Effect on charge transfer and charge recombination by insertion of a naphthalene-based bridge in molecular dyads based on borondipyrromethene (Bodipy)

Article abstract of DOI:10.1002/cphc.201200510

The photophysical properties of two related dyads based on a N,N-dimethylaniline donor coupled to a fully-alkylated boron dipyrromethene (Bodipy) acceptor are described. In one dyad, BD1, the donor unit is attached directly to the Bodipy group, whereas in the second dyad, BD2, a naphthalene spacer separates the two units. Cyclic voltammograms recorded for the two dyads in deoxygenated MeCN containing a background electrolyte are consistent with the reversible one-electron oxidation of the N,N-dimethylaniline group and the reversible one-electron reduction of the Bodipy nucleus. There is a reasonable driving force (ΔG_CT) for photoinduced charge transfer from the N,N-dimethylaniline to the Bodipy segment in MeCN. The charge-transfer state is formed for BD1 extremely fast (1.5 ps), but decays over 140 ps to partially restore the ground state. On the other hand, the charge-transfer state for BD2 is formed more slowly, but it decays extremely rapidly. Charge recombination for both dyads leads to a partial triplet formation on the Bodipy group. The naphthalene spacer group is extremely efficient at promoting back electron transfer.

Full text of DOI:10.1002/cphc.201200510

DOI: 10.1002/cphc.201200510
Effect on Charge Transfer and Charge Recombination by
Insertion of a Naphthalene-Based Bridge in Molecular
Dyads Based on Borondipyrromethene (Bodipy)
Andrew C. Benniston,*[a] Sophie Clift,[a] Jerry Hagon,[a] Helge Lemmetyinen,[b]
Nikolai V. Tkachenko,*[b] William Clegg,[c] and Ross W. Harrington[c]
The photophysical properties of two related dyads based on
a N,N-dimethylaniline donor coupled to a fully-alkylated boron
dipyrromethene (Bodipy) acceptor are described. In one dyad,
BD1, the donor unit is attached directly to the Bodipy group,
whereas in the second dyad, BD2, a naphthalene spacer sepa-
rates the two units. Cyclic voltammograms recorded for the
two dyads in deoxygenated MeCN containing a background
electrolyte are consistent with the reversible one-electron oxi-
dation of the N,N-dimethylaniline group and the reversible
one-electron reduction of the Bodipy nucleus. There is a rea-
sonable driving force (DGCT) for photoinduced charge transfer
from the N,N-dimethylaniline to the Bodipy segment in MeCN.
The charge-transfer state is formed for BD1 extremely fast
(1.5 ps), but decays over 140 ps to partially restore the ground
state. On the other hand, the charge-transfer state for BD2 is
formed more slowly, but it decays extremely rapidly. Charge re-
combination for both dyads leads to a partial triplet formation
on the Bodipy group. The naphthalene spacer group is ex-
tremely efficient at promoting back electron transfer.
1. Introduction
The ultimate aim of bio-inspired artificial photosynthesis is to
create a fully operational system, capable of efficient capture
of natural sunlight and its storage in the form of chemical
energy.[1] As with any scientific jigsaw puzzle, there are many
pieces that need to fall into place before any real working pro-
totype is to emerge. The primary reaction in natural photosyn-
thesis is the light-induced separation of charge across a mem-
brane involving embedded pigments and transferable quinone
(Q) electron acceptors.[2] Mimicking this primary charge-separa-
tion process in custom-built molecular systems has been in-
tensely studied over many years.[3] In many cases the molecular
architectures were designed to promote a cascade of electron-
transfer events, so as to separate charge both spatially and
temporally.[4] Successful systems are especially notable from
the groups of Gust,[5] Osuka,[6] Sauvage[7], and many others.[8]
With the complexity in design comes the need to perform
multiple synthetic procedures and purifications, generally
yielding a small amount of material at the end. At the other
extreme; very basic systems, though far easier to manufacture,
do not generally offer long-lived charge-transfer (CT) charac-
ter.[9] In discussing a long-lived photo-generated species we
should identify its spin character. Triplet CT states are likely to
live for long times but store less energy,[10] whereas singlet CT
states generally show the opposite behavior.[11]
well into the visible region. 2) Triplet formation is unfavoured
from the first-excited singlet state by inter-system-crossing.
3) The redox chemistry of the unit is reversible and the radical
anion/cations are robust. 4) The synthetic procedures are well
established to produce workable quantities of material in good
yields.[13]
In our earlier employed dyads the quinone unit was used as
electron acceptor, and positioned with or without a phenylene
spacer in two positions on the Bodipy core.[14] The photoin-
+
C
ꢀC
duced CT state produced comprised BD ꢀQ , but lifetimes
were too short for practical considerations. The alternative
methodology is to use the Bodipy unit as an electron acceptor
and to provide a suitable electron donor (D) in proximity. In
[a] Prof. A. C. Benniston, S. Clift, Dr. J. Hagon
Molecular Photonics Laboratory, School of Chemistry
Newcastle University
Newcastle upon Tyne, NE1 7RU (UK)
Fax: (+44)1912226929
E-mail: a.c.benniston@ncl.ac.uk
[b] Prof. H. Lemmetyinen, Dr. N. V. Tkachenko
Department of Chemistry and Bioengineering
Tampere University of Technology
PO Box 541, 33101 (Finland)
E-mail: nikolai.tkachenko@tut.fi
[c] Prof. W. Clegg, Dr. R. W. Harrington
Crystallography Laboratory, School of Chemistry
Newcastle University
In an attempt to identify basic molecular dyads for the effi-
cient capture of sunlight, and with a capacity to form singlet
CT states, our attention was drawn to the boron dipyrrome-
thene [Bodipy (BD)] chromophore.[12] The basic unit has
a number of positive traits. 1) It is highly colored, absorbing
Newcastle upon Tyne, NE1 7RU (UK)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cphc.201200510.
