Silver(I) Complexes of Diphenylpyridines: Crystal Structures, Luminescence Studies, Theoretical Insights, and Biological Activities

Article abstract of DOI:10.1002/cplu.201600620

A series of simple two-coordinated cationic silver(I) complexes, namely, [Ag{4-(4-R¹-phenyl)-2,6-diphenylpyridine}₂]X (X=ClO₄ ^?, BF₄ ^?, or SO₃CF₃ ^?), with different electron-donating or -withdrawing groups (e.g., R¹=N(Me)₂, Me, H, Cl, and Br) on the phenyl ring, were successfully prepared. Extensive characterization of these complexes by various NMR spectroscopy techniques and mass spectrometry was further corroborated by single-crystal XRD analyses. Detailed photophysical investigations of [Ag{4-(4-N,N-dimethylaminophenyl)-2,6-diphenylpyridine}₂]ClO₄ (C1) displayed a strong room-temperature fluorescence in solution with an anomalously high luminescence quantum yield of 0.83. The effects of distinct substituent groups (C2–C5), π-conjugated aromatic rings (C6 and C7), and anions (C8 and C9) on the photoluminescence properties were evaluated. Furthermore, DFT and time-dependent DFT calculations were performed to discern the composition of the excited state, as well as to confirm the obtained relative emission energies upon substitution with electronically different ligands. These results indicated that the strong electron-donating substituent of N,N-dimethylamine played an important role in the unprecedented high luminescence quantum yield of C1. In addition, preliminary antimicrobial studies and confocal microscopy fluorescent imaging of HeLa cells labeled with these complexes reveal their potential applications in biological activities.

Full text of DOI:10.1002/cplu.201600620

DOI: 10.1002/cplu.201600620
Full Papers
Silver(I) Complexes of Diphenylpyridines: Crystal
Structures, Luminescence Studies, Theoretical Insights,
and Biological Activities
[a]
[b]
[a]
[a]
[c]
De-Hui Wang,* Yuan Zhang, Yu-Tong Wang, Hui-Yan Feng, Yong Chen, and
De-Zhi Zhao
[a]
A series of simple two-coordinated cationic silver(I) complexes,
groups (C2–C5), p-conjugated aromatic rings (C6 and C7), and
anions (C8 and C9) on the photoluminescence properties were
evaluated. Furthermore, DFT and time-dependent DFT calcula-
tions were performed to discern the composition of the excit-
ed state, as well as to confirm the obtained relative emission
energies upon substitution with electronically different ligands.
These results indicated that the strong electron-donating sub-
stituent of N,N-dimethylamine played an important role in the
unprecedented high luminescence quantum yield of C1. In ad-
dition, preliminary antimicrobial studies and confocal microsco-
py fluorescent imaging of HeLa cells labeled with these com-
plexes reveal their potential applications in biological activities.
1
À
,
4
namely, [Ag{4-(4-R -phenyl)-2,6-diphenylpyridine} ]X (X=ClO
2
À
À
BF4 , or SO CF ), with different electron-donating or -with-
3
3
1
drawing groups (e.g., R =N(Me) , Me, H, Cl, and Br) on the
2
phenyl ring, were successfully prepared. Extensive characteriza-
tion of these complexes by various NMR spectroscopy tech-
niques and mass spectrometry was further corroborated by
single-crystal XRD analyses. Detailed photophysical investiga-
tions of [Ag{4-(4-N,N-dimethylaminophenyl)-2,6-diphenylpyridi-
ne} ]ClO4 (C1) displayed a strong room-temperature fluores-
2
cence in solution with an anomalously high luminescence
quantum yield of 0.83. The effects of distinct substituent
Introduction
The design and construction of coinage metal complexes with
able connecting node for the construction of coordination
polymers owing to its two-coordination diversity and flexibili-
10
d -monovalent ions is an area of intense research activity be-
cause of their rich photophysical properties and potential ap-
[
4]
ty. The majority of strategies to obtain high luminescence
quantum yields (QYs) of silver(I) complexes in solution so far
have relied on silver(I) sensory materials and silver(I) N-hetero-
[
1]
plications. Recently, synthetic strategies for constructing func-
1
0
tional complexes of d metals with various structures and lu-
minescence properties have been well established. In particu-
lar, considerable attention has been given to the design of lu-
[
5]
cyclic carbene complexes. In addition, poor stability has hin-
dered the investigation of these reported luminescent silver(I)
complexes, in contrast to extensively explored copper(I) and
gold(I) complexes. Therefore, the development of silver(I) com-
plexes with strong fluorescent emission and excellent stability
in solution at room temperature has been a formidable chal-
lenge yet to be achieved.
[
2]
minescent copper(I) and gold(I) complexes in solution.
However, only a handful of luminescent silver(I) complexes in
solution have been reported owing to their nonemissive
[
3]
nature in solution. Recently, a great deal of research has been
focused on their weakly fluorescence properties in low-temper-
[3]
ature glassy solutions or in the solid state. Moreover, most sil-
ver(I) complexes were designed to investigate the properties
of special structures because silver(I) is a favorable and fashion-
The C^N^C-type ligands are those derived from cyclometa-
lated ligands. The C^N^C-based complexes with high lumines-
cence QYs were pioneered by von Zelewsky and developed by
[
6]
the groups of Rourke, Che, Yam, and others. Accordingly, the
successful applications of precious metals encouraged us to
further extend this excellent research. Herein, we report the
synthesis, crystal structures, luminescence studies, and biologi-
[
a] Prof. D.-H. Wang, Y.-T. Wang, H.-Y. Feng, Prof. D.-Z. Zhao
College of Chemistry
Chemical Engineering and Environmental Engineering
Liaoning Shihua University, Fushun 113001 (P. R. China)
E-mail: dhwang@lnpu.edu.cn
1
+
cal activities of [Ag{4-(4-R -phenyl)-2,6-diphenylpyridine} ]
2
+
complexes. The pyridine derivative was used as the Ag recep-
tor unit, whereas the chromophore of N,N-dimethylamine
(DMA) unit was chosen as the strong electron-donating sub-
stituent. These necessary units were introduced as trigger sites
dhuiwang@aliyun.com
[
b] Dr. Y. Zhang
Liaoning Institute for Food Control
Shenyang 110015 (P. R. China)
+
[
c] Prof. Y. Chen
to achieve efficient Ag coordination and a strongly fluores-
Technical Institute of Physics and Chemistry
Chinese Academy of Sciences
Beijing 100190 (P. R. China)
cent emission in solution. The unprecedentedly high lumines-
cence QY of C1 can be attributed to intraligand charge-transfer
I
(
ILCT) transitions with some mixed Ag 5p orbital perturbation
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/cplu.201600620.
