New highly fluorescent silver complexes and their thin films obtained by spin coating method

Article abstract of DOI:10.1039/c8nj03801e

New silver complexes with ligands obtained from 2(2-pirydyl)ethylamine (epy) or o-phenylenediamine and series of aldehydes were obtained and characterized by their crystalline, molecular and spectroscopic (UV-Vis, fluorescence, IR, NMR) features. Molecula

Full text of DOI:10.1039/c8nj03801e

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New highly fluorescent silver complexes and their
thin films obtained by spin coating method†
Cite this: New J. Chem., 2018,
42, 18559
a
a
a
Magdalena Barwiolek, * Andrzej Wojtczak, Anna Kozakiewicz,
a
a
b
a
Robert Szczesny, Magdalena Babinska, Lukasz Skowronski and Edward Szlyk
New silver complexes with ligands obtained from 2(2-pirydyl)ethylamine (epy) or o-phenylenediamine
and series of aldehydes were obtained and characterized by their crystalline, molecular and spectro-
scopic (UV-Vis, fluorescence, IR, NMR) features. Molecular architecture of the synthesized complexes
+
3ꢀ
[Ag
2
(epy(5-Brthca))
2
(NO
3
2
) ] 1 and [Ag(bzim-anthr)
2
] NO
2 has been determined by X-ray crystal
1
structure analysis. H NMR NOE diff. spectra of Ag complexes reveal different ligand conformations in
the solution and the solid state (crystal structure). Thin (13–113 nm average thickness), homogenous
layers of the studied silver(I) complexes were obtained by the spin coating method. The new materials
were also characterized with AFM, SEM/EDX and ellipsometry. Calculated band-gap energies related to
conformational transitions in layers of complexes 3, 1 and 2 on Si(111) were 2.83 ꢁ 0.02 eV (438 ꢁ 3 nm),
2
.90 ꢁ 0.01 eV (428 ꢁ 2 nm) and 2.95 ꢁ 0.02 eV (420 ꢁ 3 nm), respectively. The fluorescence bands of
Received 30th July 2018,
Accepted 9th October 2018
layers observed in the range 470–580 nm revealed significant intensity and were bathochromically shifted
in comparison with those observed for the complexes solutions. Influence of polarity of the solvent on
fluorescence properties of the obtained complexes was studied. The highest quantum yield of the fluores-
DOI: 10.1039/c8nj03801e
2
cence was achieved in methylene chloride for 2 with f = 0.52 (lex = 335–385 nm) and is related to the
rsc.li/njc
anthracene moiety.
1
5–18
19,20
devices development
related to their fluorescence properties.
Introduction
21
Reported benzimidazole derivatives were used in dye syntheses, or
Study of silver(I) complexes with N-donor ligands are of interest in advanced materials fabrication, such as nonlinear optics (NLO)
due to their unique structural features and potential applica- and organic light-emitting diodes (OLED).
Silver range of heterocycles, benzimidazoles were also applied in material
complexes reveal a large variety of coordination architectures, science in fuel cell research, or in pharmaceutical industry.
22
23,24
Amongst the vast
1
,2
tions in various fields of science and technology.
25
structural variability correlated with various anions or solvents,
Metallic silver layers and silver(I) compounds are used in the
3
–8
and unusual luminescence properties.
The dependence preparation of materials for applications in electronic, photo-
26
between structural architecture and fluorescence properties of graphic and imaging applications. Moreover, silver(I) complexes
mono and multisilver(I) compounds was also studied. The were reported as components of molecular materials with non-
2
7
influence of coordination geometry and distance between silver linear optical (NLO) properties. Thin layers of metals or metal
6
,9
centers was also investigated.
complexes, including those with Ag(I) ions, are prepared by
Moreover, silver(I) complexes with benzimidazole derivatives different deposition techniques, e.g. chemical vapour deposition
1
0
11
exhibited antimicrobial, antimalarial, antitubercular activity,
(CVD), metal–organic chemical vapour deposition (MOCVD),
1
1–14
28–30
and photoluminescence and catalytic properties.
Benzimid- laser ablation, sonication, spin coating, etc.
These materials
azole and anthracene systems found applications in optical exhibit luminescence and can be used as conductors, semi-
conductors, in organic light emitting diodes (OLED) and even
3
1,32
a
as drug transporters.
The spin coating technique was used
Faculty of Chemistry, Nicolas Copernicus University in Torun, 87-100 Torun,
Poland. E-mail: mbarwiolek@umk.pl; Fax: +48 (56) 611 45 16;
Tel: +48 (56) 611 45 16
for preparation of thin films of silver on TiN-coated oxidized Si
wafers, where [(nBu P)AgO C(CH ) CO Ag(nBu )] and [(nBu P) -
AgO C(CH ) CO Ag(nBu ) ] were used as precursors. For [(nBu P)-
AgO C(CH ) CO Ag(nBu )] complex, conductive silver films were
obtained with a film resistivity of 3.4 mO cm. The film thickness
mean value of about 2.5 mm) was not homogeneous, and EDX
analysis showed traces of oxygen and carbon in the layers.
3
2
2 2
2
3
3
2
b
Institute of Mathematics and Physics, UTP University of Science and Technology,
2
2 2
2
3 2
3
8
5-796 Bydgoszcz, Poland
2
2 2
2
3
†
Electronic supplementary information (ESI) available: NMR spectra of com-
plexes, UV-Vis spectra of complexes, details of ligands synthesis, AFM images of
layers. CCDC 1504448 and 1834866. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c8nj03801e
(
31
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We have noted that silver(I) complexes derived from 2-thio-
pheneethylamine with 3-methyl-2-thiophenecarboxaldehyde or
-bromo-2-thiophenecarboxaldehyde, and thin films prepared
from them by spin and dip coating techniques, exhibited signi-
5
3
3
ficant fluorescence in the range 463–474 nm (lex = 361 nm).
