Promising fast energy transfer system via an easy synthesis: Bodipy-porphyrin dyads connected via a cyanuric chloride bridge, their synthesis, and electrochemical and photophysical investigations

Article abstract of DOI:10.1021/ic201052k

The boron dipyrrin (Bodipy) chromophore was combined with either a free-base or a Zn porphyrin moiety (H₂P and ZnP respectively), via an easy synthesis involving a cyanuric chloride bridging unit, yielding dyads Bodipy-H₂P (4) and Bo

Full text of DOI:10.1021/ic201052k

ARTICLE
pubs.acs.org/IC
Promising Fast Energy Transfer System via an Easy Synthesis:
BodipyꢀPorphyrin Dyads Connected via a Cyanuric Chloride Bridge,
Their Synthesis, and Electrochemical and Photophysical
Investigations^
Theodore Lazarides,† Georgios Charalambidis,† Alexandra Vuillamy,†,§ Marius Rꢀeglier,*,§
Emmanuel Klontzas,† Georgios Froudakis,† Susanne Kuhri,‡ Dirk M. Guldi,*,‡ and
Athanassios G. Coutsolelos*,†
†Laboratory of Bioinorganic Chemistry, Chemistry Department, University of Crete, Voutes Campus, P.O. Box 2208, 71003 Heraklion,
Crete, Greece
‡Department of Chemistry and Pharmacy, Interdisciplinary Center for Molecular Materials (ICMM), Friedrich-Alexander-Universitaet
of Erlangen-Nuernberg, Egerlandstrasse 3, 91058 Erlangen, Germany
§ISM2 UMR 6263, CNRS, Aix-Marseille Universitꢀe, 13397 Marseille, Cedex 20, France
S Supporting Information
b
ABSTRACT: The boron dipyrrin (Bodipy) chromophore was
combined with either a free-base or a Zn porphyrin moiety
(H2P and ZnP respectively), via an easy synthesis involving a
cyanuric chloride bridging unit, yielding dyads Bodipy-H2P (4)
and Bodipy-ZnP (5). The photophysical properties of Bodipy-
H2P (4) and Bodipy-ZnP (5) were investigated by UV-Vis
absorption and emission spectroscopy, cyclic voltammetry, and
femtosecond transient absorption spectroscopy. The compar-
ison of the absorption spectra and cyclic voltammograms of
dyads Bodipy-H2P (4) and Bodipy-ZnP (5) with those of their
model compounds Bodipy, H2P, and ZnP shows that the
spectroscopic and electrochemical properties of the constituent
chromophores are essentially retained in the dyads indicating
negligible interaction between them in the ground state. In addition, luminescence and transient absorption experiments show that
excitation of the Bodipy unit in Bodipy-H2P (4) and BodipyꢀZnP (5) into its first singlet excited state results in rapid Bodipy to
porphyrin energy transferꢀk4 = 2.9 ꢁ 1010
s
ꢀ1 and k5 = 2.2 ꢁ 1010
s
ꢀ1 for Bodipy-H2P (4) and Bodipy-ZnP (5), respectively-
generating the first porphyrin-based singlet excited state. The porphyrin-based singlet excited states give rise to fluorescence or
undergo intersystem crossing to the corresponding triplet excited states. The title complexes could also be used as precursors for
further substitution on the third chlorine atom on the cyanuric acid moiety.
’ INTRODUCTION
their light-harvesting potential, porphyrins can be derivatized
with highly absorbing antenna chromophores, which act as
sensitizers of the porphyrin-based excited state. The boron
dipyrrin, Bodipy, chromophore is particularly effective as an
antenna group for porphyrins and phthalocyanines as it com-
bines a high fluorescence quantum yield, a relatively long lifetime,
a suitable excited state energy, and excellent photostability.8
A Bodipyꢀporphyrin conjugate, with an acetylide group as
bridge, has recently shown improved power conversion effi-
ciency, η, compared to its parent porphyrin chromophore when
used as a sensitizer in a DSSC.9 In search of alternative ways to
combine Bodipy and porphyrin chromophores we explored the
Molecular systems combining two or more chromophores
have enjoyed continuing interest in their photophysical and
photochemical properties due to their potential for important
applications ranging from light harvesting and storage to sensors
and optoelectronic devices.1ꢀ3 Recent developments in the field
of dye-sensitized solar cells (DSSCs) have shown that porphyrin
derivatives are good candidates for photovoltaic applications due
to their highly reducing excited state which leads to efficient
electron injection into metal oxide semiconductors such as ZnO
or TiO2.4ꢀ6 However, despite the favorable excited state redox
potentials of porphyrins, their relatively modest molar absorp-
tivity values in the low-energy visible region (maximum ε of
∼20 000 Mꢀ1 cmꢀ1 at 500ꢀ700 nm)7 represent a significant
drawback for their use in photocurrent generation. To improve
Received: May 18, 2011
Published: August 16, 2011
r
2011 American Chemical Society
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Scheme 1. Synthesis of the Compounds Discussed in This Paper (1ꢀ5)
use of cyanuric chloride as a bridging unit. Cyanuric chloride has
been used as the linker for the preparation of porphyrinic dimers
and oligomers10ꢀ12 and porphyrinꢀoxadiazole dyads13 and for
the immobilization of porphyrins on resin beads.14 In this work,
we describe the facile synthesis of Bodipyꢀporphyrin dyads
using cyanuric chloride as the linker. The dyads are studied
photophysically by steady state and time-resolved absorption and
emission spectroscopy to investigate the dynamics and mechan-
ism of Bodipy to porphyrin energy transfer, which are discussed
along with the potential importance of such dyads as sensitizers
in solar energy conversion schemes.
