Rapid energy transfer in cascade-type bodipy dyes

Article abstract of DOI:10.1021/ja0631448

Three new molecular dyads, comprising a bora-3a,4a-diaza-s-indacene (Bodipy) dye linked to two aromatic polycycles via the boron center, have been synthesized and fully characterized. The polycyclic compounds are either pyrene or perylene, or a mixture of both. Whereas the absorption spectral profiles contain important contributions from each of the subunits, fluorescence occurs exclusively from the Bodipy fragment. Intramolecular excitation energy transfer is extremely efficient in each case, even though spectral overlap integrals for the pyrene-based system are modest. Although these polycycles are sterically congested, molecular dynamics simulations indicate that they are in dynamic motion, and this hinders proper computation of the orientation factors for Foerster-type energy transfer. These new dyes, especially the mixed polycycle system, greatly extend the range of excitation wavelengths that can be used for fluorescence microscopy.

Full text of DOI:10.1021/ja0631448

Published on Web 07/28/2006
Rapid Energy Transfer in Cascade-Type Bodipy Dyes
Anthony Harriman,*,† Guillaume Izzet,† and Raymond Ziessel*,‡
Contribution from the Molecular Photonics Laboratory, School of Natural Sciences, Bedson
Building, UniVersity of Newcastle, Newcastle upon Tyne NE1 7RU, United Kingdom, and
Laboratoire de Chimie Mole´culaire, Ecole de Chimie, Polyme`res et Mate´riaux, CNRS, 25 rue
Becquerel, 67087 Strasbourg Cedex 02, France
Received May 5, 2006; E-mail: anthony.harriman@ncl.ac.uk; ziessel@chimie.u-strasbg.fr
Abstract: Three new molecular dyads, comprising a bora-3a,4a-diaza-s-indacene (Bodipy) dye linked to
two aromatic polycycles via the boron center, have been synthesized and fully characterized. The polycyclic
compounds are either pyrene or perylene, or a mixture of both. Whereas the absorption spectral profiles
contain important contributions from each of the subunits, fluorescence occurs exclusively from the Bodipy
fragment. Intramolecular excitation energy transfer is extremely efficient in each case, even though spectral
overlap integrals for the pyrene-based system are modest. Although these polycycles are sterically
congested, molecular dynamics simulations indicate that they are in dynamic motion, and this hinders proper
computation of the orientation factors for Fo¨rster-type energy transfer. These new dyes, especially the
mixed polycycle system, greatly extend the range of excitation wavelengths that can be used for fluorescence
microscopy.
Introduction
presents problems when using the dye as a fluorescent reagent
in biotechnology.12 Indeed, it has become increasingly clear that
An inordinately wide range of bora-3a,4a-diaza-s-indacene
(Bodipy) dyes have been reported in the literature and used for
such diverse applications as biolabels,1 artificial light harvesters,2
sensitizers for solar cells,3 fluorescent sensors,4,5 electrolumi-
nescent agents,6 laser dyes,7 stains for DNA,8 drug delivery
templates,9 and electron-transfer reagents.10 A common feature
for all such applications concerns the intense fluorescence and
favorable photophysics of the Bodipy family of dyes.11 A noted
weakness of Bodipy dyes, and indeed most other classes of
intensely fluorescent dyes, is the small Stokes’ shift, which
dyes based on a single chromophore are inadequate for purposes
such as flow cytometry, fluorescence labeling, and chemical
sensing because of the closeness between excitation and
detection wavelengths.13 This realization has led to the develop-
ment of dual-chromophore dyes that possess extended “virtual”
Stokes’ shifts.14 Several Bodipy-based dual-chromophore dyes
have been reported in recent years wherein aromatic polycycles
have been appended to the organic framework. These reagents
operate by virtue of efficient intramolecular excitation energy
transfer from the polycycle to the Bodipy dye, followed by
regular fluorescence from the dye. In this way, the energy gap
between excitation and detection wavelengths can be increased
by at least 10-fold.15
† University of Newcastle.
‡ Ecole de Chimie, Polyme`res, Mate´riaux.
(1) Haugland, R. P. In The Handbook: A Guide to Fluorescent Probes and
Labelling Technologies, 10th ed.; Spence, M. T. Z., Ed.; Molecular
Probes: Eugene, OR, 2005.
The most popular aromatic polycycles have been anthracene
and pyrene, but in all cases these ancillary chromophores have
been attached to the organic core and the difluoroboron unit
has been retained.16,17 Both through-space and through-bond
energy-transfer processes have been described, with the poly-
(2) Lokey, G. E.; Hanes, M. S.; Bailey, S. T.; Shearer, J. D. M.; Zhang, Y.-Z.;
Wittmershaus, B. P. Presented at the 225th ACS National Meeting, New
Orleans, LA, March 23-27, 2003.
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Tkachenko, N. V.; Lemmetyinen, H.; Fukuzumi, S. J. Phys. Chem. B 2005,
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Biology: AdVanced Methods and their Applications to Membranes,
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10868
J. AM. CHEM. SOC. 2006, 128, 10868-10875
10.1021/ja0631448 CCC: $33.50 © 2006 American Chemical Society

Rapid Energy Transfer in Cascade-Type Dyes
A R T I C L E S
4-bora-3a,4a-diaza-s-indacene18 (abbreviated herein as F-Bodipy),
1-ethynylpyrene,19 and 1-ethynylperylene20 were prepared and purified
according to literature procedures. n-Butyl lithium was titrated prior
to use according to a literature procedure.21 All reactions were carried
out under dry argon using Schlenk-tube and vacuum-line techniques.
