Synthesis of a Zn-salen resorcinarene-based cavitand and its fluorescence response to nitro compounds

Article abstract of DOI:10.1080/10610278.2013.852673

The synthesis, spectroscopic characterisation, conformational switching and fluorescence quenching efficiency of a resorcinarene-based cavitand containing Zn-salen (Zn-Cav) are reported. Synthesis of Zn-Cav was accomplished by the condensation of a quinoxaline derivatised with Zn-salen and a resorcinarene-based cavitand containing three quinoxalines. ¹H NMR spectroscopy confirmed that in DMSO, chloroform and acetone Zn-Cav resides in the vase conformation. The molecular geometry of Zn-Cav selectively changes from vase to kite under acidic conditions. Detection by fluorescence quenching of nitro-containing molecules, such as 4-nitrotoluene, 2,4-dinitrotoluene and 2,3-dimethyl-2,3-dinitrobutane was explored by spectrofluorimetry. It was found that the fluorescence of Zn-Cav is efficiently quenched by nitroaromatic compounds.

Full text of DOI:10.1080/10610278.2013.852673

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Synthesis of a Zn-salen resorcinarene-based cavitand
and its fluorescence response to nitro compounds
Yujin Leeab, Hyo Geun Kooab, Seung Pyo Jangab, Alan B. Erdmanna, Samantha S. Vernettia,
Cheal Kimb & Roger G. Harrisona
a Department of Chemistry and Biochemistry, Brigham Young University, Provo, UT 84602,
USA
b Department of Fine Chemistry, Seoul National University of Science & Technology, , Seoul
139-743, Republic of Korea
Published online: 18 Nov 2013.
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To cite this article: Yujin Lee, Hyo Geun Koo, Seung Pyo Jang, Alan B. Erdmann, Samantha S. Vernetti, Cheal Kim & Roger
G. Harrison (2014) Synthesis of a Zn-salen resorcinarene-based cavitand and its fluorescence response to nitro compounds,
Supramolecular Chemistry, 26:3-4, 245-250, DOI: 10.1080/10610278.2013.852673
To link to this article: http://dx.doi.org/10.1080/10610278.2013.852673
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Supramolecular Chemistry, 2014
Vol. 26, Nos. 3–4, 245–250, http://dx.doi.org/10.1080/10610278.2013.852673
Synthesis of a Zn-salen resorcinarene-based cavitand and its fluorescence response to
nitro compounds
Yujin Leea,b, Hyo Geun Kooa,b, Seung Pyo Janga,b, Alan B. Erdmanna, Samantha S. Vernettia,
Cheal Kimb and Roger G. Harrisona*
b
aDepartment of Chemistry and Biochemistry, Brigham Young University, Provo, UT 84602, USA; Department of Fine Chemistry,
Seoul National University of Science & Technology, Seoul 139-743, Republic of Korea
(Received 3 July 2013; accepted 2 October 2013)
The synthesis, spectroscopic characterisation, conformational switching and fluorescence quenching efficiency of a
resorcinarene-based cavitand containing Zn-salen (Zn-Cav) are reported. Synthesis of Zn-Cav was accomplished by the
condensation of a quinoxaline derivatised with Zn-salen and a resorcinarene-based cavitand containing three quinoxalines.
1H NMR spectroscopy confirmed that in DMSO, chloroform and acetone Zn-Cav resides in the vase conformation. The
molecular geometry of Zn-Cav selectively changes from vase to kite under acidic conditions. Detection by fluorescence
quenching of nitro-containing molecules, such as 4-nitrotoluene, 2,4-dinitrotoluene and 2,3-dimethyl-2,3-dinitrobutane was
explored by spectrofluorimetry. It was found that the fluorescence of Zn-Cav is efficiently quenched by nitroaromatic
compounds.
Keywords: molecular recognition; fluorescence quenching; dinitrotoluene
Introduction
due to fluorescence being very sensitive (7). Explosives
containing nitro groups and the volatile additive 2,3-
dimethyl-2,3-dinitrobutane (DMNB) need to be detected
due to their use in bombs and as an explosive additive.
Detecting these by fluorescence sensors would potentially
give a quick response to their presence (8). Zn-salen
complexes have optical properties, such as fluorescence,
that might be useful in sensor materials (9). In fact, their
use in detecting explosive type compounds has been
initiated (10). Cavitands functionalised with fluorescent
molecules might be aided by their molecular recognition
sites in the detection of nitro-containing explosives.