Also silver(I) complexes with bis(benzimidazole): [Ag (1,3-bis-
benzimidazol-1-ylmethyl)benzene) (1,4-ndc) , {[Ag(1,3-bis(5,6-
dimethylbenzimidazole-1-ylmethyl)benzene)]ꢂ(1,4-Hndc)ꢂH O}
and {[Ag(1,4-bis(5,6-dimethylbenzimidazole)butane)ꢂ(H O)]ꢂ(1,4-
Hndc)} (1,4-H ndc = 1,4-naphthalenedicarboxylic acid) obtained
under hydrothermal conditions revealed red shifted luminescence.
The fluorescence properties of silver complexes, the possi-
4
(
2
2 n
]
Fig. 1 A: Asymmetric unit of the crystal structure of dinuclear Ag(I) com-
plex 1. Atoms constituting the asymmetric unit are labeled. Ellipsoids are
plotted at 30% probability level. B: Perspective of the complex molecule
along the plane of coordination.
2
n
,
2
n
2
34
bility of using the spin coating technique for deposition of each L1 ligand is tridentate. In this architecture, both Ag(I) ions
silver(I) complexes and the likely fluorescent behaviour of have a distorted tetrahedral coordination environment, and
obtained layers prompted us to synthesize a series of silver(I)
4
t = 0.6 is interpreted. The architecture of the molecule might
complexes obtained from different benzimidazoles, and Schiff be described as two-stairs system. Each stair is formed by the
base synthetized from 2-(2-pirydyl)ethylamine and 5-bromo- pyridine moiety and Ag ion of a asymmetric unit and the
2-thiophenecarboxaldehyde. For innovative applications of bromotiophene moiety of the symmetry related ligand. The nitrate
benzimidazole and anthracene, we planned to synthesize mole- ion is positioned on the lower stair of such pairs, limiting the axial
cules that contain one or both of these units linked at pre- access to the Ag ion.
determined positions.
The valence geometry of L1 ligand is typical for such
The new silver(I) complexes were used as precursors for thin systems. The S12–C11 and S12–C13 distances are 1.723(3) and
layer deposition and their fluorescence and optical properties 1.717(3) Å, respectively. The C13–Br14 bond length of 1.870(4) Å
3
3
were studied.
is well matched with those in similar ligands.
Analysis of structure 1 revealed the intermolecular C–Hꢂ ꢂ ꢂO
interactions, between C5–H and C10–H groups of pyridine ring or
Results and discussion
X-ray
ꢀ
imine moiety and oxygen atoms of NO3 ions. The C5ꢂꢂꢂO2[x ꢀ 1,
y, z] and C10ꢂ ꢂ ꢂO3[ꢀx, ꢀy + 2, ꢀz + 2] distances were 3.469(3)
and 3.221(3) Å, respectively. The C–Hꢂ ꢂ ꢂO interactions were also
Crystal data and structure refinement for complexes 1 and 2 are
listed in Table 4, whereas selected bond lengths and angles in
are Table 1.
ꢀ
detected between the C7–H, C8–H and C15–H group and NO
3
ions, with a distances of C7ꢂ ꢂ ꢂO1[ꢀx ꢀ 1, ꢀy + 2, ꢀz + 2] of
3
.415(4) Å, C8ꢂ ꢂ ꢂO1[ꢀx ꢀ 1, ꢀy + 2, ꢀz + 2] of 3.249(3) Å and
X-ray crystal structure of complex [Ag (epy(5-Brthca)) (NO ) ] 1.
2
2
3 2
C15ꢂ ꢂ ꢂO3[x + 1, y ꢀ 1, z] of 3.321(3) Å. pꢂ ꢂ ꢂp interactions are
The asymmetric unit of 1 consists of half of the dinuclear Ag(I)
complex. This unit converts to the dinuclear complex molecule
by the inversion symmetry [ꢀx ꢀ 1, ꢀy + 2, ꢀz + 2] (Fig. 1). Each
Ag ion is bound to imine N9 atom of L1 and pyridine N1 atom of
observed between pyridine rings N1–C6, the pꢂ ꢂ ꢂp[ꢀ1 ꢀ x, 2 ꢀ y,
1
3
ꢀ z], with distance between the ring gravity centers being
.831(2) Å. Also, pꢂ ꢂ ꢂp intermolecular interactions between
thiophene ring C11–C16 and pyridine ring N1–C6[1 + x, y, z]
are found, with a pꢂ ꢂ ꢂp distance of 3.755(2) Å.
the symmetrically related ligand. In structure 1, an interaction
ꢀ
between Ag and oxygen atom of NO
3
ion is observed, with the
2
3ꢀ
X-ray crystal structure of complex [Ag(bzim-anthr) ] NO 2.
+
Ag1–O1 distance being 2.628(3) Å. Moreover, an interaction
between Ag and S atoms of the brominated thiophene is
observed, with Ag1–S12 distance of 2.9975(9) Å. Consequently,
The asymmetric unit in the structure 2 consists of the complex
+
2
cation [Ag(bzim-anthr) ] and the nitrate anion (Fig. 2).
The coordination sphere of Ag is almost linear, with an
N2–Ag–N21 angle of 175.27(8)1 (Fig. 2). There is no statistically
significant difference between multiple Ag–N bonds, with
Ag1–N1 and Ag1–N21 distances of 2.0995(19) and 2.1023(18) Å,
respectively.
Table 1 Selected bond lengths [Å] and angles [1] for 1 and 2
1
2
Bond lengths [Å]
Ag1–N1
Ag1–N9
Ag1–O1
Ag1–S12
The valence geometry of ligands is similar to that found in
the structures of free benzimidazole and anthracene moieties.
However, the effect of coordination via N1 is clearly visible.
