Inclusion of two push-pull N -methylpyridinium salts in anionic surfactant solutions: A comprehensive photophysical investigation

Article abstract of DOI:10.1021/acs.jpcb.5b02336

Two N-methylpyridinium salts with push-pull properties have been investigated in the aqueous solution of anionic micelles of sodium dodecyl sulfate (SDS) and potassium p-(octyloxy)benzenesulfonate (pOoBSK) surfactants. These molecules are known to be extr

Full text of DOI:10.1021/acs.jpcb.5b02336

Article
pubs.acs.org/JPCB
Inclusion of Two Push−Pull N‑Methylpyridinium Salts in Anionic
Surfactant Solutions: A Comprehensive Photophysical Investigation
Alessio Cesaretti,*,† Benedetta Carlotti,† Giuseppe Consiglio,‡ Tiziana Del Giacco,† Anna Spalletti,†
and Fausto Elisei†
†Department of Chemistry, Biology and Biotechnology and Centro di Eccellenza sui Materiali Innovativi Nanostrutturati (CEMIN),
University of Perugia, Perugia, Italy
‡Department of Industrial Engineering, University of Catania, viale Andrea Doria 6, I-95125 Catania, Italy
ABSTRACT: Two N-methylpyridinium salts with push−pull
properties have been investigated in the aqueous solution of
anionic micelles of sodium dodecyl sulfate (SDS) and potassium
p-(octyloxy)benzenesulfonate (pOoBSK) surfactants. These
molecules are known to be extremely sensitive to the local
environment, with their absorption spectrum being subjected to a
net negative solvatochromism. These compounds are also
characterized by an excited state deactivation strictly dependent
on the physical properties of the chemical surrounding, with the formation of intramolecular charge-transfer (ICT) states
accordingly stabilized. Thanks to steady-state and femtosecond resolved spectroscopic techniques, the photophysical properties
of these molecules in the presence of anionic micelles have been fully characterized and an efficient permeation within the
micellar aggregates can thus be inferred. The extent of the changes in the photophysical properties of these molecules (with
respect to what is observed in water) is an indicator of the medium experienced in the nanoheterogeneous solutions: enhanced
fluorescence emissions, reduced Stokes shifts and slowed-down excited state decays strongly confirm the confinement within a
scarcely polar and restraining environment. The slightly different behavior shown in the two types of micelles can be ascribed to a
peculiar interaction between the aromatic moiety of the surfactant and that of the cations. Additionally, the inclusion promotes
the solubilization of these poorly water-soluble salts, which is alluring in their promising use as DNA binders for antitumor
purposes. Thus, the anionic micelles allowed the solubilization of the pyridinium salts under investigation, which in turn allowed
the characterization of the nonhomogeneous medium established by the micellar aggregates.
water-soluble molecules. This ability is essential when dealing
with compounds of potential pharmacological interest. Micellar
aggregates have therefore been largely employed as drug
vehicles, for they make the drugs ready for use and help
controlling their trajectory in the human body for a targeted
drug delivery.25−30
Moreover, the effect of the polarity and viscosity of the
solvent on the spectral and photophysical properties of
pyridinium salts has been broadly investigated. Many groups
have studied the major influence of the environment on the
steady state and dynamic properties of these molecules, both in
homogeneous and microheterogeneous media.31−35 Particu-
larly, our group has focused its attention on some molecules
belonging to this family, namely, the trans isomers of the 2-D-
vinyl-1-methylpyridinium cation, where D represents an
electron donor group, which is the 4-dimethylaminophenyl
(1) and 3,4,5-trimethoxyphenyl (2), respectively (Scheme
1).36,37 A substantial negative solvatochromism has been
found in the absorption spectrum of these molecules, whereas
INTRODUCTION
■
Stilbene-like compounds bearing electron donor/acceptor (D/
A) units are of the utmost interest because of their
multipurpose applications.1−10 A long-term project of our
group has been indeed devoted to quaternized azastilbene
molecules with push−pull nature, where the acceptor character
is bestowed upon one moiety of the molecule by the
methylpyridinium unit and specific substituents on the other
end bring electron donor properties.11−14 These push−pull
compounds could be exploited as optoelectronics/photonics
materials for their nonlinear optical (NLO) properties15 and as
anticancer drugs for their ability to form complexes with
DNA.16 This latter feature has been the object of a number of
works, where N-methylpyridinium salts and analogous
compounds were found to bind the DNA macromolecule.17−21
This finding is extremely promising in light of a potential
antitumor activity performed by these compounds, but the
sparing water solubility of these molecules is an issue to be
reckoned with, which might be overcome by means of
surfactant or polymeric micelle vehicles.22−24
Micelles, being de facto nearly spherical surfactant aggregates
formed in aqueous solution, are indeed extremely appealing
systems for their ability to swallow molecules within their
structure, providing a useful tool for the dissolution of poorly
Received: March 10, 2015
Revised: April 30, 2015
Published: May 6, 2015
© 2015 American Chemical Society
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self-organized media provided by two anionic surfactants: the
commercial sodium dodecyl sulfate (SDS) and the laboratory-
synthesized potassium p-(octyloxy)benzenesulfonate
(pOoBSK). Resorting to both conventional stationary and
ultrafast time-resolved spectroscopic techniques, the inves-
tigation aims at a twofold purpose: (1) proving the inclusion of
the push−pull pyridinium salts within the micellar aggregates
and (2) taking full advantage of the sensitivity of these chemical
species toward the properties of the medium in order to
characterize the chemical makeup experienced by the molecule.
