From Red to Green Luminescence via Surface Functionalization. Effect of 2-(5-Mercaptothien-2-yl)-8-(thien-2-yl)-5-hexylthieno[3,4- c]pyrrole-4,6-dione Ligands on the Photoluminescence of Alloyed Ag-In-Zn-S Nanocrystals

Article abstract of DOI:10.1021/acs.inorgchem.0c02468

A semiconducting molecule containing a thiol anchor group, namely 2-(5-mercaptothien-2-yl)-8-(thien-2-yl)-5-hexylthieno[3,4-c]pyrrole-4,6-dione (abbreviated as D-A-D-SH), was designed, synthesized, and used as a ligand in nonstoichiometric quaternary nanocrystals of composition Ag1.0In3.1Zn1.0S4.0(S6.1) to give an inorganic/organic hybrid. Detailed NMR studies indicate that D-A-D-SH ligands are present in two coordination spheres in the organic part of the hybrid: (i) inner in which the ligand molecules form direct bonds with the nanocrystal surface and (ii) outer in which the ligand molecules do not form direct bonds with the inorganic core. Exchange of the initial ligands (stearic acid and 1-aminooctadecane) for D-A-D-SH induces a distinct change of the photoluminescence. Efficient red luminescence of nanocrystals capped with initial ligands (λmax = 720 nm, quantum yield = 67%) is totally quenched and green luminescence characteristic of the ligand appears (λmax = 508 nm, quantum yield = 10%). This change of the photoluminescence mechanism can be clarified by a combination of electrochemical and spectroscopic investigations. It can be demonstrated by cyclic voltammetry that new states appear in the hybrid as a consequence of D-A-D-SH binding to the nanocrystals surface. These states are located below the nanocrystal LUMO and above its HOMO, respectively. They are concurrent to deeper donor and acceptor states governing the red luminescence. As a result, energy transfer from the nanocrystal HOMO and LUMO levels to the ligand states takes place, leading to effective quenching of the red luminescence and appearance of the green one.

Full text of DOI:10.1021/acs.inorgchem.0c02468

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From Red to Green Luminescence via Surface Functionalization.
Effect of 2‑(5-Mercaptothien-2-yl)-8-(thien-2-yl)-5-
hexylthieno[3,4‑c]pyrrole-4,6-dione Ligands on the
Photoluminescence of Alloyed Ag−In−Zn−S Nanocrystals
́
́
Patrycja Kowalik,* Piotr Bujak,* Zbigniew Wrobel, Mateusz Penkala, Kamil Kotwica, Anna Maron,
and Adam Pron
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ABSTRACT: A semiconducting molecule containing a thiol anchor
group, namely 2-(5-mercaptothien-2-yl)-8-(thien-2-yl)-5-hexylthieno-
[3,4-c]pyrrole-4,6-dione (abbreviated as D-A-D-SH), was designed,
synthesized, and used as a ligand in nonstoichiometric quaternary
nanocrystals of composition Ag1.0In3.1Zn1.0S4.0(S6.1) to give an
inorganic/organic hybrid. Detailed NMR studies indicate that D-A-
D-SH ligands are present in two coordination spheres in the organic
part of the hybrid: (i) inner in which the ligand molecules form direct
bonds with the nanocrystal surface and (ii) outer in which the ligand
molecules do not form direct bonds with the inorganic core. Exchange
of the initial ligands (stearic acid and 1-aminooctadecane) for D-A-D-
SH induces a distinct change of the photoluminescence. Efficient red
luminescence of nanocrystals capped with initial ligands (λmax = 720 nm, quantum yield = 67%) is totally quenched and green
luminescence characteristic of the ligand appears (λmax = 508 nm, quantum yield = 10%). This change of the photoluminescence
mechanism can be clarified by a combination of electrochemical and spectroscopic investigations. It can be demonstrated by cyclic
voltammetry that new states appear in the hybrid as a consequence of D-A-D-SH binding to the nanocrystals surface. These states
are located below the nanocrystal LUMO and above its HOMO, respectively. They are concurrent to deeper donor and acceptor
states governing the red luminescence. As a result, energy transfer from the nanocrystal HOMO and LUMO levels to the ligand
states takes place, leading to effective quenching of the red luminescence and appearance of the green one.
alization. Moreover, many of them can be processed from
popular solvents.22−24 However, the overwhelming majority of
organic electroactive compounds do not contain appropriate
functional groups capable of binding to the nanocrystal surface.
Such functionalization is necessary because the preparations of
classical blends by mixing nonfunctionalized organic semi-
conductors with inorganic nanocrystals frequently lead to
uncontrollable phase separation.25 Thus, new designs of
electroactive molecules have to be elaborated if they are
planned to serve as nanocrystals capping ligands, involving,
among others, introduction of alkyl-type solubilizing groups
and functional groups which could form stable bonds with
nanocrystal core surfacial atoms (ions).2
INTRODUCTION
■
Semiconductor nanocrystals can be considered as inorganic/
organic hybrids consisting of an inorganic core and surfacial
capping ligands which ensure their colloidal stability. Primary
surfacial ligands, that is, ligands introduced during the
nanocrystals synthesis, can be further modified or exchanged
for new ligands providing a desired functionality.1−8 Detailed
studies of the interactions at the inorganic core−ligands
interface are of crucial importance since they determine basic
physical properties of the resulting hybrids. There are
numerous literature examples of the effect of ligands and
nanocrystal core compositions on their energy gap, positions of
valence and conduction bands, photoluminescence, and their
quantum yields (QYs).9−14 Hybrids containing ligands of
organic semiconductor nature are especially interesting since
they can exhibit tunable photo- and electroactivity.2,15−21
Potential capping ligands can be searched among low and high
molecular mass electroactive compounds whose ionization
potential (IP), electron affinity (EA), band gap, absorption,
and emission spectra can be tuned by appropriate function-
Received: August 18, 2020
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It should be noted that, to date, the vast majority of papers
devoted to the design of functional ligands and the ligand
exchange dealt with nanocrystals of binary semiconductors
such as CdSe, CdS, PbSe, and PbS. Recent progress in the
synthesis of ternary and quaternary nanocrystals, including
strongly nonstoichiometric alloyed ones, has not induced
parallel intensive research on the design of “tailor-made”
photo- and electroactive ligands suitable for these nanocrystals
as well as on the exchange of primary ligands for functionalized
ones. This type of research is highly desirable in the case of
quaternary Ag−In−Zn−S nanocrystals, especially, because by
changing their composition it is possible to tune their efficient
luminescence over the whole visible spectral range. This
strongly facilitates their application in electronics26−29 and
biomedical sciences.30−35 The analysis of ligand exchange-
induced photoluminescence changes is especially important in
this respect. In the case of binary CdSe nanocrystals the
luminescence is governed by the classical radiative recombi-
nation mechanism 1S(e) → 1S(h). On the contrary, the
luminescence of alloyed quaternary Ag−In−Zn−S nanocryst-
als proceeds via the donor−acceptor radiative recombination
mechanism, associated with the presence of point defects in
the nanocrystal core.36
characteristic of alloyed Ag−In−Zn−S nanocrystals and
consistent with our previous findings.38,39 They emitted red
light (λmax = 720 nm, QY = 67%).