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A. C. Benniston et al.
this case the photoinduced CT state produced comprises
dard cross-coupling conditions. Hence, compound 1 was pro-
duced in a manner similar to that for BD1, bearing a reactive
aryl bromide for Suzuki coupling.[21] The desired compound
BD2 was produced in overall 68% yield as a red solid. All com-
pounds were fully characterised by standard analytical tech-
niques (e.g. NMR spectroscopy, mass spectrometry, melting
point).
+
ꢀC
C
BD ꢀD There are several examples in the literature where
this basic strategy is used, especially in the field of sensors
using a dialkylamino substituent as donor.[15] Detailed studies
of the photophysical properties of the compounds are less
common.[16] The most basic system is BD1 (Scheme 1) compris-
ing a N,N-dimethylaniline group linked directly to the Bodipy
Single crystals were obtained for BD1 and BD2 and subject-
ed to X-ray structure determinations. The molecular structures
of the two compounds are displayed in Figure 1. The basic fea-
Scheme 1. Reagents and Conditions: a) DCM, TFA, RT; b) DDQ, N,N-diisopro-
pylethylamine, BF3·Et2O; c) PdCl2(PPh3)2, Na2CO3 (2.0m), THF, reflux.
core. In a similar derivative, deactivation of the Bodipy excited
state was shown to be efficient.[17] Based on this observation
we speculated (wrongly) that any formed CT state would be
extremely short-lived. In order to separate the donor and ac-
ceptor groups a naphthalene unit was incorporated in a new
dyad, BD2 (Scheme 1). Synthetically, such a procedure repre-
sented no challenge, but the excited-state dynamics for BD2
proved to be more interesting. The slower CT-state formation
seen for the dyad BD2 compared to BD1 by naphthalene in-
sertion is counteracted by ultrafast charge recombination. The
naphthalene spacer appears to be an efficient conduit for back
electron transfer.
Figure 1. X-ray crystallographically-determined molecular structures for BD2
(top) and BD1 (bottom). Hydrogen atoms are omitted for clarity.
2. Results and Discussion
tures of the Bodipy segment are in agreement with other
structures in the literature.[22] From the structure determina-
tions it was possible to obtain the accurate distance between
the donor (dimethylamine) and acceptor (dipyrrin) groups for
the two dyads. The center-to-center distances (as shown in
Figure 1) are calculated from the nitrogen atom to the centroid
of the middle ring in the dipyrrin core. The insertion of the
naphthalene group roughly results in a 60% increase in dis-
tance between the donor and acceptor units. The steric con-
straints imposed by the two 1,7-dimethyl groups mean that
the N,N-dimethylaniline group is almost orthogonal to the
plane of the dipyrrin (79.28) for BD1. In structure BD2 it is the
naphthalene group that is nearly orthogonal (82.48), and as
a consequence the N,N-dimethylaniline subunit is less con-
strained. The group is now, at least in the solid state, more in
plane with the dipyrrin group. In solution, the N,N-dimethylani-
2.2.1. Synthesis
Despite the plethora of Bodipy derivatives in the literature,[18]
very little has been reported on the synthesis and characteriza-
tion of BD1. The non-alkylated version is known as well as the
diethyl and carboxylic acid derivatives.[19] We were especially
interested in the fully-alkylated version, since this imparts
good electrochemical stability to the final Bodipy dye, and af-
fords some protection from sacrificial breakdown reactions
with free radicals.[20] Following standard conditions the cou-
pling of p-dimethylaminobenzaldehyde with 2,4-dimethyl-3-
ethylpyrrole followed by oxidation and chelation to the BF2
unit afforded BD1 in 22% yield as a red solid. To construct
BD2, the basic strategy was to introduce the naphthalene unit
at the beginning and link the donor group at the end by stan-
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Electronic effects on Bodipy by the Insertion of a Naphtalene-Based Bridge
line could rotate freely, however, evidence from 19F NMR spec-
troscopy (see the Supporting Information) show that the naph-
thalene subunit is simply rocking on its axis and not spinning
freely.
HOMOs for the two dyads is likely due to the insertion of the
naphthalene group. The HOMO–LUMO gap of 2.81 eV for BD2
represents the energy associated with a one-electron transfer
from the Me2N (electron donor) to the Bodipy (electron accept-
or). The corresponding energy gap for BD1 is 3.10 eV (i.e.
HOMOꢀ1 to LUMO). The calculated energy gaps are rather
large since there is no solvation contributing to the stabiliza-
tion. However, the general trend of a larger energy gap for
BD1 compared to BD2 is consistent with our electrochemistry
experiments (Section 2.2.3). It is also worth noting that the en-
ergies of the HOMOꢀ1 (BD1) and the HOMO (BD2) differ by
a modulus of 240 mV.
