Intramolecular charge transfer of push-pull pyridinium salts in the singlet manifold

Article abstract of DOI:10.1021/jp407342q

The solvent effect on the photophysical and photochemical properties of the iodides of three trans (E) isomers of 2-D-vinyl,1-methylpyridinium, where D is a donor group (4-dimethylaminophenyl, 3,4,5-trimethoxyphenyl and 1-pyrenyl), was studied by stationary and transient absorption techniques. The results obtained allowed the negative solvatochromism and relaxation pathways of the excited states in the singlet manifold to be reasonably interpreted. Resorting to ultrafast absorption techniques and DFT calculations allowed information on the excited state dynamics and the role of the solvent-controlled intramolecular charge transfer (ICT) processes to be obtained. The structure-dependent excited state dynamics in nonpolar solvents, where the ICT is slower than solvent rearrangement, and in polar solvents, where an opposite situation is operative, was thus explained. The push-pull character of the three compounds, particularly the anilino-derivative, suggests their potential application in optoelectronics.

Full text of DOI:10.1021/jp407342q

Article
pubs.acs.org/JPCA
Intramolecular Charge Transfer of Push−Pull Pyridinium Salts in the
Singlet Manifold
Benedetta Carlotti,† Giuseppe Consiglio,‡ Fausto Elisei,† Cosimo G. Fortuna,‡ Ugo Mazzucato,†
and Anna Spalletti*,†
†Department of Chemistry, Biology and Biotechnology and Centro di Eccellenza sui Materiali Innovativi Nanostrutturati (CEMIN),
University of Perugia, via Elce di Sotto 8, 06123 Perugia, Italy
‡Department of Chemical Sciences, University of Catania, viale Andrea Doria 6, 95125 Catania, Italy
S
* Supporting Information
ABSTRACT: The solvent effect on the photophysical and photochemical
properties of the iodides of three trans (E) isomers of 2-D-vinyl,1-
methylpyridinium, where D is a donor group (4-dimethylaminophenyl,
3,4,5-trimethoxyphenyl and 1-pyrenyl), was studied by stationary and
transient absorption techniques. The results obtained allowed the negative
solvatochromism and relaxation pathways of the excited states in the
singlet manifold to be reasonably interpreted. Resorting to ultrafast
absorption techniques and DFT calculations allowed information on the
excited state dynamics and the role of the solvent-controlled intramolecular
charge transfer (ICT) processes to be obtained. The structure-dependent
excited state dynamics in nonpolar solvents, where the ICT is slower than
solvent rearrangement, and in polar solvents, where an opposite situation is
operative, was thus explained. The push−pull character of the three
compounds, particularly the anilino-derivative, suggests their potential
application in optoelectronics.
and DFT calculations. The photobehavior of the polar 1
(usually named by its acronym o-DASPMI) and its positional
isomer p-DASPMI, were previously studied in various
laboratories by stationary and ultrafast spectroscopic techni-
ques.9,16−30 For 1, a negative solvatochromism and a very
modest, if any, positive fluosolvatochromism were reported.
The solvent-dependent excited state dynamics, which involves
internal rotation around double (cis−trans isomerization) and
single (formation of twisted rotamers) bonds, was discussed
using different approaches. Some authors mainly emphasized
the effects of solvent dynamics reflected by the fastest decay
components25,26,28,29 while others gave more attention to the
concomitant evolution of the excited solute on the basis of a
three-state model.23,24,27 These authors attributed the multi-
exponential emission specifically to the contributions of the
locally excited (LE) state, formed by the relaxation of the
Franck−Condon state and precursor of a second state (still
planar), produced by further intramolecular charge transfer
(ICT), generally favored in polar solvents, and a successive
third state formed in polar solvents by the twisting of the p-
dimethylanilino or N-methylpyridinium group around the
quasi-single bond with the ethene bridge (TICT).23,24,27 As
pointed out in previous papers23,29 both aspects, the relaxation
1. INTRODUCTION
In the framework of our long-term project on the internal
rotation around double and single bonds, we have recently been
concerned with the study of quaternized azastilbenes1−4 since
methyl-pyridinium salts and analogous compounds could be of
interest for potential applications in the medical fields, due to
their ability to form complexes with DNA,5,6 and in
optoelectronics/photonics as nonlinear optical (NLO) materi-
als.7 In particular, dimethylamino-derivatives have also been
used as fluorescent sensors8 since the introduction of the
anilino group can affect significantly the quantum yields of the
competitive reactive (trans → cis photoisomerization) and
radiative deactivation.9
Solvent polarity is expected to substantially influence the
photobehavior of these ionic dipolar compounds where
electron-donor groups and electron-deficient pyridinium units
are present. The positive solvatochromism of push−pull
systems is well documented10−14 whereas the negative
solvatochromism, where the molecular dipole moment is
expected to decrease under excitation, has been less
studied.8,9,12,15 In this paper the effect of solvent polarity on
the spectral and photophysical properties of two stilbazolium
salts and a pyrenyl-analogue, namely the iodides of the trans
(E) isomers of the 2-D-vinyl,1-methylpyridinium cations where
D is a donor group [4-dimethylaminophenyl, (1), 3,4,5-
trimethoxyphenyl (2) and 1-pyrenyl (3)] (Scheme 1), were
investigated by stationary and transient absorption techniques
Received: July 24, 2013
Revised: April 29, 2014
Published: April 29, 2014
© 2014 American Chemical Society
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Scheme 1. Molecular Structures of the Trans (E) Cations of the Iodides of 2-D-Ethenyl-1-methylpyridinium (D = 4-
(Dimethylamino)phenyl (1), 3,4,5-Trimethoxyphenyl (2), and 1-Pyrenyl (3)
of solvent dipoles and the detection of several emitting states,
have to be taken into account when describing the emission
properties of these kind of dyes.
stirred during the experiments to avoid photoproduct
interferences. Transient absorption data were analyzed using
the Surface Xplorer PRO (Ultrafast Systems) software where it
was possible to perform singular value decomposition35,36 of
the 3D surface into the principal components (spectra and
kinetics) and to perform global analysis (giving lifetimes and
decay associated spectra (DAS) of the detected transients)37
with a number of components determined on the basis of best
fitting procedure. Target analysis assuming several successive
steps and resulting in the species associated spectra (SAS) was
also used to globally fit the acquired data by using Glotaran.38
Only consecutive first order reactions were considered.
