Siloxy-substituted cyclopentadiene showing aggregation-enhanced emission: An application of cycloaddition of isolable dialkylsilylene

Article abstract of DOI:10.1021/om3005178

Cycloaddition of an isolable dialkylsilylene converted nonemissive 2,3,4,5-tetraphenylcyclopentadienone to an emissive siloxycyclopentadiene, which shows aggregation-enhanced emission behavior with a light blue fluorescence (λ_em = 474 nm, φ_F = 0.11) in the solid state rather than in solution.

Full text of DOI:10.1021/om3005178

Communication
pubs.acs.org/Organometallics
Siloxy-Substituted Cyclopentadiene Showing Aggregation-Enhanced
Emission: An Application of Cycloaddition of Isolable Dialkylsilylene
Shintaro Ishida,*,† Kenya Uchida,† Tsunenobu Onodera,‡ Hidetoshi Oikawa,‡ Mitsuo Kira,†
and Takeaki Iwamoto*,†
†Department of Chemistry, Graduate School of Science, Tohoku University, Aramakiazaaoba 6-3, Aoba-ku, Sendai 980-8578, Japan
‡Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan
S
* Supporting Information
ABSTRACT: Cycloaddition of an isolable dialkylsilylene
converted nonemissive 2,3,4,5-tetraphenylcyclopentadienone
to an emissive siloxycyclopentadiene, which shows aggrega-
tion-enhanced emission behavior with a light blue fluorescence
(λem = 474 nm, ΦF = 0.11) in the solid state rather than in
solution.
lthough molecular luminescence is generally quenched in
the condensed phase, some polyarylated compounds such
Scheme 1. Plausible Reaction Mechanism
A
as cyclopentadiene, silacyclopentadienes (siloles), fulvenes, and
butadienes show unusual strong emission in the solid state
rather than in solution. This unique emission behavior has been
rationalized by suppression of the undesired nonradiative
quenching process involving rotation of peripheral aryl rings
and π−π stacking of the central aromatic core in the solid state
and is called “aggregation-enhanced emission (AEE)” or
“aggregation-induced emission (AIE)”.1,2 In recent years,
AEE-active molecules have received much attention because
of their advantage for emissive functional materials.
The AEE-active molecules are often synthesized by transition
metal catalyzed reactions. However, because the residual
transition metals occasionally cause trouble in their usage as
materials,3 non-transition metal routes for the synthesis are
increasingly demanded. In this aspect, cycloaddition of silylenes
(silicon version of carbenes) may have an advantage for the
synthesis of the AEE active molecules, because dialkylsilylene 1
compound toward aromatic ketones and other substrates
without transition metal catalysts. In addition, the bulky
dialkylsilyl group may serve as a good isolator between
aromatic cores of the silylene-incorporated AEE molecules.5−7
Recently, dihydrocyclopenta[a]indene 2, which shows AEE
behavior, has been synthesized by palladium-catalyzed trime-
rization of diphenylacetylene.2g The skeletal structure of 2 is
quite similar to 3, synthesized by Belzner and co-workers using
the reaction of a base-stabilized transient diarylsilylene [Ar2Si:,
Ar = 2-(dimethylaminomethyl)phenyl] with 2,3,4,5-tetraphe-
nylcyclopentadienone (tetracyclone), while its luminescence
behavior has not been reported.6c These studies prompted us to
investigate the reaction of dialkylsilylene 1 with tetracyclone.8
The reaction gives the desired cycloadduct 4 in a high yield,
while siloxyfulvene 5 (a 2:1 adduct of 1 with tetracyclone) is
obtained as a sole byproduct depending on the conditions.
Compound 4 shows remarkable AEE in the solid state.
(RH Si:, Chart 1)4 is now available as an isolable but reactive
2
Chart 1
Received: June 11, 2012
Published: August 9, 2012
© 2012 American Chemical Society
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Communication
Figure 1. Molecular structure of 4. Thermal ellipsoids are drawn at the
30% probability level. Selected bond lengths (Å): Si1−O1 =
1.6976(17), Si1−C7 = 1.878(3), O1−C1 = 1.359(3), C1−C2 =
1.473(3), C1−C5 = 1.352(4), C2−C3 = 1.355(4), C3−C4 =
1.520(4), C4−C5 = 1.516(3), C5−C6 = 1.454(3), C6−C7 =
1.419(4), C6−C11 = 1.399(4), C7−C8 = 1.400(3), C8−C9 =
1.390(4), C9−C10 = 1.382(4), C10−C11 = 1.387(4).
