Self-association and nitroaromatic-induced deaggregation of pyrene substituted pyridine amides

Article abstract of DOI:10.1021/ja411672f

The self-assembly features of the bis-pyrene methyl amide functionalized pyridine and benzene "tweezers" 1 and 2 were studied in organic solution and in the solid state. These systems were found to display remarkably different self-association features an

Full text of DOI:10.1021/ja411672f

Article
pubs.acs.org/JACS
Self-Association and Nitroaromatic-Induced Deaggregation of
Pyrene Substituted Pyridine Amides
Sung Kuk Kim,† Jong Min Lim,‡ Tuhin Pradhan,§ Hyo Sung Jung,§ Vincent M. Lynch,†
Jong Seung Kim,*,§ Dongho Kim,*,‡ and Jonathan L. Sessler*,†,‡
†Department of Chemistry, The University of Texas at Austin, 105 East 24th Street, Stop A5300, Austin, Texas 78712-1224, United
States
‡Department of Chemistry, Yonsei University, Seoul 120-749, Korea
§Department of Chemistry, Korea University, Seoul 136-701, Korea
S
* Supporting Information
ABSTRACT: The self-assembly features of the bis-pyrene
methyl amide functionalized pyridine and benzene “tweezers”
1 and 2 were studied in organic solution and in the solid state.
These systems were found to display remarkably different self-
association features and optical properties, which was
rationalized by control experiments using compounds bearing
pyrenemethyl esters, alkyl groups, or a single pyrene
substituent (3−6). As dilute solutions in chloroform, tweezers
1 displays both pyrene monomer and excimer emission
features reflecting intramolecular contacts between the pyrene
subunits. At higher concentrations in chloroform, as well as in the solid state, tweezers 1 self-assembles to form a linear
supramolecular polymer. In contrast, tweezers 2 does not interact in an intermolecular fashion and photoexcitation produces
emission features characteristic of a pyrene monomer. DFT (density functional theory) and TDDFT (time dependent density
functional theory) calculations revealed that the lowest vertical transitions are forbidden and that S1 of 1 is an emissive state. In
contrast to 1 and 2, both pyrene-free control systems 5 and 6 were found to form linearly self-assembled supramolecular arrays in
the solid state, albeit of differing structure. Upon exposure to trinitrobenzene (TNB), the self-assembled structures formed from
1 undergo deaggregation to form TNB complexes. This change is reflected in both an easily discernible color change and a
quenching of the fluorescence emission intensity. Changes in the optical features were also seen in the case of 2. However,
notable differences between these two ostensibly similar systems were seen.
on a central benzene core ring are capable of forming
supramolecular polymers or dendrimers in highly concentrated
solution.4 These findings led us to consider that small variations
in the structure of ostensibly similar molecular tweezers might
have a significant impact on their ability to undergo self-
assembly and to recognize targeted substrates. To test this
hypothesis, we have sought to probe the recognition and self-
assembly features of two structurally related bis-pyrene
molecular tweezers, 1 and 2, that contain central pyridine
and benzene cores, respectively (see Chart 1). Also studied
were control systems 3−6 (Chart 1) that bear pyrenemethyl
esters or alkyl substituents, or contain only a single pyrene
subunit. Compounds 1 and 2 are known compounds.5
However, to the best of our knowledge, their self-assembly
features have yet to be examined. Here, we report detailed
optical, structural, and 1H NMR spectroscopic studies of
compounds 1 and 2 and detail the ability of 1 to form self-
assembled structures in solution and in the solid state. We also
INTRODUCTION
■
Structurally defined cleft-like molecules containing two parallel
subunits capable of interacting with substrates, so-called
molecular tweezers, have received increasing attention over
the years.1 They have attracted interest for use in ion and small
molecule recognition,1a,b,d,f,h in biomimetic chemistry,1e,g and as
components of molecular machines,1c supramolecular poly-
mers, and dendrimers.1i Early investigations by Zimmerman
and co-workers established that rigid molecular tweezers based
on electron rich acridine building blocks were able to bind the
electron deficient guest 2,4,7-trinitrofluorenone within their
2
́
open slot-like cavities. More recently, Martın and co-workers
reported that the molecular tweezers consisting of two
extended tetrathiafulvalene (exTTF) units linked by a flexible
spacer, such as an isophthalate, recognize C60 via electrostatic
interactions involving the exTTF subunits and the relatively
electron deficient fullerene.3 In this case, the tweezers adjust
their conformations so as to interact with the C60 guest via
three different binding modes depending on the choice of
́
solvent. In addition, the Martın group reported that molecular
Received: November 15, 2013
Published: December 11, 2013
tweezers bearing both an electron donor and electron acceptors
© 2013 American Chemical Society
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(≥0.5 mM) in CDCl3 and in the solid state, as inferred from 1H
NMR spectroscopic measurements and a single crystal X-ray
diffraction analysis, respectively.11 These techniques were also
used to study the TNB-triggered disassembly process and the
nature of the resulting complex, 1·TNB.
