A design concept of long-wavelength fluorescent analogs of rhodamine dyes: Replacement of oxygen with silicon atom

Article abstract of DOI:10.1039/b718544h

Replacement of the oxygen with a silicon atom on the rhodamine framework produces a strong red-emission fluorophore which has a high molar extinction coefficient and 90 nm red shift relative to rhodamine dye PY. The Royal Society of Chemistry.

Full text of DOI:10.1039/b718544h

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A design concept of long-wavelength fluorescent analogs of rhodamine
dyes: replacement of oxygen with silicon atom
Meiyan Fu,a Yi Xiao,*a Xuhong Qian,*b Defeng Zhaoa and Yufang Xub
Received (in Cambridge, UK) 30th November 2007, Accepted 22nd January 2008
First published as an Advance Article on the web 14th February 2008
DOI: 10.1039/b718544h
Replacement of the oxygen with a silicon atom on the rhodamine
framework produces a strong red-emission fluorophore which
has a high molar extinction coefficient and 90 nm red shift
relative to rhodamine dye PY.
(OLEDs)9 and organic field-effect transistors (OFETs).10
Some green emission and even red emission silole derivatives
have been reported very recently.11 Most silaanthracene com-
pounds show relatively weak absorption bands in the range of
270–550 nm and emission bands centered around 550 nm,12
and have poor stability at room temperature except those with
bulky substituents on the silicon atom.13 So far, there is no
report on designing long-wavelength fluorophores by directly
utilizing the spectral tuning (lowering LUMO energy) ability
of silicon atom. Encouraged by the significant achievements
on siloles and silaanthracenes, we believed it was feasible to
obtain a long-wavelength and strongly emitting silaanthracene
derivative using the silicon atom as the functional element to
modify the rhodamine framework.
Long-wavelength fluorescent dyes have been extensively ap-
plied in life science in recent years.1 The advantage of long-
wavelength fluorophores over the shorter ones, lies in that they
can effectively avoid the interference from the biological back-
ground fluorescence. However, among numerous fluoro-
phores, long-wavelength ones with high fluorescence
quantum yields amount to an extremely small portion.
Although a few novel long wavelength fluorophores have been
developed,2 most of them are based on structural modification
of conventional short wavelength fluorophores3 in which the
rhodamine type cationic dyes (as shown in Scheme 1) play a
very important role. The rhodamines are widely used as
fluorescent probes and molecular markers in chemistry and
biochemistry.4 However, the absorption and emission wave-
lengths of most rhodamine derivatives are well below 600 nm.
In order to get longer wavelength rhodamine derivatives,
many efforts have been made, including extending the con-
jugation by fusing heteroaromatic ring moieties to the rhoda-
mine precursor,5 substituting the central carbon by nitrogen,6
or introducing electron-withdrawing groups, such as cyano,
on the central carbon atom,7 etc. Analogs developed by the
above methods have some disadvantages such as synthetic
difficulties, decrease in absorption or fluorescence intensity,
and so on. Another important strategy to get tunable spectro-
scopic properties of rhodamine-relative dyes is to replace the
oxygen bridge atom by other elements, such as N, C, S, Se and
Te. However, only C substitution leads to a 50 nm batho-
chromic shift, other substitutions resulting in small batho-
chromic shift or even hypochromic shift.8
In this paper, the first red-emission silaanthracene fluoro-
2,7-N,N,N0,N0-tetramethyl-9-dimethyl-10-hydro-9-
phore:
silaanthracene (TMDHS) was synthesized as shown in Scheme
2.14 From the starting material, 3-bromoaniline, a two-step
procedure15 was adopted to prepare bis(2-bromo-4-N,N-
dimethylphenyl)methane. Then, from this key intermediate,
a facile one-pot synthesis process, the consecutive operations
of lithiation, silylanization and oxidation, gave TMDHS with
a good yield up to 80%. Unlike many other silaanthracene
compounds, TMDHS is stable under ambient conditions.
With the rhodamine-like cationic dye framework, TMDHS
exhibits intriguingly strong absorption and emission. The
properties of the TMDHS were studied in comparison with
the rhodamine counterpart (R = H, R1 = Me, commercial
name pyronine Y, PY).
The spectral properties of TMDHS and PY are shown in
Fig. 1 and Table 1. TMDHS has very strong and sharp
absorption and emission bands in the red region of the
spectrum. For example, in dichloromethane, the maximum
absorption wavelength is at 641 nm, with a 90 nm batho-
chromic shift compared with PY. The extinction coefficient (e)
is very high, 105 Mꢀ1 cmꢀ1, and twice that of PY (e E 5 ꢁ 104
Mꢀ1 cmꢀ1). The emission band peaked at 659 nm is very
narrow (30 nm FWHM). The fluorescence quantum yield (FF)
Recently, the silico-cyclic conjugated compounds, siloles
and silaanthracenes, (shown in Scheme 1), have attracted
much attention on their syntheses and properties. Siloles,
which have extremely low LUMO energy, fast electron mobi-
lity and unique aggregation induced emission (AIE) proper-
ties, are extensively used in organic electroluminescent devices
