Coaxing solid-state phosphorescence from tellurophenes

Article abstract of DOI:10.1002/anie.201307373

The synthesis of the first examples of tellurophenes exhibiting phosphorescence in the solid state and under ambient conditions (room temperature and in air) is reported. Each of these main-group-element-based emitters feature pinacolboronates (BPin) as r

Full text of DOI:10.1002/anie.201307373

Angewandte
Chemie
DOI: 10.1002/anie.201307373
Emissive Tellurophenes
Coaxing Solid-State Phosphorescence from Tellurophenes**
Gang He, William Torres Delgado, Devon J. Schatz, Christian Merten, Arash Mohammadpour,
Lorenz Mayr, Michael J. Ferguson, Robert McDonald, Alex Brown, Karthik Shankar,* and
Eric Rivard*
Abstract: The synthesis of the first examples of tellurophenes
exhibiting phosphorescence in the solid state and under
ambient conditions (room temperature and in air) is reported.
Each of these main-group-element-based emitters feature
pinacolboronates (BPin) as ring-appended side groups. The
nature of the luminescence observed was also investigated
using computational methods.
gaps, deeper LUMO levels, and potentially higher charge
carrier mobilities relative to their well-studied thiophene
counterparts.[5] Notably, the Seferos and Rivard groups have
prepared a series of polytellurophenes for consideration as
photovoltaic materials with the aid of improved synthetic
approaches.[6] In each case, the reported polytellurophenes
were non-emissive owing to the “heavy-metal effect”[7]
whereby heavy elements encourage singlet-to-triplet crossing,
leading to the formation long-lived triplet excited states that
are prone to internal (self) or external (oxygen or solvent-
mediated) quenching. For example, the Navarrete group
noted that Te-substituted benzodichalcogenophenes were
non-emissive, whereas fluorescence was observed for the
lighter S and Se analogues.[8] Herein, we report that the
previously known boronic ester-capped tellurophene B-Te-6-
B[6c] displays intense green phosphorescence in the solid state
(Scheme 1[9]). Furthermore, B-Te-6-B readily forms emissive
T
he discovery of new phosphorescent materials that are
efficient light emitters in the solid state is of paramount
importance in advancing organic light-emitting diode
(OLED) technologies, as higher internal emission quantum
efficiencies can be obtained in relation to ubiquitous fluo-
rescent systems (100% vs. 25%).[1] However, molecules that
exhibit efficient emission in solution are often non-lumines-
cent in the solid state owing to aggregation-caused quenching
(ACQ).[2] As the ACQ effect is detrimental to many practical
applications (for example, requiring dilution of the emitter in
a host material for OLEDs), there have been numerous
efforts to tackle this problem. In 2001, Tang and co-workers
uncovered aggregation-induced emission (AIE), whereby
a restriction of intramolecular motions in the solid state
suppresses the ACQ effect by limiting non-radiative decay
pathways, leading to intense emission in both the solid and
aggregated states;[3] however in the vast majority of reported
cases, only fluorescence was noted.[4]
Tellurophenes are attracting increasing attention from the
scientific community with respect to the development of new
optoelectronic materials as a result of their lower optical band
[*] Dr. G. He, W. Torres Delgado, D. J. Schatz, Dr. C. Merten,
M. J. Ferguson, R. McDonald, Prof. Dr. A. Brown, Prof. Dr. E. Rivard
Dept. of Chemistry, University of Alberta
11227 Saskatchewan Dr., Edmonton, Alberta, T6G2G2 (Canada)
E-mail: erivard@ualberta.ca
A. Mohammadpour, L. Mayr, Prof. Dr. K. Shankar
Dept. of Electrical and Computer Engineering, University of Alberta
Edmonton, Alberta, T6G 2V4 (Canada)
Scheme 1. Synthesis of pinacolboronate-capped tellurophenes, and
emission characteristics of B-Te-6-B in solution and in the solid state.
