Investigation of two-photon absorption properties in new A-D-A compounds emitting blue and yellow fluorescence

Article abstract of DOI:10.1016/j.molstruc.2015.03.036

Three new acceptor-donor-acceptor compounds (LBQ, DBQ, BYQ) were synthesized and characterized by infrared, hydrogen nuclear magnetic resonance, mass spectrometry and elemental analysis. Their photophysical properties were investigated including linear ab

Full text of DOI:10.1016/j.molstruc.2015.03.036

Journal of Molecular Structure 1093 (2015) 33–38
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Investigation of two-photon absorption properties in new A–D–A
compounds emitting blue and yellow fluorescence
a
b
b
Fan Jin a, Zhi-Bin Cai a, , Jiu-Qiang Huang , Sheng-Li Li , Yu-Peng Tian
⇑
a College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, PR China
b Department of Chemistry, Anhui Province Key Laboratory of Functional Inorganic Materials, Anhui University, Hefei 230039, PR China
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Three new A–D–A structure compounds based on pyridine were synthesized.
The comprehensive photophysical properties of three compounds were investigated.
The structure–properties relationship was discussed in various solvents.
BYQ has a good solubility in water.
BYQ shows promising applications in pharmaceutical and biological fields.
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new acceptor–donor–acceptor compounds (LBQ, DBQ, BYQ) were synthesized and characterized
by infrared, hydrogen nuclear magnetic resonance, mass spectrometry and elemental analysis. Their
photophysical properties were investigated including linear absorption, single-photon excited fluores-
cence, fluorescence quantum yield and two-photon absorption. These compounds in CH2Cl2 exhibit good
fluorescence quantum yield which are 0.36, 0.26, and 0.25 and the largest two-photon absorption cross-
section which are 48, 36, and 181 GM respectively. Under the excitation of Ti: sapphire laser with a pulse
width of 140 fs, LBQ and DBQ emit blue two-photon excited fluorescence (TPEF), while BYQ emits bright
Received 29 January 2015
Received in revised form 11 March 2015
Accepted 23 March 2015
Available online 27 March 2015
Keywords:
Two-photon absorption
Two-photon excited fluorescence
Acceptor–donor–acceptor compound
Synthesis
yellow TPEF. BYQ has a good solubility in water and the
r can be as large as 130 GM, so it shows promis-
ing applications in many pharmaceutical and biological fields.
Ó 2015 Elsevier B.V. All rights reserved.
Introduction
In order to improve
models were designed, including donor–
A) type [17,18], donor– -bridge–donor (D–
acceptor– -bridge–acceptor (A– –A) type [21], donor–acceptor–
donor (D–A–D)/acceptor–donor–acceptor (A–D–A) type [22–24],
and arborization type [25,26]. Among them, the (D–A–D)/(A–D–
A) type have some characteristics such as large intramolecular
charge transfer (ICT) and small energy band gap which lead to
r
values of materials, a variety of structural
-bridge–acceptor (D–
–D) type [19,20],
p
p–
Two-photon absorption (TPA) can be defined as simultaneous
absorption of two photons through virtual states in a medium,
and it belongs to the category of the third-order nonlinear optical
effect. In 1931, Maria Goeppert-Mayer [1] theoretically predicted
the existence of it in her doctoral thesis. However, the laser had
not been invented at that time, it was difficult to get the necessary
of two-photon absorption’s light intensity. Until the 1860s, Kaiser
[2] proved this absorption process existed through the actual
p
p
p
p
enhanced
of (D–A–D)/(A–D–A) type whose
tude of 10ꢁ47
r
, Chen [27] and Ray et al. [28] designed the molecules
r
were up to an order of magni-
experiment. TPA cross-section (
r) is the most important parameter
.
of TPA properties, TPA materials with large
r
have many potential
In this work, three new A–D–A molecules (LBQ, DBQ, BYQ) were
focused on their synthesis and TPA properties. Their structures
were characterized by IR, 1H NMR, MS, and elemental analysis.
