Tellurophenes with delocalized π-systems and their extended valence adducts

Article abstract of DOI:10.1021/ja210763n

The π-conjugated 2,5-substituted tellurophene compounds 2,5-bis(2-(9,9-dihexylfluorene))tellurophene (1) and 2,5-diphenyltellurophene (3) were synthesized through ring closing reactions of 1,4-substituted butadiyne. The oxidative addition of Br₂ to tellurophene compounds 1 and 3 was studied through absorption spectroscopy, NMR, electrochemistry, X-ray crystallography, and density functional theory (DFT) calculations. When Br ₂ adds to the tellurium center the absorption spectrum shifts to a lower energy. From electrochemistry and DFT calculations we show that this is caused by lowering the lowest unoccupied orbital. The highest occupied orbital is also lowered, but to a lesser extent. This shift in absorption spectrum and lowering of the oxidation potential can provide a method to modify tellurophene containing materials. The two-electron oxidative addition is promising for catalyzing energy storage reactions.

Full text of DOI:10.1021/ja210763n

Article
pubs.acs.org/JACS
Tellurophenes with Delocalized π-Systems and Their Extended
Valence Adducts
Theresa M. McCormick, Ashlee A. Jahnke, Alan J. Lough, and Dwight S. Seferos*
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, M5S 3H6 Canada
S
* Supporting Information
ABSTRACT: The π-conjugated 2,5-substituted tellurophene
compounds 2,5-bis(2-(9,9-dihexylfluorene))tellurophene (1)
and 2,5-diphenyltellurophene (3) were synthesized through
ring closing reactions of 1,4-substituted butadiyne. The
oxidative addition of Br2 to tellurophene compounds 1 and
3 was studied through absorption spectroscopy, NMR,
electrochemistry, X-ray crystallography, and density functional
theory (DFT) calculations. When Br2 adds to the tellurium
center the absorption spectrum shifts to a lower energy. From
electrochemistry and DFT calculations we show that this is caused by lowering the lowest unoccupied orbital. The highest
occupied orbital is also lowered, but to a lesser extent. This shift in absorption spectrum and lowering of the oxidation potential
can provide a method to modify tellurophene containing materials. The two-electron oxidative addition is promising for
catalyzing energy storage reactions.
when these compounds were exposed to Br2, which appears to
be the result of Br2 coordination at their tellurium centers. The
observed change in absorption properties of the tellurophene
polymer upon exposure to Br2 suggests a change in electronic
distribution in the polymer, a property that could have several
applications and advantages. Moreover, this shows that a single
well-defined tellurophene compound can serve as an
intermediate to produce a plurality of optoelectronic
compounds. Specifically, through molecular engineering we
can design and alter low energy sites in the polymer through
specific chemical reactions. The reversible, two-electron
oxidative addition reactions of X2 at the tellurium center are
especially important due to the potential use in energy storage
reactions such as conversion of HX to H2 and X2.30−33
Herein, we present the first detailed investigation of the
optoelectronic properties of tellurophene-based small mole-
cules with extended π-systems and their extended valence
adducts. Using carefully designed and synthesized telluro-
phenes, we fully characterize the structural, crystallographic,
and optoelectronic properties of these compounds and their Br2
adducts. By combining experimental results, density functional
theory (DFT) calculations, and time-dependent DFT calcu-
lations we are able to discover the origin of the shift in optical
and electronic properties of this new class of optoelectronic
materials.
INTRODUCTION
■
The metalloid tellurium is a versatile element that forms the
basis for several important materials, including catalysts, dyes,
and optoelectronic compounds.1−10 Tellurophene, the tellu-
rium analog of the widely studied five-member heterocycle
thiophene, however has not been extensively explored. This is
particularly true of tellurophene-containing π-conjugated
materials, even though they have certain advantages relative
to their lighter analogs due to the unique chemistry of
tellurium. These advantages include red-shifted optical
absorption properties,11−13 unique solid-state supramolecular
structures through tellurium−tellurium interactions,14−16 and
high polarizability.17,18
As research in π-conjugated materials has turned to more
complex delocalized π-systems, and hence more lengthy
synthetic pathways, there is a need to develop materials with
properties that can be tuned postsynthesis. Examples of
postsynthesis modification include complexation to form
Lewis acid adducts,19−21 metal ion coordination,22−24 depro-
tonation at acid sites,25 or ion exchange reactions.26,27
Tellurophene-containing π-conjugated materials are important
in this regard as well, because of the diverse chemistry of this
heteroatom. For example, tellurophenes (or similar tellurium-
containing heterocycles) can extend their valence to form
stable, isolatable, coordinated intermediates through oxidative
addition reactions.28
RESULTS AND DISCUSSION
Molecular Design and Synthesis. To understand both
the electronic and structural basis for the change in optical
■
Recently, we described the synthesis of a novel, bifunctional
tellurophene monomer, as well as the conditions required to
prepare polytellurophenes by palladium-catalyzed solution-
based methods.29 This synthesis prepares well-defined
polytellurophenes that are stable, processable materials.
Interestingly, we observed a dramatic shift in optical properties
Received: November 29, 2011
Published: January 27, 2012
© 2012 American Chemical Society
3542
dx.doi.org/10.1021/ja210763n | J. Am. Chem. Soc. 2012, 134, 3542−3548

Journal of the American Chemical Society
Article
a
Scheme 1. Synthetic Route to Compounds 1−4
a
TMS is trimethylsilyl; TBAF is tetrabutylammonium fluoride.
properties when tellurophenes with delocalized π-systems
coordinate with bromine, two compounds were synthesized
and their Br2 adducts were prepared. One compound is
designed with an extended delocalized π-system 2,5-bis(2-(9,9-
dihexylfluorene))tellurophene (1). The known compound 2,5-
diphenyl-tellurophene (3)34−37 was also used for ease of
crystallization to verify the geometry of bromine addition to the
tellurium center. Importantly, the 2,5-substituted tellurophenes
1 and 3 can be accessed through functionalized butadiyne
precursors, reacted with sodium telluride, allowing for a
straightforward convergent synthesis (Scheme 1). The syn-
thesis begins with the deprotection of bis(trimethylsilyl)-
butadiyne by treatment with tetrabutylammonium fluoride in
the presence of CuI and Pd(PPh3)4 and subsequent addition of
an aryl iodide precursor to afford the desired 1,4-substituted
butadiyne.38 Sodium telluride was prepared39 in refluxing
ethanol by the slow addition of sodium borohydride to
tellurium powder until the solution changed from a gray
suspension, to purple, and finally a colorless solution. To afford
the ring-closed tellurophene target compounds, a tert-butanol
solution of 1,4-substituted butadiyne was added to the solution
of sodium telluride followed by isolation and purification by
column chromatography.