E-mail: kshankar@ualberta.ca
[**] This work was supported by the Natural Sciences and Engineering
Research Council (NSERC) of Canada, the Canada Foundation for
Innovation, Alberta Innovates-Technologies Futures (New Faculty
Award to E.R. and scholarship to A.M.), and the Canada School for
Energy and the Environment. C.M. acknowledges the Alexander von
Humboldt foundation for a Feodor Lynen Postdoctoral fellowship.
We also gratefully acknowledge access to the computing facilities
provided by the Western Canada Research Grid (Westgrid) and
Compute/Calcul Canada. We also thank Prof. Al Meldrum for useful
discussions and Zhenyu Yang for SEM measurements.
host-free films that can be cast directly from THF solution.
The development of inexpensive tellurophene phosphors is of
added interest given that most phosphorescent materials
feature precious metal Ir or Pt complexes diluted in accom-
panying host materials.[10] To the best of our knowledge, this is
the first example of luminescence from a tellurophene under
ambient conditions,[11] and the photophysical data of com-
parative inorganic rings are provided to extract the nature of
light emission from B-Te-6-B.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201307373.
Angew. Chem. Int. Ed. 2014, 53, 4587 –4591
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4587

.
Angewandte
Communications
The synthesis of B-Te-6-B is outlined in Scheme 1 and
involves the initial formation of the zirconacycle, B-Zr-6-B,
followed by an efficient Zr/Te transmetalation reaction with
bipy·TeCl2[12] to give B-Te-6-B as a pale yellow-green solid.[6c]
B-Te-6-B is non-emissive when dissolved in organic solvents
such as THF, but in the solid state shows bright green
luminescence (lemis. = 535 nm; Scheme 1). Furthermore, B-Te-
6-B undergoes a dramatic change from a non-emissive state in
THF to highly emissive aggregated states in THF/water
mixtures when the water content exceeded 80 vol%
(Figure 1); this observation is a hallmark of the AIE effect.[3c]
phosphorescence. This observation is noteworthy given that
efficient phosphorescence in neat films at room temperature
is generally very difficult to achieve, as triplet excited states
are highly susceptible to intermolecular triplet–triplet anni-
hilation and quenching by oxygen.[1d,14]
To probe the structural requirements for light emission,
metallacycle transfer chemistry[15] was used to prepare
a tellurophene with a modified cycloalkane group B-Te-5-B
(Scheme 1), and
a tellurophene containing four ring-
appended pinacolboronate esters 4B-Te [Eq. (1)].[9,16,17] Nei-
ther of these heterocycles exhibited luminescence in organic
solvent; however, aggregation-induced emission (AIE) was
noted in both cases with emission maxima (520 and 516 nm
for B-Te-5-B and 4B-Te, respectively; Figure 2) that were
slightly blue-shifted in relation to B-Te-6-B (lemis. = 535 nm).
Figure 2. PL spectra of B-Te-5-B, B-Te-6-B, and 4B-Te in THF/water
(5:95) mixtures; [tellurophenes]=600 mm (lexcit. =365 nm).
