Unprecedented laser action from energy transfer in multichromophoric BODIPY cassettes

Article abstract of DOI:10.1039/c1cc13874j

A cassette molecule, featuring direct integration of two donor BODIPY units to one acceptor BODIPY unit, was conveniently developed as the first highly "through-bond energy transfer" (TBET) laser dye. This multicolor absorbing dye exhibited highly efficie

Full text of DOI:10.1039/c1cc13874j

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COMMUNICATION
Unprecedented laser action from energy transfer in multichromophoric
BODIPY cassettesw
Yi Xiao,*a Dakui Zhang,a Xuhong Qian,*b Angel Costela,c Inmaculada Garcia-Moreno,*c
Virginia Martin,c M. Eugenia Perez-Ojeda,c Jorge Banuelos,*d Leire Gartziad and
d
´
Inigo Lopez Arbeloa
Received 29th June 2011, Accepted 1st September 2011
DOI: 10.1039/c1cc13874j
A cassette molecule, featuring direct integration of two donor
BODIPY units to one acceptor BODIPY unit, was conveniently
developed as the first highly ‘‘through-bond energy transfer’’
(TBET) laser dye. This multicolor absorbing dye exhibited
highly efficient and photostable laser action under drastic pumping
conditions.
donor–acceptor distance and favours ultrafast excited energy
transfer. The strong steric congestion blocks the twist of the
single bond spacer and forces the connected two parts to take
a perpendicular orientation, avoiding the system to behave as
a single conjugated dye.
The absorption spectrum of ET-2 is essentially equal to the
sum of the absorption bands of the free BDP-1 donor (500 nm)
and BDP-2 acceptor (570 nm) components (Fig. 1). This
indicates that the ET-2 fluorophore behaves as a cassette since
both chromophoric units are electronically decoupled in
the ground state and remain as individual patterns in the
multichromophoric compound, according to the theoretically
predicted optimized ground-state geometry (Fig. 1, inset). The
fluorescence spectrum of ET-2 under excitation at 470 nm,
where the donor absorbs intensively while the acceptor hardly
absorbs, results in a very weak residual emission near 510 nm
from the donor and a strong emission centred at 600 nm from
the acceptor (Fig. 1). Thus, the shift between fluorescence and
absorption maxima is higher than 6000 cmÀ1 exciting the
cassette at the S0 - S1 donor transition. In addition, the
corresponding excitation spectrum of ET-2, recorded moni-
toring the emission in the acceptor region (660 nm), perfectly
matches the corresponding absorption spectrum and clearly
shows the absorption bands of the parent chromophores.
Renewed interest has been focused on the design and synthesis
of fluorophores with enhanced optical properties for newer
applications in medical, analytic, physics, and biophysics
science and technology.1 Although a wealth of organic dyes
are known, unfortunately they still have to overcome some
limitations related to: (1) low absorption over broad spectral
regions and (2) high photobleaching rate, which becomes
especially important under drastic excitation conditions such
as those required in single molecule detection methods.2 An
attractive approach to overcome these drawbacks has been the
straightforward synthesis of energy transfer cassettes to
span the absorption spectra of dyes, which result in large
pseudo-Stokes shifts and, not least important, in improved
photostability.3 This powerful strategy has been exploited in
light-harvesting systems and sensors but never focused to
enhance lasing properties for advanced applications.
To address this issue, we have synthesized a rigid energy
transfer cassette, ET-2, via a facile and efficient strategy
tandem Sonogashira coupling (Chart 1 and ESIw). Different
from other previous multichromophoric dyes with donor and
acceptor connected by flexible and relatively long chains,3
trifluorophoric ET-2 features compact integration of two
donor BODIPY (BDP-1) units to one acceptor BODIPY
(BDP-2) by a single C–C bond which leads to a short
These results indicate
a highly efficient intramolecular
excitation energy transfer from the donor to the acceptor.
Fluorescence quantum yield data provide also solid evidence
for this energy transfer phenomena. Since the corresponding
bands from the donor and acceptor units are resolved in the
ET-2 fluorescence spectrum, separate quantum yields (F) for
a State Key Laboratory of Fine Chemicals, Dalian University of
Technology, Dalian 116012, China. E-mail: xiaoyi@dlut.edu.cn;
Fax: 86 411 84986251; Tel: 86 411 84986252
b Shanghai Key Laboratory of Chemical Biology, East China
University of Science and Technology, Shanghai 200237, China.