[
7]
character. In addition, the introduction of different electron-
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donating or -withdrawing groups (C2–C5), anions (C8 and C9),
and pendant aromatic groups (C6 and C7) was taken advant-
age of to evaluate the effect of chemical structures on the
spectral properties. Interestingly, the bright-blue region (l
ꢀ
450–550 nm), high QY, and visible-light excitation of C1 pro-
vided an opportunity to use this complex as a bioimaging
agent for cells. To the best of our knowledge, complex C1 is
one of the brightest silver(I) complexes in solution at room
temperature with a high luminescence QY.
Results and Discussion
Synthesis and characterization of L1–L6 and C1–C9
Various methods have been published for the synthesis of
[
8]
Scheme 2. Synthesis of complexes C1–C9. Reagents and conditions:
i) CH Cl /CH OH, room temperature.
2 2 3
C^N^C derivatives. Herein, we adopted the one-step method
[8a]
1
reported by Collin et al. The 4-(4-R -phenyl)-2,6-diphenylpyri-
dine derivatives (L1–L6) were prepared through the 1,4-Mi-
chael addition reaction of the unsaturated ketone intermediate
tion of the ligands to the silver(I) center. Furthermore, the reso-
nances did not shift upon dilution of the solutions from 0.40
to 0.01m. The concentration dependence of absorption spectra
confirmed that the complexes remained intact over the con-
1
produced by condensation of 4-R -benzaldehyde and aceto-
phenone (or 2-acetonaphthone) in good yield (Scheme 1).
À6
centration range of 0.4 to 1”10 m (Figure S32 in the Sup-
porting Information). Extensive characterization of the com-
plexes, including intermediate products, has been performed
by means of NMR spectroscopy and mass spectrometry (MS).
Single-crystal XRD studies
Single-crystal XRD studies performed for C1–C5 further corro-
borated the structural assignment from the NMR spectroscopy
and MS studies. The perspective views of C1–C5 are shown in
Figure 1. Single-crystal structural analysis reveals that the sil-
ver(I) center is coordinated to two pyridine nitrogen atoms
from two distinct L1–L5 ligands in a slightly distorted linear
geometry with N-Ag-N bond angles of 168.8(3)8 for C1,
Scheme 1. Synthesis of the ligands L1–L6 and chemical structures of L1–L7.
1
69.8(2)8 for C2, 173.4(1)8 for C3, 168.8(2)8 for C4, and
Reagents and conditions: i) acetophenone or 1-(2-naphthalenyl)ethanone/4-
1
174.9(3)8 for C5 (Table 1). In the solid state, these compounds
are asymmetric. Steric hindrance of the pendant phenyl or
naphthalene groups prevents the two pyridyl rings from being
coplanar, and the dihedral angle between the two pyridyl
planes is approximately 778. Meanwhile, steric hindrance of the
pendant aromatic groups effectively encapsulates the silver(I)
cation. The spatial arrangement of the side aromatic groups
around silver is clearly demonstrated by viewing the molecule
along the NÀAgÀN axis (Figure 2), which clearly shows how
R -benzaldehyde (2:1), acetamide, reflux, 2 h.
With the aim of studying the effect of chemical structures of
the organic ligands on the properties of these silver(I) com-
plexes, and hence, obtain the structure–property relationship,
different moieties with distinct properties of electronic clouds
were introduced into the C^N^C-type ligands. The reaction of
stoichiometric amounts of AgX (X=ClO , BF , and SO CF ) with
4
4
3
3
I
L1–L7 in CH Cl or mixed solvents of CH Cl /CH OH afforded
the Ag cation is protected in the middle of the complexes.
2
2
2
2
3
1
light lemonade-colored or colorless crystals of [Ag{4-(4-R -
Complexes C1–C9 were stable in air and in common organic
solvents for extended periods and no signs of degradation
were observed. This phenomenon is consistent with the en-
capsulated structures. Short AgÀC distances of 3.039 to
3.155 Š have also been observed for these complexes. It is sup-
posed that weak AgÀC contacts should be responsible for the
unique stability, although these values are longer than those
reported in the literature, for example, 2.363(5)–2.647(6) Š in
[Ag(toluene) {Al(OR) }] (R=C(H)(CF ) ) and 2.385(5)–2.410(5) Š
phenyl)-2,6-diphenylpyridine} X] (C1–C9; ꢀ85%) upon recrys-
2
1
tallization from CH Cl /diethyl ether (Scheme 2). H NMR spec-
2
2
troscopy studies performed on the final products revealed
characteristic resonances for almost all of the protons on the li-
1
gands. Compared with the H NMR spectrum of the free
ligand, silver(I) binding caused significant upfield shifts of
almost all of the proton signals (Figures S17–S31 in the Sup-
2
porting Information), especially for the protons on the R
2
4
3 2
9]
[
groups (dꢁ0.33 ppm); this strongly suggested the coordina-
in [Ag(ethene) {Al(OR) }] (R=C(CF ) ).
3 4 3 3
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Figure 2. The encapsulation structure of C1, showing the spatial orientation
I
of the side aromatic groups around the central Ag cation and a view along
the NÀAgÀN axis.
3
.762 Š in C1. Complexes C2–C5 exhibited similar weak cyclic
p–p interactions.
Electronic absorption and luminescence properties
Electronic absorption
The electronic absorption of complexes in CH Cl2 obey the
2
À6
À4
Beer–Lambert law in the concentration range of 10 –10
m
(
Figure S32 in the Supporting Information). Important photo-
physical data for the complexes are summarized in Table 2. For
ligand L1, the absorption bands at l=258, 308, and 388 nm
come from the p!p* transitions (Figure S12 in the Supporting
Information). As depicted in Figure 3, complexes C2–C5, con-
taining different substituents, consist of one group with the
same electronic transitions. The intensive absorption bands in
the UV region (l<300 nm) are assigned to the intraligand p!
p* transition. More significantly, however, the spectrum of C1,
with a DMA substituent, consists of another two broad, low-
energy bands at l=390 and 475 nm (eꢀ61010 and
I
Figure 1. X-ray crystal structures of Ag complexes C1–C5. Thermal displace-
ment ellipsoids are drawn at the 30% probability level. The anions, solvent
molecules, and hydrogen atoms are omitted for clarity.