Chart 1. Structures of Compounds 1−6
RESULTS AND DISCUSSION
■
Compounds 1 and 2 were recently reported by Colquhoun and
co-workers as possible receptors for the recognition of specific
sequences present on linear templates containing various
chemical motifs.5f,h As part of this effort, the single crystal X-
ray structure of compound 1 was solved. As published, the
structure revealed a monomeric unit with nonparallel pyrene
subunits.5e Our own examination of compound 1 led us to
consider that the combined presence of two different types of
aromatic subunits (pyridine and pyrene) and amide linkages
would make it likely that this particular compound would
undergo aggregation under conditions where hydrogen
bonding or aromatic donor−acceptor interactions would be
stabilized. Separate from these considerations, we envisioned
that these same interactions could be modulated via the
addition of electron deficient guests, such as TNB. To test this
hypothesis, we have reprepared compound 1 and reanalyzed its
X-ray structure. We have also prepared control compounds (3−
6), including those containing pyrenemethyl esters, a single
pyrene, or no pyrene at all. As detailed below, in the case of 1,
but not 2, self-assembly takes place in the solid state and at
typical NMR concentrations in CDCl3. Moreover, in accord
with our design expectations, exposure to test NAEs, such as
TNB, leads to deaggregation and a considerable change in both
the supramolecular structure and the fluorescence intensity.
The synthesis of compounds 1−6 is summarized in Schemes
S1−S4 in the Supporting Information. Briefly, these systems
were prepared via the simple reaction of pyrene methylamine
hydrochloride, 1-pyrenemethanol, or propylamine with the
requisite acid chloride in the presence of triethylamine or
pyridine.
A single crystal of compound 1 suitable for single crystal X-
ray diffraction analysis was obtained by subjecting 1 to slow
evaporation from a mixture of chloroform and methanol. The
resulting structure is shown in Figure 1. This structure reveals
the presence of an ordered polymeric arrangement within the
crystal lattice. This extended array is characterized by both close
intermolecular amide−pyridine separations and short pyrene−
pyridine and pyrene−pyrene contacts. Presumably, these close
contacts reflect underlying stabilizing effects, including NH−N
hydrogen bonds and π−π interactions. The net result is that in
the solid state the ground state pyrene−pyrene contacts are
exclusively intermolecular, rather than intramolecular, in nature.
This is not true for the excited state interactions in solution
(vide infra).
report the structural and optical changes that result from the
recognition of trinitrobenzene (TNB) and trinitrotoluene
(TNT).
Both 1 and 2 contain a pair of linking amide groups, a
functionality that has been exploited extensively in the
construction of self-assembled materials stabilized via inter-
molecular hydrogen bonding interactions.6 However, as
detailed further below, “tweezers” 1 and 2 were found to
display remarkably different self-assembly and fluorescence
emission features. These differences are further manifest in
their interactions with electron deficient nitroaromatic
explosives (NAEs). For instance, tweezers 1 self-associates to
produce a linear oligomer both in chloroform solution and in
the solid state; it also undergoes analyte-induced deaggregation
when exposed to trinitrobenzene (TNB).7−9 In contrast, no
evidence of intermolecular interaction was found in the case of
the phenyl-linked bis-pyrene tweezers compound 2. Nor was it
found to interact strongly with NAEs. The different behavior
seen for compounds 1 and 2 is rationalized on the basis of a
control experiment involving the pyrene-free systems 5 and 6.
Both compounds 5 and 6, which contain pyridine and phenyl
cores, respectively, undergo self-assembly to form linear
supramolecular polymers albeit ones that differ in terms of
their specific structures.
As detailed below, it was found that the small structural
variation between tweezers 1 and 2 has a large impact on their
optical properties, as well as their self-assembly characteristics,
and their ability to bind electron deficient guests. For instance,
whereas no evidence of appreciable excimer formation is seen
in the phenyl linked tweezers 2, the corresponding pyridine-
linked congener 1 gives rise to characteristic pyrene excimer
emission features in the fluorescence spectrum (i.e., λmax = 470
nm) when subject to photoexcitation in chloroform at
concentrations of ≤20 μM.10a On the basis of control studies
involving 3−6 (vide infra), as well as dilution and solvent
polarity studies, this excimer emission is thought to be
intramolecular, as opposed to intermolecular, in origin and
made possible as the result of internal amide−pyridine NH−N
hydrogen bonds.10b,c Hydrogen bonding also plays a critical
role in the aggregation seen for 1, but not 2. In the case of 1,
evidence for self-assembly is seen at higher concentrations
Evidence for self-association in solution came from the
observation that the chemical shift values of various protons
present in tweezers 1, especially those associated with the
aromatic subunits, proved concentration dependent when the
1H NMR spectra were recorded in CDCl3 (cf. Figure 2 and
Figure S1 in the Supporting Information). As can be seen from
an inspection of Figure 2 and Figure S1 in the Supporting
Information, increasing the concentration of 1 over the 0.5−
14.4 mM concentration range leads to upfield shifts in the
signals for both the pyridine and pyrene CH resonances, with
the shifts for the Ha and Hb resonances being particularly
dramatic. In addition, the signal ascribed to the bridging
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(CF3)2CHOH in CDCl3 (cf. Figures S5 and S6 in the
Supporting Information), leading us to propose that even in the
presence of protic solvents compound 1 still aggregates to form
self-associated supramolecular species.
