Energy- and charge-transfer processes in a perylene-BODIPY-pyridine tripartite array

Article abstract of DOI:10.1002/ejoc.200800159

A novel boron dipyrromethene (BODIPY) dye has been synthesized in which the F atoms, usually bound to the boron center, have been replaced with 1-ethynylperylene units and a 4-pyridine residue is attached at the meso-position. The perylene units function as photon collectors over the wavelength range from 350 to 480 nm. Despite an unfavorable spectral overlap integral, rapid energy transfer takes place from the singlet-excited state of the perylene unit to the adjacent BODIPY residue, which is itself strongly fluorescent. The mean energy-transfer time is 7 ± 2 ps at room temperature. The dominant mechanism for the energy-transfer process is Dexter-type electron exchange, with Foerster-type dipole-dipole interactions accounting for less than 10% of the total transfer probability. There are no indications for light-induced electron transfer in this system, although there is evidence for a nonradiative decay channel not normally seen for F-type BODIPY dyes. This new escape route is further exposed by the application of high pressure. The meso-pyridine group is a passive bystander until protons are added to the system. Then, protonation of the pyridine N atom leads to complete extinction of fluorescence from the BODIPY dye and slight recovery of fluorescence from the perylene units. Quenching of BODIPY-based fluorescence is due to chargetransfer to the pyridinium unit whereas the re-appearance of perylene-based emission is caused by a reduction in the Foerster overlap integral upon protonation. Other cations, most notably zinc(II) ions, bind to the pyridine N-atom and induce similar effects but the resultant conjugate is weakly fluorescent. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejoc.200800159

FULL PAPER
DOI: 10.1002/ejoc.200800159
Energy- and Charge-Transfer Processes in a Perylene–BODIPY–Pyridine
Tripartite Array
Mohammed A. H. Alamiry,[a] Anthony Harriman,*[a] Laura J. Mallon,[a] Gilles Ulrich,[b] and
Raymond Ziessel*[b]
Keywords: Energy transfer / Perylene / Fluorescence / Chemical sensor / Boron dipyrromethene dye
A novel boron dipyrromethene (BODIPY) dye has been syn-
thesized in which the F atoms, usually bound to the boron
center, have been replaced with 1-ethynylperylene units and
a 4-pyridine residue is attached at the meso-position. The
perylene units function as photon collectors over the wave-
length range from 350 to 480 nm. Despite an unfavorable
spectral overlap integral, rapid energy transfer takes place
from the singlet-excited state of the perylene unit to the adja-
cent BODIPY residue, which is itself strongly fluorescent.
The mean energy-transfer time is 7Ϯ2 ps at room tempera-
ture. The dominant mechanism for the energy-transfer pro-
cess is Dexter-type electron exchange, with Förster-type di-
pole–dipole interactions accounting for less than 10% of the
total transfer probability. There are no indications for light-
induced electron transfer in this system, although there is
evidence for a nonradiative decay channel not normally seen
for F-type BODIPY dyes. This new escape route is further
exposed by the application of high pressure. The meso-pyr-
idine group is a passive bystander until protons are added to
the system. Then, protonation of the pyridine N atom leads
to complete extinction of fluorescence from the BODIPY dye
and slight recovery of fluorescence from the perylene units.
Quenching of BODIPY-based fluorescence is due to charge-
transfer to the pyridinium unit whereas the re-appearance of
perylene-based emission is caused by a reduction in the
Förster overlap integral upon protonation. Other cations,
most notably zinc(II) ions, bind to the pyridine N-atom and
induce similar effects but the resultant conjugate is weakly
fluorescent.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
the targeted substrate, but it is clear that the recognition
event must be accompanied by definitive on-off signaling.
We now report on a general purpose fluorescent sensor de-
signed specifically to monitor high concentrations of cat-
ions in an effluent stream. Such systems are intended as
companion monitors for probes that detect low and/or me-
dium range levels of pollutants and should be considered
as the final overload device for an integrated network of
sensors. That is to say, a sensitive probe sends out a warning
signal that a pollutant is present. A network of secondary
sensors monitors the rising concentration of pollutant until
the point when the final sensor is triggered. At this point,
a termination signal is sent to the operations panel.
As a major breakthrough over the past few years, boron
dipyrromethene dyes have emerged as a versatile, robust
and easily modified class of fluorescent reagent.[6] These
materials display little inclination to undergo intersystem
crossing to the triplet manifold and exhibit fluorescence
properties that are relatively insensitive to changes in the
local environment. Fluorescence quantum yields are usually
high but the excited-state lifetimes are too short to be affec-
ted by the presence of dissolved oxygen. A problem found
for earlier derivatives in that the Stokes’ shift is too small
for practical exploitation of the dye as a fluorescent probe
can be overcome by covalent attachment[7] of secondary
Introduction
The last few decades have witnessed the rationale devel-
opment of many different luminescent sensors for the sensi-
tive, and sometimes selective, recognition of certain chemi-
cal species in solution. A tremendous wealth of information
has accrued and some invaluable lessons have been learned
with respect to the basic design principles.[1–3] Most notable
among the major strategies used to introduce an on/off
switching device has been light-induced electron transfer,
usually involving an amine donor covalently attached close
to a fluorescent molecule.[4] Two complementary ap-
proaches can be considered within this overall scheme.
