Synthesis, optical properties, and crystal structures of dithienostannoles

Article abstract of DOI:10.1021/om400213q

Reactions of dichlorodiphenylstannane with 3,3′-dilithio-5,5′- bis(trimethylsilyl)-2,2′-bithiophene and 3,3′-dilithio-2,2′- di(benzo[b]thiophene) gave 1,1-diphenyl-3,6-bis(trimethylsilyl)dithienostannole (DTSn1) and di(benzo[b]thieno)-1,1-diphenylstannole (DTSn2), respectively. Optical properties of the dithienostannoles and the results of DFT calculations on a model suggested an in-phase interaction between Sn σ* and bithiophene π* orbitals, stabilizing the LUMO. Interestingly, DTSn1 showed crystallization-enhanced emission and the photoluminescence (PL) quantum yield of the crystals was determined to be higher by about 20 times than that in the amorphous and solution phase, while DTSn2 exhibited moderate PL efficiency both as crystals and in solution. Crystal structures of the dithienostannoles were determined by X-ray diffraction studies, which showed the differences in the molecular packing between DTSn1 and DTSn2, being responsible for their different PL properties in the crystal state.

Full text of DOI:10.1021/om400213q

Article
pubs.acs.org/Organometallics
Synthesis, Optical Properties, and Crystal Structures of
Dithienostannoles
Daiki Tanaka,† Joji Ohshita,*,† Yousuke Ooyama,† Norifumi Kobayashi,‡ Hideyuki Higashimura,‡
Takayuki Nakanishi,§ and Yasuchika Hasegawa§
†Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
‡Advanced Materials Research Laboratory, Sumitomo Chemical Co. Ltd., 6 Kitahara, Tsukuba 300-3294, Japan
§Division of Materials Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan
S
* Supporting Information
ABSTRACT: Reactions of dichlorodiphenylstannane with 3,3′-dilithio-5,5′-
bis(trimethylsilyl)-2,2′-bithiophene and 3,3′-dilithio-2,2′-di(benzo[b]-
thiophene) gave 1,1-diphenyl-3,6-bis(trimethylsilyl)dithienostannole
(DTSn1) and di(benzo[b]thieno)-1,1-diphenylstannole (DTSn2), respec-
tively. Optical properties of the dithienostannoles and the results of DFT
calculations on a model suggested an in-phase interaction between Sn σ* and
bithiophene π* orbitals, stabilizing the LUMO. Interestingly, DTSn1 showed
crystallization-enhanced emission and the photoluminescence (PL) quantum
yield of the crystals was determined to be higher by about 20 times than that in
the amorphous and solution phase, while DTSn2 exhibited moderate PL
efficiency both as crystals and in solution. Crystal structures of the
dithienostannoles were determined by X-ray diffraction studies, which showed
the differences in the molecular packing between DTSn1 and DTSn2, being
responsible for their different PL properties in the crystal state.
Chart 1. Dithienostannoles Reported (a) by Saito et al.,8 (b)
1. INTRODUCTION
by Shimizu et al.,9 and (c) by Us10
Heteroatom-bridged 2,2′-bithiophene derivatives have been
extensively studied as functional materials, in which the highly
planar tricyclic structure enhances π conjugation.1 In addition,
electronic effects of the bridging heteroatom often provide the
desirable manipulation of the bithiophene electronic state. Of
those, group 14 dithienometalloles, dithienosilole (DTS)2 and
-germole (DTG),3 are of particular interest, because of their
unique electronic states. In-phase interactions between the
metal σ* and bithiophene π* orbitals lower the LUMO energy
level to further minimize the HOMO−LUMO energy gap.2a,3a,4
Currently, DTS and DTG are widely used as the building units
of functional π-conjugated polymers and oligomers that can be
applied to the active materials of organic transistors5 and
photovoltaic cells.3,6 Highly photo- and electroluminescent
DTS compounds have also been developed to date.7
On the other hand, much less is known about tin-bridged
bithiophene, namely dithienostannole (DTSn). Saito and co-
workers have reported the synthesis of tin-bridged 3,3′-
bithiophenes (Chart 1a).8 For 2,2′-bithiophene derivatives,
however, there has been only one report by Shimizu et al.9
They prepared di(benzo[b]thieno)-1,1-dimethylstannole
(Chart 1b) and demonstrated its potential utility as a starting
material for polyannulated bithiophene derivatives, but no
detailed properties of this DTSn were provided. In this paper,
we report the synthesis, optical properties, and crystal
structures of two new DTSn compounds, 1,1-diphenyl-3,6-
bis(trimethylsilyl)dithienostannole (DTSn1) and di(benzo[b]-
thieno)-1,1-diphenylstannole (DTSn2) that exhibit σ*−π*
conjugation, similarly to DTS and DTG. Crystallization-
enhanced emission of DTSn1 is also described.
