Key words
thiophene supramolecular assemblies - synthesis - non-covalent interactions - functional
self-assembly - nanoparticles
1. Introduction
Despite the numerous investigations carried out over the last three decades, thiophene-based
compounds continue to arouse wide interest for their easy synthesis and multiple tunable
physicochemical and biological properties. Applications spreading from medicinal and
pharmaceutical chemistry to materials chemistry, organic (bio)electronics and sensing
have been reported.[1] Several papers highlight the importance of elucidating the interplay between molecular
structure, supramolecular interactions and crucial properties such as charge transport
and light emission in order to optimize the possible applications. A remarkable characteristic
of the thiophene ring – one of the most commonly found building blocks in semiconducting
conjugated materials – is the possibility to be functionalized in different positions
with different groups. This is one of the main factors at the origin of the great
chemical diversity of thiophene derivatives. [Scheme 1] illustrates the various positions where substituents can be grafted. The most studied
functionalization type is that with substituents at positions 2–5, in particular with
alkyl chains. However, owing to its diffuse and very polarizable electrons, the sulfur
atom may be hypervalent, i.e., it can accommodate more electrons than the eight ones
associated with filled s and p shells and form a variety of groups with coordination
number four and six. In materials chemistry only sulfur functionalization with oxygen
has been explored so far (at position 1), giving rise to S,S-dioxides and S-oxides
as illustrated in [Scheme 1].
Scheme 1 Molecular structure and numbering scheme of thiophene, and functionalization positions.
Whatever their nature and ring position is, the substituents determine the type of
intra- and intermolecular interactions hence the supramolecular structure generated
under given experimental conditions, the optoelectronic properties and the type of
application in devices. A peculiarity of the thiophene ring is its ‘plasticity’, a
concept defined in 1993 and derived from the observation that in oligothiophene (OT)
single crystals the thiophene ring always displays an irregular pentagonal geometry.[2] The plasticity of thiophene was defined as the capability of the ring to be deformed
in order to relieve steric strain by means of small and progressive angle and bond
changes while maintaining the planar geometry. Once again, this property was ascribed
to the easily distorted clouds of electrons of the sulfur atom, which are at the origin
of the great tendency of thiophene-based molecules to form aggregates through numerous
different inter- and intramolecular non-bonding interactions with neighboring molecules.
Because of the intrinsic plasticity of thiophene coupled to the low energy barriers
to carbon–carbon inter-ring rotations,[3] thiophene-based supramolecular architectures are difficult to predict only on the
basis of the structure of the composing molecules. The most diverse ways of aggregation
are possible, largely depending on the environment and the experimental conditions.
[Figure 1] shows some significant examples of how thiophene molecules can self-assemble/self-organize
or even co-assemble with neighbouring molecules.
Figure 1 A) Optical micrographs under polarized (right) and unpolarized (left) light of a
melted drop of compound T6. Reprinted with permission from Ref. [5] (SI). Copyright 2004 American Chemical Society. B) AFM image of α-quinquethiophene
melt-quenched on glass. Reprinted with permission from Ref. [6]. Copyright 2003 American Chemical Society. C) AFM image of a microfiber formed inside
live neuroblastoma cells by spontaneous co-assembly of the protein vimentin with the green fluorescent thiophene-based dye DTTO.
Adapted with permission from Ref. [8]. Copyright 2015 Royal Society of Chemistry.
[Figure 1A] shows the optical micrographs under polarized and unpolarized light of a melted
drop of α-sexithiophene, one of the most investigated thiophene oligomers for its
supramolecular structures in relation to charge transport in field-effect transistors
(T6, molecular structure in [Scheme 2]).[4] Under polarized light, Maltese crosses, typical of liquid-crystalline nematic phases,
are visible[5] (see Supporting Information of ref. [16]). [Figure 1B] shows the atomic force microscopy (AFM) image of thin films prepared by vacuum evaporation
of powders of α-quinquethiophene (T5, molecular structure in [Scheme 2]) displaying uniaxially aligned stripes and self-affine (fractal) morphology, i.e., the same type of hierarchical self-organization over
several orders of magnitude in scale.[6] Note that many natural and metallic surfaces display self-affinity over several
scale orders.[7]
[Figure 1C] displays the AFM image of a microfiber formed inside live neuroblastoma cells[8] by spontaneous co-assembly of the protein vimentin with the green fluorescent thiophene-based dye DTTO
(3,5-dimethyl-2,3′-bis(phenyl)dithieno[3,2-b;2′,3′-d]thiophene-4,4-dioxide; molecular structure in [Scheme 3]
[9]). Being green fluorescent, DTTO confers green fluorescence to the microfiber, which
consequently can be isolated from the cell and analyzed. Chernyatina et al. reported
the calculated structure of the vimentin dimer[10] which displays an astonishing similarity to the AFM image of the very peculiar shape
of the microfiber. This suggests that there is a fractal growth of the co-assembled supramolecular structure of the microfiber over a great number of orders
of magnitude in scale. [Scheme 2] shows the pattern for the synthesis of T5 and T6,[5] generating the supramolecular structures in panels A and B of [Figure 1], while [Scheme 3] illustrates the synthesis of DTTO[9] used in the co-assembly of vimentin–DTTO bioconjugate shown in [Figure 1C].
Scheme 2 Microwave-assisted synthesis of α-quinquethiophene (T5) and α-sexithiophene (T6) from commercial bithiophene 1. Adapted with permission from Ref. [5]. Copyright 2004 American Chemical Society. Reagent and conditions: (i) NIS, DMF, overnight. −20 °C; (ii) PdCl2dppf, basic alumina/KF, 2-thienylboronic acid, MW 5 min, max temp 80 °C; (iii) NIS,
CH2Cl2/AcOH, overnight, rt; (iv) PdCl2dppf, toluene/methanol, 2-thienylboronic acid, MW 10 min, max temp 70 °C; (v) NBS,
DMF; (vi) PdCl2dppf, toluene/methanol, Bis(pinacolato)diboron, MW 10 min, max temp 70 °C.