This synthetic pathway is important because the ring closing
reaction40,41 and the formation of tellurophene occur in the
final step, avoiding functionalization of the tellurophene ring,
which can degrade under harsh reaction conditions. Purification
by silica gel chromatography afforded the products in
reasonable yields (11−55%) and in high purity. The
tellurophene−dibromide adducts (2 and 4) were then prepared
by the addition of 1 equiv of Br2 to a carbon tetrachloride
solution of the parent compounds. The composition of all
products were verified by 1H and 13C NMR and high-resolution
mass-spectrometry or elemental analysis (see the Experimental
Section).
Figure 1. ORTEP drawing of compound 4 shown at 50% probability
with hydrogens removed for clarity.
151.5° for 4A and 4B, respectively (see Supporting
Information). This appears to be caused by solid state
interactions of the phenyl rings. In the structure, we observe
a seesaw configuration around the tellurium, the average Te−Br
bond length is 2.673 Å, and the bond angle is 175.33(5)°. Due
to the five-member ring the C−Te−C bond angle in compound
4 is 82.0(5)°. The tellurophene ring has an average Te−C bond
length of 2.12 Å, which is slightly longer than that of previously
reported tellurophene compounds where the tellurium is not
coordinated to bromine.42 A similar change is observed in the
C−C bond lengths. The C2−C3 average bond length is 1.33 Å,
decreased slightly from that of previously reported non-
brominated compounds. While the C3−C4 length is increased
to 1.46(2) Å, indicating a significant degree of bond alternation
in the ring, it is representative of a decrease in aromaticity
compared to previously reported nonbrominated tellurophene
compounds, and an increase in Te−C bond length.
The conversion of 1 to 2 can be monitored by NMR. The
single peak from the protons on the 3 and 4 position of the
tellurophene shifts from 7.99 ppm in 1 to 7.55 ppm in
compound 2 (Figure 2). This decreasing value of the resonance
peak indicates that the tellurophene ring in 1 is more
deshielded than 2, which is also consistent with a loss of
aromaticity when 1 is converted to 2. When fractions of molar
equivalents of Br2 are added, NMR shows a clean conversion
from 1 to 2 with no peak broadening. The integration of the
peak at 7.99 ppm (0.75) and the doublet at 7.95 ppm (0.25)
correlate to the amount of Br2 added, supporting a 1:1 addition
of Br2 to compound 1.
To examine the coordination of bromine to the tellurium
center the structure of 2,5-bis(phenyl)dibromotellurophene (4)
was determined by X-ray crystallography. X-ray quality crystals
form as red blocks when methanol is allowed to slowly diffuse
into a dichloromethane solution containing 4 (Figure 1). The
structure reveals that two molecules of 4 (4A and 4B)
crystallize in each unit cell. The largest difference between the
two molecules is the dihedral angle between the tellurophene
and phenyl rings, which are 160.9° and 161.1°, and 174.4° and
Optoelectronic Properties. With the structure upon Br2
addition determined, the effect on the electronic properties was
investigated. For these studies, we focused on compounds 1
3543
dx.doi.org/10.1021/ja210763n | J. Am. Chem. Soc. 2012, 134, 3542−3548

Journal of the American Chemical Society
Article
The effect of Br2 addition on the photoluminescence
properties of 1 and 2 was examined next. Emission from
room temperature solutions of 1 and 2 is not observed, likely
due to the presence of the heavy tellurium atom; however,
frozen glasses of 1 and 2 in 2-methyl-THF (measured at 77 K)
display vibrationally resolved luminescence (Figure 3). The
heavy atom facilitates intersystem crossing to a triplet state;
thus we hypothesize that the luminescence is phosphorescence
since emission from these compounds is observed neither at
room temperature nor in oxygenated solutions. At room
temperature nonradiative decay pathways prevent emission.
Likewise, triplet ground-state oxygen can cause deactivation of
this state and quench emission. At 77 K the emission of
compound 2 is red-shifted relative to compound 1, which is
consistent with the absorption spectra and a lower energy
excited-state to ground-state transition for 2, relative to 1.
Next, cyclic voltametry was used to determine the positions
of the frontier orbitals of these compounds; for clarity all values
are reported vs vacuum (see Table 1). The energy of the
HOMO level was experimentally found from the first oxidation
peak. Compound 1 has reversible oxidizations at −5.70 V and
−6.04 V. Both of these waves correspond to a one-electron
oxidation with a nearly ideal peak splitting of 64 and 60 mV,
respectively. Similar reversible oxidations are observed in other
fluorene compounds with extended π-systems43 but are not
observed in tellurophene compounds including bitellurophene
or tritellurophene42 and benzo[1,2-b:4,5-b′]ditellurophene.44
This suggests that these waves are due to the oxidation of the
extended π-system, rather than the metal center. No reduction
peak was observed for 1. Using the optical HOMO−LUMO
gap of 3.10 eV, taken from the rise of the low energy absorption
peak, the reduction should be near −2.6 V; this value is well
beyond the electrochemical window of our experiments.
