Synthesis, structure, electrochemistry, and electrochemiluminescence of thienyltriazoles

Article abstract of DOI:10.1021/jo300802h

Four blue-emitting thienyltriazoles with desired N and O coordination atoms were prepared in high yield via click chemistry for potential incorporation into metal complexes. Three of their crystal structures were determined by X-ray crystallography. The electrochemical properties, electronic structures of these thienyltriazoles, 1-4, and their correlations were studied using cyclic voltammetry and differential pulse voltammetry techniques along with density function theory (DFT) calculations. All of the compounds underwent irreversible redox reactions, leading to unstable electrogenerated radical cations and anions. Electrochemical gaps determined from the differences between first formal reduction and oxidation reactions were correlated to HOMO-LUMO energy gaps obtained from UV-vis spectroscopy and the DFT calculations as well as energies of excited states measured from photoluminescence spectroscopy. We observed weak electrochemiluminescence (ECL) from annihilation of these thienyltriazole radicals in acetonitrile containing 0.1 M tetra-n-butylammonium perchlorate as electrolyte. An enhancement in ECL efficiency ranging from 0.16 to 0.50% was observed upon addition of benzoyl peroxide as a coreactant in the above electrolyte solutions. The generation of excimers in solutions of 1-4 was observed as seen by the red-shift in ECL maxima relative to their corresponding photoluminescence peak wavelengths. Our work is of importance for the development of efficient blue-emitting fluorophores via click chemistry that could be potential luminophores in metal complexes.

Full text of DOI:10.1021/jo300802h

Article
pubs.acs.org/joc
Synthesis, Structure, Electrochemistry, and
Electrochemiluminescence of Thienyltriazoles
Kalen N. Swanick, Jacquelyn T. Price, Nathan D. Jones, and Zhifeng Ding*
Department of Chemistry, The University of Western Ontario, 1151 Richmond Street, London, Ontario, Canada N6A 5B7
S
* Supporting Information
ABSTRACT: Four blue-emitting thienyltriazoles with desired N
and O coordination atoms were prepared in high yield via click
chemistry for potential incorporation into metal complexes. Three of
their crystal structures were determined by X-ray crystallography.
The electrochemical properties, electronic structures of these
thienyltriazoles, 1−4, and their correlations were studied using
cyclic voltammetry and differential pulse voltammetry techniques
along with density function theory (DFT) calculations. All of the
compounds underwent irreversible redox reactions, leading to
unstable electrogenerated radical cations and anions. Electrochemical
gaps determined from the differences between first formal reduction
and oxidation reactions were correlated to HOMO−LUMO energy gaps obtained from UV−vis spectroscopy and the DFT
calculations as well as energies of excited states measured from photoluminescence spectroscopy. We observed weak
electrochemiluminescence (ECL) from annihilation of these thienyltriazole radicals in acetonitrile containing 0.1 M tetra-n-
butylammonium perchlorate as electrolyte. An enhancement in ECL efficiency ranging from 0.16 to 0.50% was observed upon
addition of benzoyl peroxide as a coreactant in the above electrolyte solutions. The generation of excimers in solutions of 1−4
was observed as seen by the red-shift in ECL maxima relative to their corresponding photoluminescence peak wavelengths. Our
work is of importance for the development of efficient blue-emitting fluorophores via click chemistry that could be potential
luminophores in metal complexes.
group. Varying the conjugation in the thiophene ring system
or adding a phenyl substituent between the thiophene-triazole
backbone and the hydroxyl group might potentially modulate
the ECL efficiencies before complexation with metals. Click
chemistry with simple small molecules allows us to achieve the
desired blue emission from the thiophene-triazoles (our first
stage) while the hydroxyl group along with the triazole afford us
potential N−O-containing metal complex emitters (our second
stage). In this context, four thienyltriazoles, 1−4, have been
synthesized in this study as seen in Schemes 1 and 2. Note that
our thienyltriazoles are blue luminescent.
Electrochemiluminescence or electrogenerated chemilumi-
nescence, ECL, generates luminescence in solution,16 while
electroluminescent devices generate luminescence in the solid
state. ECL involves light emission that is produced by an
energetic electron-transfer reaction between electrochemically
generated radicals in the vicinity of an electrode.16 Two general
methods for producing ECL are “annihilation” and “coreactant”
reactions.16 In annihilation systems, radical cations and anions
are generated in solution and light emission results when they
combine. Coreactant studies are useful when a system does not
give stable radical cations or anions. Coreactant intermediates
are either strong reducing agents in oxidative-reduction ECL or
INTRODUCTION
■
Recently, there has been increasing interest in materials for
electrochemiluminescent sensors and organic light-emitting
diodes, OLEDs, for applications in television and lighting
technology.1 Organic luminescence is also a promising
technology for the fabrication of flat panel displays.2 The
design of new luminophores has been driven by the
requirement for RGB full color displays and white light
illumination applications.3−7 The emission color tuning in Alq3,
tris(8-hydroxyquinolinato)aluminum, analogues has been in-
vestigated with extended conjugated chromophores8 and by
modifying the N−O containing compounds by the addition of
electron-withdrawing, EWG, or electron-donating groups,
EDG.3,8−10 Our group has studied the behavior of some
triazole-modified deoxycytidine analogues in previous work.11
In this work, we synthesized and studied the spectroscopic
properties and electrochemistry of four potential blue-emitting
thienyltriazoles with N in the thienyltriazole ring and O in a
methoxy or phenol group attached to the trizole acting as
possible coordination atoms to form metal complexes. Our
attention has been focused on designing modified thienyl-
triazoles to tune deep-blue fluorescence using “click chem-
istry”.12−15 The modulation of our thienyltriazoles is based on
the reaction of azidothiophenes with terminal alkynes with a
hydroxyl group to form a triazole ring, thus creating a
fluorescent multiconjugated ring system with a hydroxyl
Received: April 20, 2012
Published: June 5, 2012
© 2012 American Chemical Society
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Scheme 1. General Procedure for the Synthesis of
Thienyltriazoles 1 and 2 via Cu(I)-Catalyzed Huisgen 1,3-
Dipolar Cycloaddition
RESULTS AND DISCUSSION
■
Synthesis of Thienyltriazoles. Our focus has been on
synthesis of thiophene-based luminescent molecules using the
copper alkyne−azide cycloaddition, CuAAC, approach (click
chemistry),12−15 which chelates to Al and Zn for possible
subsequent enhancement in fluorescence. The basic synthetic
approach to our thienyltriazoles 1−4 has been outlined in
Schemes 1 and 2. The triazole moiety links electron donor and
acceptor units, leading to an inclusion of a luminescent
emission via charge-transfer processes. The precursors were
readily available and were easily converted to 1−4 using
published procedures. In fact, compounds 1−4 were prepared
using the “click” reaction between 3-azidothiophene, or 4-
azido-2,2′-bithiophene, with 2-progargyl alcohol or 2-ethynyl-
phenol. The “click” reactions had mild reaction conditions and
required little product isolation and no chromatography.
