Energy transfer by way of an exciplex intermediate in flexible boron dipyrromethene-based allosteric architectures

Article abstract of DOI:10.1021/jp106626v

We have designed and synthesized a series of modular, dual-color dyes comprising a conventional boron dipyrromethene (Bodipy) dye, as a yellow emitter, and a Bodipy dye possessing extended conjugation that functions as a red emitter. A flexible tether of variable length, built from ethylene glycol residues, connects the terminal dyes. A critical design element of this type of dyad relates to a secondary amine linkage interposed between the conventional Bodipy and the tether. Cyclic voltammetry shows both Bodipy dyes to be electroactive and indicates that the secondary amine is quite easily oxidized. The ensuing fluorescence quenching is best explained in terms of the rapid formation of an intermediate charge-transfer state. In fact, exciplex-type emission is observed in weakly polar solvents and over a critical temperature range. In the dual-color dyes, direct excitation of the yellow emitter results in the appearance of red fluorescence, indicating that the exciplex is likely involved in the energy-transfer event, and provides for a virtual Stokes shift of 5000 cm^-1. Replacing the red emitter with a higher energy absorber (namely, pyrene) facilitates the collection of near-UV light and extends the virtual Stokes shift to 8000 cm^-1. Modulation of the efficacy of intramolecular energy transfer is achieved by preorganization of the connector in the presence of certain cations. This latter behavior, which is fully reversible, corresponds to an artificial allosteric effect.

Full text of DOI:10.1021/jp106626v

J. Phys. Chem. A 2010, 114, 10515–10522
10515
Energy Transfer by Way of an Exciplex Intermediate in Flexible Boron
Dipyrromethene-Based Allosteric Architectures
Soumyaditya Mula,† Kristopher Elliott,‡ Anthony Harriman,*,‡ and Raymond Ziessel*,†
Laboratoire de Chimie Organique et Spectroscopies AVance´es (LCOSA), Ecole Europe´enne de Chimie,
Polyme`res et Mate´riaux, UniVersite´ de Strasbourg, 25 rue Becquerel, 67087 Strasbourg Cedex 02, France, and
Molecular Photonics Laboratory, School of Chemistry, Bedson Building, Newcastle UniVersity, Newcastle upon
Tyne NE1 7RU, United Kingdom
ReceiVed: July 16, 2010; ReVised Manuscript ReceiVed: August 30, 2010
We have designed and synthesized a series of modular, dual-color dyes comprising a conventional boron
dipyrromethene (Bodipy) dye, as a yellow emitter, and a Bodipy dye possessing extended conjugation that
functions as a red emitter. A flexible tether of variable length, built from ethylene glycol residues, connects
the terminal dyes. A critical design element of this type of dyad relates to a secondary amine linkage interposed
between the conventional Bodipy and the tether. Cyclic voltammetry shows both Bodipy dyes to be electroactive
and indicates that the secondary amine is quite easily oxidized. The ensuing fluorescence quenching is best
explained in terms of the rapid formation of an intermediate charge-transfer state. In fact, exciplex-type emission
is observed in weakly polar solvents and over a critical temperature range. In the dual-color dyes, direct
excitation of the yellow emitter results in the appearance of red fluorescence, indicating that the exciplex is
likely involved in the energy-transfer event, and provides for a virtual Stokes shift of 5000 cm-1. Replacing
the red emitter with a higher energy absorber (namely, pyrene) facilitates the collection of near-UV light and
extends the virtual Stokes shift to 8000 cm-1. Modulation of the efficacy of intramolecular energy transfer is
achieved by preorganization of the connector in the presence of certain cations. This latter behavior, which
is fully reversible, corresponds to an artificial allosteric effect.
