Synthesis, electrochemical, thermal and photophysical characterization of photoactive discotic dyes based on the tris-[1,2,4]-triazolo-[1,3,5]-triazine core

Article abstract of DOI:10.1016/j.dyepig.2016.06.041

This work presents the synthesis and characterization of compounds based on the heterocycle tris-[1,2,4]-triazolo-[1,3,5]-triazine (TTT) obtained by a new building block that can be applied to afford different discotic materials in one step. The thermal p

Full text of DOI:10.1016/j.dyepig.2016.06.041

Dyes and Pigments xxx (2016) 1e8
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis, electrochemical, thermal and photophysical
characterization of photoactive discotic dyes based on the tris-[1,2,4]-
triazolo-[1,3,5]-triazine core
a
a
a
Alexandre Gonçalves Dal-B oꢀ , Georgina Gisel L oꢀ pez Cisneros , Rodrigo Cercena ,
a
a
c
c
Jackson Mendes , Letícia Matos da Silveira , Eduardo Zapp , Kelvin Guessi Domiciano ,
Rodrigo da Costa Duarte , Fabiano Severo Rodembusch , Tiago Elias Allievi Frizon
b
b
a, *
a
Universidade do Extremo Sul Catarinense, Av. Universit aꢀ ria, 1105, CP 3167, CEP 88806-000, Criciúma, SC, Brazil
b
Grupo de Pesquisa em Fotoquímica Org a^ nica Aplicada, Instituto de Química/UFRGS, Av. Bento Gonçalves, 9500, CEP 91501-970, Porto Alegre, RS, Brazil
Universidade Federal de Santa Catarina, UFSC e Campus Ararangu aꢀ , Rodovia Governador Jorge Lacerda, 3201 e Km 35,4, Jardim das Avenidas, CEP
8906072, Ararangu aꢀ , SC, Brazil
c
8
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 April 2016
Received in revised form
This work presents the synthesis and characterization of compounds based on the heterocycle tris-
[
1,2,4]-triazolo-[1,3,5]-triazine (TTT) obtained by a new building block that can be applied to afford
different discotic materials in one step. The thermal properties were evaluated by TGA and DSC and
indicate very thermal stable compounds. The compounds exhibited absorption in the UV-B region
17 June 2016
Accepted 24 June 2016
Available online xxx
(
~280 nm) with molar absorptivity coefficients and radiative rate constants arising from spin and
symmetry allowed * electronic transitions. An emission located in the UV-A region (~363 nm) was
observed, which was most likely due to a normal emission because no evidence of a charge-transfer
mechanism in the excited state was observed. Spin-coated films indicate the presence of -stacking in
pep
Keywords:
Tris-[1,2,4]-triazolo-[1,3,5]-triazine
p
compounds presenting lower alkyl chains. Cyclic voltammetry measurements showed reduction and
oxidation processes. The HOMO energy was obtained from the oxidation potential corresponding and the
LUMO energy was found by subtracting its optical band gap from the HOMO value. In addition, in situ
UV-Vis measurements were employed to evaluate the changes in the absorption spectra due to the
changes in the electronic structure of the conjugated molecules under oxidative and reductive potentials.
p
-stacking
Fluorescence
Thermal stability
©
2016 Elsevier Ltd. All rights reserved.
1
. Introduction
luminescence [4,5]. Detection technology based on light-emitting
molecules and semiconductors has been widely studied due to
the low power consumption required, low cost and ease of use in
sensors. In the presence of external agents, such as temperature
and electric field, the orientation of these materials is altered,
producing an optical signal that can be easily visualized [6]. Organic
Several detection technologies based on different physico-
chemical signal transmission mechanisms, such as colorimetry,
metal oxide based devices, electrochemistry, chemiluminescence
and semiconductor devices, have been developed in recent years
[1]. However, these technologies are limited in their application
materials containing p conjugated bridges in its structure allow for
due to lack of sensitivity, selectivity, or portability [2]. The most
widely used techniques in optical sensors are optical absorption
and luminescence [3]. As a means of controlling absorption and/or
luminescence, molecular design plays a crucial role in providing the
ability to control the properties of molecules via functionalization;
for example, introducing heterocycles may confer high
photo- or electro-luminescence and charge transportation (organic
semiconductor), thereby providing the ability to reversibly respond
to external stimuli, such as chemical, electro-chemical, or photo-
chemical stimuli.
These features make organic semiconductors good candidates
for applications in molecular electronics, such as photoconductors,
liquid crystals, organic light emitting diodes, solar cells and optical
sensors [7e10]. In this context, discotic functional materials,
discovered by Chandrasekhar et al. in 1977 [11], are now viewed as
*
Corresponding author.
E-mail address: tiagofrizon@unesc.net (T.E.A. Frizon).
http://dx.doi.org/10.1016/j.dyepig.2016.06.041
143-7208/© 2016 Elsevier Ltd. All rights reserved.
