Vectorial photoinduced energy transfer between boron-dipyrromethene (bodipy) chromophores across a fluorene bridge

Article abstract of DOI:10.1002/chem.201000466

A series of novel multichromophoric, luminescent compounds has been prepared, and their absorption spectra, luminescence properties (both at 77 K in rigid matrix and at 298 K in fluid solution), and photoinduced intercomponent energy-transfer processes have been studied. The series contains two new multichromophoric systems 1 and 2, each one containing two different boron-dipyrromethene (Bodipy) subunits and one bridging fluorene species, and two fluorene-Bodipy bichromophoric species, 6 and 7. Three monochromophoric compounds, 3, 4, and 5, used as precursors in the synthetic process, were also fully characterized. The absorption spectra of the multichromophoric compounds are roughly the summation of the absorption spectra of their individual components, thus demonstrating the supramolecular nature of the assemblies. Luminescence studies show that quantitative energy transfer occurs in 6 and 7 from the fluorene chromophore to the Bodipy dyes. Luminescence studies, complemented by transient-absorption spectroscopy studies, also indicate that efficient inter-Bodipy energy transfer across the rigid fluorene spacer takes place in 1 and 2, with rate constants, evaluated by several experimental methods, between 2.0 and 7.0 × 10⁹S^-1. Such an inter-Bodipy energy transfer appears to be governed by the Foerster mechanism. By taking advantage of the presence of various protonable sites in the substituents of the lower-energy Bodipy subunit of 1 and 2, the effect of protonation on the energy-transfer rates has also been investigated. The results suggest that control of energytransfer rate and efficiency of interBodipy energy transfer in this type of systems can be achieved by an external, reversible input.

Full text of DOI:10.1002/chem.201000466

DOI: 10.1002/chem.201000466
Vectorial Photoinduced Energy Transfer Between Boron–Dipyrromethene
(Bodipy) Chromophores Across a Fluorene Bridge**
Fausto Puntoriero,[a] Francesco Nastasi,[a] Sebastiano Campagna,*[a] Thomas Bura,[b] and
Raymond Ziessel*[b]
Abstract: A series of novel multichro-
mophoric, luminescent compounds has
been prepared, and their absorption
spectra, luminescence properties (both
at 77 K in rigid matrix and at 298 K in
fluid solution), and photoinduced inter-
component energy-transfer processes
have been studied. The series contains
two new multichromophoric systems 1
and 2, each one containing two differ-
ent boron–dipyrromethene (Bodipy)
subunits and one bridging fluorene spe-
cies, and two fluorene–Bodipy bichro-
multichromophoric compounds are
roughly the summation of the absorp-
tion spectra of their individual compo-
nents, thus demonstrating the supra-
molecular nature of the assemblies. Lu-
minescence studies show that quantita-
tive energy transfer occurs in 6 and 7
from the fluorene chromophore to the
Bodipy dyes. Luminescence studies,
complemented by transient-absorption
spectroscopy studies, also indicate that
efficient inter-Bodipy energy transfer
across the rigid fluorene spacer takes
place in 1 and 2, with rate constants,
evaluated by several experimental
methods, between 2.0 and 7.0ꢀ109 sꢀ1.
Such an inter-Bodipy energy transfer
appears to be governed by the Fçrster
mechanism. By taking advantage of the
presence of various protonable sites in
the substituents of the lower-energy
Bodipy subunit of 1 and 2, the effect of
protonation on the energy-transfer
rates has also been investigated. The
results suggest that control of energy-
transfer rate and efficiency of inter-
Bodipy energy transfer in this type of
systems can be achieved by an exter-
nal, reversible input.
mophoric species,
6 and 7. Three
monochromophoric compounds, 3, 4,
ACHTUNGTRENNUNG
Keywords: boron · energy transfer ·
fluorene · luminescence · multicom-
ponent reactions
and 5, used as precursors in the syn-
thetic process, were also fully charac-
terized. The absorption spectra of the
Introduction
[a] F. Puntoriero, F. Nastasi, S. Campagna
Dipartimento di Chimica Inorganica
Photoinduced energy transfer has been studied for several
decades[1] and continues to attract increasing attention since
it plays key roles in both natural and artificial systems deal-
ing with the conversion of light energy into more concen-
trated forms of energy (i.e., electrical[2] and chemical
energy[3]) and/or information technology.[4]
Chimica Analitica e Chimica Fisica
Universitꢁ di Messina and Centro Interuniversitario
per la Conversione Chimica dell’Energia Solare (sede di Messina)
Via Sperone 31, 98166 Vill. S. Agata, Messina (Italy)
Fax : (+39)090-393756
E-mail: campagna@unime.it
[b] T. Bura, R. Ziessel
Laboratoire de Chimie Organique et Spectroscopies Avancꢂes
(LCOSA)
Centre National de la Recherche Scientifique (CNRS)
Ecole de Chimie, Polymꢃres Matꢂriaux (ECPM)
Universitꢂ de Strasbourg (UdS)
25 rue Becquerel, 67087 Strasbourg Cedex 02 (France)
Fax : (+33)390-242689
E-mail: ziessel@unistra.fr
Recently, much of that attention has been concentrated
on organized, supramolecular assemblies, in particular on
multichromophoric systems in which vectorial electronic
energy transfer (EET) is maximized across a rigid spacer
that keeps the chromophores far apart.[5] The role of the
rigid spacer is not limited to a structural one, but it can also
be active in assisting the EET process by means of superex-
change,[6] particularly if the Dexter electron exchange mech-
anism[7] significantly contributes to the process. Further-
more, multichromophoric assemblies for vectorial EET in
which the efficiency and rate of the photoinduced energy-
transfer process can be controlled are highly desirable, since
Homepage: http://www-lmspc.u-strasbg.fr/lcosa
[**] Bodipy=4,4-difluoro-4-bora-3a,4a-diaza-s-indacene.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000466. It includes Figures
SI1–SI4, which show a comparison of absorption spectra, excitation
spectra, and titration processes.
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FULL PAPER
such systems could have a large importance for future devel-
opments in several areas, including information technology
based on controlled light impulse.
wires,[13] multichromophoric scaffoldings dedicated to direc-
tional energy transfer,[14] solar cells,[15] fluorescence-sensing
materials,[16] probes for biolabeling,[17] as well as laser appli-
cations.[18] In some limited cases, engineering allowed for the
production of two-photon-absorbing dyes dedicated to cell
imaging.[19] Recently, new types of liquid crystals,[20] fluores-
cent organogelators,[21] and controlled polymers have been
produced by tailoring specific decoration at the periphery of
the dyes.[22]
Here we report two new multicomponent species, 1 and 2
(see structural formulae in Scheme 1), each of them contain-
ing two different Bodipy dyes connected by a rigid fluorene
bridging subunit. The absorption spectra and luminescence
properties of the multichromophoric assemblies and the vec-
Over the last decade, fluorene, oligofluorene, and poly-
fluorene derivatives have emerged as a promising class of
blue-light-emitting materials for use in polymer-based emis-
sive displays, organic light-emitting devices, and solar cells
mostly because of high photoluminescent and electrolumi-
nescent yields.[8,9] These properties could easily be tuned
through chemical structure modification, and an impressive
set of chemical tools are now available to modify the fluo-
rene backbone at will.[10] Along these lines, we have previ-
ously prepared fluorene derivatives chemically modified in
the 2,7-substitution positions by protected alkyne frag-
ments.[11] These stable building
blocks appeared interesting in
the construction of phosphores-
cent architectures based on
transition metals. In the present
contribution, the central fluo-
rene subunit has been modified
in the central 9,9’-substitution
position to connect a short eth-
yleneglycol chain and to insure
good solvent solubility and to
import polarity, a key tenet for
purification purposes. The p-
conjugated tethers were intro-
duced at the 2,7-position by
means of acetylene chemistry
using palladium(0) cross-cou-
pling reactions.
