Quasi-one-dimensional electronic systems formed from boron dipyrromethene (BODIPY) dyes

Article abstract of DOI:10.1002/chem.201001142

Synthetic strategies have been devised that allow the rational design and isolation of highly coloured boron dipyrromethene (BODIPY) dyes that absorb across much of the visible region. Each dye has an aryl polycycle (usually pyrene or perylene) connected to the central BODIPY core through a conjugated tether at the 3,5-positions. Both mono- and difunctionalised derivatives are accessible, in certain cases containing both pyrene and perylene residues. For all new compounds, the photophysical properties have been recorded in solution at ambient temperature and in a glassy matrix at 77 K. The presence of the aryl polycycle(s) affects the absorption and emission maxima of the BODIPY nucleus, thereby confirming that these units are coupled electronically. Indeed, the band maxima and oscillator strengths depend on the conjugation length of the entire molecule, whereas there is no sign of fluorescence from the polycycle. As a consequence, the radiative rate constant tends to increase with each added appendage. The nature of the linkage (styryl, ethenyl, or ethynyl) also exerts an effect on the photophysical properties and, in particular, the absorption spectrum is perturbed in the region of the aryl polycycle. The perylene-containing BODIPY derivatives absorb over a wide spectral range and emit in the far-red region in almost quantitative yield. A notable exception to this generic behaviour is provided by the anthracenyl derivative, which exhibits charge-transfer absorption and emission spectra in weakly polar media at ambient temperature. Regular BODIPY-like behaviour is restored in a glassy matrix at 77 K. Overall, these new dyes represent an important addition to the range of strongly absorbing and emitting reagents that could be used as solar concentrators. Getting bigger all the time! Appending aryl polycycles to an expanded boron dipyrromethene nucleus leads to giant chromophores that emit at long wavelengths, while absorbing over most of the visible region (see figure). All spectroscopic indicators point to a shared electronic system that is fully delocalised over the π system. These new materials hold promise for use as organic solar concentrators or in light-conversion devices.

Full text of DOI:10.1002/chem.201001142

DOI: 10.1002/chem.201001142
Quasi-One-Dimensional Electronic Systems Formed from Boron
Dipyrromethene (BODIPY) Dyes
Raymond Ziessel,*[a] Sandra Rihn,[a] and Anthony Harriman*[b]
Abstract: Synthetic strategies have
and emission maxima of the BODIPY
nucleus, thereby confirming that these
units are coupled electronically.
Indeed, the band maxima and oscillator
strengths depend on the conjugation
length of the entire molecule, whereas
there is no sign of fluorescence from
the polycycle. As a consequence, the
radiative rate constant tends to in-
crease with each added appendage.
The nature of the linkage (styryl, eth-
enyl, or ethynyl) also exerts an effect
on the photophysical properties and, in
particular, the absorption spectrum is
been devised that allow the rational
design and isolation of highly coloured
boron dipyrromethene (BODIPY)
dyes that absorb across much of the
visible region. Each dye has an aryl
polycycle (usually pyrene or perylene)
connected to the central BODIPY core
through a conjugated tether at the 3,5-
positions. Both mono- and difunction-
alised derivatives are accessible, in cer-
tain cases containing both pyrene and
perylene residues. For all new com-
pounds, the photophysical properties
have been recorded in solution at am-
perturbed in the region of the aryl
polycycle. The perylene-containing
BODIPY derivatives absorb over a
wide spectral range and emit in the far-
red region in almost quantitative yield.
A notable exception to this generic be-
haviour is provided by the anthracenyl
derivative, which exhibits charge-trans-
fer absorption and emission spectra in
weakly polar media at ambient temper-
ature. Regular BODIPY-like behaviour
is restored in a glassy matrix at 77 K.
Overall, these new dyes represent an
important addition to the range of
strongly absorbing and emitting re-
agents that could be used as solar con-
centrators.
Keywords: dyes/pigments · energy
transfer · fluorescence · sensitizers ·
synthesis design
bient temperature and in
a glassy
matrix at 77 K. The presence of the
aryl polycycle(s) affects the absorption
Introduction
organo-gels and to solar concentrators, have been proposed
for such materials.[1] Among the many diverse types of fluo-
rophore in common usage, boron dipyrromethene
(BODIPY) dyes have come to prominence in recent years[2]
and there has been a tremendous growth in the number of
publications and patents describing this class of com-
pound.[2,3] In particular, considerable research effort has
been devoted to the synthesis[4] and photophysical examina-
tion[5] of BODIPY dyes equipped with ancillary photon col-
lectors, such as pyrene.[6] The motivation for the develop-
ment of these materials stems largely from the need to en-
hance the (virtual) Stokesꢀ shift,[7] thereby optimising the
performance in flow cytometry and fluorescence imaging
technology, and to facilitate the harvesting of a larger frac-
tion of solar energy, thereby helping to engineer improved
photon collectors.[8] A common feature of such materials is
that the BODIPY nucleus and the appended light harvester,
usually an aryl polycycle, remain in weak electronic commu-
nication such that each unit retains its own identity, even
though intramolecular energy transfer might be extremely
fast.[9] We now describe an alternative approach to the fabri-
There appears to be an almost insatiable market for highly
fluorescent dyes, especially for systems in which the basic
fluorophore is robust, easily functionalised and resistant to
triplet formation. Numerous applications, ranging from fluo-
rescent bio-labels and sensors, to anti-forgery devices, to
[a] Dr. R. Ziessel, S. Rihn
Laboratoire de Chimie Molꢁculaire et
Spectroscopies Avancꢁes (LCOSA)
Ecole Europꢁenne de Chimie, Polymꢂres et Matꢁriaux, CNRS
25 rue Becquerel, 67087 Strasbourg Cedex 02 (France)
Fax : (+33)368852761
E-mail: ziessel@unistra.fr
Homepage: http://www-lmspc.u-strasbg.fr/lcosa
[b] Prof. Dr. A. Harriman
Molecular Photonics Laboratory, School of Chemistry
Bedson Bldg, Newcastle University
Newcastle upon Tyne, NE1 7RU (UK)
Fax : (+44)191-222-8660
E-mail: anthony.harriman@ncl.ac.uk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001142.
11942
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Chem. Eur. J. 2010, 16, 11942 – 11953

FULL PAPER
cation of next-generation BODIPY dyes that absorb over
much of the visible spectral range and fluoresce in the far-
red region. Our strategy involves the synthesis of BODIPY
dyes wherein the ancillary photon collector forms part of
the overall conjugated network running throughout the dye
molecule. One such series of compounds has styryl-based
connections at the 3,5-positions of the dipyrromethene
core,[10] which ensure excellent conjugation across the mole-
cule. In seeking to extend and generalise this work, two ad-
ditional series of compounds having the polycycle attached
to the dipyrromethene nucleus through a vinyl or an acety-
lene linkage have also been prepared. Finally, we report a
notable exception to the generic principle of increased p-
conjugation for the BODIPY family of dyes.
