Improving quantum efficiencies of siloles and silole-derived butadiene chromophores through structural tuning

Article abstract of DOI:10.1002/anie.200461844

A brighter future for siloles. Luminescent silole chromophores with the highest quantum efficiencies reported for a fully substituted monomeric silole are generated by altering the steric properties of the substituent groups. Treatment of 1,1-dimethylsilo

Full text of DOI:10.1002/anie.200461844

Communications
Luminescent Materials
manipulation of the C2, C5, and Si substituents. Increased
chromophore rigidity often leads to improved luminescence
by dually facilitating electron delocalization and minimizing
vibrational–rotational events in the excited state.[8] With
consideration of the silole nucleus, it seemed likely that
increasing steric bulk about silicon and the 2,5-substituents
would increase the energy barriers for non-emissive decay
processes and ultimately result in increased photolumines-
cence. Thus, we explored synthetically practical silole mod-
ifications intended to impart “rigidity” to analogues of the
parent chromophore 1a. Herein, we report the structural
optimization of a series of siloles ultimately resulting in the
first highly luminescent 3,4-diphenylsilole chromophores.
The dimethylsilolene chromophores (4a and 5a) were
prepared by Negishi cross-coupling reactions with dibromide
3[6] (Scheme 1). These new siloles were crystalline solids that
were purified by recrystallization from the crude reaction
mixtures.
Improving Quantum Efficiencies of Siloles and
Silole-Derived Butadiene Chromophores through
Structural Tuning**
Andrew J. Boydston and Brian L. Pagenkopf*
Over the last decade, Tamaoꢀs endo-endo cycloreduction of
bis(phenylethynyl)silanes has emerged as a powerful tool for
generating 2,5-dilithiosiloles that can be trapped by various
electrophiles in situ.[1] The strategy has been used for the
construction of various silole-containing monomeric, oligo-
meric, and polymeric systems,[2] but a significant structural
limitation for cycloreduction is the requirement for arene
rings at the alkyne termini. Unfortunately, 3,4-diarylsiloles
are virtually non-emissive in solution, exhibiting low quantum
efficiencies from 0.1–11%.[3,4] Their poor lumines-
cence has been ascribed directly to effects arising from
3,4-substitution, given that the corresponding unsub-
stituted systems consistently show greater emission
intensities.[5] The lackluster luminescence of 3,4-diary-
lsilole chromophores condemns them to a dark future,
which is unfortunate given the ease with which they
can be prepared by cycloreduction methods.
We recently observed from silole 1a a curiously
high quantum efficiency of 9%,[6] and in our studies of
conjugated silole oligomers[7] we observed a 20%
quantum efficiency from the silole-capped extended
Scheme 1. Synthesis of 1,1-dimethylsilole fluorophores.
chromophore 2. The emission intensities of 1a and 2
suggested that silole luminescence attenuation classically
ascribed to the 3,4-diphenyl rings can be circumvented by
For the synthesis of di-tert-butylsilolene systems
(Scheme 2) we found that isolation of dibromide intermediate
7 was not necessary and good yields were obtained by
carrying out the entire reaction sequence from silane 6
through to siloles 1b, 4b, or 5b. As with 4a and 5a, this
simple process provided the desired siloles as crystalline
solids that could be purified by recrystallization from the
crude product mixtures.
Chlorosilole 10 was prepared via known intermediate
9[6] in 72% overall yield from silane 8 (Scheme 3). Cross-
coupling with phenylacetylene provided asymmetric
“mixed” silole 11a in 86% isolated yield after recrystal-
lization. Similarly, bromosilole 12 was used en route to
“mixed” silole 11b.
We found that treatment of the 1,1-dimethylsiloles with
Bu4NF in THF accomplished stereospecific[9] desilylation in
nearly quantitative yield after only a few minutes at room
temperature (Scheme 4). As can be seen from X-ray crystal
structures (Figure 1), the backbone of the chromophore
achieves a nearly completely planar orientation upon removal
of the silolene moiety and, as expected, the butadiene moiety
adopts an s-trans conformation. The efficiency and simplicity
of this desilylation method provided 1c, 4c, and 5c essentially
gratis.[10]
All of the new silole chromophores absorb in the visible
region (Table 1) and have moderate molar absorptivities. In
comparison with 1a, siloles bearing larger arenes (11a, 4a,
and 5a) display sequential bathochromic shifts in absorption
maxima consistent with having greater electron density.
[*] A. J. Boydston, Prof. B. L. Pagenkopf
Department of Chemistry and Biochemistry
The University of Texas at Austin
1 University Station A5300, Austin, TX 78712-0165 (USA)
Fax: (+1)512-471-8696
E-mail: pagenkopf@mail.utexas.edu
[**] We are grateful to the Robert A. Welch Foundation and donors of the
American Chemical Society Petroleum Research fund for partial
financial support of this research. A.J.B. thanks the Robert A. Welch
Foundation for a Welch Summer Research Fellowship. We are
indebted to Dr. Vincent Lynch for determination of crystal
structures. We thank FMC Lithium, a division of FMC Corporation,
for a generous donation of tBu2Si(OTf)2.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
6336
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200461844
Angew. Chem. Int. Ed. 2004, 43, 6336 –6338

