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interesting molecular structure, that is, two of the three arms,
bromobithienyl units are located in a parallel fashion to adopt
p
-
stacking in the crystal structure (Chart 2). Therefore, we had in-
terest in the crystal structure of compound 3, and attempted to
carry out X-ray crystallographic analysis. However, all attempts to
obtain single crystals that can be used for X-ray analysis by
recrystallization of 3 were unsuccessful. The fine powder, but not
single crystal was always obtained. Accordingly, we focused on
theoretical treatment to elucidate the structure of 3.
2.1. Theoretical calculations
To examine stable structures of 3 illustrated in Scheme 1, the
density functional theory (DFT) calculations were carried out using
the Gaussian 03 program package [8]. The Becke-three-parameter-
LeeeYangeParr hybrid functional [9e11] was employed. The 6-31G
and 6-31G(d) basis sets were used for C and H atoms and Si and S
atoms, respectively. With the optimized structure, the singlet exci-
tation energy was evaluated with the time-dependent DFT method.
It has been found that many stable conformations are present
for this molecule. For example, in a bithiophene unit arising from
bonding of two thienyl groups, syn and anti conformations are
generated per a bithiophene unit. We calculated all-syn and all-anti
conformers as the representative, and their optimized structures
are shown in Fig. 4 (see Supplementary material). The all-anti
conformer is more stable by 22 kJ molꢀ1 than the all-syn conformer.
The energies for the other intermediate conformers (including both
syn and anti parts) are expected to be laid in this range.
For the all-anti conformer, the singlet excitation energies were
evaluated for ten of the lowest excitations between the occupied and
vacant MO’s, as shown in Table 1. The excitations with oscillation
strength smaller than 0.1 are omitted. For the ten excitations, the
excitation energies ranges from 353.2 to 334.7 nm, and well agree
with the experimental value of 336 nm. The third excitation has the
largest oscillation strength (0.94), which is composed of five electron
transfers from the occupied MO’s to the vacant MO’s. Fig. 5 shows the
orbital contour plots for the four typical MO’s (MO#359, #360, #361,
and #364) which have larger coefficients, i.e., leading contribution to
this excitation (see Supplementary material). Among the four MO’s,
the destination MO of excitation is solely the LUMO (#364), whereas
the origin MO’s of excitation are distributed to #359, #360, and #361,
and the HOMO (#363) is not included. As can be seen in Fig. 5, this
palladium gave the H-shaped molecule 3 in 52% yield as light yel-
low solids.
The structure of compound 3 was verified by spectrometric
analysis. The mass spectrum for 3 shows the parent ion at m/z 1366,
corresponding to the calculated molecular weight of C66H86Si6S10
.
The 1H NMR spectrum for 3 shows a singlet signal at 0.30 ppm due
to dimethylsilyl protons, 0.93 ppm attributable to tert-butyl protons
and 0.96 ppm attributed to the methyl protons on the silicon atom,
as well as the signals due to the thienyl ring protons. The 13C NMR
spectrum for 3 shows two signals at ꢀ5.0 and 0.2 ppm due to
methyl carbons on the silicon atoms, two signals at 16.9 and
26.3 ppm attributable to tert-butyl carbons, together with twelve
signals attributed to thienyl ring carbons at 125.1e144.6 ppm. The
29Si NMR spectrum for 3 shows signals at ꢀ25.2 and 1.0 ppm,
respectively (see Experimental section).
excitation is the pep type, and the electron transfer occurs from the
central bithiophene unit (#359), left units (#360) and right units
(#361) to the central bithiophene unit (#364). The Si atoms are not
contributed to the excitation, and the contribution from S atoms
seems to be smaller than that from C atoms.
In conclusion, the H-shaped molecule 3 with the silylbithienylene
groups was synthesized by using SuzukieMiyaura cross-coupling
reaction. Compound 3 showed the absorption maximum at 336 nm
and the emission maxima at 377 and 387 nm in dioxane. Inpowder, it
exhibited the emission at 392 nm. The excitation energies calculated
for the all-anti conformer well reproduced the experimental value.
The absorption and fluorescence spectra of compound 3 are pre-
sented in Figs.1e3 (see Supplementary material). Compound 3 shows
the absorption maximum at lmax ¼ 336 nm with a molar absorption
coefficient (ε) of 113,000 Mꢀ1 cmꢀ1 in dioxane. The fluorescence
spectrum of 3 excited at 336 nm in dioxane displays the emission
maxima at 377 and 387 nm. In the case of excitation at 336 nm, the
fluorescence quantum yield of 3 in dioxane amounts to FF ¼ 48%.
We have reported the synthesis and optical properties of starlike
compound 4 which is the viscous yellow liquid with an organo-
silicon core and bithienylene arms [4]. The absorption and fluo-
rescence maximum wavelengths and quantum yields of 3 in
solution are very similar to those of compound 4 (lmax,abs ¼ 335 nm,
lmax,F ¼ 374 nm, FF ¼ 50%) (Chart 1).
3. Experimental
3.1. General procedures
Compound 3 in powder exhibits blue emission (lmax ¼ 392 nm)
with the quantum yield of 7%. It is well-known that the fluorophors
show a strong tendency in the solid state to cause aggregations that
result in fluorescence quenching in general.
All reactions were performed under an atmosphere of dry ni-
trogen. NMR spectra were recorded on a JNM-LA300 spectrometer
and JNM-LA500 spectrometer. Low-resolution mass spectra were
measured on a JEOL Model JMS-700 instrument. High-resolution
mass spectra were obtained from LTQ Orbitrap XL. UVevisible ab-
sorption spectra were measured with a JASCO V-560 spectrometer.
Previously, we investigated X-ray crystallographic analysis of a
starlike
molecule,
tris[(50-bromo-2,20-bithiophen-5-yl)dime-
thylsilyl]methylsilane, and found that this compound has the
Fluorescence spectra were measured with
a JASCO FP-777
Naka, Akinobu
