In search of efficient solubilizing groups for liquid and luminescent oligo(phenylene-thiophene) chromophores

Article abstract of DOI:10.1039/d0tc04536e

In this work, the synthesis of oligomers having a rigid conjugated 4,4′-bis(2-thienyl)biphenyl fragment end-capped with various types of solubilizing groups (SGs), such as either alkyl or alkylsilyl or alkyl-oligodimethylsiloxane, has been reported. The comprehensive study of their thermal and optical properties as well as rheology in comparison to model highly crystalline oligomers with simple either hexyl or trimethylsilyl SGs allowed us to elucidate structure-property correlations and find the most powerful type of SG in terms of liquefaction for them. It was revealed that oligomers with long and branched alkyl SGs still retain high crystallinity, whereas oligomers with alkyl-oligodimethylsiloxane SGs combine very low glass transition temperatures (up to -111 °C) with a liquid-crystalline behaviour. The alkylsilyl SGs were found to be the most efficient, since the oligomers end-capped with trihexyl- and tri(2-butyloctyl)silyl SGs are liquid and have low values of both the glass transition temperature (up to -60 °C) and viscosity (up to 1.94 Pa s). All the oligomers prepared have similar optical absorption/luminescence spectra and high values of photoluminescence quantum yield in solution (90-95%) without a significant impact of the SG type. In the neat films, the type of SG has a huge impact on the shape and maxima of the absorption and luminescence spectra as well as the photoluminescence efficiency. Among this series of molecules, oligomers with alkylsilyl SGs demonstrate the highest values of photoluminescence quantum yield in the neat form (24-61%) and close to the solution optical characteristics, which indicates their strong capability to suppress aggregation of molecules in the bulk. Thus, for the first time liquid luminescent thiophene/phenylene co-oligomers were reported and the solubilizing capabilities of some of the most promising types of SG were comprehensively investigated and compared to each other. The results obtained can be used as a guideline for the design of functional materials based on conjugated oligomers with a tunable and controllable phase behaviour, solubility and optical properties in the neat state.

Full text of DOI:10.1039/d0tc04536e

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Page 1 of 16
Journal of Materials Chemistry C
In search of efficient solubilizing groups for liquid and luminescent oligo(phenylene-
View Article Online
DOI: 10.1039/D0TC04536E
thiophene) chromophores
Yuriy N. Luponosov1*, Dmitry O. Balakirev1, Ivan V. Dyadishchev,1 Alexander N. Solodukhin1,
Marina A. Obrezkova1, Evgeniya A. Svidchenko1, Nikolay M. Surin,1 Sergei A. Ponomarenko1
1Enikolopov Institute of Synthetic Polymeric Materials of the Russian Academy of Sciences,
Profsoyuznaya st. 70, Moscow 117393, Russia
*Corresponding author e-mail: luponosov@ispm.ru (Dr. Yu. Luponosov)
Abstract
In this work, the synthesis of oligomers having rigid conjugated 4,4'-bis(2-thienyl)biphenyl
fragment end-capped with various types of solubilizing groups (SGs), such as either alkyl or
alkylsilyl or alkyl-oligodimethylsiloxane are reported. The comprehensive study of their thermal
and optical properties as well as rheology in comparison to model highly crystalline oligomers
with simple either hexyl or trimethylsilyl SGs allowed us to elucidate structure-properties
correlations and find the most powerful type of SG in terms of liquefaction for them. It was
revealed that oligomers with long and branched alkyl SGs still remain high crystallinity, whereas
oligomers with alkyl-oligodimethylsiloxane SGs combine very low glass transition temperatures
(up to −111 ºC) with a residual crystallinity. The alkylsilyl SGs were found to be the most efficient,
since the oligomers end-capped with trihexyl- and tri(2-butyloctyl)silyl SGs are liquid and have
low values of both the glass transition temperature (up to −48 ºC) and viscosity (up to 1.98 Pa·s).
