Synthesis and photo-electro-thermal characterization of non-symmetrical 4,7-dibromobenzo[c][1,2,5]thiadiazole derivatives

Article abstract of DOI:10.1016/j.dyepig.2020.108703

This work describes the synthesis of new fluorophores based on non-symmetric 4,7-dibromobenzo[c][1,2,5]thiadiazole derivatives with benzene ethynyl tetrazole and propargyl carbazole as terminal groups. The new compounds were obtained using the Sonogashira

Full text of DOI:10.1016/j.dyepig.2020.108703

Journal Pre-proof
Synthesis and photo-electro-thermal characterization of non-symmetrical 4,7-
dibromobenzo[c][1,2,5]thiadiazole derivatives
Rodrigo da Costa Duarte, Rodrigo Cercená, Fabiano da Silveira Santos, Luís
Gustavo Teixeira Alves Duarte, Teresa Dib Zambon Atvars, Eduardo Zapp, Daniela
Brondani, Fabiano Severo Rodembusch, Alexandre Gonçalves Dal-Bó
PII:
S0143-7208(20)31400-5
DOI:
https://doi.org/10.1016/j.dyepig.2020.108703
DYPI 108703
Reference:
To appear in: Dyes and Pigments
Received Date: 29 April 2020
Revised Date: 7 July 2020
Accepted Date: 8 July 2020
Please cite this article as: da Costa Duarte R, Cercená R, da Silveira Santos F, Teixeira Alves Duarte
LG, Zambon Atvars TD, Zapp E, Brondani D, Rodembusch FS, Dal-Bó AG, Synthesis and photo-electro-
thermal characterization of non-symmetrical 4,7-dibromobenzo[c][1,2,5]thiadiazole derivatives, Dyes and
Pigments, https://doi.org/10.1016/j.dyepig.2020.108703.
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Author contributions
Rodrigo da Costa Duarte: Conceptualization; Formal analysis; Roles/Writing -
original draft.
Rodrigo Cercená: Formal analysis; Roles/Writing - original draft.
Fabiano da Silveira Santos: Conceptualization; Formal analysis; Roles/Writing -
original draft.
Luís Gustavo Teixeira Alves Duarte: Formal analysis; Roles/Writing - original
draft.
Teresa Dib Zambon Atvars: Supervision; Writing - review & editing.
Eduardo Zapp: Formal analysis; Roles/Writing - original draft.
Daniela Brondani: Formal analysis; Roles/Writing - original draft.
Fabiano Severo Rodembusch: Conceptualization; Methodology; Supervision;
Writing - review & editing.
Alexandre Gonçalves Dal-Bó: Funding acquisition; Project administration;
Supervision; Resources.

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Synthesis and photo-electro-thermal characterization of non-symmetrical
4,7-dibromobenzo[c][1,2,5]thiadiazole derivatives
a
a
a
Rodrigo da Costa Duarte, Rodrigo Cercená, Fabiano da Silveira Santos, Luís
b
b
c
Gustavo Teixeira Alves Duarte, Teresa Dib Zambon Atvars, Eduardo Zapp,
c
d
Daniela Brondani, Fabiano Severo Rodembusch, Alexandre Gonçalves Dal-
a
Bó *
a
Universidade do Extremo Sul Catarinense (UNESC). Av. Universitária, 1105.
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CEP 88806-000. Criciúma-SC, Brazil.
b
Chemistry Institute, University of Campinas, Campinas, 13083-970, Brazil
c
Universidade Federal de Santa Catarina (UFSC). Rua João Pessoa, 2750.
CEP 89036-256. Blumenau, SC, Brazil
d
Grupo de Pesquisa em Fotoquímica Orgânica Aplicada. Instituto de Química
(UFRGS), Av. Bento Gonçalves, 9500, CEP 91501-970. Porto Alegre-RS,
Brazil.
*Corresponding author: Universidade do Extremo Sul Catarinense (UNESC).
Av. Universitária, 1105. CEP 88806-000. Criciúma-SC, Brazil. Tel.: +55 (48)
3431 2775. E-mail address: adalbo@unesc.net
Abstract
This work describes the synthesis of new fluorophores based on non-symmetric
4,7-dibromobenzo[c][1,2,5]thiadiazole derivatives with benzene ethynyl
tetrazole and propargyl carbazole as terminal groups. The new compounds
were obtained using the Sonogashira cross-coupling reaction and produced
1