Stirring “click” reactions overnight at 35 °C resulted in high
yields of 1−4 (81−96%). Recrystallization provided single
crystals for 1, 2, and 4, suitable for X-ray diffraction. The
Scheme 2. General Procedure for the Synthesis of
Thienyltriazoles 3 and 4 via Cu(I)-Catalyzed Huisgen 1,3-
Dipolar Cycloaddition
identities of 1−4 were confirmed by H and 13C{1H} NMR
1
spectra and HRMS. At this time, products 1−4 are new
compounds.
Crystal Structures. An ORTEP representation of 1 has
been shown in Figure 1a. The solid-state structure of 1 displays
a high degree of planarity in the bithiophene ring system, with a
torsion angle between the bithiophene rings, S(1)−C(1)−
C(5)−C(6), of only 0.80°. There was twisting of the triazole
ring system relative to the bithiophene ring system. A torsion
angle of 41.42° around the ring junctions of C(8)−C(7)−
N(1)−C(10) was observed between the triazole ring and its
adjacent thiophene ring. In the solid state, molecules of 1 pack
in a head-to-tail arrangement with their bithiophene ring
systems approximately parallel and separated by a distance of
3.60 Å, between C(3) and C(6), Figure S1 (Supporting
Information). When looking at three crystallographically
adjacent molecules, there may be an H-bonding interaction
between N(3) and the H-atom associated with O(1) a distance
of 2.78 Å between the heavy atoms.
strong oxidizing agents in reductive-oxidation ECL.16 Benzoyl
peroxide (BPO) is a common coreactant and produces a strong
oxidizing agent when reduced; this has been selected for the
purpose of our studies.16 Previously, we have studied the ECL
of iridium(III) complexes containing aryltriazoles that emitted
blue light.17 We wanted to investigate if the prepared
thienyltriazole ligands were electrochemiluminescent. Here,
we report the electrochemical properties of 1−4, their ECL
spectra and efficiencies in annihilation and coreactant pathways,
and correlations of structures and these properties. Further-
more, ECL18−24 is of importance for possible applications in
biosensors, OLED displays, optoelectronics, microelectronics,
and bioanalytical chemistry.1,3,18,20,25−30
An ORTEP representation of 2 has been shown in Figure 1b.
There was a high degree of planarity in the thiophene−triazole
ring system of 2 with a small torsion angle of 4.10° between the
Figure 1. ORTEP representations of (a) 1, (b) 2, and (c) 4 (30% probability ellipsoids, H-atoms removed for clarity, expect −OH protons) with
rings approximately parallel to the page.
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two ring systems, C(3)−C(2)−N(1)−N(2). In the solid state,
molecules of 2 pack in a head-to-tail arrangement with their
ring systems approximately parallel and separated by a distance
of 3.40 Å, between N(3) and C(15), Figure S2 (Supporting
Information). There may be an H-bonding interaction between
N(3) and the H-atom associated with O(1) from adjacent
molecules with a distance of 2.81 Å between the heavy atoms.
An ORTEP representation of 4 has been depicted in Figure
1c. The molecules of 4 display a high degree of planarity
throughout the three ring systems. There was probably an
intramolecular hydrogen bond between one of the triazole N-
atoms and the phenol OH group: the N(2)−O(1) distance was
2.63 Å, which was significantly less than the sum of the van der
Waals radii of N and O (2.74 Å). The torsion angle between
the thiophene and triazole rings C(1)−C(2)−N(1)−C(3) was
7.34° and the triazole and phenol rings N(2)−C(4)−C(5)−
C(11) was 1.80°. In the solid state, molecules of 4 π-stack with
all three ring systems approximately parallel and separated by a
distance of 3.31 Å, between N(2) and C(10), Figure S3
(Supporting Information).
estimated the potentials using the equation developed for a
reversible system.
ΔE
2
E0′ = Emax
+
(1)
Upon scanning the potential from −2.289 to 2.810 V, a peak
potential was observed at 1.894 V for the oxidation reaction,
top solid wave in Figure 3a, which was close to the peak
potential obtained by CV. The formal potential for the
oxidation was calculated as 1.919 V (Table 1) from eq 1.
DPV provides better visibility than the CV in Figure 2 and
could easily access the formal oxidation potential. As
demonstrated from the crystal structure, Figure 1a, the
bithiophene ring plane has a torsion angle as large as 41.42°
relative the triazole ring. This led to a decrease in the
conjugation length, a low delocalization of the charge upon
losing an electron and therefore a low stability of the generated
radical cation. Furthermore, we observed that the HOMO
electron density of 1, inset in Figure 3a, was distributed mostly
on the bithiophene chromophore and minor on the triazole
ring using density functional theory (DFT). The electron
density extended less than expected from our molecule design
on this conjugated ring system.