Thus, binding of the analyte can curtail electron transfer,
thereby leading to restoration of the emission inherent to
the chromophore,[5] or the opposite behavior can be engine-
ered so that binding switches off the emission.[4] Each route
has benefits and disadvantages, according to the nature of
[a] Molecular Photonics Laboratory, School of Natural Sciences,
Bedson Building, Newcastle University,
Newcastle upon Tyne, NE1 7RU, UK
E-mail: anthony.harriman@ncl.ac.uk
[b] Laboratoire de Chimie Moléculaire, Centre National de la Re-
cherche Scientifique (CNRS), École de Chimie, Polymères, Ma-
tériaux (ECPM),
25 rue Becquerel, 67087 Strasbourg Cedex 02, France
2774
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CT Processes in a Perylene–BODIPY–Pyridine Tripartite Array
chromophores that expand the range of excitation wave- cedures afforded the analytically pure dyes 2 and 3 in mod-
lengths. In this regard, it is important to note the benefits est yields. The compounds 1 and 2 are used as reference
associated with functionalization at the boron center.[8] In materials by which to better understand the properties of
the sensor described below, we use this latter strategy to the perylene-based dye 3 (Scheme 1).
attach ethynylated perylene units to the boron dipyrrome-
thene (BODIPY) dye for precisely this reason.
Photophysics of 3
Regarding the recognition unit, we return to a molecular
dyad reported recently in which the BODIPY dye is
equipped with a meso N-methylpyridinium unit.[9] This elec-
tron-affinic residue quenches fluorescence from the BOD-
IPY dye due to the onset of a light-induced charge-transfer
process that is unavailable to the simpler pyridine derivative.
Our presumption here is that the N-methyl group might be
replaced with cationic species, including protons, which will
also serve as quenchers for the BODIPY emission. Since
pyridine is not a particularly effective coordination reagent,
we might expect the stability constants for complexation of
cations from solution to be relatively low. If so, this fulfills
the basic design element essential for a fluorescent termin-
ator of the type highlighted above. We now describe the
synthesis, photophysical properties and preliminary binding
characteristics of a prototypic dye of this general class.
The absorption spectrum of 3 in CH2Cl2 (Figure 1)
shows a transition located at ca. 465 nm that can be as-
signed to the perylene S0ǞS1 transition.[11] Also present in
the absorption spectrum is the S0ǞS1 transition of the
BODIPY chromophore that is centered at ca. 525 nm.[12]
The S0ǞS2 transition associated with the BODIPY unit lies
underneath the more intense perylene S0ǞS1 bands.[13] The
absorption bands are sharp, well-resolved and located at
similar positions to those found for appropriate reference
compounds, which suggests that the perylene and BODIPY
units are not in strong electronic communication. This find-
ing is in agreement with earlier work carried out with B-
linked molecular dyads.[6] Close comparison of the absorp-
tion spectrum of 3 with those of the respective reference
compounds indicates that the perylene S0ǞS1 band un-
dergoes a red shift of ca. 2 nm while the S0ǞS1 band for
the BODIPY unit is red-shifted by ca. 6 nm. On integration
of the molar absorption coefficients measured for 1 and 3
it can be concluded that the presence of the perylene units
leads to a 2.6-fold increase in the number of photons that
Results and Discussion
Synthesis
The synthesis of compound 1 has been described pre- can be collected.
viously and involves condensation of kryptopyrrole with 4-
formylpyridine.[10] An acceptable yield is obtained on a rou-
tine basis using 4-(chlorocarbonyl)pyridine as the starting
point for a one-pot synthesis. Two pyrrole molecules are
condensed over four days with the acetyl chloride, to form
the corresponding dipyrromethene hydrochloride salts. De-
protonation with a tertiary amine, followed by addition of
boron difluoride, allows formation of the boron complex 1
in 40% yield. Substitution of the two fluorine atoms of the
boradiazaindacene dye 1 is achieved by addition of the cor-
responding acetylenic Grignard reagent and subsequent
heating at 60 °C. Work-up and standard purification pro-
Figure 1. Absorption (black) and emission (grey) spectra recorded
for 3 in CH2Cl2 solution at room temperature. The excitation wave-
length used for the fluorescence spectrum was 408 nm.
The fluorescence spectrum recorded for 3 in CH2Cl2
(Figure 1) shows emission from the BODIPY unit, centered
at 540 nm,[12] but some residual fluorescence from the per-
ylene unit[11] can be seen within the window between 450
and 500 nm. The fluorescence quantum yield (ΦF) mea-
sured for the BODIPY unit in CH2Cl2 was found to be
0.31, which is somewhat reduced relative to 1 (ΦF = 0.58).