Introduction
CHART 1: Simple Illustration of an Artificial Allosteric
Effect As Applied to Intramolecular EET in a Two-Color
The successful realization of artificial photosynthesis,1 whereby
sunlight is converted to chemical potential in abiotic but
bioinspired molecular arrays, requires the close cooperation
between two separate photosystems.2 The first such system must
collect photons across a broad spectral range before directing
the harvested electronic energy to a remote site. The second
photosystem drives successive electron-transfer reactions aimed
at fuel production.3 Although in principle it should be possible
to design integrated chemical networks based on a single
photosystem, it seems most unlikely that any single chro-
mophore will possess the optimal features for both photon
collection and electron transfer.4 As such, we have focused
attention on the light-harvesting apparatus5 and designed several
rigid arrays capable of absorbing light over most of the visible
range.6 These systems display highly efficient energy transfer
among disparate chromophores until concentrating the excitation
energy at a terminal acceptor. Various transfer mechanisms
coexist but can be resolved by way of ultrafast fluorescence
spectroscopy. At a purely fundamental level, some of these
systems can be used to test the validity of Fo¨rster’s theory of
electronic energy transfer at short separation distances7 and to
better define the resistance of molecular-scale wires for conduct-
ing electronic charge.6a It is notable that minor alteration of the
conformation or composition of the system can cause an
important perturbation of the energy-transfer rates.8
Dye Caused by Introduction of a Cation (M+) Able To
Bind to the Connecting Tether
A particular problem associated with this rational approach
to the construction of abiotic photon collectors relates to the
protracted synthesis needed to bring about an energy-transfer
cascade.6 While interesting from an elementary viewpoint, such
an elaborate synthesis presents difficulties for scaleup and/or
repair of damaged modules. Replacing the rigid connectors with
flexible analogues might simplify the preparative aspects but
tends to curtail through-bond electron exchange.9 This means
that energy transfer must occur via the Fo¨rster dipole-dipole
mechanism,10 for which the choice of chromophore is more
restricted. A potential way around this specific problem might
be to impose some kind of allosteric effect11 on the connecting
chain, as illustrated in Chart 1. Allosterism, either positive or
negative, is ubiquitous in nature, where biological events must
be regulated toward chemical or physical stress from the outside
world.12 The simple concept introduced here is to link two dyes,
each based on difluoroboradiaza-s-indacenes, commonly termed
Bodipy, having different colors and being linked by a polyeth-
* To whom correspondence should be addressed. E-mail: anthony.
harriman@ncl.ac.uk (A.H.); ziessel@unistra.fr (R.Z.).
† Universite´ de Strasbourg.
‡ Newcastle University.
10.1021/jp106626v  2010 American Chemical Society
Published on Web 09/13/2010

10516 J. Phys. Chem. A, Vol. 114, No. 39, 2010
Mula et al.
CHART 2: Reference Compounds Used To Aid Understanding of the Redox and Optical Properties
SCHEME 1a
ylene glycol chain. Electronic energy transfer (EET) between
the terminals should be promoted by folding the chain into a
closed conformation.
In seeking to involve the connecting chain in the transfer
process, something other than a cation-induced folding process
might be necessary. Now, earlier work13 has shown that a
fluorescent charge-transfer state could be formed from a Bodipy
dye equipped with a weak redox-active unit at the meso position.
The latter site is crucial because of the need to hold the reactants
in an orthogonal geometry.14 We now consider the possibility
to use such an intermediate as a relay for intramolecular energy
transfer. Since it is well-known15 that an amine will form a
fluorescent exciplex with many different types of aryl poly-
cycles, a logical starting point would seem to hinge on the design
of a molecular architecture comprising two disparate Bodipy
dyes, an allosteric chain and a meso-linked arylamine. To
properly test the idea, chains of differing length are required.
a Reagents and conditions: (i) aqueous NaHCO3, dodecyl sulfate,
corresponding tosylate, C2H4Cl2, 80 °C, 24 h; (ii) [Pd(PPh3)4] (6 mol
%), benzene, Et3N, 50 °C; (iii) [Pd(PPh3)4] (6 mol %), benzene,
(iPr)2NH, 70 °C.
We now expand on this generic idea and describe intramolecular
energy transfer taking place in molecular systems subjected to
exciplex formation.16 There are few, if any, reported cases where
an exciplex is directly involved in energy transfer.17
TABLE 1: Electrochemical Properties of Selected Dyesa
Results and Discussion
compd
Eox°/V (∆E/mV)
Ered°/V (∆E/mV)
The routine synthesis and characterization of the reference
compounds sketched in Chart 2 are described in the Supporting
Information.