Electrochemistry and Its Correlation to Crystal and
Electronic Structures. The cyclic voltammogram (CV) of 1 is
shown in Figure 2 in acetonitrile solution containing 0.1 M
When the potential scanning direction was reversed, bottom
solid wave in Figure 3a, a peak potential was observed at
−1.905 V in the cathodic region, indicating the injection of an
electron to the LUMO reduction reaction with a calculated
formal reduction potential of −1.930 V (Table 1). The LUMO
electron density again was delocalized on the bithiophene ring,
inset in Figure 3a.
A small cathodic peak was observed in the cathodic DPV
scan, top curve in Figure 3a, representing the instability of the
radical cation. At the beginning of the DPV scan, radical cations
were generated in the vicinity of the electrode. Upon scanning
more negative, these anions, if they were still there, were
rereduced to the neutral form. The lower peak current than that
of the corresponding anodic peak in the anodic scan described
well the irreversibility of the oxidation or the low stability of the
radical cations. Similarly, a smaller anodic peak in the anodic
DPV, top wave in Figure 3a, than that of the corresponding
cathodic peak, bottom curve in Figure 3a, illustrated the
irreversibility of the reduction or the low stability of the radical
anions.
The electron promotion from HOMO to LUMO was
delocalized mainly on the bithiophene, which might cause an
inefficient luminescent emission. The electrochemical gap
determined from the first oxidation and reduction potentials
(ΔE = Eox0′ − Ered0′) read 3.85 eV in Table 1, which agrees very
well with electronic energy gap values of 4.03 eV in MeOH and
3.91 eV in DMF determined from UV−vis spectra, Table 2, and
Figures S4−S5 (Supporting Information). It was noticed that
the electrochemical gap could be obtained by differentiating the
first oxidation and reduction peak potentials, which read 3.80
eV, very similar to that (3.85 eV) from the estimated formal
redox potentials. The energy gap between the excited state and
the ground state was evaluated from the photoluminescence
spectrum, Figure S6 (Supporting Information), was 3.37 eV
(Table 2), a value that was smaller than the electrochemical gap
of 3.85 eV (Table 1). This implied a nonemissive relaxation
from the electron promotion to the excited state.
Figure 2. Cyclic voltammogram (red) and electrochemilumines-
cence−voltage curve (green) of 1 with a scan rate of 0.1 V s−1 and a
potential range between 2.740 and −2.358 V.
tetra-n-butylammonium perchlorate as supporting electrolyte.
When the potential was scanned from 0.000 to 2.740 V, 1
underwent oxidation at a peak potential of 1.828 V, becoming a
radical cation. The oxidation process was irreversible in the CV
since there was no return peak in the reverse potential scan
from 2.740 to 0.000 V. Upon scanning in negative potential
range, 1 was reduced to a radical anion showing a cathodic peak
at −2.092 V. The radical anions were not stable as well since
there was no anodic wave in the reverse potential scan. It
should be noted that new peaks appeared in the middle of the
potential window after one cycle of the potential scan. This
indicated that some chemical reactions from the generated
radicals occurred after the electrochemical reactions, which
agreed well with their stabilities illustrated by the CV in Figure
2.
In order to assess the redox property with less noise,
differential pulse voltammetry (DPV) was used in the potential
range of 2.810 and −2.289 V in the same supporting
electrolyte, Figure 3. The formal potentials of 1 can be
determined from eq 1,31,32 where Emax is the peak potential in
the DPV and ΔE is the pulse height (50 mV), we have
Table 1 summarizes the electrochemical data of the other
three thienyltriazoles, where their formal oxidation and
reduction potentials are listed. The DPV of 2−4, Figure 3b−
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Figure 3. Differential pulse voltammogram and representations of the calculated HOMO and LUMO of (a) BiTTM (1) from 2.810 to −2.289 V,
(b) TTM (2) from 2.683 to −1.976 V, (c) BiTTP (3) from 2.626 to −2.174 V, and (d) TTP (4) from 1.975 to −1.772 V in ACN containing 0.1 M
TBAP as supporting electrolyte with a scan rate of 0.1 V s−1. Gaussian-09 (B3LYP/6-31+G*) was used for calculations of 1−4 in hartrees/particle at
T = 289.15 K, P = 1 atm.
d, shows irreversible oxidation and reduction processes in all
cases. However, even though the processes were irreversible,
the radical cations and anions were still generated in solution as
seen from an increase in photocurrent in Figure 2. The
irreversible processes led to great instability of the radical
cations and anions, from which only a few excited species were
generated through electron transfer. Ultimately, weak ECL was
observed as a result. When the potential was scanned from
−1.976 to 2.683 V in the electrolyte solution containing 2, the
radical cation was generated at a potential more positive than
its formal potential for the oxidation reaction, estimated as
2.296 V, top solid wave in Figure 3b. The crystal structure
showed very good planarity, and the HOMO electron density
of 2 was delocalized on the thienyltriazole according to the
DFT calculation, Figure 3b. However, 2 still showed a higher
oxidation potential than 1. This is because 1 might have a
planar structure in solution and therefore a high degree of
conjugation. Scanning from 2.683 to −1.976 V, 2 underwent
reduction with a formal reduction potential of about −1.549 V,
generating a radical anion of 2, bottom solid wave in Figure 3b.
The electron density for the LUMO of 2 was well delocalized
on the thiophene and triazole rings and partially on the
hydroxyl group, Figure 3b, leading a facile electron injection
and therefore a less negative formal potential, −1.549 V, than
that of 1, −1.930. The HOMO−LUMO gap of 2 from the DFT
calculation is 5.27 eV, Table 1, and is much higher than the
electrochemical gap, 3.84 eV, but is compatible with the
electronic gap from the absorption spectrum, Table 2 and
Figure S4 (Supporting Information). The installation of the
second thiophene ring in 1, relative to 2, shows a trend of
Table 1. Redox Peak Potentials, Estimated Formal
Potentials, and Electrochemical Gap along with HOMO−
LUMO Energy Gaps Obtained by DFT Calculations
Ep,a
oxidation
(V)
Ep,c
reduction
(V)
a
a
a
a
0
b
0
Eox
′
Ered
′
ΔE
theor energy
c
(V)
(V)
(eV)
gap (eV)
1
2
3
4
1.894
2.271
2.126
1.687
−1.905
−1.524
−1.906
−1.460
1.919
2.296
2.151
1.712
−1.930
−1.549
−1.931
−1.485
b
3.85
3.84
4.08
3.19
4.22
5.27
3.92
4.22
a
In V vs Ag/Ag+ at 0.1 V s−1 scan rate. Energies determined from
DPV first oxidation and reduction peak potentials data (ΔE = Eox0′ −
c
Ered0′). Energy gap values obtained from the DFT/B3LYP/6-31+G*
calculations.