The use of 4,4-difluoro-4-
bora-3a,4a-diaza-s-indacene
(Bodipy) dyes has recently
become very popular as a result
of their valuable optical proper-
ties.[12] Among these, pro-
nounced stability, a high ab-
sorption coefficient in the visi-
ble portion of the electromag-
netic spectrum, narrow emis-
sion profiles, and outstanding
emission quantum yields ap-
proaching 100% are available
in the best cases. A cornerstone
of these particular dyes is the
possibility to react the methyl
residue at the ortho position of
the dipyrromethene core, to in-
crease the p conjugation, and
to adequately tune absorption
and fluorescence properties.
From these capabilities numer-
ous applications have emerged,
such as use in solar concentra-
Scheme 1. i) Compound 3 (1 equiv), [Pd(PPh3)4] (10 mol%), C6H6/Et3N, 60 8C, 15 h, 25% of 6b, 21% of 6;
ii) compound 5 (1 equiv), [Pd(PPh3)4] (10 mol%), C6H6/Et3N, 60 8C, 15 h, 42%; iii) 4-dimethylbenzaldehyde
tors, artificial molecular scale (1 equiv), toluene, piperidine, Dean Stark, trace p-TsOH, 45%.
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8833

S. Campagna, R. Ziessel et al.
torial photoinduced energy-transfer processes that occur be-
tween Bodipy subunits in and across the bis-
ACHTUNGTRENNUNG(ethynyl)fluorene bridge have been investigated, together
1
2
with the absorption spectra and luminescence properties of
the four model compounds 4, 5, 6, and 7, also shown in
Scheme 1. Furthermore, by taking advantage of the possible
protonation of the pendant aminostyryl substituents of the
acceptor Bodipy subunits of 1 and 2, we also show that a re-
versible perturbation (protonation/deprotonation) can affect
the rate constant and efficiency of the EET process in the
studied compounds.
Results
The target compounds 1 and 2 were prepared as shown in
Scheme 1. The pivotal diethynylfluorene compound 4 was
prepared in three steps from 2,7-dibromofluorene and car-
ries two ethyleneglycol chains at the central five-membered
ring. The cross-coupling reaction between compound 4 and
the Bodipy dye 3[23] is promoted by palladium(0) in the pres-
ence of a base, which quenches the nascent acid generated
during the reaction. This statistical reaction afforded the
mono-Bodipy dye 6 and the disubstituted derivative 6b.
Both compounds were easily separated by column chroma-
tography thanks to the presence of two short ethyleneglycol
chains. A second cross-coupling was realized using the blue
dye 5[24] and the fluorene-substituted Bodipy dye 6 under
our standard experimental conditions, thereby providing the
mixed dye 1 in 42% after purification. The bis-Bodipy dye
6b was reacted with 4-dimethylaminobenzaldehyde to
afford the mixed dye 2 in 45% isolated yield. This Knoeve-
nagel reaction,[25] catalyzed by piperidine under heating at
reflux and following protocols established earlier,[26] was in-
spired by previous work on linear Bodipy molecular-scale
wires.[27] Noteworthy is the fact that no degradation or mul-
tisubstitution was observed under standard conditions. Here
the polarity is imported by the dimethylamino fragment,
which facilitates the purification process. As would be ex-
pected by the type of condensation employed, an E confor-
mation of the double bond was found as revealed by the ob-
served J=16.4 Hz proton–proton coupling constant in the
proton NMR spectra.[28] Finally, the model compound 7 was
prepared in 40% by cross-coupling the blue dye 5 to com-
pound 2-ethynyl-7-trimethylsilylacetylene-9,9-bis[2-(2-meth-
Scheme 2. i) Compound 5 (1 equiv), [Pd(PPh3)4] (10 mol%), THF/Et3N,
608C, 15 h, 40%.
Table 1. Absorption and luminescence data of the studied complexes.
Data are for samples in acetonitrile unless otherwise stated.
Compound Absorption[a]
Luminescence
298 K
lmax [nm]
77 K[b]
(e [mꢀ1 cmꢀ1]) lmax [nm] t [ns]
F
lmax [nm] t [ns]
1
2
600 (75000)
500 (75000)
355 (88000)
605 (75000)
500 (75000)
355 (92000)
326 (55000)
302 (51000)
600 (75000)
385 (17000)
290 (34000)
500 (70000)
335 (60000)
230 (37000)
600 (75000)
340 (65000)
290 (42000)
695
510
–
725
510
–
2.22[c] 0.26[d]
630
510
–
655
510
–
3.30
0.40
–
4.00
0.45
–
0.16[e] 0.06[f]
–
–
0.80[c] 0.15[d]
0.20[e] 0.07[f]
–
–
4
5
333
1.51
0.70
332
1.60
695
510
695
2.00
1.80
2.00
0.26
0.39
0.26
633
510
633
3.80
5.30
3.20
ACHTUNGTRENNUNGoxyethoxy)ethyl]fluorene, itself obtained as a side product
6
7
from the deprotection process that leads to compound 4
(Scheme 2). These new compounds were characterized un-
ambiguously by NMR spectroscopic studies, ESIMS, as well
as by elemental analysis, which support the supposed stoi-
chiometry when double substitution is feasible.
The absorption spectra of solutions of compounds 1, 2, 4,
5, 6, and 7 in acetonitrile at room temperature show intense
bands both in the UV and in the visible region (with the ex-
ception of 4, which only exhibits absorption in the UV
region). In most cases, the absorption bands are structured.
The relevant data is collected in Table 1, and Figure 1,
[a] Only the maxima of the main absorption bands are given. [b] In butyr-
onitrile rigid matrix; please note that all the data are identical to those
obtained in MeOH/EtOH 4:1 (v/v), not shown. [c] Measured in the
lower-energy emission band. [d] Quantum yield obtained by exciting in
the lowest-energy absorption band. [e] Measured in the higher-energy
emission band. [f] Quantum yield obtained by exciting at 480 nm, at
which point absorption is almost exclusively due to the higher-energy
Bodipy dye (see text).
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Bodipy Chromophores
FULL PAPER
energy emission slightly increased when short-wavelength
excitation is used, whereas the luminescence spectra are in-
dependent from excitation wavelength for the other studied
compounds. Figure 4 shows the room temperature emission
spectra of 4, 6, and 7, Figure 5 shows the room temperature
emission spectra of 1 and 2, excited at different wavelengths,
and Figure 6 shows the 77 K emission spectra of all com-
pounds. Luminescence spectra maxima, luminescence life-
times, and quantum yields of all the studied compounds are
gathered in Table 1.
Figure 1. Absorption spectra of solutions of 1 (c) and 5 (a) in aceto-
nitrile.
Figure 2, and Figure 3 show the absorption spectra of all the
compounds.
Figure 4. Emission spectra of solutions of 4 (g), 6 (c), and 7 (a)
in acetonitrile at room temperature.
Figure 2. Absorption spectra of solutions of 4 (a), 6 (g), and 7
(c) in acetonitrile.
Figure 3. Absorption spectra of solutions of 2 (c) and 6 (c) in aceto-
nitrile.
All the studied compounds exhibit luminescence both at
room temperature in acetonitrile and at 77 K in butyroni-
trile and MeOH/EtOH 4:1 (v/v) rigid matrices. In all cases,
compounds 4–7 show a single luminescence feature, which
in most cases exhibits a vibrational progression with mono-
exponential decays; both compounds 1 and 2 exhibit two lu-
minescence features, which decay with different lifetimes.
For the latter two compounds, luminescence spectra are also
slightly excitation-wavelength dependent, with the higher-
Figure 5. Normalized emission spectra of 1 (top panel) and 2 (bottom) in
acetonitrile at room temperature. Excitation is 480 nm (c) or 335/
350 nm (a).