Figure 3. Emission spectra of 4 in THF−water mixed solvents
(colored solid lines) and normalized emission spectrum in the solid
state (dotted black line). These spectra were obtained by excitation at
each λmax in solution and 350 nm in the solid state.
similarly to the reaction of Belzner’s silylene with tetracyclo-
ne,5c while the [1+4] cycloaddition of 7, having a reactive
cyclohexadiene moiety, with another silylene 1 would give 5.11
The molecular structure of 4 was unequivocally determined
by X-ray structural analysis, as shown in Figure 1. A
cyclopentadiene moiety with a saturated C4 atom exists in 4
with C1−C5 and C2−C3 lengths of 1.352(4) and 1.355(4) Å
and C1−C2, C3−C4, and C4−C5 lengths of 1.473(3),
1.520(4), and 1.516(3) Å, respectively. The cyclopentadiene
ring and the silylated benzene ring are almost coplanar, with an
interplane angle of 0.84°, indicating effective conjugation
between the cyclopentadiene and the phenyl ring. As shown
in Figure 2, 4 forms a lamellar structure consisting of a bulky
2,2,5,5-tetrakis(trimethylsilyl)-1-silacyclopentane moiety
(RH Si:) and π-systems (given in red) in the solid state. Each
2
phenylcyclopentadiene core of 4 is well segregated by the
peripheral phenyl rings. Although the shortest intermolecular
C···C distance was 3.298 Å in the crystal (Figure S7, in the
Supporting Information), no significant π−π stacking exists.
The RH SiO unit works as an effective isolator of the aromatic
2
core.
The UV−vis absorption spectrum of 4 in THF (Figure S1,
Supporting Information) showed an absorption band at 374
nm (ε = 1.56 × 104) assignable to the π → π* (HOMO →
LUMO) transition.12 With increasing the water content,
absorption tailing due to light scattering indicating the
aggregation of 4 was observed, but the transition band was
not significantly shifted. As expected, compound 4 shows
typical aggregation-enhanced emission behavior, as shown in
Figure 3. The very weak fluorescence of 4 in THF solution is
observed at 474 nm with a quantum yield (ΦF) of 0.25% when
the π → π* (HOMO → LUMO) transition band of 4 at 374
nm is excited. Emission intensity of 4 in a THF−water (1:9)
mixed solvent, however, is enhanced by 21 times compared to
that in THF solution (Figure S2, in the Supporting
Information). A fine powder of 4 on a glass plate displays
much more intense light blue fluorescence at 476 nm with a ΦF
of 11%, which is 40 times enhanced compared to that in the
THF solution. Notably, the emission band shapes of 4 in
various solvents and in the solid state are similar to each other,
which is consistent with the well π-segregated packing structure
Figure 2. Packing diagram of 4 along the b-axis. Tetraphenylcyclo-
pentadiene units are shown in red.
When 1 was added to a toluene solution of one equivalent of
tetracyclone, the color of the solution turned from purple of
tatracyclone to dark red. After usual workup, fluorescent, pale
yellow solid of 4 was isolated in 81% yield with a trace amount
of unprecedented 1:2 adduct 5, having siloxypentafulvene and
7-silanorbornene moieties, as a red-brown solid. The yield of 5
increased to 46% by the reaction of two equivalents of 1 with
tetracyclone in hexane.9
A plausible formation mechanism of 4 and 5 is displayed in
Scheme 1. Formation of 4 and 5 suggests the existence of π-
extended fulvene 7 as a relatively long-lived intermediate. Thus,
ring closure of carbonyl silaylide 6 formed by complexation of 1
with tetracyclone would afford π-extended fulvene 7.10 Thermal
rearomatization of 7 via hydrogen shift would provide 4,
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Communication
of 4, as shown in Figure 2.13 The emission behavior of 4 can be
explained by the reported AEE mechanism; nonradiative decay
owing to rotation of phenyl rings and π−π stacking would be
suppressed in the solid state by the peripheral phenyl rings and
the bulky silylene moiety.1,2
C.; Wang, T.; Tu, Q.; Bekos, E.; Richardson, D.; Eckert, J.; Cui, J. Org.
Process Res. Dev. 2011, 15, 1371 , and references therein.
(4) Kira, M.; Ishida, S.; Iwamoto, T.; Kabuto, C. J. Am. Chem. Soc.
1999, 121, 9722.
(5) For recent reviews on silylenes, see: (a) Driess, M.;
Grutzmacher, H. Angew. Chem., Int. Ed. Engl. 1996, 35, 828.
̈
The emission properties (λmax and ΦF) of 4 are similar to
those of dihydrocyclopenta[a]indene 2 (λmax = 458 nm, ΦF (in
1:9 THF−water mixed solvent) = 11.7%).2g Although the
emission band of 4 was red-shifted by 20 nm compared to that
of 2, probably due to the electronic effect of the siloxy group
RH SiO connected to the π-core, the bulky RH SiO unit does
(b) Jutzi, P.; Burford, N. Chem. Rev. 1999, 99, 969. (c) Tokitoh, N.;
Okazaki, R. Coord. Chem. Rev. 2000, 210, 251. (d) Gaspar, P. P.; West,
R. In The Chemistry of Organic Silicon Compounds; Rappoport, Z.;
Apeloig, Y., Eds.; John Wiley & Sons: Chichester, 1998; Vol. 2, p 2463.
(e) Weidenbruch, M. J. Organomet. Chem. 2002, 646, 39.
(f) Weidenbruch, M. Organometallics 2003, 22, 4348. (g) Hill, N. J.;
West, R. J. Organomet. Chem. 2004, 689, 4165. (h) Mizuhata, Y.;
Sasamori, T.; Tokitoh, N. Chem. Rev. 2009, 109, 3479. (i) Kira, M.
