Substituent dependent fluorescence response of diazacrown-based PET sensors

Article abstract of DOI:10.1016/j.tet.2008.05.006

The sensing ability of three 1,10-diaza-18-crown-6 based sensors bearing a coumarin fluorophore was studied and compared with the analogous monoaza-18-crown-6 sensor. Coordination experiments between hosts and organic ammonium salts with varying steric demand were studied using fluorescence and ¹H NMR spectroscopy. We have found that the fluorescent signal generated on complexation was greatly influenced by the structure of the sensor. Surprisingly, in two cases, we observed the decrease of fluorescence on complexation, which was attributed to changes in the conformational dynamics of the sensors on complexation.

Full text of DOI:10.1016/j.tet.2008.05.006

Tetrahedron 64 (2008) 6191–6195
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Substituent dependent fluorescence response of diazacrown-based PET sensors
a
b
a
c
b
a,
*
´
´
´
´
´
´
´
Krisztina Nagy , Szabolcs Beni , Zoltan Szakacs , Attila C. Benyei , Bela Noszal , Peter Kele
,
a,
*
´
Andras Kotschy
a
´
´
´
´
´ ´
Institute of Chemistry, Eo¨tvo¨s Lorand University, Pazmany Peter setany 1/A, H-1117 Budapest, Hungary
HAS Research Group for Drugs of Abuse, Department of Pharmaceutical Chemistry, Semmelweis University, Hogyes Endre utca 9, H-1092 Budapest, Hungary
c Department of Chemistry, Laboratory for X-ray Diffraction, University of Debrecen, PO Box 7, H-4010 Debrecen, Hungary
b
}
a r t i c l e i n f o
a b s t r a c t
Article history:
The sensing ability of three 1,10-diaza-18-crown-6 based sensors bearing a coumarin fluorophore was
studied and compared with the analogous monoaza-18-crown-6 sensor. Coordination experiments be-
tween hosts and organic ammonium salts with varying steric demand were studied using fluorescence
and 1H NMR spectroscopy. We have found that the fluorescent signal generated on complexation was
greatly influenced by the structure of the sensor. Surprisingly, in two cases, we observed the decrease of
fluorescence on complexation, which was attributed to changes in the conformational dynamics of the
sensors on complexation.
Received 12 February 2008
Received in revised form 16 April 2008
Accepted 1 May 2008
Available online 3 May 2008
Dedicated to Professor Ka´lma´n Medzih-
radszky on the occasion of his 80th birthday
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
guests. These experiments have pointed out that the conforma-
tional mobility of the donor site’s surroundings has a profound
A large number of today’s fluorescent chemosensory systems
exploit the photoinduced electron transfer (PET) phenomenon.1
Guest binding to the receptor unit changes the redox potential of an
adjacent donor site, which reverses its ability to quench the fluo-
rescence of the fluorophore part resulting in measurable fluores-
cence enhancement (OFF–ON systems). The generality and
efficiency of this switching process led to the common use of PET
sensors in numerous fields from simple ion sensing to logic gates.2
The effect of guest binding on the PET process is mostly ratio-
nalized by the guest’s coordinative interaction with the donor site.
There are certain cases for common PET sensors, however, where
the formation of secondary interactions between the guest and the
donor site in the host does not give a satisfactory explanation for
the observed change of fluorescence.3 This suggests that in these
sensors change in the conformational dynamics of the receptor part
might also play a significant role in signal generation, which has
been studied for some unique structures.4 The role of steric per-
turbation and conformational constraints at binding site has rarely
been investigated in detail so far.5
effect on its signaling potential.6 Our continuing efforts at un-
derstanding signal generation in common sensor types to track
down the effects of conformational dynamics on their signal gen-
eration process are presented herein.
