Water-induced fluorescence quenching of mono- and dicyanoanilines

Article abstract of DOI:10.1016/j.cplett.2006.03.073

Photophysical properties of monocyano- (2-, 3-, and 4-cyano) and dicyano- (3,4-, 3,5-, 2,3-, 2,4-, 2,5-, and 2,6-dicyano) anilines are investigated by fluorescence measurements. All the monocyanoanilines are virtually nonfluorescent in water (quantum yield < 0.01); however, in nonaqueous solvents (cyclohexane, acetonitrile and ethanol), the fluorescence quantum yield is enhanced substantially. In contrast, dicyanoanilines investigated are highly fluorescent both in aqueous and nonaqueous environments. The photophysical data and MO calculations suggest that conformational changes in the amino group and variation of hydrogen-bonding interactions between the solute and solvent water upon electronic excitation are responsible for the water quenching in the monocyanoanilines.

Full text of DOI:10.1016/j.cplett.2006.03.073

Chemical Physics Letters 423 (2006) 306–311
www.elsevier.com/locate/cplett
Water-induced fluorescence quenching of mono- and dicyanoanilines
*
Juro Oshima, Toshitada Yoshihara, Seiji Tobita
Department of Chemistry, Gunma University, Kiryu, Gunma 376-8515, Japan
Received 25 December 2005; in final form 2 March 2006
Available online 3 April 2006
Abstract
Photophysical properties of monocyano- (2-, 3-, and 4-cyano) and dicyano- (3,4-, 3,5-, 2,3-, 2,4-, 2,5-, and 2,6-dicyano) anilines are
investigated by fluorescence measurements. All the monocyanoanilines are virtually nonfluorescent in water (quantum yield < 0.01);
however, in nonaqueous solvents (cyclohexane, acetonitrile and ethanol), the fluorescence quantum yield is enhanced substantially. In
contrast, dicyanoanilines investigated are highly fluorescent both in aqueous and nonaqueous environments. The photophysical data
and MO calculations suggest that conformational changes in the amino group and variation of hydrogen-bonding interactions between
the solute and solvent water upon electronic excitation are responsible for the water quenching in the monocyanoanilines.
ꢀ 2006 Elsevier B.V. All rights reserved.
1. Introduction
quenching due to solvent water. Since 3-ANCN possesses
the first absorption band at much longer wavelength as
compared with fluorescent amino acids (tryptophan, tyro-
sine, and phenylalanine [8]), the environment-sensitive fluo-
rescent properties of 3-ANCN can be applied to new
fluorescent probe for proteins.
The present study was undertaken with the aim of eluci-
dating the photophysical properties of a series of dicyano-
anilines (see Scheme 1) in aqueous and nonaqueous
solvents with particular interest in their applicability to
environment-sensitive fluorescent probe. Comparison of
the photophysical properties of the dicyanoanilines with
those of monocyanoanilines is made, and the origin of
the water-induced fluorescence quenching in cyanoanilines
are discussed with the aid of MO calculations.
Aromatic amines exhibit unique fluorescent behavior in
solution. One of the most remarkable features is occurrence
of the so-called dual fluorescence in polar environment.
Since the discovery of dual fluorescence in 4-(dimethyla-
mino)benzonitrile (DMABN) by Lippert et al. [1], a consid-
erable amount of experimental and theoretical works have
been carried out on the photophysical properties of
DMABN and related compounds [2] to reveal the molecu-
lar mechanism of the dual fluorescence. Special attention
has been paid to geometrical changes accompanying the
adiabatic relaxation from the locally excited state to the
intramolecular charge transfer (ICT) state which is respon-
sible for the long-wavelength component of the dual fluo-
rescence [3–5].
As another noticeable photophysical property of aniline
derivatives, we have recently found that some mono-substi-
tuted anilines exhibit significant fluorescence quenching in
water [6,7]. Among the aniline derivatives investigated, 3-
cyanoaniline (3-ANCN) showed the most remarkable
2. Experimental
3,4- and 2,3-Dicyanoanilines were synthesized from the
corresponding nitrophthalic acids. 3,5-, 2,4-, and 2,6-Dicy-
anoanilines were synthesized from the corresponding
nitroisophthalic acids. 2,5-Dicyanoaniline was synthesized
from the corresponding nitroterephthalic acids. Nitroph-
thalic acids, nitroisophthalic acids, and nitroterephthalic
*
Corresponding author. Fax: +81 277 30 1213.
