Optical properties of thiophene-containing liquid crystalline and hybrid liquid crystalline materials

Article abstract of DOI:10.1039/c4nj00298a

We report new mesogens made of thiophene and stilbene moieties as main building blocks that exhibit nematic and/or smectic phases. The liquid crystalline properties of their thiophene/azobenzene analogues have been found to be strongly suppressed compared to their thiophene/stilbene analogues. In twinned "bent-core" thiophene/azobenzene compound relatively strong fluorescence was found. A new hybrid material built of AuNPs coated with a thiophene-containing mesogenic ligand that forms a smectic phase is shown. In solutions of thiophene-mesogen coated AuNPs an unusual decrease of the ligand fluorescence decay rate was observed. Fluorescence lifetime increase is ascribed to two-point anchoring of ligand molecules resulting in molecule orientation tangential to the AuNP surface. UV-Visible absorption and fluorescence data of all synthesised compounds are presented. the Partner Organisations 2014.

Full text of DOI:10.1039/c4nj00298a

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Optical properties of thiophene-containing liquid
crystalline and hybrid liquid crystalline materials†
Cite this: New J. Chem., 2014,
38, 2927
Jerzy Romiszewski,a Zita Puterova-Tokarova,*ab Jozef Mieczkowskia and
´
´
Ewa Goreckaa
We report new mesogens made of thiophene and stilbene moieties as main building blocks that exhibit
nematic and/or smectic phases. The liquid crystalline properties of their thiophene/azobenzene analogues
have been found to be strongly suppressed compared to their thiophene/stilbene analogues. In twinned
‘‘bent-core’’ thiophene/azobenzene compound relatively strong fluorescence was found. A new hybrid
material built of AuNPs coated with a thiophene-containing mesogenic ligand that forms a smectic phase
is shown. In solutions of thiophene-mesogen coated AuNPs an unusual decrease of the ligand
fluorescence decay rate was observed. Fluorescence lifetime increase is ascribed to two-point anchoring
of ligand molecules resulting in molecule orientation tangential to the AuNP surface. UV-Visible absorption
and fluorescence data of all synthesised compounds are presented.
Received (in Montpellier, France)
28th February 2014,
Accepted 26th March 2014
DOI: 10.1039/c4nj00298a
www.rsc.org/njc
devices10 and for image storing applications.11 Stilbenes also
belong to systems that show reversible photoinduced cis–trans
Introduction
Conjugated materials have received much attention as they offer isomerization,12 exhibit strong blue fluorescence13 and as pro-
new possibilities for devices combining desired optical, electrical mesogenic entities in host thiophene-containing structures can
and mechanical properties. Thiophenes and their functional deri- introduce mesomorphic properties.14 The optical anisotropy of
vatives are among the variety of heterocyclic compounds used as such LC systems allows the generation of polarized light in
key structural units in optoelectronic devices1 such as field-effect OLEDs and polarized fluorescence which are of interest for
transistors (OFETs),2,3 organic-light emitting diodes (OLEDs),4,5 security labels.14–18 Advantage of both, azobenzene and stilbene
photovoltaic cells6 and chemosensors.1,7 In fact, performance of elements, is their acceptable synthetic accessibility requiring
such functional material is strongly dependent on its molecular only straightforward purification steps.19,20 Their insertion into
structure and organization. Major efforts are presently being made thiophene-based materials can lead to formation of a variety of
in utilizing the liquid-crystalline (LC) state to organize p-conjugated mesomorphic structures with possible application for new types of
semiconducting organic molecules in such a way that high charge optoelectronic devices. These discoveries have prompted our con-
carrier mobilities can be achieved. As attractive as thiophene- tinuous research on development of novel mesogenic compounds
containing materials are for their physical properties their synthesis with the 2-aminothiophene core as a main structural feature
is often challenging.7 Alternating p-conjugated thiophene units containing a stilbene or an azobenzene moiety. Along with inves-
with azobenzene and stilbene moieties can be used for changing tigations on mesogenic properties of target thiophene/stilbene and
and enhancing optical and/or conducting properties of compounds thiophene/azobenzene compounds the recent introduction of an
as well as for introducing light-responsive properties. Azobenzene aminothiophene core helped us further to develop a mesogenic
groups undergo a rapid cis–trans reversible isomerization by photo- thiol-ended ligand for functionalization of gold nanoparticles
irradiation.8,9 Therefore, azobenzenes are typically used as photo- (AuNPs). Recently much interest is being placed in the use of
chromic switching elements for example in holographic recording liquid crystals (LCs) for inducing controlled and eventually stimuli-
responsive self-assembly of AuNPs21–23 as well as for combining
desired properties exhibited by both substrates that could result in
new unique materials.24–26 Functionalized AuNPs are key materials
in nanochemistry and nanomaterials science with numerous
applications ranging from opto-electronics,27 catalysis28 to energy
conversion devices.29 Particle size and interparticle distance
dependencies of optical properties30–32 in resulting AuNP based
materials make them very promising building blocks that never-
theless require an appropriate method for controlled assembling.
