Strongly luminescent and liquid-crystalline π-conjugated 2-methyl[1,2,3]benzotriazoles with a linear donor-acceptor-donor structure

Article abstract of DOI:10.1016/j.molliq.2020.113616

Symmetrical 2-methyl[1,2,3]benzotriazole (BZT) derivatives with elongated peripheral units connected via acetylenic triple bonds present calamitic thermotropic mesomorphism with nematic and smectic phases. They show intense photoluminescence with near-unity quantum yields in solution. The peripheral groups significantly influence the excited-state lifetime. Fluorescence quenching is observed in the presence of C₆₀, testifying of charge transfer to the fullerene acceptor. The varying sterical demand of the different substituents considerably influences the efficiency of the charge transfer induced fluorescence quenching. HOMO, LUMO and band gap energies ranged from ?5.15 to ?5.97 eV (ionization potential), ?2.47 to ?2.96 eV (electron affinity) and 2.68 to 3.08 eV (optical band gap).

Full text of DOI:10.1016/j.molliq.2020.113616

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Strongly luminescent and liquid-crystalline π-conjugated
2-methyl[1,2,3]benzotriazoles with a linear donor-acceptor-donor
structure
Elias Regis da Silva, Abad Roger Castillo Hinojosa, Juliana
Eccher, Michele Duarte Tonet, Daniela Brondani, Eduardo Zapp,
Sergio Fernando Curcio, Bruna Bueno Postacchini, Thiago
Cazati, AndréAlexandre Vieira
PII:
S0167-7322(20)32116-4
DOI:
https://doi.org/10.1016/j.molliq.2020.113616
MOLLIQ 113616
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
6 April 2020
9 June 2020
15 June 2020
Please cite this article as: E.R. da Silva, A.R.C. Hinojosa, J. Eccher, et al., Strongly
luminescent and liquid-crystalline π-conjugated 2-methyl[1,2,3]benzotriazoles with a
linear donor-acceptor-donor structure, Journal of Molecular Liquids (2020),
https://doi.org/10.1016/j.molliq.2020.113616
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Strongly Luminescent and Liquid-Crystalline
π-Conjugated 2-Methyl[1,2,3]Benzotriazoles
with a linear Donor-Acceptor-Donor Structure
Elias Regis da Silvaa, Abad Roger Castillo Hinojosaa, Juliana Eccher b, Michele
Duarte Tonet b, Daniela Brondanic, Eduardo Zappc, Sergio Fernando Curciod, Bruna
Bueno Postacchinid, Thiago Cazatid and André Alexandre Vieiraa
aInstituto de Química, Universidade Federal da Bahia, 40170-115.
Salvador, Bahia, Brazil.
bDepartamento de Física, Universidade Federal de Santa Catarina, 88040-900,
Florianópolis, SC, Brazil.
cUniversidade Federal de Santa Catarina, Campus Blumenau, 89065-300, SC,
Brazil.
dDepartamento de Física, Universidade Federal de Ouro Preto, 35400-000 Ouro
Preto, MG, Brazil.
Keywords: 1,2,3-Benzotriazole, luminescence, liquid crystals, charge transfer.
*email: Vieira.andre@ufba.br
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Abstract
Symmetrical 2-methyl[1,2,3]benzotriazole (BZT) derivatives with elongated
peripheral units connected via acetylenic triple bonds present calamitic thermotropic
mesomorphism with nematic and smectic phases. They show intense
photoluminescence with near-unity quantum yields in solution. The peripheral groups
significantly influence the excited-state lifetime. Fluorescence quenching is observed
in the presence of C60, testifying of charge transfer to the fullerene acceptor. The
varying sterical demand of the different substituents considerably influences the
efficiency of the charge transfer induced fluorescence quenching. HOMO, LUMO
and band gap energies ranged from –5.15 to –5.97 eV (ionization potential), −2.47 to
–2.96 eV (electron affinity) and 2.68 to 3.08 eV (optical band gap).
