Intramolecular charge transfer with 4-(N-phenylamino)benzoic acid. The N-phenyl amino conjugation effect

Article abstract of DOI:10.1016/S0009-2614(03)00372-5

4-(N-phenylamino)benzoic acid (PhABA) was synthesized and its fluorescence spectra were recorded. In aprotic polar solvents, PhABA emitted strongly Stokes-shifted single-banded fluorescence with practically the same wavelength as that of 4-(N,N-diphenylam

Full text of DOI:10.1016/S0009-2614(03)00372-5

Chemical Physics Letters 372 (2003) 104–113
www.elsevier.com/locate/cplett
Intramolecular charge transfer with
4-(N-phenylamino)benzoic acid. The N-phenyl
amino conjugation effect
a
a, ,2
*
Li-Hua Ma , Zhao-Bin Chen b,*,1, Yun-Bao Jiang
a
Department of Chemistry and the MOE Key Laboratory of Analytical Sciences, Xiamen University, Xiamen 361005, China
b
Department of Chemistry, Shanxi University, Taiyuan 030006, China
Received 2 December 2002; in final form 24 February 2003
Abstract
4-(N-phenylamino)benzoic acid (PhABA) was synthesized and its fluorescence spectra were recorded. In aprotic
polar solvents, PhABA emitted strongly Stokes-shifted single-banded fluorescence with practically the same wavelength
as that of 4-(N,N-diphenylamino)benzoic acid (DPhABA), indicative of the ICT character of the emissive state. 4-(N-
isopropylamino)benzoic acid (iPrABA) showed single band emission however, sluggish response to the solvent polarity.
In alkanols dual fluorescence of PhABA and DPhABA was observed but with different dependencies on alkanol
structures. We concluded that ICT occurring with PhABA was due to the N-phenyl/amino conjugation effect. The work
could be of help in understanding speciality of anilino moiety as electron donor compared to aliphatic amino group.
Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction
investigations of the influence of electron donor
and/or acceptor on the CT process are then of
great importance for the understanding of the CT
photophysics.
4-(N,N-dialkylsubstituted)aminobenzoic acids
(DAABA) emit dual fluorescence originated from
LE and CT states in solvents more polar than
alkanes, while the 4-(N-mono-n-alkyl) substituted
derivatives such as methyl, ethyl, 1-butyl, 1-hexyl
(AABA) show single emission from LE insensible
to solvent polarity [15,16].
Since the elucidation of dual fluorescence of
4-(dimethylamino)benzonitrile (DMABN) [1,2],
the properties of this remarkable molecule and its
dual fluorescent derivatives have fascinated
chemists, and tremendous efforts [3–14] have been
devoted to understanding their behavior during
the process from the initially locally excited (LE)
to the intramolecular charge transfer (ICT). The
In comparison to many researches on the N-
alkyl substituted aminobenzoic acid, the N-phenyl
* Corresponding authors.
E-mail addresses: zchen@sxu.edu.cn (Z.-B. Chen), ybjiang@
xmu.edu.cn (Y.-B. Jiang).
ꢀ
substituted counterparts are limited. Kakas [17]
1
and Greiner [18] pointed out that for triphenyl-
amine and its derivatives, there is conjugation
Fax: +86-351-7010688.
Fax: +86-592-2185662.
2
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0009-2614(03)00372-5

L.-H. Ma et al. / Chemical Physics Letters 372 (2003) 104–113
105
between the nitrogen lone pair electrons and the
phenyl p-electrons. The whole molecule is a new
chromophore with characteristic absorption and
emission spectra. N-phenyl vs N-alkyl substitution
of aromatic amines leads to superior performance
in material chemistry (hyperpolarizability [19],
hole transport [20,21], electroluminescence [21,22],
etc.). 4-Aminodiphenylamine is a model of the
polymer polyaniline [23] and research in [23]
has been extended in another publication [24].
But few studies have been devoted to the effect
of N-phenyl substitution on the excited-state
behavior.
