Ultrafast energy transfer in triptycene-grafted bodipy scaffoldings

Article abstract of DOI:10.1002/chem.201300413

A series of new compounds in which various Bodipy dyes are grafted logically on triptycene rigid structures are synthesized and characterized, and their absorption spectra and photophysical properties are studied, also by pump-probe transient absorption spectroscopy. The studied compounds are: the mono-Bodipy species TA, TB, and TC (where A, B, and C identify different Bodipy subunits absorbing and emitting at different wavelengths), the multichromophore species TA₃, which bears three identical A subunits, and the three multichromophoric species TAB, TBC, and TABC, all of them containing at least two different types of Bodipy subunits. The triptycene moiety plays the role of a rigid scaffold, keeping the various dyes at predetermined distances and allowing for a three-dimensional structural arrangement of the multichromophoric species. The absorption spectra of the multichromophoric Bodipy species are essentially additive, indicating that negligible inter-chromophoric interaction takes place at the ground state. Luminescence properties and transient absorption spectroscopy indicate that a very fast (on the picosecond time scale) and efficient photoinduced energy transfer occurs in all the multi-Bodipy species, with the lower-energy Bodipy subunits of each multi-Bodipy compounds playing the role of an electronic energy collector. In TAB, an energy transfer from the A-type Bodipy subunit to the B-type one takes place with a rate constant of 1.6×10¹⁰ s^-1, whereas in TBC an energy transfer from the B-type Bodipy subunit to the C-type subunit is bi-exponential, exhibiting rate constants of 1.7×10¹¹ and 1.9×10 ¹⁰ s^-1; the possible presence of different conformers with different donor-acceptor distances in this bichromophoric species is proposed to cause the bi-exponential energy-transfer process. Interpretation of the intricate energy-transfer pathways occurring in TABC is made with the help of the processes identified in the bichromophoric compounds. In all cases, the measured energy-transfer rate constants agree with a Foerster mechanism for the energy-transfer processes. Copyright

Full text of DOI:10.1002/chem.201300413

FULL PAPER
DOI: 10.1002/chem.201300413
Ultrafast Energy Transfer in Triptycene-Grafted Bodipy Scaffoldings**
Thomas Bura,[a] Francesco Nastasi,[b] Fausto Puntoriero,[b] Sebastiano Campagna,*[b] and
Raymond Ziessel*[a]
Abstract: A series of new compounds
in which various Bodipy dyes are graft-
ed logically on triptycene rigid struc-
tures are synthesized and character-
ized, and their absorption spectra and
photophysical properties are studied,
also by pump-probe transient absorp-
tion spectroscopy. The studied com-
pounds are: the mono-Bodipy species
TA, TB, and TC (where A, B, and C
identify different Bodipy subunits ab-
sorbing and emitting at different wave-
lengths), the multichromophore species
TA3, which bears three identical A sub-
units, and the three multichromophoric
species TAB, TBC, and TABC, all of
them containing at least two different
types of Bodipy subunits. The tripty-
cene moiety plays the role of a rigid
scaffold, keeping the various dyes at
predetermined distances and allowing
for a three-dimensional structural ar-
rangement of the multichromophoric
species. The absorption spectra of the
multichromophoric Bodipy species are
essentially additive, indicating that neg-
ligible inter-chromophoric interaction
takes place at the ground state. Lumi-
nescence properties and transient ab-
sorption spectroscopy indicate that a
very fast (on the picosecond time
scale) and efficient photoinduced
energy transfer occurs in all the multi-
Bodipy species, with the lower-energy
Bodipy subunits of each multi-Bodipy
compounds playing the role of an elec-
tronic energy collector. In TAB, an
energy transfer from the A-type
Bodipy subunit to the B-type one takes
place with a rate constant of 1.6ꢀ
1010 sꢀ1, whereas in TBC an energy
transfer from the B-type Bodipy sub-
AHCTUNGTRENNuGUN nit to the C-type subunit is bi-expo-
nential, exhibiting rate constants of
1.7ꢀ1011 and 1.9ꢀ1010 sꢀ1; the possible
presence of different conformers with
different donor–acceptor distances in
this bichromophoric species is pro-
posed to cause the bi-exponential
energy-transfer process. Interpretation
of the intricate energy-transfer path-
ways occurring in TABC is made with
the help of the processes identified in
the bichromophoric compounds. In all
cases, the measured energy-transfer
rate constants agree with a Fçrster
mechanism for the energy-transfer
processes.
Keywords: bipyridines · Bodipy ·
dyes/pigments · energy transfer ·
fluorescence · triptycene
Introduction
ACHTUNGTRENuNUGN nits are of interest for the design of luminescence sensors
in biological systems when a large wavelength separation
between the exciting light pulse and the analyzing light
signal is required.[4]
Multichromophoric species have been extensively investigat-
ed in the last decades.[1] These species are interesting for
solar energy conversion purposes, because suitably designed
multichromophore assemblies can play the role of light-har-
vesting antenna components for artificial photosynthesis
(i.e., direct solar fuels production),[2] and in dye-sensitized
solar cells (i.e., for conversion of solar light into electrici-
ty).[3] Moreover, multichromophoric species featuring photo-
induced energy transfer between donor and acceptor sub-
Recently, Bodipy dyes (i.e., difluoroborondipyrromethene
species, also named bora-indacene) have emerged as effi-
cient and tunable light-absorption and luminescent spe-
cies.[5–9] As a consequence, multi-Bodipy systems have been
prepared and extensively investigated.[10–12] In particular,
these studies have provided useful information on various
aspects of intercomponent energy transfer in multichromo-
phoric assemblies, such as external modulation of energy
transfer processes[13] and the effect of appropriate alignment
of transition dipole moment vectors on the energy transfer
efficiency,[14] and have uncovered limitations of simplified
Fçrster theories in predicting energy transfer rate constants
for fixed geometries[15] and visualizing nanomechanical
properties of molecular-scale bridges.[16]
Various pre-organized platforms have been scrutinized for
the connection of Bodipy dyes. Direct linkage of Bodipy
dyes or linking through an unsaturated spacer form basically
1D systems.[11,13] Two-dimensional systems are more planar
and are constructed around soluble truxene scaffoldings[10]
or flat Bodipy as the central core.[17] The use of 3D plat-
[a] T. Bura, Dr. R. Ziessel
Laboratoire de Chimie Organique
et Spectroscopies Avancꢁes (LCOSA)
ECPM, ICPESS-UMR7515 rue Becquerel
67087 Strasbourg, Cedex 02 (France)
E-mail: ziessel@unistra.fr
[b] Dr. F. Nastasi, Dr. F. Puntoriero, Dr. S. Campagna
Department of Chemical Sciences, University of Messina
via F. Stagno dꢂAlcontres 31, I-98166 Messina (Italy)
E-mail: campagna@unime.it
[**] Bodipy=difluoroborondipyrromethene.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300413.