a Department of Chemistry, Warsaw University, Pasteura 1, 02-093 Warsaw, Poland.
E-mail: z.puterova@chem.uw.edu.pl; Tel: +4822 8220211
b
´
Department of Chemistry, University of SS Cyril and Methodius, Nam. J. Herdu 2,
917-01 Trnava, Slovakia. E-mail: zita.tokarova@ucm.sk; Fax: +4213 35921403;
Tel: +4213 35921408
† Electronic supplementary information (ESI) available: Synthesis, experimental
details of methods used for synthesis, characterization of all obtained com-
pounds, figures of UV-Vis absorption and fluorescence spectra of all compounds.
See DOI: 10.1039/c4nj00298a
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We describe, herein, the synthesis, liquid-crystalline, optical on a Bruker GADDS system equipped with a VANTEC 2000 area
behaviour and supramolecular organization in a novel type of detector and a hot stage controlled with a Linkam controller and on
hybrid AuNP grafted with amidothiophene-based soft layers a BrukerNanostar equipped with a Vantec 2000 detector. CuKa
(Fig. 1). The mesogenic and optical properties in thiophene/ radiation was used. Phase transition temperatures, enthalpies and
stilbene (Fig. 2) and thiophene/azobenzene compounds (Fig. 3) are melting points were determined by differential scanning calorimetry
investigated and compared, if possible. Preparation of a substi- measurements on a TA Instruments DSC Q 200 calorimeter. In
tuted 2-aminothiophene core as a main element in the structure of case that DSC measurements were inconclusive (no clear transition
all designed compounds was carried out by the Gewald reaction.33 signal) or impossible (sample degradation accompanying heating)
The availability of reagents and the mild reaction conditions all transition temperatures were determined by POM observation.
contribute to the versatility of this reaction, where by choosing Absorption spectra were recorded using a Shimadzu UV-3100 PC
suitable substrates the structure of the final thiophene-containing spectrometer for solutions of appropriate compounds in dichloro-
material is precisely predicted.14,33
methane (c B 5 ꢀ 10ꢁ5 mol dmꢁ3). Fluorescence measurements
were performed for dilute solutions in dichloromethane using a
FluoroLog HORIBA JobinYvon spectrometer equipped with a TBX-04
PMT detector. Fluorescence lifetimes were calculated from fluores-
cence decay curves obtained by the time-correlated single-photon
counting method using the same FluoroLog HORIBA JobinYvon
instrument with a NanoLED excitation source.
Experimental
Synthesis of all presented compounds is described in the ESI†
(Schemes S1 and S2). Gold NPs were synthesized according to the
Brust method, using n-decanethiol to passivate the metal surface.
The diameter of the metal particle core was determined from the
small angle X-ray scattering of NPs in toluene solution. In the next
Synthesis
step, part of the n-alkylthiol molecules was exchanged for the meso- In our study we have synthesized three series of compounds:
genic molecules of compound L2 (Fig. 1) by the Murray reaction. The series A – stilbene containing thiophenes M1–M5 (Fig. 2); series B –
efficiency of the ligand exchange reaction was unambiguously azobenzene containing thiophenes M6–M9 (Fig. 3) and series C –
confirmed by 1H NMR spectra: broadening of signals coming from thiol-ended ligand L2, which was used for the preparation of gold
ligands covalently attached to the metal surface was observed. No nanoparticles Au@L2, and its bromo-substituted precursor L1
traces of free ligands were found. Identification of the liquid crystal- (Fig. 1). The synthesis of M1–M5 required preparation of a stilbene
line phases was based on the type of texture exhibited by the com- fragment20 terminated by the long alkyloxy tail (C12, C8) and for
pound. The textures were observed using a polarizing optical micro- compounds M6–M9 substituted azobenzene with an incorporated
scope (POM) Zeiss Axio Imager A2m equipped with a Linkam hot octyloxy chain was prepared in advance.19 The 2-aminothiophene-
stage. The identification was confirmed by X-ray studies carried out 3-carbonitrile as a main part of the central core in compounds M1,
Fig. 1 Molecular structures of the ligand used as secondary grafting layer L2 and its brominated precursor L1. Schematic structure of grafted
nanoparticle Au@L2.