Figure 2. In the aromatic region of the 1H NMR spectra of compound
1, compound 1 with 0.25 mol equiv of Br2 and compound 1 with 1
mol equiv of Br2 added, the proton on the tellurophene ring is shown
with an * for compound 1 and with a # for compound 2. The spectra
were acquired in CCl4 with benzene inserts.
and 2 because they have an extended π-system and optical
properties that lie in the visible region. Accordingly, 1 has a
strong absorption band at 390 nm (ε = 4.5 × 104 L cm−1
mol−1). Upon slow conversion of 1 to 2 (by the addition of Br2
in 0.2 mol equiv aliquots) the band at 390 nm decreases, and
two new bands, one at 470 nm and one at 320 nm, appear
(Figure 3). This absorption titration experiment reveals two
Unlike 1, the oxidation of the brominated compound 2 is not
reversible. The onset of oxidation occurs at −5.87 V. There is
also a small oxidation shoulder at −6.20 V and an oxidation at
−6.79 V. All of these values indicate that the HOMO of
compound 2 is lower in energy than compound 1. Additionally,
compound 2 undergoes irreversible reduction at −3.04. Based
on previous reports of halogenated tellurium-containing
compounds, this first reduction peak is likely due to a
debromination reaction.45 Taking the optical HOMO−
LUMO gap as the onset of absorption, the gap for 1 is 3.10
eV and for 2 is 2.46 eV. For compound 2, taking the HOMO as
the first oxidation potential and LUMO as the reduction
potential the electrochemical energy gap is slightly larger at
2.79 eV (Table 1). The optical and electrochemical HOMO−
LUMO gaps are in general agreement. Overall, this series of
experiments has shown that both the HOMO and LUMO
levels become more low-lying upon bromine addition.
Figure 3. Absorption spectra of compounds 1 and 2 in dichloro-
methane at 1.3 × 10−5 M (solid lines). Normalized emission spectra of
compounds 1 and 2 at 77 K in frozen 2-methyl-THF glass (dashed
lines).
concurrent isosbestic points, showing a clean conversion of 1 to
2. Two isosbestic points are observed because the starting
compound has a single strong absorption and the product has
two absorption bands. The reaction equilibrates after the
addition of 1 equiv of Br2 when the titration is carried out in a
carbon tetrachloride solution (see Supporting Information).
We also note that the dual band absorption appears to be
characteristic of Br2-adducts 2 and 4 (see Supporting
Information); its origin will be discussed in detail below.
Table 1. Experimental and Calculated Optical and Electrochemical Data (eV)
a
d
e
d
f
g
h
Compound
Oxexp(V)
Oxcal
Redexp(V)
Redcal
GapEchem
GapOptical
GapCalcd
3.46
b
c
1
2
−5.70, −6.04
−6.20, −6.79
−5.08
−5.58
−
−3.44
−1.62
−2.79
−
2.76
3.10
2.46
2.79
a
b
c
d
Structures are given in Scheme 1. E1/2 from reversible oxidation peaks. Eox from maximum current of irreverible oxidation peak. Calculated
HOMO or LUMO energy level. Ered from maximum current of irreverible reduction peak. Difference of oxidation and reduction potentials. Onset
of low energy absorption band. Difference between calculated HOMO and LUMO energy levels.
e
f
g
h
3544
dx.doi.org/10.1021/ja210763n | J. Am. Chem. Soc. 2012, 134, 3542−3548

Journal of the American Chemical Society
Article
COMPUTATIONAL STUDIES
■
To elucidate the electronic structure responsible for the observed
optoelectronic properties, a series of DFT calculations were carried
out. The geometries of compounds 1′, 2′ (analogous to compounds 1
and 2 with methyl groups in place of the hexyl chains), 9,9-
dimethlyfluorene, tellurophene, and dibromotellurophene were
optimized using the Gaussian 09 software package. All geometries
were optimized to a minimum as verified by frequency calculations.
The extent of conjugation in 1′ was determined by comparing the
electron density of the frontier orbitals with individual tellurophene
and fluorene units. We will first describe the frontier orbitals of
Figure 5. Calculated frontier orbitals of tellurophene, fluorene, and
compound 1′ with an isocontour value of 0.05.
dibromotellurophene. The LUMO of 2′ is stabilized by 1.71 eV
compared to compound 1′, resulting in an overall smaller calculated
HOMO−LUMO gap in 2′. Taken together, these calculations indicate
that the red shift in absorption upon bromination is at least partly a
result of the involvement of the bromine atoms in the delocalized
electronic structure, as well as a redistribution of electron density
(discussed in detail below).
To understand optical absorption properties, time-dependent DFT
(TD-DFT) calculations were carried out on both compounds 1′ and 2′
Figure 4. Calculated frontier orbitals of tellurophene, fluorene, and
compound 1′ with an isocontour value of 0.05.
tellurophene and fluorene (Figure 4). The HOMO of tellurophene is
π in nature with significant electron density on the tellurium atom and
the carbons C3−C4. The LUMO of tellurophene has d-orbital
character from the tellurium atom (Ted), and the LUMO+1 is the first
unoccupied π* orbital. The HOMO and LUMO of 9,9-dimethyl-
fluorene are delocalized π and π* orbitals respectively. When
tellurophene is coupled to 9,9-dimethylfluorene in the 2 and 5
positions (compound 1′) the electron density is further delocalized
over the entire π-system. This delocalization is supported by the
narrowing of the calculated HOMO−LUMO gap for compound 1′
(the HOMO becomes higher in energy, and the LUMO becomes
lower in energy) compared to tellurophene and 9,9-dimethylfluorene.
Unlike the unsubstituted tellurophene, the HOMO of 1′ does not have
electron density on the tellurium atom, and rather the electron density
resides on the carbon atoms in the ring. In the HOMO−1, the
electron density does reside on the tellurium atom. The LUMO for 1′
is a delocalized π* orbital, and the LUMO+1 contains the tellurium d-
character, which is switched in order compared to tellurophene.