Transient absorption spectra have been performed on so-
ACHUTNGRENlNUG uACTHUNGRTENNUtG ions of 1 and 2 in acetonitrile at room temperature, on
exciting at 400 nm (see Figure 7 for compound 1). For both
1 and 2, two main bleachings are formed immediately after
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S. Campagna, R. Ziessel et al.
Figure 6. Emission spectra of the studied compounds in a butyronitrile
rigid matrix at 77 K. a) 1 (c) and 5 (a). b) 4 (g), 6 (a), and 7
(c). c) 2 (c) and 6 (a). Compounds 1 and 2 are excited at
480 nm. The spectra of the other species are independent of excitation
wavelength.
Figure 7. Top panel: transient absorption spectra of 1 in acetonitrile (ex-
citation 400 nm). The spectra registered between 0.5 and 200 ps are
shown. In the bottom panel, kinetic traces at 650 and 500 nm (complete
time range) are shown.
excitation: the bleaching at higher energy (in the range 450–
520 nm) is recovered on the picosecond timescale and simul-
taneously the bleaching at lower energy (in the range 560–
610 nm) increases, coupled with a transient absorption grow-
ing at wavelengths longer than 620 nm. The low-energy
bleaching is then recovered on a longer timescale (ns), to-
gether with the decay of transient absorption at wavelengths
longer than 620 nm.
In particular, a careful comparison of the absorption spec-
tra (see Figures 1–3) allows us to state that the fluorene
chromophore 4 (which is also present in 2, 6, and 7) has its
main absorption in the range 260–350 nm; the Bodipy spe-
cies that does not contain aminostyryl substituents, present
in 6 (and also present in the multichromophoric species 1
and 2), exhibits its main absorption feature in the region
around 450–520 nm; the aminostyryl-substituted Bodipy
chromophore, which is present in 1, 5, 7, and although
slightly modified in 2, has its main absorption band in the
520–650 nm range. However, whereas the bands assigned to
the absorption features of fluorene derivative 4 and to the
Bodipy chromophores that do not bear aminostyryl substitu-
ents are clearly structured, it can be noted that the absorp-
tion bands due to the aminostyryl-substituted Bodipy chro-
mophore(s) are broader and the structural vibration is less
pronounced. This suggests some charge-transfer (CT) contri-
bution to the Bodipy-based lowest-energy absorption band
of 1, 2, 5, and 7. However, such a contribution is probably
relatively weak, as reported by Baruah et al.[29] for the quite
related dye A, and definitely less pronounced than in the
truxene-containing Bodipy B, the lowest-energy absorption
Discussion
Absorption spectra: Because of the large values of molar
absorption (Table 1), the main absorption bands of the com-
pounds can be assigned to spin-allowed p–p* transitions. It
appears that the absorption spectra of the multichromophor-
ic species are roughly the sum of the absorption spectra of
the individual components (an example is shown in Fig-
ure SI-1 of the Supporting Information, which shows a com-
parison of the absorption spectrum of 1 with that obtained
by the sum of the absorption spectra of 5 and 6).
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Bodipy Chromophores
FULL PAPER
cence properties of 6 and 7, the bis-ethynylfluorene dye
does not exhibit any luminescence, since it is efficiently
quenched by energy transfer to the two peripheral Bodipy
chromophores. In both 1 and 2, two emission bands are evi-
dent (Table 1, Figure 5), which corresponds to the emission
of the higher-energy (non-aminostyryl-substituted) and
lower-energy (aminostyryl-substituted) Bodipy subunits.
However, both in 1 and 2, emission from the higher-energy
Bodipy chromophore derived from 6 is strongly reduced rel-
ative to the corresponding emission of 6 (see Table 1). For
example, in 1, whereas the luminescence quantum yield of
the lower-energy chromophore (calculated by integrating
the emission intensity between 600 and 900 nm and exciting
at 580 nm, in which absorption is exclusively due to the ami-
nostyryl-substituted Bodipy subunit) is 0.26, identical to that
of the parent species 7, the emission quantum yield of the
higher-energy Bodipy (calculated by integrating the emis-
sion intensity in the range 480–600 nm) is 0.06 upon exciting
at 480 nm (at which point absorption is almost exclusively
due to the higher-energy Bodipy) and is reduced to 0.03
upon exciting at 335 nm (at which excitation is largely fluo-
rene-based, which can deactivate to both the peripheral
Bodipy subunits), which is in any case quite reduced relative
to the quantum yield of 6 (0.39). The room-temperature life-
time of the 500 nm emission band is 160 ps, which is also
quite shorter than the luminescence lifetime of 6 (1.8 ns).
Since the excitation spectra of 1 and 2 collected at the emis-
sion wavelength of the lowest-energy Bodipy subunit(s)
fairly overlap with the respective absorption spectra (see
Figure SI-2 in the Supporting Information), we can attribute
the reduced emission of the higher-energy Bodipy subunit in
1 and 2 to an efficient, vectorial photoinduced energy trans-
fer to the lower-energy Bodipy subunit(s) across the fluo-
rene bridge.
band of which is of dominant CT nature and in fact peaks at
noticeably lower energy of 710 nm (in dichloromethane).[14]
Luminescence properties and photoinduced energy-transfer
processes: The luminescence exhibited by 4 is straightfor-
wardly assigned to fluorescence from its lowest-energy p–p*
levels, as indicated by the small Stokes shifts (see Table 1,
Figure 4), the structured shape of the emission band, the
short lifetime, the high quantum yield, and the indepen-
ACHTUNGTRENNUNGdence of the maximum of the emission on passing from a
room-temperature solution in acetonitrile to 77 K (Table 1,
Figure 6); a better resolved vibrational structure is also evi-
dent in the emission spectrum at 77 K, in full agreement
with the p–p* emission assignment. The luminescence of 5
would also be straightforward to assign, as this species is a
monochromophoric compound that contains an aminostyryl-
substituted Bodipy molecule: however, the room-tempera-
ture emission spectrum of this species is broader than those
reported for “non-aminostyryl-substituted” Bodipy mole-
cules,[14] and is significantly blueshifted on passing from
room-temperature fluid solution to 77 K rigid matrix
(Table 1). Such features allow us to attribute the Bodipy-
based emission of 5 to an excited state with a significant CT
nature (from the amino group to the Bodipy subunit), as
also reported for other aminostyryl-substituted Bodipy chro-
mophores.[14,29]
As far as the rate constants of the energy-transfer process-
es in 1 and 2 are concerned, they can be calculated from the
experimental luminescence lifetime and quantum yield data
by Equations (1) and (2):
Both compounds 6 and 7 contain two chromophores, the
fluorene and the Bodipy subunits, therefore in principle
they could exhibit two emission features. However, in both
cases only the lowest-lying, Bodipy-based emission is found,
both at room temperature and at 77 K (Table 1, Figure 4
and Figure 6). Excitation spectra of 6 and 7 (see the Sup-
porting Information) show contributions from both fluo-
rene-based and Bodipy-based absorption bands, so it can be
assumed that quantitative energy transfer from the higher-
energy fluorene subunit to the lower-energy Bodipy subunit
takes place in these species. The emissive excited state of 7,
analogously to what was mentioned above for 5, also exhib-
its a CT character, at least at room temperature.
Compounds 1 and 2 are both made of three chromo-
phores, a bridging fluorene subunit, and two peripheral
Bodipy-based chromophores (see structural formulae in
Scheme 1). In both cases, the aminostyryl-substituted
Bodipy molecule is the lowest-lying chromophore of the
whole assembly. As expected on the basis of the lumines-
F0
ꢀ 1
F
ð1Þ
ð2Þ
ken
ken
¼
¼
t0
1
1
ꢀ
t0 t0
In Equations (1) and (2), F0 and F are the unquenched
and quenched quantum yields, respectively, and t0 and t’ are
the unquenched and quenched lifetimes, respectively.[30]
From Equation (1), the rate constant ken of the higher-
energy Bodipy to lower-energy Bodipy energy transfer in 1
is 3.1ꢀ109 sꢀ1, whereas by using Equation (2), the resulting
ken is 5.7ꢀ109 sꢀ1 (see Table 2). On considering the experi-
mental uncertainties of lifetime (10%) and quantum yield
(20%) measurements, such values appear to be in accepta-
ble agreement with one another. As far as 2 is concerned,
the calculated ken values are 2.5ꢀ109 and 4.5ꢀ109 sꢀ1 when
Equations (1) and (2), respectively, are used. Also in this
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S. Campagna, R. Ziessel et al.