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forms is less common and some examples are focused on
dendrimers[18] and fullerenes.[19] This is the reason why trip-
tycene, a 3D rigid structure, caught our attention. Triptycene
belongs to the family of iptycene and consists of three
phenyl rings separated by a saturated [2.2.2]biclycloocta-
triene bridgehead framework.[20] This 3D-shape-persistent
structure has several interesting applications in the field of:
1) molecular machines (e.g., gyroscopes, compasses,[21]
brakes,[22] or ratchets[23]), due to restricted rotation of the
molecule preventing interdigitation; 2) molecular balances[24]
for evaluation of p–p and other non-covalent interactions;
3) material sciences, such as polymers and liquid crystals for
large internal free volumes providing large porosity for gas
absorption;[25] and 4) crystal engineering in host–guest su-
pramolecular chemistry.[26] The rigid triptycene core also has
applications in coordination chemistry, catalysis for stabiliz-
ing highly reactive intermediates[27] and sterically bent com-
plexes,[28] and electrophosphorescence.[29] Furthermore, elec-
tron and energy transfer in rigid triptycene-bipyridine metal
complexes[30] and in porphyrin-based dyads and triads for
charge separation[31] has also been investigated. Studies on
triptycene-bridged donors/acceptors have demonstrated that
electronic interactions occur through the s-bridged
system,[32] and that no homoconjugation effect is present in
the p-conjugated oligomers.[33]
Abstract in French: Une nouvelle sꢀries de sondes Bodipy ont
ꢀtꢀ construites sur une plateforme triptycene T prꢀalablement
fonctionnalisꢀe. Lꢁapproche synthꢀtique a consistꢀe ꢂ construire
dꢁabord un triptycene modifiꢀ avec trois Bodipy identiques TA3
et ꢂ sꢀlectivement transformer les unitꢀs existantes en des com-
posꢀs ayant des couleurs diffꢀrentes (A, B, et C). De nombreux
composꢀs modꢃles ont ainsi pu Þtre prꢀparꢀs tels que TA, TB,
TC, TAB, TBC. La molꢀcule cible TABC possꢃde un ensemble
de chromophores capable de transfꢀrer leur ꢀnergie sur la
sonde de plus basse ꢀnergie et ses processus en cascade permet-
tent de concentrer les photons sur un ꢀmetteur unique localisꢀ ꢂ
basse ꢀnergie. Le spectre dꢁabsorption de TABC est une combi-
naison linꢀaire de lꢁabsorption individuelle des quatre modules
dꢀmontrant une faible interaction dans lꢁꢀtat fondamental. Les
transferts dꢁꢀnergie sont extrÞmement rapides (picoseconde) et
ont ꢀtꢀ ꢀtudiꢀs par absorption transitoire. Plus prꢀcisꢀment dans
le modꢃle TAB le transfert dꢁꢀnergie de A vers B a lieu en 60 ps
tandis que le transfert de B vers C dans TBC se fait en 6 et
50 ps. La prꢀsence de diffꢀrents conformꢃres a ꢀtꢀ invoquꢀe
pour expliquer le dꢀclin bi-exponentiel. Lꢁinterprꢀtation des
phꢀnomꢃnes de transfert dꢁꢀnergie dans TABC a ꢀtꢀ faite ꢂ
lꢁaide des composꢀs modꢃles et donne une image rꢀaliste de
lꢁensemble des processus impliquꢀs lors dꢁune irradiation. Lꢁen-
semble de ces donnꢀes sont en accord avec un transfert dꢁꢀner-
gie par rꢀsonnance liꢀ ꢂ une superposition spectrale de lꢁꢀmis-
sion du donneur et de lꢁabsorption de lꢁaccepteur (thꢀorie de
Fçrster). Ces rꢀsultats montrent les progrꢃs rꢀalisables dans la
concentration de photons et la monochromatisation de la lum-
Herein, we report the synthesis, characterization, and
study of the absorption spectra and photophysical properties
(including pump-probe transient absorption spectroscopy)
of a new series of compounds in which various Bodipy dyes
are grafted on a preorganized triptycene rigid structure. The
triptycene moiety plays the role of a rigid scaffold, keeping
the various dyes at predetermined distances. The com-
pounds whose spectroscopic and photophysical properties
have been investigated are: the mono-Bodipy species TA,
TB, and TC (for simplicity, the Bodipy subunits contained
are named A, B, and C, respectively, in descending order
based on their excited-state energy), the multichromophoric
species TA3, which contains three identical A subunits, and
the three multichromophoric species TAB, TBC, and TABC,
all of which contain at least two different types of Bodipy
subunits (see Schemes 1–3 for their structural formulas).
The results indicate that very fast and efficient energy trans-
fer takes place in all the multi-Bodipy species studied here.
Comparison of the energy-transfer rate constants with those
reported in the literature for some selected multi-Bodipy
species have also been made.
ACHTUNGTRENNUNGiꢃre dans des molꢁcules tridimensionnelles.
Abstract in Italian: Eꢁ stata sintetizzata e caratterizzata una
nuova famiglia di antenne costituite da diversi frammenti cro-
moforici Bodipy ancorati su un core tripticenico. Sono stati stu-
diati i loro spettri di assorbimento e le loro proprietꢂ foto-
ACHTUNGTRENNUNGfisiche, anche tramite spettroscopia pump-probe di assorbimen-
to transiente. Oltre ai sistemi multicromoforici TAB, TBC, e
TABC (A, B, e C identificano differenti subunitꢂ Bodipy, che
assorbono ed emettono a diversa energia) sono state preparate e
studiate sia le specie modello monocromoforiche TA, TB e TC
sia la specie multicromoforica TA3, contenente tre Bodipy iden-
tici. Il frammento tripticenico ha il ruolo fondamentale di per-
mettere un arrangiamento topologicamente controllato e rigido,
nelle tre dimensioni, dei diversi cromofori. Gli spettri di assor-
bimento delle specie multicromoforiche sono essenzialmente
additivi, indicando che allo stato fondamentale l’interazione in-
tercromoforica ꢃ trascurabile. Le proprietꢂ di luminescenza e la
spettroscopia di assorbimento transiente hanno messo in eviden-
za che in tutti i sistemi multicromoforici investigati si verificano
processi di trasferimento di energia elettronica fotoindotti, parti-
colamente efficenti e veloci (nella scala temporale dei ps) verso
la subunitꢂ a piꢄ bassa energia (trappola di energia) di ciascun
sistema multi-Bodipy. In particolare, nel sistema TAB il trasferi-
mento di energia dalla subunitꢂ di tipo A a quella di tipo B av-
viene con una costante di velocitꢂ pari a 1.6ꢅ1010 sꢀ1, mentre in
TBC il trasferimento da B a C ꢃ biesponenziale, con costanti di
velocitꢂ pari a 1.7ꢅ1011 e 1.9ꢅ1010 sꢀ1. Tale comportamento po-
trebbe essere razionalizzato grazie alla presenza, per tale
specie, di diversi conformeri con differenti distanze donatore -
accettore. I risultati ottenuti per i sistemi bicromoforici hanno
permesso di interpretare le complesse dinamiche di decadimen-
to degli stati eccitati caratteristiche della specie multicromofori-
ca TABC. Per tutte le specie investigate, i risultati ottenuti speri-
mentalmente sono in accordo con un meccanismo di trasferi-
mento energetico fotoindotto di tipo Fçrster.