LBQ and DBQ are composed of pyridine and a formyl group as
terminal acceptors (A) and two methoxy groups as central donors
(D), while BYQ has pyridinium as a stronger electron acceptor. The
applications, including two-photon fluorescence imaging [3–5],
optical limiting [6–8], three-dimensional (3D) data storage [9,10],
3D microfabrication [11–13], and photodynamic therapy [14–16].
p
-conjugated electron systems of these three molecules are rela-
⇑
Corresponding author.
tively small. In general, molecules possessing small conjugated
http://dx.doi.org/10.1016/j.molstruc.2015.03.036
0022-2860/Ó 2015 Elsevier B.V. All rights reserved.

34
F. Jin et al. / Journal of Molecular Structure 1093 (2015) 33–38
Br
C H Br
2
5
N
CH
3
C H
N
CH
3
2
5
OCH
OCH
3
3
OCH
OCH
3
3
CHO
CH Br
2
(HCHO)n
HBr
N (CH )
4
2 6
OHC
OCH
BrH C
2
CH3COOH/H2O
OCH
OCH
3
3
2
1
3
2
CH
3
LBQ
DBQ
CHO
N
(CH3CO)2O/CH3COOH
N
H CO
3
OCH
3
2
N
N
CH
3
CHO
(CH3CO)2O/CH3COOH
H CO
3
Br
OCH
2
3
Br
C H
2
N
5
BYQ
C H
N
CH
3
2
5
KOH
CHO
H CO
3
Scheme 1. Synthesis of the target compounds (LBQ, DBQ, BYQ).
systems exhibit short wavelength emission. Emission wavelength
exhibits red shifts by increasing the conjugated systems. LBQ and
DBQ emit blue fluorescence accordingly, however, BYQ realizes a
leap of emission wavelength through the salitying of pyridine
acceptor rather than increasing the conjugated system, which
emits bright yellow fluorescence. What is more, BYQ has a good
spectrophotometer with the maximum absorption wavelengths
as the excitation wavelengths. The fluorescence quantum yields
were determined using fluorescein in 0.1 mol Lꢁ1 sodium hydrox-
ide as the standard. The TPEF spectra were measured using a fem-
tosecond laser pulse and Ti: sapphire system (680–1080 nm,
80 MHz, 140 fs) as the light source.
solubility in water, and its
r can be as large as 130 GM in H2O,
so it shows promising applications in selective, recognition, and
analysis in medical diagnostics, pathogen recognition or
fundamental research, biological probes and other pharmaceutical
and biological fields [29].
Synthesis
1,4-Bis(bromomethyl)-2,5-dimethoxybenzene (1)
A
mixture of 1,4-dimethoxybenzene (8.28 g, 60 mmol),
paraformaldehyde (8.10 g, 270 mmol) and acetic acid (150 mL)
was heated to 60 °C, then 40% HBr (40 mL) was added to it using
a pressure equalizing funnel and the reaction mixture was stirred
for 4 h, cooled to room temperature and refrigerated overnight at
0 °C. The solid was filtered and dissolved in CH2Cl2, repeatedly
washed with saturated sodium bicarbonate, then the solvent was
evaporated under reduced pressure and the residue was recrystal-
lized from CHCl3 to give white needle crystals (13.65 g, 70.2%).
m.p. = 197–199 °C (Lit. m.p. = 197–198 °C [31]); 1H NMR
(500 MHz, CDCl3) d: 6.89 (s, 2H), 4.55 (s, 4H), 3.89 (s, 6H); FT-IR
(KBr) v/cmꢁ1: 3044, 2962, 2934, 2834, 1509, 1461, 1404, 1319,
1229, 1205, 1040, 874, 718, 664, 551.
Experimental
Materials and instruments
1-Ethyl-4-methylpyridinium bromide was synthesized accord-
ing to the literature procedures [30]. Other materials were com-
mercially available and were used without further purification.
Melting points were measured on an X-4 micromelting point
apparatus without correction. 1H NMR spectra were collected on
a Bruker AVANCE III 500 apparatus, with TMS as internal standard
and DMSO-d6 and CDCl3 as solvent. FT-IR spectra were recorded on
a Thermo Nicolet 6700 spectrometer using KBr pellets. Mass spec-
tra were taken on a Therm LCQ TM Deca XP plus ion trap mass
spectrometry instrument. Elemental analyses were conducted on
a Thermo Finnigan Flash EA 1112 apparatus.
