Synthesis and specific solvatochromism of D-π-A type pyridinium dye

Article abstract of DOI:10.1016/j.tet.2012.08.010

Donor-π-acceptor (D-π-A) type pyridinium dye bearing an iodide ion as the counter anion that has been newly synthesized showed specific solvatochromism, leading to large bathochromic shift of absorption band in halogenated solvents: the bathochromic shifts of the D-π-A type pyridinium dye in halogenated solvents are larger than those of the non-halogenated solvents of low ε_r values. Investigation of absorption spectral measurement, ¹H NMR measurements, and semi-empirical molecular calculations (AM1 and INDO/S using the SCRF Onsager Model) revealed that the intramolecular charge transfer (ICT) characteristics of the D-π-A type pyridinium dye became stronger in the halogenated solvents. On the basis of the experimental results and the theoretical calculations, the influences of halogenated solvent on the large bathochromic shift of D-π-A type pyridinium dye are discussed. 2012 Elsevier Science. All rights reserved.

Full text of DOI:10.1016/j.tet.2012.08.010

Tetrahedron 68 (2012) 8577e8580
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis and specific solvatochromism of DepeA type pyridinium dye
*
Yousuke Ooyama , Kohei Kushimoto, Yuichiro Oda, Daisuke Tokita, Naoya Yamaguchi, Shogo Inoue,
*
Tomoya Nagano, Yutaka Harima, Joji Ohshita
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 July 2012
Received in revised form 3 August 2012
Accepted 4 August 2012
Available online 10 August 2012
Donore
been newly synthesized showed specific solvatochromism, leading to large bathochromic shift of ab-
sorption band in halogenated solvents: the bathochromic shifts of the De eA type pyridinium dye in
r values. Investigation
peacceptor (DepeA) type pyridinium dye bearing an iodide ion as the counter anion that has
p
halogenated solvents are larger than those of the non-halogenated solvents of low
3
of absorption spectral measurement, 1H NMR measurements, and semi-empirical molecular calculations
(AM1 and INDO/S using the SCRF Onsager Model) revealed that the intramolecular charge transfer (ICT)
Keywords:
characteristics of the DepeA type pyridinium dye became stronger in the halogenated solvents. On the
Pyridinium dye
Solvatochromism
Donorepieacceptor system
Halogenated solvent
Carbazole
basis of the experimental results and the theoretical calculations, the influences of halogenated solvent
on the large bathochromic shift of De
p
eA type pyridinium dye are discussed. Ó2012 Elsevier Science. All
rights reserved.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
those in non-halogenated solvents of similar
3
values, such as
r
toluene and hexane.12 Upto the present time, however, there have
been few efforts to clarify the influences of halogenated solvent on
the large bathochromic shift, although the specific solvatochrom-
ism is of great interest from an academic viewpoint, together with
a practical importance, such as development of sensors for halo-
genated organic compounds.
Solvatochromic dyes have received considerable attention be-
cause of a fundamental interest in photochemistry and their ap-
plication as environmental probe.1 Donore
peacceptor (DepeA)
type pyridinium dyes having diphenyl or dialkyl amino group as
electron donor and pyridinium ring as electron acceptor linked by
a
p
-conjugated bridge, and bearing a halide anion are known well
to show negative solvatochromism (i.e., bathochromic shift of ab-
sorption band with decreasing dielectric constant ( r) of solvent
In this paper, we describe the specific solvatochromic behaviors
of novel DepeA type pyridinium dye 2 bearing an iodide ion as the
counter anion: the dye 2 possesses stronger intramolecular charge
transfer (ICT) characteristics, and the bathochromic shifts of ICT
band in halogenated solvents are larger than those of the non-
3
(solvent polarity), in other words, with increasing solvent polarity,
the absorption band shows a hypsochromic shift), which have been
studied systematically by many researchers.2e15 It has been spec-
ulated from absorption spectra and theoretical calculations that the
negative solvatochromism would be ascribed to the change of the
electronic ground-state structure of the pyridinium dye on
changing of solvent polarity; with increasing solvent polarity the
ground state is more stabilized than that of excited state because
the ground state is more dipolar than the excited state, and this
produces a hypsochromic shift.1,6,7 Recently, we have mentioned
halogenated solvents of low
3
r
values (Scheme 1). The influences
of halogenated solvents on the large bathochromic shifts are
studied on the basis of 1H NMR measurements and semi-empirical
molecular calculations (AM1 and INDO/S using the SCRF Onsager
Model). These results revealed that the ICT characteristics of 2 in
halogenated solvents, which are induced by the formation of hal-
ogenehalide anion interaction between iodide ion of the counter
anion and the halogen atom of halogenated solvents, are re-
another interesting feature of the heteropolycyclic De
peA type
sponsible for the large bathochromic shifts of ICT band of DepeA
pyridinium dyes is that the bathochromic shifts in halogenated
solvents, such as dichloromethane and chloroform are larger than
type pyridinium dye in halogenated solvents.