For photochemical measurements, a xenon lamp coupled
with an interferential filter (λexc = 436 or 325 nm) and
potassium ferrioxalate in 0.1 N sulfuric acid as actinometer were
used. The photoreaction (solute concentration ∼10−4 M) was
monitored by HPLC using a Waters apparatus equipped with a
Phenomenex Jupiter C18−300 Å (4.6 × 250 mm; 5 μm)
column and an UV detector. W/MeCN mixtures with some
addition of trifluoroacetic acid were used as eluent. The
monitoring wavelength was at the isosbestic point between
trans and cis absorption spectra, detected by irradiating the
trans isomer in the eluent mixture. The conversion percentage
was held below 10% to avoid the competition from the back
photoreaction and the experimental error on the photo-
isomerization quantum yield is about 10%. All measurements
were performed at room temperature.
The main aim of the present work is 2-fold. Using ultrafast
techniques and with the help of DFT calculations we obtain
information on the excited state behavior of 1 in the singlet
manifold and in a wide series of solvents of different polarity
thus allowing the solvent-controlled ICT process and its
dynamics to be exhaustively interpreted. Moreover, we extend
the measurements to the new 2 and 3 analogues, to compare
the behavior of the three compounds having electron donor
properties of different efficiency, in order to better understand
the role of the ICT process in these molecules and the
competition between the solvent organization and the charge
movement in the solutes. Contrary to what generally reported
in the literature, the new results indicate that the relaxed ICT
state (or TICT, in the case of compounds 1 and 2) reflects a
back localization of the positive charge on the pyridinium
group.
2. EXPERIMENTAL SECTION
2.1. Materials. The investigated compounds 1−3 are
shown in Scheme 1. They were synthesized as iodide salts at
the Catania Laboratory following the procedure described in
previous works.5,6
Measurements were performed in various organic solvents of
different polarity: chloroform (CHCl3), dichloromethane
(DCM), 1,2-dichloroethane (DCE), acetone (Ac), acetonitrile
(MeCN), dimethyl sulfoxide (DMSO), ethanol (EtOH),
methanol (MeOH), 2-propanol (2-PrOH), water (W), and
glycerol (Gly). All solvents were from Fluka (spectrophoto-
metric grade) and were used without further purification.
2.2. Photophysical and Photochemical Measure-
ments. A PerkinElmer Lambda 800 spectrophotometer was
used for absorption measurements. The fluorescence spectra
were measured by a Spex Fluorolog-2 F112AI spectrofluorim-
eter. Dilute solutions (absorbance <0.1 at the excitation
wavelength, λexc) were used for fluorimetric measurements.
The fluorescence quantum yield (ϕF, uncertainty 10%) was
determined at λexc corresponding to the maximum of the first
absorption band (λmax). Tetracene (ϕF = 0.1731 in aerated
cyclohexane, CH) and 9,10-diphenylanthracene (ϕF = 0.90,32
in deaerated CH) were used as fluorimetric standards.
A detailed description of the experimental setup for ultrafast
spectroscopic and kinetic measurements was previously
reported.33,34 The 400 nm excitation pulses of ca. 60 fs were
generated by an amplified Ti:sapphire laser system (Spectra
Physics, Mountain View, CA). The transient absorption set up
(Helios, Ultrafast Systems) is characterized by temporal
resolution of ca. 150 fs and spectral resolution of 1.5 nm. All
measurements were carried out under magic angle in a 2 mm
cell at an absorbance of about 0.5 at 400 nm. The solution was
2.3. Theoretical Calculations. Simple theoretical calcu-
lations of the electronic spectra, conformational equilibria and
heats of formation of the cations were carried out using the
HyperChem computational package (version 7.5). The
computed transition energies and oscillator strengths were
obtained by ZINDO/S using optimized geometries (according
to PM3 method) including 400 (20 × 20) single excited
configurations.
Mulliken charges and dipole moments in the ground and
excited singlet state (both in the vertical and relaxed
conformation) were obtained by quantum-mechanical calcu-
lations using Gaussian 09 package39 by taking also into account
the solvent effect in the ground and excited states through the
conductor polarizable continuum model (CPMC).40 Density
functional theory (DFT) B3LYP/3-21G* was used to optimize
the ground state geometry of the substrates while the lowest
excited singlet states were characterized by time dependent
DFT excited-state calculations (TD-DFT/B3LYP/3-21G*).
Dipole moments were obtained by the Mulliken atomic
charges.41,42
As generally applied to these kinds of salts,43,44 the
calculations refer to the monochromophoric cations. To
confirm the validity of this assumption we also did some
calculations using the B3LYP/3-21G* method on the reference
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Table 1. Spectral Properties of 1 in Solvents of Increasing ETN
Parameters
compound 1 that allowed the counterion to be considered (see
section 3 in the Supporting Information).
solvent
ETN
λabs/nm
λem/nm
Δν/cm−1
3. RESULTS AND DISCUSSION
CHCl3
DCM
DCE
0.259
0.309
0.327
0.355
0.441
0.472
0.546
0.654
0.762
0.787
0.827
0.896
1.0
484
500
497
458
460
456
469
467
462
464
460
451
438
577
590
594
603
610
600
583
589
590
588
585
585
589
3330
3090
3285
5250
5350
5260
4170
4435
4695
4545
4645
5080
5850
3.1. Spectral Properties and Solvatochromism. The
absorption and emission spectra of 1−3 were measured in a
large series of solvents as a function of the ENT parameter, based
on the pyridinium N-phenolate betaine.45
The salts were considered completely dissolved as free ions
surrounded by solvent without any clear evidence of ion-pair
formation. In a detailed study on the para-isomer of compound
1, the presence of ion-pair was evidenced in solvents of ENT <
0.3.46 On the basis of this result, some contribution of ion pairs
could be expected in CHCl3 only (ENT = 0.259). In fact, some
anomalies found in the behavior of 1−3 in CHCl3, such as the
blue shift of the absorption maximum with respect to DCM or
a too low ϕF (see below), could be related to partial
contribution of ion-pairs.
Ac
DMSO
MeCN
2-PrOH
EtOH
MeOH
MeOH/Gly (50/50)
W/EtOH (50/50)
W/EtOH (70/30)
W
Generally the spectra show nonstructured, intense and bell-
shaped bands (Figure 1 shows an example in DCE). The
Figure 1. Normalized absorption and emission spectra of the
investigated compounds in DCE.