E-mail address: tobita@chem.gunma-u.ac.jp (S. Tobita).
0009-2614/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2006.03.073

J. Oshima et al. / Chemical Physics Letters 423 (2006) 306–311
307
3. Results and discussion
NH2
NH2
NH2
CN
3.1. Spectral properties
CN
CN
Fig. 1 illustrates the absorption and fluorescence spectra
of aniline and monocyanoanilines in water. According to
Platt’s notation [13], the longest-wavelength band with
the maximum at 279 nm and the second absorption band
extending to shorter wavelengths of aniline can be assigned
to the transitions to 1Lb- and 1La-type benzenic states from
the ground state, respectively [14]. In aniline, the transition
2-ANCN
3-ANCN
4-ANCN
NH2
NH2
NH2
CN
CN
CN
NC
CN
1
moment of the Lb state is directed perpendicularly to the
CN
substituent axis, and that of the 1La state is in the substitu-
ent axis. The absorption (kambasx) and fluorescence (kmfluaox) max-
ima, extinction coefficient of the first absorption maximum
3,4-ANCN
3,5-ANCN
2,3-ANCN
ꢀ
(emax), and Stokes shifts (Dm) of each compound are shown
NH2
NH2
NH2
CN
CN
NC
CN
in Table 1.
It can be seen from Fig. 1 that the first absorption band
and fluorescence band of 2-ANCN and 3-ANCN shift
markedly to the red compared to those of aniline, suggest-
ing that charge migration from the amino group to the
cyano moiety occurs upon electronic excitation. This
results from strong mixing between Lb and La states in
2-ANCN and 3-ANCN. In contrast, such a red-shift is
not seen in the first absorption band of 4-ANCN, and a sig-
nificant red-shift is noticed in the second absorption band
with an extinction coefficient much larger than the first
absorption band (see Fig. 1 and Table 1). The absorption
spectrum of 4-ANCN taken in cyclohexane (Fig. 1e) shows
NC
CN
2,4-ANCN
2,5-ANCN
2,6-ANCN
Scheme 1. Structures of the compounds used in this study and their
abbreviations.
1
1
acid were converted to the corresponding diamido com-
pounds by using SOCl2 and aqueous NH3 solution [9].
The diamino compounds obtained were dehydrated by
using POCl3 to produce dicyanonitrobenzenes which were
then reduced to dicynoanilines using Fe and HCl. Cyclo-
hexane (CH; Aldrich, spectrophotometric grade) was used
as received. Acetonitrile (MeCN; Kanto) and ethanol
(EtOH) were purified by distillation. 2,2,2-Trifluoroethanol
(TFE; Kishida) and n-heptane (Hep; Kishida) were purified
by distillation after passing through a silica-gel column.
Wavelength (nm)
250
500
(a) AN
400
300
Deionized water was purified by
(Millipore).
a Milli-Q Labo
(b) 2-ANCN
(c) 3-ANCN
The absorption and fluorescence spectra were measured
with a spectrophotometer (JASCO, Ubest-50) and a spec-
trofluorometer (Hitachi, F-4010), respectively. The fluores-
cence quantum yield (Uf) was determined by using a 1.0 N
H2SO4 aqueous solution of quinine bisulfate (Uf = 0.546
[10]) or a cyclohexane solution of aniline (Uf = 0.17 [11])
as a standard.
The fluorescence lifetimes were determined based on the
time-correlated single-photon counting method. The third
harmonic (266 nm, FWHM ꢀ 250 fs) of a mode-locked
Ti:sapphire laser (Spectra Physics) or a nanosecond pulsed
discharge lamp (Edinburgh Analytical Instruments) was
used as the excitation source [6].