Scheme 3 Synthesis of 3,5-dimethyl-2,3′-bis(phenyl)dithieno[3,2-b;2′,3′-d]thiophene-4,4-dioxide
(DTTO). Adapted with permission from Ref. [9a]. Copyright 2011 American Chemical Society. Reagents and conditions: i) Bis(tri-n-butyltin) sulfide, Pd(PPh3)4., toluene, 130 °C, 90%; ii) n-BuLi, CuCl2, ethyl ether, 0 °C, 50%; iii) 3-chloro-perbenzoic acid, CH2Cl2, 70%; iv) 2 mmol of NBS, CH3COOH/CH2Cl2, US, 99%; v) tributyl(phenyl)stannane 5%,
Pd(PPh3)4, toluene, MW, 80 °C, 95%.
It is worth noting that the preparation of materials with reproducible optoelectronic
features requires absence of any trace of byproducts or contaminants. Such a purity
degree requires generally several purification steps or alternative eco-friendly synthetic
patterns, as in the case of T5 and T6, which have also been prepared in the absence of solvent[11] or in aqueous media using silica and chitosan-supported Pd catalysts.[12]
2. Supramolecular Organization may Impart New Functions to the System
2. Supramolecular Organization may Impart New Functions to the System
When a supramolecular structure is formed under given experimental conditions, it
may acquire new properties different from those pertaining to the building molecules
and enable new applications. A few examples illustrating this point are shown below.
2.1 White Organic Light Emitting Diode from a Single Compound
The molecular structure of the compound in question, namely 3,5-dimethyl-2,6-bis(dimesitylboryl)-dithieno[3,2-b:2′,3′-d]thiophene, is reported in [Figure 2A].
Figure 2 A) Molecular structure and UV-vis and PL of 3,5-dimethyl-2,6-bis(dimesitylboryl)-dithieno[3,2-b:2′,3′-d]thiophene 1 in solution (a) and in the solid state (b). B) (a) Molecular structure of two interacting
molecules forming a cross-like dimer. (b) Intermediated neglect of differential overlap/single
configuration interaction (INDO/SCI) excitation energy shifts due to intermolecular
interactions. The scale on the right corresponds to calculated excitation energy shifts.
C) (a) CIE coordinates of EL emission. (b) Luminance and current density versus the
applied voltage of the ITO/PEDOT:PSS/1/LiF/Al device. Inset: photo of a working OLED. Adapted with permission from Ref.
[13]. Copyright 2005 Wiley-VCH.
The borylated derivative was prepared by adding a solution of dimesitylboron fluoride
in THF to a solution of 3,5-dimethyl-dithieno[3,2-b:2′,3′-d]thiophene/BuLi in the same solvent.[13] It is processable in common organic solvents and emits blue light in solution and
white light in the solid state. By the aid of optical measurements, theoretical calculations
and the comparison with structurally similar systems, we demonstrated that the white
emission in the solid state is the result of the addition of the intrinsic blue-green
emission of the isolated molecule with the red emission of cross-like dimers formed
by self-assembly of 1 in the solid state, as shown in [Figure 2B]. The key element for the dimer formation is the boryl group, in which boron has
a vacant p-orbital and then can act as an electron acceptor through p–π* conjugation
with adjacent electron-donor groups, particularly effective in the excited state.
There is extensive literature on the structural features and the peculiar electronic
properties of boron derivatives.[14] Using a spin-coated thin film of 1, a single-layer white-emitting diode was fabricated. The electroluminescence (EL)
spectrum showed similarity with the absorption spectrum in the solid state; however,
the additional low-energy peak at 680 nm was much more intense, indicating a larger
population in the aggregated state following electrical injection. The CIE (Commission
Internationale de l'Éclairage) coordinates (0.33, 0.42) indicated a clear white light
emission of the light-emitting diode (LED), characterized by high stability to the
applied voltage ([Figure 2C]). Note that this is the first example of a white-emitting LED obtained from a single
material spontaneously self-assembling in the solid state. Despite the great development
of white LEDs having as active materials mixtures of compounds emitting different
colors (blue/yellow and blue/green/red emitters) under various conditions, there are
still very few organic molecules capable of producing white EL under electroexcitation
in organic LEDs.[15] An intriguing example – very recently reported[15a] – concerns quinazoline-based white light emitters. The molecular structure of compound
2PQ − PTZ, having donor–π-acceptor configuration with quinazoline (PQ) as acceptor
and phenothiazine (PTZ) as donor separated by a phenyl spacer, is shown in [Scheme 4] together with the corresponding synthetic procedure.
Scheme 4 Synthesis of quinazoline-based compound 2PQ–PTZ. Adapted with permission from Ref.
[15a]. Copyright 2020 American Chemical Society.
In the single crystal of 2PQ − PTZ, two different conformational isomers, named by
the authors ‘axial’ and ‘equatorial’, were found to coexist in a 1:1 proportion. The
isomers are depicted in [Figure 3A]. According to the authors, the single crystal of 2PQ − PTZ is the first experimental
example of the coexisting (not interconverting) ‘ax’ and ‘eq’ conformers in rigid
organic conjugated molecules. A detailed photophysical study of the two conformational
isomers is described, supported by theoretical calculations. In thin films, using
the appropriate matrix, ‘ax’ and ‘eq’ conformers emitted blue and orange light, respectively,
providing complementary colors for white light emission. A white light-emitting LEDs
based on 2PQ − PTZ and displaying 0.32, 0.34 CIE coordinates could be fabricated (panel
B) with external quantum efficiency amounting to 10.1%, which was the highest performance
reported until then.[15a]
Figure 3 A) ‘Axial’ and ‘equatorial’ conformational isomers present in single crystals of
2PQ–PTZ. B) Characterization and photographs of white LED. (a) LED fabrication process.
(b) Spectrum of LED of 2PQ − PTZ/PMMA onto the surface of LED chip. (c) Photographs
of the white luminescence when LED is turned on. Adapted with permission from Ref.
[15a]. Copyright 2020 American Chemical Society.
