Photophysical properties of izomeric N-chlorobenzyl substituted (E)-2′ (3′-or 4′)-hydroxy-4-stilbazolium chlorides in alcohols

Article abstract of DOI:10.1039/c0cp02587a

Absorption and steady-state fluorescence spectra of nine N-p-(m- and o-) chlorobenzyl substituted (E)-2′-(3′ and 4′)-hydroxy-4- stilbazolium chlorides belonging to the hemicyanine class of compounds were studied in extra dry alcohols of different polarity. Derivatives with 2′-hydroxy or 4′-hydroxy substituent in the benzene moiety of stilbazol molecule displayed negative solvatochromizm. On the other hand, the excited state decay of compounds with a 3′-hydroxy group in the benzene moiety was dominated by non-radiative processes in protic solvents. Solutions of each of the compounds are yellow in extra dry solvents, red in solvents with small amount of water and yellow again if more water is added. The absorbance and steady-state fluorescence methods were used to explain the protonation/deprotonation processes for N-p-chlorobenzyl-(E)-4′-hydroxy-4- stilbazolium chloride and its zwitterionic or quinoid form in 2-propanol responsible for these phenomena. the Owner Societies 2011.

Full text of DOI:10.1039/c0cp02587a

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PAPER
Photophysical properties of izomeric N-chlorobenzyl substituted (E)-20
(30-or 40)-hydroxy-4-stilbazolium chlorides in alcoholsw
Dorota Pruka$a,*a Wies$aw Pruka$a,b Igor Khmelinskiic and Marek Sikorskia
Received 18th November 2010, Accepted 3rd February 2011
DOI: 10.1039/c0cp02587a
Absorption and steady-state fluorescence spectra of nine N-p-(m- and o-) chlorobenzyl substituted
(E)-20-(30 and 40)-hydroxy-4-stilbazolium chlorides belonging to the hemicyanine class of
compounds were studied in extra dry alcohols of different polarity. Derivatives with 20-hydroxy or
40-hydroxy substituent in the benzene moiety of stilbazol molecule displayed negative
solvatochromizm. On the other hand, the excited state decay of compounds with a 30-hydroxy
group in the benzene moiety was dominated by non-radiative processes in protic solvents.
Solutions of each of the compounds are yellow in extra dry solvents, red in solvents with small
amount of water and yellow again if more water is added. The absorbance and steady-state
fluorescence methods were used to explain the protonation/deprotonation processes for
N-p-chlorobenzyl-(E)-40-hydroxy-4-stilbazolium chloride and its zwitterionic or quinoid form in
2-propanol responsible for these phenomena.
salts (hemicyanine dyes) have taken great attention recently,
Introduction
owing to their propensity to form H-aggregates spontaneously
both at interfaces11,12 and in solution.13,14 The unusual prop-
(E)-20-(or 40)-hydroxy-4-stilbazolium chlorides belong to the
hemicyanine class of compounds. Such compounds possess a
erties of these H-aggregates, especially the large molecular
donor-p-system-acceptor structure where the acceptor moiety
is uncharged or charged like in quaternary derivatives of
hyperpolarizability of hemicyanines, have opened the possibility
of fabricating nonlinear optical devices from them. The spectral
substituted pyridine. Their different photophysical properties
behaviour of studied compounds [1–9] presented on Fig. 1
as a high molar extinction coefficient or high fluorescence
depends on the possibility of conjunction between electron-
intensity cause their wide applications in many fields. They are
donating hydroxy substituent and electron-accepting quaternary
applied as near-infrared laser dyes, fluorescent labeling agents
heterocyclic nitrogen. Compounds [1–3] and [7–9] are composed
for proteins,1 fluorescent thermometer,2 molecular electronics,3,4
by a donor group (hydroxy group) and an acceptor group
Langmuir films,5,6 and photoinitiated polymerization.7,8
(substituted pyridinium moiety), connected by an ethylene
Molecules of hemicyanine dyes are known as having great
bridge, possessing an extended conjugated p-electron system.