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good yields. The benzothiadiazole derivatives presented absorption in the
ultraviolet–visible region (391-400 nm) ascribed to spin and symmetry allowed
1
electronic transitions ππ*. The main fluorescence emission located at 460-500
nm was observed with solvatochromism up to 40 nm, indicating higher
electronic delocalization in the excited state. Electrochemically, the compounds
are electroactive in both the anodic and cathodic potential range. The
magnitude of oxidation potentials ranged from 1.54 to 1.61 V vs. NHE and
reduction potentials ranged from -2.00 to -2.05 V vs. NHE. The HOMO and
LUMO energies were calculated using cyclic voltammetry data. HOMO energies
ranged from -6.16 to -6.23 eV and EA energies ranged from -2.57 to -2.62 eV,
and thus, the electrochemical band gap of compounds ranged from 3.56 to 3.66
eV. Thermogravimetric analysis showed that the compounds presented good
thermal stability up to 200 °C. The DFT and TD-DFT calculations were
performed at the B3LYP/6-31G level of theory. The results show that the dipole
moment of the molecules varies tremendously from the electronic ground to
excited state, ensuring a charge transfer character.
Keywords: 2,1,3-benzothiadiazole, carbazole, fluorescence, solvatochromism,
intramolecular charge transfer
1. Introduction
Photoactive compounds based on the D-π-A-π-D system derived from
the 4,7-dibromobenzo[c][1,2,5]thiadiazole (BTD) motif have been extensively
studied in recent years due to their photophysical and optoelectronic properties
[1,2], which can be tailored according to the electronic properties of the donor
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and acceptor substituents present in positions 4 and 7 of the BTD moiety [3,4].
In this sense, different synthetic methodologies can be found in the literature for
structural modification and consequent tuning of the BTD nucleus, leading to
changes in its organoelectronic properties. These methodologies involve
Sonogashira and Suzuki cross-coupling reactions, aromatic formulation,
amination reactions, click reactions, and others [1]. Thus, a wide range of
synthetic methodologies have been presented to date regarding the different
applications available for these compounds, such as in organic photovoltaic
cells (OPVs) [5-11], organic light-emitting diodes (OLEDs) [12-14], organic field-
effect transistors (OFETs) [15], and organic thin-film transistors (OTFTs)
[16,17].
On the other hand, there is a class of non-symmetrical BTDs that can
end the possibility of C2 symmetry [18-20]. These derivatives have been cited
to be involved in very interesting applications, such as sensors for metal
detection [21], solar cells sensitized by organic dyes (DSSCs) [22], enhanced
two-photon absorption cross sections [23], as well as to present a large Stokes
shift and high fluorescent emission [21]. In general, non-symmetrical BTD
derivatives are structurally characterized by the chemical architecture D-π-A,
allowing energy levels for the LUMO orbital above the conduction band of
semiconductors such as TiO (~4.0 eV vs vacuum). The latter provides a driving
2
force for these compounds to inject electrons into their conduction band [24].
Despite the almost absent in the literature, studies regarding the synthesis of
non-symmetric derivatives of BTD are of great importance, since new important
effects in their photophysical, thermal, and electrochemical properties can be
obtained by inserting new substituents into their structure.
3