E-mail: xhqian@ecuct.edu.cn
c
´
Instituto de Quımica-Fısica ‘‘Rocasolano’’, CSIC, Serrano 119,
28006 Madrid, Spain. E-mail: iqrfm84@iqfr.csic.es
´
d
´
Dpto Quımica Fısica, Universidad del Paıs Vasco (UPV/EHU), 644,
48080 Bilbao, Spain. E-mail: jorge.banuelos@ehu.es
´
´
w Electronic supplementary information (ESI) available: Experimental
procedures and full characterizations data. See DOI: 10.1039/c1cc13874j
Chart 1 Molecular structures of ET-2 cassette and donor BDP-1 and
acceptor BDP-2 units.
c
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transition moments in the ET-2 molecule eliminates the
orientation term k in the Forster equation.5 Thus, even though
¨
the spectral overlap between the donor emission and the
acceptor absorption is good, the dipole–dipole contribution
to the intramolecular energy transfer should be negligible, and,
consequently, this process should take place via a through-
bond mechanism.6
Time-resolved emission techniques allow getting insight into
the dynamics of the energy transfer process. The acceptor
fluorescence time profile in the ET-2 cassette appears to be
monoexponential and is essentially identical to that of the free
BDP-2 dye. However, the lifetime of the donor, 3.55 ns, is
drastically affected by the presence of the acceptor, since upon
binding to BDP-2 in ET-2, its fluorescence decay becomes
multiexponential (Table 1). The lowest contributions corres-
pond to a long lifetime (3.17 ns), assigned to the residual
donor emission, and to an intermediate lifetime (1.68 ns)
whose assignment will be discussed later. The main contri-
bution corresponds to very short lifetime (B20 ps), in the limit
of our single-photon counter time-resolution, which is
assigned to the energy transfer process. This fast emission
from the donor together to the highly increased fluorescence
quantum yield from the acceptor (59%), confirm the high
Fig. 1 Absorption (black), fluorescence (lexc = 470 nm, black) and
excitation (lem = 660 nm, green) spectra of ET-2, together with
transition moment orientations. The absorption spectra of BDP-1
(blue) and BDP-2 (red) are also shown.
both emissions can be defined and calculated (Table 1). The
fluorescence quantum yield of ET-2 (0.59) is almost inde-
pendent of the excitation wavelength, within the donor and
acceptor absorption ranges, and is quite similar to that
exhibited by the free BDP-2 component excited at 575 nm.
The absence of the fluorescence quantum yield dependence
suggests almost complete energy transfer from the donor
to the acceptor. Furthermore, taking into account that the
BDP-2 dye excited at 470 nm exhibits a fluorescence quantum
yield near zero, the ET-2 dye allows reaching a significant
‘‘fluorescence enhancement’’, defined as the ratio of the fluores-
cence intensity of the cassette to that of corresponding acceptor
excited at the short wavelength. The rigidity of the ET-2
cassette plays a main role since this avoids the deactivation
via internal conversion mechanism and enhances the efficiency
of the energy transfer process. So, the cassette ET-2 not only
extents the absorption of BDP-2 into the blue spectral region
but also converts it in a more sensible chromophore activated
for two donor fragments.
efficiency of the energy transfer (kET B 5 Â 1010
s
À1) via a
through-bond mechanism.6 Scanning the gated fluorescence
spectrum of ET-2 at different delay times following excitation
at 470 nm produces a sequence of time-resolved emission
spectra (Fig. 2), which show a decay of the donor emission,
quenched by this fast energy transfer, and the growth of the
acceptor emission.
The photophysics of the ET-2 cassette is sensitive to its
concentration, which allows the unambiguous assignment of
the intermediate lifetime observed above. The analysis of its
fluorescence, upon excitation at 470 nm and at high concen-
tration (0.7 mM), reveals an increase in the donor/acceptor
emission ratio with respect to that recorded in diluted solutions
together to a different weight of each lifetime in the multi-
exponential fit of the decay curve (Table 1). This result could
be indicating that some energy may be transferred through an
In the ET-2 cassette, the donor and acceptor units con-
nected by conjugated linker may transfer energy via different
pathways,4 including ‘‘through-space’’ energy transfer as well
as other pathways usually referred to as ‘‘through-bond’’
energy transfer. To ascertain the contribution of each to the
whole energy transfer process may not be a simple matter,
especially in cassettes with donor and acceptor fragments
relatively close and with flexible linkers. As shown in Fig. 1,
the perpendicular disposition of the donor and acceptor
intermolecular Forster resonance process (inter-FRET) via
¨
dipole–dipole coupling assuming randomly distributed neigh-
boring transition moments. Hence, the overall excitation
energy transfer in ET-2 should result from the contribution
of intramolecular through-bond and intermolecular through-
space mechanisms. Therefore, at high concentration, the
lifetime component assigned to the through-bond energy
transfer increases, from 20 to 100 ps, decreasing its contribution,
Table 1 Photophysical properties in ethyl acetatea
F
Dye
e/labs
lexc
lem
t/ns
BDP-1
BDP-2
ET-2
71600 (503) 470
65800 (568) 575
127000 (503) 470
68200 (569)
510
600
510 B0
0.66 3.55
0.57 3.78
0.020 (92%)
1.68 (5%)
3.17 (3%)
(2 mM)
470
575
470
600
600
510
0.59 3.46
0.59 3.46
ET-2
(0.7 mM)
0.103 (27%)
1.91 (54%)
3.58 (19%)
3.48
470/575 600
e/MÀ1cmÀ1: molar absorption; labs, lexc, lem (nm): absorption,
excitation and emission wavelengths, F: fluorescence quantum yield;
t: fluorescence lifetime.