The predominant mechanism of AIE is believed to be the
restriction of intramolecular rotations (RIR).[3c] However, we
cannot simply explain the phosphorescence in our telluro-
phene rings entirely by an RIR process given the involvement
of singlet-to-triplet crossing. To better understand the role of
Te in the noted light emission, we studied the photolumines-
cence of a homologous series containing the butadiene
analogues, B-H-4-B, B-H-5-B, and B-H-6-B, along with the
phosphorus-, sulfur-, and selenium-containing heterocycles B-
PPh-6-B, B-S-6-B, and B-Se-6-B (Figure 3).[6c] Each of these
species are non-emissive both in the aggregated/solid state
with the exception of T-H-T (see below). Moreover the
brominated tellurophene B-TeBr2-B was prepared[9,17] by
combining B-Te-6-B with Br2, and this TeIV heterocycle was
also shown to be non-emissive. These results indicate that the
Figure 1. a) Photoluminescence (PL) spectra of B-Te-6-B in different
THF/water ratios; b) emission intensity of B-Te-6-B as the THF/water
ratio is altered. Inset: aggregates under UV light (lexcit. =365 nm); [B-
Te-6-B]=600 mm. See the Supporting Information, Figures S15 and S16
for SEM images of the aggregates.[9]
The emission profile is independent of the aggregation state,
and we were able to make emissive films of B-Te-6-B (1–
10 wt%) in PMMA (Supporting Information, Figure S13).[9]
A weighted mean luminescence lifetime of 166 ms was found
for pure films of B-Te-6-B drop-coated from THF (Support-
ing Information, Figure S17 and Table S12)[9] (quantum yield
F ꢀ 11.5 %), indicating that light emission is phosphores-
cence.[13] Furthermore, the large Stokes shift (170 nm) found
in B-Te-6-B lends added support for the presence of
4588
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 4587 –4591

Angewandte
Chemie
at boron was confirmed by in situ 11B NMR spectroscopy
wherein the signal at 31 ppm owing to B-Te-6-B shifted to
a new resonance at 6 ppm upon adding two equiv of FÀ,
consistent with the presence of four-coordinate fluoroborate
environments (FB-Te-6-BF; Supporting Information, Fig-
ure S25).[9,19] Importantly, the new fluoride-bound product
was non-emissive both in solution and in the solid state
indicating that empty p-orbitals within the BPin units are
likely involved in the light emission of B-Te-6-B. The boron-
bound fluorides in FB-Te-6-BF dissociate in water, enabling
the free pinacolboronate B-Te-6-B to be recovered quantita-
tively upon extraction of the mixture with Et2O.
We did encounter AIE in another chalcogenophene
system T-H-T (Supporting Information, Figure S24);[9,17]
however, some informative differences in the luminescent
properties relative to B-Te-6-B were noted. First, T-H-T
exhibits discernable aggregation-induced blue light emission
(lemis. = 448 nm) when a 30:70 THF/water ratio was reached
(Supporting Information, Figure S24). Secondly, lifetime
measurements show that the emission transpires by fluores-
cence (t = 1.8 ns),[3a] reinforcing the unique role that the
heavy element Te has in promoting single–triplet crossing and
subsequent phosphorescence-based light emission in the
solid/aggregated state.
Figure 3. Structural variants of B-Te-6-B investigated.
TeII center in B-Te-6-B is playing an active role in the solid-
state phosphorescence.
Tellurophenes were also synthesized wherein the pinacol-
boronate esters (BPin) were replaced by thiophene groups (T-
Te-T and T-TeBr2-T; Figure 3).[9,17] Again, no luminescence
was detected, implying a cooperative interaction between the
TeII centers and the adjacent BPin groups[18] in B-Te-5-B, B-
Te-6-B, and 4B-Te is leading to efficient light emission in the
solid state. We also attempted to prepare a tellurophene
wherein the capping BPin groups were replaced by BMes2
The previously reported structure of B-Te-6-B[6c] and
those of the new luminophores B-Te-5-B and 4B-Te[9,17] each
contain at least one coplanar BPin group with respect to the
À
central tellurophene unit. The intermolecular Te Te contacts
in these compounds all lie outside of 5.5 ꢁ (Supporting
Information, Figure S27),[9] and could partially explain the
retention of phosphorescence by the suppression of intermo-
lecular quenching. To probe the phosphorescence in B-Te-6-B
further, a series of TD-DFT computations were carried out at
the B3LYP/6-31G(2d,p) [LANL2DZ for Te] level of theory[21]
(Supporting Information, Figure S31). The calculated UV/Vis
spectrum for B-Te-6-B (Supporting Information, Fig-
ure S30)[9] indicates that the dominant absorption occurring
around 350 nm (lexcit. = 365 nm for phosphorescence) is due to
a HOMO to LUMO electronic transition. The HOMO of B-
Te-6-B has significant participation from a Te lone pair (p-
orbital), while the LUMO is a delocalized p-type orbital with
(Mes = 2,4,6-Me3C6H2); however, attempts to couple the
[9,17]
À ꢁ À
À ꢁ À
diyne Mes2B C C (CH2)4 C C BMes2
(Scheme 1) have failed thus far.