2.2 Light-Emitting Transistor and Bicolor Light-Emitting Diode from a Single Compound
The molecular structure and the synthesis of the material in question, namely 2,6-bis-(5′-hexyl-[2,2′]bithiophen-5-yl)-3,5-dimethyl-dithieno[3,2-b;2′,3′-d]thio-thiophene,
DTT7Me,[16] are reported in [Scheme 5].
Scheme 5 Synthesis of 2,6-bis-(5′-hexyl [2,2′]bithiophen-5-yl)-3,5-dimethyl-dithieno[3,2-b;2′,3′-d]thiophene,
DTT7Me. Adapted with permission from Ref. [16]. Copyright 2006 Wiley-VCH.
Based on our previous experience on thiophene oligomers, DTT7Me was designed in order
to be soluble in common organic solvents, to possess great conformational flexibility
and display good charge transport (because of the rigid dithienothiophene inner core)
and good photoluminescence (PL; thanks to the presence of the methyl groups keeping
the molecules sufficiently apart from each other). Exploiting the conformational flexibility
and aggregation capabilities of this compound under different experimental conditions,
a light-emitting transistor (LET) and a bicolor bipixelated light emitting diode could
be fabricated.
2.2.1 Light Emitting Transistor from DTT7Me
A LET is a multifunctional device combining good charge transport with good EL.[17] These conditions are not often simultaneously fulfilled since materials having high
charge mobility generally also display efficient π–π stacking, which in turn causes
PL quenching because of nonradiative decay processes due to strong intermolecular
interactions. A single-layer LET was prepared using both a vacuum-evaporated and a
drop-cast film of DTT7Me from toluene, both films displaying a high degree of order.
[Figure 4A(a–c)] shows the AFM images of vacuum-evaporated films of DTT7Me of different thicknesses
on SiO2, while [Figure 4A (d, e)] shows laser scanning confocal microscopy (LSCM) fluorescence images of a 20-nm-thick
vacuum-evaporated film (d) and of a drop-cast film on SiO2 (e).
Figure 4 A) AFM micrographs (4.5 lm × 4.5 lm) of DTT7Me films grown on SiO2 at a nominal deposition flux of 0.4 Å s−1 having nominal thicknesses of a) 3, b) 6, and c) 20 nm. The root-mean-square roughnesses
of the films are 1.4, 1.9, and 2.5 nm, respectively. Laser scanning confocal microscopy
(LSCM) fluorescence images (100 lm × 100 lm) of d) a 20-nm-thick vacuum-sublimed film
and e) a drop cast film, on SiO2. B) XRD pattern of DTT7Me: a) a 20-nm-thick vacuum-sublimed film (F = 0.4 Å s−1) on SiO2; b) a cast film on SiO2, and c) the precursor powder. Adapted with permission from Ref. [16]. Copyright 2006 Wiley-VCH.
Both AFM and LSCM images show that the films are characterized by a high degree of
surface coverage. The thickest vacuum-evaporated film is formed by large grains covering
the entire surface of the substrate (c) and displays a spatially continuous red luminescent
signal (d). [Figure 4B] shows the X-ray diffraction profile of the crystalline powder of DTT7Me (c) together
with the profile of the vacuum-evaporated film of 20 nm thickness (a) and of the drop-cast
film (b). The (c) profile indicates a high crystalline degree and a highly ordered
structure of DTT7Me powders. Some of the peaks present in the powder profile are also
found in the X-ray pattern of the cast film (b). On the contrary, none of the two
peaks present in the profile of the vacuum-evaporated film is present in DTT7Me powder,
indicating that the cast film (b) and the vacuum-evaporated one (c) belong to two
different polymorphs, probably generated by different inter-ring angles of the terminal
bithiophenes. The inset in panel B shows the proposed model for the supramolecular
organization of the vacuum-evaporated film. LETs were fabricated using vacuum-evaporated
films, as well as drop-cast films were grown on gold source–drain electrodes. The
output optoelectronic characteristics are shown in [Figure 5A] and [B], respectively, where the left y-axis reports the drain–source current I
ds and the right axis the EL.
Figure 5 Optoelectronic output characteristics of DTT7Me-based OLETs. Drain–source current
(I
ds) on the left y-axis, and EL on the right y-axis, vs. drain–source voltage (V
ds), recorded at various gate–source voltages (V
gs). A) Vacuum-sublimed film (thickness 20 nm, F = 0.4 Å s−1), W/L = 42,000/10 (lm/lm). B) Drop-cast film, W/L= 42,000/6 (lm/lm); W = channel width, L = channel length. Adapted with permission from Ref. [16]. Copyright 2006 Wiley-VCH.
When the holes injected from the source electrode and moving along the channel meet
the electrons near the drain electrode, they recombine radiatively in both cases and
both intensity of emitted light and photocurrent increase with the increasing source–drain
and gate voltages. Thus, both vacuum-sublimed and cast films generate EL under an
appropriate applied voltage with very similar optical output characteristics. It is
to be noted that this is the first example of an organic LET obtained by employing
a non-polymeric material and the first having the active film obtained by drop casting.
2.2.2 Bicolor Organic Light-Emitting Diode from DTT7Me
Using the same multifunctional compound, DTT7Me, presented in the previous paragraph
and by means of an approach based on the surface-tension-driven (STD) lithography,[18] a bicolor bipixelated LED was fabricated.[19] Thanks to the STD technique, which allows the control of the molecular behavior
of solutions at the solid/liquid interface, it was possible to simultaneously manipulate
the conformation and the aggregation pattern of DTT7Me. By means of a single-step
bottom-up procedure, it was possible to control both the conformation and self-assembly
modalities of DTT7Me, hence the electro-optical properties of the compound in different
positions of the substrate. [Figure 6] illustrates step by step the procedure employed.
Figure 6 Sketch of the STD deposition technique used for the realization of the DTT7Me bicolor
pixelated OLED. A drop of dilute solutions of DTT7Me is deposited on a template mesh,
set on a glass/ITO substrate; during the solvent evaporation, the DTT7Me molecules
move both under and inside the mesh; the interplay of the energetic conditions at
the different interfaces drives the molecular self-assembly and realizes a bicoloured
pixelated structure; the OLED is realized on the bicolor pattern. Adapted with permission
from Ref. [19b]. Copyright 2010 American Chemical Society.