A question might be raised at this point as to how to es-
tablish the difference between (1) rapid intramolecular elec-
tronic energy transfer (EET) from the appended polycycle
to the BODIPY dye and (2) internal conversion between
singlet-excited states resident on the same giant molecule.
In this respect, it should be noted that sub-ps EET has al-
ready been reported[11] for several closely linked BODIPY–
anthracene dyads, and the rates were rationalised in terms
of mutual orientation factors. One key point to bear in mind
is that to properly examine intramolecular EET in extended
p-systems, it is necessary that the presence of the substituent
does not perturb the molecular orbital, or transition dipole
moment vector, of the BODIPY dye. In the event of strong
electronic coupling persisting, it is important to consider in-
ternal conversion between coupled electronic states of the
same system. For the dyes described here, the latter case
seems to dominate.
ꢀ
Scheme 1. Key: (i) HC C
diisopropylamine, [Pd(PPh3)2Cl2] (6 mol%), CuI (10 mol%), RT, 2 days;
(ii) 3-ethynylperylene (1 equiv), THF, diisopropylamine, [Pd(PPh3)4]
(6 mol%), 608C, 12 h; (iii) 1-ethynylpyrene (1 equiv), THF, diisopropyl-
amine, [Pd(PPh3)4] (6 mol%), 608C, 10 h; (iv) 3-ethynylperylene
(2.2 equiv) or 1-ethynylpyrene (2.2 equiv), THF, diisopropylamine, [Pd-
(PPh3)4] (6 mol%), 608C, 18 h.
ACHTUGNTRNEN(UNG CH2)4COOEt (1.5 equiv), benzene/THF (20:1),
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
Results and Discussion
AHCTUNGTRENNUNG
Synthesis and characterisation: The target compound com-
prising fused BODIPY (B), perylene (Pe) and pyrene (Py)
residues, hereafter abbreviated as PeBPy, could be routinely
prepared as illustrated in Scheme 1. To aid interpretation of
the photophysical properties, related compounds were also
prepared and fully characterised as also shown in Scheme 1.
The relatively insoluble starting material B1, containing one
iodo and two bromo residues, requires selective functionali-
sation at the most reactive site (i.e., the iodo group) to
impart solubility and polarity to facilitate the purification
procedure. Cross-coupling of ethyl 6-heptynoate with pre-
cursor B1 provided B in 78% yield. The selectivity was
checked by NMR spectroscopy, while EI-MS provided clear
evidence for a molecular peak with an isotopic profile corre-
sponding to B (see the Supporting Information). We next
examined the connection of 3-ethynylperylene[12] using clas-
sical Pd0-promoted cross-coupling reactions. The use of one
equivalent of 3-ethynylperylene gave a mixture of two com-
pounds, PeB and Pe2B, which could be separated and were
isolated in yields of 42% and 39%, respectively. Notably,
the symmetrical model compounds, Pe2B and Py2B, could be
produced in high yields (>85%) under similar conditions
AHCTUNGTRENNUNG
but using the ethynyl derivatives in excess. The final cou-
pling reaction involved attachment of a 1-ethynylpyrene[13]
residue to the pre-organised PeB dye (Scheme 1).
The chemical structures and molecular compositions of
these new green dyes have been unambiguously assigned on
the basis of NMR spectroscopy, mass spectrometry and ele-
1
mental analyses. The well-defined H NMR spectra exclude
the presence of aggregates. Interestingly, grafting a perylene
moiety onto B leading to PeB provides a characteristic de-
shielded doublet at d=8.32 ppm and two b-pyrrolic proton
signals at d=6.63 and 6.85 ppm.[9a] Subsequent attachment
of the pyrene unit introduces an additional doublet at d=
8.75 ppm, while two singlets at d=6.70 and 6.92 ppm are
still present. Such behaviour is indicative of a strong elec-
tronic interaction between the aryl residues and the dipyrro-
methene core.[9b] For the symmetrically substituted Pe2B and
Py2B dyes, only one doublet is observed, this appearing at
d=8.35 ppm for Pe and at d=8.72 ppm for Py. In each case,
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11943

R. Ziessel, A. Harriman, and S. Rihn
a singlet is observed for the b-pyrrolic protons at d=6.75
and 6.98 ppm, respectively. The characteristic H,H coupling
constant of 16.3 Hz is diagnostic of an E conformation of
the double bonds,[14] which remains unchanged during subse-
quent chemical transformations. Note that 13C NMR spec-
troscopy can be used to follow the build up of the ethyny-
lene connections because of the highly characteristic signals
seen in the range d=80–100 ppm.
In an effort to expand this series of fused BODIPY dyes,
we set out to eliminate the ethynylphenyl linkage as a
means of decreasing the degree of conformational heteroge-
neity (Scheme 2). Thus, a Knoevenagel reaction[15] was used
to condense C with 1-pyrenecarboxaldehyde. The use of var-
ious BODIPY/pyrene ratios permitted forcing the reaction
in favour of either the mono- or disubstituted compounds
PyC or Py2C, respectively. The poor solubility of Py2C facili-
tated its separation from PyC, which could then be purified
by column chromatography. A similar synthetic procedure
was used to produce the reference tolyl derivative Tol2C and
the anthracenyl compound Ant2C (Scheme 2). In the latter
cases, preparation of the monosubstituted derivatives
proved to be very difficult because of their similar solubili-
ties and polarities to those of the disubstituted compounds.
Having omitted the ethynylphenyl relay, it was of interest
to compare the coupling properties of ethene and ethyne
linkages. This was done by reference to the pyrene-contain-
ing derivatives. To reach this goal without modifying the
BODIPY skeleton, we tried first to convert the ethenyl PyC
and Py2C derivatives into the ethynyl-linked compounds by
addition of Br2 to the double bonds, followed by elimination
of HBr under basic conditions. Despite varying the experi-
mental conditions (solvent, temperature, microwave irradia-
tion, etc.), these experiments failed to give viable quantities
of the desired compounds. Alternatively, we employed a re-
cently developed protocol that enabled us to produce the di-
chloro-substituted BODIPY D (Scheme 3).[16,17] It has been
Scheme 3. Key: (i) 1-ethynylpyrene (1 equiv), THF, triethylamine, [Pd-
AHCTUNGTERN(GNUN PPh3)2Cl2] (10 mol%), CuI (12 mol%), RT, about 6 h.
established previously that replacement of both chloro
groups by alkynyl functions can be brought about in the
presence of palladium(0) catalysts.[17,18] In this specific case,
the phenyliodo moieties in the BODIPY series B and C had
to be replaced by the less reactive phenylbromo group, and
the two methyl groups in the 1,7-positions had to be omitted
to avoid steric crowding. After some trials, the mono- and
disubstituted alkynylpyrene derivatives PyD and Py2D were
isolated from D as starting material.