In the benzimidazole moiety of two ligands, the C11–N2 and
C31–N22 distances are 1.345(3) and 1.347(3) Å, respectively,
while those involving the coordinated atoms C11–N1 and
C31–N21 are significantly shorter (1.325(3) and 1.329(3) Å, respec-
tively). In complex 2, C11–N1–C12 and C31–N21–C32 angles are
2.172(3)
2.141(3)
2.628(3)
2.9975(9)
Ag1–N1
Ag1–N21
2.0995(19)
2.1023(18)
Angle [1]
N9–Ag1–N1
N9–Ag1–O1
N1–Ag1–O1
N9–Ag1–S12
N1–Ag1–S12
O1–Ag1–S12
166.91(11)
98.79(10)
94.30(10)
74.77(7)
101.18(8)
107.74(7)
N1–Ag1–N21
175.27(8)
105.87(19) and 106.1(2)1, respectively. In contrast, the C11–N2–
C17 and C21–N21–C27 angles on the non-coordinated nitrogen
1
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Fig. 2 Asymmetric unit of the crystal structure of 2. Ellipsoids are plotted
at 30% probability level.
atoms are significantly larger (107.9(2) and 107.84(19)1, respec-
tively). Coordination also affects the relative position of ring
moieties in the ligands, with the ligand N1–C11–C9 angle
and its equivalents being larger by approximately 3–41 than
N2–C11–C9 angles. That difference seems to result from a repul-
sion between the bulky anthracene moiety and the remaining
Ag(bzim-anth) fragment of the complex ion. Meanwhile, the
dihedral angle between anthryl and benzimidazolyl moieties in
Fig. 3 Structural formula and atoms numbering scheme in 2-(anthracene-
9
-yl)-1H-benzo[d]imidazole L2, 2-(2H-benzimidazol-2-yl)-4-bromo-6-
methoxyphenol L3 and 2-(2,5-dimethoxyphenyl)-1H-benzimidazole L4.
2
are 61.85(7) and 65.81(7)1. The dihedral angle between two
benzimidazolyl rings in 2 is 73.89(8)1, and that between two In a series of NOE experiments, H–N protons at 9.11 ppm H(3b)
anthracene moieties is 53.09(6)1.
in 2, 10.43 ppm H(3) in 3 and at 12.50 ppm H(3) in 4 were
The anthracene moieties are bent along the C1A–C4A and saturated (cods of ligands correspond to Fig. 3). Surprisingly,
C5A–C8A bonds, with the respective dihedral angles between there was no NOE effect observed in spectra of complexes 2 and
neighboring six-membered rings being 2.33(13) and 2.37(12)1, 3 when N–H(3) protons at 9.11 ppm 2, at 13.63 ppm 3 were
and the resulting angle between the peripheral rings is 4.11(13)1. saturated, possibly because the nearest protons H8 and H7 in
For the second ligand, the analogous bend along C21A–C24A anthracene moiety and H5b in phenyl ring are most probably
3
and C25A–C28A results in the corresponding dihedral angles of out of sp nitrogen–H distance. In the benzimidazole moiety of
1.87(12), 3.53(12) and 4.55(13)1.
two ligands, the C11–N2 and C31–N22 distances were 1.345(3)
In the crystal packing, pꢂ ꢂ ꢂp interaction is found between the and 1.347(3) Å, respectively. That resulted in the increased
anthryl six-membered rings C1A–C9 and C5–C6[ꢀx, 1 ꢀ y, ꢀz] distances between N–H bond and hydrogens H8, H7 and H5b
with the distance between the ring gravity centers of 3.9478(15) Å. hydrogens, leading to the lack of NOE effect. Moreover, the
C–Hꢂ ꢂ ꢂp interactions are found between C7–H7A and C21–C22 NOE results of 2 suggest identical conformations of the com-
fragment, C16–H16A and benzimidazole C33–C37[1/2 + x, plex in solution and in solid state. X-ray analysis confirmed the
3
/2 ꢀ y, 1/2 + z], C30–H30A and C1A–C9[1 + x, y, z], and C33– lack of Hꢂ ꢂ ꢂH interaction between N–H and other H atoms
H33A and C25–C26[ꢀ1 + x, y, z], with H distances to the ring in solid state as well. In 4, the enhancement of 7,8 protons
gravity centers being 2.82, 2.81, 2.75 and 2.86 Å, respectively. The signals after irradiation of N–H(3) protons (10.60%) was noted.
H-bonds between benzimidazolyl N–H and nitrates are detected Additionally, the saturation of H7,8 protons in 4 led to the
N2–H2Bꢂꢂ ꢂO52[x ꢀ 1/2, ꢀy + 3/2, z + 1/2] and N2–H2Bꢂ ꢂ ꢂO54[x ꢀ enhancement of H6,9 (21%) signals; moreover, an interaction
1
/2, ꢀy + 3/2, z + 1/2], as well as in N22–H22Nꢂ ꢂ ꢂO52 and N22– between H15 and H13 protons (74%) was observed as well. The
H22Nꢂ ꢂ ꢂO53, with the Nꢂꢂ ꢂO distances being 2.986(3), 2.930(3), latter can be related to the deformation of the three hydrogen
.721(3) and 3.227(3) Å, respectively.
atoms below or above the two phenyl rings, and to the experi-
ment with OCH signal saturation, where the interaction with
2
3
H(13) and H(7,8) was registered.
The NOE analyses of the silver complexes show that the
ligands adopt different conformations in the solution in com-
NMR analysis
NOE experiment
To determine the impact of substituents on deformation of the parison to the crystal structures. Since such changes would include
coordination plane and position of the neighboring atoms, breaking of intramolecular HꢂꢂꢂH interactions and overcoming the
NOE difference spectra of the studied complexes (2–4) were rotational energy barriers, the energy necessary for such transi-
ꢀ1
measured. Because of the low solubility of complex 1 in tions should not exceed 100 kcal mol . This is consistent with our
common solvents the NOE spectra of 1 were not registered. results of ellipsometry discussed further in the text.
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Fluorescence and UV-Vis studies
systems in the ligand due to the presence of ethylene spacer.
This hypothesis is further supported by our unpublished
results, which show that in complexes 2 and 4, fluorescence
bands originate from transitions: benzimidazole to phenyl ring,
benzimidazole to benzimidazole and phenyl to phenyl ring. In
contrast, in 3, the fluorescence bands could be from charge
transfer transitions from the benzimidazole to the phenyl
group and from bromine towards the benzimidazole.