3
7a
8
+1.04 (irrev); +1.21 (90)
+0.98 (irrev); +1.12 (90)
+0.74 (60); +1.08 (70);
+1.58 (irrev)
+0.72 (60); +0.98 (irrev);
+1.11 (irrev); +1.22 (irrev);
+1.57 (irrev)
-1.30 (70)
-1.27 (70)
-0.97 (60); -1.90 (irrev)
The synthesis of the target compounds involves alkylation
of the amino group of 4,4-difluoro-1,3,5,7-tetramethyl-8-anilino-
4-bora-3a,4a-diaza-s-indacene (2) with a polyethylene glycol-
based reagent bearing two disparate functionalities: The tosylate
derivative is known to facilitate nucleophilic substitution,
whereas the propargylic function provides the reactive site
needed for cross-linking to aryl-based residues via a palladium-
promoted reaction. A biphasic reaction in water using NaHCO3
as the base, 1,2-dichloroethane as the nonmiscible solvent, and
dodecyl sulfate as the phase transfer agent was found to provide
the best conditions (Scheme 1). Though the yield was modest,
the reaction furnished the mono-N-substituted products 7a-7c,
but with a linker suitable for subsequent substitution chemistry.
To obtain the target compounds and to explore the synthetic
versatility of compounds 7a-7c, the isolated derivatives were
subjected to cross-coupling with 4,4-difluoro-3,7-bis(p-meth-
oxystyryl)-1,7-dimethyl-8-(p-iodophenyl)-4-bora-3a,4a-diaza-s-
indacene (8). Specifically, Sonogashira coupling reactions
promoted by palladium(0) were employed to furnish 9a-c in
satisfactory yield (Scheme 1). Finally, dye 7a was treated with
1-bromopyrene (10) to obtain the flexibly linked pyrene
derivative 11.
9a
-1.01 (70); -1.29 (74);
-1.88 (irrev)
a Potential determined by cyclic voltammetry in deoxygenated
CH2Cl2 solutions, containing 0.1 M TBAPF6, at a solute concen-
tration of (1-5) × 10-3 M, at 20 °C. Potentials were standardized
with ferrocene (Fc) as the internal reference and converted to SCE
assuming that E1/2 (Fc/Fc+) ) 0.38 V (∆Ep ) 70 mV) SCE. The
error in half-wave potentials is (10 mV. When the redox potential
is irreversible, the peak potential (Eap/cp) is quoted. All waves are
monoelectronic unless otherwise specified.
tion of the Bodipy unit;18 the derived reduction potentials (Ered°)
and separations between forward and reverse peaks (∆E) are
provided in Table 1. In the case of 8, which bears two styryl
arms, reduction is easier by ca. 300 mV compared to that of
both 3 and 7a, while there is a second reductive step that
corresponds to formation of the π-dianion. In the anodic portion
of the cyclic voltammograms, an irreversible peak is observed
together with a reversible wave that could be assigned to one-
electron oxidation of the Bodipy unit;18 the respective oxidation
potentials (Eox°) and peak separations are also given in Table
1. The additional irreversible peak observed for 3 and 7a is
due to the one-electron oxidation of the secondary amine center;
it is recognized that such processes are often irreversible.19 As
might be expected, the anodic portion of the cyclic voltammo-
Electrochemistry. Selected electrochemical properties mea-
sured for 3, 7a, 8, and 9a are gathered in Table 1. For each
compound, one reversible cathodic wave was observed in the
cyclic voltammograms and assigned to the one-electron reduc-

Energy Transfer from an Exciplex
J. Phys. Chem. A, Vol. 114, No. 39, 2010 10517
TABLE 2: Photophysical Properties of Selected Amino-Bodipy Dyes Recorded in Diethyl Ether at Room Temperature
compd
λ
abs/nm
ε
max/M-1 cm-1
λflu/nm
ΦF
τS1/ns
τS2/ns
2a
3
5
500
500
641
504
500
500
500
80000
86000
102000
99000
103000
98000
74000
510
513
656
514
515
515
514
0.38
0.017
0.32
3.1
0.16
5.8
2.3
6
0.38
3.0
7a
7b
7c
0.027
0.053
0.060
0.24
0.48
0.51
1.9
1.7
1.5
a All values for 2 were determined in anhydrous CH2Cl2.
gram recorded for 8 is more crowded than for the other
compounds. On the basis of the electrochemical properties
recorded for similar analogues of the distyryl dye, the first
oxidation step is assigned to removal of one electron from the
Bodipy unit. Two additional waves observed at higher potential
are now assigned to oxidation of the styryl residue and to
removal of a second electron from the Bodipy nucleus,
respectively.
