Through-bond energy transfer cassettes based on coumarin-Bodipy/distyryl Bodipy dyads with efficient energy efficiences and large pseudo-Stokes' shifts

Article abstract of DOI:10.1039/c1jm12503f

The energy transfer cassettes composed of coumarin and Bodipy as well as NIR distyryl Bodipys were designed to capture photonic energy and convert it to longer wavelength fluorescence emission; large pseudo-Stokes' shifts (up to 410 nm) and efficient ener

Full text of DOI:10.1039/c1jm12503f

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Through-bond energy transfer cassettes based on coumarin–Bodipy/distyryl
Bodipy dyads with efficient energy efficiences and large pseudo-Stokes’ shifts†
*
Yun Zhao, Yan Zhang, Xin Lv, Yunlong Liu, Maliang Chen, Pi Wang, Jing Liu and Wei Guo
Received 2nd June 2011, Accepted 20th July 2011
DOI: 10.1039/c1jm12503f
providing more opportunities to develop EET systems on the base of
the TBET platform. The high energy transfer efficiencies, fast energy
transfer rates and large pseudo-Stokes’ shifts make TBET systems
considerably attractive, and enable their applications in many fields,
such as optical materials,11 photosynthetic models,12 biotechnology10
and chemosensors.13 However, a few TBET systems that involve the
near-infrared (NIR) dyes were reported though NIR dyes have found
wide applications in various fields, such as laser printing, information
storage, displays,14 in vivo imaging15 and solar power conversion.16
Distyryl boron dipyrromethenes (Bodipys) are versatile NIR dyes
of which the absorption and emission properties can be easily tuned
by chemical modification of the p systems.17 Some of the desirable
characteristics of these dyes include strong absorption in the NIR
region, high fluorescence quantum yield, and good solubility and
stability in many solvent systems. As a result, they have been widely
studied as fluorescent probes for metal ions,18 building blocks for
artificial photosynthetic models,19 and photosensitisers for photody-
namic therapy20 and dye-sensitised solar cells.21 Herein, we present
a series of TBET cassettes (5–8, Scheme 1) that combines the prop-
erties of coumarin and Bodipy as well as NIR distyryl Bodipy dyes in
a donor–acceptor chemical architecture. In these cassettes, a rigid and
conjugated phenyl moiety was selected as a linker that may prevent
the donor and acceptor moieties from becoming planar (Fig. S1,
ESI†) and facilitate the TBET process.10 These cassettes display
The energy transfer cassettes composed of coumarin and Bodipy as
well as NIR distyryl Bodipys were designed to capture photonic
energy and convert it to longer wavelength fluorescence emission;
large pseudo-Stokes’ shifts (up to 410 nm) and efficient energy
transfer efficiencies (up to 98.6%) as well as a marked antenna effect
were demonstrated.
Excitation energy transfer (EET) is a vital process in nature, and
primarily responsible for ensuring that the reaction-center complex is
adequately supplied with photons.1 For example, photosynthetic
organisms make use of chromophore-rich antenna proteins to funnel
the captured light energy to the reaction center where it initiates
a multistep electron-transfer reaction, leading eventually to the
oxidation of water to oxygen and the fixation of carbon dioxide.2
Energy transfer has also found numerous important applications in
many fields, such as measuring distances in biological materials,3
improved efficacy of organic light-emitting diodes,4 chemical
sensors,5 advanced photochromic devices,6 and sensitized solar cells.7
Essentially, EET can take place via two pathways (Fig. 1): fluo-
rescence resonance energy transfer (FRET) and through-bond energy
transfer (TBET). FRET dyads8 are, generally, linked by a noncon-
jugated spacer, and the single excitation wavelength of a donor flu-
orophore results in emission of the acceptor at a longer wavelength.
The efficiency of FRET is primarily controlled by two parameters:9
(1) the distance between the energy donor and acceptor fluorophores,
and (2) the spectral overlap between the emission spectrum of the
energy donor and the absorption spectrum of the energy acceptor.
Virtually, the strong overlap requirement in FRET systems places
a limitation on the flexibility of selection of the donors or acceptors,
and therefore limits the number of FRET-based EET systems to
some extent. By contrast, in TBET systems,10–13 the donor and the
acceptor fragments are joined via a conjugated spacer (for example
phenyl and ethynylphenyl), but steric effects prevent them from
becoming flat and conjugated. Compared with FRET, TBET is not
limited by the known constraint of intense spectral overlap between
the donor emission and the acceptor absorption in FRET, thereby
School of Chemistry and Chemical Engineering, Shanxi University,
Taiyuan, 030006, China. E-mail: guow@sxu.edu.cn
Fig. 1 (a) FRET and TBET cassettes. (b) Energy level diagram illus-
trating the photophysical processes of EET. Excitation of the donor
results in EET to the higher excited state of the acceptor. This state is
rapidly converted to S1, from which the acceptor emission is observed.