a State Key Laboratory of Fine Chemicals Dalian University of
Technology, Zhongshan Road 158, Dalian, China.
E-mail: xiaoyi@chem.dlut.edu.cn; Fax: +86-411-83673488;
Tel: +86-411-88993870
b Laboratory of Chemical Biology East China University of Science
and Technology, Meilong Road 130 Shanghai.
E-mail: xhqian@ecust.edu.cn
Scheme 1 The molecular structures of rhodamines, siloles and
silaanthracenes.
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Fig. 2 Cyclic voltammetry of TMDHS and PY. The CV was carried
out at a glass carbon electrode in dichloromethane solution containing
0.1 M tetrabutylammonium hexafluorophosphate as electrolyte, Pt
wire as auxiliary electrode and Ag/AgNO3 as the reference electrode.
The concentration of the compounds used in this experiment was
Scheme 2 The synthesis process of TMDHS.
1 mM and the scan rate was 100 mV sꢀ1
.
through the bridge atom replacement strategy. To understand
the effect of silicon on the spectral properties, electrochemical
measurement was applied. The cyclic voltammetry (CV)
curves of TMDHS and PY are similar (Fig. 2). Both show a
pair of reversible oxidation peaks and an irreversible reductive
peak. The electrochemical data are listed in Table 2. The
original oxidation potential, the average of the anodic and
cathodic peak potentials of TMDHS, is located at 1.00 V (vs.
Fc/Fc+) and gives a HOMO value of ꢀ5.50 eV. The original
reductive potential investigated as the onset potential is at
ꢀ0.58 V and gives a LUMO value of ꢀ3.92 eV. Similarly, PY
has an oxidation potential at 1.14 V and a reductive potential
at ꢀ0.76 V, which gives a HOMO value of ꢀ5.64 eV and a
LUMO value of ꢀ3.74 eV. The energy gap values Eg of the
two compounds are 1.58 and 1.90 eV, respectively, which is
consistent with the optical results Eog obtained from the
absorption onset wavelength. Compared with PY, the HOMO
of TMDHS increases by 0.14 eV, while the LUMO of
TMDHS decreases by 0.18 eV. This result indicates that the
substitution of oxygen by silicon affects the HOMO and
LUMO energy level simultaneously, and thus a smaller energy
gap is obtained and large red shift is achieved. Similar to that
of siloles,16 the low-lying LUMO energy level of the TMDHS
is caused by the special s*–p* conjugation, with the contribu-
tion of the s* of the silicon atom and p* of the adjacent
carbons. The HOMO energy increase of the TMDHS is
ascribed to the inductive electron-donating effect of the
dimethylsilyl group relative to oxygen.
Fig. 1 The absorption (a) and emission (b) spectra of 11.5 mM
TMDHS (blue line) and PY (magenta line) in CH2Cl2.
Table 1 The spectral properties of TMDHS in different solvents
a
FF
Solvent
labs/nm
e/Mꢀ1 cmꢀ1
lem/nm
Dichloromethane
Acetone
Ethanol
641
637
636
634
102 623
103 285
43 646
64 191
659
660
655
653
0.39
0.29
0.26
0.18
Water
a
Quantum yields were measured using rhodamine B (FF = 0.49 in
ethanol) as the standard.
of TMDHS is 0.39 in CH2Cl2 and 0.18 in water. The values of e
and FF decrease in protic solvents considerably. Considering a
similar tendency was also observed for PY, we speculated that
such a special solvent effect might involve the interactions of
solvent molecules with the electron-deficient central CH posi-
tion of the cationic dyes, where the positive charge is partially
localized. We believed that such a solvent effect could be
eliminated by introducing blocking substituents on the carbon
atom, such as aryl groups as in the case of the rhodamines. The
high quantum yield and the long absorption and emission
wavelength of the TMDHS confirmed our design concept. To
the best of our knowledge, this is the first time where the
introduction of a silicon atom to the key position of a
conventional dye has successfully tuned the spectral properties.
It is amazing that, from rhodamine dye PY to TMDHS, a
single atom change leads to a large red shift up to 90 nm, and
such a significant effect has never been obtained before
In summary, TMDHS, a new strong fluorescence silaan-
thracene dye in the red region has been developed through
substitution of the rhodamine oxygen bridge atom by silicon,
which results in a 90 nm red shift. This is the first case where
silicon atom is adopted as the functional element into the key
position of a conventional excellent dye to successfully mod-
ulate the spectral properties. The electrochemical studies
Table 2 Electrochemical dataa of TMDHS and PY
Compd.
Eox/V
Ered/V
EHOMO/eV
ELUMO/eV
Eg/eV
labs/nm
Eog/eV
PY
TMDHS
1.14
1.00
ꢀ0.76
ꢀ0.58
ꢀ5.64
ꢀ5.50
ꢀ3.74
ꢀ3.92
1.90
1.58
552
641
1.82
1.61
a
The electrochemical data were estimated on the basis of ferrocene (measured E1/2FC = 0.30 V vs. Ag/AgNO3), and Eox = 1/2(Epa + Epc). EHOMO
and ELUMO energy levels were calculated according to EHOMO/ELUMO = ꢀ4.8 ꢀ e(Eox/red ꢀ E1/2 FC) eV, respectively.17
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indicate that the smaller energy gap of TMDHS results from
both energy decrease of LUMO and energy increase of
HOMO. Further derivation on TMDHS for efficient sensors
is now in progress.
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(no. 20406004, 20572012, 20536010).
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1782 | Chem. Commun., 2008, 1780–1782