Further details on their synthesis and characterization can be
found in the Experimental Section.
NMR Spectroscopy. Confirmation of the molecular formulas
from the coupling of the cyanuric chloride on the monoamino
porphyrin to the final derivatives metalated with zinc or not has
been followed by 1H NMR and 13C NMR experiments. There is
no significant chemical shift displacement observed during the
transformations with respect to the used starting materials.
Figure 1 depicts the proton NMR spectra of compounds 4 and
5. However, except the presence of all protons for each entity,
several chemical shifts of the key protons in the different steps
have been observed. The presence of the cyanuric group between
the monoamino porphyrin and the amino Bodipy could be
confirmed by the displacement of the chemical shift of the
phenyl group protons at higher frequencies. Attachment of the
Bodipy influences its pyrrolic protons: from a singlet correspond-
ing to two protons, two singlet peaks have been observed (see
Supporting Information). Finally, after the metalation reaction
the peak at ꢀ2.76 ppm, which corresponds to the two free
protons of the porphyrin ring, disappears.
Electrochemistry. The electrochemical properties of the two
Bodipyꢀporphyrin conjugates, 4 and 5, and their model com-
pounds, 1, 2, and 3, were investigated by cyclic and square wave
voltammetry in dichloromethane. The redox data are collected
in Table 1, and some representative cyclic voltammograms are
depicted in Figure 2. All data are reported vs the ferrocene/
ferrocenium couple (Fc/Fc+). Comparison of the cyclic voltam-
mograms of dyads 4 and 5 with those of their model compounds
and the electrochemical data of Bodipy28,29 and porphyrin7
derivatives reported in the literature shows that the two reversible
processes occurring in the range 0.35ꢀ0.52 and 0.71ꢀ0.76 V vs
’ RESULTS AND DISCUSSION
Synthesis and Characterization. The synthesis of the com-
pounds discussed in this paper is shown in Scheme 1. Reaction
of BDPNH2 with cyanuric chloride under low-temperature
conditions (0 ꢀC to room temperature) affords the monoadduct
1, which is stable enough to be purified by column chromatog-
raphy. Similarly, reaction of TPPNH2 with cyanuric chloride
leads to the precursor 2,11,27 which can be readily converted
to the corresponding Zn complex, 3, by treatment with Zn-
(CH3COO)2 2H2O. The Bodipyꢀporphyrin conjugate 4 can
3
be obtained by reaction of 2 with BDPNH2 at 60 ꢀC. At this
higher temperature, the second chloride of the cyanuric chloride
group can be substituted by an amine.11 Reaction of the free
base porphyrin derivative 4 with Zn(CH3COO)2 2H2O readily
3
affords the corresponding Zn derivative 5. The new compounds
1ꢀ5 were characterized on the basis of their 1H and 13C NMR
spectra, MALDI-TOF mass spectra, and elemental analyses.
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Figure 1. 1H NMR spectrum of 5 (a) and 4 (b). The asterisks (*) denote solvent peaks.
Table 1. Redox Data Reported vs Fc/Fc+ in Dichloromethane
Using Tetrabutylammonium Hexafluorophosphate 0.1 M as
Supporting Electrolytea
E1/2Ox1/V
(ΔEp/V)b
compound
E1/2Ox2/ V
E1/2Red1/ V
E1/2Red2/ V
ꢀ2.07(0.07)
2
3
1
4
5
0.51(0.07)
0.37(0.07)
0.76(0.08)
0.52(0.06)
0.35(0.06)
0.79(0.07)
0.68(0.07)
ꢀ1.71(0.07)
ꢀ1.82(0.07)
ꢀ1.74(0.08)
ꢀ1.70(0.08)
ꢀ1.65(0.07)
0.76(0.15)
0.71(0.11)
ꢀ2.09(0.07)
ꢀ1.80(0.06)
a All reported waves are reversible (ΔEp of the Fc/Fc+ couple is 0.07 V).
b The anodicꢀcathodic peak separations are given in parentheses.
Fc/Fc+ (Table 1 and Figure 2) are due to the first porphyrin-
based oxidation and to the overlapping waves of the second
porphyrin and Bodipy-based oxidations, respectively. In addi-
tion, the dyad 4 shows two successive reversible processes at
ꢀ1.70 and ꢀ2.09 V vs Fc/Fc+, attributed to the overlapping
Bodipy and first porphyrin-based reductions and to the second
porphyrin-based reduction, respectively. In 5, the Bodipy and
first porphyrin-based oxidations appear as two closely spaced
reversible waves at ꢀ1.65 and ꢀ1.80 V, respectively. The
anticipated second porphyrin-based reduction of 5 is probably
obscured due to its occurrence beyond the solvent window,7 a
behavior that is also seen in model compound 3.
Photophysical Properties. Electronic Absorption Spectra.
The electronic absorption spectra of 4 and 5 and the model
compounds 1, 2, and 3 are shown in Figure 3. A collection of
absorption data can be found in Table 2. The electronic spectrum
of 1 (Figure 3a) is dominated by an intense absorption band at ca.