À1
3
À1
1
990 mol dm cm ); these bands were tentatively assigned
to charge-transfer transitions. The shapes of the spectra and
extinction coefficients of complexes C1–C5 are similar, except
that of C1, which exhibits a stronger and broader low-energy
charge-transfer absorption.
Table 1. Selected bond lengths [Š] and angles [8] for C1–C5.
C1
C2
C3
C4
C5
Ag1ÀN1
2.129(7)
2.149(7)
168.8(3)
2.164(4)
2.162(4)
169.8(2)
2.174(2)
2.175(2)
173.4(1)
2.163(3)
2.158(3)
168.8(2)
2.181(6)
2.176(6)
174.9(3)
Ag1ÀN2
N1-Ag1-N2
Emission
In L1, the strong electron-donating property of the substituent,
DMA, makes it facile to form an ILCT transition from the DMA
(p) moiety to the pyridine moiety (p*; Figure S15 in the Sup-
porting Information). Ligand L1 alone showed a weak emission
with lmax =409 nm (Figure 4a). When the silver(I) ion coordi-
nates to two pyridine nitrogen atoms from two distinct L1 li-
The stacking diagrams of C1–C5 are depicted in Figures S1–
S5 in the Supporting Information. As shown in Figure S1 in the
Supporting Information, the pyridyl ring and 4-DMA–phenyl
plane from two distinct molecules of L1 organize in a head-to-
tail fashion with offset face-to-face cyclic p–p interactions at
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DMA portion increases. Some 4-methyl-4’-[p-(dime-
thylamino)-a-styryl]-2,2’-bipyridine and 4’-aryl-terpyri-
dine ligands with amino substituents are proven to
Table 2. Spectroscopic and photophysical properties of C1–C9.
Compd Medium
T [K])
l
abs [nm]
l
em [nm]
QY
(F
À1
3
À1
(
(e [mol dm cm ])
(t [ns])
f
)
[10]
possess ILCT transitions. Most luminescent silver(I)
C1
C2
C3
C4
C5
C6
C7
C8
C9
L1
CH Cl
solid (298)
glass (77)
2
2
(298) 288 (23110), 393 (61010), 470 sh (1990) 475 (2.42)
0.83
complexes are merely emissive in the solid state or
exhibit intense emissions in glassy solutions. Interest-
ingly, complex C1 exhibits an emission not only in
the solid state, but also in solution at room tempera-
ture (Figure 4a). Complex C1 exhibited a strong pure
blue photoluminescence with an emission maximum
at l=475 nm upon excitation at l=391 nm in
494
471, 550 sh
CH
2
Cl
2
(298) 255 (76900), 307 sh (20360)
368 (6.18)
384
0.27
solid (298)
glass (77)
380, 446, 478
CH
2
Cl
2
(298) 257 (52140), 290 (45120)
368 (55.48), 391 0.25
380, 504 sh
CH Cl at room temperature (Figure S13 in the Sup-
solid (298)
glass (77)
2
2
372, 455, 485
porting Information). The measurements of fluores-
cence lifetimes of C1 and L1 are almost identical
CH
2
Cl
2
(298) 258 (62390)
368 (3.87)
386, 511 sh
388, 477
0.19
0.18
0.32
0.79
0.82
0.80
0.45
(
2.42 and 2.46 ns in CH Cl at room temperature, re-
2 2
solid (298)
glass (77)
spectively), which reveals that the emission bands for
C1 may mainly originate from electron transitions of
the ligand. The nanosecond excited-state lifetimes
are strongly indicative of the pure fluorescence
nature of the emission.
CH
2
Cl
2
(298) 255 (77660), 310 sh (22080)
368 (28.6)
391
solid (298)
glass (77)
386, 507
CH
2
Cl
2
(298) 266 (51820), 290 (45970), 319 (47218)
462 (12.21)
453
It is worth noting that solid C1 exhibits a significant
redshift (ca. 25 nm) in comparison with a solution of
C1 in CH Cl , and there is a slight shoulder at l=
solid (298)
glass (77)
589
2
2
5
50 nm for C1 in the solid state, which disappears in
CH
2
Cl
2
(298) 257 (66970)
446 (5.66)
437
solid (298)
glass (77)
solution. These phenomena may be ascribed to p–p
412, 426, 548
stacking in the solid state because these weak inter-
[
11]
actions may be destroyed in solution.
CH
2
Cl
2
(298) 286 (35800), 342 (37500), 393 (58170)
475 (5.78)
482
We were surprised to find that C1 had an unprece-
dentedly high luminescence QY. Relative QYs were
determined by the standard method with quinine
solid (298)
glass (77)
471, 550 sh
CH
2
Cl
2
(298) 286 (38629), 342 (36760), 393 (59590)
475 (3.17)
525
[12]
sulfate in 0.1n H SO as a reference (F =0.546).
2
4
f
solid (298)
glass (77)
The QYs of L1 and C1 in CH Cl are 0.45 and 0.83, re-
498, 559
2
2
spectively. Complex C1 exhibited a strong emission
with an emission maximum at l=503 nm upon exci-
tation at l=475 nm in the solid state. The emission
energy of C1 is bathochromatically shifted by 50 nm
2 2
CH Cl (298) 258 (42178), 309 (39463), 388 sh (38017) 409 (2.46)
with respect to that of L1 (Figure 4a). Therefore, these versatile
characteristics, together with the lifetime in the nanosecond
range and the color of the emission in the pure blue region,
make C1 very attractive for future studies, including practical
applications as fluorescent dopants for organic light-emitting
diodes (OLEDs), dye-sensitized solar cells, biosensors, and non-
[
13]
linear optical materials.