Table 2. Absorption and Photoluminescence Spectroscopic
Data of 1−4
a
absorption
photoluminescence
Stokes shift
Abs
Ex
Em
d
λmax
λmax
λmax
Em
λmax(PL) − λmax(Abs)
(nm)
Abs (eV) (nm) (nm) (eV)
(nm)
b
1
307
/313
4.03/3.91
344
367
3.37
54
c
b
2
3
4
252
4.92
4.10
366
337
337
453
357
358
2.73
3.47
3.46
201
55
c
302
291
b
4.26
67
a
b
c
d
PL in MeOH. Abs in MeOH. Abs in DMF. Energies determined
from PL data.
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Table 3. ECL Spectroscopic Data of 1−4
b
annihilation
coreactant
shift
a
a
a
a
scanning QE (%)
pulsing QE (%)
scanning QE (%)
pulsing QE (%)
λmax (nm)
λmax(ECL) − λmax(Em) (nm)
1
2
3
4
0.11
0.17
0.50
1.11
0.00
0.00
0.00
0.01
0.50
0.16
0.35
0.40
0.40
0.01
0.08
544
185
91
554
546
189
84
0.02
422/576
a
b
ECL quantum efficiencies (QE) measured in ACN relative to DPA (Φ = 6.1% in ACN).36 The shift represents the difference between the ECL
and PL emission wavelengths in nm.
increasing conjugation resulting in a decrease in energy
between the HOMO and LUMO, from 5.27 eV in 2 to 4.22
eV in 1 from Table 1. When looking at the electrochemical
behavior of 2, the electrochemical gap of 3.84 eV varied from its
PL emission of 2.73 eV. This may be due to fast rotation
between the thiophene and triazole rings, as seen in the
disorder of the thiophene ring in the crystal of 2.
as we extended the conjugation in the compound. From DPV,
we observed a decrease in the electrochemical gap of
compound 2 of 3.84 eV to compound 4 of 3.19 eV. A decrease
in energy was not observed between 1 and 3, however this
trend of decreasing energy with increased conjugation was seen
between 1 and 2.
ECL via Annihilation. In Figure 2, the CV of 1 is overlaid
with the ECL photocurrent−voltage curve recorded simulta-
neously. Weak light emission from the thienyltriazoles during
potential scanning was observed. The proposed mechanism for
the observed ECL in 1 can be seen in eqs 2−5 and follows the
annihilation pathway. When scanning from zero to positive
potential, the radical cation of 1 was generated.
For compound 3, the radical cation formed when the
potential was scanned from −2.174 to 2.626 V with its
estimated formal oxidation potential of 2.151 V, top solid wave,
Figure 3c. When the potential was scanned from 2.626 to
−2.174 V, the radical anion was generated with its formal
potential of about −1.931 V, bottom solid wave in Figure 3c.
From the HOMO in Figure 3c, the electron density of 3
resided mostly on the bithienyltriazole ring system. In the
LUMO, the electron density was delocalized on the phenol and
triazole rings, with very little contribution from the bithiophene
chromophore. From Table 1, we observed a decrease in the
theoretical energy gap with the installation of the phenol ring to
the bithienyltriazole compound. While the increase in
conjugation in 3 decreased the energy gap from 4.22 eV in 1
to 3.92 eV in 3 based on DFT calculations, the electrochemical
gap increased slightly from 3.85 to 4.08 eV. This discrepancy
might be due to the planar geometry used in DFT calculations
and incomplete planarity in electrolyte solution. For absorption,
the energy was 4.10 eV, similar to the DFT energy of 3,
however there was no noticeable difference in energy between
1 and 3 for absorption. Compound 3 did not readily dissolve in
MeOH, and was analyzed instead in DMF (see Figure S5,
Supporting Information). Only one band was observed for both
1 and 3 due to the solvent cutoff of DMF at approximately 270
nm. The addition of the bithiophene and phenol aromatic
systems to compound 3 increases the absorption intensities and
selectively tunes the wavelengths relative to 1 and 2. The
electrochemical gap of 3, 4.08 eV, was compatible with the
electronic gap but larger than its PL emission energy of 3.47 eV.
This again implies that nonemissive relaxation from the
electron promotion to excited stated has occurred.
Lastly, compound 4 was scanned from −1.772 to 1.975 V,
which generated the radical cation with its formal potential of
1.712 V, top solid wave in Figure 3d. From DFT, the HOMO
electron density of 4 was delocalized on the entire compound,
with the major contribution from the chromophore, very
similar to that of 2. The radical anion was formed when the
potential was scanned from 1.975 to −1.772 V with its formal
potential of −1.485 V, bottom solid wave, Figure 3d. The
LUMO of 4 showed the electron density on the triazole and
phenol rings. Comparing 2 with 4, the theoretical energy gap of
4 was 4.22 eV, which was in good agreement with the
absorption energy 4.26 eV, of 2. From PL, the energy for 4 was
3.46 eV while the energy was 3.19 eV from the electrochemical
gap of 4. The decrease in energy between 2 and 4 was expected
BiTTM → BiTTM•+ + e−
(2)
The potential was then scanned from the positive to the
negative region. This generated the radical anion of 1 as seen in
eq 3.
BiTTM + e− → BiTTM•−
(3)
Through this annihilation pathway, the radical cation and
anion generates an excited species of 1 and a ground-state
species of 1, eq 4. The excited thienyltriazole species will return
to its ground-state and emit light, eq 5, as observed in the ECL
photocurrent in Figure 2.