500 nm attributed to the lowest energy spin-allowed πꢀπ*
Figure 2. Cyclic voltammograms of a) 3 and b) 5. The reversible couple
of ferrocene (Fc/Fc+) is also shown. The voltages are vs the saturated
calomel electrode (SCE).
transition of the Bodipy moiety.30 The spectrum of 1 also shows
absorption bands of lower intensity in the 280ꢀ380 nm region
attributed to higher energy πꢀπ* transitions of the Bodipy
group.30 The UVꢀVis absorption spectra of 2 and 3 (Figure 3b
and 3c), as expected for TPP and ZnTPP-type chromophores,
exhibit intense Soret bands at ca. 420 nm and weaker Q bands in
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the 500ꢀ700 nm region (i.e., four Q bands for the free-base
porphyrin 4 and two Q bands for the Zn complex 5).7
Comparison of the spectra of the dyads 4 and 5 with those of
their model compounds 1, 2, and 3 shows that they are essentially
the sum of their constituent chromophores. This indicates
negligible electronic interactions between the porphyrin and
Bodipy units in the ground states of the dyads.
Emission Spectra. Compounds 1ꢀ5 were studied by steady
state and time-resolved emission spectroscopy in dichloro-
methane at ambient temperature. A collection of emission
maxima, quantum yields, and lifetimes is given in Table 2. Some
representative emission and excitation spectra can be seen in
Figures 4 and 5. The Bodipy derivative 1 shows the intense
fluorescence of a Bodipy dye30 at 513 nm with a lifetime, τ1, of
3.0 ns. The porphyrin derivatives 2 and 3 exhibit the character-
istic fluorescence of free-base and Zn porphyrins with emission
maxima at 650 and 712 nm for 2 (τ2 = 8.8 ns) and at 596 and
645 nm for 3 (τ3 = 1.8 ns). When the dyad 4 is irradiated at
486 nm, corresponding to selective excitation of the Bodipy unit,
it shows emission from the porphyrin chromophore at 650 and
714 nm (Figure 4a) in addition to residual fluorescence from the
Bodipy group at 512 nm.30 The Bodipy-based emission in 4 is
quenched by ca. 98% when compared to the emission measured
for the model compound 1. In addition, the excitation spectrum
of 4 monitoring at the emission of the porphyrin moiety at
652 nm shows a pronounced Bodipy absorption feature at ca.
500 nm (Figure 5a). This is an indication of Bodipyꢀporphyrin
Figure 3. UVꢀVis absorption spectra of (a) 1, (b) 2 and 4
(normalized), and (c) 3 and 5 (normalized) in dichloromethane.
Figure 4. Room temperature emission spectra in dichloromethane of
(a) 4 and (b) 5 upon selective excitation of the Bodipy unit at 476 nm.
Table 2. Summary of Spectroscopic Data for Compounds 1ꢀ5
absorption
emission
compound
λmax/ nm (ε/ Mꢀ1cmꢀ1
)
λmax/ nm
513
Φ
τ/ ns
1
2
3
4
5
502(75 600); 342(9400); 277(33 000)
0.58
0.11
0.032
3.0
418(478 000); 514(19 000); 549(9000); 589 (6000); 645 (4500)
419(585 000); 542(20 400); 583(6000)
650; 712
8.8
596; 645
1.8
419(419 000); 500(74 800); 550(8300); 589(5200); 645(4200)
420(434 000); 500(75 300); 546(21 500); 583(7200)
512; 650; 714
513; 595; 645
8.7a
1.9a
a Measured at the emission wavelengh of a porphyrin moiety using a bandpass filter centered at 670 nm with a bandwidth of ∼50 nm.
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Figure 5. Room temperature excitation spectra of (a) 4 monitoring at
652 nm and (b) 5 monitoring at 654 nm.
energy transfer following excitation of the Bodipy unit. Dyad 5
shows analogous behavior to its nonmetalated counterpart, 4, as
can be seen in the corresponding emission and excitation
spectra in Figures 4b and 5b. Excitation of 5 at 486 nm results
in dual emission corresponding to the residual fluorescence of
the Bodipy unit at ca. 512 nm (quenched by ca. 96% compared
to model compound 1) and to the Zn porphyrin chromophore
at 596 and 645 nm. In the case of 5, the residual Bodipy-based
fluorescence is more intense than the sensitized emission of the
porphyrin unit. This is mainly due to the intrinsically lower
luminescence quantum yield of zinc porphyrins compared to
their parent macrocycles.16 The lifetime of the residual Bodipy-
based fluorescence—and therefore the rate of Bodipy to
porphyrin energy transfer—in 4 and 5 could not be measured
with our experimental setup. However, femtosecond transient
absorption measurements discussed below allowed us to ana-
lyze the dynamic behavior of 4 and 5 at the picosecond
time scale.
Transient Absorption Spectroscopy. Femtosecond transient
absorption spectroscopy was used to obtain further insight into
the excited state interactions in dyads 4 and 5 and to corro-
borate the proposed energy transfer process. Toward this end,
1ꢀ5 were probed with either 420 or 490 nm excitation to
excite separately the porphyrin or Bodipy chromophore,
respectively.