The formation of self-assembled aggregated structures from
compound 1 in solution was further supported by a nuclear
Overhauser effect spectroscopy (NOESY) NMR study, which
revealed NOE correlations between Hc and two protons of the
pyrene subunit (Supporting Information, Figure S7). In
addition, spectra from diffusion-ordered spectroscopy
(DOSY) NMR measured at two different concentrations of
compound 1 confirmed the expectation that the weight-average
diffusion coefficient of 1 at a higher concentration (16 mM)
was 14% smaller than the corresponding value recorded at a
lower concentration (3.0 mM) (Supporting Information,
Figures S8−S10). This can be taken as evidence that a more
aggregated, presumably oligomeric, structure is stabilized at
higher concentrations.
Consistent with its proposed supramolecular character, the
self-assembled oligomer formed from compound 1 was found
to undergo deaggregation with a dramatic change in structure
when exposed to NAEs, such as TNB and TNT. Initial
evidence for this structural change caused by TNB came from a
single crystal X-ray diffraction analysis (Figure 3). Suitable
crystals were obtained by allowing a dichloromethane/hexane
solution of compound 1 to undergo slow evaporation in the
presence of excess TNB. The resulting structure was found to
be a 1:1 complex, 1·TNB, wherein a TNB molecule is
intercalated between the two pyrene subunits units that
comprise the “tweezers” present in receptor 1 (Figure 3). On
the basis of the observed structural parameters, this solid state
TNB complex is stabilized by hydrogen bonding interactions
between the NHs and one of the TNB nitro groups, as well as
by apparent donor−acceptor interactions. Intermolecular π−π
contacts involving adjacent pyrene moieties appear to stabilize
further the overall structure (Figure 3).
Figure 1. Views of the single crystal X-ray structure of 1 showing (a)
the monomer present within the overall extended arrangement, (b) a
truncated fragment found within the crystal lattice (C, gray; O, red; N,
light blue; H, light gray), and (c) a more extended view of the
supramolecular polymeric structure seen in the solid state.
methylene protons (Hc) shifts to higher field as the
concentration of compound 1 is increased (Supporting
Information, Figure S2). By contrast, the signal assigned to
the amide NH protons shifts to lower field as the concentration
is increased, a finding that is ascribed to an incipient
intermolecular hydrogen bonding interaction between the
NH protons and the carbonyl oxygen atom of another
molecule.
Plots of the chemical shift changes for Hb and Hc as a
function of concentration revealed no obvious breaks in what
appears to be a monotonic variation (Supporting Information,
Figures S3 and S4). These findings are fully consistent with a
concentration-dependent aggregation process and the forma-
tion of a self-assembled structure wherein Ha and Hb are
“nestled” within the tweezers-like cleft present in 1 and thus
subject to the ring current effects of (at least) a nearby pyrene
moiety; the result is formation of oligomeric species of varying
length and size that mirror in terms of basic structure what is
seen in the solid state (vide supra).11 Similar spectral features
were observed in the presence of 10% methanol-d4 and 10%
Further evidence for the proposed tweezers-like interactions
between compound 1 and TNB and the substrate recognition-
induced deaggregation of the initial self-associated form of 1
came from 1H NMR analyses carried out in CDCl3. Specifically,
upon titration of 1 with TNB, the proton signals corresponding
1
Figure 2. Partial H NMR spectra of compound 1 recorded at different concentrations in CDCl3.
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1
normally observed at δ = 9.40 ppm in the H NMR spectrum
(Supporting Information, Figure S8), shifts to δ = 7.00 ppm
(Figure 4b). The changes in the TNB and pyrene signals are
taken as evidence that a charge transfer complex, 1·TNB, is
formed between TNB and tweezers 1 and that this complex
exists as the dominant species under most solution phase
conditions (Supporting Information, Figure S11).
Similar 1H NMR spectral changes were observed when TNB
was replaced by TNT (trinitrotoluene). We thus suggest that
TNT also forms a charge transfer complex with compound 1.
In both cases, the presence of the NAEs prevents 1 from
forming a self-associated supramolecular polymer (Supporting
Information, Figure S12). The 1H NMR titration curves
corresponding to the interaction of 1 with TNB and TNT are
shown in Figure S13 in the Supporting Information. The
resulting binding constants for a 1:1 interaction with 1 were
determined to be 7.32 × 104 and 7.69 × 102 M−1 in the cases of
TNB and TNT, respectively (Supporting Information, Table
S1).