BiTTM•+ + BiTTM•− → BiTTM + BiTTM
*
(4)
*
BiTTM → BiTTM + hν1
(5)
The ECL light was detected in the negative potential region.
The stability of the radicals can be determined from where we
observed light emission. The radical cations were more stable
than the radical anions of this thienyltriazole compound
because we only see ECL light after scanning from positive
to negative potential. Compounds 2−4 demonstrated similar
weak ECL emission.
Quantitatively, ECL efficiency can be calculated as the
number of photons emitted per redox event relative to
diphenylanthracene (DPA).33−35 The ECL efficiencies were
low, as seen in Table 3; however, the thienyltriazoles 1 and 3
displayed higher ECL efficiencies than 2 and 4 due to their
extended π conjugation. The ECL efficiencies for compounds
1−4 were 0.11% for 1, 0.17% for 2, 0.50% for 3, and 1.11% for
4. CV of 1−4 shows irreversible processes indicating the poor
radical stability.
Annihilation pulsing efficiencies were essentially zero, not
efficient, because the radicals were not stable in solution as seen
in the irreversible redox reactions, see Table 3. Since the
compounds were not efficient through annihilation pulsing, the
ECL spectrum for each compound was not obtained.
ECL via Coreactant. ECL was enhanced when BPO was
added to the solutions of compounds 1−4 as observed in
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Figure 4. Cyclic voltammogram (red) and ECL−voltage curve (green) of (a) BiTTM (1) from 0.000 to −1.989 V, (b) TTM (2) from 0.000 to
−2.145 V, (c) BiTTP (3) from and 0.000 to −2.074 V, and (d) TTP (4) from 0.000 to −2.021 V in ACN containing 5.0 × 10−3 M BPO and 0.1 M
TBAP as supporting electrolyte with a scan rate of 0.1 V s−1.
Figure 5. ECL spectra of 1−4 in ACN containing 5.0 × 10−3 M BPO and 0.1 M TBAP as supporting electrolyte, pulsing for t = 60 s between
potential ranges from (a) 0.000 to −1.989 V for 1, (b) 0.000 to −2.145 V for 2, (c) 0.000 to −2.074 V for 3, and (d) 0.000 to −2.021 V for 4. ECL
intensities were normalized by their respective peak heights.
Figure 4. Adding a coreactant to the solution was useful by
generating ECL using a single potential step, one directional
potential scanning at an electrode thus overcoming the limited
potential window of the solvent, reducing the time dely for the
meeting of radical anions and cations and therefore enhancing
the ECL light emission by generating radicals in solution.16,37
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The proposed mechanism for the observed ECL in 1, Figure
4a, with BPO is stated in eqs 6−9 and follows the coreactant
pathway. When scanning from 0.000 V to more negative
potential, BPO was first reduced to its radical anion, BPO•−, at
−0.760 V, as seen in eq 6.
in the ground state with one in the excited state due to the π
conjugation of the compound.33 When compound 1 was in
solution with the excited species of 1, BiTTM*, an excimer of
1, (BiTTM)2*, eq 10, could be generated.
*
*
BiTTM + BiTTM → (BiTTM)2
(10)
BPO + e− → BPO•−
(6)
Another route would be the generation of the excimer of 1,
(BiTTM)2*, when the radical cation and anion of 1 were in
solution. Instead of generating an excited species of 1 and a
ground-state species of 1, as seen in eq 4, it could form the
excimer of 1, (BiTTM)2*, eq 11.
This radical rapidly decomposes to generate a strong
•
oxidizing radical, C6H5CO2 , eq 7.
−
•
BPO•− → C6H5CO2 + C6H5CO2
(7)
•
In eq 8, the C6H5CO2 radical reacts with 1 and generates
the radical cation of 1, BiTTM•+.
BiTTM·+ + BiTTM·− → (BiTTM)2
*
(11)
•
−
C6H5CO2 + BiTTM → BiTTM•+ + C6H5CO2
The excimer of 1, (BiTTM)2*, would relax down to its
ground state and emit light as seen in eq 12.
(8)
Upon further reduction, the radical anion of 1 was observed
at −1.883 V, eq 9.
*
(BiTTM)2 → 2BiTTM + hν2
(12)
BiTTM + e− → BiTTM•−
(9)
Excimer formation would be an alternate route the results in
light emission. By changing the substituents on the compounds,
adding an additional thiophene ring or by adding a phenol ring,
the PL and ECL coreactant emissions were tuned based on the
conjugated system. ECL peak wavelengths of 1 and 3 were the
furthest red-shifted from their PL spectra. The ECL peak
wavlength of 3 was observed at 546 nm with a red-shift of 189
nm relative to its PL peak wavelength. Compound 2 shows a
red-shift of 91 nm and compound 4 demonstrates a red-shift of
84 nm. From Table 3, we can conclude that excimer formation
was observed in each ECL system with BPO for all compounds
studied when pulsing the electrode potential. The largest red-
shift in wavelength was observed in 1 and 3, the two
compounds that had the bithiophene ring system. The addition
of the phenol ring system to 4 changed the ECL excimer
emission from 554 nm in 2 to 576 nm in 4; thus, the extended
conjugation in 4 increased the formation of excimers in solution
for ECL measurements.
The radical cation and anion of 1 combine and form the
excited species of 1, BiTTM*, similar to eq 4 with the
annihilation mechanism, and eventually relax back down to its
ground state and light is emitted, eq 5.
A large increase in photocurrent in Figure 4a for 1,
approximately 90 nA, was observed using BPO. Comparing it
to the photocurrent seen in annihilation, Figure 2, with
approximately 1 nA, BPO significantly increased the amount of
light detected. Compounds 2−4 in the same coreactant system
showed increased intensities of photocurrent relative to that in
the annihilation systems when extending the conjugation in the
thienyltriazole ring system. ECL intensity generally increased
with the addition of BPO to each system as seen in Table 3.