I n F i g u r e 1 ,
a
p o w d e r d i ff r a c t o g r a m o f
Ag1.0In3.1Zn1.0S4.0(S6.1) nanocrystals is presented together
In this report, we describe the design and fabrication of a
hybrid consisting of the quaternary Ag−In−Zn−S core capped
with organic semiconductor-type ligands, namely, 2-(5-
mercaptothien-2-yl)-8-(thien-2-yl)-5-hexylthieno[3,4-c]-
pyrrole-4,6-dione. Spectroscopic and electrochemical proper-
ties of this hybrid are analyzed. We also demonstrate that the
exchange of primary, spectroscopically inactive ligands for the
conjugated ones induces a hypsochromic shift of the
photoluminescence peak by over 200 nm, that is, from the
red region of the spectrum to the green one.
Figure 1. TEM image of Ag1.0In3.1Zn1.0S4.0(S6.1) nanocrystals (d = 5.8
0.9 nm, n = 200). An XRD pattern of these nanocrystals is
presented in the inset.
with their representative TEM image. The alloyed nature of
the obtained nanocrystals was confirmed by the positions of
the observed Bragg reflections which were intermediate
between these characteristic of orthorhombic AgInS2
(JCPDS 00-025-1328) and those of hexagonal ZnS (JCPDS
00-036-1450). Broadening of the registered peaks was
consistent with small size of the prepared nanocrystals clearly
evidenced by TEM. Additionally, in Figure S3 a representative
HR-TEM image of the obtained nanocrystals is presented
together with a histogram showing their size distribution.
Primary ligands were identified by using a procedure
previously elaborated by our group and consisting of
controlled dissolution of the nanocrystals inorganic core and
NMR analysis of the organic residue.40 The obtained 1H NMR
spectrum (see Figure S4) revealed the presence of two types of
ligands: stearic acid originating from the precursor of zinc and
1-aminooctadecane formed through hydrogenation of OLA in
the course of the preparation of nanocrystals.39
Among organic semiconductors, which could be considered
as candidates for photo- and electroactive surfacial ligands of
inorganic semiconductors nanocrystals, donor−acceptor com-
pounds deserve a special attention. This is due to the fact that
their band gap as well as HOMO and LUMO energies can be
precisely tuned by selection of donor and acceptor units of
appropriate strength. In particular, increasing the accepting
ability of “A” units results in lowering of the LUMO level,
whereas strengthening of the electron donating effect of “D”
units results in rising of the HOMO level.20,22,41−43
RESULTS AND DISCUSSION
■
The presented research was focused on surface functionaliza-
tion of alloyed Ag−In−Zn−S with photo- and electroactive
ligands. In our previous research we elaborated new
procedures of fabricating these quaternary nanoparticles of
different compositions which exhibited strong and tunable
photoluminescence covering green and red regions of the
spectrum (QY in the range 48−67%).37−39 We also developed
an efficient method of exchanging primary hydrophobic ligands
for hydrophilic ones such as 11-mercaptoundecanoic acid
(MUA), for example. In subsequent steps bioactive objects
could be grafted to these capping ligands such as transferrin,
doxorubicin, unsymmetrical bisacridine derivatives, and so on,
providing new hybrids suitable for medical applications.34,35
Ag−In−Zn−S nanocrystals studied in this research were
prepared by using a reaction mixture that consisted of silver
nitrate, indium(III) chloride, zinc stearate, and 1-dodecane-
thiol (DDT) dissolved in 1-octadecene (ODE) in molar ratios
AgNO3/InCl3/zinc stearate/DDT = 1.0/3.4/3.6/5.6. To
initiate the reaction, sulfur dissolved in oleylamine (S/OLA)
was injected to this mixture at 150 °C.38,39 The resulting
nanoparticles were spherical in shape (diameter, d = 5.8 0.9
nm) and showed the composition of Ag1.0In3.1Zn1.0S4.0(S6.1) as
determined by EDS (the value in parentheses indicates the
stoichiometric content of sulfur with respect to the content of
Ag, In, and Zn; for EDS data see Figure S1). Additional
characterization was performed by using XPS (see Figure S2
where the survey and high-resolution (Ag 3d, In 3d, Zn 2p, and
S 2p) spectra are presented). The recorded spectra are
As an electro- and photoactive ligand we selected thieno-
[3,4-c]pyrrole-4,6-dione (TPD), which was used as a building
block in numerous low44−46 and high molecular weight organic
semiconductors.47−52 A strong advantage of TPD is associated
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Figure 2. Chemical structures of the (a) 2,8-bis(thien-2-yl)-5-hexylthieno[3,4-c]pyrrole-4,6-dione (D-A-D) and (b) electroactive ligand 2-(5-
mercaptothien-2-yl)-8-(thien-2-yl)-5-hexylthieno[3,4-c]pyrrole-4,6-dione (D-A-D-SH) and their synthetic pathways.
with the presence of an alkyl group in the dione subunit which
increases its solubility and stability as a ligand. Functionaliza-
tion of this molecule with two thienyl groups at 2- and 8-
positions yielded a donor−acceptor−donor compound, 2,8-
bis(thien-2-yl)-5-hexylthieno[3,4-c]pyrrole-4,6-dione, abbrevi-
ated in the subsequent text as D-A-D. This molecule contained
no anchor functional group capable of binding to the
nanocrystal surface; therefore, it was additionally function-
alized with a thiol group (−SH). The selection of this
functional group was rationalized by the versatility of this
anchor group with respect to semiconductor nanocrystals
synthesis. In addition, it assured a stable bond with the
nanocrystal surficial atoms. An amine anchor group could in
principle be considered as an alternative. However, the binding
capability of a given amine depends on its basicity; that is,
more basic aliphatic amines form stronger bonds than aromatic
ones which are weaker bases.40 Introduction of a −CH2NH2
group to D-A-D would lead to efficient binding of the ligand to
the nanocrystal surface, but at the same time the presence of a
methylene group would result in a conjugation break between
the nanocrystal and the ligand. Thus, a thiol-functionalized
semiconductor ligand, namely 2-(5-mercaptothien-2-yl)-8-
(thien-2-yl)-5-hexylthieno[3,4-c]pyrrole-4,6-dione (D-A-D-
SH), better provided the anchoring capability while retaining
the conjugation.
