Reversal of the intramolecular charge transfer in p-dimethylaminobenzanilides by amido anilino substitution

Article abstract of DOI:10.1021/jp026369n

p-Dimethylaminobenzanilides with a para or meta substituent at the amido anilino moiety were designed to generate a series of dual fluorescent molecules of variable electron acceptors. Ab initio calculations indicated that the anilino substitution did not

Full text of DOI:10.1021/jp026369n

12432
J. Phys. Chem. B 2002, 106, 12432-12440
Reversal of the Intramolecular Charge Transfer in p-Dimethylaminobenzanilides by Amido
Anilino Substitution
Xuan Zhang,† Chao-Jie Wang,‡ Li-Hong Liu,† and Yun-Bao Jiang*,†
Department of Chemistry and the MOE Key Laboratory of Analytical Sciences, Xiamen UniVersity,
Xiamen 361005, China, and Department of Chemistry and the State Key Laboratory for
Physical Chemistry of Solid Surfaces, Xiamen UniVersity, Xiamen 361005, China
ReceiVed: June 24, 2002; In Final Form: September 25, 2002
p-Dimethylaminobenzanilides with a para or meta substituent at the amido anilino moiety were designed to
generate a series of dual fluorescent molecules of variable electron acceptors. Ab initio calculations indicated
that the anilino substitution did not lead to an obvious change in the ground-state structures of the fluorophores,
1
and the H NMR signal of the amido -NH proton was found to experience a linear downfield shift with
increasing σ of the substitutent, supporting the fact that the amido anilino moiety was indeed varied comparably
by the substitution. The intramolecular charge transfer dual fluorescence was indeed observed in solvents
over a large polarity range from the nonpolar cyclohexane (CHX) through diethyl ether (DEE) and
tetrahydrofuran (THF) to highly polar acetonitrile (ACN). It was found that the CT emission shifted to the
blue with increasing electron-withdrawing ability of the amido anilino substituent up to a Hammett constant
σ of ca. +0.39 and to the red at higher σ. The solvent polarity variation did not change the σ value at which
the CT emission shift direction reverses. Similar variation profiles were also observed with the CT to LE
emission intensity ratio and the total fluorescence quantum yield. It was concluded that the CT direction in
p-dimethylaminobenzanilides was reversed by the amido anilino substitution, and the CT occurs from amido
anilino to benzoyl moiety at low σ, whereas at high σ, the CT switches to that from dimethylamino to the
benzanilide moiety, which was also supported from the Hartree-Fock calculations. This finding provides an
alternative method based on substitutent effect for identifying the charge-transfer direction in multiple charge-
transfer systems. The results suggested that the anilino group could be a much stronger electron donor than
an aliphatic amino group, which would be of use in designing electron donor substituted molecules. In both
cases the dependences of the CT emission energy against σ of the substituent at the amido aniline phenyl
ring were found to be much stronger than that in the ester counterparts of p-dimethylaminobenzanilides. This
interesting σ dependence in the CT emission would be of significance in developing new fluorescent sensors
based on electron donor/acceptor variations in p-dimethylaminobenzanilides.
1. Introduction
SCHEME 1: Molecular Structures of
p-Dimethylaminobenzamides (1), Phenyl
p-Dimethylaminobenzoates (2),
For understanding of the intramolecular charge transfer (CT)
photophysics of the DMABN-like dual fluorescent electron
donor/acceptor para-substituted benzenes, investigations of the
influence of the electron donor and/or acceptor have been an
important strategy.1-8 It has been shown that, although a
comparable variation in the dialkylamino electron donor group
by changing alkyl substituents is successful, with the electron
acceptor, replacing the cyano group with other electron-
withdrawn groups such as an ester and amide is not feasible
because of the accompanied change in the CT photophysics,
e.g., DMABN versus its ester derivative,2,6 and the steric effect.8
Rettig et al.8 have designed a series of p-dimethylaminobenza-
mides (1, Scheme 1) attempting to decrease the electron-
accepting ability of the acceptor by changing the amide group
from -C(O)NH2 through -C(O)N(CH3)2 to -C(O)N(C2H5)2.
Accordingly, they indeed observed a systematic blue shift of
p-Dimethylaminobenzanilides (3), and Benzanilides (4)a
a Substituents in 3 are p-CH3 (a), m-CH3 (b), H (c), p-F (d), p-Cl
(e), m-Cl (f), m-Br (g), p-CO2CH3 (h), p-COCH3 (i), and p-NO2 (j),
and those in 4 are p-OCH3 (a), p-CH3 (b), H (c), p-F (d), p-Cl (e),
p-CO2CH3 (f), p-COCH3 (g), and p-NO2 (h), respectively.
the CT emission and the decrease in the CT to LE intensity
ratio. Meanwhile, the accompanied steric contribution due to
the alkyl substitutions in the amide moiety that influenced the
conjugation of the amide moiety with the phenyl ring made it
difficult to abstract the real electronic polar effect. We have
* To whom correspondence should be addressed. Phone/Fax: +86 592
2185662. E-mail: ybjiang@xmu.edu.cn.
† Department of Chemistry and the MOE Key Laboratory of Analytical
Sciences, Xiamen University.
‡ Department of Chemistry and the State Key Laboratory for Physical
Chemistry of Solid Surfaces, Xiamen University.