Results and Discussion
Synthesis: The pivotal compound 2,6,14-triiodotriptycene
(T), was synthesized in three steps from commercially avail-
able triptycene according to a procedure previously de-
AHCTUNGTREGsNNUN cribed by Zhang and Chen (Scheme S1 in the Supporting
Information).[34] First, triptycene was reacted with nitric acid
while heating at reflux to give 2,6,14- and 2,7,14-trinitrotrip-
tycene (64 and 21% yield, respectively). These two re-
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Energy Transfer in Bodipy Scaffoldings
FULL PAPER
regioselective Knoevenagel re-
actions. The first condensation
between TA3
methoxyACHTUGNTERNeNUNG th-
and
para-
AHCTUNGERTGoNNUN xyethoxybenzaldehyde
(1 equiv) gave compound TA2B
in a respectable yield of 24%.
Here the polarity and solubility
imported by the ethyleneglycol
chain facilitated the purification
process. This was clearly ob-
served by the color of the reac-
tion mixture, which turned ma-
genta rather than deep-blue, as
would be expected by forma-
tion of a double vinyl function
on the same Bodipy core.[35]
This was further confirmed by
proton NMR spectroscopy, in
which
a new singlet at d=
6.84 ppm (1H) for the b-pyrrol-
ic proton (labeled e; see
Scheme 3 for the proton assign-
Scheme 1. Preparation of the Bodipy subunits A, B, and C. i) [PdACHTUNRGTNEUNG(PPh3)2Cl2], CuI, trimethylsilylacetylene Et3N,
C6H6, 508C, 18 h; ii) MeOH, K2CO3, RT.
ment) is in close proximity to
gioisomers were separated by column chromatography on
silica gel with an appropriate solvent and were unambigu-
ously characterized by 1H and 13C NMR spectroscopy.
Indeed, the 13C NMR spectrum of the symmetrical isomer
2,7,14-trinitrotriptycene shows only six signals for the aro-
matic carbon atoms (as expected for a C3 symmetry),
whereas the derivative 2,6,14-trinitrotriptycene exhibits
eleven signals, as found for T. Reduction of the trinitro de-
the vinyl function, compared to the singlet at d=6.13 ppm
(5H) for the other b-pyrrolic protons.[36] Note that for TA3 a
single signal is found at d=6.13 ppm (protons b, 6H; Fig-
ure 1A). Confirmation of the single substitution was ob-
tained by dissymmetrization of the singlets at d=1.45 and
2.50 ppm (for TA3), which were assigned to the methyl
groups a, c and d (Figure 1B). All other signals for the eth-
yleneglycol chain and the phenyl rings are in keeping with
this statement.
rivative by using PdACHTUNGTRENNUNG(10%)/C and H2 gave the corresponding
triaminotriptycene in 90% yield. Finally, a Sandmeyer reac-
tion under standard conditions provided the 2,6,14-triiodo-
triptycene (T) in 55% yield. The three different modules A,
B, and C were prepared as shown in Scheme 1.
Finally, the condensation of TA2B with the bis-bipyridine
aldehyde 8 allowed the preparation of the multichromophor-
ic target dye TABC in 19% yield (Scheme 3); again, the
condensation was regioselective, causing monovinyl derivati-
zation on a second tetramethyl-Bodipy side. This serendipi-
tous discovery is likely due to the stronger acidity of the
methyl groups of the tetramethyl-Bodipy group with respect
to a monovinyl methyl group. It is not excluded that steric
congestion might also be part of the reason for selective
monocondensation on each site. Proton NMR spectroscopy
was again used for the characterization of the final structure.
In particular, two additional singlets are observed at d=
6.80 ppm (1H), assigned to the b-pyrrolic proton (g), and at
d=6.07 ppm (1H), assigned to the b-pyrrolic proton (b’’;
Figure 1C). In parallel, the methyl signals are split according
to the multiplicity expected by the dissymmetrization of the
structure (inset in Figure 1C). The de-shielded bipyridine
proton and ethyleneglycol proton signals are in keeping
with the step-by-step monofunctionalization. Interestingly,
The ethynyl function was grafted on the iodophenyl deriv-
ative by a cross-coupling reaction with trimethylsillylacety-
lene and promoted by in situ prepared Pd0 precursors. After-
wards, the trimethylsilyl (TMS) group was deprotected by
using K2CO3 in methanol to afford the desired module with
excellent yields in each case (Scheme 1). Cross-coupling of
these modules to the preorganized platform T provided the
reference dyes TA, TB, and TC in expected yields for a stat-
istical cross-coupling reaction with three reactive functions
(Scheme 2). The side products of double and triple cross-
coupling reactions were not isolated.
Likewise, cross-coupling between TB and A, or TC and B
gave the TAB and TBC dyads in 37 and 20% yield, respec-
tively. Again, the side products TA2B and TB2C were not
isolated. Unfortunately, the final compound TABC could
not be isolated by reacting TBA or TBC with dyes C or A,
respectively, due to the presence of an intractable mixture
of compounds.
ꢀ
the C H bridgehead protons 9 and 10 are weakly split (d=
5.88 and 5.89 ppm) in case of TABC, whereas they resonate
as singlets at d=5.87 ppm in the model compounds TA2B
and TA3 (Figure 1C).
After some trials, the best way to prepare the target
TABC started from the triply substituted dye TA3 by using
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S. Campagna, R. Ziessel et al.
Scheme 2. Preparation of the multichromophoric species TAB and TBC. i) [PdACHTGNUTRENNUNG
dine, p-TsOH (Ts=tosyl), 1408C.
