Impact of terminal substituents on the electronic, vibrational and optical properties of thiophene-phenylene co-oligomers

Article abstract of DOI:10.1039/c9cp00910h

Owing to the combination of efficient charge transport and bright luminescence, thiophene-phenylene co-oligomers (TPCOs) are promising materials for organic light-emitting devices such as diodes, transistors and lasers. The synthetic flexibility of TPCOs enables facile tuning of their properties. In this study, we address the effect of various electron-donating and electron-withdrawing symmetric terminal substituents (fluorine, methyl, trifluoromethyl, methoxy, tert-butyl, and trimethylsilyl) on frontier orbitals, charge distribution, static polarizabilities, molecular vibrations, bandgaps and photoluminescence quantum yields of 5,5′-diphenyl-2,2′-bithiophene (PTTP). By combining DFT calculations with cyclic voltammetry and absorption, photoluminescence, and Raman spectroscopies, we show that symmetric terminal substitution tunes the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energies of TPCOs within a range of ～0.7 eV, shifts the frequencies of the vibrational modes associated with the phenyl rings, changes the photoluminescence quantum yield by about two-fold and slightly changes the bandgap by ～0.1 eV. We demonstrate that these effects are governed by two factors: the Hammet constant of the substituents and their involvement in the π-conjugation/hyperconjugation described by the effective conjugation length of the substituted oligomer. A detailed picture underlying the effect of the terminal substituents on the electronic, vibrational and optical properties of TPCOs is presented. Overall, the unraveled relationships between the structure and the properties of the substituted PTTPs should facilitate a rational design of π-conjugated (co-)oligomers for efficient organic optoelectronic devices.

Full text of DOI:10.1039/c9cp00910h

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K. Nuraliev, E. Feldman, D. Maslennikov, O. Borshchev, M. S. Skorotetcky, N. M. Surin, M. S. Kazantsev, S.
Ponomarenko and D. Paraschuk, Phys. Chem. Chem. Phys., 2019, DOI: 10.1039/C9CP00910H.
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Page 1 of 17
Physical Chemistry Chemical Physics
Impact of terminal substituents on electronic, vibrational and optical
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DOI: 10.1039/C9CP00910H
properties of thiophene-phenylene co-oligomers
Andrey Yu. Sosorev1,2, Muzaffar K. Nuraliev1, Elizaveta V. Feldman1, Dmitry R. Maslennikov1,2, Oleg V.
Borshchev3, Maxim S. Skorotetcky3, Nikolay M. Surin3, Maxim S. Kazantsev4,5, Sergei A. Ponomarenko3,6*
Dmitry Yu. Paraschuk1**
,
1Faculty of Physics and International Laser Center, Lomonosov Moscow State University, Leninskie Gory
1/62, Moscow 119991, Russia
2Institute of Spectroscopy, Russian Academy of Sciences, Fizicheskaya 5, Troitsk, Moscow 108840, Russia
3Enikolopov Institute of Synthetic Polymeric Materials, Russian Academy of Science, Profsoyuznaya 70,
Moscow 117393, Russia
4N.N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Lavrentieva 9, Novosibirsk 630090, Russia
5Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russia
6Department of Chemistry, Lomonosov Moscow State University, Leninskie Gory 1/3, Moscow 119991,
Russia
Abstract
Owing to combination of efficient charge transport and bright luminescence, thiophene-phenylene co-
oligomers (TPCOs) are promising materials for organic light-emitting devices such as diodes, transistors
and lasers. The synthetic flexibility of TPCOs enables facile tuning of their properties. In this study, we
address the effect of various electron-donating and electron-withdrawing symmetric terminal
substituents (fluorine, methyl, trifluoromethyl, methoxy, tert-butyl, and trimethylsilyl) on frontier
orbitals, charge distribution, static polarizabilities, molecular vibrations, bandgaps and
photoluminescence quantum yields of 5,5’-diphenyl-2,2’-bithiopene (PTTP). By combining DFT
calculations with cyclic voltammetry, absorption, photoluminescence, and Raman spectroscopies, we
show that symmetric terminal substitution tunes the highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) energies of the TPCO within the range of ~0.7 eV, shifts the
frequencies of the vibrational modes associated with the phenyl rings, changes the photoluminescence
quantum yield in about twice and slightly changes the bandgap by ~0.1 eV. We demonstrate that these
effects are governed by two factors: the Hammet constant of the substituents and their involvement into
the π-conjugation/hyperconjugation described by the effective conjugation length of the substituted
oligomer. The microscopic picture underlying the effect of terminal substituents on the electronic,
vibrational and optical properties of TPCOs is presented. Overall, the unraveled relationships between the
structure and the properties of the substituted PTTPs should facilitate the rational design of π-conjugated
(co-)oligomers for efficient organic optoelectronic devices.
1. Introduction
Organic optoelectronic devices, e.g., organic light-emitting diodes (OLEDs), organic light-emitting
transistors (OLETs) and electric-driven lasers, require materials with high charge-carrier mobility and
luminescence efficiency. However, combination of these two properties is rarely observed. Thiophene-
phenylene co-oligomers (TPCOs) constitute one of a few lucky exceptions — they show
photoluminescence quantum yield (PLQY) up to ~80-90 % in the solid state1,2 approaching that in solution
along with considerable charge-carrier mobility.1,3 Their benefits are also facile crystallization from either
vapor or solution phase, with large4 and high-quality5 single crystals as an outcome. Various organic
1