Contrary to that, compounds 4–6, while having the same
tendency to association in their ground state. Interaction
between the chromophores has been explained by McRae
donor–acceptor group, don’t form a conjugated p-system,
and thus have very distinct absorption spectra.
and Kasha in terms of exciton coupling theory, in which the
excited state of the aggregate splits into two energy levels. The
transition to the upper excited state is allowed in the case of
face-to-face (parallel) dimers and to the lower state for head-to-
tail (linear) dimers.9 The parallel dimer is characterized by a
hypsochromically shifted absorption band (H-aggregate) and
the linear dimer by a bathochromically shifted absorption
band (J-aggregate).10 Molecular assemblies of stilbazolium
a Laboratory of Applied Photochemistry, Faculty of Chemistry,
´
A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland.
E-mail: dprukala@amu.edu.pl
b Department of Organometalic Chemistry, Faculty of Chemistry,
´
A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland
c Universidade do Algarve, FCT, DQF, and CIQA, Campus de
Gambelas, 8005–139 Faro, Portugal
w Dedicated to Professor Bogdan Marciniec on the occasion of his
70th birthday.
Fig. 1 Structures of the compounds studied.
c
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for stilbazolium salts because the ground state is more stabilized
by polar solvents than the excited state.
Presently, we studied spectral and photophysical properties
of a series of such compounds in alcoholic solutions, with their
structures shown in Fig. 1. These compounds possess the
resonance forms demonstrated in Fig. 3. Such forms are
possible for compounds 1–3 and 7–9, but not for compounds
4–6. The excitation is believed to involve transition from a
ground state, its structure resembling the neutral charge-
separated form, to a higher-energy quinoid form (with different
location of the charge) in the excited state.
Fig. 2 Two limiting resonance structures of the stilbazolium cation.
Thick arrows show the dipole moments corresponding to the struc-
tures according to Nakanishi.
The photochemical behavior of D-p-A+ Xꢀ salts has been
presented by H. Nakanishi et al. for the p-hydroxystilbazolium
cations (Fig. 2).15 According to their semiempirical calculations,
they conclude that the dipole moment is smaller in the excited
state in compared to the ground state, with its direction
switching to the opposite. They found that positive charge is
localized on the oxygen atom in the excited state and on the
nitrogen atom in the ground state.
Note that the N-substituted derivatives of (E)-azastilbenes
are also known to possess a variety of biological activities;19–21
in particular, all of the presently investigated compounds
showed antimicrobial activity.22
The solvatochromism of hemicyanine dyes may be
explained in two ways. The first, proposed by Neckers et al.,16
based on calculations of the HOMO and the LUMO for
p-(dimethylamino)stilbazolium salts, assumes that the positive
charge of the cation is localized on the pyridinium ion in the
ground state, changing its location in the excited state due to
intramolecular charge transfer (CT) to the oxygen atom. The
anion in the excited state follows the cation while the relaxation
processes occur and the excited state is stabilized primarily by
the position of this anion. In this case the solvent polarity has
an important role. Such explanation is accepted by other
authors.17
Experimental
Compounds 1–9 were synthesized according to the literature.23
Their physicochemical properties have been described else-
where.22 Compound 1a—the merocyanine (betaine)—was
synthesized by the addition of aqueous solution of 1M NaOH
to aqueous solution of compound 1. The precipitated dark-red
solid was collected by filtration under vacuum, washed by
large amounts of water, and compound 1a was recrystallised
from a mixture of ethanol and water. After collection by
filtration the solid was dried under reduced pressure using a
very efficient pump. All solutions were prepared on the same
day when their absorbance and steady-state emission spectra
were recorded.