Scheme 2. Key: (i) 1-pyrenecarboxaldehyde (1 equiv), toluene, piperidine,
p-TsOH trace, reflux, Dean–Stark; (ii) 1-pyrenecarboxaldehyde
(2.5 equiv), toluene, piperidine, p-TsOH trace, reflux, Dean–Stark; (iii) 4-
toluenecarboxyaldehyde (3 equiv), toluene, piperidine, p-TsOH trace,
For the new vinyl-based molecules, the number of reso-
nances corresponding to the b-pyrrolic protons (labelled 4
reflux,
Dean–Stark;
(iv) 10-methylanthracene-9-carboxaldehyde
(3.5 equiv), toluene, piperidine, p-TsOH trace, reflux, Dean–Stark.
11944
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Chem. Eur. J. 2010, 16, 11942 – 11953

Boron Dipyrromethene Dyes with Extended p-Systems
FULL PAPER
and 4’) is diagnostic of the level of substitution (i.e., mono-
versus di-). For the former species, two doublets of an AB
pattern are found in the range d=6.95 to 6.90 ppm, whereas
a typically narrow AB system is found in the disubstituted
case; this situation holds true for PyC, Py2C, Tol2C and
Ant2C. In each of the vinyl-based molecules, the observed
16 Hz proton–proton coupling constant is in keeping with an
E conformation of the double bonds,[14] a situation consis-
tent with the type of condensation reaction used.[15] For both
PyD and Py2D, the degeneracy of the AB quartet arising
from the b- and g-pyrrolic protons (labelled 3 and 3’) is di-
agnostic of the degree of substitution. For the monosubsti-
tuted PyD, two AB quartets are seen, with that at d=
6.7 ppm having a relatively large coupling constant of 44 Hz
and being representative of the chloro-substituted ring, and
the second, narrow AB quartet seen at d=6.9 ppm corre-
sponding to the alkyne-substituted pyrrole site. It is also in-
formative to note the deshielding of the proton in the a-po-
sition of the pyrene residue caused by its proximity to the
alkyne tether; its integration further confirmed the degree
of substitution of the BODIPY nucleus (Figure 1).
ACHTUNGTRENNUNG
Figure 2. Absorption spectra recorded for the various styryl BODIPY
dyes in MTHF at room temperature: B (black), PeB (grey), Pe2B (open
circles), Py2B (solid dots) and PeBPy (open triangles).
Photophysics of the styryl derivatives B: Absorption spectra
recorded for the various fused dyes in methyltetrahydrofu-
the BODIPY nucleus can be recognised in the far-red
region of the spectrum.[19] Typically, these transitions appear
as a set of overlapping bands with a pronounced 0,0 peak
(lABS) in the region 640–685 nm (Table 1). Notably, the ace-
tylenic function attached through the central meso position
has no effect on the position or strength of this transition.
However, the nature and number of polycycles fitted to the
styryl residues has a strong influence on the position of the
1
0,0 peak (n0,0) and on the half-width (Dn = ) of the individual
2
bands that comprise the relevant absorption envelope
(Table 2); these values have been derived by fitting the
entire absorption transition to a series of Gaussian compo-
Table 1. Photophysical properties recorded for the styryl-bridged dyes in
MTHF at room temperature.
[c]
Cmpd lABS
eMAX
lFLU
[nm][b]
SS
[cmꢁ1
]
FF
tS
kRAD/
[a]
[nm]
A
]
N
[ns][d] 108[e,f]
B
629,
579
646,
596
661,
610
659,
608
81375
(0.41)
110630
(0.67)
128450
(1.08)
121200
(0.82)
641
664
683
675
680
300
420
465
360
400
0.75 4.5
0.76 4.1
0.86 3.6
0.80 3.9
1.65
(1.15)
1.85
(1.75)
2.40
(2.60)
2.05
G
PeB
Pe2B
Py2B
(2.05)
PeBPy 662,
606
124400
(0.97)
0.83 3.75 2.20
(2.15)
[a] Molar absorption coefficient for the lowest-energy absorption band
with the corresponding oscillator strength given in parentheses. [b] Inde-
pendent of excitation wavelength. [c] Direct excitation into the BODIPY
absorption transition at 590 nm. [d] Excitation at 410, 470 and/or 590 nm.
[e] Radiative rate constant given in units of sꢁ1. [f] Given in parentheses
is the radiative rate constant calculated from the Strickler–Berg expres-
sion.
Figure 1. Typical proton NMR spectra (400 MHz) for PyD (top) and
Py2D (bottom) in [D2]dichloromethane at RT. For proton labelling, see
Scheme 3; py refers to the pyrene protons.
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11945

R. Ziessel, A. Harriman, and S. Rihn
Table 2. Parameters derived from spectral curve-fitting of room tempera-
ture data.
perylene, showing that the appended BODIPY core influen-
ces the electronic properties of the perylene chromophore.
The same conclusion is reached for the Py residue. Thus,
p,p* transitions localised on the Py unit are evident as a
series of bands extending between 370 and 450 nm that re-
mains unaffected by replacing one pyrene with perylene.
This absorption envelope is markedly different from that
found for 1-ethynylpyrene and also for the molecular dyad
formed by replacing the fluorine atoms with 1-ethynylpyr-
ene residues.[20] Again, the conclusion is that the polycycle
and BODIPY nucleus are in strong electronic contact.
This impression is strengthened by consideration of the
molar absorption coefficients (eMAX) determined for the
BODIPY-based transition located at long wavelengths
(Table 1). The magnitude of eMAX evolves steadily with in-
creasing molecular length. There is a more pronounced in-
crease in the corresponding oscillator strength (f) for this
transition, caused by the increased half-widths and eMAX
values. Indeed, f increases from 0.41 for B to 1.08 for the
most extended system, Pe2B (Table 1). This is clear evidence
for electronic coupling between the aryl polycycle and the
BODIPY core.