Interestingly, each of the anodic and cathodic waves observed
for the precursors 7a and 8 were present in the assembled dye
9a. In particular, the two reversible, cathodic waves separated
by 280 mV could be assigned to successive reduction of the
two Bodipy-based terminals. These units are too remote to
interact in an electronic sense or to offer electrostatic perturba-
tions. The anodic portion of the voltammogram recorded for
9a is crowded with waves, but the first two processes could be
assigned to oxidation of the modified Bodipy nucleus and of
the amine center, respectively. Similar cyclic voltammograms
were recorded for the yellow dyes 7b and 7c and for the dual
dyes 9b and 9c. These data are in keeping with a molecular
dyad in which the two partners remain in electronic isolation.
Photophysics of the Amino-Bodipy Dyes. The primary
amino derivatives 2 and 5 display absorption spectra typical of
Bodipy units with the relevant degree of π-conjugation,20
possessing intense absorption maxima (λmax) at 500 and 641
nm, respectively (Table 2). The molar absorption coefficients
(εmax) measured at the peak maxima are in line with values
recorded for related compounds.20 Both dyes fluoresce in
solution, exhibiting clear emission maxima (λflu) at 510 and 656
nm, respectively, with reasonably high quantum yields (ΦF)
measured at room temperature (Table 2). Fluorescence decay
profiles remain monoexponential for both compounds and can
be analyzed to obtain the corresponding emission lifetimes (τS),
which are collected in Table 2. There is no indication for light-
induced electron transfer in any solvent and no evidence for
the involvement of charge-transfer states. Alkylation of the
amino function, forming 3 (bearing an ethyl group) or 7a-c
(functionalized with polyethylene glycol residues of varying
length) results in a pronounced decrease of ΦF in diethyl ether
(Et2O) at room temperature (Table 2). Individual ΦF values show
a modest sensitivity toward the length of the tether in Et2O and
increase slightly for longer chain lengths. In each case, the
corrected excitation spectrum remains in good agreement with
the absorption spectrum, the latter being little affected by
alkylation. The reduced ΦF values are not a consequence of
poor solubility or aggregation but are a genuine reflection of
the system. Furthermore, the emission spectra recorded in Et2O
show the presence of an additional fluorescence profile centered
at long wavelength that appears as a broad and featureless band
(Figure 1). The emission spectral profile is independent of the
sample concentration. Throughout this set of compounds,
fluorescence decay profiles could not be analyzed satisfactorily
in terms of single-exponential fits but followed dual-exponential
Figure 1. Fluorescence spectrum recorded for 7a in diethyl ether at
room temperature.
kinetics. The derived lifetimes, τS1 and τS2, are collected in Table
2. To explore this behavior, detailed studies were undertaken
with 7a in a range of solvents.
The effect of the solvent polarity, as expressed in terms of
the Pekar factor (f), on the photophysical properties of 7a are
illustrated by the data compiled in Table 3. In cyclohexane,
Bodipy-like emission is observed, ΦF remains similar to that
observed for 2, and decay curves are monoexponential at all
monitoring wavelengths. A slight increase in f, however, leads
to a decrease in ΦF and also in τS and in the appearance of a
structureless fluorescence band seen at long wavelength. This
general behavior continues as f increases, and it is notable that,
whereas λflu remains quite insensitive to changes in the nature
of the solvent, the emission maximum for the longer wavelength
band moves toward lower energy with increasing f. Both
lifetimes decrease as f increases, although to differing degrees.
A further feature of increasing f is that the featureless emission
band becomes increasingly difficult to resolve from the baseline
without using curve-fitting routines. This latter fluorescence band
has the appearance expected for an emissive exciplex.21
Monitoring at long wavelength leads to the conclusion that the
exciplex decays via monoexponential kinetics with a lifetime
closely comparable to τS2. Critical examination of the emission
from the Bodipy-like band around 515 nm indicates that this
species decays via dual-exponential kinetics corresponding to
τS1 and τS2 in all solvents except cyclohexane. Fitting the
exciplex emission profile to a Gaussian band shape allows
estimation of its quantum yield (Φex) and the peak position (νex),
as can be seen from Table 3. There is a linear correlation
between νex and f, as is often observed for emissive exciplexes,22
with the notable exception of dioxane (Figure 2). Compounds
3, 7b, and 7c show behavior remarkably similar to that
summarized for 7a.
