Intramolecular excimer formation and delayed fluorescence in sterically constrained pyrene dimers

Article abstract of DOI:10.1002/chem.200601498

The synthesis is described for a series of five molecular dyads comprising pyrene-based terminals covalently linked through a 1,3-disubstituted phenylene spacer. The extent of through-space communication between the pyrene units is modulated by steric int

Full text of DOI:10.1002/chem.200601498

FULL PAPER
DOI: 10.1002/chem.200601498
Intramolecular Excimer Formation and Delayed Fluorescence in Sterically
Constrained Pyrene Dimers
Andrew C. Benniston,* Anthony Harriman,* Sarah L. Howell, Craig A. Sams, and
Yong-Gang Zhi[a]
Abstract: The synthesis is described for
a series of five molecular dyads com-
prising pyrene-based terminals cova-
lently linked through a 1,3-disubstitut-
ed phenylene spacer. The extent of
through-space communication between
the pyrene units is modulated by steric
interactions imposed by bulky moieties
attached at the 6,8-positions of each
pyrene unit. For the control compound,
only hydrogen atoms occupy the 6,8
positions (DP1), whereas the remain-
ing compounds incorporate ethynylene
groups terminated with either triisopro-
pylsilyl (DP2), 1-tert-butylbenzene
(DP3), 2,6-di-tert-butylbenzene (DP4)
pends markedly on the molecular
structure; DP1 is heavily biased in
favour of the excimer and DP4 is en-
riched with monomer fluorescence.
Photophysical properties, including
laser induced and delayed fluorescence
data, are reported for each compound.
Delayed fluorescence occurs by both
intramolecular and bimolecular steps,
but these events take place on different
timescales. The possibility is raised for
using intramolecular triplet–triplet an-
nihilation as a means of molecular
imaging.
or
1-tert-butyl-3,5-dimethylbenzene
(DP5) units. Each compound shows a
mixture of monomer and excimer fluo-
rescence in fluid solution at room tem-
perature, but only monomer emission
in a glassy matrix at 77 K. The ratio of
monomer to excimer fluorescence de-
Keywords: excimers · fluorescence ·
imaging agents · pyrene · steric
hindrance
Introduction
units are usually layered in a staggered fashion so as to
lower repulsive intermolecular forces,[5] whereas the T-
shaped, edge-to-face orientation is particularly abundant in
the stacking of protein sidechains.[6] A common feature of
all p-stacking interactions is the close proximity of the reac-
tants. The onset of p stacking can be explored by using
NMR spectroscopy,[7] while quantum chemical calculations
also provide important insight into the binding motif.[8] It is
rare for p stacking to be the only form of intermolecular as-
sociation, and is often found in conjunction with solvopho-
bicity, charge-transfer effects, hydrogen bonding and/or co-
ordinative bonding. Delineating the actual effects of p stack-
ing can be a difficult problem. With the growing interest in
molecular electronics, and the understanding that p stacks
form good conduits for electronic charge,[9–11] the signifi-
cance of p stacking seems certain to increase.
p Stacking is an integral part of supramolecular chemistry[1]
and provides an invaluable means by which to induce self-
association of aromatic subunits, planar heterocycles and un-
saturated groups.[2] Double-stranded DNA, including the
phenomenon of intercalation, provides the most readily rec-
ognised example of p stacking; indeed, such interactions
play key roles in protein folding.[3] It is also recognised that
p stacking is a major feature of many crystal structures, in-
cluding certain enzymes. The mutual orientation of the
stacking units is of crucial importance in determining the
stability of the self-assembled entity, and the two most
common arrangements, are “face-to-face” and “edge-to-
face” geometries.[4] In the face-to-face orientation, the two
Excimer fluorescence forms the basis of many analytical
probes[12–15] and can also be used to monitor close interac-
tions between aromatic units. The most popular fluorophore
is pyrene,[16] and innumerable examples of inter- and intra-
molecular excimer formation involving pyrene residues have
been reported.[17] The main advantage of this method over
NMR spectroscopy is that very low solute concentrations
can be used, hence favouring intramolecular association.
[a] Dr. A. C. Benniston, Prof. A. Harriman, Dr. S. L. Howell,
Dr. C. A. Sams, Dr. Y.-G. Zhi
Molecular Photonics Laboratory
School of Natural Sciences (Chemistry), Newcastle University
Newcastle upon Tyne, NE1 7RU (UK)
Fax : (+44)191-222-6929
E-mail: anthony.harriman@ncl.ac.uk
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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A
associates with a ground-state molecule. The photon is then
delocalised over the conjugate to show a net change in fluo-
rescence spectral profile and lifetime.[16] The resultant pho-
tophysical properties can be used to derive structural infor-
mation about the conjugate and, in turn, can lead to an im-
proved understanding of p-stacking interactions. In particu-
lar, intramolecular excimer fluorescence is a strong indicator
of the spatial proximity of the relevant subunits given that
such emission occurs solely from folded rotamers. Here, we
examine the role of bulky blocking groups as a means to
control the self-association of pyrene units linked by 1,3-di-
A
p-stacking tendency of the aromatic units, as evidenced by
excimer fluorescence, against the inhibitory effects of the
side chains. Such studies form an essential part of the pro-
tein-folding problem[18] and might lead to the design of next-
generation foldamers[19] that possess predetermined elec-
tronic properties. With respect to these properties, it is well
known[16] that pyrene displays delayed fluorescence by way
of triplet–triplet annihilation. A key feature of such P-type
delayed fluorescence is that the emission lifetime is unusual-
ly long and is set by the timescale for diffusive encounters
between pyrene molecules in the triplet state. This could
form the basis of an advanced type of fluorescence imag-
ing[20] as the slow decay could be used to discriminate this
emission from scattered excitation light and background
fluorescence. As such, an integral part of this research pro-
gramme is to evaluate the sterically congested pyrene
dimers and their ability to undergo intramolecular triplet–
triplet annihilation on a relatively slow timescale.
Results and Discussion
Scheme 1. i) MeOH, H2SO4 reflux, 24 h; ii) LiAlH4, Et2O, reflux 2 h;
iii) NaH, THF, 608C; a,a’-dibromo-m-xylene, reflux, 12 h; iv) Br2, nitro-
Synthesis: Pyrene is used extensively in the field of chemical
sensors[21] and exhibits an intense structured fluorescence
profile in the visible spectral region (lmax =400 nm) and a
fluorescence lifetime of around 120 ns.[16] In fluid solution,
at modest concentrations, the interaction of two pyrene
units to form a face-to-face dimer accounts for the observed
excimer emission centred at lꢀ485 nm. Our devised strat-
egy to control excimer formation involves attaching bulky
groups to the 6,8-positions of each pyrene nucleus
(Scheme 1). The control dyad, DP1, contains no sterically
demanding groups and was used as a reference for intramo-
lecular excimer emission.
The starting point for the synthesis was pyrene-2-carbox-
ylic acid 1, which is readily prepared in a pure form
(Scheme 1). This material was converted to the correspond-
ing ester, 2, by heating the reaction at reflux for 24 h in
methanol containing concentrated H2SO4. Ester 2 was re-
duced to the corresponding alcohol, 3, by using LiAlH4 in
diethyl ether. Deprotonation of 3 with NaH in dry tetrahy-
drofuran (THF), followed by reaction with a,a’-dibromo-m-
xylene, gave the reference compound, DP1, in a respectable
95% yield after workup and purification.
benzene, 608C; v) [PdCl2(PPh3)2], CuI, THF, Et3N , 558C, 24 h; vi) Li-
A
AlH4, Et2O, 25 8C, 1 h.
