Towards unimolecular luminescent solar concentrators: Bodipy-based dendritic energy-transfer cascade with panchromatic absorption and monochromatized emission

Article abstract of DOI:10.1002/anie.201104846

A polymer-embedded dendritic, bodipy-based panchromatic absorber with a built-in energy gradient concentrates incident solar radiation at a terminal chromophore, resulting in a monochromatized emission directed to the sides of the polymer waveguide (see p

Full text of DOI:10.1002/anie.201104846

DOI: 10.1002/anie.201104846
Dendritic Solar Concentrators
Towards Unimolecular Luminescent Solar Concentrators: Bodipy-
Based Dendritic Energy-Transfer Cascade with Panchromatic
Absorption and Monochromatized Emission**
O. Altan Bozdemir, Sundus Erbas-Cakmak, O. Oner Ekiz, Aykutlu Dana, and
Engin U. Akkaya*
Today, efficient and effective utilization of solar energy is a
high-priority target and is expected to be even more so in the
near future.[1] For the large-scale exploitation of the stellar
energy source, cost is always the major prohibitive item. The
use of polycrystalline silicon,[2] amorphous thin films of
silicon,[3] or alternative semiconducting materials such as
Cu(In,Ga)Se2 (CIGS),[4] together with dye-sensitized solar
cells[5] already have or are expected to have big impacts on the
production costs, but more effort in all aspects of the solar
energy transduction is needed. One approach is to break
down this massive problem into relatively easily addressable
components, such as absorption of solar photons and con-
version of absorbed solar energy into electricity. Installation
and transmission of the produced electrical energy are two
other components, which are essentially engineering prob-
lems. For the efficient absorption of the solar radiation
component, it has been known for some time that even
without major changes in solar cell design, it should be
possible to obtain substantial enhancements by making use of
solar concentrators.[6] Optical solar concentrators have been
around for the last four or five decades, however, overheating
is always a troublesome issue, with an additional need for
solar tracking with most optical concentrators.[7] Luminescent
solar concentrators on the other hand seem to be more
promising.[8] Conversion of the incident solar radiation into
monochromatized light is expected to lead to a large
enhancement in the efficiency of solar cells. Key features of
the luminescent solar concentrators are the dispersed dye or
dyes in a transparent waveguide. Through total internal
reflection, reemitted light is trapped within a plastic or glass
matrix, and photovoltaic units are fixed to the sides through
which the light is channelled out. The advantages are striking:
no tracking or cooling is needed and much smaller areas have
to be covered by expensive solar-cell components. However,
such concentrators are not free from problems; self absorp-
tion of the emitted light is a major problem.[9] Recently a
different luminescent concentrator design that made use of a
mixture of dyes in amorphous thin films placed in a tandem
design with one terminal absorber was reported.[10] The other
two dyes absorb light at different wavelengths and are
expected to transfer the excitation energy to the terminal
absorber. The intermolecular Fçrster energy transfer (FRET)
was invoked as the operational mechanism of the energy
transfer. With the assumption of efficient intermolecular
energy transfer in the solid (gel) phase, the only emission will
be at the longer wavelength region with large pseudo-Stokes
shifts, thus minimizing self-absorption.
The intermolecular energy-transfer efficiency is an impor-
tant limiting factor that requires high concentrations of the
dyes for optimal results, but higher concentrations will lead to
larger losses caused by self-absorption.[9] Herein, we propose
that this apparent dilemma can be addressed at least in
principle, by replacing a cocktail of dyes with a dendritic light-
harvesting energy gradient with a core molecule as the
terminal absorber and emitter. In the dendritic system,
energy-transfer efficiency will remain high, regardless of its
concentration within the matrix.
Unimolecular energy gradients have been reported pre-
viously[11] with a number of peripheral antenna molecules and
a core chromophore absorbing at a longer wavelength.