with “Cp2Zr”
Given that the BPin groups contain empty p-orbitals at
boron that could mediate light emission from the excited
state, we explored the luminescence of B-Te-6-B in the
presence of exogenous fluoride donors.[19] Initially the reac-
tion between B-Te-6-B and [nBu4N]F was studied in THF by
UV/Vis spectroscopy (Figure 4), and the resulting spectral
profiles show the conversion of B-Te-6-B into a new product,
with an overall FÀ binding constant of about 3 ꢀ 107 mÀ1
(Supporting Information, Figure S26).[20] Binding of fluoride
À
significant B C p-orbital overlap (Supporting Information,
Figures S32 and S33). On examining the relative computed
energies for the singlet (Sn) and triplet (Tn) states for the S, Se,
and Te analogues (Figure 5), one striking difference was
noted in the tellurophene, B-Te-6-B. In this Te heterocycle,
there is a triplet state (T3) which is nearly degenerate with the
singlet excited state (S1), while in the S and Se analogues, the
T3 state is about 1 eV higher in energy. Thus it appears that B-
Te-6-B has energetically well-matched states to enable
efficient singlet–triplet crossing to occur; once T3 is popu-
lated, then relaxation through the triplet manifold to the T1
state is possible, followed by emission.
Lastly, B-Te-6-B can act as a solid-state on/off sensor for
organic vapor. A sample of B-Te-6-B drop-coated on a TLC
plate shows bright green luminescence when irradiated with
a hand-held UV lamp (lexcit. = 365 nm) at room temperature.
This emission is quenched when the sample is exposed to
organic vapor (for example THF, hexanes, Et2O, and ben-
Figure 4. UV/Vis absorption spectra upon titrating B-Te-6-B (60 mm)
with [nBu4N]F (TBAF) in THF.
Angew. Chem. Int. Ed. 2014, 53, 4587 –4591
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 4589

.
Angewandte
Communications
Keywords: boronic esters · main-group elements ·
.
solid-state phosphorescence · tellurophenes
[1] a) M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson,
S. R. Forrest, Appl. Phys. Lett. 1999, 75, 4 – 6; b) R. C. Evans, P.
Douglas, C. J. Winscom, Coord. Chem. Rev. 2006, 250, 2093 –
2126; c) Y. You, S. Y. Park, Dalton Trans. 2009, 1267 – 1282; d) O.
Bolton, K. Lee, H. J. Kim, K. Y. Lin, J. Kim, Nat. Chem. 2011, 3,
205 – 210; e) J. Xu, A. Takai, Y. Kobayashi, M. Takeuchi, Chem.
Commun. 2013, 49, 8447 – 8449; f) J. Ohshita, S. Matsui, R.
Yamamoto, T. Mizumo, Y. Ooyama, Y. Harima, T. Murafuji, K.
Tao, Y. Kuramochi, T. Kaikoh, H. Higashimura, Organometallics
2010, 29, 3239 – 3241.
[2] J. B. Birks, Photophysics of Aromatic Molecules Wiley, London,
1970.
[3] a) Y. Hong, J. W. Y. Lam, B. Z. Tang, Chem. Commun. 2009,
4332 – 4353; b) J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen,
C. Qiu, H. S. Kwok, X. Zhan, Y. Liu, D. Zhu, B. Z. Tang, Chem.
Commun. 2001, 1740 – 1741; c) Y. Hong, J. W. Y. Lam, B. Z.
Tang, Chem. Soc. Rev. 2011, 40, 5361 – 5388.
Figure 5. Computed vertical excitation energies to both singlet and
triplet states at the TD-B3LYP/6-31G(2d,p) [LANL2DZ for Te] level of
theory in the gas-phase, for B-S-6-B, B-Se-6-B, and B-Te-6-B.
[4] a) B.-K. An, S. H. Gihm, J. W. Chung, C. R. Park, S.-K. Kwon,
S. Y. Park, J. Am. Chem. Soc. 2009, 131, 3950 – 3957; b) N. B.
zene; Figure 6); furthermore, removing B-Te-6-B from the
organic vapor leads to renewal of light emission, demonstrat-
ing that solvent detection is reversible.