0
Please cite this article in press as: Dal-B oꢀ AG, et al., Synthesis, electrochemical, thermal and photophysical characterization of photoactive
discotic dyes based on the tris-[1,2,4]-triazolo-[1,3,5]-triazine core, Dyes and Pigments (2016), http://dx.doi.org/10.1016/j.dyepig.2016.06.041

2
A.G. Dal-B oꢀ et al. / Dyes and Pigments xxx (2016) 1e8
the basis of a new generation of organic semiconductor compounds
12]. Discotic functional materials offer the advantage of highly
Optically transparent gold mesh was employed as the working
electrode, a platinum wire was used as the auxiliary electrode, and
[
þ
ordered structures, which enable processing of the optical signals
and storing of the electro-optic information, as well as excellent
load carrying and thermal stability [13]. Combining these charac-
teristics in a single molecule can enable new compounds with high
charge mobility combined with light emission capability in the
solid state or in solution; such compounds have potential applica-
tions in electroluminescent devices [6,14]. The last decade has seen
growing interest in the synthesis of discotic compounds containing
the heterocycle tris-[1,2,4]-triazolo-[1,3,5]-triazine (TTT) [15e18]
because such discotic compounds are good candidates for use in
functional discotic materials with luminescence and load carrying
capacity. Some examples in the literature show highly stable phases
for widely conjugated and luminescent compounds by adding to
the ends of a greater number of aliphatic chains.
Ag/Ag was employed as the reference electrode. Tetrabuty-
ꢀ1
lammonium hexafluorophosphate (0.1 mol
L
in dichloro-
methane) was employed as the electrolyte. The spectra of the tris-
[1,2,4]-triazolo-[1,3,5]-triazines were obtained by a constant po-
tential with the aid of a coupled potentiostat. Low-resolution mass
spectra were recorded on a Bruker Daltonics Autoflex apparatus for
MALDI and on a Waters Micromass ZQ spectrometer for ESI/ApCI-
þ
ITD (Esquire 3000 Bruker Daltonics). The thermal properties
were investigated using differential scanning calorimetry on DSC-
2
50 and SDT Q600 TA Instruments using a N atmosphere with a
ꢀ1
ꢁ
ꢀ1
flow rate of 100 mL min and a heating rate of 10 C min . The
DSC curves were performed using two heating-cooling cycles in
order to obtain the thermal transitions and the glass transition
temperatures, respectively.
In this manner, new discotic compounds containing heterocyclic
tris-[1,2,4]-triazolo-[1,3,5]-triazine (TTT) with a variable number of
flexible alkoxy chains were synthesized and characterized. In
addition, we studied the thermal, electrochemical and photo-
physical properties of the synthesized molecules.
2.2. Synthesis
2.2.1. 5-(4-hidroxyphenyl)tetrazole (1)
4-Cyanophenol (10.01 g, 84 mmol), NaN
3
(16.38 g, 252 mmol),
NH Cl (13.48 g, 252 mmol) and 100 mL of DMF were added to a
4
2
. Experimental
round bottom flask. The mixture was kept under heating and stir-
ꢁ
ring at 125 C for 20 h. Next, the solution was poured into 300 mL of
ꢀ1
2.1. Materials and methods
water and then acidified to pH z 1 using HCl (6 mol L ). The
precipitate formed was filtered and recrystallized from water,
ꢁ
All reagents were used as received. The solvents were purified
yielding 10.4 g (77%) of a crystalline orange solid. M.p.: 237e239 C.
FTIR (
ꢀ
1
before use, according to the procedure in the literature [19]. FTIR
spectra were recorded on a Shimadzu IR Prestige-21 with a reso-
nmax/cm ): 3367, 3099, 3065, 3021, 2937, 2847, 2743, 2632,
2494, 1647, 1615, 1599, 1515, 1471, 1415, 1379, 1280, 1249, 1181, 1154,
ꢀ1
1
13
1
lution of 4 cm using KBr pellets. The H and C NMR spectra were
recorded in DMSO-d or CDCl at 400 and 100 MHz, respectively.
The chemical shifts ( ) are reported in parts per million (ppm)
relative to TMS, and the coupling constants J are reported in Hertz
Hz). Spectroscopic grade solvents were used in the photophysical
6
1061, 1025, 995, 918, 842, 792. H NMR (DMSO-d , 400 MHz): 3.52
6
3
(broad, 1H, tet-H), 6.98 (d, J ¼ 8.9 Hz, 2H, AreH), 7.89 (d, J ¼ 8.9 Hz,
13
d
2H, AreH), 10.23 (broad, 1H, AreOH).