We then synthesized PhABA, DPhABA, iPr-
ABA and ethyl 4-(N-phenylamino)benzoate (PhA-
BEt) to provide new clues for the understanding of
superiority of anilino moiety as an electron donor
to an aliphatic amino group. A particular goal is to
elucidate the nature of ICT enhanced by the N-
phenyl amino conjugation effect.
vestigated molecules were optimized by AM1 cal-
culations [26].
3. Results and discussion
3.1. Spectral behavior of PhABA and DPhABA in
aprotic solvents
3.1.1. Absorption spectra
The absorption spectra of PhABA displayed a
single broad band with a maximum of ca. 319 nm
in different solvents, whereas DPhABA had two
bands, one broad band at 335 nm and the other
slightly narrower band at 300 nm with vibration
structure (Table 1). This difference in the absorp-
tion spectra may be attributed to a larger separa-
tion between the two lowest absorption bands (La
and Lb) [27]. The second band of DPhABA
around 300 nm is the consequence of an electronic
transition mainly localized in the triphenylamine
moiety [28,29].
2. Experimental
3.1.2. Fluorescence spectra
PhABA was synthesized by refluxing ethyl
4-(N-acetylamino)benzoate and iodobenzene. 1H
NMR (DMSO-d6/TMS): d(ppm) 6.94(m, 1H),
7.05(d, 2H), 7.18(d, 2H), 7.32(m, 2H), 7.78(d, 2H),
8.72(s, 1H), 12.26(s, 1H). DPhABA was achieved
by oxidation of 4-(N,N-diphenylamino)benzalde-
Compared to the diverse in ground-state, the
fluorescence spectra of DPhABA and PhABA
differed much in the same aprotic solvent. The
fluorescence spectra of PhABA (Scheme 1) in
aprotic solvents of varying polarity are depicted in
Fig. 1a. PhABA, though among the N-mono-
substituted derivatives of p-aminobenzoic acid
(ABA), showed also the single emission band,
however, red-shifted greatly with increasing the
solvent polarity, together with gradually widening
of the half-peak width (Table 1), in contrast to a
polarity insensitive nature of the UV–Vis absorp-
tion and fluorescence excitation spectra (the figures
were abbreviated). For example, the emission
wavelength was red-shifted for 115 nm from 371
nm in cyclohexane (CHX) to 486 nm in acetonitrile
(ACN) (Table 1). This is ascribed to the ICT nature
of the emissive state. It should be pointed out that
the maximal emission wavelength and the shape
were independent of concentration and excitation
wavelength from 280 to 340 nm. The solvent effect
of PhABA differs much from that of AABA, which
emits the short wavelength fluorescence solely from
LE state hardly dependent on solvent polarity [15].
1
hyde. HNMR ðCDCl3=TMSÞ: d(ppm) 7.90 (d,
2H), 7.32(t, 4H), 7.15(m, 6H), 6.99(d, 2H). iPr-
ABA was obtained by the reaction of 4-amino-
benzoic acid with 2-bromopropane. 1HNMR
(DMSO-d6/TMS): d(ppm) 1.14(d, 6H), 3.60(m,
1H), 6.26(d, 1H), 6.54(d, 2H), 7.65(d, 2H), 11.90(s,
1H). Organic solvents for spectral investigations
were purified before use.
Absorption and corrected fluorescence spectra
were recorded on Beckman DU-7400 absorption
spectrophotometer and Hitachi F-4500 fluores-
cence spectrometer, respectively. The fluorescence
quantum yields of the aerated sample solutions
were measured by comparison with a quinine sul-
fate solution (0.546 in 0:5 mol lÀ1 H2SO4 [25]) of
equal optical density at the excitation wavelength.