A drop of DTT7Me dissolved in toluene was deposited on a micrometer-sized grid, placed
on a glass/ITO substrate, to induce geometrical confinement. During the evaporation
of the solvent, the DTT7Me molecules move under and inside the meshes of the grid.
The energy conditions at the different interfaces control molecular conformation and
self-assembly, leading to two alternating emitting regions, orange and red, on which
the bicolour pixelated organic LED was fabricated.[19b] Through AFM, confocal microscopy, X-ray diffraction of the patterned films and theoretical
calculations, we were able to demonstrate that the red regions correspond to a crystalline
film and a fully planar conformation of DTT7Me, while the green regions correspond
to an amorphous film where DTT7Me is present as a series of distorted conformations
of the terminal bithiophenes.[19a]
[Figure 7 (a, b)] displays the structure of the fabricated bipixelated LED, i.e., the different layers
composing the device, including the DTT7Me active layer, together with the corresponding
energy level diagram. [Figure 7c] shows the scanning confocal microscopy image of the green and red patterns of the
DTT7Me layer, while [Figure 7d] shows the three-dimensional reconstruction of the spatially resolved PL emission.
Finally, [Figure 7e] shows the PL spectra collected by confocal microscopy inside and under the grid
mesh. The bicolor micro-pixelated LED of [Figure 7] is the first example of a bicolor device obtained from a single molecular material
employing a single-step procedure. Thanks to the conformational flexibility and spontaneous
self-assembly properties of the thiophene oligomer DTT7Me, the formation of two regularly
alternating regions on a micrometer scale emitting light of two different colors from
a single material could be achieved.
Figure 7 (a) Structure of the device ITO//PEDOT:PSS//CBP//DTT7ME//BCP//LiF/Al and (b) the
simplified energy level diagram. (c) Multichannel laser scanning confocal microscopy
image of the DTT7Me layer of OLED patterned by STD technique. (d) 3D reconstruction
of the spatially resolved PL emission, obtained by x–z optical sections of confocal scans. (e) PL spectra collected by confocal microscopy
in zones inside (dashed line) and under (dotted line) the grid mesh. The scale bars
in (c) and (d) correspond to 10 µm. Reprinted with permission from Ref. [19b]. Copyright 2004 American Chemical Society.
2.3 Random Laser from T5COx
Thanks to their relevant PL properties, thiophene-based materials have been thoroughly
investigated as active materials in lasing applications.[20] Ghofraniha et al. demonstrated that a functionalized quinquethiophene, namely 3,3′,4′′′,3′′′′-tetra-cyclohexyl-3′′,4′′-di(n-hexyl)-2,2′:5′,2′′:5′′,2′′′:5′′′′,2′′′′-quinquethiophene-1′′,1′′-dioxide (T5OCx)
([Figure 8a]), when confined in patterns with different shapes is capable of producing a tunable
random laser emission.[21] A random laser is an optical device based on an active molecular layer in which
defects, aggregates or external beads act as scattering centers inducing light amplification.[22] As depicted in [Figure 8], the patterning of the light emitter T5OCx, achieved by spontaneous molecular self-assembly
driven by STD lithography, induces the formation of well-defined nanoaggregates in
spatially restricted environments. Ring, stripe and pixel microarray structures were
realized by depositing few microliters of a solution of T5OCx in dichloromethane on
different stamps used as geometrical constraints ([Figure 8b–j]). This approach allowed tuning dimensions and distribution density of the nanoaggregates
while permitting to finely control some of the physical parameters in a confined system.
Additionally, changes in the temperature during the solvent evaporation process led
to a further tuning of the size and spatial distribution of the aggregates. Indeed,
faster (slower) evaporation rates result in smaller (larger) aggregates. The authors
demonstrated that self-assembled T5OCx nanoaggregates behave as scattering centers
leading to the fabrication of one-component organic lasers without employing an external
resonator and with the desired shape by using soft lithographic techniques ([Figure 8l–n]). Particularly, improved lasing properties were achieved in devices in which more
packed and larger supramolecular nanostructures were present.
Figure 8 a) Schematic illustration of T5OCx. b–d) Sketches of the different bottom-up lithographic
techniques employed to pattern the oligomer: ring (b, e), stripe (c, d) and squared
pixels (d, g). h–j) optical images of the fabricated samples. A drop of T5OCx solution
in CH2Cl2 is poured at the centre of the related template placed on a glass substrate. The
average thickness of all samples is about 1 μm. Scale bars: 100 μm (h, j) and 50 μm
(i). K) A sketch of the random laser set-up. l–n) Emission spectra of a portion of
the ring-shaped (l), stripe-shaped (m), and single pixel (n). Adapted with permission
from Ref. [21]. Copyright 2013 Wiley-VCH.
3. Supramolecular and Optoelectronic Properties of Oligothiophene-S,S-dioxides
3. Supramolecular and Optoelectronic Properties of Oligothiophene-S,S-dioxides
Oligothiophene-S,S-dioxides (OTOs) are OTs where one or more of the thiophene rings
bear two oxygen atoms at the ipso-position, i.e., at the sulfur atom (see [Scheme 1]).[23] The oxidation of sulfur in a substituted thiophene ring was first obtained by using
the complex HOF·CH3CN (Rozen's reagent), prepared by passing F2 through a mixture of CH3CN and H2O.[24] Subsequently the oxidation was performed by using m-chloroperoxybenzoic acid as the oxidant and the preparation was extended to various
OTOs (see, for example, [Schemes 6] and [7]).[23a]
Scheme 6 Synthesis of oligothiophene-S,S-dioxide units containing one single inner thiophene-S,S-dioxide
unit. Adapted with permission from Ref. [23a]. Copyright 2004 American Chemical Society.
Scheme 7 Synthesis of dimers, trimers, tetramers, and pentamers containing from one to three
thiophene-S,S-dioxide units. Adapted with permission from Ref. [23a]. Copyright 1998 American Chemical Society.