Another explanation of the specific photophysical properties
of stilbazolium salts was given by Rettig et al.,18 who studied
o-, m- and p-(dimethylamino)stilbazolium salts. According to
these authors, the excited state of stilbazolium molecule
involves the locally excited state and the TICT (twisted
intramolecular charge transfer) state. In the latter, twists in
both the double bond and in the adjacent single bonds result in
states comparable in energy to that of planar conformation,
while the twist of the dimethylamino group leads to an
energetically higher TICT state. Charge separation existing
in the perpendicular form causes fluorescence occurring from
such molecules to depend on solvent polarity. According to
these two mechanisms, negative solvatochromizm is observed
Solvents: methanol, ethanol, 2-propanol, spectroscopic
grade, all from Aldrich, were used after drying. Solutions
prepared with as-received solvents with contents of water from
0.033% for methanol and 0.05% for ethanol were colored red.
Karl–Fischer titrations were performed with 2-propanol and
demonstrated the presence of water in a concentration of 55 ppm
(0.0055% water) in dry 2-propanol and 270 ppm (0.027%
water) in as-received 2-propanol. LC-grade water was prepared
by purification of osmotically treated water in an Elgastat
UHP system (High Wycombe, UK). All glassware was carefully
dried in all experiments with dry solvents.
Fig. 3 The possible resonance forms of compounds 1–3 and 7–9.
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Titration experiment of compound 1 in 2-propanol containing
water (0.027%) illustrated on Fig. 11 were carried out by
adding small amounts (2–10 ml) of the 2-propanol with a
microsyringe to closed quartz cuvettes containing the
compound 1 solution with the concentration of 3.04 ꢁ 10ꢀ5 M.
The final concentration was 0.86 ꢁ 10ꢀ5 M. After each
addition the UV–vis spectra were taken.
were recorded in dry methanol, ethanol and 2-propanol. Some
of these absorption spectra for compounds 1–9 in methanol
are shown in Fig. 4. Table 1 presents absorption parameters
of compounds 1–9 in different solvents, including the wave-
lengths and molar absorption coefficient of the absorption
maxima. The absorption spectra of these compounds show
one (for compounds 1–6) or two bands (for compounds 7–9).
The position of the –Cl substituent in the benzyl moiety does
not affect the position of the absorption maximum. This is due
to the fact that the substituent is out of conjugation with the
chromophore because of the presence of an insulating methylene
group. Still, the position of the –Cl substituent in the benzyl
moiety is affecting the molar absorption coefficient. Indeed,
the molar absorption coefficient is the largest for –Cl
substituent in ortho position in the chlorobenzyl moiety, and
the smallest for –Cl in para position, for compounds 1–3 in
methanol, ethanol and 2-propanol and for compounds 7–9 in
methanol, considering molecules with the same position of the
–OH group.
Titration experiments were also performed in 2-propanol
(0.027%)–water systems, using the solution of 1 with the
concentration of 4.15 ꢁ 10ꢀ5 M and adding pure water to
the cuvette to reach concentration of 4.0 ꢁ 10ꢀ5 M. The water
contents were in the range of 0.027% to 3.2%.
Solutions of each of the compounds are yellow in dry
solvents, red in solvents with small amount of water when
new red shifted band appeared due to deprotonation process.
They stay yellow again if more osmotically cleaned water is
added. The pure, osmotically cleaned water was acidic because
of atmospheric carbon dioxide dissolved in it. This caused a
re-protonation of the studied solutes.
UV–vis absorption spectra were recorded on a Varian Cary
5E spectrophotometer and Jobin Yvon-Spex Fluorolog 3
spectrofluorometer. Steady-state fluorescence excitation and
emission spectra wee recorded on a Jobin Yvon-Spex Fluorolog
3 spectrofluorometer.
The absorption maximum is blue-shifted with increased
solvent polarity for all compounds investigated, independently
of possibility of quinoid structure existing in the excited state.
Such negative solvatochromizm is characteristic for compounds
with their ground state more polar than the Franck–Condon
excited state. This effect could be also explained by lowering
of energy of hydrogen bonds in the excited state. However,
the dependence is linear, indicating that only non-specific
interactions affect the absorbance spectra. The largest lmax
occurs in 2-propanol, for all of the compounds investigated. This
solvent, having the lowest polarity among the solvents used,
has got the weakest H-bond donating and the strongest
H-bond accepting capacity, as described by a and b-scales.25,26
The strongest solvatochromic effect is observed for compounds
1–3 (about 800 cmꢀ1), however, solvatochromizm is relatively
weak in all of the compounds studied.