To further elucidate the unprecedented high luminescence
QY of C1, four substituents with different electron-donating or
-
withdrawing properties were decorated on the 4-position of
1
the phenyl ring (R ). Indeed, the resultant complexes C2–C5
gave very weak photoluminescence with emission maxima at l
ꢀ
375 nm upon excitation at l=275 nm in CH Cl at room
2 2
À
temperature (Figure 4b). When the ClO anion of C1 was re-
4
À
À
placed by BF4 (C8) and SO CF3 (C9), the emission maxima re-
3
mained almost intact (Figure 4c); this revealed that the anions
had a negligible influence on the luminescence properties. On
Figure 3. UV/Vis absorption spectra of C1-C5.
2
the other hand, the side aromatic groups (R ) of C1 were re-
placed by naphthalene groups (C6) or ÀH (C7), which were de-
signed and prepared to investigate the influence of steric hin-
drance by side aromatic groups (Figure 4d). Clearly, the fluo-
gands, the energy level of the p* orbital of the pyridine part
decreases, whereas the energy level of the p orbital of the
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rescence emission was successfully induced concomitant with
a redshift in the emission energy owing to the increasing steric
hindrance effect associated with side aromatic groups. Accord-
ingly, solid C6 exhibits a significant redshift (ꢀ100 nm) in com-
parison with a solution of C6 in CH Cl (Figure S10 in the Sup-
2
2
porting Information). Complex C7 (F =0.79) exhibited a similar
f
luminescence QY to that of C1. However, complex C7 was un-
stable in air for extended periods owing to a loss of the char-
acter of the encapsulated structure only. On the other hand,
complex C6 showed a surprisingly low QY (F =0.32) because
f
of the increasing probability of strong p–p stacking, intermo-
lecular weak contacts, and hydrogen-bond interactions, which
would reduce the fluorescence emission. From this vantage
point, it should be noted that the strong electron-donating
substituent of DMA moieties play an important role in the un-
precedented high luminescence QY of C1. The solid-state emis-
sions of these complexes in the glassy state at 77 K vary from
lmax =400 to 600 nm, but no distinct relationship is evident be-
tween molecular structures and emission energies (Figure S11
in the Supporting Information).
Computational study
Absorption spectra
On the basis of optimized ground-state geometry or X-ray
crystallography results, a systemic evaluation of the per-
formance of the computation model on the TDDFT//B3LYP/
GENECP(6-31G**) step was performed based on the calculation
of the vertical absorption energy lmax values of C1 and L1. The
solvent effect was simulated by using the polarizable continu-
[
18]
um model (PCM) incorporated in the Gaussian 09 program.
The excitation energies of singlet transitions with oscillator
strengths greater than 0.01 and bands greater than l=300 nm
are listed in Table S6 in the Supporting Information. The calcu-
lated results provide reasonably similar results to those of the
experimental data. Compared with L1, complex C1, with an
electron-donating DMA group, produces strong, broad absorp-
tions (larger redshift of 120 nm) in the visible region at l=390
À1
3
À1
and 475 nm (eꢀ61010 and 1990 mol dm cm , respectively)
in CH Cl at 298 K (Figure 3). As shown in Figures S14 and S15
2
2
in the Supporting Information, time-dependent (TD) DFT calcu-
lations of C1 and L1 elucidated that the emergence of these
bands might be due to a significant participation of the pend-
ant DMA group in the HOMO. From the molecular orbital (MO)
patterns, the HOMO and HOMOÀ1 of the complex C1 typically
have major contributions from p orbitals of the 4-dimethylami-
nophenyl group from one independent L1, whereas the
LUMOs are predominantly located on the p* orbitals of the
two pyridine rings from distinct L1 ligands and unnoticeable
pp orbitals of the metal centers. Thus, the lowest energy
HOMO!LUMO transitions associated with C1 have clear-cut
predominantly ligand-centered features (with ILCT characters)
À5
2 2
Figure 4. a) Emission spectra of L1 and C1 in CH Cl (1”10 m) or in the
solid state. Inset: photographic image of solution C1 under 365 nm irradia-
tion. b) Normalized emission spectra of C1–C5. c) Emission spectra of C1, C8,
and C9. d) Emission spectra of C1, C6, and C7. The emission spectra were ac-
À5
quired in CH
2
Cl
2
(1”10 m) at 298 K. The excitation wavelength for each
I
together with some Ag 5p orbital perturbation character. The
complex is at the peak position.
new HOMO!LUMO transitions give rise to broad, low-energy
bands that match well with those observed experimentally,
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with predicted wavelengths of l=389.57, 463.64, 472.46, and
16 nm (experimental values l=390 and ꢀ475 nm).
changes to absorption maxima. Therefore, structural modifica-
tion is pivotal for tuning the excited-state properties. From this
vantage point, it should be noted that the emission band for
complex C1 may mainly originate from the electron transition
of the ligand. The strong fluorescence was due to reduced
electron-charge density at the pyridine site after binding with
5
Emission spectra
To investigate the emission of C1, the geometry of the singlet
I
Ag , which reduced the push–pull nature of the ILCT excited
excited state (S ) was optimized and the excitation based on
1
state of L1 (caused by electron-donating DMA and electron-
the S1 geometry was calculated in CH Cl at the PCM/TD-
2
2
[
10b]
withdrawing pyridine), as reported by Goodall and Williams
PBE1PBE/LANL2DZ level. The calculated emission bands great-
er than l=400 nm, oscillator strength greater than 0.001, and
transition character are summarized in Table S7 in the Support-
ing Information. Results reveal that the simulated fluorescence
wavelength of C1 is l=466.75 nm, which is in fairly good
agreement with experimental data (l=475 nm). The absence
of an emissive S state (S !S transition with f=0.0001) sug-
[
15]
and Sun et al.
Biology activities
To examine whether the ability of high luminescence QY is pre-
served in vivo, HeLa cells were used to detect relative cytotox-
icity in live cells. Indeed, HeLa cells incubated with C1 (5 mm)
for 15 min at room temperature showed a bright-blue intracel-
lular fluorescence (corresponding to the luminescence proper-
ties of C1). The cells remained viable and no apparent toxicity
or side effects were observed throughout the imaging experi-
ments, which suggested that C1 was cell permeable (Figure 6).
1
1
0
[
14]
gests that radiative S !S decay is not possible (dark state).
1
0
The first allowed transition is S !S with f=0.1491. As shown
2
0
in Figure 5, the distributions of the frontier molecular orbitals
1
Figure 5. Natural orbitals of C1 involved in excitation to the S state.
