Photophysics and excited-state proton transfer of 2′-hydroxy-2-trans- styrylquinoline

Article abstract of DOI:10.1016/j.cplett.2005.11.009

2-StQ-2OH possessing a proton donor group (hydroxy) and a proton acceptor group (quinoline) was synthesized in order to study the influence of structural effects on the excited-state proton transfer (ESPT) and tautomerization. The ESPT were observed for ground-state hydrogen-bonding complex between 2-StQ-2OH and 2,2,2-trifluoroethanol (TFE) or with triethylamine. In aqueous solution, ESPT can only be observed in α-cyclodextrin (α-CD) inclusion complex. The dielectric constant values of the α-CD cavities probed by 2-StQ-2OH and 2-StQ-2OMe are 8.5 and 6.3, respectively, because of the difference in size of the probe molecule inside the cavity.

Full text of DOI:10.1016/j.cplett.2005.11.009

Chemical Physics Letters 418 (2006) 397–401
www.elsevier.com/locate/cplett
Photophysics and excited-state proton transfer of
0
2
-hydroxy-2-trans-styrylquinoline
a,
a
b
*
Shun-Li Wang , Tzu-Wei Yeh , Tong-Ing Ho
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 27 August 2005; in final form 16 October 2005
Available online 23 November 2005
Abstract
-StQ-2OH possessing a proton donor group (hydroxy) and a proton acceptor group (quinoline) was synthesized in order to study the
2
influence of structural effects on the excited-state proton transfer (ESPT) and tautomerization.
The ESPT were observed for ground-state hydrogen-bonding complex between 2-StQ-2OH and 2,2,2-trifluoroethanol (TFE) or with
triethylamine. In aqueous solution, ESPT can only be observed in a-cyclodextrin (a-CD) inclusion complex. The dielectric constant val-
ues of the a-CD cavities probed by 2-StQ-2OH and 2-StQ-2OMe are 8.5 and 6.3, respectively, because of the difference in size of the
probe molecule inside the cavity.
ꢀ
2005 Elsevier B.V. All rights reserved.
1
. Introduction
different relative proportions of the zwitterionic and quino-
noid froms of the tautomer can be observed in the excited
state by the coupling between proton transfer (PT) and
electron transfer [10,11].
Proton transfer and electron transfer represent two of
the most fundamental processes involved in biological
and chemical reactions [1–3]. For some bifunctional com-
pounds, which possess a proton donor group and a proton
acceptor group, undergo a dramatic change in their acidity
and basicity upon photoexcitation [4,5]. When these two
groups possess opposite excited-state acidity (pK*) tenden-
cies, excitation may promote protonation and deprotona-
tion to yield a tautomeric species [6,7]. Depending on the
distance between these two groups, various interesting pho-
toinduced processes can occur, including intramolecular
proton transfer via H-bonded vicinal groups, concerted
biprotonic transfer within a doubly H-bonded dimmer or
by a bridge of solvent molecules between two distinct
groups [8,9], double proton transfer at very distant groups.
If intramolecular charge-transfer (ICT) rate is fast enough,
Among bifunctional molecules, hydroxyquinoline
derivatives are extensively studied. For hydroxyquinoline
derivatives, the two prototropic groups were located on
the same aromatic ring; therefore, there are no structural
factor to influence the occurrence of intramolecular elec-
tron transfer. However, if the two prototropic groups
were not located on a same aromatic ring, whether the
ICT rate is still fast enough to induce the occurrence
of photoinduced tautomerization? In this study, a model
0
compound, 2 -hydroxy-2-trans-styrylquinoline (2-StQ-
2OH), that the two prototropic groups were separated
by three conjugated double bonds was synthesized to
study the influence of structure on excited-state ICT
and PT. Since coplanar is important for ICT through
conjugation along the pi-electronic system, the occurrence
of photoinduced tautomerization for 2-StQ-2OH was
decided not only by the distance of donor and acceptor
units but also on the ICT rate influenced by the struc-
tural change.
*
Corresponding author. Fax: +886 5 2717899.
E-mail address: wangshunli@mail.ncyu.edu.tw (S.-L. Wang).