Inspection of a molecular model of DP1 suggests that in-
corporation of bulky substituents at the 6,8-positions should
hinder face-to-face interactions between the pyrene units.
Selective bromination of 2 using Br2 in nitrobenzene afford-
ed the appropriately functionalised pyrene derivative 4 in
96% yield. This compound was used as the main building
block for attachment of acetylene-based bulky groups 6a–
6d by using standard cross-coupling procedures. Selective
reduction of the ester group within 6a–6d using LiAlH4 af-
forded the desired alcohols, 7a–7d, in good yields. Using the
same procedure as for the synthesis of DP1, alcohols 7a–7d
were reacted with a,a’-dibromo-m-xylene to afford dyads
DP2–DP5. These new compounds were fully characterised
by means of NMR spectroscopy, mass spectrometry and ele-
mental analysis. All of the compounds are reasonably solu-
ble in most organic solvents at ambient temperature.
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Constrained Pyrene Dimers
FULL PAPER
Photophysical properties of the reference compound DP1:
The absorption spectrum recorded for the reference system,
DP1, in cyclohexane is as expected for a non-interacting
pyrene dimer.[16] In particular, the lowest energy absorption
bands appear as a vibrational progression stretching from
l=340 to 300 nm, but on closer inspection the 0,0 transition
corresponds to a very weak band at 365 nm (Figure 1).
uation is found for DP1 in both ethanol and acetonitrile.
Progressively diluting the sample up to a factor of 104 has
no observable effect on the spectral profile and, in particu-
lar, does not shift the balance towards monomer emission.
This finding confirms that the excimer arises from intramo-
lecular association of two pyrene units and is, therefore, a
folded rotamer. In a frozen ethanol glass at 77 K, however,
only monomer fluorescence is observed (Figure 1). This sit-
uation is consistent with excimer formation taking place
after excitation, as opposed to illumination of a ground-state
dimer. In all cases, the corrected excitation spectrum gave a
good match to the absorption spectrum.
Observation of this latter band is important because it indi-
In deoxygenated cyclohexane, fluorescence from pyrene
decays by means of single exponential kinetics with a life-
time of ꢀ120 ns but more complex behaviour was found for
DP1 under the same conditions (Table 1). Examination of
the monomer emission at lꢀ380 nm shows that fluores-
cence decay profiles, IF(t), can be satisfactorily described as
the sum of two exponential components, according to Equa-
tion (1) (Figure 2). Here, A refers to the fractional ampli-
Figure 1. Absorption (black) and fluorescence (light grey) spectra record-
ed for DP1 in cyclohexane at RT and the fluorescence spectrum (dark
grey) recorded in a glassy ethanol matrix at 77 K.
cates that the lowest-energy, singlet-excited state (S1) has
1Lb character. As such, we would expect a modest fluores-
cence quantum yield but a long fluorescence lifetime.[22] The
absorption spectrum is insensitive to changes in solute con-
centration or solvent polarity. Fluorescence spectra recorded
for dilute solutions of DP1 show two regions of interest.
There is a set of partially resolved bands across the l=360–
410 nm region but the spectrum is dominated by a feature-
less band centred at 485 nm (Figure 1). Based on earlier
work with simpler pyrene derivatives,[23] the higher-energy
bands can be assigned to monomer (i.e., S1) fluorescence,
whereas the lower-energy band can be attributed to an exci-
mer. The total fluorescence quantum yield (FF) measured in
deoxygenated cyclohexane at 208C is 0.25 (Table 1); the
ratio of integrated monomer (M) and excimer (E) fluores-
cence spectral profiles resolved from the steady-state spec-
trum (SM/E) is 25-fold in favour of the excimer. The same sit-
Figure 2. Fluorescence decay profiles recorded by time-correlated, single-
photon counting for DP1 in deoxygenated cyclohexane at RT, showing
emission for the monomer (dark grey), excimer (light grey) and the in-
strumental response function (black).
tude of a particular component with a lifetime t; under
these conditions, A1/A2 ꢀ17. Increasing the number of expo-
nential terms did not improve the quality of the fit as
judged by the usual statistical criteria.[24] This is perhaps due
to the phenylene spacer restricting the range of available
conformations. Monitoring the excimer fluorescence at
l=470 nm also requires analysis of dual-exponential kinet-
ics, in line with Equation (2) (Figure 2). In this case, the
signal grows-in after the excitation pulse before decaying.
Within the margin of experimental error, tM1 =tE1 and
tM2 =tE2. Slightly different lifetimes were obtained in other
solvents, but the basic behaviour remained as described
above.
Table 1. Photophysical properties recorded for the various pyrene dimers
in deoxygenated cyclohexane at 208C.
Property
DP1
DP2
DP3
DP4
DP5
FF
0.25
3.5
45
0.04
100
–
0.78
4.4
43
2.5
110
1.9
1.00
55
0.38
2.1
30
1.9
95
2.0
0.42
33
0.88
6.3
17
7.0
90
2.1
0.34
120
0.65
3.4
28
1.8
95
2.0
0.40
45
tM1 [ns]
tM2 [ns]
SM/E
tT [ms]
tION [ms]
FDF
ꢀ
ꢁ
ꢀ
ꢁ
t
tM1
t
tM2
–
–
IMF ðtÞ ¼ A1exp
À
þ A2exp
À
ð1Þ
tDF [ns]
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4667

A. C. Benniston, A. Harriman et al.
ꢀ
ꢁ
ꢀ
ꢁ
t
tE2
t
tE1
IMF ðtÞ ¼ G2exp
À
þ G1exp
À
ð2Þ
Photophysical properties of DP2–DP5: The presence of al-
kynylene substituents on the pyrene units induces a signifi-
cant redshift for the lowest-energy absorption bands
(Figure 3), although the spectral profile remains similar to
Based on the wealth of prior experimental studies pertain-
ing to intramolecular excimer formation,[25] the kinetic data
can be explained in terms of Scheme 2. The shorter lifetime
Scheme 2. Proposed pathways for decay of the monomer (M) and exci-
mer (E) to the ground state (GS). The derived parameters are listed in
Table 2.
(tM1), relative to that recorded for pyrene, reflects the dy-
namics of the diffusional processes needed for self-associa-
tion. This process is reversible and rate constants for the for-
mation (kF) and dissociation (kD) of the excimer can be ex-
tracted from the time-resolved fluorescence data. The
longer lifetime (tM2) refers to the decay of the equilibrated
mixture of monomer and excimer, and can be used to derive
the rate constants for the inherent decay of the monomer
(kM) and excimer (kE). These latter processes comprise both
radiative and non-radiative events. The radiative rate con-
stant for excimer emission (kR(E)) was calculated by iterative
reconstitution of the steady-state spectrum. The derived
values are compiled in Table 2. Notably, triplet formation is
inefficient for DP1, although the triplet lifetime (tT) is quite
long in deoxygenated cyclohexane (Table 1). Despite the ap-
parent simplicity of Scheme 2, and the assumptions that un-
derlie it, several useful conclusions can be derived that
could be instructive in understanding the other systems.