Two methods of the synthesis of thieno[3,4-c]pyrrole-4,6-
dione (TPD) have been developed to date. In the classical
approach thiophene is used as the starting substrate.53,54
Alternatively, the Gewald reaction can be used for the
preparation of an appropriate thiophene derivative which
upon condensation yields the target product.55 In the research
presented here the classical method was used for the
preparation of D-A-D (Figure 2a). Thiophene was first
transformed into thiophene-3,4-dicarboxylic acid, which upon
condensation with an aliphatic amine (e.g., n-C6H13NH2)
yielded the core of TPD. The detailed procedure of the TPD
core preparation can be found in the Supporting Information.
Bromination of this core using NBS followed by Suzuki
coupling with thiophene-2-boronic acid pinacol ester yielded
D-A-D.56 D-A-D was transformed into its thiol derivative,
namely 2-(5-mercaptothien-2-yl)-8-(thien-2-yl)-5-hexylthieno-
[3,4-c]pyrrole-4,6-dione (D-A-D-SH) (Figure 2b), in a two-
step process. In the first step −SCN was introduced by using
NH4SCN/bromodimethylsulfonium bromide57 and then trans-
formed into −SH in the presence of H2O/HCl.58
Chemical identities of D-A-D and D-A-D-SH were
1
confirmed by NMR spectroscopy. In particular, the H NMR
spectrum of D-A-D revealed the presence of a triplet at 3.55
ppm corresponding to the CH2N moiety. In the aromatic part
of the spectrum three doublets appeared at 6.65, 6.73, and 8.13
ppm which could be ascribed to protons of the thienyl rings.
Two characteristic signals at 38.6 and 162.4 ppm, present in
the 13C NMR spectrum, could unequivocally be attributed to
the CH2N and CO groups of the acceptor unit. Detailed
1
1
analysis of H and 13C NMR data, including H−1H COSY
and 1H−13C HMQC results, is presented in Figure S5.
Functionalization of D-A-D with −SH to yield D-A-D-SH
1
perturbs its symmetry. As a result, in the H NMR spectrum
two additional doublets appear at 6.93 and 7.87 ppm
originating from coupling of thienyl protons with that of the
thiol group (J = 3.9 Hz). Because the second thienyl ring
remains unsubstituted, its protons give rise to three doublets at
1
1
6.64, 6.74, and 8.11 ppm. In Figure S6, the H and H−1H
COSY NMR spectra of D-A-D-SH are presented together with
detailed signals attributions. Introduction of −SH to (D-A-D)
significantly reduces the solubility of the resulting D-A-D-SH.
For this reason no good quality 13C NMR spectrum of this
compound could be registered.
The target inorganic/organic hybrid, that is, Ag−In−Zn−S/
D-A-D-SH, was obtained through exchange of primary ligands
for D-A-D-SH. In all preparations Ag1.0In3.1Zn1.0S4.0(S6.1)
nanocrystals originating from the same batch were used (see
the Experimental Section for a detailed description of the
ligand exchange). The ligand exchange had no effect on the
nanocrystals size (d = 6.0
0.8 nm) as well as on the
composition of the inorganic core (Ag1.0In3.1Zn1.0S4.0(S6.1) (see
Figure S7 for their TEM image and EDS spectrum).
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Upon ligand exchange the colloidal stability of the studied
nanocrystals remained essentially unchanged despite limited
solubility of free D-A-D-SH as compared to D-A-D. This can
be considered as a manifestation of the formation of a stable
bond between the nanocrystal surface Ag1.0In3.1Zn1.0S4.0(S6.1)
and D-A-D-SH which affects the ligand polarity and facilitates
the dispersion of the resulting nanohybrids in nonpolar or
weakly polar solvents.
to ca. 1/9. This distinct difference is caused by different
protons relaxation times for these two types of ligands. Protons
of surface-bound ligands (∼3.9 ppm) relax more slowly as
compared to protons of free ligands (∼3.5 ppm). If the pulse
repeat sequence is kept constant (1 s), each particular pulse
excites a smaller number of nuclei in the surface-bound ligands
as compared to ligands of the second coordination sphere.
Upon increasing repetition of pulses, this difference grows
giving rise to a decrease of the I3.9ppm/I3.5ppm ratio. A similar
effect is also observed for other NMR signals of the aromatic
region, registered for Ag−In−Zn−S/D-A-D-SH. The signal at
7.91 ppm corresponds to protons of the thienyl group in the
ligand bound to the nanocrystal surface, whereas the doublet at
7.87 ppm is attributed to the corresponding protons in ligands
of the second coordination sphere. The I7.91ppm/I7.87ppm ratio
decreases from 1/5 for 16 scans to 1/27 for 128 scans (Figure
S9).
For the identification of the chemical nature of ligands and
their mode of binding to nanocrystals, diffusion-ordered NMR
or nuclear Overhauser effect spectroscopies are used.59 Their
efficacy was proven in the analysis of binding of amine- or acid-
type ligands to several binary nanocrystals such as CdS, CdSe,
and CdTe.60,61 In this research we propose a different
1
approach based on the analysis of H and 13C NMR and
1H−1H COSY and 1H−13C HMQC spectra presented in
1
Figures S8 and S9. In the H NMR spectrum registered in
The presented results unequivocally show that a fraction of
D-A-D-SH molecules form direct bonds with the nanocrystal
surface (first coordination sphere). The second coordination
sphere consists of D-A-D-SH molecules which do not form
direct bonds with the Ag1.0In3.1Zn1.0S4.0(S6.1) nanocrystals
inorganic core. The coexistence of two (inner and outer)
coordination spheres has previously been reported for other
types of ligand-stabilized nanocrystals.62
C6D6, two signals characteristic of D-A-D-SH, namely triplets
at 3.53 and 3.95 ppm, attributable to the CH2N group, could
be distinguished. In Figure 3, aliphatic regions of the 1H NMR
In a detailed study electrochemical and spectroscopic
properties of the hybrid Ag−In−Zn−S/D-A-D-SH) were
compared with those determined for the same nanocrystals
(Ag1.0In3.1Zn1.0S4.0(S6.1)) capped with primary ligands, the
organic semiconductor (D-A-D), and its derivative containing
the anchor group (D-A-D-SH).