10.1021/jp026369n CCC: $22.00 © 2002 American Chemical Society
Published on Web 11/13/2002

CT in p-Dimethylaminobenzanilides
J. Phys. Chem. B, Vol. 106, No. 48, 2002 12433
chosen an alternative way by designing phenyl p-dimethylami-
nobenzoates (2, Scheme 1) hoping to comparably vary the
electron acceptor by introducing a substituent at the ester phenyl
ring thereby avoiding the possible steric effect. Indeed, we
showed that the substituent did not affect the ground-state
structure of the ester and observed that the CT emission energy
varied linearly with the Hammett constant of the substituent.9
In that case, however, the substituent tuning efficiency was not
high with the Hammett linear slope value lower than 0.2. A
possible reason could be the lower conjugation extent between
the phenoxy oxygen atom with its phenyl ring as suggested by
a high dihedral angle of around 50°.9 In benzanilide, the
conjugation extent of the anilino nitrogen atom with the phenyl
ring could be much higher because a nearly planar structure
was shown for the aniline moiety.10 It was therefore expected
that in the amide counterparts of phenyl p-dimethylaminoben-
zoates, p-dimethylaminobenzanilides (3, Scheme 1), the amido
phenyl substituent might have a higher efficiency in influencing
the CT behavior if similar CT photophysics applies to these
two kinds of molecules.
With p-dimethylaminobenzanilides, however, an additional
CT reaction channel might exist as that with benzanilides (4, X
) H and other para substitutnets, Scheme 1), in which the CT
occurs from the anilino donor to the benzoyl acceptor.10-15 The
CT state of benzanilide (4) has also been suggested to be of the
twisted configuration10,12,15 as is done with p-dimethylami-
nobenzamides. In particular, the CT emission could even be
seen in a nonpolar solvent,10-15 which might suggest a low
activation energy of the CT process in 4. Therefore, with 3,
there could be two competitive charge-transfer processes.
Results with p-dimethylaminobenzamides (1) showed that,
with increasing electron donating strength of the amido moiety,
from -NH2 through -N(CH3)2 to -N(C2H5)2, which leads to
the decrease in the electron accepting ability of the benzamide
acceptor, the CT emission shifted to the blue together with a
decrease in the CT to LE intensity ratio, whereas the CT
direction remained unchanged as from the dimethylamino donor
to the benzamide acceptor.8 Similar charge transfer was also
assigned in a secondary amide derivative of 1 (-C(O)-
NHCH2C5H5N16) that has an amide moiety sterically similar to
1a. Electrochemically, the anilino group is a stronger electron
donor than the diethylamino group according to the oxidation
excitation wavelength was 305 nm. Fluorescence quantum yields
were measured using quinine sulfate as the standard (0.546 in
0.5 mol L-1 H2SO418). The absorption spectra were taken on a
Beckman DU-7400B or Varian Cary 300 absorption spectro-
photometer using a 1 cm quartz cell. IR spectra were taken on
a Nicolet FT-IR 360 with a KBr pellet, and 1H NMR data were
acquired in DMSO-d6 on a Varian Unity+ 500 MHz NMR
spectrometer using TMS as an internal reference.
All ab initio calculations were performed on a DELL PC
using the Gaussian 98 suit of programs.19a Ground-state
structures were fully optimized at the Hartree-Fock (HF) self-
consistent field level. The internally stored pseudopotentials and
LANL1DZ basis sets were used to all sorts of atoms in
p-dimethylaminobenzanilides (3, Scheme 1). The singlet excited-
state energies were calculated by means of the time-dependent
Hartree-Fock (TDHF)19b method on HF optimized ground-state
structures.
2.2. Materials. p-Aminobenzoic acid and substituented
anilines were purchased from Shanghai Chemical Reagents
Company, China National Chemicals Group (Shanghai, China)
and used for synthesis without further purification. p-Dimethyl-
aminobenzoic acid was prepared from the reaction of p-
aminobenzoic acid with dimethyl sulfate.20 Anilino-substituted
benzanilides (4, Scheme 1) were synthesized by procedures
described previously.15 Solvents for spectroscopic investigations
were purified before use and checked to have no fluorescent
impurity at the used excitation wavelength. All spectra were
measured at a sample concentration of ca.10-5 mol L-1
.
2.3. General Procedures for the Synthesis of p-Dimethyl-
aminobenzanilides (3). p-Dimethylaminobenzoic acid and the
corresponding substituted aniline were heated to melt or to reflux
in acetonitrile and to which POCl3 was dropped. After about
0.5 h, the residue was cooled to room temperature and washed
with dilute aqueous NaOH, and a yellow precipitate was
collected by filtration and was washed alternatively by dilute
HCl and NaOH. The crude products were subject to repeated
recrystallizations from acetone to give the desired compounds
in 50-80% overall yield based on p-dimethylaminobenzoic acid.
4-Dimethylamino-4′-methylbenzanilide (3a). IR (KBr, cm-1):
1
3354, 3037, 2921, 1639, 1606. H NMR (500 MHz, DMSO-
d6): δ (ppm) 2.265 (s, 3H), 2.996 (s, 6H), 6.751 (d, 2H, J ) 9
Hz), 7.116 (d, 2H, J ) 8 Hz), 7.635 (d, 2H, J ) 8.5 Hz), 7.850
(d, 2H, J ) 9 Hz), 9.776 (s, 1H).
potentials of aniline (Eox ) 0.96 V) and diethylamine (Eox
)
1.31 V).17 This suggests the possibility, but does not necessarily
mean, that the charge transfer with p-dimethylaminobenzanilides
(3) would be that which occurs with benzanilides (4, Scheme
1) from the amido anilino moiety to the dimethylaminobenzoyl
moiety, with the direction of the charge transfer with 3, hence,
remaining to be investigated.
4-Dimethylamino-3′-methylbenzanilide (3b). IR (KBr, cm-1):
1
3327, 3041, 2921, 1644, 1610. H NMR (500 MHz, DMSO-
d6): δ (ppm) 2.295 (s, 3H), 2.999 (s, 6H), 6.756 (d, 2H, J )
8.5 Hz), 6.866 (d, 1H, J ) 7 Hz), 7.190 (t, 1H, J ) 8 Hz),
7.553 (d, 1H, J ) 8 Hz), 7.602 (s, 1H), 7.857 (d, 2H, J ) 9
Hz), 9.774 (s, 1H).