The study hereinafter discussed intends to describe the non-
heterogeneous multicomponent system under investigation by
combining the solubilizing abilities of the anionic micelles with
the close dependence between the photophysical properties of
pyridinium salts and the physical properties of the sensed
environment. In doing so, a clear picture of the push−pull
pyridinium salts within the solubilizing micellar carriers can
thus be drawn.
Scheme 1. Molecular Structures of the trans (E) Isomers of
the Iodides of 2-D-Ethenyl-1-methylpyridinium
a
a
D = 4-(dimethylamino)phenyl (1) and 3,4,5-trimethoxyphenyl (2).
MATERIALS AND METHODS
■
Materials. The investigated molecules are depicted in
Scheme 1. They were synthesized as iodide salts for previous
works,36,37 following a procedure which has already been
described elsewhere.1,16 All the spectroscopic measurements
were carried out on samples prepared in deionized water as
solvent (ELGA grade) in the presence of two anionic
surfactants: sodium dodecyl sulfate (SDS) and potassium p-
(octyloxy)benzenesulfonate (pOoBSK) (Scheme 2). With the
their emission is affected to a minor degree by the solvent
polarity.36
These spectral features have been explained by taking into
account the different nature of the electronic states presumably
involved in the absorption and emission processes, whose
character has also been suggested by quantum-mechanical
calculations.36,38 The ground state is defined by a large dipole
moment, owing to the positive charge localized on the strong
electron-withdrawing cationic pyridinium ring, while the excited
Franck−Condon state shows a much lower dipole moment,
because of a charge migration from the D to the A unit. A third
state is involved in the deactivation of these molecules in what
is an intramolecular charge transfer (ICT) state, where the
charge moves back from A to D, thus enhancing the charge
separation and the dipole moment of the excited ICT state. Its
dipole moment resembling that of the polar ground state
justifies the invariance of the fluorescence band energy
irrespective of solvent polarity, even though its molecular
structure has ignited ongoing controversy. In solvents with high
polarity and in the presence of suitable D substituents, the
formation of this state is thought to come with the twisting of
the N-methylpyridinium group around the quasi-single bond
with the ethene bridge (TICT).36 When a TICT state is
formed, the back-charge transfer is extremely efficient and the
internal conversion becomes the main deactivation channel for
this excited state. Depending on the efficiency of the D unit, its
hindrance and solvent physical properties, the relative level of
stability between ground and excited states and also between
the locally excited (LE) and ICT states can therefore be
considerably altered; additionally, utterly different spectral
photophysical and kinetic properties can be retrieved. The
Stokes shift experiences a drastic increase in highly polar
solvents where the polar ground state gets stabilized moving the
absorption spectrum to higher energies; the fluorescence
quantum yield is enhanced by up to 2 orders of magnitude,
as a result of increased viscosity and/or reduced polarity which
limit the ICT process; the singlet excited state dynamics is
greatly affected, with lifetimes ranging from a few picoseconds
in water for those molecules allowing an efficient charge
separation via plausible TICT formation to hundreds of
picoseconds in viscous and nonpolar media.