Experimental Procedure and Characterization. The pyrene and
perylene lithio derivatives were prepared in two separate Schlenk flasks
at -78 °C. To a stirred, degassed solution of 1-ethynylpyrene (0.154
g, 0.680 mmol) in anhydrous THF (10 mL), n-BuLi titrated at 1.49 N
(0.460 mL, 0.680 mmol) was added dropwise, and the mixture was
stirred at -78 °C for 1.5 h. In a separate Schlenk tube, 1-ethynyl-
perylene (0.188 g, 0.680 mmol) was dissolved in anhydrous THF (10
mL), n-BuLi titrated at 1.49 N (0.460 mL, 0.680 mmol) was added
dropwise, and the mixture was stirred at -78 °C for 1 h. After that
time, the resulting pyrene anion was transferred via cannula to the
perylene solution kept at -78 °C. The resulting solution was transferred
rapidly to the difluoroboradiazaindacene precursor (0.180 g, 0.566
mmol) dissolved in anhydrous THF (10 mL) and maintained at room
temperature. The mixture was stirred at room temperature for 5 min,
until complete consumption of the starting material was observed by
TLC. Water (1 mL) was then added, and the solution was extracted
with CH2Cl2 (3 × 50 mL). The organic phase was dried over cotton-
wool. After evaporation, the crude mixture was purified by column
chromatography on flash silica using cyclohexane and increasing
amounts of ethyl acetate (1-20%). The crude reaction mixture was
dispersed on neutral alumina and deposited as a solid on the top of the
column. The pure compounds were isolated by slow evaporation of
dichloromethane from a mixture of dichloromethane, methanol (trace),
and hexane.
Figure 1. Structural formulas and abbreviations used for the Bodipy-based
cascade-type dyes studied in this work.
cycle being attached at different sites on the indacene back-
bone.16 Many of these dual-chromophore dyes have connected
the subunits by way of ethynylene linkers so as to ensure good
electronic communication.17 We now seek to expand on this
subject by introducing a new class of dual-chromophore dyes
in which the polycycles replace the fluorine atoms. This strategy
places the substituent close to the indacene core and allows for
facile synthesis of asymmetrical derivatives bearing two different
polycycles. For the compounds described herein, connection to
the boron center is made via a single ethyne group. It will be
shown that intramolecular singlet energy transfer is extremely
efficient and that, in the case of a mixed pyrene/perylene system,
the fluorescence excitation spectrum covers most of the region
between 200 and 540 nm.
Characterization of the Compounds. 4,4′-Bis(pyrenyl-1-ethynyl)-
1,3,5,7,8-pentamethyl-2,6-diethyl-4-bora-3a,4a-diaza-s-indacene (1).
1
3
Isolated yield: 20%, 0.099 g. H NMR (CDCl3): δ ) 8.75 (d, 2H, J
) 9.0 Hz), 8.16-7.96 (m, 16H), 3.11 (s, 6H), 2.74 (s, 3H), 2.56 (q,
4H, J ) 7.5 Hz), 2.45 (s, 6H), 1.17 (t, 6H, J ) 7.5 Hz). 13C {1H}
NMR (CDCl3, 100 MHz): δ ) 152.1, 140.0, 134.7, 132.8, 132.1, 131.4,
131.3, 130.5, 130.4, 129.7, 127.8, 127.43, 127.38, 126.4, 126.0, 125.3,
125.11, 125.08, 124.61, 124.57, 124.4, 94.4, 17.6, 17.4, 15.2, 14.8,
14.5. IR (KBr mull): ν ) 2960 (s), 2293 (m), 2169 (w), 1599 (s),
1430 (s), 1184 (s), 978 (s) cm-1. FAB+ (m/z, nature of peak, relative
intensity): 731.2, [M + H]+, 100. Anal. Calcd for C54H43BN2 (Mr )
730.74): C, 88.76; H, 5.93; N, 3.83. Found: C, 88.57; H, 5.77; N,
3.65.
3
3
Experimental Methods
4,4′-Bis(perylen-3-ethynyl)-1,3,5,7,8-pentamethyl-2,6-diethyl-4-
bora-3a,4a-diaza-s-indacene (2). Isolated yield: 16%, 0.066 g.
Prepared according to the general procedure with 1-ethynylperylene
(0.045 g, 0.16 mmol) in 5 mL of THF, 93 µL of n-butyl lithium (1.74
M in n-hexane), and 1 (0.026 g, 0.082 mmol) in 10 mL of THF.
Complete consumption of the starting material was observed after 30
min. The chromatography was performed on silica (CH2Cl2/cyclohex-
ane, 20:80), and recrystallization gave 0.02 g of 6 (15% yield). 1H NMR
(CDCl3): δ ) 8.37 (d, 2H, 3J ) 8.1 Hz), 8.21-8.13 (m, 6H), 8.08 (d,
2H, 3J ) 8.1 Hz), 7.67-7.65 (m, 4H), 7.61 (d, 2H, 3J ) 7.9 Hz), 7.51-
Structural formulas for the molecular systems studied herein are
given in Figure 1. The new compounds were synthesized and character-
ized by NMR, fast-atom bombardment mass spectra in positive mode
(FAB+), Fourier transform infrared (FT-IR), UV-visible spectroscopy,
and elemental analysis.
Synthesis. General Methods. The 400 (1H) and 100.3 (13C) MHz
NMR spectra were recorded at room temperature using perdeuterated
solvents as internal standard: δ (H) in ppm relative to residual protiated
solvent; δ (C) in ppm relative to the solvent. A fast-atom bombardment
ZAB-HF-VB analytical apparatus, operated in positive mode, was used
for mass spectral studies with m-nitrobenzyl alcohol (m-NBA) as matrix.
FT-IR spectra were recorded on the neat liquids or as thin films,
prepared with a drop of dichloromethane and evaporated to dryness
on KBr pellets. Chromatographic purification was conducted using
standardized flash silica gel. Thin-layer chromatography (TLC) was
performed on aluminum oxide or silica plates coated with fluorescent
indicator. All mixtures of solvents are given as volume/volume (v/v)
ratios.