In the present paper, we will show that, with the synthesized
series of p-dimethylaminobezanilides (3, Scheme 1) with a
substituent at the amido anilino moiety ranging from electron
donating to withdrawing, the CT is not unidirectional. We found
that, although the electron density at the amido nitrogen atom
indeed experienced a monotonic decrease with increasing
electron withdrawing ability of the substituent, the CT emission
shifted initially to the blue and then to the red when the
substituent was highly electron accepting. This observation
clearly indicated that the CT direction with 3 was reversed by
amido anilino substitution.
4-Dimethylaminobenzanilide (3c). IR (KBr, cm-1): 3315,
1
3038, 2916, 1634, 1604. H NMR (500 MHz, DMSO-d6): δ
(ppm) 3.003 (s, 6H), 6.761 (d, 2H, J ) 8.5 Hz), 7.045 (t, 1H,
J ) 7.5 Hz), 7.314 (t, 2H, J ) 8 Hz), 7.760 (d, 2H, J ) 8.5
Hz), 7.864 (d, 2H, J ) 9 Hz), 9.859 (s, 1H).
4-Dimethylamino-4′-fluorobenzanilide (3d). IR (KBr, cm-1):
1
3334, 3062, 2825, 1650, 1608. H NMR (500 MHz, DMSO-
d6): δ (ppm) 3.000 (s, 6H), 6.759 (d, 2H, J ) 9 Hz), 7.156 (t,
2H, J ) 9 Hz), 7.757-7.785 (m, 2H), 7.855 (d, 2H, J ) 8.5
Hz), 9.916 (s, 1H).
4-Dimethylamino-4′-chlorobenzanilide (3e). IR (KBr, cm-1):
1
2. Experimental Section
3344, 3093, 2923, 1653, 1608. H NMR (500 MHz, DMSO-
2.1. Experimental Techniques. Corrected fluorescence
spectra were recorded on a Hitachi F-4500 fluorescence spec-
trophotometer using excitation and emission slits of 5 nm. The
d6): δ (ppm) 3.003 (s, 6H), 6.760 (d, 2H, J ) 9 Hz), 7.370 (d,
2H, J ) 9 Hz), 7.805 (d, 2H, J ) 9 Hz), 7.855 (d, 2H, J ) 9
Hz), 9.985 (s, 1H).

12434 J. Phys. Chem. B, Vol. 106, No. 48, 2002
Zhang et al.
TABLE 1: Calculated Dihedral Angles and Bond Lengths of
3 in the Ground Statea
SCHEME 2: Selected Dihedral Angels
dihedral angels, °b
bond lengths, Å
θ1 θ2 θ3 θ4 θ5 θ6 C(O)-N CdO Ph-N Ph-C(O)
3a 1.3 2.5 176.7 172.3 20.2 17.5 1.370 1.234 1.420 1.490
3b 1.6 2.1 176.6 172.3 20.1 17.5 1.371 1.237 1.419 1.490
3c 1.3 2.3 176.7 172.3 20.1 17.4 1.372 1.237 1.419 1.489
3d 1.2 2.5 176.8 172.4 19.5 16.9 1.373 1.237 1.418 1.488
3e 0.8 2.6 176.7 172.6 18.7 16.2 1.374 1.236 1.416 1.487
3f 0.9 2.5 176.8 172.3 19.2 16.3 1.376 1.235 1.414 1.487
3g 1.2 2.3 176.8 172.5 19.2 16.5 1.375 1.236 1.415 1.487
3h 1.6 1.9 176.4 172.2 19.5 16.8 1.377 1.235 1.411 1.488
3i 1.8 1.8 176.5 172.2 19.4 16.7 1.377 1.235 1.411 1.487
3j 1.6 1.8 176.4 172.2 18.6 15.9 1.382 1.234 1.405 1.484
a Molecular structures of 3 shown in Scheme 1. b Dihedral angels
defined in Scheme 2.
4-Dimethylamino-3′-chlorobenzanilide (3f). IR (KBr, cm-1):
1
3322, 3060, 2893, 1645, 1612. H NMR (500 MHz, DMSO-
d6): δ (ppm) 3.007 (s, 6H), 6.767 (d, 2H, J ) 8.5 Hz), 7.098
(d, J ) 8.5 Hz, 1H), 7.344 (t, 1H, J ) 8.5 Hz), 7.699 (d, 1H,
J ) 8 Hz), 7.860 (d, 2H, J ) 9 Hz), 7.971 (s, 1H), 10.012 (s,
1H).
4-Dimethylamino-3′-bromobenzanilide (3g). IR (KBr, cm-1):
1
3321, 3070, 2892, 1644, 1610. H NMR (500 MHz, DMSO-
d6): δ (ppm) 3.006 (s, 6H), 6.765 (d, 2H, J ) 9 Hz), 7.230 (d,
1H, J ) 8 Hz), 7.284 (t, 1H, J ) 8 Hz), 7.749 (d, 1H, J ) 7.5
Hz), 7.858 (d, 2H, J ) 8.5 Hz), 8.105 (s, 1H), 9.998 (s, 1H).
4-Dimethylamino-4′-methoxycarbonylbenzanilide (3h). IR
1
(KBr, cm-1): 3336, 3078, 2946, 1710, 1654, 1608. H NMR
(500 MHz, DMSO-d6): δ (ppm) 3.012 (s, 6H), 3.830 (s, 3H),
6.772 (d, 2H, J ) 9.5 Hz), 7.882 (d, 2H, J ) 9 Hz), 7.933 (s,
4H), 10.181 (s, 1H).
1
Figure 1. Linear dependence of the H NMR chemical shift of the
amido -NH proton against the Hammett constant21 of the substituent
at the amido aniline phenyl ring in p-dimethylaminobenzanilides. The
substituents are (a) p-CH3, (b) m-CH3, (c) H, (d) p-F, (e) p-Cl, (f) m-Cl,
(g) m-Br, (h) p-CO2CH3, (i) p-COCH3, and (j) p-NO2, respectively.
The NMR spectra were taken in DMSO-d6 using tetramethylsilicate
(TMS) as internal standard.