2. Results and discussion
Four 18-crown-6 based sensors were selected for the present
study: 13b contains an azacrown host unit and an attached cou-
marin fluorophore, while 2a–c have a 1,10-diazacrown core with
either two coumarin units (2a)7 or pendant coumaryl and benzyl
(2b) or tert-butoxycarbonyl-methyl groups (2c) (Fig. 2). According
to the classical working hypothesis, the electron-donating nitrogen
atoms of the azacrown moieties quench the luminescence of the
attached coumarins as long as they are not perturbed by any
O
nO
In our recent report, we have introduced a new sensor design,
which used the influence of conformational dynamics on the PET
process (Fig. 1).6a These new sensors gave comparable or even
greater signals in the presence of guest molecules with the salient
feature of their increased sensitivity toward the steric demand of
O
O
O
N
n = 1,2
MeO
O
O
* Corresponding authors. Tel.: þ36 1 209 0555x1610; fax: þ36 1 372 2909.
E-mail addresses: kelep@elte.hu (P. Kele), kotschy@elte.hu (A. Kotschy).
Figure. 1. Sensors where signal generation are invoked by changes in the conforma-
tional dynamics of the molecule.
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.05.006

6192
K. Nagy et al. / Tetrahedron 64 (2008) 6191–6195
Table 1
O
O
O
O
O
O
O
O
Stability constants (log K) of complexes (uncertainties in parenthesis are estimated
standard deviations of the last significant unit)
R'
R''-NH3 ClO4
N
N
N
O
1
2a
2b
R
R
3
3a
3d
3e
6.0 (1)
4.62 (3)
5.23 (8)
3.53 (3)
2.75 (8)
2.84 (8)
4.54 (2)
4.53 (3)
5.06 (5)
1
2
n
3a R" = Bu
3b R" = Cy
3c R" = iPr
3d R" = tBu
R =
2a R' = R
2b R' = -Bn
2c R' = -CO2 Bu
and the measured binding constants are collected in Table 1. Since
log K>5 values are accessible only by competition titrations,10 in
experiments including the high-affinity host 1, the host was added
to the corresponding ammonium complex of a medium-strength
binder host (e.g., 2a).
t
t
3e R" = CH2 Bu
MeO
O
O
Figure. 2. The studied PET sensors and guests.
The NMR titrations show that, of the studied sensors, 1 has the
highest affinity toward the ammonium salts, and 2a the lowest. In
the case of 2b, the determined stability constants lie between those
of 1 and 2a, although for ammonium salts of increased steric de-
mand the experimental values are closer to 1. It is more difficult to
establish a trend in the binding strength of the different ammo-
nium salts with the same sensor, although we might state that 3d
has a smaller binding constant in each case than the other salts.
To establish the efficiency of the PET process in the studied
sensors (that also corresponds to the highest available signal on
complexation), the fluorescence enhancements (FEs) observed on
their protonation using excess HBF4 were examined. We compared
the fluorescence of the sensor molecules and their protonated
forms, where the PET process is completely blocked (Table 2).
Sensors bearing a diazacrown moiety have all shown increased
signaling potential, compared to 1, regardless of the number of the
fluorophore units. The fluorescence enhancements in the 2 series
were 3–4 times higher than that of 1. The lower signaling ability of
1 originates in the less efficient fluorescence quenching by PET in
its non-protonated form (compared to 2a–c). This finding is in
agreement with the earlier observations on phenanthridinyl,11 and
pyrenylmethyl12 substituted mono- and diazacrown systems. The
rationale for this peculiarity might either invoke the self-quenching
of the fluorescence in 2a–c having syn-aligned chromophores or
explain the differences on the basis of restricted conformational
mobility around the donor sites in 2a–c. The syn-alignment of the
coumarin units is unlikely since by measuring the fluorescence of
2a at various concentrations we observed no sign of excimer for-
mation.13 Moreover, the X-ray structure14 of 2a (Fig. 4) also in-
dicates the preferential trans-alignment of the fluorophores in the
solid phase. The molecule has an inversion center, which is mir-
rored by the space group symmetry having half of the molecule in
the asymmetric unit.
secondary interaction. On complexation, hydrogen bonding or
protonation, the redox potential of the donor nitrogen is increased,
weakening its donating capability that leads to an increase in the
fluorescence intensity.