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Therefore, this study reports the rational synthesis of non-symmetric
compounds based on the 4,7-dibromobenzo[c][1,2,5]thiadiazole unit containing
benzene ethynyl tetrazole and propargyl carbazole as terminal groups in the D-
π-A-π-D system. The carbazole unit is an important substituent in organic
electronics, acting as a hole-transporting moiety bonded to the electron
acceptor moiety BTD. The non-symmetrical structure was proposed due to the
probable increase on its polarity allowed by the intramolecular charge transfer
character in this electronic system. In addition, their photophysical, thermal, and
electrochemical characterization were also evaluated and discussed.
2. Experimental
2.1 Materials and methods
4,7-Dibromobenzo[c]-1,2,5-thiadiazole, 1-bromooctane, 1-bromodecane,
1-bromododecane, triphenylphosphine, copper iodide, 4-bromobenzonitrile, 2-
methyl-3-butyn-2-ol, bis(triphenylphosphine)palladium(II)dichloride, and 9H-
carbazole-9-ethanol were purchased from Sigma-Aldrich. Sodium azide (NaN3)
was acquired from Synth. The solvents and reagents were used as received or
purified using standard procedures [25]. Silica Gel 60 F254 was used for thin
layer chromatography, and silica Gel 60 Å (70-230 mesh) was used for column
1
13
chromatography. Proton and carbon nuclear magnetic resonance ( H and C
NMR) were recorded in CDCl at 400 and 100 MHz, respectively. The chemical
3
shifts (δ) are reported in parts per million (ppm) relative to TMS (0.00 ppm), and
the coupling constants J are reported in Hertz (Hz). The compounds were also
-1
characterized using Fourier transform infrared (FTIR) spectra (600-4000 cm )
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recorded by a Shimadzu IR Prestige-21 spectrophotometer with a resolution of
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4 cm using KBr pellets. High resolution mass spectra (HRMS) were recorded
on a Micromass Q-Tof spectrometer, using electrospray ionization (ESI). UV–
vis absorption spectra were obtained using a Shimadzu UV-2450
spectrophotometer. Steady-state fluorescence spectra were obtained using a
Shimadzu RF-5301PC spectrofluorometer. In addition, 5.0 nm slits were used
for excitation and emission. For emission spectra, the absorption maximum
wavelength as excitation radiation was used. The quantum yield of fluorescence
was measured at 25 °C using coumarin 153 in ethanol as the quantum yield
standard [26]. Excited state dynamics were performed by the acquisition of the
fluorescence spectra using an ISS single photon counting spectrofluorometer
(model PC1) in solution with λ =290 nm. Fluorescence decays were acquired
exc
using the time-correlated single-photon counting (TCSPC) technique and an
Edinburgh Analytical Instruments FL 900 spectrofluorometer with an MCP-PMT
Hamamatsu R3809U-50 with a pulsed light emitting diode operating at λ =290
exc
nm (model EPLED-290, pulse width 875.7 ps). The instrument response
®
function was recorded using a Ludox sample. The emission decays were fitted
as exponential functions using the software FAST, as expressed in Eq. (1):
ꢈ
ꢇꢉ
ꢀꢁꢂꢃ = ∑ꢋ ꢄꢅꢆ
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ꢊ
, Eq. (1)
ꢅꢌꢍ
Where I(t) is the signal intensity sum of the electronic excited species weighted
by ꢄ (pre-exponential factor) and with a lifetime constant τi. For all fluorescence
ꢎ
decays, 10,000 counts were accumulated in the peak channel to determine the
2
lifetime. The value of χ and a visual inspection of the residuals and the
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autocorrelation function were used to determine the quality of the fit. The
residuals of all curves are presented as supplementary data.
The electrochemical measurements were performed using
a
PalmSens3 potentiostat/galvanostat (Palm Instruments BV). Cyclic voltammetry
-1
(CV) experiments were performed at a scan rate of 50 mV·s in solutions of
-1
0.01 mol·L tetra-n-butylammonium hexafluorophosphate (TBAPF ) using
6
+
CH Cl as the supporting electrolyte and the ferrocene/ferricenium (Fc/Fc )
2
2
redox couple as an internal reference. A three-electrode cell was used,
comprising a glassy carbon electrode (GCE) as the working electrode, a
+
platinum wire as the counter electrode, and an Ag/Ag electrode (0.01 M AgNO
3
in acetonitrile) as the reference. Before each measurement, the cell was
deoxygenated by purging with argon. The thermal stability of compounds was
investigated using thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC). Thermogravimetric measurements were performed using a
-1
TG 50 Shimadzu thermal analyzer with a heating rate of 10 °C·min from 25 to
900 °C under a nitrogen gas atmosphere. DSC analysis were performed using a
-1
Netzsch 3500 Sirius with a heating rate of 10 °C∙min from 25 to 200 °C under
a nitrogen gas atmosphere.
Calculations of the electronic ground and excited state geometries,
frontier molecular orbitals (HOMO/LUMO), and vertical transitions were
performed using density functional theory and its time-dependent counterpart
(DFT/TDDFT) methods using B3LYP and 6-31G basis sets in ORCA 4.0 [27,28]
and Avogadro 1.2 software [29]. The ground state structure was further
optimized using B3LYP with D3BJ correction, from which the frontier molecular
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orbitals (HOMO/LUMO) were extracted, along with the employment of CAM-
B3LYP-D3BJ for obtention of the vertical transitions.
2.2 Synthesis
2.2.1 Synthesis of 9-(prop-2-yn-1-yl)-9H-carbazole 1
Carbazole derivative 1 was prepared as previously described in the
literature [30]. Initially, in a round-bottom flask, a mixture of carbazole (1.0 g, 6.0
mmol) and potassium hydroxide (0.67 g, 12.0 mmol) in DMF (15 mL) was
prepared under stirring in an ice bath. After 5 min, propargyl bromide (1.42 g,
12.0 mmol) was slowly added; the final mixture was stirred for 24 h at 25 °C.
After this period, the solvent was concentrated under vacuum and the crude
mixture was adsorbed on silica gel and purified by column chromatography
using 2% ethyl acetate in hexane (v/v) as an eluent.
9-(Prop-2-yn-1-yl)-9H-carbazole 1, white solid, yield 95%, mp 123-124 °C. FTIR
-1
1
(NaCl) cm : 3263, 3040, 2928, 2848, 2117, 1454. H NMR (CDCl , 400 MHz)
3
ppm: 8.15 (2H, d, J = 8.0 Hz), 7.73 (4H, m), 7.32 (2H, m), 5.06 (2H, d, J = 4.0
13
Hz), 2.29 (1H, t, J = 4.0 Hz). C NMR (CDCl , 100 MHz) ppm: 139.9, 125.9,
3
+
123.3, 120.4, 119.6, 108.9, 77.2, 72.2, 32.3. HRMS (ESI) found: [M+H]
+
206.0983; molecular formula C H N requires [M+H] 206.0970.
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12
2.2.2 Synthesis of 4-(3-(9H-carbazol-9-yl)prop-1-yn-1-yl)-7-
bromobenzo[c][1,2,5]thiadiazole 3
Dichloromethane (20 mL) and triethylamine (5 mL) previously degassed
with argon for 15 min were added to a round-bottom flask. Then, compound 2