Fig. 2 Chemical structures of studied stilbene-containing thiophenes.
Fig. 3 Chemical structures of studied azobenzene-containing thiophenes.
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M2 and M6 was synthesized utilizing the fourth version of the For the detailed synthesis and characterization of all com-
Gewald reaction.34 Under standard reaction conditions using pounds see ESI.†
N-bromosuccinimide the corresponding 5-bromo-substituted
derivatives M3 and M7 were formed.35 New stilbene-
Mesomorphic properties and X-ray structural studies
containing bithiophene M4 and its azobenzene homologue All of the obtained stilbene-containing compounds form liquid-
M8 were synthesised employing palladium-catalysed coupling crystalline phases. Mesomorphic behaviour of compounds
reactions.36 2,5-Diamino-thiophene-3,4-dicarbonitrile as a core M1–M4 is typical for rod-like molecules, exhibiting nematic
unit in M5 and M9 was obtainable from tetracyanoethylene and smectic A phases with relatively high (above 150 1C) melt-
according to Middleton.37 The central unit in L1 and L2, the ing and clearing temperatures. Compound M5 despite its bent-
biphenyl-substituted-2-aminothiophene, was formed via
a
core shape forms a non-polar smectic C phase at elevated
modified Gewald reaction sequence published by us recently.14 temperatures (above 300 1C). Because of high temperatures
Generally, the Gewald reaction provides broad synthetic poten- and moderate stability of M5 compound it was not possible to
tial in material chemistry in synthesis of thiophene-containing confirm liquid-crystalline behaviour by XRD or DSC and the
metamaterials14 and optoelectronics.33 The methylester group phase determination relied on POM observations. Among azo-
in a position next to the amino group stabilizes the 2-amino- compounds only M7 compound with bromine in the C-5 posi-
thiophene core and enables subsequent reaction steps coupled tion of the thiophene ring showed mesomorphic properties;
with amino group substitution. It has to be noticed that it exhibits a monotropic nematic phase. For the L1 precursor
substituted 2-aminothiophenes are stable only if the electron- no LC phase was observed, while for thiol L2 a monotropic
withdrawing group is present in the position C-3 adjacent to the smectic A phase was found. Mesomorphic properties; transition
amino group in C-2 of the five-membered ring.33,34 Exchanging enthalpies and temperatures for all studied materials are sum-
bromine in L1 with the thiol group in L2 was conducted marized in Table 1.
according to the pioneering work38 with hexamethyldisilathiane
(HMDT) as a thiolating agent.
Although all stilbene-containing compounds show meso-
morphic properties, only M3 and M4 exhibit a relatively broad
Passivated gold nanoclusters were obtained by the Brust– temperature range (ca. 20 1C) of the LC phase. The stability
Schiffrin method39 using n-decanethiol. The particles with range of LC phases increases with elongation of the alkyl chain
simple n-alkylthiol (B55 thiol molecules per metal particle) (from M1 to M4) and also upon introduction of halogen atoms
did not form any long-range structure.40 The radius of the (M2, M3) or the additional thiophene ring (M4). Unfortunately
gold core, B1.0 ꢂ 0.2 nm, was determined by small-angle M4 undergoes partial decomposition at clearing temperature.