Note also that simple hemicyanine dyes like 1-methyl-
4-(40-hydroxystyryl)pyridinium salts undergo photoisomerisation
from trans- to cis- isomer.24 This process may be difficult to
monitor, especially because both isomers may absorb at the
same wavelength, being only distinguishable by the values of
their molar absorption coefficient. Therefore, we always
checked whether the absorption spectra in the beginning and
in the end of the experiments were the same, noting no
isomerization for any of the compounds studied. All experi-
ments were carried out at room temperature.
Results and discussion
Fluorescence spectra of compounds 1–9
Absorption spectra of compounds 1–9 in dry solvents
Table 2 presents emission parameters of compounds 1–9 in dry
methanol, ethanol and 2-propanol, including the excitation
and emission maximum wavelengths.
The UV–vis absorption spectra for N-p-(m- or o-)chlorobenzyl
substituted chlorides of (E)-40 (30- or 20)-hydroxystilbazoles-4
Fig. 4 Absorption spectra of compounds 1–9 in methanol.
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Table 1 Absorption parameters for compounds 1–9 in dry solvents
max/nm, (e/dm3 molꢀ1cmꢀ1
The dependence of the steady-state Stokes shift for
compounds 1–3 and 7–9 on solvent polarity is shown in
Fig. 6. We used the solvent polarity scale based on Onsager’s
reaction field theory.27
l
)
MeOH
EtOH
2-PrOH
Compound
1
2
3
4
5
6
7
403 (27 500)
403 (31 300)
405 (36 400)
358 (23 000)
360 (26 900)
360 (13 600)
394 (18 500)
340 (10 300)
395 (25 200)
340 (13 500)
395 (33 200)
342 (17 864)
406 (30 000)
407 (31 300)
407 (41 000)
362 (24 800)
362 (25 700)
363 (15 800)
401 (16 500)
345 (8900)
400 (25 100)
343 (13 000)
401 (14 900)
345 (7800)
409 (32 300)
411 (33 700)
410 (34 300)
365 (20 800)
365 (24 000)
365 (18 700)
404 (18 100)
348 (10 200)
405 (25 100)
345 (12 700)
404 (17 300)
348 (10 000)
e ꢀ 1
n2 ꢀ 1
Df ¼
ꢀ
ð1Þ
2e þ 1 2n2 þ 1
The values of the Stokes shift are dependent on the position
of the –OH substituent in stilbazolium moiety; i.e. for
compounds 1–3 with the –OH group in the para position of
the benzene moiety they are in the range of 4.5 ꢁ 103 to 6 ꢁ 103
cmꢀ1. These shifts are larger for compounds 7–9 with the –OH
group in the ortho position of the benzene moiety, being in the
8
9
range of 6 ꢁ 103 to 8 ꢁ 103 cmꢀ1
.
As we can conclude from Table 2 the plot of the emission
maxima vs. polarity of the solvents is not linear but this plot of
the absorbance maxima is much more linear. So the specific
interactions between solvent and solute occur rather in the
excited state than in the ground state. The Stokes shifts are
always larger in methanol as compared to other solvents. The
linearity of plots of this shift vs. solvent polarity function is
often regarded as evidence for the dominant importance of
general solvent effects in the spectral shifts.27
Table
compounds 1–9
2 Fluorescence maxima and excitation wavelengths for
lem/nm, (lexc/nm)
Compound
MeOH
EtOH
2-PrOH
1
2
3
4
5
6
7
8
9
524 (410)
523 (400)
523 (405)
—
521 (410)
520 (405)
519 (410)
—
515 (410)
517 (420)
518 (430)
—
—
—
—
—
—
—
Now, negative solvatochromizm in the absorption spectra
and batochromic shifts of the emission bands with increased
solvent polarity are well known manifestations of the charge-
transfer character of the solvent-relaxed emissive state.28
The fluorescence quantum yields of compounds 1–3 and 7–9
in dry alcohols are presented in Table 3. The quantum yields
are rather small, with the largest values observed for compounds
1 and 3 in 2-propanol.