4-Dimethylamino-4′-methylcarbonylbenzanilide (3i). IR (KBr,
1
cm-1): 3340, 3093, 2921, 1665, 1654, 1605. H NMR (500
MHz, DMSO-d6): δ (ppm) 2.539 (s, 3H), 3.013 (s, 6H), 6.774
(d, 2H, J ) 8.5 Hz), 7.889 (d, 2H, J ) 9 Hz), 7.939 (s, 4H),
10.172 (s, 1H).
4-Dimethylamino-4′-nitrobenzanilide (3j). IR (KBr, cm-1):
not undergo obvious variation within the substitution series. This
means that the substituent effect, if any, would originate from
the substituent electron polar effect.
The 1H NMR signals of the amido -NH protons were
monitored and correlated to the Hammett substituent constant,
σ.21 We observed that, with increasing electron-withdrawing
1
3401, 3087, 2910, 1651, 1604. H NMR (500 MHz, DMSO-
d6): δ (ppm) 3.021 (s, 6H), 6.784 (d, 2H, J ) 8.5 Hz), 7.899
(d, 2H, J ) 8.5 Hz), 8.058 (d, 2H, J ) 9 Hz), 8.239 (d, 2H, J
) 9 Hz), 10.432 (s, 1H).
3. Results and Discussion
1
ability of the substituent, the H NMR signal experienced a
continuous downfield shift, which was indicated by a nice linear
dependence of the chemical shift against σ with a slope of
+0.66, Figure 1. This means that a monotonic decrease in the
electron density at the amido -NH nitrogen atom occurs with
increasing electron-withdrawing ability of the substituent.
Therefore, in case that the CT in 3 is identified as that in 1, the
electron acceptors in 3 are indeed varied comparably by the
amido anilino substitution.
3.2. Absorption and Fluorescence Spectra. The absorption
spectra of p-dimethylaminobenzanilides (3, Scheme 1) in
cyclohexane (CHX), diethyl ether (DEE), tetrahydrofuran (THF),
and acetonitrile (ACN) of increasing polarity were recorded and
parts of the spectral parameters were summarized in Table 2. It
was found from Table 2 that the absorption spectra of all of the
10 p-dimethylaminobenzanilide derivatives (3a-3j) had a main
intense band peaked at 306-330 nm that slightly shifted to the
red with increasing solvent polarity. The observed molar
absorption coefficients at the order of magnitude of 104 mol-1
L cm-1 (Table 2) were indicative of the (π, π*) transition
3.1. Ground-State Structures of p-Dimethylaminobenza-
nilides (3). In p-dimethylaminobenzanilides (3a-3j, Scheme
1), the substituent was introduced in the amido anilino phenyl
ring at the para or meta position but not at the ortho position to
avoid the possible steric effect. To confirm that no substantial
steric effect was introduced by the substitution, the ground-
state structures of 3 were optimized by the ab initio method.
The calculations indicated that 3c in the trans conformation was
lower for ca.3.8 kcal mol-1 in energy than that in the cis
conformation at Hartree-Fock (HF) level which included the
zero-point energy correction. The following calculations were
therefore only carried out on the trans amides. The calculated
ground-state conformations of 3 were almost planar at the amido
anilino moiety as seen from the dihedral angels θ1, θ2, θ3, and
θ4 (Scheme 2 and Table 1) and have large amide benzoyl
dihedral angels around 20° (θ5 and θ6, Scheme 2 and Table 1).
It was seen from Table 1 that the amido anilino substitution
did not lead to discernible changes in the dihedral angels (<2°)
and bond lengths, revealing that the ground-state structures did

CT in p-Dimethylaminobenzanilides
J. Phys. Chem. B, Vol. 106, No. 48, 2002 12435
TABLE 2: Absorption and Fluorescence Spectral Data for 3
in Organic Solvents
absorption b
fluorescence
λmax
,
ꢀ, 104
λ
LE/λCT,
nm
X
solventa
nm
L mol-1 cm-1
Φd
3a p-CH3
CHX
DEE
THF
ACN
CHX
DEE
THF
ACN
CHX
DEE
THF
ACN
CHX
DEE
THF
ACN
CHX
DEE
THF
ACN
CHX
DEE
THF
ACN
CHX
DEE
THF
ACN
306
2.15
3.39
3.02
2.62
2.56
3.35
2.96
3.47
3.52
4.06
3.73
3.79
2.74
3.50
3.31
3.31
3.91
4.23
4.22
4.17
3.08
3.60
3.70
3.83
3.84
4.78
4.52
4.42
4.93
5.53
5.27
5.65
3.07
3.75
3.97
3.75
340/465 0.0015
345/518 0.0034
351/532 0.0028
364/502 0.0027
340/452 0.0013
342/498 0.0048
353/518 0.0044
367/502 0.0063
338/448 0.0011
344/494 0.0036
352/500 0.0061
367/494 0.0107
340/452 0.0011
345/500 0.0034
354/504 0.0053
367/492 0.0104
338/435 0.0011
346/481 0.0050
356/490 0.0101
370/498 0.0171
307
311
312
306
306
311
314
307
307
311
314
307
307
311
314
309
310
315
317
310
311
314
318
310
312
317
318
318
319
324
326
321
323
328
331
3b m-CH3
3c H
3d p-F
Figure 2. Plots of the absorption maximum of 3 against the Hammett
constant of the substituent at the amido anilino phenyl ring. Solvent:
CHX, cyclohexane; DEE, diethyl ether; THF, tetrahydrofuran; and
ACN, acetonitrile.