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(0.250 g, 0.85 mmol), Pd(PPh ) Cl (0.03 g, 0.042 mmol), and CuI (3.8 mg, 0.02
3 2 2
mmol) were added and stirred at 60 °C for 15 min. Next, compound 1 (0.087 g,
0.42 mmol) was added to the mixture, and the reaction was maintained under
an argon atmosphere for 3 h at 60 °C. After this period, the reaction mixture
was treated with 50 mL of an NH Cl saturated solution and extracted with
4
dichloromethane (3 x 50 mL), and the combined organic layers were dried
(MgSO ) and concentrated under vacuum. The crude was purified by column
4
chromatography on silica gel using hexane/dichloromethane 95:5 as an eluent.
4-(3-(9H-Carbazol-9-yl)prop-1-yn-1-yl)-7-bromobenzo[c][1,2,5]thiadiazole
3,
−1
yellow solid, yield 51%, mp 197-198 °C. FTIR (NaCl) cm : 3012, 2954, 2814,
1
1620, 1486. H NMR (CDCl , 400 MHz) ppm: 8.14 (2H, d, J = 8.0 Hz), 7.73 (1H,
3
d, J = 8.0 Hz), 7.64 (2H, d, J = 8.0 Hz), 7.54 (2H, m), 7.45 (1H, d, J = 8.0 Hz),
13
7.31 (2H, m), 5.44 (2H, s). C NMR (CDCl , 100 MHz) ppm: 154.1, 152.9,
3
139.9, 133.6, 131.7, 126.0, 123.3, 120.5, 119.6, 115.5, 115.2, 108.9, 90.8, 79.2,
+
33.5. HRMS (ESI) found: [M+H] 418.0001; molecular formula C H 3BrN S
21
1
3
+
requires [M+H] 418.0014.
2.2.3 General procedure for the synthesis of non-symmetrical BTDs 5a-c
Dichloromethane (30 mL) and triethylamine (5 mL) previously degassed
with argon for 15 min were added to a round-bottom flask. Then, compound 3
(0.1 g, 0.23 mmol), Pd(PPh ) Cl (0.016 g, 0.023 mmol), and CuI (0.002 g,
3
2
2
0.023 mmol) were added and stirred at 60 °C for 15 min. Next, compound 4a
(0.065 g, 0.23 mol) was added to the mixture, and the reaction was maintained
under an argon atmosphere for 2 h at 60 °C. After this period, the reaction
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mixture was treated with 50 mL of an NH Cl saturated solution and extracted
4
with dichloromethane (3 x 50 mL), and the combined organic layers were dried
(MgSO ) and concentrated under vacuum. The crude was purified by column
4
chromatography on silica gel using hexane/dichloromethane 1:1 (v/v), as an
eluent. The same procedure was used for the synthesis of compounds 5b-c.
4-(3-(9H-Carbazol-9-yl)prop-1-yn-1-yl)-7-(1-octanetetrazole-4-ethynylbenzene)-
2,1,3-benzothiadiazole 5a, yellow solid, yield 47%, mp 147-151 °C. FTIR (NaCl)
−1
1
cm : 3020, 2919, 2840, 2201, 1442. H NMR (CDCl , 400 MHz) ppm: 8.20 (2H,
3
d, J = 8.0 Hz), 8.15 (2H, d, J = 8.0 Hz), 7.78 (2H, d, J = 8.0 Hz), 7.70 (1H, d, J =
8.0 Hz), 7.65 (2H, d, J = 8.0 Hz), 7.55 (3H, m), 7.31 (2H, m), 5.48 (2H, s), 4.67
13
(2H, t, J = 8.0 Hz), 2.08 (2H, m), 1.28 (10H, m), 0.90 (3H, t, J = 8.0 Hz). C
NMR (CDCl , 100 MHz) ppm: 164.3, 154.3, 154.1, 139.9, 133.2, 132.5, 128.0,
3
126.7, 126.0, 124.2, 123.3, 120.4, 119.6, 117.3, 116.3, 108.9, 97.0, 91.5, 86.6,
79.8, 53.3, 33.5, 31.7, 29.7, 29.4, 29.0, 28.8, 26.3, 22.6, 14.0. HRMS (ESI)
+
+
found: [M+H] 620.2584; molecular formula C H N S requires [M+H]
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620.2596.
4-(3-(9H-Carbazol-9-yl)prop-1-yn-1-yl)-7-(1-decanetetrazole-4-ethynylbenzene)-
2,1,3-benzothiadiazole 5b, yellow solid, yield 40%, mp 144-149 °C. FTIR (NaCl)
−1
1
cm : 3010, 2916, 2842, 2209, 1455. H NMR (CDCl , 400 MHz) ppm: 8.20 (2H,
3
d, J = 8.0 Hz), 8.15 (2H, d, J = 8.0 Hz), 7.78 (2H, d, J = 8.0 Hz), 7.71 (1H, d, J =
8.0 Hz), 7.67 (1H, d, J = 8.0 Hz), 7.57 (3H, m), 7.32 (3H, m), 5.48 (2H, s), 4.67
13
(2H, t, J = 6.0 Hz), 2.08 (2H, m), 1.28 (14H, m), 0.90 (3H, t, J = 8.0 Hz). C
NMR (CDCl , 100 MHz) ppm: 164.3, 154.3, 154.1, 139.9, 133.2, 132.5, 132.4,
3
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128.0, 126.7, 126.0, 124.2, 123.3, 120.5, 119.6, 117.3, 116.3, 108.9, 97.0, 91.5,
86.6, 79.8, 53.3, 33.5, 31.8, 29.7, 29.4, 29.3, 29.2, 28.9, 26.3, 22.6, 14.1.
+
HRMS (ESI) found: [M+H] 648.2900; molecular formula C H N S requires
40
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7
+
[M+H] 648.2909.
4-(3-(9H-Carbazol-9-yl)prop-1-yn-1-yl)-7-(1-dodecanetetrazole-4-
ethynylbenzene)-2,1,3-benzothiadiazole 5c, yellow solid, yield 41%, mp 142-
−1
1
146 °C. FTIR (NaCl) cm : 3015, 2918, 2849, 2209, 1459. H NMR (CDCl , 400
3
MHz) ppm: 8.20 (2H, d, J = 8.0 Hz), 8.15 (2H, d, J = 8.0 Hz), 7.78 (2H, d, J =
8.0 Hz), 7.71 (1H, d, J = 8.0 Hz), 7.66 (2H, d, J = 8.0 Hz), 7.55 (3H, m), 7.32
(2H, m), 5.48 (2H, s), 4.67 (2H, t, J = 6.0 Hz), 2.08 (2H, m), 1.28 (18H, m), 0.90
13
(3H, t, J = 8.0 Hz). C NMR (CDCl , 100 MHz) ppm: 164.3, 154.3, 154.1, 139.9,
3
133.2, 132.5, 132.4, 128.0, 126.7, 126.0, 124.2, 123.3, 120.5, 119.6, 117.3,
116.3, 108.9, 97.0, 91.5, 86.6, 79.8, 53.3, 33.5, 31.9, 29.7, 29.6, 29.5, 29.4,
+
29.3, 28.9, 22.7, 14.1. HRMS (ESI) found: [M+H] 676.3251; molecular formula
+
C H N S requires [M+H] 676.3222.
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3. Results and discussion
3.1 Synthesis
In order to obtain the desired non-symmetrical compounds 5a-c, the side
groups carbazole 1 [30] and the tetrazoles 4a-c [31,32] were prepared as
previously described in the literature. In general, compound 1 was synthesized
through the nucleophilic substitution reaction between carbazole and propargyl
bromide. Potassium hydroxide was used as the base and DMF as the solvent
without the need for heating. The tetrazoles 1-alkanetetrazole-4-
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ethynylbenzenes 4a-c were obtained by the formation of the tetrazole ring from
the reaction of 4-bromobenzonitrile with sodium azide and ammonium chloride
in the DMF solution to form the 5-(4-bromophenyl)tetrazole. The latter was
alkylated with the respective 1-bromoalkane in the presence of potassium
carbonate in acetone, forming the derivative 5-(4-bromophenyl)-2-
alkanetetrazole, which reacted with the 2-methyl-3-butyne-2-ol using the
Sonogashira coupling reaction under an inert argon atmosphere to produce an
alcohol intermediate. The latter was unprotected when using sodium hydroxide
in toluene to form the terminal alkynes 4a-c.
Compound
3
was obtained by mono-coupling between the
benzothiadiazole (2) with carbazole 1 in the molar ratio 1:2 (carbazole 1:
benzothiadiazole 2). This step was performed according to the Sonogashira
reaction conditions using copper iodide, bis(triphenylphosphine)palladium
chloride, and a dichloromethane/triethylamine mixture as the refluxing solvent
(Scheme 1). Compound 3 was purified using column chromatography and
obtained in moderate yields (46%). Sequentially, the non-symmetric compounds
5a-c were synthesized by reacting equimolar amounts of compound 3 with the
terminal alkynes 4a-c also according to the Sonogashira reaction conditions.
The desired compounds were purified using column chromatography and
obtained in moderate yields (54%). Spectroscopic characterizations were
performed for all intermediates and final compounds, and the results are
presented in the supplementary material.
11