3
7.43 (m, 6H), 3.00 (s, 6H), 2.71 (s, 3H), 2.52 (q, 4H, J ) 7.5 Hz),
3
2.43 (s, 6H), 1.13 (t, 6H, J ) 7.5 Hz). 13C{1H} NMR (CDCl3): δ )
152.1, 135.4, 134.8, 134.7, 132.9, 131.5, 131.4, 131.3, 130.6, 130.5,
128.7, 127.9, 127.1, 127.0, 126.7, 123.0, 120.63, 120.57, 120.4, 119.9,
17.6, 17.5, 15.3, 14.9, 14.5. IR (KBr): ν ) 2922 (m), 2125 (m), 2172
(w), 1618 (m), 1554 (m), 1430 (s), 1184 (s), 1122 (s), 977 (m), 876
(m), 767 (m). FAB+ (m/z, nature of peak, relative intensity): 831.1,
(18) Shah, M.; Thangaraj, K.; Soong, M. L.; Wolford, L.; Boyer, J. H.; Politzer,
I. R.; Pavlopoulos, T. G. Heteroat. Chem. 1990, 1, 389.
(19) (a) Harriman, A.; Hissler, M.; Khatyr, A.; Ziessel, R. Chem. Commun. 1999,
735. (b) Hissler, M.; Harriman, A.; Khatyr, A.; Ziessel, R. Chem. Eur. J.
1999, 11, 3366.
(20) Inouye, M.; Hyodo, Y.; Nakazumi, H. J. Org. Chem. 1999, 64, 2704.
(21) Suffert, J. J. Org. Chem. 1989, 54, 509.
Materials. Tetrahydrofuran was distilled over sodium and ben-
zophenone. Samples of 4,4-difluoro-1,3,5,7,8-pentamethyl-2,4-diethyl-
(17) Ziessel, R.; Goze, C.; Ulrich, G.; Ce´sario, M.; Retailleau, P.; Harriman,
A.; Rostron, J. P. Chem. Eur. J. 2005, 11, 7366.
9
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A R T I C L E S
Harriman et al.
[M + H]+, 100. Anal. Calcd for C62H47BN2‚1/2CH2Cl2 (Mr ) 830.86
+ 42.46): C, 85.96; H, 5.54; N, 3.21. Found: C, 85.42; H, 5.51; N,
3.21.
beam was combined with the excitation pulse and used as the diagnostic
beam. The two beams were directed to different parts of the entrance
slit of a cooled charge-coupled device (CCD) detector and used to
calculate differential absorbance values. The CCD shutter was kept open
for 1 s, and the accumulated spectra were averaged. This procedure
was repeated until about 100 individual spectra had been averaged.
Time-resolved spectra were recorded with a delay line stepped in
increments of 100 fs. The sample, possessing an absorbance of ca. 1 at
420 nm, was flowed through a quartz cuvette (optical path length ) 2
mm) and maintained under an atmosphere of N2.
Electrochemical studies employed cyclic voltammetry with a con-
ventional three-electrode system using a BAS CV-50W voltammetric
analyzer equipped with a Pt microdisk (2 mm2) working electrode and
a platinum wire counter electrode. Ferrocene was used as an internal
standard and was calibrated against a saturated calomel reference
electrode (SCE) separated from the electrolysis cell by a glass frit
presoaked with electrolyte solution. Solutions contained the electrode-
active substrate (ca. 1.5 × 10-3 M) in solvent previously deoxygenated
with anhydrous nitrogen and with tetra-n-butylammonium hexafluo-
rophosphate (0.1 M) as supporting electrolyte. The quoted half-wave
potentials were reproducible to within (15 mV.
4-(Pyrenyl-1-ethynyl)-4′-(perylen-3-ethynyl)-1,3,5,7,8-pentamethyl-
2,6-diethyl-4-bora-3a,4a-diaza-s-indacene (3). Isolated yield: 28%,
1
3
0.149 g. H NMR (CDCl3): δ ) 8.64 (d, 1H, J ) 9.0 Hz), 8.34 (d,
1H, 3J ) 8.0 Hz), 8.24 (t, 1H, 3J ) 8.0 Hz), 8.20-8.02 (m, 10H), 7.85
(d, 1H, J ) 9.0 Hz), 7.62 (t, 1H, J ) 8.0 Hz), 7.55-7.48 (m, 5H),
3.05 (s, 3H), 3.01 (s, 3H), 2.80 (s, 3H), 2.60 (q, 2H, 3J ) 7.4 Hz), 2.57
3
3
3
3
(q, 2H, J ) 7.4 Hz), 2.51 (s, 3H), 2.49 (s, 3H), 1.19 (t, 3H, J ) 7.4
Hz), 1.16 (t, 3H, J ) 7.4 Hz). 13C{1H} NMR (CDCl3): δ ) 152.2,
3
140.1, 137.9, 135.5, 134.7, 134.6, 132.9, 132.3, 131.5, 131.4, 131.3,
130.6, 130.5, 130.4, 129.7, 128.8, 127.8, 127.6, 127.4, 127.3, 126.7,
126.4, 126.1, 125.3, 125.2, 125.0, 124.6, 124.5, 124.3, 124.2, 124.1,
119.8, 94.5, 17.7, 17.6, 17.4, 17.3, 15.3, 15.2, 14.8, 14.7, 14.5. IR
(KBr): ν ) 3043 (m), 2960 (m), 2926 (m), 2868 (m), 2161 (m), 1597
(m), 1555 (s), 1479 (s), 1386 (m), 1360 (m), 1323 (m), 1184 (s), 1122
(m), 1062 (m), 977 (s), 846 (m). FAB+ (m/z, nature of peak, relative
intensity): 781.2, [M + H]+, 100. Anal. Calcd for C58H45BN2 (Mr )
780.80): C, 89.22; H, 5.81; N, 3.59. Found: C, 88.94; H, 5.67; N,
3.41.