1
Dn =
1
2
Cmpd
n0,0
[cmꢁ1
nFLU
[cmꢁ1
Dn =
SD
hwM
[cmꢁ1
hwL
[cmꢁ1
]
2
[a]
[b]
G
]
G
]
R
]
[cmꢁ1
]
N
]
U
B
15598 540
15440 630
15065 755
15120 675
15605 530
15050 575
14450 700
14805 660
14705 700
0.35 1335
0.38 1365
0.36 1360
0.35 1345
0.36 1365
495
575
680
645
670
PeB
Pe2B
Py2B
PeBPy 15090 700
[a] Half-width of fitted absorption band. [b] Half-width of fitted fluores-
cence band.
nents. This analysis indicates that the absorption transition
is coupled to an averaged low-frequency torsional mode
that increases systematically with increasing molecular size;
for example, this term increases from 456 cmꢁ1 for B to
596 cmꢁ1 for Pe2B. As the total number of peripheral double
bonds increases, there is a progressive shift of n0,0 towards
1
lower energy and a substantial increase in Dn = (Figure 3,
2
Fluorescence is readily observed from these dyes
(Figure 4) and, in each case, the 0,0 transition (lFLU) occurs
at slightly lower energy than that of the corresponding ab-
sorption transition. Stokesꢀ shifts (SS) tend to be rather
small (Table 1),[21] indicating that the geometry does not
change significantly upon excitation. Furthermore, the
change in emission maximum tracks the shifts noted for the
absorption band and lFLU moves progressively towards
lower energy on increasing the effective size of the ancillary
photon collector (Figures 3 and 4). The effect on the emis-
sion maximum is more pronounced, by a factor of about
50%, than that seen for the corresponding absorption band,
indicating that electronic coupling is stronger for the excited
state manifold than for the corresponding ground state. Ad-
ditional changes are seen for the fluorescence quantum
Figure 3. Effect of the change in the number of peripheral double bonds
(n) on the magnitude of the spectral shifts for the 0,0 transitions for both
absorption and emission spectra recorded in MTHF at room tempera-
ture.
Table 2); note that each pyrene and perylene residue con-
tributes seven and nine additional peripheral double bonds,
respectively, to the p-conjugated network. The basic
BODIPY core also displays a strong absorption band over
the 350–450 nm range that seems to be due to a combination
of two or more transitions. Interestingly, the sharp absorp-
tion transition seen at around 360 nm, attributable to the
S0–S3 transition of the BODIPY nucleus, also shifts progres-
sively towards lower energy as the size of the substituent is
increased. The corresponding S0–S2 absorption transition,
seen as a weak shoulder at about 420 nm, becomes obscured
by the more intense p,p* transitions due to the aromatic
polycycle.
The perylene unit present in both PeB and Pe2B displays
strong absorption bands at around 450 and 470 nm, together
with a transition localised at around 360 nm. These two ab-
sorption envelopes are little affected by the number of pery-
lene units present or by the replacement of one perylene
with pyrene. Nonetheless, the bands are red-shifted and
broadened significantly with respect to those of 1-ethynyl-
Figure 4. Fluorescence spectra recorded for the various fused BODIPY
*
dyes in MTHF at room temperature: B ( ), PeB (&), Pe2B (black), Py2B
~
(grey) and PeBPy ( ).
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Boron Dipyrromethene Dyes with Extended p-Systems
FULL PAPER
yields (FF), which tend to increase as the emission maxi-
mum moves towards lower energy, and for the fluorescence
lifetimes (tS), which follow the opposite general trend
(Table 1). For each compound, there is reasonably good
mirror symmetry between the fluorescence and (lowest-
energy) absorption spectral patterns and the excitation spec-
tra match well with the absorption spectra over the entire
wavelength range. This latter observation is consistent with
tronic-vibronic coupling, whereas a value exceeding unity is
taken to indicate strong coupling.
The low-energy region of the absorption spectrum record-
ed for B in MTHF shows fine structure due to a combina-
tion of low- (hwL =456 cmꢁ1) and medium-frequency (hwM =
1430 cmꢁ1) vibrational modes. Similar patterns are seen in
the fluorescence and excitation spectra, suggesting that the
absorption spectral envelope is due to a single transition.
Stepwise cooling of B in MTHF leads to a progressive red
shift of both the absorption and emission maxima, with the
Stokesꢀ shift decreasing steadily as the temperature is low-
all absorbed photons being channelled to the emissive state.
[22]
The radiative rate constants (kRAD
)
increase as the emis-
sion maximum moves towards lower energy and remain in
excellent agreement with the corresponding value calculated
from the Strickler–Berg expression.[23] The increase in kRAD
can be traced to the effect of the substituent on the oscilla-
tor strength. As a result, the anticipated fall in kRAD as the
mean emission wavenumber decreases is offset by the in-
creased oscillator strength and FF tends to increase as the
series evolves. Again, this finding points clearly to coupling
between the termini and the central BODIPY core. It is
also notable that the rate constants (kNR =(1ꢁFF)/tS)) for
nonradiative decay of the BODIPY-based excited singlet
state tend to decrease with increasing size of the substituent,
in apparent contradiction to the energy-gap law.[24] We will
return to this point later.
ered until reaching the glassy matrix, for which SS=
ꢁ1
1
190 cm . The emission band half-width (Dn = ) decreases no-
2
ticeably upon cooling, especially between 295 and 200 K,
and reaches a value of 270 cmꢁ1 at 77 K. The red shifts can
be explained in terms of structural distortions being damp-
ened at low temperature, such that there is an extension of
the conjugation length. Spectral analysis of the low-tempera-
ture data shows that hwM splits into two modes of 1450 and
1200 cmꢁ1, whereas hwL appears as a progression of low-fre-
quency modes of about 310 cmꢁ1. Thermal fluctuations at
higher temperatures lead to the observed broadening of the
spectral bands.[27] It is noticeable that the excitation spectra
show identical effects.
Spectral deconvolution of the total emission envelope re-
corded for B implies a minimum of six Gaussian-shaped
The same type of temperature-dependent spectral analysis
was applied to the functionalised compounds in MTHF. Es-
sentially similar behaviour was observed in each case, al-
though the band half-widths remain larger for the deriva-
tives than for B and, even at 77 K, the spectra were not so
well resolved. This situation is illustrated for Pe2B in
Figure 5. Thus, the emission band half-width observed at
77 K is 520 cmꢁ1, whereas the coupled hwL has a value of
460 cmꢁ1, both parameters being significantly larger than
those determined for B. Furthermore, hwM splits into sepa-
rate vibronic modes of 980 and 1345 cmꢁ1 at lower tempera-
tures, at which thermal broadening is greatly diminished.