2,5-Dimethoxy-1,4-benzenedicarboxaldehyde (2)
A mixture of hexamethylenetetramine (12.60 g, 90 mmol), 1
(9.72 g, 30 mmol) and CHCl3 (200 mL) was heated under reflux
for 3.5 h, the solvent was evaporated under reduced pressure and
50% acetic acid (200 mL) was added to the residue, heated under
reflux for 4.5 h, then concentrated hydrochloric acid (10 mL) was
The linear absorption spectra were measured on a Shimadzu
UV-2550 UV–visible spectrophotometer. The SPEF spectra mea-
surements were performed using
a RF-5301PC fluorescence

F. Jin et al. / Journal of Molecular Structure 1093 (2015) 33–38
35
Table 1
Linear and nonlinear optical properties of the target compounds (LBQ, DBQ, BYQ).
e
Compounds
LBQ
Solvents
D
md cmꢁ1
/
rg GM
SPEF
max
c
TPEF
nm
max
f
abs
a
k
nm
k
k
nm
10ꢁ4eb molꢁ1 L cmꢁ1
max
Toluene
THF
CH2Cl2
DMF
324
325
323
322
321
1.03
1.10
1.67
2.78
2.99
444
446
455
456
458
8342
8348
9078
9126
9316
0.16
0.19
0.36
0.21
0.24
459
467
26
48
CH3CN
DBQ
BYQ
Toluene
THF
CH2Cl2
DMF
312
311
306
307
307
1.46
1.72
2.17
3.12
3.38
446
448
460
463
465
9630
9833
10941
10975
11068
0.09
0.11
0.26
0.15
0.17
463
472
19
36
CH3CN
Toluene
THF
CH2Cl2
DMF
CH3CN
H2O
342
342
343
344
343
335
1.52
2.04
2.89
3.59
3.80
5.41
554
557
561
565
566
558
11189
11287
11329
11371
11487
11930
0.08
0.10
0.25
0.12
0.06
0.01
573
582
97
181
572
130
a
b
c
d
e
f
Maximum linear absorption wavelength, c = 1 ꢂ 10ꢁ5 mol Lꢁ1
.
Molar absorption coefficients.
Maximum single-photon excited fluorescence wavelength, c = 1 ꢂ 10ꢁ7 mol Lꢁ1
.
Stokes shift.
Fluorescence quantum yield were measured using fluoresceine as the standard in 0.1 mol Lꢁ1 sodium hydroxide. ð/fluorescein ¼ 0:90Þ [36].
Maximum two-photon excited fluorescence wavelength, c = 1 ꢂ 10ꢁ3 mol Lꢁ1
.
g
Two-photon absorption cross-section, 1 GM = 1 ꢂ 10ꢁ50 cm4 s photonꢁ1
.
yellow needle crystals (1.69 g, 62.8%). m.p = 110–112 °C 1H NMR
(500 MHz, DMSO-d6): d 10.34 (s, 1H), 8.62 (d, J = 3.9 Hz, 1H), 7.99
(d, J = 16.2 Hz, 1H), 7.83 (t, J = 7.6 Hz, 1H), 7.61 (d, J = 16.2 Hz,
1H), 7.60 (s, 1H), 7.55 (d, J = 7.8 Hz, 1H), 7.31 (dd, J1 = 7.5 Hz,
J2 = 4.8 Hz, 1H), 7.28 (s, 1H), 3.99 (s, 3H), 3.89 (s, 3H); FT-IR (KBr)
v/cmꢁ1: 3051, 2921, 2872, 1670, 1603, 1466, 1406, 1214, 1123,
1041, 970, 870, 770, 685 cmꢁ1; ESI-MS: m/z (%): 270.1 (100)
[M + H]+; elemental analysis calcd (%) for C16H15NO3: C 71.36, H
5.61, N 5.20; found: C 71.48, H 5.69, N 5.41.