In Figure 4 a representative cyclic voltammogram of
Ag1.0In3.1Zn1.0S4.0(S6.1) nanocrystals capped with primary
1
Figure 3. H NMR spectra (in the range 3.0−5.0 ppm) of the 2,8-
bis(thien-2-yl)-5-hexylthieno[3,4-c]pyrrole-4,6-dione (D-A-D) (a), 2-
(5-mercaptothien-2-yl)-8-(thien-2-yl)-5-hexylthieno[3,4-c]pyrrole-
4,6-dione (D-A-D-SH) (b), and hybrid Ag−In−Zn−S/D-A-D-SH
recorded with 16 scans (c) and 128 scans (d) in benzene-d6 at 298 K.
Figure 4. Cyclic voltammograms of Ag1.0In3.1Zn1.0S4.0(S6.1) nano-
crystals capped with initial ligands (stearic acid and 1-amino-
octadecane). Electrolyte: 0.1 M Bu4NBF4/CH2Cl2, reference
electrode Ag/0.1 M AgNO3 in acetonitrile, scan rate = 50 mV/s.
spectra of D-A-D and D-A-D-SH are compared with those of
Ag−In−Zn−S/D-A-D-SH recorded after 16 and 128 scans,
respectively. The presence of these two above-mentioned
peaks indicates that two types of ligands coexist in the hybrid:
(i) strongly bound to the nanocrystal surface (chemical shift
∼3.9 ppm) and (ii) ligands from the second (outer)
coordination sphere whose chemical shift (∼3.5 ppm) is
close to that registered for “free” D-A-D-SH ligands. The
coexistence of two coordination spheres is further corrobo-
rated by the evolution of the spectrum of Ag−In−Zn−S/D-A-
D-SH with increasing number of scans. Upon its increase from
16 to 128, the I3.9ppm/I3.5ppm signals ratio decreases from ca. 1/2
ligands (stearic acid and 1-aminooctane) and dispersed in
0.1 M Bu4NBF4/CH2Cl2 is presented. The observed oxidation
and reduction processes are irreversible. From the potentials of
the oxidation (1.28 V vs Fc/Fc+) and reduction (−1.49 V vs
Fc/Fc+) onsets the ionization potential (IP) and electron
affinity (EA) can be calculated knowing the potential of the
Fc/Fc+ redox couple on the absolute potential scale (eqs 1 and
2):63−65
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Figure 5. Cyclic voltammograms of 2-(5-mercaptothien-2-yl)-8-(thien-2-yl)-5-hexylthieno[3,4-c]pyrrole-4,6-dione (D-A-D-SH). Electrolyte: 0.1 M
Bu4NBF4/CH2Cl2, reference electrode Ag/0.1 M AgNO3 in acetonitrile, scan rate = 50 mV/s.
Figure 6. UV−vis−NIR (left column) and photoluminescence (right column) spectra of 2-(5-mercaptothien-2-yl)-8-(thien-2-yl)-5-
hexylthieno[3,4-c]pyrrole-4,6-dione (D-A-D-SH) (orange lines), Ag−In−Zn−S capped with initial ligands (red lines), and ligand (D-A-D-SH)
(green lines) in toluene.
whereas its lower edge to In 5s5p hybridized with S 3p
IP (eV) = |e|(Eox(onset) + 4.8)
(1)
orbitals.71 A decrease of the Ag/In ratio to ca. 0.3 in
EA (eV) = −|e|(Ered(onset) + 4.8)
nonstoichiometric Ag−In−S nanocrystals results in lowering of
the valence band and an increase of the band gap.72 The band
gap of macrocrystalline hexagonal ZnS is equal to 3.68 eV.73
Thus, an increase of the Ag/Zn ratio in Ag−In−Zn−S
quaternary nanocrystals (to ∼1.0 in the nanocrystals described
in this research) results in an increase of the band gap. To sum
up, a combination of all phenomena described above leads to
widening of the band gap, which in the case of
Ag1.0In3.1Zn1.0S4.0(S6.1) nanocrystals reaches 2.60 eV.
Ag1.0In3.1Zn1.0S4.0(S6.1) nanocrystals capped with primary
ligands (stearic acid and 1-aminooctadecane) emit red light
(λmax = 720 nm, QY = 67%). The photoluminescence of
alloyed Ag−In−Zn−S nanocrystals does not follow the simple
radiative recombination 1S(e) → 1S(h) mechanism, known for
CdSe nanocrystals, for example. The observed large values of
the Stokes shift ∼170 nm and the FWHM (full width at half-
maximum) of the photoluminescence peak seem to indicate
the donor−acceptor mechanism of radiative recombination in
this type of nanocrystal.36
(2)
IP and EA values determined for Ag1.0In3.1Zn1.0S4.0(S6.1)
capped with primary ligands are 6.1 and −3.3 eV, respectively,
which correspond to an electrochemical band gap Egel = 2.8
eV.
The optical band gap of the same nanocrystals, Egopt, was
determined from their UV−vis−NIR spectrum by using the
relationship (Ahν)2 vs hν (Figure S10). The obtained value of
2.60 eV is lower than the electrochemical one. This difference
originates from Coulombic interactions between the created
charges; thus Egel = Egopt + ΔEJ (where ΔEJ is the Coulombic
interaction energy).66 The value of ΔEJ = 0.2 eV obtained for
Ag1.0In3.1Zn1.0S4.0(S6.1) capped with primary ligands is in the
typical range for the majority of inorganic semiconductor
nanocrystals.67 The measured value of the optical band gap
(Egopt = 2.60 eV) is mainly governed by the nanocrystals
nonstoichiometric composition (Ag1.0In3.1Zn1.0S4.0(S6.1)) and
in particular by the Ag/In and Ag/Zn ratios. The quantum
confinement effect also interferes. The gap is significantly
larger than that of stoichiometric macrocrystalline AgInS2
(Egopt = 1.98 eV).68−70 The upper edge of its valence band
corresponds to a hybrid of S 3p and Ag 4d orbitals hybrid,
The same set of characterization techniques was applied to
D-A-D-SH, the second component of the investigated hybrid.