Table 2. Rate constants ken [sꢀ1
energy-transfer process in 1 and 2 in acetonitrile at room temperature
and at 77 K in a rigid matrix (sixth column), calculated by different meth-
ods, and other relevant data.
] of the photoinduced inter-Bodipy
corded 400 fs after the flash (see Figure 7): the bleaching
around 500 nm is clearly assigned to the excited state involv-
ing the higher-energy Bodipy subunit, and the bleaching at
about 600 nm is assigned to the excited state that involves
the lower-energy, aminostyryl-substituted Bodipy subunit.
This transient spectrum evolves with time, thus leading to a
recovery of the 500 nm bleach coupled with an enhanced
bleaching at 600 nm. These two spectral changes are clearly
concomitant, as shown by the almost identical time con-
stants measured for the 500 nm bleaching recovery (130 ps)
and for the 580 nm bleaching formation (170 ps). These re-
sults demonstrate that inter-Bodipy energy transfer takes
place with a rate constant of about 6.7ꢀ109 sꢀ1 (150 ps),
which is in quite good agreement with luminescence data
(see above; energy-transfer rate constants, derived from dif-
ferent methods, are collected in Table 2). Moreover, a tran-
sient absorption grows at a longer wavelength with a time
constant (160 ps) that correlates with the energy-transfer
process: this transient is therefore assigned to the lowest-
energy Bodipy-based excited state of CT nature. The transi-
ent absorption spectrum of 1 successively decays on the
nanosecond timescale, in agreement with luminescence data
(Table 1). Figure 8 schematizes the EET process that occurs
in 1.
ken
ken (transi-
ken
JF [cm6][d] ken
[Eq. (2)]
DG
[Eq. (2)][a] ent data)[b]
[Eq. (3)][c]
[eV][e]
77 K
1
2
5.7ꢀ109
4.5ꢀ109
6.7ꢀ109
2.8ꢀ109
9.7ꢀ109
9.9ꢀ109
1.10ꢀ10ꢀ13 2.3ꢀ109
1.12ꢀ10ꢀ13 2.0ꢀ109
0.46
0.54
[a] From luminescence experiments. Equation (1) yields slightly slower
rate constants, but values from Equation (2) are considered more reliable
(see text and ref. [31]), therefore these are the values derived from lumi-
nescence properties that we reported in Table 1. [b] From transient ab-
sorption spectroscopy. [c] From theory (approximated Fçrster equation).
In the approximation, donor–acceptor distance is assumed to be 26.85 ꢅ,
taking into account the CT nature of the acceptor chromophore (for de-
tails, see text). [d] Spectral overlap integral used for the theoretical calcu-
lation of ken expressed as in Equation (4). [e] Driving force of the energy
transfer, approximated from the 77 K emission maxima of the partners
(Table 1).
case, the agreement between the lifetime- and quantum-
yield-derived values is considered acceptable.[31]
Inter-Bodipy photoinduced energy transfer in 1 and 2 also
takes place at 77 K in a rigid matrix. Indeed, luminescence
of the higher-energy Bodipy is significantly reduced in life-
time in the multi-Bodipy spe-
cies relative to the model com-
pound 6 (see Table 1). Rate
constants of the intercompo-
nent quenching process, calcu-
lated by using Equation (2)
(Table 2), are of the same order
of magnitude as those mea-
sured at room temperature and
confirm the energy-transfer
nature of the quenching mecha-
nism.
Definitive support for the oc-
currence of the inter-Bodipy
energy transfer in 1 and 2 are
offered by transient absorption
spectroscopy (Figure 7). Be-
cause of technical reasons, we
were forced to perform ultra-
fast excitation only at 400 nm:
at this wavelength both the pe-
ripheral Bodipy chromophores
that are present in 1 and 2
Figure 8. Energy-level diagram schematizing the energy-transfer process in 1. GS stands for ground state. In
the excited states, the asterisk indicates the Bodipy subunit at which the excited-state energy is located. Solid
and dashed lines stand for radiative and radiationless transitions, respectively. The excited-state energies are
approximated to the 77 K emission maxima (see Table 1) of the two Bodipy subunits.
absorb, therefore selective exci-
tation is not possible. However,
clear-cut conclusions could still
be drawn. Ultrafast excitation
of 1 at 400 nm leads to formation of both excited states that
involve the two peripheral Bodipy dyes. The central fluo-
rene chromophore is not excited since its absorption is neg-
ligible at this wavelength. The two excited states of the dif-
ferent Bodipy chromophores are signaled by the appearance
of two bleachings in the differential absorption spectra re-
An essentially identical situation can be discussed for 2
(not shown). In this case, the rate constant for the energy-
transfer process derived from transient data is 2.8ꢀ109 sꢀ1,
still in fair agreement with luminescence data.
As far as the mechanisms of the energy-transfer processes
that occur in the studied compounds are concerned, we
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Bodipy Chromophores
FULL PAPER
refer to the two main mechanisms usually invoked to ex-
plain EET between weakly coupled partners, that is, the
Fçrster-type (Coulombic) resonance[32] and the Dexter-type
electron exchange.7
almost identical for 1 and 2 (1.10ꢀ10ꢀ13 and 1.12ꢀ10ꢀ13 cm6,
respectively).
With the above-mentioned approximations, Equation (3)
yields 2.9ꢀ1010 and 3.0ꢀ1010 sꢀ1 as kr values for 1 and 2
(Table 2), respectively. These rate constants are in both
cases substantially larger than the experimental values. Such
a discrepancy could mainly be due to the donor–acceptor
distance we assumed between the Bodipy subunits in 1 and
2: in fact, the acceptor partner of the energy-transfer pro-
cess in both 1 and 2 (which is also partly responsible for the
absorption spectra of the acceptor partner) is an excited
state that has a significant CT level, so it is “delocalized” be-
tween the amino group and dipyrromethene framework of
the aminostyryl-substituted Bodipy chromophore. This cer-
tainly modifies the donor–acceptor distance parameter:[36] if
we add to the distance of separation between the carbon
atoms that connect two pyrrole molecules in different
Bodipy dyes mentioned above, halfway between the dipyr-
romethene framework and the amino group a donor–accept-
or distance of 26.85 ꢅ is obtained, and by using this value as
the corrected rAB, the calculated Fçrster rate constants for
the energy-transfer processes that occur in 1 and 2 result in
values of 9.7ꢀ109 and 9.9ꢀ109 sꢀ1, respectively, which are in
much better agreement with the experimental values (see
Table 2). However, whereas it appears that approximations
on the calculated Fçrster rate constants by using Equa-
tion (3) limit the precise determination of such values, it can
be safely stated that the mechanism of the inter-Bodipy
EET in the studied species is Coulombic, Fçrster-type
energy transfer.
The luminescence data of 6 and 7 show that the energy-
transfer processes that occur in these species (that is, EET
from the fluorene-based excited state to the lower-energy
Bodipy-based chromophore(s)) are extremely fast, since
the luminescence of the fluorene subunit, reported in 4, is
totally quenched, with concomitant sensitization of the
Bodipy emission (see above). Since the diethynylfluorene
and Bodipy(s) chromophores are quite close and directly
linked (see Scheme 1), we assume that the Dexter electron
exchange mechanism is responsible for the fast energy-trans-
fer processes, although a Fçrster contribution cannot be ex-
cluded.
