation of singlet oxygen
As electron-rich substrate, 2,3-dihydropyran could react with
singlet oxygen to yield characteristic photooxidation products,

2728
F. Gao et al. / Tetrahedron 69 (2013) 2720e2732
Table 5
which in turn could determine the photosensitized generation of
singlet oxygen. Under the filtered tungsteneiodine lamp or fem-
tosecond near-IR laser irradiation, oxygen gas was kept bubbling
into the quartz cell containing the solutions of singlet oxygen
photosensitizer and 2,3-dihydropyran. It is assumed that the triplet
states of the photosensitizers could be produced via intersystem
crossing (ISC) from the singlet states, and then singlet oxygen is
produced by the energy transfer from the triple states of the pho-
tosensitizers to the ground state of molecular oxygen (Scheme 2a).
Representative 1O2-photooxidation products detected by GCeMS
confirm photosensitized generation of singlet oxygen by C1eC8
under one- and two-photon irradiation (shown in Scheme 2b). The
ratio of the photooxidation products 4-formoxybutanal to dihy-
dropyrone was dependent on the solvent polarity.37 Non-polar
solvents were favorable for the production of 4-formoxybutanal
via ene reaction, while dihydropyrone was mainly yielded in po-
lar solvents. Representative triplet ESR peaks of nitro-oxide radial
of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO),38 which was
produced from 1O2-photooxidation of TEMP, demonstrate further
singlet oxygen generation by C1eC8 (see Supplementary data). No
superoxide could be determined from the photooxidation products
and ESR experiments. While enough efficient singlet oxygen
quencher such as 1,4-diazobicyclo[2.2.2]octane (DBACO) was added
into the above organic photochemical systems, 1O2-photooxidation
products or ESR signal of TEMPO could not be obtained. These re-
sults suggest that singlet oxygen could be produced by the energy
transfer between the triplet state of the photosensitizers and the
ground state oxygen irradiated by regular lamp (one-photon pro-
cess) and near-IR femtosecond laser (two-photon process).
Singlet oxygen quantum yields of the photosensitizers in various solvents
Photosensitizers Singlet oxygen quantum yields
Benzene
EtOAc
THF
CH3CN
G1
G2
C1
0.261
0.293
0.308
0.369
0.334
0.387
0.401
0.422
0.323
0.367
0.416
0.453
0.413
0.462
0.535
0.571
0.354
0.395
0.433
0.492
0.432
0.493
0.576
0.591
0.634
0.635
0.656
0.715
0.715
0.731
0.768
0.871
C2
C3
C4
C5
C6
C7
C8
from the triple states of the photosensitizers and the ground state
of molecular oxygen. For instance, the quantum yield of singlet
oxygen of C1 was measured as 0.354 in THF, while the quantum
yield of singlet oxygen of C4 was improved roughly 1.4 times
(0.492, in THF). C5eC8 have higher singlet oxygen quantum yields
than C1eC4 (ca. 1.2e1.3 times) in various solvents. Although single
oxygen quantum yields of C1eC8 could be improved at some ex-
tents by bromine atoms, respectively, TPA cross-sections are de-
creased more remarkably by more bromine substitution (see Fig. 9).
However, singlet oxygen quantum yields and TPA cross-sections of
C5eC8 could be improved simultaneously by side-chained struc-
ture. Singlet oxygen quantum yields could be enhanced in polar
solvents. It could be supposed that the polarity could be favorable
for the formation of (
excited states are characterized with internal charge nature, thus
1
p,
p
*
)
of these photosensitizers since the
3
(p,
p ) could be easily formed in polar solvents accordingly.
*
Scheme 2. Singlet oxygen generation under one- and two-photon process and photooxidation of 2,3-dihydropyran.
Singlet oxygen quantum yield reflects quantitatively the effi-
3. Conclusions
ciency of singlet oxygen generated by the excited-state photosen-
sitizer. One-photon singlet oxygen quantum yields of these
photosensitizers were determined by the photobleaching method of
9,10-diphenylanthracene (DPA) (see Supplementary data). It could
be deduced from Kasha’s rule that the yield of singlet oxygen is
irrelevant to the method by which initial excitation is achieved.19
Hence, the quantum yield of singlet oxygen generated by two-
photon excitation is assumed to be identical to that produced from
one-photon excitation.19 Although there are some controversies,39 it
is accepted widely that two-photon singlet oxygen quantum yields
are shown positive interrelationship with one-photon singlet
oxygen quantum yields.