Typically, they are characterized in solution. In this work
however, we explicitly targeted an energy cascade system SC
composed of bodipy dyes (see below) with varying degrees of
substitution with styryl groups. This approach will ensure
strong absorption in most parts of the visible spectrum,
however, through efficient energy-transfer processes, emis-
sion is expected to originate only from the terminal absorber.
An optimal solar cell placed on the sides of the matrix is
expected to produce efficient and cost-effective conversion.
In addition, we wanted to demonstrate the efficiency of every
single step of cascading energy transfers; to that end we
synthesized energy-transfer modules of ET-1, ET-2, and ET-3.
Bodipy dyes are highly versatile chromophores[12] and can
be conveniently derivatized[13] to span the entire visible
spectrum and beyond, showing exceptional photochemical
and photophysical qualities. These properties of Bodipy dyes,
including sharp absorption and emission maxima, were
previously exploited in energy-transfer modules. In our
design, the goals were to optimize the absorption in a large
part of the visible spectrum and also the conversion to
emission centered at 672 nm, which is ideally suited for
[*] Dr. O. Altan Bozdemir, S. Erbas-Cakmak, O. O. Ekiz, Dr. A. Dana,
Prof. Dr. E. U. Akkaya
UNAM-Institute of Materials Science and Nanotechnology
Bilkent University, Ankara 06800 (Turkey)
E-mail: eua@fen.bilkent.edu.tr
Prof. Dr. E. U. Akkaya
Department of Chemistry, Bilkent University
06800 Ankara (Turkey)
[**] We are grateful for funding by BOREN, Turkish academy of Sciences
(TUBA), and State Planning Organization (DPT).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201104846.
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
10907

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efficient photovoltaic conversion when coupled to a GaAs or
InGaAs solar cell.
The absorption, emission, and excitation spectra of the
dendritic concentrator and the reference energy-transfer
modules were acquired in chloroform. Absorption spectra
show bands in accordance with the number and the type of
chromophore units. The absorption maximum for peripheral
bodipy units is located at 527 nm, the one for intermediate
monostyryl-bodipy units at 590 nm, and the one for core
distyryl-bodipy at 655 nm, respectively (Figure 1). Eight
peripheral antenna bodipy chromophores present a large
absorption cross-section (540000mÀ1 cmÀ1) in the middle of
The envisioned synthesis makes use of a convergent
dendrimer build-up approach (Scheme 1) with strategically
placed Huisgen-type click components (terminal alkyne and
azide groups). This approach not only allows the synthesis of
final target compounds but also various modules of energy
transfer can be synthesized in a straightforward sequence of
reactions. Synthesis details and the compound structures are
given in the electronic Supporting Information.
Scheme 1. Synthesis and schematic representations of ET molecules and SC. a) piperidine, AcOH, benzene, reflux; b) CuSO4·5H2O, Et3N, Sodium
Ascorbate, THF:H2O; c) NaN3, B18C6, acetonitrile, 608C. Synthetic procedures for each aldehyde, compounds 5, 6, and all other reaction details
and explicit structures can be found in the Supporting Information.
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Angew. Chem. Int. Ed. 2011, 50, 10907 –10912

Figure 1. Absorption spectra (in CHCl3) of the target solar concentrator
SC and separate chromophoric units Bod (dashed), mono-BOD-2
(dot), di-BOD-2(dashed-dot) functionalized with “clickable” groups.
Concentrations of the dyes were adjusted so that the dyes have equal
absorbance at the maxima.
Figure 3. Schematic representations of ET modules and the solar
concentrator SC with relevant energy-transfer-efficiency values. Calcu-
lations are based on either quantum yield (ET-2, ET-3, SC) or lifetime
(ET-1) changes.
the visible range, whereas four monostyryl-bodipy dyes
absorb in the yellow-orange region of the visible spectrum.
Emission spectra collected by exciting the chromophores at
different wavelengths show energy funneling in accordance
with our design goals. Excitation spectra obtained at 675 nm
show direct evidence of energy transfer with sharp shorter-
wavelength maxima that correspond to energy flow from the
peripheral antenna units to the red-emitting core (Figure 2).