2.2.2. Molecular Orbital Calculations
The spatial localisation of the highest occupied molecular orbi-
tal (HOMO) and the lowest unoccupied molecular orbital
(LUMO) for both dyads were calculated using the B3LYP/6-
311G density functional theory (DFT) method and Gauss-
ian 03.[23] The orbitals are represented in Figure 2. For both mo-
2.2.3. Electrochemistry
Cyclic-voltammetry experiments
were carried out for the two de-
rivatives in dry MeCN containing
0.2m
tetra-n-butylammonium
tetrafluoroborate (TBATFB) as
background electrolyte. The oxi-
dative scan segment for the vol-
tammogram of BD2 contained
a
one-electron quasi-reversible
wave at E1/2 = +0.84 V (90 mV)
versus Ag/AgCl associated with
the oxidation of the amine. On
further scanning a quasi-reversi-
ble wave corresponding to the
oxidation of the Bodipy site was
observed at
E
1/2 = +1.09 V (140 mV) versus
Ag/AgCl. In the reductive poten-
tial window one-electron
a
quasi-reversible wave was seen
at E1/2 =ꢀ1.13 V versus Ag/AgCl,
and is associated with the reduc-
tion of the Bodipy group (see
the Supporting Information). The
energy difference (DE) between
the potentials for oxidation of
the amine and reduction of the
Bodipy was 1.97 V. The solubility
of BD1 in MeCN is rather poor
and hampered the collection of
Figure 2. HOMO-1, HOMO and LUMO for the BD1 (left) and BD2 (right) as calculated using DFT (B3LYP) and the
6-311G basis set. Energies are in eV.
lecular dyads the LUMO is exclusively localised on the dipyrro-
methene segment. The nature of the appended aromatic in
the meso position does not affect the energy of the orbital sig-
nificantly. The HOMO for BD1 is localised on the dipyrrome-
thene segment, whereas the HOMOꢀ1 is located on the N,N-
dimethylaniline group. The difference in energy between the
orbitals is rather modest (160 mV). In comparison, the DFT-cal-
culated HOMO for BD2 is localised on the N,N-dimethylaniline
group, with very little electron density residing on the inter-
vening naphthalene unit. The HOMOꢀ1 resides on the dipyrro-
methene, and again the difference in energy between the two
orbitals is small (136 mV). The variance in ordering of the
a good-quality cyclic voltammogram. The oxidative scan for
BD1 was dominated by two one-electron quasi-reversible
waves located at +1.13 V (90 mV) and +1.31 V (190 mV)
versus Ag/AgCl. The first wave was the oxidation of the amine
and the second was the oxidation of the Bodipy group. The
single wave observed at E1/2 =ꢀ1.03 V (80 mV) on reductive
scanning was again the reduction at the Bodipy site. The corre-
sponding value for DE was 2.16 V. It is noted that DEBD1
>
DEBD2. The difference in potentials for the oxidation of the
amine (EBD1ꢀEBD2) is 290 mV, which is in good agreement with
the corresponding computer-calculated value.
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A. C. Benniston et al.
2.2.4. Absorption and Fluorescence
The electronic absorption spectra for BD1 and BD2 in dilute
MeCN are shown in Figure 3. The typical sharp strong absorp-
tion band associated with the Bodipy-centered S0-S1 electronic
transition is observed at 520 and 525 nm for BD1 and BD2, re-
spectively. The small red shift of 5 nm in lABS for BD2 is likely
Figure 4. Top: Transient-absorption decay components obtained from three-
exponential fit of pump-probe data of BD1 in MeCN. Corresponding time
Figure 3. Room-temperature absorption spectra recorded for BD1 (gray) and
BD2 (black) in dilute MeCN.
~
&
^
constants are 1.5 ps ( ), 140 ps ( ), and roughly 1 ns ( ). Bottom: Time-re-
solved transient-absorption spectra at a number of delay times obtained
after group-velocity dispersion compensation.
the result of a partial conjugation with the naphthalene unit.
The much weaker Bodipy-based S0-S2 absorption profile for
BD2 is obscured by the p-p* electronic transitions of the naph-
thalene group. Extremely weak room-temperature fluorescence
was observed from dilute MeCN solutions of highly purified
samples of BD1 or BD2. The emission profiles were reasonably
good mirror images of the absorption bands. The Stokes shifts
were again typical for Bodipy-like chromophores.[24] Upper
limits of fluorescence quantum yields (fFLU) for BD1 and BD2
were 0.0003 and 0.002, respectively. Fluorescence lifetimes (ts)
were too short to be measured by the conventional time-cor-
related single-photon-counting technique. As discussed later,
femtosecond up-conversion spectroscopy provided reliable
values for ts. Selected data for the two dyads are collected in
the Supporting Information.
stimulated emission. With a time constant of 1.5 ps, the nega-
tive absorption evolves into a transient-absorption band with
maximum at around 580 nm. By comparison to previous re-
sults this new feature was assigned to the Bodipy-based radical
anion.[25] The band relaxes with a time constant of 140 ps.
However, after relaxation of the band, some bleaching of the
ground-state absorption band remains, indicating that roughly
one quarter of initially excited molecules are still in an excited
state.
The time profile of the transient-absorption evolution at
580 nm and fluorescence decay of the sample measured using
up-conversion techniques are presented in Figure 5. The
strong almost instant negative absorption at 580 nm is trans-
formed into a positive absorption with the same time constant
as the fluorescence decay of the sample. This supports that
the negative absorption originates from stimulated emission,
as well as the fact that the charge-separated state is formed
from a singlet excited state.