Figure 2. Normalized absorption and emission spectra of 1 in solvents
of different polarity.
spectral behavior of 1 in a few solvents was already reported in
previous papers16−20,22−29 but was here revisited to extend the
range of solvents examined (see section 3.5 on the ultrafast
measurements) and to compare the results for the three
compounds under the same experimental conditions. The
absorption spectra of 1 shift notably toward the blue on
increasing the solvent polarity (Table 1 and Figure 2) in
agreement with literature results.16−20,22−29 On passing from a
nonpolar and nonprotic DCM to the polar and protic W, a
significant blue shift of about 60 nm was observed. It should be
noted that hydrogen-bonding solvents, hampering the displace-
ment of charge, can also contribute to the blue shift of the
absorption spectrum. This could offer another explanation as to
why the λmabasxvalues in CHCl3 do not fit the trend as a function
of the ENT parameter.17,21
In contrast to the absorption trend, the weak emission,
centered at ∼600 nm, is influenced very little by the solvent.
This behavior can be considered normal for ionic systems
having a large dipole moment in the ground state that decreases
under excitation, as found by the quantum-mechanical
calculations (see below).
irradiation and further after relaxation to the TICT states giving
rise to a highly Stokes-shifted fluorescence (strong positive
fluosolvatochromism).
High absorption coefficients were measured in nonpolar
solvents (39700 and 40900 M−1 cm−1 in CHCl3 and DCM,
respectively) while a much lower value was found in W (25700
M−1 cm−1), probably due to the increased CT character in
polar solvents (see section 3.5).
An analysis of the full width at half-maximum (fwhm) of the
first absorption band (Δν1/2) of 1 showed that it gradually
̅
increases from 3240 to 6060 cm−1 on going from DCM to W
(being 5090 cm−1 in 7/3 W/EtOH mixture and 4080 cm−1 in
MeOH) . This behavior is typical of a net CT on excitation.12,50
However, the Δν1/2 of the emission band was affected much
̅
less by the solvent; it only increased by 500 cm−1 on going from
DCM to W.
The dual- (or multi-) emission, reported in previous
works,24−28 was not evidenced in the stationary fluorescence
spectrum of 1, probably because of similar energies (LE-S0 and
TICT-S0) involved in the deactivation, but the presence of at
least two emitting states of different nature cannot be excluded
and would be in agreement with the strong decrease of the
The behavior of our ionic compounds is in a net contrast to
the better known corresponding neutral [4-(dimethylamino)-
styryl]pyridines47−49 where the dipole moment increases under
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a
Table 2. Spectral Properties of 2 and 3 in Solvents of Increasing ENT Parameters
2
3
solvent
λabs/nm
λem/nm
Δν/cm−1
λabs/nm
λem/nm
Δν/cm−1
CHCl3
DCM
390
393
393
369
378
375
371
369
362
353
535
559
553
555
543
548
554
544
539
546
6950
7690
7362
9082
8039
8418
8904
8718
9071
10014
400sh, 457
400sh, 463
400sh, 462
393sh, 429
397sh, 444
397sh, 440
394sh, 434
394sh, 436
393sh, 431
392sh, 420
569
588
590
585
582
582
585
580
581
586
4307
4590
4696
6216
5340
5545
5947
5694
5990
6745
DCE
MeCN
2-PrOH
EtOH
MeOH
MeOH/Gly(50/50)
W/EtOH(50/50)
W
a
sh means shoulder.
fluorescence quantum yield and kinetic constant (see below,
Table 6) on increasing the solvent polarity that implies also
small spectral shifts (not parallel to those observed in
absorption).
Table 2 and Figures 3 and 4 show the spectral parameters of
2 and 3 in a limited number of solvents. The trimethoxy-
Figure 4. Normalized absorption and emission spectra of 3 in solvents
of different polarity.
The absorption and emission spectra of compounds 1-3 in a
viscous medium (MeOH/Gly, 50/50, v/v) are reported in
Figure 1SI (Supporting Information) and compared with those
in MeOH (similar solvent polarity). These spectra showed that
the solvent viscosity does not affect the spectral shape and
position that are mainly determined by the polarity.
Figure 3. Normalized absorption and emission spectra of 2 in solvents
of different polarity.
Quantum-mechanical calculations by semiempirical methods,
reported in Table 1SI for the three compounds, gave spectral
results in substantial agreement with the available experimental
data (present work and literature data for 1). The results
showed that the first transition (of ππ* nature and mainly
described by HOMO−LUMO configuration) is totally
localized in the cation with a partial charge-transfer character
(Scheme 1SI).
To further justify the neglect of the counterion we compared
the experimental absorption spectrum of compound 1 in
MeCN and DCM with those in the same solvents after elution
by HPLC (see Figure 2SI) which causes an anion exchange of
I−, replaced (most likely) by Cl−. The hypsochromic band
around 245 nm (characteristic of I−) is missing after elution,
whereas the main bathochromic band is the same before and
after elution in both solvents.
substituted compound 2 absorbed at much shorter wave-
lengths, almost 100 nm, with respect to the dimethylamino
analogue (λmax = 390 and 353 nm; εmax = 23700 and 24800 M−1
cm−1 in CHCl3 and W, respectively), while in the pyrenyl-
derivative 3 the main band shifted by ca. 35 nm to the blue
(λmax = 462 nm and εmax = 23800 M−1 cm−1 in DCE). This
result was expected due to the reduced donor ability of the
pyrenyl and trimethoxyphenyl moieties (see section 3.2) and
the consequent reduced CT character of the transition.
The absorption spectrum of 3 shifted hypsochromically in
polar solvents (λmax = 420 nm and εmax = 21530 M−1 cm−1 in
W) with a shoulder around 395 nm, that was affected little by
the solvent. It was assigned to the π,π* transition that is
probably localized at the aromatic group. As found for 1, the
emission spectra of 2 and 3 are quite insensitive to the solvent
polarity. Even for them, an absorption shift toward the blue was
found in CHCl3 compared with the spectrum in DCM.