Absorption spectra: The absorption spectra of the studied
compounds in the solvents used (acetonitrile (AN) and 1,2-
dichloroethane (DCE)) are dominated by the absorption of
the Bodipy subunits, as expected, because the triptycene
moieties do not absorb significantly in the visible region and
their UV absorption is not comparable with the molar ab-
lowed p–p* transitions; however, a significant charge-trans-
fer (CT) character is expected to be present in the lower-
energy band of TC.[5,10,13]
The absorption spectra of the multiple Bodipy-decorated,
triptycene-based compounds are essentially the superposi-
tion of the absorption of the individual chromophoric sub-
sorption of the Bodipy unit. All the absorption spectra data
are collected in Table 1, and Figures 2 and 3 show the ab-
sorption spectra of some compounds in DCE. The absorp-
tion spectra of representative compounds in AN are shown
in the Supporting Information (Figure S25 in the Supporting
Information).
ACHTUNGTRENNUNG
For the mono-Bodipy species, the absorption spectrum of
TA, essentially due to the absorption of its Bodipy subunit
A, is at a maximum at about l=500 nm, the absorption
spectrum of TB, essentially due to its Bodipy subunit B,
peaks at about l=580 nm, and the absorption spectrum of
TC, mainly due to the Bodipy chromophore C, is at a maxi-
mum at about l=605 nm. In all cases, the visible absorption
of the compounds is assigned to Bodipy-centered spin-al-
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Energy Transfer in Bodipy Scaffoldings
FULL PAPER
essentially solvent-independent
for TA, TA3, and TB within ex-
perimental uncertainty. On the
contrary, for TC, which contains
the Bodipy dye C, the lowest-
energy absorption band is
weakly solvent dependent but
the emission spectrum is signifi-
cantly blue-shifted on passing
from acetonitrile to DCE; in
fact, the emission maximum
(Table 1) moves from l=
705 nm (acetonitrile) to l=
665 nm (DCE). This agrees
with the already-mentioned sig-
nificant CT character of the
corresponding electronic transi-
tion, with the N,N’-bis(bipyri-
AHCTUNGTRENGNdUN yl)-aminophenyl group playing
the role of the electron-donor
subunit and the Bodipy moiety
acting as the electron acceptor,
as is already known for this and
similar subunits in analogous
species.[5,10–13] Because acetoni-
trile is a more polar solvent
than DCE, the CT excited state
is more stabilized in acetonitrile
and therefore the emission
spectrum is significantly sensi-
tive to the solvent polarity. The
large red-shift of the emission
in acetonitrile also suggests the
presence of noticeable nuclear
reorganization in the CT state
Figure 1. Proton NMR spectra of a) TA3, b) TA2B, and c) TABC in [D6]acetone. For the sake of clarity, the
spectra have been cut below d=3.2 ppm. The insets in dashed lines are expansions of the methyl region.
Luminescence properties: All the compounds exhibit in-
tense Bodipy-based emissions, which, in the multi-Bodipy
species, mainly involves the singlet p–p* excited state of the
Bodipy unit with its excited state at the lowest energy. How-
ever, less-intense emission bands at higher energies also
appear in all multi-Bodipy compounds except TA3, for
which excitation is performed at wavelengths corresponding
to the absorption of the high-energy Bodipy components.
Table 1 shows the luminescence data of the compounds at
room temperature in acetonitrile and DCE. Figures 4 and 5
display the emission spectra of most of the compounds in
DCE. Emission spectra of other representative compounds
are reported in the Supporting Information (Figure S26 in
the Supporting Information).
Let us start our discussion from the species containing
only one type of Bodipy dye. From the data in Table 1, it is
clear that for the species containing the A and B Bodipy
chromophores—that is, TA, TA3, and TB—passing from
acetonitrile to DCE solution only has a small effect on the
absorption and emission band energies; this agrees with the
essentially pure p–p* nature of the corresponding transi-
tions. Also, the emission quantum yields and lifetimes are
in polar solvents before emission. The nature of the solvent
also affects the emission lifetime of TC (Table 1), which is
reduced in acetonitrile in comparison to DCE; because the
emission quantum yield is essentially constant on moving
from acetonitrile to DCE, it can be concluded that both the
non-radiative and the radiative decay rate constant are
larger in acetonitrile than in DCE for this compound.
Let us move to the multichromophoric species containing
different types of Bodipy dyes. For the compounds TAB and
TBC, in both acetonitrile and DCE solutions the dominant
emission band (at about l=585 nm for TAB and at l=705
or 665 nm for TBC depending on the solvent, see the discus-
sion above for TC) is clearly due to the excited state of the
lowest-energy Bodipy subunit (see Figures 4 and 5) at any
excitation wavelength.1The wavelength of the minor emis-
sion components at higher energy (namely, at l=510 nm for
TAB and at about l=590 nm for TBC), whose lifetimes are
close to the detection limit of our equipment (140 ps), indi-
1
For any excitation wavelengths here—as well as in other parts of this
paper—we refer to excitation performed at wavelengths corresponding
to the absorption bands of the various Bodipy components on multi-
Bodipy species.
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S. Campagna, R. Ziessel et al.
Scheme 3. Preparation of the multichromophoric species TABC. i) Toluene, piperidine, p-TsOH, 1408C.
cate that such high-energy emission corresponds to the emis-
sion of the higher-energy Bodipy units in each multichromo-
phoric species (Table 1). These are assigned to partially
quenched higher-energy Bodipy chromophores present in
the multi-Bodipy species.
As far as TABC is concerned, the emission spectrum of
this compound (Figure 5c) in both acetonitrile and DCE
solutions is dominated by the emission of the C-based chro-
mophore at any excitation wavelength. However, upon exci-
tation at wavelengths shorter than l=500 nm, the emission
spectrum also shows a small component at l=510 nm, cor-
responding to the partially quenched excited state of the A-
type subunit, with an apparent lifetime (e.g., 140 ps in aceto-
nitrile) coincident with the experimental limit. A non-negli-
gible emission from the B-type dye is also present at l=
580 nm, but its lifetime cannot be recorded with our appara-
tus.
excitation spectrum of TABC is shown in the inset of Fig-
ure 5c; it is clear that emission at l=680 nm, mainly involv-
ing the lower-lying C subunit, is also contributed from exci-
tation centered in the A and B chromophores.
Because the lifetimes of the partially quenched emission
components in the multicomponent species are less reliable
(in all cases they are too close to our equipment limit), cal-
culations of the energy-transfer rate constants cannot be
made on the basis of the emission data. Comments on inter-
chromophore energy transfer rate constants are therefore
demanded to transient absorption spectroscopy results (see
later).
Pump-probe transient absorption spectroscopy: The transi-
ent absorption spectra of the mono-Bodipy species TA, TB,
and TC and of the multi-Bodipy compounds TAB, TBC,
and TABC have been investigated in 1,2-dichloroethane at
room temperature by using l=400 nm as the excitation
wavelength. All the spectra were investigated after 600 fs
from the laser pulse. It was not possible to perform analo-
gous experiments in acetonitrile, because some of the com-
pounds were not stable under laser light in this medium. For
Similarly to what is reported in the literature for other
multi-Bodipy species,[5,10–15] quenching of the higher-energy
Bodipy subunits in TAB, TBC, and TABC is attributed to
an energy transfer to the lower-energy Bodipy subunits. Ex-
citation spectra confirm this attribution. As an example, the
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technical problems, excitation at different wavelengths than
l=400 nm was also not available.