2.2.5. Time-Resolved Studies
The steady-state measurements performed on both dyads
were consistent with efficient excited-state deactivation. To
obtain a more comprehensive temporal picture of events, fem-
tosecond time-resolved transient-absorption experiments were
carried out. The samples were excited with a 70 fs laser pulse
delivered at 385 nm and 50–70 time-resolved spectra were re-
corded in a 200–1000 ps time frame depending on the relaxa-
tion time of the sample. The data were fitted globally to
obtain relaxation-time constants and the decay-component
spectra associated with them. Results of the measurements for
BD1 in MeCN are presented in Figure 4. At a very early time
delay there is a clear bleach effect centered at 521 nm. In addi-
tion, there is a negative transient absorption in the range of
540–600 nm, where virtually no ground-state absorption of the
sample is observed. This negative absorption arises from
A similar transient absorption experiment was performed
with BD2 in MeCN but the results turned out to be difficult to
analyse and the measurements were repeated in a number of
solvents including DMF, THF, and toluene. The results of pump-
probe measurements of BD2 in DMF and comparison of the
transient absorption time profile at 580 nm with the fluores-
cence decay are presented in Figures 6 and 7.
The transient absorption signal for BD2 decays much faster
than that for BD1, but the emission decay relaxes more slowly
for BD2 than for BD1. At the same time the number of inter-
mediate states in relaxation for the excited state of BD2 is sim-
ilar to that for BD1. A few reactions take place in the same
timescale of a few picoseconds, which makes quantitative anal-
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Electronic effects on Bodipy by the Insertion of a Naphtalene-Based Bridge
Figure 5. Transient-absorption time profiles at 580 nm (top) and fluores-
cence decay at 550 nm (bottom) of BD1 recorded in MeCN.
Figure 7. Transient-absorption time profiles at 580 nm (top) and fluores-
cence decay at 550 nm (bottom) of BD2 recorded in DMF.
fluorescence decay and the disappearance of negative absorp-
tion in the 540–600 nm part of the spectrum. The lifetime of
the charge-transfer state (CTS) is only 12 ps, and the small dif-
ference in formation- and decay-time constants makes the
band at 580 nm almost invisible. Still, close inspection of the
transient-absorption kinetics at 580 nm shows that after simul-
taneous relaxation of stimulated (negative) absorption and
fluorescence a positive absorption is formed (Figure 7).
Two other differences between the transient absorption re-
sponses of BD1 and BD2 were that for the latter there was
a short-lived component, 0.1 ps, but practically no long-lived
component. The short-lived component could be attributed to
internal conversion from the second-excited singlet state to
the first. This is a fast and almost unresolved process, since the
time resolution of the instrument used was 0.2 ps. The internal
conversion is probably even faster in the case of BD1, which
precluded its detection for this compound. The virtually miss-
ing long-lived component in the case of BD2 was explained by
fast charge recombination and is discussed in Section 2.2.6.
In THF the differences between charge-separation and -re-
combination time constants were larger for BD2 and the char-
acteristic Bodipy-based radical anion band in the 550–620 nm
region could be seen better (see the Supporting Information
and comments therein). Additionally, all features of the time-
resolved spectra of BD2 in MeCN, DMSO and THF were similar
and generally the same conclusions could be drawn for the
transient states formed in all three solvents. Unfortunately,
careful comparison of photoinduced reactions in BD1 and BD2
in THF and other tested solvents of moderate polarity was pre-
cluded by the poor solubility of BD1, therefore the following
discussion is mainly limited to polar solvents.
Figure 6. Top: Transient-absorption decay components obtained from three-
exponential fit of pump-probe data of BD2 in DMF. Bottom: Time-resolved
transient-absorption spectra at a number of delay times obtained after
group-velocity dispersion compensation.
ysis a difficult task. However, the shape of the 12 ps compo-
nent for BD2 resembles that of the 140 ps component for
BD1, showing an absorption band around 580 nm. This band
could be attributed to the charge-separated state. The electron
transfer takes place at the singlet excited state with a time
constant close to 7 ps. This time constant could be seen in the
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A. C. Benniston et al.
2.2.6. Interpretation
a spin-forbidden process and therefore presumably slow. Inter-
system crossing from the Bodipy S1 to T1 state is known to be
in the order of 106 sꢀ1.[27] The energy of the triplet state is
about 1.6 eV above the ground state.
Based on molecular-modeling calculations and electrochemis-
try results a basic model for singlet excited-state deactivation
for the two dyads was proposed (Figure 8). In the polar solvent
Transient-spectroscopic evidence for BD1 supported that
1
the CTS is formed extremely fast (kCTS =7ꢁ1011
s
ꢀ1). The rate
1
for decay of the CTS (second decay component) is considera-
bly slower (6.4ꢁ109 sꢀ1). Discrimination between rates for
charge separation and charge recombination is good (~100),
especially considering the close proximity of the donor to the
acceptor. It is worth noting that a similar effect was witnessed
for a closely-spaced pyridinium-based Bodipy dyad.[28] The
value for kDREC is around 5.4ꢁ109 sꢀ1 for a driving force (DGDREC
)
of ꢀ2.11 eV, putting the electron-transfer process well into the
Marcus-inverted region. The long-lived component in the tran-
sient records is assigned to the triplet localised on the Bodipy
segment. Similar observations have been reported previous-
ly.[29] Thus, part of the process in charge recombination leads
to triplet formation, which is known to occur readily in orthog-
onally arranged geometries.[30] Given the yield of the Bodipy
triplet the value for k1 (or k3) is about 1ꢁ109 sꢀ1. The driving
force (DGTF) for this process is in the order of about ꢀ0.5 eV,
and electron transfer is likely to occur in the so-called Marcus-
normal region.