In Figure 5 representative cyclic voltammograms, registered in
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the negative (vs Fc/Fc+) and positive potential ranges, are
presented. D-A-D-SH undergoes irreversible oxidation and
irreversible reduction. The potentials of the reduction and
oxidation onsets are used for the determination of IP and EA
according to eqs 1 and 2. The reduction of D-A-D-SH starts at
Ered(onset) = −1.40 V (vs Fc/Fc+). Its oxidation of begins at
Eox(onset) = 0.87 V (vs Fc/Fc+). The EA and IP values,
calculated on the basis of the electrochemical data, are −3.40
and 5.70 eV, respectively. These values yield the electro-
chemical band gap, Egel = 2.30 eV.
reduction process onset is only slightly higher (by 30 mV) than
that of the free ligand. Ionization potential and electron affinity
of the hybrid were calculated according to eqs 1 and 2, yielding
the following values: EA = −3.43 eV, IP = 5.64 eV, and Egel =
2.20 eV. Thus, the exchange of initial ligands for D-A-D-SH
lowered the band gap by 0.60 eV.
The UV−vis−NIR spectrum of the hybrid is dominated by
the absorption peak originating from the D-A-D-SH ligand
which is very similar to the corresponding peak of the free
ligand as far as its position and shape are concerned (λmax
=
In Figure 6 absorption and emission spectra of D-A-D-SH
are presented. The absorption band peaked at 399 nm is
inhomogeneously broadened toward longer wavelengths. Its
optical band gap, Egopt, is equal to 2.60 eV. D-A-D-SH is a
rather weak luminophore (QY = 2.5%); its large Stoke shift of
126 nm should be noted, since the emission band shows its
maximum at 525 nm. Compared to D-A-D-SH, before
introduction of an −SH group, the D-A-D emitted blue light
(λmax = 473 nm, QY = 10%; see Figure S11).
The next question to be considered is how the electro-
c h e m i c a l a n d s p e c t r o s c o p i c p r o p e r t i e s o f
Ag1.0In3.1Zn1.0S4.0(S6.1) and D-A-D-SH are altered upon the
formation of their hybrid. Figure 7 presents a representative
399 nm, Egopt = 2.6 eV; see Figure 6). The most pronounced
effect induced by the ligand exchange concerns the photo-
luminescence. As already mentioned, Ag1.0In3.1Zn1.0S4.0(S6.1)
nanocrystals are efficient luminophores emitting red light (λmax
= 720 nm, QY = 67%). Upon exchange of primary ligands for
D-A-D-SH this red photoluminescence is totally quenched,
and new green photoluminescence appears (λmax = 508 nm),
originating from the ligand. The emission peak is slightly
hypsochromically shifted as compared to the corresponding
peak of the free ligand. Its QY = 10% is 4-fold higher than QY
of free D-A-D-SH ligands. To determine the mechanism of the
observed changes, the emission spectra for complexes of D-A-
D-SH ligand with Ag+, In3+, and Zn2+ cations were recorded.
The complexes were obtained by adding the solution of the
ligand in toluene to the appropriate salts (AgNO3, InCl3, and
zinc stearate). Comparing the emission spectrum of the free
ligand with those recorded for the formed complexes, a change
in the position of the emission peak could be observed (see
Figure S12). Only in the case of the D-A-D-SH complex with
Ag+ partial extinction of the emission was detected. The
performed studies indicated that luminescent properties of the
Ag−In−Zn−S/D-A-D-SH hybrid were a result of interactions
between the nanocrystal and the ligand. Thus, they did not
originate from the formation of bonds between the ligands and
specific surfacial cations of the nanocrystal.
Results of electrochemical and spectroscopic investigations
obtained for Ag1.0In3.1Zn1.0S4.0(S6.1) capped with initial ligands,
free ligand D-A-D-SH and Ag1.0In3.1Zn1.0S4.2(S6.1)/D-A-D-SH
hybrid, are summarized in Table 1.
To explain the effect of the emission color change upon
introducing D-A-D-SH ligands to Ag1.0In3.1Zn1.0S4.0(S6.1), it is
instructive to briefly discuss the already-mentioned donor−
acceptor mechanism of radiative recombination 1S(e) →
1S(h) in nonstoichiometric Ag−In−Zn−S nanocrystals capped
with optically and electrochemically inactive primary ligands.
The photoluminescence QY is in this case strongly related to
the presence of point defects in the inorganic core which favor
the emission. These are Ag vacancies (VAg) and S interstitials
(Si) as acceptor levels and S vacancies (VS) and Ag interstitials
(Agi) as donor levels.74,75 These donor and acceptor levels act
as traps for charge carriers leading in the next step to radiative
Figure 7. Cyclic voltammograms of Ag1.0In3.1Zn1.0S4.2(S6.1) nano-
crystals capped with ligand (D-A-D-SH). Electrolyte: 0.1 M
Bu4NBF4/CH2Cl2, reference electrode Ag/0.1 M AgNO3 in
acetonitrile, scan rate = 50 mV/s.
cyclic voltammogram of the Ag−In−Zn−S/D-A-D-SH hybrid.
