Synthesis and characterization of (Di)benzosilaphenalenes

Article abstract of DOI:10.1246/cl.170939

Solid-state fluorescent materials were prepared by the annulation of silylated polycyclic aromatic hydrocarbons (PAHs) with internal alkynes catalyzed by [RuH₂(CO)(PPh₃)₃]. Single-crystal X-ray analysis revealed that the a

Full text of DOI:10.1246/cl.170939

CL-170939
Received: October 6, 2017 | Accepted: November 7, 2017 | Web Released: November 15, 2017
Synthesis and Characterization of (Di)Benzosilaphenalenes
Yuichiro Tokoro* and Toshiyuki Oyama*
Department of Advanced Materials Chemistry, Faculty of Engineering, Yokohama National University,
79-5 Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa 240-8501, Japan
E-mail: tokoro-yuichirou-zv@ynu.ac.jp (Y. Tokoro), oyama-toshiyuki-wz@ynu.ac.jp (T. Oyama)
Solid-state fluorescent materials were prepared by the
annulation of silylated polycyclic aromatic hydrocarbons
(PAHs) with internal alkynes catalyzed by [RuH2(CO)(PPh3)3].
Single-crystal X-ray analysis revealed that the annulation
proceeded through C-H cleavage at the peri-positions and
afforded slightly strained six-membered rings containing silicon.
In UV-vis absorption and photoluminescence experiments,
bathochromic shifts of the annulation product as compared with
those of the corresponding substrates were observed.
Keywords: Benzosilaphenalene
| Solid-state fluorescent |
Polycyclic aromatic hydrocarbon (PAH)
Polycyclic aromatic hydrocarbons (PAHs) and their deriv-
atives are attractive molecules because of their potential
application in field-effect transistors1 and photosensitizers.2
Introduction of substituents and expansion of the π-conjugated
system are proposed as strategies to tune the aggregation
properties, color, emission, and conductivity of PAHs. Silicon-
containing substituents are often used to modify the packing,3
color and fluorescence quantum yields.4 For instance, some
silylanthracenes show higher fluorescence quantum yields and
red-shifted emission as compared with anthracene.4a Fusion of
silicon-containing rings and aromatic hydrocarbon rings has also
been found to be effective in perturbing the electronic structure.5
Silacyclopentadiene moieties, which show n-type conductivity
due to their σ*-π* conjugation, have been condensed to PAHs
and their properties investigated in detail. The utility of the
silyl groups and silacyclopentadiene moieties have encouraged
synthetic chemists to develop direct C-H silylation of PAHs.6
In contrast to the five-membered rings containing silicon,
the effects of fused six-membered rings containing silicon on
the properties of PAH have rarely been investigated probably
due to the difficult synthesis. 1-Sila-1H-phenalene is composed
of three fused six-membered rings. The conventional synthetic
procedures for these compounds require high temperature (ca.
650 °C)7 or complex pentacoordinate silicates.8 Recently, one of
the authors developed a direct and mild synthetic route to 1-sila-
1H-phenalene. Ruthenium-catalyzed annulation of hydrosilyl-
naphthalenes with internal alkynes afforded 1-sila-1H-phenalene
in good yield.9 Moreover, the annulation could result in the
addition of bulky regions to the substrate. Bulkiness around
the chromophore is an important factor to obtain solid-state
emission of organic molecules.10 In this study, we subjected
PAHs such as silylated anthracene, pyrene, and phenanthrene
to ruthenium-catalyzed annulation conditions. The use of such
large π-conjugated systems would allow for the investigation
of absorption and emission properties in the visible region.
Silylated anthracenes, pyrenes, and phenanthrenes were
prepared from the brominated precursors via lithium-bromine
exchange and subsequent trapping by the chlorosilanes. In the
Scheme 1. Scope of the annulation of silylated PAHs and
internal alkynes. Isolated yields are reported. Yields determined
by H NMR are reported within parentheses.
1
presence of [RuH2(CO)(PPh3)3], annulation of the silylated
arenes with alkynes proceeded smoothly at 125 °C through C-H
cleavage at the peri-positions (Scheme 1).11 Annulation of 9-
dimethylsilylanthracene 1a with diphenylacetylene (2a) gave
product 3aa in 71% isolated yield and 84% NMR yield.
The reactivity of 1a was higher than that of 1-dimethylsilyl-
naphthalene under conditions similar to those mentioned in the
previous report (59% GC yield).9 The high reactivity of 1a could
be related to two-fold peri-positions, which increases the
probability of activation of an active C-H bond by the
ruthenium catalyst. The diethylsilyl group (1b) and diphenylsilyl
group (1d) were also tolerated in the annulation. Diisopropyl-
silylanthracene (1c), however, was recovered after being sub-
jected to the annulation conditions. Considering 1-diisopropyl-
silylnaphthalene gave the annulation product in the previous
work (44% isolated yield),9 it appears that the bulky 9-anthryl
group may partially disturb the annulation without affecting
the C-H cleavage process. Diphenylacetylenes with isopropoxy
groups (2c) and with trifluoromethyl groups (2d) at the para-
positions were also effective as the annulation partners of
silylanthracene 1. Instead of the silylanthracenes, silylpyrenes
130 | Chem. Lett. 2018, 47, 130–133 | doi:10.1246/cl.170939
© 2018 The Chemical Society of Japan