0
009-2614/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2005.11.009

398
S.-L. Wang et al. / Chemical Physics Letters 418 (2006) 397–401
2
. Experimental
(HBC) with triethylamine in the ground state. The emission
spectra display a different figure compared with absorption
spectra. Apart from the emission band corresponding to 2-
StQ-2OH with kmax at 402 nm (normal band) decreases
continuously with increasing concentration of triethyl-
amine, another emission with a band maximum at
530 nm develops in intensity with increasing triethylamine
concentration. Isoemissive points in the emission spectra
are observed at 477 nm, which indicate the existence of
more than one emitting species. Excitation spectra of the
normal band and the new band are similar with the absorp-
tion spectra of 2-StQ-2OH without and with triethylamine
respectively (data not shown). Therefore, the normal band
is exclusively due to free 2-StQ-2OH, and ground state
HBC contribute to the new emission band. Since the Stokes
2
.1. Materials
0
Compound 2 -hydroxy-2-trans-styrylquinoline (2-StQ-
2
OH) was synthesized by reacting 2-hydroxybenzaldehyde
with Quinalide. The mixtures were refluxed in acetic anhy-
dride catalyzed by zinc chloride for 2 h [12]. The solid
product was hydrolyzed in basic aqueous solution (with
0
1
0% methanol) and recrystallized from methanol. 2 -meth-
oxy-2-trans-styrylquinoline (2-StQ-2OMe) was synthesized
by reacting 2-methoxybenzaldehyde with Quinalide. The
mixtures were refluxed in acetic anhydride catalyzed by
zinc chloride for 2 h. The solid product was recrystallized
from ether.
All the solvents were Uvasol grade from Merk or spec-
trophotometric grade from Aldrich and were used as
received.
ꢀ1
shift of this new band is quite large (9144 cm ), it is likely
that in the excited state this H-bonded complex may be
present as contact ion-pair by excited-state proton transfer
(ESPT) because of very high photoacidity of the hydroxylic
2
.2. Method
proton [13,14].
The absorption and emission spectra of compound
2-StQ-2OH in various ratios of mixed solvents between
dichloromethane and TFE are shown in Fig. 2. The
absorption band displayed a shoulder in a region 400–
450 nm with a decrease in the absorbance. This change
might be from the hydrogen bonding between TFE with
nitrogen atom of quinoline ring. The normal emission band
displayed a little red shift when the TFE ratio is increased
because TFEÕs polarity is larger than dichloromethane. A
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
ꢀ
5
spectrometer. Sample with concentrations of 1.0 · 10
–
ꢀ5
1
.5 · 10 M were used for the measurements.
3
. Results and discussion
ꢀ
1
The absorption spectra and fluorescence spectra of com-
new band at 507 nm with large Stokes shift (9262 cm )
pound 2-StQ-2OH in dichloromethane with varying con-
centration of triethylamine are shown in Fig. 1. The
absorption maxima of 2-StQ-2OH in dichloromethane
are at about 345 and 355 nm. Upon addition of triethyl-
amine to the solution, the absorption spectra of 2-StQ-
was observed at higher TFE content solvent. This emission
band was attributed to the ground state HBC undergo
ESPT and leads to contact ion-pair formation in dichloro-
methane because a high photobasicity of the quinoline in
the excited state [15].
2
OH do not change appreciably, except for a small increase
in the optical density towards longer wavelength. Since
-StQ-2OH possesses an acidic hydroxyl group, it is rea-
sonable for 2-StQ-2OH to form hydrogen bonded complex
The ground state pK of 2-StQ-2OH are 5.6 (quinoline
a
ring) and 9.2 (hydroxyl group), and its acidity in the excited
ꢁ
2
state ðpK Þ recorded by Forster cycle are 10.5 (quinoline
a
ring) and 3.8 (hydroxyl group), respectively. The quinoline
a
e
a*
e*
a
f
a*
f*
f*
e*
a*
a*
2
50
300
350
400
450
500
550
600
280
330
380
430
480
530
580
630
Wavelength (nm)
Wavelength (nm)
Fig. 2. Absorption and emission spectra of compound 2-StQ-2OH in
different mixed solvents between dichloromethane and 2,2,2-trifluoroeth-
Fig. 1. Absorption and emission spectra of compound 2-StQ-2OH in
dichloromethane with various concentration of triethylamine: (a) 0 M, (b)
.01 M, (c) 0.02 M, (d) 0.03 M, (e) 0.04 M, (f) 0.05 M (a*, b*, c*, d*, e*, f*
anol (TFE): (a) pure dichloromethane, (b) with 20% (V/V
40% (V/V ) TFE, (d) with 60% (V/V ) TFE, (e) pure TFE (a*, b*, c*, d*,
e* are emission spectra, EXC = 350 nm).
