Synthesis, crystal structures, one/two-photon optical properties and bioimaging application of two organic molecules with D-A and D-π-A models containing 6-phenyl-2,2′-bipyridine

Article abstract of DOI:10.1039/c7nj04812b

Two novel organic compounds with D-A (L1) and D-π-A (L2) models containing 4-(N,N-dipropylamino)phenyl [donor group (D)] and 6-phenyl-2,2′-bipyridine [acceptor (A)] groups were synthesized and characterized by IR, ¹H NMR, ¹³C NMR, MS and single crystal X-ray diffraction. Their photophysical properties were systematically investigated using both experiments and theoretic calculations. It was found that the two-photon action cross-section (δΦ) of the D-π-A model molecule (δΦ_L2 = 119 GM) was approximately three times larger than that of the D-A type molecule (δΦ_L1 = 43 GM). Furthermore, their cytotoxicity and applications in bioimaging were determined. The results showed that L1 and L2 could be used as cytoplasmic dyes with high stability and low cytotoxicity.

Full text of DOI:10.1039/c7nj04812b

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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ARTICLE TYPE
Synthesis, crystal structures, one/two-photon optical properties and
application for bioimaging of two organic molecules with D-A and D-π-
A models containing 6-phenyl-2,2’-bipyridine
Zhaodi Liu,a,b Fuying Hao,a Jie Liu,a Yingzhong Zhu,b Wei Du,b Qiong Zhang,b Jieying Wu*b
5
Received (in XXX, XXX) Xth XXXXXXXXX 2017, Accepted Xth XXXXXXXXX 2017
DOI: 10.1039/b000000x
Two novel organic compounds with D-A (L1) and D-π-A (L2) model, containing 4-(N,N-
dipropylamino)phenyl [donor group (D)] and 6-phenyl-2,2’-bipyridine [acceptor (A)] groups, were
synthesized and characterized by IR, 1H NMR, 13C NMR, Ms and single crystal X-ray diffraction
10 determination. Their photophysical properties were systematically investigated relying on both
experiments and theoretic calculations. It was found that the two-photon action cross-section (Φδ) of D-π-
A model molecule (ΦδL2 = 119 GM) is approximately three times larger than that of D-A type molecule
(ΦδL1 = 43 GM). The cytotoxicity and applications in bioimaging for them were carried out. The results
showed that L1 and L2 could be used as cytoplamic dyes with high stability as well as low cytotoxicity.
15
which was shown below. i) The styryl was introduced to extend
Introduction
the molecular conjugated length as well as adjust the molecular
50 planarity. ii) The electron acceptor ability of 2,2’-dipyridyl was
modified by 6-position phenyl group. iii) Propyl group (long
aliphatic chain) would result a good solubility. Based on their
crystal structure information, photophysical properties were
systematically studied by experimental method and theoretical
55 calculation. More importantly, easy protonation as well as large
2P action cross section make the compounds target the
intracellular acidic organelle under two-photon confocal
microscopy possible.
Two-photon absorbing materials have attracted attention
because of their appealing and numerous relevant biophotonic
applications such as fluorescence imaging in live tissue,
20 photodynamic cancer therapy, and biosensor1. In two-photon
absorption process, molecules utilized two near-infrared (NIR)
photons as excitation source to generate one photon with higher
energy2. This is therefore a definite advantage to access a deep
penetration depth with less intrinsic scattering in heavily
25 scattering biological tissue media than visible light3. Besides, the
deeper penetration depth of long wavelength irradiation into
issues, t the technique of two-photon excited fluorescence would
provide a high spatial resolution owing to low background noise
and high photon density within the scope of λ34. Herein, the
30 advantages not only lie in the increased penetration depth in
tissue but also in the reduced background fluorescence of the
imaging collected.
Results and Discussion
60
The common two-photon absorbing materials could be roughly
divided to organic molecules, organic/inorganic complexes, and
35 nanoparticals5. Among them, organic molecules presented
significant benefits, such as tunable molecular structure, tunable
molecular weight, and terminal modification. Hence, organic
molecules could be served as building blocks for materials used
in nonlinear optics. To date, the main strategies used in designing
40 two-photon absorbing organic molecules are strengthening the
ability of donor/acceptor group, extending the conjugated length,
adjusting the planarity of molecule, or increasing the branched
dipole or quadrupoles6. Furthermore, the selection of terminal
group would also lead to positive effects7. For example, organic
45 molecule with long aliphatic group would lead a better solubility.