6
C NMR (DMSO-d ,
100 MHz): 115.3, 116.8, 129.4, 155.4, 160.8.
(
study. The UV-Vis absorption spectra in solution and in spin-coated
films were acquired on a Shimadzu UV-2450 spectrophotometer,
and the steady-state fluorescence spectra were measured on a
Shimadzu spectrofluorometer model RF-5301PC. Spin coated films
containing the tris-[1,2,4]-triazolo-[1,3,5]-triazines were prepared
under vacuum using a Glove Box MBRAUN model MB 200B; silicon
plates were used as the substrates for the films. The quantum yield
2.2.2. 5-(4-acetoxyphenyl)tetrazole (2)
5-(4-hydroxyphenyl)tetrazole (1) (5.02 g, 31 mmol) and 80 mL
of water were added to a round bottom flask. To this suspension
ꢀ
1
was added aqueous NaOH (3 mol L ) under magnetic stirring and
at room temperature until the phenol is completely dissolved. Next,
the solution was cooled in an ice bath, and then glacial acetic an-
hydride (3.37 g, 33 mmol) was slowly added. After complete
addition, the system was stirred for 5 min in an ice bath and 15 min
at room temperature. The suspension was then poured into 300 mL
of ice/water, acidified to pH z 2, and then filtered. Recrystallization
from water yielded 5 g (98%) of a crystalline white solid. M.p.:
of fluorescence (
method. Quinine sulfate (QS) in H
FFL) was determined by applying the dilute optical
ꢀ
1
2
SO
4
(0.5 mol L ) (FFL ¼ 0.55)
was used as the quantum yield standard [20]. The uncertainties
related to the quantum yield of fluorescence (DFFL) measurements
using QS are in order of 6% [21,22]. The absorption spectra of the
spin-coated films were measured on a Shimadzu UV2450PC spec-
trophotometer using an ISR-2200 integrating sphere. The baseline
ꢁ
ꢁ
ꢀ1
182e183 C (dec.) (lit. 182 C) [10]. FTIR (nmax/cm ): 3072, 3017,
2924, 2859, 2775, 2724, 2630, 2485, 1756 (C]O), 1614, 1501, 1440,
1407, 1364, 1288, 1212, 1170, 1054, 1026, 1007, 994, 913, 850, 751. H
1
in the solid state was obtained using BaSO
4
. The fluorescence
NMR (DMSO-d
6 3
, 400 MHz): 2.33 (s, 3H, COOCH ), 3.61 (broad, 1H,
spectra of the films were obtained with a solid sample holder at an
angle designed to limit the reflected excitation beam from the
emission monochromator.
tet-H), 7.41 (d, J ¼ 8.8 Hz, 2H, AreH), 8.10 (d, J ¼ 8.8 Hz, 2H, AreH).
13
C NMR (DMSO-d
6
, 100 MHz): 21.5, 122.4, 123.7, 129.1, 153.2, 160.0,
169.7.
The electrochemical measurements were performed using a
Biologic potentiostat Model SP-200. The cyclic voltammetry ex-
2.2.3. Tris-(4-hydroxyphen-1-yl)-[1,2,4]-triazolo-[1,3,5]-triazine
(4)
In a 3 neck round bottom flask equipped with condenser and
under inert Ar atmosphere, a mixture of 5-(4-acetoxyphenyl)tet-
razole (2) (1.84 g, 9 mmol), cyanuric chloride (0.55 g, 3 mmol) and
ꢀ
1
periments were performed using 0.1 mg mL solutions of the TTTs
ꢀ1
in 0.1 mol L
tetra-n-butyl ammonium hexafluorophosphate
(
TBAPF
6
) in dry dichloromethane as the supporting electrolyte, and
þ
the ferrocene/ferricenium (Fc/Fc ) redox couple was employed as
the internal reference. The experiments were performed using a
standard three-electrode cell with a circular glassy carbon elec-
anhydrous K
under vigorous stirring and reflux for 48 h. The hot mixture was
filtered on Büchner funnel during washing with CH Cl
2 3
CO (4.98 g, 36 mmol) in 80 mL butanone was kept
þ
ꢀ1
trode, a Pt-wire counter electrode and Ag/Ag (AgNO
3
0.01 mol L
2
2
(3 ꢂ 20 mL).
in acetonitrile) as the reference electrode. All measurements were
conducted in an electrolyte solution that had been purged with
The protecting group acetyl was removed by adding 20 mL of
distilled water and 30 mL of concentrated HCl to compound (3). The
mixture was kept to reflux for 24 h and then cooled to room tem-
purified N
formed on
2
. The spectroelectrochemical measurements were per-
Shimadzu UV-1800 spectrofluorophotometer.
ꢀ1
a
perature and basified with NaOH solution (1 mol L ). The solid
Please cite this article in press as: Dal-B oꢀ AG, et al., Synthesis, electrochemical, thermal and photophysical characterization of photoactive
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A.G. Dal-B oꢀ et al. / Dyes and Pigments xxx (2016) 1e8
3
formed was filtered and washed with water to give the desired
elimination. The reaction product undergoes ring thermolysis, fol-
lowed by nitrogen elimination generating an iminonitrile, which
cyclizes to produce the desired heterocycle [23,24]. Compound 4
was obtained in 60% yield by acid catalysed deprotection of the
acetyl groups of precursor 3.