1
The scenario for BD2 is rather different. The CTS is formed
much more slowly, about1.4ꢁ1011 sꢀ1, and relaxes much faster,
about 0.8ꢁ1011 sꢀ1. The slower charge transfer can be expect-
ed, considering the longer distance between the donor and ac-
ceptor parts, but the faster charge recombination requires
a deeper analysis to be understood. In the frame of the semi-
classical Marcus electron-transfer theory [Eq. (2)] the reaction-
rate constant is:
Figure 8. Simplified energy diagram showing possible contributions to excit-
ed state deactivation of the Bodipy and the formed CTS.
MeCN the fully-formed singlet charge-transfer states (1CTS) are
situated at about 220 mV (BD1) and 390 mV (BD2) below the
respective Bodipy S1 states. The Gibbs free-energy changes
(DGCT) for the formation of the 1CTS, calculated using Equa-
tion (1), are ꢀ234 meV (BD1) and ꢀ374 meV (BD2).
ꢀ
ꢁ
2
2 p3=2 V2
ðDG þ lÞ
4 lkBT
pffiffiffiffiffiffiffiffiffi
ket
¼
exp ꢀ
ð2Þ
h
lkBT
DGCT ¼ FðEDꢀEAÞꢀE00 þ W
ð1Þ
Here, ED and EA are the oxidation and reduction potentials
for the donor and acceptor respectively. E00 is the mid-point in
the crossing of the absorption and fluorescence spectra for the
donor (BD1=2.34 eV, BD2=2.31 eV), and W is the Coulombic
work term. In MeCN these terms are only ꢀ54 meV (BD1) and
ꢀ34 meV (BD2) for the two dyads.
where h is the Planck constant, kB is the Boltzmann constant,
DG is the reaction free energy, l is the reorganization energy,
and V is the electronic coupling matrix element. For the for-
ward- and back-electron transfers the reorganization energy
and electronic coupling can be assumed to be the same, there-
fore the ratio of the charge-separation and -recombination
rate constants depends only on the reorganization energy, l.
Therefore, the reorganisation energy can be evaluated from
known charge-transfer and -recombination rate constants, and
free energies of the states.[31] The calculations provided 1.06 eV
and 1.14 eV as the reorganisation energies for BD1 and BD2,
respectively. These estimations agree with the expectation of
a higher reorganisation energy for BD2 relative to that of BD1
since the charge-transfer distance is greater for BD2. Also the
values of the energies are reasonable for this type of dyads.
The energies can be used to draw Franck-Condon energy dia-
grams for the two compounds in polar solvent, as presented
in Figure 9 (see the Supporting Information for calculation de-
tails). The essential difference in position of the CTS energy
profiles is that for BD2 the profile is shifted down (lower free
3
On spin considerations a lower-lying CTS must also be con-
sidered. If the 1CTS–3CTS energy gap is small, given by 2J,
where J is the spin–spin exchange integral, then the intersys-
tem-crossing rate (k2) may be extremely fast.[26] This scenario
opens up an efficient pathway to populate the triplet state on
the Bodipy segment (rate=k3). Direct population of the
1
Bodipy triplet state from the CTS is another feasible option
(rate=k1), as well as direct restoration of the ground state
(rate=kDREC). The value for k1 depends very much on the orien-
tation between the orbitals on the dimethylaniline and the
Bodipy. In an orthogonal arrangement the charge-transfer pro-
cess is accompanied by a change in the orbital angular mo-
mentum which facilitates the necessary spin flip to form a trip-
let. Direct recombination from 3CTS to the ground state is
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Electronic effects on Bodipy by the Insertion of a Naphtalene-Based Bridge
a conversion rate from the CTS to the Bodipy triplet state of
about 2ꢁ109 sꢀ1. The mechanism of conversion can be rela-
tively complex, that is through changing multiplicity of the
CTS first, followed by relaxation to the lower-lying locally-excit-
ed triplet state of the Bodipy chromophore. The triplet state is
virtually non-existent in the excitation relaxation of BD2, as
can be seen from the weak intensity of the long-lived compo-
nent in the transient-absorption measurements (Figure 6).
There are at least two reasons for this. First of all, the conver-
sion to the triplet state competes with the charge recombina-
tion which returns the system directly to the ground state.
Since the charge recombination is much faster for BD2, the rel-
ative amount of molecules relaxing through the triplet state is
smaller, even if the rate constants for this reaction pathway are
the same as for BD1. Secondly, the reaction rates for the relax-
ation through the triplet are expected to be slower for BD2
than those for BD1, since the potential barrier is higher and
the electronic coupling is smaller.
Figure 9. Franck-Condon energy diagram for BD1 (a) and BD2 (c) in
polar solvent. GS=ground state.
energy of the CTS) and away from the excited state (higher re-
organisation energy), relative to that of BD1.
3. Conclusions
The potential barrier for the charge separation is somewhat
smaller for BD2, which could accelerate the electron transfer
as compared to BD1. The distance between the donor and ac-
ceptor is essentially larger for BD2, resulting in a weaker elec-
tronic coupling and a slowing down of the electron-transfer
rate. The net result is roughly a four times slower electron-
transfer rate for BD2. For the charge recombination the key
factor is a much lower potential barrier in the case of BD2. The
barrier for charge recombination is virtually equal to that of
charge separation, according to the small difference in the re-
action-rate constants. In comparison to BD1, a hypothetical
slowing down for charge recombination, due to smaller elec-
tronic coupling (longer CT distance), is over-compensated by
the much lower charge-recombination potential energy barrier,
making the reaction much faster for BD2.