Firstly, formation of the excimer (kF) is reasonably fast but
reversible dissociation (kD) is slow. Secondly, on the assump-
tion that kM can be equated to that for pyrene (kM =8.3”
106 sÀ1), the excimer decays almost exclusively by routes
that do not involve repopulation of the monomer excited
state.
Figure 3. Absorption (black) and fluorescence (light grey) spectra record-
ed for DP2 in cyclohexane at RT and the fluorescence spectrum (dark
grey) recorded in a glassy ethanol matrix at 77 K.
1
that of DP1. For these compounds, the S1 state has La char-
acter; typical properties being fluorescence quantum yields
approaching unity, but relatively short fluorescence life-
times.[22] The position of the 0,0 transition (l00) varies slight-
ly among the compounds (Table 3), which must be due to a
weak electronic effect associated with the substitution pat-
tern. This behaviour indicates a rather subtle effect imposed
by the substituent. The fluorescence spectrum shows a mix-
ture of monomer and excimer bands (Figures 3 and 4), with
both fluorescence and absorption spectra being redshifted
with respect to DP1. Electronic effects also affect the
maxima of the monomer (lM) and excimer (lE) fluorescence
peaks. The absence of any spectral changes upon serial dilu-
tion confirms that excimer fluorescence is the result of intra-
molecular associations between the pyrene units, in both cy-
clohexane and ethanol at ambient temperature. At 77 K,
however, only monomer fluorescence is observed (Figure 3).
Corrected excitation spectra were found to agree with the
corresponding absorption spectra across the near UV
region. The energy difference between the respective
maxima of monomer and excimer emission bands provides a
crude measure of the free energy of stabilisation (DGGS) of
Table 2. Kinetic and thermodynamic properties derived for the various
pyrene dimers in deoxygenated cyclohexane on the basis of Scheme 2.
Table 3. Spectroscopic properties recorded for the various pyrene dimers
in cyclohexane, together with the molar volumes of the substituent
groups.
Property
DP1
DP2
DP3
DP4
DP5
k1 ”108 [sÀ1
]
]
]
2.9
2.7
0.2
97.2
17
0.83
2.4
0.06
0.04
–
1.0
3.5
0.34
0.27
0.70
76.9
3.8
12.5
4.1
0.21
1.5
1.70
1.62
0.80
90.9
20
12.5
3.1
0.05
1.4
kF ”108 [sÀ1
0.96
0.60
88.9
15.7
12.5
1.7
0.06
1.2
0.5
3.44
0.70
94.7
49
12.5
3.2
0.02
1.1
2.5
[b]
DGGS [kJmolÀ1
]
VMOL
kD ”107 [sÀ1
A1 [%]
K
Cmpd
l00 [nm]
lM [nm]
lE [nm]
DP1
DP2
DP3
DP4
DP5
337[a]
392
410
403
414
377
395
422
420
427
471
523
525
514
532
63.3
74.1
55.6
52.1
55.3
–
125.8
90.2
134.5
117.5
kM ”107 [sÀ1
]
]
kE ”107 [sÀ1
a
kR(E) ”107 [sÀ1
]
[a] Refers to the first intense absorption transition. [b] Molar volume of
the substituent in units of cm3 molÀ1
kNR(E) ”107 [sÀ1
]
2.6
1.7
.
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Constrained Pyrene Dimers
FULL PAPER
Figure 4. Comparison of the fluorescence spectra recorded for DP3
(black), DP4 (dark grey) and DP5 (light grey) in deoxygenated cyclohex-
ane at RT.
Figure 5. Fluorescence decay profiles recorded by time-correlated, single-
photon counting for DP4 in deoxygenated cyclohexane at RT, showing
emission for the monomer (dark grey), excimer (light grey) and the in-
strumental response function (black).
the excimer (Table 3). The derived values for DP3–DP5 are
comparable, as might be expected on the basis of their simi-
lar structures. These excimers are subject to far less stabili-
sation than DP1, in which the pyrene nuclei can come into
closer contact. However, DP2 shows a considerably higher
level of stabilisation. In each case the ratio (SM/E) of the
quantum yields for monomer and excimer fluorescence in
the steady-state spectrum is considerably higher than that
for DP1. Indeed, there is a surprisingly large variation in
SM/E that does not appear to be related to the size of the
blocking group, as illustrated by the relative molar volume
[26]
(VMOL
)
(Table 1). We note that DP4 shows little tendency
to form an excimer.
The photophysical properties derived for the pyrene
dimers in deoxygenated cyclohexane are listed in Table 1.
The overall fluorescence quantum yields (FF) vary some-
what throughout the series but remain high due, at least in
part, to the high radiative rate constants inherent to ethyny-
lated pyrene fluorophores.[27] In each case, fluorescence
from the monomer decays through dual-exponential kinet-
ics, in accordance with Equation (1), and the two lifetimes,
tM1 and tM2, are given in Table 1. Fluorescence from the ex-
cimer grows-in with a lifetime comparable to tM1 before de-
caying with a lifetime similar to tM2 (Figure 5). At wave-
lengths at which only monomer fluorescence is detected, the
fractional contribution of the shorter-lived species (A1)
greatly exceeds that (A2 =1ÀA1) of the longer-lived species
(Table 2). However, because tM (8.2 ns) is in the range ex-
pected for an ethynylated pyrene-based 1La state (4<t<
10 ns),[27] it cannot be assumed that tM relates to the time
taken to establish the equilibrium between monomer and
excimer species. Indeed, time-gated fluorescence spectra
show that SM/E decreases steadily with time over the first
few nanoseconds until reaching a constant value.
Figure 6. Differential triplet–triplet absorption spectra recorded after
laser flash excitation of a solution of DP4 in deoxygenated cyclohexane.
state bleaching can be observed at l<400 nm in addition to
A
pronounced absorption in the range 400–600 nm. At modest
laser powers, the signal decays by complex kinetics that can
be approximated as the sum of two exponential components.
The shorter-lived species has a lifetime of ꢀ2 ms in each
case. The longer-lived transient corresponds to a lifetime
(tT) of ꢀ100 ms in the absence of oxygen (Table 1). Addition
of oxygen quenches the entire signal but, in the presence of
iodoethane (10% v/v) the yield of the longer-lived species is
increased, while the shorter-lived species decreases in signif-
icance. At very low laser power, only the longer-lived spe-
cies is observed. On this basis, the longer-lived transient can
A
be assigned to the pyrene-based triplet excited state.[28]
The shorter-lived component (tION), which exhibits a dif-
ferent spectrum to the triplet state (Figure 6), is observed
only at relatively high laser power and must, therefore, be a
consequence of a multi-photon effect. A probable assign-
ment for this species is the pyrene p-radical cation formed
by photo-ionisation,[29] but this possibility was not explored
in detail. In the event that the shorter-lived species is the
result of two-photon ionisation, the second photon must be
Transient differential absorption spectra were recorded
following laser excitation (l=355 nm; full width at half
maximum (fwhm)=4 ns) of solutions of the pyrene dimers
in deoxygenated cyclohexane. The derived spectra show cer-
tain common features (Figure 6). Consequently, ground-
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4669

A. C. Benniston, A. Harriman et al.
absorbed by the excimer as identical behaviour is found for
DP1. The net result of these various processes is to mini-
mise the yield of the triplet excited state.