2. Results and discussion
* Corresponding authors. Tel.: þ81 82 424 7689; fax: þ81 82 424 5494; e-mail
addresses: yooyama@hiroshima-u.ac.jp (Y. Ooyama), jo@hiroshima-u.ac.jp (J.
Ohshita).
The synthetic pathway of 2 is shown in Scheme 1. We used
diphenyl-[7-(5-pyridin-4-yl-thiophen-2-yl)-9H-carbazol-2-yl]-
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.08.010

8578
Y. Ooyama et al. / Tetrahedron 68 (2012) 8577e8580
40000
30000
20000
10000
0
in dichloromethane
in dibromomethane
in diiodomethane
1-Iodobutane
CH3CN
N
N
S
N
H
in THF
1
in DMSO
N
N
S
I
N
H
2
300 400 500 600 700 800
Wavelength/nm
Scheme 1. Synthesis of DepeA type pyridinium dye 2.
Fig. 1. Absorption spectra of 2 in various solvents.
amine 116 as starting material. The reaction of 1 with 1-iodobutane
gave the De eA type pyridinium dye 2 (87% yield).
p
The solvatochromic spectral data of 2 are summarized in Table 1
and the spectra of 2 in various solvents are shown in Fig. 1. The
pyridinium dye 2 shows absorption maxima at around 370 nm
(p/p* transition) and at around 460e530 nm (ICT excitation from
diphenylamino group to pyridinium ring). The ICT band exhibits
a strong negative solvatochromism (Table 1). With decreasing sol-
(a)
(b)
(c)
(d)
(e)
vent polarity from DMSO (
ICT band shifts from 460 nm to 499 nm. It is worthy to note here
that the bathochromic shifts of ICT band are prominent in halo-
3
¼47.24) to cyclohexane ( ¼2.02), the
3
r
r
Fig. 2. Solvatochromic properties of 2 in various solvents: (a) THF, (b) DMSO, (c) di-
chloromethane, (d) dibromomethane, and (e) diiodomethane.
contributable to stronger ICT characteristics, which are induced by
the hydrogen bonding between hydroxyl group of alcohol and io-
dide ion as the counter anion of 2.
genated solvents: although the
3
value of dibromomethane
r
(
3
¼7.77) is very close to that of tetrahydrofuran ( ¼7.52), the ICT
3
r
r
band of 2 in dibromomethane occurs at 497 nm compared with
467 nm in THF. Fig. 2 shows color changes of 2 in various solvents.
Moreover, we found that the bathochromic shifts of ICT band in
iodinated solvents, such as iodobenzene and diiodomethane are
larger than those in chlorinated and brominated solvents. On the
To elucidate the influences of solvent polarity on the ICT bands
of 2, semi-empirical MO calculations of 2 were carried out by the
INDO/S method18 using the SCRF Onsager Model after geometrical
optimizations using the MOPAC/AM1 method.19 The calculated
absorption data are collected in Table 2. For all the solvents used,
the calculations show that the first excitation bands of 2 are mainly
assigned to a transition from HOMO to LUMO, where HOMO is
mostly localized on the diphenylaminoecarbazole moiety and
other hand, among polar solvents ( r>20), the ICT bands of 2 in
3
alcohol solvents appear at wavelengths longer than that of 2 in the
aprotic solvents, such as acetone, acetonitrile, and DMSO. It was
suggested that the bathochromic shifts of ICT band in alcohols are
Table 1
Table 2
Solvatochromic spectral data of 2
Calculated absorption spectra for 2
abs
max
max/Mꢀ1 cmꢀ1
)
No.
Solvent
lmax/nm
CI componenta
a
b
No.