† Electronic supplementary information (ESI) available: Experimental
procedure, characterization data, 1H and 13C NMR spectra, and
HRMS for 5–8. See DOI: 10.1039/c1jm12503f
13168 | J. Mater. Chem., 2011, 21, 13168–13171
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Fig. 3 The emission spectra of cassettes 5–8 (excited at the coumarin
donor unit and the core Bodipy units, respectively) and the model donor
3 in ethanol. The efficient energy transfer and the marked antenna effect
are displayed. The concentration of all the compounds is 5 mM.
with DDQ and treatment with BF3–OEt2. The cassettes 6 and 7 were
obtained by the reaction of compound 5 with the corresponding
aldehydes in toluene, using piperidine, glacial acetic acid, and Mg
(ClO4)2 as catalyst. The cassette 8 is the doubly protonated species of
7, and could be easily prepared by treatment of an ethanolic solution
of 7 with ethanol–HCl. These new coumarin–Bodipy compounds
Scheme 1 Synthesis of the coumarin–Bodipys EET cassettes 5–8.
efficient energy transfer efficiencies (up to 98.6%) and a large pseudo-
Stokes’ shift (up to 410 nm) as well as a remarkable antenna effect (up
to 5.5-fold). Specially, cassettes 6–8 work via excitation of a coumarin
donor with blue light (340 nm) followed by efficient energy transfer to
variable distyryl Bodipys-based acceptors, which then emit at the
NIR region varied from 632 to 750 nm. In addition, this TBET
platform was also applied to develop an NIR pH probe.
were fully characterized by H and 13C NMR spectroscopy, and
1
HRMS (ESI†).
As expected, the absorption spectra of cassettes 5–8 reflect the
presence of the separate coumarin and Bodipy units (Fig. 2). They all
have a coumarin donor component, and hence they absorb around
340 nm (the p–p* transitions of coumarin groups). The bands at 501,
641, 700 and 620 nm for 5, 6, 7 and 8, respectively, are attributed to
the S0 / S1 Bodipy transition. For 6 and 7, the high energy parts of
the spectra at 370 nm and 415 nm, respectively, can be assigned to
intramolecular charge transfer (ICT) bands due to electron-donating
methoxy group and dimethyl amino group. In fact, the band at 415
nm of 7 disappeared on protonation, suggesting its ICT property, as
shown in absorption spectra of 8. The above results reveal that there
are very weak electronic interactions between the donor and the
acceptor in the ground state, and the compounds 5–8 behave as
cassettes instead of planar, totally conjugated dyes.
Compound 4, 3-(4-formylphenyl)-coumarin, was prepared
according to a literature procedure.10f As shown in Scheme 1, the
cassette 5 was synthesized by condensation of compound 4 with 2,4-
dimethylpyrrole with added TFA as catalyst followed by oxidation
Fig. 2 Normalized absorption, excitation and emission spectra of
cassettes 5–8 [(a)–(d), respectively] in EtOH. The concentration of all the
compounds is 5 mM. The excitation wavelength for 5–8 is 340 nm.
Fig. 4 The visual emission color of cassettes 5–8 when excited at a hand-
held 365 nm UV lamp. The concentration of all the compounds is 5 mM.
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Table 1 Photophysical properties of 5–8a
Compound
lmax/nm (log 3)
lemb/nm
Ff
ETE (%) f
Dlg
5
6
7
8
340(4.42), 501(4.85)
370(4.88), 641(5.03)
337(4.67), 415(4.54), 700(4.86)
344(4.97), 620(5.00)
512
658
750
632
0.49c
0.36d
—
0.56d
96.0
98.5
98.5
98.6
172
318
410
292
e
a
Measurements recorded in ethanol. b The maximal emission of the cassettes. c Quantum yield estimated with fluorescein as standard (Ff ¼ 0.79 in 0.1 N
NaOH). d Fluorescence quantum yields were determined using rhodamine B (Ff ¼ 0.7 in EtOH) as a standard. e No appropriate standard due to the
range of wavelengths. f The energy transfer efficiency was calculated from {100 ꢀ [1 ꢁ (fluorescence intensity of the donor in the cassette)/(fluorescence
intensity of the free donor)]%}. g Pseudo-Stokes’ shifts of the cassettes 5–8.