2. RESULTS AND DISCUSSION
Reactions of dichlorodiphenylstannane with 3,3′-dilithio-5,5′-
bis(trimethylsilyl)-2,2′-bithiophene and 3,3′-dilithio-2,2′-bi-
(benzo[b]thiophene) gave dithienostannoles DTSn1 and
DTSn2, respectively, as light yellow solids (Scheme 1). We
recently demonstrated that spirobi(dithienostannole) was
thermally less stable than the dimer based on the DFT
calculations, likely due to the ring strain of the spirobi-
Received: April 3, 2013
Published: July 25, 2013
© 2013 American Chemical Society
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in Table 1. The absorption maximum of DTSn1 in THF
appeared at 350 nm (Figure 2), which was slightly blue-shifted
Scheme 1. Synthesis of DTSn1 and DTSn2
Table 1. Optical Properties of DTSn1 and DTSn2
PL λ /nm
emab
,
(Φf)
a
UV abs λmax/nm (ε/L mol−1
cm−1
)
DTSn1
DTSn2
350 (25846)
solution
crystal
410 (0.009)
422 (0.556)
411 (0.028)
431 (0.296)
454 (0.214)
(dithienostannole) system.10 In fact, the attempted preparation
of tetrakis(trimethylsilyl)spirobi(dithienostannole) by a 2:1
reaction of bis(trimethylsilyl)dilithiobithiophene and tin
tetrachloride provided no spiro compound but gave its
macrocyclic dimer arising from 4:2 coupling (Chart 1c), in
contrast to similar reactions with silicon and germanium
tetrachloride, which gave the corresponding spiro-condensed
DTS and DTG derivatives in good yields. However, the present
reaction proceeded cleanly to give DTSn1 and DTSn2 in 64%
and 57% yields, respectively. They are stable enough to be
handled without special care under ambient conditions. We
carried out DSC (differential scanning calorimetry) of DTSn1
and DTSn2 to investigate their thermal properties. They
showed irreversible DSC profiles, and no evident peaks were
observed in the cooling process after melting (Figure 1). That
amorphous
solution
crystal
369 (14600)
a
b
In THF. Excited at the absorption maximum in solution.
Figure 2. UV and PL spectra of DTSn1 (top) and DTSn2 (bottom).
from that of DTS1 (λmax 356 nm)4 and nearly the same as that
of DTG1 (λmax 350 nm),10 reported previously (Chart 2),
Chart 2. DTS and DTG Derivatives
Figure 1. DSC curves of DTSn1 (top) and DTSn2 (bottom).
no decomposition had occurred by melting DTSn1 was
confirmed by the UV−vis absorption spectrum of the sample
after a cycle from room temperature to 160 °C (Figure S-7,
Supporting Information), indicative of the formation of the
stable amorphous phase of DTSn1 upon cooling. In contrast,
the absorption spectrum of DTSn2 once melted revealed a new
broad band approximately at 540 nm, together with a major
band ascribed to the original sample, indicating that DTSn2
had undergone partial decomposition upon melting (Figure S-
8, Supporting Information), although no DSC peaks were
found up to the melting point.