Relative to the standard, DPA, a slight decrease in the efficiency
appears. This was due to the fact that the efficiency of DPA was
significantly higher with BPO relative to its efficiency without
the coreactant. ECL was enhanced when using BPO and the
efficiencies for compounds 1−4 were 0.50% for 1, 0.16% for 2,
0.35% for 3, and 0.40% for 4.
Pulsing experiments with coreactant resulted in slightly
higher efficiencies than in the annihilation mechanism as seen
in Table 3. When BPO was added to the solutions, there was a
small increase in efficiencies for 1 at 0.40%, 2 at 0.01%, 3 at
0.08%, and 4 at 0.02% because pulsing generates the radicals
faster therefore reducing the decay of the unstable radicals.
Being able to detect photocurrent even with these very low
efficiencies allowed our group to acquire the ECL spectra for
compounds 1−4 as seen in Figure 5.
ECL Spectra and Their Correlation to PL Spectra. The
ECL spectra of compounds 1−4 containing BPO were
obtained, Figure 5, and their peak wavelengths have been
listed in Table 3. Pulsing between the potential ranges for each
compound in the cathodic regions, thus reducing each
compound, generated the excited species and the emission
was acquired and recorded. The ECL spectra were red-shifted
relative to the PL spectra as seen in Table 2. The ECL spectrum
of 1 showed a maximum wavelength at 554 nm in ACN
whereas its PL maximum was at 367 nm in MeOH. The
difference of 185 nm could be due to the formation of excimers
in solution during annihilation and coreactant studies.
CONCLUSIONS
■
We synthesized four blue-emitting thienyltriazoles 1−4 that
were characterized using H and 13C{1H} NMR spectroscopy
1
and high-resolution mass spectrometry. Crystal structures of 1,
2, and 4 were determined by X-ray diffraction. While 1 showed
a torsion angle of 41.42° between the bithiophene and triazole
rings, 2 and 4 possessed greater planarity. All four of the
compounds underwent irreversible redox reactions to generate
electrochemically unstable radical anions and cations. Based on
DFT calculations, the majority of electron density resided on
the chromophore for the LUMOs of 1−4, and mostly in the
triazole hydroxyl/phenol ring systems for the HOMOs, with
the exception of the HOMO of 1, where the electron density
was delocalized on the bithienyltriazole ring system. Electro-
chemical gaps determined from the differences between first
formal reduction and oxidation reactions correlated well to
HOMO−LUMO energy gaps obtained from UV−visible
spectroscopy and the DFT calculations as well as energies of
excited states measured from photoluminescence spectroscopy.
They demonstrated a trend dependent on the conjugation
length and planarity. ECL efficiencies for annihilation of
electrogenerated radicals were determined to be 0.11% for 1,
0.17% for 2, 0.50% for 3, and 1.11% for 4, relative to that of
DPA. Upon addition of BPO as the coreactant, ECL intensities
were enhanced approximately 90 times for 1, 20 times for 2,
100 times for 3, and 10 times for 4, bearing efficiencies of
It was possible that excimers form in solution because they
were excited states of dimers that can be observed in ECL of
organic compounds.38,39 Excimers can form from dimerization
of radical cations and anions or from stacking of the monomer
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Synthesis of 2-[1-(2,2′-Bithien-4-yl)-1H-1,2,3-triazol-4-yl]-
phenol (3, BiTTP). To a solution of 4-azido-2,2′-bithiophene (0.33
g, 1.6 mmol), 2-ethynylphenol (0.19 g, 1.6 mmol), and tert-butyl
alcohol/H2O (1:1, 12 mL) was added CuSO4 (0.041 g, 0.25 mmol) at
room temperature and the mixture stirred for 5 min. Sodium ascorbate
(0.016 g, 0.079 mmol) was then added. The solution was initially
yellow, and the reaction mixture was stirred overnight at 35 °C. After
72 h, the solution was yellow-brown. The product was extracted with
ethyl acetate (3 × 10 mL) and washed with H2O (3 × 10 mL). The
combined organic phase, brown, was washed with ethyl acetate (10
mL) and brine (10 mL), dried (MgSO4), and concentrated under
vacuum. The product was dissolved in MeOH (10 mL) and then
concentrated under vacuum to afford 0.42 g (81% yield) of 3, light
brown powder, mp 180−184 °C. HRMS: calcd for C16H11N3OS2
([M]) 325.0344, found 325.0336.
Synthesis of 2-[1-(3-Thienyl)-1H-1,2,3-triazol-4-yl]phenol (4,
TTP). To a solution of 3-azidothiophene (0.28 g, 2.28 mmol), 2-
ethynylphenol (0.27 g, 2.28 mmol), and tert-butyl alcohol/H2O (1:1,
10 mL) was added CuSO4 (0.058 g, 0.36 mmol) at room temperature
and the mixture stirred for 5 min. Sodium ascorbate (0.023 g, 0.11
mmol) was then added. The solution was initially yellow. After being
stirred at 35 °C for 18 h, the solution turned brown-green with a
precipitate. The product was extracted with ethyl acetate (3 × 10 mL)
and washed with H2O (3 × 10 mL). The combined organic phase,
brown solution, was washed with ethyl acetate (10 mL) and brine (10
mL), dried (MgSO4), and concentrated under vacuum to afford 0.46 g
(84% yield) of 4, light brown powder, mp 170−176 °C. HRMS: calcd
for C12H9N3OS ([M]) 243.0466, found 243.0462.
0.50% for 1, 0.16% for 2, 0.35% for 3, and 0.40% for 4, relative
to DPA in the coreactant system. The ECL spectra of 1−4 were
acquired ranging from 544 nm for 1, 554 nm for 2, 546 nm for
3, and 576 nm for 4. The radicals electrogenerated in solution
followed the dimerization and electron transfer pathways,
leading to excimers. A red-shift was observed for all four
compounds in the ECL spectra relative to the corresponding
PL spectra. Monomer ECL emission was only observed for 4.
ECL is a valuable, quick, and cost-effective technique that
requires a small quantity of compound while being highly
sensitive and selective.