In the cases of 2 and 3, the singlet excited states—formed
essentially right after the laser pulse—include a net decrease of
the absorption around 550 nm, a region which is dominated by
the strong free-base porphyrin (H2P) and Zn porphyrin (ZnP)
ground state absorption. This suggests consumption of H2P and
ZnP as a result of converting the singlet ground states of 2 and 3
to the corresponding singlet excited states. An immediately
formed absorption accompanies the bleach. For example, the
H2P-based singlet excited state in 2 exhibits a new absorption
band between 660 and 730 nm. The singlet transients produced
Figure 6. (a) Differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (490 nm, 150 nJ) of 1 in
toluene with several time delays between 0 and 100 ps at room
temperature. (b) Timeꢀabsorption profiles of the spectra at 505 nm,
monitoring the intersystem crossing dynamics.
upon photoexcitation decay with lifetimes of 9.8 (2, H2P) and
2.1 ns (3, ZnP) via intersystem crossing to the corresponding
triplet manifold. The main spectral features of the latter state
include maxima at 780 (2, H2P) and 840 nm (3, ZnP).
The corresponding femtosecond experiments with 1 reveal
upon 490 nm laser excitation (Figure 6) the instantaneous
formation of the Bodipy-based singlet excited state features.
Here, the transient absorption spectra are dominated by a
pronounced bleach between 450 and 600 nm, which is due to
depletion of the singlet ground state. In the 400 and 700 nm
regions additional maxima seem to emerge. Overall, all of the
aforementioned Bodipy-based singlet excited state features are
long lived (i.e., 3.2 ns). Implicit is a slow intersystem crossing to
the corresponding triplet manifold.
The spectral features that are seen immediately (i.e., 1 ps)
upon excitation of 5 at 490 nm (Figure 7) are almost super-
imposable with those recorded for the Bodipy singlet excited
state, vide supra. The lifetime of the Bodipy singlet excited state
is, however, greatly diminished by the presence of the covalently
attached ZnP chromophore. Formation of a new transient
develops as a result of the rapid decay (i.e., 45 ps) of the
Bodipy-based singlet excited state instead of the much slower
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Figure 7. (a) Differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (490 nm, 150 nJ) of 5 in
toluene with several time delays between 0 and 100 ps at room
temperature. (b) Timeꢀabsorption profiles of the spectra at 455 and
550 nm, monitoring the intramolecular energy transfer.
Figure 8. (a) Differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (490 nm, 150 nJ) of 4 in
toluene with several time delays between 0 and 100 ps at room
temperature. (b) Timeꢀabsorption profiles of the spectra at 440 and
550 nm, monitoring the intramolecular energy transfer.
intersystem crossing to the triplet excited state. The new
transient resembles the singlet excited state characteristics of
ZnP, that is, maxima at 455, 580, and 620 nm as well as minima at
<430 and 555 nm. This suggests involvement of the ZnP singlet
excited state in an intramolecular energy transfer, occurring
between the photoexcited Bodipy unit and the singlet ground
state of ZnP. The kinetics of Bodipy to porphyrin energy transfer
were determined by exponential fitting of the ZnP singlet excited
Intersystem crossing is the ultimate fate of the H2P singlet
excited state. However, the correspondingly formed triplet
excited state is only discernible on the nanosecond time scale,
due to the fact that the intersystem crossing occurs within the
10 ns time window. The most characteristic feature of the H2P
triplet excited state is a 780 nm maximum.
In reference experiments, 4 and 5 were excited at 420 nm.
However, due to the dominating H2P and ZnP-based absorp-
tions at the 420 nm excitation wavelength, no appreciable
Bodipy-based features were seen at any time following excitation.
Instead, only the H2P and ZnP characteristics emerge.31
In summary, as shown in the Jablonski diagram of Figure 9,
excitation of 4 and 5 into their Bodipy-based πꢀπ* excited states
is followed by energy transfer to the lowest-lying porphyrin-
based singlet excited states with rate constants of k4 = 2.9 ꢁ
state formation (k5 = 2.2 ꢁ 1010
s
ꢀ1) as seen in Figure 7. On a
longer time scale, the fate of the ZnP-based singlet excited state is
identical to that known for Zn porphyrins—intersystem cross-
ing, driven by a strong spinꢀorbit coupling, governs the trans-
formation (i.e., 2.1 ns) of the singlet into the triplet excited state.
The latter was identified by a long-lived (i.e., 100 μs) and strongly
absorbing tripletꢀtriplet maximum at 840 nm.
Upon excitation of 4 at 490 nm (Figure 8), the same Bodipy-
based singlet excited state features are seen early on (i.e., 1 ps). In
line with the photoreactivity of 5, the Bodipy singlet excited state
in 4 decays rapidly (i.e., 34 ps) in the presence of the H2P
chromophore. Importantly, maxima at 440, 570, 620, and 680 nm
as well as minima at <430, 550, 590, and 680 nm are exact
matches of the H2P singlet excite state and, in turn, confirm the
transduction of singlet excited state energy. The rate constant of
1010 sꢀ1 and k5 = 2.2 ꢁ 1010
s
ꢀ1, respectively. The slightly faster
energy transfer rate measured in dyad 4 correlates well with the
larger spectral overlap compared to that in dyad 5, see discussion
below. The observed Bodipy to porphyrin energy transfer
rates in 4 and 5 are considerably faster than those measured in
other Bodipyꢀporphyrin dyads with nonconjugated bridges32
(∼9 ꢁ 109 sꢀ1) and comparable with those found in dyads of this
type connected through an amide linkage8 or a rigid conjugated
energy transfer in 4 is k4 = 2.9 ꢁ 1010
s
ꢀ1, as shown in Figure 8.
ethynyl bridge33 (∼3ꢀ5 ꢁ 1010
s
ꢀ1).