UV-Vis absorption and emission spectra of the reported Ag(I)
complexes were studied at room temperature in acetonitrile
(Fig. S1, ESI†) and methylene chloride solutions. In case of
complex 1, the UV-Vis and fluorescence spectra were measured
only in acetonitrile because of low solubility in methylene
chloride. UV-Vis spectra revealed the emission bands for 1 at:
371, 302 and 230 nm, 2 at: 283, 347, 383, 364 nm, 3 at: 328, 303
In methylene chloride, the emission bands of 3 and 4 were
detected at 430 nm and 410 nm (lex = 360 nm), respectively,
while in the spectrum of complex 2, two strong emission bands
were registered at 420 nm and 550 nm (lex = 380 nm) (Fig. 5).
Spectra of complexes exhibit an increase in the fluorescence
intensity and bathochromic shifts, which is rarely noted for
Ag(I) complexes and is important for their future applications as
fluorescence probes. In complex 2, interactions occur between
the anthracene p electrons system and the imidazole nitrogen
atoms or the C–N carbon atoms. Moreover, in 2, Ag(I) is coordi-
nated to the imine N atom adjacent to the anthracene moiety,
which suggests that the electron density redistribution might be
responsible for the observed high fluorescence quantum yield
and the splitting of fluorescence band. The shift in emission
bands towards higher wavelengths in methylene chloride can
correlate with the changes in molecular geometry of the excited
state, which implies a decrease in the resonance energy and a red
and 291 nm and 4 at: 498, 326, 301 nm (MeCN). The bands
registered in the region 230–303 nm are related to intra-ligand
p - p* transitions in aromatic rings, while those between 326–
371 nm are related with p - p* transitions in –CQN– group.
Complex 1 exhibits emission at 460 nm (lex =355 nm,
MeCN), and complex 2 gives strong luminescence at lem
76 nm (lex = 355 nm), whereas in the spectrum of 3, an intense
band at lem = 490 nm was noted (lex = 350 nm) (Fig. 4). Complex
=
4
4
exhibits strong emission band at 410 nm after excitation at
370 nm, and a band at 370 nm when lex = 335 nm. The intensity
of luminescence for complex 1 is relatively low in comparison
to other complexes. This may be due to the ligand conforma-
tion in 1 and the distance between Agꢂ ꢂ ꢂAg centers of 4.356(2) Å.
The architecture of molecule 1 was described as a two-stairs
system. As a stair is formed by the pyridine moiety and Ag ion of
one asymmetric unit and another by the bromotiophene moiety
of the symmetrical unit ligand, any interactions between silver
ions were not detected. The long distance between silver centers
and the lack of intermolecular interactions prevents the Agꢂ ꢂ ꢂAg
3
5
shift of the fluorescence band. For complex 3, the opposite
effect was noted, which can be caused by the presence of methoxy
groups and bromine atoms in the structure. The fluorescence of
all complexes depends on solvent polarity, which is important
,24 because tailored emission of materials can lead to potential
application as optical sensors. Similar behavior was observed
1
6
interactions enhanced emission as observed previously. The
influence of coordination of the ligand on the fluorescence
6
properties of disilver complexes has been already observed.
Photoluminescence properties were closely associated with the
local environments around metal ions. Moreover, the presence
of pꢂ ꢂ ꢂp intermolecular interactions between thiophene and
pyridine rings enhanced the luminescence properties of 1, as
emission mostly results from ligand charge transfer transitions.
The oxidation state of silver ions has minor influence on the
fluorescence properties of studied compounds. Additionally,
the quantum yield of fluorescence for complex 1 is the lowest in
comparison to those of 2–4 silver compounds. This can be a
result of the presence of two silver ions, which are known to
quench the fluorescence, or conformational changes around Ag
central ions in molecule 1 leading to flattening of the molecule.
Yet another possibility could be weak coupling between the ring
3
5
for 2-(phenyl-1H-benzo[d]imidazol-2-yl)phenol.
The enhancement of luminescence for silver complexes
relative to the free ligand (paper in preparation) can be related
to the ligand coordination (N–Ag–N bonds) resulting in con-
formational rigidity and reduction of the non-radiative transi-
3
6
tions of the excited state. The same bathochromic shift was
3
previously observed by us in case of [Ag(thet(9-anthraceneca)) ]NO ,
2
37
where thet = 2-thiopheneethylamine. Very high intensity of the
fluorescence bands, observed at room temperature, is an important
and unusual phenomenon for silver(I) compounds. It should be
Fig. 4 Fluorescence spectra of complexes in acetonitrile solutions,
Fig. 5 Fluorescence spectra of complexes in methylene chloride solu-
tions, lex = 380 nm for 2, 360 nm for 3 and for 4.
l
ex = 385 nm for 1, 355 nm for 2, 340 nm for 3 and 335 nm for 4.
1
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Table 2 Relevant photophysical data of studied compounds, (lem, lex in nm, the emission band was observed for the layers of complex 3,
f total fluorescence quantum yield)
compared to complex 2 (lex = 282 nm). This is opposite to our
observations for the solutions of complexes, and can be attributed
Compound
Solvent
l
ex
l
em
f
to the thickness of these layers. The film of 3, PMMA/Si is thicker
than that of 2, PMMA/Si, which causes a higher concentration of
0.78 the silver complex in the layer, and consequently, higher fluores-
1
2
Acetonitrile
Acetonitrile
Methylene chloride
Acetonitrile
Methylene chloride
Acetonitrile
385
355
380
350
360
360
460
476
470, 560
490
430
0.1
0.53
3
4
0.71
0.61
0.56
cence intensity. The least intense fluorescence bands were regis-
tered for the layers of 1 and is compatible with the fluorescence
410
3
335
35
370
370
0.50 emission observed for these compounds in solutions. All these
Methylene chloride
0.62
layers displayed even distributions of the complexes over the
silicon surface. The highest intensity band was registered for the
layer obtained at 2500 rpm spin speed, with coating time of 30 s.
noted that luminescent silver(I) compounds usually exhibit emis-
3
8,39
40–43
sion at low temperature,
and only a few monomeric
and
Thin layers of complexes
4
4,45
polymeric
silver(I) complexes have been reported to date,
The morphology and the surface roughness of these thin films
have been investigated by SEM and AFM techniques. To study
the chemical composition of the films, EDS analyses were con-
ducted for all samples.