0
) TFE, (c) with
0
0
0
are emission spectra, EXC = 350 nm).

S.-L. Wang et al. / Chemical Physics Letters 418 (2006) 397–401
399
ring and hydroxy group of 2-StQ-2OH undergo an obvious
change in their basicity and acidity in the excited state.
Therefore, the ground state HBC undergoes a proton-
transfer reaction in the excited state, as shown in Figs. 1
and 2. Since the quinoline ring and the hydroxy group of
reaction in the excited state. Unfortunately, the anionic
and cationic forms of 2-StQ-2OH are not fluorescent in
aqueous solution. Therefore, we can not monitor any
excited-state proton transfer in aqueous solution.
The emission spectra of 2-StQ-2OH and 2-StQ-2OMe
with various concentration of a-CD in pH 7 aqueous solu-
tion were shown in Fig. 4. Although free 2-StQ-2OH and
free 2-StQ-2OMe show similar figure in absorption and
emission spectra, their encapsulated complex with a-CD
displayed a different result. The emission spectrum of 2-
StQ-2OMe encapsulated in a-CD displayed a little blue
shift (447 nm) compared with the free one (453 nm) in
pH 7 aqueous solution, and the emission intensity only
increase obviously with the addition of a-CD. The normal
emission of 2-StQ-2OH in pH 7 aqueous solution was
located at 462 mm, but the emission spectrum of encapsu-
lated complex between 2-StQ-2OH and a-CD produce two
strong emission peaks at 410 (S band) and 515 (L band)
2
-StQ-2OH possess opposite excited-state acidity (pK*)
tendencies, it is interesting to see whether excitation may
stimulate the occurrence of tautomerization to yield a zwit-
terions or a quinonoid form.
Because the distance between the quinoline ring and the
hydroxy group are not in close proximity, photoinduced
tautomerization of 2-StQ-2OH can only occur through a
bridge of solvent molecules between these two groups or
by double proton transfer with water molecules. In this
work, photoinduced tautomerization of 2-StQ-2OH can
not be observed in cyclohexane whatever guest molecule
(
methanol or acetic acid molecule) are used. There are
two conjugated single bonds existed between the quinoline
group and hydroxy group. We regard that the rotation of
the single-bond retarded the correct conformation to
undergo the photoinduced tautomerization by a bridge of
solvent molecules. Moreover, the rotation of the single-
bond destroyed the conjugated pi-electronic systems, and
influenced the photoinduced ICT reaction rate. Therefore,
in contrast to hydroxyquinoline derivatives, the 2-StQ-
b
2
OH does not behave like a strong photobase and
photoacid.
The absorption and emission spectra of 2-StQ-2OH in
d
c
pH 7 aqueous solution were shown in Fig. 3. In order to
verify the influence of hydrogen bonding with water mole-
cules on hydroxy group, the absorption and emission spec-
tra of 2-StQ-2OMe were also shown in Fig. 3. Because the
donor ability of methoxy group is similar with hydroxy
group, these two compounds will display similar absorp-
tion if there are no specific interaction occurred on hydroxy
group. From Fig. 3, we can conclude there are no detected
ground-state tautomerization reaction happen in pH 7
aqueous solution. Because the quinoline ring and the
hydroxy group of 2-StQ-2OH change their acidity greatly
in excited state, it is important to monitor the acid–base
a
× 5
460
360
410
510
560
610
Wavelength (nm)
Fig. 4. Emission spectra of compounds 2-StQ-2OH and 2-StQ-2OMe in
pH 7 aqueous solution with various concentration of a-cyclodextrin. (a) 2-
StQ-2OH without a-cyclodextrin, (b) 2-StQ-2OH with 0.01 M a-cyclo-
dextrin, (c) 2-StQ-2OMe without a-cyclodextrin, (d) 2-StQ-2OMe with
0
.04 M a-cyclodextrin, (EXC = 350 nm).