The addition of TNB to a chloroform solution of 1 results in
an easily discernible color change from colorless to red (Figure
5). The addition of methanol to the red solution containing 1·
Figure 3. (top) Two different views of the donor−acceptor complex 1
TNB as deduced from a single crystal X-ray diffraction analysis.
(bottom) Partial view of the extended structure seen in the crystal
lattice (C, gray; O, red; N, light blue; H, light gray).
to Ha and Hb were seen to shift to lower field (Figure 4). This
finding is ascribed to the deshielding of protons Ha and Hb that
results when the pyridine ring from one unit of 1 that is initially
present in the tweezers-like cleft of another unit of 1 within the
self-assembled form is displaced by TNB. The NH proton
signals were also seen to shift to lower field during the titration,
a finding that is consistent with the intermolecular hydrogen
bonding interactions present in the supramoleular polymeric
form of 1 being replaced by ones involving the nitro groups of
the TNB bound within the complex 1·TNB.
Coincident with the above spectral changes, the proton
signals of the pyrene moiety of compound 1 were seen to shift
to higher field as the concentration of TNB increased (Figure
4). Likewise, the proton signal of TNB also shifted upfield in
the presence of compound 1. For example, after the addition of
0.14 equiv of TNB to 1 in CDCl3, the TNB proton resonance,
Figure 5. Color changes observed upon the addition of TNB and
TNT to otherwise identical solutions of compound 1 in chloroform:
(a) free 1 (1.0 mM), (b) 1 + TNB (3.0 equiv), and (c) 1 + TNT (3.0
equiv).
1
Figure 4. Partial H NMR spectra corresponding to the titration of compound 1 with TNB (trinitrobenzene) in CDCl3.
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TNB leads to a loss of color (Supporting Information, Figure
S14). Presumably, this latter change reflects the fact that
solvation of TNB by methanol leads to decomplexation of 1·
TNB and re-formation of the supramolecular polymer formed
via self-assembly of 1. The underlying chemistry is illustrated in
Figure 6 and is supported by the fact that a single crystal of 1
time, the weight-average diffusion coefficient of the TNB guest
recorded in the presence of receptor 1 was found to be ca. 6%
smaller than that of TNB alone (Supporting Information,
Figure S17). Taken together, these findings are consistent with
the conclusion drawn above, namely that the addition of TNB
or TNT to chloroform solutions of 1 leads to the formation of a
discrete TNB complex and, as a consequence, serves to break
up the self-assembled oligomer that would otherwise exist.
In contrast to what was seen for compound 1, no appreciable
1
change was seen in the H NMR spectra of the phenyl linked
pyrene tweezers 2 measured at different concentrations in 10%
(CF3)2CHOH in CDCl3 (Supporting Information, Figure S18).
We take this as evidence that compound 2 does not form a
supramolecular polymer under these solution phase conditions.
We suggest that the difference between the phenyl tweezers 2
and the pyridine congener 1 reflects the critical role that the
intramolecular pyridine N−amide NH hydrogen bonding
interactions play in (1) preorganizing the two pyrene subunits
to form a cleft and (2) orienting the amide carbonyl oxygen
atoms outward such that they can interact with another
molecule of 1. The net result is a conformation that is
predisposed to form a supramolecular polymer in the case of 1,
but not 2.
Figure 6. Schematic depiction of the supramolecular polymeric
structure formed from tweezers 1 and the proposed deaggregation
induced via the addition of TNB or TNT as seen in the solid state via
single crystal X-ray diffraction analysis.
To explore the effect of the tweezers-like nature of 1 on the
formation of a supramolecular oligomer, the pyridine
dipyrenemethyl ester tweezers 3 and the monopyrene amide
system 4 were prepared and subject to a concentration-
grown in the presence of excess TNB and methanol proved to
have the same structure as crystals grown in the absence of
TNB, as discussed above and presented in Figure 1.
1
dependent H NMR spectroscopic analysis. In these cases, no
1
effect of concentration was seen in the H NMR spectrum
Further evidence for the initial formation of a discrete TNB
complex of compound 1 followed by deaggregation of its
supramolecular polymeric structure came from NOESY and
DOSY NMR spectral studies carried out in CDCl3. In the
NOESY spectrum, NOE correlations between the TNB
protons and the protons of the pyrene subunits, as well as
with the amide NH protons, were observed (Supporting
Information, Figure S15). In the case of the DOSY NMR
spectral studies (cf. Supporting Information, Figures S9 and
S16), it was found that the weight-average diffusion coefficient
of 1 measured (3.0 mM in CDCl3) in the presence of TNB (2.0
equiv) was ca. 14% larger than that in its absence. At the same
(Supporting Information, Figures S19 and S20). This finding
provides further support for the conclusion that both NH
protons in 1 are essential for stabilizing a self-associated
oligomeric structure.