In this work, we design and synthesized two novel two-photon
absorbing organic molecules. The three strategies were utilized
Scheme 1. The molecular structures of L1 and L2.
To search for novel two-photon absorption materials, we
design and synthesize D-A and D-π-A organic molecules based
on 6-phenyl-2,2’-bipyridine (Scheme 1). The detailed synthetic
65 strategy of L1 and L2 are outlined in Scheme S1. the 1,5-
dicarbonyl compounds (1) were obtained using the general
solvent-free methodology reported by Cave and Raston11. 4-
(N,N-dipropylamino)benzaldehyde, 4’-(p-Tolyl)-6-phenyl-2,2’-
bipyridine (2) were synthesized according to procedures
70 described in the literatures, respectively12.
3.1 Crystal structures of L1, L2
Crystal data and experimental details for the compounds are
listed in Table 1. The crystal structures of L1 and L2 are shown in
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358 nm (L1) to 395 nm (L2).
Fig. 1.
L1, consisted of 6-phenyl-2,2’-bipyridine (A: acceptor) and
4-(N,N-dipropylamino) phenyl (D: donor) moieties which formed
D-A molecular type, crystalized in monoclinic system with P21/n
As described in Fig. S5 and S6, the emission bands were
sensitive to the polarity of solvents. When changed the solvent
from toluene to N,N-dimethyl formamide, the emission band
5
space group. The molecule possessed a good planarity with 40 would red shift from 416 to 503 nm for L1, and 471 to 554 nm
for L2, respectively. The conjugation length of molecule would
distinctly affect their emission spectra. By extending the length of
π-conjugated bridge, a red shift, with near 60 nm wavelength,
would be achieved from L2 to L1 in the same solvent. The
45 fluorescence emission intensity of two compounds in different
dihedral angle of 35.5° (R1 and R4), 20.5° (R1 and R3), and 5.7°
(R1 and R2), respectively. Meanwhile, the primary bond lengths
were located between normal C-C/C-N and C=C/C=N bond
which confirm a good delocalized π-electron in molecule. The
10 crystal data of L2 were similar to that of L1, except for the D-π-A
Table 2. Summarized the data obtained for L1, L2 in different
solvents.
Table 1. Crystallographic data for L1, L2.
Compound
L1
L2
[g]
ν
/cm-1
[c]
λabs λem
Kr[f] Knr
ꢀ
Φ[d] τ/ns[e]
Empirical formula
Crystal system
Space group
T(K)
C28H29N3
Monoclinic
P21/n
C36H35N3
Monoclinic
P21/c
Compound
Solvent
Toluene
nm[a] nm[b]
(108s-1) (108s-1)
352 416 4370 0.94 0.69 0.99 0.68
Dichloromethane 354 459 6462 0.59 0.76 1.28 0.91
Tetrahydrofuran 353 452 6204 0.59 0.56 0.49 25.15
296(2)
296(2)
a (Å)
b (Å)
c (Å)
9.7012(1)
15.2517(2)
15.9105(2)
99.164(2)
2324.1(5)
4
1.165
872
0.069
0.3 × 0.3 ×0.1
0.0767
11.401(5)
12.131(6)
20.955(1)
97.285(5)
2875(2)
4
1.178
1088
0.069
0.2 × 0.1 ×0.1
0.0631
Ethyl acetate
Ethanol
Acetonitrile
N,N-dimethyl
formamide
Toluene
350 461 6879 0.45 0.56 0.82 0.98
352 501 8449 0.02 0.39 0.31 1.21
356 506 8327 0.20 0.58 0.49 0.69
L1
β
(°)
367 503 7367 0.42 0.44 0.99 0.68
V (Å3)
Z
384 471 4810 0.76 0.04 7.31 2.31
Dc(g cm-3)
F (000)
ꢀ (mm-1)
Crystal size (mm)
R1
Dichloromethane 386 523 6786 0.42 0.89 2.20 3.09
Tetrahydrofuran 390 516 6261 0.39 0.75 2.22 3.49
Ethyl acetate
Ethanol
Acetonitrile
N,N-dimethyl
formamide
382 513 6684 0.47 0.62 1.20 22.61
387 531 7007 0.05 0.42 2.90 3.26
388 554 7722 0.15 0.93 0.77 4.42
L2
397 553 7138 0.21 0.32 0.91 3.38
wR2
0.2203
0.1812
0.579 and
0.294
[a] Peak position of the longest absorption band. [b] Peak position of OPEF,
excited at the absorption maximum. [c] Stokes’ shift. [d] Quantun yields
determined. [e] The fitted fluorescence lifetime. [f] The radiative decay rate
constant. [g] Nonradiative decay rate constant.