Solvent
3
r
l
/nm (
3
1
2
3
4
5
6
7
8
Cyclohexane
1,4-Dioxane
Tetrachloromethane
Toluene
2.02
2.22
2.24
2.38
4.40
4.59
4.81
5.32
5.45
5.47
5.69
6.09
6.27
6.97
7.28
7.32
7.52
7.77
8.93
20.80
21.10
25.3
33
499 (e)c
1
2
3
4
5
6
7
8
Cyclohexane
1,4-Dioxane
Tetrachloromethane
Toluene
457
454
449
443
416
414
419
408
413
416
414
419
414
412
416
414
414
410
414
410
411
410
411
410
408
407
HOMO/LUMO (50%)
HOMO/LUMO (51%)
HOMO/LUMO (52%)
HOMO/LUMO (53%)
HOMO/LUMO (72%)
HOMO/LUMO (73%)
HOMO/LUMO (75%)
HOMO/LUMO (77%)
HOMO/LUMO (77%)
HOMO/LUMO (78%)
HOMO/LUMO (78%)
HOMO/LUMO (79%)
HOMO/LUMO (80%)
HOMO/LUMO (82%)
HOMO/LUMO (82%)
HOMO/LUMO (82%)
HOMO/LUMO (84%)
HOMO/LUMO (83%)
HOMO/LUMO (83%)
HOMO/LUMO (89%)
HOMO/LUMO (89%)
HOMO/LUMO (90%)
HOMO/LUMO (90%)
HOMO/LUMO (90%)
HOMO/LUMO (90%)
HOMO/LUMO (90%)
470 (7600)
478 (e)c
481 (e)c
Bromoform
Iodobenzene
Chloroform
Diiodomethane
Bromobenzene
Fluorobenzene
Chlorobenzene
Ethyl acetate
1-Iodobutane
Iodomethane
1-Chlorobutane
1-Bromobutane
THF
Dibromomethane
Dichloromethane
1-Propanol
Acetone
Ethanol
Methanol
Acetonitrile
DMF
502 (27,800)
Bromoform
Iodobenzene
Chloroform
Diiodomethane
Bromobenzene
Fluorobenzene
Chlorobenzene
Ethyl acetate
1-Iodobutane
Iodomethane
1-Chlorobutane
1-Bromobutane
THF
Dibromomethane
Dichloromethane
1-Propanol
Acetone
Ethanol
Methanol
Acetonitrile
DMF
507 (e)c
497 (29,300)
527 (24,800)
486 (e)c
9
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
469 (e)c
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
494 (e)c
470 (32,000)
493 (e)c
497 (20,300)
480 (e)c
474 (e)c
467 (22,800)
497 (28,600)
502 (24,500)
484 (34,300)
461 (33,000)
476 (33,300)
465 (5700)
456 (33,600)
459 (32,200)
460 (32,600)
36.64
36.72
47.24
DMSO
DMSO
a
a
Dielectric constant (Ref. 17).
The longest wavelength absorption maximum.
Poorly soluble.
The transition is shown by an arrow from one orbital to another, followed by its
percentage CI (configuration interaction) component (see Table S1 in Supplemen-
tary data for HOMO and LUMO energy levels).
b
c

Y. Ooyama et al. / Tetrahedron 68 (2012) 8577e8580
8579
LUMOs are mostly localized on the thienylepyridinium moiety
(a)
(Fig. 3a and b). The changes in the calculated electron density ac-
companying the first electron excitation are shown in Fig. 3c, which
reveals that the ICT occurs from the diphenylaminoecarbazole
moiety to the pyridinium moiety. The calculation demonstrates
that with decreasing solvent polarity from DMSO to cyclohexane
the ICT bands show bathochromic shifts from 407 nm to 457 nm
(Table 2). In order to see the influence of solvent polarity on the ICT
24
26
E
21
23
17
25
10
2
E
3EE 12
22
16
15
13
20
4E
9
E
Non-halogenated solvents
Fluorinated solvent
Chlorinated solvents
Brominated solvents
Iodinated solvents
Alcohols
11
1
18
19
7
E5
6
14
~
bands more explicitly, the wavenumbers (v) of the experimental
8
and calculated absorption maxima for the ICT bands of 2 are plotted
against the
3
r
value of solvent in Fig. 4. The plot of the calculated
0
5
10 15 20 25 30 35 40 45 50
absorption maximum against
3
r
shows that the wavenumber in-
creases dramatically with the increase in
3
r value from cyclohexane
εr
(
3
¼2.02) to dichloromethane ( ¼8.93), while in the range of
3
3
r
r
r
26
(b)
10, 11, 13,
25
8
value between dichloromethane (
the wavenumber is increased gradually (Fig. 4b). On the other hand,
3
r
¼8.93) and DMSO ( ¼47.24)
3
r
22
20
23
9E
14, 15, 16,
18, 19
E
E
6
5
7
E
24
E EE EE17
E
21
E E
the profile of the plot of experimental absorption maximum against
E E 12
3 r differs considerably from that of the calculation: the ICT bands of
Non-halogenated solvents
Fluorinated solvent
Chlorinated solvents
Brominated solvents
Iodinated solvents
Alcohols
2 in halogeneted solvents are located in the low-wavenumber
range (Fig. 4a). However, when the plots of halogeneted solvents
and alcohol solvents are excluded form the plot of Fig. 4a, the
profile of the plot of Fig. 4a is similar to that of Fig. 4b. This result
shows that with increasing solvent polarity of non-halogenated
solvents, the ICT band of 2 shows a hypsochromic shift, that is,
the dye 2 in non-halogenated solvents exhibits a normal negative
solvatochromism.