indicating the similarity of the electronic states of these group
14 dithienometalloles, regardless of the bridging element (i.e.,
metal = Si, Ge, Sn). DTSn2 showed the maximum at 369 nm,
again a wavelength slightly shorter than that of DTS2 (λmax 382
nm).11 The quantum chemical calculations on model molecules
(BT, DTS0, DTG0, and DTSn0) at the B3LYP/LanL2DZ
The optical properties of DTSn1 and DTSn2 were first
examined in solution with respect to the UV absorption and
photoluminescence (PL) spectra, and the data are summarized
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level of theory suggested similar HOMO and LUMO energy
levels of dithienometalloles, regardless of the bridging element
(Figure 3 and Table 2),3a in accordance with the experimental
Table 3. TD-DFT Calculations on DTSn1 and DTSn2 at the
B3LYP/LanL2DZ Level
a
b
b
crystal
optimized
transition energy/
eV (wavelength/
nm)
transition energy/
eV (wavelength/
nm)
oscillator
strength
oscillator
strength
DTSn1
DTSn2
3.68 (336)
4.19 (296)
4.54 (273)
4.59 (270)
4.71 (263)
3.40 (364)
4.05 (306)
4.08 (304)
4.22 (294)
4.48 (277)
0.4036
0.0225
0.0249
0.0022
0.0019
0.3726
0.1221
0.1028
0.0357
0.0088
3.62 (342)
4.19 (296)
4.49 (276)
4.64 (267)
4.65 (267)
3.21 (386)
3.86 (322)
3.89 (319)
3.96 (313)
4.28 (290)
0.3699
0.0523
0.0019
0.0000
0.0152
0.4152
0.0901
0.0820
0.0523
0.0012
Figure 3. Structures of model compounds and HOMO and LUMO
profiles of DTSn0 based on DFT calculations at the B3LYP/LanL2DZ
level.
Table 2. HOMO and LUMO Energy Levels of
Dithienometalloles Derived from DFT Calculations at the
B3LYP/LanL2DZ Level
a
Five low-energy transitions were calculated, and those with oscillator
b
strengths larger than 0.1 are presented. Definitions: crystal, calculated
on the crystal structure determined by X-ray diffraction analysis;
optimized, calculated on optimized geometry at the B3LYP/LanL2DZ
level.
a
a
b
BT
DTS0
DTG0
DTSn0
HOMO/eV
LUMO/eV
−5.81
−1.72
4.09
−5.74
−1.83
3.91
−5.74
−1.81
3.93
−5.72
−1.80
3.92
HOMO and LUMO profiles are seen between them (Figure 4).
In addition, major transitions that are ascribed to HOMO →
Δ(LUMO−HOMO)/eV
a
b
See ref 3a. Present work; see Figure 1.
observation. The σ*−π* interaction is clearly seen in the
LUMO of DTSn0, similarly to DTS0 and DTG0. All these
dithienometalloles were predicted to possess band gaps smaller
than that of nonbridged BT, as expected.
PL spectra of DTSn1 and DTSn2 in THF revealed maxima
at 410 and 431 nm, respectively. Interestingly, DTSn1
exhibited a much lower PL quantum yield in comparison to
DTS1 (λem 425 nm, Φ = 0.69) and DTG1 (λem 409 nm, Φ =
0.71), which have the same substituents.10 This is likely due to
the long Sn−C bonds that promote molecular vibration, thus
facilitating nonradiative decay processes. The heavy-atom
effects of Sn may be also responsible for the lower Φ value
of DTSn1. However, we did not observe any phosphorescence
from DTSn1 and DTSn2 and have no clear data to show how
the heavy-atom effects affect the PL properties. A similar
decrease of fluorescence quantum efficiencies of group 14
dibenzometallole derivatives on going from metal = Si to metal
= Sn was reported previously.12 The PL quantum yield of
DTSn1 was also lower than that of DTSn2, possibly due to the
presence of two flexible trimethylsilyl groups, which provides a
path for nonradiative relaxation. It is also likely that the two
benzoannulated units in DTSn2 hinder rotation of the phenyl
groups on the stannole ring, thus suppressing nonradiative
pathways in DTSn2. The PL spectra were measured also as
crystals, revealing maxima slightly red-shifted from those in
solution (Table 1 and Figure 2). The shift for DTSn2
depending on the states (in solution or as crystal) was larger
than that of DTSn1, presumably due to the higher degree of
intermolecular interaction of DTSn2 in the solid state, as
indicated by its crystal structure analysis (vide infra). One
might consider the possibility that conformational freezing in
the solid states would change the electronic states of these
dithienostannole derivatives. However, no significant changes
of photo transition patterns depending on the geometry
(crystal geometry or optimized gas-phase geometry) were
provided by TD-DFT calculations on the real molecules of
DTSn1 and DTSn2 (Table 3). No evident differences of
Figure 4. HOMO and LUMO profiles of DTSn1 based on optimized
and crystal strucures derived from DFT calculations at th eB3LYP/
LanL2DZ level.