3e p-Cl
3f m-Cl
3g m-Br
338/-
0.0031
348/445 0.0103
358/455 0.0263
373/496 0.0319
338/-
0.0033
348/445 0.0108
358/455 0.0256
373/496 0.0367
3h p-CO2CH3 CHX
343/-
356/-
0.0148
0.0512
DEE
THF
ACN
386/470 0.1984
370/516 0.0209
∼345 c/- 0.0006
356/470 0.0010
368/468 0.0158
378/550 0.0017
3i p-COCH3 CHX
DEE
THF
ACN
CHX
DEE
THF
ACN
3j p-NO2
337 (278) 1.10 (0.42) ∼360 c/- <0.0001
345 (283) 2.85 (1.18) 388/536 0.0006
353 (290) 2.60 (1.35) 400/-
354 (294) 2.46 (1.28) 430/-
0.0003
0.0002
Figure 3. Normalized fluorescence spectra of 3a in solvents of varied
polarity. Solvent: CHX, cyclohexane; DEE, diethyl ether; THF,
tetrahydrofuran; and AAE, ethyl acetate.
a Solvent: CHX, cyclohexane; DEE, diethyl ether; THF, tetrahy-
drofuran; ACN, acetonitrile. b Parameters of the second absorption band
are given in the parentheses. c Uncertainty due to poor solubility.
d Fluorescence quantum yield.
transfer character of the emissive state. The dual fluorescence
was hence assigned to the LE and the CT states,1-3,8,9
respectively. It was found, however, that with part of the
p-dimethylaminobenzanilides 3 dual fluorescence could be
observed even in nonpolar solvent cyclohexane (Figure 4a),
whereas with p-dimethylaminobenzamides, no such dual fluo-
rescence but only the LE emission was shown in the nonpolar
solvent,8 suggesting that the involved CT process with the
former molecules would have a very low activation energy. Most
strikingly, it was found in Figure 4 that the long-wavelength
CT emission shifted to the blue when the amido moiety became
increasingly electron-withdrawing as the substituent on the
amido aniline became more electron demanding from para-
methyl (3a) to meta-Cl (Br) (3f and 3g), suggesting that the
substituent is located at the electron donor moiety. With 3h-
3j, no CT-like long-wavelength emission was observed in the
nonpolar solvent, whereas in polar solvents such as DEE and
THF, the long-wavelength CT emission turned to shift to the
red with increasing electron withdrawing ability of the sub-
stituent as expected in the cases of p-dimethylaminobenzamides
18 and in 2, the ester counterparts of 3 (Scheme 1).9 In Figure
5, the CT emission energies were plotted against the Hammett
substituent constants in which a clear turn was found at
character. Within the molecule series of 3a-3j, the absorption
spectrum maximum in each solvent was found to remain nearly
constant for 3a-3d and underwent a very slight red-shift until
3g with a substituent σ of +0.39, which was followed by a
substantial red-shift at higher σ (Table 2). This is clearly
indicated in Figure 2 in which the absorption maximum was
plotted against the Hammett substituent constant. The substantial
red-shift in the absorption maximum with increasing σ of higher
than +0.39 suggests that a ground-state charge transfer toward
the substituted amido anilino moiety in 3 might occur. At σ
lower than +0.39, there was practically no ground-state charge
transfer as the absorption maximum remained almost unchanged
with amido anilino substitution.
The fluorescence spectra were therefore recorded. Indeed, as
expected for the p-dimethylaminobenzamide derivatives (1,
Scheme 1), dual fluorescence was observed for p-dimethylami-
nobenzanilides (3, Scheme 1) in most cases, see Table 2 and
Figures 3 and 4. In Figure 3, it was shown as an example that,
although the short-wavelength emission of 3a underwent minor
red-shift, the long-wavelength emission shifted dramatically to
the red with increasing solvent polarity, pointing to the charge-

12436 J. Phys. Chem. B, Vol. 106, No. 48, 2002
Zhang et al.
TABLE 3: TDHF Calculated Singlet Excited-State Energy,
Absorption Maximum Wavelength, Oscillator Strength, and
Transition Compositions
∆E,
λcalc.
,
λscaled,
eVa
nmb
nmc
fd
nøm(%)e
3a S1 5.3925 230
S2 5.6361 220
S3 5.8013 214
3b S1 5.3996 230
S2 5.6371 220
S3 5.8918 210
3c S1 5.3941 230
S2 5.6364 220
S3 5.8941 210
3d S1 5.3823 230
S2 5.6335 220
S3 5.8854 211
3e S1 5.3646 231
S2 5.6316 220
S3 5.8727 211
3f S1 5.3685 231
S2 5.6327 220
S3 5.9119 210
3g S1 5.3696 231
S2 5.6330 220
S3 5.8828 211
3h S1 5.2859 235
S2 5.5707 223
S3 5.6451 220
3i S1 4.6060 269
S2 5.2513 236
S3 5.5173 225
3j S1 4.2843 289
S2 4.6934 264
S3 5.0842 244
310
297
288
310
297
283
310
297
283
310
297
284
311
297
284
311
297
283
311
297
284
317
301
297
363
318
303
390
356
329
0.6660 0ø0(69)
0.0660 1ø0(52), 0ø3(23), 1ø1(21)
0.0485 2ø1(36), 2ø0(30), 3ø2(19)
0.6109 0ø0(73)
0.0586 1ø0(62), 0ø3(23)
0.0499 2ø2(30), 0ø1(26), 3ø1(21)
0.6177 0ø0(75)
0.0593 1ø0(64), 0ø3(23)
0.0691 0ø1(26), 2ø2(21), 3ø1(16)
0.6051 0ø0(78)
0.0542 2ø0(69), 0ø2(14)
0.0492 1ø1(63), 1ø0(12)
0.6858 0ø0(79)
0.0564 2ø0(65), 0ø3(22)
0.0467 1ø1(53), 3ø2(17)
0.6434 0ø0(74)
0.0530 1ø0(66), 0ø3(22)
0.0619 0ø5(28), 0ø1(22), 2ø2(17)
0.6492 0ø0(76)
0.0540 2ø0(43), 1ø0(24), 0ø3(22)
0.0697 1ø1(26), 0ø1(21), 3ø2(12)
0.9988 0ø0(50), 1ø0(19)
0.0842 0ø1(43), 1ø0(30)
0.0058 2ø0(62), 1ø3(15)
0.0001 0ø4(46), 1ø4(22), 4ø4(22)
1.0075 0ø0(43), 0ø1(22), 1ø0(16)
0.0286 0ø1(43), 1ø0(35)
0.0000 0ø8(75), 4ø8(16)
0.0005 0ø9(76), 4ø9(15)
1.0275 0ø1(58), 1ø0(18), 0ø0(16)
a Singlet excited-state energy. b Singlet excited-state energy given
in wavelength. c Scaled by a factor of 1.35. d Oscillator strength. e The
percentage of a transition from the (HOMO - m)th orbital to the
(LUMO + n)th orbital.