(d) 4-ANCN
(e) 4-ANCN in CH
x 3
Molecular orbital (MO) calculations were performed
by using the GAUSSIAN-03 software package [12] on a per-
sonal computer. The molecular geometries of the ground
state were optimized at B3LYP/6-31G* level and those
of the excited state were optimized at CIS/6-31+G* level.
All optimized structures were confirmed by frequency
analysis.
20
30
40
Wavenumber (103 cm-1)
Fig. 1. Absorption and fluorescence spectra of (a) AN, (b) 2-ANCN, (c) 3-
ANCN, (d) 4-ANCN in H2O, and (e) 4-ANCN in CH at 293 K. The
dashed line in (e) was obtained by subtracting the short-wavelength part of
the second absorption band from the original absorption spectrum.

308
J. Oshima et al. / Chemical Physics Letters 423 (2006) 306–311
Table 1
Spectral properties and photophysical parameters of mono- and dicyanoanilines in solution at 293 K
emax (Mꢁ1 cmꢁ1
)
k
(nm)
Dm (10 cm
Uf
sf (ns)
kf (107 sꢁ1
)
knr (108 sꢁ1
)
abs
max
fluo
max
3
ꢁ1
)
ꢀ
Compound
Solvent
k
(nm)
2-ANCN (pKa = 0.9)c
CH
314
321
325
314
315
348
368
377
376
383
3.11
3.98
4.24
5.25
5.64
0.10
0.26
0.34
0.007
0.005
2.8
5.8
6.9
0.18
0.11
3.4
4.5
4.9
4.1
4.5
3.2
1.3
1.0
55
MeCN
EtOH
TFE
4550
H2O
3900
2900
91
3-ANCN (pKa = 2.8)d
4-ANCN (pKa = 1.7)d
CH
310
318
321
300
307
343
374
384
380
392
3.10
4.71
5.11
7.00
7.06
0.062
0.21
0.19
0.0054
0.0019
2.4
7.3
6.2
0.11
0.045
2.6
2.8
3.1
4.8
4.4
3.9
1.1
1.3
87
230
MeCN
EtOH
TFE
H2O
2330
CH
283
272
276
264
283
323
338
341
335
344
4.38
0.13
0.14
0.041
0.0052
0.0046
5.6
3.3
0.68
0.13
0.067
2.4
4.4
6.0
4.0
6.9
1.6
2.6
14
77
150
MeCN
EtOH
TFE
24250
7.18a
6.91a
8.03a
6.27
H2O
18000
3,4-ANCN (pKa = 0.1)c
3,5-ANCN (pKa = 0.9)c
2,3-ANCNb (pKa = ꢁ0.5)c
2,4-ANCNb (pKa = ꢁ1.2)c
2,5-ANCNb (pKa = ꢁ0.4)c
Hep
316
328
335
326
353
384
394
401
3.32
4.45
4.47
5.74
0.03
0.22
0.26
0.09
2.5
4.9
6.8
1.5
1.3
4.5
3.8
5.8
4.0
1.6
1.1
6.1
MeCN
EtOH
H2O
5300
5000
Hep
325
340
346
330
360
392
402
408
2.99
3.90
4.03
5.79
0.05
0.11
0.12
0.14
2.3
4.1
5.8
5.8
2.1
2.7
2.1
2.4
4.2
2.2
1.5
1.5
MeCN
EtOH
H2O
3100
2650
Hep
334
349
354
346
374
404
414
419
3.20
3.90
4.09
5.04
0.10
0.51
0.66
0.65
2.9
11
14
3.6
4.6
4.6
4.4
3.1
0.4
0.2
0.2
MeCN
EtOH
H2O
5350
4300
5300
15
Hep
325
326
347
325
350
367
389
377
4.22
3.43
3.11
4.24
0.10
0.14
0.17
0.16
3.3
3.6
4.3
4.0
2.9
4.0
3.9
4.0
2.8
2.4
1.9
2.1
MeCN
EtOH
H2O
Hep
335
352
358
349
374
401
412
418
3.11
3.47
3.66
4.73
0.15
0.56
0.67
0.53
3.9
13
16
3.8
4.2
4.3
3.2
2.2
3.3
0.2
0.3
MeCN
EtOH
H2O
17
2,6-ANCN (pKa = ꢁ2.7)c
Hep
332
343
347
344
365
382
391
394
2.72
2.98
3.24
3.69
0.17
0.29
0.37
0.40
2.9
5.1
6.9
7.2
5.8
5.6
5.3
5.5
2.9
1.4
0.9
0.8
MeCN
EtOH
H2O
6800
6800
a
Stokes shifts for the S2 S0 absorption bands.
b
c
The extinction coefficient of these compounds in H2O could not be determined because of the low solubility in water.