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Introduction
The electronic properties of linear π-conjugated molecules have acquired a growing
importance in various areas of chemistry and condensed matter physics. These
molecular materials represent the simplest models of molecular wires and have
contributed to the emergence of the concepts of molecular electronics [1, 2]. In
particular, the π-conjugated compounds based on donor-acceptor (D-A) or donor-
acceptor-donor (D–A–D) types are widely investigated as active materials [3],
because the band gap and electro-optical properties can be tuned by judicious
choice of the fluorophore’s donor and acceptor moieties. The fusion of D–A–D units
leads to the formation of highly anisotropic and rigid structures which are beneficial
to obtain calamitic liquid crystals. Luminescent liquid crystals (LLCs) have gained
enormous interest in optoelectronic applications [4-6], as they can self-organize in
periodic arrangements leading to good charge transport properties, while retaining
fluidity that allows self-healing features and dynamic properties [7-10]. Moreover,
LLCs are particularly engaging for applications involving polarised light [11, 12].
Fused heterocyclic compounds are valuable scaffolds which play an important role
as structural and functional units in LLCs. The benzazoles such as 2,1,3-
benzoxadiazole (BOD), 2,1,3-benzothiadiazole (BTD) and 2,1,3-benzoselenadiazole
(BSD) have played an important role as acceptor building blocks in the development
of linear π-conjugated materials for applications in organic electronic devices such
as organic field effect transistors [13], organic light emitting diodes [14, 15] and
organic solar cells [16, 17]. All these π-conjugated benzazoles have characteristic
strong fluorescence properties in both solution and the solid state, large Stokes shifts
and good photostabilities [18-20].
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A much less explored benzazolic system is 1,2,3-benzotriazole (BTZ). Benzotriazole
is a moderate electron acceptor moiety due to its polarizable imine unit [21-25].
Sanchez and Prieto [26, 27] synthesized T-shaped structures of 2H-benzo[d][1,2,3]-
triazoles and studied their photophysical properties. These BTZ materials exhibited
optical waveguiding properties and the possibility of tunable emission and band gap.
More recently Zade [28] prepared a series of D-A-D type molecules based on
benzazoles modifying the central heteroatom (N, O, S and Se) and observed that the
1,2,3-benzotriazole derivative showed a superior luminescence quantum yield
compared to its O, S and Se homologs.
However, there are no reports for linear π-conjugated molecules with LC properties
based on 1,2,3-benzotriazoles. In this work, we report the synthesis, characterization
and study of thermal properties of symmetrical 2-methyl-1,2,3-benzotriazole
derivatives with different peripheral units connected to BTZ core via acetylenic triple
bonds. The peripheral groups were chosen in order to explore the geometrical limits
to mesogenic behavior and to tune the photophysical and electrochemical properties.
Results and discussion
Synthesis
A stepwise synthetic protocol has been followed to develop the final
compounds based on 2-methyl[1,2,3]-benzotriazoles. A series of π-conjugated rod-
shaped D–π-A–π-D molecules was synthesized with various peripheral units
following a Sonogashira cross-coupling protocol as shown in Scheme 1.
4,7-Dibromo-2,1,3-benzothiadiazole 1 was selected as key starting material for a
sulfur extrusion reaction using sodium borohydride and catalytic cobalt chloride in a
mixture of ethanol/tetrahydrofuran to obtain the diamine 2 in 92% yield. The resulting
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3,6-dibromobenzene-1,2-diamine was subjected to reaction with sodium nitrite in
acetic acid to afford 4,7-dibromo-1,2,3-benzotriazole 3 in 91% yield. The alkylation of
this benzotriazole with iodomethane and potassium carbonate in dimethylformamide
gave both 1-methyl-[1,2,3]-benzotriazole and 2-methyl-[1,2,3]-benzotriazole, and the
latter building block 4 was easily isolated by column chromatography. In parallel, the
aryl acetylenes 5b-g were prepared from the respective aryl halides via palladium
catalyzed cross-coupling using 2-methyl-3-butyn-2-ol, followed by elimination of the
protective isopropanol group [29]. The final molecules 6a-g containing the 2-
methyl[1,2,3]benzotriazole core were obtained via the Sonogashira coupling reaction
between one equiv. of the 4,7-dibromo-2-methyl[1,2,3]benzotriazole and two equiv.
of the corresponding alkyne 5a-g, using bis(-triphenylphosphine)palladium (II)
dichloride, triphenylphosphine and copper iodide in triethylamine and
tetrahydrofuran. After purification by column chromatography, the desired molecules
were obtained in satisfying yields (58-79%).
Scheme 1. Synthesis of the rod-shaped benzotriazoles 6a-g.
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1
All of the final compounds synthesized (6a-g) were characterized by H and 13C
NMR and IR spectroscopy and APPI mass spectrometry.