The electronic coupling discussed above can also be esti-
mated from Equation (2) for a known reaction-rate constant
and the free and reorganisation energies.[32] The calculations
yielded 690 and 25 cmꢀ1 for BD1 and BD2, respectively. These
values are reasonable, and accounting for the difference in
donor–acceptor distance between the dyads, which is 4.3 ꢂ.
The expected decrement factor in the exponential dependence
of the electron-transfer rate constant on the distance is 0.8 ꢂꢀ1,
which is again a reasonable value for a phenylene-like linker
between the donor and the acceptor. The semi-classical ET
theory is known to overestimate the rate constants in the in-
verted Marcus regime. In this particular case, not only a quanti-
tative picture predicted by the theory explains the observed
differences between BD1 and BD2 fairly well, but also the
qualitative estimations are self-consistent and agree with simi-
lar estimations made for analogous systems.
The naphthalene unit as a spacer appears to be a good con-
duit for supporting CTS formation, at least in cases where the
donor is N,N-dimethylaniline and the acceptor is Bodipy. Un-
fortunately, the orbitals and their energies for the bridge
appear to be ideally suited to promote rapid charge recombi-
nation.[33] Clearly, this situation is not conducive for the crea-
1
tion of a long-lived CTS in a simple molecular dyad. The ques-
tion to ask is: what is the effect of adding additional units be-
tween the donor and acceptor? The next in the series is the bi-
naphthalene group, which is itself interesting since it is chiral
by restricted rotation. The separation distance will increase by
around 40% and kCTS should decrease to about 1ꢁ1010 sꢀ1 as-
suming a simple exponential decay model. Of course, a secon-
dary consideration is the twist angle (q) in the binaphthalene,
since electron-transfer rates are known to be highly dependent
on q.[34] We expect to explore the role of the binaphthalene in
promoting electron transfer in a new dyad currently in prepa-
ration in the group.
Experimental Section
Materials
Bulk chemicals were purchased at the highest purity possible from
Aldrich Chemical Co. and used as received unless otherwise stated.
Tetra-n-butylammonium tetrafluoroborate (TBATFB) purchased
from Fluka was recrystallized several times from methanol and
dried thoroughly under vacuum before being stored in a desiccator.
Standard solvents were dried by literature methods before being
distilled and stored under nitrogen over 4 ꢂ molecular sieves.
Spectroscopic-grade solvents were used in all fast-kinetic experi-
ments and fluorescence/absorption-spectroscopy measurements.
There is an indication in Section 2.2.5 that charge recombi-
nation results in triplet localisation on the Bodipy group. The
long-lived component in the transient-absorption measure-
ments of BD1 (>1 ns, Figure 4) is attributed to the triplet
state. The relative bleaching of the ground-state absorption as-
sociated with this component is roughly 25%, which gives
Instrumentation
1H and 13C NMR spectra were recorded with either Bruker AVANCE
300 MHz, JEOL 400 MHz, or JEOL Lambda 500 MHz spectrometers.
11B and 19F NMR spectra were recorded using the 400 MHz spec-
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A. C. Benniston et al.
trometer. Chemical shifts for 1H and 13C NMR spectra are referenced
relative to the residual deuterated solvent. The 11B NMR chemical
shift is referenced relative to BF3·Et2O (d=0), and the 19F NMR
chemical shift is given relative to CFCl3 (d=0). Routine mass spec-
tra and elemental analyses were obtained using in-house facilities.
MALDI mass spectra were recorded at the EPSRC-sponsored Mass
Spectrometry Service at Swansea. Absorption spectra were record-
ed using a Hitachi U3310 spectrophotometer and corrected fluo-
rescence spectra were recorded using a Lambda Advanced F 4500
spectrometer. Uncorrected melting points were measured using
a Stuart SMP11 apparatus and typically carried out twice to check
for consistency in the readings. Crystallographic data for BD1 were
collected at Beamline 19 of Diamond Light Source with synchro-
tron radiation, whereas data for BD2 were collected on an Agilent
analysis at several different wavelengths. Up-conversion fluores-
cence lifetimes were obtained by fitting the single-photon-count-
ing data to different kinetic models using a variable Gaussian in-
strument response function. Analysis was attempted using mono-
to tri-exponentials and the stretched exponential function. Best fits
were judged by the usual methods of remaining residuals and
sigma value.