3.2. Dipole Moments. The cations of these salts are
dipolar A+−π−D systems, where the pyridinium unit A+ is the
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Table 3. Mulliken Charges and Dipole Moments of 1−3 in DCM Calculated by DFT/B3LYP/3-21G* (S0) and TD-DFT/
B3LYP/3-21G* (S1) together with the Dihedral Angles θ (C9−C8-C1−C2) and φ (C8−C9-C10-C11) Showing Rotation of Parts C
and A, Respectively
Scheme 2. Calculated Molecular Structure and Dipole Moment Orientation in the Ground State (S0) and in the FC (S1,FC) and
Relaxed (S1,ICT) Excited Singlet States for 1−3 in DCM
strong acceptor and the dimethylaminophenyl D the strong
donor, while the trimethoxyphenyl and pyrenyl groups are D
units of moderate donor efficiency, as indicated by their higher
halfwave reduction potential (+0.81, +1.42 and +1.16 V vs SCE
for 1, 2 and 3, respectively).51 Obviously the dipole moment for
an ion is undefined since it depends on the choice of the
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Scheme 3. Conformational Equilibrium of Cation 1
coordinate origin. It is usually convenient to setup a “working
definition” based on the center of gravity of the system.52
Considering that the ICT involving the π-system of the
monochromophoric cation is the largely prevailing electronic
effect for these salts, one can compare the charge transferred
from the D to the A unit upon excitation51,52 neglecting the
counterion.44 Thus, the dipole moment of the cation can be
defined just like that of conventional polar molecules.43,44 The
dipole moments of Table 3 were computed for the ground (S0)
and excited FC (S1,FC) states and for the relaxed ICT (or
TICT) state formed after completion of the charge movement
in the excited state (S1,ICT). Table 3 reports also the charge
densities calculated for the three moieties of the cation, namely
the donor (part A), ethene bridge (part B) and methylpyr-
idinium (part C) groups, as shown in Table 3 for 1. Calculated
and a complete charge shift can be induced by polar solvents,
particularly for 1 and 2 where twisting around single bonds
favors the localization of the positive charge on the
methylpiridinium moiety (C in Table 3) again. This polar
TICT state is characterized by a high rate of formation as well
as by a quick decay (internal conversion, IC) to the ground
state.43,55,56 This explains why fluosolvatochromism is rarely
observed in ionic species where the TICT formation manifests
itself through fluorescence quenching. Interestingly, the final
increase of the dipole moment leads to a computed μICT value
slightly higher than μS because the twisting in the excited state
0
assures a better localization of the positive charge with respect
to the planar ground state. The particular behavior found for 3,
where twisting does not occur and the dipole moment does not
change direction under excitation, leads to μICT ≅ μS and
0
molecular structure and dipole moment orientation in S0, S1,FC
,
explains why in this compound other relaxation processes
become more competitive, particularly photoisomerization.
Our interpretation of a final S1 state with the positive charge
again localized at the pyridinium moiety appears in contrast
with the information available in the abundant literature on 1.
Direct calculations of the charge localization in the relaxed
TICT state are not available in previous works, but it can be
argued from the reports of different groups that the TICT
formation is considered to imply a localization of the positive
charge on the twisted donor.8,25,27 On the contrary, the present
calculations (Mulliken charges, molecular structures and dipole
moment orientation) seem to clearly indicate that the positive
charge in TICT of 1 is returned to the pyridinum moiety. A
similar behavior has been found for the twisted compound 2
and for the planar 3.
and S1,ICT states for 1−3 in DCM are reported in Scheme 2.
Further computational results for 1 in DCM are shown in
Table 2SI and Scheme 2SI.
Relatively high values (μS = 6.9, 14.6, and 17.7 D in DCM
0
for 1, 2 and 3, respectively, see Table 3) were found for the
dipole moment of the most stable ground state conformer
(section 3.3) due to the positive charge largely localized on the
pyridinium group (see the Mulliken charges in Table 3).
As shown by the MOs in Scheme 1SI and the charge
densities in Table 3 the CT character of the transition under
light absorption leads to an electron transfer toward the
pyridinium group, which reduces the polar character of the FC
excited state (μFC = 3.3, 4.9, and 4.5 D in DCM for 1, 2, and 3,
respectively). These results are similar to those reported for the
para-isomer of 153 and analogous compounds.54
3.3. Conformational Equilibria. In principle, the flexible
molecules under investigation could exist in fluid solution at
room temperature as an equilibrium mixture of conforma-
tions58,59 that originate from the rotation around the quasi-
single bonds between the aryl groups and the double bonds.
The formation enthalpy (ΔHof ) calculated by PM3 method and
reported in Table 1SI, gave information on the most stable
conformer. Compound 1 exists as an equilibrium between two
conformers completely shifted toward 1A at room temperature
(Scheme 3). This is in agreement with a λexc-independent
emission spectrum as expected from the coincidence between
absorption and fluorescence excitation spectra (see Figure 3SI
for 1 in DCM, as an example).
Indeed, the rotation of the methylpyridinium group around
the quasi-single bond with the central bridge leads to a steric
interaction between the methyl group and the ethenic
hydrogen, which is particularly strong in the 1B conformer.
Similar results were obtained for the conformers of 2 and 3.
Even for them, the computed formation enthalpies indicate that
the conformers shown in Scheme 1 (2A and 3B of Table 1SI)
are expected to be the prevalent species in our solutions.
The dipole moment component along the principal
molecular axis (μx, Table 3) changes direction upon excitation
only in the case of 1 and 2, but the total dipole moment (μTot
,
last column of Table 3) of the S1,FC state is reduced for all the
three compounds. The relaxed conformation of the S1,ICT state
has a twisted geometry in the case of 1 and 2 (the twisting
concerning the rotation of methylpiridinium and trimethox-
yphenyl groups, respectively) whereas the pyrenyl-derivative
retains a planar configuration. The μTot of the relaxed S1
increases and returns to the same direction of the ground
state (Table 3) pointing to a relaxation that involves a back
charge transfer from C to A moieties. In fact, the positive
Mulliken charge of the C moiety decreases on excitation and
increases again during the relaxation process toward the
optimized geometry of S1.
It should be recalled that while in the nonionic compounds a
charge separation is needed for the formation of the D+−π−A−
structure in the perpendicular TICT conformation, in the ionic
ones a charge shift, favored in polar solvents, is sufficient.24,55 In
the present case the planar and moderately polar state
produced by absorption undergoes a back charge migration
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Table 4. Fluorescence and Trans → Cis Photoisomerization Quantum Yields of 1−3 in Various Solvents
1
2
3
solvent
ϕF × 102
ϕt→c
0.16
ϕF × 102
ϕt→c
ϕF × 102
ϕt→c
CHCl3
DCM
5
14
13
0.2
0.1
1.2
3.2
1.3
2.1
2.2
1.5
3.4
0.6
0.3
0.07
1.0
3.3
6.0
5.3
6.7
7.3
6.9
7.2
5.4
2.8
a
a
a
0.16 (0.09)
0.03 (0.025)
0.30 (0.20)
0.50
DCE
MeCN
<0.001
0.14
2-PrOH
EtOH
1.0
0.4
1.9
0.064
0.22
0.60
MeOH
MeOH/Gly 50/50
W/EtOH
W
0.1
<0.001
0.054
0.12
a
Values in parentheses refer to aerated solutions.