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2
Scheme 1. Synthesis of non-symmetrical BTD based compounds 5a-c, where
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286
287
(i) and (ii) are CH Cl , PdCl (PPh ) , CuI, Et N.
2
2
2
3 2
3
3.2 Photophysical characterization
The UV–vis absorption and steady-state fluorescence emission spectra
were used to investigate the photophysical properties of the obtained non-
symmetrical BTDs. The compounds were investigated in solutions of different
-6
organic solvents (10 M). The relevant data are summarized in Table 1. From
the UV–Vis spectra of compounds 5a-c shown in Figure 1, absorption maxima
located between 391-400 nm can be observed. Although changes in the solvent
polarity allowed a solvatochromic effect of ~8 nm, no clear tendency was
observed. The latter indicates that these compounds present an almost absent
charge transfer in the ground state.
12

2
88
89
2
Figure 1. UV–Vis absorption spectra in solution of non-symmetrical BTDs 5a-c
-
6
290
291
292
293
294
295
296
297
298
299
300
301
in different organic solvents [ca. 10 M].
The molar absorptivity coefficients were obtained using the Beer–
Lambert law, and they are presented in Table 1. The Strickler–Berg relations
were used to calculate the oscillator strength (f ) presented in Eq. (2) and the
e
0
theoretical rate constant for emission (k ) presented in Eq. (3) [33]. In addition,
e
0
e
0
0
e
k was used to calculate the pure radiative lifetime τ , defined as 1/k [34], and
its values are also shown in Table 1.
ꢏꢐ ≈ 4.3ꢑ10ꢇꢒ ꢓ ꢔꢕꢖ
̅ Eq. (2)
ꢘ
ꢇꢒ
ꢙ
ꢘ
ꢗ ≈ 2.88ꢑ10 ꢖ
̅
ꢓ ꢔꢕꢖ
̅
Eq. (3)
ꢐ
13

302
303
304
305
306
307
308
309
310
311
312
313
314
315
The oscillator strength values at approximately 0.71-0.95 indicated highly
probable electronic transitions. In addition, the molar absorptivity coefficient
4
-1
-1
8
-
values of ~10 M ·cm , as well as calculated radiative rate constants of ~10 s
1
, indicated spin- and symmetry-allowed electronic transitions, which could be
1
-9
related to ππ* transitions. The comparable constant radiative lifetime ~10 s
also indicated that absorptions populated the same excited state. The structural
variation of the compounds, with aliphatic chains from C to C in the tetrazolic
8
12
ring, was observed to not significantly influence the behavior in the ground
state. In order to obtain the fluorescence emission spectra, the absorption
maxima wavelengths were used as excitation radiation. The obtained data are
summarized in Table 1. The respective fluorescence spectra of the non-
symmetrical BTDs 5a-c in different organic solvents are shown in Figure 2.
14

