Design, synthesis, linear and nonlinear photophysical properties of novel pyrimidine-based imidazole derivatives

Article abstract of DOI:10.1039/c5nj02874d

Novel donor-π-acceptor (D-π-A) and donor-π-acceptor-π-donor (D-π-A-π-D) type pyrimidine imidazole derivatives with flexible ether chains (L1 and L2) have been efficiently synthesized through improved Knoevenagel condensation and Ullmann reactions with high yields. Based on systematic photophysical investigations and theoretical calculations, the structure-property relationships can be described as follows: (1) the linear and nonlinear optical properties of the target chromophores change regularly with increasing the number of branches and the polarity of the solvents. (2) The single-substituted chromophore L2 exhibited a remarkable negative solvato-kinetic effect, while the double-substituted chromophore L1 showed a positive solvato-kinetic effect. Significant bathochromic shifting of the emission spectra and larger Stokes shifts were observed in polar solvents. (3) The two-photon absorption (TPA) cross-section results further demonstrated that their TPA cross section values (δ) increase notably with increasing branch number, and the presence of high π-delocalization could induce large size-scalable TPA enhancements. (4) By comprehensively considering the optical performance, cytotoxicity and solubility, L1 was identified as the better candidate for living cell (HepG2) imaging.

Full text of DOI:10.1039/c5nj02874d

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Design, synthesis, linear and nonlinear photophysical properties of
novel pyrimidine-based imidazole derivatives
Received 00th January 20xx,
Accepted 00th January 20xx
Qiong Zhanga,b, Lei Luoc, Hong Xua, Zhangjun Hud, Caroline Brommessond, Jieying Wua, Zhaoqi
Sunb*, Yupeng Tiana* and Kajsa Uvdald
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel donor-π-acceptor (D-π-A) and donor-π-acceptor-π-donor (D-π-A-π-D) type pyrimidine imidazole derivatives with
flexible ether oxygen chain molecules (L1 and L2) have been efficiently synthesized through improved Knoevenagel
condensation and Ullmann reactions with high yield. Based on systematic photophysical investigations and theoretical
calculations, the structure-property relationships can be drawn as follows: (1) The linear and nonlinear optical properties
of the target chromophores change regularly with increasing the number of branches and the polarity of solvents. (2) One-
substituted chromophore L2 exhibited remarkable negative solvato-kinetic effect, while two-substituted chromophore L1
showed positive solvato-kinetic effect. Significant bathochromic shifting of the emission spectra and larger Stokes shifts
were observed in polar solvents. (3) The two-photon absorption (TPA) cross-section results further demonstrated that
their TPA cross section values (δ) increase notably with increasing branch number, as well as the presence of high πꢀ
delocalization could induce large size-scalable TPA enhancements. (4) Comprehensively considered the optical
performance, cytotoxicity and solubility, L1 was identified to be the better candidate for living cells (HepG2) imaging.
Introduction
storage [7-9], 3D microfabrication [10,11] and so on. To realize
these applications, recent research encompassing synthesis,
Two-photon excitation (TPE) has become particular practical
significance because of the development of highly efficient two-
photon absorbing (TPA) materials and their applications. Organic
materials with striking TPA effect and large TPA cross- section have
attracted considerable attention. Novel large two-photon-active
materials, which play an important role in the current field of the
non-linear optical materials, have attracted more and more
attentions from chemists, material scientists and biophysicists,
owing to their applications in many fields, such as optical limiting
[1,2], two-photon photodynamic therapy [3,4], two-photon excited
fluorescence microscopy [5,6], three-dimensional(3D) optical data
structures and theory have revealed the importance of certain basic
structural motifs for TPA-active organic materials [12–17]. Many
pioneers believe and demonstrate the conjugation length, donor
and acceptor, and planarity of the center are important parameters
for symmetric charge displacement upon excitation, which enhance
ultimately the TPA cross-section [18–22]. By combining experiments
with theory, Prasad et al. [20] and Goodson et al. [24] found that
triphenylamine-cored branched molecules with alkene π-bridges
show exceptionally large TPA cross-sections. Cho et al. [25] have
shown theoretically that, in branched molecules, the TPA cross-
section increases as the strength of the donor–acceptor interaction
increases.