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ꢀ1
acceptor, τD
ꢀ
ꢁ
6
1
Rc
kF ¼
ð3Þ
τD
r
where r is the distance between the two chromophores and Rc is
given by eq 4
!
1=6
9000 ln 10 k2 ϕ
DJF
3
3
3
ð4Þ
Rc ¼
128 π5 N n4
3
3
3
where k2 is an orientation factor for the two dipoles, ϕD is the
luminescence quantum yield of the donor, N is Avogadro’s
number, n is the refractive index of the solvent, and
Figure 9. Jablonski diagram relevant to the photophysics of 4 and 5. 1P*
and P* denote the lowest singlet and triplet porphyrin-based excited
3
Z
states, respectively.
ε ðνÞf ðνÞ
̅
̅
A
D
ν4
JF ¼
dv
ð5Þ
̅
̅
The porphyrin-based singlet excited states formed after
Bodipy to porphyrin energy transfer undergo deactivation either
to the ground state by radiative decay (fluorescence) or to the
corresponding porphyrin-based triplet states by intersystem
crossing. The porphyrin-based triplet excited states in 4 and 5
lie, however, lower than the corresponding Bodipy-based triplet
excited state, so “back transfer” from the porphyrin to the Bodipy
triplet is not possible in these cases. Such “back transfer” was
observed in the study of dyads combining a Bodipy chromophore
with a PtII(diimine)(dithiolate) moiety.34 Thus, the porphyrin-
based triplet excited states of 4 and 5 can be readily generated by
initial excitation of the Bodipy part of the dyads and could act as
effective electron donors in photocurrent or photocatalytic H2
production schemes.35
Discussion on Energy Transfer Mechanism. In general, the
mechanism of energy transfer between a donor and an acceptor
can be described on the basis of two limiting mechanisms.
(i) A double-electron exchange involving one electron from
the LUMO of the excited donor to the empty LUMO of
the acceptor with a simultaneous transfer of another
electron from the HOMO of the acceptor to the half-filled
HOMO of the donor (Dexter mechanism).36
is the F€orster spectral overlap integral calculated from the area
normalized emission spectrum of the donor, f (ν), and the
absorption spectrum of the acceptor, ε (ν).
̅
D
̅
A
Regarding the possibility of through-bond Dexter-type36
Bodipy to porphyrin energy transfer in 4 and 5, it should be
investigated if the bridging unit can effectively mediate orbital
interactions between the two chromophores.38 As can be seen in
Figure 10, the results of a DFT calculation on a geometrically
optimized model of dyad 4 show that the frontier orbitals are
localized either on the Bodipy (HOMOꢀ1 and LUMO) or on
the porphyrin (HOMO and LUMO+1) parts of the molecule
and do not possess any bridge character.39 On this basis, in
addition to the UVꢀVis absorption data of the dyads, we expect
no significant orbital interactions between the two chromo-
phores and the bridging units in 4 and 5. Furthermore, the spec-
troscopic data measured for the model compounds 1, 2, and 3
allowed us to estimate the Dexter overlap integrals in 4 and 5 at
JD/4 = 0.21 eVꢀ1 and JD/5 = 0.12 eVꢀ1, respectively. According to
eq 1, the electronic interaction factors for 4 and 5 should
be H4 = 31 cmꢀ1 and H5 = 35 cmꢀ1 to reproduce the experimen-
tally measured energy transfer rates of k4 = 2.9 ꢁ 1010 sꢀ1 and
k5 = 2.2 ꢁ 1010 sꢀ1. Such values seem to be too high considering
the poor orbital interactions between the constituent chromo-
phores in 4 and 5.40ꢀ42 Therefore, photoinduced energy transfer
by the Dexter mechanism is considered highly unlikely for dyads
4 and 5.
(ii) An excitation transfer as a result of dipoleꢀdipole inter-
actions between the energy donor and the acceptor
(F€orster mechanism).37
The rate constant of energy transfer by the Dexter mechanism,
kD, is given by eq 1
The spectroscopic overlap integrals for Coulombic F€orster-
type Bodipy to porphyrin energy transfer for 4 and 5 were
estimated at JF/4 = 8.6 ꢁ 10ꢀ14 cm6 and JF/5= 5.7 ꢁ 10ꢀ14 cm6
using the experimentally determined spectroscopic parameters
of the model compounds 1, 2, and 3. The corresponding critical
radii for 4 and 5 were found to be Rc/4 = 42.3 Å and Rc/5 = 39.5 Å
assuming a dynamically averaged orientation (k2 = 2/3)37,43 of
the constituent chromophores in the dyads. Such an assumption
is valid given the high degree of rotational freedom and flexibility
of the bridging unit. According to eq 3, the interchromophoric
distances in 4 and 5 are estimated at r4 = 20.1 Å and r5 = 19.6 Å
using the measured energy transfer rates of k4 = 2.9 ꢁ 1010 sꢀ1
4π2H2
kD
¼
JD
ð1Þ
h
where h is Planck’s constant, H is the intercomponent electronic
interaction parameter, and JD is the Dexter spectral overlap
integral calculated from the area normalized donor emission
spectrum, f (ν), and the area normalized acceptor absorption
̅
D
spectrum expressed by its extinction coefficient, ε (ν), both
̅
A
taken on an energy scale, i.e., wavenumbers, ν, as shown in eq 2
̅
Z
JD
¼
ε ðνÞf ðνÞdν
ð2Þ
̅
̅
̅
A
D
and k5 = 2.2 ꢁ 1010
s
ꢀ1. The results of a geometry optimization
on a model of dyad 4 (Figure 11) show that the interchromo-
phoric distance (taken as the distance from the center of the
porphyrin ring to the boron atom of the Bodipy moiety) in the
minimum energy conformation, where the bridging unit adopts
a fully extended configuration, is 22.2 Å. However, the bridging
The rate constant of energy transfer by the F€orster mechanism,
kF, given by eq 3, can be written in terms of the critical transfer
radius Rc, which corresponds to the interchromophoric distance
at which kF equals the decay rate of the donor in the absence of an
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Figure 10. Frontier orbitals of dyad 4.
unit can easily undergo conformational rearrangements due to
free rotation about the dihedral angles D1 and D2 (Figure 11).