To examine the strength of the complexation between selected
guests and sensors, we conducted a series of 1H NMR spectroscopic
experiments. The stoichiometry of the complexes between
n-butylammonium perchlorate (3a) and 1 or 2a (Fig. 3, inset) was
established as 1:1 by continuous variation method (Job’s plot).8
We also determined the stability constants for selected sensor (1
and 2a,b)–guest (3a,d,e) complexes. Among these complexes, me-
dium-strength binder hosts (2a,b) were studied by direct chemical
shift titrations with the corresponding guest in CH2Cl2/CDCl3/
CD3OD 90/9/1 v/v/v%. Figure 3 shows the representative results9
A
c3a/c2a
4.53
2.04
1.36
0.74
0.23
0.0
ppm
3.9 3.8 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3.0 2.9 2.8
B
Since the conformational mobility of the donor site’s sur-
roundings has a profound effect on its fluorescence quenching, we
presume that restricting the conformational freedom of the crown
moiety statistically increases the occurrence of such conformers,
where the donor site is more efficient in quenching the fluores-
cence. The decreased signaling potential of 1 is the result of the fact
that one bulky substituent causes less constraint on the confor-
mational mobility of the 18-membered ring system than two of
them (as in 2a–c).
Next, we conducted a series of coordination experiments with
different organic ammonium salts including n-butyl, cyclohexyl,
isopropyl, tert-butyl, and neopentylammonium perchlorates (3a–e,
respectively). The ammonium salts were selected on the basis of
3.70
3.69
3.68
3.67
3.66
3.65
3.64
0.03
0.02
0.01
0
0
0.25 0.5 0.75
1
c
3a / (c3a + c2a)
Table 2
0
1
2
3
4
5
Fluorescence enhancement observed on blocking the PET process in 1 and 2a–c by
protonation of the sensor molecules (see Supplementary data for experimental
details)
c3a / c2a
Figure 3. (A) 1H NMR (600 MHz) partial spectral series obtained in the titration of
0.5 mM 2a with 3.8 mM 3a (CH2Cl2/CDCl3/CD3OD 90/9/1, 25 ꢀC). (B) Chemical shift of
crown O–CH2 of 2a upon titration with 3a. Inset: Job’s plot for the same chemical shift
versus guest mole fraction (c2aþc3a¼1 mM).
Sensor
FE
1
2a
2b
2c
38
135
149
169

K. Nagy et al. / Tetrahedron 64 (2008) 6191–6195
6193
conformational control of the electron transfer process, an area
little studied so far in related systems.5,15 Based on the measured
complex stabilities, we can state that the relative amount of com-
plexed sensors for 2a is lower than that for 1 for a given guest,
therefore, the higher signal for 2a cannot originate solely in sec-
ondary host–guest interactions. We presume that guest binding in
2a invokes a more drastic change of the conformational dynamics
of the crown ether unit, and this decreases the fluorescence
quenching ability of the ring nitrogen atom.
In 1, the guest can approach the macrocycle without encoun-
tering steric hindrance of the chromophore, therefore, conforma-
tional changes in this sensor are mainly induced by the crown
ether–guest coordinative interaction.16 Accepting that in its pre-
ferred conformation the fluorophore units in 2a are located on the
opposite sides of the azacrown unit, sterically hindering the ap-
proach of the guest from both sides, it is evident that conforma-
tional changes on guest binding have to be more significant than
that for 1 since the movement of at least one of the coumarins is
a prerequisite for guest capturing. The change in the conforma-
tional characteristics of the macrocycle can lead to a significant
decrease in the efficiency of the PET process and a highly increased
fluorescence signal.6 Superposition of this dynamic effect on the
effect of the secondary interactions (that are similar for both sen-
sors) results in an increased FE.
Figure 4. X-ray structure of 2a.
their similar binding modes but various steric demands. The
measured fluorescence enhancements (FEs) for the different host–
guest combinations are summarized in Figure 5. Comparing sensors
1 and 2a, the measured FE values with guests 3a–e show a similar
pattern, the values in the 2a series being significantly higher.