Pyrimidine, as a heterocycle molecule, has high electron affinity
and good coplanarity making it an appropriate building block in
construction of chromophores for nonlinear optical materials. [26-
27]. Recently, several TPA chromophores utilizing pyrimidine as an
electron-withdrawing central core substituent have been
synthesized, and fairly good values of TPA cross section have been
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measured for them[28]. Meanwhile, pyrimidine are found in many reported in parts per million (ppm) relative to TMS (0 ppm) and
natural products which are the important constituents of a number coupling constants in Hz. Splitting patterns were described as
of modern drugs. It exhibited an adequate safety, tolerability and singlet (s), doublet (d), triplet (t), quartet (q), or multiplet (m). The
clinical efficacy profile in preclinical and clinical trials[29]. The mass spectra were obtained on a Bruker Autoflex III smartbeam
methyl groups located in position 4 and 6 of the pyrimidine core mass spectrometer and a Finnigan LCQ Spectrometer.
have a well-known reactivity,which can easily undergo reactions
2 Optical measurements
with an aromatic aldehyde under solvent-free conditions.On the
other hand, Flexible ether oxygen chain can not only increase the
The one-photon absorption (OPA) spectra were recorded on a
solubility of the molecule, but also enhance the extent of electron
SPECORD S600 spectrophotometer. The one-photon excited
delocalization and ability of the TPA compound to donate electrons.
fluorescence (OPEF) spectra measurements were performed using a
Hitachi F-7000 fluorescence spectrophotometer. The OPA and OPEF
Furthermore, combined with previous work[30], two novel
spectra of compounds L1 and L2 were measured in seven organic
pyrimidine derivatives with flexible ether oxygen chain are
solvents of different polarities with a concentration of 1.0 ×10-6 mol
investigated in this work. When these molecules have similar
L-1. The quartz cuvettes used had a 1 cm path length. The
framework, such as bent-shaped D-π-A-π-D chromophore
fluorescence quantum yields (φ) were determined by using
molecules, one can observe that the nature of peripheral
fluorescein as the reference according to the literature method
substituting groups (D) have significant influence on their
[31].Quantum yields were corrected as follows:
fluorescence properties. In this work, it can be safely concluded
that
a novel Λ-type pyrimidine-based imidazole quadrupolar
A
rηs2Ds
Φs = Φr (
)
A
sηr2Dr
derivative consisting of a strong pyrimidine electron-accepting
center and an electron-donating end group linked through a π-
conjugated bridge have proved to be excellent TPA materials, which
demonstrate that good coplanarity of the conjugated system, high
πꢀdelocalization, and additional cooperative enhancement of the
two branches play an important role in the non-linear optical
materials. Finally, we found successfully that L1 to be an efficient TP
imaging reporter for detection of organelle- and tumor cell with
high spatial resolution in HepG2 cells.
Where the s and r indices designate the sample and reference
samples, respectively; A is the absorbance at λexc; h is the average
refractive index of the appropriate solution; and D is the integrated
area under the corrected emission spectrum.
For time-resolved fluorescence measurements, the fluorescence
signals were collimated and focused onto the entrance slit of a
monochromator with the output plane equipped with
a
photomultiplier tube (HORIBA HuoroMax-4P). The decays were
analyzed by the ‘least-squares’ method. The quality of the
Experimental section
1 General procedure
exponential fits was evaluated by the goodness of fit ( χ2 ).
Chemicals were purchased and used as received. Every solvent
was purified by conventional methods beforehand. The solvents
were dried and distilled according to standard procedures. The
synthetic routes for the compounds are presented in Figure 1. All of
the reactions were monitored by TLC (silica gel plates, GF254). Silica
gel 60 (200–300 mesh) was used for column chromatography. IR
TPA cross-section (δ): TPEF spectra were measured using
femtosecond laser pulse and Ti: sapphire system (680-1080 nm, 80
MHz, 140 fs) as the light source. All measurements were carried out
in air at room temperature. The sample was at a concentration of
1.0 ×10-3 mol L-1. The intensities of the TPEF spectra of the
reference and the sample were determined at their excitation
wavelengths. TPA cross-sections were measured using two-photon
induced fluorescence measurement technique. The TPA cross-
spectra were recorded with
a Nicolet FT-IR NEXUS 870
spectrometer (KBr discs) in the 4000–400 cm-1 region. 1H and 13C
NMR spectra were recorded on a 400 or 500 MHz NMR instrument
using CDCl3 or (CD3)2SO as the solvent. Chemical shifts were
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sections (δ) are determined by comparing their TPEF to that of ([M+H]+
fluorescein in solvents, according to the following equation [32]: 43H60N6O8:C,65.46;H,7.67;N,10.65;Found: C,65.41; H, 7.67;N,10.66.
,100).
Anal.
Calcd.