The emission spectra of cassettes 5–8 are displayed in Fig. 3. When
cassettes 5–8 in ethanol were excited at 340 nm, the emission of the
coumarin energy donor around 408 nm was almost completely
quenched when compared with that of model coumarin 3, only the
characteristic emission bands of the acceptor Bodipys (512, 658, 750,
and 632 nm, respectively, for 5, 6, 7, and 8) were observed. The weak
emission at 750 nm for 7 is likely because the photoinduced electron
transfer (PET) from the dimethylamino nitrogen to the excited state
Bodipy moiety quenches the emission. This is demonstrated by the
emission spectra of its protonated product 8, where a strong and
hypsochromic-shift emission at 632 nm was observed due to the loss
of lone pair delocalization and inhibition of PET. The visual emission
color of cassettes 5–8 when excited at a hand-held 365 nm UV lamp is
shown in Fig. 4, which is consistent with the above analysis. In
addition, the excitation spectra obtained by collecting emission data
of 5, 6, 7, and 8 at 512, 658, 750, and 632 nm, respectively, match the
absorption spectrum well (Fig. 2), indicating that all transitions
participate in the emission process and the energy transfer
can occur from the coumarin donor to the Bodipy acceptors.
The energy-transfer efficiencies were calculated based on the
equation: E ¼ {100 ꢀ [1 ꢁ (fluorescence intensity of the donor in the
cassette)/(fluorescence intensity of the free donor)]%},10g to be 96.0%,
98.5%, 98.5%, and 98.6%, respectively, confirming the efficient energy
transfer in cassettes 5–8. In addition, it was noted that when the
coumarin donor moiety in 5–8 was excited, the fluorescence intensi-
ties of Bodipys emission were significantly higher than that obtained
by direct excitation of the Bodipy acceptors, suggesting the marked
antenna effect.8h The antenna effect quantified in this way for 5–8 is
3.0-, 1.6-, 2.7-, and 5.5-fold, respectively.
The photophysical data of the cassettes 5–8 are given in Table 1.
Indeed, in these cassettes, the energy is absorbed at wavelengths
corresponding to the donor fragment and emitted at wavelengths
governed by the acceptor part, hence large pseudo-Stokes’ shifts (up
to 410 nm) were observed, which is, to the best of our knowledge,
much larger than those of the reported TBET cassettes,10 as well as
those of the typical FRET-based systems.8
Due to the large differences of the emission properties between
cassette 7 and its protonated product 8, cassette 7 is expected to be
a sensitive pH probe. As shown in Fig. 5, with decreasing the pH
value from 7.5 to 1.5, the emission at 750 nm gradually decreased;
concomitantly, the emission at 632 nm was drastically enhanced due
to the inhibition of the ICT and PET processes upon protonation of
the basic dimethyl amino groups. The fluorescence quantum yield
finally reaches a value of 0.56, and the fluorescence of the solution
also turned to bright red. The change of fluorescence intensity at
632 nm based on the pH variation is over 1460-fold, suggesting the
high sensitivity of 7 to a minor pH change. There was a good linearity
between the fluorescence intensity and pH in the range from 1.9 to 3.4
(Fig. 5b). The regression equation was YF ¼ 1.91 ꢀ 106 ꢁ 5.65 ꢀ
105XpH with a linear coefficient of 0.992.
In summary, we have synthesized and characterized a series of
coumarin–Bodipy/distyryl Bodipy EET cassettes. The important
features of this class of EET cassettes are the efficient energy transfer
efficiencies, and the large pseudo-Stokes-shifts, as well as the
remarkable antenna effect. In addition, this EET platform was also
applied to develop an NIR pH probe.
We would like to thank the Natural Science Foundation of China
(NSFC No. 20772073 and 21072121) and the Natural Science
Foundation of Shanxi Province (2008011015) for financial support.
Notes and references
Fig. 5 (a) pH-dependence of the fluorescence intensity of cassette 7
(5 mM) with pH decrease from 7.0 to 1.5. Spectra were obtained with
excitation at 340 nm in aqueous solution containing 80% ethanol as a co-
solvent. (b) The linear relationship of fluorescence intensity at 632 nm
and varying pH values from 3.4 to 1.9.
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