Angewandte
Chemie
4c Dlmax = ꢀ64 nm, 5a!5c Dlmax
=
ꢀ66 nm, Table 1) as a result of the loss of
butadiene tethering and electronic inter-
actions with the silicon atom. Increasing
the size of the alkyl groups on the terminal
arenes (i.e. 4c and 5c vs. 1c) also causes
longer wavelength absorption maxima and
decreased molar absorptivity for the buta-
dienes. Notably, blue emission was
obtained from both 4c and 5c, which
emit with 20% and 25% efficiency, respec-
tively.
Scheme 2. Synthesis of 1,1-di-tert-butylsilole fluorophores; NBS=1-bromo-2,5-pyrrolidine-
dione.
These structural modifications resulted
in significant stepwise increases in quan-
tum efficiencies (Ff) with minimal impact
on emission wavelengths (Figure 2,
Table 1). Compared with the parent silole
1a (Ff = 9%), emission intensities
improved when larger terminal arenes
were employed. Replacing
a
single
phenyl group on 1a (Ff = 9%) with a
mesityl (11a, Ff = 10) had a marginal
effect, but exchanging both phenyl groups
for mesityl groups results in a marked
increase in luminescence (4a, Ff = 30%).
Replacing the mesityl groups with triiso-
propylphenyl groups gave only slight
enhancement (5a, Ff = 41%). Substituting
the dimethylsilolene for a di-tert-butylsilo-
lene has a greater influence on Ff than
changes to the terminal arene units. For the
parent silole 1a (Ff = 9%), the exchange
brought Ff to 25% (1b). Incorporation of
Scheme 3. Synthesis of asymmetric silole fluorophores; NCP=N-chlorophthalimide.
a single mesityl group again lead to only a modest
improvement (11b, Ff = 32%), but when two mesityl
groups were used in combination with the di-tert-
butylsilolene (4b) a quantum efficiency of 63% was
observed. No further increase was observed, however,
with silole 5b (Ff = 56%). To our knowledge, these
quantum efficiencies are the highest ever reported for
3,4-diphenylsilole chromophores in solution.
Scheme 4. Protiodesilylation for synthesis of butadiene chromophores.
Table 1: Electronic absorption and emission data in solution.
Silole
Butadiene
Absorption
lmax log(e)
Emission
lmax
Ff[b]
1a
1b
4a
4b
5a
5b
11a
11b
–
–
–
–
–
–
–
–
–
429
428
443
439
447
439
436
434
363
379
381
4.74
4.36
4.49
4.48
4.43
4.51
4.42
4.43
5.03
4.99
4.85
520
516
532
0.09
0.25
0.30
0.63
0.41
0.56
0.10
0.32
0.02[c]
0.20[c]
0.25[c]
Figure 1. X-ray structures of siloles 1c (left), 4c (middle), and
5c (right).
529 (536)[a]
523
507 (512)[a]
532
522
428
448
426
Increasing steric bulk about silicon alone causes a small
hypsochromic shift in the absorption maxima (1a vs. 1b, 4a vs.
4b, 5a vs. 5b, 11a vs. 11b). Relative to the least hindered
silole 1a, a hypochromic shift (decrease in absorption
intensity) is observed for each of the bulkier systems
suggesting a decrease in planarity. In all cases, desilylation
causes nearly equivalent hypsochromic shifts (that is, blue
shifts) in absorption maxima (1a!1c Dlmax = ꢀ66 nm; 4a!
1c
4c
5c
–
–
[a] Emission maxima of thin films given in parentheses. [b] Determined
with reference to fluorescein unless stated otherwise. [c] Determined
with reference to 9,10-diphenylanthracene.
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Communications
H. Chen, Q. Chengfeng, H. S. Kwok, X. Zhan, Y. Lui, D. Zhu,
B. Z. Tang, Chem. Commun. 2001, 1740 – 1741; d) J. Chen, Z.
Xie, J. W. Y. Lam, C. C. W. Law, B. Z. Tang, Macromolecules
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[4] To our knowledge, at its current state of development the