Largest difference peak and 0.639 and
hole (e Å-3) 0.525
-
-
Fig. 1 Crystal structure of L1 (left) and L2 (right). Hydrogen
15 atoms have been omitted for clarity.
molecular type that provided a larger delocalized conjugated
system. In the crystal structure, L2 have a better planarity than
L1 with dihedral angels 35.5° (R1 and R4), 19.0° (R1 and
R3),and 3.7° (R1 and R2), respectively. All these results indicated
20 that L1 and L2 might be in favor of good nonlinear optical
properties.
3.2 Spectroscopic properties
The absorption and emission spectra of L1 and L2 were
presented in Fig. 2. Meanwhile, the corresponding photophyscial
25 data were summarized in Table 2.
Fig. 2. (a) The Uv-vis absorption spectra and (b) emission spectra
50 of L1 and L2 in DMF with concentration 1 × 10-5 ml/L. (c)
Frontier molecular orbitals of L1 (left), and L2 (right) (optimized
geometry).
3.2.1. Linear photophysical properties
solvent was consistent to their fluorescence quantum yield and
lifetime. With increasing the polarity of solvent, the quantum
55 yield and lifetime decreased. These results could be ascribed to
reducing the radiative decay rate (in connection with the red-
shifted emission) or increasing the non-radiative decay rates
(Table 2). Such phenomenon could also be observed in other
related push-pull terpyridine-based chromophores13.
In the absorption spectra, L1 and L2 presented two intense
bands in the range of 270 nm to 500 nm. Among them, the band
located at ~280 nm could be ascribed as π-π* transition of
30 terpyridine moiety, while the band located in lower energy range
(310-500 nm) could be attributed to intramolecular charge
transfer of molecule. A weak solvatochormism effect could be
observed with increasing the polarity of the solvents, revealing a
fairly small dipole moment at ground states. Due to the extending
35 conjugation of molecule, the absorption band was red shift from
60
TD-DFT calculation was employed to further confirm the
photophyscial properties of L1 and L2 (Fig. 2c S5 and S6, Table
2
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respectively (equation 1). The large change in the dipole moment
on excitation is typical for ICT processes, which implies they
have an extremely polar structure in the excited, corresponding to
their linear and nonlinear optical properties. Moreover, the large
45 value of L1 maybe due to the size of the molecule and the
efficient charge separation15.
3 and S1). Firstly, the orbital analysis exhibit that the highest
occupied molecular orbitals (HOMO) with π character MOs were
essentially composed of the 4-(N,N-dipropylamino) phenyl.
Meanwhile, the lowest unoccupied molecular orbital (LUMO)
distributions were localized over the entire molecule except the
phenyl of 6-phenyl-2,2’-bipyridine moiety. Secondly, the lower
energy bands in UV-vis absorption spectra of L1 and L2 (> 350
nm) should be assigned to the HOMO→LUMO, revealing an
intramolecular charge transfer form donor group to accepter
5
3.2.2. Two-photon excited fluorescence spectra and 2PA cross
sections
Since there is no linear absorption in the wavelength range
50 690-900 nm for L1, L2 (Fig. 2), therefore, upon excitation from
690 to 910 nm, it is impossible to produce single-photon-excited
up-converted fluorescence. Hence, the frequency up-converted
fluorescence or absorption spectra appeared upon excitation with
a tunable laser in this range should be attributed to a multi-photon
55 process. The multi-photon excited emission spectra of L1 and L2
were presented in Fig. S7. The logarithmic plot of the
fluorescence integral (830 nm for L2) showed a good linear
relationship with that of the pumped powers with slope 2.13 for
L2, which indicated a two-photon excited mechanism.
