Fluorescence enhancement of Di-p-tolyl viologen by complexation in cucurbit[7]uril

Article abstract of DOI:10.1021/ja206833z

A viologen derivative, 1,1′-di-p-tolyl-(4,4′-bipyridine)-1, 1′-diium dichloride (DTV²⁺), was studied in solution and encapsulated in cucurbit[7]uril (CB7), a macrocyclic host. Upon encapsulation, DTV²⁺ exhibited dramatically enhanced

Full text of DOI:10.1021/ja206833z

Article
pubs.acs.org/JACS
Fluorescence Enhancement of Di-p-tolyl Viologen by Complexation
in Cucurbit[7]uril
Marina Freitag, Lars Gundlach,† Piotr Piotrowiak, and Elena Galoppini*
Chemistry Department, Rutgers University, 73 Warren Street, Newark, New Jersey 07102, United States
S
* Supporting Information
ABSTRACT: A viologen derivative, 1,1′-di-p-tolyl-(4,4′-bipyridine)-1,1′-diium dichloride
(DTV2+), was studied in solution and encapsulated in cucurbit[7]uril (CB7), a
macrocyclic host. Upon encapsulation, DTV2+ exhibited dramatically enhanced
fluorescence. Aqueous solutions of DTV2+ were weakly fluorescent (Φ = 0.01, τ < 20
ps), whereas the emission of the DTV2+@2CB7 complex was enhanced by 1 order of
magnitude (Φ = 0.12, τ = 0.7 ns) and blue-shifted by 35 nm. Similar properties were
observed in the presence of NaCl. DTV2+ in a poly(methyl methacrylate) matrix was
fluorescent with a spectrum similar to that observed for the complex in solution. 1H NMR
and UV−vis titrations indicated that the DTV2+@2CB7 complex is formed in aqueous
solutions with complexation constants K1 = (1.2 0.3) × 104 M−1 and K2= (1.0 0.4) ×
104 M−1 in water. Density functional theory and configuration interaction singles
calculations suggested that the hindrance of the rotational relaxation of the S1 state of
DTV2+ caused by encapsulation within the host or a polymer matrix plays a key role in the
observed emission enhancement. The absorption and emission spectra of DTV2+@2CB7 in water exhibited a large Stokes shift
(ΔSt ∼ 9000 cm−1) and no fine structure. DTV2+ is a good electron acceptor [E°(DTV2+/DTV•+) = −0.30 V vs Ag/AgCl] and a
strong photooxidant [E°(DTV*2+/DTV•+) = 0.09 V vs NHE]).
the past.36−38 It has frequently been reported that methyl
INTRODUCTION
■
viologen does not fluoresce in solution.39−42 Reports of
fluorescence are controversial, partly because highly fluorescent
pyridone impurities may be present in viologen samples.43
Fluorescence was reported for MV2+ embedded in zeolites44
and for 2,7-dimethylthieno(2,3-c:5,4-c′)dipyridinium,45 a meth-
yl viologen with a thienyl bridge locking the pyridinium rings in
a rigid planar structure. The fluorescence and excited-state
properties of MV2+ were thoroughly characterized for the first
time a decade ago by Kohler and co-workers.46 The dynamics
of the S1 excited state was found to be strongly dependent on
the solvent, with a fluorescence quantum yield (Φ) of ∼0.03
and a decay lifetime (τ) of ∼1 ns in acetonitrile, in contrast
with a decay lifetime of several picoseconds in water and even
faster decay in methanol. In water, the fast excited-state decay
was ascribed to a nonradiative channel. In methanol, quenching
of the S1 excited state of MV2+ involves electron transfer from
the solvent.46
Viologens (1,1′-disubstituted-4,4′-bipyridinium salts)1 have
wide-ranging applications including electrochromic displays,2−5
molecular electronics,6,7 and redox sensors8,9 and are used in
the design of cyclophane,10−13 rotaxane,14−16 and cate-
nane17−19 molecular hosts. They are common herbicides
because of their phytotoxicity and bind strongly to DNA.20
Viologens are excellent electron acceptors and, in a fast and
reversible redox process, form a radical cation that is stable and
intensely colored.1,2 Because of these properties, viologens are
widely used as acceptors in electron-transfer studies21−23 and as
components of novel electrochromics.2−4 Also, they have been
employed as redox mediators in photocatalytic hydrogen
production24−26 as well as in enzymatic activity studies.27,28
Methyl viologen (1,1′-dimethyl-4,4′-bipyridine-1,1′-diium,
MV2+) dichloride forms a stable complex with cucurbit[7]uril
(CB7), a water-soluble macrocyclic host consisting of seven
glycoluril units that has a portal diameter of 5.4 Å, an inner
diameter of 7.3 Å, and a high affinity for positively charged
compounds (Figure 1).29−31 The host−guest chemistry, the
complexation constant (Kc ∼ 105 M−1), and the redox
properties of the MV2+@CB7 complex have been extensively
investigated.32−34 More recently, we reported the electro-
chromic properties of CB7 complexes of MV2+ and 1-methyl-1′-
p-tolyl-4,4′-bipyridine-1,1′-diium (MTV2+) physisorbed on
nanoparticle TiO2 thin films.35
This paper describes an unprecedented fluorescence