3
16
17
3
Figure 2. Normalized steady-state fluorescence emission spectra in solution of
-
6
318
319
320
321
322
323
324
325
326
327
328
329
non-symmetrical BTDs 5a-c in different organic solvents [ca. 10 M].
The studied compounds showed the main fluorescence emission located
-1
between 460 and 500 nm, with a relatively large Stokes shift (3836-5707 cm ).
Changes in the solvent polarity indicated a solvatochromic effect of 21 nm (5a),
40 nm (5b), and 11 nm (5c). The values of the Stokes shift combined with the
solvatochromic effect show a relationship between the emission band and the
solvent polarity, indicating a small charge transfer characteristic in the excited
state. The compounds showed fluorescence quantum yields below 10%, as
previously observed in similar compounds [31]. Excitation spectra were
acquired in different organic solvents using the fluorescence emission maxima
as the observation wavelength (Figure 3). These data are also summarized in
15

330
331
332
333
334
335
336
337
Table 1. The excitation spectrum indicates the wavelengths that will induce the
emission of light by the molecule from the lowest excited state in a monitored
wavelength and shows the S -S band. The compounds showed similar excited
o
1
spectra and presented the lowest energy absorption features around 410 nm.
Regarding the excited state, the profile in the excitation and emission spectra
also were similar. On the other hand, the fluorescence quantum yield was
higher in the compound with the shorter carbonic chain.
3
38
39
3
Figure 3. Excitation spectra in solution of non-symmetrical BTDs 5a-c in
-6
3
40
41
different organic solvents [ca. 10 M].
3
16

4
Table 1. Photophysical data of compounds 5a-c where λabs is the absorption maxima (nm), ε is the molar extinction coefficient (10
-1
-1
0
e
8
-1
0
M ·cm ), f is the calculated oscillator strength, k is the calculated radiative rate constant (10 s ),
τ
is the calculated pure
-1
radiative lifetime (ns), λem and λexc are the emission and excitation maxima (nm), ∆λST is the Stokes shift (nm/cm ), and QY is the
fluorescence quantum yield (%).
0
0
Compound
Solvent
,4-Dioxane
λabs
ε
f
ke
τ
λex
λem
∆λST
QY
1
394
400
393
392
395
392
398
389
396
391
394
398
394
395
391
7.94
5.82
6.73
8.95
7.75
4.35
4.48
4.58
5.75
7.00
7.91
3.74
4.40
7.43
5.42
1.36
1.04
1.20
1.62
1.45
0.82
0.28
0.85
1.10
0.28
0.94
0.71
0.75
0.95
0.88
8.77
6.53
7.75
10.5
9.30
5.32
1.74
5.63
7.03
1.81
6.04
4.49
4.83
6.06
5.73
1.14
1.53
1.29
0.94
1.08
1.88
5.74
1.78
1.42
5.54
1.66
2.23
2.07
1.65
1.75
415
417
410
411
412
422
426
420
431
421
416
415
413
429
414
479
482
491
480
470
476
490
500
493
460
478
484
489
484
487
85/4504
82/4253
98/5079
88/4677
75/4040
84/4502
92/4717
111/5707
97/4969
69/3836
84/4460
86/4464
95/4931
89/4655
96/5042
2.07
3.03
2.01
3.00
2.69
5.44
2.05
1.56
2.83
2.12
3.90
1.91
1.47
1.36
1.58
Dichloromethane
Methanol
5
5
5
a
b
c
Acetonitrile
Dimethylsulfoxide
1
,4-Dioxane
Dichloromethane
Methanol
Acetonitrile
Dimethylsulfoxide
1
,4-Dioxane
Dichloromethane
Methanol
Acetonitrile
Dimethylsulfoxide
17

Due to the presence of a carbazole moiety in the BTD structures, the
excited state photophysics of the studied compounds was thoroughly
investigated, where their emission spectra were also acquired by exciting them
at λ =290 nm. By doing so, an emission band around λ =350 nm was also
exc
em
observed, indicating that the systems are not completely conjugated (Figure 4).
Figure 4. (a-c) Steady-state fluorescence emission spectra in solution of non-
-6
symmetrical BTDs 5a-c, respectively, in different organic solvents [ca. 10 M]
under an excitation of 290 nm. (d) PL spectrum of 9H-carbazole-9-ethanol
(Aldrich) is also presented at an optical density ~0.1 to simulate the carbazole
motif photophysics.
18

Hence, to further understand the deactivation dynamics in these
compounds, the fluorescence decays were determined using the TCSPC
technique by exciting the molecules at λ =290 nm with a pulsed excitation
exc
source. Figure 5 presents the fluorescence decay curves of the non-
symmetrical BTD 5b. The additional compounds showed similar behavior and
are presented as supplementary data. The relevant data is summarized in
Table 2.
Figure 5. Fluorescence decays and respective exponential fit for the non-
symmetrical BTD 5b at the (a) short and (b) long emission wavelengths.
In general, three different lifetime constants were obtained for the three
compounds in all studied solvents. The decays obtained at the emission band
from the carbazole unit were adjusted as mono and biexponential functions,
with the longest component belonging to the carbazole unit. This assignment
was performed after comparing the decays with the fluorescence lifetime of 9H-
19

