Synthesis and optical properties of stable gallafluorene derivatives: Investigation of their emission via triplet states

Article abstract of DOI:10.1021/ja400287y

We designed and synthesized air-and moisture-stable gallafluorenes in which two benzene rings were bridged by the four-coordinate gallium atoms. The series of gallafluorenes were prepared by introducing electron-donating and -withdrawing groups through Su

Full text of DOI:10.1021/ja400287y

Communication
pubs.acs.org/JACS
Synthesis and Optical Properties of Stable Gallafluorene Derivatives:
Investigation of Their Emission via Triplet States
Takuya Matsumoto, Kazuo Tanaka, and Yoshiki Chujo*
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
S
* Supporting Information
atoms to increase their stabilities toward air and moisture by
ABSTRACT: We designed and synthesized air- and
moisture-stable gallafluorenes in which two benzene
rings were bridged by the four-coordinate gallium atoms.
The series of gallafluorenes were prepared by introducing
electron-donating and -withdrawing groups through
Suzuki−Miyaura coupling reactions. The gallafluorenes
showed unique emissions via their triplet states in the
presence of B(C6F5)3. These emissions were obtained via
the triplet exciplex of gallafluorene and B(C6F5)3.
introducing 2,4-di-tert-butyl-6-[(dimethylamino)methyl]phenyl
(Mamx) as a protecting group.15 The gallium atoms were
stabilized kinetically by the tert-butyl groups and thermody-
namically by the coordination of the nitrogen atom. Moreover,
the high stabilities enabled us to study their unique optical
properties. Especially in the presence of B(C6F5)3, emissions via
the triplet exciplexes16 of the gallafluorenes and B(C6F5)3 were
observed. To the best of our knowledge, this is the first report
to reveal the optical properties of gallafluorenes.
The synthetic schemes for the gallafluorenes are illustrated in
Schemes 1 and 2. The appropriate diiodide-substituted
luorenes have attracted attention as building blocks for
Fconstructing highly efficient optoelectronic materials that
can be used in, for example, electroluminescence displays1 and
photovoltaic cells1a,2 because of their strong emission proper-
ties and high charge carrier ability. The introduction of a variety
of heteroatoms (nitrogen,1a,3 silicon,1a,4 boron,5,6 sulfur,7
germanium,8 etc.) into the bridgehead position leads to various
inherent characteristics. Thus, the exploration of synthetic
methods for fluorenes and their new functions are of great
significance to comprehend their electronic structures, which
may lead to applications of these materials for advanced
devices.
Scheme 1. Synthesis of Gaf-H and Gaf-Br
The optical and electronic properties of heterocyclic
compounds containing group 13 elements are recent topics
with high relevance. In particular, boron-containing hetero-
cycles and conjugated systems have been vigorously studied in
terms of antiaromaticity,5,9 unique optical properties,5,6e,10 and
electron-carrier ability.5,10d−f,11 To compare the electronic
structures and induce other functions, replacement with heavier
elements in the same periodic group often offers interesting
examples.12 Moreover, the heavy-atom-bridged compounds
have great possibilities for applications as phosphorescent or
electron-conducting materials. However, there have been only a
few reports on dibenzoheteroles containing aluminum13 or
gallium14 because their poor stabilities have made it difficult to
evaluate the characteristics of these compounds. Although
Cowley and co-workers synthesized gallafluorene and deter-
mined the structure by single-crystal X-ray diffraction (XRD)
analysis,14 the optical and electrochemical properties of
gallafluorenes were not mentioned. We focused on the use of
gallium as the bridging element in heterofluorenes because of
the relatively higher stability of organogallium compounds than
organoaluminum ones.
Scheme 2. Synthesis of Gaf-a−d
biphenyls were treated with n-BuLi followed by (Mamx)GaCl2.