enhancement observed in aqueous solutions of complexes
formed by CB7 and 1,1′-di-p-tolyl-4,4′-bipyridine-1,1′-diium
(DTV2+, 1) dichloride (Figure 1). The viologen DTV2+, which
carries two p-tolyl groups on the quaternized nitrogens, was
synthesized as part of our interest in the electrochromic
properties of viologen@CB7 complexes bound to TiO2.35 We
noticed that solutions of DTV2+ were weakly fluorescent and
While the redox chemistry of viologen is well-characterized,
the excited-state deactivation mechanism of photoexcited
viologen in fluid solutions has been the object of debate in
Received: July 21, 2011
Published: February 9, 2012
© 2012 American Chemical Society
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Article
of alkyl viologen@CB7 complexes has been extensively
investigated,31−35 fluorescence has never been reported. This
suggests that the p-tolyl group is important for the observed
fluorescence enhancement. The reason for the observed
fluorescence enhancement and the influence of the p-tolyl
group are the main focus of this work.
In this work, we studied the complexation of DTV2+ in CB7
in aqueous solutions to determine the complexation constants,
the stoichiometry of the complex, the electrochemical proper-
ties, and the steady-state and time-resolved fluorescence
properties of DTV2+ and its complexes with CB7. The
experiments were complemented with density functional theory
(DFT) and configuration interaction singles (CIS) calculations
of the S1 and T1 excited states of DTV2+. In addition, we
investigated the effect of excess NaCl on the complexation of
DTV2+ in CB7 and the resulting emission. Cucurbiturils have
multiple carbonyl binding sites for positive ions and tend to
complex strongly with salts,53,54 especially NaCl, for which the
complexation constant is ∼120 M−1.55,56 The presence of salts
influences the complexation equilibria and, in some cases, the
emissive properties of the chromophoric guests.57,58
Figure 1. (top) Molecular structure of DTV2+, the viologen derivative
studied in this work. The counterion is chloride. (bottom) Molecular
structure and dimensions of CB7. The structures of CB7 (top and side
view) are not in scale.
that encapsulation in CB7 produced a strong blue emission.
Similar behavior was observed for MTV2+,35 a viologen with
one methyl group and one p-tolyl group on the quaternized
nitrogens, while no fluorescence emission enhancement upon
encapsulation in CB7 was observed for MV2+ or phenyl
viologen. First, the possibility that this effect was due to an
emissive impurity had to be ruled out. Although the presence of
traces of impurities could not be excluded completely,
comparisons of different batches of DTV2+ after repeated
purification steps, combined with quantum yield and emission
lifetime studies, elemental analysis, and titration experiments
with CB7, confirmed that the observed fluorescence enhance-
ment reported here is caused by encapsulation of DTV2+.
The emissive properties of DTV2+@CB7 are interesting for a
number of reasons. To the best of our knowledge, this is the
first example of fluorescence enhancement of a viologen
derivative induced by encapsulation in a molecular host. In one
of the first studies of fluorophore@CB7 complexes, Nau and
co-workers reported that encapsulation of 2,3-
diazabicyclo[2.2.2]oct-2-ene (DBO) in CB7 resulted in a 2-
fold increase in emission lifetime caused by the host shielding
the guest from quenchers (e.g., oxygen).47 More recently,
emission enhancement of complexes of CBs with a variety of
fluorescent dyes has attracted considerable attention for
potential applications as fluorescent probes for biological
systems,48 in fluorescence lifetime imaging microscopy
(FLIM),49 as water-soluble sensors,34 and in supramolecular
photochemistry studies.47,50,51 The ability to extend this type of
host−guest photochemistry to viologens is an important
development, especially because we previously demonstrated
that CB7 and its viologen complexes bind to nanostructured
metal oxide films that were used to prepare fully reversible
electrochromic windows.35 Hybrid organic/inorganic layers
prepared from emissive CB7 complexes can extend the possible
applications to light-emitting diodes. The advantages of the
host−guest chemistry (inhibited quenching, chemical stability,
etc.) are important for fundamental charge-transfer studies at
semiconductor interfaces.52
EXPERIMENTAL SECTION
■
1H NMR Titration in Figure 2. Stock solutions of DTV2+ (1 mM)
and CB7 (2 mM) were prepared in D2O. The stock solutions were
combined in an NMR tube to obtain the desired host:guest ratio. The
same experiment was repeated in the presence of 50 mM NaCl.
