Symmetry-breaking intramolecular charge transfer in the excited state of meso-linked BODIPY dyads

Article abstract of DOI:10.1039/c1cc12260f

We report the synthesis and characterization of symmetric BODIPY dyads where the chromophores are attached at the meso position, using either a phenylene bridge or direct linkage. Both molecules undergo symmetry-breaking intramolecular charge transfer in the excited state, and the directly linked dyad serves as a visible-light-absorbing analogue of 9,9′-bianthryl.

Full text of DOI:10.1039/c1cc12260f

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COMMUNICATION
Symmetry-breaking intramolecular charge transfer in the excited state of
meso-linked BODIPY dyadsw
Matthew T. Whited, Niral M. Patel, Sean T. Roberts, Kathryn Allen, Peter I. Djurovich,
Stephen E. Bradforth and Mark E. Thompson*
Received 19th April 2011, Accepted 30th October 2011
DOI: 10.1039/c1cc12260f
We report the synthesis and characterization of symmetric
BODIPY dyads where the chromophores are attached at the
meso position, using either a phenylene bridge or direct linkage.
Both molecules undergo symmetry-breaking intramolecular
charge transfer in the excited state, and the directly linked dyad
serves as a visible-light-absorbing analogue of 9,90-bianthryl.
Fig. 1 BODIPY dyads 1 and 2.
The phenylene bridge is canted at an angle of 471 relative to the
Photoinduced electron transfer reactions are crucially important
BODIPY planes, suggesting minimal steric encumbrance to
for energy storage processes in both biological and photovoltaic
partial rotation of the BODIPY units with respect to the linker.
systems. In the photosynthetic reaction center, electron transfer
Thus, electronic superexchange, which requires interaction of
from the ‘‘special pair’’ is preceded by ultrafast formation of
the BODIPY and phenylene p-orbitals, should be possible
an intradimer charge-transfer state via symmetry breaking.1
across the phenylene bridge.
However, model compounds capable of such symmetry-breaking
Absorption spectra of 1 in solvents of varying polarity are
intramolecular charge transfer (ICT) are largely confined to
nearly identical to the model compound 3,5-Me2BODIPY–Ph,
biacenes absorbing predominantly ultraviolet wavelengths.2–4
indicating minimal ground-state interaction or excitonic coupling
The 9,90-bianthryl molecule is the best studied system of this
between the chromophores of 1 (Fig. 2). Emission spectra of
sort,5 and the emissive nature of its excited state provides a useful
1 display small Stokes shifts that are nearly invariant in all
probe to study the nature of symmetry-breaking ICT.
solvents. However, the photoluminescence quantum efficiencies
During the course of our recent studies on charge and energy
(F) vary between 0.05 and 0.1 and drop precipitously in the most
transfer reactions in BODIPY–porphyrin hybrids,6 we have
polar solvents (Table 1). In contrast, for 3,5-Me2BODIPY–Ph
prepared a series of molecules that allow the investigation of
the value of F is 0.29 in cyclohexane, and declines to 0.17 in
interactions between multiple covalently tethered BODIPY
acetonitrile.8 The lower values of F for 1 indicate the possible
units. Reported here are the synthesis and unusual symmetry-
formation of a nonemissive charge-transfer state that entails
breaking ICT properties of symmetrical BODIPY dyads in
some degree of symmetry breaking since the BODIPY units
which the units are connected through the meso position either
are identical. The similarity in both emission spectra (see ESIw)
indirectly by an intervening phenylene or directly through a
and F for a range of nonpolar and weakly polar solvents suggests
C–C bond. The directly linked dyad is particularly interesting as
that the rate of formation of the charge transfer state is not
it has excited-state properties that mimic behaviour found in
dependent on solvent polarity, up to a polarity index of 41
9,90-bianthryl.
(Table 1).
The first compound investigated was a phenylene-bridged
BODIPY dyad 1 (Fig. 1, synthesis is given in the ESI). A similar
dyad was previously reported by Akkaya as a precursor to
multichromophoric ion sensors.7 Analysis of 1 by single-crystal
X-ray diffraction reveals two coplanar BODIPY units rendered
identical by a crystallographic center of symmetry (see ESIw).
Department of Chemistry and the Center for Energy Nanoscience,
University of Southern California, Los Angeles, CA 90089-0744,
USA. E-mail: met@usc.edu; Fax: +1 213 740 8594;
Tel: +1 213 740 6402
w Electronic supplementary information (ESI) available: Synthetic
procedures, electrochemical data, transient absorption, and crystal-
lographic data (CCDC 820543). See DOI: 10.1039/c1cc12260f
Fig. 2 Normalized absorption and emission spectra of dyad 1 and
absorption spectrum of 3,5-Me2BODIPY–Ph in CH2Cl2. The LUMO
surface of 1 is also shown.
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284 Chem. Commun., 2012, 48, 284–286
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Table 1 Photophysical properties of dyads 1 and 2 in various solvents