Table 5 shows that singlet oxygen quantum yields of the pho-
tosensitizers are enhanced gradually by the substitution of bromine
atoms, which reflects that heavy atom effect would be favorable for
the yield of the excited triplet states of photosensitizers. This also
indicates that singlet oxygen is produced by the energy transfer
To be summarized, this article reports a variety of linear and side-
chained TPA singlet oxygen photosensitizers containing bromine
atoms. Singlet crystal analysis suggests that the molecular co-
planarity of the photosensitizers is reduced by the substitution of
bromine atoms. One- and two-photon absorption and emission
properties have shown the dependence on the numbers of bromine
atoms. The side-chained photosensitizers exhibit much stronger
photoluminescence in modest polar solvents, although they contain
more bromine atoms than the linear photosensitizers. Singlet oxy-
gen quantum yields of the photosensitizers are improved at certain
extents by the bromine atoms, but TPA cross-sections are reduced
remarkably. On the other hand, singlet oxygen quantum yields and
TPA cross-sections of the photosensitizers could be improved by
the side-chained construction. Construction of dendric or hyper-
branched derivatives with a few numbers of bromine atoms as po-
tential singlet oxygen photosensitizers could be efficient chemical

F. Gao et al. / Tetrahedron 69 (2013) 2720e2732
2729
Z
ꢀ
ꢁ
approach to realize TPA PDT. This study presents strong evidences
that the numbers of bromine atoms could be optimized to obtain
ideal TPA photosensitizers for the generation of singlet oxygen.
In conclusion, the result shown herein would benefit to develop
new TPA singlet oxygen photosensitizers, and for near-IR TPA
PDTeventually. Singlet oxygen generation by these photosensitizers
under femtosecond near-IR laser was confirmed by chemical
approach.
n20A0 If lf
d
lf
Vf ¼ V0f
(1)
Z
ꢀ
ꢁ
n2A If0 lf
d
lf
in which n0 and n represent the refractive indices of the solvents, A0
and A denote the optical densities at excitation wavelength, Ff and
V0f are the quantum yields, and the integrals mean the area of the
fluorescence bands for the reference and sample, respectively.
4. Experimental section
4.3. Two-photon spectral determination
4.1. Materials and characterization
Two-photon excited fluorescence spectra, pumped by Ti:sap-
phire femtosecond 700e880 nm laser of Spectra-Physics Ltd (Tsu-
nami modedlocked, 80 MHz, <130 fs, average power ꢃ700 mW)
tuned at 20 nm step, were registered on Ocean Optics USB2000 CCD
camera. Up-conversion fluorescence method was utilized to de-
Organic solvents obtained from Chongqing Medical and Chem-
ical Corporation were further purified by standard methods.40 The
other chemicals and reagents were purchased from Aldrich unless
otherwise specified. All photosensitizers, 4-nitro-40-(phenyl-
methyl-oxy)-diphenylethylene (C1), 4-nitro-40-(400-bromo-phenyl-
methyl-oxy)-diphenylethylene(C2), 4-nitro-40-(300,500-dibromo-phe
nyl-methyl-oxy)-diphenylethylene (C3), 4-nitro-40-(20,300, 40,500,600-
pentabromo-phenyl-methyl-oxy)-diphenylethylene (C4), 4-nitro-
30,40-bis(phenyl-methyl-oxy)-diphenylethylene (C5), 4-nitro-30,40-
bis(400-bromo-phenyl-methyl-oxy)-stilbene (C6), 4-nitro-30,40-
bis(300,500-dibromo-phenyl-methyl-oxy)-diphenylethylene (C7), and
4-nitro-30,40-bis(200,300,400,500, 60-pentabromo-phenyl-methyl-oxy)-
diphenylethylene (C8) were synthesized in our laboratory, which
were presented in Scheme 3. The photosensitizers containing
bromine atoms were reported firstly in this work.
termine TPA cross-section (
s
) using 5ꢂ10ꢁ4 mol/L fluorescein in
0.1 mol/L solution of NaOH as reference.43 The sample was bubbled
with nitrogen for 15 min to eliminate oxygen before the detection.