After reaction for 1 day, the stable gallafluorenes Gaf-H and
Gaf-Br were obtained (Scheme 1). The series of modified
gallafluorenes Gaf-a−d with electron-withdrawing or -donating
groups were prepared in good yields via Suzuki−Miyaura
coupling with Gaf-Br in the presence of the catalyst formed
from tris(dibenzylideneacetone)dipalladium [Pd2(dba)3] and 2-
(2′,4′,6′-triisopropylbiphenyl)dicyclohexylphosphine (X-Phos)
(Scheme 2). All of the gallafluorene derivatives showed high
Herein we designed and synthesized a series of gallafluorenes
with electron-donating and -withdrawing groups and inves-
tigated their optical properties. We modified their gallium
Received: January 10, 2013
Published: March 12, 2013
© 2013 American Chemical Society
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Journal of the American Chemical Society
Communication
durability toward common synthetic procedures such as
extraction with water, column chromatography with acidic
silica gels, and recrystallization under ambient atmosphere. In
addition, unexpected degradation was hardly observed during
the subsequent measurements of optical properties. Thus, it can
be said that the gallafluorenes used in these experiments should
have good resistance to environmental changes.
red-shifted absorptions and emissions from Gaf-a−d relative to
Gaf-H could originate from extension of the π conjugation due
to the introduction of the substituents. The quantum yield of
Gaf-H was much smaller than those of Gaf-a−d. The low
quantum yields could be explained by two factors: First, smaller
oscillator strengths of the transitions in Gaf-H than those in
Gaf-a−d were obtained from theoretical calculations,18
indicating that the synthesized gallium complexes could have
intrinsically low emission properties. Second, phosphorescence
was observed from the PL measurements at 77 K.19 This result
suggests that decay of the singlet excited states via intersystem
crossing could occur, leading to low emissions of the gallium
complexes.
The molecular structure of Gaf-H was determined by single
crystal XRD analysis (Figure 1). A small C1−C6−C7−C12
Figure 3 shows the PL spectrum of 10 mM Gaf-H in
benzene solution after the addition of an equivalent amount of
Figure 1. ORTEP drawing of Gaf-H (50% probability for thermal
ellipsoids). H atoms have been omitted for clarity.
dihedral angle of 1.06° was obtained, indicating that the
fluorene moiety forms a nearly planar structure. In addition, the
sum of the C−Ga−C angles was 355.4°, showing that the
gallium center is nearly planar rather than tetrahedral. The
average Ga1−N1 bond length of 2.112 Å is significantly shorter
than the sum of van der Waals’ radii of gallium and nitrogen
(3.42 Å). These data indicate that the gallium atom in Gaf-H
has a four-coordinate structure. We evaluated the stability of
the dative bond between the nitrogen and gallium atoms by
variable-temperature NMR analysis. The spectra were subtly
changed over the temperature range from 24 to 90 °C.17 This
result suggests that the nitrogen atom is strongly coordinated to
the gallium atom.
The optical properties of the gallafluorene derivatives Gaf-H
and Gaf-a−d were investigated by UV−vis absorption and
photoluminescence (PL) spectroscopy in CHCl3 solution
(Figure 2). Gaf-H showed a weak absorption peak at 282 nm
assigned to the S0 → Sn (n = 1, 2) transitions.18 The absorption
peaks of Gaf-a−d were shifted by ca. 50 nm. In the PL spectra,
bathochromic shifts of the peak maxima in the spectra of Gaf-
a−d relative to that of Gaf-H (λFL = 369 nm) were found. The
Figure 3. (a) PL spectra of 10 mM Gaf-H + B (orange), Gaf-H alone
(black), and B alone (blue) in benzene solution excited at 282 nm. (b)
PL spectrum obtained with a cutoff filter below 400 nm.
Table 1. PL Spectral Data for Gallafluorene Derivatives and
B in Benzene Solution (10 mM Gaf-H, λex = 282 nm; 1 mM
Gaf-a−d, λex = 360 nm)
λPL (nm)
τ1/2 (μs)
χ2
Gaf-H
575
0.152 (4.3%)
1.33 (95.7%)
1.17
Gaf-a
Gaf-b
Gaf-c
Gaf-d
635
654
684
581
12.5
0.99
0.95
1.01
0.91
5.38
5.34
0.047 (0.9%)
0.221 (99.1%)
B(C6F5)3 (B). The optical properties are listed in Table 1. A
new emission band with a peak at 575 nm appeared upon
addition of B. The lifetime of this new emission was 1.33 μs. In
contrast, the lifetimes of Gaf-H and B are <5 ns. The 1H NMR
spectra of Gaf-H were less significantly changed upon addition
of B. In a 0.5 mM solution in benzene, the absorption spectrum
of the mixture of Gaf-H and B was similar to the sum of the
absorption spectra of the two components.20 These data show
that a weak interaction between Gaf-H and B occurs. In other
words, the interaction that induced the new emission band
should take place mainly in the excited state. After oxygen was
bubbled through the mixture of Gaf-H and B, the emission
band at 575 nm disappeared. These results suggest that this
emission band originates via their triplet excited states.