Physical Chemistry Chemical Physics
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(opto)electronic devices based on TPCOs, including light-emitting diodes (OLEDs),6 solar cells,7 field-effect
transistors (OFETs),3 light-emitting transistors (OLETs)1,2 and lasers8 have been demonstrated. Moreover,
View Article Online
bendable9 and monolayer10,11 devices with TPCO-based active layers have recently DbOeIe: 1n0.1r0e3p9/oCr9tCePd0.09T1h0He
synthetical flexibility of TPCOs enables their prompt chemical modification, e.g., introduction of electron-
donating or electron-withdrawing substituents.3 For several oligomer and co-oligomer series, e.g., for
distyrylbenzenes12 and furan-phenylene co-oligomers,9,13 it was shown that substitution can dramatically
alter the properties of both the isolated molecules and the corresponding solid-state phase. Importantly,
many of the structural, electronic, and optical properties of organic semiconductors are directly related
to those of the molecules constituting them. For instance, the emission spectra and PLQYs of the ultrapure
TPCO crystals14 correspond well to those of the isolated TPCOs (in solution). Though a large number of
TPCOs have been synthesized and studied to date,3, 10, 15, 16 the impact of their molecular structure on the
electronic and optical properties has not been systematically investigated yet. Such a study is highly
desired since it can reveal important structure-property relationships for the rational design of TPCOs and
related materials for organic optoelectronics.
Combination of experiment and quantum-chemical modeling is the most appropriate way for
systematic studies of the organic semiconductors at the molecular level.17,18 Modeling provides excessive
information about various molecular properties; however, it is frequently model-dependent and hence is
not always reliable.19, 20 On the contrary, experiment often provides very limited and indirect information;
nevertheless, the experimental data can verify the modeling results. Thus, combination of theory and
experiment is highly desired to establish the structure-properties relations in organic semiconductor
materials.
In this work, we perform a combined experimental and theoretical study of TPCO series with a
5,5’-diphenyl-2,2’-bithiopene (PTTP) π-conjugated core and various symmetric terminal substituents
including electron-donating or electron-withdrawing ones (Fig. 1): fluorine (F), methyl (Me),
trifluoromethyl (TFM), methoxy (MeO), tert-butyl (tertBu), and trimethylsilyl (TMS). The impact of these
substituents on different molecular properties including the frontier orbital energies, vibrational spectra,
bandgaps and PLQY is addressed with the use of cyclic voltammetry (CV), optical absorption,
photoluminescence (PL), and Raman spectroscopies combined with density functional theory (DFT)
calculations. The results obtained allowed us to draw a microscopic picture of the effect of terminal
substituents on the electronic, vibrational and optical properties of TPCOs, which is important for the
rational design of TPCOs and other linear π-conjugated systems.
2. Methods
2.1. Materials
Chemical structures of the TPCOs investigated are shown in Fig. 1; for Raman measurements, para-
quaterphenyl (4P) and 2,2’:5’,2’’:5’’,2’’’-quaterthiophene (4T) were also used. Purification of the target
compounds was made by a vacuum sublimation. Gel permission chromatography (GPC) analysis showed
that all of them are monodisperse individual compounds. The purity and molecular structures of all
1
synthesized oligomers were confirmed by H NMR spectroscopy, GPC and elemental analysis; see
Supporting information (SI) for details. 13C NMR measurements were performed for tertBu-PTTP-tertBu
only because of its good solubility in CDCl3.
2.2. Experimental details
Cyclic voltammetry (CV) measurements were performed in CH2Cl2 solution (concentration of ~0.1 g/l) by
using a computer controlled potentiostat (P-8nano, Elins) in combination with three-electrode cell
(Gamry); 0.1 M tetrabutylammonium hexafluorophosphate was used as a supporting electrolyte. The Pt,
2