LUMO for the crystal geometries of DTSn1 and DTSn2 are
calculated to be blue-shifted from those in gas phase,
respectively, in contrast to experimental observations that
DTSn1 and DTSn2 showed higher energy absorption maxima
in solution in comparison to those in crystals. These results
clearly indicate that the dependence of optical properties of
DTSn1 and DTSn2 on the state (solution, amorphous, or
crystal) does not arise from changes in single molecular
structures.
To our surprise, the PL quantum yield of the crystals of
DTSn1 was approximately 20 times higher than that in
solution. In the amorphous phase, DTSn1 exhibited weak
emission with Φ = 0.028, similar to that in solution, as
presented in Figure 5. Thus, reversible crystallization-enhanced
emission was readily performed by melting/cooling of the
crystal sample to form the amorphous state, followed by its
recrystallization to recover the crystals. DTSn2 showed
moderate quantum efficiencies, regardless of the states (in
solution or as crystal). The emission lifetime of DTSn2 as
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Figure 6 depicts the crystal structures of DTSn1 and DTSn2,
and Table 4 summarizes the selected bond distances and angles.
They show similar structural parameters for the dithienostan-
nole core fragments with Sn−C(Ph) and Sn−C(stannole)
bond distances between 2.128(3) and 2.144(3) Å, which are in
the standard range for Sn−C(sp2) bonds13 and are similar to
those reported for 3,3′-dithienostannole.8 The endocyclic C−
Sn−C angles are 84.08(9) and 83.6(4)°, respectively, smaller
than that reported for a dithienosilole (DTS3 in Chart 2),4b
reflecting the longer Sn−C bond distances relative to Si−C
bonds. These acute angles may be indicative of ring strain in the
dithienostannole system. Figure 6 also represents the molecular
packing of the dithienostannoles. For DTSn1, the dithienos-
tannole fluorophores are efficiently separated by phenyl and
trimethylsilyl groups, thus preventing aggregation-induced
quenching of the fluorophore, a phenomenon often observed
for organic PL materials in the solid state. π−π stacking
contacts between phenyl rings are the only intermolecular
interactions observed in this structure. These interactions
suppress motion of the phenyl rings but do not seem to have an
influence on the electronic states of DTSn1, as no significant
electron density is localized on the phenyl substituents (Figure
4). In contrast, DTSn2 showed intermolecular face-to-face π−π
contacts between the dithienostannole core fragments, with the
shortest interplane distance being only 3.8 Å. These short
Figure 5. PL spectra of DTSn1. The insets are photos of DTSn1 in
the crystal and amorphous states under UV irradiation (365 nm).
crystals was also measured to be 2.6 ns, which was shorter than
but of the same order as that of DTS1 (4.5 ns). The lifetime of
DTSn1 could not be determined exactly, because the lifetime
was shorter than the pulse width, but it was estimated to be
approximately 0.35 ns. Although the reason for the relatively
short lifetimes of DTSn1 and DTSn2 is still unclear, this clearly
indicates that the dithienostannole emission is based on
fluorescence, not phosphorescence.
Figure 6. Crystal structures (top) and molecular packings (bottom) of DTSn1 (left) and DTSn2 (right).
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Table 4. Selected Bond Distances and Angles of DTS3 and DTSn1
bond distance/Å
bond angle/deg
inner ring C−M−C C(Ph)−M−C(Ph)
C(metallole)−M
C(Ph)−M
DTSn1
DTSn2
2.135(3), 2.143(3)
2.144(9), 2.131(9)
1.868(3), 1.875(3)
2.128(3), 2.136(3)
2.137(9), 2.118(9)
1.859(3), 1.865(3)
84.08(9)
83.6(4)
92.0(1)
110.65(9)
105.9(4)
112.5(1)
a
DTS3
a
See ref 4b.