For
C
(E)-4-(2-(2-(1H-imidazol-1-yl)pyrimidin-4-yl)vinyl)-N,N-bis(2-(2-
methoxyethoxy)ethyl)aniline L2
Φref cref nref
F
δ
= δref
Φ
c
n Fref
1.01 g (9 mmol) t-BuOK was placed into a dry mortar and well
milled into powder with heating, 2-(1H-imidazol-1-yl)-4-
methylpyrimidine M5 (0.48 g, 3 mmol, seen in SI) and 4-(bis(2-(2-
methoxyethoxy)ethyl)amino)benzaldehyde M4 (1.00 g, 3 mmol,
seen in SI) were added to the dry mortar and milled vigorously
under the IR LED for about 5 min. The sticky crude product was
obtained and was dissolved with CH2Cl2, extracted with water
(3×100 mL). Then it was dried with anhydrous MgSO4, filtered,
concentrated. The crude product was purified by chromatography
on a silica gel with petroleum (bp 60-90oC)/ethyl acetate (1:1 by
volume) as eluent to yield 0.97 g of red target sticky product. Yield:
69%. 1H-NMR: (400 MHz, CD3COCD3), δ (ppm): 3.30 (s, 6H) ,
3.48~3.50 (t, J =4.6 Hz, 4H), 3.58~3.60 (t, J = 4.6 Hz, 4H), 3.67 (s, 8H)
Here, the subscripts ref stands for the reference molecule. δ is the
TPA cross-section value. c is the concentration of solution. n is the
refractive index of the solution. F is the TPEF integral intensities of
the solution emitted at the exciting wavelength, and F is the
fluorescence quantum yield. The δref value of reference was taken
from the literature [33].
3 General procedure for the preparation of L1 and L2
L1 and L2 were synthesized by the reactions shown in Fig. 1
4,4'-(1E,1'E)-2,2'-(2-(1H-imidazol-1-yl)pyrimidine-4,6-
diyl)bis(ethene-2,1-diyl)bis(N,N-bis(2-(2-
methoxyethoxy)ethyl)aniline) L1
，
6.82 (d, J = 8.8 Hz, 2H), 7.02 (d, J = 15.6 Hz, 1H), 7.11 (s, 1H),7.32
0.67 g (6 mmol) t-BuOK was placed into a dry mortar and well
milled into powder with heating, then 2-(1H-imidazol-1-yl)-4,6-
dimethylpyrimidine M6 (0.17 g, 1 mmol, seen in SI) and 4-(bis(2-(2-
methoxyethoxy)ethyl)amino)benzaldehyde M4 (1.30 g, 4 mmol,
seen in SI) were added and milled vigorously for about 5 min. The
sticky crude product was obtained. The mixture was dissolved with
CH2Cl2 and extracted with water (ca. 3×100 mL). Then it was dried
with anhydrous MgSO4, filtered, concentrated. The crude product
was purified by chromatography on a silica gel with ethyl acetate
/ethanol mixture as the eluent. The red sticky product (0.51g) was
(d, J = 5.2 Hz, 1H), 7.58 (d, J = 8.4 Hz, 2H), 8.01 (s, 1H), 8.09 (d, J =
15.6 Hz, 1H), 8.61 (d, J = 5.2 Hz, 1H), 8.63 (s, 1H). 13C-NMR
(CD3COCD3, 100 MHz, δ/ppm): 51.74, 58.93, 69.25, 71.23, 72.72,
112.6, 116.8, 117.5, 120.0, 123.9, 130.7, 131.1, 136.7, 140.2, 150.5,
155.4, 159.7, 166.2. MS
([M+H]+, 28), 490.42 ([M+Na]+, 83)
([M+H]+,
: C25H33N5O4, 467.56, Found: 468.42
；
MALDI-TOF, m/z (%): 468.26
100).
Anal. Calcd. For
C25H33N5O4:C,64.22;H,7.11;N,14.98;Found:C,65.20; H, 7.11;N,10.99.
Results and discussion
1 Synthesis
collected. Yield
：
65%. IR (KBr, cm-1) selected bands: 3425 (w), 3074
(m), 3037 (s), 2922 (s), 2622 (w), 1602 (s), 1568 (s), 1519 (s), 1468
(s), 1424 (s), 1376 (s), 1322 (s), 1235 (s), 1183 (s), 1109 (s), 1023 (s),
978 (s), 889 (s), 836 (s), 810 (s), 750 (m), 724 (w), 685 (m), 655 (m),
565 (w), 521 (s). 1H-NMR: (400 MHz, CD3COCD3), δ (ppm): 3.31 (s,
12H), 3.52 (d, J = 4 Hz, 8H), 3.61 (d, J = 4 Hz, 8H), 3.69 (s, 16H), 6.85
(d, J = 8 Hz, 4H), 7.00 (d, J = 16 Hz, 2H), 7.27 (s, 1H), 7.59 (d, J = 8 Hz,
4H), 8.06 (d, J = 16 Hz, 2H), 7.12 (s, 1H), 8.11 (s, 1H), 8.73 (s, 1H).