Molecular orbital calculations were made on energy-minimized
conformations calculated by Gaussian 03 using the parametrized
semiempirical AM1 method and checking for imaginary frequencies.26
Several different starting geometries were sampled. Such AM1 calcula-
tions are far from definitive for boron-containing compounds but have
been found to adequately model cyclic boron ethers.27 Parameters for
boron were taken from the literature.28 These calculations were used
to generate energy-minimized geometries and transition dipole moments
for the dyes in vacuo. Molecular dynamics simulations (MDS) were
performed with INSIGHT-II29 running on a Silicon Graphics O2
workstation. Structures were drawn in the Builder module, and partial
charges were assigned using the ESFF force field.30 Energy minimiza-
tion was carried out with the Discover-3 module using the conjugate
gradient method with a cutoff of 9.5 Å until the maximum derivative
was less than 0.0005 kcal/Å. The energy-minimized geometries were
used as the starting points for the MDS studies. Each MDS run consisted
of an initial 10 ps of equilibration using the velocity scaling method,
followed by 100 ps of production dynamics. During the latter stage,
the temperature averaged 300 K, with a standard deviation of 4.8 K.
Data points were sampled each 10 fs of simulation time.
Spectroscopic Studies. Spectrophotometric grade solvents were
purchased from Aldrich Chemical Co. and used as received. Absorption
and fluorescence spectra were recorded using a Hitachi U3300
spectrophotometer and a fully corrected Yvon-Jobin Fluorolog Tau-3
spectrofluorimeter, respectively. Low-temperature emission spectra were
taken using an immersion-well liquid N2 Dewar. Emission quantum
yields were measured in N2-purged 2-methyltetrahydrofuran (MeTHF)
relative to Rhodamine 6G (in methanol)22 or Coumarin 153 (in
cyclohexane or ethanol/water 1:1).23 Corrections were made for changes
in refractive index24 or density as required. Fluorescence lifetimes were
recorded using the phase modulation method on an Yvon-Jobin
Fluorolog Tau-3 Lifetime System, with the instrumental response
function being measured against a solution of Ludox in distilled water.
Additional lifetime measurements were made with a PTI XenoFlash
system. All solutions used for fluorescence spectral measurements were
optically dilute and were used in conjunction with nonemissive glass
cutoff filters.
Flash photolysis studies were made with an Applied Photophysics
Ltd. LKS60 instrument. Excitation was made with 4-ns pulses at 532
nm, delivered with a frequency-doubled, Q-switched Nd:YAG laser,
while detection was made at 90° using a pulsed, high-intensity Xe arc
lamp. The signal was detected with a fast-response photomultiplier tube
after passage through a high-radiance monochromator. Transient
differential absorption spectra were recorded point-by-point, with five
individual records being averaged at each wavelength. Kinetic measure-
ments were made after averaging 50 individual records using global
analysis methods. The sample was purged with N2 before use. For some
studies, iodomethane (10% v/v) was added before photolysis. The laser
intensity was calibrated by reference to the triplet state of zinc meso-
tetraphenylporphyrin in deoxygenated toluene.25
Results and Discussion
Synthesis and Characterization. The three target compounds
were prepared in a single reaction using the difluoro derivative
(F-Bodipy) and half an equivalent each of 1-lithioethynylpyrene
and 1-lithioethynylperylene. Due to comparable reactivity, it
was anticipated that a statistical ratio of the dyes would be
formed. Separation of the three compounds was successfully
realized on several flash chromatography columns using ad-
equate mobile phases, followed by multiple recrystallizations.
The fingerprint of these molecules is given by the proton NMR
spectra. In particular, compound 1 exhibits a characteristic
doublet at 8.75 ppm, whereas for 2 a doublet is found at 8.37
ppm. In both cases, the integrations of these peaks correspond
Fast transient spectroscopy was made by pump-probe techniques
using femtosecond pulses delivered from a Ti:sapphire generator
amplified with a multipass amplifier pumped via the second harmonic
of a Q-switched Nd:YAG laser. The amplified pulse energies varied
from 0.3 to 0.5 mJ, and the repetition rate was kept at 10 Hz. Part of
the beam (ca. 20%) was focused onto a second harmonic generator in
order to produce the excitation pulse. The residual output was directed
onto a 4-mm sapphire plate so as to create a white light continuum for
detection purposes. The continuum was collimated and split into two
equal beams. The first beam was used as reference, while the second
(26) Frisch, M. J.; et al. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford,
CT, 2004.
(27) Kuznetsov, V. V. J. Struct. Chem. 2001, 42, 494.
(28) (a) Dewar, M. J. S.; Reynolds, C. H. J. Comput. Chem. 1986, 2, 140. (b)
Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. P. J. Am. Chem.
Soc. 1985, 107, 3902. (c) Dewar, M. J. S.; McKee, M. L.; Rzepa, H. S. J.
Am. Chem. Soc. 1978, 100, 3607. (d) Dewar, M. J. S.; Thiel, W. J. Am.
Chem. Soc. 1977, 99, 4899. (e) Dewar, M. J. S.; Jie, C.; Zoebisch, E. G.
Organometallics 1988, 7, 513.
(22) Olmsted, J., III. J. Phys. Chem. 1979, 83, 2581.
(23) Jones, G., II; Rahman, M. A. J. Phys. Chem. 1994, 98, 13028.
(24) Eaton, D. F. Pure Appl. Chem. 1988, 60, 1107.
(25) (a) Hurley, J. K.; Sinai, N.; Linschitz, H. Photochem. Photobiol. 1983, 38,
9. (b) Carmichael, I.; Hug, G. L. J. Phys. Chem. Ref. Data 1986, 15, 1.
(29) INSIGHT-II; Accelrys Software Inc.: Cambridge, UK, 2001-2006.
