Excited-state proton transfer and excited-state de-hydrogen bonding of the push-pull styryl system

Article abstract of DOI:10.1016/j.cplett.2005.08.146

The excited-state reaction of push-pull styryl system was studied in this work. In the excited state, compounds of 2-StP-NMe₂, 2-StQ-NMe ₂, 4-StQ-NMe₂ and 4-StP-NMe₂ possess a sufficiently large acidity change f

Full text of DOI:10.1016/j.cplett.2005.08.146

Chemical Physics Letters 415 (2005) 217–222
www.elsevier.com/locate/cplett
Excited-state proton transfer and excited-state de-hydrogen bonding
of the push–pull styryl system
a,
a
b
a
Shun-Li Wang *, Guo-Yi Gao , Tong-Ing Ho , Li-Yu Yang
a
Department of Applied Chemistry, National ChiaYi University, 300 University Road, ChiaYi, 600, Taiwan, Republic of China
b
Department of Chemistry, National Taiwan University, Taipei, Taiwan, Republic of China
Received 18 May 2005; in final form 14 August 2005
Available online 26 September 2005
Abstract
The excited-state reaction of push–pull styryl system was studied in this work. In the excited state, compounds of 2-StP-NMe2, 2-StQ-
NMe2, 4-StQ-NMe2 and 4-StP-NMe2 possess a sufficiently large acidity change for the proton transfer process to occur. For compounds
2-StT-NMe2 and 2-StN-NMe2 the acidity change is too small to induce the proton transfer process. A new type of excited-state de-hydro-
gen bond (ESDHB) process was observed in this study. It is possible to explore the polarity and HB effect of solvent simultaneously by
using the ESDHB mechanism.
ꢀ 2005 Elsevier B.V. All rights reserved.
1. Introduction
accept a second proton which produces a large blue-shift
at the absorption and emission peak. Since the basicity of
the N,N-dimethylaniline group is reduced upon photoexci-
tation, the excited-state deprotonation (ESDP) process was
observed in a doubly protonated form of 2-StQ-NMe2 in
dichloromethane solvent.
It is well known that the basicity of the acceptor will be
increased and the basicity of the donor will be decreased
upon photoexcitation because of the larger dipole moment
in the excited state for ICT compounds [4,5]. This explains
the ESDP of 2-StQ-NMe2. However, the influence of vari-
ous acceptors to the process of ESDP is still unknown. In
this work compounds with different acceptors in a push–
pull p-N,N-dimethylamino styryl system were synthesized
to resolve this problem, as shown in Scheme 1.
The photophysical aspects of intramolecular charge
transfer (ICT) continue to attract attention for their impor-
tance in chemistry and biochemistry [1]. The ICT state of-
ten differs from the parent ground state in their geometry
and electronic structure. For compounds such as push–pull
stilbene system where the donor and the acceptor moiety
are joined by a chemical bond the electron transfer occurs
through a process called twisted intramolecular charge
transfer (TICT) model [2]. For compounds with TICT,
the dipole moment of the excited state is sensitive to the
environment [2]. We have previously reported a study of
excited-state deprotonation of p-N,N-dimethylamino-2-sty-
rylquinoline (2-StQ-NMe2) system [3]. There are two basic
sites in 2-StQ-NMe2, quinoline and N,N-dimethylaniline
groups. The basicity of quinoline nitrogen atom is higher
than the basicity of the N,N-dimethylaniline site. When
the quinoline group is protonated, the absorption and
emission peak shows a large red-shift. However, at a higher
acid concentration the N,N-dimethylaniline group can
Acid–base reactions generally follow a three-step mecha-
nism:
association
PT
dissociation
ꢁ
þ
ꢁ
þ
*
*
*
B þ H–A )
BꢀꢀꢀHA ) BH ꢀꢀꢀA
)
Bh þ A
HB
IP
freeions
where HB is a hydrogen bonding complex, IP is an ion-
pair complex, and PT is proton transfer. Protons can be
transferred from Brønsted acids (A–H) to bases (B) via
hydrogen-bonds (HB) and ionic complexes (IP), depending
*
Corresponding author. Fax: +886 5 2717901.
E-mail address: wangshunli@mail.ncyu.edu.tw (S.-L. Wang).
0009-2614/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2005.08.146