Finally, temperature-dependence studies made in butyronitrile
(BuCN) show complex behavior. As mentioned above, fluo-
rescence from the exciplex is difficult to resolve from the
baseline at ambient temperature but becomes more pronounced
on cooling (Figure 3). In fact, the exciplex emits strongly only

10518 J. Phys. Chem. A, Vol. 114, No. 39, 2010
Mula et al.
TABLE 3: Effect of the Solvent Polarity on the Photophysical Properties of 7a Recorded at Room Temperature
a
solvent
CHXb
f
ΦF
τS1/ps
Φex
τS2/ns
νex/cm-1
0
0.46
4200
165
1050
700
240
85
52
43
70
50
dioxane
0.022
0.097
0.148
0.163
0.200
0.203
0.209
0.215
0.221
0.281
0.305
0.019
0.115
0.080
0.027
0.006
0.005
0.005
0.008
0.003
0.002
0.002
0.0070
0.0090
0.0080
0.0065
0.0041
0.0029
0.0025
0.0020
0.0016
0.0009
0.0005
1.34
2.05
1.80
1.75
1.70
1.54
1.35
1.20
0.96
0.65
0.48
16845
17975
17285
17125
16065
15850
15690
15820
15850
14550
13800
Bu2Oc
CHCl3
Et2O
EtOAcd
MTHFe
THFf
CH2Cl2
CH2ClCH2Cl
BuCN
41
37
CH3CN
a Refers to fluorescence from the S1 state of the Bodipy unit. b Cyclohexane. c Dibutyl ether. d Ethyl acetate. e Methyltetrahydrofuran.
f Tetrahydrofuran.
The emission lifetimes were recorded for 7b under identical
conditions. Starting first with τS2, this being the fluorescence
lifetime of the exciplex, we find that this parameter decreases
slightly with decreasing temperature due to the accompanying
increase in f. This effect is kept small, however, because of the
offsetting inhibition of thermal repopulation of the Bodipy S1
state. As the temperature approaches 165 K, which corresponds
to the freezing point of BuCN, there is a sharp increase in τex
that continues until vitrification of the solvent. At 77 K, τex
Figure 2. Effect of the solvent polarity on the maximum of the
reaches a maximum value of 34 ns. Over the same temperature
exciplex-like emission band. Note that the point for dioxane has been
range, τS1, which can be taken to represent the lifetime of the
omitted.
Bodipy S1 state, decreases from 41 ps at room temperature to
34 ps at 170 K and then begins to increase in line with the
changes in f. At 77 K, τS1 attains a value of 250 ps. Monitoring
at 515 nm, where the exciplex does not emit, shows that the
fractional contribution made by the longer lived component
becomes progressively less significant as the temperature falls.
Such behavior is to be expected if the S1 state and the exciplex
are in thermal equilibrium.25 Because of the temperature-induced
changes in f, it is not possible to comment on whether charge
transfer is thermally activated.
On the above basis, we assign the short-lived emission to
the S1 state localized on the Bodipy fluorophore while the
longer-lived species is attributed to an exciplex intermediate.
The latter is weakly emissive, except in low-polarity solvents
or in a frozen glass. At room temperature in CH2Cl2 electro-
chemical data can be used to conclude that intramolecular
electron transfer to the first excited singlet state of the Bodipy
unit is highly unlikely in 2 and 5. Alkylation lowers the
oxidation potential of the amine and thereby brings the energy
of the exciplex closer to that of the excited state, especially for
7a-c. The length of the tether is not expected to affect the
energy of the exciplex, although there are small perturbations
of the peak potential for the anodic wave (Table 1) among these
derivatives.