Figure 4. Normalized fluorescence spectra of 3 in (a) cyclohexane
and (b) diethyl ether. The substituents in 3 are indicated in the inset.
higher σ, it turns out to be the p-dimethylaminobenzamide-like
charge transfer from dimethylamino group to the rest of the
molecule as assumed for 1.1,2,8 Therefore, the present finding
on the substituent effect provides an alternative method in
identifying the charge-transfer direction in a system where
multiple charge-transfer possibilities exist. It is also of signifi-
cance to note that the charge transfer in p-dimethylaminoben-
zanilides 3a-3e even with an electron-withdrawing substituent
at the amido anilino group (p-F and p-Cl in 3d and 3e,
respectively) is not that which occurred in p-dimethylaminoben-
zamides (1), suggesting that the anilino group is indeed a
stronger electron donor than the aliphatic amino groups. Similar
observations were also made in several other systems,22-24
which would be of use in designing electron donor/acceptor
substituted charge-transfer molecules.
3.3. Switching of the Intramolecular Charge Transfer.
Previously, we have shown that the charge-transfer emission
energy, hνmax(CT), varied linearly with the Hammett constant
of the substituent in the electron acceptor or donor of substituted-
phenyl p-dimethylaminobenzoates9 and benzanilides,15 eq 1,
provided that the singlet excited-state energy, E0,0, and the
ground-state repulsion energy, δErep, or their difference are
constant within the molecule series. In eq 1, R and T are the
gas constant and absolute temperature, respectively, and F is
the reaction constant as defined in the classic Hammett linear
free energy formula:
Figure 5. Plots of the CT emission energy versus the Hammett
substituent constant.
σ of around +0.39. No such turn was found with benzanilides
4 without the dimethylamino group (Scheme 1) whose CT
emission in CHX was found to shift to the blue with increasing
electron-withdrawing ability of the substituent,15b and a linear
dependence was found between the CT emission energy and
the Hammett substituent constant with a slope of +0.41 eV and
an intercept of 2.46 (γ ) 0.9364, n ) 7). It was thus clear that
at a σ of +0.39 the direction of charge transfer in p-
dimethylaminobenzanilides 3 was reversed. At a σ lower than
+0.39, the charge transfer with 3 was the benzanilide-like charge
transfer from the aniline to benzoyl moiety,10-15 whereas at
hνmax(CT) ) -2.303RTFσ + E0,0 - δErep + constant (1)
With 3 at a σ lower than +0.39, it was observed that the

CT in p-Dimethylaminobenzanilides
J. Phys. Chem. B, Vol. 106, No. 48, 2002 12437
SCHEME 3: MOLDEN25 Plots of the HOMO and LUMO Orbitals of 3
absorption maximum and the LE emission energy (Table 2 and
Figure 2) were nearly constant within the 3a-3e series,
suggesting that the singlet excited-state energy is almost
constant. δErep could be reasonably assumed to be constant for
3a-3e because of their similar structure. Equation 1 thus holds
for 3a-3e. We found from Figure 5 that in CHX, DEE, and
THF, the linear slopes were 0.41, 0.40, and 0.49 eV, respec-
tively, which are also close to that found for 4 in CHX (0.41
eV), indicating that the reaction constant F is practically
independent of solvent polarity. This means that, with 3a-3e
in these solvents of increasing polarity, only the benzanilide-
like CT reaction occurs, without the contribution of the other,
the p-dimethylaminobenzamide-like, CT reaction. In the highly
polar solvent ACN, however, the corresponding linear slope of
0.07 eV is much lower. As in a more polar solvent, the charge
transfer is enhanced, which would lead to a higher linear slope;15
the present observation of a much lower slope in ACN would
mean that the benzanilide-like CT reaction in 3a-3e is mixed
with the p-dimethylaminobenzamide-like CT reaction, which
was further supported by the fact that in several cases the CT
emission in ACN was more blue-shifted than that in less polar
solvent THF, because the p-dimethylaminobenzamide-like CT
emission8 locates at a shorter wavelength than the benzanilide-
like CT emission.15

12438 J. Phys. Chem. B, Vol. 106, No. 48, 2002
Zhang et al.
Figure 6. Variations of (a) the CT to LE emission intensity ratio and (b) the total fluorescence quantum yield with substituent constant.
With 3h-3j, the p-dimethylaminobenzamide-like CT reaction
occurred (Figure 5). A credible analysis of the Hammett constant
dependence of the CT emission energy according to eq 1 was
not possible because of the limited data points and the variation
of E0,0 with the substituent (see the data shown in Figure 2 and
Table 2). The apparent dependence (Figure 5), however, is
higher than those of their ester counterparts 2 in which linear
dependences were found with slope values lower than 0.2.9 In
3h-3j, the electron acceptors (benzanilide moieties) were
comparable as shown with 2,9 because the substituent at the
amido anilino phenyl ring of 3h-3j hardly introduced any steric
effect whereas it changed the electron-withdrawing ability but
not the identity of the benzamide acceptor.