The design of molecular receptors, combining guest
binding and detection, continues to be an important area
of research. Resorcinarene-based cavitands have become
established as one of the classes of molecular receptors.
They have a cavity surrounded by four binding regions,
which may act in cooperative guest binding. They can also
be functionalised with groups that elongate their cavities
and result in enclosed cavities. Their ability to have groups
attached to them has resulted in cavitands with hydrogen
bonding moieties, chiral groups and metal ions (1).
Resorcinarenes have cavities that can be switched
between open and enclosed. Conformational switching can
be induced by changes in temperature (2) and pH (3), with
the kite conformation being preferred at low temperatures
and low pH values. The development of molecular sensors,
receptors and machines is based on the switching between
the vase conformation, with a deep cavity for guest binding
and the kite conformation with a flat surface (4). Nuclear
magnetic resonance spectroscopy is usually used to
monitor the recognition of guest binding to host and
molecular switching between kite and vase (5).
In this work, we present the synthesis and character-
isation of a resorcinarene-based cavitand (Zn-Cav) that
contains one Zn-salen and three other quinoxaline groups.
There is one previous example of a resorcinarene that
contains a Zn-salen moiety; it was successfully used in
catalysis (11). Our molecule is synthesised by the
condensation of a specially prepared Zn-salen quinoxaline
with a resorcinarene that contains three quinoxaline arms.
1H NMR spectra confirm that Zn-Cav is in the vase
conformation in DMSO, chloroform and acetone, as its
methine protons appear around 5.5 ppm. Zn-Cav absorbs
visible light and fluoresces with a high quantum yield. It
has several molecular recognition properties: (1) a cavity
provided from the vase conformation of the cavitand; (2) a
Zn2þ that provides a binding site for Lewis base guest
molecules and (3) a Zn-salen group, which has excellent
Cavitands, with their molecular recognition sites, have
shown the ability to bind a variety of molecules (6).
1
Although H NMR is an excellent technique to monitor
host–guest interactions, it is not feasible for the monitoring
of large numbers of samples. Molecules that show a change
in fluorescence are used extensively to detect molecules,
*Corresponding author. Email: roger_harrison@byu.edu
q 2013 Taylor & Francis

246
Y. Lee et al.
O2N
NO2
H2N
NH2
O2N
NO2
NO2
NO2
NH2
b
c
d
a
N
N
N
N
N
N
N
N
H2N
Cl
Cl
Cl
Cl
HO
OH
HO
OH
1
3
4
5
2
O
O
H2N
NH2
Zn
OH
N
N
O
e
+
+ Zn(CH3COO)2
N
N
N
N
Cl
Cl
Cl
Cl
6
Scheme 1. Synthesis of Zn-salen quinoxaline. (a) Oxalic acid (1.5 equiv.), H2O, HCl, 1158C, 21 h. (b) KNO3, H2SO4, 08C for 1 h and
then at r.t. for 18 h. (c) POCl3, N-N-dimethylaniline, 1108C, 3 h. (d) SnCl2, HCl, EtOH, r.t., 1 h. (e) MeOH, r.t., 18 h.
photophysical properties. It was found that the fluorescence
of Zn-Cav was efficiently quenched by 4-nitrotoluene
(NT), 2,4-dinitrotoluene (DNT) and DMNB.
to form the Zn2þ containing dichloroquinoxaline (6).
Molecule 6 absorbs light at 320 and 461 nm and fluoresces
at 476 nm in DMSO with a quantum yield of 0.51.
1
The H NMR of 6 shows six peaks, with the imine
protons at 9.19 ppm. The hydroxyl proton is not present in
the NMR spectrum and displaced when Zn2þ bonds,
resulting in a neutral molecule. The structure of this
molecule is predicted to be similar to a Zn-salen complex
in the literature (10). In the solid state, the Zn is most likely
positioned a little above the plane of the four coordinating
atoms and is five coordinate, with its fifth coordination site
occupied by O from a DMSO molecule. Thus, the Zn
would be in a pseudosquare pyramidal geometry.