More recently, S-oxides (where sulfur bears only one oxygen) and mixed S,S-dioxides/S-oxides
have been synthesized employing a novel methodology for the facile synthesis of oligo-
and polythiophene-S-oxides and -S,S-dioxides under mild conditions by use of enabling
technologies such as ultrasound and microwave assistance ([Scheme 8]).[23b]
Scheme 8 Synthesis of oligo- and polythiophene-containing thiophene-S-oxide or thiophene-S,S-dioxide
or mixed thiophene-S-oxide/S,S-dioxide units. Adapted with permission from Ref. [23b]. Copyright 2016 Wiley-VCH. Reagent and conditions: i) 2 equiv H2O2 30 wt.%, CH2Cl2/CF3COOH (2:1, v:v); ii) 1 equiv H2O2 30 wt.%, CH2Cl2/CF3COOH (2:1, v:v); iii) 2,5-bis(tributylstannyl)thiophene, Pd(PPh3)4, toluene, reflux overnight; iv) 2-(tributylstannyl)thiophene, Pd(PPh3)4, toluene, reflux overnight; v) N-bromosuccinimide (1 equiv), CH2Cl2/CH3COOH (1:1, v:v), US; vi) n-BuLi, SnBu3Cl, THF(dry), −78 °C; vii) bis(pinacolato)diboron, Pd(dppf)Cl2, NaHCO3, THF/H2O (2/1), MW, 80 °C; viii) Pd(PPh3)4, toluene, reflux overnight.
OTOs display smaller energy gaps, higher electron affinities and ionization energies
than the corresponding OTs.[25] Most OTOs are characterized by bright fluorescence in the solid state with high
quantum yields (QYs), as shown in [Figure 9] displaying the emission of cast films of some OTOs containing a single central thiophene-S,S-dioxide
(TDO).[26]
Figure 9 Cast films (from chloroform) of some oligothiophene-S,S-dioxides containing a single
central thiophene-S,S-dioxide under UV excitation (λ
exc = 363 nm). Reprinted with permission from Ref. [26]. Copyright 2000 American Chemical Society.
However, these compounds display a very low emission QY in solution and high emission
QY in the solid state; hence, they are a special class of aggregation-induced emission
materials that are nonemissive in solution but intensively emissive upon aggregation
in the solid state.[26] Concerning the relationship between supramolecular and optoelectronic properties,
the most investigated so far are OTOs with one single central thiophene sulfonyl group.
Structural modifications strongly impact on aggregation capabilities, which depend
on the interplay of several intra- and intermolecular interactions. As a consequence,
crucial features such as absorption and emission, redox properties, frontier energy
levels and charge carrier mobility are modified with respect to the starting systems.
While conventional OTs tend to aggregate mainly on the basis of π–π stacking and van
der Waals interactions, in TDOs, OTOs, the presence of the oxygen atoms generates
a large number of intra- and intermolecular hydrogen bonds contributing to the packing
forces and allows the molecules to stay away from each other. In consequence, most
OTs show very low PL QYs in the solid state because of prevailing radiationless decay
pathways caused by strong intermolecular interactions, while most OTOs display significant
QYs in the solid state.
[Figure 10] shows the single-crystal X-ray structures of trimer, pentamer, and heptamer having
the same central alkylated TDO core but a different number of adjacent unsubstituted
thienyl groups. Mainly due to the competing effects of dipolar SO2
…SO2 intermolecular interactions, intramolecular and intermolecular C–H…O hydrogen bonds and S…S interactions, compounds 1–3 display very different packing characteristics. In trimer 1 the molecules organize in a chiral form by spontaneous resolution and form a chiral
single crystal. The conformation is very distorted, and the crystal packing shows
a remarkable lack of short intermolecular contacts. Pentamer 2 displays a packing mode close to the ‘herringbone’ motif found in planar (or quasi-planar)
conventional oligomers. In contrast to trimer 1, several very short van der Waals contacts – C…C, C…O, S…S, rarely observed in OTs – are present in it. The molecules of heptamer 3 are quasi-planar (except for the out-of-plane structure, due to the presence of the
oxygen atoms of the sulfonyl group) and stack in parallel layers related by a crystallographic
inversion center. There are many short S…O, S…C, C…O, C…C and S…S intermolecular contacts, while layers of molecules interact via short C–H…O intermolecular contacts.
Figure 10 Molecular structure and perspective view of the crystal packing of trimer 1; molecular structure and projection of the crystal packing down the b-cell axis of pentamer 2; molecular structure and perspective view of the crystal packing of heptamer 3. Adapted with permission from Ref. [28]. Copyright 2000 American Chemical Society.
The absolute PL QYs of the crystalline powders of compounds 1, 2 and 3, (measured with an integrating sphere) are 0.45, 0.12 and 0.02, respectively. The
trend, which is different from that in solution, was accounted for by the supramolecular
organization of the three compounds in single crystals, hence establishing a correlation
between PL efficiencies and packing modalities.[27] Contrary to the behavior in the solid state, in solution (CH2Cl2) all three compounds display PL QYs amounting to less than 0.5,[28] as is the case of most OTOs with one single central TDO. The different PL properties
of OTOs with respect to OTs are the result of the different molecular structures and
supramolecular properties but also of the mechanisms for energy relaxation to the
ground state. Photophysical and theoretical[29] studies show that sulfur dioxygenation causes a dramatic change in photophysics
with respect to nonoxygenated thiophene oligomer. The relaxation dynamics to the ground
state are related to conformational effects and solvent viscosity but not to solvent
polarity despite the presence of the polar SO2 group. Moreover, of the two most important nonradiative decay mechanisms, internal
conversion and intersystem crossing, only the former gives a significant contribution,
contrary to what is observed in nonoxygenated oligomers. Strong intermolecular interactions
in the solid state favor charge transport but are unfavorable to PL. An interesting
compromise from the two opposing factors is found in the pentamer with one central
dioxygenated unit and different alkyl substituents. The increase in electron affinity,
induced by the dioxide functionalization in OTO, confers to charge carriers increased
n-character thus allowing the fabrication of LEDs much more efficient than those obtained
using conventional OTs. Multicolor LEDs with EL covering the entire visible range
down to near-infrared were prepared using α-quinquethiophene derivatives.[30]
[Figure 11] shows the LED obtained using a thermally evaporated neat film of the α-quinquethiophene-S,S-dioxide[31] T5COX (see Section 2.3) using the p-i-n technology, based on the deposition of highly conductive p- and n-doped transport layers.[32] The doping of the p and n transport materials is realized by means of electron acceptor molecules, such as
2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), and electron donor
alkali metals, such as lithium or cesium ([Figure 11A]). As shown by the data reported in [Figure 11(B, C)], luminances of 10,000 cd/m2 were obtained with a voltage of 9 V, about 30 times the best value reported for a
bilayer configuration. Moreover, aging measurements showed device lifetimes of about
108 and 2200 h at 100 and 3200 cd/m2 of starting luminances, respectively.