TPA cross-sections of the photosensitizers were determined by the
following equations:44
sTPE
s
¼
(2)
(3)
FF
TPEccal ncal
S
sTPE
¼
scal
c
n Scal
Scheme 3. Synthesis routes of the photosensitizers in this study.
Nuclear magnetic resonance (NMR) spectra were performed at
the room temperature on a Bruker 500 MHz apparatus using tet-
ramethylsilane (TMS) as an internal standard. Elemental analysis
was determined with a CE440 elemental analysis meter from
Exeter Analytical Inc. Beijing Fukai melting point apparatus was
utilized to measure the melting points of the photosensitizers.
where s
denotes two-photon absorption section, sTPE represents
two-photon excited crossing section, c is concentration of reference
and sample molecules, n is refractive index of the solvent, and S
denotes two-photon up-conversion fluorescence intensity, cal
represents reference.
We also detected two-photon fluorescence spectra excited by
various pumped powers of near-IR laser in order to remove satu-
ration photophysical processes and ensure two-photon excited
fluorescence intensity being quadratically dependent on excitation
intensity, the excitation powers in TPA cross-section measurements
were limited below 130 mW.
4.2. One-photon spectral determination
Cintra spectrophotometer was employed to measure ultraviolet/
visible absorption spectra (1ꢂ10ꢁ5 mol/L). Shimadzu RF-531PC
spectrofluorophotonmeter was utilized to detect emission spectra
(1ꢂ10ꢁ5 mol/L). Fluorescence quantum yields (
F
) of the photo-
4.4. Molecular modeling
sensitizers herein were detected with rodamin 6G in ethanol (F,
0.94, 1ꢂ10ꢁ6 to 1ꢂ10ꢁ5 mol/L) or quinoline sulfate in 0.1 mol/L
Gaussian 03 program package was utilized for the molecular
geometry optimization. All calculations were performed with 6-
31G** basis set. The geometry optimization in the ground states of
the photosensitizers was performed with HF method and at DFT
level using B3LYP method both,45e48 while the combination of HF
and CIS was employed to optimize the geometries of the first sin-
glet excited state (S1) of the photosensitizers. Although CIS method
H2SO4 (F
, 0.55) as the references.41 The optical density of the
photosensitizers at the excited wavelength was lower than 0.1 to
reduce errors of the fluorescence quantum yields. According to
references,42 the fluorescence quantum yields of a compound in
various solvents with different polarities were determined based
on the following equation:

2730
F. Gao et al. / Tetrahedron 69 (2013) 2720e2732
could provide reliable geometries and force-fields, it is supposed to
produce too high excitation energies (ca. 1 eV).45 To correct the
errors and introduce the dynamic electron correlation, DFT and
TDDFT (time-dependence DFT) were performed to predict energies
at HF and CIS optimized geometries for S0 and S1 states, re-
spectively, such as DFT//HF or TDDFT//CIS (denoted as single-point
calculation//optimization method).48 Herein, TDDFT//CIS was per-
formed to analyze the emission spectra, and TDDFT//HF was used to
calculate the absorption spectra.
photooxygenating system sensitized by the reference and by the
sample, respectively. Herein, DCA was used as a reference.
4.7. Synthesis
The photosensitizers C1eC8 were synthesized based on routine
routes in our laboratory (Scheme 3).
General synthesis description of title compounds. Heat conden-
sation reaction of p-nitro-phenylacetic acid and corresponding
benzaldehyde at 140 ꢀC using base as catalyst yielded 4-nitro-40-
hydroxy-diphenylethylene or 4-nitro-30, 40-dihydroxy-diphenyl-
ethylene, which was dissolved in 18-C-6/K2CO3/dry acetone solu-
tion containing bromine-substituted benzyl bromide. The reactant
mixture was stirred at room temperature under argon for 1 day. The
solution was obtained by the filtration and the solvent was evap-
orated under vacuum. The obtained mixture was dissolved in CHCl3
and washed with water. The organic layer was dried with anhy-
drous sodium sulfate and then concentrated. Further purification
was performed by column chromatography and then twice re-
crystallization with benzene to get the title compounds.