Scheme 2. Structures of the Anionic Surfactants Used: (A)
Sodium-dodecylsulfate (SDS) and (B) Potassium p-
(Octyloxy)benzenesulfonate (pOoBSK)
critical micellar concentration being 8 × 10−3 and 1 × 10−3
M
for SDS39 and pOoBSK, respectively, micelle containing
solutions were obtained with a surfactant concentration of 3
× 10−2 M. The SDS surfactant was purchased from Sigma-
Aldrich and purified twice by crystallization from methanol−
acetone mixture, while the pOoBSK surfactant was synthesized
and purified in our laboratories according to a slightly different
procedure with respect to the one previously reported in the
literature.40−42
The strategy we adopted, which considers the two steps of
the reaction proposed by Bunton et al.40 in reverse order, leads
to the formation of potassium p-(octyloxy)benzenesulfonate
(pOoBSK) avoiding the formation of other organic salts as side
products, which would be otherwise difficult to remove. First, p-
(octyloxy)benzene was prepared according to the Williamson
synthesis by reacting sodium phenoxide with 1-bromooctane in
ethanol to reflux for 12 h. After workup, the oil obtained was
purified by reduced pressure distillation to give p-(octyloxy)-
1
benzene as a colorless oil. B.p.: 165−167 °C (20 mmHg); H
NMR (CDCl3, 200 MHz) δ 7.26 (m, 2H), 6.89 (m, 3H), 3.94
(t, 2H), 1.77 (m, 2H), 1.43 (m, 10H), 0.87 (tr, 3H). pOoBSK
was hence prepared from p-(octyloxy)benzene as follows.
In that respect, the present paper is intended as a detailed
photophysical study of the aforementioned molecules in the
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Concentrated sulfuric acid (10 mL) was added dropwise to p-
(octyloxy)benzene (10.0 g, 0.048 mol), stirred and heated to
60−65 °C in an oil bath. After the resulting mixture had been
stirred for 12 h, a very viscous yellow-orange liquid was
obtained, which was mixed with CCl4 (60 mL) and cooled at 4
°C. The small liquid phase formed on the bottom was
separated and CCl4 was completely removed under a vacuum.
The residue was dissolved in petroleum ether and allowed to
stand at −20 °C affording to obtain p-(octyloxy)-
benzenesulfonic acid as a white crystalline solid, which was
filtered and collected with a Buchner funnel. The salt
(pOoBSK) was precipitated as a white solid by mixing a
solution of p-(octyloxy)benzenesulfonic acid with an equimolar
solution of KOH and then dissolved in hot ethanol and left to
crystallize. The crystalline solid (12.5 g, 80%) was then
collected by filtration under a vacuum, washed with cold
ethanol and acetone, and then dried under a vacuum. M.p.: 120
°C (mesophase), 285 °C (dec); 1H NMR (D2O, 200 MHz) δ =
0.74 (tr, 3H), δ = 1.32 (m, 10H), δ = 1.42 (m, 2H), δ = 3.55 (t,
2H), δ = 6.55 (d, 2H), δ = 7.47 (d, 2H). cmc: 1.4 × 10−2 M at
35 °C; α = 0.33.
Photophysical Measurements. Steady-state absorption
and emission spectra were recorded with a PerkinElmer
Lambda 800 spectrophotometer and a Fluorolog-2 (Spex,
F112AI) spectrophotofluorometer, respectively. The latter
gives back fluorescence emission spectra taking into account
both the monochromator response and the detector sensitivity.
Fluorescence quantum yields (experimental error of ca. 7%)
were determined from the emission spectra of samples whose
absorbance at the excitation wavelength was lower than 0.1, to
have a linear relation between the absorbance and the emitted
intensity and avoid self-absorption effects. Tetracene (ϕF = 0.17
in air-equilibrated cyclohexane) and 9,10-diphenylanthracene
(ϕF = 0.73 in air-equilibrated cyclohexane)43 were used as
fluorimetric standards for the determination of the fluorescence
quantum yields of compounds 1 and 2, respectively, in the
micellar environment.
The experimental setup for ultrafast spectroscopic and
kinetic measurements was widely described elsewhere.44−46
The 400 nm excitation pulses of ca. 40 fs were generated by an
amplified Ti:sapphire laser system (Spectra Physics, Mountain
View, CA). The transient absorption setup (Helios, Ultrafast
Systems, Sarasota, FL) is characterized by a temporal resolution
of ca. 150 fs and a spectral resolution of 1.5 nm. Probe pulses
for optical measurements were produced by passing a small
portion of the 800 nm light through an optical delay line (with
a time window of 3200 ps) and focusing it into a 2 mm thick
sapphire or CaF2 window to generate a white-light continuum
in the 475−800 nm or 400−625 nm range, respectively,
depending on the spectral region of interest. The chirp inside
the sample cell was determined by measuring the laser-induced
Kerr signal of water. All the measurements were carried out
under magic angle conditions, in a 2 mm cell and with an
absorbance ranging from 0.3 to 1.0 at 400 nm. The samples
were stirred during the measurements to prevent the
interferences generated by the photoreaction of the molecules.