Likewise, the excited-singlet-state lifetime (τS = 2.6 ns) for
the BODIPY unit remains about half of those recorded for
the reference compounds 1 (τS = 5.4 ns) and 2 (τS = 4.9 ns).
Even so, the decay traces fit well to mono-exponential ki-
Scheme 1. i) Kryptopyrrole (2 equiv.), CH2Cl2 room temp.; ii)
TEA, BF3·OEt2; iii) R–ϵ–MgBr (2.5 equiv.), THF, 60 °C.
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netics. The radiative rate constant (kRAD) found for 3 is
1.3ϫ108 s–1 and is in excellent agreement with that calcu-
lated from the Strickler–Berg expression[14] (kRAD
=
1.2ϫ108 s–1). Fluorescence from the BODIPY unit shows
good mirror symmetry with the corresponding absorption
spectral profile and the Stokes’ shift is modest. The cor-
rected fluorescence excitation spectrum agrees well with the
absorption spectrum recorded over the region from 250 to
520 nm and it is clear that photons absorbed by the per-
ylene units are transferred to the BODIPY unit with high
efficiency. Nonetheless there is a clear sense that 3 is subject
to an additional nonradiative channel that remains closed
to the conventional F-BODIPY dyes and that somehow in-
volves the perylene units.[6] The exact nature of this new
decay route is unknown but might involve large scale tor-
sional motion of the appendages because the effect appears
to increase proportionally with the bulk of the B-based sub-
stituents. More work is required to better clarify this effect,
which is currently being examined by high-pressure studies
on derivatives not able to undergo intramolecular energy
transfer.
The small residual fluorescence from the perylene units
(Figure 1) corresponds to an amount of around 0.1% of
that found for 1-ethynylperylene in CH2Cl2 at room tem-
perature (ΦF = 0.79) with excitation at 435 nm. Time-corre-
lated, single-photon counting studies with a temporal reso-
lution of ca. 50 ps and using a variety of emission and exci-
tation wavelengths failed to detect any emission from the
perylene unit in CH2Cl2, indicating that τS Ͻ40 ps. Similar
results were found in other solvents, including methyltetra-
hydrofuran, cyclohexane and acetonitrile, and at all suitable
excitation wavelengths. From this, it can be deduced that
efficient energy transfer occurs from the perylene units to
the nearby BODIPY residue.[15–17] On the basis of the time-
resolved emission studies, the rate constant for intramolecu-
lar energy transfer (kEnT) must exceed ca. 2.5ϫ1010 s–1.
Using improved temporal resolution, kEnT was found to
be 1.4ϫ1011 s–1 in CH2Cl2 solution at room temperature.
This latter study involved excitation of the perylene unit at
420 nm with a sub-ps laser pulse (FWHM = 0.22 ps) and
monitoring bleaching of the S0–S1 absorption transition as-
sociated with the BODIPY unit (Figure 2). It is clear that
the bulk of the bleaching signal occurs on time scales much
longer than that of the excitation pulse. The bleaching ki-
netics are not well described by a single exponential step,
however, and the quoted rate constant is the mean value
that corresponds to the reciprocal of the time taken for the
signal to reach 1/e of its initial value. The mean lifetime
found for the singlet-excited state of the perylene unit was
7Ϯ2 ps. The BODIPY S1 state shows a small amount (i.e.,
Figure 2. A kinetic trace showing depletion of the ground state of
the BODIPY unit following laser excitation into one of the per-
ylene residues at 420 nm. The trace was recorded at 515 nm.
appendages. Thus, Burgess et al.[16] have reported rapid
electron exchange for certain meso-substituted BODIPY
dyes while Harriman et al.[17] have described dipole–dipole
energy transfer for some B-substituted BODIPY dyes. For
3, there is strong spectral overlap between fluorescence
from the perylene unit and absorption by the BODIPY resi-
due. The Förster overlap integral (JF) calculated from the
reference compounds is 1.16ϫ10–13 mol–1 cm6. The average
center-to-center separation distance derived from molecular
dynamics simulations[17] (MDS) is 15.6 Å, while the quan-
tum yield (ΦF = 0.79) and excited-singlet lifetime (τS
=
4.5 ns) of the donor are known from separate studies. Fluc-
tuations around the connecting B–C bond and slight struc-
tural distortions associated with the perylene residue cause
a modest range of Förster-type orientation factors (κ). The
mean κ value obtained from MDS runs is 0.21, which is
rather small and much below the value (κ2 = 0.67) appropri-
ate for random orientations. Using these various values in
conjunction with the Förster expression leads to an esti-
mated rate constant for dipole–dipole energy transfer (kF)
of 1.3ϫ1010 s–1. Although very high, this rate constant is
too low to satisfactorily account for the measured value. We
conclude, therefore, that intramolecular energy transfer in 3
involves both electron exchange (kD = 12.7ϫ1010 s–1) and
dipole–dipole (kF = 1.3ϫ1010 s–1) mechanisms. For the spe-
cific molecule under consideration, it appears that Dexter-
type electron exchange is by far the dominant mechanism.