Energy transfer modules (ET1–3) have been helpful in
establishing the efficiency of energy transfer along the
cascade. Thus, we determined maximal energy-transfer effi-
ciencies of 98% in the module ET-1 based on the decrease in
excited-state lifetimes, 91% for ET-2, and 93% in ET-3, based
on changes in the quantum yields of the energy-donor
moieties (Figure 3).
most shorter wavelength absorbing chromophores transfer
energy to the mono-styryl and distyryl core unit with a total
efficiency of 97%. Intermediate monostyryls are also efficient
in transferring energy to the core unit (90%). The emission
spectrum of the SC shows residual peaks corresponding to
direct emission from the outer chromophores. However, this
emission leak is insignificant compared with the emission
peaks of the FRET-decoupled “free” bodipy dyes of com-
parable structure in the dye mixture (Figure 4). Table 1 lists
SC also shows highly efficient energy-transfer cascades
between the spectrally divergent chromophores. The outer-
Figure 4. Emission versus excitation spectra: a) dye mixture (BOD, mono-
BOD-2, and di-BOD-2) and b) SC (2.5 mm) in epoxy resin. Dye concen-
trations were adjusted to have absorbance values equal to that of SC at
the peak values.
relevant photophysical parameters for energy transfer.
Energy-donor moieties show decreased quantum yields with
concomitant reduction in the emission lifetime as expected.
Another important parameter to be considered for solar
concentrators is the self-absorption within the dye assembly,
which is often quantified by a factor S. This factor is the ratio
of the absorbance of the energy donor at its maximum value
to the absorbance at the emission maximum of the acceptor
Figure 2. Excitation spectra of SC (solid) and the energy-transfer
modules ET-1 (dashed), ET-2 (dot), ET-3 (dashed-dot). Emission data
were collected for excitation at 605 nm in the case of ET-1 and 675 nm
for the other ET modules and SC.
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Table 1: Molar absorption coefficients, emission lifetimes, absorption, and emission maxima, energy-transfer efficiencies and rate constants of the
dendrimer SC and relevant compounds synthesized herein.
[b]
[b,c]
[b]
[d]
[d]
Dye
labs
[nm]
e
lF
fF[a]
[lexc(nm)]
t1
t2
t3
KET
eET
[mÀ1 cmÀ1
]
[nm]
[ns]
[ns] (%)
[ns]
[ꢀ10À9 sÀ1
]
BOD
526
590
590
651
653
527
590
590
654
590
656
527
590
655
76000
59000
63000
71000
72000
120000
61000
132000
64000
270000
88000
540000
299000
82000
539
605
605
670
672
539
605
605
672
605
672
539
605
672
0.80 (488)
0.85 (550)
0.89 (550)
0.67 (610)
0.68 (610)
0.14 (488)
0.39 (550)
0.08 (550)
0.42 (610)
0.06 (550)
0.31 (610)
0.02 (488)
0.09 (550)
0.32 (610)
4.30
–
–
–
–
–
–
–
–
–
4.97
–
–
–
–
–
–
–
–
–
–
0.98
0.91
0.93
mono-BOD-1
mono-BOD-2
di-BOD-1
di-BOD-2
ET-1
5.57
4.57
–
–
5.27
–
0.07
11.42
ET-2
ET-3
SC
–
2.75 (36)
5.45 (64)
1.85 (13)
4.28 (87)
0.87 (12)
4.16 (88)
5.67
4.54
4.46
2.21
2.90
–
0.11
7.52[e]
1.97[f]
0.97[e]
0.90[f]
[a] Quantum yields were calculated using rhodamine G6 (excitation at 488 nm in H2O), sulforhodamine 101 (excitation at 550 nm in EtOH) and cresyl
violet (excitation at 610 nm in MeOH) as standard chromophores. Integration values for each FF data point was obtained by selecting the area under
the corresponding emission maximum. [b] The dye laser excitations were carried out at 495, 609, and 667 nm for t1, t2, and t3 respectively. [c] The two
different values are caused by two exponential decay models, contribution percentages to decay are shown in parentheses. Decay paths with
contributions less than 1% were neglected. [d] Steady-state approach based on change in quantum yields was used for ET-2, ET-3, SC (excitation at
590 nm). For all other compounds, a time-resolved approach based on decrease in lifetime was used. [e] The calculation was carried out using the
change in t1. [f] The calculation was carried out using the change in fF at 605 nm.