13C-NMR (CD3COCD3, 100MHz), δ (ppm): 51.76, 58.95, 69.28, 71.24,
72.73, 112.61, 114.14, 117.65, 120.7, 124.2, 130.5, 130.8, 136.89,
139.1, 150.3, 155.3, 166.0. MS: C43H60N6O8, 788.97, Found: 789.45
Synthetic routes of L1 and L2 and their intermediates are depicted
in Fig. 1 and Fig. S1. 4-methylbenzene-1-sulfonyl chloride and 2-
methoxyethanol are available commercially. The intermediates M5
and M6 were produced by improved Ullmann reactions in good
yield. Compound L1 and L2 were produced by the modified
Knoevenagel condensation in the solid phase with t-BuOK as base.
([M+H]+, 46), 811.43 ([M+Na]+, 75)
；MALDI-TOF, m/z (%): 789.29
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Fig. 1. The molecular structures of L1 and L2.
π* transitions in differing solvents, which is in accordance with
theoretical calculations. In addition, these compounds show a
pronounced positive solvatochromism [34], resulting in a red-shift
of the UV-Vis absorption bands in DMF compared with cyclohexane.
Their absorption wavelengths in cyclohexane are located at 429 for
2 Theoretical calculations
Fig. 2 gives straightforward representations of the electron density
distribution. Orbital analysis exhibits that the highest occupied
molecular orbital (HOMO) is comprised of
π orbitals localized on L1 and 403 nm for L2, whereas in DMF red-shifted to 444 and 412
nm, respectively. Such solvent-induced shifts are usually
interpreted in terms of the different solvation interactions between
the polar groups and the solvent, and mainly depend on solvent
polarity. The positive solvatochromism is an indicative of a larger
stabilization of the excited state as compared to the ground state
the benzene and vinyl group. Also the lowest unoccupied molecular
orbital (LUMO) distribution is localized in the pyrimidine orbitals
with abundant conjugated π-bridge. Hereby, there are relatively
strong π-donor interactions between flexible ether oxygen chain
and the pyrimidine center, and the electron density in each
molecule is very similar. As shown in Fig. 2 and Table S1, the lowest
energy absorption band can be assigned to ICT between the
nitrogen atom of the amino group and the pyrimidine ring.
Basically, the calculated singlet-singlet transitions in all
chromophores are in reasonable agreement with the experimental
by
a polar solvent, suggesting that a significant charge
redistribution takes place upon excitation, which is in accordance
with a multidimensional intramolecular charge transfer (MDICT)
occurring between the core and the peripheral groups [35].
Moreover, as show in the Fig. 3 and Table 1, the one-photon
absorption peak position (λmax) of L1 and L2 show an increasing
tendency as polarity of the solvent increase, except in
dichloromethane. The OPEF maxima are obtained at their maxima
excitation wavelengths in eight different solvents, from which one
can see that their emission maxima (λmaxem) of L1 and L2 were
slightly red shifted with increasing solvent polarity and all the
derivatives show definite solvatochromic behavior; for example, for
compound L1, the peak wavelength shift from 473nm (cyclohexane)
to 557nm (DMF), and for compound L2, from 445 nm (cyclohexane)
to 523 nm (DMF) indicating that this charge separation increases in
the excited state, resulting in a larger dipole moment than that in
the ground state, which can be explained the sensitivity of the
emission spectra of these chromophores to solvent polarity
accordingly [36] (supported by DFT calculations). Furthermore, the
emission of compound (L1) in cyclohexane reveal vibronic
definition, and show a progressive red-shift and loss of vibronic
structure as the solvent polarity increases[37]. Fig. S2a shows the
OPEF intensities of the compound (L1) decrease as polarity of the
solvent increase due to solvato-kinetic effect [38]. This can be
explained by the fact the larger change in the dipole moment
between the excited and the ground state of L1 compared with L2,
which means that a strong intramolecular charge transfer existed
during the photoinduced process. However, for compound (L2), the
OPEF intensities increase with polarity of the solvent increase (Fig.