Liquid crystalline behavior
The mesomorphic properties of the compounds 6a-g were investigated by polarized
optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray
diffraction (XRD). The transition temperatures and enthalpy values are given in
Table 1.
Table 1. Phase transition temperatures (°C) and enthalpies (kJ·mol−1) for
compounds 6a-g.
Compound Transition T / °C, heating Tdec. /°C b
(H/kJ.mol-1) a
6a
6b
Cr-Iso
153-154
340
380
391
Cr-SmC
SmC-Iso
143 (48.0)
146 (broad)
6c
6d
Cr-Iso
102 (79.4)
Cr(I)-Cr(II)
Cr(II)-Iso
45 (13.5)
336
384
167 (43.4)
6e
6f
Cr-Iso
78 (46.2)
Cr-SmC
SmC-N
N-Iso
160 (38.9)
203 (5.5)
386
369
209 (broad)
Cr-SmC
SmC-N
N-Iso
172 (34.2)
246 (1.8)
261 (2.1)
6g
a Determined by POM and DSC measurements (10 ºC/min) on the second heating cycle. b Determined
by TGA, onset of decomposition in nitrogen (10 ºC/min). c Cr = Crystal, N = Nematic, SmC = Smectic
C, Iso = Isotropic liquid.
It is generally observed in liquid crystals that lateral substitution has a greater effect
on melting points (i.e. Crystal-to-LC transition temperatures) than terminal
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substitution [30]. Thus, the 2-methyl-1,2,3-benzotriazole system was designed to
produce steric repulsion by the methyl group and hinder close parallel packing,
thereby increasing the free volume and lowering the melting point. As lateral alkyl
substitution is known to suppress quickly the mesogenic phase range even with
moderate alkyl length and number [31], we did not attempt to replace the lateral
group by a more voluminous substituent on the nitrogen, but instead aimed at
keeping the melting and clearing transitions at moderate temperatures by employing
relatively long dodecyl moieties as terminal chains. Further selective lowering of the
melting point, to obtain liquid crystalline behavior at or near room temperature, might
be predictable by introducing of a plurality of terminal chain lengths. The POM
analysis allowed to identify that only 6b, 6f and 6g displayed enantiotropic liquid
crystalline behavior (Figure 1) with decreased melting and clearing temperatures
compared to similar benzothiadiazoles [32]. The textures observed by POM on
cooling 6f and 6g from the isotropic phase were identified as corresponding to
nematic (N) and smectic C (SmC) phases [30].
Figure 1. Polarized optical photomicrographs (100x magnification) of compounds:
(a) 6f at 139 ºC and (b) 6g at 255 ºC on cooling.
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The cooling of these samples from the isotropic phase induced the nucleation of
birefringent liquid droplets, which later merge to give the schlieren texture typical of a
nematic mesophase. On continuing the cooling of samples, the nematic phase
transits slowly to a striated texture like a wave, characterizing a N–SmC transition.
Further cooling of these compounds leads to a broken fan-shaped texture of the
SmC mesophase.
Figure 2 shows the thermogram obtained by DSC analysis of second heating and
cooling at 10 °C/min and the transitions for sample 6f. The inserted frame Figure 2b
displays the POM texture of the nucleating nematic phase.
Figure 2. (a) DSC results for compound 6f (scan rate 10 ºC/min); (b) The inset
shows a POM image obtained during the Iso-N transition of 6f at 202 ºC on cooling.
A single mesophase was observed with 6b by POM. On cooling, this material
exhibited a colorful texture from the isotropic that quickly gives way to the crystalline
state. During the second heating cycle, the fusion for the mesophase occurred as a
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fast wave, revealing the same texture. The transition to this mesophase can be
observed by differential scanning calorimetry (DSC) during cooling as well as during
heating of the material (see ESI). During heating the transition is observed as a
shoulder at 146ºC and during cooling a slight separation of the transitions is
observed at 138ºC and 134ºC. The energy involved in the process at 138ºC is about
3.4 kJ/mol-1, a typical value for a liquid-to-smectic transition. The XRD study was
essential to confirm the mesophase.
To explore the limits of the mesogenic potential of the diarylethynyl-BTZ structure 6,
we introduced lateral substituents on the terminal aryl units, and found that the
introduction of lateral substituents such as nitro (6d) or alkoxy (6c and 6e) does
efficiently suppress the liquid crystalline behavior.
