Synthesis of Diphenylchalcogenophene Diboronic Acid Bis(pinacol) Esters and Halogen Photoelimination from Tellurium by Triplet–Triplet Annihilation

Article abstract of DOI:10.1002/cplu.201600335

The synthesis of diphenylthiophene-, diphenylselenophene-, and diphenyltellurophene-based diboronic bis(pinacol) esters and their optoelectronic properties is reported. The addition of bromine to the borylated diaryltellurophene and subsequent photoelimination are investigated. The photochemical quantum yield of bromine photoelimination is (9.7±0.2) % at a 4.4 m trap (2,3-dimethyl-1,3-butadiene) concentration. It is found that the bromine photoelimination reaction can also be driven by the incorporation of a triplet sensitizer through a triplet–triplet annihilation process.

Full text of DOI:10.1002/cplu.201600335

Accepted Article
Title: The Synthesis of Diphenylchalcogenophene Diboronic Acid Bis(pinacol) Es-
ters and Halogen Photoelimination from Te by Triplet-Triplet Annihilation.
Authors: Peng-Fe Li; Elisa I. Carrera; Dwight S. Seferos
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10.1002/cplu.201600335
The Synthesis of Diphenylchalcogenophene Diboronic Acid Bis(pinacol) Esters and
Halogen Photoelimination from Te by Triplet-Triplet Annihilation.
Peng-Fei Li,[a] Elisa I. Carrera,[a] and Dwight S. Seferos*[a]
[a] Dr. P. F. Li, E. I. Carrera, and Prof. D. S. Seferos
Department of Chemistry, University of Toronto
80 St. George, Toronto, ON M5S 3H6 (Canada)
E-mail: dseferos@chem.utoronto.ca
Homepage: http://www.chem.utoronto.ca/staff/seferos/
Abstract: We report the synthesis of diphenylthiophene-, diphenylselenophene-, and
diphenyltellurophene-based diboronic bis(pinacol) esters and their optoelectronic properties. The
addition of bromine to the borylated diaryltellurophene and subsequent photoelimination were
investigated. The photochemical quantum yield of bromine photoelimination is 9.7 ± 0.2% at a
4.4 M trap (2,3-dimethyl-1,3-butadiene) concentration. We found that the bromine
photoelimination reaction can also be driven by the incorporation of a triplet sensitizer through a
triplet-triplet annihilation process.
Visible light driven reactions have attracted much interest in various fields in recent years,
such as photoredox catalysis and artificial photosynthesis.1 Several metal complexes can
efficiently photoeliminate halogens, which is becoming important for light-driven fuel
production.2 The metalloid nature of tellurium provides a versatile platform to study
photoelimination of tellurium(IV), which could find the potential use in energy storage reactions
such as conversion of HX to H2 and X2.3 We discovered that visible light can drive the
photoelimination of π-conjugated tellurophenes.4 For example, green light can induce
photoelimination of bromine from isoindigo-functionalized tellurophenes.4b The photochemical
quantum yield of this system is strongly related to the nature of the substituted groups. Increased
photochemical quantum yields have been achieved for
a
series of substituted
diphenyltellurophenes. However, it is difficult to drive this reaction with longer wavelength
visible light, because this would require a molecule with highly extended conjugation.
Sensitized triplet-triplet annihilation (TTA) has emerged as an efficient way to achieve photon
upconversion, a process by which low-energy photons are converted to high-energy ones.5 In the
TTA process, a triplet sensitizer, which is excited by low energy photons, transfers its triplet
energy to an acceptor molecule. Two triplet-excited acceptors then undergo TTA to form one
ground state and one singlet excited state molecule, which can then emit a higher energy photon.
Inspired by this process, we reasoned that it would be possible to drive the halogenated
tellurophene photolysis reaction with longer wavelength light. Triplet states of tellurium
containing compounds are accessible due to the heavy atom effect imparted by tellurium. Laser
flash photolysis studies on substituted diphenyltellurophenes showed that intersystem crossing is
competitive with halogen photoelimination from the photoactive singlet excited state.4e Here, we
report a high-yielding synthesis of new boronic ester capped diphenylthiophene-,
diphenylselenophene-, and diphenyltellurophene. We investigate the optoelectronic properties of
these compounds, and for the first time we show that low energy halogen photoelimination from
tellurophene can be accelerated by triplet-triplet annihilation.
Thiophene- and selenophene-containing polymers are the most successful classes of
conjugated materials and have been the focus of optoelectronic investigations.6 Tellurophene, the
heaviest known group 16 heterocycle, should prove even more useful for optoelectronic