When tellurophene is converted to the dibromine adduct both the
HOMO and LUMO decrease in energy (Figure 5). The HOMO has
electron density on the bromine atoms, with contribution from the p
orbitals (Brp). The LUMO is delocalized over the π-system and also
includes Brp orbitals. When dibromotellurophene is coupled to
fluorene in the 2 and 5 positions (compound 2′) the HOMO of the
complex is a delocalized π orbital, similar in electron density
distribution to compound 1′, but destabilized by 0.51 eV. The
LUMO of 2′, however, is a tellurophene π* orbital with contribution
from the two Brp orbitals, which resembles the LUMO of
Figure 6. Calculated frontier orbitals of compounds 1 and 2 showing
the calculated transitions.
(Figure 6). These calculations were then used to determine the
orbitals involved in the transitions that make up the absorption
spectra. The first 20 singlet and triplet states were calculated. For
compound 1′ these calculations predict a state at 3.1 eV (399 nm, f =
1.6; where f is the oscillator strength of the transition, and a value over
0.1 is considered significant) consisting of a HOMO to LUMO
transition involving the π and π* orbitals. The single strong transition
is consistent with the optical absorption spectrum of 1 (see Supporting
Information).
3545
dx.doi.org/10.1021/ja210763n | J. Am. Chem. Soc. 2012, 134, 3542−3548

Journal of the American Chemical Society
Article
The TD-DFT calculations for 2′ predict two states with high
oscillator strengths, one low in energy (2.3 eV; 539 nm) and one high
in energy (3.5 eV; 351 nm). The low energy state at 539 nm (f = 0.44)
is a HOMO to LUMO transition involving the π and π*-Brp orbitals.
The high energy state at 351 nm (f = 1.02) is mainly (90%) comprised
of a HOMO to LUMO+2 transition, where the LUMO+2 involves the
π* orbital with a contribution from both the Tep and Brp orbitals. Just
as in 1′, the TD-DFT calculations on 2′ match the experimental
absorption spectrum of 2, which has peaks at 470 and 320 nm.
Overall, the unique features of the tellurophene compound 2 are the
red-shift in optical properties, the low-lying HOMO and LUMO levels,
and the dual band absorption profile. DFT calculations reveal that a
large reorganization of the electron distribution occurs during the 1 to
2 transformation. In the HOMO, both compounds have a delocalized
electronic structure as indicated by orbital contribution across the π-
system of the entire molecule. In 1′ the LUMO is also very delocalized.
In 2′, however, the unoccupied orbitals appear to be much more
localized on the tellurium ring. We postulate that this is similar to the
formation of a ground−excited state charge transfer (C-T) complex.
Indeed, previous work on polymers with charge-transfer π-systems has
shown that they typically have narrow optical HOMO−LUMO gaps,
dual bands in their absorption spectra, and low-lying frontier orbtials
relative to π-systems that do not have a strong C-T.46 Conversion of 1
to 2 engineers a discrete C-T in the π-system.
To gain insight into how the tellurophenes described herein differ
from previous tellurium compounds, DFT and TD-DFT calculations
were performed on a six member tellurium-containing heterocycle that
exhibits a blue shift in absorption upon Br2 addition, 4-[4-
(dimethylamino) phenyl]-2,6-dimethyltelluropyrylium chloride (see
Supporting Information).45,47−49 This telluropyrylium dye is cationic
and contains two six-member rings, one containing tellurium, the
other containing an amine group. Here, the parent (nonbrominated)
compound has a HOMO that is delocalized throughout the π-system,
including the p orbital of the tellurium atom, and a LUMO that is
delocalized over the π*-system. Conversion to the dibromine adduct
stabilizes the HOMO; however this also excludes the tellurium from
the π-system. This results in a disruption of the conjugation and causes
a predicted blue shift in absorption properties, which matches the
experimental data. The TD-DFT calculations show that the lowest
energy singlet transition with a high oscillator strength is a transition
from the HOMO to the LUMO for the parent compound (π−π*) at
491 nm (f = 0.88). In the dibrominated compound, however, the first
singlet state with a strong oscillator strength (f = 0.80) occurs at 458
nm with almost equal contributions from the HOMO to LUMO
transition and HOMO−5 to LUMO transition, which is a π-Brp and
Brp orbital to π* orbital transition. The mixture of the low-lying Brp
orbitals results in the blue-shifted absorption spectrum of the dibromo
adducts. Pertelluranes, five-member rings containing tellurium that is
ionically bound to an oxygen in the ring, also exhibit a bathochromic
shift in absorption upon bromination,50,51 which is due to a
destabilization of the HOMO energy level.52
and also to modify optical and electronic properties by
engineering discrete low-energy sites into the π-system. Due
to their optoelectronic properties and reactivity, these novel
compounds have potential use as semiconducting materials.
Moreover, two-electron oxidative addition to tellurophenes is
promising for catalyzing energy storage reactions. The relatively
straightforward synthesis reported here should inspire
researchers to begin to design and develop tellurophenes as
catalysts as well as optoelectronic materials.
EXPERIMENTAL SECTION
■
General Considerations. All reagents were used as received
unless otherwise noted. Tellurium, periodic acid dehydrate, potassium
iodide (KI), bromohexane, bis(trimethylsilyl)butadiyne, palladium
tetrakis(triphenylphospine) (Pd(PPh3)4), tetrabutylammonium fluo-
ride, and bromine were purchased from Sigma-Aldrich. Acetic acid,
sulphuric acid, iodine, NaCO3, MgSO4, dichloromethane, hexanes,
DMSO, KOH, and sodium thiosulfate were purchased from Fisher
Scientific. Fluorene, CuI, sodium borohydride, and iodobenzene were
purchased from Alfa Aesar. Silica gel was purchased from Silicycle.
The compounds 2-iodo-fluorene,53 2-iodo-9,9-dihexylfluorene,54
and 2,5-bis(phenyl)tellurophene35 were synthesized according to
literature procedures with minor modifications as indicated below.