Table 1. Photophysical properties of 1 and 3
(a)
(b)
(c)
Compd.
- - -
abs/nma,b (¾)c em,chloroform/nma,d (ΦF)e em,powder/nmd (ΦF)e
1a
1d
1e
1f
390 (770)
393 (970)
396, 419 (0.50)
401, 423 (0.66)
378, 396 (0.08)
379, 397 (0.11)
352, 368 (0.10)
434, 455 (0.36)
435, 459 (0.20)
496 (0.16)
431, 455 (0.42)
395, 454 (0.04)
391, 412 (0.13)
377, 433 (0.02)
427, 447 (0.46)
429, 448 (0.29)
493 (0.30)
467 (0.22)
369 (0.06)
455, 482 (0.29)
466, 494 (0.20)
521 (0.36)
453, 482 (0.22)
418, 484 (0.26)
436 (0.14)
348 (4720)
351 (4370)
301 (1220)
419 (1280)
423 (1010)
418 (1250)
419 (1290)
370 (3810)
372 (3820)
338 (1650)
1g
3aa
3db
3ac
3ad
3ea
3fb
3ga
419 (0.37)
a10 ¯M in chloroform. Observed absorption maximum at the longest
wavelength. cMolar extinction coefficient (M¹1 cm¹1). dObserved
fluorescent maxima. eFluorescent quantum yield determined by a
calibrated integrating sphere system.
b
Figure 1. X-ray crystal structures of (a) 3aa, (b) 3ea, and (c)
3ga. Thermal ellipsoids are set to 50% probability level.
Hydrogen atoms are omitted for clarity. HOMA values of the
individual rings are reported below the crystal structures. As
two sets of molecules were contained in the crystal of 3ga, an
averaged HOMA value of 3ga is reported here.
(1e and 1f) and a silylphenanthrene (1g) were also used as
silylated substrates to afford 3ea, 3fb, and 3ga. The yields were
lower than those obtained with silylanthracenes but similar
to those obtained with silylnaphthalenes. Comparison of the
annulation results from the silylated hydrocarbons suggested that
the 9-anthryl group with two peri-positions was suitable for this
reaction. The efficiency of C-H cleavage at the peri-position
may influence the yields of the annulation products.
The structures of 3aa, 3ea, and 3ga were revealed by single-
crystal X-ray analysis (Figure 1).12 As expected, the ruthenium-
catalyzed annulation gave the six-membered rings containing
silicon. The fused ring systems were almost planar. As a result
of annulation, the bond lengths of the additional moieties by
annulation fell within the typical C-C, C=C, and C-Si range,
indicating weak π-delocalization effects through the moiety. The
interior C-C-C (or Si) angles of the silicon-containing rings
were larger than 120° and the C-Si-C angles were smaller than
109°. The deviation from the ideal angles for C(sp2) and Si(sp3)
suggested some strain in the silicon-containing rings. In order to
evaluate the aromaticity of the carbon rings from substrates 1,
HOMA indexes were calculated using the bond lengths.13 While
the HOMA values of rings far from the silicon-containing rings
were similar to those of the parent polycyclic aromatic hydro-
carbons, such as anthracene, pyrene, and phenanthrene,14 rings
directly fused to the silicon-containing ring tended to show small
HOMA values. The decrease in the HOMA values implied that
the strain in the silicon-containing rings affected the structures
of the adjacent rings.
Figure 2. UV-vis absorption (solid line) and photolumines-
cence (dotted line) spectra of 1a in chloroform (blue), 3aa in
chloroform (red), and 3aa in powder state (green).
regardless of the parent rings and substituents (Figures S1-S5).
In particular, substituents on the additional ring fused to the
anthracene core hardly affected the absorption properties.
Photoluminescence peaks of 3aa were observed at 434 and
455 nm. The luminescence peak shift from 1a to 3aa was similar
to the absorption peak shifts, although the Stokes shift of 3aa
was slightly larger than that of 1a. The vibronic structure in the
luminescence spectra of 3aa suggested that the fused ring system
was rigid even in the excited state. The photoluminescence
spectra obtained for 3db and 3ad were almost the same as that
obtained for 3aa and the trend coincided with their absorption
spectra. It is noteworthy that the (4-isopropoxy)phenyl-substi-
tuted product (3ac) showed red-shifted and broadened photo-
luminescence spectra.
The UV-vis and photoluminescence properties of the
obtained compounds in CHCl3 were investigated to elucidate
the effect of π-extension from 1 to 3 (Table 1). The bath-
ochromic shift of the absorption peaks from 1a to 3aa implied
effective π-conjugation between the parent anthracene moiety
and the silicon-containing ring (Figure 2). A vibronic structure
was observed in the absorption band around 400 nm for 3aa. The
structure resembled that of 1a, indicating that the anthracene
core mainly contributed to the absorption of 3aa. Similar
bathochromic shifts and vibronic behavior were observed in
UV-vis absorption spectra of the other annulation products
Solid-state photoluminescence data of 1 and 3 were also
collected (Figures 2 and S1-S6). The silylanthracenes and their
annulation products in solid state showed 20-35 nm bathochro-
mic shift as compared with those in solution state and a similar
vibronic structure. Moreover, moderate fluorescence quantum
yields of 1a, 1d, 3aa, 3db, and 3ad were obtained in the solid
state and the yield of 3ac almost doubled upon solidification.
These luminescence properties indicated that the silyl groups
and additional rings effectively restrained π-π stacking of the
anthracene cores leading to broadened peaks and quenching.
Aggregation-induced emission enhancement (AIEE) behavior
Chem. Lett. 2018, 47, 130–133 | doi:10.1246/cl.170939
© 2018 The Chemical Society of Japan | 131