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applications. However, due to synthetic challenges, their development lags behind their lighter
counterparts.7 In order to develop tellurophene-containing materials, the unsubstituted
tellurophene must be efficiently functionalized. Aryl-boronic acid derivatives have been key
intermediates in organic chemistry, which are stable, nontoxic, and very soluble.8 In addition,
boronic acids provide special binding sites, which is quite unique compared with other functional
groups.9 Despite the utility and advantages of using Suzuki coupling reactions, boronic acid
derivatives of the group 16 heterocycles are sparse in the literature, except for some thiophene
derivatives10 and only a few recent examples of tellurophene.11 Very recently, we reported
hydrogen-bonded organic frameworks based on thiophene, selenophene, and tellurophene
capped with N-methyliminodiacetic acid (MIDA) boronates.12
Our study began with the synthesis of the precursors to diboronic bis(pinacol) esters of
diphenyltellurophene. Tellurophene was synthesized via a ring-closing reaction between sodium
telluride and diacetylene.13 The conditions of the diacetylene production step are critical to this
gas-liquid reaction. We used DMSO as the diacetylene generation solvent, and added THF to the
sodium telluride preparation reaction, which increased the yield of tellurophene to around 75%
based on tellurium loading with good reproducibility (Scheme 1a).
Scheme 1. The modified synthesis of tellurophene and the synthesis of
diphenylchalcogenophene diboronic acid bis(pinacol) esters 1, 2 and 3.
With an efficient synthetic procedure, tellurophene was transformed to 2,5-
bis(trimethyltin)tellurophene by the previously reported method.14 Diphenylthiophene-,
diphenylselenophene-, and diphenyltellurophene-based diboronic esters (1, 2, and 3,
respectively) were synthesized by a Pd(PPh3)4 catalyzed Stille-coupling reaction of 4-
iodophenylboronic acid pinacol ester (2.5 equiv.) with the corresponding 2,5-
o
bis(trimethyltin)heterocycle (1.0 equiv.) in degassed toluene at 110 C overnight under an argon
atmosphere (Scheme 1b). The crude product was recrystallized directly from hot toluene,
affording yellow fiber-like solids of 1, 2, and 3, respectively in 82-90% yield. The melting point
of this series of compounds increases moving from 1 (255-256 oC) to 2 (282-283 oC) to 3 (>300
oC). The heterocycle proton signal shifts downfield from 1 (7.37 ppm) to 2 (7.53 ppm) to 3 (7.91
ppm) due to decreased shielding with a longer C-Te bond. On the contrary, the boron signal
shifts upfield from 1 (32.04 ppm) to 2 (31.32 ppm) to 3 (30.14 ppm) due to higher electron
density of heavier chalcogenophene.

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Figure 1. (a) Solution absorption spectra of 1, 2 and 3 in chloroform; (b) cyclic voltammograms
in acetonitrile vs. Ag/Ag+ reference; (c) excitation spectra; (d) emission spectra in chloroform.
Optical absorption spectra were measured in chloroform solution to determine the effect of
heavy atom substitution on electronic properties (Figure 1a). We observe that the absorption
maximum (λmax) of this series shifts to longer wavelengths (lower energy) as one moves from 1
(345 nm) to 2 (351 nm) to 3 (361 nm). This trend is further illustrated by the energy of the
optical band gap (measured from the absorption onset) of 1 (3.15 eV), 2 (3.08 eV), and 3 (2.97
eV). The red-shift on moving from S to Se to Te is also observed in excitation spectra (Figure
1c). At the same time, the intensity of excitation spectra is decreased upon moving down the
group. The emission spectra are a mirror image of the excitation spectra, which also displays a
red-shift from 1 (412 nm) to 2 (424 nm) to 3 (439 nm) (Figure 1d). Similar to the excitation
spectra, the intensity of the emission spectra is also attenuated upon moving down the group. The
quantum yields of 1 to 3 are 41 %, 2 % and < 1 %, which were measured by using an integrating
sphere. Since 1 has the highest quantum yield in this series, its fluorescence lifetime was
obtained by time-correlated single photon counting. The lifetime of 1 is 0.64 ns (Supporting
Information, Figure S14). Next, cyclic voltammetry was used to determine the positions of the
frontier orbitals of these compounds (Figure 1b). The energy of the HOMO level was estimated
from the first oxidation peak. 1 and 2 have reversible oxidization waves at 1.24 V and 1.23 V vs.
Ag/Ag+ reference. A similar reversible oxidation wave was not observed for 3, which has an
oxidation potential at 1.16 V vs. Ag/Ag+ reference. The irreversible oxidation is possibly due to
the formation of dihydroxyl tellurophene or telluroxide species, which is hard to reduced.4c The
oxidation potential shifts to a more negative value for 3, which indicates that incorporation of Te
results in more facile oxidation compared to S and Se.
To understand the results of the optical characterization, the geometry and electronic structure
of 1, 2, and 3 were calculated using density functional theory (DFT). Geometry optimizations
were performed using the Gaussian 09 program at the B3LYP level of theory with the 6-
311+G(d) basis set for C, H, O, B, and S, and the LanL2DZ basis set for Se and Te. The frontier