Result and discussion
The synthesis of Zn-Cav required making a Zn-salen
group and bonding it to a resorcinarene with one open
bridging site. The Zn-salen group was prepared by first
synthesising diaminodichloroquinoxaline (Scheme 1). To
make this quinoxaline, nitrodiaminobenzene was reacted
with oxalic acid to form dihyroxynitroquinoxaline, after
which another nitro group was added, the hydroxyl groups
were exchanged for chlorines and the nitro groups were
reduced to amines. With the amino groups next to each
other, the molecule was condensed with two equivalents of
2-hydroxy-3-methylbenzaldehyde and Zn(CH3COO)2
The Zn-salen-containing resorcinarene (Zn-Cav) was
synthesised by bonding 6 to a resorcinarene with three
quinoxaline groups (7) (Scheme 2). The resorcinarene with
three quinoxaline groups was prepared by adding two
O
O
Zn
N
N
O
O
N
Zn
N
N
N
N
O
N
N
N
N
H
N
O
N
N
N
N
N
N
O
O
O
O
HO
O
O
O
O
+
O
O
O
O
OH
a
N
N
H
Cl
Cl
H
H
6
7
Zn-Res
Scheme 2. Synthesis of Zn-Cav. (a) K2CO3, dry DMSO, 508C, 48 h.

Supramolecular Chemistry
247
Figure 1. 1H NMR spectrum of Zn-salen resorcinarene in DMSO. The aromatic region shows protons with 14 different chemical shifts.
The methine proton from the foot comes at 5.7 ppm.
equivalents of dichloroquinoxaline to resorcinarene and
then isolating it from the mixture (12). The addition of Zn-
salen to the resorcinarene went as anticipated and the Zn-
salen group closed the macrocycle. The 1H NMR spectrum
showed a multitude of peaks in the aromatic region
(Figure 1). Zn-Cav has a mirror plane of symmetry
bisecting the Zn and one of the quinoxaline arms opposite
the Zn-salen. This results in 17 different proton resonances,
14 of which are in the aromatic region ranging from 6.5 to
9.5 ppm. Of major interest is the resonance for the methine
proton on the foot of the resorcinarene, because it gives the
conformation of the resorcinarene. This proton comes at
5.69 ppm in DMSO and thus the resorcinarene is in the vase
conformation. Resorcinarenes in the vase form have
methine protons that appear at around 5.5 ppm, whereas
when in the kite form their resonances are upfield at around
4.0 ppm (2b, 13). As with the Zn-salen, Zn-Cav absorbs
visible light and fluoresces at a high quantum yield
(Figure 2, Øem ¼ 0.59). Its absorption bands at 320 and
458 nm and its emission band at 468 nm are all near to
where they were for the Zn-salen and are thus not affected
by the resorcinarene.
acetone, the methine resonances of Zn-Cav come at
around 5.5 ppm and thus Zn-Cav has a vase conformation.
However, acidifying a chloroform solution of Zn-Cav
with trifluoroacetic acid at room temperature led to the
methine peak moving from 5.35 to 4.00 ppm. This vase to
kite conversion most likely arises as a result of the
protonation of the basic quinoxaline N atoms, which when
protonated repel each other due to their positive charge
(12). Unlike in chloroform, lowering the pH of DMSO and
acetone solutions did not cause the methine to change
chemical shift.
Recognising the intense fluorescence of Zn-Cav, we
tested its ability to act as a sensor for nitro-containing
compounds. When a solution of Zn-Cav was exposed to 4-
NT, the Zn-Cav emission peak at 480 nm was drastically
reduced (Figure 3). This fluorescence quenching was not
dependent on solvent and occurred in acetonitrile,
chloroform and DMSO. Other nitro compounds, such as
DMNB, also quenched the fluorescence of Zn-Cav.
Plotting the diminishing fluorescence as a function of
amount of nitro compound and calculating the slope, we
obtained Stern–Volmer constants. The constants for NT
and DNT with Zn-Cav were similar in magnitude and
around 100 M21 (4-NT: 107 M21, 2,4-DNT: 110 M21).
Although Zn-Cav resides in the vase conformation, it
changes to the kite at low pH. In DMSO, chloroform and
Figure 2. (Colour online) Absorption (left) and emission (right) spectra of Zn-Cav. The solvent was DMSO and the excitation was at
400 nm.

248
Y. Lee et al.
500 MHz instrument. Mass spectra were taken on an
Agilent TOF ESI MS instrument in positive mode unless
otherwise stated. Absorbance measurements were taken on
a Hewlett Packard 8453 spectrophotometer. Steady-state
luminescence measurements were taken using a quartz
cuvette with a Photon Technology International (PTI)
Bryte Box fluorometer.