All the oligomers prepared have similar optical absorption/luminescence spectra and high values
of photoluminescence quantum yield in solution (up to 90%) without a significant impact of the
SG type on that. In the neat films, the type of SG has a huge impact on the shape and maxima of
absorption and luminescence spectra as well as the photoluminescence efficiency. Among this
series of molecules, oligomers with alkylsilyl SGs demonstrate the highest values of
photoluminescence quantum yield in the neat form (24% – 61%) and close to the solution optical
characteristics, which indicate their strong capability to suppress aggregation of molecules in the
bulk. Thus, for the first time liquid luminescent thiophene/phenylene co-oligomers were reported
and a solubilizing capability of some of the most perspective types of SG was comprehensively
investigated and compared to each other. The results obtained can be used as a guideline for design
of functional materials based on conjugated oligomers with a tunable and controllable phase
behaviour, solubility and optical properties in the neat state.
1

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Page 2 of 16
1. Introduction
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DOI: 10.1039/D0TC04536E
Solvent-free nonvolatile room temperature molecular liquids based on luminescent organic
π-conjugated molecules represent a new generation of luminescent soft matter with unique
properties [1,2]. Molecules of liquid organic chromophores (LOCs) consist of a π-conjugated
fragment and an efficient solubilizing group (SG), which allows them to be fluid at room
temperature. LOCs have a number of outstanding features over the traditional chromophores such
as an easy and ecologically friendly processability at room temperature, unlimited deformability
with no impact in their photophysical properties, isotropy of optical and electrical properties, etc.
Moreover, LOCs can serve as a fluorescent matrix the properties of which can be easily changed
and tuned by addition of functional organic or inorganic dopants [2-6]. LOCs have been actively
investigated over the last decade and used as functional materials for various applications [2].
Examples of LOCs applications include luminescent inks[7], liquid semiconductors[8-10],
sensors[11-13], organic lasers[10-15], organic light-emitting devices[16-20], memory
devices[21], photon upconversion[22,23], solar energy storage systems[23], liquid electret
devices[24], etc.
Although some examples of LOCs have been reported many years ago, a real interest to
their investigation and application has been appeared recently, due to a fast development of novel
generation of optoelectronic devices based on organic materials. Nowadays, various organic both
luminescent and non-luminescent LOCs such as porphyrins,[25,26] phthalocyanines,[27],
carbazoles[16, 28], fluorenes[10], pyrelenes[29], arylamines[9,] oligo(p-phenylenevinylene)s[7],
coumarines[30], azo compounds[31], etc. have been reported. A number of approaches to suppress
crystallinity and decrease inter- and intramolecular interactions of LOCs have been applied [2,
29]. The most often used approaches include change of SG position, its length or branching degree.
Alkyl fragments can be considered as the simplest and the most often used SG to design
LOCs[1,2]. Besides them, several examples of LOCs with short oligodimethylsiloxane [9,10,27]
and oligo(ethylene oxide)[32,33] SGs have been also reported. Although alkylsilyl groups are
known for a long time, we were able to find only a single example of their utilization for the design
of LOCs [34]. We believe that alkylsilyl groups are a promising and powerful tool to design LOCs
especially for those having extended and rigid conjugated backbone. Indeed, one can expect that
the solubilizing ability of the alkylsilyl group may be up to three times more efficient as compared
to the alkyl group, since the silicon atom possess three alkyls instead of one.
In this work, in order to investigate and compare a liquefaction power of alkylsilyl group
in the design of LOCs we synthesized a series of molecules based on rigid conjugated 4,4'-bis(2-
thienyl)biphenyl (TPPT) [35,36] backbone with various types of terminal SGs attached to 5,5'-
positions of thiophene rings (Figure 1), such as alkyl (Alk), alkyl-oligodimethylsiloxane (Alk-SiO)
2

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Journal of Materials Chemistry C
and alkylsilyl (Alk-Si). To the best of our knowledge, this work is the first example, in which
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DOI: 10.1039/D0TC04536E
various and the most efficient types of SGs are investigated and compared to each other in terms
of liquefaction ability and impact on the optical and thermal properties as well as rheology. Thus,
this work can be considered as a guide for researchers to design LOCs and tune their properties.
Moreover, to the best of our knowledge, the LOCs developed in this work will be the first example
of liquid luminescent thiophene/phenylene co-oligomers reported so far.
Figure 1. Chemical structures of the oligomers based on TPPT fragment with terminal
alkyl, alkyl-oligodimethylsiloxane and alkylsilyl SGs.
2. Results and discussion
2.1 Synthesis
The synthesis of the molecules based on TPPT fragment with different SGs can be divided
into three main stages (Fig. 2).
3

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Page 4 of 16
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DOI: 10.1039/D0TC04536E
Figure 2. Scheme of TPPT-based oligomers synthesis.