The time-resolved fluorescence data were analysed ac-
cording to Scheme 2 to expose the individual rate constants
(see the Supporting Information for a detailed description
of the methodology used for data analysis) and the main re-
sults are collected in Table 2. Here, k1 refers to the rate con-
stant for fluorescence quenching, whereas kF and kD, respec-
tively, refer to the rate constants for the formation and re-
versible dissociation of the excimer. These last two values
can be used to calculate the equilibrium constant, K, for the
excimer and the fraction of monomer, a, present in the
equilibrium mixture. It is apparent that kF decreases linearly
with increasing molar volume of the substituent (see the
Supporting Information). Given that kD is essentially inde-
pendent of VMOL, it follows that both K and a are sensitive
to the volume of the substituent. These effects are probably
associated with changes in the diffusion coefficients of the
pyrene units and/or the frequency with which suitable geo-
metries are reached upon collision between units. Across
the series, there is a 13-fold variation in kF, and the results
indicate that the inefficient excimer formation noted for
DP4 arises because of the bulky substituent. The excimer
decays by way of thermal repopulation of S1, thereby giving
Figure 7. Delayed fluorescence spectra recorded at high laser intensity in
^
deoxygenated cyclohexane containing 10% v/v iodoethane; DP2 ( ),
~
*
DP3 ( ), DP4 (&), DP5 ( ).
rise to delayed fluorescence, in addition to radiative (kR(E)
)
Scheme 3. Proposed routes leading to triplet–triplet annihilation and sub-
sequent intramolecular and intermolecular delayed fluorescence.
and non-radiative (kNR(E) pathways. The derived kR(E)
)
values tend toward an average value of 1.3”107 sÀ1 but the
corresponding kNR(E) values show a crude linear correlation
with DGGS (see Supporting Information). This latter trend
serves to control the fluorescence yield and lifetime for the
excimer and it is notable that DP2 forms the most fluores-
cent excimer.
Triplet–triplet annihilation: The triplet yield can be in-
creased dramatically by addition of iodoethane (10% v/v),
although the triplet lifetime is shortened to around 10 ms by
the external heavy-atom effect.[30] Under these conditions,
delayed fluorescence can be observed following exposure to
intense laser pulses (l=355 nm; fwhm=4 ns) in deoxygen-
ated cyclohexane (Figure 7).[31] The resultant spectral profile
is similar to that of the equilibrium mixture of the monomer
and excimer.[32] The yield increases with increasing laser
power and corresponds to a two-photon process (see the
Supporting Information). Under the same conditions, tran-
Figure 8. Decay profiles recorded for the delayed fluorescence observed
with DP4 in deoxygenated cyclohexane using a high laser intensity at
concentrations of 2 mm (dark grey) and 50 mm (black). Note, the signals
have been normalised for clarity of presentation.
A
goes triplet–triplet annihilation (TTA).[33] Therefore, the
source of the delayed fluorescence can be traced to the in-
teraction between two triplet states according to Scheme 3.
Decay of the delayed fluorescence involves biphasic ki-
netics in each case, although the spectral profile does not
change with time (Figure 8). The longer-lived component,
which makes a relatively minor contribution to the total
signal, has a half-life of ꢀ5 ms at the highest available laser
power but this value depends on the initial triplet concentra-
tion. The latter can be varied by dilution or by modulation
of the laser power. Systematic changes in triplet concentra-
tion indicate that the longer-lived component is the result of
a bimolecular process and that the rate constant for diffu-
sional encounter of two triplet states (kTTA
)
is
ꢀ1”109 mÀ1 sÀ1 in deoxygenated cyclohexane at 208C. This
is well below the diffusion-controlled rate limit, suggesting
that many encounters are needed before the appropriate ge-
ometry is found. The kinetics of intermolecular triplet–trip-
let annihilation have been considered in terms of spin-re-
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Constrained Pyrene Dimers
FULL PAPER
striction rules and the mechanism for such delayed fluores-
cence has been resolved,[34] including the observation of
both monomer and excimer emission.[35] We can consider
that these pyrene dimers behave similarly in terms of their
bimolecular interactions. Identical results were obtained for
each compound within the series DP2–DP5. However, it
was not possible to make meaningful measurements for
DP1 because of the poor absorption profile around the
available laser-excitation wavelengths.
The shorter-lived component could be approximated to
first-order kinetics and the derived lifetimes (tDF) are com-
piled in Table 1. There is considerable variation among the
series, in which DP4 displays a particularly long lifetime for
the delayed fluorescence. In contrast, DP2 and DP3 show
tDF values close to those found for excimer emission. These
lifetimes are independent of the initial triplet concentration
and clearly correspond to a unimolecular process. There is a
good correlation between kF, the rate constant for formation
of the excimer, and the inverse of tDF (see Supporting Infor-
mation). The gradient of the resultant linear plot is ꢀ0.3, in-
dicating that TTA is subject to more restrictions than is exci-
mer formation. Prior work[34,35] has established that the rate
of bimolecular TTA is slower than the diffusion-controlled
rate limit, in which the fractional reduction is dependent on
both the solvent and solute. Such behaviour is consistent
with spin-restriction rules.
For the compounds considered here the majority of the
detected delayed fluorescence arises from the unimolecular
pathway, although it should be realised that intramolecular
TTA involves processes other than delayed fluorescence,
such as that denoted by kTT in Scheme 3. The relative quan-
tum yields for delayed fluorescence (FDF) also vary through-
out the series (Table 1) but can be well explained in terms
of the kR(E) and kNR(E) values derived earlier. Therefore,
DP2 gives the highest yield of delayed fluorescence under
standardised conditions and this compound is characterised
by a relatively low kNR(E) value. Furthermore, DP4 shows a
much higher kNR(E) value and a correspondingly low yield of
delayed fluorescence. It should be noted that spin-restriction
rules lower the rate constant for direct formation of an ex-
cited singlet state by means of TTA to around one-ninth of
the diffusion-controlled limit.[34,35]
contribution (a) of monomer present at equilibrium. This
latter term influences the fluorescence spectral profile, par-
ticularly the delayed fluorescence, and makes an important
contribution towards the lifetime of the excimer. Finally, the
role of the ethynylene groups should not be overlooked
given that these cause inversion of the La and Lb excited
states and thereby impose high radiative rate constants.
These rate constants are carried through to the excimer and
there is a surprising consistency in the derived kR(E) values
(Table 2). However, the lifetime of the excimer is controlled
by a combination of reverse population of the S1 state and
non-radiative decay. Whereas kD depends weakly on the
nature of the substituent, there is a more profound variation
in the derived kNR(E) values, which strongly affects the fluo-
rescence yield of the excimer. It is clear that kNR(E) does not
depend on the size of the substituent but does show a linear
1
1
relationship with DGGS
.
The reason why DP2 forms such a stable excimer is un-
clear; it cannot be a simple effect of stereochemistry, as
DP1 is far less sterically constrained, and must involve an
electronic effect. Accordingly, this effect must reflect some
property of the silicon atom. Simple molecular-orbital calcu-
lations made at the AM1 level indicate that the interposed
silicon atom restricts both the HOMO and LUMO to the
ethynylated pyrene residues, whereas more extended deloc-
alisation is inherent to the other derivatives. To what effect
this influences excimer emission is not obvious at present,
but it seems to be a rather fortunate consequence of this
type of blocking group that could be exploited in future pro-
totypes.