c
6
5
4
3
2
1
0
d
b
e
b
a
a
1
.5
6.5
pH
11.5
b
f
a
g
3
60
410
460
510
560
610
Wavelength (nm)
2
50
350
450
550
650
Fig. 5. Emission spectra of compound 2-StQ-2OH with 0.01 M of a-
cyclodextrin in different pH aqueous solutions. (a) pH 4.0, (b) pH 5.0, (c)
pH 7.0, (d) pH 8.0, (e) pH 9.0, (f) pH 10.0, (g) pH 11.0. Inset figure: the
relation between pH and intensity ratio of I515 nm/I410nm. (EXC =
350 nm).
Wavelength (nm)
Fig. 3. Absorption and emission spectra of compounds 2-StQ-2OH and
-StQ-2OMe in pH 7 aqueous solution. (solid line: absorption, dotted line:
emission): (a) 2-StQ-2OH, (b) 2-StQ-2OMe. (EXC = 350 nm).
2

400
S.-L. Wang et al. / Chemical Physics Letters 418 (2006) 397–401
N
N
O
HO
H3C
Scheme 1.
nm. The excited-state dipole moment of 2-StQ-2OH and 2-
StQ-2OMe are 4.2 and 14.1 D, respectively, measured by
the solvatochromic effect on the position of absorption
and emission spectra, so the position of emission band will
depends on solvent polarity. It is well known that the
polarity of a-CD cavity is low compared with pure water;
therefore, the blue-shift emission (S band) was assigned
as the emission of encapsulated complex of a-CD-2-StQ-
Here, K is equilibrium constant and n is complex stoichi-
0
ometry, I and I are the fluorescence intensities in the pres-
f
f
ence and absence of a-CD, respectively, and a is an
experimental constant. The K value for the a-CD/2-StQ-
2OH (2:1 stoichiometry) and a-CD/2-StQ-2OMe (2:1 stoi-
chiometry) inclusion complex is evaluated to be 11600 and
ꢀ
2
3
3260 mol dm , respectively.
The polarity of the a-CD cavity (dielectric constant, e)
probed by the encapsulated complex of a-CD/2-StQ-2OH
is about 8.5 using solvatochromic effect of emission spec-
tra. But the e values of a-CD cavity probed by the encap-
sulated complex of a-CD/2-StQ-2OMe was evaluated
about 6.3. The complexes were shown in Scheme 1. The
contradiction about the e values of a-CD cavity may be
from different size between 2-StQ-2OH and 2-StQ-2OMe
fitted into a-CD cavity. Therefore, the location of the
probe molecule inside the cavity is quite critical for the
evaluation of the dielectric constant values of the cyclodex-
trin cavities [19,20].
2
OH.
The emission spectra of 2-StQ-2OH with the addition of
a-CD in various pH values solution were display in Fig. 5.
The relation between intensity ratio of Lband/Sband and pH
value was inserted into Fig. 5. It is obviously that the ratio
of Lband/Sband increase at higher pH value. The identical
excitation spectra of these two peak (S band and L band)
suggested that they were from a same ground-state species.
Therefore, we regarded that the emission source of L
band was from the anionic forms of 2-StQ-2OH. Because
the acidity of hydroxy group increases in the excited state,
an excited-state deprotonation occurred in neutral 2-StQ-
2
OH when it encapsulated into cyclodextrin cavity. At high
4. Conclusion
pH value solution, the ESDP reaction was accelerated, so
the intensity ratio of Lband/Sband increased. Although the
intensity ratio of Lband/Sband increased at high pH value
solution, the absolute intensity of L band at neutral solu-
tion is higher than high pH value solution. Therefore, we
regard that the anion form of 2-StQ-2OH can not form
the inclusion complex with a-CD. Only when the neutral
2-StQ-2OH possessing a proton donor group (hydroxy)
and a proton acceptor group (quinoline), undergo a dra-
matic change in their acidity and basicity upon photoexci-
tation. The ESPT reactions were observed in solvent with
good hydrogen-bond donor (TFE) or ground-state hydro-
gen-bond complex with TEA. However, the rotation of the
single-bond retarded the correct conformation needed to
proceed the photoinduced tautomerization by a bridge of
solvent molecules. In aqueous solution, ESPT reaction
can only happen in a-CD inclusion complex. Different
probe molecule can occupy different site of the cavity, thus
the e values of a-CD cavities probed by 2-StQ-2OH and
2-StQ-2OMe are 8.5 and 6.3, respectively.