λab in the two major absorption bands observed.
Fig. 2 Representation of calculated HOMO and LUMO orbitals of L1
and L2.
3 Photophysical properties
Fig. 3 and Table 1 lists the UV-Visable and fluorescence spectra
of compounds L1 and L2 in several solvents with differing polarities
(cyclohexane, benzene, dichloromethane, THF, ethyl acetate,
ethanol, acetonitrile, DMF). The concentration of the solution is
1.0×10-6mol L-1
.
Solvatochromism
From L1 to L2, the large molar absorption coefficients (log S2b), showing negative solvate-kinetic effect. As is well-known, the
εmax>>4) of the maximum peaks suggested the presence of the π– negative solvate-kinetic effect is caused by different factors such as
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biradicaloid charge transfer [38], the proximity effect [38], and
others [39].
Lippert-Mataga plot
The magnitudes of dipole moment change (µe-µg) are
proportional to the one photon oscillator strengths, so their values
can be calculated from the linear absorption spectrum. However,
(µe-µg) is rarely determined experimentally, so the design of
efficient 2PA compounds has proceeded empirically, with guidance
from theoretical calculations of transition moments. As seen in
Table S2 and Table 1, the fluorescence spectra showed moderate
Stokes shifts depending on the solvent polarity. Further, the
solvatochromism is observed, suggesting a large change in dipole
moment between ground and excited states. The Lippert-Mataga
equation is the most widely used to evaluate the dipole moment
changes of the dyes with photoexcitation [41].
Fig.
3 Single-photon absorption and Single-photon excited
2ꢁf
ꢁ
ν
=
(ꢀe − ꢀ
g )2 + b
fluorescence spectrum of L1 and L2 in benzene and acetonitrile
(c=1×10-6mol L-1).
4
πε0hca3
ε
2ε
−1 n2 −1
ꢁf =
−
+1 2n2 +1
Comparison of branching number
in which Δν
＝νabs - νex stands for the Stokes shifts, νabs and νex are
As showed in Fig. S2 and Table 1, from L2 to L1, the one-
photon absorption maxima in benzene and in acetonitrile show
regular red shifts as the branch number increases. These can be
assigned to the more highly extended π-delocalization and a certain
coupling between the branches in L1 which can lower the band
gaps obviously. Furthermore, the absorption coefficients (εmax) of
the maxima increase consistently and significantly and the OPEF
spectral intensities are also vary regularly upon increasing the
branch number. The intensities of the OPEF of L2 and L1 follow the
pattern L1 > L2 in the low polar solvents (benzene) with the ratios
of 17.2:1 which is not consistent with branch number [40], while
the intensities of the OPEF decrease in the high polar solvents
(CH3CN) from compound (L2) to compound (L1) with the ratios of
1:1.3. These can be explained by negative solvato-kinetic effect of
L2. Besides, seen from Table 1, the stokes shifts show
monotonically decreasing tendency as the branch number
increases, indicating the lone pair electrons on the nitrogen atom of
L1 may have delocalized onto the conjugated system and increase
intra-molecular charge transfer which could reduce the
nonradiative decay channels. These results suggest that compound
(L1) is typical D-π-A-π-D chromophores and the nature of peripheral
substituting groups (D) have significant influence on their single-
photon optical properties.
absorption and emission (cm-1), h is Planck’s constant, c is the speed
of light in a vacuum, a is the Onsager radius and b is a constant. Δf
is the orientation polarizability, μe and μg are the dipole moments of
the emissive and ground states, respectively, and ε0 is the
permittivity of the vacuum. (μe -μg)2 is proportional to the slope of
the Lippert–Mataga plot. Plots of the Stokes shifts as a function of
the solvent polarity factor Δf are shown in Fig. 4. Only the data
involving the aprotic solvents are shown because application of this
analysis with solvents where specific solute-solvent interactions are
present is not appropriate. As shown in Fig. 4, the Lippert–Mataga
plot of L1 and L2 give a large slope, which implies larger dipole
moment changes with photoexcitation. The slope of the best-fit line
is related to the dipole moment change between the ground and
excited states (μe -μg). The slopes of two lines are different: 6086,
7109 cm-1 for compounds L1 and L2, respectively. So the values of
μe -μg were calculated as 12.3 D for L1, 10.3 D for L2, respectively.
The values of the complexes indicate that the molecule in the
excited state has an extremely polar structure, an enhanced 2PA
response of L1 was found compared with L2, which is in good
agreement with their nonlinear optical properties as shown in Table
2.