We can rationalize the photophysical properties in terms of
Scheme 2, where the exciplex plays a key role. Thus, excitation
of the Bodipy chromophore in 3 or 7a-c in a weakly polar
solvent results in fast charge transfer from the lone pair on the
amino group to the excited state. The resultant exciplex is
relatively long lived and able to repopulate the S1 state by way
of setting up a thermal equilibrium. The rates of formation and
decay of the exciplex in Et2O, and also in BuCN, show a
moderate dependence on the length of the tether (Table 2). This
latter effect is most likely associated with a conformational
change that must accompany exciplex formation.16,26 Indeed, it
is known that the maximum stabilization of the exciplex will
occur when the lone pair is aligned with the π-orbitals on the
Figure 3. Effect of lowering the temperature on the emission spectrum
recorded for 7a in butyronitrile. The arrow indicates the spectral shifts
as the temperature decreases friom 295 to 77 K.
as the temperature approaches the freezing point of the solvent,
and over the same temperature range, νex moves sharply toward
higher energy. This situation is consistent with freezing of the
solvent, causing a significant decrease in the magnitude of f,23
specifically because the dielectric constant changes precipitously
as the density of the solvent increases.24 Surprisingly, exciplex
emission is observed for 7a in a frozen glass at 77 K. Under
identical conditions, Bodipy-like fluorescence increases in
intensity without undergoing a substantial change in λflu. The
absorption spectrum shows only a slight (i.e., 3 nm) blue shift
and band narrowing over the same temperature range. At 77 K,
Φex reaches a maximum value of 0.19 while ΦF increases to
only 0.03; the low ΦF indicates that charge transfer still occurs
in the glassy matrix at 77 K. Similar behavior is observed for
3, 7b, and 7c under comparable conditions, and exciplex
fluorescence is found in BuCN near and below the freezing
point.

Energy Transfer from an Exciplex
J. Phys. Chem. A, Vol. 114, No. 39, 2010 10519
SCHEME 2: Proposed Pathways for Deactivation of the
Bodipy Excited Singlet State Following Selective
Excitationa
excitation into the yellow-emitting Bodipy unit gives rise to
weak fluorescence centered at about 515 nm, together with
fluorescence from the opposite terminal (Figure 5). The basic
photophysical properties recorded for both terminals in Et2O
are compiled in Table 4. Comparative excitation spectra recorded
for emission at 730 nm confirm that photons collected by the
yellow-emitting Bodipy chromophore are transferred to the red
emitter.
It is usual practice to compute the probability for EET (PEET
)
by comparing fluorescence lifetimes recorded with and without
the acceptor being in place. Unfortunately, the complex emission
decay kinetics associated with the Bodipy unit presents problems
when this strategy is applied here. Instead, PEET values were
determined by comparing29 the excitation spectra with the
corresponding absorption spectra recorded in the same solvent
and with the same slit width. The values derived in Et2O are
collected in Table 4 and show only a modest dependence on
the length of the tether. Indeed, PEET decreases from 28% for
the shortest tether to 21% for the longest linker. We presume
that this drop in EET efficacy relates to the need for large-
scale diffusional motion to bring the terminals into close
proximity;30 that is to say, a folded conformation is required
for efficient EET.31 There is a small effect of solvent on the
measured PEET, with values of 8%, 31%, and 38%, respectively,
being found for 9a in acetonitrile, dioxane, and chloroform.
These variations most likely relate to the average conformational
distribution present in that solvent and also to the lifetime of
the donor. The longer analogues show a similar trend but are
always somewhat less efficacious than 9a under the same
conditions. Protonation of the amine by exposure to HCl vapor
causes a 40% reduction in PEET in each case in Et2O, this
representing a mild form of negative allosterism. Under these
latter conditions, the exciplex is not formed and it seems likely
that the molecular conformation will change. As such, it can
be argued that the exciplex assists EET by preorganizing the
geometry to some extent.
a CT ) charge transfer, CR ) charge recombination, and ISC )
intersystem crossing.
Figure 4. Triplet-triplet differential absorption spectra recorded after
laser excitation (λ ) 490 nm; fwhm ) 4 ns) of 7a in deoxygenated
butyronitrile at room temperature. Individual traces were recorded at
different times after the excitation pulse.