This situation is reminiscent of the meso-anthracene substi-
tuted BODIPY dyes described by Burgess et al.[16]
Effect of High Pressure
Further insight into the nature of the additional nonradi-
10%) of decay over 40 ps but the residual signal is not deac- ative decay channel available to 3 but closed to conventional
tivated on time scales less than several hundred picose- F-BODIPY dyes was sought from fluorescence studies per-
conds.
formed at high applied pressure. This approach was taken
The two mechanisms normally invoked to account for because of suspicions that the large appendages might im-
electronic energy transfer are the Förster (dipole–dipole, pose changes in the geometry of the BODIPY unit, espe-
through space) and Dexter (electron exchange, through cially at the excited-state level.[18] It was found that the fluo-
bond) processes. Both these mechanisms have been ob- rescence intensity for the corresponding F-BODIPY deriva-
served with BODIPY dyes equipped with aryl hydrocarbon tive fitted with a meso-phenyl group increased slightly when
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CT Processes in a Perylene–BODIPY–Pyridine Tripartite Array
a solution in methyltetrahydrofuran (MTHF) was exposed a few comments are in order here. A possible explanation
to high pressure. This effect was quite modest, however, and has the perylene appendages acting as levers by which to
corresponds to an increase of ca. 5% for an applied pres- perturb the geometry around the upper rim of the dipyrrin
sure of 550 MPa. It is caused by a small reduction in the unit. A similar effect has been described[21] for F-BODIPY
volume of the solvent.[19] There was also a red shift in the dyes lacking the bulky alkyl groups on the pyrrole nucleus.
emission peak, amounting to ca. 5 nm at 550 MPa, due to Such structural distortion allows the meso-phenyl ring to
the pressure-induced change in solvent polarisability.[20] rotate in the excited state and, by way of holes in the poten-
Surprisingly, applying pressure to a dilute solution of 3 in tial surface, facilitates fast nonradiative decay to the ground
MTHF causes a serious decrease in fluorescence from the state.[22] We invoke a similar effect here as the reason why
BODIPY unit but a small increase in fluorescence from the 3 is somewhat less fluorescent than expected. It then re-
perylene units (Figure 3). This experiment refers to exci- mains for the amount of structural distortion to increase
tation into the perylene subunits at 408 nm. It is noticeable systematically with increasing pressure. This is a mechanical
that an isoemissive point at 522 nm is preserved during the effect.
pressure changes and that the system is both reversible and
reproducible.
Protonation of 3
Earlier work[9] has shown that alkylation of the pyridine
N atom present in 1 switches-on a light-induced charge-
transfer reaction in which the N-alkyl pyridinium unit acts
as acceptor and the singlet-excited state of the BODIPY
unit functions as donor. The charge-transfer state (CTS) is
weakly fluorescent. Addition of protons to a solution of
3 (Scheme 2) results in extinction of fluorescence from the
BODIPY unit (Figure 4). This effect is independent of the
nature of the proton source and insensitive to the choice of
solvent. The protonated analogue does not fluoresce, at
least with any appreciable yield or lifetime. Protonation,
which is assumed to take place at the pyridine N atom, is
fully reversible and does not cause chemical damage to the
molecule. Inappropriate counteranions, however, facilitate
Figure 3. The effect of applied pressure (in even steps up to
precipitation of the protonated species. The data collected
from sets of fluorescence spectral titrations allow calcula-
tion of the stability constant (β) for the monoprotonated
species as being 400Ϯ20 –1 in methyltetrahydrofuran solu-
tion. Higher-order protonation was not observed during
these titrations. Accurate fitting of the data could be
achieved with the fluorescence quantum yield for the pro-
tonated species being set equal to zero; that is to say, only
the pyridine form fluoresces to any significant degree. Con-
comitant with fluorescence quenching, the absorption spec-
trum recorded for the BODIPY unit undergoes a red shift
of ca. 12 nm but that of the perylene unit remains unaffec-
ted.
550 MPa) on the fluorescence spectrum of 3 in MTHF, following
excitation at 408 nm. With increasing pressure, the signal attributed
to perylene increases while that due to BODIPY decreases.
The enhanced fluorescence from the perylene units seen
at high pressure must be a consequence of a decreased rate
of intramolecular energy transfer. In turn, this situation
arises because of a pressure-induced change in the molecu-
lar geometry, the precise nature of which cannot be ex-
plained on the basis of these experiments alone. This is be-
cause a change in geometry could easily affect the probabili-
ties of both Förster and Dexter mechanisms while a similar
effect would result from freezing the structure into the low-
est-energy conformation. The increase in perylene-based
emission is a factor of ca. 10-fold at 550 MPa in MTHF.