Transient absorption data were analyzed using the Surface
Xplorer PRO (Ultrafast Systems)47,48 and Glotaran softwares.49
The first allows to perform the singular value deconvolution of
the 3D surface into principal components (spectra and
kinetics) followed by global analysis (giving lifetimes with an
error of ≈ 10% and decay associated spectra, DAS, of the
detected transients).50 Glotaran was used to perform the target
analysis assuming successive steps in order to carry out the
global fit of the acquired data and provide the species associated
spectra (SAS), which allow to avoid precursor−successor
dynamics of the transients interfering with the calculation of
their relative spectral shapes.49
Triplet−triplet absorption spectra and triplet lifetimes (τT) of
the molecules under investigation in the presence of micellar
aggregates were measured by a nanosecond laser flash
photolysis setup previously described (Nd:YAG Continuum,
Surelite II, third harmonics, λexc = 355 nm, pulse width ca. 7 ns
and energy ≤1 mJ pulse−1).51,52 The transient spectra were
obtained by monitoring the change of transient absorption over
the 300−900 nm spectral range and averaging at least 10 decays
at each wavelength. The setup was calibrated by an optically
matched solution of benzophenone (ϕT = 1 and εT = 6500 M−1
cm−1 at the corresponding absorption maximum, λmax = 520
nm) in acetonitrile.53
In order to minimize the interference arising from the
formation of photoproducts during the measurements, the
deoxygenated solutions (∼25 mL) of the investigated
compounds were flowed through the quartz cell using a
continuous-flow system. The experimental errors were
estimated to be about 10% for τT and 15% for the factor
ϕT × εT. The quenching constants of the triplets by molecular
oxygen were evaluated by applying the Stern−Volmer equation
to the triplet lifetime measured in deaerated and air-
equilibrated solutions (oxygen concentration in water: 2.9 ×
10−4 M). All the ns-transient absorption measurements were
carried out at room temperature on solutions which were
deaerated by bubbling pure nitrogen.
RESULTS AND DISCUSSION
■
Absorption and Fluorescence Properties. The absorp-
tion and fluorescence properties of the pyridinium salts were
studied in deionized water (where they are known to be
completely dissolved without ion-pair formation)36 in the
presence of anionic SDS and pOoBSK surfactants. An anionic
sulfate head and a 12-carbon aliphatic chain characterize the
former, while the latter has a shorter methylenic chain (eight
carbons) bonded through an oxygen atom to an aromatic ring
in the para position with respect to the sulfonate headgroup.
Resorting to anionic surfactants was a deliberate choice in order
to favor the interaction between the cationic pyridinium salts
and the anionic micellar aggregates. Moreover, the synthesis of
the pOoBSK aimed at fostering the interaction between the
surfactant and the two compounds by virtue of the aromaticity
featured by either molecular structures.
The absorption and emission spectra of compounds 1 and 2
in highly polar water (W) and low polar dichloromethane
(DCM) and in the presence of the two different surfactants are
shown in Figure 1, and their absorption and fluorescence
properties are listed in Table 1. Since these molecules are
sensitive to changes in both the polarity and viscosity of the
medium in which they are dissolved, the position of the
absorption and emission bands together with the fluorescence
intensities are a clear sign of the environment experienced by
the molecules.36
In nonmicellar solution, compounds 1 and 2 show
nonstructured bell-shaped absorption bands, which undergo a
negative solvatochromism; i.e., the band shifts toward lower
energies by decreasing the solvent polarity. As for the
fluorescence spectra of the two molecules, the emission band
shape and position remain almost insensitive to polarity,
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Moreover, a specific interaction between the additional
aromatic ring of the pOoBSK surfactant and the aromatic
portion of the pyridinium salts, presumably by π-stacking,
strictly confines the latter inside the micellar aggregate. This
π−π interaction is reflected in the fluorescence maximum
position (see Table 1), which is shifted toward higher energies
in pOoBSK micelles when compared with SDS aggregates (as
opposed to the greater effect provided by this latter
environment on the absorption spectrum). Hence, the emission
maximum serves as an indicator of the less efficient ICT
occurring in the excited states of 1 and 2 in the presence of
pOoBSK micelles, by virtue of the aforementioned π−π
interaction. The fluorescence quantum yields, sensible to the
combined effect of viscosity and polarity, are indeed enhanced
in both kinds of micelles for both compounds (Table 1).
In order to relate the changes in the spectral properties
induced by the micelles to an effective inclusion within the
micellar aggregate, spectroscopic and fluorimetric titrations
were performed starting from aqueous solutions of the
compounds and then adding increasing amounts of surfactant
to reach a concentration far exceeding its cmc (Figures 2 and
Figure 1. Normalized absorption and emission spectra of 1 (upper
graph) and 2 (lower graph) in W, in DCM, and in the presence of
pOoBSK (0.1 M) and SDS (0.1 M) solutions.
although the fluorescence quantum yield decreases by 2 orders
of magnitude for compound 1 and almost 1 order of magnitude
for compound 2 when going from DCM to the more polar W.