Electrochemistry. Cyclic voltammograms were collected on a BAS
CV27 potentiostat. The measurements were conducted on 0.5 mM
DTV2+ aqueous solutions with 0.1 M phosphate buffer (pH 7.3),
following literature methods.33 The solutions were deaerated before
and during the measurements by bubbling nitrogen in a standard
three-electrode arrangement with a glassy carbon electrode (2 mm
diameter), a Pt gauze auxiliary electrode, and a Ag/AgCl(3.0 M NaCl)
reference electrode. The scan rate was 100 mV/s at a sensitivity of 100
mA/V between −0.1 and −0.5 V. All values are reported in volts
versus Ag/AgCl.
Spectroscopic Measurements. UV−vis absorbance spectra were
collected at room temperature on a Varian Cary 500 spectropho-
tometer.
Steady-state fluorescence spectra were acquired and recorded at
room temperature on a Varian Cary Eclipse fluorescence spectropho-
tometer calibrated with a standard NIST tungsten halogen lamp.
Fluorescence quantum yields (Φ) were calculated using eq 1:
2
⎛
⎜
⎜
⎝
⎞
⎟
⎟
⎠
⎛
⎝
⎞⎛
⎠⎝
⎞
⎠
η
I
A
sample
sample
ref
⎜
⎜
⎟⎜
⎟⎜
⎟
⎟
Φ
= Φ
ref
sample
2
A
I
η
sample
ref
ref
(1)
where Asample and Aref are the absorbances at λex = 320 nm for the
sample and reference, ηsample and ηref are the refractive indexes of the
solvent used for the sample and reference solutions, and Isample and Iref
are the integrated emission intensities of the sample and reference,
respectively. Stilbene 420 (2,2″-([1,1′-biphenyl]-4,4′-diyldi-2,1-
ethenediyl)bisbenzenesulfonic acid disodium salt) in ethanol was
used as the reference (Φref = 0.52 in water and 0.80 in 9:1 ethanol/
water),59 with integration over the 375−600 range. No significant
changes in the emission quantum yield or fluorescence lifetime
measurements were observed in deaerated solutions (freeze−pump−
thaw) or in the presence of air, so all of the reported spectra were
collected in the presence of air.
Time-resolved fluorescence measurements were collected after
excitation with 320 nm, ∼30 fs pulses (∼2 nJ) from a Ti:sapphire-
pumped non-collinear optical parametric amplifier. Picosecond time-
resolved measurements were performed in a 10 mm cuvette. The
fluorescence was collected via a lens and focused onto a Hamamatsu
H5783-01 photomultiplier. For some of the measurements, a 500 nm
The fact that DTV2+ exhibited enhanced fluorescence in CB7
while methyl viologen and the structurally similar phenyl
viologen did not is intriguing. While the host−guest chemistry
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band-pass filter was employed (40 nm fwhm). The signal was recorded
with a Becker & Hickl PCS 150 sampling card and a PC (50 s
sampling time). Femtosecond time-resolved measurements were
performed in a 1 mm cuvette. The fluorescence was detected in a
Kerr-gated fluorescence spectrometer.60
The titration experiments shown in Figures 4b and 5a were
conducted by recording the integrated emission spectra of DTV2+ (2
mL of a 5 μM aqueous solution) upon addition of 20 μL aliquots of a
100 μM CB7 solution in water.
Preparation of DTV2+ in PMMA Polymer Matrix Films.
Polymer coatings were prepared by dissolving poly(methyl meth-
acrylate) (PMMA) in formic acid (10 wt %) and adding DTV2+ (0.8
wt %) to the solution. Formic acid was a good spin-coating solvent for
both PMMA and DTV2+. A SCS|G3P-8 Spin-Coat System (Specialty
Coating Systems, Inc.) was used to prepare thin films on a 1 cm × 1
cm cleaned glass slide at 2000 rpm. The resulting PMMA films
contained ∼8 wt % DTV2+.
DFT and CIS calculations. Geometry optimization and spectral
calculations were done using the Spartan’10 software package
(Wavefunction, Inc.). The DFT geometry optimizations of the S0
ground state and the T1 triplet state of the dication were performed
using the B3LYP pseudopotential and a 31-G* basis set. Optimization
of the S1 state and the calculation of the S0−S1 transition energies at
the equilibrium geometries of the S0 and S1 states were performed at
the CIS level utilizing the same 31-G* basis set.