The question now arises as to whether the exciplex is involved
in the EET process as a relay or merely as a spectator that serves
to prolong the lifetime of the Bodipy S1 state via thermal
repopulation. There is increased spectral overlap17d between
exciplex emission and absorption by the expanded Bodipy dye
that will favor Fo¨rster EET when compared to the case with a
conventional Bodipy dye as the donor. Furthermore, τex
measured at long wavelength in Et2O is shortened considerably
with respect to the corresponding compound lacking the terminal
acceptor (e.g., compare 7a with 9a), which is entirely consistent
with the exciplex fulfilling the role of primary donor. The τex
values derived from global analysis across a wide spectral range
are collected in Table 4 and tend toward a limiting value of
1.25 ns. The shortening of the lifetime can be interpreted in
terms of fast EET to the S1 state localized on the styryl-bearing
Bodipy unit, for which the rate constant (kXT) can be estimated
(Table 4). This behavior is most unusual. There are prior reports
of EET leading to exciplex formation17f and one report of
exciplex-exciplex annihilation,17a but we are unaware of any
prior description of EET from an exciplex intermediate. In
considering the mechanism of this EET step, we note that Eox
for the acceptor Bodipy (Eox ) 0.72 V vs SSCE) lies below
that of the amine (Eox ) 0.98 V vs SSCE), ignoring the fact
that the latter is irreversible in electrochemical terms. As such,
EET might proceed by way of charge-transfer interactions along
the lines illustrated in Scheme 3.
Figure 5. Absorption (black curve) and fluorescence (gray curve)
recorded for 9a in diethyl ether at room temperature. The excitation
wavelength was 490 nm.
benzene ring,27 and this requires that a subtle geometry change
accompanies hybridization of the N atom.28 Decay of the
exciplex results in partial formation of the Bodipy-based triplet
excited state, as detected by nanosecond laser flash photolysis
in deoxygenated BuCN (Figure 4).
Energy Transfer between the Terminals. Having estab-
lished the photophysics of the secondary amino derivatives of
the yellow-emitting Bodipy unit, attention now turns to the
possibility of engineering EET to the red-emitting Bodipy
derivative. For the dual dyes 9a-c, the absorption spectra appear
as linear combinations of the absorption bands of the individual
components while fluorescence can be detected from both
terminals (Figure 5). The energy gap between the first excited
singlet states localized on these Bodipy-based terminals is 0.54
eV (i.e., 4355 cm-1). As such, the exciplex is situated at an
energy that is intermediate between those of the terminals. Now,
illumination into the styryl-bearing Bodipy unit present in 9a-c
results in strong fluorescence from that unit with ΦF and τS
remarkably similar to those recorded for 5. In contrast, direct
Addition of potassium ions to a solution of 9a-c in Et2O or
BuCN had no obvious effect on PEET; hence, there is no

10520 J. Phys. Chem. A, Vol. 114, No. 39, 2010
Mula et al.
TABLE 4: Photophysical Properties of the Dual-Dye Systems Recorded in Et2O at Room Temperature
compd
λ
abs/nm
ε
mzx/M-1 cm-1
λflu/nm
ΦF
τS/ns (τex/ns)
PEET/%
kXT/107 s-1
9a
646
500
372
646
501
372
646
500
371
120000
86000
80000
112000
77000
74000
107000
73000
71000
668
514
0.51
0.014
6.0
(1.25)
28
2.7
9b
9c
662
512
0.49
0.030
6.0
(1.30)
25
21
1.7
1.6
665
514
0.50
0.038
5.0
(1.20)
SCHEME 3: Proposed Route for Intramolecular EET
Involving the Exciplex as a Direct Intermediate
SCHEME 4: Pictorial Representation of the Events That
Follow from Selective Illumination of the Pyrene
Chromophore in 11a
allosteric behavior under these conditions. Adding ammonium
hexafluorophosphate to a solution of 9c in CHCl3 caused an
increase in PEET to 92%, which can be explained in terms of
the cation folding up the polyethylene glycol chain upon
hydrogen bonding to the oxygen atoms. This positive allosteric
effect, which brings the terminals into closer proximity, is less
pronounced for 9b (PEET increases from 28% to 80%) and
nonexistent for 9a (PEET decreases from 26% to 22%). For the
longer analogue 9c, addition of dimethyl sulfoxide disrupted
the hydrogen-bonding network formed in the presence of
ammonium ions with concomitant loss of the positive allosterism.