When the two compounds are dissolved in aqueous
surfactant solution, significant spectral alterations are revealed:
both micelles are responsible for a bathochromic shift of the
absorption and a little hypsochromic shift of the emission band
with respect to pure water. Interestingly, each surfactant
solution induces peculiar spectral modifications for 1 and 2:
SDS micelles, probably providing a less polar environment for
these compounds, cause larger shifts in the absorption
spectrum; pOoBSK micelles are instead responsible for greater
changes in the position of the emission band.
Figure 2. Absorption (left panels) and emission (right panels) spectra
of 1 in aqueous solution, alone and in the presence of increasing
concentration of the two surfactants (up to 0.03 M): SDS (upper
panels, [1] = 3.1 × 10−6 M) and pOoBSK (lower panels, [1] = 3.6 ×
10−6 M). The spectra recorded before the cmc of SDS and pOoBSK
are represented as green lines.
It is likely that π−π interactions among the phenyl units of
the pOoBSK surfactant monomers define a more compact
micellar aggregate, characterized either by reduced water
content or more structured water, thus acting as a more
viscous environment for the pyridinium salts (vide infra).
Table 1. Spectral Properties and Fluorescence Quantum Yields of Compounds 1 and 2 in DCM, SDS Solution (0.1 M),
pOoBSK Solution (0.1 M), and W
1
2
medium
λabs (nm)
λem (nm)
Δν (cm−1
)
ϕF
λabs (nm)
λem (nm)
Δν (cm−1
)
ϕF
a
DCM
500
468
460
438
590
585
580
589
3090
4270
4530
5850
0.14
393
368
366
353
559
535
530
546
7690
8480
0.012
0.048
0.034
0.003
SDS solution
0.026
0.027
0.001
pOoBSK solution
8520
a
W
10010
a
Data retrieved from ref 36.
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3). In the presence of both surfactants, a double trend for all of
the compounds is always revealed throughout the titrations, i.e.,
unchanged (cf. colored spectra in Figures 2 and 3). Beyond the
cmc of the two surfactants, major changes are revealed in both
absorption and emission properties: the absorption band
undergoes a larger red-shift and the absorbance increases,
while the fluorescence experiences a considerable hyperchromic
blue-shift (cf. black spectra in Figures 2 and 3). This evidence
suggests the occurrence of two distinct events: (1) at low
surfactant concentrations, a one-to-one electrostatic interaction
between the negatively charged surfactant head and the cationic
pyridinium unit, with the resultant formation of a poorly
soluble ion pair as indicated by the detection of scattered light
at long wavelengths; (2) beyond the cmc, the intercalation of
the pyridinium salts inside the micelles, which brings about the
red-shift of the absorption and the enhancement of the
fluorescence emission, as the micelles provide a less polar
and more viscous environment.54,55
Femtosecond Transient Absorption Measurements.
Femtosecond transient absorption measurements were carried
out on aqueous solutions of the two compounds under
investigation in the presence of micelles of SDS and pOoBSK.
The transient absorption decays were acquired upon excitation
with laser pulses centered at 400 nm over a time window of
3200 ps. On the basis of the spectral range of interest for
detecting the transient signals of each compound, we resorted
to a sapphire and a CaF2 window to generate the probing light
in the case of 1 and 2, respectively.
The physical properties of the medium have already proven
to have a discernible effect on the excited state dynamics of the
two compounds, leading to two limiting cases: (a) intra-
molecular charge transfer (ICT) controlled deactivation and
(b) solvent controlled deactivation.36 In case a, the Franck−
Condon (FC) state, populated by direct absorption of the
excitation light, leads to a relaxed locally excited state (LE),
defined by a decreased dipole moment with respect to the
highly polar ground state. The LE state then evolves into an
ICT state, by back-transferring the positive charge to the
methylpyridinium unit, thus increasing the dipole moment of
Figure 3. Absorption (left panels) and emission (right panels) spectra
of 2 in aqueous solution, alone and in the presence of increasing
concentration of the two surfactants (up to 0.03 M): SDS (upper
panels, [2] = 3.4 × 10−6 M) and pOoBSK (lower panels, [2] = 4.3 ×
10−6 M). The spectra recorded before the cmc of SDS and pOoBSK
are represented as blue lines.
before reaching the cmc and after the formation of some
micelles in the solution.