NMR data were taken on Varian Unityþ 500 MHz
spectrometer. Ground-state structures of the in-

106
L.-H. Ma et al. / Chemical Physics Letters 372 (2003) 104–113
Table 1
The maximal absorption wavelength kambasx, emission wavelength kmfluax, fluorescence quantum yield U, and full-width at half maximum
(FWHM) of PhABA and DPhABA in aprotic solvents
abs
max
flu
max
Molecule
PhABA
Solvent
ET [30]
k
(nm)
k
(nm)
U
FWHM
(cmÀ1
)
(kcal molÀ1
)
CHX
DEE
30.9
34.5
36.0
37.4
38.1
39.1
40.7
45.6
316
320
321
319
320
320
316
316
371
413
425
431
438
466
473
486
0.365
0.308
0.316
0.195
0.166
0.105
0.125
0.033
4577
5014
5023
5032
5070
5102
5118
5410
DOX
THF
EtAc
CHCl3
CH2C12
ACN
DPhABA
CHX
DEE
30.9
34.5
36.0
37.4
38.1
39.1
40.7
45.6
342, 299
321, 302
334, 303
332, 303
331, 302
340, 305
339, 304
330, 302
401
412
423
428
434
465
472
481
0.228
0.284
0.317
0.341
0.354
0.509
0.468
0.215
3588
3909
4029
4100
4131
4217
4248
4664
DOX
THF
EtAc
CHCl3
CH2Cl2
ACN
iPrABA
CHX
DEE
30.9
34.5
36.0
37.4
38.1
39.1
40.7
45.6
324
331
332
334
334
335
343
352
3588
3909
4029
4100
4131
4217
4248
4664
DOX
THF
EtAc
CHCl3
CH2Cl2
ACN
Scheme 1. Chemical structures of interested ABA derivatives.
To understand further the luminescence be-
havior of PhABA, DPhABA (Scheme 1) as one of
the 4-(N,N-disubstituted)aminobenzoic acid de-
rivatives which should possess ICT characteristics
was synthesized. Fig. 1b shows the fluorescence
spectra of DPhABA in the same series of aprotic

L.-H. Ma et al. / Chemical Physics Letters 372 (2003) 104–113
107
Two assumptions are proposed for the ICT
emission of PhABA. One is the twisting of
N-phenylamino with respect to the benzoic acid
moiety which resulted from steric hindrance ac-
cording to TICT model, just as in the case of the
ICT model compound 3,5-dimethyl-4-(N,N-dim-
ethylamino)benzonitrile (MMD) (Scheme 2) [2,3],
the other is the N-phenyl amino conjugated effect
[28].
3.2. Molecular structures of PhABA and DPhABA
The ground-state structures of PhABA and
DPhABA were optimized using AM1 calculation.
The optimized structures are shown in Scheme 2
and the structural parameters are reported in Ta-
ble 2. For PhABA, the dihedral angle of C15–N1–
C6–C7 equals 1.4°, whereas C16–C15–N1–C6 of
53.3° is found. For DPhABA, the dihedral angle of
C15–N1–C6–C7 is 26.8°, of C28–N1–C6–C7 is
27.2°, of C27–C28–N1–C6 is 40.5° and of C14–
C15–N1–C6 is 38.9°. The twist of the amino group
leads to a small lengthening of the N-phenyl bond
by the theory calculation. The N1–C6 bond
Fig. 1. The fluorescence spectra of (a) PhABA and (b) DPh-
ABA in organic solvents. From left to right the solvents
are cyclohexane (CHX), diethyl ether (DEE), dioxane (DOX),
tetrahydrofuran (THF), ethyl acetate (EtAc), chloroform
(CHCl3), dichloromethane ðCH2Cl2Þ, and acetonitrile (ACN),
respectively.
solvents. DPhABA exhibits sole emission band,
which is red-shifted with increasing solvent polar-
ity (Fig. 1b, Table 1). It is significant that the
maximal emission wavelengths ðkfmluax) of DPhABA
and PhABA are almost the same in each polar
solvent such as diethyl ether (DEE), tetrahydro-
furan (THF), dioxane (DOX), chloroform
(CHCl3), dichloromethane ðCH2Cl2Þ and ACN.