In order to probe the effect of the pyrene subunits on the
self-assembly process observed in the case of 1, two additional
tweezers-type molecules, 5 and 6, were prepared. These
systems contain propyl groups instead of the pyrene subunits
present in 1 and 2. Proton NMR spectroscopic analyses carried
out in CDCl3 revealed that the proton signals of the amide NH
protons resonate at lower field in the case of the pyridine-
Figure 7. (a and b) Front views and (c) side view of single crystal X-ray structure of 5. (d and e) Partial views of the extended arrangement seen in
the crystal lattice (C, gray; O, red; N, light blue; H, light gray).
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Figure 8. (a and b) Front views and (c) side view of the single crystal X-ray structure of 6. (d and e) Partial view of the extended arrangement seen
in the crystal lattice (C, gray; O, red; N, light blue; H, light gray).
containing compound 5 than they do for the phenyl-lined
compound 6. As in the cases of 1 and 2 discussed above, this
difference is attributed to intramolecular amide NH to pyridine
N hydrogen bonding interactions being present in 5 but not 6
(cf. Supporting Information, Figures S20 and S21). Never-
theless, for both 5 and 6 gradual downfield shifts in the proton
signals of the amide NHs were observed as the concentration
was increased, as deduced from 1H NMR spectroscopic
analyses (Supporting Information, Figures S21 and S22).
The observation of chemical shifts that change with
concentration leads us to propose that both 5 and 6 self-
aggregate to form supramolecular structures. Support for this
contention came from single crystal X-ray diffraction studies.
Single crystals suitable for these latter analyses were obtained
by subjecting compounds 5 and 6 to slow evaporation from
dichloromethane/methanol and water/methanol mixtures,
respectively. The resulting structures, shown in Figures 7 and
8, revealed that the conformations of these two species are
quite different in the solid state. In the case of compound 5, the
NH protons of the amide groups are in the same plane and are
oriented toward the central nitrogen atom (Figure 7). In
analogy to what was seen for compound 1, this conformation,
which appears stabilized by hydrogen bonding interactions,
facilitates the formation of a “head-to-tail” supramolecular
polymer. On the other hand, the two NH protons of 6 do not
reside within the same plane, presumably reflecting steric
repulsion from the C1 proton of the bridging phenyl ring. In
fact, the two amide protons are not coplanar and point away
from one another (Figure 8). Perhaps as the result of this local
conformational effect, compound 6 exists in the form of a “face-
to-face” supramolecular polymer in the solid state. This self-
assembled arrangement appears to be stabilized entirely by
intermolecular hydrogen bonds.
The different inter- and intramolecular interactions observed
in compounds 1−6 in the solid state and in CDCl3 led us to
investigate how the inferred structural changes affected the
optical properties of compounds 1 and 2. Toward this end, the
absorbance and steady state fluorescence emission spectra of 1
and 2 were first recorded in chloroform at concentrations of ca.
6 and 20 μM, respectively. As can be seen from an inspection of
Figure 9, three dominant bands are seen in the absorption
Figure 9. Normalized steady state absorption (solid lines) and
fluorescence emission spectra (dotted lines) of 1 (red) and 2 (blue)
recorded in chloroform. The excitation wavelength is 346 nm and the
sample concentration is 20 μM, respectively.
spectra of 1 (and 2); these appear at 316 (317), 329 (329), and
346 (346) nm, respectively. Figure 9 serves to highlight the fact
that compound 2 gives rise to only pyrene monomer emission
features (λmax = 375 and 400 nm) when irradiated at 346 nm in
chloroform. No substantial changes in these emission features
were seen when other excitation wavelengths (e.g., 317 and 329
nm), corresponding to other absorption maxima, were
employed. Nor was the emission spectrum found to be
strongly solvent dependent, with little significant change seen
when a 1:9 mixture of chloroform−methanol was employed.
This stands in contrast to what is seen for 1. Here, in addition
to relatively weak monomer features, a broad structureless band
The self-association behavior of compound 6 inferred in
solution and observed directly in the solid state stands in
contrast to what was seen for compound 2 and leads us to infer
that the bulky pyrene substitutents present in 2, but not 6, serve
to inhibit self-association.
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Figure 10. Fluorescence decay profiles of (a) monomer and (b) excimer emissions of 1 monitored at λ = 400 nm (monomer emission) and λ = 500
nm (excimer emission), respectively. The excitation wavelength was 355 nm and the solvent was chloroform.
Figure 11. Fluorescence changes of (a) 1 and (b) 2 (5.0 μM, respectively) seen upon the addition of TNB (0, 10, 30, 100, 200, 300, 400, 500, 900
equiv) in chloroform with excitation at 343 nm. Also shown are the UV−vis spectra of (c) 1 and (d) 2 (1.0 mM, respectively) recorded in the
presence of TNB (0, 1, 2, 3, 4, 5 equiv) in chloroform.
is seen at longer wavelengths (λmax = 470 nm). On the basis of
prior studies involving chemosensors having pyrene moieties,
we attribute this latter feature to an excimer emission (Figure
9).10
pyrene emission was found. This monomer-like behavior is
reflected in the fluorescence lifetime value of 7.2 ns
(Supporting Information, Figure S23).