carbazole-9-ethanol in 1,4-dioxane (τ =10.00 ns) (data not shown, see
2
supplementary material). The curves acquired at the red-shifted bands were
only adjusted by biexponential decays, and the shortest components were
always similar to the ones found for the decays acquired at λ =350 nm,
em
indicating spectral overlap between the two emission bands. Finally, the other
two lifetime constants were assigned based on the results from theoretical
predictions. The shortest component found on both sets of decays were
attributed to the benzothiadiazole moiety (τ ), because it also emits in the blue
1
region of the electromagnetic spectrum [2,35], and the longest one observed in
the decays of the red-shifted bands were attributed to the conjugated system
benzothiadiazole-tetrazole (τ ). The latter is due to the intramolecular charge
3
transfer process, as demonstrated by theoretical predictions. The obtention of
bipolar host materials presenting intramolecular charge transfer is an interesting
strategy for improvement of OLED devices [36], allowing these compounds to
be envisaged for such technological application.
20

Table 2. Time-resolved photophysical data of compounds 5a-c and 9H-
carbazole-9-ethanol, so-called carbazole, where λem is the emission maxima
(nm) at the excitation wavelength of 290 nm, τ is the experimental fluorescence
lifetime (ns), A is the respective pre-exponential factor, and χ² indicates the
quality of the fit.
a
b
c
Compound Solvent
λem τ1
50 1.36 18 9.46 82
79 1.88 77 -
50 - 10.2 100 -
76 4.15 50 - 6.67 50 1.053
50 3.05 12 7.17 88 1.151
56 2.99 41 - 4.87 59 1.188
50 - 11.1 100 - 1.113
76 2.03 68 - 7.29 32 1.168
50 - 9.17 100 - 1.316
62 5.50 64 - 5.88 36 1.149
50 - 11.4 100 - 1.194
50 2.87 24 - 4.86 76 1.168
50 2.22 18 9.18 82 1.470
80 2.63 32 - 5.32 68 1.114
50 - 10.9 100 - 1.117
81 2.12 70 8.17 30 1.307
50 - 11.6 100 - 1.240
62 4.00 67 - 9.11 33 1.057
50 - 12.00 100 - 1.109
81 2.39 16 - 5.75 84 1.146
50 - 8.12 100 - 1.196
51 3.57 90 - 7.20 10 0.984
50 10.4 100 - 1.175
81 1.98 77 - 6.95 23 1.154
357 - 10.0 100 - 1.180
BTD fluorophore; Carbazole fluorophore; BTD-tetrazole fluorophore
A τ
A2 τ3
A3 χ²
1.089
4.06 23 1.014
1.098
1
2
3
4
3
4
3
4
3
4
3
4
3
4
3
4
3
4
3
4
3
4
3
4
3
4
-
-
1
,4-Dioxane
-
-
-
Methanol
Acetonitrile
Dimethylsulfoxide
-
5
5
5
a
b
c
-
-
-
-
-
-
-
-
1
,4-Dioxane
-
-
-
Methanol
-
-
-
Acetonitrile
-
-
-
Dimethylsulfoxide
-
-
1
,4-Dioxane
-
-
-
Methanol
-
-
-
Acetonitrile
-
-
-
Dimethylsulfoxide
-
Carbazole 1,4-Dioxane
-
-
a
b
c
21

3
.3 Theoretical calculations
The structure of the chromophore was optimized by DFT under vacuum
using a methyl group as the substituent on the alkyl chains, because it will not
affect the main molecular electronic structure (Figure 6). The ground state
geometry presents a dihedral angle of 179.6º formed on the saturated carbon
between carbazole and the alkyne unit and a dipole moment of µ = 4.19
g
Debye.
Figure 6. (a) Top and (b) lateral view of the optimized geometry at the ground
state of a non-symmetrical BTD used as a model of the synthesized compounds
5
a-c.
Despite the presence of this saturated carbon, predictions of frontier
molecular orbitals indicated a charge transfer character from the carbazole unit
toward the benzothiadiazole (HOMO ‰ LUMO transition). Specifically, the
conjugated framework evolving from the tetrazolic moiety and benzothiadiazole
moieties became clear after the analysis of other sets of molecular orbitals
(Figure 7). The charge transfer character was finally concluded from
22

optimization of the excited state structure (TD-DFT), which possess a dipole
moment of µ = 25.0 Debye (∆µ = 20.81 Debye). Considering the predicted
e
electronic transitions, vertical excitations of λabs = 392 nm and 280 nm with
biggest oscillator strengths (f=0.86 and 0.68, respectively) were identified,
matching with experimental values of carbazole and benzothiazole-tetrazole
moieties.
Figure 7. HOMO (left) and LUMO (right) orbitals of a non-symmetrical BTD
used as a model of the synthesized compounds 5a-c.
3
.4 Electrochemical characterization
Figure 8 revealed that compounds 5a-c are electrochemically active in
the studied potential range. At more negative potential values, the compounds
exhibit a well-defined reduction process with an onset potential of -2.00 to -2.05
+
V vs. Ag/Ag , which may be related to the formation of anionic radicals [31]. The
scans performed at more positive potential values showed an irreversible
+
oxidation process with an onset potential of 1.54 to 1.61 V vs. Ag/Ag due to the
23