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Physical Chemistry Chemical Physics
Pt wire and Ag/AgCl were used as working, counter and reference electrodes, respectively. The
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measurements were standardized by measuring the redox potential of ferrocene afDteOrI: 1e0a.1c0h39c/Co9mCPp0o09u1n0Hd
analysis. The HOMO energy levels were estimated using the onset oxidation potentials according to
equation: Ehomo= -(Eoxonset+4.8) (eV). Raman measurements in oligomer powders were conducted using a
Raman microscope (inVia, Renishaw) with an excitation wavelength of 633 nm provided by a He-Ne laser.
Absorption spectra were recorded using a spectrophotometer (UV-2501PC, Shimadzu) in a standard 10-
mm-thick photometric quartz cuvette filled with THF solutions with the concentrations of 10-5 M. A
scanning spectrofluorimeter (ALS01M)21 was used for the registration of PL spectra. The PL excitation
wavelengths were set to the absorption maxima. PL measurements were performed in 90°-geometry for
several optical densities in the range from 0.06 to 0.12 absorbance units in a 10-mm-thick quartz cuvette.
The PLQY was measured by comparing the integral PL intensity of 10-6 M diluted solutions of TPCOs in
tetrahydrofuran (THF) with the integral PL intensity of the standard as described elsewhere.22 As a
standard in measuring the PLQY (Ф), a diluted solution of 1,4-bis(5-phenyloxazol-2-yl)benzene (POPOP) in
cyclohexane (Ф=1) was used.
2.3. Computational details
All DFT calculations were performed using GAMESS package23,24 at B3LYP/6-31G(d,p) level, which provides
a reasonable tradeoff between the calculation time and accuracy. HOMO/LUMO energies, optical
bandgaps and oscillator strengths were also calculated using 6-311G(d,p) basis set; the comparison of the
results for the two basis sets is given in SI, Section S2. The geometry of the isolated molecules was fully
optimized, and its correspondence to the previous calculations was checked (see Fig. S2).25 The frequency
analysis was used to check the absence of imaginary frequencies in the optimized structures. The electron
density at various atoms for HOMO/LUMO was estimated as the sum of the squared coefficients for all
the basis vectors centered at these atoms in the HOMO/LUMO wavefunctions. Raman spectra were
calculated in the off-resonance regime from the static polarizabilities. Time-dependent DFT (TDDFT)
calculations implemented in GAMESS package by Chiba et al.26 were performed to address the optical
properties of the studied molecules:27 energies of the lowest excited states (optical bandgaps) and
oscillator strengths for absorption and luminescence.
3. Results and discussion
3.1. Synthesis
H,
PTTP
F,
F-PTTP-F
CH3,
Me-PTTP-Me
R
S
CF3,
TFM-PTTP-TFM
MeO-PTTP-OMe
R =
S
OCH3,
R
C(CH3)3, tertBu-PTTP-tertBu
Si(CH3)3, TMS-PTTP-TMS
Fig. 1. Chemical structure of the TPCOs investigated.
For preparation of 5,5’-diphenyl-2,2’-bithiophene (PTTP), 5,5′-bis[4-methylphenyl]-2,2′-bithiophene (Me-
PTTP-Me), 5,5′-bis[4-methoxylphenyl]-2,2′-bithiophene (MeO-PTTP-OMe) and 5,5′-bis(4-tert-
butylphenyl)-2,2′-bithiophene (tertBu-PTTP-tertBu) novel synthetic schemes were elaborated. The
oligomers with methyl (Me-PTTP-Me) and methoxyl terminal groups (MeO-PTTP-OMe) were prepared by
the Kumada coupling reaction of 5,5'-dibromo-2,2'-bithiophene with p-tolylmagnesium bromide or
p-anisole magnesium bromide, respectively (Fig. S1a). The reaction yield was 83% for Me-PTTP-Me and
86% for MeO-PTTP-OMe (as determined by GPC analysis), and the isolated yield was 63% and 58%,
3

Physical Chemistry Chemical Physics
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respectively. In both cases, purification was carried out by column chromatography on silica gel followed
View Article Online
DOI: 10.1039/C9CP00910H
by vacuum sublimation.
PTTP and tertBu-PTTP-tertBu were prepared in two steps (Fig. S1b). First, 2-phenylthiophene (PT)
and 2-(4-tert-butylphenyl)thiophene (tertBu-PT) were synthesized by the Kumada reaction between
2-thienylmagnesium bromide, prepared in situ from 2-bromothiophene, magnesium, and bromobenzene
or 1-bromo-4-tert-butylbenzene, respectively. Second, PTTP and tertBu-PTTP-tertBu were obtained by
lithiation of PT or tertBu-PT followed by oxidative coupling reaction in presence of CuCl2 (Fig. S1b). These
reactions proceeded with high efficiency, with the yields between 78% and 84% (as determined by GPC
analysis of the reaction mixture). The synthetic details are given in the SI.
5,5′-bis[4-fluorophenyl]-2,2′-bithiophene (F-PTTP-F), 5,5′-Bis[4-(trifluoromethyl)phenyl]-2,2′-
bithiophene (TFM-PTTP-TFM) and 5,5′-bis(4-trimethylsilyl-phenyl)-2,2′-bithiophene (TMS-PTTP-TMS)
were synthesized according to the methods described previously.5 4P and 4T were synthesized according
to the methods described in Refs. [28] and [29], respectively.
3.2. Electronic properties
3.2.1. Frontier orbitals
Fig. 2 collates the HOMO energies for the investigated TPCOs estimated from CV analysis (see SI, Section
S3, for details) and calculated by DFT at B3LYP/6-31G(d,p) level. The experimental and theoretical data,
except for fluorine-containing substituents (F and TFM), are in good agreement considering a systematic
bias of ~0.2 eV for calculated HOMO energies, which can be attributed to the solvent effect: the
calculations were performed for isolated molecules, while the experimental data were recorded for
solutions. To check this hypothesis, we also performed the calculations in solution (CH2Cl2) using a
polarizable continuum model (SI, Section S2). In the latter case, for all the TPCOs except for TFM-PTTP-
TFM, the HOMO energies indeed decreased by ~0.1 eV as compared to the isolated molecules (Fig. S4a).
However, the observed differences between the calculated and experimental HOMO energies became
not systematic, which probably should be assigned to oversimplified treatment of the solvent effect in the
polarizable continuum model. Because of this, we will use below the calculations for isolated molecules,
which involve less approximations. The second source of the difference between the theoretical and
experimental HOMO energies is in the moderate size of the basis set used, 6-31G(d,p). Using the extended
basis set (6-311G(d,p)) provided the HOMO energies closer to the experimental data, as shown in SI,
Section S2. However, the vibrational analysis with this extended basis set yielded imaginary frequencies,
which we could not eliminate. Since vibrational properties constitute an essential part of this study, we
will use below the data calculated with the 6-31G(d,p) basis set. Comparison of the molecular properties
calculated with the 6-31G(d,p) and 6-311G(d,p) basis sets (SI, Section S2) shows that the trends discussed
below are similar for both basis sets. Thus, the extension of the basis set does not affect the conclusions
of this study.
For the fluorine-containing substituents (F and TFM), the relative HOMO energies as compared to
PTTP are in less agreement with the experiment than for the other studied compounds (considering the
abovementioned systematic bias of ~0.2 eV). Specifically, F increases the experimental HOMO energy as
compared to the unsubstituted TPCO (PTTP) by ~0.1 eV but should decrease it according to the
calculations. For TFM, the experimental decrease of the HOMO energy as compared to PTTP is reproduced
by the calculations; however, the calculated decrease is stronger and amounts to 0.43 eV (0.1 eV in the
experiment). Considering well-known strong electron-withdrawing properties of F- and TFM-substituents,
which should decrease the HOMO energy,30 we assume that the deviation of the CV data from the
calculated HOMO energies is because of the essentially different charge distribution patterns for the
fluorine-containing TPCOs as compared to the other TPCOs studied: F and TFM groups bear large negative
electrical charges (Fig. 3c) in contrast to positive charges at the substituents for the other investigated
4