546 (395)
538 (400)
546 (400)
539 (430)
538 (420)
547 (410)
536 (400)
536 (420)
537 (400)
The absorption and the corrected fluorescence excitation
spectra agree well with each other, with the exception of
compounds 7–9, where the shorter-wavelength absorption
bands do not overlap with the fluorescence excitation bands.
Typical fluorescence emission spectra of compounds 1–3 and
7–9 in dry alcohols show a single band of Gaussian shape, with
the exact position depending on the solvent. There is a red shift
in the emission maxima for all six molecules in alcohols, as the
solvent polarity increases from 2-propanol to methanol. This
is the reverse of the direction of the shift observed in the
absorbance spectra and indicated that the emitting state is
more polar than the Franck–Condon ground state.
Absorption and fluorescence spectra of compounds 1–9 in
solvents containing water
When the investigated compounds 1–3 and 7–9 were dissolved
in alcohols containing small amounts of water (0.027% H2O
in 2-propanol), a different batochromically shifted band
appeared in their absorption spectra. Such long-wavelength
bands appear also in insufficiently dry acetonitrile, acetone,
dioxane and DMSO solutions (with more than 0.005% of
water; spectra not shown). In contrast, no changes were
observed for compounds 4–6. The intensity and maximum of
this new band strongly depend on the compound, on the
solvent and on water contents. Thus, solutions with small
amounts of water were colored red, while extra dry solutions
were yellow. Fig. 7 presents the examples of the absorption
and fluorescence emission photographs of solutions of
compounds 1–3 and 7–9 in 2-propanol with 0.027% water
and after addition of 0.02 ml of H2O to each cuvette.
The strongest bathochromic shift is observed in methanolic
solutions, indicating an additional change of the energy of
hydrogen bonds in the excited state as compared to the ground
state. Typical emission spectra in methanol are shown in
Fig. 5.
The behavior of compounds 4–6 is distinctly different from
the others, as they show very low emission in protic solvents,
with their excited state decaying non-radiatively, involving the
hydrogen bond interactions. However, in acetonitrile—a polar
but aprotic solvent—their emission spectra are intensive with
their fluorescence quantum yield in the range from 0.014 to
0.019; and their Stokes shift is very large (spectra not shown).
Additionally they can not form a quinoid structure, because
the n electrons of the oxygen atom are not conjugated to the
extended double-bond p-electron system. The compounds 4–6
have different structures and electron densities than other
compounds. It means that in both ground and excited states
they can not form an intramolecular charge transfer state.
We interpret this new red-shifted band as characteristic for
the deprotonated hemicyanine that appears in the presence of
water. Indeed, hemicyanines containing the –OH group shows
amphiphilic properties;29 this should be especially true for
compounds 1–9, which can be considered salts of a strong
acid and a weak base. The deprotonation reaction for compound
1 can be explained as shown in Fig. 8. Fig. 8 also presents
structures and common names of the compound 1 in its
protonated and deprotonated forms.
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in addition to the spectrum of compound 1 (hemicyanine dye-
protonated form) in 2-propanol containing 0.027% of water.
As we see, the positions of the bands in the UV–vis spectra
of these two compounds are almost equal, and the more
red-shifted band is broader. Therefore, we believe that this
new band of compound 1 belongs to the deprotonated form of
hemicyanine. The absorption spectra of 1a in dry 2-propanol
solutions reveal two bands: the shorter wavelength band at
lmax 417 nm is similar to that observed for the protonated
species (1) in water-containing solvent at lmax 416 nm; the
longer wavelength band has its maximum at 561 nm and 562 nm,
respectively. Several explanations were proposed for these two
bands in the UV–vis spectrum of para-hydroxystilbazolium
(merocyanine) dyes. According to Gruda et al.31 the short-
wavelength band in protic solvents and in protic solvent-water
mixtures of stilbazolium merocyanines is attributed to the
protonated form, as merocyanine can abstract proton from
the water or form H-bonds with water or alcohols, while the
long-wavelength absorption is attributed to the free base.