6-Nitro-2,3-dihydroxyquinoxaline (2)
To 30 ml 1:1 H2O/HCl was added 4-nitro-o-phenylene-
diamine (7.480 g, 0.048 mol) and oxalic acid (6.602 g,
0.073 mol). The solution was stirred 21 h under reflux
at 1158C. A grey precipitate formed which then was
filtered and washed in 2 £ 10 ml H2O, 2 £ 10 ml ethanol
and 2 £ 10 ml ether. The precipitate was dried under
vacuum and weighed 10.67 g (94%). 1H NMR (300 MHz,
DMSO-d6, 258C): d 12.36 (1H, s, OH), 12.16 (1H, s, OH),
7.97 (2H, m, ArH), 7.24 (1H, d, J ¼ 8.5, ArH) ppm. 13C
NMR (500 MHz, DMSO-d6, 258C): d 155.6, 155.1, 142.5,
132.1, 126.5, 119.0, 115.9, 110.7 ppm. HRMS (ESI
negative): m/z calcd for C8H5N3O4 2 Hþ: 206.03; found
206.04.
Figure 3. (Colour online) Fluorescence quenching of Zn-Cav
by NT in acetonitrile. NT concentrations from the top spectrum to
the bottom were 0, 0.010, 0.050 and 0.10 M.
The Stern–Volmer constant for DMNB was much lower
and was 4.3 M21. The nitro compounds were also titrated
with 6 and Stern–Volmer constants were calculated. The
constants were similar to those for Zn-Cav and were
108 M21 (4-NT), 203 M21 (2,4-DNT) and 3.2 M21
(DMNB). The binding constants of the free arm are
similar to those of the cavitand, and thus the cavitand does
not enhance fluorescence quenching. This is probably due
to solvent, such as DMSO, occupying the cavity of
Zn-Cav. These constants of Zn-Cav are comparable
to those of other Zn-salen compounds (10). The
fluorescence quenching is a dynamic process, which
most likely involves the nitro compounds bombarding with
the Zn-salen moieties, resulting in energy transfer.
6,7-Dinitro-2,3-dihydroxyquinoxaline (3)
KNO3 (10.67 g, 0.051 mol) was dissolved in 45 ml of
concentrated H2SO4 and 6-nitro-2,3-dihydroxyquinoxa-
line (10.15 g, 0.049 mol) was slowly added to it at 08C
while stirring. The solid dissolved and the solution was
stirred for 1 h. The reaction was then brought to room
temperature and allowed to stir for 18 h. The reaction
solution was then poured into 100 ml of stirred ice water
and a yellow precipitate formed. The precipitate was
filtered and washed 2 £ 20 ml H2O, 2 £ 20 ml ethanol and
2 £ 20 ml ether. The solid was dried under vacuum and
Conclusion
A resorcinarene-based cavitand containing a Zn-salen
group has been synthesised and characterised. Along with
the Zn-salen functional group, the cavitand has three
quinoxalines. In organic solvents, such as DMSO, acetone
and chloroform, the cavitand is in the vase conformation,
but its conformation changes to the kite under acidic
conditions. Due to the Zn-salen arm, the cavitand absorbs
visible light and fluoresces. The fluorescence of the
cavitand is quenched by 4-NT, 2,4-DNT and DMNB and
thus could be beneficial to detect nitro-containing
compounds.
1
weighed 9.89 g (76 %). H NMR (300 MHz, DMSO-d6,
258C): d 12.50 (2H, s, OH), 7.72 (2H, s, ArH) ppm. 13C
NMR (500 MHz, DMSO-d6, 258C): d 155.2, 137.4, 130.0,
112.2 ppm. HRMS (ESI negative): m/z calcd for
C8H4N4O6 2 Hþ: 251.01; found 251.02.
2,3-Dichloro-6,7-dinitroquinoxaline (4)
2,3-Dihydroxy-6,7-dinitroquinoxaline (6.80 g, 0.027 mol)
and 3.0 ml of N,N-dimethylaniline were added to 18 ml
POCl3 and the mixture was brought to reflux at 1108C for
18 h. The red solution was cooled and poured into 150 ml
of stirred ice water, upon which a red precipitate formed.
The precipitate was filtered, washed with 5 £ 30 ml of
water and dried under vacuum. The precipitate was then
extracted eight times in 100 ml of benzene, and the
benzene solution was filtered and evaporated to a red
crystalline solid (5.18 g, 66.4%). 1H NMR (300 MHz,
Experimental
General
All reagents were used directly as supplied. 2,3-
Dichloroquinoxaline (7) and the methyl-footed resorcinar-
ene (2) were prepared following the literature procedures.