Figure 2. UV−vis absorption spectra (solid lines) and PL spectra
(dashed lines, excited at λmabasx) of gallafluorene derivatives in CHCl3
solution (Gaf-H, 1.0 × 10−4 M; Gaf-a−d, 1.0 × 10−5 M).
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Communication
(d) Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.; Neagu-
Plesu, R.; Belletete, M.; Durocher, G.; Tao, Y.; Leclerc, M. J. Am.
Chem. Soc. 2008, 130, 732. (e) Blouin, N.; Michaud, A.; Leclerc, M.
Adv. Mater. 2007, 19, 2295.
(4) (a) Chen, J.; Cao, Y. Macromol. Rapid Commun. 2007, 28, 1714.
(b) Lu, G.; Usta, H.; Risko, C.; Wang, L.; Facchetti, A.; Ratner, M. A.;
Marks, T. J. J. Am. Chem. Soc. 2008, 130, 7670. (c) Usta, H.; Lu, G.;
Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2006, 128, 9034. (d) Chan,
K. L.; McKiernan, M. J.; Towns, C. R.; Holmes, A. B. J. Am. Chem. Soc.
2005, 127, 7662.
(5) (a) Yamaguchi, S.; Wakamiya, A. Pure Appl. Chem. 2006, 78,
1413. (b) Yamaguchi, S.; Tamao, K. Chem. Lett. 2005, 34, 2.
(6) (a) Tokoro, Y.; Nagai, A.; Tanaka, K.; Chujo, Y. Macromol. Rapid
Commun. 2012, 33, 550. (b) Ribero, C.; Brogueira, P.; Lvareda, G.;
Carvalho, C. N.; Amaral, A.; Santos, L.; Morgado, J.; Scherf, U.;
Moreover, at lower concentrations (<0.2 mM), this long-
lifetime emission was not obtained. In addition, the emission
was undetectable in relatively polar solvents such as CHCl3 and
CH2Cl2. These data suggest that this emission band could be
caused by formation of the triplet exciplex16 between Gaf-H
and B.
The mixtures of the gallafluorene derivatives Gaf-a−d and B
showed weak emission bands at around 600 nm (Table 1). As
the gallafluorene derivative was substituted with more electron-
donating groups, the emission band showed more bath-
ochromic shifts by 100 nm. The energy band gaps were
narrowed by the introduction of electron-donating groups.
These results also support the triplet exciplex formation with B.
In summary, we have reported the synthesis and optical
properties of a series of gallafluorenes with various functional
groups. The formation of four-coordinated gallium atoms
stabilized the gallafluorenes toward air and moisture. In
addition, the gallafluorene derivatives showed unique emissions
via their triplet states, particularly in the presence of B(C6F5)3.
We are continuing to survey their optical and electronic
characteristics. These gallafluorenes have promise to be key
materials for fabricating highly functional optoelectronic
devices.
Bonifac
V. D. B.; Morgado, J.; Scherf, U. J. Polym. Sci., Part A: Polym. Chem.
2008, 46, 2878. (d) Briere, J.-F.; Cote, M. J. Phys. Chem. B 2004, 108,
́ ́
io, V. D. B. Biosens. Bioelectron. 2010, 26, 1662. (c) Bonifacio,
̀
̂
́
3123. (e) Yamaguchi, S.; Shirasaka, T.; Akiyama, S.; Tamao, K. J. Am.
Chem. Soc. 2002, 124, 8816.
(7) (a) Chen, J.; Angelici, R. J. Coord. Chem. Rev. 2000, 206, 63.
(b) Li, X.-F.; Chi, Z.-G.; Xu, B.-J.; Li, H.-Y.; Zhang, X.-Q.; Zhou, W.;
Zhang, Y.; Liu, S.-W.; Xu, J.-R. J. Fluoresc. 2011, 21, 1969. (c) Liu, S.;
He, P.; Wang, H.; Shi, J.; Gong, M. J. Lumin. 2010, 130, 855.