ꢁ
product. Yield: 2.6 g (60%) of a light beige solid. M.p.: 120 C (dec.)
ꢀ
1
1
FTIR (
DMSO-d
J ¼ 8.44 Hz, Ar), 4,60 (broad, OH). C NMR (DMSO-d
75.7, 161.5, 159.6, 127.6, 122.0, 116.2.
n
max/cm ): 3448, 1610, 1585, 1487, 1259, 1180, 831. H NMR
(
6
, 400 MHz): 8.60 (d, 6H, J ¼ 8.43 Hz, Ar), 7.65 (d, 6H,
13
6
, 100 MHz):
1
Finally, to obtain the compounds 5e7, an alkylation with three
different alkyl bromides containing 8, 10, or 12 carbon atoms was
performed via Williamson reaction in the presence of potassium
carbonate to afford the final compounds with yields higher than
60%.
2
.2.4. General procedure to synthesize (5e7)
Tris-(4-hydroxyphenyl-1-yl)-[1,2,4]-triazolo-[1,3,5]-triazine (4)
3.01 g, 6.3 mmol), K CO (2.76 g, 20.0 mmol), 19.5 mmol of the
corresponding alkyl bromides and a catalytic amount of KI in
50 mL of butanone were added into a round bottom flask. The
mixture was refluxed under vigorous stirring for 20 h. The K CO
(
2
3
All compounds were characterized by 1H and 13C NMR spec-
troscopies (Figs. S1e8). For compounds 5e7 the obtained NMR
1
2
3
3
spectra in CDCl showed a very similar profile (data not shown). For
was filtered, and then the solvent was evaporated at reduced
pressure. The obtained product was extracted with a mixture of
ethyl acetate/water. The organic phase was dried with sodium
sulfate (Na SO ). After filtration and solvent evaporation, the
2 4
resulting solid was purified by column chromatography using silica
example, compound 7 exhibits two doublets with a coupling con-
stant of 8.79 Hz centered at approximately 8.10 and 7.07 ppm
relative areas to six hydrogens for each signal (AB system in Fig. S7),
which is related to the phenyl rings attached to the TTT core. In
addition, a triplet located at approximately 4.0 ppm with coupling
constant of 6.74 Hz is observed, which is related to the six hydro-
gens (c) present in the methylene groups adjacent to the oxygen
atoms in the alkyl portion of the molecule. In the region of 1.8 to
0.8 ppm, a set of signals related to 69 hydrogens is observed, which
gel and dichloromethane as eluent.
2
.2.4.1. Tris-(4-octyloxyphen-1-yl)-[1,2,4]-triazolo-[1,3,5]-triazine
ꢁ
(
5). Yield: 60% of a dark beige solid. M.p.: 97e98 C. FTIR (
n
max
/
ꢀ
1
1
13
cm ): 2924, 2854, 1595, 1483, 1255, 1180, 833. H NMR (CDCl
3
,
corresponds to the alkyl moiety of compound 7. In the C NMR
4
4
00 MHz): 8.08 (d, 6H, J ¼ 9.06 Hz, Ar), 7.05 (d, 6H, J ¼ 8.81 Hz, Ar),
.05 (t, 6H, J ¼ 6.55 Hz, eOCH e), 1.86 (quint, 6H, J ¼ 8.06 Hz,
CH e), 1.51e1.33 (m, 30H, eCH
e), 0.93 (t, 9H, J ¼ 7.05 Hz,
, 100 MHz): 162.0, 150.8, 140.5, 131.8, 115.8,
spectrum (data not shown, Fig. S8), six different aromatic carbons
were found, of which, the signals at 150.7 and 140.4 ppm were
highlighted from the TTT system.
2
eOCH
eCH ). C NMR (CDCl
3 3
2
2
2
13
1
C
14.5, 68.2, 31.9, 29.4, 26.1, 22.7, 14.2. ESI Anal. Calcd. for
3.2. Photophysical characterization
þ
H
48 63
N
9
O
3
: m/z 814.1; found m/z 814.6 [MþH] .
Fig. 1 presents the electronic spectra of the TTTs 5e7 in
dichloromethane. The relevant data from the UV-Vis absorption
spectroscopy are summarized in Table 1. All compounds presented
similar photophysical behavior in DMSO and DMF (Figs. S24e27).