10 group. Thirdly, the emissions of L1 and L2 originated from
LUMO→HOMO transition with allowed emissions at 341 nm for
L1 and 451 nm for L2. All these results were correspondent to
the experimental data.
As discussed above, an increase in solvent polarity induced a
15 marked red shift of the emission band, which is assigned to an
intramolecular charge-transfer (ICT) property. The positive
solvatochromic properties were analyzed following the Lippert–
Mataga model to evaluate the dipole moment changes of the dyes
with photoexcitation14:
2ꢁf
60
In principle，the excited energy level of a molecule under the
one- and two-photon excitation should be the same, which lead to
similar emission spectra, but, some slight difference often
appeared due to the different excitation process. In this paper, the
1PEF and 2PEF band of L2 are almost the same but red-shift for
20
(1)
ꢁ
ν
=
(ꢀe − ꢀ
g )2 + b
4
πε0hca3
Table 3. Calculated UV-vis absorption properties, including
wavelength (nm), excitation energy (eV), oscillator strengths, and
major contribution for L1, L2.
65 about 22nm is observed for L1. The different excitation processes
(1PA and/or 2PA) and different excited energy level may
responsible for the phenomenon. Under the same experimental
condition, the intensity of L2 was stronger than that of L1,
originating from larger conjugated system. The two-photon action
70 cross-sections of L1 and L2 were calculated by two-photon
excited fluorescence
Experimental
Calculated data
f
Compound
E
chara
cter
λ
λ
(oscillatorcomposition
(eV)
max(nm) max(nm)
strengths)
109-110
(H-L)
136-137
(H-L)
367
397
362
412
3.42 0.11
3.01 1.04
ICT
ICT
L1
L2
Table 4. Calculated two-photon absorption cross-section of L1
and L2
δ (GM)a
Compounds The best nonlinear absorption wavelength (nm)
672
827
146
910
L1
L2
a 1 GM = 1 × 10-50 cm4s/photon
25 Fig. 3. Lippert -Mataga regressions of L1 and L2 with transition
dipole moments 17.7 D and 15.7 D, respectively.
ε
−1
n2 −1
(2)
ꢁf =
−
2ε
+1 2n2 +1
75 Fig. 4. Two-photon action absorption cross sections of L1, L2.
in which ∆ν = νabs - νem stands for Stokes shift, νabs and νem are
absorption and emission (cm-1), is the Planck’s constant, c is
h
method (Fig. 4). The maximum two-photon action cross- section
(Φδ) values were 43 GM at 720 nm for L1, and 119 GM at 830
nm for L2, indicating a low energy excited wavelength in near-IR
region. To further elucidate these results, the theory calculation
80 was carried out for the two-photon absorption cross-sections of
them. The related nonlinear optical data were summarized in
Table 4. As it described, the maximum 2PA cross-section value
was 146GM at 672 nm for L1, while that was 910 GM at 827 nm
for L2.
30 the velocity of light in vacuum, a is the Onsager radius and b is a
constant. ∆f 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. Even if this model is
35 limited to a point dipole approximation, but if hydrogen bonds
between solvent (ethanol) and dichloromethane are excluded, a
good linear correlation with relationship (1) was found Fig. 3.
The slopes of the lines are 12149 and 9590 cm-1 for
compounds L1 and L2, respectively. So the values of (µe-µg)
40 were calculated as 17.7 D, 15.7 D for compounds L1 and L2,
85 3.3 Cytotoxicity analysis and imaging application
In order to explore their potential biological applications,
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cytotoxicity tests of L1 and L2 were firstly evaluated using the
human cervical carcinoma cells (HepG2) by MTT assay. Fig. 5
directly presented the cytotoxicity of compounds to HepG2 cells
which cultured with compounds for 24 h. As it described, L1 and
L2 presented low invasive properties even the concentration up to
80 ꢀM (~ 80 %). This result indicated that the L1 and L2 showed
hypotoxicity to HepG2 cells implying that they could be safely
used for further biological studies. Additionally, with the same
functional group, L2 displayed a lower rate of cell apoptosis than
meant that L1 and L2 could be used as cytoplasmic dyes in bio-
imaging. The staining mechanism was starting from the
cytoplasm and could be explained as below. First, the pyridine
moiety of two compounds could be protonized in a mild acidic
40 envrionment (Fig. S12) which would lead to localized in acidic
organelle, such as lysosome and mitochondria. Afterward, the
compounds possessed lipid solubility which would lead to the
coloration for endoplasmic reticulum.