After the first additions of surfactant into the solution, a
reduction of the absorption intensity is detected along with a
small bathochromic band shift (which is more apparent with
pOoBSK), while the fluorescence spectrum stays essentially
Figure 4. Pump−probe absorption spectroscopy of 1 within SDS (0.1 M) (left graph) and pOoBSK (0.1 M) (right graph) solutions (λexc = 400
nm): (A) contour plot of the experimental data, (B) time-resolved absorption spectra (the first two in dashed lines) recorded at increasing delays
after the laser pulse (inset: decay kinetics at meaningful wavelengths, with a linear scale for the first picoseconds and a log scale for longer times), and
(C) species associated spectra (SAS) calculated by target analysis.
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Figure 5. Pump−probe absorption spectroscopy of 2 within SDS (0.1 M) (left graph) and pOoBSK (0.1 M) (right graph) solutions (λexc = 400
nm): (A) contour plot of the experimental data, (B) time-resolved absorption spectra (the first ones in dashed lines) recorded at increasing delays
after the laser pulse (inset: decay kinetics at meaningful wavelengths, with a linear scale for the first picoseconds and a log scale for longer times), and
(C) species associated spectra (SAS) calculated by target analysis.
Table 2. Excited State Spectral and Kinetic Properties of Compounds 1 and 2 in DCM, SDS (0.1 M) Solution, pOoBSK (0.1 M)
a
Solution, and W Obtained by Femtosecond Transient Absorption Spectroscopy (λexc = 400 nm)
1
2
medium
λ (nm)
<540(−)
585(−)
550(+), 605(−)
580(+)
τ (ps)
λ (nm)
τ (ps)
assignment
b
DCM
0.70
52
520(−), 635(+)
555(−)
0.68
10
Solv.
1LE*
1ICT*
T1
134
rest
0.61
23
575(−)
172
SDS solution
540(−), 615(+)
590(−)
535(+), 605(−)
450(+), 505(−), >555(+)
450(+), 540(−)
445(+), 550(−)
435(+), 585(+)
450(+), 520(−)
445(+), 540(−)
445(+), 545(−)
560(+)
0.69
8.6
Solv.
1LE*
1ICT*
T1
97
220
rest
0.69
30
pOoBSK solution
535(−), 610(+)
585(−)
525(+), 600(−)
0.39
30
Solv.
1LE*
1ICT*
Solv.i
Solv.d/1LE*
1ICT*
120
200
0.24
0.91
25
b
W
560(−)
520(+), 615(−)
0.70
4.4
515(−), 630(+)
<510(+), 580(−)
a
Spectral properties refer to the species associated spectra (SAS) calculated by target analysis. The symbols (+) and (−) stand for positive and
b
negative signals, respectively. Data retrieved from ref 36.
the relaxed excited state. Through quantum-mechanical
calculations, the latter was found to be twisted (TICT) in the
case of compounds 1 and 2 in polar solvents.36 Hence, in polar
environments, the two molecules experience lower energy
barriers during the ICT process and lose the net separation
between the LE and ICT states. The latter is directly populated
along with solvent relaxation (case b). The excited state
deactivation scheme of the two molecules is therefore very
revealing about the environment they can experience.
Looking at the time-resolved spectra of the compounds
(panel B of Figures 4 and 5), large spectral shifts are observed:
negative signals of stimulated emission shift to lower energies
over time. These substantial evolutions reflect the stabilization
of the excited states occurring together with their deactivation:
larger shifts mean more energetically stabilized excited states,
that is to say a more efficient intramolecular charge transfer.
The spectral evolution of compound 1 (Figure 4) is described
by a broad region of negative signal, in correspondence to its
stationary fluorescence spectral range, and it is thus assigned to
the stimulated emission promoted by the white-light
continuum probe. This negative band moves toward lower
energies while decaying within hundreds of picoseconds.
Compound 2 (Figure 5) shows a significant stimulated
emission signal in the 500−600 nm range, red-shifting during
the measurements, and a strong transient absorption band at
shorter wavelengths; both bands fully decay in less than 1000
ps. Similar spectra were recorded in pOoBSK micelles for the
two molecules.