For instance, as shown in Figure 9b, a minimum configuration
is found at which the interchromophoric distance is 11.2 Å.
Consequently, in solution, dyads 4 and 5 adopt a variety of
conformations between the two extremes shown in Figure 11,
probably favoring the more extended conformations of the
bridging unit. Therefore, the measured energy transfer rates in
4 and 5 fit well to the F€orster mechanism and reflect the flexibility
of the bridging unit.
’ EXPERIMENTAL SECTION
Photophysical Measurements. UVꢀVis absorption spectra
were measured on a Shimadzu UV-1700 spectrophotometer using
10 mm path-length cuvettes. The emission spectra were measured on
a JASCO FP-6500 fluorescence spectrophotometer equipped with a
red-sensitive WRE-343 photomultiplier tube (wavelength range 200ꢀ
850 nm). Quantum yields were determined from corrected emission
spectra following the standard methods15 using meso-tetraphenylpor-
phyrin(TPP) (Φ = 0.11 in toluene16) or zinc meso-tetraphenylporphyrin
(ZnTPP) (Φ = 0.03 in toluene16) as standards. Emission lifetimes were
determined by the time-correlated single-photon counting technique
using an Edinburgh Instruments mini-tau lifetime spectrophotometer
equipped with an EPL 405 pulsed diode laser at 406.0 nm with a pulse
width of 71.52 ps and a pulse period of 200 ns and a high-speed red-
sensitive photomultiplier tube (H5773-04) as detector. Calculations of
F€orster and Dexter spectral overlap integrals and critical radii for F€orster
energy transfer were performed using the photochemCAD computer
program version 2.1.17 Femtosecond transient absorption studies were
performed with 420 and 387 nm laser pulses (1 kHz, 150 fs pulse width)
from an amplified Ti:Sapphire laser system (Clark-MXR, Inc. CPA
2101); the laser energy was 200 nJ.
Figure 11. Optimized molecular geometries of the two lowest energy
conformations of 4 predicted from DFT calculations. D1 and D2
(yellow line) correspond to the dihedral angles that were varied during
semiempirical calculations. Carbon, nitrogen, chlorine, boron, fluorine,
and hydrogen atoms are shown in gray, blue, green, purple, cyan,
and white, respectively.
Computational Details. We used quantum chemistry semiempi-
rical calculations to locate the minimum energy conformations of the
molecule by performing a grid calculation in which we varied
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simultaneously two coordinates corresponding to the dihedral angles
D1 and D2 (Figure 11). We used these methods in order to extract
important qualitative relations in much shorter computational times
than any ab initio method. These calculations were done at the PM618,19
semiempirical level with the MOPAC2009,20 version 9.259W, package.
Calculations resulted in formation of a 2D contour plot of the varied
coordinates with respect to the total energy, from which we were able to
find the two lowest conformations in energy. The latest conformations
were further explored by performing more sophisticated quantum
chemistry calculations within the framework of density functional theory
(DFT). Density functional theory in the resolution of identity (RI)
approximation21 was applied in our calculations. The B-P8622 exchange-
correlation functional along with the def2-SVP23 basis set (with the
corresponding auxiliary basis set for the RI approximation) was used. All
structures were optimized without any symmetry constraints, and the
optimized minimum-energy structures were verified as stationary points
on the potential energy surface by performing numerical harmonic
vibrational frequency calculation. DFT calculations were performed
with the TURBOMOLE program.24
839.1172. Anal. Calcd for C47H28Cl2N8Zn: C, 67.12; H, 3.36; N, 13.32.
Found: C, 67.24; H, 3.45; N, 13.21.
4,4-Difluoro-8-[4-(4,6-dichloro-1,3,5-triazin-2ylamino)-
phenyl]-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene
(1). A 0.085 g (0.25 mmol) amount of BDPNH2 was added to a
suspension of 0.173 g (1.25 mmol) of anhydrous K2CO3 in dry THF
(10 mL). The mixture was stirred in an ice bath at 0 ꢀC under an
atmosphere of dry nitrogen for ∼5 min before adding 0.055 g (0.3
mmol) of cyanuric chloride. The resulting mixture was allowed to slowly
reach room temperature while being stirred. Completion of the reaction
was confirmed by TLC (silica gel, dichloromethane/hexanes 4:1). A
stream of nitrogen removed the solvent, the solid residue was subjected
to column chromatography (silica gel, dichloromethane/hexanes 4:1),
and the principal bright orange band was collected. The desired product
was isolated as an orange powder. Yield: 0.080 g (64%). 1H NMR (500
MHz, CDCl3): δ 7.76 (d, J = 8.5 Hz, 2H), 7.68 (s, 1H), 7.33 (d, J = 8.5
Hz, 2H), 5.99 (s, 2H), 2.56 (s, 6H), 1.43 ppm (s, 6H). 13C NMR (75
MHz, CDCl3): δ 164.1, 156.0, 143.1, 140.6, 137.0, 132.2, 131.6, 129.4,
121.5, 121.2, 14.8 ppm. UVꢀVis(CH2Cl2): λmax (ε, molꢀ1 dm3 cmꢀ1
)
Materials. TPPNH225 and BDPNH226 (Scheme 1) were prepared
using published procedures. All other chemicals and solvents were
purchased from the usual commercial sources and used as received
unless otherwise stated. Tetrahydrofuran was freshly distilled from Na/
benzophenone, and anhydrous K2CO3 was ground in a mortar and
pestle and dried in an oven at 150 ꢀC overnight prior to use.