The first stage includes preparation of various monofunctionalized SGs precursors (Fig. 2,
I). Synthesis of monofunctional alkylchlorosilanes (3a-c) includes two steps. First, Grignard
reagents were prepared from commercially available alkyl bromides (1a-c) followed by their
reacting with freshly distilled trichlorosilane to give alkylhydrosilanes (2a-c) in the yields of 91-
96%. Alkylchlorosilanes (3a-c) were obtained by refluxing of 2a-c in anhydrous carbon
tetrachloride at the presence of PdCl2 as a catalyst[37,38]. Synthesis of 1,1,1,3,3,5,5,7,7,9,9,11,11-
tridecamethylhexasiloxane (6) was carried out by condensation of 1,1,3,3,5,5,7,7-
octamethyltetrasiloxan-1-ol[39] (4) with chloropentamethyldisiloxane[40] (5) in 68% yield
(Figure 2, I).
The second stage consists of attachment of SGs or their parts to thiophene followed by
preparation of organoboron precursors (Fig. 2, II). Synthesis of thiophene precursors with SGs
(7a-g) was carried out by coupling reaction between a thiophene lithium derivative and either
4

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Journal of Materials Chemistry C
commercial 1-bromoalkanes or alkylchlorosilanes (3a-c). Organoboron precursors (8a-g, 10) were
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DOI: 10.1039/D0TC04536E
obtained by lithiation reaction of either 7a-g or 2-but-3-en-1-ylthiophene (9) with n-butyl lithium
followed by coupling with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (IPTMDOB) in
the yields of 86-97%.
Finally, oligomers with alkyl (TPPT-Hex, TPPT-Dodec, TPPT-BuOct) and alkylsilyl
SGs (TPPT-Si-Me, TPPT-Si-Hex, TPPT-Si-MeHex,TPPT-Si-BuOct, were obtained by Suzuki
cross-coupling reaction between boronic esters (8a-g) and 4,4'-dibromobiphenyl (Figure 2, III). It
should be noted that the synthesis of TPPTSi--Me via palladium-catalyzed direct arylation has
been reported previously [41].
The synthesis of TPPT-Bu-3SiO and TPPT-Bu-6SiO having alkyl-oligodimethylsiloxane
groups included two sequential reaction steps. First, Suzuki cross-coupling reaction between 10
and 4,4'-dibromobiphenyl gave 4,4'-(5-but-3-en-1-yl-thiophene-2-yl)biphenyl (11) in 72 % yield.
Second, the hydrosilylation reaction of 11 with commercially available 1,1,1,3,5,5,5-
heptamethylsiloxane
or
its
longer
linear
analogue,
1,1,1,3,3,5,5,7,7,9,9,11,11-
tridecamethylhexasiloxane (6), gave rise to TPPT-Bu-3SiO or TPPT-Bu-6SiO, respectively. All
the target oligomers were purified and their chemical structure and high purity were proved by 1H,
13C, 29Si NMR spectroscopy, MALDI–TOF mass spectrometry and elemental analysis (see
Electronic supporting information (ESI), Fig. S1 – Fig. S67).
2.2 Thermal properties and rheology.
Methods of thermogravimetric analysis (TGA) and differential scanning calorimetry
(DSC) were applied in order to reveal the effect of SG on the thermal stability and phase behaviour
of the novel luminophores and the data obtained are summarized in Table 1. TGA data showed
that all the materials have high thermal and thermal-oxidative stability, since their values of the
destruction temperature (Td), measured as 5% weight loss of samples, exceed 275°C in air and
under inert atmosphere (Figure 3, Table 1). Comparing the effect of SG on the thermal-oxidative
stability one can see that Td values in air for all the materials are quite similar. In the inert
atmosphere the difference between the molecules stability is a more pronounced. In general,
thermal stability of the molecules with Alk-Si groups was found to be slightly higher as compared
to those of their analogs with Alk and Alk-SiO groups. The exception only is the molecule with
trimethylsilyl groups (TPPT-Si-Me) having the lowest Td value among this series of molecules,
which can be explained by its lower molecular weight allowing sublimation of the material before
the thermal destruction begins.
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Page 6 of 16
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DOI: 10.1039/D0TC04536E
Figure 3. TGA curves of TPPT-based oligomers in air (a) and under inert atmosphere (b).