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molecular orbitals are very similar in electronic distribution for all three compounds, except the
HOMOs display decreased electron density on the central ring with heavier elements (Figure 2
and Supporting Information, Figure S15). The HOMO energy gradually increases from 1 (-5.66
eV) to 2 (-5.62 eV) to 3 (-5.60 eV), while the LUMO energy gradually decreases from 1 (-1.82
eV) to 2 (-1.83 eV) to 3 (-1.85 eV), leading to an overall narrowing of the calculated HOMO-
LUMO gap from 3.84 to 3.75 eV.
Figure 2. DFT optimized structures (top), and molecular orbital diagrams of the LUMOs
(middle) and HOMOs (bottom; isovalue of 0.02) of 3 (left) and 3-Br (right).
Due to its metalloid nature, the incorporation of tellurium into the heterocycle imparts
reactivity that is unique to tellurophene compared to the thiophene and selenophene counterparts.
In particular, tellurium can be readily oxidized to form hypervalent adducts involving oxygen or
halogens. 3 was oxidized in the presence of bromine. Increasing titration with Br2 in CHCl3 leads
to a decrease in the absorption corresponding to the parent tellurophene accompanied by an
increase in absorption at both higher- and lower-energy wavelengths (Figure 3b). Clear
isosbestic points indicate clean conversion to the brominated compound. The red-shift in the
lower-energy λmax of the brominated compound relative to that of the parent compound is less
pronounced for 3 bearing boronic acid pinacol esters (80 nm) than those bearing electron
donating groups, like t-butyl, t-butoxy, and diethylamino, but close to electron withdrawing
groups like pentafluorophenyl and trifluoromethyl.4e Complete conversion to the brominated
1
product was achieved using 1 equiv. of Br2 as determined by H NMR spectroscopy. 3-Br was
also synthesized on a preparative scale and isolated (Supporting Information).
To understand this optical absorption shift, geometry optimizations and time-dependent DFT
(TD-DFT) calculations were carried out on both 3 and the brominated compound (3-Br) using
the Gaussian 09 program. The first 20 singlet excited states were calculated. For compound 3,
these calculations predict an excitation at 3.28 eV (378 nm, f = 1.1; where f is the oscillator
strength of the transition) consisting of a HOMO to LUMO transition involving delocalized π
and π* orbitals. The single strong transition is consistent with the optical absorption spectrum of
3 (Supporting Information, Figure S16). The TD-DFT calculations for 3-Br predict two main
excitations with high oscillator strengths, one low in energy (2.10 eV; 590 nm) and one high in
energy (3.79 eV; 327 nm). The low energy state at 590 nm (f = 0.14) is a HOMO to LUMO

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transition which has a charge-transfer character from a delocalized π state to the π*-pBr orbitals.
The high-energy state at 327 nm (f = 0.88) is mainly comprised of a HOMO to LUMO+2
transition. The calculated spectra are overall red-shifted for both 3 and 3-Br compared to those
determined experimentally (λmax = 294 and 442 nm, respectively), however the trends and shapes
of the spectra are consistent.
Figure 3. (a) Bromination of 3 and photolysis of 3-Br; (b) optical absorption spectra from
bromine titration experiments for 3 in CHCl3; (c) photolysis of 3-Br under irradiation with a 450
nm LED.
Photolysis experiments were carried out on the brominated compound by irradiating within the
lowest-energy absorption band. The reactions were monitored using optical absorption
spectroscopy. Irradiation of 3-Br at 450 nm in the presence of 3 M DMBD led to spectral
changes consistent with the loss of bromine. The photolysis product spectrum is quite close to
the parent compound which demonstrates clean conversion to the reduced tellurophene. 1H NMR
spectra also show the clean photolysis of 3-Br. The efficiency of photoelimination was evaluated
by determining the photochemical quantum yield, Φp, using potassium ferrioxalate
actinometry.15 3-Br has a photochemical quantum yield of 5.2% at a DMBD concentration of 2
M, which is slightly lower than the unsubstituted diphenyltellurophene (8.5%), but significantly
higher than the t-butyl substituted tellurophene (1.6%), demonstrating that the boronic ester has a
slight electron-donating effect. A concentration-dependent quantum yield study was also carried
out at various trap concentrations. A linear dependence of the quantum yield on the trap
concentration was observed (Supporting Information, Figure S22), with a moderate value of 9.7
± 0.2% at a DMBD concentration of 4.4 M. In addition, the photoelimination quantum yield was
measured under different irradiation wavelengths. At 1.75 M trap concentration, the quantum
yield is 4.65% ± 0.04%, 4.61% ± 0.26% and 7.31% ± 0.31% at 450 nm, 365 nm and 297 nm,
respectively (Supporting Information, Figure S23 and S24). The quantum yield at 297 nm is
significantly higher than at longer wavelength, which is a sign that an additional
photoelimination pathway may be accessed through higher energy excited states. Indeed, the