Figure 6. Confocal fluorescence images of HeLa cells (lex =390 nm). a) A
bright-field image of the cells in (b). b) Cells supplemented with C1 for
15 min.
(
FMOs) for C1, the HOMO and HOMO-1, are located on one in-
dependent molecule of L1, whereas the LUMO and LUMO+
are mainly localized on the p* orbitals of the two pyridine
I
1
The AgÀN bond-containing complexes also exhibit biologi-
[16]
rings from distinct L1 ligands with unnoticeable p orbitals of
cal properties, including antibacterial activity.
Two novel
p
the metal centers. By comparison, the shape of the HOMO and
LUMO involved in the S state is similar to that of the S state
Gram-positive bacteria (Bacillus subtilis, Micrococcus luteus), one
Gram-negative microorganism (Escherichia coli), and one
Fungal pathogen (Candida albicans) were selected to evaluate
the antimicrobial activity of complexes C1–C9 (Table 3). Stan-
dard antibacterial drugs tetracycline and erythromycin were
used as positive controls. The antimicrobial activity of these
complexes was measured by using the broth microdilution
method. Simple silver(I) complexes C1–C9 exhibited broad-
spectrum activity against Gram-negative bacteria and yeasts,
whereas Gram-positive bacteria (B. subtilis, M. luteus) were re-
sistant to these complexes. Interestingly, complexes C3, C4,
and C9 exhibited broad-spectrum activity against Gram-posi-
tive and -negative bacteria and yeasts. The highest activity
against Gram-negative bacteria were recorded with E. coli, fol-
lowed by yeast C. albicans, and Gram-positive bacteria M.
luteus. The three complexes were more active than the positive
controls in most cases, which suggested that the presence of
1
0
(
Figure S14 in the Supporting Information). The distribution of
the FMOs suggests the ILCT nature of the emission process in
C1.
The vertical emission energies of C2–C5 have also been in-
vestigated in their excited-state (S ) optimized structures
1
(
Table S7 in the Supporting Information). Impressively, the
computed emission maxima, lmax,em (l=380–386 nm), are in
good agreement with experimental data for all complexes. For
I
C2–C5, the LUMOs are mainly the empty p orbital of Ag, to-
gether with some p* orbitals of the two pyridine rings from
distinct L2–L4 ligands (Figure S16 in the Supporting Informa-
tion). On the basis of the results of these calculations, the
nature of the excited state can be assigned tentatively to ILCT
and ligand–ligand charge-transfer transitions, mixed with
metal–ligand charge-transfer character, for complexes C2–C5.
Comparisons of C2–C5 reveal charge-transfer abilities balanced
with lower efficiency. Moving from C1 to C2–C5, there is
a clear blueshift in lmax,em with the introduction of weak elec-
tron-donating or -withdrawing substituents, similar to that of
I
AgÀN could enhance their antimicrobial activity significantly.
In addition, it is worth mentioning that C4 and C9 exhibit
higher levels of antibacterial activity against all four bacteria.
These results suggest that the introduction of a halogen group
ChemPlusChem 2017, 82, 323 – 332
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328
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Full Papers
1
13
H and C NMR spectra were recorded by using a DRX 400 FT-NMR
Table 3. Antimicrobial activity of C1–C9.
spectrometer with chemical shifts reported in ppm (in [D ]DMSO
6
[
a]
À3
or MeOD, with tetramethylsilane (TMS) as an internal standard).
Electron impact (EI) and fast atom bombardment (FAB) mass spec-
tra were obtained by using a Finnigan Mat 95 mass spectrometer.
Steady-state luminescence emission spectra were recorded by
using a SPEX Fluorolog-2 Model F111 fluorescence spectrophotom-
eter. For all fluorescent measurements, both excitation and emis-
sion slit widths were 1 nm. Optical absorption spectra were mea-
sured by using a Hewlett Packard 8453 spectrometer or a Hitachi
U-0080D spectrometer at room temperature. To facilitate compari-
son, all absorption and emission spectra were recorded in CH Cl
Compound
Dose of MIC [mgcm ]
M. luteus
B. subtilis
E. coli
C. albicans
C1
C2
C3
C4
C5
C6
C7
C8
C9
128
256
64
64
64
256
256
128
64
128
16
64
128
32
16
64
64
64
128
32
16
16
32
16
64
32
16
16
8
32
32
32
8
32
32
32
16
8
2
2
at 298 K and as an alcoholic glass (CH OH/C H OH=1/4) at 77 K.
3
2
5
AgClO
tetracycline
4
128
16
64
8
64
8
16
Fluorescence lifetimes were recorded by using an Edinburgh Ana-
lytical Instruments F900 device. Luminescent QYs were referenced
to quinine sulfate in 0.1n H SO (F =0.546) and extrapolated to
[b]
[b]
erythromycin
8
8
16
2
4
f
[
a] MIC=minimum inhibitory concentration; this is the lowest concentra-
infinite dilution (estimated error ꢂ3%). The intrinsic excited-state
tion of compound that inhibits 100% growth of the microorganism.
[
lifetimes (t ) in CH Cl were obtained from the Stern–Volmer equa-
0
2
2
b] Compounds used as positive controls.
[14]
tion. All spectroscopic measurements were performed at least in
triplicate and averaged.
À
and SO CF3 could enhance selectivity at the same time, as re-
3
[17]
ported by Massoud et al.
X-ray structure determinations
Crystals mounted in a glass capillary were used for data collection
at 288C on a MAR diffractometer with a 300 mm image plate de-
tector using graphite monochromatized Mo-Ka radiation (l=
Conclusion
A prominent class of simple fluorescent C^N^C-type silver(I)
complexes with different moieties was designed and synthe-
sized for the first time. Single-crystal X-ray structural analyses
were executed on C1–C5 to reveal encapsulated silver(I) struc-
tures with a slightly distorted linear geometry in crystal lattices.
The photophysical properties were measured in solution at
room temperature and discussed in the context of computa-
tional analysis. Interestingly, complex C1 exhibited an emission
not only in the solid state, but also in solution, at room tem-
perature with an incredible QY after exact measurements (over
0
.71073 Š). Data collection was made with 1.58 oscillation step of
f, 5 or 10 minutes exposure time and scanner distance at 120 mm.