Page 5 of 17
Physical Chemistry Chemical Physics
oligomers. Therefore, F-PTTP-F and TFM-PTTP-TFM have a larger quadrupole moment, which could
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strongly modify the arrangement of solvent molecular dipoles (CH2Cl2). As a result, the solvent
DOI: 10.1039/C9CP00910H
reorganization energies for F-PTTP-F and TFM-PTTP-TFM (and their corresponding cations) will
substantially differ from those of the other TPCOs studied. These changes could facilitate electron
detachment from the molecule. However, detailed investigation of the solvation effects on the CV data is
beyond the scope of this work.
As Fig. 2a shows, TFM results in the largest decrease of both the HOMO and LUMO energies (0.43
eV in theory, 0.1 eV in experiment), whereas MeO results in the largest increase of these energies (0.27
eV in calculations, 0.28 eV in experiment). Me and tertBu substituents also increase the energies of the
frontier orbitals, although to a lesser extent (0.1 eV in theory, ~0.2 eV in experiment) than MeO group.
From the calculations, TMS has a negligible effect on the HOMO and LUMO energies, while F slightly
decreases both HOMO and LUMO energies. Fig. 2b shows that the calculated HOMO values are in
excellent correspondence with the Hammet constants of the substituents,30 σ, which describe the ability
of the group to donate/withdraw the electron density to/from the conjugated core. The similar
correspondence is observed for LUMO (SI, Fig. S4b). This means that the HOMO and LUMO energies of a
terminal-substituted PTTP can be rather accurately predicted just from the σ values of the substituents.
Fig. 2c-h presents the HOMO and LUMO patterns, as well as the histograms of the estimated HOMO
electron densities at various atoms, for PTTP and its derivatives showing the strongest effect of the
substituents – MeO-PTTP-MeO and TFM-PTTP-TFM. It clearly shows that both the HOMO and LUMO are
concentrated at the PTTP core, mostly at the thiophene rings. Specifically, outermost -carbon atoms of
the thiophene rings (labeled as 2 and 5 in Fig. 2c) bear the largest π-electron density (HOMO and LUMO).
Among the phenyl ring atoms, the largest π-electron density is located at the terminal atom (labeled as 9
in Fig. 2c). As a result of hyperconjugation,31 the substituents bear small π-electron (HOMO and LUMO)
density, which explains the fact that the substituents slightly affect the HOMO/LUMO and optical
bandgaps (see Fig. 7b). Although the substituents (except for MeO) are formally not involved into the π-
conjugated system, the small π-electron density at the substituents for the oligomers investigated affects
significantly the radiative decay rate and PLQY as shown below.
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a)
b)
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DOI: 10.1039/C9CP00910H
-1
-2
-3
-4
-5
MeO
Me
LUMO
-4.8
-5.0
-5.2
-5.4
tertBu
H
TMS
F
HOMO
TFM
e
u
F
S
H
O
M
-0.2
0.0
0.2
0.4
0.6
B
e
M
F
t
M
r
T
T
M
e
t
Hammet constant
Fig. 2. Effect of symmetric terminal substituents of PTTP on the frontier orbitals. (a) Calculated HOMO (red)
and LUMO (blue) energies and experimental HOMO energies (black). Dashed lines indicate the calculated
HOMO and LUMO levels of unsubstituted PTTP. The TPCOs in the investigated series are arranged in order of
increasing the molecular length, from the shortest (PTTP) to the longest (TMS-PTTP-TMS). (b) Correlation
between the calculated HOMO energy and Hammet constant of the substituent (for para-substitution, taken
from Ref. [30]). The line is a linear fit. (c-h) HOMO patterns (c,e,g) and estimated electron density at HOMO
(d,f,h) for PTTP, MeO-PTTP-MeO and TFM-PTTP-TFM. The bars indicate the HOMO electron density. Atoms
belonging to thiophene rings, phenyl (phenylene for the substituted PTTPs) rings, and substituents are
highlighted with blue, green, and red frames, respectively.
6