The thin layers of complex 1 were obtained on the activated
silicon substrate (first covered with one drop of PMMA). (Fig. 7),
after which the silver solvent was spun on the Si(111) surface
which exhibit luminescent properties at room temperature.
Quantum yields of the fluorescence in acetonitrile and
methylene chloride are high for all complexes except for 1
4
6–48
(f
(MeCN) = 0.50–0.71 and f(CH Cl ) = 0.61–0.78)
(Table 2).
2
2
The highest quantum yield of fluorescence was observed for
complex 2 due to the presence of anthracene ring in the
structure. Generally, the quantum yields of studied ligands
have decreased along with a decrease in solvent polarity. For
complexes 2 and 4, the yield of fluorescence increased in
methylene chloride. The polarity of solvent influences the
redistribution of electrons and consequently, stabilization of
the excited state in MeCN. The opposite effect was found for 3.
Fluorescence properties of the obtained materials were also
studied. Upon excitation between 228 and 320 nm, the emis-
sion band between 470 and 580 nm from intraligand p - p*
transitions was noted. Bathochromic shifts in the fluorescence
bands of layers in comparison to those for the compound
solutions were noted (Fig. 6).
(the procedure was repeated four times). In case of complexes 2
and 3, mixtures of toluene and 2 or 3 compound were used. EDS
results confirmed the presence of silver, nitrogen and carbon
from silver complexes in the layers. Mappings of the films
indicate uniformly distributed Ag(I) complexes over the Si(111)
surface (Fig. 8c). The layers are homogenous, and the discontinuity
of the surface was only sporadically noted. This was confirmed
by covering the 1/PMMA/Si(111), 2/PMMA/Si(111) and 3/PMMA/
Si(111) materials with a gold layer (Fig. 8a and b).
This can be a result of different pathways of non-radiative
transitions and may be due to the reduced conformational
flexibility of ligand in the complex. This reduction results from
the restraints imposed by the substrate surface. During compar-
ison of fluorescence properties of the new layers with our pre-
33
viously obtained data, shifts in emission bands towards longer
wavelengths is noted. This can be connected with the presence of
Fig. 7 SEM image of 1/PMMA/Si(111) layer. (1 in PMMA: 4.0 mg of 1 dis-
solved in 10 cm toluene, + 100 mL 5% PMMA, at 2500 rpm for 30 s, and
3
benzimidazole rings in complexes structures. Highest intensity of 3000 rpm for 10 s. The sequence was repeated 4 times).
Fig. 8 SEM image after application of a nanometer gold with palladium
Fig. 6 Fluorescence spectra of thin layers of complexes 1, 2 and 3 on Si,
ex = 282 nm.
layer of (a) 1, PMMA/Si layer covered with gold, and (b) 3, PMMA/Si, magn.
100 000. (c and d) EDS mappings of 3, PMMA/Si Ag-2.65, Br 1.58%.
l
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consistency to the optical constants, the complex dielectric
functions of Ag complex layers were parameterized using a
4
9,50
sum of Gaussian oscillators:
m
X
ꢀ
ꢁ
~
e ¼ e1
þ
Gauss A ; E ; Br ;
j
(3)
j
j
j¼1
Fig. 9 AFM image of 1, PMMA/Si(111), 2500 rpm 30 s, 5000 rpm 20 s ꢃ 4. where, e
N j j
is the high frequency dielectric constant, A , E and
Br are the amplitude, energy and broadening of the j-th absorp-
j
tion band, respectively.
Two dimensional (2D) and three dimensional (3D) AFM
Fig. 10 shows the experimental C and D azimuths for the 2,
PMMA/Si. Results for the two other samples also exhibit a very
good agreement between the raw and calculated data. Thick-
nesses for the examined films are 15.8 ꢁ 0.1 nm, 3.8 ꢁ 0.1 nm
and 10.4 ꢁ 0.1 nm for layers of 1, 2 and 3 complexes on Si,
respectively.
2
images scanned over a surface area of 1 ꢃ 1 mm show a
uniform and dense morphology (Fig. 9 and Fig. S6, ESI†). The
phase images indicated a single compound covers Si(111) sur-
face in all cases. The root-mean square (RMS) parameters were
calculated from AFM images. The average roughness parameters
equaled R = 1.59 nm and R = 1.22 nm for 1, PMMA/Si, and
q
a
The real components of the complex refractive indexes and
extinction coefficients determined for the samples are pre-
sented in Fig. 11, where the optical constants of PMMA (taken
from ref. 49) are presented for a comparison. The value of n for
PMMA (a matrix material for the new samples) is about 1.5 in
the non-absorbing spectral range (l 4 300 nm) and exhibits a
normal dispersion in the presenting spectral range. At wave-
lengths longer than 500 nm, the refractive index of Ag complex
layers is higher than that for PMMA, and the highest value
R
q
= 2.24 nm and R
a
= 1.25 nm for 2, PMMA/Si, which indicated
smooth layers of materials. This smooth morphology reveals that
the organic–inorganic hybrid coatings are well deposited on Si
substrates by spin-coating method.
The average depth of the layer equals 13.23 nm for 1, PMMA/Si
and 51 nm for 2, PMMA/Si (Fig. 9 and Fig. S6, ESI†).
In case of 3, PMMA/Si(111), the roughness parameters are
R = 3.76 nm and R = 2.45 nm, whereas the average depth of the
q
a
layer equaled 113 nm. This material also displays homogeneity of
the thin, smooth layers and equally distributed compound over the
silicon surface. The new layers are thinner than those previously
(about 1.65) is for the sample 3, while the lowest (about 1.52) is
for the sample 1. At shorter wavelengths (o500 nm), the absorp-
tion bands are visible. These features were reproduced using
a sum of Gaussian oscillators. Energies (and wavelengths) of
particular oscillators are summarized in Table 3. The extinction
coefficient spectra of Ag complex films presented in Fig. 11b show
two regions of absorption bands (300–400 nm and o300 nm)
associated with p - p* and n - p* transitions in the aromatic
rings and CQN group, respectively. To determine the band
obtained by us ([Ag(thet(3-methca))
2
(NO
3
)]/Si, [Ag(thet(5-Brthca))
2
-
33
(NO )]/Si), thickness (150–400 nm). Also, the values of roughness
3
parameters of the new materials are lower in comparison with
previously observed roughness parameters of [Ag(thet(3-methca))2-
(NO )]/Si, [Ag(thet(5-Brthca)) (NO )]/Si layers (8.17–21.7 nm).