Bearing in mind that the binding constants for 1 are 2 orders of
magnitude higher than that for 2a, and that the binding of the
ammonium salts proceeds in a similar manner, the increased sig-
naling potential of 2a is striking.
Further evidence of the different measures of guest induced
structural alterations in 1 and 2a is provided by comparing the
changes in the 1H NMR chemical shifts of selected nuclei (Table 3).
Evidently, the guests have a more profound effect on the crown
frame in the case of 2a, which is in agreement with the results of
the fluorescence measurements.
The similar pattern in the enhancement values observed for 1
and 2a can be rationalized on the basis of the proton-donating
abilities (acidity) of guests 3a–d. The order of the FE values for both
sensors correlates with the proton-donating abilities of 3a–d, since
the stronger the proton-donating ability of the ammonium ion is,
the more it depletes the electron density of the donor nitrogen
atom. Although the literature data on the acidity of 3e show some
variation, the measured FE values for 3e in the 1 and 2a series are
most likely in line with the hydrogen-bond donor ability of the
neopentylammonium moiety.
To extend our study, the complex formation between sensors 2b
and 2c and ammonium salts 3a–e was also examined in detail.
Since the signaling potentials of 2b and 2c are similar to 2a (see
Table 2), we expected
a similar complexation behavior. In-
terestingly, upon complexation of ammonium salts 3a–e the fluo-
rescence of 2b and 2c showed a moderate but persistent decrease
(Fig. 5). The observation that such a PET sensor gives a fluorescence
decrease (quenching) upon guest binding is quite unprecedented
and could not be rationalized on the basis of secondary in-
teractions. Consideration of the structural differences between 2b,c
and 2a led to the assumption that in 2b and 2c the ring nitrogen
atom next to the coumarin unit does not participate in the
complexation.
The fact that 2a shows increased FE values points to the exis-
tence of a signal amplification process, which might originate in the
The reason behind this is that the benzylic- and the tert-
butoxycarbonyl-methyl-amine moieties are significantly more ba-
sic than the coumarylmethyl-amine moiety.17 Assuming that guest
binding in 2b and 2c occurs preferentially to the more basic ni-
trogen, this directs guest coordination opposite to the fluorophore’s
side, which leads only to the conformational changes in the mac-
rocycle and no direct secondary interaction between the guest and
the coumarylmethyl nitrogen. This means that the changed redox
properties of the said nitrogen atom are the result of changes in the
conformational dynamics of its environment. The observed
changes in fluorescence therefore stem from the altered confor-
mational dynamics of the sensor. Such spatial arrangements might
become favorable around the coumarylmethyl nitrogen on guest
binding (e.g., an arrangement of atoms similar to that of pyrroli-
dine) that lead to a more efficient fluorescence quenching by PET.6
3a
5
3b
3c
3d
4
3e
3
2
1
0
Table 3
Maximal changes of 1H NMR chemical shifts (Dd) of 1 and 2a in the presence of
5 equiv of guests added (the studied signals are that of N–CH2)
1
2a
2b
2c
Compounds
1
2a
3a
3e
0.052
0.061
0.112
0.228
Figure 5. Enhancement of the fluorescent signal of sensors 1 and 2a–c on binding
ammonium salts 3a–e in 1% MeOH in CH2Cl2.

6194
K. Nagy et al. / Tetrahedron 64 (2008) 6191–6195
The foundation of this reasoning is the hypothesis that the
as a yellow oil (24%). Rf¼0.20 (10% MeOH in CH2Cl2). IR (neat)
guests do differentiate the two nitrogen atoms in 2b and 2c. To
provide experimental support for this, 2c (where the various N-CH2
signals could be differentiated) was titrated with acid and the
process was monitored by 1H NMR. On the addition of acid, the
signal next to the ester unit showed a more significant change than
the signal of the methylene group next to the coumarin unit. It is
evident from this experiment that the protonation in this system
occurs preferentially next to the tert-butoxycarbonyl-methyl moi-
ety, supporting our assumption (for more details see Supplemen-
tary data).
n
¼1610, 1715, 2860, 3059 cmꢁ1
.