The situation is different for the inter-Bodipy energy-
transfer processes that occur in 1 and 2. In these cases,
donor and acceptor chromophores are spatially separated by
a bridge (the fluorene subunit) and the long-range Fçrster
mechanism seems probable. Theoretical values for the
energy-transfer rate constant by means of the Fçrster mech-
anism can be approximately obtained by using Equa-
tion (3),[33] in which keFn is the rate constant of the energy-
transfer process; K is an orientation factor that accounts for
the directional nature of the dipole–dipole interaction (K2 is
2
= for random orientation); F and t are the luminescence
3
quantum yield and lifetime of the donor, respectively; n is
the solvent refractive index; rAB is the distance (in ꢅ) be-
tween donor and acceptor; and JF [see Eq. (4)] is the Fçrster
overlap integral between the luminescence spectrum of the
ꢀ
donor, F(v), and the absorption spectrum of the acceptor,
ꢀ
Protonation effects on the inter-Bodipy energy-transfer pro-
cesses: Bodipy compounds that contain aminostyryl groups
are known to exhibit luminescence properties that strongly
depend, reversibly, on protonation.[14,28,29,37] This is the case
for the Bodipy species A and B, formerly discussed, the
emissions of which are significantly blueshifted and en-
hanced upon proton addition, both in solution and in the
solid state.[28,37] For such species, the lowest-energy absorp-
tion band also moves to higher energies upon protona-
tion.[28,29] Since the absorption features of the aminostyryl
Bodipy molecules have important roles in determining the
energy-transfer rate constants in 1 and 2,[38] it can be fore-
seen that the addition of protons can affect the efficiency of
the photoinduced EET processes in 1 and 2, so protons
could play the role of promoters (or inhibitors) of the
energy-transfer process, with the final result of opening the
possibility of controlling the process by an external, reversi-
ble input.
The addition of triflic acid to a solution of 2 in acetonitrile
(concentration: 3ꢀ10ꢀ6 m) leads to absorption and emission
spectra changes that indicate complete protonation of the
amino group at a 4:1 acid/2 molar ratio; titration indeed evi-
dences a series of isosbestic points, with the lowest-energy
absorption band (Figure 9) moving to the blue, as expect-
ed.[28,29] The luminescence spectrum also moves to higher
energy and the intensity of the low-energy emission (excita-
ꢀ1
e(v), on an energy scale (cm ).
K2F
n4rA6 Bt
kFen ¼ 8:8 ꢁ 10ꢀ25
JF
ð3Þ
ð4Þ
R
FðnÞeðnÞ=n4dn
R
JF ¼
FðnÞdn
It should be noted that Equation (3) (and its usual ap-
proximations mentioned above, including the average figure
assumed for the dipole orientation factor) is a roughly ap-
proximated model of the Coulombic interaction between
weakly interacting chromophores, and although this level of
approximation allows in most cases satisfactory results, it
can require modifications in several cases.[13,34,35]
For the inter-Bodipy energy transfer in 1 and 2, in a first
approximation for the donor–acceptor distance rAB we as-
sumed a value of 22.4 ꢅ, which is the space distance be-
tween the center of the two dipyrromethene subunits, calcu-
lated from energy-minimized computer-generated structures.
More precisely, it is the distance between the carbon atoms
that connect two pyrroles in different dipyrromethene dyes.
Since each Bodipy subunit is freely rotating along the triple-
bond axis, a random value for the orientation factor (0.667)
is assumed. The spectral overlap integral JF results are
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8839

S. Campagna, R. Ziessel et al.
es: in the first one, until a 2:1 acid/5 molar ratio was
reached, several isosbestic points were maintained
(Figure 10), and the main absorption modification took
Figure 9. Absorption spectra changes of 2 upon protonation. Concentra-
tion of 2 is 3ꢀ10ꢀ6 m; in the panel, the molar ratio between added triflic
acid and 2 is reported. The inset shows the changes on emission spectra
(excitation at an isosbestic point, 480 nm) before and after titration.
Figure 10. Absorption spectra changes of 5 upon protonation in acetoni-
trile. Concentration of 5 is 9.4ꢀ10ꢀ6 m; in the panel, the molar ratio be-
tween added triflic acid and 2 is reported. The inset shows the changes
on emission spectra (excitation at an isosbestic point, 595 nm.)
tion wavelength, 480 nm) is significantly enhanced (Figure 9,
inset). Interestingly, the absorption spectral features con-
nected to the higher-energy Bodipy at about 500 nm are not
modified appreciably by protonation, so confirming the ex-
pectation that the protonation process here monitored does
not involve the higher-energy Bodipy. However, Specfit
analysis of the titration followed by means of absorption
spectrum changes indicates that two protons are involved,
thereby forming a 2·(2H)2+ species. The two association
constants k1 and k2 of the two protonation processes are ap-
parently so close that they cannot be solved, and the global
value for logk (=logk1 +logk2) for the 2·(2H)2+ species
thus formed is 13.9 (with (k1 +k2)=8ꢀ1013 mꢀ2).
The luminescence lifetime of the higher-emission compo-
nent of 2 is reduced upon protonation. In particular, the life-
time of the 500 nm emission, which was 200 ps in the ab-
sence of protons (see Table 1), become 140 ps for the
2·(2H)2+ species, which suggests an increase in energy-trans-
fer rate constant (in spite of the reduced driving force, due
to the shift to higher energy of the acceptor chromophore as
shown by the inset of Figure 9), which in the protonated
compound results to be 6.5ꢀ109 sꢀ1 by using Equation (2)
(to be compared with the 4.5ꢀ109 sꢀ1 value obtained for the
unprotonated species from emission lifetime measurements;
the same method, based on Equation (2), is used for calcu-
lating energy-transfer rate constants, to have a homogeneous
comparison). Luminescence quantum yield of the chromo-
phore emissive at 500 nm is also affected by protonation: in
fact, it decreases to about 60% of the value of the unproto-
nated compound (0.04 vs. 0.07). Therefore, luminescence
data indicate that inter-Bodipy photoinduced energy trans-
fer in 2 is accelerated by about 40% upon protonation.
The situation is quite different for 1. Since this species
contains an aminostyryl-substituted Bodipy that bears addi-
tional, potentially protonable units such as the two bipyri-
dine moieties, before analyzing the effect of protonation on
the properties of the multi-Bodipy compound 1, we per-
formed an acid titration of its model species 5.
place in the range 250–350 nm, which is the absorption
range typical of spin-allowed p–p* transitions that involve
bipyridine moieties.[39] In particular, the band maximizing at
287 nm in the starting compound moves to lower-energy and
peaks at 315 nm upon acid addition. Simultaneously, there is
only a small change in the emission spectrum shape, but the
emission quantum yield decreases to 0.17 at the end of this
titration step (it was 0.26 before acid addition; excitation
wavelength was at 595 nm, at an isosbestic point). These
data suggest that protonation does not take place at the
amino group, but it involves the bipyridine moieties.[40] Also
in this case, Specfit analysis indicates that two protons are
involved: we assume that each proton is involved in the pro-
tonation of a single bipyridine molecule, with negligible in-
teraction between the protonated sites. The global value for
logk (=logk1 +logk2) for the 5·(2H)2+ species thus formed
is 13.5 (with (k1 +k2)=3ꢀ1013 mꢀ2).
For acid/5 molar ratios larger than 2:1, a further process,
characterized by several isosbestics, takes place (see Fig-
ure SI-3 in the Supporting Information), which in any case
requires a large excess amount of acid (about 100:1 acid/
compound ratio) to be completed. In this successive process,
the main spectral changes involve a (slight) blueshift of the
low-energy absorption band, and a blueshift of the emission
band, with enhanced emission quantum yield (Figure SI-3,
inset). These changes can be rationalized on the assumption
that further protonation of 5 takes place, and it involves the
amino group.