Table 1. Thermal and rheology properties of TPPT-based oligomers
air
Td / TdN2, Tg,
T1,°C
T2, °C
Ti, °C
η,
Ea,
State
at r.t.
Compounds
°C
oC
(ΔH, J/g) (ΔH, J/g) (ΔH, J/g)
Pa·s kJ·mol-1
TPPT-Hex
286/290
276/327
288/315 -41
283/280
309/356 -54
–
–
69(19)
99(73)
261(38) 270(26) solid
221(25) 228(41) solid
–
–
–
–
–
–
–
–
45
–
48
–
–
TPPT-Dodec
TPPT-BuOct
TPPT-Si-Me
TPPT-Si-Hex
TPPT-Si-MeHex 289/379 -22
TPPT-Si-BuOct 283/376 -60
TPPT-Bu-3SiO 285/315 -64
TPPT- Bu-6SiO 283/333 -111
-
75
–
–
–
-
42(44)
60(14)
solid
–
202
240(70) solid
–
–
–
-
–
liquid 1.94
solid
liquid 5.84
solid
solid
53(27)
–
95(6)
85(3)
–
–
–
-
-
Notes: Td is the decomposition temperature calculated for 5% weight-loss; Tg is a glass transition temperature; T1 is
a temperature of the first endothermic transition (suggested as alkyl melting); T2 is a temperature of the second
endothermic transition; Ti is an isotropization peak temperature; In parentheses the corresponding value of transition
enthalpy (ΔH,J·g-1) is given; η is a dynamic viscosity; Ea is an activation energy of viscous flow.
Comparison of DSC data for the series of molecules allowed us evaluating the efficiency
degree of SGs for reducing crystallinity of TPPT fragment. The first and second heating DSC
scans are shown in Fig. 4 and in Fig S68a (see ESI), respectively. As can be seen from these data
some molecules demonstrate several endothermic transitions and some of them have relatively
low values of enthalpy (ΔH), which indicate on a liquid crystalline behaviour and was confirmed
by polarizing optical microscopy (Fig. S69).
As expected, TPPT-Hex having the shortest aliphatic fragment within the series of
molecules with alkyl SGs was found to be a material with high values of both temperature and
enthalpy of phase transitions (Table 2). Moreover, the DSC heating scan of TPPT-Hex shows
several endothermic peaks, where the first (T1) at 69ºC with ΔH of 19 J/g could be attributed to
the melting of alkyl chains, the second one (T2) at 261ºC with ΔH of 38 J/g – to the transition from
a liquid crystalline smectic phase (Sm1) to the other smectic phase (Sm2) (Fig. S69) followed by
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Journal of Materials Chemistry C
the final isotropization (Ti) at 270°C. TPPT-Dodec has similar to TPPT-Hex phase behaviour.
View Article Online
DOI: 10.1039/D0TC04536E
However the doubling of alkyl chain length in the SGs from hexyl to dodecyl leads to an increase
of the temperature for the first endothermic transition responsible for alkyl chain melting and to
decrease for the subsequent Sm1 to Sm2 transition and final isotropization. The similar trend of
further temperature decreasing of the liquid crystal phase transitions was found to be also for
TPPT-BuOct. However the change of the linear dodecyl group at TPPT fragment by 2-butyl-octyl
one being the alkyl with the same number of carbon atoms but having a branched architecture,
leads to the appearance of a glass transition at -41°C (Fig S68b) instead of the alkyls melting
observed previously for TPPT-Dodec and TPPT-Hex molecules.
Figure 4. The first heating DSC scans of TPPT-based oligomers.
The usage of alkylsilyl groups leads to a change in the phase behaviour of the TPPT
molecules. While the model TPPT-Si-Me being the first representative from the series of
molecules with Alk-Si groups is a crystalline material with a relatively high Tm at 240°C and ∆Hm
of 70 J/g, the subsequent analogue with longer hexyl groups (TPPT-Si-Hex) is a liquid material
with a low glass transition temperature (Tg) of -54 °C. This is also in a sharp contrast to TPPT-
Hex analogue being a solid material with high-temperature phase transitions. Surprisingly, the
replacement of hexyl chains by 3,5,5-trimethylhexyl ones at the silicon atom leads to a solid
aggregate state for TPPT-MeHex with increased Tg value of -22° and melting peak at 53°C (∆Hm
= 27 J/g). This fact may be explained by a branched character of 3,5,5-trimethylhexyl groups
capable to form an interpenetrating network in the bulk state and thus increase intermolecular
interactions. However, the absence of an endothermic peak on the second heating DSC scans
indicates that the crystalline phase does not recover from a melt (Fig. S68), which often happens
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Page 8 of 16
for branched molecules [42]. The phase behaviour of TPPT-Si-BuOct was found to be quite
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DOI: 10.1039/D0TC04536E
similar to that of TPPT-Si-Hex, since this material is also a liquid with a glass transition at -60°C
and no melting peaks on the DSC scan.