1H NMR (CDCl3 vs TMS)
d¼2.72–
2.87 (8H, m, 4ꢂ–NCH2(crown)), 3.46–3.70 (18H, m, 8ꢂCH2(crown)
þ
CH2–), 3.77 (2þ3H, s, CH2–, CH3O–), 6.43 (1H, s, CHC3(coumarin)),
6.70–6.79 (2H, m, CHC6,C8(coumarin)), 7.10–7.30 (5H, m, Ar), 7.69 (1H,
d, J¼8.8 Hz, CHC5(coumarin)). 13C NMR (CDCl3 vs TMS)
¼53.3, 53.5,
d
54.3, 55.5, 56.4, 59.5, 69.4, 69.5, 69.7, 70.5, 100.5, 111.3, 111.9, 112.3,
125.6, 126.9, 128.0, 128.9, 138.6, 153.8, 155.3, 161.4, 162.2. HRMS
calcd for C30H41N2O7: 541.2914 (MþH)þ, found: 541.2953.
4.2.2. Synthesis of 1-[(7-methoxy)-4-ylmethylcoumarin]-1,10-
diaza-18-crown-6
3. Conclusions
A mixture of 1,10-diazacrown-6 (0.300 g, 1.14 mmol), cesium
carbonate (0.441 g, 2.28 mmol), and potassium iodide (0.010 g,
0.06 mmol) in dry THF (20 mL) was heated to reflux and a solution
of 4-(chloromethyl)-7-methoxycoumarin (0.272 g, 1.21 mmol) in
THF (40 mL) was added over 2.5 h. After the addition was complete,
the reaction mixture was refluxed for 18 h. The reaction mixture
was allowed to cool to rt and then it was filtered through a pad of
Celite. The filtrate was concentrated in vacuo. Product was purified
on silica gel using acetone and acetone–triethylamine (10:1 v/v%)
as eluent to give the title compound as a yellow oil, 0.130 g (25%).
In summary, the present studyddirected at comparing the
fluorescent signal generation in different diazacrown-based sen-
sorsdrevealed that the changes in the conformational mobility of
these sensors induced by guest binding have a profound effect on
their signaling. We demonstrated that the diazacrown ether based
sensor with two coumarin fluorophore units gives an increased
fluorescent signal on the complexation of alkylammonium ions
compared with the appropriate monoazacrown ether based sensor.
Very surprisingly, studying the other two similar sensor molecules
we observed signal weakening on sensing, a behavior quite unique
so far. The origin of the altered signaling (i.e., increase in some cases
and decrease in others) was linked to the changes in the sensors’
conformational dynamics on complexation.
Moreover, these results confirmed that effects derived from
conformational dynamics are present in sensors of very simple
design and are not a peculiarity coupled with complex frameworks.
Exploiting the effect of conformational mobility on signal genera-
tion, one might be able to devise sensors that have enhanced
sensitivity toward the steric demand of the guests, a possibility the
investigation of which is underway in our laboratory.
Rf¼0.11 (10% MeOH in CH2Cl2). IR (neat)
n
¼1608, 1713, 2864,
3380 cmꢁ1 1H NMR (CDCl3 vs TMS)
.
d
¼2.71 (4H, t, J¼4.7 Hz,
2ꢂNCH2(crown)), 2.78 (4H, t, J¼5.4 Hz, 2ꢂNCH2(crown)), 3.42–3.60
(17H, m, 8ꢂCH2(crown)þNH–), 3.76 (3H, s, CH3O–), 3.80 (2H, s, CH2–),
6.37 (1H, s, CHC3(coumarin)), 6.69 (1H, d, J¼2.4 Hz, CHC8(coumarin)), 6.69
(1H, dd, J¼2.5, 8.9 Hz, CHC6(coumarin)), 7.74 (1H, d, J¼8.9 Hz,
CHC5(coumarin)). 13C NMR (CDCl3 vs TMS)
69.9, 70.1, 70.7, 100.4, 111.2, 111.8, 112.3, 125.8, 153.9, 155.3, 161.4,
162.1. HRMS calcd for C23H35N2O7: 451.2439 (MþH)þ, found:
451.2446.