Form the Strickler-Berg relationship, it is possible to determine the
2
.2.4.2. Tris-(4-decyloxyphen-1-yl)-[1,2,4]-triazolo-[1,3,5]-triazine
ꢁ
(
6). Yield: 63% of a pale yellow solid. M.p.: 89e90 C. FTIR (
n
max
/
ꢀ
1
1
cm ): 2924, 2852, 1610, 1595,1485, 1255, 1180, 833. H NMR
CDCl
, 400 MHz): 8.05 (d, 6H, J ¼ 8.79 Hz, Ar), 7.02 (d, 6H,
J ¼ 8.79 Hz, Ar), 4.01 (t, 6H, J ¼ 6.44 Hz, eOCH e), 1.81 (quint, 6H,
J ¼ 6.44, eOCH CH e), 1.49e1.24 (m, 42H, eCH e), 0.87 (t, 9H,
, 100 MHz): 162.0, 150.8, 140.5,
(
3
0
0
2
pure radiative lifetimes (
e
t ) and the radiative rate constants (k )
2
2
2
[25]. In this relationship, the pure radiative lifetime can be defined
13
0
J ¼ 7.03 Hz, eCH
3
). C NMR (CDCl
3
e e
as 1/k , and the oscillator strength (f ) can be obtained [26].
131.8, 115.7, 114.5, 68.3, 32.0, 29.6, 29.4, 26.1, 22.7, 14.2. ESI Anal.
The tris-[1,2,4]-triazolo-[1,3,5]-triazines present absorption
maxima in the UV-B region, below 300 nm (~280 nm), which is
attributed to electronic transitions in the heteroaromatic portion of
the molecule, as already observed in the parent compounds
[16e18,27]. The molar absorptivity coefficient (ε) values for both
þ
Calcd. for C54
H
75
N
9
O
3
: m/z 898.2; found m/z 898.7 [MþH] .
2.2.4.3. Tris-(4-dodecyloxyphen-1-yl)-[1,2,4]-triazolo-[1,3,5]-triazine
ꢁ
(
7). Yield: 65% of a pale white solid. M.p.: 84e85 C. FTIR (
n
max
/
ꢀ
1
1
cm ): 2918, 2848, 1610, 1585, 1487, 1259, 1180, 831. H NMR
CDCl
, 400 MHz): 8.10 (d, 6H, J ¼ 8.79 Hz, Ar), 7.07 (d, 6H,
J ¼ 8.9 Hz, Ar), 4.06 (t, 6H, J ¼ 6.74 Hz, eOCH e), 1.85 (quint, 6H,
J ¼ 7.91 Hz, eOCH CH e), 1.51 (m, 6H, eCH e), 1.31 (broad, 48H,
eCH ). C NMR (CDCl , 100 MHz):
e), 0.91 (t, 9H, J ¼ 6.74 Hz, eCH
62.0,150.8,140.4,131.8,115.7,114.4, 68.2, 31.9, 29.7, 29.4, 26.0, 22.7,
absorption bands as well as the calculated radiative rate constants
0
(
3
(k
e
) for all of the compounds indicate that the spin and symmetry
1
2
allowed electronic transitions were related to pp* transitions,
where high values to the oscillator strength could be calculated,
probably related to the rigidity of the TTT core. The absence of a
shoulder on the absorption spectra located at higher wavelengths
(>300 nm), as well as any evidence of solvatochromism allowed us
to conclude that these compounds do not present an ICT character
2
2
2
13
2
3
3
1
14.1. ESI Anal. Calcd. for C60
87
H N
9
O
3
: m/z 982.4; found m/z 982.8
þ
[MþH] .
in the ground state [15]. Similar radiative lifetime values were
0
3
. Results and discussion
obtained (
t
~ 1 ns) for all of the compounds, indicating that after
radiation absorption, the fluorophores populate the same excited
state.
3.1. Synthesis and spectroscopic characterization
The fluorescence emission spectra of tris-[1,2,4]-triazolo-[1,3,5]-
triazines 5e7 in dichloromethane are also shown in Fig. 1. These
compounds exhibited a similar photophysical behavior in both
DMSO and DMF. The emission curves were obtained by exciting the
compounds at the absorption maxima wavelength. The relevant
data from fluorescence emission spectroscopy are listed in Table 1.
The changes in the chemical structure of the fluorophores do not
appear to play a key role in the fluorescence maxima location, since
the TTTs 5e7 present fluorescence emission in the UV-A region
(~363 nm). Furthermore, the absent solvatochromism from the
emission curves (Dlem ¼ 1 nm) are indicating that no charge-
The synthetic route for the preparation of all compounds is
shown in Scheme 1.
The intermediate 1 containing the tetrazole ring was obtained
from 4-cyanophenol under reflux with sodium azide and ammo-
nium chloride in N,N-dimethylformamide. The next step was the
acetylation of compound 1 using acetic anhydride and sodium
hydroxide to yield compound 2. To obtain the compound contain-
ing the tris-[1,2,4]-triazolo-[1,3,5]-triazine core (3), a procedure
proposed by Huisgen was used [23]. This step involves the reaction
of a tetrazole ring with cyanuric chloride, followed by HCl
Please cite this article in press as: Dal-B oꢀ AG, et al., Synthesis, electrochemical, thermal and photophysical characterization of photoactive
discotic dyes based on the tris-[1,2,4]-triazolo-[1,3,5]-triazine core, Dyes and Pigments (2016), http://dx.doi.org/10.1016/j.dyepig.2016.06.041

4
A.G. Dal-B oꢀ et al. / Dyes and Pigments xxx (2016) 1e8
Scheme 1. Synthetic route to obtain the tris-[1,2,4]-triazolo-[1,3,5]-triazines 5e7.