To understand the reversal of the charge transfer direction,
the electronic structures of the vertically (Franck-Condon, FC)
excited singlet states of 3 in the gas phase were investigated by
means of TDHF calculations. The calculated transition energies
from the FC ground state to the FC excited state are reported
in Table 3. The TDHF calculated transition energies were seen
to decrease with increasing electron-withdrawing ability of the
substituent at the amido anilino moiety, which is in general in
agreement with the experimental observation of a red-shift in
the absorption spectra (Table 2). The TDHF calculated values
correspond to the (0,0) transitions and thus are at higher energies
than the experimental values.10c Scaling the calculated absorption
maxima by a factor of 1.35 provides values that are in good
agreement with the experimental data obtained in nonpolar
solvent cyclohexane, see Tables 2 and 3. It was found from
Table 3 that, with only several exceptions in 3i and 3j, the most
allowed transition indexed by the highest oscillator strength was
that from the S0 to S1 states and corresponded mainly to the
transition from HOMO to LUMO as suggested by the highest
percentage value of 0ø0. The π-orbital characters of HOMO and
LUMO, however, were found to undergo interesting variations
along the 3a-3j variations, see Scheme 3. It was seen in Scheme
3 that the π electron in the HOMO orbital of 3a was delocalized
in the whole molecule and became increasingly localized on
the dimethylaminobenzoyl moiety with increasing electron-
withdrawing ability of the substituent (3b-3e) until completely
localized on the dimethylaminobenzoyl moiety (3f-3j). Mean-
while, the π electron in the LUMO orbital, despite delocalized
but with higher density at the dimethylaminobenzoyl moiety in
3a, became increasingly localized on the amido anilino moiety.
It thus follows that upon transition from HOMO to LUMO,
charge transfer occurs from the amido anilino moiety to the
the dimethylaminobenzoyl moiety with 3a and gradually
changes to that from dimethylaminobenzoyl to the amido anilino
moiety with, for example, 3f-3j. The turn in the CT direction
occurs around 3f and 3g (Scheme 3). This is in good agreement
with the observations made in the substituent dependence of
the CT emission. It thus appears from the calculations that an
important reason for the CT direction reversal is the thermo-

CT in p-Dimethylaminobenzanilides
J. Phys. Chem. B, Vol. 106, No. 48, 2002 12439
dynamics of the CT process. This is also in agreement with the
experimental observations made in nonpolar to moderately polar
solvents (CHX, DEE, and THF) in which the CT reaction
identity remained unchanged. In a highly polar solvent such as
ACN, the kinetic factor seems to start to contribute to govern
the CT process, Figure 5.
nobenzamide-like CT reaction occurred, the substitutent constant
dependence of the CT emission energy of 3 was found, as
expected, to be higher than that in their ester counterparts 2.9
The observation that with the majority of 3 the charge transfer
is in the opposite direction to that in 1, even when the amido
anilino phenyl ring in 3 is substituted by an electron withdrawing
substituent, points to the difference of aniline as an electron
donor against alkylamine that aniline is actually a much stronger
electron donor. This would be of use in designing electron
donor/acceptor substituted CT sensing systems.
p-Dimethylaminobenzanilides are dual fluorescent, and the
CT emission wavelength, the total fluorescence quantum yield,
and the CT to LE emission intensity ratio undergo interesting
variations when the amido anilino substituent is varied, which
indicates that the 3-based dual fluorescent chemosensors could
be constructed. The CT direction reversal behavior in 3 suggests
that it could be an alternative framework for constructing D-A-
D′ balance-like fluorescence sensors.26,27
p-Dimethylaminobenzanilides (3) would thus stand as an
alternative framework for designing D-A-D “balance”-like
fluorescence chemosensors.26,27 Indeed, the total fluorescence
quantum yield and the CT to LE intensity ratio of 3, in addition
to the CT emission wavelength (Figure 5), underwent a similar
switch along with the substituent variations (Figure 6), which
could be of use in designing 3-based sensors based on electron
donating/accepting ability variations. In this regard, it is
important to point out that p-dimethylaminobenzanilides are dual
fluorescent with strong LE emission in both CT reactions, which
could be seen from a comparison of parts a and b of Figure 6.
With benzanilides, however, it is known that the LE emission
is very weak, and that even up to now doubt still exists that it
is due to impurity or photoproducts.10-15 It is also worth noting
in Figure 6b that the quantum yields in nonpolar solvent CHX
are lower than those in polar solvents, and at the lower σ range
when the benzanilide-like CT occurs, the total quantum yield
increases with increasing σ. This is an indication that the (n,π*)
transition is one of the nonradiative deactivation channels of
the benzanilide derivatives.8,10
Acknowledgment. We thank the National Natural Science
Foundation of China (Grant Nos. 29975023 and 20175020), the
Ministry of Education (MOE) of China, and the Natural Science
Foundation of Fujian Province for support of this work. The
Varian Cary 300 UV-vis absorption spectrophotometer was
purchased under the support of the German Volkswagenstiftung
(Grant No. I/77 072).
References and Notes
4. Conclusions
(1) Grabowski, Z. R.; Rotkiewicz, K.; Siemiarczuk, A.; Cowley, D.
J.; Baumann, W. NouV. J. Chim. 1979, 3, 443.
(2) Rettig, W. Angew. Chem., Int. Ed. Engl. 1986, 25, 971.
(3) Zachariasse, K. A.; von der Haar, T.; Hebecker, A.; Leinhos, U.;
Ku¨hnle, W. Pure Appl. Chem. 1993, 65, 1745.
(4) Schuddeboom, W.; Jonker, S. A.; Warman, J. M.; Leinhos, U.;
Ku¨hnle, W.; Zachariasse, K. A. J. Phys. Chem. 1992, 96, 10809.
(5) Il′chev, Y.; Ku¨hnle, W.; Zachariasse, K. A. J. Phys. Chem. A 1998,
102, 5670.
(6) Rettig, W.; Zietz, B. Chem. Phys. Lett. 2000, 317, 187.
(7) Dobkowski, J.; Wo´jcik, J.; Koz´min´ski, W.; Kolos, R.; Waluk, J.;
Michl J. J Am. Chem. Soc., 2002, 124, 2406.
(8) Braun, D.; Rettig, W.; Delmond, S.; Le´tard, J.-F.; Lapouyade, R.