Figure 11 A) Structures of the tested OLEDs: (a) undoped device, (b) p-doped device, and (c) p-i-n device. (d) Energy levels of the p-i-n device and chemical structures of materials used. B) Current density (full symbols)
and luminance (open symbols) vs. voltage for device A (square), device B (circle),
and device C (triangle). Inset: EL spectrum of device C. C) Experimental lifetime
curve starting from 6500 cd/m2 (continuous curve), SED extrapolation (dashed curve); shown in the right corner is
the SED equation. L(t) is the luminance at time t, L
0 is the initial luminance, t coincides with the decay time, and β is called the dispersion factor, which is related to the shape of the curve; this
last parameter does not vary for different current densities measured in the same
structure. The half-lifetime t
1/2 is related to t by the equation: t1/2 = (Τ/β)ln 2. Inset: lifetime vs. luminance; SED-extrapolated lifetime at different
starting luminance (full symbols) and accelerated lifetime extrapolation (continuous
curve) in the middle of the filled area. The filled area envelopes the incertitudes
of the extrapolated data. Adapted with permission from Ref. [31a]. Copyright 2009 American Institute of Physics.
Recently, a major contribution to the understanding of the optoelectronic and charge
transport properties of dioxygenated OTs has come from Campos and coworkers.[33] Choosing a model series of oligomers characterized by one to four TDO groups flanked
on each side by a terminal thiophene, they were able to demonstrate that the prevailing
charge carriers change from holes to electrons as the number of TDO units present
in the backbone increases.[33a] Along the series from the trimer to the octamer, the absorption maximum is progressively
red-shifted and the energy gap decreases from 2.1 to 1.4 eV. However, this large decrease
is mainly due to the LUMO orbital, the HOMO remaining almost unchanged. In consequence,
on passing from the trimer to the octamer, the nature of the prevailing charge carrier
changes from p- to n-, i.e., from holes to electrons as the number of TDO units present in the oligomer
increases. It is to note that the control of charge carriers obtained on varying the
length of the oligomer was unprecedented. The flexibility of the thiophene ring coupled
to its ipso- di-oxidated and mono-oxidated counterparts is confirmed by our recent results[23b] by means of the fabrication of thin-film field-effect transistors demonstrating
that while a conventional polythiophene has prevailing p-type charge transport characteristics, the corresponding polymer bearing alternating
thiophene-S-oxide groups has ambipolar charge transport properties, whereas the corresponding
polymer with TDO groups has prevalently n-type charge transport carriers.
4. Colloidal Nanoparticles formed by Self-Assembly of Thiophene-Based Polymers
4. Colloidal Nanoparticles formed by Self-Assembly of Thiophene-Based Polymers
In the field of chemical nanotechnology, an important role is played by nanoparticles
(NPs) obtained by self-assembly of functional π-conjugated polymers for their biological
applications – in particular as carriers for the delivery of therapeutic molecules
– and for their use in organic electronics.[34] Recently, NPs of poly(3-hexylthiophene), P3HT-NPs, have successfully been employed
for in vitro and in vivo applications.[35] The successful use of colloidal P3HT-NPs in field-effect transistors has also been
reported.[36] Below we describe two recent examples from our own work concerning the preparation
and the optoelectronic characterization of novel thiophene-based oxygenated NPs (Section
4.1) and application of NPs of P3HT-NPs in flexible solid-state electrochromic devices
(ECDs; Section 4.2).
4.1 Spherical Nanoparticles of Poly(3-hexyl)thiophene Containing Thiophene-S,S-dioxide
Units
Di Maria et al.[37] have described the preparation and optoelectronic characterization of novel NPs
of P3HT containing variable amounts of TDO units. The NPs were prepared either by
first synthesizing P3HT NPs and then oxygenating the NPs with Rozen's reagent[24] or by first oxygenating P3HT with Rozen's reagent and then preparing the NPs, as
illustrated in [Scheme 9].
Scheme 9 Different strategies combining Rozen's reagent (HOF·CH3CN) and nanoprecipitation techniques to obtain well-defined functionalized TDO-nanoparticles:
(i) PTDO-NPs and (ii) core − shell P3HT@PTDO-NPs. Reprinted with permission from Ref.
[37]. Copyright 2017 American Chemical Society.