(d) Zhang, X.; Sun, Y.; Yu, X.; Zhang, B.; Huang, B.; Jiang, M. Synth.
Met. 2009, 159, 2491.
(8) (a) Alem, S.; Tse, S. C.; Tao, Y.; Leclerc, M. Macromolecules
2010, 43, 2328. (b) Chen, R.; Zhu, R.; Zheng, C.; Liu, S.; Fan, Q.;
Huang, W. Sci. China, Ser. B: Chem. 2009, 52, 212. (c) West, R. Pure
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and spectral data for all new
compounds, crystallographic data for Gaf-H (CIF), and results
of theoretical calculations. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
Appl. Chem. 2008, 80, 563. (d) Liu, Y.; Ballweg, D.; Muller, T.; Guzei,
̈
I. A.; Clark, R. W.; West, R. J. Am. Chem. Soc. 2002, 124, 12174.
(9) (a) Braunschweig, H.; Kupfer, T. Chem. Commun. 2011, 47,
10903. (b) Iida, A.; Yamaguchi, S. J. Am. Chem. Soc. 2011, 133, 6952.
(c) Fan, C.; Piers, W. E.; Parvez, M. Angew. Chem., Int. Ed. 2009, 48,
2955. (d) So, C.-W.; Watanabe, D.; Wakamiya, A.; Yamaguchi, S.
AUTHOR INFORMATION
Corresponding Author
chujo@chujo.synchem.kyoto-u.ac.jp
Notes
■
́
Organometallics 2008, 27, 3496. (e) Braunschweig, H.; Fernandez, I.;
Frenking, G.; Kupfer, T. Angew. Chem., Int. Ed. 2008, 47, 1951.
(f) Schleyer, P. v. R.; Freeman, P. K.; Jiao, H.; Goldfuß, B. Angew.
Chem., Int. Ed. Engl. 1995, 34, 337. (g) Eisch, J. J.; Galle, J. E.; Kozima,
S. J. Am. Chem. Soc. 1986, 108, 379.
The authors declare no competing financial interest.
(10) (a) Schwedler, S.; Eickhoff, D.; Brockhinke, R.; Cherian, D.;
Weber, L.; Brockhinke, A. Phys. Chem. Chem. Phys. 2011, 13, 9301.
(b) Muhammad, S.; Janjua, M. R. S. A.; Su, Z. J. Phys. Chem. C 2009,
113, 12551. (c) Agou, T.; Kobayashi, J.; Kawashima, T. Inorg. Chem.
2006, 45, 9137. (d) Braunschweig, H.; Damme, A.; Jimenez-Halla, J.
ACKNOWLEDGMENTS
■
This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas “New Polymeric Materials Based
on Element-Blocks (No. 2401)” (24102013) from the Ministry
of Education, Culture, Sports, Science, and Technology, Japan.
O. C.; Horl, C.; Krummenacher, I.; Kupfer, T.; Mailander, L.; Radacki,
̈
̈
K. J. Am. Chem. Soc. 2012, 134, 20169. (e) Araki, T.; Fukazawa, A.;
Yamaguchi, S. Angew. Chem., Int. Ed. 2012, 51, 5484. (f) Agou, T.;
Kobayashi, J.; Kawashima, T. Chem.Eur. J. 2007, 13, 8051.
(11) (a) Braunschweig, H.; Breher, F.; Chiu, C.-W.; Gamon, D.;
Nied, D.; Radacki, K. Angew. Chem., Int. Ed. 2010, 49, 8975.
(b) Wakamiya, A.; Mishima, K.; Ekawa, K.; Yamaguchi, S. Chem.
Commun. 2008, 579.
REFERENCES
■
(1) (a) Beaupre,
́
S.; Boudreault, P.-L. T.; Leclerc, M. Adv. Mater.
2010, 22, E6. (b) Scherf, U.; List, E. J. W. Adv. Mater. 2002, 14, 477.