Compound TA: The transient spectrum of TA shows a
bleach in the region l=460–600 nm, corresponding to the
Table 1. Absorption and emission data recorded at room temperature.[a]
Absorption
Luminescence
lem [nm]
lmax [nm] (e [mꢀ1 cmꢀ1])
F
t [ns]
Figure 3. Absorption spectra of TAB (a), TBC (g), and TABC
(c) in DCE.
TA
500 (75000)
505 (79000)
500 (225000)
505 (235000)
602 (75000)
609 (99000)
568 (92000)
577 (118000)
500 (82000)
568 (92000)
505 (91600)
577 (118000)
568 (128000)
602 (76000)
577 (170000)
609 (102000)
501 (87600)
568 (125700)
602 (77400)
505 (97000)
577 (170000)
609 (102000)
510
515
510
515
705
665
580
590
0.55
0.60
2.4
2.7
TA3
TC
0.59
0.26
0.24
0.99
0.99
0.99
2.8
2.0
3.8
3.8
TB
3.9
TAB
580, 510
3.7[b]
590, 515
0.99
0.26
0.25
0.26
4.1[c]
2.0[d]
3.9[e]
2.0[f]
TBC
705, 580
665, 590
Figure 4. Luminescence spectra of TA (a), TB (g), and TC (c) in
DCE at room temperature.
TABC
705, 580, 510
665, 599, 515
0.23
3.9[g]
[a] For each species, the data in the first row, in italics, are in acetonitrile
and the data in the second row, in plain text, refer to 1,2-dichloroethane;
compounds are labeled as given in Schemes 2 and 3; excitation was per-
formed at l=408 nm. [b] A shorter lifetime component was recorded at
l=510 nm (A-based quenched emission). Its apparent value (0.2 ns) is
close to the limit of the equipment, so it cannot be considered the real re-
sidual lifetime of the
A subunit. [c] A shorter lifetime component
(0.25 ns) was recorded at l=520 nm, see also footnote [b]. [d] A shorter
lifetime component was recorded at l=580 nm (B-based quenched emis-
sion), see footnote [b]. [e] The reported lifetime was recorded at l=
640 nm. The lifetime of the emission at l=590 nm could not be recorded
with our equipment. [f] A shorter lifetime component (140 ps) was re-
corded at 510 nm (A-based quenched emission), see footnote [b]. It was
not possible to record a short emission lifetime component at l=580 nm.
[g] This lifetime refers to the l=609 nm emission. The lifetimes of the
emission contributions at l=592 and 520 nm were under the detection
limit of our equipment.
Figure 5. Luminescence spectra of a) TAB, b) TBC, and c) TABC in
DCE. Excitation wavelength in all the cases was l=408 nm. The inset in
panel c is the excitation spectrum of TABC, emission wavelength l=
680 nm; its comparison with the absorption spectrum of TABC (Figure 3)
confirms an efficient inter-Bodipy energy transfer.
Figure 2. Absorption spectra of TA (a), TB (g), and TC (c) in
DCE.
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S. Campagna, R. Ziessel et al.
ground-state absorption at lowest energy (see Figure 6). The
transient spectrum seems to directly decay mono-exponen-
tially to the ground state in the time window explored; how-
ever, the lifetime of the excited state cannot be measured
Figure 8. Transient absorption spectra of TC, shorter time range (see
text).
Figure 6. Transient absorption spectra of TA.
with care by transient absorption data, as expected, because
the upper time limit of our experimental setup (3.2 ns) is
comparable to the excited-state lifetime measured by lumi-
nescence decay (2.7 ns in DCE).
Figure 9. Transient absorption spectra of TC, intermediate time range
(see text).
Compound TB: The transient spectrum of TB exhibits a
bleach in the l=550–660 nm range, with a maximum at
about l=575 nm and transient absorption bands at wave-
lengths shorter than l=550 nm and longer than l=670 nm
(Figure 7). Similarly to what was found for TA, even the
transient spectrum of TB directly decays to the ground state
with an apparent mono-exponential process on a longer
Figure 10. Transient absorption spectra of TC, longer time range (see
text).
680 nm. This process, because of the relatively small spectral
changes and time regime, is assigned to vibrational relaxa-
tion of the initially prepared Franck–Condon state. Succes-
sively, within the time range 2–500 ps (Figure 9), the bleach
in the region l=630–720 nm tended to disappear, until a
transient peak at about l=650 nm was clearly formed. For
time delays longer than 800 ps from the laser pulse
(Figure 10), the transient spectrum decayed mono-exponen-
tially within the available time range without any further
spectral change, which is in fair agreement with emission
data. The nature of the processes occurring in the time
frame 2–500 ps in the excited-state decay of TC was not in-
vestigated here; however, it seems likely that the excited-
state decay process of TC deals with some structural rear-
rangement involving the polypyridine moiety, because the
transient absorption spectra of analogous Bodipy species
containing the amino group but missing the polypyridine
fragments do not exhibit such complex behavior. The intrin-
sic excited-state decay of TC has to be taken into account
when discussing the intercomponent energy transfer process-
es in the multichromophoric species containing this type of
subunit.
Figure 7. Transient absorption spectra of TB.
time scale than the experimentally investigated window,
which is in agreement with the emission lifetime of TB
(3.9 ns in DCE).