4
E
3
E
2
E 1
E
0
5
10 15 20 25 30 35 40 45 50
εr
Fig. 4. Wavenumber (n) of (a) the experimental and (b) calculated absorption maxi-
mum for the ICT bands of 2 versus dielectric constant ( 3 ) of solvent. The numbers
r
correspond to those of Tables 1 and 2, respectively. The circles in black, blue, and red
show non-halogenated solvents, alcohol solvents, and halogenated solvents,
respectively.
non-halogenated solvents, and in CD2Cl2 as the halogenated sol-
vent were performed (Fig. 5). Remarkable differences were ob-
served between the halogenated and non-halogenated solvents:
the chemical shifts of Ha, Hb, Hc, and Hd, show upfield shifts on
changing solvents from DMSO-d6 or THF-d8 to CD2Cl2. Furthermore,
the NH of carbazole ring varies from 11.2 ppm in DMSO-d6 and
10.3 ppm in THF-d8 to 8.6 ppm in CD2Cl2. It is worth noting that
these chemical shifts in CD2Cl2 show significantly upfield shifts
compared to those in THF-d8, although the
3
r
value of CH2Cl2 is
close to that of THF. It is apparent that the observed upfield shifts in
the halogenated solvents are responsible for a shift of the electronic
structure due to strong ICT characteristics of 2 in the halogenated
solvents. Awwadi et al. have demonstrated by theoretical and
crystallographic studies for halogen-halide anion interaction
Fig. 3. (a) HOMO and (b) LUMO of 2. The red and blue lobes denote the positive and
negative phases of the coefficients of the molecular orbitals. The size of each lobe is
proportional to the MO coefficient. (c) Calculated electron density changes accompa-
nying the first electronic excitation of 2. The black and white lobes signify decrease and
increase in electron density accompanying the electronic transition, respectively. Their
areas indicate the magnitude of the electron density change. (Light blue, green, blue,
red, and gold balls correspond to hydrogen, carbon, nitrogen, oxygen, and sulfur atoms,
respectively.).
l
m
g
f
i
k
j
c d
S
a b
N
N
A significant difference between the experimental and the cal-
culated absorption maxima for 2 indicates that the electronic
ground-state structure of 2 in halogenated solvent does not mainly
I
N
H
e
h
2
depend on the r values of halogenated solvents but hints that the
3
(a)DMSO-d6
h k
m
l
interactions of halogenated solvent molecules with dye molecules
contribute to the large bathochromic shifts of ICT band in haloge-
nated solvents. Thus, the absorption spectral measurements of 2 in
mixed solvents of non-halogenated solvent (acetonitrile or THF)
and halogenated solvent (chloroform or dibromomethane) were
performed. We have plotted the wavenumber of the absorption
maximum against the mixture composition. However, the plots
were almost linear in all cases, we could not obtain useful in-
formation for specific interactions between the dye molecule and
the halogenated or non-halogenated solvents.
NH
b
a
c
d e
g
i
j
f
11.2 ppm
(b)THF-d8
NH
10.3 ppm
(c)CD2Cl2
NH
In order to investigate the influence of solvent on the electronic
ground-state structure of 2, the 1H NMR measurements in DMSO-
d6 of a high
3
r
value and THF-d8 of a relatively low
3
r
value as the
Fig. 5. 1H NMR spectra of 2 in (a) DMSO-d6, (b) THF-d8, and (c) CD2Cl2.

8580
Y. Ooyama et al. / Tetrahedron 68 (2012) 8577e8580
between the halide anion and the halogen of halogenated solvents
that the energy of interaction between iodine atom of iodinated
solvents and halide anion is larger than that of interaction between
chloride atom in chlorinated solvents or bromide atom in bromi-
nated solvents, and halide anion.20 In fact, the bathochromic shifts
of ICT band of 2 in iodinated solvents, such as iodobenzene and
diiodomethane are larger than those in chlorinated and bromi-
nated solvents (Table 1 and Fig. 1). Therefore, it was suggested that
the enhanced ICT characteristics of 2 in halogenated solvents,
which is induced by the formation of halogenehalide anion in-
teraction between iodide ion as the counter anion and the halogen
atom of halogenated solvents, are responsible for the large bath-
d
¼13.5, 19.0, 32.7, 59.5, 105.9, 108.0, 116.5, 117.4, 117.9, 120.7, 121.6,
121.9, 123.2, 123.9, 124.2, 126.2, 128.9, 129.7, 134.2, 135.0, 140.5,
142.1, 144.6, 146.6, 147.78, 147.84, 152.9 ppm. HRMS (ESI) : calcd for
[MꢀIþ] 550.23115; found 550.23151.