(30) Shi, S.; Yan, L.; Yang, Y.; Fisher-Shaulsky, J.; Thacher, T. J. Comput.
Chem. 2003, 24, 1059.
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Rapid Energy Transfer in Cascade-Type Dyes
A R T I C L E S
to the two protons located at the R position of the ethynyl tether
and lying in the deshielding cone of the indacene core. As
expected for the mixed pyrene/perylene dye 3, two doublets,
each integrating for one proton, were found at 8.64 and 8.34
ppm. Furthermore, owing to the asymmetry introduced by two
different polyaromatic nuclei, all methyl groups of the pyrrole
rings are split, whereas the central methyl group resonates as a
singlet at 2.80 ppm. Also noteworthy is the quaternary ethynyl
carbon which resonates at 94.4 ppm for 1, at 119.9 ppm for 2,
and as two singlets at 94.5 and 119.8 ppm for 3.
Electrochemistry. Cyclic voltammetry studies were carried
out in deoxygenated dichloromethane containing excess tetra-
N-butylammonium hexafluorophosphate as supporting electro-
lyte. Electrode processes observed for the asymmetric derivative
3 could be assigned on the basis of studies made with the
symmetrical compounds 1 and 2, and with the parent F-Bodipy
dye. On oxidative scans, three reversible steps could be resolved
with half-wave potentials (E1/2) of 0.81, 0.91, and 1.32 V vs
SCE, respectively. The first process corresponds to one-electron
oxidation of the Bodipy unit,31 whereas the second one-electron
oxidation refers to removal of an electron from the perylene
unit.32 The pyrene unit undergoes one-electron oxidation at
higher potentials and is responsible for the third oxidative
wave.17,19 For 3, it was possible to resolve two reversible, one-
electron reduction processes. The first step, with E1/2 ) -1.54
V vs SCE, is due to reduction of the Bodipy unit, while the
second step, occurring with E1/2 ) -1.76 V vs SCE, is due to
reduction of the perylene group. It was not possible to observed
reduction of the pyrene unit within the given electrochemical
window.
The electrochemical behavior is in keeping with the relative
electron densities and delocalization abilities of the polycycles.
Relative to the parent F-Bodipy dye, it is notable that E-Bodipy
derivatives (referring to ethynyl-substituted Bodipy dyes) are
easier to oxidize by about 100 mV but more difficult to reduce
by about 120 mV. It is also significant to note that these
electrochemical results predict that both the HOMO and the
LUMO will be centered on the Bodipy unit. Used in conjunction
with the photophysical data (see later), these E1/2 values indicate
that intramolecular electron transfer from or to the Bodipy
excited singlet state is thermodynamically unfavorable for both
polycycles. Minor thermodynamic driving forces are available
for both oxidation and reduction of the Bodipy unit by the
perylene excited singlet state and for reduction of the Bodipy
unit by the excited singlet state of the pyrene fragment.33
Photophysical Studies. Absorption spectra recorded for the
three new dyes display a common feature representative of the
Figure 2. Absorption spectra recorded for the three dyes in MeTHF
solution. Spectra for compounds 1 and 2 are shown as solid lines, with the
pyrene bands being due to 1 and the perylene bands being due to 2. The
spectrum recorded for 3 is shown as a dotted line. Concentrations were ca.
10 µM.
Figure 3. Normalized fluorescence spectra recorded for the three dyes in
MeTHF solution. Individual spectra are overlaid to show their close
comparability. The excitation wavelength was 480 nm in each case, and
concentrations were around 2 µM.
lowest-energy transition associated with the Bodipy residue
(Figure 2). In each case, this feature appears as a strong S0 f
S1 transition centered (λmax) at 525 nm (ꢀmax ) 80 000 M-1
cm-1). For 1, there is also a weaker, broad S0 f S2 band
centered at 375 nm, but this transition is obscured in the other
compounds.34 Each polycycle shows a series of well-resolved
absorption bands at higher energy, while 3 displays both sets
of bands. There is no obvious sign of electronic coupling
between the subunits, and the absorption spectrum recorded for
3 is exactly as expected on the basis of spectra recorded for 1
and 2 (Figure 2). Fluorescence, centered at 530 nm, is readily
observed in fluid solution at ambient temperature and shows
good mirror symmetry with the S0 f S1 absorption transition
(Figure 3). The small Stokes’ shift (ca. 550 cm-1) implies there
is little change in geometry or polarity between ground and
excited states.
The fluorescence spectra are identical throughout the series
and entirely consistent with fluorescence from the Bodipy
subunit (Figure 3).35 For each dye, the fluorescence signal was
found to decay by way of first-order kinetics with a lifetime
(τF) of 6.5 ( 0.1 ns in deoxygenated MeTHF solution at room
temperature. The fluorescence quantum yield (ΦF) was found
to be 0.90 ( 0.05 under these conditions, in each case. It is
clear that the appended polycycle does not perturb the photo-
physical properties of the Bodipy dye. For these compounds,
the radiative rate constants calculated from the Strickler-Berg
expression36 agree well with those derived by experiment. Both
(31) (a) Ulrich, G.; Ziessel, R. Synlett 2004, 439. (b) Ulrich, G.; Ziessel, R. J.
Org. Chem. 2004, 69, 2070. (c) Ulrich, G.; Ziessel, R. Tetrahedron Lett.
2004, 45, 1949. (d) Ziessel, R.; Bonardi, L.; Retailleau, P.; Ulrich, G. J.
Org. Chem. 2006, 71, 3093.
(32) Salbeck, J.; Kunkely, H.; Langhals, H.; Saalfrank, R. W.; Daub, J. Chimia
1989, 43, 6.
(33) The oxidation and reduction potentials, respectively, for the first excited
singlet state of the Bodipy unit are calculated to be -1.55 and 0.82 V vs
SCE. On this basis, we can rule out light-induced electron-transfer processes
involving the Bodipy excited state and either of the polycycles. The
reduction potential for the excited singlet state of the perylene unit is
calculated to be 0.89 V vs SCE, such that this species might be expected
to oxidize the Bodipy unit (∆G° ) -0.08 eV, ignoring electrostatic effects).