These refined values support the conclusions drawn from
analysis of the room temperature spectra.
components, sharing
a common half-width of around
530 cmꢁ1. This analysis further indicates that both medium-
(hwM =1335 cmꢁ1) and low-frequency (hwL =495 cmꢁ1) vi-
brational modes are coupled to nonradiative decay of the
lowest-energy singlet-excited state at room temperature
(Table 2). The monoperylene derivative, PeB, exhibits a
somewhat broader emission profile, but retains both
medium- and low-frequency modes; it is notable that com-
parable half-widths are found for the absorption and emis-
sion spectral profiles for each dye. Spectral broadening, as
1
measured in terms of the derived Dn = term, increases as the
2
overall size of the substituent increases, but hwM tends to
remain fairly constant at around 1360 cmꢁ1. This latter value
may be suggestive of a C=C bending vibration being cou-
pled to nonradiative decay. Interestingly, the coupled hwL
parameter increases with increasing bulk of the substituent
but, throughout the series, is notably higher than that de-
rived from the absorption transition. This vibronic mode is
most likely associated with torsional motion of the styryl
linkage(s), with the magnitude increasing with increasing
electron delocalisation.[25] In contrast, the Huang–Rhys
factor[26] (SD) is insensitive to the size of the appendage
(Table 2). This latter finding suggests to us that SD is essen-
tially determined by the nature of the connection at the
BODIPY unit and this feature does not change throughout
the series. In molecular spectroscopy, the Franck–Condon
overlap factors establish the probability of particular elec-
tronic-vibronic transitions and SD gives a quantitative appre-
ciation of the strength of this interaction.[25] An SD value of
less than 0.1 can be considered as representing weak elec-
Figure 5. Effect of decreasing temperature on the a) excitation and
b) fluorescence spectra recorded for Pe2B in MTHF: temperatures 295,
245, 195, 145 and 95 K.
Chem. Eur. J. 2010, 16, 11942 – 11953
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11947

R. Ziessel, A. Harriman, and S. Rihn
Despite strenuous efforts, we were unable to resolve fluo-
rescence from the polycyclic appendage, although the isolat-
ed polycycle itself is strongly emissive. Indeed, the maxi-
mum FF value for the Pe and Py units in the various multi-
component dyes was estimated to be <10ꢁ5, while the corre-
sponding tS values must be <30 ps. Interrogation of Pe2B in
MTHF by ultrafast transient spectroscopy following direct
excitation of the Pe unit showed the first excited singlet
state on the BODIPY dye to be formed within 300 fs. This
is significantly faster than previously found for related dyes
in which the subunits are interconnected either through the
boron atom or at the meso position.[6] Such behaviour could
result from very fast intramolecular energy transfer, assum-
ing that the Pe unit forms a discrete excited state that is de-
coupled from the BODIPY manifold, or by way of fast in-
ternal conversion, in the event that the molecule functions
as a single chromophore.
intensity of this latter band is reduced by a factor of about
three for Tol2C relative to B. For PyC and Py2C, high-energy
absorption transitions appear at 395 and 430 nm, respective-
ly, which may contain a contribution from the pyrene chro-
mophore. Excitation spectra match absorption spectral pro-
files recorded across the window from 250 to 700 nm. As
above, it was not possible to observe fluorescence that could
be attributed to the pyrene residue for either PyC or Py2C.
As noted above, FF remains high for these ethenylene-
based compounds and tends to increase with increasing
extent of p conjugation (Table 3). The radiative rate con-
stants, being in good accord with those calculated from the
Strickler–Berg expression,[23] show a similar increase along
the series, but the excited-state lifetimes decrease as the
emission maximum shifts towards lower energy. This generic
behaviour is comparable to that described above.
Spectral curve-fitting routines were carried out as de-
scribed earlier to ascertain the properties that are character-
istic of these ethenylene-linked derivatives, and the results
are collected in Table 4. As expected, Tol2C closely resem-
Photophysics of the ethenylene derivatives C: Further stud-
ies were carried out to assess the contribution made by the
connecting benzene ring. Firstly, it was noted that the termi-
nal iodine atoms present in series C have no effect on the
photophysical properties since those recorded for Tol2C
were essentially the same as those recorded for compound
B (Table 3). More significantly, marked blue shifts were ob-
served for both the absorption and fluorescence maxima re-
corded for the monoethenyl derivative PyC, which can be
attributed to the shortened conjugation pathway. Increasing
the extent of p conjugation causes a significant red shift in
both the absorption and emission maxima and elevates the
molar absorption coefficient (and related oscillator
strength). The S0–S2 transitions found for both B and Tol2C
appear as weak, broad absorption bands with maxima at
around 400 nm. There is an additional absorption transition,
S0–S3, at 360 nm that appears as a narrow band. Notably, the
Table 4. Parameters derived from spectral curve-fitting of room tempera-
ture data.
1
Dn =
1
2
Cmpd n0,0
[cmꢁ1
nFLU
[cmꢁ1
Dn =
SD
hwM
hwL
2
[a]
[b]
]
[cmꢁ1
]
]
[cmꢁ1
]
[cmꢁ1
]
[cmꢁ1
]
PyC
Tol2C 15800
Py2C 14485
Ant2C 15630 1855
PyD 16390 1180
Py2D 14425 1005
16765
550
610
740
16195
15495
13870
14695
15245
13810
630
585
620
785
1265
745
0.46 1320
0.31 1300
0.33 1340
0.69 NA
0.39 1275
0.53 1350
580
615
805
680
700
575
[a] Half-width of fitted absorption band. [b] Half-width of fitted fluores-
cence band.
bles B and there are no notable differences between these
two derivatives. The monopyrene analogue, PyC, shows the
highest S factor and requires inclusion of both medium- and
low-frequency vibronic terms, the latter being the lowest of
all the coupled vibrational modes extracted from the spec-
tral fits. Upon extending the degree of p conjugation by
moving to Py2C, there is a pronounced decrease in S and an
increase in hwL. The generic trend seems to agree with that
derived for the B series, such that the bridging ethynylben-
zene residue has little direct effect on the photophysical
properties of the dye. In particular, there is no great change
in the band half-widths and this is interpreted in terms of
the internal flexibility remaining similar for the two sets of
compounds.
To probe this latter effect in more detail, the emission
spectra were recorded in an MTHF glass at 77 K. In each
case, the band maximum undergoes a pronounced red shift
and narrowing on cooling. For example, lFLU moves to
738 nm for Py2C, while the band half-width decreases to
330 cmꢁ1 (Figure 6). The magnitude of the red shift is more
exaggerated than that found for Py2B (i.e., 315 cmꢁ1 com-
pared with 230 cmꢁ1), whereas the relative decrease in the
Table 3. Photophysical properties recorded for the ethenylene- and ethy-
nylene-bridged dyes in MTHF at room temperature.