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to multiphoton excited fluorescence. As shown in Fig. S4, a log-log
plot of the excited fluorescence signal versus excited light power. It
provides direct evidence for the squared dependence of excited
fluorescence power and input laser intensity upon excitation with a
tunable laser in this, therefore, it should be safely assigned to two-
photon excited fluorescence (TPEF) for L1 and L2.
By tuning the pump wavelengths incrementally from 720 to 860nm
with a step size of 10 nm while keeping the input power fixed and
then recording TPEF intensity, TPEF spectra are obtained; the TPEF
spectra of L1 and L2 in benzene are shown in Fig.5. It can be seen
that the peak position of the TPEF spectra of L1 and L2 are
independent of the excitation wavelengths (from 720 to 860nm, Fig
S5). The similarities between TPEF and OPEF indicate that both the
emissions for a given compound are from the same excited state,
though their initial Frank-Condon states may be different. As shown
in Fig. 5, the TPEF intensities of these compounds are remarkably
stronger than their corresponding OPEF intensities and the TPEF
peak wavelengths around 500 and 600 nm of L1 with much higher
concentration in benzene are evidently red-shifted by 34-40nm in
comparison with those of OPEF in the much lower concentration
solution, which can be explained by the reabsorption effect. The
short wavelength side of the two-photon fluorescence was
reabsorbed by the solution and red-shifts of the fluorescence
spectra were readily observed [42]. Moreover, Fig. S6 shows
obvious red-shift in the two-photon fluorescence maxima as the
polarity of solvent increases of L1 and L2. For L1, The intensities of
the TPEF decrease with solvent polarity increase, exhibiting an
obvious positive solvate-Kinetic effect. On the contrary, for L2, the
intensities of the TPEF increase generally, showing the negative
solvate-kinetic effect. Furthermore, It can be seen from Fig. S7 that
the intensities of the TPEF increases obviously with the ratios of
1.0:9.3 as the branch number increases, which is a result of
extended π-delocalization of L1.
Fig. 4 Lippert–Mataga plots for compounds L1 and L2.
Fluorescence lifetime and quantum yield
To get more insight into the radiative and nonradiative decay
processes of L1 and L2, we also conducted time-resolved lifetime
experiments (Table 1, Fig. S3).With increasing polarity of the
solvents, fluorescence lifetime of the compounds increase
generally. These can be explained by the stronger solute/solvent
interaction at the excited state comparing with that at the ground
state, which indicates that the increasing polarity of the excited
state increases. The energy level can be lowered greatly by
increasing dipole-dipole interaction between the solute and
solvent. The quantum yields (Φ) of the compounds L1 and L2 in
different solvents are determined by using Fluorescein as
a
standard. As shown in Table 1, the quantum yields of compound L1
decrease generally as the polarity of solvent increases except in
cyclohexane and benzene, showing the positive solvato-kinetic
effect. Note that the quantum yields of L1 obtained in cyclohexane
are smaller than that in benzene, because of the negative solvato-
kinetic effect. On the contrary, for compound L2, the quantum yield
increases generally with solvent polarity, exhibiting negative
solvato-kinetic effect.
4 Two-photon excited luminescence (2PEF) spectral properties
It was found that there is no linear absorption in the wavelength
range 680~1080nm, indicating that there is no energy level
corresponding to an electron transition in the spectral range.
Therefore, upon excitation from 680 to 1080 nm, it is impossible to
produce single-photon-excited up-converted fluorescence. If
frequency upconverted fluorescence occurs upon excitation with a
tunable laser in the range 680-1080 nm, it can be safely attributed
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Fig.5 The single-photon and two-photon excited fluorescence spectra of L1 in
benzene(c=1×10-3mol L-1).
Table 1 Photophysical properties of chromophores L1 and L2 in several of different polar solvents.
a
c
Δνi
εb
d
h
τ
λmax
λmax
Φ
Solvents
0.51
1.04
1.67
2.02
1.64
0.69
0.22
1.42
0.09
cyclohexane
benzene
429
442
461
449
445
446
449
444
7.97
1.98
2.36
2.56
2.41
2.25
2.52
9.77
473
506
544
529
526
567
566
557
0.59
0.65
0.35
0.46
0.42
0.11
0.10
0.17
2196
2887
3310
3393
3461
4635
4604
4569
dichloromethane
THF
L1
ethyl acetate
ethanol
acetonitrile
DMF
cyclohexane
benzene
403
410
415
413
402
409
402
412
9.64
9.06
9.73
9.08
9.35
9.20
9.86
1.12
455
478
514
506
500
531
523
523
0.04
0.09
0.14
0.19
0.11
0.24
0.25
0.23
2867
3500
4670
4450
4876
5617
5755
5151
0.24
0.65
dichloromethane
THF
0.61
0.44
L2
ethyl acetate
ethanol
0.87
0.79
acetonitrile
DMF
0.96
a Absorption peak position in nm (1×10-5 mol L-1). b Maximum molar absorbance in 104 mol-1 L cm-1.