There remain some doubts about the reality of zwitterionic or
quinoid forms of merocyanines in solutions. Different authors
mentioned that the quinoid form plays an important role in the
resonance hybrid and suggested its existence in solutions even in
the ground state (Fig. 8). The existence of the quinoid form was
confirmed, for example, in hemicyanine dye DiA (N-methyl-
4-[4-dihexadecylamino]styryl pyridinium iodide) in n-alcohols
(n = 1. . .11) by studying its time-resolved behavior.32 Addi-
tionally, Steiner and co-workers33 provided good evidence of
the quinoid form in aqueous solutions. Some authors34 suggest
another interpretation, proposing that the ideal-polymethinic
structure of merocyanines is in equilibrium with quinoid and
zwitterionic forms in solutions. On the other hand Fujita35
proposed that 1-alkyl-4-(4-hydroxystyryl) pyridinium halides
take the protonated form in acidic solutions, whereas two kinds
of structures are possible in basic solutions: one is a zwitterionic
form and the other is a quinoid form. The former appears in
polar solvents such as alcohols, while the latter appears in non-
polar environments such as chloroform or dichloromethane.
Generally, the equilibrium between such forms strongly depends
on environmental condition such as pH and on the solvent
polarity and/or hydrogen bond interactions.
Fig. 5 Emission spectra of compounds 1–9 in methanol, lex = 400 nm.
Fig. 6 Plots of Stokes shift vs. solvent polarity function.
Table 3 The fluorescent quantum yield FF of compounds 1–9 in dry
alcohols
Compounds
FF (MeOH)
FF (EtOH)
FF (i-PrOH)
1
2
3
4
5
6
7
8
9
0.009
0.009
0.010
—
—
—
0.009
0.010
0.007
0.007
0.006
0.007
—
—
—
0.007
0.006
0.007
0.015
0.009
0.015
—
—
—
0.009
0.010
0.011
Although the first band can be attributed to the protonated
form in compound 1, such explanation fails for compound 1a;
therefore, we suggest that very similar structure and electron
density distribution exist in the protonated compound 1 and
its hydrogen-bonded counterpart—compound 1a in both the
ground and the excited states, as we infer from the similarity of
spectra in the Fig. 9.
Recently other authors in a very interesting study have
arrived to similar conclusions. They found that conjugated
bases like CNꢀ and Fꢀ anions can abstract the proton from
the –OH group in protonated hydroxystilbazolium betaines
dissolved in acetonitrile.30
The second longer-wavelength band of 1a can be attributed
to the free base in its quinoid or zwitterionic form.
However the values of lmax about 560 nm in compounds 1
and 1a rather indicate a quinoid form, as this value is similar
to those published previously.35 Indirect evidence is the fact
that the compounds 4–6, which can’t assume the quinoid form,
neither show the long-wavelength band in water containing
solvents. The zwitterionic forms of compounds 1 and 4 are
presented in Fig. 10. These were obtained by addition of 5 ml
of aqueous solution of 4 M NaOH to the 3 ml of 2-propanol
solution of compounds 1 and 4. In these conditions we can
Subsequently, compounds 1 and 1a were chosen for the
more detailed studies. Pure free base was obtained as described
in the experimental part to study the similarities in absorbance
and emission spectra of protonated and deprotonated forms of
hemicyanine. We compared the spectra of compound 1 with
these of deprotonated dye 1a, as shown in Fig. 9, presenting
absorption (A), emission spectra (B) and excitation (C) spectra
of compound 1a in dry 2-propanol at several concentrations,
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Fig. 7 Solutions of compounds 1–3 and 7–9: A – in 2-propanol with 0.027% water; B – after addition of 0.02 ml of H2O to each cuvette, under
visible light. C – fluorescence emission, compounds 1–3 and 7–9, from left to right.