was observed for the phenanthrene-based compounds. The
fluorescence quantum yield of 3ga in the solid state (ΦF = 0.37)
was 18 times higher than that in the solution state (ΦF = 0.02),
although the parent silylphenanthrene (1g) showed low fluores-
cence quantum yield in both the solution (ΦF = 0.10) and solid
states (ΦF = 0.06). Fluorescence peaks of silylpyrenes (1e and
1f) in the solid state largely shifted to longer wavelengths as
compared with those in the solution state. The large bath-
ochromic shift indicated π-π stacking of 1e and 1f in the solid
state. In contrast, bathochromic shifts of 3ea and 3fb upon
solidification were as small as those of the other annulation
compounds, which implied that bulky groups installed by the
annulation effectively suppressed the π-π stacking of pyrenes
(Figures S2 and S3). The packing structures of 3aa, 3ea, and
3ga obtained by single-crystal X-ray analysis might support the
emission behavior in the solid state. There was no π-π stacking
but many CH-π interactions in the crystals.
The contribution from the additional silicon-containing ring
to the UV-vis absorption properties was also investigated by
the TD-DFT calculation at the ωB97X-D/def2-SVPD level.15
The calculated structures were optimized at the ωB97X-D/
def2-SV(P) level, and the fused core structures were in good
agreement with the crystal structures. The TD-DFT calculation
revealed that absorption at the longest wavelengths in 1a, 3aa,
3ea, and 3ga corresponded to the HOMO-LUMO transitions.
The calculated peaks of 1a and 3aa were 329 and 347 nm,
respectively. Although the calculated wavelengths were shorter
than those obtained from UV-vis absorption experiments,
the red-shift of 3aa as compared with 1a was qualitatively
reproduced by calculation. The red-shift might be explained
by π-delocalization (Figure 3). The HOMO and LUMO of 3aa
were extended from the anthracene moiety to the two additional
carbon atoms. A similar delocalization was also seen in 3ea and
3ga (Figure S7). Although little contribution of silicon to the
frontier orbitals of 3aa and 3ea was observed, the LUMO of
3ga was extended to the silicon atom, probably due to the close
energy levels between the π* orbitals of the hydrocarbon moiety
and the σ* orbital of the silicon atom.
In conclusion, ruthenium-catalyzed annulation of silylated
anthracene, pyrene, and phenanthrenes with internal alkynes
proceeded smoothly through the C-H cleavage at the peri-
position. Single-crystal X-ray analysis revealed that the six-
membered rings containing silicon in the products had slight
strain and tended to reduce the aromatic character of the adjacent
hydrocarbon rings. Ring fusion induced about 30-nm bath-
ochromic shift in the absorption and emission spectra, except
for 3ac, which showed a larger bathochromic shift (77 nm as
compared with 1a) and broadening in the photoluminescence
spectra. Moderate fluorescence quantum yields were obtained
even in the solid state of the annulation products probably due to
difficult π-π stacking.
This research was supported financially by Grants-in-Aid
for Young Scientists (B) (No. 16K17872) from the JSPS. The
authors would like to thank Prof. Yoshitaka Yamaguchi for the
kind assistance in the X-ray analysis.
Supporting Information is available on http://dx.doi.org/
10.1246/cl.170939.
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Chem. Lett. 2018, 47, 130–133 | doi:10.1246/cl.170939
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