3 2 3
Concluding from the presented results, the spin-coating procedure
provides thin, homogeneous films with the complexes uniformly
spread over the substrate surface. The best coverage results were
obtained for the following spin coating parameters: Spin speed of
2500 rpm, time of covering at 30 s, and 3000 rpm for 30 s. Any
further increase in deposition time or spin speed leads to for-
mation of heterogeneous films, in which crystallites appear. The
layers exhibit fluorescence, which in conjunction with their homo-
geneity, make them promising candidates for use in solar cells.
Ellipsometric analysis of the thin films
To determine the complex refractive index ( n˜ ) and thickness of Fig. 10 Experimental (dashed lines) and model based (solid lines) plots for
2
the deposited films, the four phase optical model of sample was (a) C and (b) D azimuths for the 2/Si (w = 2.6).
considered: Si (substrate)/SiO (native oxide)/Ag complex layer.
2
The complex refractive index is then,
n˜ = n + ik,
(1)
where, n is the real part of the complex refractive index and k is
the extinction coefficient directly associated with the complex
dielectric function ( ~e ) of a material:
2
~
e = n˜ .
(2)
Optical constants of Si and SiO
database of optical constants. To provide the Kramers–Kronig extinction coefficients of the layers.
2
were taken from the
Fig. 11 (a) Real component of the complex refractive index and (b) the
4
9
1
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g
Table 3 The band-gap energy (E ) and energies of Gaussian oscillators
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
Sample
E
g
(eV nm
)
E
1
(eV nm
)
E
2
(eV nm
)
E
3
(eV nm
)
E
4
(eV nm
)
E
5
(eV nm
)
1
/Si
2.90 ꢁ 0.01/428 ꢁ 2 3.04 ꢁ 0.08/408 ꢁ 11 3.25 ꢁ 0.11/381 ꢁ 13 3.59 ꢁ 0.24/345 ꢁ 23 4.15 ꢁ 0.15/299 ꢁ 11 7.99 ꢁ 0.55/
155 ꢁ 11
2/Si
/Si
2.95 ꢁ 0.02/420 ꢁ 3 3.19 ꢁ 0.01/389 ꢁ 1
4.87 ꢁ 0.02/255 ꢁ 1
13*
—
—
—
3
2.83 ꢁ 0.02/438 ꢁ 3 3.14 ꢁ 0.13/395 ꢁ 16 3.32 ꢁ 0.17/373 ꢁ 17 3.82 ꢁ 0.12/325 ꢁ 10 6.52 ꢁ 0.33/190 ꢁ 10
using Vario EL III Elemental analyzer. Layers of complexes were
deposited on Si(111) wafers (2 cm ꢃ 2 cm), B500 nm thick,
using spin coating technique. Precursors were dissolved in
toluene with 5% solution of PMMA and deposited on Si(111)
using a spin coater (Laurell 650 SZ). The morphology and com-
position of obtained materials were analysed with a scanning
electron microscope, (SEM) LEO Electron Microscopy Ltd,
England, model 21430 VP equipped with secondary electrons
(
(
SE) detectors and an energy dispersive X-ray spectrometer
EDX) Quantax with a detector XFlash 4010 (Bruker AXS micro-
analysis GmbH). Morphology of the layers was also studied
using SEM/FIB (scanning electron microscope/focused ion beam)
Quanta 3D FEG equipped with gold and palladium sputter
SC7620. Atomic force microscopy (AFM) images were captured
in the tapping mode with a Multi Mode Nano Scope IIIa (Veeco
Fig. 12 The Tauc plot of Ag complex layers (1, 2, 3 means films of
complexes 1, 2 and 3, respectively, deposited on Si(111)).
gap-energy of the examined specimens, the Tauc method was Digital Instrument) microscope.
5
1
used. To extract value of the band gap-energy, the relation
(
The ellipsometric azimuths C and D were measured for
m%
ahn) versus hn should be plotted, where a is the absorption three angles of incidence (651, 701 and 751) by means of the
coefficient, hn is the photon energy and m is a parameter V-VASE device (J.A. Woollam Co., Inc.) in the spectral range
associated with the type of transition: m = 1/2 for a direct from 0.62 eV to 6.5 eV (191–2000 nm).
allowed transition, m = 3/2 for a direct forbidden transition,
m = 2 for an indirect allowed transition and m = 3 for an indirect squared error (w ) defined as:
Model quantities were varied to minimize the reduced mean
2
49,50
forbidden transition. For the prepared films, m = 1/2 was chosen.
The Tauc plot for the Ag complex layers is presented in Fig. 12.
The band-gap energies are 2.83 ꢁ 0.02 eV (438 ꢁ 3 nm), 2.90 ꢁ
2
!
!3
2
2
mod
Cj
exp
j
mod
Dj
exp
j
X
ꢀ C
ꢀ D
1
2
4
5
w ¼
þ
;
(4)
C
D
N ꢀ P
s
s
j
j
j
0
.01 eV (428 ꢁ 2 nm) and 2.95 ꢁ 0.02 eV (420 ꢁ 3 nm) for
compounds 3, 1 and 2 on Si(111), respectively.
where, N and P are the total number of data points and
the number of fitted model parameters, respectively. C
are experimental data, while C
the values obtained during the fitting procedure. sj and s are
exp
j
and
exp
mod
j
mod
D
j
and D
j
correspond to
Experimental
Materials
C
D
j
standard deviations of C and D data. The fit procedure was
4
9
o-Phenylenediamine (96%), 2-(2-pirydyl)ethylamine (epy) (95%), performed using WASE32 software (J. A. Woollam Co., Inc.).