RESULTS AND DISCUSSION
■
Synthesis. There is one report in the literature of
DTV2+(1),61 but the synthesis and properties of 1 have not
been reported. DTV2+ was synthesized in 30% yield in two
steps from 4,4′-bipyridine (2) using the Zincke reaction,62
which converts pyridines into pyridinium salts (Zincke salts) by
the reaction of 2,4-dinitrochlorobenzene (3) and an aniline
derivative (Scheme 1; also see the Supporting Information).63
1,1′-Bis(2,4-dinitrophenyl)-(4,4′-bipyridine)-1,1′-diium chloride
(4) was obtained in 44% yield by reaction of 3 with 2. Reaction
of p-toluidine (5) with 4 yielded the dichloride salt of 1 in 90%
yield. DTV2+, a very pale yellow powder, is soluble in protic,
polar solvents such as water, ethanol, and methanol.
1
Figure 2. (top) Viologen region of the H NMR spectra (in D2O) of
DTV2+ alone and after addition of 0.5, 1.0, 2.0, and 3.0 equiv of CB7.
The 3−6 ppm region and the CB7 and solvent signals have been
1H NMR Study of the Complexation of DTV2+ with
CB7. The formation of DTV2+@CB7 host−guest complexes
1
1
omitted for clarity. (middle) Viologen region of the H NMR spectra
was monitored by H NMR titrations in D2O (Figure 2, top).
(in 0.05 M NaCl in D2O) of DTV2+ alone and after addition of 0.5,
1.0, 2.0, and 3.0 equiv of CB7. (bottom) Chemical shift differences for
DTV2+ upon complexation with 2 equiv of CB7 in D2O. Similar shifts
were observed in the presence of NaCl.
The complexation in CB7 resulted in significant encapsulation-
induced chemical shifts of the viologen guest protons.33 The
same experiment was repeated in the presence of a large excess
of sodium chloride (0.05 M NaCl) (Figure 2, middle).
Upon addition of 0.5 equiv of CB7, the tolyl group’s doublets
(Hc and Hb) broadened considerably, and all of the signals
assigned to the tolyl group (Ha, Hb, and Hc) exhibited a
considerable upfield shift (Figure 2, top). The Δδ pattern
suggests that CB7 preferentially encapsulates the p-tolyl moiety
(Figure 2, bottom). We made a similar observation with
MTV2+,35 and this might be explained by the propensity of the
hydrophobic inner cavity of CB7 to accommodate the
hydrophobic tolyl unit in an aqueous environment.
The spectrum of the 1:2 guest:host (G:H) ratio was sharper,
with the protons of the p-tolyl groups exhibiting the largest
shifts (ΔδH = −0.97 ppm, ΔδH = −0.77 ppm, and ΔδH
=
b
c
a
−0.50 ppm) and the 4,4′-bipyridyl protons Hd and He
exhibiting shifts that are typically observed upon encapsulation
of viologens in CB7 (Figure 2).33 Additional amounts of CB7
did not result in significant spectral changes, indicative of the
complete encapsulation of DTV2+, starting from the 1:2 G:H
ratio. The spectral changes in the CB7 region (shown in Figure
S3 in the Supporting Information) also suggested a 1:2 G:H
complex, as the symmetry of the two rims of each CB7 host was
disrupted. In the presence of a large excess of NaCl, the
spectrum of free DTV2+ was unchanged (Figure 2, middle).
Upon addition of CB7, the proton signals broadened and
became sharp at a G:H ratio of 1:3, suggesting that binding is
A remarkable broadening of the 4,4′-bipyridyl proton signals
at the initial stages of the titration with CB7 were observed. A
1
similar broadening of the H NMR spectra for CB7 complexes
of viologens has been reported previously by Kaifer and co-
workers.64−66 A plausible explanation for this behavior is that
when the guest is in excess, there is a dynamic distribution of
species during the time scale of the NMR experiment. Such
equilibria could include shuttling of the host along the
backbone of DTV2+, partial threading, or a complexation/
decomplexation process.
1
weaker in the presence of NaCl. In summary, the H NMR
spectral evolution during titration with increasing amounts of
host indicates that a dynamic distribution of various complex
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Scheme 1. Synthesis of DTV2+
association of a second host molecule with the 1:1 complex to
form the 1:2 complex (eq 3) with complexation constant K2.