4.4.1. Photosensitized generation of single oxygen in one- and two-
photon irradiation. Oxygen-saturated solution containing the
photosensitizers and the substrates in quartz cells was irradiated by
the filtered tungsteneiodine lamp and near-IR laser, respectively.
Photooxidation reaction of 1O2 was kept at the room temperature
via condenser water and strong fan to remove thermo-reaction, and
UV light was endured to be blocked. After the irradiation, the
oxygen-saturated solution was checked immediately by GCeMS.
The products were identified using known samples and literature
mass spectral data and retention times.
4.7.1. Photosensitizer C1. 1H NMR (CDCl3, 500 MHz)
d (ppm): 5.11 (s,
4.5. Preparation of single crystals and X-ray crystallography
2H, AreCH2eO), 7.01 (t, J¼7.8 Hz, 3H, AreH, eCH]CH), 7.22 (d,
J¼16.5 Hz, 1H, ]CHCH), 7.34 (t, J¼7.6 Hz, 1H, AreH), 7.40 (t,
J¼7.5 Hz, 2H, AreH), 7.45 (d, J¼7.0 Hz, 2H, AreH), 7.50 (d, J¼8.5 Hz,
2H, AreH), 7.60 (d, J¼8.5 Hz, 2H, AreH), 8.20 (d, J¼8.5 Hz, 2H,
Successfully growing of single crystals of the title compounds
was carried out with slow volatilization of the solvent of their
benzene solution at room temperature.
X-ray single crystal diffraction was employed to measure the
crystal structures of the title compounds. XRD data were registered
AreH). 13C NMR(CDCl3, 125 MHz)
d (ppm): 70.105, 115.278, 124.159,
126.522, 127.473, 128.119, 128.440, 128.659, 129.219, 132.864,
136.636, 144.263, 146.451, 159.441. Yellow solid, melting point:
197e198 ꢀC, yield: 55%. Anal. Calcd for C21H17NO3: C, 76.12; H, 5.17;
N, 4.23. Found: C, 75.92; H, 5.22; N, 4.36.
by a Bruker-AXS CCD area detector equipped with diffractometer
ꢀ
with Mo K
a
(
l
¼0.71073 A) at 298 K. A suitable single crystal was
mounted inside a glass fiber capillary. The structure of the title
compound was analyzed by direct methods and refined by full-
matrix least squares on F2. All the hydrogen atoms were added in
their calculated positions and all the non-hydrogen atoms were
refined with anisotropic temperature factors. SHELXS97 was used
to disclosure the structure, and SHELTL was used to refine the
structure.49,50
4.7.2. Photosensitizer C2. 1H NMR (CDCl3, 500 MHz)
d (ppm):
8.195e8.212 (d, J¼8.5 Hz, 2H, AreH), 7.587e7.603 (d, J¼8.0 Hz, 2H,
AreH), 7.483e7.531 (m, 4H, AreH), 7.307e7.360 (2H, AreH),
7.200e7.233 (d, 1H, eCH]CHe), 6.960e7.027 (m, 3H, AreH, eCH]
CHe), 5.153 (s, 2H, AreCH2eO). 13C NMR (DMSO-d6, 125 MHz)
d
(ppm): 68.453, 115.201, 120.976, 124.024, 126.891, 128.622,
129.265, 129.826, 131.361, 132.922, 136.358, 144.450, 145.804,
158.655. Yellow solid, melting point: 216.4e217.5 ꢀC, yield: 65%.
Anal. Calcd for C21H16BrNO3: C, 61.48; H, 3.93; N, 3.41. Found: C,
61.53; H, 3.84; N, 3.37.