277 (33 000), 342 (9400), 502 nm (75 600). HRMS (MALDI-TOF):
m/z calcd for C22H19BCl2F2N6 + H+: 487.1188 [M + H]+. Found:
487.1173. Anal. Calcd for C22H19BCl2F2N6: C, 54.24; H, 3.93; N, 17.25.
Found: C, 54.11; H, 3.75; N, 17.41.
5-{4-[4-Chloro-6-(aminophenyl-4-(4,4-difluoro-8-(1,3,5,7-
tetramethyl-4-bora-3a,4a-diaza-s-indacene)))-1,3,5-triazin-
2-ylamino]phenyl}-10,15,20-triphenylporphyrin (4). A 0.055
g amount of 2 (0.071 mmol) and 0.024 g (0.071 mmol) of BDPNH2
were added to a suspension of 0.039 g (0.284 mmol) of anhydrous
K2CO3 in dry THF (10 mL) in a Schlenck tube under an atmosphere
of dry nitrogen. The tube was sealed, and the mixture was stirred at
60 ꢀC for 24 h. At the end of the reaction the mixture was filtered to
remove the K2CO3 and the solvent was removed in a rotary evaporator.
The crude product was subjected to column chromatography (silica gel,
dichloromethane), and the principal orange-red band was collected.
The product was isolated as a red-purple powder. Yield: 0.048 g (61%).
1H NMR (500 MHz, CDCl3): δ 8.85 (s, 8H), 8.22 (s, 8H), 7.97 (s, 2H),
7.76 (s, 12H), 7.58 (s, 1H), 7.26 (s, 2H), 5.99 (s br, 1H), 5.65 (s br, 1H),
2.57 (s br, 3H), 2.27 (s br, 3H), 1.41 (s, 6H), ꢀ2.76 ppm (s, 2H). 13C
NMR (75 MHz, CDCl3): δ 164.5, 155.6, 143.0, 142.3, 138.6,
137.1, 135.2, 134.7, 131.6, 129.0, 127.9, 126.9, 121.4, 120.5, 120.4,
5-[4-(4,6-Dichloro-1,3,5-triazin-2-ylamino)phenyl]-10,15,
20-triphenyl-porphyrin (2).27. A 0.070 g (0.11 mmol) amount of
TPPNH2 was added to a suspension of 0.076 g (0.55 mmol) of
anhydrous K2CO3 in dry THF (10 mL). The mixture was stirred in
an ice bath at 0 ꢀC under an atmosphere of dry nitrogen for ∼5 min
before adding 0.024 g (0.13 mmol) of cyanuric chloride. The resulting
mixture was allowed to slowly reach room temperature while being
stirred. Completion of the reaction was followed and confirmed by TLC
(silica gel, dichloromethane/hexanes 4:1). A stream of nitrogen re-
moved the solvent, the solid residue was subjected to column chroma-
tography (silica gel, dichloromethane/hexanes 4:1), and the principal
purple band was collected. The desired product was isolated as a purple
powder. Yield: 0.072 g (80%). 1H NMR (500 MHz, CDCl3): δ 8.87 (m,
8H), 8.23 (m, 8H), 7.88 (d, J = 8.5 Hz, 2H), 7.82ꢀ7.74 (m, 10H), ꢀ2.74
ppm (s, 2H). 13C NMR (75 MHz, CDCl3): δ 164.2, 142.1, 139.6, 135.6,
135.3, 134.6, 131.3, 127.8, 126.7, 120.4, 120.3, 119.1, 118.7 ppm.
UVꢀVis (CH2Cl2): λmax (ε, molꢀ1 dm3 cmꢀ1) 418 (478 000), 514
(19 000), 549 (9000), 589 (6000), 645 nm (4500). HRMS (MALDI-
TOF): m/z calcd for C47H30Cl2N8 + H+: 777.2049 [M + H]+. Found:
777.2061. Anal. Calcd for C47H30Cl2N8: C, 72.59; H, 3.89; N, 14.41.
Found: C, 72.68; H, 3.65; N, 14.55.
118.9, 14.8, 14.6 ppm. UVꢀVis (CH2Cl2): λmax (ε, molꢀ1 dm3 cmꢀ1
)
419 (419 000), 500 (74 800), 550 (8300), 589 (5200), 645 nm
(4200). HRMS (MALDI-TOF): m/z calcd for C66H49BClF2N11
+
H+: 1080.4000 [M + H]+. Found: 1080.4012. Anal. Calcd for
C66H49BClF2N11: C, 73.37; H, 4.57; N, 14.26. Found: C, 73.28; H,
4.67; N, 14.12.