The corresponding oxidation potential for the perylene unit is -1.73 V vs
SCE, such that reduction of the Bodipy unit is a possibility (∆G° ) -0.19
eV, ignoring electrostatic effects). For pyrene, the oxidation potential for
the excited singlet state is estimated to be -1.89 V vs SCE, such that, in
principle, this species might be expected to reduce both the Bodipy (∆G°
) -0.35 eV) and perylene (∆G° ) -0.13 eV) units.
(34) Karolin, J.; Johansson, L. B.-A.; Strandberg, L.; Ny, T. J. Am. Chem. Soc.
1994, 116, 7801.
(35) Lo´pez Arbeloa, F.; Ban˜uelos, J.; Martinez, V.; Arbeloa, T.; Lo´pez Arbeloa,
I. Int. ReV. Phys. Chem. 2005, 24, 339.
(36) Strickler, S. J.; Berg, R. A. J. Chem. Phys. 1962, 37, 814.
9
J. AM. CHEM. SOC. VOL. 128, NO. 33, 2006 10871

A R T I C L E S
Harriman et al.
Table 1. Parameters Used To Calculate the Rate Constants for
Fo¨rster-Type Energy Transfer in the Three New Dyes
ΦF and τF were found to be independent of temperature in
deoxygenated MeTHF over the range from 300 to 80 K. Triplet
state formation remains highly inefficient under these conditions,
and, in the absence of a perturbing heavy-atom solvent,
intersystem crossing does not compete with radiative decay of
the first-excited singlet state. The triplet quantum yields are
estimated from nanosecond laser flash photolysis to be less than
0.01.
parameter
1a
2b
3c
d
ΦF
0.86
55
7.7
0.63
4.1
8.5
0.86
55
11.5
3.5
τF/nsd
R
CC/Åe
JF /10-14 mol-1 cm6
1.4
13.0
0.14
K
0.20
0.95
kF /s-1
4.7 × 1010
1.7 × 1012
5.0 × 1010
a Refers to singlet energy transfer from pyrene to the S2 state localized
on the Bodipy unit. b Refers to singlet energy transfer from perylene to the
S1 state localized on the Bodipy unit. c Refers to singlet energy transfer
from pyrene to perylene. d Fluorescence quantum yield and lifetime for
1-trimethylsilylacetylenepyrene and 1-triethylsilylacetyleneperylene. e Av-
erage center-to-center separation distance derived from the MDS runs.
The corrected fluorescence excitation spectra recorded for
compounds 1-3 were found to be in excellent agreement with
the corresponding absorption spectra over the range 250-530
nm. No fluorescence could be detected from either of the
polycycles, and, compared to suitable reference materials, it is
clear that ΦF for the pyrene and perylene units in these
compounds is <10-4. Time-resolved fluorescence studies made
with near-UV excitation confirmed that the fluorescent states
associated with the polycycles have lifetimes less than 50 ps.
On this basis, it appears that photons collected by the appended
polycycles are transferred quantitatively to the Bodipy unit. The
rate constants for intramolecular excitation energy transfer must
exceed 2 × 1010 s-1. Such behavior has been noted before,16,17
especially with anthracene and pyrene appendages. With pyrene
attached at the pseudo-meso position, it was concluded that
intramolecular energy transfer occurred by a through-space
process, with the overlap integral being increased due to transfer
to the S2 state localized on the Bodipy unit.17 The situation was
not so clear with a series of anthracene-based dual-chromophore
dyes, and both through-space and through-bond interactions
were considered.16
Figure 4. Illustration of the relevant transition dipole moments involved
in the calculation of the orientation factor for 1. The dipole moments µA
and µD are indicated.
Fo1rster-Type Energy Transfer. The close proximity be-
tween the Bodipy and polycyclic units should favor dipole-
dipole energy transfer, although the ethynylene connector could
act as a good conduit for through-bond interactions.37 The rate
constant (kF) for Fo¨rster-type energy transfer can be expressed
in terms of eq 1,
synthetic protocols and involve pyrene and perylene units
bearing a single ethyne group at the relevant site. Photophysical
properties were measured for these compounds in deoxygenated
MeTHF at room temperature and are given in Table 1. The
spectral overlap integral refers to the reduced fluorescence
spectra (FD) for the reference compounds, converted into
wavenumber (ν), and the normalized absorption spectrum of
the Bodipy unit displayed in terms of the molar absorption
coefficient (ꢀA). The derived JF values are collected in Table 1,
together with the corresponding RCC values obtained from
molecular modeling studies.
It is seen that the photophysical properties of the reference
donors are remarkably different, especially the fluorescence
lifetimes. The more extended perylene-based compound, which
emits at longer wavelength, gives the larger center-to-center
separation and the higher overlap integral. This latter value refers
to spectral overlap with the S0 f S1 absorption transition of
the Bodipy unit for 2 but to the S0 f S2 transition for the pyrene-
based system 1. This difference accounts for the large discrep-
ancy between the derived JF values. To compute the orientation
factors, it is necessary to establish the geometries for the isolated
molecules. Indeed, K can be expressed in terms of eq 2,
(8.8 × 10-25)ΦF K 2
kF )
JF
(1)
n4 τF RCC
6
where
∞F (ν) ꢀ (ν) ν-4 dν
∫
D
A
0
JF )
∞F (ν) dν
∫
D
0
n is the refractive index of the surrounding solvent and both
ΦF and τF refer to the polycyclic donor when isolated from the
Bodipy acceptor. Here, RCC is the center-to-center separation
distance between the reactants, K is their mutual orientation
factor, and JF is the spectral overlap integral.38 The key factors
involved in evaluating this expression are (i) identifying an
appropriate reference compound with which to compute JF, (ii)
determining a realistic separation distance, and (iii) predicting
the orientation factor. Given the fact that the rates of intramo-
lecular energy transfer are very fast, the main problem concerns
calculating appropriate values for K. Suitable reference com-
pounds, used to measure ΦF and τF, are available from the
K ) cos φ - 3 cos θD cos θA
(2)
where θD and θA are the angles between the transition dipole
moments of the donor and acceptor, respectively, with the
molecular axis and φ is the angle between the two dipoles
(Figure 4). Information about these angles was sought from
molecular modeling studies made for 1 and 2.