Fig. 1. UV-Vis absorption (left) and fluorescence emission (right) spectra for the tris-[1,2,4]-triazolo-[1,3,5]-triazines 5e7 in dichloromethane. Photophysical data for the precursor 4
is also presented for comparison (dotted lines).
Table 1
4
ꢀ1
ꢀ1
Relevant photophysical data of the UV-Vis spectra of 5e7, where
oscillator strength, k
l
abs is the absorption maxima (nm), ε is the molar absorptivity (ꢂ10 L mol cm ), f
e
is the calculated
0
9
ꢀ1
0
e
(ꢂ10 ) is the calculated radiative rate constant (s ),
t
is the inherent emission lifetime (ns), lem is the emission maxima (nm), DlST is the Stokes shift
ꢀ
1
(
nm/cm ), and
F
FL is the fluorescence quantum yield.
0
t0
Solvent
Compound
l
abs
ε
f
e
k
e
l
em
DlST
F
FL
Dimethylformamide
Dimethylsulfoxide
Dichloromethane
5
6
7
5
6
7
5
6
7
282
282
282
282
282
282
288
288
288
6.0
3.3
1.7
4.8
3.4
3.0
2.8
4.8
4.7
0.958
0.873
0.591
0.905
0.897
0.792
0.966
0.917
0.939
12.05
10.98
0.744
11.38
11.28
0.997
11.65
11.06
11.32
0.83
0.91
1.34
0.88
0.89
1.00
0.86
0.90
0.88
362
362
362
363
363
363
363
363
363
80/7836
80/7836
80/7836
81/7912
81/7912
81/7912
75/7174
75/7174
75/7174
0.10
0.10
0.10
0.09
0.13
0.12
0.15
0.22
0.23
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A.G. Dal-B oꢀ et al. / Dyes and Pigments xxx (2016) 1e8
5
transfer takes place in the excited state. On the other hand, in so-
lution the alkoxy groups seems to play a significant role on the
excited singlet state deactivation due to (i) the relative large Stokes
shift (75e81 nm), although any evidence of charge transfer char-
acter was observed, which can be related to a non-radiactive energy
loss due to conformational relaxation of the alkoxy chains present
in the TTTs; and (ii) the low values to the fluorescence quantum
yields (0.09e0.23), which can be related to non-radiactive path-
ways probably due to the free bond rotation of the benzene ring
and alkoxy chains [17].
obtained in solution (~360 nm). A higher fluorescence emission
could be observed in compound 7, which has the higher alkyl chain.
In contrast, spin-coated films containing compound 5, which pre-
sents the lower alkyl chain, show a very weak fluorescence emis-
sion, similar to that observed for the parent compound 4 (Fig. 3c).
As the formation of excimers via
suppress the emission in solids in similar compounds [16], the
fluorescence emission spectra indicate that the -stacking was
sterically inhibited in compound 7. In this manner, the weak
emission from compounds 4 and 5 indicate that this electronic
interaction was not effectively suppressed and the formation of
excimers via stacking occurs in the excited state. In addition, it
p-stacking are well known to
p
p
Compared to those of the solution spectra, the absorption
maxima of the spin-coated films presented significant changes,
showing a clear dependence on the alkyl chain length of the
studied compounds (Fig. 2). The intense absorption located at
p-p
could be observed that the weaker the fluorescence emission is, the
higher the fluorescence emission maxima is, as expected.
2
50 nm can probably be ascribed to the tristriazolotriazine moiety
because it is known that spin-coated films can present blue-shifted
bands relative to the maxima observed in solution [16]. In addition,
a red shifted shoulder is observed in the DRUV spectra located at
3.3. Thermal characterization
The thermal behavior of compounds 5e7 was investigated using
differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA). The relevant data from DSC analysis are summa-
rized in Table 2.
3
37 nm (5), 328 nm (6) and 290 nm (7). Despite the absence of
liquid crystalline behavior in these compounds, these results sug-
gest the formation of aggregates in the ground state, probably via
p-stacking between the heteroaromatic units. A higher alkyl chain
The thermal behaviors of the studied compounds 5e7 by DSC
showed a similar tendency, as shown in Fig. 3. An increase of the
aliphatic chain leads to a decrease in their respective melting points
does not allow a better interaction between the aromatic moieties,
thereby increasing the absorption energy of the aggregate. In
contrast, a lower alkyl chain allows a better interaction between the
ꢁ ꢁ
of 92.7 C (5) to 85.5 C (7). These compounds are also found to
p
systems, resulting in absorption at longer wavelengths.
remain in a super-cooled state after fusing, i.e., cooling the material
from its isotropic liquid state between two glass slides, no crys-
tallization of the material is observed. For compound 7 (Fig. S18),
during the first heating and cooling cycle, two transitions were
The spin-coated films presented fluorescence intensity in the
UV-A-blue regions with maxima located at 461 nm (5), 408 nm (6)
and 380 nm (7), which are red shifted relative to the values
Fig. 2. Photophysical characterization of spin coated films containing tris-[1,2,4]-triazolo-[1,3,5]-triazines 5e7: (a) normalized DRUV, (b) transmittance and (c) fluorescence
emission spectra. The inset presents the normalized emission of compounds 5e7. In all spectra, precursor 4 is also presented for comparison (dot lines).