3.4. Fluorescence and Photoisomerization Quantum
compounds where a fluorescence decrease is generally coupled
with a favored photoisomerization.60 This probably reflects the
fact that in these push−pull compounds in polar solvents one
of the two moieties attached to the ethene bridge (anilino or
pyridinium group) undergoes a twisting around the single bond
with the bridge toward perpendicularity with the formation of
the polar TICT state, which in turn undergoes a fast
radiationless IC to the ground state. Such behavior is generally
assumed for stilbenoid systems including 1 and is supported by
the early observation that the rigid analogues have much higher
fluorescence yields.8,23 Instead, the increase of both processes
in low polarity solvents may reflect competitive emission and
ISC to the triplet manifold,57 the latter being followed by
diabatic isomerization. The more abundant emissive/reactive
decay found for 2 and particularly for 3 even in polar solvents
indicates that slower ICT processes are operative.
Yields. The quantum yields of the radiative and reactive
deactivation pathways of 1−3 in solvents of different ENT are
reported in Table 4. The ϕF values of 1 are rather low in the
less-polar solvents and decrease further in polar solvents in
agreement with the values reported in alcoholic media.20,23
A
similar, although reduced, trend with the ENT parameter was
observed for 2. A particularly strange behavior was observed in
CHCl3 where the lowest emission yield was measured, probably
due to the cited role of ion-pairs in this solvent.48 A different
trend was found for 3 where the ϕF value remained practically
unchanged, again with the exception of CHCl3. The ϕF values
were not influenced by molecular oxygen as expected from the
very short lifetimes of the lowest excited singlet states (tens or
hundreds of ps, see also section 3.5).
The fluorescence quantum yield increased for 1 and 2 in a
more viscous medium (MeOH/Gly in Table 4) but remained
unchanged for 3 in agreement with twisted (1 and 2) and
planar (3) conformations calculated for the relaxed S1 state.
The irradiation of the trans isomer of 1-3 produced the cis
isomer (detected by HPLC) as the only photoproduct (its
absorption spectrum is reported in Figures 4SI-6SI for 1−3
compounds). The trans → cis photoisomerization of these
compounds occurs by the common diabatic mechanism. This
implies twisting around the central double bond toward an
energy minimum at the perpendicular configuration in the
singlet or triplet manifold, followed by S1 → S0 internal
conversion (IC) or T1 → S0 intersystem crossing (ISC) and
relaxation to the ground-state trans and cis isomers in roughly a
1:1 branching ratio.60 The photoreaction yield (ϕt→c) was
negligible in polar solvents but increased substantially in
nonpolar solvents for 1 (as reported for the analogous p-
DASPMI9) and in all the solvents examined for 2. The increase
was even greater for 3 but in this case solubility problems
limited the number of media investigated. The high reactivity of
3 compared to the photostability of 1-styrylpyrene61 indicates
that the pyridinium group strongly reduces the energy barrier
to the twisting around the ethene double bond toward the cis
geometry.
3.5. Femtosecond Transient Absorption. The transient
absorption measurements of compounds 1−3 were carried out
in various solvents of different polarity upon excitation with
laser pulses at 400 nm. Figures 5−7 show a contour plot of the
experimental data (panel A) and the main time-resolved
absorption spectra and kinetics recorded at significant wave-
lengths (panel B), together with the species associated spectra
(SAS) and kinetic properties of the main components obtained
by target analysis (panel C) for 1 in DCM and for 2 and 3 in
MeOH. Spectral results extended to several other solvents
(shown in Figures 7SI-19SI) and data analysis are summarized
in Table 5. In particular, the latter shows the time constant τ of
each detected transient together with the wavelengths where
positive and negative signals, corresponding to decay and
growth, respectively, were obtained. In the last column the
assignment of the different transients is reported. Fittings of the
transient absorption at meaningful wavelengths for 1−3 in
MeOH and 1 in DCM in all the investigated ps range and at
early times (2 ps) are reported, as an example, in Figure 20SI to
show the goodness of our fitting models.
Compound 1. The spectrum of 1 in DCM (Figure 5) shows
a negative broad band of stimulated emission, formed just after
the laser pulse. This band shifts from <540 to 565 nm and
grows during the first 0.8 ps; then it shifts from 565 to 600 nm
and decays within 390 ps to form a positive transient
absorption (rest in Table 5) detected around 580 nm and
assigned to the triplet state on the basis of laser flash photolysis
results.57 The data analysis for 1 in DCM (Table 5) showed
that three components are present in addition to rest: one with
a short lifetime (S) of 0.70 ps, a second one with a medium
decay time (M) of 52 ps and a third one with a longer decay
In contrast to the case of the emissive deactivation, the
photoreaction was efficiently quenched by oxygen for 1 in
aerated DCM (see Table 4) pointing to a contribution of ISC
followed by isomerization in the triplet manifold. This finding is
in agreement with the substantial triplet population found for
this compound in DCM.57 The negligible yield of both
emissive/reactive deactivation pathways of 1 in polar solvents
differs from the results reported for nonionic stilbenoid
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Figure 6. Pump−probe absorption spectroscopy of 2 in MeOH (λexc
=
Figure 5. Pump−probe absorption spectroscopy of 1 in DCM (λexc
=
400 nm): (A) contour plot of the experimental data, (B) time-resolved
absorption spectra recorded 0 (a), 0.1 (b), 0.2 (c), 0.6 (d), 1.4 (e), 3.1
(f), 11 (g), 28 (h), 66 (i), 126 (j) and 766 (k) ps after the laser pulse
(inset: decay kinetics recorded at meaningful wavelengths, with a linear
+ log scale), and (C) species associated spectra (SAS) calculated by
target analysis.
400 nm): (A) contour plot of the experimental data, (B) time-resolved
absorption spectra recorded 0.15 (a), 0.45 (b), 0.80(c), 1.2 (d), 2.8
(e), 5.0 (f), 9.2 (g), 17 (h), 38 (i), 56 (j), 97 (k), 170 (l) and 390 (m)
ps after the laser pulse (inset: decay kinetics recorded at meaningful
wavelengths, with a linear scale for the first picoseconds and log scale
for the higher times), and (C) species associated spectra (SAS)
calculated by target analysis.
excitation indicating that intramolecular processes and solvent
relaxation may occur simultaneously25,62 and that the ICT
trajectory is controlled (probably induced) by the solvation
process on a ps time scale.