[c]
Cmpd lABS
eMAX
[mꢁ1 cmꢁ1
lFLU
[nm][b]
SS
FF
tS
kRAD/
[a]
[nm]
]
[cmꢁ1
]
[ns][d] 108[e,f]
PyC
598
93600
(0.62)
101200
(0.76)
121700
(1.05)
123400
(1.13)
83200
(0.69)
99500
(0.76)
618
645
720
714
664
727
540
345
645
935
1360
660
0.64 4.0
0.75 4.05
0.80 3.6
0.038 0.35
0.34 2.1
0.54 3.0
1.60
(1.65)
1.85
(2.00)
2.20
(2.30)
1.10
Tol2C 631
Py2C 691
Ant2C 632
PyD
609
1.60
(1.65)
1.80
Py2D 692
(1.75)
[a] Molar absorption coefficient for the lowest-energy absorption band
with the corresponding oscillator strength given in parentheses. [b] Inde-
pendent of excitation wavelength. [c] Direct excitation into the BODIPY
absorption transition at 590 nm. [d] Excitation at 410, 470 and/or 590 nm.
[e] Radiative rate constant given in units of sꢁ1. [f] Given in parentheses
is the radiative rate constant calculated from the Strickler–Berg expres-
sion.
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Boron Dipyrromethene Dyes with Extended p-Systems
FULL PAPER
sponding bis-ethynyl derivative described by Boens et al.[19]
This latter point is clear indication that the electronic levels
of BODIPY and pyrene are coupled for Py2D. Furthermore,
the individual absorption bands are broadened compared
with those of the other compounds, with a half-width of
about 1000 cmꢁ1, which might indicate variable degrees of p
conjugation for the ground-state species.
Fluorescence is readily observed for Py2D in MTHF at
room temperature, this being fully supported by excitation
spectra, with the emission maximum being located at
727 nm (Figure 7). This corresponds to the relatively large
Figure 6. Fluorescence spectra recorded for PyC in MTHF at room tem-
perature (grey curve) and 77 K (black curve).
half-width remains comparable. As noted above, the
medium-frequency vibration coupled to the excited-state
decay for the C-series is split at 77 K. Thus, for Py2C at 77 K
we find that hwM appears as a pair of vibrations with indi-
vidual values of 1100 and 1350 cmꢁ1. Under the same condi-
tions, hwL takes on a value of 350 cmꢁ1. For all three etheny-
lene derivatives, there is a small but significant increase in
the magnitude of SD on cooling to 77 K, with the derived
values being 0.55, 0.33 and 0.39 for PyC, Tol2C and Py2C, re-
spectively. These perturbed SD factors are a consequence of
the reduced hwL values, which themselves are taken to re-
flect an increase in the degree of p conjugation due to pla-
narisation of the molecule at low temperatures.[28] For Py2B
at 77 K, the SD factor is 0.36 compared to a room tempera-
ture value of 0.34, such that the geometry of this compound
is little affected by cooling.
Figure 7. Fluorescence spectra recorded for Py2D in MTHF at room tem-
perature (grey curve) and 77 K (black curve).
Stokesꢀ shift of 660 cmꢁ1. The FF remains reasonably high,
given the low energy of the emission maximum,[22] and the
lifetime is comparable to those recorded for the other deriv-
atives. As found throughout this work, both medium- and
low-frequency vibrations are coupled to deactivation of the
excited state and the derived hwM term remains closely com-
parable to those found for the other derivatives. At room
temperature, the corresponding hwL is slightly smaller than
those found for the ethenyl and styryl compounds. There is,
however, a marked increase in SD for Py2D relative to the
other derivatives that must arise from the different type of
connection at the BODIPY unit. On cooling to 77 K, the
emission spectrum narrows and becomes better resolved
(Figure 7). The fluorescence maximum shifts to 723 nm, rep-
resenting a blue shift of 40 cmꢁ1, in contrast to the com-
pounds described above for which the glassy matrix favours
a red shift. At 77 K, SD decreases substantially to 0.20 while
Photophysics of the ethynylene derivatives D: To further ex-
plore the effects of conformational disorder on fused
BODIPY dyes, a few compounds were prepared in which an
ethynylene linkage replaced the ethenylene group used for
the other derivatives. Before making any critical compari-
sons, it should be noted that the monopyrene derivative
PyD has a chlorine atom at the 5-position and this might
affect the photophysical properties by virtue of the heavy-
atom effect[29] and/or electronic factors. Indeed, it is notable
that both the absorption and fluorescence bands are rela-
tively broad, whereas FF is somewhat lower than might be
expected (Tables 3 and 4). The key comparator, therefore, is
the bis-pyrene analogue Py2D and we have examined the
properties of this compound with respect to those of Py2B
and Py2C. Absorption and emission spectra were recorded
in MTHF at room temperature and the relevant data are
collected in Tables 3 and 4. For Py2D, the oscillator strength
is modest, whereas the pyrene-based absorption transitions
are clearly visible in the near-UV region. The lowest-energy
absorption band, this being associated with the BODIPY
chromophore, appears at 692 nm, which is considerably red-
shifted with respect to all other spectra reported here; it
represents a red shift of some 80 nm relative to the corre-
1
the individual peaks become slightly narrower (Dn = =
2
685 cmꢁ1); note that the impression of a narrower emission
profile at 77 K is caused by the change in SD rather than
Dn = . Both hwM (=1030 cmꢁ1) and hwL (=435 cmꢁ1) de-
1
2
crease at 77 K relative to room temperature, but there is no
splitting, as observed above. As such, there are important
differences in the behaviour of the ethynylene derivatives
relative to the other families.
Chem. Eur. J. 2010, 16, 11942 – 11953
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11949

R. Ziessel, A. Harriman, and S. Rihn
Comparison of the bis-pyrenyl derivatives: The optical prop-
erties recorded for these extended BODIPY dyes are fully
consistent with strong electronic interactions between the
peripheral aryl polycycle(s) and the central BODIPY core.
To compare the efficiencies of the various spacer groups in
promoting this interaction, we now compare the properties
of the three bis-pyrenyl compounds, namely Py2B, Py2C and
Py2D, measured in MTHF. As a starting point, we contrast
the relevant absorption spectral profiles (Figure 8). It can be
Figure 9. Comparison of fluorescence spectra recorded for Py2B (black),
Py2C (grey) and Py2D (open circles) in MTHF at room temperature. The
spectral profiles are insensitive to changes in excitation wavelength.
BODIPY unit than is found for an ethenylene group. The
kRAD values are set by the confines of the Strickler–Berg ex-
pression,[23] but variations in kNR are more difficult to ex-
plain. This term increases slightly on going from Py2B
(kNR =5.1ꢄ107 sꢁ1) to Py2C (kNR =5.8ꢄ107 sꢁ1), despite the
fall in the HOMO–LUMO energy gap, but undergoes a
threefold increase on going to Py2D (kNR =15.3ꢄ107 sꢁ1). On
cooling to 77 K, Py2D shows a blue-shifted emission spec-
trum, in contrast to the red shifts observed for both Py2B
and Py2C. Furthermore, the glassy matrix has the effect of
splitting the accompanying vibronic modes for Py2B and
Py2C but not for Py2D. Again, Py2D shows a reduced SD at
77 K in contrast to all the other compounds. Thus, the
ethyne linker behaves differently to its ethene counterpart.