a
Fig. 2 Normalized time-resolved emission spectra of ET-2 at different
times after excitation; 0.06, 0.3, 1, 2 and 4 ns.
c
11514 Chem. Commun., 2011, 47, 11513–11515
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Table 2 Laser properties (efficiency, Z and maximum output wavelength lout/nm) of ET-2 and BDP-2 dependent on pump sources and solvents
ET-2
BDP-2
500
532
570
500
532
570
l
pump/nm
1.4 Â 10À4
8.0 Â 10À4
2.2 Â 10À4
1.2 Â 10À3
4.0 Â 10À4
1.6 Â 10À4
c/M
Solvent
Z
lout
Z
lout
Z
lout
Z
lout
Z
lout
Z
lout
Ethyl acetate
THF
CH2Cl2
32%
21%
26%
609
612
613
32%
—
23%
624
—
624
39%
24%
28%
610
615
616
15%
10%
11%
622
612
623
35%
24%
30%
619
622
619
36%
24%
28%
610
619
616
from 92 to 27%, to the total fluorescence decay curve of ET-2
at 510 nm. At the same time, the contribution of the longest
lifetime increases, according to the observed increase of the
donor emission, which could be related to donor–donor
energy migration (see ESIw). Even more significant is the
increase of the intermediate lifetime, attributed to the donor–
acceptor inter-FRET process, which appears also in diluted
solution but with a ten times lower contribution (Table 1). The
lifetime assignment is also confirmed adding free donor or
acceptor to highly concentrated solutions of ET-2 (ESIw).
The optimized photophysics of the ET-2 cassette leads to its
enhanced behaviour as a laser dye compared to that of the
donor and acceptor components, since it lases efficiently under
pumping at three different relevant wavelengths; 500, 532 and
570 nm (Table 2). Upon pumping at 500 nm, the laser proper-
ties of BDP-2 are inferior due to its lower absorption in this
region, which requires a concentration 10 times higher than
that of ET-2 (Fig. 3a). But even at this concentration, or
because such a high concentration is required, the highest laser
efficiency of BDP-2 is only 15%, less than half the efficiency
reached with ET-2 (32%). In addition, under excitation at
500 nm, the lasing threshold of BDP-2 (0.9 mJ) is much higher
than that of ET-2 (0.4 mJ). Moreover, ET-2 exhibits excellent
photostability, even better than that of BDP-2, since ET-2
maintains 100% of its initial emission after 1 Â 105 pump
pulses at 10 Hz repetition rate, while BDP-2 loses 10% of its
initial emission under the same pumping conditions (Fig. 3b).
These encouraging results led us to initiate a systematic
investigation on the applications of ET-2 in solid lasers. In a
preliminary study, this dye dissolved in PMMA and pumped
at 500 nm lases at 620 nm with an efficiency of 24% and high
photostability, maintaining 93% of its initial emission after
1 Â 105 pump pulses.
In summary, the highly straightforward synthesis of an
efficient ‘‘through-bond energy transfer’’ cassette allows
enhancing significantly the already valuable photophysical
and laser properties of BODIPYs. Compared to ‘‘unmodified’’
BODIPY dyes, the new multichromophoric system spans the
absorption spectrum into a broad region, exhibiting high
molar extinction coefficients with enhanced fluorescence
emission and increased Stokes shifts. This enhanced photo-
physics assures an unprecedented highly efficient and photo-
stable laser action under drastic pumping conditions from a
cassette in both liquid solutions and solid-state, suggesting
that the new photonic system could perform outstandingly in
advanced applications in optoelectronics and biophotonics.
Y. X. thanks National Natural Science Foundation of China
(No. 20876022) and the Fundamental Research Funds for the
Central Universities (No. DUT10ZD114), and I. G.-M. and
I. L. A. thank Spanish MICINN (MAT2010-20464-C04-01 and
-C04-4, respectively, and TRACE2009-0144) for financial support.
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Fig. 3 (a) Laser efficiency of ET-2 (black) and BDP-2 (red) depen-
dent on the concentrations in ethyl acetate (lpump = 500 nm)
(b) normalized laser output of ET-2and BDP-2 as a function of the
number of pump pulses at 532 nm, 5.5 mJ and 10 Hz repetition rate.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11513–11515 11515