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Physical Chemistry Chemical Physics
3.2.2. Charge distribution, conjugation length and molecular polarizability
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DOI: 10.1039/C9CP00910H
a)
4.2
4.0
3.8
b)
120
100
80
60
40
20
0
H
F
Me
TFM
MeO
tertBu
TMS
Fig. 3. Comparison of charge distribution in Fig. 4. Calculated conjugation lengths (a) and static
unsubstituted, MeO, and TFM substituted electronic polarizabilities along the longest molecular
PTTP. (a) Mulliken charges for PTTP. (b,c) axis (b) for the TPCOs investigated.
Difference in Mulliken charges for MeO-
PTTP-MeO (b) and TFM-PTTP-TFM (c) as
compared to PTTP. Red (blue) color denotes
positive (negative) Mulliken charge. Note
that color scheme is different for panels (a)
and (b,c): dark blue (red) corresponds to
-0.25e (+0.25e) at (a) but to -0.025e (+0.025e)
at (b) and (c). Black labels at (b,c) denote the
difference between the Mulliken charge for
substituted and unsubstituted PTTPs; grey
labels denote the Mulliken charges for
substituents atoms.
To get a deeper understanding of the effect of substituents on the electronic properties of substituted
PTTP, we analyzed changes in the charge distribution, effective conjugation length of the molecule, and
molecular polarizability with substitution. Fig. 3 compares the Mulliken charges for PTTP and its
substituted counterparts. In PTTP, the hydrogen and sulfur atoms bear positive charges, while the carbon
atoms are negatively charged. As follows from Fig. 3b,c, terminal substitution affects mostly the charge
density at the aromatic rings adjacent to the substituents (phenylenes) and has a negligible impact on the
more distant aromatic rings (thiophenes). Fig. 3b also shows that MeO substitution provides an extra
negative charge to the conjugated core so that the electron affinity and ionization potential decrease, and
the frontier orbital energies increase (Fig. 2a). On the contrary, TFM substitution (Fig. 3c) results in extra
positive charge on the conjugated core – this should increase the electron affinity and ionization potential,
i.e., decrease the HOMO and LUMO energies as observed in Fig. 2a.
To quantify the involvement of the substituents into π-conjugation/hyperconjugation, we
c2r2
c2
, where ci are the
i
estimated the conjugation lengths for HOMO and LUMO as
lH ,L



i
i
i
i
coefficients of the i-th atom wavefunctions in the corresponding molecular orbitals (see Fig. S5b), and
then obtained the effective conjugation length as l  lH lL . Fig. 4a presents l for the investigated
7

Physical Chemistry Chemical Physics
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TPCOs. It increases from PTTP to TMS-PTTP-TMS, i.e., with the increase of the substituent size and hence
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the molecule length. For MeO-PTTP-MeO, l is larger than for all the other investigated TPCOs, which
DOI: 10.1039/C9CP00910H
should be attributed to a significant involvement of the oxygen atoms of MeO groups into π-conjugation
(see Fig. 2e,f and Fig. S5a).
Fig. 4b shows the calculated static electronic polarizabilities of PTTP and its substituted
counterparts. The polarizability gradually increases within the investigated series from PTTP to TMS-PTTP-
TMS. This fact can be naturally assigned to the l increase. Noteworthily, in contrast to l, the polarizability
does not reach maximum for MeO-PTTP-MeO, probably because of an additional contribution in the
polarizability of the bulky tertBu and TMS groups to the overall polarizability of the molecule. For TPCOs
with the similar sizes of the substituents – PTTP and F-PTTP-F, as well as Me-PTTP-Me and TFM-PTTP-TFM,
– the polarizabilities are nearly equal. The variation of the polarizability in the TPCOs series affects the
radiative lifetime as described below.
3.3. Vibrational properties
Changes in the vibrational spectra with substitution can elucidate changes in the molecular geometry and
π-electron density distribution, since the frequencies of the vibrational modes are determined by the
force constants sensitive to the molecular geometry and charge density, whereas the Raman intensities
are determined by the vibrational modulation of the π-conjugated electronic system.32, 33 In this section,
first of all, we compare Raman spectra of PTTP and oligomers comprised of either only thiophene or only
phenyl (phenylene) aromatic rings – quaterphenyl (4P) and quaterthiophene (4T), and distinguish the
vibrational modes with dominant motion of thiophene rings from that of phenyl (phenylene) ones. Then,
we analyze the effect of the substituents on the Raman frequencies/intensities.
3.3.1. Thiophene- and phenyl-associated vibrational modes
Fig. 5 collates the experimental and calculated Raman spectra for 4P, 4T, and PTTP in the frequency range
of 900-1700 cm-1, which contains the most part of the intensive Raman modes. The experimental and
theoretical spectra are in good agreement strongly supporting the reliability of the calculated molecular
geometries and vibrational modes. To match the theoretical and experimental spectra, the calculated
vibrational frequencies were multiplied by a factor of 0.97. Such vibrational frequency scaling is a common
practice for DFT calculations, and the scaling factor chosen is close to the values of 0.961-0.969
recommended for the DFT functional used in this study (B3LYP).34 The scaled calculated vibrational
frequencies will be used below.
As follows from Fig. 5, seven modes are the most intensive in the PTTP Raman spectrum. Among
them, positions and relative intensities of the experimental (calculated) bands at ~1060 (1052) and ~1462
(1463) cm-1 are similar to the bands of 4T, whereas those at ~1000(985) and 1600 (1606) cm-1 are close to
the bands of 4P. The atomic vibrational displacements (vibrational pattern) shown in Fig. S9 indicate that
all these modes correspond to the collective motion of atoms in the conjugated molecular core. These
displacements confirm that the bands at ~1060 (1052) and ~1462 (1463) cm-1 of PTTP are indeed
associated mainly with vibration of the thiophene rings (similar to 4T), whereas bands at ~1000 (985) and
1600 (1606) cm-1 are associated with vibration of the phenyl moieties (similar to 4P). On the other hand,
some PTTP modes, e.g., at ~1440 (1441) and 1500 (1494) cm-1, incorporate comparable contributions from
the vibrations of both phenyl and thiophene moieties.
To quantify the contributions of vibrations of thiophene and phenyl moieties to the i-th vibrational
mode of PTTP, we introduce the quantities
8