Restricted energy transfer would account for a modest drop
in fluorescence from the BODIPY unit but the actual de-
crease observed is far too high to be explained solely in this
way. The residual BODIPY-based emission is red-shifted by
ca. 8 nm, again indicating the change in polarisability of the
solvent,[20] but not significantly broadened. Related experi-
ments show that the absorption spectrum undergoes a red
shift of 6 nm but no observable decrease. Loss of fluores-
cence, therefore, is not due to marked effects imposed at the
ground-state level.
A full description of this pressure-sensitive nonradiative
decay channel must await a more detailed investigation but Scheme 2.
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excited singlet state of the BODIPY unit and charge trans-
fer occurs on a time scale of ca. 5 ps.[9] Cyclic voltammetry
studies performed with the protonated analogue showed the
reduction step to be irreversible but did confirm that intra-
molecular charge transfer is likely to compete with radiative
decay of the BODIPY-based singlet-excited state, at least on
thermodynamic grounds. Laser excitation into the perylene
unit at 420 nm leads to rapid bleaching of the S0–S1 absorp-
tion transition localized on the BODIPY dye. Again, the
mean lifetime for the energy-transfer step was of the order
of 7Ϯ2 ps. In this case, however, the S1 state localized on
the BODIPY unit reverts to the ground state with a lifetime
of 42Ϯ4 ps (Figure 5). This is substantially shorter than
that found for either the parent compound 3 (τS = 2.6 ns)
or the charge-transfer state observed for the N-methylpyr-
idinium analogue (τCTS = 0.52 ns in acetonitrile).[9] On the
basis that quenching of the BODIPY S1 state occurs exclu-
sively by way of intramolecular charge transfer, we can es-
tablish the rate constant for this step as being 2.4ϫ1010 s–1.
Since the resultant charge-transfer state cannot be resolved
in the decay records, it follows that charge recombination
must occur on a faster time scale. The remarkable difference
in lifetimes for the charge-transfer states found for methyl-
Figure 4. The fluorescence spectrum of 3 in methyltetrahydrofuran
solution recorded as a function of incremental additions of triflic
acid. The insert shows a 10-fold expanded version of the perylene
fluorescence region, as observed in the presence of excess acid.
An interesting feature of the protonation of 3 in methyl- ated (τCTS = 0.52 ns) and protonated (τCTS Ͻ 40 ps species),
tetrahydrofuran solution relates to the partial recovery of allowing for the different nature of the BODIPY unit (i.e.,
fluorescence associated with the perylene unit (Figure 4), 1 vs. 3), indicates that the N–H group must promote rapid
similar to that seen at high pressure. Thus, when the pyr- nonradiative deactivation of the charge-transfer state. This
idine unit is protonated the fluorescence quantum yield for observation is in line with the energy-gap law.[23]
emission from the perylene unit is raised from 0.1% to
0.5%. This fivefold increase in ΦF is highly reproducible
and essentially independent of the nature of the solvent.
The fluorescence enhancement can be traced to a decreased
Förster overlap integral caused by the red-shifted absorp-
tion spectral profile for the BODIPY-based acceptor. In-
deed, protonation of the pyridine N atom reduces JF by
ca. 15% and thereby minimizes the contribution of dipole–
dipole energy transfer. The calculated rate constant for
Förster-type energy transfer (kF = 1.1ϫ1010 s–1) following
protonation agrees very well with the decrease in the overall
rate constant on the assumption that the Dexter process
remains unaffected. Although the effect is small, the ability
to switch on fluorescence from the ancillary light harvester
Figure 5. Kinetic trace recorded at 520 nm following laser exci-
during the sensing process holds promise for the future de-
tation of the protonated form of 3 and showing the depletion
sign on improved orthogonal probes. It might be noted that
precisely the same effect is found with weaker acids, includ-
ing trifluoroacetic acid.
of the ground state of the BODIPY unit. The insert shows an
expanded version of the early time period.
The mechanism whereby 3 registers the presence of pro-
tons in solution can be explained in terms of an internal
Several other BODIPY-based dyes are known to func-
charge-transfer process that is available only to the proton- tion as pH indicators whereby protonation switches off a
ated species. This behavior is similar to that noted pre- charge-transfer reaction and restores fluorescence.[24] Com-
viously for the N-alkylated derivative of 1,[9] except that the pound 3 complements these probes by functioning primar-
protonated species is non-fluorescent. Alkylation or proton- ily in acid solution and using protonation to switch off the
ation of the pyridine N atom raises the reduction potential radiative process. Many of the other proton sensors employ
for one-electron reduction of this moiety to a much less an electron-donating amine as the protonation site and ben-
negative value and thereby switches on the light-induced efit from high binding constants (e.g., β Ͼ 105 –1). Such
charge-transfer step. Following methylation of the pyridine dyes are useful for measuring low concentrations of acid
N atom, there is a thermodynamic driving force of ca. whereas 3 will operate best in proton-rich environments.
0.12 eV for reduction of the pyridinium unit by the first- This is precisely in accord with our main design requisite.