Figure 8. Comparison of absorption spectra recorded for Py2B (black),
Py2C (grey) and Py2D (open circles) in MTHF.
seen that, whereas Py2B exhibits what might be considered
to be a conventional spectrum with limited coupling be-
tween the subunits, the spectra recorded for both Py2C and
Py2D show some unusual features. The lowest-energy transi-
tion, being closely associated with the BODIPY chromo-
phore, is red-shifted and broadened, although eMAX remains
similar in each case. For Py2B, the lowest-energy pyrene-
based absorption band appears as a series of sharp peaks be-
tween 430 and 320 nm, which arises from two or more over-
lapping transitions. For Py2C, these individual transitions are
split into two well-resolved sets of bands centred at 430 and
340 nm, respectively. More significant perturbations are seen
for Py2D, for which the pyrene-based chromophore exhibits
a set of absorption transitions in the region from 550 to
440 nm, in addition to a stronger series of optical bands ap-
pearing between 420 and 320 nm. This splitting, and the in-
version of the intensity levels, of the pyrene-based absorp-
tion bands ensures that Py2D is an effective photon collector
over much of the visible range.
An exception to the rule—Ant2C: The ethenylene-bridged
bis-anthracene derivative, Ant2C, is insoluble in nonpolar
solvents, but dissolves slowly in MTHF to give a blue-col-
oured solution. The resultant absorption spectrum is quite
unlike those recorded for the other derivatives and consists
of
a broad, featureless transition centred at 632 nm
(Figure 10). A similar spectrum is observed in dibutyl ether
(Bu2O), this being the least polar solvent that dissolves the
compound. Spectral deconvolution indicates that the absorp-
tion transition is best described in terms of a progression of
three Gaussian-shaped bands of common half-width (full
The fluorescence spectra confirm the reduced HOMO–
LUMO energy gaps for Py2C and Py2D relative to Py2B and
show the relative broadening of the bands for the former
compounds (Figure 9). The pronounced spectral broadening
apparent for Py2D is a consequence of the significant contri-
bution made by hwL. Indeed, for this latter compound it is
noticeable that hwL takes on a particularly small value. Most
of the other derived properties (Tables 2 and 4) remain com-
parable for the various compounds, except for SD, which is
much higher for Py2D. This latter finding can be attributed
to the ethynylene group being more strongly coupled to the
Figure 10. Absorption (black curve) and fluorescence (grey curve) spec-
tra recorded for Ant2C in dibutyl ether.
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Boron Dipyrromethene Dyes with Extended p-Systems
FULL PAPER
width at half maximum (FWHM)=1855 cmꢁ1). Notably, the
half-width greatly exceeds those measured for any other
compound studied here. The maximum of the lowest-energy
Gaussian band lies at 15630 cmꢁ1 in MTHF, and is only
slightly dependent on the nature of the solvent, at least in
weakly polar media. The absorption band has the general
appearance of a charge-transfer transition. In Bu2O, weak
fluorescence can be observed from Ant2C at ambient tem-
perature, with
a broad maximum centred at 698 nm
(Figure 10). Under these conditions, FF was found to be
0.075, although spectral deconvolution is consistent with a
series of relatively broad Gaussian bands (Table 4). Similar,
but weaker, emission is observed in MTHF (lFLU =715 nm),
whereas the fluorescence becomes increasingly weaker and
red-shifted as the polarity of the solvent is increased. The
emission, like the corresponding absorption profile, is best
described as being of charge-transfer character. The fluores-
cence lifetime measured in MTHF is 0.35 ns, where FF is
0.038 (Table 3).
Temperature-dependent fluorescence studies were per-
formed on Ant2C in butyronitrile (BuCN). At room temper-
ature, the weak emission (FF =0.025) is centred at 722 nm,
but this shifts to the red upon cooling. Thus, between 298
and 180 K, the emission maximum moves progressively to-
wards 728 nm, while there is a threefold increase in FF. The
red shift can be attributed to the temperature-induced in-
crease in the static dielectric constant of BuCN, whereas the
increased intensity points to an activated decay route. As
the solvent begins to freeze, there is an abrupt shift towards
the blue region and a pronounced increase in the emission
intensity. During this transformation, the fluorescence pro-
file acquires the features expected for a BODIPY-based
dye, with the maximum appearing at 710 nm for the dye
within the glassy matrix. Excitation spectra recorded over
the same temperature range confirm that the charge-transfer
band, evident at ambient temperature, firstly moves slightly
towards the red and then begins to evolve into a regular
BODIPY-like transition as the solvent starts to freeze. The
charge-transfer character is switched off in the glassy
matrix, no doubt because of the rigidity and nonpolar envi-
ronment, and normal BODIPY behaviour prevails. The ex-
citation and emission maxima are consistent with those re-
corded for the other compounds in a glassy matrix.
Figure 11. Cyclic voltammogram recorded for Ant2C in CH2Cl2 contain-
ing background electrolyte and at a scan rate on 100 mVsꢁ1
.
thracene moiety. Since this process is clearly a one-electron
step, it follows that reduction of the second anthracene unit
is suppressed, presumably because of electronic coupling
along the molecular axis. The first oxidative wave corre-
sponds to two overlapping peaks, each due to addition of
one electron. The overall process is poorly reversible. The
two half-wave potentials, derived from curve-fitting of the
entire peak, are 0.42 and 0.47 V versus Fc/Fc+. We attribute
this process to the successive oxidation of each anthracene
residue, with the splitting of 50 mV being taken as evidence
for long-range electron delocalisation along the molecular
backbone. There is a further two-electron oxidation peak
centred at 0.89 V versus Fc/Fc+, which is assigned to oxida-
tion of the BODIPY core.
This interpretation of the cyclic voltammogram is consis-
tent with the observation of intramolecular charge-transfer
interactions in polar media. The situation, however, differs
from that observed for other BODIPY–anthracene molecu-
lar dyads[11] built around conventional BODIPY units. Here,
the two chromophores remain in electronic isolation but dis-
play fast intramolecular energy transfer. This is a clear indi-
cation of the effectiveness of the ethenylene linkage in pro-
viding electronic coupling between the residues.