Fig. 8 Protonated and deprotonated forms of compound 1 (1a).
observe the spectra of zwitterionic (deprotonated) forms of
compounds 1 and 4, which are unable to assume the quinoid
form. The maximum of the absorbance band of this form
lmax = 511 nm (for compound 1) is moved to the blue by
about 50 nm as compared to the quinoid form. The maximum
of the emission band is moved to the red by about 10 nm.
These results are very similar to those for the zwitterionic form
of 1-alkyl-4-(4-hydroxystyryl) halide in ethanol solutions con-
taining NaOH. The maxima of the absorption and emission
bands are at lmax = 520 nm and lmax = 580 nm respectively.
Many authors proposed aggregation phenomena of hydroxy-
styryl hemicyanines in different solvents, manifested as two (or
more) bands in UV–vis absorption spectra. Formation of
aggregates in solvents is also often responsible for additional
bands in merocyanine solutions. The aggregation phenomena
were eliminated for compound 1a by studying the absorption
as the function of concentration. The Lambert–Beer’s law was
found valid in the range from 7.8 ꢁ 10ꢀ6 M to 2.9 ꢁ 10ꢀ6 M.
Compound 1 presented a different case. The absorption and
emission spectra of compound 1 in 2-propanol with 0.027%
water at different concentrations are presented in Fig. 11.
As we see, the ratio of the long-wavelength absorption band
to the short-wavelength band decreases with growing concen-
tration of the compound 1. This is usually interpreted as
evidence for aggregation, with equilibrium between monomeric
and dimeric forms,36 from the UV–vis spectra of compound 1
in dry solvent and from the above considerations. Thus, we
need to take into account the relative concentrations of water
(15 ꢁ 10ꢀ3 M) and of the solute in our system. The relative
amount of water was increased by a factor of 4 as the
concentration of the compound 1 decreased, forcing a more
complete deprotonation of the hemicyanine. The equilibrium
between these two forms can be seen as an isosbestic point at
the 461 nm on the extinction coefficient plot (Fig. 11A).
The absorption and emission spectra on Fig. 12 show the effects
of the addition of the extra pure water to the solution of 1 in
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Fig. 9 (A)-Absorption spectra of compound 1a in dry 2-propanol (concentration from 2.9 ꢁ 10ꢀ6 to 7.8 ꢁ 10ꢀ6) and of compound 1
(concentration 1.1 ꢁ 10ꢀ5 in 2-propanol contains 0.027% of water, (B)-emission spectra of compound 1a (front-face mode), (C)-excitation spectra
of compound 1a in concentrations from 2.9 ꢁ 10ꢀ6 M to 7.8 ꢁ 10ꢀ6 M (front-face mode). Arrows shows the increasing concentration.
Fig. 10 The normalized absorption spectra (A) and emission spectra (B) (for excitation at 510 nm and 540 nm respectively) of zwitterionic form of
compound 1 and 4.
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Fig. 11 The absorption (A) and emission spectra (B for excitation at 410 nm, C for excitation at 570 nm) of compound 1 in concentrations from
3.04 ꢁ 10ꢀ5 M to 0.86 ꢁ 10ꢀ5 M in 2-propanol containing 0.027% water. Arrows indicate increasing concentration.
2-propanol containing 0.027% of water. The absorbance of the
first short–wavelength band increases, although no significant
changes are evident in the second long–wavelength band from
0.027 to 0.16% of water. At higher H2O concentrations the first
band grew still more, while the second rapidly decreased to zero.
The maximum of the first band shifted from 413 to 412 nm for
0.027 to 0.16% H2O and from 412 nm to 409 nm in the range
from 0.15 to 3.2% H2O. This effect can be caused by changes in
solvation. The maximum of the second band shifted from 561 to
552 nm upon water addition. The equilibrium between the
protonated and deprotonated forms in this experiment can be
seen as one well-defined isosbestic point at 461 nm. The proto-
nation of the 1 in extra pure water should be caused by acidic
nature of such water (pH 6.0) resulting from atmospheric carbon
dioxide.
Similar emission spectra were obtained for excitation at 420
and 560 nm, in right angle (RA) and front face (FF) modes.