(37) Harriman, A.; Ziessel, R. In Carbon-Rich Compounds; Haley, M. M.,
Tykwinski, R. R., Eds.; Wiley-VCH: Weinheim, 2006; p 26.
(38) Fo¨rster, T. Discuss. Faraday Soc. 1959, 27, 7.
9
10872 J. AM. CHEM. SOC. VOL. 128, NO. 33, 2006

Rapid Energy Transfer in Cascade-Type Dyes
A R T I C L E S
Figure 5. Representation of the major conformational exchanges available to 1 as indicated by semiempirical molecular modeling studies.
Figure 6. Left: Representation of the main torsion angles of the pyrene units with respect to the molecular axis. Right: Variation of Ri and âi during a
MDS run (red, i ) 1; blue, i ) 2). The torsion angles are given in absolute values.
Molecular Modeling Studies Carried Out with 1. Semiem-
pirical molecular orbital calculations made at the AM1 level
indicate that 1 adopts many different conformations that vary
only slightly in energy (i.e., less than 0.1 eV). For each
conformation, the energy-minimized structures for both ground
and first-excited singlet states were closely comparable. Con-
formational heterogeneity can be considered to arise from two
major internal rotations. The first grouping arises from twisting
of the ethyl group (represented in blue in Figure 5) and has no
observable effect on the orientation factor or separation distance.
The second family of conformers arises from rotation around
the connecting ethynylene group (represented in red in Figure
5) and has a profound effect on the orientation factor, but not
the center-to-center separation. In particular, this rotation alters
the orientation of the pyrene dipole moment with respect to the
molecular axis. This effect could be seen clearly from molecular
dynamics simulations.
In the MDS runs, attention was focused on the mutual ori-
entation of the pyrene and Bodipy units. The relative orientation
of the pyrene fragment can be described by the angle R and the
dihedral angle â shown in Figure 6. The MDS results clearly
indicate high internal flexibility around the ethynylene connec-
tor, since R and â vary over a wide range at 300 K, as shown
in Figure 6. The barrier to rotation around an ethynylene con-
necting bond is known to be very small,39 and the MDS results
tend to confirm this realization. Despite the apparent stereo-
chemical crowding caused by the bulky pyrene residue, internal
rotation is facile and might be expected to take place on a time
scale comparable with that of intramolecular energy transfer.
The actual center-to-center separation distance remains fairly
constant at 7.7 Å throughout the MDS run, and there is no
indication for buckling of the indacene core, as happens for
certain Bodipy derivatives.40 Here, the structural integrity of
(39) Amini, A.; Harriman, A. Phys. Chem. Chem. Phys. 2003, 5, 1344.
9
J. AM. CHEM. SOC. VOL. 128, NO. 33, 2006 10873

A R T I C L E S
Harriman et al.
three representative conformations of 2 are given in Figure 8.
Again, the computational studies found essentially the same
geometry for ground and first-excited singlet states, while MDS
runs confirmed the high degree of internal flexibility. The larger
polycycle reduces the variation in geometry relative to 1, but
even so the orientation factor varies considerably (Figure 7).
The mean K value derived from these studies is 0.14, and this
allows calculation of the mean Fo¨rster rate constant as 1.7 ×
1012 s-1. The higher kF with respect to 1 (Table 1) arises from
the improved spectral overlap integral since perylene fluores-
cence overlaps with the S0 f S1 absorption transition localized
on the Bodipy unit. The very fast rate of intramolecular energy
transfer is fully consistent with the reported excitation spectra
and with the noted absence of perylene-like fluorescence. The
shorter singlet lifetime of the perylene-like reference compound
and the enhanced JF value compensate for the longer center-
to-center distance. Excitation energy transfer from perylene to
Bodipy is expected to be quantitative, as found experimentally.
Excitation Energy Transfer in 2 and 3. The absorption
spectral profile characteristic of the pyrene-like unit in 1 and 3
is outside the range where the excited-state dynamics can be
probed by ultrafast laser spectroscopy. As such, it has not been
possible to confirm the fast rates of intramolecular energy
transfer from pyrene to Bodipy, or to perylene, by direct
observation. Perylene, however, absorbs at longer wavelength
and can be interrogated by subpicosecond laser pulses delivered
at 420 nm. The experimental setup allowed excitation of 2 in
MeTHF solution with short (fwhm ) 130 fs) laser pulses at
420 nm, where only the perylene unit absorbs. The course of
reaction was monitored by following the rate of bleaching of
the Bodipy S0 f S1 absorption transition at 535 nm. It was
observed that bleaching of the Bodipy chromophore occurred
after the laser pulse and on a time scale of a few picoseconds
(Figure 9). The kinetic trace did not correspond to a single-
exponential curve, but the mean lifetime ( τ ) for the bleaching
process was ca. 0.7 ps. This seems entirely consistent with the
results of the Fo¨rster calculations and confirms the observation
that the perylene unit in 2 does not fluoresce with appreciable
yield. The same experiment carried out with 3 gave a mean
lifetime of 0.8 ps. In each case, the bleaching curve gave a
reasonable fit to stretched exponential kinetics, as expressed in
terms of eq 3,
Figure 7. Statistical distribution of the orientation factor calculated from
MDS runs for 1 (top panel) and 2 (bottom panel).
the compound is preserved, the indacene unit being planar, and
the only serious geometrical changes are those depicted in Figure
5.