d
¼49.4, 53.8, 55.4, 56.3,
4.2.3. Synthesis of 2c
A mixture of 1-[(7-methoxy)-4-ylmethylcoumarin]-1,10-diaza-
18-crown-6 (0.110 g, 0.24 mmol) and triethylamine (0.69 mL) in dry
toluene (3 mL) was heated to reflux and then a solution of tert-butyl
chloroacetate (0.051 g, 0.34 mmol) in toluene (3 mL) was added
dropwise over 3 min. Catalytic amount of KI (10 mg, 0.06 mmol)
was added to the reaction mixture and was refluxed for 18 h. After
cooling to rt, the mixture was filtered and concentrated in vacuo.
The residue was dissolved in CH2Cl2 (10 mL) and washed with
water (2ꢂ10 mL). The organic layer was dried over MgSO4 and
concentrated in vacuo, and then purified on silica gel (acetone then
acetone/Et3N 9:1 v/v%) to give 2c (80 mg, 58%) as a yellow oil.
4. Experimental
4.1. General
Unless otherwise indicated, all starting materials were obtained
from commercial suppliers (Aldrich, Fisher, Merck) and were used
without further purification. Analytical thin-layer chromatography
(TLC) was performed on Polygram SIL G/UV 254 pre-coated plastic
TLC plates with 0.25 mm silica gel from Macherey–Nagel & Co.
Silica gel column chromatography was carried out with Flash silica
gel (0.040–0.063 mm) from Merck. The 1H and 13C NMR spectra
were recorded on a Bruker DRX-250 spectrometer. Chemical shifts
Rf¼0.18 (10% MeOH in CH2Cl2). IR (neat)
n
¼1610, 1717, 2865 cmꢁ1
.
1H NMR (CDCl3 vs TMS)
d
¼1.43 (9H, s, 3ꢂCH3), 2.84 (4H, t, J¼5.5 Hz,
(d
) are presented in parts per million using TMS as internal stan-
2ꢂ–NCH2(crown)), 2.94 (4H, t, J¼5.5 Hz, 2ꢂ–NCH2(crown)), 3.36 (2H, s,
CH2–), 3.52–3.65 (16H, m, 8ꢂCH2(crown)), 3.82 (2H, s, CH2–);, 3.84
(3H, s, CH3O–);, 6.45 (1H, s, CHC3(coumarin)), 6.76–6.84 (2H, m,
CHC6,C8(coumarin)), 7.76 (1H, d, J¼8.5 Hz, CHC5(coumarin)). 13C NMR
dard. Coupling constants (J) are reported in hertz (Hz). Splitting
patterns are designated as s (singlet), d (doublet), t (triplet), m
(multiplet), and dd (doublet doublet). IR spectra were obtained on
a Bruker IFS55 spectrometer on a single-reflection diamond ATR
unit. High-resolution mass spectrometric analyses were performed
on a Bruker MicroTOF-Q equipment using electrospray ionization
by the Laboratory for Mass Spectrometry, Department of Applied
Chemistry, University of Debrecen, Debrecen, Hungary.
(CDCl3 vs TMS)
d
¼28.1, 54.0, 54.4, 55.6, 56.6, 57.0, 69.9, 70.1, 70.6,
70.7, 80.7, 100.6, 111.5, 112.0, 112.4, 125.8, 153.9, 155.5, 161.5, 162.3,
170.9. HRMS calcd for C29H45N2O9: 565.3125 (MþH)þ, found:
565.3141.
4.2.4. Crystallographic characterization of 2a
4.2. Synthesis of sensors 2b and 2c
Compound 2a was prepared by the literature method.7 Single
crystals were grown by very slow evaporation of ethanol solution.