130 images were collected. The images were interpreted and inten-
sity was integrated with the program DENZO. All structures were
solved by direct methods employing SHELXS-97 program on a PC.
Ag and many non-H atoms were located according to the direct
methods. The positions of the other non-hydrogen atoms were
found after successful refinement by full-matrix least-squares using
the SHELXL-97 program suite. One phenyl ring in C2, C4, and per-
chlorate anion in C3 were found to be disordered into two differ-
ent orientation. In the final stage of least-squares refinement, disor-
dered C atoms were refined isotropically, other non-H atoms were
refined anisotropically. The positions of H atoms were calculated
based on riding mode with thermal parameters equal to 1.2 times
that of the associated C atoms, and participated in the calculation
of final R-indices. Details of each structure are given in Table 4.
0
.80) in CH Cl . To the best of our knowledge, complex C1 is
2 2
one of the brightest silver(I) complexes in solution at room
temperature. Data for comparison of C2–C9, TDDFT calcula-
tions and lifetime measurements, revealed that the emission of
C1 was mainly due to the ILCT transition. The absorption and
emission spectra of C1–C5 and L1 were also investigated by
means of TDDFT. The calculated absorption and fluorescence
wavelengths showed a good correlation with those obtained
experimentally. The robust silver(I) complex of C1 can serve as
effective sky-blue fluorescence for cell imaging. Preliminary an-
timicrobial studies against four microorganisms (B. subtilis, M.
luteus, C. albicans, and E. coli) showed that the above silver(I)
complexes were active antimicrobial and antifungal agents.
Further studies on the antimicrobial activity and excellent QY
in degassed solutions, as well as applications as a silver(I) che-
mosensor and OLEDs, will be reported in due course.
CCDC 1444572, 1444573, 1444574, 1444575, and 1453967 contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre.
Calculation details
All theoretical calculations were performed in Virtual Laboratory
for Computational Chemistry, CNIC, CAS. All calculations were per-
[18]
formed with the Gaussian 09 program. B3LYP calculations were
performed by using the 6-31G**(d, p) basis set for the main-group
elements and effective core potentials (ECPs), such as LANL2DZ,
for the silver atom. We employed DFT with no symmetry con-
straints to investigate the optimized ground-state (S ) geometries
of the ligands and complexes (GENECP) with the three-parameter
hybrid functional (B3LYP/6-31G**). Based on the energy-minimized
0
Experimental Section
Reagents and instruments
or X-ray structures, excited-state (S ) geometrical optimization was
1
All materials were used as received. The solvents used for synthesis
were of analytical grade. 4-(4-N,N-Dimethylaminophenyl)pyridine
performed by using the TDDFT method. Fluorescence emission en-
ergies were computed from TDDFT calculations based on the opti-
mized geometry or X-ray crystallography of the lowest excited
(L7) was purchased from Alfa Aesar, A Johnson Matthey Company.
ChemPlusChem 2017, 82, 323 – 332
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329
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
benzaldehyde (25.7 mmol) was heated to
Table 4. Crystal data and structure refinements for C1–C5.
C1 C2 C3
reflux for 2 h and then cooled to 1208C,
3
and NaOH (22.5 g in 50 cm of water) was
C4
C5
added. After a further 2 h, the solution was
cooled to room temperature. The suspen-
sion was diluted with water and extracted
with dichloromethane. After evaporation of
the solvent, the residue was washed with
petroleum ether to give products with
yields of 40–50%.
Formula
Formula
weight
Space
group
Crystal
system
a [Š]
C
51
H
46AgCl
3
N
4
O
4
C
49
H
40AgCl
3
N
2
O
4
C
46
H
34AgClN
2
O
4
C
47
H
34AgCl
5
N
2
O
4
C
47
H
34AgBr
2 3 2 4
Cl N O
993.14
935.05
822.07
975.88
1064.80
P2
1
/n
P2
1
/n
P2
1
/n
P2
1
/n
1
P2 /n
Monoclinic
Monoclinic
Monoclinic
Monoclinic
Monoclinic
10.814(3)
23.407(6)
19.149(5)
90
103.317(4)
90
4716(2)
4
298(2)
1.399
10.280(2)
21.340(4)
19.781(4)
90
94.54(3)
90
4325.8(15)
4
301(2)
1.436
15.347(3)
15.608(3)
16.201(3)
90
98.08(3)
90
3842.2(13)
4
301(2)
1.421
10.295(2)
21.182(4)
19.831(4)
90
94.48(3)
90
4311.3(15)
4
301(2)
1.503
16.657(3)
14.444(3)
19.061(4)
90
107.17(3)
90
4381.6(15)
4
301(2)
1.614
4
-(4-N,N-Dimethylaminophenyl)-2,6-di-
b [Š]
c [Š]
1
phenylpyridine (L1): H NMR (400 MHz,
CDCl ): d=8.19 (d, J=7.1 Hz, 4H; H ), 7.87
a [deg]
b [deg]
g [deg]
3
Ar
(s, 2H; H ), 7.70 (d, J=8.9 Hz, 2H; H ), 7.51
Ar Ar
(t, J=7.1 Hz, 4H; H ), 7.43 (t, J=7.3 Hz,
Ar
3
V [Š ]
2
3
H; H ), 6.84 (d, J=8.9 Hz, 2H; H ),
Ar Ar
Z
T [K]
.05 ppm (s, 6H; Me); ESI-MS: m/z: 351.4
+
[M+H] .
1
calc
À3
[
gcm
]
4-(4-Tolyl)-2,6-diphenylpyridine
(L2):
H NMR (400 MHz, CDCl ): d=8.20 (d, J=
q
rang [deg] 1.40–25.01
1.91–25.65
0.698
0.885
8139
0.0758
504
2.15–25.65
0.641
0.771
7081
0.0515
505
2.06–25.63
0.824
0.904
7672
0.0528
502
2.01–25.67
2.513
0.788
8203
0.1318
527
1
3
À1
m [mm
]
0.646
1.005
7
.1 Hz, 4H; H ), 7.88 (s, 2H; H ), 7.66 (d,
Ar Ar
GOF
J=8.1 Hz, 2H; H ), 7.52 (t, J=6.9 Hz, 4H;
Ar
no. unique 8144
0.1697
H ), 7.44 (t, J=7.1 Hz, 2H; H ), 7.34 (d, J=
Ar
Ar
R
int
8
.0 Hz, 2H; H ), 2.45 ppm (s, 3H; Me); ESI-
Ar
no. param- 568
eters
+
MS: m/z: 322.4 [M+H] .