X-ray scattering (SAXS) experiments for particles dissolved in We assume that due to the exchanging of the CQC bond by the
toluene.38,40 In a ligand-exchange Murray reaction41 n-decanethiol NQN bond the liquid crystalline properties of azo-compounds
molecules were then substituted by mesogenic thiophene- M6, M8, M9 are hindered.43 The lateral chloro-substitution on
containing ligand L2 with the thiol group pending at the long the benzene ring acts as a supplier of p-electrons lowering the
C-10 alkyl chain (Fig. 1). The rate of exchange depends strongly dipole moment of the molecule, causing the decrease of the
on the Murray reaction conditions,42 and thus the temperature melting point of rod-like and banana-shaped stilbene-containing
and concentration of the incoming ligand were chosen to mesogens;44–47 whereas the dipole moment of trans azobenzenes
ensure that B50% of the exchange ligand ratio was obtained. is equal to zero regardless of the substitution.48–50 Compound
This result indicates that between 25 and 40 mesogenic groups M7, with a bromo-substituent on the thiophene ring, is the only
were attached to a single gold particle. It was found previously azo-derivative among studied that shows mesomorphic proper-
that the type of nanoparticle organization is not affected by the ties; it forms a monotropic nematic phase 10 1C beneath the
number of mesogenic pendants within the limits given above.38 melting point.
Table 1 Mesogenic properties, phase-transition temperatures (in 1C) and enthalpies (in parentheses, kJ molꢁ1), and layer thicknesses in the smectic A
phase (in Å)
Comp.
T/1C [DH/kJ molꢁ1
]
Layer thickness in SmA phase [Å]
M1
M2
M3
M4
M5
M6
M7
M8
M9
L1
Cr 181.6 [30.58] Iso 180 [*]a SmA 178.78 [31.4] Cr
Cr 158.6 [20.4] N 164.4 [0.1] Iso 163.7 [0.7] N 150.62 [22.82] Cr
35.8
—
—
33.7
—
—
—
—
—
—
Cr 158.2 [29.1] N 167.8 [0.55] Iso 170.4 [1.11] N [29.6] Cr
Cr 61.7 [19.9] SmA 188.0 [0.47] N 242.9 [0.15] Iso 235.2 [3.49] Nb
Cr 310 [*] SmCc
No LC
Cr 211.6 [34.8] Iso 201.9 [0.2] N 198.0 [27.0] Cr
No LC
No LC
No LC
L2
Cr 75.7 [40.8] Iso 53.2 [0.5] SmA 31.9 [30.0] Cr
35.1
a
b
Detected only by optical and X-ray measurements, [*] no data available. Probably due to partial decomposition on heating only iso-nematic
c
transition is clear in the cooling cycle. Phase transition is accompanied by decomposition, transition temperature based on POM observation, [*]
no data available.
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The Au@L2 system was designed with the aim of producing
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The gold nanoparticles coated with the L2 ligand show a
a gold nanoparticle-based LC with a relatively simple coating lamellar phase (Fig. 5). Long-range positional order for the
layer that could be compatible for doping with M-series com- structure formed by hybrid Au@L2 was confirmed by X-ray
pounds or could be used as a dopant for these compounds.
studies; two sharp, commensurate XRD signals in a small angle
Ligand L2 forms a monotropic SmA phase with a layer region indicate the presence of a lamellar structure with the
thickness of B35.1 Å which is close to the length of a fully layer thickness d = 52.2 Å (Fig. 4) that is only weakly dependent
stretched molecule (Fig. 4). Due to the presence of a –SH on temperature. The measured layer periodicity for the Au@L2
terminal group (Fig. 1) it can also be introduced as a secondary system is much larger than observed for ligand L2 in the
ligand, by partial exchanging of a primary alkyl ligand at the smectic phase. The mean distance between the gold particles
metal surface of AuNPs.
inside the layers, calculated from the position of the diffuse
signal, is 37.3 Å. Taking into account the diameter of metal
particles (B10 Å), molecular lengths of n-decanethiol and L2
molecules, one can propose the model of the structure in which
gold particles are arranged in the sublayers separated by
organic sublayers. The organic sublayers are made of L2
molecules that are collected below and above metal cores.
Ligands attached to the particles from neighbouring metal
sublayers intertwine with one another, whereas n-alkyl ligands
are evenly distributed around gold centres and determine the
distance between gold particles in the sublayer. Lack of measur-
able birefringence of the structure shows that there is no
orientational order of L2 ligands in the organic sublayer. Heating
of the Au@L2 above 160 1C causes decomposition; such decom-
position prevents observation of isotropic melt.
Fig. 4 XRD pattern of the L2 ligand in the SmA phase just below clearing
temperature, and the scheme showing arrangement of molecules in the
SmA phase and the molecular structure of L2.