On the basis of the above observations alone, it was not
possible to assign the excimer emission of 1 as originating from
an intermolecular aggregate, such as observed under the
Support for the emission assignments made in the cases of 1
and 2 came from fluorescence lifetime measurements carried
out using time-correlated single photon counting (TCSPC).
Since 1 reveals an emission feature ascribable to an excimer,
along with characteristic features of a pyrene monomer, two
different wavelengths regions were monitored in the context of
these lifetime studies, namely 400 and 500 nm. While the
excimer-derived fluorescence emission is characterized by a
13.6 ns time component, the monomer band decays with two
time components (1.8 and 12.3 ns, respectively). Further, an
additional rise time component of 2 ns (40%) was observed for
the decay profile associated with the monomeric emission
(Figure 10). We thus ascribe the 2 ns time component to
excimer formation within the tweezers conformation. Very
different behavior was seen for 2. Here, only a monomer-like
1
conditions of the H NMR spectroscopic analyses discussed
above, or from an intramolecular excimer formation between
the two pyrene moieties present in a single tweezers molecule.
On the other hand, the fact that no evidence of excimer
emission was seen in the case of 2 or the control compound 4
(Figure 9 and Figure S24 in the Supporting Information) led us
to consider that the excimer, no matter how it was formed, was
stabilized by amide−pyridine NH−N hydrogen bonds. Thus, a
series of dilution and solvent polarity studies were undertaken
in an effort to distinguish between these two limiting scenarios
(i.e., intra- vs intermolecular excimer formation).
It was found that the ratio of fluorescence intensities
(monomer emission/excimer emission) remained constant at
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Figure 12. Relative fluorescence quenching seen for (a) 1 and (b) 2 (5.0 μM, respectively) upon addition of various guest molecules (PhCH3, PhF,
PhCO2Et, PhNCS, PhCN, PhNO2, DNB, DNT, TNB, TNT; 200 equiv of each) in chloroform. The findings are shown in bar graph form for the
fluorescence intensity of the complexes (of generalized form 1·guest) at 470 nm for under excitation at 343 nm.
Figure 13. Transient absorption spectra and decay profiles of (a and b) 1·TNB, (c and d) 1·TNT, and (e and f) 1·NT in chloroform. Excitation
wavelengths are 500 and 570 nm for the TNB, TNT, and NT complexes, respectively.
different concentrations (Supporting Information, Figure S25),
leading us to conclude that the excimer emission around 470
nm originates from an intramolecular interaction between the
two pyrenes within the same molecule rather than an
intermolecular effect.10 This conclusion was further supported
by solvent polarity studies wherein the ratio of monomer
emission/excimer emission gradually increased with increasing
polarity. Such a finding is expected if, as is reasonable, the
addition of methanol molecules serves to preclude the
formation of intramolecular hydrogen bonds (Supporting
Information, Figure S26). The absorption bands were also
found to shift toward shorter wavelength with increasing
polarity; presumably, this reflects decreased intramolecular
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contact between the two pyrene subunits (Supporting
Information, Figure S26).
features in the visible spectral region (450−800 nm) and strong
ESA bands at 476 nm that are ascribed to a pyrene cation
radical.14 Consistent with the quenched fluorescence signals,
the excited state dynamics of tweezers−electron acceptor
complexes display relatively fast decay profiles. While the free
receptor 1 displays fluorescence lifetimes on the order of 12.3−
13.6 ns (vide supra), the excited state decay profiles of the
complexes are characterized by relatively fast decay dynamics,
namely 18 and 20 ps for 1·TNB and 1·TNT, respectively.
These findings are taken as being indicative of complete or
partial electron transfer from one of the photoexcited pyrene
moieties present in 1 to the nitroaromatic guest, which acts as
an acceptor.
The excited state dynamics of the present tweezers were also
investigated in the presence of other electron accepting
molecules, including naphthalene tetracarboxylic anhydride
(NT). In the case of this latter electron deficient system, the
transient dynamics of 1·NT are characterized by spectral
features similar to those seen in the case of the nitroaromatic
guests. Moreover, the excited state lifetime was found to be 110
ps. On the basis of the slight differences in spectral features and
excited state lifetimes seen for the various complexes, we
propose that the electron transfer and charge recombination
processes depend, as would be expected, on the specific choice
of acceptor.
Geometry optimizations of 1, 2, 1·TNB, and 2·TNB
complexes were performed for the ground state (shown in
Supporting Information, Figure S27) using density functional
theory (DFT) method with Becke’s three-parameter hybrid
exchange functional and Lee−Yang−Parr gradient corrected
correlation functional (B3LYP) and the 6-31G(d) basis set.