EXPERIMENTAL SECTION
■
General Methods. For synthesis and characterization, all reagents
were purchased from commercial sources and used as supplied unless
otherwise indicated. All experiments were conducted in air unless
otherwise noted. Reactions that were carried out under an atmosphere
of Ar were conducted using standard Schlenk techniques. Thin-layer
chromatography was performed using 250 μm silica gel glass-backed
plates and visualized by UV light. Flash column chromatography was
performed using SiliaFlash P60, 40−63 μm (D50) 60 Å, silica gel. All
solvent mixtures were reported as volume ratios. Melting points were
obtained using a Fisher-John melting point apparatus and reported
uncorrected. The 1H and 13C{1H} NMR data were recorded on a 400
MHz spectrometer at room temperature. High resolution mass
spectrometry (HRMS) data were collected using electron spray
(ESI) time-of-flight technique.
Infrared Measurements. Infrared spectroscopy measurements
were recorded using a Fourier transform infrared spectrometer;
samples were run between KBr plates. The wavenumbers,values, were
reported in cm−1; peak intensities were given as follows: strong (s),
medium (m), and weak (w).
General Procedure for the Synthesis of Thienyltriazoles, 1−
4, via Cu(I) Catalyzed Huisgen 1,3-Dipolar Cycloaddition. The
following compounds were synthesized on the basis of previous
literature: 3-azidothiophene,30 4-azido-2,2′-bithiophene,30 and 2-
ethynylphenol.40 Compounds 1−4 were made by reacting either 2-
propargyl alcohol (Scheme 1) or 2-ethynylphenol (Scheme 2) with
one of the two corresponding thienylazides in the presence of a Cu(I)
catalyst, from CuSO4 and sodium ascorbate, in a 1:1 tert-butyl alcohol/
H2O solution. The products 1−4 were isolated and then purified for
electrochemical and spectroscopic analyses and for X-ray crystallog-
raphy of 1, 2, and 4.
Synthesis of [1-(2,2′-Bithien-4-yl)-1H-1,2,3-triazol-4-yl]-
methanol (1, BiTTM). To a solution of 4-azido-2,2′-bithiophene
(0.80 g, 3.8 mmol), 2-propargyl alcohol (0.22 g, 3.8 mmol), and tert-
butyl alcohol/H2O (1:1, 6 mL) was added CuSO4 (0.098 g, 0.62
mmol) at room temperature and the mixture stirred for 5 min. Sodium
ascorbate (0.038 g, 0.19 mmol) was then added. The solution was
initially yellow, and the reaction mixture was stirred overnight at 35
°C. After 16 h, the solution was yellow-brown. The product was
extracted with ethyl acetate (3 × 10 mL) and washed with H2O (3 ×
10 mL). The combined organic phase, brown, was washed with ethyl
acetate (10 mL) and brine (10 mL), dried (MgSO4), and concentrated
under vacuum to afford 0.82 g (81% yield) of 1, light brown powder,
mp 140−143 °C. HRMS: calcd for C11H9N3OS2 ([M]) 263.0187,
found 263.0186.
Synthesis of [1-(3-Thienyl)-1H-1,2,3-triazol-4-yl]methanol (2,
TTM). To a solution of 3-azidothiophene (0.064 g, 0.51 mmol), 2-
propargyl alcohol (0.037 g, 0.66 mmol), and tert-butyl alcohol/H2O
(1:1, 2 mL) was added CuSO4 (0.017 g, 0.11 mmol) at room
temperature and the mixture stirred for 5 min. Sodium ascorbate
(0.007 g, 0.03 mmol) was then added. The solution was initially
yellow, and the reaction mixture was stirred overnight at 35 °C. After
16 h, the solution was yellow-brown. The product was extracted with
ethyl acetate (3 × 10 mL) and washed with H2O (3 × 10 mL). The
combined organic phase, yellow, was washed with ethyl acetate (10
mL) and brine (10 mL), dried (MgSO4), and concentrated under
vacuum. The product was dissolved in ethyl acetate and methanol.
Hexanes was added to the solution, which was left for recrystallization
over a period of 5 days. This afforded 0.089 g (96% yield) of 2, golden
crystals, mp 127−130 °C. HRMS: calcd for C7H7N3OS ([M])
181.0310, found 181.0308.
UV−vis and Photoluminescence Measurements. UV−vis
absorption (Abs) and photoluminecescnece (PL) spectra were
recorded over the range of 200−800 nm using a fluorimeter with a
xenon flash lamp and PMT detector interfaced to a computer
workstation. Quartz cuvettes were used using a 1 cm path length cell at
25 °C. Spectroscopic grade methanol (MeOH) was used as received.
CV and ECL Measurements. For electrochemical studies, 9,10-
diphenylanthracene (DPA, 97%), benzoyl peroxide (BPO, reagent
grade, ≥98%), ferrocene (Fc, 98%), and supporting electrolyte, tetra-n-
butylammonium perchlorate (TBAP, electrochemical grade) were
used as received. All solutions were prepared using anhydrous
acetonitrile (ACN, 99.8%) in a Sure Seal bottle that was immediately
transferred into an N2-filled drybox prior to use.