The results of target analysis are shown in panel C of the
concerning figures and listed in Table 2, together with data
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Table 3. Triplet Spectral and Kinetic Properties of 1 and 2 in W, SDS (0.1 M) Solution, and DCM Obtained by Nanosecond-
Laser Flash Photolysis (λexc = 355 nm)
1
2
medium
λmax (nm)
τT (μs)
ϕT × εT (M−1 cm−1
)
kox (M−1 s−1
1.2 × 109
)
λmax (nm)
τT (μs)
ϕT × εT (M−1 cm−1
)
kox (M−1 s−1
)
a
DCM
600
860
590
740
570
730
1.6
1.2
5130
450
5
930
SDS solution
100
0.67
14
380
250
3.1 × 108
8.0 × 109
1.5 × 109
2.9 × 1010
440
690
430
710
120
1.6
200
160
1.1 × 108
1.0 × 1010
2.2 × 109
1.1 × 1010
a
W
125
0.47
0.41
a
Data retrieved from ref 37.
referring to the same molecules in the polar aqueous solution
(W) and in the much less polar DCM.
molecules under investigation in the presence of both
surfactant micelles. Table 3 shows the results referring to
SDS solutions, compared to the data collected in W and in
DCM.
The deactivation of the triplet state of the two compounds in
the presence of SDS micelles showed a behavior resembling the
one previously detected in water,37 although they experience a
scarcely polar environment: the transient absorption spectra in
micelles and those in aqueous solution are much more similar
than those in solvents such as acetonitrile or DCM. The
deactivation kinetics is well described by two components with
independent decays and different spectral shapes. As shown in
Figures 6 and 7 for compounds 1 and 2, respectively, the
The global and target analysis pointed out the contribution
of three transients to the relaxation dynamics in both micellar
media for both compounds: the shortest component (S) is
assigned to a fast solvent equilibration, as the one previously
seen in both DCM and W (e.g., for compound 1, τS = 0.61 and
0.39 ps in SDS and pOoBSK, respectively, and τS = 0.70 ps in
homogeneous solution). In fact, anionic surfactant micelles are
expected to provide the methylpyridinium derivatives with a
rather thin hydration layer, keeping them exposed to the
solvent to a certain degree.56 After the solvation has occurred,
in aqueous solution, the deactivation process is reported to be
extremely efficient with a time constant of 4.4 ps for 1 and 25
ps for 2.36 This transient is assigned to a twisted ICT state,
populated from the LE state during solvent relaxation (case b).
However, in the presence of micelles and just like what has
been observed in nonpolar solvents (as reported for DCM),36
two distinct electronic states are detected: an intermediate
component assigned to the relaxed LE state and a longer-living
component assigned to an ICT state. The detection of these
two distinct states offers evidence of the inclusion of the
molecule within the micellar aggregate, where the less polar
environment is responsible for a net separation between the
two states (ICT-controlled deactivation, case a).
The comparison between the two micellar media reveals
longer time constants for the intermediate component when
the pOoBSK is considered (τLE = 30 vs 23 ps for compound 1
and τLE = 30 vs 8.6 ps for compound 2 in pOoBSK and SDS,
respectively). These findings can be adequately explained by
taking into account the surfactant structures and the aromatic
ring of the pOoBSK molecule. The π-stacking interaction
between the surfactant and the two compounds has been
already invoked for the rationalization of the differences in the
alterations of the photophysical properties induced by the two
kinds of micelles: this interaction supposedly causes the LE
lifetime to lengthen by slowing down the back-charge transfer
and hindering the twisting which would lead to the fast
formation of the ICT state.
Despite the difference in the donor unit of the two
compounds, especially reflected in the peculiar fluorescence
quantum yields (larger for 2 than 1) and excited state lifetimes
(longer for 2 than 1) observed in pure aqueous solution, the
two pyridinium cations exhibit similar photophysical behavior
when dissolved in the same surfactant solution. These
interesting results unravel that the nature of the surfactant,
defining the microenvironment experienced by the probes, is
the key factor in determining their excited state properties.
Flash Photolysis Measurements. Nanosecond-resolved
laser flash photolysis measurements were performed on the two
Figure 6. Transient absorption spectroscopy of 1 in SDS (0.1 M)
solution (λexc = 355 nm): (A) contour plot of the experimental data,
(B) time-resolved absorption spectra recorded at increasing delays
after the laser pulse (inset: decay kinetics at meaningful wavelengths),
and (C) decay associated spectra (DAS) of the decay components
obtained by SVD and global analysis.
longer-living component, which absorbs at shorter wavelengths,
has been already assigned to the T1→Tn transient absorption,
whereas the fast component shows a broad band at lower
energies and is associated with the hydrated electron formed by
photoinduced ejection from the donor moiety of 1 and 2.37
In spite of the similarities with the spectral evolutions
acquired in water, evidence of the inclusion of the molecules
inside the micellar aggregate can still be found: (i) the time
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Article
surprising that the micellar environment causes photoisomeri-
zation to occur, especially in the case of pOoBSK micelles,
where the aforementioned interaction between the phenyl ring
of the surfactant and the donor moiety of the push−pull
compounds might limit the intramolecular charge transfer.