The thermal stability of the compounds 6a-g was investigated by thermogravimetric
analysis (TGA) under nitrogen atmosphere. The initial thermal decomposition
temperatures (Tdec, at 5% weight loss) of all the seven compounds were above
330ºC, indicating a good thermal stability in all cases. The nitro derivative 6d had the
lowest decomposition temperature (Tdec = 336°C), compared to the other five (Tdec
369-391°C).
=
X-ray Diffraction studies
In order to verify the liquid crystal structure of the BTZ compounds 6b, 6f and 6g,
XRD analysis was performed on cooling from the isotropic state at temperatures
within the presumed smectic mesophase range. The diffractograms showed in the
low angle region two reflection peaks, assigned to the Miller indexes (d001) and (d002),
where the ratio of the associated interlayer distance d001/d002 ∼ 2 is in agreement
with a lamellar structure, characteristic of smectic mesophases. Figure 3 shows a
representative diffractogram for 6f.
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Figure 3. XRD patterns for 6f collected at 160 ºC in the SmC mesophase.
The intense peak in the low-angle region (d001), testifies in all three cases of an
interlayer spacing that is more than a fifth smaller than the length of the extended
molecules, confirming, together with the characteristic POM textures, the transition
enthalpies and the absence of diffraction peaks other than d001 and d002, the tilted
SmC nature of the mesophase (Table 2).
Table 2 Structural parameters of 6b, 6f and 6g.
Compound L (Å)a d001 SmC (Å)b
6b
6f
6g
47.8
50.6
51.8
37.1
40.3
40.2
a Estimated by ChemBio3D Ultra Software.
b Experiment taken on cooling from isotropic phase.
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Photophysical properties
Absorption and fluorescence
The photophysical properties of the 2-methyl[1,2,3]benzotriazoles in solution are
depicted in Table 3. Absorption and fluorescence spectra of 6a-g were taken in
diluted solutions of chloroform (10-5 M). Representative spectra for the final
compounds are given in Figure 4. All molecules exhibited absorptions of between
325 and 450 nm, with the maxima at around 390 nm. These absorption bands are
assigned to π–π* transitions due to their high molar absorption coefficients (ε = 3.5 –
4.8 × 104 Lmol−1cm−1).
Table 3. Summary of photophysical properties of 6a–g in chloroform solution.
Absorptionb Emission
λmax (nm)
(ε/ 104)a
f /
Stokes
λmax (nm)b,c
ΦPL
Ns
Kr / (ns)-1 Knr / (ns)-1
Sample
Shift (cm-1)
± 0.01
6a
6b
6c
6d
6e
6f
369 (4.1)
383 (4.0)
389 (4.4)
376 (4.8)
373 (4.0)
395 (3.5)
398 (3.9)
404; 424
434; 454
460
2348
3068
3968
---
2419
2794
3939
0.99 1.31
0.99 1.49
0.90 1.59
0.76
0.66
0.57
---
0.73
0.79
0.21
0.01
0.01
0.06
---
0.03
0.07
0.33
---
---
---
410; 430
444; 464
472
0.96 1.32
0.92 1.16
0.39 1.86
6g
b
a Units = L mol−1cm−1; chloroform solution (1.0 xˑ10−5 mol L−1); c excited at maximum absorption. Kr
and Knr are the radiative and non-radiative rate constant, evaluated by Kr = ΦPL/f and Knr = (1-
ΦPL)/f, respectively. The excited lifetimes, f, were measured at the maxima of the emission intensity
after excitation at 401 nm.
All of the final benzotriazoles, with the exception of nitro derivative 6d, exhibited
intense fluorescence when excited at the maximum absorption wavelength in
solution. The presence of the nitro group completely quenched the fluorescence of
molecule 6d in solution. This behavior points to an efficient intersystem crossing
process promoted by the strong electron-withdrawing power of the -NO2 group [33].
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For the other compounds the maximum emission peaks varied between 404 to 472
nm (Figure 4b).
Figure 4. Absorbance (left) and emission (right) spectra for compounds 6a-g in
chloroform solution (10-5 mol/L).