Page 9 of 17
Physical Chemistry Chemical Physics
mjr2
mjr2


ij
ij
View Article Online
j,thiophene
j, phenyl
DOI: 10.1039/C9CP00910H
T 
P 
,
,
(1)
i
i
m r2
m r2


j
ij
j
ij
j,all
j,all
where mj is the mass of atom j and Δrij is the vibrational displacement of this atom for the studied
vibrational mode. Summation in the numerator of Ti and Pi runs over carbon and sulfur atoms in thiophene
and phenyl rings, respectively; summation in the denominators runs over all the carbon and sulfur atoms
in the molecule. We omitted summation over hydrogens as they are practically not involved in the π-
conjugation/hyperconjugation. The larger the Ti (Pi) value, the larger the contribution from the atoms of
the thiophene (phenyl) rings to the given vibrational mode. We will refer to the modes with dominating
contribution of thiophene atoms motion (large Ti) as to “thiophene modes”, and to that with dominating
contribution of phenyl atoms motion (large Pi) as to “phenyl modes”. The calculated values of Ti and Pi are
shown in Fig. 5 as blue and red bars, respectively.
1
a) 4P
0
1
b) 4T
0
1
c) PTTP
0
1000
1200
1400
Raman shift, cm-1
1600
Fig. 5. Experimental (line) and calculated (bars) Raman spectra of 4P (a), 4T (b), PTTP (c). The
experimental spectra were recorded for polycrystalline powders, the calculations were performed for
isolated molecules. The calculated intensity bars for PTTP are divided to show the contribution of
thiophene (blue) and phenyl (red) atoms motions to the corresponding modes. Dashed lines are guides
to the eye, they indicate positions of the most intensive calculated Raman bands for 4P and 4T for their
comparison with PTTP.
9

Physical Chemistry Chemical Physics
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View Article Online
DOI: 10.1039/C9CP00910H
3.3.2. Impact of substituents on Raman spectra
Fig. 6 illustrates the effect of substituents on the Raman spectra of the TPCOs studied. The calculated and
experimental frequencies as well as the corresponding relative intensities are in close agreement. Most
of the PTTP modes are retained in the spectra of the substituted oligomers; however, some modes show
considerable frequency and intensity variations with the substituent.
The most pronounced changes were observed for PTTP experimental (calculated) Raman bands
at ~1000 (982), 1440 (1441), 1500 (1494) and 1600 (1606) cm-1. All these modes have large Pi and small Ti
(see Fig. 5), whereas the modes with large Ti and small Pi are much less sensitive to substituents. In the
experimental Raman spectra of the substituted PTTPs (Fig. 6a), the former two modes (~1000 and 1440
cm-1) are significantly shifted (see SI, Section S4) and nearly disappeared. Similar changes were observed
in calculations except for MeO-PTTP-MeO. These two modes in PTTP contain significant contributions
from the motion of the atoms at the terminals of PTTP, namely C and H, as shown in Fig. S10. Therefore,
the substituents restrict the motion of the terminal C atoms resulting in changes in the corresponding
vibrational patterns. The modes at 1500 (1494) and 1600 (1606) cm-1 of PTTP do not show such a
pronounced reduction in the intensity but demonstrate significant frequency shifts (Fig. 6a). Fig 6b shows
that the shift of the peak frequencies for experimental (calculated) bands at ~1500 (1494) cm-1 and 1600
(1606) cm-1 correlates well with the absolute value of the Hammet constant of the substituent, ||: these
frequencies increase for either electron-donating (Me, MeO, tertBu) or electron-withdrawing (F, TFM)
substituents. The frequency increase can be attributed to the altered charge distribution at the phenylene
moieties, which makes the structure more quinoid-like. In fact, as shown for both the strongest electron-
donating/withdrawing (MeO/TFM) substituents in Table S1, the bond lengths between atoms 7-8 and 10-
11 shorten, whereas those between atoms 8-9 and 9-10 (see Fig. 2c for atoms numbers) lengthen. In
contrast, the bond lengths within the thiophene rings are unaltered. The results obtained confirm our
suggestion that terminal substituents affect mostly the π-electron density on the adjacent phenylene
rings, while the electronic structure of the thiophene rings remains nearly unaffected.
10

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Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C9CP00910H
a)
b)
1615
mode ~1600 cm-1
TFM
1610
1605
1600
1595
TMS
Me
MeO
tertBu
F
tertBu
MeO
H
TMS
mode ~1500 cm-1
1520
1515
1510
1505
1500
TFM
Me
F
D
TFM
MeO
tertBu
F
Me
TMS
H
H
0.0 0.1 0.2 0.3 0.4 0.5 0.6
1000
1200
1400
1600
Raman shift (cm-1)
||
Fig. 6. The effect of substituents on the Raman spectra of PTTP oligomers. (a) Experimental (lines) and
calculated (bars) Raman spectra. The modes showing a pronounced frequency shift with substitution
are labeled with arrows. (b) Positions of the experimental Raman bands at ~1600 (top) and 1500 cm-1
(bottom) as a function of the absolute value of the Hammet constant, ||.
3.4. Optical properties
3.4.1. Absorption and photoluminescence spectra
Energy, eV
a)
b)
EHL
Eagbs
Elgum
3.6
3.4
3.2
3.0
2.8
2.6
2.4
1.0
1.0
0.8
0.6
0.4
0.2
0.0
3
2
1
0
0.8
0.6
0.4
0.2
0.0
H
F
MeO
350
400
450
500
e
u
F
S
H
O
M
B
e
M
F
t
M
r
T
T
M
Wavelength, nm
e
t
Fig. 7. (a) Experimental absorption and PL spectra of PTTP, F-PTTP-F and MeO-PTTP-MeO in THF
solution. (b) Calculated HOMO-LUMO gaps, EHL (red), and optical bandgaps for absorption (blue) and
luminescence (green). Dashed lines are guides to the eye showing EHL and Еg for PTTP.
11