Chem. Commun. 2010, 46, 2893.
2
2
not affect the AEE nature of tetraphenylcyclopentadiene
significantly, as expected by the crystal structure.
In summary, we have demonstrated that cycloaddition of
silylene 1 with tetracyclone giving a siloxycyclopentadiene is a
promising synthetic route of AEE molecules without transition
metal catalysts. Further investigation is in progress.
(6) Cycloadditions of transient silylenes with aromatic or unsaturated
carbonyl compounds: (a) Ando, W.; Ikeno, M.; Sekiguchi, A. J. Am.
Chem. Soc. 1977, 99, 6447. (b) Ando, W.; Ikeno, M. Chem. Commun.
1979, 655. (c) Belzner, J.; Ihmels, H.; Pauletto, L.; Noltemeyer, M. J.
Org. Chem. 1996, 61, 3315. (d) Sakai, N.; Fukushima, T.; Minakata, S.;
Ryu, I.; Komatsu, M. Chem. Commun. 1999, 1857. (e) Sakai, N.;
Fukushima, T.; Okada, A.; Ohashi, S.; Minakata, S.; Komatsu, M. J.
Organomet. Chem. 2003, 686, 368. (f) Calad, S. A.; Woerpel, K. A. J.
Am. Chem. Soc. 2005, 127, 2046. (g) Calad, S. A.; Woerpel, K. A. Org.
Lett. 2007, 9, 1037. (h) Howard, B. E.; Woerpel, K. A. Org. Lett. 2007,
9, 4651. (i) Bourque, L. E.; Woerpel, K. A. Org. Lett. 2008, 10, 5257.
(j) Minakata, S.; Ohashi, S.; Amano, Y.; Oderaotoshi, Y.; Komatsu, M.
Synthesis 2007, 16, 2481.
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthetic details and physical data of 4 and 5; theoretical
studies of model compounds for assignment of the UV−vis
absorption bands. This material is available free of charge via
the Internet at http://pubs.acs.org.
(7) Reactions of isolable silylenes with aromatic or unsaturated
carbonyl compounds: (a) Jutzi, P.; Eikenberg, D.; Bunte, E. A.;
Mohrke, A.; Neumann, B.; Stammler, H. G. Organometallics 1996, 15,
1930. (b) Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F. Organo-
metallics 1997, 16, 4861. (c) Ghadwal, R. S.; Sen, S. S.; Roesky, H. W.;
Granitzka, M.; Kratzert, D.; Merkel, S.; Stalke, D. Angew. Chem., Int.
Ed. 2010, 49, 3952. (d) Azhakar, R.; Sarish, H. P.; Tavcar, G.; Roesky,
H. W.; Hey, J.; Stalke, D.; Koley, D. Inorg. Chem. 2011, 50, 3028.
(e) Azhakar, R.; Ghadwal, R. S.; Roesky, H. W.; Hey, J.; Stalke, D.
Organometallics 2011, 30, 3853.
(8) Reports on the cycloaddition of 1 with unsaturated bonds:
(a) Ishida, S.; Iwamoto, T.; Kira, M. Organometallics 2010, 29, 5526.
(b) Ishida, S.; Iwamoto, T.; Kira, M. Heteroat. Chem. 2011, 22, 432.
(9) Molecular structure and packing diagram of 5 determined by X-
ray structural analysis are shown in Figures S8 and S9 in the
Supporting Information. Similarly to 4, the lamellar structure was
observed for the crystal of 5. Compound 5 shows a weak absorption at
447 nm (ε = 769) (HOMO → LUMO transition) and an intense band
assignable to the HOMO−1 → LUMO transition at 398 nm in hexane
solution. Although some dibenzofulvenes showed AEE behavior,2e
compound 5 does not show detectable fluorescence both in solution
and in the solid state, probably due to its small molar extinction
coefficient.
(10) In our previous report, the silicon atom of the dimethylsilylene−
cyclopentadienone complex, a cyclopentadienone silaylide, was found
to be positively charged by computational studies, suggesting
electrophilic addition of the silicon to an intramolecular aromatic
ring would be reasonable. For details, see ref 8a.
(11) Intermediate 7 was not observed during the reaction. The
details of the isomerization of 7 to 4 via formally [1,7] hydrogen shift
remains open.
(12) Assignment of UV−vis absorption bands are based on the TD-
DFT calculations of the model compounds described in the
Supporting Information.
(13) The emission spectrum of powdery 4 has two less-resolved
maxima, which may be attributed to emission from a different phase
such as crystalline and amorphous phases and/or Davydov splitting.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: sishida@m.tohoku.ac.jp (S.I.), iwamoto@m.tohoku.ac.
jp (T.I.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported in part by KAKENHI [Nos.
21750035 (S. I.), 22550028 (M. K.), and 24655024 (T. I.)].
The authors thank Dr. Eunsang Kwon and Satomi Ahiko
(Research and Analytical Center for Giant Molecules, Graduate
School of Science, Tohoku University) for elemental analyses.
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