Electronic transition energies and oscillator strengths were
calculated using time dependent density functional theory
(TDDFT) at the B3LYP/6-31G(d) level of theory for 1 and 2,
and at the B3LYP/6-31+G(d) level of theory for 1·TNB and 2·
TNB complexes. All the computations were carried out using
the Gaussian 09 program package.15
The optimized structure of 1 is comparable with its crystal
structure. However, the distance between the two pyrene
moieties is smaller in the optimized geometry of 1 than it is in
the crystal structure (∼6.5 vs 7.72 Å). This could reflect the fact
that solid state effects (such as polarization and intermolecular
packing forces) have been neglected in the calculations. The
corresponding distance in 2 (between carbon 1 of the two
pyrene moieties, ∼8.5 Å) is also larger than that in 1 (∼6.5 Å).
Formation of H-bonds (2.39 and 2.33 Å) between the amide
proton and the pyridyl nitrogen in 1 is considered responsible
for these differences. As noted above, such interactions are
absent in 2.
Addition of TNB to solutions of 1 or 2 leads to a quenching
of the fluorescence intensity. Both the monomer and the
excimer-based emission intensities decrease with increasing
TNB concentrations as shown in Figure 11. From the crystal
structure of the 1·TNB complex (shown in Figure 3) and the
associated NMR spectroscopic analyses (vide supra), we infer
that TNB “slides” into the cleft provided by the two pyrene
moieties, thereby stabilizing a supramolecular donor−acceptor
complex. Such a donor−acceptor complex is expected to be
devoid of appreciable emission due to photoinduced charge
transfer (PICT) from the pyrene moiety to TNB, as is indeed
seen by experiment.
DFT (density functional theory) calculations provide
support for the notion that the complexes formed between 1
and TNB are thermodynamically stable (by ∼2.99 kcal/mol in
the gas phase; cf. Supporting Information). For these
calculations, the BSSE (basis set superposition error) was
corrected via the counterpoise method.12 These theoretical
analyses are consistent with the crystallographic data, which
revealed the presence of two hydrogen bonds formed between
the two amide hydrogens and one of the NO2 groups in TNB.
The calculated H-bond distances are 2.21 and 2.24 Å,
respectively (H-bonds are defined according to the suggested
geometry cutoffs for the D−H···A hydrogen bonds,13 i.e., H···A
distances < 3.0 Å and D−H···A angles > 110°). As detailed
below, experimental analyses reveal that fluorescence quench-
ing does indeed occur in the presence of TNB.
A charge transfer (CT) band is observed around 465 nm in
the absorption spectra for both 1 and 2 at higher
concentrations (1 mM) in solution (as shown in Figure 11).
The band (CT) intensity increases with increasing TNB
concentration, and accounts for the observed color change from
colorless to yellow or red (Figure 5).9 The increase in the CT
band intensity and the concurrent decrease in the excimer
emission intensity with increasing TNB concentrations is
consistent with the excimer being intramolecular in nature.
Several other nitroaromatic compounds, such as nitro-
benzene (NB), dinitrobenzene (DNB), dinitrotoluene
(DNT), and TNT, were also found to form donor−acceptor
complexes with 1 and 2. The stability of these complexes
increases as the number of electron withdrawing nitro (NO2)
groups in the aromatic ring increases (Figure 12). The
quenching efficiency of various aromatic compounds is
summarized in Figure 12. Taken in concert, these results
provide support for the conclusion that 1 and 2 differ in how
they respond to electron deficient aromatic compounds. More
precisely, compound 1 distinguishes mono-, di-, and trinitroar-
omatic compounds, whereas tweezers 2 fails to interact
appreciably with any of these species. The greater efficiency
of compound 1 as a recognition unit for nitroaromatic
substrates is reflected in the fact that its excimer emission is
easily “switched” on and off as the result of binding-induced
structural changes, whereas in 2 the optical features derive
almost exclusively from a monomer−monomer donor−accept-
or effect.
Using the optimized structure, the frontier molecular orbitals
of the two tweezer systems were calculated. The results are
summarized in Figure S28 in the Supporting Information. In
both systems, the electron density in the highest occupied
molecular orbital (HOMO) and the HOMO − 1 is mainly
localized in one of the pyrene moieties. The hole density in the
lowest unoccupied molecular orbital (LUMO) is confined
within a pyrene moiety in 2, whereas in 1 the hole density is
delocalized through the pyrene moiety to the pyridine ring and
over the whole molecule in the LUMO and LUMO + 1,
respectively. The nitrogen atom in the pyridine ring plays a
significant role in mediating this delocalization, as inferred from
an analysis of molecular orbital coefficients.