218
S.-L. Wang et al. / Chemical Physics Letters 415 (2005) 217–222
S
N
N
2-StT-NMe2
2-StN-NMe2
2-StP-NMe2
N
N
N
N
N
N
N
N
4-StQ-NMe2
2-StQ-NMe2
4-StP-NMe2
Scheme 1.
on the relative acidity of H–A, the basicity of B, and the
solvation capability of the surrounding medium [6,7]. The
main factor, which determines the position of the acid/base
equilibrium, is the differential solvation of the HB and IP
complex. In dichloromethane protonation reaction will
produce an IP complex because the solvation can not sup-
port the formation of free ions. The ESDP process forms a
HB complex from an IP complex in the excited state [8–11].
Besides studying the influence of photoexcitation to a
ground-state IP complex, we would like to study the influ-
ence of photoexcitation to a ground-state HB complex.
The influence of hydrogen-bonds (HB) to the absorption
and emission maxima is usually less than protonation but
in the same direction. Therefore, the HB interaction at an
acceptor of an ICT compound causes a red-shift in the
absorption and emission maxima, whereas the interaction
at a donor site produces the opposite effect [12]. The extent
of shift depends on the HB ability of solvent. The second
part of this work will measure the spectra of 2-StN-NMe2
in various ratios of dichloromethane and trifluoroethanol
(TFE) solvents to explore the influence of photoexcitation
to a ground-state HB complex.
from Merk or spectrophotometric grade from Aldrich and
were used as received.
2.2. Method
UV–Vis absorption spectra were recorded on a Perkin–
Elmer Lambda 40 UV/Vis spectrometer and fluorescence
spectra were obtained with a Hitachi F-4500 fluorescence
spectrometer. Sample with concentrations of 1.5 · 10ꢁ5 M
were used for the measurements. The hydrochloric acid
was dissolved in an ethyl acetate (EA) solution and was
added to the organic phase at a volume of less than 3%.
All acidic solvents were used when as fresh as possible.
3. Results and discussion
3.1. Factors which influence excited-state deprotonation
The absorption spectra and fluorescence spectra of
compound 4-StP-NMe2 in dichloromethane are shown in
Fig. 1. In a neutral solution, the absorption and emission
maxima of 4-StP-NMe2 occurred at 363 and 457 nm,
respectively, but were shifted to 456 and 560 nm, respec-
tively after 5 · 10ꢁ5 M HCl was added. The pyridine nitro-
gen atom is more basic than the N,N-dimethylaniline site
because of ICT. Therefore when the pyridine group is pro-
tonated, the electron pulling ability will be stronger and
will show a larger red-shift in the absorption and emission
spectra. When more acid was added, the double proton-
ation induced an enormous blue-shift in the absorption
and emission maxima. The absorption maxima shifted
from 456 to 325 nm and the emission maxima (D band)
shifted from 560 to 388 nm. Even though 4-StP-NMe2
became double protonated at higher acid concentration,
the emission of a monoprotonated species (M band) is still
very strong, as seen in Fig. 1. The similarities between the
M band and the D band excitation spectra (data not
shown) explain that these two emission bands are from
the same source. The excitation spectra indicated that an
ESDP process occurred at the N,N-dimethylaniline site.
The basicity of N,N-dimethylaniline site was reduced upon
photoexcitation. Besides 4-StP-NMe2, the ESDP process
was observed in double-protonated 2-StQ-NMe2, 4-StQ-
NMe2 and 2-StP-NMe2 in dichloromethane. However,
The ultimate purpose of this work is to show the possi-
bility of these compounds in probing of the microenviron-
ment [13,14], which can detect the polarity and HB effect of
interest at the same time [15,16].
2. Experimental
2.1. Materials
Compounds 2-StP-NMe2, 4-StP-NMe2, 2-StQ-NMe2,
and 4-StQ-NMe2, were synthesized by reacting p-N,N-dim-
ethylamino benzaldehyde with either a-picoline, c-picoline,
quinalide, or 4-methylquinoline. The mixture were refluxed
in acetic anhydride catalyzed by zinc chloride for two to
four hours [17]. The solid product was purified by column
chromatography and recrystallized from ethyl acetate.
2-StT-NMe2 and 2-StN-NMe2 were prepared by Wittig
condensation of p-N,N-dimethylamino benzaldehyde with
2-methylenethiophene or 2-methylenenaphthalene triethyl
phosphonium ylides. The solid product was recrystallized
from dichloromethane. All the solvents were Uvasol grade