lmax,abs
(nm)
lmax,em
(nm)
ET(30)a
F
Solvent
(kcal molꢀ1
)
1
2
1
2
1
2
Cyclohexane
Toluene
2-MeTHF
Chloroform
Dichloromethane 40.7
DMF
Acetonitrile
30.9
33.9
36.5
39.1
513 523 538 564
515 526 540 574
511 524 540 620
515 526 538 585
0.052 0.78
0.095 0.62
0.046 0.18
0.097 0.35
0.069 0.087
o0.001 —b
o0.001 —b
513 530 538 651
b
43.2
45.6
512 530 536
508 526 531
—
—
b
a
Solvent polarity index.10 b PL quantum yield could not be accurately
determined due to overlap with BODIPY monomer impurity emission.
The potential for 1 to undergo symmetry-breaking ICT was
examined by electrochemistry. Cyclic voltammetry of 1 revealed
a reversible reduction (E1/2 = ꢀ1.37 V) and an irreversible
oxidation (Epa = 0.94 V, both vs. Fc+/Fc). The corresponding
values for 3,5-Me2BODIPY–Ph are Epc = ꢀ1.5 V and Epa = 0.90 V
(both irreversible). The difference between oxidation and reduc-
tion values (2.31 V) of 1 indicates that the S1 state (E00 = 2.38 eV
in cyclohexane) should be energetic enough to undergo ICT, as
previously discussed by Zander and Rettig.9 DFT calculations
(B3LYP/6-31g*) show both the HOMO (ꢀ5.68 eV) and LUMO
energies (ꢀ2.88 eV) of 1 are stabilized relative to the respective
values in 3,5-Me2BODIPY–Ph (ꢀ5.51 eV and ꢀ2.56 eV). The
increase in the electron affinity of 1 is likely due to delocalization
across the phenylene bridge as shown in the surface contours of
the LUMO (see Fig. 2 and ESIw).
Support for the formation of an ICT state in polar solvents
was provided by femtosecond transient absorption measure-
ments (Fig. 3). Excitation of 1 at 509 nm in acetonitrile
populates the S1 state, as reflected by the appearance of a
stimulated emission band from 525–600 nm that matches the
S1 emission lineshape. Over the course of 10 ps, this band
disappears concomitant with the rise of a weak induced
absorption band peaked at 545 nm that matches absorption
spectra reported for the BODIPY radical anion and cation.11
A global fit to the data (Fig. 3b and ESIw) yields a rate of
4.8 ps for the formation of this ICT state. Subsequently, all
transient spectral features decay with a rate constant of 34 ps,
indicating a fast nonradiative return to the S0 state, consistent
with asymmetric dyads incorporating a BODIPY acceptor.11
On the other hand, excitation of 1 in toluene leads to for-
mation of an S1 state that decays at a rate consistent with the
lifetime determined from emission studies (t = 860 ps).
The importance of twisting and other structural changes of ICT
excited states in donor/acceptor molecules has been extensively
explored.3 Additionally, rotation of meso-aryl substituents relative
to BODIPY chromophores has previously been invoked as a
major pathway for nonradiative deactivation.12 In the present
case, facile rotation of the phenylene bridge in 1 is also what likely
allows the ICT state to undergo ultrafast direct surface crossing to
the ground state. Thus, we aimed to extend these studies using
dyad 2 (synthesis is given in the ESI), with the two BODIPY
units linked directly at the meso position, significantly restricting
rotational freedom. We have been unable to obtain X-ray
quality crystals of 2, however, structure minimization using
DFT (B3LYP/6-31g*) methods indicates that the planar
Fig. 3 (a) Ultrafast transient absorption spectra of 1 after excitation
at 509 nm in acetonitrile. (b) Time domain slices of the ground state
bleach (red) and BODIPY radical absorption (blue) for dyad 1 (filled
squares; bleach: 507 nm x0.55, induced absorption: 550 nm x2.5) & 2
(open circles; bleach: 527 nm ꢁ2.5, induced absorption: 590 nm) with
predicted traces based on a kinetic model (dashed black).
BODIPY units of 2 have local geometries similar to those of 1,
with BODIPY units canted at a dihedral angle of 711.
Cyclic voltammagrams of 2 in CH2Cl2 display two well
resolved reversible reduction peaks (E1/2 = ꢀ1.38 V, ꢀ1.19 V
vs. Fc+/Fc) and an irreversible oxidation peak (Epa = 0.965 V
vs. Fc+/Fc). DFT calculations show that the HOMO energy
of 2 (ꢀ5.61 eV) is similar to 1 but the LUMO (ꢀ2.97 eV) is
stabilized due to conjugation across the meso linkage (see Fig. 4
and ESIw). The electrochemical gap of 2 (Epa ꢀ E1/2 = 2.15 V)
is 100 mV less than the E00 energy of the S1 state (E00 = 2.25 eV
in cyclohexane), suggesting that the energetics are more
favourable for ICT formation in 2 than for dyad 1.
Absorption spectra of 2 are nearly invariant across several
solvents and are similar to that of 1 and other BODIPY
chromophores.13 Slight splitting of the primary (S0 - S1)
absorption band at 530 nm indicates a modest degree of
exciton coupling between the BODIPY units.14 Fluorescence
spectra, on the other hand, are dramatically affected by solvent. A
progressive red-shift in the emission wavelength is observed with
increasing solvent polarity, accompanied by a concomitant
Fig. 4 Absorption spectrum of 2 in CH2Cl2 and emission spectra of 2
in solvents of varying polarity. The LUMO surface of 2 is also shown.
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Chem. Commun., 2012, 48, 284–286 285