2
-StQ-2OH molecule was included inside the a-CD, follow-
ing an excited-state deprotonation reaction, we can record
the emission of the anion form of 2-StQ-2OH. We conclude
that the excited-state prototropic behavior of 2-StQ-2OH is
similar with 6-hydroxyquinoline-N-oxides and 10-hydroxy-
camptothecin and does not involve the aromatic quinolin-
ium nitrogen moiety as a strong photobase in the excited
state, but only deprotonation of the hydroxy group [16,17].
The stoichiometry and equilibrium constant of inclusion
complex can be obtained by Benesi–Hildebrand equation:
References
[
[
[
1] W.R. Browne, N.M. OÕBoyle, J.J. McGarvey, J.G. Vos, Chem. Soc.
Rev. 34 (2005) 641.
2] M.C. Jimenez, M.A. Miranda, R. Tormos, Chem. Soc. Rev. 34 (2005)
783.
[
18]
1
1
1
¼
f
þ
ð1Þ
0
ꢁ
ꢁ
n
ðI ꢀ I Þ
a
ða K ½a ꢀ CDꢂ Þ
f
3] J.A. Mayer, Annu. Rev. Phys. Chem. 55 (2004) 363.

S.-L. Wang et al. / Chemical Physics Letters 418 (2006) 397–401
401
[
4] A. Kyrychenko, J. Herbich, M. Izydorzak, M. Gil, J. Dobkowski, F.
Wu, R.P. Thummel, J. Waluk, Isr. J. Chem. 39 (1999) 308.
[13] M.S. Mehata, H.C. Joshi, H.B. Tripathi, Chem. Phys. Lett. 366
(2002) 628.
[
[
5] L.M. Tolbert, K.M. Solntsev, Acc. Chem. Res. 35 (2002) 19.
6] H.J. Park, O.H. Kwon, C.S. Ah, D.J. Jang, J. Phys. Chem. B 109
[14] F.D. Lewis, L.E. Sinks, W. Weigel, M.C. Sajimon, E.M. Crompton,
J. Phys. Chem. A 109 (2005) 2443.
(
2005) 3938.
[15] S.L. Wang, T.C. Lee, T.I. Ho, J. Photochem. Photobiol. A 151 (2002)
21.
[16] K.M. Solntsev, C.E. Clower, L.M. Tolbert, D. Huppert, J. Am.
Chem. Soc. 127 (2005) 8534.
[
[
7] T.G. Kim, M.R. Topp, J. Phys. Chem. A 108 (2004) 10060.
8] C.Y. Wei, W.S. Yu, P.T. Chou, F.T. Hung, C.P. Chang, T.C. Lin, J.
Phys. Chem. B 102 (1998) 1053.
[
9] Y. Matsumoto, T. Ebata, N. Mikami, J. Phys. Chem. A 106 (2002)
5
[17] K.M. Solntsev, E.N. Sullivan, L.M. Tolbert, S. Ashkenazi, P.
Leiderman, D. Huppert, J. Am. Chem. Soc. 126 (2004) 12701.
[18] S. Hamai, A. Nakamura, in: H.S. Nalwa (Ed.), Handbook of
Photochemistry and Photobiology, vol. 2, American Scientific Pub-
lishers, 2003, p. 59 (Chapter 2).
591.
10] E. Bardez, A. Chatelain, B. Larrey, B. Valeur, J. Phys. Chem. 98
1994) 2357.
[
(
[
[
11] E. Bardez, Isr. J. Chem. 39 (1999) 319.
12] S.L. Wang, T.I. Ho, J. Photochem. Photobiol. A 135 (2000)
[19] S. Hamai, J. Phys. Chem. 94 (1990) 2595.
119.
[20] K.A. Connors, Chem. Rev. 97 (1997) 1325.