With the information on the influence of acid addition on
the properties of model 5 in our hands, we performed titra-
tion of 1 (concentration of 1, 3.3ꢀ10ꢀ6 m). Similarly to what
happens for 5, for acid/compound ratios smaller than 2:1, a
first process takes place in 1; this process is clearly evi-
denced by several isosbestic points in the absorption spectra
The addition of acid to a solution of 5 in acetonitrile (con-
centration: 9.4ꢀ10ꢀ6 m) evidenced two consecutive process-
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Bodipy Chromophores
FULL PAPER
and by a net change in the absorption spectrum area con-
nected with bipyridine absorption (see Figure 11). Simulta-
neously, the low-energy emission at about 690 nm decreases
Figure 11. Absorption spectra changes of 1 upon protonation in acetoni-
trile. Concentration of 1 is 3.3ꢀ10ꢀ6 m; in the panel, the molar ratio be-
tween added triflic acid and 2 is reported. The inset shows the changes
on emission spectra (excitation at an isosbestic point, 330 nm).
Figure 12. Illustration of modification in the spectral overlap integral JF
upon protonation of 2. The solid line is the emission spectrum of the
donor; the dashed line is the calculated absorption spectrum of the ac-
ceptor, obtained by subtracting the absorption spectrum of 2—in its un-
protonated (top) and protonated (bottom) forms—the absorption spec-
trum of 3. Changes in the absorption spectrum area upon protonation
are relevant.
in intensity, reaching a quantum yield of 0.17 (in comparison
to the original value of 0.26, see above). Interestingly, the
high-energy emission at about 500 nm does not change ap-
preciably during the titration (see Figure 11, inset). The ti-
tration process, by analogy with the changes occurring in 5,
can be identified as protonation of each bipyridine moiety.
In the case of 1, however, the rate constant and efficiency of
the inter-Bodipy energy transfer are not significantly per-
turbed by protonation, which is different from that found
for 2. Specfit calculation yields a value of logk (=logk1 +
logk2) for the 1·(2H)2+ species thus formed of 13.1 (with
(k1 +k2)=1.2ꢀ1013 mꢀ2). Successively, a large excess of acid
produces absorption changes that can be assigned to proto-
nation of the aminostyryl group, with concomitant blueshift
and enhancement of the low-energy emission (see Figure SI-
4 in the Supporting Information).
The difference between 1 and 2, as far as the influence of
the initial protonation on the inter-Bodipy photoinduced
energy transfer is concerned, would be connected with the
changes in the (donor emission/acceptor absorption) spec-
tral overlap JF: protonation of the aminostyryl substituent of
the acceptor dye in 2 induces a better spectral overlap JF (it
becomes 2.36ꢀ10ꢀ13 cm6, to be compared with the value of
1.12ꢀ10ꢀ13 cm6 of the unprotonated species mentioned
above; see Figure 12), with a concomitant increased efficien-
cy of the energy-transfer process. In 1, the additional bipyri-
dines efficiently compete with the amino group of the ami-
nostyryl substituent for proton coordination, so that pertur-
bation of the spectral overlap JF results is negligible, at least
for moderate acid/compound ratios (Figure 13), and as a
consequence the effect of protonation on the energy-trans-
fer efficiency is also negligible.
Figure 13. Illustration of modification in the spectral overlap integral JF
upon protonation in 1. The solid line is the emission spectrum of the
donor; the dashed line is the absorption spectrum of the acceptor (the
spectra of unprotonated and protonated 7 are used to simulate the ac-
ceptor absorbance in 1). Changes in the relevant absorption spectrum
area were negligible. Compare with Figure 12.
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8841

S. Campagna, R. Ziessel et al.
800 nm). Effective time resolution is around 200 fs; temporal chirp over
the white-light 450–750 nm range is around 150 fs; and the temporal
window of the optical delay stage is 0–3200 ps. The time-resolved data
were analyzed using the Ultrafast Systems Surface Explorer Pro soft-
ware.
Conclusion
Two new multichromophoric systems 1 and 2, each one con-
taining two different Bodipy dye subunits and one bridging
fluorene species, have been prepared, along with two fluo-
rene–Bodipy bichromophoric species, 6 and 7, and three
monomeric models, 3, 4, and 5. The absorption spectra of
the multichromophoric compounds are roughly the summa-
tion of the absorption spectra of the individual components,
thereby demonstrating the supramolecular nature of the as-
semblies. Quantitative energy transfer from fluorene to
Bodipy subunits occurs in 6 and 7, and efficient (>90%)
photoinduced energy transfer also occurs between the two
Bodipy subunits of each multichromophoric species 1 and 2,
across the rigid fluorene bridge, with rate constants in the
109–1010 sꢀ1 time range, by means of the Fçrster mechanism.
Since some Bodipy dyes bear protonable groups such as
aminostyryl substituents, and some of them also carry bipyr-
idine groups, protonation has significant effects on the ab-
sorption spectra and luminescence properties of such spe-
cies. More interestingly, the inter-Bodipy energy-transfer
rate and efficiency in 2 increased upon the addition of acid,
although at a moderate extent (about 40%), whereas acid
addition has a negligible effect on the energy-transfer pro-
cesses that occur in 1, at least for low acid/1 molar ratio
(<2). This different behavior is due to the presence in 1 of
bipyridine residues, which successfully compete with the
aminostyryl moiety for protonation.
Experimental uncertainties are as follows: absorption and luminescence
maxima, 2 nm; emission quantum yields, 20%; emission lifetimes, 10%;
transient absorption decay and rise rates, 10%.
Materials: 2,7-Dibromo-9,9-bis[2-(2-methoxyethoxy)ethyl]fluorene,[43] 8-
(4-iodophenyl)-1,3,5,7-tetramethyl-4,4difluoro-4bora-3a,4a-diaza-s-inda-
cene[23] were prepared according to the literature.
Synthesis of 2,7-di(trimethylsilylacetylene)-9,9-bis[2-(2-methoxyethoxy)-
AHCTUNGTERGeNNUN thyl]fluorene: 2,7-Dibromo-9,9-bis[2-(2-methoxyethoxy)ethyl]fluorene
(370 mg, 0.7 mmol) was dissolved in THF (20 mL) and triethylamine
(5 mL) in a Schlenk tube. After bubbling argon through the mixture for
30 min, [Pd(PPh3)2Cl2] (50 mg, 0.07 mmol), trimethylsilylacetylene
(0.3 mL, 2.1 mmol), and CuI (14 mg, 0.07 mmol) were added and the
mixture stirred at room temperature for 48 h. The solvent was evaporat-
ed and the residue treated with dichloromethane, washed with water and
brine, filtered over hydroscopic cotton wool, and rotary evaporated. The
residue was purified by column chromatography on silica gel and eluted
with ethyl acetate/petroleum ether (20:80 v/v) to give 2,7-di(trimethylsily-
lacetylene)-9,9-bis[2-(2-methoxyethoxy)ethyl]fluorene (162 mg, 40%)
and 2-bromo-7-trimethylsilylacetylene-9,9-bis[2-(2-methoxyethoxy)ethyl]-
fluorene (160 mg, 40%).
2,7-Di(trimethylsilylacetylene)-9,9-bis[2-(2-methoxyethoxy)ethyl]fluor-
ene: 1H NMR (CDCl3, 200 MHz): d=0.28 (s, 18H), 2.38 (t, 3J=7.32 Hz,
4H), 2.69 (t, 3J=7.67 Hz, 4H) 3.12–3.17 (m, 4H), 3.26–3.31 (m, 10H)
7.51 ppm (AB system,
J
AB =7.63 Hz, uod=24.13 Hz, 4H); 13C NMR
(CDCl3, 75 MHz): d=0.12, 39.72, 51.37, 59.13, 66.89, 70.06, 71.85, 95.09,
105.70, 120.05, 122.38, 127.02, 131.62, 140.17, 149.33 ppm; EIMS: m/z
(%): 562.1 (100) [M]+; elemental analysis calcd (%) for C33H46O4Si2
(Mr =562.29): C 70.41, H 8.24; found: C 70.32, H 8.18.