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broadening and decrease in F (Fig. 4 and Table 1). The spectra
indicate that 2 has a nonpolar ground state and a significantly
higher dipole moment in the excited state, even though the two
constituent chromophores are identical. Similar behaviour is
observed for the 9,90-bianthryl molecule.5,15
photovoltaic devices; studies that exploit these properties are
currently underway in our laboratories.
We acknowledge financial support from the Center for
Advanced Molecular Photovoltaics (CAMP) (KUS-C1-015-21)
of the King Abdullah University of Science and Technology
and Global Photonic Corporation. Femtosecond transient
absorption measurements were performed with support from
the Department of Energy’s Energy Frontier Research Center
program (Award: DE-SC0001011). STR acknowledges an ACC-F
fellowship from the National Science Foundation (CHE-0937015).
In cyclohexane, dyad 2 exhibits a simple first-order lumines-
cence decay (t = 9.3 ns). In contrast, a biexponential decay is
observed in CH2Cl2, comprised of a fast component (o200 ps)
accompanied by a longer-lived (ca. 7 ns) decay. Thus, excita-
tion of the S1 state in 2 leads to the population of a new
emissive state. In acetonitrile, we find that the nonradiative
decay rate of this new state (knr = 1.5 ꢁ 109 sꢀ1) is 20 times
slower than that of 1 in the same solvent (see ESI). Given how
the emission of 2 depends on solvent polarity, this new state
likely corresponds to an emissive ICT state populated by
solvent-induced symmetry breaking.5
Notes and references
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Femtosecond transient absorption spectroscopy in acetonitrile
was used to further illuminate the charge-transfer behaviour of 2
in polar media. The S1 state observed upon excitation at 509 nm
ꢀ1
evolves within our experimental time resolution (kIC r 170 fs)
to produce an excited state that absorbs at 590 nm, consistent
with the formation of a BODIPY radical anion.11 However, the
spectral features associated with the ICT state of 2 persist an
order of magnitude longer (krꢀ1 = 650 ps, Fig. 3b) than those
of 1, relaxing with a rate that matches the emissive lifetime of 2
in acetonitrile (see ESI). We propose that steric congestion in
dyad 2 hinders rotation to a surface crossing region, thus
inhibiting nonradiative charge recombination.
Although several biacenes display similar luminescent
properties,2 to the best of our knowledge 2 represents the first
example of a dyad that combines symmetry-breaking formation
of an emissive ICT state with intense absorption in the visible
region of the spectrum. While porphyrins are in many respects
related to dipyrrins, the meso-linked porphyrin analogues of 2
do not undergo symmetry-breaking ICT because formation of
such an excited state is endothermic with respect to the S1
state.16 BODIPY dyads directly linked at the a- or b positions
also do not exhibit this sort of emissive behaviour.17 However,
Benniston et al. have reported a hybrid of 2 and 9,90-bianthryl,
a meso-linked 9-anthracenyl–BODIPY compound, that readily
forms an emissive ICT state in polar solvents.18
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In conclusion, excitation of BODIPY dyads 1 and 2 leads to
formation of ICT states in polar media by solvent-induced
symmetry breaking. The further presence of strong absorption
at visible wavelengths enables these molecules to mimic features
seen in the photosynthetic reaction center. Model systems that
possess both these characteristics are rare.4 Differing degrees of
rotational freedom in the dyads significantly alter the behavior
of the ICT state. Whereas dyad 1 undergoes rapid nonradiative
decay to the ground state, the more hindered dyad 2 has a long-
lived ICT state with moderate-to-high fluorescence quantum
efficiency. The excited state properties of the dyads reported
here could prove useful in facilitating charge separation in
14 A. C. Benniston, G. Copley, A. Harriman, D. Howgego,
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c
286 Chem. Commun., 2012, 48, 284–286
This journal is The Royal Society of Chemistry 2012