Determined from the pH dependence of absorption spectra.
From Ref. [15].
d
more clearly, the relative positions of the S1 and S2 states
[16]. Comparison of the absorption and fluorescence spec-
tra of AN and 4-ANCN in water reveals their similar spec-
tral positions, suggesting that the lowest excited singlet
extinction coefficient increases moderately. The absorption
and fluorescence spectra of 2,4-ANCN are blue-shifted and
those of 2,5-ANCN are red-shifted, compared to other five
compounds. It is also noted from Table 1 that the Stokes
shifts of dicyanoanilines are smaller than those of
monocyanoanilines.
The spectral properties of mono- and dicyanoanilines
were also investigated in organic solvents with different
polarity as summarized in Table 1, where dicyanoanilines
were not sufficiently soluble in cyclohexane, so that hep-
tane was used as nonpolar solvent instead of cyclohexane.
In all the monocyanoanilines, the first absorption band
exhibits red-shift upon changing solvent from CH to
1
state of 4-ANCN retains the Lb character. In the evalua-
tion of the Stokes shift for 4-ANCN in H2O, the first
absorption maximum was assumed to be equal to that in
cyclohexane (see Fig. 1d, e)
The absorption and fluorescence spectra of dicyanoani-
lines are displayed in Fig. 2. In comparison with monocy-
anoanilines, the lowest energy absorption maxima of
dicyanoanilines are further shifted to the red by introduc-
tion of two cyano groups into the aromatic ring, and the

J. Oshima et al. / Chemical Physics Letters 423 (2006) 306–311
309
calculated from Eq. (2) is extremely large (2.3 · 1010 sꢁ1).
Similar quenching observed in strongly hydrogen-donating
solvent TFE suggests that hydrogen-bonding interactions
between the amino group and TFE or H2O are responsible
for the quenching. According to time-resolved photoacou-
stic experiments and transient absorption measurements
for 3-ANCN in water, the triplet yield was negligibly small
at 293 K [6], i.e., the fast nonradiative process of 3-ANCN
in water is attributable almost exclusively to internal
conversion.
Wavelength (nm)
400 300
250
500
(a) 3,4-ANCN
(b) 3,5-ANCN
(c) 2,3-ANCN
The fluorescence lifetime of 3-ANCN in water depended
on temperature. For the temperature dependence, the fol-
lowing relation can be assumed:
ꢀ
ꢁ
(d) 2,4-ANCN
(e) 2,5-ANCN
1
ꢁEa
¼ kf þ k0nr þ A exp
ð3Þ
sf
RT
where k0nr denotes the temperature-independent nonradia-
tive rate constant and the last term in Eq. (3) shows the
temperature dependence of nonradiative process with an
activation energy Ea and the frequency factor A. Since
the fluorescence lifetime of 3-ANCN in water is extremely
short, it was assumed that ð1=sf Þ ꢂ kf þ k0nr. The tempera-
ture dependence of the fluorescence lifetime of 3-ANCN in
water is shown in Fig. 3. The Ea and A values were ob-
(f) 2,6-ANCN
tained as 11 kJ molꢁ1 and 1.8 · 1012 ꢁ1, respectively.
s
20
30
Wavenumber (103 cm-1)
40
In contrast to monocyanoanilines, all the dicyanoani-
lines investigated are highly fluorescent both in aqueous
and nonaqueous solvents. Significant quenching in water
is not recognized for these compounds, except for slight
quenching in 3,4-ANCN, and the environment-sensitive
fluorescent behavior as observed in monocyanoanilines is
no longer seen for dicyanoanilines.