Intramolecular triplet–triplet annihilation leads to delayed
fluorescence that persists for unusually long times.[31,33]
There is a clear correlation between tDF and kF, such that
both parameters are set by steric factors. This finding sug-
gests that TTA requires orbital contact between the poly-
cycles. In each case, the spectral profile recorded for delayed
fluorescence corresponds to that characterised by time-
gated emission spectra as belonging to the equilibrated mix-
ture. Except for DP4, this spectrum is dominated by emis-
sion from the excimer. This is a useful property for a fluo-
rescent sensor because it helps to separate excitation and
emission wavelengths and thereby minimise contamination
by scatter. The temporal profile offered by such delayed
fluorescence is a great bonus with respect to avoiding back-
ground fluorescence and scattering effects. As such, it will
be interesting to evaluate next-generation prototypes as a
novel form of fluorescence imaging. Detailed studies are on-
going in our laboratory.
Role of the blocking groups: Seemingly minor variations in
the nature of the blocking groups cause serious perturbation
of the photophysical properties of both the monomer and
excimer and also affect the yield and lifetime of the delayed
fluorescence. Excimer formation is observed in each case
but the steady-state yields vary considerably throughout the
series. This effect can be traced to variations in kF and there
is a direct correlation between kF and the molar volume of
the blocking group (see the Supporting Information).
Indeed, kF varies by a factor of 13 among the series. This
simple relationship is a clear testimony of the need for the
pyrene units to form a p stack to stabilise the excimer. As a
consequence of the steric effect on kF, there is a correspond-
ing modulation of K and, more importantly, the fractional
Experimental Section
Materials: All chemicals were purchased from commercial sources and
were used as received. Solvents for synthesis were dried by using stan-
dard literature methods[36] before being distilled and stored under nitro-
gen over 4 Š molecular sieves. 1H and 13C NMR spectra were recorded
by using a JEOL Lambda 500 MHz or Bruker AVANCE 300 MHz spec-
trometer. Within the NMR data B and P represent Benzene and Pyrene
Chem. Eur. J. 2007, 13, 4665 – 4674
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4671

A. C. Benniston, A. Harriman et al.
respectively. Routine mass spectra and elemental analyses data were ob-
tained by using in-house facilities. The starting materials pyrene-2-car-
boxylic acid (1),[37] 5b,[38] 5c[39] and 5d[40] were prepared by using litera-
ture methods and purified extensively before use.
at 608C for a further 4 h, cooled to RT and left to stand overnight. The
resultant solid was filtered, washed with cold EtOH, then Et2O and dried
in air to give a pale-yellow solid (0.34 g, 96%). EI-MS: m/z: calcd for
C18H10Br2O2: 418; found: 418 [M]+, 387 [MÀOCH3]+, 359
[MÀCO2CH3]+. (Because of the poor solubility of the compound, no
NMR data were collected.)
Methods: Absorption spectra were recorded by using a Hitachi U3310
spectrophotometer. All fluorescence studies were carried out by using a
Yvon-Jobin fluorolog tau-3 spectrometer. Fluorescence spectra were cor-
rected for spectral imperfections by using a standard lamp. Measure-
ments were made by using optically dilute solutions after that had been
deoxygenated by purging with dried N2. Fluorescence quantum yields
were measured with respect to 9,10-dimethylanthracene in deoxygenated
cyclohexane.[41] Corrected excitation spectra were also recorded under
optically dilute conditions. Fluorescence lifetimes were measured by
using time-correlated, single-photon counting conditions following excita-
tion by a laser diode at l=340 nm. After deconvolution of the instru-
ment response function, the temporal resolution of our setup was
ꢀ100 ps. Low-temperature studies were carried out by using an immer-
sion-well Dewar filled with liquid N2.
Preparation of 6,8-di(isopropylsilylprop-1-ynyl)pyrene-2-carboxylic acid
methyl ester (6a): Ester 4 (0.42 g, 1.00 mmol), copper(I) iodide (14 mg,
0.074 mmol) and dichlorobis(triphenylphosphine)palladium(II) (77 mg,
0.11 mmol) were combined in a three-neck flask flushed with nitrogen.
The solids were dissolved in THF (60 mL) containing Et3N(20 mL), and
the solution thoroughly purged with nitrogen for 1 h. Triisopropylsilyl
acetylene 5a (0.82 mL, 3 mmol) was added dropwise through a syringe to
the stirred solution. After the solution had been stirred for 20 min, the
temperature of the reaction mixture was raised to 55–608C and main-
tained for 24 h. The solvent was removed and the residue was extracted
with CH2Cl2 (120 mL), washed with H2O (2”30 mL) and dried over
MgSO4. Following filtration, the volatile component was removed and
the crude material was purified by means of column chromatography on
silica gel by using ethyl acetate/petroleum ether (1:8 then 1:2) as the
eluant to give a white solid (0.62 g, 99%). 1H NMR (300 MHz, CDCl3,
258C): d=8.79 (s, 2H; 1H, 3H), 8.57 (d, J=9.1 Hz, 2H; 5H, 9H), 8.25 (s,
1H; 7H), 8.18 (d, J=9.1 Hz, 2H; 4H, 10H), 4.03 (s, 3H; OMe), 1.18–
1.17 ppm (m, 42H; iPr); 13C NMR (75 MHz, CDCl3, 258C): d=167.62,
135.21, 133.15, 131.47, 129.64, 128.52, 127.04, 126.67, 126.62, 124.53,
119.00, 105.14, 98.29, 52.67, 19.13, 11.94 ppm; EI-MS: m/z: calcd for
C40H52O2Si2: 620; found: 620 [M]+, 589 [M-OCH3]+.
Laser flash photolysis studies were carried out by using an Applied Pho-
tophysics LKS 60 instrument. An excitation beam was generated at
l355 nm by using a frequency-tripled, Nd–YAG laser. The pulse width
was ꢀ4 ns. A high-intensity, pulsed Xe arc lamp was used for the moni-
toring beam, and was kept perpendicular to the excitation beam. The
monitoring beam was passed through a high-radiance monochromator
and was detected with a fast response photo multiplying tube (PMT).
Transient differential absorption spectra were recorded point-by-point
with five separate records being averaged at each wavelength. Kinetic
data were obtained by averaging about 100 individual records collected
at a particular wavelength. Solutions for flash photolysis were prepared
that have an absorbance of ꢀ0.20 at l=355 nm. All solutions were de-
oxygenated prior to the experiments by purging with dried N2.
Preparation of [6,8-di(isopropylsilylprop-1-ynyl)pyren-2-yl]-methanol
(7a): Ester 6a (1.0 g, 1.6 mmol) was dissolved in dry Et2O (80 mL) in
three-neck flask under a nitrogen atmosphere. LiAlH4 (0.1 g, 2.64 mmol)
was added carefully to the stirred solution, and then stirred for a further
1 h. The mixture was added to ethyl acetate (100 mL) containing H2O, to
which dilute HCl was added carefully. The organic phase was washed
with H2O (2”30 mL), dried over MgSO4 and evaporated to give a resi-
due that was purified by means of column chromatography on silica gel
by using ethyl acetate/petroleum ether (1:3) as the eluant to afford a
yellow-green solid (0.94 g, 98%). 1H NMR (300 MHz, CDCl3, 25 8C):
Preparation of pyrene-2-carboxylic acid methyl ester (2): Pyrene-2-car-
boxylic acid, 1 (0.20 g, 0.81 mmol) was dissolved in MeOH (40 mL) con-
taining concentrated H2SO4 (1 mL) and was heated at reflux for 24 h.