1
The known compounds were characterized by H NMR and were in
agreement with literature.
Absorption spectra were recorded using a Varian Cary 5000
spectrometer. Emission spectra were recorded on a PTI fluorimeter
with a photodiode detector and xenon arc lamp source. NMR spectra
were recorded on a Varian Mercury 400 spectrometer (400 MHz).
Masses were determined on a Waters GCT Premier ToF mass
spectrometer (EI). Electrochemistry was performed with a BASi
Epsilon potentiostat.
Computations. Calculations were performed using the Gaussian
program,55 utilizing DFT with the B3LYP level of theory.56,57 The
basis set 6-31(d) was used for C and H atoms, and LAND2Z was used
for tellurium and bromine atoms.58 The structures were generated
using Gausview 5.0 and optimized to a minimum. From the optimized
geometry the first 20 singlet and triplet transitions were calculated
using time-dependent DFT.59
Cyclic Voltammetry. In 5 mL of dichloromethane, 5 mg of
compound 1 or 2 were dissolved, along with 0.19 g of
tetrabutylammonium hexaflorophosphate electrolyte. The working
electrode was a 2 mm Pt-disk, the counter electrode was a Pt wire
electrode, and the pseudoreference electrode was a Ag wire, with a
ferrocene internal reference and a scan rate of 500 mV/s.
Titrations. A 1.3 × 10−5 M solution of 1 in CCl4 was prepared and
transferred to a 3 mL cuvette. A 1.95 × 10−4 M solution of Br2 was
added in 20 μL aliquots, or 0.1 mol equiv. The absorption spectrum
was recorded at each point after 5 min of stirring.
Synthesis. 2-Iodofluorene.53 Fluorene (10 g, 60 mmol) was
dissolved in 100 mL of a boiling mixture of acetic acid, water,
and sulphuric acid (100:20:3). The mixture was cooled to 65
°C, and periodic acid dehydrate (2.3 g, 10 mmol) and iodine
(5.1 g, 20 mmol) were added. The red solution was stirred
overnight, and upon cooling to room temperature a yellow
solid was observed. Dichloromethane was added to dissolve the
solid. The organic layer was washed with saturated NaHCO3
solutions (3 × 50 mL) and then dried with MgSO4. The solvent
was evaporated under reduced pressure. The resulting solid was
recrystallized three times from hot hexanes (5.85 g, 30.2%
CONCLUSIONS
■
Tellurophenes are a relatively unrexplored class of compounds,
particularly those with extended π-systems. Through the
synthesis of 2,5-substituted tellurophene small molecules, 2,5-
bis(2-(9,9-dihexylfluorene))tellurophene (1), and 2,5-diphenyl-
tellurophene (3), as well as the Br2 adducts 2 and 4, we
understand how the optical and electronic properties of
telluorophenes with extended π-systems are controlled by the
coordination environment at the tellurium center. Conversion
of 1 to 2 changes the measured optical absorption spectrum
and oxidation potential of the compound. Namely, compound
2 has a red shift in optical properties, more low-lying HOMO
and LUMO levels, and dual as opposed to single absorption
profile, when compared with 1. Br2 addition lowers the HOMO
and as such is a potentially useful way to increase stability to
oxidation, which is a concern for these types of heterocycles,
1
yield). H NMR (400 MHz, CDCl3): δ 3.88 (s, 2H), 7.34 (m,
1H), 7.39 (m, 1H), 7.55 (m, 2H), 7.71 (m, 1H), 7.77 (m, 1H),
7.90 (m, 1H) ppm. Spectroscopy was identical to that of the
previous report of this compound.
2-Iodo-9,9-dihexylfluorene.54 A solution of 2-iodofluorene (1.7 g,
5.7 mmol) in DMSO was cooled in an ice bath and treated with KOH
(0.92 g, 16 mmol) and KI (0.13 g, 7.7 mmol) to afford a red solution.
After the KOH was fully dissolved, bromohexane (1.6 mL, 11 mmol)
3546
dx.doi.org/10.1021/ja210763n | J. Am. Chem. Soc. 2012, 134, 3542−3548

Journal of the American Chemical Society
Article
was added, and the solution was allowed to warm to room temperature
and stir overnight. The solution was extracted with dichloromethane
and washed with a 10% solution of sodium thiosulfate, followed by
brine and water. The organic layer was dried with MgSO4, and the
solvent was removed under reduced pressure. Column chromatog-
raphy (silica gel, hexanes) afforded the title compound as a yellow oil
(1.9 g, 81% yield). 1H (400 MHz, CDCl3): δ 0.58−0.63 (m, 4H), 0.76
(t, J = 6.75 Hz, 6H), 1.04−1.16 (m, 12 H), 1.90−1.96 (m, 4 H), 7.31−
7.38 (m, 3H), 7.44−7.46 (m, 1H), 7.64−7.69 (m, 1H). Spectroscopy
was identical to that of the previous report of this compound.
mmol), and iodobenzene (2.0 g, 9.8 mmol) were added to a Schlenk
bomb and degassed by three pump purge cycles. Dry, degassed toluene
was added (20 mL), followed by tetrabutylammonium fluoride (13
mL, 1 M in THF, 13 mmol). The reaction mixture turned deep red in
color and was sealed, heated to 50 °C, and stirred overnight. The
reaction mixture was diluted with dichloromethane and washed three
times with brine. The organic layer was dried with MgSO4, and the
solvent was removed under reduced pressure. Column chromatog-
raphy (silica gel, hexanes) afforded the title compound, a white
1
crystalline solid (0.18 g, 21% yield). H (400 MHz, CDCl3): δ 7.33−
7.39 (m, 6H), 7.53−7.56 (m 4H). 13C NMR (CDCl3, 100 MHz): δ
73.9, 81.5, 121.8, 128.4, 129.2, 132.5. TOF MS EI+ C16H10. Expected:
202.0783. Found: 202.0784. Δ = 0.5 ppm.
1,4-Bis(2-(9,9-dihexylfluorene))buta-1,3-diyne. Bis(trimethylsilyl)-
butadiyne (0.10 g, 0.52 mmol), Pd(PPh3)4 (0.059 g, 0.052 mmol), CuI
(0.019 g, 0.10 mmol), and 2-iodo-9,9-dihexylfluorene (0.50 g, 1.1
mmol) were added to a Schlenk bomb and degassed by three pump
purge cycles. Dry, degassed toluene was added, followed by
tetrabutylammonium fluoride (1.6 mL, 1 M in THF, 1.6 mmol).