In both cases, the NPs were prepared by the nanoprecipitation method, i.e., by first
dissolving the polymer in THF (in the absence of surfactants) and then adding this
solution into distilled water under rapid stirring.[38] The different preparation modalities afforded different NPs. Those obtained from
oxygenation of preformed NPs of P3HT were core-shell NPs (P3HT@PTDO-NPs in [Scheme 9]) having the oxygenated polymer arranged in the external shell and the non-oxygenated
polymer in the inner core. On the contrary, those obtained after oxygenation of P3HT
(PTDO-NPs in [Scheme 9]) had the oxygen randomly distributed inside and outside the entire volume of the
NPs. By employing in both cases variable amounts of the oxidant, a fine tuning of
NP dimensions and optoelectronic properties was achieved. [Scheme 9] shows that the dimensions of PTDO-NPs could be modulated in the range of 60–150 nm,
becoming progressively smaller as the oxygenation degree of the polymer increased,
a result probably due to different inter- and intramolecular interactions promoted
by oxygen during NP formation. By contrast, the dimensions of P3HT@PTDO-NPs, obtained
by oxygenation of preformed P3HT-NPs, were always nearly 300 nm. The z-potential values indicated that the stability of the colloidal solutions of NPs increased
as the number of TDO groups present in the NPs increased, the solutions being stable
for months without any sign of precipitation. Transmission electron microscopy and
AFM images reported in [Figure 12] (panels A[a, b] and B[a, b]) show the spherical morphology of the NPs. The figure
also displays the IR spectra and X-ray photoelectron spectra of PTDO-NPs and P3HT@PTDO-NPs
obtained with the maximum amount of oxidant employed. The IR spectra show the presence
of the peaks typical of TDO units present in OTOs hence confirming that the oxygenation
has occurred at the sulfur atom (panel A[c]). Also, X-ray photoelectron spectroscopy
measurements are in agreement with the proposed structure for the different types
of NPs (panel B[c]). Indeed, while the spectrum of pristine P3HT-NPs displays only
one peak corresponding to the binding energy of the S − C bond, the spectrum of core − shell
P3HT@PTDO-NPs displays three peaks centered at different binding energies and indicating
the presence of three non-equivalent sulfur atoms, one due to metal–sulfur interactions
and the other two corresponding to the oxidation of the thiophene sulfur atom. The
control of the oxygenation degree also allows the fine modulation of the optical properties
of the NPs. Passing from P3HT-NPs to PTDO-NPs, a blue-shift of the maximum wavelength
absorption was observed – ascribed to the smaller size of the NPs causing a decrease
in polymer conjugation length – paralleled by a progressive decrease in the intensity
of fluorescence emission as the amount of oxygen increased. On the contrary, going
from P3HT-NPs to core–shell P3HT@PTDO-NPs, only small variations in the UV-vis and
fluorescence spectra are observed, in agreement with the fact that oxygenation is
limited to the outer shell of the NPs.
Figure 12 A) (a) Low-magnification TEM micrograph of a PTDO1-NP sample with its particle size histogram in the inset. (b) AFM image of PTDO1-NPs. (c) Infrared spectra in the region between 900 and 1400 cm−1 and 2750 − 3000 cm−1 of PTDO-NPs (PTDO0.25-NPs, green line; PTDO0.5-NPs, blue line; PTDO1-NPs, red line) compared to P3HT-NPs (black line). B) SEM image of P3HT-NPs (a) and
P3HT@PTDO1NPs (b) obtained after addition of 1 equiv of HOF·CH3CN to P3HT-NPs. (c) X-ray photoelectron spectra of P3HT-NPs (□) and P3HT@ PTDO1-NPs (○). Scale bar, 1 μm. Adapted with permission from Ref. [37]. Copyright 2017 American Chemical Society.
[Figure 13] compares the cyclovoltammetries of P3HT-NPs, PTDO-NPs and P3HT@PTDO-NPs. The figure
shows that the different structures and morphologies of the NPs have a striking effect
on their electrochemical properties (panel A). The data show that the structuration
into NPs causes the decrease (on the order of hundreds of mV) of the HOMO–LUMO energy
gap with respect to the starting polymer. More importantly, both the oxidation and
reduction potentials are largely affected by the number of TDO units present in the
NPs. In PTDO-NPs, as the number of TDO units increases, the oxidation potential increases
while the reduction potential becomes less negative, as already observed in OTOs,[25a] leading to low band-gap NPs. By contrast, in core–shell P3HT@PTDO-NPs, the increasing
oxygenation of the external shell causes only a negligible variation in the oxidation
potential but a remarkable change in the reduction potential which becomes progressively
less negative. This is in agreement with the fact that the inner core of the NPs is
made of pure P3HT and only a few external layers are oxygenated. Consequently, the
energy gap of core–shell P3HT@PTDO-NPs is lower than that of PTDO-NPs and P3HT-NPs
(panel B). At the maximum oxygenation degree, P3HT@PTDO-NPs displayed an energy gap
of 1.42 eV while PTDO-NPs at maximum oxygenation displayed an energy gap of 2.67 eV,
both to be compared to the value of pure-shell P3HT-NPs, 1.54 eV. The high electron
affinity of P3HT@PTDO-NPs suggests that they may display n-type semiconductor behavior, in line with what has already been observed in polythiophene-S,S-dioxides.[23b]
[33a] Kelvin Probe measurements, also reported in [Figure 13C], confirm this to be the case. Kelvin Probe measurements allow the simultaneous investigation
of the structural and electronic properties of a surface with nano-scale resolution.[39] The formation of charges is revealed by a shift of the surface photovoltage under
illumination with respect to the dark. Charge carrier generation and separation under
illumination, after dissociation of excitons, were measured for the three types of
NPs. [Figure 13C] shows the data for P3HT@PTDO-NPs with different oxygenation degrees and, for comparison,
also for non-oxygenated P3HT-NPs. It is seen that in core–shell P3HT@PTDO-NPs the
parameter V
light − V
dark, i.e., approximately the charge density of the NPs, increases with the oxygenation
degree up to a maximum of nearly 100 mV (five times the value for P3HT-NPs), suggesting
that beyond a given oxygenation degree in the outer shell no better charge separation
may be achieved.
Figure 13 (A) Cyclic voltammetries of P3HT-NPs (red line), P3HT@PTDO-NPs (black line), and
PTDO-NPs (orange line). (B) Schematic representation of the cross-section of core − shell
P3HT@PTDO-NPs made of PTDO in the shell and P3HT in the core and its corresponding
energy levels. (C) Measured surface photovoltage values corresponding to different
nanoparticles (red circle, P3HT-NPs; black circle, P3HT@PTDO-NPs). Adapted with permission
from Ref. [37]. Copyright 2017 American Chemical Society.