Because melting temperatures of M-series compounds are
close to decomposition temperature of Au@L2 material compo-
sites were prepared by dissolving both Au@L2 and M4 com-
pounds in appropriate solvent and slow evaporation of the
solvent. The XRD measurements prove that only a limited amount
of M4 compound can be incorporated into the Au@L2 matrix and
the rest of the M-compound precipitates in the crystalline state.
The X-ray pattern of the mixture shows signals coming from the
layer structure formed by NPs and a few sharp reflections (the
signal at 3.4 deg overlaps with the peak of second harmonic of
NPs smectic phase) coming from the crystal structure of M4
(Fig. 6). The layer thickness of the composite is unchanged
compared to pure Au@L2 material but the in-plane distance
between NPs slightly increased, which is probably due to the
introduction of some of the dopant molecules and/or secondary
grafting molecules (L2) between the AuNPs within the layer. The
composite melts at lower temperature (B130 1C), however, even
in the melt the excess of the M4 phase stays separated. We believe
that slight modifications of both ligand (L2) and dopant mole-
cules should improve stability of composites, paving the way for
light responsible AuNP LCs.
Fig. 5 Integrated X-ray pattern obtained for Au@L2 at 140 1C together
with schematic drawing showing arrangement of the NPs. The intense
signal results from the layered structure with thickness d = 52.2 Å; weaker,
diffuse signal (d = 37.3 Å) results from liquid-like in-plane order of gold
particles inside the layer.
Optical properties: UV-Visible absorption and fluorescence
For all studied compounds, ligands as well as for Au@L2 gold
particles, UV-Visible absorption spectra, fluorescence spectra
and fluorescence lifetime were measured. Results are sum-
marized in Table 2.
Stilbene derivatives (Fig. 7) M1–M5 exhibit a single optical
absorption band at around 350 nm and a fluorescence band
at around 445 nm. The twinning of molecular structure
(compound M5) does not change much the optical absorp-
tion spectra but introduces a considerable redshift in the
Fig. 6 Integrated XRD patterns of Au@L2 and the composite of Au@L2
with M4 compound. In the inset is the magnified region at higher angles,
showing the signals from crystal structure of M4.
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Table 2 UV-Visible absorption and fluorescence data for all prepared
compounds
UV-Visible
lmax
Fluorescence
e
Eogpt lmax
t
Relative
amplitude [%]
Comp. [nm] [dm3 cmꢁ1 molꢁ1
]
[eV] [nm] [ns]
M1
M2
M3
M4
M5
350.5
349.5
347.0
359.0
355.0
25 200
40 400
40 500
23 580
26 200
3.11 451
3.14 438
3.10 438
2.85 443
2.92 516
0.16
97.4
2.1
93.2
6.5
98.7
1.3
91.2
8.4
67.6
27.6
—
—
—
84.2
12.0
92.3
6.7
95.1
3.4
48.6
43.6
7.8
0.50
0.20
0.13
0.40
10.8
0.49
0.28
1.4
0.17
—
—
—
1.60
0.72
0.17
1.70
0.16
1.50
0.17
1.50
8.70
Fig. 7 UV-Visible absorption (solid line) and fluorescence (dashed line)
spectra of M1–M5 compounds.
M6
M7
M8
M9
369.5
371.0
374.5
365.5
27 900
27 200
5940
2.34
2.34
2.27
—
—
—
17 190
2.34 510
2.79 392
2.82 392
L1
L2
267.5
268.0
21 100
29 500
—
Au@L2 267.5
—
371
Measurements taken in dichloromethane at room temperature, typical
concentration c = 5 ꢀ 10ꢁ5 mol dmꢁ3
.
fluorescence spectrum, emission is observed at 516 nm.