Excitation at about 420 nm gives a strong emission band with
a maximum at about 515 nm, with the shape being close to the
mirror image to the absorption band for both compounds. The
Stokes shifts between the absorption and emission maxima
for this band for compounds 1 and 1a are 4716 cmꢀ1 and
4583 cmꢀ1, respectively.
The excitation at the second absorption band of these
compounds at 560 nm gives a broad and weak band, with
the maximum at 610 nm in dry 2-propanol solution of 1a, and
609 nm in 0.027% water 2-propanol solutions of compound 1.
The Stokes shift is smaller, 1437 and 1432 cmꢀ1, respectively,
indicating a strong reduction of the dipole moment upon
excitation.
The fluorescence spectra of compound 1 in 2-propanol
containing 0.027% of water and compound 1a in dry
2-propanol show key similarities as we can see in Fig. 9,
11 and 12.
Donor–acceptor molecules undergo a reaction from an
initially prepared locally excited state (LE) to an intramolecular
charge transfer (ICT) state, which generally show a dual LE
and ICT emission.37,38
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Fig. 12 The absorption (A) and emission spectra (B,C) of compound 1 upon water titration. (B)-The emission spectra when the solution was
excited at 410 nm, and (C)-when it was excited at 570 nm. The concentration of water was from 0.027 to 3.2%, the concentration of the solute was
in the range of 4.04 ꢁ 10ꢀ5–4.15 ꢁ 10ꢀ5 M.
Fig. 13 The excitation spectra of compound 1 at several selected water concentrations. A—emission observed at 520 nm, B—emission observed at
610 nm.
The shorter wavelength emission originated from the decay
of the LE excited state and the longer wavelength emission
originated from ICT state (Fig. 9B). Again the longer wavelength
emission band is a result of irradiation of compound 1a with
lower and higher wavelengths as we can see on Fig. 9C. Thus
we can conclude that the LE state is the precursor of the ICT
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state. Additionally the second longer wavelength emission is
probably due to direct excitation of the ICT state from the S0
ground state, bypassing the LE state.39
increasing with solvent polarity, which can be explained by a
strong redistribution of electron density upon excitation.
Compounds 1–3 and 7–9 change color from yellow to red in
alcohols contain even small amounts of water. Comparing
absorption and emission spectra of compound 1 (hemicyanine)
and its deprotonated counterpart compound 1a, we found that
these changes are due to protonation/deprotonation processes
leading to the existence of two forms in solutions. One of them
is the protonated form and the second is probably the quinoid,
deprotonated form. Examination of spectra measured in
2-propanol lead to the conclusion that these two forms exist
in equilibrium in 2-propanol containing from about 0.27 to
3.2% of water.
As we see, the maxima of both emission bands for compounds
1 in water-containing 2-propanol and for 1a in extra-dry
2-propanol are almost the same. Moreover, these bands have
not changed their position when the concentration of these
compounds was changed or when water was added. Experi-
mentally, the differences between the intensities were larger
than those apparent in Fig. 9, 11 and 12, as different slit widths
were used for different spectra.
Let us discuss the emission spectra in Fig. 11 and 12. While
the absorbance of the first band increases with gradual addition
of water (Fig. 12), the intensity of the emission band decreases
till the absorbance reaches the highest value, implying a
reduction in the emission quantum yield. Further on, the
absorbance in this band decreases while the emission increases.
While some authors observed a decrease of the value of the
quantum yield with an increase in solvent polarity, attributing
it to properties of compounds with hemicyanine structures,
which undergo ICT transition upon excitation,40 such expla-
nation is insufficient in our case. As regards the second
emission band, its intensity also decreases with the addition
of water, in synchrony with that of the second absorption
band. We see the same happening in Fig. 11, as the quantum
yield of the increasing form diminishes. Such phenomena are
characteristic for forms being in equilibrium in both ground
Finally, compounds 1–3 and 7–9 are fluorescent both as
water-containing alcoholic solutions and dry solutions, emit-
ting green fluorescence.
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