Physical Chemistry Chemical Physics
Page 12 of 17
Fig. 7a presents the experimental absorption and PL spectra of PTTP and its substituted counterparts with
View Article Online
the widest (F) and the narrowest (MeO) bandgaps. Experimental spectra of the otherDiOnIv: 1e0s.1t0ig39a/tCe9dCPT0P09C1O0Hs
are given in Fig. S12. All the absorption (PL) spectra have very similar shapes: the absorption spectra are
unstructured, while the PL spectra show a vibrational structure with a few vibrational subbands. The
relative intensities of the subbands do not change significantly with substitution. Therefore, the nearly
identical spectral shapes for all the TPCOs studied indicate that the substituents do not considerably affect
the molecular geometry both in the ground (S0) and in the first excited (S1) states.
The substituents slightly affect the bandgap, Eg: the differences in absorption and emission
maxima positions for the investigated TPCOs amount to 20 nm (0.1 eV). This is in good agreement with
our calculations, which predict that the HOMO-LUMO gap (Fig. 2a) and Eg (Fig. 7b) are slightly affected by
the substituents. The experimental and calculated Eg values for the investigated TPCOs are in the
reasonable correspondence with each other (see Fig. S13). Some discrepancies should be attributed to
the limited accuracy of the DFT technique, which hardly exceeds 25 meV.20 The narrowest experimental
absorption and emission bandgaps within the investigated series are observed for TMS-PTTP-TMS and
MeO-PTTP-MeO (see Fig. 7 and Fig. S12), whereas PTTP and F-PTTP-F show the widest bandgaps. The DFT
results for the HOMO-LUMO gaps and optical bandgaps shown in Figs. 2a and 7b are in line with the
experimental data, except for lower calculated absorption Eg for tertBu-PTTP-tertBu and TFM-PTTP-TFM
than for MeO-PTTP-MeO. The possible reason of the latter discrepancy can be a more planar
conformation of MeO-PTTP-MeO in solution (experiment) than in vacuum (calculations); this suggestion
is in line with the fact that the calculations reproduce much narrower luminescence Eg (which is associated
with the nearly planar excited-state geometry) for this compound as compared to the other TPCOs
studied. Low Eg for MeO-PTTP-MeO and TMS-PTTP-TMS most likely stem from the noticeable involvement
of MeO and TMS groups into π-conjugation and hyperconjugation, respectively; this results in the longest
effective conjugation length, l, for these TPCOs (see Fig. 2g,h and Fig. 4a). On the contrary, F-PTTP-F and
PTTP have the shortest l values. Among the investigated TPCOs, only F substituent resulted in a blueshift
of the experimental absorption and PL spectra, which was reproduced by the calculations for absorption.
The moderate effect of substituents on Eg and the HOMO-LUMO gap (variation in the range of ~0.1-0.2
eV) can be explained by small π-electron density at the substituents as follows from Fig. 2c-h, Fig. S3 and
Fig. S5.
3.4.2. Radiative decay rates and PL quantum yields
Involvement of the substituents into a π-conjugated system of PTTP affects also the oscillator
strengths, f, of the S0→S1 (absorption) and S1→S0 (luminescence) transitions:
,
(2)
where me is the electron mass, ħ is the reduced Planck constant, and d is the corresponding transition
dipole moment. The calculated f values are shown in Fig. 8a. They are the lowest ones for the
unsubstituted and F-substituted PTTPs and the highest ones for tertBu- and TMS-substituted
counterparts. The f values increase with the molecular polarizability (Fig. 4b), which in turn increases with
the effective conjugation length of the TPCO due to the presence (albeit weak) of π-electron density at
the substituents (see Fig. 2c-h and Fig. S5). From the calculated Eg and f values, the radiative decay rates
kr were obtained according to formula:
,
(3)
12

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Physical Chemistry Chemical Physics
where ε0 is the vacuum permittivity, e is the elementary charge, and c is the speed of light. Fig. 8b presents
View Ar8ticle Online8
the calculated kr values for the investigated TPCO series. These values are in the range of 3.8·10 –5·10
DOI: 10.1039/C9CP00910H
s-1, which is in reasonable correspondence with the available experimental data: kr=4.3·108 s-1 for PTTP35
and 6.5·108 s-1 for TMS-PTTP-TMS.36 Note that higher kr for TMS-PTTP-TMS than for PTTP obtained in the
experiment is reproduced in the calculations. As follows from Fig. 8b, the substituents slightly affect the
kr values (up to 20%), which can be attributed to different involvement of the substituents into the
π-conjugated system and hence the effective conjugation length (which affects f as mentioned above).
Like the f values, the calculated kr values are the lowest ones for unsubstituted and F-substituted TPCOs
and the highest ones for tertBu- and TMS-substituted TPCOs.
PLQY, Φ, is governed by the interplay of the radiative decay quantified by kr and various non-
radiative decay pathways characterized by rate constant knr:
kr
 