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CT Processes in a Perylene–BODIPY–Pyridine Tripartite Array
Coordination of Metal Cations
Many different cations will coordinate to the pyridine N
atom so as to form a stable complex, for which the binding
constants vary over a very wide range. There is little, if any,
selectivity for the binding event. It was noted that 3 forms
a ground-state complex with a selection of metal cations. In
each case, complexation affected the absorption and fluo-
rescence spectra. We now describe the results observed on
addition of zinc(II) ions to a solution of 3 in methyltetra-
hydrofuran as being representative of these cations. Thus,
in the presence of a small excess of zinc perchlorate, the
absorption spectrum recorded for 3 shows a pronounced
red shift and broadening for the bands associated with the
BODIPY unit. This shift is on the order of 10 nm but little
change is observed for the transitions localized on the per-
ylene unit (Figure 6). The fluorescence yield, measured af-
ter excitation into the BODIPY unit, is decreased in the
presence of zinc(II) ions and reaches a limiting value on
addition of a large excess of the salt that corresponds to ca.
30% of that of the non-coordinated compound. Interest-
ingly, and unlike the situation that follows from protonation
of 3, the residual fluorescence spectrum is red-shifted by ca.
10 nm (Figure 7). This latter spectrum is characteristic of
the zinc(II) – 3 conjugate. The fluorescence lifetime re-
corded for this complex is 1.2 ns. Again, there is a definite
increase in fluorescence from the perylene unit due to the
reduced Förster spectral overlap integral associated with the
red-shifted absorption spectrum of the BODIPY-based ac-
ceptor. In this case, the spectral shift is sufficient to virtually
extinguish the Förster contribution.
Figure 7. Effect of added zinc(II) cations on the fluorescence spec-
trum recorded for 3 in methyltetrahydrofuran; before addition (grey
curve) and after addition (black curve) of zinc perchlorate.
emission spectrum does not change on addition of excess
zinc(II) cations and is independent of the nature of the
counterion. Cyclic voltammetry shows that light-induced
charge-transfer is unlikely to take place for the zinc(II) com-
plex, at least on thermodynamic grounds. Consequently, the
observed effect of coordination of the metal ion to the pyr-
idine N atom can be attributed to an electronic factor. The
somewhat enhanced rate of decay of the fluorescent conju-
gate is possibly because of spin-orbit coupling associated
with the zinc(II) ion.
Conclusions
The ancillary perylene units appended to the previously
reported BODIPY-based dye 1 allows harvesting of pho-
tons in the 350–480 nm window, where the dye is relatively
transparent. These photon collectors, being attached by
ethyne linkers to the B center, display fast energy transfer
to the BODIPY core via both through-bond and through-
space mechanisms. As reported before, the presence of large
polycyclic hydrocarbons in place of the usual F atoms leads
to a modest increase in the rate of nonradiative decay of the
excited singlet state of the BODIPY unit.[22] This process is
highly sensitive to applied pressure, although the actual de-
tails for this effect are unclear at present. The polycycle is
free to rotate around the connecting ethyne group, but there
is poor alignment of the transition dipoles and this mini-
mizes Förster-type energy transfer to the BODIPY unit.
This effect is offset, however, by the effective electronic
coupling supplied by the linkage that facilitates fast
through-bond energy transfer. This realization opens the
way to design improved photon collectors built from mul-
tiple units that combine to cover much of the spectral
Figure 6. Effect of the presence of zinc(II) cations on the absorp-
tion spectrum recorded for 3 in methyltetrahydrofuran; before ad-
dition (grey curve) and after addition (black curve) of zinc perchlo-
rate.
In marked contrast to the protonated species, the conju- range.[26] It should also be noted that the conjugation length
gate formed between 3 and zinc(II) cations fluoresces in of the BODIPY dye is extended when suitable groups are
fluid solution at room temperature.[25] The emission spec- attached to the pyrrole rings.[27,28] In this way, most of the
trum is red-shifted and broadened, in agreement with the visible region can be harvested. This simple strategy serves
modified absorption spectrum, while the excitation spec- to enlarge the effective Stokes’ shift and provides a range
trum shows good correspondence with the absorption spec- of excitation wavelengths that can be utilized for sensor
trum recorded over the visible and near-UV regions. The technology.
Eur. J. Org. Chem. 2008, 2774–2782
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A. Harriman, R. Ziessel et al.
FULL PAPER
m/z (%) = 382.2 (30) [M + H+], 381.2 (100) [M], 362.3 (20) [M –
F]+. C22H26BF2N3 (381.27): calcd. C 69.30, H 6.87, N 11.02; found
C 69.10, H 6.61, N 10.82.