Fig. 2. Absorption and fluorescence spectra of (a) 3,4-ANCN, (b) 3,5-
ANCN, (c) 2,3-ANCN, (d) 2,4-ANCN, (e) 2,5-ANCN, and (f) 2,6-ANCN
in H2O at 293 K.
MeCN and EtOH, i.e. with increasing solvent polarity. In
H2O, however, the first absorption maximum appears at
shorter wavelengths compared with those in MeCN and
EtOH, and the most pronounced blue-shift is seen for
3-ANCN in water. This can be explained in terms of hydro-
gen-bonding interaction between the amino group and
water molecule which inhibits the electronic interactions
between the lone-pair electrons in the amino group and
the aromatic p-system.
3.3. Water-induced fluorescence quenching
As described in the previous section, the fluorescence of
monocyanoanilines is significantly quenched in water; the
magnitude of the nonradiative rate is in the following
order: 3-ANCN > 4-ANCN > 2-ANCN. In contrast to
3.2. Photophysical properties
10.5
The fluorescence decay curves of all the compounds in
Scheme 1 followed single exponential kinetics. The photo-
physical parameters of monocyanoanilines and dicyanoani-
lines obtained in different solvents are summarized in Table
1, where the radiative (kf) and nonradiative (knr) rate con-
stants were estimated by using the following relations:
10.4
τ
10.3
10.2
kf ¼ Uf =sf
knr ¼ ð1 ꢁ Uf Þ=sf
ð1Þ
ð2Þ
It is noted from Table 1 that all the monocyanoanilines
are highly fluorescent in CH, MeCN and EtOH, however
in TFE and H2O the fluorescence quantum yield (Uf) and
lifetime (sf) decrease substantially and a particularly large
quenching is seen in water. The nonradiative rate in water
3.2
3.4
3.6
1/T (10-3 K-1)
Fig. 3. Temperature dependence of the fluorescence lifetime of 3-ANCN
in water.

310
J. Oshima et al. / Chemical Physics Letters 423 (2006) 306–311
monocyanoanilines, dicyanoanilines do not show such a
significant quenching due to solvent water. In order to clar-
ify the origin of the water-induced fluorescence quenching,
we examined the correlation of the nonradiative rate con-
stant of mono- and dicyanoanilines with the basicity con-
stant pKb in the ground state, pyramidal angle (h) of the
amino group, and the Stokes shift. Here the pKb values
were calculated from the pKa values listed in Table 1 by
using a relationship pKa + pKb = 14. The pKb values are
related to hydrogen-bonding ability of cyanoanilines, i.e.,
the molecules with a smaller pKb value are expected to have
stronger proton-accepting ability, thus favoring the hydro-
gen bond formation with water molecule.
(a)
2
1
0
k
12
14
16
pKb
(b)
The pyramidal angle (h) of the amino group influences
the electronic conjugation between the amino lone-pair
electrons and the aromatic p-system. The calculated pyra-
midal angles in the geometry-optimized S0(h) and S1(h*)
states of cyanoanilines are collected in Table 2, where the
values reported by Pirowska et al. (HF/6-31++G** and
CIS/6-31++G** calculations) [17] are also shown in paren-
theses for comparison. The optimized geometries obtained
by our calculations (B3LYP/6-31G* and CIS/6-31+G*)
are almost consistent with the results of Pirowska et al.
except for h* for 4-ANCN, predicting a nonplanar and pla-
nar equilibrium geometry of the amino group for the S0
and S1 states, respectively. The calculated pyramidal angle
h of 4-ANCN (34.4ꢁ) is in good agreement with the exper-
imental value (34ꢁ) according to an X-ray analysis [18]. The
Stokes shift is associated with changes in solute dipole
moment between S0 and S1 states and the resultant varia-
tion in the solute–solvent interactions.
H
H
θ
2
1
0
N
C
k
0
20
40
Δ
θ
(deg)
(c)
2
1
0
k
The relationship between knr and the values of pKb,
*
ꢀ
Dh(=h ꢁ h ) and Dm in cyanoanilines is depicted in Fig. 4.