Compound TC: The initial transient spectrum of TC exhib-
its a bleach in the region l=580–720 nm and a transient at
l<550 nm. Contrary to the spectra of TA and TB, the tran-
sient spectrum of TC does not decay directly to the ground
state, but undergoes a series of successive processes (see
Figures 8–10). In the time frame 0.6–2 ps (Figure 8), the
bleach at about l=610 nm is partially recovered and the
bleach at wavelengths longer than l=650 nm increases, sug-
gesting the presence of a transient absorption in the l=600–
750 nm region—partially obscured by the strong bleach-
ing—which is modified in this time range. Isosbestic points
at about l=650 and 575 nm hold during this process, the
time constant (t, the reciprocal of the rate constant) of
which is about 1 ps, calculated both at l=610 and at
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of TC (see Figure 8), so it is assigned to vibrational relaxa-
tion within the C subunit. The successive process involves
recovery of the bleach at l=575 nm coupled with a bleach
at a slightly higher wavelength than l=610 nm (Figure 12,
transient absorption spectra between 1.29 and 187 ps). Sev-
eral isosbestic points hold during this time range, for exam-
ple at l=702 nm. This process is biphasic, with a faster com-
ponent having a time constant of (5.8ꢁ1) ps and a slower
component having a time constant of (52ꢁ7) ps, As the
decay of the C-based subunit exhibits components in this
time range (see Figure 9), one tends to attribute at least one
of these decay components of TBC to an intrinsic decay of
the C subunit. However, the spectral changes at l=575 nm,
which are monotonic within the overall 1.3–187 ps range,
suggest that decay of the B subunit predominates (note that
a B-to-C energy transfer would impose recovery of the
bleaching, whereas an intrinsic decay within C would led to
an increase of the bleach in this time range, see Figure 9),
and the simultaneous increase of the bleach at l=610 nm
supports the attribution of both components to an energy
transfer from the excited state of the B subunit to the excit-
ed state of the C subunit. The possible presence of two con-
formers in which the B and C dyes have different inter-chro-
mophoric separation could justify the bi-exponential nature
of the energy-transfer process. Once produced, TB(*C)
decays with a complex behavior, analogously to that of
T(*C).
Figure 11. Transient absorption spectra of TAB. The inset shows the ki-
*
netics of the bleach recovery at l=505 nm ( ) and the bleach increase at
^
l=578 nm ( ). For details, see text.
Compound TAB: The bleaches present at about l=505 and
580 nm in the transient absorption spectrum of TAB are in-
dicative of the simultaneous excitation of the A and B
Bodipy dyes, respectively, in TAB (see Figure 11). In the
time frame 1–90 ps, the transient absorption spectrum
evolves, with the recovery of the bleach at l=505 nm and a
(slight) increase and red-shift of the bleach at about l=
580 nm. The recovery of the bleach at l=505 nm indicates
the decay of the A-centered excited state, whereas the con-
comitant changes in the l=580 nm bleach suggest further
production of the B-centered excited state. Isosbestic points
remain at about l=660 and 570 nm. The time constants
measured for the spectral changes at l=505 [(63ꢁ10) ps]
and 578 nm [(62ꢁ22) ps] are nearly identical (Figure 11,
inset), signaling an energy transfer from the excited state of
the A subunit to the excited state of the B subunit of TAB
(i.e., T(*A)B-to-TA(*B) energy transfer). Then, the pro-
duced transient spectrum decays to the ground state with an
apparent mono-exponential process, the time domain of
which is out of the explored window.
Compound TABC: In the initially formed transient absorp-
tion spectrum of this compound (Figure 13), the bleachings
at l=505, at 575, and in the l=600–700 nm range can be
assumed as signatures for the presence of T(*A)BC,
TA(*B)C, and TAB(*C), respectively. To investigate the
possible intercomponent energy-transfer processes, it is con-
venient to analyze the fate of the bleachings at l=575 and
Compound TBC: The initial bleaching at about l=575 nm
and that in the l=600–700 nm range are taken as signatures
of the T(*B)C and TB(*C) states, respectively (see
Figure 12, compared with Figures 7 and 8). In the time
range 330 fs–1.30 ps, the bleaching in the range l=560–
640 nm is partially recovered, whereas the bleaching at
Figure 13. Transient absorption spectra and normalized kinetic decays of
TABC (inset), shorter time range. The focus here is on the energy trans-
fer from the B-centered to the C-centered excited states (see text).
at 505 nm separately. After an initial spectral evolution in
the fs time scale (not shown), which appears to be almost at
an end after 1 ps [(t=720ꢁ100) fs] and recalls the fast pro-
Figure 12. Transient absorption spectra of TBC.
longer wavelengths increases with an isosbestic point kept at
l=642 nm. This process, which occurs with t=(700ꢁ90) fs,
appears to be qualitatively similar to the fast decay process
ACHTUNGTRENcNUGN ess occurring in TC and TBC (thus is assigned to vibration-
al relaxation within the C-based unit), the bleaching at l=
575 nm, typical of a B-based excited state, is recovered by a
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S. Campagna, R. Ziessel et al.
Table 2. Rate constants (calculated values from Equation (1) and experi-
mental data from transient absorption spectroscopy measurements) of
the intercomponent energy-transfer processes occurring in the multi-
Bodipy species.[a]
biphasic process (Figure 13). The faster component has a
rate constant of (3.3ꢁ0.2)ꢀ1011 sꢀ1 [time constant, (3ꢁ
0.4) ps]. This process is simultaneous to an increased bleach
at l=610 nm, having the same rate constant (Figure 13,
inset). The slower component of the l=575 nm bleach re-
covery takes place with a time constant of (56ꢁ6) ps (see
Figure 14).2These processes are very similar, both from a
Energy-transfer
(involved subunits)
JF
ken
]
ken
]
[b]
[c]
G
[sꢀ1
G
TAB
TBC
A !B
4.3ꢀ10ꢀ13
6.8ꢀ10ꢀ13
7.4ꢀ1010
1.6ꢀ1011
1.6ꢀ1010
1.7ꢀ1011
1.9ꢀ1010
assumed as
for TAB
3.3ꢀ1011
1.8ꢀ1010
1.3ꢀ1010[e]
B!C[d]
TABC
A!B
B!C[d]
A!C
as for TAB
as for TBC
as for TAB
as for TAB
as for TBC
5.5ꢀ1010
[a] The solvent was 1,2-dichloroethane; for the parameters used in Equa-
tion (1), see text. [b] Calculated by using Equation (1). [c] Obtained from
the transient absorption data. [d] The B-to-C energy-transfer process is
experimentally biphasic, see main text. [e] This is an estimated value, ob-
tained from the difference between the decay of the A-based excited
state in TABC and the decay of the corresponding chromophore subunit
in TAB, see text.
Figure 14. Transient absorption spectra and normalized kinetic decays of
TABC (inset), longer time range. The focus here is on the energy transfer
from the B-centered to the C-centered excited states (see text).
1) Energy transfer from A to B subunits in TAB takes
place with t=62 ps (rate constant, ken =1.6ꢀ1010 sꢀ1)3.
2) Energy transfer from B to C subunits in TBC takes place
bi-exponentially with time constants of about 6 and 52 ps
(i.e., ken =1.7ꢀ1011 and 1.9ꢀ1010 sꢀ1, respectively); the
possible presence of two conformers of TBC is likely re-
sponsible for this biphasic behavior.