CAUTION: Diacetylene gas is formed and is flammable and can form
explosive mixtures with air. Care should be taken with the
deprotection of bis(trimethylsilyl)butadiyne. The reaction mixture
turned deep red in color, was sealed and heated to 50 °C, and stirred
overnight. The reaction mixture was diluted with dichloromethane and
washed three times with brine, the organic layer was dried with
MgSO4, and the solvent was removed under reduced pressure.
Column chromatography (silica gel, hexanes) afforded a white solid
2,5-Bis(phenyl)tellurophene (3).35 Tellurium powder (380 mg, 2.9
mmol) was suspended in 20 mL of degassed ethanol. A sodium
borohydride (0.26 g, 6.9 mmol) solution in 9:1 ethanol/water (50
mL) was slowly added to the refluxing tellurium suspension. After the
solution turned from gray to purple to colorless a solution of 1,4-
bis(phenyl)buta-1,3-diyne (0.20 g, 0.98 mmol) in n-butanol (100 mL)
was slowly added after the solution was cooled to 50 °C. The reaction
was stirred overnight at 50 °C. The mixture was stirred under a flow of
air for 20 min to oxidize the unreacted sodium telluride and filtered
through Celite. The solvent was removed under reduced pressure. The
residue was dissolved in dichloromethane and washed three times with
brine. The organic layer was dried with MgSO4, and the solvent was
removed under reduced pressure. Column chromatography (silica gel,
hexanes) afforded the title compound, a light yellow crystalline solid
1
(0.30 g, 80% yield). H (400 MHz, CDCl3): δ 0.61−0.64 (m, 4 H),
0.79 (t, J1 = 7.1 Hz, 6H), 1.04−1.16 (m, 12H), 1.97 (t, J1 = 8.3 Hz,
4H), 7.34−7.36 (m, 3H), 7.53 (d, J1 = 1.7 Hz, 1H), 7.54 (d, J1 = 1.4
Hz, 1H), 7.67 (d, J1 = 8.4 Hz, 1H), 7.71 (dd, J1 = 7.3 Hz, J2 = 2.2 Hz,
1H). 13C NMR (CDCl3, 100 MHz): 13.0, 22.6, 23.7, 29.7, 31.5, 40.3,
55.1, 74.1, 83.1, 119.7, 119.9, 120.2, 122.9, 126.9, 126.9, 127.8, 131.5,
140.1, 142.3, 150.8, 151.2. TOF MS EI+ C54H66. Expected: 714.5165.
Found: 714.5179. Δ = 2.0 ppm.
1
(35 mg, 11% yield). H (400 MHz, CDCl3): δ 7.29−7.31 (m, 2H),
7.34−7.38 (m, 4H), 7.49−7.51 (m, 4H), 7.84 (s, JTe = 5.5 Hz, 1H).
13C NMR (CDCl3, 100 MHz): δ 126.7, 127.6, 129.0, 133.9, 139.9,
148.3. TOF MS EI+ C54H68Te. Expected: 335.00795. Found:
335.00902. Δ = 3.2 ppm.
2,5-Bis(phenyl)dibromotellurophene (4). Compound 3 (8.0 mg,
0.024 mmol) was dissolved in 5 mL of CCl4. A solution of bromine
(0.5 mL of Br2 in 100 mL of CCl4) was prepared, and 0.25 mL was
added (0.024 mmol, 1 equiv). The mixture was layered with methanol
and was refrigerated for 3 days. Orange-red plate crystals were
collected and examined by single crystal X-ray diffraction (7.5 mg,
2,5-Bis(2-(9,9-dihexylfluorene))tellurophene (1). Tellurium pow-
der (71 mg, 0.56 mmol) was suspended in 10 mL of degassed ethanol.
A sodium borohydride (0.20 g, 5.3 mmol) solution in 9:1 ethanol/
water (50 mL) was slowly added to the refluxing tellurium suspension.
After the solution turned from gray to purple to colorless a solution of
1,4-bis(2-(9,9-dihexylfluorene))buta-1,3-diyne (0.15 g, 0.27 mmol) in
n-butanol (100 mL) was slowly added after the solution was cooled to
60 °C. The reaction was stirred overnight at 60 °C. The mixture was
stirred under a flow of air for 20 min to oxidize the unreacted sodium
telluride and filtered through Celite. The solvent was removed under
reduced pressure. The residue was dissolved in dichloromethane and
washed three times with brine. The organic layer was dried with
MgSO4, and the solvent was removed under reduced pressure.
Column chromatography (silica gel, hexanes) afforded a yellow
powder (45 mg, 55% yield). 1H (400 MHz, CDCl3): δ 0.64−0.74 (m,
4H), 0.79 (t, J1 = 7.02 Hz, 6H), 1.06−1.17 (m, 12H), 2.00 (t, J1 = 8.38
Hz, 4H), 7.32−7.37 (m, 3H), 7.47−7.49 (m, 2H), 7.66−7.68 (m, 1H),
1
63.3% yield). H (400 MHz, CDCl3): δ 7.44 (s, JTe = 5.5 Hz, 2H),
7.49−7.51 (m, 3H), 7.63−7.65 (m, 4H). 13C NMR (CDCl3, 100
MHz): δ 126.3, 127.5, 128.2, 129.7, 130.7, 138.5. TOF MS EI+
C16H12TeBr. Expected: 412.91846. Found: 412.91826. Δ = 0.5 ppm.