Molecular dynamics calculations on the fastest decays have
shown that the solvent response function can be bimodal
containing a very fast (inertial motion) component and a red-
shifted, much slower (diffusive) component.63 These should
correspond to our S lifetimes (inertial solvation, Solvi, in all
solvents) and M lifetimes (diffusive solvation, Solvd, in polar
solvents), reflecting the different stages of solvation of the
solute.
The role of a biphasic vibrational cooling (VC) process in a
few hundreds of fs (solute−solvent energy transfer) and in a
few ps (solvent−solvent transfer), as recently reported for two
UV dyes (2,5-diphenyloxazole and p-terphenyl),64 can offer an
alternative explanation.65 The latter could be operative due to
the significant vibrational excess energy at λexc = 400 nm. For
this reason, the option VC was introduced for the shortest
transients observed in all solvents (see last column of Table 5).
The planar and moderately polar LE state described in the
literature (evidenced in our ultrafast measurements only in less
polar solvents for 1 and 2 and in all solvents for 3) can be
considered to lie in a minimum of the potential energy surfaces
with similar geometry and dipole moment of the FC state but
vibrationally relaxed. The observation that the solvent depend-
ent dynamics is similar for the three compounds investigated
(see below) and is also quite similar for the bridged and
unbridged o-DASPMI and analogous ionic styryl dyes,28,63
supports the conclusion that in polar solvents the shortest living
S transient, measured at short wavelengths, is related to inertial
time (L) of 134 ps. Spectral shapes associated with each
component (Figure 5, panel C) were obtained: the transients S,
M and L show bands of negative signal centered at <540, 585,
and 605 nm, respectively, the two latter in the region of the
steady state fluorescence emission spectrum (where the
different components are probably buried within the apparently
single emission band).24
Similar behaviors with different time constants and analogous
spectral shapes were found for 1 in other solvents of different
polarity (see Figures 7SI-11SI and Table 5). The data analysis
in polar solvents reveals three components, generally of shorter
time constants if compared with those in less-polar solvents,
and the disappearance of the transient rest, in agreement with
strong decrease of the triplet yield observed in MeCN by ns
measurements.57 Only two components characterized by the
shortest lifetimes (0.7 and 4.4 ps) were detected in the most
polar W (see Figure 11SI). Moreover, in polar solvents the
transient L showed a spectral shape also characterized by an
absorption band at wavelengths around 535 nm (see Figures
7SI and 9SI-11SI). This was not observed in CHCl3, DCM and
DCE because of the presence of the red-shifted ground state
absorption.
In general, our results are in reasonable/good agreement
with those of previous studies in CHCl3 (34 and 79 ps),27 W (6
ps)27 and EtOH (4 and 62 ps;27 ∼1, ∼6, and 40 ps28) even if in
some cases the literature data do not include the S transient,
due to insufficient resolution time of the equipment.
In agreement with previous results,23−30 the solvent
relaxation is accompanied by a progressive red shift of the
stimulated emission band during the first ps after laser
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finding can explain the higher fluorescence (greater contribu-
tion from LE) and/or ISC57 yields found in these solvents.
Compounds 2 and 3. Analogous studies were carried out
for compounds 2 and 3. The results of SVD and global analysis
are also reported in Table 5.
The behavior of the trimethoxy analogue 2 (Figure 6 and
Figures 12SI-15SI) is rather similar to that of 1 showing a
stimulated emission band red shifting with time. The transient
absorption band is out of the investigated spectral range (below
470 nm) in analogy with the hypsochromically shifted
absorption observed for the ground state of 2 with respect to
1. The main difference was found in the longer τM and τL values
which decrease less with increasing the medium polarity. On
going from PrOH to W, τL decreases by a factor of 25 for 1 but
only by a factor of 6 for 2. This behavior is expected
considering the diminished donor properties of the trimethoxy
substitution of 2, which leads to a reduced push−pull character
of this compound. It can be noted that the longer τM value of 2
with respect to 1, particularly evident in nonprotic and
medium-polarity solvents (Ac, DMSO and MeCN) indicates
an ICT process that is slower than diffusive solvation as in less-
polar solvents.
An interesting behavior was found for 3, particularly in
solvents of medium and high polarity (see Figures 7, 16SI-19SI
and Table 5), since four components resulted from the fitting
and the ICT efficiency was further reduced according to the
computed planar conformation. Figure 7 shows the spectra in
MeOH, where the initial absorption band at 620 nm shifts to
540 nm in around 8 ps. This shift is accompanied by the
formation of a negative stimulated emission signal around 630
nm. The formed spectrum decays to zero within 1300 ps. The
two components with the shortest lifetimes (S and S′)
correspond to the inertial and diffusive solvation parameters.
The time constants for M and L were generally found to be
longer than those obtained for 1 and 2 in the same solvents (for
example, the τL value is 19, 93, and 310 ps in MeCN for 1, 2,
and 3, respectively). This result indicates that the slower ICT
for 3, not accompanied by twisting (see Scheme 2 and Table
3), makes the two processes (ICT and diffusive solvation)
separable in polar solvents, thereby strengthening the transient
assignments for 1 and 2. The reduced ICT character of 3 is also
reflected by the reduced effect of the solvent polarity on the
time constants of the M and L transients that remain of the
same order of magnitude (tens and hundreds of ps,
respectively) in all the investigated solvents.
Figure 7. Pump−probe absorption spectroscopy of 3 in MeOH (λexc
=
400 nm): (A) contour plot of the experimental data, (B) time-resolved
absorption spectra recorded 0.1 (a), 0.2 (b), 0.4 (c), 0.7 (d), 1.0 (e),
1.5 (f), 1.9 (g), 2.9 (h), 7.8 (i), 34 (j), 78 (k), 128 (l), 214 (m), 354
(n), 534 (o) and 1884 (p) ps after the laser pulse (inset: decay kinetics
recorded at meaningful wavelengths, with a linear + log scale), and (C)
species associated spectra (SAS) calculated by target analysis.
solvation, while the intermediate lifetime of M is due to
diffusive solvation. In fact, the lifetimes of our S/M transients in
the polar media (e.g., 0.3/1.2 ps in Ac, 0.19/0.75 in MeCN and
0.22/2.9 in MeOH) are in good agreement with those reported
for the polar dynamics of Coumarin 153 (0.187/1.09 ps in Ac,
0.089/0.63 in MeCN and 0.28/3.2 in MeOH).63 The decay in
these solvents is characterized by very small, if any, fluorescence
and ISC57 yields and shorter τL values, which reflect the
increased efficiency of the return to the ground state by IC. In
fact, the presence of TICT states is evidenced by the net
fluorescence quenching.