The molecules with Alk-SiO groups, namely TPPT-Bu-3SiO and TPPT-Bu-6SiO,
demonstrate the lowest Tg values in this series of molecules, -64°C and -111 °C, respectively,
which is originated from a flexible oligodimethylsiloxane fragments nature [40]. Although these
data indicate that the solubilizing ability of the latter is much better than that of alkyl SGs, it was
found to be slightly insufficient to suppress completely π-π interactions of TPPT fragments in the
neat state. One can see an endothermic phase transition on the DSC scan at 95°C for TPPT-Bu-
3SiO and at 85°C for TPPT-Bu-6SiO with rather low enthalpy melting values of 6 and 3 J/g,
respectively with the typical for hexagonal columnar mesophase textures [43-45]. Similar to
TPPT-Si-BuOct the TPPT-Bu-3SiO and TPPT-Bu-6SiO molecules showed only a glass
transition process on the second DCS heating scan (Fig S68).
Thus, among the series of molecules investigated only TPPT-Si-Hex and TPPT-Si-BuOct
were found to be in liquid aggregate state at room temperature (Fig. 5).
Figure 5. Photographs of TPPT-based oligomers on glass substrates under (a) day light and (b)
365 nm UV light; Photograph of TPPT-Si-Hex under (c) day light and (d) 365 nm UV light; (e)
dependence of viscosity on shear rate (flow curves) at various temperatures for TPPT-Si-Hex
and TPPT-Si-BuOct.
The fluid behaviour of TPPT-Si-Hex and TPPT-Si-BuOct were studied by the rotational
viscometry technique. The dependence of the luminophores flow curves for various temperatures
8

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Journal of Materials Chemistry C
in coordinates shear rate – viscosity (η) are presented in Figure 5e. One can see that both materials
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DOI: 10.1039/D0TC04536E
are Newtonian liquids, since the viscosity is independent of the shear rate. The viscosity of TPPT-
Si-BuOct at all temperatures was found to be 5.84 Pa·s which is almost three times higher as
compared to that of TPPT-Si-Hex (1.94 Pa·s) due to a longer and branched alkyls of the former.
Moreover, using the temperature-viscosity plots for the molecules (Fig. S70) and Arrhenius
equation, the activation energy of viscous flow (Ea) was evaluated (Table 1). The fact of slightly
higher Ea value for TPPT-Si-BuOct (48 kJ·mol-1) as compared to TPPT-Si-Hex (45 kJ·mol-1)
confirms the abovementioned conclusion about the influence of branched alkyls on the viscosity
increase.
Therefore by introducing various end groups the intermolecular interactions for
oligo(phenylene-thiophene)s can be finely tuned thus regulating their aggregate state, thermal
stability, phase behaviour and viscosity.
2.3 Optical properties.
The optical properties of novel luminophores were studied by absorption and luminescent
spectroscopy in both the diluted THF solutions and in the neat state (Table 2, Fig. 6). For a more
detailed comparison, average values were calculated for all main optical parameters in solution
(see TPPT-SGmean).
Deviations between values of absorption and photoluminescent parameters of the
molecules with different substituents in their diluted THF solutions are within the measurement
error (Fig. 6a, Table 2). The absorption and luminescence spectra contour for THF solutions
remains almost unchanged when replacing one terminal substituent with the other one having
different architecture or type. The photoluminescence quantum yield (PLQY) in solution is very
high (up to 95%) and similar for all the molecules. Thus, the type of SG does not have any
noticeable effect on the optical and photophysical properties of their solutions.