Twisting of the pyrene unit around the ethyne connector has
the effect of orienting the transition dipole moment of the donor
and thereby has a major influence on the size of the orientation
factor. Both semiempirical calculations and the MDS studies
indicate that a wide variety of conformations should be expected
at ambient temperature. Although these calculations were made
in vacuo, it seems unlikely that solvent molecules will com-
pletely dampen the internal oscillations. As such, K was
calculated for each conformation found in the MDS runs,
allowing for the frequency of occupation (Figure 7). The mean
value for K is 0.2, which when used in the Fo¨rster equation
predicts a mean rate of dipole-dipole energy transfer of 4.7 ×
1010 s-1. This mean rate is high, given the rather limited overlap
integral, and consistent with the lack of fluorescence from the
pyrene unit. The Fo¨rster mechanism is insensitive to changes
in temperature; again, this is in agreement with the observation
that pyrene-like emission is not seen in a frozen glass at 77 K.
Molecular Modeling Studies Carried Out with 2. Semiem-
pirical calculations also indicate that many conformations are
available for 2 that differ by only minor variations in energy;
a
A(t) ) A0 e-(t/ τ )
(3)
suggesting that internal rotation of the perylene unit is slower
than energy transfer. Here, the stretching exponent (R) has a
value of ca. 0.65 and presumably represents the distribution of
conformers that do not equilibrate on the relevant time scale.
Figure 8. Representative conformations calculated for 2 by semiempirical methods.
9
10874 J. AM. CHEM. SOC. VOL. 128, NO. 33, 2006

Rapid Energy Transfer in Cascade-Type Dyes
A R T I C L E S
light harvester for near-UV photons. In this respect, the
asymmetric derivative 3 is the most attractive dye, since it
collects photons across most of the accessible spectral range.
This compound fluoresces strongly when dispersed in polymeric
media and acts as a highly efficient solar concentrator. It also
provides for a large “virtual” Stokes’ shift, displaying several
clear absorption peaks that are useful as markers for chemical
sensors. The fluorescence quantum yield is independent of
temperature and excitation wavelength and relatively insensitive
to changes in the polarity of the surrounding medium. This dye
looks to be a useful addition to the Bodipy family.
Figure 9. Typical kinetic trace showing bleaching of the Bodipy chro-
mophore at 535 nm following laser excitation into the perylene unit for 2.
The signal has been normalized and fit to eq 3 with R ) 0.65 and τ ) 0.7
ps. The red line is the best fit according to a linear least-squares analysis.
Intramolecular excitation energy transfer appears to be
consistent with the Fo¨rster dipole-dipole mechanism,16,17 at
least for the perylene-based chromophore. It is possible that the
rate of energy transfer is augmented by Dexter-type through-
bond interactions, given the short separation and conjugated
ethynylene linker. The latter is known to be an effective bridge
for electron-exchange processes.37 The ability to tunnel through
the central boron atom is unknown, however, and it has not
been necessary to invoke the Dexter mechanism in our work.
Concluding Remarks
The new dual-chromophore dyes introduced herein display
extremely fast intramolecular energy transfer from the poly-
cycles to the Bodipy residue. Similar behavior has been noted
before for polycycles attached to the indacene core, and the rates
appear comparable for the different systems. There are no
obvious indications for competing intramolecular electron
transfer, despite the modest thermodynamic driving forces for
certain processes.33 It is possible that the weak solvent polarity,
relatively long through-space distances, and/or poor conductivity
of the B atom inhibit electron transfer and thereby favor energy
transfer. The ease of synthesis and, in particular, the ability to
isolate asymmetrical derivatives are clear advantages of the
boron-substituted dyes. We have studied both pyrene- and
perylene-based systems, each connected to the boron center via
a single ethyne group; these dyes can be conveniently described
as E-Bodipy dyes so as to readily distinguish them from the
more common F-Bodipy dyes bearing two B-F bonds. There
is no reason why the synthesis cannot be extended to include
other polycyclic substituents nor why cross-functionalized dyes,
having additional substituents covalently linked at the indacene
backbone, cannot be prepared. An interesting feature of these
E-Bodipy dyes is that the substituent does not affect the
photophysical properties of the Bodipy unit; this is in marked
contrast to the corresponding F-Bodipy dyes, where the absorp-
tion and fluorescence spectral profiles can be tuned over a wide
range. The ethynyl substituent merely functions as an ancillary
The final point of interest concerns the possibility of setting
up a cascade effect in the asymmetric derivative 3. Here, photons
absorbed by the pyrene unit can be transferred directly to the
Bodipy S2 state, in accordance with our calculations, or to the
perylene unit. The latter is perfectly placed to act as an energy
acceptor. The Fo¨rster-type overlap integral is high, the center-
to-center separation distance is modest, and the mean orientation
factor is acceptable (Table 1). Model calculations, using the
reference compounds employed above, predict that the Fo¨rster-
type rate of energy transfer from pyrene to perylene in 3 is on
the order of 5 × 1010 s-1. As such, we might expect photons
absorbed by the pyrene unit to be channeled to perylene or
Bodipy units with equal probability. Unfortunately, limited
experimental facilities, in terms of restricted excitation wave-
lengths, prevent direct observation of any such cascade effect.
Acknowledgment. We thank the EPSRC (EP/C007727/1),
the CNRS, the University of Newcastle, and the Centre National
de la Recherche Scientifique (CNRS) for financial support of
this work.
Supporting Information Available: Complete ref 26. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(40) Li, F.; Yang, S. I.; Ciringh, Y.; Seth, J.; Martin, C. H., III; Singh, D. L.;
Kim, D.; Birge, R. R.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Am.
Chem. Soc. 1998, 120, 10001.
JA0631448
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