Some fs measurements on compounds 1-3 in solvents of
increasing viscosity and similar dielectric constant (MeOH/
glycerol mixtures) were performed and the results are collected
in Table 7SI. They showed three components for 1 and 2 and
four for 3 (Figures 21SI-23SI) as in the less viscous media
(MeOH) but the time constants increased with the solvent
viscosity, as expected for solvent-controlled processes, thus
supporting the assignment of the fast transients to inertial and
diffusive solvation. The time constant of the longer-lived
transient of 1 in MeOH/glycerol mixture (50/50) is 4-fold the
value measured in MeOH and twice in the case of 2, while the
time constant of 3 remains practically unchanged in agreement
Generally the τL values were found to decrease as the solvent
polarity increased (from 134 ps in DCM to 4.4 ps in W). This
trend becomes clearer if the lifetimes in solvents of the same
nature are compared, e.g. in the nonprotic Ac (28 ps) and
MeCN (19 ps) and in the protic ones, alcohols and water (94
ps in PrOH, 52 in EtOH, 24 in MeOH, and 4.4 in W).
However, in nonpolar media the shift of the positive charge
toward a complete ICT is unfavored and the diffusive solvent
orientation becomes faster in comparison. In CHCl3 and DCM,
the lifetime of the M transient is much longer than the time
constants reported for solvation dynamics (26 and 52 ps in our
measurements, to be compared with 4.15 and 1.02 ps reported
for diffusive solvation in the literature,63 which are found to be
in good agreement with the lifetimes of 2.0 and 0.7 ps obtained
for the S transient in CHCl3 and DCM, respectively). In our
opinion, this result could be the evidence that in these solvents
the M lifetime reflects the rate of the LE → ICT process which
is slower and thus becomes the rate controlling step. This
with the planar configuration of the latter in the relaxed S1,ICT
.
The comparison among some competitive deactivation
constants of 1−3, derived on the basis of τL in four solvents
of different polarity and proticity, was particularly informative
(Table 6). On going from DCM to W: (i) kF decreased notably
for 1 but remained almost unchanged for the other two
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Table 5. Spectral and Kinetic Properties of 1−3 in Solvents of Different Polarity Obtained by Ultrafast Time-Resolved
a
Absorption Spectroscopy at λexc = 400 nm
1
2
3
solvent (ENT)
λ/nm
τ/ps
λ/nm
τ/ps
0.35
λ/nm
τ/ps
transient
CHCl3 (0.259)
<540(−)
560(−)
590(−)
570(+)
2.0
26
505(−), 650 (+)
535(−)
Solv/VC
1LE*
1ICT*
22
44
79
575(−)
<480(+)
rest
0.70
rest
T1
DCM (0.309)
DCE (0.327)
<540(−)
585(−)
550(+), 605(−)
580(+)
520(−), 635(+)
555(−)
0.68
590(+)
1.3
34
Solv/VC
1LE*
1ICT*
52
134
rest
10
550(+), 640(−)
545(+), 630(−)
575(−)
172
424
T1
<540(−)
590(−)
0.96
4.2
625(+)
530(−), 640(+)
565(−)
0.17
2.3
15
150
broad
0.1
Solvi /VC
Solvd/VC
1LE*
1ICT*
T1
550(+)
18
160
605(−)
575(+)
177
rest
0.30
1.2
575(−)
540(+), 615(−)
Ac (0.355)
DMSO (0.441)
MeCN (0.472)
PrOH (0.546)
EtOH (0.654)
MeOH (0.762)
W/EtOH (0.827)
W (1)
570(−)
525(+), 605(−)
540(−)
0.64
570(+)
0.58
Solvi/VC
Solvd/VC
1LE*
580(−)
585(−)
525(−), 590(+)
5.1
101
545(+), 640(−)
540(+), 630(−)
585(+)
46
540(+), 630(−)
525(−), 600(+)
585(−)
28
360
1ICT*
0.20
3.1
1.4
0.45
3.1
45
473
0.15
0.84
32
310
<0.1
Solvi/VC
Solvd/VC
1LE*
575(+)
560(−)
585(−)
540(−)
6.0
550(+), 650(−)
545(+), 635(−)
635(+)
540(+), 625(−)
590(+)
45
160
1ICT*
0.19
0.75
0.33
Solvi/VC
Solvd/VC
1LE*
590(−)
550(+), 650(−)
540(+), 640(−)
535(+), 625(−)
650(+)
580(−)
585(−)
505(−), 645(+)
530 (−)
3.2
535(+), 625(−)
535(−)
19
1.6
18
93
0.93
16
1ICT*
Solvi/VC
Solvd/VC
1LE*
575(−)
600(+)
10
40
550(+), 635(−)
540(+), 620(−)
630(+)
535(+), 600(−)
540(−)
94
0.78
560(−)
510(−), 650(+)
535(−)
135
0.64
6.8
386
1ICT*
0.83
5.2
30
285
0.37
2.8
29
295
0.74
3.2
25
286
0.1
1.1
20
275
Solvi/VC
Solvd/VC
1LE*
580(−)
10
570(+)
550(+), 630(−)
540(+), 625(−)
630(+)
535(+), 605(−)
540(−)
52
575(−)
520(−),640(+)
550(−)
135
1ICT*
0.22
2.9
0.55
4.6
Solvi/VC
Solvd/VC
1LE*
580(−)
570(+)
545(+), 635(−)
535(+), 625(−)
615(+)
535(+), 620(−)
550(−)
24
580(−)
510(−), 645(+)
535(−)
83
1ICT*
0.23
3.4
0.34
4.8
Solvi/VC
Solvd/VC
1LE*
580(−)
565(+)
545(+), 635(−)
535(+), 630(−)
650(+)
525(+), 600(−)
570(−)
36
565(−)
535(−)
550(−)
98
1ICT*
0.24
0.91
Solvi/VC
Solvd/VC
1LE*
0.70
550(+)
540(+), 645(−)
535(+), 630(−)
520(+), 610(−)
4.4
580(−)
25
1ICT*
a
Spectral properties refer to species associated spectra (SAS) calculated by target analysis. The symbols (+) and (−) stand for positive and negative
signals, respectively.