Coordination of protons to the pyridine N atom switches
on an intramolecular charge-transfer reaction and thereby
decreases the amount of fluorescence from the BODIPY
unit. Complexation can also be followed by absorption
spectroscopy as the bands associated with the BODIPY
unit undergo a modest red shift. Because absorption transi-
tions localized on the perylene units are insensitive to cation
coordination, the spectral shift decreases the rate of
through-space electronic energy transfer and restores some
fluorescence from the perylene residues. Pyridine is a rela-
tively weak and indiscriminate coordinative group and will
bind to most cations. This situation cannot be exploited to
design sensitive and/or selective fluorescent sensors but is
ideal for a remote probe intended to provide a termination
signal if cations leak from a reactor due to a ruptured mem-
brane or seal. As a further safeguard, we note that the de-
creased fluorescence from BODIPY is accompanied by in-
creased emission from the perylene units. This type of or-
thogonal sensing has many attractions.
General Procedure for Substitution of Fluoride by Acetylide: In a
Schlenk flask, ethylmagnesium bromide (2.2 equiv.) was added to
a stirred, degassed solution of the acetylenic compound (2.5 equiv.)
in anhydrous THF held at room temp. The mixture was heated at
60 °C for 2 h. The resulting anion was then transferred via cannula
to a degassed solution of the precursor difluoroboradiazaindacene
(1 equiv.) in anhydrous THF. The solution was stirred at 60 °C for
18 h. Water was added, and the solution was extracted with
CH2Cl2. After evaporation, the organic layer was purified by col-
umn chromatography and recrystallized from CH2Cl2/hexane.
2,6-Diethyl-1,3,5,7-tetramethyl-8-(pyridin-4-yl)-4,4-bis(p-tolylethy-
nyl)4-bora-3a,4a-diaza-s-indacene (2): Prepared according to the ge-
neral procedure with 4-ethynyltoluene (0.046 mL, 0.36 mmol) in
2 mL of THF, 0.30 mL of EtMgBr (1  in THF), and 1 (0.046 g,
0.12 mmol) in 2 mL of THF. Chromatography was carried out on
silica (CH2Cl2/petroleum ether, 20:80), and followed by a
recrystallization (CH2Cl2/cyclohexane) to afford 18 mg of 2 (26%
1
yield). H NMR (CDCl3 300 MHz): δ = 8.74 (br. s, 2 H), 7.41 (br.
s, 2 H), 7.17 (AB system, 8 H, JAB = 8.0 Hz, δ = 75.2 ), 2.86 (s, 6
3
H), 2.35 (t, J = 7.5 Hz, 4 H), 2.31 (s, 6 H), 1.32 (s, 6 H) 1.02 (t,
Experimental Section
General Methods: 200.1 (1H), 300.1 (1H), 400 (1H), 50.3 (13C), 75.46
3J = 7.5 Hz, 6 H) ppm. 13C{1H} NMR (CDCl3, 75 MHz): δ =
154.7, 136.9, 136.1, 135.5, 133.4, 131.4, 128.7, 128.0, 122.3, 30.9,
(13C) and 100.3 (13C) MHz NMR spectra were recorded at room 29.6, 21.4, 17.4, 14.7, 14.1, 12.1 ppm. EI-MS: m/z (nature of peak,
temperature using the residual proton resonances in deuterated sol-
vents as internal references. The 128.4 MHz 11B NMR spectra were
recorded at room temperature with B in borosilicate glass as refer-
ence (BF3·Et2O value 3.18 ppm). Fast-atom bombardment mass
spectra were obtained using a ZAB-HF-VB-analytical apparatus
in positive mode with m-nitrobenzyl alcohol (mNBA) as matrix.
Chromatographic purification was conducted using standardized
silica gel or aluminium oxide. Thin-layer chromatography (TLC)
was performed on silica gel or aluminium oxide plates coated with
fluorescent indicator. All mixtures of solvents are given in v/v ratio.
relative intensity): 574.2 (100) [M + H]+, 458.2 (20) [M – tol–ϵ–].
C40H40BN3 (573.59): calcd. C 83.76, H 7.03, N 7.33; found C 83.45,
H 6.89, N 7.09.
2,6-Diethyl-1,3,5,7-tetramethyl-4,4-bis(1-perylenylethynyl)--8-(pyr-
idin-4-yl)-4-bora-3a,4a-diaza-s-indacene (3): Prepared according the
general procedure with 1-ethynylperylene (0.103 g, 0.37 mmol) in
2 mL of THF, 0.35 mL of EtMgBr (1  in THF), and 1 (0.062 g,
0.16 mmol) in 2 mL of THF. Chromatography was performed on
silica (CH2Cl2/petroleum ether, 20:80), and recrystallization
(CH2Cl2/cyclohexane) gave 0.04 g of 3 (27% yield). 1H NMR
(CDCl3 300 MHz): δ = 8.81 (br. s, 2 H), 8.42 (d, 3J = 8.3 Hz, 2 H),
8.22–8.08 (m, 8 H), 7.69–7.64 (m, 6 H), 7.53–7.44 (m, 8 H), 3.07
(s, 6 H), 2.44 (q, 3J = 7.5 Hz, 4 H), 1.41 (s, 6 H), 1.10 (t, 3J =
7.5 Hz, 6 H) ppm. 13C{1H} NMR (CDCl3, 75 MHz): δ = 154.7,
150.4, 145.0, 136.4, 135.9, 135.2, 134.7, 133.7, 131.4, 131.2, 131.0,
130.6, 130.5, 128.6, 128.5, 128.3, 127.9, 127.8, 126.9, 126.7, 126.6,
126.5, 124.1, 122.6, 120.5, 120.3, 119.7, 94.3, 17.4, 14.8, 14.6, 12.2
ppm. 11B {1H} NMR (CDCl3, 128 MHz): δ = –8.97 (s) ppm. EI-
MS: m/z (nature of peak, relative intensity): 893.2 (100) [M]+, 618.2
(35) [M – peryl–ϵ–]. C66H48BN3 (893.94): calcd. C 88.68, H 5.41,
N 4.70; found C 88.39, H 5.17, N 4.35.
