Synthesis and reversible control of the fluorescent properties of a divalent tin dipyrromethene

Article abstract of DOI:10.1021/ja904470m

(Figure Presented) A fluorescent chlorostannylene bearing a dipyrromethene ligand was synthesized. Its fluorescence quantum yield was low, φ_f = 0.04, probably because of the existence of the lone-pair orbital energetically close to the " orbita

Full text of DOI:10.1021/ja904470m

Published on Web 07/21/2009
Synthesis and Reversible Control of the Fluorescent Properties of a Divalent
Tin Dipyrromethene
Junji Kobayashi,* Tomokatsu Kushida, and Takayuki Kawashima*
Department of Chemistry, Graduate School of Science, The UniVersity of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan
Received June 2, 2009; E-mail: jkoba@chem.s.u-tokyo.ac.jp; takayuki@chem.s.u-tokyo.ac.jp
Stannylenes are heavy congeners of carbenes and have been
widely investigated.1 They easily dimerize without steric protection,
and bulky substituents are necessary to stabilize these species. An
alternative strategy for stabilization is the utilization of intramo-
lecular coordination by the introduction of anionic chelating ligands
such as ꢀ-diketiminate, aminotroponiminate, or amidinate. By the
introduction of these ligands, LSnCl-type compounds have been
isolated.2 Chloride is the reactive ligand and is easily replaced by
other small, reactive ligands, such as alkoxide, amide, azide, triflate,
and hydride.3 The presence of these small, reactive ligands promotes
valuable reactivities, such as the polymerization of lactide, the
cyclization of isocyanate, and the hydrostannylation of carbon
dioxide.4 These are interesting and important examples of applica-
tions of divalent tin compounds. However, they are limited to
transformations on the tin atom, and the ligand functions only as a
stabilizing group. On the other hand, Driess and co-workers5
developed the ambivalent reactivity of silylene and germylene by
using hydrohalogenated ꢀ-diketiminate ligand. These examples are
very striking because they indicate that modification of the ligand
could attach new functionality to the divalent group 14 species.
Sn(II) chemistry.2 The bond lengths and angles around the tin atom
are listed in the Supporting Information and are not much different
from those of similar Sn(II) compounds. There is an intermolecular
interaction between the tin and chlorine atoms. Thus, the tin atom
takes a pseudo-trigonal-bipyramidal structure, and 2 forms infinite
columns along the b axis of the unit cell, as shown in Figure 1b.
Figure 1. (a) ORTEP drawing of 2 (50% probability). (b) Packing structure
of 2. For selected distances and angles, see the Supporting Information.
The optical properties of 2 in benzene solution were investigated,
and the data are summarized in Table 1. Compound 2 showed a
strong absorption at 509 nm with vibrational structure, and BODIPY
(3), bearing the same ligand, showed quite similar absorption
properties (λmax ) 514 nm). On the other hand, a distinct difference
in the fluorescence properties was revealed. Steady-state fluores-
cence emission of 2 was observed at 518 nm with a small Stokes
shift. However, its quantum yield was 0.04, which is considerably
smaller than that of 3 (Φf ) 0.89; λf ) 525 nm).
In this context, we focused our attention on dipyrromethenes as
ligands of stannylenes. When these ligands are introduced to a boron
atom or transition metals, they gain excellent optical properties.6
We considered that the combination of the highly reactive stan-
nylene with the fluorescent dipyrromethene ligand could produce
a novel chromophore responsive to external stimuli. Here we report
a novel stannylene bearing the dipyrromethene ligand and reversible
control of its fluorescent properties by simple reactions on the tin
atom.
To clarify the reason for the difference in the fluorescence
properties of SnClDIPY and BODIPY, DFT calculations (B3LYP)
were performed using model compounds9 in which the mesityl
group was replaced by a xylyl group and the methyl groups on the
dipyrromethene were removed. The HOMOs and LUMOs of
BODIPY and SnClDIPY consist of π and π* orbitals, respectively,
of the dipyrromethene framework, and their orbital shapes and
energy levels are similar. The lowest-energy transitions of these
species correspond to the HOMO-LUMO transitions. For Sn-
ClDIPY, however, the lone-pair (n) orbital of the divalent tin atom
exists as the HOMO-1, and the energy difference between the
HOMO and HOMO-1 is only 0.44 eV. Thus, the n f π* transition
should be located near the lowest-energy π f π* transition and is