Preparation of BD1
TFA (2 drops) was added dropwise to a stirred solution of 2,4-di-
methyl-3-ethylpyrrole (1.88 mL, 14 mmol, 2.1 equiv.) and 4-dime-
thylaminobenzaldehyde (1.0 g, 6.7 mmol, 1.0 equiv.) in DCM
(250 mL). The reaction mixture was stirred at room temperature
until consumption of the aldehyde was complete (checked by
TLC). DDQ (1.60 g, 7.0 mmol, 1.05 equiv) was then added in
a single portion, and the reaction was left with stirring overnight
at room temperature. The following day N,N-diisopropylethylamine
(14.00 mL, 80.4 mmol, 12.0 equiv.) and BF3·Et2O (14.26 mL,
113 mmol, 16.8 equiv.) were added, and the reaction was left with
stirring for 6 h at room temperature. The reaction mixture was
washed with water (3ꢁ100 mL) and brine (3ꢁ100 mL). The sepa-
rated organic fractions were dried (MgSO4), filtered, and the sol-
vent was removed in vacuo to yield a black/dark violet residue
with a green tint. This was chromatographed on silica gel using
toluene as eluent to afford a red solid (0.648 g, 22% yield). This
solid was then frozen in ether and the solvent removed at the
pump and finally washed with petrol. Mpt.>2508C. 1H NMR
(CDCl3, 300 MHz): d=7.06 (d, J=8.7 Hz, 2H), 6.78 (d, J=8.7 Hz,
2H), 3.02, (s, 6H, Nꢀ(CH3)2), 2.53 (s, 6H), 2.31 (q, J=7.4, 4H), 1.26
(s, 6H), 0.99 ppm (t, J=7.4 Hz, 6H). 13C NMR (CDCl3, 75 MHz): d=
152.93, 150.56, 141.55, 138.58, 132.33, 131.48, 128.96, 123.06,
112.33, 40.38, 17.08, 14.64, 12.42, 11.93 ppm. 11B NMR (CDCl3,
160 MHz): d=ꢀ0.115 ppm (t, Jav =34.45 Hz). 19F NMR (CDCl3,
Techologies Gemini
A Ultra diffractometer. CCDC 886352 and
886353 contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre through www.ccdc.cam.ac.uk/
data_request/cif.
Cyclic voltammetry experiments were performed using a fully auto-
mated HCH Instruments Electrochemical Analyzer and a three elec-
trode set-up consisting of a platinum working electrode, a platinum
wire-counter electrode and a Ag/AgCl reference electrode. Ferro-
cene was used an internal standard. All studies were performed in
deoxygenated CH3CN containing TBATFB (0.2m) as background
electrolyte. The solute concentrations were typically 0.1 mm. Redox
potentials were reproducible to within ꢁ15 mV.
Time-Resolved Absorption Measurements
Femto- to pico-second time-resolved absorption spectra were col-
lected using a pump-probe technique described previously.[35] The
femtosecond pulses of a Ti-sapphire generator were amplified by
using a multipass amplifier (CDP-Avesta, Moscow, Russia) pumped
by a second harmonic of the Nd:YAG Q-switched laser (model
LF114, Solar TII, Minsk, Belorussia). The amplified pulses were used
to generate second harmonic (420 nm) for sample excitation
(pump beam) and the white light continuum for a time-resolved
spectrum detection (probe beam). The samples were placed in
1 mm rotating cuvettes, and averaging of 100 pulses at a 10 Hz
repetition rate was used to improve the signal-to-noise ratio. The
typical response time of the instrument was 150 fs (fwhm). Absorp-
tion spectra were recorded prior to and after all experiments to
check for compound degradation.
~
470 MHz): d=ꢀ145.66 (q, J=28.59 Hz,). IR (neat): n=2958, 2923,
2854 (CꢀH), 1527, 1473 (C=C, C=N), 1183 cmꢀ1 (BꢀF). EI-MS : m/z
calc. for C25H32BF2N3 =423 fnd. 424 [MH]+.
Preparation of 1
TFA (2 drops) was added dropwise to a stirred solution of 2,4-di-
methyl-3-ethylpyrrole (1.46 mL, 10.84 mmol, 2.1 equiv.) and p-bro-
monaphthaldehyde (1.21 g, 5.16 mmol, 1.0 equiv.) in DCM
(200 mL). The reaction mixture was stirred at room temperature
until the consumption of the aldehyde was complete (checked by
TLC). DDQ (1.23 g, 5.4 mmol, 1.05 equiv) was then added in
a single portion, and the reaction was left with stirring overnight
at room temperature. The following day N,N-diisopropylethylamine
(10.79 mL, 61.92 mmol, 12.0 equiv) and BF3·Et2O (10.99 mL,
86.69 mmol, 16.8 equiv.) were added, and the reaction was left
with stirring for 6 h at room temperature. The reaction mixture
was washed with water and brine. The separated organic fractions
were dried (MgSO4), filtered, and the solvent removed in vacuo to
yield a pink/purple residue with a greenish tint. The residue was
chromatographed on silica gel using toluene as eluent to afford
a purple/green solid (1.55 g, 59% yield). This solid was then frozen
in ether and the solvent removed at the pump and washed with
petrol. Mpt.=197–1988C. 1H NMR (CDCl3): d=8.34 (d, J=8.2 Hz,
1H, aromatic), 7.91 (d, J=7.3 Hz, 1H, aromatic), 7.89 (d, J=8.2 Hz,
1H, aromatic), 7.64 (t, J=7.1 Hz, 1H, aromatic), 7.51 (t, J=7.1 Hz,
1H, aromatic), 7.29 (d, J=7.3 Hz, 1H, aromatic), 2.61 (s, 6H, CH3),
2.27 (q, J=7.5 Hz, 4H, CH2CH3), 1.02 (s, 6H, CH3), 0.96 ppm (t, J=
7.5 Hz, 6H, CH2CH3). 13C NMR (CDCl3, 75 MHz): d=154.07, 137.81,
136.94, 133.24, 133.03, 132.76, 131.86, 130.97, 129.75, 127.89,
Fluorescence Lifetime Measurements
Ultrafast fluorescence decays were measured by an up-conversion
method as described previously.[35] The instrument (FOG100, CDP,
Moscow, Russia) utilizes the second harmonic (420 nm) of a 50 fs
Ti:sapphire laser (TiF50, CDP, Moscow, Russia) pumped by an Ar ion
laser (Innova 316P, Coherent). The samples were placed in a rotat-
ing disk-shaped 1 mm cuvette. A typical resolution for the instru-
ment was 150 fs (fwhm).