Materials: Samples of CH2Cl2 were distilled from P4O10 and tetra-
hydrofuran (THF) from Na/benzophenone. Kryptopyrrole,
EtMgBr, tolylacetylene, Et3N and BF3·Et2O were used as pur-
chased. 1-Ethynylperylene was obtained by a literature pro-
cedure.[29]
2,6-Diethyl-4,4-difluoro-1,3,5,7-tetramethyl-8-(4-pyridinyl)-4-bora-
3a,4a-diaza-s-indacene (1): Kryptopyrrole (330 µL, 2.4 mmol) was
stirred with 4-(chlorocarbonyl)pyridine (0.216 g, 1.2 mmol) in
CH2Cl2, at room temperature during 4 d. The solution turned deep
red. The resultant dipyrromethene solution was deprotonated with
triethylamine (1.0 mL), boron trifluoride–diethyl ether (1.2 mL)
was added, and the solution stirred for 3 h. The resulting organic
mixture was washed with a saturated NaHCO3 aqueous solution,
dried with MgSO4, and the solvent removed. Column chromatog-
raphy on alumina (Act IV, hexane/dichloromethane, with a gradi-
ent from 8:2 to 5:5) afforded the desired compound as a red powder
Spectroscopic Studies: All solvents were purchased at spectroscopic
grade from Aldrich Chemicals Co., used as received, and were
found to be free of fluorescent impurities. Absorption spectra were
measured with a Hitachi U3310 spectrophotometer, corrected with
baseline changes and converted to molar absorption coefficient
using the Beer–Lambert law, averaging over a series of dilute mea-
1
(0.184 g, 40%). M.p. 168–169 °C. H NMR (400 MHz, CDCl3): δ
3
3
= 0.99 (t, J = 7.6 Hz, 6 H), 1.32 (s, 6 H), 2.31 (q, J = 7.6 Hz, 4 surements of known concentration recorded at room temperature.
3
4
H), 2.55 (s, 6 H), 7.31 (dd, J = 4.3, J = 1.5 Hz, 2 H), 8.78 (dd, Fluorescence spectra were measured with a fully-corrected Jobin–
3J = 4.5, 4J = 1.7 Hz, 2 H) ppm. 13C{1H} NMR (100.6 MHz,
Yvon Fluorolog tau-3 spectrometer for quantitative measurements
CDCl3): δ = 12.3 (CH3), 13.0 (CH3), 15.0 (CH3), 17.5 (CH2), 124.1 and a Hitachi F-4500 fluorescence spectrometer for routine spectra.
(CH), 130.1 (Cq), 133.8 (Cq), 136.6 (Cq), 138.2 (Cq), 144.8 (Cq),
Fluorescent measurements were obtained using optically dilute
151.0 (CH), 155.2 (Cq) ppm. 11B NMR (128.4 MHz, CDCl3): δ = solutions with an absorbance less than or equal to 0.1 at the exci-
1
3.79 (t, JB-F = 32.8 Hz) ppm. UV/Vis (CH2Cl2, 23 °C): λmax (ε,
tation wavelength. Fluorescence quantum yields were measured at
room temperature in CH2Cl2 solution relative to perylene (φF
IR (KBr): ν = 1638 (s, νC=N), 1414, 1118 cm–1. MS (FAB+, mNBA): 0.79),[11] excited at 408 nm, or 1 (φF = 0.88), excited at 500 nm.[9]

–1 cm–1) = 235 (14800), 382 (6300), 501 (sh, 21000), 528 (59000).
=
˜
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CT Processes in a Perylene–BODIPY–Pyridine Tripartite Array
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Lifetime measurements were made using a PTI EasyLife spectrom-
eter using Ludox in distilled water to measure the instrumental
response function. Analysis of the experimental data consists of
deconvolution against the instrumental response function and fit-
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quartz cuvette (optical pathlength: 2 mm) and maintained under
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This work received financial support from the Engineering and
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Received: February 9, 2008
Published Online: April 15, 2008
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Eur. J. Org. Chem. 2008, 2774–2782