4.6. Determination of quantum yields of singlet oxygen
Singlet oxygen quantum yields of these sensitizers were
determined with the photobleaching method of 9,10-diphen-
ylanthracence. 9,10-Diphenylanthracene (DPA) could effectively
react with 1O2, and an obvious photobleaching occurred. This could
be observed spectrometrically for the decreasing in the 374 nm
absorption peak of DPA.51 The reaction efficiency can be expressed
as following:
4.7.3. Photosensitizer C3. 1H NMR (DMSO-d6, 500 MHz)
d (ppm):
8.210e8.228 (d, J¼9.0 Hz, 2H, AreH), 7.812e7.829 (d, J¼8.5 Hz, 3H,
AreH), 7.695e7.697 (d, J¼1.0 Hz, 2H, AreH), 7.635e7.652 (d,
J¼8.5 Hz, 2H, AreH), 7.472e7.505 (d, 1H, J¼16.5 Hz, eCH]CHe),
7.269e7.302 (d, 1H, J¼16.5 Hz, eCH]CHe), 7.072e7.089 (d, 2H,
AreH), 5.180 (s, 2H, AreCH2eO). 13C NMR (DMSO-d6, 125 MHz)
F
1O2
FAO
¼
(4)
2
d
(ppm): 67.463, 115.184, 122.499, 124.010, 126.896, 128.653,
ð
b
þ ½AꢄÞ
129.354, 129.473, 132.724, 132.839, 141.743, 144.403, 145.813,
158.341. Yellow solid, melting point: 159.3e160.2 ꢀC, yield: 62%.
Anal. Calcd for C21H15Br2NO3: C, 51.56; H, 3.09; N, 2.86. Found: C,
51.63; H, 3.14; N, 2.73.
wherein FAO and
F
are the quantum yields of DPA photo-
1O2
2
oxygenation and single oxygen formation by the sensitizer, re-
spectively. [A] is the concentration of DPA and is the ratio of the
b
chemical trapping of singlet oxygen by DPA to the physical
quenching of singlet oxygen by DPA. Thus, at the same experiment
condition, if a reference compound was used as the standard to
determine the quantum yield of singlet oxygen formation. The
following equation could be derived:
4.7.4. Photosensitizer C4. 1H NMR (DMSO-d6, 500 MHz)
d (ppm):
8.219e8.225 (d, J¼8.0 Hz, 2H, AreH), 7.813e7.830 (d, J¼8.5 Hz, 2H,
AreH), 7.600e7.617 (d, J¼8.5 Hz, 2H, AreH), 7.470e7.503 (d, 1H,
J¼8.0 Hz, eCH]CHe), 7.258e7.290 (d, 1H, J¼16.0 Hz, eCH]CHe),
6.947e6.963 (d, J¼8.0 Hz, 2H, AreH), 4.867 (s, 2H, AreCH2eO). Yellow
solid, melting point: 255.1e256.3 ꢀC, yield: 72%. Anal. Calcd for
C21H12Br5NO3: C, 34.75; H,1.67;N,1.93. Found:C, 34.83; H,1.74; N,1.84.
FX
1O2
D
AX
AS
¼
(5)
FS
D
1O2
wherein FS and FX are the quantum yields of singlet oxygen
4.7.5. Photosensitizer C5. 1H NMR (CDCl3, 500 MHz)
d (ppm): 8.19
1O2
1O2
formation by the reference and sample, respectively.
are the decreasement in the absorbance at 374 nm of the
D
AS and
D
AX
(d, J¼9.0 Hz, 2H, AreH), 7.57 (d, J¼9.0 Hz, 2H, AreH), 7.48 (d,
J¼7.5 Hz, 4H, AreH), 7.39 (d, J¼7.5 Hz, AreH), 7.33 (t, J¼2.8 Hz, 2H,

F. Gao et al. / Tetrahedron 69 (2013) 2720e2732
2731
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CHeAr), 6.95 (d, J¼6.5 Hz, 2H, AreCH]CH, AreH), 5.22 (s, 4H,
OeCH2eAr). 13C NMR (CDCl3, 125 MHz)
d (ppm): 71.119, 113.216,
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solid, melting point: 136.5e137 ꢀC, yield: 46%. Anal. Calcd for
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4.7.6. Photosensitizer C6. 1H NMR (CDCl3, 500 MHz)
d (ppm):
8.193e8.211 (d, J¼9.0 Hz, 2H, AreH), 7.574e7.591 (d, J¼8.5 Hz, 2H,
AreH), 7.493e7.523 (t, J¼7.5 Hz, 4H, AreH), 7.341e7.295 (m, 4H,
AreH), 7.163e7.083 (m, 3H, AreH, eCH]CHe), 6.960e6.9079 (m,
2H, AreH, eCH]CHe), 5.130e5.109 (d, J¼10.5 Hz, 4H, OeCH2eAr).