The solution was cooled to RT and the solvent was removed to afford a
residue that was redissolved in CH2Cl2 (80 mL), and then washed with
H2O (30 mL), Na2CO3 (30 mL) solution and H2O again. The organic
layer was separated, dried over MgSO4, filtered and evaporated. The con-
centrated residue was purified by means of column chromatography on
silica gel by using petroleum ether/ethyl acetate (1:10) as the eluant to
afford a white solid (0.21 g, 99%). 1H NMR (300 MHz, CDCl3, 258C):
d=8.80 (s, 2H; 1H, 3H, 8.19 (d, J=7.4 Hz, 2H; 6H, 8H), 8.08 (d,
J=9.0 Hz, 2H; 4H, 10H), 8.04 (d, J=9.0 Hz, 2H; 5H, 9H), 8.04 (dd,
J=7.2 Hz, J’=7.2 Hz, 1H; 7H), 4.06 ppm (s, 3H; OMe); 13C N MR
(75 MHz, CDCl3, 258C): d=167.61, 131.90, 131.05, 128.14, 127.74, 127.47,
127.02, 126.96, 125.72, 125.39, 124.45, 52.23 ppm; EI-MS: m/z: calcd for
5
9
7
1
3
d=8.53 (d, J=9.0 Hz, 2H; H, H), 8.20 (s, 1H; H), 8.15 (s, 2H; H, H),
8.11 (d, J=9.0 Hz, 2H; 4H, 10H), 5.09 (s, 2H; OCH2), 1.18–1.17 ppm (m,
42H; SiiPr); 13C NMR (75 MHz, CDCl3, 258C): d=139.52, 133.98,
132.27, 131.54, 128.95, 125.89, 124.37, 123.61, 118.23, 105.29, 97.41, 65.62,
18.86, 11.71 ppm; EI-MS: m/z: calcd for C39H52Si2O: 592; found: 592
[M]+.
General procedure for the preparation of 6b–6d with 6b as a detailed
example
Compound 6b: Ester 4 (0.418 g, 1.0 mmol), copper(I) iodide (11.4 mg,
C18H12O2: 260; found: 260 [M]+, 229 ,
A
0.060 mmol),
dichlorobis(triphenylphosphine)palladium(II)
(56 mg,
Preparation of pyren-2-yl-methanol (3): LiAlH4 (0.4 g, 11.8 mmol) was
suspended in dry Et2O (150 mL) in a three-neck flask under a dry nitro-
gen atmosphere. Ester 2 (2.1 g, 8.07 mmol) was then added as a solid.
After addition, the mixture was stirred for 3 h, and heated at reflux for a
further 2 h, cooled to RT, and poured carefully into ethyl acetate. A few
drops of H2O were added carefully, followed by a larger addition H2O
(50 mL). The aqueous phase was extracted with ethyl acetate (3”80 mL).
The combined organic phase was dried over MgSO4, filtered and evapo-
rated to afford a residue that was purified by means of column chroma-
tography on silica gel by using ethyl acetate/petroleum ether (1:6) as
eluant to give a white solid (1.87 g, 100%). 1H NMR (300 MHz, CDCl3,
258C): d=8.13 (d, J=7.2 Hz, 2H; 6H, 8H), 8.11 (s, 2H; 1H, 3H), 8.04 (d,
J=9.0 Hz, 2H; 5H, 9H), 8.00 (d, J=9.0 Hz, 2H; 4H, 10H), 7.94 (t,
J=7.2 Hz, J’=7.2 Hz, 1H; 7H), 5.08 ppm (s, 2H; OCH2); 13C N MR
(75 MHz, CDCl3, 258C): d=139.07, 131.87, 131.56, 128.08, 127.64, 126.19,
125.42, 125.05, 124.69, 123.63, 66.19 ppm; EI-MS: m/z: calcd for C17H12O:
232; found: 232 [M]+, 215 [MÀOH]+.
0.079 mmol) and Et3N(40 mL) were dissolved in THF (150 mL) in a
three-neck flask under nitrogen. The solution was purged with nitrogen
for 1 h, followed by the addition of 5b (1.24 mL, 3 mmol). The mixture
was stirred for 20 min before the temperature was raised to 558C for
24 h. Removal of the solvent afforded a residue that was extracted into
CH2Cl2 (150 mL), washed with H2O (2”40 mL), and dried over MgSO4.
The crude material, obtained after removal of the CH2Cl2, was purified
by means of column chromatography on silica gel by using ethyl acetate/
petroleum ether (1:20) as the eluant, to afford a yellow-green solid
(0.52 g, 95%). 1H NMR (300 MHz, CDCl3, 258C): d=8.86 (s, 2H; 1H,
3H), 8.67 (d, J=9.1 Hz, 2H; H, H), 8.45 (s, 1H; H), 8.23 (d, J=9.1 Hz,
2H; 4H, 10H), 7.68 (d, J=8.4 Hz, 4H; 3BH, 5BH), 7.48 (d, J=8.4 Hz, 4H;
2BH, 6BH), 4.09 (s, 3H; OMe), 1.38 ppm (s, 18H; tBu); 13C N MR
(75 MHz, CDCl3, 258C): d=167.28, 152.17, 133.94, 132.11, 131.64, 131.14,
128.89, 128.05, 126.55, 126.27, 125.54, 124.24, 120.46, 118.80, 96.12, 87.11,
52.27, 34.94, 31.26 ppm; EI-MS: m/z: calcd for C42H36O2: 572.2715;
found: 572.2710 [M]+.
5
9
7
Preparation of 6,8-dibromopyrene-2-carboxylic acid methyl ester (4):
Ester, 2, (0.22 g, 0.85 mmol) was dissolved in nitrobenzene (10 mL) and
heated to 608C. To this, a solution of bromine (0.22 mL) dissolved in ni-
trobenzene (2 mL) was added dropwise over 1 h. The solution was stirred
Compound 6c: Ester 4 (0.836 g, 2.00 mmol), copper(I) iodide (22.8 mg,
0.120 mmol)
dichlorobis(triphenylphosphine)palladium(II)
(112 mg,
0.160 mmol), THF (100 mL), Et3N(40 mL), 5c (1.07 g, 5 mmol). Column
chromatography: silica gel, ethyl acetate/petroleum ether (1:20). Yield:
4672
www.chemeurj.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 4665 – 4674

Constrained Pyrene Dimers
FULL PAPER
1
1
3
1.25 g, 91%. H NMR (300 MHz, CDCl3, 25 8C): d=8.89 (s, 2H; H, H),
CH2Cl2 (3”50 mL). The combined organic phases were washed with H2O
(2”40 mL) and dried over MgSO4. Filtration and removal of the volatile
component gave the crude material which was purified by means of
column chromatography on silica gel by using ethyl acetate/petroleum
ether (1:8) as the eluant to afford a white solid (0.81 g, 95%). 1H N MR
(300 MHz, CDCl3, 258C): d=8.18 (s, 4H; 1H, 3H), 8.02 (d, J=7.8 Hz,
5
9
7
8.74 (d, J=9.1 Hz, 2H; H, H), 8.54 (s, 1H; H), 8.29 (d, J=9.1 Hz, 2H;
4H, 10H), 7.58 (d, J=1.8 Hz, 4H; 2BH, 6BH), 7.49 (t, J=1.8 Hz, 2H; 4BH),
4.10 (s, 3H; OMe), 1.41 ppm (s, 36H; tBu); 13C NMR (75 MHz, CDCl3,
258C): d=167.39, 151.27, 134.36, 132.24, 131.35, 129.05, 128.18, 126.65,
126.56, 126.08, 124.51, 123.29, 122.43, 119.05, 97.21, 86.37, 52.36, 34.98,
31.45 ppm; EI-MS: m/z: calcd for C50H52O2: 684.3967; found: 684.3933
[M]+.