Although the effect of protonation on the inter-Bodipy
energy transfer in 2 is moderate, this result suggests a possi-
ble way to control the interchromophoric energy-transfer ef-
ficiency in similar systems by means of a reversible external
input, namely, protonation of the acceptor subunits.
2-Bromo-7-trimethylsilylacetylene-9,9-bis[2-(2-methoxyethoxy)ethyl]-
fluorene: 1H NMR (CDCl3, 200 MHz): d=0.28 (s, 9H), 2.37 (t, 3J=
7.49 Hz, 4H), 2.72 (t, 3J=7.49 Hz, 4H), 3.14–3.31 (m, 14H), 7.43–
7.59 ppm (m, 6H); 13C NMR (CDCl3, 75 MHz): d=0.10, 39.64, 51.67,
59.13, 66.88, 70.06, 71.83, 95.05, 105.61, 119.81, 121.53, 121.91, 122.34,
126.90, 126.98, 130.74, 131.68, 138.83, 139.88, 148.66, 151.63 ppm; EIMS:
m/z (%): 546.2 (100) [M]+, 544.2 (100) [M]+; elemental analysis calcd
(%) for C28H37BrO4Si (Mr =544.16): C 61.64, H 6.84; found: C 61.40, H
6.77.
Experimental Section
Synthesis of compounds 4 and 4b: Compound 2,7-di(trimethylsilylacety-
lene)-9,9-bis[2-(2-methoxyethoxy)ethyl]fluorene (157 mg, 0.28 mmol) was
dissolved in methyl alcohol (30 mL). K2CO3 (38 mg, 0.28 mmol) was
added and the mixture was stirred at room temperature for 30 min. The
mixture was then treated with water, extracted with dichloromethane,
washed with brine, and filtered over hydroscopic cotton wool. After
rotary evaporation the residue was purified by column chromatography
on silica gel and eluted with ethyl acetate/petroleum ether (40:60 v/v) to
give compounds 4b (50 mg, 36%) and 4 (68 mg, 58%).
1
General: H (200.1, 300.1, and 400 Hz) and 13C (50.3, 75.5, and 100.3 Hz)
NMR spectra were recorded at room temperature using the residual
proton resonances in deuterated solvents as internal references. The
high-resolution mass spectra were recorded using EI at 70 eV. Chromato-
graphic purification was conducted using standardized silica gel 60
(0.063–0.200 mm). Thin-layer chromatography (TLC) was performed on
silica gel plates coated with fluorescent indicator. All mixtures of solvents
are given in v/v ratio. Absorption spectra were recorded using a JASCO
V570 spectrophotometer. For luminescence spectra, a Jobin Yvon–Spex
Fluoromax P spectrofluorimeter was used, equipped with a Hamamatsu
R3896 photomultiplier, and the spectra were corrected for photomultipli-
er response using a program purchased with the fluorimeter. Lumines-
cence lifetimes were determined by time-correlated single-photon count-
ing (TCSPC) using an Edinburgh OB900 spectrometer (light pulse: Ha-
mamatsu PL2 laser diode, pulse width 59 ps at 408 nm; or nitrogen dis-
charge, pulse width at 337 nm: 2 ns). Luminescence quantum yields have
been calculated by the optically diluted method[41] with [Ru(bpy)3]2+
(bpy=2,2’-bipyridine) in air-equilibrated aqueous solution as the quan-
tum yield standard (F=0.028[42]). Time-resolved transient-absorption ex-
periments were performed using a pump–probe setup based on the Spec-
tra-Physics MAI-TAI Ti:sapphire system as the laser source and the Ul-
trafast Systems Helios spectrometer as the detector. The pump pulse was
generated using a Spectra-Physics 800 FP OPA. The probe pulse was ob-
tained by continuum generation on a sapphire plate (spectral range, 450–
3
Compound 4b: 1H NMR (CDCl3, 200 MHz): d=0.27 (s, 9H), 2.37 (t, J=
7.35 Hz, 4H), 2.70 (t, 3J=7.35 Hz, 4H), 3.13–3.30 (m, 14H), 7.43–
7.61 ppm (m, 6H); 13C NMR (CDCl3, 75 MHz): d=0.10, 39.64, 51.40,
59.10, 66.89, 70.03, 71.82, 84.21, 95.14, 105.64, 120.11, 121.29, 122.47,
127.04, 124.14, 131.62, 131.77, 140.15, 140.47, 149.33, 149.45 ppm; EIMS:
m/z (%): 490.1 (100) [M]+; elemental analysis calcd (%) for C30H38O4Si
(Mr =490.25): C 73.43, H 7.41; found: C 73.59, H 7.63.
Compound 4: 1H NMR (CDCl3, 200 MHz): d=2.37 (t, 3J=7.52 Hz, 4H),
2.73 (t, 3J=7.39 Hz, 4H), 3.13–3.30 (m, 14H), 7.54 ppm (AB system,
J
AB =7.93 Hz, uod=25.25 Hz, 4H); 13C NMR (CDCl3, 75 MHz): d=39.57,
51.44, 59.08, 66.91, 70,71.29, 84.16, 120.18 121.40, 127.17, 131.77, 140.33,
149.46 ppm; ESIMS: m/z (%): 418.1 (100) [M]+; elemental analysis calcd
(%) for C27H30O4 (Mr =418.21): C 77.48, H 7.22; found: C 77.27, H 7.04.
Synthesis of compounds 6 and 6b: 8-(4-Iodophenyl)-1,3,5,7-tetramethyl-
4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (90 mg, 0.21 mmol) and
4
(90 mg, 0.2 mmol) were dissolved in a mixture of benzene (20 mL) and
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Bodipy Chromophores
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triethylamine (5 mL). Argon was bubbled through the mixture for
30 min. [Pd(PPh3)4] (25 mg, 0.02 mmol) was added and the mixture was
stirred at 658C for 18 h. The solvent was then rotary evaporated and the
residue treated with dichloromethane, washed with water and brine, fil-
tered over hydroscopic cotton wool, and rotary evaporated. The residue
was purified by column chromatography on silica gel and eluted with
ethyl acetate/dichloromethane (10:90 v/v) to give compounds 6b (50 mg)
and 6 (33 mg).
Compound 6b: 1H NMR (CDCl3, 200 MHz): d=1.44 (s, 12H), 2.45–2.61
(m, 16H), 2.79 (t, 3J=7.30 Hz, 4H), 3.17–3.35 (m, 14H), 6.00 (s, 4H),
7.29–7.72 ppm (m, 14H); 13C NMR (CDCl3, 75 MHz): d=14.70, 29.80,
39.75, 51.49, 59.10, 67, 70.07, 71.83, 89.65, 91.45, 120.37, 121.50, 122.16,
124.19, 126.73, 128.42, 131.31, 132.42, 135.15, 140.27, 140.93, 143.12,
149.60, 155.89 ppm; ESIMS: m/z (%): 1062.2 (100) [M]+; elemental anal-
ysis calcd (%) for C65H64B2F4N4O4 (Mr =1062.5): C 73.45, H 6.07, N 5.27;
found: C 73.12, H 5.62, N 5.09.
C78H76BF2N7O4Si (Mr =1193.58): C 74.44, H 6.16, N 5.87; found: C 74.13,
H 6.36, N 5.71.
Synthesis of compound 2: In a round-bottomed flask equipped with a
Dean Stark apparatus, 4-dimethylaminobenzaldehyde (8 mg, 0.06 mmol)
and piperidine (2 mL) were added to a stirred solution of compound 6b
(69 mg , 0.06 mmol) in toluene (20 mL). The solution was heated at
reflux during 12 h. After cooling to room temperature, the mixture was
washed with water and brine. The organic phase was filtered over hydro-
scopic cotton wool and rotary evaporated. The residue was purified by
column chromatography on silica gel eluting with a gradient of ethyl ace-
tate/petroleum ether (20:80 to 60:40) to give compound 2 (32 mg, 45%).