To investigate the photoinduced excited dynamics of the
proposed tweezers−electron acceptor complexes, we carried
out femtosecond-transient absorption measurements (Figure
13). Upon photoexcitation of the charge transfer band at 500
nm, transient species derived from 1·TNB and 1·TNT are seen;
these are characterized by broad excited state absorption (ESA)
503
dx.doi.org/10.1021/ja411672f | J. Am. Chem. Soc. 2014, 136, 495−505

Journal of the American Chemical Society
Article
The calculated vertical transitions of compounds 1 and 2 are
summarized in Table S2 in the Supporting Information. The
calculated wavelengths corresponding to the vertical excitations
with the largest oscillator strengths are 343 and 345 nm for
compounds 1 and 2, respectively (Supporting Information,
Table S2 and Figure S29). These wavelengths match
exceptionally well with the strongest absorption peaks for
both 1 and 2 (346 nm, Figure 9). However, these two tweezers
molecules exhibit different features in their molecular orbital
(MO) structures: while 1 exhibits electron density that extends
into the bridging pyridine moiety, especially in its LUMOs, its
analogue 2 reveals pyrene-localized MO features. It should also
be noted that the lowest transitions of 2 are forbidden (f <
0.01) and consist mainly of transitions between two pyrene
moieties, which show independently localized electron
densities. In contrast, the lowest transitions of 1 are allowed
and contain contributions from the bridging pyridine moiety.
DFT and TDDFT calculations were carried out to explore
the underlying energetics associated with complex formation
between tweezers 1 and 2 and the test nitroaromatic substrate,
TNB. Optimized structures of the TNB complexes formed
from 1 or 2 (1·TNB and 2·TNB) are shown in Figure S27 in
the Supporting Information. The frontier orbitals in 1·TNB and
2·TNB are shown in the Supporting Information, Figures S30
and S31, respectively. The HOMOs are localized on the pyrene
moieties, while the LUMOs are localized only on TNB.
These transformations are accompanied by easy-to-visualize
color changes. Tweezers 1 displays both pyrene monomer and
excimer emissions, while 2 displays only monomer-like
fluorescence features. As inferred from studies of the control
compounds 3−6, the differences between 1 and 2 are ascribed
to the presence (1) and absence (2) of intramolecular
hydrogen bonds between the amide NH and the pyridyl
nitrogen atom. For example, in analogy to what was seen for 1,
compound 5 forms a linear supramolecular polymer in a “head-
to-tail” fashion, whereas 6 self-assembles to form a “face-to-
face” supramolecular polymer. This differing structural behavior
is ascribed to inherent conformational differences that again
reflect the presence or absence of intramolecular hydrogen
bonds. This experimental finding is fully supported by
theoretical (DFT and TDDFT) calculations.
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthetic procedures and spectral characterization data (1H,
13C NMR, and ESI mass spectra) for compounds 1−6, binding
studies, theoretical calculations, and X-ray structural data for 1
(CCDC 970248), 1·TNB (CCDC 970249), 5 (CCDC
970250), and 6 (CCDC 970251). This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
On the basis of the frontier orbitals, we propose that, upon
photoexcitation, charge transfer occurs from the pyrene
moieties to the bound TNB. This photoinduced charge transfer
process (PICT) is responsible for the fluorescence quenching
of 1 and 2 observed in the presence of TNB. The highest and
second highest values of the calculated oscillator strengths are
summarized in Table 1. The calculated wavelengths corre-
■
Corresponding Author
sessler@cm.utexas.edu; jongskim@korea.ac.kr; dongho@
yonsei.ac.kr
Author Contributions
All authors contributed to this work and declare no conflicts of
interest.
Notes
Table 1. Excitations, Oscillator Strengths, and Absorption
Wavelengths in 1·TNB and 2·TNB, Calculated Using
TDDFT Methods
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
compd
major transitions
osc strength calcd λ (nm)
The authors thank the National Science Foundation (Grant
CHE 1057904 to J.L.S. and Grant 0741973 for the X-ray
diffractometer) and the Robert A. Welch Foundation (Grant F-
1018 to J.L.S.) for financial support. This work was also
supported by the Midcareer Researcher Program (2005-
0093839) administered through the National Research
Foundation of Korea (NRF) funded by the Ministry of
Education, Science and Technology (MEST). J.S.K. is grateful
for a grant from the CRI program (No. 2009-0081566) from
the NRF.
1·TNB
HOMO − 3 → LUMO
HOMO −1 → LUMO + 1
HOMO − 2 → LUMO
HOMO − 1 → LUMO
0.005
0.012
0.004
0.007
553
831
550
879
2·TNB
sponding to the second highest oscillator strengths match well
with the observed charge transfer band (∼465 nm) in the
absorption spectra for these two complexes (Table 1 and Figure
11).
CONCLUSIONS
The present study has allowed a comparison between the
ostensibly similar tweezers-like compounds 1 and 2, and several
control systems. As established by H NMR spectral studies
and a single crystal X-ray diffraction analysis, the pyridine-
bridged tweezers 1 forms a supramolecular oligomer both in
chloroform solution and in the solid state. In contrast, the close
analogue 2, which contains a phenyl, as opposed to pyridine,
linker, does not. The supramolecular oligomer formed from 1
undergoes deaggregation upon exposure to TNB or TNT to
form 1:1 complexes with these nitroaromatic explosives.
However, the complexes are destroyed and the initial
aggregated structure is re-formed upon the subsequent addition
of methanol to chloroform solutions of 1·TNB or 1·TNT.
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