3) In the multichromophoric TABC species, energy transfer
from B to C is quite similar to the corresponding process
in TBC, exhibiting time constants of about 3 ps (ken =
3.3ꢀ1011 sꢀ1) and 56 ps (ken =1.8ꢀ1010 sꢀ1).
spectral viewpoint and as far as time constants are con-
cerned, to the first two processes exhibited by TBC and can
be assigned to an energy transfer from TA(*B)C to
TAB(*C). Bleach recovery at l=505 nm takes place with a
time constant of (34ꢁ4) ps and is accompanied by a small
increase of the bleach at l=610 nm (Figure 15). On longer
4) In TABC, the excited state of A can decay by energy
transfer to both B and C. Assuming that the energy
transfer from A to B in TABC is identical to that found
in TAB—that is, it occurs with ken =1.6ꢀ1010 sꢀ1—this as-
sumption must be taken with care because some (al-
though small) kinetic difference between B-to-C energy-
transfer processes in TBC and TABC is indeed present.
The difference in the time constants for the disappear-
ance of the A-based excited state in TAB (62 ps, see
above) and TABC (34 ps) is attributed to a contribution
from direct energy transfer from the A subunit to C.
Direct energy transfer from A to C in TABC is therefore
assumed to occur with a rate constant of about ken =1.3ꢀ
1010 sꢀ1.4 Note that the energy transfer from the A to the
Figure 15. Transient absorption spectra and normalized kinetic decays of
TABC (inset), intermediate time range. The focus here is here on the dis-
appearance of the A-based excited state (see text).
time scales, the prepared TAB(*C) decays to the ground
state with the typical processes already evidenced for TC
(spectra not shown).
3
In principle, the rate constant of the energy-transfer process is not
given by the reciprocal of the time constant of the decay, because the
intrinsic decay of the chromophore also contributes to this later pro-
cess. However, because the intrinsic decays of the A and B chromo-
phores (i.e., the emission lifetimes of TA and TB) are negligible com-
pared to the excited state decays of the A-based and B-based chromo-
phores in multicomponent species, the rate constants of the intercom-
ponent energy transfer are approximated to the reciprocal of the re-
spective decay time constants for all the energy-transfer processes dis-
cussed here.
Interchromophoric energy transfer in multi-Bodipy species:
From the above-reported transient absorption data, the fol-
lowing conclusions can be summarized and are collected in
Table 2:
2
The expected concomitant increase of the bleach at l=610 nm during
4
this slower process is probably balanced by the intrinsic decay of the C-
based subunit, which has components in the same time scale. As a con-
sequence, the l=610 nm bleach is almost constant in this time scale
(see insets of Figures 14 and 15).
The time constant of 34 ps for the decay of the A-centered excited
state in TABC, equivalent to a decay rate constant of 2.9ꢀ1010 sꢀ1
,
would therefore be factorized into two energy-transfer components,
that is, [kenA(A-to-B)+kenA(A-to-C)].
C
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B subunit in TABC should be followed by energy trans-
fer from B to C. Because both the components of the
energy-transfer process from B to C in TABC are faster
than energy transfer from A to B, the intermediate
TA(*B)C in the two-step, cascade energy-transfer path-
way from A to C cannot be experimentally proven.
In Equation (1), other relevant parameters beside JF are:
n, the refraction index of the medium (for 1,2-dichloro-
ethane, n=1.4448), K, a statistical orientation factor, usually
considered as 2/3 (this is the value we also assumed here,
where free rotation of the dyes around the triple-bond axis
connecting each dye to the triptycene scaffold is allowed,
and some flexibility of the overall systems cannot be exclud-
ed), F and t, the unquenched emission quantum yield and
the lifetime of the donor (taken as the emission quantum
yields and the lifetimes of the parent TA and TB species, re-
ported in Table 1), and rAB, the distance (in [ꢄ]) between
the donor and acceptor dipoles. The rAB is probably one of
the most difficult parameters to quantify for species in solu-
tion. For the multi-Bodipy species studied here, we approxi-
mated rAB to the computed distance between the methene
carbon atoms of different chromophores across the tripty-
cene framework (22.9 ꢄ). By using this approximate donor–
acceptor distance, the rate constants for the Fçrster energy
transfer reported in Table 2 have been obtained. The calcu-
lated values have the same order of magnitude as the re-
spective experimental values (for the B-to-C energy-transfer
process, the calculated values are compared to the faster
rate constant), so, considering the approximation used, Fçr-
ster energy transfer is assumed to be an effective mechanism
for the intercomponent energy-transfer processes in these
compounds.
It should be noted that the energy-transfer rate constants
measured for the present multi-Bodipy species are close to
those obtained for star-shaped multi-Bodipy arrays made of
similar Bodipy dyes arranged on a truxene core, also featur-
ing comparable donor–acceptor distances.[10] This confirms
the structural role of the triptycene scaffold and its “inno-
cent” role—from an electronic point of view—for the inter-
Bodipy energy-transfer mechanism (as well as for the trux-
ene scaffold in the formerly studied species), according to
the Fçrster theory. On the contrary, energy-transfer rate
constants of one order of magnitude slower have been re-
ported for inter-Bodipy dyes in roughly linear dichromo-
phoric species containing a fluorene spacer, in spite of simi-
lar (or slightly smaller) donor–acceptor distances.[13] In this
latter case, however, the Fçrster overlap integrals [Eq. (2)]
between the donor emission and the acceptor absorption
spectra were smaller and therefore probably responsible for
the reduced energy-transfer rate constants.
Figure 16. Schematic of the excited states and decay (upon population of
the excited state involving the highest-energy Bodipy subunit) of the
multi-Bodipy compounds TAB, TBC, and TABC. The asterisk denotes
where the excitation is localized in each level. Wavy, dashed lines indi-
cate energy-transfer processes and solid lines indicate radiative decays;
radiative decays from higher-energy Bodipy subunits are neglected, as
well as radiationless decays to the ground state. Energy-transfer time
constants are shown (time constants in the grey background are estimat-
ed, not measured, see text). The energy of the excited-state levels are not
to scale.
The above discussion is summarized in Figure 16, which
shows a schematic of the excited-state levels and decays of
the multi-Bodipy species studied here.
In multi-Bodipy systems, inter-Bodipy energy transfer
usually takes place through a Fçrster mechanism.[5,10–15]
simplified Fçrster equation is shown in Equation (1):
A
K2F
n4rA6 Bt
kFen ¼ 8:8 ꢂ 10ꢀ25
JF
ð1Þ
Finally, the bi-exponential energy transfer from the B-cen-
tered subunit to the C-centered subunit in TBC and TABC,
which is attributed to the existence of more than one con-
former, warrants some further comment. We propose a ten-
tative explanation based on the following discussion. Both B
and C Bodipy dyes are asymmetric with respect to the main
Bodipy framework because they have a single pyrrole ring
substituted by a conjugate moiety (see Schemes 1–3). The
lower-lying excited states of both B- and C-containing chro-
mophore subunits—which are the excited states involved in
the energy-transfer processes—are therefore expected to
partially extend over these (conjugated) substituted frag-
ments; this is also supported by the red shift of their absorp-
In Equation (1), JF is the Fçrster overall integral between
¯
the luminescence spectrum of the donor, F(V), and the ab-
¯
sorption spectrum of the acceptor, e(V), on an energy scale
(in [cmꢀ1]). The expression for JF is given in Equation (2),
and the JF values calculated by this equation for energy
transfer from A-to-B, A-to-C, and B-to-C subunits of the
multicomponent species studied herein are reported in
Table 2.