[a]
R
1
0.0842
0.1498
0.0566
0.1458
0.0389
0.1033
0.0533
0.1490
0.0641
0.1445
1
2,4,6-Triphenylpyridine
(L3):
H NMR
[
2
a]
wR
(
400 MHz, CDCl ): d=8.21 (d, J=7.7 Hz, 4H;
3
2
2
2
2 2 1/2
[
a] I>2s(I). R
1
=Sj jF
o
j–jF
c
j jSjF
o
j. wR
2
={S[w(F
o
–F
c
) ]/S[w(F
o
) ]}
.
H ), 7.90 (s, 2H; H ), 7.75 (d, J=7.6 Hz, 2H;
Ar Ar
H ), 7.43–7.56 ppm (m, 9H; H ). ESI-MS: m/
Ar
Ar
+
z: 308.4 [M+H] .
1
state in dichloromethane. To account for solvation, TDDFT calcula-
tions were combined with the PCM, which emerged as an effective
tool to treat bulk solvent effects for both the ground and excited
states. The contour plots of MOs were obtained with the Gaussi-
an 09 viewing program.
4
-(4-Chlorophenyl)-2,6-diphenylpyridine (L4): H NMR (400 MHz,
CDCl ): d=8.20 (d, J=7.0 Hz, 4H; H ), 7.85 (s, 2H; H ), 7.69 (d, J=
.6 Hz, 2H; H ), 7.50–7.54 (m, 6H; H ), 7.46 ppm (t, J=7.2 Hz, 2H;
Ar Ar
3
Ar
Ar
8
+
H ); ESI-MS: m/z: 351.4 [M+H] .
Ar
1
4
-(4-Bromophenyl)-2,6-diphenylpyridine (L5): H NMR (400 MHz,
CDCl ): d=8.19 (d, J=7.1 Hz, 4H; H ), 7.85 (s, 2H; H ), 7.67 (d, J=
3
Ar
Ar
Details of live-cell imaging and antibacterial activities
8.6 Hz, 2H; HAr), 7.62 (d, J=8.5 Hz, 2H; HAr), 7.52 (t, J=7.0 Hz, 4H;
+
H ) 7.46 ppm (t, J=7.1 Hz, 2H; H ); ESI-MS: m/z: 388.4 [M+H] .
Ar ,
Ar
HeLa cells were cultured in 1640 medium supplemented with 10%
fetal calf serum (FCS; Invitrogen). Cells were seeded in 24-well flat-
bottomed plates for Nikon eclipase TE2000-5 inverted fluorescence
microscopy. After 12 h, HeLa cells were incubated with 5 mm C1 (in
the culture medium containing 0.5% DMSO) for 15 min at 378C
4
-(4-N,N-Dimethylaminophenyl)-2,6-di(naphthalen-2-yl)pyridine
1
(L6): H NMR (400 MHz, CDCl ): d=8.69 (s, 2H; H ), 8.42 (dd, J=
3 Ar
8
7
6
.6, 1.8 Hz, 2H; H ), 8.00–8.05 (m, 6H; H ), 7.90–7.92 (m, 2H; H ),
.52–7.55 (m, 4H; H ), 6.88 (d, J=8.8 Hz, 2H; H ), 3.07 ppm (s,
Ar Ar Ar
Ar
Ar
+
H; Me); ESI-MS: m/z: 451.3 [M+H] .
under 5% CO and then washed with phosphate-buffered saline
2
(
PBS) three times. The fluorescence imaging of intracellular C1 in
HeLa cells was observed under Nikon eclipase TE2000-5 inverted
fluorescence microscopy with a 20” objective lens (excited with
blue light).
Synthesis of C1–C9
Antibacterial activities of C1–C9 were evaluated by using the broth
Caution! Perchlorate salts are potentially explosive and should be
handled with care and in small amounts.
microdilution method. The highest concentration used was
À3
2
56 mgcm . Bacteria were cultured in Mueller Hinton broth
1
(
(
CM0405) for 24 h at 358C and yeast in glucose peptone broth
05602) for 48 h at 308C. The MIC value corresponded to the
Reaction mixtures of AgX (X=ClO , SO CF , and BF ) and 4-(4-R -
4
3
3
4
phenyl)-2,6-diphenylpyridine ligands in a 1:2 molar ratio in CH Cl
2
2
lowest concentration that inhibited bacterial growth (Table 3).
(30 mL) or mixtures (total 30 mL) of CH Cl and CH OH were stirred
2 2 3
at room temperature for several hours, filtered, and concentrated
to 5 mL. Addition of diethyl ether gave colorless solids. The crude
products were redissolved in CH Cl ; recrystallization by slow diffu-
Synthesis and characterization
2
2
sion of diethyl ether into solutions in CH Cl gave light lemonade-
2
2
Synthesis of L1–L6
colored or colorless single crystals suitable for X-ray structure anal-
ysis. Complexes C1, C2, and C3 were soluble in most organic sol-
vents. Complexes C4–C7 were slightly soluble in CDCl3 or
A mixture of acetamide (45.5 g, 0.77 mol), ammonium acetate
1
(
29.3 g, 0.38 mol), acetophenone (6.0 mL, 51.5 mmol), and 4-R -
ChemPlusChem 2017, 82, 323 – 332
www.chempluschem.org
330
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
13
[
D ]DMSO, but the solubility was too low to allow for C NMR
6
Conflict of interest
spectroscopy measurements.
[
(
Ag{4-(4-N,N-dimethylaminophenyl)-2,6-diphenylpyridine} ]ClO
The authors declare no conflict of interest.