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystal structure of 4, crystal data and structure refinement of
compound 4, selected bond lengths and angles of compound 4,
Stern−Volmer plot for the addition of Br2 to 1, absorption
spectrum of compounds 3 and 4, cyclic voltammogram for
compounds 1 and 2, calculated and experimental absorption of
1 and 2, frontier orbitals for telluropyrillium dyes, geometry
coordinates for optimized structures, and complete citation for
ref 55. This material is available free of charge via the Internet
at http://pubs.acs.org.
7.72 (dt, J1 = 7.0 Hz, J2 = 1.2 Hz, 1H), 7.93 (s, JTe = 20.3 Hz, 1H). 13
C
NMR (CDCl3, 100 MHz): δ 14.0, 22.6, 23.7, 29.7, 31.5, 40.4, 55.1,
119.7, 120.1, 120.6, 122.9, 126.2, 126.8, 127.1, 133.6, 138.8, 140.6,
140.9, 148.7, 151.0, 151.6. TOF MS EI+ C54H68Te. Expected:
864.4383. Found: 846.4407. Δ = 2.8 ppm.
2,5-Bis(2-(9,9-dihexylfluorene))dibromotellurophene (2). 2,5-Bis-
(2-(9,9-dihexylfluorene))tellurophene (0.10 g, 0.12 mmol) was
dissolved in 5 mL of CCl4. A solution of bromine (0.1 mL of Br2 in
1.0 mL of CCl4) was prepared, and 60 μL were added (0.12 mmol).
The mixture was layered with methanol and refrigerated for 3 days to
AUTHOR INFORMATION
■
Corresponding Author
dseferos@chem.utoronto.ca
1
obtain red crystals collected by filtration (72% yield). H (400 MHz,
CDCl3): δ. 0.70 (m, J = 8.38 Hz, 4H), 0.78 (t, J = 7.02 Hz, 6H), 1.02−
1.20 (m, 12H), 2.02 (dd, J = 9.45, 7.11 Hz, 4H), 7.35−7.41 (m, 3H)
7.49 (s, 1H), 7.55 (d, J = 1.36 Hz, 1H), 7.61 (dd, J = 7.99, 1.75 Hz,
1H), 7.75 (d, J = 3.51 Hz, 1H), 7.79 (d, J = 7.80 Hz, 1H). 13C NMR
(CDCl3, 100 MHz): δ 14.0, 22.5, 23.8, 29.6, 31.4, 40.2, 55.4, 120.3,
120.8, 121.49. 123.1, 127.1, 127.1, 128.1, 131.5, 137.5, 139.8, 143.9,
151.4, 152.3, 163.1. CHN: C54H68TeBr2 Expected: C, 64.57; H, 6.82.
Found: C, 64.0; H, 6.73.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the University of Toronto,
NSERC, the CFI and the Ontario Research Fund. We would
like to thank Dmitry Pichugin and Marius Kapp for their help
with the benzene insert NMR.
1,4-Bis(phenyl)buta-1,3-diyne. Bis(trimethylsilyl)butadiyne (0.84
g, 4.26 mmol), Pd(PPh3)4 (0.49 g, 0.43 mmol), CuI (0.16 g, 0.85
3547
dx.doi.org/10.1021/ja210763n | J. Am. Chem. Soc. 2012, 134, 3542−3548

Journal of the American Chemical Society
Article
(38) Caution: Diacetylene gas is formed and is flammable and can
form explosive mixtures with air. Care should be taken with the
deprotection of bis(trimethylsilyl)butadiyne.
(39) Sweat, D. P.; Stephens, C. E. J. Organomet. Chem. 2008, 693,
2463−2464.
REFERENCES
■
(1) Detty, M. R.; Luss, H. R. Organometallics 1986, 5, 2250−2256.
(2) Leonard, K. A.; Zhou, F.; Detty, M. R. Organometallics 1996, 15,
4285−4292.
(3) Higgs, D. E.; Nelen, M. I.; Detty, M. R. Org. Lett. 2001, 3, 349−
(40) Mack, W. Angew. Chem., Int. Ed. Engl. 1966, 5, 896−896.
(41) Rhoden, C. R. B.; Zeni, G. Org. Biomol. Chem. 2011, 9, 1301−
1313.
352.
(4) Butcher, T. S.; Detty, M. R. J. Org. Chem. 1999, 64, 5677−5681.
(5) Abe, M.; Detty, M. R.; Gerlits, O. O.; Sukumaran, D. K.
Organometallics 2004, 23, 4513−4518.
(6) Zhang, B.; Hou, W.; Ye, X.; Fu, S.; Xie, Y. Adv. Funct. Mater.
2007, 17, 486−492.
(7) Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A.
P. Nat. Mater. 2003, 2, 382−385.
(8) Chu, J.; Li, X.; Xu, P. J. Mater. Chem. 2011, 21, 11283−11287.
(9) Gamboa, S. A.; Sebastian, P. J.; Mathew, X.; Nguyen-Cong, H.;
Chartier, P. Sol. Energy Mater. Sol. Cells 1999, 59, 115−124.
(10) Smith, A. M.; Nie, S. Acc. Chem. Res. 2010, 43, 190−200.
(11) Narita, Y.; Hagiri, I.; Takahashi, N.; Takeda, K. Jpn. J. Appl. Phys.
1 2004, 43, 4248−4258.
(42) Inoue, S.; Jigami, T.; Nozoe, H.; Otsubo, T.; Ogura, F.
Tetrahedron Lett. 1994, 35, 8009−8012.
(43) Promarak, V.; Punkvuang, A.; Jungsuttiwong, S.; Saengsuwan,
S.; Sudyoadsuk, T.; Keawin, T. Tetrahedron Lett. 2007, 48, 3661−
3665.