˘
Shustova, B. D. McCarthy, M. Dinca, J. Am. Chem. Soc. 2011,
133, 20126 – 20129; c) S. J. Toal, K. A. Jones, D. Magde, W. C.
Trogler, J. Am. Chem. Soc. 2005, 127, 11661 – 11665; d) Y. Xu, L.
Chen, Z. Guo, A. Nagai, D. Jiang, J. Am. Chem. Soc. 2011, 133,
17622 – 17625; e) G. He, H. Peng, T. Liu, M. Yang, Y. Zhang, Y.
Fang, J. Mater. Chem. 2009, 19, 7347 – 7353; f) M. Shimizu, Y.
Takeda, M. Higashi, T. Hiyama, Angew. Chem. 2009, 121, 3707 –
3710; Angew. Chem. Int. Ed. 2009, 48, 3653 – 3656.
[5] A. A. Jahnke, D. S. Seferos, Macromol. Rapid Commun. 2011,
32, 943 – 951.
[6] a) A. A. Jahnke, G. W. Howe, D. S. Seferos, Angew. Chem. 2010,
122, 10338 – 10342; Angew. Chem. Int. Ed. 2010, 49, 10140 –
10144; b) G. L. Gibson, T. M. McCormick, D. S. Seferos, J. Am.
Chem. Soc. 2012, 134, 539 – 547; c) G. He, L. Kang, W.
Torres Delgado, O. Shynkaruk, M. J. Ferguson, R. McDonald,
E. Rivard, J. Am. Chem. Soc. 2013, 135, 5360 – 5363; d) A. A.
Jahnke, B. Djukic, T. M. McCormick, E. Buchaca Domingo, C.
Hellmann, Y. Lee, D. S. Seferos, J. Am. Chem. Soc. 2013, 135,
951 – 954.
Figure 6. On/off luminescence switching of B-Te-6-B drop-coated on
a TLC plate: without vapor (left) and in vapor (CH2Cl2) (right) under
UV (365 nm) illumination at room temperature.
[7] a) Y. Koide, M. Kawaguchi, Y. Urano, K. Hanaoka, T. Komatsu,
M. Abo, T. Terai, T. Nagano, Chem. Commun. 2012, 48, 3091 –
3093; b) T. M. McCormick, A. A. Jahnke, A. J. Lough, D. S.
Seferos, J. Am. Chem. Soc. 2012, 134, 3542 – 3548.
[8] J. Casado, M. Moreno Oliva, M. C. Ruiz Delgado, R. Ponce Or-
tiz, J. Joaquꢂn Quirante, J. T. Lꢃpez Navarrete, K. Takimiya, T.
Otsubo, J. Phys. Chem. A 2006, 110, 7422 – 7430.
In conclusion, we report the unusual solid/aggregated-
state phosphorescence within a series of tellurophenes bear-
ing pinacolboronate (BPin) side groups. Our studies indicate
that both TeII and proximal BPin units are required for the
observed room temperature phosphorescence and investiga-
tions by computational methods are presented. Furthermore,
we show that reversible on/off sensing of organic vapor is
possible and incorporation of these Te materials in polymer
matrices (for example PMMA) leads to retention of the light-
emitting properties. As our molecular emitters form high
quality films directly from solution, future work will involve
integrating these materials into host-free electroluminescent
devices, and structural modification (by known cross-coupling
strategies)[6c] to access a wider color emission palette involv-
ing this new class of molecular main-group solid-state emitter.
[9] See the Supporting Information for details.
[10] a) Q. Zhao, F. Li, C. Huang, Chem. Soc. Rev. 2010, 39, 3007 –
3030; b) M. T. Whited, P. I. Djurovich, S. T. Roberts, A. C.
Durrell, C. W. Schlenker, S. E. Bradforth, M. E. Thompson, J.
Am. Chem. Soc. 2011, 133, 88 – 96.