13C NMR (DMSO-d6, 125 MHz)
d (ppm): 150.238, 149.611, 147.090,
143.539, 136.377, 136.185, 132.708, 131.939, 129.245, 129.202,
126.631, 125.246, 124.189, 114.958, 113.738, 70.812, 70.364. Yellow
solid, melting point: 159.2e161.1 ꢀC, yield: 50%. Anal. Calcd for
C28H21Br2NO4: C, 56.49; H, 3.56; N, 2.35. Found: C, 56.43; H, 3.45;
N, 2.47.
11. Gottschaldt, M.; Schubert, U. S.; Rau, S.; Yano, S.; Vos, J. G.; Kroll, T.; Clement, J.;
Hilger, I. ChemBioChem 2010, 11, 649.
12. Silva, E. F. F.; Serpa, C.; Da˛browski, J. M.; Monteiro, C. J. P.; Formosinho, S. J.;
Stochel, G.; Urbanska, K.; Simoe, S.; Pereira, M. M.; Arnaut, L. G. Chem.dEur. J.
2009, 30, 9273.
4.7.7. Photosensitizer C7. 1H NMR (CDCl3, 500 MHz)
d (ppm):
13. Pawlicki, M.; Collins, H. A.; Denning, R. G.; Anderson, H. L. Angew. Chem., Int. Ed.
2009, 48, 3244.
8.208e8.226 (d, J¼9.0 Hz, 2H, AreH), 7.599e7.642 (m, 4H, AreH),
7.572 (s, 2H, AreH), 7.536 (s, 2H, AreH), 7.189e7.134 (m, 3H, eCH]
CHe, AreH), 6.993e6.920 (m, 2H, AreH, eCH]CHe), 5.130e5.109
(d, 4H, AreCH2eO). Yellow solid, melting point: 217e219 ꢀC, yield:
40%. Anal. Calcd for C28H19Br4NO4: C, 44.86; H, 2.54; N, 1.86. Found;
C, 44.95; H, 2.46; N, 1.97.
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4.7.8. Photosensitizer C8. 1H NMR (CDCl3, 500 MHz)
d (ppm):
17. (a) Drobizhev, M.; Stepanenko, Y.; Dzenis, Y.; Karotki, A.; Rebane, A.; Taylor, P.
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8.230e8.213 (d, J¼8.5 Hz, 2H, AreH), 7.612e7.630 (d, J¼9 Hz, 2H,
AreH), 7.363 (s, 2H, AreH), 7.202 (s, 1H, eCH]CHe), 7.061e7.053
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H, 1.16; N, 1.05.
Acknowledgements
We appreciate financial support from the Fundamental Research
Funds for the Central Universities (CDJZR10220006). We also thank
the support from Chongqing Natural Science Committees
(CSTC2012jjB50007 and CSTC2010BB0216). F.G. thanks the warm
encouragements from the Ministry of Education, China (NCET-10-
0876). In particular, we thank the warm encouragements from
National Science Foundation of China (NSFC) and Key Laboratory of
Photochemical Conversion and Optoelectronic Materials, TIPC,
Chinese Academy of Sciences.
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Supplementary data
CCDC 821856, 885014, and 885015 contain the supplementary
crystallographic date for this work. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html or
from the Cambridge Crystallographic Data Center (12, Union Road,
Cambridge CB2 1EZ, UK; fax: þ44 1223 336033). The crystallo-
graphic details, crystal molecular view of C2eC4, normalized ab-
sorption and emission spectra, and more molecular optimization
results were included in the supplementary data. Supplementary
data associated with this article can be found in the online version,
at http://dx.doi.org/10.1016/j.tet.2013.02.010.
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