The potential of the oxidation onset (0.84 V vs Fc/Fc+) is only
slightly lowered (by 30 mV) with respect to the corresponding
potential registered for the free ligand. This strongly indicates
that the oxidation process takes place at the organic (ligand)
part of the hybrid. On the other hand, the potential of the
Table 1. Redox Potentials (vs Fc/Fc+), Electrochemically Determined Ionization Potential (IP) and Electron Affinity (EA),
Electrochemical and Optical Band Gaps, Maxima of the Photoluminescence Bands (PL), Quantum Yields (QY) for Ag−In−
Zn−S Nanocrystals Capped with Initial Ligands, D-A-D-SH and the Hybrid, i.e., Ag−In−Zn−S Nanocrystals Capped with
Ligand D-A-D-SH
(V) (Fc/Fc+)
E
onset
(V) (Fc/Fc+)
IP (eV)
EA (eV)
Egel (eV)
Egopt (eV)
PL (nm)
QY (%)
ox
onset
red
E
AgInZnS
D-A-D-SH
hybrid
1.28
0.87
0.84
−1.49
−1.40
−1.37
6.1
5.7
5.6
−3.3
−3.4
−3.4
2.8
2.3
2.2
2.6
2.6
2.6
720
525
508
67.0
2.5
10.0
F
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Article
recombination, which is significantly lower in energy than the
band gap.72,76−81
shape with a clear maximum at ∼580 nm and a shoulder near
∼440 nm, coinciding with the onset of the increasing
absorbance in the spectrum of Ag−In−Zn−S stabilized with
primary ligands. The excitation spectrum of free D-A-D-SH is
characterized by a rather narrow band peaked at 330 nm. The
corresponding excitation spectrum of Ag−In−Zn−S/D-A-D-
SH hybrid consists of two narrow peaks at 340 and 440 nm;
the first one nearly coincides with the corresponding peak of
the free ligand, whereas the second one closely matches the
absorption spectrum of the hybrid (compare Figure 6 and
Figure S13). Thus, these spectroscopic investigations clearly
indicate that two mechanisms are involved in the luminescence
generation in Ag−In−Zn−S/D-A-D-SH nanocrystalsdirect
excitation of the ligand and the excitation of nanocrystals
followed by the energy transfer from the nanocrystal to the
ligand, as schematically depicted in Figure 8b. An evident
manifestation of this additional mechanism is a 4-fold increase
of QY from 2.5% in free D-A-D-SH to 10% in the nanocrystal
surface bound ligands.
In Ag1.0In3.1Zn1.0S4.0(S6.1) nanocrystals capped with initial
ligands are characterized by the Ag/In ∼ 0.3 and a strong
deficit of sulfur (compare to the experimentally determined
sulfur content (S4.0) and the theoretical one calculated for the
determined content of metals (S6.1)). This nanocrystal
composition should result in the presence of appropriate
donor and acceptor states favoring the above-mentioned
donor−acceptor radiative recombination mechanism. This
reasoning is fully consistent with the spectroscopic results.
Their spectroscopically determined Egopt is equal to 2.6 eV,
whereas their luminescence band is peaked at 720 nm (red
light) and is characterized by a large Stokes shift (∼170 nm), a
large FWHM value (∼150 nm), and a high photoluminescence
QY (67%). The observed radiative recombination involving
donor and acceptor states corresponds to ΔE ∼ 1.7 eV. This is
schematically presented in Figure 8a where IP and EA are
converted to HOMO and LUMO, according to Koopmans’
theorem.
Moreover, for Ag−In−Zn−S capped with initial ligands, the
free D-A-D-SH ligand and the hybrid (Ag−In−Zn−S/D-A-D-
SH) photoluminescence decay profiles were recorded (see
Figure S14). The decay curves (λexc = 405 nm) were
multiexponential in nature. They could be fitted via the
following equation:
l
o
|
o
l
o
|
o
t
τ1
t
τ2
o
o
o
o
I(t) = A1 expm− } + A2 expm−
}
o
o
o
o
o
o
o
o
(3)
n
~
n
~
where τ1 and τ2 represent the decay time of the photo-
luminescence and A1 and A2 (Table 2) represent the relative
Table 2. Biexponential Fitting Results for
Photoluminescence Decay Profiles of Ag−In−Zn−S
Nanocrystals Capped with Initial Ligands, 2-(5-
Mercaptothien-2-yl)-8-(thien-2-yl)-5-hexylthieno[3,4-
c]pyrrole-4,6-dione (D-A-D-SH), and Ag−In−Zn-S/D-A-D-
SH Hybrid
Figure 8. Photoluminescence mechanisms in alloyed Ag−In−Zn−S
nanocrystals capped with initial ligands (a) and electroactive ligands
(D-A-D-SH) (b).
τ1 (ns)
A1 (%)
τ2 (ns)
A2 (%) lifetime (ns)
AgInZnS
D-A-D-SH
hybrid
677.82
0.53
0.52
56.82
95.60
96.08
1949.82
1.94
43.18
4.40
3.92
1227.00
0.59
1.75
0.57
Upon exchange of inactive primary ligands for D-A-D-SH, a
distinct change of the emission color from red (720 nm) to
green (508 nm) occurs, indicating that the ligand states are
involved in the emission. The emission band is hypsochromi-
cally shifted by 17 nm as compared to the corresponding band
of the free ligand. The red emission characteristic of
nanocrystals capped with primary ligands is totally quenched.
The exchange of inactive primary ligands for D-A-D-SH
introduces additional states located at −3.4 and −5.6 eV, that
is, below the Ag1.0In3.1Zn1.0S4.0(S6.1) nanocrystals’ LUMO level
and above their HOMO level, respectively. Thus, ligand states
can act as traps for photogenerated charge carriers. Therefore,
in addition to direct ligand excitation as in free D-A-D-SH,
another photoluminescence mechanism is envisioned involving
charge and energy transfer from the inorganic core to the
ligand as schematically depicted in Figure 8b. The above
outlined mechanism is corroborated by the excitation spectra
recorded for D-A-D-SH, Ag−In−Zn−S nanocrystals capped
with initial ligands, and the Ag−In−Zn−S/D-A-D-SH hybrid
(see Figure S13). The excitation spectrum of nanocrystals
capped with initial ligands is broad and has a rather complex
weight of the decay components at t = 0.82−84 Based on the
performed calculations, Ag−In−Zn−S nanocrystals capped
with initial ligands were characterized by a relatively long
average photoluminescence lifetime of 1227 ns, that is, in the
range typical of stoichiometric AgInS2 and alloyed AgInS2−
ZnS nanocrystals.72,85 However, in the case of free D-A-D-SH
ligands and the Ag−In−Zn−S/D-A-D-SH hybrid, significantly
shorter and practically the same values of the photo-
luminescence lifetime (0.59 and 0.57 ns, respectively) were
obtained, unequivocally proving a change in the photo-
luminescence mechanism occurring upon the exchange of
the primary ligands for D-A-D-SH (Figure 8b).