[11] Phosphorescence has been detected in benzo- and dibenzotel-
lurophenes in frozen EtOH and n-pentane (77 K): M. Zander, G.
Kirsh, Z. Naturforsch. A 1989, 44, 205 – 209.
[12] J. L. Dutton, G. J. Farrar, M. J. Sgro, T. L. Battista, P. J. Ragogna,
Chem. Eur. J. 2009, 15, 10263 – 10271.
[13] The reported BPin-capped tellurophenes are stable indefinitely
to light and acid (HCl) in organic solvent; exposure of these
species to strong base or acid in aqueous media leads to
degradation of the BPin groups. Photoluminescence lifetimes of
1–200 ms concomitant with relatively high emission quantum
yields (> 10%) typically arises where mixing of singlet and
triplet states occurs owing to spin–orbit coupling. We use the
term phosphorescence in this context.
Received: August 21, 2013
Revised: January 31, 2014
Published online: March 25, 2014
4590 www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 4587 –4591

Angewandte
Chemie
[14] W. Z. Yuan, X. Y. Shen, H. Zhao, J. W. Y. Lam, L. Tang, P. Lu, C.
Wang, Y. Liu, Z. Wang, Q. Zheng, J. Z. Sun, Y. Ma, B. Z. Tang, J.
Phys. Chem. C 2010, 114, 6090 – 6099.
[15] For selected reports on metallacycle transfer chemistry, see:
a) P. J. Fagan, W. A. Nugent, J. Am. Chem. Soc. 1988, 110, 2310 –
2312; b) B. L. Lucht, S. S. H. Mao, T. D. Tilley, J. Am. Chem. Soc.
1998, 120, 4354 – 4365; c) J. R. Nitschke, S. Zꢄrcher, T. D. Tilley,
J. Am. Chem. Soc. 2000, 122, 10345 – 10352.
uk/data_request/cif. For full crystallographic details, see the
Supporting Information.
[18] a) N. Matsumi, K. Naka, Y. Chujo, J. Am. Chem. Soc. 1998, 120,
5112 – 5113; b) S. Yamaguchi, S. Akiyama, K. Tamao, J. Am.
Chem. Soc. 2001, 123, 11372 – 11375; c) S. Yamaguchi, T.
Shirasaka, S. Akiyama, K. Tamao, J. Am. Chem. Soc. 2002,
124, 8816 – 8817; d) T. Neumann, Y. Dienes, T. Baumgartner,
Org. Lett. 2006, 8, 495 – 497; e) F. Jꢅkle, Chem. Rev. 2010, 110,
3985 – 4022.
[19] a) M. Nicolas, B. Fabre, G. Marchand, J. Simonet, Eur. J. Org.
Chem. 2000, 1703 – 1710; b) A. Oehlke, A. A. Auer, I. Jahre, B.
Walfort, T. Rꢄffer, P. Zoufalꢆ, H. Lang, S. Spange, J. Org. Chem.
2007, 72, 4328 – 4339.
[20] a) K. A. Connors, Binding Constants: The Measurement of
Molecular Complex Stability Constants, Wiley, New York,
1987; b) Y. Sun, Z. M. Hudson, Y. Rao, S. Wang, Inorg. Chem.
2011, 50, 3373 – 3378.
[21] See the Supporting Information for further computational
details.
[16] For the recent preparation of 4B-Zr, see: K. Altenburger, P.
Arndt, A. Spannenberg, W. Baumann, U. Rosenthal, Eur. J.
Inorg. Chem. 2013, 3200 – 3205.
[17] CCDC 956585 (B-Zr-4-B), 956586 (B-Zr-5-B), 956587 (B-Te-5-
B), 956588 (B-TeBr2-6-B), 956589 (B-PPh-6-B), 956590 (T-H-T),
ꢁ
956591 (T-Te-6-T), 956592 (4B-Zr), 956953 (4B-Te), Mes2B-C
ꢁ
C-(CH2)4-C C-BMes2 (983510), and 984482 (B-Te-6-B room
temp.) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
Angew. Chem. Int. Ed. 2014, 53, 4587 –4591
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
4591