4.2 Spherical Nanoparticles of Poly(3-hexylthiophene-2,5-diyl) for Electrochromic
Devices
Electrochromism is a reversible change of the color of materials under an applied
external voltage.[40] Conjugated polymers are among the most important electrochromic materials for application
in devices. In turn, polythiophenes are amongst the most studied electrochromic polymers,
owing to their significant color contrast, considerable conductivity, chemical stability,
and color tunability obtained by structural changes via organic synthesis. Several
polythiophenes with tailored electrochromic properties have been reported giving good
performance in devices, in particular poly(3,4-ethylenedioxythiophenes) and its derivatives.[41] It is known that the morphology of the active film may strongly affect the performance
of ECDs. In particular, improved results are obtained employing nanostructured thin
films.[42] We found that employing nanostructured films obtained by deposition of P3HT-NPs
by spread-casting, it was possible to fabricate ECDs of P3HT displaying improved performance
with respect to what was obtained with cast films from chloroform ([Figure 14]).[43] The P3HT samples employed for the fabrication of the ECDs were prepared with the
same methodology employed for the preparation of highly pure P3HT-NPs employed as
fluorescent probes for cellular studies.[44]
[Figure 14] shows the characterization of P3HT obtained by oxidative polymerization of 3-hexyl-thiophene
together with a sketch illustrating the formation and the centrifugation of the NPs
obtained by nanoprecipitation (panels A and B). In the proton spectrum, the peaks
pertaining to the four triads characteristic of P3HT[44]
[45] are visible.
Figure 14 A) (a) Synthesis of P3HT by oxidative polymerization with FeCl3. (b) Normalized PL and UV-Vis spectra. (c) 1H-NMR spectra in CHCl3 showing the presence of the different configurational triads. Reprinted with permission
from Ref. [44] (Figure S1). Copyright 2017 Royal Society of Chemistry. B) Preparation of P3HT-NPs
by use of the reprecipitation method and differential centrifugation. Adapted with
permission from Ref. [44]. Copyright 2017 Royal Society of Chemistry. C) Spray-casting of the water dispersion
of P3HT-NPs on a flexible PET-ITO substrate by means of an air-gun sprayer. Adapted
with permission from Ref. [43]. Copyright 2020 American Chemical Society.
Grazing-incidence wide-angle X-ray scattering measurements on P3HT-NPs of 100, 200,
and 400 nm size spray coated on silicon support showed that large, randomly oriented,
crystalline domains were present in all NPs, independent of their size. ECDs were
fabricated employing spray-casted films from P3HT-NPs suspensions in water deposited
on flexible PET-ITO substrates with the aid of an air-gun sprayer (see [Figure 14C]) and compared to analogous devices employing P3HT cast films from chloroform. All
devices show reversible color transitions between red and light blue by applying a
−1.5/1.5 V voltage, as shown in [Figure 15A]. Performance parameters were evaluated on the basis of switching time, charge consumption
(Q), optical contrast (ΔT) and improved durability (cycling). It was found that devices fabricated with P3HT-NPs
with size of 100 nm displayed enhanced properties with respect to ECDs having P3HT-NPs
of larger size as well as with respect to ECDs with thin films deposited from chloroform.
Figure 15 A) Electrochromic device with spray-coated 100 nm P3HT-NPs at different redox states.
Inner square reduced (left) and oxidized (right) by applying1.5/−1.5 V. B) (A, B)
Transmittance (black) and current (red) data obtained from the characterization of
the device using P3HT as cast film and P3HT-NPs of 100 nm, respectively. C) (A, B)
Comparison in transmittance of the devices using P3HT thin film and P3HT-NPs (100 nm),
as prepared and after 1000 cycles, respectively. D) (C) Changes in transmittance of
P3HT thin film and P3HT 100 nm NP devices during the cycling measurement and (D) Optical
contrast evolution (in ΔT) of the P3HT thin film and P3HT 100 nm NP devices during the 1000 cycles performed.
In all measurements, 1.5 V was used for oxidation and −1.5 V was used for reduction
(1 cycle = 10 s). Adapted with permission from Ref. [43]. Copyright 2020 American Chemical Society.
[Figure 15B] shows a plot of transmittance and current data from the device employing a thin
film of P3HT and the device with a film of NPs of 100 nm, the data being largely in
favor of the latter. The same is true for the comparison in transmittance of the devices
as prepared and after 1000 cycles (panel C) and changes in transmittance during the
cycling measurement or optical contrast evolution during the 1000 cycles performed
(panel D). Finally, a significant result of this work is that employing films of NPs
of P3HT dispersed in water is not only a way to obtain more performant ECDs but also
an environmentally sustainable procedure, avoiding toxic organic solvent vapors for
the users and with a much lower environmental impact.
Conclusions and Outlook
In this short review, we have reported a few examples taken from our own work aimed
to show that thiophene derivatives are a rich source of molecular entities whose supramolecular
aggregation allows the fabrication of a variety of highly performant functional optoelectronic
devices. Thiophene derivatives can generate a multiplicity of supramolecular structures,
hardly predictable on the mere structure of the molecular components but programmable
through the use of molecules containing sufficient information in their covalent framework
to promote their spontaneous supramolecular organization into functional superstructures
via non-covalent interactions (noncovalent interactional algorithms according to J. M. Lehn's definition[46]), as in the case of fluorescent and conductive nanostructured microfibers.[47] Thus, to face up to new opportunities in the field of supramolecular optoelectronic
materials, there is an urgent need for innovative thiophene-based compounds capable
to give rise to supramolecular architectures with programmed functions. In parallel,
new synthetic methodologies should be developed through more ecofriendly and low-cost
procedures such as, for example, use of aqueous solvents and catalysts without polluting
metals. Finally, in the present context, it should be reminded that besides the works
concerning thiophene derivatives – from ourselves and the many researchers working
in the field – there is plenty of excellent examples of supramolecular structures
and applications from other classes of conjugated oligomers and polymers. Nevertheless,
the peculiarity of thiophene derivatives stems from their numerous applications in
biological and medical fields, which make it reasonable to foresee that there will
soon be a convergence of materials chemistry, biology, and medicine towards the development
of sophisticated materials and optoelectronic devices spontaneously assembled inside
living organisms.