To this was added dropwise a 1.64 M solution of n-BuLi/hexane (2.00
mL, 3.40 mmol) at −78 °C over a period of 30 min. After the mixture
was stirred at this temperature for 30 min, 0.64 g (1.80 mmol) of
diphenyltin dichloride in 5 mL of ether was slowly added to the
mixture. The mixture was further stirred at −78 °C for 1 h and
warmed to room temperature. To the resulting mixture was added 20
mL of hexane to precipitate inorganic salts, which were removed by
filtration. After evaporation of the solvent from the filtrate, the residue
was subjected to preparative GPC with toluene as eluent to give 0.52 g
(57% yield) of DTSn2 as a light yellow solid: 1H NMR (δ in CDCl3)
7.29−7.43 (m, 10H, benzothiophene, m- and p-phenyl protons),
7.66−7.69 (m, 4H, o-phenyl protons), 7.77−7.79 (m, 2H,
benzothiophene protons), 7.91−7.93 (m, 2H, benzothiophene); 13C
NMR (δ in CDCl3) 122.99, 124.18, 124.79, 125.17, 129.15, 130.01,
135.68, 136.56, 137.08, 142.99, 143.51, 149.21; 119Sn NMR (δ in
CDCl3 from SnMe4) −145.72; high-resolution APCI/MS calcd for
C28H19S2Sn m/z 538.99447 ([M + H+]) found m/z 538.99430 ([M +
H+]). Single crystals of DTSn1 suitable for the X-ray diffraction study
were obtained by solvent diffusion crystallization from toluene/
methanol.
contacts most certainly provide a pathway for at least
nonradiative quenching of the excited state to enhance the
nonradiative decay. Thus, somewhat serendipitously, DTSn2
has the same PL efficiency in the solid state and in solution.
3. CONCLUSIONS
In summary, we prepared the two new dithienostannoles
DTSn1 and DTSn2 and studied their optical properties and
crystal structures. Interestingly, clear and reversible crystal-
lization-enhanced emission was observed for DTSn1. There has
been only a limited number of reports about compounds that
show crystallization-enhanced emission,14 and this is the first
example of a thiophene-based compound. Studies to explore
the functionalities and chemical modification of DTSn are
underway.
4. EXPERIMENTAL SECTION
General Considerations. All reactions were carried out under a
dry argon atmosphere. THF and ether used as the reaction solvents
were dried over calcium hydride and were stored over activated
molecular sieves 4A under an argon atmosphere until use. The starting
compounds 5,5′-bis(trimethylsilyl)-3,3′-dibromo-2,2′-bithiophene4a
and 3,3′-dibromo-2,2′-bibenzo[b]thiophene11 were prepared as
ASSOCIATED CONTENT
* Supporting Information
CIF files, tables, and figures giving crystallographic data, details
of DFT calculations, and NMR spectra of DTSn1 and DTSn2.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
1
reported in the literature. H and 13C NMR spectra were recorded
on Varian System500 and MR400 spectrometers. UV and PL spectra
were measured on Hitachi U-3210 and F-4500 spectrophotometers,
respectively. Measurements of high-resolution APCI and ESI mass
spectra were carried out using a Thermo Fisher Scientific LTQ
Orbitrap XL instrument at the Natural Science Center for Basic
Research and Development (N-BARD), Hiroshima University.
Emission life times were measured on a Spec Fluorolog-3 ps
instrument with a nanoLED 370 nm light source, (pulse width 1.5 ns).
Synthesis of DTSn1. In a 100 mL two-necked flask fitted with a 10
mL dropping funnel were placed 5,5′-bis(trimethylsilyl)-3,3′-dibromo-
2,2′-bithiophene (2.20 g, 4.70 mmol) and ether (80 mL). To this was
added dropwise a solution of 1.64 M n-BuLi/hexane (5.70 mL, 9.40
mmol) at −78 °C over a period of 1 h. After the mixture was stirred at
this temperature for 30 min, 1.61 g (4.70 mmol) of diphenyltin
dichloride in 10 mL of ether for 10 min was slowly added to the
mixture. The mixture was further stirred at −78 °C for 1 h and
warmed to room temperature. To the resulting mixture was added 50
mL of hexane to precipitate inorganic salts, which were removed by
filtration. After evaporation of the solvent from the filtrate, the residue
was subjected to preparative gel permeation chromatography (GPC)
with toluene as eluent to give 1.75 g (64% yield) of DTSn1 as a
AUTHOR INFORMATION
Corresponding Author
*J.O.: tel, +81-82-424-7743; fax, +81-82-424-5494; e-mail, jo@
hiroshima-u.ac.jp.
■
Notes
The authors declare no competing financial interest.
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)” (24102005) of The Ministry of
Education, Culture, Sports, Science, and Technology of Japan.
We thank Prof. M. Saito (Saitama University) for his helpful
suggestion for the synthesis of the present dithienostannoles.
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■
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