d
c Peak position of SPEF in nm (1.0×10-5 mol L-1), excited at the absorption maximum. Quantun
e
yields determined by using coumarin 307 (Φ = 0.56) (1.0×10-5 mol L-1) as the standard.
fluorescence liftime (ns). f Stokes shift in cm-1.
Table 2 Two-photon fluorescence properties of chromophores L1 and L2 in four polar solvents.
a
b
λmax
λmax
δc
Solvents
L1
benzene
532
840
2844
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dichloromethane
558
555
565
508
524
522
545
810
840
840
840
870
860
870
3502
3335
2844
842
THF
DMF
benzene
dichloromethane
THF
1815
1273
842
L2
DMF
a TPEF peak position in nm pumped by femtosecond laser pulses at 300 mw at their
maximum excitation wavelength. b 2PA maximum excitation wavelength. c 2PA cross
section in GM.
The two-photon absorption (TPA) spectra of L1 and L2 complexes
are determined in the wavelength range 680-1080 nm in benzene
(1.0 × 10-3 mol L-1) by the two-photon-induced fluorescence
method with fluorescein as the standard (Fig. 6, Table 2), using
two-photon induced fluorescence measurement technique
described in the experimental section. The experiments were
conducted in the femtosecond regime, thereby preventing
contribution from linear nonresonant absorption or from
excitedstate absorption. As shown in Fig. 6 and Table 2, the two-
photon absorption cross sections (δ) are dependent over
excitation wavelengths (from 720 to 860nm). The details of
determination conditions and theoretical calculations are given in
the Experimental Section. The largest δ is 2844 GM (1GM = 10-50
cm4 s photon-1) in benzene solution for L1, which is similar to
Goodson’s value [21]. The maximum two-photon absorption
cross-section is 842 GM for L2 in benzene solution which is in line
with the results of theoretical calculation about Δμ, as mentioned
above. Compared with L2, L1 bearing the much larger 2PA cross-
section may imply that extending πꢀconjugation domain
with Eher oxygen chains is higher than the ehyl substituent
pyrimidine derivative Z2 (1879 in benzene), such a difference must
be due to varied substituent group of the pyrimidine-based
chromophores, which may arise from the strong electron-donating
effect of L1.
Fig.6 TPA cross section of L1 and L2 in benzene (c=1×10-3mol L-1)
versus excitation wavelengths.
The TPA cross-section values (δ) increases obviously as the
branch number increases with the ratios of 1.0:3.5 from L2 and L1
(Fig. 6). The 2PA cross-section δ/MW is divided by the molecular
weight, varies in a proportion of 1.0:2.0. This means that the unit
molecular weight enables enhanced TPA cross-section values as
the branch number increases, which leads to the indication of
some interactions between branches in the molecule. The solvent-
dependent of the two-photon absorption cross section value was
reported by Bazan et al. [44] and our group [45]. Fig. S8 and Table
accomplished by increasing the number of branches of
a
chromophore molecule based on the structural motif of a multi-
branched olefin could be a useful approach toward enhanced
molecular nonlinear absorptivities. With respect to our system, L1
accomplished
a large 2PA cross-section (2844 GM) which
attributed to the fact that the branched chromophore has an
extended π-conjugated system compared to that of L2 [43].
Compared with our previous work[30], the 2PA cross-section of L1
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2 reveals that the two chromophores exhibit a discernible TPA
spectral feature in different solvents. L2 in high polar solvent
(DMF) shows its maxima TPA cross-section of 1192 GM. Increasing
the polarity of solvents leads to a remarkable increase of TPA
cross-section due to negative solvate-Kinetic. For compound (L1),
the maxima TPA cross-section values (3357 GM) were observed in
the solvent with moderate polarity (THF) [46]. These data indicate
effective DMSO content did not exceed 1%. These data show that
HepG2 cells show near 100% viability following 24 h of treatment
with 10 μM L1. Higher concentrations result in decreased cell
survival, with 65% and 53% viability observed following 24 h of
treatment with 40 μM L1 and L2 (Fig. S9). These cytotoxicity tests
show that sub- and low-micromolar concentrations of L1 are
related low toxic over a period of at least 24 h and could be safely
that
electronic
excitation
augments
pyrimidine-based
used for further biological studies (Fig. 7). A co-staining
chromophores electron density via an internal charge transfer
mechanism due to solvent effects. Consequently, compound L1
has the cooperative effects in both linear and non-linear TPA
process, which has been verified by Beljonne’s exciton model [47].