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Page 10 of 16
Table 2. Optical and photophysical properties of TTPT-based oligomers with various SGs in
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DOI: 10.1039/D0TC04536E
THF solution and neat state.
solution in THF
neat state
Compound
λabs,
εmax
,
ν1/2,
λlum,
nm
PLQY,
%
λabs,
nm
-
ν1/2,
cm-1
-
λlum,
nm
404/424
PLQY,
nm L(mol·cm)-1 cm-1
%
63
37
61
24
11
13
11
3
TPPT-Si-Me
340
55000
68000
59000
65000
54300
66000
66000
59000
61000
4600 386/407
4550 389/409
4600 388/409
4650 391/411
4600 387/406
4700 388/408
4650 389/409
4500 388/408
4700 388/408
95
94
91
94
90
93
91
90
TPPT-Si-Hex 342
TPPT-Si-MeHex 342
TPPT-Si-BuOct 344
344 4600 395/416
339 5500 398/419
344 4600 394/414
–
–
335 10000 430/453
342 7600 419/440
334 5900 422/443
TPPT-Hex
341
˃10000 431/455
˃10000 431/457
TPPT-Dodec 341
TPPT-BuOct 342
TPPT-Bu-3SiO 341
TPPT-Bu-6SiO 342
95
5
average values of the measured parameters
342
±2
61500
±6500
4600 389/409
±100 ±2
93
±2
*
TPPT-SGmean
–
–
–
–
Notes: * TPPT-SG acronym is used to illustrate average values of optical parameters in THF solution; λabs
is an absorption maximum; εmax is a molar extinction coefficient; λabs is a photoluminescence spectrum
maximum; ν1/2 is a half-width of the absorption spectrum; PLQY is a photoluminescence quantum yield.
Figure 6. Normalized absorption and photoluminescence spectra of TPPT-based compounds with
various SGs in THF solutions (a) and in neat state of molecules with alkylsilyl (b), alkyl (c) and
oligodimethylsiloxane (d) SGs in comparison to TPPT-SGmean (average optical spectrum in THF
solution).
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DOI: 10.1039/D0TC04536E
However, optical properties in the neat state of TPPT-based molecules significantly depend
on the type of SG (Table 2, Fig.6b-d).
Considering molecules with long alkylsilyl SGs (TPPT-Si-Hex, TPPT-Si-MeHex,
TPPT-Si-BuOc) one can see that both their absorption (λabs) and photoluminescence (λlum)

maximum as well as the half-width of the absorption spectrum ( 1/ 2 ) are very similar to those
of the average spectrum and characteristics in solution (compare to TPPT-SGmean) (Fig. 6b). Since
according to DSC the longer alkylsilyl substituents prevent the formation of a crystalline phase, it
leads to an increased amount of molecules in the amorphous phase. As a result, the absorption and
luminescence spectra in the neat state of TPPT-Si-Hex, TPPT-Si-MeHex and TPPT-Si-BuOct
almost coincide with the spectra of their solutions. The slight bathochromic shift of their
luminescence spectrum in the neat state compared to the solution can be explained by changing of
the environment field. The slightly red-shifted and broadened absorption and luminescence spectra
of TPPT-Si-MeHex indicate a stronger π-π interaction between molecules as compared to TPPT-
Si-Hex and TPPT-Si-BuOct, which is in a good agreement with the DSC data described above.
It was not possible to obtain an absorption spectrum for TPPT-Si-Me in the neat state, due to a
strong reflection and scattering from its polycrystalline film. Besides the fact that luminescence
spectrum of the most crystalline TPPT-Si-Me material is pronouncedly red-shifted, the above
described patterns in the luminescence spectrum are typical for this molecule as well (Figure 6).
PLQY values for all neat oligomers with alkylsilyl groups were found to be relatively high
and ranged from 24% to 63%. The PLQY decrease in the neat state as compared to solution and
the difference of photoluminescence efficiency for the series of molecules with Alk-Si SGs can be
explained by both a different environment field and an incomplete correspondence of the molecule
conformation in the bulk to that in the solution. The latter means that molecules in the neat state
can have different torsion angles between adjacent units in TPPT backbone as compared to the
solution, which in turn leads to some changes in the structure of electronic transitions and affects
the luminescence efficiency. It is also known that bulky substituents affect the torsion angles in
TPPT molecules [46], thus one can expect that the torsion angles in the neat state can be slightly
varied within the series of molecules with Alk-Si SGs having different bulkiness of terminal
substituents.