Reverse Energy Transfer Using the Pyrene Derivative. For
the pyrene-linked dye 11 in BuCN direct excitation into the
pyrene residue at 362 nm results in a rather weak fluorescence
characteristic of the pyrene unit that corresponds to ca. 6% of
the fluorescence yield recorded for 1-ethynylpyrene under
identical conditions. The emission lifetime measured for the
pyrene fluorophore is ca. 0.8 ns, compared to a value of 13.0
ns found for 1-ethynylpyrene. Under the same conditions,
fluorescence from the Bodipy unit could be seen clearly (Figure
6); this signal decays as described above. Comparative excitation
a Key: (a) normal case, (b) after protonation of the amine, (c) addition
of Fe2+ to the acidified solution.
spectra indicate that EET occurs with a probability of around
45%. Since the overall quenching of pyrene-based emission is
almost 95%, it follows that there is an additional nonradiative
decay route responsible for about 50% of the initial pyrene S1
state. This latter process most likely involves electron transfer
from the amine to the excited singlet state of pyrene, for which
the thermodynamic driving force is ca. 0.26 eV.
Protonation of this amino moiety by exposure to trace
amounts of HCl vapor causes an increase in emission from
Bodipy, by curtailing exciplex formation, and increases the
excited-state lifetime for the pyrene residue to 1.8 ns (Scheme
4). In contrast, adding a small amount of Fe2+ to the solution
decreases τS for pyrene to only 220 ps while raising PEET to ca.
80%, without perturbing the spectral properties. This latter effect
must be due to preorganization of the connecting chain so as to
bring the terminals into closer proximity (Scheme 4). Addition
of trace quantities of both acid and Fe2+ elevates PEET to 95%
while rendering Bodipy emission almost quantitative. Neutral-
ization of an acidic solution restores emission to the level seen
before addition of acid, while removal of Fe2+ with adventitious
2,2′-bipyridine reverses this effect.
Since EET is restricted to dipole-dipole interactions in these
systems, the level of PEET is set by the separation distance10b
and by the spectral overlap integral.32 Pyrene-based fluorescence
overlaps only weakly with the lowest energy absorption
transition localized on Bodipy, but overlap is more pronounced
with the transition to the upper singlet excited state. Following
EET from pyrene to Bodipy, rapid internal conversion33 will
establish the first excited singlet state localized on Bodipy prior
to exciplex formation taking place in the absence of added
Figure 6. Absorption (black curve) and fluorescence (gray curve)
spectra recorded for 11 in butyronitrile at room temperature. The
excitation wavelength was 355 nm.

Energy Transfer from an Exciplex
J. Phys. Chem. A, Vol. 114, No. 39, 2010 10521
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protons. For 11 in BuCN at room temperature, the rate constant
for intramolecular EET can be estimated as being ca. 6 × 108
s-1. From the computed critical distance for EET (RCC ) 12.3
Å), the average separation distance between the reactants in the
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Conclusion
In this work we describe two separate effects relating to the
probability of electronic energy transfer in flexibly linked
molecular dyads.
First, the conformation of the connecting chain can be
modulated, at least to a modest degree, by external factors.
Cation complexation is the obvious choice for this goal34 and
leads to negative or positive allosterism. The effect becomes
more pronounced as the chain length increases and could be
dramatic for long chains bearing multiple oxygen atoms. There
are, of course, other ways to fold up a connecting chain, and
these could be used to switch on/off EET in suitably designed
dyads. Allosterism of this type is reversible upon removal of
the cation or curtailment of complexation. These are not fast
events, however, since they require addition of further reagents.
Rapid switching of the molecular conformation could be realized
by incorporating an electro- or photoactive component in the
chain.35
The second effect illustrated here relates to involvement of
an exciplex as a relay in long-range EET. This is novel behavior
that, according to Scheme 3, might demand particular thermo-
dynamic conditions. Formation of the exciplex imposes geo-
metrical changes at the donor center, and in turn, this causes
secondary alterations in the alignment of the connecting chain.
Such changes are subtle but sufficient to give rise to positive
allosterism. Here, the geometry change is fast and does not need
the addition of extra reagents. An interesting feature in this
respect is that the lifetime of the exciplex can greatly exceed
that of the precursor locally excited state;16 for 9c, we have
observed τex as high as 34 ns whereas τS1 does not exceed 5 ns.
Such behavior should permit secondary photochemical events
to compete effectively with radiative and/or nonradiative
deactivation of the excited state. As such, the exciplex might
be used to enhance EET in rationally designed dyads.
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Acknowledgment. This work was supported by the CNRS
and Newcastle University. S.M. is grateful to the French
Embassy in India for the award of a Sandwich-PhD Scholarship
and also to the Bhabha Atomic Research Centre. Dr. Gilles
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Supporting Information Available: Full experimental
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