ꢁ
lengths are 1.394 and 1.404 A for PhABA and
DPhABA, respectively. As compared with ICT
model compound MMD (Scheme 2) which shows
only CT band owing to the great steric effect by
methyls, the corresponding dihedral angles of C8–
N1–C6–C7 and C9–N1–C6–C7 are 51.5° and
The only difference occurs in nonpolar solvent
flu
max
ꢁ
87.3°, respectively, with 1.430 A bond length of
CHX in which the k
of PhABA (371 nm) is
N1–C6. Though the AM1 method is rough, the
results of computation show at least that the steric
hindrance effects of PhABA and DPhABA are
different, both lower than that of MMD. The steric
hindrance effect, therefore, is not the key ac-
counting for the ICT emission behavior of PhABA
in polar solvents.
obviously shorter than that of DPhABA (401 nm).
In addition, the half-peak width of PhABA in each
solvent is ca. 1000 cmÀ1 broader than that of
DPhABA (Table 1). The introduction of another
phenyl in DPhABA made the amine less distorted,
which resulted in the red-shift of absorption and
fluorescence in CHX (Table 1) and the narrower
band of the emissive fluorescence [28]. The fluo-
rescence quantum yields of PhABA are increas-
ingly lower with increasing solvent polarity (0.365
in CHX compared to 0.033 in ACN), whilst sol-
vent polarity has far less effect on the quantum
yields of DPhABA (0.228 in CHX and 0.215 in
ACN). Despite the differences, the similarity of the
emission wavelength of PhABA and DPhABA in
aprotic polar solvents supported the ICT nature of
PhABA emissive state.
3.3. Counter evidences of N-phenyl amino conjuga-
tion
3.3.1. The emission characters of iPrABA
We postulated that the role of N-phenyl amino
conjugation could be proved or disproved con-
clusively by studying the fluorescence spectra of
iPrABA (Scheme 1) (Fig. 2). iPrABA was selected
because its steric structure around amino group

108
L.-H. Ma et al. / Chemical Physics Letters 372 (2003) 104–113
Scheme 2. AM1 optimized ground-state structures of related compounds.
was expected to be similar to that of PhABA,
whereas its lone pair electrons at the amino ni-
trogen was far less delocalized when compared to
that of PhABA. The amino conjugation would be
disrupted and hence the role of N-phenyl amino
conjugation in ICT process would be proved to
some extent if no CT of iPrABA was discerned.
flu
Fig. 2 and Table 1 showed that the k of iPrABA
max

L.-H. Ma et al. / Chemical Physics Letters 372 (2003) 104–113
109
Table 2
AM1 calculated dihedral angle at the amino conjunction moiety, bond length, ground-state dipole moment lg, the measured CT state
dipole moment lCe T and Onsager radius R
c
ꢁ
Bond length (A)
ꢁ
R (A)
Molecule
PhABA
Dihedral anglea (°)
lg (D)
lCe T (D)b
C16–C15–N1–C6
53.3
C15–N1–C6–C7
1.4
N1–C6
1.394
N1–C6
N1–C15
1.410
N1–C15
N1–H26
0.998
N1–C28
3.73
17.4
4.26
4.54
DPhABA
C28–N1–C6–C7
27.2
C15–N1–C6–C5
26.8
3.49
3.09
18.4
C27–C28–N1–C6
40.5
C14–C15–N1–C6
38.9
1.404
1.417
1.417
PhABEt
iPrABA
MMD
C10–C15–N1–C6
53.6
C15–N1–C6–C7
1.6
N1–C6
1.396
N1–C15
1.408
N1–H19
0.995
17.7
4.36
4.09
d
C9–N1–C6–C7
21.5
N1–C6
1.392
N1–C9
1.450
N1–H4.36
0.998
9.6
C9–N1–C6–C7
87.3
C8–N1–C6–C7
51.5
N1–C6
1.430
N1–C9
1.441
N1–C8
1.441
a The atom numbers are indicated in Scheme 2.
b Measured from solvatochromic data, see details in the text.
c Calculated at HF/STO-3G level.
d The dipole moment of LE state.
was blue-shifted compared to that of PhABA (Fig.