2,5-Dimethoxy-4-[(1E)-2-(4-pyridinyl)ethenyl]benzaldehyde (DBQ)
The Synthesis of this compound was similar to LBQ. Light yel-
low needle crystals. Yield 71.3%. m.p. = 144–148 °C 1H NMR
(500 MHz, DMSO-d6): d 10.34 (s, 1H), 8.59 (d, J = 5.9 Hz, 2H), 7.70
(d, J = 16.6 Hz, 1H), 7.59 (s, 1H), 7.58 (d, J = 5.9 Hz, 2H), 7.54 (d,
J = 16.6 Hz, 1H), 7.29 (s, 1H), 3.98 (s, 3H), 3.89 (s, 3H); FT-IR (KBr)
v/cmꢁ1: 3064, 2947 2873, 1681, 1600, 1485, 1408, 1215, 1124
1035, 971, 879, 771, 691 cmꢁ1; ESI-MS: m/z (%): 269.8 (100)
[M + H]+; elemental analysis calcd (%) for C16H15NO3: C 71.36, H
5.61, N 5.20; found: C 71.59, H 5.78, N 5.47.
Fig. 1. UV–visible absorption spectra of the target compounds (LBQ, DBQ, BYQ) in
CH2Cl2 (c = 1 ꢂ 10ꢁ5 mol Lꢁ1).
added to the mixture, cooled to room temperature, extracted with
CHCl3 and dried with anhydrous sodium sulfate. after the solvent
was evaporated under reduced pressure and the residue was
recrystallized from ethyl alcohol to give bright yellow needle crys-
tals (1.98 g, 34.0%). m.p. = 202–203 °C (Lit. m.p. = 201 °C [32]); 1H
NMR (500 MHz, CDCl3) d: 10.52 (s, 2H), 7.47 (s, 2H), 3.96 (s, 6H);
FT-IR (KBr) v/cmꢁ1: 3047, 2985, 2953, 2870, 1679, 1483, 1411,
1396, 1301, 1214, 1130, 1027, 878, 660.
1-Ethyl-4-[(1E)-2-(2,5-dimethoxy-4-formylphenyl)ethenyl] pyridinium
bro- mide (BYQ)
A flask fitted with a magnetic stirrer and condenser was charged
with 1-ethyl-4-methylpyridinium bromide (2.01 g, 10 mmol), 2
(1.94 g, 10 mmol), piperidine (0.85 g, 10 mmol) and methyl alcohol
(50 mL). The mixture was heated under reflux for 7 h, then cooled
to room temperature. The mixture was filtered and the filtrate was
evaporated by rotary evaporators. The solid was purified by col-
umn chromatography on silica gel using petroleum ether/ethyl
acetate (1:5) as eluent to give a orange–yellow crystalline powder
(2.11 g, 55.7%). m.p. = 255–257 °C 1H NMR (500 MHz, DMSO-d6): d
10.37 (s, 1H), 9.00 (d, J = 6.9 Hz, 2H), 8.31 (d, J = 6.8 Hz, 2H), 8.09 (d,
J = 16.5 Hz, 1H), 7.77 (d, J = 16.5 Hz, 1H), 7.63 (s, 1H), 7.35 (s, 1H),
4.57 (q, 2H), 3.99 (s, 3H), 3.93 (s, 3H), 1.55 (t, J = 7.3 Hz, 3H); FT-
IR (KBr) v/cmꢁ1: 3084, 2941, 2897, 1685, 1618, 1490, 1410, 1211,
1123, 1041, 978, 873, 692 cmꢁ1; ESI-MS: m/z (%): 298.3 (100)
[M]+; elemental analysis calcd (%) for C18H20BrNO3: C 57.15, H
5.33, N 3.70; found: C 57.43, H 5.52, N 3.96.
2,5-Dimethoxy-4-[(1E)-2-(2-pyridinyl)ethenyl]benzaldehyde (LBQ)
A flask fitted with a magnetic stirrer and condenser was charged
with 2-methyl-py-ridine (0.93 g, 10 mmol), 2 (1.94 g, 10 mmol),
acetic anhydride (3 mL), and acetic acid (1.5 mL). The mixture
was heated under reflux for 6 h, then cooled to room temperature.