Conclusion
The impression here is that Ant2C acts as an intramolecu-
lar charge-transfer system, in marked contrast to the other
dyes, for which p,p* interactions dominate. The charge-
transfer character is lost at 77 K in a glassy matrix because
of the sharp decrease in local polarity. Support for this asser-
tion is provided by consideration of the cyclic voltammo-
gram recorded for Ant2C in CH2Cl2 (Figure 11). Thus, two
peaks are seen upon reductive scans. The first reduction
wave is quasi-reversible and corresponds to a one-electron
half-wave potential of ꢁ1.33 V versus Fc/Fc+ (where Fc
refers to ferrocene). This process is attributed to the addi-
tion of one electron to the BODIPY nucleus. The second
step, having a peak at ꢁ2.03 V versus Fc/Fc+, is irreversible
and is attributed to the addition of one electron to an an-
These highly conjugated, BODIPY-type dyes operate as
quasi-one-dimensional electronic systems, in which extended
delocalisation of the p-electron backbone serves to decrease
the HOMO–LUMO energy gap. There is a corresponding
modification of the oscillator strength for the BODIPY-
based transition found at low energy and of the position and
shape of transitions associated with the appended aryl poly-
cycle. Only the BODIPY-based, lowest-energy excited state
emits and the radiative rate constant is fully consistent with
the Strickler–Berg expression.[22] This relationship takes no
direct account of the nature of the chromophore or of the
conjugation length. However, the nonradiative rate constant
might be expected to be more sensitive to structural changes
Chem. Eur. J. 2010, 16, 11942 – 11953
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11951

R. Ziessel, A. Harriman, and S. Rihn
connected with the effective conjugation length. The latter
is not easily established, especially for compounds of the
type described here, despite its obvious importance for con-
ducting polymers.[30] Recently, a rudimentary definition of
the effective conjugation length (Ap) made use of Equa-
tion (1), in which b is an unspecified constant taken to equal
unity.[31] Although there is a smooth evolution of FF with
the Ap value computed from Equation (1), this is not a
useful definition of the effective conjugation length for mo-
lecular systems. Indeed, there is no global correlation be-
tween Ap and, for example, the emission maximum, al-
though the data derived for the B series do show a smooth
evolution. There is the additional problem that kRAD increas-
es with decreasing nFLU, as expected, but only within a given
series of compounds. On the other hand, the corresponding
nonradiative rate constant, kNR, shows no obvious correla-
tion with either nFLU or Ap. The impression, therefore, is
that each series must be treated separately.
weigh the generic tendency to extend the p-electron conju-
gation length if suitable electron-donating and -withdrawing
substituents are present. This behaviour introduces difficul-
ties for compound design and also raises the possibility that
dark charge-transfer states might contribute to the overall
photophysical properties at slightly elevated temperatures.
The situation is further complicated by the realisation that
BODIPY can function as both an electron acceptor and
donor with appropriate complementary reagents. Thus,
charge-transfer effects have to be taken into account before
designing the molecular structure. Despite this complication,
it is now possible to identify putative BODIPY dyes for use
in the far-red and near-IR regions that will be highly fluores-
cent. The most extreme example of this behaviour, at least
from the new compounds reported here, concerns Pe2B. This
compound fluoresces at 755 nm with a quantum yield of 0.8
and a fluorescence lifetime of 3.6 ns. It does not aggregate
in solution and looks to be a highly attractive addition to
the BODIPY family. No fluorescence can be detected from
the perylene units and the BODIPY-like S1 state is formed
within 0.35 ps after excitation into the perylene chromo-
phore. It is likely that the emission wavelength could be
pushed further towards the near-IR region by extending the
conjugation or omitting the bridging phenylene rings.
kRAD
kNR
¼ ebA
p
ð1Þ
It is instructive at this point to compare the photophysical
properties recorded for this series of fused BODIPYs with
those measured for related BODIPY dyes in which the
polycycle is attached to the boron centre, PyBPy.[20] In this
latter case, the two units remain in electronic isolation and
varying the size of the hydrocarbon has no effect on the op-
tical properties of the BODIPY unit. Rapid electronic
energy transfer occurs from the polycycle to the BODIPY
core,[9] but the fluorescence lifetime and quantum yield of
the BODIPY fluorophore are unperturbed by the nature of
the substituent, provided that an appropriate solvent is used.
On this basis, we can identify the boron linkage as being a
poor electronic mediator. It is also known[2,6] that attaching
the polycycle at the meso position does not perturb the pho-
tophysical properties of the BODIPY dye PymB, in part be-
cause the 1,7-methyl groups enforce a near-perpendicular ar-
rangement of the BODIPY plane and the meso-aryl residue.
It is only when the connection is made via conjugated
groups at the 3,5-linkage that such effects are observed.
Finally, some comment is justified with regard to the fact
that Ant2C behaves differently to the other compounds
studied here. It appears that charge-transfer effects out-
Experimental Section
A full description of the synthesis and characterisation of all new com-
pounds is given as part of the Supporting Information. Methyltetrahydro-
furan was purchased as spectroscopic grade from Aldrich Chemicals Co.;
it was used as received and found to be free from fluorescent impurities.
Absorption spectra were measured with a Hitachi U3310 spectrophotom-
eter and fluorescence spectra were measured with a fully corrected
Jobin–Yvon Fluorolog tau-3 spectrofluorimeter. Fluorescence measure-
ments were made using optically dilute solutions having an absorbance
less than or equal to 0.05 at the excitation wavelength. Fluorescence
quantum yields were measured relative to cresyl violet in methanol,
taking due account of concentration effects, and were corrected for
changes in refractive index. Lifetime measurements were made with a
PTI EasyLife spectrometer using Ludox in distilled water to determine
the instrumental response function to be used for subsequent deconvolu-
tion. Fluorescence was isolated with a high radiance monochromator
after passing through suitable cut-off filters. The resolution of this instru-
ment was approximately 150 ps after deconvolution. Further improve-
ment in temporal resolution was achieved using a high-intensity, ultra-
short laser diode as excitation source and a cooled Hamamatsu R3809U-
50 photodetector in conjunction with a Becker and Hickl HFAC-26–01
pre-amplifier, applying time-correlated, single-photon counting methods.
This set-up provided a wide range of excitation wavelengths and reduced
the temporal resolution to about 30 ps. Low-temperature studies were
performed with an Oxford Instruments Optistat DN cryostat; all solu-
tions were purged with dried nitrogen before undertaking the measure-
ments. Molar absorption coefficients were determined by weighing small
samples of solid material using a micro-balance. At least three separate
measurements were made in each case. Oscillator strengths and radiative
lifetimes were calculated from signal-averaged spectral data.
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Chem. Eur. J. 2010, 16, 11942 – 11953

Boron Dipyrromethene Dyes with Extended p-Systems
FULL PAPER
Hochstrasser, M. L. Metzker, K. Burgess, Chem. Eur. J. 2006, 12,
7816–7826.
Acknowledgements
[12] M. Inouye, Y. Hyodo, H. Nakazumi, J. Org. Chem. 1999, 64, 2704–
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Received: April 29, 2010
Published online: September 8, 2010
Chem. Eur. J. 2010, 16, 11942 – 11953
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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11953