4H; H, H), 7.99 (s, 8H; H, H, H, 10H), 7.98 (m, 2H; H), 7.52 (s, 1H;
2pH), 7.48 (m, 3H; 4pH, 5pH, 6pH), 5.02 (s, 4H; OCH2Pyr), 4.72 ppm (s,
4H; PhCH2O); 13C NMR (75 MHz, CDCl3, 258C): d=139.05, 136.50,
131.71, 131.54, 128.93, 127.94, 127.68, 127.53, 126.12, 125.33, 125.05,
124.69, 124.50, 73.07, 72.79 ppm; EI-MS: m/z: calcd for C42H30O2: 566;
found: 566 [M]+; elemental analysis calcd (%) for C42H30O2: C 89.02,
H 5.65; found: C 88.44, H 5.28.
6
8
4
5
9
7
Compound 6d: Ester 4 (1.672 g, 4 mmol), copper(I) iodide (45.6 mg,
0.24 mmol),
dichlorobis(triphenylphosphine)palladium(II)
(224 mg,
0.32 mmol), THF (150 mL), Et3N(40 mL), 5d (2.00 g, 10 mmol). Column
chromatography: silica gel, ethyl acetate/petroleum (1:6). Yield: 2.19 g,
87%. 1H NMR (300 MHz, CDCl3, 258C): d=8.84 (s, 2H; 1H, 3H), 8.69
(d, J=9.1 Hz, 2H; 5H, 9H), 8.39 (s, 1H; 7H), 8.22 (d, J=9.1 Hz, 2H; 4H,
10H), 7.19 (s, 4H; 3BH, 5BH), 4.09 (s, 3H; OMe), 2.71 (s, 12H; Me),
1.36 ppm (s, 18H; tBu); 13C NMR (75 MHz, CDCl3, 258C): d=167.37,
151.72, 140.18, 133.88, 131.86, 131.33, 128.98, 128.19, 126.58, 126.48,
124.57, 124.17, 120.25, 119.57, 95.26, 94.18, 52.34, 34.70, 31.29, 21.71 ppm;
EI-MS: m/z: calcd for C46H44O2: 628.3341; found: 628.3329 [M]+.
Compound DP2: Compound 7a (1.20 g, 2.02 mmol), Et2O (500 mL),
NaH (0.072 g, 3 mmol), a,a’-dibromo-m-xylene (0.275 g, 1.01 mmol),
reflux 2 d. Column chromatography: silica gel, CH2Cl2/petroleum ether
(1:4). Yield: 0.68 g, 52%. 1H NMR (300 MHz, CDCl3, 258C): d=8.59 (d,
J=9.1 Hz, 4H; 5H, 9H), 8.26 (s, 2H; 7H), 8.21 (s, 4H; 1H, 3H), 8.17 (d,
J=9.1 Hz, 4H; 4H, 10H), 7.52 (s, 1H; 2pH), 7.43 (s, 3H; 4pH, 5pH, 6pH),
5.02 (s, 4H; OCH2Pyr), 4.74 (s, 4H; OCH2Ph), 1.16–1.15 ppm (m, 84H;
SiiPr); 13C NMR (75 MHz, CDCl3, 258C): d=138.69, 137.02, 133.96,
132.29, 131.45, 129.02, 128.70, 127.34, 127.28, 125.83, 125.22, 124.39,
123.68, 118.17, 105.29, 97.31, 72.68, 72.62, 18.86, 11.68 ppm; EI-MS: m/z:
calcd for C86H110Si4O2: 1286; found: 1286 [M]+; elemental analysis calcd
(%) for C86H110Si4O2: C 80.19, H 8.61; found: C 80.08, H 8.63.
General procedure for the preparation of 7b–7d with 7b as a detailed
example
Compound 7b: Ester 6b (0.35 g, 0.61 mmol) was dissolved in dry Et2O
(150 mL) in a three-neck flask under an atmosphere of nitrogen. To this
solution LiAlH4 (34.8 mg, 0.92 mmol) was added slowly and the mixture
was stirred for 1 h at RT. The mixture was added to wet ethyl acetate. To
this solution, dilute HCl was carefully added. The separated organic
phase was washed with H2O and dried over MgSO4. The residue that re-
mained after filtration and removal of the volatile component, was puri-
fied by means of column chromatography by using ethyl acetate/petrole-
um ether (1:2) as eluant to afford a yellow-green solid (0.31 g, 93%).
1H NMR (300 MHz, CDCl3, 258C): d=8.65 (dd, J=1.4 Hz, J’=9.1 Hz,
2H; 5H, 9 H), 8.39 (d, J=1.0 Hz, 1H; 7H), 8.19 (s, 2H; 1H, 3H), 8.14 (dd,
J=1.4, 9.1 Hz, 2H; 4H, 10H), 7.68 (d, J=8.3 Hz, 4H; 3BH, 5BH), 7.48 (d,
J=8.3 Hz, 4H; 2BH, 6BH), 5.15 (s, 2H; OCH2), 1.31 ppm (s, 18H; tBu);
13C NMR (75 MHz, CDCl3, 258C): d=152.06, 139.44, 133.08, 131.60,
128.63, 125.89, 125.53, 124.55, 124.21, 123.96, 120.56, 118.38, 95.73, 87.40,
65.65, 34.94, 31.27 ppm; EI-MS: m/z: calcd for C41H36O: 544.2766; found:
544.2772 [M]+.
Compound DP3: Compound 7b (0.327 g, 0.6 mmol), NaH (0.056 g,
2 mmol), THF (100 mL), a,a’-dibromo-m-xylene (81.6 mg, 0.3 mmol).
Column chromatography: silica gel, CH2Cl2/petroleum ether (1:4). Yield:
0.26 g, 73%. 1H NMR (300 MHz, CDCl3, 258C): d=8.48 (d, J=9.1 Hz,
4H; 5H, 9H), 8.32 (s, 2H; 7H), 8.10 (s, 4H; 1H, 3H), 7.99 (d, J=9.1 Hz,
4H; 4H, 10H), 7.68 (d, J=8.4 Hz, 8H; 3BH, 5BH), 7.58 (s, 1H; 2PhH), 7.48
(d, J=8.4 Hz, 8H; 2BH, 6BH), 7.42 (s, 3H; 4PH, 5PH, 6PH), 4.95 (s, 4H;
OCH2Pyr), 4.74 (s, 4H; OCH2Ph), 1.39 ppm (s, 36H; tBu); 13C N MR
(75 MHz, CDCl3, 258C): d=151.90, 138.79, 136.86, 132.99, 131.60, 131.55,
131.40, 128.62, 128.57, 127.21, 127.16, 125.68, 125.48, 124.99, 124.40,
123.66, 120.64, 118.20, 95.62, 87.51, 72.66, 72.57, 34.89, 31.26 ppm; EI-
MS: m/z: calcd for C90H78O2: 1191.6; found: 1192 [M]+; elemental analy-
sis calcd (%) for C90H78O2: C 90.72, H 6.60; found: C 91.49, H 6.70.