The extra enhancement in TPA response for L1 can be achieved
when the core allows significant electronic coupling rather than
L2.
experiment with Lysotracker® was performed to further
determine that whether L1 was internalized with lysochondria (Fig.
7 (d)). At the same time, TPM images were obtained from HepG2
cells incubated with L1, owing to its relatively low toxicity toward
live cells, high quantum yield, large TPA cross-section and long
fluorescent lifetime. Fluorescent images of confocal microscopy
and TPM of HepG2 cells labeled with L1 were captured. A bright-
field image (Fig. 7(a)) of each cell was taken immediately prior to
the one-photon and TPM imaging. The green red channel clearly
reveals HepG2 cells successfully uptake L1, and interestingly, the
luminescence located in cell cytosol (Fig. 7(b) and (c)). The unusual
two-photon luminescent properties and the low cytotoxicity of L1
make it a potential candidate as novel luminescence material for
live cell imaging.
Conclusion
Fig. 7 (a) Bright-field image of HepG2 cells. (b) One-photon image
of HepG2 cells incubated with 20 μL1 after 60 min of incubation,
washed by PBS buffer. λex = 458 nm (emission wavelength from
525 to 625 nm). (c) Two-photon image of HepG2 cells incubated
with 20 μL after 60 min of incubation, washed by PBS buffer. λex =
840 nm (emission wavelength from 500 to 600 nm). (d) One-
photon image of HepG2 cells incubated with Lysotracker®. λex =
633 nm (emission wavelength from 660 to 700 nm). (f) The overlay
of (a) to (d). (g) The overlay of (a) to (c). (h,i) Co-localization
micrograph profile prove that the co-staining region which is
located in cell cytosol.
In brief, two pyrimidine derivatives with D–π-A–π-D and D–π-
A configuration have been synthesized by convenient method. The
chromophores showed good one and two-photon excited
fluorescence behaviour and interesting solvatochromic properties.
The centrosymmetric charge transfer D–π-A–π-D structural
chromophore L1 exhibited relatively large TPA cross-section,
strong two-photon-induced fluorescence. The TPA cross-section of
the molecules tend to tone up with the increasing of conjugation
length and the donor strength. The compound L1 with a lower
molecular weight exhibits higher quantum yield, larger 2PA cross
section in the near-infrared region, higher photo-stability, lower
toxicity and brighter two-photon fluorescent bioimaging, was
5. Cell image and cytotoxicity assay
To search for the potential application of L1 and L2 in bioimaging,
cytotoxicities of the chromophores were measured toward the
human cervical carcinoma cells HepG2. Because of low aqueous
solubility, the tested compounds were dissolved in DMSO first and
then serially diluted in complete culture medium such that the
successfully applied to
a two-photon fluorescent probe for
labeling the cytoplasm in live cells. We believe that the design
method and fabrication strategy are potentially applicable to the
derivatization
of
analogous
two-photon
absorption
chromophores.
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Journal Name
Acknowledgement
T.Scherzer, J. Stampfl, and R. Liska, Macromolecules., 2013,
46, 352-361.
This work was supported by grants from the National Natural
[9] J. Lott, C. Ryan, B. Valle, J. R. Johnson III, D. A. Schiraldi, J.
Science Foundation of China (21501001, 21271004, 51372003,
51432001, 21271003, 51072001, 81503009 and 51272001),
National Key Basic Research Program 973 (2013CB632705),
Ministry of Education Funded Projects Focus on returned overseas
scholar, Program for New Century Excellent Talents in University
(China) and Doctoral Program Foundation of Ministry of Education
of China (20113401110004), China Postdoctoral Science
Foundation (2015M571912), the Swedish Research Council (VR)
(621-2013-5357) and Swedish Government strategic faculty grant
in material science (SFO, MATLIU) in Advanced Functional
Materials (AFM), the Wenner-Gren Foundations.
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Design, synthesis, linear and nonlinear photophysical properties of
novel pyrimidine-based imidazole derivatives
L1 exhibited relatively large TPA cross-section rather than L2, which can be
applied for labeling the cytoplasm in live cells.