R
ꢀ ꢁ ꢀ ꢁ
ꢀ
ꢀ
ꢀ4 ꢀ
F V e V =V dV
R
ꢀ ꢁ
JF ¼
ð2Þ
ꢀ
ꢀ
F V dV
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S. Campagna, R. Ziessel et al.
tion and emission spectra with respect to those of TA.5 This
suggests that the donor–acceptor distance (and therefore the
rate constant) for energy-transfer processes involving B- and
C-containing chromophores also depends on the relative ori-
entation of pyrrole-conjugate substituents. Whereas rotation
of the A-type Bodipy subunit around the triple bond con-
necting it to the triptycene central moiety (or rotation of the
Bodipy framework with respect to the phenyl ring linked to
the triple bond) does not significantly modify the position of
this chromophore with respect to the other chromophores in
multi-Bodipy assemblies, rotation of the B-type and C-type
chromophores around the triple bond connecting them with
the triptycene moiety (or with respect to the phenyl ring
linked to the triple bond) can substantially change the rela-
tive distance between their respective pyrrole substituents in
multi-Bodipy species. This can strongly affect the effective
donor–acceptor distance rAB, leading to slower energy-trans-
fer rate constant for TBC and TABC species having particu-
larly distant B and C dyes and to conformers in which the
effective through-space donor–acceptor distance—consider-
ing that the excited-state dipoles can involve the conjugate
pyrrole substituents—can be smaller, so yielding different
energy-transfer rate constants.
induced energy transfer occurs in all the multi-Bodipy spe-
cies, with the lower-energy Bodipy subunits of each multi-
Bodipy compound playing the role of the electronic energy
collector. The interpretation of the intricate energy-transfer
pathways occurring in the multicomponent TABC species is
made with the help of the processes identified in TAB and
TBC bichromophoric compounds. In all cases, the measured
energy-transfer rate constants agree with a Fçrster mecha-
nism for the energy-transfer processes.
Acknowledgements
The authors thank the Italian MIUR(Project codes: PRIN-
20085ZXFEEand FIRB-RBAP11C58Y (Nanosolar)) for financial sup-
port. We also acknowledge the CNRS for provision of financial support
and the Universitꢁ de Strasbourg for research facilities. F.P. and F.N. also
thank the University of Messina for special funds for young researchers.
[1] The literature on this topic is too vast to be exhaustively cited; for
selected papers relevant to this work, see: a) D. Gust, T. A. Moore,
A. L. Moore, Acc. Chem. Res. 2001, 34, 40; b) D. Holten, D. F.
Bocian, J. S. Lindsey, Acc. Chem. Res. 2002, 35, 57; c) S. Serroni, S.
Campagna, F. Puntoriero, C. Di Pietro, F. Loiseau, N. D. McClena-
ghan, Chem. Soc. Rev. 2001, 30, 367; d) D. M. Guldi, Chem. Soc.
Rev. 2002, 31, 22; e) S. Fukuzumi, Org. Biomol. Chem. 2003, 1, 609;
f) N. Armaroli, Photochem. Photobiol. Sci. 2003, 2, 73; g) T. S. Bala-
ban, Acc. Chem. Res. 2005, 38, 612; h) W. B. Davis, M. A. Ratner,
M. R. Wasielewski, J. Am. Chem. Soc. 2001, 123, 7877; i) M. T. Indel-
li, C. Chiorboli, F. Scandola, E. Iengo, P. Osswald, F. Wꢅrthner, J.
Phys. Chem. B 2010, 114, 14495; j) B. Albinsson, J. Martensson, J.
Photochem. Photobiol. C 2008, 9, 138; k) T. Weil, T. Vosch, J. Hof-
Conclusion
A series of new compounds in which various Bodipy dyes
are grafted on triptycene rigid structures have been synthe-
sized and characterized, and their absorption spectra and
photophysical properties have been investigated. The new
compounds also include several multi-Bodipy species, most
of them containing different chromophores. Step-by-step
grafting of the Bodipy modules A, B, and/or C to the preor-
ganized triptycene platform is feasible with two modules.
For the linkage of three different Bodipy dyes leading to
TABC, an original strategy was devised based on successive
regioselective monosubstitution on a preformed homo-tris-
Bodipy dye TA3. In the multi-Bodipy species studied here,
the triptycene moiety plays the role of a rigid scaffold, keep-
ing the various dyes at predetermined distances and allow-
ing for a three-dimensional structural arrangement of the
chromophores, guaranteeing the supramolecular nature of
the arrays. The absorption spectra of the multichromophoric
Bodipy species are essentially additive, indicating that negli-
gible inter-chromophoric interaction takes place at the
ground state, and are dominated by the absorption proper-
ties of the individual Bodipy dyes. Luminescence properties
and transient absorption spectroscopy indicate that very fast
(on the picosecond time scale) and efficient (>90%) photo-
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5
Note that this also means that the center of the dipole in B-type and C-
type chromophores should be slightly delocalized over the pyrrole sub-
stituent, so the distance rAB for A-to-B and A-to-C energy transfer
should be slightly longer than 22.9 ꢄ, as used in the text. This could jus-
tify the smaller experimental values of rate constants for A-to-B and
A-to-C energy transfer in comparison with their calculated values, for
which we used 22.9 ꢄ as the donor–acceptor distance (see Table 2).
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Energy Transfer in Bodipy Scaffoldings
FULL PAPER
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Received: February 1, 2013
Published online: && &&, 0000
Chem. Eur. J. 2013, 00, 0 – 0
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www.chemeurj.org
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S. Campagna, R. Ziessel et al.
Fast, faster, ultrafast: Grafting three
identical Bodipy units on a triptycene
core allowed regioselective transforma-
tion of a single Bodipy through a Kno-
venagel reaction. The use of two differ-
ent aromatic aldehydes produced a
triptycene platform modified by three
different Bodipy dyes. Luminescence
properties and transient absorption
spectroscopy indicate a fast (ps) step-
wise energy transfer from higher- to
lower-energy Bodipy units (see figure).
Bodipy Dyes
T. Bura, F. Nastasi, F. Puntoriero,
S. Campagna,* R. Ziessel* .. &&&& — &&&&
Ultrafast Energy Transfer in Tripty-
cene-Grafted Bodipy Scaffoldings
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Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!