Azo-compounds M6–M9 are characterised by the main absorp-
tion band at 370 nm (p–p* transition in trans form) with a
small tail without very clearly differentiated peaks at longer
wavelength. From the data fit one can deduce the position of
the second absorption band at around 465 nm. Twinning of
the structure (compound M9) leads only to a very slight blue-
shift of absorption to 366 nm (Fig. 8). Stilbene compounds
M1–M4 are characterized by fluorescence in the blue region
with fast, single exponential decay (with lifetime below 500 ps)
whereas M5 exhibits more complex, multiexponential decay,
its major component having a lifetime of B1.5 ns. For azo
materials M6–M8, fluorescence is strongly quenched. Con-
trary to this typical behaviour, for the material M9 with two
azo-units, relatively strong fluorescence at 510 nm was
observed with a decay time of 1.6 ns. The UV-Visible absorp-
tion measurements performed for M6–M8 (Fig. 9) during,
or shortly after irradiation with UV light (350–380 nm) showed
typical spectrum changes related to photo-induced cis-
isomerisation, i.e. a sharp decrease of intensity of the absorp-
tion band due to p–p* transition in trans form at 370 nm,
occurrence of a new absorption band with maximum at
322 nm due to p–p* transition in cis-form and increased
intensity of the absorption band due to n–p* transition in
cis-form at 442 nm. For bis-azo compound M9, even after
longer UV irradiation absorption at 370 nm remains but also
increases in absorption at ca. 460 and 300 nm, as typical for
trans–cis transition, are observed. Still irradiation of M9 with
visible light reproduces original shape of the absorption
spectrum (Fig. 10).
Fig. 8 UV-Visible absorption (solid line) and fluorescence (dashed line)
spectra of M7.
Fig. 9 UV-Visible absorption spectra of M6 before and after UV irradiation.
Fig. 10 UV-Visible absorption spectra of M9 before and after UV irradiation.
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The absorption band of L1 and L2 compounds due to atoms (2 versus 1 in other compounds in this series – M1–M4)
coupling of biphenyl and thiophene moieties is red shifted and therefore larger dipole moment for M3 as well as increased
with respect to components and exhibits maximum absorption length of the aromatic core (by introduction of the second thienyl
at 268 nm and fluorescence at 392 nm with very short decay unit) in M4. The same extension of the aromatic core leads to very
time of ca. 170 ps. Also ligand L2 attached to gold particles high melting temperature in M5. Although both azobenzene
shows fluorescence with a slight blue shift (371 nm). This is and stilbene units are often used as promesogenic entities54,55
rather unusual observation since in most cases proximity of in M6–M9 – that are close structural azo-analogues of compounds
the metal surface strongly affects fluorescence of the attached M1–M5, only M7 displays the mesogenic phase. This behaviour is
ligand,51,52 resulting in fluorescence quenching and shortening in line with a general tendency for lowering of nematic–isotropic
of fluorescence lifetime. Contrary to this general behaviour, for phase transition temperature upon exchanging CQC by the
ligand L2 attached to the gold nanoparticles (Au@L2), fluores- NQN unit in R1–Ph–XQX–Ph–R2 compounds (X = C, N).43
cence decay time becomes significantly longer than for the free Generally, the influence of halo-substitution on mesophase stabi-
ligand (Fig. 11), due to appearance of a new decay term with a lity is ambiguous and probably introduction of halogen would
lifetime of 1.5 ns.38 Fluorescence lifetime increase in the proxi- have only minor impact on the formation of mesophases in M6,
mity of a metal surface was theoretically predicted already over M8 and M9.43,56–58 Still the introduction of bromine into the
10 years ago51,52 but to the best of our knowledge such an effect a-position of the thiophene ring in M7 leads to the formation of a
was reported only once so far and for a fluorophore structurally monotropic nematic phase at relatively high temperature (200 1C).
constrained to align parallel to the AuNP surface.53
Therefore the influence of a lateral halogen substituent on
azobenzenes cannot be completely neglected and could become
a topic of further scrutiny.
Stilbene derivatives exhibit, as typical for this group of
materials, optical properties with a single strong absorption band
at around 350 nm and a fluorescence band at around 440 nm. For
M5 compound the fluorescence band shifts to 516 nm and
fluorescence decay time increases, due to the extending of the
mesogenic core by additional aromatic rings. Single azo-group
derivatives M6–M8 are not fluorescent, such behaviour is typical
for azo compounds.43 These materials show also a typical absor-
bance spectrum consisting of a high intensity absorption band at
around 370 nm due to p–p* transition in trans-form and a small
intensity tail at longer wavelengths. The absorption tail is mostly
due to the weak n–p* transition for trans-form. Quite unusual is
relatively strong fluorescence observed for azo-compound M9.
Changes in the absorption spectrum upon UV irradiation
indicate that only part of azo groups can undergo cis–trans
isomerisation whereas the rest remains in trans form.
Fig. 11 Fluorescence decay for free ligand L2 and nanoparticle Au@L2.
Bumps on the L2 fluorescence decay signal are experimental artefacts due
to characteristic time of the excitation signal.