Please cite this article in press as: Dal-B oꢀ AG, et al., Synthesis, electrochemical, thermal and photophysical characterization of photoactive
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6
A.G. Dal-B oꢀ et al. / Dyes and Pigments xxx (2016) 1e8
ꢁ
ꢀ1
.
Fig. 3. First (left) and second (right) heating-cooling cycles of the DSC thermograms obtained for the compounds 5e7 at 10 C min
Table 2
The overall degradation process indicates that for compounds
e7, a main degradation event occurs with mass loss of approxi-
mately 70% at an initial decomposition temperature higher than
d
Thermal properties of the final compounds 5e7, where T is the decomposition
temperature, T
crystal phases and the isotropic liquid phase, respectively and
associated to the thermal event (J g ).
5
g
is the glass transition temperature, and CrI & Cr II and I are the
H is the enthalpy
D
ꢁ
ꢁ
ꢀ
1
400 C and a high maximum degradation rate located at 450 C.
Based on this phenomenon, the alkylation reaction appears to
promote the thermal stability of these compounds because pre-
cursor 4 showed a significant decrease of the thermal stability with
Transitionsa
(
DH/J g
ꢀ1
)
T
( C)
ꢁ
b
ꢁ
d
T ( C)
c
Compound
g
CrI 73.6 C (ꢀ4.87) CrII 92.7 ꢁC (ꢀ6.37) I
ꢁ
ꢁ
41.6
36.3
36.7
396.1
398.2
392.8
5
6
7
ꢁ
CrI 72.3 C (ꢀ4.47) CrII 85.8 C (ꢀ2.61) I
ꢁ
ꢁ
two decomposition processes located at 100 C and 250 C.
CrI 59.4 C (ꢀ1.81) CrII 85.5 ꢁC (ꢀ11.59) I
ꢁ
a
ꢁ
ꢀ1
Determined using DSC on heating at 10 C min .
Glass transition temperature measured using DSC.
Temperature of decomposition obtained by TGA in atmosphere of
C min
3
.4. Electrochemical characterization
b
c
2
N at
ꢁ
ꢀ1
It can be observed from the cyclic voltammogram curves for
1
0
.
compound 7 that a distinct anodic and cathodic behavior occurs
Fig. 5). A similar behavior was observed for compounds 5e6
(Figs. S20e21).
(
ꢁ
observed; the first is a crystal-crystal transition at 59.4 C, and the
ꢁ
second is the melting of the solid material at 85.5 C (endothermic
peak). In the later cooling, no exothermic peak related to crystal-
lization is observed. In the second cycle, only one glass transition
A reduction processes were observed located at peak potential
þ
of ꢀ1.12 V, ꢀ1.23 V and ꢀ1.37 V versus Ag/Ag , and oxidation
processes located at peak potential of 1.55 V, 1.47 V and 1.60 V
ꢁ
þ
can be observed at 36.7 C, which is typical for amorphous
versus Ag/Ag for 5, 6, and 7, respectively. The results of the elec-
materials.
trochemical study suggested that the tristriazolotriazine group
potentially has electron-transporting characteristics [16]. The TTT
series showed a more negative reduction with an increase of the
The degradation temperatures of 4e7 were evaluated by Ther-
mogravimetric Analysis (TGA) under inert N atmosphere (Fig. 4).
2
To better visualize the thermal events, the DTG curves for all of the
compounds were obtained from the TGA curves.
Fig. 5. Cyclic voltammogram of compound 7 on a glassy carbon electrode in 0.1
ꢀ
1
Fig. 4. Thermograms (TGA) and DTG curves of compounds 4e7.
TBAPF /CH Cl at 100 mV s . The reduction region is also presented (inset).
6 2 2
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A.G. Dal-B oꢀ et al. / Dyes and Pigments xxx (2016) 1e8
7
Table 3
electrochemical data evaluated from these studies are summarized
and compared in Table 3.
Optical and electrochemical properties of compounds 5e7, Optical and electro-
chemical properties of compounds 5e7, where Eox is the onset potential of oxida-
Compared to the optical excitation process, the electrochemical
oxidation or reduction of the studied TTT series are significantly
influenced by their dielectric environments and the Coulomb in-
teractions associated with charge injection processes [28]. Spec-
trolectrochemical measurements were performed to obtain
information about the species behavior in the oxidized and reduced
form. The applied potentials were chosen before the peak, on the
peak and after the peak position observed in the cyclic voltam-
metry (Fig. 5). In the corresponding spectrum obtained on OCP
(Fig. 6) for compound 7, a strong absorption peak is observed in the
range of approximately 240 nme340 nm, which is attributed to
pp* electronic transitions in the heteroaromatic portion of the
molecule due to the high molar absorption coefficient [15,27].