Electrochemical Preparation. Cyclic voltammetry (CV), differ-
ential pulse voltammetry (DPV), and electrogenerated chemilumi-
nescence (ECL) experiments were conducted using a 2 mm diameter
Pt disk inlaid in a glass sheath as the working electrode (WE), a coiled
Pt wire as the counter electrode (CE), and a coiled Ag wire as the
quasireference electrode (RE), respectively. Prior to each experiment,
the WE was polished with a felt polishing pad using 1.0, 0.3, and 0.05
μm alumina suspensions for 5 min each to obtain a mirror surface that
was then washed with copious amounts of deionized water. The WE
was then electrochemically polished using a 0.1 M aqueous solution of
H2SO4 by scanning 400 times between the potentials of 1.400 and
−0.600 V at a scan rate of 0.5 V s−1 for a cleaner, more reproducible Pt
surface.28 Finally, the WE was rinsed with deionized water and then
dried under a stream of Ar gas at room temperature. The CE and RE
were rinsed with acetone, sonicated in acetone for 15 min, and then
thoroughly rinsed with deionized water. The electrodes were dried at
100 °C for 5 min then left to cool to room temperature. The
electrochemical cell was rinsed with acetone and deionized water, then
immersed in a base bath of 5% KOH in isopropanol for 4 h, rinsed
with deionized water, immersed in an acid bath of 1% HCl for 4 h, and
then thoroughly rinsed with deionized water. The cell was dried at 100
°C overnight then cooled to room temperature. All solutions for
electrochemical experiments were prepared in a glass electrochemical
cell inside an N2-filled drybox. The solutions of 1−4 ranged in
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concentration from 2.0 to 2.7 × 10−3 M in ACN containing 0.1 M
TBAP, the supporting electrolyte. For coreactant systems, 5.0 × 10−3
M BPO was added to each solution of 1−4. These experiments were
performed outside of the drybox. The electrodes were immersed in
solution and connected by copper wire inserted through the cap. After
completion of each experiment, the cell potential obtained was
calibrated using Fc as the internal standard. The potentials were
normalized to NHE using Fc/Fc+. The redox potential of Fc/Fc+ in
ACN was taken as 0.400 V vs NHE.41,42
Electrochemical Instrumentation. CV is a technique that is used
to measure the current during the process of linearly changing the
potential between two limits on the working electrode at a given scan
rate.28 For all CV experiments, the potential windows range from
2.810 to −2.290 V for annihilation systems and from 0.000 and
−2.150 V for coreactant systems. The experimental parameters for
CVs were as followed: 0.000 V initial potential, positive or negative
initial scan polarity, 0.1 V s−1 scan rate, 4 sweep segments, 0.001 V
sample interval, 2 s quiet time, 1−5 × 10−5 AV−1 sensitivity. In DPV
experiments, the differential current is taken as the current at the end
of the pulse minus the current seen just prior to the pulse as the
applied potential advances from one pulse to the next pulse.31 For the
purpose of our experiments, four DPVs were taken for each
compound, two for anodic scans (0.000 to 2.810 V and 2.810 to
0.000 V) and two for cathodic scans (−2.290 to 0.000 V and 0.000 to
−2.290 V), respectively. The experimental parameters for DPVs were
as followed: 0.004 V increments, 0.05 V amplitude, 0.5 s pulse width,
0.0167 s sampling width, 0.2 s pulse period, 2 s quiet time, 1−5 × 10−5
AV−1 sensitivity.
ECL Instrumentation. The electrochemical cell had a flat Pyrex
window at the bottom for detection generated from the WE and was
sealed with a Teflon cap with a rubber O-ring for CV, DPV, and ECL
measurements. The CV and ECL data were obtained using a
electrochemical analyzer coupled with a photomultiplier tube
(PMT) held at −750 V with a high voltage power supply. In the
vicinity of the WE, ECL was generated and collected by the PMT
under the flat Pyrex window at the bottom of the cell. The
photocurrent from the PMT, which represents the ECL intensity,
transformed this signal using a picoammeter/voltage source. The
potential and current signals from the electrochemical workstation
were sent through a data acquisition system (DAQ board) to the
computer. The data acquisition system was controlled from a
homemade LabVIEW program. The electrochemical cell was
positioned inside the PMT to detect light emission for ECL pulsing
experiments. The PMT was connected to the picoammeter/voltage
source for signal conversion. A potentiostat was connected to an
Universal Programmer. The data acquisition system was controlled
from another homemade LabVIEW program. Current, potential and
ECL signals were recorded simultaneously with the computer
acquisition system. Pulsing the WE between the first oxidation and
reduction peak potentials improved the ECL signals with a pulse width
of 0.1 s or 10 Hz. The photosensitivity was set between 2 and 20 nA
for annihilation systems and 200 nA for coreactant systems. The ECL
spectra were obtained using a spectrometer containing a charge-
coupled device (CCD) camera that was cooled to −55 °C prior to use
and connected to the computer. Similar to the pulsing experiments,
the samples were pulsed between the first oxidation and reduction
peak potentials, between 0.000 and −2.150 V, at 10 Hz. The exposure
time of the spectra was set to 60 s for coreactant systems. A program
recorded the intensities. Vertical lines/spikes seen in the spectra were
cosmic rays from the CCD spectrometer.
Theoretical Calculations. The ground-state structures of 1−4
were optimized from the crystal structures, 1−3, by using density
functional theory43 (DFT) with B3LYP/6-31+G* in hartrees/particle
at T = 289.15 K, P = 1 Atm. Frequency calculations were also executed
at the same level of theory as the optimizations, and the vibrational
data confirmed that the structures were indeed true minima on the
potential energy surface because there were no imaginary frequencies
listed in the vibrational analysis. The crystal structure and optimized
Cartesian coordinates of 1−4 have been given in the Supporting
Information, Figures S8−S16.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Crystallographic data, H and 13C{1H} NMR spectra, mass
spectrometry spectra, photophysical spectra, and Cartesian
coordinates. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*Tel: 519-661-2111 x86161. Fax: 519-661-3022. E-mail:
zfding@uwo.ca.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Natural Sciences and Engineering Research
Council (NSERC, Canada), Ontario Center of Excellence, and
Canada Foundation for Innovation for generous financial
support, Dr. R. H. E. Hudson, Dr. P. J. Ragogna, and Dr. D. W.
Shoesmith for use of their instruments (The University of
Western Ontario), Dr. Viktor N. Staroverov and Allison L.
Brazeau for their assistance in using the Gaussian-09 program,
X-ray Structures Western (Aneta Borecki), Mass Spectrometry
(Doug Hairsine), SHARCNET, NMR Facility (Mathew
Willans), and Electronics Shop (John Vanstone and Jon
Aukema) for their quality technical service, and The University
of Western Ontario and the Department of Chemistry for
support of graduate research.
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