It should be noted that the observed luminescence color
change is inherently associated with the presence of the D-A-
D- unit since optically inactive ligands with an −SH anchor
group cause only minimal changes in emission spectra of
nonstoichiometric Ag−In−Zn−S nanocrystals.34,35,39
G
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Article
2-(5-Thiocyanatothien-2-yl)-8-(thien-2-yl)-5-hexylthieno-
[3,4-c]pyrrole-4,6-dione (7). 7 was obtained by using a
modification of procedures described in ref 57. 2,8-Bis(thien-2-yl)-
5-(n-hexyl)thieno[3,4-c]pyrrole-4,6-dione (1.25 g, 3.13 mmol) was
dissolved in 60 mL of anhydrous acetonitrile and 15 mL of
chloroform. NH4SCN (0.36 g, 4.7 mmol) and bromodimethyl-
sulfonium bromide (0.74 g, 3.1 mmol) were added, and the reaction
mixture was stirred at room temperature for 9 days under an argon
atmosphere. After removing the solvents the residue was dissolved in
chloroform (100 mL), washed with water, dried over anhydrous
sodium sulfate, evaporated, and dried under vacuum. The crude
product was purified by column chromatography on silica gel with
chloroform to chloroform/methanol (v/v 10/1) as eluent to give a
thiocyanato product (0.77 g, 1.7 mmol, 54%) Caution! Reaction
generates odorous dimethyl sulfide. All operations should be performed in
a well-ventilated hood. The effluent should be treated with bleach before
disposal.
CONCLUSIONS
■
The donor−acceptor−donor semiconductor compound,
namely 2-(5-mercaptothien-2-yl)-8-(thien-2-yl)-5-hexylthieno-
[3,4-c]pyrrole-4,6-dione (abbreviated as D-A-D-SH), was
designed, synthesized, and used as a capping ligand in
nonstoichiometric Ag−In−Zn−S nanocrystals. D-A-D-SH is
a weak luminophore emitting green light (λmax = 525 nm, QY =
2.5%). Despite limited solubility, it readily exchanges nano-
crystals initial ligands (stearic acid and 1-aminooctadecane).
Binding D-A-D-SH to the nanocrystals surface changes its
polarity improving the colloidal stability of the obtained
Ag1.0In3.1Zn1.0S4.2(S6.1)/D-A-D-SH hybrid. It also profoundly
changes the luminescent properties of the system. Efficient red
luminescence, characteristic of nanocrystals capped with initial
ligands (λmax = 720 nm, QY = 67%), is totally quenched, and
green luminescence originating from the ligands appears at 508
nm, exhibiting a 4-fold increase QY value (10%) as compared
to that of free D-A-D-SH. This luminescence can proceed by
two pathways: (i) by direct excitation of the ligand as in the
case of free D-A-D-SH or (ii) by charge and energy transfer
from the nanocrystal core to the new states introduced to the
system through surface binding of the optically active ligand.
2-(5-Mercaptothien-2-yl)-8-(thien-2-yl)-5-hexylthieno[3,4-
c]pyrrole-4,6-dione (D-A-D-SH). D-A-D-SH was obtained by using
a modification of procedures described in ref 58. To a solution of 2-
(5-thiocyanatothien-2-yl)-8-(thien-2-yl)-5-hexylthieno[3,4-c]pyrrole-
4,6-dione (0.53 g, 1.15 mmol) in acetone (75 mL) and water (0.3
mL) tributylphosphine (0.5 mL, 2.30 mmol) was added dropwise.
After stirring for 10 min, 3 mL of water and 1 mL of aqueous HCl
(10%) were added. The resulting reaction mixture was stirred at room
temperature overnight. After the solvents were removed, the residue
was dissolved in chloroform (100 mL), washed with water, dried over
anhydrous sodium sulfate, evaporated, and dried under vacuum.
Purification was performed by column chromatography using silica gel
and chloroform to chloroform/methanol (v/v, 10/1) as eluent
followed by recrystallization from butyl acetate gave 2-(5-
mercaptothien-2-yl)-8-(thien-2-yl)-5-hexylthieno[3,4-c]pyrrole-4,6-
EXPERIMENTAL SECTION
■
Materials. N-Bromosuccinimide (NBS, 99%), thiophene-2-bor-
onic acid pinacol ester (98%), Pd(PPh3)4 (99%), K2CO3 (99%),
NH4SCN (97%), bromodimethylsulfonium bromide (95%), tribu-
tylphosphine (97%), acetonitrile (99%), butyl acetate (99%), N,N-
dimethylformamide (99%), and benzene-d6 (100%, 99.6 atom % D)
were supplied by Sigma-Aldrich.
Preparation of 2,8-Dibromo-5-(n-hexyl)thieno[3,4-c]-
pyrrole-4,6-dione (6).54 5-(n-Hexyl)thieno[3,4-c]pyrrole-4,6-dione
(TPD) (0.66 g, 2.8 mmol) was dissolved in 4.0 mL of concentrated
sulfuric acid and 13.0 mL of trifluoroacetic acid. N-Bromosuccinimide
(NBS) (1.42 g, 8.0 mmol) was added in one portion, and the reaction
mixture was stirred for 12 h at room temperature. The brown solution
was then diluted with 200 mL of water and extracted with
dichloromethane. The organic phase was dried over anhydrous
magnesium sulfate and evaporated to afford the crude product as
orange crystals. Purification by column chromatography using silica
gel and toluene/dichloromethane/heptane (3:3:1) as eluent followed
by recrystallization from aqueous ethanol gave 2,8-dibromo-5-(n-
hexyl)thieno[3,4-c]pyrrole-4,6-dione (0.99 g, 2.5 mmol, 90%) as
white crystals.
1
dione (0.24 g, 0.55 mmol, 48%) as orange crystals. H NMR (400
MHz, C6D6): δ = 0.84 ppm (t, J = 6.9 Hz, 3H, CH3), 1.14−1.25 (m,
6H, 3 × CH2), 1.57−1.64 (m, 2H, CH2CH2N), 3.53 (t, J = 7.4 Hz,
2H, CH2N), 6.64 (dd, J = 5.0 Hz, J = 3.8 Hz, 1H, CH), 6.74 (dd, J =
5.0 Hz, J = 1.0 Hz, 1H, CH), 6.93 (d, J = 3.9 Hz, 1H, CH), 7.87 (d, J
= 3.9 Hz, 1H, CH), 8.11 (dd, J = 3.8 Hz, J = 1.0 Hz, 1H, CH).