1H NMR (CD2Cl2, 400 MHz): d=1.47 (s, 3H), 1.51 (s, 6H), 2.41 (t, 3J=
3
7.38 Hz, 4H), 2.52 (s, 6H), 2.55 (s, 3H), 2.82 (t, J=7.18 Hz, 4H), 3.03 (s,
6H), 3.16–318 (m, 4H), 3.22 (s, 6H), 3.24–3.27 (m, 6H), 6.02 (s, 1H),
6.04 (s,1H), 6.65 (s, 1H), 6.71 (d, 3J=9 Hz, 2H), 7.24 (d, 3J=15.9 Hz,
1H), 7.33–7.39 (m, 5H), 7.50 (d, 3J=9 Hz, 2H), 7.59 (dt, 3J=7.8 Hz, 4J=
1.64 Hz, 2H), 7.68–7.75 ppm (m, 8H); 13C NMR (CDCl3, 400 MHz): d=
14.56, 15.65, 14.97, 39.69, 51.42, 59.04, 66.92, 70.01, 71.77, 89.56, 89.71,
91.29, 91.40, 117.93, 120.28, 120.80, 121.42, 122.06, 122.15, 123.93, 124.12,
126.65, 128.34, 128.75, 129.33, 131.25, 132.23, 132.34, 132.91, 135.07,
135.47, 140.17, 140.23, 140.86, 142.63, 143.06, 149.52, 155.82 ppm; ESIMS:
m/z (%): 1193.4 (100) [M]+; elemental analysis calcd (%) for
C74H73B2F4N5O4 (Mr =1193.58): C 74.44, H 6.16, N 5.87; found: C 74.21,
H 5.72, N 5.59.
Compound 6: 1H NMR (CDCl3, 200 MHz): d=1.44 (s, 6H), 2.41 (t, 3J=
3
7.30 Hz, 4H), 2.56 (s, 6H), 2.76 (t, J=7.30 Hz, 4H), 3.17–3.32 (m, 14H),
6 (s, 2H), 7.29–7.71 ppm (m, 10H); 13C NMR (CDCl3, 75 MHz): d=
14.73, 29.83, 39.69, 51.50, 59.13, 66.98, 70.07, 71.85, 84.21, 89.60, 91.47,
120.21, 120.38, 121.46, 122.17, 124.22, 126.76, 127.20, 128.43, 131.28,
131.86, 132.44, 135.16, 140.26, 140.44, 140.96, 143.15, 149.46,149.66,
155.92 ppm; EIMS: m/z (%): 740.3 (100) [M]+; elemental analysis calcd
(%) for C46H47BF2N2O4 (Mr =740.36): C 74.59, H 6.40, N 3.78; found: C
74.38, H 6.18, N 3.56.
Synthesis of compound 1: Compounds 5 (85 mg, 96 mmol) and 6 (59 mg,
80 mmol) were dissolved in a mixture of benzene (20 mL) and triethyl-
ACHTUNGTRENNUNGamine (5 mL). Argon was bubbled through the mixture for 30 min.
Acknowledgements
[Pd(PPh3)4] (10 mg, 8 mmol) was added and the mixture was stirred at
658C for 18 h. The solvent was then rotary evaporated and the residue
treated with dichloromethane, washed with water and brine, filtered over
hydroscopic cotton wool, and rotary evaporated. The residue was purified
by column chromatography on silica gel and eluted with dichlorome-
thane/petroleum ether (60:40 v/v) to give compound 1 (50 mg). 1H NMR
(CD2Cl2, 400 MHz): d=1.45 (s, 3H), 1.48 (s, 9H), 2.39 (t, 3J=6.98 Hz,
4H), 2.50 (s, 9H), 2.79 (t, 3J=7.3 Hz, 4H), 3.13–3.24 (m, 16H), 5.00 (s,
4H), 5.99 (s, 1H), 6.02 (s, 2H), 6.59 (s, 1H), 7.21–7.82 (m, 28H), 8.30 (d,
3J=8 Hz, 2H), 8.41 (d, 3J=7.8 Hz, 2H), 8.63 ppm (d, 3J=3.7 Hz, 2H);
13C NMR (CDCl3, 75 MHz): d=14.63, 14.72, 15.02, 29.82, 39.78, 51.53,
57.58, 59.11, 67.03, 70.10, 71.86, 89.65, 89.80, 91.38, 91.51, 112.92, 115.14,
117.96, 119.70, 120.37, 120.85, 121.11, 121.38, 122.16, 122.25, 123.88,
124.02, 124.23, 125.85, 126.75, 128.45, 128.84, 129.48, 131.33, 132.31,
132.44, 132.95, 135.18, 135.59, 137.07, 137,54, 137.89, 138.27, 140.27,
140.33, 140.96, 142.71, 143.14, 149.33, 149.66, 153.71, 154.84, 155.92,
156.21, 156.33, 157.88 ppm; EIMS: m/z (%): 1501.5 (100) [M]+; elemen-
tal analysis calcd (%) for C94H85B2F4N9O4 (Mr =1501.68): C 75.15, H
5.10, N 8.39; found: C 74.87, H 5.44, N 8.19.
The authors thank the University of Messina (Progetti di Ricerca di
Ateneo, PRA), the International Centre for Tropical Agriculture (CIAT)
laboratory of the University of Messina, the MIUR (PRIN projects), and
the Centre national de la Recherche Scientifique for financial support
and research facilities.
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Synthesis of compound 7: Compounds 5 (98 mg, 0.11 mmol) and 4b
(45 mg, 92 mmol) were dissolved in a mixture of benzene (20 mL) and
triethylamine (5 mL). Argon was bubbled through the mixture for
30 min. [Pd(PPh3)4] (10 mg, 8 mmol) was added and the mixture was
stirred at 658C for 18 h. The solvent was then rotary evaporated and the
residue treated with dichloromethane, washed with water and brine, fil-
tered over hydroscopic cotton wool, and rotary evaporated. The residue
was purified by column chromatography on alumina and eluted with di-
chloromethane (100%) to give compound 7 (40 mg, 40%). 1H NMR
(CDCl3, 300 MHz): d=0.28 (s, 9H), 1.47 (s, 3H), 1.50 (s, 3H), 2.43 (t,
3J=7.3 Hz, 4H), 2.55 (s, 3H), 2.75 (t, 3J=7.3 Hz, 4H), 3.16–3.31 (m,
16H), 5.00 (s, 4H), 5.97 (s, 1H), 6.58 (s, 1H), 7.15–7.33 (m, 8H, overlap
with solvent), 7.45–7.86 (m, 16H), 8.32 (d, 3J=7.9 Hz, 2H), 8.41 (d, 3J=
7.9 Hz, 2H), 8.70 ppm (d, 3J=4.1 Hz, 2H); 13C NMR (CDCl3, 400 MHz):
d=0.13, 14.19, 14.64, 15.04, 22.48, 34.28, 39.78, 51.47, 57.60, 59.14, 66.98,
70.10, 71.88, 89.69, 91.44, 105.70, 112.95, 117.96, 119.87, 120.11, 120.32,
121.29, 121.58, 122.13, 122.52, 123.99, 124.08, 125.95, 126.75, 127.07,
128.84, 129.49, 131.27, 131.71, 132.33, 135.57, 137.48, 138, 140.17, 140.35,
142.29, 142.75, 149.05, 149.37, 149.66, 153.75, 154.83, 155.98, 157.97 ppm;
EIMS: m/z (%): 1193.4 (100) [M]+; elemental analysis calcd (%) for
Chem. Eur. J. 2010, 16, 8832 – 8845
ꢄ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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8844
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Chem. Eur. J. 2010, 16, 8832 – 8845

Bodipy Chromophores
FULL PAPER
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Received: February 22, 2010
Published online: June 23, 2010
Chem. Eur. J. 2010, 16, 8832 – 8845
ꢄ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8845