9
3
5
9
-anthracenecarboxaldehyde (anthr) (97%), 5-bromo-2-hydroxy-
-methoxybenzaldehyde (97%), 2,5-dimethoxybenzaldehyde (99%),
X-ray crystallography
-bromo-2-thiophenecarboxaldehyde (5-Brthca) (95%), AgNO
3
4
Crystal structures of [Ag (epy(5-Brthca)) (NO ) ] 1 and
2 2 3 2
+
ꢀ
9.0% were purchased from Aldrich and used without further [Ag(bzim-anthr)
] NO
2. Crystals of 1 and 2 were obtained
2
3
purification.
from ethanol solution after slow evaporation. X-ray diffraction
data were collected at 293(2) K with an Oxford Sapphire CCD
diffractometer using Mo Ka radiation l = 0.71073 Å and o–2y
Methods and instrumentation
IR spectra were captured on a Bruker instrument using ATR method. Structures were solved by the Patterson method and
ꢀ
1
1
13
2
technique in the range 70–4000 cm
were collected with a Bruker AMX-400 spectrometer (400 MHz) the use of SHELX2014 and SHELX2017 program packages.
in DMSO-d6 against TMS. UV-Vis absorption spectra were Analytical absorption corrections were applied (CrysAlis version
. H, C NMR spectra refined with the full-matrix least-squares method on F with
5
2
5
3
recorded using a Hitachi spectrophotometer, in CH CN or 171.38.43 package of programs Rigaku OD, 2015 ). Hydrogen
3
ꢀ
5
CH Cl (1 ꢃ 10 M) solution. The fluorescence spectra were atoms were located from the difference electron density maps
2
2
recorded on a spectrofluorometer Gilden photonics 700 in the and their positions were constrained in the refinement with
ꢀ4
ꢀ3
range 900–200 nm (at 5 ꢃ 10 mol dm in MeCN or CH
2
Cl
2
appropriate AFIX commands as implemented in SHELX. Data
solution or silicon slides). Elemental analyses were carried out collection and refinement processes are summarized in Table 4.
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Table 4 Crystal data, structure refinement and CCDC deposition numbers for 1 and 2
1
2
CCDC
1504448
11AgBrN
465.08
293(2)
0.71073
Triclinic
P1%
8.7498(4)
9.3160(6)
10.2463(6)
70.602(6)
84.715(4)
68.444(6)
732.27(8)
2
1834866
C H28AgN O
42 5 3
758.56
293(2)
0.71073
Empirical formula
Formula weight
Temperature, K
Wavelength, Å
Crystal system
Space group
a, Å
b, Å
c, Å
a, deg
b, deg
g, deg
Volume, Å
Z
Density (calculated), Mg m
Absorption coefficient, mm
12
C H
3
O
3
S
Monoclinic
P2 /n
1
8.7674(5)
18.6110(11)
20.4404(10)
90
94.806(4)
90
3323.5(3)
4
1.516
ꢀ
ꢀ
3
2.109
4.259
1
0.657
F(000)
452
1544
2.189 to 28.527
23 871
Theta range for data collection, deg
Reflections collected
2.483 to 28.123
4369
Independent reflections
Completeness to theta
3185 [R(int) = 0.0256]
25.0001
7662 [R(int) = 0.0554]
25.2421
100.0%
100.0%
Max./min. transmission
1.000/0.4075
3185/0/190
1.010
0.811/0.409
7662/0/460
0.814
Data/restraints/parameters
2
Goodness-of-fit on F
Final R indices [I 4 2sigma(I)]
R indices (all data)
Largest diff. peak/hole, e Å
R
R
1
= 0.0393, wR
= 0.0449, wR
2
= 0.1079
= 0.1106
R
R
1
= 0.0347, wR
= 0.0920, wR
2
= 0.0533
= 0.0608
1
2
1
2
ꢀ3
1.055/ꢀ1.025
0.257/ꢀ0.354
The structural data of 1 and 2 have been deposited with the the resultant yellow filtrate was slowly evaporated to dryness.
Cambridge Crystallographic Data Centre, and the CCDC numbers Yellow crystals suitable for X-ray data collection were obtained
are 1504448 for 1 and 1834866 for 2.†
from the ethanol solution. Yield = 61%. M.p. = 240–270 1C.
Ag(L2) ]NO 2. Calc. elemental composition for C42 28AgN
C 66.49, H 3.72, N 9.23, Ag 14.21%. Found composition: C
[
2
3
H
5 3
O :
Synthesis of the ligands and complexes
1
Details of ligands syntheses are found in ESI.†
66.12, H 4.02, N 9.29, Ag 14.19%. H NMR (400 MHz, DMSO-d
6
)
d 9.11 (s, 1H, H3b), 8.35 (d, J = 7.7 Hz, 2H), 7.98 (m, 2H), 7.74
Synthesis of complexes
13
(
m, 2H), 7.71 (m, 6H); C NMR (400 MHz, DMSO-d ) d 149.6
6
AgNO + L1 - Ag L1 (NO )
3 2
C2, 136.1 C9, 134.7 C4b, 9b, 133.4 C1A, 132.3 C8A, 131.8 C10,
131.5 C4,5, 130.4 C1, 130.0 C8, 128.2 C4A, 127.7 C5A, 127.2
C3.6, 125.8 C2,7, 122.5 C8b, 122.2 C5b [ppm]. (Fig. 3) ATR IR:
3
2
2
AgNO + L - AgL NO
3
2
3
L = L2–L4
[
Ag
synthesized in the following manner: 0.2738 g (1.6118 mmol) 1283, 1259 n(C–N), 1178 [cm ]. UV-Vis l (Å) [nm]: 383(0.357),
AgNO was added to 0.4758 g (1.6118 mmol) of ligand L1 364(0.379), 347(0.271), 283(2.53). CH Cl : 255(2.67), 349(0.26),
dissolved in ethanol. Synthesis was carried out for 3 hours 366(0.32), 385(0.30).
under reflux. Yellow crystals suitable for X-ray data collection [Ag(L3) ]NO 3 and [Ag(L4) ]NO 4 were synthesized in the
were obtained from ethanol solution. Calc. elemental composi- same manner as 2.
tion for C H N O S Br Ag : C 27.34, H 2.10, N 10.63, Ag [Ag(L3) ]NO 3. Calc. composition for C28
0.47%. Observed composition was: C 27.19, H 2.20, N 10.46, 41.61, H 2.74, N 8.66, Ag 13.35%. Found composition: C 41.18,
2 2 3 2
(L1) ](NO ) 1. The dinuclear complex of Ag 1 was 2936 n(C–HAr), 1667 n(CQC), 1488 n(CQN), 1464, 1334 n(CQC),
ꢀ1
3
2
2
2
3
2
3
2
4
22
8
12
2
2
2
2
3
22 5 7 2
H N O Br Ag: C
2
1
Ag 20.45%. Yield = 48%.