method is limited to phenyl groups.
[5] a) S. Yamaguchi, K. Tamao, J. Chem. Soc. Dalton Trans. 1998,
3693 – 3702; b) M. Katkevics, S. Yamaguchi, A. Toshimitsu, K.
Tamao, Organometallics 1998, 17, 5796 – 5800.
[6] A. J. Boydston, Y. Yin, B. L. Pagenkopf, J. Am. Chem. Soc. 2004,
126, 3724 – 3725.
[7] A. J. Boydston, Y. Yin, B. L. Pagenkopf, J. Am. Chem. Soc. 2004,
126, 10350 – 10354.
[8] X.-C. Li and S. C. Moratti in Photonic Polymeric System:
Fundamentals, Methods and Applications, (Eds.: D. L. Wise,
G. E. Wnek, D. J. Trantolo, T. M. Cooper, J. D. Gresser), Marcel
Dekker, New York, 1998, chap. 10, pp. 335 – 371.
Figure 2. Photoluminescence spectra in CH2Cl2 at room temperature.
4b (c), 5b (c), 5a (c), 11b (c), 4a (c), 1b (c),
11a (c), 1a (c), 5c (c), 4c (c), 1c (c). Inset: normal-
ized emission spectra of thin films of 4b (dashed line) and 5b (solid
line).
[9] Determined by 1H NMR spectroscopy and X-ray analysis.
[10] Similar silole desilylations have been reported using 50 equiv of
KOH in refluxing PhCH3/H2O for approximately 20 h, see
ref. [3a].
[11] a) H. Sohn, M. J. Sailor, D. Magde, W. C. Trogler, J. Am. Chem.
Soc. 2003, 125, 3821 – 3830; b) H. Sohn, R. M. Calhoun, M. J.
Sailor, W. C. Trogler, Angew. Chem. 2001, 113, 2162 – 2163;
Angew. Chem. Int. Ed. 2001, 40, 2104 – 2105.
[12] Thin films were prepared onto glass substrates by spin-coating
solutions of the recrystallized siloles.
Although the solution photoluminescence (PL) efficiency
is a good first measure of performance and is paramount in
areas such as nitroaromatic detection,[11] the solid-state
behavior will more likely dictate the feasibility of deploying
these compounds in electronic devices. In this regard, thin-
films of 4b and 5b were prepared by spin-coating,[12] and their
PL spectra examined (Figure 2, inset). Intense green light
emission was observed from the thin film of each fluorophore
and the emission spectra revealed slight bathochromic shifts
in the solid state relative to the solution PL spectra.
In conclusion, simple structural modifications have pro-
vided new luminescent silole chromophores with the highest
quantum efficiencies for fully substituted monomeric siloles
in solution. Importantly, these discoveries welcome siloles
prepared by practical cycloreduction methods into the fold of
useful fluorophores. The PL intensity was selectively modified
without impact on the emission wavelength. This work
establishes structure–property relationships that refute the
notion that 3,4-disubstituted siloles possess intrinsically low
quantum efficiencies, and illustrates enhanced performance
through substituent tuning at C2, C5, and Si.
Received: August 31, 2004
Keywords: alkynes · fluorescence spectroscopy · luminescence ·
.
silicon · siloles
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11715 – 11722.
[2] a) S. Yamaguchi, K. Tamao, J. Organomet. Chem. 2002, 653,
223 – 228; b) M. Hissler, P. W. Dyer, R. RØau, Coord. Chem. Rev.
2003, 244, 1 – 44.
[3] a) S. Yamaguchi, T. Endo, M. Uchida, T. Izumizawa, K.
Furukawa, K. Tamao, Chem. Eur. J. 2000, 6, 1683 – 1692; b) J.
Lee, Q.-D. Liu, M. Motala, J. Dane, J. Gao, Y. Kang, S. Wang,
Chem. Mater. 2004, 16, 1869 – 1877. Increased quantum yields
(21%) have been obtained from 3,4-diarylsiloles by aggregation-
induced emission, see: c) J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng,
6338
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2004, 43, 6336 –6338