New liquid crystalline hybrid material was obtained by
exchanging part of the n-decanethiol primary grafting layer
Results and discussion
The 2-aminothiophene ring as a main structural element was at gold nanoparticles by mesogenic thiophene-biphenyl com-
introduced into two series of rod-like and bent-core compounds pound (L2). To the best of our knowledge, aminothiophene
differing in the structure of the side fragment – stilbene M1–M5 chemistry was not applied for the preparation of such nano-
vs. azobenzene M6–M9. Their mesogenic and optical properties materials so far. Advantage of this thiophene-based ligand besides
were investigated and compared, if possible. Ordered structures acting as secondary grafting for gold nanoparticles is its ability to
are promoted by planarity and aromatic character of the stilbene form smectic A mesophase at relatively low transition temperature
unit in the mesogenic core. Previously we have reported investi- (52.2 1C, Table 1). The gold nanoparticle with a thick deformable
gations on stilbene-containing thiophenes possessing mesogenic organic layer formed lamellar structure built of metal particle rich
properties with high transition temperatures.14 In order to lower sublayers divided by organic sublayers however with no intrinsic
the clearing temperatures, herein we have prepared a stilbene orientational ordering of ligands. The unusual elongation of
fragment with a chloro-substituent adjacent to the octyloxy chain fluorescence decay time of Au@L2 solutions in comparison to
on the benzene ring (M2–M5). Incorporation of the lateral halo- solutions of the free ligand is rather surprising as for the radial
gen substituent into the benzene ring of rod-like and banana- distributed ligand a contrary effect is expected.51,52 On the other
shaped mesogens containing stilbenes is a common procedure to hand for ligands aligned parallel to the metal surface, as
decrease their melting points.44–47 Nevertheless high stability of predicted theoretically, fluorescence decay time becomes longer
the crystalline phase in M1–M5 results in a relatively narrow than for ligands aligned perpendicular to the gold surface.51,52
range of LC phases. Higher stability of LC phases formed by M3 Such an effect was observed for the dithiol ligand (4-acetamido-
and M4 may be rationalised by a higher number of halogen 40-maleimidylstilbene-2,20-dithiol) whose parallel alignment to
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Paper
the AuNP surface was enforced by double point anchoring to periodicity only weakly dependent on temperature. The ligand
the surface.53 Although the L2 ligand contains one thiol group itself was found to form a SmA phase at relatively low-
in its structure two point anchoring to the AuNP surface is also temperatures. The absorption and fluorescence of the free
possible. It is known that thiophene derivatives exhibit the ligand are red shifted with respect to the ligand attached to the
ability to bind to the gold surface via a sulfur atom, forming gold surface with slight blue shift. Unique elongation of fluores-
self-assembled monolayers on planar surfaces and partially cence decay time of AuNPs compared to the free ligand was
coating Au nanoparticles59 thus one can expect that at least observed for the first time, when thiophene grafting is used.
part of L2 molecules is bound to the AuNP surface both via a
thiol group and a sulfur atom of the thiophene group. Au@L2
fluorescence decay can be fairly well described by a two
Acknowledgements
exponential model with almost equal share of fast (t = 0.17 ns)
Presented work was supported by FNP Project TEAM/2010-5/4:
and slow (t = 1.5 ns) decay. As the time constant of the fast decay is
‘‘Self-assembly of functionalized inorganic–organic liquid crystal-
almost the same as that measured for free ligand molecules, one
line hybrids for multifunctional nanomaterials.’’ Support of the
can propose a model in which there are two types of L2 particles
Foundation for Polish Science under MPD programme cofinanced
on the AuNP surface: (1) only thiol bound, on average radially
by the EU European Regional Development Fund is greatly
oriented and exhibiting a fluorescence decay rate close to that of
acknowledged. The support of the Ministry of Education of the
free L2; (2) bound by thiol and thiophene and thus oriented
Slovak Republic – VEGA 1/0073/13 is also acknowledged.
inclined towards the AuNP surface with a slower fluorescence
decay. Such an arrangement in which there is no clearly prefer-
enced orientation of ligand particles would also imply none or very
low birefringence. This is in agreement with our observation in a
References
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bound via thiol as well as a thiophene group resulting in rather
tangential than planar average orientation of ligand molecules.
Further research to validate this assumption is underway.
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