In contrast, optical properties in the neat state of the molecules with either alkyl or
alkylsiloxane SGs differ significantly from those in the solution (Fig.6c and Fig.6d). Comparing
to solution, their absorption spectra were found to be broadened and even unstructured in the case
of molecules with alkyl SGs, whereas their photoluminescence spectra remained relatively
structured but red shifted by 2000 - 2600 cm-1. Among the molecules with alkyl SGs, a more
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defined spectrum structure appeared in the absorption of TPPT-BuOct, which indicates a better
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DOI: 10.1039/D0TC04536E
shielding ability of branched 2-butyl-octyl groups from π-π interactions in the bulk as compared
to linear hexyl and dodecyl ones. In addition, both a more defined absorption spectrum structure
and slight hypochromic shift of the luminescence spectra for TPPT-Bu-3SiO and TPPT-Bu-6SiO
molecules with alkyl-oligodimethylsiloxane SGs indicate a higher solubilizing ability of these
groups as compared to the alkyl one, which is also in a good agreement with the DSC data. PLQY
for TPPT molecules with alkyl and alkylsiloxane SGs in the neat state was found to be quite low:
11-13% and 3-5%, respectively.
One can see that the highest photoluminescence efficiency in the neat state is observed for
the series of molecules with alkylsilyl groups. It is known that in the TPPT fragment the torsion
angle between the thiophene and phenyl rings affects the optical properties [46]. Thus, as a possible
reason to explain higher PLQY of molecules with alkylsilyl SGs can be assumed that these
compounds during the transition from the solution to a solid state may maintain the conformation
of molecules close to that in solution. This assumption needs further confirmation by X-ray
analysis, which is however quite challenging due to the predominantly amorphous character of
these materials.
Conclusions
To conclude, a series of luminescent oligomers based on 4,4'-bis(2-thienyl)biphenyl π-conjugated
fragment with various type of SGs have been synthesized. The influence of the type, length and
branching degree of SGs on thermal stability, aggregate state, phase behaviour, rheology and
optical properties of the oligomers were comprehensively studied. It was revealed that the ability
to suppress intermolecular interactions in the neat state increases in the row Alk <Alk-SiO < Alk-
Si. As a result, while molecules with alkyl SGs (TPPT-Hex, TPPT-Dodec, TPPT-BuOct) remain
a pronounced crystallinity, molecules with alkyl-oligodimethylsiloxane SGs (TPPT-Bu-3SiO,
TPPT-Bu-6SiO) demonstrate low glass transition temperatures (up to -110 °C) and lower
temperature and enthalpy of mesophase transitions. The most interesting results were found for
the series of oligomers with alkylsilyl SGs (TPPT-Si-Me, TPPT-Si-Hex, TPPT-Si-Dodec,
TPPT-Si-BuOct) since they not only have similar to their solutions absorption and luminescent
spectra and much higher PLQY (up to ≈60%) in the neat state, but also some of them (TPPT-Si-
Hex and TPPT-Si-BuOct) are liquids with a low viscosity (up to 1.98 Pa·s). One more benefit of
alkylsilyl solubilizing groups is that they slightly increase the thermal stability of the oligomers.
Thus, the results obtained clearly indicate that alkylsilyl groups are a powerful tool for liquefaction
of chromophores with a highly crystalline and rigid conjugated core. In this work, for the first time
liquid luminescent oligo(phenylene-thiophene)s were reported and solubilizing capability of some
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of the most perspective types of SG was comprehensively investigated and compared to each other.
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DOI: 10.1039/D0TC04536E
The results can be used as a guideline for researchers to choose appropriate SG in order to design
luminescent materials based on conjugated oligomers with a desirable and controllable phase
behaviour, solubility and optical properties in the neat state.
Acknowledgments. The reported study was funded by the Russian Foundation for Basic Research
according to research project no. 18-33-20224. NMR and UV-VIS absorption and luminescence
spectra were recorded using the equipment of Collaborative Access Center ‘Center for Polymer
Research’ of Enikolopov Institute of Synthetic Polymeric Materials of the Russian Academy of
Sciences with the financial support from the Ministry of Science and Higher Education of the
Russian Federation. Authors acknowledge P. V. Dmitryakov for the help with DSC / TGA
experiments and Dr. V.G. Vasilyev for rheology measurements.
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DOI: 10.1039/D0TC04536E
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