4.3. Computational methods
The semi-empirical calculations were carried out with the
WinMOPAC Ver. 3.9 package (Fujitsu, Chiba, Japan). Geometry cal-
culation of compound 2 in the ground state was made using the
AM1 method.19 The geometry was completely optimized (keyword
PRECISE) by the eigenvector following routine (keyword EF). Ex-
perimental absorption spectra of the compound was compared
with their absorption data by the semi-empirical method INDO/S
(intermediate neglect of differential overlap/spectroscopic)18 using
the SCRF Onsager Model. All INDO/S calculations were performed
using single excitation full SCF/CI (self-consistent field/configura-
tion interaction), which includes the configuration with one elec-
tron excited from any occupied orbital to any unoccupied orbital,
where 100 configurations were considered [keyword CI (10 10)].
ochromic shifts of ICT band of DepeA type pyridinium dye in ha-
logenated solvents.
3. Conclusions
Novel De
counter anion has been synthesized and its solvatochromic proper-
ties were investigated. The De eA pyridinium dye showed specific
peA type pyridinium dye bearing an iodide ion as the
p
solvatochromism, leading to large bathochromic shift of absorption
band in halogenated solvents: the ICT band of the pyridinium dye in
Acknowledgements
halogenated solvent is independent of
3 r values of halogenated sol-
vents, although the De eA pyridinium dye exhibited a general
negative solvatochromism in non-halogenated solvents. The exper-
imental results and the theoretical calculations revealed that the
p
This work was supported by Grants-in-Aid for Scientific Re-
search (B) (23350097) and for Young Scientist (B) (22750179) from
the Japan Society for the Promotion of Science (JSPS), and by a re-
search grant from Kurita Water and Environment Foundation.
enhanced ICT characteristics of the DepeA pyridinium dye in halo-
genated solvents, which are induced by the formation of halogen-
ehalide anion interaction between iodide ion of the counter anion
andthehalogenatomofhalogenated solvents, are responsible for the
Supplementary data
large bathochromic shifts of ICT band of De
in halogenated solvents. Further studies on the specific sol-
vatochromism of De eA type pyridinium dyes bearing various
peA type pyridinium dye
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2012.08.010.
p
counter anions are now in progress to gain greater insight into the
influences of halogen-counter anion interaction between the
counter anion of pyridinium dye and the halogen atom of haloge-
nated solvents on the electronic structures and the ICTcharacteristics
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17. CRC Handbook of Chemistry and Physics, 83rd ed.; Lide, D. R., Ed.; CRC: 2002.
18. (a) Ridley, J. E.; Zerner, M. C. Theor. Chim. Acta 1973, 32, 111e134; (b) Ridley, J. E.;
Zerner, M. C. Theor. Chim. Acta 1976, 42, 223e236; (c) Bacon, A. D.; Zerner, M. C.
Theor. Chim. Acta 1979, 53, 21e54.
to reprecipitation from CH2Cl2ehexane to give 2 (0.206 g, yieꢀld1
ꢁ
~
87%) as dark brown solids; mp 180e183 C. IR (ATR): v¼1629 cm
1H NMR (500 MHz, DMSO-d6):
d
¼0.92 (t, J¼7.0 Hz, 3H), 1.28e1.35
(m, 2H), 1.86e1.92 (m, 2H), 4.49 (t, J¼7.0 Hz, 2H), 6.87 (d, J¼8.0 Hz,
1H), 7.04e7.08 (m, 7H), 7.30e7.33 (m, 4H), 7.59 (d, J¼8.0 Hz, 1H),
7.80 (s, 1H), 7.88 (d, J¼4.0 Hz, 1H), 8.03 (d, J¼8.0 Hz, 1H), 8.11 (d,
J¼8.0 Hz, 1H), 8.31 (d, J¼4.0 Hz, 1H), 8.34 (d, J¼7.0 Hz, 2H), 8.94 (d,
J¼7.0 Hz, 2H), 11.2 (s, ꢀNH, 1H) ppm 13C NMR (500 MHz, DMSO-d6):
19. Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. Soc. 1985,
107, 3902e3909.
20. Botrel, A.; Beuze, A.; Jacques, P.; Strub, H. J. Phys. Chem. A 2007, 111, 2319e2328.