.
(4)
kr  knr
The experimental Φ data for the investigated TPCOs plotted in Fig. 8b as a function of kr is well described
by Eq. (4) with the best-fit knr value (2.3±0.2)·109 s-1, which is ~4 times higher than kr. These values are in
line with the experimental knr values: 2.9·109 s-1 for PTTP35 and 2.6·109 s-1 for TMS-PTTP-TMS.36 However,
knr calculated directly using Eq. (4) from kr and Φ for the substituted PTTP studied slightly differ as shown
in Fig. S14. Specifically, the lowest knr is observed for TMS-PTTP-TMS (1.77·109 s-1) and tertBu-PTTP-tertBu
(1.95·109 s-1), while the highest knr is observed for Me-PTTP-Me (3.1·109 s-1) and F-PTTP-F (2.9·109 s-1).
Probably, the lowest knr for TMS-PTTP-TMS and tertBu-PTTP-tertBu stem from the strongest π-electron
delocalization among the TPCOs studied. The stronger delocalization could decrease the internal
conversion and intersystem crossing rates since it decreases the Huang-Rhys factors (reorganization
energy)37 and the charge density at heavier atoms (sulfur); however, detailed investigation of the non-
radiative deactivation pathways is beyond the scope of the current study.
a)
b)
absorption
luminescence
1.6
1.2
0.8
0.4
0.0
25
20
15
10
f
TMS
tertBu

TFM
Me
MeO
H
F
4.0
4.2
4.4
kr, 108 s-1
4.6
4.8
H
F
Me
TFM MeO tertBu TMS
Fig. 8. Impact of terminal substituents of PTTP on the oscillator strength and PL quantum yield. (a)
Calculated oscillator strengths of S0→S1 (black) and S1→S0 (red) transitions. (b) Experimental PLQY (Φ)
as a function of the calculated luminescence rate. The red line is the fit with Eq. (4), and the green line
is a linear fit.
3.5. Extension to other TPCOs
To check whether our conclusions on the effect of terminal substituents are relevant for other
TPCOs, we addressed the frontier orbitals, optical bandgaps and oscillator strengths for TPCOs with 5-ring
conjugated core, PTPTP. TPCOs with this conjugated core are actively studied in OFETs and light-emitting
devices.3, 38, 39 The results are given in SI, Section S4. As follows from Fig. S15, the impact of the substituents
13

Physical Chemistry Chemical Physics
Page 14 of 17
on the HOMO and LUMO energies of the substituted PTPTPs is quite similar to that on the studied PTTPs
View Article Online
and correlates well with the Hammet constants of the substituents. Specifically, TFM- and MeO-
DOI: 10.1039/C9CP00910H
substituted PTPTPs show the lowest and highest HOMO/LUMO energies, respectively, in line with the
results for the PTTP series. Analogously, the substituents alter the Eg and f values of PTPTPs in a similar
fashion as in the PTTPs studied (see Fig. S16); e.g., the largest f is observed for TMS-PTPTP-TMS. The
positive impact of TMS group on the f value is also in line with the observed increase in PLQY upon
substitution of PTPTP.40 Although further studies on other π-conjugated oligomers series are required, we
anticipate that the revealed effects of the substituents are general for this class of organic semiconductor
materials.
4. Conclusions
The impact of various terminal substituents on the frontier orbitals, charge distribution, molecular
polarizability, Raman spectra, optical bandgaps, radiative decay rates and PLQY was investigated for
TPCOs series with 5,5’-diphenyl-2,2’-bithiopene (PTTPs) π-conjugated core. Electron-donating or electron-
withdrawing substituents resulted in tuning of the HOMO and LUMO energy levels within about 0.7 eV in
the calculations and 0.4 eV in the experiment in accordance with the Hammet constants of the
substituents. The substitution considerably changed the electron density at the phenylenes but did not
affect it at the central 2,2’-bithiophene moiety. As a result, vibrational (Raman-active) modes associated
mainly with the motion of the phenyl rings showed a significant frequency shift with substitution, while
those related mostly to the thiophene rings remained nearly unaffected. The substituents were found to
be weakly involved in the π-conjugated system of the PTTP core, which resulted in weak changes in the
absorption and emission maxima (within 0.1 eV) with substitution. However, subtle spreading of the
frontier orbitals over the substituents can nearly double the photoluminescence quantum yield as
observed for TMS- and tertBu-substituted PTTPs. Thus, two main factors governing the impact of the
substituent were identified: the Hammet constant of the substituent and its involvement in π-conjugation
(or hyperconjugation) quantified by the effective conjugation length of the substituted molecule. The
revealed effect of the terminal substituents is anticipated to be general for various other TPCOs and
related co-oligomers, e.g. furan-phenylene ones,9,41 making the established structure-property
relationships valuable for the rational design of functional materials for optoelectronic applications.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
Synthetic work was made in the framework of Leading science school (grant NSh-5698.2018.3). Synthesis
and characterization of TMS-PTTP-TMS and tertBu-PTTP-tertBu was supported by Russian Foundation for
Basic Research (project #18-33-20050). Experimental studies and modeling of the Raman and luminescent
properties were supported by Russian Foundation for Basic Research (project #17-02-00841). Theoretical
work on molecular orbitals, charge distribution and polarizabilities was supported by Russian Science
Foundation (project #18-72-10165). Electrochemical study was supported by state assignment of Russian
Ministry of Science and Higher Education. The study was conducted using equipment purchased under
the Lomonosov Moscow State University Program of Development.
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DOI: 10.1039/C9CP00910H
TOC entry
Two main factors governing the effect of terminal substituents on the properties of thiophene-
phenylene co-oligomers are revealed.