[HG]
[H][G]
[G] + [H] ⇄ [HG]
K =
1
(2)
[H G]
[HG][H]
2
[HG] + [H] ⇄ [H G]
K =
2
2
(3)
ΔAobs, the absorption change upon titration of DTV2+ with
CB7, was determined using eq 4:69
2
ε
[G] K [H] + 2ε
[G] K K [H]
2
Δ[HG] 0 1 Δ[H G] 0 1 2
2
ΔA
=
obs
1 + K [H] + K K [H]
1 2
(4)
1
were determined as in Eq.4, where εΔ[HG] and εΔ[H2G] are the
extinction coefficients measured at the Host: Guest ratios 1:1
and 2:1, respectively.69 The complexation constants in water
calculated using this method were K1 = (1.2 0.3) × 104 M−1
and K2 = (1.0
0.4) × 104 M−1. We did not calculate the
complexation constant in the presence of excess NaCl because
of the complexity of the equilibria involving an additional
component and the uncertainty in the number of sodium ions
coordinating to the host.
UV−Vis Absorption Spectra and Steady-State and
Time-Resolved Fluorescence of DTV2+@2CB7. The
absorption (λmax = 335 nm) and emission (λem = 470 nm)
spectra of DTV2+@2CB7 in water exhibited an exceptionally
large Stokes shift (ΔSt ∼ 9000 cm−1), and no fine structure was
present in the room-temperature fluorescence spectrum
(Figure 4a).
Aqueous solutions of DTV2+ in the absence of the host
exhibited a low fluorescence quantum yield (Φ = 0.01), as
shown in Table 1 and Figure 4b. Upon addition of CB7
aliquots, however, the emission spectra showed a progressive
fluorescence enhancement and a blue shift of the emission
band. The emission spectra stabilized at a G:H ratio of about
1:2, at which point further addition of the host did not result in
increased emission. Overall, the encapsulation in the host
resulted in a 1 order of magnitude enhancement of the
emission yield and a blue shift of approximately 35 nm in the
maximum (λem from 505 to 470 nm). Figure 5a shows the
titration curves of DTV2+ with CB7 in aqueous solution and in
the presence of excess NaCl. The integrated emission intensity
at each titration step was plotted against the number of
equivalents of CB7. The maximum emission intensity was
reached at a G:H ratio of 1:2 and is qualitatively consistent with
the observations shown in Figure 4b. The stoichiometry of the
complex was determined by the continuous variation method70
(Job plot, shown in Figure 5b) using emission spectroscopy.
The total molar concentration of host and guest was held
constant at 0.6 mM, and the mole fractions were incrementally
varied, while the integrated fluorescence intensity was
monitored. The titration plots in Figure 5 suggest that the
maximum enhancement was observed for the fully encapsulated
DTV2+ and that the stoichiometry of the complex is G:H = 1:2,
Figure 3. Experimental absorbance changes (ΔAobs) at λ = 335 nm
2+
■
( ) and the fit to eq 4 (red solid line) in the titration of 5 μM DTV
with CB7 in water.
species is observed when the host is in substoichiometric
amount and that full encapsulation of DTV2+ requires a G:H
ratio of at least 1:2 to form predominantly the DTV2+@2CB7
complex.
Determination of the Complexation Constant of
DTV2+@2CB7 in Water. Changes in the absorption spectra
are frequently used to study complexation constants of CBs
complexes.67,68 The complexation constant for the 1:2 complex
of DTV2+ (the guest, G) with CB7 (the host, H) was
determined by monitoring the UV−vis spectral changes at 335
nm (ΔAobs in eq 4 and Figure S9) upon titration of a 5 μM
solution of viologen (2 mL) by addition of 5 μL aliquots of a 2
mM aqueous solution of CB7. The data points were plotted
against the concentration of CB7 and fitted as shown in Figure
3. For the complexation constant calculations for the 1:2 G:H
complex, we employed eqs 2−4, as recently described by
Thordarson.69 This approach involves the formation of a 1:1
complex (eq 2) with complexation constant K1 followed by
1
consistent with the H NMR spectral changes. Overall, the
experiments suggest that in this case the presence of excess
NaCl did not result in dramatic changes of the fluorescence or
complexation stoichiometry. However, it is widely acknowl-
edged that in studies involving CB hosts, it is preferable to
control the concentration of salts, or at least to assess the effect
of their presence on the complexation equilibria.71,72,57,73
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Figure 5. (a) Fluorescence emission titration curve of 5 μM DTV2+
■
●
with CB7 in water ( , blue line) and in presence of 0.05 M NaCl ( ,
dashed black line). λex = 350 nm. (b) Job plot for the complexation of
CB7 in DTV2+. The total concentration [CB7] + [DTV2+] was
maintained at 0.6 mM.
Figure 4. (a) Normalized absorption and emission spectra of DTV2+@
2CB7 in aqueous solution. (b) Emission spectra of DTV2+ (5 μM)
upon addition of CB7 (concentration range: 1−15 μM). λex = 350 nm.
Inset: Solutions of (left) DTV2+ and (right) DTV2+@2CB7 exposed to
black light.