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LUMO + 2 (which is involved in this higher energy transition) also displays significant Te-Br
antibonding character similar to that of the LUMO (Supporting Information, Figure S25).
We hypothesized that the photoactive singlet excited state could be accessed via a TTA
mechanism, which would allow for low-energy photolysis. Octaethylporphinepalladium(II)
(PdOEP) is a sensitizer that is widely used in the TTA upconversion process.5h We found that
bromine photoelimination could be efficiently driven by a 532 nm green laser with PdOEP
(Figure 4a). In the absence of PdOEP, 3-Br can be slowly photolyzed by a 532 nm green
(Supporting Information, Figure S26), due to the absorbance tail that reaches this wavelength.
However, upon addition of the triplet sensitizer the rate of photolysis is dramatically increased
(Figure 4b). Based on the molar absorption coefficient and absorbance change over time, the
initial photolysis rate was calculated to be 0.06 to 0.39 μM/s at 0.02 to 0.10 mM 3-Br. We
propose that the photolysis rate is enhanced due to triplet energy transfer from the sensitizer to 3-
Br, followed by more efficient TTA to the singlet excited state at higher concentration. A TTA
process is supported by two key experimental results: 1) In the presence of sensitizer and O2, the
photolysis rate is significantly hampered (Figure 4b), confirming the involvement of triplet
species; 2) The initial photolysis rate has a square dependence on the concentration of 3-Br
(Supporting Information, Figure S28), which is a sign of the bimolecular nature of TTA. In
addition, the TD-DFT-calculated singlet and triplet energies of PdOEP and 3-Br satisfy the
energy requirements for the TTA process (Supporting Information, Figure S29). Although we
could not quantitatively determine Φp and make a direct comparison to Φp at 450 nm due to the
wavelength range limitation of potassium ferrioxalate actinometry, this work demonstrates that
triplet-triplet annihilation can be used to effect halogen photoelimination.
Figure 4. (a) Photolysis of 6.0 × 10-5 M chloroform solution 3-Br with 2.5 × 10-5 M sensitizer
PdOEP under irradiation of a 530 nm green laser (30 mW) at 0.2 M DMBD; (b) comparison of
the photolysis with and without sensitizer, and under air saturated conditions, monitored at 450
nm.
In conclusion, we report the improved synthesis of tellurophene, and a facile, synthesis of
diphenylthiophene-, diphenylselenophene-, and diphenyltellurophene-based diboronic esters.
The optoelectronic properties of this series of compounds were investigated, which demonstrated
characteristic heavy element effects. The bromine addition and photoelimination reactions of the
tellurophene compound were also studied. Boronic ester-substituted biphenyltellurophene has a
significantly higher photochemical quantum yield than diphenyltellurophenes with electron-

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donating functional groups. Additionally, we demonstrate for the first time that the halogen
photoelimination reaction can be driven with green light by a triplet-triplet annihilation process.
Extending this TTA process to other functionalized tellurophenes with higher quantum yields
should allow for the combination of high efficiency and low energy halogen photoelimination.
Acknowledgements
D.S.S. is grateful to NSERC, DuPont for a Young Professor Grant, and the Alfred P. Sloan
Foundation for a research fellowship in chemistry. E.I.C. is grateful to NSERC for a CSG D and
to the Walter Sumner Foundation. We thank Prof. Rebecca Jockusch for the 532 nm green laser
and Dr. Fengjia Fan for the mearsurement of quantum yields and fluorescence lifetime.
Keywords: triplet-triplet annihilation • tellurophene • bromine addition • photoelimination
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Entry for the Table of Contents
Fire at the bromine. A facile synthesis of diphenylchalcogenophene diboronic acid bis(pinacol)
esters and their optoelectronic properties were reported. Bromine addition and photoelimination
of the tellurophene compound were investigated. We demonstrate that photoelimination can be
driven through triplet-triplet annihilation.