J. Phys. Chem. A 1997, 101, 6836.
(9) Huang, W.; Zhang, X.; Ma, L.-H.; Wang, C.-J.; Jiang, Y.-B. Chem.
Phys. Lett. 2002, 352, 401.
(10) (a) Lewis, F. D.; Long, T. M. J. Phys. Chem. A 1998, 102,
5327. (b) Lewis, F. D.; Liu, W. Z. J. Phys. Chem. A 1999, 103, 9678. (c)
Lewis, F. D.; Liu, W. Z. J. Phys. Chem. A 2002, 106, 1976.
(11) (a) Tang, G.-Q.; MacInnis, J.; Kasha, M. J. Am. Chem. Soc. 1987,
109, 2531. (b) Heldt, J.; Gormin, D.; Kasha, M. J. Am. Chem. Soc. 1988,
110, 8255. (c) Heldt, J.; Gormin, D.; Kasha, M. Chem. Phys. Lett. 1988,
150, 433. (d) Heldt, J.; Gormin, D.; Kasha, M. Chem. Phys. 1989, 136,
321.
(12) Azumaya, I.; Kagechika, H.; Fujiwara, Y.; Itoh, M.; Yamaguchi,
K.; Shudo, K. J. Am. Chem. Soc. 1991, 113, 2833.
(13) (a) Heldt, J.; Heldt, J. R.; Szatan, E. J. J. Photochem. Photobiol.
A: Chem. 1999, 121, 91. (b) Brozis, M.; Heldt, J.; Heldt, J. R. J. Photochem.
Photobiol. A: Chem. 1999, 128, 39.
(14) (a) Lucht, S.; Stumpe, J.; Rutloh, M. J. Fluorescence 1998, 8, 153.
(b) Lucht, S.; Stumpe, J. J. Lumin. 2000, 91, 203.
(15) (a) Zhang, X.; Sun, X.-Y.; Wang, C.-J.; Jiang, Y.-B. J. Phys. Chem.
A 2002, 106, 5577. (b) Zhang, X.; Ma, L.-H.; Sun, X.-Y.; Wang, C.-J.;
Wu, F.-Y.; Li, Z.; Jiang, Y.-B. Chem. J. Chin. UniVersity 2002, 23,
1701.
The intramolecular charge-transfer dual fluorescence of
p-dimethylaminobenzanilides (3) in a variety of solvents was
investigated. Compound 3 represent a nice combination of
p-dimethylaminobenzamide (1) and benzanilide (4) that emit
dual fluorescence but with charge-transfer reactions of different
direction. The 1H NMR chemical shift of the amido -NH proton
was found to shift linearly to downfield with increasing
substituent constant, and ab initio calculations indicated that
the para or meta substitution at the amido anilino moiety did
not result in an obvious change in the ground-state structure of
p-dimethylaminobenzanilides (3). This points to the comparable
variation of the amido moiety in 3 by the substitution. With 3,
the subsituent dependence in the dual fluorescence in a variety
of solvents, however, clearly indicated that the charge-transfer
direction was reversed by substituent variation, which was found
in the plots of the CT emission energy against the Hammett
substituent constant that showed a turn at a σ of +0.39. In 2,
the ester counterparts of 3, and in 4, the derivatives of 3 without
dimethylamino group, a single linear correlation was found along
with similar substituent variation. It was concluded that the CT
direction in p-dimethylaminobenzanilides (3) was from the
amido anilino moiety to the dimthylaminobenzoyl moiety (the
benzanilide-like) when the amido anilino substituent had lower
σ, whereas with a substituent of higher σ, the CT in 3 turned
out to be p-dimethylaminobenzamide-like from the dimethyl-
amino electron donor. Ab initio calculations supported this
conclusion. Therefore, the substituent effect served as an
alternative method in judging the charge-transfer direction in
case that multiple charge-transfer possibilities exist.
(16) Malval, J.-P.; Lapouyade, R. HelV. Chim. Acta 2001, 84,
2439.
In the lower σ range, the substituent constant dependence of
the CT emission energy of 3 in CHX, DEE, and THF was found
to be similar to that of 4 in CHX, indicating that in nonpolar to
moderately polar solvents the benzanilide-like CT reaction in
3 was not mixed by the other CT reaction. In a highly polar
solvent, the benzanilide-like CT reaction was shown to be mixed
by the other CT reaction. In the case when the p-dimethylami-
(17) (a) Jacques, P.; Allonas, X.; Dossot, M. J. Photochem. Photobiol.,
A: Chem. 2001, 142, 91. (b) Jacques, P.; Allonas, X. Chem. Phys. Lett.
1995, 233, 533.
(18) Demas, J. N.; Crobys, G. A. J. Phys. Chem. 1971, 75, 991.
(19) (a) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.,
Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.;

12440 J. Phys. Chem. B, Vol. 106, No. 48, 2002
Zhang et al.
Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.;
Malick,D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-
Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe,
M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J.
L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98; Gaussian,
Inc.: Pittsburgh, PA, 1998.
(22) Demeter, A.; Be´rces, T.; Zachariasse, K. A. J. Phys. Chem. A 2001,
105, 4611.
(23) Kapelle, S.; Rettig, W.; Lapouyade, R. Chem. Phys. Lett. 2001,
348, 416.
(24) Yang, J.-S.; Chiou, S.-Y.; Liau, K.-L. J. Am. Chem. Soc. 2002,
124, 2518.
(25) For details of MOLDEN, the reader is directed to http://www.
cmbi.kun.nl/∼schaft /molden/molden.html.
(26) Crochet, P.; Malval, J.-P.; Lapouyade, R. Chem. Commun. 2000,
(20) Jiang, Y.-B. J. Photochem. Photobiol. A: Chem. 1994, 78,
125.
289.
(27) Rurack, K.; Rettig, W.; Resch-Genger, U. Chem. Commun. 2000,
(21) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1997, 91, 1165.
407.