Data analysis
Time-resolved transient-absorption data were manipulated using
a freely available software package. In a typical analysis the whole
collection of differential absorption spectra were inspected over
the full time scale, and decay kinetics obtained at two specifically
chosen wavelengths using an appropriate number of exponentials
and instrument response function. Lifetimes obtained by a least-
squares fit to the kinetic model were also checked by a global
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Electronic effects on Bodipy by the Insertion of a Naphtalene-Based Bridge
[6] A. Osuka, S. Nakajima, K. Maruyama, N. Mataga, T. Asahi, I. Yamazaki, Y.
127.83, 127.23, 126.42, 125.57, 123.72, 16.85, 14.42, 12.41,
11.14 ppm. 11B NMR (160 MHz, CDCl3) d=0.00 (t, Jav =32.0 Hz).
19F NMR (470 MHz, CDCl3) d=ꢀ145.6 ppm (broadened multiplet).
Nishimura, T. Ohno, K. Nozakild, J. Am. Chem. Soc. 1993, 115, 4577–
4589.
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~
IR (neat): n=2963, 2928, 2870 (CꢀH), 1534, 1474 (C=C, C=N),
1182 cmꢀ1 (BꢀF).
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ton, A. Harriman, ChemPhysChem 2005, 6, 2251–2260.
Preparation of BD2
A solution in THF (20mL) of 1 (0.15g, 0.295mmol, 1.0 eq), 4-dime-
thylaminophenyl boronic acid (0.10g, 0.620mmol, 2.1 eqiv.) and
sodium carbonate (0.093g, 0.885mmol, 3 eqiv.) in water was de-
gassed for an hour prior to addition of the catalyst, tetrakis(triphe-
nylphosphine) palladium(0) (0.02g, 0.024mmol, 0.08 eqiv.). The re-
action was then heated with continuous stirring overnight at 608C.
The following morning the crude reaction mixture was left to cool
before being filtered through a glass pipette plugged with cotton
wool to remove the unreacted catalyst. The residual solvent was
then removed under vacuum and the crude material taken up in
DCM, washed with sodium carbonate solution (0.6m) and dried
over sodium sulfate. It was then filtered and the DCM removed in
vacuo to yield a black/dark violet residue with a green tint. The
residue was chromatographed on silica gel using toluene as eluent
to afford a red solid (0.11g, 68% yield). This solid was then frozen
in ether and the solvent removed at the pump and washed with
petrol. Mpt.>2508C. 1H NMR (CDCl3, 300 MHz): d=8.11 (d, J=
7.5 Hz, 2H), 7.87 (d, J=7.5 Hz, 2H), 7.46, (m, 4H), 6.91 (d, J=9 Hz,
2H), 3.07 (s, 6H, Nꢀ(CH3)2), 2.59 (s, 6H), 2.28 (q, J=7.3, 4H), 1.07 (s,
6H), 0.97 ppm (t, J=7.3 Hz, 6H). 13C NMR (CDCl3, 75 MHz): d=
11.25, 12.56, 14.61, 17.05, 40.56, 112.21, 125.49, 125.76, 125.95,
126.14, 126.27, 126.53, 126.65, 130.94, 131.43, 131.74, 131.93,
132.43, 132.61, 138.34, 138.99, 141.80, 149.95, 153.68 ppm. 11B NMR
(CDCl3, 160 MHz): d=0.0192 ppm (t, Jav =33.22 Hz). 19F NMR (CDCl3,
470 MHz): d=ꢀ145.39 ppm (broadened multiplet). EI-MS: m/z calc.
for C35H38BF2N3 =549 fnd. 550 [MH]+.
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We thank the Engineering and Physical Sciences Research Council
(EPSRC) for financial support (EP/G04094X/1 and EP/F03637X/1)
and Diamond Light Source for synchrotron diffraction facilities.
The EPSRC-sponsored Mass Spectrometry Service at Swansea is
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Keywords: charge
photophysics · spectroscopy · triplet formation
transfer
·
charge
separation
·
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Received: June 23, 2012
Revised: July 24, 2012
[33] W. D. Oosterbaan, C. Koper, T. W. Braam, F. J. Hoogesteger, J. J. Piet,
B. A. J. Jansen, C. A. van Walree, H. J. van Ramesdonk, M. Goes, J. W. Ver-
Published online on && &&, 2012
&10
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www.chemphyschem.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 0000, 00, 1 – 11
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These are not the final page numbers!

ARTICLES
A. C. Benniston,* S. Clift, J. Hagon,
H. Lemmetyinen, N. V. Tkachenko,*
W. Clegg, R. W. Harrington
&& – &&
Effect on Charge Transfer and Charge
Recombination by Insertion of
a Naphthalene-Based Bridge in
Molecular Dyads Based on
Borondipyrromethene (Bodipy)
A bridge too far: Despite the fact that
insertion of the naphthalene group into
the Bodipy dyad (see picture, right op-
posite) increases the distance between
the donor and acceptor, charge recom-
bination is much faster when compared
to the simple dyad (see picture, left op-
posite).
ChemPhysChem 0000, 00, 1 – 11
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org
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11
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These are not the final page numbers!