The homolog 6a with unsubstituted terminal phenyl groups expectedly showed the
shortest-wavelength emission. The presence of electron donor units such as alkoxy
(6b; 6c) and piperazine (6g) connected to bis(phenylethynyl)BTZ resulted in red
shifts of 45, 55 and 65 nm, respectively. Meta-alkoxy substituted 6e showed a red
shift of only 6 nm due to the lesser donor effect of a meta substituent compared to its
para analog. The Stokes shifts are in the range of 2348-3968 cm-1, with a small
overlap region between absorption and emission. The unsubstituted and alkoxy-
substituted homologs 6a, 6b, 6c, 6e as well as the naphthylene homolog 6f exhibited
fluorescence quantum yields (Φ) between 0.90 and 0.99 (see Table 3). Besides the
non-fluorescent nitro-derivative 6d, the only compound that exhibited lower
photoluminescence efficiency was 6g (0.39) with a piperazine donor group. The
strongly donating phenylpiperazine portion results in a stronger intramolecular
charge transfer (ICT), which induces fluorescence at higher wavelengths, a stronger
color dependence on solvent polarity and lower fluorescence quantum yields
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probably related to the formation of twisted intramolecular charge-transfer (TICT)
states. [34]. The excellent quantum yield values for 6a, 6b, 6c, 6e and 6f are
superior to those of the previously reported 2-benzo[1,2,3]triazoles [26, 27]. The high
fluorescence quantum yields may result from the nonbonding orbital of the nitrogen
atom being perpendicular to the plane of the ring, which allows it to overlap with the
π orbitals on the adjacent atoms to enhance the system's conjugation [33].
All homologs except 6d showed mono-exponential fluorescence decay with lifetimes in
the nanosecond timescale (Table 3 and SI), characteristic of monomeric emission. The
singlet excited-state lifetimes (f) of compounds 6a-g were very similar to each other,
with a modest increase of lifetime with increasing donor effect in the order 6a (no
alkoxy) – 6e (meta) – 6b (para) – 6c (para + meta) – 6g (piperazine). 6f, with its
conjugation elongated by the naphthyl group, showed the shortest lifetime. The
fluorescence decay curves are shown in the supporting information (See ESI).
Moreover, the radiative and non-radiative rate constants of these systems, in
chloroform solution, are also affected by the nature of peripheral groups (see Table
2). 6f with a conjugation elongated by the naphthyl group showed the highest
radiative rate constants, whilst the lowest non-radiative rate constants are exhibited
by molecules with peripheral alkoxy donor groups (6b, 6c and 6e). The piperazine
homolog 6g showed the lowest radiative rate constants and the highest non-radiative
rate constants.
The benzotriazoles 6a-g have also been studied in the form of thin films prepared by
spin-coating. In general, the films exhibited hypsochromic shifts between 24 nm and
43 nm for the absorption bands, while the emission showed bathochromic shifts in
the range of 6 and 65 nm in relation to the solution. Due to aggregation-induced
quenching, the fluorescence quantum yields of the thin films, between 11 and 22%,
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were much lower than in solution, except for 6b, which with 43% maintained a rather
high quantum yield in the condensed film. (see ESI).
Solvatochromic properties
In order to study the solvatochromic effect, the absorption and fluorescence spectra
of 6a-g were taken in diluted solutions of heptane, toluene, chloroform,
tetrahydrofuran, acetone, dimethylformamide and acetonitrile. The absorption and
emission spectra of 6g and 6c in various solvents are shown in Figure 5. The
photophysical properties of compound 6c in solution of several solvents are
summarized in Table 4. All the samples presented well-resolved absorption and
emission bands. Only small spectral variations are observed for the absorption
spectra, regardless of the polarity of the solvents. However, there is a prominent red
shift in the emission peaks with increasing solvent polarity (See ESI), which is
greatest for the homolog 6g with the strongest donor group.
44nm
DMF
Acetone
CHCl3
THF
Toluene
Heptane
400
450
500
550
600
650
Wavelength (nm)
Figure 5. Normalized absorbance and emission spectra
for 6g in different solvents (left). Normalized emission spectra for 6c in different
solvents (right).
The maximum emission bands are shifted to longer wavelengths by increasing the
solvent polarity, e.g., from 446 nm (hexane) to 567 nm (DMF) in the case of
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piperazine derivative 6g. The increase of the Stokes' shift with increase of the
dielectric constant of the solvent indicates a stabilization of the excited states by the
polar solvents [35]. This behavior implies that the excited state of these compounds
is more polar than the ground state.