S.-L. Wang et al. / Chemical Physics Letters 415 (2005) 217–222
219
c*
a*
a
c
b*
b
250
300
350
400
450
500
550
600
650
wavelength(nm)
Fig. 1. The absorption and emission spectra of compound 4-StP-NMe2 in dichloromethane under different conditions (solid line: absorption, dotted line:
emission): (a) neutral solvent, (b) with 3.3 · 10ꢁ5 M HCl, (c) with 3.3 · 10ꢁ3 M HCl. (a*, b*, c* are emission spectra, EXC = 330 nm.)
compounds 2-StN-NMe2 and 2-StT-NMe2 displayed a dif-
ferent phenomenon.
The basicity difference between the excited state and ground
state (DpKa) of different compounds is listed in Table 1.
Compounds of 2-StP-NMe2, 2-StQ-NMe2, 4-StQ-NMe2
and 4-StP-NMe2 which show ESDP processes possess a
large value of DpKa, For compounds without ESDP pro-
cess, such as 2-StT-NMe2 and 2-StN-NMe2, the value of
DpKa is smaller. The ESDP process was induced by a large
change in basicity between the ground state and the excited
state. For compounds 2-StT-NMe2 and 2-StN-NMe2, the
value of DpKa is too small to induce the ESDP process from
the protonated species. Therefore, we conclude that the
value of DpKa is important in determining the occurrence
of ESDP process.
The absorption and fluorescence spectra of compound
2-StN-NMe2 in dichloromethane are shown in Fig. 2.
The absorption maxima shows a blue-shift from 367 to
323 nm and the emission maxima blue-shifts from 460 to
391 nm when HCl is added to the solution. Unlike 4-StP-
NMe2, our experiments did not produce ESDP in 2-StN-
NMe2. Compounds 2-StT-NMe2 and 2-StN-NMe2 exhibit
similar behavior. For ICT compounds, the basicity of the
donor decreases upon photoexcitation due to the larger
dipole moment in the excited state.
By using of the Forster cycle the excited-state acidity con-
stant ðpKꢂaÞ can be predicted from the ground-state (pKa)
ꢁ
value and the excitation energy difference ðEA ꢁ EAH
Þ
3.2. Excited-state de-hydrogen bonding
between the unprotonated and protonated forms by the
following equation [18]
The absorption and emission spectra of compound 2-
StN-NMe2 in various ratios of mixed solvents between
dichloromethane and trifluoroethanol (TFE) are shown in
Fig. 3. The absorption maxima displayed a blue-shift when
pKꢂa ¼ pKa þ ðEA ꢁ EAHÞ=2:3RT;
ð1Þ
ð2Þ
ꢁ
DpKa ¼ pKꢂa ꢁ pKa ¼ ðEA ꢁ EAHÞ=2:3RT.
ꢁ
a*
b
a
b*
250
300
350
400
wavelength(nm)
450
500
550
Fig. 2. The absorption and emission spectra of compound 2-StN-NMe2 in dichloromethane under different conditions (solid line: absorption, dotted line:
emission): (a) neutral solvent, (b) with 2.2 · 10ꢁ3 M HCl. (a*, b* are emission spectra, EXC = 320 nm.)