Exchange of Initial Ligands for 2-(5-Mercaptothien-2-yl)-8-
(thien-2-yl)-5-hexylthieno[3,4-c]pyrrole-4,6-dione (D-A-D-SH).
A mixture consisting of colloidal solution of Ag−In−Zn−S nano-
crystals capped with initial ligands (∼200 mg in 10 mL of toluene)
and D-A-D-SH (100 mg, 0.23 mmol) was stirred at room temperature
for 12 h. The nanocrystals were precipitated with acetone,
centrifuged, and redispersed in toluene (or hexane, chloroform, or
dichloromethane). The use of a large excess of D-A-D-SH resulted in
binding of the largest number of the target ligands to the nanocrystals
surface and assured the reproducibility of the optical and electro-
chemical properties of the resulting hybrids.
2,8-Bis(thien-2-yl)-5-(n-hexyl)thieno[3,4-c]pyrrole-4,6-
dione (D-A-D). D-A-D was obtained by using a modification of
procedures described in ref 56. A mixture of 2,8-dibromo-5-(n-
hexyl)thieno[3,4-c]pyrrole-4,6-dione (1.48 g, 3.0 mmol), thiophene-
2-boronic acid pinacol ester (2.30 g, 6.2 mmol), Pd(PPh3)4 (0.35 g,
7.5 mmol), K2CO3 (2.76 g, 20 mmol), and DMF (45 mL) was stirred
at 110 °C for 2 h under an argon atmosphere. After cooling to room
temperature, the reaction mixture was poured into saturated sodium
chloride solution (60 mL) and then extracted with ethyl acetate (2 ×
60 mL). The combined organic extracts were washed with water and
dried over anhydrous sodium sulfate. After the solvent was removed,
the residue was purified by column chromatography on silica gel with
hexane/ethyl acetate (v/v 25/1 to 9/1) as eluent to give D-A-D as a
green solid (0.78 g, 1.9 mmol, 65%). 1H NMR (400 MHz, C6D6): δ =
0.83 ppm (t, J = 6.9 Hz, 3H, CH3), 1.14−1.25 (m, 6H, 3 × CH2),
1.57−1.65 (m, 2H, CH2CH2N), 3.55 (t, J = 7.4 Hz, 2H, CH2N), 6.65
(dd, J = 5.0 Hz, J = 3.8 Hz, 2H, 2 × CH), 6.73 (dd, J = 5.0 Hz, J = 1.0
Hz, 2H, 2 × CH), 8.13 (dd, J = 3.8 Hz, J = 1.0 Hz, 2H, 2 × CH). 13C
NMR (100 MHz, C6D6): δ = 14.2 ppm (CH3), 22.9 (CH2), 26.9
(CH2), 28.9 (CH2), 31.7 (CH2), 38.6 (CH2N), 127.8 (2 × CH),
128.2 (2 × CH), 129.5 (2 × C), 130.2 (2 × CH), 133.1 (2 × C),
136.0 (2 × C), 162.4 (CO).
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02468.
Preparation of alloyed Ag−In−Zn−S nanocrystals,
synthesis of thieno[3,4-c]pyrrole-4,6-dione (TPD),
characterization methods, XPS spectra and HR-TEM
image of Ag−In−Zn−S nanocrystals capped with initial
ligands, EDS spectra and TEM images of Ag−In−Zn−S
nanocrystals capped with initial ligands and D-A-D-SH,
1
1
1H,13C, and H−1H COSY and H−13C HMQC NMR
spectra recorded for the initial ligands, D-A-D, D-A-D-
SH, and Ag−In−Zn−S/D-A-D-SH hybrid, band gap
calculation, UV−vis−NIR and photoluminescence spec-
tra of D-A-D, photoluminescence excitation and decay
curves of D-A-D-SH and Ag−In−Zn−S nanocrystals
capped with initial ligands and D-A-D-SH (PDF)
H
https://dx.doi.org/10.1021/acs.inorgchem.0c02468
Inorg. Chem. XXXX, XXX, XXX−XXX

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pubs.acs.org/IC
Article
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AUTHOR INFORMATION
■
Corresponding Authors
Patrycja Kowalik − Faculty of Chemistry, Warsaw University of
Technology, 00-664 Warsaw, Poland; Faculty of Chemistry,
University of Warsaw, PL-02-093 Warsaw, Poland;
Email: pkowalik@ch.pw.edu.pl
Piotr Bujak − Faculty of Chemistry, Warsaw University of
Technology, 00-664 Warsaw, Poland; orcid.org/0000-
0003-2162-961X; Email: piotrbujakchem@poczta.onet.pl
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S.; Sargent, E. H. A Charge-Orbital Balance Picture of Doping in
Colloidal Quantum Dot Solids. ACS Nano 2012, 6, 8448−8455.
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Zbigniew Wrobel − Institute of Organic Chemistry, Polish
Academy of Sciences, 01-224 Warsaw, Poland
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Grossman, J. C.; Bulovic, V. Energy Level Modification in Lead
Mateusz Penkala − Institute of Chemistry, Faculty of Science
and Technology, University of Silesia, 40-007 Katowice, Poland
Kamil Kotwica − Faculty of Chemistry, Warsaw University of
Technology, 00-664 Warsaw, Poland; Institute of Physical
Chemistry, Polish Academy of Sciences, 01-224 Warsaw, Poland
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Nano 2014, 8, 5863−5872.
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Anna Maron − Institute of Chemistry, Faculty of Science and
Technology, University of Silesia, 40-007 Katowice, Poland
Adam Pron − Faculty of Chemistry, Warsaw University of
Technology, 00-664 Warsaw, Poland
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02468
Author Contributions
P.K. and P.B. designed the concepts; P.K., Z.W., M.P., K.K.,
and A.M. performed the experiments; P.K., M.P., and P.B.
discussed the results; P.K., P.B., and A.P. supervised the
manuscript.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the National Science Centre of
Poland, Grant 2015/17/B/ST4/03837. P.K. additionally
acknowledges financial support from the Project TRI-BIO-
CHEM which is implemented under the Operational Program
Knowledge Education Development 2014-2020 cofinanced by
the European Social Fund. M.P. and A.M. acknowledge partial
support of the University of Silesia.
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