2
4
1
C1): H NMR (400 MHz, CDCl ): d=7.77 (s, 4H; H ), 7.68 (d, J=
3
Ar
9
.0 Hz, 4H; H ), 7.49–7.55 (m, 20H; H ), 6.79 (d, J=9.0 Hz, 4H;
Ar Ar
1
3
Keywords: biological activity · fluorescence · luminescence ·
silver · structure–activity relationships
H ), 3.06 ppm (s, 12H; Me); C NMR (101 MHz, CDCl ): d=158.2,
1
4
Ar
3
52.5, 152.1, 141.6, 130.4, 129.8, 128.3, 126.7, 122.2, 118.6, 112.2,
+
0.2 ppm; ESI-MS: m/z: 809.2 [M+ClO ] ; elemental analysis calcd
4
(
%) for C H AgClN O : C 66.12, H 4.88, N 6.17; found: C 66.16, H
50 44 4 4
4
.93, N 6.12.
[
[
[
1] a) J. C. Garrison, W. J. Y. Youngs, Chem. Rev. 2005, 105, 3978–4008;
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1
[
Ag{4-(4-tolyl)-2,6-diphenylpyridine} ]ClO
(C2):
H NMR
2
4
(
400 MHz, CDCl ): d=7.84 (s, 4H; H ), 7.66 (d, J=8.2 Hz, 4H; H ),
3
Ar
Ar
3445–3458; c) Z.-Y. Cao, X. Wang, C. Tan, X.-L. Zhao, J. Zhou, K. Ding, J.
7
8
.61 (d, J=7.9 Hz, 8H; H ), 7.52–7.55 (m, 12H; H ), 7.35 (d, J=
Ar Ar
.0 Hz, 4H; H ), 2.44 ppm (s, 6H; H ); C NMR (101 MHz, CDCl3):
Am. Chem. Soc. 2013, 135, 8197; d) E. C.-C. Cheng, W.-Y. Lo, T. K.-M. Lee,
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Yuan, Y. Yu, Q. Zhang, D. T. Leong, J. Y. Lee, J. Xie, J. Am. Chem. Soc.
1
3
Ar
Ar
d=159.5, 152.9, 141.1, 141.0, 133.3, 130.6, 130.3, 128.9, 127.2,
1
+
26.7, 120.3, 21.4 ppm; ESI-MS: m/z: 751.1 [M+ClO ] .
4
1
[
Ag{2,4,6-triphenylpyridine} ]ClO (C3): H NMR (400 MHz, CDCl ):
2
4
3
d=7.86 (s, 4H; H ), 7.76 (d, J=8.0 Hz, 4H; H ), 7.66 (d, J=7.8 Hz,
8
d=161.8, 153.0, 141.1, 136.4, 130.7, 130.5, 129.9, 128.5, 127.4,
1
Ar
Ar
1
3
H; H ), 7.52–7.56 ppm (m, 18H; H ); C NMR (101 MHz, CDCl3):
Ar Ar
+
26.7, 120.6 ppm; ESI-MS: m/z: 723.1 [M+ClO ] .
4
2012, 134, 16662–16670; d) J. Schneider, Y.-A. Lee, J. Perez, W. W. Bren-
1
[
Ag{4-(4-chlorophenyl)-2,6-diphenylpyridine} ]ClO (C4): H NMR
2 4
nessel, C. Flaschenriem, R. Eisenberg, Inorg. Chem. 2008, 47, 957–968;
e) K. M.-C. Wong, L.-L. Hung, W. H. Lam, N. Zhu, V. W.-W. Yam, J. Am.
Chem. Soc. 2007, 129, 4350–4365.
(
400 MHz, CDCl ): d=7.84 (s, 4H; H ), 7.70–7.73 (m, 12H; H ),
3 Ar Ar
+
7
.51–7.56 ppm (m, 16H; H ); ESI-MS: m/z: 791.0 [M+ClO ] .
Ar 4
3] a) V. J. Catalano, H. M. Kar, J. Garnas, Angew. Chem. Int. Ed. 1999, 38,
1
[
Ag{4-(4-bromophenyl)-2,6-diphenylpyridine} ]ClO4 (C5): H NMR
2
1979–1982; Angew. Chem. 1999, 111, 2083–2086; b) X.-C. Huang, S.-L.
(
(
(
400 MHz, CDCl ): d=7.84 (s, 4H; H ), 7.76–7.78 (m, 8H; H ), 7.69
3 Ar Ar
d, J=8.7 Hz, 4H; H ), 7.63 (d, J=8.7 Hz, 4H; H ), 7.55–7.57 ppm
m, 12H; H ); ESI-MS: m/z: 880.9 [M+ClO ] .
Zheng, J.-P. Zhang, X.-M. Chen, Eur. J. Inorg. Chem. 2004, 1024–1029;
c) C. Seward, W.-L. Jia, R.-Y. Wang, G. D. Enright, S. Wang, Angew. Chem.
Int. Ed. 2004, 43, 2933–2936; Angew. Chem. 2004, 116, 2993–2996;
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Lee, K.-T. Wong, R.-T. Chen, Y.-M. Cheng, P.-T. Chou, Inorg. Chem. 2004,
43, 4781–4783; e) C.-C. Hsu, C.-C. Lin, P.-T. Chou, C.-H. Lai, C.-W. Hsu, C.-
H. Lin, Y. Chi, J. Am. Chem. Soc. 2012, 134, 7715–7724; f) R. B. Thomas,
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Leung, D. L. Phillips, N. Zhu, Z.-Y. Zhou, J. Am. Chem. Soc. 2003, 125,
Ar
Ar
+
Ar
4
[
Ag{4-(4-N,N-dimethylaminophenyl)-2,6-di(naphthalen-2-yl)pyri-
1
dine} ]ClO4 (C6): H NMR (400 MHz, CDCl ): d=7.94 (s, 4H; H ),
2
3
Ar
7
8
6
1
.78–7.85 (m, 8H; H ), 7.72 (d, J=8.4 Hz, 4H; H ), 7.60 (d, J=
Ar Ar
.8 Hz, 4H; H ), 7.51–7.55 (m, 8H; H ), 7.46 (d, J=8.4 Hz, 4H; H ),
.71 (d, J=8.8 Hz, 4H; H ), 3.06 ppm (s, 12H; HMe); ESI-MS: m/z:
009.5 [M+ClO ] .
Ar
Ar
Ar
Ar
+
4
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2
4
(
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400 MHz, MeOD): d=8.57 (d, J=5.2 Hz, 4H; H ), 7.58 (d, J=
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Manuscript received: December 7, 2016
Revised: January 28, 2017
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