considered to cause fluorescence quenching in SnClDIPY. If the
energy level of the n orbital were to be decreased substantially,
the resulting tin dipyrromethene compounds would be expected to
gain good fluorescence properties. DFT calculations on the cationic
species Sn+DIPY predicted that the dipyrromethene π orbital still
exists at a higher level (HOMO-2), whereas the n orbital lies at a
much lower level (HOMO-6), so the energy difference between
these two orbitals is increased to 2.02 eV. These predictions
encouraged us to synthesize the cationic species in order to enhance
the fluorescence emission.
A stannylene bearing a dipyrromethene ligand was readily
synthesized by the reaction of an in situ-generated lithium dipyr-
rinato7 with SnCl2 (Scheme 1). SnClDIPY (2) was obtained as an
orange solid in 74% yield. Compound 2 is relatively stable in air
in the solid state, although it decomposed gradually in solution under
the open atmosphere. The 119Sn NMR spectrum of 2 showed a signal
at -246.7 ppm corresponding to the divalent tin atom, which
suffered from intramolecular coordination.2
The structure of 2 was determined by X-ray crystallographic
analysis. The ORTEP drawing of 2 is shown in Figure 1a.8 The tin
center adopts a pyramidal structure, which is fairly common in
Scheme 1. Synthesis of SnClDIPY (2)a
a (a) n-BuLi (1.0 equiv), -78 °C, THF; (b) SnCl2 (1.0 equiv), r.t. (74%
over two steps).
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10836 J. AM. CHEM. SOC. 2009, 131, 10836–10837
10.1021/ja904470m CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Reaction of 2 with AgOTfa
Table 1. Optical Properties of Tin Dipyrromethenes 2 and 4 and
BODIPY (3)a
b
λmax (nm)
ꢀ (M-1 cm-1
)
λf (nm)
Φf
2
3
4
509
514
513
8.54 × 104
9.71 × 104
6.46 × 104
518
525
524
0.04
0.89
0.42
a In benzene. b Absolute quantum yields.
a (a) AgOTf (1.0 equiv), r.t., toluene (89%); (b) Bu4NCl (1.0 equiv),
r.t., toluene (85% as estimated by H NMR).
1
was low compared with that of BODIPY. However, its fluorescence
emission was reversibly controlled by the dechlorination and
chlorination reactions. As a preliminary result, the increase of
fluorescence emission was selective for AgOTf. Further investiga-
tion of applications of this molecular sensor is now under way.
Chlorostannylenes have been reported to be easily converted to
cationic tin species by silver salt metathesis.3c Indeed, the reaction
of 2 with 1 equiv of AgOTf in toluene led to the quantitative
formation of [SnDIPY][OTf] (4), which showed a bright-green
fluorescence emission (Scheme 2). Compound 4 is an air- and
moisture-sensitive orange solid. A signal for 4 was not observed
in the 119Sn NMR spectrum, but its structure was finally determined
by X-ray crystallographic analysis. The ORTEP drawing of 4 is
shown in Figure 2a.10 In the crystalline state, the triflate ligand is
attached to the tin atom as a bridging ligand, and 4 forms a
polymeric chain (Figure 2b). The Sn-O distances varied from 2.420
to 2.534 Å, which are much longer than the sum of the covalent
radii of Sn and O (2.14 Å), indicating that the Sn-O interactions
are not strong.
Acknowledgment. This work was partially supported by the
Global COE Program (Chemistry Innovation through Cooperation
of Science and Engineering) and Scientific Research, MEXT, Japan.
We thank Tosoh Finechem Corp. for the generous gifts of
alkyllithiums.
Supporting Information Available: Experimental synthetic details
and spectroscopic data for 2 and 4, crystallographic data for 2 and 4
(CIF), computational details for the model compounds, and complete
ref 9. This material is available free of charge via the Internet at http://
pubs.acs.org.
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Figure 2. (a) ORTEP drawing of 4 (50% probability). (b) Packing structure
of 4. For selected distances and angles, see the Supporting Information.
UV-vis spectra of 4 showed a strong absorption maximum at
513 nm with a vibrational structure assignable to the π f π*
transition of the dipyrromethene framework. The absorption
property of 4 was the same as that of 2, indicating that the electronic
structure of the dipyrromethene moiety of 4 was almost the same
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emission at 524 nm; its quantum yield was estimated as 0.42. Thus,
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In summary, we have succeeded in synthesizing SnClDIPY (2)
as a fluorescent stannylene. The fluorescence quantum yield of 2
JA904470M
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