Table 1. Selected Photophysical Properties of Aqueous
Solutions of DTV2+ and its CB7 Complexes
a
b
DTV2+:CB7
λem (nm)
Φ
τ (ns)
1:0
1:1
1:2
1:3
505
482
470
470
0.01
0.08
0.12
0.12
<0.02 0.004
0.4 0.08
0.7 0.14
0.7 0.14
Quantum yields for DTV2+ were measured in the presence of
0, 1, 2, and 3 equiv of CB7, in water, as shown in Table 1. The
formation of the CB7 complexes resulted in a significant
fluorescence quantum yield and lifetime enhancement. The
quantum yield of DTV2+ was Φ = 0.01, whereas the quantum
yield of the CB7 complex increased up to Φ = 0.12, depending
on the G:H ratio (Table 1).
The fluorescence lifetime of 2 μM aqueous solutions of
DTV2+ was <20 ps, which was within the time resolution of the
instrument. Bleaching of the DTV2+ sample occurred during
the measurement, and this was ascribed to sample degradation.
This is consistent with the observation that aqueous solutions
of samples left in the laboratory environment for prolonged
periods of time (weeks) were no longer emissive. Upon
addition of 1 and 2 equiv of CB7, the emission lifetime for the
complexes was dependent on the DTV2+@CB7 complex
a
Stilbene 420 was used as the reference (see the Experimental
Section). λex = 320 for the reference and the samples. The uncertainty
b
in the Φ values is 0.01. λex = 350 nm.
lifetime is quantitatively consistent with the increase in
emission intensity. The time-resolved experiments were
repeated in the presence of a large excess of NaCl (0.05 M),
and the results were, within the experimental error, identical to
what was observed in water.
To explore the influence of conformation restrictions of
DTV2+ on the emission properties, DTV2+ was mixed with
PMMA polymer and spin-coated into films. The emission
spectra of DTV2+ in a PMMA matrix (Figure 6) showed a blue-
shifted, structureless emission band (λem = 465 nm) similar to
that observed upon encapsulation into CB7.
stoichiometry and determined to be τ1:1 = 0.4 ns and τ1:2
=
0.7 ns (Figure S10 and Table 1). The increase in fluorescence
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Figure 8. Most notably, the length of bond “d” connecting the
nearly coplanar p-tolyl moiety to the viologen core decreases
Figure 8. Illustration of the bond lengths in the S1 state of DTV2+. The
last p-tolyl ring’s bond lengths are all identical. Bonds whose lengths
change by more than 0.025 Å compared with those in the S0 state are
shown in color (contraction in red and lengthening in green), and
bonds in black are all identical in length. A similar illustration for the
T1 state is also shown. For bond lengths, see Table 2.
Figure 6. Emission spectrum of DTV2+ (8.0 wt %) in a PMMA
polymer matrix.
from 1.45 Å to a double-bond-like 1.34 Å. The complete set of
calculated bond lengths in the S0, S1, and T1 states is given in
Table 2.
DFT and CIS Calculations. DFT and CIS calculations
showed that the formation of the S1 state and the subsequent
intersystem crossing to the T1 state are associated with major
changes of the dihedral angles between the four aromatic rings
of the DTV2+ molecule, as shown in Figure 7, top. Such large-
The formation of the T1 state is associated with another large
geometry change. In this case, the viologen core adopts a
completely planar quinonoid structure, while both p-tolyl
groups are rotated out-of-plane considerably. The large
geometry changes associated with the S0 → S1 and S1 → T1
transitions are accompanied by large intramolecular reorganiza-
tion energies [λ(S0−S1) = 0.74 eV and λ(S1−T1) = 0.40 eV,
respectively]. The particularly large value of λ(S0−S1) explains
the lack of vibronic structure in the absorption and emission
spectra of DTV2+.
Consistent with the experimental observations of the changes
in the emission spectra of DTV2+ in solution and in a confining
medium (PMMA or a host), the calculations predicted a 0.74
eV red shift of the S1−S0 transition upon relaxation from the
Franck−Condon state to the optimum S1 geometry. The blue
shift of the emission maximum is therefore the result of going
from a fluid to a confining medium, which prevents such
relaxation.74 The blue shift could also be explained by the
different polarities experienced by DTV2+ in CB7 and in water.