220
S.-L. Wang et al. / Chemical Physics Letters 415 (2005) 217–222
Table 1
an obvious change in absorption and emission spectra
[12]. Neither the absorption nor the emission spectra of
the 2-StN-NMe2 were affected by the presence of triethyl-
amine. Therefore, the influence of TFE to 2-StN-NMe2 is
seen mainly the HB interaction.
The absorption maxima and the basicity difference between the excited
state and the ground state (DpKa) of unprotonated (Aꢁ) and protonated
(AH) forms of different compounds in water
a
ꢁ
Compound
EA (nm)
EAH (nm)
Absorption maxima
DpKa
Absorption maxima
The reaction barrier of the proton transfer is higher than
the ESDHB process. The value of DpKa for 2-StN-NMe2 is
not enough to stimulate the occurrence of ESPT, but for
the ESDHB process, the smaller reaction barrier of the
ESDHB process can be observed in this study.
2-StN-NMe2
2-StT-NMe2
2-StQ-NMe2
2-StP-NMe2
4-StQ-NMe2
4-StP-NMe2
a
344.5
346.5
485.0
428.0
494.5
444.5
317.5
323.5
366.0
324.5
365.0
320.5
ꢁ5.2
ꢁ4.3
ꢁ14.0
ꢁ15.6
ꢁ15.1
ꢁ18.3
Basicity difference between the excited state and the ground state.
3.3. Solvatochromism of 2-StN-NMe2
the ratio of TFE in the mixed solvent was increased, whereas
the emission maxima exhibited an opposite behavior. Since
TFE is a protic solvent, it is reasonable to conclude that
hydrogen-bonds form between the N,N-dimethylaniline site
(donor site) of compound 2-StN-NMe2 and the solvent
molecules. Therefore, the absorption maxima displayed a
blue-shift when the ratio of TFE is increased in the mixed
solvent. However, it was found that the emission maxima
exhibited a red-shift instead of a blue-shift. This phenome-
non can be explained by the ground-state hydrogen bonding
complex practicing a excited-state de-hydrogen bonding
(ESDHB) process because of the basicity of N,N-dimethy-
laniline site decreasing upon photoexcitation. The influence
of the HB effect was counteracted in the excited state, so the
emission maxima was dominated by solvent polarity. An
increase in polarity stabilizes the ICT state by solvation,
hence the emission maxima undergoes a red-shift with an
increase of solvent polarity. Since the polarity of TFE is
higher than dichloromethane, the emission maxima displays
a red-shift with an increase of the solvents polarity when
ESDHB occurs in the excited state.
It is well known that solvent-dependent spectral shifts
occur from either general or specific solvents effects. The
first effect is the interaction of the fluorophore with the
reactive field induced by the surrounding solvent. Specific
effects result from the short-range interaction between the
fluorophore with one or more solvent molecules [18,19],
such as the hydrogen bond. Generally speaking, the Stokes
shift of the ideal chromophore is supposedly determined by
only nonspecific solvation. However, the interactions from
HB are known to lead to deviations from ideal polarity
correlation. To distinguish between the two effects, it is
important to quantify the relative contribution from these
two effects. For compounds with acidic (or basic) sites, it is
difficult to separate these two effects when the solvent is a
hydrogen bond acceptor, or hydrogen bond donor. There-
fore, it is our goal to find a fluorophore that can simulta-
neously measure nonspecific and specific effects, such as
HB effects.
The HB interaction of 2-StN-NMe2 in the ground state
will exhibit a blue-shift at absorption maxima, but the
ESDHB process will counteract this influence. The value
of the blue-shift in the absorption maxima was influenced
by HB, but the emission maxima only responded to pure
nonspecific interaction. Thus, 2-StN-NMe2 will produce a
large StokesÕ-shift fluorescence in a hydrogen-bond solvent.
In order to counteract the influence of protonation, a
large quantity (0.01 M) of triethylamine was added. If pro-
tonation occurred in 2-StN-NMe2 the addition of triethyl-
amine induced a deprotonation process, which produced
c
e* f*
b
g f e d
a
g*
a* b* c* d*
270
320
370
420
470
520
wavelength(nm)
Fig. 3. The absorption and emission spectra of compound 2-StN-NMe2 in different mixed solvents (v/v0) between CH2Cl2 and CF3CH2OH. (solid line:
absorption, dotted line: emission): (a) CH2Cl2 100%, (b) CH2Cl2 80%, (c) CH2Cl2 60%, (d) CH2Cl2 40%, (e) CH2Cl2 20%, (f) CH2Cl2 0%, (g) with
1.0 · 10ꢁ3 M HCl in CH2Cl2 (a*, b*, c*, d*, e*, f*, g* are emission spectra, EXC = 340 nm.).