dye (terminal chromophore). The outer, unmodified bodipy
units thus give an S factor of 10000 in SC; which is a truly
remarkable value, showing the potential of bodipy-based
dendritic cascades in solar concentrator design.
To further illustrate the superiority of the unimolecular
energy-cascade design, we acquired comparative excitation
versus emission spectra for the SC and a collection of the
individual dyes (BOD, mono-BOD-2, and di-BOD-2) at
absorbance values equal to that of SC at their respective
maxima in epoxy resin waveguide slabs (Figure 4). The data
clearly shows energy transfer from the periphery to the core
in SC with almost total annihilation of the peripheral
emission. With the simple mixture of dyes in a polymer
matrix, most of the emission originates from the shortest-
wavelength dye without much of an energy transfer (Fig-
ure 4a). Emission and excitation spectra of SC and ET
molecules in solution (Figure S2–S14) and in epoxy resin
(Figure S15–S17) can be found in the Supporting Information.
The emission from the dye mixture and the molecular
concentrator SC embedded in an epoxy slab was investigated
as a function of the distance between excitation and emission
positions. The refractive index of the slab was n = 1.5 and the
emitted light was partially captured in the slab by total
internal reflection. In each absorption/emission event, about
75% of the light is retained in the slab, and approximately
25% escapes. The slab was excited by a 0.5 mm diameter laser
beam at 532 nm wavelength and the emitted light was
collected at the edge of the slab by an optical fiber connected
to a spectrometer (Figure 5a). As the position of the
excitation spot is changed away from the edge, the collected
Figure 5. a) Pictorial representation of the experimental setup for the
light shows changes both in intensity and in spectral
acquisition of excitation-distance-dependant spectra. b,c) Monte Carlo
simulation and d,e) experimental results showing the distance depend-
distribution. It is experimentally observed that for the dye
mixture, caused by self-absorption and escape losses, spectral
conversion is limited and the output spectrum is concentrated
ence of emission spectra of the dye mixture (BOD, mono-BOD-2, and
di-BOD-2) and equally absorbing SC both in clear epoxy resin.
10910 www.angewandte.org
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Received: July 12, 2011
Published online: September 22, 2011
mainly to the emission of the peripheral bodipy dyes around
550 nm (Figure 5b). For the SC however, it is clear that
spectral conversion is more efficient and low-energy light
(around 700 nm) is more effectively retained in the slab and
delivered to the sides (Figure 5b). Monte Carlo (MC) analysis
was used to estimate the spectra of photon fields inside dye-
doped slabs (see the Supporting Information for details). MC
analysis qualitatively predicts the distance-dependent spectral
distribution for the dye mixture without energy transfer
(Figure 5d) and for SC with intramolecular energy transfer
(Figure 5e). It must be noted however, that in the MC
simulations, absolute energy-transfer efficiencies for excita-
tion energy transfer (EET) processes are found to be about
30% by steady-state or time-resolved measurements as
opposed to 95% calculated. The energy-transfer efficiencies
of 30–40% estimated by fitting of MC analysis results to
measured spectra (Figure 4a, b, 5b, and c) highlight the fact
that calculations based on just the changes in donor-emission
lifetime and quantum yields clearly overestimate the EET
efficiency, as many other nonradiative modes of de-excitation
are widely ignored.
Keywords: bodipy · dendrimers · energy transfer ·
.
light harvesting · solar cells
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