Furthermore, the calculations revealed that the oscillator
strength of the S1 → S0 transition diminishes from 0.79 in the
geometry of the ground state to 0.20 in the relaxed geometry of
the S1 state. This 4-fold reduction cannot fully account for the
essentially complete quenching of the fluorescence of DTV2+ in
water. An oscillator strength of 0.2 in the fully relaxed geometry
would lead to substantial emission unless the system undergoes
further nonradiative decay. We postulate that the low emission
quantum yield and the very short (<20 ps) excited-state lifetime
can be ascribed to rapid intersystem crossing and internal
conversion occurring on a time scale that is similar to or only
slightly longer than the S1 relaxation. Any hindrance or
retardation of the rotation of the four rings of DTV2+, as for
instance by a polymer matrix or confinement in a host, would
slow the intersystem crossing and internal conversion and
result in lengthening of the S1 lifetime and increased
fluorescence quantum yield, as observed in the reported
experiments. The 4-fold reduction in the oscillator strength
does not fully account for the experimentally observed
fluorescence quantum yields of the free and encapsulated
DTV2+, which differ by a factor of nearly 15. Clearly, DTV2+ in
Figure 7. Optimized structures of the S0, S1, and T1 states of DTV2+,
indicating the inter-ring dihedral angles as well as the λ(S0−S1) and
λ(S1−T1) intramolecular reorganization energies. The dashed boxes
indicate the almost planar geometries of three consecutive rings in the
S1 state and the two bipyridyl rings in the T1 state.
amplitude relaxation processes are certainly hindered and
considerably slowed in a PMMA matrix as well as upon
encapsulation in the CB7 host. The ground-state equilibrium
conformation of DTV2+, with a 34.4° dihedral angle of the
viologen core, is close to the 35° dihedral angle for the ground
state of the isoelectronic biphenyl. In the relaxed S1 state, the
twofold symmetry of the system is broken, reflecting the
intermolecular charge-transfer nature of the transition. The
viologen core and one of the p-tolyl moieties are more planar
and form a strongly coupled sequence of three rings, while the
fourth p-tolyl moiety is twisted (65.5°) with respect to the rest
of the molecule (Figure 7). The respective dihedral angle
changes in the S0 → S1 transition are shown in Figure 7.
The planarized section of the ring sequence in the S1 state
adopts a pronounced quinonoid structure as illustrated in
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a
Table 2. Calculated C−C and C−N Bond Lengths of the Conjugated Core of DTV2+ in the S0, S1, and T1 Electronic States
a
b
c
d
e
f
g
h
g′
f′
e′
d′
c′
b′
a′
S0
S1
T1
1.41
1.42
1.42
1.39
1.36
1.38
1.40
1.44
1.42
1.45
1.34
1.42
1.36
1.42
1.39
1.38
1.34
1.36
1.41
1.43
1.44
1.48
1.45
1.42
1.41
1.41
1.44
1.38
1.37
1.36
1.36
1.34
1.39
1.45
1.46
1.42
1.40
1.39
1.41
1.39
1.38
1.38
1.41
1.40
1.42
a
All values in Å. See Figure 8 for labeling of the bonds.
aqueous solution is subject to additional nonradiative
deactivation processes, such as a rearrangement of the solvation
shell, which are also hindered upon encapsulation. The chloride
counterion is more tightly associated to DTV2+ in aqueous
solutions than in the complex46 and could also play a role in
modulating the emission yield. For this reason, it could be
interesting to study the effect of softer counterions.
The absorption and fluorescence spectra of the DTV•+@
2CB7 complex are shown in Figure 9 a,b, and those for the free
In conclusion, the excited-state rotational dynamics of the
DTV2+ guest appears to play the key role in the observed
enhancement and blue shift of the emission and is augmented
by solvation and ion-association effects. Precedent for this effect
can be found in the study by Bhasikuttan, Nau, and co-workers
of the dye Brilliant Green, a triphenylmethane derivative, where
restriction of intramolecular bond rotations between the aryl
rings induced by complexation with CB7 and a protein resulted
in enhanced fluorescence.75
Electrochemistry. The one-electron reduction process of
viologens (Scheme 2) is fast and reversible, and the radical
Scheme 2. One-Electron Reduction of MV2+ (R = Me) and
DTV2+ (R = p-Tolyl)
cation is intensely blue colored.1,76 DTV2+ showed the same
reversibility and a more positive reduction potential, indicating
that DTV2+ is a better electron acceptor than methyl viologen
(Figure S6 and Table 3). It should be noted, however, that the
Table 3. Voltammetric Parameters for Free and
Encapsulated MV2+ and DTV2+
b
compound
E10/2 (V)
a
MV2+
−0.66
−0.68
−0.35
−0.36
a
MV2+@CB7
DTV2+
DTV2+@2CB7
a
Data from ref 35. The experiments from the literature were
Figure 9. (a) Absorption and (b) emission spectra of DTV2+@2CB7
in water before (black solid line) and after (red dashed line) one-
electron reduction to the radical cation DTV•+@2CB7 by application
of −1.0 V. λexc = 350 nm.