1a) and red-shifted slightly with increasing solvent
polarity, from 324 nm in CHX to 343 nm in ACN
(Table 1). The solvent effect findings of iPrABA
and PhABA differ much, which could be seen
more clearly by the plots of the emission energies
hmmax against polarity scale f À f 0 (Fig. 3). The
red-shift with increasing solvent polarity experi-
enced by the emission band of iPrABA is ap-
proximately of the same magnitude as that found
for the LE band of 4-(N-methylamino)benzenitrile
(MABN) and DMABN [16]. The solvent polarity
dependence of the emission bands of PhABA and
DPhABA is just like that of the CT band of
DMABN, clearly demonstrating the dramatic
difference between the emission characters of
PhABA and iPrABA. The CT state dipole mo-
ments were estimated from the solvatochromic
Fig. 3. Solvatochromic plots of the energies hmmax of the LE
fluorescence of MABN, iPrABA, DMABN and also of the ICT
emission of DMABN, DPhABA, PhABA and PhABEt. f À f 0
is solvent polarity parameter. The data in chloroform were not
included in the linear fitting.
Fig. 2. The fluorescence spectra of iPrABA in solvents of dif-
ferent polarity.

110
L.-H. Ma et al. / Chemical Physics Letters 372 (2003) 104–113
3.3.2. Fluorescence spectra of PhABA and DPh-
ABA in protic alkanols
data following a reported method [16] by plotting
the CT emission energies against those of DMA-
BN in corresponding solvents. The ground and CT
state dipole moments of DMABN, 6.6 and 17 D,
respectively, were taken [23]. The radiuses of
PhABA, DPhABA and iPrABA were calculated at
We assumed that if the excited-state delocalized
electron shared by conjugated system of nitrogen
and phenyl moiety that means a tremendously
larger CT character for the N-phenyl derivatives in
the lowest excited-state was right, changing solvent
from aprotic to protic such as methanol (MeOH),
the hydrogen bond between nitrogen of PhABA or
DPhABA and hydroxyl of MeOHwould decrease
conjugation effect between nitrogen and N-phenyl.
Fluorescence of PhABA and DPhABA from LE
might emerge.
The fluorescence spectra of PhABA (a) and
DPhABA (b) in alkanol solvents were recorded
(Fig. 4) and the spectral data were in Table 3. The
emission band was found greatly blue-shifted. As
for PhABA in MeOH( f À f 0 is 0.309), only an
ꢁ
HF/STO-3G level to be 4.26, 4.54 and 4.09 A
(Table 2), respectively. The excited-state dipole
moments of PhABA, DPhABA and iPrABA are
17.4, 18.4 and 9.6, respectively. However, it should
be noted that the oxidation potential of aniline
(1 V) is lower than that of N-alkylamine (1.4 V)
favoring thermodynamically the intramolecular
electron transfer in PhABA. We therefore
provided another forceful counter evidence to il-
luminate the N-phenyl amino conjugation, the
fluorescence spectra of PhABA and DPhABA in
protic solvents.
Fig. 4. The fluorescence spectra of (a) PhABA and (b) DPhABA in alkanols. From left to right the solvents are methanol (MeOH),
ethanol (EtOH), normal propanol (n-PrOH), normal butanol (n-BuOH), iso-propanol (i-PrOH), iso-butanol (i-BuOH), and tert-
butanol (t-BuOH), respectively.

L.-H. Ma et al. / Chemical Physics Letters 372 (2003) 104–113
111
emission band located at ca. 430 nm was observed,
compared with 486 nm in ACN (f À f 0 is 0.305).
The result is in agreement with the fact that MeOH
is the easiest alkanol to form hydrogen bond with
nitrogen [30]. Changing the solvent from MeOHto
ethanol (EtOH), normal propanol (n-PrOH),
normal butanol (n-BuOH), iso-propanol (i-
PrOH), iso-butanol (i-BuOH) and tert-butanol (t-
BuOH), with decreased ability to form hydrogen
bond, would decrease the ability of alkanol hy-
droxyl to affect N-phenyl amino conjugation, thus
relatively enhancement of ICT emission compo-
nent would be observed. It was found in Fig. 4a
that the ICT emission of PhABA did not emerge in
solvents of MeOH, EtOH and n-PrOH until the
solvent was n-BuOH. The intensity ratio of long
wavelength to short wavelength bands was in-
creasingly larger when the solvent became more
difficult to form hydrogen bond owing to the steric
hindrance. For example, in t-BuOHthat was the
most difficult to form hydrogen bond owing to
large steric effect introduced by methyls, the
emission consisted mainly of ICT state. The simi-
lar results were obtained with DPhABA. Dual
fluorescence of DPhABA was observed in EtOH,
the position of the short wavelength band was just
about the same with DPhABA in CHX, while the
long wavelength band was the ICT emission (Fig.