The excited state lifetimes of 6c in different solvents are shown in Table 4. The
sample exhibited single lifetimes in the nanosecond range in all solvents studied,
suggesting that emissions arise only from fluorescent monomers of 6c. In heptane,
toluene and THF solution, the fluorescence decay exhibited a single excited state
lifetime of 1.1 to 1.3 ns. For polar solvents (DMF and acetone), 6c exhibited slightly
longer lifetimes in range of 1.6 to 1.8 ns (see ESI for decay curves).
Table 4. Photophysical properties of compound 6c in solution of different solvents.
a,b
b,c
Solvents
Stokes Shift
(cm-1)
abs
em
1
(ns)
(nm) (nm)
Heptane 386
Toluene 391
424
2322
1.12
435
454
440
460
468
2587
3681
3046
4235
4339
1.15
1.59
1.30
1.63
1.75
CHCl3
THF
389
388
Acetone 385
DMF 389
aUnits = L mol−1cm−1; bchloroform solution (1.0 xˑ10−5 mol L−1); cexcited at maximum absorption
The influence of solvent polarity on the optical properties of a fluorophore can be
estimated using the Lippert-Mataga equation [36]. The plot of the Stokes shifts (Δν)
as a function of orientation polarizability (Δf) for 6b is shown in Figure 6. The
increase of Δν with an increase of the solvent polarity leads to a linear correlation of
0.84 for this compound. The other derivatives also showed good values of linear
correlation. (See ESI).
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Figure 6. Changes in the Stokes shifts (Δν) of 6b versus (Δf) of various solvents.
The calculated values for the slope of the linear correlation for all the benzotriazoles
are shown in Table 5. A much steeper slope (13400 cm−1) was obtained for 6g,
indicating that its dipole moment increase from the ground state to the excited state
is more prominent than in to the other compounds.
Table 5. Data obtained by treatment of the Lippert-Mataga correlations.
Compounds Slope Linear correlation
6a
6b
6c
6d
6e
6f
771
4871
7139
---
0.88
0.84
0.91
---
1323
7027
13400
0.95
0.79
0.95
6g
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Mechanism of photoinduced charge separation
Benzazole-based fluorophores have attracted interest due the potential applications
in organic photovoltaic cells [37]. Aware of this phenomenon, the photoinduced
charge transfer process was investigated through fluorescence extinction
experiments carried out with benzotriazoles in solution. The fluorescence samples in
chloroform were gradually quenched by doping with fullerene C60 (Figure 7).
This suggests that electron transfer from photoexcited benzotriazole to fullerene C60
occurs in the excited state.[38, 39] Moreover, the dependence of the fluorescence
intensity on the C60 concentration is linear, in accordance with the Stern-Volmer
equation:
F0/F = 1 + Ksv [C] = 1+ kq.0[C]
Eq. (1)
where F0 and F represent the fluorescence intensity in the absence and present of
C60, respectively, Ksv is the Stern-Volmer quenching constant, [C] is the
concentration of C60 and kq is the bimolecular rate constant and 0 is the lifetime in
absence of C60.
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Figure 7. (a) Emission spectra for 6a-c and 6e-g (1.8 × 10-5 M) in chloroform with
concentration of C60: 0.0 (blank), 1.0 equiv. (1), 2.0 equiv. (2), 3.0 equiv. (3), 4.0
equiv. (4), 5.0 equiv. (5), 6.0 equiv. (6), 7.0 equiv. (7), 8.0 equiv. (8), 9.0 equiv. (9).
The insets show the Stern-Volmer plots for these samples.
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Thus, the Stern-Volmer quenching constant Ksv and the bimolecular rate constant
kq for the benzotriazoles were obtained from the fitting of Stern-Volmer plot by Eq. 1
(Table 6). The values of Ksv range from 15.65 x 103 M-1 to 4.96 x 103 M-1 suggesting
that the photoinduced charge separation between the fluorescent compound and C60
can be tuned by appropriate peripheral groups [39-41].
Here, the absence of alkoxy chains proved to be beneficial, leading to the highest
values of Ksv and kq. Possibly the alkoxy chains affect the approach of BTZ and C60
in solution and thus attenuate the quenching processes, since the bimolecular rate
constant, kq, reflects the accessibility of the fluorophore to the quencher. This
shielding effect is confirmed when 6c is compared with 6b, 6e or 6f, as 6c with four
alkoxy chains showed the lowest kq value of the series. The pattern of alkoxy
substitution also influenced the constant, as meta-substituted 6e had a higher kq
than 6b substituted in para position.