It is apparent from Fig. 4 that the nonradiative rates of
monocyanoanilines are correlated very well to the values
4
5
6
7
Δ (103 cm-1)
ꢀ
of pKb, Dh, and Dm, whereas the knr of dicyanoanilines does
Fig. 4. Plot of nonradiative rate constant (knr) vs. (a) pKa in the ground
state, (b) variation in the pyramidal angle (Dh), and (c) Stokes shift (Dꢀm)
for monocyanoanilines (m) and dicyanoanilines (d). The definition of the
pyramidal angle is schematically shown in the inset in (b).
not depend significantly on these physical parameters.
Even in the dicyanoanilines having the values of pKb, Dh,
and Dꢀm comparable to those of monocyanoanilines, sub-
stantial increase in knr is not seen. The pKb values of mon-
ocyanoanilines are much smaller than those of
dicyanoanilines. This suggests that the hydrogen-bonding
interactions between the amino group and water molecule
in the ground state are more pronounced in the monocy-
ꢀ
anoanilines. The Dh and Dm of monocyanoanilines are
Table 2
Pyramidal angles of mono- and dicyanoanilines in the S0(h) and S1(h*)
state obtained by B3LYP/6-31G** and CIS/6-31+G* calculations,
respectively
much larger than those of dicyanoanilines. It can therefore
be inferred that the water-induced fluorescence quenching
of monocyanoanilines is related to the geometry change
of the amino group and variation in the hydrogen-bonding
interactions between solute and water molecules upon elec-
tronic excitation. The activation energy (11 kJ molꢁ1) for
the water-induced fluorescence quenching of 3-ANCN
would correspond to the barrier for reorganization of sol-
vent molecules including hydrogen bond.
As a similar compound showing significant water-
induced fluorescence quenching, 1,8-anilinonaphthalene-
sulfonate (1,8-ANS) is well known [19–21]. The fluorescence
lifetime of 1,8-ANS varies between about 300 ps in water
Compound
h (degr)
h* (degr)
2-ANCN
3-ANCN
4-ANCN
3,4-ANCN
3,5-ANCN
2,3-ANCN
2,4-ANCN
2,5-ANCN
2,6-ANCN
30.2 (31.8)
38.2 (39.4)
34.4 (35.5)
34.4
30.3 (37.4)
24.0
15.9
24.4
0.6 (0.0)
0.2 (0.0)
0.4 (0.0)
23.7 (0.0)
0.2
0.2 (0.0)
0.3
0.2
0.0
0.4 (0.0)
Values in parentheses: From Ref. [17].

J. Oshima et al. / Chemical Physics Letters 423 (2006) 306–311
311
and 13.8 ns in ethanol [19]. It is also reported that in pure
Appendix A. Supplementary data
water the fluorescence lifetime of 1,8-ANS is independent
of temperature above 0 ꢁC, showing almost no activation
barrier and low frequency factor (ca. 109 sꢁ1) for the water
quenching [19,21]. Ebbesen and Ghiron have reported that
static quenching by water occurs within the time scales of
the singlet excited state lifetime [19]. It is apparent that a
distinct quenching mechanism due to water operates in
monocyanoanilines.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.cplett.2006.03.073.
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The present results show that the dicyanoanilines inves-
tigated are highly fluorescent both in aqueous and non-
aqueous solutions and the fluorescence properties
(intensity and wavelength) are not environment-sensitive.
Although the first absorption band of dicyanoanilines
extends longer wavelengths and has higher extinction coef-
ficients compared to those of monocyanoanilines, dicyano-
anilines would not be applicable to fluorescent probe for
investigating the dynamics of biological substances. In con-
trast to dicyanoanilines, the fluorescence properties of all
the monocyanoanilines depend strongly on the environ-
ment. Among the monocyanoanilines, 3-cyanoaniline
showed the most remarkable fluorescence quenching in
water. Since the molecular size of the monocyanoanilines
is much smaller than that of the currently used fluorescent
probes, they can be a novel chromopholic moiety of fluo-
rescent probe molecules.
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This work was partly supported by a Grant-in-Aid from
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