After concentrated hydrochloric acid (15 mL) was added, the mix-
ture was filtered. The filtrate was neutralized with 30% aqueous
sodium hydroxide solution (30 mL) and gave a precipitate. The pre-
cipitate was purified by column chromatography on silica gel using
petroleum ether/ethyl acetate (10:1) as eluent to give bright

36
F. Jin et al. / Journal of Molecular Structure 1093 (2015) 33–38
reaction of 1,4-bis(bromomethyl)-2,5-dimethoxybenzne (1) with
hexamethylenetetramine in chloroform, followed by hydrolysis
in acetic acid. LBQ and DBQ were synthesized by the acid con-
densation of 2-methyl-pyridine and 4-methyl-pyridine with 2,
while BYQ was synthesized by the alkaline condensation reaction
of 1-ethyl-4-methylpyridinium bromide with 2. In 1H NMR spectra,
compared with these three compounds, one can see chemical shift
(d) of BYQ is higher than these of LBQ and DBQ, it indicates that
pyridinium’s electron-accepting ability is stronger than that of
pyridine. The (E)-configurations of the C–C double bonds were cer-
tified by the coupling constants 3J (H, H) = 16.2–16.6 Hz for the ole-
finic AB spin systems.
Linear absorption and single-photon excited fluorescence (SPEF)
The comprehensive photophysical properties of the compounds
(LBQ, DBQ, and BYQ) in various solvents with different polarity are
listed in Table 1. One can see that absorption properties of three
compounds show little dependence on the polarity of the solvents.
With increasing polarity of the solvents from toluene to CH3CN, the
absorption peak position of LBQ, DBQ, and BYQ almost have no
variation, but the absorption peak position of BYQ in water exhi-
bits blue shift. This blue shift can be attributed to hydrogen bond-
ing formed by n electron and water in the ground state S0, which
results in the increase of the energy gap between S0 and the excited
state S1. As shown in Fig. 1 and listed in Table 1, compared the
ꢀ
ꢁ
abs
maximum absorption wavelength
k
of three compounds, we
max
abs
find the
k
exhibits obvious red shift from DBQ to BYQ
max
(BYQ > LBQ > DBQ). The reason for this characteristic is that the
electron-accepting ability of pyridinium is stronger than that of
2-pyridine, which is stronger than that of 4-pyridine. Among them,
the molar absorption coefficients (e) of BYQ is higher than the
other two, it shows that the ability of absorbing light of BYQ is
the strongest.
As shown in Fig. 2, all these compounds emit SPEF between 400
and 600 nm under the excitation at their maximum absorption
À
Á
SPEF
max
wavelengths. The SPEF maxima
k
and the stokes shifts (Dv)
of LBQ, DBQ, and BYQ increase with increasing polarity of the sol-
vents from toluene to CH3CN. We can explain the slight red shifts
are that strong solvent–solute dipole–dipole interactions, a mani-
festation of the large dipole moment and orientational polarizabil-
ity. An increased dipole–dipole interaction between the solute and
solvent leads to a lower energy level. H2O is a solvent whose
Fig. 2. SPEF spectra of the target compounds (LBQ, DBQ, BYQ) in various solvents
(c = 1 ꢂ 10ꢁ7 mol Lꢁ1).
Results and discussion
Synthesis and characterization
The new target compounds LBQ, DBQ, and BYQ were synthe-
sized according to Scheme 1. 2,5-Dimethoxy-1,4-benzenedicarbox-
aldehyde (2) was obtained according to Sommelet’s procedure by
Fig. 3. SPEF spectra and fluorescence photos of the target compounds (LBQ, DBQ,
BYQ) in CH2Cl2 (c = 1 ꢂ 10ꢁ7 mol Lꢁ1).

F. Jin et al. / Journal of Molecular Structure 1093 (2015) 33–38
37
from 680 nm to 910 nm in Fig. 4. The
r
is obtained by comparing
the two-photon excited fluorescence (TPEF) intensity of the sample
with that of a reference compound by the following Eq. (1) [33]:
Fs /r nr c
Fr /s ns cs
rs
¼
r rr
ð1Þ
where the subscripts s and r denote the sample and the reference
compound. F and / represent the TPEF integral intensity and the
SPEF quantum yield. n and c are the refractive index and the con-
centration of the solution. In this work, we selected fluorescein in
0.1 mol Lꢁ1 sodium hydroxide (c = 1 ꢂ 10ꢁ3 mol Lꢁ1) as the refer-
ence (r = 36 GM [33]).