Table 6. Values of Stern-Volmer constant, Ksv, and bimolecular rate
constant, kq, obtained from the fitting of Stern-Volmer plot by Eq. 1 for 6a-g
in CHCl3 with C60.
Compounds
Ksv / 103 M-1
15.65
6.31
kq / 1011 M-1s-1
119.47
42.35
0 / ns
6a
6b
6c
6d
6e
6f
1.31 ± 0.01
1.49 ± 0.01
1.59 ± 0.01
---
1.32 ± 0.01
1.16 ± 0.01
1.86 ± 0.02
4.96
---
9.57
5.46
31.19
---
72.50
47.07
6g
5.45
29.30
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Electrochemical behavior
The electrochemical properties of the benzotriazoles derivatives were investigated
by cyclic voltammetry (CV) and the data of electrochemical properties of 6a-g are
summarized in Table 7. The cyclic voltammograms reflect the modifications of the
donor substitution pattern by changes in the HOMO and LUMO values as well as the
width of the band gaps, showing that the redox properties are sensitive to the
electronic influence of the peripheral groups.
The cyclic voltamograms of 6a-g (see ESI) exhibit reduction processes in the range
of -1.08 V to -1.93V vs. Ag/Ag+ (or -0.80 V to -1.68 V vs. NHE). The charge transfer
process at negative potentials involves a quasi-reversible reduction of the
benzotriazole ring, which is indicated by the fact that the current versus potential
curves have the same shape for all compounds. Scanning at positive potential
values, the materials showed irreversible oxidation processes, whose potentials
were significantly more substituent-dependent than the reduction potentials, leading
to a prominent variation of the band gap energies. When an electron-donating
alkoxyl group is inserted into the 2-methyl-4,7-bis(phenylethynyl)-[1,2,3]benzotriazole
core, the oxidation of 6b, 6c and 6e begins at less anodic values, and the reduction
process begins at more cathodic values compared to molecule 6a. A similar effect
observed with 6f due to the elongation of the aromatic portion. The insertion of a
nitro group, due to the electron withdrawing effect of the -NO2 group, caused an
increase in the oxidation potential of compound 6d compared to other molecules that
had electron-donating substituents. When a piperazine group is inserted (6g), the
donating effect is most pronounced, yielding the lowest oxidation potential observed
for the entire series.
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Table 7. Optical and electrochemical properties of compounds 6a-g where
is the onset potential
of oxidation,
is the onset potential of reduction against NHE, IP (HOMO) is the ionization
potential, EA (LUMO) is the electron affinity, is the electrochemical band gap, λonset is the absorption
onset wavelength and
is the optical band gap
Compounds
6d
Parameter
6a
6b
6c
6e
6f
6g
(V)a
1.50
-1.60
1.26
-1.66
1.19
-1.68
1.53
1.44
-1.62
1.24
-1.58
0.71
-1.67
(V)b
-0.80
IP
(eV)c
(HOMO)
-5.94
-5.70
-5.63
-5.97
-5.88
-5.68
-5.15
EA (LUMO)
(eV)d
-2.86
402
-2.77
423
-2.76
432
-2.96
411
-2.83
406
-2.85
437
-2.47
463
λonset (nm)e
(eV)f
3.08
2.93
2.87
3.01
3.05
2.83
2.68
a
+
+
Notes: First oxidation process:
(vs. NHE) =
(vs. Ag/Ag ) + 0.25; b First reduction process:
c
e
+ 4.44) eV; d EA = – ( (+ - IP) eV;
(vs. NHE) =
(vs. Ag/Ag ) + 0.25; IP = – (
f

onset: absorption onsets wavelength; Optical band gap calculated from the low energy edge of the
absorption spectrum ( = 1240 / λonset).
Comparing the results obtained with benzotriazole-based and similar
benzochalcogenodiazole derivatives [19, 42, 43], the values obtained for the LUMO
energies show less negative values for benzotriazoles than for benzoxadiazoles, due
to the presence of a less electronegative oxygen atom that reduces the capacity for
accepting electrons [28]. The BTZ derivatives also showed slightly lower HOMO
energies than the benzoxadiazoles. Compared to derivatives containing sulfur and
selenium, higher HOMO energies were observed for the benzotriazoles. The
evaluated benzotriazole molecules possess band gaps in the range of 2.68 to 3.08
eV, which are close to the other emitting materials applied in the construction of the
light-emitting diode (LED) and organic light-emitting diode (OLED) [44, 45]. The
HOMO energy levels of our BTZ derivatives are close to the most well-known hole-
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transporting materials used in the construction of OLED devices. Thus, these
compounds may have potential for optoelectronic applications [46].