S.-L. Wang et al. / Chemical Physics Letters 415 (2005) 217–222
221
The absorption maxima and emission maxima of 2-StN-
NMe2 in different solvents are collected in Table 2. It is
seen that the absorption maxima does not vary with differ-
ent solvents. Although the absorption maxima was insensi-
tive to solvent polarity, the emission maxima showed a
significant bathochromic shift with increasing solvent
polarity. The relationship between the Stokes shift of 2-
StN-NMe2 to different solvent polarity are shown in
Table 2
The absorption and emission maxima of compound 2-StN-NMe2 in different solvents
Solvent
fea
a-Scaleb
Absorption
kmax (nm)
Emission
kmax (nm)
Stokes
shift (cmꢁ1
Pure HB
effect (cmꢁ1
)
)
n-Hexane
Cyclohexane
Toluene
Di-n-butyl ether
Diethyl ether
Ethyl acetate
THF
CH2Cl2
Acetone
DMF
CH3CN
0.1849
0.2023
0.2396
0.2905
0.345
356.6
358.2
365.4
359.8
359.0
362.2
364.6
367.0
363.0
367.0
363.0
371.8
409.2
413.8
430.0
424.4
432.4
448.4
455.6
460.0
471.8
478.6
477.8
490.0
3604
3751
4111
4230
4728
5308
5478
5509
6353
6354
6619
6488
0.385
0.4072
0.4156
0.4646
0.480
0.4803
0.4841
DMSO
Ethanol
Ethylene glycol
Methanol
Glycerol
Trifluoroethanol
0.47
0.86
0.9
0.98
1.21
1.51
359.8
364.2
359.0
333.5
328.6
458.8
477.8
472.2
477.2
497.6
5997
6528
6678
9029
10336
ꢁ274
153
334
2620
4041
0.4803
0.4774
0.484
0.4724
Acronyms. Tetrahydrofuran (THF), dimethyl formamide (DMF), dimethyl sulfoxide (DMSO).
a
f(e), Kirkwood function (= (e ꢁ 1)/(2e + 1)); e, dielectric constant.
b
Hydrogen-bond acidity [20].
a
11000
10000
aprotic solvent
Pure HB effect
9000
8000
7000
6000
5000
4000
3000
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
Solvent polarity parameter, f(ε)=(ε-1)/(2ε+1)
b
11000
10000
9000
8000
7000
6000
5000
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
Hydrogen Bond Acidity (α)
Fig. 4. The relationship between the Stokes shift of compound 2-StN-NMe2 to different solvent polarity function. (a) Kirkwood function (fe = (e ꢁ 1)/
(2e + 1)). (b) Hydrogen bond acidity.

222
S.-L. Wang et al. / Chemical Physics Letters 415 (2005) 217–222
Fig. 4. For an aprotic solvent the Kirkwood function
(fe = (e ꢁ 1)/(2e + 1)), a solvation scale to measure nonspe-
cific interaction, is used in Fig. 4a. For a protic solvent the
hydrogen bond acidity, a, developed by Taft is used to ex-
plore the influence of HB to the Stokes shift [20]. A good
practice the proton transfer process in the excited state.
For compounds 2-StT-NMe2 and 2-StN-NMe2, the value
of DpKa is too small to induce the ESDP process. Thus only
an ESDHB process, which possesses a smaller reaction bar-
rier than ESDP, was observed in this study. Using this
mechanism, we can probe the polarity and HB effect of
an interesting microenvironment at the same time.
linear correlation exists between the Stokes shift (cmꢁ1
)
and different solvent polarity functions, except for the pro-
tic solvent, as seen in Fig. 4a. But in Fig. 4b, a good linear
correlation exists between the Stokes shift and hydrogen
bond acidity. The correlation indicates that nonspecific
interaction possesses a dominant importance in aprotic sol-
vents, but HB interactions do exist between 2-StN-NMe2
molecules and protic solvents.
The value of the Stokes shift obtained in protic solvents
was higher than the one measured in aprotic solvents. The
HB interaction between N,N-dimethylaniline site and pro-
tic solvents was stronger in the ground state, leading to
hypsochromic shifts in the absorption maxima with
increasing a. This bond breaks after photoexcitation, so
the emission maxima are not seriously influenced by the
HB interaction, resulting in a large Stokes shift.
By calculating the deviation from linearity of protic sol-
vents, we can quantify the influence of HB, as shown in
Fig. 4a. These values are listed in Table 2. Unlike the pro-
tonation effect, the steric effect is significant for HB interac-
tions [12]. Table 2 shows that the HB interaction contributes
a positive Stokes shift only when the hydrogen bond acidity
is higher than the ethylene glycol. This means that the
ground-state HB interaction is not important for ethanol
because of its lower HB acidity and its bulky steric effect.
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4. Conclusion
Compounds of 2-StP-NMe2, 2-StQ-NMe2, 4-StQ-NMe2,
and 4-StP-NMe2, which possesses a large value of DpKa
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