As shown in Fig. 4, the wavelength of LBQ, DBQ, and BYQ at
maximum values in CH2Cl2 are 680 nm, 770 nm and 700 nm,
respectively. The peak values of LBQ and DBQ are 48 and
r
r
36 GM, which are not the best values due to our wavelength of
light source ranges from 680 nm to 1080 nm. This range cannot
Fig. 4. Two-photon absorption cross sections of the target compounds (LBQ, DBQ,
BYQ) in CH2Cl2 (c = 1 ꢂ 10ꢁ3 mol Lꢁ1).
abs
reach their relative twofold k . The
r of BYQ which is as large
max
as 181 GM in CH2Cl2 is higher than the other two obviously, the
reason is also electron-accepting ability of pyridinium is stronger
than pyridine and it is easy to realize the polarization transition
under the light induced. BYQ has a good solubility in water and
polarity is stronger than CH3CN, however, the SPEF of BYQ in H2O
exhibits slight blue shift. The reason is the influence of water via
hydrogen bonding, too. In Table 1, we find the fluorescence quan-
tum yields (/) of three compounds are maximum in CH2Cl2, it is
possible that CH2Cl2 has special solvation effect which increases
its
r can be as large as 130 GM in H2O, it shows promising applica-
tions in pharmaceutical and biological fields.
As shown in Fig. 5, the two-photon fluorescence intensity of
three compounds in CH2Cl2 is the strongest among three solvents,
the reason is similar to the SPEF. In water, two-photon fluorescence
molecular structure rigidity and coplanarity. As shown in Fig. 3,
SPEF
max
compared these three compounds, one can see the k
increases
intensity has
a bit decrease compared with the other two
from LBQ to BYQ (BYQ > DBQ > LBQ). For example, in CH2Cl2 solu-
SPEF
solvents, the reason is the hydrogen bonds consuming some
energy from the excited state molecules [34]. Compared with the
tion, the k
of BYQ presenting bright yellow fluorescence at
max
561 nm is red-shifted by about 100 nm relative to that of LBQ
and DBQ presenting blue fluorescence at about 460 nm. This can
be attributed to the stronger electron-accepting ability of
pyridinium which reduces the energy gap between S0 and S1.
À
Á
TPEF
max
SPEF
max
TPEF
of three compounds, the k
max
TPEF maxima
k
and the k
exhibits red shifts, this can be explained by the effect of reabsorp-
tion for the linear absorption band. It has a slight overlap with the
emission band and our two-photon fluorescence experiments were
carried out in concentrated solutions that made reabsorption
significant.
Two-photon properties
BYQ in CH2Cl2 were pumped by femtosecond laser pulses at dif-
ferent pump intensities with 700 nm excitation wavelength. Fig. 6
shows a log–log plot of the excited fluorescence signal vs. excited
light power. The slope value of BYQ is 2.02. It provides direct evi-
dence for the squared dependence of excited fluorescence intensity
Methods for
r measurements include nonlinear transmission
method, Z-scan technology, two-photon induced fluorescence
method and two-photon transient absorption spectroscopy
method. Three compounds (LBQ, DBQ, BYQ) were measured by
the two-photon induced fluorescence method. The
the target compounds in CH2Cl2 were recorded under 500 mW
r values of
and input laser power, suggesting
mechanism.
a two-photon excitation
Fig. 6. Output fluorescence intensity (Iout) vs. the square of input laser power (Iin
for BYQ, excitation carried out at 700 nm, with c = 1 ꢂ 10ꢁ3 mol Lꢁ1 in CH2Cl2.
)
Fig. 5. TPEF spectra of BYQ in various solvents (c = 1 ꢂ 10ꢁ3 mol Lꢁ1).

38
F. Jin et al. / Journal of Molecular Structure 1093 (2015) 33–38
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In this work, three new A–D–A structure compounds containing
pyridine and pyridinium were synthesized. The linear absorption,
single-photon excited fluorescence, fluorescence quantum yield,
two-photon absorption of three compounds were investigated.
The structure–properties relationship was discussed in various sol-
vents. Three compounds in CH2Cl2 exhibit the largest fluorescence
quantum yield which are 0.36, 0.26, and 0.25 and the largest two-
photon absorption cross-section which are 48, 36, and 181 GM
respectively. LBQ and DBQ emit blue two-photon excited fluores-
cence, while BYQ emits bright yellow TPEF. What is more the
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water-solubility of BYQ whose
r is 130 GM provides the possibility
for selective, recognition, and analysis in medical diagnostics,
pathogen recognition or fundamental research, more importantly,
it is anticipated to provide insights into small molecule–protein
and protein–protein recognition at the molecular level [35].
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