Conclusions
In summary, 2-methyl-BTZ derivatives can present calamitic liquid-crystalline phases
as long as the molecule has a high anisometry, with both nematic and smectic C
mesophases being observed with two alkoxyarylethinyl or alkylpiperazinylarylethinyl
substituents. The decrease on melting and clearing points by lateral substitution
occurred in the longest molecules [31]. The presence of lateral group in the shortest
aromatic portion did not allow the formation of mesomorphism, probably due to the
steric effect and the dependence on length-to-width ratio. An additional decrease of
the melting temperatures or suppression of crystallization in ambient conditions is
possible by entropy increase through mixing several homologues with different
terminal arylene moieties and different alkyl chain lengths. The peripheral groups
significantly influence photophysical properties of the benzotriazole derivatives,
especially the fluorescence intensity and emission wavelength (λEm = 404-472 nm).
All the final benzotriazoles exhibit intense fluorescence in solution, except for the
nitro-derivative. Near-unity quantum yields (ΦPL = 0.90-0.99) were observed for the
homologs with alkoxyarylethynyl peripheral groups. Their excited-state lifetimes of
the bis(phenylethinyl)-BTZ homologs, in the nanosecond timescale, increase
following the order: unsubstituted (6a), meta- (6e), para- (6b), para- + meta-alkoxy
(6c), alkylpiperazinyl (6g). The emission of the BTZ series is strongly
solvatochromic, the most sensitive sample being 6g. The fluorescence intensity
decreases in the presence of C60, indicating photoinduced charge separation, and
the steric demand of the peripheral groups affects these charge transfer processes,
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the highest values of Ksv and kq being observed in the absence of flexible alkyl tails.
The electrochemically determined HOMO and LUMO energy levels revealed
moderate band gaps, showing potential for electro-optical applications.
Acknowledgments
This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal
de Nível Superior - Brasil (CAPES) - Finance Code 001. We are grateful to CNPq
(MCTI/CNPq Nº 422750/2018-2 Universal), INCT/INEO, PROPPI/UFOP (Nº
23109.004080/2019-88) and FAPESB (PIBIC) for financial support, and to CEBIME-
UFSC for the HRMS analysis. We thank Dr. Harald Bock (CRPP Bordeaux) for
fruitful discussions.
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Author Statement
Strongly Luminescent and Liquid-Crystalline π-Conjugated 2-
Methyl[1,2,3]Benzotriazoles with a linear Donor-Acceptor-Donor Structure
The authors of this work meet all three of the following conditions in accordance with
the ―Consensus Statement on Surgery Journals Authorship—2005‖:
1) Authors make substantial contributions to conception and design, and/or
acquisition of data, and/or analysis and interpretation of data; 2) Authors participate
in drafting the article or revising it critically for important intellectual content; and 3)
Authors give final approval of the version to be submitted and any revised version.
Below are listed the activities performed by each author of the work.
Investigation: Elias Regis da Silva, Abad Roger Castillo Hinojosa, Michele Duarte
Tonet, Sergio Fernando Curcio, Daniela Brondani.
Data Curation: Juliana Eccher, Eduardo Zapp, Bruna Bueno Postacchini, Thiago
Cazati.
Writing - Review & Editing: Juliana Eccher, Eduardo Zapp, Thiago Cazati, André
Alexandre Vieira.
Project administration and Writing: André Alexandre Vieira
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27

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Strongly Luminescent and Liquid-Crystalline π-Conjugated
2-Methyl[1,2,3]Benzotriazoles with a linear Donor-
Acceptor-Donor Structure
Elias Regis da Silvaa, Abad Roger Castillo Hinojosaa, Juliana Eccherb, Michele
Duarte Tonetb, Daniela Brondanic, Eduardo Zappc, Sergio Fernando Curciod, Bruna
Bueno Postacchinid, Thiago Cazatid and André Alexandre Vieiraa
Highlights
 A novel luminescent liquid crystals based on the 2-
methyl[1,2,3]benzotriazole.
 The molecules presented calamitic mesomorphism.
 Compounds exhibited strong photoluminescence in solution and solid
film.
 Doping with C60 suggested charge transfer, being dependent on the
π-conjugate core.
28

Figure 1