(c) Neher, D. Macromol. Rapid Commun. 2001, 22, 1365. (d) Gross,
M.; Muller, D. C.; Nothofer, H.-G.; Scherf, U.; Neher, D.; Brauchle,
C.; Meerholz, K. Nature 2000, 405, 661. (e) Yu, W. L.; Pei, J.; Cao, Y.;
Huang, W.; Heeger, A. J. Chem. Commun. 1999, 1837.
(12) (a) 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.
(b) Mizuhata, Y.; Sasamori, T.; Nagahora, N.; Watanabe, Y.;
Furukawa, Y.; Tokitoh, N. Dalton Trans. 2008, 4409. (c) Kurita, J.;
Ishii, M.; Yasuike, S.; Tsuchiya, T. Chem. Pharm. Bull. 1994, 42, 1437.
(d) Kurita, J.; Ishii, M.; Yasuike, S.; Tsuchiya, T. J. Chem. Soc., Chem.
Commun. 1993, 1309−1310.
(13) (a) Eisch, J. J.; Kaska, W. C. J. Am. Chem. Soc. 1966, 88, 2976.
(b) Eisch, J. J.; Kaska, W. C. J. Am. Chem. Soc. 1962, 84, 1501.
(14) Decken, A.; Gabbaï, F. P.; Cowley, A. H. Inorg. Chem. 1995, 34,
3853.
(2) (a) Li, C.; Liu, M.; Pschirer, N. G.; Baumgarten, M.; Mullen, K.
Chem. Rev. 2010, 110, 6817. (b) Zhang, F.; Mammo, W.; Andersson,
L. M.; Admassie, S.; Andersson, M. R.; Inganas, O. Adv. Mater. 2006,
̈
̈
18, 2169. (c) Padhy, H.; Huang, J. H.; Sahu, D.; Patra, D.; Kekuda, D.;
Chu, C. W.; Lin, H. C. J. Polym. Sci., Part A: Polym. Chem. 2010, 48,
4823. (d) Cho, N. S.; Park, J. H.; Lee, S. K.; Lee, J.; Shim, H. K.; Park,
M. J.; Hwang, D. H.; Jung, B. J. Macromolecules 2006, 39, 177.
(e) Ashraf, R. S.; Hoppe, H.; Shahid, M.; Gobsch, G.; Sensfuss, S.;
Klemm, E. J. Polym. Sci., Part A: Polym. Chem 2006, 44, 6952.
(3) (a) Morin, J.-F.; Leclerc, M.; Ades, D.; Siove, A. Macromol. Rapid
̀
Commun. 2005, 26, 761. (b) Zou, Y. P.; Gendron, D.; Badrou-Aïch, R.;
Najari, A.; Ye, T.; Leclerc, M. Macromolecules 2009, 42, 2891. (c) Park,
(15) (a) Yoshifuji, M.; Kamijo, K.; Toyota, K. Tetrahedron Lett. 1994,
35, 3971. (b) Bagh, B.; Schatte, G.; Green, J. C.; Muller, J. J. Am. Chem.
̈
S. H.; Roy, A.; Beaupre,
D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photonics 2009, 3, 297.
́
S.; Cho, S.; Coates, N.; Moon, J. S.; Moses,
Soc. 2012, 134, 7924. (c) Bagh, B.; Gilroy, J. B.; Staubitz, A.; Muller, J.
J. Am. Chem. Soc. 2010, 132, 1794.
̈
4213
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Journal of the American Chemical Society
Communication
(16) (a) Frederichs, B.; Straerk, H. Chem. Phys. Lett. 2008, 460, 116.
(b) Chen, S.-C.; Fang, T.-S. Chem. Phys. Lett. 2007, 450, 65.
(c) Cocchi, M.; Virgili, D.; Sabatini, C.; Kalinowski, J. Chem. Phys. Lett.
2006, 421, 351. (d) Kolosov, D.; Adamovich, V.; Djurovich, P.;
Thompson, M. E.; Adachi, C. J. Am. Chem. Soc. 2002, 124, 9945.
(e) Jana, P.; Mallik, G. K.; Ganguly, T.; Banerjee, S. B. Chem. Phys.
Lett. 1990, 172, 158.
(17) See Figure S14 in the Supporting Information.
(18) See Table S2 in the Supporting Information.
(19) See Figure S20 in the Supporting Information.
(20) See Figure S22 in the Supporting Information.
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