NMR spectra were obtained from a Varian 300 or

Supramolecular Chemistry
249
DMSO-d6, 258C): d 9.08 (2H, s, ArH) ppm. 13C NMR
(500 MHz, DMSO-d6, 258C): d 151.4, 143.5, 140.9,
126.3 ppm. HRMS (ESI, ethyl acetate): m/z calcd for
C8H2Cl2N4O4·C4H8O2: 376.00; found 376.34.
(0,832 g, 6.02 mmol) was added and stirring under N2 was
continued for 18 h at room temperature and for 6 h at 508C.
After cooling, the brown solution was added to H2O
(50 ml) and the pH was adjusted to 6–7 by addition of 1 M
HCl. The pink precipitate that formed was isolated by
filtration, washed with H2O (50 ml) and dried under
vacuum. Column chromatograph (200 g of SiO2 in 250 ml
capacity column flask; CH2Cl2/AcOEt 95:5 ! 85:5)
afforded resorcinarenes with four quinoxalines (827 mg),
three quinoxalines (133 mg) and two quinoxalines
(191 mg). The mass spectrum and 1H NMR spectra
matched the reported values for the resorcinarene with
three quinoxalines: Rf (SiO2; CH2Cl2/AcOEt 90:10): 0.15.
1H NMR (500 MHz, DMSO-d6, 258C): d 1.81–1.82 (3H,
m), 1.92–1.93 (6H, m), 2.00–2.04 (3H, m), 4.51–4.53
(4H, m), 4.51–4.53 (1H, m), 5.60–5.61 (2H, m), 5.75 (1H,
m), 6.960 (2H, m), 7.64–47.67 (2H, m), 7.75–7.77 (4H,
m), 7.81–7.83 (2H, m), 8.02–8.03 (2H, m), 8.09–8.11
(2H, m), 8.11 (2H, s), 9.82 (2H, s) ppm. HRMS (ESI): m/z
calcd for C56H38N6O8 þ Hþ: 923.28; found 923.29.
2,3-Dichloro-6,7-diaminoquinoxaline (5)
2,3-Dichloro-6,7-dinitroquinoxaline (2.6 g, 0.0090 mol)
and SnCl2·2H2O (12.56 g, 0.054 mol) were dissolved in
60 ml ethanol and stirred for 30 min, after which 120 drops
of HCl were added from a Pasteur pipet. After the reaction
mixture was stirred for 18 h, it was evaporated until no
more solvent could be pulled off (about 1/10 of total
solution remained) and poured into 50 ml ice water. The
brown precipitate that was formed was filtered, washed in
6 £ 30 ml water and air-dried. It was then extracted with
methanol until no more solid dissolved and the methanol
solution evaporated to dryness to yield 1.44 g (70.0 %). 1H
NMR (300 MHz, DMSO-d6, 258C): d 6.81 (2H, s, ArH),
6.13 (4H, s, NH) ppm. 13C NMR (500 MHz, DMSO-d6,
258C): d 143.5, 138.1, 137.4, 104.0 ppm. HRMS (ESI
negative): m/z calcd for C8H6Cl2N4 2 Hþ: 227.00; found
227.01.
Synthesis of Zn-Cav
To resorcinarene 7 (0.067 g, 0.067 mmol) and 6 (0.036 g,
0.067 mmol) in dry DMSO (2.5 ml), K2CO3 (0.012 g,
0.084 mmol) was added and the mixture stirred under N2 at
508C for 48 h. After addition of H2O to the mixture, an
orange precipitate formed, which was isolated by
filtration, washed with H2O and dried under vacuum to
yield 0.048 g (52%). 1H NMR (500 MHz, DMSO-d6,
258C): d 1.91–1.95 (12H, m, CH3), 2.27 (6H, s, CH3), 5.70
(4H, m, CH), 6.50 (2H, m, ArsalenH), 6.94 (2H, m,
ArquinH), 7.28 (2H, m, ArsalenH), 7.40 (2H, m, ArsalenH),
7.59 (2H, m, ArquinH), 7.60 (2H, m, ArquinH), 7.79–7.81
(2H, m, ArquinH), 7.87–7.88 (2H, m, ArquinH), 7.95 (4H, s,
ArresH), 8.02 (2H, s, ArresH), 8.09 (2H, s, ArresH), 8.15–
8.16 (2H, m, ArquinH), 8.41 (2H, s, ArsalenH), 9.54 (2H, s,
NCH). 13C NMR (500 MHz, DMSO-d6, 258C): d 172.4,
165.2, 152.3, 152.1, 151.7, 142.4, 139.5, 139.4, 138.7,
137.4, 135.4, 134.8, 131.1, 129.9, 129.3, 128.0, 127.8,
125.5, 119.0, 118.3, 113.4, 28.9, 18.9, 18.7, 17.3 ppm.