conducted in aqueous 0.1 M phosphate buffer (pH 7.3) using an
Ag/AgCl (3.0 M NaCl) electrode. Half-wave potentials for the first
reduction process vs saturated Ag/AgCl (3.0 M NaCl) reference
electrode in 0.1 M aqueous phosphate buffer (pH 7.3).
b
radical cation DTV•+ are shown in Figures S7 and S8. The
absorption spectrum of DTV•+@2CB7 is identical to the
current−potential curves are distorted from fully reversible
shapes, probably because of partial precipitation of the reduced
form on the electrode surface. The shift to a more negative
potential in the cyclic voltammogram of DTV2+ upon
encapsulation in CB7 (Figure S6), although very small and
possibly close to an experimental error, is consistent with
similar observations for other viologens35,34 and can be ascribed
to a decreased affinity of the radical cation for CB7 relative to
that of the dication.34
spectrum of free DTV•+ and considerably blue-shifted (λmax
=
400 nm) relative to the spectra of the free methyl viologen
radical cation and of MV•+complexed in CB7 (λmax = 600 nm).
The fluorescence spectra of the one-electron-reduced complex
DTV•+@2CB7 (Figure 9b) and of free DTV•+ (Figure S8)
indicate that DTV•+ is not emissive.
Finally, DTV2+@2CB7 is a strong photooxidant. The singlet-
excited-state reduction potential, E°(DTV2+*/DTV•+), was
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Article
estimated to be +2.93 eV vs NHE on the basis of the singlet
energy, E00 = 3.09 eV, and the ground-state reduction potential,
E°(DTV2+@2CB7/DTV•+@2CB7) = −0.156 V vs NHE. This
is consistent with the singlet-excited-state reduction potential
for methyl viologen reported by Kohler,46 which was estimated
to be +3.65 eV vs NHE.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Synthesis and characterization of 1, H NMR spectra of CB7
and DTV2+, UV−vis spectra for the determination of binding
1
constants, H NMR spectra of the CB7 region, absorption and
emission spectra of DTV•+, cyclic voltammograms of DTV2+
and DTV2+@2CB7, and experimental procedure for the Job
plot. This material is available free of charge via the Internet at
http://pubs.acs.org.
CONCLUSIONS
■
We have presented the first example of fluorescence enhance-
ment of p-tolyl viologen, DTV2+, upon encapsulation in CB7 in
aqueous solutions or casting in PMMA films. The encapsula-
tion process and the stoichiometry of the complex were
AUTHOR INFORMATION
■
Corresponding Author
galoppin@rutgers.edu
1
determined by H NMR spectroscopy and Job plot analysis.
The 1H NMR spectra in aqueous solutions and in the presence
of excess NaCl indicated that DTV2+ forms a 1:2 G:H complex
with CB7. Smaller H:G ratios resulted in broadening of the
NMR spectra, suggesting a dynamic distribution of species
during the time scale of the NMR experiment.
Present Address
†Department of Chemistry and Biochemistry, University of
Delaware, Newark, DE 19716.
ACKNOWLEDGMENTS
The DTV2+@2CB7 complexes exhibited a fluorescence
enhancement of about 1 order of magnitude relative to free
DTV2+. Simultaneously, the emission lifetime increased from
<20 ps to 0.7 ns. A blue shift of ∼35 nm was observed upon
addition of increasing amounts of CB7 host (from 0 to 2 equiv)
to DTV2+. The presence of NaCl or oxygen did not
significantly change the emissive behavior of the complex.
The fluorescence emission of DTV2+ and the blue shift were
also observed in PMMA films.
Such enhancement had not been observed for alkyl or phenyl
viologen complexes of CB7, suggesting that the structure of the
quaternizing moieties, in this case the p-tolyl units, is important.
DFT and CIS calculations predicted that the relaxed S1 state
has a strongly coupled sequence of three nearly coplanar rings
(one p-tolyl ring and the two rings of the 4,4′-bipyridium core)
exhibiting a quinonoid structure with alternating bond lengths.
Encapsulation in the host or casting in a film prevents the
system from adopting this geometry and results in dramatically
increased emission.
■
E.G. thanks Prof. Cornelia Bohne for a discussion of the role of
NaCl in CB7 complexation processes and Ms. Yan Cao for
quantum yield measurements with Stilbene 420. The authors
thank the American Chemical Society Petroleum Research
Fund (Grant 46663-AC10) for support. The FT-IR-ATR
spectra, time-resolved measurements, and DFT calculations
were made possible by funding by the Office of Basic Energy
Sciences of the U.S. Department of Energy through Grant DE-
FG02-01ER15256. P.P. thanks NSFR Grant # 0923345 for
support.
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