4b). Also, the intensity ratio of long wavelength to
short wavelength bands was increasingly larger
(Table 3) when the solvent became more difficult
to form hydrogen bond.
Just as in aprotic solvents, the half-peak width
of PhABA was larger than that of DPhABA. The
fluorescence maximum wavelengths of PhABA
and DPhABA in different alkanol solvents were
almost the same except in EtOHand n-PrOH. It is
easily interpreted that DPhABA, possessing two
phenyl substituents at the amino group, is more
difficult to form hydrogen bond with alkanols than
PhABA. The hydrogen bond effect for DPhABA is
hence not so great as that for PhABA.
Yoon and Kim et al. [31,32] have reported the
hydrogen-bonding effect on the twisted intramo-
lecular electron transfer (TICT). In [32], the effect
of hydrogen bonding between hydroxyl group of
SiO2 colloid (polarity of its surface is similar to
that of EtOH) and p-N,N-dimethylaminobenzoic
acid (DMABA) on the latterÕs TICT was investi-
gated in detail. The SiO2-concentration dependent
intensity ratio of the TICT to the LE emission
clearly showed that an enhancement of the TICT
emission occurred upon addition of up to 0.3 lM
of SiO2 colloid whereas a reduction in that ratio
took place in the presence of excess amount of
SiO2 colloid. The authors [32] considered that the
Table 3
The emission wavelength k, CT to LE intensity ratio, ICT=ILE, the total quantum yield Uf ;total the absorption wavelength kambasx of PhABA
and DPhABA in alkanols
abs
max
Molecule
PhABA
Solvent
ET [30]
f À f 0
kCT
(nm)
kLE
(nm)
I
CT=ILE
Uf ;total
k
(nm)
(kcal molÀ1
)
MeOH55.4
EtOH51.9
0.309
0.289
0.274
0.264
0.276
0.266
0.251
–
–
432
406
391
391
371
–
–
0.028
0.036
0.031
315
324
321
320
324
n-PrOH50.7
n-BuOH50.2
i-PrOH48.6
i-BuOH47.2
t-BuOH43.3
–
–
477
1.0
0.8
0.030
0.052
0.057
0.074
471
479
460
–
–
7.9
16
324
324
DPhABA
MeOH55.4
EtOH51.9
0.309
0.289
0.274
0.264
0.276
0.266
0.251
–
430
–
0.033
0.064
0.073
0.116
0.227
0.320
0.601
308
331, 300
332, 300
487
479
478
476
396
396
380
380
0.8
0.5
4.2
5.9
n-PrOH50.7
n-BuOH50.2
i-PrOH48.6
i-BuOH47.2
t-BuOH43.3
332, 300
333, 300
334, 300
335, 300
479
456
–
–
45
152

112
L.-H. Ma et al. / Chemical Physics Letters 372 (2003) 104–113
main reason for the emission enhancement and
reduction was the different hydrogen-bonding
sites. When SiO2 concentration was low, hydrogen
bonding was suggested to occur between the car-
boxylic group of DMABA and SiO2 colloid. At
higher SiO2-colloid concentration, the hydrogen
bonding was between the amino nitrogen of
DMABA and the colloid. The latter is consistent
with our conclusion that the hydrogen bonding is
between the amino nitrogen of PhABA or DPh-
ABA and the hydroxyl group of alkanol because
the molar concentration of hydroxyl group is ob-
viously high when alcohol is chosen as the solvent.