{5-[4-[4-Chloro-6-(aminophenyl-4-(4,4-difluoro-8-(1,3,5,7-
tetramethyl-4-bora-3a,4a-diaza-s-indacene)))-1,3,5-triazin-2-
ylamino]phenyl]-10,15,20-triphenylporphyrinato}zinc(II) (5).
A 0.020 g (0.018 mmol) amount of 4 was dissolved in 3:1 dichloro-
methane/methanol (10 mL), and 0.040 g (0.180 mmol) of Zn(CH3-
{5-[4-(4,6-Dichloro-1,3,5-triazin-2-ylamino)phenyl]-10,-
15,20-triphenyl-porphyrinato}zinc(II) (3). A 0.020 g amount of 2
(0.025 mmol) was dissolved in 3:1 dichloromethane/methanol
(10 mL), and 0.055 g (0.25 mmol) of Zn(CH3COO)2 2H2O was
3
added. The resulting mixture was stirred at room temperature overnight.
At the end of the reaction the solvent was removed in a rotary evaporator
and the resulting residue was partitioned between dichloromethane
(5 mL) and water (5 mL). The organic layer was further washed with
5 mL of water and dried with Na2SO4. The solvent was removed, and
the residue was passed through a short column of silica eluting
with dichloromethane. The desired product was isolated as red-purple
powder. Yield: 0.018 g (84%). 1H NMR (500 MHz, CDCl3): δ 1H NMR
COO)2 2H2O was added. The resulting mixture was stirred at room
3
temperature overnight. Atthe end of thereaction thesolvent was removed
in a rotary evaporator and the resulting residue was partitioned between
dichloromethane (5 mL) and water (5 mL). The organic layer was further
washed with 5 mL of water and dried with Na2SO4. The solvent was
removed, and the residue was passed through a short column of silica
eluting with dichloromethane. The desired product was isolated as a red-
purple powder. Yield: 0.016 g (78%). 1H NMR (300 MHz, CDCl3): δ
8.95 (s, 8H), 8.22 (m, 8H), 7.96 (d, J = 8.1 Hz, 2H), 7.77 (m, 12H), 7.59
(s br, 1H), 7.23 (m, 2H), 5.96 (s br, 1H), 5.67 (s br, 1H), 2.55 (s br, 3H),
2.25 (s br, 3H), 1.42 ppm (m, 6H). 13C NMR (75 MHz, CDCl3): δ 164.6,
155.6, 150.4, 142.9, 138.1, 136.9, 135.0, 134.6, 132.2, 129.0, 127.7,
126.7, 121.3, 118.9, 14.8 ppm. UVꢀVis (CH2Cl2): λmax (ε, molꢀ1
(500 MHz, CDCl3/1% pyridine-d5):
δ 8.78 (m, 8H), 8.11
(m, 8H), 7.73 (s br, 2H), 7.57 ppm (m, 10H). 13C NMR (75 MHz,
CDCl3/1% pyridine-d5): δ 149.7, 142.9, 136.0, 134.5, 131.8, 127.2,
126.4, 120.6, 119.6 ppm. UVꢀVis (CH2Cl2): λmax (ε, molꢀ1 dm3 cmꢀ1
m/z calcd for C47H28Cl2N8Zn + H+: 839.1184 [M + H]+. Found:
)
419 (585 000), 542 (20 400), 583 nm (6000). HRMS (MALDI-TOF):
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dm3 cmꢀ1) 420 (434 000), 500 (75300), 546 (21500), 583 nm (7200).
HRMS (MALDI-TOF): m/z calcd for C66H47BClF2N11Zn + H+:
1142.3135 [M + H]+. Found: 1142.3148. Anal. Calcd for C66H47-
BClF2N11Zn: C, 69.30; H, 4.14; N, 13.47. Found: C, 69.19; H, 4.27;
N, 13.67.
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’ CONCLUSIONS
We report the synthesis of dyads 4 and 5 which combine the
Bodipy chromophore with either a free-base or a Zn porphyrin
moiety linked to eachother via a cyanuric chloride bridge. The
photophysical behavior of 4 and 5 was studied by UVꢀVis
absorption and emission spectroscopy, cyclic voltammetry and
femptosecond transient absorption spectroscopy. Comparison
of the absorption spectra and cyclic voltammograms of dyads
4 and 5 with those of their model compounds 1, 2, and 3 shows
that the spectroscopic and electrochemical properties of the
constituent chromophores are essentially retained in the dyads,
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’ ASSOCIATED CONTENT
S
Supporting Information. Complementary data of differ-
b
ential absorption spectra and their time-absorption profiles, as
well as the 1H-NMR, 13C-NMR of the title compounds 1, 2, 3, 4,
and 5. This material is available free of charge via the Internet at
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’ DEDICATION
^This work is dedicated to the memory of our colleague and
friend Nikos Manoussakis.
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: marius.reglier@univ-cezanne.fr (M.R.); dirk.guldi@
chemie.uni-erlangen.de (D.M.G.); coutsole@chemistry.uoc.gr
(A.G.C.).
’ ACKNOWLEDGMENT
The European Commission funded this research by FP7-REG-
POT-2008-1, Project BIOSOLENUTI No 229927, Heraklitos
grant from Ministry of Education, and GSRT. In addition, financial
support from the DFG (Cluster of Excellence and SFB 583) and
the BMBF is greatly acknowledged.
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