5
10 L1.
Conclusions
Cell uptake was the essential prerequisite in bio-imaging, pre-
experiments indicated that two compounds had successfully
penetrated the cellular membrane and associated with some
organelles of the cells. Subsequently, the cells were cultured with
15 L1 and L2 for 30 min, after that a higher resolution live-cell
micrograph (63×) was captured. As shown in Fig. 6, the emission
signals of L1 and L2 were both mainly located in the cytoplasm
45 In conclusion, structures, one/two photon properties of two
different types (D-A: L1 and D-π-A: L2) molecules contained 6-
phenyl-2,2’-bipyridine were systematically investigated. The
relationships between the structures and photophysical properties
for them can be understood both the experimentally and
50 theoretically. It has been found that the two-photon absorption
cross sections of L2 are larger than that of L1, which should be
attributed to the enlarged conjugation of L2. Considering the
chemical properties of two compounds, the bio-application and
the staining mechanism have been investigated in detailed.
55 Moreover, the two kinds of chromophores are N^N^C-
cyclometalated ligands which are suitable for preparing
cyclometalated platinum(II) and ruthenium(II) complexes to
studied their 2PA properties and the related studies are currently
underway in our group.
60 Acknowledgments
This work was supported by the National Natural Science
Foundation of China (51372003, 51432001, 51672002, and
21701025), and Excellent Young Talents Fund Program of
Higher Education Institutions of Anhui Province (No.
65 gxyq2017039).
Fig. 5 MTT assay of HepG2 cells treated with L1, L2 at different
20 concentrations for 24 h.
Notes and references
a
Department of chemistry and materials engineering, Fuyang Normal
College, Fuyang, 230039, P. R. China.
b Department of Chemistry, Anhui University and Key Laboratory of
70 Functional Inorganic Materials of Chemistry of Anhui Province, Hefei
230039, P. R. China.
E-mail: jywu1957@163.com
Electronic Supplementary Information (ESI) available: [†Crystallographic
75 date reported in this paper has been deposited with the Cambridge
Crystallographic Date Center and allocated the deposition numbers
CCDC 971264–971265. Copies of these information may be obtained
free of charge from the Director, CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (fax: +44-1223/336-033; e-mail: deposit@ccdc.cam.ac.uk).
80 Detailed experimental procedures; X-ray crystallography; fluorescence
lifetime measurement; photophysical data of all the compounds; figures].
See DOI: 10.1039/b000000x/
Fig. 6 Confocal microscopy imaging of L1 (upper), L2 (lower).
1PEF: one-photon fluorescence imaging micrograph; 2PEF: the
two-photon excited fluorescence imaging micrograph; DIC: the
25 bright field; Merge: the overlay micrograph of 1PEF, 2PEF, and
DIC. Scale bar = 10 ꢀm.
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New Journal of Chemistry
Page 6 of 6
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DOI: 10.1039/C7NJ04812B
Two novel organic compounds with DꢀA (L1) and DꢀπꢀA (L2) model, containing 4ꢀ(N,Nꢀdipropylamino)phenyl
[donor group (D)] and 6ꢀphenylꢀ2,2’ꢀbipyridine [acceptor (A)] groups, were synthesized and characterized by IR,
1H NMR, 13C NMR, Ms and single crystal Xꢀray diffraction determination. Their photophysical properties were
systematically investigated relying on both experiments and theoretic calculations. It was found that the
twoꢀphoton action crossꢀsection (Φδ) of DꢀπꢀA model molecule (ΦδL2 = 119 GM) is approximately three times
larger than that of DꢀA type molecule (ΦδL1 = 43 GM). The cytotoxicity and applications in bioimaging for them
were carried out. The results showed that L1 and L2 could be used as cytoplamic dyes with high stability as well
as low cytotoxicity.