In order to know clearly whether the hydrogen-
bonding effect on N-phenyl (PhABA, DPhABA)
and N-alkyl (DMABA) was the same or not, we
investigated the effect of minute amount of EtOH
in aprotic nonpolar solvents with these com-
pounds. Fig. 5 shows the maximal emission
wavelength (PhABA for example) as a function of
EtOHvolume fraction in DEE. It was observed
that the maximal emission wavelength of PhABA,
an N-phenyl substituted ABA, in DEE shifted to
the red when a small amount of EtOHwas intro-
duced and remained constant over a medium
EtOHconcentration range that was followed by a
substantial blue-shift at much higher EtOHcon-
centration. In addition, the maximal emission
wavelength of PhABA in absolute EtOHis 396 nm
(Fig. 4). It should be noted that the hydrogen bond
introduced by SiO2 colloid affected only the in-
tensity ratio of the dual fluorescence of DMABA
while the emission wavelengths of both the LE and
CT bands remained almost constant [32]. That
means in the case of DMABA, no large blue or
red-shift of the emission bands is observed upon
hydrogen-bonding. The results lead to the con-
clusion that the hydrogen-bonding effect on the
emission of DMABA, an N-alkyl substituted
ABA, is different from that of PhABA, an
N-phenyl substituted ABA. This, on the other side,
supports our conclusion that it is the amino ni-
trogen of PhABA or DPhABA and the hydroxyl
group of the alcohol solvent that form hydrogen
bonds which hinder the N-phenyl/amino conjuga-
tion and hence lead to abnormally large blue-shift
in the CT emission in alcohol solvents compared
to that in aprotic solvent of similar polarity.
PhABEt (Scheme 1) was synthesized in order to
highlight further the hydrogen bond would be
between nitrogen of DPhABA or PhABA and
hydroxyl of alkanol. Similar phenomena were
observed with PhABEt (Scheme 2) that had dipole
moments of 17.7 (Table 2). For PhABEt, the
emission wavelengths in different aprotic solvents
were just as those for PhABA and DPhABA ex-
cept 356 nm in CHX, clearly seen in Fig. 3.
4. Conclusions
The singlet emission band of PhABA in polar
aprotic solvents is found to be typical of ICT
character, which is quite different from the N-alkyl
derivative. Introduction of another N-phenyl to
PhABA, leading to is DPhABA, has little effect on
the maximal emission wavelength and shape in
polar aprotic solvents whilst much difference oc-
curs in absorption spectra. We concluded that the
delocalization of lone pair electrons of nitrogen
conjugated to p electron of phenyl group might be
the direct reason for the anomalous emission be-
havior of PhABA. Two approaches were used to
disrupt the conjugation. The first approach in-
volved the synthesis of a new model compound, in
which N-phenyl of PhABA was placed by N-iso-
propyl. It is appealing that the emission behavior
of iPrABA differs much to that of PhABA. In the
second one, alkanols were introduced as the protic
Fig. 5. The maximal emission wavelength of PhABA in DEE as
a function of EtOHvolume fraction.

L.-H. Ma et al. / Chemical Physics Letters 372 (2003) 104–113
113
[10] A.B.J. Parusel, W. Rettig, W. Sudholt, J. Phys. Chem. A
106 (2002) 804.
solvent hydroxyl of which would form hydrogen
bond with nitrogen of N-phenyl derivatives. Dual
fluorescence was observed for both PhABA and
DPhABA but with different dependencies on alk-
anol steric structures. The described experiments
were designed counter evidently to ascertain that
ICT could be enhanced by N-phenyl amino con-
jugation, which was supplement to the active re-
searches on N-phenyl effect [28,33,34] and offered a
new hint for constructing CT-based fluorescent
chemosensors.
[11] J. Dobkowski, J. Wojcik, W. Kozminski, R. Kolos, J.
Waluk, J. Michl, J. Am. Chem. Soc. 124 (2002) 2406.
[12] L.R. Lin, Y.B. Jiang, Sci. China (Ser. B) 43 (2000) 295.
[13] W. Huang, X. Zhang, L.H. Ma, C.J. Wang, Y.B. Jiang,
Chem. Phys. Lett. 352 (2002) 401.
[14] S. Zilberg, Y. Haas, J. Phys. Chem. A 106 (2002) 1.