4.2.1. Synthesis of 2b
Colorless block (0.3ꢂ0.26ꢂ0.2 mm) crystals of
C17H21N1O5,
4-(Chloromethyl)-7-methoxycoumarin (0.360 g, 1.60 mmol)
and 1-benzyl-1,10-diaza-18-crown-618 (0.360 g, 1.02 mmol) were
refluxed in CH2Cl2 (20 mL) in the presence of triethylamine
(0.42 mL) for 12 h. The reaction mixture was allowed to cool to rt
and concentrated in vacuo. The product was purified by column
chromatography (SiO2, CH2Cl2/MeOH 50:1 v/v%) to give 0.130 g 2b
M¼319.35, triclinic, a¼9.3744(10) Å, b¼9.851(3) Å, c¼10.528(4) Å,
a
¼64.24(1),
b
¼66.06(1),
g
¼69.81(1), V¼783.3(4) Å3, Z¼2, space
group: Pꢁ1 (no. 2), rcalcd¼1.354 g cmꢁ3. Data were collected at
293(1) K, Enraf Nonius MACH3 diffractometer, Mo K
a radiation
l
¼0.71073 Å,
u
ꢁ2
q
motion, qmax¼25.3ꢀ, 3141 measured, 1587 re-
flections were unique with I>2s(I), decay: 1%. The structure was

K. Nagy et al. / Tetrahedron 64 (2008) 6191–6195
6195
solved using the SIR-92 software19 and refined on F2 using SHELX-
97 program,20 publication material was prepared with the WINGX-
97 suite,21 R(F)¼0.057 and wR(F2)¼0.137 for 2844 reflections, 209
parameters. Residual electron density: 0.19/ꢁ0.16 e/Å3. Non-hy-
drogen atoms were refined anisotropic, while hydrogen atoms
were placed into geometric positions. Orientation of methyl groups
was refined using a riding model. Additional crystallographic in-
formation is provided in the deposited CIF.
4. (a) Cotlet, M.; Masuo, S.; Luo, G.; Hofkens, J.; van der Auweraer, M.; Verhoeven,
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Lindsey, J. S.; Holten, D. J. Am. Chem. Soc. 2004, 126, 2664–2665.
5. (a) Schmittel, M.; Heng-Wei, L. Angew. Chem., Int. Ed. 2007, 46, 893–896; (b)
Baruah, M.; Qin, W.; Valle´e, R. A. L.; Beljonne, D.; Rohand, T.; Dehaen, W.; Boens,
N. Org. Lett. 2005, 7, 4377–4380; (c) McFarland, S. A.; Finney, N. S. J. Am. Chem.
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Acknowledgements
Financial support of the Hugarian Scientific Research Fund
(OTKA F047125, F048739, K73804) and KPI (GVOP-4.2.1-358/2004)
7. Orbulescu, J.; Kele, P.; Kotschy, A.; Leblanc, R. M. J. Mater. Chem. 2005, 30, 3084–
3088.
8. Connors, K. A. Binding Constants; Wiley-Interscience: New York, NY, 1987;
pp 24–28. See Supplementary data for details.
¨
is gratefully acknowledged. A.C.B. is grateful for Oveges Fellowship
from the Hungarian Research and Technology Fund.
9. For more examples see Figures S1 and S2 in Supplementary data.
10. Fielding, L. Tetrahedron 2000, 56, 6151–6170.
Supplementary data
11. Alihodzic, S.; Zinic, M.; Klaic, B.; Kiralj, R.; Kojic-Prodic, B.; Herceg, M.; Cimer-
man, Z. Tetrahedron Lett. 1993, 34, 8345–8348.
Supplementary data available: NMR spectra of compounds,
details of 1H NMR titrations, and fluorescence studies. Supple-
mentary data associated with this article can be found in the online
version at doi:10.1016/j.tet.2008.05.006.
12. Kubo, K.; Sakurai, T. Chem. Lett. 1996, 11, 959–960.
13. Fluorescence measurements in the 10ꢁ7 M to 10ꢁ3 M concentration range
rule out the significance of intramolecular quenching. For details see Supple-
mentary data.
14. Crystallographic data for the structure 2a in this paper have been deposited
with the Cambridge Crystallographic Data Centre as a supplementary publi-
cation number CCDC 654040. Copies of the data can be obtained, free of charge,
on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax: þ44 1223
336033 or e-mail: deposit@ccdc.com.ac.uk].
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