Compound DP4: Compound 7c (0.74 g, 1.13 mmol), THF (150 mL),
NaH (0.10 g, 4.2 mmol), a,a’-dibromo-m-xylene (0.139 g, 0.512 mmol).
Column chromatography: silica gel, CH2Cl2/petroleum ether (1:4). Yield:
0.40 g, 54%. 1H NMR (300 MHz, CDCl3, 258C): d=8.69 (d, J=9.1 Hz,
4H; 5H, 9H), 8.49 (s, 2H; 7H), 8.25 (s, 4H; 1H, 3H), 8.21 (d, J=9.1 Hz,
4H; 4H, 10H), 7.49 (d, J=1.8 Hz, 8H; 2BH, 6BH), 7.49 (t, J=1.8 Hz, 4H;
4BH), 7.46 (s, 1H, 2pH), 7.44 (d, J=1.9 Hz, 3H; BH), 5.04 (s, 4H;
OCH2Pyr), 4.76 (s, 4H; OCH2Ph), 1.41 ppm (s, 72H; tBu); 13C N MR
(75 MHz, CDCl3, 258C): d=151.19, 138.72, 136.98, 133.40, 131.68, 131,61,
128.73, 127.37, 127.28, 126.06, 125.99, 125.14, 124.66, 123.91, 123.11,
122.60, 121.21, 118.46, 96.72, 86.73, 72.68, 65.76, 34.96, 31.57 ppm; EI-
MS: m/z: calcd for C106H110O2: 1416.0; found: 1420 [M]+; elemental anal-
ysis calcd (%) for C106H110O2: C 89.91, H, 7.83; found: C 89.89, H 8.13.
Compound 7c: Ester 6c (0.85 g, 1.2 mmol), LiAlH4 (70.7 mg, 1.86 mmol),
Et2O (200 mL). Column chromatography: silica gel, ethyl acetate/petrole-
um ether (1:4). Yield: 0.60 g, 74%. 1H NMR (300 MHz, CDCl3, 258C):
d=8.58 (d, J=9.1 Hz, 2H; H, H), 8.48 (s, 1H; H), 8.09 (s, 2H; H, H),
8.06 (d, J=9.1 Hz, 2H; 4H, 10H), 7.63 (d, J=1.8 Hz, 4H; 2BH, 6BH), 7.53
(t, J=1.8 Hz, 2H; 4BH), 5.11 (s, 2H; OCH2), 1.45 ppm (s, 36H; tBu);
13C NMR (75 MHz, CDCl3, 258C): d=151.20, 139.38, 133.28, 131.56,
131.52, 128.57, 126.04, 125.84, 124.46, 124.11, 123.64, 123.13, 122.58,
118.36, 96.73, 86.72, 65.58, 34.95, 31.45 ppm; EI-MS: m/z calcd for
C49H54O: 658 4175; found: 658.4183 [M]+.
5
9
7
1
3
Compound 7d: Ester 6d (0.20 g, 0.32 mmol), LiAlH4 (18.1 mg,
0.480 mmol), Et2O (100 mL). Column chromatography: silica gel, ethyl
acetate/petroleum ether (1:2). Yield 0.19 g, 99%. 1H NMR (300 MHz,
CDCl3, 258C): d=8.65 (d, J=9.0 Hz, 2H; 5H, 9H), 8.34 (s, 1H; 7H), 8.17
(s, 2H; 1H, 3H), 8.14 (d, J=9.1 Hz, 2H; 4H, 10H), 7.19 (s, 4H; 3BH, 5BH),
5.14 (s, 2H; OCH2), 2.71 (s, 12H; Me), 1.36 ppm (s, 18H; tBu); 13C N MR
(75 MHz, CDCl3, 258C): d=151.55, 140.12, 139.45, 132.96, 131.71, 131.28,
128.62, 126.01, 124.75, 124.14, 123.93, 120.38, 119.07, 95.58, 93.73,65.67,
34.68, 31.29, 21.73 ppm; EI-MS: m/z calcd for C45H44O: 600.3392; found:
600.3411 [M]+.
Compound DP5: Compound 7d (0.50 g, 0.83 mmol), THF (150 mL),
NaH (0.072 g, 3 mmol) a,a’-dibromo-m-xylene (0.113 g, 0.416 mmol).
Column chromatography: silica gel, CH2Cl2/petroleum ether (1:4). Yield:
0.42 g, 77%. 1H NMR (300 MHz, CDCl3, 258C): d=8.59 (d, J=9.0 Hz,
4H; 5H, 9H), 8.31 (s, 2H; 7H), 8.16 (s, 4H, 1H, 3H), 7.08 (d, J=9.1 Hz,
2H; 4H, 10H), 7.59 (s, 1H; 2PH), 7.43 (s, 3H; 4PH, 5PH, 6PH), 7.18 (s, 8H;
3BH, 5BH), 5.00 (s, 4H; OCH2Pyr), 4.77 (s, 4H; OCH2Ph), 2.68 (s, 24H;
Me), 1.37 ppm (s, 36H; tBu); 13C NMR (75 MHz, CDCl3, 258C):
d=151.47, 140.91, 138.77, 136.93, 132.92, 131.56, 131.25, 128.68, 128.61,
127.30, 127.24, 125.88, 125.04, 124.66, 124.10, 123.89, 120.44, 118.98, 95.67,
93.67, 72.66, 72.61, 34.67, 31.30, 27.71 ppm; EI-MS: m/z: calcd for
C98H94O2: 1303.8; found: 1304 [M]+; elemental analysis calcd (%) for
C98H94O2: C 90.28, H 7.27; found: C 90.19, H 7.31.
General procedure for the preparation of target compounds with DPP1
as a detailed example
Compound DP1: NaH (0.144 g, 6 mmol) was added to a solution of pyr-
enyl-2-methanol,
3 (0.696 g, 3 mmol) dissolved in dry THF (50 mL)
under nitrogen. The solution was stirred for 1 h at 608C and then cooled
to RT. a,a’-dibromo-m-xylene (0.396 g, 1.5 mmol) was then added to this
solution, which was stirred for a further 2 h, and then heated at reflux
overnight. The reaction mixture was cooled to RT and poured into ethyl
acetate (150 mL), followed by careful addition of H2O (60 mL). The or-
ganic layer was separated, and the aqueous phase was extracted with
Chem. Eur. J. 2007, 13, 4665 – 4674
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4673

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Acknowledgements
We thank EPSRC (EP/D001194) and Newcastle University for financial
support of this work. The EPSRC-sponsored Mass Spectrometry Service
at Swansea is thanked for recording the mass spectra.
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Published online: February 7, 2007
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