(44) Takimiya, K.; Konda, Y.; Ebata, H.; Niihara, N.; Otsubo, T. J.
Org. Chem. 2005, 70, 10569−10571.
(45) Detty, M. R.; Friedman, A. E.; McMillan, M. Organometallics
1994, 13, 3338−3345.
(46) Beaujuge, P. M.; Amb, C. M.; Reynolds, J. R. Acc. Chem. Res.
2010, 43, 1396−1407.
(47) Detty, M. R.; Frade, T. M. Organometallics 1993, 12, 2496−
(12) Narita, Y.; Takeda, K Jpn. J. Appl. Phys. 1 2006, 45, 2628−2642.
(13) Muranaka, A.; Yasuike, S.; Liu, C. Y.; Kurita, J.; Kakusawa, N.;
Tsuchiya, T.; Okuda, M.; Kobayashi, N.; Matsumoto, Y.; Yoshida, K.;
Hashizume, D.; Uchiyama, M. J. Phys. Chem. A 2009, 113, 464−473.
(14) McCullough, R. D.; Kok, G. B.; Lerstrup, K. A.; Cowan, D. O. J.
Am. Chem. Soc. 1987, 109, 4115−4116.
2504.
(48) Detty, M. R.; Friedman, A. E. Organometallics 1994, 13, 533−
540.
(49) Engman, L.; Lind, J.; Merenyi, G. J. Phys. Chem. 1994, 98,
3174−3182.
(50) Detty, M. R.; Luss, H. R. J. Org. Chem. 1983, 48, 5149−5151.
(51) Detty, M. R.; Lenhart, W. C.; Gassman, P. G.; Callstrom, M. R.
Organometallics 1989, 8, 866−870.
(15) Bendikov, M.; Wudl, F.; Perepichka, D. F. Chem. Rev. 2004, 104,
4891−4946.
(16) Wudl, F.; Aharon-Shalom, E. J. Am. Chem. Soc. 1982, 104,
1154−1156.
(52) Detty, M. R.; Luss, H. R.; McKelvey, J. M.; Geer, S. M. J. Org.
Chem. 1986, 51, 1692−1700.
(17) Millefiori, S.; Alparone, A. Phys. Chem. Chem. Phys. 2000, 2,
2495−2501.
(53) Lee, S. H.; Nakamura, T.; Tsutsui, T. Org. Lett. 2001, 3, 2005−
2007.
(18) Jansik, B.; Schimmelpfennig, B.; Norman, P.; Macak, P.; Agren,
H.; Ohta, K. THEOCHEM 2003, 633, 237−246.
(19) Welch, G. C.; Coffin, R.; Peet, J.; Bazan, G. C. J. Am. Chem. Soc.
2009, 131, 10802−10803.
(54) Chen, C. H.; Hsieh, C. H.; Dubosc, M.; Cheng, Y. J.; Hsu, C. S.
Macromolecules 2010, 43, 697−708.
(55) Frisch, M. J., et al. Gaussian 09, revision A.1; Gaussian, Inc.:
Wallingford, CT, 2009.
(56) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652.
(57) Becke, A. D. J. Chem. Phys. 1996, 104, 1040−1046.
(58) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270−283.
(59) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett. 1996, 256,
454−464.
(20) Job, A.; Wakamiya, A.; Kehr, G.; Erker, G.; Yamaguchi, S. Org.
Lett. 2010, 12, 5470−5473.
(21) Welch, G. C.; Bazan, G. C. J. Am. Chem. Soc. 2011, 133, 4632−
4644.
(22) Shultz, A. M.; Farha, O. K.; Hupp, J. T.; Nguyen, S. T. Chem. Sci.
2011, 2, 686−689.
(23) Wang, B.; Wasielewski, M. R. J. Am. Chem. Soc. 1997, 119, 12−
21.
(24) Thomas, S. W.; Joly, G. D.; Swager, T. M. Chem. Rev. 2007, 107,
1339−1386.
(25) Kappaun, S.; Scheiber, H.; Trattnig, R.; Zojer, E.; List, E. J. W.;
Slugovc, C. Chem. Commun. 2008, 5170−5172.
(26) Yang, R.; Wu, H.; Cao, Y.; Bazan, G. C. J. Am. Chem. Soc. 2006,
128, 14422−14423.
(27) Yang, R.; Garcia, A.; Korystov, D.; Mikhailovsky, A.; Bazan, G.
C.; Nguyen, T. Q. J. Am. Chem. Soc. 2006, 128, 16532−16539.
(28) McCullough, J. D. Inorg. Chem. 1975, 14, 2285−2286.
(29) Jahnke, A. A.; Howe, G. W.; Seferos, D. S. Angew. Chem., Int. Ed.
2010, 49, 10140−10144.
(30) Cook, T. R.; Esswein, A. J.; Nocera, D. G. J. Am. Chem. Soc.
2007, 129, 10094−10095.
(31) Cook, T. R.; Surendranath, Y.; Nocera, D. G. J. Am. Chem. Soc.
2009, 131, 28−29.
(32) Lee, C. H.; Cook, T. R.; Nocera, D. G. Inorg. Chem. 2011, 50,
714−716.
(33) Teets, T. S.; Nocera, D. G. J. Am. Chem. Soc. 2009, 131, 7411−
7420.
(34) Sweat, D. P.; Stephens, C. E. Synthesis 2009, 3214−3218.
(35) Davies, A. G.; Schiesser, C. H. J. Organomet. Chem. 1990, 389,
301−313.
(36) Potapov, V. A.; Amosova, S. V.; Doron’kina, I. V. Chem.
Heterocycl. Compd. 2002, 38, 259−260.
(37) Potapov, V. A.; Amosova, S. V.; Doron’kina, I. V.; Zinchenko, S.
V. Sulfur Lett. 2001, 24, 275−279.
3548
dx.doi.org/10.1021/ja210763n | J. Am. Chem. Soc. 2012, 134, 3542−3548