[15] H. Chen, Ph.D. thesis, Xiamen University, China, 1999.
[16] K.A. Zachariasse, M. Grobys, E. Tauer, Chem. Phys. Lett.
274 (1997) 372.
ꢂ
[17] I. Janic, M. Kakas, J. Mol. Struct. 114 (1984) 249.
[18] R. Sander, V. Stumpflen, J.H. Wendorff, A. Greiner,
ꢀ
€
Macromolecules 29 (1996) 7705.
[19] C.M. Whitaker, E.V. Patterson, K.L. Kott, R.J. McMa-
hon, J. Am. Chem. Soc. 118 (1996) 9966.
[20] Z.G. Liu, H. Nazare, Syn. Met. 111 (2000) 47.
[21] A. Yamamori, C. Adachi, T. Koyama, Y. Taniguchi, J.
Appl. Phys. 86 (1999) 4369.
Acknowledgements
The support of National Natural Science
Foundation of China (NSFC Grants Nos.
29975023 and 20175020) is gratefully acknowl-
edged.
[22] D. Kolosov, V. Adamovich, P. Djurovich, M.E. Thomp-
son, C. Adachi, J. Am. Chem. Soc. 124 (2002) 9945.
[23] S. Kapelle, W. Rettig, R. Lapouyade, Chem. Phys. Lett.
348 (2001) 416.
[24] S. Kapelle, W. Rettig, R. Lapouyade, Photochem. Photo-
biol. Sci. 1 (2002) 492.
References
[25] J.N. Demas, G.A. Crobys, J. Phys. Chem. 75 (1971)
991.
[1] E. Lippert, W. Luder, H. Boos, in: A. Mangini (Ed.),
Advances in Molecular Spectroscopy, Pergamon, Oxford,
1962, p. 443.
[26] M.J. Frisch et al., GAUSSIAN 94, Gaussian, Inc, Pitts-
burgh, PA, 1995.
[27] A.B.J. Parusel, W. Rettig, W. Sudholt, J. Phys. Chem. A
106 (2002) 804.
[2] Z.R. Grabowski, K. Rotkiewicz, A. Siemiarczuk, D.J.
Cowley, W. Baumann, Nouv. J. Chim. 3 (1979) 443.
[3] W. Rettig, Angew. Chem. Int. Ed. Engl. 25 (1986) 971.
[28] J.S. Yang, S.Y. Chiou, K.L. Liau, J. Am. Chem. Soc. 124
(2002) 2518.
€
[29] R.P. Subrayan, J.W. Kampf, P.G. Rasmussen, J. Org.
Chem. 59 (1994) 4341.
[4] Y. IlÕichev, W. Kuhnle, K.A. Zachariasse, J. Phys. Chem.
A. 102 (1998) 5670.
[5] C. Cornelissen, W. Rettig, J. Phys. Chem. A 102 (2000) 7754.
[6] W. Rettig, B. Zietz, Chem. Phys. Lett. 317 (2000) 187.
[7] C. Ma, W.M. Kwok, P. Matousek, A.W. Parker, D.
Phillips, W.T. Toner, M. Towrie, J. Phys. Chem. A 106
(2002) 3294.
[30] D.H. Waldeck, Chem. Rev. 91 (1991) 415.
[31] Y.H. Kim, D.W. Cho, M. Yoon, D. Kim, J. Phys. Chem.
100 (1996) 15670.
[32] Y.H. Kim, H.W. Cheon, M. Yoon, N.W. Song, D. Kim,
Chem. Phys. Lett. 264 (1997) 673.
[33] T. Verbiest, D.M. Burland, M.C. Jurich, V.Y. Lee, R.D.
Miller, W. Volksen, Science 268 (1995) 1604.
[34] S.R. Marder, J.W. Perry, Science 263 (1994) 1706.
[8] S. Dorairaj, H.J. Kim, J. Phys. Chem. A 106 (2002) 2322.
[9] M.M. Martin, P. Plaza, P. Changenet-Barret, A. Sie-
miarczuk, J. Phys. Chem. A 106 (2002) 2351.