Table 6. Kinetic Constants (108 s−1) of Some Deactivation Pathways of the Excited Singlet Manifold
1
2
3
solvent
kF
10
kIC
kLE→CT
kF
kIC
kLE→CT
kF
kIC
kLE→CT
DCM
MeCN
EtOH
W
44
520
190
>13000
>1000
0.70
1.4
54
76
1000
3130
0.78
1.7
294
313
333
500
1.0
2.1
2.3
−
−
26
190
1.6
40
>1500
>11 000
2.6
2270
>14000
1.0
355
1.0
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compounds, its values (on the order of 108 s−1) indicating an
allowed transition; (ii) kIC increased by almost two and 1 order
of magnitude for 1 and 2, respectively, while it was much
smaller for 3 where the reactive process largely prevails; (iii)
kLE→CT increased strongly for 1 and 2 but only weakly for 3.
fluorescence yield and time constants) and a planar ICT state
in the case of 3 (no effect observed).
In conclusion, the overall results allowed an interesting
analysis of the simultaneous role of the solvation and the ICT
process on the behavior of the solvent−solute system for
compounds with different push−pull properties to be
performed, as summarized in Scheme 4.
4. CONCLUSIONS
The experimental and theoretical results of the present work
allowed the mechanism of the excited state deactivation of the
push−pull compounds 1−3 to be reasonably explained. The
results reported in the literature for the strongest push−pull
compound 1 were generally confirmed and some open
questions satisfactorily answered.
Scheme 4. Sketch of the Relaxation Dynamics in Two
Limiting Cases: (A) CT Controlled (1ICT* is Twisted for 1
and 2 and Planar for 3); (B) Solvent Controlled
Computed values of the formation enthalpies showed that
the conformational equilibrium in solutions is completely (for
1) and largely (for 2 and 3) shifted toward one of the possible
conformers. The weak radiative and reactive decays of the most
stable conformer of 1 are sizable in little-polar solvents only,
less for 2, and decrease further in more polar solvents. On the
contrary, the small fluorescence yield of 3 changes little with
the solvents and its high photoisomerization yield indicates that
the less ICT character opens the way to competitive reactive
decay.
The ultrafast measurements, extended to a large series of
solvents, confirmed previous results for 1 regarding the
occurrence, during the lifetime of the excited state, of fast
ICT toward a twisted conformation accompanied by a strong
fluorescence decrease in polar solvents.
A satisfactory description of the charge motion in the excited
state was obtained with the help of calculations (improved in
this revised version using more reliable computational methods,
as suggested by one of the reviewers).
The positive charge, which is mainly localized on the
acceptor pyridinium group in the ground state, moves toward
the dimethylanilino donor group under excitation reaching in
the FC state a distribution extended over the whole molecule,
thus explaining the decrease in the dipole moment under
excitation and the negative solvatochromism. Fast relaxation of
the FC state (possibly involving both vibrational cooling and
solvation) forms the LE state with a geometry close to that of
S0, whose further relaxation leads to back transfer of the
positive charge to the final TICT state with a μICT value similar
This led us to understand the different behavior of the
excited state dynamics in moderately polar solvents for 1 and 2,
where the charge movement is slower than the solvent
rearrangement (CT controlled), and in polar solvents, where
the situation is inverted (solvent controlled) and a fast
1
barrierless relaxation toward the final TICT* state occurs.
The dynamics of 3 is CT controlled (A side of Scheme 4) in all
the investigated solvents due to the presence of the weaker
electron donor pyrene group.
It should be noted that the sensitivity of the spectral behavior
of these compounds to the solvent properties and to the light
excitation suggests their suitability for use as molecular probes
and as elements to elaborate molecular logic units.
ASSOCIATED CONTENT
* Supporting Information
■
to μS . We believe that the LE state (stable intermediate
S
0
Additional information on spectrophotometric and fluorimetric
measurements, quantum-mechanical calculations, and femto-
second transient absorption in different solvents is available free
of charge via the Internet at http://pubs.acs.org.
observed in fs measurements) is separated from TICT by a
small energy barrier only in the case of compound 1 in the less-
polar solvents CHCl3 and DCM (for 2 even in more polar
solvents, until to MeCN, and for 3 in all solvents) whereas in
more polar solvents the FC state directly relaxes giving back
charge transfer to TICT.
The comparison with the behavior of the two new analogues
2 and 3 helped the transient assignment and allowed the effect
of a reduced efficiency of the donor moiety on the extent and
dynamics of the ICT process to be evaluated. For compound 2,
whose emission yield decreases less in W, and the ICT
character is less pronounced, a twisting of the trimethoxyphenyl
moiety seems to be favored by calculations. For 3, whose
emission remains practically unchanged in the polar solvent, the
charge separation is not accompanied by twisting.
AUTHOR INFORMATION
Corresponding Author
*(A.S.) E-mail: anna.spalletti@unipg.it. Telephone:
+390755855575. Fax: +390755855598.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors thank MIUR (Ministero dell’Universita
Ricerca, Rome, Italy), University of Perugia (PRIN 2010-2011
No. 2010FM738P), for fundings. B.C. also thanks the Regione
Umbria (POR FSE 2007-2013, Perugia, Italy) for a fellowship.
The authors are grateful to Prof. Eric Vauthey and Dr. Arnulf
■
̀
e della
The results obtained in the viscous solvent, that are expected
to hinder the torsional movements, are in agreement with the
formation of a TICT state for 1 and 2 (increase of both the
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(16) Matsui, M.; Kawamura, S.; Shibata, K.; Muramatsu, H. Synthesis
and Characterization of Mono-, Bis-, and Trissubstituted Pyridinium
and Pyrylium Dyes. Bull. Chem. Soc. Jpn. 1992, 65, 71−74.
(17) AZ Al-Ansari, I. Interaction of Solvent with the Ground and
Excited State of 2- and 4-[4-(Dimethylamino)Styryl]-1-Alkylpyridi-
nium Iodides: an Absorption and Fluorescence Study. Bull. Soc. Chim.
Fr. 1997, 134, 593−599.
Rosspeintner for suggestions useful to the interpretation of the
femtosecond measurements. The authors thank also Mr. Danilo
Pannacci for his technical assistance in HPLC measurements
and Dr. Serena Vannucci and Dr. Marta Moroni for their
contributions to the first part of this work during their thesis for
the first-level degree in Chemistry.
(18) Gawinecki, R.; Trzebiatowska, K. The Effect of the Amino
Group on the Spectral Properties of Substituted Styrylpyridinium
Salts. Dyes Pigm. 2000, 45, 103−107.
(19) Mishra, A.; Behera, R. K.; Behera, P. K.; Mishra, B. K.; Becera,
G. B. Cyanines During the 1990s: A Review. Chem. Rev. 2000, 100,
1973−2011.
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