electrochemical formation of the cation radical. The HOMO and LUMO energy
levels of these compounds were determined by the ionization potential (IP) and
electron affinity (EA), respectively, which could be correlated with
electrochemical processes accessed by CV, where the potentials were
+
standardized with a Fc/Fc couple. The IP and EA values were calculated using
the empirical Eqs. (4) and (5) [37]:
ꢛꢞꢟꢠꢡ
IP = ꢚꢁEꢛꢜꢝ ꢢ 4.44ꢃ eV, Eq. (4)
ꢛꢞꢟꢠꢡ
EA = ꢚꢁEꢣꢠꢤ ꢢ 4.44ꢃ eV, Eq. (5)
ꢛꢞꢟꢠꢡ
ꢛꢞꢟꢠꢡ
represent the oxidation onset potential and
ꢣꢠꢤ
where Eꢛꢜꢝ
and E
reduction onset potential, respectively. The electrochemical data obtained from
these calculations are summarized and compared in Table 3.
Figure 8. Cyclic voltammogram of a glassy carbon electrode in 0.1
-1
TBAPF /CH Cl at 50 mV∙s of non-symmetrical BTDs 5a-c.
6
2
2
24

Table 3. Electrochemical properties of non-symmetrical BTDs 5a-c, where
ꢛꢞꢟꢠꢡ
ꢛꢜꢝ
ꢛꢞꢟꢠꢡ
E
is the onset potential of oxidation, Eꢣꢠꢤ
is the onset potential
of reduction, IP (HOMO) is the ionization potential, EA (LUMO) is the electron
affinity, and Eꢠꢨꢠ is the electrochemical band gap.
ꢥꢦꢧ
Non-symmetrical BTDs
Parameters
5
a
5b
5c
ꢛꢞꢟꢠꢡ
ꢣꢠꢤ
a
E
E
(V)
(V)
1.79
1.72
-1.84
-6.16
-2.60
1.77
-1.82
-6.21
-2.62
ꢛꢞꢟꢠꢡ
ꢣꢠꢤ
a
-1.87
-6.23
-2.57
b
IP (HOMO) (eV)
c
EA (LUMO) (eV)
eV)
(
3
.66
3.56
3.59
a
b
c
versus NHE; calculated using Eq. (3); calculated using Eq. (4).
A comparison of the data presented in Table 1 with the electrochemical
properties of non-symmetrical BTDs 5a-c indicates that modifying the alkyl
tetrazole moiety affects the band gap energies of these molecules. The different
electrochemical behaviors resulting from modifying fluorophores derivatives
consequently cause changes in the HOMO and LUMO values, as well as the
band gaps values of the molecules, showing that the electrochemical properties
have changed due to the structural variation of the compounds, with aliphatic
chain C , C , and C in the tetrazolic ring.
8
10
12
3
.5 Thermal characterization
The DSC analyses of the of non-symmetrical BTDs 5a-c are presented in
Figures 9a-c, respectively. It can be observed that the melting temperature (Tm)
during the first heating cycle was found for compound 5a at ~145 °C (∆H = -
m
25

-1
-1
4
5.26 J∙g ) and for compound 5b at ~65 °C (∆H = -2.21 J∙g ), which is
m
probably
related
to
the
removal
of
the
residual
solvent
-1
(
hexane/dichloromethane) at ~122 °C (∆H = -15.22 J∙g ) and ~143 °C (∆H = -
m
m
-1
4
2.71 J∙g ), respectively, indicating a possible polymorphism of the structure
due to the presence of the solvent. For compound 5c the melting temperature
-1
during the first heating cycle was at ~128 °C (∆H = -64.66 J∙g ) [38]. During the
m
second heating cycle, the studied compounds presented different curves with
cold crystallization (T ) of the amorphous materials and melting temperature
cc
(
T ). T and T values for compound 5a were found at ~92 °C (∆H = 11.97
m
cc
m
cc
-1
-1
J∙g ) and ~113 °C (∆H = -5.23 J∙g ), for compound 5b at ~76 °C (∆H = 5.89
m
cc
-1
-1
J∙g ) and ~112 °C (∆H = -4.78 J∙g ), and for compound 5c at ~73 °C (∆H =
m
cc
-1
-1
7
.83 J∙g ) and ~103 °C (∆H = -3.17 J∙g ), respectively. Compound 5b was the
m
only one that presented a crystallization temperature (T ) during cooling at ~76
cr
-1
°
C (∆H = 2.594 J∙g ), probably due to crystallization of the first crystalline
cr
phase formed (polymorphism). The changes in the values of T and ∆H during
m
m
the second heating could be related to morphological changes after
recrystallization [39].
26

Figure 9. DSC curves (exo up) of non-symmetrical BTDs (a) 5a, (b) 5b, and (c)
-1
5
c at a heating rate of 10 °C∙min from 25 to 200 °C under a nitrogen gas
-1
atmosphere and (d) TGA curves with a heating rate of 10 °C∙min from 25-900
°
C under a nitrogen gas atmosphere.
The thermogravimetric curves (TGA) of the studied non-symmetrical
BTDs presented three main steps of weight loss, as depicted in Figure 9d. The
DTG curves are presented as supplementary material. The first step between
1
00 and 160 °C corresponds to the removal of water molecules. After that, the
compounds start to decompose at ~200 °C. The maximum degradation
temperature (T ) in the second step is located at ~280 °C and the third step at
d
~450 °C. These results indicate a relatively high thermal stability of the
compounds [40].
27