CONCLUSIONS
■
The effect of the organized medium provided by some anionic
micelles on the photophysics of the trans isomers of two push−
pull pyridinium salts was fully characterized through steady-
state and time-resolved spectroscopic techniques. By combining
all the information gathered from the new experiments
performed and the thorough knowledge of these molecules
acquired from the literature and previous studies, we gained a
deep insight into the nanoheterogeneous environment
experienced by the pyridinium salts.
Both molecules were found to be readily solubilized by the
anionic micellar aggregates. The electrostatic attraction
between the negatively charged surfactant heads and the
positively charged pyridinium ring proved to be the driving
force behind the prompt interaction leading to an efficient
inclusion within the micelles. The low polarity and confining
conditions provided by the micellar aggregate are responsible
for the distinctive spectral and photophysical properties shown
by these molecules. The absorption spectrum undergoes a
major red-shift and the fluorescence quantum yield is enhanced
by at least 1 order of magnitude in the case of both compounds
if compared to the pure water solution. A comparison between
the two surfactants revealed a reduced efficiency of the
intramolecular charge-transfer process operative in the excited
state in the case of the aromatic pOoBSK with respect to the
SDS surfactant, as a consequence of the π,π interaction between
the phenyl ring of the surfactant and those of the
methylpyridinium salts.
Moreover, the excited-state dynamics in the singlet and
triplet manifold are instrumental, since the ICT state
deactivation channels and its formation from the locally excited
(LE) state or via direct solvent relaxation depend on the
polarity and viscosity features of the chemical surroundings.
The detection of a transient associated with the LE state and
the slowed down deactivation of the ICT singlet state, together
with a significant concomitant population of the triplet state,
are all indicators of a micellar medium characterized by lower
polarity and confining properties.
Figure 7. Transient absorption spectroscopy of 2 in SDS (0.1 M)
solution (λexc = 355 nm): (A) contour plot of the experimental data,
(B) time-resolved absorption spectra recorded at increasing delays
after the laser pulse (inset: decay kinetics at meaningful wavelengths),
and (C) decay associated spectra (DAS) of the decay components
obtained by SVD and global analysis.
constant of the longer-living T1 transient (hundreds of
microseconds) is generally longer than the same component
detected in aqueous solution; (ii) its absorption band is slightly
red-shifted comparing to the same in pure water; (iii) its
formation quantum yield (ϕT) increases according to a reduced
polarity effect; (iv) it is quenched by molecular oxygen at a
slower rate than the diffusional one (kox ∼ 108 M−1 s−1). The
concentration of oxygen inside a micellar aggregate is indeed
expected to be lower than that in the bulk aqueous solution,57
and a less efficient oxygen quenching is thus envisaged. On the
other hand, the transient associated with the hydrated electron
is probably ejected out of the micelle and into the aqueous
medium and thus strongly affected by oxygen (kox ∼ 1010 M−1
s−1), as it was previously found in the experiments performed in
homogeneous aqueous solution.37 However, the lifetime
lengthening of this transient is supposedly due to the hindered
recombination with the radical species, which might still be
placed within the micellar aggregate.
These two methylpyridinium salts proved therefore to be a
telling probe of the local environment. Their inclusion within
the anionic micelles could also be exploited for their
solubilization and targeted delivery, in order to put to use
their potential pharmacological activity.
Differently, these compounds in pOoBSK micelles proved to
be extremely reactive upon laser irradiation and a significant
formation of photoproducts was revealed, thus affecting the
reliability of the acquired data.
The reason underlying this behavior is provided by the
competitive deactivation pathways. The photoisomerization
around the central double bond is an operative decay channel
for the trans isomers of the pyridinium salts, notwithstanding
being negligible in solvent of higher polarity. The photoreaction
competes with the intramolecular charge transfer, thus
becoming a minor deactivation pathway when the efficiency
of the ICT state formation and the subsequent internal
conversion to the ground state are enhanced (i.e., increasing the
solvent polarity or the charge separation). Even the inclusion
within the micellar aggregate should induce a less efficient
charge transfer, thus fostering both the isomerization in the
singlet manifold and the triplet formation, the latter
contributing to the isomerization as well.37 Thus, it is not
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: alex.cesaretti14@gmail.com. Phone: +39 0755855574.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank MIUR (Ministero dell’Universita
̀
e della
Ricerca, Rome, Italy) and University of Perugia (PRIN 2010−
2011, no. 2010FM738P, and FIRB 2013, no. RBFR13PSB6) for
funding.
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