Under the reduction potential the TTT series showed suppression
and displacement of the absorption band to higher energy values
p a
tion, Ered is the onset potential of reduction, I (HOMO) is the ionization potential, E
(
LUMO) is the electron affinity, E
g
is the band gap, and
lonset is the absorption onset
wavelength.
Parameters
Compounds
5
6
7
E
E
ox (V) vs. NHEa
1.60
1.56
1.64
a
red (V) vs. NHE
ꢀ0.53
ꢀ6.04
336
3.69
ꢀ2.35
ꢀ0.72
ꢀ6.00
334
3.71
ꢀ2.29
ꢀ0.76
ꢀ6.08
334
3.70
ꢀ2.38
b
I
p
(HOMO) (eV)
onset (nm)c
l
E
E
(eV)d
(LUMO) (eV)
g
a
e
Notes.
a
þ
E
ox/red (vs. NHE) ¼ Eox/red (vs. Ag/Ag ) þ 0.266.
b
c
I
p
¼ ꢀ(Eox þ 4,44) eV.
lonset: absorption onsets wavelength.
d
Optical band gap calculated on the low energetic edge of the absorption spec-
¼ 1240/
¼ E ꢀ I
(Fig. 6 and Figs. SI22-23). The oxidized species of 5e7 present dif-
trum (E
g
lonset).
e
E
g
a
p
.
ferences in the absorption; when submitted to oxidizing potentials,
the compounds showed suppression and displacement of the ab-
sorption band to lower energy values (Fig. 6 and Fig. SI22-23).
alkyl chain; this behavior results in higher LUMO levels [17]. The
cyclic voltammogram of compound 4 (data not shown, Fig. S19)
was obtained for comparison since it was used as a building block
for the TTT series; the same voltammetric processes were observed,
but at a lower potential, and a new oxidation process was observed,
probably due to oxidation of the hydroxyl group. In addition, the
onset potentials of oxidation (Eox) and reduction (Ered) could be
directly correlated to electron transfer at the HOMO (ionization
potential) and LUMO (electron affinity), respectively. Therefore, Ag/
4
. Conclusions
A series of compounds based in heterocycle tris-[1,2,4]-triazolo-
[
1,3,5]-triazine (TTT) was synthesized and characterized using FTIR,
H and C NMR and mass spectrometry (MS). Compounds 5e7
1
13
exhibited absorption maxima located around 280 nm, with molar
absorptivity coefficient (ε) values related to pp* electronic transi-
tions. Changes in the solvent polarity did not cause a significant
solvatochromic effect (~6 nm), indicating a nearly absent charge-
transfer state in the ground state. Compounds 5e7 exhibited
emissions in the UV-A region (~360 nm), where the different alkoxy
groups do not play a fundamental role in photophysics of these
compounds in solution. A Stokes shift (75e81 nm) can be related to
a non-radiative energy loss in the excited state, probably due to the
þ
Ag was used as the reference, and the potential found was
þ
normalized to NHE, using the Fc/Fc couple as an internal standard
[25].
The electron affinity and ionization potential of 5e7 were
calculated based on the optical and electrochemical data and are
shown in the Table 3. The optical band gaps of TTTs 5e7 were ob-
tained from the onset values of band in the UV-vis spectra,
measured in the open circuit potential (OCP), as shown for com-
pound 7 (Fig. 6). Once again, the parent compounds 5e6 presented
a similar behaviour (Figs. S22e23). It was not employed the
reduction potential obtained electrochemically in the calculation of
the band gap. In this case, the process was not very pronounced,
which would generate errors in the calculation of the LUMO energy
value. Thus, the HOMO energy was obtained from the onset po-
tential oxidation corresponding and the LUMO was found by sub-
tracting its optical band gap from the HOMO value. The optical and
alkoxy chains (conformational relaxation). Molecular
p-stacking
was inhibited in spin-coated films prepared using compound 7,
improving the prospects for application of this compound as a good
solid state emitter.
Spectroelectrochemical measurements showed changes in the
absorption spectra due to changes in electronic structure under
oxidation and reduction conditions. The electrochemical band gaps
confirmed that the energy level required for the materials is
compatible for building electronic devices.
Fig. 6. Spectroelectrochemistry in different potential regions for 7: (left) reduction potential, (right) oxidation potential, and (inset) OCP.
Please cite this article in press as: Dal-B oꢀ AG, et al., Synthesis, electrochemical, thermal and photophysical characterization of photoactive
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discotic dyes based on the tris-[1,2,4]-triazolo-[1,3,5]-triazine core, Dyes and Pigments (2016), http://dx.doi.org/10.1016/j.dyepig.2016.06.041