HRMS (ESI): m/z calcd for C80H52N10O10Zn·8H2O þ Hþ:
1521.40; found 1521.95. Elemental analysis calcd (%) for
C80H52N10O10Zn·C2H6OS·7H2O 1582.95: C 62.22, H
4.58, N 8.85; found C 62.00, H 4.66, N 8.66. (Water and
Zn-salen complex (6)
Compound 6 was synthesised by a procedure similar to
related compounds (14). 2-Hydroxy-3-methylbenzalde-
hyde (0.365 ml, 3.2 mmol) and Zn(OAc)2·2H2O (0.358 g,
1.6 mmol) were added to methanol (40 ml) and the mixture
was stirred for 30 min at room temperature. Then 2,3-
dichloro-6,7-diaminoquinoxaline (0.366 g, 1.6 mmol) was
added to the solution. The solution was stirred for 28 h and
the resulting deep orange precipitate was collected by
filtration, washed 3 £ 20 ml with ice-cold methanol and
1
dried under vacuum. Yield: 0.430 g (50.8%). H NMR
(300 MHz, DMSO-d6, 258C): d 9.20 (2H, s, NCH), 8.48
(2H, s, ArH), 7.34 (2H, m, ArH), 7.26 (2H, m, ArH), 6.46
(2H, m, ArH), 2.20 (6H, s, CH3) ppm. 13C NMR
(500 MHz, DMSO-d6, 258C): d 172.6, 166.4, 144.5, 144.0,
139.9, 135.4, 135.0, 131.1, 118.8, 113.7, 113.4, 17.3 ppm.
HRMS (ESI): m/z calcd for C24H16N4O2Zn þ Hþ: 528.99;
found 529.00.
1
DMSO were verified by H NMR.)
Preparation of resorcinarene with 3 quinoxalines (7)
Resorcinarenes with different numbers of quinoxalines
were obtained by using procedures reported in the
literature and the resorcinarene containing three quinox-
alines was isolated from the mixture (12). To a solution of
resorcinarene-based methyl bowl (1.65 g, 3.01 mmol) in
dry DMSO (40 ml), K2CO3 (0.416 g, 3.01 mmol) and 2,3-
dichloroquinoxaline (1.20 g, 6.02 mmol) were added and
the mixture was stirred for 1 h under N2. More K2CO3
Fluorescence quenching
Absorbance and fluorescence measurements were col-
lected in a 1 cm path length quartz cuvette in DMSO,
chloroform and acetonitrile. Quinine sulfate in 0.1 M
H2SO4 was used as a standard and exited at 400 nm and the
emission monitored at 450 nm (Ø ¼ 0.54) (15, 16).
Solutions of Zn-Cav and 6 were also excited at 400 nm

250
Y. Lee et al.
Lamb, J.D.; Harrison, R.G. Org. Biomol. Chem. 2012, 10,
7392–7401. (c) Hooley, R.J.; Biros, S.M.; Rebek, J., Jr.
Angew. Chem. Int. Ed. 2006, 45, 3517–3519; (d) Purse, B.
W.; Gissot, A.; Rebek, J., Jr. J. Am. Chem. Soc. 2005, 127,
11222–11223.
and monitored at lem ¼ 470–480 nm. The resulting
fluorescence intensities were plotted against solution
absorbance to generate a linear plot to calculate the
quantum yield of the compound in different solvent
environments. Analysis of the normalised fluorescence
intensity (Io/I) as a function of increasing quencher
concentration ([Q]) was well described by the Stern–
Volmer equation, Io/I ¼ 1 þ Ksv[Q], for all titrations.
(7) Pochorovski, I.; Breiten, B.; Schweizer, W.B.; Diederich, F.
Chem. Eur. J. 2010, 16, 12590–12602.
(8) (a) Pramanik, S.; Zheng, C.; Zhang, X.; Emge, T.J.; Li, J.
J. Am. Chem. Soc. 2011, 133, 4153–4155; (b) Sanchez, J.
C.; Trogler, W.C. J. Mater. Chem. 2008, 18, 3143–3156; (c)
Narayanan, A.; Varnavski, O.P.; Swager, T.M.; Goodson, T.
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Supporting Information Available
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NMR spectra and Stern–Volmer plots are available via the
Internet.
Acknowledgement
We acknowledge Brigham Young University for the financial
support and the NSF for instrument funding.
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