H 2.44, N 8.39, Ag 13.25%. (48% yield), m.p. = 115 1C. H NMR
) d 8.37 (d, HCQN, 1H), 8.27 (400 MHz, DMSO-d ) d 13.63 (s, 2H, OH, NH), 7.86 (d, J = 2.4 Hz,
1
H NMR (400 MHz, DMSO-d
6
6
(
3
2
HCQN, 1H), 7.21 (m, 2H), 7.16 (m, 2H), 3.77 (t, J = 7.0 Hz, 2H), 1H, H13), 7.77 (m, 2H, H6,9), 7.44 (m, 2H, H7, H8), 7.37 (d, J =
1
3
.21 (t, J = 14.7 Hz, 2H) [ppm]; IR (KBr): 3008, 2924 n(C–HAr), 2 Hz, 1H, H15), 3.99 (s, OCH
3
); C NMR (400 MHz, DMSO-d
6
)
ꢀ
1
851 n CH , 1631 n(CQN), 1477, 1385 n(CQC) [cm ]. UV-Vis d 150.1 C2, 147.4 C(12, 14), 125.0 C(8,7), 121.3 C(13), 117.9 C(11),
2
l (Å) [nm] in MeCN: 371(0.08), 302(0.05), 230(0.22).
115.0 C(6,9), 113.0 C(5), 112.10 C(4), 110.7 C(10), 57.0 OCH [ppm].
[
Ag(L2) ]NO 2. A solution of AgNO (0.030 g, 0.17 mmol) in (Fig. 3). FT-IR (KBr): 3085 n(CQH)Ar, 1623 n(CQC), 1557, 1504,
2
3
3
5
0
mL of ethanol was added to a solution of L2 (0.050 g, 1457 n(CQN), 1445, 1341, 1308, 1267 n(C–N), 1250, 1237, 1215
ꢀ1
.17 mmol) in the same solvent and stirred at 78 1C in the n(C–N), 876[cm ]. UV-Vis l (Å) [nm]: In MeCN: 328(0.147),
Cl : 229(0.26), 305(0.154) nm.
dark for 2 days. The light yellow precipitate was filtered off and 291(0.200), 303(0.229) nm, in CH
2
2
1
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Ag(L4)
Paper
[
2
]NO
3
4. Calc. composition for C30H N O
28 5 7
Ag: C 53.11, shifted to a larger extent. Using a spin coating process thin
H 4.15, N 10.32, Ag 15.89%. Found composition: C 53.09, layers of the silver complexes were obtained. The band-gap
1
H 4.20, N 10.46, Ag 15.45%. (48% yield), m.p. = 195 1C. H NMR energies were 2.83 ꢁ 0.02 eV (438 ꢁ 3 nm), 2.90 ꢁ 0.01 eV
(
400 MHz, DMSO-d ) d 12.50 (s, 1H, NH), 7.77 (d, J = 3.2 Hz, 1H, (428 ꢁ 2 nm) and 2.95 ꢁ 0.02 eV (420 ꢁ 3 nm) for compounds 3,
6
H(9)), 7.72 (m, 2H, H6,8), 7.33 (m, 2H, H7, 12), 7.23 (d, J = 3.2 Hz, 1 and 2, respectively, with PMMA on Si(111). Such gaps might
1
H, H15), 7.15 (m, 1H, H13), 3.94 (s, OCH
3
), 3.81 (s, OCH
) d 153.6 C2, 151.7 C12, 149.18 molecular interactions during non-radiative transitions of the
C14, 132.0 C(10), 128.0 C(15), 122.8 C(7), 123.5 C7, 118.5 C8, excited state. This is consistent with our results from NOE
14.7 C11, 114.1 C12, 112.4 C6, 56.7 OCH , 56.1 OCH [ppm]. experiments. The new materials exhibited fluorescence in the
Fig. 3) FT-IR (ATR): 3074 n(C–HAr), 1627 n(CQC), 1574, 1528, range 470 to 580 nm after excitation between 228 and 368 nm.
3
). be due to conformational changes and breaking of the intra-
1
3
C NMR (400 MHz, DMSO-d
6
1
(
3
3
1
1
494 n(CQN), 1452, 1360 n(CQC ), 1337, 1307, 1276 n(C–N), This is important as it indicates the possibility of applying
Ar
225, 1200, 1187, 1043 n(C–N), 1020 [cm ]. UV-Vis l (Å) [nm]: these materials in solar cells.
ꢀ1
In MeCN: 498(0.05), 326(0.21), 301(0.22) nm, in CH Cl :
2
2
228(0.20), 303(0.21), 328(0.18) nm.
Conflicts of interest
Thin films preparation
There are no conflicts to declare.
Thin layers of complexes were deposited by a spin coating tech-
nique, using rotation speeds ranging from 2000 to 5000 rpm, to
modify film thickness, homogeneity and optical properties. The
obtained materials were characterized with SEM/EDX, AFM,
spectroscopic ellipsometry and fluorescence spectroscopy.
Two different procedures of spin-coating were carried out. In
the first procedure, silicon was covered with PMMA (polymethyl
Acknowledgements
We would like to thank the National Science Centre Poland for
financial support (grant no 2013/09/B/ST5/03509).
methacrylate) solution, and then, the complex solution was References
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2
3
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nitrogen atoms of two different 2-anthracene benzimidazole
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